A Periodic DFT Study of Isobutene Chemisorption in Proton-Exchanged Zeolites:  Dependence of Reactivity on the Zeolite Framework Structure

Isobutene chemisorption within proton-exchanged zeolites is investigated using periodic d. functional theory method. This allows us to consider the effect of the zeolite micropore dimension to reactivity. The isobutene reaction pathways that proceed through primary and tertiary carbocation-like transition states have been investigated. The results agree with predicted reactivity trends. Activation energies of isobutene chemisorption are estd. to be around 100 and 25 kJ/mol for primary and tertiary transition states, resp. Destabilization of transition state complexes and products are as obsd. before. Interestingly, because of the steric constraints, the chemisorbed alkoxy species appeared to become as unstable as protonated hydrocarbons. The more significant result is the correlation of the zeolite micropore dimension with activation energies. Fluctuations of the activation energies are obsd. as a function of the match of the transition state structures with the zeolite cavities. We define a limit to the applicability of the semiempirical Polaniy-Evans-Bronsted relation in zeolite catalysis.