and Polynuclear Iron Hydroxy Complexes in Brown ... - ACS Publications

Mar 22, 2005 - Chemistry Department, La Trobe University, Victoria, 3086, Australia ... Iron hydroxy complexes were added to brown coal by a stepwise ...
1 downloads 0 Views 276KB Size
Download PDF
Energy & Fuels 2005, 19, 1047-1055

1047

Studies of Mono- and Polynuclear Iron Hydroxy Complexes in Brown Coal George Domazetis,* Monthida Raoarun, and Bruce D. James Chemistry Department, La Trobe University, Victoria, 3086, Australia

John Liesegang Physics Department, La Trobe University, Victoria, 3086, Australia Received January 1, 2005. Revised Manuscript Received February 11, 2005

Iron hydroxy complexes were added to brown coal by a stepwise pH adjustment of solutions of Fe(III) mixed with coal. The amounts of NaOH used to adjust the pH and the amount of Fe(III) added to coal were consistent with the addition of both mono- and polynuclear iron complexes to coal. Analysis of the bulk sample and of the sample sieved into smaller and larger particles, and SEM-EDX examination of large and small particles, revealed an uneven distribution of iron, with higher concentrations in smaller coal particles. XPS and TOF-SIMS data show that monoand polynuclear iron species form in the coal samples. Computer molecular modeling has illustrated the structure and bonding of the mono- and polynuclear species in brown coal and the variation in the relative distribution of these species as the amount of added iron is increased. Computer models have also been used to show similarities between low-temperature pyrolysis of brown coal containing large amounts of iron and the thermal decomposition of iron carboxylates.

Introduction The use of catalysts in the gasification of low-rank coals has the potential to improve the yield of hydrogen and carbon monoxide and offers one of the cleanest options for power generation. An improved yield of hydrogen and carbon monoxide has been attributed to catalytic gasification,1 and steam gasification of char containing inorganics has been reported to improve the gaseous yield, including that of methane.2 Calcium and iron, when added to low-rank coals, have been shown to exert an additional impact on the steam reformation of tars and char conversion, and catalytic activity has been discussed for iron and compounds added to Loy Yang coal (Victoria, Australia).3,4 A clearer understanding of the nature of the inorganic species added to brown coal is critically important for an understanding of any catalytic activity that may be observed, as it is necessary to distinguish between catalytic from noncatalytic events attributed to the added inorganic species. Iron complexes as catalysts have been discussed5 and are of particular interest for * Corresponding author. Telephone: 61 3 9479 2811. Fax: 61 3 9479 1399. E-mail: [email protected]. (1) Lee, J. M.; Kim, Y. J.; Kin, S. D. Appl. Therm. Eng. 1998, 18, 1013-1024. (2) Timpe, R. C.; Kulas, R. W.; Hauserman, W. B.; Sharma, R. K.; Olson, E. S.; Willson, W. G. Int. J. Hydrogen Energy 1997, 22, 487492. (3) Yamashita, H.; Ohtsuka, Y.; Yoshida, S.; Tomita, A. Energy Fuels 1989, 3, 686-692. (4) Ohtsuka, Y.; Asami, K. Catal. Today 1997, 39, 111-125. (5) Huttinger, K. J.; Adler, J.; Hermann, G. In Carbon and Coal Gasification; Figueiredo, J. L., Moulijn, J. A., Eds.; NATO ASI Series E, No. 105; Martinus Nijhoff: Dordrecht, The Netherlands, 1986; pp 213-229.

industrial gasification of low-rank coals. Studies of catalytic gasification, however, have been difficult because of the heterogeneous nature of coal, the diverse methods of adding inorganics to coal, and the variety of inorganic complexes added to these coals as catalysts. We have shown that a variety of aqueous inorganic species may be added to brown coal and that these may exert pronounced effects on brown coal heated under inert and reactive atmospheres.6 Studies of the Mo¨ssbauer spectra of Fe2+ and Fe3+ species in brown coal have led to the suggestion that octahedral iron complexes are found in brown coal, with additional bonding between carboxylate iron complexes and other Fe complexes in close proximity via H-bonded water molecules.7 A variety of structures of iron complexes with oxo- and carboxyl ligands has been reported,8 for example, [(H2O)5Fe(µ-O)Fe(OH2)5]4+. We have also used computer molecular modeling to study main group and transition inorganic species within a molecular model of brown coal and have shown that a variety of species may be added to brown coal, their addition dependent on pH, rate of uptake, and concentration of the inorganic complex in the mixture of solution and brown coal.6,9 In this paper, experimental data on the addition of various amounts of iron to brown coal samples have been used to show that it is possible for several aqua (6) Domazetis, G.; Liesegang, J.; James, B. D. Fuel Process. Technol. 2005, 86, 463-486. (7) Bocquet, S.; Cashion, J. D.; Cook, P. S. Phys. Chem. Miner. 1998, 25, 328-337. (8) Junk, P. C.; McCool, B. J.; Moubaraki, B.; Murray, K. S.; Spiccia, L.; Cashion, J. D.; Steed, J. W. J. Chem. Soc., Dalton Trans. 2002, 1024-1029. (9) Domazetis, G.; James, B. D. Org. Geochem. Submitted for publication, 2004.

