Short contact time oxidative dehydrogenation of propane

Short contact time oxidative dehydrogenation of propane has been investigated over a Pt/10%Rh gauze catalyst and a catalyst consisting of VMgO supported on a monolithic substrate, The main focus has been on obtaining information about the roles of homogeneous and heterogeneous reactions by comparing the behavior of the two catalysts with blank runs with no active catalyst in the reactor. The reaction was studied at temperatures ranging from 500 to 950 degreesC and with propane/oxygen ratios between 1.1 and 2.3 at atmospheric pressure. It was observed that ethene. methane and synthesis gas (with a hydrogen/carbon monoxide ratio close to 1.0) were the main products: at high conversions of propane. Propene becomes increasingly more important at lower conversions, especially in the presence of an active VMgO catalyst. Very low selectivities to ethene and high selectivities to carbon oxides were obtained with the VMgO catalyst at low conversions of propane (<20%). At high temperatures and high conversions no significant effect of the presence of neither the noble metal catalyst nor the VMgO catalyst was found. Small amounts of ethyne, propyne and benzene were always observed in the product gas. The conversion of oxygen and the selectivity to carbon oxides were higher over the Pt/10%Rh gauze and the VMgO catalyst than in the empty reactor, particularly at the lower conversions of propane. One important role of both catalysts seems to he to ignite and to facilitate the internal production of heat for the gas phase formation of alkenes. The effect of varying the flow rate on the performance of the VMgO catalysts indicates that the heterogeneous reactions are subject to transport limitations as would be expected at these conditions. (C) 2001 Elsevier Science B.V. All rights reserved.