Catalysis In Microemulsions Research Articles

Fast‐track process development at mini‐plant scale for the production of long‐chained amines using homogeneous catalysis in microemulsions

Fast‐track process development at mini‐plant scale for the production of long‐chained amines using homogeneous catalysis in microemulsions

The esterification in cyclohexane/DBSA/water microemulsion system

The esterification of straight-chain, branched-chain, unsaturated and aromatic acid with alcohol in cyclehexane/dodecylbenzenesulfonic acid (DBSA)/water microemulsion have been investigated. The results prove that DBSA microemulsion system has superiority for these organic reactions in enhancing the reaction rate and equilibrium conversion even in the relatively low temperature. The shorter the alkyl chain is, the faster the reaction rate becomes. The effects of the solvents species, reactant concentration and the w 0 ([H 2O]/[surfactant]) value on the reaction conversion have also been studied. The results show that solvent species has a little effect on the conversion, the conversion decreases with the increasing of reactant concentration, conversion has the largest value at certain w 0 value. The superiority of DBSA microemulsion for organic reaction is due to its dual catalysis. On one hand, DBSA acts as a Brønsted acid to catalyze the reaction; on the other hand, it can form microemulsion as surfactant to enlarge interface area and entrap water produced during esterification reaction, we called it “microemulsion catalysis”. DBSA microemulsion should also be applicable to the etherification, thioetherification reaction, and other organic dehydration reactions.

Organic Reactions in Microemulsions

AbstractMicroemulsions, i.e., thermodynamically stable, one‐phase mixtures of oil, water and surfactants, are useful as reaction media as a way to overcome the reactant incompatibility problem that one frequently encounters in organic synthesis. The use of a microemulsion can be seen as an alternative to phase‐transfer catalysis, i.e., use of a two‐phase system with added phase‐transfer agent. The microemulsion approach and phase‐transfer catalysis can also be combined, i.e., the reaction can be performed in a microemulsion in the presence of a small amount of a phase‐transfer agent. A very high reaction rate may then be obtained. The reaction rate in a microemulsion is often influenced by the charge at the interface and this charge depends on the surfactant used. For instance, reactions involving negatively charged species will be accelerated by the use of a cationic surfactant. This effect is analogous to micellar catalysis and may be termed microemulsion catalysis. The acceleration is more pronounced when the surfactant counterion is small and weakly polarizable. Microemulsions can also be used to induce regiospecificity of organic reactions. Organic molecules with one more polar and one less polar end will accumulate at the oil‐water interface of microemulsions. They will orient at the interface so that the polar part of the molecule extends into the water domain and the nonpolar part extends into the hydrocarbon domain. A water‐soluble reagent will react from the “water side”, i.e., attack the polar part of the amphiphilic molecule, and a reagent soluble in hydrocarbon will react at the other end of the amphiphilic molecule. This review demonstrates that microemulsions can be a useful for (i) overcoming reactant incompatibility, (ii) speeding up reactions of one polar and one apolar reactant (microemulsion catalysis), and (iii) inducing regiospecificity. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Nucleophilic substitution reaction of carboxylate and phosphate esters with hydroxamate ions in microemulsions

A kinetic study of the nucleophilic substitution reaction of hydroxamate ions with p-nitrophenyl acetate (carboxylate ester) and p-nitrophenyl diphenyl phosphate (phosphate ester) has been performed in cationic microemulsion prepared from combinations of weight percentage of cationic surfactants (CTAB, CTACl and CPC), co-surfactant n-butanol, n-hexane (oil) and water. The pseudo-first-order rate constants increases with increasing water content. Significant ‘microemulsion catalysis’ have been observed for the reaction of PNPDPP with the hydroxamate ions over the reactions in aqueous solution. Kinetic model for nucleophilic substitution reactions, have been discussed.

Reactivity of furfural–cysteine model reaction in food-grade five-component nonionic O/W microemulsions

In our present study, a model Maillard furfural/cysteine reaction was chosen for exploring and understanding the different aspects that control the reactivity in five-component fully dilutable U-type food-grade oil/water (O/W) microemulsions of R(+)-limonene/ethanol/water/propylene glycol/Tween 60. It was found that these self-assembled nanosized structured systems are capable of enhancing the rate of Maillard reactions in which the selectivity and the reactivity are controlled by the composition of the interface and its curvature. The sulfur-containing flavor formation is very efficient if carried out in those O/W microemulsions (modified oil-swollen nanoreactors) and the reaction yields high amounts of key aroma compounds. The proposed Tween 60-based O/W systems show strong preference for the formation of 2-(2-furanyl)-thiazolidine (the main product). The O/W reaction medium enhances furfural–cysteine reaction rate ( microemulsion catalysis) in comparison to the water-based reaction. It was found that the initial reaction rate ( V 0) is affected by the solubilized aqueous phase content. The furfural–cysteine reaction is affected also by the process conditions (temperature, pH, time of reaction). These results suggest that O/W microemulsions are a promising choice for generation of flavors and food products.

