Solid support catalysts cross-linking

As anticipated in the introduction, cross-linked polymers swell, to variable extent, when put in contact with liquids. Therefore, the working state of any cross-linked organic pol5uner under solid-liquid conditions, no matter if it is a catalyst, a support or a carrier for solid state S5mthesis, is the swollen state. In macroreticular CFPs swelling does not involve the whole polymeric mass it is... 

With a view to producing catalysts that can easily be removed from reaction products, typical phase-transfer catalysts such as onium salts, crown ethers, and cryptands have been immobilized on polymer supports. The use of such catalysts in liquid-liquid and liquid-solid two-phase systems has been described as triphase catalysis (Regen, 1975, 1977). Cinquini et al. (1976) have compared the activities of catalysts consisting of ligands bound to chloromethylated polystyrene cross-linked with 2 or 4% divinylbenzene and having different densities of catalytic sites ([126], [127], [ 132]—[ 135]) in the... 

This chapter will focus exclusively on cross-linked vinyl polymer supports either in a spherical bead or resin form, or in some other macroscopic format. These essentially insoluble materials lead to considerably simplified reaction work-up and product isolation procedures when used e.g. in solid phase synthesis or as catalyst or... 

Macroporous and isoporous polystyrene supports have been used for onium ion catalysts in attempts to overcome intraparticle diffusional limitations on catalyst activity. A macroporous polymer may be defined as one which retains significant porosity in the dry state68-71 . The terms macroporous and macroreticular are synonomous in this review. Macroreticular is the term used by the Rohm and Haas Company to describe macroporous ion exchange resins and adsorbents 108). The terms microporous and gel have been used for cross-linked polymers which have no macropores. Both terms can be confusing. The micropores are the solvent-filled spaces between polymer chains in a swollen network. They have dimensions of one or a few molecular diameters. When swollen by solvent a macroporous polymer has both solvent-filled macropores and micropores created by the solvent within the network. A gel is defined as a solvent-swollen polymer network. It is a macroscopic solid, since it does not flow, and a microscopic liquid, since the solvent molecules and polymer chains are mobile within the network. Thus a solvent-swollen macroporous polymer is also microporous and is a gel. Non-macroporous is a better term for the polymers usually called microporous or gels. A sample of 200/400 mesh spherical non-macroporous polystyrene beads has a surface area of about 0.1 m2/g. Macroporous polystyrenes can have surface areas up to 1000 m2/g. 

The immobilization of the enzyme, the redox catalyst, and sometimes also the cofactor can also take place at a solid support different from the electrode so that the components can be recovered within a solid-bed reactor (a column filled with the enzyme-containing particles) or by a filter plate or membrane. The immobilization of enzymes at solid supports or by the foraiation of cross-linked enzyme crystals can sometimes also enhance the enzyme stability. This concept has the advantage of the ease of separation but the disadvantage of diffusional limitations due to the heterogeneity of the reactions between the enzyme and the substrate and the cofactor or the redox catalyst. Additionally, the number of available redox centers is usually limited. 

The immobilization of catalysts or catalyst precursors on solid supports is a common technique for simplifying reaction procedures and/or increasing the stability of the catalyst. The homogeneous MTO catalyst can be transformed into a heterogeneous system in a number of different ways. In a recent approach by Saladino and coworkers, poly(4-vinylpyridine) and poly(4-vinylpyridine) //-oxides were used as the catalyst carrier. The MTO-catalyst obtained from 25% cross-linked poly(4-vinylpyridine) with divinylbenzene proved to catalyze efficiently the formation of even hydrolytically sensitive epoxides in the presence of aqueous hydrogen peroxide (Scheme 11). The catalyst could be recycled up to 5 times without any significant loss of activity. 

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