Nonradical Reactions during Coal Conversion. A Search for Synchronous 1,4-Hydrogen Addition as a Precursor to Radical Reactions Anna Korda, † John W. Larsen,* ,† Shona C. Martin, ‡ Ajay K. Saini, ‡ Harold H. Schobert,* ,‡ and Chunshan Song ‡ Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, and The Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 Received July 6, 1999 Previous studies on the relative reactivity of H 2 and tetralin have shown greater hydrogen transfer from H 2 to coal than from tetralin to coal at 350 °C. These results are not consistent with a radical pathway for hydrogen addition and require the existence of a nonradical route. We explored the possibility that this nonradical route is synchronous 1,4-H 2 addition, which could occur either with acenes or with dihydroxybenzenes. Reactions of Illinois No. 6 coal with added phenanthrene, anthracene, or m-dihydroxybenzene provide no evidence in support of this addition reaction. The 1,4-addition of H 2 to phenols produces carbonyl compounds. Reactions of Wyodak coal, which should have a higher population of phenolic groups than Illinois No. 6, show no evidence for a correlation of CO production with H 2 utilization or for changes in carbonyl group population that would be consistent with 1,4-addition to phenols. In light of these negative findings, the likely nonradical pathway would then seem to be catalysis by mineral matter. This possibility was probed by comparing reactions of untreated and demineralized Wyodak. The untreated coal, in the absence of solvent, gives higher liquid yields and proportionately more oils and asphaltenes at the expense of preasphaltenes than does the demineralized coal. This indicates a role for the mineral matter in H 2 utilization. Introduction In 1990, Kabe and co-workers published some inter- esting and provocative data on the relative reactivity of Datong coal with H 2 and tetralin. 1 They clearly demonstrated greater hydrogen transfer to coal from H 2 than from tetralin at 300 °C and 350 °C, and comparable transfer at 400 °C. Hydrogen exchange between coal and H 2 is greater than between coal and tetralin at 350 °C and 400 °C. Huang has shown that, for a Texas subbituminous C coal, hydrogen consumption from H 2 is greater than from tetralin at 350 °C (though both are low); however, in her system this was not the case at 400 °C. 2 These results are fascinating and important because they are difficult to explain using radical reaction pathways. This behavior is not universal; for example, Wandoan coal behaves differently. 3 The H-H bond strength is 104 kcal/mol, while the benzylic C-H bond strength in tetralin is about 82 kcal/ mol. 4-6 It is inconceivable that a radical formed at 300 °C would break a 104 kcal/mol bond in preference to an 82 kcal/mol bond. Yet this is required to explain these results using radical reactions. If all of the 22 kcal/mol difference in bond dissociation energy is preserved in the transition state for radical hydrogen abstraction, then tetralin will be favored over H 2 as a hydrogen atom source by about 10 8 .H 2 competes with tetralin at 450 °C only because it participates in a radical chain reaction while tetralin does not. 7 Two arguments can be used to show that H 2 will be less competitive with tetralin in free radical reactions at lower temperatures. As the radicals from the coal get more stable, they become more selective and react more with tetralin and less with H 2 . In an experimental demonstration of this, lowering the bond strength of the molecule forming the initial radical from 56 kcal/mol to 50 kcal/mol lowers the amount of reaction with H 2 in an H 2 -tetralin mixture from 36 to 30%. 7 A bond having a half-life of 30 min at 350 °C has a strength of 46 kcal/mol, so its reaction with H 2 would be less than 3% of its reaction with tetralin. Kabe’s data demand a nonradical pathway for the reaction of H 2 with coals. There are several possible reaction pathways to consider. We shall address them, beginning with 1,4-addition of H 2 to dienes. The synchronous 1,2-addition of H 2 to alkenes (1) is an orbital symmetry forbidden reaction that must be catalyzed. The synchronous 1,4-addition to dienes (2) † Department of Chemistry. ‡ The Energy Institute. (1) Kabe, T.; Yamamoto, K.; Ueda, K.; Horimatsu, T. Fuel Proc. Technol. 1990, 25, 45. (2) Huang, L. Ph.D. Dissertation, The Pennsylvania State Univer- sity, University Park, PA, 1995. (3) Ishihara, A.; Morita, S.; Kabe, T. Fuel 1995, 74, 63. (4) Berkowitz, J.; Ellison, G. B.; Gutman, D. J. Phys. Chem. 1994, 98, 2744. (5) Kamiya, Y.; Futamura, S.; Mizuki, T.; Kajioka, M.; Koshi, K. Fuel Proc. Technol. 1986, 14, 79. (6) Franz, J. A.; Camaioni, D. M. J. Org. Chem. 1980, 45, 5247. (7) Vernon, L. Fuel 1980, 59, 102. 545 Energy & Fuels 2000, 14, 545-551 10.1021/ef990146n CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000