Adsorption behavior of N-2 water, C-6 hydrocarbons, and bulkier benzene derivative (TMB) on Na-X zeolite and its K+-, Rb+-, and Cs+-exchanged analogues

The hydrothermal crystallization of X-type zeolite with a Si/Al ratio of 1. 15 was achieved from the Na2O-Al2O3-SiO2-H2O system at 368 K under static c onditions. The post-synthesis modification was carried out by a conventiona l ion-exchange technique to obtain K+-, Rb+-, and Cs+-exchanged samples wit h different degrees of exchange. All the samples were characterized using c hemical analysis, IR, SEM, powder XRD, low-temperature nitrogen adsorption, and equilibrium sorption uptakes of different probe molecules. The relativ e intensities of the XRD peaks of cation-exchanged zeolite were found to be affected to different extents, depending on the nature and the concentrati on of nonframework cationic size, without any shift in the positions of ref lection. The sorptive properties of the K-, Rb-, and Cs-exchanged samples w ere studied using nitrogen, water, and different C-6 hydrocarbons including bulkier benzene derivative 1,3,5-trimethylbenzene (TMB) as probe molecules . The trend observed in chemical potential estimated as a function of nitro gen coverage indicates different sorption selectivity because of difference s in the cationic size and population. Sorption uptake kinetics for probe m olecules such as water, n-hexane, cyclohexane, benzene, and TMB were also s tudied. The samples with higher degrees of exchange and/or cationic size ha ve shown a decrease in hydrophilic character due to the formation of irregu lar networks of water molecules connected with preadsorbed water molecules, framework oxygen ions, and nonframework cations. Among Cg hydrocarbons inc luding TMB, the benzene molecule is found to be the most promising probe fo r the estimation of openness of structure and surface heterogeneity as well . (C) 2001 Academic Press.