10.1021/ef050001i CCC: $30.25 © 2005 American Chemical Society Published on Web 03/22/2005

1048

Energy & Fuels, Vol. 19, No. 3, 2005

iron complexes, including multinuclear iron species, to be added to brown coal. Characterization of the iron species is based on solution data, SEM-EDX, XPS, and TOF-SIMS studies. Factors affecting the chemical interactions of various iron species with brown coal will be illustrated using computer molecular modeling. Application of molecular models to low-temperature pyrolysis of samples of brown coal with added iron species will be discussed. Experimental Section Coal. A solid block of German brown coal containing minimal inherent mineral particles (supplied by Rheinbraun GBT, Germany) was initially crushed using jaw crushers and then pulverized with an IKA-Werke MF10 impact analytical mill until the coal passed through a mesh 10 sieve (1.68-mm opening). A relatively large batch of this coal sample was thoroughly mixed and acid washed to an ash content of ∼0.1% (db). The acid-washing method has been outlined previously.6 After being thoroughly mixed and air-dried to a final moisture content of 28.4 wt %, this batch was used for the preparation of all iron-containing samples. Adding Iron(III) to Coal. A 0.3 M stock solution of Fe(NO3)3 was prepared using AR grade Fe(NO3)3‚9H2O, and a 0.2 M solution of NaOH with AR grade NaOH. A known volume of the Fe(III) solution (equivalent to the amount of iron added to the coal) was diluted to 500 mL (pH ∼1.4) and the pH adjusted to about 2, by slow addition of 0.2 M NaOH. The volume of NaOH used and the pH of the Fe(III) solution were measured. Acid-washed brown coal (78.0 g) was then added to the Fe(III) solution. The pH of the coal/solution mixture dropped, and the pH value was measured when it had reached a constant value. The 0.2 M NaOH was then slowly added to the stirred mixture until the pH had increased to the original value of the solution prior to the addition of coal. The mixture of coal and Fe(III) solution was stirred continuously until the pH value remained constant, usually for >2 h, and the amount of NaOH used in adjusting the pH was measured. The coal was filtered off and the pH of the clear solution was measured. A small sample of the clear solution was tested using atomic absorption spectroscopy to determine the iron remaining. The procedure was then repeated, until the atomic absorption result showed negligible iron remained in the solution. The pH values of the solution and the mixture and the volume of NaOH used at every step were measured. The coal was finally separated and washed thoroughly using excess distilled water and then air-dried and stored in polystyrene jars. The volumes of NaOH used to adjust the pH of the iron solution (prior to adding coal), and the amount of iron remaining in solution, were used in calculations of ratios of [OH] to [Fe] for the solution, indicative of the iron-hydroxy species in solution (see discussion below). The total volume of the iron solution was carefully monitored as some of the solution was taken for analysis after each step. The amounts of NaOH used to neutralize the acid released after the coal was added to the iron solution for each step were also measured and used in calculations as indicators of the type of iron species added to the coal on the basis of the protons released from coal. Mass balance calculations were performed by analyzing a small portion of the clear iron solution after each addition step to determine the concentration of the iron in the known volume. The difference between the amount of iron in the solution at each step and that of the total iron used initially was assumed to be equal to the iron added to the coal. The amount of iron in the coal after each addition was also determined by acid extraction. In all cases, some iron was lost during analysis of solution samples used to monitor the uptake of iron during each step in addition. Usually, 0.2 wt % or less

