Micelle-bound catalysts

Diffusion of Micelle-Bound Catalysts Models for Diffusion to Electrodes... 

PA anion radical rapidly reduced 4-bromobiphenyl (4-BB) to biphenyl in 0.1 H CTAB with an enhanced rate compared to isotropic solvent (Table 1). Quantitative bulk electrolytic reduction of 0.02 mmol of 4-BB in 25 mb 0.1 M CTAB was effected on stirred mercury pool electrodes in 2.5 h with 20 % decomposition of the catalyst. Time for complete conversion to biphenyl and amount of catalyst decomposed were significantly smaller compared to similar experiments in surfactant-free N,N-dimethyl-formamide (DMF) . Diffusion controlled CV and chronocoulometric data for 0.2 mM 9-PA in 0.1 H CTAB were used to obtain an apparent diffusion coefficient (D ) of lO cm s-. This is much too large to attribute to a diffusing micelle-bound species. Furthermore, at scan rates (v) below 5 mV s i, CV s for the 9-PA anion radical were not diffusion controlled as at higher v, but had a symmetric peak shape attributable to a thick surfactant layer at the surface of the electrode. Thus, at the potential required (-2.2 V vs SCE) to reduce 9-PA in 0.1 M CTAB, the catalytic reduction of 4-BB takes place in a thick, spontaneously organized surfactant film on the electrode surface. In addition to voltammetric results , support for existence of a thick film comes from differential capacitance, ellipsometry , and reflectance infrared spectroscopy . 

In the foregoing sections, the rate-enhancing effect of alkylammonium micelles has been extensively described. Similar effects can be expected for bilayer membranes of dialkylammonium salts. In addition, specific catalytic processes may be realized in this new system by taking advantage of the peculiar membrane structure. For example, catalyst molecules which are anisotropically bound to the membrane may act in very specific manners, and the liquid-crystalline nature of the bilayer membrane should provide unique microenvironments for catalysis. These are particularly interesting in relation to the mode of action of membrane-bound enzymes. 

The kinetic behavior of these systems is consistent with the supposition that substrate and/or catalyst molecules are freely moving around among the micelles and the bilayer vesicles much faster than the rate of reaction. However, Kunitake and Sakamoto (1978) showed that the rate of the intravesicle reaction was much faster than that of the intervesicle reaction, when p-nitrophenyl palmitate was used as substrate. Table 6 compares the rates of the intra- and intervesicle reactions in 2C12N+2C, bilayer and in CTAB micelles. A large rate difference (> 200-fold) was found in the bilayer system for the combination of cholest-Im and p-nitrophenyl palmitate. Slow transfer among vesicles of tightly bound p-nitrophenyl palmitate causes the rate difference. 

The catalytic principle of micelles as depicted in Fig. 6.2, is based on the ability to solubilize hydrophobic compounds in the miceUar interior so the micelles can act as reaction vessels on a nanometer scale, as so-called nanoreactors [14, 15]. The catalytic complex is also solubihzed in the hydrophobic part of the micellar core or even bound to it Thus, the substrate (S) and the catalyst (C) are enclosed in an appropriate environment In contrast to biphasic catalysis no transport of the organic starting material to the active catalyst species is necessary and therefore no transport limitation of the reaction wiU be observed. As a consequence, the conversion of very hydrophobic substrates in pure water is feasible and aU the advantages mentioned above, which are associated with the use of water as medium, are given. Often there is an even higher reaction rate observed in miceUar catalysis than in conventional monophasic catalytic systems because of the smaller reaction volume of the miceUar reactor and the higher reactant concentration, respectively. This enhanced reactivity of encapsulated substrates is generally described as micellar catalysis [16, 17]. Due to the similarity to enzyme catalysis, micelle and enzyme catalysis have sometimes been correlated in literature [18]. 

Artificial enzymes with metal ions can also hydrolyze phosphate esters (alkaline phosphatase is such a natural zinc enzyme). We examined the hydrolysis of p-nitro-phenyfdiphenylphosphate (29) by zinc complex 30, and also saw that in a micelle the related complex 31 was an even more effective catalyst [118]. Again the most likely mechanism is the bifunctional Zn-OH acting as both a Lewis acid and a hydroxide nucleophile, as in many zinc enzymes. By attaching the zinc complex 30 to one or two cyclodextrins, we saw even better catalysis with these full enzyme mimics [119]. A catalyst based on 25 - in which a bound La3+ cooperates with H202, not water - accelerates the cleavage of bis-p-nitrophenyl phosphate by over 108-fold relative to uncatalyzed hydrolysis [120]. This is an enormous acceleration. 

Figure 3 is a schematic representation of the concepts to generate a homogeneously soluble catalyst retainable in a membrane reactor, either bound to a polymeric backbone (cf. Section 3.1.1.3), or a dendrimer (cf. Section 3.2.2), or embedded in a micelle (cf. Section 3.1.11). 

Kunitake and his coworkers have investigated bifunctional polymer catalysts(29) in micelles(30) and polysoaps.(3L) The bifunctional interaction between a hydroxamate nucleophile and a neighboring imidazole group at the catalytic site is similar to the charge relay system of serine proteases. This interaction leads to remarkably accelerated acylation and deacylation processes. In the hydrophobic environment of cationic micelles, where the reactivity of anionic nucleophiles are remarkably enhanced, the overall catalytic efficiency exceeded even that of a-ch3rmotrypsin at pH 8.(30) Micellar monofunctional catalysts, nonmicellar bifunctional catalysts, and polysoap-bound bifunctional catalysts were less effective. 

Poly(ethylene oxide)s are the only water-soluble polymers which can be terminally functionalized and from which we can obtain complexes bound to the polymer tail. Thereby, several problems encountered in producing conventional polymer-immobilized catalysts can be obviated. The metal complexes synthesized retain the properties of, on the one hand, homogeneous low molecular weight metal complexes, and on the other, poly(ethylene oxide)s or ethylene oxide-propylene oxide block copol)miers. Among these properties are, first of all, water solubility and also the ability to concentrate nonpolar substances in polymer globules or micelles formed by polymer ligands. 

The sol-gel method is also used to make very fine spherical particles of oxides. By structuring the solvent with surface-active solutes, other forms can also be realized during condensation of the monomeric reactant molecules to form a solid particle. Figure 8.16 shows that normal or inverse micelles or liquid crystals (liquids having long-distance order) can be formed in such solutes. Micelles are small domains in a liquid that are bounded by a layer of surface-active molecules. In these domains the solid is condensed and the microstructure of the precipitated solid is affected by the micelle boundaries. Monodisperse colloidal metal particles (as model catalyst) have been made in solvents that have been structured with surfactants. In the concentration domains where liquid crystals obtain highly porous crystalline oxides can be condensed. After calcination such solids can attain specific surface areas up to 1000 m /g. Micro-organisms use structured solutions when they precipitate calcite, hematite and silica particles. 

The resulting alkoxyl and peroxyl radicals increase the autoxidation reaction rates of initiation and propagation phases, since the rate of cleavage of hydroperoxides by metal ions is much faster than the formation of radicals ab inicio. Metal ions and non-ionised salts may react in this way (Figure 3.65). Of the metals bound in complexes, some are effective, but some are ineffective. Metals may also become less effective in the presence of fats if micelles are formed. The catalyst for the oxidation of Kpids may be bound iron complexes. Iron bound in haem pigments has the same catalytic activity as the ions Fe + a Fe +, in aqueous solutions it is even more active, as it catalyses the cleavage of hydroperoxides as follows ... 

---