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This paper is an overview of the calculations performed on various molecular models of char containing iron species, developed from a brown coal model...
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Energy & Fuels 2007, 21, 2531-2542

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Semiempirical and Density Functional Theory Molecular Modeling of Brown Coal Chars with Iron Species and H2, CO Formation George Domazetis,* Monthida Raoarun, and Bruce D. James Chemistry Department, La Trobe UniVersity, Victoria, 3086, Australia ReceiVed March 13, 2007. ReVised Manuscript ReceiVed May 15, 2007

This paper is an overview of the calculations performed on various molecular models of char containing iron species, developed from a brown coal model with iron complexes, to mimic pyrolysis over 200-700 °C. The semiempirical (SE) optimization of four char models and density functional theory (DFT) calculations on the fourth smaller model were assessed on the basis of calculated heats of formation, total energy, bond lengths and angles, and partial charges. The formation of hydrogen was examined via H abstraction from [O-H] and [C-H] groups by iron clusters in char to form the hydride, followed by the dihydride, and accompanied by [C-O-Fe] and [C-Fe] bond formation. Single-point self-consistent field DFT and SE calculations for relevant structures containing iron clusters indicated that H abstraction and hydrogen formation were energetically favored. Two routes were examined for CO formation: via the adsorption of FeO and Fe2O on graphite followed by a loss of CO and via decomposition of the newly formed [C-O-Fe] group. The latter was shown to be the likely route. A concerted reaction for hydride formation during CO formation has been discussed and a reaction scheme for H2 and CO suggested; the chemistry for H2 and CO from catalytic steam gasification is briefly discussed.

Introduction Studies of metal-mediated pyrolysis and the gasification of low-rank coals require the characterization of inorganic species added to the macromolecular structure of the coals and their forms in char. To this end, we have systematically studied the addition and characterization of inorganic complexes to lowrank coals, particularly iron hydroxyl complexes, and have examined the decomposition reactions involving coal oxygen functional groups coordinated to iron hydroxyl complexes during low-temperature pyrolysis.1-4 Semiempirical (SE) molecular modeling and experimental methods have shown that the chemistry during low-temperature pyrolysis includes the transformation of the iron hydroxyl complexes into Fe(III)/Fe(II) oxides and ultimately into Fe(0). The present work extends our effort to include SE and density functional theory (DFT) molecular modeling of char containing iron complexes and examines the formation of H2 and CO from models of char that contain iron species. The formation of CO2 and CO during the initial stages of pyrolysis involves the thermal breakdown of coal oxygen functional ligands coordinated to the inorganic complex. As the temperature increases, the amount of CO increases during pyrolysis, and significant amounts of H2O, H2, and CH4 are * Corresponding author. Tel.: 61 3 9479 2811. Fax: 61 3 9479 1399. E-mail: [email protected]. (1) Domazetis, G.; Raoarun, M.; James, B. D. Studies of Mono- and Poly-nuclear Iron Hydroxyl Complexes in Brown Coal. Energy Fuels 2005, 19, 1047-1055. (2) Domazetis, G.; Liesegang, J.; James, B. D. Studies of inorganics added to low-rank coals for catalytic gasification. Fuel Process. Technol. 2005, 86, 463-486. (3) Domazetis, G.; James, B. D. Molecular Models of Low Rank Coals Incorporating Metal Containing Species. Org. Geochem. 2006, 37, 244259. (4) Domazetis, G.; Raoarun, M.; James, B. D. Low-Temperature Pyrolysis of Brown Coal and Brown Coal Containing Iron Hydroxyl Complexes. Energy Fuels 2006, 20, 1997-2007.