Domazetis et al. of the total iron remained in the final solution after the final step; the mass balance accounted for 92-95 wt % of the total iron used in the experiment. Experiments were performed to test possible effects of the time taken to add iron to coal particles at given pH values. This involved stirring the coal/solution mixture for extended periods of time (the maximum time taken for an experiment was ∼2 months) for a given pH value. The pH value of the mixture was stable after a few hours of stirring. The amount of iron added by stirring overnight, or after more then a week, was not measurably different, nor was it possible to detect any measurable difference in the distribution of coal between larger coal particles and smaller ones attributed to longer periods of time used during each step. Experiments were performed to test the effects of the initial iron solution pH (and subsequent adjusted pH of the solution/ coal mixture to the original value) on the addition of iron. For example, NaOH was slowly added to a 0.05 M solution of iron nitrate, care taken to avoid a precipitate, to reach a pH between 3 and 4. A known amount of coal was then added to the clear solution. The pH of the mixture dropped and was then readjusted to the original value by adding NaOH, as described previously. At this pH, most of the iron in the solution was added to the coal during the first step (see discussion). The remaining iron was added in subsequent steps. Addition of NaOH to a Coal/Water Mixture. One hundred grams of brown coal was mixed with sufficient distilled water to equal 500 mL and the mixture was stirred using a Teflon coated magnetic stirrer. Sufficient 0.10 M NaOH solution was added to the coal/water using a buret to cause a 0.1 increase in the pH change of the coal/water mixture. Once the pH was stable, another aliquot of NaOH was added to again increase the pH by 0.2; this was repeated until the pH of the mixture reached 7. Atomic Absorption Spectrometry (AAS). Iron and sodium in the treated coal were acid extracted for analysis by AAS with a GBS 933 Spectrometer for sodium at 589.0 nm and for iron at 248.3 nm. Complete extraction of iron was achieved and the coal sample, after acid extraction, provided the same amount of ash (∼0.1 wt %) as was present in the coal sample prior to iron addition. Scanning Electron Microscopy-Energy Dispersive X-ray Microanalysis (SEM-EDX). A JEOL model 840A instrument with Link Analytical Pentafet X-ray energy dispersive detector, using the thin window, allowed for the detection of carbon and oxygen. The microscope was operated at 20 kV with the detector resolution being 144 eV. Samples were covered with either 20 nm of carbon or approximately the same thickness of platinum. Samples were dried in a desiccator. A variety of sample preparation procedures was used including pressed pellets and epoxy mounted samples, which were either polished or microtomed to reveal the particle interior. Single particles were also microtomed to reveal the particle exterior. Pellets were prepared by grinding the sample and pelletizing using a Perkin-Elmer hydraulic press at a pressure of 6 tons/m2 for 2 min. X-ray Diffraction (XRD). Finely ground samples of the coals, chars, and ashes were analyzed using a Siemens Kristalloflex D5000 Diffractometer and graphite monochromator over the 2θ analysis range of 4°-70° using Cu KR radiation. XRD peaks were analyzed using Macdiff 4.2.5 software. Time-of-Flight-Secondary Ion Mass Spectrometry (TOF-SIMS). TOF-SIMS spectra were obtained using a TOF-SIMS IV (Ion-TOF GmbH, Germany) instrument with a reflectron analyzer, a Ga+ ion source (25 keV), and a pulsed electron flood source for charge neutralization. The pressure in the analysis chamber was typically less than 3 × 10-8 mbar. The primary pulsed ion beam current was 2.5 pA and the primary dose was lower than 1.0 × 1013 ions/cm2. All experi-

