Solvent Regeneration in Direct Synthesis of Alkoxysilanes

A procedure was developed for regeneration of Termolan, solvent used in synthesis of alkoxysilanes, involving its treatment with ethanol to remove alkoxysilanes.     

Solvent-free synthesis of reactive inorganic precursors for preparation of organic/inorganic hybrid materials

Prehydrolyzed-condensed precursors containing amino or glycido groups were prepared via sol gel process using various alkoxysilanes in the bulk, without addition of solvent in any step of their preparation. The influence of the experimental set-up, the functionality and ratio of alkoxysilanes, and type of catalyst, on the structure buildup was studied. In the case of amino precursors, the sol–gel process was carried out at weak basic conditions while in the case of glycido precursors the sol–gel process was catalyzed by acid or neutral pH. The sol–gel process was monitored by ²⁹Si NMR in solution and the structure of the prehydrolysed-condensed precursors was characterized by small-angle X-Ray scattering. The systems with high content of tetraethoxysilane led to the fast gel formation. In weak acid medium tetraethoxysilane formed larger, more condensed species as well as small structures (based on Q ₁, Q ₂ and Q ₃ species) with silanol groups. Strong acidic conditions led to the fast formation of insoluble silica particles in liquid (sol) phase containing monomeric alkoxysilanes. The most suitable precursor formulations based on the alkoxysilanes with amino groups, as well as the most efficient set-up, were selected and used to prepare hybrid organic/inorganic networks based on an epoxy matrix. These networks were characterized using dynamic mechanical analysis.     

Tetrafluoroborate salts as site-selective promoters for sol-gel synthesis of mesoporous silica

Tetrafluoroborate ion (BF(4)(-)) serves as a powerful and better-behaved promoter than fluoride ion (F(-)) for hydrolytic condensation of alkoxysilanes, such as tetraethoxy orthosilicate, in aqueous media containing amphiphiles with onium ion headgroups as templates, affording thermally and hydrothermally stable mesoporous silica. According to (19)F NMR spectral profiles, BF(4)(-) is localized on a positive-charged micellar surface, thereby allowing a site-selective growth of the silica framework. The resulting porous silica has an ordered hexagonal structure with a well-developed and thick silicate wall. Even without calcination, the condensation with BF(4)(-) as the promoter progresses to a large extent to furnish a [Si(OSi-)(4)]/([HOSi(OSi-)(3)] + [(HO)(2)Si(OSi-)(2)]) ratio of 6.2, which is greater than that of mesoporous silica formed without BF(4)(-) before (1.5) and even after calcination (3.5) to promote thermal condensation in the solid state.     

硅氧烷的水解-缩聚反应动力学

综述了硅氧烷的水解动力学的主要影响因素(硅氧烷上的基团和水解反应催化剂等)、单组分以及多组分硅氧烷缩聚反应动力学、硅氧烷共缩聚产物结构控制策略。     

On the mechanism derived from kinetic solvent effects of Grignard reactions with silanes

In this communication we present the results of initial kinetic studies in which we have established that alkoxysilanes and chlorosilanes react with Grignard reagents in entirely different ways. The Grignard reaction with alkoxysilanes consists of replacement of a donor molecule at the magnesium centre by silane, followed by a subsequent rearrangement of the complex to the products. Chlorosilanes react without solvent molecule replacement.     

Phase Separation in Sol-Gel System Containing Mixture of 3- and 4-Functional Alkoxysilanes

The phase separation behavior of gelling systems containing the mixture of 3-functional and 4-functional alkoxysilanes has been investigated. The relation between the starting composition and resultant macroporous morphology was examined using tetramethoxysilane (TMOS) and vinyltrimethoxysilane (VTMS) as starting alkoxysilanes, formamide (FA) as an additive, under an acidic condition. Up to TMOS:VTMS molar ratio of 0.5:0.5, the phase relation remained almost unchanged from that of pure TMOS system which exhibits morphology with well-defined co-continuous macropores in a very limited concentration region. On the VTMS-rich side typically TMOS:VTMS = 0.2:0.8, however, the co-continuous macroporous morphology was obtained in a broader composition range than those of either pure TMOS or VTMS system. A dome-like pseudo binary region was obtained with the two-phase region extending toward FA-rich direction. The domain size and pore volume of the gels with macroporous morphology could be controlled by alkoxide:water ratio and total solvent fraction, respectively.     

