Do zeolite precursor species really exist in aqueous synthesis media?

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.

Understanding formation thermodynamics of structurally diverse zeolite oligomers with first principles calculations

The mechanisms of many zeolitic processes, including nucleation and interzeolite transformation, are not fully understood owing to complex growth mixtures that obfuscate in situ monitoring of molecular events. In this work, we provide insights into zeolite chemistry by investigating the formation thermodynamics of small zeolitic species using first principles calculations. We systematically study how formation energies of pure-silicate and aluminosilicate species differ by structure type and size, temperature, and the presence of alkali or alkaline earth metal cations (Na⁺, K⁺, and Ca²⁺). Highly condensed (cage-like) species are found to be strongly preferred to simple rings in the pure-silicate system, and this thermodynamic preference increases with temperature. Introducing aluminum leads to more favorable formation thermodynamics for all species. Moreover, for species with a low Si/Al ratio (≤2), a thermodynamic preference does not exist among structure types; instead, a pool of diverse aluminosilicate structures compete in formation. Metal cation effects strongly depend on the presence of aluminum, cage size, cation type, and location, since each of these factors can alter electrostatic interactions between cations and zeolitic species. We reveal that confined metal cations may destabilize pure-silicate cages due to localized interactions; conversely, they stabilize aluminosilicates due to strong cation-framework attractions in sufficiently large cages. Importantly, this work rationalizes a series of experimental observations and can potentially guide efforts for controlling zeolite nucleation/crystallization processes.

Influence of the Zeolite ZSM-22 Precursor on a UF-PES Selective Substrate Layer for Salts Rejection

Fabrication of the ZSM-22/Polyethersulfone (ZSM-22/PES) membranes as selective salt filters represent a growing membrane technological area in separation with the potential of high economic reward based on its low energy requirements. The incorporation of ZSM-22 zeolite material as additives into the PES polymer matrix has the prospective advantage of combining both the zeolite and polymer features while overcoming the limitations associated with both materials. This work investigated the influence of the nature of the silica precursor on ZSM-22 zeolite hydrothermally synthesised using colloidal (C60) and fumed (C60) silica to Si/Al of 60. The successful synthesis of the highly crystalline zeolitic materials was confirmed through XRD, FTIR, and SEM with EDX. The ZSM-22 additives were directly dispersed into a PES polymeric matrix to form a casting solution for the preparation of the ZSM-22/PES selective substrate layers via a phase inversion method for salts rejection. The polymeric PES was selected as an organic network in which the content of the ZSM-22 zeolite (ranging between 0 and 1.0 wt.%), was obtained and characterised by XRD, FTIR, and SEM analysis, as well as water contact angle (WCA) measurement and dead-end filtration cell. The phase inversion preparation method has induced the resulting ZSM-22/PES NF substrates anisotropy, as attributed to a high water flux to the above 700 L·m−2·h−1; high selectivity and rejection of salts to above 80% is revealed by the obtained results. The materials also exhibited improved antifouling behavior to above 70% flux recovery ratios. As such, the nature of the silica precursor influences ZSM-22 zeolite synthesis as a potential additive in the PES polymer matrix and led to the enhanced performance of the pure PES ultrafiltration membrane.

Stability and behaviour in aqueous solutions of the anionic cubic silsesquioxane substituted with tetramethylammonium

The aqueous behaviour of the anionic octa-tetramethylammonium substituted cubic silsesquioxane, [N(CH3)4]8[Si8O20], was studied with quantitative 29Si-NMR. This molecule partially fragments in aqueous solutions, forming several smaller entities. The most abundant silica species are the monomer, dimer, cyclic trimer, cyclic tetramer and double three-ring. Higher concentrations are required in order to prevent complete fragmentation of the cubic structure. Additives such as alcohols and tetraalkylammonium salts have a stabilising effect on the cubic silsesquioxane, unlike sodium salts that destabilise it. A high concentration solution, containing the non-fragmented molecule as well as entities resulting from fragmentations, was investigated with neutron scattering coupled with modelling, using empirical potential structure refinement (EPSR). The modelling reveals that TMA+ ions coordinates to all different silica species, with approximately three TMA+ per cube. These are located at the faces of the cube.

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.

Role of subcolloidal (nanosized) precursor species in the early stage of the crystallization of zeolites in heterogeneous systems

A critical analysis was carried out for the purpose of understanding the role of subcolloidal (nanosized) (alumino)silicate precursor species in the early stage of crystallization of zeolites in heterogeneous systems (hydrogels). The formation and evolution of these subcolloidal species in both the solid and the liquid phases were investigated by various experimental methods such a scanning electron microscopy (SEM, FE-SEM), transmission electron microscopy, atomic force microscopy, particle size analysis, pH measurement, atomic absorption spectroscopy, and dynamic light scattering, after careful separation of intermediates from reaction mixture by two-step centrifugation treatment. The results revealed that a chain of processes (i) the formation of low-molecular-weight (LMW) silicate species, by dissolution of Al-enriched amorphous silica, and their aggregation into about 3 nm sized primary precursor species (PPSs), (ii) the formation of larger (∼3 to ∼15 nm sized) silicate precursor species (LSPSs) by a rapid aggregation/coalescence of PPSs, (iii) the formation of "gel" (primary amorphous precursor) by a random aggregation of LSPSs at room temperature, and (iv) the formation of the worm-like particles (secondary amorphous precursor) occurred in the solid phase during heating of the reaction mixture (hydrogel) from room temperature to 170 °C. It is interesting that almost the same processes occur in the liquid phase but with decreased rate according to the relative low concentration of LMW silicate species. With the above described findings, it is highly expected that the manipulation of crystallization pathway through controlling the formation/evolution of precursor species in the initial stage of the process can be achieved.

