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

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 (C2H5delta--Hdelta+), 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 Zndelta+-Odelta- pair; (ii) formation of ethene from the alkyl group bound to Zn; and (iii) formation of dihydrogen from the Bronsted 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 (C2H5delta+-Hdelta-), 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)).