Effects of the cation and Si/Al ratio on CH3I adsorption by faujasite zeolites

Abstract

In this study, the effects of some specific zeolitic parameters on the methyl iodide retention properties of metal-exchanged faujasites (FAU) of X and Y types were investigated. In the first part, the nature of exchangeable cation (H+, Na+, Cu+/Cu2+, Ag+, Pb2+) is discussed. In order to link the materials properties with their adsorption behavior, the state and location of exchanged transition metals after calcination at 500 °C were first probed using N2 sorptiometry at 77 K, DRS-UV–Vis spectroscopy, XRD and FTIR of adsorbed CO and NO. Adsorption capacities at breakthrough and saturation, deduced from CH3I breakthrough experiments performed at 100 °C, are then discussed. From the I/M ratio computed at saturation, it was found that the efficiency of iodine trapping follows the order: Cu+/Cu2+ > Ag+ ≫ Pb2+ > Na+ > H+. This order of efficiency roughly follows the trend of the different cations to form stable MIx precipitates from CH3I. Nevertheless, Ag/Y is preferred for nuclear application over Cu/Y, due to the undesirable CH3I oxidation to I2 catalyzed by copper sites. The effect of the Si/Al ratio is then discussed for Ag/Y zeolites having similar silver content (23 wt%). It is found that this parameter has a high impact on silver oxidation state and dispersion and also on the CH3I adsorption/desorption behavior.