Short contact time oxidative dehydrogenation of propane

Short contact time oxidative dehydrogenation of propane has been investigat ed over a Pt/10%Rh gauze catalyst and a catalyst consisting of VMgO support ed on a monolithic substrate, The main focus has been on obtaining informat ion 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 t o 950 degreesC and with propane/oxygen ratios between 1.1 and 2.3 at atmosp heric pressure. It was observed that ethene. methane and synthesis gas (wit h a hydrogen/carbon monoxide ratio close to 1.0) were the main products: at high conversions of propane. Propene becomes increasingly more important a t lower conversions, especially in the presence of an active VMgO catalyst. Very low selectivities to ethene and high selectivities to carbon oxides w ere obtained with the VMgO catalyst at low conversions of propane (<20%). A t high temperatures and high conversions no significant effect of the prese nce of neither the noble metal catalyst nor the VMgO catalyst was found. Sm all amounts of ethyne, propyne and benzene were always observed in the prod uct 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 rea ctor, particularly at the lower conversions of propane. One important role of both catalysts seems to he to ignite and to facilitate the internal prod uction of heat for the gas phase formation of alkenes. The effect of varyin g the flow rate on the performance of the VMgO catalysts indicates that the heterogeneous reactions are subject to transport limitations as would be e xpected at these conditions. (C) 2001 Elsevier Science B.V. All rights rese rved.