Accurate Prediction of Hydrocarbon Interactions with Zeolites Utilizing Improved Exchange-Correlation Functionals and QM/MM Methods: Benchmark Calculations of Adsorption Enthalpies and Application to Ethene Methylation by Methanol

摘要

The adsorption enthalpy of light hydrocarbon molecules in both acidic and neutral zeolite MFI has been investigated with a range of computational methods. The role of cluster model size and density functional theory methodology is examined by comparison with high quality ab initio wave function theory MP2 results and experimentally determined heats of adsorption. The commonly applied B3LYP functional performs poorly in benchmark studies due to an inadequate description of intermolecular interactions. The functionals ωB97X-D and M06-2X predict adsorption enthalpies consistent with experimental values over three classes of adsorbates. A hybrid quantum mechanics/molecular mechanics (QM/ MM) method is required to converge calculated thermochemical properties with respect to cluster model size in a manner that is computationally efficient. The accuracy of both QM and QM/MM methods is highly sensitive to choice of level of theory and cluster size, requiring the use of large basis sets, large cluster models, and a density functional capable of capturing intermolecular interactions for achieving the desired chemical accuracy of 2 kcal/mol with respect to experimentally determined adsorption enthalpies and activation barriers. The computational effort for performing QM/MM simulations is considerably lower than that of similar quality QM results, and allows for the chemically accurate simulation of chemical reactions occurring in zeolites in a manner that is computationally cost efficient without sacrificing accuracy. The resulting QM/MM procedure is applied to study the reaction of ethene methylation by methanol.