Brønsted Acid Sites in HSAPO-34 and Chabazite: An Ab Initio Structural Study
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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.
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