Effects of Water and Molecular Hydrogen on Heat Treatment of

1200

Energy & Fuels 1998, 12, 1200-1211

Effects of Water and Molecular Hydrogen on Heat Treatment of Turkish Low-Rank Coals† Levent Artok,*,‡ Harold H. Schobert,*,§ Masakatsu Nomura,| Oktay Erbatur,| and Koh Kidena| Department of Chemistry, Ç ukurova University, Adana 01330, Turkey, Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, and Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan Received March 11, 1998. Revised Manuscript Received August 20, 1998

Two low-rank Turkish coals, Tunç bilek (TB) subbituminous and Goÿnu¨k (GN) lignite, were subjected to low-severity heat treatment with or without added water, under N2 or H2 atmospheres at 285-330 °C, both as original coals and after demineralization with HCl(aq)/HF(aq) treatment. The samples processed in H2-H2O combination seemed to be more dissociated or decomposed than those processed in N2-H2O or under H2 without water. Gas analyses and spectroscopy of samples clearly indicated an effect of water in enhancing decarboxylation reactions and that oxygen rejection from the coals was mainly due to CO2 formation. Water also enhanced the cleavage of aryl-ether bonds. These cleavage reactions are more pronounced in GN coal than in TB coal, probably due to a higher concentration of activated ether structures in the former coal. The disruption of noncovalent interactions is considered mainly responsible for the extensive depolymerization of TB coal. The role of H2 in these heat treatment conditions is considered to be predominantly hydrogenation of polyaromatic sites with the help of mineral matter and scavenging radicals, which arose from cleavage of weak aliphatic ether bonds. The coal samples treated in H2-H2O donated more hydrogen to anthracene during their co-pyrolysis, attributed mainly to a reduced tendency of the treated samples to undergo radical-generating reactions.

Introduction The initial stage of some coal conversion processes, e.g., liquefaction and pyrolysis or carbonization, involves bond-cleavage reactions that lead to a progressive reduction in molecular weight. Since low-molecularweight distillable liquid products are usually the aim of liquefaction and pyrolysis processes, thermally induced formation of active radical fragments must be accompanied by their prompt stabilization by an externally added, or by an internal, hydrogen source. Otherwise, the high reactivity of these fragments for radicalradical combination and radical-addition reactions would bring about highly cross-linked refractory and highmolecular-weight products. Low-rank coals are commonly considered to be thermally reactive in heat treatment because they contain high concentrations of thermally labile functional groups or relatively weak cross-links. Thereby, they may decompose more readily under liquefaction or pyrolysis conditions. However, the high reactivity of these coals could result in the generation of large concentrations of highly reactive species. Minimizing the participation of such reactive species in retrogressive condensation and polymerization reactions † Presented in part at the 9th International Conference on Coal Science, September 7-12, 1997, Essen, Germany. ‡ Present address: Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan. § The Pennsylvania State University. || Osaka University.

could necessitate reaction conditions that may not be economically desirable, such as using highly active, dispersed catalysts or very effective hydrogen-donor solvents, which may represent a high-operating cost for the process. In carbonization processes, the intrinsically labile hydrogen atoms of high-rank coals can be partly transferred to stabilize radical fragments following thermally induced bond-cleavage reactions. Subsequently, molecular units having more freedom of motion melt and the overall structure reorients itself, accompanied or followed by gas evolution and resolidification steps. Many reports in the literature indicate that the high-temperature chemistry of low-rank coals is greatly affected by the inability of hydrogen intrinsic to the coal to effectively cap radicals. Suuberg and co-workers showed that low-rank coals are more prone to cross-linking reactions than coals of higher rank;1 this may be due to the generation of reactive free radicals that cannot be capped by intrinsic hydrogen. Saini et al. concluded that a good hydrogen donor is necessary to prevent retrogressive reactions in Wyodak subbituminous coal.2 The in situ ESR study of Petrakis et al. explicitly indicates that intrinsic hydrogen atoms cannot compensate for radical formation.3 Rudnick and Tueting’s study of liquefaction of Belle Ayre coal showed, by ESR, (1) Suuberg, E. M.; Lee, D.; Larsen, J. W. Fuel 1985, 64, 1668. (2) Saini, A. K.; Coleman, M. M.; Song, C.; Schobert, H. H. Energy Fuels 1993, 7, 328. (3) Petrakis, L.; Grandy, D. W. Fuel 1982, 61, 21.

10.1021/ef980049e CCC: $15.00 © 1998 American Chemical Society Published on Web 10/15/1998

Heat Treatment of Turkish Low-Rank Coals

a higher radical concentration for the product from reaction with a solvent with a low hydrogen content relative to a solvent with a high hydrogen content.4 Since intrinsic hydrogen atoms within low-rank coals cannot adequately stabilize the large amount of reactive radical fragments formed, such coals convert to a highly cross-linked isotropic char rather than passing through a plastic zone. Low-rank coals are abundant in oxygen functional groups, such as -OCH3, -OH, and -COOH. Reactive radicals formed by thermolysis of these groups are considered to contribute to the cross-linking reactions. For -OH functionalities, both radical5-7 and ionic mechanisms have been proposed.8 Nevertheless, there are arguments as to whether -COOH groups take part in condensation reactions. Carboxyl groups were correlated with cross-linking reactions in various pyrolysis tests1,9,10 and successive radical decarboxylation and condensation reactions were considered for the formation of cross-links. Recently, the role of carboxyl groups in cross-linking reactions has been brought into question, and it has been proposed that these groups may not be involved in condensation reactions during coal conversion processes.11-13 However, Artok and Schobert14 have found that aryl and alkyl acids can undergo condensation with neighboring carboxyl or hydroxyl groups, through ionic pathways, to form anhydride and ester structures respectively, to a significant extent, and subsequently these condensation products can participate in radical reactions to form relatively more condensed and polymeric structures.14,15 For conversion of low-rank coals to value-added products, a strategy involving the alteration of their structures, in which a treated coal would be less susceptible to cross-linking reactions prior to conversion, could be of use. Examples of such strategies would include controlled cleavage of relatively weak bonds and subsequent stabilization, defunctionalization, or conversion of functional groups that are prone to condensation reactions to less reactive forms or increasing the intrinsic amount of labile hydrogen. As examples of such pretreatment methods, heating under molecular hydrogen with or without added solvent or catalyst,16-18 heating in CO-H2O in the pres(4) Rudnick, L. R.; Tueting, D. Fuel 1984, 63, 153. (5) Poutsma, M. L.; Dyer, G. W. J. Org. Chem. 1982, 47, 3367. (6) McMillen, D. F.; Chang, S.-J.; Nigenda, S. E.; Malhotra, R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1985, 30 (4), 414. (7) Trewella, M. J.; Grint, A. Fuel 1988, 67, 1135. (8) Siskin, M.; Brons, G.; Vaughn, S. N.; Katritzky; Balasubramanian, M.; Greenhill, J. V. Energy Fuels 1993, 7, 589. (9) Serio, M. A.; Hamblen, D. A.; Markham, J. R.; Solomon, P. R. Energy Fuels 1987, 1, 138. (10) Ibarra, J. W.; Moliner, R.; Gavilan, M. P. Fuel 1991, 70, 408. (11) Manion, J. A.; McMillen, D. F.; Malhotra, R. Energy Fuels 1996, 10, 776. (12) Eskay, T. P.; Britt, P. F.; Buchanan, A. C., III Energy Fuels 1996, 10, 1257. (13) Eskay, T. P.; Britt, P. F.; Buchanan, A. C., III Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 1997, 42 (1), 20. (14) Artok, L.; Schobert, H. H. J. Anal. Appl. Pyrol. 1998, submitted for publication. (15) Eskay, T. P.; Britt, P. F.; Buchanan, A. C., III Energy Fuels 1997, 11, 1278. (16) Derbyshire, F. J.; Davis, A.; Epstein, M.; Stansberry, P. G. Fuel 1986, 65, 1233. (17) Nishioka, M.; Laird, W.; Bendale, P. G.; Zeli, R. A. Energy Fuels 1994, 8, 643. (18) Inukai, Y.; Arita, S.; Hirosue, H. Energy Fuels 1995, 9, 67.

