Aggregation Mechanism of Nanoslabs with Zeolite MFI-Type Structure

The Current Understanding of Mechanistic Pathways in Zeolite Crystallization

Zeolite catalysts and adsorbents have been an integral part of many commercial processes and are projected to play a significant role in emerging technologies to address the changing energy and environmental landscapes. The ability to rationally design zeolites with tailored properties relies on a fundamental understanding of crystallization pathways to strategically manipulate processes of nucleation and growth. The complexity of zeolite growth media engenders a diversity of crystallization mechanisms that can manifest at different synthesis stages. In this review, we discuss the current understanding of classical and nonclassical pathways associated with the formation of (alumino)silicate zeolites. We begin with a brief overview of zeolite history and seminal advancements, followed by a comprehensive discussion of different classes of zeolite precursors with respect to their methods of assembly and physicochemical properties. The following two sections provide detailed discussions of nucleation and growth pathways wherein we emphasize general trends and highlight specific observations for select zeolite framework types. We then close with conclusions and future outlook to summarize key hypotheses, current knowledge gaps, and potential opportunities to guide zeolite synthesis toward a more exact science.

Hierarchical COK-X Materials for Applications in Catalysis and Adsorptive Separation and Controlled Release

Over the years, COK has developed a family of silicate materials and metal–organic framework hybrids with hierarchical porosity and functionality, coined zeogrids, zeotiles, and COK-x (stemming from the Flemish name of the laboratory “Centrum voor Oppervlaktechemie en Katalyse”). Several of these materials have unique features relevant to heterogeneous catalysis, molecular separation, and controlled release and found applications in the field of green chemistry, environmental protection, and pharmaceutical formulation. Discovery of a new material typically occurs by serendipity, but the research was always guided by hypothesis. This review provides insight in the process of tuning initial research hypotheses to match material properties to specific applications. This review describes the synthesis, structure, properties, and applications of 12 different materials. Some have simple synthesis protocols, facilitating upscaling and reproduction and rendering them attractive also in this respect.

Polymorph Selection in Zeolite Synthesis Occurs after Nucleation

Zeolites are porous crystals with extensive polymorphism. The hydrothermal synthesis of zeolites is a multistage process involving amorphous precursors that evolve continuously in solubility and local order toward those of the crystal. These results pose several questions: Why does a first-order transition appear as a continuous transformation? At which stage is the polymorph selected? How large are the barriers and critical sizes for zeolite nucleation? Here we address these questions using nucleation theory with experimental data. We find that the nucleation barriers and critical zeolite nuclei are extremely small at temperatures of hydrothermal synthesis, resulting in spinodal-like crystallization that produces a mosaic of tiny zeolitic crystallites that compete to grow inside each glassy precursor nanoparticle. The subnanometer size of the critical nuclei reveals that the selection between zeolite polymorphs occurs after the nucleation stage, during the growth and coarsening of the crystals around the excluded volume of the structure-directing agents.

Molecular level nucleation mechanisms of hierarchical MFI and MOR zeolite structures via non-stochastic pathways

Understanding the chemical mechanism of crystal nucleation at the molecular level is crucial for the design of architectural structures of valuable materials in the future. In this study, it has been revealed that amorphous silicate precursors, which play a role in the nucleation processes of zeolitic frameworks, can be regularly fragmented in mass spectroscopy due to the hydroxyl functional groups in their molecular structures. In this way, by using the mass spectra acquired from LDI-TOF MS, the systematic evolution stages of a common 1D precursor converting to the 3D unit cells of MFI and MOR zeolite structures observed in the same reaction medium were constructed through a nucleation mechanism at the molecular level for the first time. Here we show a novel nucleation pathway that does not occur via stochastic assembly of atoms or distinct building blocks by molecular recognition. Each of the proposed nucleation mechanisms of these different frameworks carrying structural similarities is from different combinations of sequential self-attaching intramolecular covalent couplings of identical origin precursors. The dynamic molecular structure capable of forming finite building units of target frameworks during the nucleation process of this precursor, which is the polymerized form of simple 6-membered siloxane chains, has been arranged around structure directing agents before a hydrothermal reaction.

