MOLECULAR MECHANICS TO MODEL COAL CHARSTRUCTURES AND DFT TO MODEL THEIR

REACTIVITY WITH CO2 GAS FOR SYNTHETIC GASPRODUCTION

a Coal Research Group, School of Chemical and Minerals Engineering, North-West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom, 2520, South Africa.c John and Willie Leone Family Department of Energy and Mineral Engineering, The EMS Energy Institute, The Pennsylvania State University, University Park, PA 16802, USAd Laboratory of Applied Molecular Modelling, Chemical Resource Beneficiation Focus Area, North-West University, Potchefstroom 2520, South Africae Chemistry Department, La Trobe University, Melbourne, VIC 3086, Australia

Mokone J. Robertsa, Raymond C. Eversona, Hein W. J. P. Neomagusa, Jonathan P. Mathewsc, George Domazetise, Cornelia G.C.E. van Sittertd

CONTENTS

• Background and motivation• Char characterisation • Construction and properties of large-scale molecular

structures of chars using molecular mechanics • Reactivity modelling of chars using quantum mechanics • Modelling of the fundamental char-CO2 reaction mechanism • Conclusions• Acknowledgements

2

BACKGROUND AND MOTIVATION

• The generation of char is generally an important intermediate step in coal conversion processes, e.g., gasification1

• Coal chars can be described on mineral matter free basis– as polyaromatic hydrocarbons (PAHs) with a network structure – in which hetero atoms (O, N and S) are dispersed2

• Exploring the structure of chars at an atomic scale is vital to facilitate understanding of the relationship between char structure and reactivity (with CO2 in this investigation).

1. Sadhukhan 2009.Fuel Processing Technology 90, 692–7002. Chen et al. 2011. Ind. Eng. Chem. Res, 50, 2562–2568

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CHARACTERISATION

TECHNIQUE STRUCTURAL INFORMATION

SPECIFIC INFORMATION

Petrographic analysis on parent coal (PSD =0.3-1.0 mm)

None v% Inertinite and vitrinite

Standard analysis Bulk properties wt% C H O N S

Density measurements Physical properties Helium density

XRD Structural ordering of carbons

% Aromaticity

NMR Structural parameters %Aromaticity

HRTEM Surface structure Size distribution of aromatic fractionBasic construction requirements

4

REACTIVITY MEASUREMENTS5,6,7

• Thermax 500 TGA* supplied by Thermo Fisher Scientific, RSA

• Char-CO2 gasification experiment using 100% CO2 and -75 μm PSD.• TGA data were evaluated using • Reactivity was determined by random pore model (RPM)

5. Everson et al. 2008. Fuel 87(15-16): 3403-3408.6. Everson et al. 2013. Fuel, 2013. 109:148-156.7. Hattingh et al. 2011. Fuel Processing Technology. 92(10): 2048-2054.* TGA = Thermogravimetric analyser

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CHARACTERATION RESULTS

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CHARACTERISATION RESULTS WERE USED IN STRUCTURAL AND REACTIVITY MODELLING

PROCESSES USING MOLECULAR AND QUANTUM MECHANICS TECHNIQUES,

RESPECTIVELY.

MOLECULAR MODELLING

• University's HPC cluster and National CHPC

• Accelrys Material Studio 6.0

• Amorphous Cell for 3D constructions

• Forcite for structural geometries and density calculations

• DREIDING forcefield

• PCFF force field for aromaticity

• Perl scripting for model characterisation

MOLECULAR MECHANICS FACILITIES MADE AVAILABLE TO THE USER FOR THE STRUCTURAL MODELLING

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MOLECULAR MODELLING

STRUCTURAL CONSTRUCTION COMMENCED WITH AROMATIC STRUCTURES FROM THE

HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPE (HRTEM)

8

RAW IMAGE FROM COAL CHAR

Lattice fringe length = 51 Å

IMAGE PROCESSING8 OF HRTEM MICROGRAPHS

IMAGE AFTER SKELETONISATION

8. Sharma et al. 1999. Fuel, 1999. 78(10): p. 1203-1212.9

ANALYSED AND INTEPRETED AS AROMATIC CARBON RAFTS9

Lattice Fringe length = 54 Å

9. Mathews et al. 2010. Fuel 89 1461–1469

Average length = 54 Å

Min. length = 39.959 Å Max. length = 67.713 Å

PARALLELOGRAM CATENATIONS9

Max.length

Min.length

10

HRTEM: AVERAGE AROMATIC RAFT SIZE DISTRIBUTION

11

INITIAL H/C RATIOS FROM PAHs DISTRIBUTION

12

Example: A few samples (3x3 – 25x25) from aromatic carbon rafts size distribution from the HRTEM

MOLECULAR MODELLING

• To produce geometric representations according to the shapes of the lattice fringes of chars from the HRTEM.

