Influence of the size of extraframework monovalent cations in X-type zeolite on their thermal behavior

Pure and well crystalline NaX zeolite with Si/Al=1.5 was prepared by hydrothermal crystallization from the Na2O–Al2O3–SiO2–H2O system. The hydrothermal crystallization was carried out under autogeneous pressure and at static condition at 368 K. The post-synthesis modification was achieved by following conventional ion-exchange technique to obtain K+, Rb+ and Cs+-exchanged forms of NaX zeolite. Compositional and structural investigations of parent sample and the samples with nearly equal population of cations with different sizes were performed by independent methods such as chemical analysis, IR, SEM, powder XRD, low temperature nitrogen adsorption. Thermal analyses data for these samples were obtained by TG/DTG/DTA and employed to evaluate the water desorption behavior. The dehydration of zeolite-type X and its modified forms was found to follow the first-order kinetics irrespective of the size of monovalent extraframework cations. Depending upon the size of extraframework cations, activation energy was found to vary from 25.4 to 27.3 kJ mol−1. Enthalpy change (ΔH) values for dehydration process calculated from differential scanning calorimetric (DSC) data was found to follow the sequence NaX