Silicate Framework Research Articles

Investigation into the long-term alkali resistance of basalt fibers

To more accurately simulate the alkaline environment of basalt fiber in cement-based materials, this study innovatively employs the benchmark cement supernatant as the alkaline solution for the long-term corrosion testing of basalt fiber (BF). Additionally, through mechanical performance testing of mortar containing basalt fiber at different ages, the study reveals the corrosion behavior of basalt fiber in an alkaline environment. The results indicate that basalt fiber undergoes corrosion in the benchmark cement supernatant, where the OH− ions in the solution react with SiO2 in the basalt fibers, thereby disrupting the silicate framework of the fibers and causing pitting corrosion on the fiber surface. The black and white binarization analysis of the SEM images of BF7 after 360d erosion revealed that the corrosion pits covered 31.50 % of the fiber surface area, indicating severe corrosion. Furthermore, the study on the long-term mechanical properties of basalt fiber mortar demonstrated an initial increase in performance, followed by a gradual decline as the curing age progressed. Notably, after 360d of curing, the flexural strength of basalt fiber mortar was lower than that at 30d. For instance, the 360d flexural strength of BF1-0.1 and BF7-0.1 decreased by 18.85 % and 14.54 %, respectively, compared to 30d. This decline is primarily attributed to the corrosion of basalt fibers in alkaline environments, leading to a decrease in their mechanical properties. Therefore, it is essential to enhance the alkali resistance of basalt fibers when used in cement-based materials.

Sulfide and Fe-Ti-P liquid immiscibility in the Ni-Cu-Co ovoid deposit of the Voisey’s Bay complex, Labrador, Canada

AbstractIn the Voisey’s Bay complex, sulfide-matrix breccias developed through the percolation of dense sulfide melt, leading to the displacement of the silicate melt within partially molten silicate-matrix breccias. In these sulfide matrix-breccias, hydrous silicate rims are commonly present at the interface between the sulfide matrix and the silicate framework. Multiple lines of evidence support a magmatic origin of these hornblende-biotite rims, which was largely coeval with the emplacement of the sulfide melt in the magmatic breccias. The formation of the hornblende-biotite rims required the addition of alkalis and water that could not have entirely been sourced from either the sulfide melt or the silicate framework. Through the integration of compositional maps with major and trace element analyses of the main accessory minerals, we propose that the critical components required for the development of the hydrous silicate rims in sulfide-matrix breccias originated from an immiscible Fe-Ti-P melt. Distinct textural and compositional features of apatite, hercynite, ilmenite and magnetite support the presence of small amounts of Fe-Ti-P melt in the sulfide melt. This Fe-Ti-P melt likely formed through melt immiscibility in the early stages of the development of the Voisey’s Bay complex, and was transported in the magma conduits together with the sulfide melt.

Assessment of acid catalytic properties of ferrosilicate MFI zeolite by methanol-to-hydrocarbon conversion.

Four representative synthetic methods were employed to prepare Fe-containing siliceous MFI zeolites. The obtained Fe-MFI zeolites exhibited markedly different catalytic performances in the methanol-to-hydrocarbon (MTH) conversion reaction depending on the type of Fe incorporation within the siliceous framework. The catalytically active Brønsted acid sites were analyzed using pyridine adsorption experiments combined with Fourier transform infrared spectroscopy, providing characteristic signal intensities according to the acid-base interactions. Based on the MTH conversion results and acidity analyses, a suitable synthetic method was identified for the incorporation of Fe within the MFI zeolite framework. However, compared to other catalytic reactions, structural analyses by transmission electron microscopy, ultraviolet-visible spectroscopy, and X-ray absorption spectroscopy were much less conclusive.

Rotherkopfite, KNa2(Fe2+2.5Ti4+1.5)Fe2+(Si4O12)2, a new neptunite-group mineral without essential lithium, from Rother Kopf, Eifel volcanic fields, Germany