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.

Tailoring the physicochemical properties of zeolite catalysts

Here we summarize our recent findings in the area of zeolite synthesis, focusing on pathways to control crystallization in the absence of organics, tailoring crystal habit with growth modifiers, and pioneering techniques in zeolite surface science to elucidate the mechanisms of growth.

Calculation of the 29Si NMR chemical shifts of aqueous silicate species

A DFT methodology for calculating (29)Si NMR chemical shifts of silicate species typically present prior to nucleation in zeolite synthesis solutions, incorporating solvent effects through an implicit representation is presented. We demonstrate how our methodology can reproduce the experimentally observed spectra and, by comparison to well characterized peaks in two different experimental studies, demonstrate the transferability and robustness of the methodology. We discuss certain cases in which caution must be exercised when implicit solvent representations are used for calculating silicate cluster geometries: those cases in which intramolecular hydrogen bonding can play a significant role in the geometry. A number of reassignments of previous tentative experimental assignments are proposed, and we also make assignments for the challenging substituted four-ring species. We present all of our computed chemical shift for previously observed species together with a number of other viable silicate clusters to serve as a reference point for future experimental studies.

Reaction-induced phase separation of bis(triethoxysilyl)ethane upon sol-gel polymerization in acidic conditions

While organically bridged alkoxysilane precursors such as bis(triethoxysilyl)ethane (BTESE) find increasing use in materials synthesis, their polymerization still has not been subject to detailed kinetic investigations. One factor complicating the measurement and interpretation of the behavior of these monomers is their tendency to aggregate during polymerization into small clusters or particles. Here, the phase behavior and kinetics of BTESE during polymerization in acidic water-ethanol solutions are investigated in situ using (29)Si NMR spectroscopy. Based on macroscopic observation of the colloidal stability of the reacting solutions, a ternary pseudo-phase diagram is constructed and solutions that seem from a macroscopic point of view to be favorable for kinetic investigation are probed in situ using (29)Si NMR. However, even when the solutions remain optically clear, the polymerization mixtures are sometimes prone to gradual loss of NMR signal. The rate of signal loss is found to be dependent on the initial composition as well as the pH of the reacting mixtures. We speculate that this phenomenon is caused by microphase separation of reaction intermediates formed early in the polymerization process. This phenomenon is likely to affect the formation and distribution of oligomers in the solution that eventually react together to form a material.

Methods for in situ spectroscopic probing of the synthesis of a zeolite

Unraveling the crystallization mechanism of zeolites remains an increasingly important challenge in chemistry. During the last decade, in situ spectroscopic methods have provided an unprecedented level of detail of the underlying molecular mechanisms and their kinetics. Magnetic resonance, vibrational and X-ray absorption techniques have emerged as principal tools for the in situ observation of crystallization. In this tutorial review, we discuss how these in situ methods have contributed to our understanding of the complex and diverse molecular processes that govern zeolite crystallization.

Insights into the crystal growth mechanisms of zeolites from combined experimental imaging and theoretical studies

Detailed investigations into surface mediated crystal growth at zeolite external surfaces are presented. High resolution TEM is able to directly resolve surface and bulk crystallographic features and the unusual surface structural features are interpreted from simulation work. The growth of the double 4 ring is found to be a crucial and rate-determining step in the surface mediated, post-nucleation crystal growth mechanism of zeolite Beta C. Growth of 4 rings is found to be more favourable on fast growing rather than slow growing faces, explaining the relative growth rate of crystal faces in this materials. Similarly, the terminating structures of zeolite Y/Faujasite can be partly explained by considering the condensation of 6 ring and double 6 ring species at the crystal surface. Whilst 4-ring and double 4 rings are known solution species, 6 rings and double 6 rings are not, and hence it is speculated that post-nucleation crystal growth may involve competition between primary building unit and secondary building unit mediated crystal growth mechanisms.

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.

Detailed Study on the Use of Electrospray Mass Spectrometry To Investigate Speciation in Concentrated Silicate Solutions

Silicate speciation in aqueous solutions containing tetraalkylammonium hydroxide as template is examined by electrospray mass spectrometry. A thorough study has been carried out to define and optimize the conditions of analysis for a highly concentrated reaction solution of inorganic species--in this case, silicate oligomers--by using different ion source designs. The results reveal specific advantages with respect to the detected species. Condensation of monomers leads to oligomeric units that condense further in different reaction steps to larger species. Potential gas-phase reactions that can disturb characterization of the formed silicate species were intensively investigated and characterized. One reaction that plays a key role is the alkoxylation of Si-OH groups that is caused by two different reactions: first, a reaction of methanolate from the solvent and, second, by the involvement of the template salt, tetramethylammonium hydroxide.

Prospects of transition interface sampling simulations for the theoretical study of zeolite synthesis

The transition interface sampling (TIS) technique allows large free energy barriers to be overcome within reasonable simulation time, which is impossible for straightforward molecular dynamics. Still, the method does not impose an artificial driving force, but it surmounts the timescale problem by an importance sampling of true dynamical pathways. Recently, it was shown that the efficiency of TIS when calculating reaction rates is less sensitive to the choice of reaction coordinate than those of the standard free energy based techniques. This could be an important advantage in complex systems for which a good reaction coordinate is usually very difficult to find. We explain the principles of this method and discuss some of the promising applications related to zeolite formation.