Kinetics of the grignard reaction with silanes in diethyl ether and ether-toluene mixtures

Kinetics of the reactions of butylmagnesium chloride and phenylmagnesium bromide with tetraethoxysilane and methyltrichlorosilane was investigated in diethyl ether and diethyl ether-toluene mixtures. Replacement of ether by toluene significantly accelerates the reaction with alkoxysilanes, while no effect was found for the reaction with chlorosilanes. We established that the reaction with alkoxysilanes consists of replacement of a donor molecule at the magnesium center by the silane followed by subsequent rearrangement of the complex to products through a four-center transition state. Chlorosilanes react differently without solvent molecule replacement but also via a four-center transition state. Large negative activation entropies are consistent with formation of cyclic transition states. Small activation enthalpy values together with remarkable exothermicity point to early transition states of the reactions.     

Evaluation of physicochemical properties of a new group of SiO2/silane/POSS hybrid materials

Preparation of a new group of hybrid fillers, of SiO₂/silane/oligomeric silsesquioxane type, characterised by specific desirable physicochemical properties, was studied. Synthetic SiO₂ was precipitated by the emulsion method. At first, as a result of improved adhesion between SiO₂ and selected POSS compound, SiO₂ surface was functionalised with alkoxysilanes containing characteristics functional groups. Functionalised SiO₂ was used in the process of hybrid filler preparation according to hydrolytic condensation using methacryl POSS® mixture. To evaluate potential application of such fillers, SiO₂ systems, bifunctionalised using innovative method, were thoroughly characterised to determine their physicochemical properties as well as the effectiveness of functionalisation with silanes and POSS compound. Proposed method of SiO₂ surface modification using selected alkoxysilanes and oligosilsesquioxanes is innovative and gives very promising results. Bifunctionalisation of inorganic fillers with those compounds will substantially extended the range of their applications and probably will lead to improvement of mechanical properties of final polymer composites and reduction in the cost of their production which is the main feature of this research. Copyright © 2013 John Wiley & Sons, Ltd.     

Selective Formation of Alkoxychlorosilanes and Organotrialkoxysilane with Four Different Substituents by Intermolecular Exchange Reaction

Alkoxychlorosilanes are scientifically and industrially important toward preparing silicone and silica as well as preparation of siloxane‐based nanomaterials by stepwise reactions of Si?OR (R=alkyl) and Si?Cl groups. Intermolecular exchange of alkoxy and chloro groups between alkoxysilanes and chlorosilanes (functional group exchange reaction) provides an efficient and environmentally benign route to alkoxychlorosilanes. BiCl₃ as a Lewis acid catalyst can promote the functional group exchange reactions more efficiently than conventional acid catalysts. Higher reactivity has been observed for chlorosilanes with smaller numbers of Si?CH₃ groups and for alkoxysilanes with larger numbers of Si?CH₃ groups. The reaction mechanism is proposed and selective syntheses of alkoxychlorosilanes are demonstrated. These findings also enable us to synthesize an organotrialkoxysilane with four different substituents.     

Surface characteristics and wetting properties of sol–gel coated base paper

Two hybrid coatings synthesized by using alkoxysilanes as precursors in a sol–gel process, differing from each other in terms of the organic components in alkoxysilanes, have been developed to improve the water repellent properties of base paper. The sol–gel‐coated base paper samples were characterized by scanning electron microscopy, atomic force microscopy, confocal laser scanning microscopy, X‐ray photoelectron spectroscopy, time‐of‐flight secondary ion mass spectrometry, and contact angle measurements. The sol–gel coatings were found to clearly change the surface properties of base paper. Thin coating layers were formed on base paper surfaces. The topographical data indicated the formation of discontinuous thin films; the time‐of‐flight secondary ion mass spectrometry analyses confirmed that the coatings were covering the fibres but only partially covered the fibre–fibre intersections. Water and the subsequent heat treatment used as a reference treatment reduced the surface roughness and porosity and slightly changed the surface chemistry of the base paper. The wettability and absorptivity of base paper was clearly reduced by the applied coatings. Copyright © 2011 John Wiley & Sons, Ltd.     

Asymmetric hydrosilylation of ketones catalysed by a chiral rhodium complex

Synthesis of optically active alkoxysilanes by asymmetric hydrosilylation of ketones, in the presence of a chiral phosphine—rhodium complex, is described; optical yields up to 46% are observed.     

