Showing papers on "Silica gel published in 2003"

Peering into the self-assembly of surfactant templated thin-film silica mesophases.

Abstract: It is now recognized that self-assembly is a powerful synthetic approach to the fabrication of nanostructures with feature sizes smaller than achievable with state of the art lithography and with a complexity approaching that of biological systems. For example, recent research has shown that silica/ surfactant self-assembly combined with evaporation (so-called evaporation induced self-assembly EISA) can direct the formation of porous and composite thin-film mesostructures characterized by precise periodic arrangements of inorganic and organic constituents on the 1-50-nm scale. Despite the potential utility of these films for a diverse range of applications such as sensors, membranes, catalysts, waveguides, lasers, nano-fluidic systems, and low dielectric constant (so-called low k) insulators, the mechanism of EISA is not yet completely understood. Here, using time-resolved grazing incidence small-angle X-ray scattering (GISAXS) combined with gravimetric analysis and infrared spectroscopy, we structurally and compositionally characterize in situ the evaporation induced self-assembly of a homogeneous silica/surfactant/solvent solution into a highly ordered surfactant-templated mesostructure. Using CTAB (cetyltrimethylammonium bromide) as the structure-directing surfactant, a two-dimensional (2-D) hexagonal thin-film mesophase (p6mm) with cylinder axes oriented parallel to the substrate surface forms from an incipient lamellar mesophase through a correlated micellar intermediate. Comparison with the corresponding CTAB/water/ alcohol system (prepared without silica) shows that, for acidic conditions in which the siloxane condensation rate is minimized, the hydrophilic and nonvolatile silicic acid components replace water maintaining a fluidlike state that avoids kinetic barriers to self-assembly.

A Fast Way for Preparing Crack-free Mesostructured Silica Monolith

Abstract: Large-sized, crack-free silica monoliths with highly ordered mesostructure are prepared by a fast and easy way via liquid-paraffin-medium protected solvent evaporation. By employing the inert liquid paraffin as the morphology “protector”, cracks of the materials can be successfully avoided and the processing time can be reduced to 8 h. The block copolymer−silica composite monoliths are transparent and crack-free with a large size. The mesoporous silica monoliths have been characterized by small-angle powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and nitrogen adsorption, which show that the materials have a highly ordered hexagonal mesostructure of space group p6mm and narrow pore size distribution, with a mean pore diameter of 5.65 nm. In addition, metal ions can be easily doped into the monoliths, indicating potential optical, electronic, magnetic, and catalytic properties. This fast synthetic method is valuable for the applications of mesostructured silica monoliths in optics a...

The Michael addition of indoles to alpha,beta-unsaturated ketones catalyzed by CeCl3.7H2O-NaI combination supported on silica gel.

Abstract: Alkylation of indoles by means of the Michael addition has been the subject of a number of investigation. It is well established that regioselectivity in the additions of indoles to electron-deficient alkenes is strongly controlled by the reaction medium. In a continuation of the work on developing greener and cleaner technologies, the cerium(III) chloride heptahydrate and sodium iodide combination supported on silica gel catalyzes the alkylation of various indoles with α,β-unsaturated ketones giving 3-(3-oxoalkyl)indole derivatives in good yields. The substitution on the indole nucleus occurred exclusively at the 3-position, and N-alkylation products have not been observed.

Organic−Inorganic Polymer Hybrids Using Polyoxazoline Initiated by Functionalized Silsesquioxane

Abstract: New hybrid polyoxazolines (CubePOZO) were synthesized by ring-opening polymerization of 2-methyl-2-oxazoline initiated by functionalized polyhedral oligomeric silsesquioxane (POSS) with various feed ratios. The hybrid micelles derived from CubePOZO with both seven hydrophobic cyclopentyl groups of POSS and polyoxazoline (POZO) as a hydrophilic segment were prepared in an aqueous phase. Compared to POZO initiated by cyclohexyl iodide or methyl p-toluenesulfonate, increased hydrophobicity caused by the structure of POSS in CubePOZO played a major role in the micelle formation. This depended on the amount of POSS incorporated into the POZO. The CubePOZO100 and CubePOZO200 formed micelles in an aqueous phase with critical micelle concentrations (cmc) of 55 and 100 mg/L, respectively. Transparent polymer hybrids, homogeneous dispersion of hydrophobic POSS bonded covalently to POZO in the silica gel matrix at the molecular level, were obtained through hydrogen-bonding interaction between CubePOZO and silica gel...

