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.