Energy & Fuels, Vol. 12, No. 6, 1998 1201

ence of basic catalysts19 or with steam,20-22 and solvent swelling of coals23-27 were reported to be effective in enhancing conversion and improving the quality of liquid products in liquefaction reactions. Steam treatment was also found effective in the improvement of extraction28,29 and pyrolysis yields.21,28-31 In solventswelling pretreatment, coal exposed to an effective swelling solvent has been considered to be more accessible to catalyst, molecular hydrogen, and donor solvent.23-27 Depending on the conditions and the type of additives, combinations of the cleavage of relatively weak bonds, an increase of the labile hydrogen content in coal, and some extent of defunctionalization and depolymerization reactions may be anticipated in the former case. However, little is known about the effect of hydrogen and water on the structural changes in coal during heat treatment at relatively low temperatures. Brandes et al. suggested that water might have caused the rupturing of ether bonds by the fact that the hydroxyl content of the Illinois No. 6 coal increased when steam treated at 340 °C.32 Artok and Erbatur found that Tunç bilek (TB) subbituminous coal gained caking and plastic properties when treated at 285-330 °C in H2-H2O combination33 and that coke samples with a high strength were produced from blends containing up to 30% pretreated coal and a coking coal.34 The treatments in N2-H2O or under H2 only were not sufficient to achieve this. Despite the known beneficial effect of the pretreatment of coals at relatively low temperatures, the sort of reactions that are induced or hindered by the influence of water and hydrogen and the structural changes occurring during heat treatment are not yet clear. In this work, we have explored the effect of H2 and water, both independently and in combination, on the structural changes and the properties of two low-rank coals at relatively low temperatures. To clarify the questions cited above, two Turkish low-rank coals, Goÿnu¨k (GN) lignite and TB subbituminous coal, were heat-treated between 285 and 330 °C, with or without added water, under H2 or N2 atmospheres. Detailed examination of the treated coal samples and of their various properties showed that these coals are undergoing remarkable changes. (19) Lim, S. C.; Rathbone, R. F.; Rubel, A. M.; Givens, E. N.; Derbyshire, F. J. Energy Fuels 1994, 8, 294. (20) Bienkowski, P. R.; Narayan, R.; Greenkorn, R. A.; Chao, K. C. Ind. Eng. Chem. Res. 1987, 26, 202. (21) Serio, M. A.; Kroo, E.; Solomorn, P. R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36, 7. (22) Pollack, N. R.; Warzinski, R. P Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36, 15. (23) Rincon, J. N.; Cruz, S. Fuel 1988, 67, 1162. (24) Joseph, J. T. Fuel 1991, 70, 139. (25) Joseph, J. T. Fuel 1991, 70, 459. (26) Artok, L.; Davis, A.: Mitchell, G. D.; Schobert, H. H. Fuel 1992, 71, 981. (27) Artok, L.; Davis, A.: Mitchell, G. D.; Schobert, H. H. Energy Fuels 1993, 7, 67. (28) Graff, R. A.; Brandes, S. D. Energy Fuels 1987, 1, 84. (29) Mapstone, J. O. Energy Fuels 1991, 5, 695. (30) Serio, M. A.; Kroo, E.; Solomon, P. R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 37, 1681. (31) Ross, D. S.; Hirschon, A.; Tse, D. S.; Loo, B. H. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 1990, 35, 352. (32) Brandes, S. D.; Graff, R. A.; Gorbaty, M. L.; Siskin, M. Energy Fuels 1989, 3, 494. (33) Artok, L.; Erbatur, O. Unpublished results, Adana, Turkey, 1997. (34) Artok, L.; Erbatur, O. Unpublished results, Adana, Turkey, 1996.