Insight into the Formation of Nanostructured MFI Sheets and MEL Needles Driven by Molecular Recognition

Mesoporous and nanostructured zeolite-based catalysts experience prolonged lifetimes due to increased mass transfer and reduced micropore obstruction by coke formation as compared to their bulky microporous counterparts. Diquaternary ammonium structure-directing agents (SDAs) can be used to synthesize hierarchical MFI sheet-like and MEL needle-like zeolites. An explanation of the underlying molecular-level details of the synthesis of these nanostructured zeolites is presented on the basis of non-covalent interactions between the template and zeolite surfaces as well as silicate oligomers studied by means of classical molecular dynamics. Use was made of Si₁₁ and Si₃₃ silicate oligomers that contain structural features of the framework to be formed as originally proposed by the Leuven group. Molecular recognition is driven by a combination of strong electrostatic and weaker dispersion interactions. An analysis of the early stage of zeolite formation is necessary, as the template adsorption energies in the fully formed zeolite crystals cannot explain the preferential growth of the MFI sheets or MEL needles. Specifically, it is found that the differences in dispersion interactions between the SDA alkyl chains and the silicate oligomers are decisive in the formation of particular zeolite structures.

Reversible room temperature ammonia gas absorption in pore water of microporous silica-alumina for sensing applications

Microporous silica and silica-alumina powders exhibit a reversible uptake and release of ammonia gas from water vapor containing gas mixtures at ambient temperature, with capacities of 0.9 and 2.0 mmol g-1 in the presence of 100 ppm and 1000 ppm NH3, respectively. The ammonia trapping mechanism was revealed using a combination of direct excitation 1H MAS, 1H-1H EXSY and 1H DQ-SQ NMR spectroscopy, indicating that the major part of the captured ammonia is blended in the hydrogen bonded water network in the pores of the adsorbent. A small fraction is irreversibly bound as result of protonation and chemisorption. While common ammonia adsorbents need thermal regeneration, microporous silica-alumina can be regenerated by sweeping with dry gas at ambient temperature, desorbing the physisorbed fraction together with occluded water. As carbon dioxide does not interfere with the ammonia absorption process, this reversible absorption process of ammonia gas at ambient temperature is particularly attractive for sensor applications.

Synthesis of Chainlike ZSM-5 Zeolites: Determination of Synthesis Parameters, Mechanism of Chainlike Morphology Formation, and Their Performance in Selective Adsorption of Xylene Isomers

Chainlike zeolites are advantageous to various applications as a catalyst or an adsorbent with specific selectivity; however, it is often very difficult to get desired morphology due to the complexity of zeolite synthesis process. In this work, appropriate parameters for the synthesis of perfect chainlike ZSM-5 zeolites were well determined, which illustrates that the chain length can be controlled by the composition of synthesis mixture, the amount of residual alcohol in the synthesis system, and the crystallization time. Moreover, the mechanism of chainlike crystal growth was investigated by analyzing the surface species during the synthesis process, with the help of density functional theory (DFT) calculation. The results indicate that the formation of disk crystals with proper dimension and flat surface having abundant hydroxyl groups is crucial to the growth of chainlike ZSM-5 crystals; the condensation of Si-OH groups on the (010) facet is energetically more favorable than that on other facets, leading to the growth of MFI crystals along the b-orientation. Through finely tuning the multifarious synthesis parameters, chainlike ZSM-5 zeolites with controllable length in b-orientation are obtained without using any other extra organic additives except the necessary template agent such as tetrapropylammonium hydroxide (TPAOH). Owing to the increased tortuosity of pore channels in the chainlike ZSM-5, the difference between p-xylene and o/m-xylenes in their adsorption behavior and diffusivity is greatly enhanced. These results help to clarify the formation mechanism of zeolites with chainlike morphology and then bring forward an effective approach to get zeolite materials with specific properties in adsorption and catalysis.

Monitoring early zeolite formation via in situ electrochemical impedance spectroscopy

Hitherto zeolite formation has not been fully understood. Although electrochemical impedance spectroscopy has proven to be a versatile tool for characterizing ionic solutions, it was never used for monitoring zeolite growth. We show here that EIS can quantitatively monitor zeolite formation, especially during crucial early steps where other methods fall short.

A design concept of amphiphilic molecules for directing hierarchical porous zeolite

This review describes a design concept of novel amphiphilic molecules for a one-step preparation of hierarchically porous zeolites containing mesopores with certain orders.

