Mesoporous ceramic membrane

J.M. Hofman-Ziiter, Chemical and Thermal Stability of (Modified) Mesoporous Ceramic Membranes , PhD Thesis, University of Twente, 1995. 

The most demanding support requirements are those for ultra thin micro-porous gas separation membranes, which are currently being developed in several research organisations worldwide including ECN (Petten, the Netherlands). In principle, a mesoporous Knudsen or UP membrane can serve as support for these membranes if the defect density in the substrate surface, i.e. the mesoporous layer, is low enough. Indeed, the quality of the Knudsen or UF membrane as support for a microporous gas separation membrane should be higher than is usually needed for the UF or Knudsen function [4]. This means that not every mesoporous ceramic membrane is a suitable support for micro-porous or dense amorphous gas separation membranes. 

Finally, organic additives can affect the thermal properties (phase transformation behaviour) of mesoporous ceramic membranes. Ziiter [15] reported the transformation from monoclinic to tetragonal particles and back as a function of the amount of PVA. 

J.M. Ziiter-Hofman, Chemical and thermal stability of (modified) mesoporous ceramic membranes. PhD Thesis, University of Twente, Enschede, the Netherlands. 

S. Roy Chowdhury, P. T. Witte, D. H. A. Blank, P. L. Alsters, J. E. ten Elshof, Recovery of homogeneous polyoxometalate catalysts from aqueous and organic media by a mesoporous ceramic membrane without loss of catalytic activity, Chem. Eur.. 12 (2006) 3061. 

Key words mesoporous ceramic membranes, alumina, titania, zirconia, membrane reactors for dehydrogenation reactions. 

For the transport of gas mixtures, the generalised Maxwell-Stefan equation (Krishna and WesseUngh, 1997) has been widely adopted to describe multi-component diffusion. Although quantitative descriptions of gas diffusion in various microporous or mesoporous ceramic membranes based on statistical mechanics theory (Oyama et al., 2004) or molecular dynamic simulation (Krishna, 2009) have been reported, the prediction of mixed gas permeation in porous ceramic membranes remains a challenging task, due to the difficulty in generating an accurate description of the porous network of the membrane. 

Finally, in chapter 9, conclusions are drawn and suggestions made for further research on (steam-stable) molecular sieving silica membranes or mesoporous y-alumina membranes. Though not all of the project objectives were obtained, progress was made in the synthesis of micro- and mesoporous membranes. Especially the development of steam stable membranes may be a large step forward in the development of ceramic membranes. 

The porous structure of ceramic supports and membranes can be first described using the lUPAC classification on porous materials. Thus, macroporous ceramic membranes (pore diameter >50 nm) deposited on ceramic, carbon, or metallic porous supports are used for cross-flow microfiltration. These membranes are obtained by two successive ceramic processing techniques extrusion of ceramic pastes to produce cylindrical-shaped macroporous supports and slip-casting of ceramic powder slurries to obtain the supported microfiltration layer [2]. For ultrafiltration membranes, an additional mesoporous ceramic layer (2 nm<pore diameter <50 nm) is deposited, most often by the solgel process [11]. Ceramic nanofilters are produced in the same way by depositing a very thin microporous membrane (pore diameter <2 nm) on the ultrafiltration layer [4]. Two categories of micropores are distinguished the supermicropores >0.7 nm and the ultramicropores <0.7 nm. 

Microfiltration and ultrafiltration are the two main filtration techniques for which ceramic membranes have been widely used to date. As described in Section 6.2.1.2, MF and UF ceramic membranes exhibit macro- and mesoporous structure, respectively, which result from packing and sintering of ceramic particles. Liquid flow in such porous media is convective in nature and the simplest description of permeation flux, J, is given by the Darcy s equation [20] ... 

Basic mechanisms involved in gas and vapor separation using ceramic membranes are schematized in Figure 6.14. In general, single gas permeation mechanisms in a porous ceramic membrane of thickness depend on the ratio of the number of molecule-molecule collisions to that of the molecule-wall collisions. In membranes with large mesopores and macropores the separation selectivity is weak. The number of intermolecular collisions is strongly dominant and gas transport in the porosity is described as a viscous flow that can be quantified by a Hagen-Poiseuille type law ... 

