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Hydration-dehydration behavior and thermodynamics of chabazite
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Dr Claire Fialips
Fialips CI, Carey JW, Bish DL
Geochimica et Cosmochimica Acta
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Equilibrium in the chabazite-H2O system was investigated by isothermal thermogravimetric analysis over a large range of temperatures (from 23 to 315°C) and H2O-vapor pressures (from 0.03 to 28 mbar). Thermodynamic analysis of the phase-equilibrium data revealed the existence of three energetically distinct types of H2O, referred to as S-1, S-2, and S-3. At 23°C and 26 mbar of H2O-vapor pressure, chabazite has maximum H2O occupancies of 8.2, 11.1, and 3.1 wt.% for S-1, S-2, and S-3, respectively. During dehydration, S-1 H2O is lost first, followed by S-2 H2O and then S-3 H2O, with significant overlap for S-1 and S-2 as well as S-2 and S-3. The thermodynamics of chabazite-H2O were modeled using three independent equilibrium formulations for S-1, S-2, and S-3. These formulations yielded standard-state molar Gibbs free energy of hydration of -21.8 +/- 0.6, -52.1 +/- 1.8, and -111.7 +/- 6.7 kJ/mol for S-1, S-2, and S-3. Standard-state molar enthalpies of hydration for each type of H2O are -65.6 +/- 0.5, -100.1 +/- 1.6, and -156.9 +/- 6.2 kJ/mol, respectively. Integral molar values for the Gibbs free energy of hydration for each type of H2O are -19.0 +/- 0.7, -40.1 +/- 2.1, and -76.9 +/- 9.6 kJ/mol, respectively. Integral molar values for the enthalpy of hydration for each type of H2O are -62.8 +/- 0.6, -88.1 +/- 1.9, and -122.2 +/- 9.3 kJ/mol, respectively. Integration of the predicted total partial molar enthalpy of hydration for all three types of H2O over the full H2O content of chabazite gave an integral molar enthalpy of -39.65 +/- 9.3 kJ/mol relative to liquid water. The thermodynamic data obtained for the hydration of natural chabazite were used to predict the hydration state of chemically similar chabazites under various temperatures and PH2O, ranging from 25 to 400°C and from 10^5 to 10^4 bars.
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