Surfactant micelles, catalytic reactions

Trialkylamines are used as additives in the telomerization of butadiene and water in a two-phase system (103). The catalyst comprises a palladium salt and tppms or tppts. The amines may build cationic surfactants under catalytic conditions and be capable of micelle formation. The products include up to five telomerization products (alcohols, alkenes, and ethers), and thus the reaction is nonselective. 

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). 

This work has shown that hydrothermal treatment strongly affects the texture of disordered wormhole-like mesoporous catalyst supports, and in particular the ability of the hydrophilic oxyethylene head of the non-ionic surfectant to adopt different conformations with temperature. These compounds show a remarkable thermal resistance up to 800°C, suitable for a series of catalytic reactions. On the other hand, ordered hexagonal CMI-1 can be directly obtained by lowering the surfactant concentration below the domain of existence of hexagonally ordered micelles in solution. 

The effect of humic materials on the photolytic micellar system was evaluated in DR s photodegradation. DR solubilized within Tween 80 micellar solution with or without humic materials was determined. In order to calculate the quantum yield, the molar absorptivity of DR was determined by spectrophotometry. The determination of the quantum yield and reaction rates was examined through a pseudo first-order decay rate expression. Quenching and catalytic effects resulting from the humic substances were examined through Stem-Volmer analysis. A reaction mechanism of photolytic decay of DR solubilized within surfactant micelles in the presence of various amount of humic materials was proposed for this purpose. The effect of high and low concentration of humic materials has been accounted for by a designed model. 

One of the most successful asymmetric catalytic reactions is the asymmetric hydrogenation of amino acid precursors by means of optically active rho-dium(I)phosphine or phosphinite complexes [55]. Usually, the reaction is carried out in methanol as solvent. When water is used the activity and enantiose-lectivity decrease significantly [16], but the addition of micelle forming surfactants leads to a solubilization of catalyst and substrate and increases activity and enantioselectivity. The results are somewhat better than the ones obtained with methanol as solvent [56]. Table 2 shows the effect with different types of surfactants. 

Recently, mesoporous sieve-materials such as MCM-41 and FSM-16 have been synthesized by using different micelle surfactant templates such as alkyltrimethyl ammonium salts. - " These materials consist of ordered mesoporous channels of 20 100 A diameter, which are much larger than those in the conventional zeolites such as ZSM-5, AlPO-5, and NaY, as depicted in Fig. 2. They are potential hosts for the inclusion of organometallic complexes and nanoparticles that will align with the ordered channels of the mesoporous materials and are accessible to larger substrates in the catalytic reactions. 

Little information is available on microemulsion-mediated synthesis of rhodium particles. Considering the importance of Rh nanoparticles in catalytic reactions, Kishida et al [426] developed a method using microemulsions. The reverse micelle was prepared with the surfactant NP-5 and cyclohexane as the continuous phase. An aqueous solution of rhodium chloride was solubilized in the micelle and hydrazine directly added to it at 25°C. The average particle size of rhodium thus obtained was about 3 nm. Kishida et al. [427] later extended the method to the use of a variety of non-ionic and ionic surfactants (C-15, i.e. polyoxyethylene(15)cetyl ether, L-23, i.e. polyoxyethylene(23)lauryl ether, NP-5 and NaAOT), as also cyclohexane or 1-hexanol (according to necessity) as the continuous phase. The reactants remained the same, i.e. rhodium chloride and hydrazine hydrate. In addition, the rhodium particles thus synthesized were coated with silica via hydrolysis-polycondensation of tetraethyl orthosilicate. The size of Rh varied in the range 1.5-4.0 nm in a typical case, a 4 nm particle was covered with a 14 nm thick layer of silica. 

The collection of nanoparticles entrapped within the core of the micelles from surfactants is essential to subsequently conduct catalytic reactions using these particles. Today, two of the most common methods of the collection of nanoarticles prepared in water-in-scC02 microemulsions are reducing pressure and rapid expansion method. 

Additives such as surfactants or salts in aqueous solvent have marked influence on the solvation properties of reacting species. Selke, Oehme, and coworkers have reported that the use of micelle-forming additives has beneficial effect on the catalyst activity and selectivity. In several cases, micelle acts both as a solubilizing agent for poorly soluble organic substrates and as reaction medium for the catalytic reaction. 

The kinetics of catalytic reduction of 4-BB in CTAB solutions involves micellar catalysis of the electron transfer (ET) between 9-PA anion radical and 4-BB (Equation 2) in a thick layer of surfactant at the electrode. The effective rate constant for this ET in 0.1 M CTAB increased more than three orders of magnitude compared (Table 1) to the same reaction in surfactant-free DMF. The rate-determining step in CTAB was not Equation 2 as in DMF. In CTAB decomposition of the 4-BB anion radical (Equation 3) became kinetically important. The major cause of the kinetic alterations was compartmentalization of the reactants in high concentrations in surfactant aggregates at the surface of the electrode. The same catalytic reaction was not as successful in non-ionic igepal micelles, which did not provide good stabilization for 9-PA anion radicals. 

Unlike metallomicelles, fnnctional surfactant micelles compared to nonfunctional snrfactant micelles exhibit rate enhancement of many orders. The reason for snch apparent characteristic difference between the catalytic efficiency of metallomicelles and functional snrfactant micelles is the choice of the reference reactions nsed to evaluate the catalytic efficiency of metallomicelles and func-... 