1202 Energy & Fuels, Vol. 12, No. 6, 1998

Experimental Section TB subbituminous (containing 19.4% ash on a dry basis; 73.9% C, 5.5% H, 2.5% N, 5.2% S, and 12.9% O (by difference) on daf basis) and GN lignite (containing 13.8% ash on dry basis; 69.3% C, 4.2% H, 2.4% N, 1.9% S, and 22.1% O (by difference) on daf basis) were used in this study, in both raw and demineralized forms. Demineralization was carried out by treating the coal samples successively with 5 N HCl and 40% HF solutions for 1 h each at 60-70 °C. After demineralization, the ash yield was 4.6% for TB and 0.91% for GN coal. The heat treatment of the coal samples was carried out in 25-mL microautoclave reactors for 90 min. Four grams of vacuum-dried (at 110 °C) coal sample (-60 mesh) was charged to the reactor for each experiment. For experiments with added water, the ratio of coal to water was kept at 2, based on earlier work of Song et al. with Wyodak coal.35 Reaction temperatures in the range 200-350 °C have been shown by many investigators to be appropriate for pretreatment of lowrank coals.16-18,36 The pressure of the gas charged (H2 or N2) was 1000 psi at room temperature. The reactor pressure increased to ∼1500 psi at 330 °C for reactions in the absence of water and ∼2150 psi for reactions in the presence of water. (Under these conditions, water is in the gas phase.) A 90min reaction time was chosen on the basis of earlier work that showed the longer the reaction time, the more improved were the caking properties.37 At the end of the reaction period, the reactor was immersed into water to ensure fast cooling. The gaseous products were collected for further analyses. Gas analyses were performed using a packed Carbosieve column and thermal conductivity detector on a Perkin-Elmer gas chromatograph. Spectroscopic analyses of the pretreated samples were performed by means of FTIR and SPE/MAS C13 techniques. FTIR spectra were obtained on a Digilab FTS-6-A system. Spectra were recorded by co-adding 300 scans (interferograms) at a resolution of 2 cm-1. To avoid spectral artifacts from moisture in the samples or in the laboratory, KBr pellets of all samples were prepared on the same day, dried at 100 °C in a vacuum oven, placed in a common desiccator, and analyzed on the same day. The sample chamber of the instrument was continuously purged with dry nitrogen to avoid moisture adsorption. SPE/MAS C13 NMR was recorded on a Chemagnetics CMX-300 (75 MHz) with 10 kHz MAS with parameters of 100 s of pulse delay, 45° pulse width, and 15002000 scan numbers. Curve fitting of the spectra was carried out by dividing the spectra into 12 Gaussian curves38 using commercially available NMR data processing software, MacAlice (version 2.0, JEOL DATUM). Microscopic investigations of the raw and treated samples were done using a Zeiss polarized reflected light microscope. The solubilities of the coals were determined by extraction with pyridine. The raw and treated coals were heated at 420 °C with anthracene for 5 min to evaluate the hydrogen-donor ability (HDA) following the method reported by Kidena et al.38 Briefly, the mixture of coal and anthracene (100 mg each) was reacted in an evacuated and sealed silica tube at 420 °C for 5 min. The reaction contents were washed with CH2Cl2 and filtered. The CH2Cl2 solubles were analyzed by a Shimadzu 14A gas chromatograph using a SBP-1 capillary column (0.25 mm × 25m). The amount of donatable hydrogen was estimated from the amount of hydrogenated anthracene molecules (9,10-dihydroanthracene and 1,2,3,4-tetrahydroanthracene). (35) Song, C.; Saini, A. K.; McConnie, J. Proc. 8th Intl. Conf. Coal Sci. 1995, 1391. (36) Wham, R. M. Fuel 1987, 66, 283. (37) Artok, L.; Erbatur, O. Unpublished results, Adana, Turkey, 1998. (38) Kidena, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 672.

Artok et al. Standard deviations for the pyridine solubility measurements were (4%. The error in the elemental analyses is (2%.

Results and Discussion Dissociation of Coals. All of the recovered solid products of TB coal after heat treatment in H2-H2O combination appeared as agglomerated lumps, the samples being harder the higher the treatment temperature. In contrast, samples processed under hydrogen but without water, or in the N2-H2O system, were in the form of discrete particles. These observations indicate that in the H2-H2O system, a different chemistry is going on. No clear differences were observed among the pretreated GN coal samples, implying that these two coals respond in different ways to the pretreatment procedure. Morphological changes induced by heat treatment can be more clearly observed by means of reflected light microscopy. Original TB coal is highly liptinitic, and its vitrinite content is mainly composed of two distinctive components: one of high reflectance (telocollinite) and the other of low reflectance (desmocollinite), Figure 1a.33 When TB coal was treated at 305 °C under H2 only, there was no indication of a modification of the particles. The particles preserved their original angular shape (Figure 1b). After treatment in H2-H2O combination, even at lower (285 °C) temperature, there were clearly visible changes in the morphology of coal particles. The particles were swollen and had begun to agglomerate, indicating an effect of H2-H2O combination in disrupting cross-links and initiating dissociation of the structure (Figure 1c). (Throughout this discussion we use the term “cross-link” to include both covalent bonds and associative forces such as hydrogen bonds or π-π interactions as some other investigators have done;39,40 our use of the term thus has a broader meaning than is used by some investigators who do not consider noncovalent interactions as cross-links, e.g., ref. 41.) Liptinitic macerals remained intact with this treatment, although most were modified in shape. After treatment at 305 °C in H2-H2O, the coal had the appearance of large and coherent masses and boundaries of particles were indistinguishable, implying significant changes in the structure, possibly resulting in a more dissociated structure (Figure 1d). The product recovered from the reaction at 330 °C in H2-H2O combination was of a planar shape and the pores were larger (Figure 1e). The appearance of particles that show evidence of swelling, flow, or melting is indicative of structural changes (including conformational changes) within the coal. The pathways from the unreacted coal to the final stage seen under the microscope after reactions in H2/ H2O have to involve disruption of interactionsscovalent or noncovalentsso that the coal could be mobilized. Microscopy cannot indicate the type of bonds cleaved. Further, while some bond disruption is occurring during reaction, the formation of new bonds or associations via condensation reactions cannot be excluded. The reduced solubility after reaction at 285 °C compared to unreacted coal (discussed below) may indicate that the average (39) Larsen, J. W.; Gurevich, I. Energy Fuels 1996, 10, 1269. (40) Larsen, J. W.; Flowers, R. A.; Hall, P. J.; Carlson, G. Energy Fuels 1997, 11, 998.



Figure 1. Photomicrographs of (a) raw and processed TB coals: (b) without water at 305 °C; (c) with water at 285 °C; (d) with water at 305 °C; (e) with water at 330 °C. Heat treatments were done under H2.

Figure 2. Pyridine solubility of raw and processed coal samples. DM: demineralized.

cross-link (covalent and noncovalent) of the coal is not effectively reduced by this treatment. This would corroborate earlier work of Nishioka.42 From microscopic examination, the sample obtained after treatment at 305 °C in N2-H2O consisted of fine nonagglomerated particles and showed no evidence of an alteration in the morphology that reveals synergism arising from H2H2O combination in degradation of the coal. No differences between the pretreated GN coals were observed under the microscope; all appeared microscopically unmodified.

From the consideration that a less cross-linked structure, i.e., one more dissociated, would have more extractability in organic solvents, the raw and heattreated samples were subjected to pyridine extraction to obtain an indication of the disruption or dissociation of the macromolecular structure. In general, coals treated in H2-H2O afforded a higher pyridine solubility than the others (Figure 2), in agreement with the conclusions drawn from the microscopic examination, that treatment in H2-H2O produced a more degraded coal. Treating TB coal at 285 °C in H2-H2O did not


Artok et al.

