Alkyl Silicates Research Articles

Enhancing Electron Donor-Acceptor Complex Photoactivation with a Stable Perylene Diimide Metal-Organic Framework.

Electron donor-acceptor complexes are commonly employed to facilitate photoinduced radical-mediated organic reactions. However, achieving these photochemical processes with catalytic amounts of donors or acceptors can be challenging, especially when aiming to reduce catalyst loadings. Herein, we have unveiled a framework-based heterogenization approach that significantly enhances the photoredox activity of perylene diimide species in radical addition reactions with alkyl silicates by promoting faster and more efficient electron donor-acceptor complex formation. Besides offering broad substrate scope in alkene hydroalkylation, the newly developed heterogeneous photocatalysis substantially improves the catalyst turnover numbers in comparison to previous homogeneous photocatalytic systems and demonstrates outstanding catalyst recyclability. These research findings pave the way for the advancement of various efficient and practical organic transformations using framework-supported organocatalysts.

Unprecedented single-electron-transfer reduction-based N → C acyl migration reactions of imides enabled by redox-neutral photocatalysis

N → C acyl migration: in the absence of a base, redox-neutral photocatalyzed acyl migration reactions have been realized via the reaction of N-vinylimides with alkyl radicals derived from alkyl silicates.

Regime Switch in the Dual-Catalyzed Coupling of Alkyl Silicates with Aryl Bromides.

Metallaphotoredox catalyzed cross-coupling of an arylbromide (Ar-Br) with an alkyl bis-catecholato silicate (R-Si⊖) has been analyzed in depth using a continuum of analytical techniques (EPR, fluorine NMR, electrochemistry, photophysics) and modeling (micro-kinetics and DFT calculations). These studies converged on the impact of four control parameters consisting in the initial concentrations of the iridium photocatalyst ([Ir]0), nickel precatalyst ([Ni]0) and silicate ([R-Si⊖]0) as well as light intensity I0 for an efficient reaction between Ar-Br and R-Si⊖. More precisely, two regimes were found to be possibly at play. The first one relies on an equimolar consumption of Ar-Br with R-Si⊖ smoothly leading to Ar-R, with no side-product from R-Si⊖ and a second one in which R-Si⊖ is simultaneously coupled to Ar-Br and degraded to R-H. This integrative approach could serve as a case study for the investigation of other metallaphotoredox catalysis manifolds of synthetic significance.

Photocatalytic Cross-Coupling of Alkyl Silicates with Aryl or Alkenyl Bromides

Photocatalytic Cross-Coupling of Alkyl Silicates with Aryl or Alkenyl Bromides

Radical addition-polar termination cascade: efficient strategy for photoredox-neutral-catalysed cyclopropanation and Giese-type reactions of alkenyl N-methyliminodiacetyl boronates

Silicate meets boronate: efficient cyclopropanation and Giese-type reactions of alkenyl N-methyliminodiacetyl boronates with alkyl silicates have been realized via a photoredox-neutral-catalysed radical-polar crossover process.

Directed Silica Co-Deposition by Highly Oxidized Silver: Enhanced Stability and Versatility of Silver Oxynitrate

Novel silver compounds in higher oxidation states, Ag (II) and Ag (III), have emerged as desirable alternatives to existing forms of antimicrobial silver compounds. Offering enhanced efficacy without sacrificing biocompatibility. Unique physiochemical characteristics associated with higher oxidation state silver confer desirable therapeutic traits. However, these same characteristics create challenges in terms of long-term stability and chemical compatibility with conventional biomedical materials. Core-shell methodologies, utilizing silica as a mesoporous or amorphous shell, have been adopted to enhance the stability of reactive active ingredients or cores. These methodologies commonly utilize controlled condensation of silicic acids in non-aqueous media by way of hydrolyzing alkyl silicates: the Stöber process or modified processes thereof. However, these strategies are not conducive to cores of higher oxidation state silver wherein hydroxyl organic precursors and by-products are incompatible with strong oxidizing agents. Addressing these challenges, we present a strategy herein for the preparation of a self-directed silver oxynitrate-silica, Ag7NO11:SiO2, framework. The method described utilizes pH gradients generated from the oxidation reaction of soluble silver, Ag (I), with a strong oxidizing agent/alkaline silicate media to facilitate spatial control over the protonation and subsequent condensation of silicic acid from aqueous solution. The resulting Ag7NO11:SiO2 framework confers enhanced long term and thermal stability to silver oxynitrate without impairing aqueous degradation profiles or subsequent antimicrobial and antibiofilm activities.

