Chiral Mesoporous Silicates Immobilizing Titanium Dioxide for Catalytic Asymmetric Epoxidation of Alkenes

Two kinds of titanium-containing silicates were prepared by doping of titanium salt and chiral additive in a sol-gel process, which were further modified by chiral sulfonyl chloride in order for catalytic asymmetric epoxidation of alkenes. These titanium-containing materials had good porosities, ordered pore size distributions, high titanium incorporation yields, ordered morphologies, as well as internal chiral configurations. Particularly, they contained a lot of titanium dioxide particles according to TEM. In catalysis, the titanium silicates showed good conversion of alkenes, satisfactory yields and e.e. values of epoxides. The chiral inducing synergy appeared between silicate matrix and attached ligand, which was significant for transformations of styrene and α-methylstyrene. Moreover, iodosylbenzene, tert-butyl hydroperoxide and hydrogen peroxide were all promising oxidants. In addition, the present titanium catalysts showed satisfactory recycling behaviors and chemical stabilities. This work would contribute to the design of efficient chiral catalysts.     

Spectroscopic Study of CuO/CoO Catalysts Supported by Si-Al-Y Zeolite Matrices Prepared by Two Sol-Gel Methods

Two different sol-gel methods for silica-aluminates have been used for catalyst preparation, one based on inorganic precursors, aluminum nitrate and sodium silicate, and a second based on alkoxides, aluminum tri-sec-butoxide (ATB) and tetra-ethoxy-silane (TEOS). A third element, Y-zeolite was incorporated in both of the gel matrices during the process, in order to alter the acid properties of the catalyst. Although the final compound has approximately the same chemical composition, the crystalline structures of the two catalysts are substantially different. The support made from inorganic precursor yielded a more hydroxylated surface, whereas the support based on organic precursors readily incorporated the metals into the network. The support from organic precursors showed almost total dehydroxylation. The Co-Cu complexes formed at the catalyst surface differ greatly, depending on the preparation method.     

Preparation of Macroporous Sol-Gel Bioglass Using PVA Particles as Pore Former

Bioactive glasses are well known for their bone regeneration ability. Sol-gel bioglasses have many advantages comparing to melt derived bioglasses. 3-D scaffold prepared by sol-gel method is a promising substrate material for bone tissue engineering and large scale bone repair. But it is very difficult to produce macroporous sol-gel bioglasses with pores larger than 100 m. In this work, a series sol-gel bioglasses with macropores larger than 100 m are produced by adding PVA particles into the sol as pore former. The texture feature of this material is obtained by nitrogen adsorption/desorption analysis. Pore size distribution is collected by intrusion mercury porosimetry. SEM (Scanning Electron Microscope) was used to observe the pore structure of these samples. In these bioglasses, the diameter of the pore created by pore former vary from 100 to 300 m. In vitro tests were used to evaluate the bioactivity of these sol-gel bioglasses. The samples prepared in this work show satisfactory handle resistance and good bioactivity.     

Synthesis of Highly Porous Organic/Inorganic Hybrids by Ambient Pressure Sol-Gel Processing

This paper reports the synthesis of highly porous organic/inorganic hybrids by a two-step acid-base catalyzed sol-gel process and ambient pressure drying. In the method organic and inorganic precursors are copolymerized so as to incorporate organic ligands into the solid network. The two-step acid-base catalyzed process was used to prevent phase segregation during the hydrolysis and co-condensation of organic and inorganic precursors. The organic ligands incorporated into the solid gel network modify the surface chemistry. Thus, the wetting angle is significantly increased so that the collapse of the gel network is greatly reduced upon the removal of pore fluid during drying. Organic/inorganic hybrids with BET surface areas above 1250 m2/g, porosities above 75% and pore sizes of 8 nm have been synthesized.     

Fluoroalkoxides as Molecular Precursors of Fluoride Materials by the Sol-Gel Process

Numerous works have reported the preparation of pure oxide materials using the sol-gel process. The purpose of this paper is to show that it is possible to obtain pure fluoride homometallic or heterometallic materials at room temperature by the hydrolysis of tailored molecular precursors.The work is focussed on alkaline-earth and lanthanide fluoroalkoxides ([M(OR)n]n where OR = fluoroalkoxo group) and the characterization of the final products of their hydrolysis. The molecular precursors were characterized by FT-Raman and the final powders by XRD and EDXRMA.The formation of these fluoride materials involves fluoride organic by-products. The reactivity of these organic compounds on various substrates in terms of surface modification has been studied.     

