一种新型有机-无机杂化介孔碱性催化材料的合成与表征

通过直接合成方法, 制备了胺基功能化的HMS型有机无机杂化介孔碱性催化材料(Amx-HMS).采用粉末X射线衍射分析、透射电镜、氮气吸附-脱附、29Si固体核磁共振、 红外光谱和元素分析等方法对合成材料进行了表征. 通过典型的2'-羟基苯基甲基酮和苯甲醛缩合制备黄烷酮的反应对其碱催化活性中心进行了表征.     

Preparation of co‐polymethyl(alkoxy)siloxanes by acid‐catalyzed controlled hydrolytic copolycondensation of methyl(trialkoxy)silane and tetraalkoxysilane

Polymethyl(alkoxy)siloxane copolymers, poly(MTES‐co‐TEOS), and poly(MTMS‐co‐TMOS), are prepared by acid‐catalyzed controlled hydrolytic co‐polycondensation of methyl(trialkoxy)silane MeSi(OR)₃ (R = Et (MTES) and Me (MTMS)) and tetra‐alkoxysilane Si(OR)₄ (R = Et (TEOS) and Me (TMOS)), respectively. The products are purified by fractional precipitation to provide polymethyl(alkoxy)siloxane copolymers with molecular weight 1000–10,000 (poly(MTES‐co‐TEOS)) or 1700–100,000 (poly(MTMS‐co‐TMOS)) that are stable to self‐condensation. These polymers are soluble in common organic solvents except for hexane, and form flexible and transparent free‐standing films with a tensile strength of 4.0–10.0 MPa. The structure of the polymethyl(alkoxy)siloxane copolymers is thought to be a random or a block co‐polymer. They are found to provide coating films with an adhesive strength up to 10, a refractive index of 1.36–1.40, and a dielectric constant of 3.5–3.6. The products also show better weathering stability than polyethoxysiloxane due to the hydrolytic polycondensation of TEOS. Field emission‐scanning electron micrography analysis reveals that coating films are composed of a micro‐phase separated structure. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4732–4741     

Hydrolysis and Polymerization of Dimethyldiethoxysilane, Methyltrimethoxysilane and Tetramethoxysilane in Presence of Aluminum Acetylacetonate. A Complex Catalyst for the Formation of Siloxanes

Hydrolysis and polymerization of dimethyldiethoxysilane (DMDE), methyltrimethoxysilane (MTMS) and tetramethoxysilane (TMOS) in the presence of aluminum acetylacetonate (Al(acac)₃) have been investigated by infrared and NMR spectroscopy. In the absence of acidic catalyst, Al(acac)₃ catalyzes the hydrolysis of all the silanes. The catalytic activity of Al(acac)₃ is less than that of HNO₃, but larger than that of NH₃. The hydrolysis rate increases with increasing concentration of Al(acac)₃ in DMDE. The hydrolysis of TMOS occurs rapidly after an inductive period, which becomes longer with addition of Al(acac)₃. The results are explained by assuming an Al(acac)₃ catalyzed hydrolysis and a silanol catalyzed hydrolysis. The addition of Al(acac)₃ causes changes in polymerization of the resultant silanols. In DMDE and MTMS, it stabilizes the silanols at the early stage, and then enhances their polymerization. The polymerization in TMOS leads to the formation of precipitates that have a high degree of polymerization. The polymerization appears to proceed via a deprotonation mechanism including transfer of protons from silanols to Al(acac)₃. The present results strongly suggest that, besides acids and bases, metal complexes can be used as catalysts for the formation of siloxanes under ambient conditions.     

Sol-gel polycondensation of methyltrimethoxysilane in ethanol studied by <Superscript>29</Superscript>Si NMR spectroscopy using a two-step acid/base procedure

Sol-gel polymerization of methyltrimethoxysilane (MTMS) in ethanol using a two-step acid/base catalyzed procedure (B2) is followed by ²⁹Si NMR spectroscopy. Analysis of the structural evolution of the B2 system shows that esterification of monomeric and end silicon species is rate-limited while that of linear and cyclic species is able to reach pseudoequilibrium in the second basic step. Condensation reactivity is reduced with increasing network connectivity, however, to a much less degree under B2 conditions than MTMS polymerization under acidic conditions. Steric effects as well as many other factors are attributed to this trend. The concentration of cyclic and polycyclic species of the B2 system is nearly 3 times lower compared to the acid-catalyzed system. The empirical degree of condensation at the gel point is determined to be 0.88. The effects of cyclization and phase separation on MTMS gelation are discussed for both B2 and acid-catalyzed systems. Based on these results it is believed that MTMS-based gels form for B2 and not acid-catalyzed conditions due to reduced cyclization, rapid hydrolysis and condensation, effective use of functional groups, and effective contribution of branched and polycyclic species as crosslinking points to connect polymeric chains in the B2 system.     

