Micellar Catalysts Research Articles

How Droplets Can Accelerate Reactions─Coacervate Protocells as Catalytic Microcompartments.

ConspectusCoacervates are droplets formed by liquid-liquid phase separation (LLPS) and are often used as model protocells-primitive cell-like compartments that could have aided the emergence of life. Their continued presence as membraneless organelles in modern cells gives further credit to their relevance. The local physicochemical environment inside coacervates is distinctly different from the surrounding dilute solution and offers an interesting microenvironment for prebiotic reactions. Coacervates can selectively take up reactants and enhance their effective concentration, stabilize products, destabilize reactants and lower transition states, and can therefore play a similar role as micellar catalysts in providing rate enhancement and selectivity in reaction outcome. Rate enhancement and selectivity must have been essential for the origins of life by enabling chemical reactions to occur at appreciable rates and overcoming competition from hydrolysis.In this Accounts, we dissect the mechanisms by which coacervate protocells can accelerate reactions and provide selectivity. These mechanisms can similarly be exploited by membraneless organelles to control cellular processes. First, coacervates can affect the local concentration of reactants and accelerate reactions by copartitioning of reactants or exclusion of a product or inhibitor. Second, the local environment inside the coacervate can change the energy landscape for reactions taking place inside the droplets. The coacervate is more apolar than the surrounding solution and often rich in charged moieties, which can affect the stability of reactants, transition states and products. The crowded nature of the droplets can favor complexation of large molecules such as ribozymes. Their locally different proton and water activity can facilitate reactions involving a (de)protonation step, condensation reactions and reactions that are sensitive to hydrolysis. Not only the coacervate core, but also the surface can accelerate reactions and provides an interesting site for chemical reactions with gradients in pH, water activity and charge. The coacervate is often rich in catalytic amino acids and can localize catalysts like divalent metal ions, leading to further rate enhancement inside the droplets. Lastly, these coacervate properties can favor certain reaction pathways, and thereby give selectivity over the reaction outcome.These mechanisms are further illustrated with a case study on ribozyme reactions inside coacervates, for which there is a fine balance between concentration and reactivity that can be tuned by the coacervate composition. Furthermore, coacervates can both catalyze ribozyme reactions and provide product selectivity, demonstrating that coacervates could have functioned as enzyme-like catalytic microcompartments at the origins of life.

Micellar catalysis: Polymer bound palladium catalyst for carbon-carbon coupling reactions in water.

Metallosurfactants, defined here as hydrophobic metal-containing groups embedded in hydrophilic units when dispersed in water, emanate in the formation of metallomicelles. This approach continues to attract great interest for its ability to serve as micellar catalysts for various metal-mediated chemical transformations in water. Indeed, relevant to green chemistry, micellar catalysis plays a preeminent function as a replacement for organic solvents in a variety of chemical reactions. There are several methods for the interaction of metal complexes (catalysts or catalyst precursors) and surfactants for producing micellar aggregates. A very effective manner for achieving this involves the direct bonding of the metal center to the amphiphilic polymeric materials. Herein, we describe the synthesis of a metallosurfactant containing a palladium complex covalently incorporated into a CO2-based triblock polycarbonate derived using a dicarboxylic acid chain-transfer agent. This amphiphilic polycarbonate was shown to self-assemble in water to provide uniform and spherical micelles, where the catalytic metal center is located in the hydrophobic portion of the micelle. The resulting metallosurfactant was demonstrated to effectively catalyze carbon-carbon coupling reactions at very low catalyst loadings.

Fabrication and application of amphiphilic polyoxometalate catalyst (CTA)nH5-nPMo10V2O40 for transformation of lignin into aromatic chemicals

Conversion of renewable lignin into bio-aromatic chemicals offers a sustainable pathway to increase biorefinery profitability. However, the catalytic transformation of lignin into monomers remains a highly challenging task due to the complexity and stability of the lignin structure. In this study, a series of micellar molybdovanadophosphoric polyoxometalate (POM) catalysts, (CTA)nH5-nPMo10V2O40 (n = 1–5), were prepared by the ion exchange method and applied as oxidative catalysts for birch lignin depolymerization. These catalysts showed efficient cleavage of C-O/C-C bonds in lignin, and the introduction of an amphiphilic structure facilitated the generation of monomer products. The best catalytic activity was observed at 150 °C within 150 min under a 1.5 MPa oxygen atmosphere over (CTA)1H4PMo10V2O40, which yielded a maximum lignin oil yield of 48.7 % and lignin monomer yield of 13.5 %. We also employed phenolic and nonphenolic lignin dimer model compounds to explore the reaction pathway and demonstrated the selective cleavage of CC and/or CO lignin bonds. Moreover, these micellar catalysts have excellent recyclability and stability as heterogeneous catalysts, which can be used up to five times. The application of amphiphilic polyoxometalate catalysts facilitates the valorization of lignin, and we expect to develop a novel and practical strategy for harvesting aromatic compounds.

