Molecular models of brown coal containing inorganic species

Organic Geochemistry Organic Geochemistry 37 (2006) 244–259 www.elsevier.com/locate/orggeochem

Molecular models of brown coal containing inorganic species George Domazetis *, Bruce D. James Department of Chemistry, La Trobe University, Vic., 3086, Australia Received 4 March 2004; accepted 8 July 2005 (returned to author for revision 8 October 2004) Available online 7 December 2005

Abstract Molecular models of inorganic solution species, brown coal and brown coal containing inorganic species have been optimised using the computer-aided molecular package for Windows, CAChe, and the general-purpose semi-empirical molecular package MOPAC. The coal models were based on elemental composition and the experimentally measured distribution of aliphatic carbon, aromatic carbons, and oxygen functional groups. The calculated partial charges and bond polarities of the model are consistent with the hydrophilic properties of brown coal. Water molecules within the model exert the major stabilisation through H-bonds and electrostatic interactions. Calculated values of bond lengths and angles of the inorganic models were in good agreement with reported values, but variations occur because they are modelled as isolated structures. Brown coal models with NaCl, Na+, Mg2+ and Ca2+ are stabilised when the respective cations have been located in spaces that reduce steric hindrance and are surrounded by oxygen functional groups, carboxyl anions and water molecules. Coal models with octahedral Fe(III) or Ni(II) complexes may form stable structures bonded to carboxyl groups, but mono-nuclear complexes with bi-dentate carboxyl ligands usually form distorted structures. Polynuclear complexes form energetically favoured structures in coal, especially when carboxyl ligands form mono-dentate bonds to the metal centres. Reported experimental data on iron and nickel complexes, and for polymeric iron species in brown coal, are consistent with the modelling results.
2005 Elsevier Ltd. All rights reserved.

1. Introduction Coal is a substance derived from a complex accumulation of plant remains in swamps and its heterogeneous nature precludes a definitive molecular structure. As a result, numerous attempts have been made to provide a simpler molecular model for the *

Corresponding author. Tel.: +61 3 9479 2811; fax: +61 3 9479 1399. E-mail address: [email protected] (G. Domazetis).

insoluble organic matter. Models for bituminous coals have included aromatic/hydroaromatic structures in which clusters of aromatic groups have been connected by various ether or aliphatic links. Models for low-rank coals are of greater heterogeneity, especially when humic acids, waxes and other materials are considered as components of brown coal. Ohkawa et al. (1997) considered various approaches for constructing coal molecular structure and proposed a knowledge and partial structure evaluation method. Ad hoc models of low-rank coals, based on elemental composition, carbon distribution and

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2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2005.07.006

G. Domazetis, B.D. James / Organic Geochemistry 37 (2006) 244–259

ratio of one-, two-, and three-ring fused aromatics, and oxygen distribution, have been developed, as for example by Hu¨ttinger and Michenfelder (1987) and Domazetis (2001). Discussions of the structures of lignin and the coalification process by Stout et al. (1988) and Levine (1993) point to central characteristics, such as the distribution of p-hydroxyphenylpropane, guaiacyl, syringyl units, and linkages such as arylglycerol-b-aryl, phenylcoumaran and ether, as reported by Dorrestijn et al. (2000). The degradation processes of wood modify functional groups and split inter-unit linkages, while the gradual degradation of polysaccharides is followed by a slow biotransformation of the lignin macromolecule by depolymerisation, demethylation, de-methoxylation and further defunctionalisation. The formation of carboxyl functional groups during coalification may occur as a result of easier oxidation of the C1 carbon in lignin-like structures. Models derived from structural studies of coalified wood found in brown coals deposits have been reported by Hatcher and co-workers (1982, 1988, 1990, 1994, 1997) and Faulon et al. (1994). A helical template for low rank coal models has been proposed by Hatcher and Faulon (1994). Our major interest has been to develop a model of brown coal suitable for studies of the interaction of inorganic species with oxygen functional groups in the coal. To this end, we have developed and compared two models; one that encapsulates experimentally measured properties and another based on studies of coalified wood similar to that reported by Hatcher and co-workers. Studies of the chemical interactions of various inorganic species have often been underpinned by a notion that an
ion-exchange takes place between aqueous inorganic species and oxygen functional groups effective in coal. The interactions between coal functional groups and various inorganic species, however, are far more complex than suggested by the concept of ion exchange. The inorganic species that may be added to the coal matrix span almost the entire range of metal complexes and such species may well be modified by their environment within the coal. Thus, salts such as NaCl and MgCl2 may be added as a solution of dissolved anions and cations to the wet coal matrix, while acid–base chemistry may be used to add cations to the coal matrix containing carboxyl anions. Transition metal complexes can be added to brown coal, such as for example, the mononuclear and polymeric iron species reported by Domazetis et al. (2005a,b). Davies

245

et al. (1997) have discussed sites in humic acids having different affinities for metal complex formation and brown coal similarly is expected to contain different binding sites and affinities. As a result, numerous chemical interactions involving a variety of chemical species need to be invoked when considering the addition of inorganic species within brown coal. The interactions of inorganics with hydrophilic low-rank coals are relevant to a number of coal utilisation processes, including the removal of inorganics to produce ultra-low ash coal, subsequent addition of selected inorganics, including Fe, Ni, species and studies of reaction pathways of metal mediated pyrolysis and low temperature catalytic gasification. This paper presents results of computer molecular modelling of brown coal, and studies of the interactions of the aqua complexes of Na+, Mg2+, Ca2+, Fe3+ and Ni2+ with carboxyl and phenoxy functional groups of the brown coal model. 2. Computer optimisation of model structures Molecular models of up to 200 atoms were initially developed using the Advanced Chemistry Development Inc., ChemSketch 5.111 software package. Structures up to a molecular weight of 20,000 were developed with the Fujitsu CAChe 5.04 suite of programs, using molecular mechanics (MM), followed by semi-empirical quantum mechanical treatment. Optimisation of large molecules was also performed using MOPAC2002 at the Australian Partnership for Advanced Computing-High Performance Computing Facility (APAC-HPCF). MM is an empirical method for modelling the molecular geometry that treats molecules as balls connected by springs. Interactions include electrostatic, hydrogen bonding, van der Waals, bond stretching, angle bending, bond torsions, and the chemical environment (i.e., atom hybridisation). The augmented MM2 and MM3 force field (FF) parameters provided in CAChe are derived from experimental data. Detailed discussions of MM are presented by Hinchliffe (1996), Comba (1993), Rappe´ and Casewit (1997) and Boeyens and Comba (2001). Deeth (2001) discusses the challenges to MM techniques posed by transition metal complexes and points out that regular geometries do not pose significant technical difficulties, but less regular four-, five-, and six-coordinate species, where valence

