Catalytic Upgrading of Methane to Higher Hydrocarbon in a Nonoxidative Chemical Conversion
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2016 American Chemical Society. Discovery of large shale gas reserves in recent years resulted in the reduction of natural gas price. In order to convert methane in a direct and energy efficient route, nonoxidative catalytic conversion is a potentially attractive option which includes an activation of methane molecules at low temperature. The oxide of transition metals such as Mo, Fe, V, W, Cr, Zn, and Cu have been studied as catalysts for methane conversion, where usually the conversion is lower than 20% and the operating temperature needed is above 800 C which causes coking, thus resulting in an early catalyst deactivation. In this work, a noble transition metal, ruthenium, has been chosen as the catalyst with the objective to decrease the methane activation temperature, increase the stability, and also achieve higher conversion than other transition metal catalysts. The catalyst was prepared as 1.5 or 3.0 wt % ruthenium loading on zeolite (i.e., ZSM-5) and silica supports separately to compare the effect of metal loading and metal-support combination on the methane conversion reaction, wherein the operating temperature was varied from 500 to 800 C. From online GC and FT-IR analyses of the gas products, it was observed that, on the 3.0 wt % Ru/ZSM-5 catalyst bed, a rise in methane conversion took place at 700 C, where heavy hydrocarbon molecules from C4 to C10 were produced, whereas for the 3.0 wt % Ru/SiO2 catalyst bed, methane conversion was found to be low even at 800 C and no significant production of higher hydrocarbon molecules was observed. The catalyst bed of 3.0 wt % Ru/ZSM-5 produced some aromatic compounds in liquid product. This could be attributed to the special framework structure in the ZSM-5 catalyst which influenced the formation of cyclic higher hydrocarbon molecules as product after methane is being activated on the surface of the ruthenium metal catalyst. Ruthenium supported on ZSM-5 also produced methyl radicals in a considerable amount at above 700 C. Furthermore, the origin of the lower-temperature activation effect on transition metals was examined with the density functional theory analyses, which suggests that the zeolite structure lowers the activation energies more than the silica structure by inducing more negative charge on C atom of methane.