Comparative Multi-Stage TG-DSC Study of K+, Na+, Ca2+ and Mg2+-Exchanged Clinoptilolite Forms

A multi-stage TG-DSC approach consisting of five heating/holding and five cooling/holding stages within one experiment in the temperature range 20-320 °C was applied to investigate the dehydration/hydration processes in K⁺, Na⁺, Ca²⁺, and Mg²⁺ clinoptilolite forms. The influence of extra-framework cations on the parameters characterizing these processes (such as mass changes, dehydration and hydration heats calculated per gram zeolite, amounts of water molecules leaving and entering the structure, and enthalpy values calculated per mol water) was established. The values of molar enthalpy of dehydration for different cationic clinoptilolite forms increase in different ways with temperature increasing (within the framework of 50-120 kJ mol^-1). The data on the molar enthalpy are in good agreement with the distributions of the two types of water molecules-weakly bound to cations and water molecules coordinating cations in the applied crystal chemical models of the cationic exchange samples. The data obtained for water molecules and their molar enthalpies of dehydration for the various cationic forms are useful in studying the sorption of water vapor and other sorbates, in choosing a desiccant and an object to dry at room conditions, etc. The first data on the hydration energy of sequentially added water molecules in a dynamic cooling mode in the temperature range 320-20 °C were obtained.