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) spectr oscopy. A migration of benzene molecules adsorbed on the cations towar d the 12R windows upon co-adsorption of NH3 is evidenced. On removal o f NH3 from the system, remigration of benzene from the 12R windows tow ard the cations is observed. This migration-remigration corresponds to a competition between the cations and the 12R windows for benzene. Th e quantitative study reveals that in the presence of ammonia, the numb er of 12R windows able to interact with benzene molecules remains 3 pe r 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 prea dsorbed 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 sh ows clearly that NH3 interacts with the cations of the Cs(Na)EMT zeoli te through nitrogen atoms. However, the interaction of NH3 with framew ork oxygen, as found in the case of H2O, is not detectable by IR spect roscopy because of its weakness or the presence of the preadsorbed ben zene on this zeolite. These interactions decrease the Lewis acidity of cations and increase the negative charge of oxygen atoms, which in tu rn lower the interaction of benzene molecules with the cations but enh ance the interaction with the 12R windows. The overall order of the in teraction strength of benzene with cations and 12R windows and of ammo nia 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 o n the chemical and structural properties of benzene and 12R windows, c ould be considered as an evident example of molecular recognition effe ct in the field of zeolites.