Oxygen-Free Conversion of Methane to Higher Alkanes through an Isothermal Two-Step Reaction on Ruthenium

Abstract

Ruthenium dispersed on silica is able to chemisorb CH4at temperatures significantly lower than EUROPT-1 (see the previous articles). At the temperatures used (≥80°C), H2desorption parallels CH4chemisorption but no C2H6is observed. During the following temperature programmed desorption under flowing argon, CH4is removed through a wide range of temperature (from room temperature to 300°C) with a first contribution peaking at less than 100°C. Very small amounts of CH4are desorbed after an adsorption carried out atT≥180°C, due to strong dehydrogenation of the adspecies. Subsequent temperature programmed surface reaction of the remaining adspecies with hydrogen displays upto four CH4peaks at well defined temperatures (ranging from ≈60 to ≈340°C), accompanied by a negligible formation of ethane. No Cγwas formed. The total amount of adsorbed CH4and the average H/C ratio of the corresponding adspecies can be derived from these experiments. In a separate set of experiments, CH4is switched to H2at the end of the exposure step, the temperature being fixed. An immediate formation of alkanes ranging from C1to C6is then evidenced. A sizeable fraction of the chemisorbed layer can so be homologated to higher alkanes. The influences of the various operating factors are reported. In particular a neat maximum of the C2+production versus temperature (at 160°C) is evidenced and is clearly due to the adverse hydrogenolysis reactions, efficiently catalyzed by Ru. All the results can be interpreted in complete similarity with the Pt case.