Comparison of dye adsorption by mesoporous hybrid gels: understanding the interactions between dyes and gel surfaces

Without using any templating agents, mesoporous hybrid gels were prepared using mixtures of tetraethoxysilane (TEOS) with n-propyltriethoxysilane (PTES), bis(trimethoxysilyl)hexane (TSH), or bis(trimethoxysilylpropyl)amine (TSPA) as precursors. Fourier transform infrared (FTIR), N2 adsorption/desorption, thermogravimetry (TG), point of zero charge (PZC), and water vapor adsorption measurements were used to characterize the gels. The adsorption of methyl orange (MO), methyl red (MR), bromocresol purple (BP), phenol red (PR), neutral red (NR), and brilliant blue FCF (BBF) by the gels in both 0.01 M HCl and 0.01 M NaOH solutions was compared comprehensively. The gel derived from TEOS/TSH (with -(CH2)6- groups, Gel 2) has the largest specific surface area (695 m2 g(-1)), the smallest pore volume (0.564 cm3 g(-1)), and the smallest average pore size (3.7 nm). The gels derived form TEOS/PTES (with -(CH2)2CH3 groups, Gel 1), and TEOS/TSPA (with -(CH2)3NH(CH2)3- groups, Gel 3) have similar textual properties. The PZC of Gels 1, 2, and 3 was estimated to be 6.28, 6.20, and 6.88, respectively. Gel 3 has the highest PZC due to the presence of -NH- groups. In general, Gel 2 shows the highest dye adsorption among all the gels in both acidic and basic solutions. All the dyes except NR have much lower adsorption in basic solutions than in acidic solutions. In acidic solutions Gels 1 and 2 have similar adsorption trends for the dyes, except for BP, with NR having the highest adsorption, and PR the lowest adsorption. Gel 3 presents a different trend from Gels 1 and 2, with BBF having the highest adsorption, and MR the lowest adsorption. In basic solutions the order of dye adsorption by all the gels is shown to follow the sequence NR>MR approximately BBF>MO>BP approximately PR. The adsorption results can be explained by considering the textural properties of the gels and the interactions between the gel surfaces and the dyes, which include hydrogen bonding, electrostatic, and hydrophobic interactions.     

X-Ray Diffraction Studies of Sol-Gel Derived ORMOSILs Based on Combinations of Tetramethoxysilane and Trimethoxysilane

ORMOSILs have been prepared in the series TMOSx·MTMS(100 – x) (where TMOS is tetramethoxysilane; MTMS is methyltrimethoxysilane; x is mol% silane with respect to total silane for 0 x 100) by means of acid catalyzed, sol-gel processing. After drying at 60°C, small bulk samples were obtained of excellent optical clarity. Powder X-ray diffraction (XRD) patterns, in the range of 5 to 60°2, were compared with that of fused silica. All the prepared samples were amorphous. Fused silica exhibits one broad peak, d2 centered at d-spacing 4.12 Å. For the TMOS100 silica xerogel, the analogous broad peak had shifted slightly, to be centered at 3.88 Å; and remained in about the same position as x was decreased for the series TMOSx·MTMS(100 – x). In addition, a second, broad peak, d1, was observed for the ORMOSIL series centered at the d-spacing 8.7 Å for MTMS100 (i.e., x = 0) and increasing smoothly as x was increased, reaching 11.3 Å for x = 70, and >11.3 Å for x > 70. The intensity of d1 was found to have trebled, relative to the intensity of d2, on increasing the organic character of the matrix from TMOS70·MTMS30 to MTMS100.The d2 peak appearing at about 4 Å for both fused silica and the ORMOSILs is assumed to be associated with the spacing between silicon atoms connected by means of an oxygen bridge. The Si–O–Si angle for silica xerogels is known to depend upon the nature of the sol-gel processing and is bigger than that of fused silica.The d1 peak may be associated with the spacing between silicons attached to methyl groups and indicative of channels of methyl groups in the structure. Alternatively, the d1 peak may have its origin in a preferred, discrete structural unit in the matrix for instance cubane based on a octameric silicon arrangement.     

