High ethane dehydrogenation activity of [GaH] 2+[sbnd]Al pair sites in Ga/H-[Al]ZSM-5: A DFT thermochemical analysis of the catalytic sites under reaction conditions

Abstract

Through a full thermochemical analysis at reaction conditions, we studied ethane dehydrogenation activity of extra-framework [GaH] 2+ species in Ga/H-[Al]ZSM-5 at the 6–31g(d,p)/B3LYP level. We provide a theoretical evidence to support the proposition of [GaH] + near-pair framework–Al sites as being the active catalytic sites for ethane dehydrogenation. Dehydrogenation activity is governed by the reducibility of [GaH] 2+Z 2− d site in the presence of H 2, which is largely dependent on a zeolite ring structure consisting of two framework Al atoms. The optimum sites are defined by the intersection of linear Brønsted–Evans–Polanyi relations between the activation energy of the structure sensitive steps and enthalpy of reduction for the catalytic site. A comparison of the carbenium and alkyl activation mechanism indicates that the activity of the alkyl activation mechanism will be limited by the removal of highly activated ( Δ E act ∼ 55 – 60 kcal / mol ) ethene from [HGaC 2H 5] + species. Our thermochemical analysis also indicates that at higher temperatures, the pair Al sites with larger Al Al distances become more prevalent, increasing the number of optimum catalytic sites. This work suggests a potential optimal Si/Al ratio for a given Ga loading.