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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.

The preparation of nanosized zeolites is critical for applications where mass-transport limitations within microporous networks hinder their performance. Often the ability to generate ultrasmall zeolite crystals is dependent upon the use of expensive organics with limited commercial relevance. Herein, we report the generation of zeolite L crystals with uniform sizes less than 30 nm using a facile, organic-free method. Time-resolved analysis of precursor assembly and evolution during nonclassical crystallization highlights key differences among silicon sources. Our findings reveal that a homogenous dispersion of potassium ions throughout silicate precursors leads to the formation of a metastable nonporous phase, which undergoes an intercrystalline transformation to zeolite L. The generation of highly interdispersed alkali-silicate precursors is seemingly critical to enhancing the rate of nucleation and facilitating the formation of ultrasmall crystal.

Application of ultrasound in crystallization has showed improved process characteristics. Although several attempts have been made in the past to study the sono-crystallization kinetics, only nucleation and crystal growth were considered, neglecting breakage and agglomeration of crystals. In this study, an attempt is made for the estimation of the kinetic parameters of all the phenomena occurring simultaneously during sono-crystallization. For this, both conventional and ultrasonic crystallization of KSO-water system has been reported. Sono-crystallization experiments were carried out using ultrasonic horn operating at 20 kHz frequency. Reduction in the induction time, reduction in metastable zone width (MSZW), narrowing of crystal size distribution (CSD) were the key observations of sono-crystallization experiments. Population balance equations (PBE) were used to model the crystallization system and the various kinetic parameters have been estimated. The kinetic parameters obtained for conventional crystallization and sonocrystallization were compared. The estimated parameters suggest an increase in nucleation and breakage rate during sono-crystallization. Growth rates were observed to be of the same order of magnitude for both conventional and sonocrystallization. While agglomeration during sono-crystallization was found to be negligible.

Crystal engineering relies upon the ability to predictively control intermolecular interactions during the assembly of crystalline materials in a manner that leads to a desired (and predetermined) set of properties. Economics, scalability, and ease of design must be leveraged with techniques that manipulate the thermodynamics and kinetics of crystal nucleation and growth. It is often challenging to exact simultaneous control over multiple physicochemical properties, such as crystal size, habit, chirality, polymorph, and composition. Engineered materials often rely upon postsynthesis (top-down) processes to introduce properties that would otherwise be challenging to attain through direct (bottom-up) approaches. We discuss the application of crystal engineering to heterogeneous catalysts with a focus on four general themes: ( a) tailored nanocrystal size, ( b) controlled environments surrounding active sites, ( c) tuned morphology with well-defined facets, and ( d) hierarchical materials with disparate pore size and active site distributions. We focus on nonporous materials, including metals and metal oxides, and two classes of porous materials: zeolites and metal organic frameworks. We review novel synthesis methods involving synergistic experimental and computational design approaches, the challenges facing catalyst development, and opportunities for future advancement in crystal engineering.

The treatment of zeolites with surfactants in alkaline media is an effective and versatile technique to impart intracrystalline well-defined mesoporosity in these materials. In this study, the dynamics of surface reconstruction that occurs during the treatment of USY zeolite by surfactant-templating was monitored in situ by atomic force microscopy. The development of surfactant-templated mesoporosity and the concurrent healing of defects that are characteristic of steamed zeolites occur in less than one hour at room temperature, which emphasizes the low energy barriers needed to reorganize the crystalline structure of this zeolite. This transformation was also followed by X-ray diffraction, N 2 adsorption, and TEM analysis of ultramicrotomed samples to confirm that the rapid formation of surfactant-templated mesoporosity and the reconstruction of the zeolite crystals occur not only on the surface of the zeolite, but homogeneously throughout the whole zeolite. This process involves a significant and rapid breaking and re-formation of bonds; however, the zeolite does not dissolve during this process as solids recovery at any given time of the treatment is approximately 100% and the concentration of soluble Si or Al species in the liquid is negligible. Parametric analysis revealed that excessive NaOH leads to the partial transformation of zeolite into an amorphous mesoporous solid,