Membrane Emulsification Using Sol-Gel Derived Macroporous Silica Glass

A macroporous silica glass membrane with continuous cylindrical pores was prepared by a sol-gel process using phase separation. The applicability of the sol-gel derived macroporous silica to the membrane emulsification process was evaluated in comparison with a conventional SPG (Silasu Porous Glass) membrane.Aqueous colloidal silica in one side of the membrane was emulsified through the pores under an applied pressure to a toluene bath containing surfactant. With the sol-gel and SPG membranes with respective median pore diameters of 0.6 and 1.0 m, emulsions with almost the same droplet size centered around 3 m were obtained. The permeation rate of the sol-gel derived membrane was about 1.6 times faster than SPG, which reflected higher pore volume of the former one.     

Some Recent Developments in Aqueous Sol-Gel Processing

In this overview we describe the recent use of aqueous sol-gel processing for the preparation of ceramics that have very different end-uses. Zirconia nanofiltration membranes with 50% rejection of solutes at a molecular weight of about 1000 have been produced using zirconia sols containing inorganic polymeric particles and evaluated on the pilot-plant scale. Microporous alumina- and zirconia-pillared clays having a large and hydrothermally stable interlayer spacing (2 nm) and specific surface area (approximately 400 m2 g–1) have been prepared using similar polymeric sols. These have been produced on the 20 kg scale and evaluated for catalytic applications. Cathodoluminescent phosphor powders, based on the doped-yttrium aluminium gallium oxide system, have been synthesized for high resolution displays using a combination of aqueous sol-gel precursors and aerosol techniques to produce particles of controlled size and shape. Finally, ceramic stains for decorating ceramic bodies have been synthesized using this method, not only in powder form but also for direct application to ceramic ware by ink-jet printing. These examples illustrate the versatility of aqueous sol-gel processing for the preparation of a wide range of ceramic compositions and forms.     

Encapsulation of Water-Soluble Dye in Spherical Sol-Gel Silica Matrices

The water-soluble, Lithol rubine B, dye was encapsulated into silica microspheres matrices. Encapsulation has been carried out by sol-gel process of W/O microemulsions formed from sodium silicate and dye aqueous solution in cyclohexane medium. The average particle size could be tailored from 1–10 m, depending on the processing parameter such as homogenizing speed in the formation of W/O emulsion, the weight ratio of water to oil, and concentration of sodium silicate solution, etc. The pore size of dye-doped silica microspheres was measured by nitrogen adsorption-desorption isotherms. The leaching behavior of dye entrapped in silica matrices was investigated by UV/VIS and UV diffuse reflectance spectroscopy for the extract and solid powders after immersion for 24 h in water. The doping of GPTS (3-glycidoxypropyltrimethoxysilane) in sodium silicate and dye mixture solution greatly enhanced the stability against leaching of the dye. It was ascribed that GPTS serves simultaneously as an intermediate for the chemical bonding between the dye and silica, and as an agent for the formation of hybrid sol responsible for the shrinkage of pore size.     

Sol-Gel Derived Titania/γ-Glycidoxypropyltrimethoxysilane and Methyltrimethoxysilane Hybrid Materials for Optical Waveguides

We report here on titania/organically modified silane hybrid materials produced by the sol-gel technique for optical waveguide applications. Acid catalyzed solutions of -glycidoxypropyltrimethoxysilane and methyltrimethoxysilane mixed with tetrapropylorthotitanate have been used as precursors for the hybrid materials. Waveguide films with a thickness about 1.3-m have been prepared on a silicon substrate by a single-coating process and low-temperature heat treatment. Atomic force microscopy (AFM), thermal gravimetric analysis (TGA), UV-visible spectroscopy (UV-VIS), and Fourier transform infrared (FTIR) spectroscopy have been used to investigate the optical and structural properties of these waveguide films. The results have shown that dense, pore-free, and highly transparent films can be obtained by low temperature heat treatment. The planar waveguide propagation loss of the hybrid films has also been measured.     

Use of Functional Dendritic Macromolecules for the Design of Metal Oxo Based Hybrid Materials

The elaboration of porous materials by using metal alkoxides (Ce(O-ⁱPr)₄, Ti(OR)₄) as inorganic precursors and acid functionalised dendrimers as organic templates is reported. Chelation by COOH groups present at the dendrimer surface was used to control the reactivity of the metal oxide precursors, creating at the same time anchoring points for the nucleation of a gel phase. The bicontinuous gels resulting after solvent evaporation present a sponge-like structure with pore size ranging from 10 to 30 nm. The effect of the metal/dendrimer ratio, the sol preparation procedure and aging conditions were studied by XRD, FTIR, TEM, SEM and BET.     

