Showing papers on "Silica gel published in 1993"
TL;DR: In this article, the freezing of water adsorbed on high surface area materials such as silica gel, controlled-pore glass, and activated charcoal is investigated with NMR methods.
163 citations
TL;DR: In this article, the process of apatite formation on silica gel was investigated by means of thin-film X-ray diffraction, Fourier-transformed infrared reflection spectroscopy and scanning electron microscopic observation of the surface of the silica, as well as the measurement of changes in the ion concentration of the fluid.
Abstract: It has been proposed that a hydrated silica plays an important role in forming a biologically active apatite layer on the surfaces of bioactive glasses and glass-ceramics in the body Recent experiments have shown that a silica hydrogel actually induces apatite formation on its surface in a simulated body fluid (SBF) In the present study the process of apatite formation on silica gel was investigated by means of thin-film X-ray diffraction, Fourier-transformed infrared reflection spectroscopy and scanning electron microscopic observation of the surface of the silica gel, as well as the measurement of changes in the ion concentration of the fluid It was found that the induction period for the apatite nucleation on the surface of the silica gel was about 6 days Once the apatite nuclei were formed they grew, taking a spherulitic form by consuming the calcium and phosphate ions from the surrounding fluid Each spherulite consisted of a lot of flake that clustered into a petal-like morphology The flake was carbonate-containing hydroxyapatite of small-crystallites and/or defective structure The Ca/P ratio of the apatite was estimated as 15-16 Thus, the apatite formed was able to induce secondary nucleation of the apatite
158 citations
TL;DR: It is concluded that sol-gel prepared silica is an efficient apatite inducer and the morphology of the hydroxyapatite deposit is determined by factors of the fluid such as pH, Ca/P molar ratio, Mg and F concentrations.
134 citations
TL;DR: In this paper, a surface bonding scheme for monolayers on silica was designed to combine the superior hydrolytic stability of self-assembled monlayers with the selectivity and reproducibility of monomeric stationary phases.
Abstract: A new surface bonding scheme for monolayers on silica recently reported was designed to combine the superior hydrolytic stability of self-assembled monolayers with the selectivity and reproducibility of monomeric stationary phases. In this work, the scheme is shown to work very effectively for chromatographic silica gel. A mixture of CH[sub 3](CH[sub 2])[sub 17]SlCl[sub 3] and CH[sub 3](CH[sub 2])[sub 2]SlCl[sub 3] is horizontally polymerized onto 40- and 50-[mu]m-diameter porous silica gel particles. [sup 13]C NMR and microanalysis are used to establish that a coverage consistent with horizontal polymerization is achieved. The chromatographic retention behavior of a set of test solutes is shown to be similar to that for conventional monomeric C[sub 18] phases. The repeated synthesis of the new phase is shown to give reproducible retention times for benzo[a]pyrene and aniline. The hydrolytic stability is demonstrated to be greatly improved over that of a monomeric C[sub 18] stationary phase: the new phase is stable to at least 2,400 column volumes of pH 1.8 solution and stable to at least 900 column volumes of pH 10.0 solution. 20 refs., 6 figs., 3 tabs.
133 citations
TL;DR: Both the tin(IV) and indium(III) metalloporphyrin-based stationary phases exhibit useful selectivity for the separation of aromatic sulfonates and aromatic carboxylates.
Abstract: A tetraphenylporphyrin-based stationary phase for high-performance liquid chromatography is prepared by attaching a p-carboxyphenyl derivative of the porphyrin to aminopropyl silica gel through an amide bond. This stationary-phase support is used for the reversed-phase separation of several polycyclic aromatic hydrocarbons and is shown to exhibit novel shape selectivity toward planar aromatic solutes. Subsequent metalation of the immobilized tetraphenylporphyrin with either tin(IV) or indium(III) results in a stationary phase with anion-exchange characteristics. Both the tin(IV) and indium(III) metalloporphyrin-based stationary phases exhibit useful selectivity for the separation of aromatic sulfonates and aromatic carboxylates.
126 citations
Patent•
08 Apr 1993TL;DR: In this paper, a trialkylaluminum compound with a transition metal metallocene was used for the polymerization of olefins and a method for its production.