Functionalization on Silica Gel with Allylsilanes. A New Method of Covalent Attachment of Organic Functional Groups on Silica Gel

Abstract: Treatment of an (allyl)organosilane with silica gel in refluxing toluene brought about deallylation forming an Si−O−Si bond with the silicon on the silica gel. This Si−O−Si bond formation provides us with a new reliable method for the functionalization of a silica gel surface.

Rheological study of the stabilization of magnetizable colloidal suspensions by addition of silica nanoparticles

Abstract: An experimental investigation is described on the stability of magnetorheological fluids (MRFs) consisting of iron suspensions in silicone oil with a thixotropic agent (silica nanoparticles) as stabilizer. The rheological properties were investigated using a commercial rheometer with a parallel-plate measuring cell. Several kinds of experiments were performed in steady-state, oscillatory, and transient regimes. The effects of the volume fraction of magnetic particles, the concentration of silica, magnetic flux density, B, and waiting time after preshear on the rheology of the MRFs were considered. Steady-state measurements demonstrated that our systems only display plastic behavior, for which a yield stress, σy, is appreciable, for the highest iron concentrations and/or magnetic fields. The yield stress was found to be independent of the magnetic flux density when the concentration of silica particles was large enough (> ∼20 g/L). This is a manifestation of the entrapment of iron particles in the silica gel. The adhesion of silica on iron particles by acid-base (proton donation) reactions between both colloids in apolar media is also investigated as another mechanism that hinders the aggregation among iron particles under the external field action. For the same reasons, σy ceased to scale as B2, or to increase with iron volume fraction, for such a threshold silica concentration. Oscillometric determinations were performed at a frequency of 1 Hz, and the complex viscosity was found to increase with B due to structure formation as a result of magnetic particle–particle interactions. In agreement with steady-state results, if the concentration of silica is sufficiently large, the complex viscosity reaches high values, but independent of magnetic flux density. Creep-recovery experiments are particularly sensitive to MRF stability, because the interplay between iron and silica concentrations, magnetic flux density, and waiting time after preshear, led to a broad range of behaviors, ranging from liquid-like to almost elastic solid.An experimental investigation is described on the stability of magnetorheological fluids (MRFs) consisting of iron suspensions in silicone oil with a thixotropic agent (silica nanoparticles) as stabilizer. The rheological properties were investigated using a commercial rheometer with a parallel-plate measuring cell. Several kinds of experiments were performed in steady-state, oscillatory, and transient regimes. The effects of the volume fraction of magnetic particles, the concentration of silica, magnetic flux density, B, and waiting time after preshear on the rheology of the MRFs were considered. Steady-state measurements demonstrated that our systems only display plastic behavior, for which a yield stress, σy, is appreciable, for the highest iron concentrations and/or magnetic fields. The yield stress was found to be independent of the magnetic flux density when the concentration of silica particles was large enough (> ∼20 g/L). This is a manifestation of the entrapment of iron particles in the silica g...

