Activation of alkanes on zeolites: A DFT study of hydride abstraction and dehydrogenation on extra‐framework aluminum species

Abstract

AbstractAn AlOH2+ · T6 cluster was used to represent, at zeroth order, an extra‐framework aluminum species coordinated with the zeolite Y structure. This cluster was used to study the hydride abstraction from alkanes, at B3LYP/6‐311++G(d,p)//B3LYP/6‐31G(d,p) level, to form an alkoxide and an Al‐hydride · T6 species. It was found that all reactions were endothermic with respect to the isolated reactants and presented high activation energy. The endothermicity and activation energy depend on the type of hydride being abstracted. Methane showed the highest values (ΔH = +41.5 kcal/mol and ΔH‡ = 91.7 kcal/mol); isobutane has the lowest (ΔH = +36.8 kcal/mol and ΔH‡ = 46.7 kcal/mol). Dehydrogenation to form an alkene and an adsorbed hydrogen molecule on the AlOH2+ · T6 cluster was also investigated and computed slightly more endothermic than the respective hydride abstraction. However, dehydrogenation showed a significantly lower activation enthalpy, especially for linear alkanes. For isobutane the results indicated a similar energy barrier for hydride abstraction and dehydrogenation. Larger, more realistic clusters and periodic reactivity studies are nevertheless needed to describe, in a more refined way, the energetics of these relevant catalytic reactions in zeolitic systems. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005