Abstract: This invention relates to a supported catalyst composition useful in the polymerization of olefins and to a method for its production. The catalyst is formed by first reacting a trialkylaluminum compound with a transition metal metallocene, and subsequently reacting the resulting material with the undehydrated support material to form a solid product containing at least 20 grams of support material per milimole of transition metal. The resulting supported catalyst has a level of activity comparable to that of conventionally made supported catalysts which are achieved at active metal loadings which are reduced from the loadings used in the previously known metallocene-alumoxane catalysts by at least 20 %, and preferably 50 %. The invention particularly relates to the use of undehydrated silica gel containing from 6 to 20 per cent by weight adsorbed water as the catalyst support material.
121 citations
Patent•
08 Nov 1993TL;DR: In this paper, a process for the separation of taxanes, particularly taxol, cephalomanine, baccatin III and deacetylbaccatinIII, from ornamental yew tissue is described.
Abstract: A process for the separation of taxanes, particularly taxol, cephalomanine, baccatin III and deacetylbaccatin III, from ornamental yew tissue is described. The process involves a specific solvent mixture in water and 50 to 95 percent by volume methanol, ethanol or acetone and treatment of the resulting extract using activated carbon, preferably charcoal. The taxanes are preferably separated from the crude extract by a normal phase chromatographic step which preferably is through vacuum and then medium pressure column, chromatographic separation, using inexpensive silica gel as an absorbent. The silica gel is regenerated by heating in a furnace above about 500 °C to remove adsorbed organic materials.
117 citations
Patent•
14 Oct 1993
TL;DR: In this article, a vapor phase corrosion inhibitor-desiccant composite comprising silica gel granules coated with a component is selected from a formulation comprising anhydrous molybdates such as ammonium dimolybdate, sodium molyb date, mixed with benzotriazole and sodium nitrate.
Abstract: A vapor phase corrosion inhibitor-desiccant composite comprising silica gel granules coated with a vapor phase corrosion inhibitor component. The corrosion inhibitor component is selected from a formulation comprising anhydrous molybdates such as ammonium dimolybdate, sodium molybdate and amine molybdates mixed with benzotriazole and sodium nitrate, or from a formulation comprising amine benzoates, amine nitrates and benzotriazole. The composites can be impregnated into foam, extruded with polyolefin films which can additionally be laminated with metallized second film, or encapsulated in an air-permeable container. The corrosion inhibitor formulations have vapor pressures which provide ongoing corrosion protection for susceptible articles situated favorably with respect to the composite.
111 citations
TL;DR: A survey of polymers immobilized on silica gels and of various experimental procedures which can be used for composite sorbent synthesis is given in this article, where the authors also present two main approaches to the preparation of such composite sorbents.
Abstract: A variety of stationary phases for HPLC can be prepared by immobilization of organic polymers on silica gels. Such silica-polymer adsorbents combine the strength of an inorganic matrix with the selectivity and chemical inertness of organic resins. Moreover, effective shielding of residual silanols and the increase of column packing stability can also be achieved by modification of silica gels with organic polymers. There are two main approaches to the preparation of such composite sorbents. The first includes various routes for coating the silica surface with a polymer layer and the second is by filling the silica pores with a polymer network. A survey of polymers immobilized on silica gels and of various experimental procedures which can be used for composite sorbent synthesis is given in this review.
108 citations
TL;DR: In this paper, the deuterium exchange with D 2 O at 25 and 100 o C was investigated and only 3.0% of the single silanols was not deuterated at 25 o C.
Abstract: Silica gels were deuterium-exchanged with D 2 O at 25 and 100 o C. 29 Si CP-MAS NMR results indicate that for a silica gel with lower surface area, 9.3% of the single silanols remains undeuterated at 25 o C and 4.0% of the single silanols remains undeuterated at 100 o C. For a higher surface-area silica gel, only 3.0% of the single silanols was not deuterated at 25 o C. In all deuterium-exchanged samples of this study, there is little, if any, evidence of internal geminal silanols
107 citations
TL;DR: Using 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, silicon coordinated by five oxygens has recently been found in a number of alkali silicate glasses quenched from liquids both at high and ambient pressure.
