Effects of synthesis parameters on the microstructure and phase structure of porous 316L stainless steel supported TiO2 membranes

Porous stainless steel (PSS) supported TiO₂ membrane was synthesized from colloidal TiO₂ sol by the sol–gel technique. Morphology and phase structure of the obtained membranes were regulated through optimizing the synthesis parameters including organic binders, aging periods of the parent sol and concentrations of the casting solutions as well as the sintering temperatures. Polyvinyl alcohol (PVA) 1750 was found to be feasible to fabricate TiO₂ membrane with relatively flat surface and homogeneous morphology without crack. The aging period of the parent sol, which was revealed to be very important to the morphology of the particles deposited in the membranes on PSS, was decided to be 24 h. The concentration, under which the membranes could avoid macro-pores and have a uniform thickness of approximately 8 μm, was regulated to 0.0036 mol/l. Besides, a homogeneous microstructure with grain sizes of 0.08–0.2 μm was obtained in the membrane with a pure rutile phase when sintered at 850 °C. The obtained PSS supported TiO₂ membrane with homogeneous microstructure and rutile phase may be very promising for practical applications.     

The search of a homogeneously dispersed material—the art of handling the organic polymer/metal oxide interface

The combination of organic polymers with the sol–gel process represents one major route for the preparation of inorganic–organic hybrid materials. One challenge in this kind of materials is to overcome macroscopic phase separation. This review gives an overview of concepts that lead to homogeneously dispersed materials by tailoring the inorganic–organic interface.     

Microporous silica membranes deposited on porous supports by filtration

Water based particulate silica sols have been coated onto Anodisc® filters by filtration. The membranes prepared by this technique are more uniform than those formed by slip-casting. The average diameter of the silica particles used in these studies is 6 nm. Unsupported silica membranes formed from these sols have a microporous structure. The adhesion between silica and the alumina support is influenced by the sol pH. Coating thickness can be controlled by the concentration and volume of the sol filtered. Polyvinyl alcohol (PVA) was used to improve adhesion and to prevent cracking during drying. When the PVA/SiO₂ ratio by weight is less than 20%, the membranes retain their microporosity after firing. The membranes prepared by this filtration method have their pore size in Knudsen diffusion range.     

Preparation and characterization of silica—polyimide composite membranes coated on porous tubes for CO2 separation

Silica-polyimide microcomposite membranes were prepared on γ-alumina-coated α-alumina support tubes, and their gas permeation properties were evaluated with He, N₂ and CO₂. Smoothing of the substrate surface and hybridization of silica and polyamic acid were both effective to form defect-free thin composite membranes. The CO₂ permeance of a membrane with a silica content of 68 wt% was one order of magnitude higher than that of a polyimide membrane having the same thickness. The permselectivity of CO₂ to N₂ was 30 at 30°C and 13 at 100°C. Contributions of the silica and polyimide phases to permeance of the composite membrane were analyzed with a two-phase permeation model. The effective thickness of the rate-controlling polyimide phase was less than one-tenth of the total thickness of the silica-polyimide membrane.     

Cobalt-doped silica membranes for gas separation

Cobalt-doped silica membranes were synthesized using tetraethyl orthosilicate-derived sol mixed with cobalt nitrate hexahydrate. The cobalt-doped silica structural characterization showed the formation of crystalline Co₃O₄ and silanol groups upon calcination. The metal oxide phase was sequentially reduced at high temperature in rich hydrogen atmosphere resulting in the production of high quality membranes. The cobalt concentration was almost constant throughout the film depth, though the silica to cobalt ratio changed from 33:1 at the surface to 7:1 at the interface with the alumina layer. It is possible that cobalt has more affinity to alumina, thus forming CoOAl₂O₃. The He/N₂ selectivities reached 350 and 570 at 160 °C for dry and 100 °C wet gas testing, respectively. Subsequent exposure to water vapour, the membranes was regenerated under dry gas condition and He/N₂ selectivities significantly improved to 1100. The permeation of gases generally followed a temperature dependency flux or activated transport, with best helium permeation and activation energy results of 9.5 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ and 15 kJ mol⁻¹. Exposure of the membranes to water vapour led to a reduction in the permeation of nitrogen, attributed to water adsorption and structural changes of the silica matrix. However, the overall integrity of the cobalt-doped silica membrane was retained, given an indication that cobalt was able to counteract to some extent the effect of water on the silica matrix. These results show the potential for metal doping to create membranes suited for industrial gas separation.     

