Monomeric Silica Research Articles

Thermodynamic properties and revised Helgeson-Kirkham-Flowers equations of state parameters of the hydrated and dehydrated monomeric silica species at t = 0.01–600oC, P = 1–3000 bars, ρH2O = 0.35–1.1 g cm−3, and Im= 0 m

The thermodynamic properties and revised Helgeson-Kirkham-Flowers equations of state (r-H-K-F EoS) parameters of the hydrated (Si(OH)4(aq), SiO(OH)3– and SiO2(OH)22−) and corresponding dehydrated (SiO2(aq), HSiO3− and SiO32−) monomeric silica species are important to describe the pH, composition, temperature, and pressure dependence of formation/breakdown reactions of all silicon-bearing compounds globally. Experimental log10 equilbrium constant, K values describing the formation reactions of these hydrated and dehydrated monomeric silica species were therefore compiled from the literature, extrapolated to zero ionic strength by specific ion interaction theory as required and used to derive their thermodynamic properties and r-H-K-F EoS parameters.Consideration of all formation reactions in the same study provides a collective, internally consistent update to the thermodynamic properties and r-H-K-F EoS parameters of the monomeric silica species that are able to provide satisfactory matches to the available experimental log10 K values at t = 0.01–600oC, P = 1–3000 bars, ρH2O = 0.35–1.1 g cm−3, and zero ionic strength. These temperature and pressure limits comfortably bracket t = 0.01–100oC and P = 1–270 bars relevant to the geological disposal of radioactive wastes at depths of up to 1 km.Updates to the thermodynamic properties of silicon-bearing compounds in all of the available geochemical thermodynamic databases are necessary, especially if reaction properties are used or given. Internal consistency between the hydrated and dehydrated species means that the hydrated species alone can be used as entries in geochemical thermodynamic databases.

Silica polymerization and nanocolloid nucleation and growth kinetics in aqueous solutions

Amorphous silica precipitation from aqueous solutions at low temperatures (<100 °C) involves the initial nucleation and growth of silica polymers followed by aggregation, cementation and recrystallization to form opaline silica. In this study, silica polymerization was studied at temperature 23–80 °C, pH 2.48–9.65, and ionic strength 0.01–0.53 mol kg−1. Measured changes in the concentration of monomeric silica (SiO2(mono)) were fit assuming the polymerization reaction is fourth-order with respect to the difference between the concentration of silicic acid (Si(OH)4(aq)) and its concentration at equilibrium (Si(OH)4(aq)eq) with respect to amorphous silica:r=-k4AsmSi(OH)4(aq)-mSi(OH)4(aq)eq4where r is the overall rate of the reaction, As is the specific surface area of the formed polymers, calculated assuming particle-size-dependent solubility and a spherical polymer geometry, and k4 is the rate constant. The rate constant at zero ionic strength is given by:k0=Aexp-Ea/RTaOH-nwhere A is a pre-exponential constant (0.3822 mmol−3 cm−2 s−1), Ea is the activation energy (29.87 kJ mol−1), and n is equal to −0.80. The overall rate constant as a function of ionic strength (I) is given by:k4=k0Imwhere m is equal to 0.65. The rate constant increases by approximately six orders of magnitude from pH of 2 to 10 and one order of magnitude between temperature ∼20 and 80 °C. However, hydrolysis of silicic acid (Si(OH)4(aq)) at pH above ∼8 decreases the relative abundance of Si(OH)4(aq) while increasing the solubility of amorphous silica, thereby decreasing the rate of silica polymerization. The inferred reaction order is consistent with a reaction mechanism rate-limited by the formation of the cyclic tetramer across a wide range of pH. The time-dependence of the specific surface area during polymerization reflects the formation of critical nuclei towards the beginning of the polymerization process and Ostwald ripening of larger polymers towards the end of the polymerization process. The developed rate equation can predict rates of silica polymerization across a wide range of aqueous solution compositions.

Molecular Design of Functional Polymers for Silica Scale Inhibition.

