Christian Guizard

Porous ceramic membranes for catalytic reactors — overview and new ideas

Abstract The membrane reactor (MR) concept, combining in the same unit a conversion effect (catalyst) and a separation effect (membrane), already showed various potential benefits (increased reaction rate, selectivity and yield) for a range of reactions involving the membrane as extractor, distributor or contactor. Due to the generally severe conditions of heterogeneous catalysis, most MR applications use inorganic membranes, which can be dense or porous, inert or catalytically active. After a rapid overview of the working concepts of MRs, the main types of porous ceramic membranes, which have been developed for MR applications, are reported and discussed (characteristics and limitations). Starting from these general basis, our objective is to put recent developments into focus, with a special emphasis on porous composite infiltrated membranes and related synthesis methods. Some new ideas currently explored in our group, such as the ‘chemical valve membrane’ concept and the interest of nanophase materials for oxygen transport, will be also developed. An attempt in addressing the future developments of porous membranes for MRs will be finally proposed.

Metastable and unstable cellular solidification of colloidal suspensions

Colloidal particles are often seen as big atoms that can be directly observed in real space. They are therefore becoming increasingly important as model systems to study processes of interest in condensed-matter physics such as melting, freezing and glass transitions. The solidification of colloidal suspensions has long been a puzzling phenomenon with many unexplained features. Here, we demonstrate and rationalize the existence of instability and metastability domains in cellular solidification of colloidal suspensions, by direct in situ high-resolution X-ray radiography and tomography observations. We explain such interface instabilities by a partial Brownian diffusion of the particles leading to constitutional supercooling situations. Processing under unstable conditions leads to localized and global kinetic instabilities of the solid/liquid interface, affecting the crystal morphology and particle redistribution behaviour.

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.

In Situ X-Ray Radiography and Tomography Observations of the Solidification of Aqueous Alumina Particle Suspensions—Part I: Initial Instants

This paper investigates the behaviour of colloidal suspensions of alumina particles during directional solidification, by in situ high-resolution observations using X-ray radiography and tomography. This second part is focussed on the evolution of ice crystals during steady state growth (in terms of interface velocity) and on the particles redistribution taking place in this regime. In particular, it is shown that diffusion cannot determine the concentration profile and the particles redistribution in this regime of interface velocities (20-40 microns/s); constitutional supercooling arguments cannot be invoked to interpret particles redistribution. Particles are redistributed by a direct interaction with the moving solidification interface. Several parameters controlling the particles redistribution were identified, namely the interface velocity, the particle size, the shape of the ice crystals and the orientation relationships between the crystals and the temperature gradient.

Gas separation properties of hybrid imide–siloxane copolymers with various silica contents

Abstract Hybrid polyimide–siloxane copolymers containing different proportion of silica have been prepared by polycondensation, imidisation and sol–gel coreaction of pyromellitic dianhydride, aminoalkoxysilane and tetramethoxysilane. Two types of aminoalkoxysilane (aminopropyltrimethoxysilane and aminopropylmethyldiethoxysilane) able to provide a bonding between the inorganic and the organic phase were used. Materials obtained were characterized by various techniques (thermogravimetric analysis, infrared spectrometry and 29 Si NMR) in order to study their molecular structure. The presence of methyl side-groups linked to the silicon of APrMDEOS precursors inhibit the formation of OH free or OH linked bonds in the material. Gas permeation measurements were obtained at various temperatures for hydrogen, carbon dioxide and nitrogen. The permeability properties of the two materials vary with the siloxane content and the nature of the aminosiloxane. Thus, the presence of methyl side-groups in the APrMDEOS materials and the network crosslinking are a limiting factor for the gas permeation. The gas transport is thermally activated and the activation energy values vary with the siloxane content of the materials.

Synthesis conditions for hexagonal mesoporous silica layers

Hexagonal mesoporous silica layers were prepared by the sol–gel route using silicon alkoxides as silica precursors and alkyltrimethylammonium bromides to form the templating liquid crystal mesophase. The synthesis conditions required to obtain well-ordered crack-free layers were investigated. Thin layers exhibiting these properties were deposited from diluted sols. X-Ray diffraction enabled characterisation of the thermal evolution of their ordered structure and crystalline texture. Their porosity was experimentally measured from nitrogen adsorption–desorption analyses carried out directly on the thin layers. Two main synthesis parameters were identified. The first is the ageing time of the sol before deposition: 29Si NMR showed that the disappearance of the ordered structure in the layers is related to the appearance of the Q3 species in the sol. The second important synthesis parameter is the surfactant volume fraction in the medium after the removal of the volatile components. Using hexadecyltrimethylammonium bromide, well-ordered hexagonal layers were obtained for surfactant volume fractions ranging from 0.5 to 0.65, as expected from the corresponding water–surfactant binary diagram. For surfactants with shorter alkyl chains, the domain of existence of the hexagonal layers shifts to higher surfactant volume fractions in agreement with the evolution previously observed on the water–surfactant binary diagrams.

Polyimide-siloxane hybrid materials : Influence of coupling agents addition on microstructure and properties

Polyimide-silica hybrid materials were prepared by polycondensation, polyimidization, and sol-gel coreactions. Several aminoalkoxysilane precursors providing a chemical bonding between the inorganic and the organic phase were used. During the synthesis, the molecular structure of the materials prepared with these precursors were followed at various steps by 29Si-NMR (nuclear magnetic resonance). The final materials were characterized at the macroscopic level by thermogravimetric analysis and scanning electron microscopy and at the molecular level by infrared spectrometry and 29Si-MAS-NMR spectroscopy. It has been demonstrated that the properties of the organic sidegroups of the bonding agent are determinent on the properties of the final material.

Characteristics and performance in the oxidative dehydrogenation of propane of MFI and V-MFI zeolite membranes

Original V-MFI zeolite membranes in/on alumina tubes have been prepared, characterized and tested for the oxidative dehydrogenation of propane. The morphological and textural characteristics of the V-MFI membrane were found to be very similar to those of V-free MFI membranes. The insertion of vanadium species has been checked by elemental analysis, XRD, FTIR and ESR. The single gas permeance studies showed that both MFI and V-MFI membranes were free of macrodefects and provided an activated transport for both C3H8 and O2 above 400°C, with a Knudsen-like permselectivity. Both MFI and V-MFI membranes were found to produce propene with about 40% selectivity but with higher O2 and C3H8 conversions for the V-MFI. In the studied conditions, between 550 and 650°C, the oxygen distributor configuration was not found to improve the reactor performance compared to the flow-through one.

Hybrid organic-inorganic fixed site dibenzo 18-crown-6 complexant membranes

New heteropolysiloxane (HPS) materials containing the 2,3,11,12-bis[4-(10-aminodecylcarbonyl)]-benzo-18-crown-6 and 2,3,11,12-bis[4-(2-aminoethylcarbonyl)]-benzo-18-crown-6 compounds chemically linked in a reticulate silica matrix were synthesized by the sol-gel process. The preparation and the characterization of the HPS precursors are reported here. From these materials, dialysis membranes used for the facilitated competitive transport of silver/copper ions are obtained. The selectivity of the membranes is improved by the incorporation of the complexing molecules which allow a better solubility of silver ions in the membrane. In accordance with the experimental results, a dual transport mechanism is proposed, based on a reversible binding equilibrium solute/complexing site and a solute diffusion between two complexing sites.