Barrierless Crystallization in Glassy Precursors Drives Zeolite Formation

Zeolites are crystalline, microporous silicates widely used in catalysis and separations, yet the molecular mechanisms of their formation remain unresolved. Experiments indicate that hydrothermal synthesis of zeolites from solution proceeds through amorphous nanoaggregates that gradually evolve in ordering and solubility to the zeolite, suggesting a continuous amorphous to crystal transformation. Here, we combine molecular simulations, advanced algorithms to identify zeolite order, and computer vision to elucidate the pathway of zeolite formation from solution to nanocrystal. We show that at hydrothermal synthesis conditions, the transformation of precursor aggregates into zeolite is barrierless and proceeds via continuous crystallization within a glassy matrix. Our temperature-size phase diagram for the precursor aggregates reveals that the lines of zeolite-amorphous equilibrium and of maximum crystallization rate converge for nanoparticle diameters of ~3 nm and temperatures of ~200 °C, eliminating the nucleation barrier and terminating the first-order amorphous-to-zeolite transition below that temperature. Zeolite-like pores and short-range order emerge early within glassy precursors, well before detectable crystallinity appears in simulated or experimental TEM images. These findings explain the puzzling continuous character of zeolite crystallization and the catalytic activity of X-ray amorphous, protozeolite, and embryonic zeolite intermediates. The observed mechanism parallels behavior in other nanoscale systems, such as water in nanodroplets, where finite-size effects also lead to the suppression of the first-order liquid-crystal transitions. Our results provide a unifying framework for understanding barrierless crystallization in zeolites and suggest that continuous crystallization may govern the formation of other nanoparticle systems, including minerals and oxides synthesized far below their bulk melting points.