PLASMONIC PHOTOCATALYSTS OF

SUPPORTED GOLD NANOPARTICLES FOR

ORGANIC CONVERSIONS

Xingguang ZHANG

[Bachelor of Chemical Engineering and Technology, Guizhou University;

Master of Industrial Catalysis, Nanjing University of Technology]

Thesis completed under the supervision of Dr. Xuebin Ke and Prof. Huaiyong Zhu,

and submitted to Queensland University of Technology, in fulfilment of the

requirements of the degree of

Doctor of Philosophy

School of Chemistry, Physics and Mechanical Engineering

Faculty of Science and Engineering

Queensland University of Technology

February, 2014

i

Keywords

Photocatalysis

Visible light

Gold nanoparticle

Plasmonic photocatalysts

Localised surface plasmon resonance

Electric near-field enhancement

Zeolite

Polarised electrostatic fields

Photooxidation

Selective oxidation

Photoreduction

Selective reduction

Acetalisation

Transesterification

Molecular polarity

Alcohol

Aldehyde

Ketone

Epoxide

ii

Abstract

In photocatalysis, one of the greatest challenges is to design novel photocatalysts

that can efficiently harvest ―green‖ sunlight to drive or enhance chemical reactions.

Compared with traditional TiO2 semiconductors that can only utilise UV light (< 4%

of sunlight), plasmonic photocatalysts of supported gold nanoparticles (Au-NPs)

possess distinct advantages. Au-NPs can absorb visible light (about 43% of sunlight)

owing to the localised surface plasmon resonance (LSPR) effect that amplifies the

electromagnetic field of incident light (|E0|), leading to an intensified electromagnetic

field (|E|). This near-field enhancement phenomenon is well-known as the ―electric

near-field enhancement‖ effect. The two distinct features play an important role in

determining the catalytic performance of plasmonic photocatalysts.

In this thesis, plasmonic photocatalysts of supported Au-NPs were investigated

for several important organic transformations under visible light irradiation at

ambient temperature. The selective reduction of nitroaromatics to azo compounds,

azobenzene to hydroazobenzene, ketones to alcohols, and epoxides to alkenes, the

selective oxidation of aromatic alcohols to aldehydes, and the acetalisation of

aldehydes/ketones to acetals/ketals were studied with the aim to explore the catalytic

performance of new plasmonic photocatalysts.

Firstly, the selective reduction of the aforementioned four types of reduction

reactions proceeded effectively on plasmonic photocatalysts of supported Au-NPs on

Al2O3, CeO2, TiO2, ZrO2, or zeolite Y, in which Au-NPs provided catalytic centres.

The reactive efficiency of the photocatalysts primarily depended on two factors: the

light absorption efficiency and the reduction ability to activate reactants. The light

absorption ability by Au-NPs determined the reduction ability of the photocatalysts;

iii

specifically, light with shorter wavelengths could excite the free conduction electrons

in Au-NPs to higher energy levels, thus inducing reduction with more negative

reduction potentials. Moreover, the photoexcited Au-NPs could grab hydrogen from

the solvent, isopropanol, to form Au-H species on the gold surfaces. These active

Au-H species then reacted with the N=O, N=N, C=O double bonds or epoxide bonds

of reactants to weaken them by the energetic electrons of Au-NPs to produce the

final reductive products. The reduction ability of the Au-H species depended on the

energy of the energetic electrons in Au-NPs: the higher the energy of electrons was,

the stronger the reduction ability of Au-H species was, indicating that the reduction

ability of the photocatalysts could be tuned by controlling the light wavelength.

Secondly, the selective oxidation of aromatic alcohols to aldehydes proceeded

effectively on plasmonic photocatalysts of Au/zeolites with high selectivity (99%), in

which Au-NPs provided catalytic centres. The kinetic study indicated that the

photooxidation under visible light irradiation required much less activation energy to

initiate the reactions compared with that of the thermal-driven reaction. The

molecular polarities of aromatic alcohols were demonstrated to correlate with

photocatalytic performance, because of the different strengths of interaction with

electronically polarised Au-NPs. Moreover, the adsorption capability of zeolite

supports facilitated the concentration of reactants from the solvent.

Thirdly, the acetalisation of aldehydes/ketones to give acetals/ketals exhibited

enhanced catalytic performance on plasmonic photocatalysts of Au/zeolites, in which

zeolites provided catalytic centres. Zeolites possess strong Brønsted acid sites and

polarised electrostatic fields (PEF) created by extra-framework cations. The PEF

could be further intensified by the electric near-field enhancement of Au-NPs. The

acetalisation reaction was chosen as the model reaction performed on Au/MZSM-5

iv

and MZSM-5 (M=H+, Na

+, Ca

2+, or La

3+). Density functional theory (DFT)

calculations confirmed that the intensified PEF played a critical role in stretching the

C=O bonds of the reactants of benzaldehyde and cyclohexanone, to enlarge their

molecular polarities. The reactants with a larger C=O bond polarity could be

activated more easily by active sites of H+, thus boosting the reaction rate.

Overall, this research program illustrates a new way of harvesting visible light

using plasmonic photocatalysts of supported Au-NPs. These findings may evoke

intensive research interests on plasmonic metal nanostructures and diverse catalytic

supports (e.g., zeolites) to take advantage of ―green‖ sunlight for plasmonic devices,

molecular electronics, energy storage, and catalysis.

v

List of publications

Journal publications:

1. Xingguang Zhang, Xuebin Ke*, and Huaiyong Zhu. Zeolite-supported gold

nanoparticles for selective photooxidation of aromatic alcohols under visible-

light irradiation. Chem. Eur. J. 2012, 18, 8048-8056. (Highlighted paper)

2. Xingguang Zhang, Xuebin Ke*, Zhanfeng Zheng, Hongwei Liu, and Huaiyong

Zhu*. TiO2 nanofibers of different crystal phases for transesterification of

alcohols with dimethyl carbonate. Appl. Catal. B 2014, 150-151, 330-337.

3. Xingguang Zhang, Xuebin Ke*, Aijun Du, and Huaiyong Zhu. Plasmonic

nanostructures to enhance catalytic performance of zeolites under visible light.

Sci. Rep. 2014, 4(3805), 1-6.

4. Xuebin Ke*, Xingguang Zhang, Jian Zhao, Sarina Sarina, John Barry, and

Huaiyong Zhu*. Selective reductions using visible light photocatalysts of

supported gold nanoparticles. Green Chem. 2013, 15, 236-244.

5. Xuebin Ke*, Xingguang Zhang, Hongwei Liu, Song Xue, and Huaiyong Zhu*.

Efficient catalysts of zeolite nanocrystals grown with a preferred orientation on

nanofibres. Chem. Commun. 2013, 2013, 49, 9866-9868.

6. Xuebin Ke, Sarina Sarina, Jian Zhao, Xingguang Zhang, Jin Chang, and

Huaiyong Zhu*. Tuning the reduction power of supported gold nanoparticle

photocatalysts for selective reductions by manipulating the wavelength of visible

light irradiation. Chem. Commun. 2012, 48, 3509-3511.

vi

Conference presentation:

1. Xingguang Zhang, Xuebin Ke*, and Huaiyong Zhu. Photocatalysts of zeolite

supported gold nanoparticles for selective oxidation of aromatic alcohols to

aldehydes. The 6th

International Conference on Gold Science, Technology, and

its Applications, 2012, Tokyo, Japan

2. Xuebin Ke, Xingguang Zhang, Sarina Sarina, and Huaiyong Zhu*.

Photocatalytic reductions activated by supported gold nanoparticles. The 6th

International Conference on Gold Science, Technology, and its Applications,

2012, Tokyo, Japan

3. Sarina Sarina, Xuebin Ke, Jian Zhao, Xingguang Zhang, and Huaiyong Zhu*.

Contribution of light irradiation to reduction of activation energy in gold

nanoparticle photocatalysed reactions. The 6th

International Conference on Gold

Science, Technology, and its Applications, 2012, Tokyo, Japan

vii

Table of contents

Keywords……………………………………………………………………….……..i

Abstract…………………………………………………………………………….....ii

List of publications…………………………………………………………………...v

Table of contents…………………………………………………………………….vii

List of figures……….…………………………………………………..……………ix

List of abbreviations..…………………………………………………..……………..I

Statement of original authorship……………………………………………………..II

Acknowledgements……………………………………………………………...…..III

Chapter 1: Introduction……………………………………………………….……1

Chapter 2: Literature review……………………………………………….………9

2.1 Plasmonic gold photocatalysts…………...……………………………………….9

2.1.1 Localised surface plasmon resonance……………………………………...…9

2.1.2 Electric near-field enhancement………………………………………….....11

2.1.3 Applications of plasmonic gold photocatalysts………………...…………..12

2.2 Zeolite catalysts……………………………………………………………..…..17

2.2.1 Zeolite solid-acid catalysts………………………………………...…………..17

2.2.2 Zeolite-based photocatalysts……………...……………………………….…..18

2.2.3 Polarised electrostatic fields…………………………………………….…..20

2.2.4 Applications of zeolite catalysts…………………………………………..…..21

2.3 Summary…………………….…………………………………………………..25

Chapter 3 Selective reduction reaction………………………………………...…35

Introductory remarks……………………………………..………………….………35

Article 1: Selective reductions using visible light photocatalysts of supported gold

viii

nanoparticles.....….…………………………………...…..………………….………39

Chapter 4 Selective oxidation reaction…………………………………………...51

Introductory remarks……………………………………..………………….………51

Article 2: Zeolite-supported gold nanoparticles for selective photooxidation of aromatic

alcohols under visible-light irradiation………………...……………...……….………55

Chapter 5 Acetalisation reaction………………………………………………..64

5.1 Acetalisation reaction on Au/zeolite photocatalysts………………………..64

Introductory remarks……………………………………..………………….………64

Article 3: Plasmonic nanostructures to enhance catalytic performance of zeolites under

visible light………….………………………...…………..………………….………69

5.2 Acetalisation reaction on zeolite catalysts…………….………………..……..85

Introductory remarks……………………………………..………………….………85

Article 4: Efficient catalysts of zeolite nanocrystals grown with a preferred orientation on

nanofibres...………………….....…………………………….………………………90

Chapter 6 Conclusion…………………………………………………………...…96

Chapter 7 Limitations and future work…...………………………………...…99

7.1 Limitations and possible solutions…………………………………………....99

7.2 Future work……………………………………………………….............……101

Appendix

Introductory remarks……………….…………………….………………….……107

Article 5: TiO2 nanofibres of different crystal phases for transesterification of alcohols with

dimethyl carbonate …....……………….……………………….…………….…….112

-

ix

List of figures (in Chapter 2: Literature review)

Figures Page

Figure 1. The scheme of the electromagnetic field of light and its

interaction with Au-NPs: A dipole is induced, which oscillates in phase

with the electromagnetic field of the incoming light.

9

Figure 2: The LSPR bands of (a) Au, Ag, and Cu nanoparticles; (b) Ag

nanostructures with different shapes; (c) Ag nanocubes with different sizes;

and (d) Au-Ag alloy with different compositions.

10

Figure 3 The FDTD (finite-difference time-domain) simulation results of

(a) distribution of the enhanced electric field intensity at the LSPR peak

wavelength (420 nm) around a 75 nm Ag nanocube; (b) quantified electric

field intensity as a function of distance, d, along the dashed line indicated

in (a); (c) distribution of the electric field intensity between two 75 nm Ag

nanocubes separated 1 nm away (one cube is rotated 45°); (d) quantified

value of electric field intensity as a function of dash-indicated d in (c).

11

Figure 4: The proposed mechanism of the photoelectrochemical process

on Au/TiO2. Charges are separated at a visible-light-irradiated gold

nanoparticle-TiO2 system.

13

Figure 5. The schematic mechanism of the photocatalytic reduction of

nitroaromatic compounds. Au-H reacts with N-O bonds to form Au-OH

species that subsequently decompose to release dioxygen molecules and

give Au-H species.

16

Figure 6. The mechanism of the acetalisation of cyclohexanone catalysed

by solid acid catalysts (H+).

23

Figure 7: Visible light induces charge transfer between ethane and oxygen

molecules to form a transient ethane radical and a superoxide species (O2-).

24

I

List of abbreviations

NPs: Nanoparticles

NFs: Nanofibres

LSPR: Localised surface plasmon resonance

PEF: Polarised electrostatic fields

ENFE: Electric near-field enhancement

DFT: Density functional theory

VOCs: Volatile organic compounds

TON: Turnover number

TOF: Turnover frequency

XRD: X-ray diffraction

BET: Brunauer-Emmett-Teller

GC: Gas chromatography

MS: Mass spectrometry

IES: FT-IR emission spectroscopy

ATR: Attenuated total reflectance

NMR: Nuclear magnetic resonance spectroscopy

SEM: Scanning electron microscopy

EDS: Energy dispersive X-ray spectroscopy

TEM: Transmission electron microscopy

TG: Thermogravimetric analysis

UV/Vis: Ultraviolet-visible spectra

XPS: X-ray photoelectron spectroscopy

QUT Verified Signature

III

Acknowledgements

I would love to express sincere gratitude and great appreciation to my

supervisors, Dr. Xubin Ke and Prof. Huaiyong Zhu, for their meticulous guidance,

generous support and great patience towards the completion of this work.

Grateful acknowledgements must go to Dr. Zhanfeng Zheng, Dr. Jian Zhao,

Dr. Sarina Sarina, Mr. Qi Xiao, Miss Fathima Sifani Zavahir, Dr. Hongwei Liu, Prof.

Aijun Du, Prof. Xianliang Sheng, Prof. Jianfeng Jia, Prof. Eric Waclawik, Prof.

Jingyi Li and Dr. Juan Yang, for their kind collaboration, advice and valuable

comments and suggestions, particularly for the method of conducting the research.

Sincere thanks also go to the students: Arixin Bo, Yiming Huang, Chao Chen, who

always give me hands in conducting the laboratory work.

My sincere appreciations also extend to Dr. Wayde Martens, Dr. Llew Rintoul,

Dr. Chris Carvalho, Mr. Eric Martinez, Mrs. Leonora Newby, Dr. Peter Hines, Dr.

Thor Böstrom, Dr. Jamie Riches, Dr. Hui Diao, Dr. Tony Raftery, and other

technicians who have provided assistance with instrumentation. Special thanks go to

Dr. Barry Wood (UQ) for the help with the sample characterisation by XPS spectra.

I wish to thank QUT for offering me the full scholarship. Thanks also go to the

Queensland State Government for the Smart Futures Fellowship and the Australian

Research Council (ARC) for the funding of research.

Finally, I would love to thank my family - my parents and my wife, for their

love, support, encouragement and understanding throughout my work.

1

Chapter 1: Introduction

Catalysis plays an indispensable part in materials science, chemical production,

energy storage, and environmental remediation. Conventional heterogeneous

catalysis heavily relies on high operating temperatures because of large activation

energy barriers and thus suffers from numerous negative side effects: low energy

efficiencies for inherent exothermic reactions, quick deactivation of catalytic centres,

poor selectivity for most partial redox reactions, and dangerous operating

conditions[1-3]

.

Photocatalysis offers great promise in its ability to lower reaction temperature

to the ambient degree thus opens new avenues in developing energy-efficient

catalysts to harvest sunlight - the ―green‖ and abundant solar energy. Photocatalysis

is also a promising approach to fulfil the goals of green chemistry which aims at

designing chemical products or processes to reduce or eliminate the use and

generation of hazardous substances. Solar energy is a clean abundant energy source,

in particular, the energy of sunlight incident on the earth is about 10, 000 times more

than the current energy consumption in the world[4]

. However, the light absorption

efficiency of heterogeneous photocatalysts has long been restricting the

photocatalytic capability for commercial availability. For instance, traditional

photocatalysts almost exclusively focus on TiO2-based semiconductors that can acess

only UV light (< 4% of sunlight)[5]

and thus cannot take advantage of visible light

(400-800 nm in wavelength). Visible light constitutes approximately 43% of solar

energy[6]

; therefore, photocatalysts that can harness visible light are highly desirable.

Recent years have seen development of a new family of plasmonic photocatalysts

of supported gold, silver, or copper nanoparticles, a feature of which is the effect of

2

collective and coherent oscillation between the metal’s free conduction electrons and

the incident photons at the surface of NPs due to the localised surface plasmon

resonance (LSPR) irradiated by visible light[7-9]

. This LSPR effect has been applied

to boost catalytic performance for a variety of important organic transformations

under visible light irradiation, particularly, for the oxidation of organic contaminants

and aromatic alcohols[10]

. However, the neglected photoreduction is rarely reported

by researchers. Given that reduction is one of the major processes for the synthesis of

fine chemicals, it will be of great interest if photocatalytic reduction reactions can be

applied to synthesise fine chemicals under visible light irradiation. Currently, reports

on the topic of photocatalytic reduction are sparse, focusing on the reduction of

nitric-aromatics using cadmium sulphide or modified titania as photocatalysts[11]

.

The core issue regarding photocatalytic reduction is to devise efficient photocatalysts

that can catalyse the reduction reactions with superior product selectivity. Hopefully,

plasmonic photocatalysts of supported Au-NPs can catalyse reduction reactions;

therefore, photocatalytic reactions that are driven by sunlight can be developed.

Moreover, supports play a critical role in modifying the catalytic performance of

supported Au-NPs. For instance, zeolites are crystalline aluminosilicates with rigid

microporous structures and well-defined channels at molecular dimensions. The

contribution of zeolites to the catalytic performance of photocatalysts of supported

Au-NPs has not been clarified, even though the mechanism investigation into their

functions for the aromatic alcohol oxidation in thermal reactions has been

examined[12]

. Li and co-workers report that zeolites supported Au and Au-Pd NPs

can catalyse the oxidation of aromatic alcohols[13]

. The reactions are carried out at

100oC and two-bar O2 pressure, but the conversions are very low and side reactions

are observed. Therefore, under visible light irradiation, zeolites supported Au-NPs

3

probably function more efficiently, because the adsorption ability of zeolites supports

can help concentrate reactants from the solvent.

