Hydrophobic catalytic reactions

The first one is a general methodology developed by Abu-Reziq et al [26,27] for the conversion of fully hydrophobic catalytic reactions - the catalyst, the substrate, and the product, are all hydrophobic - into a catalytic reaction that is carried out in water, eliminating the need for organic solvents. The method is based on a three-phase system composed of an emulsion (oil in water of the substrate and product molecule) and a solid (the catalysts), and was termed the EST (emulsion/solid transport) process. The idea (Figure 31.11) relies on the transport of hydrophobic substrates to an entrapped catalyst, and the transport of the resulting product from the catalyst porous solid back into the bulk. Specifically, the catalyst is entrapped inside a hydrophobically modified porous sol-gel matrix the hydrophobic substrate for that catalyst is emulsified in water in the presence of a suitable surfactant and the powdered catalytic sol-gel material is dispersed in that emulsion. Upon contact of the surfactant with the hydrophobic interface of the sol-gel matrix, it reorients and spills the substrate into the pores... 

Upon reaction, the heterogenized catalyst can be easily separated from the reaction mixture by filtration and then recycled. The hydro-phobic substrate is microemulsified in water and subjected to an orga-nometallic catalyst, which is entrapped within a partially hydrophobized sol-gel matrix. The surfactant molecules, which carry the hydrophobic substrate, adsorb/desorb reversibly on the surface of the sol-gel matrix breaking the micellar structure, spilling their substrate load into the porous medium that contains the catalyst. A catalytic reaction then takes place within the ceramic material to form the desired products that are extracted by the desorbing surfactant, carrying the emulsified product back into the solution. 

After fimctionahsation, the matrix is usually further sUylated using MesSiCl in order to remove the residual Si-OH groups on the surface of the mesoporous material, which are assumed to be unfavourable for catalytic reactions. Indeed, the adsorption characteristics of MCM-41 for polar molecules greatly depend upon the concentration of surface silanol groups [43]. Modification of MCM by silylation with MesSiCl makes the surface more hydrophobic. Moreover, stability with respect to moisture and mechanical compression is also improved [44]. 

Water in oil microemulsions with reverse micelles provide an interesting alternative to normal organic solvents in enzyme catalysis with hydrophobic substrates. Reverse micelles are useful microreactors because they can host proteins like enzymes. Catalytic reactions with water insoluble substrates can occur at the large internal water-oil interface inside the microemulsion. The activity and stability of biomolecules can be controlled, mainly by the concentration of water in these media. With the exact knowledge of the phase behaviom" and the corresponding activity of enzymes the application of these media can lead to favomable effects compared to aqueous systems, like hyperactivity or increased stability of the enzymes. 

The water-shell-model, strictly speaking, will only apply to very hydrophilic enzymes which do not contain hydrophobic parts. Many enzymes, like lipases, are surface active and interact with the internal interface of a microemulsion. In fact, lipases need a hydrophobic surface in order to give the substrate access to the active site of the enzyme. Nevertheless, Zaks and Klibanov found out that it is often not necessary to have a monolayer of water on the enzyme surface in order to perform a catalytic reaction in an organic solvent [98]. 

Ionic liquids are quite simply liquids that are composed entirely of ions [96, 97]. They are generally salts of organic cations, e.g. tetraalkylammonium, alkylpyridi-nium, 1,3-dialkylimidazolium, tetraalkylphosphonium (Fig. 7.28). Room temperature ionic liquids exhibit certain properties which make them attractive media for performing green catalytic reactions. They have essentially no vapor pressure and are thermally robust with liquid ranges of e.g. 300 °C, compared to 100 °C for water. Polarity and hydrophilicity/hydrophobicity can be tuned by a suitable combination of cation and anion, which has earned them the accolade, designer solvents . 

