DISPLACEMENT OF ADSORBED BENZENE FROM CATIONIC SITES TO 12R APERTURESIN NAEMT UPON COADSORPTION OF NH3

Changes in the location of benzene upon coadsorption of NH3 and the st rength of benzene/zeolite and NH3/zeolite interactions in NaEMT zeolit e have been evaluated by a study of CH out-of-plane vibration range of absorbed benzene using infrared spectroscopy. A successive migration of benzene previously adsorbed on cationic sites to the 12R window sit es has been evidenced as the amount of NH, introduced in the infrared cell increases. With the addition of NH3 in the system, NH, molecules might interact with the framework atoms and/or remaining Na+ cations. Th is very likely results in a modification or redistribution of charg es on framework atoms and probably gives a sufficient increase in the basicity of oxygen atoms of 12R windows to interact with benzene. The 12R windows become adsorption sites for benzene. On removal of NH3 fro m the system, however, the benzene adsorbed on 12R windows remigrates to cationic sites. The quantitative study on the amount of benzene ads orbed on 12R window sites shows that only a part of, but not all of, 1 2R window sites existing in NaEMT can interact with benzene molecules upon coadsorption of NH,. The present work illustrates that (1) the be nzene molecules might adsorb only selectively on the six elliptical 12 R windows, and (2) two other 12R windows in circular form should very likely not be adsorption sites for benzene since there is no suitable geometric compatibility between the benzene molecule and this kind of 12R aperture. This selective adsorption of benzene on the different 12 R windows suggests that other than chemical parameter (the basicity of the oxygen atoms of the 12R windows), structural factors should inter vene in directing benzene adsorption on 12R apertures. A molecular rec ognition effect has been revealed in benzene adsorption on NaEMT zeoli te.