PERIODIC OPERATION OF A CATALYST AS A MEANS OF OVERCOMING A THERMODYNAMIC CONSTRAINT - THE CASE OF METHANE HOMOLOGATION ON METALS

Periodic operation of a catalyst can be a way of overcoming a thermodynamic constraint. Homologation of methane is thermodynamically disfavored. However, a two-step procedure using metal catalysts under non-oxidative conditions allows the thermodynamic limitations to be circumvented. Metal catalysts, such as Pt, Co and Ru are exposed first to methane and then to hydrogen. In their dual-temperature procedure, van Santen et al. carry out the first step with dilute methane at a high temperature (usually 725 K), which allows the endothermic decomposition of methane to take place. Part of the C deposits may yield higher alkanes up to C-4-C-5 through the following hydrogenation at a much lower temperature (368 K). In contrast, we carry out these two steps at atmospheric pressure and at the same but moderate temperature (usually less than 570 K). In this case, chemisorption of methane is accompanied by release of hydrogen whereas coupling of H-deficient CH, adspecies may take place. Numerous higher alkanes up to C-7-C-8 are then removed by supplying hydrogen at ordinary pressure and at the same temperature as that of the first step. The driving force can be found in the energy which has to be supplied in order to compress part of the dilute hydrogen removed in the first step to make it usable in the second one. The influence of some key factors is studied.