Synthesis of Functional Bis(pentafluoroethyl)silanes (C2F5)2SiX2, with X=H,F, Cl,Br, OPh,and O2CCF3

As recently shown, the introduction of pentafluoroethyl functionalities into silicon compounds is of general interest due to an enhanced Lewis acidity of the resulting species. By this means, the synthesis of previously inaccessible hypervalent silicon derivatives is enabled. 1 While an easy access to tris(pentafluoroethyl)silanes has already been published, synthetic strategies for the selective preparation of bis derivatives are yet unknown. In this contribution, a convenient protocol for the synthesis of functional bis(pentafluoroethyl)silicon compounds is presented. These compounds represent precursors for the synthesis of pentafluoroethylated polysiloxanes. 2 Furthermore, they prove to be resistant to oxonium cations, which is a key feature for the preparation of stable pentafluoroethylsilic acids. 3 Treatment of dichlorodiphenoxysilane with in situ generated pentafluoroethyl lithium leads to the corresponding bis(pentafluoroethyl)silane in high yields. (C₂F₅)₂Si(OPh)₂ serves as a starting material for further functionalized bis(pentafluoroethyl)silanes. These silanes have been isolated and their reactivity towards N bases studied. The pronounced Lewis acidity of the obtained compounds has been documented by the formation of octahedral adducts with nitrogen donors such as 1,10‐phenanthroline and acetonitrile.     

Optimization of Ormosil Films for Optical Sensor Applications

Recent work has indicated that Ormosil films, fabricated from organically modified precursors, produce better sensor performance for some specific applications, compared to films fabricated from conventional sol-gel precursors such as TEOS or TMOS. This paper aims to compare film properties and sensor behavior for films fabricated from tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) silica precursors and both methyltrimethoxysilane (MTMS) and methyltriethoxysilane (MTES) organically modified precursors. Microstructural differences, for example, porosity changes due to the different precursor backbone structures, are interrogated by monitoring oxygen gas and aqueous-phase sensor response. Oxygen sensing using these films is enabled by incorporating in the films an oxygen-sensitive ruthenium dye whose fluorescence is quenched in the presence of oxygen. Film properties such as thickness, thickness stabilization time, as well as sensor response, are discussed in terms of relative hydrolysis and condensation behavior for the different precursors. Film hydrophobicity, an issue which has been identified as being of crucial importance for optimum dissolved oxygen sensor response, is discussed and contact angle measurements are used to investigate the degree of hydrophobicity for different film types. The main motivation for this work is film optimization for optical gas-phase and dissolved oxygen sensors.     

Adsorption kinetics of an organic dye by wet hybrid gel monoliths

Wet hybrid gel monoliths are prepared with bis(trimethoxysilylpropyl)amine (TSPA) or the mixture of TSPA with n-propyltriethoxysilane (PTES) or bis(trimethoxysilyl)hexane (TSH) or tetraethoxysilane (TEOS) as precursors. The adsorption kinetics of an organic dye (erioglaucine disodium salt, EDS) by the gel monoliths in aqueous solutions is studied comprehensively. The effects of temperature, pH, and ionic strength on the adsorption kinetics are investigated. Kinetic studies show that in general the kinetic data are well described by the pseudo second-order kinetic model. Initial adsorption rate increases with the increase in temperature, but decreases with the increase in solution pH and ionic strength. The adsorption activation energy is found to be 17–51 kJ mol⁻¹ under our experimental conditions. The internal diffusion of the dye into the hybrid gels appears to be the rate-limiting step of the overall adsorption process. The adsorption is promoted by hydrogen bonding, hydrophobic and electrostatic attractions in acidic or neutral solutions, suppressed by the electrostatic repulsion in basic solutions and by the ionic exchange competition of Cl⁻ with the dye anions in solutions with a high NaCl concentration. After adsorption for 165 h, all the gel monoliths present a linear shrinkage less than 10%.     

