Bronsted acid sites in HSAPO-34 and chabazite:: An ab initio structural study

The structure of Bronsted acid sites and the effects of the Al --> Si and Si --> P substitution in chabazite and AlPO4-34 frameworks, respectively, have been studied by ab initio plane-wave calculations on periodic models. Geometry distortions remain well localized to the nearest neighborhood of the Bronsted acid site. The Si-O(H) and Al-O(H) distances of the common Si-O(H)-Al moiety are significantly different in the two kinds of materials. Calculated OH stretching frequencies of acidic protons, which could be assigned to the low- and high-frequency OH bands, in agreement with experimental infrared spectra of H-SSZ-13 (high silica chabazite) and HSAPO-34, correlate with OH bond lengths and Si-O(H)-Al angles.