Brønsted sites in zeolitic catalysts. An ab initio study of local geometries and of the barrier for proton jumps between neighbouring sites

Abstract

Complete geometry optimizations were performed on the equilibrium structure (C s) of the H 3SiOHAl(OH) 2OSiH 3 model of bridged hydroxyls in zeolites as well as on the transition structure (C 2v) for moving the proton within this model from one SiOAl site to the other. The calculations (DZP basis set, 190 basis functions) were carried out with the direct SCF method on a workstation. Reduced models, H 2OAl(OH) 2OH and H 2OAlH 2OH, showed very similar potentials for such proton motion. For these models second energy derivatives were obtained analytically and single-point calculations including electron correlation (CPF method, TZ2P basis set) were made. The final estimate of the energy barrier for proton jumps is 52±10 kJ/mol.