Activation of ethane in Zn-exchanged zeolites: a theoretical study

Abstract

A theoretical study of the mechanism of ethane dehydrogenation catalysed by Zn-doped zeolites was undertaken. The catalyst was modelled by the ring cluster [Al₂Si₂O₄H₈]²⁻ coordinated with the Zn²⁺ ion. The results obtained indicate that the reaction proceeds via a mechanism starting from the ‘‘alkyl’’ rupture of the ethane C–H bond (C₂H₅^δ−–H^δ+), and the zinc cation acts as an acceptor of the alkyl group. The catalytic cycle for the ‘‘alkyl’’ activation consists of three elementary steps: (i) rupture of ethane C–H bond on the Zn^δ+–O^δ− pair; (ii) formation of ethene from the alkyl group bound to Zn; and (iii) formation of dihydrogen from the Brønsted proton and hydrogen bound to Zn. The computed activation energies for these three steps are 18.4, 53.4 and 20.5 kcal mol⁻¹, respectively. The alternative mechanism starting from the ‘‘carbenium ’’ activation (C₂H₅^δ+–H^δ−), with the zinc cation abstracting the hydride ion, is unfavourable because of the high activation barrier for the first step (67.6 kcal mol⁻¹).