Isotopic exchange of deuterium in zeolites
The information obtainable from the study of deuterium isotope exchange in zeolites by i.r. and m.s. methods is discussed. The conditions necessary for high sensitivity determination of OH groups by gas-phase analysis are mentioned together with a determination of the hydroxyl number in the 3 types of OH groups found in HNaY zeolite by solid-phase analysis. The advantage of the i.r. method in the study of hydroxyl heterogeneity in isotopic exchange is compared with the limited success of the m.s. method. The evaluation of the rates of homo- and hetero-exchange and their dependences on cationic exchange, stabilization and the pretreatment of Mn+HY zeolites are shown. The activation of hydrogen molecules seems to be the slowest step in the exchange which proceeds by a single-step mechanism. The OH groups alone do not activate hydrogen molecules as opposed to Lewis sites formed by dehydroxylation and cations and/or metal particles in suitable concentrations at suitable locations.