Theoretical determination of proton affinity differences in zeolites

An important factor in zeolite catalysis is the proton affinity, i.e., the energy required to remove a proton from the zeolite lattice. Differences in proton affinity are expected to influence the catalytic activity of acid sites, making the catalytically active sites inhomogeneous (within one zeolite framework) and dependent on zeolite-framework type. In this study influences of both composition (aluminum content) and structure on proton affinity are examined using both ab initio quantum chemistry and classical force field methods. Changes in the zeolite's aluminum content have a very large influence on the proton affinity of neighboring protons, due to modification of the covalent binding properties of the Si-OH-Al bridge. The structurally induced differences in proton affinity are calculated to be approximately 0.8 eV. These differences in proton abstraction energy can be correlated with structural properties of the all-silica lattice, specifically with the Si-O distances. The zeolites faujasite and ZSM-5 are explicitly discussed.