The kinetic data are essentially always treated using the pseudophase model, regarding the micellar solution as consisting of two separate phases. The simplest case of micellar catalysis applies to unimolecTilar reactions where the catalytic effect depends on the efficiency of bindirg of the reactant to the micelle (quantified by the partition coefficient, P) and the rate constant of the reaction in the micellar pseudophase (k ) and in the aqueous phase (k ). Menger and Portnoy have developed a model, treating micelles as enzyme-like particles, that allows the evaluation of all three parameters from the dependence of the observed rate constant on the concentration of surfactant". ... 

The catalytic effect on unimolecular reactions can be attributed exclusively to the local medium effect. For more complicated bimolecular or higher-order reactions, the rate of the reaction is affected by an additional parameter the local concentration of the reacting species in or at the micelle. Also for higher-order reactions the pseudophase model is usually adopted (Figure 5.2). However, in these systems the dependence of the rate on the concentration of surfactant does not allow direct estimation of all of the rate constants and partition coefficients involved. Generally independent assessment of at least one of the partition coefficients is required before the other relevant parameters can be accessed. 

Inspired by the many hydrolytically-active metallo enzymes encountered in nature, extensive studies have been performed on so-called metallo micelles. These investigations usually focus on mixed micelles of a common surfactant together with a special chelating surfactant that exhibits a high affinity for transition-metal ions. These aggregates can have remarkable catalytic effects on the hydrolysis of activated carboxylic acid esters, phosphate esters and amides. In these reactions the exact role of the metal ion is not clear and may vary from one system to another. However, there are strong indications that the major function of the metal ion is the coordination of hydroxide anion in the Stem region of the micelle where it is in the proximity of the micelle-bound substrate. The first report of catalysis of a hydrolysis reaction by me tall omi cell es stems from 1978. In the years that... 

It turned out that the dodecylsulfate surfactants Co(DS)i Ni(DS)2, Cu(DS)2 and Zn(DS)2 containing catalytically active counterions are extremely potent catalysts for the Diels-Alder reaction between 5.1 and 5.2 (see Scheme 5.1). The physical properties of these micelles have been described in the literature and a small number of catalytic studies have been reported. The influence of Cu(DS)2 micelles on the kinetics of quenching of a photoexcited species has been investigated. Interestingly, Kobayashi recently employed surfactants in scandium triflate catalysed aldol reactions". Robinson et al. have demonshuted that the interaction between metal ions and ligand at the surface of dodecylsulfate micelles can be extremely efficient. ... 

The catalytic activity of micelles bearing catalytically active metal counterions (Lewis acid-surfactant combined catalysts, LASCs) on Diels-Alder reactions was recently investigated [72a, 76]. 

Hydroaminomethylation of alkenes occurred to give both n- and /. so aliphatic amines catalyzed by [Rh(cod)Cl]2 and [Ir(cod)Cl]2 with TPPTS in aqueous NH3 with CO/H2 in an autoclave. The ratio of n-and /.soprimary amines ranged from 96 4 to 84 16.178 The catalytic hydroaminomethylation of long-chain alkenes with dimethylamine can be catalyzed by a water-soluble rhodium-phosphine complex, RhCl(CO) (Tppts)2 [TPPTS P(m-C6H4S03Na)3], in an aqueous-organic two-phase system in the presence of the cationic surfactant cetyltrimethy-lammonium bromide (CTAB) (Eq. 3.43). The addition of the cationic surfactant CTAB accelerated the reaction due to the micelle effect.179... 

The DBSA-system is also applicable for the dithioacetalization of aldehdyes and ketones with 1,2-ethanedithiol to give the corresponding dithioacetals (Scheme 5.4, d). Increasing the reaction temperature decreases the yield of the products. Interestingly, increases in the concentration of the surfactant also decrease the yield of products formed, while shortening the alkyl chain of the surfactant abolishes its catalytic activity. Optical microscopy shows the formation of micelles, which are proposed to form hydrophobic environments and decrease the effective concentration of water and facilitate the dehydrative condensation reactions. 

Other cationic surfactants such as TTAB, DTAB, DODAB, STAC, CEDAB, and DDDAB have been used in CL reactions with less frequency. Thus, tetradecyltrimethylammonium bromide [TTAB] has been used to increase the sensitivity of the method to determine Fe(II) and total Fe based on the catalytic action of Fe(II) in the oxidation of luminol with hydrogen peroxide in an alkaline medium [47], While other surfactants such as HTAB, hexadecylpiridinium bromide (HPB), Brij-35, and SDS do not enhance the CL intensity, TTAB shows a maximum enhancement at a concentration of 2.7 X 10 2 M (Fig. 11). At the same time it was found that the catalytic effect of Fe(II) is extremely efficient in the presence of citric acid. With regard to the mechanism of the reaction, it is thought that Fe(II) forms an anionic complex with citric acid, being later concentrated on the surface of the TTAB cationic micelle. The complex reacts with the hydrogen peroxide to form hydroxy radical or superoxide ion on the... 

An interesting extension of aqueous solution radiolysis involved solutions of sodium dodecyl sulphate in the presence of MNP. Spin adducts of secondary alkyl radicals were detected provided that the critical micelle concentration of the surfactant was exceeded. Whilst it was rather loosely concluded that there is a marked catalytic effect of micelles on the rates of reaction of radicals with nitroso spin traps , no single origin of this effect could be clearly identified (Bakalik and Thomas, 1977). 

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