A theoretical explanation of solvent effects in zeolite catalysis

In a previous study of solid acid catalysis (Nature (1998) 389, 832) we showed that the catalytic activity of zeolites could be increased by the coadsorption of "solvent" molecules, such as nitromethane. These coadsorbates do not participate directly in the reaction, but alter the environment within the zeolite such that reactivity is increased. In this work we provide further theoretical explanation of the increased reactivity observed upon coadsorption. We first use density functional theory (DFT) to study the proton affinity of acetone, and complexes of acetone with propane, bromomethane, nitromethane, nitroethane, nitropropane, and acetonitrile. We find that the proton affinity of acetone in the complexes is much higher than for acetone alone. Optimizations and frequency calculations at the B3LYP/6-311++G** level predict proton affinity increases that range from 0.9 kcal/mol for the acetone/propane complex to 12.8 kcal/mol for the acetone/acetonitrile complex. The increase in proton affinity due to the coadsorbed molecules is one of the causes of the increased reactivity observed experimentally. We also used DFT (B3LYP/DZVP2) to optimize the geometry of acetone and the acetone-nitromethane complex in contact with a cluster model of HZSM-5. There is greater proton transfer from the zeolite to acetone when nitromethane is present, as is reflected in the shorter distance between the acidic zeolite proton and the carbonyl carbon of acetone. Predictions of H-1 and C-13 NMR isotropic chemical shifts also indicate increased proton transfer to acetone in the presence of nitromethane. This further demonstrates how coadsorbates promote reactivity.