PRODUCTION OF OLEFINS BY OXIDATIVE DEHYDROGENATION OF PROPANE AND BUTANE OVER MONOLITHS AT SHORT-CONTACT TIMES

The autothermal production of olefins from propane or n-butane by oxidative dehydrogenation and cracking in air or oxygen at atmospheric pressure over noble metal coated ceramic foam monoliths at contact times of similar to 5 milliseconds has been studied. On Pt, synthesis gas (CO and H-2) dominates near its stoichiometry, while olefin production dominates at higher fuel-to-oxygen ratios. No carbon buildup is observed, and catalysts exhibit no deactivation over at least several days. On Rh, primarily synthesis gas is produced under these conditions, while on Pd, carbon deposition rapidly deactivates the catalyst. We observe up to 65% selectivity to olefins at nearly 100% conversion of propane or n-butane with a catalyst contact time of 5 ms. Ethylene selectivity is maximized by increasing the reaction temperature, either by preheating the reactants or by using oxygen enriched air. Propylene selectivity is maximized by lower temperature and shorter catalyst contact time. Very small amounts of alkanes and higher molecular weight species are obtained, suggesting that a homogeneous pyrolysis mechanism is not occurring. A very simple reaction mechanism appears to explain the observed product distribution. Reactions are initiated by oxidative dehydrogenation of the alkane by adsorbed oxygen to form a surface alkyl. On Pt, beta-hydrogen and beta-aIkyl elimination reactions of adsorbed alkyl dominate which lead to olefin production rather than cracking to C-s and H-s. (C) 1994 Academic Press, Inc.