Formation of hydronium ions on Brønsted sites in zeolitic catalysts: a quantum-chemical ab initio study

Abstract

A new structure for the surface complex of water on bridged hydroxyl sites is proposed which is stablized by two hydrogen bonds. The energy for the formation of a surface hydronium is estimated to be 0.5±20 kJ/mol. This conclusion is based on quantum-chemical ab initio SCF calculations (DZP basis set, geometries optimized) on the complex of water with the H 3SiOH-Al(OH) 2OSiH 3 model of a bridged hydroxyl site and on the complex of the hydronium ion with the [H 3SiOAl(OH) 2OSiH 3]-model of a negatively charged framework site. Electron-correlation contributions are estimated by the MP2 method on smaller models (SiH 3 groups replaced by H). For the latter models, second energy derivatives are also obtained. A direct SCF code was employed on workstations.