observed. These events can precede gasification chemistry (e.g., char gasification with O2 or H2O) and include the transformation of the iron hydroxyl into various Fe(III)/(II) and Fe(0) complexes. Such iron species may subsequently be involved in catalytic activity. The modeling approach adopted in our studies commenced with a molecular model of brown coal, and this was transformed into a char model by removing functional groups to mimic the loss of oxygen functional groups during pyrolysis and also the transformation of iron hydroxyl species into iron oxides and reduced iron species. Calculated data obtained from the transformation of the model of coal into char have been compared with experimental data, that is, elemental composition, percentage weight loss, the ratio CO2/CO measured during pyrolysis at given temperatures, iron hydroxyl species, and the various Fe(III)/Fe(II) oxides and Fe(0) moieties, over the temperature range 200-700 °C, as discussed previously.4 The molecular modeling theory used in our studies has been SE with the PM5 Hamiltonian (SE-PM5) for large molecule models and DFT for smaller char models with iron clusters FemOn (n ) 1-4 and m ) 0-3), and also a char/graphite model containing the CtC group, reported by a number of workers for comparison with our char models.5-9 The approach in this work has been to use relative changes in the calculated heat of (5) Zhu, Z.; Lu, G. Q.; Finnerty, J.; Yang, R. T. Electronic structure methods applied to gas-carbon reactions. Carbon 2003, 41, 635-658. (6) Sendt, K.; Haynes, B. S. Density functional study of the chemisorption of O2 on the armchair surface of graphite. Proc. Combust. Inst. 2005, 30, 2141-2149. (7) Haynes. B. S. A turnover model for carbon reactivity I. Development. Combust. Flame 2001, 126, 1421-1432. (8) Ma, X.; Wang, Q.; Cis, L-Q.; Cermignani, W.; Schobert, H. H.; Pantano, C. G. Semi-empirical Studies on Electronic Structures of a Borondoped Graphene Layer - Implications on the Oxidation Mechanism. Carbon 1997, 35, 1517-1525. (9) Chan, N.; Yang, R. T. Ab initio Molecular Orbital Calculations on Graphite: Selection of Molecular Systems and Model Chemistry. Carbon 1998, 36, 1061-1070.

10.1021/ef070129v CCC: $37.00 © 2007 American Chemical Society Published on Web 07/14/2007

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formation of the compound from its elements in their standard state (∆Hf), partial charges of char and iron species, bond lengths, and angles as a basis for assessing the molecular structures in reaction sequences for hydrogen and carbon monoxide formation. In this paper, we discuss (i) the modeling of brown coal char formation, (ii) molecular models of char containing various iron species, and (iii) reaction routes for H2 and CO formation from char containing iron clusters. The chemistry of H2 and CO formation from catalytic steam gasification is also briefly discussed. Computer Molecular Models Molecular models of low-rank coals have been discussed previously, and the brown coal molecule, discussed below, was modified into molecular structures of brown coal char by removing functional groups, to mimic the loss of carboxyl, carbonyl, ether, and hydroxyl groups, observed during the pyrolysis of brown coal.3,4 The same coal model with iron hydroxyl complexes was also transformed in a similar manner into molecular structures of brown coal char with iron oxide, but in this case, the carboxyl and hydroxyl groups coordinated to the iron centers were lost as CO2 and CO, and the hydroxyl groups within the iron complex were lost as water molecules;4 for example, [Fe6(OH)14]4+ in the initial coal model lost (OH) groups as H2O and changed into [(Fe3+)4(Fe2+)2(O2-)8] for the char model. The weight percent loss, elemental analysis, and CO2/CO ratio were calculated for each of the resulting char molecular models and compared with experimental data for char samples from pyrolysis at particular temperatures; the models, experimental data, and loss of functional groups have been discussed previously.1,2,4,10 Molecular structures were optimized using molecular mechanics, single-point self-consistent field calculations using the PM5 Hamiltonian (1scf-PM5), semiempirical calculations using the PM5 Hamiltonian (SE-PM5), single-point self-consistent field calculations using DFT (1scf-DFT), and DFT. Calculations for small molecules were performed with the Fujitsu CAChe ab initio 5.04 software package and for large molecules with MOPAC200211a at the Australian Partnership for Advanced Computing National Facility (APAC-NF). DFT calculations of small molecules were performed with the CAChe 5.04 DGauss 4.1/UC-4.1 program, using Becke ‘88: Perdew & Wang ‘91 theory, with a triple-ζ-valence uncontracted 63321/531/41 Gaussian basis set (DZVP, A1), and a Li-Rn pseudopotential which includes relativistic effects for heavy atoms.11b DFT calculations were also carried out at the Victorian Partnership for Advanced Computing facility (VPAC) using the Schro¨dinger Jaguar package at the B3LYP level of theory (exact Hartree-Fock, Slater local exchange functional, Becke’s 1988 nonlocal gradient correction; the correlation is the Vosko-WilkNusair local functional and Lee-Yang-Parr local and nonlocal functionals) and using lacvp** or lacvp3** basis sets, which include effective core potentials (ECPs) for Fe developed at Los Alamos National Laboratory; the basis set is valence-only, containing the highest s and p shells for main group atoms and the highest s, p, and d shells for transition metals including the outermost core orbitals; the 631G basis set developed by Pople and co-workers was used for atoms not described by ECPs. Polarization functions were used on all atoms except for transition metals, and the effective core potentials include one-electron mass-velocity and relativistic corrections.12a The geometries of three smaller molecular structures of char, and char with the cluster Fe3, were optimized using DFT(10) Domazetis, G.; Raoarun, M.; James, B. D.; Liesegang, J.; Pigram, P. J.; Brack. N.; Glaisher, R. Analytical and Characterization Studies of Organic and Inorganic Species in Brown Coal. Energy Fuels 2006, 20, 1556-1564. (11) (a) Stewart, J. J. P. MOPAC 2002, version 2.5.2; Fujitsu Ltd: Tokyo, Japan. (b) CAChe ab initio, version 5.04; Fujitsu Ltd: Tokyo, Japan, 20002002; Oxford Molecular Ltd: Oxford, U.K., 1989-2000. (12) (a) Jaguar, version 6.5; (b) MacroModel, version 9.1; Schro¨dinger, LLC: New York, 2005.