Hydrolysis-oxidation of cellulose to formic acid in the presence of micellar vanadium-containing molybdophosphoric heteropoly acids

OxFA-processing of inedible cellulose is considered as a perspective way for the production of biogenic formic acid (FA) which appeared to be a promising source for sustainable energy. In our work, the one-pot hydrolysis-oxidation of cellulose to FA was studied in the presence of the micellar acidic catalysts based on Mo–V–P heteropoly acids [Cn-MIM]y/H3+x-yPMo12-xVx (n = 6, 10 and 16; x = 1, 2 and 4; y = 2–6). The influence of amount of V, substitution of H+ by [Cn-MIM] and the length of alkyl group Cn of the cation was revealed. The optimization of the reaction conditions was also performed. To our knowledge, heterogeneous catalysts of such a composition were applied in cellulose hydrolysis-oxidation to FA for the first time. In the presence of C10-MIM/HPMo11V catalyst, the optimal reaction conditions were determined: 10 g/L of cellulose, 1.25–10 g/L of catalyst, 5 MPa of gas mixture (O2/N2 - 20/80 vol/vol) and 170–180 °C. Decreasing the alkyl group length of Cn-MIM+ cation or V content into framework of heteropoly anion were revealed to rise up the yield of the target product. These correlations were caused by the following factors. First, the decreasing acidity of micellar catalysts. Senond, the change in distribution of electron density in heteropoly anion due to the interaction between hydrogen atoms of alkyl group of Cn-MIM cation and oxygen atom of heteropoly anion. Third, blocking of V active sites for reagents. Influence of acidity on catalytic properties of micellar catalysts was also revealed in the course of screening samples with the different degree of protons substitution by C16-MIM+. The reaction rate was demonstrated to rise with increasing degree of protons substitution by C16-MIM + cation in C16-MIM/HyPMo11V and C16-MIM/HyPMo8V4 samples. The yield of FA up to 45 mol% can be reached over micellar HPA catalysts studied.

Recyclable Polyethylene Glycol Ubiquinol Succinate-Bound N-Heterocyclic Carbene–Gold(I) Complexes: Sustainable Micellar Catalysis in Water

An efficient synthesis of three amphiphilic polyethylene glycol ubiquinol succinate (PQS)-bound N-heterocyclic carbene (NHC)–gold(I) complexes as micellar catalysts in bulk water was developed. The surfactant-bound gold catalysts were applied in three different cyclization reactions of acetylenic or allenic substrates, with the main focus being the gold-catalyzed dehydrative cyclization of alkynediols to furans. An effective recycling of the designer surfactant-bound NHC–gold(I) catalysts in water was demonstrated for all cyclization reactions. With sodium dodecyl sulfate as the cosurfactant, a significant acceleration of the catalytic reactions was observed. Mechanistic investigations of the gold-catalyzed dehydrative cyclization of alkynediols point to possible causes for this acceleration effect.

Halloysite nanotubes as nanoreactors for heterogeneous micellar catalysis

HypothesisElectrostatic attractions between the anionic head group of sodium alkylsulphates and the positively charged inner surface of halloysite nanotubes (HNTs) drive to the formation of tubular inorganic micelles, which might be employed as nanoreactors for the confinement of non polar compounds in aqueous media. On this basis, sodium alkylsulphates/halloysite hybrids could be efficient nanocatalysts for organic reactions occurring in water. ExperimentsSodium decylsulphate (NaDeS) and sodium dodecylsulphate (NaDS) were selected for the functionalization of the halloysite cavity. The composition, the structure and the surface charge properties of the hybrid nanotubes were determined. The actual formation of inorganic micelles was explored by studying the microviscosity and polarity characteristics of the surfactant modified nanotubes through fluorescence spectroscopy experiments using DiPyme as probe. The performances of the sodium alkylsulphates/halloysite composites as micellar catalysts for the Belousov-Zhabotinsky (BZ) reaction were investigated. FindingsThe halloysite functionalization with sodium alkylsulphates generated the formation of hydrophobic microdomains with an enhanced microviscosity. Compared to the surfactant conventional micelles, the functionalized nanotubes induced larger enhancements on the rate constant of the BZ reaction. This is the first report on the surfactant/halloysite hybrids showing their efficiencies as reusable nanocatalysts, which are dependent on their peculiar microviscosity and polarity properties.

Soft nanosystems based on hydroxypiperidinium surfactants as adjuvants and micellar catalysts

Quantitative parameters characterizing aggregation behaviour and wetting effect of 1-(2-hydroxyethyl)-piperidinium surfactants have been obtained. Their ability to act as adjuvants in herbicidal compositions based on clopyralid has been revealed. The kinetics of alkaline hydrolysis of paraoxon and armine, organophosphorus ecotoxicants, in the surfactant solutions has been explored and it has demonstrated more than 100-fold micellar rate enhancement.

