Chemoselectivity during propene hydrogenation reaction over H-ZSM-5 zeolite: Insights from theoretical calculations

A comprehensive study has been made on product selectivity and detailed reaction pathway invoked during propene hydrogenation reaction over H-ZSM-5 zeolite by DFT calculations based on an extended 64T cluster model. Accordingly, structural parameters and activation barrier heights as well as reaction energies can be deduced from the optimized adsorption complexes. By comparing with the results predicted based on simplified 8T cluster models, it is indicative that the carbonium ion transition states are effectively stabilized by the long-range electrostatic effects provoked by the zeolite framework. The stability of the propoxide complexes and the effects of acid strength on reaction mechanism were also investigated. It is found that the propene hydrogenation reaction is strictly kinetically controlled and the formation of i-propoxide is intrinsically much more favorable than n-propoxide over zeolites, particularly those with higher acidic strengths.