Selective Photoactivation of Carbon-Hydrogen Bonds in Paraffinic Hydrocarbons. Dimerization of Alkanes

Article abstract of DOI:10.1021/j100223a024

When contacted with a silver-loaded zeolite Y, containing both isolated silver atoms and very low nuclearity cationic silver clusters, under broad-band irradiation (220-300 nm) and at room temperature and atmospheric pressure, low molecular weight paraffinic hydrocarbons undergo a selective dimerization, namely, methane leads to ethane, ethane to n-butane, and propane to hexanes.The species thought to be responsible for the activation of the C-H bond of the paraffinic hydrocarbons is suggested to be a lattice V-center obtained in the zeolite under UV irradiation, the formation of which is enhanced by the presence of small charged silver clusters (bifunctional reduction of electron-hole recombination processes).The V-centers are both accessible to and highly reactive toward alkanes C_nH_2n+2, Thereby abstracting a hydrogen atom from the alkane to form a lattice hydroxy species and the respective gaseous alkyl radical C_nH_2n+1, which in the presence of the reactant alkane rapidly dimerizes with high selectivity toward the product alkane, C_2nH_4n+2.The reaction conditions lead neither to a breakage of the C-C bond of the alkane to an appreciable extent nor to its dehydrogenation to the alkene.The alkane photodimerization reaction on silver loaded zeolite Y, namely, 2C_nH_2n+2+hν->C_2nH_4n+2+2H_ads is intriguing in that it bears a number of similarities to the water photosplitting reaction, namely, 2H2O+hν->O2+4H_ads on the same substrate.In both cases, lattice reaction with the alkane or water reagents predominates, making the major product generating channel stoichiometric rather than catalytic.