• To commence the adjustment of atomic H/C, O/C, N/C & S/C ratios.

• e.g. trimming of 11x11 aromatic raft as shown:

TRIMMING TECHNIQUES10,11,12

10. Niekerk et al. 2010. Fuel 89(1): p. 73-82. 11. Weimershaus et al. 2013 Current Opinion in Immunology 25(1): p. 90-96. 12. Heifetz et al. 2003. Protein Engineering 16(3): p. 179-185.

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HETERO ATOMS IN COAL CHARS13,14,15,16

Thiophenic-Sether-O Quaternary-Ncarbonyl-O Pyridinic-N

(a) (b)

13. Fletcher et al. 1992. Energy & Fuels, 6, 643-65014. Pels et al. 1995. Carbon 33 (11), 1641-165315. Kelemen et al. 1998. Energy & Fuels, 12, 159-17316. Liu et al. 2007. Fuel, 86 , 360–366

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A suitable number of molecules with individual geometries was used to form large-scale 3D molecular structures.

LARGE-SCALE 3D MOLECULAR STRUCTURES: MODELLING PROCESS

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3D construction at 0.1 g.cm-3 from a combination of molecules

Energy minimisation of the 3D structure

Annealing calculations at 25-1000 ℃, 3.0 GPa over 20 cycles

Molecular Dynamics at 25 ℃ and 3.0 GPa on a frame of 1.798 g.cm-

3

Automatic/manual Atomic Force field calculations

LARGE SCALE MODELS IN 3D: CPK* DISPLAYED STYLE

16

C = green H = white O = redN = blueS = yellow

INERTINITE CHAR MODELSelected for d002, Lc and Nave (-)(subjective measurements)

Ave. values (Å)

d002 = 3.4Lc = 15.0La = 35.0Nave(-) = 4

LARGE SCALE MODELS IN 3D: CPK* DISPLAYED STYLE

17* CPK = space-filling model

Default view

C = greenH = whiteO = redN = blueS = yellow

VITRINITE CHAR MODELd002, Lc , La and Nave (-)(subjective measurements)

Ave. values (Å)d002 = 4.4Lc = 16.9La = 31.6Nave (-) = 4

EXPERIMENTAL & MODELLING DATA COMPARED (XRD)

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Property Inertinite char Vitrinite char

Experimental Modelling Experimental Modelling

Inter-layer spacing, d002 (Å) 3.493 3.4 3.508 4.4

Crystallite height, Lc (Å) 12.19 15.0 11.76 16.9

Crystallite diameter, La (11) (Å)

39.39 35.0 32.47 31.9

Average number of aromatic layers Nave (-)

4.489 4 4.352 4

EXPERIMENTAL & MODELLING DATA COMPARED

PropertyExperimental Model Experimental Model

Total molecules 21 37Total atoms 1130 1142 1162 1171Total C 1000 1000 1000 1000Total H 104 105 123 125Total O 7 14 21 21Total N 18 22 15 22Total S 1 1 3 3H/C atomic ratio 0.10 0.10 0.12 0.12O/C atomic ratio 0.01 0.01 0.02 0.02N/C atomic ratio 0.02 0.02 0.02 0.02S/C atomic ratio 0.001 0.001 0.003 0.003Helium density (g.cm-3) 1.87 1.87 1.82 1.82f a (%) (from XRD) 96.0 96.0 95.0 95.0Formula C1000H104O7N18S1 C1000H105O14N22S1 C1000H123O21N15S3 C1000H125O21N22S3

Inertinite char Vitrinite char

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Note that NMR results could not be obtained because of the extensive line broadening phenomena which prevented accurate calculation of structural and lattice parameters17,18

17. Solum et al. 2001. Energy & Fuels. 15(4): p. 961-971.18. Perry et al. 2000. Proceedings of the Combustion Institute. 28(2): p. 2313-2319.

DFT* REACTIVITY MODELLING

• Accelrys Material Studio • Spin unrestricted DFT calculations (DMol3 module) • Generalised gradient approximation of PW91• Basis set: Double numerical by polarisation (DNP)• Thermal smearing used to improve SCF convergence• Calculations included:

– Geometry Optimisation (GeomOpt)– Single-point energy (1-scf) – Transition state (TS) theory

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QUANTUM MECHANICS FACILITIES MADE AVAILABLE TO THE USER FOR THE REACTIVITY MODELLING

* DFT = density functional theory. Offers highly accurate results with theoretical soundness. Has very high but justifiable computational costs

3x3 4x4

5x5

Simplified char models sampled from the large scale models without the trimming and hetero atoms effects were selected for reactivity modelling because of DFT size limitations.