The plasmon-mediated processes fundamentally differ in their catalytic

mechanisms compared with those occuring on traditional semiconductors or with

processes driven merely by thermal energy. Moreover, the LSPR effect of plasmonic

NPs can amplify the electromagnetic field intensity of incident light (|E0|), resulting

in an enhanced electric field (|E|) on the nanometre scale[7, 9]

. This near-field

enhanced interaction is well-known as the ―electric near-field enhancement (ENFE)‖

effect and has motivated numerous theoretical and experimental studies, and

advanced technological applications, such as surface-enhanced Raman scattering[14]

,

sensors[15, 16]

, plasmonic devices[17, 18]

and photovoltaic solar cells[19, 20]

. In

photocatalysis, Ag-NPs covered by the SiO2 shell and embedded in TiO2 particles

have been tried to degrade methyl blue, and better catalytic performance was

observed as a result of the electric near-field amplitude on the surface of Ag-NPs[21]

.

However, the inert SiO2 shell, which is used to prevent the Ag core from being

oxidised, impedes the light absorption efficiency in photocatalysis.

Zeolites are important catalysts themselves and have extensive applications in

catalysis, separation, and adsorption. Besides possessing regular microporous

structures, high surface areas, shape-selectivity, zeolites possess unique solid acidity.

Moreover, the strong polarised electrostatic fields (PEF, about 1-10 V/nm) created by

extra-framework cations have the power to polarise molecules adsorbed on zeolite

surfaces or confined in the porous matrix (e.g., host-guest structures)[22-24]

. In

principle, the PEF can reduce the energy required to facilitate the electron transfer or

to activate reactants, thereby initiating reactions by moderate heating or visible-light

excitation. The charge-transfer properties from hydrocarbons to molecular oxygen

4

(O2) have been investigated on cation-treated zeolites, such as ZSM-5, Y and beta,

with visible light irradiation in the selective oxidisation of toluene, propane,

cyclohexane, and small alkenes[25-27]

. These studies confirm that the PEF of extra-

framework cations can lower the charge-transfer excitation energy required from

hydrocarbons to O2. However, the products cannot be quantitatively measured, which

results from the low visible light absorption efficiency of zeolites. This triggers a

logical speculation that improved catalytic performance can be expected if the PEF

of cations in zeolites are further intensified to a sufficiently large extent.

In this thesis, three promising findings of plasmonic photocatalysts are reported:

firstly, enhanced catalytic performance of supported Au-NPs for the selective

reduction of four types of reduction reactions, in which Au-NPs provide the catalytic

centres and supports make no contribution to catalytic performance (Article 1:

Selective reductions using visible light photocatalysts of supported gold

nanoparticles); secondly, improved catalytic performance of supported Au-NPs for

the selective oxidation of aromatic alcohols to aldehydes, in which Au-NPs provide

the catalytic centres and the zeolite supports contribute to adsorbing reactants

(Article 2: Zeolite-supported gold nanoparticles for selective photooxidation of

aromatic alcohols under visible-light irradiation); thirdly, boosted catalytic

performance of zeolites for acetalisation of aldehydes/ketones by plasmonic effect of

Au-NPs, in which zeolites provide catalytic centres (Article 3: Plasmonic

nanostructures to enhance catalytic performance of zeolites under visible light). In

addition, the acetalisation is also investigated on zeolite-grafted TiO2 nanofibres

under thermal conditions (Article 4: Efficient catalysts of zeolite nanocrystals grown

with a preferred orientation on nanofibres), showing that the transformation between

aromatic alcohols and dimethyl carbonate can occur on pure TiO2 nanofibres without

5

zeolite nanocrystals. Therefore, the study of ―transesterification of alcohols on TiO2

nanofibres with different crystal phases‖ is also performed as a periphery study and

attached in the ―Appendix‖ section (Article 5: Selective catalysis of alcohols with

dimethyl carbonate using TiO2 nanofibres of different crystal phases).

In Chapter 2 (Literature Review), the theoretical background, the recent

application of plasmonic gold photocatalysts and zeolite catalysts in organic

conversions, and the future development of these fields are summarised to provide an

outline of the scientific novelty and significance of this research program.

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7

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8

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9

Chapter 2: Literature review

2.1 Plasmonic gold photocatalysts

2.1.1 Localised surface plasmon resonance

Materials in the size of nanometre scale (nanomaterials) can exhibit very different

physicochemical properties from bulky ones. Plasmonic metal nanostructures

generally refer to nanomaterials of gold, silver, and copper nanoparticles, nanorods,

nanowires, and nanocubes, because they can strongly interact with the incident light

owing to the localised surface plasmon resonance (LSPR) effect[1-3]

. To illustrate, the

electromagnetic field of incident light can cause the free electrons in gold

nanoparticles (Au-NPs) to oscillate. As the front wave of visible light passes, the

electron density in the Au-NPs are polarised to one side of the surfaces, and the free

conduction electrons collectively and coherently oscillate in resonance with the

light’s frequency, causing a dynamic dipole oscillator, as shown in Figure 1.

Figure 1. The scheme of the electromagnetic field of visible light and its interaction with Au-

NPs: A dipole oscillator is induced, which resonates in phase with the electromagnetic field

of the incoming visible light.

Visible light usually undergoes intraband LSPR absorption (electrons in 6sp

band), which is different from interband absorption irradiated by ultraviolet light

(electrons oscillate from 5d to 6sp)[4]

. These energetic electrons may return to their

Electric field

Magnetic field

Visible light

Electron clouds

Au(δ+-δ-)

dipole oscillator -

E

-

- - -

+ + +

+ + +

- -

+

10

thermal equilibrium states and release heat to the lattice and the surrounding media,

or can be grabbed by reactants to reduce them to initiate chemical reactions.

If the size, shape, and surrounding materials of the Au-NPs change, then the

electric field density of the surface of Au-NPs changes, leading to a change in the

oscillation frequency of electrons and in the LSPR peak positions. For instance, the

free conduction electrons in Au-NPs can readily travel through the material, where

the mean free path is about 50 nm. If the size of Au-NPs prepared is smaller than this

dimension, then no scattering occurs from the gold bulk, and all interactions are on

the gold surfaces. In general, if the size of Au-NPs produced by chemical reduction

of gold salts are smaller than half the wavelength of the incident light, then they can

exhibit the LSPR effect[3]

. Moreover, gold, silver, and copper NPs and their alloys

show different LSPR peak positions, as given in Figure 2. All these properties afford

a variety of adjustable photocatalytic performance of plasmonic nanostructures.

Figure 2: LSPR bands of (a) Au, Ag, and Cu nanoparticles; (b) Ag nanostructures with

different shapes; (c) Ag nanocubes with different sizes; and (d) Au-Ag alloy with different

compositions. Sets (a)-(c) are reproduced from Ref 2, and the set (d) is from Ref 5.

11

2.1.2 Electric near-field enhancement

An important characteristic of the LSPR effect of Au-NPs is that it can amplify

the electromagnetic field intensity of incident light (|E0|) and results in an enhanced

electric field (|E|) on the nanometre scale. This LSPR-induced near-field interaction

is called ―electric near-field enhancement‖ (ENFE). This enhancement effect is

affected by the spatially non-homogeneous distribution of plasmons on NPs and

attenuates exponentially within the spacing range of 10 - 50 nanometres away from

the metal surfaces, relying on the size, shape, and dielectric environments[2]

. The

near-field enhancement factor (|E|/|E0|) is as large as 103 at the surface of individual

NPs, whereas it can reach 106 at the junction of interparticles, as shown in Figure 3

with Ag-NPs being examples[2, 6]

.

Figure 3 FDTD (finite-difference time-domain) simulation results of (a) distribution of the

enhanced electric field intensity at the LSPR peak wavelength (420 nm) around a 75 nm Ag

nanocube; (b) quantified electric field intensity as a function of distance, d, along the dashed

line indicated in (a); (c) distribution of the electric field intensity between two 75 nm Ag

nanocubes separated 1 nm away (one cube is rotated 45°); (d) quantified value of electric

field intensity as a function of dash-indicated d in (c). Reproduced from Ref 2.

12

The nano-gap regions where the maximum ENFE occurs are referred to as ―hot

spots‖, playing a critical role in modifying the physiochemical, optical, and reactive

properties of supports. Thus, the enhancement effect has been successfully applied in

surface-enhanced Raman scattering (SERS), sensitive photodetection[7]

, heat-assisted

magnetic recording[8]

, and photovoltaic solar cells[9]

. In photocatalysis, Awazu and

co-workers embed Ag-NPs in TiO2 particles to degrade the methyl blue, showing

that the electric near-field amplitude on the surface of Ag-NPs affords better catalytic

performance[10]

.

2.1.3 Applications of plasmonic gold photocatalysts

The unique capacity of plasmonic nanostructures to activate molecules by photo-

excited electron clouds, by electric near-field enhancement, by scattering

electromagnetic radiation, or by converting the energy of photons into heat, makes

them suitable for various applications. The applications include fluorescence[11]

,

enhanced Rayleigh (Mie) scattering[12]

, SERS[13]

, water splitting[14]

, degradation of

dye pollutants[15]

, and organic photovoltaics[9]

. Particularly, they can be applied in

the organic conversions that play a critical role in the chemical industry to produce

important precursors/intermediates (e.g., carbonyl compounds of aldehydes/ketones

oxidised from alcohols) for the synthesis of drugs, vitamins, and fragrances. Here the

main focus of the applications of plasmonic gold photocatalysts in this section is the

redox reactions either driven, or enhanced by visible light on plasmonic

photocatalysts of supported Au-NPs.

(1) Selective oxidation reactions

Many studies have been devoted to the mild oxidation ability of supported Au-

NPs, such as thiol to disulfide[16]

, methanol to formic acid[17]

, ethanol to acetic

acid[17]

, isopropanol to acetone[18]

, benzyl alcohol to benzaldehyde[19]

and benzene to

13

phenol[20]

. In contrast to only thermal-energy-driven oxidation with unselective

processes, high selectivity can be obtained under ambient conditions in

photocatalysis.

Among these oxidation reactions, the selective aerobic oxidation of alcohols to

carbonyl compounds with O2 has received attention from the perspective of green

chemistry, particularly on the plasmonic photocatalysts. An early study on the

oxidation of alcohols was Tian and co-workers’ report[17]

, which investigated the

oxidation of methanol and ethanol on Au/TiO2 photocatalysts under visible light

irradiation. The authors stated that the Au-NPs were photoexcited to form the

energetic electrons that were transferred to the TiO2 conduction band. Then

compensating electrons captured from a donor (e.g., Fe2+

Fe3+

) in the solution

transferred to Au-NPs. They also predicted that this system could be potentially

applied in photovoltaic fuel cells. The general mechanism is illustrated in Figure 4.

Figure 4: The proposed mechanism of the photoelectrochemical process on Au/TiO2.

Charges are separated at a visible-light-irradiated gold nanoparticle-TiO2 system.

Reproduced from Ref 17.

On the basis of this system, Naya and co-workers designed heterosupramolecular

photocatalysts consisting of Au/TiO2 and cationic surfactant admicelles. Such

photocatalysts exhibited high activity for the chemoselective oxidation of cinnamyl

alcohol to cinnamaldehyde under visible light irradiation. The authors also

14

demonstrated that the LSPR effect triggered the electron transfer from Au-NPs to

TiO2[16]

. In addition, a recent study on the oxidation of aromatic alcohols on Au-NPs

loaded on the interface of anatase/rutile particles also derived a similar mechanism

whereby Au-NPs located at the anatase/rutile interface facilitated an efficient

electron transfer to TiO2 under visible light irradiation[19]

. All the above-mentioned

studies indicate that TiO2 supports play an essential role in the oxidation of alcohols

under visible light irradiation because of the charge separation and electron transfer

processes, which involve the participation of the electron-hole pairs in TiO2.

However, the aforementioned studies ignore that the Au-NPs themselves are also

efficient photocatalytic materials. For instance, Chen and co-workers’s study

demonstrated that the oxidation of volatile organic contaminants (VOCs), including

CO, CH3OH and HCHO, proceeded effectively on the photocatalysts of Au/ZrO2,

Au/SiO2, and Au/Y zeolite[4, 21]

without using TiO2 photocatalysts. ZrO2 and SiO2

powders were chosen as supports because their band gaps (ca. 5.0 eV and ca. 9.0 eV,

respectively[22]

) are much larger than the energies of photons of visible light (less

than 3.0 eV). It is impossible for the Au-NPs to narrow the band gaps of them

enough for visible light photons to be absorbed or to excite electrons in them.

Therefore, the catalytic activity of these photocatalysts is not caused by the same

mechanism which occurs on semiconductor (TiO2) photocatalysts, but by the LSPR

effect of Au-NPs instead. This phenomenon also applies to other photocatalytic

systems. For instance, plasmonic photocatalysts Au/CeO2 were found to be efficient

for the selective oxidation of aromatic alcohols to aldehydes/ketones in aqueous

suspensions under the irradiation of LED green light[23]

. Au/CeO2 photocatalysts

exhibited high chemoselectivity both in the selective oxidation of aminobenzyl

alcohol (benzyl alcohol with an amino group) to aminobenzaldehyde and in the

15

intermolecular selective oxidation of benzyl alcohol to benzaldehyde in the presence

of aniline. This discovery also confirms that the photocatalytic performance of

Au/CeO2 originates from the LSPR effect of Au-NPs.

(2) Selective reduction reactions

Compared with the selective oxidation processes, the selective reduction reactions

are rarely seen in reports, probably because of the strong ability of Au-NPs to adsorb

and activate O2, thus necessitating the removal of O2 from the reaction system[24]

.

Nevertheless, the unique photocatalytic performance of Au-NPs can always motivate

scientists to explore more applications of plasmonic gold photocatalysts. Zhu and co-

workers found that Au/ZrO2 (3wt%) exhibited high performance in reducing

nitrobenzenes to azobenzene compounds. The overall photocatalytic reaction process

is described as the following[25]

:

8 Ph-NO2 + 2 CH3-CH(OH)-CH3 + Au-NPs

4 Ph-N=N-Ph + 2 CH3-C(=O)-CH3 +2 Au-H + 2 Au-OH +7 O2

As shown in Figure 5, photoexcited Au-NPs can grab hydrogen from the solvent,

isopropyl alcohol, to form Au-H species, in which KOH can assist the abstraction of

the hydrogen atom from isopropyl alcohol. Next, the key step is the cleavage of the

N-O bonds. Under visible light irradiation, the energetic electrons on Au-NPs

induced by the LSPR effect interact strongly with the electrophilic nitro groups of

nitrobenzene, thereby facilitating the breakage of the N-O bonds by Au-H species.

The resulting Au-H species subsequently decompose to give O2 and Au-H species.

Azobenzene is formed as the intermediate in the thermal hydrogenation of

nitrobenzene. This study demonstrated that it is possible to directly transform an

unstable intermediate in a thermal reaction to the wanted product by a photocatalytic

16

process and highlights that the plasmonic Au-NPs can complete all these processes

without the participation of catalyst supports.

Figure 5. The schematic mechanism of the photocatalytic reduction of nitroaromatic

compounds. Au-H species react with N-O bonds to form Au-OH species that subsequently

decompose to release dioxygen molecules and give Au-H species. Reproduced from Ref 25.

On the other hand, Au-NPs supported on TiO2 perform differently. Tanaka and

co-workers report that Au/TiO2 can photocatalytically reduce nitrobenzene to aniline

in the isopropanol suspension under visible light irradiation. The whole process is as

the following[26]

:

Ph-NO2 + 3 CH3-CH(OH)-CH3 Ph-NH2 + 3 CH3-C(=O)-CH3 +2 H2O

They found that the photocatalytic performance (reaction rate and stoichiometry)

was considerably enhanced by functionalisation of Au/TiO2 with Ag-NPs as a co-

catalyst. Ag-free Au/TiO2 sample showed a much lower reaction rate of nitrobenzene

consumption and aniline formation, compared with the Au/TiO2-Ag sample, and this

was probably because the energetic electrons tended to accumulate in the TiO2

particles because of the relatively large rate of hole scavenging by isopropanol. This

process affected the formation of Au-H species that were critical intermediates to

interact with the N-O bonds of nitrobenzene[25]

.

17

All these studies demonstrate that the selective redox reactions can proceed

effectively on plasmonic photocatalysts. Therefore, it should be scientifically

innovative and practically significant to explore more applications of plasmonic

photocatalysts, especially combined with traditional catalysts, such as zeolite solid-

acid catalysts, in other organic conversions.

2.2 Zeolite catalysts

Zeolites are crystalline aluminosilicates with regular porous structures of

molecular dimensions, high surface areas, shape selectivity, and strong adsorptivity,

widely applied in catalysis, separation, adsorption, and ion-exchange. Here the main

focus will concentrate on the solid-acidic properties of zeolites as catalysts in the

acetalisation reactions and the photochemical properties of zeolites as photocatalysts

in the oxidation of hydrocarbons.

2.2.1 Zeolite solid-acid catalysts

Zeolites possess an abundance of Brønsted and Lewis acid sites. The Brønsted

acid sites play a critical role in numerous organic conversions, and the acidity is

influenced by several factors, including the density/concentration, strength,

distribution and location of acid sites[27]

.

The generation of Brønsted solid acidity is related to the terminal and bridging

silanols on the zeolite surfaces, which can be introduced by several approaches.

