Controlling crystal polymorphism in organic-free synthesis of Na-zeolites.

ABSTRACT

Controlling polymorphism is critical in areas such as pharmaceuticals, biomineralization, and catalysis. Notably, the formation of unwanted polymorphs is a ubiquitous problem in zeolite synthesis. In this study, we propose a new platform for controlling polymorphism in organic-free Na-zeolite synthesis that enables crystal composition and properties to be tailored without sacrificing crystal phase purity. Through systematic adjustment of multiple synthesis parameters, we identified ternary (kinetic) phase diagrams at specific compositions (i.e., Si, Al, and NaOH mole fractions) using colloidal silica and sodium aluminate. Our studies identify multiple stages of zeolite phase transformations involving the framework types FAU, LTA, EMT, GIS, SOD, ANA, CAN, and JBW. We report an initial amorphous-to-crystalline transition of core-shell particles (silica core and alumina shell) to low-density framework types and their subsequent transformation to more dense structures with increasing temperature and/or time. We show that reduced water content facilitates the formation of structures such as EMT that are challenging to synthesize in organic-free media and reduces the synthesis temperature required to achieve higher-density framework types. A hypothesis is proposed for the sequence of phase transformations that is consistent with the Ostwald rule of stages, wherein metastable structures dissolve and recrystallize into more thermodynamically stable structures. The ternary diagrams developed here are a broadly applicable platform for rational design that offers an alternative to time- and cost-intensive methods of ad hoc parameter selection without a priori knowledge of crystal phase behavior.