21

DFT* REACTIVITY MODELLING: RATIONAL OF MODELS USED

FUKUI FUNCTION19,20,21

The Fukui Function () is among the most basic and commonly used reactivity indicators.

is defined according to reactivity governing the

• nucleophilic attack () • electrophillic attack ()• radical attack ()

• It is a property used during the 1-scf calculations • The larger the = the higher the reactivity

19. Sablon et al. 2009. Journal of Chemical Theory and Computation 5 (5): p. 1245-1253.20. Bultinck et al. 2007. The Journal of Chemical Physics 127 (3): p. 034102.21. Fukui et al. 1970. Springer Berlin Heidelberg. p. 1-85. 22

DFT* REACTIVITY MODELLING: ACTIVE SITES

The results showed that:1. Each edge C had a value.2. Their occupied different levels, e.g., edge C at the tip (Ct) > zigzag edge next to Ct (Cz) > armchair

edge Cs (Cr) > zigzag edge intermediate between Cz and Cr (Czi).

3x3 edge C C1 C2 C6 C7 C13 C14 C17 C18 C19 C23 C27 C28 C29 C30

f + (r) Hirshfield 0.034 0.031 0.031 0.026 0.028 0.028 0.026 0.028 0.028 0.026 0.026 0.031 0.034 0.031

4x4 edge C C1 C2 C6 C7 C14 C15 C16 C21 C22 C28 C29 C31 C40 C43

f + (r) Hirshfield 0.023 0.019 0.019 0.015 0.015 0.018 0.019 0.015 0.015 0.019 0.018 0.016 0.016 0.0164x4 edge C C45 C46 C47 C48

f + (r) Hirshfield 0.016 0.021 0.024 0.021

5x5 edge C C1 C2 C6 C7 C14 C15 C21 C22 C25 C26 C33 C34 C35 C49

f + (r) Hirshfield 0.02 0.017 0.017 0.013 0.012 0.012 0.015 0.015 0.013 0.012 0.012 0.015 0.015 0.0125x5 edge C C54 C55 C62 C65 C66 C68 C69 C70

f + (r) Hirshfield 0.012 0.012 0.013 0.012 0.013 0.017 0.02 0.017

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DFT* REACTIVITY MODELLING: ACTIVE SITES

3x3Ct

Czi

Cr1 Cr2

Cz

4x4Ct

Czi

Cr1 Cr2

Cz

Czi

5x5Ct

Czi

Cr1 Cr2

Cz

Czi

Czi

This mixture of valuesat the edge carbon sites of char models possibly represented preferred (or less stable) and less preferred sites (or more stable) active sites

: Ct > Cz > Cr1 and Cr2 > Czi

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DFT* REACTIVITY MODELLING: ACTIVE SITES

𝑅𝑒𝑎𝑐𝑡𝑖𝑣𝑖𝑡𝑦 ¿Since all edge were active sites, the was expressed as:

ratio vs size of char molecules

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In summary, the:

DFT* REACTIVITY MODELLING: ACTIVE SITES

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

3x3 4x4 5x5

Rea

ctiv

ity r

atio

(f+)

Size of char model

REACTIVITY-ATOMIC STRUCTURE RELATIONSHIP

Fig.2 Size distribution of char molecules Fig.3 TGA reactivity of chars (RPM)

1. The ratio decreases with increasing size of char molecules. 2. Structural results showed that the two chars were similar except that

inertinite char had high distribution of large molecules than vitrinite chars (Fig. 2).

3. Reactivity experiments showed that inertinite chars recorded lower reactivity than vitrinite chars (Fig. 3).

4. Hence an important contribution to understand the structural-reactivity relationship of coal chars derived from inertinite- and vitrinite-rich coals

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Inertinite charsVitrinite chars Inertinite chars

Vitrinite chars

• DFT: Fundamental CO2-char reaction mechanism22,23 • Active sites (C*) exposed by C-H breakdown.

1. Adsorption of CO2.2. Dissociation of CO2 gas molecule.3. Desorption of CO as a dissociation product, leaving O-

complex.4. Disintegration reaction where 6C……5C6. Desorption of CO as a gasification product.

7. End of simplified gasification reaction, where,

8. ………………………….(2)

.

DFT REACTIVITY WITH CO2

22. Frederick et al. 1993. Ind. & Eng. Chemistry Research, 32, 1747-1753.23. Moulijn, et al. 2010. Carbon, 33, 1155-1165.