Mono- and Polynuclear Iron Hydroxy Complexes ments were performed using a cycle time of 100 ms. The mass resolving power was typically greater than 7500 at m/z ) 27. Positive and negative spectra were acquired from a 100 mm × 100 mm area. Samples of coal were used both as powder and as a pellet. Pellets were prepared as in SEM-EDX. The identification of the relevant peaks in the spectra has been performed using principal component analysis that finds the mass combinations that best describe the data, with the results displayed to allow classification of similar species. X-ray Photoelectron Spectroscopy (XPS). XPS experiments were performed using a Kratos Axis Ultra spectrometer (Kratos Analytical, U.K.) with monochromatized Al KR radiation (1486.6 eV) operating at 150 W. Both survey and highresolution region spectra were recorded. The spectrometer energy scale was calibrated using the Au 4f7/2 photoelectron peak at electron binding energy EB ) 83.98 eV. Spectra were charge-corrected with reference to C-C species at EB ) 285.0 eV. Such surface charge neutralization was used to effect a slight improvement in peak resolution. The analysis area was 700 µm × 300 µm. Spectra were quantified using Kratos XPS elemental sensitivity factor data after background subtraction and the fitting of Gaussian (70%)/Lorentzian (30%) component peaks. The full width at half-maximum (fwhm) of the peaks was maintained constant within a chosen region spectrum for all components. Atomic concentrations were recalculated by assuming that coal contained 5 wt % of hydrogen, and uncertainties for all fitted spectra were estimated to be (10 at. % of the measured concentrations. Molecular Models of Iron Complexes in Brown Coal. Details of computer molecular modeling studies are found elsewhere;6,9 briefly, molecular models of brown coal have been used to study the interactions of aqua species of Na+, Ca2+, Mg2+, Fe3+, and Ni2+. In this work, such models have been used to evaluate the coordination and stereochemistry of iron in the macroligand and the amounts of iron that may be added. Models were optimized with the software packages CAChe 5.04 and MOPAC2002, using molecular mechanics and single-point energy self-consistent field calculations with the PM5 Hamiltonian (MM/1SCF-PM5). The elemental composition of the coal model was typical for brown coals (C, 66.6 wt %; H, 5.5 wt %; N, 0.9 wt %; O, 26.9 wt %). Bond lengths and bond angles in the optimized models were those typical of all atoms present in the organic and inorganic moieties. The calculated properties of the coal model were Caromatic/Ctot ) 0.63; Haromatic/Htot ) 0.2; distribution of oxygen functional groups were carboxyl 23%, phenolic 30%-35%, methoxy 7%, ether and aliphatic hydroxy ∼4% and 18%, carbonyl 11%-15%. These have been compared to experimentally determined values6,9 for brown coal, typically Caromatic/Ctot 0.57-0.65; Haromatic/Htot ∼0.3; elemental composition: C, 67.8%; H, 4.9%; N, 0.6%; O, 26.4%; S, 0.3%; distribution of oxygen functional groups: carboxyl, 17%-23%; phenolic, 35%-38%; methoxy, ∼12%; ether and aliphatic hydroxyl, ∼4%; carbonyl, ∼23%. The iron species used in examining models were Fe3+, [Fe2(OH)2]4+, [Fe(OH)]2+, [Fe3(OH)4]5+, [Fe(OH)2]+, [Fe5(OH)12]3+, [Fe4(OH)10]2+, [Fe5(OH)13]2+, [Fe5(OH)12(H2O)2]3+, and [Fe7(OH)16(H2O)5]5+. Low-Temperature Pyrolysis. Samples consisted of acidwashed coal and coal with known amounts of iron. These were placed in a porcelain boat and dried in an atmosphere of nitrogen by heating in a tube furnace at 120-150 °C to a constant weight. The known weight of dry coal was then heated to a constant weight at a specific temperature between 200 and 400 °C; the char was then allowed to cool to room temperature under nitrogen and weighed to determine total weight loss. Samples were stored under nitrogen or in a desiccator for later use in SEM-EDX, XRD, and XPS studies.

Energy & Fuels, Vol. 19, No. 3, 2005 1049 The gases were sampled in a standard gas cell for analysis using a Perkin-Elmer model 1600-X FTIR instrument. The FTIR cell was calibrated with standard mixtures of CO and CO2.

