Hydrocracking and Hydroisomerization of Long-Chain Alkanes and Polyolefins over Metal-Promoted Anion-Modified Zirconium Oxides

Abstract

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 ZrO 2 modified by anion-derived groups such as SO 4 and WO 3 and promoted with hydrogenation metals such as Pt or Ni. Depending on reaction temperature and time, high yields of C 5 −C 12 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 (C 7+ ) 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 WO 3 are more stable than those containing SO 4, retaining their anionic groups in reactions at severe reducing conditions [300+ °C, 500−1200 psig (cold) H 2 ]. XANES analysis of the Pt/ZrO 2 /WO 3 catalyst indicated that both Pt and W maintained their zerovalent (Pt 0 ) and hexavalent (W 6+ ) states, respectively, during alkane hydrocracking as well as during recalcination in air.