Silica polymerization, which involves the condensation reaction of silicic acid, is a fundamental process with wide-ranging implications in biological systems, material synthesis, and scale formation. The formation of a silica-based scale poses significant technological challenges to energy-efficient operations in various industrial processes, including heat exchangers and water treatment membranes. Despite the common strategy of applying functional polymers for inhibiting silica polymerization, the underlying mechanisms of inhibition remain elusive. In this study, we synthesized a series of nitrogen-containing polymers as silica inhibitors and elucidated the role of their molecular structures in stabilizing silicic acids. Polymers with both charged amine and uncharged amide groups in their backbones exhibit superior inhibition performance, retaining up to 430 ppm of reactive silica intact for 8 h under neutral pH conditions. In contrast, monomers of these amine/amide-containing polymers as well as polymers containing only amine or amide functionalities present insignificant inhibition. Molecular dynamics simulations reveal strong binding between the deprotonated silicic acid and a polymer when the amine groups in the polymer are protonated. Notably, an extended chain conformation of the polymer is crucial to prevent proximity between the interacting monomeric silica species, thereby facilitating effective silica inhibition. Furthermore, the hydrophobic nature of alkyl segments in polymer chains disrupts the hydration shell around the polymer, resulting in enhanced binding with ionized silicic acid precursors compared to monomers. Our findings provide novel mechanistic insights into the stabilization of silicic acids with functional polymers, highlighting the molecular design principles of effective inhibitors for silica polymerization.

Monomeric silica supported interaction of TOSMIC with highly functionalized imines: A green approach to azetidines via ABB‐type cycloaddition reaction

AbstractThe reaction of TOSMIC with highly functionalized imines has been reported. The use of monomeric silica as a catalyst for the reaction has been reported for the first time. The main product of the reported green methodology, formed by the sequential attack of the two TOSMIC units upon the carbon–nitrogen double bond of highly functionalized imines, has been identified as bis(tosylmethyl)azetidine, a four member N‐heterocyclic system. The scope of the reaction concerning functionalized imines and TOSMIC reactivity has been studied and determined, keeping in view the advantage of using TOSMIC as component B for the ABB‐type cycloaddition reactions.

Microbial Fe cycling in a simulated Precambrian ocean environment: Implications for secondary mineral (trans)formation and deposition during BIF genesis

Banded Iron Formations (BIFs) are ancient marine chemical sediments that contain various Fe-bearing minerals such as hematite (Fe2O3), magnetite (Fe3O4), siderite (FeCO3) and a variety of FeII-/FeIII-silicates. The prevailing opinion is that primary Fe(III) (oxyhydr)oxides, such as ferrihydrite (simplified formula of Fe(OH)3), were precipitated from the ocean’s photic zone by marine plankton, and a fraction of these minerals was subsequently transformed into secondary magnetite and siderite by dissimilatory Fe(III)-reducing bacteria (DIRB). However, aside from broad estimates, it is currently unknown what fraction of the primary Fe(III) minerals was sedimented to the seafloor where it was eventually lithified, and what fraction was reduced by DIRB in the water column, thus forming a microbial Fe cycle in the water column. To test this, we conducted Fe cycling experiments with marine phototrophic Fe(II)-oxidizing bacteria and DIRB under conditions mimicking the Precambrian ocean water column with elevated Fe(II) and Si concentrations. We followed secondary mineral formation over three consecutive redox cycles (oxidation followed by reduction) over a time interval of up to 58 days to determine which mineral phases would ultimately have settled as BIF forming sediments. We used wet geochemical methods to follow Fe speciation, measured dissolved silica and volatile fatty acid (VFA) concentrations, determined cell-mineral associations using fluorescence and electron microscopy, and characterized the mineralogy of the precipitates using 57Fe-Moessbauer spectroscopy and X-ray diffraction (XRD). Our results showed that both the absence of silica and an increasing number of Fe cycles favored the formation of more crystalline minerals, such as goethite (α-FeOOH). However, in the presence of high concentrations of monomeric silica, as suggested for ancient oceans (2.2 mM), only short-range ordered (SRO) Fe(III) minerals such as ferrihydrite were observed. These did not transform into the more thermodynamically stable goethite during repeated Fe cycling. Interestingly, no magnetite formed in any of the setups. Instead, increasing Si concentrations favored the formation of increasing quantities of Fe(II) minerals. Microscopy revealed a tight association between microbial biomass and minerals formed. Dissolved silica analysis showed the removal of Si from solution congruent with Fe(II) oxidation and a release of Si during Fe(III) reduction. Together, these results suggest an important role of co-precipitated biomass as well as silica for secondary mineral formation by either constraining crystal growth and/or inhibiting Fe(II)-induced mineral transformation. Overall, our results imply that microbial Fe cycling during settling of primary ferrihydrite through the photic zone in a Precambrian ocean would have resulted in the partial transformation of ferrihydrite into secondary Fe(II) mineral phases in the water column. This would have resulted in the accumulation of mixtures of a ferrihydrite-silica composite and Fe(II) minerals in the initial BIF forming sediments.