Hydrophobic silica aerogels prepared via rapid supercritical extraction

Hydrophobic silica aerogels have been prepared using the rapid supercritical extraction (RSCE) technique. The RSCE technique is a one-step methanol supercritical extraction method for producing aerogel monoliths in 3 to 8 h. Standard aerogels were prepared from a tetramethoxysilane (TMOS) recipe with a molar ratio of TMOS:MeOH:H₂O:NH₄OH of 1.0:12.0:4.0:7.4 × 10⁻³. Hydrophobic aerogels were prepared using the same recipe except the TMOS was replaced with a mixture of TMOS and one of the following organosilane co-precursors: methytrimethoxysilane (MTMS), ethyltrimethoxysilane (ETMS), or propyltrimeth-oxysilane (PTMS). Results show that, by increasing the amount of catalyst and increasing gelation time, monolithic aerogels can be prepared out of volume mixtures including up to 75% MTMS, 50% ETMS or 50% PTMS in 7.5–15 h. As the amount of co-precursor is increased the aerogels become more hydrophobic (sessile tests with water droplets yield contact angles up to 155°) and less transparent (transmission through a 12.2-mm thick sample decreases from 83 to 50% at 800 nm). The skeletal and bulk density decrease and the surface area increases (550–760 m²/g) when TMOS is substituted with increasing amounts of MTMS. The amount of co-precursor does not affect the thermal conductivity. SEM imaging shows significant differences in the nanostructure for the most hydrophobic surfaces.     

Hydrophobicity and drag reduction properties of surfaces coated with silica aerogels and xerogels

Superhydrophobic surfaces have application in self-cleaning, anti-fouling and drag reduction. Most superhydrophobic surfaces are constructed using complex fabrication methods. An alternative method is to use sol–gel methods to make hydrophobic aerogel and xerogel surfaces. In this work, hydrophobic silica aerogels and xerogels were made from the silica precursors tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) in volume ratios MTMS/TMOS of 0–75 % using a base-catalyzed recipe. Overall hydrophobicity was assessed using contact angle measurements on surfaces prepared from crushed aerogel and xerogel powders. The surfaces made from aerogels were super-hydrophobic (with contact angles of 167°–170°) for all levels of MTMS (10–75 %). Of the xerogel-coated surfaces, those made with 50 % MTMS were hydrophobic and with 75 % MTMS were superhydrophobic. Chemical hydrophobicity was assessed using Fourier transform infrared spectroscopy, which showed evidence of Si–CH₃ and Si–C bonds in the aerogels and xerogels made with MTMS. Morphological hydrophobicity was assessed using SEM imaging and gas adsorption. The drag characteristics of the aerogel- and xerogel-coated surfaces were measured using a rotational viscometer. Under laminar flow conditions all of the hydrophobic aerogel-coated surfaces (10–75 % MTMS) were capable of capturing an air bubble, thereby reducing the drag on a horizontal rotating surface by 20–30 %. Of the xerogel-coated surfaces, only the one made from 75 % MTMS could capture a bubble, which led to 27 % drag reduction. These results imply that morphological differences between silica aerogels and xerogels, rather than any differences in their chemical hydrophobicity, give rise to the observed differences in hydrophobicity and drag reduction for the sol–gel-coated surfaces.     

A Spectroscopic Study of the Effects of Various Solvents and Sol-Gel Hosts on the Chemical and Photochemical Properties of Thionin and Nile Blue A

Tetraethyl orthosilicate (TEOS)-based gels were doped with two optically active organic indicators, thionin and nile blue A. Before trapping in a sol-gel host, thionin and nile blue A were both evaluated for solvent and protonation effects on their spectral properties. Only extreme pH values provided by HCl, NaOH, and NH₄OH produced new absorption and/or fluorescence bands. Introduction of nile blue A into alkaline environments (0.1N NaOH, NH₄OH) results in the appearance of a broad absorption band centered near 520 nm whereas highly acidic environments (1N HCl) show a reduction of the 635 nm absorption peak accompanied by an absorption band located near 460 nm. A marked decrease is observed in the optical density of thionin in 1N HCl solution which results in a reduction in the fluorescence intensity. The absorption and fluorescence spectra also reveal a decrease in a pH 11 solution of NH₄OH as compared to neutral conditions. Both dyes formed dimers when the sol-gel host, initially synthesized with TEOS, was organically modified with methyltrimethoxysilane (MTMS). However, thionin dimers were present in all silica-based sol-gel compositions, as evidenced by the absorption and fluorescence spectra. Substitution of MTMS for some of the TEOS in the gel matrix resulted in blue shifts in the absorption and fluorescence spectra of nile blue A. The absorption peak shifted 50 nm to 596 nm whereas the fluorescence shifted around 40 nm to 635 nm. These blue shifts resulted from the reduced polarity of the silica-based xerogel. Thionin also exhibited shifts in its absorption and fluorescence spectra with organic modification by MTMS. The absorption shifted approximately 3 nm to 595 nm while the fluorescence maximum decreased 7 nm to 630 nm. The blue shifts in the spectra of thionin with additions of MTMS were attributed to surface sites that altered the molecular structure of the adsorbed thionin molecules.     

