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

A review of sodium silicate solutions: Structure, gelation, and syneresis

Sodium silicate solutions, also known as waterglass, have been found to have remarkable utility in a variety of applications. The cumulative weight of evidence from 70 years of varied analysis indicates that silicate solutions consist of a wide range of species, from monomers through oligomers, up to colloids. Moreover, the structure and distribution of these species are greatly dependent upon many parameters, such as solute concentrations, silica to alkali ratio, pH, and temperature. The most interesting and characteristic property of silicate solutions is their ability to form silica gels. Overall, despite extensive research using different spectroscopic and scattering techniques, many questions related to sodium silicate's dynamic structure, stability, polymerization, and gelation remain difficult to answer. The multitude of simultaneous reactions which restructure the silicate species at the atomic scale in response to variation in solution and environmental parameters, makes it difficult to investigate the individual events using only experimental data. Molecular modelling provides an alternative way to study the unknown areas in the aqueous silicate and silica gel systems, generating key insights into the chemical reactions at microscopic length scales. However, sufficient sampling remains a challenge for the practical use of molecular simulation for these systems. Based on both experimental and modelling studies, this review provides a detailed discussion over the structure and speciation of sodium silicate solutions, their gelation mechanism and kinetics, and the syneresis phenomenon. The goal is not only to review the current level of understanding of sodium silicate solutions, silica gels and characterization techniques suitable for studying them, but also to identify the gaps in the literature and open up opportunities for advancing knowledge about these complex systems. We believe that the future direction of research should be toward correlating atomistic, molecular, and meso-scale level details of interactions and reactions in silicate solution and establishing a fundamental understanding of its gelation mechanism and kinetics. We believe that this knowledge could eliminate the "trial and error" approach in manufacturing, and improve structural control in the synthesis of important materials derived from these solutions, such as silica gels and zeolites.

Does Water Enable Porosity in Aluminosilicate Zeolites? Porous Frameworks versus Dense Minerals

Recently identified zeolite precursors consisting of concentrated, hyposolvated homogeneous alkalisilicate liquids, hydrated silicate ionic liquids (HSIL), minimize correlation of synthesis variables and enable one to isolate and examine the impact of complex parameters such as water content on zeolite crystallization. HSIL are highly concentrated, homogeneous liquids containing water as a reactant rather than bulk solvent. This simplifies elucidation of the role of water during zeolite synthesis. Hydrothermal treatment at 170 °C of Al-doped potassium HSIL with chemical composition 0.5SiO₂:1KOH:xH₂O:0.013Al₂O₃ yields porous merlinoite (MER) zeolite when H₂O/KOH exceeds 4 and dense, anhydrous megakalsilite when H₂O/KOH is lower. Solid phase products and precursor liquids were fully characterized using XRD, SEM, NMR, TGA, and ICP analysis. Phase selectivity is discussed in terms of cation hydration as the mechanism, allowing a spatial cation arrangement enabling the formation of pores. Under water deficient conditions, the entropic penalty of cation hydration in the solid is large and cations need to be entirely coordinated by framework oxygens, leading to dense, anhydrous networks. Hence, the water activity in the synthesis medium and the affinity of a cation to either coordinate to water or to aluminosilicate decides whether a porous, hydrated, or a dense, anhydrous framework is formed.

Application of Multinuclear MAS NMR for the in situ Monitoring of Hydrothermal Synthesis of Zeolites

In situ MAS NMR studies on the monitoring of hydrothermal synthesis of zeolites are reviewed. The first part of the review contains information on the experimental techniques used for the in situ NMR studies in static and MAS conditions. In the second part, the main capabilities of the in situ ¹ H, ¹¹ B, ¹³ C, ¹⁴ N, ¹⁹ F, ²³ Na, ²⁷ Al, ²⁹ Si and ³¹ P MAS NMR for the elucidation of the mechanism of hydrothermal synthesis of zeolites are examined and the data on NMR lines identification are summarized. In the last part the main application areas of the techniques are considered and illustrated with examples taken from the mechanistic studies of zeolites A, X, MFI and BEA synthesis. A cross-reference index between the materials studied, the experimental approaches used, the mechanistic information obtained, and the corresponding literature sources is established.

