Preparation of Inorganic Membranes

One company (Carre Inc., Seneca, SC) has exploited this technology by selling plants utilizing porous stainless steel tubes and the know-how to apply the hydrous Zr(IV) oxide dynamic membrane. When the membrane becomes fouled, it is stripped off with acid and a new membrane applied using cross-flow of the Zr(IV) oxide slurry feed. 

Union Carbide went one step further-sintering the hydrous Zr(IV) oxide dynamic membrane in place on a porous carbon tube at elevated temperatures. This resulted in a high-temperature (300°F) inorganic membrane which is resistant to a pH of 1-14. Further, the membrane may be cleaned with organic solvents that would dissolve conventional polymer membranes. This technology was subsequently licensed to SFEC (Societe de Fabrication d Elements Cata-lytiques) in Bollene, France and to Gaston County Filtration Systems in Stanley, NC. 

Ceramic Membranes. Alumina membranes for UF have been introduced by Ceraver (Tarbes, France) (membrane division recently purchased by Alcoa). The Norton Co. (Worcester, MA) is reportedly about to introduce an alumina UF membrane in addition to its current MF membrane. 

The methods used to make these membranes are diverse and highly proprietary. However, one method for preparing gamma-alumina membrane films on porous supports (which can be made from alpha-alumina) has been reported by Dutch researchers.13 They use a sol/gel technique with Boehmite (7-AIO-OH) as the precursor because it can be easily dispersed with acids. 

Final pore size Thermal Treatment 1-10 micron calcination on parous support r 

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Sol-gel is one of the most useful techniques for preparation of inorganic membranes with fine pores in the nanometer range (1-5 nm). The sol is a stable suspension of colloidal solid particles within soft uniform solution. The gel was obtained by hydrolysis with open reflux in 24 hours at 85-90 °C. The advantage of sol-gel technology is the ability to produce... 

The separation layer, either porous or dense, can be formed using different methods such as sol-gel and template routes, hydrothermal synthesis, chemical vapor deposition (CVD), or physical sputtering, depending on the membrane material and its application. These membrane preparation methods will be described in the following chapters of this book for different membranes and membrane reactors. We note that the preparation of inorganic membranes involves a multi-step high-temperature treatment process. Therefore, inorganic membranes are much more expensive than polymeric ones. 

Although the pyrolysis of organic materials (organic hollow fibers) is used in the commercialization of a new family of inorganic membranes (Fleming 1988) there are only a few descriptions in the open literature. Koresh and Soffer (1980, 1986, 1987) have published a series of articles on this subject. There is also a paper by Bird and Trimm (1983) which is based on a previously described preparation procedure of Trimm and Cooper (1970, 1973). 

The separation efficiency (e.g. permselectivity and permeability) of inorganic membranes depends, to a large extent, on the microstructural features of the membrane/support composites such as pore size and its distribution, pore shape, porosity and tortuosity. The microstructures (as a result of the various preparation methods and the processing conditions discussed in Chapter 2) and the membrane/support geometry will be described in some detail, particularly for commercial inorganic membranes. Other material-related membrane properties will be taken into consideration for specific separation applications. For example, the issues of chemical resistance and surface interaction of the membrane material and the physical nature of the module packing materials in relation to the membranes will be addressed. 

While considerable progress has been made in the preparation of ceramic membranes by sol-gel processing, the development of membranes from hybrid polymers is in its infancy (see also Section V). This is, nevertheless, a very promising area of development, because the possibility of forming mechanically stable membranes by inorganic polycondensation is implemented by the possibility to incorporate organic functions. 

To facilitate discussions on the preparation methods, characteristics and applications of inorganic membranes in the following chapters, some terminologies related to the types of membranes according to the combined structures of the separating and support layers, if applicable, will be introduced. 

The processes discussed so far produce various types of inorganic membranes in one production process prior to applications and the membrane structures are fixed to the supports in the case of composite membranes. There are, however, other special types of inorganic membranes that are prepared either by a second process to modify the... 

The preparation and fabrication methods and their conditions described in Chapter 3 dictate the general characteristics of the membranes produced which, in turn, affect their performance as separators or reactors. Physical, chemical and surface properties of inorganic membranes will be described in detail without going into discussions on specific applications which will be treated in later chapters. Therefore, much of this chapter is devoted to characterization techniques and the general characteristics data that they generate. 

In this chapter, different aspects of the sol-gel process have been described which were applied to the synthesis of inorganic membrane materials. Macro-, meso- or microporous as well as almost dense materials can be obtained depending on the preparation method. Some limitations in the control of pore size and pore size distribution are attached to conventional sol-gel methods, namely the colloidal and the polymeric routes, frequently used for the S5mthesis of inorganic membrane materials. Thanks to recent advances in sol-gel processing, a number of these limitations have been overcome introducing new concepts in the preparation of membranes with tailor-made porous textures. 

V.M. Linkov, R.D. Sanderson and E.P. Jacobs, Preparation of inorganic hollow-fibre membrane and composite hollow-fibre carbon membranes, in Y.H. Ma (Ed.), Proceedings of 3rd International Conference on Inorganic Membranes (ICIM3), July 10-14, 1994, Worcester, MA, USA. Distributed by Worcester Polytechnic Institute, 100 Institute Rd. Worcester, MA 01609, USA. pp. 471-481. 

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... 

Zeolites are used as detergent builders, adsorbents, and catalysts. In the past decade, we saw the development of a variety of zeoiite membranes, and a number of investigators reported on the preparation of such membranes and their applications to a variety of separation systems. These research activities are motivated by features common to inorganic membranes, such as thermal resistance and resistance to organic solvents, and features unique to zeolite materials, such as molecular sieving, selective adsorption, and catalytic activity.In this article, the discussion will be restricted to zeolite membranes for use in separation and catalysis. First, an overview is presented on recent progress in zeolite membranes, followed by a discussion of our research activities. 

Chang CS, Ni HS, Suen SY, Tseng WC, Chiu HCh, Chou CP. Preparation of inorganic-organic anion-exchange membranes and their application in plasmid DNA and RNA separation. J. Membr. Sci. 2008 311 336-348. 

Homok V, Erdohelyi A, Dekany I. Preparation of ultrathin membranes by layer-by-layer (LBL) deposition of oppositely charged inorganic colloids. Colloid Polym Sci 2006 284(6) 611-619. 

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