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Energy & Fuels 1996, 10, 1163-1170
1163
Hydrocracking and Hydroisomerization of Long-Chain Alkanes and Polyolefins over Metal-Promoted Anion-Modified Zirconium Oxides K. R. Venkatesh,† J. Hu,‡ W. Wang,§ G. D. Holder, J. W. Tierney, and I. Wender* Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 Received March 25, 1996X
Previous work on the hydrocracking and hydroisomerization of alkanes over metal-impregnated anion-modified zirconium oxides (AZOs) is extended to long-chain alkanes, from n-heptane to high molecular weight polyolefins, using ZrO2 modified by anion-derived groups such as SO4 and WO3 and promoted with hydrogenation metals such as Pt or Ni. Depending on reaction temperature and time, high yields of C5-C12 isoalkanes or a mixture of gases with high selectivities to isobutane and isopentane can be produced. The products do not contain olefins, aromatics, or alkanes of carbon number higher than the feed. The iso/normal ratios of the alkane products obtained are significantly higher than those predicted by isomerization equilibria at the reaction conditions. It appears that higher (C7+) alkane hydrocracking over metal-promoted AZOs may not proceed via the conventional bifunctional mechanism involving initial dehydrogenation to an olefinic intermediate. The AZOs did not sinter or agglomerate during the hydrocracking reactions as indicated by particle size measurements. AZOs containing WO3 are more stable than those containing SO4, retaining their anionic groups in reactions at severe reducing conditions [300+ °C, 500-1200 psig (cold) H2]. XANES analysis of the Pt/ZrO2/WO3 catalyst indicated that both Pt and W maintained their zerovalent (Pt0) and hexavalent (W6+) states, respectively, during alkane hydrocracking as well as during recalcination in air.
Introduction The desire to produce highly isomerized alkanes for high-octane transportation fuels has led to the use of bifunctional catalysts containing dispersed transition metals on acidic supports for alkane hydrocracking.1 Bifunctional catalysts consisting of hydrogenation components such as sulfided Ni, Mo, or W promoted on acidic supports such as Al2O3 or SiO2-Al2O3 have been used; they crack n-alkanes to a significant extent but result in little isomerization.2-4 Noble metals, such as Pt and Pd, are strong hydrogenation catalysts which balance the acidity of supports and were reported5,6 to result in high selectivities to isomerized products in alkane hydrocracking. Weitkamp7 indicated that the amounts of isomerized and normal alkanes formed in n-alkane hydrocracking depend on the relative strengths of the metal and acidic functions. Over the past two decades, a variety of Pt-promoted zeolites have been used as n-alkane hydrocracking catalysts and have * Author to whom correspondence should be addressed. † Present address: Calgon Carbon Corp., P.O. Box 717, Pittsburgh, PA 15230. ‡ Present address: Hydrocarbon Technologies Inc., P.O. Box 6047, Lawrenceville, NJ 08648. § Present address: 8 Rue Klingenthal, 67200 Strasbourg, France. X Abstract published in Advance ACS Abstracts, October 1, 1996. (1) Sullivan, R. F.; Meyer, J. A. ACS Symp. Ser. 1975, No. 20, 28. (2) Flinn, R. A.; Larson, O. A.; Beuther, H. Ind. Eng. Chem. 1960, 52, 153-156. (3) Archibald, R. C.; Greensfelder, B. A.; Holzman, G.; Rowe, D. H. Ind. Eng. Chem. 1960, 52, 745-750. (4) Voorhies, A.; Smith, W. M. Advances in Petroleum Chemistry and Refining; Interscience Publishers: New York, 1964; Vol. 8. (5) Van Hook, W. A.; Emmett, P. H. J. Am. Chem. Soc. 1962, 84, 4410. (6) Conradt, H. L.; Garwood, W. E. Ind. Eng. Chem. 1960, 52, 113. (7) Weitkamp, J. ACS Symp. Ser. 1975, No. 20, 1.
shown good selectivity to isoalkanes and increased resistance to heteroatom impurities.8-10 Strong solid acid catalysts have high activity in cracking and skeletal isomerization of n-alkanes11-13 and could serve as effective hydrocracking catalysts, especially when promoted by strong hydrogenation metals such as Pt and Ni. The metal-promoted anionmodified zirconium oxides (AZOs) such as Pt/ZrO2/SO4 and Pt/ZrO2/WO3, are strong solid acids with an active hydrogenation function; they could potentially replace the conventional bifunctional catalysts. There is increased interest in these catalysts because they have high activity at relatively low temperatures (