Reaction Kinetics Regulated Formation of Short-Range Order in an Amorphous Matrix during Zeolite Crystallization

The crystallization of zeolites, a disorder-to-order transformation of aluminosilicates, has not been thoroughly understood because the nucleation events in the amorphous matrix are difficult to recognize from the diverse structural changes, especially for the dense hydrogel systems. Therefore, relationships between the synthesis conditions, the generated amorphous species, and the crystallization behavior of zeolites remain unclear. Herein, by comparatively investigating the structural evolution of the aluminosilicate matrix in a dense hydrogel system when different Si reactants (fumed silica and silicate solution) are employed, we demonstrate that the reactivity of the reactants and the kinetics of the condensation reaction is critical to the formation of short-range order in an amorphous matrix, which greatly influences the nucleation frequency of zeolites. It was revealed that an amorphous solid containing plentiful Al-rich four-membered rings and Si-rich six-membered rings could be produced when fumed silica gradually reacted with sodium aluminate solution at 80 °C. It is considered that the interaction between these rings promotes the construction of the essential building units of zeolite X (FAU). In contrast, a complex aluminosilicate matrix was formed immediately when sodium silicate solution was mixed with sodium aluminate solution due to the intense condensation reaction. Furthermore, this complex matrix became more stable when the reactant mixture was hydrothermally treated at 80 °C, which significantly impedes the crystallization process. Aging the reactant mixture at ambient temperature before heating, instead, facilitated the formation of short-range order in the amorphous matrix, which increases the nucleation frequency of zeolites.

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.

Properties of hydrated TiO2 and SiO2 nanoclusters: dependence on size, temperature and water vapour pressure

Nanoscale titania (TiO₂) and silica (SiO₂) are massively produced technologically important nanomaterials used in a wide range of technological applications where nano-titania is the active component (e.g. water splitting, pollution remediation, self-cleaning coatings). Generally, these applications entail contact with water and a degree of hydration of these nano-oxides. Although the hydration of nano-silica has been fairly well studied, the corresponding level of microscopic understanding for nano-titania is severely lacking. Here, using accurate electronic structure calculations we perform a detailed and comprehensive study of the hydration of titania nanoclusters. Firstly, using global optimisation, we establish the most energetically stable structures of a set of (TiO₂)_M(H₂O)_N nanoclusters with sizes ranging through M = 4-16 and with N/M ratios of ≤ 1.0. Using this extensive dataset we investigate how the structures, energy gaps, and thermodynamic stabilities of these species depend on size, temperature and water vapour pressure. To provide a broader chemical context for our study we also provide this full set of data for the respective set of (SiO₂)_M(H₂O)_N nanoclusters which we use to compare and contrast their properties with those of nano-titania. Our broad systematic study thus provides a comparative and foundational reference study for a thorough understanding of how hydration affects the structure, energetics and properties of both nano-SiO₂ and nano-TiO₂.

Cyclodextrin-Driven Formation of Double Six-Ring (D6R) Silicate Cage: NMR Spectroscopic Characterization from Solution to Crystals

