Influence of the Hydrogenation Step on Selectivity during the Nonoxidative Oligomerization of Methane to Alkanes on Pt/SiO2Catalysts (EUROPt-1)

Abstract

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°C on the reference Pt/SiO2catalyst 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°C their reactivities toward hydrogen are similar. Among alkanes up to C5, 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 andn-pentane are produced during the first minutes of reaction, while on a sintered catalyst the initial production inn-pentane is negligible. The release ofn-pentane during an intermediate purge with inert gas on EUROPt-1 shows that C–C bonds can form already during methane adsorption, leading to C5precursors on specific active sites of this catalyst maybe coordinately unsaturated platinum atoms. A model of formation of C5precursors is proposed by analogy with the organometallic chemistry of molecular hydrocarbon platinacycles. The subsequent production of alkanes (C2>C3>C4>C5) 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 C6–C8compounds.