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electronic effects play a significant role in determining the molecular structure (e.g., Jahn Teller effect), pose a challenging problem. In the present studies, results from MM using augmented MM3 FF have been used to optimise regular structures and the calculated bond lengths and angles for transition metal structures have been compared with those from published data, to ensure these conform to well understood structures. The Fujitsu CAChe 5.04 for Windows (www.cachesoftware.com) is a computer-aided chemistry modelling package able to carry out optimisation of structures, spectra and reactions, using MM, and MOPAC2002 that includes the MOZYME algorithms, with the semi-empirical Hamiltonians MNDO, MINDO/3, AM1, PM3, PM5, and MNDO-d. The general-purpose semiempirical quantum mechanics package MOPAC2002, by Stewart (1999), can be used for the study of chemical properties of molecules in the gas, solution or solid-state. Calculations provide the electron density, molecular orbitals, electrostatic potential, partial charges and bond orders of the molecules. The MOZYME algorithms in MOPAC 2002 use localised molecular orbitals to lower the timedependency to N1.4 from the N3 required for diagonalisation and for construction of the density matrix (N = number of atoms, orbital or electrons). Only closed shell models are allowed in MOZYME. Computer optimisation of small open shell models complexes was carried out using MM3, MOPACPM5. Large models (where computer resources required are excessive) were studied using extended Hu¨ckel (EH) and single point energy self-consistent field (1SCF) calculations. The 1SCF method calculates the electron density and partial charges of every atom generated by a MOPAC/PM5 wavefunction for the chemical sample, and the total energy for a molecule, with structure co-ordinates provided by MM optimisation. EH provides an all-valence-electron empirical approximation for solving the electronic Schro¨dinger equation. The computation treats all valence electrons and calculates the electronic wavefunction of the structure. All orbitals are based on Gaussian expansions of Slater-type orbitals. Double-zeta basis sets are used for d- and f-functions. Data of total energy, partial charges, bond lengths and bond angles obtained from the same computational techniques have been used for direct comparison between molecular models of brown coal with and without inorganic species. Modelling

data were obtained for: (a) the coal molecular model, (b) the same model with a given number of water molecules, xH2O, (c) the coal model (a) with the added inorganics, but without water molecules, and (d) the coal model with the xH2O in (b) and with the added inorganic. The major modelling difficulties in this work stem from considering single molecules in a vacuum, or as isolated structures. Large molecular models may provide a more realistic environment for the inorganic complexes within the models, and this has been carried out for a few models containing Mg2+ and Ca2+ using MOZYME, but this treatment has been limited because the computer resources required for the higher level MOPAC-PM5 calculations of these models are excessive. The iron and nickel complexes in coal have been modelled as octahedral structures, bonding to carboxyl groups as either bi-dentate or mono-dentate. Some or all of the coordinated water molecules were replaced by coal carboxyl and phenoxy-groups, and these subsequently formed H-bonds with coal functional groups when the structure was optimised. Consequently, most of the coal models used to directly compare changes in the total energy of the molecules with and without the inorganic complex contained the same number of water molecules. The species and variations in bonding of ligands to iron and nickel complexes can lead to a large number of models and as a consequence, initial studies and assessments of these models were conducted using less computer intensive MM and EH techniques. Models of structures that were not severely distorted were further optimised using MOZYME-PM5 and/or 1SCF-PM5. Coulson partial charges on atoms were obtained from MOZYME, Coulson and Mulliken partial charges from MOPAC and 1SCF, and Mulliken partial charges from EH. Structures that proved difficult to study because of excessive computer resources, or other difficulties (such as for example, Fe(III) structures that could not be optimised, discussed in Section 3.3), were studied using a smaller 3-D brown coal model. This model was constructed from two of the unit molecules shown in Fig. 1. The bond lengths and bond angles for this smaller coal model were the same as those in the larger models, but the Fe(III) in the coal constitutes a larger percentage of the total mass of the model and steric effects were relatively greater.

G. Domazetis, B.D. James / Organic Geochemistry 37 (2006) 244–259

247

HO O HO O HO

O HO

H3C

H3C OH

HO

HO OH

O OH OH

OH

O

OH

O OH

HO OH

O

HO

NH HO

HO HO

H3C

Fig. 1. Structure used to develop 3-D models for brown coal.

2.1. Molecular model of brown coal Two molecular models for brown coals have been examined in these studies. One model was based mainly on the transformations of lignin discussed by Hatcher (1990) for coalified wood, with the addition of a humic acid block discussed by Davies et al. (1997). The result was a large 3-D structure consisting of four molecules tightly held by extensive H-bonding. The elemental composition of this model was similar to a typical analysis of brown coal but had higher phenolic oxygen content (single molecule: C219 H228 O69N2; MW 3992.1; four units for 3D structure, MW 15968.4; C 65.9%, H 5.8%, O 27.7%, N 0.7%); functional groups: carboxyl 20%, phenolic 53%, methoxy 6%, ether 4%, and carbonyl 10%, remainder aliphatic hydroxy and quinone; Car/Ctotal = 0.63. Models with water varying up to 21 wt% were optimised using MOZYME-PM5. The model held water molecules on the outer portions of the structure and also in the spaces between the aggregated units, with the greater number of phenolic groups contributing to very strong H-bonding between the molecules and made it an extremely difficult 3-D structure to use for the modelling studies with inorganic species.

The large size of this also required excessive computer resources and consequently has not been considered further in the present paper. The second model of brown coal was based on the experimentally determined values of elemental composition, distribution of aliphatic and aromatic carbon, and oxygen distributed amongst carboxyl, phenolic, ether, ester and methoxy groups. This model did not contain the various lignin linkages, nor waxes and long chain alkyl groups, and did not resemble a helix structure. It was developed by linking the structure shown in Fig. 1 (C86H87NO26, MW 1550.6) into the 3-D molecule shown in Fig. 2 using hydrocarbon and ether linkages (C258H256N2O78S, MW 4664.8). The properties for this molecular model were similar to those of brown coals: Car/Ctot = 0.63 and Har/Htot = 0.2; elemental composition C 66.4%; H 5.5%; N 0.6%; O 26.8%; S 0.7%; distribution of oxygen: carboxyl O(COOH) 21%; phenolic O(OH) 30–35%; methoxy O(O–CH3) 7%; ether and aliphatic hydroxy O(R–OH)
4% and 18%; carbonyl O(RC@O) 11– 15%. The experimentally determined values for brown coal reported by Verheyen and Perry (1991) are typically: Car/Ctot 0.57-0.65; Har/Htot
0.3; elemental composition: C 67.8%; H 4.9%; N 0.61%; O

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Fig. 2. Structure of the 3-D model of brown coal (H atoms removed) constructed from three of the molecules shown in Fig. 1.

