Showing papers on "Silica gel published in 1996"

The Fumed Silica Surface: A Study by NMR

Abstract: High-resolution solid-state NMR techniques were used to investigate the surface structure of Cab-O-Sil fumed silica. 1H NMR results obtained from CRAMPS, MAS-only and relaxation studies reveal the existence of both hydrogen-bonded silanols and isolated silanols on the Cab-O-Sil surface. A systematic dehydration study of fumed silica was carried out, with results on the quantity of each type of silanol on the surface at various dehydration stages. 29Si CP-MAS experiments, including CP spin dynamics studies and various other relaxation studies, were employed to probe hydrogen bonding and the local structural environments of various hydroxyl groups of silica surfaces. 29Si CP-MAS experiments on water-treated and deuterium-exchanged Cab-O-Sil indicate the existence of interparticle silanols and internal silanols in fumed silica. 1H and 29Si NMR show that for fumed silica both isolated and hydrogen-bonded silanols are present on the surface of an untreated sample, in contrast to the case of silica gel, where a...

Selective water sorbents for multiple applications, 5. LiBr confined in mesopores of silica gel: Sorption properties

Abstract: This paper presents sorption properties of a selective water sorbent based on mesoporous KSKG silica gel as a host matrix and lithium bromide as a hygroscopic salt. Sorption isobars, isochores and isotherms measured at T=40–120°C and partial vapor pressures of 7.5–81.0 mbar indicated two types of water sorption: 1) formation of a solid crystalline LiBr monohydrate at low amounts of sorbed water, and 2) vapor absorption by the salt solution at higher sorptions. Sorption properties of the LiBr monohydrate are found to change significantly due to salt impregnation into the mesoporous silica gel, whereas the solution confinement to the mesopores did not change its water sorption properties as compared to the bulk solution. Desorption curves follow a first order kinetics in the temperature range of 60–130°C at different vapor pressures.

Adsorption of fluoride, phosphate, and arsenate ions on lanthanum- impregnated silica gel

Abstract: A lanthanum-impregnated silica gel has been developed for the removal of fluoride, phosphate, and arsenate ions by adsorption The interaction between silica gel and lanthanum ion was maximum at a final pH of 6 The removal of fluoride and arsenate ions by adsorption on the lanthanum-impregnated silica gel was more than 99 9% at neutral pH from initial concentration of 055 and 02 mmol/L, respectively The removal of phosphate ion was 95% at an initial concentration of 05 mmol/L at neutral pH Arsenite ion was not adsorbed on the material The rate of adsorption of the anions followed the first-order reaction and fit in the Lagergren equation The adsorption of each anion followed the Langmuir isotherm Other anions such as Cl{sup -}, BR{sup -}, I{sup -}, NO{sub 3}{sup -}, and SO{sub 4}{sup 2-} did not interfere with the adsorption A column study was conducted for the removal of these anions at a fixed flow rate of 05 mL/min at pH approximately 7 These anions were removed by more than 999% at initial first or second fraction, and the column was regenerated at pH 85 The method was applied for the removal of these anions from synthetic and high-tech industrial wastewatersmore » 30 refs, 8 figs, 1 tab« less

Adsorption on new and modified inorganic sorbents

Abstract: Part 1 Preparation, structure and properties of new and modified inorganic sorbents: computational studies on the design of synthetic sorbents for selective adsorption of molecules, R. Vetrivel et al controlled porosity glasses (CPGs) as adsorbents, molecular sieves, ion-exchanges and support materials, A.L. Dawidowicz influence of pH of precipitation of hydroxides on the structure of co-precipitated adsorbents, V.S. Komarov colloidal silicas, S. Kondo complex carbon-mineral adsorbents - preparation, surface properties and their modification, R. Leboda and A. Dabrowski solid-phase hydrosilylation reactions with participation of modified silica surface, V.A. Tertykh and L.A. Belyakova structure and molecular organization of bonded layers of chemically modified silicas, A. Yu et al the chemical basis of surface modification technology of silica and alumina by molecular layering method, A.A. Malygin et al kinetics of organic compounds chemisorption from the gas phase on oxides surface, V.I. Bogillo functionalized polysiloxane sorbents - preparation, structure, properties and use, Yu L. Zub and R.V. Parish surface structure and molecular adsorption of apatites, T. Ishikawa surface chemistry and adsorption properties of Al13 colloids, J.Y. Bottero and J.M. Cases. Part 2 Adsorption from gaseous phase: Computer simulation of adsorption on amorphous oxide surface, V. Bakaev and W. Steele on the nature of the energetic heterogeneity of water/oxide interface in adsorption phenomena occurring at oxide surface, W. Rudzinski et al energetic heterogeneity of porous inorganic oxides - adsorption and chromatographic studies, M. Jaroniec adsorption kinetics on real surfaces, G.F. Cerofolini inverse gas chromatography in the examination of acid-base and some other properties of solid materials, A. Voekel chemical and morphological characteristics of inorganic sorbents with respect to gas adsorption, E. Papirer and H. Balard structure and properties of the films formed by organic substances on silica gel surface, investigations by inverse gas chromatography (IGC), J. Rayss the use of gas chromatography to study the adsorption from gaseous phase at the finite dilution, F.J. Lopez Garzon and M. Domingo Garcia molecular statistic and gas chromatographic study of hydrocarbons adsorption on the modified layer silicates and silica in the Henry region, Yu I. Tarasevich et al micropore filling mechanism in inorganic sorbents, K. Kaneko phase transitions in adsorbed layers, A. Patrykiejew drying of gases and liquids by activated alumina, S. Sircar et al. Part 3 Adsorption from solution: characterization of inorganic sorbents by means of adsorption at the liquid-solid interface, A. Dabrowski et al study of adsorption from solutions by chromatography, V. Ya Davydov equilibria of adsorption from solutions on the silica surface, V.A. Tertykh and V.V. Yanishpolskii adsorption from dilute solutions - some novel aspects, P. Nikitas (Part contents)

