C. Guizard

The sol-gel approach to prepare candidate microporous inorganic membranes for membrane reactors

Abstract In catalytic membrane reactors, the membrane can play a multiple role such as promotion of an integrated catalytic reaction but also partitioning of the reactants and separation of the products. For these applications permselective membranes may be used that can work at high temperature (gas or liquid phase), in organic solvents or in contact with biomass. One type of possible membranes are the ceramic membranes which exhibit a connected microporosity. This kind of membrane can at present be made using the latest developments in sol-gel processing. We used this process to prepare various microporous ceramic membranes in aqueous media: ZrO2, Al2O3, LaOCl; and in organic media: SiO2, ZrO2. Attention has been paid to the microstructure of these membranes in order to identify the main parameters governing the evolution of these membrane materials during their preparation. The main feature of these membranes is the presence of non-aggregated nanosized particles after calcination. A peptization reaction and/or a chemical modification of the precursors has been shown to be of prime interest in the control of the precursor reactivity and in limiting the condensation rate so as to avoid inhomogeneous and rough products. Non-aggregated ceramic particles down to 4 nm in size can thus be obtained and result in microporous membrane formation.

Synthesis and characterization of microporous zirconia powders: Application in nanofilters and nanofiltration characteristics

Abstract Mineral nanofiltration membranes are not commercially available because it is difficult to generate connected micropores in an inorganic material. Recent advances in sol-gel chemistry can be applied for the preparation of mineral oxide particles allowing the formation of a microporous structure after sintering. Zirconia has been chosen as a membrane material because of its chemical and thermal stabilities. Initially powders were synthesized and characterized to determine the optimal synthesis conditions for application in the preparation of membranes. Magnesium oxide stabilized zirconia (13 mol.%) demonstrated high surface areas leading to elevated surface charge densities. Crack-free coating could be deposited on ceramic multichannel substrates. The development of these membranes has permitted both model solute rejections in the nanofiltration range (73.2% B 12 vitamin and 54.5% saccharose), high water permeability (12.31/h m 2 bar) and high fluxes with a 19-channel configuration. The high surface charge density of the membrane material ensured a large divalent anion rejection (66.3% sulfates) and a low rejection of monovalent anions (31.5% chlorides).

Silica membranes by the sol―gel process

Abstract The sol-gel process allows the preparation of thin microporous layers. This technique has been used to prepare silica membranes with controlled thickness (from 2 to 10 μm) and pore diameters (from 6 to 20 nm). The preparation conditions are described and the marked influence of the drying conditions on the final properties of the membranes is pointed out. The filtration performance of the membrane reveals an important interaction with aqueous media.

A microporous zirconia membrane prepared by the sol—gel process from zirconyl oxalate

Abstract We have prepared a xirconia membrane by the sol—gel process, that is potentially active in the low-ultrafiltration or high-nanofiltration range. The precursor is a dried gel available as a powder, which leads to a sol when added to water. Coatings of tubular ceramic supports by slip-casting have been successfully performed. Characterization was made by scanning electron microscopy, nitrogen adsorption—desorption and filtration of dextran. This zirconia membrane has pores of ∼4 nm diameter and also retention properties for dextran molecules of molecular weight higher than 10,000 daltons.

Effect of boric acid addition in colloidal sol-gel derived SiC precursors

Abstract Sol-gel derived precursors are known to be suitable for the formation of submicron and homogeneous SiC powders. From colloidal silica and saccharose, we obtained SiC powders whose grain size is 100nm in diameter and which are easily sinterable with boron containing additives. Furthermore, we have pointed out that boric acid added in the original colloidal sols permits to increase considerably the carbothermal reduction yield and the powder crystallinity.

A comparison of immobilization sol-gel methods for an optical chemical hydrazine sensor

Optical-fibre chemical sensors are of great interest in nuclear fuel reprocessing, because of remote, real-time measurements and lack of waste. An intrinsic optical chemical sensor for hydrazine has been developed to optimize its concentration in the Purex process. p-Dimethyl-aminobenzaldehyde (p.DMAB) has been chosen as the reactive molecule. This paper reports on immobilization sol-gel methods to synthesize host matrices for p.DMAB. Two ways of immobilization are investigated: physical entrapment in a microporous structure and covalent bonding in a mesoporous one. The physical characteristics and the analytical performances of such sensors are reported. The sensing scheme of the sensor depends on several parameters. The mass transfer of the analyte between the solution and the matrix is governed by the diffusion process and influences the response time of the sensor. The method of immobilization of the reactive molecule and the nature of the host matrix govern the lifetime of the sensor. The third parameter, which is not often mentioned, relies on the interactions between the diffused species and the surface of the host matrix, which modify its response time and its reversibility.

Performance of stabilized zirconia nanofilters under dynamic conditions

The nanofiltration performance of stabilized zirconia nanofilters was tested under dynamic conditions in an electrolytic solution. Interaction between nanostructured materials and solutes may modify the rejection properties of such membranes in the dynamic range of the experiments. Optimal filtration performance can only be realized by a proper understanding of these interactions. At high pressures, a decrease in electrolyte rejection is observed and the membranes show a reduction in hydraulic permeability. Two approaches (one thermodynamic and the other electrokinetic) are considered to explain these phenomena, i.e. the concentration polarization model and the electroviscosity model. A combination of these two approaches is successfully developed to explain the observed nanofilter behaviour at high pressures in the case of electrolytes. Further investigations were performed using an uncharged solute (vitamin B12) in an electrolyte buffer. Although a reduction in permeability was observed at high pressures, there was no important decrease in vitamin B12 rejection because the optimal transmembrane pressure (where the rejection reaches a maximum) was above the pressure range studied experimentally or did not allow the rejection reducing effects of the concentration polarization layer to be observed more clearly.

Influence of the interface on the filtration performance of nanostructured zirconia ceramic

Abstract Nanostructured stabilized zirconia (13 mol.% MgO) has been synthesized by the sol-gel process to prepare a ceramic nanofiltration membrane. N 2 adsorption and X-ray diffraction analysis have been carried out on this material to determine the membrane nanostructure. Size and charge rejections of the nanofilter have been determined by filtration experiments using different model solutes. Performances of nanofiltration properties of the stabilized zirconia nanofilter have been characterized for various pH. At the nanometer scale, interactions between the membrane nanostructured material and solutes are very important. Proper understanding and explanation of these phenomena are necessary to optimize filtration performances. Results have been interpreted according to measurements performed by electrophoretic mobility on the stabilized zirconia powder which was used as the microporous membrane material. The amphoteric behavior of the nanostructured zirconia layer, which determines the filtration properties of the membrane, has been observed by electrolyte rejection measurements as a function of pH. The high surface charge density (0.4 μC/cm 2 ) of this zirconia has permitted 90% sulfate rejection at high pH values. Membrane-solute characterizations have shown the influence of anion specific adsorption at the zirconia surface. A pH memory effect, induced by proton exchanges at the membrane-solution interface, has also been identified and clearly explained.