Insights from molecular simulation of local structure of acid site on heterogeneous catalyst

Abstract

Local structure and acidic properties of catalytic active sites on metal oxides and zeolites were studied using molecular simulation methods, DFT (density functional theory) and ONIOM. Strong acidity of sulfate modified ZrO2 surface could be explained by local structure incorporating co-adsorbed SO3 and H2O molecules on the surface. The structure around Ti ion in MFI zeolite framework was studied. Relative stability of Ti atom at each T site was investigated using a large cluster model. The local structure of Brønsted acid sites on beta (BEA) zeolite was studied in detail. It has been revealed that the proton in a 6-oxygen ring interacts with two oxygen atoms to form hydrogen bond. These hydrogen bonds stabilize the proton in the framework, so that the proton shows very weak acidity. Finally, the acidity of OH group ensemble on a pure quartz surface was studied. Interestingly, the proton in the OH group ensemble shows strong acidity, suggesting that addition of atoms such as Al is not necessary to induce acid site in a silicate structure.