Abstract: Using 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, silicon coordinated by five oxygens has recently been found in a number of alkali silicate glasses quenched from liquids both at high and ambient pressure. In this report, both the presence of this species in other ambient-pressure samples and the increase in its concentration with fictive temperature are confirmed. However, it has not been possible to detect this species in a hydrated cesium silicate glass sample or in a silica gel, the latter at a detection limit of about 0.02%. Also as previously noted for high-pressure samples, it is found that five-coordinated Si is more abundant in K2Si4O9 than in Na2Si4O9 glass. Given the effects of alkali size on the coordination and bonding to oxygen known from crystalline silicates, this finding supports a mechanism for the formation of the high-coordinate species that involves the consumption of non-bridging oxygens.
TL;DR: Hydroxyapatite induction by a synthesized pure silica hydrogel was examined in various simulated body fluids having different magnesium, calcium, and phosphate ion concentrations as well as pH values and the results provide the basic knowledge for designing new bioactive materials.
Abstract: Hydroxyapatite induction by a synthesized pure silica hydrogel was examined in various simulated body fluids (SBFs) having different magnesium, calcium, and phosphate ion concentrations as well as pH values. The silica hydrogel generated biologically active apatite on its surface by taking up calcium and phosphorous ionic groups from a surrounding SBF that was prepared to emulate the human plasma in inorganic composition. The induction period for apatite nucleation on the surface of the silica was largely decreased with the addition of a small amount of the calcium or phosphate ions to the SBF and with an increase in pH, but increased with the addition of magnesium ion. Bioactivity of bioactive materials like Bioglass® and glass-ceramic A-W was well interpreted in terms of the rate of apatite formation reflected in these results. Moreover, the results provide the basic knowledge for designing new bioactive materials. © 1993 John Wiley & Sons, Inc.
TL;DR: In this paper, the authors explain the textural features of mesothermal gold-quartz veins by the initial precipitation of amorphous silica gel (colloid), which subsequently crystallizes to quartz.
Abstract: Some of the textural features of mesothermal gold-quartz veins may be best explained by the initial precipitation of amorphous silica gel (colloid), which subsequently crystallizes to quartz. This can occur in brittle-ductile shear zones where a significant fluid-pressure drop occurs during stick-slip failure. Such a process rapidly supersaturates the hydrothermal fluid with respect to amorphous silica, which precipitates instead of quartz, owing to favorable kinetics. Depressurization also commonly leads to fluid unmixing and destabilization of soluble gold complexes. However, the presence of colloidal silica can stabilize gold colloid, allowing further transport of particulate gold in suspension in the hydrothermal fluid. Silica gel would be highly unstable under mesothermal conditions and would undergo rapid syneresis and crystallization to form quartz; solid impurities would tend to be expelled toward grain boundaries. This model can account for the primary anhedral aggregate textures typical of mesothermal quartz veins, the concentration of gold along grain boundaries and the formation of discrete gold nuggets, and the rare occurrence of low-order silica polymorphs and relict spheroidal structures. The transport of gold in colloidal form may be one reason for the frequently consistent bulk grade distribution in gold-quartz vein systems over many hundreds of metres (in some cases kilometres) of depth. In addition, the formation of charged colloidal particles may help to explain the attraction of gold grains to specific mineral surfaces.
TL;DR: In this article, an empirical correlation between the 129Xe NMR chemical shift and the parameters of the porous structure of a number of silica gels is characterized by adsorbed xenon.
Abstract: The porous structure of a number of silica gels is characterized by 129Xe NMR of adsorbed xenon. On the basis of experimental data for the samples with well defined structure, an empirical correlation between the 129Xe NMR chemical shift and the parameters of the porous structure is proposed. The energetic parameters of xenon adsorption have been obtained from the temperature dependence of the chemical shifts. The possibilities and limitations of 129Xe NMR spectroscopy of adsorbed xenon for studying the porous structure of silica gel are discussed.
TL;DR: In this paper, the structural and energetic changes of alkoxide silica during densification were followed in situ using photoacoustic Fourier transform infrared spectroscopy (PAS-FTIR), transposed temperature drop calorimetry, and drop solution calculus.
