Showing papers on "Silica gel published in 2014"

An overview on silica aerogels synthesis and different mechanical reinforcing strategies

Abstract: Silica aerogels are lightweight and highly porous materials, with a three-dimensional network of silica particles, which are obtained by extracting the liquid phase of silica gels under supercritical conditions. Due to their outstanding characteristics, such as extremely low thermal conductivity, low density, high porosity and high specific surface area, they have found excellent potential application for thermal insulation systems in aeronautical/aerospace and earthly domains, for environment clean up and protection, heat storage devices, transparent windows systems, thickening agents in paints, etc. However, native silica aerogels are fragile and sensitive at relatively low stresses, which limit their application. More durable aerogels, with higher strength and stiffness, can be obtained by proper selection of the silane precursors, and constructing the silica inorganic networks by compounding them with different organic polymers or different fiber networks. Recent studies showed that adding flexible organic polymers to the hydroxyl groups on the silica gel surface would be an effective mechanical reinforcing method of silica aerogels. More versatile polymer reinforcement approach can be readily achieved if proper functional groups are introduced on the surface of silica aerogels and then co-polymerized with appropriate organic monomers. The mechanical reinforced silica aerogels, with their very open texture, can be an outstanding thermal insulator material for different industrial and aerospace applications. This paper presents a review of the literature on the methods for mechanical reinforcing of silica aerogels and discusses the recent achievements in improving the strength and elastic response of native silica aerogels along with cost effectiveness of each methodology.

Synthesis and performance of highly dispersed Cu/SiO2 catalysts for the hydrogenolysis of glycerol

Abstract: The performance of Cu/SiO2 (commercial silica gel, SBA-15 and SBA-15 treated at 900 °C) catalysts for the hydrogenolysis of glycerol to propylene glycol is investigated with emphasis on the stability characteristics. Cu catalysts with large crystals, small monodisperse crystallites or a highly dispersed XRD amorphous copper phase were obtained after calcination in stagnant air, in a flow of NO/N2 or a flow of air, respectively. Analysis by XRD, N2O surface oxidation and TEM confirmed the variation of the Cu specific surface area by the calcination conditions and the type of silica support used. The different dispersion characteristics resulted in different activities (20–50% glycerol conversion), while all the catalysts proved to be highly selective towards propylene glycol (92–97%). Present results indicate that glycerol hydrogenolysis over Cu-based catalysts is a structure sensitive reaction as significant variations in initial TOF were observed as a function of varying Cu crystallites. It is shown here that the presence of a solvent greatly influences the intrinsic reaction rate and the nature of structure sensitivity. The deactivation behaviour of all catalysts was studied, and based on detailed characterization of the spent samples it was attributed to Cu sintering and the presence of strongly adsorbed species on the catalytic surface. The 18 wt%Cu/silica gel (air) catalyst presented only moderate deactivation (∼20%) while the catalyst supported on SBA-15 calcined at 900 °C (SBA900C) proved to be the most stable with negligible deactivation after three consecutive runs.

Removal of hexavalent chromium ions using polyaniline/silica gel composite

Abstract: In this study, a composite composed of polyaniline and silica gel was successfully prepared via in situ polymerization. The prepared polyaniline/silica gel (PANI/SiO 2 ) composite was used as an adsorbent for the removal of Cr(VI) ions from the aqueous solutions. Structure and morphology of the composite were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscope equipped with an energy dispersive X-ray analyzer (SEM-EDX). A batch adsorption system was applied to study the ability of the adsorbent to remove Cr(VI) ions from aqueous solution. Results revealed that the PANI/SiO 2 composite requires minimum contact time as 60 min, pH 4.2, a dosage of 0.1 g and 50 mg/L as an initial concentration of Cr(VI) ions for the maximum removal capacity at 303 K. Langmuir and Freundlich adsorption isotherm models were studied to describe isotherm constants. The Cr(VI) ion uptake by the composite follows Freundlich isotherm. The maximum Cr(VI) ion adsorption capacity of PANI/SiO 2 composite was found to be 63.41 mg/g at 303 K. Thermodynamic parameter studies concluded that the nature of Cr(VI) ion adsorption was spontaneous and endothermic. The kinetic study revealed that the adsorption of Cr(VI) ions by the composite follows the pseudo-second-order and pore diffusion models. The mechanism was mainly driven by both ion exchange and adsorption coupled reduction.