26.4%; S 0.32%. The distribution of oxygen is O(COOH) 17–23%; O(OH) 35–38%; O(O–CH3)
12%; O(R–OH)
4%; O(RC@O)
23%. 2.2. Inorganic complexes 2.2.1. Isolated main group ions Models for solution species of sodium and calcium could only realistically be based on ionic species with a hydration cluster of six, and for magnesium a hydration cluster of four or six. Models of cations with hydration spheres suffer from difficulties related to the effects of first and second hydration spheres, discussed by Derepas et al. (2002), and from additional difficulties related to the structure of water and the interactions of cations in water, discussed by Dutkiewicz and Jakubowska

(2002). These matters may be illustrated by considering the results for [Na(H2O)6]+. MM modelling for [Na(H2O)6]+ provided a regular octahedral structure with long Na. . .OH2 bond distances in which electrostatic and H-bonding were the major contributions to the stability. MOPAC-PM5 optimisation, however, resulted in a distorted octahedral configuration of the water molecules. A number of such studies were carried out on distorted structures, and these compared with undistorted octahedral configurations in which values of O–H bond angles and Na  OH2 distances were
locked. The results of the various methods employed are shown in Table 1. Overall, similar values for partial charges were obtained from MOPAC-PM5, MOZYME and 1SCF methods for the same structure. The partial

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249

Table 1 Calculated nearest atom distances and partial charges for isolated [Na(H2O)6]+ Method

MM

MOZYME-PM5

Structure

Regular

Na  OH2 Partial charge

˚ 3.67–3.97 A –

Distorted ˚ 2.67 A

a

MOPAC-PM5 Locked

Distorted ˚ 2.22 A

˚ 2.45–2.46 A +0.48 (+0.61)a

+0.60 (+0.72)a

+0.43 (+0.60)a

Locked ˚ 2.45–2.46 A +0.45 (+0.61)a

Mulliken partial charge.

charges on the cation were lower when Na  OH2 distances were smaller. The Na  OH2 distances of ˚ are comparable to those reported by 2.22–2.67 A Derepas et al. (2002) and similar to the Na  OH2 ˚ for an octahedral structure distances of 2.38–2.67 A reported by Huheey (1975). Attempts to model sodium chloride as two regular octahedral species [Na(H2O)6]+ and [Cl(H2O)6] were unsuccessful, and instead MM3 and MOPAC treatments optimised the water molecules into an irregular configuration. Examples of three models are shown in Table 2. In model 1, water molecules surround the cation and anion, in model 2, water forms an irregular cluster about the cation and anion, with most of the water molecules situated between the ions, and model 3 provided the lowest energy by distributing four water molecules between the cation and anion, and the remainder surrounding Na+ and Cl. Structures of hydrated calcium [Ca(H2O)6]2+ were modelled as distorted octahedral and also as a
locked regular octahedral species. For magnesium, however, three structures were used: (1) a 2þ locked undistorted octahedral ½MgðH2 OÞ6 , (2) distorted octahedral, and (3) a distorted tetrahedral

[Mg(H2O)4 Æ 2H2O]2+. Modelling results for calcium and magnesium structures are summarised in Table 3. Numerous other configurations for the magnesium structures were considered, and generally, the partial charge on the cation varied from +0.75 to +1.97, the lower partial charges being observed in models with shorter Mg  OH2 distances of 1.92– ˚ and the larger values for distances of 2.7– 1.96 A ˚ 4.8 A. 2.2.2. Iron and nickel species Transition metals have been modelled as mono and polymeric solution species and thus involve a number of oxy-hydroxy metal solution species. The octahedral [Fe(H2O)6]3+ is the major mononuclear solution metal complex at low pH, with minor amounts of [Fe(OH)(H2O)5]2+ and [Fe(OH)2(H2O)4]+. Polymeric iron species modelled in these studies include [Fe2(l–OH)2(H2O)8]4+ and [Fe3(III)(l–OH)6(H2O)6]3+. The octahedral complex [Ni(H2O)6]2+ and the polymeric nickel hydroxy complex [Ni2(l–OH)2(H2O)8]2+ was used for these studies. While low pH values favour the mono-iron octahedral complexes, Greenwood and Earnshaw

Table 2 Calculated nearest atom distances and partial charges for isolated [Na(H2O)6]+ and [Cl(H2O)6] ˚) ˚) ˚) Na  Cl (A Na  OH2 (A Cl  HOH (A Partial charge Na+ Model 1 Model 2 Model 3

6.3 3.36 3.96


4 2.4–2.5 2.53–2.62

2.45 2.2 2.20–2.51

+0.95 +0.46 +0.35

Partial charge Cl 0.91 0.86 0.85

Table 3 2+ Calculated nearest atom distances, angles and partial charges for isolated [Ca(H2O)6]2+, ½MgðH2 OÞ2þ 6 , and [Mg(H2O)4 Æ 2H2O] ˚) Model M  OH2 (A H2O  M  OH2 (deg) Partial charge [Ca(H2O)6]2+ (distorted) [Ca(H2O)6]2+ (locked) ½MgðH2 OÞ2þ 6 (locked) ½MgðH2 OÞ2þ 6 (distorted octahedral) [Mg(H2O)4 Æ 2H2O]2+ (dist. tetrahedral) a

Mulliken partial charge.

2.19–2.20 3.88–3.91 1.98–1.99 1.95–1.98 sixth H2O, 3.9 1.93, 1.96, 1.92, 2.3 (4.3, 3.8)

163.2, 194.8, 83.7, 92.3 173.9, 88.9 179.2, 179.6, 89.9 160–176 132.9, 83.7, 109.8,142.1

+1.09 +1.87 +0.69 +0.75 +0.93

(+1.33)a (+1.89)a (+0.88)a (+0.93)a (+1.06)a

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G. Domazetis, B.D. James / Organic Geochemistry 37 (2006) 244–259

Table 4 ˚ ) and angles (deg) for iron complexes containing Bond lengths (A Fe–O bonds [Fe2OCl2(O2CPh)L(H2O)2]Cl, Seddon et al. (2000) ˚) Bond lengths (A Fe(1)–O(3) Fe(1)–O(4) Fe(1)–O(18) Cl(2)–Fe(1)–O(3) Cl(2)–Fe(1)–O(4)

2.158 1.761 2.016 178.69 93.50

Bond angles (deg) O(3)–Fe(1)–O(4) O(3)–Fe(1)–O(18) O(4)–Fe(1)–O(18) Cl(2)–Fe(1)–N(5) Cl(2)–Fe(1)–O(18)

87.78 87.15 98.39 89.82 92.95

[Fe2(bpmp)-(pba)2][BF4]2, Manago et al. (1999) ˚) Bond lengths (A Fe(1)–O(1) Fe(1)–O(2) Fe(1)–O(3) Fe(1)–Oav Fe(2)–O(1) Fe(2)–O(4) Fe(2)–O(5) Fe(2)–Oav

