Effect of sintering atmosphere on the pore-structure stability of cerium-doped nanostructured alumina

Pore-structure stability of pure and Ce-doped alumina in air and argon atmospheres was studied using DTA, TGA, N2 ads./des. and XRD with a view to understand the importance of the ionic size of the dopant cation on the pore-structure stability of alumina. The ionic size effect was studied by heat treating the Ce-alumina system in both oxidizing and reducing atmospheres to have Ce4+ (87 pm) and Ce3+ (106 pm) respectively. No compound formation between Ce and alumina was observed. In the case of pure alumina there is a drastic reduction in porosity during the transformation to α-alumina. Ce-doped alumina has a higher DSC transformation temperature corresponding to the α-alumina transformation compared to pure alumina. Ce-doped alumina showed higher pore-structure stability compared with pure alumina and the stability was relatively higher in reducing atmosphere (higher Ce3+/Ce4+ ratio, higher effective ionic size) compared with oxidizing conditions (lower Ce3+/Ce4+ ratio, lower effective ionic size)

High performance hydrogen selective membranes prepared using rapid processing method

The production of hydrogen using membrane based reforming technology is one important application where the membrane is required to withstand high temperatures and pressures to achieve maximum efficiency from an equilibriumlimited reaction. Microporous silica membranes offer a viable alternative to polymer and metal composite membranes. The processing of silica membranes including sol-gel and chemical vapor deposition (CVD) methods are discussed adequately in the literature. The Sol-gel method allows for the precise control of pore structure while CVD results in chemically homogenous deposits inside the porous substrates to yield better selectivity. The main problem associated with these methods is the long processing times to prepare these membranes, resulting in increased cost of production for the processing of large batches. Here we report an advance in membrane processing that could drastically reduce membrane processing time without compromising performance

Lithium silicate based membranes for high temperature CO2 separation

Processing and characterisation of a novel membrane system for CO2 separation is detailed. The membrane was made of Lithium Orthosilicate (Li4SiO4), which has potential to react with CO2 molecules reversibly at high temperature. Using the membrane, a separation factor of 5.5 was measured between CO2 and N2 gas molecules at a temperature of 525C. The gas permeance value through the membrane at 525C was around 10-8 mol/m2 s Pa. Ionic diffusion through the liquid phase electrolyte and solid phase skeleton, produced by the reaction between CO2 and Li4SiO4,was suggested to assist the selective permeance of CO2. This facilitation effect was experimentally identified by examining the variation in CO2 flux with partial pressure. Oxygen ion conductivity through the skeleton material, Li2SiO3, is reportedly very high and therefore charge balance should have been achieved by O2- transfer through the skeleton

Preparation and structure of microporous silica membranes

Silica sols have been prepared in an alcoholic solution by hydrolysis and condensation of TEOS (tetra-ethyl-ortho-silicate) molecules as a function of water and nitric acid concentration. The polymers are intended as precursors for ceramic, gas separation membranes. These molecules show fractal behavior as determined by SAXS (Small Angle X-ray Scattering). Microporosity of dried and calcined silica polymers is determined by N2-adsorption at 77 K. Fractal dimension and porosity increase with increasing acid concentration. Both the sol structure and the drying kinetics determine the porosity values. N2-adsorption isotherms are not very suitable for the determination of pore size distributions of microporous silica

Synthesis, characterisation and gas permeation studies on microporous silica and alumina-silica membranes for separation of propane and propylene

Microporous silica membranes are known to exhibit molecular sieving effects. However, separation of nearly equal sized molecules is difficult to carry out by size exclusion. Introducing sorption selectivity and keeping the kinetics favourable to facilitate a good contribution of permeation from sorption is a possible solution to enhance selectivity of adsorbing molecules. Results are presented in this paper on the synthesis of a microporous silica membrane with commendable permselectivity between helium and propylene. Modifications are performed on the membrane to improve its almost non-selective nature to propylene/propane mixtures to give practical separation values. Gas separation results on the modified membranes are presented. Surface selectivity on the newly added alumina surface layer is identified as the helping mechanism in realising this separation

Amine impregnated porous silica gel sorbents synthesized from water-glass precursors for CO2 capturing

