Macroporous supports

Other immobilization methods are based on chemical and physical binding to soHd supports, eg, polysaccharides, polymers, glass, and other chemically and physically stable materials, which are usually modified with functional groups such as amine, carboxy, epoxy, phenyl, or alkane to enable covalent coupling to amino acid side chains on the enzyme surface. These supports may be macroporous, with pore diameters in the range 30—300 nm, to facihtate accommodation of enzyme within a support particle. Ionic and nonionic adsorption to macroporous supports is a gentle, simple, and often efficient method. Use of powdered enzyme, or enzyme precipitated on inert supports, may be adequate for use in nonaqueous media. Entrapment in polysaccharide/polymer gels is used for both cells and isolated enzymes. 

Figure 7 shows that N2 permeability strongly depends on the pore size. For the macroporous support (curve 1) Poiseuille flow occurs, leading to an increase of the permeance... 

Richey, J. S., Optimal pH conditions for ion exchangers on macroporous supports, Meth. Enzymol., 104, 223, 1984. 

The low metal content in VGO admits to handle the occurring demetallization reactions by using either guard beds or an appropriate catalyst at the top of the bed. Catalysts with broader tolerance to metals have been developed the improvement commonly consists of a macroporous support. 

Figure 6.6 ULtrafiLtration separates molecules based on size and shape, (a) Diagrammatic representation of a typical laboratory-scale ultrafiltration system. The sample (e.g. crude protein solution) is placed in the ultrafiltration chamber, where it sits directly above the ultrafilter membrane. The membrane, in turn, sits on a macroporous support to provide it with mechanical strength. Pressure is then applied (usually in the form of an inert gas), as shown. Molecules larger than the pore diameter (e.g. large proteins) are retained on the upstream side of the ultrafilter membrane. However, smaller molecules (particularly water molecules) are easily forced through the pores, thus effectively concentrating the protein solution (see also (b)). Membranes that display different pore sizes, i.e. have different molecular mass cut-off points, can be manufactured, (c) Photographic representation of an industrial-scale ultrafiltration system (photograph courtesy of Elga Ltd, UK)...

Inorganic membranes employed in reaction/transport studies were either in tubular form (a single membrane tube incorporating an inner tube side and an outer shell side in double pipe configuration or as multichannel monolith) or plate-shaped disks as shown in Figure 7.1 (Shinji et al. 1982, Zaspalis et al. 1990, Cussler 1988). For increased mechanical resistance the thin porous (usually mesoporous) membrane layers are usually supported on top of macroporous supports (pores 1-lS /im), very often via an intermediate porous layer, with pore size 100-1500 nm, (Keizer and Burggraaf 1988). 

Porous ceramic membrane layers are formed on top of macroporous supports, for enhanced mechanical resistance. The flow through the support may consist of contributions due to both Knudsen-diffusion and convective nonseparative flow. Supports with large pores are preferred due to their low resistance to the flow. Supports with high resistance to the flow decrease the effective pressure drop over the membrane separation layer, thus diminishing the separation efficiency of the membrane (van Vuren et al. 1987). For this reason in a membrane reactor it is more effective to place the reaction (catalytic) zone at the top layer side of the membrane while purging at the support side of the membrane. 

The palladium-catalyzed coupling of boronic acids with aryl and alkenyl halides, the Suzuki reaction, is one of the most efficient C-C cross-coupling processes used in reactions on polymeric supports. These coupling reactions requires only gentle heating to 60-80 °C and the boronic acids used are nontoxic and stable towards air and water. The mild reaction conditions have made this reaction a powerful and widely used tool in the organic synthesis. When the Suzuki reaction is transferred to a solid support, the boronic add can be immobilized or used as a liquid reactant Carboni and Carreaux recently reported the preparation of the macroporous support that can be employed to efficiently immobilize and transform functionalized arylboronic adds (Scheme 3.12) [107, 246, 247]. 

Another important consideration was the choice of resin. The use of Hmb was developed using resins composed of polydimethylacrylamide polymerized within the pores of a solid, macroporous support, either Pepsyn or polyhipe, both are exemplified below. The use of Hmb involves the switching of solvents from DMF to dichloromethane the resins mentioned have excellent swelling properties in both solvents. However, some commercial polystyrene supports have given poor results because of the need to switch solvents, which can cause problems due to resin shrinkage. 1 ... 