Abstract. Rotherkopfite, KNa2(Fe2.52+Ti1.54+)Fe2+(Si4O12)2, is a new member of the neptunite group, from Rother Kopf, Roth, near Gerolstein, Eifel volcanic fields, Rhineland-Palatinate, Germany. It is found in cavities in a quartz–sanidine xenolith embedded in a vesicular alkaline basalt and is associated with fluorophlogopite and an amphibole supergroup mineral that is zoned from potassic-magnesio-fluoro-arfvedsonite on the exterior to potassic-fluoro-richterite in the core. It is presumed to have formed as the result of contact metasomatism of the xenolith by the alkaline basalt melt. Rotherkopfite occurs as brownish-red equant or tabular crystals, up to about 0.2 mm in maximum dimension. The mineral has a light-orange streak, a vitreous lustre, a Mohs hardness of ∼4.5, a brittle tenacity, a curved fracture and a density of 3.20(2) g cm−3. Optically, rotherkopfite crystals are biaxial (+), with α=1.668(5), β=1.678(5), γ=1.720(5) (white light) and 2V(meas) = 53.2(6)°. The empirical formula from electron microprobe analyses, secondary ion mass spectrometry and structure refinement is (K0.87Na0.20)Σ1.07(Na1.99Ca0.01)Σ2.00M1+M2(Fe1.662+Ti1.48Mg0.79Mn0.02)Σ3.95M3(Fe0.642+Li0.16Ti0.15Al0.01)Σ0.96(Si8.00O24). Rotherkopfite is monoclinic with space group C2/c and unit-cell parameters a=16.4599(17), b=12.5457(6), c=10.0487(7) Å, β=115.669(7)°, V=1870.3(3) Å3 and Z=4. The crystal structure (R1=0.0268 for 1324 reflections with I>2σI) is based on two interwoven three-dimensional frameworks: (1) a silicate framework made up of pyroxene-like chains of corner-sharing SiO4 tetrahedra and (2) an octahedral framework made up of chains of edge-sharing metal–oxygen octahedra. The two interwoven frameworks are bound to one another by corner sharing. K and Na are hosted in channels in the combined framework.

The intensification of moisture resistance over MFI zeolite via heteroatom incorporation for diluted CO2 capture

Herein, copper and manganese heteroatoms were incorporated into the siliceous MFI frameworks with high dispersity. When employed in the CO2/N2 binary gas separation, a delicate trade-off for the CO2 adsorption capacity, adsorption heat and moisture resistance could be realized on the heteroatom-incorporated MFI zeolites, especially the Cu-incorporated one. In order to confirm the role that heteroatoms played in CO2 adsorption, the textural analyses were then conduced, which proved that the enhanced microporosity may be at the heart of the promoted CO2 capacity. Next to that, the in situ DRIFTs spectra were continuously tracked during the dynamic adsorption process. Compared to the commercial aluminosilicate analogue, the MFI zeolites with Cu/Mn heteroatoms possesses weaker electrostatic field intensity, which in consequence resulted in their stronger moisture resistance, even in presence of humidity up to 90 RH%. As such, a bottom-up adjustment for the adsorption properties of zeolites in practical environment containing competitive moisture impurity was shown.

Spectroscopic Characterization of Ti Sites in MWW Zeolite in Presence of Hydrogen Peroxide

AbstractTi‐MWW zeolite is a promising catalyst for partial oxidation reactions. In the present work, a Ti‐MWW sample with high TiO2 loading was synthesized. It was revealed that the Ti insertion in the purely siliceous MWW framework mainly occurs during the post treatment washing with HNO3, when the double structure directing agent synthetic method is used. By exploiting carbon monoxide, acetonitrile, pyridine and ammonia as probe molecules in infrared spectroscopy and diffuse reflectance ultraviolet spectroscopy, the acidic behavior of the Ti sites and the interaction of the Ti sites with hydrogen peroxide (H2O2) were revealed. The Ti Lewis acid sites showed stronger acidity than the one observed in Titanium Silicalite‐1 (TS‐1), with MFI framework. This is coherent with previously reported results, suggesting that a significative fraction of the Ti sites in Ti‐MWW are TiOH(SiO)3 instead of Ti(OSi)4. The Ti‐peroxo and ‐hydroperoxo complexes formed upon H2O2 were shown to be more labile than in TS‐1 and they were shown to be completely reversible upon calcination.

Synthesis of Chabazite Zeolite via Faujasite Conversion Using 1‐(1‐Methylpropyl)‐4‐aza‐1‐azoniabicyclo[2.2.2]octane Hydroxide as Cost‐Effective Structure‐Directing Agent