Influence of dirt and coating deterioration on the aging of solar reflectance of high-reflectance paint

Cool-roof coatings have received considerable attention worldwide as a solution for the “heat island” effect in urban areas. The solar reflectance of roofs coated with high-reflectance paint is reduced by dirt. We continuously measured the solar reflectance of a roof coated with high-reflectance paint with and without a self-cleaning function for more than two years. The solar reflectance of self-cleaning paint decreased by 0.5% per month over two years, due to the effect of dirt. The solar reflectance of conventional paint decreased by 4% per month over four months after coating and then reached a value similar to that for the self-cleaning paint for two years due to deterioration. With a slight decrease in solar reflectance, the self-cleaning function is effective in mitigating the heat island effect and obtaining continuous energy-savings. We applied the alkyl silicate system as self-cleaning for high-reflectance paint, the solar reflectance was measured continuously at 1 minute intervals, and the actual state of the fluctuation in reflectance over the short and long terms was analyzed with the influence of the altitude of the sun. The deterioration of the coating was also investigated by gloss retention and chalking test.

Study on aging of solar reflectance of the self-cleaning high reflectance coating

The self-cleaning system consisting of waterborne two-component acrylic silicon polymers and alkyl silicate is used as a highly effective self-cleaning coating. This system is useful in the formation of a self-cleaning topcoat that effectively maintains the high solar reflectance of a cool roof after coating. This capability was confirmed by an outdoor exposure test that compared self-cleaning and non-self-cleaning coatings. The energy savings of the self-cleaning coating for cooling were estimated via a cooling load calculation.

Effect of silica nanoparticles in mixed matrix membranes for pervaporation dehydration of acetic acid aqueous solution: plant-inspired dewatering systems

Clean technology in dehydration of acetic acid using pervaporative separations inspired by plant dewatering systems is studied in this work. The novel polyphenylsulfone-based membranes, modified with silica nanoparticles, were used to dehydrate a binary mixture of acetic acid and water. The Stöber process was applied for the synthesis of monodispersed silica colloids by hydrolysis of alkyl silicates and condensation of silicic acid in alcoholic solution. The surface free energy of the synthesized silica particles was altered through a silanization treatment with hexamethyldisilazane to convert the surface hydroxyl groups to trimethylsilyl[–Si(CH3)3] groups. The synthesized particles and the modified polyphenylsulfone based membranes were characterized using dynamic light scattering, scanning electron microscopy, atomic force microscopy, Fourier-transform infrared and macroscopic contact angle measurements. Pervaporation was used for the dehydration of 70 wt% acetic acid at 70 °C using nine different membrane variations. Inspired by the natural dewatering in plants containing silica, the performance of the synthetic silica nanoparticles filled-membranes was investigated. Similar to plants, the incorporation of silica nanoparticles in these membranes significantly affected the dewatering process. It was found that at higher concentrations of silica, an improvement of membrane properties was observed, particularly in terms of selectivity. Besides that, the hydrophilicity and surface roughness increases when sufficient amounts of silanols groups were present on the membrane surface. It was concluded that more hydrophilic silica should be first produced and incorporated into the membranes for efficient removal of water from acetic acid. This can be achieved by controlling the composition ratio of ammonium hydroxide, water and ethanol for the nanoparticle fabrication. The effect of different nanoparticle types on membrane morphology and pervaporation performance is also discussed in detail in this work.

