Molecular modelling and catalytic properties of ammonia adsorption on Brønsted acid sites

Abstract

The catalytic properties of ammonia adsorption on zeolite clusters have been investigated within the framework of the ab initio self-consistent field method. Full optimization of structures has been carried out at the DZP and TZ2P levels of theory. Two different types of ammonia adsorption on zeolite framework sites are proposed. In one of these, the covalent structures H 3SiOHA1(OH) 2SiH 3…NH 3 are stabilized on the bridging OH group by two cyclic hydrogen bonds with a binding energy of −12.80 kcal mol −1, which is different from the singly bonded structures previously suggested. The other structure is a type of ion-pair structure [H 3SiOA1(OH) 2SiH 3] −[NH 4] +, in which the ammonium cation forms two very strong hydrogen bonds towards the negatively charged framework site. The conversion energy of the ammonia adsorption structure H 3SiOHA1(OH) 2SiH 3…NH 3 to the ion-pair structure is about −4.63 kcal mol −1. Comparison with the hydrogen halide and silanol (H 3SiOH) complexes of ammonia show that H 3SiOHA1(OH) 2SiH 3/NH 3 is at least as strongly acidic as HF.