Mechanism and poisoning of the molecular redistribution reaction of alkanes with a dual-functional catalyst system

The disproportionation of alkanes was carried out over a dual-functional catalyst comprising platinum-on-alumina mixed with tungsten oxide-on-silica. The reaction involves formation of low concentrations of olefinic intermediates over a dehydrogenation catalyst followed by disproportionation of these olefins over a disproportionation catalyst. Product olefins are rehydrogenated to the corresponding paraffins. A series of layered bed experiments involving the individual catalyst components was performed to confirm this sequence of reactions. Because a complex mixture of reacting species and products is generated from a single reactant and cross reactions occur between different classes of hydrocarbons, the overall process is termed molecular redistribution rather than simply disproportionation (1). In this system, straight-chain alkanes are more reactive than branched, with pure isobutane being only slightly reactive. With mixtures of n-butane and isobutane, the former reacts not only with itself but also with isobutane, in which case isopentane is a major product. The molecular redistribution catalyst system is extremely sensitive to poisoning by impurities, and the catalyst must be protected by a suitable guard chamber in the feed line. An excess of either olefins or hydrogen over the low concentrations formed as reaction intermediates inhibits the reaction. Catalyst stability increases with decreasing temperature and with increasing pressure and is also influenced by the ratio of the two catalyst components.