Firstly, ion-exchange of sodium-form zeolites by ammonium salts followed by

calcinations: NH4+ + Na

+-zeolite NH4

+-zeolite + Na

+ H

+-zeolite + Na

+ + NH3

(the direct ion-exchange of zeolites with mineral acids (e.g., HCl) is unfavoured,

because it damages the zeolite framework by dealumination or even leads to

complete collapse). Secondly, water molecules are adsorbed and electrically

18

polarised owing to the presence of polarised electrostatic fields of the extra-

framework cations, this process helps form bridging hydroxyl groups (Si-O(H)-Al) to

provide Brønsted acid sites (H+): M

n+ + n(T-O-T) + nH2O M(OH)n + n(T-O(H

+)-

T)[28]

. Terminal silanol Si(OH) and extra lattice Al(OH) hydroxyl groups also serve

as weaker Brønsted acid sites and have been investigated in several studies by the

solid-sate IR and 1H-NMR spectroscopy

[29, 30]. In solution, the Brønsted acid sites can

be detected by the colorimetric method that is based on the difference in the

electronic absorption properties of neutral and cationic forms of probe molecules,

such as retinol and retinyl Schiff base. The density of Brønsted acid sites in a given

zeolite host is related to the framework Al sites, and thus a full understanding of

zeolite acidity would involve a detailed analysis of the precise location of the

catalytic centres. For instance, whether they are on external surfaces or inside pores,

whether they are in large pores or small cavities, and whether they are evenly

distributed or concentrated. An increasing number of reports are emerging[31, 32]

, and

given the importance of Brønsted acidity of zeolites, its exploration remains a hot

and important field in the foreseeable future.

2.2.2 Zeolite-based photocatalysts

In recent years, interest has arisen in the employment of zeolites in

photochemistry for several reasons[33, 34]

:

(a) Full photochemical stability, large thermal and chemical inertness, and

transparency to UV/Vis radiations above 240 nm, allow a certain percent of the

excitation light to get into the opaque solid powders and reach the channels and

cages.

(b) Ability of the zeolite framework to actively participate in electron transfer

processes as an either electron acceptor or electron donor[35]

.

19

(c) The possibility of altering the chemical composition of the framework and

surface charges allows the introduction of active sites that make these molecular

sieves photoactive. For instance, one can synthesise zeolites to make them have

photocatalytic activity by incorporation of heteroatoms into the framework, such as

Ti rather than Si or Al.

(d) Property of ion-exchange to change non-framework positions. The net

negative charges on the aluminosilicate framework are balanced by counter ions like

Na+, K

+ or other alkali and alkaline earth metal ions. These counter ions introduced

in the pores play a very important role in introducing photocatalytic properties and

can be readily exchanged by noble or transitional metal ions, providing an

opportunity for modifying the photocatalytic property of zeolites.

(e) Ability of accommodating photoactive guests within the internal voids of

zeolite pores. For instance, semiconductor-incorporated zeolites[36]

, have gained great

importance as potential photocatalysts. Such assembly of multi-component host-

guest systems mainly constitute antenna and relays of natural photosynthetic centres,

including inorganic metal oxide clusters and organic electron-transfer

photosensitisers, since the guest becomes significantly stabilised by incorporation.

(f) High adsorptive ability of zeolites for organic compounds in solution, which

can concentrate reagents in the proximity of the photosensitiser. This strong

adsorptivity contributes to the success of the photocatalytic reactions[37]

. In some

cases, the photosensitiser absorbs light and reaches an excited state, and this state

offers the potential to transfer energy to substrates; hence, the substrates become

electronically excited even if the substrates have not directly absorbed light.

(g) Amphoteric properties which means the co-existence of acid and basic sites.

The three coordinated aluminium sites on the framework and non-framework

20

aluminium sites are normally considered to be Lewis acid sites. Additionally, charge

compensating cations present in the pores of zeolite act as Lewis acids, and the

framework oxygen represents a base. In particular, the oxygen atoms adjacent to Al

(Si–O–Al oxygen) are more basic because of a larger negative charge on the oxygen.

The Lewis acidity is connected to the electron-accepting property and the Lewis

basicity to the electron donating property[34]

.

(h) Modulation of the micropolarity and the polarising strength of zeolite interior

by varying the nature of internal charge balancing cations (charge density) and the

size of channels. The latter factor can lead to dramatic changes and modifications in

the electronic states and conformational mobility of guests within zeolites[38]

.

All these unique characteristics make zeolite-based materials attractive to

scientists in the field of photocatalysis. Particularly, the polarised electrostatic fields

in the proximity of extra-framework cations play a critical role in activating

molecules adsorbed on surfaces, or encapsulated within cavities. This could provide

an opportunity to work cooperatively with the plasmonic Au-NPs because of their

dynamic dipole oscillators induced by the LSPR effect.

2.2.3 Polarised electrostatic field of extra-framework cations in zeolites

Zeolite frameworks feature T-O-T bonds, of which T refers to Si, Al, or other

heteroatoms (no Al-O-Al sequences according to the Loewenstein rule[39]

), and the

T-O-T bonds constitute tetrahedra of [SiO4]4-

and [AlO4]5-

. Isomorphous substitution

of Si with Al creates negative charges on the oxygens of framework and requires

charge-balanced protons or cations of Na+ or K

+ that can be ion-exchanged by other

cations, such as Ag+, Ca

2+ and Ce

3+. Compared with the well-understood mechanism

of reactions catalysed by Brønsted acid sites, the function of extra-framework cations

remains a topic of debate, particularly regarding the polarised electrostatic fields in

21

the vicinity of them (in general, 1-10 V/nm[40]

). In principle, the polarised

electrostatic fields can polarise molecules confined in the microporous matrix (e.g.,

host-guest structures). This could activate adsorbed molecules to initiate reactions

under mild heating or visible light irradiation conditions. In some reports, extra

electrostatic fields have been found to affect the conformational change of flexible

molecules, thus leading to the change in the molecular polarity[41]

. In weak

electrostatic fields compared to the intramolecular fields on the order of volts per

angstrom (V/Å), atoms and molecules can be polarised to form an electrostatic field-

induced dipole moment and a gain in energy; hence, new pathways in chemical

reactions may be established. A study shows that the extra electrostatic field of 12-15

V/nm obviously helps activate the reactants of CO/O2 and enhances the oxidation

process on small gold crystals (20-30 nm). This effect is considered to be associated

with the formation of partially charged gold surfaces induced by the extra

electrostatic field[42]

.

Given the importance of molecular polarity in controlling molecular behaviour

and chemical reactions[43]

, if photocatalysts can modify the molecular polarities, for

instance, by providing a strong electrostatic field, they should offer great promise in

terms of manipulating their photocatalytic performance. Therefore, it is reasonable to

speculate that the electrostatic fields around dipole oscillators of Au-NPs, denoted as

Au(δ+-δ-)

, and the electric near-field enhancement may intensify the polarised

electrostatic fields of zeolites to afford a stronger electrostatic field that can directly

enhance the molecular polarities to improve their reaction rate.

2.2.4 Applications of zeolite catalysts

(1) As solid-acid catalysts for acetalisation reactions

22

Protection of the carbonyl groups is an important technique in multi-step

organic synthesis and can be achieved by employing several types of reagents:

alcohols, diols, orthoesters, oxiranes, or acetals[44]

. Among these, the acetalisation of

aldehydes/ketones with alcohols is a primary step, because products of acetals/ketals

have higher stability towards strong bases or oxidants, Grignard reagent, lithium

aluminium hydride, and esterification reagents[45]

. Moreover, acetals have found

direct applications as fragrances in cosmetics, as additives in food and beverage

industry, and as pharmaceuticals in synthesising enantiometrically pure

compounds[46]

.

The acetalisation is traditionally catalysed by strong/intermediate Brønsted or

Lewis acids, such as HCl, H2SO4, ZnCl2, FeCl3 and p-toluene sulphonic acid[47, 48]

.

These processes involve expensive reagents, tedious work-up procedure,

neutralisation of the strongly acidic media, and production of harmful wastes. These

are disadvantages that make these processes inefficient and uneconomical.

Consequently, there is a demand for environmentally-friendly acid catalysts to

synthesise acetals/ketals under mild conditions. Heterogeneous catalysts offer

promise in these syntheses owing to their ease of separation, high purity,

recyclability, and reusability. Different heterogeneous solid-acid catalysts have been-

developed, such as montmorillonite clay[49]

, sulphated zirconia (SO42-

/ZrO2) and

sulphated titania (SO42-

/TiO2)[50]

. Zeolites are important solid-acid catalysts and

perform efficiently in acetalisation reactions. For example, Al-MCM-41 is for the

acetalisation of cyclohexanone with methanol, ethylene glycol; and pentaerythritol[51]

cation-treated Y zeoiltes are for the acetalisation of cyclohexanone, acetophenone,

and benzophenone with methanol[52]

. The mechanism of the acetalisation of

23

aldehydes/ketones is generally recognised to be the Brønsted-acid catalysed process,

such as the acetalisation of cyclohexanone with methanol, as illustrated in Figure 6.

Despite the obvious advantages that zeolte catalysts have, the catalytic activity

is still not very high and requires elevated operating temperatures and long reaction

times. Thus, optimisation of these processes remains under investigation.

Figure 6. General mechanism of the acetalisation of cyclohexanone catalysed by solid-acid

catalysts (H+). Reproduced from Ref 58.

(2) As photocatalysts for oxidation of hydrocarbons

Partially oxidised hydrocarbons are important building blocks of synthetic fibres

and plastics, and intermediates for manufacturing chemicals[53]

. The present partial

oxidation processes involve O2 which is the best oxidant commercially because of

the very large scale of these processes. Most hydrocarbon merchants give rise to a

wide range of oxidised products because of the high operating temperature; thus,

large amounts of energy are spent on separation of the desired product from side

products. Simultaneously this causes a heavy environmental burden[54]

.

Zeolite photocatalysts hold great promise in the partial oxidation of hydrocarbons,

such as small alkanes, alkenes and aromatics. Hydrocarbons and O2 can be activated

when a gas mixture is loaded into zeolite cages under visible light irradiation[55]

. The

critical oxidation components in this system are extra-framework alkali or alkaline-

earth cations in the zeolites. The highly polarised electrostatic fields in the vicinity of

24

these cations (see Figure 7)[53]

may reduce the activation energy required to activate

hydrocarbon-oxygen charge transfer.

Figure 7: Visible light induces charge transfer between ethane and oxygen molecules to

form a transient ethane radical and the superoxide species (O2-). Reproduced from Ref 53.

Xu and co-workers report that the zeolite Y with different contents of Ca2+

cations

can activate propane oxidation to produce acetone under mild heating[56]

. The

oxidation rate improved considerably when the content of Ca2+

cations increased,

demonstrating that the reaction required hydrocarbon-oxygen encounters in locations

of the high electrostatic fields that are close to the Ca2+

cations. The authors proposed

a mechanism that once charge transfer occurred, the highly acidic hydrocarbon

radical cations are ready to transfer a proton to O2 to form a hydroperoxide molecule

through coupling of the alkyl and HOO* radicals. A water molecule is eliminated

during this process to yield the desired carbonyl product. For the selective oxidation

of ethane (an attractive reaction for commercial use due to the mild C-H bond

activation, see the Figure 7), the intermediate is ethyl hydroperoxide, and the final

product is acetaldehyde. Another reaction accessible by this system is the oxidation

of toluene to benzaldehyde by O2, and this reaction remains a great challenge. Sun

and co-workers tried barium(2+)- and calcium(2+)-exchanged zeolite Y under the

visible light irradiation[57]

; however, like photooxidation of other hydrocarbons, the

products could not be quantitatively measured. The poor yield was primarily due to

25

the low visible light absorption efficiency of zeolites. Therefore, more efforts are

needed to optimise the oxidation of hydrocarbons on zeolite photocatalysts under the

visible light irradiation.

2.3 Summary

These reports mentioned in the literature review demonstrate that plasmonic

nanostructures, particularly Au-NPs, are effective photocatalysts in the selective

oxidation and the selective reduction under visible light irradiation. Their adjustable

size, shape, and their alloys of Ag, Cu, Pt, and Pd, particularly the LSPR effect and

the electric near-field enhancement, make them attractive not only for chemical

production, but also for plasmonic devices, photovoltaic cells, and detectors.

Supports of gold photocatalysts such as zeolites are a class of well-known solid-acid

catalysts that possess strong adsorptivity, singular shape-selectivity, and unique

polarised electrostatic fields near extra-framework cations. Wide applications are

envisaged for these materials in adsorption, separation, ion-exchange, and catalysis.

In catalysis, for instance, the solid-acid catalysed acetalisation of aldehydes/ketones

to acetals/ketals is an important organic transformation to protect carbonyl groups in

multi-step organic synthesis. Zeolite solid-acid catalysts are considered to be ideal

substitutes for liquid-acid catalysts for their ease of separation, minimal waste, and

their reusability. Plasmonic photocatalysts of Au/zeolites offer a great potential to

achieve acetalisation in a photocatalytic way at ambient conditions, thus saving non-

renewable energy. An investigation into the mechanism of these photocatalytic

systems is expected to provide insights into understanding the light-material

interaction mechanism and to promote development of novel photocatalysts for other

important reactions either driven, or enhanced by visible light irradiation.

26

These properties of plasmonic photocatalysts afford a great promise for their

applications in a wide range, such as in materials science, production of fine

chemicals, energy storage, and environmental remediation. Research achievements

on plasmonic photocatalysts may eventually help develop efficient commercial

photocatalysts that can take advantage of sunlight, the most abundant and ―green‖

energy in the world.

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29

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33

56. J. Xu, B. L. Mojet, J. G. van Ommen, and L. Lefferts, ―Effect of Ca2+

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35

Chapter 3: Selective reduction reaction - Selective reductions using

visible light photocatalysts of supported gold nanoparticles

Introductory remarks

This chapter reports that Au-NPs supported on Al2O3, CeO2, TiO2, ZrO2, or

zeolite Y enhance the catalytic performance in the selective reduction of

nitroaromatics to azo compounds, azobenzene to hydroazobenzene, ketones to

alcohols, and epoxides to alkenes at ambient temperatures under visible light

illumination (or simulated sunlight). The article describing the discoveries was

published in the journal of ―Green Chemistry” in 2012[1]

.

The selective reduction is an important process in organic synthesis, energy

storage and environmental chemistry, such as production of chemical intermediates,

development of solar panels and degradation of organic pollutants. If these processes

can be achieved in a photocatalytic way, then it will be promising to synthesise

organic chemicals with significant benefits, such as better selectivity to the desired

product via a green approach because the photocatalytic reactions often occur at

ambient conditions. Moreover, it opens a potential way to develop the sunlight-

driven photocatalytic processes in the chemical community[2]

.

This study investigated the reduction activity of photocatalysts of supported Au-

NPs induced by the LSPR effect. Other studies have reported that the same above-

mentioned reactions can be driven by heat energy[3-8]

. However, this study showed

that these reactions exhibited much high conversions of reactants under visible light

irradiation compared with the thermal-driven processes (other reaction conditions

were identical), clearly demonstrating the contribution of visible light irradiation.

36

The reactivity of Au-NPs depended on two basic factors: (i) the light absorption

ability of the photocatalysts to absorb visible light, and (ii) the reduction ability of

the photocatalysts to reduce reactants. The light absorption of Au-NPs resulted from

the LSPR effect under visible light irradiation and was directly correlated to the

reduction ability of Au-NPs. Visible light with shorter wavelengths could excite the

free conduction electrons of Au-NPs to higher energy levels, thereby inducing

reduction of reactants with more negative reduction potentials. This discovery

demonstrated that the reduction capability of the gold photocatalysts could be

adjusted by tuning the wavelength of incident light.

References

1. X. B. Ke, X. G. Zhang, J. Zhao, S. Sarina, J. Barry, and H. Y. Zhu, ―Selective

reductions using visible light photocatalysts of supported gold nanoparticles,‖

Green Chem. 15, 236-244 (2013).

2. I. Kirm, F. Medina, J. E. Sueiras, P. Salagre, and Y. Cesteros, ―Hydrogenation of

styrene oxide in the presence of supported platinum catalysts to produce 2-

phenylethanol,‖ J. Mol. Catal. A 261, 98-103 (2007).

3. X. B. Chen, L. Liu, P. Y. Yu, and S. S. Mao, ―Increasing solar absorption for

photocatalysis with black hydrogenated titanium dioxide nanocrystals,‖ Science,

331, 746-750 (2011).

4. T. Mitsudome, A. Noujima, Y. Mikami, T. Mizugaki, K. Jitsukawa, and K.

Kaneda, ―Supported gold and silver nanoparticles for catalytic deoxygenation of

epoxides into alkenes,‖ Angew. Chem. Int. Ed. 49, 5545-5548 (2010).

37

5. F. Z. Su, L. He, J. Ni, Y. Cao, H. Y. He, and K. N. Fan, ―Efficient and

chemoselective reduction of carbonyl compounds with supported gold catalysts

under transfer hydrogenation conditions,‖ Chem. Commun. 3531 (2008).

6. Y. Zhu, H. F. Qian, B. A. Drake, and R. C. Jin, ―Atomically precise Au25(SR)18

nanoparticles as catalysts for the selective hydrogenation of α, β-unsaturated

ketones and aldehydes,‖ Angew. Chem. Int. Ed. 49, 1295-1298 (2010).

7. E. Merino, ―Synthesis of azobenzenes: the coloured pieces of molecular

materials,‖ Chem. Soc. Rev. 40, 3835-3853 (2011).

8. A. Grirrane, A. Corma, and H. Carcía, ―Gold-catalysed synthesis of aromatic azo

compounds from anilines and nitroaromatics,‖ Science, 322, 1661-1664 (2008).

38

Statement of contribution

The authors listed below have certified that:

1. They meet the criteria for authorship in that they have participated in the

conception, execution, or interpretation, of at least that part of the publication in

their field of expertise;

2. They have public responsibility for their part of the publication, except for the

responsible author who accepts overall responsibility for the publication;

3. There are no other authors of the publication according to these criteria;

4. Potential conflicts of interest have been disclosed to (a) granting bodies, (b) the

editor or publisher of journals or other publications, and (c) the head of the

responsible academic unit, and

5. They agree to the use of the publication in the student’s thesis and its publication

on the Australian Digital Thesis database consistent with any limitations set by

publisher requirements.

In the case of this chapter:

Selective reductions using visible light photocatalysts of supported gold

nanoparticles

Xuebin Ke, Xingguang Zhang, Jian Zhao, Sarina Sarina, John Barry, and Huaiyong Zhu

Published in the journal: Green Chemistry, 2013, 15, 236-244

51

Chapter 4: Selective oxidation reaction - Zeolite-supported gold

nanoparticles for selective photooxidation of aromatic alcohols

under visible-light irradiation

Introductory remarks

This chapter reports that Au-NPs supported on zeolites can oxidise benzyl alcohol

and its derivatives into product aldehydes with high selectivity (99%) under visible

light irradiation at ambient temperature. The article was published in the journal of

―Chemistry – A European Journal”[1]

.