Domazetis et al. B3LYP and lacvp**; in this case, the very large computer resources required for this task necessitated limited medium-accuracy computations. Our goal has been to develop specific molecular models that could be used to examine reactions leading to H2 and CO formation from char containing iron complexes, and also the reactions of these molecules with H2O, as models for catalytic steam gasification; consequently, an exhaustive study of all configurations, electronic states, and transition species, for all models of chars with inorganic species, is outside the scope of our work. Conformational analyses for particular structures were initially performed using CAChe 5.04 Cornflex and the Monte Carlo multiple minimum method with Schro¨dinger’s MacroModel 9.1.12b Details have been provided in the discussion of particular conformations of molecules and their impact on the calculated data. SE geometric optimization using the default options in MOPAC was obtained for molecular models of brown coal and brown coal chars. Calculations for structures of char containing iron complexes were complicated, however, by multielectron configuration interactions (MECI), and such calculations for the large structures required excessively large computer wall time. Consequently, only a few smaller models containing iron complexes were examined using MECI, and for these molecules, the singlet state was the lowestenergy configuration, with singlet and triplet states separated by 1 to 2 eV. It is noted that the structures formed during pyrolysis are of chemically reactive systems at elevated temperatures, and it is obvious that SE results of structures optimized to a ground state can only be used to provide a relative indication of energetically favored geometries. It was observed that a number of molecular structures used to examine hydrogen and carbon monoxide reactions were relatively stable molecules, and thus the calculated ∆Hf, and the total energy for these, may be used as a relative measure of energetically favored structures provided the data were for the same molecule that had undergone a simple internal transformation. Our approach has been simplified in this way: we have also used data obtained from 1scf-PM5, 1scf-DFT, and SE calculations, to ensure that the relative comparison was reasonable. The default option for structure optimization in MOPAC was the Baker’s Eigen Following geometry optimization method; results from the Broyden-Fletcher-Goldfarb-Shanno method were occasionally used for larger structures. A number of options were examined in the SE treatment, including relaxing the default conditions for geometry optimization and removing safety checks in MOPAC. In virtually all molecular models containing iron complexes, removing safety checks often resulted in a catastrophic failure, while in other cases, the structure was optimized with very short distances between coal oxygen functional groups, and also between organic functional groups and iron complexes. This behavior was not observed for the organic molecular models of coal and char, but only these models containing iron species; additionally, this behavior was not observed when similar molecular models of char, but without hydroxyl or phenolic groups, and containing the same iron complexes, were optimized. It is likely, therefore, that the behavior observed for these structures stemmed from the SE treatment of hydrogen bonds and coordination bonds formed in these models. The structures that were optimized using DFT did not display this behavior. SE results were obtained for these difficult structures by modifying the options available in MOPAC that vary the minimum allowed ratio for energy change. In minimum energy searches, it is usually desirable that the energy decreases in each iteration; MOPAC searches for lower energy changes for rigid systems by stipulating values for a maximum(RMAX) and minimum- (RMIN) allowed ratio for energy change. In cases where the optimization may terminate before the stationary point has been reached, the trust radius may be set to a low value, or set to zero, and RMIN may be set to a negative value to allow the program to continue searching for a stationary point with steps that allow the energy to increase as well as decrease. The data obtained from SE calculations include ∆Hf, total energy, bond lengths, bond angles, bond orders, partial charges, and contributions of σ and π components to bonding with iron clusters.