Soft nanosystems based on hydroxypiperidinium surfactants as adjuvants and micellar catalysts

Quantitative parameters characterizing aggregation behaviour and wetting effect of 1-(2-hydroxyethyl)-piperidinium surfactants have been obtained. Their ability to act as adjuvants in herbicidal compositions based on clopyralid has been revealed. The kinetics of alkaline hydrolysis of paraoxon and armine, organophosphorus ecotoxicants, in the surfactant solutions has been explored and it has demonstrated more than 100-fold micellar rate enhancement.

Modulation of aggregation behavior, antimicrobial properties and catalytic activity of piperidinium surfactants by modifying their head group with a polar fragment

To establish the effect of surfactants structure on their functional activity a number of alkyl-N-hydroxyethylpiperidinium bromides with different length of the hydrophobic tail were synthesized and characterized. Aggregation parameters of these surfactants were determined using tensiometry and fluorimetry. It was shown that the piperidinium surfactants, including those functionalized with OH groups, show a high antimicrobial effect at concentrations lower than critical micelle concentration for both bacteria and fungi, with the low hemolytic activity revealed. In addition, these surfactants were studied as micellar catalysts in alkaline hydrolysis of carboxylic acid esters. Micellar catalytic effect increases with an growth in the length of the alkyl tail, as well as with an increase in the number of hydroxy groups in surfactant molecule and can exceed two orders of magnitude.

Chemoselective synthesis of long-chain alkyl-H-phosphinic acids via one-pot alkylation/oxidation of red phosphorus with alkyl-PEGs as recyclable micellar catalysts.

Long-chain n-alkyl-H-phosphinic acids (Alk = C4-C18) are chemoselectively synthesized in yields up to 90% via the direct one-pot alkylation/oxidation of red phosphorus (Pn) in the multi-phase alkyl bromide/KOH/H2O/toluene system with alkyl-PEG recyclable micellar catalysts, which demonstrate good recyclability.

Micellar Organocatalysis Using Smart Polymer Supports: Influence of Thermoresponsive Self-Assembly on Catalytic Activity.

Micellar catalysts with a switchable core are attractive materials in organic synthesis. However, little is known about the role of the shell forming block on the performance of the catalyst. Thermoresponsive block copolymers based on poly(N-isopropylacrylamide-co-vinyl-4,4-dimethylazlactone) attached to different permanently hydrophilic blocks, namely poly(ethylene glycol), poly(N,N-dimethylacrylamide), and poly(2,3-dihydroxypropyl acrylate), were successfully synthesized via reversible addition/fragmentation chain transfer radical polymerization (RAFT). Post-polymerization attachment of an amino-functionalized L-prolineamide using the azlactone ring-opening reaction afforded functionalized thermoresponsive block copolymers. Temperature-induced aggregation of the functionalized block copolymers was studied using dynamic light scattering. It was shown that the chemical structure of the permanently hydrophilic block significantly affected the size of the polymer self-assemblies. The functionalized block copolymers were subjected to an aldol reaction between p-nitrobenzaldehyde and cyclohexanone in water. Upon temperature-induced aggregation, an increase in conversion was observed. The enantioselectivity of the polymer-bound organocatalyst improved with an increasing hydrophilic/hydrophobic interface as a result of the different stability of the polymer aggregates.

Ionic liquid surfactants as multitasking micellar catalysts for epoxidations in water

Surface-active imidazolium tungstate ILs are multitask epoxidation catalysts in water using H2O2 as oxidant. The micelles solubilize olefins in the aqueous phase and catalyze the epoxidation. Organophosphonic acids greatly increase the reaction rate.

Kinetic Model of Two‐Phase Epoxidation with Ionic Liquids as Micellar Catalysts

AbstractThe biphasic catalytic epoxidation of cyclooctene using the ionic liquid (IL) 1,2‐dimethyl‐3‐octyl‐imidazolium perrhenate ([OMMIM]ReO4) as micellar catalyst and H2O2 as oxidant was investigated. Kinetic experiments were carried out in the intrinsic kinetic regime as proved by variation of stirring rate and temperature. Variation of catalyst concentration allowed for determination of the critical micellar concentration (CMC) of the catalytic IL. The effect of substrate concentrations on the reaction rate was also assessed. Based on the experiments, a kinetic model adapted from enzyme catalysis was proposed to account for the micellar reaction environment. The model takes into account the onset of micelle formation at the CMC. The application of the kinetic model illustrated the good agreement with the experimental data. The model will be applied to other micellar epoxidation reactions and for the design of an appropriate reaction setup in the future.