1.943 2.007 1.948 1.966 2.075 2.036 2.130 2.080

Bond angles (deg) O(1)–Fe–O(2) O(1)–Fe–O(3) O(1)–Fe–N(1) O(1)–Fe–N(3) O(3)–Fe–N(2)

87.5 101.4 90.7 162.4 167.0

trans-Bis(quinoline-2-carboxylato)bis(propanol)iron(II), Dobrzyn´ska et al. (2004) ˚) Bond lengths (A Fe–O(1) Fe–O(3) Fe–N(1) O(1)–Fe–N O(3)–Fe–N

2.033 2.170 2.246 102.88 85.96

Bond angles (deg) (1)–Fe–O(3) O(1)–Fe–O(3)i O(1)–Fe–N O(3)–Fe–N O(1)–Fe–N

91.14 88.86 77.12 94.04 102.88

L = 1,2-bis(2,2 0 -bipyridyl-6-yl)ethane. Hbpmp = 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenol. Hbpa = 5-phenylvaleric acid.

(1997) report that higher pH values favour octahedral polymeric l-hydroxy-aqua Fe(III) complexes. Domazetis et al. (2005a,b) have discussed addition

of transition metal complexes to brown coal by step-wise adjustment of the pH of a mixture of a solution of the metal salt with brown coal. Details of mono- and polymeric iron hydroxy species added to brown coal have been discussed by Domazetis et al. (2005a,b). The octahedral complex [Ni(H2O)6]2+ is reported to be the predominant species at low pH by Greenwood and Earnshaw (1997); Baes and Mesmer (1976); and Kolski et al. (1969) report polymeric species at higher pH values. Bond lengths and bond angles from single crystal structure studies of iron complexes containing oxygen-ligands, shown in Table 4, have been compared to calculated values for model iron-hydroxy structures in Table 5. Single crystal structures of these poly-nuclear iron complexes with bridging hydroxy groups have not been reported and instead values of Fe O coordinated bonds reported by Junk et al. (1999) for a crown ether adduct of a l-oxo-bridged Fe(III) aqua dimer has been used to compare with the calculated bond lengths. The bond lengths and angles for the iron complexes are within the range reported for Fe–O and Fe O bonds in octahedral complexes. Variations in the Fe  Fe distance of ˚ are observed for the di-iron between 3.0 and 3.4 A and tri-iron complexes. These are related to the variation in the OH–Fe OH
bite angle which ranged from 68 to 83. Additionally, the relative positions of hydrogens in water and hydroxyl molecules may vary and this leads in some cases to distorted structures. These atoms can be
locked into a specific geometry during the calculation. For example, MOPAC-PM5 calculations [Fe2(lOH)2(H2O)8]4+ with the locked H atoms gave a

Table 5 Calculated bond lengths, bond angles and partial charges for isolated iron complexes Iron complex [Fe(H2O)6]3+

[Fe2(OH)4(H2O)6]2+

a

Method MM3

MOPAC-PM5

MM3-1SCF-PM5

EH

˚) Bond lengths (A Bond angles (deg)

2.11 89.8–90.3, 180.0

2.11 89.8–90.3, 180.0

2.11 89.8–90.2, 180.0, 179.9

Partial charges

–

1.97, 2.08 88.7, 89.0, 87.8, 179.3, 178.9 +0.91 (+1.40)a

+0.86 (+1.40)

+1.74a

˚) Bond lengths (A Fe–O and Fe O ˚) Fe  Fe (A Bond angles O ! Fe–O (deg) Bond angles O–Fe O (deg) Partial charges

1.90, 2.16, 2.17

1.80, 2.05, 1.81, 2.07, 2.03, 2.09, 2.05 3.38 68.5, 81.1, 80.2, 96.5, 99.0, 92.3 174.6, 166.5, 179.1, 175.5, 176.6, 168.1 +0.91, +0.91

1.90, 2.16, 2.17

1.87, 2.13, 2.14

3.08 82.0, 90.2, 89.4, 89.5, 89.7, 82.2 178.8, 176.6, 175.8, 175.9, 179.0 +0.86, +0.95

3.01 82.7, 90.4, 88.5, 92.2, 89.5, 95.4 173.9, 176.1, 178.1, 175.4, 176.5 +1.60, +1.71a

Mulliken partial charges.

3.01 81.8, 89.6, 94.0, 89.9, 82.9 179.3, 176.5, 175.8, 175.9 –

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Table 6 Calculated bond lengths, bond angles and partial charges for isolated nickel complexes Nickel complex

[Ni(H2O)6]

2+

[Ni2(OH)2(H2O)6]2+

Method MM3

MOPAC-PM5

MM3-1SCF-PM5

EH

˚) Bond lengths (A Bond angles (deg) Partial charges

2.09 89.9, 90.2, 180.0 –

2.04, 2.02 86.5, 94.1, 178.3, 176.1 +0.69 (+1.10)a

2.11 89.9, 90.2, 180.0 +0.57 (+0.89)a

2.09 89.9, 90.2, 180.0 +1.49a

˚) Bond lengths (A ˚) Ni  Ni (A Bond angles (deg)

1.85, 2.10, 2.12 2.98 82.9, 97.9, 80.4, 173.5, 177.9 –

1.85, 2.10, 2.09 3.01 82.4, 80.7, 92.6, 172.3, 178.3 +0.41, +0.43, (+0.80, +0.76)a

1.90, 2.16, 2.17 2.99 82.3, 95.6, 87.0, 177.4, 177.9 +0.45, +0.45 (+0.75, +0.75)a

1.85, 2.09, 2.11 2.97 83.0, 91.0, 81.2, 172.6, 178.5 +1.45, +1.45a

Partial charges a

Mulliken partial charges.