Figure 3. Photomicrographs of pyridine-insoluble (a) raw and processed TB coals: at (b) 285 °C, (c) 305 °C, and (d) 330 °C. Heat treatments were done in the presence of water and under H2. The scales denote 20 µm.

improve its solubility. At 305 °C, the coal pretreated in H2-H2O combination afforded a higher pyridine solubility (36%, daf) compared to the respective samples treated in H2-dry or N2-H2O systems. The solubility of the sample after treatment in N2-H2O was only slightly higher than that in H2 only. The solubility of TB coal was increased greatly, to ∼79% (daf), when treated in H2-H2O at 330 °C, while the sample treated in the absence of water gave only 22% extractability (daf). This suggests a profound effect of water in an extensive reduction or disruption of cross-links; corroboration is provided by the greatly altered physical appearance seen by microscopy and by the spectroscopic data discussed below. The acid-leached TB coal (DMTB) was slightly more soluble than the original coal, probably due to its lessassociated character after removal of divalent metal cations that could bridge between functional groups (e.g., -COO-M2+-OOC-). Treatment of this sample in the H2-H2O system also improved its solubility significantly, to ∼68%, but this is somewhat less than that of coal without acid treatment. This difference may be due to loss of the catalytic effect of mineral matter or adverse changes in the macromolecular structure caused by the acid treatment. GN coal was almost insoluble in pyridine, reflecting its highly covalent cross-linked nature. Treatment in H2 in the absence of water, even at 330 °C, did not improve pyridine extractability, whereas treating the samples in H2-H2O increased the solubility to ∼24% at 305 °C and ∼35% at 330 °C (daf basis), suggesting a significant extent of depolymerization. The fact that the sample pretreated in N2-H2O at 330 °C also gave remarkable solubility in pyridine (∼20%, daf) but significantly less than that of H2-H2O indicates synergism arising from H2-H2O combination. Microscopic examinations of nonextractable (i.e., pyridine-insoluble) raw and pretreated samples were also carried out. Changes in the morphological structure of TB coal were observed after successive pyridine extraction and vacuum-drying procedures (Figure 3a). The irreversibly swollen insoluble particles show some agglomeration that is indicative of substantial disruption

of noncovalent associative forces and subsequent formation of new ones during the removal of pyridine. The insoluble fraction of the sample after treatment at 285 °C in H2-H2O combination appeared nonagglomerated, similar to original coal (Figure 3b). This indicates that the agglomerated structure is due to formation of a fluid phase that acts as cement between the particles and is soluble in pyridine. However, the insoluble fraction of the sample treated at 305 °C in H2H2O still appeared to be swollen and agglomerated (Figure 3c), even though most of the cement-like material was removed during the extraction. This may imply that during the heat treatment, the more dissociated portion of the coal is soluble in pyridine and the portion that is less dissociated or more concentrated in crosslinked units is not capable of dissolving but swells and reorients its structure. During the drying period, new noncovalent associations can occur between the surfaces of particles. The sample treated at higher temperature, 330 °C, is highly pyridine soluble (79% daf), and the insoluble remnant likely consists of highly cross-linked materials (Figure 3d). Structural Changes. The fact that both coals, demineralized or not, afforded significantly higher CO2 and less CO gaseous products with added water than in reactions performed in the absence of water (at all temperatures) clearly shows that water activates some facile CO2 formation reaction (Figure 4). Song et al. also observed that addition of water substantially increased CO2 but observed reduced CO formation in both catalytic and noncatalytic liquefaction experiments.40 CO formation in the absence of water is so low that the increase in CO2 cannot be accounted for by a decrease in CO. The demineralized coals produced slightly lower CO2 than did raw coals in the corresponding reactions, although the trend regarding the effect of water was similar, which may imply some catalytic role of mineral matter in decarboxylation reactions. Figure 5 shows the FTIR spectra of the raw coals and the samples heat treated at 330 °C with or without water. When GN coal was treated, the intensity of the -CdO stretch at 1700 cm-1 and, in parallel, -OH stretching band, as well, decreased with treatment,



Figure 4. Effect of preheat treatment conditions on the formation of gaseous products from raw and demineralized coals.

Figure 6. FTIR spectra of (a) raw and (b) heat-treated coal. Heat treatment was done with water under H2 and at 330 °C.

Figure 5. FTIR spectra of GN and demineralized GN (DMGN) coals with or without heat treatment. Heat treatments were done at 330 °C and under H2: (a) raw coal; (b) without water; (c) with water.

indicating the occurrence of decarboxylation. The intensities of these bands were reduced to a greater extent when the coal was treated in the presence of water than in the absence of water, indicating that water significantly promoted decarboxylation. The carbonyl stretch-

ing peak for the demineralized GN coal is more discernible. A more apparent reduction of this band upon pretreatment, and the catalytic effect of water in this reduction, can be seen explicitly. Some variations in the range of C-O stretching and O-H bending (11501250 cm-1) also can be observed after heat treatment. These results correlate well with the tendency for CO2 formation in that the greater decarboxylation resulted in higher CO2 formation. We have mentioned in the Experimental Section the precautions taken to avoid confounding the results with adventitious moisture; if there was any contamination by moisture, the probability of obtaining the same trend in the reduction of -OH and -CdO intensities would be very low. The decrease of the carboxyl groups from TB coal was relatively less, as judged from the variation of the carbonyl and hydroxyl bands in the FTIR spectra of the raw and treated coal samples (Figure 6) and from CO2 formation during pretreatment experiments. In a recent study it has been determined that below 400 °C the unactivated benzoic acid and its sodium and calcium salts are rather stable and that their decarboxylation reactions are very slow both with and without water whereas an activated-ring acid, 4-hydroxybenzoic acid, was completely decarboxylated even at 300 °C in the presence of water (its conversion was only 38% in the absence of water) and condensation to esters took place


Artok et al. Scheme 1

Table 1. Elemental Analyses of Solid Products and Calculated Elemental Mass Balance Values coal TB

DMTB GN

DMGN

temp, °C

gas

water

H/C

O/Ca

285 305 305 305 330 330 330 330 305 305 330 330 330 330 330

H2 H2 H2 N2 H2 H2 H2 H2 H2 H2 H2 H2 N2 H2 H2

+ + + + + + + + +

0.894 0.942 0.923 0.957 0.898 0.918 0.979 0.874 0.897 0.935 0.770 0.743 0.750 0.773 0.827 0.698 0.751 0.701 0.714

