ADSORPTION OF WATER ON ZEOLITES OF DIFFERENT TYPES

We have investigated the interaction of water with Naf-ion exchanged z eolites of different structures (LTA, FAU, ERI, MOR and MFI) by means of temperature-programmed desorption (TPD). The non-isothermal desorpt ion of water shows, depending on the zeolite type, differently structu red desorption profiles. In every case the profiles have, however, two main ranges. Using a regularization method, desorption energy distrib ution functions have been calculated. The desorption energy distributi ons between 42-60 kJ mol(-1), which can be attributed to a non-specifi c interaction of water, show two clearly distinguished energy ranges. The water desorption behaviour of this range correlates with the elect ronegativity of the zeolites and the average charge of the lattice oxy gen atoms calculated by means of the electronegativity equalization me thod (EEM). The part of the desorption energy distributions in the ran ge of 60-90 kJ mol(-1), reflecting interactions of water with Na+ cati ons, shows two more or less pronounced maxima. In agreement with vibra tional spectroscopic studies in the far infrared region, it may be con cluded that all samples under study possess at least two different cat ion sites.