Periodic Mesoporous Organosilica Materials: Self‐Assembly of Carbamothioic Acid‐Bridged Organosilane Precursors

A new series of carbamothioic acid‐containing periodic mesoporous organosilica (PMO) materials has been synthesized by a direct cocondensation method, in which an organosilica precursor N,S‐bis[3‐(triethoxysilyl)propyl]carbamothioic acid (MI) is treated with tetraethyl orthosilicate (TEOS), and the nonionic surfactant Pluronic 123 (P123) is used as a template under acidic conditions in the presence of inorganic additives. Moreover, the synthesis of the PMO material consisting of the MI precursor without TEOS has been realized. These novel PMO materials have large surface areas, well‐ordered mesoporous structures, hollow fiberlike morphologies, and thick walls. They are also structurally well‐ordered with a high organosilica precursor content, and the carbamothioic acid groups are thermally stable up to 250 °C. Furthermore, the organosilica materials exhibit hydrothermal stability in basic solution.     

The performance of hybrid monolithic silica capillary columns prepared by changing feed ratios of tetramethoxysilane and methyltrimethoxysilane

The effect of a feed ratio of methyltrimethoxysilane (MTMS) to tetramethoxysilane (TMOS) was studied to improve the performance of a hybrid monolithic silica capillary column with 100-μm i.d. in HPLC in a range MTMS/TMOS (v/v) = 10/90–25/75. The domain size was also varied by adjusting the amount of PEG to control permeability (K = 2.8 × 10⁻¹⁴–6.9 × 10⁻¹⁴ m²). Evaluation of the performance for those capillary columns following octadecylsilylation proved an increase in retention factor (k) and a decrease in steric selectivity α(triphenylene/ortho-terphenyl) with the increase in MTMS content in the feed. The effect of the feed ratio was also observed in porosity and hydrophobic property of the C18 stationary phase from the results of size exclusion chromatography (SEC) and reversed phase characterization. The monolithic silica capillary columns prepared under new preparation conditions were able to produce a plate height of 4.6–6.0 μm for hexylbenzene in a mobile phase acetonitrile/water = 80/20 at a linear velocity of 2 mm/s. Consequently, it was possible to prepare hybrid monolithic silica capillary columns with higher performance than those reported previously while maintaining the retention factors in a similar range by reducing the MTMS/TMOS ratio and increasing the total silane concentration in feed.     

Formation of silica nanoparticles in microemulsions

Silica nanoparticles for controlled release applications have been produced by the reaction of tetramethylorthosilicate (TMOS) inside the water droplets of a water-in-oil microemulsion, under both acidic (pH 1.05) and basic (pH 10.85) conditions. In-situ FTIR measurements show that the addition of TMOS to the microemulsion results in the formation of silica as TMOS, preferentially located in the oil phase, diffuses into the water droplets. Once in the hydrophilic domain, hydrolysis occurs rapidly as a result of the high local concentration of water. Varying the pH of the water droplets from 1.05 to 10.85, however, considerably slows the hydrolysis reaction of TMOS. The formation of a dense silica network occurs rapidly under basic conditions, with IR indicating the slower formation of more disordered silica in acid. SAXS analysis of the evolving particles shows that approximately 11 nm spheres are formed under basic conditions; these are stabilized by a water/surfactant layer on the particle surface during formation. Under acidic conditions, highly uniform approximately 5 nm spheres are formed, which appear to be retained within the water droplets (approximately 6 nm diameter) and form an ordered micelle nanoparticle structure that exhibits sufficient longer-range order to generate a peak in the scattering at q approximately equal to 0.05 A-1. Nitrogen adsorption analysis reveals that high surface area (510 m2/g) particles with an average pore size of 1 nm are formed at pH 1.05. In contrast, base synthesis results in low surface area particles with negligible internal porosity.     

Rapid Silylation of a Glass Surface: Choice of Reagent and Effect of Experimental Parameters on Hydrophobicity

The silylation of the surface of sodium borosilicate glass was investigated under conditions that might be applied to the fabrication of ion-selective microelectrodes of the liquid membrane type such as are used in physiology, Silylation was carried out for 15 min at different temperatures and with six different silanes. The hydrophobicity was measured and found to be greatest with reaction temperatures of 200–350°, the effectiveness of the different silylating agents being in the (increasing) order: trimethylchlorosilane, tributylchlorosilane, (dimethylamino)trimethylsilane, hexamethyldisilazane, dimethyldichlorosilane, bis(dimethylamino)dimethylsilane. The results give evidence of a catalytic effect of basic amine groups and show that the reactivity of the surface of freshly drawn glass is increased by acid leaching.     