Mechanisms of hydroxyapatite formation on porous gel-silica substrates

Abstract: The variables affecting the nucleation and crystallization of biological hydroxy carbonate apatite (HCA) on porous gel-silica substrates are examined. Texture is the critical variable with the rate of HCA formation increasing as pore size and pore volume increase, with pore sizes >2 nm required to achieve rapid kinetics (4–6 days) of fully crystallized HCA. The concentration of silanol groups on the silica surface does not control the rate of HCA formation although metastable surface defects, such as trisiloxane rings, may be involved in HCA nucleation.

Probing hydrogen bonding and the local environment of silanols on silica surfaces via nuclear spin cross polarization dynamics

Abstract: By studying 1H→29Si cross-polarization dynamics of two untreated and two “dry” silica gel samples (one evacuated at 25 °C and one evacuated at 200 °C), we find that all the surface silanols on the two untreated silicas are hydrogen bonded, either to the hydroxyl groups of adjacent silanol(s) or to water molecule(s). About 46% and 47% of the geminal silanols and 53% and 58% of the single silanols that were hydrogen bonded only to water in the two untreated silicas become non-hydrogen bonded on the two “dry” silica surfaces, but the remainder of the silanols of the untreated silicas (i.e., those hydrogen bonded to other silanols) remain hydrogen bonded to other silanols upon drying. The ratio of the number of hydrogen-bonding single silanols to the number of hydrogen-bonding geminal silanols is 17-to-1 for a Fisher silica surface evacuated at 25 °C and 16-to-1 for a Baker silica surface evacuated at 200 °C. These results can be explained in terms of a generalized silica surface model based on the β-cristoba...

Silver nanocrystals in silica by sol-gel processing

Abstract: Silver nanocrystal doped silica films were prepared by the sol-gel process. The sol was prepared from 1:0.12:12:0.2:6:7 molar ratios of Si(OC2H5)4:AgNO3:H2O:HNO3:C3nH7OH:C4iH9OH. The glassy, highly transparent film with high dopant concentration (silver/silicon atomic ratio = 0.12) was successfully prepared by the dip-coating method. After drying in air at 60°C for 30 min, samples were heat-treated, in air, at 300, 350, 400, 450, 500 and 550°C using 30–100 min soaking periods for each step in a cumulative heating procedure. Measurements on the films were made by ultraviolet-visible and infrared spectroscopy, X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry and transmission electron microscopy. Mechanisms of silver colloid formation in the densified silica matrix with respect to the thermal treatment are discussed. To understand the formation of silver nanocrystals from the silver silicate network, the corresponding doped bulk gel samples were analyzed by X-ray diffraction and differential scanning calorimetry.

Preparation of silica gel-bonded amylose through enzyme-catalyzed polymerization and chiral recognition ability of its phenylcarbamate derivative in HPLC.

Abstract: Amylose was prepared by enzymatic polymerization of α-d-glucose 1-phosphate dipotassium catalyzed by a phosphorylase using two kinds of the primers derived from maltopentaose, and then it was chemi...

Photochemical oxidation of phenanthrene sorbed on silica gel

Abstract: There have been relatively few detailed studies of PAH photochemical degradation mechanisms and products at solid/air interfaces under controlled conditions. Results from mechanistic studies on par...