Embryonic ZSM-5 zeolites: zeolitic materials with superior catalytic activity in 1,3,5-triisopropylbenzene dealkylation

Embryonic ZSM-5 zeolites have more accessible microporosity than their zeolite offsprings and are therefore better catalysts to convert bulky molecules.

3D Study of the Morphology and Dynamics of Zeolite Nucleation

The principle aspects and constraints of the dynamics and kinetics of zeolite nucleation in hydrogel systems are analyzed on the basis of a model Na-rich aluminosilicate system. A detailed time-series EMT-type zeolite crystallization study in the model hydrogel system was performed to elucidate the topological and temporal aspects of zeolite nucleation. A comprehensive set of analytical tools and methods was employed to analyze the gel evolution and complement the primary methods of transmission electron microscopy (TEM) and nuclear magnetic resonance (NMR) spectroscopy. TEM tomography reveals that the initial gel particles exhibit a core-shell structure. Zeolite nucleation is topologically limited to this shell structure and the kinetics of nucleation is controlled by the shell integrity. The induction period extends to the moment when the shell is consumed and the bulk solution can react with the core of the gel particles. These new findings, in particular the importance of the gel particle shell in zeolite nucleation, can be used to control the growth process and properties of zeolites formed in hydrogels.

In situ imaging of silicalite-1 surface growth reveals the mechanism of crystallization

The growth mechanism of silicalite-1 (MFI zeolite) is juxtaposed between classical models that postulate silica molecules as primary growth units and nonclassical pathways based on the aggregation of metastable silica nanoparticle precursors. Although experimental evidence gathered over the past two decades suggests that precursor attachment is the dominant pathway, direct validation of this hypothesis and the relative roles of molecular and precursor species has remained elusive. We present an in situ study of silicalite-1 crystallization at characteristic synthesis conditions. Using time-resolved atomic force microscopy images, we observed silica precursor attachment to crystal surfaces, followed by concomitant structural rearrangement and three-dimensional growth by accretion of silica molecules. We confirm that silicalite-1 growth occurs via the addition of both silica molecules and precursors, bridging classical and nonclassical mechanisms.

Ordered Porous Nanomaterials: The Merit of Small

This paper will introduce the reader to some of the “classical” and “new” families of ordered porous materials which have arisen throughout the past decades and/or years. From what is perhaps the best-known family of zeolites, which even now to this day is under constant research, to the exciting new family of hierarchical porous materials, the number of strategies, structures, porous textures, and potential applications grows with every passing day. We will attempt to put these new families into perspective from a synthetic and applied point of view in order to give the reader as broad a perspective as possible into these exciting materials.

Effects of ultrasound on the synthesis of silicalite-1 nanocrystals

Application of power ultrasound, offers potential in the degree of control over the preparation and properties of nanocrystalline zeolites, which have become increasingly important due to their diverse emerging applications. Synthesis of silicalite-1 nanocrystals from a clear solution was carried out at 348 K in the absence and presence of ultrasound of 300 and 600 W, in an attempt to investigate the effects of sonication, in this respect. Variation of the particle size and particle size distribution was followed with respect to time using a laser light scattering device with a detector set to collect back-scattered light at an angle of 173°. Product yield was determined and the crystallinity was analyzed by X-ray diffraction for selected samples collected during the syntheses. Nucleation, particle growth and crystallization rates all increased as a result of the application of ultrasound and highly crystalline silicalite-1 of smaller average particle diameter could be obtained at shorter synthesis times. The particle size distributions of the product populations, however, remained similar for similar average particle sizes. The rate of increase in yield was also speeded up in the presence of ultrasound, while the final product yield was not affected. Increasing the power of ultrasound, from 300 to 600 W, increased the particle growth rate and the crystalline domain size, and decreased both the final particle diameter and the time required for the particle growth to reach completion, while its effect on nucleation was unclear.

Nascent zeolite frameworks grown from amorphous gels – identification and prospects for crystal engineering

From the standpoint of designing microporous frameworks with desired pore diameter and dimensionality, and, especially, the optimization of crystal habit, the crystal engineering of new zeolites must be based on concepts/procedures different from those appropriate for the design of organic crystals. This is because the structure building units proposed by Barrer and co-workers are probably not instrumental for framework construction, thus eliminating the important ‘synthon’ approach used for the construction of innovative organic solids. With some variant of the Flanigen model for crystal growth via one SiO2 unit at a time, the best approach to zeolite crytal growth engineering appears to occur indirectly via structure directing agents that can also be modified to influence the emergent crystal habit. Prospects for identifying frameworks emerging from synthesis gels are also discussed in this review, revealing that the use of radial distribution functions is less informative than for the analysis of silicate glasses.