Cu‐exchanged chabazite zeolite is the state‐of‐the‐art used on‐board catalyst for NOx selective reduction by ammonium (NH3‐SCR), and the zeolite is currently generated in the presence of N,N,N‐trimethyl‐1‐adamantylammonium hydroxide, serving as organic structure‐directing agent (OSDA). The expensiveness of this OSDA increases the catalyst production cost, while its low degradability brings about environmental concerns in post treatment of waste water. Here, a new type OSDA, 1‐(1‐methylpropyl)‐4‐aza‐1‐azoniabicyclo[2.2.2]octane hydroxide, is proposed to direct Faujasite‐to‐Chabazite inter‐zeolite conversion and generate chabazite with Si/Al ratio up to 7.2. The obtained zeolite possesses high hydrothermal stability for the high crystallinity, siliceous framework and low concentrations of Si(2Si, 2Al) sites. The Cu‐form catalyst exhibits better catalytic performance in standard NH3‐SCR process for the tiny crystallite size (35‐258 nm) and formation of highly dispersed copper sites. The synthesis offers a new type of OSDA to fabricate zeolites and expands the toolkit to regulate their physicochemical properties.

The role of defects in high-silica zeolite hydrolysis and framework healing

The stability of high silica zeolites under standard laboratory and mild steaming conditions is understood to be due to the lack of hydrophilic Al-O-Si moieties, which can be targeted by water via hydrolysis reactions with relatively low barriers. However, in hydrophobic high-silica and siliceous frameworks, the specific interactions between water and the internal framework sites are incompletely understood. In particular, the behaviour of common internal defects, including partial hydrolysis species and silanol nests are not established, despite their expected role in accelerating the decomposition of the framework. In this work, we utilise machine learning potentials combined with density functional calculations to rigorously sample the hydrolysis processes in siliceous zeolites with topologies CHA and MFI under low water conditions and quantify the effect of defects. Internal silanol defect sites are found to accelerate zeolite decomposition primarily by bypassing the initial, high-barrier hydrolysis step. Subsequent steps proceed with lower reaction barriers. However, all reaction steps are found to be highly activated, and unlikely to occur rapidly at room temperature under conditions of low internal water concentration. Exchange-healing routes, which reverse hydrolysis while incorporating oxygen from water molecules are found to be competitive along the entire hydrolysis pathway in CHA, providing an additional source of stabilization against hydrolytic decomposition.

Exploring the Interfacial Hydrogen Transfer between Pt and the Siliceous Framework and Its Promotional Effect on the Isotope Catalytic Exchange.

Interfacial hydrogen transfer between metal particles and catalyst supports is a ubiquitous phenomenon in heterogeneous catalysis, and this occurrence on reducible supports has been established, yet controversies remain about how hydrogen transfer can take place on nonreducible supports, such as silica. Herein, highly dispersed Pt clusters supported on a series of porous silica materials with zeolitic or/and amorphous frameworks were prepared to interrogate the nature of hydrogen transfer and its promotional effect on H2-HDO isotope catalytic exchange. The formation of zeolitic frameworks upon these porous silica supports by hydrothermal crystallization greatly promotes the interfacial hydrogen bidirectional migration between metal clusters and supports. Benefiting from this transfer effect, the isotope exchange rate is enhanced by 10 times compared to that on the amorphous counterpart (e.g., Pt/SBA-15). In situ spectroscopic and theoretical studies suggest that the defective silanols formed within the zeolite framework serve as the reactive sites to bind HDO or H2O by hydrogen bonds. Under the electrostatic attraction interaction, the D of hydrogen-bonded HDO scrambles to the Pt site and the dissociated H on Pt simultaneously spills back to the electronegative oxygen atom of adsorbed water to attain H-D isotope exchange with an energy barrier of 0.43 eV. The reverse spillover D on Pt combines with the other H on Pt to form HD in the effluent. We anticipate that these findings are able to improve our understanding of hydrogen transfer between metal and silica supports and favor the catalyst design for the hydrogen-involving reaction.

Crystal structures and X-ray powder diffraction data for AAlGe2O6 synthetic leucite analogs (A = K, Rb, Cs)

Leucites are tetrahedrally coordinated silicate framework structures with some of the silicon framework cations that are partially replaced by divalent or trivalent cations. These structures have general formulae A2BSi5O12 and ACSi2O6, where A is a monovalent alkali metal cation, B is a divalent cation, and C is a trivalent cation. There are also leucite analogs with analogous tetrahedrally coordinated germanate framework structures. These have general formulae A2BGe5O12 and ACGe2O6. In this paper, the Rietveld refinements of three synthetic Ge-leucite analogs with stoichiometries of AAlGe2O6 (A = K, Rb, Cs) are discussed. KAlGe2O6 is I41/a tetragonal and is isostructural with KAlSi2O6. RbAlGe2O6 and CsAlGe2O6 are $I\bar{4}3d$ cubic and are isostructural with KBSi2O6.