Organic Reactions in Microemulsions

AbstractFor Abstract see ChemInform Abstract in Full Text.

Furfural-cysteine model reaction in food grade nonionic oil/water microemulsions for selective flavor formation.

The thermal reaction between cysteine and furfural was investigated at 65 degrees C in five-component food grade oil/water (O/W) microemulsions of R-(+)-limonene/ethanol, EtOH/water/propylene glycol, PG/Tween 60 as apart of a systematic study on the generation of aroma compounds by utilizing structured W/O and O/W fluids. The furfural-cysteine reaction led to the formation of unique aroma compounds such as 2-furfurylthiol (FFT), 2-(2-furanyl)thiazolidine (main reaction product), 2-(2-furanyl)thiazoline, and N-(2-mercaptovinyl)-2-(2-furanyl)thiazolidine. These products were determined and characterized by GC-MS. Enhancement in flavor formation is termed "microemulsion catalysis". The chemical reaction occurs preferably at the interfacial film, and therefore a pseudophase model was assumed to explain the enhanced flavor formation. The product internal composition is dictated by process conditions such as temperature, time, pH, and mainly the nature of the interface. Increasing water/PG ratio leads to a dramatic increase in the initial reaction rate (V(0)). V(0) increased linearly as a function of the aqueous phase content, which could be due to the increase in the interfacial concentration of furfural. Microemulsions offer a new reaction medium to produce selective aroma compounds and to optimize their formation.

Nanoparticle Catalysis in Microemulsions: Oxidation ofN,N-Dimethyl-p-phenylenediamine by Cobalt(III) Pentaammine Chloride Catalyzed by Colloidal Palladium in Water/AOT/n-Heptane Microemulsions

The kinetics have been studied of the reaction between p-Me2NC6H4NH2 (DMPPD) and Co(NH3)5Cl2+, a process of relevance in color photography, in a water/AOT/n-heptane microemulsion catalyzed by nanoparticles of colloidal palladium. The rate of formation of the colored N,N-dimethyl-p-semiquinonediimine (S+) was found to be first order in Co(NH3)5Cl2+ and almost zero order in DMPPD and to increase linearly with the concentration of palladium. Studies over the 10−45 °C temperature range gave activation energies which increased markedly as the temperature fell. Voltammograms of the oxidation of DMPPD and the reduction of Co(NH3)5Cl2+ using a rotating Pd electrode were carried out to investigate the catalytic mechanism. Evidence was obtained of DMPPD adsorption on the metal surface which indicated that the rate-determining step in the catalysis was diffusion of Co(NH3)5Cl2+ through a layer of DMPPD adsorbed on the palladium particles in the water pools.

Electrochemical Phase-Transfer Catalysis in Microemulsions: Carbene Formation

Dicholocarbene can be generated at a carbon cathode from dichlorodibromomethane in microemulsions. The carbene then reacts with the organic constituent of the microemulsion to generate 7,7-dichlorobicycloheptane. A variety of reaction conditions were investigated, and the best conditions give a current efficiency of 80%.

Mechanism of stereoselective production of trans-1-decalone by electrochemical catalysis in microemulsions

Abstract Microemulsions made from water, oil and surfactant are attractive alternatives to organic solvents for mediated electrochemical synthesis. Cyclization of 2-(4-bromobutyl)-2-cyclohexen-1-one (1) to 1-decalone (2) mediated by an electrochemically generated cobalt(I) complex favored the trans- form over cis-1-decalone by about 93:7 in microemulsions, but poorer stereoselectivity was obtained in organic solvents. Microemulsion composition, surfactant type (cationic or anionic), or chirality of the mediator did not significantly influence the trans/cis ratio of 1-decalone. Selective formation of trans-1-decalone in microemulsions is attributed to equilibration of isomers via keto-enol tautomerization catalyzed by hydroxide ions formed by co-electrolysis of water during the reaction. Trans-1-decalone was efficiently produced in 2 h electrolyses in microemulsions with large volume fractions of water. Organic solvents with trace amounts of water gave little stereoselectivity over similar periods, but longer equilibration times produced larger proportions of trans-1-decalone.