New amphiphilic multiheterocycle: Micelle-forming properties and effect on the reactivity of phosphorus acid esters

Supramolecular systems based on a novel tetracationic amphiphilic multiheterocycle have been studied by tensiometry, conductometry, pH-metry, spectrophotometry, and dynamic and electrophoretic light scattering. The critical micelle concentration of the system has been determined (0.4 mM), and the possibility of open and closed association models realization has been demonstrated. A high solubilizing ability of the aggregates toward hydrophobic guest species has been revealed. Micellar catalysts based on the new multiheterocycle have shown substrate specificity in the hydrolysis of phosphonates possessing different hydrophobicities.

Erratum to: “Self-assembling catalytic systems based on new amphiphile containing purine fragment, exhibiting substrate specificity in hydrolysis of phosphorus acids esters”

Micellar catalytic systems based on new purine-containing amphiphile for hydrolytic decomposition of organophosphorus toxicants have been prepared. The aggregation threshold of the system has been estimated by means of tensiometry, conductometry, spectrophotometry, and dynamic light scattering. The solubilizing action of the amphiphile towards the hydrophobic dye has been demonstrated. Kinetic studies of hydrolysis of phosphonates differing in the hydrophobicity have disclosed the substrate specificity of the micellar catalysts; inhibition instead of catalysis has been observed in the case of the less hydrophobic substrate.

Unsupported micellar palladium nanoparticles for biphasic hydrogenation and isomerization of hydrophobic allylic alcohols in water

This article presents the evaluation of water-soluble palladium nanoparticles with hydrophobic active sites that are ideal for the biphasic colloidal catalysis of water-insoluble organic substrates in aqueous solution. Palladium nanoparticles stabilized with ω-carboxylate-functionalized alkanethiolate are first synthesized using ω-carboxylate-S-alkylthiosulfate as their ligand precursor. The biphasic catalysis is carried out for the reaction of hydrophobic allylic alcohols without using any additional mixing solvent or surfactant, which results in the complete consumption of substrates under the atmospheric pressure of H2 gas and at room temperature in less than 24h. Systematic investigations on the influence of pH and substrate size are also performed to examine the utility of these thiolate-capped palladium nanoparticles as structurally stable and water-soluble micellar catalysts for the biphasic reaction.

Study of the nanocatalysis effect on the strength formation of reactive powder concrete

Changes in the rate of cement hydration affect not only the technological characteristics of concrete mixes, but also cause changes in the rate of concrete compressive strength formation. The paper examines the effect of micellar solutions, consisting of micelle-forming surfactants or a mixture of micelle-forming surfactants and conventional molecular surfactant. The study found that the micellar solutions change the nature and the ratio of hydration products in the cement stone, but do not change the type of cement hydration products that is act as micellar catalysts. This significantly increases the rate of strength formation of reactive powder concretes, both at initial and later hardening stages. It is proved that the micellar catalysis can be used to control hardening and strength formation of cement, thereby reducing the time to achieve the design strength of concrete. Thus, concrete gains the strength of 50 MPa for 4 days with the use of the micellar catalysis and the same strength for 28 days without additives.

Surfactant Assistant Enhancement of Bioremediation Rate for Hexavalent Chromium by Water Algae

Cr(VI) in waste water is a serious pollutant in diverse environmental conditions due to its toxicity and carcinogenicity. Bioremediation can alternatively be used to the conventional chemical treatments. Bioremediation of toxic hexavalent chromium is done by reduction of hexavalent chromium to nontoxic trivalent chromium in consequence of oxidation of organic components present in the water extract of water algae. The rate of bioremediation is increased by anionic surfactant sodium dodecylsulphate (SDS) and neutral surfactant Triton-X-100 (TX-100). In the present study micellar catalysts are used as surfactans. SDS is found to be most suitable among the catalyst. All the observations were recorded using UV-Vis, fluorescenece and FTIR spectrophotometer.

Cooperative effects of ruthenium micellar catalysts and added surfactants in transfer hydrogenation of ketones in water

Added surfactants S boost the performances of surface-active catalysts RuLn (n = 8, 16) in transfer hydrogenation of hydrophobic ketones in water by forming mixed micelles.

Electrostatic immobilization of substrate and polyoxotungstate catalyst at the surface of micelles for enhanced reaction efficiency in water

Polyoxometalate (POM)-based micellar catalysts were prepared by assembling the Keggin tri-anion [PW12O40]3− and (L)-N-acetylmethioninate salts of Gemini surfactants in water. Contrary to conventional catalytic systems, we propose a new approach in which the POM catalyst and the substrate to be oxidized ((L)-N-acetylmethioninate) are both localized at the surface of micelles by electrostatic interaction before the reaction. The high local concentration of reactants due to their confinement at the surface of micelles enhances the reaction kinetics. Catalyst recovery experiments showed that, after two cycles of reactions, the activity of the catalysts was not changed.