Identification and isolation of secondary building units (SBUs) from synthesis media of zeolites still represent a challenging task for chemists. The cage structure anion Si12O3012− known as the double six-ring (D6R) was synthesized from α-cyclodextrin (α-CD) mediated alkaline silicate solutions and conditions of its stability and reactivity in aqueous solution were studied by using nuclear magnetic resonance (NMR) spectroscopy. A single crystal X-ray diffraction (XRD) analysis revealed a novel polymorph of the hybrid complex K12Si12O30·2α-CD·nD2O (n ≈ 30–40), which crystallizes in the orthorhombic C2221 space group symmetry with a = 14.841(4) Å, b = 25.855(6) Å, and c = 41.91(1) Å. The supramolecular adduct of the silicate anion sandwiched by two α-CDs forms a perfect symmetry matching the H-bonding donor-acceptor system between the organic macrocycle and the D6R unit. The driving force of such a hybrid assembly has found to be strongly dependent on the nature of the cation present as large alkali counter ions (K+, Rb+ and Cs+), which stabilize the D6R structure acting as templates. Lastly, we provided the first 29Si MAS NMR measurement of 3Q Si in an isolated D6R unit that allows the verification of the linear correlation between the chemical shift and <SiOSi> bond angle for 3Q Si species in DnR cages (n = 3, 4, 6).

Correlating geminal 2JSi–O–Si couplings to structure in framework silicates

The dependence of a ²⁹Si geminal J coupling across the inter-tetrahedral linkage on local structure was examined using first-principles DFT calculations.

Recent developments in solid-state NMR spectroscopy of crystalline microporous materials

Microporous materials, having pores and channels on the same size scale as small to medium molecules, have found many important applications in current technologies, including catalysis, gas separation and drug storage and delivery. Many of their properties and functions are related to their detailed local structure, such as the type and distribution of active sites within the pores, and the specific structures of these active sites. Solid-state NMR spectroscopy has a strong track record of providing the requisite detailed atomic-level insight into the structures of microporous materials, in addition to being able to probe dynamic processes occurring on timescales spanning many orders of magnitude (i.e., from s to ps). In this Perspective, we provide a brief review of some of the basic experimental approaches used in solid-state NMR spectroscopy of microporous materials, and then discuss some more recent advances in this field, particularly those applied to the study of crystalline materials such as zeolites and metal-organic frameworks. These advances include improved software for aiding spectral interpretation, the development of the NMR-crystallography approach to structure determination, new routes for the synthesis of isotopically-labelled materials, methods for the characterisation of host-guest interactions, and methodologies suitable for observing NMR spectra of paramagnetic microporous materials. Finally, we discuss possible future directions, which we believe will have the greatest impact on the field over the coming years.

An overview of the fundamentals of the chemistry of silica with relevance to biosilicification and technological advances

Biomineral formation is widespread in nature, and occurs in bacteria, single-celled protists, plants, invertebrates, and vertebrates. Minerals formed in the biological environment often show unusual physical properties (e.g. strength, degree of hydration) and often have structures that exhibit order on many length scales. Biosilica, found in single-celled organisms through to higher plants and primitive animals (sponges), is formed from an environment that is undersaturated with respect to silicon, and under conditions of approximately neutral pH and relatively low temperatures of 4-40 °C compared to those used industrially. Formation of the mineral may occur intracellularly or extracellularly, and specific biochemical locations for mineral deposition that include lipids, proteins and carbohydrates are known. In most cases, the formation of the mineral phase is linked to cellular processes, an understanding of which could lead to the design of new materials for biomedical, optical and other applications. In this contribution, we describe the aqueous chemistry of silica, from uncondensed monomers through to colloidal particles and 3D structures, that is relevant to the environment from which the biomineral forms. We then describe the chemistry of silica formation from alkoxides such as tetraethoxysilane, as this and other silanes have been used to study the chemistry of silica formation using silicatein, and such precursors are often used in the preparation of silicas for technological applications. The focus of this article is on the methods, experimental and computational, by which the process of silica formation can be studied, with an emphasis on speciation.

Crystal growth of nanoporous metal organic frameworks

Nanoporous metal organic frameworks (MOFs) form one of the newest families of crystalline nanoporous material that is receiving worldwide attention. Successful use of MOFs for application requires not only development of new materials but also a need to control their crystal properties such as size, morphology, and defect concentration. An understanding of the crystal growth processes is necessary in order to aid development of routes to control such properties of the crystallites. In this Perspective article we aim to provide a short overview of the current work and understanding concerning the nucleation and growth processes of nanoporous MOFs and how this work may be expanded upon to further our comprehension of this subject. We also focus heavily on in situ studies that provide real time information on the developing materials and generally provide the most conclusive findings on the processes under investigation.