Results and Discussion Addition of Fe(III) Species to Brown Coal. Coal samples with low, medium, and high amounts of iron have been prepared by using consecutively higher concentrations of iron. The respective amounts of iron in each of the coal samples, on the basis of the dry weight of the coal (and sodium from the NaOH used in the preparation) were as follows: Fe, 1.9% (Na, 0.1%); Fe, 1.7% (Na, 0.09%); Fe, 4.5% (Na, 0.4%); Fe, 4.4% (Na, 0.3%); Fe, 7.8% (Na, 0.5%); Fe, 8.5% (Na, 0.7%); Fe, 10.9% (Na, 0.7%); Fe, 11.26% (Na, 0.8%); Fe, 12.4% (Na, 1.7%). Typically, the initial pH of the solution of iron nitrate was ∼1.4, and it was then adjusted to pH ∼2 prior to adding the coal (amount of NaOH measured). After the coal was added, the pH dropped to ∼1.5 and it was then readjusted to the original value (the amount of NaOH used in this step also measured) and the solution was separated from the coal by filtration. The pH of the clear solution was again adjusted to a higher value and the procedure repeated. In this way, the amount of NaOH used to adjust the pH of the iron solution, and the NaOH used to neutralize acid released after iron was added to the coal, was known. The amount of iron added to the coal for each step was calculated by subtracting the iron remaining in solution from the total iron. Care was taken to ensure that a precipitate of iron hydroxide did not form at any time. The addition was completed when the concentration of iron in the filtered solution was negligibly low. The final pH value for each addition of iron to coal was about 4. The amount of iron in the coal samples was also checked from time to time by taking a portion of the treated coal, washing it thoroughly, and determining the iron by acid extraction. At completion, the treated coal was washed thoroughly and the amount of iron in the sample was obtained by acid extraction and AAS analysis. The mass balance calculations accounted for 91-95 wt % of the total iron used at the commencement of each addition, while the amount remaining in the water after the final addition was 95% of iron in solution was used, adding a total of 10.9% iron in coal. The overall value for [OH-]: [Fe] was ∼2.3:1, indicating that a mixture of mono- and polynuclear iron species was added to coal. (b) In the first step, the pH of the iron solution was adjusted to ∼3.4, requiring a larger amount of NaOH. The coal was then added with stirring, and the drop in pH was fairly low, requiring a smaller amount of NaOH to readjust to the original pH of ∼3.4. In this first step, 91.7 wt % of iron in solution was added to the coal, and the remainder in another two steps. Phreeqc calculations of the iron solution at pH 3.4 indicated that 92 wt % of the iron was present as [Fe3(OH)4]5+ and [Fe2(OH)2]4+. Calculated values of B and B* were indicative of polymeric species of the type [Fex(OH)y](3x-y)+ and [FexOz(OH)y](3x-2z-y)+. The amount of NaOH used to readjust the pH of the solution in the first step, after mixing with coal (chemistry similar to eq 3), could also be rationalized by considering species of the type [Fex(OH)y](3x-y)+. Here, the parameters may range from x ) 4, y ) 11 to x ) 3, y ) 13 to satisfy the observed [OH-]:[Fe] ratios for two experiments of 0.33 and 0.25. For example, if x ) 4 and y ) 11, the species [Fe4(OH)11]+ would release one [H3O]+ when added to coal, providing a ratio of [Fe]:[OH] of 0.25. The sequence of events may be represented as 4Fe3+ + 11(OH)- f [Fe4(OH)11]+ + coal(COOH) + H2O f coal{(COO)Fe4(OH)11} + H3O+. The data indicate that at higher pH a larger proportion of the iron exists as polymeric hydroxyl species, and these require a smaller amount of NaOH to neutralize the resulting acid after the iron species have been added. This treatment of the solution data, however, is based on the assumption that once iron hydroxy complexes are formed in solution, they do not undergo any changes in composition within the coal matrix, with the

1052

Energy & Fuels, Vol. 19, No. 3, 2005

Domazetis et al.

Figure 5. SEM-EDX analyses of coal particles coated with platinum: (a) acid-washed coal particles (700×), (b) a large woody particle with added Fe(III) (40×), (c) the center of a microtomed large particle from sample b (80×), and (d) elemental distribution from a small particle (400×).

exception of forming bonds with carboxyl groups and coordination bonds with available oxygen functional groups. The molecular models discussed below clarify this assumption by showing that molecular molecules are energetically favored with multinuclear iron complexes that adopt a nondistorted octahedral stereochemistry. The solution data allow for a classification of aqua mono- and multinuclear iron species in the iron/coal solution mixture, described by eqs 1, 2a-2d, and 3, but the data cannot conclusively determine the exact nature of all species of iron present in the coal particles. XPS examination of coal particles with relatively large amounts of iron complexes has detected iron and inorganic hydroxide, consistent with the proposed hydroxyl-iron species. Di-iron oxy fragments were also observed in TOF-SIMS examination of these coal

samples, consistent with polynuclear iron complexes in treated coal samples. SEM-EDX, XPS, and TOF-SIMS. The SEM-EDX spectra of particles of acid-washed coal consist of C and O peaks, while that of coal particles with added iron contain an additional peak due to Fe, with increased relative intensity of O. SEM-EDX data have been obtained from the surfaces of small and large particles, and numerous areas of such particles were analyzed to provide an estimate of the variation of Fe in the coal particles. The appearance of larger coal particles was often woody. The SEM-EDX data show a considerable variation in the amounts of iron-hydroxy species observed near the surface and the interior of large coal particles, illustrated in Figure 5. The C, O, and Fe elemental distributions were observed from the surfaces

