CHARACTERIZATION OF THE DEALUMINATION EFFECT INTO H-FAUJASITES BY ADSORPTION .1. THE WATER MOLECULE AS A STRUCTURAL ALUMINUM ION-SELECTIVE PROBE

The adsorption and desorption isotherms of water vapor are drawn at 25-degrees-C for dealuminated HY zeolites upon framework Si/Al ratio. The isotherms are compared to that of the parent NaY zeolite studied in a wide range of temperatures and filling coefficients (0.03 < theta < 1).The isotherm changes in shape from type I to type IV with an hysteresis loop changing from type H4 to type H2, as the Si/Al ratio increases. The Polanyi-Dubinin theory is used to determine the volume accessible to water. It decreases with increasing Si/Al ratios, down to zero at a Si/Al ratio of 35. Such a result is accounted for by the adsorption on the hydrophillic centers, which are the cations (H+) associated with the structural aluminum ions, each cation being coordinated on average by 9 H2O.