ISOBUTANE DEHYDROGENATION ON CHROMIA ZIRCONIA CATALYSTS/

Isobutane dehydrogenation was studied on CrOx/ZrO2 (ZC) catalysts (0.0 4 to 1.9% chromium by weight) in a flow apparatus at 673-873 K. Up to 823 K, molar selectivity to isobutene was greater than or equal to 95% , while the only gaseous by-products were propene, methane and traces of propane. No skeletal. isomerization occurred up to 873 K. A deactiv ation was observed due to the formation of coke. The activity decay wa s considerably delayed by adding a few percent hydrogen to the reactan t mixture. In all cases, catalytic activity was completely recovered b y treating the catalyst with flowing oxygen at 773 K over 22 reaction- regeneration cycles. The catalyst surface was conditioned mainly by th e reaction atmosphere, so that sample reduction with carbon monoxide a t 623 K before catalysis did not influence the activity above 773 K. T he turnover frequency per total surface chromium atom was found to be independent of surface chromium concentration at all given temperature s, which indicated a mononuclear active site. A comparison with chromi a/silica and chromia/gamma-alumina samples of similar chromium content revealed a higher activity for the zirconia-based catalysts. The over all picture shows close similarities with that of propane dehydrogenat ion previously studied in our laboratory on the same catalytic system. It is concluded that the site previously proposed for propane dehydro genation is also responsible for the activity in the dehydrogenation o f isobutane; this site involves a mononuclear Cr-III species with two coordinative vacancies and, most probably, an adjacent O2- ion of the support.