In summary, one can see that separation selectivity for gas and vapor molecules depends on the category of pores (mesopores, supermicropores, and ultramicropores) and on the related transport mechanisms. Either size effect or preferential adsorption effect (irrespective of molecular dimension) is involved in selective separation of multicomponent mixtures. The membrane separation selectivity for two gases is usually expressed either as the ratio between the two pure gas permeation fluxes (ideal selectivity) or between each gas permeation flux measured from the mixture of the two gases (real selectivity). More detailed information on gas and vapor transport in porous ceramic membranes can be found in Ref. [24]. 

Binkerd CR, Ma YH, Moser WR, and Dixon AG. An experimental study of the oxidative coupling of methane in porous ceramic radial-flow catalytic membrane reactors. Proceedings of ICIM4 (Inorganic Membranes), Gatlinburg, TN D.E. Fain (ed.), 1996 441-450. Yeung AKL, Sebastian JM, and Varma A. Mesoporous alumina membranes synthesis, characterization, thermal stability and nonuniform distribution of catalyst. J. Membr. Sci. 1997 131 9-28. 

A combination of characterization techniques for the pore structure of mesoporous membranes is presented. Equilibrium and dynamic methods have been performed for the characterisation of model membranes with well-defined structure while three-dimensional network models, combined with aspects from percolation theory can be employed to obtain structural information on the porous network topology as well as on the pore shape. Furthermore, the application of ceramic membranes in separations of condensable from noncondensable vapors is explored both theoretically and experimentally. 

Fig. 2.6. Schematic representation of an as)mimetric (composite) ceramic membrane. 1. Porous support (1-15 pm pores) 2. intermediate layer(s) (pore diameter dp = 100-1500 run) 3. mesoporous separation layer (dp = 3-100 run) 4. Modification of 3 to microporous sepeiratian layer (dp=0.5-2 nm).

Fig. 12.12. Influence of zeta-potential (Stem-layer thickness 1) and Streaming-potential (electrokinematic flow) on ion rejection and volume flux for porous ceramic membranes exhibiting negatively charged pore walls. Cases of micropores (nanofiltration), mesopores (ultrafiltration) and macropores...

In proton exchange membrane fuel cells, perhaps the most divulgate type of fuel cells, a proton-conducting polymer membrane acts as the electrolyte separating the anode and cathode sides. Porous anaodic alumina (Bocchetta et al., 2007) and mesoporous anastase ceramic membranes have been recently introduced in this field (Mioc et al., 1997 Colomer and Anderson, 2001 Colomer, 2006). 

Naszalyi, L. et al., Sol-gel-derived mesoporous SiOj/ZnO active coating and development of multifunctional ceramic membranes, Separ. Purif. Technol.. 59, 304, 2008. 

This chapter focuses on the chemical processing of ceramic membranes, which has to date constituted the major part of inorganic membrane development. Before going further into the ceramic aspect, it is important to understand the requirements for ceramic membrane materials in terms of porous structure, chemical composition, and shape. In separation technologies based on permselective membranes, the difference in filtered species ranges from micrometer-sized particles to nanometer-sized species, such as molecular solutes or gas molecules. One can see that the connected porosity of the membrane must be adapted to the class of products to be separated. For this reason, ceramic membrane manufacture is concerned with macropores above 0.1 pm in diameter for microfiltration, mesopores ranging from 0.1 pm to 2 nm for ultrafiltration, and nanopores less than 2 nm in diameter for nanofiltration, per-vaporation, or gas separation. Dense membranes are also of interest for gas... 

Ceramic membranes normally have an asymmetrical structure composed of at least two, normally three, different porosity levels. Indeed, before applying the active top layer, a mesoporous intermediate layer is often appHed in order to reduce the surface roughness. A macroporous support ensures mechanical stahility. Ceramic membranes generally show a higher chemical, structural and thermal stability. They do not deform under pressure, do not swell and are deaned easily [13]. 

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