Covalently Linked Scaffold of Cobalt Corrins on Graphite for Electrochemical Catalysis in Microemulsions

Catalytic films were constructed by covalently binding poly-l-lysine (PLL) onto oxidized carbon electrodes and then forming covalent amide linkages from PLL to the cobalt corrin vitamin B12 hexacarboxylic acid [B12(COOH)6]. Covalent bonds from electrode to PLL and PLL to B12(COOH)6 provided good stability in microemulsions. PLL−B12(COOH)6 films gave reversible electron transfer for the Co(II)/Co(I) redox couple and exhibited characteristic voltammetric features of surface-confined electrochemistry. Formal potentials of the Co(II)/Co(I) couple in the films were controlled by the concentration of electrolyte in the fluid and by Coulombic interactions with surfactant. PLL−B12(COOH)6 films demonstrated excellent catalytic activity in microemulsions for the reduction of vicinal dihalides to olefins, for dechlorination of trichloroacetic acid, and for alkylation of an activated olefin. Turnover numbers for conversion of dibromocyclohexane to cyclohexene in microemulsions were 3-fold larger for PLL−B12(COOH)6 on...

Catalytic Electrochemical Synthesis Using Nanocrystalline Titanium Dioxide Cathodes in Microemulsions

Compared to similar catalysts dissolved in solution, electrolysis using a cobalt complex chemisorbed onto nanocrystalline titanium dioxide cathodes was catalytically more efficient in a microemulsion for the debromination of dibromocyclohexane, and the stereoselective cyclization of 2-(4-bromobutyl)-2-cylohexen-1-one to 1-decalone. 1,2-Dibromocyclohexane was converted to cyclohexene with 100 h-1 turnover using TiO2 electrodes coated with vitamin B12 hexacarboxylate. This represented a 5-fold increase in the turnover number compared to the same reaction catalyzed using vitamin B12 in solution on a bare carbon electrode. Cyclization of 2-(4-bromobutyl)-2-cyclohexen-1-one in the microemulsion gave similar trans:cis ratios (14−18) for product 1-decalone for both soluble and adsorbed catalyst systems, but more than 100-fold larger turnover number using the coated TiO2 electrodes. In DMF, the coated TiO2 gives little stereoselectivity for this reaction, and poorer turnover than that in the microemulsion. Binding vitamin B12 hexacarboxylate onto TiO2 electrodes improves electron-transfer rates and catalytic efficiency while retaining essential features of cobalt complex-mediated catalysis in microemulsions.

Electrochemical catalysis in microemulsions. Dynamics and organic synthesis

Abstract Microemulsions are clear microheterogeneous fluids made from water, oil, and surfactant. They provide cheaper and less toxic alternatives to organic solvents. In this paper, we review progress in understanding reaction dynamics and designing mediated electrochemical syntheses in microemulsions. Rates of mediated electrochemical reactions in conductive microemulsions can be enhanced by preconcentration of reactants in a dynamic film on the electrode surface, or more generally by controlling the formal potential of the mediator in the fluid. The electrode|fluid interface in microemulsions probably consists of a dynamic layer of surfactant molecules packed more loosely on the electrode than in aqueous micellar solutions. Microemulsions afforded good yields of carbon–carbon addition products in reactions mediated by the cobalt complex vitamin B 12 . Excellent stereoselective control in microemulsions made with the cationic surfactant cetyltrimethylammonium bromide was demonstrated for the catalytic cyclization of 2-(4-bromobutyl)-2-cyclohexen-1-one to 1-decalone. Electrochemical synthesis in microemulsions may be a viable future approach to environmentally friendly methods of producing organic chemicals.

ChemInform Abstract: Liquid/Liquid Interface as a Model System for Studying Electrochemical Catalysis in Microemulsions. Reduction of trans‐1,2‐Dibromocyclohexane with Vitamin B12.

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Liquid/Liquid Interface as a Model System for Studying Electrochemical Catalysis in Microemulsions. Reduction of trans-1,2-Dibromocyclohexane with Vitamin B12

A complex electrochemical catalytic reaction was investigated at the interface between water and benzonitrile as a model for interfacial chemistry in microemulsions. Structures similar to those between oil−water microphases in microemulsions were created by using the interface between two immiscible electrolyte solutions (ITIES). While interfacial area in a microemulsion can be uncertain, the ITIES is well-defined and can be used to evaluate relevant heterogeneous interfacial kinetics. The reaction between the Co(I) form of vitamin B12 generated electrochemically in the water phase, and trans-1,2-dibromocyclohexane (DBCH) in benzonitrile was probed directly at the ITIES by scanning electrochemical microscopy (SECM). Apparent heterogeneous rate constants for the interfacial reaction were extracted from SECM current−distance curves. Influences of reactant concentration, potential drop across the ITIES, and adsorbed surfactants were investigated. Results suggest that the kinetics of reduction of DBCH by B12 ...

