Energetics of formation and hydration of ion-exchanged zeolite Y

Cationic variants of zeolite Y (LiY, NaY, KY, RbY, CsY, CaY, LaY and HY) were prepared via aqueous ion exchange. High-temperature calorimetry was used to study the integral hydration enthalpy and the enthalpy of formation from the constituent oxides. For the alkali cation-exchanged zeolites, the total energetic contribution by hydration decreases in the order LiY > NaY > RbY > CsY. However, the average hydration enthalpy per mole of water has little dependence on the nature of the exchanged cations. In the presence of smaller cations (higher ionic potential), water molecules can be packed more efficiently in zeolite cavities, and the average hydration number (H₂O/M) is higher. The low hydration enthalpy of KY relative to the trend of the zeolites exchanged with other alkali cations may be due to the absence of a stable cation-water arrangement in sodalite cages. The enthalpy of formation of a zeolite from its constituent oxides becomes more exothermic as the basicity of the exchanged cations increases. For the alkali cation exchanged zeolites, the enthalpies of formation can be correlated to the average ionic potential, (Z/r)(av). The standard formation enthalpies of the ion-exchanged zeolites from constituent elements at 25°C were derived by combining the calorimetric data with literature data. The hydration enthalpies and enthalpies of formation of CaY, LaY and HY are also presented. (C) 2000 Elsevier Science B.V. All rights reserved.