The selective oxidation of aromatic alcohols to corresponding aldehydes is an

important process in the production of fine chemicals, because aldehydes are

valuable components in the industry of perfume, dyes, pharmaceuticals, and

agrochemicals[2, 3]

. In particular, benzaldehyde is the most extensively investigated. It

is traditionally produced by hydrolysis of benzyl chloride or by oxidation of toluene[4,

5]; however, this process often leads to trace of chlorine impurities and a substantial

quantity of waste. If the selective oxidation of aromatic alcohols can be achieved in a

photocatalytic way, then it will be promising to improve the catalytic performance

such as higher selectivity, because the photocatalytic reactions often happen at

ambient conditions.

This study showed that the LSPR effect of Au-NPs, the adsorption capability of

zeolite supports, and the molecular polarities of aromatic alcohols all contributed to

the photocatalytic performance. The kinetic study demonstrated that the

photooxidation under visible light irradiation required less activation energy

compared with that of the thermal-driven reaction. Moreover, the influence of light

52

intensity, wavelength range, and the role of molecular oxygen were investigated in

detail. This study illustrates the potential of Au/zeolite photocatalysts for the

selective oxidation reaction driven by sunlight at ambient temperatures. The

mechanistic study may provide practical guidelines for developing new catalytic

materials for ―green‖ and energy-saving chemical processes.

References

1. X. G. Zhang, X. B. Ke, and H. Y. Zhu, ―Zeolite-supported gold nanoparticles for

selective photooxidation of aromatic alcohols under visible-light irradiation,‖

Chem. Euro. J. 18, 8048-8056 (2012).

2. N. Lingaiah, K. M. Reddy, N. S. Babu, K. N. Rao, I. Suryanarayana, and P. S. S.

Prasad, ―Aerobic selective oxidation of benzyl alcohol over vanadium substituted

ammonium salt of 12-molybdophosphoric acid,‖ Catal. Commun. 7, 245-250

(2006).

3. R. A. Sheldon, and J. K. Kochi, ―Metal-catalysed oxidations of organic

compounds,‖ Academic Press, New York (1981).

4. H. Sun, F. Blatter, and H. Frei, ―Selective oxidation of toluene to benzaldehyde

by O2 with visible light in barium(2+)- and calcium(2+)-exchanged zeolite Y,‖ J.

Am. Chem. Soc. 116, 7951-7952 (1994).

5. Y. Mao, and A. Bakac, ―Photocatalytic oxidation of toluene to benzaldehyde by

molecular oxygen,‖ J. Phys. Chem. 100, 4219-4223 (1996).

53

Statement of contribution

The authors listed below have certified that:

1. They meet the criteria for authorship in that they have participated in the

conception, execution, or interpretation, of at least that part of the publication in

their field of expertise;

2. They have public responsibility for their part of the publication, except for the

responsible author who accepts overall responsibility for the publication;

3. There are no other authors of the publication according to these criteria;

4. Potential conflicts of interest have been disclosed to (a) granting bodies, (b) the

editor or publisher of journals or other publications, and (c) the head of the

responsible academic unit, and

5. They agree to the use of the publication in the student’s thesis and its publication

on the Australian Digital Thesis database consistent with any limitations set by

publisher requirements.

In the case of this chapter:

Zeolite-supported gold nanoparticles for selective photooxidation of aromatic

alcohols under visible-light irradiation

Xingguang Zhang, Xuebin Ke, and Huaiyong Zhu

Published in the journal: Chemistry-A European Journal, 2012, 18, 8048-8056

halla

Due to copyright restrictions, the published version of this journal article is not available here. Please view the published version online at: http://dx.doi.org/10.1002/chem.201200368

64

Chapter 5: Acetalisation reaction

5.1 Acetalisation reaction on Au/zeolite photocatalysts - Plasmonic

nanostructures to enhance catalytic performance of zeolites under

visible light

Introductory remarks

This chapter reports an effective way to harvest visible light to boost zeolite

catalysis by using plasmonic Au-NPs supported on the MFI zeolites. Zeolites possess

strong Brønsted acid sites (H+) and polarised electrostatic fields that created by extra-

framework cations as outline in Chapter 2. The polarised electrostatic fields can be

further intensified by the electric near-field enhancement of Au-NPs, which results

from the LSPR effect upon visible light irradiation. The intensified polarised

electrostatic fields play a critical role in polarising or interacting with reactants

adsorbed on zeolite surfaces or confined in zeolite pores. This study demonstrated a

cascade enhancement from visible light to zeolites via the LSPR-induced electric

near-field enhancement which functioned as a ―relay‖ to intensify the polarised

electrostatic fields to boost the catalytic performance of acetalisation reactions. This

research was published in the journal of ―Scientific Reports‖[1]

.

The acetalisation reaction plays an important part in the multi-step organic

synthesis to produce multi-functional organic molecules[2]

. Two acetalisation

reactions were selected as model reactions that were performed on Au/MZSM-5 and

MZSM-5 (M=H+, Na

+, Ca

2+, or La

3+): the condensation of benzaldehyde with 1-

pentanol and the methylation of cyclohexanone with methanol[3, 4]

. Density

functional theory (DFT) calculations confirmed that the intensified polarised

65

electrostatic fields can stretch the C=O bonds of reactants (benzaldehyde and

cyclohexanone), thus allowing them to be activated more efficiently by active sites

(H+) to boost the reaction rates.

This discovery should draw huge attention to plasmonic metal nanostructures

modified zeolitic catalysts, because the enhancement effect holds great potential in

enlarging the molecular polarity of adsorbates. This stretching effect can influence

the activity and the electrostatic behaviour of molecules, which have broad

applications in organic synthesis, ionic liquids, nanofiltration membranes, molecular

electronics, and molecular self-assembly processes[5-7]

. Moreover, new applications

can be expected in sensors, solar panels, water splitting, drug delivery, and

environmental remediation.

References

1. X. G. Zhang, X. B. Ke, A. J. Du, and Huaiyong Zhu, ―Plasmonic nanostructures

to enhance catalytic performance of zeolites under visible light,‖ Sci. Rep.

4(3805), 1-6 (2014).

2. B. Thomas, S. Prathapan, and S. Sugunan, ―Synthesis of dimethyl acetal of

ketones: design of solid acid catalysts for one-pot acetalisation reaction,‖

Micropor. Mesopor. Mater. 80, 65-72 (2005).

3. F. J. Liu, T. Willhammar, L. Wang, L. F. Zhu, Q. Sun, and X. J. Meng, W.

Carrillo-Cabrera, X. D. Zou, and F. S. Xiao, ―ZSM-5 zeolite single crystals with

b-axis-aligned mesoporous channels as an efficient catalyst for conversion of

bulky organic molecules,‖ J. Am, Chem. Soc. 134, 4557-4560 (2012).

66

4. M. Iwamoto, Y. Tanaka, N. Sawamura, and S. Namba, ―Remarkable effect of

pore size on the catalytic activity of mesoporous silica for the acetalisation of

cyclohexanone with methanol,‖ J. Am. Chem. Soc. 125, 13032-13033 (2003).

5. B. Van der Bruggen, J. Schaep, D. Wilms, and C. Vandecasteele, ―Influence of

molecular size, polarity and charge on the retention of organic molecules by

nanofiltration,‖ J. Membrane Sci. 156, 29-41 (1999).

6. P. A. Lewis, C. E. Inman, F. Maya, J. M. Tour, J. E. Hutchison, and P. S. Weiss,

―Molecular engineering of the polarity and interactions of molecular electronic

switches,‖ J. Am. Chem. Soc. 127, 17421-17426 (2005).

7. A. S. Singh, and S.-S. Sun, ―Dynamic self-assembly of molecular capsules via

solvent polarity controlled reversible binding of nitrate anions with C3 symmetric

tripodal receptors,‖ Chem. Commun. 47, 8563-8565 (2011).

67

Statement of contribution

The authors listed below have certified that:

1. They meet the criteria for authorship in that they have participated in the

conception, execution, or interpretation, of at least that part of the publication in

their field of expertise;

2. They have public responsibility for their part of the publication, except for the

responsible author who accepts overall responsibility for the publication;

3. There are no other authors of the publication according to these criteria;

4. Potential conflicts of interest have been disclosed to (a) granting bodies, (b) the

editor or publisher of journals or other publications, and (c) the head of the

responsible academic unit, and

5. They agree to the use of the publication in the student’s thesis and its publication

on the Australian Digital Thesis database consistent with any limitations set by

publisher requirements.

In the case of this chapter:

Plasmonic nanostructures to enhance catalytic performance of zeolites under

visible light

Xingguang Zhang, Xuebin Ke, Aijun Du, and Huaiyong Zhu

Published in the journal: Scientific Reports, 2014, 4(3805), 1-6.

Plasmonic nanostructures to enhancecatalytic performance of zeolites undervisible lightXingguang Zhang, Xuebin Ke, Aijun Du & Huaiyong Zhu

School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, QLD 4001, Australia.

Light absorption efficiency of heterogeneous catalysts has restricted their photocatalytic capability forcommercially important organic synthesis. Here, we report a way of harvesting visible light efficiently toboost zeolite catalysis by means of plasmonic gold nanoparticles (Au-NPs) supported on zeolites. Zeolitespossess strong Brønsted acids and polarized electric fields created by extra-framework cations. Thepolarized electric fields can be further intensified by the electric near-field enhancement of Au-NPs, whichresults from the localized surface plasmon resonance (LSPR) upon visible light irradiation. The acetalizationreaction was selected as a showcase performed on MZSM-5 and Au/MZSM-5 (M 5 H1, Na1, Ca21, or La31).The density functional theory (DFT) calculations confirmed that the intensified polarized electric fieldsplayed a critical role in stretching the C5O bond of the reactants of benzaldehyde to enlarge their molecularpolarities, thus allowing reactants to be activated more efficiently by catalytic centers so as to boost thereaction rates. This discovery should evoke intensive research interest on plasmonic metals and diversezeolites with an aim to take advantage of sunlight for plasmonic devices, molecular electronics, energystorage, and catalysis.

Photocatalysis has significantly advanced in developing efficient catalysts to harvest the ‘‘green’’ solar energyat ambient conditions. Recent years have seen a new family of plasmonic photocatalysts of gold, silver, orcopper nanoparticles (NPs), which features a collective and coherent oscillation of the free conduction

electrons with the incident photons at the surface of NPs due to the localized surface plasmon resonance (LSPR)irradiated by visible light1–3. This LSPR effect has been applied to boost catalytic performance in various import-ant organic transformations under the irradiation of visible light: oxidation of organic contaminants and aromaticalcohols, and reduction of nitroaromatic compounds, ketones and epoxides on Au-NPs4–7; epoxidation of ethyl-ene on Ag-NPs and of propylene on Cu-NPs8,9. Very recently, Au-Pd alloys are also founded to be efficientphotocatalysts for Suzuki coupling reactions and oxidation of benzylamine10,11. These discoveries demonstratethat the plasmon-mediated processes fundamentally differ in catalytic mechanisms compared with those ontraditional semiconductors. Moreover, the LSPR effect of plasmonic NPs can amplify the electromagnetic fieldintensity of incident light (jE0j) and results in an enhanced electric field (jEj) of 3–6 orders of magnitude3,12. Thisnear-field enhanced interaction is well-known as the ‘‘electric near-field enhancement (ENFE)’’ effect and hasmotivated numerous intensive studies, such as surface-enhanced Raman scattering13, sensors14–16, plasmonicdevices17–19 and solar cells20,21. In photocatalysis, Ag-NPs covered by the SiO2 shell and embedded in TiO2

particles have been tried to degrade methyl blue and exhibit better catalytic performance owing to the electricfield amplitude effect of Ag-NPs22. However, the inert SiO2 shell, which is used to prevent the Ag core fromoxidation, constraints the light absorption efficiency in photocatalysis.

Zeolites possess regular microporous structures, high surface areas, shape-selectivity, and unique solid acidity,having extensive applications in catalysis, separation, and adsorption. Particularly, the strong polarized electro-static fields (PEF, 1–10 V/nm) created by extra-framework cations have the power to polarize molecules adsorbedon surfaces or confined in the porous matrix (e.g. host-guest structures)23–25. In principle, the PEF can reduce theenergy consumption required to facilitate the electron transfer or to activate reactants, thereby initiating reactionsby moderate heating or visible-light excitation. The charge-transfer properties from hydrocarbons to molecularoxygen (O2) have been investigated on ion-exchanged zeolites, such as ZSM-5, Y and Beta, with visible lightirradiation in the selective oxidization of toluene, propane, cyclohexane, and small alkenes26–29. These studiesverify that the PEF can lower the charge-transfer excitation energy from hydrocarbons to O2. However, the

OPEN

SUBJECT AREAS:PHOTOCATALYSIS

HETEROGENEOUS CATALYSIS

PHOTOCHEMISTRY

CHEMICAL ENGINEERING

Received29 October 2013

Accepted23 December 2013

Published22 January 2014

Correspondence andrequests for materials

should be addressed toX.B.K. (x.ke@qut.edu.

au)

SCIENTIFIC REPORTS | 4 : 3805 | DOI: 10.1038/srep03805 1

products cannot be quantitatively measured because of low produc-tivity, which results from the low absorption efficiency of zeolites ofvisible light.

Plasmonic Au-NPs are loaded on ion-exchanged ZSM-5 zeolites(Au/MZSM-5, M 5 H1, Na1, Ca21, or La31) as ‘‘antenna’’ to effi-ciently absorb visible light. This study for the first time reports acascade enhancement from visible light to zeolites via the LSPR-induced ENFE which functions as a ‘‘relay’’ to intensify the PEF ofcations in zeolites to boost the catalytic performance. The acetaliza-tion of benzaldehyde with 1-pentanol is selected as the model reac-tion30–34. The present research demonstrated that the ENFE ofAu-NPs could intensify the PEF of MZSM-5 so as to facilitate theeffective activation of the reactant of benzaldehyde by stretching theC5O bonds, thus improving the conversion of the acetalization.

To illustrate this enhancing process, as shown in Figure 1, Au-NPsform dynamic Au(d1, d2) dipole moments owing to the LSPR effectupon visible light irradiation, simultaneously inducing the ENFEaround the surface of Au-NPs2,3. The ENFE intensifies the PEF ofextra-framework cations in MZSM-5, which enables the intensifiedPEF to stretch the C5O bond of reactants of benzaldehyde. Thestretched reactants display larger molecular polarities and can beactivated more efficiently by catalytic active centers of H1, therebyaccelerating reactions with alcohols35. The Brønsted acid sites origin-ate from terminal hydroxyl groups, and extra-framework cations:M21 1 H2O 1 Si-O-Al R M(OH)1 1 Si-O(H1)-Al24,36.

Table 1 revealed that the acetalization between benzaldehyde and1-pentanol proceeded effectively on Au/MZSM-5 under visible lightirradiation. The catalytic conversions improved significantly on Au/NaZSM-5, Au/CaZSM-5, and Au/LaZSM-5; particularly, the conver-sion increased from 21.5% on CaZSM-5 to 62.2% on Au/CaZSM-5(an exception for HZSM-5 will be interpreted later). However, withlight off (see SI, Table S1, Section S1), the catalytic conversions ofMZSM-5 and Au/MZSM-5 showed fewer differences, indicating Au-NPs themselves were not active centers under dark conditions.Moreover, visible light irradiation showed negligible influence onthe activity of MZSM-5 compared with those under dark conditions.These results confirmed that visible light and plasmonic Au-NPswere necessities to sharply boost the acetalization reaction.

The differences in conversions on MZSM-5 indicated that extra-framework cations markedly influenced the catalytic activity.

HZSM-5 exhibited the highest conversion of 53.8%, whereasNaZSM-5, CaZSM-5, and LaZSM-5 exhibited lower conversions(just better than Silicalite-1) primarily because of their relativelyfewer active centers (H1). The change in Brønsted acidity could beproved by the spectra of FT-IR spectroscopy - attenuated total reflec-tance (ATR) which also confirmed that Brønsted acids preserved wellafter loading of gold (see SI, Figure. S1, Section S2).

Moreover, the optical properties of catalysts were investigated byUV/Vis spectra (Figure 2a), showing that MZSM-5 had no mea-surable absorption of visible light (.420 nm), whereas Au/MZSM-5 strongly absorbed visible light owing to the LSPR effect of Au-NPs,with the absorption peaks being about 525–565 nm. The crystalstructures of Au/MZSM-5 were detected by X-ray diffraction(XRD) patterns (Figure 2b), confirming that the MFI structures pre-served well. The oxidation state of gold was analyzed by X-ray photo-electronic spectra (XPS) (Figure 2c) which identified that the bindingenergies of Au4f7/2 electrons (84.1 eV) and Au4f5/2 electrons(87.7 eV) were in agreement with reports for Au-NPs37. In addition,XPS showed that the gold content were similar on Au/MZSM-5, theSi/Al ratio was close to 14, and that the ion-exchange level was highbecause Na1 ions were undetectable in CaZSM-5 and LaZSM-5.Transmission-electron-microscopy (TEM) images provided directinformation of the size (6–18 nm), shape (pseudospherical) and dis-tribution of Au-NPs on zeolites (see SI, Figure. S3, Section S3).

