Showing papers on "Silica gel published in 1989"

Transparent silica gel–PMMA composites

Abstract: Transparent silica gel–polymer composites have been prepared by the impregnation of porous gels with organic monomer and polymerization in situ. The relative amount of each phase was adjusted by varying the porosity of the silica gel prior to impregnation. These materials constitute a new class of transparent composites. Properties, such as density, refractive index, modulus of rupture, compressive strength, abrasion rate, and Vickers hardness, have been measured over the compositional range of 100% silica to 100% polymethyl methacrylate (PMMA).

Method for utilizing triethylaluminum to prepare an alumoxane support for an active metallocene catalyst

Abstract: of EP0368644This invention relates to a process for preparing a supported metallocene alumoxane catalyst for use in the polymerization of olefins. The invention particularly relates to the use of silica gel containing from about 6 to about 20 per cent by weight adsorbed water as the catalyst support material. It has been found that such silica gel may be safely added to a solution, of trialkylaluminum to form by direct reaction with the adsorbed water content of the silica gel catalyst support material the majority of the alumoxane component requirement of the catalyst system. An alkylalumoxane coated silica gel is formed to which a trimethylaluminum treated metallocene may be added and the resulting material dried to free-flowing powder. The dry free flowing powder may then be used as a supported metallocene alumoxane catalyst complex for gas phase or liquid phase polymerization of olefins. The supported catalyst produced by such procedure has an activity at least comparable to a catalyst of similar composition which is produced exclusively with trimethylaluminum as the alumoxane forming aluminum alkyl.

2,4-Dinitrophenylhydrazine-coated silica gel cartridge method for determination of formaldehyde in air: identification of an ozone interference

Abstract: Two versions of the 2,4-dinitrophenylhydrazine method, a coated silica gel cartridge (solid) and acetonitrile impinger (solvent based), were used simultaneously to sample varied concentrations of ozone (0-770 ppb) and formaldehyde (20-140 ppb). Ozone was found to be a negative interference in the determination of formaldehyde by the 2,4-dinitrophenylhydrazine-coated silica gel cartridge method. At 120 ppb of ozone, formaldehyde at 40 ppb was under-reported by the cartridge method by 34% and at 300 ppb of ozone, formaldehyde measurements were 61% low. Greater losses were seen at higher ozone concentrations. Impinger sampling (2,4-DNPH in acetonitrile) showed no formaldehyde losses due to ozone.

Amino groups immobilized on silica gel: an efficient and reusable heterogeneous catalyst for the Knoevenagel condensation

Abstract: Silica gel functionalized with amino groups is a useful insoluble catalyst for the Knoevenagel condensation: the reaction can be carried out under continuous-flow conditions and good yields are obtained when aromatic aldehydes, cyclohexanone, and acetophenone react with ethyl acetoacetate, ethyl cyanoacetate, and malononitrile. Lower yields are obtained in the case of ethyl benzoylacetate and acetylacetone; this fact and the easy dehydration of the aldol intermediate strongly suggest the participation of the residual free silanol groups of the matrix in the catalysis mechanism.

Study of structural evolution of silica gel using 1H and 29Si NMR

Abstract: Silica sols and gels were prepared by hydrolysis and condensation of tetraethyl orthosilicate (TEOS) under acid conditions. By use of 29Si and 1H NMR the chemical evolution of the system was followed through gelation, aging, and drying at temperatures ⩽ 50 °C. The effects of higher H2O/TEOS ratios, refluxing, and stripping on the promotion of short- and long-term condensation and gelation were characterized. The extent of crosslinking (i.e., the number of siloxane bonds) was found to increase very slowly after gelation. The driest gels still retained about one SiOH per two Si atoms. The proton spectra showed that the dynamic state of the protons in dried gels correlates well with the residual water content, but not with the details of the drying treatment: when the moisture level exceeds 0.5 H2O per Si atom, the water is uniformly distributed throughout the drying gel network, where it provides a mechanism for high diffusional mobility of all the silanol and water protons. When less than 0.2 free-water molecules per Si remain in the gel, the hydroxyls of both SiOH and residual H2O are immobilized in the silica network. The bulk structure of the dried gel skeleton resembles the surface layer of colloidal silica particles.

