OXIDATIVE DEHYDROGENATION OF ISOBUTANE OVER MONOLITHS AT SHORT-CONTACT TIMES

Isobutylene can be produced with high selectivity and conversion by oxidative dehydrogenation of isobutane in air or oxygen over a ceramic foam monolith coated with Pt at contact times of similar to 5 ms in an atmospheric pressure reactor operating at 800 to 900 degrees C. Total olefin selectivities up to 80% (40% isobutylene, 40% propylene) at 60% conversion of isobutane are achieved in an autothermal process with higher reactant conversion when the reactants are preheated up to 400 degrees C. No carbon build-up is observed at molar isobutane-to-oxygen feed ratios up to 2.5, and the catalyst shows no deactivation over at least several weeks of operation. Maximum isobutylene selectivity occurs at a fuel-to-oxygen ratio of 1.4 at contact times less than or equal to 5 ms, while propylene yield is maximized at longer contact times and at higher fuel-to-oxygen ratios. The total selectivity to methane, ethane, and ethylene is always much less than the total selectivity to isobutylene and propylene. A simple reaction mechanism can explain these products. Surface oxygen abstracts a hydrogen from isobutane resulting in an isobutyl group on the surface. On the Pt surface, beta-elimination reactions than take place, leading to either isobutylene or propylene production. The small amounts of smaller hydrocarbons compared to n-butane oxidation or to homogeneous cracking are explained by the absence of a beta-alkyl group on the isobutyl species adsorbed at a 3 degrees carbon. (C) 1995 Academic Press, Inc.