A New Manufacturing Process to Obtain Thermoluminescent Dosimeters Using Sol-Gel Method

A new manufacturing process using sol-gel method to obtain thermoluminescent dosimeters—TLDs is described. A LiF/SiO₂ thermoluminescent dosimeter in the form of reusable solid chip was prepared using an acid-catalyzed sol-gel process, with tetramethoxysilane (TMOS) and LiF powder as precursors. In comparison with the manufacturing process of typical lithium fluoride based dosimeters, the method optimizes the manufacturing process. The new dosimeter has a dose response over a dose range varying from 0.1 Gy to 160 Gy and can be used to detect higher doses using the typical commercial TLD readers without special adjustments. Furthermore, the process may be used for the preparation of other TLDs using a simple chemical processing.     

Phase Development in Sol Derived Microporous Membranes for Gas Permselectivity

A sol-gel route has been used to produce a nanoporous membrane for the separation of greenhouse gases from power plants at elevated temperatures (700–900 K). The membrane has a controlled pore size distribution and is multiphasic, in order to prevent grain growth and pore coarsening. The alumina/titania diphasic ceramic was obtained from alkoxide precursors with additional magnesium or lanthanum doping to stabilize low temperature phases and to provide possible chemisorption of CO2. It was found that both Mg and La stabilized the -alumina phase to higher temperature by as much as 200 K, although Mg is less effective.     

Small Diameter (3 μm), Large Pore (1500 Å) Octadecylsilica for Capillary Electrochromatography

An ODS packing material was specially designed for CEC to achieve high efficiency. The size of the particles was approximately 3.2 m based on scanning electron microscopy, and the average pore size of the particles was 1500 Å by mercury intrusion porosimetry measurement. The packing materials were used in sol-gel bonded continuous-bed columns. The performance of columns containing these 3 m, 1500 Å ODS particles was evaluated for CEC and compared to 3 m, 80 Å ODS particles. The electroosmotic velocity (EOF) for the 1500 Å ODS material was slightly greater than that for the 80 Å ODS material. An efficiency of 410,000 plates m^–1 was obtained for unretained thiourea with a 30 cm × 75 m i.d. 1500 Å ODS column, which was more than 2 times higher than the efficiency obtained from the 80 Å ODS column.Dedicated to Professor K. Jinno on the occasion of his 60th, birthday.     

Sol-Gel Derived Nanocomposites and Nanoporous Oxide Powders and Related Coatings for the Reversible Chemisorption of Hydrogen Sulfide

Two types of nanocomposites and nanoporous powders and related coatings were prepared by the sol-gel route. These silica-based materials contain dispersed reactive oxides, ZnO and ZnCr₂O₄, respectively. Experiments evidenced their ability of reversible chemisorption of H₂S as ZnS. Their attractive porous characteristics (small pore size 2–2.5 nm, high specific surface area 900–1100 m²· g^–1, high porosity 50–60%) are not significantly modified during the successive treatments of H₂S chemisorption and oxide regeneration. These preliminary results encourage to pursue this study which aims at the preparation of nanofilters for the desulfurization of gas mixtures.     

Microstructural Analysis of the Effects of Poly(ethylene glycol) on an Acid Catalyzed Sol-Gel Derived Ceramic Material

Ceramic materials have been derived from an acid catalyzed sol-gel process. The addition of different molecular weights and concentrations of polyethylene glycol (PEG) to the sol mixture modifies the phase behaviour of the sol-gel process. The resulting gel is burned at 973 K to make porous ceramic materials. Nitrogen adsorption-desorption isotherms are used to assess the effects of PEG on the internal structure of the burned ceramic material. These isotherms indicate an extensive pore network exists consisting of micropores and mesopores. In the micropore region of the isotherms, the _S-plot analysis reveals changes in specific primary micropore volumes, specific total pore volumes, specific external surface areas and specific SPE surface area when PEG is added in the sol-gel process. The average pore width and the overall mesopore size distribution curves shift to higher pore size values and ranges on addition of PEG to the sol-gel mixture. The presence of PEG during the sol-gel process leads to an apparent narrowing of the micropore size distribution. The results of this work clearly indicate that the molecular weight and the concentration of a polymer, such as PEG, influences the eventual internal structure of a ceramic after burning.     

Synthesis of PDMS-Based Porous Materials for Biomedical Applications

Polydimethylsiloxane (PDMS) and tetraethoxysilane (TEOS)-based porous organically modified silicates (ORMOSILs) for biomedical applications were synthesized through a sol-gel process, using sucrose particles as templates. These materials were characterized by ²⁹Si CP-MAS NMR spectroscopy, thin film X-ray diffraction, and scanning electron microscopy. Their bioactivity was evaluated using a simulated body fluid (SBF) of Kokubo recipe. These materials had a bimodal porous structure with pores of 300–500 m and 10–50 m in diameter. NMR showed that the silanol groups of the PDMS chain cross-linked to silica derived from the hydrolysis and condensation of TEOS. The samples containing Ca(II) exhibited apatite deposition on the pore walls within 3 days in SBF.     

