Acid strength of zeolitic Bronsted sites-Dependence on dielectric properties

摘要

The dependence of deprotonation energies of zeolitic Bronsted acid sites on the dielectric constants of these materials is investigated using quantum chemical calculations. The deprotonation energy decreases as the reciprocal value of average static dielectric constant of the zeolite framework increases with a correlation coefficient R-2 equal to 0.74 (252 data points for 19 zeolite frameworks). We decompose the deprotonation energy of these nanoporous materials into an intrinsic deprotonation energy and a proton "solvation" energy within zeolites which is the origin of the observed 1/epsilon dependence. The intrinsic deprotonation energy is shown to be a superior acidity descriptor. In agreement with experiments, it indicates that two-dimensional zeolites (nanosheets) are not more acidic than three-dimensional ones (bulk materials). We use our large data set to examine the dependence of the intrinsic deprotonation energy on structure parameters (Al-O-Si bond angle, O-H bond distance, confinement coefficient) and on the structure relaxation energy but for none of them a correlation has been found.