NATURE OF ACIDIC SITES OF THE LEWIS TYPE AT THE SURFACE OF MICA AND RELATED ALUMINOSILICATE MATERIALS

This paper describes the results of theoretical calculations of the desorption energies of water, ammonia, and pyridine from model acidic surface sites of aluminosilicate materials of the Bronsted (Type I) and Lewis types (II and V). We have borrowed our description and nomenclature of surface sites from Bhattacharyya (Langmuir 1992,8,2284) and compared our theoretical results to his results from temperature programmed desorption (TPD) experiments of ammonia from mica surfaces. We conclude that only Lewis sites of type V, i.e., involving an electron-deficient coordinatively unsatured tetrahedral silicon atom as the adsorption center, are able to bind ammonia with energies (110-250 kJ/mol) comparable to those observed experimentally (76-122 kJ/mol). Coordinatively unsatured tetrahedral aluminum centers do not bind Lewis bases. Bronsted sites develop binding energies lower than 10 kj/mol. In this context, pyridine is a stronger Lewis base than both ammonia and water.