MOLECULAR MECHANICS TO MODEL COAL CHAR STRUCTURES AND DFT TO MODEL THEIR REACTIVITY WITH CO 2 GAS FOR SYNTHETIC GAS PRO

1. MOLECULAR MECHANICS TO MODEL COAL CHARSTRUCTURES AND DFT TO MODEL THEIRREACTIVITY WITH CO2 GAS FOR SYNTHETIC GASPRODUCTION Mokone J. Robertsa, Raymond C. Eversona, Hein W. J. P. Neomagusa, Jonathan P. Mathewsc, George Domazetise, Cornelia G.C.E. van Sittertd a Coal Research Group, School of Chemical and Minerals Engineering, North-West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom, 2520, South Africa. cJohn and Willie Leone Family Department of Energy and Mineral Engineering, The EMS Energy Institute, The Pennsylvania State University, University Park, PA 16802, USA d Laboratory of Applied Molecular Modelling, Chemical Resource Beneficiation Focus Area, North-West University, Potchefstroom 2520, South Africa e Chemistry Department, La Trobe University, Melbourne, VIC 3086, Australia

2. 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

3. 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 polyaromatichydrocarbons (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, 692–700 2. Chen et al. 2011. Ind. Eng. Chem. Res, 50, 2562–2568

4. CHARACTERISATION

5. REACTIVITY MEASUREMENTS5,6,7 • Thermax 500 TGA* supplied by Thermo Fisher Scientific, RSA • Char-CO2gasification 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.Hattinghet al.2011. Fuel Processing Technology. 92(10): 2048-2054. * TGA = Thermogravimetricanalyser

6. CHARACTERATION RESULTS CHARACTERISATION RESULTS WERE USED IN STRUCTURAL AND REACTIVITY MODELLING PROCESSES USING MOLECULAR AND QUANTUM MECHANICS TECHNIQUES, RESPECTIVELY.

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

8. MOLECULAR MODELLING STRUCTURAL CONSTRUCTION COMMENCED WITH AROMATIC STRUCTURES FROM THE HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPE (HRTEM)

9. IMAGE PROCESSING8 OF HRTEM MICROGRAPHS IMAGE AFTER SKELETONISATION RAW IMAGE FROM COAL CHAR Lattice fringe length = 51 Å 8. Sharma et al. 1999. Fuel, 1999. 78(10): p. 1203-1212.

10. ANALYSED AND INTEPRETED AS AROMATIC CARBON RAFTS9 PARALLELOGRAM CATENATIONS9 Lattice Fringe length = 54 Å Average length = 54 Å Max.length Min.length Min. length = 39.959 Å Max. length = 67.713 Å 9. Mathews et al. 2010. Fuel 89 1461–1469

11. HRTEM: AVERAGE AROMATIC RAFT SIZE DISTRIBUTION

12. INITIAL H/C RATIOS FROM PAHs DISTRIBUTION Example: A few samples (3x3 – 25x25) from aromatic carbon rafts size distribution from the HRTEM

13. MOLECULAR MODELLING TRIMMING TECHNIQUES10,11,12 • 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: 10. Niekerket al. 2010. Fuel 89(1): p. 73-82. 11. Weimershauset al. 2013 Current Opinion in Immunology 25(1): p. 90-96. 12. Heifetz et al.2003. Protein Engineering 16(3): p. 179-185.

14. HETERO ATOMS IN COAL CHARS13,14,15,16 Pyridinic-N ether-O carbonyl-O Thiophenic-S Quaternary-N A suitable number of molecules with individual geometries was used to form large-scale 3D molecular structures. 13. Fletcher et al. 1992. Energy & Fuels, 6, 643-650 14. Pelset al. 1995. Carbon 33 (11), 1641-1653 15. Kelemenet al.1998. Energy & Fuels, 12, 159-173 16. Liu et al. 2007. Fuel, 86 , 360–366 (a) (b)

15. LARGE-SCALE 3D MOLECULAR STRUCTURES: MODELLING PROCESS 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

16. LARGE SCALE MODELS IN 3D: CPK* DISPLAYED STYLE INERTINITE CHAR MODEL Selected for d002, Lcand Nave (-) (subjective measurements) C = green H = white O = red N = blue S = yellow Ave. values (Å) d002 = 3.4 Lc = 15.0 La = 35.0 Nave(-) = 4

17. LARGE SCALE MODELS IN 3D: CPK* DISPLAYED STYLE VITRINITE CHAR MODEL d002, Lc , La and Nave (-) (subjective measurements) Ave. values (Å) d002 = 4.4 Lc = 16.9 La = 31.6 Nave(-) = 4 C = green H = white O = red N = blue S = yellow Default view * CPK = space-filling model

18. EXPERIMENTAL & MODELLING DATA COMPARED (XRD)

19. EXPERIMENTAL & MODELLING DATA COMPARED 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. Solumet al.2001. Energy & Fuels. 15(4): p. 961-971. 18. Perry et al.2000. Proceedings of the Combustion Institute. 28(2): p. 2313-2319.

