Mesoporous inorganic membranes

Figure 7-19. Mass transport mechanisms for gas and vt or molecules in micro and mesoporous inorganic membranes.

Inorganic membranes employed in reaction/transport studies were either in tubular form (a single membrane tube incorporating an inner tube side and an outer shell side in double pipe configuration or as multichannel monolith) or plate-shaped disks as shown in Figure 7.1 (Shinji et al. 1982, Zaspalis et al. 1990, Cussler 1988). For increased mechanical resistance the thin porous (usually mesoporous) membrane layers are usually supported on top of macroporous supports (pores 1-lS /im), very often via an intermediate porous layer, with pore size 100-1500 nm, (Keizer and Burggraaf 1988). 

Numerous studies on inorganic membranes have shown that the separation factor is limited and not far from that predicted by the Knudsen diffusion. This primarily reflects the current status of material developments of inorganic membranes. The majority of commercial and developmental inorganic membranes contain macropoies or mesopores. These pore sizes fall within the dominant regime of Knudsen diffusion which is of limited use for many gas separation applications from the standpoint of process economics. To break this barrier, finer pore sizes or transport mechanisms more effective than Knudsen diffusion for gas separation is essential. 

Bose F, Ayral A, and Guizard C. Mesoporous anatase coatings for coupling membrane separation and photocatalysed reactions. Proceedings of the Eight International Conference on Inorganic Membranes, Cincinnati, OH, July 18-22, 2004 114—117. 

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. 

There are relatively few studies dealing with adsorption on microporous inorganic membranes. Except the work by Ma and his co-workers and Burggraaf and his co-workers, few studies on the interrelation between adsorption and permeation have been reported. The extremely thin membrane layer on a relatively thick membrane support makes the adsorption measurement rather difficult. Neither gravimetric nor volumetric technique will provide sufficient accuracy for the measurement due to the extremely small fraction of the membrane layer in a supported membrane. Nevertheless, adsorption measurements can give important information on pore sizes and permeation mechanisms in microporous membranes. This section will examine the adsorption of gases on microporous membranes and of liquids on mesoporous and macroporous membranes. 

This is a combined organic-inorganic membrane that comprises a macropor-ous a-alumina substrate (tubular or multichannel), an intermediate mesopor-ous inorganic titanium oxide layer (thickness 1 pm) and a microporous Nafion polymer top-layer (thickness less than 0.1 pm). The overall performance and... 

Hydrogen selective inorganic membranes can be mesoporous (2 nm < pore diameter < 50 nm ceramic, glass or carbon) microporous (pore diameter < 2 nm ceramic, carbon or zeolite) or dense (ceramic or metal). These membranes can be used from ambient temperatures up to about 600°C for mesoporous materials, up to about 500°C for microporous inorganic membranes and up to about 800°C for dense inorganic membranes [14-16]. These temperatures are only a rough indication, because of the different materials which can be used and the test conditions at which the membranes have to operate. 

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... 

Silica is also employed to prepare microporous inorganic membranes suitable for gas separation. De Vos et al. [163] reported the preparation of silica membranes with a very low defect concentration. They employed a sol-gel synthesis starting from tetraethylorhosilicate. These membranes consist of a microporous layer on top of a supported mesoporous y-Al203 membrane. The support layer provides mechanical strength to the selective silica top layer. The prepared membranes have a thick... 

As described earlier, most of the studies on photocontrolled transport phenomenon focused on azobenzene-doped organic platforms such as planar lipid membranes and spherical vesicles, which have an intrinsic disadvantage. Those lipid membranes and vesicles are delicate and unstable, thus limiting their practical applications. Incorporation of the azobenzene moieties into a robust inorganic matrix greatly enhances the system stability and facilitates the device fabrication. For example, azobenzene moieties precisely positioned onto the pore surfaces of mesoporous silica membranes enable novel photocontrolled transport in the resulting composite materials (Liu et al., 2004). 

In the viewpoint of the morphology and/or membrane structure categorization, the inorganic membranes can be also subdivided into ceramics and metallic. In particular, ceramics membranes differ according to their pore diameter in microporous d < 2 nm), mesoporous (2 nm < < 50 nm) and macroporous >... 

One of the features of inorganic membranes is their controlled pore structure. Anodic aluminum oxide membranes have uniform cylindrical pores, and were applied to an investigation of the analysis of transport mechanism [ 12]. Another route involves the application of a micelle template to membrane preparation [44]. Cubic mesoporous silica (MCM48) membranes were prepared on a stainless steel supports [45] to possible applications for filtration membranes and membrane reactions. 

The conductivity tests were carried out for a number of P(VDF-CTFE)/ Nafion/Inorganic membranes 3D phase zirconium phosphate, 3D porous titanosili-cate, amorphous zirconium phosphate (AZP), mesoporous alumina, and a silica-based commercial molecular sieve (MS). Figure 12.12 shows the conductivity of various PEM composite membranes under various humidity conditions. The inorganic phase... 

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