Molecular recognition effect in benzene adsorption on the 12R window zeolites

The location and amounts of adsorbed benzene on NaEMT and KL zeolites, with and without coadsorption of ammonia, have been studied by IR spectroscopy, and compared with other 12R window zeolites. When benzene is present alone, only adsorption of benzene onto the cations was observed for these two zeolites. A higher amount of Na+ cations which are accessible to benzene, observed in NaEMT compared with NaY and NaYd, should result from migration of the Na+ cations from the hexagonal prisms and sodalite cages to the large cages. However no migration of Na+ cations can occur in faujasites. Upon coadsorption of ammonia, a migration of preadsorbed benzene molecules from the cationic sites to the 12R aperture sites has been evidenced in NaEMT, as also observed in NaX, NaY, NaYd and HY, but this was not observed in KL. The absence of the benzene adsorption on the 12R windows in KL, even in the presence of ammonia, and the selective adsorption of benzene on six elliptical 12R windows in NaEMT suggest that factors, other than purely chemical (basicity of the framework oxygen atoms) intervene in the location of benzene on the 12R windows of zeolites. The present IR results reveal that adsorption of benzene is governed by a molecular recognition effect, in which the substrate and absorbent assume adapted chemical and structural properties, such as in enzyme-substrate systems.