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The assembly and structural evolution of amorphous precursors during zeolite crystallization is an important area of interest owing to their putative roles in the nucleation and growth of aluminosilicate microporous materials. Precursors range in complexity from oligomeric molecules and colloidal particles to gels comprised of heterogeneous silica and alumina domains. The physical state of precursors in most zeolite syntheses is generally not well understood; however, it is evident that the physicochemical properties of precursors depend on a wide range of conditions that include (but are not limited to) the selection of reagents, the composition of growth mixtures, the methods of preparation, and the use of inorganic and/or organic structure-directing agents. The fact that precursors evolve in size, shape, and/or microstructure during the course of nucleation and potentially throughout crystallization leads to questions pertaining to their mode of action in the formation of zeolites. This also highlights the diversity of species that are present in growth media, thus rendering the topic of zeolite synthesis essentially a black box to those attempting to better understand the fundamental role(s) of precursors. In this Article, we discuss the wide variety of precursors encountered in the synthesis of various framework types, emphasizing their complex physical states and the thermodynamic and kinetic factors that govern their heterogeneity. E lucidating the mechanisms of zeolite crystallization is complex owing in large part to the vast number of species present in synthesis mixtures. 1,2 This is a contributing factor to the challenges associated with zeolite crystal engineering where it is difficult to design materials with predetermined physicochemical properties without sufficient knowledge of how synthesis variables can be tailored to mediate crystal growth. 3 The ubiquitous presence of amorphous precursors throughout nucleation and growth make zeolites quintessential examples of materials that grow via nonclassical pathways, which include crystallization by particle attachment. 4−7 This rapidly emerging area is garnering considerable attention owing to the expanding list of materials that show evidence of growth via multifaceted pathways. 8−13 Knowledge of nonclassical mechanisms, however, is rather limited due to inadequate analytical techniques available to observe dynamic processes of growth in situ with sufficient spatiotemporal resolution. In this perspective Article, we highlight the various routes leading to the assembly and evolution of amorphous precursors in zeolite synthesis wherein it is recognized that changes in conditions, most notably the selection of silica/alumina sources and room temperature aging protocols, can significantly influence polymorphism, crystallization kinetics, and the properties of zeolites, among other factors. Here, we address the physical state 51 of precursors with an emphasis on the appropriate use of the 52 word "gel" to properly convey the heterogeneity of these species...

Conventional methods to prepare hierarchical zeolites depend upon the use of organic structure‐directing agents and often require multiple synthesis steps with limited product yield and Brønsted acid concentration. Here, it is shown that the use of MEL‐ or MFI‐type zeolites as crystalline seeds induces the spontaneous formation of self‐pillared pentasil zeolites, thus avoiding the use of any organic or branching template for the crystallization of these hierarchical structures. The mechanism of formation is evaluated by time‐resolved electron microscopy to provide evidence for the heterogeneous nucleation and growth of sequentially branched nanosheets from amorphous precursors. The resulting hierarchical zeolites have large external surface area and high percentages of external acid sites, which markedly improves their catalytic performance in the Friedel–Crafts alkylation and methanol to hydrocarbons reactions. These findings highlight a facile, commercially viable synthesis method to reduce mass‐transport limitations and improve the performance of zeolite catalysts.

A combination of bulk crystallization studies and molecular modelling are used to elucidate the role of dual inorganic/organic SDAs in ZSM-5 synthesis. Our findings reveal unexpected synergistic effects on crystallization times and physicochemical properties.