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

Methane oligomerization to alkanes can be accomplished on supported platinum via a two-step procedure: formation of carbonaceous species on the metallic surface by methane adsorption, followed by hydrogenation of these species. Temperature-programmed oxidation (TPO) experiments performed after hydrogenation steps of various durations show that the hydrogenation of a carbonaceous 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, corresponding to one-half of the surface carbon reactive toward hydrogen; linear and branched alkanes are produced from the other half of the reactive carbonaceous species. On EUROPt-1, mainly ethane and n-pentane are produced during the first minutes of reaction, while on a sintered catalyst the initial production in n-pentane is negligible. The release of n-pentane during an intermediate purge with inert gas on EUROPt-1 shows that C-C bonds can form already during methane adsorption, leading to C-5 precursors on specific active sites of this catalyst maybe coordinately unsaturated platinum atoms. A model of formation of C-5 precursors is proposed by analogy with the organometallic chemistry of molecular hydrocarbon platinacycles. The subsequent production of alkanes (C-2 > C-3 > C-4 > C-5) could be described through a statistical model of dynamic coupling between carbonaceous species involving hydrogen, rather than by hydrogenolysis of heavier carbonaceous species. However, this latter mechanism is likely to predominate for the production of C-6-C-8 compounds. (C) 1999 Academic Press.