Diverse Physical States of Amorphous Precursors in Zeolite Synthesis

Abstract

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...

On the key role of aluminium and other heteroatoms during interzeolite conversion synthesis

“…1,4 IZC provides both its Si and Al in one chemical entity, in contrast with conventional syntheses which typically contain a separate silicate and aluminate source. With amorphous or soluble (non-zeolitic) sources, the order of reactant addition plays a vital role on the heterogeneous synthesis mixtures obtained at elevated temperatures, and thus ultimately the crystal formation, 49 as changes in this order have been reported to alter the pathways of formation. [50][51][52] Such uctuations related to chemical entities are not expected in single source (true) IZC, thus reducing the complexity of the system.…”

“…Acknowledging the importance of physical states and its heterogeneity in zeolite crystallization kinetics is therefore very important. 49 Fig. 7 summarizes the dissolution behaviour of IZC and conventional sources and their resulting physical states as hypothesized from the key role of dissolved Al.…”

Titanosilicate zeolite precursors for highly efficient oxidation reactions

“…59 Although the secondary amorphous phases demonstrate some spectroscopic and size-exclusive characteristics of zeolites, the lack of long-range periodicity of a crystalline zeolite lattice makes it amorphous and unsuitable for X-ray diffraction (XRD). [22][23][24][25] Therefore, these zeolite precursors are also regarded as X-ray amorphous zeolites or embryonic zeolites.…”

Low-cost synthesis of nanoaggregate SAPO-34 and its application in the catalytic alcoholysis of furfuryl alcohol

“…First, after hydrolyzing in the alkaline solution, the aluminum and silicon precursors exist as Al(OH)4  and SiO2(OH)  , respectively; these species will probably combine with each other and form aluminosilicates with Si-O-Al bonds [36]. The aluminosilicates, with Si-O-Al bonds as the basic units of zeolite, will remain undissolved and will be inclined to produce more secondary units, including oligomers, amorphous particles, and even nanocrystallites, which can lead to a rapid and massive formation of nuclei [37][38][39]. This can be proved from the XRD and SEM results (Figs.…”

“…After the addition of TEAOH, alumina in these colloidal suspensions will be deposited on the surface of the silica particles, which dramatically slows breakage of the Si-O-Si bonds. This can extend the time frame for reaching thermodynamic equilibrium or create a metastable state whereby the solution never reaches equilibrium prior to the onset of zeolite nucleation; that is, this can retard the rate of nucleation [39]. In this system, crystal growth occurs via a layer-by-layer mechanism that typically leads to well-defined crystals [41].…”