OXYGEN-FREE CONVERSION OF METHANE TO HIGHER HYDROCARBONS THROUGH A DUAL-TEMPERATURE 2-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 (approx imate to 150-300 degrees C), methane can be converted to higher alkane s through subsequent hydrogenation at the same temperature. On both me tals too elevated temperatures must be avoided because of adverse hydr ogenolysis 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), f ollowed by hydrogenations at a given temperature, T-H2, ranging from r oom temperature to TCH4 in a set of successive experiments. On both me tals, strong favoring of the selectivity to heavier alkanes resulted f rom the lowering of T-H2. On Pt, the total amount of homologated metha ne was an increasing function of T-H2. Conversely, on Ru the homologat ed 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 adspeci es can grow upon exposure to CH4 and it is suggested that the differen ce between the two metals has essentially to be linked to their differ ent hydrogenolysis activities. The high hydrogenolysis activity of Ru caused the final decrease of the C2+ production beyond some critical v alue of T-H2 The mediocre hydrogenolysis activity of Pt did not preven t the total C2+ production from monotonically increasing versus T-H2 i n the range of temperature explored. It could even bring a positive co ntribution by allowing some heavy (and not easily removed) surface pre cursors to be cut into lighter and more quickly retrieved species. (C) 1998 Academic Press.