20. 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 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

21. DFT* REACTIVITY MODELLING: RATIONAL OF MODELS USED 3x3 4x4 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. 5x5

22. DFT* REACTIVITY MODELLING: ACTIVE SITES 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. Bultincket al. 2007. The Journal of Chemical Physics 127 (3): p. 034102. 21. Fukui et al. 1970. Springer Berlin Heidelberg. p. 1-85.

23. DFT* REACTIVITY MODELLING: ACTIVE SITES The results showed that: Each edge C had a value. 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).

24. DFT* REACTIVITY MODELLING: ACTIVE SITES Cr1 Cr2 Cr1 Cr2 Czi Czi Czi Cz Cz 4x4 Ct Ct 3x3 : Ct > Cz > Cr1 and Cr2 > Czi Cr1 Cr2 Czi Czi Czi Cz This mixture of values at the edge carbon sites of char models possibly represented preferred (or less stable) and less preferred sites (or more stable) active sites Ct 5x5

25. DFT* REACTIVITY MODELLING: ACTIVE SITES Since all edge were active sites, the was expressed as: ratio vs size of char molecules In summary, the:

26. REACTIVITY-ATOMIC STRUCTURE RELATIONSHIP Inertinite chars Inertinite chars Vitrinite chars Vitrinite chars Fig.2 Size distribution of char molecules Fig.3 TGA reactivity of chars (RPM) The ratio decreases with increasing size of char molecules. Structural results showed that the two chars were similar except that inertinite char had high distribution of large molecules than vitrinite chars (Fig. 2). Reactivity experiments showed that inertinite chars recorded lower reactivity than vitrinite chars (Fig. 3). Hence an important contribution to understand the structural-reactivity relationship of coal chars derived from inertinite- and vitrinite-rich coals

27. DFT REACTIVITY WITH CO2 • DFT: Fundamental CO2-char reaction mechanism22,23 • Active sites (C*) exposed by C-H breakdown. • Adsorption of CO2. • Dissociation of CO2 gas molecule. • Desorption of CO as a dissociation product, leaving O-complex. • Disintegration reaction where 6C……5C 6. Desorption of CO as a gasification product. • End of simplified gasification reaction, where, • ………………………….(2) . 22. Frederick et al. 1993. Ind. & Eng. Chemistry Research, 32, 1747-1753. 23. Moulijn, et al. 2010. Carbon, 33, 1155-1165.

28. CALCULATED ENRGIES NEEDED FOR THE C-H BOND BREAKDOWN Cr1 Cr2 Czi Cz Ct The active sites with highest and 2nd highest were chosen to form a Ct-Cz (C-C) edge to model the fundamental reaction mechanism

29. DFT GEOM_OPT REACTION CONFIGURATIONS ON Ct - Cz EDGE O1 C1 O2 C2 CO2 adsorption CO2 approaching Disintegration of C-ring to form CO CO2 dissociation to form CO and O

30. REACTION MECHANISM ON Ct - Cz EDGE RESULTS OF GEOM.OPT* CONFIGURATIONS These bond lengths results showed that reaction mechanism of CO2with char model proceeded favorably, from adsorption to the 2nd formation of CO

31. SIMPLIFIED ENTHALPY CHANGES FOR THE REACTION MECANISM Config.4 Config.3 Config.1 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.

32. CONCLUSIONS • 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.

33. ACKNOWLEDGEMENTS • Colleagues • SANERI • DST • Universities (Wits, UCT, SU, PSU, RU, UKZN, LaTrobe, Nottingham) • National CHPC and NWU HPC • Coal mining industry

34. THANK YOU

35. 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 than C31-O32 = 0.3654, and C27-O33 = 0.3659 more stable than C31-O27 = 0.3999 Therefore O-complex on C28 and CO could form, but the CO could adsorb onto C27

36. LARGE SCALE MODELS IN 3D: CPK* DISPLAYED STYLE INERTINITE CHAR MODEL Ave. values (Å) d002 = 3.4 Lc = 15.0 La = 35.0 Nave(-) = 4 C = green, H = white, O = red, N = blue S = yellow * CPK = space-filling model

37. LARGE SCALE MODELS IN 3D*: d002, Nave (-) and Lc MEASUREMENTS INERTINITE CHAR MODEL Default view * Ball and stick

38. LARGE SCALE MODELS IN 3D*: d002, Nave (-) and Lc MEASUREMENTS VITRINITE CHAR MODEL * Ball and stick

39. SIMPLIFIED ENTHALPY CHANGES FOR THE REACTION MECANISM 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.5 Config.1 Config.2 Config.3 N.B. Configuration 4 can represent gasification process since the char lattice carbon is allocated to the O-complex to form gaseous CO molecule.

40. 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?

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