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Energy & Fuels 1998, 12, 660-661
Communications Pyrolysis of a Polymeric Model of Aromatic Carboxylic Acids in Low-Rank Coal Phillip F. Britt,*,† William S. Mungall,‡ and A. C. Buchanan, III† Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6197, and Department of Chemistry, Hope College, Holland, Michigan 49423 Received December 1, 1997. Revised Manuscript Received March 25, 1998 Cross-linking reactions associated with oxygen functional groups in low-rank coals limit the thermochemical conversion to liquid products by leading to char formation.1,2 Carboxyl groups have been implicated in lowtemperature cross-linking reactions, and CO2 evolution from carboxylates appears to be the key indicator of crosslinking reactions in pyrolysis and liquefaction.2b Solomon et al. have also modeled the pyrolytic loss of solvent swelling in coal by including one cross-link for every CO2 evolved.2a However, based on the limited mechanistic information available in the literature on the decarboxylation of aromatic carboxylic acids, it was not apparent how decarboxylation would lead to cross-linking.3,4 Recent investigations into the pyrolysis of model benzoic acid derivatives3 and 1,2-(3,3′-dicarboxyphenyl)ethane4b at 350-425 °C discovered that decarboxylation of free aromatic carboxylic acids proceeds primarily by an acidpromoted cationic pathway which does not produce crosslinked products. However, simple compounds may not adequately model the complex chemistry and physical environment found in coal. Moreover, a small amount of cross-linking, which could go undetected in these nonpolymeric models, could have a large influence on the solvent swelling of a complex macromolecule such as coal. Therefore, to investigate the impact of the macromolecular structure of coal on the reaction pathways of free aromatic carboxylic acids, we report the pyrolysis of poly(1,3-xylylene-co-5-carboxy-1,3-xylylene), 1, as a polymeric model of carboxylic acids in low-rank coal and compare its reactivity to the O-methylated derivative, 3, and the
unsubstituted homopolymer, poly(1,3-xylylene), 2. As opposed to simple model compound studies,3,4 significant amounts of cross-linking were observed (as tetrahydrofuran (THF) insoluble residues) in the pyrolysis of 1 under †
Oak Ridge National Laboratory. Hope College. (1) Derbyshire, F.; Davis, A.; Lin, R. Energy Fuels 1989, 3, 431. (2) (a) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. Energy Fuels 1990, 4, 42 and references therein. (b) Serio, M. A.; Kroo, E.; Chapernay, S.; Solomon, P. R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (3), 1021. (c) Suuberg, E. M.; Lee, D.; Larsen, J. W. Fuel 1985, 64, 1668. (d) Joseph, T. J.; Forrai, T. R. Fuel 1992, 71, 75. ‡
specific conditions where the volatile products were removed from the residue. These conditions were found to enhance the formation of anhydrides which act as low temperature (>250 °C) cross-links. At higher temperatures, anhydrides can decompose under the free-radical reaction conditions to form robust aryl-aryl cross-links.4a The extent of cross-linking was found to be strongly dependent on the pyrolysis conditions, i.e., open vs closed system. The synthesis and characterization of 1-3 have been previously described.5 The concentration of carboxylic acids in 1, 2.3 acids per 100 carbons, is similar to that reported for Beulah-Zap coal, 2.2 acids per 100 carbons.6 At a heating rate of 5 °C min-1, the general shape of the weight loss curves (TGA) for 1, 2, and 3 are similar, with depolymerization occurring between 425 and 500 °C. However, the carboxylated polymer, 1, forms ca. 2.4 times more char upon heating to 800 °C than 2 but methylation of 1 decreases the yield of char from 24 to 16 wt %. Evolution profiles of volatile hydrocarbons for 1 and 2 are similar by TG-MS analysis.8 At a heating rate of 10 °C min-1, hydrocarbon evolution starts at 480-500 °C and reaches a maximum at 530 °C. However, in the pyrolysis of 1, CO2 evolution starts at 410 °C, prior to depolymerization of the polymer backbone, and reaches a maximum at 520 °C. Overall, the pyrolysis behavior of 1 is analogous to that reported for low-rank coals in which CO2 evolution occurs prior to tar evolution and Omethylation reduces cross-linking (char yield).2 The bulk of the experimental data on the influence of functional groups on cross-linking reactions during coal pyrolysis has been obtained under conditions in which the volatile products were removed from the reaction zone for analysis by FTIR.2 In analogy, the pyrolysis of 1-3 (3) Manion, J. A.; McMillen, D. F.; Malhotra, R. Energy Fuels 1996, 10, 776. (4) (a) Eskay, T. P.; Britt, P. F.; Buchanan, A. C., III. Energy Fuels 1997, 11, 1278. (b) Eskay, T. P.; Britt, P. F.; Buchanan, A. C., III. Energy Fuels 1996, 10, 1257. (5) Mungall, W. S.; Britt, P. F.; Buchanan, A. C., III. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42 (1), 26 and references therein. (6) Jung, B.; Stachel, S. J.; Calkins, W. H. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36 (3), 869. (7) A small weight loss (ca. 2%) was observed for 1 between 100 and 300 °C. Although no significant amount of water was observed by TGMS analysis, small amounts of water would be hard to detect as a consequence of the experimental design. Quantitative conversion of the carboxylic acids in 1 to anhydrides would produce a weight loss of
