Migration of adsorbed benzene molecules from cations to 12R windows in the large cages of Cs(Na)EMT zeolite upon coadsorption of NH3

Benzene adsorption behavior in Cs(Na)EMT zeolite upon co-adsorption of NH3 was studied with in situ Fourier transform infrared (FTIR) spectroscopy. A migration of benzene molecules adsorbed on the cations toward the 12R windows upon co-adsorption of NH3 is evidenced. On removal of NH3 from the system, remigration of benzene from the 12R windows toward the cations is observed. This migration-remigration corresponds to a competition between the cations and the 12R windows for benzene. The quantitative study reveals that in the presence of ammonia, the number of 12R windows able to interact with benzene molecules remains 3 per unit cell and the presence of NH3 does not result in an increase in this number. The ammonia adsorption capacity of Cs(Na)EMT zeolite preadsorbed with 9.0 molecules per unit cell (m/uc) of benzene is similar to 15 m/uc, however, only 7.1 m/uc of benzene can be retained by this zeolite in the presence of a high pressure of NH3. The present work shows clearly that NH3 interacts with the cations of the Cs(Na)EMT zeolite through nitrogen atoms. However, the interaction of NH3 with framework oxygen, as found in the case of H2O, is not detectable by IR spectroscopy because of its weakness or the presence of the preadsorbed benzene on this zeolite. These interactions decrease the Lewis acidity of cations and increase the negative charge of oxygen atoms, which in turn lower the interaction of benzene molecules with the cations but enhance the interaction with the 12R windows. The overall order of the interaction strength of benzene with cations and 12R windows and of ammonia with this zeolite is benzene-Cs(Na)EMT (benzene-cations > benzene-12R windows) > NH3-Cs(Na)EMT. The adsorption or fixation of benzene on the 12R windows, a phenomenon of multiple interactions that depends on the chemical and structural properties of benzene and 12R windows, could be considered as an evident example of molecular recognition effect in the field of zeolites.