Kinetic Studies of Reactions of Hydroxyporphyrins on Silicate Surface

Construction of the monolayer film of semiconductive and redox-active porphyrins on the metal oxides such as silicate or titanates attracts interests for possible applications of the process in fabricating organic electronic devices, organic catalysts, and organic-inorganic hybrid materials. Porphyrins bearing linker functional groups such as hydroxy or bromo groups in the peripheral meso-phenyl groups were reacted with the surface silanol groups of silicate glass at 80 to 240 oC in a solid-solid reaction to obtain the monolayer film of porphyrin on the silicate glass. The rates of the reaction were determined by visible spectroscopic studies and they decreased in the order: primary alcohol > primary bromide ~ tertiary alcohol. In addition to the structures of the linker groups, melting points of porphyrins also affect the reactivity of the porphyrin. Porphyrins with long alkyl chains reacted faster than those without long alkyl chains, because the long alkyl groups would facilitate molecular motion to reach the transition state of the reaction. Hydrolysis of the porphyrin monolayer prepared from primary alcohol in 1 M aq. HCl at 50 oC proceeded in a similar rate to that prepared from tertiary alcohol. Base catalyzed hydrolysis in 1 M aq. NH3 at 30 oC of the porphyrin monolayer prepared from tertiary alcohol proceeded 20 times slower than that prepared from the primary alcohol. In the literature, it is well established that the hydrolysis of the silicate ester bond proceeds via the Si-O bond cleavage, since the silicon atom can form a penta-coodinated structure, and much more reactive than carbon atom toward a substitution reaction. Hydrolysis of the porphyrin monolayer prepared from the tertiary alcohol in H2 18O afforded 18O-labelled tertiary alcohol, indicating that there was a covalent bond between the porphyrin and silicate glass, and the C-O bond was cleaved in the acid catalyzed hydrolysis reaction of the tertiary alkyl silicate ester on the silicate glass.

Development of a New Adhesive Reagent for Shell Mold Having High Adhesive Strength

A new adhesive reagent has been developed for the fabrication of shell mold with the adhesive strength under high temperature. The new adhesive reagent was prepared by mixing inorganic precursors of alkyl silicate and sodium alkoxide, starting particle composed of silica (SiO2) and alumina (Al2O3) used as substrate in the conventional mold process, and colloidal silica. Inorganic precursor was used to increase the adhesive strength of mold, resulting from the glass phase generated by the sol-gel reaction and glassification of precursors. In addition, colloidal silica and starting particle were added to control the viscosity of adhesive reagent, inducing the availability of adhesive reagent. In the case of mold sample adjoined by the new adhesive reagent the glass phase is well formed at the interface between adhesive reagent and substrate, compared with those made with the conventional adhesive reagents such as a mortar and a sand bond. This leads to the increase in the adhesive strength of shell mold, preventing the collapse of the mold at a high temperature during casting process. Therefore, the new adhesive reagent could desirably prepare shell mold without an additional process to enhance the adhesive strength in the conventional casting process.

Synthesis and Dispersion Stabilization of Indium Tin Oxide Nanopowders by Coprecipitation and Sol-Gel Method for Transparent and Conductive Films

Indium tin oxide (ITO) nanopowders were synthesized by coprecipitation and the sol-gel method to prepare a stable dispersion of ITO nano-colloid for antistatic coating of a display panel. The colloidal dispersions were prepared by attrition process with a vibratory milling apparatus using a suitable dispersant in organic solvent. The ITO coating solution was spin-coated on a glass panel followed by the deposition of partially hydrolyzed alkyl silicate as an over-coat layer. The double-layered coating films were characterized by measuring the sheet resistance and reflectance spectrum for antistatic and antireflective properties.

Low-temperature immobilization of liquid radioactive waste using silica from concentrated hydrothermal solutions

Experiments on vitrification of simulated liquid radioactive waste (LRW) were performed. To obtain solid glass-like phases, LRW components were treated with aqueous-alcoholic hydrochloric acid solutions containing hydrolyzates of alkyl silicates and aqueous silica sols obtained by membrane concentration of a natural hydrothermal solution. The pH of the mixture was 1.5–4. The characteristics of the solid samples obtained were studied by X-ray phase analysis, thermogravimetry, and scanning electron microscopy. A procedure was developed for low-temperature (5–60°C) immobilizaiton of low-level LRW using aqueous silica sols.