Organic and bioorganic reactions in microemulsions

During the last two decades reactions in microemulsions have developed into an emerging technology. In most instances oil-continuous microemulsions (w/o systems) have been used and the water droplets have proven useful as “minireactors” for various types of syntheses. This review discusses recent advances in the fields of organic and bioorganic reactions in microemulsions. In preparative organic synthesis microemulsions are of interest to overcome incompatibility problems between nonpolar organic compounds and inorganic salts. For this purpose, microemulsions can be regarded as an alternative to two-phase systems with added phase transfer reagents. Properly formulated microemulsions may also accelerate organic reactions, various mechanisms of such rate enhancements are discussed. Transition from a homogeneous solvent system to a microemulsion may also affect the regioselectivity of organic reactions due to orientation of reactants at the oil-water interface. In bioorganic synthesis, microemulsions are of interest as a reaction medium for several reasons: (i) nonpolar substrates can be dissolved in high concentrations, (ii) thermodynamic equilibria of condensation/hydrolysis reactions can be shifted by adjusting water content, (iii) enzymes are sometimes found to be more stable and more active than in aqueous buffer. Lipases are the most widely used enzymes and reactions have been performed in different types of microemulsion systems. This review presents general aspects of enzymatic catalysis in microemulsions followed by a discussion of recent advances in preparative work focusing on lipase catalyzed processes.

Enzymatic catalysis in microemulsions: Enzyme reuse and product recovery

A technique for enzyme reuse and product recovery from enzymatic catalysis in microemulsions is demonstrated. The enzymatic reaction is performed in a homogeneous isotropic microemulsion; AOT (sodium bis-(2-ethyl- hexyl)sulfosuccinate)/isooctane/buffer or C(12)E(5)(penta ethylene glycol dodecyl ether)/heptane/buffer. By small temperature changes the systems are shifted to two phase regions, where an oil-rich phase, containing the product, coexists with a water-rich phase containing surfactant and enzyme. The oil-rich phase may be replaced by an oil solution containing new substrate. Thus, the reaction may be continued and the enzyme reused. This procedure was repeated nine times in the present study. Data on phase behavior in presence and in absence of protein, partitioning of the components and a radioactive-labelled protein between the phases, and the repeated use of horse liver alcohol dehydrogenase (HLADH) in the microemulsions are presented.

Reactions in microemulsion media. Borohydride reduction of mono- and dicarbonyl compounds

Microemulsions were prepared at 26° from mixtures of hexanes ( O), a 50: 1 (w/w) solution ( W) of 0.1 M KOH-NaBH 4, and a 1.23:1 (w/w) mixture ( S) of hexadecyltrimethylammonium bromide (HTABr) and 1-butanol. A pseudoternary phase map contained a significant microemulsion (μE) region, and μE's A and B (60:35:5 and 20:10:70 S:W:O, respectively) were used for reduction of several monocarbonyl compounds [benzophenone ( 1a), benzaldehyde ( 2a), acetophenone ( 3a), and 1-phenyl-1-octadecanone ( 4a)], an α,β-unsaturated ketone [ trans-4-phenyl-3-buten-2-one ( 6a)], and a diketone [4-(4'-benzoylphenyl)-2-butanone ( 7a)]at 26°. For comparison purposes, reductions were also performed in aqueous 2-propanol (2-PrOH A and 2-PrOH B) prepared by the substitution of 2-propanol for the S and O components of μE's A and B. Generally, the reductions were slightly faster in the microemulsion media than in the corresponding aqueous 2-propanol media. The significantly slower reduction of 4a relative to that of 3a in μE B indicated that the interphase is the reactive site. With enone 6a, the influence of microemulsions on the competition between 1,2- and 1,4-reduction was determined. In μE's A and B there was 8% and ll% 1,4-reduction, respectively, whereas in 2-PrOH A and B there was only a trace. With diketone 7a, the reactivity of the aromatic carbonyl group relative to that of the aliphatic carbonyl group increased on going from 2-PrOH A and B to μE's A and B, respectively. For the sodium borohydride reduction of ketones, microemulsion catalysis is more effective than phase transfer catalysis or the use of a tetraalkylammonium borohydride in a hydrocarbon solvent.