Migration of zeolite-encapsulated subnanometre platinum clusters via reactive neural network potentials.

The migration of atoms and small clusters is an important process in sub-nanometre scale heterogeneous catalysis, affecting activity, accessibility and deactivation through sintering. Control of migration can be partially achieved via encapsulation of sub-nanometre metal particles into porous media such as zeolites. However, a general understanding of the migration mechanisms and their sensitivity to particle size and framework environment is lacking. Here, we extend the time-scale and sampling of atomistic simulations of platinum cluster diffusion in siliceous zeolite frameworks, by introducing a reactive neural network potential of density functional quality. We observe that Pt atoms migrate in a qualitatively different manner from clusters, occupying the dense region of the framework and avoiding the free pore space. We also find that for cage-like zeolite CHA there exists a maximum in self diffusivity for the Pt dimer beyond which, confinement effects hinder intercage migration. By extending the quality of sampling, NNP-based methods allow for the discovery of novel dynamical processes at the atomistic scale, bringing modelling closer to operando experimental characterization of catalytic materials.

Hydrothermal Conversion of Fly Ash into Monomineralic Zeolite Synthesis for Biodiesel Production

Abstract Fly ashes as a residue from combustion processes of coal in coal-fired power stations can be applied for zeolite formation. The zeolite synthesis has considerable effect on their structure following further catalytic use. The formation of analcime is guided by the operation of the silicate framework in the company of Na+, [Al(OH)4]−, [H2SiO4]2− species, coming from fly ash processing. Here, we have highlighted the importance of conducting the coal fly ash waste management towards monomineralic zeolite synthesis, in the form of powder analcime. It is the first report on powder analcime production without fly ash residues, confirmed by SEM and XRD analysis. The obtained analcime-zeolite was further explored as biodiesel catalyst based on base-catalyzed transesterification process. The effect of analcime catalyst usage on the biodiesel yield was determined with observed 97.2 % conversion efficiency under the concentration of 4%wt at 230 °C, with three times reusable analcime catalysts. Coal fly ash-derived monominerals on a large scale can significantly contribute to the sustainability goals and efficient waste management.

Decorated Vesicles as Prebiont Systems (a Hypothesis).

Decorated vesicles in deep, seafloor basalts form abiotically, but show at least four life-analogous features, which makes them a candidate for origin of life research. These features are a physical enclosure, carbon-assimilatory catalysts, semi-permeable boundaries, and a source of usable energy. The nanometer-to-micron-sized spherules on the inner walls of decorated vesicles are proposed to function as mineral proto-enzymes. Chemically, these structures resemble synthetic FeS clusters shown to convert CO2, CO and H2 into methane, formate, and acetate. Secondary phyllosilicate minerals line the vesicles' inner walls and can span openings in the vesicles and thus can act as molecular sieves between the vesicles' interior and the surrounding aquifer. Lastly, basalt glass in the vesicle walls takes up protons, which replace cations in the silicate framework. This results in an inward proton flux, reciprocal outward flux of metal cations, more alkaline pH inside the vesicle than outside, and production of more phyllosilicates. Such life-like features could have been exploited to move decorated vesicles toward protolife systems. Decorated vesicles are proposed as study models of prebiotic systems that are expected to have existed on the early Earth and Earth-like exoplanets. Their analysis can lead to better understanding of changes in planetary geocycles during the origin of life.

A convenient and user-friendly hydrometallurgical process for preparing porous siliceous adsorbents using ascharite tailings: Mechanism of structural change and evaluation of adsorption properties

Ascharite tailings are discarded by the boron industry due to their low boron content and high magnesium content. Additionally, the large-scale storage of these tailings contributes to resource wastage and environmental pollution. The present study successfully prepared a siliceous adsorbent through the combined use of mechanical and acid activation methods. The tailings underwent acid treatment under mild conditions, leading to the dissolution of magnesium while maintaining the integrity of the silicate framework. This process resulted in the formation of a mesoporous structure primarily composed of slit-shaped pores. The specific surface area reached 234.7 m2 g−1, while the pore volume reached 0.31 cm3 g−1. The maximum adsorption capacity for methylene blue was 0.66 mg m−2 per unit of surface area. The pollutants displayed a flat orientation at equilibrium, with a maximum tilt angle of approximately 53°. The adsorption rate was limited due to the decrease in available adsorption sites. However, mechanical activation increased the density of active silanol sites from 2.76 to 4.34 mg m−2, significantly weakening the shielding effect on other potential sites. Thus, tailings resources can be effectively converted into valuable adsorbent materials using a convenient and controllable hydrometallurgical process.