Flame‐retardant mechanism of silica: Effects of resin molecular weight

Abstract: The effects of resin molecular weight on the flame-retardant mechanism of silica were studied with two different molecular weights of poly(methyl methacrylate) (PMMA), 122,000 and 996,000 g/mol, and two silicas, fused silica with a small surface area and silica gel with a large surface area. A total of six different samples were studied, with a mass fraction of 10% silica. The mass loss rate of the six samples in nitrogen and the heat release rate from burn- ing in air were measured at an external radiant flux of 40 kW/m 2 . The addition of silica gel to the low-molecular- weight PMMA significantly reduced the mass loss rate and heat release rate; addition to the high-molecular-weight PMMA provided the largest reductions of these quantities in this study. For fused silica, some reduction in mass loss rate and heat release rate was observed when it was added to the high-molecular-weight PMMA; addition to the low-molec- ular-weight PMMA did not reduce either loss rate. Chemical analysis of the collected residues and observation of the sample surface during gasification reveal the accumulation of silica near the surface; the larger its coverage over the sample surface was, less the mass loss rate and heat release rate were. Both the level of accumulation and its surface coverage depended strongly not only on the silica charac- teristics but also on the melt viscosity of the PMMA. © 2002

Preparation of Ni/SiO2 catalyst with high thermal stability for CO2-reforming of CH4

Abstract: Silica-supported nickel (Ni/SiO 2 ) catalysts were prepared by homogeneous precipitation of nickel hydroxide in a wet silica gel derived from a sol–gel process (HPG). Their microstructures and their catalytic performance for the CO 2 -reforming of CH 4 were investigated in comparison with those of Ni/SiO 2 prepared by a conventional incipient wet impregnation process (IMP). The HPG process consists of two successive steps: gelation of silica in the presence of nickel nitrate and urea at 20 °C, followed by aging at higher temperature, typically at 80 °C, to induce thermal decomposition of the urea. The nickel metal surface area of the HPG catalysts was larger than that of IMP catalysts at each calcination temperature. Nickel species in HPG catalysts are expected to be entrapped in the network of silica with high dispersion, while those in IMP catalysts are expected to aggregate on the surface of silica. The characteristic structure in HPG catalysts is considered to be formed through concurrence of dissolution–reprecipitation of silica and entrapment of nickel species into the network. Such processes are promoted in a basic condition led by NH 3 formation during thermal decomposition of urea at 80 °C. The amount of carbon deposited and the catalytic deactivation degree in HPG catalysts were smaller than those in IMP catalysts. Thermal stability of HPG catalysts was much higher than that of IMP catalysts and catalytic activities of the former were higher than those of the latter. This originates in the highly-dispersed nickel metal particles in the rigid network of silica for HPG catalysts.

Liquid chromatography under critical and limiting conditions: A survey of experimental systems for synthetic polymers

Abstract: At the interface between the entropic size exclusion separation and the enthalpy-dominated liquid adsorption chromatography it is possible, experimentally, to identify conditions where polymer samples elute independent of their molar mass. These critical conditions function according to theory by compensating the entropy and enthalpy of the separation. For a series of common and specialty polymers, the mobile phase compositions and temperature which provide the molar mass independent elution behavior, for a given stationary phase, have been extracted from literature and summarized herein. This collection may help to select, or forecast, suitable LC systems, when an application of liquid chromatography under critical conditions for a polymer is required. Correlations between properties of solvents, sorbents and polymers, such as solubility parameters, eluotropic strength and Mark-Houwink constants have been extracted from the collected data. Specifically, solubity parameters of critical mobile phases corresponding to a pair polymer-sorbent are in a majority of cases very similar. The elution strength of the first component of a critical binary eluent correlates linearly with the volume percent of the second component, especially, when an identical silica gel is applied. However, the critical conditions are independent of the thermodynamic quality of the solvent. Under limiting conditions, the mobile phase may be even a precipitant or a strong adsorption promoting liquid for an injected polymer. Possibilities for increase of the upper molar mass separation limit are outlined and influence of sample solvent on elution behavior is described. Applications of liquid chromatography under conditions of enthalpy-entropy compensation for separation of homo- and copolymers are also, briefly, summarized.