Mono- and Polynuclear Iron Hydroxy Complexes

of acid-washed coal particles, the surfaces of large woody particles, the center of microtomed large coal woody particles, and from small coal particles. Variations in iron content often differed from particle to particle, but variation in iron was not as great among small coal particles. The SEM-EDX results were consistent with the variation in the total iron of two batches of treated coal obtained by sieving into large and small coal particles (Table 1). Numerous SEM-EDX results from large woody particles show that most of the added iron species were on the outer area of the particles, decreasing to small amounts in the interior. Samples of coal and iron solution that were mixed for over 2 weeks contained large particles showing similar concentration variations. XPS and TOF-SIMS have been used to identify the major elements on the surface of coal samples,13-15 and binding energies have been assigned to inorganic oxygen for several inorganic oxides and hydroxides.16 The XPS region scans of numerous coal samples containing varying amounts of iron have identified the oxygen attributed to the coal (O 1s binding energy 533.8 and 532.0 eV) and to the iron complex (O 1s binding energy 530.5 eV). The region scan of coal samples with FeH3O2- > Fe2O2H- > Fe2O3-. The oxy- and hydroxyl- iron fragments from the iron-containing coal samples may be compared to the fragments FeO2-, FeO3-, and FeO2H‚O- reported for the TOF-SIMS of crystals of iron sulfate containing water of hydration and in which the multinuclear oxy- sulfur-iron fragments Fe2O3‚HSO4- and Fe(OH)SO4‚Fe2O3‚O- were also observed.19 Although fragments of iron-containing organic ligands have been reported for TOF-SIMS of ferrous materials coated with a variety of lubricants containing carboxyl groups,20 similar iron-containing organic fragments were not observed in the data from our coal samples. Molecular Models of Iron Complexes in Brown Coal. Molecular models have been used as a basis for (i) calculating the total amount of iron that can be bound to all the carboxyl groups in coal, (ii) providing the stereochemistry of the various iron complexes in coal, (iii) calculating the total energy of coal models with and without iron complexes, to indicate if coal models with particular iron complexes were energetically favored, and (iv) calculating the number of OH molecules in the Fe(III)-hydroxy complex in coal, to provide the [OH]: [Fe] ratio of iron species for comparison with experimental values. The results for the amounts of mononuclear iron complexes needed to bond to all of the available carboxyl groups in the coal model were Fe3+, 3.5 wt %; [Fe(OH)]2+, 5.1 wt %; and [Fe(OH)2]+, 9.2 wt %. Coal models with these species providing iron ranging from 10 wt % in the coal, consistent with the measured amounts of iron added to coal. For example, MM3/1SCF-PM5 optimization provided an energetically favored structure, C172H188N2O69Fe6, MWt 3722.431, in which the six carboxyl groups acted as two bidentate and four monodentate ligands to [Fe2(OH)2]4+ and [Fe4(OH)10(H2O)2]2+, giving an overall atomic ratio OH: Fe of 2:1, with partial charges on each of the iron centers of +1.12, +1.1 and +0.87, +0.91, +0.84, +0.97, and all bond lengths and angles for these iron complexes normal for iron hydroxyl compounds. Other models included (a) a coal model with sufficient [Fe5(OH)12]3+ to bind with all carboxyl groups and provide ∼13 wt % of Fe and (b) a large coal model containing several of the iron species present over the pH range discussed in this report. Model b was a large molecular model, consisting of four coal molecules held together by hydrogen bonds and electrostatic interactions and containing a mixture of mono- and polynuclear iron complexes to a total Fe ∼17 wt %. Because of its large size, the model could only be optimized using MM. The model experienced low steric hindrance and was optimized to a structure with a lower total energy compared to the same coal model without iron. The polymeric iron complexes occupied spaces between the coal molecules. This molecular structure was stabilized by extensive hydrogen bonding between adjacent coal molecules and water molecules or hydroxy groups of the iron complex. The smaller mono- and dinuclear iron species in this model were bound to carboxyl ligands in the interior of each coal molecule. All carboxyl groups formed bonds with the iron species, and phenol groups formed coordination bonds to iron species, displacing the coordinated water molecules on iron. The atomic ratio OH:Fe averaged over the monoand multi-iron complexes was typically 2.1:1. These studies provide information on iron species that can be added to brown coal under laboratory conditions. The exact nature of the iron species in mined brown coal may differ from the species discussed in this report