Molecular Modeling of Brown Coal Chars

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Three SE calculations were carried out: (i) using default options in MOPAC, (ii) without MECI capabilities, and (iii) stipulating lower values for the trust radius (including zero) and a negative value for RMIN. MOPAC provides Wyberg indices as bond orders that mirror the simple ideas of single, double, and triple bonds; bond orders of less than 0.2 are indicative of “no bond”; the bonds matrix is split into σ-π-δ components, and the net charges, or partial charge, on each atom from MOPAC were the Coulson charges, while for specific models, Mulliken populations and partial charges were also computed. DFT and 1scf-DFT data included total energy, bond lengths and angles, Mulliken atomic charges, and bond order, and if needed, Mayer total atomic valence and bond order.

Results and Discussion Char Formation. The molecular model of brown coal was transformed into the various char molecular models by progressively performing the following steps: (a) loss of carboxyl groups to form CO2; (b) loss of carboxyl groups to form CO; (c) loss of carbonyl groups to form CO; (d) loss of hydroxyl groups from phenol functional groups to form CO and H2O, and associated reactions; and (e) loss of “links” comprising ester and aliphatic groups to form CO, H2O, H2, and hydrocarbons (e.g., CH4, CH2dCH2). The changes made to the coal molecular model, progressing from step a through to step e, were consistent with the pyrolysis chemistry observed at progressively higher temperatures. The calculated data from each of the transformations of the coal molecular model usually compared well with the experimental data obtained from the pyrolysis of acid-washed brown coal.4 Coal molecular models containing iron hydroxyl species, however, were lower in total oxygen content compared to those amounts measured for these char samples. This was due to the simplifying assumptions adopted for these models, in that all hydroxyl and coordinated water molecules were removed instantly as water molecules, resulting in a lower total oxygen content for the molecular model. Experimentally, however, at the relatively low temperatures of these studies, it is likely some iron hydroxyl groups, and water molecules strongly coordinated to iron hydroxyl, remain in the char, and this would provide a larger amount of total oxygen in the sample. The transformations of the molecular model of brown coal containing iron hydroxyl complexes, into char models with iron oxide species, also followed steps a to e, but the carboxyl and hydroxyl groups bound to the iron complex were initially lost. In cases when functional groups that linked units in the 3D structure were removed, the resulting char molecular models fragmented, and these molecular structures were optimized as single char models consisting of the resulting large molecular fragments. The molecular modeling results are for three chars, with and without iron hydroxyl/oxide complexes, and are compared to chars obtained experimentally from the pyrolysis of brown coal at temperatures of ∼300, ∼450, and ∼600700 °C. Above 700 °C, our interest is in oxygen/steam catalytic gasification chemistry, which will be discussed in detail in subsequent publications. Molecular Models. The properties of the brown coal molecular model, a molecular formula (MF) of [C263H239NO90] and a formula weight (FW) of 4853.734, are similar to properties reported as typical for brown coal; that is, molecular model: Car/Ctot ) 0.6 and Har/Htot ) 0.2 (ar ) aromatic, tot ) total); elemental composition: C 65.1%, H 5.0%, N 0.3%, O 29.7%; distribution of oxygen: carboxyl O(COOH) ) 21%, phenolic O(OH) ) 34%, methoxy O(O-CH3) ) 7%, methyl, ether, and aliphatic hydroxy O(R-OH) ) ∼4% and 18%, carbonyl O(RC)O) ) 11%, other ) 5%. The measured values vary considerably due to the

