Inorganic membranes asymmetric

A completely different type of inorganic membrane also has its origin in the nuclear industry the asymmetric alumina membranes obtained by the anodic oxidation of an aluminum sheet were first developed for uranium enrichment... 

Zeolite/polymer mixed-matrix membranes can be fabricated into dense film, asymmetric flat sheet, or asymmetric hollow fiber. Similar to commercial polymer membranes, mixed-matrix membranes need to have an asymmetric membrane geometry with a thin selective skin layer on a porous support layer to be commercially viable. The skin layer should be made from a zeohte/polymer mixed-matrix material to provide the membrane high selectivity, but the non-selective porous support layer can be made from the zeohte/polymer mixed-matrix material, a pure polymer membrane material, or an inorganic membrane material. 

Membranes are classified as organic or inorganic, taking into account the material used for their syntheses porous or dense, based on the porosity of the material applied and symmetric and asymmetric for a membrane made of a single porous or dense material or for a membrane made of a porous support and a dense end, respectively [16,64], We are fundamentally interested here in asymmetric inorganic membranes made of a porous end to bring mechanical stability to the membrane and made of alumina, silica, carbon, zeolites, and other materials, and a dense end to give selectivity to the membrane (see Chapter 10). However, we also analyze the performance of porous polymers. 

FIGURE 6.7 Scanning electronic microscopy images of inorganic membrane porous structures (a) asymmetric alumina (b) asymmetric carbon structure (c) homogeneous alumina structure and (d) homogeneous glass structure. 

P. Pradanos, A. Hernandez, J. I. Calvo, and F. Tejerina, Mechanisms of protein fouling in cross-flow UF through an asymmetric inorganic membrane. J. Membr. Sci. 114, 115-126 (1996). 

An inorganic membrane can be described as an asymmetric porous ceramic formed by a macroporous support with successive thin layers deposited on it. The support provides mechanical resistance to the medium. The successive layers are active in microfiltration (MF), ultrafiltration (UF) or nanofiltration (NF), depending on their pore diameters. 

Pt(IV) than Pd(n) [49], An interesting system, with asymmetric inorganic membranes, was used for selective metal ion separation. The membrane phase was a self-assembled monolayer of alkyl thiols as a hydrophobic phase for a trialkyl phosphate and phosphine oxide-based metal ion carrier. This organic mixture was attached on alumina porous supports with thin layers of gold. The thin membrane layer gave high fluxes and high selectivity, while metal ions transport was carrier limited [50]. 

Catalytically active supported ionic liquid membranes were used for propylene/propane vapor mixture separation. In this case, the ionic Hquid was immobilized in the pores of an asymmetric ceramic support, displaying sufficient permeability, good selectivity, and long-term stabUity [51]. Porous inorganic membranes were also used as a support for chiral-selective liquid membranes. For this purpose, porous tubular ceramic membranes were impregnated with 3-cyclodextrin polymer. Such SLMs were used for separation of enantiomers of racemic pharmaceutical chlorthahdone [52]. 

Most inorganic membrane supports exhibit a tubular shape. This is a well-adapted geometry for cross-flow filtration in which the feed stream is circulated across the surface of the membrane and the permeated flux passes through the membrane in a perpendicular direction. Stainless steel, carbon, and ceramic are the most frequently used materials in the preparation of supports. As shown in Fig. 2, tubes or multichannel substrates can act as membrane supports. A well-designed support must be mechanically strong, and its resistance to fluid flow must be very low. Aiming at enhancing flux performances, multilayered substrates have been prepared that exhibit an asymmetric structure... 

Anisotropic (asymmetric) Defines a particular type of ultrastructure of microporous membranes. The surface of the membrane where separation occurs is more dense than the rest of the membrane body. The pore diameter increases in a direction perpendicular to the membrane surface, with the pore opening near the separation surface being smaller than the pore opening on the bottom of the membrane. This skin layer is typically present in polymeric membranes made by the phase-inversion process. Some asymmetry is also present in many inorganic membranes. 

UltrafUtration membranes are commonly asymmetric (skinned) polymeric membranes prepared by the phase inversion process. Materials commercially made into membranes include cellulose nitrate, cellulose acetate, polysulfone, aramids, polyvinylidene fluoride, and acrylonitrile polymers and copolymers. Inorganic membranes of hydrous zirconium oxide dqrosited on a tubular carbon liking are alM commercially available. 

Flat-sheet asymmetric-skinned membranes made from synthetic polymers (also copolymers and blends), track-etched polymer membranes, inorganic membranes with inorganic porous supports and inorganic colloids such as Zr02 or alumina with appropriate binders, and melt-spun thermal inversion membranes (e.g., hollow-fiber membranes) are in current use. The great majority of analytically important UF membranes belong to the first type. They are usually made of polycarbonate, cellulose (esters), polyamide, polysulfone, poly(ethylene terephtha-late), etc. 

Torres et al. (1994) studied nitrobenzene hydrogenation on an asymmetric catalytic porous tubular membrane where a thin catalytic layer (ca. 3 pm) is supported by a thick (ca 1.1 mm) macroporous inert tube and they developed a model using experimental kinetics. They showed that by using an inorganic membrane the most beneficial configuration to the overall reaction kinetics is when the gas phase flows directly on the catalytic layer while the liquid phase flows along the inert support. It should be noted that the support was impregnated by the liquid phase. 

Kaizer K, Verweiji H (1996) Progress in inorganic membranes. Chemtech 26 (1) 37-41 Soria R (1995) Overview on industrial membranes. Catal Today 25 (3-4) 285-290 Hsieh HP (1988) Inorganic membranes. AIChE Symp Ser 84 (261) 1-18 Fuertes AB, Centeno TA(1995) Preparation of supported asymmetric carbon molecular sieve membranes. J Membr Sci 144 (1-2) 105-111... 

Figure 1.5 (a) Schematic representation of the asymmetric inorganic membrane and... 

As inorganic membranes have a multi-layered asymmetric structure consisting of porous support, intermediate layers and a selective separation layer, the fabrication of inorganic membranes is a multi-step process, as illustrated in Figure 1.8. 

As pointed out by Nunes and Peinemann [108], inorganic membranes are usually preferred because many processes at the industrial level are carried out at high temperature. However, polymeric membranes can be used for H2/hydrocarbon separation in the platformer off gases from refineries and for CO2 separation in coal plants. Polymeric manbranes for GS can be symmetric or asymmetric, but should have a dense selective layer. Three types of membrane structures can be employed (1) homogeneous dense manbranes (symmetric) (2) integrally skinned asymmetric membranes and (3) composite membranes. 

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