Hierarchical Nanomanufacturing: From Shaped Zeolite Nanoparticles to High-Performance Separation Membranes

Despite more than a decade of intense research on the high-resolution selectivity of thin zeolite films as alternatives to energy-intensive industrial separations, membranes consisting of intergrown, oriented zeolite crystals have fallen short of gaining wide commercial application. Factors including poor performance, high cost, and difficulties in scale up have contributed to this, and have also stunted their application in other niche markets. Until recently, rational design of these materials was limited because of the elusive mechanism of zeolite growth, and forced more empirical approaches. New understanding of zeolite growth along with recent advances in the molecular engineering of crystal microstructure and morphology, assembly of crystal monolayers, and synthesis of ordered films constitute a strong foundation for meeting stringent industrial demands in the future. Together with new processing capabilities, such a foundation should make it possible to synthesize commercially viable zeolite membranes through hierarchical approaches. Such advances open exciting prospects beyond the realm of separations for assembly of novel and complex functional materials including molecular sensors, mechanically stable dielectrics, and novel reaction-diffusion devices.

Revisiting the identification of structural units in aqueous silicate solutions by two-dimensional silicon-29 INADEQUATE

(29)Si-(29)Si INADEQUATE experiments have been successfully used on sodium aqueous silicate partially enriched (19% of (29)Si isotope) solutions in order to gain connectivity information and to help in spectral assignments. The structures of almost all known species from the literature were confirmed by this technique. Moreover, accurate scalar J couplings were extracted. The two-dimensional (2D) spectra demonstrated additional cross correlations for some Si-Si bonds never reported before, representing additional oligomeric species. They reveal five silicate cages with the following connectivity sets: Q(2)-Q(2)-Q(3) or Q(3Delta)-Q(3Delta)-Q(3) for one species, Q(2)-Q(2) or Q(3Delta)-Q(3Delta) for three species (Q(3Delta) indicates a three-connected silicon in a three-membered cycle), and Q(3)-Q(3) for one species. All possible molecular graphs for anions containing silicon up to 11 atoms, matching these sequential connnectivities, were enumerated. Additionally, a direct comparison between resonances observed in the 2D experiments with that obtained in the 1D spectrum allows us to assign single-sited silicate anions. Only a few species reported before were not detected, most probably due to their too low concentration in our solutions.

Evolution of self-assembled silica-tetrapropylammonium nanoparticles at elevated temperatures

The time evolution of silica nanoparticles in solutions of tetrapropylammonium (TPA) has been studied using a combination of small-angle scattering, conductivity, and pH measurements to provide the first comprehensive analysis of nanoparticle structural and compositional changes at elevated temperatures. We have found that silica-TPA nanoparticles subjected to hydrothermal treatment (70-90 degrees C) grow via an Ostwald ripening mechanism with growth rates that depend on both pH and temperature. Small-angle X-ray (SAXS) and neutron (SANS) scattering confirm that the core-shell structure of the particles, initially present at room temperature, is maintained during heating, but an evolution toward sphericity is evidenced especially at high values of pH. SAXS absolute intensity calculations were utilized to calculate the changes in nanoparticle composition and concentration over time. These changes along with the conductivity and pH measurements and SANS contrast matching studies indicate that, upon heating, TPA becomes embedded in the core of nanoparticles giving rise to more zeolitic-looking nanomaterials.