Now one may wonder why visible light coupled with Au-NPscould boost the reaction on Au/MZSM-5. Here we hypothesize thatthe ENFE of Au-NPs directly intensifies the PEF of cations to a largeextent that the intensified PEF can polarize reactants of benzalde-hyde and 1-pentanol to facilitate activation of them by catalytic activecenters of H1. From the experimental perspective, Au/HZSM-5 andHZSM-5 exhibited similar catalytic activities with light on, as shownin Table 1, indicating that Au-NPs neither served as extra activecenters nor interacted directly with the reactants to accelerate thereaction rate even though the LSPR-induced dipole moment ofAu(d1, d2) could in theory interact with the reactants. In this case,there should exist other key factors that affected the interaction withreactants. The density functional theory (DFT, see SI, Section S4)calculations unambiguously demonstrated that increasing thestrength of external electric field stretched the C5O bond of benzal-dehyde, whereas had no effect on 1-pentanol, probably because themolecular polarity of benzaldehyde (dipole momentm5 3.00 Debye)is larger than that of 1-pentanol (1.70 Debye). The relationshipbetween the C5O bond length and the electric field strength waslinear (Figure 3a). In detail, the C5O bond lengths of benzaldehydewere 1.218 A, 1.223 A, 1.229 A, 1.236 A, and 1.245 A, whenthe electric field strengths were 0 V/nm, 2.58 V/nm, 5.16 V/nm,7.74 V/nm, and 10.32 V/nm, respectively. Here the electric fieldstrength used in simulation were on the same order of magnitudeas that reported for HZSM-5 (3.0 V/nm), NaZSM-5 (6.2 V/nm),CaZSM-5 (8.3 V/nm), and LaZSM-5 (10.7 V/nm) (see SI, TableS2, Section S5)23,38. However, the experimental results revealed thatthe PEF of MZSM-5 without Au-NPs had no influence on the con-versions, which indicated that the PEF of MZSM-5 was not strongenough to boost the reaction rate even with light on.

These results trigger a logical deduction that further increasing theelectrostatic field strength may make distinct difference. And it hasbeen underpinned by the experimental evidence that the existence ofan external electric field of 12–15 V/nm obviously helps activate thereactants of CO/O2 and enhances the oxidation process on small goldcrystals (20–30 nm). This effect is considered to be associated withthe formation of partially charged gold surface39. Furthermore, it hasbeen reported that the near-field enhancement factor (jEj/jE0j) ofplasmonic NPs can reach as large as 102 at the surface of individualNPs and 103 at the junction of interparticles (hot spots)40,41, and thatthe ENFE attenuates exponentially within the spacing range of 10–50nanometers away from metal surfaces, depending on the metal

Figure 1 | The acetalization on Au/MZSM-5 (M 5 H1, Na1, Ca21, orLa31) under the irradiation of visible light. ENFE means the electric near-

field enhancement of Au-NPs. PEF means the polarized electro-static fields

of extra-framework cations in zeolites.

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crystal size and shape, and the dielectric environments1. Therefore, itbecomes convincing that the ENFE may directly intensify the PEF ofcations in MZSM-5 to a large extent so as to stretch the C5O bond ofreactants, thereby boosting the reaction rate.

The intensified PEF of H1, Na1, Ca21, and La31 differed in boost-ing the catalytic activities on Au/MZSM-5. The largest net increase inthe conversion of acetalization reaction, which means the conversionon Au/MZSM-5 deducts the conversion on its counterpart ofMZSM-5, occurred between Au/CaZSM-5 and CaZSM-5, and thelowest between Au/HZSM-5 and HZSM-5 (Figure 3b, calculatedfrom Table 1). It has been reported that the PEF strength of a singlecation within a given zeolite host is influenced by multi-factors, suchas Si/Al ratio, charge/radius ratio, cation charges and cation density(see SI, Table S2, Section S5). As for Au/HZSM-5 and HZSM-5, theoriginal PEF strength of H1 is ca. 3.0 V/nm and the intensified PEFstrength of H1 might be still too weak to stretch the C5O bond ofbenzaldehyde, thus showing the lowest net increase in conversion.The intensified PEF strength of Na1 (originally ca. 6.2 V/nm), Ca21

(originally ca. 8.3 V/nm), or La31 (originally ca. 10.7 V/nm) shouldbe sufficiently strong to extend the C5O bond of benzaldehyde toaccelerate the reaction. Interestingly, La31 ions which had the stron-gest electrostatic field showed a lower net increase of conversion thanNa1 ions and Ca21 ions. This phenomenon probably resulted fromthe difference in cation density within MZSM-5 after ion-exchangebecause one La31 ion corresponds to three Na1 in terms of compens-ating for the negatively-charged framework sites (see SI; Section S5,Table S3, and Section S6); therefore, the number of La31 (La31/Al 5

0.09) was smaller in MZSM-5 compared with the number of Na1

(Na1/Al 5 0.43) or Ca21 (Ca21/Al 5 0.25), thereby exhibiting a lowernet conversion between Au/LaZSM-5 and LaZSM-5. One may arguethat the acidity of zeolites plays a more important role than PEF inthe acetalization reaction because Brønsted acid sites (H1) are thecatalytic centers. However, this study conveys a clear message thatthe intensified PEF by ENFE does contribute to improving the cata-lytic performances of zeolites by affecting the molecular polarities ofreactants.

The impacts of light intensity, the range of wavelength, and reac-tion temperature on the acetalization reaction were investigated onCaZSM-5 and Au/CaZSM-5 with light on. As shown in Figure 4a, theincrease of light intensity improved the catalytic activity on Au/CaZSM-5, but not on CaZSM-5. This result indicated that raisingthe light intensity could afford a much stronger ENFE on Au/CaZSM-5 thus contributing more to boosting the catalytic activity.The light wavelength was also investigated using glass filters to blockphotons with wavelengths below the filter threshold. For example,the cut-off wavelength of 420 nm means that the light with wave-length smaller than 420 nm was blocked. Figure 4b showed that thelight wavelength significantly influenced the catalytic activity of Au/CaZSM-5, but not on CaZSM-5. This result demonstrated a wave-length-selective enhancement on Au/CaZSM-5, and proved that vis-ible light that could induce the ENFE effect of Au-NPs made a largercontribution to boost the catalytic activity. Raising the reaction tem-perature increased the conversions both on CaZSM-5 and Au/CaZSM-5 (Figure 4c). The conversion on Au/CaZSM-5 was muchhigher than that on CaZSM-5 at the same temperature. The slightdecrease in selectivity could be ascribed to the oxidation of

Table 1 | Catalytic performances of the acetalization between benzaldehyde and 1-pentanol on MZSM-5 and Au/MZSM-5 (M 5 H1, Na1,Ca21, or La31) under visible light irradiation

Catalyst Conv.a (%) Sele.b (%) Yield (%) Reaction rate (1023 molg21 h21)

CaZSM-5 21.5 .99 21.3 4.37Au/CaZSM-5 62.2 99.1 61.6 12.79NaZSM-5 30.9 .99 31.6 6.28Au/NaZSM-5 52.5 97.8 51.3 10.80LaZSM-5 48.7 .99 48.2 9.90Au/LaZSM-5 58.4 96.5 56.4 12.01HZSM-5 53.8 .99 53.3 10.93Au/HZSM-5 55.8 98.7 55.1 11.48Silicalite-1 9.3 .99 9.2 1.91Au/Silicalite-1 11.5 96.5 11.1 2.37a: the conversion of benzaldehyde.b: the selectivity towards pentyloxy(phenyl)methanol. Reaction conditions: benzaldehyde (0.65 g), 1-pentanol (4.40 g); temperature (60uC), time (6 h); light intensity (0.5 W/cm2), wavelength range(420–800 nm); catalyst (0.05 g); atmosphere (air). No detectable reaction without catalysts.

Figure 2 | Characterizations of Au/Silicalite-1 and Au/MZSM-5 (M 5 H1, Na1, Ca21, or La31): (a) UV/Vis spectra, (b) XRD patterns, and (c) XPSspectra.

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SCIENTIFIC REPORTS | 4 : 3805 | DOI: 10.1038/srep03805 3

benzaldehyde by oxygen inevitably adsorbed on catalysts42,43. Theseresults suggested the boosting effect of ENFE of Au-NPs held atdifferent temperatures, and that the largest net increase of conversionwas at 60uC. The reusability test shows that CaZSM-5 and Au/CaZSM-5 were very stable with above 95% retentions of originalconversions after five runs (see SI, Section S7). Furthermore, theacetalization reaction between cyclohexanone and methanol showedsimilar cascade enhancement effect from visible light to zeoiltes (seeSI, Section S8)31,44.

In summary, the present study unravels a cascade enhancementeffect from visible light to zeolite catalysts bridged via the ENFE ofAu-NPs. The enhancement effect holds great potential in modifyingthe molecular polarities which affect the activity and electrostaticbehavior of molecules and have broad applications in organicsynthesis, ionic liquids, nanofiltration membranes, molecular elec-tronics, and molecular self-assembly technology by means of non-covalent interactions45–47. This study should draw attention to anumber of critical issues following this topic, considering more than200 types of zeolites (IZA Framework Type Codes), and variousplasmonic metal (or alloy) nanostructures together with their wideapplications in sensing, solar panels, solar water splitting, photoche-mical synthesis, drug delivery, industrial processes of cracking andisomerization, and environmental remediation.

MethodsPreparation and ion-exchange of zeolites. All chemicals used for preparing zeoliteswere purchased from Sigma-Aldrich without further treatment. Silicalite-1 and ZSM-5 were synthesized by hydrothermal methods and were calcined at 550uC for 5 h withthe step of increasing temperature being 5uC/min to remove organic templates.

MZSM-5 (M 5 H1, Na1, Ca21, or La31) were obtained by standard ion-exchangeprocedures from the as-synthesized zeolites at 90uC for 3 h for each run, three timesin total, under continuous stirring with 0.1 mol/L NH4NO3 aqueous solution toprepare HZSM-5, 0.1 mol/L sodium chloride (NaCl) aqueous solution to prepareNaZSM-5, 0.1 mol/L calcium chloride (CaCl2) aqueous solution to prepare CaZSM-5, or 0.1 mol/L lanthanum(III) chloride (LaCl3) aqueous solution to prepare LaZSM-5. After each run of ion-exchange, the samples were washed thoroughly withdeionized water and then calcinations were conducted at 400uC for 3 hours.

Preparation of zeolites supported gold catalysts. Zeolites supported catalysts (Au/zeolite) catalysts were prepared by a reduction method. Typically, 1.25 g of zeolitefine powders were dispersed into 50 mL of 3.8 3 1023 mol/L aqueous solution ofchloroauric acid (HAuCl4). Then 0.125 g of poly(vinyl alcohol) (PVA) was dispersedin 10 mL of deionized water; and the PVA solutions were added into the mixture ofzeolite and HAuCl4 solution under stirring for 0.5 h. To this suspension, 0.5 mL of0.38 mol/L aqueous solution of tetrakis(hydroxymethyl) phosphonium chloride(THPC) was added drop by drop as the reducing agent, followed by adding 2.25 mLof 0.38 mol/L sodium hydroxide aqueous solution (NaOH). The mixture was stirredcontinuously for 2 h and aged statically for 24 h. Finally, the solid was washed withdeionized water three times and ethanol once; and the obtained solids were dried at60uC for 16 h. The dried solids were used directly as photocatalysts, denoted as Au/zeolite catalysts.

Characterization of catalysts. X-ray diffraction (XRD) patterns of the samples weredetected on a Philips PANalytical X’Pert PRO diffractometer using Cu Ka radiation(l 5 1.5418 A) at 40 kV and 40 mA. The diffraction data were collected from 5u to75uwith a resolution being 0.01u (2h). UV/Visible (UV/Vis) spectra were recorded ona Cary 5000 UV/Vis-NIR Spectrophotometer in the wavelength range of 200–800 nm. The XPS data were recorded on an ESCALAB 250 spectrometer and Al Karadiation was used as the X-ray source. The C1s peak at 284.8 eV was used as areference for the calibration of the binding energy scale. Transmission electronmicroscopy (TEM) images were taken with a Philips CM200 Transmission electronmicroscope employing an accelerating voltage of 200 kV. The specimens were finepowders deposited onto a copper microgrid coated with a holey carbon film. Thediffuse reflectance The FT-IR spectra were recorded on Nicolet Nexus 870 IR

Figure 3 | (a) The relationship between the C5O bond length (Angstrom) of benzaldehyde and the intensity of extra electrostatic fields simulated by the

density functional theory (DFT) method. (b) The net increase of the conversion of the condensation reaction between benzaldehyde and 1-pentanol was

calculated from Table 1. The net increase means the conversion on Au/MZSM-5 deducts the conversion on its counterpart of MZSM-5 (M 5 H1, Na1,

Ca21, or La31).

Figure 4 | The trend of conversion and selectivity of the acetalization between benzaldehyde and 1-pentanol varies as the function of light intensity (a),

cut-off wavelength (b) (e.g. 420 nm means the light wavelength ,420 nm is cut off, so the light used is from 420–800 nm), and the reaction temperature.

(c) Reaction conditions: benzaldehyde (0.65 g), 1-pentanol (4.40 g), Au/ZSM-5 (0.05 g), atmosphere (argon), reaction temperature 60uC, and reaction

time 6 h.

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SCIENTIFIC REPORTS | 4 : 3805 | DOI: 10.1038/srep03805 4

spectrophotometer equipped with a deuterated triglycine sulfate (DTGS) detectorand a Diamond Attenuated Total Reflectance (ATR) Smart Accessory. 128 scans werecollected for each measurement over the spectral range of 4000–650 cm21 with aresolution of 4 cm21.

Catalytic test of Au/zeolites. All the raw chemicals were purchased from Sigma-Aldrich and used without further treatment. The batch reactions were conducted in around-bottomed 50-mL transparent glass flask equipped with a sealed spigot and amagnetic stirrer. The reaction temperature was controlled by a portable air-conditioner in a sealed box. The flask was irradiated with a 500-Walt Halogen lamp,the UV light was removed by using a glass filter to cut off the light with the wavelengthbeing shorter than 420 nm to avoid the possibility of direct inter-band transition ingold. The intensity of light source was from 0 (light off) to 0.505 W/cm2. Aliquots(0.5 mL) were collected at given time intervals and filtrated through a Millipore filter(pore size 0.45 mm) to remove the catalyst particles. Control experiments wereperformed without light irradiation, maintaining the other conditions identical. Thefiltrates were analyzed by a Gas Chromatography (HP6890, HP-5 column) and GC-MS (6890-5793, HP-5MS column) was used to determine the components.Quantification of the products was obtained from the peak area ratios of the productand its corresponding reactant. The typical details of the chosen two reactions were asthe following: the acetalization between benzaldehyde and 1-pentanol. Typically,0.65 g of benzaldehyde was added into 4.40 g of 1-pentanol, and then 0.05 g catalystwas involved. The reaction temperature was controlled at 60uC. The acetalization ofcyclohexanone with methanol produced dimethyl acetals. Typically, 1.0 g ofcyclohexanone was mixed with 10 mL of methanol, and then 0.05 g of catalyst wasadded into this mixture. The reaction temperature was controlled at 40uC, andsamples were withdrawn every 2 h.

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AcknowledgmentsThis research is supported by the Australian Research Council (ARC) and X. Ke is indebtedto QUT and the Queensland State Government for a Smart Futures Fellowship.

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SCIENTIFIC REPORTS | 4 : 3805 | DOI: 10.1038/srep03805 5

Author contributionsX.Z. conducted the experiments, sample characterization and data analysis. X.K. designedthe experiments and prepared parts of the materials. A.D. conducted DFT calculations andhelped revise the manuscript. H.Z. proposed the study and contributed importantsuggestions. The manuscript was written by X.Z. and X.K.

Additional informationSupplementary information accompanies this paper at http://www.nature.com/scientificreports

Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Zhang, X.G., Ke, X.B., Du, A.J. & Zhu, H.Y. Plasmonicnanostructures to enhance catalytic performance of zeolites under visible light. Sci. Rep. 4,3805; DOI:10.1038/srep03805 (2014).

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported license. To view a copy of this license,

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SCIENTIFIC REPORTS | 4 : 3805 | DOI: 10.1038/srep03805 6

p1

Supporting Information

Plasmonic nanostructures to enhance catalytic performance of zeolites under visible

light

Xingguang Zhang, Xuebin Ke,* Aijun Du, and Huaiyong Zhu

Table of Contents

Section 1. Control experiments of the acetalization between benzaldehyde and 1-pentanol (Table S1) p2

Section 2. Analysis of Brønsted acid sites on catalysts from the IR-ATR results (Figure. S1, Figure. S2) p2

Section 3. TEM images and the distribution of Au-NPs (Figure. S3) p4

Section 4. The computer simulation method p4

Section 5. Polarized electrostatic fields (PEF) of zeolites p5

5.1 Calculation of the PEF of cations in zeolites (Table S2)

5.2. Influential factors on the PEF of cations in zeolites

Section 6. XPS analyses of the gold content, the Si/Al ratio, and the cation contents (Table S3) p6

Section 7 Reusability test of the catalyst CaZSM-5 and Au/CaZSM-5 (Figure. S4) p7

Section 8. The acetalization between cyclohexanone and methanol (Table S4, Table S5, Figure. S5) p7

References p9

p2

Section S1. Control experiments of the acetalization between benzaldehyde and 1-pentanol

Typically, 0.65 g of benzaldehyde was added into 4.40 g of 1-pentanol, and then 0.05 g catalyst was added. The reaction

temperature was kept at 60oC in a silicone oil bath. The results were listed in Table S1.

Table S1 | Catalytic performances for acetalization between benzaldehyde and 1-pentanol on MZSM-5 and Au/MZSM-5 (M=H+, Na

+, Ca

2+, or La

3+)

with light off.

Catalyst Conversiona (%) Selectivity

b (%) Yield (%) Reaction rate (10

-3 molg

-1h

-1)

CaZSM-5 19.7 >99 19.5 4.00

Au/CaZSM-5 22.3 >99 22.1 4.53

NaZSM-5 31.7 >99 31.4 6.44

Au/NaZSM-5 36.9 >99 36.5 7.50

LaZSM-5 49.8 >99 49.3 10.12

Au/LaZSM-5 47.7 >99 47.2 9.69

HZSM-5 55.2 >99 54.6 11.21

Au/HZSM-5 54.3 >99 53.8 11.03

Silicalite-1 8.6 >99 8.5 1.75

Au/Silicalite-1 9.7 >99 9.6 1.97

a: Conversion refers to the conversion of benzaldehyde.

b: Selectivity refers to the selectivity towards pentyloxy(phenyl)methanol. Reaction

conditions: of benzaldehyde (0.65 g) and 1-pentanol (4.40 g); catalyst (0.05 g); atmosphere (air); temperature (60oC); time (6 h). No reaction was

detectable without catalysts.

