Current Understandings and Future Opportunities Related to the Structure Direction in Zeolite Synthesis

Porous materials are intrinsically metastable structures owing to the presence of voids, which form in opposition to thermodynamic driving forces of crystallization, with the aid of structure-directing agents. For some materials, such as metal-organic frameworks, the solvent can play a critical space-filling role to prevent structural collapse. For zeolites, the role of structure direction is multifaceted. The packing of inorganic or organic cations within pores can function in a space-filling or templating role to stabilize channels or cages, while the positive charge is necessary to counterbalance the negative charge imposed by heteroatoms or defects in zeolite frameworks. Fundamental understanding of the structure direction in zeolite synthesis is relatively limited, with few predictive tools available to help guide the selection of synthesis parameters. In this article, we review what is known about zeolite structure direction in terms of its impact on crystallization and material properties, and opportunities for future advancement through synergistic combinations of experimental, computational, and data analytic techniques. Here, we provide an overview of both traditional and emerging methods employing organics, inorganics, and combinations thereof to stabilize zeolite polymorphs and tailor their physicochemical properties. These approaches are described with respect to their thermodynamic and kinetic impact on mechanisms of zeolite formation.