Furthermore, it is reported that metal catalysts usually require a basic pH for sufficient activity for the aerobic oxidation of alcohols in water [85]. Thus, Biffis found that the productivity of the Au nanoclusters can be enhanced by running the reaction at basic pH (pH 9.9) in their system. However, further enhancement of the pH of the reaction solution was not possible, due to the precipitation of catalyst caused by the increased hydrophobicity of the microgel through deprotonation of the polymer-bound amino groups [84]. In our work, we tried to carry out the catalytic reaction... 

The formation of reverse micelles and water-ln-oll (w/o) mlcroemulslons In liquid hydrocarbons using the surfactant sodium bis(2-ethylhexyl) sulfosucclnate (AOT) has been widely studied (2m3). In nonpolar liquid solvents, these molecular aggregates generally consist of 3- to 20-nanometer-dlameter, roughly spherical shells of surfactant molecules surrounding a polar core, which Is typically an aqueous solution. This combination of hydrophilic, hydrophobic, and Interfaclal environments In one solvent has created potential applications In separations (4.5), chromatography ( ), and catalytic reactions (2). 

The initiation step involves the generation of a radical site away from the active site which transforms substrate. Early ESR studies of Type I holoenzymes revealed a persistent tyrosyl radical in the holoenzyme, and for a long time this was erroneously thought to participate in the catalytic reaction. Type I holoenzymes have an 2p2 quaternary structure, with the active site in the so-called R1 subunit (the large 2 homodimer). The tyrosyl radical in fact resides on the R2 subunit (the small p2 homodimer), the C-terminus of which fits in a hydrophobic pocket in the R1 subunit. From the tyrosyl radical, the unpaired electron is transferred through a chain of hydrogen-bonded tyrosines. The tyrosyl residue is generated by a redox centre with two octahedrally-coordinated... 

Within the last 30 years, micro emulsions have also become increasingly significant in industry. Besides their application in the enhanced oil recovery (see Section 10.2 in Chapter 10), they are used in cosmetics and pharmaceuticals (see Chapter 8), washing processes (see Section 10.3 in Chapter 10), chemical reactions (nano-particle synthesis (see Chapter 6)), polymerisations (see Chapter 7) and catalytic reactions (see Chapter 5). In practical applications, micro emulsions are usually multicomponent mixtures for which formulation rules had to be found (see Chapter 3). Salt solutions and other polar solvents or monomers can be used as hydrophilic component. The hydrophobic component, usually referred to as oil, may be an alkane, a triglyceride, a supercritical fluid, a monomer or a mixture thereof. Industrially used amphiphiles include soaps as well as medium-chained alcohols and amphiphilic polymers, respectively, which serve as co-surfactant. 

The residues lining the hydrophobic active site pocket are listed in Table IX. There are only two polar residues, Ser-48 and Thr-178, in the part of this pocket where the catalytic reaction occurs. The other residues create a hydrophobic environment for the zinc atom, the nicotinamide moiety and the bound substrate. No obvious role for Thr-178 in the catalytic mechanism can be deduced from the structure. 

The methodology of solid phase peptide synthesis (SPPS) [65, 66] has been credited with the award of 1984 Nobel Prize in chemistry [67] to its inventor, Bruce R. Merrifield of the Rockefeller University. At the heart of the SPPS lies an insoluble polymer support or gel , which renders the synthetic peptide intermediates insoluble, and hence readily separable from excess reagents and by-products. In addition to peptide synthesis, beaded polymer gels are also being studied for a number of other synthetic and catalytic reactions [2]. Ideally, the polymer support should be chemically inert and not interfere with the chemistry under investigation. The provision of chemical inertiKss presents no difficulty, but the backbone structure of the polymer may profoundly influence the course of the reaction on the polymer support. This topic has attracted considerable interest, particularly in relation to the properties of polystyrene (nonpolar, hydrophobic), polydimethylacrylamide (polar, hydrophilic), and copoIy(styrene-dimethylaciylamide) (polar-nonpolar, amphiphilic) (see later). 

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