Synergistic effect of plasmacatalyst and ozone in a pulsed corona discharge reactor on the decomposition of organic pollutants in water

Abstract: Plasmacatalytic effects of ?-alumina, ?-alumina, and silica gel in a pulsed corona discharge (PCD) reactor on the decomposition of aqueous methylene blue are described. Methylene blue concentration in the effluent was reduced to 23% of the inlet concentration by PCDs in water. Under the same experimental conditions, addition of ?-alumina further reduced it to 8%, ?-alumina to 4%, and silica gel to below the detection limits. PCDs with silica gel were run for >100?h in a continuous flow reactor and methylene blue in the effluent remained below the detection limit. A hybrid system of plasmacatalysis and ozonation is also described. Phenol concentration in the effluent was reduced to 84% of the inlet concentration by PCDs in water. Under the same experimental conditions, addition of either silica gel or ozone further reduced it to around 35%, and simultaneous addition of silica gel and ozone to 14% of inlet concentration. Decolourization of pre-adsorbed methylene blue on silica gel has been demonstrated. Adsorption and stabilization of the chemically active species on silica gel was indicated by experimental evidence. A significant improvement in the rate of decomposition of organic pollutants in water has been realized by hybridizing plasmacatalysis and ozonation in a PCD reactor.

Surface Polymerization of Aniline on Silica Gel

Abstract: The surface polymerization of aniline producing polyaniline nanofilms on surfaces in contact with the reaction mixture has been investigated. The hypothesis that aniline polymerization is heterogeneously catalyzed by the surfaces has been tested. This concept is applicable to planar surfaces and also to surfaces provided by microparticles. The introduction of silica gel, a substrate with a large specific surface area, led to the acceleration of polyaniline formation. The effect of silica gel concentration, particle size, and porosity is discussed, and the courses of polymerization in the presence of porous and compact spheres are compared. It can be concluded that the initiation of aniline polymerization is promoted by adsorption of oligomeric intermediates at the surface. The surface polymerization, giving rise to polyaniline coatings, is thus preferred to precipitation polymerization in the aqueous phase that produces a polyaniline precipitate. The coating of the silica gel surface with polyaniline was ...

Synthesis and characterisation of ibuprofen-anchored MCM-41 silica and silica gel

Abstract: A non-steroidal anti-inflammatory drug (ibuprofen) has been anchored inside the mesoporous channels of MCM-41-type silica and on a silica gel surface. The relevant anchoring procedure through an ester function has been investigated. It uses the epoxide ring opening of 3-glycidoxypropylsilane grafted on the silica surface by the carboxylic-group-containing ibuprofen. The control of the surface modification and of the anchoring efficiency was achieved by comparison of spectroscopic data with those obtained using homogeneous counterparts. The use of nanostructured silica allowed an accurate verification of the different surface modifications and also a higher drug loading.

Determination of Cu, Ni, and Zn in fuel ethanol by FAAS after enrichment in column packed with 2-aminothiazole-modified silica gel.

Abstract: This work describes the synthesis and characterization of 2-aminothiazole-modified silica gel (SiAT), as well as its application for preconcentration (in batch and column technique) of Cu(II), Ni(II) and Zn(II) in ethanol medium. The adsorption capacities of SiAT determined for each metal ion were (mmol g–1): Cu(II)=1.20, Ni(II)=1.10 and Zn(II)=0.90. In addition, results obtained in flow experiments, showed a recovery of ca. 100% of the metal ions adsorbed in a column packed with 500 mg of SiAT. The eluent was 2.0 mol L–1 HCl. The sorption–desorption of the studied metal ions made possible the development of a preconcentration method for metal ions at trace level in fuel ethanol using flame AAS for their quantification.

Measuring and modeling supercritical adsorption in porous solids : carbon dioxide on 13X zeolite and on silica gel

Abstract: Supercritical adsorption of carbon dioxide on 13X zeolite pellets and on silica gel is studied experimentally and theoretically. A gravimetric apparatus with provision for in situ density measureme...