Fabrication of membranes

A further application of this technology will certainly be the fabrication of membranes of these materials. Membrane reactors have shown great utility in many systems, where one component of a reaction mixture can be separated by permeation through a membrane, thus driving a reaction forwards, by continuous separation. Such continuous processes can themselves save a great deal of waste. 

In addition to packed and wall-coated systems, numerous researchers have investigated the fabrication of membranes, within microchannels, in which catalytic material can be incorporated. Employing a protocol developed by Kenis et al. (1999), Uozumi et al. (2006) deposited a poly(acryla-mide)-triarylphosphane palladium membrane (PA-TAP-Pd) (1.3 pm (wide), 0.37 mmol g-1 Pd) within a glass microchannel [100 pm (wide) x40pm (deep) x 1.4 cm (long)]. Once formed, the membrane was used to catalyze a series of Suzuki-Miyaura C-C bond-forming reactions, the results of which are summarized in Table 21. 

In this paper an approach has been presented that will facilitate selection and evaluation of possible membrane materials. Having selected potential membrane materials for desired separations, steric considerations are taken Into account In membrane preparation. This approach avoids starting with and subsequently modifying aqueous-separation membranes, allows a vast spectrum of polymers to be considered as potential membrane materials and focuses the selection process. A brief review of membrane material evaluation procedures have been discussed with emphasis on those techniques which do not require the fabrication of membranes. Finally, the survey of materials evaluated for possible membrane use Indicates both the Interest In this field and the need for appropriate material selection and evaluation procedures. The ideas presented here will continue to grow In value In the future as membranes are called upon to achieve more difficult separations in an energy efficient fashion. 

Mechanical integrity is one of the most important prerequisites for fuel cell membranes in terms of handhng and fabrication of membrane electrode assemblies, and to offer a durable material. Robust fuel cell membranes are required because of the presence of mechanical and swelling stresses in the application [172]. Moreover, membranes should possess some degree of elasticity or elongation to prevent crack formation. 

In the literature, many articles reporting different types of glassy polymers that have been used for the fabrication of membrane-based gas separation equipment can... 

There is currently a renewed interest in the use of electrospinning techniques for the fabrication of membranes. Chapter 8 reviews the use of this versatile technique for the production of nanofiber webs or membranes. The chemical and physical properties of nanofiber manbrane surfaces play an important role in their application to filtration, biomedical materials, tissue engineering scaffolds, drug delivery... 

The fabrication of membranes to work in MC operations has to meet some important structural and chemical requirements, including controlled pore size and narrow pore distribution, along with high porosity and controlled mutual membrane-liquid interactions. The achievement of these targets is often reliant on the combination of suitable materials with well-addressed tailoring procedures. 

Fabrication of membrane-type partial oxidation reformer and its reforming properties... 

Successful application of this principle for separation depends on the fabrication of membranes with controlled and uniform pore sizes. This was done by Rao, Sircar, and co-workers by controlled pyrolysis of PVDC supported on macrop-orous alumina tubes. A schematic of their CMS membrane and the principle of separation by the membrane is shown in Figure 5.29. The membrane has pores of uniform sizes in the range between 5-10 A. The pore sizes can be tailored for different separations. 

Liquid-membrane electrodes include classical ion-exchange, liquid ion-exchange, and electroneutral ionophore-based liquid monbrane electrodes. Of particular interest are systems where the ion-exchanging compounds are dissolved macrocyclic compounds that have a strong selectivity to alkali metals. The stability of the formed complexes in nonpolar solvents far exceeds that found in water and allows for the fabrication of membrane-free micropipettes where the nonpolar/water interface is the membrane. Unfortunately, this leads to higher resistance than that exhibited by crystalline micropipettes and requires the addition of lipophilic salt to the nonpolar solvent to decrease the pipette resistance. 

Water transport capability (high water flux) from the cathode to the anode These properties have to be assured under a wide range of temperature and humidity (—30-120°C, nominal 0-100% relative humidity (RH)) considering the fabrication of membrane electrode assemblies (MEAs)... 

Fabrication of Membrane Electrode Assembly for Carbon Nanotubes and Nanofibers-based Catalysts... 

Casting by solvent evaporation is a commonly used procedure for fabrication of membranes based on organic polymers. It is probably the most widely used technique for polybenzimidazole membrane preparation for high-temperature polymer electrolyte membrane fuel cells. After casting, doping with phosphoric acid provides proton conductivity to the membrane. 

Amphiphilic Pluronic triblock copolymers of two blocks of poly-(ethylene oxide) (PEO) and poly(propylene oxide) in between have worth as both the surface modifier and pore former in the fabrication of membranes (77). The effect of Pluronics with different molecular architectures and contents as a pore forming additive for the fabrication of poly(ethersulfone) ultrafiltration hollow fibers has been investigated. 

Kim et al. presented a simple, fast and practical method to vertically ahgn CNTs on a porous support using a combination of self-assembly and filtration method. The authors claimed that this method can be easily scaled up to large surface areas that may allow the fabrication of membranes for practical applications [8],... 

Morikawa, H., Mitsui, T., Hamagami, J. and Kanamura, K. (2002) Fabrication of membrane electrode assembly for micro fuel cell by using electrophoretic deposition process. Electrochemistry 70, 937-939. 

One of the recent attempts to patent polyphosphazenes for fuel cells is an application filed by Honda Motor Co., Ltd., Tokyo on January 20, 2005 [52]. The invention deals with the fabrication of membranes composed of highly sidfonated polyalkylphenoxyphosphazenes, for possible use in a hydrogen/air or direct methanol fuel cell. Methods of synthesizing polyphosphazenes with an lEC as high as 4.9 mmol/g are described and the proton conductivity of the resulting films is presented. 

Different polyelectrolyte multilayers have been used in the fabrication of membranes for different applications such as fuel cells or separation [16]. The multilayers have evidenced good performance in the separation of several species when they are used as separation membranes [62, 226]. The modification of separation membranes by PEMs for pervaporation or ultrafiltration applications has been widely developed to obtain a better performance of the existing media [62, 227]. Rmaile and Schlenoff [232] have designed chiral multilayers for separation of optical active compounds. Also the fabrication of membranes for selective ion separation have been developed using the LbL approach [233]. Daiko et al. [234] built PEMs optimized for proton transport in fuel cells. 

Polymer hydrogels due to their unusual properties may be used as sensors, artificial muscles, drug delivery systems. The ability of polymer hydrogels to absorb the huge amounts of water have ensured the successful using of these polymers in medicine, agriculture, membrane technologies. Interpolymer complexes at present are widely used for fabrication of membranes for separation and pervaporation purposes (15,16). 

Poly(p-phenylene oxide)s or poly(2,6-dimethyl-l,4-phenylene oxide) (PPO) is also an attractive polymer for the fabrication of membranes for its outstanding film-forming... 

Recently, PVDF has become a more popular material to produce hydrophobic membranes through phase inversion processes, mainly for membrane contactor and MD applications. It is preferred to other more hydrophobic polymers, such as polypropylene and polytetrafluoroethylene, because of its excellent combination of properties and its solubility in common organic solvents. Furthermore, the excellent thermal stability of PVDF has made it interesting as a membrane material in a wide range of industrial applications. In addition, unlike other crystalline polymers, PVDF exhibits thermodynamic compatibility with other polymers, such as poly(methyl methacrylate) (PMM A), over a wide range of blend compositions, which can be useful in the fabrication of membrane with desired properties. PVDF can be further chemically modified to obtain specific functions. In addition, it can be cross-linked when subjected to electron beam radiation or gamma radiation. 

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