STUDY ON SURFACE ENERGY PARAMETERS AND MORPHOLOGY …

九州大学学術情報リポジトリKyushu University Institutional Repository

STUDY ON SURFACE ENERGY PARAMETERS ANDMORPHOLOGY OF PROMISING ADSORBENT MATERIALS

エム, エル, パラシュ

http://hdl.handle.net/2324/4110543

出版情報：九州大学, 2020, 博士（学術）, 課程博士バージョン：権利関係：

STUDY ON SURFACE ENERGY

PARAMETERS AND MORPHOLOGY OF

PROMISING ADSORBENT MATERIALS

Dissertation

Doctor of Philosophy

by

M L Palash

M. Eng. (KU, Japan), M. Sc. (DU, Bangladesh)

Department of Energy and Environmental Engineering

Interdisciplinary Graduate School of Engineering Sciences

Kyushu University

Japan

May 2020

STUDY ON SURFACE ENERGY PARAMETERS

AND MORPHOLOGY OF PROMISING

ADSORBENT MATERIALS

A dissertation submitted in partial fulfillment of the requirements for

the award of the degree of

Doctor of Philosophy

by

M L Palash

M. Eng. (KU, Japan), M. Sc. (DU, Bangladesh)

Supervisor: Professor Bidyut Baran Saha

Department of Energy and Environmental Engineering

Interdisciplinary Graduate School of Engineering Sciences

Kyushu University

Japan

May 2020

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Summary

The demand for adsorption technologies is rapidly rising in various applications due to

its applicability in utilizing low-temperature heat sources (industrial waste heat or solar heat)

and environmentally friendly refrigerators (H2O, CO2, NH3, CH4O, etc.). The application area

includes but not limited to refrigeration & heat pumping, water production & treatment, air

conditioning & thermal comfort, and thermal energy storage. The critical component of the

adsorption technologies is the porous materials, known as adsorbents. The morphological

features of the adsorbents require to have some distinctive features like high surface area, meso

or microporosity, and optimum affinity towards refrigerants to become suitable for the

adsorption-based systems. Many promising adsorbents (silica gel, activated carbons, metal-

organic frameworks) are already synthesized having the mentioned features; still, there is no

clear breakthrough in adsorption systems found. One of the reasons behind this is the existing

gap between material science (MS) and applied thermal engineering (ATE).

This research gap predominantly depends on the existence of the intermediate

characterization technique, which supposes to relate the morphological features with the

adsorption phenomenon. For example, the material scientist explains the new adsorbents by

using surface area, pore size distribution, thermal/cycle stability to provide implicit indications

for the ATE scientists to understand the applicability of these adsorbents for their targeted

systems. Afterward, these adsorbents are further characterized by employing

volumetric/gravimetric adsorption techniques to measure the adsorption characteristics, such

as adsorption isotherms and kinetics measurements. Based on the adsorption pairs, the

isotherms are categorized into six major parts with two subparts (according to the International

Union of Pure and Applied Chemistry (IUPAC)) and shows an indirect relationship with the

morphological features of the adsorbents. Although the isotherm modeling has been reported

widely in the literature, however, it still lacks a universal approach in predicting the adsorption

isotherms of all available types. Recently, it is found that the surface energy of the adsorbents

plays a vital role in developing a universal model for all eight major types of isotherms.

Therefore, it is assumed that surface energy might be the predicted intermediate

characterization technique that can mitigate the existing gap up to some reasonable level

between MS and ATE.

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The surface energy is a distinctive feature of adsorbents, which is rarely measured;

indeed, it carries essential information for the adsorption process. The only surface

morphological characterization does not provide the full feature of a surface; it is vitally

required to measure the surface activities which can be performed by surface energy

measurements. The total surface energy is divided into two components; dispersive and

specific. Dispersive surface energy is related to van der Waals interaction between the

adsorbate and adsorbent molecules, whereas specific surface energy depends on the acid-base

interactions. The separate measurement of these two components carries useful information;

however, the related research is hardly found in the literature.

From the above perspective, this thesis emphasizes the novel characterization techniques

that can employ in developing promising adsorbents for adsorption-based systems. The

research stresses several factors, firstly, extracting the morphological and surface activity

information of the promising adsorbents using novel characterizing techniques. Secondly,

finding a thermodynamic relationship between surface activities with the texture properties.

Finally, it includes the synthesis and characterization of metal-organic frameworks to enhance

the performance of adsorption chillers, which concludes with an insight of the enhancement

from the surface energy point of view.

In this research, atomic force microscopy (AFM) was used for extracting surface 3D

images, which is further employs to calculate the surface porosity information. AFM is an

excellent equipment for measuring texture information, which has many advanced features for

taking both the qualitative and quantitative data of the surface texture. However, it still not

convenient in measuring adsorbents having highly rough (> 10 nm) surfaces. Therefore, a novel

approach is developed to measure the adsorbent surfaces to generate height images. Provided

3D visualization exploits the surfaces with many distinctive features that are rarely found in

the literature. Additionally, the height data was used to detect the surface pores by utilizing

watershed segmentation. These detected pores are counted and compared with the N2-

adsorption techniques for understanding the differences and integrability.

The surface energy analysis in the infinite dilution was performed by Inverse Gas

Chromatography (IGC) technique to measure dispersive and specific components for various

promising silica gels (RD silica gel, Chromatorex, Home silica gel, and B-type silica gel). In

the IGC experiment, the silica gels were placed in 3 mm columns in a stationary phase. Three

non-polar solvents (Hexane, Heptane, Octane) and Five polar solvents (Ethanol,

dichloromethane, acetone, ethyl acetate, acetone) were carried through the column by helium

gas. From this experiment, the dispersive component is found dominating, and the highest

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value was observed for RD granular silica gel. The trend of dispersive surface energy follows

the variation of surface area. It was interestingly found that porosity also influences the

dispersive component of the surface energy. Despite having a similar surface area, surface

energy is higher for RD silica gel than chromatorex. It is predicted that the higher the surface

energy in the infinite dilution, the higher will be the adsorption uptake because surfaces contain

high energy sites that might contribute significantly to the adsorption process. However, high

surface energy might generate high isosteric heat that can consequently reduce the adsorption

uptake. These results led to further experiments on the activated carbon-based adsorbents,

which exhibits high surface energy (>200 mJ m-2) than the silica gels (<100 mJ m-2).

The experiments conducted on the activated carbons (Maxsorb III, WPT-AC, H2-treated

Maxsorb) were slightly different from that of silica gels. Here, the measurement of isosteric

heat and isotherms in the Henry region were targeted to calculate the energetic behaviors of a

single component adsorbate-adsorbent system (ethanol-activated carbon pair) in terms of

enthalpy and entropy. A thermodynamic trend is established between the specific entropy and

the Henry’s law constant including the pore volume of adsorbents, and one can predict the

isosteric heats and adsorbent-pore-size for activated carbon + ethanol system by extending the

proposed linear trend, which is predicted to significantly contribute in tailoring the adsorbent

materials for the design of adsorption bed with a minimal or maximum driving force depending

on the types of heat transformation applications. However, the improvement mostly depends

on the modification of the pores and surface area, which might limit the tailoring efficiency.

On the other hand, another class of adsorbents, namely, metal-organic frameworks (MOFs), is

heightening interest in the field of adsorption due to its distinctive inherent properties. The

interior decoration of MOFs can be modified targeting the application area. As can be seen

from the mentioned studies, the morphological variation brings huge impacts on surface energy

and adsorption properties; it was assumed that modification of MOFs might deliver

extraordinary results on the adsorption process.

A rigorous review of promising MOFs and their modification were performed to select

the suitable MOF for adsorption chiller applications. After synthesizing a wide variety of

MOFs (Aluminium Fumarate, MIL-101 (Fe), MIL-100(Fe), MOF-74 (Co), MOF-74(Ni),

CAU-10H, HKAUST-1), aluminum fumarate was selected for analysis and modification. A

modified protocol was used to develop a tailored MOF developed in our laboratory (SMOF) to

enhance the yield and water adsorption uptake, which is then doped with various concentration

metal ions (Fe, Co) to observe the variation on water adsorption isotherms. Interestingly found

that the newly synthesized SMOF exhibits significantly high uptake and shifts the water

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adsorption isotherms towards the lower pressure region. Both the findings are crucial for the

development of adsorption chillers; the analysis shows that SMOF improves the specific

cooling efficiency (SCE) up to 150%.

To deliver an insight into the improvement, the surface energy of the SMOFs was

conducted in the lower pressure region. A comparison of surface energy components between

the SMOFs reveals that the dispersive surface energy of the doped SMOFs was significantly

improved. However, the improvement of the specific surface energy was not compelling

considering dispersive surface energy. The doped ions only contributed to tailoring the

morphological properties.

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Acknowledgments

This thesis is an outcome of a challenging yet fun-filled journey at Kyushu University,

which was made possible by the support, guidance, and encouragement from several

individuals. It is a wonderful opportunity for me to express my sincere thanks and gratitude to

all of them.

To begin with, I thank my advisor Professor Bidyut Baran Saha, for his support and

encouragement throughout my doctoral work. The success of the present study is due to his

wise counsel and timely advice on prioritizing the tasks at hand. He provided me with a

comfortable environment at the workplace and much-needed freedom to carry out my research

work. Being a resourceful man, he has been instrumental in connecting me with the right person

at various phases of my Ph.D. Interactions with him have helped widen my perspective in

multiple aspects of life. I hope to continue these interactions and get opportunities to work with

him in the future as well.

I would like to express my gratitude to my mentor Professor Takahiko Miyazaki

invaluable suggestions and encouragement throughout my entire study life in Japan.

I am indebted to Associate professor Kyaw Thu for his inspiring guidance whenever

asked for. He is an excellent young professor having the superior ability of multitasking, which

makes him my role model for the rest of my academic career. I would also like to express my

appreciation to Dr. Sivasankaran Harish for providing valuable suggestions with admirable

guidance from the very beginning of my doctoral study.

I am also grateful to Dr. Animesh Pal, Mr. Tahmid Hasan Rupam, and Ms. Israt Jahan

for their valuable assistance and for providing me the opportunity to discuss the technical

results of this work at any time.

I am also thankful to Professor Munim Kumar Barai for hosting the domestic internship

at Ritsumeikan Asia Pacific University, Oita, Japan. I am very grateful to Professor Anutosh

Chakraborty to give me the opportunity to work with him at Nanyang Technological

University (NTU), Singapore, and for teaching me the modern technique of synthesizing

promising metal-organic frameworks.

I am also indebted to Professor Bidyut Baran Saha, Professor Takahiko Miyazaki,

Professor Kazuhide Ito, and Associate Professor Jin Miyawaki for evaluating this work and

for their valuable feedback.

I am thankful to all of my present and former laboratory members and staff for their help

and kind cooperation. In particular, I must thank Late Professor Mahfuza Sharifa Sultana,

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Dr. Kutub Uddin, Dr. Sourav Mitra, Dr. Amirul Islam, Dr. Mahbubul Muttakin, Mr. Sampad

Ghosh, Mr. Kaiser Ahmed Rocky, Mr. Matiar Rahman, Ms. Mahua Jahan Alam, Mr. Perera

Colombatantirige Uthpala Amoda, Mr. Shamal Chandra Karmaker, Mr. Mir Shariful Islam,

Mr. Hosan Shahadat, Mr. Ye Lei, and Mr. Hisham Maher Abdelwahab Mohamed for making

me feel comfortable in the lab and for supporting me in various ways.

I would also like to special thanks to Mrs. Tandra Bhuiyan Saha, wife of my

Supervisor, for giving advice, suggestions, and taking care during my stay in Japan. Because

of her wise suggestions, my Japan life was very smooth, funny, and enjoyable.

I wish to express my heartfelt indebtedness Advanced Graduate Program in Global

Strategy for Green Asia (GA), IGSES, Kyushu University for providing scholarship, and all

other facilities required in this study. I would like to acknowledge I2CNER at Kyushu

University for the access to their experimental facilities. I am also grateful to all staff of Green

Asia and IGSES for supporting me in various ways during my stay at Kyushu University. I

consider myself fortunate to have enjoyed the opportunity of working at Kyushu University

under the supervision of honorable Professor Bidyut Baran Saha. I am grateful to the authority

of the University of Dhaka, Bangladesh, for granting the study leave and giving me the

opportunity to study at Kyushu University. Special thanks to technical staff, Mr. Shoji Hirano

and Ms. Yoka Hara, Ms. Yoshiko Kano, and Ms. Yuku Hayashi, for helping me in various

ways during my stay in Japan.

I would like to express my gratefulness to my father, Late Md. Fazlul Haque, my

inspiration, and an exemplary role model. I express my humble obligation to my affectionate

and loving mother, sister, brother-in-law, and all the family members for their love, inspiration,

and prayers for me. In particular, I am thankful and would like to express my gratitude to my

mother, Mrs. Khawla Khatun, sister Dr. Ayesha Akhter, brother-in-law Dr. Md. Rafiqul

Islam, and my elder brother like maternal uncle Mr. M. A. Mamoon for everything they have

done for me.

Finally, I am thankful to my angels Arisha Mariam, Aariz Faizan, and Aarib Faiyaz,

for keeping me happy and energetic throughout this amazing journey. And I would like to

dedicate this thesis to my beloved Mahfuza Faruquee, whose immense sacrifice and constant

encouragement made all of these possible.

M L Palash

Kyushu University, Japan

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List of Publications

Awards

1. “Silver Award-Young Researcher Award on Energy Research” given by the

president of Kyushu University under the "Kyushu University Platform of Inter-

/Transdisciplinary Energy Research", Jan. 28, 2020, Fukuoka, Japan.

2. “President’s Award” from the President of Kyushu University for producing most

excellent results as a team in the Kyushu University Challenge and Creation Project,

Mar. 29, 2019.

3. “HULT Prize Japan National Winner” for the most innovative idea entitled

“Distributed Cold Storage for Vegetables and Life Saving Drugs Without Electricity”,

Osaka, Japan, 20th May 2018.

4. “QREC C&C Challenge”, won the challenge as a team member of S-cube, Kyushu

University, May 13, 2018, Fukuoka, Japan.

5. “Best Poster award”, for conference paper presented at International Exchange and

Innovation conference on Engineering & Sciences, Oct. 18-19, 2018, Fukuoka, Japan.

6. “QREC J.O.C Challenge”, won the challenge as a team member of S-cube, Kyushu

University, April 24, 2018, Fukuoka, Japan.

7. “QREC GC&C Challenge”, won the challenge as a team member of S-cube, Kyushu

University, Jan. 16 2018, Fukuoka, Japan.

8. “Bronze Award-Young Researcher Award on Energy Research” given by the

president of Kyushu University under the "Kyushu University Platform of Inter-

/Transdisciplinary Energy Research", Jan. 30, 2018, Fukuoka, Japan.

9. “Hult Prize Kyushu University Championship, Japan, as a team leader of S-cube,

we have won the national championship of Japan for our project titled as “Distributed

system for the farmers of Bangladesh”, Dec. 17, 2017, Fukuoka, Japan.

Journal Publications

1. M. L. Palash, Israt Jahan, Tahmid Hasan Rupam, Sivasankaran Harish, Bidyut Baran

Saha, “Novel technique for improving the water adsorption isotherms of metal-organic

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frameworks for performance enhancement of adsorption driven chillers”, Inorganica

Chimica Acta, 501 (2020).

2. M. L. Palash, Sourav Mitra, Shivasankaran Harish, Kyaw Thu, Bidyut Baran Saha,

“An approach for quantitative analysis of pore size distribution of silica gel using

atomic force microscopy”, International Journal of Refrigeration, 105, pp. 72-79

(2019).

3. Israt Jahan, M. A. Islam, M. L. Palash, Kaiser Ahmed Rocky, Tahmid Hasan Rupam,

Bidyut Baran Saha, “Experimental study on the influence of metal doping on

thermophysical properties of porous aluminum fumarate”, Heat Transfer Engineering,

42 (13-14), 2020. (Accepted)

4. M. L. Palash, Animesh Pal, Tahmid Hasan Rupam, Bidyut Baran Saha, “Surface

characterization of different particulate silica gels by inverse gas chromatography at

infinite dilution”, Colloids and Surfaces A: Physicochemical and Engineering Aspects,

603, 125209.

Presentation at an International Conference

1. M. L. Palash*, Animesh Pal, Bidyut Baran Saha, “Investigation of surface energy of

porous adsorbents”, 5th International exchange and Innovation Conference on

Engineering & Sciences (IEICES 2019), pp. 32-33, Oct 24-25, 2019, Fukuoka, Japan.

2. Tahmid Hasan Rupam*, M. L. Palash, Israt Jahan, Bidyut Baran Saha, “Adsorption

characteristic of aluminium fumarate metal-organic frameworks”, 5th International

exchange and Innovation Conference on Engineering & Sciences (IEICES 2019), pp.

34-35 Oct 24-25, 2019, Fukuoka, Japan.

3. T. H. Rupam*, M. L. Palash, Israt Jahan, S. Bhaumik and B. B. Saha "Shifting of

adsorption isotherm induced by transitional metal doping in aluminum fumarate"

International Conference on “Water, Energy and Biodiversity (WEB) for Sustainable

Development of BIMSTEC Countries (WEB for BIMSTEC-2019)” Agartala, Tripura,

India, 12-14 December 2019. (Best Oral Presentation Award).

4. M. L. Palash*, Animesh Pal, Kyaw Thu, Bidyut Baran Saha, “Study on Surface

Characteristics of Various Adsorbents using Inverse Gas Chromatography”, 5th

International Conference on Polygeneration (ICP 2019), May 15-17, 2019, Fukuoka,

Japan.

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5. B. B. Saha*, M. L. Palash, “Metal-organic Frameworks as adsorbents for heat pump

applications”, the International Workshop on Environmental Engineering 2019, Nago,

Okinawa, Japan, June 25-28, 2019.

6. T. H. Rupam*, M. L. Palash, I. Jahan, S. Harish and B. B. Saha "Green Synthesis and

Adsorption characterisation of an aluminum based metal organic framework" Proc. of

21st Cross Straits Symposium on Energy and Environmental Science and Technology

(CSS-EEST), Shanghai Jiao Tong University, Shanghai, China, 24-27 November 2019.

7. M. L. Palash*, Kyaw Thu, Bidyut Baran Saha, “Qualitative and Quantitative

characterization of nano-porous materials” International Exchange and Innovation

Conference on Engineering & Sciences, Oct. 18-19, 2018, Fukuoka, Japan.

8. M. L. Palash*, Kyaw Thu, Bidyut Baran Saha, “Surface Characterization of Porous

Materials for Adsorption Cooling Systems”, International Conference on Material

Science and Semiconductor Devices, Sept. 7-8, 2018, Dhaka, Bangladesh.

9. M. L. Palash*, S. Mitra, S. Harish, Kyaw Thu, K. Takahashi, B.B. Saha “An Approach

for Quantitative Analysis of Pore Size Distribution of Silica Gel Using Atomic Force

Microscopy”, International Sorption Heat Pump Conference (ISHPC 2017), Aug. 7-10,

2017, Tokyo, Japan.

* Presenting Author.

Presentation at a Domestic Conference and Symposium

1. M. L. Palash*, B.B. Saha, “Surface energy characterization of various porous

adsorbents”. Hydrogenius and I2CNER Joint Research Symposium, Kyushu

University, Fukuoka, Japan, Jan 31 2020. [Oral]

2. M. L. Palash*, Tahmid Hasan Rupam, Israt Zahan, Bidyut Baran Saha,

“Functionalization of porous material for developing adsorption-based portable passive

water harvester”, Energy week 2020, Kyushu University, Fukuoka, Japan, Jan 28, 2020.

3. M. L. Palash*, Tahmid Hasan Rupam, Israt Jahan, Bidyut Baran Saha, “Study on

Metal-organic Frameworks (MOFs) as Energy Materials for Adsorption-based Heat

Pumps”, National Conference on Physics 2019, Dhaka Bangladesh, MS-07, pp. 42, Feb

07-09, 2019.

4. M. L. Palash*, Animesh Pal, Kyaw Thu, Bidyut Baran Saha, “Study on Surface Energy

Components to Develop Functional Materials for Heating/Cooling Applications”,

I2CNER Annual Symposium 2019, Kyushu University , Fukuoka, Japan, Jan. 31, 2019

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5. M. L. Palash*, Sourav Mitra, Kyaw Thu, Koji Takahashi, Bidyut Baran Saha,

“Implementing direct imaging technique for quantitative analysis of surface porosity

of mesoporous adsorbents”, Q-PIT Annual Symposium 2018, Kyushu University,

Fukuoka, japan.

6. M. L. Palash*, Kyaw Thu, Bidyut Baran Saha, “Quantitative and qualitative

characterization of nanoporous materials”, 47th International exchange and Innovation

Conference on Engineering & Sciences (IEICES 2018), Kyushu University, Japan, Oct.

18-19, 2018.

7. M. L. Palash* “Topographic analysis of silica gel imaged with Scanning Probe

Microscopy”, FY2016 Green Asia Program Short-term Fieldwork in Taiwan, Jan. 17-

19, 2017, Kaohsiung, Taiwan.

8. M. L. Palash*, S. Mitra, K. Thu, B. B. Saha, “Study of In-situ and Ex-situ Porosity Of

Mesoporous Silica Gel”, International Forum for Green Asia 2017, Kyushu University,

November, 2017, Fukuoka, Japan.

9. M. L. Palash*, S. Mitra, S. Harish, Kyaw Thu, B. B. Saha, “Topographic analysis of

silica gel imaged with atomic force microscopy”, The 18th Cross-Straits Symposium on

Energy and Environmental Science & Technology, pp. 47-48, Dec. 4-6, 2016,

Shanghai, China.

* Presenting Author.

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Contents

Summary ............................................................................................................................. 3

Acknowledgements .............................................................................................................. 7

List of Publications .............................................................................................................. 9

List of Figures.................................................................................................................... 17

List of Tables ..................................................................................................................... 21

Chapter 1 Introduction ..................................................................................................... 22

1.1 Background .................................................................................................................. 22

1.2 Adsorption cooling system ........................................................................................... 24

1.2.1 Principle of adsorption ........................................................................................ 24

1.2.2 Working principle of adsorption cooling system .................................................. 24

1.3 Adsorbents for cooling application ............................................................................... 26

1.3.1 Silicate ................................................................................................................ 27

1.3.2 Zeolite ................................................................................................................. 28

1.3.3 Activated carbon (AC) ........................................................................................ 29

1.3.4 Metal-organic frameworks (MOFs) ..................................................................... 29

1.4 Characterization techniques .......................................................................................... 31

1.5 Enhancing performance ................................................................................................ 33

1.5.1 Metal coating/doping: ......................................................................................... 33

1.5.2 Composite ........................................................................................................... 33

1.6 Motivation.. .................................................................................................................. 34

1.7 Aims and objectives of the thesis .................................................................................. 37

1.8 Organization of the thesis ............................................................................................. 38

Chapter 2 Overview of Modern Characterization Techniques ....................................... 41

2.1 Adsorption.. ................................................................................................................. 41

2.1.1 Basic of the adsorption process ........................................................................... 41

2.1.2 Factor influencing adsorption process.................................................................. 42

2.2 Heat transfer applications of adsorption ........................................................................ 43

2.3 3D-imaging using Atomic Force Microscopy (AFM) ................................................... 45

2.4 Characterization at zero coverage using Inverse Gas Chromatography (IGC) ............... 51

2.4.1 Components of IGC ............................................................................................ 51

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2.4.2 Probe molecule detection technique..................................................................... 52

2.4.3 Operation modes ................................................................................................. 54

2.4.4 Applications of IGC ............................................................................................ 54

2.5 Approaches of this thesis .............................................................................................. 56

Chapter 3 Morphological Study of Porous Materials using Atomic Force Microscopy . 58

3.1 Material….. .................................................................................................................. 58

3.2 Experimental apparatus and procedure ......................................................................... 58

3.2.1 Experimental apparatus ....................................................................................... 59

3.2.2 Generation of three-dimensional images .............................................................. 60

3.2.3 Measuring conditions .......................................................................................... 61

3.2.4 Measurement procedure ...................................................................................... 62

3.2.5 Error analysis and minimization .......................................................................... 63

3.2.6 Image processing................................................................................................. 65

3.3 Results and Discussions ............................................................................................... 65

3.3.1 Porous properties................................................................................................. 67

3.3.2 Qualitative study of the surface using SPM ......................................................... 69

3.3.3 Quantitative analysis of topographic images ........................................................ 70

3.4 Conclusions .................................................................................................................. 74

Chapter 4 Surface Energy Characterisation of Different Porous adsorbents by Inverse

Gas Chromatography ....................................................................................................... 75

4.1 Introduction .................................................................................................................. 75

4.2 Theory…… .................................................................................................................. 77

4.3 Materials… .................................................................................................................. 81

4.4 Experimental ................................................................................................................ 81

4.5 Results and discussion .................................................................................................. 83

4.5.1 Surface energies .................................................................................................. 83

4.5.2 Gibbs free energy of polar components ............................................................... 87

4.5.3 Effect of morphology and comparison among various adsorbents ........................ 90

4.6 Conclusions .................................................................................................................. 91

Chapter 5 Experimental Investigation of Adsorption Isotherms and Heat of Adsorption

at Henry Region for Activated Carbon/Ethanol Pairs..................................................... 93

5.1 Introduction .................................................................................................................. 93

5.2 Material….. .................................................................................................................. 94

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5.3 Experimental ................................................................................................................ 95

5.4 Results and discussion .................................................................................................. 96

5.4.1 Isotherms at Henry region ................................................................................... 96

5.4.2 The heat of adsorption at zero coverage ............................................................. 104

5.4.3 Entropy modeling .............................................................................................. 108

5.5 Conclusions ................................................................................................................ 113

Chapter 6 Novel Technique for Improving Water Adsorption Isotherms of Metal-organic

Frameworks..................................................................................................................... 115

6.1 Introduction ................................................................................................................ 115

6.2 Experimental .............................................................................................................. 117

6.2.1 Material and synthesis ....................................................................................... 117

6.2.2 Material Characterization .................................................................................. 118

6.3 Results and Discussion ............................................................................................... 119

6.3.1 Physical Properties ............................................................................................ 119

6.3.2 Adsorption isotherms ........................................................................................ 123

6.4 Conclusions ................................................................................................................ 126

Chapter 7 Study on Surface Activities of Improved Metal-doped Metal-organic

Frameworks..................................................................................................................... 127

7.1 Materials synthesis and preparation ............................................................................ 127

7.2 Experimental procedure.............................................................................................. 128

7.3 Results and discussion ................................................................................................ 129

7.3.1 Morphological characterization ......................................................................... 129

7.3.2 Water Adsorption isotherms .............................................................................. 129

7.3.3 Surface energies of studied SMOF samples ....................................................... 132

7.3.4 Specific Gibbs free energy ................................................................................ 137

7.4 Comparative analysis ................................................................................................. 139

7.5 Conclusions ................................................................................................................ 141

Chapter 8 Overall conclusions and recommendations .................................................. 142

8.1 Overall conclusions .................................................................................................... 142

8.2 Recommendations ...................................................................................................... 146

References........................................................................................................................ 148

Appendix A. Three-dimensional images of various adsorbents .................................... 164

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Appendix B. Example of surface energy date measured for RD granular silica gel using

Inverse Gas Chromatography ........................................................................................ 166

Appendix C. Henry region isotherm data of activated carbon/ethanol pairs ............... 170

Appendix D. SMOF synthesis and water adsorption isotherms .................................... 172

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List of Figures

Fig. 1.1. (a) Schematic of adsorption cooling system (b) adsorption-desorption isotherm at

different temperatures (T1<T2). ........................................................................25

Fig. 1.2. Composite C has a more significant difference between AB and DC line, which is a

measure of effective uptake. The effective uptake of the composite is more

significant than Maxsorb III [14]. (Reprinted with the permission of

publisher) .........................................................................................................27

Fig. 1.3. Water adsorption isotherm at 25℃ for various MOFs . .........................................31

Fig. 1.4. Fundamental queries that motivates to pursue this thesis work. .............................35

Fig. 1.5. Prediction of the role of surface energy on Type I isotherms . ...............................36

Fig. 1.6. Preliminary target was to improve surface energy in the lower pressure region where

surface energy is higher....................................................................................38

Fig. 2.1. A general view of the adsorption process. .............................................................42

Fig. 2.2. Influential factors of the adsorption process, (a) textural properties, (b) surface

energy. .............................................................................................................43

Fig. 2.3. Application of adsorption technologies. A) building energy management B)

decarbonization in the industrial sectors. ..........................................................44

Fig. 2.4. The fundamental operation of Scanning Probe Microscopy (a) tip-surface analogy

with a blind man’s walking. (b) schematic of probe surface interaction. ...........46

Fig. 2.5. Built-in optical lever a) schematic of optical lever b) laser path in SPM-9700. ......46

Fig. 2.6. The schematic diagram of SPM-9700 operated in contact mode . .........................47

Fig. 2.7. Modes of operation of Scanning Probe Microscopy. .............................................48

Fig. 2.8. Experimental procedure to extract AFM images. ..................................................49

Fig. 2.9. SPM images of Silica gel a) position of the cantilever on sample b) Tilted raw image

c) Flatten image d) 3D image after flattening. ..................................................50

Fig. 2.10. SPM images of activated carbon a) Phase image b) height image c) 3D image....50

Fig. 2.11. Schematic diagram of Inverse Gas Chromatography. ..........................................52

Fig. 2.12. A typical chromatographic peak in IGC generated on the Maxsorb III sample by

ethanol probe molecule for 0.01 fractional coverage at 303 K. The probes are

injected using the pulse method. .......................................................................53

Fig. 2.13. Approaches of this thesis. ...................................................................................57

Fig. 3.1. Schematic diagram of SPM system operated in phase mode. ................................60

18 | P a g e

Fig. 3.2. Scanning pattern of SPM a) raster scanning pattern b) trace and retrace line graph

confirming reproducibility c) line graph showing corrupt scanning. .................61

Fig. 3.3. Scanning spherical silica gel using AFM a) schematic of the surface of silica gel and

the influence of set point to optimize the movement of SPM probe. B) 3D

topographic image of a silica gel surface with line profile showing height variation

along a flat surface. ..........................................................................................62

Fig. 3.4. Error measurement procedure, (a) 3D view of the standard calibration sample and its

corresponding line graph, (b) measured height of individual pitches of TGG1. .64

Fig. 3.5. BET-N2 experiment onto various silica gel (a) N2 adsorption/desorption isotherm (b)

NLDFT pore size distribution...........................................................................68

Fig. 3.6. Extended 3D images of various porous materials. .................................................69

Fig. 3.7. Filtering high topographic information a) raw image which contains large height

variation, b) corresponding frequencies of spatial image, c) image after applying

filter d) zoomed raw image, e) zoomed filtered image shows the existence of

similar curvatures as in the raw image. .............................................................71

Fig. 3.8. AFM experiment on various silica gel (a) variation of different drop size to determine

the effect on the surface area and the skewness values are shown in each bar graph,

(b) pore size distribution is showing surface porosity. ......................................72

Fig. 3.9. Pore size distribution of different porous material, (a) Silica-Alumina (b)

Acetaminophen. ...............................................................................................74

Fig. 4.1. Schematic diagram of Inverse Gas Chromatography. ............................................82

Fig. 4.2. (a) Retention volume (VN) of alkanes (b) fitted alkane series obtained with RD

granular silica gel. ............................................................................................84

Fig. 4.3. Adsorption uptake of (a) Heptane and (b) Dichloromethane onto different types of

silica gels. ........................................................................................................85

Fig. 4.4. Comparison of (a) dispersive; (b) specific; and (c) total surface energy among the

different types of silica gels. .............................................................................87

Fig. 4.5. The typical diagram for determining the specific Gibbs free energy (ΔGSP) by

polarization method for RD granular silica gel at 0.03 coverage. ......................88

Fig. 4.6. Gibbs free energy changes of adsorption for polar probes (a) surface coverage of

0.03 and (b) surface coverage of 0.05. ..............................................................89

Fig. 4.7. Correlation between the dispersive component of surface energy and morphological

characteristics. (a) variation with specific surface area (b) variation with pore size

distribution. ......................................................................................................90

19 | P a g e

Fig. 4.8. Comparison of dispersive surface energy among the various adsorbents. (1st 4 silica

gels (measured data); SBA-16, SBA-15; Maxsorb III ; A-20 (measured data); and

Chemviron F400, Norit SA4). ..........................................................................91

Fig. 5.1. Schematic diagram of IGC-SEA equipment. .........................................................96

Fig. 5.2. Simplified illustration of step 1 to measure isotherm (Maxsorb III/ethanol pair at 303

K) ....................................................................................................................97


K) ....................................................................................................................98


K) ....................................................................................................................99

Fig. 5.5. Ethanol adsorption on Maxsorb III at Henry region. ........................................... 100

Fig. 5.6. Ethanol adsorption on WPT-AC at Henry region. ............................................... 100

Fig. 5.7. Ethanol adsorption on M-AC at Henry region. .................................................... 101

Fig. 5.8. Ethanol adsorption on H2-Maxsorb III at Henry region. ...................................... 101

Fig. 5.9. Comparison of Henry’s constant for different pairs............................................. 103

Fig. 5.10. Determination of heat of adsorption for Maxsorb III/ethanol pairs. Experiment was

conducted two times to confirm the data regeneration ability. ........................ 105

Fig. 5.11. Determination plot of heat of adsorption for WPT-AC/ethanol pairs. ................ 105

Fig. 5.12. Determination of heat of adsorption for M-AC/ethanol pairs. ........................... 106

Fig. 5.13. Determination of heat of adsorption for H2-Maxsorb/ethanol pairs. .................. 106

Fig. 5.14. Comparison of heat of adsorption for all studied samples at different surface

coverage. ....................................................................................................... 107

Fig. 5.15. Comparison of measured heat of adsorption with the theoretically found values

addressed at various literature. ....................................................................... 108

Fig. 5.16. The relation between adsorbed phase specific entropy and the ratio of Henry

constant and total pore volume of adsorbents at 328 K temperature. ............... 111

Fig. 5.17. The relation between adsorbed phase specific entropy against surface coverage at

328 K temperature.......................................................................................... 112

Fig. 6.1. Scanning electron micrography (SEM) of different SMOFs (a) SMOF (b)10% Co-

SMOF (c) 10% Ni-SMOF. ............................................................................. 120

Fig. 6.2. XRD of different MOF samples. ......................................................................... 121

Fig. 6.3. TGA of SMOF samples. ..................................................................................... 122

Fig. 6.4. The pore size distribution of studied samples. ..................................................... 123

Fig. 6.5. Water adsorption isotherms of different samples................................................. 124

20 | P a g e

Fig. 6.6. Enhanced uptake of doped aluminum fumarate. The water uptake is significantly

increased in the lower pressure region. ........................................................... 125

Fig. 6.7. Comparison of △q (Effective uptake) between RD silica gel/water and SMOF/water.

...................................................................................................................... 126

Fig. 7.1. Water desorption isotherms of the studied SMOFs at 303 K temperature. ........... 130

Fig. 7.2. Water desorption isotherms of the studied SMOFs at 333 K temperature. ........... 131

Fig. 7.3. Retention volume vs coverage for alkanes of all the studied samples. ................. 133

Fig. 7.4. Uptakes of the polar probes on 10% Co-SMOFs at 513 K temperature for 0.005

coverage ........................................................................................................ 134

Fig. 7.5. The dispersive surface energy of all studied samples. ......................................... 135

Fig. 7.6. The specific surface energy of studied samples. .................................................. 136

Fig. 7.7. The total surface energy of all studied samples ................................................... 137

Fig. 7.8. Comparison of Work of adhesion for different adsorbent/water pairs at 0.1 coverage.

...................................................................................................................... 141

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List of Tables

Table 3.1. Comparison of various advantages of atomic force microscopy with other

techniques. .......................................................................................................66

Table 3.2. Porous properties of studied silica gel. ...............................................................67

Table 3.3. Pore count in 1µm2 area. ....................................................................................73

Table 4.1. Porous properties of the selected silica gel samples . ..........................................81

Table 4.2. Physicochemical properties of molecular probes employed for conducting surface

energy experiment. ...........................................................................................82

Table 5.1. Porous properties of the studied AC samples. ....................................................95

Table 5.2. Henry’s constant for different pairs at different temperature............................. 102

Table 5.3. Heat of adsorption of all the studied samples. .................................................. 107

Table 5.4. Summary of Henry constant and specific entropy at 328 K. ............................. 110

Table 6.1. Porous properties of different SMOFs .............................................................. 119

Table 7.1. The doping concentration of the doped SMOFs. .............................................. 127

Table 7.2. Porous properties of the studied samples .......................................................... 129

Table 7.3. Specific Gibbs free energy of studied samples ................................................. 138

Table 7.4. Summary of work of cohesion and work of adhesion of SMOFs/water pair. .... 140

22 | P a g e

Chapter 1

Introduction Equation Chapter 1 Section 1

The research on adsorption cooling and heat pump system got momentum after observing

the worldwide energy crisis along with the obligation of international protocols, which limits

the production and utilization of CFCs and HCFCs as refrigerants. The adsorption heat pump

(AHP) is found to be a safe and viable alternative to the mechanical vapor compression system

because of the following advantageous features: (i) ability to use effectively low-grade waste

heat or solar heat of temperature below 100°C as the driving heat source, (ii) it can utilize

natural and/or alternative refrigerants having zero or negligible global warming potential, (iii)

no moving parts, (iv) does not require electricity other than the heat transfer fluid pumps and

electromagnetic valves, and (v) require negligible maintenance. However, due to having lower

Coefficient of Performance (COP) and bulkier size make it less compatible with the vapor

compression system. The possible way to improve the performance of adsorption chiller is by

introducing new adsorbent materials with distinctive features. This chapter explains the

motivation of this thesis to adsorption heat pump systems. The objectives and outline of this

thesis are presented in the latter part of the chapter.

1.1 Background

Maintaining a proper thermal environment is not optional, but it becomes essential for

human life because it influences productivity and health. Thermal comfort has a close relation

to various aspects of human behavior; providing a cool environment is the most crucial part of

achieving this [1]. Air conditioning and refrigeration are a useful technique ever used in

developed countries to maintain the standard of thermal comfort for occupants of buildings. In

developing countries, the use of cooling is becoming popular day by day. The increasing

interest in the cooling system to maintain thermal comfortability has an adverse effect on

energy consumption. It is estimated that the conventional vapor compression refrigeration

system used for air conditioning consumes 45 percent of the total electricity consumed in

commercial and household buildings [2]. On top of that 15 percent of the total electricity

consumed in the world are by refrigeration and air-conditioning [3].

The developing world plays a significant role in increasing the demand for air-

conditioning and refrigeration, which is alone anticipated to surge 33-fold by 2100 because of

the rise of income and urbanization. Because of this fact, within 2060, people will use more

CHAPTER 1 INTRODUCTION

23 | P a g e

energy for cooling than heating [4]. The conventional system used for cooling applications uses

a vapor compression cycle, which has a two-fold environment pollution nature. A large amount

of electricity consumed by the system is produced by burning fossil fuel; on the other hand, the

refrigerants used in the system are not environment friendly. The global warming potential of

these gases is, in some cases, more than 4000 times dangerous than carbon dioxide. For

example, diesel driven refrigerator on food trailers releases 30 times more hazardous matter

and six-time more nitrogen oxide than the engine drives the vehicle.