Although less discussed in the technical and scientific literature, permeate-side concentration polarisation may also become a problem when using thin selective films that require macroporous supports for mechanical stability [13]. 

The GDL is located on the back of the CL in order to improve gas distribution and water management in the cell. This layer has to be porous to the reacting gases, must have good electronic conductivity, and has to be hydrophobic so that the liquid produced water does not saturate the electrode structure and reduce the permeability of gases. The GDL needs to be resilient and the material of choice for the PEMFC is usually carbon fiber, paper or cloth, with a typical thickness of 0.2-0.5mm [74,75], This macroporous support layer is coated with a thin layer of carbon black mixed with a dispersed hydrophobic polymer, such as P I LL, in order to make it hydrophobic. This latter compound can, however, reduce the electronic conductivity of the GDL, and limit the three-phase boundary access. 

All in all, little has been reported on the reactions taking place for impregnated [Pt2+(NH3)4](N03 )2 on macroporous supports. Moreover, the relation of the pretreatment to the final particle size distribution is rarely investigated. In our view, knowledge of the reactions occurring during pretreatment is a crucial step towards the development of a process leading to uniform small particle sizes. 

A common and well-known method to prepare silica membranes with molecular sieving properties is sol-gel coating [3-5], With this technique, microporous silica layers with a pore-size of about 0.5 nm are dip-coated on top of supported y-alumina membranes. The supports are porous a-alumina disks with pore diameters in the range from 100-200 nm. On top of these macroporous supports a 3 pm thick mesoporous y-alumina layer is coated, with a pore size of 3 nm. 

Two characteristics of macroporous supports are worth mentioning. First, the transfer of specific reaction conditions from solution to the macroporous support should be easier because the influence of swelling in different solvents and diffusion rates, typical for low-cross-linked PS resins, is not relevant. Second, this support can be washed more easily than the classical PS resins. A study of the retention of biphenyl in hydrophobic PS and AP-PS resins treated with solutions of biphenyl in methylene chloride (26) shows how after two identical wash cycles AP-PS retains 0.01% of biphenyl while hydrophobic PS retains 2.63% of the same impurity. This is due to the... 

One important consideration of the macroporous supports that is often overlooked is that they do not possess the high efficiencies of conventional supports when operated at < 300 cm/h flow velocities deemed optimal for the latter supports. As the flow velocities are increased, the efficiency of conventional supports deteriorates while that of the macroporous supports varies little. However, at flow velocities of 3500 cm/h, the chromatographic behaviour of both supports is comparable. 

In the case of the smallest pores (mesopores and micropores), the developed area is very large and the permeability is very low. Thus the thickness of the separative layer must be thin enough to reach attractive fluxes with experimentally acceptable transmembrane pressure. On the other hand, the mechanical strength of the membrane must be large enough to withstand the applied pressure. These considerations led to the concept of asymmetric structure based on macroporous support and successive layers with decreasing thickness and pore size (Table 25.3 Figure 25.1). 

The porous structure of ceramic supports and membranes can be first described using the lUPAC classification on porous materials. Thus, macroporous ceramic membranes (pore diameter >50 nm) deposited on ceramic, carbon, or metallic porous supports are used for cross-flow microfiltration. These membranes are obtained by two successive ceramic processing techniques extrusion of ceramic pastes to produce cylindrical-shaped macroporous supports and slip-casting of ceramic powder slurries to obtain the supported microfiltration layer [2]. For ultrafiltration membranes, an additional mesoporous ceramic layer (2 nm<pore diameter <50 nm) is deposited, most often by the solgel process [11]. Ceramic nanofilters are produced in the same way by depositing a very thin microporous membrane (pore diameter <2 nm) on the ultrafiltration layer [4]. Two categories of micropores are distinguished the supermicropores >0.7 nm and the ultramicropores <0.7 nm. 

FIGURE 6.10 Different membrane concepts for oxygen-ion conducting membranes, (a) Dense mixed conducting membrane top-layer supported on an asymmetric macroporous support (b) dense self-supported mixed conducting membrane with graded porous interfaces and (c) solid electrolyte cell (oxygen pump). 

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