Influence of Silica Types on Synthesis and Performance of Amine–Silica Hybrid Materials Used for CO2 Capture

Abstract: Amine–silica hybrid materials have been investigated extensively in terms of their suitability for postcombustion CO2 capture. However, research on how the silica types affects the synthesis and performance of amine–silica hybrid materials is scarce. In this study, four types of commonly used and representative silica including precipitated silica, fumed silica, MCM-41, and silica gel are used to synthesize a series of comparable materials by grafting a silane onto them. We undertake a porosity analysis of plain silica and the amine–silica hybrid materials and determined the CO2 adsorption performance of amine–silica hybrid materials. The results suggest that precipitated silica is a superior and promising support material for amine–silica hybrid materials synthesis by grafting. The amine–silica hybrid material supporting with precipitated silica possesses relatively high amine content, exhibits good porosity, and obtains the highest CO2 adsorption capacity and amine efficiency compared to those of three ...

Effect of iron precursor on the Fenton-like activity of Fe2O3/mesoporous silica catalysts prepared under mild conditions

Abstract: Fe2O3/mesoporous silica catalysts were prepared by adding Fe2+ or Fe3+ ions, as an iron precursor, during the synthesis of silica gel. Low angle XRD data showed that both iron precursors affected the hexagonal ordered arrays of silica, which were more perturbed by Fe2+ than by Fe3+ ions, as confirmed by TEM images. Based on the Mossbauer spectroscopy, XPS, EPR and NH3-TPD data was possible to identify the iron species supported on the silica matrix are constituted of hematite and maghemite. The iron precursor strongly changed the specific area of the materials from 1406 m2 g−1, in pure silica sample, to 1165 and 799 m2 g−1 for the Fe3-MS and Fe2-MS samples, respectively, but the pore size distribution was maintained in the range of 15–40 A in both samples. These Fe-containing silica nanoparticles showed good catalytic activity to degrade methylene blue in aqueous medium. However, the Fe3-MS sample was more active than the Fe2-MS sample because the iron in the first is more exposed, which facilitated the accessibility to the active Fenton-like sites. These synthesized samples showed to be more stable against the Fe leaching than samples prepared by simple impregnation of hematite on the silica surface. This was because the strong interaction of iron and silica matrix was achieved by adding iron in the course of the synthesis of silica gel.

Selective synthesis of sulfoxides and sulfones from sulfides using silica bromide as the heterogeneous promoter and hydrogen peroxide as the terminal oxidant

Abstract: Silica bromide as a heterogeneous promoter and reagent is prepared from the reaction of silica gel with PBr3 as a non-hydroscopic, filterable, cheap, and stable yellowish powder that can be stored for months The results show that silica bromide is a suitable and efficient promoter for the chemoselective oxidation of sulfides to the corresponding sulfoxides or sulfones in the presence of 30% H2O2 in acetonitrile The excellent yields, heterogeneous conditions, simplicity, compatibility with a variety of functionalities, and ease of isolation of the products make our procedure a practical alternative

Fluorescent boronic acid polymer grafted on silica particles for affinity separation of saccharides.

Abstract: Boronic acid affinity gels are important for effective separation of biological active cis-diols, and are finding applications both in biotech industry and in biomedical research areas. To increase the efficacy of boronate affinity separation, it is interesting to introduce repeating boronic acid units in flexible polymer chains attached on solid materials. In this work, we synthesize polymer brushes containing boronic acid repeating units on silica gels using surface-initiated atom transfer radical polymerization (ATRP). A fluorescent boronic acid monomer is first prepared from an azide-tagged fluorogenic boronic acid and an alkyne-containing acrylate by Cu(I)-catalyzed 1,3-dipolar cycloaddition reaction (the CuAAC click chemistry). The boronic acid monomer is then grafted to the surface of silica gel modified with an ATRP initiator. The obtained composite material contains boronic acid polymer brushes on surface and shows favorable saccharide binding capability under physiological pH conditions, and displays interesting fluorescence intensity change upon binding fructose and glucose. In addition to saccharide binding, the flexible polymer brushes on silica also enable fast separation of a model glycoprotein based on selective boronate affinity interaction. The synthetic approach and the composite functional material developed in this work should open new opportunities for high efficiency detection, separation, and analysis of not only simple saccharides, but also glycopeptides and large glycoproteins.