Highlighting the importance of an energy-efficient environment-friendly heat-driven

sorption system has the immense potential [5] as a viable alternative to the electricity-driven

vapor compression system. The expedient part of the sorption system is, it replaces the vapor

compression cycle by a “thermal compressor”. In the absorption system, the sorbent part is

liquid, and in adsorption, it is solid. Adsorption cooling systems can be driven by low-grade

waste heat or solar heat. However, the absorption cooling system required high-grade waste

heat, which can be found in steam or even combustion. Compare to absorption systems,

adsorption chiller is in several steps forward, considering no possibilities of crystallization,

corrosion, hazardous leaks. These features make this system superior in air conditioning and

heat recovery system [6,7]. Although initially, the primary application was restricted to

industrial sectors, solar-driven adsorption is gradually gaining attention in domestic

refrigeration and cooling sectors [8].

There are many challenges still to overcome in adsorption cooling systems because of

having various technical constraints. It cannot be considered for direct-fired because of the

requirement of an ample amount of primary energy, only waste or solar heat driven are proven

to be beneficial. It still has a lower COP compare to absorption and conventional vapor

compression system. The larger size also makes it challenging to become cost-competitive to

other technologies. There are three principal areas for successful implementation of adsorption

cooling systems; the adsorbent selection/tailoring, device construction, and advanced cycles.

Generally, they are highly correlated and dependent on operational conditions. Particularly, the

careful design of adsorbents ideal for particular adsorption cooling applications is of primary

importance [9]. Therefore, this thesis targeted the characterization and tailoring of adsorbents

to contribute to the primary challenge for improving the performance of adsorption chillers.


24 | P a g e

1.2 Adsorption cooling system

1.2.1 Principle of adsorption

The crucial part of an adsorption process is a porous solid which provides an identical

large surface area (>500 m2 g-1) and large pore volumes. These two phenomena lead to sizeable

adsorptive capacity [10]. The surface of the solid material is usually unsaturated and

unbalanced. When the surface is brought into contact with the gas, an interaction is materialized

between the unbalanced molecular forces at the surface and the gas molecular forces. The

reason behind that is solid surface tends to satisfy these residual forces by attracting and

retaining on its surface to the molecules, atoms, or ions of the gas. This phenomenon results in

a higher concentration of the gas or liquid in the near vicinity of the solid surface than in the

bulk gas or vapor phase, despite the nature of the gas or vapor [11]. The process is known as

adsorption.

Depending on the constraining force during the adsorption, this process may take place

in two ways; physisorption and chemisorption. In the Physisorption process, the adsorbate

molecules are attracted to the adsorbent surface by the weak van der Waals force, which is

similar to the molecular forces of cohesion. There are not any changes in the chemical

composition of the adsorption pair. The chemisorption process involves valence forces arising

from the sharing of electrons between the adsorbent and the adsorbate atoms. These forces

result in a chemical reaction and forming a complex surface compound. The forces of these

formed bonds are much stronger than the Van der Waals force. It should be mentioned that the

adsorptive action is physical for almost all solid adsorbents, which are commonly used in

adsorption cooling systems [12].

1.2.2 Working principle of adsorption cooling system

The adsorption cooling system works similarly as the conventional refrigerator, except it

has no mechanical compressor. This mechanical part is replaced by the thermal compressor,

which is shown in Fig. 1.1(a). The adsorbent containing the refrigerant is heated by the solar

collectors or waste heat, and the adsorbed refrigerant is expulsed as vapor and condenses in the

condenser. The condensed refrigerant is then transferred to the evaporator via the expansion

valve, leading to a lower pressure area. The refrigerant evaporates in the evaporator, taking up

the heat from the chilled water and finally adsorbed in the cooled adsorbent part. Coldwater

then evacuates the adsorption heat. Continuous cooling can be achieved by the assistance of

two separate adsorption beds.


25 | P a g e

Fig. 1.1. (a) Schematic of adsorption cooling system, (b) adsorption-desorption isotherm at

different temperatures (T1<T2).

The adsorption isotherms corresponding to the operation of adsorption chiller is shown

in Fig. 1.1(b). For understanding the operation, the process is taken for two different

temperatures. During the operation, it is undergone by four processes; pre-heating, desorption,

pre-cooling, and adsorption.

In the pre-heating process (1 to 2), the adsorbent is isolated from the condenser and

evaporator by closing the valves (VE and VC). The adsorber temperature increases continuously

by the assistance of an external heat source. This is the beginning of the desorption stage; hence

no desorption occurs; only the pressure increases from PE to PC.

In the desorption process (2 to 3), the heating is continued, and the valve VC is kept open

to allow desorbed refrigerant to enter the condenser chamber by keeping the pressure constant.

The desorbed refrigerant vapor liquified inside the condenser and return to the evaporator

through the expansion valve. During this process, the adsorbate/refrigerant amount in the

adsorbent reduced from W1 to W2.

In the pre-cooling process (3 to 4), the adsorber is cooled by the cooling load, and the

valves VE, VC are closed to stop the refrigerant exchange in the adsorber chamber. The

Evaporator

Adsorber

bed

Condenser

Desorber

bed

Cooling Room

THW IN

THW

O UT

TCW IN

TCW

OUT

cooling water supply

Hot water

supply

PC

cooling water supply

PEValve VE

TCH INTCH O UT

PC to

PE

PE to PC

Valve Vc

W

P

T2

T1

W1

1

PE

Ad

sorp

tio

n

2Pre-heating

PC

3

Deso

rpti

on

4

Pre-coolingW2

(a) (b)


26 | P a g e

temperature is decreased; as a consequence, the pressure of the adsorber chamber dropped from

PC to PE.

In the adsorption process (4 to 1), the pressure is kept constant at PE, and the adsorber

bed is connected to the evaporator through valve VE. The refrigerant vapor enters the

evaporator chamber and adsorbs by the adsorbent in the adsorber bed. During the adsorption

process, the adsorbate uptake increases from W2 to W1.

Therefore, during the adsorption process, the refrigerant is evaporated by gaining heat

from the environment, and hence the cooling effect of this cycle occurs. Conversely, the heating

effect occurs when the refrigerant is condensed by releasing heat to the environment.

The adsorption isotherms, shown in Fig. 1.1(b) are the key characteristics discussed in

this thesis. It will be rigorously analyzed in a different situation and will try to find a correlation

with the different properties of adsorbents.

1.3 Adsorbents for cooling application

A large number of porous material are addressed by the different scientific community

which is available by naturally and synthetically [12,13]. However, not all these porous

materials are suitable for the cooling application. One of the essential properties of these

materials can guide to select suitable adsorbent materials is sorption capacity. The strong

interaction between adsorbate and adsorbent leads to high sorption capacity, whereas weak

interaction leads to low capacity. For a cooling application, moderate affinity forces are

required between adsorbent and adsorbate. This affinity can be calculated by the boundary

condition of the refrigeration cycle. Observing the isosters both for adsorption and desorption

often, it is possible to select the effective adsorbent. For example, effective uptake measured

from the claperon diagram is useful to predict the usefulness of the adsorbent in adsorption

cooling applications. It is preferable to having efficient uptake having substantial value. For

example, consolidated composite activated carbon shows more considerable effective uptake

than the pristine in an experiment using ethanol pair. Fig. 1.2 shows an experimental result

depicting the effectiveness of new composite materials. Here, composite C comprises of more

considerable effective uptake, which makes it potential for cooling application. Several

promising materials for cooling applications are included below.


27 | P a g e

Fig. 1.2. Composite C has a more significant difference between AB and DC line, which is a

measure of effective uptake. The effective uptake of the composite is more significant than

Maxsorb III [14]. (Reprinted with the permission of the publisher)

1.3.1 Silicate

Mesoporous silicates, especially synthetic silica gels, are now considered as the most

popular adsorbent in cooling applications [15–18]. Silica gel is formed by the agglomeration

of primary silica particles. The specific surface area (SSA), pore size distribution (PSD), and

pore volume are determined by the size of the silica particle and packing process. The surface

morphology, along with amorphous texture, can be determined by various processing

phenomena such as sol-gel transition, temperature, duration of drying of gel, the concentration

of precursors, pH at hydrolysis, etc. [12]. The regular silica gel typically has 2 nm pore

dimension, whereas low, dense silica gel has 15-20 nm. Pore volume varies from 0.3 to 1.5

cm3 g-1.

The surface chemistry of silica gel depends on the amount of –OH and –OR group

existed. These groups also determine the hydrophilicity and hydrophobicity of the material. By

maintaining the ratio, it is possible to prepare silica gel for specific adsorbates. For example,

for water adsorption, hydrophilicity gives an added advantage.


28 | P a g e

Other than silica gel, there are other forms of silicates available. One of these is silica

aerogels, which is a particular type of amorphous silicates with an open system [19]. For gas

transport, its built-in nature of having meso- and macro-pores gives an extra advantage. This

type of silicates has a lower bulk density, which leads to low mechanical strength and low

thermal conductivity. Aerogels are merely used in cooling systems due to having high relative

pressure and vapor capillary condensation.

Another promising type of silica gel is ordered silicates, which comprise regular system

channels and uniform pore dimensions [20,21]. Usually, the silicates are irregular shaped, and

these types are regularly shaped. For instance, MCM-41 has a hexagonal array of pore size.

Due to having mesopores, these types of silica gel adsorb water and methanol at very high P/Po

for adsorption chiller. Nevertheless, this uniform ordered structure can be used as hosts for

various composite adsorbents [22–24].

Silicates have a significant disadvantage in having a lack of strong hydrophilic property,

which makes the chiller system gigantic. Despite these silicates are still popular in industries

because it is commercially available, cheap and stable. Furthermore, regeneration is possible

at a very low temperature, typically below 45℃ in multistage cycles [25].

1.3.2 Zeolite

Zeolite is another class of adsorbent which is used in adsorption application vastly.

Zeolites in nature are more hydrophobic than silica gels. These materials contain negatively

charged aluminosilicate, which makes the host framework balanced by various counter cations.

Around 200 types of zeolite framework has been addressed by the various scientific community

[12,26]. The adsorption property of these materials is adjustable by changing the

aluminum/silicon ratio [27]. Hydrophilic nature can be enhanced by increasing this ratio. A

favorite example of this type of zeolites is type A [28]. The water uptake of these type zeolites

are very high and can be achievable in low relative pressure. It exhibits a strong interaction

with water molecules with electrostatic fields and the balancing cations, which is advantageous

for gas drying. However, this nature is not useful for adsorption cooling applications. For a

cooling application, property tuning is required, such as molecular sieving is often adopted.

Another tool for tuning is post-synthetic ion exchange [29]. To use water as a working pair

with zeolite, popularly used materials are zeolite 4A and zeolite NaX [30–32]. However, zeolite

shows either high or very low affinity to water, which is not advantageous for the cooling

application.


29 | P a g e

1.3.3 Activated carbon (AC)

Activated carbon is a favorite material for gas separation and purification because having

a large surface area and significant porosity. It also has potential application in cooling systems;

many studies have been adopted with various working pairs [31,33–35]. Besides, the

theoretical study of effects on the physical properties of carbon on cooling performance has

been studied in Ref. [34]. In this article, the condition for having the best performance optimum

physical condition of AC is shown. Researchers also develop an optimum process to develop

activated carbon for heat pump applications [36]. Most of the research has been adopted in

finding an optimal adsorbate-adsorbent pair for a cooling application that is covered in the later

section.

1.3.4 Metal-organic frameworks (MOFs)

MOFs are considered as the most potential porous material in heat pump applications

because of having noticeable micropore volume, surface area, and a wide range of flexibility

on modifying the internal structure [37–41]. For a cooling application, the principal focus is on

water adsorption properties and storage capacities. These properties are strongly related to

specific pore volume. Recently the specific pore volumes of MIL-101 have been addressed 2

cm3 g-1 [42]. Besides having this high porous MOF, Omar et al. introduce a new generation of

MOF having 7000 m2 g-1 BET surface area having pore volume 4.4 cm3 g-1 [43]. Additionally,

geometric flexibility, such as a reversible change in the structure and physical response to

adsorbate changes. Based on surface area and pore volume MOFs are far more superior than

zeolites, silica gel, and porous carbons. However, stability in the adsorption and desorption

cycle depicts the weakness of MOFs in the cooling application. Some of the popular MOFs

which have potential in adsorption cooling are listed below.

i) HKUST-1: The molecular structure consists of Cu-BTC structure, where BTC is

benzene-1,3,5-tri-carboxylate. This MOF is researched for different applications along with as

a model MOF for simulation studies to cognize interactions between guests and framework

[45]. Strong influence has been observed in unsaturated Cu sites, which are potential for

adsorption [46]. Additionally, water adsorption on HKUST-1 shows promising results [47]. By

taking a different sample of these MOF water adsorption characteristics are analyzed, and

maximum uptake is observed 0.55 g g-1 for p/po=0.90 [48]. The overall measured range for

water uptake is between 0.3-0.55 g g-1, which is promising compared to silica gel and zeolite.


30 | P a g e

This high uptake makes HKUST-1 promising for the cooling application. However, it has

demerits in hydrothermal stability, which can be improved by a new synthesis method [49].

ii) ISE-1: This MOF synthesized in earlier stages, which shows better performance for

low-temperature heating and cooling application. The water uptake is relatively smaller; the

maximum uptake reported is 0.21 g g-1. One positive side is water stability, which makes it

promising for the cooling application.

iii) MIL-100 and MIL-101: These MOFs are named after the Institut Lavoisier’s Material

section, which is considered the most promising applicant for the cooling application. The

water adsorption experiment is reported at 0.939 g g-1. This experiment was conveyed using

MIL-101 under typical conditions, adsorption at 40℃, desorption at 90℃, and vapor pressure

were 5.6 kPa [50]. Many other reported water uptakes for MIL-101(Cr) is 1.37 g g-1, 1.43 g g-

1, which are considerably higher than traditional adsorbents use in cooling systems. Similarly,

MIL-100(Cr) also provides higher uptake (0.8 g g-1) [51]. Only a small hysteresis is shown in

MIL-100 compare to MIL-101. That means, MIL-100 contains more pores in the microporous

region than MIL-101. The authors also run adsorption cycles for 2000 times and found no

declination in the adsorption process [51]. Therefore, accumulating all the advantages, MIL-

100 is found preferable than MIL-101 for adsorption chiller application.

iv) Basolite/aluminum fumarate: Sigma Aldrich co. Commercially developed this

material, more specifically A100 and F300. Basolite A100 is identical to of MIL-53(Cr), the

commercial production of this type of MOF shows higher adsorption capacity [52]. Adsorption

isotherm shows a delayed increase due to having a narrow and open pore. Narrow pores are

advantageous for the adsorption of water molecules [53].

v) CAU-10H: Aluminium isophthalate is a new type of MOF that shows high water

adsorption and stability, which is advantageous for adsorption cooling applications. It is found

that thermal stability is higher than other reported MOFs; it was stable for several thousand

adsorptions and desorption cycles while working fluid water is used [54]. The water uptake of

0.26 g g−1 for the coated sample is lower compared to that of other decent and stable MOFs,

like aluminum-fumarate coating (0.35 g g−1). There are MOFs that have higher water capacities

like UiO-66 (0.45 g g−1), MIL-100(Fe) (0.65 g g−1), but have demerits from the perspective of

cycling.


31 | P a g e

A comparative analysis between different MOFs is done to find the impact of its S-shaped

isotherms in water adsorption, which is shown in Fig. 1.3 [55]. It is found that at saturated

pressure, MIL-101 shows higher uptake, probably because of having higher surface area and

larger pores. Unlike the porous carbons, MOFs have two different dominating ranges of pore

size. It is also found that there is a significant relationship between water uptake with the second

peak of pore size distribution.

Fig. 1.3. Water adsorption isotherm at 25℃ for various MOFs [55].

1.4 Characterization techniques

Characterization of adsorbent carries outmost importance prior to understanding the

adsorption process. The equilibrium adsorption uptake and the instantaneous uptake are the

two most fundamental characteristics to determine the applicability of the adsorption pairs for

a pair. The equilibrium uptake or the capacity of any pair is generally represented by an

adsorption isotherm on a pressure versus uptake plot. On the other hand, the instantaneous

uptake exhibits the time required to achieve a targeted uptake or noted as uptake speed. Besides

these two crucial characterizations, the morphological properties of adsorbents carry crucial

information to correlate the adsorption process with the adsorbent's inherent properties.


32 | P a g e

The morphological characterization is the primary task that is performed by material

scientists. Particle size, surface area and porosity are the three key characteristics that control

various properties of materials such as adsorption, filter-ability, flowability, agglomeration, the

storage capacity of fluids and gas. In the case of gas adsorption, which has a major application

in energy-efficient chiller systems, specific surface area (SSA) and pore size distribution(PSD)

is needed to determine quantitatively [56]. Larger SSA means higher adsorption where PSD is

directly related to the response of adsorbent to adsorbate. For example, activated carbon is

considered a suitable adsorbent because of having high SSA, which is about 3000 m2 g-1 [57].

Similarly, different PSD of adsorbents contribute to varying adsorption kinetics for

specific adsorption. Adsorption kinetics of ethanol onto activated carbon and mesoporous silica

gel is completely different due to PSD [58]. Therefore, measuring these characteristics is

practiced prior to much research.

Following the high impact of these types of characterization, there are various methods

existed to measure SSA and PSD. Gas adsorption & expansion, imaging, and light scattering

are the most common techniques for measuring these characteristics. In gas adsorption, a

reference gas, usually Nitrogen (N2), is used as an adsorbate, which is sent to the sample in a

low-pressure condition [59]. Since N2 is used below its critical point, capillary condensation is

considered as an essential factor that provides information related to pore size distribution [60].

The scattering method uses small or ultra small-angle scattering to determine the particle size

and porosity.

On the other hand, imaging techniques are more of a straight forward method, which

takes the images of the surface to extract the topographic information of the surface. A wide

range of imaging methods is available to determine the type of porosity of different porous

materials. Optical Microscopy and Transmission Electron Microscopy(TEM)[61], Scanning

Electron Microscopy (SEM) [62], X-ray spectroscopy, Nuclear Magnetic Resonance Imaging

(NMRI) are the notable techniques in this category. Atomic Force Microscopy is a considerably

new technique which has an identical feature of providing height based information of surface

[63]. This feature can be used to identify pores in the nanometre range, and additionally, pore

volume can be calculated.

However, morphological properties are not capable of explaining the surface activities.

However, several thermal properties like the heat of adsorption of a particular pair, specific

heat capacity, and thermal conductivity of adsorbents are often measured to predict the surface

activities. These measurements are not adequate to understand the surface activities on the

adsorption process. For example, the heat of adsorption measurement can only predict the


33 | P a g e

surface interaction; it lacks on identifying the source of interactions. It can not measure the

dispersive and the electrostatic interaction separately. On the other hand, surface energy

measurement provided by the inverse gas chromatography technique can identify both the

interaction separately [64].

It is of utmost importance to characterize the surface energy component separately. The

dispersive component is related to the van der Waals attraction, and the specific/electrostatic

interaction includes polarization, dipole, acid-base, and magnetic[65]. It is found that in

activated carbon materials, the dispersive surface energy is dominating, whereas, in silica gel,

both the surface energy components are domination [66,67].

1.5 Enhancing performance

Weak thermal conductivity and mass transfer properties are the major problems that are

prior required to solve the problem in making the cooling system more profitable and viable.

Various approaches are adopted to improve the material properties which are listed below:

1.5.1 Metal coating/doping:

Thermal conductivity enhancement of the adsorbent is one effective way of improving

the heat transfer in adsorption systems and thereby speeding up the process of

adsorption/desorption. By doping high thermally conductive materials to adsorbents is a

straightforward attempt to increase thermal conductivity [68]. It is found that about 2-25%

conductivity is improved by adding copper powder into activated carbon. Another group uses

graphite blocks with silica gel and found significant improvement in both heat and mass

transfer [69]. It is also possible to increase thermal conductivity by introducing metal coating.

By using thermal gradient synthesis, a polycrystalline layer of MOFs can be grown, which

shows the stable coating of HKUST-1which shows high thermal coupling to the substrate [70].

Coating with the sputtering method of copper on RD type silica gel also found 45% higher than

the pristine one. However, it increases the specific heat [71].

1.5.2 Composite

It is possible to improve adsorption characteristics by making composite materials form

porous adsorbents. Guillenminot et al. [72] found significant improvement in both thermal

conductivity and heat transfer coefficient by adding copper foam on zeolite (35-65 copper –

zeolite ratio). For activated carbon, adding expanded graphite and binder shows a proportional

relation with thermal conductivity with graphite content [14,25].


34 | P a g e

1.5.2.1 Adsorbate/adsorbent pair:

Besides improving the properties of the material, it is possible to improve adsorption

performance by selecting proper pairs. The selection of adsorption pairs largely depends on

the chemical, physical, and thermodynamic properties [73]. A comparative study has been done

by Younes et al. [74] with various adsorbent/adsorbate pairs. He showed for activated carbon

uptake varies 0.6-1.03 kg kg-1 when methanol is taken as the adsorbate. The uptake increases

when ethanol is used rather than methanol. The highest uptake is observed for activated carbon/

CO2 pair, which is 1.73 kg kg-1 [75].

1.5.2.2 Surface treatment:

Surface functionalities are one of the significant, influential parameters of adsorption

characteristics. Kil et al. [76] studied the influence of the different concentrations of oxygen

contents on adsorption of ethanol on activated carbon. They found that an abundance of oxygen

content decreased the ethanol adsorption and lowered the adsorption equilibrium time.

1.6 Motivation

The above study provides that the adsorption cooling system is a promising technique for

remediation of continuous degradation of the environmental condition due to conventional

refrigeration systems. However, until now, there is no visible breakthrough in the adsorption

heat pump is. The bottleneck behind this situation is the introduction of suitable pairs. For

several decades a lot of researchers are finding the solution; still, adsorption systems are

commercially not feasible. It is believed that the breakthrough might come from the material

side. An extremely well adsorbent/adsorbate pair having high uptake capacity and kinetics

might increase the coefficient of performance (COP), which is now significantly low. To assist

this finding of new pairs throughout the thesis, several non-conventional techniques are

adopted. Fig. 1.4 illustrates the fundamentals queries of the adsorption process that motivates

this thesis work.

First of all, the work focuses on taking the surface images to understand the adsorbent

properties. Besides adopting the conventional techniques of imaging like SEM and TEM, this

is the very first time the AFM technique is applied to extract the surface images. The motivation

for using AFM comes from the fundamental drawbacks of conventional techniques. For

example, TEM is not suitable for taking images of adsorbents in a practical situation; a lot of

pre-processing is required before taking the image. Similarly, both the SEM and TEM requires

to operate in the vacuum condition that might not predict the situation of the surface in the


35 | P a g e

practical condition. On the other hand, AFM can be operated in ambient pressure and

temperature and generate three-dimensional images that can provide the actual visual

understanding of the surfaces in the real scenario. Prior to conduct the thorough analysis,

reviewing the articles, it is found that AFM provides identical information like height profile

that can be used to understand the pore shapes, roughness which is absent in conventional

imaging techniques.

Fig. 1.4. Fundamental queries that motivates to pursue this thesis work.

Secondly, besides the morphological properties, one most essential properties of

adsorption are the interaction between the molecules of adsorbent and adsorbate. The

interactions often generalized using the heat of adsorption, which can not explain the source

and nature of the interactions. Furthermore, Ng et al. [77] predict that surface energy of the

adsorbent plays the critical role of the adoption phenomenon, and they have provided a

universal model using surface energy to relate the adsorbent-adsorbate pair properties with the

IUPAC isotherms. In their work, they have extracted the surface energy by theoretical analysis

and assumes the surface as a distribution of heterogenous energy sites rather than distributed

porosity. The direct relationship between the surface energy and isotherms motivates this thesis

work to find the preliminary information of surface energy. However, it is challenging to study

the surface energy component for all the pressure region. Therefore, it is assumed that in the

lower pressure region, there might be no adsorbate-adsorbate interaction, and this thesis work

focuses on this region. Fig. 1.5 shows the concept of extracting surface energy in the lower

pressure region where Henry’s law is applicable. To perform this extraction, inverse gas

chromatography (IGC) techniques are promising, because it uses standard polar and non-polar

H HO

H HO

H HO Q1: How the adsorbent

surface actually looks

like?

Q2: How can we define

and measure the

interaction?

Q3: Can we relate the

interactions with surface

morphology?Q4: Can we tailor material

to improve adsorption?

Q5: If the improvement is

possible then what is the

reason behind that?

Chapter 3

Chapter 4

Chapter 5

Chapter 6

Chapter 7


36 | P a g e

gas probes which can be controlled to elude with a significantly lower concentration that can

mimic the infinite dilution to extract the surface energy profiles in very lower pressure region.

Fig. 1.5. Prediction of the role of surface energy on Type I isotherms [77].

Thirdly, Henry region analysis provides an understanding of the limits of the adsorption

process by providing the fundamental interaction characteristics of the adsorption profile. One

important property is entropy, which is related to input energy. Targeting entropy, the

relationship with the pore volume might reveal the initial understanding of adsorption. As the

IGC technique provides the ability of extraction information at Henry region, the study of

isotherms, heat of adsorption, and entropy might reveal interesting data for the adsorption

process.

Fourthly, the adsorption process severely depends on the surface area of the adsorbents.

Earlier of this chapter, it is shown that metal-organic frameworks (MOFs) are breaking all the

records of adsorbent surfaces and forwarding to provide extremely surface area (more than

10000 m2 g-1). It is predicted that MOF will be the next generation adsorbent material that

might break the bottleneck of the adsorption process. Besides, tailoring MOFs might give us

the ability to optimize the adsorption process. For example, S-shapes isotherms are suitable for

adsorption chillers. Therefore, understanding the process of shaping the isotherms of MOF-

water pairs might bring significant importance for adsorption chiller systems.

Heterogeneous Surfaces

Distributed Surface Energy

Pressure

Up

tak

e

Surface Energy

Type – I isotherm

Surface coverage

En

erg

y


37 | P a g e

Finally, shaping of MOF isotherms and the surface energy relation with the shaping is

promising to understand the functionalization process. This understanding will assist the

material scientists in developing new MOFs for adsorption chillers and other adsorption-based

applications like water harvesting.

1.7 Aims and objectives of the thesis

The aims of this thesis to explore the five fundamental questions illustrated in Fig. 1.4

by adopting non-conventional approaches. AFM technologies conducted the exploration of the

first query by extracting three-dimensional information of the surfaces. Later, surface energy,

entropy, isotherms, the heat of adsorptions of traditional adsorbents are aimed to analyzed using

IGC techniques. Based on the findings, a modified MOFs are targeted to synthesis to improve

the water adsorption isotherms. Generally, the target of this work focused on the improvement

of surface energy profile at the lower pressure region shown in Fig. 1.6. It is evident that the

increase of surface energy in the lower pressure region will improve the initial uptake of the

adsorption isotherm. However, it is not known yet what will be the effect on kinetics. The

targets of this thesis are summarized as below:

1. To find an alternative representation of surface images of adsorbents to

understand the behavior in the adsorption process.

2. To quantify the porous properties using the height data of the surface

morphology.

3. To understand the surface energy of different porous adsorbents and quantify the

dispersive and specific components of energetic characteristics.

4. To analyze the surface energy of silica gels, because of silica gels inherent both

the dispersive energy components generated from van der Waals and electrostatic

forces.

5. To conduct a comparative analysis of surface energy of activated carbons, silica

gel and metal-organic frameworks to provide additional information for

developing functional materials.

6. To investigate the Henry region isotherms of different activated carbon materials.

7. To investigate the heat of adsorption at zero coverage to identify the limit of

adsorption for different materials.

8. To find the relationship between the adsorbent morphology and surface activities

with thermodynamic modeling.

9. To validate the modeling with the experimental data


38 | P a g e

10. To perform synthesis and tailoring of metal-organic frameworks and tailoring for

shaping the adsorption isotherms focusing on shifting the isotherms into lower

pressure regions.

11. To shift the isotherms of MOF-water pair towards hydrophilic zone at lower

pressure and temperature. Furthermore, shifting it towards the hydrophobic zone

at higher pressure and temperature to increase the effective uptake.

12. To analyze the shifted isotherms using surface energy and find out the reason

behind that shifting.

Fig. 1.6. The preliminary target was to improve surface energy in the lower pressure region

where surface energy is comparatively higher.

1.8 Organization of the thesis

The thesis comprises eight chapters describing the various experiments characterization

of different adsorbent materials focusing on finding non-conventional surface properties like

three-dimensional images, surface energies, etc. Furthermore, modification tailoring of

Pressure

Up

tak

e

Targeted surface energy

Type – I isotherm

Current surface energy distribution

Current isotherm shape and uptake

Expected uptake and isotherm shift of tailored adsorbent


39 | P a g e

aluminum fumarate is included to enhance the water adsorption characteristics, which are later

analyzed with surface energy and work of adhesion. The arrangement of the thesis is as follows:

Chapter 1: Introduction

Chapter 2: Overview of Modern Characterization Techniques

Chapter 3: Morphological Study of Porous Materials using Atomic Force Microscopy

Chapter 4: Surface Energy Characterisation of Different Porous Adsorbents by Inverse

Gas Chromatography

Chapter 5: Experimental Investigation of Adsorption Isotherms and Heat of Adsorption

at Henry Region for Activated Carbon/Ethanol Pairs

Chapter 6: Novel Technique for Improving Water Adsorption Isotherms of Metal-

organic Frameworks

Chapter 7: Study on Surface Activities of Improved Metal-doped Metal-organic

Frameworks

Chapter 8: Overall conclusions and recommendations

The key points of each chapter are given as follows:

Chapter 1 gives an overview of the thesis with a concise literature review. It also presents

the aims and objectives of the current research work. The chapter ends with the organization

of the thesis.

Chapter 2 presents the general introduction of the modern characterization techniques

used in the present works. It has two major parts; i) introduction and operational principle of

atomic force microscopy, ii) overview of measuring technique of surface energy components

using inverse gas chromatography technique.

Chapter 3 describes the measurement processes of surface morphology of different

adsorbents, which includes various silica gels, activated carbons, etc. Using AFM, the three-

dimensional surface images are extracted and presented in various forms. The measured height

data is processed using watershed segmentation method, and the irregular pores are counted to

measure the pore size distribution. The pore size distribution data is then compared with the

conventional method.

Chapter 4 presents the experimental studies of surface energy components of various

adsorbents. Silica gels are the primarily focused adsorbents for the surface energy

measurements. It is found that dispersive and specific surface energy is dominant in the silica

gel samples. However, activated carbons are dominated by only the dispersive component of

the surface energy.


40 | P a g e

Chapter 5 presents the experimental procedures to measure the energetic parameters of

activated carbon/ethanol pairs in the Henry region. The experimental method is inspired by the

surface energy measurement described in chapter 4. The experiments are carried out using

inverse gas chromatography method at adsorption temperature ranging from 303K to 353 K.

This chapter comprises of three major parts: i) Measurement of Henry region isotherms, ii)

measurement of zero coverage isosteric heat and iii) modeling of specific entropy with pore

volume.

Chapter 6 describes the green synthesis process of aluminum fumarate with improved

adsorption characteristics. Furthermore, it provides an overview of the in-situ doping process

of MOFs that further enhances the adsorption characteristics.

Chapter 7 provides insights into the improvement of adsorption characteristics described

in chapter 6 using surface energy analysis. The surface energy components are compared with

the water adsorption isotherms to find an implicit relation between them. The chapter ends with

an intense comparison of work of adhesion of various promising adsorbents.

In chapter 8, the thesis ends with the significant findings where the originality and

contribution of the author and recommendations for future improvements have been made. It

is concluded with the in-situ doping are promising for tailoring metal-organic frameworks,

which can provide the ability to control the shape of adsorption isotherms. Furthermore, it is

observed that surface energy can be used as an intermediate characterization technique to

mitigate the gap of understanding between MS and ATE scientists. The explanation of

improvement of SMOF is possible with surface energy analysis where morphological analysis

failed.

41 | P a g e

Chapter 2

Overview of Modern Characterization

Techniques Equation Chapter 2 Section 1

This chapter provides an overview of the concepts of the adsorption process and new

characterization techniques adopted in this thesis. Adsorption is a promising field for energy

conservation systems, especially related to heat transfer applications. The fundamental

properties of adsorption can be used to transfer energy to various forms. To understand the

transfer, it is important to have a clear idea of the factor influencing the adsorption process. For

example, surface morphology and activities of adsorbents are the main two factors influencing

the adsorption process. There are many techniques to characterize these properties. In this

chapter, two new techniques of characterization are shortly described, which are not yet

established in the adsorption research, even though having a lot of potentialities. One is Atomic

Force Microscopy (AFM), and another one is Inverse Gas Chromatography (IGC). The former

is advantageous for taking 3D images of the surfaces in the ambient conditions, and the later

one is suitable for measuring surface energies.

2.1 Adsorption

2.1.1 Basic of the adsorption process

Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface

of a solid or a liquid forming a molecular or atomic film. The molecular film is termed as

adsorbate, and the solid or liquid where the event occurs is as adsorbent. In the thesis work, the

targeted adsorbents are solids, and the adsorbates are in vapor or in the gaseous phase. The

forces involved in the adsorption process are weak and often possible to reverse the process by

providing external forces. Generally, the adsorption process is categorized into two

fundamental parts; i) physical adsorption, ii) chemisorption. Physical adsorption is the cause

of weak intermolecular interaction between the adsorbate and adsorbent. Whereas, more strong

interactions are involved in chemisorption. Due to the weak intermolecular attraction, the

physical adsorption is considered as a reversible process. The reversible nature of the physical

adsorption leads to many heat transfer applications, which include refrigeration, desalination,

gas separation, etc. A typical adsorption process is shown in Fig. 2.1.

CHAPTER 2 OVERVIEW OF MODERN CHARACTERIZATION TECHNIQUES

42 | P a g e

Fig. 2.1. A general view of the adsorption process.

The adsorption process is often misunderstood by the absorption process, which is

entirely different. In the absorption process, the gas mixture is brought to contact with liquid

for the purpose of dissolving one or more components of the mixture into the liquid. Therefore,

the absorption process is a bulk phenomenon, and the adsorption is completely a surface base

one. Because of the gas mixture completely dissolve into the material in the absorption process,

the separation is limited by the solubilities of the involved gas. On the other hand, the separation

process in adsorption only depends on the energy required to detach the adsorbate from the

adsorbent surface.

2.1.2 Factor influencing adsorption process

All most all the application related to heat transfer application is depended on the physical

adsorption rather than chemisorption. The forces involved in physical adsorption include van

der Waals forces and electrostatic interactions comprising polarization, dipole, and quadruple

interactions [78]. The van der Waals interaction is a common property of physical adsorption,

whereas the electrostatic interaction appears only when the adsorbent surface contains ionic

structures. The electrostatic attraction will be termed as a specific attraction later in this thesis.

On the other hand, the van der Waals attraction is generally known as a dispersive attraction.

These forces are significantly depending on the fundamental characteristics of adsorbate-

adsorbent pairs; however, the major role is played by the adsorbents.

The most influential part of the adsorbent materials that control the adsorption process is

the textural properties like surface area, average pore width, pore-volume, pore size

distribution, etc. (Fig. 2.2 (a)). The surface area provides the space/sites for adsorbate to stick

Adsorbate

Adsorbent

Monolayer adsorption

Multi-layer adsorption

Adsorption

Desorption Physical

adsorption Chemisorption

Adsorbent


43 | P a g e

with the adsorbent. It is evident that the higher the surface area and pore volume, the higher

the possibility of adsorption uptake [10]. The pore size distribution indicates the possibility of

the various types of the adsorption process, which are standardized by the International Union

of Pure and Applied Chemistry (IUPAC) [79]. For example, the adsorbents contain micropores

(<2 nm) often prone to show Type I characteristics, whereas mesoporous materials (2 nm to 50

nm) shows Type IV characteristics.

Fig. 2.2. Influential factors of the adsorption process, (a) textural properties, (b) surface energy.

The second most influential factor is of adsorption process is the surface chemistry.

Surface chemistry leads to surface activities that can not be defined only by the textural

property. For example, surface morphology can predict the probable isotherms but limited to

provide an explanation of the surface activities. In that case, the surface energy is the key to

explain the insights of the adsorption process Fig. 2.2 (b). Unlike the textual representation of

the surface, surface energy provides the heterogeneous distribution of the surface.

2.2 Heat transfer applications of adsorption

The adsorption process is gaining its interest in the heat transfer applications because of

its ability to operate with a low-grade heat as driving energy. Initially, the waste heat was

Heterogeneous Surfaces

Distributed Surface EnergySu

rfac

e En

ergy

Micropore < 2 nm

Macropore >

50 nm

Mesopore 2-50 nm

(a) (b)


44 | P a g e

thought of as primary source energy; however, recently, solar energy is increasing its

potentiality to become an important source of driving energy.

Fig. 2.3. Application of adsorption technologies. A) building energy management B)

decarbonization in the industrial sectors.

For building energy management, adsorption based rotary wheels can efficiently make

enthalpy recovery from the exhaust air of the conditioned spaces to reduce the energy

consumption for fresh air handling. Passive solar-powered adsorption-based dehumidifiers

integrated into building envelopes (roof, wall, window, etc.) are very promising to achieve

zero-energy building. Utilization of adsorbents achieving electricity-to-chemical energy at

night and chemical energy-to-heat at day for space heating is an effective way to reduce the

electricity bill in terms of peak and valley time price. Meanwhile, desiccant-coated heat

exchangers have demonstrated a great future to dramatically improve the energy efficiency of

typical air conditioners. In the case of industrial decarbonization, low temperature (~50˚C)

waste heat from cooling water of the data center can drive the adsorption chiller to lower the

cooling needs. Similarly, the waste heat from the engine exhaust of the truck and fish boat also

can power the adsorption systems to make cooling and ice, respectively. Meanwhile, the giant

waste heat in oil, iron, and power industry can be recovered cheaply by adsorption technologies

to achieve cogeneration.

Although this field is up-and-coming, it still faces enormous challenges. In the last

decade, several novel adsorbents have been reported by MS scientists for AHC with great

predicted performances, but ATE scientists challenge their real performances, which are not

always so optimistic. This overestimation is mainly because the analysis was based on limited


45 | P a g e

data on the adsorption equilibrium, often just one adsorption isotherm under room temperature

and several issues important for actual AHC applications were not covered.

To provide more information to the MS and ATE scientists, more information about the

adsorbents are required. Therefore, this thesis work focuses on the characterization of

adsorbents using modern tools like Atomic Force Microscopy (AFM) and Inverse Gas

Chromatography (IGC) to provide the non-conventional information that might contribute in

the mitigation of the gap.

2.3 3D-imaging using Atomic Force Microscopy (AFM)

The imaging technique is the preliminary characterization to understand the textural

property of the adsorbents. Unlike conventional imaging techniques, AFM provides three-

dimensional morphological properties, which are essential for understanding the surface

properties of the adsorbents. Additionally, it is possible to extract the surface pore information

to predict the internal pore shapes of the adsorbents. In this thesis work, Chapter 3 provides the

detail of the AFM process for collecting important information on the adsorbent surface.