Domazetis et al.

because of the different geochemical conditions during coal formation. The amount of iron in mined brown coal is relatively small, for example, Victorian brown coal typically contains 0.5 wt % Fe in dry coal.21 The presence of other soluble species, including NaCl, in brown coal deposits may further complicate the chemistry as Cl- can act as a ligand to iron complexes. Ferric oxyhydroxide complexes in Victorian brown coal have been proposed on the basis of Mo¨ssbauer spectroscopy, as mononuclear octahedral iron complexes situated between micellelike organic moieties, with a variety of oxygen ligands (including carboxyl groups) and perhaps cations situated as next-near neighbors.7 It is difficult to envisage this arrangement for a significant amount of iron added to clean coal. The conditions used in adding iron to coal for such studies would appear to favor the addition of a mixture of mono- and polynuclear iron species such as [Fe(H2O)6]3+, [FeOH]2+, and [Fe2(OH)2(H2O)8]4+, while impregnating coal (and then removing bulk water) with a solution of iron salts such as Fe(NO3)3 or FeCl3, or adjusting the pH of such a mixture to between 2 and 11, would produce coals with a mixture of crystalline salts, iron-hydroxy species, and precipitated iron hydroxide.3,22,23 Speculation has been reported on iron hydroxyl species and an assumed dependence of gasification mechanisms (and iron intermediates during gasification) on the pH used during the addition or mixing of various iron salts with coals.24-26 Low-Temperature Pyrolysis. Although a detailed treatment of pyrolysis results is outside the scope of the present discussion, computer molecular modeling of brown coal iron species has provided useful insights into the pyrolysis chemistry, on the basis of the assumption that in coal samples with >10 wt % total Fe, all of the carboxyl groups in coal are present as iron carboxylate complexes. Data from our pyrolysis experiments includes the FTIR spectra of gases from acid-washed coal and coal with increasing amounts of iron. When heated over the temperature range 200-350 °C, the acid-washed coal yielded a mixture of CO, CO2, CH4, and H2O, with a relatively high proportion of CO to CO2. Under the same conditions, coal containing iron yielded a gas mixture of CO and CO2 (and H2O); the amount of CO2 relative to CO increased with increasing amount of iron in coal, until gases from coal samples with >10 wt % Fe provided CO2 without detectable amounts of CO. The thermal chemistry of the coal samples with >10 wt % Fe is thus considered analogous to that reported for iron carboxylates.27,28 Such chemistry includes the elimination of a carboxyl group bound to iron to yield CO2, the formation of a carbon-centered radical, and the reduc(21) Brockway, D. J.; Higgins, R. S. The Science of Victorian Brown Coal; Durie, R. A., Ed.; Butterworth Heinemann: London, 1991; pp 247-278. (22) Tanaka, S.; U-emura, T.; Ishizaki, K.; Nagayoshi, K.; Ikenaga, N.; Ohme, H.; Suzuki, T. Energy Fuels 1995, 9, 45-52. (23) Ozaki, J.; Nishiyama, Y.; Cashion, J. D.; Brown, L. J. Fuel 1999, 78, 489-499. (24) Yamashita, H.; Ohtsuka, Y.; Yoshida, S.; Akira Tomita, A. Energy Fuels 1989, 3, 686-692. (25) Yamashita, H.; Yoshida, S.; Tomita, A. Energy Fuels 1991, 5, 52-57. (26) Yamashita, H.; Tomita, A. Ind. Eng. Chem. Res. 1993, 32, 409415. (27) (a) Bassi, P. S.; Randhawa, B. S.; Jamwal, H. S. Thermochim. Acta 1983, 62, 209-216. (b) Bassi, P. S.; Randhawa, B. S. Thermochim. Acta 1983, 65, 1-8. (28) Morando, P. J.; Piacquadio, N. H.; Blesa, M. A. Thermochim. Acta 1987, 117, 325-330.

