Microporous inorganic membranes

The sol-gel process involves the transition of a system from a liquid "sol" (mostly colloidal) into a solid "gel" phase (11). By applying this methodology, it is possible to fabricate ceramic or glass materials in a wide variety of forms ultrafine or spherical-shaped powders, thin film coatings, ceramic fibers, microporous inorganic membranes, monolithic ceramics and glasses, or extremely porous aerogel materials. 

Although this technique has been frequently used to characterize microporous organic membranes, it has not been applied to microporous inorganic membranes. Moreover, this method provides only the average pore size but not the pore size distribution. 

Knudsen diffusion may take place in a microporous inorganic membrane or through pinholes in dense polymeric membranes. It may also take place in a mixed matrix membrane with insufficient adhesion between the phases. 

Morooka S and Kusakabe K. Microporous inorganic membranes for gas separation. MRS Bulletin March 1999 25-28. 

Microporous inorganic membranes have pores that can be tuned to the molecular size. This enables zeolite membranes to carry out separations (i.e., the separation of isomer compounds) that are not possible with membranes in which only Knudsen selectivity is possible. Moreover, zeolite microporous membranes can compete with traditional energy costly separation methods, such us distillation of mixmres of close boiling point components, separation of mixtures of low concentration, and azeotropic distillation. 

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. 

A. Julbe, C. Guizard, A. Larbot, L. Cot and A.G. Fendler, The sol-gel approach to prepare candidate microporous inorganic membranes for membranes reactors. /. Membr. Sci., 77 (1993) 137. 

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. 

A sol of metal alkoxides has been used to prepare inorganic ion exchange membranes. Metal alkoxides are easily hydrolyzed to form a metal hydroxide sol in the presence of acid. The sol is then repeatedly coated on a microporous inorganic membrane (alumina substrate) and heated to form a metal oxide on the substrate, which is an inorganic ion exchanger.139 Though this is also a heterogeneous membrane, some reported membranes show lithium ion permselectivity in electrodialysis.140... 

As mentioned earlier, new developments in gas separation, catalysis, and petroleum industries have stimulated interest in microporous inorganic membranes suitable for high-temperature applications and with the property of high... 

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... 

Sommer S and Melin T. Performance evaluation of microporous inorganic membranes in the dehydration of industrial solvents. Chem. Eng. Process. 2005 44 1138-1156. 

One disadvantage of microporous inorganic membranes is that they are porous. They can never produce 100% pure gas streams as can thin-film-palladium or ion-transport membranes. However, when microporous membranes are coupled with pressure swing adsorption (PSA), the combined system can produce 100% hydrogen. In this scenario, PSA would only be required to separate the final 1% of the impurities, and the coupling of the two technologies should result in a very compact and efficient separation system. 

The parameters and consequently the efficiency of PV strongly depends on the properties of the membrane material. Common membrane materials are various dense polymers and microporous inorganic membranes (zeolithes, silica,. ..) either with hydrophilic or organophilic character. Furthermore composite membranes offer the possibility to combine different materials for the dense active layer and the porous support layer. Besides membrane material fluid hydrodynamics influences the efficiency of separation. The pressure drop especially on the permeate side reduces the driving force of the most permeating components. 

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