The operation principle of AFM is simple in a manner that total operation design on

detecting interactive forces between tip and sample surface. This operation can be thought like

a blind man walking with a stick (Fig. 2.4). A blind man uses his stick to feel his path rather

than see it. If there is an artifact or variation on the surface, he can detect and change his stick

position by mental feedback. Similarly, here probe is the “blind man’s” stick, and the road is

the sample surface. To get a surface image, the probe is moved along a designated path and

registers its force interaction. Feedback is used to change its vertical movement. The value of

feedback contains critical information on the surface profile. It is a matter of design in which

value is significant and how to move the probe along the surface. Depending on this design,

different modes exist.


46 | P a g e

Fig. 2.4. The fundamental operation of Scanning Probe Microscopy (a) tip-surface analogy

with a blind man’s walking. (b) schematic of probe surface interaction.

Force sensors are used to measure the force interaction [80], among them tunneling tip

sensor [81], interferometer crystal oscillator [82], piezoelectric sensor [83] are most

mentionable. However, optical lever sensors are widely used sensors in SPM because of the

advent of the microfabricated cantilever. The working principle of an optical lever sensor is

illustrated in Fig. 2.5. Due to interactive force, the cantilever (it is a flat plate which contains a

sharp probe/tip) bends. This bending is detected using a laser beam, which is reflected on the

backside of the cantilever and incident upon a four-segmented photodetector. The deflection is

prior calibrated with a known sample to measure unknown surface topography.

Fig. 2.5. Built-in optical lever a) schematic of optical lever b) laser path in SPM-9700.

After the detection of forces, it is required to move the tip along the surface, which

requires a three-dimensional movement. In this case, piezoelectric transducers are used. Piezo

(b)(a)

(b)(a)


47 | P a g e

materials have a distinct feature of changing its shape in accordance with the applied potential.

For example, the expansion coefficient of a single crystal piezo device is 0.1 nm per applied

voltage [84]. Our equipment SPM-9700 uses sample scanning technique, SPM sample is

mounted on an x-y-z scanner, and force sensor remains fixed. As mentioned earlier, the force

sensor is optical lever based; variation is detected the laser deflection. In Fig. 2.6, there is an

illustration of the sample which is mounted on the cylinder-shaped x-y-z piezoelectric scanner.

The x-y scanning follows the raster scanning pattern, and for z-direction, it uses feedback from

the optical sensor to keep tip safe. The schematic diagram of the operation principle of SPM-

9700 is drawn in Fig. 2.6, which is operated in contact mode [85].

The sample is mounted on a piezoelectric scanner, which is scanned by a sharp probe.

Due to force interaction, the tip holder (cantilever) bends and deflects the laser. The sample

continuously moves through x-y axes in a raster pattern; no feedback is used in this case (open

loop). However, the z-direction movement is controlled by feedback (close loop), which is

generated by the photodetector. This operation is controlled by a Personal Computer (PC)

where live images also visible after quick processing.

Fig. 2.6. The schematic diagram of SPM-9700 operated in contact mode [85].

The range of available modes is the key advantage of modern SPM. The wide variety of

experiments can be performed by setting up the modes, which makes SPM a versatile, powerful

tool. All the modes are beyond the scope of this experiment; here, only topographic modes are


48 | P a g e

discussed. In topographic mode, there are mainly three kinds of modes which are: a) contact

mode, b) non-contact mode, and c) tapping mode. In contact mode, tip touches the sample

surface and the detection mainly due to repulsive force (Fig. 2.7). In non-contact mode tip

always keep a suitable distance from the surface and the variation detected by an attractive

force. Not to mention, here, the cantilever is initially vibrated near to its resonant frequency,

and the deflection is found due to attractive force changes the vibrating frequency. The

detection of these changes categorizes the SPM. In our work, we have used Amplitude

Modulation SPM, detects the variation of amplitudes, and generates data using average values.

Besides, there is Frequency Modulated SPM, which is not used in this work. The last mode is

tapping mode, which is used both the features of the previous modes. In this mode, the

cantilever is vibrated continuously and keep as close as possible to the surface. In SPM-9700,

non-contact and tapping mode combinedly termed as the dynamic mode. In our work, we have

used dynamic mode because our samples are of different properties, and this mode is generally

optimized for safe use and high-resolution image.

Fig. 2.7. Modes of operation of Scanning Probe Microscopy.

The scanning process for adsorbents is quite challenging; generally, there are significant

sections (Fig. 2.8). The first one is sample preparation, which is essential because SPM-9700

is designed for flat sample analysis. We have taken flat, amorphous, spherical, and 2-D

material, which is treated differently prior to observation. The second part is setting the SPM

equipment, which includes selecting appropriate observation mode, setting setpoint, and

scanning parameters. Though is SPM-9700 is amplitude modulation based AFM, the variation


49 | P a g e

of amplitude is essential to determine. Setpoint plays a vital role in this part; error in setting

may distort the image and even may cause breaking the cantilever. Scanning parameter

selection includes finding the appropriate scanning location, area, z-parameter values etc. Most

of the cases, these values are set by a trial and error analysis basis. The third part is image

processing, which is required to get acceptable surface images. The collected image data may

have tilted, drifted, or even contains thermal noises. Image processing is, in this case, useful to

mitigate these effects.

Fig. 2.8. Experimental procedure to extract AFM images.

The scanning process for silica gels quite challenging because of the high roughness and

spherical shapes of the samples. At first, silica gel samples are cleaned with ethanol to remove

dust and other impurities. In this process, some ethanol might have been absorbed, which is

removed by vacuum heating for two hours at 85oC. Adhesive tape is used to tautly stick the

silica gel particle with AFM sample holder. The optical microscope has been used to locate the

scanning location. To avoid spherical shape problem optimized scanning area was taken. RD

type silica gel consists of pores in the mesoporous region (above 2 nm and below 50 nm).

Considering this, the surface scanning is taken below 5 μm2 areas. Fig. 2.9 shows the images

of Silica gel. The first one is the position of the cantilever on the silica gel; the top and the less

coercive surface is ideal for selecting the scanning area. After the scanning finished the second

images is found, which have severe tilt problem. To mitigate the tilt problem, postprocessing

such as filtering and flattening is required. Fig. 2.9(c) is the ideal image of the surface, which

is corrected by the flattening process. Orthogonal scanning is done to avoid the drift problem.

[86]

Sample preparation •Different procedures are taken for different samples

Observation using SPM-9700

•selecting observation mode (Phase mode)

•setting set point value

•setting scan area and scanning parameter

•scanning

Image processing

•Image correction and flattening

•Noise elemination and spatial frequency filteration


50 | P a g e

Fig. 2.9. SPM images of Silica gel a) position of the cantilever on sample b) Tilted raw image

c) Flatten image d) 3D image after flattening.

To get an SPM image of activated carbon, which is in powder form, has been

consolidated under a hydraulic press using 5 MPa [87]. Then the adsorbed water is removed

by vacuum heating for 3 hours at 200°C temperature. As the surface is very coercive, the

operating point is taken as large as possible. This value is reduced to optimum value gradually

by trial and error basis. To get a clear image, the P and I value of the feedback controller is also

taken care of.

Firstly, the phase images are searched as the features are more visible in-phase image

(Fig. 2.10(a)), when the acceptable image [88] is found, then the parameters are changed to

find the height image(Fig. 2.10(b)). Here, the phase image acts as a guiding image. The features

of the height image are becoming visible after the flattening operation. From the live

monitoring, it is challenging to find the features of this sample. The 3D image is drawn after

the postprocessing (Fig. 2.10(c)). The explanation of the postprocessing is beyond the scope of

the thesis.

Fig. 2.10. SPM images of activated carbon a) Phase image b) height image c) 3D image.

(a) (b) (c) (d)

(a)(b) (c)


51 | P a g e

2.4 Characterization at zero coverage using Inverse Gas Chromatography (IGC)

Inverse Gas Chromatography (IGC) is a gas-solid technique for characterizing surface

and bulk properties of powders, particulates, fibers, films, and semi-solids. A series of vapor

pulses are injected through a column packed with the sample of interest. Unlike traditional

analytical gas chromatography, IGC is a physical chemistry technique using vapor probes with

known properties to characterize the unknown surface/ bulk properties of the solid sample.

The most interesting features of IGC are; i) it can be operated in both infinite and finite

dilution, ii) it can be used to measure surface characteristics at ambient temperature, iii)

designed to measure surface energy influenced by different components of adsorbent surface.

Infinite dilution is a term used for significantly low coverage where the adsorbate pressure is

in the Henry region. The probe injection technique and the detection technique are identical

parts of IGC, which make it possible to operate in a zero coverage region. Throughout this

thesis work, measurements in Henry region were performed with a rigorous analysis of

corresponding surface energies, which are not possible to use conventional thermogravimetric

and volumetric analysis.

2.4.1 Components of IGC

The main component of the IGC instruments is similar to conventional Gas

Chromatography (GC) equipment. It consists of an oven, column, solvent reservoir, detector,

mass flow controller, and a controller computer (Fig. 2.11). However, the column used in IGC

is different from that of GC; here, a cylindrical shaped glass column is used. However, in

several kinds of literature, different materials are found to use rather than glasses [89–91].

Another difference from GC is, in IGC, the sample is kept in the stationary phase, and the

probes are in the mobile phase. Samples/adsorbents are packed inside the glass column and

sealed with glass fiber wools. Due to this, there is no extra special sample preparation required.

Compare to other energy analysis, IGC requires minimum preparation for samples [92]. During

the experiment of surface energy measurements, a low concentration of well-characterized

single gas or vapor of a volatile substance is injected from the solvent reservoirs and carried

out by as inert gas through the sample column. The direction of the flow is illustrated in Fig.

2.11. This vapor/gas is termed as a probe molecule, the characteristics such as polarity, acidity,

molecular are, and the electron donor/acceptor numbers are known. The related chrematistics

of the adsorbents kept in the stationary can be analyzed by extracting the retention data (time

and volume).


52 | P a g e

Fig. 2.11. Schematic diagram of Inverse Gas Chromatography.

2.4.2 Probe molecule detection technique

The constant flow of the carrier gas is used to pass the probe molecules through the

column. During the passing, the probe molecules are adsorbed on the sample surface and

eventually eluded by the help of the inert carrier gas. It is assumed that the adsorption and

desorption equilibrium of the probe molecule on the stationary surface is established.

Generally, to achieve the equilibrium, two injection modes are used; i) pulse injection mode ii)

frontal injection mode [93]. In the pulse technique, a set volume of the probe is injected into

the carrier gas to be passed through the column, thus encounter the material under investigation.

On the other hand, probe molecules flow continuously through the column. Therefore, The

pulse technique is applicable for the systems that obtain equilibrium very quickly, and the

frontal technique is suitable for the system having slow rating equilibria [64].


53 | P a g e

Fig. 2.12. A typical chromatographic peak in IGC generated on the Maxsorb III sample by

ethanol probe molecule for 0.01 fractional coverage at 303 K. The probes are injected using

the pulse method.

Detection of the eluded probe molecules is one of the identical features of IGC, where

priority is given to selectivity and sensitivity factors. There are mainly two types of detectors

used in IGC; i) Flame ionization detector (FID), ii) Thermal conductivity detector (TCD). The

FID measures the concentration of organics by ionizing the probe molecules with flame. It is

highly sensitive to hydrocarbons; the major problem is detecting water. On the other hand,

TCD us the difference in thermal conductivity difference between the different components of

a mixture that makes it detect a wide variety of mixtures. Several types of research are already

addressed, comparing these two detectors [94,95]. In this work, the FID detectors are used as

the experiments were conducted in a lower pressure region where sensitivity is essential. From

FID signal, retention time (tR) and retention volume (VN) is measured for every probe molecule.

Deadtime (t0), another crucial data is extracted, which is required to calculate the actual

retention time. Deadtime indicates the time required for a probe molecule to travel the column

without any interaction (Fig. 2.12). The difference between tR and t0 indicates the time taken by

0

1000

2000

3000

4000

5000

6000

0 0.5 1 1.5 2 2.5

Norm

ali

zed

FID

sig

nal

[µV

]

Time [min]

Ethanol

Methane


54 | P a g e

the probe molecules to be adsorbed and eluded from the surface. The higher the difference, the

higher the interactive force if the retention volume remains constant.

2.4.3 Operation modes

Two chromatographic conditions can be used to operate the IGC experiments; i) infinite

dilution and ii) finite concentration. Infinite dilution is also known as zero surface coverage

condition, which is suitable for investigating the surface energy and the heat of adsorption of

particulate samples [96]. In this condition, a deficient concentration of the probe molecule is

injected and carried to the adsorption column targeting to adsorb a significantly lower quantity

of the probe molecules onto the surface. Due to the small amount of adsorption, it is assumed

that the adsorption will only occur in the higher energy sites of the surface. Because of the high

sensitivity of the FID detector (approximately 10-9 mol), it is possible to detect the lower

quantity of eluded probe molecules. In an ideal condition, it is expected that there is no probe-

probe interaction that leads to acquiring linear adsorption isotherm and symmetrical

chromatographic peak [94,97,98]. In finite concentration IGC, it is possible to obtain

adsorption isotherms where conventional measurements like volumetric analysis have shown

some limitations [99]. The limitation of the volumetric analysis is inherited due to the use of a

large quantity of adsorbent creates more void space and leads to the uncertainty in mitigating

dead zone. On the other hand, in IGC the concentration of the adsorbate is achieved by

introducing a large quantity of probe molecule to chromatographic system intend to occupy all

the targeted sites of the surface.

2.4.4 Applications of IGC

After the measurement of retention time (tR) and dead retention time (t0), the net retention

volume is calculated. The interaction time indicates the difference between retention time and

dead retention time.

Generally, 𝐹𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 =𝑉𝑜𝑙𝑢𝑚𝑒

𝑡𝑖𝑚𝑒

So, 𝐶𝑎𝑟𝑟𝑖𝑒𝑟 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 =𝐶𝑎𝑟𝑟𝑖𝑒𝑟 𝑣𝑜𝑙𝑢𝑚𝑒

𝐼𝑛𝑡𝑒𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒

That means, 0

Nc

R

VF

t t

Therefore, 0( )N c RV F t t (2.1)


55 | P a g e

In the above relation, James-martin, correction fraction j is used to mitigate the error that

occurred due to the variation of pressure drop and packing density.

Here

2

3

( ) 13

2 ( ) 1

IN

OUT

IN

OUT

PP

jP

P

, INP and OUTP are the inlet and outlet pressure, respectively.

So, 0. ( )N c RV j F t t

Sometimes, retention volume ( NV ) is presented by specific volume 0

gV [ml g-1]

0

0

273.15. ( )g c R

s

jV F t t

m K (2.2)

Where sm is the molecular mass of the probe molecule, and K is the test temperature. The

retention time and volume all are related to surface area and surface energy.

2.4.4.1 Isotherm measurement

Isotherms are the primary data that is measured by the IGC from where the surface area

can be calculated. The unique ability of IGC is to measure the isotherm at Henry region at

ambient temperature. However, in the case of measuring the surface area using these isotherms

are limited to lower surface area (<200 m2 g-1). The reason behind that, the probe gas used in

IGC has a larger molecular size than nitrogen. Despite the measurement of surface area, IGC

is an identical tool for measuring the Henry region isotherms.

The isotherms are usually measured using the pulse injection method, and the retention

time is detected using peak max value for a symmetric peak. If the peak is asymmetric, then

the ECP method is applied. The theoretical treatment for isotherm measurement from

chromatographic peak has been derived from Cremer and Huber approach [100,101]. The

corresponding pressure (P) and uptake (n) are calculated using the following two equations:

273.15

. . ..

cLoop inj

c c Loop

hP V P

F A T (2.3)

1 N

s

Vn dp

m RT (2.4)

Where ch is the height of the chromatographic peak, cA is the area of the peak, LoopV and LoopT is

the volume and temperature of the injection loop, and injP is the partial pressure of the loop.

As the isotherm data was measured in a very low concentration and the FID peak is

almost symmetrical, it is considered that the isotherms are in the Henry region. The interaction


56 | P a g e

occurs between the vapor phase probe molecule and the high-energy sites of the solid surface,

which are also useful to measure the thermodynamic paraments with the highest sensitivity.

The range of infinite dilution depends on the nature of the probe molecule and the heterogeneity

of the surface. The polar probe molecules usually adsorbed in heterogeneous surfaces and

generate asymmetric peaks even though the smallest injection size. Therefore, there is some

debate on the originality of representing the Henry region condition, but still, these

measurements are useful for practical consideration [102].

2.4.4.2 Surface energy measurement

Surface energy is one of the commonly measured characteristics by IGC to depict the

energetic situation of the adsorbent surface. The surface energy of solid is analogous to the

surface tension of a liquid, which indicates the amount of energy required to form a surface of

solid material in reversible conditions. In practical understanding, surface energy is a measure

of the reactive component of the surface. The higher the surface energy, the higher is the

reactive component. It influences the catalytic activity and the particle-particle interactions

[98,103]. Generally, the dispersive and specific surface energy are measured using IGC. The

dispersive surface energy is related to van der Waals interaction, whereas the specific

component is related to acid-base interaction [66]. The detail of surface energy measurement

procedures is described in chapter 3.

2.5 Approaches of this thesis

This thesis focuses on the improvement of adsorbent materials based on their influential

factors on the adsorption process. Initially, it is focused on finding an alternate characterization

technique that can provide additional information on the adsorbent surfaces which are absent

in the conventional techniques. However, more stress is given to finding suitable

characterization techniques for measuring surface activities, such as surface energy, entropy

and heat of adsorption. To measure these properties, inverse gas chromatography techniques

are rigorously used. To converge the focus, the analysis is confined to the Henry region. Based

on the analysis of surface energy, aluminum fumarate is modified using in-situ doping, and the

corresponding water uptake is analyzed. The uptake variation is further analyzed using surface

energy to find influencing factors. Fig. 2.13 illustrates the conceptual approaches of the thesis.

AFM and IGC are used to understand the additional information of the surfaces which are not

commonly available in the literature. Using the information generated from these additional


57 | P a g e

data, a different strategy is adopted to synthesize metal-organic frameworks, namely SMOFs.

The performance of the newly developed SMOFs is then studied with the application in water

adsorption systems. The results are analyzed with IGC to find the insights of improvement of

water adsorption characteristics.

Fig. 2.13. Approaches of this thesis.

Finding surface properties of

promising adsorbents to extract

information not available in

literature.

Characterization01

Building concept of intermediate

characterization technique to

mitigate the gap between MS and

ATE.

Understanding

the gap 02

Synthesis of new functional

adsorbents depending on the

results of characterization.Synthesis 03

Analysis of newly synthesized

materials to understand the

factors influencing the

improvements.

Insights04

Development of

functional porous

adsorbents

58 | P a g e

Chapter 3

Morphological Study of Porous Materials

using Atomic Force Microscopy Equation Chapter 3 Section 1

This chapter discusses a direct approach for assessing irregularly shaped pore distribution

of porous materials using Atomic Force Microscopy (AFM). Using the tapping mode, AFM is

employed to obtain the surface topographic information of the porous materials commonly

used in adsorption applications. Three different types of silica gels are investigated by an image

processing technique for the pore size-related information. Mesopores in a specified region is

visualized then quantified and counted using 2-D Fast Fourier Transformation (2D FFT)

technique. The results obtained by the AFM technique are then compared with the BET surface

area and pore size distribution (NLDFT) extracted from the N2 adsorption isotherms.

3.1 Material

Silica gel samples from several sources are used in this study; the first type of silica gel

is collected form Fuji silica Ltd (SS1), which is popularly used in an adsorption-based chiller

system. The second sample is a generic silica gel commercially used for humidity control in

food preservation (SS2). The last sample is CaCl2 coated silica gel, which is used for laboratory

applications (SS3). The unique feature of this silica gel type is its property to change its color

during the adsorption process.

3.2 Experimental apparatus and procedure

In this study, surface imaging of silica gel is carried out using a scanning probe

microscopy (SPM-9700, Shimadzu Corp., Japan). Atomic Force Microscopy (AFM) is one

type of SPM. Unlike the other microscopy techniques, SPM uses a sharp probe to measure the

interactive forces between the tip of the probe and the molecules on the surface of a sample.

The variation of the interactive forces contains a broad range of information on surfaces, such

as topography, viscosity, elasticity, surface energy, etc. By selecting the probe type and mode

of operation, different kinds of information can be obtained. For instance, silicon tip is useful

with the dynamic mode of operation for measuring the topographic variation of samples,

whereas in the nano-indentation testing diamond tip with contact mode is preferable. In this

CHAPTER 3

MORPHOLOGICAL STUDY OF POROUS MATERIALS USING

ATOMIC FORCE MICROSCOPY

59 | P a g e

study, tips made of monolithic silicon (NCHR-20, Nanoworld AG, Switzerland) are used with

phase mode to determine the three-dimensional topographic images.

3.2.1 Experimental apparatus

A schematic diagram of the SPM system operated in-phase mode is illustrated in Fig.

3.1. In this figure, the sample holder is shown in the center position where the sample is placed;

the precise feed of the holder is controlled by the piezoelectric scanner. In this equipment, the

movement of the cantilever is fixed to a lateral axis, and this movement is restricted only to

bring the cantilever near the sample. During scanning, cantilever remains in a fixed position,

and the sample moves in a controlled manner using the piezo scanner. The horizontal

movement is maintained by the parameters set before scanning, and the lateral movement is

controlled by a feedback controller. Lateral movement serves two crucial tasks; one is it keeps

the probe at a safe distance from the sample surface, and the second one is it carries the

information of forces acting vertically. SPM can be operated in various modes, and each mode

does a different task. For example, only topographic data can be retrieved during the operation

of phase mode; here, tips are usually moving very near to the sample surface to measure the

repulsive force between tip and sample. On the other hand, in non-contact mode, the probe

keeps a distance to measure the attractive force between tip and sample. Another favorite mode

is intermittent contact mode, where tip taps the surface and generates phase and topographic

data. In this mode, which is also known as phase mode, the cantilever is excited near to its

resonance frequency to measure the amount force generated due to both the attraction and

repulsion between sample and tip, which changes in accordance with tip-sample separation

distance[104]. These interactive forces shift the cantilever frequency, which can be detected by

an optical lever sensor. In amplitude-modulated SPM, the output signal contains noise, which

is eliminated by the lock-in detector. The equipment used in this study includes two lock-in

detectors to generate two separate signals comprising sine and cosine components to find the

phase differences.

CHAPTER 3



60 | P a g e

Fig. 3.1. The schematic diagram of the SPM system operated in phase mode.

3.2.2 Generation of three-dimensional images

SPM uses its probe to scan the surface of the sample in a raster scanning pattern, and the

scanning parameters like resolution, scanning speed can be set before the scanning process. In

this experiment, a scanning parameter was set to a resolution of 512 × 512 pixels with 1Hz

scanning speed. In other words, in each direction (x and y), 512 points per line are scanned

(Fig. 3.2(a)). Each line is scanned twice to confirm the reproducibility (Fig. 3.2(b,c)). Each

scanning point provides height information from the set point, which is registered in

accordance with its position, and this information is used to generate 3D topographic images.

X directional scanning often termed as “fast scanning direction” and Y directional as the slow

scanning direction. Due to slow scanning towards Y direction, drift error is introduced, which

changes the images [86].

CHAPTER 3



61 | P a g e

Fig. 3.2. Scanning pattern of SPM a) raster scanning pattern b) trace and retrace line graph

confirming reproducibility c) line graph showing corrupt scanning.

3.2.3 Measuring conditions

Silica gel samples are spherical, mesoporous and contain coercive surface, which makes

it challenging to capture images using SPM. Therefore, a careful and methodical approach to

carry out the experiments is necessary. The foremost step is a selection of suitable cantilever

capable of scanning the coercive surface of silica gel. A backside coated with Aluminum was

found to be ideal as it ensures that the detector can receive a strong signal in the form of intense

reflected LASER light. This signal reception allows for high accuracy in measuring the surface

variation with minimum artifacts. In the phase mode of operation, one of the most important

factors is to set the cantilever operating while investigating the porous surface, having a very

high specific surface area. The ideal frequency is the nearest smallest frequency situated in the

steepest descent path in the frequency vs. detector voltage curve. After many trials, it is found

that for high topographic variation samples, it is better to take the operating point as the lowest

possible frequency near the resonant frequency. Additionally, the amplitude of cantilever is

required to set to maximum to get initial images, which is optimized afterward by monitoring

the live images. If the set amplitude is larger than the optimized amplitude, the topographic

image may suffer from the lower resolution in the z-direction. On the other hand, a smaller set

512 points

51

2 li

ne

s

Fast scanning

Slo

w s

can

nin

g

Trace

Retrace

(a)

(b)

(c)

Fast scanning

CHAPTER 3



62 | P a g e

amplitude generates images of pores having inaccurate depth. The process is illustrated in Fig.

3.3(a), path 2 is preferable to others. This condition leads to the finding if the scanning area is

between 200 nm × 200 nm to 5 µm × 5 µm, the Mesopores become visible with a probe having

8 nm radius. As the primary focus of this study is to detect and count the pores, the scanning

area is set to 1.2 µm × 1.2 µm, which is optimum for this analysis. This setting allows us to

have an adequate amount of pores having proper depth in a single image, which is explained

thoroughly in a later section. Fig. 3.3(b) shows the actual scenario of performed scanning.

Fig. 3.3. Scanning spherical silica gel using AFM a) schematic of the surface of silica gel and

the influence of set point to optimize the movement of the SPM probe. B) 3D topographic

image of a silica gel surface with line profile showing height variation along a flat surface.

3.2.4 Measurement procedure

At first, the samples are slowly degassed at 85°C temperature and stored in a desiccator.

The chamber of the SPM equipment was maintained with fixed humidity (30% RH) and

temperature (40°C) to have similar surrounding conditions for scanning. Samples are attached

to the magnetic sample holder with double-sided adhesive tape. If the sample is not tightly

mounted on the sample holder, it will tend to move during scanning, which results in distorted

images. In SPM-9700, there are two kinds of approaches available; one is fast and another

slower. In a fast approach, cantilever comes closer to the sample using stepper motor controlled

devices. On the other hand, the slow approach uses a piezoelectric controller, which has a

500 nm

(a)

(b)

Path 1

Path 2

Path 3

500 nm

69.9 nm

Fist scanning direction

He

ight

0 nm

CHAPTER 3



63 | P a g e

precision in the micrometer range. If repeated scanning is required, a slow approach is

recommended, the fast approach is only used in the very beginning and in the time of removing

the sample. The samples are scanned in phase mode, which provides minute topography and

differences in properties on the sample surface. Another advantage of phase mode is active

scanning, which is its ability to track surface variation, which is often not visible in dynamic

mode. For example, minute topographic variation becomes visible after image processing, such

as flattening, but phase images can still detect the difference. This phase signals only show the

property variation through the XY plan, no significant effect on the properties along the z-axis.

However, it can generate substantial signal variation while it encounters edges. For this reason,

in our experiment phase, images are used only to detect a minor change of the surface

topography. Raw images are then undergone by several image processing to extract a clear

view.

3.2.5 Error analysis and minimization

The error generated in SPM is a result of quantum and chaotic phenomena. The source

of quantum phenomena is the interaction between tip and sample, whereas chaotic phenomena

are associated with the dynamic behavior of measurement technique and the control system

used in SPM equipment. Therefore, the overall uncertainty effects are linked with the

instantaneous phenomena both from fundamental physical limits (quantum), chaos, and various

kinds of instabilities, including technical faults in the microscope. It is complex to determine

the cumulative effects of the errors by analyzing individual sources and apply in the

measurement to understand data integrity. Therefore, a more simplistic approach was adopted

in this work, which can be termed as “dimensional measurement” error correction.

Dimensional measurement is vital in nanometrology because, in this method, standard three-

dimensional calibration gratings are used to measure the errors as well as the uncertainty

directly from the image data.

In this work, a standard 3D grating (TGG1, Tipsnano OÜ, Estonia) was used to determine

both the lateral and vertical variances. Generally, for the evaluation of uncertainty, an average

of many individual period measurements is taken from the grating profile. Additionally, the

grating itself is prone to have dimensional errors generated from the synthesis process. In the

case of TGG1, the dimensional error of the lateral period is ±0.05 μm, whereas the length is

3.0 μm. Similarly, the height of the period is 1.5±0.05 μm. Using the height measurement of

CHAPTER 3



64 | P a g e

the various periods of the 3D calibration gratings, it is possible to evaluate the uncertainty

satisfactorily. The uncertainty of the weighted average can be measured using the following

equation:

2

1

21

( )

1( )

ni

i in

i i

x

x

(3.1)

here i is a local variance belonging to the measured height value ix . Similarly, the

variance of this average can be expressed as the following equation:

2

2

21

2

1

1 1 ( )

11( )

ni

nx

i i

ii

x x

n

(3.2)

These equations can be used to calculate the root mean square deviation of the final

calibration grating pitch volume of the calibration grating pitch value. The measurement

process is illustrated in Fig. 3.4.

Fig. 3.4. Error measurement procedure, (a) 3D view of the standard calibration sample and its

corresponding line graph, (b) measured height of individual pitches of TGG1.

CHAPTER 3



65 | P a g e

Using the above equations, the weighted average of the height measurement is 1440 nm,

and the variance of this mean is 56 nm, which is 0.025% of the mean. Compare to the actual

measurement; the variance is negligible. The SPM-9700 equipment includes a build-in

calibration system, which provides the ability to correct the height measurement for each

sample. In this work, mean correction is used for every measurement prior to image processing.

3.2.6 Image processing

Post-processing of SPM data is carried out to obtain designated features. In order to

calculate the height, image leveling is required. As the pores are irregular shaped, choice of

wrong leveling methods may eliminate valuable information of the pores. For this purpose, a

plane fit leveling technique is used wherein a standard fitting determines the inner surface, and

then this surface is subtracted from individual data. Sometimes the inclination of the surface is

increased when a line of best fit differs from the profile inclination. After completion of this

flattening process, images are further treated by 1D Fourier Transformation (1D-FFT) to

eliminate the line noises. Then a filter is applied to select the noise frequencies, and finally, the

inverse transformation is used to attain a filtered image. Third-party software like Gwyddion

provides a 1D FFT filter to remove the noise frequencies by suppressing corresponding values

to neighboring frequencies [105]. Gwyddion (Version 2.49, 2017) is used in the present

investigation to remove horizontal line noise by using vertical direction 1D FFT filter.

3.3 Results and Discussions

AFM shows many identical features over BET and other imaging techniques, which are

advantageous for the characterization of porous materials. The most useful one is the extraction

of three-dimensional images which contain various important topographic information. For

instance, it is possible to measure the roughness of the surface using AFM. This surface

roughness data has a close relationship with adsorption isotherms; even the isosteric heat tends

to affect mostly by surface roughness in low the density region [106]. Besides, the

thermodynamic formulation of different adsorbent materials shows the dependency of water

uptake on a particular pore structure [107]. AFM can generate 3D data, which can be used to

better understanding of pore structures and shapes along with surface area measurements.

A conventional technique for measurement of pore size distribution and surface area is

by using gas adsorption technique, which is an indirect technique. The measured surface area

is not the actual surface area, rather tagged as BET surface area. Furthermore, the pore size

CHAPTER 3



66 | P a g e

distribution is estimated, which might vary in accordance with DFT modeling techniques. On

the other hand, AFM provides a direct measurement of surface area and counting of the surface

pore numbers even in ambient conditions. It’s vital for adsorption related research to analyze

the pore size distribution in ambient pressure and temperature, which is not possible in

conventional gas adsorption measurement. Table 3.1. provides a brief comparison of the AFM

technique with gas adsorption and other imaging techniques. In the case of traditional imaging

processes like TEM and SEM, there is no option for extracting height profile, which makes it

difficult for the detection of irregular pores and measuring surface area. In this study, the height

data from AFM is processed by watershed segmentation for detecting pores which are not

applicable for TEM/SEM data. However, a key limitation of AFM over gas adsorption is the

processed sample area often doesn’t represent the bulk properties. This limited scanning area

problem can be mitigated by taking multiple images at different locations of the same sample.

Table 3.1. Comparison of various advantages of atomic force microscopy with other

techniques.

Remarks Gas Adsorption TEM/SEM AFM

Mode of analysis of

surface morphology

Indirect Direct Direct

Sample treatment Not required Required Not required

Sample thickness

requirement

Not required TEM sample

thickness should be

at the nanometer

level.

Not required.

Required environment for

sample analysis

Low-level

vacuum is

initially required

High level of

vacuum condition

is required

No vacuum

condition is

required

Tracking of

morphological variation

on the different ambient

condition

Not possible Not possible Possible

Extraction of three-

dimensional

morphological data

Not possible Not possible Possible

Quantitative data analysis Possible Not possible Possible

CHAPTER 3



67 | P a g e

3.3.1 Porous properties

Porous properties, namely BET surface area, total pore volume and PSD of studied silica

gel has been estimated using N2 adsorption. The estimated surface area and pore volume of the

three different types of silica gel are shown in Table 3.2. The first two (SS1, SS2) of the silica

gels have a very high surface area compare to SS3. Similar differences are also observed in the

case of the pore volume. The last silica gel sample has a lower BET surface area, which can be

attributed to the impregnation of CaCl2.

Table 3.2. Porous properties of studied silica gel.

Type BET surface area [m2 g-1] Pore volume [cm3 g-1]

Silica gel-SS1 798±18 0.415



Silica-Alumina 206 0.59

Acetaminophen 0.2 -

N2 adsorption and desorption isotherms show significant differences in studied samples

(Fig. 3.5 (a)). For instance, N2 adsorption increases in accordance with the rise of partial

pressure, which is high for SS1 and SS2. Furthermore, SS3 shows hysteresis, which is absent

in the isotherm of SS1 and SS2. These dissimilarities are reflected in the PSD analysis done by

the NLDFT method, which is illustrated in Fig. 3.5(b).

CHAPTER 3



68 | P a g e

Fig. 3.5. BET-N2 experiment onto various silica gel (a) N2 adsorption/desorption isotherm (b)

NLDFT pore size distribution.

0

50

100

150

200

250

300

350

0 0.2 0.4 0.6 0.8 1

Qu

an

tity

ad

sorb

ed

[cm

³ g

-1]

Relative pressure [-]

Adsorption

desorption

SS1

SS2

SS3

Adsorption

Desorption

(a)

0

5

10

15

20

25

30

35

40

1 3 5 7 9

Increm

en

tal

area (

m²

g-1

)

Pore width (nm)

SS1

SS2

SS3

(b)

CHAPTER 3



69 | P a g e

3.3.2 Qualitative study of the surface using SPM

Using SPM, it is possible to generate three-dimensional images of pore distribution on

the surface, which can further enhance our understanding of porous surfaces. These 3D images

not only provide a pictorial presentation of the surface but also comprises essential information

that is absent in other visualization techniques. Representative flattened 3D images of the

surface of studied samples are shown in Fig. 3.6. All these images are obtained for a fixed

scanning area of 1µm2. Sample SS1 and SS2 have low variation in height, the maximum height

is around 100 nm, whereas, SS3 has about 150 nm. One of the probable reasons is the silica sol

in SS1 and SS2 have smaller dimensions leading to more minor undulations on the surface.

However, little cracks are visible in sample SS1, which are not visible in SS2. Sample SS3 has

a mountain-like artifact, which makes the surface very rough. From the 3D images, various

information can be extracted, which includes surface area and surface volume. Surprisingly,

SS2 shows a larger surface area (1.20046 µm2) compared to other samples. N2 adsorption data

showed SS2 has a lower BET surface area then SS1. The probable reason behind this

anomalous observation is the surface area measured by imaging is the total surface area, which

includes all kinds of topographic variation.

Fig. 3.6. Extended 3D images of various porous materials.

SS1 SS2 SS3

Silica-AluminaAcetaminophen

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70 | P a g e

3.3.3 Quantitative analysis of topographic images

In quantitative analysis, a different approach is adopted. Unlike the qualitative study, the

image is post-processed to filter out some of the topographic information. The extracted

information contains only the high topographic variation parts, which have a minimal

contribution to constituting Mesopores. The process of filtering is illustrated in Fig. 3.7(a). At

first, the SPM images, which are in a spatial domain converted (Fig. 3.7(a)) to the frequency

domain by FFT (Fig. 3.7(b)). In this converted image frequencies having high topographic

variation resides in the central region and contains peak intensity. These peaks can be selected

and removed. The filtered image can be transferred to the spatial domain by Inverse Fourier

Transformation (Fig. 3.7(c)). The effect can be compared visually by zooming the images. For

example, high topographic variation visible in Fig. 3.7(c) are absent in Fig. 3.7(e). Another

way to compare the filtering process is by analyzing the roughness. For instance, the former

figure has skewness 0.17 and Kurtosis -0.214. Skewness is the third statistical moment that

qualifies the asymmetry of the height distribution; here, a positive value means surface consists

of higher peaks rather than deeper valleys. Kurtosis is the fourth statistical moment that

qualifies the flatness of the height distribution as well as the width of the height distribution.

Usually, kurtosis value 3.0 means the distribution corresponds to a Gaussian distribution, and

the picks and valleys are sharp. Here the values of skewness and kurtosis indicate the pores are

in a minority region compare to cracks and large valleys, which makes it difficult to detect.

However, the later figure comprises different values, skewness -0.18 and kurtosis 3.3. This

comparison of statistical parameters indicates the pores are dominating and have sharp shapes.

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71 | P a g e

Fig. 3.7. Filtering high topographic information a) raw image which contains large height

variation, b) corresponding frequencies of the spatial image, c) image after applying filter d)

zoomed raw image, e) zoomed filtered image shows the existence of similar curvatures as in

the raw image.

The pores have irregular shapes that are difficult to detect by the traditional thresholding

algorithm. In this case, the watershed segmentation method is preferable because it usually

arbitrarily identifies local minima and finds the pore boundaries afterward [108]. Here the

problem of determining the bottom of pores is assumed as the problem of finding the local

minima of the surface. For convenience, the process can be explained in two steps. Firstly, to

find the location at each point of the surface, the virtual water drops are placed and compared

with other points to determine which drops reach the lowest point. In case of drops not reaching

the lowest minima, follow the steepest descent path to reach the local minima, which can be

controlled by step number. These result lakes of different sizes filling the surface depressions

and smaller lakes are removed considering as these are constituted by noises. The remaining

lakes are marked and used to identify the pores for the segmentation. In segmentation, virtual

drops are continuously supplied to fill the local minima. This site is controlled by a parameter

called pixel drop size. Finally, the pore boundaries and pore are determined by five different

scenarios and masked for a further process such as counting, measuring surface area, volume,

etc. [109]. In this experiment, various pixel drop size is used to find the proper skewness

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72 | P a g e

parameter. In Fig. 3.8(a) variation of surface area and skewness are shown for various silica

gel samples. It is found that for SS1, SS2, and SS3 silica gel, the skewness decreases until drop

size reaches 1.01. This statistical parameter stipulates that at the turning point, the identified

pores remain in the valley region, but after that, it expanded to the open flat areas, which may

not constitute the region of pores. Considering this, in the case of pore size determination, the

before mentioned drop sizes are taken.

Fig. 3.8. AFM experiment on various silica gel (a) variation of different drop sizes to determine

the effect on the surface area and the skewness values are shown in each bar graph, (b) pore

size distribution is showing surface porosity.

The masked pore regions are counted, and normalized PSD is shown in Fig. 3.8(b). Mean

radius data is considered as the pores are concave shaped, and this data indicates the mean

distance from the pore center of mass to its boundary. The trend of surface porosity shows a

significant peak in the distribution at 2–4 nm on the surface of every sample. However, SS2

another minor peak at around 12 nm whereas SS3 shows multiple small peaks in the porosity

distribution curve. This indicates slight heterogeneity in porosity for SS2 and SS3 samples.

