Location of benzene in NaBeta zeolite upon coadsorption of ammonia and methylamine: A further confirmation of molecular recognition effect in benzene adsorption in 12R window zeolites

摘要

The adsorption of a single component such as ammonia and methylamine in NaBeta zeolite has been first investigated by means of in-situ infrared spectroscopy, and the effect of coadsorption of ammonia and methylamine on the location of benzene has been then checked. The benzene adsorption behavior in NaBeta zeolite with or without the presence of coadsorbates has been correlated with structural and chemical properties of zeolite. The hypothesis of the molecular recognition effect in benzene adsorption has been further verified. The present work shows that ammonia and methylamine can interact not only with Na+ ions via the lone pair on nitrogen atoms but also with the large number of silanols present in NaBeta zeolite. The interaction strength of ammonia, methylamine, and benzene with NaBeta ranks in the order of methylamine/NaBeta > benzene/NaBeta > ammonia/NaBeta. It is found that the interaction of ammonia and methylamine with NaBeta causes an important modification of lattice parameters, indicating the deformation of zeolite framework. Despite this deformation of zeolite framework upon coadsorption of either ammonia or methylamine, only adsorption of benzene in Na+ ions and no change in the location of benzene are observed. The 12R windows in NaBeta remain less favorable sites for benzene adsorption, being contrary to what we observed in KL zeolite upon coadsorption of methylamine, in NaEMT and HY upon coadsorption of ammonia, and in NaY upon adsorption of benzene alone. The present work confirms further again that the adsorption of benzene in 12R window zeolites is governed by a molecular recognition effect. The location of benzene in 12R windows of zeolites, which is a phenomenon of multiple interaction, i.e., six hydrogen atoms of benzene with six oxygen atoms of 12R window, is possible only if the structural and chemical characters are compatible between the benzene molecule and 12R window. In regard to the benzene adsorption properties, 12R window zeolites can be divided into four different categories. This classification could be quite useful in the design of new catalysts and adsorbents for industrial treatments of aromatics. [References: 61]