72.2
bite angle of the hydroxy ligands, and ˚ , and partial charges on Fe  Fe distance of 3.29 A Fe of +0.91, +0.91 (Mulliken +1.34) (Table 5). The Fe(III)/Fe(II) dimer was optimised with similar bond lengths and angles [Fe2(l-OH)2(OH)(H2O)7]2+, and partial charges of Fe(II) +0.62 and Fe(III)+0.89 (Mulliken Fe(II) +1.17, Fe(III) +1.37). Results for a tri-Fe(III) complex were also similar to the di-Fe(III) structure, with the exception of the bite angles varying from 51 to 78, and partial charges on the three Fe(III) of +0.92, +0.95, and +0.90, (Mulliken +1.32, +1.48, and +1.31). Modelling results for the octahedral [Ni(H2O)6]2+ and [Ni2(OH)2(H2O)6]2+ complexes are shown in Table 6. These bond lengths and angles may be compared to values reported by Liu et al. (2002) for the complex [NiL2(H2O)4] (where L = 1,4-dihydropyrazine-2,3-dione-5,6-dicarboxyl˚ ), ate): bond lengths Ni OH2, 2.041–2.059 (A ˚ Ni–O = 2.046 (A) (mono-dentate carboxylate), bond angles O–Ni O, 93.4, 84.5. The bond lengths and angles for [Ni(OH)(H2O)5]+ were all similar to these values. Optimisation of the di-nickel species [Ni2(OH)2(H2O)8]2+ with MOPAC was sensitive to the positions of the hydrogen atoms on the bridging hydroxyl groups, as noted for the iron complex, and optimisation of the octahedral structure with locked position for MOPAC calculations was carried out. 3. Results and discussion MM3/MOZYME-PM5 treatment of the brown coal model shown in Fig. 2 provided bond lengths and angles within the accepted ranges for organic structures. The calculated partial charges on hydro-

gen of functional groups were, typically: +0.34 (carboxylic), +0.33 (phenolic), +0.20 (aromatic), +0.06 (aliphatic). The calculated partial charges on the oxygen functional groups were typically: (i) carboxyl (C@O) 0.50, 0.48, 0.39; (C–OH) 0.39, 0.38, 0.38, (ii) aliphatic methoxy 0.35 and 0.33, aliphatic OH 0.48, 0.44, and (iii) phenolic 0.35 to 0.38. The largest positive charges for hydrogen were on carboxyl and phenol groups, indicative of the acidity of these groups. The hydrophilic properties of the molecule arise from the ionic character of the molecule, due to the oxygen functional groups in brown coal that participate in: (i) inter-molecular and intra-molecular H-bond formation, (ii) H-bonding with water molecules, and (iii) interactions with inorganic species within the macromolecule. The partial charges on oxygen functional groups, and associated carbon atoms in the coal molecule, increase when water molecules are added to the coal model (e.g., 0.53 for some carbonyl oxygen and +0.40 for carbons) indicating polarity may have increased due to the water molecules. The acidity of carboxyl and phenol hydrogens, as indicated by their partial charges, does not change significantly with the addition of water. The total energy of the brown coal molecules with varying amounts of water has been calculated using MOZYME-PM5 and 1SCF/PM5. Reproducibility for each method has been obtained at better than 0.002% of the total energy, while agreement between the two methods for the same molecular model has been better than 0.01% of the calculated total energy. Changes in the total energy of particular molecules have been used as indicators of the relative stabilities of these various models. Thus, a decrease in the total energy of a coal model with added inorganic, compared with the same model

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without the inorganic, indicates an enhanced stability of that molecular model. The largest decrease in the calculated total energy of the molecular structure was observed when water molecules were added. For example, comparing the total energies of the molecular models (MOZYME-PM5 results) (i) [brown coal], (ii) [brown coal Æ 70H2O], and (iii) [brown coal(COO Na+ Na+Cl). 70H2O], shows the total energy of model (ii) was
30% lower relative to the total energy of the brown coal model without water, while the difference in the total energies between model (ii) and model (iii) was 0.4% lower for model (iii). The breakdown of the various contributions to the total energy of the model from MM3 calculations show the structures enhanced stability was due to H-bonding and electrostatic interactions, while steric crowding destabilised the molecule. Almost all brown coal models containing Na+, Mg2+ and Ca2+ were optimised with partial charges and bond lengths similar to the lowest energy configuration obtained for the aqua complexes discussed in Section 2.2.1. 3.1. Models of brown coal with NaCl, Na, Mg and Ca In practice, inorganic species are added to brown coal using a number of methods, discussed by Ohtsuka and Asami (1997), Shirai et al. (1997) and Domazetis et al. (2005a,b), such as impregnation of the coal with a dissolved inorganic salt, precipitation of a hydroxide within the coal matrix, mixing a paste of calcium carbonate and wet brown coal to facilitate an acid/base reaction, or adjustment of the pH of a mixture of coal and a solution containing the inorganic salt. NaCl has been added to the brown coal model as Na+ and Cl, with varying numbers of water molecules to mimic the hydration clusters of the solution species. The water molecules from this model form H-bonds with the coal functional groups. Models Table 7 Modelling results for brown coal with Na+Cl and Na+ ˚) Model Na  O (A +

˚) Cl  HO (A



Coal(Na Cl ) Coal(Na+Cl) Æ 10H2O  þ CoalðCO 2 Þð2Na Þ Cl   þ CoalðCO2 Þð2Na ÞCl  30H2 O

2.44, 2.43, 2.79, 2.52 2.28, 2.36, 2.51, 2.4–2.6 2.66, 2.52, 2.44, 2.50, .51, 2.59

þ CoalðCO 2 ÞðNa Þ þ ÞðNa Þ  10H2 O CoalðCO 2

2.52, 2.47, 2.46, 2.56 2.47, 2.49, 2.50, 2.64

a

with sodium, calcium and magnesium cations were initially placed adjacent to carboxyl anions within the coal molecule to resemble an acid/base reaction. Results of changes in the total energy, partial charges, bond distances and bond angles of the various brown coal models with sodium are shown in Table 7. The simplest model for brown coal is that containing NaCl. In practice, NaCl can generally be removed from brown coal by washing with water. Modelling shows the greatest change in the total energy of the model results from the addition of water molecules, indicating NaCl is present essentially as a solution in the water held by the brown coal molecule. A mixture of Na+ and NaCl is of practical importance because washing only with water removes NaCl, leaving the remaining Na+ in the coal. The results for [brown coal(COO) (2Na+) Cl] show Na+ cations were weakly held within the polar environment of brown coal, with the total energy of the model lower by 0.7% compared to that of the brown coal model. The [brown coal (COO)(2Na+)(Cl).70H2O], however, was greatly stabilised by water molecules. The cations were situated in
pockets, or spaces with lower steric hindrance within the coal molecule. Suitably situated carboxyl ligands acted as weak bi-dentate ligands. The two Na+ ions were surrounded by phenolic and carboxyl groups, while Cl forms weak Cl  H interactions with hydrogens in phenolic groups and water molecules. Stable structures for [Coal(COO)(Na+)] were obtained with the Na+ located close to a carboxyl cation, acting as a weak bi-dentate ligand, with such models with water molecules also showing water molecules provide most of the increase in the stability of the molecule. This example mimics both acid–base chemistry, and also the concept of ion exchange, as the structure may be described as containing Na+ forming weak ionic bonds with a carboxyl cation. A second carboxyl group, however, is also within a similar distance to

Increased stability is relative to the brown coal model.