Mono- and Polynuclear Iron Hydroxy Complexes

tion of an Fe(III) to Fe(II). Additional reaction pathways for coals with less iron may take place to yield a gaseous mixture of CO, CO2, H2, and H2O; further, the elimination of a water molecule from hydroxyl-iron complexes with the formation of iron oxides occurs at higher temperatures.29-31 Using the model, we have summarized the chemistry of CO2 formation, simplified in eq 4 by assuming all of the iron is present as the dinuclear octahedral complex, with each Fe(III) bonded to two monodentate carboxylate groups and the remaining coordination sites occupied by hydroxyl groups from the coal matrix. The coal model with an [Fe(III)Fe(III)] hydroxyl complex was calculated to be energetically favored by 3.4% relative to the coal model without iron (the iron complex contained normal bond lengths and angles, and the Fe(III) partial charges were +1.04 and +1.09). The elimination of a carboxyl group then occurs with a reduction of one Fe(III) to Fe(II), the formation of a carbon-centered radical, and the remaining carboxyl group now acting as a bidentate ligand to maintain the octahedral complex about the Fe(II):

coal{[(COO)2(Fe(III))(µ-OH)2(Fe(III))(COO)2]} f coal[•]{[(COO)(Fe(II))(µ-OH)2(Fe(III))(COO)2]} + CO2 (4) where the symbol [•] is used to signify a C centered free radical. Experimental data from the pyrolysis of the coal samples in support of this chemistry include the detection of reduced iron as Fe3O4 in the XRD patterns of char and ash and the XPS detection of inorganic carbonate associated with the iron in char.6 Iron carbonate has been reported from the decomposition of iron propionate and butyrate.27 Fe3O4 has been reported in the XRD of char produced by the steam gasification of Loy Yang coal with added iron,4,26 while Fe3O4 and FeOOH are some of the species reported from studies based on Mo¨ssbauer, XRD, X-ray absorption spectra, and extended X-ray absorption fine structure spectroscopy of Loy Yang coal heated under nitrogen and from steam gasification.24-26 The pyrolysis and carbonization of Loy Yang coal with added iron has been reported to include increased formation of CO2 and CO (compared with that from acid-washed coal) and organic radicals, while EPR signals from iron have been detected from carbonized sampled of coal with iron,23 consistent with the chemistry illustrated by eq 4. (29) Koga, N.; Takemoto, S.; Okada, S.; Tanaka, H. Thermochim. Acta 1995, 254, 193-207. (30) Koga, N.; Okadaa, S.; Nakamurab, T.; Tanaka, H. Thermochim. Acta 1995, 267, 195-208. (31) Das, R. P.; Anand, S. Int. J. Miner. Process. 1996, 48, 159168.

Energy & Fuels, Vol. 19, No. 3, 2005 1055

The iron-mediated chemistry, including the formation of an intermediate with a carbon-centered radical, would lead to further radical chemistry involving several chemical pathways, usually dependent on the nature of the radical organic groups formed; for example, if the elimination of the carboxyl group in eq 4 formed a moiety similar to PhCH2CH2•, elimination of a hydrogen radical and the formation of a CdC double bond may occur.32 Molecular models have been constructed to study such reaction sequences. The proposed coal molecule with the Fe(II)Fe(III) hydroxyl (after the elimination of a carboxylate group) and a CdC bond formed by elimination of a H radical was energetically favored by 1.8% relative to the original coal model without iron. The octahedral structure of the Fe(II) center has been maintained by changing the monodentate into a bidentate carboxyl, all bond lengths and angles were normal values, and the partial charge on Fe(II) was +0.73 and on Fe(III) was +0.82. Molecular modeling of other structures relevant to the pyrolysis chemistry of coal with added iron has been carried out, and although this area of research is continuing, the present example illustrates that computer molecular modeling of brown coal with ironhydroxy species is useful in investigating reaction pathways of low-temperature pyrolysis. Conclusion (1) Solution data have indicated that mono- and polynuclear Fe(III) species have been added to brown coal. (2) Mono- and dinuclear Fe(III) species have been identified from XPS and TOF-SIMS of coal samples. (3) Computer molecular modeling has been used to illustrate how mono- and polynuclear iron complexes may be present in brown coal. (4) Molecular modeling has been useful in examining some low-temperature pyrolysis reactions of samples of brown coal containing iron species bound to carboxyl groups. Acknowledgment. We gratefully acknowledge Dr. Robert Glaisher, Department Physics, for SEM-EDX and XRD; Dr. Narelle Brack, Centre for Materials and Surface Sciences, for XPS and TOF-SIMS; and the Australian Partnership for Advanced Computing National Facility under the Merit Allocation Scheme and the Victorian Partnership for Advanced Computing for access to computer molecular modeling resources. Rheinbraun GBT kindly supplied samples of German brown coal. EF050001I (32) Poutsma, M. L. J. Anal. Appl. Pyrolysis 2000, 54, 5-35.