The numerical values of pore count data are shown in Table 3.3. SS1 has the most substantial

number of pores on the surface, amongst which 238 mesopores are counted within the scanned

1µm2 of surface area. SS3 has the least pore counts among all the silica gel samples, which are

expected due to the impregnation of CaCl2. Interestingly, the trend in the number of pores

counted by the present method has a direct correlation with the surface area measured by the

BET-N2 method. SS1 has the highest BET-surface area, and so is the number of pores counted

0

5

10

15

20

25

30

0.51 1.01 2.01 3.01

Increase

d s

urfa

ce a

rea [

%]

Pixel drop size [%]

SS1 SS2 SS3

-1.2

8

-2.5

0

-1.9

0

-1.4

9

-2.6

9

-1.9

1

-1.4

4

-2.6

1

-1.5

7

-1.3

6

-2.2

7

-1.2

8

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 10 20 30 40 50

No

rm

ali

zed

po

re c

ou

nt

Mean pore radius [nm]

SS1

SS2

SS3

(a) (b)

CHAPTER 3



73 | P a g e

in this analysis. Whereas, SS3 has the lowest BET-surface area and also the least amount of

pores obtained by the surface analysis using AFM.

Table 3.3. Pore count in 1µm2 area.

Sample Total pore count Mesopores (pore width 2–50 nm)

SS1 305 238

SS2 257 207

SS3 208 140

Silica-Alumina 145 131

Acetaminophen 21 10

Furthermore, comparing the PSD obtained from AFM analysis (Fig. 3.8(b)) and as

predicted by NLDFT based BET-N2 method (Fig. 3.5 (b)), it is apparent that the two

distributions agree qualitatively. The majority of the pores present in all the samples are

<10nm. However, the NLDFT based BET-N2 method shows firm bi-disperse behavior,

whereas the PSD, unlike the PSD, measured by AFM. This dissimilarity may be due to the fact

that AFM shows the surface pore distribution, whereas the NLDFT based method predicts the

PSD for both external and internal pores. Nevertheless, a close agreement in PSD between the

two shows that AFM can be an alternative method to visualize and count the pores and not just

rely on the predicted values using the BET-N2 adsorption method.

This study is further extended to porous materials having a lower surface area than silica

gel to observe the rangeability of the AFM measurement technique. Fig. 3.6 shows the three-

dimensional topographic images of commercially existing two porous materials, namely Silica-

Alumina and Acetaminophen. The porous properties from BET and AFM measurements of

these samples are included in Table 3.2 and Table 3.3, respectively. Comparing these two

samples, Acetaminophen has the lowest BET surface area. Fig. 3.9shows that AFM is capable

of detecting a significantly smaller number of pores on the surface for such samples.

In the case of PSD measurement using AFM, Silica-Alumina shows a quite similar PSD

of studied silica gel (Fig. 3.9(a)). The surface structure of this sample is dominated by

mesopores ranging from 15 nm to 40 nm. On the other hand, a well-known medical sample,

namely Acetaminophen, having low BET, shows a wide range of PSD (Fig. 3.9(b)).

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74 | P a g e

Fig. 3.9. The pore size distribution of different porous material, (a) Silica-Alumina (b)

Acetaminophen.

3.4 Conclusions

The principal findings from the present studies can be summarized as:

(1) surface porosity measurement of porous materials like silica gel using direct imaging

technique has been investigated, (2) Using height images from Atomic Force Microscope,

depth base pore detection was established to identify irregular pores on the surface (3) the

maxima of PSD using the present technique agreed satisfactorily with the well-established

NLDFT method which proves the accuracy of the studied 3D imaging method (4) the observed

results showed that the homogenous PSD is dominating on the silica gel surface (5) total pore

count of the studied three silica gel samples found to be higher with the higher surface area

samples.

(a) (b)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 20 40 60 80 100

Norm

ali

zed

pore

cou

nt


Silica-Alumina

0

0.02

0.04

0.06

0.08

0.1

0.12

0 50 100 150 200 250 300

Norm

ali

zed

pore

cou

nt


Acetaminophen

75 | P a g e

Chapter 4

Surface Energy Characterisation of

Different Porous adsorbents by Inverse Gas

Chromatography Equation Chapter 4 Section 1

Porous silica gels are the well-established adsorbent materials used in adsorption cooling,

dehumidification, gas separation, and desalination applications. To understand the adsorption

characteristics, morphological characterization of the adsorbent is widely used. However, the

surface activities of the adsorbent material, which are the most influential parameters for the

adsorption process, is still unknown. To determine the surface activities, surface energy

analysis has been performed for four commercially available silica gels (RD granular silica gel,

Chromatorex, Home silica gel and B-type silica gel) using inverse gas chromatography

technique. The experiment is conducted at infinite dilution (0.008 to 0.1 coverage) with a fixed

flowrate (30 sccm) of helium gas. The result shows that RD silica gel has the highest value of

total surface energy for all the coverages. Several comparative studies with relevant insights

are also presented to reveal the possible field of improvement.

4.1 Introduction

Adsorption technologies hold great potential in energy conservation and conversion in

accordance with gas storage and heat transfer applications. Consequently, a considerable

number of researches has been conducted in this field in the past decade. As a result,

multifarious types of adsorbents are introduced with distinctive properties, such as silica gel,

activated carbon, zeolite, etc. Among these adsorbents, silica-gel exhibits excellent features

like high water adsorption capacity [110] along with long-time cycle stability [111], which

enabled silica-gel/water adsorption pair to be frequently used in commercial applications like

adsorption cooling [112,113], dehumidification [114,115], desalination [116–118], and gas

separation [119,120]. As solid-gas adsorption is a surface phenomenon, the characterization of

the surface properties of the adsorbent materials is vital to understand the performance before

applying in all the fields of applications. Morphological characterizations such as BET surface

area, pore size distribution, pore volume, etc. are the well-established methods of

CHAPTER 4

SURFACE ENERGY CHARACTERISATION OF DIFFERENT

POROUS ADSORBENTS BY INVERSE GAS CHROMATOGRAPHY

76 | P a g e

characterizing an adsorbent using nitrogen adsorption and mercury intrusion porosimetry.

However, these characterizations are simply the geometric parameters that do not provide any

information about surface activity; thus, they cannot truly explain the adsorption process. The

surface properties of the adsorbents are the crucial parameters that are mainly responsible for

any adsorbent’s affinity towards a particular adsorbate. For this reason, silica-gels are good

adsorbent for moisture adsorption while activated carbons are not. The fundamental knowledge

of the interaction between the adsorbent’s surface and the adsorbate molecules is vital for

further development of the functional porous materials. Therefore, the estimation of surface

chemistry is essential to understand the adsorption characteristics properly.

The surface activities can be estimated using surface energy measurement, which

provides information about dispersive and specific (acid-base) interactions between the

adsorbate molecule and the adsorbent surface. For example, surfaces having a higher value of

surface energy is more likely to become wet than the lower one [121]. In addition, the surface

energy is higher in the lower coverage region, indicating the possibility of high energy

dissipation during the adsorption process. There are many articles available that have explained

the fundamentals of physical adsorption using the heat of adsorption; however, there is hardly

any literature found that provides meaningful insights regarding the sources of this energy

dissipation in the form of released heat [122,123]. In this case, surface energy could provide

the different component associated with the energy dissipation. For instance, dispersive surface

energy is related to van der Waals attraction, whereas specific energy is to acid-base attraction

[64]. In carbon-based adsorbents, the dispersive surface energy is higher than specific surface

energy, indicating that in the adsorption process, the heat of adsorption is generated from the

van der Waals attraction between the probe and the adsorbent’s surface molecules [66].

Additionally, in adsorption related applications, the shape of the isotherms plays a vital role,

like as S-shaped isotherms are more suitable for adsorption heat pump applications [124]. A

universal model has recently been proposed to relate the shape of the isotherms with the surface

energy distribution, indicating the importance of the measurement of surface energy [77].

There are several methods used to measure the surface energy, such as Sessile Drop (SD),

Atomic Force Microscopy (AFM), Wilhelmy Plate (WP), Inverse Gas Chromatography (IGC),

etc. Among them, the IGC method is used for particulate samples and widely accepted

technique suitable for measuring surface energy component of the porous adsorbents

[125,126]. In this technique the adsorbent is kept in the stationary phase and several polar and

non-polar solvents are used to measure the surface energy components. Recently, IGC

CHAPTER 4



77 | P a g e

techniques are employed successfully to measure the surface properties of various modified

silicates [67,97,127].

The aim of the present work is to investigate the surface energy characteristics of various

silica gels with different porous properties. For this purpose, five polar and three non-polar

probes have been used for the measurement. The Dorris-Gray method is applied to determine

the dispersive component of the surface energy, and the polarization method is used for

determining the specific Gibbs free energy. Furthermore, a rigorous comparative analysis with

different carbon-based adsorbent has been presented to understand the surface energy variation

due to the change of adsorbent types.

4.2 Theory

Surface energy measurement was performed in infinite dilution, where the minimum

concentration of the probe molecule was injected. In infinite dilution, it is considered that the

interaction between adsorbate is negligible, and the adsorption process takes place in Henry’s

region [128]. The experimental results of IGC provide chromatographic data, which is then

used to calculate the surface energy components. The total surface energy has two components:

dispersive and specific. The dispersive component is calculated for Schultz or Dorris-Gray

method, whereas a specific component is calculated using the polarization method. The

necessary equations related to the calculation are presented in this section.

From the IGC experiment, the chromatograph results contain the information of retention

time (tR), which indicates the time required to generate a peak resulting from an interaction

between molecules and sample surface. However, this time includes the non-interaction time

of the probe molecules while residing inside the column. To measure the non-interaction time

period (t0), methane is purged through the column. Finally, the actual retention time can be

written as tR-t0. Using this time interval, retention volume VN can be measured. The retention

volume is the volume of carrier gas required to elude the probe gases from the sample. The

flow rate ( CF ) of the carrier gas has a direct relationship with retention time and volume.

Carrier gas flow rate, 0

NC

R

VF

t t

(4.1)

Or simply, 0( )N C RV F t t (4.2)

A correction factor known as the James-martin correction factor (j) is used to mitigate the

variation of VN due to the pressure drop and packing density.

CHAPTER 4



78 | P a g e

Where,

2

3

( ) 13

2( ) 1

IN

OUT

IN

OUT

P

Pj

P

P

(4.3)

here, INP and OUTP are the inlet and outlet pressure of the probe gases, respectively.

So, 0( )N C RV j F t t (4.4)

Sometimes, the retention volume can be written as the form of specific retention volume 0

gV

(ml.g-1) at 273.5K.

0

0

273.15( ) ( )g C R

s

jV F t t

m K (4.5)

According to thermodynamic, the interaction between adsorbate and adsorbent can be written as

follows:

0 0 lnads des NG G RT V C (4.6)

where, 0

adsG and 0

desG are the changes of molar gas free energy for adsorption and desorption

process, R is the gas constant (8.314 J K-1), and T is the absolute temperature (K). The constant C

represents the constant due to the differences of reference states. The two components of surface

energies (Dispersive and Specific) constitute the Gibbs free energy of adsorption ( 0

adsG ) [129].

Therefore, 0 D SP

ads ads adsG G G (4.7)

When only alkanes are used, it is assumed that there is no significant interaction due to specific

components,

so, 0 D

ads adsG G ( SP

adsG =0) (4.8)

In this case, the quantity of Gibbs free energy depends on the number of carbon molecules that

existed in the alkane probe molecule [126]. So, the free energy of adsorption can be written as:

0

ads A cross AG N a W (4.9)

where AN represents the Avogadro’s number (mol-1), crossa is the cross-sectional area of the

alkane probe molecule (m2) and AW is the work of adhesion (mJ.m-2). The work of adhesion has an

explicit relationship with the surface energy, especially with the dispersive surface energies of the

interacting surfaces[130].

2 D D

A S lW (4.10)

Where, D

S is the dispersive surface energy (mJ.m-2) of solid and D

l is the dispersive surface

energy (mJ.m-2) of probe molecules. Using Eq. (4.6), Eq. (4.9) and Eq. (4.10) it can be written as

follows:

CHAPTER 4



79 | P a g e

ln 2 D D

N A cross S lR T V N a C (4.11)

The equation (4.11) represents the relationship with experimentally found retention volume ( NV

) with the surface energy components. In classical thermodynamics, the reference state is three

dimensional, whereas the adsorption process is a surface-based phenomenon which is two dimensional.

The constant C represents the state difference constant, which is eliminated by taking the differences

between successive adsorption of alkanes. The slope of the plot . .ln NR T V as a function of D

la a

series of alkanes provides the quantitative value of dispersive surface energy ( D

S ) of the solid samples

kept in the stationary phase. The plot is also known as alkane line; it is further used for determining the

specific component of the surface energy (Schultz method) [131]. When the polar probe is used, the

points of the Gibbs free energy of these probes do not lie on the alkane lines. The vertical distance

between these points with the alkane line indicates the specific component of the surface energy.

Another method of measuring surface energy is the Dorris-Gray method, which is based on the

fact that the free energy of alkanes varies in a linear way with the number of carbon atom[132]. Here,

the slope of a plot between Gibbs free energy with carbon number provides the free energy component

of one methyl molecule, and it is assumed that the Gibbs free energy of desorption of each methylene

group is equal to the work of adhesion between the solid phase and mobile phase (hydrocarbon).

Hence, 1 2 4

2

2 2

( )

( ). .ln

n n

n n

C HCH Nads C H

N

VG K T

V

(4.12)

And the equation (4.10) can be written as, 2 2

2 D D

CH S CHW (4.13)

Similar to the equation (4.9) it can be presented as follows:

2 2

0

CH A CH AG N a W (4.14)

Finally, the dispersive surface energy of the stationary phase (solid) can be written as:

2

2 2

0

21.( )

4 .

CHD

S D

CH A CH

G

N a

(4.15)

The specific component can be measured from the Schultz method using the Von Oss, Chaudhury

and Good (vOCG) approach, where the specific free energy of two monopolar probes is used. The

equation of calculating the specific surface energy is as follows:

. .2( )S

A cross l s l sG N a (4.16)

The base and acidic parameters are measured respectively from the SG values of two different

monopolar polar probes. The equation (4.16) is reduced by Owens and Wendt by taking the two

distinguish monopolar acidic (dichloromethane) and basic (ethyl acetate) [133].

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80 | P a g e

2. .SP

s s s

(4.17)

However, the specific surface energy of the stationary is sensitive in selecting the reference

polarity. In the vOCG approach, the reference used is water, where : ratio becomes equal to unity

because of the amphoteric nature of water. This leads to the cause of “basicity catastrophe” [134]. To

mitigate the problem, Della Volpe and Shiboni et al. [135] suggested a new scale considering water as

acidic rather than amphoteric. Therefore, the scale of acidity to basicity creates a problem of identifying

the mono-polarity of the probes, which leads the measurement of specific surface energy to only use in

comparison rather than quantifying.

From the Dorris-Gray method, it is not possible to measure the specific surface energy; for this

purpose, the polarization method is used [136]. The polarizability ( ) of a molecule is a microscopic

phenomenon, which has two components; (i) electronic polarizability ( e ) and (ii) atomic polarizability

( a ). The atomic polarizability is comparatively very small, which is often neglected in that case

e , which has a relationship with macroscopic molar deformation polarization ( DP , cm3.mol-1).

Here,

2

2

4 1

3 2D e

n MP N

n

(4.18)

where M and n are the molar mass (g.mol-1), the molar volume (cm3g-1) and the refractive

index, respectively. These values are measurable and identical for a particular probe. For example,

corresponding values of M, and n for n-heptane are 100.21 g mol-1, 0.6838 cm3 g-1 and 1.3876,

respectively [136]. The molar deformation polarization ( DP ) can be related with the RT.ln (VN) with

the following equation:

1 2ln ( )SP

N Dl DSRT V C C P P G (4.19)

where C1 and C2 are two constants relating to similar reference states and DlP DSP are the molar

deformation polarization of probe and the stationary phase, respectively. If the alkanes are used for

analysis, the specific attraction ( SPG ) is negligible. In that case, the equation (4.19) can be written as:

1 2ln N Dl DSRT V C C P P (4.20)

The plot ln NRT V as a function of DlP forms a straight line and the slope of the plot is equal to

2 DSC P which is proportional to the alkane lines presented in the equation (4.11). In the case of polar

probes, the values ln NRT V result from both the dispersive and specific interactions. Therefore, the

vertical distance between the ln NRT V point to the straight line indicates the specific component of the

Gibbs free energy due to the interaction between the polar probe and the stationary phase.

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81 | P a g e

4.3 Materials

Four different types of silica gels (Fuji Silysia Chemical Ltd., Japan) having various

porous properties are used in this work to investigate the surface energy components. The

porous properties of the studied silica gels are presented in Table 4.1. All the silica gel sample

shows two significant peaks in the pore size distribution. Notably, RD silica gel and

Chromatorex both have two peaks, one in the microporous region and another one in the

mesoporous region [137].

Table 4.1. Porous properties of the selected silica gel samples [137].

Silica gel type Particle

diameter

(mm)

BET surface

area

(m2 g-1)

Total pore

volume

(cm3 g-1)

Pore size

distribution

Peak

(nm)

Range

(nm)

RD silica gel

granular D ≈ 0.7 to 1.18 775 ± 9 0.392

0.74,

1.18

0.68 – 0.86,

1.09 – 5.18

B-type silica

gel

D ≈ 1.7 to 4.0 487 ± 2 0.803

1.40,

6.90,

1.20 – 1.70,

2.40 – 14.0

Home silica gel D ≥ 0.5 565 ± 2 0.700 1.40,

5.40

1.20 – 1.80,

2.40 – 9.80

Chromatorex

silica gel

D ≈ 0.075 to

0.5 753 ± 6 0.422

0.73,

1.18

0.68 – 0.85,

1.10 – 6.98

4.4 Experimental

Inverse gas chromatography experiments were conducted using IGC equipment supplied

by Surface Measurement Systems, UK. In the experiment, the probe gases were carried out by

a carrier gas (Helium) and adsorbed in the adsorbent surface, which were then eluded from the

surface using the same carrier gas. The eluded probe gas was detected through the Flame

Ionization Detector (FID), and the chromatographic peak was acquired. The schematic diagram

of the IGC equipment is illustrated in Fig. 4.1.

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82 | P a g e

Fig. 4.1. Schematic diagram of Inverse Gas Chromatography.

Five different polar solvents and three non-polar solvents were used as probe molecules;

the properties of the probes are summarized in Table 4.2. The solvents were placed in the

chamber as liquid and evaporated by providing an adequate amount of heat.

Table 4.2. Physicochemical properties of molecular probes employed for conducting surface

energy experiments [66].

Molecular probes

Cross-sectional area [m2]

Dispersive surface tension [J m-2]

Molecular mass [g mol-1]

AN* [kJ mol

-1]

DN [kJ mol-1]

Polar

Acetone 3.40E-19 0.0165 58.08 10.46 71.13

Acetonitrile 2.14E-19 0.0275 41.05 35.64 58.99

Dichloromethane 2.45E-19 0.0245 84.93 16.32 0

Ethanol 3.53E-19 0.0211 46.04 43.10 79.50

Ethyl acetate 3.30E-19 0.0196 88.11 6.28 71.55

Non-

polar

Hexane 5.15E-19 0.0184 86.18 - -

Heptane 5.73E-19 0.0203 100.21 - -

Octane 6.30E-19 0.0213 114.23 - -

DN represents the donor number of the polar probes, and the AN* is the

acceptor number corrected with the van der Waals interactions.

All the experiments were conducted under a similar carrier gas flow rate (30 sccm) and

column temperature (363 K). However, in the case of RD granular silica gel, the experiment

MASS FLOW CONTROLLER

PROBE GAS INJECTION

CONTROLLER

‘Dry’ carrier gas

‘Wet’ carrier gas

Pulse injecting

FID

Detected pulse

Isotherm

Desorbed gas

Packed sample

SAMPLECOLUMN

PROBE GAS RESERVOIR

CARRIER GAS

RESERVOIR

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83 | P a g e

was conducted at 373 K column temperature. Due to the experimental limitations, the

chromatographic peaks were not available at that temperature. To conduct the experiment in

infinite dilution, the minimal surface coverage was maintained, and the targeted surface

coverages were 0.008 to 0.1.

4.5 Results and discussion

4.5.1 Surface energies

The total surface energy of adsorbents is influenced by the nature of the material, origin,

and the functional group existed on the material. The typical IGC experiment provides

information on the retention time of the probe molecule and the retention time of the non-

interacting carrier gas (Helium). These two values of retention times were used to measure the

retention volume employing Eq. (4.5) And the corresponding plot of retention volume versus

the surface coverage for RD granular silica gel is illustrated in Fig. 4.2(a). According to this

plot, the retention volume continuously decreases with the increase of surface coverage. The

reseason behind that is the interactions between the probe molecules and the surface become

weaker with the increase of surface coverages. Therefore, less amount of carrier gas was

required, and consequently, retention volume was decreased. Similar characteristics were

found for other studied silica gels.

Generally, Schulz and Dorris-Gray methods are used to determine the dispersive

component of the surface energy. The detail of these models is described in the previous

section. According to the theory, the Schultz method is applicable at room temperature and

does not provide any solution for the different dimensional reference problem. On the other

hand, Dorris-Gray is applicable in high temperature, and surface energy measurement is free

form reference dimension variation problem [138,139]. Therefore, the corresponding Eq.

(4.15) was employed to calculate the dispersive surface energy. The plot of ln NRT V versus

carbon number for RD granular silica gel and non-polar (alkane) probes shows linear trends

(R2>0.99), which is illustrated in Fig. 4.2(b). These kinds of alkane lines are acquired for all

other studied samples, and the surface energy calculated in a similar fashion.

CHAPTER 4



84 | P a g e

Fig. 4.2. (a) Retention volume (VN) of alkanes (b) fitted alkane series obtained with RD

granular silica gel.

500

700

900

1100

1300

1500

1700

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11

Rete

nti

on

volu

me (

VN

) [m

l g

-1]

Surface coverage [-]

Hexane

Heptane

Octane

(a)

R² = 0.999

R² = 0.9983

R² = 0.9984

R² = 0.9993

R² = 0.999

R² = 0.9928

8000

10000

12000

14000

16000

18000

20000

22000

5 6 7 8 9

RT

ln

VN

[J m

ol-1

]

Carbon number [-]

0.008

0.01

0.03

0.05

0.09

0.1


Hexane

Heptane

Octane(b)

CHAPTER 4



85 | P a g e

Another interesting measurement that can be performed is the isotherm analysis in

Henry’s region. Generally, it is quite difficult to acquire the isotherm data in the lower pressure

region where uptake vs. pressure plot is linear. In the Henry region, the isotherm plot provides

information on the strength of affinity between adsorbate and adsorbent. The quantity of

pressure and uptake can be calculated from Eq. (4.21) and (4.22), respectively [101].

273.15i PEAK

PEAK

n R Hp

F A

(4.21)

Where in is the number of moles injected, R is the universal gas constant, HPEAK is the

FID signal peak, F is the carrier gas flow rate, and APEAK is the peak area of the corresponding

FID signal.

0

1p

Nn V dPR T

(4.22)

here, VN is the retention volume.

The experimentally found isotherm data for one non-polar (Heptane), and one polar probe

is illustrated in Fig. 4.3. The experiment is conducted for RD granular silica gel at 373 K and

other studied samples at 363 K. Therefore, the uptake for RD granular silica gel is lower than

that of Cromatorex silica gel. The uptakes for both probe solvents are higher for chromatorex

silica gel than others. This data follows the surface area variation of the studied samples. The

surface area of Home silica gel and B-type nearly similar, which results in the similar trends in

the solvent uptake values.

Fig. 4.3. Adsorption uptake of (a) Heptane and (b) Dichloromethane onto different types of

silica gels.

0

10

20

30

40

50

60

70

80

90

0 0.01 0.02 0.03 0.04 0.05 0.06

Up

tak

e [m

g g

-1]


RD granular silica gel

Home silica gel

Chromatorex silica gel

B-type silica gel

0

5

10

15

20

25

30

35

40

45

0 0.005 0.01 0.015 0.02

Up

tak

e [m

g g

-1]



Home silica gel


B-type silica gel

(b)(a)

CHAPTER 4



86 | P a g e

The dispersive surface energy, specific surface energy and the total surface energy for all

the studied silica gels are summarized in Fig. 4.4. For specific surface, energy determination

Della Volpe scale was employed where two non-polar probes ethyl acetate (l

475.67 mJ m-

2 and l

0 mJ m-2) and dichloromethane (l

0 mJ m-2 and l

124.58 mJ m-2 ) were used

in the IGC experiment [66]. Not to mention, the surface energy values decrease with the

increase of surface coverage. This indicates that the surfaces of studied silica gels are

energetically heterogeneous. Furthermore, the dispersive surface energies of RD granular silica

gel and chromatorex are higher than the other two silica gels having a lower surface area,

indicating a direct relationship between dispersive surface energy and surface area. However,

this trend doesn’t suitable for specific surface energy. Home silica gel exhibits moderately

higher specific surface energy while having the lowest surface area among the studied silica

gels. In the case of total surface area, RD granular silica gel is dominating; however, the surface

energy values for chromatorex aligns with the other two types of silica gels. This might be the

result of higher specific surface energies of the home and B-type silica gels.

0

20

40

60

80

100

120

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11

Dis

persi

ve s

urfa

ce e

nergy [

mJ m

-2]


Home silica gel


B-type silica gel


(a)

CHAPTER 4



87 | P a g e

Fig. 4.4. Comparison of (a) dispersive; (b) specific; and (c) total surface energy among the

different types of silica gels.

4.5.2 Gibbs free energy of polar components

Gibbs free energy of the polar probes was determined by the Polarization method. The

plot RTlnVn versus PDl for RD granular silica gel at 0.03 coverage is presented in Fig. 4.5

0

5

10

15

20

25

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11

Sp

ecif

ic s

urfa

ce e

nergy [

mJ m

-2]


Home silica gel


B-type silica gel


(b)

0

20

40

60

80

100

120

140

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11

To

tal

surfa

ce e

nerg

y [

mJ

m-2

]


Home silica gel


B-type silica gel


(c)

CHAPTER 4



88 | P a g e

using the Eq. (4.23). All polar probes are located above the alkane line indicating that the

studied samples contain reactive surfaces. Furthermore, all the polar probes exhibit the higher

value of specific Gibbs free energy except dichloromethane, which has dominating l

value

indicating the nature of the surface is slightly basic at 0.3 coverage.

Fig. 4.5. The typical diagram for determining the specific Gibbs free energy (ΔGSP) by

polarization method for RD granular silica gel at 0.03 coverage.

To provide a detailed explanation of the different surface properties, the specific Gibbs

free energy of all the studied samples at two different coverages are summarized in Fig. 4.6.

The silica gel samples exhibit similar trends of Gibbs free energy; however, the specific affinity

of chromatorex towards ethyl acetate ( l

) is comparatively lower than others. Though the

surface area of chromatorex is almost similar to RD granular silica gel, there is a significant

difference in specific energy observed. In case of energy variation due to an increase of surface

coverage, a decreasing trend was detected for all studied samples.

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35 40 45

RT

lnV

n [

KJ m

ol-1

]

PDl [cm3 mol-1]

ΔGEthyl acetate

Hexane

Heptane

Octane

Dichloromethane

Acetonitrile

Ethanol

Acetone

Ethyhl acetateMethod: Polarization

Coverage: 0.03

CHAPTER 4



89 | P a g e

Fig. 4.6. Gibbs free energy changes of adsorption for polar probes (a) surface coverage of

0.03 and (b) surface coverage of 0.05.

26.6

1

28

.66

28

.84

29

.4

9.9

1

25

.06

23.5

4

22.3

5

20.0

2

8.7

9

17.8

8 22.4

8

24

.22

24.3

9

7.6

21.3

2

23.7

5

23

.91

23.6

3

9.2

9

0

5

10

15

20

25

30

35

Eth

yle

acet

ate

Aceto

ne

Aceto

nit

rile

Eth

an

ol

Dic

hlo

rom

e

than

e

Gib

bs

free e

nergy o

f ad

sorp

tion

, Δ

G [

kJ m

ol-1

] RD silica gel Home silica gel

Chromatorex silica gel B-type silica gel(a)

24

.68

26.4

3

26.8

2

28.5

6

7.9

6

24.3

4

21.9

9

21.3

6

19.3

8

8.1

5

14

20

.12

22.3

4

21.9

9

6.5

6

17

.41

20.9

2

22

.94

22.3

7

8.3

4

0

5

10

15

20

25

30

35

Eth

yle

acet

ate

Aceto

ne

Aceto

nit

rile

Eth

an

ol

Dic

hlo

rom

e

than

e

Gib

bs

free e

nergy o

f ad

sorp

tion

, Δ

G [

kJ m

ol-1

] RD silica gel Home silica gel

Chromatorex silica gel B-type silica gel(b)

CHAPTER 4



90 | P a g e

4.5.3 Effect of morphology and comparison among various adsorbents

The morphological characteristic of the studied silica gels was determined by the nitrogen

adsorption technique (Table 4.1). The specific surface area of the silica gel samples is plotted

against the dispersive surface energy to determine the relationship between these two important

characteristics (Fig. 4.7 (a)). It is important to note that the silica gels having a higher surface

area exhibits a higher value of dispersive surface energy. This is because a higher surface area

indicates a high probability of having additional energy sites. On the contrary, the surface areas

of RD granular silica gel and Chromatorex are almost similar; however, the surface energy of

Chromatorex is significantly lower than the RD granular silica gel. This lower value of

dispersive surface energy could be explained form the pore diameter point of view (Fig. 4.7

(b)). Generally, in mesoporous silica gels, mesopores had no effect on the alkane groups

because of large pore diameters [140]. The double field effect is not applicable to the alkanes;

however, micro-pores might show some influence. In the case of RD silica gel, the range of

the pore distribution is from 1.09 – 5.18 nm, whereas in Chromatorex, this range is from 1.10

– 6.98 nm. As the pore size distribution of Chromatorex is slightly aligned in the mesoporous

region, the double-wall effect is less effective than RD granular silica gel.

Fig. 4.7. Correlation between the dispersive component of surface energy and morphological

characteristics. (a) variation with specific surface area, (b) variation with pore size distribution.

Fig. 4.8 represents a comparative illustration of the dispersive surface energy of various

adsorbents. All the carbonaceous samples show a significant high value of dispersive surface

energy than that of silica gel samples. One of the reasons behind that the alkane samples shows

high affinity towards carbonaceous materials than silica-based one. Besides, carbonaceous

0

10

20

30

40

50

60

70

80

90

100

487 565 753 775

Dis

per

siv

e su

rfa

ce e

ner

gy [

mJ

m-2

]

Surface area [m2 g-1]

0.03 (surface coverage)

0.05 (surface coverage) RD

silica gel

Chro

mato

rex

B-ty

pe silica g

el

Ho

me silica g

el

(a)

0

10

20

30

40

50

60

70

80

90

100

0.68 – 0.86,

1.09 – 5.18

0.68 – 0.85,

1.10 – 6.98

1.20 – 1.80,

2.40 – 9.80

1.20 – 1.70,

2.40 – 14.0

Dis

per

siv

e su

rfa

ce e

ner

gy [

mJ

m-2

]

Pore size distribution range [nm]



RD

silica gel

Chro

mato

rex

Ho

me silica g

el

B-ty

pe silica g

el

(b)

CHAPTER 4



91 | P a g e

materials pose rich surface characteristics, such as large surface area and dominating

micropores. These texture properties not only increase the energy sites but also shows a double

wall effect. All the studied silica gel samples have dispersive surface energy value below 100

mJ m-2, but due to many surface functional groups, these sample shows a higher value of

specific surface energy than carbonaceous samples. However, silica gels are well-established

adsorbents for water, only lacking large surface area that affects the adsorption uptake. Surface

energy study shows that it is possible to increase the specific surface energy that might improve

the uptake capacity.

Fig. 4.8. Comparison of dispersive surface energy among the various adsorbents. (1st 4 silica

gels (measured data); SBA-16, SBA-15 [140]; Maxsorb III [66]; A-20 (measured data); and

Chemviron F400, Norit SA4 [125]).

4.6 Conclusions

The dispersive and specific surface energy of commercial silica gels are investigated to

find the possible improvement factors. Silica gels are well-known adsorbents used for

93

.14

59.6

4

49

.98

41

.8

71

.93

56.0

3

213

.98

21

0.0

3

20

8.2

12

5.3

50

50

100

150

200

250

Dis

persi

ve s

urfa

ce e

nerg

y [

mJ

m-2

]

Porous silica gelsPorous activated

carbons

CHAPTER 4



92 | P a g e

multifarious applications; however, due to moderate surface area, adsorption uptake is not

good. This study shows that the dispersive component of the surface energy is implicitly

correlated to texture properties like surface area and pore size distribution. RD silica gel has a

higher value of surface energy components. However, chromatorex, while having a similar

surface texture of RD silica gel, shows significantly lower values of surface components. A

possible explanation of the lower surface energy is the domination of mesoporous pores in

chromatorex rather than that of RD silica gel. In this study, the additional adsorption isotherms

at Henry region are presented to find the molecular affinity between the adsorbate and

adsorbent molecules. The specific Gibbs free energy for all studied silica gel also presented to

determine the surface activities. The specific Gibbs free energies for all the probe solvents have

almost similar trends for all studied silica gels where the dispersive surface energy varies

significantly. This finding reveals the crucial information to improve the surface characteristics

of silica gel for further development. It is expected that by increasing the specific surface

energy, it is possible to improve the adsorption uptake where there is a limitation of increasing

the specific surface area.

93 | P a g e

Chapter 5

Experimental Investigation of Adsorption

Isotherms and Heat of Adsorption at Henry

Region for Activated Carbon/Ethanol Pairs Equation Chapter 5 Section 1

This chapter presents a thermodynamic formulation to capture the relationship between

the adsorption interaction energy and adsorbent-morphological property, which can be further

extended into surface coverage. Employing the inverse gas chromatography method, the

adsorbate uptakes are measured in Henry’s law region, from which the energetic behaviors of

a single component adsorbate-adsorbent system are calculated in terms of the enthalpy and the

entropy of adsorbed phase in very low pressures. A thermodynamic trend is established

between the specific entropy and Henry’s law constant, including the pore volume of

adsorbents, and one can predict the isosteric heats and adsorbent’s pore size for activated

carbon + ethanol system by extending the proposed linear trend. These findings could

significantly contribute to tailoring the adsorbent materials for the design of an adsorption bed

with a minimal or maximum driving force depending on the types of heat transformation

applications.

5.1 Introduction

Isosteric heat at Henry region provides the necessary information about the optimum pore

size to which the adsorbate molecules adsorb most strongly [122]. At the Henry region, the

partial pressure of the adsorbate is very low, which results in a low concentration of adsorbate

molecules. As there are fewer adsorbate molecules, the adsorbate-adsorbate interaction can be

considered negligible when compared with the adsorbent/adsorbate interaction. Moreover,

every adsorbate molecule gets an equal opportunity to explore the adsorbent surface without

any effect of its surroundings before adsorbing onto one of the high energy sites [141].

Therefore, the isotherm in the Henry region shows a linear relationship between the adsorption

uptake and the partial pressure of the adsorbate gas, where the slope of the line is defined as

the Henry constant [142]. In other words, Henry constant is the fundamental parameter that

CHAPTER 5

EXPERIMENTAL INVESTIGATION OF ADSORPTION ISOTHERMS

AND HEAT OF ADSORPTION AT HENRY REGION FOR

ACTIVATED CARBON/ETHANOL PAIRS

94 | P a g e

describes the primary interactions (both van der Walls and columbic) between the adsorbent

surface and adsorbate molecules.

Both the enthalpy and entropy of the adsorbed phase are considered two major factors to

characterize the adsorption phenomena. The first one accounts for the released heat during

adsorption whilst the second one indicates the driving force to provide a pivot between the

equilibrium and non-equilibrium conditions of an adsorption system. These two driving forces

have an insightful impact on adsorption based cooling, heating, refrigeration, and gas

separation/storage applications [123,143–145]. Despite having a profound contribution to the

energetics of adsorption, the adsorbed phase entropy for any adsorbent/adsorbate pair remains

difficult to predict [146]. Moreover, the knowledge of adsorbed phase entropy as a function of

surface coverage indicates the adsorbed phase molar volumes, which are useful in describing

adsorption data in the high-pressure region using potential theory [147]. Regardless of having

vital importance in the studied field, there is hardly any experimental data available in the zero-

coverage region.

The chapter begins with the ethanol adsorption on four different activated carbons,

namely Maxsorb III, WPT-AC, M-AC, and H2-treated Maxsorb III in Henry’s region (i.e., at

very low pressure). Based on isotherms data, Henry’s law coefficient is calculated, and the

driving force related to the enthalpy and entropy is predicted. Employing the proposed

formulation, a thermodynamic trend is presented, which dispels the confusions between

adsorbed-phase-entropy and Henry’s law coefficient with the type of adsorbent structure.

5.2 Material

Activated carbon (AC) based materials were used in this study, which exhibits high

surface area promising for adsorption applications. The studied materials are i) Maxsorb III

(as-received, highly porous activated carbon powder supplied by Kansai Coke & Chemical Co.

Ltd. Japan; ii) Waste palm trunk AC (WPT-AC), synthesized through carbonization process

and activated using KOH [148]; iii) Mangrove AC (M-AC), synthesized through carbonization

process and also activated using KOH [148]. iv) H2 treated Maxsorb II was prepared by flowing

H2 at reducing the atmosphere at 600˚C [66]. The porous properties of the studied samples are

summarized in Table 5.1.

CHAPTER 5




95 | P a g e

Table 5.1. Porous properties of the studied AC samples[66,148].

Adsorbent Total surface area [m2 g-1] Pore volume [cm3 g-1]

Maxsorb III (as-received) 3299 1.72

WPT-AC 2927 2.51

M-AC 2924 2.18

H2-Maxsorb III 3019 1.48

5.3 Experimental

The experiments for measuring Henry region isotherms and zero-coverage heat of

adsorption for ACs-ethanol pairs were carried out using Inverse Gas Chromatography-Surface

Energy Analyzer (iGC-SEA) [Surface Measurement Systems Ltd. UK]. The major component

of the equipment is an oven (20-150˚C); two sample columns; twelve solute reservoirs (for six

polar and six non-polar solvents); a flame ionization detector (FID); a mass flow controller;

and a computer as the processor and controller. The schematic diagram of the equipment is

shown in Fig. 5.1. The activated carbon samples of 2.8-3.5 mg were filled into typical sterile

glass columns (30 cm length, 3mm inner diameter). Both ends of the columns were plugged

with silane-treated glass wool, and for mitigating the samples sticking on the walls, mechanical

vibration was used. Before measurement, all the samples were conditioned at 140˚C for 3 h.

Helium was used as the carrier gas to assist the mobility of the purged gases. Initially, methane

was purged and carried using the carrier gas through the column for six minutes to measure the

dead time. The sharp peak of methane was used to confirm the initialization of the working

environment.

CHAPTER 5




96 | P a g e

Fig. 5.1. Schematic diagram of IGC-SEA equipment.