A structural approach to bone adhering of bioactive glasses

Abstract: Some silicate glasses exhibit the property of bonding to bone. In vivo tests show that a layer of silica gel is always formed on the glass surface when the bonding is successful. From a comparison with the silicon chemistry of diatoms and connective tissues it is apparent that silicate chelates form an essential step in the formation and mineralization of hard tissues. The same kind of chelating is also demonstrated for silicate anions in binary melts. It is suggested that in implants, the silica gel formed on the glass surface not only acts as a chelating agent, but also is flexible enough to supply the correct atomic distances required by the crystal structure of bone apatite. The phosphate phase used in most bioactive glasses is suggested to act as a source for a phosphate buffer, which adjusts the surface acidity of the glass implant.

Efficient tunable solid-state laser near 630 nm using sulforhodamine 640-doped silica gel

Abstract: Lasing is observed near 630 nm in a sulforhodamine 640-doped silica gel pumped by frequency-doubled radiation from a Q-switched Nd: YAG laser. A conversion efficiency of 20% is obtained. The laser wavelength is tunable over 40 nm.

Silica gel dissolution in aqueous alkali metal hydroxides studied by 29SiNMR

Abstract: The rate of silica gel dissolution in aqueous alkaline media was investigated using 29Si nuclear magnetic resonance spectroscopy. A profound difference of alkali metal hydroxides on the dissolution rate of amorphous silica gel was observed. The dissolution rate increases in order LiOH ≈ CsaOH) < (RbOH ≈ NaOH) < KOH, as was confirmed by the β = silicomolybdate complexation method. Silica gel dissolution involved formation of monomeric silicic acid, Q0. The monomeric anions oligomerise into dimer species, which in turn form cyclic and lineartrimer species. The structure of highly polymerised silicate species depends on the alkali metal cation, i.e. low pH silicate solutions have structurally different silicate species as a function of alkali metal hydroxide, as is shown by 29Si-NMR spectroscopy. Potassium cations gave rise to more polymerised silica species compared with other alkali metal cations studied. On the contrary, when lithium hydroxide and silica gel are mixed in a molar SiO2/Li2O = 1:1, a microcrystalline phase is formed which consists of lithium silicate crystals. Crystallisation of the lithium silicate proceeds via monomeric silica being in solution.

Effect of shrinkage on the modulus of silica gel

Abstract: The shear modulus of alkoxide-derived silica gels was measured during aging and drying; base- and acid-catalyzed gels were examined, and gave qualitatively similar results. There is a continual increase in shear modulus (G) during syneresis, which is most rapid for the first two weeks after gelation. If the gel is soaked in a liquid containing more water than the pore liquor, it shrinks and stiffens more rapidly; base-catalyzed gels exhibit phase separation (apparently from segregation of residual partially condensed TEOS). If evaporation is prevented, G only increases by a factor of ∼ 2 upon aging from 1 to 12 months. When the liquid is allowed to evaporate, G increases by about 3 orders of magnitude as the gel contracts by ∼ 50% linearly. There is an approximate exponential relation between G and the radial strain that applies about as well for samples aged in various solutions as for samples dried in air. The relation between G and strain (contraction) is nearly independent of drying rate when the linear strain exceeds ∼ 10%. The modulus of rupture rises by about 2.5 orders of magnitude during drying.

