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 <i>n</i>-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 activation energies can be determined accurately with this method, although heats of adsorption were found to be higher in magnitude relative to experiments, which is most likely mainly caused by the MM dispersion parameters for the zeolite framework atoms. Intrinsic activation energies calculated for reactions at the T10 site are higher than those at T12 owing to differences in interaction of the substrate with the acid site as well as with the zeolite framework, demonstrating that Brønsted acid sites in H-MFI are not equivalent for these reactions. Apparent activation energies, determined from calculated intrinsic activation energies and experimentally measured heats of adsorption taken from the literature, are in excellent agreement with experimental results