The measurement condition for Henry isotherms and zero coverage heat of adsorption is

different. For isotherms measurement, Ethanol is first evaporated from the polar probe chamber

and carried to the FID detector through the sample column. During this process, ethanol is

adsorbed in the activated carbon samples and eventually eluded by helium gas. The ethanol

adsorption for all the studied samples was conducted at 303 K to 343 K temperatures. The

fractional surface coverage ( n/nm= 0.001 to 0.055) was taken as lowest as possible to

determine the isotherms at the Henry region. The corresponding relative pressure (P/P0) ranged

from 0 to 0.015. In the case of the heat of adsorption measurement, ethanol adsorption was

conducted for various temperature conditions (303 K to 373 K) at a wide range of fractional

coverage ranges from (m/nm= 0.1 to 0.5).


5.4.1 Isotherms at Henry region

In the IGC experiment, the measurement can be configured in two ways; i) frontal ii)

pulse experiment. In the frontal experiment, a probe molecule is added continuously to the

carrier gas, whereas in pulse experiment, a single injection of a certain amount of vapor is

added with the carrier gas. The vapor is then transported to the IGC column, where it adsorbed

MASS FLOW CONTROLLER

PROBE GAS INJECTION

CONTROLLER

‘Dry’ carrier gas

‘Wet’ carrier gas

Pulse injecting

FID

Detected pulse

Desorbed gas

Packed sample

SAMPLECOLUMN

PROBE GAS RESERVOIR

CARRIER GAS

RESERVOIR

ANALYSIS UNIT

CHAPTER 5




97 | P a g e

in the sample. The adsorbed amount then eluded from the sample by the carrier gas, and in the

ideal case, an equilibrium state is reached. In the frontal experiment, a breakthrough curve is

formed. However, in the pulse experiment, a peak is generated in the FID detector. The shape

of the peak depends strongly on the shape of the solid-vapor sorption isotherm. The resultant

peak of the Maxsorb III/ethanol at 303 K is shown in Fig. 5.2 (Step 1). This is the first step of

the isotherm measurement. The shape is not a perfect symmetric due to the highly

heterogeneous nature of the activated carbon; however, this was the measurement that had been

taken in the lower pressure region. Fig. 5.2 is a simplified version of the core measurements,

where more equilibrium points are involved. Each of the presented curves of Fig. 5.2 represents

the chromatographic peak for a particularly targeted coverage.


K)

Measurement of equilibrium partial pressure (P) is the second step of isotherm

calculation (Step 2). The detail of the calculation to determine the isotherm is given by Cremer

and Huber [100,101]. The equilibrium partial pressure (P), for each concentration (or coverage)

150

650

1150

1650

2150

0 5 10 15 20 25 30

FID

sig

na

l [μ

V]

Time [min]

0.05

0.04

0.03

0.02

0.01

STEP 1

CHAPTER 5




98 | P a g e

of vapor in the column, can be calculated from the chromatographic peak (Fig. 5.2) using

Eq.(5.1).

273.15. . .

.

cLoop inj

c c Loop

hP V P

F A T (5.1)

Where ch is the height of the chromatographic peak, cA is the area of the peak, LoopV and

LoopT is the volume and temperature of the injection loop, and injP is the partial pressure of the

loop. The calculated partial pressure of the Masxorb III/ethanol pair for 303 K temperature is

illustrated in Fig. 5.3.


K)

In the next step, the adsorbed amount is calculated from the corresponding retention

volume (Step 3). The adsorbed amount, n, and therefore the adsorption isotherm for each probe

vapor can be calculated by integration of NV versus P, which can be expressed by the Eq. (5.2)

. The calculated isotherms are shown in Fig. 5.4.

11100

11300

11500

11700

11900

0 20 40 60 80

Rete

nti

on

vo

lum

e [

ml

g-1

]

Partial pressure [Pa]

0.01

0.02

0.01

0.02

0.03

0.04

0.05

STEP 2

CHAPTER 5




99 | P a g e

1 N

s

Vn dp

m RT (5.2)


K)

Based on these calculations, the isotherms for the Maxsorb III/ethanol pairs are measured

for various temperatures (303 K, 313 K, 323 K, 333 K, 343 K, 383 K) which are shown in Fig.

5.5. The coverages are extended from 0.001 to 0.055 for collecting the information at wide

pressure region. The corresponding partial pressure ranges from 0.67 Pa to 1531.64 Pa. For

convenience, the adsorbed amount is presented in mg g-1 and pressure in relative pressure,

which is converted using conventional unit conversation methods. Besides, the isotherms for

Maxsorb III/ethanol pairs, three more pairs, WPT-AC/ethanol pair (Fig. 5.6), M-AC/ethanol

pair (Fig. 5.7), and H2-Maxsorb III (Fig. 5.8) are studied and corresponding figures are

presented. All the isotherms exhibit linear behavior, which supports the nature of the isotherms

at the Henry region. The H2-treated Maxsorb III/ethanol pair exhibits steeper uptake than that

of the parent Maxsorb III and WPT-AC due to the availability of excess hydroxyl group, which

enhances the surface energy contributing to initial adsorption [66].

0

50

100

150

200

250

300

0 10 20 30 40 50 60 70

Am

ou

nt

ad

sorb

ed

[µ

Mol g

-1]

Partial pressure [Pa]

0.01

0.02

0.03

0.04

0.05STEP 3

CHAPTER 5




100 | P a g e

Fig. 5.5. Ethanol adsorption on Maxsorb III at Henry region.

Fig. 5.6. Ethanol adsorption on WPT-AC at Henry region.

0

3

6

9

12

15

0 0.003 0.006 0.009 0.012 0.015

Eth

an

ol

up

tak

e [

mg g

-1]

Relative pressure [P/Po]

T=30 Deg C

T=40 Deg C

T=50Deg C

T=60 Deg C

T=70 Deg C

T=80 Deg C

0

3

6

9

12

15

0 0.003 0.006 0.009 0.012 0.015

Eth

an

ol

up

tak

e [

mg g

-1]

Relative pressure [P/P0]

T=30 Deg C

T=40 Deg C

T=50 Deg C

T=60 Deg C

T=70 Deg C

T=80 Deg C

CHAPTER 5




101 | P a g e

Fig. 5.7. Ethanol adsorption on M-AC at Henry region.

Fig. 5.8. Ethanol adsorption on H2-Maxsorb III at Henry region.

0

3

6

9

12

15

18

0 0.003 0.006 0.009 0.012 0.015

Eth

an

ol

up

tak

e [

mg g

-1]


T=30 Deg C

T=40 Deg C

T= 50 Deg C

T= 60 Deg C

T= 70 Deg C

T= 80 Deg C

0

3

6

9

12

15

0 0.003 0.006 0.009 0.012 0.015

Eth

an

ol

up

tak

e [

mg g

-1]

Relative pressure [P/Po]

T=30 Deg C

T=40 Deg C

T= 50 Deg C

T= 60 Deg C

T= 70 Deg C

T= 80 Deg C

CHAPTER 5




102 | P a g e

The equilibrium relationship between the solution and the adsorbed phase and the

equilibrium data of specific adsorbent-adsorbate systems give the limit of performance of the

adsorption operation. For the simplest case of pure adsorption, the equilibrium relationship can

be expressed as

( , )q f p T (5.3)

where, the q is the amount of adsorption, p is the pressure, and T is temperature related

to the adsorbed phase. Therefore, at a given temperature, q is a function of p. In a fixed

temperature, the amount of adsorption can be expressed as the following equation:

.Hq K P (5.4)

Where, HK is Henry’s constant. The isotherms are linear, and origin is going. Based on

the equation, the calculated values HK for all the studied samples are summarized in Table

5.2. The regression coefficient (R2) of the linearity is above 0.999 for all the studied samples.

It is observed that the values HK are decreased significantly by the increase in temperature.

The H2-Maxsorb III has the highest value HK at the lower temperature and continues its values

greater than the other pairs in the corresponding temperature. As HK is the fundamental

parameter for describing the primary interactions involving both the van der Waals and

columbic interactions between adsorbent surface and adsorbate molecules, it can be interpreted

that the treatment of Maxsorb III by hydrogen significantly increase the interactions. In the

case of M-AC, which is prepared by raw wood collected from mangrove forest has superior

interaction ability than the WPT-AC.

Table 5.2. Henry’s constant for different pairs at different temperature.

Maxsorb III WPT-AC M-AC H2-Maxsorb

Temp Henry

constant R2

Henry

constant R2

Henry

constant R2

Henry

constant R2

[K] [μmol g-

1 Pa-1] [-]

[μmol g-1

Pa-1] [-]

[μmol g-1

Pa-1] [-]

[μmol g-1

Pa-1] [-]

303 3.8683 1 2.5779 0.99 4.2231 0.99 4.5141 0.99

313 1.8161 0.99 1.2847 0.99 2.0547 0.99 2.1601 0.99

323 0.9271 1 0.6813 0.99 1.0258 0.99 1.1031 0.99

333 0.5017 1 0.4012 0.99 0.5585 0.99 0.5885 0.99

343 0.2935 1 0.2449 0.99 0.3223 0.99 0.3319 0.99

353 0.1828 1 0.1584 0.99 0.1998 0.99 0.1999 0.99

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103 | P a g e

More clearer comparison can be presented by Fig. 5.9, where all the calculated values

HK are presented. It is observed that the Henry’s constant is significantly decrease with the

increase of the temperature. The values decrease about tenfold for 353 K than that of 303 K.

This indicates the suitable property for adsorption chiller applications. In the adsorption chiller,

the adsorption process is performed at a lower temperature, whereas the desorption process is

at a higher temperature.

Fig. 5.9. Comparison of Henry’s constant for different pairs.

The significance of Henry’s constant can be explained by using the sticking constant

(β). The sticking coefficient is defined as that the fraction of the total number of impinging

particles that stick or remain adsorbed [149]. The thermodynamic framework has been

addressed by relating the sticking constant with HK as expressed below [150]:

2H B

s

K mK T

A

(5.5)

0

1

2

3

4

5

303 313 323 333 343 353

Hen

ry c

on

stan

t [µ

mol

g-1

Pa

-1]

Temperature [K]

Maxsorb III

WPT-AC

M-AC

H2-Maxsorb

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104 | P a g e

Where, m is the molecular mass of the adsorbate, is the average residence time of a

molecule on the adsorbent, sA is the specific surface area of the adsorbent, BK is the Boltzmann

constant and T is the corresponding temperature. From the Eq. (5.5), it is clearly understandable

that Henry’s constant influence the sticking constant or influences the adsorption process

significantly.

5.4.2 The heat of adsorption at zero coverage

The isosteric heats of adsorption of the components of a gas mixture are key

thermodynamic variables for the design of practical gas separation processes. It determines the

extents of adsorbent temperature changes within the adsorber during the adsorption

(exothermic) and desorption (endothermic) steps of the processes [151]. The adsorbent

temperature is a crucial variable in determining the local adsorption equilibria and kinetics on

the adsorbent, which ultimately governs the sorption performance of the processes. The

theoretical maximum of the isosteric heat of adsorption ( 0H ) is found in Henry’s region,

which is the characteristic parameter of a particular pair.

In the measurement 0H for the studied four pairs using the IGC method, several

isotherms are generated for each sample at the Henry region. Unlike the isotherm measurement

shown in the previous section, here for each pair, several isosteric lines were generated for

different coverage with varying the temperature. The measured isosteric lines are linear and

show decreasing trends with the increase of inverse of temperature. The Heat of adsorption

0H is then calculated using the Clausius-Clapeyron equation [Eq. (5.6)].

0 ln

1

H M P

R

T

(5.6)

Where, M is the molecular mass of the adsorbate, and R is the Universal gas constant.

The right side of the Eq. (5.6) Represent the slope of the isosteric lines shown in Fig. 5.10 for

Maxsorb III/ethanol pairs. The similar isosteric lines for WPT-AC/ethanol pair, M-AC/ethanol

pair, and H2-Maxsorb III are shown in Fig. 5.11, Fig. 5.12 and Fig. 5.13, respectively. The

data is measured twice to confirm the reproducibility, and all the data points coincide perfectly

(see Fig. 5.10).

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105 | P a g e

Fig. 5.10. Determination of heat of adsorption for Maxsorb III/ethanol pairs. The experiment

was conducted two times to confirm the data regeneration ability.

Fig. 5.11. Determination plot of heat of adsorption for WPT-AC/ethanol pairs.

y = -6078.9x + 19.447

R² = 0.9949

y = -6015.6x + 18.752

R² = 0.9936

y = -6066.5x + 17.728

R² = 0.9935

y = -6046.5x + 19.123

R² = 0.9945

y = -6012.9x + 18.322

R² = 0.9937

y = -6043.9x + 17.654

R² = 0.9938

y = -6010x + 18.3

R² = 0.9934

y = -6006.2x + 18.714

R² = 0.9943

y = -6031.3x + 19.061

R² = 0.9947

y = -6089x + 19.452

R² = 0.9951

-5

-4

-3

-2

-1

0

1

2

3

4

0.0026 0.0027 0.0028 0.0029 0.003 0.0031 0.0032 0.0033 0.0034

ln P

[k

Pa]

1/T [K-1]

0.01

0.020.030.040.05

y = -5570.7x + 16.143

R² = 0.9878

y = -5326.3x + 16.313

R² = 0.9856

y = -5286.5x + 16.675

R² = 0.9851

y = -5324.4x + 17.097

R² = 0.9853

y = -5330.3x + 17.348

R² = 0.9871

-3

-2

-1

0

1

2

3

4

2.5E-3 2.7E-3 2.8E-3 3.0E-3 3.1E-3 3.3E-3 3.4E-3

ln P

[k

Pa

]

1/T [K-1]

0.01

0.02

0.03

0.04

0.05

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106 | P a g e

Fig. 5.12. Determination of heat of adsorption for M-AC/ethanol pairs.

Fig. 5.13. Determination of heat of adsorption for H2-Maxsorb/ethanol pairs.

y = -6199.1x + 18.048

R² = 0.9966

y = -6155.5x + 18.694

R² = 0.9967

y = -6155.6x + 19.12

R² = 0.9973

y = -6194.9x + 19.529

R² = 0.998

y = -6223.2x + 19.835

R² = 0.9976

-3

-2

-1

0

1

2

3

4

2.5E-3 2.7E-3 2.8E-3 3.0E-3 3.1E-3 3.3E-3 3.4E-3

ln P

[k

Pa

]

1/T [K-1]

0.01

0.02

0.03

0.040.05

y = -6364.6x + 18.474

R² = 0.9977

y = -6362.2x + 19.244

R² = 0.9975

y = -6395.8x + 19.772

R² = 0.9978

y = -6395.2x + 20.069

R² = 0.9984

y = -6463.8x + 20.483

R² = 0.9985

-3

-2

-1

0

1

2

3

4

2.5E-3 2.7E-3 2.8E-3 3.0E-3 3.1E-3 3.3E-3 3.4E-3

ln P

[k

Pa

]

1/T [K-1]

0.01

0.02

0.03

0.040.05

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107 | P a g e

The calculated values 0H are illustrated in Fig. 5.14 and summarized in Table 5.3. The

calculated values 0H for all the studied pairs remain constant over a certain error percentage

(1% to 5%). This indicates the probable highest measured value for the studied pairs.

Table 5.3. The heat of adsorption of all the studied samples.

Coverage

Heat of adsorption

Maxsorb III M-AC WPT-AC H2-Maxsorb III

[kJ kg-1

] [kJ

mol-1

] [kJ kg

-1]

[kJ mol-

1 ]

[kJ kg-

1]

[kJ

mol-1

] [kJ kg

-1]

[kJ

mol-1

]

0.01 1098.04 50.51 1122.03 51.61 1008.3 46.38 1151.99 52.99

0.02 1088.34 50.04 1114.14 51.25 964.06 44.35 1151.56 52.97

0.03 1088.82 50.01 1114.16 51.25 956.86 44.02 1157.64 53.25

0.04 1094.42 50.22 1121.27 51.58 963.72 44.33 1157.53 53.25

0.05 1100.28 50.7 1126.39 51.81 964.78 44.38 1169.95 53.82

Fig. 5.14. Comparison of the heat of adsorption for all studied samples at different surface

coverage.

42

44

46

48

50

52

54

0.00 0.01 0.02 0.03 0.04 0.05 0.06

Qst

[kJ m

ol-1

]


Maxsorb III

H2-Maxsorb III

WPT-AC

M-AC

CHAPTER 5




108 | P a g e

The experimentally found heat of adsorptions at zero coverage are compared with the

theoretically found values. The average values of the studied samples are higher than the other

literature values except that for WPT-AC (Fig. 5.15). In this experiment, it is also realized that

the zero coverage values might not the correct one as the values fluctuate along with the

coverage variation.

Fig. 5.15. Comparison of measured heat of adsorption with the theoretically found values

addressed at various literature [148,152].

5.4.3 Entropy modeling

One of the primary focus of this work is to find an explicit relationship between the

adsorption phenomenon with the physical properties of adsorbents and adsorbates, which are

essential for the development of engineered materials. For these purposes, the theoretical

modeling of a relationship between the adsorption phenomenon with morphological property

is developed.

When an adsorbate molecule gets adsorbed inside the pore of an adsorbent, the Gibbs

free energy of the adsorbent surface alters. During physical adsorption, this energy decreases

as the adsorption phenomenon is exothermic in nature. This released heat is the driving force

of adsorption and can be represented by the opposing enthalpic and entropic effects as can be

expressed by, ΔG0 = ΔH0 + TΔS0 where ∆G0 defines the Gibbs free energy in kJ kg-1 and

0

200

400

600

800

1000

1200

1400

M-AC WPT-AC Maxsorb III H2-Maxsorb III

Heat

of

ad

sorp

tion

[k

J k

g-1

]

this work other literature

CHAPTER 5




109 | P a g e

consists of two terms; one is related to the chemical potential of the adsorbed gas (𝜇), and the

other term is for the grand potential of solid adsorbent (∆Ω). Neglecting the solid adsorbent

potential and considering the adsorption at very low pressure with finite adsorbate loading e,

we can write:

0 0

aT S H m (5.7)

0 0

a a

S HT

m m

(5.8)

0

0 0 lnP

T s h RTP

(5.9)

where, chemical potential, 0

lnP

RTP

(5.10)

In low pressure region, the adsorbed phase can be considered as the ideal gas phase, and

hence we can use the ideal gas equation of state:

0

p aP V m RT (5.11)

0 a

p

m RTP

V (5.12)

In this region, the adsorption isotherm is usually modeled using Henry’s adsorption

isotherm model. This model predicts a linear relation between adsorbate uptake with increasing

vapor pressure.

Hq K P (5.13)

where, KH is Henry’s constant.

Assuming that, Henry’s isotherm is applicable for the whole pressure region, the

maximum uptake can be predicted as,

0

m Hq K P (5.14)

where, 0P is the saturation pressure of the adsorbate at adsorption temperature.

Therefore, 0

m

q P

q P is defined as surface coverage or fractional uptake.

Moreover,

0

a H a H

ap p

s

P m RT K m RT K

mP V q V

M

(5.15)

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110 | P a g e

0

H

P

P K RT

P (5.16)

here P is specific pore volume. Eq. (5.9) can be rewritten as,

0 0 ln H

P

K RTT s h RT

(5.17)

0

0 ln H

P

h K RTs R

T

(5.18)

Eq. (5.18) can also be expressed as a function of surface coverage as,

0 0

exph T s

RT

(5.19)

Eq. (5.18) was used to calculate the adsorbed phase specific entropy for adsorption pairs

consisting of different types of activated carbons and ethanol at 328 K, which are furnished in

Fig. 5.16. The specific entropy was plotted against the ratio of Henry constant, and the pore

volume, a linear relation between them, was found with an R2 value of 0.9992. Fig. 5.16 shows

the graphical interpretation of this phenomenon.

Table 5.4. Summary of Henry's constant and specific entropy at 328 K.

Adsorbent/adsorbate

pairs

Temperature

[K]

Isosteric

heat, Δh0

[kJ mol-1]

Pore

volume,

υp[148]

[cm3 kg-1]

Henry

constant,

KH

[kg kg-1

kPa-1]

Specific

entropy,

Δs0

[kJ kg-1 K-

1]

Maxsorb III/ethanol 328 54.23 1.70 0.0589 2.217

WPT-AC/ethanol 49.05 2.51 0.0411 1.934

M-AC/ethanol 53.13 2.18 0.0661 2.1670

H2-treated Maxsorb

III/ethanol

55.58 1.73 0.0682 2.304

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111 | P a g e

Fig. 5.16. The relation between adsorbed phase specific entropy and the ratio of Henry constant

and total pore volume of adsorbents at 328 K temperature.

The ratio of Henry’s constant to the pore volume bears the unit kg kJ-1, which is the

opposite unit of energy kJ kg-1, suggesting a possible relation between the energetics of

adsorption and the ratio term. Despite that, Fig. 5.16 reveals several other interesting facts. The

relation between adsorbed phase entropy and the ratio of Henry’s constant and total pore

volume of the adsorbent can be mathematically expressed as,

0 0.112 / 1.8354H ps K (5.20)

This straightforward relation indicates that for ethanol adsorption on carbonaceous

adsorbents, the ideal entropy should be 1.8353 kJ kg-1. This suggests that at 1.8354 kJ kg-1 is

the minimum entropy that is required for ethanol adsorption on carbon-based adsorbents.

Below that, no ethanol adsorption will occur as the positive effects of adsorbate/adsorbent

interaction is zero or KH = 0. This ideal value is not temperature specific. Moreover, the

adsorbed phase entropy is proportional to the Henry constant for a given pore volume. Higher

KH means steeper uptake. For two isotherms having different Henry constant value (with same

pore volume for both adsorbents), the steeper isotherm would possess a higher value of entropy.

This is because the steeper isotherm at a particular pressure; there would be more adsorbate (as

y = 0.0112x + 1.8354

R² = 0.9993

1.80

1.90

2.00

2.10

2.20

2.30

2.40

0 10 20 30 40 50

Sp

ecif

ic e

ntr

op

y, Δ

s0 [

kJ

kg

-1K

-1]

KH/υp [kg kJ-1]

WPT-AC/ethanol

H2-treated Maxsorb III/ethanol

Maxsorb III/ethanol

M-AC/ethanol

CHAPTER 5




112 | P a g e

steeper isotherm means higher uptake) in the pores when compared with the other isotherm,

which has a lower value of Henry’s constant. More adsorbate molecules in the pores would

result in a higher chance of interaction between them, resulting in more entropy. Adsorbed

phase specific entropy also decreases with increasing pore volume, as for highly porous

activated carbons, higher pore volume means more space for the adsorbate molecules to get

adsorbed. Larger space for the adsorbates molecules reduces the interaction among themselves,

resulting in a decrease in specific entropy.

Adsorbed phase-specific entropy was also plotted against the surface coverage in Fig.

5.17. The values shown in the figure are a bit over predicted as it was assumed that Henry’s

law region applied for the whole pressure region (maximum uptake was calculated using Henry

isotherm); the specific entropy was found decreasing with increasing fractional coverage for

all the AC/ethanol pairs. The decreasing trend was observed because inside the adsorbent with

increasing uptake, the number of adsorbate molecules increases while the number of adsorption

sites decreases. Therefore, for additional adsorbate molecules, the choice for adsorption site

4

4.2

4.4

4.6

4.8

5

0.000 0.002 0.004 0.006 0.008 0.010 0.012

Sp

ecif

ic e

ntr

op

y, Δ

s0 [k

J k

g-1

K-1

]


WPT-AC/ethanol

Maxsorb III/ethanol

Series3

M-AC/ethanol

H2-treated Maxsorb III/ethanol

Fig. 5.17. The relation between adsorbed phase specific entropy against surface coverage at

328 K temperature.

CHAPTER 5




113 | P a g e

becomes lesser and lesser (as some of the sites are already occupied by previous adsorbate

molecules.). As a result, the newly coming adsorbate molecules adsorption sites become more

and more predicted, and hence the chaotic nature of adsorption becomes more and more

predictable, causing the entropy to decrease.

5.5 Conclusions

In this chapter comprises of three major findings: i) study of adsorption isotherms at

henry region ii) study of zero coverage heat of adsorption and iii) modeling of relationships

between specific entropy with pore volume which concludes with a novel finding of

characteristics specific entropy for activated carbon/ethanol pairs.

i) Study of adsorption isotherms at the henry region: Using IGC, the measured isotherms

in lower pressure region (relative pressure < 0.015) are linear, indicating the successful

measurement in Henry region. The isotherms are highly temperature sensitive, which can be

found by analysis of Henry’s constant values. The highest Henry constant value was found for

H2-Maxsorb III (4.5141 μmol g-1 Pa-1) at 303 K temperature and the lowest one for WPT-AC

(2.5779 μmol g-1 Pa-1). Comparing to parent Maxsorb III the H2-Maxsorb III exhibits a higher

affinity towards ethanol at Henry region. Furthermore, Henry's constant values depend on the

raw materials for producing activated carbons, which is depicted form the variation of KH

among WPT-AC and M-AC.

ii) Study of zero coverage heat of adsorption: Zero coverage heat of adsorption of the

studied samples is measured using the Clausius-Clapeyron equation, by plotting

experimentally found lnP values against T-1 values. The slope of the plots at different

temperature indicates the zero coverage heat of adsorption (0H ) for the studied pairs. The

measured values of heat of adsorption at different coverages are nearly linear with minimal

error range (1%), except WPT-AC (5%). If the error range is acceptable, then the measured

0H for Maxsorb III, M-AC, WPT-AC and H2- Maxsorb III are 50.35 kJ mol-1, 51.5 kJ mol-1,

44.5 kJ mol-1 and 53.3 kJ mol-1, respectively.

iii) Measurement of characteristic specific entropy for activated carbons/ethanol pairs: It

is interestingly observed that all the plot of specific entropy against the KH/υP follows a linear

trend and cut in the vertical axis at for at 1.8354 kJ kg-1 indicating the values of ideal entropy

CHAPTER 5




114 | P a g e

of adsorption process for carbonadoes materials. Below that, no ethanol adsorption will occur

as the positive effects of adsorbate/adsorbent interaction is zero or KH = 0.

115 | P a g e

Chapter 6

Novel Technique for Improving Water

Adsorption Isotherms of Metal-organic

Frameworks Equation Chapter 6 Section 1

Thermally driven adsorption-driven chillers (ADCs) is the emerging technology for

reducing primary energy consumption and greenhouse gas emission by utilizing solar energy

and waste heat from various sources. The key element of these ADCs is the adsorbent materials;

the performance of the systems heavily depends upon the properties of the adsorbents. Metal-

organic frameworks (MOFs) are becoming the most promising adsorbent for having a high

surface area, tunability, and generating S-shaped isotherms while pairing with water. In this

study, MOF aluminum fumarate was synthesized using a novel environmental-friendly route

and doped with two different metallic ions (Co2+, Ni2+). Various material characterization

experiments were completed to check structural integrity. The idea of metallic doping is

influenced by the results of the previous chapters. It is assumed that the replacement of the

central metallic ion of parent MOF might impact on the change of specific surface energy due

to electrostatic variation. Additionally, a comparative investigation of water adsorption at 30˚C

and 60˚C were reported for commercial aluminum fumarate and three new synthesized

samples. The investigation shows a significant improvement of water uptake in the lower

relative pressure region (P/Po <0.3) for all the synthesized and doped samples.

6.1 Introduction

During the past years, the use of cooling systems is intensifying due to rapid urbanization

and global warming. This trend will increase in many folds, especially in the higher income

countries, as well as in the developing countries for intense industrialization [153]. The

conventional systems used to mitigate the demand for cooling are based on electrical

compressors, heavily depend on the hydrochlorofluorocarbons (HCFCs), and hydrogenated

CFCs as refrigerants. These refrigerants are toxic, flammable, and harmful to the environment

[154]. Additionally, the electrical compressors require electrical energy to operate, which

implicitly increases the use of fossil fuel-based energy production. On the other hand,

CHAPTER 6

NOVEL TECHNIQUE FOR IMPROVING WATER ADSORPTION

ISOTHERMS OF METAL-ORGANIC FRAMEWORKS

116 | P a g e

adsorption chillers, an alternative to conventional chillers, runs on thermal compressors where

waste heat is the primary source of driving energy, and the refrigerants for this system are

environmentally benign. These chillers have the ability to utilize low-grade waste heat, which

can be collected from various sources, such as industries, solar collectors, etc. [155]. Low-

grade waste is used in the regeneration cycle to elude the refrigerants from a porous material,

and the cooling effect is achieved by the refrigeration cycles. As a refrigerant, water is

promising because of having high evaporation enthalpy. Nevertheless, the commercially

available adsorbent materials like silica gel, zeolites exhibit low water uptake capacity and

higher desorption temperature [156,157].

A new class of crystalline porous material, metal-organic frameworks (MOFs), is

becoming promising for their extreme microporosity, tunable structural design, and

coexistence of hydrophobicity and hydrophilicity in the same structure [43]. At this moment,

MOF-based water-assisted adsorption chillers are gaining interest in the researchers. However,

these chillers have several pre-requisites to achieve enhanced performance. Firstly, high

surface area and microporosity are essential for maximizing the water uptake. Recently

addressed MOFs having a BET surface area more than 7000 m2 g-1 is an indication of the

potentiality of using MOFs in the adsorption cooling applications [43,158]. Secondly, water

stability is essential to ensure the prolonged exposure of water in the adsorption-desorption

cycles, especially for the gaseous phase [159]. Many MOFs are introduced, claiming that those

are highly stable during the presence of water at various phases, and even show strong cycle

stability for heat transfer applications [54,160]. Finally, sufficient pore hydrophilicity is

essential for pore filling and water nucleation below the relative pressure (P/Po) of 0.3 for

achieving the enhanced performance of adsorption cooling systems. A good number of MOFs

show S-shaped distinctive water sorption characteristics, which are advantageous for steep

water uptake in the lower pressure region [124,161,162]. However, it is challenging to

introduce all the mentioned features in a particular MOF. One way to introduce all the features

in one single MOF is by doping the addressed stable MOF with various dopants [163]. In-situ

doping of metal ions has possible advantages for their inherent affinity with water, which might

influence the pore hydrophilicity, hence the improvement of water uptake in the relative

pressure region. In addition, to ensure the extensive use of MOFs in real-life applications, it is

required to synthesis the MOF ecofriendly.

Therefore, in this work, we have synthesized aluminum fumarate, which is

hydrothermally stable, addressed by various researchers [159,164,165]. We have synthesized

CHAPTER 6



117 | P a g e

aluminum fumarate (SMOF) without using hazardous material like DMF; instead, water is used

as an alternative. As Co2+ and Ni2+ have a moderate affinity with water, these metallic ions are

added during the synthesis to observe the influence in water adsorption. The synthesized

SMOFs are characterized by XRD, SEM, and N2 adsorption isotherm analysis. To evaluate the

doping effect on water adsorption behavior, the thermogravimetric experiment has been

performed.

6.2 Experimental

6.2.1 Material and synthesis

6.2.1.1 Materials

Aluminum sulfate octadecahydrate (Al2(SO4)3.18H2O, 57.5%, Waco Pure Chemical

Industries), fumaric acid (COOH CH═CH COOH, 98%, Waco Pure Chemical Industries) and

sodium hydroxide (NaOH, Sigma Aldrich co) were used for the synthesis of Aluminum

Fumarate. For doping of Ni2+ ion and Co2+ ion nickel chloride salt (NiCl2, 98%, Sigma Aldrich

co) and anhydrous cobalt chloride salt (CoCl2, 97%, Sigma Aldrich co) were used. Deionized

water was used as a solvent. All the reagents were of analytical grade and were used as it is

supplied without further purification. Commercial ALF is collected from Bry-Air Int., prepared

following the similar route used in BASOLITE A520 by BASF.

6.2.1.2 Synthesis

Synthesis of pure aluminum fumarate: Pure aluminum fumarate was synthesized

according to the process described by Leung et al. [166]. In a glass beaker, 7 gm of

Al2(SO4)3.18H2O (0.0105 mol) was dissolved in 30 ml of deionized water at room temperature

and heated to 60°C using an electric heater. Next, 2.63 gm of fumaric acid (0.0227 mol) and

2.73 gm of NaOH (0.0683 mol) were mixed in 39 ml of water and dissolved by stirring. NaOH

helps in the deprotonation of fumaric acid. The solution was heated to 60°C and was added to

the solution of fumaric acid dropwise with stirring at 250 rpm for 16 minutes. White milk like

the suspension was observed. The product was separated from the reaction mixture by

centrifugation at 5000 rpm for 20 minutes and washed with water three times to remove any

unreacted reagents. The product was collected in a glass petri dish and dried in a vacuum oven

at 100°C for 8 hours. The product was powdered using a mortar and pestle. Finally, the powder

was activated at 150°C in a vacuum oven for 8 hours. Visually, the powder is white.

CHAPTER 6



118 | P a g e

Synthesis of 10% Ni-SMOF: The synthesis of Ni-doped aluminum fumarate was

conducted the same way as that of aluminum fumarate as described above where a mixture of

NiCl2 (0.3594 gm) solution and Al2(SO4)3.18H2O (6.2999 gm) solution (10 wt% Ni salt+ 90

wt% Al salt) was used in place of pure aluminum sulfate. A light green colour product was

obtained.

Synthesis of 10% Co-SMOF: Co-doped aluminum fumarate was produced following

the same procedure of Ni-doped Aluminum fumarate. 0.3594 gm of CoCl2 was dissolved in

water and mixed with the solution of Al2(SO4)3.18H2O (6.299 gm). The next steps were

similar. A light pink colour of the product was obtained.

6.2.2 Material Characterization

The microstructure and morphologies of the samples were determined by JEOL JSM-

7900F FESEM (Field Emission Scanning Electron Microscopy) operated at 3 kV. Powder X-

ray diffraction (PXRD) analysis was carried out on a Rigaku SmartLab 9 kW AMK using

monochromatic CuKα radiation with a step size of 0.02˚; the operating wavelength was 1.54 Å

at 40 kV and 30mA. Thermal stability analysis was performed on a TG/DTA 7300 equipment

supplied by Hitachi High-Technologies, Japan. In this experiment, 4 to 6mg samples were

analyzed with a continuous flow nitrogen gas at a flow rate of 270 ml min-1. Thermal stability

was measurements were carried out of a temperature range of 40 to 500˚C at a constant heating

rate of 10˚C/min. The pore size, pore volume, and specific surface area of samples were

measured by N2 adsorption/desorption isotherms at 77 K. The samples were degassed at 120˚C

for 3h prior to N2 adsorption measurement. The pore size distribution is determined using

NLDFT (Non-Localized Density Functional Theory) from the N2 adsorption isotherms.

Water adsorption isotherms on commercial aluminum fumarate, SMOF, and 10% Ni and

Co-doped SMOFs were measured using a thermogravimetric analyzer (TGA). In this

experiment, a magnetic suspension adsorption measurement unit (Rubotherm-MSB-VGS2)

supplied by BEL Japan Inc. for all the samples, 65 mg of samples were put on the sample

holder. Then the samples were put into the measuring chamber of the magnetic suspension

balance section. The samples were regenerated at 100 °C under a vacuum condition for 4 hours.

Then the samples were made to cool down until they reach the adsorption temperature. After

that, the vacuum facility is disconnected from the measuring chamber. The measuring chamber,

CHAPTER 6



119 | P a g e

then it connected with the evaporator chamber temperature of the evaporator, was controlled

using an oil bath. The pressure of the measuring chamber increased rapidly until it reached the

corresponding set evaporator temperature. After that, the adsorbent mass was recorded until it

reached equilibrium uptake. The evaporator is then disconnected from the adsorption chamber,

where its temperature increases to a particular value to create a new evaporation pressure for

the next adsorption uptake measurement. These steps were repeated for all the isotherms

measurements.

Table 6.1. Porous properties of different SMOFs

Sample BET surface area

[m2 g-1]

Pore volume

[cm3 g-1]

Average Pore radius

[Å]

Commercial ALF 600.33 - 11.93

SMOF 874.10 0.37 10.84

10% Ni-SMOF 745.68 0.3 11.17

10% Co-SMOF 751.56 0.31 11.20

6.3 Results and Discussion

6.3.1 Physical Properties

The morphology and particle size of synthesized SMOFs analyzed with FESEM are

shown in Fig. 6.1. All the samples exhibit similar size and fiber like nanostructures. These

microstructures amalgamated to spherical macro domains. The size of spherical shaped

macrodomains is about 100-200 nm. It is not clear from the images the exact location of the

micropores. However, there is a high possibility of the existence of the micropores in the voids

created by the nano-sized fiber structures. There is no effect of doping found in the SEM

images, which indicates no structural distortion is occurred due to the insertion of 10% of

doping materials.

CHAPTER 6



120 | P a g e

Fig. 6.1. Scanning electron micrography (SEM) of different SMOFs (a) SMOF (b)10% Co-

SMOF (c) 10% Ni-SMOF.

The Powder X-ray diffraction pattern of synthesized SMOFs is shown in Fig. 6.2. All the

three SMOFs have similar peaks at 2θ= 10.5˚,15˚, 21˚, 32 ˚ and 43˚ which supports the

addressed standard peaks of aluminum fumarate [167,168]. Additionally, identical peaks of the

synthesized SMOFs indicates that there is no structural deformation due to the addition of

doping materials. However, there is some peak broadening in the XRD pattern is observed due

to the change in crystal size. The average crystallite size of SMOF, 10% Ni-SMOF, and 10%

Co-SMOF are 12.14 nm, 11.16 nm, and 10.58 nm, respectively. The crystallite size is measured

using the Scherrer Equation:

d= K λ

FWHM (2ϴ) . cos ϴ (6.1)

Here, d is the crystallite size, K is a constant which is a function of crystal shape but

generally taken as 0.89, λ is the wavelength of the incident X-ray, FWHM is Full Width Half

Maxima value of peaks at 2θ and θ is the diffraction angle.

1μm

1μm 1μm

(a) (b) (c)

1μm

CHAPTER 6



121 | P a g e

Fig. 6.2. XRD of different MOF samples.

Thermogravimetric analysis (TGA) curves for SMOF and doped samples are shown in

Fig. 6.3 exhibits two main steps of weight loss. The first step of weight loss (60-120˚C) can be

depicted as the loss of guest molecules. The second step of weight loss occurs due to the

decomposition of the organic linkers in different onset temperature. It is observed that the Tonset

for SMOF, 10% Ni SMOF, and 10% Co SMOF are 415˚C, 388˚C, and 380˚C. Comparing to

three samples, initial weight loss of 10% Co SMOF is higher than than other samples, this is

because of the higher water uptake capacity of this sample. From this analysis, it can be

concluded that all the samples have good thermal stability and suitable for adsorption heat

pump applications.

0

50000

100000

150000

200000

250000

300000

10 20 30 40 50 60 70

2θ (deg)

Norm

ali

zed

in

ten

sity

10% Co-SMOF

10% Ni-SMOF

SMOF

Commercial ALF

CHAPTER 6



122 | P a g e

Fig. 6.3. TGA of SMOF samples.

Pore size distribution (PSD) curve of SMOFs analyzed with the NLDFT model illustrated

in Fig. 6.4, and the results are presented in Table 6.1. A majority portion of the pores resides

in the micropore regions for all samples. Commercial ALF and SMOF have two dominant pore

sizes below 15Å. On the other hand, doped SMOFs show no similar distribution, rather pore

size distribution spread towards the upper micropore regions. The probable reason behind this

is the doping materials reform the micropore regions.