Green Manufacturing of Silyl-Phosphate for Use in 3D NAND Flash Memory Fabrication

Three-dimensional NAND flash memory featuring dozens of vertically stacked memory cells is the state-of-the-art technology for most storage platforms. To fabricate three-dimensional (3D) NAND memory, lateral etching of the Si₃N₄ layer over SiO₂ is an essential step that is conducted through a wet etching process using a phosphoric acid-based etchant. Silyl-phosphate or highly selective nitride serves as an etching solution additive to control the SiO₂ layer dissolution rate. However, silyl-phosphate is prepared with an expensive monomeric silica precursor and at high reaction temperatures and generates environmentally harmful byproduct gases, such as HCl, HF, and CH₃OH. This study demonstrates that silyl-phosphate can be prepared using low-cost polymeric silica under a mild reaction temperature by changing the characteristic acidity of phosphoric acid. The possibility of tuning the phosphoric acid acidity was first studied by molecular dynamics simulations, and phosphoric acids with stronger acidity were prepared by the evaporation of water from H₃PO₄ (85%). The concentrated phosphoric acid enabled a fast reaction of polymeric silica and phosphate at a low reaction temperature (80 °C). The obtained silyl-phosphate lowered the SiO₂ layer dissolution rate, thereby yielding a Si₃N₄/SiO₂ layer etching ratio of up to 940. The proposed method offers an environmentally friendly production process for special chemicals used in 3D NAND flash memory fabrication.

Surface roughness affects early stages of silica scale formation more strongly than chemical and structural properties of the substrate

Precipitation of amorphous silica (SiO2) in geothermal power plants has been shown to occur via homogeneous nucleation in the separated water as well as heterogeneous nucleation on pre-existing surfaces. While the factors facilitating homogeneous nucleation are well known, the effect of surface properties on the heterogeneous pathway are less well understood. We investigated the precipitation of amorphous silica onto different surfaces by placing coupons of opal (= mirroring previously deposited silica), volcanic glass (= common reservoir rocks) and corrosion-resistant carbon steel (= geothermal pipelines) inside the pipelines of the Hellisheiði power plant (SW-Iceland) where they were in contact with a silica-supersaturated geothermal liquid (800 ppm SiO2, 60–120 °C) for up to 10 weeks. Our results showed that the similarities in chemical composition and structure of opal and volcanic glass to the amorphous silica were less important in facilitating nucleation than the rough surface of the carbon steel. However, once the nuclei had formed, their growth was independent of the surface material and only controlled by deployment length, temperature and the concentration of monomeric silica in the separated water. Thus, over time a continuous, botryoidal silica layer formed on all coupons. This suggests that surface properties are not crucial in developing better mitigation strategies against amorphous silica scaling.

Silica extraction from hydrothermal heat carrier by coagulation

We have carried out the experiments on coagulation and precipitation of silica from the separate of Verkhne-Mutnovsky geothermal electric power plant (Mutnovskoe hydrothermal field). Silica precipitation from the water solution was conducted in two ways: by adding slaked lime CaO and by electrocoagulation treatment on aluminium electrodes. Silica precipitation with addition of slaked lime was carried out under laboratory conditions at the temperature of 20 and 96 °C. Colloid and monomeric silica concentrations, solution pH, flakes sizes and physical-chemical properties of material precipitated with different quantities of added lime were determined. We also carried out series of experiments on solution treatment of mixed type: addition lime with freshly precipitated flakes, and addition lime with sea water. In the experiments on electric coagulation, current intensity and distance between electrodes were changed. Dependence of silica concentration on treatment duration was studied. Quantities of electric energy and anode aluminium consumed for reducing of silica concentration to a certain level were obtained. The results of our experiments can be used for developing of technological scheme of silica precipitation and utilization optimal for Verkhne-Mutnovsky geothermal electric power plant.

