Energetics of the Local Environment of Structure-Directing Agents Influence Zeolite Synthesis

Abstract

Organic structure-directing agents (SDAs) play a crucial role in the synthesis of zeolites like SSZ-13 (CHA structure), with novel frameworks and compositions. Zeolite crystallization is aided by the removal of the hydrophobic SDA organocations from an aqueous environment and their incorporation into an emerging silicate framework. This study combined several experimental approaches to understand the influence on nucleation to form SSZ-13 as the size of the SDA was changed. We studied crystallization rates, then modeled the SDA fit in the product to look at correlations for the rates, and then performed the measurement of SDA zeolite filling in the products. Then, we moved to determine the driving force for the transfer of this same series of organic SDAs, (C/N + = 7−16) from water to chloroform, the latter a proxy for a much less hydrophilic zeolitic environment. Calorimetric measurements of dissolution enthalpy for each SDA in water and chloroform provided enthalpy data and the distribution coefficients for the transfer reaction were measured, both at room temperature. From these experiments, the corresponding transfer enthalpy, entropy, and free energy were calculated. The thermodynamic parameters of the transfer process depend on the C/N + ratio, location, and environment of the charge in the SDA structure and are a good measure of the ability of the SDA to make the target zeolite, SSZ-13 (CHA). However, the use of a particular, hydrophilic SDA as a counter-example in this particular zeolite synthesis also produced rapid crystallization rates and demonstrated that the opportunity to initiate nucleation, by virtue of docking of the SDA coupled with best SDA-fit, may provide the optimum use of the SDA, providing a stronger factor than the solution transfer thermodynamics.

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.…”