50

60

70

80

90

100

40 90 140 190 240 290 340 390 440 490

Weig

ht

[%]

Temperature [˚C]

SMOF

10% Ni SMOF

10% Co SMOF

CHAPTER 6



123 | P a g e

Fig. 6.4. The pore size distribution of studied samples.

6.3.2 Adsorption isotherms

Water adsorption isotherms were measured for all the samples. The results are shown in

Fig. 6.5. In all cases, we obtained S-shaped isotherms that are the desired ones for adsorption

chiller applications. The S-shaped isotherms enable us to have a greater difference in

adsorption-desorption uptake. However, uptake of our synthesized SMOF was higher than that

of the commercially available sample. Moreover, for the synthesized sample, the isotherm also

shifted towards a lower pressure region, making it more suitable for practical application. This

is because of the lower the pressure, the fewer chances of leakage. In the case of the doped

samples, the uptake is comparatively higher. The reason behind SMOF having higher uptake

could be related to the increase of surface area. Whereas, in the case of the doped samples, the

dopant atoms might have caused an increase in water adsorption affinity in the samples. The

doping of Ni2+ and Co2+ which results in mixed valance metallic spices. Both the metallic ions

are transition metal with relatively larger in size compare to Al3+ (smaller ionic radius).

Therefore, there is a high chance of creation of open metal sites which have the potentiality to

enhance the adsorption characteristics [169].

0

0.01

0.02

0.03

0.04

0.05

0.06

0

0.1

0.2

0.3

0.4

0.5

0 5 10 15 20

dV

(r)

Pore Width (Å)

10% Co SMOF

10% Ni SMOF

SMOF

Commercital ALF

CHAPTER 6



124 | P a g e

Fig. 6.5. Water adsorption isotherms of different samples.

The isotherms also shifted more towards the lower pressure region. Fig. 6.6 explicitly

shows the amount of uptake increased in each pressure region. Generally, most of the MOFs

shows type-V isotherms containing three identical uptake rates. At a lower concentration of

adsorbate pressure, the uptake rate is slow, followed by a steep rise at medium concentration.

At higher concentrations, the uptake follows a steady horizontal path towards saturation [170].

The reason behind the existence of three regions is the existence of lower energy sites at a

nominal percentage [77]. It is possible to shift the isotherms in the lower concentration by

increasing the higher energy sites. There is a high chance that the doping of metallic ions

increases the high energy sites of the synthesized samples. These increased energy sites

influence the rise of water uptake below the 0.3 partial pressure. The significance of this finding

is vital for adsorption chiller design to operate using the low temperature driving heat energy.

Otherwise, there will be some restrictions on operating conditions, which leads to lower

efficiency [124]. For instance, adsorbents showing steep water uptake at P/P0= 0.30-0.45 are

only usable for evaporation temperature greater than 10-15˚C, which limits the available

cooling temperature lower than 15˚C. In addition, condensation temperature requires a value

of more than 30˚C; both these limits reduce the wide applicability of adsorption chillers.

Therefore, this finding carries the significance of metallic doing to enhance the performance of

MOFs for adsorption chillers.

0

0.1

0.2

0.3

0.4

0.5

0 0.2 0.4 0.6 0.8

Up

tak

e [g

g-1

]


SMOF

10% Co-SMOF

10% Ni-SMOF

Commercial ALF

0

0.1

0.2

0.3

0.1 0.15 0.2 0.25 0.3 0.35

Up

tak

e [g

g-1

]


CHAPTER 6



125 | P a g e

Fig. 6.6. Enhanced uptake of doped aluminum fumarate. The water uptake is significantly

increased in the lower pressure region.

The enhanced performance of aluminum fumarate based adsorption chillers over silica

gel based adsorption chillers can be shown using the effective uptake difference measured from

water uptake isotherms. According to the definition of effective or net adsorption uptake (Δq)

is the difference between the maximum and minimum uptake at a fixed operating condition.

The importance of effective uptake can be understood from the following equation [171]:

𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝐸𝑓𝑓𝑒𝑐𝑡(𝑆𝐶𝐸) = (𝑞𝑚𝑎𝑥 − 𝑞𝑚𝑖𝑛)[ℎ𝑓𝑔(𝑇𝑒) − ∫ 𝑑ℎ𝑓𝑇𝑐

𝑇𝑒] (6.2)

Here, ℎ𝑓𝑔 is the latent heat of vaporization, ℎ𝑓 is fluid enthalpy, 𝑇𝑒 and 𝑇𝑐 are the

temperature of the evaporator and condenser, respectively. And qmax and qmin are the water

uptake at the maximum and minimum operating pressures, respectively. All the parameters are

relatively constant for a fixed operating condition, except for the uptake difference. From the

water adsorption isotherms, the calculated effective uptake for SMOF is 0.214, whereas, for

silica gel, it is about 0.13 (Fig. 6.7) . Therefore, in the case of SMOF based chiller, the uptake-

offtake difference (effective uptake) will be higher than most of the traditional silica-gel/water

and zeolite/water based chillers. One of the reasons for the higher effective uptake of SMOF is

0

0.3

0.6

0.9

1.2

1 2 3

Wate

r u

pta

ke [

gg

-1]


Commercial ALF

SMOF

10% Ni-SMOF

10% Co-SMOF

0.3 0.6 0.9

CHAPTER 6



126 | P a g e

having S-shaped isotherms. This larger difference in uptake and offtake will make the system

more efficient by enabling it to achieve the desired cooling temperature in a fewer number of

cycles compared to other water based chillers.

Fig. 6.7. Comparison of △q (Effective uptake) between RD silica gel/water and SMOF/water.

6.4 Conclusions

In this study, we have shown that we have successfully synthesized aluminum fumarate

using a green synthesis approach. The XRD and SEM images matched well with the published

literature confirming its reliability. The pore size distribution matched well with the

commercial sample. In our synthesis process, we managed to increase the surface area by about

25 % than the commercial sample. Because of that, water uptake also increased. However, this

study shows that the isotherms can be shifted towards the lower pressure region with the

introduction of dopant atoms. This could lead to a new scope of research, as MOFs properties

are highly tunable. The various ways of modification can help in shaping the isotherms as well

as shifting them to our desired ones.

0

0.1

0.2

0.3

0.4

0.5

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Ad

sorp

tion

up

tak

e [

g g

-1]

Relative Pressure, P/Ps [-]

RD silica gel/water

SMOF/water

Working

region for adsorption

cooling system

△qSMOF=0.214△qRD silica gel=0.13

127 | P a g e

Chapter 7

Study on Surface Activities of Improved

Metal-doped Metal-organic Frameworks Equation Chapter 7 Section 1

The concept of this chapter is inspired by the results obtained from the surface energy

analysis (Chapter 4) and the improved water uptake characteristic of SMOFs (Chapter 6). It is

found that the incorporation of different metallic ions into SMOFs improves the water uptake

capacity and shifts the water adsorption isotherms towards hydrophilic regions, which are

significant findings for adsorption related applications. The incorporation of the metallic ions

inspired by chapter 4, targeting to improve the specific surface energy that might influence

water uptake capacity. The assumption was correct, and that leads to analyzing the

corresponding surface energy of the improved SMOFs. This chapter thus provides a rigorous

analysis of surface energy components of the doped SMOFs with an additional comparison

with other promising adsorbents. Furthermore, the study of the work of adhesions is also

included to understand the affinity variation due to doping.

7.1 Materials synthesis and preparation

All the studied samples are synthesized using a hydrothermal process, and the detailed

process is described in chapter 6. For the doping process, the following concentration of the

NiCl2 and CoCl2 salt were used:

Table 7.1. The doping concentration of the doped SMOFs.

Sample name Al2(SO4)3.18H2O CoCl2 NiCl2

10% Co-SMOF 6.2999 g 0.3594 g -

20% Co-SMOF 5.599 g 0.7188 -

10% Ni-SMOF 6.299 g - 0.3594 g

20% Ni-SMOF 5.599 g - 0.7188

The samples were preserved in a sealed container to mitigate the adsorption of water

vapor from the environment. However, all the samples were degassed at 120˚C before

CHAPTER 7

STUDY ON SURFACE ACTIVITIES OF IMPROVED METAL-

DOPED METAL-ORGANIC FRAMEWORKS

128 | P a g e

performing characterization processes. To evaporate, most of the vapor content from the

samples degassing process was performed in a closed chamber having pressure below 1 MPa.

7.2 Experimental procedure

The pore size, pore volume, and specific surface area of samples were measured by N2

adsorption/desorption isotherms at 77 K. The samples were degassed at 120˚C for 3h prior to

N2 adsorption measurement. The pore size distribution is determined using NLDFT (Non-

Localized Density Functional Theory) from the N2 adsorption isotherms.

Water adsorption isotherms on SMOF samples were measured using a thermogravimetric

analyzer (TGA). In this experiment, a magnetic suspension adsorption measurement unit

(Rubotherm-MSB-VGS2) supplied by BEL Japan Inc. for all the samples, 65 mg of samples

were put on the sample holder. Then the samples were put into the measuring chamber of the

magnetic suspension balance section. The samples were regenerated at 100 °C under a vacuum

condition for 4 hours. Then the samples were made to cool down until they reach the adsorption

temperature. After that, the vacuum facility is disconnected from the measuring chamber. The

measuring chamber, then it connected with the evaporator chamber temperature of the

evaporator, was controlled using an oil bath. The pressure of the measuring chamber rapidly

increased until it reached the corresponding set evaporator temperature. After that, the

adsorbent mass was recorded until it reached equilibrium uptake. The evaporator is then

disconnected from the adsorption chamber, where its temperature increases to a particular value

to create a new evaporation pressure for the next adsorption uptake measurement. These steps

were repeated for all the isotherms measurements.

The IGC experiment was performed by pulse or frontal technique in infinite dilution. In

the pulse experiment, a small concentration of the probe molecule is injected, and the amount

is controlled using the targeted surface coverage. In this experiment, targeted surface coverage

was considerably small compared to the total surface coverage to maintain the experiment in

zero coverage/ infinite dilution. The pulse was then transported by the mobile phase (helium

gas) to the column containing SMOFs in the stationary phase. The columns were made of glass,

sterilized, and used only once for one sample to avoid contamination. Columns were filled with

3-5 mg of samples packed with glass wools at the top and bottom of the samples that the mobile

phase can pass through the column without any kind of adsorption in the glass wools. Before

conducting the pulse experiment, the samples are conditioned for 3 hours at 423 K with a

CHAPTER 7



129 | P a g e

helium carrier gas flow rate of 30 sccm to remove pre-adsorbed vapor contents. Afterward,

methane is purged through the column twice for 3 minutes each to measure the dead retention

time (t0). The targeted surface coverages were from 0.005 to 0.01, which were theoretically

measured from the monolayer coverage theorem [60]. The schematic of the experimental setup

is presented in Fig. 2.11. The interactions were measured using non-polar (hexane, heptane,

and octane) and polar probes (ethanol, dichloromethane, acetone, acetonitrile, and ethyl

acetate) by pulse injection through the column at 413 K temperature and helium flow rate of

30 sccm. The desorbed probe molecules were detected as chromatographic peak using a flame

ionization detector.


7.3.1 Morphological characterization

BET surface area is the fundamental morphological feature that provides the estimation

of the possible area for available energy sites. It was also used in the IGC experiments to

calculate the target coverage area. The measured surface area is presented in Table 7.2. Among

the studied samples, SMOF exhibits the highest surface area, and it continuously decreases

with the increase of doping concentration. It was observed that, for the 10% increase of the

doping concentration, there were about 50 m2 g-1 reductions of surface area from the parent

SMOF and the trend was continued for 20% doping concentration.

Table 7.2. Porous properties of the studied samples

Material Surface area

[m2 g-1]

Pore size

[Å]

Pore volume

[cm3 g-1]

SMOF 874.10 10.84 0.371

SMOF 10% Co 751.56 11.2 0.307

SMOF 20% Co 705.384 10.80 0.305

SMOF 10 % Ni 745.68 11.17 0.302

SMOF 20 % Ni 696.97 - -

7.3.2 Water Adsorption isotherms

Water adsorption isotherm was measured for all the SMOF samples. This chapter is

considering only the desorption isotherms at 303 K and 343 K to correlate these with the surface

energy components. The desorption isotherms are more suitable to compare as the IGC

CHAPTER 7



130 | P a g e

generates data-based detecting the desorbed probe molecules. Additionally, the lower pressure

region is more comparable with the measured surface energy than the higher pressure region.

In the surface energy analysis, the measurement was targeted in the homogeneous region, i.e.,

in the Henry region where surface coverage (θ=n/nm) were considered below 0.01. Therefore,

the lower pressure region of the water adsorption should have below 1% of the total uptake.

From chapter 4, it is found that the maximum measured water uptake was above 1 g g-1 at the

higher-pressure region. Considering this, the upper value of the uptake requires to be below

0.01 g g-1. Unfortunately, it is not possible to measure such lower value by using traditional

isotherm measurement systems. However, to understand the trend of water adsorption

isotherm, the lowest possible points were considered, which are shown in Fig. 7.1 and Fig. 7.2.

In both the temperature range, 20% Co-SMOFs exhibit the highest values for uptakes compare

to others. On the other hand, water uptake values of parent SMOF are lower than the doped

samples. However, 10% Co-SMOF and 10% Ni-SMOF alternate their positions in the different

values of temperature.

Fig. 7.1. Water desorption isotherms of the studied SMOFs at 303 K temperature.

The desorption trend of isotherm at 303 K showed in Fig. 7.1 indicates the requirement

of the highest input energy for 10% Co-SMOF as in the measured lowest point; there are a

0

0.03

0.06

0.09

0.12

0.15

0.18

0 0.03 0.06 0.09 0.12 0.15 0.18

Wa

ter u

pta

ke [

g g

-1]


SMOF 10% Co-SMOF

10% Ni-SMOF 20% Co-SMOF

CHAPTER 7



131 | P a g e

significant number of the adsorbate exists. The adsorption mechanism of MOFs is different

from the other traditional adsorbents. There are three types of adsorption mechanisms might

happen in MOF; (i) chemisorption on open (or coordinatively unsaturated, CUS) metal sites

via coordination bonds, (ii) reversible layer or cluster adsorption, or continuous pore filing,

and (iii) irreversible capillary condensation [172,173]. Adsorption in the CUS is considered to

have significant interaction, which requires a large amount of surface energy. In the 20% Co-

SMOF, the concentration of the cobalt ions is higher than the others leads to the high possibility

of a large number of coordination bonding with the water molecule with cobalt. That might

lead to high remaining uptake of water in the lower pressure region. The medium concentration

of cobalt and Ni in the other two SMOFs supports this statement, which continues doping fewer

parent SMOFs with a low amount of coordination bonding. The remaining uptakes are

significantly reduced in the higher desorption temperature shown in Fig. 7.2. All the isotherm

trends showing origin going behavior, indicating the high desorption rate due to higher

temperature value. The metallic vibration is higher for high temperature, that might desorb the

water content faster than that of the lower temperature excitation.

Fig. 7.2. Water desorption isotherms of the studied SMOFs at 333 K temperature.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0 0.03 0.06 0.09 0.12 0.15 0.18

Wate

r u

pta

ke [

g g

-1]


SMOF 10% Co-SMOF

10% Ni-SMOF 20% Co-SMOF

CHAPTER 7



132 | P a g e

7.3.3 Surface energies of studied SMOF samples

The surface energy of a solid material depends on the nature, origin, and surface

composition of the material and test temperature. Generally, from the IGC experiment, two

types of fundamental data are measured; i) Retention time (tR) and ii) Retention volume (VN).

Retention time represents the required time to generate a peak resulting from an interaction

between the probe molecule and the sample surface. On the other hand, retention volume is a

measure of the amount of carrier gas required to elude the probe gases from the sample surface.

The higher the interaction, the higher the amount of carrier gas is required to elude the probe

molecule from the surface. The retention volumes of all the studied samples for non-polar

probes are illustrated in Fig. 7.3. For all the studied samples, the highest amount of carrier gas

was required to elude octane. On the contrary, the lowest amount of carrier gas is required for

hexane. The number of carbons is higher in the octane, which increases the chain that interacts

with the surfaces. The samples having a higher percentage of doping concentration requires

more helium gas to elude non-polar probes from the surface. It is not clear from Fig. 7.3, what

kind of interaction is behind this rise of required retention volume. However, the retention

volumes are almost showing constant values over the variation of coverage, indicating the

studied area was taken in the homogenous region.

CHAPTER 7



133 | P a g e

Fig. 7.3. Retention volume vs. coverage for alkanes of all the studied samples.

In the pulse method, it is possible to measure the adsorption isotherms at the Henry region

because of the uptake measurement techniques. Unlike the conventional techniques where

volume or mass of the adsorbed phase are directly measured. Due to the significant variance in

the amount of the adsorbed phase molecules with the adsorbent, the accurate measure of the

adsorbate is not possible in the Henry region. However, in IGC, the adsorbed amount is

measured using the FID detector, where the desorbed molecules are detected after ionizing.

The resolution for the counting is possible to take in the nano-scale. In IGC to measure the

isotherm, at first, the equilibrium partial pressure, P, for each concentration of probe molecule

in the column can be calculated from the chromatographic shape via the Eq. (7.1) [174].

273.15

. . ..

cLoop inj

c c Loop

hP V P

F A T (7.1)

0.0E+0

1.0E+4

2.0E+4

3.0E+4

4.0E+4

5.0E+4

6.0E+4

0.004 0.005 0.006 0.007 0.008 0.009 0.01 0.011

Rete

nti

on

Volu

me (

Vn

) [m

l g

-1]

Surface Coverage [-]

SMOF

Hexane

Heptane

Octane

0.0E+0

5.0E+4

1.0E+5

1.5E+5

2.0E+5

2.5E+5

0.004 0.005 0.006 0.007 0.008 0.009 0.01 0.011

Rete

nti

on

Volu

me (

Vn)

[ml

g-1

]


SMOF 10% Co

Hexane

Heptane

Octane

0.0E+0

5.0E+4

1.0E+5

1.5E+5

2.0E+5

2.5E+5

3.0E+5

3.5E+5

4.0E+5

4.5E+5

0.004 0.005 0.006 0.007 0.008 0.009 0.01 0.011

Rete

nti

on

Volu

me (

Vn)

[ml

g-1

]


SMOF 20% Co

Hexane

Heptane

Octane

0.0E+0

1.0E+4

2.0E+4

3.0E+4

4.0E+4

5.0E+4

6.0E+4

0.004 0.005 0.006 0.007 0.008 0.009 0.01 0.011

Rete

nti

on

Volu

me (

Vn)

[ml

g-1

]


SMOF 10% Ni

Hexane

Heptane

Octane

0.0E+0

2.0E+4

4.0E+4

6.0E+4

8.0E+4

1.0E+5

1.2E+5

1.4E+5

0.004 0.005 0.006 0.007 0.008 0.009 0.01 0.011

Rete

nti

on

Volu

me (

Vn)

[ml

g-1

]


SMOF 20% Ni

Hexane

Heptane

Octane

CHAPTER 7



134 | P a g e

where, ch is the chromatogram peak height cA is the chromatogram peak area, LoopV is the

injection loop volume, LoopT is the injection loop temperature, and injP is the partial pressure of

the solute inside the injection loop. The adsorbed amount, n, and the adsorption isotherms for

each probe molecule can be obtained by integrating NV with respect to P.

1

.

N

s

Vn dP

m R T (7.2)

where, sm is the adsorbent. To mitigate the ambiguity from here all the measured

isotherms will be indicated as desorption isotherms as the isotherms are formed after the

desorption process. The desorption isotherms in the Henry region for all the polar probes used

in this study are shown in Fig. 7.4 for 10% Co-SMOF. The slope of the desorption isotherm

represents Henry’s constant (KH), which indicates the affinity of the probe molecule towards

the adsorbent surface.

Fig. 7.4. Uptakes of the polar probes on 10% Co-SMOFs at 513 K temperature for 0.005

coverage

The technique of measuring the dispersive and specific surface energy is explained in

chapter 3. The measured dispersive surface energy is shown in Fig. 7.5. The dispersive surface

y = 438820x

R² = 0.6747y = 45129x

R² = 0.8984

y = 6839.8x

R² = 0.9029

y = 41552x

R² = 0.9755

y = 10370x

R² = 0.6768

0

0.2

0.4

0.6

0.8

1

1.2

0.E+0 2.E-5 4.E-5 6.E-5 8.E-5 1.E-4 1.E-4 1.E-4

Up

tak

e [

mg

g-1

]


Ethyl acetateAcetoneAcetonitrileEthanolDicloromethane

CHAPTER 7



135 | P a g e

energy of all the doped SMOFs has remained constant over the change of surface coverage.

However, the dispersive surface energy of parent SMOF decreases significantly with the

increase of surface coverage. This decrease indicates the surface of SMOF is heterogenous in

the studies coverage. On the other hand, the constant value of the surface energies in the doped

SMOFs indicates the increase of the dispersive surface energy in a higher coverage. Generally,

the dispersive surface energy depends on the surface morphological structure [140]. The

increase of dispersive surface energy due to the doping indicating the morphological changes

in the doped SMOFs. Nevertheless, the increase of surface area influences the surge the

dispersive surface energy because the higher surface area provides more available adsorption

sites. However, the doped SMOFs exhibits higher dispersive surface energy despite having a

comparatively lower surface area( see Table 7.2). One possible reason behind that is the

removal of the aluminum ions from the structure by cobalt and nickel. Chemically, cobalt and

nickel exhibit more valance electron in their structure than the aluminum, which might

contribute to the increase of dispersive surface energy.

Fig. 7.5. The dispersive surface energy of all the studied samples.

50

100

150

200

250

300

350

0.004 0.005 0.006 0.007 0.008 0.009 0.01 0.011

Dis

per

siv

e su

rfa

ce e

ner

gy [

mJ

m-2

]


CHAPTER 7



136 | P a g e

The effect of doping is more visible in Fig. 7.5, where the specific surface energy of the

studied samples is illustrated. The specific surface energy of the 10% Ni-SMOF is higher than

that of 10% Co-SMOF, which has a similar concentration of the doped metallic ions. The

valency of nickel is higher than that of cobalt, which is reflecting on the specific surface energy

components by showing higher surface energy. Furthermore, the specific surface energy of

10% Co-SMOF is significantly higher than that of other SMOFs. There is a high probability

that the extra valency of cobalt heightened the availability of new dangling bonds for

adsorption. Besides, unlike the dispersive surface energy, the specific surface energy decreases

with the increase of surface coverage. It is further observed that the dispersive surface energy

is dominating in all the studied samples. For example, the dispersive surface energy of 20%

Co-SMOF is fifteen times higher than the specific surface energy of the same sample.

Fig. 7.6. The specific surface energy of studied samples.

The dominating trend of dispersive surface energy reflects in the total surface energy

measurement Fig. 7.7. The total surface energy clearly shows a significant impact on the water

adsorption isotherms, which are illustrated in Fig. 7.1 and Fig. 7.2. Previously, comparing with

the morphology, there was no relation found with the surface area, size, and pore volume (see

0

2

4

6

8

10

12

14

16

18

20

0.004 0.005 0.006 0.007 0.008 0.009 0.01 0.011

Sp

ecif

ic s

urf

ace

en

ergy [

mJ

m-2

]


CHAPTER 7



137 | P a g e

Table 7.2. However, it is apparent that the total surface energy influences the water desorption

isotherms. Among all the studied samples, 10% Co-SMOFs exhibit the highest value of total

surface energy and contains the highest amount of water in both the temperature (303 K, 333

K).

Fig. 7.7. The total surface energy of all studied samples

The total surface energy is almost similar for both the 10% Ni-SMOF and 10% Co-

SMOF, which reflects the water desorption isotherm trend. However, initially, the Nickel

doped SMOFs lags behind the Cobalt doped one at 333K temperature, which gradually

overcomes the lagging in the comparatively higher-pressure zone. It is still not understandable

from this study.

7.3.4 Specific Gibbs free energy

Water is an amphoteric molecule, which supposed to behave neutrally on the acid/basic

nature of the surface. However, as it is observed in Fig. 7.4 that the uptake varies significantly

due to the change of polar probes. The specific Gibbs free energy provides the initial idea on

the probable adsorption sites for different polar molecules. To measure the specific Gibbs free

50

100

150

200

250

300

350

400

0.004 0.005 0.006 0.007 0.008 0.009 0.01 0.011

To

tal

surf

ace

en

erg

y [

mJ

m-2

]


CHAPTER 7



138 | P a g e

energy, the polarization approach was adopted where molar deformation polarization (PD) was

used for the characterization process [175]. The measured specific Gibbs free energy is

summarized in Table 7.3.

Table 7.3. Specific Gibbs free energy of studied samples

Adsorbent Adsorbate

Surface coverage

0.005 0.007 0.009 0.01

Specific Gibbs free energy ΔGsp [kJ mol-1]

SM

OF

Ethyl acetate 8.63 8.76 7.21 5.52

Acetone 11.75 12.28 10.66 9.33

Acetonitrile 15.26 21.58 15.68 9.96

Ethanol 18.59 18.52 14.46 11.56

Dichloromethane 12.32 11.5 5.89 1.67

10%

Co-S

MO

F

Ethyl acetate 19.06 17.6 15.45 14.33

Acetone 18.02 17.05 15.74 15.19

Acetonitrile 20.7 19.63 18.46 17.97

Ethanol 22.43 21.37 20.62 20.34


20%

Co-S

MO

F

Ethyl acetate 19.57 18.49 17.22 17.22

Acetone 21.04 20.1 19.1 18.92

Acetonitrile 26.73 25.27 24.12 24.15

Ethanol 29.3 27.05 25.99 26.07


10%

Ni-

SM

OF

Ethyl acetate 21.27 19.68 18.52 18.3

Acetone 18.27 19.81 20.78 19.61

Acetonitrile 23.78 22.54 21.75 21.44

Ethanol 25.44 24.26 24.08 24.07

Dichloromethane 7.43 6.91 7 7.09

20%

Ni-

SM

OF

Ethyl acetate 13.98 12.27 10.9 11.04

Acetone 21.34 20.46 19.86 19.51

Acetonitrile 23.56 22.5 21.77 21.35

Ethanol 22.69 22.57 23.36 23.8


CHAPTER 7



139 | P a g e

Gibbs free energy is a measure of available surface energy that may contribute to the

adsorption process. The specific Gibbs free energy indicates the identical free energy for a

particular pair. It is observed that this free energy decreases with the increase in surface

coverage. It doesn’t directly depend on the surface area; rather, it depends on the amount and

type of functional group existed on the adsorbent/adsorbate pair. As a result, the doped SMOFs

exhibits a larger quantity of the Gibbs free energy than the parent SMOF. In the case of ethanol,

the amount is highest among all the studied samples. It is not confirmed how it is influenced

by the ethanol adsorption. However, it might be concluded in a manner that there is a high

possibility of this extra energy might contribute to increasing heat of adsorption for doped

SMOF/ethanol pairs.

7.4 Comparative analysis

The thermodynamic work of cohesion (particle-particle) and work of adhesion (particle-

liquid) is an important measure to understand the adsorption process. The higher surface energy

leads to greater cohesive forces, which influence the agglomeration and decreased load transfer

mechanism [176]. On the other hand, the work of adhesion with water measures the wettability

of the adsorbent. Therefore, the higher the value of work of adhesion creates a better

opportunity for water adsorption. The measured values are summarized in Table 7.4. Both the

works are the contribution of the dispersive and specific components. The ration of the work

of adhesion and cohesion shows the possibility of the wettability; higher is better. This ratio is

highest for SMOF, but the water uptake shown in this work is significantly lower compared to

others. The reason behind that the total work of adhesion is lower compare to others. In this

case, 20% of Co-SMOF exhibits the highest amount of work of adhesion for all the

experimented coverages. This reflects in the water adsorption isotherms. Furthermore, for all

the doped SMOFs, the ratio is almost similar; only the comparable parameter is the work of

adhesion. It is also observed that the work of cohesion is dominated by the dispersive

component, whereas for all the doped SMOFs, the work of adhesion comprises of both

components.

CHAPTER 7



140 | P a g e

Table 7.4. Summary of work of cohesion and work of adhesion of SMOFs/water pair. A

dso

rben

t

Surface

coverage

Work of Cohesion Work of adhesion

Dispersive Specific Total Dispersive Specific Total Ratio

[-] [mJ m-2] [mJ m-2] [mJ m-2] [mJ m-2] [mJ m-2] [mJ m-2] [-]

SM

OF

0.005 368.51 14.90 383.42 126.76 47.84 174.60 0.455

0.007 391.97 14.12 406.08 130.73 45.47 176.20 0.434

0.009 283.40 5.95 289.35 111.16 26.46 137.62 0.476

0.01 204.41 1.29 205.71 94.41 11.56 105.96 0.515

10%

Co

-SM

OF

0.005 507.50 18.89 526.39 148.75 43.90 192.65 0.366

0.007 500.75 16.39 517.13 147.76 40.88 188.64 0.365

0.009 492.10 14.04 506.14 146.48 37.88 184.36 0.364

0.01 489.33 12.77 502.09 146.06 36.20 182.26 0.363

20%

Co

-SM

OF

0.005 619.74 35.95 655.69 164.38 63.01 227.39 0.347

0.007 598.40 32.92 631.32 161.53 60.51 222.03 0.352

0.009 580.00 29.12 609.12 159.02 57.07 216.10 0.355

0.01 585.04 29.52 614.56 159.71 57.58 217.29 0.354

10%

Ni-

SM

OF

0.005 511.85 22.14 533.99 149.39 47.56 196.95 0.3688

0.007 497.30 19.04 516.35 147.25 44.11 191.36 0.3706

0.009 504.84 18.16 523.01 148.36 43.04 191.41 0.366

0.01 511.09 18.16 529.26 149.28 43.05 192.32 0.3634

20

% N

i-S

MO

F

0.005 511.85 22.14 533.99 149.39 47.56 196.95 0.3688

0.007 497.30 19.04 516.35 147.25 44.11 191.36 0.3706

0.009 504.84 18.16 523.01 148.36 43.04 191.41 0.366

0.01 511.09 18.16 529.26 149.28 43.05 192.32 0.3634

For further understanding, the work of adhesion was studied for different materials. For

comparison, the surface coverage remains constants; however, the experimental column

temperature was different. For RD silica-gel, it was 363 K, and for all other samples, column

temperature was kept to 413 K. The RD silica-gel and activated carbons are conventional

adsorbents, where silica gel is generally used for water adsorption. The comparative analysis

is illustrated in Fig. 7.8. It is observed that the work of adhesion is significantly lower than the

CHAPTER 7



141 | P a g e

RD silica gel. However, the specific component of the work of adhesion is almost similar to

the dispersive one. On the other hand, 20% Co-SMOF shows an imposing work of adhesion in

this coverage. Not to mention, activated carbon exhibits attractive adhesive force to water in

this lower coverage almost similar to RD silica gel, the dominating part of this force is

constituted by the dispersive force.

Fig. 7.8. Comparison of work of adhesion for different adsorbent/water pairs at 0.1 coverage.

7.5 Conclusions

The dispersive and specific surface energy of SMOFs of different concentrations of

doped metallic ions are measured with IGC to find the contribution of surface energy on water

uptake capacity. However, this study only limited to the lower pressure region. It is found that

surface energy contributes significantly to the water uptake capacity at lower pressure regions.

Surprisingly, there is no relationship found with the morphology with the water uptake

capacity. The 20%Co-SMOF exhibits the highest surface energy and the water uptake capacity.

All the surface energy components doped SMOFs increased significantly than the parent

SMOFs and contributed to the increase of water uptake capacity. Work of adhesion with water,

which is a measure of the affinity of adsorbent molecules towards the water, is also analyzed,

and it is observed that incorporation of Cobalt ion increases the work of adhesion from 105.95

mJ m-2 to 217.29 mJ m-2 indicating huge improvement affinity towards water. However, in the

practical case, the improvement is not visible due to an additional increase in the work of

cohesion.

0

50

100

150

200

250

SMOF 20% Co-

SMOF

RD Granular

Silica gel

Maxsorb III

94.4

159.71

92.25132.79

11.55

57.5863.66

20.54

Work

of

ad

hesi

on

[m

J m

-2]

Wadh (Dispersive) Wadh (Specific)

142 | P a g e

Chapter 8

Overall conclusions and recommendations Equation Chapter 8 Section 1

8.1 Overall conclusions

The principal objective of this thesis is to mitigate the gap between material synthesis

and the application in applied thermal engineering. For this purpose, from the beginning of the

thesis, the target was to find the additional characterization techniques which are hardly

available in the literature. The idea of using non-conventional but promising techniques are

used to find properties of adsorbents and pairs that might reveal intermediate information to

develop new materials for heat transfer applications like as adsorption chillers. It is observed

that surface energy is the key component of material characterization, which might be useful

for developing new functional materials. The overall findings and realization are concisely

described as follows, where findings are the interpretation of the results, and realization is the

motivation for doing further studies:

Morphological study using Atomic Force Microscopy:

Research gap: i) Direct analysis of surface porosity is hardly found in the literature,

ii) three-dimensional morphological information of the adsorbent surfaces are not

commonly available.

Studied materials: Silica gel, Acetaminophen, Silica-alumina, Maxsorb III.

Target of the study: i) conventional morphological study adopts indirect

techniques, does not provide an explicit understanding of adsorbent surfaces. Therefore,

the principal target was to study the surfaces of adsorbents directly using atomic force

microscopy, ii) Direct counting of pore size distribution, iii) visualization, and

quantitative analysis of adsorbent surfaces.

Obstacles: i) atomic force microscopy generally used for a surface having low

roughness (below 100 nm), the surface of the conventional adsorbents exhibits

moderately high surface roughness (above 500 nm), ii) identification of pores are

complicated because of having irregular shapes.

Findings: i) height images carry both the qualitative and quantitative information

of the surfaces, ii) Optimum adjustment of the operational parameters of AFM allows to

take height image data of the highly rough surface of the adsorbents, iii) Image

CHAPTER 8 OVERALL CONCLUSIONS AND RECOMMENDATIONS

143 | P a g e

processing and 2D image filtration are useful for detecting irregular shape pores, iv)

overall porosity and the surface porosity follows the similar trends.

Surface energy analysis of adsorbents:

Research gap: i) surface energy data is not commonly available for porous

adsorbents, ii) to find a relation between surface energy and adsorption, it is required to

analyze the surface energy of various adsorbents.

Target of the Study: i) to observe the variation of surface energy components

(dispersive and specific) on different adsorbents, ii) to observe the relationship between

surface energy and surface morphology.

Studied materials: RD granular silica gel, Chromatorex silica gel, Home silica gel,

B-type silica gel, Activated carbon fiber (A-20), Maxsorb III.

Obstacles: i) A similar study is hardly found in the literature, countless trial and

error analysis are required to perform before finding the optimum operation condition,

ii) porous adsorbents used in real applications comprises of large surface area which leads

to time-consuming measurement procedure for IGC experiments.

Findings: i) both the dispersive and specific surface energy are dominating in silica

gel, ii) surface area shows a direct impact on dispersive surface energy; however, no

relationship found with the specific surface energy, ii) carbonaceous materials exhibit

high dispersive surface energy.

Realization: ii) changing of morphology such as surface area, porosity has no

significant impact on specific surface energy, ii) structural modification might lead to

electrostatic variation and influence the specific surface energy, iii) Gibbs free energy on

different polar solvents are vital for preparing adsorbents for selective adsorption, iv),

unlike the carbonaceous materials, the surface of silica gel materials contains ionic

structures like as zeolites, which might be the primary source of the electrostatic

contribution of specific surface energy on adsorbing polar adsorbates.

Henry region characterization of activated carbon/ethanol pairs:

Research gap: i) experimentally found isotherms and heat of adsorption at Henry

region are not commonly available due to the limitation of conventional measurement

techniques, ii) the contribution of morphological parameters on entropy is unknown.


144 | P a g e

Target of the study: i) to measure the Henry region isotherms and Henry constant

experimentally, ii) to measure the zero-coverage heat of adsorption experimentally, iii)

to develop a model relating the entropy and

Studied materials: Maxsorb III/ethanol pair, WPT-AC/ethanol pair, M-AC/ethanol

pair, and H2-Maxsorb III/ethanol pair.

Obstacles: i) requires finding the suitable coverage range where the homogeneous

distribution of the surface energy exists, ii) requires establishing a modeling to relate the

energetic behavior with surface morphology.

Findings: i) the experimentally isotherms of the studied samples show linear nature

(R2>0.99) indicating that the measured data is in Henry region, ii) The Henry constant

ranges from 0.1584 μmol g-1 Pa-1 to 4.5141 μmol g-1 Pa-1 at various temperatures, iii) the

highest Henry constant value was found for H2-Maxsorb III (4.5141 μmol g-1 Pa-1)

temperature and the lowest on for WPT-AC (2.5779 μmol g-1 Pa-1) at 303 K, iv) zero

coverage heat of adsorption of the studied samples are measured using Clausius-

Clapeyron equation, by plotting experimentally found lnP values against T-1 values,

v) the experimentally measured zero coverage 0H for Maxsorb III, M-AC, WPT-AC,

and H2- Maxsorb III are 50.35 kJ mol-1, 51.5 kJ mol-1, 44.5 kJ mol-1, and 53.3 kJ mol-1,

respectively, vi) the plot of the specific entropy with the KH/υP shows a linear trend and

cut in the vertical axis at for at 1.8354 kJ kg-1 indicating the values of ideal entropy of

adsorption process for carbonadoes materials.

Realization: i) Henry region isotherms are the fundamental characterization

techniques that indicate the affinity between adsorbent and adsorbates; in this case, H2-

Maxsorbs exhibits the highest affinity towards ethanol. ii) the increase of affinity is

generated from the contribution of specific surface energy which was acquired by the

additional treatment of parent Maxsorb III using the flow of hydrogen at high

temperature, iii) the similar treatment is not possible for silica gel because of their lower

thermal stability, iv) new findings of the ideal entropy for carbonaceous might provide a

guide to develop new promising adsorbents; however, it is required to investigate the

adsorbents besides activated carbons.


145 | P a g e

Modification of MOFs to improve adsorption characteristics:

Research gap: i) MOFs are promising adsorbents because of having high surface

area and tailoring capability, ii) MOFs comprises of a central metal structure which can

be replaced by other metallic ions that might influence the adsorption property.

Target of the study: i) to develop new MOF using green synthesis process, ii) to

study of hydrothermal synthesis process to adopt in-situ doping for developing novel

adsorbents, iii) to shifting of S-shape isotherms towards lower pressure region, iv) to

increase the uptake capacity of MOF/water pairs.

Studied materials: Commercial aluminum fumarate, SMOF, 10% Co-SMOF, 10%

Ni-SMOF. SMOFs are newly developed MOF in our laboratory.

Obstacles: i) DMF is used in the synthesis procedure, which is toxic and harmful

to the environment, ii) replacing DMF with water generates low yields.

Findings: i) yield is increased by dropwise addition of fumaric acid with stirring at

250 rpm for 16 minutes with 60°C solution temperature, ii) synthesized MOFs exhibits

fiber shape unit clustered in a spherical lump, and the surface area increased significantly

than the commercial one, ii) No significant change in the powder XRD analysis, iii)

addition of doping concentration reduce the surface area and increase the average pore

radius, iv) the water uptake is increased significantly at the lower pressure region, and

the measured highest uptake is observed for 10% Co-SMOF which is 0.3 g g-1, v) the

calculated effective uptake of SMOF is about 1.6 times higher than silica gel in the lower

pressure region.

