Interaction of water with alkali-metal cation-exchanged X type zeolites: Atemperature-programmed desorption (TPD) and X-ray diffraction study

We have investigated the interaction of water with alkali-metal cation-exch anged X type zeolites with different Si/Al ratios by means of temperature-p rogrammed desorption (TPD). The nonisothermal desorption of water shows, de pending on the type of cation (Li+, Na+, K+, Rbf, and Csi) and the exchange degree, differently structured desorption profiles. Using a numerical regu larization method, desorption energy distribution functions have been calcu lated. The width of the distributions of about 30-50 kJ mol-l shows clearly the energetic heterogeneity of the water-zeolite interaction. All desorpti on energy distributions exhibit two characteristic ranges. Beside a small p art at lower energy values the distribution functions have a pronounced mai n peak between 55 and 65 kJ mol(-1) and a second range at higher energy val ues (65-95 kJ mol(-l)). The systematic changes with respect to position and intensity of the peaks at higher energy values show clearly the influence of the respective cation on the water desorption at higher temperatures. Th is is also supported by the fact that the position on the energy scale of t he cation-specific peak correlates linearly with the softness of the cation s. To determine the localization of these water molecules X-ray powder diff raction studies were carried out. It was found that in NaX, IM, and CsX at low loadings the water was exclusively localized between sodium ions on the SIII and SII* positions.