Catalytic Functionalization of Indoles by Copper‐Mediated Carbene Transfer

Abstract: After the first step a cyclopropane intermediate is obtained which is directly converted to indole derivatives (III), (VI), (VII), and (IX) after silica gel column chromatography.

Colorimetric sensor arrays: Interplay of geometry, substrate and immobilization

Abstract: This study addresses the interplay of geometry, substrate, and dye immobilization method on colorimetric sensor array performance. Arrays of cross-responsive dyes were exposed to either ammonia or sulfur dioxide gas at their permissible exposure levels (PEL) or their immediately dangerous to life or health (IDLH) concentrations and their colorimetric responses analyzed. Two-dimensional and linear arrays in flow cells were compared for flow path uniformity. Substrate effects were explored using arrays of 36 dyes immobilized in organically modified silica sol–gel (ormosil) formulations printed on six common substrates in three classes: impermeable (glass slides and polyethylene terephthalate), cellulose based (printer paper and chromatography paper with silica gel), and porous polymer membranes (polypropylene and polyvinylidene difluoride). The effect of immobilization of dyes in an ormosil versus in a plasticizer (i.e., a viscous semi-liquid) was also compared. The linear geometry showed a more homogeneous flow path than obtained with the two-dimensional array, which contributes to higher overall response, faster response, and better reproducibility. Arrays printed on impermeable substrates showed long response times attributed to slow diffusion of the analyte through the spot, and those printed on cellulose based substrates showed high noise caused by macroscale surface texturing. Arrays printed on porous polymer substrates showed the best spot quality and reproducibility, fastest response, and lowest noise. Finally, plasticizer and ormosils proved to be comparable immobilization matrices for colorants, and the preferred choice depends on the combination of dye, immobilization method, and substrate.

Silica functionalized propyl sulfonic acid (SiO2-Pr-SO3H): An efficient catalyst in organic reactions

Abstract: Silica gel as an accessible substrate with high surface area can be modified by propyl sulfonic acid. Silica-functionalized sulfonic acid (SiO 2 -Pr-SO 3 H) as a highly efficient heterogeneous solid acid catalyst catalyzes various organic reactions and can be removed easily, recovered and reused without significant loss of activity. In this review, preparation and application of SiO 2 -Pr-SO 3 H in organic synthesis is investigated.

Highly dispersed Cu(II), Co(II) and Ni(II) catalysts covalently immobilized on imine-modified silica for cyclohexane oxidation with hydrogen peroxide

Abstract: This paper describes the synthesis of Cu(II), Co(II) and Ni(II) catalysts immobilized on imine-functionalized silica gel through a 3-aminopropyltriethoxysilane linker. The synthesized catalysts were characterized by spectroscopic techniques, namely EDS, FTIR, UV-Vis, 29Si MAS NMR, powder XRD and ESR spectroscopy. These analytical methods evidently confirmed the formation of silica-supported catalysts. Thermal properties of catalysts were studied between 30 and 800 °C by thermogravimetric-differential thermal gravimetric (TG-DTG) analysis. The surface roughness of the silica gel was increased upon modification but without losing its lumpy shape, as evidenced by SEM investigation. Magnified SEM and AFM images both suggested the high dispersive nature of the catalysts. Cyclohexane was successfully converted into cyclohexanol and cyclohexanone by the catalysts with the aid of hydrogen peroxide (oxidant). Comparatively, Cu(II) catalyst exhibited better cyclohexane conversion than the other two catalysts. The reusable nature of the catalysts was established by performing five consecutive catalytic runs with Cu(II) catalyst. Comparatively, the present reported catalytic systems were simple, reusable and effective models for higher cyclohexane conversion with better product selectivity.