Sanyuan Yang

Energetics of formation and hydration of ion-exchanged zeolite Y

Abstract 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.

Thermodynamics of ion-exchanged and natural clinoptilolite

Abstract Natural clinoptilolite (Cpt: Na0.085K0.037Ca0.010Mg0.020Al0.182Si0.818O2 ·0.528H2O) from Castle Creek, Idaho, and its cation-exchanged variants (Na-Cpt, NaK-Cpt, K-Cpt, and Ca-Cpt) were studied by high-temperature calorimetry. The hydration enthalpy for all the clinoptilolites is about -30 kJ/mol H2O (liquid water reference state) at 25 °C. The energetic stabilization effect of hydration on each clinoptilolite can be largely correlated to its hydration capacity. The higher the average ionic potential of the extra-framework cations, the larger the hydration capacity of the clinoptilolite. This trend may be attributed to the small size as well as the efficient water-cation packing of high field strength cations in the zeolite structure. The hydration properties of these clinoptilolites are compared with those previously reported in the literature. The dehydration conditions as well as the measurement direction (dehydration of the initially hydrated sample or rehydration of the dehydrated zeolites) are important factors to control to obtain consistent thermodynamic properties for hydration. The standard enthalpy for formation of the clinoptilolites from the constituent elements at 25 °C based on two framework O atoms was obtained from the calorimetric data: -1117.57 ± 0.95 kJ/mol Cpt, -1130.05 ± 1.00 kJ/mol Na-Cpt, -1109.49 ± 1.04 kJ/mol NaK-Cpt, -1094.21 ± 1.12 kJ/mol KCpt, and -1153.78 ± 1.07 kJ/mol Ca-Cpt. Their molar entropy was determined by a summation method based on the thermodynamic properties of the component oxides. Thus the standard free energy based on two framework O atoms was derived: -1034.01 ± 1.05 kJ/mol Cpt, -1044.19 ± 1.10 kJ/mol Na-Cpt, -1027.26 ± 1.13 kJ/mol NaK-Cpt, -1014.89 ± 1.21 kJ/mol K-Cpt, and -1064.95 ± 1.16 kJ/mol Ca-Cpt.

An in situ calorimetric study of zeolite crystallization kinetics

Abstract In this in situ calorimetric method the reaction rate is derived from the rate of evolution of the heat of reaction. The rationale and the procedure to obtain the reaction rate from the recorded heat flow during crystallization are elucidated. One major advantage of this method is the continuous and accurate measurement of crystallization rates over a wide range of crystallinity. In contrast, other methods like XRD and microscopy, directly measure the mass (or volume) of the product, but not the reaction rate, at a few times and only an average crystallization rate is usually computed. Other features of the in situ calorimetric method are also discussed. A case study of the crystallization kinetics of FAU zeolite is reported. The Arrhenius activation energy for crystallization increases from 66±2.3 to 72±0.8 kJ/mol as the crystallization proceeds from ∼5% to ∼75%. This change can be correlated to the increase of the Si/Al ratio in the solution, indicating that Al-rich aluminosilicate nutrient species are more reactive in the crystal growth.