0.130 0.110 0.115 0.100 0.116 0.096 0.080 0.116 0.097 0.074 0.239 0.187 0.156 0.151 0.101 0.125 0.256 0.197 0.156

mass recoveryb

C mass balanceb

H mass balancec

O mass balancea,b

98.5 99.4 98.5 98.5 97.9 95.2

99.4 101.4 100.7 99.3 101.2 97.5

104.2 104.4 106.9 98.9 103.4 105.5

92.6 93.2 87.5 100.3 80.6 76.0

ND ND

ND ND

ND ND

ND ND

100.2 104.6 101.5 97.6 103.2

101.0 103.1 101.4 95.2 102.3

100.5 101.2 102.3 98.5 90.9

95.3 102.8 100.5 104.7 110.6

ND ND

ND ND

ND ND

ND ND

a Oxygen content of the samples was determined by difference. b Calculated on the basis the solid and gaseous products. c Calculated on the basis of the only solid product.

significantly in the latter case.14 Such activated acids could conceivably be expected to constitute the majority of aryl carboxylic groups in low-rank coals, which can be readily decarboxylated by the rate-enhancing effect of water. The effect of water in decarboxylation of naphthoic acids was also observed, although to a relatively lesser extent.14,44 In the absence of water, the conversion of these acids proceeded through anhydride formation.14 Decarboxylation mechanisms of these acids are thought to proceed through proton attack on a terminal aryl carbon,45,46 and water may assist as a proton shuttler in the system and prevent ionic condensation reactions, Scheme 1. The intrinsically acidic clay minerals may also additionally catalyze this reaction. The acid converted completely in the presence of water, whereas in the absence of water, the acid conversion was only 38.5%. The dissociation constant of water increases by more than 3 orders of magnitude from ambient to near-critical conditions.47 This allows water in the near-critical region to act as an effective acid or base catalyst.44,48,49 For example, Chandler and co-workers successfully alkylated phenolic molecules in

water at 275 °C without any added catalyst.49 Moreover, water may facilitate ionization of the acid and the formation of ionic intermediates in the mechanism proposed by Manion et al.11 Elemental analyses of the solid products and calculated C, H, and O mass balances are given in Table 1. The amounts of gaseous and solid products were taken into account to calculate elemental mass balances. Recovery of the solid material from the reactor after reaction, however, was not effective since a significant quantity of the sample remained on the inner wall of the reactor. The consideration of washing the inside of the reactor with any solvent for efficient recovery was not a viable course because fractionation of the sample would not let us determine the average structural changes during the heat treatment processes. Thus, the amount of solid product was deduced by tracing the ash content. After reaction, the product was dried in a vacuum at 100 °C and a small portion was set aside for microscopy. The remaining portion was ground and homogenized thoroughly to ensure that the ash would be equally distributed through the analytical sample.

(41) Painter, P. C. Energy Fuels 1996, 10, 1273. (42) Nishioka, M. Fuel 1992, 71, 946. (43) Song, C.; Saini, A K. Energy Fuels 1995, 9, 188. (44) Siskin, M.; Brons, G.; Vaughn, S. N.; Katritzky, A. R.; Balasubramanian, M. Energy Fuels 1990, 4, 488. (45) Brown, B. R. Quart. Rev. 1951, 5, 131. (46) Kaeding, W. W. J. Org. Chem. 1964, 29, 2556. (47) Marshall, W. L.; Franck, E. U. J. Phys. Chem. Ref. Data 1981, 10, 295. (48) Kuhlman, B.; Arnett, E. M.; Siskin, M. J. Org. Chem. 1994, 59, 3098. (49) Chandler, K.; Deng, F.; Dillow, A. K.; Liotta, C. L.; Eckert, C. A. Ind. Eng. Chem. Res. 1997, 36, 5175.

wt of solid product ) wt of coal × ash% of coal/ash% of the solid product The mass balance of carbon and oxygen was calculated by accounting for the quantity of the corresponding elements in coal (Ec), in solid product (Es), and gaseous products (Eg).

elemental mass balance ) (Es + Eg)/Ec × 100


To estimate hydrogen incorporation into the solid matrix, the hydrogen in gaseous products is omitted. Of the GN coal samples treated under hydrogen, only the one processed at 330 °C with water had a slightly higher H/C ratio than did the raw coal. However, only 98.5% H recovery indicates that the apparent enrichment of hydrogen was actually mainly due to oxygen and carbon rejection. There was a 5-20 psi increase of pressure noted in the reactor at the end of the reactions carried out under H2, but under N2, the increase was higher (40 psi). This observation, along with the hydrogen recovery was the lowest for reaction under H2, indicates the incorporation of hydrogen. After treatment in N2-H2O combination at 330 °C, the H/C ratio of the coal was reduced to 0.698 and H recovery reduced to ∼91%, indicating the existence of extensive dehydrogenation reactions likely due to dehydroaromatization, radical addition reactions, or both. (The formation of molecular hydrogen was observed by GC, but due to the similar thermal conductivity of H2 with He used as the carrier gas, its quantitative determination could not be performed.) Molecular hydrogen may have reduced these reactions by capping newly generated radicals. Approximately 62.4% of the oxygen content of GN coal was removed by reaction in H2-H2O combination at 330 °C. Oxygen recovery values were either slightly less or slightly higher than 100%, suggesting that oxygen rejection was mainly due to decarboxylation and that from hydroxyl and ether oxygen rejection is relatively little. Oxygen recovery was greater in the presence of water, in general higher than 100%. This may be due to oxygen incorporation to the solid matrix from water, possibly due to hydrolysis reactions. For example, hydrolysis of esters, anhydrides, and some ethers41,47 would introduce oxygen to the matrix. There is no direct evidence for esters or anhydrides in these specific coals; however, esters have been observed in American lignites,51,52 and there is some evidence suggesting the formation of anhydrides in American lignites as an artifact of drying.53 The fact that the calculated carbon mass recovery was slightly higher than 100% indicates a somewhat inherent error in the method. Nevertheless, as a general trend, estimation of a higher oxygen recovery for the runs performed in the presence of water cannot be attributed entirely to this inaccuracy. The oxygen balance for the treatment performed in N2-H2O combination was the highest. This may imply that H2 interferes with the oxygen-addition reactions. Although water was also effective in deoxygenating demineralized GN coal (DMGN), the oxygen content of this coal, after treatment, was higher than that of the corresponding treated GN coal. This result is also in keeping with the trends observed for CO2 formation. Treating TB coal under H2 enriched the hydrogen content of the coal at all temperatures employed, irrespective of the presence of water. There was a diminished pressure in the reactor after treatment under hydrogen, and since a pressure increase was noticed under N2, these results indicate the occurrence of hydrogenation reactions. TB coal contains relatively high pyritic sulfur (2.2%, dry basis), and an active form (50) Siskin, M.; Katritzky, A. R.; Balasubramanian, M. Energy Fuels 1991, 5, 770.