Aggregative Growth of Silicalite-1

Precursor silica nanoparticles can evolve to silicalite-1 crystals under hydrothermal conditions in the presence of tetrapropylammonium (TPA) cations. It has been proposed that in relatively dilute sols of silica, TPA, water, and ethanol, silicalite-1 growth is preceded by precursor nanoparticle evolution and then occurs by oriented aggregation. Here, we present a study of silicalite-1 crystallization in more concentrated mixtures and propose that growth follows a path similar to that taken in the dilute system. Small-angle X-ray scattering (SAXS), cryogenic transmission electron microscopy (cryo-TEM), and high-resolution transmission electron microscopy (HRTEM) were used to measure nanoparticle size and to monitor zeolite nucleation and early-stage crystal development. The precursor silica nanoparticles, present in the clear sols prior to crystal formation, were characterized using two SAXS instruments, and the influence of interparticle interactions is discussed. In addition, SAXS was used to detect the onset of secondary particle formation, and HRTEM was used to characterize their structure and morphology. Cryo-TEM allowed for in situ visual observation of the nanoparticle population. Combined results are consistent with growth by aggregation of silica nanoparticles and of the larger secondary crystallites. Finally, a unique intergrowth structure that was formed during the more advanced growth stages is reported, lending additional support for the proposal of aggregative growth.

Mesocrystals: inorganic superstructures made by highly parallel crystallization and controlled alignment

Controlled self-organization of nanoparticles can lead to new materials. The colloidal crystallization of non-spherical nanocrystals is a reaction channel in many crystallization reactions. With additives, self-organization can be stopped at an intermediary step-a mesocrystal-in which the primary units can still be identified. Mesocrystals were observed for various systems as kinetically metastable species or as intermediates in a crystallization reaction leading to single crystals with typical defects and inclusions. The control forces and mechanism of mesocrystal formation are largely unknown, but several mesocrystal properties are known. Mesocrystals are exiting examples of nonclassical crystallization, which does not proceed through ion-by-ion attachment, but by a modular nanobuilding-block route. This path makes crystallization more independent of ion products or molecular solubility, it occurs without pH or osmotic pressure changes, and opens new strategies for crystal morphogenesis.

Vanadium Silicalite-1 Nanoparticles Deposition onto the Mesoporous Walls of SBA-15. Mechanistic Insights from a Combined EPR and Raman Study

Continuous Wave (CW) and pulsed Electron Paramagnetic Resonance (EPR) spectroscopy in conjunction with Raman spectroscopy are used to investigate the properties of Vanadium Silicalite-1 (VS-1) nanoparticles dispersed onto the mesoporous walls of SBA-15 silica. The properties of the deposited zeolite nanoparticles are found to be remarkably different from those of the full grown VS-1 zeolite. Monitoring of the local VO(2+) environment in the noncalcined nanoparticles in SBA-15 reveals that, in contrast to the full grown zeolite case, these sites are highly hydrophilic. Also, the stability of the TPAOH template is found to be affected by acidification of the nanoparticles. These results promise to be of great importance in elucidating the formation mechanism of TPAOH-templated zeolitic nanoparticles and their incorporation in mesoporous silica materials.

Oriented attachment and mesocrystals: non-classical crystallization mechanisms based on nanoparticle assembly

In this review, we highlight particle based crystallization pathways leading to single crystals via mesoscopic transformation. In contrast to the classical mechanism of atom/molecule mediated growth of a single crystal, the particle mediated growth and assembly mechanisms are summarized as "non-classical crystallization", including exiting processes like oriented attachment and mesocrystal formation. Detailed investigations of non-classical crystallization mechanisms are a recent development, but evidence for these pathways is rapidly increasing in the literature. A major driving force for these investigations originates from biomineralization, because it seems that these crystallization routes are frequently applied by natural organisms. We give a non-exhaustive literature survey on these two mechanisms with a focus on recent examples and studies, which are dedicated to a mechanistic understanding. Furthermore, conditions are introduced for which these non-classical crystallization mechanisms can be expected, as they are always an alternative reaction pathway to classical crystallization.

Monitoring Temporal Evolution of Silicate Species during Hydrolysis and Condensation of Silicates Using Mass Spectrometry

The first stages of solid-state formation from solution can be crucial in determining the properties of the resulting solids. We are trying to approach prenucleation reactions of silicates from an aqueous solution containing tetraalkylammoniumhydroxides (TAAOH) and tetraalkoxysilanes (TAOS) by analyzing hydrolysis and condensation using electrospray mass spectrometry (ESI MS). Time-resolved measurements were performed using different reactor systems to show the stepwise hydrolysis of the silanes and subsequent condensation of silicate monomers via oligomers to form larger units. We approached the precipitation point by varying the pH and the concentrations of the reactants. The results show the evolution of different silicate species occurring during condensation. No defined molecular entities were identified at pH values close to precipitation, which suggests that under the conditions used, solids are probably not formed from defined building blocks.