Section S2. Analysis of Brønsted acid sites on catalysts from the IR-ATR results

The Brønsted acid sites of MZSM-5 and Au/ZSM-5 were investigated by the adsorption of 2-(hydroxymethyl) pyridine

(2-pyridinemethanol) on IR-ATR (attenuated total reflectance). The samples were prepared as the following. Typically,

0.025 g sample was heated to 400oC for 2 h in a clean crucible with a lid, and then the sample was cooled automatically to

150oC at which 0.1 mL 2-pyridinemethanol (Sigma-Aldrich) was added directly into the sample for adsorption. After

adsorbing, the IR-ATR spectra of these treated samples, untreated MZSM-5 and Au/MZSM-5, and pure 2-pyridinemethanol

were collected.

Based on the literature reports,s1, s2, s3 the band at 1530 cm-1 represented the Brønsted acid sites on which the adsorbed

species were 2- (hydroxymethyl) pyridinium ions and the band at 1415 cm-1 was assigned to Lewis acid sites on which the

adsorbed species were 2- (hydroxymethyl) pyridine complexes, as shown in Figure. S1a and S1b. Therefore, the IR-ATR

spectra could give four conclusions based on Figure. S1c and S1d) Silicalite-1 and Au/Silicalite-1 had nearly undetectable

Brønsted acids; 2) MZSM-5 had Brønsted acids with their peaks centered at around 1530 cm-1; 3) HZSM-5 had more

Brønsted acids than CaZSM-5 as indicated by the peak areas; 4) The acid sites preserved well after loading of Au-NPs.

p3

1700 1600 1500 1400 130050

60

70

80

90

100

110

(a)1415

Tra

nsm

itance (

%)

Wavenumber (cm-1)

1530

2-(hydroxymethyl)pyridine

Au/HZSM-5 + 2-(hydroxymethyl)pyridine

Au/Silicalite-1 + 2-(hydroxymethyl)pyridine

Silicalite-1 + 2-(hydroxymethyl)pyridine

HZSM-5 + 2-(hydroxymethyl)pyridine

HZSM-5

Silicalite-1

Au/Slicalite-1

Au/HZSM-5

1700 1600 1500 1400 130050

60

70

80

90

100

110

(b)14151530

Tra

nsm

itta

nce (

%)

Wavenumber (cm-1)

2-(hydroxymethyl)pyridine

Au/CaZSM-5 + 2-(hydroxymethyl)pyridine

Au/Silicalite-1 + 2-(hydroxymethyl)pyridine

Silicalite-1 + 2-(hydroxymethyl)pyridine

CaZSM-5 + 2-(hydroxymethyl)pyridine

CaZSM-5

Silicalite-1

Au/Slicalite-1

Au/CaZSM-5

1575 1550 1525 1500 1475

(c) 1530

CaZSM-5

NaZSM-5

HZSM-5

LaZSM-5

Silicalite-1

Tra

nsm

itance

Wavenumber (cm-1)

1575 1550 1525 1500 1475

(d) 1530

Au/CaZSM-5

Au/NaZSM-5

Au/HZSM-5

Au/LaZSM-5

Au/Silicalite-1

Tra

nsm

itta

nce

Wavenumber (cm-1)

Figure S1 | The ATR results of (a) Silicalite-1, Au/Silicalite-1, HZSM-5, Au/HZSM-5, pure 2-(hydroxymethyl)pyridine, and these catalysts treated

by absorbing 2--(hydroxymethyl)pyridine; (b) Silicalite-1, Au/Silicalite-1, CaZSM-5, Au/CaZSM-5, pure 2-(hydroxymethyl)pyridine, and these

catalysts treated by absorbing 2--(hydroxymethyl)pyridine; (c) and (d) Comparison of peaks centerd at 1530 cm-1

which showed the Brønsted

acids on MZSM-5 and Au/MZSM-5 (M=H+, Na

+, Ca

2+, or La

3+).

Under the same experimental schedule, pyridine was also used as the probe molecules to detect the Brønsted acids, but

no peaks of Brønsted acids were observed, probably because the interaction between pyridine and MZSM-5 was too weaker

to be detected by IR-ART at room temperature and in air atmosphere.

1700 1600 1500 1400 130020

30

40

50

60

70

80

90

100

110

Pyridine

Au/HZSM-5 + Pyridine

Au/Silicalite-1 + Pyridine

Silicalite-1 + Pyridine

HZSM-5 + Pyridine

HZSM-5

Silicalite-1

Au/Slicalite-1

Au/HZSM-5

(a)1415

Tra

nsm

itance (

%)

Wavenumber (cm-1)

1530

1700 1600 1500 1400 130020

30

40

50

60

70

80

90

100

110

(b)14151530

Tra

nsm

itta

nce (

%)

Wavenumber (cm-1)

Pyridine

Au/CaZSM-5 + Pyridine

Au/Silicalite-1 + Pyridine

Silicalite-1 + Pyridine

CaZSM-5 + Pyridine

CaZSM-5

Silicalite-1

Au/Slicalite-1

Au/CaZSM-5

Figure S2 | The ATR results of (a) Silicalite-1, Au/Silicalite-1, HZSM-5, Au/HZSM-5, pure 2-(hydroxymethyl)pyridine, and these catalysts treated

by absorbing 2--(hydroxymethyl)pyridine; (b) Silicalite-1, Au/Silicalite-1, CaZSM-5, Au/CaZSM-5, pure 2-(hydroxymethyl)pyridine, and these

catalysts treated by absorbing 2--(hydroxymethyl)pyridine.

p4

Section S3. TEM images and the distribution of Au-NPs

4 6 8 10 12 14 16 18 20 22 24 260

5

10

15

20

25

30Au/Silicalite-1

Pe

rcen

tage

(%

)

Particle size (nm) 2 4 6 8 10 12 14 16 18

0

5

10

15

20

25

30Au/CaZSM-5

Pe

rcen

tage

(%

)

Particle size (nm)

Figure S3 | TEM analyses of two typical catalysts: Au/Silicalite-1 and Au/CaZSM-5.

The Transmission-Electron-Spectroscopy (TEM) was conducted to determine the size, shape and distribution of Au-NPs

on two typical catalysts of Silicalite-1 and CaZSM-5. Figure. S3 indicated that the shape of Au-NPs was generally spherical

and that they were distributed better on Silicalite-1 and formed clusters on CaZSM-5. The size distributions were also

provided, showing that the size of Au-NPs on Silicalite-1 and CaZSM-5 was in the range of 6-18 nm.

Section S4. The computer simulation method

All the ab initio calculations were performed with the Gaussian09 package.s4 The Density Functional Theory level

method B3lYP with basis set of 6-31G(d,P) was employed.s5, s6 Geometry optimizations are first performed in the absence of

electric field for benzaldehyde and -pentanol, respectively. Then a finite field at various sizes was added along a specific axis

and all the geometries were fully relaxed in the presence of an electric field until a tight convergence reached.

Section S5. Polarized electrostatic fields (PEF) of zeolites

S5.1 Calculation of the PEF of cations in zeolites

Several reported values of the PEF of ion-exchanged ZSM-5 were given in Table S2, and two methods were used to

estimate the PEF of the other cation-treated ZSM-5.

The first method was based on the low-temperature CO adsorption bands:s7

Equation (1):E = 6.1764 × 10-4·v(CO)2 – 2.4976·v(CO) + 2516, in which:

The CO frequencies (v(CO)) was in wavenumber (for free CO, it was at 2143 cm-1)

The second method was based on the equation reported in Rybakov and coworkers study:s8

Equation (2):E = q/rn, in which:

q is the charge, for Na+, q=1, for Ca2+, q=2, and for La3+, q =3;

r is the distance from the site to the point charge q (here the radius of cation was used for simplification);

n is a factor that can be calculated from a reported zeolite. For example, NaZSM-5 is used for calculating the n factor of

the monovalent cations, and BaZSM-5 for the divalent cations. (http://boomeria.org/chemlectures/textass2/firstsemass.html).

p5

Table S2. Reported and calculated polarized electrostatic field (PEF) strength of ion-exchanged ZSM-5.

MZSM-5 Reported

PEF (V/nm)

Calculated PEF from

Equation (1) (V/nm)

v(CO)

cm-1

Calculated PEF from

Equation (2) (V/nm)

Ion Radius

(Å)

HZSM-5 3.0 s9

3.8 2170 s11

unsuitable s14

0.53

LiZSM-5 7.2 2188 s11

8.1 0.68

NaZSM-5 6.2 s9

5.3 2178 s11

6.2 0.95

KZSM-5 3.1 2166 s11

4.9 1.33

MgZSM-5 11.9 0.65

CaZSM-5 8.3 2193 s12

9.1 0.99

SrZSM-5 7.7 2190 s12

8.3 1.13

BaZSM-5 7.4 s10

7.0 2187 s12

7.4 1.35

La(III)ZSM-5 10.7 1.06

Ce(III)ZSM-5 10.9 1.03

Pd(III)ZSM-5 14.7 2221 s13

14.7 0.64

S5.2. Influential factors on the PEF of cations in zeolites

The data of PEF listed in the above Table S2 might not be absolutely precise; however, they could qualitatively reflect

the tendency of the PEF in a given zeolite host. According to the literature reports, four general conclusions can be

summarized (to be clear, these tendencies apply to a single cation in zeolite pores, namely, even though a cation has a strong

PEF, it does not necessarily mean that the net PEF within the whole zeolite host is strong because the content of cations may

be very low, depending on the Si/Al ratio).

Point 1: the PEF increases with increasing cation charges, for instance, ECaZSM-5 > ENaZSM-5. It is reported that divalent

cations generally have stronger electrostatic fields than monovalent cations because the monovalent cations, e.g. H+, or Na+

balanced the negatively charged Al sites one to one. As for divalent cations, Kazansky and co-workers propose a model for

localization of bivalent cations of zinc cations (Zn2+) in high silica zeolites with distantly placed Al atoms in the framework.

The model indicates that some of the bivalent cations Mg2+ and Ca2+ can be localized at the isolated singly negatively

charged Al occupied oxygen tetrahedra. In parallel the equivalent number of the isolated framework Al atoms remains

without compensating protons or bivalent cations, and their negative charges are compensated indirectly by coulomb

interaction with the surrounding excessively charged cationic sites. The partially compensated positive electric charges

possess unusually high chemical activity.s15, s16

Point 2: the PEF increases with increasing the charge/radius ratio for a given zeolite, for instance, EMg/ZSM-5 > ECaZSM-

5 > EBaZSM-5. Xu and co-workers find that the electrostatic field of alkaline-earth cation increases with decreasing cation

radius, which results in an increased polarization of adsorbed oxygen in the order BaY < SrY < CaY. According to this

conclusion, the electrostatic field strength of CaZSM-5 should be larger than BaZSM-5 (7.4 V/nm, reported) with other

conditions be identical. Therefore, the calculated value 8.3 V/nm is reasonable.s17, s18

Point 3: the PEF increases with increasing Si/Al ratios. The correlation between the Si/Al ratio and the PEF of zeolites

has been investigated by measurements of the heats of immersion into various organic liquids as for Na- and Ca-exchanged

zeolites. The PEF of Na-form zeolites is not much affected by the Si/Al ratio. However, as for Ca-exchanged zeolites, raising

Si/Al ratio increases the PEF, probably because univalent cations like Na+, the number of the cation and the number of the

site that form the electrostatic field decrease with an increase in the Si/Al ratio, the net PEF becomes smaller. On the other

hand, in the case of bivalent cations like Ca2+, parts of cation sites in the large cavity become to be positively charged and

others to be negatively charged with an increase in the Si/Al ratio. The polarization thus occurs, and a strong net electrostatic

p6

field is formed. This tendency seems remarkable when the Si/Al ratio is high. Nevertheless, if the Al sites are too few, e.g.

Silicalite-1 has no Al sites; the net electrostatic field strength decreases dramatically.s7,s19-s21

Therefore it is easy to understand that the electrostatic field strength of rare-earth ions of La3+ or Ce3+ is generally higher

than that of alkaline-earth ions of Ca2+or Ba2+, but the total quantity of La3+ ions is quite low because one La3+ ion

corresponds to three Na+ ions, and one and a half Ca2+ ions, thus the net electrostatic strength of the whole zeolite particle

may be not strong. To illustrate, if a zeolite had six negatively-charged sites, it required six H+/Na+ cations, or three Ca2+

cations, or two La3+ cations to compensate/balance the framework negative charges (please see the illustration below,

Scheme s1). The change from H+, Na+, Ca2+, to La3+ is not only in valences (cation charges) and strength of PEF, but also in

the number of cations (cation density), and cation/radius ratio. Therefore, the net effect of enhancement on boosting the

catalytic performance of a zeolite should be a synergetic effect of these factors. Even though the original PEF of La3+ (10.7

V/nm) is higher than that of Ca2+ (8.3 V/nm), the number of La3+ on ZSM-5 is smaller, thus leading to a lower net

enhancement effect over a given zeolite catalyst.

Scheme s1: Illustration of the compensation relationship between negatively-charged framework sites and extra-framework cations of Na+,

Ca2+

, and La3+

.

Point 4: The PEF is affected by the location cations. ZSM-5 with is characterized by a 2D pore system with straight,

parallel channels intersected by zigzag channels (the detailed 3D structure can be found on the website: http://www.iza-

structure.org/databases). Both channels consist of 10-membered rings with a diameter of 0.53 × 0.56 nm (straight channels)

and 0.51 × 0.5 nm (sinusoidal channels). IR studies of H2 and CO adsorption indicate that the cations are preferentially

located at the intersections of the sinusoidal and straight channels at the edge of a four-membered ring (site I). Additionally,

small cations, like Na+ ions are also found in small cavities above the sinusoidal channel (site II), but MAS-NMR studies

showed a low occupancy of this site (10%).s22,s23

Section S6. XPS analyses of the gold content, the Si/Al ratio, and the cation contents

The XPS results of the gold content, the Si/Al ratio, and the cation contents were given in Table S3. The MZSM-5 used

in the experiment might not have a 100% ion-exchange and might contain residual Na+, or H+ ions, but these residual ions

were negligible and undetectable. The Si/Al ratios of MZSM-5 and Au/MZSM-5 were generally remained stable and the

average value of Si/Al ratio was 13.95 for MZSM-5, and 14.00 for Au/MZSM-5. Therefore, the influence of Si/Al ratio on

the electric field strength of zeolites could be ruled out. The gold contents on MZSM-5 were similar, with the average value

Si

O O

o

O O

o Al

O

O

Si

O

O

Al

O

O

Si

O

O

Al

O

O

Si

O

O

Al

O

O

Si

O

O

Al

O

O

Si

O

O

Al

O

O

Si

O

O

Na+

Na+

Na+

Na+

Na+

Na+

Si

O O

o

O O

o Al

O

O

Si

O

O

Al

O

O

Si

O

O

Al

O

O

Si

O

O

Al

O

O

Si

O

O

Al

O

O

Si

O

O

Al

O

O

Si

O

O

Ca2+

Ca2+

Ca2+

Si

O O

o

O O

o Al

O

O

Si

O

O

Al

O

O

Si

O

O

Al

O

O

Si

O

O

Al

O

O

Si

O

O

Al

O

O

Si

O

O

Al

O

O

Si

O

O

La3+

La3+

Radii of La3+

0.106 nm

Radii of Ca2+

0.099 nm

Radii of Na+

0.095 nm (-) (-) (-) (-) (-) (-)

(-) (-) (-) (-) (-) (-)

(-) (-) (-) (-) (-) (-)

p7

being 2.19%. Likewise, the influence of the content of gold was negligible. Therefore, the primary influential factor on the

electric field strength boiled down to the extra-framework cations: charges, charge/radius ratio, and density (amount).

Table S3. The gold content on MZSM-5, Si/Al ratio of catalysts, and the content of cations on catalysts detected or calculated from the XPS

results.

Catalyst Au (wt%) Si/Al (Ratio) Cation (At%)

HZSM-5 --a 12.94 --

NaZSM-5 -- 13.32 (Na) 0.85 (Na+/Al = 0.47)

CaZSM-5 -- 14.94 (Ca) 0.41 (Ca2+

/Al = 0.23)

LaZSM-5 -- 14.66 (La) 0.17 (La3+

/Al = 0.10)

Slicalite-1 -- -- --

Au/HZSM-5 2.09 15.26 --

Au/NaZSM-5 2.40 12.70 (Na) 0.76 (Na+/Al = 0.43)

Au/CaZSM-5 2.28 14.89 (Ca) 0.38 (Ca2+

/Al = 0.25)

Au/LaZSM-5 1.98 13.16 (La) 0.15 (La3+

/Al = 0.09)

Au/Silicalite-1 2.13 -- --

a: -- refers to “undetected”

Section S7. Reusability of the catalyst CaZSM-5 and Au/CaZSM-5

The reusability of the CaZSM-5 and Au/CaZSM-5 for the acetalization between benzaldehyde and 1-pentanol was

conducted by recycling the catalyst solids after reaction and then washed thoroughly with deionized water. The recycled

catalysts were dried overnight at 60°C in a vacuum oven and used as such. These two catalysts showed quite slight changes

in the activity; meanwhile, the selectivity remained excellent (>99%) after five runs. These results indicated that CaZSM-5

and Au/CaZSM-5 were stable and reusable.

1 2 3 4 510

20

30

40

50

60

70

Au/CaZSM-5

CaZSM-5Co

nve

rsio

n (

%)

Run

Figure S4 | Test of the reusability of CaZSM-5 and Au/CaZSM-5 in the acetalization reaction between benzaldehyde and 1-pentanol after five

runs.

Section S8. The acetalization between cyclohexanone and methanol

Typically, 1.0 g of cyclohexanone was mixed with 10 mL of methanol, and then 0.05 g of catalyst was added into this

mixture. The reaction temperature was controlled at 40oC in an oil bath. The results were listed in Table S4 and S5.

