Synthesis of zeolites in the absence of organic structure-directing agents: factors governing crystal selection and polymorphism

Organic structure-directing agents (OSDAs) are commonly avoided in commercial zeolite synthesis because of the economic and environmental disadvantages associated with the synthesis and removal of organics occluded within zeolite micropores. Zeolite crystallization in OSDA-free media is the route by which microporous clays form in nature, and it is also the preferred method of producing zeolites in bulk for a wide range of applications. There are many synthesis parameters that influence zeolite crystallization, among which include the molar fractions of reagents (silica, alumina, and hydroxide ions), water content, temperature, synthesis aging and heating time, the selection of extraframework cations, the choice of silica and alumina sources, and the use of crystal seeds. In this review, we discuss zeolite framework types that form in OSDA-free solutions at these different synthesis conditions in an effort to highlight structure-property relationships while simultaneously emphasizing the areas where further studies are needed to optimize and/or discover new materials. Interestingly, fewer than 15% of the total reported zeolite structures have been prepared in the absence of OSDAs. For many of these structures, fundamental mechanisms governing their formation are not well understood. In addition, OSDA-free syntheses tend to be more susceptible to the formation of crystal polymorphs (or impurities) that can be generated through a series of structural transformations during the course of zeolite growth. Here we examine the driving forces for phase transitions and explore methods to control phase selection and polymorphism. In order to better facilitate comparisons among zeolite synthesis parameters, we have reinstituted the approach of using kinetic phase diagrams to identify conditions of phase stability.