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 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.

Impact of long-range electrostatic and dispersive interactions on theoretical predictions of adsorption and catalysis in zeolites

“…Other studies have shown that the cluster size is also critical for correctly parametrizing the long-range dispersion interactions in the MM region of the zeolite. Sharada et al [102] observed that even when using a T437 cluster with a high quality functional and basis set (ωB97X-D/6-311++G(3df,3pd)), the chosen MM parameters had to be tuned in order to accurately capture experimental adsorption data. Significant deviations were found between experimental measurements and theoretical predictions for heats of alkane adsorption with MM parameters optimized for T23 QM clusters in earlier work [25]; the use of these parameters for the prediction of adsorption energies subsequently led to overbinding, especially for the larger alkanes.…”

“…The MM parameters (static point charges and Lennard Jones parameters) and link atoms properties at the QM/MM boundary were selected to reproduce the energies of T23-T44 QM clusters. This method has been recently validated to capture the heats of molecular adsorption accurately and has been successfully applied to a range of chemical systems including alkane cracking, dehydrogenation [102,103], alkene methylation [14] and other organic systems [104][105][106] with excellent reproduction of experimentally measured activation barriers and adsorption effects in zeolites [103]. Furthermore, the most attractive feature of this approach is its cost-effectiveness.…”

Impact of Zeolite Structure on Entropic–Enthalpic Contributions to Alkane Monomolecular Cracking: An IR Operando Study

“…In particular, an accurate determination of adsorption parameters at reaction conditions is required to extract the activation parameters. Until recently, these adsorption parameters were either estimated by extrapolation of low‐temperature measurements, or by molecular simulations . The intrinsic activation energy E a derived from low temperature adsorption parameters was found insensitive to the alkane or zeolite structure.…”

The Nature and Catalytic Function of Cation Sites in Zeolites: a Computational Perspective

“…A systematic investigation was carried out on the adsorption thermodynamics and intrinsic kinetics of alkane monomolecular cracking and dehydrogenation in zeolites with different pore size and channel topology by a combination of experiment, QM/MM, and configurational‐bias Monte Carlo (CBMC) simulation ,. It was found that the intrinsic activation energies of cracking and dehydrogenation are determined by the locations of the BASs as well as the topological properties of the zeolite framework . The adsorption equilibrium constant (K ads‐H+ ) at 773 K depends strongly on the entropy changes (ΔS ads‐H+ ) but not on the changes in adsorption enthalpy (ΔH ads‐H+ ).…”