The individual and competitive hydrogenation reactions of ethene, propene and isobutene over various nickel metal-loaded 4A zeolites have been investigated. The reaction kinetics were found to be similar to those found by other workers for alkene hydrogenation over pure nickel metal. Reactions were first order in hydrogen and zero order in alkene at lower temperatures. However, the order with respect to alkene increased towards unity as temperature increased and led to a decrease in apparent activation energy with increasing temperature. Activation energies generally became negative at temperatures greater than 170°C. The catalytic reactions often exhibited pronounced induction periods. The catalytic activity during this induction period and the length of the period varied with the catalyst and reaction temperature used. An explanation of some aspects of the results found in this work is proposed in terms of the presence of two different sets of catalytically active sites. One set of sites, residing in the zeolite pores, are more reactive towards the smaller alkene molecules and do not exhibit an induction effect. The other sites, which are responsible for the observed induction effect, are equally active towards all alkenes and are located either on the surface of the zeolite crystallites or on other amorphous material present in the zeolite samples used. The reaction sites are not always associated with the presence of nickel atoms as the parent zeolite, which contained no nickel, was also reasonably active for the hydrogenation reaction. 4A zeolite has been reported as being a good shape-selective catalyst if the active centres are located within the pore structure. However, the results obtained from competitive hydrogenation in this work indicate that no selectivity for ethene hydrogenation in the presence of propene or isobutene can be obtained at temperatures below about 130°C, as the larger alkenes block the zeolite pores