Molecular imprinting of β-cyclodextrin/cholesterol template into a silica polymer for cholesterol separation

The molecular assembly formed by the inclusion complex of cholesterol in β-cyclodextrin (β-CD:chol) was used as a template for the molecular imprinting of a sol–gel polymer (MIP/β-CD:chol), produced with tetraethoxysilane (TEOS) as precursor. The MIP/β-CD:chol and pure silica matrix (PSM) were tested for the efficiency of cholesterol removal from solutions at different cholesterol concentrations (1–10 mg/mL). The adsorption tests were run at 25°C using 1% (w/v) solid/liquid suspensions during 24 h. The MIP/β-CD:chol data on cholesterol adsorption was fitted by the Langmüir isotherm model, giving a maximum adsorption capacity of 76.5 mg cholesterol/g-adsorbent. The PSM data did conform to the Langmüir model. The maximum cholesterol adsorption achieved with the PSM was higher, 251 mg/g, probably due to multilayer adsorption. The hydrophobic silica matrix, imprinted with the inclusion complex of β-CD and a target molecule, has the potential of being used as an adsorbent for other organic molecules.     

Theory of pressure-driven transport of neutral solutes and ions in porous ceramic nanofiltration membranes

Hindered transport theory and homogeneous electro-transport theory are used to calculate the limiting, high volume flux, rejection of, respectively, neutral solutes and binary electrolytes by granular porous nanofiltration membranes. For ceramic membranes prepared from metal oxides it is proposed that the membrane structural and charge parameters entering into the theory, namely the effective pore size and membrane charge density, can be estimated from independent measurements: the pore radius from the measured hydraulic radius using a model of sintered granular membranes and the effective membrane charge density from the hydraulic radius and the electrophoretic mobility measurements on the ceramic powder used to prepare the membrane. The electro-transport theory adopted here is valid when the membrane surface charge density is low enough and the pore radius is small enough for there to be strong electrical double layer overlap in the pores. Within this approximation the filtration streaming potential is also derived for binary electrolytes.     

New hierarchically porous titania monoliths for chromatographic separation media

Separation media based on hierarchically porous titania (TiO₂) monoliths for high‐performance liquid chromatography (HPLC) have been successfully fabricated by the sol–gel process of titanium alkoxide in a mild condition utilizing a chelating agent and mineral salt. The as‐gelled TiO₂ monoliths were subjected to a simple solvent exchange process from ethanol (EtOH) to H₂O followed by drying and calcination. The resultant monolithic TiO₂ columns consist of anatase crystallites with the typical specific surface area of more than 200 m²/g. The resultant monolithic TiO₂ column calcined at 200 and 400°C exhibited a good separation performance for organophosphates as well as for polar benzene derivatives in the normal‐phase mode.     

Transport properties of alkanes through ceramic thin zeolite MFI membranes

Polycrystalline randomly oriented defect free zeolite layers on porous α-Al₂O₃ supports are prepared with a thickness of less than 5 μm by in situ crystallisation of silicalite-1. The flux of alkanes is a function of the sorption and intracrystalline diffusion. In mixtures of strongly and weakly adsorbing gases and a high loadings of the strongly adsorbing molecule in the zeolite poze, the flux of the weakly adsorbing molecule is suppressed by the sorption and the mobility of the strongly adsorbing molecule resulting in pore-blocking effects. The separation of these mixtures is mainly based on the sorption and completely different from the permselectivity. At low loadings of the strongly adsorbing molecules the separation is based on the sorption and the diffusion and is the same as the permselectivity. Separation factors for the isomers of butane (n-butane/isobutane) and hexane (hexane/2,2-dimethylbutane) are respectively high (10) and very high (> 2000) at 200°C. These high separation factors are a strong evidence that the membrane shows selectivity by size-exclusion and that transport in pores larger than the zeolite MFI pores (possible defects, etc) can be neglected.     

