The website for the International Zeolite Association shows something like 250 different zeolite structures confirmed by x-ray diffraction data. A closer analysis will also show that more than half of them were discovered by using organic molecules in the synthesis. The organic molecules help to determine which eventual structure will crystallize in a certain synthesis environment and is then found to invariably closely fill the void space in the microporous product with favorable van der Waals interactions playing a seemingly important role. In this study we attempt to consider the thermodynamic contributions of the behavior of a series of organo-cations in the synthesis of zeolite SSZ-13 from a single set of reaction conditions. On the one hand we show that the better the organo-cations fill the cages of SSZ-13, the faster the crystallization rate is, and a better energetic fit in the zeolite is calculated. From a different point of view we now consider whether a driving force in successful nucleation might also be the favorable entropy change from removal of the more hydrophobic organo-cations from a high ionic strength aqueous environment, into an emerging silicate framework. The hydrophobicity of the increasingly larger organo-cations is demonstrated by their increasing partition coefficients between water and immiscible chloroform. Then, for the first time, calorimetric measurements of enthalpies of solvation, water and chloroform each, are made towards computing an entropic value in the removal of the organo-cations from water.

Energetics of the local environment of structure directing agents influence zeolite synthesis

ACS Spring 2022

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