An infrared study on the location of benzene molecules and cations in Cs+-exchanged EMT zeolite

The adsorption behavior of benzene in Cs+-exchanged EMT zeolite has been investigated using FTIR spectroscopy and compared with that of NaEMT. The benzene location has been correlated with the (local) negative charge of oxygen atoms of 12R windows, the zeolite structure, and the Al fraction in the two different 12R windows of the EMT framework. The possible location of the cations is discussed. It is shown that in Cs+-exchanged EMT zeolite, all the Cs+ ions are located in the large cages and the remaining Na+ ions are present in the sodalite cages and hexagonal prisms. It is evidenced that besides benzene molecules interacting with the Cs+ cations in the large cages, some 12R windows can also be adsorption sites for benzene. However, it is observed that benzene molecules adsorb mainly on the Na+ ions in NaEMT. The ion exchange of Na+ by Cs+ induces an increase in the (local) negative charge of oxygen atoms in the 12R windows. The oxygen atoms thus bear a charge which is sufficiently negative to interact with benzene molecules. From the changes in absorbance of the C-H out-of-plane vibration in the range 2200-1700 cm(-1) as a function of benzene loading, it is deduced that the adsorption capacity of Cs+-exchanged EMT zeolite is around 9.8+/-0.5 molecules per unit cell, of which around 6.8+/-0.5 molecules per unit cell interact with cations and around 2.9+/-0.5 molecules per unit cell are located in the 12R windows. The observation that the number of benzene molecules interacting with the cations present in the large cages is half the number of the Cs+ ions present in Cs(Na)EMT zeolite suggests that each benzene molecule is oscillating between or interacting simultaneously with two Cs+ ions. The low adsorption capacity of Cs(Na)EMT for benzene compared with NaEMT most probably results from the presence of the large Cs+ ions which reduce the accessible volume of the large cages, and the higher mobility of benzene molecules in Cs(Na)EMT. The oscillation and the high mobility of benzene molecules found in Cs(Na)EMT should be related to the weaker interaction of benzene molecules with the Cs+ ions in Cs(Ma)EMT than that with Na+ ions in NaEMT. The structural compatibility between the benzene molecule and one type of 12R window can offer an explanation why only some of the 12R windows of Cs(Na)EMT adsorb benzene molecules. (C) 1998 Elsevier Science B.V. All rights reserved.