Multiple approach to model unpaired spin density effects in H-ZSM5 zeolite with extra-framework O atom: H-abstraction reaction from methane

Abstract

The interaction of methane with an extra-framework oxygen atom in acidic zeolite (H-ZSM5) porous substrate has been investigated by means of different state of the art calculations using two model systems in their triplet spin state: zeolite was modelled both by its crystallographic structure subject to periodic boundary condition and by cluster approach in gas-phase. We have evaluated the energetics of H-abstraction reaction from methane within the (010) straight channel of acidic zeolite by different computational approaches. First by in bulk plane waves Density Functional Theory (with and without dispersion energy correction) and then, in gas-phase cluster, by typical quantum chemistry approaches based on molecular orbital theory, where electronic wave function is expanded in Gaussian basis sets, within variational or perturbative schemes. Reaction paths have been determined either by Climbing Image Nudged Elastic Band method (c-NEB) in bulk or by Intrinsic Reaction Coordinate (IRC) method in gas phase. Transition State has been characterized in cluster models using Becke-3-Lee Yang Parr (B3LYP) and BeckeHalf&HalfLeeYangParr (BHLYP) hybrid functionals. Calculated energy barriers and all results allow an interesting vis-à-vis of commonly used, alternative, often exclusive models. This study indicates that open shell systems composed of O atoms and H-ZSM5 are worth to be considered in H-abstraction from small hydrocarbons in applicative processes.