Colloidal indium tin oxide nanoparticles for transparent and conductive films

Nanosized colloidal indium tin oxide (ITO) dispersion was prepared for electrically conductive and transparent coating materials. A titanate coupling agent, isopropyl tri(N-ethylenediamino)ethyl titanate, was chosen as a dispersant for the stabilization of ITO nanoparticles in organic solvent. ITO sol was deposited on a cathode ray tube panel for antistatic or electromagnetic shielding purposes, and alkyl silicate was used for the formation of an antireflective over-coat layer. The resulting double-layered coating showed low sheet resistance, which satisfied semi-TCO regulation and low reflectance of visible light. To control the electrical and optical properties of the coating layer, the effects of secondary particle size of ITO aggregates and the dispersant concentration of ITO sol were studied. The stability of ITO sol was estimated by measuring the particle size as a function of the storage days and the aggregation of colloidal ITO dispersion with storage day was explained by depletion flocculation.

Deposition and Integration of a Novel Ultra-Low k (2.2) Material

AbstractIncreasing demands for faster chip speed and reduced power consumption are driving the semiconductor industry to develop insulating layers with lower dielectric constants. As the dielectric constant of a material is reduced, however, it becomes increasingly difficult to achieve the mechanical strength required to manufacture a multilevel interconnect. A new route to the synthesis of mesoporous silica has been demonstrated on 200 mm wafers. Silicate precursors dissolved in supercritical CO2 are infused into a block copolymer film. The polymer is then removed, but the resulting porous SiO2 replicates its ordered structure, enhancing the strength of the network. Incorporation of alkyl silicates further improves the film properties. Post-treatment to cap residual silanol groups renders the surface of the film hydrophobic and stabilizes it to air exposure. By appropriate choice of the block copolymer and other process parameters, the pore size and density can be varied and k values as low as 1.8 can be achieved. For a film with a dielectric constant of 2.25, the pore size is ∼4 nm. The hardness and modulus are 1.1 GPa and 7.8 GPa, respectively, as measured by nanoindentation. Four-point bend measurements yield fracture energies of 9.8 J/m2. More importantly, the film can withstand chemical mechanical planarization (CMP) using standard oxide polishing conditions.

Wettability alteration by aging of a gel placed within a porous medium

Wettability has a strong impact on many oil recovery processes. However, the determination and restoration of reservoir wettability in the laboratory is still problematic, due to the difficulty in reproducing reservoir conditions. Wettability changes are usually achieved in the laboratory by aging the rock samples with selected crude oils, or by chemical treatment of the solid surfaces. The objective of this work is to study the effects of wettability alteration, by aging silica gels, placed within a porous medium, on the ability of oil and water to flow through that medium. A series of experiments was performed to visualise oil and water flow through a porous medium containing silica gel. The experiments were conducted in transparent glass models in which the flow events and the fluid distribution were clearly visible and could be recorded. To form the gel in situ, an alkyl silicate gelant was used, which is soluble in an organic phase, and which, when in contact with water, reacts to convert the water phase into a gel. The newly formed silica gel is water-wet, but when aged in paraffin, interesting changes are observed over time: the surface wettability changes to finally become oil-wet, while small cracks develop within the gel structure. The gel was placed in both strongly water-wet and oil-wet models, which, combined with the gel behaviour, generated a wide range of flow and wettability conditions, i.e., water-wet, intermediate-wet, mixed-wet and oil-wet. The visual observations are complemented by measurements of end-point relative permeabilities, allowing the demonstration of the effects of wettability upon relative permeability. The fluid distribution at pore level and the hierarchy of wetting determine the flow characteristics. The high values of both oil and water relative permeabilities correspond to the strongly wetting conditions, which agrees with the traditional description of oil–water distribution within porous media. On the other hand, the intermediate-wet case has the lower oil and water relative permeability, with the oil permeability proportional to the fraction of the surface that is oil-wet. The mixed-wet cases have intermediate oil and water permeability, but show an increase in water relative permeability as the amount of water-wet surface decreases. These results have important implications in the design and performance of wellbore gel treatments, which are used to modify water and oil flow behaviour.

