A. Larbot

Inorganic membranes and solid state sciences

Abstract The latest developments in inorganic membranes are closely related to recent advances in solid state science. Sol–gel processing, plasma-enhanced chemical vapor deposition and hydrothermal synthesis are methods that can be used for inorganic membrane preparation. Innovative concepts from material science (templating effect, nanophase materials, growing of continuous zeolite layers, hybrid organic–inorganic materials) have been applied by our group to the preparation of inorganic membrane materials. Sol–gel-derived nanophase ceramic membranes are presented with current applications in nanofiltration and catalytic membrane reactors. Silica membranes with an ordered porosity, due to liquid crystal phase templating effect, are described with potential application in pervaporation. Defect-free and thermally stable zeolite membranes can be obtained through an original synthesis method, in which zeolite crystals are grown inside the pores of a support. Hybrid organic–inorganic materials with permselective properties for gas separation and facilitated transport of solutes in liquid media, have been successfully adapted to membrane applications. Potential membrane developments offered by CVD deposition techniques are also illustrated through several examples related to the preparation of purely inorganic and hybrid organic–inorganic membrane materials.

Inorganic membranes obtained by sol-gel techniques

Abstract The production scheme of thin and porous ceramic layers (inorganic membranes) is presented. Three ways are possible for obtaining inorganic membranes: one using suspended powders, and two using sol-gel techniques. The latter permits ultrafiltration membranes to be obtained. p]We expose one particular sol-gel technique viz. the destabilization of colloidal solutions, and we describe the successive steps of this process: peptization, sol viscosity adjustment, addition of binders, coating the sol layer, drying, and thermal treatment. p]At last, we report various techniques to characterize membranes: scanning electron microscopy, mercury porosimetry and permeation tests.

Rejection of mineral salts on a gamma alumina nanofiltration membrane application to environmental process

Abstract Nanofiltration of salt solutions through an alumina membrane prepared by a sol gel process was investigated: rejections depend on the charge of the ions and decrease in the order: (divalent cation, monoanion) > (monocation, monoanion) or (dication, dianion) > (monocation, dianion). The surface charge of the membrane in the investigated solutions was shown to be positive over a very large pH range. The rejection of ionic species can be rationalised by taking into account the low pore diameters of the membrane and its surface charge: as in the case of organic nanofiltration membranes, the rejection mechanism depends on the relative ratio of the coulombic, dielectric and hydration interactions between the membrane material and the ionic species. The use of nanofiltration through the alumina nanofiltration membrane for denitration of water is also presented.

Sol-gel synthesis of magnesium aluminum spinel from a heterometallic alkoxide

MgAl2O4 spinel powder of 5 to 20 nm particle size, was prepared by the hydrolysis-condensation reaction of a heterobimetallic aluminum magnesium n-butoxide modified by polyethylene glycol (PEG). The gel after hypercritical drying yielded a porous aerogel with a density of 0.2 g/cm3. Calcination of the aerogel at 700°C in air yielded a pure MgAl2O4 powder which could be hot pressed to transparency.

Salt filtration on gamma alumina nanofiltration membranes fired at two different temperatures

The performance towards filtration of different salts of two gamma alumina nanofiltration membranes, fired at 450°C and 650°C, were compared in terms of flux and rejection. The phenomenological parameters (reflection coefficient σ and overall solute permeability P) were determined for some salts.

Sol-gel transition in zirconia systems using physical and chemical processes

Abstract Two routes are described to achieve the formation of stable sols and the conversion of sols to gels in multicomponent systems with zirconia as a major component. Physical and chemical gelation have been obtained according to whether the peptization of thin powders or the polymerization of metal organic compounds were involved in the preparation of the gels. In both cases metal alkoxides of zirconium, silicon, aluminum and sodium were chosen as starting materials. Controlled hydrolysis and copolymerization of these alkoxides were performed through esterification of ethanol by acetic acid or the formation of zirconium glycoxide by reaction with ethylene glycol.

Porous texture of a titanium oxide gel: evolution as a function of medium used

Abstract Esterification reaction was used to prepare gels by hydrolysis of titanium isopropoxide. Depending on the acetic acid/alkoxide ratio, crystals, gels or precipitates can be obtained. The relation between theoretical and experimental hydrolysis ratio, h , has been determined using gas chromatography. The porous structure was measured in the case of a gel prepared with h th = 6 ( h expt = 1.27). The infrared spectra show that acetic acid acts as bidentate ligand bridging two titanium atoms. The gel texture was measured by small angle X-ray spectroscopy. If the Scherer model is applied, the mean pore radius obtained was 2.2 nm. The mean mesopore radius, measured by thermoporometry in decane, was 2.8 nm. Thermoporometry measurements were performed in water. Pore radii evolved to a stable value of 4 nm. This can be explained by hydrolysis of organic groups present in the gel.

Cobalt spinel CoAl2O4 via sol-gel process : elaboration and surface properties

Abstract In this work the preparation of cobalt spinel CoAl2O4 is described. The technique of colloidal solution destabilization was used as the sol-gel process. The powder obtained from the dried gel was characterized by thermal analysis (thermogravimetric and differential), X-ray diffraction, infrared spectroscopy, and specific surface area measurement. Crystallization of CoAl2O4 spinels occurs at about 400°C. The study of surface properties (surface charge and electrophoretic mobility) of the spinel powder fired at 600°C, in suspension in different electrolyte solutions (10−3 molar ionic strength), shows that the spinel material charge depends on the pH of the solution. The spinel sol was used for the preparation of membrane by the slip-casting method.

Grafting γ alumina microporous membranes by organosilanes:: Characterisation by pervaporation

Abstract γ alumina microporous membranes, produced from a sol–gel process, were grafted by means of chloro-, methoxy- or ethoxy-organosilanes. The grafting conditions were first tested on γ alumina powders which were characterised by means of thermogravimetric analysis and nitrogen adsorption. The membrane material was studied by means of infrared spectrometry, nitrogen adsorption and nitrogen permeation. The performances of grafted and ungrafted membranes were examined for the separation by pervaporation of binary mixtures such as water/ethanol, cyclohexane/toluene, ethanol/cyclohexane and methylethylketone/cyclohexane.

GAMMA ALUMINA NANOFILTRATION MEMBRANE APPLICATION TO THE REJECTION OF METALLIC CATIONS

Abstract A gamma alumina nanofiltration membrane, prepared by a sol-gel process, exhibits interesting properties for the filtration of metallic complexes. Experimental rejection and fluxes depend on the pH of the solution. Comparison of the distribution diagrams of the species in solution as a function of pH, established for several metallic complexes using equilibrium constants, shows that rejections and fluxes are affected by the size and the charge of the complex. High rejection ( > 95%) can be reached if the molecular weight of the complex is higher than 400. For positively charged complexes, the flux of the permeate is almost independent of the pH of the solution, whereas for negatively charged ones, the flux increases with increasing pH.