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CALCULATED ENRGIES NEEDED FOR THE C-H BOND BREAKDOWN

Ct

Czi

Cr1 Cr2

Cz

The active sites with highest and 2nd highest were chosen to form a Ct-Cz (C-C) edge to model the fundamental reaction mechanism

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Energy required Average(kcal/mol) Ct Cz Cr1 Cr2

C-H bond breakdown 124.97 125.02 124.88 124.91 124.94

Active site

DFT GEOM_OPT REACTION CONFIGURATIONS ON Ct - Cz EDGE

CO2 approaching CO2 adsorption

CO2 dissociation to form CO and O

O1 C1 O2

C2

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Disintegration of C-ring to form CO

REACTION MECHANISM ON Ct - Cz

EDGERESULTS OF GEOM.OPT* CONFIGURATIONS

These bond lengths results showed that reaction mechanism of CO2 with char model proceeded favorably, from adsorption to the 2nd formation of CO

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Reaction configurations

Ct - Cz edge active site (target Ct)

Barrier (kcal/mol)

C1-O1 C1-O2 Ct-O2 Ct-C2 Ct-Cz Ct-Cnew

1 CO2 adsorption on Ct 1.85 0.385 0.390 0.386 0.419 0.4112 CO2 dissociation 0.393 0.394 0.391 0.420 0.4143 CO formation 1 1.62 lost broken 0.407 0.438 0.4404 Decomposition lost broken 0.400 0.4345 CO formation 2 2.30 lost broken lost broken

decomposeddecomposed

Bond length (nm) (stability)

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

160.0

CO2 at a distance(Start)

CO2…..adsorbs CO…..formation 1 CO…..formation 2

Ent

halp

y ch

ange

(kJ/

mol

Ct

Cz

Cr1

31

SIMPLIFIED ENTHALPY CHANGES FOR THE REACTION MECANISM

Config.1

Config.4Config.3

Config.2

N.B. Configuration 4 can represent gasification process since the char lattice carbon is allocated to the O-complex to form gaseous CO molecule.

CONCLUSIONS

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• Molecular structures of coal chars derived from inertinite- and vitrinite-rich South African coals were constructed on the basis of experimental data.

• These structures provided possibilities to explore atomic structure-reactivity relationships.

• DFT calculations contributed to the rational behind variations in reactivity of coal chars on mineral matter free basis, using the Fukui function property.

• The carbon ring disintegration from 6 to 5 carbons and the allocation of lattice carbon to form the 2nd CO gas molecule can essentially be called a gasification process.

ACKNOWLEDGEMENTS

• Colleagues

• SANERI

• DST

• Universities (Wits, UCT, SU, PSU, RU, UKZN, LaTrobe,

Nottingham)

• National CHPC and NWU HPC

• Coal mining industry

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THANK YOU

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CALCULATIONS ON UNCAPPED CHAR MODEL

Here it was found that both the dissociative CO2 adsorption and re-adsorption of CO just formed were possible, e.g.,C28-O32 = 0.3634 more stable thanC31-O32 = 0.3654, and

C27-O33 = 0.3659 more stable thanC31-O27 = 0.3999

Therefore O-complex on C28 and CO could form, but the CO could adsorb onto C27

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LARGE SCALE MODELS IN 3D: CPK* DISPLAYED STYLE

36* CPK = space-filling model

Ave. values (Å)d002 = 3.4Lc = 15.0La = 35.0Nave (-) = 4

INERTINITE CHAR MODEL

C = green, H = white, O = red, N = blueS = yellow

LARGE SCALE MODELS IN 3D*: d002, Nave (-) and Lc MEASUREMENTS

37

Default view

INERTINITE CHAR MODEL

* Ball and stick

LARGE SCALE MODELS IN 3D*: d002, Nave (-) and Lc MEASUREMENTS

38* Ball and stick

VITRINITE CHAR MODEL

-40-20

020406080

100120140

Ent

haly

cha

nge

(kJ/

mol

)

39

SIMPLIFIED ENTHALPY CHANGES FOR THE REACTION MECANISM

Config.1

Config. 1: CO2 is introduced to 3x3 char model (start)

Config. 2: CO2 chemi-adsorbs on Ct active site

Config. 3: 1st Hidden intermediate Config. 4: 1st CO formation (dissociation)

Config. 5: 2nd Hidden intermediate

Config. 6: : 2nd CO formation

Config.6

Config.4

Config.2

N.B. Configuration 4 can represent gasification process since the char lattice carbon is allocated to the O-complex to form gaseous CO molecule.

Config.3

Config.5

RESEARCH OBJECTIVES & QUESTIONS

Objective• To present the atomic structures of chars derived from

different types of coals and their impact on reactivity with CO2 gas.

Research questions• What is the effect of the nature and origin of chars on

reactivity?• How well do predictions from structural chemistry and

molecular representations of chars compare with direct reactivity measurements?

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COAL SOURCES IDENTIFIED

Witbank coalfield

Waterberg coalfield

Map 1. from http://www.mml.co.za/docs/FET_CAPS/Platinum-grade-12-activity.pdf on 28/09/2013 at 16:25Map 2. from Pinetown et al. 2007. International Journal of Coal Geology. 70(1-3) p. 166-183.

Map 1

Map 2