Desilicated ZSM-5 zeolites for optimized xenon adsorption at very low pressure

Abstract

Selective Xe adsorption on regenerable porous materials such as activated carbons, MOFs and zeolites has been found to be promising and economically attractive. The beneficial effect of silver doping of zeolites on Xe adsorption and Xe/Kr separation has been studied for numerous zeolites such as Ag-ZSM-5, Ag-mordenite and Ag-chabazite. Among them, we had shown that Ag-exchanged ZSM-5 is the most effective material for atmospheric Xe capture and selective separation at very low pressure (ppm level). A linear tendency was observed with a ratio of 2 mol of Ag for 1 mol of strong adsorption sites. Based on this observed tendency, we hypothesize that increasing the number of exchangeable sites by lowering the Si/Al ratio could be an approach to improve silver loading and selective xenon adsorption.The aim of this study is to evaluate the impact of desilication of ZSM-5 zeolites on decreasing their Si/Al ratio and increasing the concentration of sodium-exchanged sites and, possibly, strong silver-based adsorption sites. We have shown here that light desilication of this parent ZSM-5 makes it possible to significantly increase xenon uptake at very low pressure. The desilication creates a larger concentration of Al in the zeolite and thus enables a larger concentration of silver to be loaded. Yet we found that for severe desilication, decreased xenon uptake is observed, likely due to the facilitated formation of silver particles in which the core atoms are not accessible for xenon uptake. It appears that a concentration of 0.5 mmol g−1 of strong adsorption sites is a maximum achievable value for the Ag-ZSM-5 system. Surprisingly, we found that the parent ZSM-5 zeolite prepared by organic templating yields Ag-ZSM-5 with a similar concentration of strong adsorption sites but at higher silver loading. We could hypothesize that silanol defects present in commercial zeolites may be responsible for the stabilization of silver clusters.