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 oxid ative dehydrogenation and cracking in air or oxygen at atmospheric pre ssure over noble metal coated ceramic foam monoliths at contact times of similar to 5 milliseconds has been studied. On Pt, synthesis gas (C O and H-2) dominates near its stoichiometry, while olefin production d ominates at higher fuel-to-oxygen ratios. No carbon buildup is observe d, and catalysts exhibit no deactivation over at least several days. O n Rh, primarily synthesis gas is produced under these conditions, whil e on Pd, carbon deposition rapidly deactivates the catalyst. We observ e up to 65% selectivity to olefins at nearly 100% conversion of propan e or n-butane with a catalyst contact time of 5 ms. Ethylene selectivi ty is maximized by increasing the reaction temperature, either by preh eating the reactants or by using oxygen enriched air. Propylene select ivity is maximized by lower temperature and shorter catalyst contact t ime. Very small amounts of alkanes and higher molecular weight species are obtained, suggesting that a homogeneous pyrolysis mechanism is no t occurring. A very simple reaction mechanism appears to explain the o bserved product distribution. Reactions are initiated by oxidative deh ydrogenation 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 t o C-s and H-s. (C) 1994 Academic Press, Inc.