Oxygen-free conversion of methane to higher alkanes through an isothermal two-step reaction on platinum (EUROPT-1) .2. Hydrogenation of the adspecies resulting from the chemisorption of methane

Hydrogenation of the adspecies formed from CH4 chemisorption on EUROPT-1 removes a mixture of alkanes ranging from C-1 to C-6 Or C-8, depending upon the conditions. Sizeable fractions of the CH4 initially chemisorbed can be homologized in this manner. The influences of various factors (temperature, duration of exposure, flow rate of CH4, partial pressures of CH4 or H-2) upon the quantity of homologized CH4 and the product distribution are reported. In particular, it is shown that, every other factor fixed, there exists an optimum temperature for homologizing methane. That results from the occurrence of the opposite hydrogenolysis reactions during the second step of each sequence. All the results can be interpreted by assuming that C-C bonding takes place between H-deficient CHx fragments during the chemisorption step. In the second step, H-2 saturates the alkane precursors and removes them from the surface. Fresh H-2 might not be used in the second step since more H-2 than needed is evolved during the first step. The driving force of this homologization process, conducted isothermally and in the absence of oxygen, can be seen as the energy which would then be required to compress the necessary quantity of H-2 from the low partial pressure at which it is removed in the first step up to 1 bar in order to make it able to remove alkanes from platinum. (C) 1996 Academic Press, Inc.