Adsorption behavior of benzene in NaBeta zeolite: An in-situ infrared study of molecular recognition

The adsorption behavior of benzene in NaBeta zeolite at different temperatures has been investigated by means of in-situ infrared spectroscopy. The effect of coadsorption of ammonia on the benzene location has been studied at low and high benzene loading levels. The location of Na+ ions has been discussed, and the interaction strength of benzene with NaBeta zeolite has been evaluated by desorption experiments. The present work demonstrated that benzene molecules sit mainly on Na+ ions. The large number of silanols present in this zeolite can also weakly interact with benzene molecules. Two kinds of 12R windows occurring in the Beta zeolite structure are not the preferential adsorption sites for benzene at the studied temperature range even in the presence of ammonia. Quantitative analysis of the changes in the absorbance of the CH out-of-plane vibrations with benzene loadings has shown that all the Na+ ions are accessible to benzene molecules. This led us to suggest that a part of the Na+ ions located initially in the small cages of Beta zeolite is probably attracted by benzene molecules toward and is finally located in the 12R channels. The absence of benzene molecules on the two kinds of 12R windows of NaBeta zeolite has been correlated with the calculated average basicity of framework oxygen atoms and the geometry of the two kinds of 12R windows. This results very likely from the structural incompatibility of the benzene molecules and the 12R windows of Beta zeolite. This work confirmed that the location of benzene should very likely be governed by a molecular recognition effect where the adsorbate and absorbent should have the suitable chemical and structural properties as in substrate-enzyme systems.