Asymmetric Synthesis of Silicon-Stereogenic Silanes by Copper-Catalyzed Desymmetrizing Protoboration of Vinylsilanes

The catalytic asymmetric creation of silanes with silicon stereocenters is a long-sought but underdeveloped topic, and only a handful of examples have been reported. Moreover, the construction of chiral silanes containing (more than) two stereocenters is a more arduous task and remains unexploited. We herein report an unprecedented copper-catalyzed desymmetrizing protoboration of divinyl-substituted silanes with bis(pinacolato)diboron (B₂pin₂). This method enables the facile preparation of an array of enantiomerically enriched boronate-substituted organosilanes bearing contiguous silicon and carbon stereocenters with exclusive regioselectivity and generally excellent diastereo- and enantioselectivity.     

Epoxide Opening versus Silica Condensation during Sol–Gel Hybrid Biomaterial Synthesis

Hybrid organic–inorganic solids represent an important class of engineering materials, usually prepared by sol–gel processes by cross‐reaction between organic and inorganic precursors. The choice of the two components and control of the reaction conditions (especially pH value) allow the synthesis of hybrid materials with novel properties and functionalities. 3‐Glycidoxypropyltrimethoxysilane (GPTMS) is one of the most commonly used organic silanes for hybrid‐material fabrication. Herein, the reactivity of GPTMS in water at different pH values (pH 2–11) was deeply investigated for the first time by solution‐state multinuclear NMR spectroscopic and mass spectrometric analysis. The extent of the different and competing reactions that take place as a function of the pH value was elucidated. The NMR spectroscopic and mass spectrometric data clearly indicate that the pH value determines the kinetics of epoxide hydrolysis versus silicon condensation. Under slighly acidic conditions, the epoxy‐ring hydrolysis is kinetically more favourable than the formation of the silica network. In contrast, under basic conditions, silicon condensation is the main reaction that takes place. Full characterisation of the formed intermediates was carried out by using NMR spectroscopic and mass spectrometric analysis. These results indicate that strict control of the pH values allows tuning of the reactivity of the organic and inorganic moities, thus laying the foundations for the design and synthesis of sol–gel hybrid biomaterials with tuneable properties.     

Asymmetric Synthesis of Silicon‐Stereogenic Silanes by Copper‐Catalyzed Desymmetrizing Protoboration of Vinylsilanes

The catalytic asymmetric creation of silanes with silicon stereocenters is a long‐sought but underdeveloped topic, and only a handful of examples have been reported. Moreover, the construction of chiral silanes containing (more than) two stereocenters is a more arduous task and remains unexploited. We herein report an unprecedented copper‐catalyzed desymmetrizing protoboration of divinyl‐substituted silanes with bis(pinacolato)diboron (B₂pin₂). This method enables the facile preparation of an array of enantiomerically enriched boronate‐substituted organosilanes bearing contiguous silicon and carbon stereocenters with exclusive regioselectivity and generally excellent diastereo‐ and enantioselectivity.     

Mechanical strengthening of TMOS-based alcogels by aging in silane solutions

It has been shown that aging of tetramethoxysilane (TMOS)-based alcogels in solutions of tetraethoxysilane (TEOS)/methanol (MeOH) provides new monomers to the alcogel and favorably increases the strength and stiffness of the alcogel and hence reduces the shrinkage during the subsequent drying. Load relaxation experiments have been performed to determine the shear modulus (G), Poisson's ratio (), and the permeability of wet gel rods as a function of aging time in the TEOS/MeOH solution. The modulus of rupture (MOR) and G have also been obtained from 3-point bending tests. Aging the gels in 70 vol% TEOS/MeOH causes an increase in G from 0.48 MPa to 1.8 MPa and 7.4 MPa after aging for 24 hours and 144 hours, respectively.It is shown that the drying stress is actually increased by the aging treatment, but the increase in strength of the gel is even greater; hence, strengthening of the alcogels dramatically reduces the probability of cracking during drying. Unaged gels with higher TMOS concentrations corresponding to the silica content of gels aged in TEOS solution, however, showed large shrinkage and severe cracking.