High-Pressure Organic Chemistry. 19. High-Pressure-Promoted, Silica Gel-Catalyzed Reaction of Epoxides with Nitrogen Heterocycles1,†

Abstract: The reaction of epoxides with nitrogen heterocycles such as indoles, pyrroles, imidazoles, and pyrazoles was studied under both high-pressure and silica gel-catalyzed conditions. Whereas it has been reported that the treatment of indole with styrene oxide at 10 kbar and 42 °C for 24 h gave 2-(3-indolyl)-2-phenylethanol in 56% yield, the same compound was obtained in 88% yield when the reaction was conducted on silica gel at rt for 1 week. Similarly, efficient reaction of epoxides with pyrroles, imidazoles, and pyrazoles was achieved. In terms of stereochemical features, the epoxide ring opening reaction of (R)-(+)-styrene oxide with indole was found to proceed stereoselectively in an SN2 fashion at the benzyl carbon, in either case.

Apatite formation on silica gel in simulated body fluid: Its dependence on structures of silica gels prepared in different media

Abstract: It has been shown that the prerequisite for glasses and glass-ceramics to bond to living bone is the formation of a layer of biologically active bonelike apatite on their surfaces. The hydrated silica formed on the surfaces of glasses and glass-ceramics plays an important role in nucleating the apatite. In the present study, the structure of the hydrated silica responsible for the apatite nucleation was investigated in an accellular simulated body fluid with ion concentrations nearly equal to those of human blood plasma. Three kinds of porous silica gels were prepared by hydrolysis and polycondensation of tetraethoxysilane in pure water or in aqueous solution containing polyethylene glycol or polyacrylic acid. The silica gels prepared in aqueous solution containing polyethylene glycol or polyacrylic acid had micron-size interconnected pores, whereas the gel prepared in pure water did not. All the gels contained a large volume of nanometer-size pores, almost the same amounts of silanol groups and D2 defect, and showed a high dissolution rate of the silica. Despite this, only the gel prepared in the solution containing polyethylene glycol formed the apatite on its surface in the simulated body fluid. This indicates that only a certain type of structural unit of the silanol group is responsible for the apatite nucleation.

Synthesis conditions for encapsulating cytochrome c and catalase in SiO2 sol-gel materials

Abstract: The encapsulation of biological molecules in sol-gel materials has led to the development of a new class of chemical and biomedical sensors. The influence of sol-gel synthesis conditions on the stability, chemical function, and enzymatic reactivity for horse heart cytochrome c and bovine liver catalase encapsulated in silica sol-gel materials was studied. The effects of synthesis pH, alcohol/alkoxide ratio, and buffer type were examined using UV-V is absorption spectroscopy for cytochrome c -doped materials and gasometric analysis for assays of catalase in gels. In general, the behavior of the proteins encapsulated in silica gels mirrored their behavior in solution suggesting that the stability limiting factor for encapsulating these biological molecules is determined by the solution stability of the protein. In the case of catalase, the results indicate that the silica gel stabilizes the encapsulated enzyme to higher methanol concentrations.

Hybrid Silica Gels Containing 1,3-Butadiyne Bridging Units. Thermal and Chemical Reactivity of the Organic Fragment

Abstract: Silica gel containing diyne units have been obtained from molecular organosilicon precursors and the properties associated to the very reactive unsaturated organic moieties have been explored. The sol−gel polymerization of 1,4-bis(trimethoxysilyl)-1,3-butadiyne ((MeO)3SiC⋮CC⋮CSi(OMe)3) quantitatively led to a silsesquioxane network, [O1.5SiC⋮CC⋮CSiO1.5]n, consisting of siloxanes chains with bridging diyne units. The derived xerogels were characterized by IR and 13C and 29Si CP MAS NMR spectroscopies. The major environment of the Si atom corresponded to a T2 CSi(OR)(OSi)2 substructure, and only minor Si−C bond cleavage occurred during the sol−gel condensation. The chemical reactivity of the hybrid organic−inorganic gel was studied and used as a tool for the study of the organization of the solid induced by the organic moieties. Upon heating, in the solid state, the diyne fragments undergo a polyaddition to give an ene−yne structure. The polymerization, observed in the solid state, suggests favorable arrang...

Dimer-to-monomer transformation of rhodamine 6G in sol—gel silica films

Abstract: The process of dimerization and disaggregation of rhodamine 6G in tetraethoxysilane-derived silica gel films was studied using UV-visible and fluorescence spectroscopy. Pyranine (8-hydroxy trisulfonated pyrene) was also used as a fluorescence probe to study the chemical evolution in the films during drying. Rh6G dimers were found in the as-deposited films although only monomers were observed in the sols. Larger amounts of dimers were found in the films prepared from the solutions of higher dye concentration and water content. During drying up to 150°C, the dimers changed gradually into monomers. The dimer-to-monomer transformation was explained by the evaporation of the solvent from the films.