High performance liquid chromatographic separation of taxol and related compounds from Taxus brevifolia

Abstract: Using cyano and phenyl bonded silica gel phases in the reverse phase mode, was have developed sensitive analytical high performance liquid chromatography (HPLC) methods for detecting taxol and four related taxanes, cephalomannine [II], 10-deacetylcephalomannine [III], baccatin III [IV] and 10-deacetylbaccatin III [V], in the crude methanol extract and the methylene chloride solubles partitioned from the methanol extract

Method for preparing polyethylene wax by gas phase polymerization

Abstract: This invention relates to a process for preparing polyethylene wax having a narrow molecular weight distribution in a gas phase polymerization reactor. The invention particularly relates to the process of polymerizing ethylene in the presence of a supported metallocene-alumoxane catalyst and hydrogen. The catalyst may be derived from the addition of silica gel containing about 6 to about 20 percent by weight adsorbed water to a trialkylaluminum solution. An alumoxane coated silica gel is formed to which a metallocene may be added and the resulting material dried to free flowing powder. A trimethylaluminum pretreated metallocene may be employed when the trialkylaluminum is selected from the group consisting of triethylaluminum, tripropylaluminum, tri-n-butyl-aluminum, tri-isobutylaluminum, tri(2-methylpentyl)aluminum, trihexylaluminum, tri-n-octylaluminum and tri-n-decylaluminum.

Enrichment of tocopherols in wheat germ by directly coupled supercritical fluid extraction with semipreparative supercritical fluid chromatography.

Abstract: Enrichment of tocopherol by coupled supercritical fluid extraction/preparative supercritical fluid chromatography is described. Wheat germ powder is used as the starting material and is subjected to supercritical fluid extraction with carbon dioxide. The extracted oil containing tocopherols is concentrated and trapped on a silica gel column by reducing the pressure of carbon dioxide. The trapped oil is then eluted and separated on a silica gel column of 20 mm i.d. x 20 mm length. The column effluent is fractionated by monitoring UV absorption at 290 nm. With this method, tocopherol content of the wheat germ is enriched 100-fold.

Preparation of Proton‐Conducting Amorphous Films Containing Dodecamolybdophosphoric Acid by the Sol—Gel Method

Abstract: Silica gel films containing dodecamolybdophosphoric acid, H3PMo12O40·29H2O (MPA), were prepared by the sol—gel method. Amorphous films were obtained for the compositions with 0 to 42 wt% MPA. Infrared spectra revealed that the skeleton of heteropolyanions PMo12O3−40 remained in the silica gel structure. The gel films exhibited considerably high proton conductivities in a highly humid atmosphere, and the conductivity increased with increasing MPA content. The value of conductance (G) at 1 kHz for these films was very sensitive to the variation of relative humidity (rh). The relation between log G and rh was almost linear and was reproducible on repeated runs for the gel films with 42 wt% MPA heat-treated at 200°C.

A mild selective monobromination reagent system for alkoxybenzenes; N-bromosuccinimide-silica gel.

Abstract: The monobromination of alkoxybenzenes with N-bromosuccinimide catalyzed by silica gel in carbon tetrachloride occurred preferentially at the para-position. m-t-Butylmethoxybenzene yielded 4-bromo-3...

Fluorescence spectra of 7-azaindole in the sol-gel-xerogel stages of silica

Abstract: The fluorescence spectra of 7-azaindole were studied in a mixed solution of Si(OC{sub 2}H{sub 5}){sub 4}, H{sub 2}O, and ethanol, which reacts to form silica gel glass, i.e., porous SiO{sub 2}. Significant shift in the fluorescence (380-430 nm in {lambda}{sub max}) was observed, mainly during the gel-xerogel stages. Fluorescence, fluorescence excitation, and absorption spectra indicated that 7-azaindole forms a hydrogen-bonded complex with ethanol in the solution and finally forms a complex of stronger hydrogen bonds with silanol in the xerogel. They also show that the large shift in fluorescence is attributed not to the stabilization in the ground state but to that in the excited state. We consider the large shift in the fluorescence spectra can be explained by the interaction between the 7-azaindole complex with silanol and a larger number of polar silanol groups which surround the complex. In addition, fluorescence spectra reveal that a double proton transfer reaction occurs in the sol-gel stages and, possibly, in the xerogel.