With light on, the conversion on Au/NaZSM-5, Au/CaZSM-5, and Au/LaZSM-5 improved considerably compared with

their counterparts without Au-NPs (Table S4). The boosting mechanism should resemble the aforementioned acetalization

reaction, as the DFT calculation clearly demonstrated that the C=O bond of cyclohexanone (3.25 Debye) could be stretched

p8

by extra electric fields, but the methanol molecule (1.72 Debye) was unaffected. The maximum net conversions also

appeared on Au/CaZSM-5.

Table S4 | Catalytic performances of and the acetalization between cyclohexanone and methanol on MZSM-5 and Au/MZSM-5 (M=H+, Na

+, Ca

2+,

or La3+

) under the irradiation of visible light.

Catalyst OCH3

OCH3CH3OHO

Conversiona

(%)

Selectivityb

(%)

Yield

(%)

Reaction rate

(10-3

molg-1

h-1

)

CaZSM-5 20.1 >99 19.9 20.94

Au/CaZSM-5 61.3 >99 60.7 63.85

NaZSM-5 38.9 >99 38.5 40.52

Au/NaZSM-5 56.9 >99 56.3 59.27

LaZSM-5 37.6 >99 37.2 39.16

Au/LaZSM-5 48.2 >99 47.7 50.21

HZSM-5 52.3 >99 51.8 54.48

Au/HZSM-5 53.6 >99 52.1 54.78

Silicalite-1 5.2 >99 5.2 5.42

Au/Silicalite-1 7.3 >99 7.2 7.60

a: Conversion refers to the conversion of cyclohexanone.

b: Selectivity refers to the selectivity towards dimethyl acetal (dimethoxycyclohexane).

Reaction conditions: cyclohexanone (1.0 g), methanol (10 mL), temperature, (40oC), time (6h), light intensity (0.505 W/cm

2), wavelength range

(420-800 nm); catalyst (0.05 g), atmosphere (air). No reaction was detectable without catalysts.

With light off, the conversion on Au/HZSM-5, Au/NaZSM-5, Au/CaZSM-5, and Au/LaZSM-5 did not improve much

compared with their counterparts without Au-NPs (Table S5).

Table S5 | Catalytic performances for the acetalization of cyclohexanone with methanol (ACM) on MZSM-5 and Au/MZSM-5 (M=H+, Na

+, Ca

2+, or

La3+

) with light off.

Catalyst Conversiona (%) Selectivity

b (%) Yield (%) Reaction rate (10

-3 molg

-1h

-1)

CaZSM-5 18.4 >99 18.2 19.17

Au/CaZSM-5 17.6 >99 17.4 18.34

NaZSM-5 39.7 >99 39.3 41.35

Au/NaZSM-5 38.3 >99 37.9 39.89

LaZSM-5 38.3 >99 37.9 39.89

Au/LaZSM-5 41.8 >99 41.4 43.54

HZSM-5 51.8 >99 51.3 53.96

Au/HZSM-5 50.9 >99 50.4 53.02

Slicalite-1 3.7 >99 3.7 3.79

Au/silicalite-1 5.8 >99 5.7 6.04

a: Conversion refers to the conversion of cyclohexanone.

b: Selectivity refers to the selectivity towards dimethyl acetal (dimethoxycyclohexane).

Reaction conditions: cyclohexanone (1.0 g) methanol (10 mL); catalyst (0.05 g); atmosphere (air), temperature (40oC); time (6 h). No reaction was

detectable without catalysts.

p9

0 2 4 6 8 10 121.215

1.220

1.225

1.230

1.235

1.240

1.245 (a)

O

Electric-field Strength (V/nm)

C=

O B

ond L

ength

(Å

)

H Na Ca La

0

10

20

30

40(b)

Cations

Ne

t con

ve

rsio

n (

%)

Figure S5 | (a) The relationship between the C=O bond length (Angstrom) of cyclohexanone and the intensity of extra electrostatic fields

simulated by the density functional theory (DFT) method. (b) The net conversion of the acetalization reaction between cyclohexanone and

methanol was calculated from Table 2 in the main text. The net conversion means the conversion on Au/MZSM-5 deducts the conversion on

MZSM-5 (M=H+, Na

+, Ca

2+, or La

3+).

The DFT calculations demonstrated that the C=O bond length of cyclohexanone extended with increasing the extra

electric field strength. In detail, the C=O bond lengths of benzaldehyde were 1.223 Å, 1.227 Å, 1.232 Å, 1.237 Å, and 1.242

Å, when the electric field strengths were 0 V/nm, 2.58 V/nm, 5.16 V/nm, 7.74 V/nm, and 10.32 V/nm, respectively.. The

relationship between the C=O bond length and the electric field strength was linear (see Figure. S1a). The highest net

conversion appeared on the Ca2+ (see Figure. S1b).

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1401-1407 (1996).

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reactions to produce propylene. J. Catal. 248, 29-37 (2007).

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Gaussian. Inc., Wallingford, CT 2009.

s5. Becke, A. D. Density-functional thermochemistry. III. the role of exact exchange. J. Chem. Phys. 98, 5648-5652 (1993).

s6. Lee, C. & Yang, W. Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 37, 785-

789 (1988).

s7. Panov, A. G., Larsen, R. G., Totah, N. I., Larsen, S. C. & Grassian, V. H. Photooxidation of toluene and p-xylene in cation-exchanged zeolites X, Y, ZSM-5,

and Beta: the role of zeolite physicochemical properties in product yield and selectivity. J. Phys. Chem. B 104, 5706-5714 (2000).

s8. Rybakov, A. A., Larin, A. V. & Zhidomirov, G. M. Computational differentiation of Brønsted acidity induced by alkaline earth or rare earth cations in

zeolites. Inorg. Chem. 51, 12165-12175 (2012).

s9. Dunne, J. A., Rao, M., Sircar, S., Gorte, R. J. & Myers, A. L. Calorimetric heats of adsorption and adsorption isotherms. 2. O2, N2, Ar, CO2, CH4, C2H6, and

SF6 on NaX, H-ZSM-5, and Na-ZSM-5 zeolites. Langmuir 12, 5896-5904 (1996).

s10. Xiang, Y., Larsen, S. C. & Grassian, V. H. Photooxidation of 1-alkenes in zeolites: a study of the factors that influence product selectivity and formation. J.

Am. Chem. Soc. 121, 5063-5072 (1999).

s11. Zecchina, A., Bordiga, S., Lamberti, C., Spoto, G. & Carnelli, L. Low-temperature Fourier transform infrared study of the interaction of CO with cations in

alkali-metal exchanged ZSM-5 zeolites. J. Phys. Chem. 98, 9577-9582 (1994).

s12. Kamble, V. S. & Gupta, N. M. On the role of balancing cations in the entrapment of CO in ZSM-5 zeolite at room temperature: FTIR study. Phys. Chem.

Chem. Phys. 2, 2661-2665 (2000).

s13. Chakarova, K., Ivanova, E., Hadjiivanov, K., Klissurski, D. & Knözinger, H. Co-ordination chemistry of palladium cations in Pd-H-ZSM-5 as revealed by

FTIR spectra of adsorbed and co-adsorbed probe molecules (CO and NO). Phys. Chem. Chem. Phys. 6, 3702-3709 (2004).

s14. According to Ref. s11, there is a strong interaction between H+ and the framework of ZSM-5 because hydrogen is partially covalently bonded to the

framework oxygen. As a consequence, only a part of the H+ positive charge is available, and the electric field around the proton is consequently reduced.

s15. Kazansky, V. & Serykh, A. A new charge alternating model of localization of bivalent cations in high silica zeolites with distantly placed aluminum atoms

in the framework. Micropor. Mesopor Mater. 70, 151-154 (2004).

s16. Xu, J., Mojet, B. L. & Lefferts, L. Effect of zeolite geometry for propane selective oxidation on cation electrostatic field of Ca2+ exchanged zeolites.

Micropor. Mesopor. Mater. 91, 187-195 (2006).

s17. Xu, J., Mojet, B. L., van Ommen, J. G. & Lefferts, L. Propane selective oxidation on alkaline earth exchanged zeolite Y: room temperature in situ IR study.

Phys. Chem. Chem. Phys. 5, 4407-4413 (2003).

p10

s18. Xu, J., van Ommen, J. G., Mojet, B. L. & Lefferts, L. Formation of M2+(O2)(C3H8) species in alkaline-earth-exchanged Y zeolite during propane selective

oxidation. J. Phys. Chem. B 109, 18361-18368 (2005).

s19. Tsutsumi, K. & Takahashi, H. A study of the nature of active sites on zeolites by the measurement of heat of immersion. II. Effects of silica/alumina ratio to

electrostatic-field strength of calcium-exchanged zeolites. J. Phys. Chem. 76, 110-115 (1972).

s20. Xiang, Y., Larsen, S. C. & Grassian, V. H. Photooxidation of 1-alkenes in zeolites: a study of the factors that influence product selectivity and formation. J.

Am. Chem. Soc. 121, 5063-5072 (1999).

s21. Rybakov, A. A., Larin, A. V. & Zhidomirov, G. M. Computational differentiation of Brønsted acidity induced by alkaline earth or rare earth cations in

zeolites. Inorg. Chem. 51, 12165-12175 (2012).

s22. Tsutsumi, K. & Takahashi, H. A study of the nature of active sites on zeolites by the measurement of heat of immersion. II. effects of silica/alumina ratio to

electrostatic-field Strength of calcium-exchanged zeolites. J. Phys. Chem. 76, 110-115 (1972).

s23. Xu, J., Mojet, B. L., van Ommen, J. G. & Lefferts, L. Effect of Ca2+ position in zeolite Y on selective oxidation of propane at room temperature. J. Phys.

Chem. B 108, 15728-15734 (2004).

85

5.2 Acetalisation reaction on zeolite catalysts – Efficient catalysts of

zeolite nanocrystals grown with a preferred orientation on nanofibres

Introductory remarks

This chapter reports an innovative structure—nanozeolites (as shell) grown with a

preferred orientation on ceramic nanofibres (as core). The Y-zeolite nanocrystals on

TiO2 nanofibres exhibited superior ability to catalyse acetalisation and

transesterification reactions, achieving high conversions to desired products with

selectivity of 100% under moderate conditions. This research was published in the

journal of ―Chemical communication‖[1]

.

Zeolites are important industrial catalysts that have been extensively applied in

the manufacture of fine chemicals and petrochemicals[2]

. The diffusion of reactants

within zeolite pores and the accessibility of active centres of zeolites determine the

catalytic performance of zeolite catalysts. The intra-crystalline diffusion of reactants,

intermediates, and products often adversely affects the performance when large

zeolite particles are used, especially in liquid-phase catalytic systems. Therefore,

zeolite nanocrystals have obvious advantages because more active sites can be

accessed by reactant molecules and the diffusion pathways are short[3]

. However, it is

difficult to synthesise discrete zeolite nanocrystals without aggregation, which

necessitates new synthetic methods to be developed to solve this problem.

This study demonstrated that the zeolite Y nanocrystals could be grafted on TiO2

nanofibres without aggregation[4, 5]

and that the zeolite@NFs composites exhibited

superior catalytic performance in the acetalisation reaction of cyclohexanone with

86

methanol and the transesterification of dimethyl carbonate with alcohols compared

with bulky zeolite particles and TiO2 nanofibres. Moreover, the orientation of the

zeolite nanocrystals with respect to the TiO2 substrate and the openings of the

nanozeolite micropore systems were certain, namely, with a preferred orientation

which was confirmed by the HRTEM analyses, so that the active sites of zeolites

were exposed to the reactants with a certain orientation. TiO2 NFs have a large aspect

ratio and a large external surface area[6]

, and can be easily dispersed and recovered

from in a liquid-phase system. All these advantages demonstrate that zeolite@NFs

structures are promising catalysts for the acetalisation and transesterification

reactions.

Although this study involves no plasmonic effects on enhancing the catalytic

performance, it does help further understand the influential factors, such as the size

of zeolite nanocrystals, on the acetalisation reactions and provides guidance to design

new (plasmonic) photocatalysts for the acetalisation reactions. Moreover, because of

the catalytic capacity of pure TiO2 (without zeolite Y nanocrystals) to catalyse the

transesterification of dimethyl carbonate with alcohols, we have done further work

on the transesterification on TiO2 nanofibres with different crystal phases, and the

results can be seen in the "Appendix – Transesterification reaction‖[7]

.

References

1. X. B. Ke, X. G. Zhang, H. W. Liu, S. Xue and H. Y. Zhu, ―Efficient catalysts of

zeolite nanocrystals grown with a preferred orientation on nanofibres,‖ Chem.

Commun. 49, 9866-9868 (2013).

2. B. Smit, T. L. M. Maesen, ―Towards a molecular understanding of shape

selectivity,‖ Nature 451, 671-678 (2008).

87

3. K. Varoon, X. Y. Zhang, B. Elyassi, D. D. Brewer, M. G. S.; Kumar, J. A. Lee, S.

Maheshwari, A. Mittal, C. Y. Sung, M. Cococcioni, L. F. Francis, A. V.

McCormick, K. A. Mkhoyan, M. Tsapatsis, ―Dispersible exfoliated zeolite

nanosheets and their application as a selective membrane,‖ Science 334, 72-75

(2011).

4. M. Iwamoto, Y. Tanaka, N. Sawamura, and S. Namba, ―Remarkable effect of

pore size on the catalytic activity of mesoporous silica for the acetalisation of

cyclohexanone with methanol,‖ J. Am. Chem. Soc. 125, 13032-13033 (2003).

5. P. Tundo, and M. Selva, ―The chemistry of dimethyl carbonate ,‖ Acc. Chem

Res. 35, 706-716 (2002).

6. H. Y. Zhu, X. P. Gao, Y. Lan, D. Y. Song, Y. X. Xi, and J. Cai. Zhao, ―Hydrogen

titanate nanofibres covered with anatase nanocrystals:  a delicate structure

achieved by the wet chemistry reaction of the titanate nanofibres,‖ J. Am. Chem.

Soc. 126, 8380-8381 (2004).

7. X.G. Zhang, X.B. Ke, Z.F. Zheng, H.W. Liu, and H.Y. Zhu, ―TiO2 nanofibres of

different crystal phases for transesterification of alcohols with dimethyl

carbonate,‖ Appl. Catal. B 150-151 330-337 (2014).

88

Statement of contribution

1. The authors listed below have certified that:

2. They meet the criteria for authorship in that they have participated in the

conception, execution, or interpretation, of at least that part of the publication in

their field of expertise;

3. They have public responsibility for their part of the publication, except for the

responsible author who accepts overall responsibility for the publication;

4. There are no other authors of the publication according to these criteria;

5. Potential conflicts of interest have been disclosed to (a) granting bodies, (b) the

editor or publisher of journals or other publications, and (c) the head of the

responsible academic unit, and

6. They agree to the use of the publication in the student’s thesis and its publication

on the Australian Digital Thesis database consistent with any limitations set by

publisher requirements.

In the case of this chapter:

Efficient catalysts of zeolite nanocrystals grown with a preferred orientation on

nanofibres

Xuebin Ke, Xingguang Zhang, Hongwei Liu, Song Xue, and Huaiyong Zhu

Published in the journal: Chemical Communications, 2013, 49, 9866-9868

96

6 Conclusions

In this Ph.D thesis, several new applications of plasmonic photocatalysts of

supported Au-NPs have been found in the selective reduction, the selective

oxidation, and the acetalisation reaction under visible light irradiation, and a

theoretical outline has been established on the basis of the LSPR effect and the

concurrently induced electric near-field enhancement of plasmonic nanostructures.

In the selective reduction (Article 1: Selective reductions using visible light

photocatalysts of supported gold nanoparticles), four reduction reactions:

nitroaromatics to azo compounds, azobenzene to hydroazobenzene, ketones to

alcohols, and epoxides to alkenes, proceed efficiently on the Au/CeO2 photocatalysts

under visible light irradiation or simulated sunlight. The reduction power of the

photocatalysts can be manipulated by the light wavelength. Irradiation with shorter

wavelengths can induce a more negative reduction by creating a more negative

potential. The efficiency of the photocatalytic reductions increases with the

increasing light intensity under ambient conditions. The kinetic study shows that the

activation energy can be significantly reduced under visible light irradiation

compared with that in the thermal-driven processes. The photocatalytic reduction on

Au/CeO2 follows a mechanism distinctly different from that on the semiconductor

photocatalysts and provides insights into the formation of Au-H species and their

interaction with the reactants. Such photocatalytic syntheses are sustainable and

clean as these production processes have the potential to utilise sunlight for chemical

synthesis.

In the selective oxidation (Article 2: Zeolite-supported gold nanoparticles for

selective photooxidation of aromatic alcohols under visible-light irradiation),

97

photooxidation of aromatic alcohols can proceed well on Au/zeolite catalysts with

high selectivity towards aldehydes under visible light irradiation at ambient

temperature. Experimental results show that the catalytic activities of Au/zeolite

photocatalysts are influenced by the adsorptive properties of zeolites, the particle size

and specific surface areas of Au-NPs, and the molecular polarity of aromatic

alcohols. The kinetic study shows that the visible light irradiation significantly

reduces the activation energy of the photooxidation reaction compared with the

thermal-driven reaction. The role of molecular oxygen is the hydrogen remover from

the gold surfaces. Finally, a possible mechanism based on the LSPR effect of Au-

NPs and the adsorptivity of zeolite supports is proposed. This study illustrates the

potential of Au/zeolite photocatalysts for the selective oxidation reaction driven by

sunlight at ambient temperature. The mechanistic study can provide practical

guidelines for development of new catalytic materials for ―green‖ and energy-saving

chemical processes.

The study on the acetalisation reaction unravels a cascade enhancement effect

from visible light to zeolite catalysts bridged via the electric near-field enhancement

of Au-NPs (Article 3: Plasmonic nanostructures to enhance catalytic performance of

zeolites under visible light). The polarised electrostatic fields of extra-cations in

zeolites can be further intensified by the electric near-field enhancement of Au-NP,

and the intensified polarised electrostatic fields can stretch the C=O bond of the

reactants of benzaldehyde and cyclohexanone to enlarge their molecular polarities,

thus allowing them to be activated more efficiently by catalytic centres of H+ to

boost the reaction rate. This enhancement effect offers promise in modifying the

molecular polarities which affect the activity and electrostatic behaviour of

molecules and have broad applications in organic synthesis, ionic liquids,

98

nanofiltration membranes, molecular electronics, and molecular self-assembly

processes through non-covalent interactions.

