Energy & Fuels 1993, 7, 331-333
331
Silica-Immobilized Compounds as Models for Probing Coal Pyrolysis and Hydropyrolysis Phenomena S. C. Mitchell, C. J. Lafferty, R. Garcia, and C. E. Snape* Department of Pure and Applied Chemistry, University of Strathcylde, Glasgow, G1 l X L , United Kingdom
A. C. Buchanan, III,* and P. F. Britt Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
E. Klavetter Sandia National Laboratories, Albuquerque, New Mexico 87185 Received November 6,1992. Revised Manuscript Received January 8,1993 Diphenylalkanes have been extensively used as model substrates to probe free-radical mechanisms involved in C-C bond cleavage reactions in coal 1iquefaction.l However, the fact that the macromolecular structure in coals is undoubtedly subject to highly restricted motion suggests intuitively that free-radical pathways are likely to be somewhat different from those encountered in fluid phases. Indeed, this has been confirmed through the study of silicaimmobilized diphenylalkanes where bimolecular reaction steps can be significantly perturbed compared to the corresponding fluid phase reactions.24 Thus far, immobilized diphenylalkanes have only been studied at temperatures close to 400 "C, but they also have considerable potential for probing coal and oil shale pyrolysis mechanisms at temperatures in the range 500-600 "C which are generally used to maximize tar yields in both fluidized beds and well-swept fixed The maximum temperature at which the substrates can be used will be dictated by the cleavage of the Si-O-C,,l linkage between the immobilized species and the silica. There are a number of phenomena related to coal pyrolysis, particularly under hydrogen pressure (hydropyrolysis), which cannot be adequately modeled using model compounds in the vapour phase.8 In this study, pyrolysis and hydropyrolysis of silica-immobilizedbenzene (=PhH) and diphenylmethane (=PhCHZPh or cDPM) have been conducted at temperatures up to 600 "C and pressures of 150 bar. The results demonstrate the considerable potential of these nonvolatile, well-defined solids to probe coal pyrolysis and hydropyrolysis phenomena. The results also demonstrate the separate contributions of hydrogen pressure and a dispersed sulfided Mo catalyst in promoting hydrocracking reactions in the solid state. The silica-immobilized benzene and diphenylmethane were prepared and characterized as described previously.24 (1) Poutama,M.L. Energy Fuels 1990,4(2),113 and referencestherein. (2) Buchanan, 111, A. C.; Biggs, C. A. J. Org. Chem. 1989, 54, 517. (3) Buchanan, III,A. C.;Britt,P.F.;Poutama,M.L.Prepr.Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1990, % ( I ) , 217. (4) Britt, P. F.; Buchanan, 111, A. C. J. Org. Chem. 1991, 56, 6132. (5) Snape, C. E. Fuel 1991, 70,285. (6) Gonenc, Z. S.; Gibbins, J. R.;Katheklakis, I. E.; Kandiyoti, R. Fuel 1990, 69, 383. (7) Ruben, A. M.; Coburn, T. T. Proc. 1981 Eastern US Oil Shale Symp. 1981,21. (8)Cypres, R.; Braekman-Danheux, C.; Progenaux, A. Fuel 1986,65, 1299.
The purities and surface coverages were 98.4 % and 0.36 mmol g1for =PhH, and 99.5% and 0.45 mmol g1for =PhCHZPh. The sulfided Mo catalyst was prepared from ammonium dioxydithiomolybdate as previously reported9J0and was loaded onto the substrates in methanol to give nominal Mo loadings of 2-5% wlw substrate. At the lowest catalyst loading currently employed, the mole ratio of Mo:DPM was ca. 1:2. The high-pressure apparatus for temperature programmed reduction (TPR) has been described.ll The sample (0.2-0.5 g;
