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

Characterization of Grain Boundaries in Superplastically Deformed Y-TZP Ceramics

The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZP have been investigated using surface spectroscopy, impedance analysis, and transmission electron microscopy. After sintering at low temperature (1150°C), the grain boundaries are covered by an ultrathin (1nm) yttrium-rich amorphous film. After deformation at 1200°–1300°C under low stress, some grain boundaries are no longer covered by the amorphous film. Yttrium segregation seems to occur only at wetted grain boundaries. Evidence has been found that the extent of dewetting increases with increasing applied stress

Textural evolution and phase transformation in titania membranes: Part 1. -unsupported membranes

Textural evolution in sol–gel derived nanostructured unsupported titania membranes has been studied using differential scanning calorimetry (DSC), differential thermal analysis (DTA), thermal gravimetry (TG), X-ray diffraction (XRD) and N2 adsorption. The anatase-to-rutile phase transformation kinetics were studied using the Avrami model. The precursor gel had a surface area of ca. 165 m2 g–1, which after heat treatment at 600 °C for 8 h reduced to zero. Undoped titania-gel layers transformed to more than 95% rutile after calcination at 600 °C for 8 h. The causes of surface-area reduction and pore growth were anatase crystallite growth and the enhanced sintering of rutile during transformation. Lanthanum oxide was identified as a suitable dopant for shifting the transformation temperature to ca. 850 °C. Lanthanum oxide doped titania showed an improved stability of porous texture compared to that of the undoped titania membranes

Aging and stability of microporous sol-gel-modified ceramic membranes

Aging experiments on microporous sol-gel-derived nonsupported Si02 membranes were performed. Microstructure characterization was performed using nitrogen physisorption. It is found that both chemical aging and thermal aging result in densification of the microstructure, without pore growth. The influence of aging on supported SiO2-modified membranes was investigated using gas permeation and separation experiments. As for the nonsupported materials, some densification takes place. This leads to lower permeation rates, but a strong positive effect was observed on the separation properties. This might be attributed to a decrease of the pore size. Separation factors ranging from 50 to 125 have been measured for H2/CH4 at temperatures in the order of 250°C

Synthesis and textural properties of unsupported and supported rutile (TiO2) membranes

Two approaches were postulated for improving the stability of porous texture of titania membranes: (1) retarding the phase transformation and grain growth; (2) avoiding the phase transformation. Based on the second approach, rutile membranes were made directly from a rutile sol, prepared by the precipitation of titania on SnO2 nuclei. The rutile membranes were stable up to 800 °C, with a porosity of ca. 40%, whereas normal titania membranes (starting with anatase) show very little porosity above 600 °C. Alumina substitution retards grain growth and pore growth at 850 °C for unsupported as well as supported membranes. \u

High-temperature catalyst supports and ceramic membranes: Metastability and particle packing

Parameters and/or processes responsible for the stability of catalyst supports and ceramic membranes are discussed. Two major parameters/processes were identified which are responsible for the stability of sol-gel derived nanostructured oxides at elevated temperatures. They are metastable-to-stable phase transformation and structure and packing of primary particles within the aggregate. Based on these observations, strategies to develop thermostable nanostructured oxides for high-temperature membrane and catalyst applications are discussed by taking titania and titania-alumina nanocomposites as examples

Pore Narrowing and Formation of Ultrathin Yttria-Stabilized Zirconia Layers in Ceramic Membranes by Chemical Vapor Deposition/Electrochemical Vapor Deposition

Chemical vapor deposition (CVD) and electrochemical vapor deposition (EVD) have been applied to deposit yttria-stabilized-zirconia (YSZ) on porous ceramic media. The experimental results indicate that the location of YSZ deposition can be varied from the surface of the substrates to the inside of the substrates by changing the CVD/EVD experimental conditions, i.e., the concentration ratio of the reactant vapors. The deposition width is strongly dependent on the deposition temperature used. The deposition of YSZ inside the pores resulted in pore narrowing and eventually pore closure, which was measured by using permpor-ometry. However, deposition of YSZ on top of porous ceramic substrates (outside the pores) did not result in a reduction of the average pore size. Ultrathin, dense YSZ layers on porous ceramic substrates can be obtained by suppressing the EVD layer growth process after pore closure

Textural evolution and phase transformation in titania membranes: Part 2. - Supported membranes

Nanostructural evolution and phase transformation in supported and unsupported titania membranes have been studied using Raman spectroscopy, X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). Densification of unsupported membranes started at ca. 450 °C and reached more than 97% density at 600 °C, whereas the supported membranes had a density of only ca. 70–75% even at 700 °C when calcined for 8 h. At 700 °C the average crystallite size of supported and unsupported membranes was ca. 20 and 70 nm, respectively. This behaviour is primarily attributed to the decrease in the driving force for sintering due to the stress developed during the constrained sintering of a film attached to a rigid support and to the inhibition of the reorganization process within the film, resulting in lower coordination numbers in supported membranes. Supported membranes showed a higher transformation temperature (slower rate of transformation) than did the unsupported. Supported and unsupported membranes, calcined for 8 h, transformed to ca. 90% rutile (calculated from Raman spectrum) after calcination at 850 and 650 °C, respectively. This difference in phase transformation behaviour is attributed primarily to the large stress which is developed in a constrained environment owing to the negative volume change during the anatase–rutile transformation

Ceramic Membranes by Electrochemical Vapor Deposition of Zirconia-Yttria-Terbia Layers on Porous Substrates

By means of electrochemical vapor deposition (EVD), it is possible to grow thin, dense layers of zirconia/yttria/terbiasolid solution (ZYT) on porous ceramic substrates. These layers can be used as ceramic membranes for oxygen separation.The kinetics of the EVD process, the morphology of the grown layers and their oxygen permeation properties are investigated.At a deposition temperature of 800°C, the EVD layer growth is limited by bulk electrochemical transport. At 1000°Cthe layer growth is limited by pore diffusion of the oxygen source reactant. The EVD-grown ZYT layers show columnarstructures with prismatic grains on top; the size of the grains (1 to 2 µm) increases slightly with temperature and depositiontime. ZYT is deposited mainly in the cubic doped zirconia phase. Oxygen permeation experiments show that the oxygenpermeation flux through the ZYT layers is limited by an electrochemical process. Permeation values in the order of10–8 mol/cm2 s have been observed (900–1000°C, air vs. CO/CO2)