1.85, 2.16, 2.07 1.96–2.51 2.15, 2.25, 2.31, 1.95, 2.07, 2.52 – –

Partial charges +0.40, +0.40, +0.52, +0.34, +0.47 +0.37

0.81 0.81 +0.58, 0.81 +0.37, 0.79

Increaseda stability (%) 0.6 5.7 0.7 20 0.03 5

G. Domazetis, B.D. James / Organic Geochemistry 37 (2006) 244–259

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Table 8 Modelling results of brown coal with Ca2+ and Mg2+ Model 

2+

Coal[(COO )2(Ca )] Æ 10H2O 6{Coal [(COO)2(Ca2+)] Æ 30H2O}b Coal[(COO)63(Ca2+)] Æ 10H2O Coal[(COO)2(Mg2+)] Æ 10H2O 2{Coal[(COO)2(Mg2+)] Æ 20H2O}

˚) M  O distances (A

Partial charge

Increaseda stability

2.35–2.45, 2.31–2.38, 2.35–2.70 2.28, 2.30, 2.27, 2.26, 2.25, 2.28 2.15–2.45 2.28, 2.24, 2.31, 2.36, 3.02, 3.40 1.99, 1.97, 1.94, 2.01, 2.05

+0.68 +0.90 +0.78, +0.79, +0.89 +1.07 +0.76

0.04% (5.34%) – 0.02% 0.04c (5.29%) 0.03% (5.35%)

a Increased stability relative to brown coal model with the same number of water molecules; value in parenthesis is increased stability compared to the coal model. b MM/1SCF structure optimisation. c Negative value indicates a decrease in the stability compared to the coal model.

the Na+, so that the anion is surrounded by oxygen functional groups. Data obtained for models containing Ca2+ and Mg2+, shown in Table 8, indicate that these too are stabilised in a manner similar to the models containing sodium, in that they may be slightly more stable than the coal model, but that models with water were significantly stabilised. These results are qualitatively consistent with the ease with which sodium, magnesium and calcium may be removed from a mixture of brown coal and water using weakly acidic conditions. Calcium had fitted neatly into a
pocket within the model that accommodated the cation and was surrounded by three water molecules, two bi-dentate carboxyl anions, and two phenol oxygen groups. The largest model examined in this study was a 3-D structure consisting of six coal molecules agglomerated via H-bonds and electrostatic interactions. Calcium was situated in a space between the agglomerated moieties where two carboxyl ions were well situated and steric hindrance appeared to be minimal. This model provided a distorted octahedral arrangement of oxygen ligands about calcium, with two bi-dentate carboxyl ligands and two coal oxygen functional groups. A coal model with 2.3 wt% Ca (containing three Ca2+) provided a structure in which the cations were surrounded by two bi-dentate carboxylate anions adjacent to two calcium cations and a monodentate carboxylate anion close to the third calcium cation. The remaining three carboxylate anions were not situated near any of the cations. The Ca  O distances observed in all of the models were longer than the sum of ionic radii for Ca  O of ˚ reported by Huheey (1975). 1.82 A Optimisation of [brown coal(COO)2(Mg2+) Æ xH2O] proved difficult as virtually all results either placed the cation on the outer edges of the coal molecule, or the cation was placed at unusually short

distances from atoms within the coal molecule. MM/1SCF optimisation provided a total energy that was almost the same as that of the brown coal model and in some cases slightly less stable. MM/ MOZYME-PM5 optimisation was successfully performed using a larger 3-D model {2[brown coal (COO)2(Mg2+)] Æ 20H2O}, consisting of two brown coal molecules, a total of 20 water molecules and Mg2+ situated in a
non-crowded space between the two coal molecules. The cation was surrounded by five neighbouring oxygen atoms, three from water molecules and two from coal groups. The Mg  O distances were all longer than the sum of ˚ reported by Huheey, ionic radii of Mg  O 1.75 A 1975. 3.2. Models of brown coal with Fe(III) complexes The nature of the oxygen–iron bonds is of particular interest in brown coal chemistry, as a carboxyl ligand may act as bi-dentate, mono-dentate or bridging, Fe(III)–O bonds may also form with phenol groups, and coordination bonds form with the coal functional groups and water molecules. As a consequence, a large number of Fe(III) structures can be postulated, especially when multi-nuclear iron complexes [Fe(III)x(OH)y](3xy)+ are included. An initial assessment was required to rule out unlikely structures and this was done using MM3 and EH methods. Structures which were not excessively distorted were selected for further calculations using 1SCF-PM5. The results shown in Table 9 have been selected to illustrate the overall trends observed in the relative stability of iron complexes. As a general rule, octahedral Fe(III) complexes in brown coal, in which most or all of the carboxyl groups act as bi-dentate ligands, formed distorted models. Complexes with mono-dentate carboxyl ligands and dinuclear iron complexes were less

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Model

Coordination carboxyl

˚) Fe–O bond lengths (A

Fe OR bond ˚) lengths (A

O–Fe–O bond angles (deg)

Partial charge

Increased stability

Coal[(COO)2(FeIII)(O-Ph)(H2O)3]a Coal[(COO)3(FeIII)]

Mono-dentate Bi-dentate

1.91–1.93 1.89–1.90

2.13–2.14 1.89–1.90

75.1, 77.4; 164.7, 171.5, 173.9 66.4, 65.8, 163.4, 166.1, 168.4

0.91 0.82

Coal[(COO)Fe(III)(OH)2(OHPh) (H2O)2].41H2O Coal[(COO)3(FeIII)(OHPh)]c Coal(COO)2[Fe(III)2 l-(OH)2(OH)2(OHPh)2(H2O)2]

Mono-dentate Bi-dentate Mono-dentate

2.13, 2.14 1.92, 2.17 2.15, 2.13,2.17

88.8, 90.0, 95.3, 171.5, 174.1 65.0, 64.9, 167.4, 129.1 80.4, 89.2, 93.4, 170.7, 177.5

0.93 1.08 0.99, 0.97

Coal(COO)4[Fe2 l-(OH)2(OHPh)3 (H2O)]

Mono-dentate

2.12–2.17

Bite 75.8

0.90, 1.13

3.0%

Coal(COO)2[Fe3(OH)7 (H2O)]

Mono-dentate

2.14–2.19

161.2, 85.3, 165.3, Bite 78.5

1.05, 0.92, 1.04

5.1%

Coal(COO)4 [Fe2 l-(OH)2(H2O)8]4+

–

2.12–2.17

Bite 82.0

0.66, 0.67

6.0%

Coal[Fe3(OH4)][Fe4(OH8)(H2O)3]

Mono-dentate

1.88, 1.92 1.89, 1.92 1.88, 1.92, 1.89 (Fe  Fe = 3.1) 1.85–1.92, Fe  Fe 3.2 1.88–1.92 Fe  Fe 3.2, 3.1 1.85 to 1.92 Fe  Fe 3.1 1.90–1.93, Fe  Fe 3.11–3.21

2.2% (0.6%)b Decreased stability 1.3% Negligible 4.4%

2.15–2.18

94.9, 89.8, 160.0, 169.0, Bite 75.8–78.0

0.99,1.01,0.87,1.04, 1.02,1.17,0.86

12.4%

a b c

Includes a Fe bonded with a phenol group and three co-ordinated H2O. Stability relative to [coal Æ 3H2O]. Smaller coal model.