Rapid assembly of complex 3D siloxane architectures

Few routes to well-defined 3D silicone structures exist because of their susceptibility to depolymerization/metathesis in the presence of acids or bases. The Lewis acid B(C6F5)3 can be employed to condense hydrosilanes with alkoxysilanes, producing siloxanes and alkanes (R3SiH+R'OSiR' '3 --> R3SiOSiR' '3 + R'H). We demonstrate that balancing the steric demands at both the hydrosilane and alkoxysilanes, and the careful control of reaction conditions, permits clean condensation reactions to occur in the absence of competing metathesis processes. The resulting linear or highly branched siloxane compounds can be rapidly and easily assembled into explicit, complex 3D silicone structures in high yield.     

Avoiding Carbothermal Reduction: Distillation of Alkoxysilanes from Biogenic,Green, and Sustainable Sources

The direct depolymerization of SiO₂ to distillable alkoxysilanes has been explored repeatedly without success for 85 years as an alternative to carbothermal reduction (1900 °C) to Si_met, followed by treatment with ROH. We report herein the base‐catalyzed depolymerization of SiO₂ with diols to form distillable spirocyclic alkoxysilanes and Si(OEt)₄. Thus, 2‐methyl‐2,4‐pentanediol, 2,2,4‐trimethyl‐1,3‐pentanediol, or ethylene glycol (EGH₂) react with silica sources, such as rice hull ash, in the presence of NaOH (10 %) to form H₂O and distillable spirocyclic alkoxysilanes [bis(2‐methyl‐2,4‐pentanediolato) silicate, bis(2,2,4‐trimethyl‐1,3‐pentanediolato) silicate or Si(eg)₂ polymer with 5–98 % conversion, as governed by surface area/crystallinity. Si(eg)₂ or bis(2‐methyl‐2,4‐pentanediolato) silicate reacted with EtOH and catalytic acid to give Si(OEt)₄ in 60 % yield, thus providing inexpensive routes to high‐purity precipitated or fumed silica and compounds with single Si−C bonds.     

Avoiding Carbothermal Reduction: Distillation of Alkoxysilanes from Biogenic,Green, and Sustainable Sources

The direct depolymerization of SiO₂ to distillable alkoxysilanes has been explored repeatedly without success for 85 years as an alternative to carbothermal reduction (1900 °C) to Si_met, followed by treatment with ROH. We report herein the base‐catalyzed depolymerization of SiO₂ with diols to form distillable spirocyclic alkoxysilanes and Si(OEt)₄. Thus, 2‐methyl‐2,4‐pentanediol, 2,2,4‐trimethyl‐1,3‐pentanediol, or ethylene glycol (EGH₂) react with silica sources, such as rice hull ash, in the presence of NaOH (10 %) to form H₂O and distillable spirocyclic alkoxysilanes [bis(2‐methyl‐2,4‐pentanediolato) silicate, bis(2,2,4‐trimethyl‐1,3‐pentanediolato) silicate or Si(eg)₂ polymer with 5–98 % conversion, as governed by surface area/crystallinity. Si(eg)₂ or bis(2‐methyl‐2,4‐pentanediolato) silicate reacted with EtOH and catalytic acid to give Si(OEt)₄ in 60 % yield, thus providing inexpensive routes to high‐purity precipitated or fumed silica and compounds with single Si?C bonds.     

Thalliumverbindungen als Katalysatoren für Umesterungen mit Glycidol und für Esteraustauschreaktionen mit Glycidylacetat

Thallium compounds as catalysts for transesterifications and ester exchange reactions The transesterification of di-, tri- and tetracarboxylic alkyl esters with 2,3-epoxy-propanol was investigated in the presence of various metal compounds as catalysts. It was found that only thallium compounds catalyse the reactions under mild conditions without decomposition of 2,3-epoxypropanol. Many thallium compounds also proved to be very good catalysts for the reactions of 2,3-epoxypropanol with alkoxysilanes and the ester exchange reactions of di-, tri- and tetracarboxylic alkyl esters with 2,3-epoxypropyl-acetate.     