Impregnation of Fe<sup>3+</sup> into MCM-41 Pores: Effect of Fe<sup>3+</sup> Concentration on the Weight Percent of Fe-Frameworks and Fe-Non-Frameworks

Silica from rice husks (RH) has been used as a starting ingredient in the sonication synthesis of MCM-41 (RH-MCM-41). The impregnation of Fe3+ into RH-MCM-41 pores to produce RH-MCM-41 containing Fe2O3 and Fe (denoted as Fe2O3-Fe-RH-MCM-41) was carried out by examining the effect of various Fe3+ concentrations on the weight percent of Fe-frameworks (Fe3+ that replaces Si4+ in silicate frameworks) and Fe-non-frameworks, i.e., the iron oxide formed outside the silicate frameworks. Fe2O3-Fe-RH-MCM-41 was washed with a 0.01 M HCl solution to remove Fe-non-frameworks from the materials and give Fe-RH-MCM-41 containing Fe-frameworks. The Fe content in Fe2O3-Fe-RH-MCM-41 (Fe-total) and Fe-RH-MCM-41 (Fe-frameworks) for each sample was determined by an AAS (atomic absorption spectrometer), whereas the content of Fe-non-frameworks was calculated from the difference between Fe-total and Fe-frameworks. The XRD (X-ray diffraction) pattern, N2 adsorption-desorption isotherm profile, as well as the TEM (transmission electron microscope) image clearly demonstrate that the RH-MCM-41 exhibits an ordered p6mm hexagonal mesostructure with a large specific surface area and uniform pore size. Based on the weight percents of Fe-frameworks found in each sample, it is clear that the content of Fe-non-frameworks is significantly enhanced compared to that of Fe-frameworks when the more concentrated Fe3+ is used.

Investigations on the polymorphism of K4CaSi6O15 at elevated temperatures.

In the present study, single crystals and polycrystalline material of K4CaSi6O15 were prepared from solid-state reactions between stoichiometric mixtures of the corresponding oxides/carbonates. Heat capacity (C p) measurements above room temperature using a differential scanning calorimeter indicated that two thermal effects occurred at approximately T 1=462 K and T 2=667 K, indicating the presence of structural phase transitions. The standard third-law entropy of K4CaSi6O15 was determined from low-temperature C p's measured by relaxation calorimetry using a Physical Properties Measurement System and amounts to S°(298K)=524.3±3.7 J·mol-1·K-1. For the 1st transition, the enthalpy change ΔH tr1=1.48kJ·mol and the entropy change ΔS tr1=3.25 J·mol-1·K-1, whereas ΔH tr2=3.33kJ·mol-1 and ΔS tr2=5.23 J·mol-1·K-1 were determined for the 2nd transition. The compound was further characterized by in-situ single-crystal X-ray diffraction between ambient temperature and 1063 K. At 773 K, the high-temperature phase stable above T 2 has the following basic crystallographic data: monoclinic symmetry, space group P21/c, a=6.9469(4) Å, b=9.2340(5) Å, c=12.2954(6) Å, β=93.639(3)°, V=787.13(7) Å3, Z=2. It belongs to the group of interrupted framework silicates and is based on tertiary (Q3-type) [SiO4]-tetrahedra. Together with the octahedrally coordinated Ca-cations, a three-dimensional mixed polyhedral network structure is formed, in which the remaining K-ions provide charge balance by occupying voids within the net. The intermediate temperature modification stable between T 1 and T 2 shows a (3+2)-dimensional incommensurately modulated structure that is characterized by the following q-vectors: q1=(0.057, 0.172, 0.379), q2=(-0.057, 0.172, -0.379). The crystal structures of the high- and the previously studied ambient temperature polymorph (space group Pc) are topologically equivalent and show a group-subgroup relationship. The index of the low- in the high-symmetry group is six and involves both, losses in translation as well as point group symmetry. The distortion is based on shifts of the different atom species and tilts of the 4- and 6-fold coordination polyhedra. Actually, for some of the oxygen atoms, the displacements exceed 0.5 Å. A more detailed analysis of the distortions relating to both structures has been performed using mode analysis, which revealed that the primary distortion mode transforms according to the Λ1 irreducible representation of P21/c. However, other modes with smaller distortion amplitudes are also involved.