G. Domazetis, B.D. James / Organic Geochemistry 37 (2006) 244–259

Table 9 Modelling results for brown coal with iron species

G. Domazetis, B.D. James / Organic Geochemistry 37 (2006) 244–259

sterically strained. MM3 results of the stabilities of iron complexes in coal were in the order: dinuclear > mononuclear mono-dentate carboxyl > mononuclear bi-dentate carboxyl. A self consistent field could not be achieved for a number of Fe(III) structures. In some cases, however, optimisation was not achieved because of excessive computer time, and for such structures a smaller 3D coal model was constructed with two molecules shown in Fig. 1. The overall stability of the iron containing models increases when water molecules are added to the models, as observed for the previous models. The stability of the iron models also increased when polymeric iron hydroxy complexes were added, and these species enabled all available carboxyl groups to participate in mono-dentate bond formation. Although some distortion was observed in the polymeric iron models, the bite angle (OH–Fe OH) and the Fe  Fe distances were close to the expected values that had been measured for such iron complexes. The addition of the two polymeric iron species with mono-dentate carboxyl groups, [Fe3(OH)4]5+ and [Fe4(OH)8(H2O)3]4+, provided the greatest increase in the overall stability of all the coal models considered. The structures were slightly more distorted than smaller polymeric complexes but the enhanced stability was sufficient to overcome steric constraints. The stability of this model is likely to be due to a combination of factors, including H-bonding between the ironhydroxy groups and coal functional groups, the orientation of the carboxyl groups within the coal model, and the size and length of the iron species being such that suitable carboxyl and phenoxy groups were available to form the appropriate bonding to the iron atoms. A model in which the iron formed a bond with a phenoxy group had a total energy lower than that of the coal model, indicating that it is energetically favoured. A complex containing the ionic species [Fe2(OH)2(H2O)8]4+ was also energetically favoured, this structure formed with minimal steric hindrance in which the negative carboxyl groups were situated at a large distance from the iron species. The model {Coal[(COO)4[Fe2(OH)2(OHPh)4] Æ x(H2O)} was used for a systematic study of the effect of water molecules on the total energy of the coal models (x = 0, 2 and 9). MM3 results revealed significant stabilisation by the water molecules through electrostatic and H-bonding, while steric effects about the iron structure were slightly reduced in models with more water molecules. The

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iron complex formed bonds with mono-dentate carboxyl ligands and caused the brown coal molecule to expand. 1SCF-PM5 results show the total energy decreased by 2.8% with two, and 6.6% with nine water molecules, compared with [brown coal] itself. The total energy of the model with x = 9 was 1.7% lower than that of [brown coal Æ 10H2O], showing the iron complex significantly enhanced the overall stability of the brown coal model. The partial charges on the Fe(III) centres, bond lengths and angles were comparable to those for the aqua complex (Section 2.2.2). The structure became somewhat distorted due to H-bonds between water molecules coordinated to the iron centre and coal functional groups (O–Fe OH2 bond angle decreased from 174.3 to 168.1). Models containing Fe(II)/Fe(III) polymeric iron oxy-hydroxy species were studied using smaller 3D brown coal molecules. MM3/1SCF-PM5 results of binuclear oxy-hydroxy Fe(II)/Fe(III) species indicated the model with the iron complex was >2% energetically favoured compared with the brown coal model. Bond lengths and angles are consistent with octahedral structures about the iron centres and the partial atomic charges on the iron centres were +0.73, +0.82. The increased stability observed when water was added to the models highlights the central role of water in the chemistry of brown coal. Studies of the thermodynamics of humic acids with metal species by Bryan et al. (1998) concluded the major contribution to the Gibbs energy change arises from a large increase in the entropy term while Bryan et al. (2000), indicated the dehydration of the cation may be responsible for the entropy changes. The bond lengths and angles provided by MM3 and 1SCF-PM5 calculations for coal models with Fe(III) complexes compare well with values discussed by Davies et al. (1997) for Fe(III) complexes in humic substances in which the metal ion is surrounded by six oxygen ligands at an average Fe–O ˚ . XAS data for iron comdistance of 2.01 ± 0.05 A plexes in coal by Wasserman et al. (1996) indicated a variety of iron species, and a significant portion of these were considered octahedral structures. XAFS studies by Shah et al. (1996) show the presence of octahedral ferric oxyhydroxide. Ferric oxyhydroxide complexes in Victorian brown coal have also been proposed Bocquet et al. (1998) based on Mo¨ssbauer spectroscopy, these being described as mononuclear octahedral iron complexes situated between micelle-like organic moieties, with a variety of

1.1 a

1.87, 1.86 Bi-dentate Coal[(COO)2Ni2 l-(OH)2(OHPh)2 (H2O)2] Æ 68H2O

1.91, 1.87, 1.90, 1.87 Mono-dentate Coal[(COO)2Ni2 l-(OH)2(OHPh)2 (H2O)4] Æ 66H2O

The total energy was higher than that of the corresponding coal model without nickel, and thus is not energetically favoured.

+0.18, +0.16

1.2 +0.25, +0.14

(0.5)a 0.9 1.4 0.7 0.2 3.3 +0.22 +0.22 +0.28 +0.17 +0.09 +0.25, +0.29

66.7, 65.7 (bite) 171.1, 65.7 (bite) 169.2, 84.0, 82.4, 88.2 90.4, 93.1, 94.9, 170.4, 169.1 95.6, 93.6, 93.8, 177.9, 174.5 77.6 (bite), 81.9, 88.5, 91.4, 176.4, 170.8, 171.7, 169.3 80.9 (bite), 85.0, 95.9, 98.4, 176,0, 173.4, 165.5 81.5, 67.8, 66.4 (bite), 95.5, 83.7, 100.2, 144.2, 163.2, 165.5, 167.4 1.89 2.14–2.15 2.13–2.15 2.12–2.15 2.14, 2.11, 2.10 2.12, 2.16, 2.13, 2.18 Ni  Ni 3.10 2.12, 2.13, 2.16, 2.10 Ni  Ni 3.05 1.87, 2.13, 2.11, 2.14, 2.17 Ni  Ni 3.03 Bi-dentate Bi-dentate Mono-dentate Mono-dentate Mono-dentate Mono-dentate Coal[(COO)2Ni(II)(OHPh)2] Coal[(COO)2Ni(II)(OHPh)2] Æ 6H2O Coal[(COO)2Ni(II)(OHPh)4] Æ 6H2O Coal[(COO)2Ni(II)(OHPh)(H2O)3] Æ 40H2O Coal[(COO)NiII(OH)(OHPh)(H2O)3] Æ 67H2O Coal[(COO)2Ni2 l-(OH)2(OHPh)6 (H2O)]

