Anionic Oligomerization of Ethylene over Ga/ZSM-5 Zeolite: A Theoretical Study

Reaction mechanisms for the oligomerization of ethylene on extraframework gallium sites in Ga/ZSM-5 zeolite are studied by density functional theory cluster calculations. The nature of the transition states and intermediates is analyzed by topological analysis of the electron density distribution functions in the framework of the quantum theory of atoms in molecules. Interaction of olefins with the stable intermediate [Ga3+(H-)(C2H5-)](+) of the catalytic ethane dehydrogenation and with the initial extraframework Ga+ site are considered. For the [Ga3+(H-)(C2H5-)](+) site, interaction of ethylene with the hydride ion is a facile process. Addition of ethylene to the Ga-alkyl fragment and oligomer chain growth are rather difficult and compete with ethylene desorption via the beta-H transfer reaction. A novel, alternative, and more facile reaction path for the oligomerization of olefins is proposed. This mechanism proceeds via interaction Of two C2H4 molecules with the extraframework Ga+ cation in ZSM-5 zeolite, resulting in fort-nation of vinyl and ethyl groups coordinating to gallium. Interaction of a further C2H4 molecule with the vinyl group leads to dimerization. Such an anionic oligomerization mechanism for light olefins is argued to be at least as favorable as the conventional process catalyzed by Bronsted acid sites. The anionic nature of the transition states and intermediates may result in selectivity patterns during hydrocarbon conversion over Ga/ZSM-5 zeolites completely different from the case when zeolitic Bronsted acid sites. are the catalytic sites via the conventional carbenium ion mechanism.