Chemical catalysis solid supported

The field of synthetic enzyme models encompasses attempts to prepare enzymelike functional macromolecules by chemical synthesis [30]. One particularly relevant approach to such enzyme mimics concerns dendrimers, which are treelike synthetic macromolecules with a globular shape similar to a folded protein, and useful in a range of applications including catalysis [31]. Peptide dendrimers, which, like proteins, are composed of amino acids, are particularly well suited as mimics for proteins and enzymes [32]. These dendrimers can be prepared using combinatorial chemistry methods on solid support [33], similar to those used in the context of catalyst and ligand discovery programs in chemistry [34]. Peptide dendrimers used multivalency effects at the dendrimer surface to trigger cooperativity between amino acids, as has been observed in various esterase enzyme models [35]. 

For most applications, enzymes are purified after isolation from various types of organisms and microorganisms. Unfortunately, for process application, they are then usually quite unstable and highly sensitive to reaction conditions, which results in their short operational hfetimes. Moreover, while used in chemical transformations performed in water, most enzymes operate under homogeneous catalysis conditions and, as a rule, cannot be recovered in the active form from reaction mixtures for reuse. A common approach to overcome these limitations is based on immobilization of enzymes on solid supports. As a result of such an operation, heterogeneous biocatalysts, both for the aqueous and nonaqueous procedures, are obtained. 

In heterogeneous catalysis, solids catalyze reactions of molecules in gas or solution. As solids - unless they are porous - are commonly impenetrable, catalytic reactions occur at the surface. To use the often expensive materials (e.g. platinum) in an economical way, catalysts are usually nanometer-sized particles, supported on an inert, porous structure (see Fig. 1.4). Heterogeneous catalysts are the workhorses of the chemical and petrochemical industry and we will discuss many applications of heterogeneous catalysis throughout this book. 

The forms of active components present in heterogeneous catalysts are of importance to catalysis. A supported catalyst usually consists of an active component dispersed on a support with a highly specific surface. According to current opinions (/), an active component dispersed on a support may end up in one of three forms (1) it may retain its chemical identity as a separate crystalline or amorphous phase, (2) it may form a new stoichiometric compound with the support or additive, or (3) it may dissolve in the support to give a solid solution. Examples of these forms are readily available from the literature. 

Substances which increase the rate of a chemical reaction without themselves being used up or incorporated into the finished product are called catalysts. In heterogeneous catalysis the reaction takes place on the surface of a solid support. The activity of the catalyst in this case is determined by the structure and size of the surface area as well as the way the catalyst is produced. Catalysts are not limited to immobilization on solids, they can also be introduced as homogeneous catalysts in solution. Between heterogeneous and homogeneous catalysts there is the possibility to evenly distribute small particles (dispersions) of catalyst in a liquid phase. 

A third technique that has been used to study colloidal nanocatalysis is the Colhnan test or the 3-Phase Test [16]. One reactant is chemically bound to a solid support (Phase 1) and the catalyst is chemically bound to a second solid support (Phase 2). Phases 1 and 2 are suspended in solution (Phase 3), into which a second reactant is also dissolved (not substrate bound). Direct contact between Phase 1 and Phase 2 is restricted due to the fact they are both substrate bound. If the heterogeneous catalyst in Phase 2 is required for the reaction to proceed there will not be any product formation. If the heterogeneous catalyst (Phase 2) leaches homogeneous complexes into solution (Phase 3), the reaction will proceed and in this way the nature of the active catalyst can be investigated. However, there are limitations to this experimental setup since the formation of products does not rule out heterogeneous catalysis. The formation of secondary particles from leached species is possible, so at best, this test supplies evidence for atomic leaching. 

In the previous sections the use of catalysts dissolved in ionic liquids has been documented with a variety of examples from the most recent literature. They were classified are catalytic systems based on the adoption of Strategies A, B and C, when solvent-less conditions were not adopted. In an ideal liquid-liquid biphasic system, the IL must dissolve the catalytic intermediates and, in part, the substrate to avoid that mass transfer limits reaction rates. Moreover, products should have a limited solubility in the IL to allow a facile product removal or extraction, and, possibly, the recycle of the ionic liquid-trapped catalyst. The separation of the catalyst from the products is made easier if solid support-immobilised ILs are used. The preference for a solid catalyst is dictated not only by the easier separation but also, as outlined by Mehnert in an excellent review article, " by (i) the possible use of fixed bed reactors, and (ii) the use of a limited amount of IL, a generally expensive chemical which can limit the economic viability of the process. In this section attention will be focused only on the most recent examples of solid-phase assisted catalysis using ionic liquids, following Strategy D. Examples prior to 2006 are covered in recent reviews and will not be discussed here. " ... 

In a similar vein, a series of papers published between 2002 and 2008 contains spectacular claims of highly enantioselective asymmetric additions of water to styrenes, unsaturated carboxylic acids, or simple terminal alkenes [34-Al]. The catalysts used are of the heterogeneous type and based on chiral biopolymers such as wool, gelatin, or chitosan as solid supports (sometimes in combination with silica or ion-exchange resins) that are doped with transition metal salts. This series of papers contains spectacular claims, insufficient experimental data, and erroneous chemical structures for the biopolymers used. As earlier work from the same group of authors on asymmetric catalysis on bio-polymeric supports is irreproducible [42], one is well advised to await independent confirmation of those results. 

Chiral solid catalysts usually have two functions, activation and control. The activating function ensures that the solid actually catalyzes a reaction (chemical catalysis), and the control function provides the stereochemical direction that yields the required enantiomer. Early studies were carried out with metallic catalysts supported on inherently chiral solids such as quartz, cellulose (Harada and Yoshida, 1970), and polypeptides (Akabori et al., 1956 Beamer et al., 1967), in which the metal provided the activating function and the support provided the control function. More recent emphasis has been on binding chiral molecules to nonchiral supports. 

Gas-liquid phase-transfer catalysis (GL-PTC) relies on the use of thermally stable PT catalysts adsorbed onto a solid support, which can also act as a source of the desired nucleophile. Reactions are carried out at a temperature that ensures that the catalyst is in a molten state and that reagents are in the vapor phase, and that the chemical transformation occurs in the organic microphase of molten catalyst. The products are recovered after condensation outside the reaction vessel. Only catalysts having melting points lower then the process temperature <180°C are active, but despite this limitation, GL-PTC is a versatile technique that has been applied to a number of chemical transformations. 

A particularly promising concept for designing catalysts, which combine the advantages of homogeneous and heterogeneous catalysis, involves coating a solid support with a thin layer of an IL, whereby the IL forms a second phase or is immiscible with the bulk fluids phase containing the reactants and products. Such a concept may be applicable to batch operation in the slurry phase as well as continuous operation in fixed-bed reactors (Scheme 10.2) [25, 35]. The chemical... 

Progress in many fields of metal complex catalysis is connected with immobilized catalytic systems, which are characterized by physical or chemical bonding of one of the components or a catalytic complex boimd to a solid support carrier. However, the relatively low stability of the metal to polymer bond leads, during catalysis, to a weak point in such immobilized metal complex catalysts. The immobilization of metals by chelating polymers, which ensure a stable multicenter bond between the metal, and the polymer support provides the simplest method for overcoming the above disadvantage. 

This article describes the structiual and functional properties of the polynucleotides, DNA and RNA., beginning with a description of how these polymers are synthesized through enz3rmatic catalysis or by stepwise, solid-supported chemical methods. Next, the structiual features of DNA and RNA are described, followed by the most commonly used methods for studying the properties of polynucleotides. 

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