Figure 1. Molecular model of Char1.

heterogeneous nature of brown coal:10,13 Car/Ctot ) 0.57-0.65; Har/Htot ) ∼0.3; elemental composition: C 67.8%, H 4.9%, N 0.61%, O 26.4%. The distribution of oxygen is as follows: O(COOH) 17-23%; O(OH) 35-38%; O(O-CH3) ∼12%; O(R-OH) ∼4%; O(RC)O) ∼23%. The SE calculated ∆Hf for the optimized coal molecular model structure was -3434.7 kcal (total energy -60598.2 eV). As discussed previously, structures of brown coal molecular models in which carboxyl groups formed monodentate coordination bonds with iron hydroxyl complexes, containing water molecules coordinated to iron, provided a lower energy than those for the brown coal molecular model;3 for example, the 1scf-PM5∆Hf forthebrowncoal/ironcomplex {coal[COO-]7[(Fe3Fe4)(OH)14(15H2O)]7+} was -4229.7 kcal (total energy of -71052.2 eV). The char molecular model incorporating pyrolysis steps a and b of brown coal is shown in Figure 1 (labeled Char1; MF ) C249H239NO65; FW ) 4285.595). The calculated data are comparable to those for char samples obtained at ∼300 °C: that is, the calculated weight loss on a dry basis is 12 wt %; the measured weight loss (dry basis) from acid-washed brown coal was 14 wt %; char model elemental composition: C 69.8%, H 5.6%, O 24.3%, N 0.3%; elemental analysis of the char: C 70.1%, H 4.1%, O 25.4%, N 0.3%; calculated CO2/CO ratio 3.7:1; experimental CO2/CO ratio 3.3:1. The structure was optimized (SE) with a ∆Hf of -2300.2 kcal and a total energy of -51930.6 eV; the phenyl groups formed a disordered arrangement with some two-phenyl groups adopting a parallel arrangement separated by 3.5-4 Å, which is similar to values obtained from X-ray diffraction (XRD) studies of char.14-16 The decarboxylation reactions of the iron hydroxyl complexes in low-temperature pyrolysis include the formation of carbonato, µ-oxo-iron, and ultimately Fe(II) and Fe(0) complexes.4 The chemistry for oxides and reduced iron species has been modeled by using iron oxide species containing six, five, and four iron centers, to resemble structures reported for Fe2O3, Fe3O4, and (13) Verheyen, T. V.; Perry, G. J. The Science of Victorian Brown Coal; Durie, R. A., Ed.; Butterworth Heimann: London, 1991; p 280. (14) Wertz, D. L. Interlayer Structural Models of Beulah Zap Lignite Based on Its Wide Angle X-ray Scattering. Energy Fuels 1999, 13, 513517. (15) Sahajwalla, L. L. V.; Kong, C.; Harris, D. Quantitative X-ray diffraction analysis and its application to various coals. Carbon 2001, 39, 1821-1833. (16) Feng, B.; Bhatia, S. K.; Barry, J. C. Variation of the Crystalline Structure of Coal Char during Gasification. Energy Fuels 2003, 17, 744754.