Abstract: Structural and energetic changes of alkoxide silica during densification were followed in situ using photoacoustic Fourier-transform infrared spectroscopy (PAS-FTIR), transposed temperature drop calorimetry, and drop solution calorimetry. Acid-catalyzed silica gels contained higher concentrations of adsorbed water, silanol groups, and unreacted alkoxy groups than did the base-catalyzed gels/precipitates. The surface species in acid-catalyzed gels were removed easily, allowing this material to be densified at 800°C. The densified material was both structurally and energetically equivalent to fused silica glass. Base-catalyzed gels had fewer silanol groups in the green state, but free silanol groups emerged during drying at 500°C. The base-catalyzed gels did not densify below 1000°C, maintaining a high surface area of 100 m2/g. Base-catalyzed alkoxide gels comprised strained ring structures; their IR spectra were similar to those of fumed silica, but they were energetically much less stable.
Book•
10 May 1993
TL;DR: The history and formation of Silica Gel are described in detail in this article, where the authors present a method for the synthesis of a bonded phase of the Silica gel.
Abstract: Silica Gel--Its History and Formation. The Production of Silica Gel. Test Procedures and LC Column Packing. The Silica Gel Surface. The Exclusion Properties of Silica Gel. Solvent/Solute Interactions with the Silica Gel Surface. An Introduction to Bonded Phases. The Synthesis of Bonded Phases. The Characteristics of Reverse Phases. Solute/Solvent Interactions with the Surface of a Reverse Phase. The Mechanism of Solute Retention. Appendices. Index.
TL;DR: In this article, the Cal-Ad method has elucidated three hydrogen-bonding sites of different strengths using pyridine as the basic probe molecule, which proved to be more sensitive than other characterization techniques such as temperature-programmed desorption and calorimetric techniques.
Abstract: Silica gel, under various hydration conditions, was investigated using a novel calorimetric adsorption (Cal-Ad) method. This method combines data from calorimetric and adsorption titrations of a solid surface using a small probe molecule in a noninteractive solvent. The Cal-Ad method proved to be more sensitive than other characterization techniques (e.g. temperature-programmed desorption) and calorimetric techniques. Previously, it was believed that the silica surface consisted of one type of hydrogen-bonding site. The Cal-Ad method has elucidated three hydrogen-bonding sites of different strengths using pyridine as the basic probe molecule
TL;DR: In this paper, the ill-conditioned nature of the lifetime distribution analysis was demonstrated using the fluorescence decay data of pyrene adsorbed on variously dehydroxylated silica gel surfaces.
Abstract: The ill-conditioned nature of the lifetime distribution analysis is demonstrated using the fluorescence decay data of pyrene adsorbed on variously dehydroxylated silica gel surfaces. A set of physically plausible results can be obtained only after a regularization technique is employed in the data reduction. This study indicates that a bimodal distribution appears to be suitable for describing the lifetime of pyrene adsorbed on a silica gel surface
TL;DR: In this paper, a continuous silica gel layer was first formed on the platelet which acted as a precursor phase for the crystallization of laterally oriented crystals, resulting in a random orientation of the crystals in the film.
TL;DR: A series of high-area silica gels treated with well-characterized aluminum solutions, hydrolyzed in the range OH/Al = 0-2, were analyzed by Al-27 and Si-29 solid-state NMR (static, MAS, and CP-MAS) as mentioned in this paper.
Abstract: A series of high-area silica gels treated with well-characterized aluminum solutions, hydrolyzed in the range OH/Al = 0-2, were analyzed by Al-27 and Si-29 solid-state NMR (static, MAS, and CP-MAS). The Al-27 NMR data show that even in the case of nonhydrolyzed solutions, aluminum is incorporated into the silica tetrahedral framework. For samples treated with hydrolyzed solutions, this incorporation becomes substantial (up to Si/Al = 20) and depends very little on the time of contact between solid and aluminum solution. Octahedral surface aluminum is also present; its amount increases with both degree of hydrolysis and solution contact time. In the case of nonhydrolyzed solutions, the octahedral aluminum detected on the solid is fully hydrated and mobile, whereas for hydrolyzed solutions an association of the type inner-sphere complex is formed. The Si-29 NMR results are complementary to these observations as they show, when silica is treated with hydrolyzed solutions, a redistribution of the Si, Al, and proton populations together with a loss in silanol groups. The implication of these findings are discussed in terms of both chemical modifications of the surface and lattice structural variations.
TL;DR: The simultaneous presence of alkali and ettringite in concrete has been reported by several authors as mentioned in this paper, however, the proposed interpretations of this phenomenon, and its relationship to concret...
