Anne Julbe

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.

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.

Novel B-site ordered double perovskite Ba2Bi0.1Sc0.2Co1.7O6−x for highly efficient oxygen reduction reaction

Double perovskite Ba2Bi0.1Sc0.2Co1.7O6−x (BBSC) demonstrates low polarization resistance between 600 and 750 °C due to the high oxygen reduction rate of BBSC as reflected by its large DV and k values, which are derived from the face centered cubic structure and high cobalt content.

MOF-Based Membrane Encapsulated ZnO Nanowires for Enhanced Gas Sensor Selectivity

Gas sensors are of a great interest for applications including toxic or explosive gases detection in both in-house and industrial environments, air quality monitoring, medical diagnostics, or control of food/cosmetic properties. In the area of semiconductor metal oxides (SMOs)-based sensors, a lot of effort has been devoted to improve the sensing characteristics. In this work, we report on a general methodology for improving the selectivity of SMOx nanowires sensors, based on the coverage of ZnO nanowires with a thin ZIF-8 molecular sieve membrane. The optimized ZnO@ZIF-8-based nanocomposite sensor shows markedly selective response to H2 in comparison with the pristine ZnO nanowires sensor, while showing the negligible sensing response to C7H8 and C6H6. This original MOF-membrane encapsulation strategy applied to nanowires sensor architecture pave the way for other complex 3D architectures and various types of applications requiring either gas or ion selectivity, such as biosensors, photo(catalysts), and electrodes.

Potentiality of organic solvents filtration with ceramic membranes. A comparison with polymer membranes

The objective is to identify the main interfacial interactions influencing the flow of organic solvents through ceramic oxide membranes exhibiting a submicronic porous structure, in particular ceramic nanofilters. Three microporous (rp ≤ 1 nm) composite ceramic oxide membranes (3Al2O32ZrO2, SiO2ZrO2, SiO2TiO2) prepared by the sol-gel process were investigated. Permeation experiments were operated with polar (ethanol) and non-polar (hexane, heptane, toluene) organic solvents. Ceramic oxides are considered as high energetic surfaces for non-polar liquids leading to attractive interactions and stable substrate-wetting films when el < es. In the case of a porous substrate, capillary pressure is the parameter generally used to account for liquid penetration in the pores. For submicronic pore sizes, an additional contribution to the surface energy (the disjoining pressure or the fluid density variation resulting from molecular ordering of solvent molecules at solid interfaces) may explain observed deviations to the Darcys law. Results are in good agreement with these interfacial energy concepts. The specific permeation observed for each solvent/membrane pair could be explained by the relation existing between the acid-base surface properties of ceramic oxides and the solid surface energies.

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.

Design of nanosized structures in sol-gel derived porous solids. Applications in catalyst and inorganic membrane preparation

Recent advances in sol-gel processing, with the aim of porous ceramic oxide preparation and applications in catalysis and membrane separation, are reviewed. Recent results from our group in porous solid preparation are presented. Three topics of particular interest are reported which highlight the potential of sol-gel methods to tailor microporous and mesoporous structures in solids, and also the possibility to combine catalyst and ceramic membrane properties. The first one deals with the preparation and the characterisation of supported microporous layers (e.g. with pore diameter of less than 2 nm) and the applications linked to ceramic nanofilters. The second topic illustrates improvements expected in sol-gel processing of oxide catalysts and the possibility of forming supported catalytic layers and membranes. The last one has to do with the role of surface active agents in the control of the sol-to-gel transition and in the formation of tailor-made porous structures in oxide materials.

Evaluation of sol-gel methods for the synthesis of doped-ceria environmental catalysis systems. Part I: preparation of coatings

Sol-gel technology can be employed for the synthesis of multi-component nanophase environmental catalysts with enhanced catalytic activity, controlled composition and tailor-made pore structure together with their simultaneous deposition upon porous supports. The conditions for effective deposition of nano-phase doped-ceria systems onto cordierite honeycombs through sol-gel routes were investigated. For the direct casting from the sol phase, the sol rheological characteristics were adjusted and optimized so that homogeneous smooth washcoats were obtained. In this way, integrated support-catalyst assemblies suitable for high-temperature gas-solid catalysis were prepared. To overcome the inherently low loading percentage achieved per impregnation when depositing directly the sols, the synthesis of sol-gel powders and their deposition from aqueous slurries, as well as hybrid deposition methods using sol-gel powders dispersed in sols were tested as alternatives. The hybrid sol/powder systems proved to be the most attractive by combining an effective loading procedure with desirable and fine-tuned coating characteristics.

Oxidative dehydrogenation of propane on V/Al2O3 catalytic membranes. Effect of the type of membrane and reactant feed configuration

Abstract An experimental study of the oxidative dehydrogenation of propane in a catalytic membrane reactor has been carried out. The work presented explores the influence of the type of feed arrangement and of the type of catalytic membrane on the yield to the partial oxidation product (propene). The experimental setting allowed the reaction to take place with a segregated feed, in the simultaneous presence of a bi-directional diffusive flux and a low-intensity unidirectional convective flow. Special attention has been devoted to studying the influence of the back permeation of the reactant hydrocarbon, and of procedures capable of preventing it.

Effect of non-ionic surface active agents on TEOS-derived sols, gels and materials

The sol-gel process, starting from tetraethylorthosilicate precursor, is a suitable technique for the preparation of silica thin films. The use of specific organic additives, like non ionic surface-active agents, drastically modifies the gelation process and allows the preparation of microporous materials with a high microporous volume. The effects of additives on the sol, gel and material characteristics have been investigated by several methods such as 29Si NMR, QELS, SAXS (for sols and gels), and N2 adsorption, FESEM (for fired materials). It appears that the interactions of surface active agents with TEOS derived species limit condensation reactions and particle growing. A brittle gel structure is generated which leads to highly porous microporous silica after the elimination of organic chains by thermal treatment at 450°C. The material porous texture (specific surface area, pore size distribution and porous volume) can be varied especially by varying the surface active agent chain length and quantity. This kind of sol-gel system is suitable to prepare microporous silica membranes candidate for gas separation or catalytic reactor applications.