Carbon and SiC macroscopic beads from ion-exchange resin templates

A method for preparing carbon and SiC macroscopic beads using ion-exchange resins as a macrotemplate that determines the macroshape and the pore structure of the product materials is reported. First, silicates are ion-exchanged into the resins to prevent the resin from collapsing during subsequent carbonization and allow them to be used as precursors for SiC formation. SiC is prepared via carbothermal reduction of carbon/silica composite beads obtained upon carbonization of the resin/silicate in an inert atmosphere. Finally, silica is removed by HF etching. Very high-surface area (1670-2026 m2 g-1) micro- or micro-/mesoporous carbon beads and relatively high-surface area (35-63 m2 g-1) macro- and meso-/macroporous SiC beads were prepared by the described method. The pore structure and the macroshape of the particles were controlled by the type of ion-exchange resins employed, gel or macroreticular.     

Physico-Chemical and Catalytic Study of the Co/SiO2 Catalysts

This paper is focused on the physico-chemical and catalytic properties of Co/SiO₂ catalysts. Silica-supported cobalt catalysts were prepared by sol-gel and impregnation methods and characterized by BET measurements, temperature programmed reduction (TPR_H2), X-ray diffraction (XRD), and thermogravimetry-mass spectroscopy (TG-DTA-MS). The sol-gel method of preparation leads to metal/support catalyst precursor with a homogenous distribution of metal ions into bulk silica network or on its surface. After drying the catalysts were calcined at 500, 700, and 900°C. The reducibility of the supported metal oxide phases in hydrogen was determined by TPR measurements. The influence of high temperature—atmosphere treatment on the phase composition of Co/SiO₂ catalysts was investigated by XRD and TG-DTA-MS methods. At least five crystallographic cobalt phases may exist on silica: metallic Co, CoO, Co₃O₄, and two different forms of Co₂SiO₄ cobalt silicate. Those catalysts in which cobalt was chemically bonded with silica show worse reducibility as a result of strongly bonded Co-O-Si species formed during high-temperature oxidation. The TPR measurements show that a gradual increase in the oxidation temperature (500–900°C) leads to a decrease in low-temperature hydrogen reduction effects (<600°C). The decrease of cobalt oxide reduction degree is caused by cobalt silicate formation during the oxidation at high temperature (T 1000°C). The catalysts were tested by the reforming of methane by carbon dioxide and methanation of CO₂ reactions.     

Nanorods of Various Oxides and Hierarchically Structured Mesoporous Silica by Sol-Gel Electrophoresis

In this paper, we report the template-based growth of nanorods of oxides and hierarchically structured mesoporous silica, formed by means of a combination of sol-gel processing and electrophoretic deposition. Both single metal oxides (TiO₂) and complex oxides (Pb(Zr_0.52Ti_0.48)O₃) have been grown by this method. This method has also been applied to the growth of nanorods of mesoporous silica having an ordered pore structure, where the pores are aligned parallel to the long axis of the nanorod. Uniformly sized nanorods of about 125–200 nm in diameter and 10 m in length were grown over large areas with near unidirectional alignment. Appropriate sol preparation yielded the desired stoichiometric chemical composition and crystal structure of the oxide nanorods, with a heat treatment (500–700°C for 15–30 min) for crystallization, densification and any necessary pyrolysis.     

Preparation of Porous Cordierite with Thermally Stable Pore Structure

A cordierite ceramic with a thermally stable pore structure was prepared by a simple modification of a sol-gel reaction of alkoxide precursors, synthesized from Mg metal or Mg acetate, Al(i-OPr)₃, and partially prehydrolyzed Si(OEt)₄. For aging and drying, wet cordierite gel was treated with water vapor at 150°C to strengthen the gel network through enhanced hydrolysis and condensation reactions. The cordierite xerogels showed BET surface areas between 220–410 m²/g, depending on the catalyst and treatment conditions used. In particular, water vapor treated xerogel displayed comparatively thermally stable pore characteristics, which exhibited only a 4–17% decrease in BET surface area up to 700°C, while samples prepared using the conventional sol-gel method showed a 55–91% reduction. Both the DTA and XRD patterns showed that crystallization began at 900°C leading to the -Cordierite phase and the subsequent phase transition to -cordierite at temperatures between 1050 to 1250°C.