Realization: i) in-situ doping of SMOFs significantly increases the water uptake in

the lower pressure region despite having lower surface area compare to parent SMOF, ii)

no suitable explanation can be made based on morphological change on the increase of

water uptake capacity in the lower pressure region.

Insights of the influence of in-situ doping on water uptake capacity of SMOFs:

Research gap: i) from the uptake analysis of SMOFs (explained in chapter 6), it is

observed that there is no relation of water uptake and isotherm shifting towards lower

pressure region with the morphological changes ii) study of surface energy might explain

the insight because of the ability to measure the surface activities rather than morphology.


146 | P a g e

Target of the study: i) to study the surface energy components of the synthesized

SMOFs at the lower pressure region, ii) to conduct a comparative analysis with other

adsorbents, iii) to measure work of adhesion

Studied materials: SMOF, 10% Co-SMOF, 20% Co-SMOF, 10% Ni-SMOF, 20%

Ni-SMOF, Maxsorb III, RD granular silica gel.

Obstacles: i) measuring surface energy data in the homogeneous region, ii) SMOFs

having an interesting affinity with ethanol, which significantly increases the duration of

the experiment, iii) higher chained alkanes generates dual peaks in the FID signal plot.

Findings: i) the surface energy contributes significantly to the water uptake

capacity at lower pressure regions ii) concentration of doping increases the total surface

energy, ii) 20%Co-SMOF exhibits the highest surface energy and the water uptake

capacity, despite having lowest surface area, iii) incorporation of Cobalt ion increases the

work of adhesion from 105.95 mJ.m-2 to 217.29 mJ.m-2 indicating huge improvement

affinity towards the water; however, the increase of work cohesion reduces the uptake

capacity, iv) the work of adhesion of RD granular silica gel and Maxsorb III is almost

similar, regardless of having significant differences in surface area, v) both the dispersive

and specific part of work of adhesion is dominating in RD granular silica gel, comparing

to SMOF and 20% SMOFs.

Realization: i) surface energy plays an essential role in the adsorption process, ii)

the measurements are conducted in infinite dilution; therefore, large coverage area has

not been studied, which might contain interesting information. ii) if it is possible to

improve the work of adhesion without the increase of work of cohesion might

significantly increase the adsorbate uptake.

8.2 Recommendations

The following recommendations are made by the author for further related topics covered

in this thesis:

a) AFM is a versatile tool for analyzing surfaces of porous adsorbents at ambient

temperature; it can be used at different humidity conditions that can be used to understand the

variation of surface properties due to the introduction of vapor.

b) Analysis of surface energy in the infinite dilution provides the fundamental

information of the surface activities; however, analysis at finite dilution will reveal much

interesting information.


147 | P a g e

c) The incorporation of metallic ions contributes to the increase of surface energy at

lower coverage where the homogeneous distribution of energy is considered. Therefore to get

a clear idea of the overall influence of surface energy on the adsorption process, finite dilution

analysis can be a good option.

d) It is not still clear the contribution of surface energy sites on the adsorption process.

For example, the contribution of surface energy sites on water adsorption can be measured by

IGC. The idea is the experiments can be conducted in various humid conditions, differences of

the humid and dry conditions might reveal the contribution of energy sites on water adsorption.

In that case, adsorbents having low surface area might be a convenient option for the

measurement as high surface area leads to a heterogeneous region in the very lower pressure

region.

e) The capture of volatile organic compounds (VOCs) is becoming essentials day by day.

The challenge is the low concentration of VOCs at the environments, which leads to the

requirement of very steep uptake in the lower pressure region. IGC might become a good option

for finding suitable adsorbents for capturing VOCs.

f) MOFs are becoming the promising materials for adsorption systems, tailoring of MOFs

having extreme surface area will be a good option for developing adsorbents for heat transfer

applications.

148 | P a g e

References

[1] P.O. FANGER, Thermal comfort. Analysis and applications in environmental

engineering., Danish Technical Press, 1970.

[2] Y. Fan, L. Luo, B. Souyri, Review of solar sorption refrigeration technologies:

Development and applications, Renew. Sustain. Energy Rev. 11 (2007) 1758–1775.

doi:10.1016/J.RSER.2006.01.007.

[3] W. Pridasawas, Solar-Driven Refrigeration Systems with Focus on the Ejector Cycle,

Royal Institiute of Technology, KTH, Denmark, 2006.

[4] J. Henley, World set to use more energy for cooling than heating, (2015).

https://www.theguardian.com/environment/2015/oct/26/cold-economy-cop21-global-

warming-carbon-emissions (accessed February 13, 2018).

[5] A.M. Qenawy, A.A. Mohamad, Current Technologies and Future Perspectives in Solar

Powered Adsorption Systems, in: Can. Sol. Build. Conf., 2004.

[6] D.X. Wang, A.N. Bao, W. Kunc, W. Liss, Coal power plant flue gas waste heat and

water recovery, Appl. Energy. (2012). doi:DOI 10.1016/j.apenergy.2011.10.003.

[7] K.R. Ullah, R. Saidur, H.W. Ping, R.K. Akikur, N.H. Shuvo, A review of solar thermal

refrigeration and cooling methods, Renew. Sustain. Energy Rev. 24 (2013) 499–513.

doi:10.1016/J.RSER.2013.03.024.

[8] M.A. Alghoul, M.Y. Sulaiman, B.Z. Azmi, M.A. Wahab, Advances on multi-purpose

solar adsorption systems for domestic refrigeration and water heating, Appl. Therm.

Eng. 27 (2007) 813–822. doi:10.1016/J.APPLTHERMALENG.2006.09.008.

[9] Y. Aristov, Concept of adsorbent optimal for adsorptive cooling/heating, Appl. Therm.

Eng. 72 (2014) 166–175. doi:https://doi.org/10.1016/j.applthermaleng.2014.04.077.

[10] D. Do Duong, Adsorption Analysis: Equilibria and Kinetics, World Scientific, 1998.

[11] R.C. Bansal, M. Goyal, Activated carbon adsorption, CRC Press, Boca Raton, 2005.

[12] F. Schüth, K.S.W. Sing, J. Weitkamp, Handbook of porous solids, Wiley-VCH, 2002.

http://hdl.handle.net/2324/1842737 (accessed January 22, 2018).

[13] D.W. Bruce, D. O’Hare, R.I. Walton, Porous materials, Wiley, 2011.

http://hdl.handle.net/2324/1848600 (accessed February 18, 2018).

[14] I.I. El-Sharkawy, A. Pal, T. Miyazaki, B.B. Saha, S. Koyama, A study on consolidated

composite adsorbents for cooling application, Appl. Therm. Eng. 98 (2016) 1214–1220.

doi:10.1016/J.APPLTHERMALENG.2015.12.105.

REFERENCES

149 | P a g e

[15] R.K. Iler, The chemistry of silica : solubility, polymerization, colloid and surface

properties, and biochemistry / Ralph K. Iler, Wiley, 1979.

http://hdl.handle.net/2324/1000468809 (accessed February 18, 2018).

[16] Y.I. Aristov, M.M. Tokarev, A. Freni, I.S. Glaznev, G. Restuccia, Kinetics of water

adsorption on silica Fuji Davison RD, Microporous Mesoporous Mater. 96 (2006) 65–

71. doi:10.1016/J.MICROMESO.2006.06.008.

[17] K.C. Ng, H.T. Chua, C.Y. Chung, C.H. Loke, T. Kashiwagi, A. Akisawa, B.B. Saha,

Experimental investigation of the silica gel–water adsorption isotherm characteristics,

Appl. Therm. Eng. 21 (2001) 1631–1642. doi:10.1016/S1359-4311(01)00039-4.

[18] B.B. Saha, A. Akisawa, T. Kashiwagi, Silica gel water advanced adsorption refrigeration

cycle, Energy. 22 (1997) 437–447. doi:10.1016/S0360-5442(96)00102-8.

[19] S.S. Kistler, Coherent Expanded Aerogels, Rubber Chem. Technol. (1932).

doi:10.5254/1.3539386.

[20] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W.

Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, A new

family of mesoporous molecular sieves prepared with liquid crystal templates, J. Am.

Chem. Soc. 114 (1992) 10834–10843. doi:10.1021/ja00053a020.

[21] V. Meynen, P. Cool, E.F. Vansant, Verified syntheses of mesoporous materials,

Microporous Mesoporous Mater. 125 (2009) 170–223.

doi:10.1016/J.MICROMESO.2009.03.046.

[22] I. Glaznev, I. Ponomarenko, S. Kirik, Y. Aristov, Composites CaCl2/SBA-15 for

adsorptive transformation of low temperature heat: Pore size effect, Int. J. Refrig. 34

(2011) 1244–1250. doi:10.1016/J.IJREFRIG.2011.02.007.

[23] I.A. Simonova, A. Freni, G. Restuccia, Y.I. Aristov, Water sorption on composite “silica

modified by calcium nitrate,” Microporous Mesoporous Mater. 122 (2009) 223–228.

doi:10.1016/J.MICROMESO.2009.02.034.

[24] Y.. Aristov, G. Restuccia, G. Cacciola, V.. Parmon, A family of new working materials

for solid sorption air conditioning systems, Appl. Therm. Eng. 22 (2002) 191–204.

doi:10.1016/S1359-4311(01)00072-2.

[25] A. Pal, I.I. El-Sharkawy, B.B. Saha, S. Jribi, T. Miyazaki, S. Koyama, Experimental

investigation of CO2adsorption onto a carbon based consolidated composite adsorbent

for adsorption cooling application, Appl. Therm. Eng. (2016).

doi:10.1016/j.applthermaleng.2016.08.031.

[26] International Zeolite Association, (n.d.). http://www.iza-online.org/ (accessed February

REFERENCES

150 | P a g e

19, 2018).

[27] T.J. Barton, L.M. Bull, W.G. Klemperer, D.A. Loy, B. McEnaney, M. Misono, P.A.

Monson, G. Pez, G.W. Scherer, J.C. Vartuli, O.M. Yaghi, Tailored Porous Materials,

Chem. Mater. 11 (1999) 2633–2656. doi:10.1021/cm9805929.

[28] R. Xu, Chemistry of zeolites and related porous materials : synthesis and structure, John

Wiley & Sons (Asia), 2007. http://hdl.handle.net/2324/1837049 (accessed February 19,

2018).

[29] J. Jänchen, D. Ackermann, H. Stach, W. Brösicke, Studies of the water adsorption on

Zeolites and modified mesoporous materials for seasonal storage of solar heat, Sol.

Energy. 76 (2004) 339–344. doi:10.1016/J.SOLENER.2003.07.036.

[30] R.E. Critoph, Y. Zhong, Review of trends in solid sorption refrigeration and heat

pumping technology, Proc. Inst. Mech. Eng. Part E J. Process Mech. Eng. (2005).

doi:10.1243/095440805X6982.

[31] M. Pons, F. Meunier, G. Cacciola, R.. Critoph, M. Groll, L. Puigjaner, B. Spinner, F.

Ziegler, Thermodynamic based comparison of sorption systems for cooling and heat

pumping: Comparaison des performances thermodynamique des systèmes de pompes à

chaleur à sorption dans des applications de refroidissement et de chauffage, Int. J. Refrig.

22 (1999) 5–17. doi:10.1016/S0140-7007(98)00048-6.

[32] M.A. Lambert, B.J. Jones, Review of Regenerative Adsorption Heat Pumps, J.

Thermophys. Heat Transf. 19 (2005) 471–485. doi:10.2514/1.8075.

[33] N. Douss, F. Meunier, Effect of operating temperatures on the coefficient of

performance of active carbon-methanol systems, Heat Recover. Syst. CHP. 8 (1988)

383–392. doi:10.1016/0890-4332(88)90042-7.

[34] S. Follin, V. Goetz, A. Guillot, Influence of Microporous Characteristics of Activated

Carbons on the Performance of an Adsorption Cycle for Refrigeration, Ind. Eng. Chem.

Res. 35 (1996) 2632–2639. doi:10.1021/ie950638x.

[35] Z. Tamainot-Telto, S.J. Metcalf, R.E. Critoph, Y. Zhong, R. Thorpe, Carbon–ammonia

pairs for adsorption refrigeration applications: ice making, air conditioning and heat

pumping, Int. J. Refrig. 32 (2009) 1212–1229. doi:10.1016/J.IJREFRIG.2009.01.008.

[36] J. Miyawaki, H. Kil, K. Hata, K. Ideta, T. Ohba, H. Kanoh, I. Mochida, K. Nakabayashi,

S.-H. Yoon, Development of innovative activated carbons for adsorption heat pumps,

in: Innov. Mater. Process. Energy Systerms, Midori Printing CO. LTD., Sicily, 2016:

pp. 11–14.

[37] H. Li, M. Eddaoudi, M. O’Keeffe, O.M. Yaghi, Design and synthesis of an exceptionally

REFERENCES

151 | P a g e

stable and highly porous metal-organic framework, Nature. 402 (1999) 276.

http://dx.doi.org/10.1038/46248.

[38] O.M. Yaghi, M. O’Keeffe, N.W. Ockwig, H.K. Chae, M. Eddaoudi, J. Kim, Reticular

synthesis and the design of new materials, Nature. 423 (2003) 705.

http://dx.doi.org/10.1038/nature01650.

[39] G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surblé, I. Margiolaki,

A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and

Surface Area, Science (80-. ). 309 (2005) 2040 LP – 2042.

http://science.sciencemag.org/content/309/5743/2040.abstract.

[40] R.J. Kuppler, D.J. Timmons, Q.R. Fang, J.R. Li, T.A. Makal, M.D. Young, D. Yuan, D.

Zhao, W. Zhuang, H.C. Zhou, Potential applications of metal-organic frameworks,

Coord. Chem. Rev. 253 (2009) 3042–3066. doi:10.1016/j.ccr.2009.05.019.

[41] S. Kitagawa, S. Natarajan, Targeted Fabrication of MOFs for Hybrid Functionality (Eur.

J. Inorg. Chem. 24/2010), Eur. J. Inorg. Chem. 2010 (2010) 3685–3685.

doi:10.1002/ejic.201090071.

[42] D. Tanaka, K. Nakagawa, M. Higuchi, S. Horike, Y. Kubota, T.C. Kobayashi, M.

Takata, S. Kitagawa, Kinetic Gate-Opening Process in a Flexible Porous Coordination

Polymer, Angew. Chemie. 120 (2008) 3978–3982. doi:10.1002/ange.200705822.

[43] O.K. Farha, I. Eryazici, N.C. Jeong, B.G. Hauser, C.E. Wilmer, A.A. Sarjeant, R.Q.

Snurr, S.T. Nguyen, A.Ö. Yazaydın, J.T. Hupp, Metal–Organic Framework Materials

with Ultrahigh Surface Areas: Is the Sky the Limit?, J. Am. Chem. Soc. 134 (2012)

15016–15021. doi:10.1021/ja3055639.

[44] H. Furukawa, K.E. Cordova, M. O’Keeffe, O.M. Yaghi, The Chemistry and

Applications of Metal-Organic Frameworks, Science (80-. ). 341 (2013).

http://science.sciencemag.org/content/341/6149/1230444.abstract.

[45] A. Vishnyakov, P.I. Ravikovitch, A. V Neimark, M. Bülow, Q.M. Wang, Nanopore

Structure and Sorption Properties of Cu−BTC Metal−Organic Framework, Nano Lett. 3

(2003) 713–718. doi:10.1021/nl0341281.

[46] S.K. Henninger, F.P. Schmidt, H.-M. Henning, Characterisation and improvement of

sorption materials with molecular modeling for the use in heat transformation

applications, Adsorption. 17 (2011) 833–843. doi:10.1007/s10450-011-9342-6.

[47] L. Grajciar, O. Bludský, P. Nachtigall, Water Adsorption on Coordinatively Unsaturated

Sites in CuBTC MOF, J. Phys. Chem. Lett. 1 (2010) 3354–3359.

doi:10.1021/jz101378z.

REFERENCES

152 | P a g e

[48] P. Küsgens, M. Rose, I. Senkovska, H. Fröde, A. Henschel, S. Siegle, S. Kaskel,

Characterization of metal-organic frameworks by water adsorption, Microporous

Mesoporous Mater. 120 (2009) 325–330. doi:10.1016/J.MICROMESO.2008.11.020.

[49] K. Schlichte, T. Kratzke, S. Kaskel, Improved synthesis, thermal stability and catalytic

properties of the metal-organic framework compound Cu3(BTC)2, Microporous

Mesoporous Mater. 73 (2004) 81–88. doi:10.1016/J.MICROMESO.2003.12.027.

[50] J. Ehrenmann, S.K. Henninger, C. Janiak, Water Adsorption Characteristics of MIL-101

for Heat-Transformation Applications of MOFs, Eur. J. Inorg. Chem. 2011 (2011) 471–

474. doi:10.1002/ejic.201001156.

[51] G. Akiyama, R. Matsuda, S. Kitagawa, Highly Porous and Stable Coordination

Polymers as Water Sorption Materials, Chem. Lett. 39 (2010) 360–361.

doi:10.1246/cl.2010.360.

[52] S. Bourrelly, B. Moulin, A. Rivera, G. Maurin, S. Devautour-Vinot, C. Serre, T. Devic,

P. Horcajada, A. Vimont, G. Clet, M. Daturi, J.-C. Lavalley, S. Loera-Serna, R. Denoyel,

P.L. Llewellyn, G. Férey, Explanation of the Adsorption of Polar Vapors in the Highly

Flexible Metal Organic Framework MIL-53(Cr), J. Am. Chem. Soc. 132 (2010) 9488–

9498. doi:10.1021/ja1023282.

[53] D.S. Coombes, F. Corà, C. Mellot-Draznieks, R.G. Bell, Sorption-Induced Breathing in

the Flexible Metal Organic Framework CrMIL-53: Force-Field Simulations and

Electronic Structure Analysis, J. Phys. Chem. C. 113 (2009) 544–552.

doi:10.1021/jp809408x.

[54] D. Fröhlich, E. Pantatosaki, P.D. Kolokathis, K. Markey, H. Reinsch, M. Baumgartner,

M.A. van der Veen, D.E. De Vos, N. Stock, G.K. Papadopoulos, S.K. Henninger, C.

Janiak, Water adsorption behaviour of CAU-10-H: a thorough investigation of its

structure–property relationships, J. Mater. Chem. A. 4 (2016) 11859–11869.

doi:10.1039/C6TA01757F.

[55] H.W.B. Teo, A. Chakraborty, Water Adsorption on Various Metal Organic Framework,

IOP Conf. Ser. Mater. Sci. Eng. 272 (2017) 012019. doi:10.1088/1757-

899X/272/1/012019.

[56] B.B. Saha, A. Akisawa, T. Kashiwagi, Solar/waste heat driven two-stage adsorption

chiller: The prototype, Renew. Energy. (2001). doi:10.1016/S0960-1481(00)00107-5.

[57] K. Kaneko, C. Ishii, M. Ruike, H. kuwabara, Origin of superhigh surface area and

microcrystalline graphitic structures of activated carbons, Carbon N. Y. 30 (1992) 1075–

1088. doi:10.1016/0008-6223(92)90139-N.

REFERENCES

153 | P a g e

[58] H.D. Do, D.D. Do, Effect of pore size distribution on the surface diffusivity in activated

carbon: Hybrid Dubinin-Langmuir isotherm, Adsorption. (1995).

doi:10.1007/BF00707352.

[59] J. Landers, G.Y. Gor, A. V. Neimark, Density functional theory methods for

characterization of porous materials, Colloids Surfaces A Physicochem. Eng. Asp. 437

(2013) 3–32. doi:10.1016/j.colsurfa.2013.01.007.

[60] M. Naderi, Surface Area: Brunauer-Emmett-Teller (BET), in: Prog. Filtr. Sep., 2014.

doi:10.1016/B978-0-12-384746-1.00014-8.

[61] Z.L. Wang, New Developments in Transmission Electron Microscopy for

Nanotechnology, Adv. Mater. 15 (2003) 1497–1514. doi:10.1002/adma.200300384.

[62] W. Vanderlinde, Scanning Electron Microscopy, Scan. Electron Microsc. (2011).

doi:10.1159/000265104.

[63] J.. Paredes, A. Martınez-Alonso, J.M.. Tascón, Application of scanning tunneling and

atomic force microscopies to the characterization of microporous and mesoporous

materials, Microporous Mesoporous Mater. 65 (2003) 93–126.

doi:10.1016/j.micromeso.2003.07.001.

[64] F. Thielmann, Introduction into the characterisation of porous materials by inverse gas

chromatography, J. Chromatogr. A. 1037 (2004) 115–123.

doi:10.1016/j.chroma.2004.03.060.

[65] Inverse gas chromatography (IGC) as a tool for an energetic characterisation of porous

materials, Microporous Mesoporous Mater. 209 (2015) 99–104.

doi:10.1016/J.MICROMESO.2014.08.053.

[66] A. Pal, A. Kondor, S. Mitra, K. Thu, S. Harish, B.B. Saha, On surface energy and acid–

base properties of highly porous parent and surface treated activated carbons using

inverse gas chromatography, J. Ind. Eng. Chem. (2018).

doi:10.1016/J.JIEC.2018.09.046.

[67] H. Grajek, J. Paciura-Zadrożna, Z. Witkiewicz, Chromatographic characterisation of

ordered mesoporous silicas: Part I. Surface free energy of adsorption, J. Chromatogr. A.

1217 (2010) 3105–3115. doi:https://doi.org/10.1016/j.chroma.2010.02.028.

[68] O.M.M. Eltom, A.A.M. Sayigh, A simple method to enhance thermal conductivity of

charcoal using some additives, Renew. Energy. 4 (1994) 113–118. doi:10.1016/0960-

1481(94)90072-8.

[69] T.-H. Eun, H.-K. Song, J.H. Han, K.-H. Lee, J.-N. Kim, Enhancement of heat and mass

transfer in silica-expanded graphite composite blocks for adsorption heat pumps. Part II.

REFERENCES

154 | P a g e

Cooling system using the composite blocks, Int. J. Refrig. 23 (2000) 74–81.

doi:10.1016/S0140-7007(99)00036-5.

[70] F. Jeremias, S.K. Henninger, C. Janiak, High performance metal-organic-framework

coatings obtained via thermal gradient synthesis, Chem. Commun. 48 (2012) 9708–

9710. doi:10.1039/C2CC34782B.

[71] B.B. Saha, A. Chakraborty, S. Koyama, J.B. Lee, J. He, K.C. Ng, Adsorption

characteristics of parent and copper-sputtered RD silica gels, Philos. Mag. (2007).

doi:10.1080/14786430601032386.

[72] J.J. Guilleminot, A. Choisier, J.B. Chalfen, S. Nicolas, J.L. Reymoney, Heat transfer

intensification in fixed bed adsorbers, Heat Recover. Syst. CHP. 13 (1993) 297–300.

doi:10.1016/0890-4332(93)90052-W.

[73] A.A. Askalany, M. Salem, I.M. Ismael, A.H.H. Ali, M.G. Morsy, B.B. Saha, An

overview on adsorption pairs for cooling, Renew. Sustain. Energy Rev. 19 (2013) 565–

572. doi:10.1016/J.RSER.2012.11.037.

[74] M.M. Younes, I.I. El-Sharkawy, A.E. Kabeel, B.B. Saha, A review on adsorbent-

adsorbate pairs for cooling applications, Appl. Therm. Eng. 114 (2017) 394–414.


[75] B.B. Saha, S. Jribi, S. Koyama, I.I. El-Sharkawy, Carbon Dioxide Adsorption Isotherms

on Activated Carbons, J. Chem. Eng. Data. 56 (2011) 1974–1981.

doi:10.1021/je100973t.

[76] H.-S. Kil, T. Kim, K. Hata, K. Ideta, T. Ohba, H. Kanoh, I. Mochida, S.-H. Yoon, J.

Miyawaki, Influence of surface functionalities on ethanol adsorption characteristics in

activated carbons for adsorption heat pumps, Appl. Therm. Eng. 72 (2014) 160–165.


[77] K.C. Ng, M. Burhan, M.W. Shahzad, A. Bin Ismail, A Universal Isotherm Model to

Capture Adsorption Uptake and Energy Distribution of Porous Heterogeneous Surface,

Sci. Rep. 7 (2017) 10634. doi:10.1038/s41598-017-11156-6.

[78] D.M. Ruthven, Principles of adsorption & adsorption processes, 1st ed., John Wiley &

Sons, Inc., 1984.

[79] M. Thommes, K. Kaneko, A. V Neimark, J.P. Olivier, F. Rodriguez-Reinoso, J.

Rouquerol, K.S.W. Sing, Physisorption of gases, with special reference to the evaluation

of surface area and pore size distribution (IUPAC Technical Report), Pure Appl. Chem.

87 (2015) 1051–1069. doi:https://doi.org/10.1515/pac-2014-1117.

[80] G. Binning, C.F. Quate, Atomic Force Microscope, Phys. Rev. Lett. (1986).

REFERENCES

155 | P a g e

doi:10.1103/PhysRevLett.56.930.

[81] A. Ruf, M. Abraham, M. Lacher, K. Mayr, T. Zetterer, A Miniaturised Fabry Perot AFM

Sensor, in: Solid-State Sensors Actuators, 1995 Eurosensors IX.. Transducers ’95. 8th

Int. Conf., 1995: pp. 660–663. doi:10.1109/SENSOR.1995.717316.

[82] Z. Peng, P. West, Crystal sensor for microscopy applications, Appl. Phys. Lett. (2005).

doi:10.1063/1.1846156.

[83] M.-S. Kim, J.-H. Choi, Y.-K. Park, J.-H. Kim, Atomic force microscope cantilever

calibration device for quantified force metrology at micro- or nano-scale regime: the

nano force calibrator (NFC), Metrologia. (2006). doi:10.1088/0026-1394/43/5/008.

[84] Y. Zhang, Y. Fang, X. Zhou, X. Dong, Image-based hysteresis modeling and

compensation for an AFM piezo-scanner, Asian J. Control. (2009). doi:10.1002/asjc.92.

[85] Shimadzu, Scanning Probe Microscope SPM-9700 Hardware Instruction Manual,

Kyoto, 2011.

[86] F. Marinello, P. Bariani, L. De Chiffre, E. Savio, Fast technique for AFM vertical drift

compensation, Meas. Sci. Technol. 18 (2007) 689–696. doi:10.1088/0957-

0233/18/3/019.

[87] J.I. Paredes, A. Martínez-Alonso, P.X. Hou, T. Kyotani, J.M.D. Tascón, Imaging the

structure and porosity of active carbons by scanning tunneling microscopy, Carbon N.

Y. 44 (2006) 2469–2478. doi:10.1016/j.carbon.2006.04.040.

[88] D.-W. Kim, H.-S. Kil, K. Nakabayashi, S.-H. Yoon, J. Miyawaki, Structural elucidation

of physical and chemical activation mechanisms based on the microdomain structure

model, Carbon N. Y. 114 (2017) 98–105. doi:10.1016/j.carbon.2016.11.082.

[89] E. Papirer, E. Brendlé, H. Balard, J. Dentzer, Variation of the surface properties of nickel

oxide upon heat treatment evidenced by temperature programmed desorption and

inverse gas chromatography studies, J. Mater. Sci. 35 (2000) 3573–3577.

doi:10.1023/A:1004813629876.

[90] L.J. Jallo, Y. Chen, J. Bowen, F. Etzler, R. Dave, Prediction of Inter-particle Adhesion

Force from Surface Energy and Surface Roughness, J. Adhes. Sci. Technol. 25 (2011)

367–384. doi:10.1163/016942410X525623.

[91] O. Planinšek, J. Zadnik, Š. Rozman, M. Kunaver, R. Dreu, S. Srčič, Influence of inverse

gas chromatography measurement conditions on surface energy parameters of lactose

monohydrate, Int. J. Pharm. 256 (2003) 17–23. doi:https://doi.org/10.1016/S0378-

5173(03)00058-9.

[92] J. Stapley, G. Buckton, D. Merrifield, Investigation to find a suitable reference material

REFERENCES

156 | P a g e

for use as an inverse gas chromatography system suitability test, Int. J. Pharm. 318

(2006) 22–27. doi:https://doi.org/10.1016/j.ijpharm.2006.03.011.

[93] A. Voelkel, Chapter 20 - Physicochemical Measurements (Inverse Gas

Chromatography), in: C.F.B.T.-G.C. Poole (Ed.), Elsevier, Amsterdam, 2012: pp. 477–

494. doi:https://doi.org/10.1016/B978-0-12-385540-4.00020-1.

[94] H.E. Newell, G. Buckton, D.A. Butler, F. Thielmann, D.R. Williams, The Use of Inverse

Phase Gas Chromatography to Measure the Surface Energy of Crystalline, Amorphous,

and Recently Milled Lactose, Pharm. Res. 18 (2001) 662–666.

doi:10.1023/A:1011089511959.

[95] W. Wang, Q. Hua, Y. Sha, D. Wu, S. Zheng, B. Liu, Surface properties of solid materials

measured by modified inverse gas chromatography, Talanta. 112 (2013) 69–72.

doi:https://doi.org/10.1016/j.talanta.2013.03.040.

[96] F. Thielmann, D.A. Butler, D.R. Williams, E. Baumgarten, Characterisation of

microporous materials by dynamic sorption methods, in: A. Sayari, M.B.T.-S. in S.S.

and C. Jaroniec (Eds.), Nanoporous Mater. II, Elsevier, 2000: pp. 633–638.

doi:https://doi.org/10.1016/S0167-2991(00)80266-9.

[97] P.P. Ylä-Mäihäniemi, J.Y.Y. Heng, F. Thielmann, D.R. Williams, Inverse gas

chromatographic method for measuring the dispersive surface energy distribution for

particulates, Langmuir. 24 (2008) 9551–9557. doi:10.1021/la801676n.

[98] P. Mukhopadhyay, H.P. Schreiber, Aspects of acid-base interactions and use of inverse

gas chromatography, Colloids Surfaces A Physicochem. Eng. Asp. 100 (1995) 47–71.

doi:https://doi.org/10.1016/0927-7757(95)03137-3.

[99] J. Jagiełło, G. Ligner, E. Papirer, Characterization of silicas by inverse gas

chromatography at finite concentration: Determination of the adsorption energy

distribution function, J. Colloid Interface Sci. 137 (1990) 128–136.

doi:https://doi.org/10.1016/0021-9797(90)90049-T.

[100] E. Cremer, H. Huber, Measurement of adsorption isotherms by means of high

temperature elution gas chromatography, in: Proc. Int. Symp. Gas Chromatogr., 1962:

p. 169.

[101] E. Cremer, H. Huber, Messung von Adsorptionsisothermen an Katalysatoren bei hohen

Temperaturen mit Hilfe der Gas-Festkörper-Eluierungschromatographie, Angew.

Chemie. 73 (1961) 461–465. doi:10.1002/ange.19610731303.

[102] T.I. Bakaeva, C.G. Pantano, C.E. Loope, V.A. Bakaev, Heterogeneity of the Glass Fiber

Surface from Inverse Gas Chromatography, J. Phys. Chem. B. 104 (2000) 8518–8526.

REFERENCES

157 | P a g e

doi:10.1021/jp0013457.

[103] S. Xie, X. He, Y. Yang, Effects on growth and feed utilization of Chinese longsnout

catfish Leiocassis longirostris Günther of replacement of dietary fishmeal by soybean

cake, Aquac. Nutr. 4 (1998) 187–192. doi:10.1046/j.1365-2095.1998.00069.x.

[104] B. Voigtlander, NanoScience and Technology, Springer-Verlag, 2015. doi:10.1007/978-

3-662-45240-0_14.

[105] Gwyddion [Computer Software]. 2017 Retrieved from http://gwyddion.net/, (n.d.).

[106] T. Vuong, P.A. Monson, Surface Roughness Effects in Molecular Models of Adsorption

in Heterogeneous Porous Solids, Langmuir. 14 (1998) 4880–4886.

doi:10.1021/la980033g.

[107] B. Sun, A. Chakraborty, Thermodynamic formalism of water uptakes on solid porous

adsorbents for adsorption cooling applications, Appl. Phys. Lett. 104 (2014) 201901.

doi:10.1063/1.4876922.

[108] Rafael C. Gonzalez, Richard E. Woods, Digital Image Processing, 2002.

doi:10.1017/CBO9781107415324.004.

[109] P. Klapetek, I. Ohlidal, D. Franta, A. Montaigne-Ramil, A. Bonanni, D. Stifter, H. Sitter,

Atomic force microscopy characterization of ZnTe epitaxial films, Acta Phys. Slovaca.

53 (2003) 223–230. doi:10.1143/JJAP.42.4706.

[110] H.T. Chua, K.C. Ng, A. Chakraborty, N.M. Oo, M.A. Othman, Adsorption

Characteristics of Silica Gel + Water Systems, J. Chem. Eng. Data. 47 (2002) 1177–

1181. doi:10.1021/je0255067.

[111] D.C. Wang, Z.Z. Xia, J.Y. Wu, R.Z. Wang, H. Zhai, W.D. Dou, Study of a novel silica

gel–water adsorption chiller. Part I. Design and performance prediction, Int. J. Refrig.

28 (2005) 1073–1083. doi:10.1016/J.IJREFRIG.2005.03.001.

[112] B. Choudhury, B.B. Saha, P.K. Chatterjee, J.P. Sarkar, An overview of developments in

adsorption refrigeration systems towards a sustainable way of cooling, Appl. Energy.

104 (2013) 554–567. doi:10.1016/J.APENERGY.2012.11.042.

[113] A. SAKODA, M. SUZUKI, FUNDAMENTAL STUDY ON SOLAR POWERED

ADSORPTION COOLING SYSTEM, J. Chem. Eng. Japan. 17 (1984) 52–57.

doi:10.1252/jcej.17.52.

[114] K. CHIHARA, M. SUZUKI, AIR DRYING BY PRESSURE SWING ADSORPTION,

J. Chem. Eng. Japan. 16 (1983) 293–299. doi:10.1252/jcej.16.293.

[115] A. Myat, K. Thu, N.K. Choon, The experimental investigation on the performance of a

low temperature waste heat-driven multi-bed desiccant dehumidifier (MBDD) and

REFERENCES

158 | P a g e

minimization of entropy generation, Appl. Therm. Eng. 39 (2012) 70–77.

doi:https://doi.org/10.1016/j.applthermaleng.2012.01.041.

[116] S. Mitra, P. Kumar, K. Srinivasan, P. Dutta, Development and performance studies of

an air cooled two-stage multi-bed silica-gel + water adsorption system, Int. J. Refrig. 67

(2016) 174–189. doi:https://doi.org/10.1016/j.ijrefrig.2015.10.028.

[117] K.C. Ng, K. Thu, Y. Kim, A. Chakraborty, G. Amy, Adsorption desalination: An

emerging low-cost thermal desalination method, Desalination. 308 (2013) 161–179.

doi:https://doi.org/10.1016/j.desal.2012.07.030.

[118] K. Thu, K.C. Ng, B.B. Saha, A. Chakraborty, S. Koyama, Operational strategy of

adsorption desalination systems, Int. J. Heat Mass Transf. 52 (2009) 1811–1816.

doi:https://doi.org/10.1016/j.ijheatmasstransfer.2008.10.012.

[119] S. Mokhatab, W.A. Poe, J.Y. Mak, Chapter 9 - Natural Gas Dehydration and Mercaptans

Removal, in: S. Mokhatab, W.A. Poe, J.Y.B.T.-H. of N.G.T. and P. (Fourth E. Mak

(Eds.), Gulf Professional Publishing, 2019: pp. 307–348.

doi:https://doi.org/10.1016/B978-0-12-815817-3.00009-5.

[120] H.F. Qureshi, R. Besselink, J.E. ten Elshof, A. Nijmeijer, L. Winnubst, Doped

microporous hybrid silica membranes for gas separation, J. Sol-Gel Sci. Technol. 75

(2015) 180–188. doi:10.1007/s10971-015-3687-3.

[121] K. Benzarti, C. Perruchot, M.M. Chehimi, Surface energetics of cementitious materials

and their wettability by an epoxy adhesive, Colloids Surfaces A Physicochem. Eng. Asp.

286 (2006) 78–91. doi:https://doi.org/10.1016/j.colsurfa.2006.03.007.

[122] W. Fan, A. Chakraborty, Isosteric heat of adsorption at zero coverage for AQSOA-

Z01/Z02/Z05 zeolites and water systems, Microporous Mesoporous Mater. 260 (2018)

201–207. doi:10.1016/j.micromeso.2017.10.039.

[123] A. Chakraborty, K.C. Leong, K. Thu, B.B. Saha, K.C. Ng, Theoretical insight of

adsorption cooling, Appl. Phys. Lett. 98 (2011) 221910. doi:10.1063/1.3592260.

[124] Y.I. Aristov, Challenging offers of material science for adsorption heat transformation:

A review, Appl. Therm. Eng. 50 (2013) 1610–1618.


[125] F. Thielmann, S. Reutenauer, G. Fowler, Determination of Energy Parameters of Highly

Microporous Activated Carbons by Inverse Gas Chromatography, n.d.

[126] A. Voelkel, Chapter 2.5 Inverse gas chromatography in the examination of acid-base

and some other properties of solid materials, in: A. Dąbrowski, V.A.B.T.-S. in S.S. and

C. Tertykh (Eds.), Adsorpt. New Modif. Inorg. Sorbents, Elsevier, 1996: pp. 465–477.

REFERENCES

159 | P a g e

doi:https://doi.org/10.1016/S0167-2991(06)81031-1.

[127] M. Rückriem, D. Enke, T. Hahn, Inverse gas chromatography (IGC) as a tool for an

energetic characterisation of porous materials, Microporous Mesoporous Mater. 209

(2015) 99–104. doi:10.1016/J.MICROMESO.2014.08.053.

[128] A. Legras, A. Kondor, M. Alcock, M.T. Heitzmann, R.W. Truss, Inverse gas

chromatography for natural fibre characterisation: dispersive and acid-base distribution

profiles of the surface energy, Cellulose. 24 (2017) 4691–4700. doi:10.1007/s10570-

017-1443-2.

[129] H. Balard, E. Brendle, E. Papirer, Determination of the acid-base properties of solid

surfaces using inverse gas chromatography: Advantages and limitations, Acid-Base

Interact. Relev. to Adhes. Sci. Technol. 2 (2000) 299–316.

[130] F.M. Fowkes, ATTRACTIVE FORCES AT INTERFACES, Ind. Eng. Chem. 56 (1964)

40–52. doi:10.1021/ie50660a008.

[131] J. Schultz, L. Lavielle, C. Martin, The Role of the Interface in Carbon Fibre-Epoxy

Composites, J. Adhes. 23 (1987) 45–60. doi:10.1080/00218468708080469.

[132] G.M. Dorris, D.G. Gray, Adsorption of n-alkanes at zero surface coverage on cellulose

paper and wood fibers, J. Colloid Interface Sci. 77 (1980) 353–362.

doi:https://doi.org/10.1016/0021-9797(80)90304-5.

[133] D.K. Owens, R.C. Wendt, Estimation of the surface free energy of polymers, J. Appl.

Polym. Sci. 13 (1969) 1741–1747.

[134] C. Della Volpe, D. Maniglio, S. Siboni, M. Morra, Recent theoretical and experimental

advancements in the application of van Oss–Chaudury–Good acid–base theory to the

analysis of polymer surfaces I. General aspects, J. Adhes. Sci. Technol. 17 (2003) 1477–

1505. doi:10.1163/156856103769207365.