Observation of dissolution behavior of dredged sediment in NaOH-activated system to evaluate potential activity for precursor usage

Understanding the dissolution mechanism of dredged sediment (DS) is of great importance for the usage of aluminosilicate precursor and sustainable utilization of solid waste. In this paper, a new method was proposed for evaluating the activity of a complicated system with multi-minerals. From the perspective of solid phases, the dissolution of DS detected by TESCAN Integrated Mineral Analyzer (TIMA) was characterized by the reduction of mineral phase area and element variation of individual phases. Meanwhile, the dissolved element in medium was monitored by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) from the aspect of solution. It was found that the dissolution of DS preferentially occurred at the locations with high surface energy and resulted in the reduction of phase area, and the area with low surface energy was also dissolved to a limited extent. Plagioclase and zeolite exhibit higher activity, and phyllosilicates are generally more reactive than framework silicates. The inconsistency between dissolved Al and Si from solid and those into solution reveal that the formation of initial geopolymer unit requires a higher consumption of Si than Al. This study opens up a new way of DS utilization and provides a theoretical basis for the development of a widely obtained and environmentally friendly precursor of geopolymer.

Mössbauer study of the black‐glazed Jian bowl in the Song dynasty

AbstractJian ware, also known as “Tenmoku,” is one of the famous black‐glazed porcelains in China. It was highly coveted in the Song dynasty (960–1279 AD) and was also a tribute to the royal family. The black‐glazed Jian wares are mainly made from iron‐rich clay. In this study, black‐glazed Jian bowl sherds excavated from the Song strata of Jian kiln sites were adopted as test samples. The iron phase and firing techniques of the black‐glazed Jian bowl from the Song dynasty were analyzed and discussed through Mössbauer spectroscopy on the both of body and glaze, together with X‐ray diffraction and scanning electron microscopy. According to the different iron content and the unique iron oxide phase reflected in the Mössbauer spectra, we analyzed the firing atmosphere, temperature, and other conditions of the ancient Jian bowl, as well as the difference of iron phase between the body and the glaze layer due to the collapse of the silicate framework. It provides new ideas for deciphering the firing technology and improving the synthesis of ancient black‐glazed Jian wares.

Multi-Step Nucleation of A Crystalline Silicate Framework via A Structurally Precise Prenucleation Cluster.

Hierarchical nucleation pathways are ubiquitous in the synthesis of minerals and materials. In the case of open framework lattices such as zeolites and metal-organic frameworks, pre-organized multi-ion "secondary building units" (SBUs) have long been proposed as fundamental building blocks of the forming crystals. However, detailing the progress of multi-step reaction mechanisms in going from monomeric species to stable crystals and defining the structures of the intermediate SBUs remains an unmet challenge. Combining in situ nuclear magnetic resonance, small-angle X-ray scattering, and atomic force microscopy, we show that crystallization of the framework silicate, cyclosilicate hydrate, occurs through an assembly of cubic octameric Q38polyanions formed through cross-linking and polymerization of smaller silicate monomers and other oligomers. These Q38 are stabilized by hydrogen bonds with surrounding H2O and tetramethylammonium ions (TMA+). When Q38 levels reach a threshold of~32% of the total silicate species nucleation within these clathrates occurs. Further growth proceeds through the incorporation of [(TMA)x(Q38)·nH2O](x-8) clathrate complexes into step edges on the crystals. These findings provide a clear picture of the multi-step nucleation process by which SBUs build a framework silicate lattice with implications for the synthesis of both functional materials and natural minerals.

Multi‐Step Nucleation of a Crystalline Silicate Framework via a Structurally Precise Prenucleation Cluster

AbstractHierarchical nucleation pathways are ubiquitous in the synthesis of minerals and materials. In the case of zeolites and metal–organic frameworks, pre‐organized multi‐ion “secondary building units” (SBUs) have been proposed as fundamental building blocks. However, detailing the progress of multi‐step reaction mechanisms from monomeric species to stable crystals and defining the structures of the SBUs remains an unmet challenge. Combining in situ nuclear magnetic resonance, small‐angle X‐ray scattering, and atomic force microscopy, we show that crystallization of the framework silicate, cyclosilicate hydrate, occurs through an assembly of cubic octameric Q38 polyanions formed through cross‐linking and polymerization of smaller silicate monomers and other oligomers. These Q38 are stabilized by hydrogen bonds with surrounding H2O and tetramethylammonium ions (TMA+). When Q38 levels reach a threshold of ≈32 % of the total silicate species, nucleation occurs. Further growth proceeds through the incorporation of [(TMA)x(Q38)⋅n H2O](x−8) clathrate complexes into step edges on the crystals.