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Figure 2. Model of {Char1[FeB6BOB8B]}, showing OH f Fe coordination bonds (purple atoms ) Fe, red atoms ) O).

FeO;17 additionally, a number of FemOn and Fem clusters, which have been studied using DFT methods,18-22 have also been placed within the char to model reduced and Fe(0) iron species formed in pyrolysis at higher temperatures (discussed below). The char molecular model originally shown in Figure 1, but now containing the [Fe6O8] moiety, is shown in Figure 2; MF ) [C249H239NO73Fe6], FW ) 4748.67; calculated weight loss 15 wt %; elemental composition: C 62.8%, H 5.4%, O 24.5%, Fe 7.0%; calculated CO2/CO ratio 3.7:1. The measured elemental composition of the char sample was as follows: C 56.7%, H 3.8%, O 30.2%, ash (mainly iron oxide) 8.4%; experimental weight loss 16-20%; measured CO2/CO ratio 4:1. As noted previously, the calculated composition differs from the experimental data mainly because of the lower amount of oxygen present in the molecular model, due to the simplifying assumption of instantaneous conversion of the iron hydroxyl complex into the iron oxide moiety. 1scf-PM5 data for Char1 with the various iron oxides were as follows: [Fe3O2] ∆Hf ) -1585.22 kcal (total energy -53659.30 eV); [Fe3O4] ∆Hf ) -1703.43 kcal (total energy -54216.58 eV); [Fe6O8] ∆Hf ) -1599.5 kcal (total energy -56523.97 eV). 1scf-PM5 calculations were performed for the model [C249H239NO73Fe6] using the MECI routine with 27 configurations; in these, the lowest level was the singlet, separated from the triplet by ∼1.3 eV. SE optimization using default settings in MOPAC produced a structure with very short OH‚‚‚H bonds, but calculations using the various alternate options available in MOPAC provided the following values of ∆Hf: Char1[Fe3O4], [C249H239NO69Fe3] ) -2124.7 kcal (total (17) Cornell, R. M.; Schwertmann, U. The Iron Oxides; VCH: Weinheim, Germany, 1996. (18) Gutsev, G. L.; Bauschlicher, C. W., Jr. Structural and electronic properties of iron monoxide clusters FemO and FemO- (m ) 2-6) A combined photoelectron spectroscopy and density functional theory study. J. Chem. Phys. 2003, 119, 11135-11145. (19) Gutsev, G. L.; Mochena, M. D.; Bauschlicher, C. W., Jr. Structure and Properties of Fe4 with Different Coverage by C and CO. J. Phys. Chem. A 2004, 108, 11409-11418. (20) Knickelbein, M. B.; Koretsky, G. M.; Jackson. K. A.; Pederson, M. R.; Hajnal, Z. Hydrogenated and deuterated iron clusters: Infrared spectra and density functional calculations. J. Chem. Phys. 1998, 109, 10692-10700. (21) Wang, Q.; Sun, Q.; Sakurai, M.; Yu, J. Z.; Gu, B. L.; Sumiyama, K.; Kawazoe, Y. Geometry and electronic structure of magic iron oxide clusters. Phys. ReV. B: Condens. Matter Mater. Phys. 1999, 59, 1267212677. (22) Castro, M.; Salahub. D. R. Theoretical study of the structure and binding of iron clusters: Fen (n< 5). Phys. ReV. B: Condens. Matter Mater. Phys. 1993, 47, 10955-10958.