of this mineral may have catalyzed the hydrogenation reactions. The hydrogenation activity of iron sulfide catalysts to coals at temperatures as low as 275 °C was verified previously.26 The calculated H mass balance values, which are higher than 100%, demonstrate addition of hydrogen to the solid coal matrix while lower values indicate dehydrogenation. Upon treatment at 305 °C in N2-H2O, the calculated H mass balance value being less than 100 (98.9%) indicates little dehydrogenation. Carbon mass balances were calculated as nearly 100%, indicating little loss of hydrocarbon species. Water was also effective in deoxygenation reactions of TB coal but much less so compared to those of GN coal. For the samples treated at 285 or 305 °C with or without water, O/C ratios varied between 0.1 and 0.116; these values were slightly less than that of raw coal (0.133). The effect of water in the rejection of oxygen from the coal is more remarkable at 330 °C; in this case, oxygen rejection was ∼41% in H2-H2O combination and the O/C ratio was reduced to 0.08 while that of the sample treated in the absence of water was 0.096. For the TB coal, it seems that oxygen rejection is due mainly to decarboxylation reactions and loss of aliphatic ether sites. HF-HCl treatment cannot remove pyrite from coals; thus, pyrite is in the reaction system for both the demineralized and original coals. This pyrite could be a precursor to the active catalyst pyrrhotite, although we have not determined whether the acid treatment leads to any change in the activity of the catalyst. It is well-established that acid treatment alters the reactivity toward carbonization and char formation,54,55 pyrolysis,56,57 liquefaction,58-60 and oxidation.61 Swelling tests suggest that structural changes resulting from acid extraction are minor;62 other work indicates a significant change in the elastic properties of coal but mild HCl treatment.63 Regarding the oxygen mass balance, the oxygen accounted for was less than 100% when treatments were carried out under H2. The reduction of the oxygen mass balance from 100% can be accounted for the formation of water, likely due to dehydroxylation, ether bond cleavages followed by dehydroxylation, or both. The oxygen mass balance for the sample treated in H2-H2O at 305 °C was slightly lower (87.5%) than the corresponding one treated in the absence of water (92.3%). The fact that a 100% balance of oxygen was obtained under inert N2 at 305 °C suggests a role for molecular (51) Degens, E. T. In Diagenesis of Sediments; Larsen, G., Chilingar, G. V., Eds.; Elsevier: Amsterdam, 1967; Chapter 7. (52) Schobert, H. H. Lignites of North America; Elsevier: Amsterdam, 1995; Section 3.3. (53) Ross, S. F.; Schobert, H. H. Unpublished data, Grand Forks, ND, 1985. (54) Mahajan, O. P.; Walker, P. L., Jr. Fuel 1979, 58, 333. (55) Samaras, P.; Diamadopoulos, E.; Sakellarapoulos, G. P. Carbon 1994, 32, 771. (56) Otake, Y.; Walker, P. L., Jr. Fuel 1993, 72, 139. (57) Schafer, H. N. S. Fuel 1980, 59, 302. (58) Serio, M. A.; Solomon, P. R.; Kroo, E.; Bassilakis, R.; Malhotra, R.; McMillen, D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35, 61. (59) Joseph, J. T.; Forrai, T. R. Fuel 1992, 71, 75. (60) Martin, S. C.; Schobert, H. H. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41, 967. (61) Azik, M.; Yu¨ru¨m, Y.; Gaines, A. F. Energy Fuels 1994, 8, 188. (62) Larsen, J. W.; Pan, C. S.; Shawver, S. Energy Fuels 1989, 3, 557. (63) Krezesinska, M. Energy Fuels 1997, 11, 686.


Artok et al.

Table 2. Carbon Distribution of Original and Pretreated Coalsa sample raw TB TB-H2-dry TB-H2-H2O raw GN GN-H2-dry GN-H2-H2O GN-N2-H2O

fa (aromacity) aliphatic carbon CdO carboxyl Ar-O Ar-C Ar-C,H Ar-H R-OH OR CH2, CH, q-C CH3 0.631 0.656 0.638 0.633 0.690 0.696 0.758

34.3 32.9 35.5 27.5 23.8 28.2 21.2

0 0 0 1.9 0.7 0 0

2.5 1.5 0.71 7.3 6.6 2.2 3.1

10.4 10.8 8.6 15.9 14.4 8.1 11.4

10.9 12.8 13.9 7.7 10.0 15.2 13.5

34.2 32.0 29.3 24.0 26.6 29.9 30.4

7.6 10.0 12.0 15.7 18.0 16.4 20.5

0.5 0 0 0.7 0.3 0.4 0

4.7 2.9 2.7 3.7 1.4 2.0 1.8

19.4 18.1 19.5 16.3 11.5 12.8 10.4

9.7 11.9 13.3 6.8 10.6 13.0 9.0

a All treatments were done at 330 °C. Ar-O: Oxygenated aryl carbon. Ar-C: alkyl-substituted aryl carbon. Ar-C,H: alkyl-substituted, bridgehead, and protonated aryl carbons.

Figure 7. SPE-MAS C13 NMR spectra of GN coal with or without heat treatment. Heat treatments were done at 330 °C.