1.87 1.88–1.89 1.89–1.91 1.90 1.89, 1.85 1.89, 1.91, 1.88, 1.90

Partial charge O–Ni–O bond angles (deg) Ni O bond ˚) lengths (A Ni–O bond ˚) lengths (A Coordination carboxyl Model

oxygen ligands (including carboxyl groups) and perhaps cations situated as next-near neighbours. It is difficult to envisage this arrangement for a significant amount of iron in coal, but small amounts of mono-nuclear iron species are likely to be added at low pH values. A large coal model was developed in these studies to test the distribution of relatively large amounts of iron species in brown coal over the pH range 2–4. The model consisted of four agglomerated coal molecules, containing a mixture of mono- and polynuclear iron complexes, to provide a total Fe
17 wt% of the dry coal. Because of its large size, the model could only be optimised using molecular mechanics. The resulting model was stable with low steric hindrance. The polymeric iron complexes occupied spaces between the individual coal molecules and the model was stabilised by extensive Hbonding between adjacent coal molecules, water molecules and/or hydroxy groups of the iron complex. The mono- and di- iron species were bound to carboxyl ligands in the interior of each coal molecule. Models with bridging carboxyl and carbonate groups in polymeric iron complexes are currently being considered as intermediates in studies of low temperature pyrolysis of brown coal with varying amounts of added iron species. Experimental evidence for polymeric iron species in brown coal has been obtained by Domazetis et al. (2005a,b) from the solution chemistry, scanning electron microscope–energy dispersive X-ray microanalysis (SEM-EDX), X-ray photoelectron spectroscopy (XPS) and time of flight-secondary ion mass spectrometry (TOF-SIMS) data. The distribution of iron species was shown to be uneven using SEM-EDX, especially amongst woody coal particles. XPS data provided ratios of Fe:O consistent with polymeric iron hydroxy complexes in brown coal, and di-iron oxide fragments were identified using TOF-SIMS. Models of brown coal with di-iron hydroxy species have been used in mechanistic studies of the loss of carboxyl groups to form CO2 during low temperature pyrolysis. These studies also mimic the experimentally observed formation of reduced iron oxides and organic radicals. Work is continuing on low temperature pyrolysis of brown coal with varying amounts of iron hydroxy complexes, in which computer modelling predictions of weight loss, CO2/CO ratios, and intermediate iron complexes, are compared with experimental data.

Increased stability (%)

G. Domazetis, B.D. James / Organic Geochemistry 37 (2006) 244–259

Table 10 Modelling results of brown coal with nickel species

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3.3. Models of brown coal with Ni(II) complexes Similar to the Fe(III) cases, a large number of models may also be constructed for Ni complexes and these were initially evaluated using MM3 and EH techniques. MM3/1SCF-PM5 and MOZYME-PM5 optimisation was carried out on selected models. Table 10 contains results for coal models with nickel complexes. A careful selection of carboxyl groups could provide an energetically favoured octahedral Ni(II) complexes with bi-dentate carboxyl ligands, but most structures with bidentate carboxyl ligands were less energetically favoured than ones with mono-dentate ligands. An octahedral nickel complex with bi-dentate carboxyl ligands was calculated with a higher energy than the corresponding coal molecule. A direct comparison between the calculated total energy of two models which were identical except that one contained bi-dentate and the other monodentate carboxyl ligands, shows the mono-dentate coal model was energetically favoured. The [Ni(OH)(H2O)3]+ species formed almost an ideal octahedral complex by bonding to a mono-dentate carboxyl ligand and a coordinated phenoxy ligand. Polymeric nickel hydroxy species were all energetically favoured, with mono-dentate carboxyl ligands forming less distorted octahedral structures compared with bi-dentate carboxyl ligands. The model {Coal[(COO)2Ni2(l-OH)2 (OHPh)3 (H2O)]} formed the most stable structure of those studied, with little distortion in the octahedral arrangement of the ligands. The Ni  Ni distances for the ˚ . The [Ni2(l-OH)2]2+ complexes were all at
3 A partial charges on Ni were all lower than those observed for the aqua-species, with calculated values as low as ca. +0.1. Although Shirai et al. (1997, 1999) used infrared spectroscopy to assign bi-dentate carboxyl ligands to nickel complexes in brown coal with 0.77% wt. of nickel in coal, and bridging carboxyl groups for 4% wt. of nickel, modelling of such complexes indicates the mono-dentate carboxyl ligands are more likely. Larger amounts of nickel in brown coal are likely to include poly-nuclear nickel species, such as the di-nuclear l-hydroxy complex discussed above. XANES data obtained by Shirai et al. (1999) and Murakami et al. (1995) for nickel complexes in brown coal have been interpreted as Ni surrounded by six oxygen atoms from carboxyl, H2O and OH ligands, indicating an octahedral complex.

257

4. Conclusions • Water held within the hydrophilic brown coal molecule exerts the dominant effect on the overall stability of the molecular structure. • Modelling studies of brown coal containing Na+, NaCl, Ca2+ and Mg2+ have shown energetically favoured structures are formed with lower steric crowding when these species are situated in a
space or
pocket surrounded by oxygen functional groups and water molecules. Results from larger coal models reinforce these results and show carboxyl groups may act as
bi-dentate ligands to Ca2+, and in some cases to Na+. • Optimisation of models of brown coal with iron and nickel complexes has revealed that distorted structures are less energetically favoured, while sites in the brown coal molecule in which carboxyl and phenoxy-groups can form octahedral transition metal complexes with minimal steric strain are energetically favoured. Mono-dentate carboxyl ligands to iron and nickel provide less distorted structures. • The calculated atomic partial charges on the cations usually decrease in the brown coal molecule relative to those of similar solution species; the values however, are sensitive to the configuration of the complex, including bond lengths and bond angles of the species added to coal model. • Polymeric multi-nuclear complexes are energetically favoured to form in coal. In practice, the amounts and nature of iron and nickel species added to brown coal will depend on the chemical conditions used, including the pH and concentration of the metal ion solution mixed with the brown coal. • Additional studies of the chemistry and bonding of metal complexes in brown coal, including structural studies such as carried out using XANES, will greatly improve our understanding and increasingly validate modelling results. Continued development of brown coal models, to include the heterogeneous nature of the organic components, is also required. Computer molecular modelling of brown coal with iron-hydroxy species is proving useful in investigating reaction pathways of metal mediated low temperature pyrolysis.

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