Domazetis et al.

energy -54 234.84 eV); Char1[Fe6O8], [C249H239NO73Fe6] ) -2371.2 kcal (total energy -56 556.76 eV) (∆Hf for this model without the default MECI routine was -2366.2 kcal, total energy was -56557.22 eV). The data show that these char molecular models were destabilized by the larger size of the iron oxides but were stabilized by the additional coordination bonds between coal oxygen functional groups with the iron centers. Significant ionic interactions were indicated by the increase in partial charges on the iron oxide, and the bond orders were consistent with the partially covalent and partially ionic character of the molecule. The iron oxide [(Fe3+)4(Fe2+)2(O2-)8] core consisted of four Fe(III) and two Fe(II) centers, with eight O atoms, and the remaining coordination sites were filled by neighboring hydroxyl ligands. The Fe(III) centers were octahedral and the Fe(II) centers tetrahedral; distortions in the octahedral and tetrahedral sites were observed, as discussed for iron hydroxyl in coal models.3 The calculated bond orders for the iron atoms in Char1[Fe6O8] varied from 1 to 0.4; partial charges on Fe (+) were 0.4, 0.5, 0.6, 0.7, 0.9, 0.9, and on the O bound to iron (-) were 0.8, 0.8, 0.7, 0.7, 0.7, 0.7, 0.7, and 0.6. Although partial charges on two iron centers were higher, none reflected the formal charges on the iron centers; the results were consistent with the significant ionic character of the hydrophilic molecule. Strong H‚‚‚OH interactions were evidenced by the shorter distances between some of the coal-OH ligands. A molecular model of Char1 was also formed with an iron oxide moiety resembling Wu¨stite, [(Fe5O4)]4+ (tetrahedral Fe centers, µ-oxo bonds, and four Fe-OR oxidative bonds in the {coal[(Fe5O4)(OR)4]} macromolecule). 1scf-PM5 results for this molecule were similar to those for the structure shown in Figure 2, but SE results indicated that this structure may be less energetically favored than {Char1[Fe6O8]}, shown in Figure 2. The Fe-O bond orders varied between 1.1 and 0.8. Significant ionic character was evidenced by the partial charges on iron centers of (+) 0.5-0.6 and +0.9; partial charges on oxygen bound to iron centers were (-) 0.6-0.7, and the ligand-oxygen bound to the iron centers [R-O] had values of -0.6. This molecular model was further modified into four similar molecules, by varying the formal oxidation states of the Fe, to reflect the reduction of the iron centers during pyrolysis; that is, the iron moieties were formally [FeIV(FeII)4(O2-)4]4+, [(FeII)4(O2-)2]4+, [(FeII)5(O2-)4]2+, and [FeIV(O2-)4(FeI)4]. Similar ∆Hf values were obtained for these structures, and the calculated partial charges did not reflect the formal oxidation states indicated on the iron moieties. The steric crowding observed for the structure in Figure 2 was not observed for these models. SE results for the molecular models were as follows: {Char1[FeIV(FeII)4(O2-)4]4+} ∆Hf ) -2203.5 kcal (total energy -54 990.14 eV); {Char1[FeIV(O2-)4FeI4]} ∆Hf ) -2213.68 kcal (total energy -55 043.77 eV); {Char1[(FeII)4(O2-)2]4+} ∆Hf ) -2150.17 kcal (total energy -54 033.14 eV). The structure for Char 1[Fe4O2], however, contained bonds between Fe centers and two C atoms of a phenyl group (bond orders 0.8) with bond lengths typical for single Fe-C bonds; the partial charges on these carbon atoms were more negative than those on neighboring oxygen atoms. Data for a molecular model of char formed using steps a to d (labeled Char2) resembling char at 350-450 °C were as follows: SE ∆Hf ) -1228.57 kcal (total energy ) -43298.4 eV); MF ) [C222H182N2O48]; FW ) 3645.864; elemental composition: C 73.14%, H 5.03%, N 0.77%, O 21.06%; weight loss 25 wt %; CO2/CO ratio 1:1.5. The measured weight loss at this temperature range was 20-25 wt %, and the CO2/CO ratio was 1:1.9. Although significant ring condensation was not

