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

We have investigated the interaction of water with alkali-metal cation-exchanged X type zeolites with different Si/Al ratios by means of temperature-programmed desorption (TPD). The nonisothermal desorption of water shows, depending on the type of cation (Li+, Na+, K+, Rbf, and Csi) and the exchange degree, differently structured desorption profiles. Using a numerical regularization method, desorption energy distribution functions have been calculated. The width of the distributions of about 30-50 kJ mol-l shows clearly the energetic heterogeneity of the water-zeolite interaction. All desorption energy distributions exhibit two characteristic ranges. Beside a small part at lower energy values the distribution functions have a pronounced main peak between 55 and 65 kJ mol(-1) and a second range at higher energy values (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. This is also supported by the fact that the position on the energy scale of the cation-specific peak correlates linearly with the softness of the cations. To determine the localization of these water molecules X-ray powder diffraction 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.