Guest-host interactions in high-silica zeolites

Zeolites are inorganic, crystalline, microporous solids used industrially in heterogeneous catalysis, gas separations, and ion-exchange operations. High-silica (Si/Al > 10) zeolites are synthesized with a organic molecule in the synthesis mixture. The organic molecule is referred to as a structure-directing agent (SDA) because the zeolite pore size and dimensionality often reflect the size and geometry of the organic moiety. Several studies have shown how the size, shape, and hydrophobicity of the SDA affect the product obtained from synthesis. However, the organic-inorganic coulombic forces in the synthesis gel and their role in the process of structure-direction are poorly understood. This investigation studies these coulombic forces to understand their role in structure-direction and to determine how they influence the physical properties of zeolites.; Nonasil, a clathrasil, was initially studied because it can be synthesized with several trimethylalkylammonioum compounds of different size. Variable temperature 2H NMR spectroscopy shows that none of the SDAs isotropically reorient at temperatures up to 370 K, consistent with strong forces existing between the nonasil framework and the cationic structure-directing agent. This is in contrast to nonasil samples made with electrically neutral SDAs such as 2-aminopentane, where only weak dispersive forces are observed at room temperature. The relative orientation of the cationic SDA with respect to the charge-compensating defect site was studied using two-dimensional heteronuclear correlation (HETCOR) spectroscopy. The HETCOR results show there is charge-ordering in as-made all-silica zeolites, and we believe suggest an additional role for some SDAs of controlling the location of defect sites in the zeolite framework.; Al-ZSM-12 samples synthesized with benzyltrimethylammonium cations were studied to determine if similar coulombic forces exist in as-made aluminosilicates. CP-MAS and rotational-echo double-resonance (REDOR) NMR experiments were performed on samples with selectively deuterated SDAs. The NMR results shows there is a direct spatial association between the framework aluminum and charge center of the SDA. Our results suggest a rational approach to altering the spatial distribution of trivalent framework atoms through synthesis, a goal long-sought in the zeolite science community. Our results also demonstrate the importance of organic-inorganic coulombic forces in zeolite synthesis and structure-direction.