Molecular Modeling of Brown Coal Chars

invoked in forming this structure, the proportion of aromatic groups had increased dramatically, and the proportion of two phenyl rings in the molecule had also increased. About 85% of all carbons were aromatic, and a number of CdC bonds were formed from the elimination of functional groups, for example, RCH2-CH2-COOH f RCHdCH2 + CO + H2O The char model developed after all of steps a through e were performed, mimicking char at 600-700 °C, provided a weight loss of 48 wt % (measured weight loss 45-50 wt % at ∼600 °C). This char model was designated Char3: MF ) [C177H151NO14]; FW ) 2516.1; elemental composition: C 84.5%, H 6.0%, O 8.9%, N 0.6%. All of the carboxyl groups, over half of the phenolic groups, and most of the carbonyl and ether groups had been lost. The latter also acted as links for the 3-D macromolecule, and as a result, breaking these links disrupted the 3-D structure. The pyrolysis chemistry includes the formation of carbon- and oxygen-centered radicals through hydrogen abstraction and the recombination of H radicals into H2.4 XRD indicates that the iron oxides in the char at 600-700 °C were reduced mainly to Fe(0), and consequently, these were modeled by using the iron clusters [Fe2O], [Fe3O], [Fe3], [Fe4O], and [Fe4]. Iron clusters have been studied by numerous workers.18-23 The larger-sized iron cluster in this char destabilized the molecule, consistent with the trend discussed previously for char/iron oxide models. The formation of OHfFe coordination bonds between the iron clusters and hydroxyl groups in the char molecule also caused changes to that region of the macroligand. The ∆Hf values for Char3 were as follows: 1scf-PM5 ) 51.5 kcal and SE ) -87.4 kcal (total energy -27756.2 eV). 1scf-PM5 ∆Hf values for Char3 with the following iron clusters were as follows: [Fe3O] ) 371.6 kcal, [Fe4O] ) 532.6 kcal, and [Fe4] 573.4 kcal. The SE ∆Hf values for Char3 with iron clusters were as follows: [Fe3O] ) 145.8 kcal, [Fe4O] ) 370.8 kcal, and [Fe4] ) 576.1 kcal. Data for the {Char3[Fe3O]} molecule were as follows: Fe-O bond orders 0.8 and 0.9, bond lengths 1.88 Å and 1.92 Å, and the third Fe at 2.77 Å from the cluster oxygen with a bond order < 0.1; Fe-Fe bond orders were 2.7, 1.2, and 0.5 and bond lengths were 1.74, 2.03, and 2.26 Å; one coordinate bond was typical (Fe r OH bond length of 2.06 Å, bond order 0.4), but the second Fe‚‚‚OH distance was 2.20 Å and the bond order was 3 Å; the iron species had formed two Fe-O bonds (1.93 and 1.96 Å), with Fe-O-Fe bond lengths of 1.90 and 1.76 Å, and one Fe r OH coordination bond length of 2.32 Å. Short Fe‚‚‚H distances were also observed in this structure. The large size of the [Char3/iron cluster] model required extremely large computer resources for DFT calculations, and consequently, a smaller char model was developed by reducing the size of the [Char3] model into the smaller, similar molecular model, labeled CharD: MF ) [C144H119NO10]; FW ) 2023.49; elemental composition: C 85.6%, H 5.9%, O 7.9%, N 0.7%. The [Fe3] cluster was added to this to form the molecular model [CharD(Fe3)]: MF ) [C144H119NFe3O10]; elemental composition: C 78.9%, H 5.5%, Fe 7.7%, O 7.3%, N 0.6%. Optimiza(27) Ichihashi, M.; Hanmura, T.; Kondow, T. How many metal atoms are needed to dehydrogenate an ethylene molecule on metal clusters?: Correlation between reactivity and electronic structures of Fen+, Con+, and Nin+. J. Chem. Phys. 2006, 125, 133404-133409. (28) Le Page, M. D.; James, B. R. Nickel bromide as a hydrogen transfer catalyst. Chem. Commun. 2000, 1647-1648.

tion of the CharD molecular structure, in which the internal arrangements of phenyl groups were changed relative to each other, provided SE ∆Hf values of 13.0 and 15.0 kcal, while a compacted configuration which maximized OH‚‚‚H interactions (terminated at a trust radius of