Effect of Si to Al substitution at next-nearest-neighbor sites on the acid strength 2. Low Si/Al ratio or high ammonia loading

Abstract

The acidity variations upon Si/Al substitution at next-nearest-neighbor (NNN) sites were studied by ab initio molecular orbital calculations. Our previous study on the Si/Al substitution occurring at only one NNN site (1NNN models) revealed that the effect of the substitution on the acid strength is not unique but is site-dependent [Microporous Mater. 5 (1995) 233]. In the present report, a lower Si/Al ratio was investigated using the models where two NNN sites were substituted from Si to Al (2NNN models). Even among 2NNNs, there were some cases which exhibited higher acidity than isolated Al site (0NNN). Also it was found that the Si/Al substitution at a particular site always had a similar effect on the acidity regardless of the Al occupations at other NNN sites. Furthermore, in order to examine the variation of acidity as the ammonia loading is increased, the effect of ammonia adsorption at NNN site was studied. Acidity variation was again found to be dependent upon the substitution site. The traditional explanations for the acid strength variations upon the change of Si/Al ratio from the number of Als occupying NNN sites are thus strongly doubted.