Abstract: Simultaneous presence of alkali—silica gel and ettringite in concrete has been reported by several authors. However, the proposed interpretations of this phenomenon, and its relationship to concret...
TL;DR: In this paper, the authors used 17O and 29Si MAS and CPMAS NMR spectroscopy to study the mechanism of hydration of β-C2S and the structure of calcium silicate hydrates (CSH) formed both during hydration and synthesized separately.
TL;DR: In this article, the tris(2,2'-bipyridyl)ruthenium(II) cation is incorporated into the gel layer, and the cation retains electrochemical and photoelectrochemical activity.
Abstract: Silica gel films prepared by the sol-gel route were used to modify platinum and transparent indium oxide electrodes. The silicon dioxide layer reduces electron transfer to the electrode but, due to the porous structure of the silica gel, does not completely prevent it. The tris(2,2'-bipyridyl)ruthenium(II) cation is incorporatcd into the gel layer, and the cation retains electrochemical and photoelectrochemical activity. The comparison of electrochemical and photochemical data was used to evaluate attachment of the ruthenium complex to the silica gel matrix
TL;DR: In this article, the authors report that the PYE silica, having [pi]-basic character, can provide much greater loading capacity and higher efficiency in an aromatic solvent such as benzene or toluene, permitting much easier preparative separations of carbon clusters than the reported methods.
Abstract: As the interest in the fullerenes increases, better purification methods are being sought for C[sub 60], C[sub 70], and related compounds existing in crude soot. Two major problems encountered in the preparative separation of carbon clusters by HPLC, poor peak shape and limited loading capacity, are due to the facts that the fullerenes show limited solubilities in the solvents employed and that the retention provided by the stationary phases was also limited. The silica gel bonded with nitro aromatic groups possessing [pi]-acidity showed the improved separation and the greater loading capacity than alkyl-bonded silica stationary phases, especially at high temperatures. Stationary phases containing phenyl and naphthyl groups also showed increased retention. The amount of fullerenes that can be separated under these conditions, however, was still much smaller than that of ordinary substances. Here the authors report that the PYE silica, having [pi]-basic character, can provide much greater loading capacity and higher efficiency in an aromatic solvent such as benzene or toluene, permitting much easier preparative separations of carbon clusters than the reported methods. 13 refs., 3 figs., 1 tab.
TL;DR: In this article, a transparent metal containing silica gel was synthesized by co-condensation of low concentrations of oxovanadium triisopropoxide with tetraethylorthosilicate via the sol-gel process.
Abstract: The application of the sol-gel process to the synthesis of dense oxide glasses and ceramics has been the subject of escalating interest because of the low temperatures and compositional flexibility inherent in this purely chemical approach. More recently it has been realized that undensified silica [open quotes]xerogel[close quotes] materials possess unique properties precisely because of their porous nature. The authors report the synthesis of a new transparent metal containing silica gel by the co-condensation of low concentrations of oxovanadium triisopropoxide with tetraethylorthosilicate (TEOS) via the sol-gel process.
TL;DR: In this article, the fluorescence quantum yields of six polycyclic aromatic hydrocarbons (PAHs) adsorbed on both hydroxylated and highly dehydroxylating silica gel surfaces have been measured, using a diffuse reflectance method, an integrating sphere method, and a secondary method based on diffuse reflectances.
Abstract: The fluorescence quantum yields of six polycyclic aromatic hydrocarbons (PAHs) adsorbed on both hydroxylated and highly dehydroxylated silica gel surfaces have been measured, using a diffuse reflectance method, an integrating sphere method, and a secondary method based on diffuse reflectance. Consistent results were obtained from these different procedures. Radiative and nonradiative decay rate constants of these systems were derived from these quantum yield data together with the lifetime distribution data obtained previously. These decay rate data, when compared with those obtained for the same PAHs in fluid media, appear reasonable
TL;DR: The lysozyme immobilization yields decreased on decreasing the amount of enzyme loaded, whereas the activation yields (defined as percentage ratio of immobilized active enzyme to immobilized enzyme) increased.
TL;DR: In this paper, a chemically modified electrode (CME) based on modified silica gel-containing carbon paste was developed for the determination of complex mercury ions in aqueous solution.