Interfacial chemistry of a fly ash geopolymer and aggregates

The environmental and accessibility concerns associated with the increasing use of river sand in construction have diverted the attention of scientists to alternative materials. Waste glass has been widely investigated as a replacement for sand in geopolymer concretes, and the chemical and mechanical properties of the resulting materials are investigated. Since glass is rich in amorphous silica, it is believed that the surface of glass aggregates can take part in the reaction and make a stronger geopolymer matrix compared to that of sand aggregates. An improvement of 30% in compressive strength was obtained by replacing sand aggregates with glass aggregates. While the mechanical properties support interfacial reaction with glass aggregates, the nature of the chemical bonding structure at the interface has not been revealed. In this research, synchrotron Fourier transform infrared (SR-FTIR) microspectroscopy has been used for the first time to map the distribution of the chemical bonding structures at the geopolymer interface with aggregates. While in-situ FTIR shows the development of geopolymer gel over time, SR-FTIR results show clear differences in gel chemistry around the glass compared to the sand. The study reveals that there is a relatively higher degree of silica-rich gels close to the sand aggregates and a higher extent of geopolymeric gels around glass aggregates. The alkaline content of glass makes the area surrounding the glass aggregates more suitable for maintaining monomeric silica. These small silica species help to develop a higher degree of geopolymer networks in the vicinity of the glass aggregates compared to the sand. This level of visibility is very useful for understanding the behaviour of geopolymer concretes, which is critically important for nano-engineering of materials towards a desirable distribution of the binding gels.

From amorphous to crystalline: Transformation of silica membranes into silicalite-1 (MFI) zeolite layers

The transformation of microporous, amorphous silica membranes into b-oriented silicalite-1 (MFI) zeolite layers via in-situ crystallisation was investigated. The effect of synthesis parameters, such as the type and concentration of the silica precursor, crystallisation time and temperature, on the morphology of silicalite-1 (MFI) zeolite layers was studied. By optimizing these parameters, silicalite-1 zeolite layers were formed from the already-deposited silica layers, which promotes the crystallisation from the surface in the preferred b-orientation. The use of a monomeric silica precursor, which has slower hydrolysis kinetics than a colloidal one, resulted in the formation of zeolite crystals via heterogeneous nucleation on the surface and suppressed the formation of crystal nuclei in the liquid media via homogeneous nucleation, which then would further deposit onto the surface in a random orientation. Lastly, by optimizing the crystallisation time and temperature of the synthesis, thickness, coverage and orientation of silicalite-1 zeolite layers were controlled.

Recovery of oversaturated silica from Takigami and Sumikawa geothermal brines with cationic polymer flocculants to prevent silica scale deposition

Several beaker tests for the recovery of oversaturated silica in geothermal brines were performed with cationic precipitants to prevent silica scale deposition at the Takigami and Sumikawa geothermal plants. The reaction was studied from the start of the experiment (NRT, no retention time) and for 15min retention time (15RT) at 90°C. The silica concentration at 15RT was 200mg/L lower than at NRT. These results indicate that the cationic flocculant reacts with polymeric silica rather than with monomeric silica and that 50mg/L of the cationic flocculant is sufficient to reduce the silica concentration.

Compositional and structural changes of sugarcane evaporator deposits after concentrated sodium hydroxide treatment

Fouling of industrial heat exchangers and evaporators is not only associated with energy management but also reductions in heat transfer coefficients causing loss in productivity. This paper describes the effect of NaOH solution on the removal of sugarcane factory heat exchanger deposits. Apatite, calcium oxalate, calcite, quartz, tricalcium aconitate and non-diffracting materials including amorphous silica are present in the deposits. Fourier Transform Infra-red (FTIR) spectroscopy revealed that monomeric silica species were preferential removed over oligomeric and polymeric species, while 29Si nuclear magnetic resonance (NMR) analysis not only confirmed this, but identified the types of silica units Q0 [Si(OH)4], Q1 [(OH)3*Si(OSi)], Q2 [(OH2*Si(OSi)2], Q3 [(OH)*Si(OSi)3] and Q4 [*Si(OSi)4] present, rearranged, and removed. After chemical cleaning, the scale components became truncated with indentation and contain spherical and hairy-like micro-sized particles. Also, the morphology of tetragonal CaCO4·2H2O was found to be distorted and the proportion of crystalline phases decreased through fragmentation of the scale matrix.

