Estimation of dealumination rate of ZSM-5 zeolite by adsorption of water vapor

Adsorption isotherms of water were measured at 25 degrees C for HZSM-5 zeolites with various SiO2/Al2O3 ratios. From the analysis of the adsorption isotherms by the D-R method it was found that the micropore volume W-0(H2O) accessible to water molecules increases linearly with an increase in the framework aluminums and that the number of water molecules associated with one framework aluminum is on average five. Based on these results, the number of the framework aluminums in HZSM-5 zeolites dealuminated by thermal treatment at 600-800 degrees C was estimated. It was found that the rate of the dealumination of HZSM-5 zeolite by thermal treatment is apparently second order depending on the number of framework aluminums in the zeolite structure. It became clear that water molecules are able to be used to determine the number of framework aluminums of the dealuminated HZSM-5 zeolite.