The catalytic conversion of C-1-C-n hydrocarbons to olefins and hydrogen: Microwave-assisted C-C and C-H bond activation

The gas-phase, carbon-catalyzed,microwave-promoted conversion of methane to ethylene, ethane, acetylene, and hydrogen is reported. A selection of C-1-C-4 hydrocarbons, hexadecane, and a cyclic hydrocarbon, cyclodecane, were also subjected to microwave conversion, resulting primarily in cr-olefins, ethylene, and hydrogen. For methane conversion, the products are reminiscent of those found in methane pyrolysis. Microwave-induced cleavage of the liquid hydrocarbons provides conditions for the stabilization, by rapid thermal quenching in ambient-temperature liquid reagent, of products such as terminal olefins that would be labile under conventional (thermal bath) pyrolysis reaction conditions. The reactions of long chain acyclic and cyclic hydrocarbons involve high temperatures in the region of the spark leading to a cascade of unimolecular scission reactions from initially formed biradicals from cycloalkanes or radical pairs from linear alkanes, largely to the exclusion of intermolecular radical-radical and radical-molecule reactions. The observed products are discussed in terms of the thermochemistry and dynamics of high-temperature unimolecular biradical and radical decomposition reactions, and mechanisms involving reactive surface metal sites. The reaction rates of alkanes were found to increase with the molecular weight of the reactants. Mechanistic pathways consistent with these results are discussed.