HOMOLOGATION OF METHANE UNDER NONOXIDATIVE CONDITIONS

This review presents results obtained in recent years concerning the c atalytic conversion of methane into higher hydrocarbons using metal ca talysts under non-oxidative conditions at moderate temperature. Althou gh only a limited amount of work has been carried out in this area, th e non-oxidative homologation of methane has already proved itself to b e a novel and interesting way of addressing the problem of methane upg rading. Chemisorption of methane on transition metal surfaces has been studied for long on either ill-defined surfaces or single crystals. T hese studies, reviewed here, concerned mainly the kinetics and dynamic s of chemisorption, In contrast, little has been known concerning the nature and reactivity of the adspecies. The reactivity of C-1 adspecie s originating from precursors other than methane (carbon monoxide, dia zomethane, ketene, etc.) is better understood, which can be useful in investigating the reactivity of the hydrocarbonaceous adspecies result ing from methane. Most of the work concerning homologation of methane under non-oxidative conditions has been done by the groups of Amarigli o in France and van Santen in the Netherlands. Both use two-step proce dures in which metal catalysts are exposed first to methane and then t o hydrogen. However, the procedures differ markedly in essentially two points: (i) the temperature of the first step and (ii) the pressure o f methane. The Dutch group always uses a two-temperature cycle, decomp osing dilute methane on Ru and Co at a rather elevated temperature and then carrying out hydrogenation at a much lower temperature and at at mospheric pressure, The French group, in contrast, has shown that homo logation can be performed isothermally and at a moderate temperature o n Pt, Ru and Co, using methane and hydrogen at atmospheric pressure. C onsequently, in the two procedures the nature and reactivity of the su rface species formed at the end of the exposure step are different. Wh en the exposure to methane has been carried out at a moderate temperat ure and at atmospheric pressure, C-gamma is not formed (therefore no i rreversible poisoning) and the products do not obey the Anderson-Schul tz-Flory distribution. Also, thermodynamic limitations are circumvente d by both groups but for reasons specific to each procedure. Finally, this review outlines prospects for future research and attempts briefl y to estimate the potential commercial interest of the concept.