Microporous hydrophobic support

It is well known that Nafion ionomer contains both hydrophobic and hydrophilic domains. The former domain can facilitate gas transport through permeation, and the latter can facilitate proton transfer in the CL. In this new design, the catalyst loading can be further reduced to 0.04 mg/cm in an MEA [10,11]. However, an extra hydrophobic support layer is required. This thin, microporous GDL facilitates gas transport to the CL and prevents catalyst ink bleed into the GDL during applications. It contains both carbon and PTFE and functions as an electron conductor, a heat exchanger, a water removal wick, and a CL support. 

The use of liquid membranes in analytical applications has increased in the last 20 years. As is described extensively elsewhere (Chapter 15), a liquid membrane consists of a water-immiscible organic solvent that includes a solvent extraction extractant, often with a diluent and phase modifier, impregnated in a microporous hydrophobic polymeric support and placed between two aqueous phases. One of these aqueous phases (donor phase) contains the analyte to be transported through the membrane to the second (acceptor) phase. The possibility of incorporating different specific reagents in the liquid membranes allows the separation of the analyte from the matrix to be improved and thus to achieve higher selectivity. 

This simple mass transfer model based on simplified film theory has been proposed to describe the process of facilitated transport of penicillin-G across a SLM system [53]. In the authors laboratory, CPC transport using Aliquat-336 as the carrier was studied [56] using microporous hydrophobic polypropylene membrane (Celgard 2400) support and the permeation rate was found to be controlled by diffusion across the membrane. 

Separations in two-phase systems with one immobilized interface(s) are much newer. The first paper on membrane-based solvent extraction (MBSE) published Kim [4] in 1984. However, the inventions of new methods of contacting two and three liquid phases and new types of liquid membranes have led to a significant progress in the last forty years. Separations in systems with immobilized interfaces have begun to be employed in industry. New separation processes in two- and three-phase systems with one or two immobilized L/L interfaces realized with the help of microporous hydrophobic wall(s) (support) are alternatives to classical L/L extraction and are schematically shown in Figure 23.1. Membrane-based solvent extraction (MBSE) in a two-phase system with one immobilized interface feed/solvent at the mouth of microspores of hydrophobic support is depicted in Figure 23.1a and will be discussed... 

Immobilization of lipases on hydrophobic supports has the potential to (1) preserve, and in some cases enhance, the activity of lipases over their free counterparts (2) increase their thermal stability (3) avoid contamination of the lipase-modified product with residual activity (4) increase system productivity per unit of lipase employed and (5) permit the development of continuous processes. As the affinity of lipases for hydrophobic interfaces constitutes an essential element of the mechanism by which these enzymes act, a promising reactor configuration for the use of immobilized lipases consists of a bundle of hollow fibers made from a microporous hydrophobic polymer (137). 

The last example shows that it is also feasible to use SLMs to remove and recover efficiently radioactive metals from nuclear process effluent. By using a microporous hydrophobic polypropylene hoUow-fiber supported Hquid membrane (HFSLM) consisting of extractant, tri-w-butyl phosphate (TBP) as carrier diluted with w-dodecane, actinides such as uranium (U) and plutonium (Pu) were removed [188]. It was concluded after modeling and evaluation of the process conditions that it is possible to remove more than 99% of U(VI) and Pu(IV) from process effluent in the presence of fission products when stripping reagent 0.1 M hydroxylamine hydrochloride in... 

FIG U R E 2 7.9 Schematic diagram of a spiral-type supported liquid membrane (SLM) module 1, microporous hydrophobic membrane (support) 2, mesh spacer 3, inlet pipe of feed solution 4, inlet pipe of receiver solution 5, inlet tube of organic membrane solution 6, feed solution 7, organic membrane solution and 8, receiver solution. (Reprinted from Teramoto, M. et al., Sep. Sci. TechnoL, 24, 981-999,1989. With permission from Taylor Francis Ltd.)... 

Qin et al. (2003) used SLMs for the separation of acetic acid and butyric acid from their aqueous solution. Polypropylene hfs and silicon-coated, microporous hydrophobic polypropylene membranes were used as support. In another study, Qin et al. (2002) demonstrated PV by using a liquid membrane consisting of nonvolatile hydrocarbons immobilized in the micropores of hydrophobic hfs on the outer surface of the fibers. TCE was separated and concentrated from its aqueous solution at 25°C and essentially atmospheric pressure. The feed TCE concentration was varied between 50 and 950 ppm the permeate pressure range was 0.6-70 mmHg. A 78-flber, 30-33 cm long module could achieve as much as 98% removal of TCE. It was reported by them that the hexadecane SLM was permselective for TCE the experimental selectivity was 30,000 and the intrinsic selectivity could be as high as 2 x 10 much higher than the values obtained by any solid membranes. 

Four flat microporous hydrophobic membranes, supported (M3 and M4) and non-supported (Ml and M2), were used in the tests. Their main properties are reported in Table 13.13. 

Fig. 16. Schematic presentation of the morphological features of gas diffusion electrodes for fuel cells of (A) PTFE-bonded and Pt-activatcd Hi anodes and O2 cathodes used for Oi reduction in acidic and alkaline fuel cells (a) support, (b) hydrophobic gas diffusion layer, (c) hydrophilic electrode layer, (d) electrolyte, (e) magnified schematic of PTFE-bonded soot electrode, (f) adjacent hydrophobic layer, (g) microporous soot particles, (h) gas channels (mesopores), (k) PTFE particles, (I) flooded micro- and mesopores, (B) Schematic presentation of the morphology of PTFE-bonded Raney-nickel anodes used in alkaline fuel cells ol the Siemens technology.

Hydrophilic polyvinylenediiluorate (PVDF) membranes prepared by a strong basic treatment of hydrophobic PVDF supports followed by grafting of glycine molecules onto their surface [68] showed good properties for downstream protein purification and immunoassays [69,70]. Activated acrylate coated microporous PVDF membranes (Immobilon) are also commercially available from MUlipore. 

Virtually the entire membrane manufacture today is based on laminate structures comprising a thin barrier layer deployed upon a much thicker, highly permeable support. Most are formed of compositionaUy homogeneous polysulfone, cellulose acetate, polyamides, and various fluoropolymers by phase inversion techniques in which ultrathin films of suitably permselective material are deposited on prefabricated porous support structures. Hydrophobic polymers as polyethylene, polypropylene, or polysulfone are often used as supports. A fairly comprehensive hst of microporous and ultrafiltration commercial membranes and produced companies are presented in Refs [107-109]. A review on inorganic membranes has been given in Ref. [110]. 

The hydrophobicity index HI), introduced by Weitkamp and coworkers, is most useful in this context [24]. According to the HI, benzene is expected to compete more favorably with water than with acetonitrile, acetone, or methanol for adsorption in TS-1 micropores, thus maximizing under triphase catalysis the probability of interaction with active sites. This interpretation is supported by competitive adsorption experiments which revealed that the amount of adsorbed benzene in the system TS-l-benzene-H20 was almost ten times greater than for TS-l-benzene-CH3CN [22]. 

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