Chabazite: Energetics of hydration, enthalpy of formation, and effect of cations on stability

The stability of synthetic cation-exchanged zeolites having the chabazite f ramework (CHA) and their cation-water interaction were studied using high-t emperature reaction calorimetry. Four cations (K, Na, Li, and Ca) were exch anged into CHA. The enthalpies of formation were determined for all samples , and the partial molar enthalpy of hydration was measured by varying the w ater content of one Ca-CHA. The enthalpy of formation depends strongly on the exchanged cation, becomin g more exothermic in the order Ca, Li, Na, K. The integral hydration enthal py does not depend strongly on the nature of the cation, but becomes slight ly less exothermic with increasing aluminum content, For the one Ca-CHA stu died in detail, the average enthalpy of hydration is -34.6 +/- 1.2 kJ/mol r elative to liquid water A quadratic fit to the transposed temperature drop data gives Delta (h) over bar(H2O) = -52.97 (+/-4.74) + 2.94 (+/- 0.68) n ( kJ/mol), where Delta (h) over bar(H2O) is the partial molar enthalpy of hyd ration and n is the number of water molecules per 24 oxygen formula.