Basicity of siloxanes,alkoxysilanes and ethers toward hydrogen bonding

Thermodynamic constants have been determined for hydrogen bonding of phenol to several siloxanes and alkoxysilanes, as well as to Me₃SiSCMe₃ and Me₃SiSSiMe₃. Alkoxysilanes are slightly weaker bases toward phenol than the isostructural ethers. Unstrained siloxanes have low values of △H for hydrogen bonding of 3.5–3.9 kcal/mol and are therefore distincatly weaker bases than either alkoxysilanes or ethers. Siloxanes with small SiOSi bond angles show increased basicity. Me₃SiSCMe₃ (△H 1.9 kcal/mol) and Me₃SiSSiMe₃ (△ 0.8 kcal/mol) are markedly less basic than dialkyl sulfides. The results are discussed using a molecular orbital model.     

Hybrid silica coatings on polycarbonate: enhanced properties

Hybrid silica coatings based on 3- glycidoxypropyltriethoxysilane (GPTES), tetraethylorthosilicate (TEOS) and colloidal silica were deposited on polycarbonate (PC) by the sol–gel method, in order to obtain a material with enhanced properties with respect to raw PC (mainly scratch resistance, hydrophobicity and density), and consequently reach increased durability. The necessity of performing a N₂-plasma treatment on PC (before coating deposition) was highlighted in order to obtain a good adherence between the coating and the substrate: XPS measurements showed that after treatment, nitrogenous radicals had formed on the PC surface and were able to link covalently with the sol during its deposition. Adherence was also higher when young sols (<8-day-old) were used. Different alkoxysilanes/colloidal silica ratios were tested to optimize the coating resistance: crack resistance of the coatings was found to be greater when the ratio was high. Scratch resistance of raw PC was enhanced as soon as PC was coated, irrespective of the alkoxysilanes/colloidal silica ratio or the sol ageing time. The density of the coatings was assessed by environmental ellipsometric porosimetry and found to be very high. Water contact angle measurements showed that the hydrophobicity of the coatings was inferior to raw PC. The addition in the sol of a small wt% of octyltriethoxysilane (OTES), 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FTES) and silicone surface additive (BYK-306) allowed a significant increase in hydrophobicity of the samples.     

Organic–inorganic hybrid melting gels

Melting gels are a class of organically modified silica gels that are rigid at room temperature, flow at temperature T1 and consolidate at temperature T2 (T2 > T1), when crosslinking is complete. The process of (a) softening, (b) becoming rigid and (c) re-softening can be repeated many times. Mixtures of mono-substituted alkoxysilanes and di-substituted alkoxysilanes have been studied in a systematic way to identify suitable melting gel compositions. The mixtures and the resulting melting gels have been characterized for their softening temperatures and consolidation temperatures. With an interest in using these materials for sealing microelectronics, their physical properties have been measured.     

Erratum to: Organic–inorganic hybrid melting gels

Melting gels are a class of organically modified silica gels that are rigid at room temperature, flow at temperature T1 and consolidate at temperature T2 (T2 > T1), when crosslinking is complete. The process of (a) softening, (b) becoming rigid and (c) re-softening can be repeated many times. Mixtures of mono-substituted alkoxysilanes and di-substituted alkoxysilanes have been studied in a systematic way to identify suitable melting gel compositions. The mixtures and the resulting melting gels have been characterized for their softening temperatures and consolidation temperatures. With an interest in using these materials for sealing microelectronics, their physical properties have been measured.     

Synthesis of boronate-silica hybrid affinity monolith via a one-pot process for specific capture of glycoproteins at neutral conditions

In this study, a boronate-silica hybrid affinity monolith was prepared for specific capture of glycoproteins at neutral pH condition. The monolith was synthesized via a facile one-pot procedure in a stainless steel column by concurrently mixing hydrolyzed alkoxysilanes tetramethoxysilane and vinyltrimethoxysilane, organic monomer 3-acrylamidophenylboronic acid and initiator 2,2′-azobisisobutyronitrile together. The polycondensation of alkoxysilanes and copolymerization of organic monomer and vinyl-silica monolith were carried out successively by reacting at different temperatures. After optimizing the preparation conditions, the resulting hybrid affinity monolith was systematically characterized and exhibited excellent affinity to both cis-diol-containing small molecules and glycoproteins at neutral and physiological pH, including adenosine, horseradish peroxidase, transferrin and ovalbumin. The binding capacity of ovalbumin on monolith was measured to be 2.5 mg g^?1 at pH 7.0. Furthermore, the hybrid affinity monolith was applied to the separation of transferrin from bovine serum sample at a physiological condition. Good repeatability was obtained and the relative standard deviations of retention time were 1.15 and 4.77 % (n?=?5) for run-to-run and column-to-column, respectively.