Synthesis of silica nanoparticles by modified sol–gel process: the effect of mixing modes of the reactants and drying techniques

A modified preparation of silica nanoparticles via sol–gel process was described. The ability to control the particle size and distribution was found highly dependent on mixing modes of the reactants and drying techniques. The mixture of tetraethoxysilane and ethanol followed by addition of water (Mode-A) produced monodispersed powder with an average particle size of 10.6 ± 1.40 nm with a narrow size distribution. The freeze drying technique (FD) further improved the quality of powder. In addition, the freeze dried samples have shown unique TGA decomposition steps which might be related to the well-defined structure of silica nanoparticles as compared to the heat dried samples. DSC analysis showed that FD preserved the silica surface with low shrinkage and generated remarkably well-order, narrow and bigger pore size and pore volume and also large endothermic enthalpies (ΔH _FD = −688 J g⁻¹ vs. ΔH _HD = −617 J g⁻¹) that lead to easy escape of physically adsorbed water from the pore at lower temperature.     

Fabrication of a thin palladium membrane supported in a porous ceramic substrate by chemical vapor deposition

A thin, gas-tight palladium (Pd) membrane was prepared by the counter-diffusion chemical vapor deposition (CVD) process employing palladium chloride (PdCl₂) vapor and H₂ as Pd precursors. A disk-shaped, two-layer porous ceramic membrane consisting of a fine-pore γ-Al₂O₃ top layer and a coarse-pore -Al₂O₃ substrate was used as Pd membrane support. A 0.5–1 μm thick metallic membrane was deposited in the γ-Al₂O₃ top layer very close to its surface, as verified by XRD and SEM with a backscattered electron detector. The most important parameters that affected the CVD process were reaction temperature, reactants concentrations and top layer quality. Deposition of Pd in the γ-Al₂O₃ top layer resulted in a 100- to 1000-fold reduction in He permeance of the porous substrate. The H₂ permeation flux of these membranes was in the range 0.5–1.0 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ at 350–450°C. The H₂ permeation data suggest that surface reaction steps are rate-limiting for H₂ transport through such thin membranes in the temperature range studied.     

Antiwetting silica–gelatin nanohybrid and transparent nano coatings synthesised through an aqueous sol–gel process

Hydrophobic silica-biopolymer hybrid has been synthesised using colloidal silica as the precursor for silica, gelatin as the biopolymer counterpart and vinyltrimethoxysilane as the surface modifier through a sol–gel method. The precursor sol was coated on glass substrates and further investigated for the extent of chemical modification, thermal degradation, hydrophobicity, particle size and transparency in the UV-Visible region. Stereomicrographs clearly indicate the water repellent nature of the hybrid coatings with respect to a water drop. FTIR was used to follow the presence of vinyl groups that impart hydrophobicity to the resultant hybrids. By varying the concentration of functionalizing agent, the extent of hydrophobic property could been tailored. The optimum concentration for effective surface modification of the silica–gelatin hybrid is about 50 wt%. Nearly 100% optical transmittance was obtained for silica–VTMS–gelatin hybrid coatings on glass substrates and this may have potential applications in optical devices and also for transparent biocompatible hydrophobic coatings on biological substrates such as leather.     

Application of silicate electrospun nanofibers for cell culture

Silicate produced via the sol–gel process is a biocompatible material that has high purity and high homogeneity. In this study, we evaluated the feasibility of electrospun fibers of silicate formed into silicate nonwoven fabrics (SNF) developed via the sol–gel process as substrates for substance production using Chinese hamster ovarian cells CHO-K1, and as substrates for producing drug metabolism simulators from the human cell line HepG2. We compared the adherent and proliferation profiles of the two cell types on SNF with those profiles produced on a hydroxyapatite-pulp composite fiber sheet (HAPS). During 14 days of cultivation, a greater number of CHO-K1 and HepG2 cells continued to grow on SNF compared to those on HAPS. Per unit volume, the HepG2 cells on SNF showed higher hepatic-specific functions than those on HAPS. These results demonstrate the feasibility of SNF as a cell culture substrate for substrate production, and for producing drug metabolism simulators.     