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.

Modelling nano-clusters and nucleation

We review the growing role of computational techniques in modelling the structures and properties of nano-particulate oxides and sulphides. We describe the main methods employed, including those based on both electronic structure and interatomic potential approaches. Particular attention is paid to the techniques used in searching for global minima in the energy landscape defined by the nano-particle cluster. We summarise applications to the widely studied ZnO and ZnS systems, to silica nanochemistry and to group IV oxides including TiO(2). We also consider the special case of silica cluster chemistry in solution and its importance in understanding the hydrothermal synthesis of microporous materials. The work summarised, together with related experimental studies, demonstrates a rich and varied nano-cluster chemistry for these materials.

H-Bond interactions between silicates and water during zeolite pre-nucleation

The relative strength of water-water, water-silicate and silicate-silicate interactions are studied, in order to explain the low solubility of the monomer (Si(OH)(4)), and determine the degree of dispersion of silicate clusters in solution during the hydrothermal synthesis of zeolites. We will show how the hydrogen bond interactions between water and monomeric silicate species are similar to that in pure water, whilst monomer-monomer interactions are stronger. However, when larger silicate species are also considered we find the relative hydrogen-bonding strength to follow: water-water < silicate-water < silicate-silicate. The effects of pH are also considered. The implications of the relative strength of these interactions on the formation of larger silicate species, leading to zeolite pre-nucleation, are discussed.

29Si NMR in solid state with CPMG acquisition under MAS

A remarkable enhancement of sensitivity can be often achieved in 29Si solid-state NMR by applying the well-known Carr-Purcell-Meiboom-Gill (CPMG) train of rotor-synchronized pi pulses during the detection of silicon magnetization. Here, several one- and two-dimensional (1D and 2D) techniques are used to demonstrate the capabilities of this approach. Examples include 1D 29Si{X} CPMAS spectra and 2D 29Si{X} HETCOR spectra of mesoporous silicas, zeolites and minerals, where X=1H or 27Al. Data processing methods, experimental strategies and sensitivity limits are discussed and illustrated by experiments. The mechanisms of transverse dephasing of 29Si nuclei in solids are analyzed. Fast magic angle spinning, at rates between 25 and 40 kHz, is instrumental in achieving the highest sensitivity gain in some of these experiments. In the case of 29Si-29Si double-quantum techniques, CPMG detection can be exploited to measure homonuclear J-couplings.

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.

Selective NMR Measurements of Homonuclear Scalar Couplings in Isotopically Enriched Solids

Scalar (J) couplings in solid-state NMR spectroscopy are sensitive to covalent through-bond interactions that make them informative structural probes for a wide range of complex materials. Until now, however, they have been generally unsuitable for use in isotopically enriched solids, such as proteins or many inorganic solids, because of the complications presented by multiple coupled but nonisolated spins. Such difficulties are overcome by incorporating a z-filter that results in a robust method for measuring pure J-coupling modulations between selected pairs of nuclei in an isotopically enriched spin system. The reliability of the new experimental approach is established by using numerical simulations and tested on fully (13)C-labeled polycrystalline L-alanine. It is furthermore shown to be applicable to partially enriched systems, when used in combination with a selective double-quantum (DQ) filter, as demonstrated for the measurement of (2)J((29)Si-O-(29)Si) couplings in a 50% (29)Si-enriched surfactant-templated layered silicate lacking long-range 3D crystallinity. J-coupling constants are obtained with sufficient accuracy to distinguish between different (29)Si-O-(29)Si pairs, shedding insight on the local structure of the silicate framework. The new experiment is appropriate for fully or partially enriched liquid or solid samples.