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

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 degrees C were first probed using N-2 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 degrees 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 I-2 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.