title: Energetics of sodium-calcium exchanged zeolite A.
abstract: A series of calcium-exchanged zeolite A samples with different degrees of exchange were prepared. They were characterized by powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC). High temperature oxide melt drop solution calorimetry measured the formation enthalpies of hydrated zeolites CaNa-A from constituent oxides. The water content is a linear function of the degree of exchange, ranging from 20.54% for Na-A to 23.77% for 97.9% CaNa-A. The enthalpies of formation (from oxides) at 25 °C are -74.50 ± 1.21 kJ mol(-1) TO2 for hydrated zeolite Na-A and -30.79 ± 1.64 kJ mol(-1) TO2 for hydrated zeolite 97.9% CaNa-A. Dehydration enthalpies obtained from differential scanning calorimetry are 32.0 kJ mol(-1) H2O for hydrated zeolite Na-A and 20.5 kJ mol(-1) H2O for hydrated zeolite 97.9% CaNa-A. Enthalpies of formation of Ca-exchanged zeolites A are less exothermic than for zeolite Na-A. A linear relationship between the formation enthalpy and the extent of calcium substitution was observed. The energetic effect of Ca-exchange on zeolite A is discussed with an emphasis on the complex interactions between the zeolite framework, cations, and water.
authors.affiliation: State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China.
authors.forename: H
authors.initials: H
authors.lastname: Sun
authors.forename: D
authors.initials: D
authors.lastname: Wu
authors.forename: X
authors.initials: X
authors.lastname: Guo
authors.forename: B
authors.initials: B
authors.lastname: Shen
authors.forename: A
authors.initials: A
authors.lastname: Navrotsky
mesh_terms: D002118:Calcium / Q000737:chemistry*; D002151:Calorimetry; D002152:Calorimetry, Differential Scanning; D012964:Sodium / Q000737:chemistry*; D013816:Thermodynamics; D017641:Zeolites / Q000737:chemistry*
chemical_list: D017641:Zeolites; D012964:Sodium; D002118:Calcium