[135] C.D. Volpe, S. Siboni, Some Reflections on Acid–Base Solid Surface Free Energy

Theories, J. Colloid Interface Sci. 195 (1997) 121–136.

doi:https://doi.org/10.1006/jcis.1997.5124.

[136] S. Dong, M. Brendlé, J.B. Donnet, Study of solid surface polarity by inverse gas

chromatography at infinite dilution, Chromatographia. 28 (1989) 469–472.

doi:10.1007/BF02261062.

[137] M.A. Islam, A. Pal, B.B. Saha, Experimental study on thermophysical and porous

properties of silica gels, Int. J. Refrig. 110 (2020) 277–285.

doi:10.1016/j.ijrefrig.2019.10.027.

[138] A. Kondor, C. Quellet, A. Dallos, Surface characterization of standard cotton fibres and

REFERENCES

160 | P a g e

determination of adsorption isotherms of fragrances by IGC, Surf. Interface Anal. 47

(2015) 1040–1050. doi:10.1002/sia.5811.

[139] B. Shi, Y. Wang, L. Jia, Comparison of Dorris–Gray and Schultz methods for the

calculation of surface dispersive free energy by inverse gas chromatography, J.

Chromatogr. A. 1218 (2011) 860–862.

doi:https://doi.org/10.1016/j.chroma.2010.12.050.

[140] M. Rückriem, A. Inayat, D. Enke, R. Gläser, W.-D. Einicke, R. Rockmann, Inverse gas

chromatography for determining the dispersive surface energy of porous silica, Colloids

Surfaces A Physicochem. Eng. Asp. 357 (2010) 21–26.

doi:10.1016/J.COLSURFA.2009.12.001.

[141] A. Chakraborty, B.B. Saha, S. Koyama, K.C. Ng, On the thermodynamic modeling of

the isosteric heat of adsorption and comparison with experiments, Appl. Phys. Lett. 89

(2006) 171901. doi:10.1063/1.2360925.

[142] L. Prasetyo, D.D. Do, D. Nicholson, A coherent definition of Henry constant and

isosteric heat at zero loading for adsorption in solids – An absolute accessible volume,

Chem. Eng. J. 334 (2018) 143–152. doi:10.1016/j.cej.2017.10.022.

[143] B.B. Saha, A. Chakraborty, S. Koyama, K. Srinivasan, K.C. Ng, T. Kashiwagi, P. Dutta,

Thermodynamic formalism of minimum heat source temperature for driving advanced

adsorption cooling device, Appl. Phys. Lett. 91 (2007) 111902. doi:10.1063/1.2780117.

[144] A.K. Meena, G.K. Mishra, P.K. Rai, C. Rajagopal, P.N. Nagar, Removal of heavy metal

ions from aqueous solutions using carbon aerogel as an adsorbent, J. Hazard. Mater. 122

(2005) 161–170. doi:https://doi.org/10.1016/j.jhazmat.2005.03.024.

[145] L. Deschênes, J. Lyklema, C. Danis, F. Saint-Germain, Phase transitions in polymer

monolayers: Application of the Clapeyron equation to PEO in PPO–PEO Langmuir

films, Adv. Colloid Interface Sci. 222 (2015) 199–214.

doi:https://doi.org/10.1016/j.cis.2014.11.002.

[146] P.J. Dauenhauer, O.A. Abdelrahman, A Universal Descriptor for the Entropy of

Adsorbed Molecules in Confined Spaces, ACS Cent. Sci. 4 (2018) 1235–1243.

doi:10.1021/acscentsci.8b00419.

[147] R.K. Agarwal, K.A.G. Amankwah, J.A. Schwarz, Analysis of adsorption entropies of

high pressure gas adsorption data on activated carbon, Carbon N. Y. 28 (1990) 169–174.

doi:10.1016/0008-6223(90)90110-K.

[148] A. Pal, K. Thu, S. Mitra, I.I. El-Sharkawy, B.B. Saha, H.-S. Kil, S.-H. Yoon, J.

Miyawaki, Study on biomass derived activated carbons for adsorptive heat pump

REFERENCES

161 | P a g e

application, Int. J. Heat Mass Transf. 110 (2017) 7–19.

doi:https://doi.org/10.1016/j.ijheatmasstransfer.2017.02.081.

[149] T. Smith, Effect of Surface Coverage and Temperature on the Sticking Coefficient, J.

Chem. Phys. 40 (1964) 1805–1812. doi:10.1063/1.1725409.

[150] A. Chakraborty, B.B. Saha, K.C. Ng, S. Koyama, K. Srinivasan, Theoretical Insight of

Physical Adsorption for a Single-Component Adsorbent + Adsorbate System: I.

Thermodynamic Property Surfaces, Langmuir. 25 (2009) 2204–2211.

doi:10.1021/la803289p.

[151] S. Sircar, R. Mohr, C. Ristic, M.B. Rao, Isosteric Heat of Adsorption:  Theory and

Experiment, J. Phys. Chem. B. 103 (1999) 6539–6546. doi:10.1021/jp9903817.

[152] K. Uddin, I.I. El-Sharkawy, T. Miyazaki, S. Koyama, H.-S. Kil, J. Miyawaki, S.-H.

Yoon, Adsorption characteristics of ethanol onto functional activated carbons with

controlled oxygen content, Appl. Therm. Eng. 72 (2014) 211–218.


[153] M. Sivak, Potential energy demand for cooling in the 50 largest metropolitan areas of

the world: Implications for developing countries, Energy Policy. 37 (2009) 1382–1384.

doi:https://doi.org/10.1016/j.enpol.2008.11.031.

[154] K.-H. Kim, Z.-H. Shon, H.T. Nguyen, E.-C. Jeon, A review of major

chlorofluorocarbons and their halocarbon alternatives in the air, Atmos. Environ. 45

(2011) 1369–1382. doi:https://doi.org/10.1016/j.atmosenv.2010.12.029.

[155] B.B. Saha, K.C. Ng, Advances in adsorption technology, Nova Science Publishers,

2010. http://hdl.handle.net/2324/1001434433 (accessed January 22, 2018).

[156] K.C. Ng, H.T. Chua, C.Y. Chung, C.H. Loke, T. Kashiwagi, A. Akisawa, B.B. Saha,

Experimental investigation of the silica gel–water adsorption isotherm characteristics,

Appl. Therm. Eng. 21 (2001) 1631–1642. doi:10.1016/S1359-4311(01)00039-4.

[157] S. Kayal, S. Baichuan, B.B. Saha, Adsorption characteristics of AQSOA zeolites and

water for adsorption chillers, Int. J. Heat Mass Transf. 92 (2016) 1120–1127.

doi:10.1016/J.IJHEATMASSTRANSFER.2015.09.060.

[158] J. Sahu, A. Aijaz, Q. Xu, P.K. Bharadwaj, A three-dimensional pillared-layer metal-

organic framework: Synthesis, structure and gas adsorption studies, Inorganica Chim.

Acta. 430 (2015) 193–198. doi:https://doi.org/10.1016/j.ica.2015.03.011.

[159] N.C. Burtch, H. Jasuja, K.S. Walton, Water Stability and Adsorption in Metal–Organic

Frameworks, Chem. Rev. 114 (2014) 10575–10612. doi:10.1021/cr5002589.

[160] N. Tannert, S.-J. Ernst, C. Jansen, H.-J. Bart, S.K. Henninger, C. Janiak, Evaluation of

REFERENCES

162 | P a g e

the highly stable metal–organic framework MIL-53(Al)-TDC (TDC = 2,5-

thiophenedicarboxylate) as a new and promising adsorbent for heat transformation

applications, J. Mater. Chem. A. 6 (2018) 17706–17712. doi:10.1039/C8TA04407D.

[161] H.W.B. Teo, A. Chakraborty, S. Kayal, Post synthetic modification of MIL-101(Cr) for

S-shaped isotherms and fast kinetics with water adsorption, Appl. Therm. Eng. 120

(2017) 453–462. doi:10.1016/J.APPLTHERMALENG.2017.04.018.

[162] M. Wickenheisser, F. Jeremias, S.K. Henninger, C. Janiak, Grafting of hydrophilic

ethylene glycols or ethylenediamine on coordinatively unsaturated metal sites in MIL-

100(Cr) for improved water adsorption characteristics, Inorganica Chim. Acta. 407

(2013) 145–152. doi:https://doi.org/10.1016/j.ica.2013.07.024.

[163] C.T. Lollar, J.-S. Qin, J. Pang, S. Yuan, B. Becker, H.-C. Zhou, Interior Decoration of

Stable Metal–Organic Frameworks, Langmuir. 34 (2018) 13795–13807.

doi:10.1021/acs.langmuir.8b00823.

[164] F. Jeremias, D. Frohlich, C. Janiak, S.K. Henninger, Water and methanol adsorption on

MOFs for cycling heat transformation processes, New J. Chem. 38 (2014) 1846–1852.

doi:10.1039/C3NJ01556D.

[165] N. Tannert, C. Jansen, S. Nießing, C. Janiak, Robust synthesis routes and porosity of the

Al-based metal–organic frameworks Al-fumarate, CAU-10-H and MIL-160, Dalt.

Trans. 48 (2019) 2967–2976. doi:10.1039/C8DT04688C.

[166] L. Emi, M. Ulrich, T. Natalia, M. Hendrick, C. Gerhard, B. Stefan, Process for preparing

porous metal-organic frameworks based on aluminium fumarate, US 2012/0082864 A1,

2012.

[167] F. Jeremias, D. Fröhlich, C. Janiak, S.K. Henninger, Advancement of sorption-based

heat transformation by a metal coating of highly-stable, hydrophilic aluminium fumarate

MOF, RSC Adv. 4 (2014) 24073–24082. doi:10.1039/C4RA03794D.

[168] S. Kayal, A. Chakraborty, H.W.B. Teo, Green synthesis and characterization of

aluminium fumarate metal-organic framework for heat transformation applications,

Mater. Lett. 221 (2018) 165–167. doi:https://doi.org/10.1016/j.matlet.2018.03.099.

[169] J.R. Karra, K.S. Walton, Effect of Open Metal Sites on Adsorption of Polar and

Nonpolar Molecules in Metal−Organic Framework Cu-BTC, Langmuir. 24 (2008)

8620–8626. doi:10.1021/la800803w.

[170] K.A. Cychosz, M. Thommes, Progress in the Physisorption Characterization of

Nanoporous Gas Storage Materials, Engineering. 4 (2018) 559–566.

doi:https://doi.org/10.1016/j.eng.2018.06.001.

REFERENCES

163 | P a g e

[171] F.H.M. Azahar, S. Mitra, A. Yabushita, A. Harata, B.B. Saha, K. Thu, Improved model

for the isosteric heat of adsorption and impacts on the performance of heat pump cycles,

Appl. Therm. Eng. 143 (2018) 688–700.

doi:https://doi.org/10.1016/j.applthermaleng.2018.07.131.

[172] M. V Solovyeva, L.G. Gordeeva, Y.I. Aristov, “MIL-101(Cr)–methanol” as working

pair for adsorption heat transformation cycles: Adsorbent shaping, adsorption

equilibrium and dynamics, Energy Convers. Manag. 182 (2019) 299–306.

doi:https://doi.org/10.1016/j.enconman.2018.12.065.

[173] J. Canivet, A. Fateeva, Y. Guo, B. Coasne, D. Farrusseng, Water adsorption in MOFs:

fundamentals and applications, Chem. Soc. Rev. 43 (2014) 5594–5617.

doi:10.1039/C4CS00078A.

[174] R. Ho, J.Y.Y. Heng, A review of inverse gas chromatography and its development as a

tool to characterize anisotropic surface properties of pharmaceutical solids, KONA

Powder Part. J. 30 (2012) 164–180. doi:10.14356/kona.2013016.

[175] A. Legras, A. Kondor, M.T. Heitzmann, R.W. Truss, Inverse gas chromatography for

natural fibre characterisation: Identification of the critical parameters to determine the

Brunauer–Emmett–Teller specific surface area, J. Chromatogr. A. 1425 (2015) 273–

279. doi:https://doi.org/10.1016/j.chroma.2015.11.033.

[176] S.M.S. Ltd., Determination of the Surface Energy of Nanomaterials Using Inverse Gas

Chromatography, AZoNano. (2019).

https://www.azonano.com/article.aspx?ArticleID=2609. (accessed May 5, 2020).

164 | P a g e

Appendix A. Three-dimensional images of

various adsorbents

Fig. A1. RD silica gel Fig. A2. Silica gel collected from foods

samples

Fig. A3. Blue silica gel Fig. A4. B-type silica gel

APPENDIX A

165 | P a g e

Fig. A5. Cu-sputtered silica

gel

Fig. A6. Activated carbon (Maxsorb III)

Fig. A7. Acetaminophen-medical

sample

Fig. A8. Aluminium fumarate

166 | P a g e

Appendix B. Example of surface energy date

measured for RD granular silica gel using

Inverse Gas Chromatography

Solvent

Injection

Time Duration

Target

Fractional

Surface

Coverage

Actual

Fractional

Surface

Coverage

Temperature Flow

Rate

Peak

Area

Peak

Max

(Signal)

[ms] [min] [°C] [sccm] [µV•min] [µV]

ETHYL ACETATE 834 280 0.01 0.0107 100 29.99 2130.37 84.34

ETHYL ACETATE 2559 250 0.03 0.0324 100 30.01 7236.5 476.06

ETHYL ACETATE 4114 220 0.05 0.0531 100 30 12506.71 1212.15

ETHYL ACETATE 5514 150 0.07 0.0746 100 30 18241.57 2400.85

ETHYL ACETATE 6823 90 0.09 0.0931 100 30 23294.07 4033.68

ETHYL ACETATE 7461 65 0.1 0.1005 100 29.98 25350.38 5069.12

ACETONE 432 100 0.01 0.0114 100 29.99 2582.57 277.68

ACETONE 1276 90 0.03 0.0328 100 29.99 7968.93 1364.73

ACETONE 2029 45 0.05 0.0542 100 30 13598.42 3358.02

ACETONE 2704 35 0.07 0.0744 100 29.99 19078.1 6386.29

ACETONE 3325 30 0.09 0.0931 100.01 30 24214.5 10490

ACETONE 3622 25 0.1 0.1016 100 30 26580.79 13126.44

ACETONITRILE 1387 30 0.01 0.0109 100 30.01 2524.25 1210.69

ACETONITRILE 3961 30 0.03 0.0344 100 30 8230.57 5409.21

ACETONITRILE 6307 20 0.05 0.0582 100 29.99 14097.13 11771.64

ACETONITRILE 8629 15 0.07 0.0802 100 29.99 19575.32 20575.6

ACETONITRILE 10778 15 0.09 0.1017 100 30 24972.72 31418.54

ACETONITRILE 11516 10 0.1 0.1102 100 30 27103.98 35816.26

ETHANOL 1089 160 0.01 0.0099 100 30 1053.5 263.03

ETHANOL 3318 120 0.03 0.0307 100 30.01 3555.19 1158.13

ETHANOL 5384 40 0.05 0.0505 100 30 6074.57 2270.51

ETHANOL 7339 30 0.07 0.0727 100 29.99 8991.7 4196.13

ETHANOL 9266 20 0.09 0.0934 100 29.99 11778.56 6343.87

ETHANOL 10218 15 0.1 0.1033 100 30 13134.38 7600.68

DICHLOROMETHANE 600 2 0.01 0.0094 100.01 30.01 5445.51 129895

DICHLOROMETHANE 1479 2 0.03 0.0354 100 30 15696.97 471438.4

DICHLOROMETHANE 2033 2 0.05 0.0548 100 30.02 22274.14 711167.1

DICHLOROMETHANE 2346 2 0.07 0.0677 100 30 26386.44 849380.4



APPENDIX B

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HEXANE 574 5 0.01 0.0109 100 29.99 5480.27 89105.35

HEXANE 1652 4 0.03 0.0329 100 30.02 17872.86 369368.2

HEXANE 2570 4 0.05 0.052 100 29.99 29214.75 661305.6

HEXANE 3381 3 0.07 0.0717 100 30 41185.17 995677.4

HEXANE 4139 3 0.09 0.0851 100 30.02 49493.51 1147029

HEXANE 4510 3 0.1 0.0906 100 29.99 52961.53 1151172

HEPTANE 1233 10 0.01 0.0103 100 30 4093.35 23302.73

HEPTANE 3723 9 0.03 0.0301 100 30 13563.64 103837.1

HEPTANE 5858 8 0.05 0.0495 100 30 23621.53 219673.9

HEPTANE 7967 8 0.07 0.0698 100 29.99 34706.92 368432.8

HEPTANE 10025 7 0.09 0.0892 100 29.99 45622.4 536832.6

HEPTANE 10590 7 0.1 0.0958 100 29.99 49434.82 600549.8

OCTANE 3648 15 0.01 0.0111 100 29.99 4922.78 10617.57

OCTANE 10151 15 0.03 0.0301 99.99 30 14995.96 43729.34

OCTANE 16781 10 0.05 0.0492 99.99 29.99 25956.4 94921.25

OCTANE 25960 10 0.07 0.0709 100 30 39027.18 172049.6

OCTANE 38541 8 0.09 0.0909 100 30.01 51569.74 257765.6

OCTANE 45759 8 0.1 0.1028 100 29.98 59198.97 315470.1

Solvent

Peak

Max

(Time)

Peak

Com

Peak

Com/Max

Ret

Volume

(Max)

Ret

Volume

(Com)

Net Ret

Time

(Max)

Sp. Ret

Volume

(Max)

Net Ret

Time

(Com)

[min] [min] [ml/g] [ml/g] [min] [ml/g] [min]

ETHYL ACETATE 11.228 21.738 1.936 55235.44 106941.4 11.076 54489.6 21.823

ETHYL ACETATE 5.526 14.858 2.689 27202.84 73138.2 5.375 26456.57 14.943

ETHYL ACETATE 3.302 11.228 3.4 16245.08 55238.22 3.151 15499.24 11.313

ETHYL ACETATE 2.146 9.258 4.314 10556.24 45539.38 1.994 9810.46 9.342

ETHYL ACETATE 1.534 7.567 4.931 7549.83 37228.66 1.383 6803.9 7.651

ETHYL ACETATE 1.295 6.532 5.046 6365.11 32118.87 1.143 5619.67 6.617

ACETONE 4.05 8.196 2.024 19915.62 40300.34 3.899 19170.16 8.28

ACETONE 2.189 5.694 2.601 10768.65 28008.42 2.038 10022.91 5.778

ACETONE 1.368 4.243 3.102 6729.45 20872.97 1.216 5983.6 4.327

ACETONE 0.936 3.437 3.671 4605.66 16905.99 0.785 3859.94 3.521

ACETONE 0.68 2.99 4.398 3344.25 14709.13 0.528 2598.37 3.074

ACETONE 0.585 2.65 4.532 2877.29 13039.22 0.433 2131.35 2.734

ACETONITRILE 1.035 1.934 1.87 5091.79 9520.1 0.883 4345.69 2.019

ACETONITRILE 0.71 1.518 2.139 3492.52 7470.66 0.558 2746.49 1.602

ACETONITRILE 0.542 1.301 2.402 2663.51 6396.51 0.39 1917.83 1.385

ACETONITRILE 0.433 1.067 2.463 2130.39 5247.49 0.282 1384.75 1.151

ACETONITRILE 0.365 0.954 2.616 1794.78 4695.27 0.213 1048.95 1.039

ACETONITRILE 0.353 0.949 2.687 1737.6 4669.02 0.202 991.78 1.033

ETHANOL 1.821 3.887 2.135 8958.47 19122.33 1.669 8212.64 3.971

ETHANOL 1.33 2.884 2.169 6544.2 14192.96 1.178 5797.97 2.968

APPENDIX B

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ETHANOL 1.366 2.392 1.751 6723.12 11770.93 1.215 5977.05 2.476

ETHANOL 0.873 2.107 2.413 4294.69 10363.03 0.721 3548.88 2.191

ETHANOL 0.741 1.883 2.54 3646.53 9262.22 0.59 2900.85 1.967

ETHANOL 0.691 1.769 2.558 3401.9 8702.07 0.54 2655.97 1.853

DICHLOROMETHANE 0.16 0.18 1.123 785.88 882.34 0.008 40.99 0.264






HEXANE 0.163 0.191 1.168 803.18 937.92 0.012 57.43 0.275

HEXANE 0.16 0.184 1.15 787.47 905.39 0.008 40.99 0.268

HEXANE 0.16 0.18 1.128 785.85 886.47 0.008 41.06 0.265

HEXANE 0.155 0.177 1.143 761.22 869.75 0.003 16.42 0.261

HEXANE 0.158 0.177 1.115 777.88 867.43 0.007 32.8 0.261

HEXANE 0.158 0.176 1.113 777.51 865.37 0.007 32.71 0.26

HEPTANE 0.2 0.279 1.394 983 1370.01 0.048 237.59 0.363

HEPTANE 0.185 0.253 1.37 909.24 1245.23 0.033 163.84 0.337

HEPTANE 0.177 0.238 1.349 868.91 1171.79 0.025 122.96 0.322

HEPTANE 0.173 0.228 1.318 851.41 1122.27 0.022 106.43 0.313

HEPTANE 0.17 0.221 1.301 835.34 1086.76 0.018 90.15 0.305

HEPTANE 0.168 0.219 1.303 827.86 1078.29 0.017 82.11 0.303

OCTANE 0.307 0.501 1.634 1507.92 2463.38 0.155 762.16 0.585

OCTANE 0.258 0.427 1.655 1270.38 2102.48 0.107 524.59 0.512

OCTANE 0.235 0.381 1.623 1155.43 1875.57 0.083 409.8 0.466

OCTANE 0.218 0.348 1.597 1073.74 1714.52 0.067 327.86 0.433

OCTANE 0.207 0.324 1.57 1016.93 1596.47 0.055 270.74 0.409

OCTANE 0.2 0.313 1.565 982.27 1537.7 0.048 237.42 0.397

Solvent

Sp. Ret

Volume

(Com)

Partial

Pressure

Pres Ret

Volume

(Max)

Pres Ret

Volume

(Com)

Injected

Amount

Injected

Amount

Adsorbed

Amount

Adsorbed

Amount

[ml/g] [Torr] [mMol/(g

•Torr)]

[mMol/(g•T

orr)] [mMol/g] [mMol]

(Max)

[mMol/g]

(Com)

[mMol/g]

ETHYL ACETATE 107355.7 0.0058 2.34E+00 4.61E+00 3.24E-02 2.59E-04 1.45E-02 2.77E-02

ETHYL ACETATE 73552.73 0.0292 1.14E+00 3.16E+00 9.78E-02 7.82E-04 4.11E-02 1.02E-01

ETHYL ACETATE 55652.5 0.0706 6.66E-01 2.39E+00 1.60E-01 1.28E-03 6.86E-02 2.00E-01


ETHYL ACETATE 37643 0.221 2.92E-01 1.62E+00 2.81E-01 2.25E-03 1.21E-01 4.67E-01


ACETONE 40714.43 0.0163 8.24E-01 1.75E+00 3.34E-02 2.67E-04 1.43E-02 2.00E-02

ACETONE 28422.66 0.0748 4.31E-01 1.22E+00 9.61E-02 7.69E-04 3.95E-02 9.14E-02

ACETONE 21287.27 0.1781 2.57E-01 9.15E-01 1.59E-01 1.27E-03 6.60E-02 1.86E-01

ACETONE 17320.21 0.3317 1.66E-01 7.44E-01 2.18E-01 1.75E-03 9.15E-02 3.00E-01

APPENDIX B

169 | P a g e

ACETONE 15123.44 0.537 1.12E-01 6.50E-01 2.73E-01 2.18E-03 1.14E-01 4.34E-01

ACETONE 13453.56 0.6681 9.16E-02 5.78E-01 2.98E-01 2.38E-03 1.26E-01 5.09E-01

ACETONITRILE 9934.54 0.1101 1.87E-01 4.27E-01 5.06E-02 4.04E-04 2.15E-02 4.97E-02






ETHANOL 19536.62 0.0318 3.53E-01 8.40E-01 2.80E-02 2.24E-04 1.14E-02 2.51E-02

ETHANOL 14607.46 0.1282 2.49E-01 6.28E-01 8.67E-02 6.93E-04 3.55E-02 8.57E-02

ETHANOL 12185.35 0.242 2.57E-01 5.24E-01 1.43E-01 1.14E-03 6.47E-02 1.45E-01

ETHANOL 10777.29 0.435 1.53E-01 4.63E-01 2.05E-01 1.64E-03 9.41E-02 2.35E-01

ETHANOL 9676.42 0.6451 1.25E-01 4.16E-01 2.64E-01 2.11E-03 1.20E-01 3.22E-01

ETHANOL 9116.4 0.7666 1.14E-01 3.92E-01 2.92E-01 2.33E-03 1.34E-01 3.70E-01

DICHLOROMETHANE 1296.1 4.1603 1.76E-03 5.57E-02 3.84E-02 3.07E-04 7.32E-03 2.32E-01

DICHLOROMETHANE 1256.96 19.6483 1.06E-03 5.40E-02 1.44E-01 1.15E-03 2.37E-02 1.07E+00





HEXANE 1352.16 1.5584 2.47E-03 5.81E-02 2.11E-02 1.69E-04 4.28E-03 9.12E-02

HEXANE 1320.04 5.9654 1.76E-03 5.67E-02 6.36E-02 5.09E-04 1.20E-02 3.41E-01

HEXANE 1300.17 10.3474 1.76E-03 5.59E-02 1.01E-01 8.05E-04 1.98E-02 5.86E-01

HEXANE 1283.46 15.2252 7.05E-04 5.52E-02 1.39E-01 1.11E-03 2.32E-02 8.55E-01

HEXANE 1281.3 17.3166 1.41E-03 5.51E-02 1.65E-01 1.32E-03 2.62E-02 9.70E-01

HEXANE 1279.08 17.3126 1.41E-03 5.50E-02 1.75E-01 1.40E-03 2.62E-02 9.70E-01

HEPTANE 1784.05 0.4637 1.02E-02 7.67E-02 1.79E-02 1.43E-04 4.99E-03 3.58E-02

HEPTANE 1659.27 1.8214 7.04E-03 7.13E-02 5.24E-02 4.19E-04 1.46E-02 1.33E-01

HEPTANE 1586.13 3.6345 5.28E-03 6.82E-02 8.60E-02 6.88E-04 2.41E-02 2.56E-01

HEPTANE 1536.08 5.8563 4.57E-03 6.60E-02 1.21E-01 9.71E-04 3.43E-02 4.03E-01

HEPTANE 1500.68 8.2889 3.87E-03 6.45E-02 1.55E-01 1.24E-03 4.37E-02 5.60E-01

HEPTANE 1492.53 9.1948 3.53E-03 6.41E-02 1.67E-01 1.33E-03 4.69E-02 6.18E-01

OCTANE 2877.62 0.1726 3.28E-02 1.24E-01 1.76E-02 1.41E-04 5.94E-03 2.16E-02

OCTANE 2516.74 0.6315 2.25E-02 1.08E-01 4.77E-02 3.81E-04 1.63E-02 7.12E-02

OCTANE 2289.74 1.2936 1.76E-02 9.84E-02 7.78E-02 6.23E-04 2.79E-02 1.36E-01

OCTANE 2128.83 2.2444 1.41E-02 9.15E-02 1.12E-01 8.97E-04 4.13E-02 2.23E-01

OCTANE 2010.95 3.2631 1.16E-02 8.64E-02 1.44E-01 1.15E-03 5.32E-02 3.11E-01

OCTANE 1951.43 3.9391 1.02E-02 8.39E-02 1.63E-01 1.30E-03 6.01E-02 3.68E-01

170 | P a g e

Appendix C. Henry region isotherm data of

activated carbon/ethanol pairs

Maxsorb III WPT-AC M-AC H2-Maxsorb III

Relative

Pressure

(P/P0)

Mass Adsorbed

Relative

Pressure

(P/P0)

Mass Adsorbed

Relative

Pressure

(P/P0)

Mass Adsorbed

Relative

Pressure

(P/P0)

Mass Adsorbed

[-] [mg/g] [-] [mg/g] [-] [mg/g] [-] [mg/g]

30˚C

6.4E-05 0.19 6.9E-04 0.95 4.2E-05 0.21 3.8E-05 0.22

4.6E-04 0.98 1.6E-03 2.09 4.0E-04 1.16 3.5E-04 1.11

1.1E-03 2.13 2.7E-03 3.33 1.0E-03 2.57 9.2E-04 2.49

1.7E-03 3.39 3.8E-03 4.61 1.7E-03 4.04 1.5E-03 3.86

2.5E-03 4.73 4.8E-03 5.83 2.5E-03 5.51 2.2E-03 5.18

3.2E-03 6.03 5.9E-03 7.13 3.3E-03 7.23 2.8E-03 6.46

3.8E-03 7.20 6.9E-03 8.43 4.0E-03 8.73 3.4E-03 7.79

4.4E-03 8.40 7.6E-03 9.40 4.6E-03 9.72 3.9E-03 8.97

5.0E-03 9.49 8.4E-03 10.46 5.3E-03 11.08 4.5E-03 10.19

5.6E-03 10.67 9.3E-03 11.68 5.9E-03 12.47 5.1E-03 11.49

6.3E-03 11.85 1.0E-02 13.04 6.6E-03 13.94 5.7E-03 12.69

7.2E-03 13.80 7.5E-03 15.67 6.4E-03 14.40

40˚C

8.8E-05 0.20 1.4E-04 0.26 5.4E-05 0.23 4.7E-05 0.22

5.9E-04 1.03 8.5E-04 1.09 5.3E-04 1.17 4.4E-04 1.11

1.4E-03 2.23 2.0E-03 2.29 1.3E-03 2.57 1.1E-03 2.45

2.2E-03 3.45 3.3E-03 3.57 2.2E-03 4.04 1.9E-03 3.82

3.1E-03 4.69 4.5E-03 4.86 3.1E-03 5.50 2.6E-03 5.14

3.9E-03 5.95 5.8E-03 6.13 3.9E-03 6.90 3.4E-03 6.42

4.7E-03 7.20 6.9E-03 7.35 4.7E-03 8.30 4.1E-03 7.66

5.5E-03 8.37 8.0E-03 8.53 5.5E-03 9.67 4.8E-03 8.91

6.3E-03 9.55 9.1E-03 9.74 6.3E-03 10.97 5.5E-03 10.11

7.1E-03 10.76 1.0E-02 10.88 7.1E-03 12.37 6.2E-03 11.32

7.9E-03 11.94 1.1E-02 11.91 7.9E-03 13.77 6.8E-03 12.52

8.5E-03 12.82 1.2E-02 13.07 8.8E-03 15.30 7.5E-03 13.80

50

˚C

1.2E-04 0.21 1.8E-04 0.26 8.7E-05 0.22 6.5E-05 0.23

7.4E-04 1.05 1.0E-03 1.12 7.1E-04 1.21 5.6E-04 1.17

1.7E-03 2.24 2.3E-03 2.30 1.7E-03 2.61 1.4E-03 2.49

2.7E-03 3.47 3.7E-03 3.53 2.8E-03 4.08 2.3E-03 3.88

3.6E-03 4.69 5.1E-03 4.78 3.8E-03 5.49 3.2E-03 5.16

4.6E-03 5.92 6.4E-03 6.00 4.8E-03 6.88 4.1E-03 6.50

5.6E-03 7.15 7.8E-03 7.20 5.7E-03 8.18 4.9E-03 7.71

6.6E-03 8.35 9.0E-03 8.36 6.7E-03 9.49 5.7E-03 8.95

7.5E-03 9.50 1.0E-02 9.50 7.6E-03 10.80 6.5E-03 10.14

APPENDIX C

171 | P a g e

8.4E-03 10.72 1.1E-02 10.61 8.5E-03 12.06 7.4E-03 11.36

9.3E-03 11.86 1.2E-02 11.70 9.4E-03 13.32 8.1E-03 12.45

1.0E-02 13.18 1.4E-02 12.80 1.0E-02 14.65 8.9E-03 13.58

60

˚C

1.5E-04 0.23 2.0E-04 0.26 1.1E-04 0.23 9.0E-05 0.24

8.9E-04 1.09 1.1E-03 1.13 8.4E-04 1.23 6.9E-04 1.18

2.0E-03 2.28 2.5E-03 2.34 2.0E-03 2.65 1.7E-03 2.54

3.1E-03 3.49 4.0E-03 3.62 3.2E-03 4.07 2.7E-03 3.88

4.3E-03 4.74 5.5E-03 4.84 4.4E-03 5.46 3.7E-03 5.20

5.4E-03 5.95 6.9E-03 6.05 5.5E-03 6.84 4.8E-03 6.48

6.5E-03 7.16 8.3E-03 7.25 6.6E-03 8.17 5.7E-03 7.68

7.6E-03 8.32 9.6E-03 8.42 7.7E-03 9.47 6.7E-03 8.91

8.6E-03 9.46 1.1E-02 9.55 8.8E-03 10.76 7.6E-03 10.06

9.7E-03 10.64 1.2E-02 10.68 9.9E-03 12.10 8.6E-03 11.27

1.1E-02 11.86 1.3E-02 11.79 1.1E-02 13.40 9.6E-03 12.48

1.2E-02 13.00 1.5E-02 12.87 1.2E-02 14.73 1.0E-02 13.53

70˚C

1.8E-04 0.24 2.2E-04 0.27 1.4E-04 0.25 1.2E-04 0.26

1.0E-03 1.10 1.1E-03 1.10 9.8E-04 1.27 8.7E-04 1.26

2.2E-03 2.30 2.5E-03 2.24 2.2E-03 2.67 2.0E-03 2.65

3.4E-03 3.53 4.0E-03 3.43 3.6E-03 4.09 3.3E-03 4.03

4.7E-03 4.73 5.5E-03 4.62 4.9E-03 5.48 4.5E-03 5.38

6.0E-03 5.97 6.9E-03 5.79 6.1E-03 6.82 5.7E-03 6.66

7.2E-03 7.17 8.4E-03 7.00 7.4E-03 8.13 6.8E-03 7.93

8.4E-03 8.30 9.9E-03 8.18 8.6E-03 9.40 8.0E-03 9.15

9.6E-03 9.51 1.1E-02 9.28 9.7E-03 10.65 9.1E-03 10.36

1.1E-02 10.64 1.3E-02 10.42 1.1E-02 11.99 1.0E-02 11.58

1.2E-02 11.90 1.4E-02 11.54 1.2E-02 13.33 1.1E-02 12.78

1.3E-02 12.99 1.5E-02 12.67 1.3E-02 14.59 1.2E-02 13.70

80

˚C

2.0E-04 0.25 2.1E-04 0.24 1.7E-04 0.26 1.5E-04 0.26

1.1E-03 1.11 1.1E-03 1.02 1.1E-03 1.27 9.7E-04 1.26

2.3E-03 2.31 2.3E-03 2.08 2.4E-03 2.66 2.2E-03 2.60

3.7E-03 3.54 3.7E-03 3.18 3.8E-03 4.07 3.6E-03 3.96

5.0E-03 4.76 5.1E-03 4.25 5.1E-03 5.40 4.9E-03 5.28

6.3E-03 5.94 6.4E-03 5.34 6.5E-03 6.72 6.1E-03 6.52

7.6E-03 7.12 7.8E-03 6.41 7.8E-03 8.01 7.4E-03 7.74

8.9E-03 8.30 9.2E-03 7.47 9.1E-03 9.28 8.6E-03 8.92

1.0E-02 9.48 1.1E-02 8.54 1.0E-02 10.62 9.9E-03 10.19

1.2E-02 10.64 1.2E-02 9.59 1.2E-02 11.89 1.1E-02 11.36

1.3E-02 11.80 1.3E-02 10.66 1.3E-02 13.22 1.2E-02 12.54

1.4E-02 13.00 1.5E-02 11.69 1.4E-02 14.50 1.3E-02 13.60

172 | P a g e

Appendix D. SMOF synthesis and water

adsorption isotherms

D1. Synthesis of SMOF

D2. Synthesis of 10% Ni-SMOF

2C4H4O4 + 2NaOH + Al2(SO4)3 2C4H3O5Al + Na2SO4 + 2SO3 + 2H2O Fumaric

acidSodium

hydroxideAluminium

sulphate

Aluminiumfumarate

Sodium sulphate

Reaction:

C4H3O5AlSeparation by Centrifugation

Drying Activation 65°C

Solution of Al2(SO4)3

Solution of C4H4O4

AluminiumfumarateStirring SEM image

Ni-doped Aluminium Fumarate

C4H3O5AlxNiySeparation by Centrifugation

Drying Activation65°C

Solution of

Al2(SO4)3 NiCl2

(8:2 wt%)

Solution of C4H4O4

Stirring

APPENDIX D

173 | P a g e

D3. Photographs of synthesized SMOFs

Fig. D1. Visual images of synthesized SMOFs. (a)SMOF, (b) 10% Ni-SMOF, (c) 10% Co-

SMOF (d) zoomed images of white-colored SMOF, (e) zoomed image of blue-colored 10%

Ni-SMOF, (f) zoomed image of violet-colored 10% Co-SMOF.

D4. Water adsorption/desorption isotherms of SMOFs

Water adsorption/desorption isotherms are measured for various SMOFs at 30˚C and 60˚C.

However, in the thesis work, very limited data is presented to focus on the targeted analysis.

For future reference, all the measured data is included in this section.

(a) (b) (c)

(d) (e) (f)

APPENDIX D

174 | P a g e

Fig. D2. Water adsorption/desorption isotherm of SMOF.

Fig. D3. Water adsorption/desorption isotherm of 5% Co-SMOF

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.2 0.4 0.6 0.8 1

Up

tak

e [k

g k

g-1

]


SMOF006 30C adsorption

SMOF006 30C desorption

SMOF006 60C adsorption

SMOF006 30C desorption

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1

Up

tak

e [k

g k

g-1

]


5% Co ALF 30 Deg Adsorption

5% Co ALF 30 deg Desorption

5% Co ALF 60 Deg Adsorption

5% Co ALF 60 Deg Desorption

APPENDIX D

175 | P a g e



0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1

Up

tak

e [k

g k

g-1

]


10 % Co-ALF 30 Deg Adsorption

10% Co ALF 30 Deg Desortion

10% Co ALF 60 DEG adsorption

10% Co ALF 60 DEG desorption

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1

Up

tak

e [k

g k

g-1

]


20% Co ALF 60 deg adsorption

20% Co ALF 60 Deg desorption

20% Co ALF 30 Deg adsorption

20% Co ALF 30 Deg desorption

APPENDIX D

176 | P a g e

Fig. D6. Water adsorption/desorption isotherm of 5% Ni-SMOF

Fig. D7. Water adsorption/desorption isotherm of 10% Ni-SMOF

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.2 0.4 0.6 0.8 1

Up

tak

e [k

g k

g-1

]


5% Ni ALF 30 Deg adsorption

5% Ni 30 deg desorption

5% Ni ALF 60 Deg adsorption

5% Ni ALF 60 Deg Desorption

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.2 0.4 0.6 0.8 1

Up

tak

e [k

g k

g-1

]


10% Ni ALF 30 DEG adsorption

10% Ni ALF 30 DEG desorption

10% Ni ALF 60 DEG adsorption

10 % Ni ALF 60 DEG desorption

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