Photochemistry on solid surfaces

Abstract: 1. Introduction. Introduction (I. Tanaka). General aspects of photochemistry. Search for new basic types of photochemical reactions (K. Tokumaru et al.). 2. Photochemical Techniques to Understand Photochemical and Photophysical Features on Solid Surfaces. Fluorescence and transient absorption spectra of solid surface: development of time-resolved total internal reflection spectroscopy (H. Masuhara). Laser flash photolysis on solid surfaces (F. Wilkinson, G.P. Kelly). Excimer formation with pyrenes on silica surfaces (K.A. Zachariasse). Photophysics of acridone, N-methylacridone, acridine, and pyrene adsorbed on silica gel (S. Suzuki, T. Fujii). Heterogeneous molecular environments probed by fluorophores bonded to chemically modified silica gel: Fluorescence decay measurements under a microscope (S. Hirayama et al.). Photoacoustic and fluorescence measurements of energy transfer in adsorption layers (H.D. Breuer). 3. Specific Features of Photochemical Reactions on Solid Surfaces. Photochemistry of alkyl ketones in the adsorbed state: effects of solid surfaces upon the photolysis (M. Anpo). Decomposition of azocumene on silica surfaces (J.E. Leffler, J.J. Zupancic). Photolytic and redox mechanisms for the photodecomposition of ethanoic acid adsorbed over pure and mixed oxides (M. Schiavello et al.). ESR studies of alkyl radicals adsorbed on porous vycor glass (H.D. Gesser). Chemiluminescence properties of adsorbed biacridylidenes (K. Maeda, S. Yamada). 4. New Developments of Organic Photochemistry on Solid Surfaces. Photochemistry of dibenzyl ketone adsorbed on size/shape selective faujasite zeolites: steric effects on product distributions (N.J. Turro, Z. Zhang). Photochemistry of organic cations at charged interfaces (C.A. Backer, D.G. Whitten). Electron transfer between adsorbed dye molecules and organic crystals: model character of the adsorption system for certain aspects in photosynthesis (K. Kemnitz et al.). 5. New Developments of Inorganic Photochemistry on Solid Surfaces. Inorganic photochemical reactions in low temperature matrices and in the surfaces of solids (T. Tominaga). Photochemistry of metal carbonyls physisorbed on porous vycor glass (H.D. Gafney). Photochemistry of silica-adsorbed Fe(CO) 5 (R.L. Jackson). Photopreparation of supported metal oxide and metal carbonyl catalysts (A. Morikawa, Y. Wada). 6. Laser Induced Photoreactions and Photo-CVD on Solid Surfaces. UV laser photodissociation of small molecules on solid surfaces (H. Sato, M. Kawasaki). CO 2 laser induced surface reaction (M. Kawai). Photochemical aspects of amorphous-Si nucleation by photo-CVD (H. Hada, M. Kawasaki). 7. Topics of Photochemistry on Semiconducting Materials. Photoprocesses on fractal surfaces (A. Seri-Levy et al.). New aspects in area-selective electrode reactions on illuminated semiconductors (M. Okano et al.). Photoluminescent properties of cadmium sulfide contacted with gaseous Lewis acids and bases (G.J. Meyer et al.).

