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MD Simulations with Chemical Accuracy – Alkane Reactivity in Acidic Zeolites

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If you have a question about this talk, please contact Dr Philipp Pracht.

The reduction of the carbon footprint is not possible without innovations in heterogeneous catalysis. Before a molecule can undergo conversion, it has to bind to a surface, hence, atomic understanding of adsorption on flat surfaces or in nano-sized pores is required for the rational design of new catalytic processes and materials. Adsorption strongly affects the reaction kinetics and diffusion within catalysts, but inherently poses a multiscale problem. While thermal ensembles of adsorbates at finite temperatures can only be properly described by approaches that sample the potential energy surface (PES) exhaustively, e.g. molecular dynamics (MD) simulations, a chemically accurate description of adsorbate-host interactions requires post-Hartree Fock methods. The standard approach to calculate adsorption enthalpies considers only the energetically most stable structure (“local” approach) on the PES and is based on density functional theory augmented with dispersion terms (DFT-D). This approach is used almost exclusively, despite its well-known shortcomings and large deviations from the experiments – in our case about 17 kJ mol−1. We introduce a combined hybrid MP2 :PBE+D2 MD approach that overcomes the problems. While the sampling of the configurational space is performed with MD at the PBE +D2 level, the enthalpies are based on MP2 -quality energy surfaces. We use the MP2 :PBE+D2 method to calculate MP2 corrections to adsorption enthalpies for selected snapshots and parameterize the difference between PBE +D2 and MP2 based on a physically motivated two-dimensional linear model. This approach reduces the computation time for a “MP2-quality” MD of 100,000 steps from about 340 years to 3 weeks. The new methodology yields chemically accurate adsorption enthalpies, deviating only by 1.9 kJ mol−1 for different alkanes (methane to n‑hexane) in various Brønsted acidic zeolites (H‑MFI, H‑CHA, and H‑FAU) including low and high Si/Al ratios ranging from 2.6 to 47. Insights from MD simulations in combination with accurate adsorption enthalpies are used to understand the behavior of adsorbates at experimental temperatures. Furthermore, the new method enables us to reach chemical accuracy for intrinsic enthalpy barriers as well. This is demonstrated for the monomolecular cracking reaction of linear alkanes, with rate constants improved by a factor of 50 compared to the “standard model” of computational catalysis.

This talk is part of the Lennard-Jones Centre series.

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