Fabrication of titania nanocoatings on ZnS-type phosphors using the titanium diethanolamine as the precursor in an aqueous solution

The titania nanocoatings on ZnS: Ag, Cl phosphors were successfully obtained by the sol–gel process in an aqueous solution using the titanium diethanolamine complex as the precursor. The titanium diethanolamine complex was prepared with diethanolamine (DEA) and titanium butoxide at the temperature of 120 °C. The nanocoatings synthesized by this technique were uniform and continuous with a thickness of about 10–15 nm, and the photoluminescent (PL) spectra showed that no other luminescent center was introduced. The titania coatings with an appropriate thickness were expected to prevent the degradation of ZnS phosphors in field emission displays (FEDs).     

Enhanced photoactivity of neodymium doped mesoporous titania synthesized through aqueous sol–gel method

Nanosize neodymium doped titania has been prepared by hydrolysis of titanium oxychloride followed by peptisation under acidic condition. The anatase to rutile phase transformation temperature was found to increase by 150 °C as a result of neodymium doping. The doped sample shows 10 times higher surface area than the undoped one after calcining at 700 °C. All the samples calcined at 500, 600 and 700 °C show type IV isotherm, which is characteristic of mesoporous material. The pore size distribution curves also show that the pores are in mesoporous region. Further, the neodymium doped titania shows increased photoactivity than the undoped titania with respect to decomposition of methylene blue when subjected to UV light. The transmission electron micrograph indicates that a nanocrystalline doped titania is obtained through the present method. The effect of neodymium doping on the anatase phase stability, specific surface area and photoactivity are reported.     

Synthesis and characterization of silicate polymers

In this work an inorganic polymer is developed based on Elkem microsilica and potassium hydroxide. Using experimental data and the partial charge model a model for the gelation is suggested based on the hydrolysis and condensation reactions occurring during synthesis. In addition the optimal composition of the binder system was determined using compressive strength test and solubility experiments. Based on partial charge calculations and experimental data for the hydroxide concentration and the viscosity obtained in this study it is suggested that the polymerization of the inorganic polymer is determined by the concentration of silica species. It was found that the alkalinity has a crucial effect on the condensation process. The optimal potassium hydroxide concentration used in the synthesis of the inorganic polymer was found to be around 3.5 M, which resulted in a compressive strength of the product in the region of 50 MPa.     

Sol–gel modified method for obtention of gray and pink ceramic pigments in zircon matrix

A sol–gel modified method for obtention of gray and pink ceramic pigments in zircon matrix is discussed in this work. The chromophoric powders were firstly well dispersed in the solution containing zirconium and metasilicate ions before gelation and the product ceramic pigments obtained by calcining the xerogel with mineralizers. The zircon was well crystallized at 1,050 °C with the yield about 97%. The mineralization mechanism of the zircon formation was also discussed.     

Performance and pore characterization of nanoporous carbon membranes for gas separation

The performance of a novel nanoporous carbon membrane for separation of hydrogen-hydrocarbon gas mixtures is described. The membrane selectively adsorbs hydrocarbons from hydrogen at the high pressure side and the adsorbed molecules then diffuse along the pore walls to the low pressure side. Pressure levels at thigh gh and low pressure sides of the membrane and the type and flow rate of the sweep gas at the low pressure side of the membrane were varied. The effects of these variables on the hydrogen recovery and hydrocarbon rejection by the membrane were investigated.     

Role of surface-purity in photocatalytic activity of nanocrystalline anatase–titania processed via polymer-modified sol–gel

Nanocrystalline titania powders have been synthesized via conventional and modified sol–gel using an alkoxide precursor for different R, the ratio of molar concentration of water to that of alkoxide precursor, and calcination temperature. The apparent first-order reaction rate-constant obtained for the powder synthesized via conventional sol–gel is comparable with that of commercial Degussa-P25. Conventional sol–gel has been modified using the hydroxypropyl cellulose polymer to increase the specific surface area of the photocatalyst; and hence, to further enhance its photocatalytic activity. Although higher specific surface area and smaller average nanocrystallite size have been obtained for the powders synthesized via modified sol–gel, they exhibit reduced photocatalytic activity relative to that of powders synthesized via conventional sol–gel. The deactivation of the present photocatalyst has been explained on the basis of reduced surface-purity of the powders after processing via modified sol–gel as induced by the presence of surface-residual organic compounds.