Adsorption complexes of ammonia on germanium- and gallium-modified zeolites

Abstract

Adsorption complexes of ammonia on germanium- and gallium-modified zeolites were investigated by means of ab initio calculations. Full optimization of the structures was done at the DZ, DZP, and TZ2P levels of theory. Two new models of ammonia adsorption on zeotypes are proposed. In one of these, the covalent structures are stabilized at the bridging OH group by two hydrogen bonds with binding energies of −12.20 and −12.80 kcal mol −1 for the germanium and gallium zeotypes, respectively. The other configuration is a type of the ion-pair structure, where the ammonium cation forms two very strong hydrogen bonds with the anionic zeotype clusters. The conversion energy of acid catalyst/ammonia to ion-pair complexes for zeolite, germanium and gallium zeotypes, was found to be about 2–10 kcal mol −1.