Insights into the Kinetics of Cracking and Dehydrogenation Reactions of Light Alkanes in H-MFI

Abstract

Monomolecular reactions of alkanes in H-MFI were investigated by means of a dispersion-corrected density functional, ωB97X-D, combined with a hybrid quantum mechanics/molecular mechanics (QM/MM) method applied to a cluster model of the zeolite. The cluster contains 437 tetrahedral (T) atoms, within which a T5 region containing the acid site along with the representative alkane is treated quantum mechanically. The influence of active site location on reaction energetics was examined by studying cracking and dehydrogenation reactions of n-butane at two regions in H-MFI–T12, where the proton is at the intersection of straight and sinusoidal channels, and T10, where the proton is within the sinusoidal channel. Two transition states were observed for cracking: one where the proton attacks the C–C bond and another where it attacks a C atom. Dehydrogenation proceeds via a concerted mechanism, where the transition state indicates simultaneous H2 formation and proton migration to the framework. Intrinsic activatio...