Oxygen-free conversion of methane to higher hydrocarbons through a dual-temperature two-step reaction sequence on platinum and ruthenium -: 1.: Chemisorption of CH4 at a fixed temperature

Preliminary chemisorbed on Pt and Ru at a moderate temperature (approximate to 150-300 degrees C), methane can be converted to higher alkanes through subsequent hydrogenation at the same temperature. On both metals too elevated temperatures must be avoided because of adverse hydrogenolysis reactions. Here we describe and discuss the results of dual-temperature experiments carried out on both metals and consisting of chemisorptions effected T-CH4 (less than or equal to 320 degrees C), followed by hydrogenations at a given temperature, T-H2, ranging from room temperature to TCH4 in a set of successive experiments. On both metals, strong favoring of the selectivity to heavier alkanes resulted from the lowering of T-H2. On Pt, the total amount of homologated methane was an increasing function of T-H2. Conversely, on Ru the homologated methane exhibited an optimum value versus T-H2, which could largely exceed the value corresponding to an hydrogenation conducted at T-H2 = T-CH4 (isothermal experiment). On both metal surfaces, heavy adspecies can grow upon exposure to CH4 and it is suggested that the difference between the two metals has essentially to be linked to their different hydrogenolysis activities. The high hydrogenolysis activity of Ru caused the final decrease of the C2+ production beyond some critical value of T-H2 The mediocre hydrogenolysis activity of Pt did not prevent the total C2+ production from monotonically increasing versus T-H2 in the range of temperature explored. It could even bring a positive contribution by allowing some heavy (and not easily removed) surface precursors to be cut into lighter and more quickly retrieved species. (C) 1998 Academic Press.