Molecular Beam Chemistry: Formation of Benzene and Other Higher Hydrocarbons from Small Alkanes and Alkenes in a Catalytic Supersonic Nozzle

Abstract

Large yields of higher hydrocarbons are produced by flowing C2, C3, or C4 alkanes or alkenes, typically at 80 Torr, 1000 °C, and 10 ms contact time, through a supersonic nozzle made of nickel or molybdenum. In the mass spectra of the products, CnHm, the most prominent peaks (extending to n = 12) contain even numbers of carbon atoms, but there are also substantial peaks with odd carbon atom numbers. The largest peaks have m ∼ n hydrogen atoms, but many others also appear. For n = 6 the mass spectrum indicates benzene is essentially the sole product. The total conversion of the parent beam is roughly 40−60%, of which typically 15−25% is benzene; the yields increase as C2 → C4 and are appreciably higher for alkenes. Under the same conditions, methane does not form higher hydrocarbons. Experiments with mixtures of CD4 and C2H6 show, however, that D atoms from methane pervade the higher hydrocarbon products.