To seed or not to seed? That is the question for zeolite AFI crystallization

The use of crystalline seeds in zeolite synthesis has become a more frequent method to achieve outcomes that cannot be obtained in their absence. A subset of zeolite syntheses relies on seed-assisted pathways, yet the fundamental role(s) of seeds is not well understood. Here, we systematically investigate several key factors influencing the crystallization of the all-silica AFI zeolite, which is almost exclusively prepared with zeolite seeds. Prior studies have used only two zeolites, with frameworks CHA or *BEA, as the seed (parent) to produce AFI (daughter) without providing detailed rationalizations for seed selection or mechanistic insights into the parent-to-daughter interzeolite transformations that occur in the presence of amorphous precursors. In this study, we show that AFI crystallization occurs in the presence and absence of seeds. Studies of non-seeded syntheses reveal nonclassical pathways of crystallization where nucleation occurs on the exterior surfaces of amorphous precursors, and the judicious selection of synthesis conditions can reduce crystallization times seven-fold compared to methods reported in literature. For seed-assisted syntheses, we show that the number of seeds can be expanded to a total of seven zeolite frameworks that both promote and accelerate AFI crystallization. Mechanistic pathways of AFI formation are examined using a combination of time-resolved experimental studies, molecular dynamics simulations, and zeolite-zeolite interface modeling, in which we demonstrate that seed crystal surfaces can promote nucleation through multiple routes that include heteroepitaxial growth of AFI as a result of their interaction with amorphous precursors or species in solution. Our findings also highlight the important role of inorganic cations, which is uncommon for syntheses of all-silica zeolites, but is essential for AFI crystallization with notable specificity for certain alkali metals depending on the selected organic structure-directing agent. We further demonstrate the incorporation of tetrahedral boron into the AFI zeolite framework via a non-seeded synthesis; however, attempts to incorporate other heteroatoms were unsuccessful. Overall, these studies emphasize the sensitivity of zeolite AFI synthesis to a broad range of parameters and advance our understanding of parent-daughter relationships in seed-assisted zeolite syntheses.