Zeolites as transition-metal-free hydrogenation catalysts: A theoretical mechanistic study

The B3-LYP/6-311+G(3df,2p)//B3-LYP/6-31G(d) procedure has been used to study the zeolite catalyzed hydrogenation of prototypical doubly bonded systems. Both Bronsted acid and alkali metal sites in model zeolites have been examined. For the hydrogenation of ethene, the barrier is predicted to be lowered by about 50% at the Bronsted acid sites and by about 40% at the alkali metal sites. The barriers for the hydrogenation of formimine and formaldehyde are predicted to be lowered even more substantially, with remarkably low overall barriers of 30 and 60 kJ mol^-1, respectively, at the Bronsted acid sites of the zeolites. The alkali metal sites of the zeolites are found to be not quite as effective as the Bronsted acid sites in lowering the hydrogenation barriers in these two cases, as for ethene. Comparisons are made with relevant experimental data.