Oxidative dehydrogenation (ODH) of ethane and propane feeds has been examined under a variety of temperatures, flow rates, and gas compositions in a fixed bed reactor using boron nitride-based catalysts either promoted with various metal oxides or supported on silica. Tests performed at low conversion in the presence of a large amount of N 2 diluent indicated that addition of MgO to boron nitride (BN) led to a ∼10-fold increase in alkane conversion, with a drop in the C 2 −C 3 olefin selectivity from 90% to 79%. Testing using concentrated feeds with small amounts of diluent (3%) indicated that metal-oxide promoted BN catalysts could maintain C 2 −C 3 olefin selectivities as high as 65% at high alkane conversion (∼60%). Catalyst productivity values under these conditions approached 16 kg olefins kg cat −1 h −1 . Under nondilute conditions, the catalysts appear to act as initiators of radical reactions at the surface, after which the radicals desorb into the gas phase and undergo propagation steps.
This work provides a detailed interpretation of the complex reaction mechanisms of n-butane on Pt/H-ZSM-5 catalyst in the presence of H 2 from the perspective of kinetics. In one route, n-butane conversion involved dehydrogenation on Pt to form butene and isomerization, dimerization, and cracking steps on the acid sites in the zeolite. Alternatively, butane could have underwent direct protolytic cracking on zeolite. On zeolite surfaces dominated by unoccupied Brønsted acid sites, protolytic cracking of butane was the preferred primary cracking mechanism. As the coverage of C 4 surface adsorbate increased, the routes via butene became prominent and likely dominated under practical conditions with high hydrocarbon partial pressures. The various reaction pathways all followed the Langmuir−Hinshelwood rate model. With over 90% combined selectivity of propane and ethane at nearly full butane conversion and good catalyst regenerability, this mechanism presents an effective way to convert butane fuel to chemical feedstocks.
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