Micrqporous Silica Gels from Alkylsilicate-Water Two Phase Hydrolysis

ABSTRACTMicroporous silica gels have been synthesized through a nano-particulate sol-gel route. These gels have uniformly distributed and extremely small pores(< 15Â in diameter). Hydrolysis and condensation reactions leading to these gels were carried out in an alkyl silicate-water (ammonia) two phase system. These reactions took place at the alkyl silicate droplet-water interfacial boundary. No alcohol was added. A clear, stable and uniformly distributed colloidal silica suspension having an average particle size less than 6 nm was prepared by this method. Fast hydrolysis, slow condensation and low solubility all contribute to a high supersaturation level and result in the formation of small particles. This process is consistent with classic nucleation theory. When the particles are produced under acidic rather than under basic reaction conditions, smaller particles are formed due to the slower condensation rate and lower solubility of these silica particles in acidic conditions. At the same pH, alkylsilicates having smaller alkyl groups react faster with water leading to smaller primary particles. Homogeneous nucleation conditions are achieved when the water/alkylsilicate ratio is high.

Laser Raman spectra and free and restricted rotation in phenyl silicates

Tetraphenyl silicate and related phenyl-alkyl silicates, an interesting class of compounds with applications in high vacuum technology, belong to the class of non-rigid molecules where free or restricted internal rotation of the substituent phenyl and alkyl groups provide several possible relative orientations. This may result in the formation of different phases under different conditions, providing results of fundamental interest in molecular dynamics. We have recently studied the laser Raman and infrared spectra of some of these compounds including tetraphenyl, phenyl triphenoxy, alkyl triphenyl, diphenyl dialkyl, phenyl trialkyl, and tetra alkyl silicates. Changes are observed in the spectra of these molecules with temperature, and these indicate the onset of restricted rotation at lower temperatures. These spectral changes and assignments are discussed in the paper.

Novel Wet-Process Silica Prepared from Alkyl Silicates. Part II: Performance in Reinforcing Silicone Elastomers

Abstract The wet-process hydrophobic (WPH) silica process provides a technology which allows control of surface area, structure, and surface treatment. These are the primary factors which determine the extent to which a silica can reinforce silicone elastomers. Thus, it is possible to tailor the silica to achieve specific desirable elastomer property profiles. Reinforcing capability of WPH silica is greater than that of pyrogenic silica. The unique method of passivating the silica by introducing the treating agent prior to formation of the silica eliminates the need for separate processing steps to passivate the silica and produces well treated silica, as evidenced by the exceptional shelf stability of silicone bases containing WPH silica. The optical properties of silicone elastomers reinforced with WPH silica is described in Part III of this series of papers.

Novel Wet Process Silica Prepared from Alkyl Silicates. Part III: Use in Silicone Elastomers for Optical Applications

Abstract The first two papers of this three-part series of papers dealing with a novel wet-process hydrophobic (WPH) silica technology covered the silica synthesis (Part I) and reinforcement performance (Part II) respectively. The WPH silica was highly reinforcing and provided optically clear compositions as well. The first optically clear, high consistency silicone elastomer was developed in the midnineteen fifties by Polmanteer et al., using different technology from that described in this paper. Optical clarity was accomplished in the earlier work by matching the refractive indices of the filler and polymer at 25°C, making them isorefractive at this specific temperature. This paper will discuss the unique features of this new silica technology as it relates to optically clear silicone elastomers. The term, optically clear, in the context of this paper refers to an elastomeric material, 2.54 mm in thickness, that exhibits less than 4% haze, and more than 85% transmission. It will be shown that, when using the older technology based on isorefractive compositions, optical clarity is reduced with temperature change from the isorefractive temperature, due primarily to an increase in haze. The new silica technology eliminates this problem and effectively broadens the application temperatures for optically clear, thermally stable silicone elastomers.