Low-Temperature Alkane C-H Bond Activation by Zeolites: An In Situ Solid-State NMR H/D Exchange Study for a Carbenium Concerto

Isotopic H/D exchange has been monitored by in situ MAS NMR spectroscopy of 2-[D-14]methylpentane with H-USY to probe the controversy over the alkane conversion mechanism. The probe molecule has distinct exchangeable sites with different accessibility to the zeolite surface. In the early stages of the process, the regioselectivity of exchange demonstrates that the slow step of the mechanism is controlled by a carbenium ion intermediate. At a later stage of exchange, intra-molecular hydride migrations, typical of carbenium chemistry, replace D by H also on other carbon atoms, resulting in a loss of regioselectivity. Therefore, the first and the subsequent steps of the H/D exchange proceed at this temperature through a carbenium intermediate species.