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

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.