Abstract
We have explored mechanically embedded three-layer QM/QM/MM ONIOM models for computational studies of binding in Al-substituted zeolites. In all the models considered, the high-level-theory layer consists of the adsorbate molecule and of the framework atoms within the first two coordination spheres of the Al atom and is treated at the M06-2X/6-311G(2df,p) level. For simplicity, flexibility and routine applicability, the outer, low-level-theory layer is treated with the UFF. We have modelled the intermediate-level layer quantum mechanically and investigated the performance of HF theory and of three DFT functionals, B3LYP, M06-2X and ωB97x-D, for different layer sizes and various basis sets, with and without BSSE corrections. We have studied the binding of sixteen probe molecules in H-MFI and compared the computed adsorption enthalpies with published experimental data. We have demonstrated that HF and B3LYP are inadequate for the description of the interactions between the probe molecules and the framework surrounding the metal site of the zeolite on account of their inability to capture dispersion forces. Both M06-2X and ωB97x-D on average converge within ca. 10% of the experimental values. We have further demonstrated transferability of the approach by computing the binding enthalpies of n-alkanes (C1-C8) in H-MFI, H-BEA and H-FAU, with very satisfactory agreement with experiment. The computed entropies of adsorption of n-alkanes in H-MFI are also found to be in good agreement with experimental data. Finally, we compare with published adsorption energies calculated by periodic-DFT for n-C3 to n-C6 alkanes, water and methanol in H-ZSM-5 and find very good agreement.
Highlights
Zeolites are among the most widely studied inorganic materials
By the mean signed error (MSE) of 6.7 kcal molÀ1 and the mean unsigned error (MUE) of 8.1 kcal molÀ1, we can see that the 17T quantum cluster under-binds almost systematically when we do not correct for the basis set superposition error (BSSE)
We have modelled the intermediate-level layer quantum mechanically and investigated the performance of HF theory and of three DFT functionals, B3LYP, M06-2X and oB97x-D, for different layer sizes and various basis sets, with and without BSSE corrections
Summary
Zeolites are among the most widely studied inorganic materials Their wide-ranging industrial applications (e.g., separations, catalysis)[1,2] have stimulated work which aims at optimizing the properties of existing zeolites via framework atom modifications, or at designing new zeolite-based materials for specific applications.[1,3,4,5] Electronic structure calculations, molecular dynamics, and Monte Carlo simulations have provided tremendous insights into the properties of these materials, host–guest interactions, and catalytic activity.[6,7,8,9] Accurate description of sitespecific binding is critical to the calculation of catalytic pathways and the development of new catalysts but remains a challenge for theoretical models because the host–guest interactions are determined by long-range electrostatic interactions and the long-range electron correlation effects that give rise to dispersion forces. While it is widely recognized that the extended aluminosilicate framework around the active site of a zeolite-based catalyst is as important for the description of site-specific binding as the active site itself, it has been argued that quantum mechanical description of atoms far from the active site and the substrate might not be critical.
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