Inorganic Nanowire Photocatalyst Turns Methane Into Benzene

The boom in natural gas production has presented new opportunities for chemists to figure out how to directly convert methane into longer alkanes and aromatics. In one of these efforts, a research team led by electrical engineer Zetian Mi and chemist Chao-Jun Li of McGill University, in Montreal, has developed a patented silicon-doped gallium nitride nanowire photocatalyst that for the first time efficiently converts methane to benzene and H₂ under ultraviolet light at room temperature (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja5004119). The challenge to upgrading methane is controllably activating its relatively inert C–H bonds. A standard approach is using zeolite-based catalysts at temperatures greater than 500 °C. An alternative is using light energy at lower temperature, but metal oxide photocatalysts designed so far are not efficient enough or stable enough. The McGill researchers reasoned that GaN, with its large energy band gap, might work when prepared as high-surface-area nanowires. They found that the vertical plane of the nanowires allows even exposure of carbon to gallium and hydrogen to nitrogen, providing the optimal surface for snatching and holding in place methane molecules for light-induced C–H bond splitting. The separated hydrogen forms H₂, and the remaining methyl radicals couple to make ethane. After dehydrogenation, the resulting ethylene molecules stitch themselves together to form benzene.