Experimental study on the kinetics of silica polymerization during cooling of the Bouillante geothermal fluid (Guadeloupe, French West Indies)

Despite many studies, our understanding of silica precipitation from natural waters remains limited, in particular for geothermal waters. Here we present a detailed study on the kinetics of silica polymerization as a function of fluid temperature and pH using high-temperature (250-260°C) seawater-derived geothermal fluids as those discharged from the Bouillante geothermal site. We monitored the on-site decrease in monomeric silica concentration (initial SiO2 concentration of about 600mgl−1) with time using the molybdenum blue spectrophotometric method on samples of separated water collected from the high-pressure separator at 167°C and cooled to 25, 50, 75, and 90°C and for pH values ranging from 5 to 8. During all these experiments, only silica polymerization was observed, with the formation of colloidal particles in suspension in the solutions. No scaling of amorphous silica was formed. The collected data were after modeled in order to determine the useful kinetic parameters for predicting and preventing amorphous silica precipitation in the production wells during fluid exploitation in the specific context of Bouillante. Results under the investigated experimental conditions show that the kinetics of silica precipitation is affected more strongly by pH than by temperature change. The reaction in the acidic Bouillante solution begins with a transition period that significantly decreases the kinetics of silica polymerization. Modeling the experimental data indicates that the silica polymerization up to the state of equilibrium is characterized by a 2nd-order kinetic law relative to dissolved silica; this could indicate that the polymerization is controlled mainly by the formation of dimers. However, the first hour of the experiments is better characterized by a 4th-order kinetic law, suggesting more complex polymerization reactions in the initial stage with the formation of nanocolloidal particles containing 3 to 4 monomers. In both cases, the corresponding kinetics rate constant k is linearly dependent on pH for pH between 5 and 7. The activation energy Ea for the overall reaction, calculated using the Arrhenius equation under the considered pH and temperature conditions, ranges between 41±9.8 and 54±9.6kJmol−1. To complete this work, the colloid particles formed at the end of the kinetics experiments were extracted and analyzed by SEM and TEM microscopy, X-ray Diffraction and the BET method. Results show that the size of the colloids increase and their specific surface decrease with increasing pH and are thus dependent on pH. Globally, our work provides a reliable database for understanding silica polymerization kinetics in natural geothermal brines or geologic waters characterized by a near neutral pH and moderate dissolved silica concentrations.

The behavior of silica in geothermal brine from Dieng geothermal power plant, Indonesia

Silica scaling in the Dieng geothermal power plant was investigated experimentally. We conducted two kinds of polymerization experiments to examine the behavior of silica in brine along the canals at production wellpads. Acid-treated and untreated brines were sampled along the canal to understand the effects of acidification on silica polymerization and deposition. Chemical analysis of the Dieng brine indicated high silica and salt concentrations. Silica concentrations of acid-treated brine showed that acidification successfully suppresses the deposition of silica along the canal, preventing it from fulfilling its purpose of depositing as much silica as possible. In contrast, significant silica polymerization occurred along the canal when the brine was not acidified. Batch experiments for silica polymerization at constant temperature indicated that both concentrations of total and monomeric silica decrease quickly in early times followed by gradual decrease with time.

High-Aluminum-Affinity Silica Is a Nanoparticle That Seeds Secondary Aluminosilicate Formation

Despite the importance and abundance of aluminosilicates throughout our natural surroundings, their formation at neutral pH is, surprisingly, a matter of considerable debate. From our experiments in dilute aluminum and silica containing solutions (pH ~ 7) we previously identified a silica polymer with an extraordinarily high affinity for aluminium ions (high-aluminum-affinity silica polymer, HSP). Here, further characterization shows that HSP is a colloid of approximately 2.4 nm in diameter with a mean specific surface area of about 1,000 m2 g-1 and it competes effectively with transferrin for Al(III) binding. Aluminum binding to HSP strongly inhibited its decomposition whilst the reaction rate constant for the formation of the β-silicomolybdic acid complex indicated a diameter between 3.6 and 4.1 nm for these aluminum-containing nanoparticles. Similarly, high resolution microscopic analysis of the air dried aluminum-containing silica colloid solution revealed 3.9 ± 1.3 nm sized crystalline Al-rich silica nanoparticles (ASP) with an estimated Al:Si ratio of between 2 and 3 which is close to the range of secondary aluminosilicates such as imogolite. Thus the high-aluminum-affinity silica polymer is a nanoparticle that seeds early aluminosilicate formation through highly competitive binding of Al(III) ions. In niche environments, especially in vivo, this may serve as an alternative mechanism to polyhydroxy Al(III) species binding monomeric silica to form early phase, non-toxic aluminosilicates.