DNA extraction with organic solvents in gel barrier tubes

Abstract: The extraction of nucleic acids with phenol and chloroform is the standard method to remove proteins and lipids from cellular digests. A major drawback to the use of these organic solvents is their caustic and toxic qualities. Phenol can cause severe burns while chloroform is hepatotoxic and carcinogenic. We describe in this paper the use of a special blood collection tube to isolate the offensive organic solvents. The customary use of these tubes (SST tube, Becton Dickinson, Rutherford, N.J.) is to separate serum from cells during centrifugation of blood. As blood samples are centrifuged in these collection tubes, the silica gel rises through the red cell layer and fixes itself between the red cells and serum. Similarly, we have found high molecular weight eukaryotic DNA (or plasmid preparations after alkaline lysis purification) can be safely separated from organic solvents after centrifugation. White blood cell nuclei isolated from 5 ml of EDTA anticoagulated blood according to Min (1), were resuspended in 3 ml of 0.2 M Tris pH 8.0, 0.05 M EDTA, 1 % SDS, and 100 micrograms of Proteinase K. The mixture incubated 1 hr. at 60°C was added to the silica gel containing tube, and shaken five minutes with phenol:chloroform (1:1). After a five minute centrifugation at lOOOg, the organic layer along with the proteins, became trapped beneath the gel polymer. This allowed the DNA containing aqueous phase above the gel to be simply poured out of the tube. In the usual extraction without silica gel, when DNA was aspirated with a pipet, DNA close to the interface was left behind to avoid contamination with protein. The yield of DNA using the gel barrier extraction averaged 40 per cent higher than the conventional method since there was no interface to avoid. The A26C/A28O was consistently above 1.8, showing good deproteinization (2). Precipitated DNA could be cut with restriction endonucleases after only one extraction. Also, the silica gel tube improved the nonorganic DNA extraction method of Dykes et al. (3) by trapping the precipitated protein beneath the gel polymer.

Silica gel-bound aza-crowns for the selective removal and concentration of metal ions

Abstract: Silica gel-bound macrocycles have been prepared by first hydrosilylating a macrocycle-containing alkene with triethoxysilane followed by coating and heating the resulting macrocycle-containing triethoxysilane on 60-200 mesh silica gel The macrocycle is attached to silica gel by stable hydrocarbon-ether groups The aza- crown-containing alkenes were prepared in good overall yields by convenient three- and four-step reactions using N-(2-(2-~hloroethoxy)ethyl)acetamide as a synthon The silica gel-bound macrocycles were found to be stable at aqueous pH values below 11 In numerous determinations, the log K values for the interaction of the bound- macrocycle with various cations were found to be similar to those for the analogous unbound macrocycle with the same cations in aqueous solution This result indicates that the bound- and unbound-macrocycles are solvated and form complexes in the same manner Several separations of heavy metal ions from aqueous mixtures of ions are described

A new method for immobilization of polymer-protective colloidal platinum metals via co-ordination capture with anchored ligands. Synthesis of the first example of a mercapto-containing supported metallic catalyst for hydrogenation of alkenes with high activity

Abstract: A poly(N-vinyl-2-pyrrolidone)(PVP)-protective metallic rhodium or platinum colloid interacts at room temperature with a suspension of silica gel, bearing mercapto ligands, in a 1 : 1 methanol–water system to give a well dispersed heterogeneous metallic catalyst which shows excellent activity for the hydrogenation of alkenes.

Measurement of permeability. II: Silica gel

Abstract: The permeability (D) of alkoxide-derived silica gel is measured by establishing a pressure gradient in the pore liquid in a thin (∼ 2 mm) sheet of gel. In these experiments, the layers were thin enough to minimize the viscoelastic effects discussed in Part I. For an acid-catalyzed gel (A) with a homogeneous microstructure, the order of magnitude of D is found to be in agreement with the Carman-Kozeny equation. For a base-catalyzed gel (B2) D is larger than predicted, presumably because of relatively large pores in the inhomogeneous network. The permeability drops by ∼ 2 orders of magnitude when the layers are partially dried.

Selectivity of alkyl-bonded phases in reversed-phase liquid chromatography of hydrophobic compounds

Abstract: Starting from silica gel with a purity of 99.999%, octadecyl-(ODS), octyl-(C8) and propyl-(C3) bonded silica gels were synthesized, and end-capped by trimethyl groups. The pore size was 150 A and the specific surface area was 320 m2/g. The ODS and C8 silica gels were stable in both acidic and basic solutions.