Activation of small alkanes in Ga-exchanged zeolites: A quantum chemical study of ethane dehydrogenation

Quantum chemical calculations on the mechanism of ethane dehydrogenation catalyzed by Ga-exchanged zeolites have been undertaken. Two forms of gallium, adsorbed dihydridegallium ion GaH(2)(+)Z(-) and adsorbed gallyl ion [Ga=O](+)Z(-), were considered. It was found that GaH(2)(+)Z(-) is the likely active catalyst. On the contrary, [Ga=O](+)Z(-) cannot be a working catalyst in nonoxidative conditions, because regeneration of this form is very difficult. Activation of ethane by GaH(2)(+)Z(-) occurs via an "alkyl" mechanism and the gallium atom acts as an acceptor of the ethyl group. The "carbenium" activation of ethane, with gallium abstracting a hydride ion, is much (ca. 51 kcal/mol) more difficult. The catalytic cycle for the "alkyl" activation consists of three elementary steps: (i) rupture of the ethane C-H bond; (ii) formation of dihydrogen from the Bronsted proton and hydrogen bound to Ga; (iii) formation of ethene from the ethyl group bound to Ga. The best estimates (MP2/6-311++G(2df.p)//B3LYP/6-31G*) for the activation energies of these three steps are 36.9, ca. 0, and 57.9 kcal/mol, respectively.