In this work, porous silica gel-solid beads have been made from economically affordable water-glass precursors via sol-gel nano casting technique. A stable nanometric silica sol was prepared first from water glass and studied for surface potential and sol to gel transition. A free-flow, injectable gel was obtained upon aging the sol which was then assembled into spherical silica beads in a chemical bath. A surface area of 304.7m2g-1 was obtained for water glass derived silica gel beads. These gel beads were impregnated with 3-aminopropyltrimethoxysilane (APTMS) and polyethylenimine (PEI) active functional groups at different percentages for turning the gel beads as sorbents for CO2 gas adsorption. The effect of amine loading on the thermal stability, morphology as well as porosity was studied and was correlated with CO2 adsorption values. Depending upon the amount of amine loaded in the gel support CO2 uptake was found varied. These amine modified silica gel porous adsorbents showed CO2 adsorption capacity at temperatures as low as 100°C; samples modified with 15wt% PEI had CO2 adsorption capacity of 1.16mmolg-1 at 50°C. © 2015 Elsevier B.V

Evolution of pore structure in microporous silica membranes:sol-gel procedures and strategies

Silica membranes exhibiting excellent molecular sieving capability, which would find applications in fuel‐cell electric vehicles with on‐board hydrogen generation, for example, are the aim of the sol‐gel strategies outlined here. It is shown that optimization of the sol‐gel synthesis parameters is important in order to achieve membranes with minimum defects and hence high selectivity. The preparation of the supported membranes is described and the gas permeation behavior of membranes made from different sol compositions reported

Oxidative intramolecular cyclization reactions of Cinnamayl ethers mediated by cerium (IV) ammonium nitrate (CAN): a stereoselective synthesis of 3,4-trans-disubstitutedtetrahydrofuran derivatives

Various alkoxy-cinnamyl cinnamyl ethers and an alkoxy-cinnamyl prenyl ether underwent stereoselective oxidative cyclizations on treatment with a methanolic solution of cerium(IV) ammonium nitrate to afford 3,4-trans- disubstituted tetrahydrofuran derivatives. Different products were obtained under aerobic and anaerobic conditions

ORMOSIL–ZrO2 hybrid nanocomposites and coatings on aluminium alloys for corrosion resistance; A sol-gel approach

Corrosion resistant coatings are prepared from a hybrid nanocomposite aerogel derived from tri-functional silanes, methyltrimethoxysilane (MTMS) and glycidyloxypropyl trimethoxysilane (GPTMS) and from a zirconium isopropoxide (ZIP) precursor which acts as an inorganic nano-dispersion in an organically modified silane (ORMOSIL) matrix. A series of hybrid compositions of MTMS and GPTMS are prepared in which the amount of ZIP is varied. The variations in the pH, viscosity and gelation time of the prepared compositions are monitored. The wet alcogels thus obtained are homogenized in a solvent using an ultrasonicator followed by coating the suspension on aluminium alloys and glass substrates using a dip coating unit. The prepared coatings are further dried and annealed at 400 °C for 1 h. The wet alcogels are also dried under ambient conditions for seven days resulting in hybrid nanocomposite aerogel monoliths and are calcined at 400 °C. The hybrid nanocomposites and coatings are further characterized using X-ray diffraction analysis, Fourier transform infrared spectroscopy, BET surface area analysis, X-ray photoelectron spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-visible spectroscopy, potentiodynamic polarization and EIS measurements. The hybrid nanocomposite coated aluminium alloy shows enhanced corrosion protection when compared to the uncoated aluminium alloy. The anticorrosive feature of the ORMOSIL-ZrO2hybrid nanocomposite coatings makes them an important candidate in the field of protective environment resistant coatings

Sol-gel synthesis of molecular sieving silica membranes

Polymeric silica sol was synthesized by the acid catalyzed hydrolysis and condensation of tetra-ethyl-ortho-silicate. Calcined unsupported membranes made from this sol showed microporous nature. Supported membranes on alumina were prepared by dipping and calcining. Helium showed activated diffusion with an apparent activation energy of 17 kJ mol−1. H2 permeation was comparable to that of helium under identical conditions. N2, Ar, O2, C3H6, C3H8, n-C4H10 and i-C4H10 permeation values were extremely small and therefore difficult to fit appropriate diffusion models. At 303 K hydrocarbon permeation was about 2 times higher than that of N2, Ar or O2. permselectivity around 1000 and helium permeation in the order of 10−7–10−8 mol m−2 s−1 Pa−1 were measured in the temperature range of 303–460 K. Comparison of Eact, selectivity and He and N2 permeation of different samples evidenced the dependence of nitrogen flux on processing defects. Obviously permeation rate of nitrogen molecule was insignificant through majority pores of the membrane