hydrogen in the reactions of oxygen functional groups at our reaction conditions. Accurate elemental mass balance calculations for demineralized samples could not be obtained because of their low ash values. (Oxygen rejection by water formation is not included in the oxygen balances, but gas formation-i.e., CO and CO2is included.) The trend in oxygen recoveries might be considered to be a consequence of magnifying inherent errors by calculating oxygen by difference. However, it seems unlikely that the source and magnitude of errors could be different for reactions performed in the presence of water relative to those performed without water. Figure 7 compares the SPE-MAS C13 NMR spectra of original GN coals and those samples treated at 330 °C. Carbon distributions of the samples, calculated from the areas of corresponding deconvoluted peaks, are given in Table 2. There are noticeable differences between the NMR spectra of the samples. NMR examination also provides evidence for the catalytic effect of water in decarboxylation, as observed from the decrease in the intensity of the peak at 177 ppm representing the carboxyl groups. Both samples treated under hydrogen with or without water had a similar aromaticity (0.696 and 0.69, respectively), slightly higher than that of the raw coal (0.63). In the former case, i.e., reaction without water, the increase of the aromaticity is not principally due to condensation or dehydrogenation reactions but is a result of a “concentration

effect” because of the decarboxylation reactions, as substantiated by the almost unchanged aliphatic carbon fraction. After treatment in N2-H2O, the aromaticity increased to 0.758 and the aliphatic carbon to total carbon ratio decreased from 0.275 to 0.212 because of dehydrogenation reactions, directly in keeping with the reduced hydrogen content of the sample. The absolute deviations for the carbon distribution calculations are about (2%. The precision of this method is highly dependent on the probability of matching the chemical shift of a particular carbon to the resonance range determined in the curve deconvolution method. The resonance values of the carbon distribution calculations are based on the chemical shifts observed in model compounds.64 How accurate this estimation is for a complex structure like coal is not known exactly. Nonetheless it seems highly probable that this method satisfactorily tracks changes in specific NMR resonance intensities due to a particular carbon type across the range of treatment procedures. There was a substantial reduction in the intensity of the oxygen-substituted aryl C shoulder at 153 ppm, by about a factor of 2, after treatment in H2-H2O combination. This change was negligible for the sample treated in the absence of water, the comparison providing evidence for a significant role of water in aryloxygen bond-cleavage reactions. This decrease may be accounted for by a decrease in phenolic hydroxyl groups and cleavage of aryl-ether bonds. With regard to hydroxyl groups, only some dihydroxy-funtionalized naphthalene molecules show sensitivity to the presence of water; water somewhat enhanced the removal of one hydroxyl group from the molecules at 315 °C, while monosubstituted ones were insensitive.8 From our elemental balance calculations, this kind of reaction seems to be minor for this coal. However, Siskin et al. found that activated diaryl ethers, such as 4-phenoxyphenol, are thermally unreactive at 350 °C but in aqueous medium the ether bond is cleaved readily through ionic pathways.44 Activated diaryl ethers can be expected to be abundant in low-rank coals, and ether bond cleavages in the presence of water could be a predominant reaction in this system. We have mentioned earlier that oxygen rejection from hydroxyl or ether groups is relatively low. For the reactions in the absence of water, water formation is low and waterforming reactions should be from the ether sites and from condensation of the carboxylic groups. However, in reactions done in water, it is difficult to determine whether dehydroxylation occurs or not only on the basis of NMR and elemental analysis data, due to possible (64) Yamamoto, O.; Hayamizu, K.; Yanagisawa, M.; Yabe, A.; Sugimoto, Y. J. Jpn. Inst. Energy 1994, 73, 267.


Energy & Fuels, Vol. 12, No. 6, 1998 1209 Scheme 2

Figure 8. SPE-MAS C13 NMR spectra of TB coal with or without heat treatment. Heat treatments were done at 330 °C.

oxygen incorporation into the solid from the water, Scheme 2. That is, while some dehydroxylation reactions may occur, oxygen incorporation from water may be taking place on other sites. A catalytic effect of water in the cleavage of ether bonds of 1-phenoxynaphthalene and 9-phenoxyphenanthrene molecules was also reported previously.50 The strength of the band located at 140 ppm, which is assigned as carbon-substituted aryl carbon, increased almost proportionally with the decrease of oxygenated aryl carbon. This could be due to condensation reactions and dehydrogenation of carbon-substituted alicyclic units; however, if any condensation reaction did occur, it should take place to a lesser extent than the cleavage of covalent cross-links, which we infer because the sample treated in H2-H2O gave the highest pyridine solubility. The increase of this band ultimately arises from a change in the chemical environment of carbonsubstituted aryl carbons. For instance, resonances of aryl carbons both protonated or attached to an alkyl carbon and ortho to an oxygen substituent are located ∼10-15 ppm upfield compared to non-oxygen-substituted analogues. Thereby, elimination of an oxygen group from the ring results in a shift of the resonances of alkyl-substituted and unsubstituted ortho carbons to ca. 140 and 126 ppm, respectively. Aliphatic ether carbons decrease with treatment, but this change seems to be irrespective of the presence of water. The increase of methyl groups may indicate cleavage of aliphatic C-O and C-C bonds with subsequent hydrogen capping at the freshly cleaved sites. The relatively lower increase of methyl groups during treatment under nitrogen would then be consistent with cross-linking reactions (rather than hydrogen capping) of the resulting aliphatic radicals. In the case of TB coal, the structural changes indicated by NMR seem to be minor, even with treatment in H2-H2O system (Figure 8), despite the substantial degradation of the coal that was apparent in the pyridine solubility measurements and by microscopic

examination. There was almost no change in the aryloxygen resonance region, but there seems to be a decrease of the aliphatic ethers with treatment at 330 °C in the absence of water. In the presence of water, the Ar-O band decreased by less than 2%. It is interesting to note that TB coal dissociated or decomposed to a much higher extent with a much lower degree of the cleavage of covalent bonds as compared to GN coal. This result indicates that resultant pretreated GN coal still has a relatively high number of cross-links that keep ∼66% (daf) of the coal insoluble in pyridine. Hence, we questioned whether that marked increase of the solubility of TB coal from ∼18% to ∼79% could be explained by or be consistent with only very few covalent bond cleavages. Noncovalent interactions also have to be considered with regard to their role in the association of the coal structure. However, pyridine itself is already capable of cleavage of the kinds of hydrogen bonds that may be abundant in a subbituminous coal.65 In this sense, then, different concentrations of hydrogen bonds in the two samples may not be expected to be responsible for the greatly different solubility between them, provided that all the hydrogen-bond-containing sites are accessible to pyridine. However, that accessibility may not necessarily be achieved in practice, and superheated water, with its increased solvation character,66 may be more easily transported through the pores and reach noncovalent bonds that cannot be accessed by refluxing pyridine. To summarize the agreement between NMR and FTIR, both show that heat treatment led to the diminution of the carboxyl content of the coals and that this decrease is more pronounced for the samples treated in the presence of water. The ratio of the C-H stretching intensities and C-H bending intensities at 1453 cm-1 to the C-C aromatic stretching intensity at 1610 cm-1 increased with treatment in H2-H2O. This result indicates an increase of aliphaticity in agreement with NMR and also is in keeping with the higher hydrogen consumption for TB coal treated in the presence of water. It is noteworthy that in the H2-H2O system more degradation of the structure occurs than in N2-H2O. This, again, is consistent with the argument that radical fragments from homolytic cleavage reactions of relatively weak bonds can readily undergo coupling in the N2 environment but are capped or stabilized against coupling in H2. Implications for the Behavior of Pretreated Coals in Conversion Processes. In the Introduction, we indicated possible beneficial effects of heat treatment on various coal conversion processes and particularly emphasized the role of water in this treatment. As an important and a common issue in both liquefaction and (65) Larsen, J. W.; Flowers, R. A., II; Hall, P. J. Energy Fuels 1991, 5, 998. (66) Katritzky, A. R.; Allin, S. M.; Siskin, M. Acc. Chem. Res. 1996, 29, 399.


