Showing papers on "Silica gel published in 2008"

Permeable silica shell through surface-protected etching.

Abstract: We describe a "surface-protected etching" strategy that allows convenient conversion of sol-gel derived silica into porous structures. Poly(vinyl pyrrolidone) is used to protect the near surface layer, and NaOH is used to selectively etch the interior of the silica spheres. Etching initially yields porous structures and eventually removes the core to leave behind hollow silica spheres with porous shells. This strategy is useful for constructing core-shell systems where active nanomaterials are embedded in silica shell for enhanced stability against aggregation. We experimentally demonstrate use of the surface-protected etching approach to create openings on silica shells; these openings allow dissolved chemical species to reach embedded catalytic particles to be chemically transformed while the porous shells continue to act as effective barriers against aggregation and loss of activity of the core particles. We also show that, by controlling the extent of etching, it is possible to control the permeation rate of the chemical species through the shells.

Characteristics of Superficially-Porous Silica Particles for Fast HPLC: Some Performance Comparisons with Sub-2-µm Particles

Abstract: Columns of 2.7-microm fused-core (superficially porous) Type B silica particles allow very fast separations of small molecules at pressures available in most high-performance liquid chromatography instruments. These highly-purified particles with 1.7-microm solid silica cores and 0.5-microm-thick shells of 9 nm pores exhibit efficiencies that rival those of totally porous sub-2-microm particles but at one-half to one-third of the column back pressure. This presentation describes other operating features of fused-core particle columns, including sample loading characteristics and packed bed stability. The superior mass transfer (kinetic) properties of the fused-core particles result in much-improved separation efficiency at higher mobile phase velocities, especially for > 600 molecular weight solutes.

Ordered mesoporous silicon through magnesium reduction of polymer templated silica thin films.

Abstract: This paper describes the process of making ordered mesoporous silicon (Si) thin films. The process begins with mesoporous silica (SiO2 )t hin films that are produced via evaporation induced self-assembly (EISA) using sol-gel silica precursors with a diblock copolymer template. This results in a film with a cubic lattice of 15 nm diameter pores and 10 nm thick walls. The silicon is produced through reduction of the silica thin films in a magnesium (Mg) vapor at 675 °C. Magnesium reduction preserves the ordered pore-solid architecture but replaces the dense silica walls with 10-17 nm silicon crystallites. The resulting porous silicon films are characterized by a combination of low and high angle X-ray diffraction, combined with direct SEM imaging. The result is a straightforward route to the production of ordered nanoporous silicon.

A photochemical synthesis of functionalized trans-cyclooctenes driven by metal complexation.

Abstract: Described is a scalable procedure for driving photochemical sytheses of trans-cyclooctene derivatives through metal complexation. During photoirradiation, reaction mixtures are continuously pumped through a column of a AgNO3-impregnated silica gel. The trans-cyclooctene derivative is selectively retained by the AgNO3-impregnated silica, but the cis-isomer elutes from the column back to the reaction flask, where it is photoisomerized and recirculated through the column. The method provides access to a range of usefully functionalized derivatives of trans-cyclooctene, including a derivative of 5-aza-trans-cyclooctene that underwent transannular cyclization upon treatment with bromine. The alkene stereochemistry is transferred with high fidelity to the hexahydropyrrolizine framework in the transannular cyclization.

Synthesis of Hollow Silica and Titania Nanospheres

Abstract: Hollow silica nanospheres were prepared by template growth of silica gel on monodisperse (100−500 nm) polystyrene beads coated with poly-l-lysine, followed by calcinations. A similar route was used to prepare titania nanospheres.

A Mesoporous 3D Hybrid Material with Dual Functionality for Hg2+ Detection and Adsorption

Abstract: Dual-function hybrid material U 1 was designed for simultaneous chromofluorogenic detection and removal of Hg2+ in an aqueous environment. Mesoporous material UVM-7 (MCM41 type) with homogeneously distributed pores of about 2–3 nm in size, a large specific surface area exceeding 1000 m2 g−1, and nanoscale particles was used as an inorganic support. The mesoporous solid is decorated with thiol groups that were treated with squaraine dye III to give a 2,4-bis(4-dialkylaminophenyl)-3-hydroxy-4-alkylsulfanylcyclobut-2-enone (APC) derivative that is covalently anchored to the inorganic silica matrix. The solid was characterised by various techniques including X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and nitrogen adsorption. This hybrid solid is the chemodosimeter for Hg2+ detection. Hg2+ reacts with the APC fragment in U 1 with release of the squaraine dye into the solution, which turns deep blue and fluoresces strongly. Naked-eye Hg2+ detection is thus accomplished in an easy-to-use procedure. In contrast, U 1 remains silent in the presence of other thiophilic transition metal ions, alkali and alkaline earth metal ions, or anions ubiquitously present in water such as chloride, carbonate, sulfate, and phosphate. Material U 1 acts not only as chemodosimeter that signals the presence of Hg2+ down to parts-per-billion concentrations, but at the same time is also an excellent adsorbent for the removal of mercury cations from aqueous solutions. The amount of adsorbed mercury ranges from 0.7 to 1.7 mmol g−1, depending on the degree of functionalisation. In addition, hybrid material U 1 can be regenerated for both sensing and removal purposes. As far as we know, U 1 is the first example of a promising new class of polyfunctional hybrid supports that can be used as both remediation and alarm systems by selective signalling and removal of target species of environmental importance. Model compounds based on silica gel (G 1), fumed silica (F 1), and micrometre-sized MCM-41 scaffolds (M 1) were also prepared and studied for comparative purposes.

Silica Immobilized Second Generation Hoveyda‐Grubbs: A Convenient, Recyclable and Storageable Heterogeneous Solid Catalyst

Abstract: The second generation Hoveyda-Grubbs metathesis catalyst was successfully immobilized on silica in pellet and powder form following a practical and fast synthesis procedure. The activity of the solid system is truly heterogeneous, efficient in various metathesis reaction types and stable for at least 4000 TON. Ru contamination of the products was very low (ppb levels). The successful use of the robust system has been demonstrated in a continuous reactor set-up.

Two Luminescent Molecular Hybrids Composed of Bridged Eu(III)-β-Diketone Chelates Covalently Trapped in Silica and Titanate Gels

Abstract: In the context, a kind of β-diketone, dibenzoylmethane (DBM) was first grafted with the coupling reagent 3-(triethoxysilyl)-propyl isocyanate (TESPIC), and the as-derived monomers were introduced into inorganic network through powerful covalent bonds. Subsequently, two novel chemically bonded hybrid materials, Eu(III)-modified dibenzoylmethane with silica and titanate hosts, were prepared and characterized by IR, phosphorescence, fluorescence spectroscopy, time-resolved photoluminescence spectroscopy, and scanning electron microscopy (SEM). The above spectroscopic data indicate that the modified DBM could sensitize Eu(III) ions to exhibit attracting red luminescence in the two respective hosts. It is noted that the covalently bonded silicate hybrid material presents a stronger red/orange intensity ratio, longer lifetimes, and higher quantum efficiency than a covalently bonded titanate one and europium complex of DBM, suggesting that the silica network is more suitable for the emissions of covalently bonde...