Overall, the findings in this thesis demonstrate that plasmonic photocatalysts

based on plasmonic (gold) nanostructures have great potential in the selective

reduction, the selective oxidation, and the acetalisation owing to their singular

characteristics of the LSPR effect and the electric near-field enhancement around the

particle surfaces. Considering various plasmonic metals or alloys, and numerous

catalytic supports, such as zeoiltes (213 framework types, IZA), other solid-acid

catalysts, semiconductor photocatalysts and metal/acid bifunctional catalysts, the

findings in this research thesis may inspire scientists to explore more applications of

plasmonic photocatalysts in sensing, solar panels, solar water splitting,

photochemical synthesis, drug delivery, industrial processes of cracking and

isomerisation, and environmental remediation.

99

7 Limitations and future work

7.1 Limitations and possible solutions

Even though considerable achievements have been made on plasmonic

photocatalysts, there still exist limitations and knowledge gaps in the field of

plasmonic photocatalysis, and more work needs to be done in the future to improve

the photocatalytic performance and to clarify the photocatalytic mechanism.

As for plasmonic gold photocatalysts, though significant progress has been

made, to fully unravel the LSPR effect and the induced electric near-field

enhancement, several challenges need to be addressed: (i) the electron transfer

processes are still ambiguous and under debate. Scientists are uncertain whether the

electron transfer is from metals to supports and then to reactants, or from metals to

reactants directly, or whether the interaction between catalysts and reactants is

through electron transfer or merely excitation of reactants by electron clouds without

electron migration; (ii) the measurement of the contribution of heat released by

energetic electrons that return to ground states is still challenging. The excited free

conduction electrons will release heat when they return to the ground states, and this

plasmonic photothermal effect can usually cause the temperature to increase around

metal NPs. Because the reaction rate constant exponentially depends on reaction

temperature, the plasmon-excitation-induced heat may also accelerate chemical

reactions. How to measure the contribution of heat on the surface of metal

nanocrystals remains a great challenge; and (iii) obviously the cost of plasmonic

metals is still high. The practical application of plasmonic photocatalysts is a big

issue, because the plasmonic photocatalysts heavily depend on the use of gold or

silver nanostructures. The aforementioned problems (i) and (ii) are quite difficult at

100

present, scientists are struggling to develop new techniques to detect and quantify the

electron or heat transferred in reactions. A feasible approach to measure the electron

transfer is reported in Wang and co-workers’ study[1]

; they use tetracyanoethylene

(TCNE), which is a strong electron abstractor and changes its colour after grabbing

electrons, to quantify the electrons released from the molybdenum oxides (MoOx)

supports to evaluate the catalytic activity of Au/MoOx in the oxidation of aromatic

alcohols. For the problem (iii), the practical applications of plasmonic photocatalysts

in chemical reactions necessitate inexpensive and earth-abundant metals, such as iron,

aluminium and copper. Therefore, developing photocalalysts with alloy structures

will be promising. Moreover, it is also meaningful to explore non-metal

nanomaterials that can exhibit the LSPR effect, such as indium tin oxide[2]

, tungsten

oxide[3]

and copper chalcogenide[4, 5]

. Exploration of these non-metal nanomaterials

for plasmon-enhanced chemical reactions will be interesting and worthwhile.

As for zeolite catalysts, there are also several problems for solid-acid catalysed

reactions: (i) the high operating temperature due to the large activation barriers of

reactions. To address this problem, I speculate that if the traditional reactions (e.g.,

acetalisation) could be achieved in a photocatalytic way, better catalytic performance

or low reaction temperature can be expected; (ii) the polar products/intermediates are

strongly bound to the zeolite catalysts, impeding rapid desorption of them and thus

reducing the reaction rate[6]

. Hence, fast removal of the final products/intermediates

from catalytic centres is essential. To combat this issue, a more detailed

understanding is necessary about how reactant molecules arrange themselves in

zeolite pores, for instance, around the strong electrostatic-field cations to change

their molecular polarities. Moreover, insights into the location/geometry of cation

sites may guide the design of new zeolite-based catalysts[7]

; and (iii) the mass

101

transfer limitations because of the large catalyst particles aggregated after

calcinations. A possible solution is to graft zeolite nanocrystals on a substrate (e.g.,

TiO2 nanofibres, as reported in Article 4: Efficient catalysts of zeolite nanocrystals

grown with a preferred orientation on nanofibres) to alleviate the aggregation

problem[8]

.

7.2 Future of work

The LSPR effect and the electric near-field enhancement of plasmonic

nanostructures allow for development of novel plasmonic photocatalysts that can

take advantage of visible light and show better catalytic performance than these

photocatalysts perform in reactions driven only by thermal energy under the same

conditions. More efforts need to be made on the basis of the above-mentioned

―limitations and possible solutions‖. Given the importance and phenomenal progress

that has been achieved, development of novel plasmonic photocatalysts based on

―plasmonic metal (Au or Ag) nanostructures modified traditional catalysts‖ should

be promising. To be specific, the following topics can be developed in the future:

(i) Plasmonic metal (Au or Ag) nanostructures to modify traditional TiO2-based

photocatalysts for water splitting[9]

. Production of hydrogen from water in a

photocatalytic way is a promising technology and has long been a hot topic.

However, the photo efficiency of traditional TiO2 photocatalysts is still low,

scientists are striving to modify photoelectrochemcial properties of these

conventional photocatalysts, for instance, by narrowing the band gaps, but this

decreases the driving force of a photocatalyst for redox reactions. Meanwhile,

scientists are making great efforts to develop novel photocatalytic materials, such as

porous LaTiO2N[10]

, GaN:ZnO[11]

and Ta3N5[12]

, for their enhancement of the

electrode conductivity. Recently (2011-present), the contribution of the LSPR effect

102

to enhancing water splitting receives huge attention, and photocatalysts based on

plasmonic metals include Au/TiO2[13]

, Au/N-TiO2[14]

, Au/Fe2O3[15]

, Au/ZnO2[16]

,

Au/ZnFe2O4/ZnO[17]

and Au/ZnO-CdTe[18]

. However, the light-harvesting capacity,

the enhancement effect, and the oxidation activity are still limited, probably owing to

the inefficient electronic contact between electrodes and reactants. Therefore, there is

great demand for developing new photocatlytic materials based on plasmonic

nanostructures or techniques to address those problems.

(ii) Plasmonic metal (Au or Ag) nanostructures to modify traditional zeolite-based

solid acid catalysts for the selective oxidation of hydrocarbons[19]

, alkylation of

alcohols, methylation of phenols, and carbonylation of aldehydes. I will explore

whether the dipole oscillators of Au(δ+-δ-)

can further polarise reactants to affect its

adsorption or activation to boost the reaction rate; whether the electric near-field

enhancement can function as a ―relay‖ to affect the polarised electrostatic fields of

cations in zeolites to intensify them and then to affect the photocatalytic performance

of zeolites; and whether the molecular polarities and orientations of reactions can be

controlled at moderate conditions in the zeolite pores.

(iii) Plasmonic metal (Au or Ag) nanostructures to modify traditional

metal/acid bifunctional catalysts for the isomerisation of n-paraffins in industrial

catalysis[20]

. In refineries, skeletal isomerisation of n-paraffins to give branched

isomers is an important process to enhance the octane number in the gasoline pool.

The isomerisation of C5/C6 has been used as a commercial approach to light

isoparaffins, whereas the isomerisation of C7-C9 n-paraffins remains challenging

because of concurrent cracking. The isomerisation of n-heptane usually serves as a

model on the bifunctional catalyst of Pt/Hbeta or Pt/HY, on which the mechanism

involves Pt-catalysed dehydrogenation of alkane to alkene and reversible

103

hydrogenation of branched alkenes to alkanes, and the acidity of the zeolites

promotes protonation of the intermediate alkenes and subsequent rearrangement to

form branched alkenes (the mechanism is still controversial). A big disadvantage of

this process is that n-heptane cracks largely (low selectivity) because the operating

temperature is high (200-300oC). However, the report that the catalytic performance

of epoxidation of ethylene on supported Ag-NPs and propylene on Cu-NPs can be

enhanced by visible light irradiation[21]

inspires me to improve the catalytic

performance of the isomerisation of n-heptane. Alloy structures of Au-Pt or Ag-Pt

will be supported on zeolites; I wonder whether the reaction temperature can be

lowered by using these plasmonic alloy nanostructures under the visible-light

irradiation.

References

1. F. Wang, W. Ueda, and J. Xu, ―Detection and measurement of surface electron

transfer on reduced molybdenum oxides (MoOx) and catalytic activities of

Au/MoOx,‖ Angew. Chem. Int. Ed. 51, 3883-3887 (2012).

2. M. Kanehara, H. Koike, T. Yoshinaga, and T. Teranishi, ―Indium tin oxide

nanoparticles with compositionally tunable surface plasmon resonance

frequencies in the near-IR region,‖ J. Am. Chem. Soc. 131, 17736-17737 (2009).

3. K. Manthiram, and A. P. Alivisatos, ―Tunable localised surface plasmon

resonances in tungsten oxide nanocrystals,‖ J. Am. Chem. Soc. 134, 3995-3998

(2012).

4. D. Dorfs, T. Härtling, K. Miszta, N. C. Bigall, M. R. Kim, A. Genovese, A.

Falqui, M. Povia, and L. Manna, ―Reversible tunability of the near-infrared

valence band plasmon resonance in Cu2-xSe nanocrystals,‖ J. Am. Chem. Soc.

133, 11175-11180 (2011).

104

5. S.-W. Hsu, K. On, and A. R. Tao, ―Localised surface plasmon resonances of

anisotropic semiconductor nanocrystals,‖ J. Am. Chem. Soc. 133, 19072-19075

(2011).

6. J. Xu, B. L. Mojet, J. G. van Ommen, and L. Lefferts, ―Effect of Ca2+

position in

zeolite Y on selective oxidation of propane at room temperature,‖ J. Phys.

Chem. B 108, 15728-15734 (2004).

7. J. Dědeček, Z. Sobalík, and B. Wichterlová, ―Siting and distribution of

framework aluminium atoms in silicon-rich zeolites and impact on catalysis,‖

Catal. Rev. Sci. Eng. 54, 135-223 (2012).

8. T. Cao T. Pham, H. S. Kim, and K. B. Yoon, ―Growth of uniformly oriented

silica MFI and BEA zeolite films on substrates,‖ Science, 334, 1533-1538

(2011).

9. A. Fujishima, and K. Honder, ―Electrochemical photolysis of water at a

semiconductor electrode,‖ Nature, 238, 37-38 (1972).

10. F. X. Zhang, A. Yamakata, K. Maeda, Y. Moriya, T. Takata, J. Kubota, K.

Teshima, S. Oishi, and K. Domen, ―Cobalt-modified porous single-crystalline

LaTiO2N for highly efficient water oxidation under visible light,‖ J. Am. Chem.

Soc. 134, 8348-8351 (2012).

11. K. Maeda, A. K. Xiong, T. Yoshinaga, T. Ikeda, N. Sakamoto, T. Hisatomi, M.

Takashima, D. L. Lu, M. Kanehara, T. Setoyama, T. Teranishi, and K. Domen,

―Photocatalytic overall water splitting promoted by two different co-catalysts for

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12. Y. B. Li, T. Takata, D. Cha, K. Takanabe, T. Minegishi, J. Kubota, and K.

Domen, ―Vertically Aligned Ta3N5 nanorod arrays for solar-driven

photoelectrochemical water splitting,‖ Adv. Mater. 25, 125-131 (2013).

13. Z. W. Liu, W. B. Hou, P. Pavaskar, M. Aykol, and S. B. Cronin, ―Plasmon

resonant enhancement of photocatalytic water splitting under visible

illumination,‖ Nano Lett. 11, 1111-1116 (2011).

14. D. B. Ingram, and S. Linic, ―Water splitting on composite plasmonic-

metal/semiconductor photoelectrodes: evidence for selective plasmon-induced

formation of charge carriers near the semiconductor surface,‖ J. Am. Chem. Soc.

133, 5202-5205 (2011).

15. E. Thimsen, F. L. Formal, M. Grätzel, and S. C. Warren, ―Influence of

plasmonic Au nanoparticles on the photoactivity of Fe2O3 electrodes for water

splitting,‖ Nano Lett. 11, 35-43 (2011).

16. H. M. Chen, C. K. Chen, C.-J. Chen, L.-C. Cheng, P. C. Wu, B. H. Cheng, Y. Z.

Ho, M. L. Tseng, Y.-Y. Hsu, T.-S. Chan, J.-F. Lee, R.-S. Liu, and D. P. Tsai,

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(2012).

17. A. Sheikh, A. Yengantiwar, M. Deo, S. Kelkar, and S. Ogale, ―Near-field

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system,‖ small 9, 2091-2096 (2013).

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107

Appendix – Transesterification reaction

TiO2 nanofibres of different crystal phases for transesterification of

alcohols with dimethyl carbonate

Introductory remarks

This chapter reports the novel catalysts for the transesterification of dimethyl

carbonate with alcohols using TiO2 nanofibres of different crystal phases. This study

is derived from the report (Article 4: Efficient catalysts of zeolite nanocrystals grown

with a preferred orientation on nanofibres)[1]

, in which the transesterification of

dimethyl carbonate with alcohols was discovered to occur on pure TiO2 nanofibres

without zeolite nanaocrystals. TiO2 crystals have three different crystal structures:

anatase, TiO2(B), and rutile. TiO2 with different crystal structures should have

different catalytic performances or catalytic mechanisms, and thus we dived into

details on the transesterification reaction on TiO2 as a periphery study. This research

was published in the journal of ―Applied Catalysis B: Environmental‖[2]

.

In organic conversions, methylation, carbonylation, and transesterification are

important reactions, for instance, the conversion of alcohols to methyl ethers or

methyl carbonates plays a critical role in producing organic carboxylated

intermediates[3]

. However, the traditional processes heavily hinge on the base-

promoted reactions that involve toxic, hazardous, or corrosive agents or wastes[4]

.

Therefore, new catalysts are highly demanded because of raised severe concerns on

safety and environments.

This study demonstrated that TiO2 nanofibres of different crystal phases could

effectively catalyse the transesterification of alcohols with dimethyl carbonate to

108

produce corresponding methyl carbonates. The catalytic activities correlated with the

crystal phases and adsorption of benzyl alcohol. It is interesting that the light (UV or

visible) irradiation had a negative influence on the catalytic performance of TiO2

catalysts, because partial benzyl alcohol underwent photocatalytic reaction to

produce benzaldehyde[5]

. The kinetic isotope effect (KIE) study identified the rate-

determining step and the reaction pathway. Finally, a new mechanism was proposed

on the basis of the experimental results and literature reports.

Advantages of this catalytic system included excellent selectivity (>99%), general

suitability to alcohols, convenient preparation and reusability of fibrous catalysts.

Moreover, the proposed mechanism is new, compared with those reactions occur on

the traditional zeolite-based catalysts or acid-base catalysts[5, 6]

. More significantly, It

is discovered that TiO2-based catalysts have great potential in non-photo involved

reactions even though TiO2 catalysts have been exclusively investigated as

photocatalysts after the finding of water splitting[7]

.

References

1. X. B. Ke, X. G. Zhang, H. W. Liu, S. Xue, and H. Y. Zhu, ―Efficient catalysts of

zeolite nanocrystals grown with a preferred orientation on nanofibres,‖ Chem.

Commun. 49, 9866-9868 (2013).

2. X. G. Zhang, X. B. Ke, Z. F. Zheng, H. W. Liu, and H. Y. Zhu. ―TiO2 nanofibers

of different crystal phases for transesterification of alcohols with dimethyl

carbonate,‖ Appl. Catal. B 150-151, 330-337 (2014).

3. O. Haba, I. Itakura, M. Ueda, and S. Kuze, ―Synthesis of polycarbonate from

dimethyl carbonate and bisphenol-A through a non-phosgene process,‖ J. Polym.

Sci. Part A: Polym. Chem. 37, 2087-2093 (1999).

109

4. A.-A. G. Shaik, S. Sivaram, ―Organic carbonates,‖ Chem. Rev. 96, 951-976

(1996).

5. M. Zhang, Q. Wang, C. C. Chen, L. Zang, W. H. Ma, and J. C. Zhao, ―Oxygen

atom transfer in the photocatalytic oxidation of alcohols by TiO2: Oxygen isotope

studies,‖ Angew. Chem. 68, 6081-6084 (2009).

6. P. Tundo, and M. Selva, ―The chemistry of dimethyl carbonate ,‖ Acc. Chem

Res. 35, 706-716 (2002).

7. A. Fujishima, and K. Honder, ―Electrochemical photolysis of water at a

semiconductor electrode,‖ Nature 238, 37-38 (1972).

110

Statement of contribution

1. The authors listed below have certified that:

2. They meet the criteria for authorship in that they have participated in the

conception, execution, or interpretation, of at least that part of the publication in

their field of expertise;

3. They have public responsibility for their part of the publication, except for the

responsible author who accepts overall responsibility for the publication;

4. There are no other authors of the publication according to these criteria;

5. Potential conflicts of interest have been disclosed to (a) granting bodies, (b) the

editor or publisher of journals or other publications, and (c) the head of the

responsible academic unit, and

6. They agree to the use of the publication in the student’s thesis and its publication

on the Australian Digital Thesis database consistent with any limitations set by

publisher requirements.

In the case of this chapter:

TiO2 nanofibers of different crystal phases for transesterification of alcohols

with dimethyl carbonate

Xingguang Zhang, Xuebin Ke, Zhanfeng Zheng, Hongwei Liu, and Huaiyong Zhu

Published in the journal: Applied Catalysis B, 2014, 150-151, 330-337