Shrinkage of silica gels aged in TEOS

Abstract: A model has been developed for predicting the shrinkage that occurs during drying, given knowledge of the initial pore size and modulus of the gel. The model is in good agreement with experimental results for silica gels given a variety of aging treatments. The initial pore size (used to predict the capillary pressure that drives shrinkage) can be found by making an aerogel and performing nitrogen desorption, or by calculating the effective pore size from the measured permeability; it is shown that those methods give similar results. Thus the drying shrinkage can be predicted using data obtained from the wet gel (viz. permeability and modulus). At the moment that the liquid/vapor meniscus enters a pore, the walls will be covered with a film of adsorbed liquid, which reduces the radius of the meniscus below the radius of the pore, and thereby increases the capillary pressure. The film thickness for the solvents used is not known so one assumes a thickness of 1 nm. If the presence of the layer is ignored, the theory underestimates the drying shrinkage.

Synthesis and characterization of inorganic gels in a lyotropic liquid crystal medium. Part 2.-Synthesis of silica gels in lyotropic crystal phases obtained from cationic surfactants

Abstract: Microporous silica materials with tailored porosity are synthesized by the sol–gel process using lyotropic liquid-crystal phases as templates. The starting isotropic sol is obtained by mixing tetramethoxysilane, water and an alkyltrimethylammonium bromide. The polymerization of the silica network and the formation of the amphiphilic mesophase are simultaneous and cooperative. After thermal elimination of the surfactant molecules, an ordered porous texture is maintained in the silica material. The pore size is related to the size of the templating unit and modulated by the length of the alkyl chain of the used surfactant. In this paper, the various steps of the synthesis are studied and the porous texture of the final material is characterized and discussed in terms of templating effects.

Preparation of transparent methyl-modified silica gel

Abstract: Transparent methyl-modified silica gel is produced by reacting methyltrialkoxysilane or a mixture thereof with a tetraalkoxy-, trialkoxy- or dialkoxysilane other than the methyltrialkoxysilane with a defined quantity of water at a neutral or acidic pH in the presence of a catalytically effective amount of a metal complex soluble in the methyltrialkoxysilane, and aging the resulting hydrolysate sol to form a gel. Self-sustained films of the transparent gel are produced by spreading the hydrolysate sol onto a nonadherent substrate followed by aging or drying. The gel finds use as a host material for doping optically functional molecules.

Antioxidant activity of ethanolic extract of mustard seed

Abstract: Four fractions were separated from an ethanolic extract of mustard using silica gel column chromatography. Antioxidant activity of these fractions was evaluated in a β-carotene-linoleate model system. Fractions III and IV, which possessed a high content of phenolic compounds showed higher activity than fractions I and II, which contained a lower content of phenolics. All fractions were examined by silica gel TLC. The content of phenolic acids in each fraction was confirmed by UV spectral data.

A new titanium-silica catalyst for the epoxidation of alkenes

Abstract: Silica gel treated with Ti(OiPr)4 is an efficient catalyst for the TBHP epoxidation of non-functionalized alkenes, dienes, and allylic alcohols. This catalyst is more efficient than those obtained from other titanium precursors; it must be activated at 140°C prior to use, it can be recovered and stored in the open air. The reactivity of different olefinic substrates is compared and is shown to be dependent on the structure, which allows the selective epoxidation of compounds containing different kinds of double bonds.

Distribution of organic groups in silica gels prepared from organo-alkoxysilanes

Abstract: Organically modified silicas were prepared from mixtures of tetraethylorthosilane and methyltriethoxysilane (0–40 mol%) or dimethyldiethoxysilane (0–20 mol%) by a two-step sol-gel synthesis procedure involving sol aging at pH = 1.95 and gel formation at pH = 6. The texture and structure of dry gels were inferred from N 2 adsorption, 29 Si MAS-NMR and differential thermal analysis. Also the water sorption capacity of the products was measured. The distribution of the organic groups between the silica network and the gel surface determines to a large extent texture and water sorption of resulting gels. As indicated by MAS-NMR, the exothermic peak which arises in differential thermal analysis due to combustion of organic groups contains valuable information on the distribution of organic groups. A broad peak results when methyl groups are homogeneously divided over the entire gel network whereas a narrow peak indicates preferential localization of organic functions at the gel surface. In the latter case, products with higher pore volume and lower water sorption are obtained.