Enhancement of catalytic properties of interlayer-expanded zeolite Al-MWW via the control of interlayer silylation conditions

Treatment of the zeolitic layered precursor of Al-MWW, so-called Al-MWW(P), with diethoxydimethylsilane (DEDMS) in 1.0 M HNO 3 media gave an aluminosilicate-type interlayer-expanded zeolite MWW (Al-IEZ-MWW) with expanded 12-membered ring (12-MR) micropores. By the interlayer-silylation of Al-MWW(P), the micropore diameter of interlayers with 12-MR supercages enlarged from ca. 7.0 to ca. 8.0 Å, verifying the formation of the interlayer-expanded structure. The interlayer-silylation of Al-MWW(P) under the acidic conditions with 1.0 M HNO 3 caused the penetration of DEDMS molecules into the interlayers, resulting in the formation of monomeric silica puncheons between the MWW layers. Whereas the interlayer-silylation of Al-MWW(P) by using 1.0 M HNO 3 caused remarkable dealumination of the MWW framework, the “two-step” silylation, namely, the first silylation in 0.1 M HNO 3 and the following silylation in 1.0 M NH 3, gave Al-IEZ-MWW retaining most of framework Al. The obtained Al-IEZ-MWW showed a high catalytic activity in the Friedel–Crafts acylation of anisole with acetic anhydride compared to Al-MWW as well as zeolite beta, suggesting that Al-IEZ-MWW is a useful candidate as a solid acid catalyst for the acylation reactions and other acid-catalyzed reaction of bulky molecules.

Inorganic Precipitated Silica Gel. Part 2: Fragmentation by Mechanical Energy

AbstractPrecipitated silica (SiO2) is industrially produced by mixing a silicate solution with acid in a semi‐batch process. Polycondensation of monomeric silica leads to the formation of particles which aggregate and eventually form a particulate gel. However, this is instantly fragmented by the mechanical energy input caused by the stirrer. Shrinkage and compaction of these fragments lead to the final product aggregates. It is the aim of this study to enable tailoring of the structure and size of these product particles via the process parameters. In the present paper this is achieved by varying the stirrer speed. It can be shown that there is an analogy between the behavior of the gel and that of highly viscous liquids. Unexpectedly, however, the fragment size can not be reversibly controlled by the power input. The influence of the process parameters on the strength of the gel has been described in the first part of this series [1].

Inorganic Precipitated Silica Gel. Part 1: Gelation Kinetics and Gel Properties

AbstractPrecipitated silica (SiO2) is industrially produced by mixing a silicate solution with acid in a semi‐batch process. Polycondensation of monomeric silica leads to the formation of particles which aggregate and eventually form a particulate gel. However, this is instantly fragmented by the mechanical energy input caused by the stirrer. Shrinkage and compaction of these fragments lead to the final product aggregates. It is the aim of this study to enable tailoring of the structure and size of these product particles via the process parameters. The present paper describes the influence of the parameters temperature, ionic strength, and composition, which affect the pH, on the properties of unstirred gels regarding their elasticity and thus their fragmentation behavior. Gelation time may be qualitatively estimated solely from the concentration of the feed materials.

Vapor-phase silylation for the construction of monomeric silica puncheons in the interlayer micropores of Al-MWW layered precursor

Vapor-phase silylation of Al-MWW layered precursor with SiMe(2)Cl(2) and following calcination produced an interlayer-expanded MWW structure with 12-membered ring micropores without delamination as a result of the formation of silica puncheons between the MWW sheets; strong acidity of zeolitic nature was retained.

Abstracts of Nippon Dojo-Hiryogaku Zasshi

Abstracts of Nippon Dojo-Hiryogaku Zasshi