Zeolite alkane cracking dynamics revealed by QM/MM simulations

摘要

First principles simulations provide an enlightening opportunity to examine catalytic reactivity at a molecular level. These simulations can directly lead to the engineering and control of specific catalytic processes because they provide rates and thermodynamics of elementary catalytic reactions. In this presentation, we introduce our quantum mechanical/molecular mechanical (QM/MM) simulation strategy to investigate Zeolite reactivity. While QM/MM promises to greatly improve the speed of molecular simulations, it comes with a potentially unknown cost in terms of accuracy. While many QM/MM implementations have utilized existing parameter sets that were not necessarily designed for QM/MM, we have carefully optimized MM parameters especially for Zeolite QM/MM simulations. Although geometric results are somewhat insensitive to MM parameter choice, our test set reveals that consistent reproduction of QM energies can be achieved with MM parameters optimized for QM/MM. Our parameterization is based on the ωB97X-D density functional, which provides accurate thermochemistry and includes van der Waal's interactions. We utilize our newly parameterized QM/MM setup to compute thermodynamics, reaction barriers, and reaction trajectories to reveal fundamental details of alkane cracking in H-ZSM-5 Zeolite catalysts. Dynamic reaction trajectories reveal the fascinating effects of kinetics at the reaction timescale and provide estimates of carbocation lifetimes.