Thermochemical Insights into Stability and Hydration of Ion-Exchanged Zeolite ZK-5 (KFI Framework)

Abstract

Extra-framework cations not only have a great impact on the structure of zeolites but also affect energetics and absorption of guest molecules. To investigate the stability and hydration of ion-exchanged zeolite ZK-5, high-temperature oxide melt solution calorimetry and direct water absorption calorimetry were used to reveal the influence of ion exchange on formation enthalpies and hydration processes. Formation enthalpies from oxides suggest that exchanging K + and Cs + with protons leads to destabilization of H-ZK-5, which is accompanied by the transition to an amorphous phase at high temperature. Water absorption and corresponding enthalpies are also affected by charge-compensating cations, for example, Na + ions in Na-ZK-5 promote the confinement of water molecules and generate higher water content and more exothermic absorption enthalpies. It is found that average ionic potentials are insufficient to explain formation enthalpies at different hydration levels, particularly for H-ZK-5 with complex cation forms. Instead, formation enthalpies from oxides generally become less exothermic as the distortion of the zeolite framework increases when replacing cations or absorbing/desorbing water molecules. This thermochemical study demonstrates the critical role of extra-framework cations and documents the complexity of ion exchange effects in zeolites with complicated topology and cation distribution.

Thermodynamics of Molybdenum Trioxide (MoO3) Encapsulated in Zeolite Y

“…22,23 Employing a suite of highly customized calorimeters, in collaboration with Drs Davis and Zones the Navrotsky's group pioneered research on thermodynamics of pure zeolites since 1990s, in which the cation -water -zeolite interplays of alkali and alkaline earth ion-exchanged zeolites, organic structural directing agent (OSDA)framework interactions, formation mechanisms under hydrothermal/solvothermal synthesis, and adsorption energetics of small molecules, such as water, CO 2 , and organics, were systematically investigated. [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] The general conclusions are (i) dehydrated zeolites are moderately metastable compared with their dense phase assemblages by less than 15 kJ/mol per TO 2 unit, and as the framework molar volume increases, such energetic difference tends to be more significant. The energetic stability of dehydrated alkali and alkaline earth ion-exchanged aluminosilicate zeolites is function of Si/Al ratio and charge-balancing cations.…”

“…(iii) Similarly, the energetics of OSDA -framework interactions and formation energetics under hydrothermal condition suggest moderately exothermic bonding, a product of subtly balanced enthalpy and entropy factors. [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] These studies have laid a solid foundation for zeolite thermodynamics by enabling reliable thermochemical data on natural zeolites of geochemical importance and synthetic pure zeolites applied in the petrochemical industry as sorbents, ion-exchange media and catalysts. Nevertheless, thermodynamics of zeolites with encapsulated heterocore transition metal clusters/particles, such as oxide, carbide and/or nitride clusters, has not been systematically investigated and documented.…”

Thermodynamics of molybdenum trioxide encapsulated in zeolite Y

“…(ii) Generally, hydration or adsorption of small organics is exothermic and tends to be less negative as the adsorbate loading increases. (iii) Similarly, the energetics of OSDA—framework interactions and zeolite formation energetics under hydrothermal condition suggest moderately exothermic bonding, a product of subtly balanced enthalpy and entropy factors 24–41 . These studies have laid a solid foundation for zeolite thermodynamics by enabling reliable thermochemical data on natural zeolites of geochemical importance and synthetic pure zeolites applied in the petrochemical industry as sorbents, ion‐exchange media, and catalysts.…”

Thermodynamics of Molybdenum Trioxide (MoO3) Encapsulated in Zeolite Y

“…Davis and Zones, the Navrotsky Group pioneered research on thermodynamics of pure zeolites since 1990s, in which the cation -water -zeolite interplays of alkali and alkaline earth ion-exchanged zeolites, organic structural directing agent (OSDA) -framework interactions, formation mechanisms under hydrothermal/solvothermal synthesis, and adsorption energetics of small molecules, such as water, CO 2 , and organics, were systematically investigated. [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] The general conclusions are (i) dehydrated zeolites are moderately metastable compared with their dense phase assemblages by less than 15 kJ/mol per TO 2 unit, and as the framework molar volume increases, such energetic difference tends to be more significant. The energetic stability of dehydrated alkali and alkaline earth ion-exchanged aluminosilicate zeolites is a complex function of Si/Al ratio and charge-balancing cations.…”

“…(iii) Similarly, the energetics of OSDA -framework interactions and zeolite formation energetics under hydrothermal condition suggest moderately exothermic bonding, a product of subtly balanced enthalpy and entropy factors. [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] These studies have laid a solid foundation for zeolite thermodynamics by enabling reliable thermochemical data on natural zeolites of geochemical importance and synthetic pure zeolites applied in the petrochemical industry as sorbents, ion-exchange media and catalysts. Nevertheless, thermodynamics of zeolites with encapsulated heterocore TM species, such as TMO, TMC and/or TMN clusters, has not been systematically investigated and documented.…”