Artok et al.

Figure 9. Hydrogen-donor ability (HDA) of raw and treated coal samples. Scheme 3

carbonization processes, the retrogressive cross-linking reactions must be hindered effectively. Because lowrank coals can be highly prone to this sort of reaction, it is particularly essential to reduce the tendency to cross-linking to be able to utilize these coals more effectively. Carboxyl groups are capable of undergoing ionic condensation reactions with each other or with hydroxyl groups in their vicinity to form anhydride or ester structures, respectively. These primary condensation products are then potentially involved in radical reactions that either consume the labile hydrogen atoms in coal or form strong cross-links. As evidenced from the gas analyses from the pretreatment reactions and from spectroscopic examinations of the products, water enhanced decarboxylation reactions in which subsequent condensation reactions are very limited. This mechanism is illustrated in Scheme 3 and is intended to suggest the behavior of the pretreated coals in conversion processes. The condensation of carboxylic groups with each other or hydroxyl groups may occur in pretreatment or conversion processes, the latter, in general, requiring relatively higher temperatures. These condensed sites may become involved in radical reactions during the conversion processes (not in radical reactions in pretreatment reactions occurring at 285330 °C). The C-O bond in anhydrides is a relatively strong bond (60-80 kcal/mol). Our earlier work with benzoic anhydride has shown conversions of 100% at 400 °C via radical mechanisms; this is in marked contrast to reactions of benzoic anhydride in naphthalene, which provide conversions of 7.7-11.5% at the same temperature.14

In the course of the heat treatment procedure, water remarkably promotes cleavage reactions. Molecular hydrogen mainly functions in scavenging active radical species which may have formed via homolytic cleavage of relatively weak cross-links and may also be involved in cleavage reactions to some small extent via the catalytic effect of pyrite. Thus, at higher temperatures, active radical species will be generated in lower concentration because of the reduction in the concentration or quantity of their precursors. Moreover, reduced cross-linking will give rise to a more mobile system that may melt or swell to a higher extent as compared to the raw coal, thus resulting in a higher degree of contact with donor solvent and catalyst during liquefaction reactions. Partial hydrogenation of condensed aromatics in the coal could offer a greater concentration of inherent donatable hydrogen atoms to scavenge radicals. As an implication for co-carbonization processes, the reduced tendency of heat-treated low-rank coals to early cross-linking reactions will offer their potential utilization in metallurgical coke production by blending with coking coal. The raw and pretreated coal samples were subjected to pyrolysis with anthracene to test their hydrogendonor abilities. Kidena et al. used this process to examine various coking coals and found a correlation between the development of plasticity and the quantities of donatable hydrogen in the coal.38 Figure 9 shows the HDA values of raw and pretreated coal samples. In general, samples treated in H2-H2O combination afforded higher HDAs than the samples pretreated in H2-dry or N2-H2O system, verifying a synergism arising from combination of water and H2.


The differences in HDA values can be ascribed not only to increased hydrogen content, since treatments under hydrogen with or without added water resulted in similar amounts of hydrogen absorption, but also to reduced radical reaction pathways as discussed above. It is noteworthy that the highest HDA value afforded among the coals tested by Kidena et al. was ∼1.1 mg of H2/g of daf coal by Pittstone-MM coal (85.7%C, daf),38 which is roughly similar to that by TB subbituminous coal after pretreatment at 330 °C in H2-H2O combination. The measured hydrogen-donating ability for TB in the H2-H2O system roughly doubled between 305 and 330 °C, although the corresponding increase in the H/C ratio was relatively small. Several factors may contribute to this result, including greater hydrogen absorption of the coals in the presence of water, reduced formation of radicals that consume intrinsic hydrogen, and enhanced mass transfer resulting from a fluid or semi-fluid state.


aryl carbon-ether oxygen bonds in GN coal. The activated diaryl ethers may be a useful probe for the possible ether structures in the lignite, since these ethers were highly susceptible to cleavage reactions in water while unreactive thermally.44 Although much less structural change was observed for TB coal than for GN coal upon heat treatment, the former consisted of a more dissociated structure. The observed results may be ascribed to disruption of noncovalent bonds by the effect of water. An elemental analysis revealed that hydrogen introduction to the solid matrix occurs. However, water seems to have no part in hydrogenation reactions. The extent of depolymerization of coals increases in the order H2-dry < N2-H2O < H2-H2O, this order indicating the primary effect of water in depolymerization reactions; however, hydrogen also has an explicit role in this process. The hydrogen-donor ability of the coals to anthracene also follows the same order, indicating the major role in this event is reduced radical-forming reactions during pyrolysis.

Conclusions The effects of molecular hydrogen and water on lowtemperature heat treatment of a lignite and a subbituminous coal were investigated at 285-330 °C. Spectroscopic examination of the products and analysis of the gaseous products after treatment revealed that water greatly enhanced the decarboxylation of the coals. From a comparison with earlier model compound experiments, the activated monoarylcarboxylic acids could be a good probe for representing the chemical environment of the carboxyl groups, particularly in the lignite. According to SPE-MAS C13 analyses of the raw and heat-treated coals, more structural changes occur for GN coal than TB coal. Molecular hydrogen has little effect in covalent bond-cleavage reactions, but water was determined to have a profound effect in the cleavage of

Acknowledgment. The support of The Scientific and Technical Research Council of Turkey (TUBITAK) under Contract No. of TBAG-1416 to this work and a NATO Science Postdoctoral Fellowship Program by TUBITAK to L. Artok (PSU) is gratefully acknowledged. The authors express their appreciation to Dr. Alan Davis of the Coal and Organic Petrology Laboratories of PSU for his permission to use the microscopic facilities in his laboratory and Dr. Chunshan Song for many fruitful discussions. We also thank Mr. Ronald M. Copenhaver for his assistance with reactor fabrication and other laboratory support, even when faced with considerable trouble. EF980049E

Effects of Water and Molecular Hydrogen on Heat Treatment of

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