Thermodynamics of ion-exchanged and natural clinoptilolite

Natural clinoptilolite (Cpt: Na(0.085)K0.037Ca(0.010)Mg(0.020)Al(0.182)Si(0 .818)O(2).0.528H(2)O) from Castle Creek, Idaho, and its cation-exchanged va riants (Na-Cpt, NaK-Cpt, K-Cpt, and Ca-Cpt) were studied by high-temperatur e calorimetry. The hydration enthalpy for all the clinoptilolites is about -30 kJ/mol H2O (liquid water reference state) at 25 degreesC. The energetic stabilization effect of hydration on each clinoptilolite can be largely co rrelated to its hydration capacity. The higher the average ionic potential of the extra-framework cations, the larger the hydration capacity of the cl inoptilolite. This trend may be attributed to the small size as well as the efficient water-cation packing of high field strength cations in the zeoli te structure. The hydration properties of these clinoptilolites are compare d with those previously reported in the literature. The dehydration conditi ons as well as the measurement direction (dehydration of the initially hydr ated sample or rehydration of the dehydrated zeolites) are important factor s to control to obtain consistent thermodynamic properties for hydration. The standard enthalpy for formation of the clinoptilolites from the constit uent elements at 25 degreesC based on two framework O atoms was obtained fr om the calorimetric data: -1117.57 +/- 0.95 kJ/mol Cpt, -1130.05 +/- 1.00 k J/mol Na-Cpt, -1109.49 +/- 1.04 kJ/mol NaK-Cpt, -1094.21 +/- 1.12 kJ/mol K- Cpt, and -1153.78 +/- 1.07 kJ/mol Ca-Cpt. Their molar entropy was determine d by a summation method based on the thermodynamic properties of the compon ent oxides. Thus the standard free energy based on two framework O atoms wa s derived: -1034.01 +/- 1.05 kJ/mol Cpt, -1044.19 +/- 1.10 kJ/mol Na-Cpt, - 1027.26 +/- 1.13 kJ/mol NaK-Cpt, -1014.89 +/- 1.21 kJ/mol K-Cpt, and -1064. 95 +/- 1.16 kJ/mol Ca-Cpt.