Silver ion-exchanged zeolites Y, X, and low-silica X: Observations of thermally induced cation/cluster migration and the resulting effects on the equilibrium adsorption of nitrogen

Silver is known to strongly affect the adsorptive properties of some zeolites. It is also known that thermal vacuum dehydration of some argentiferous zeolites leads to the formation of charged silver clusters within the zeolite. In this work we have synthesized silver zeolites of the types Y, X, and low-silica X. The zeolites were treated in such a way as to promote the formation of intracrystalline charged silver clusters. Equilibrium room-temperature isotherms were measured for adsorption of nitrogen for each of the zeolites after various heat treatments and dehydration. These materials were structurally characterized via Rietveld refinement using neutron powder diffraction data. Color changes upon heat treatment and subsequent X-ray photoemission spectroscopy confirmed some reduction of Ag+ --> Ag-0. The effects of various dehydration conditions, including the time, temperature, and atmosphere, on the room-temperature adsorption of nitrogen are discussed. Structural characterization, along with valence bond calculations, revealed the presence of cations in site II*, which are more active in Ag-LSX samples that were vacuum dehydrated at 450 degreesC as compared to those that were vacuum dehydrated at 350 degreesC.