Influence of the hydrogenation step on selectivity during the nonoxidativeoligomerization of methane to alkanes on Pt/SiO2 catalysts (EUROPt-1)

Methane oligomerization to alkanes can be accomplished on supported platinu m via a two-step procedure: formation of carbonaceous species on the metall ic surface by methane adsorption, followed by hydrogenation of these specie s. Temperature-programmed oxidation (TPO) experiments performed after hydro genation steps of various durations show that the hydrogenation of a carbon aceous deposit obtained at 300 degrees C on the reference Pt/SiO2 catalyst EUROPt-1 is not a fast process. Two groups of surface carbonaceous species have been characterized through their different reactivities toward oxygen, but at 300 degrees C their reactivities toward hydrogen are similar. Among alkanes up to C-5, methane is the main product of hydrogenation, correspon ding to one-half of the surface carbon reactive toward hydrogen; linear and branched alkanes are produced from the other half of the reactive carbonac eous species. On EUROPt-1, mainly ethane and n-pentane are produced during the first minutes of reaction, while on a sintered catalyst the initial pro duction in n-pentane is negligible. The release of n-pentane during an inte rmediate purge with inert gas on EUROPt-1 shows that C-C bonds can form alr eady during methane adsorption, leading to C-5 precursors on specific activ e sites of this catalyst maybe coordinately unsaturated platinum atoms. A m odel of formation of C-5 precursors is proposed by analogy with the organom etallic chemistry of molecular hydrocarbon platinacycles. The subsequent pr oduction of alkanes (C-2 > C-3 > C-4 > C-5) could be described through a st atistical model of dynamic coupling between carbonaceous species involving hydrogen, rather than by hydrogenolysis of heavier carbonaceous species. Ho wever, this latter mechanism is likely to predominate for the production of C-6-C-8 compounds. (C) 1999 Academic Press.