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, respec tively, have been studied by ab initio plane-wave calculations on peri odic models. Geometry distortions remain well localized to the nearest neighborhood of the Bronsted acid site. The Si-O(H) and Al-O(H) dista nces of the common Si-O(H)-Al moiety are significantly different in th e two kinds of materials. Calculated OH stretching frequencies of acid ic protons, which could be assigned to the low- and high-frequency OH bands, in agreement with experimental infrared spectra of H-SSZ-13 (hi gh silica chabazite) and HSAPO-34, correlate with OH bond lengths and Si-O(H)-Al angles.