Loïc Leclercq

Pickering Interfacial Catalysis for Biphasic Systems: From Emulsion Design to Green Reactions

Pickering emulsions are surfactant-free dispersions of two immiscible fluids that are kinetically stabilized by colloidal particles. For ecological reasons, these systems have undergone a resurgence of interest to mitigate the use of synthetic surfactants and solvents. Moreover, the use of colloidal particles as stabilizers provides emulsions with original properties compared to surfactant-stabilized emulsions, microemulsions, and micellar systems. Despite these specific advantages, the application of Pickering emulsions to catalysis has been rarely explored. This Minireview describes very recent examples of hybrid and composite amphiphilic materials for the design of interfacial catalysts in Pickering emulsions with special emphasis on their assets and challenges for industrially relevant biphasic reactions in fine chemistry, biofuel upgrading, and depollution.

Supramolecular effects involving the incorporation of guest substrates in imidazolium ionic liquid networks: Recent advances and future developments

The interest in ionic liquids (ILs) as green solvents has grown enormously in the last few years. ILs have been proved to be convenient and economical solvents due to their efficiencies, non-toxicities and recyclabilities. Logically, these new media are used in a wide range of organic reactions. Recent investigations of the physicochemical properties of imidazolium ILs suggest that they behave as polymeric supramolecules that cannot be seen as conventional solvent. Electrostatic, H-bonds and π-stacking interactions occur in imidazolium ILs. When various organic compounds are mixed with imidazolium ILs, ‘inclusion complexes’ are formed and complex supramolecular organisation is created. In other words, ILs can be seen as a pre-organised medium that can modify the molecular reactivity (selectivity, substrate discrimination, etc.) by the formation of ‘inclusion complexes’ between guests (reactive species) and the ‘host networks’ (ILs). This mini review is intended to describe the imidazolium IL organisation (aggregation properties, formation of ‘inclusion complexes’, etc.) and to underline some of the effects on the organic reactivity of the inclusion of the guest substrates in imidazolium ILs.

Halide-free highly-pure imidazolium triflate ionic liquids: Preparation and use in palladium-catalysed allylic alkylation

The reaction of three oxygenated heterocycles (tetrahydrofuran, tetrahydropyran, 1,4-dioxane) with trifluoromethane sulfonic anhydride in the presence of the non-nucleophilic base poly(4-vinylpyridine) affords alkylditriflates quantitatively via ring opening. The alkylditriflates react with N-alkylimidazoles providing the bis-imidazolium bis-triflate salts in high yields. Hydroxymethyl substituted oxygenated heterocycles are converted into imidazolium triflate salts without opening of the heterocycle. Two imidazolium salts were characterized by X-ray crystallographic structure determination. The halide free ionic liquids were applied successfully as solvents in the palladium catalysed allylic alkylation. A co-crystal of 15 and tetrabutylammonium triflate has also been characterized by X-ray crystallographic structure analysis.

Eco-friendly solvents and amphiphilic catalytic polyoxometalate nanoparticles: a winning combination for olefin epoxidation

Eighteen eco-friendly solvents were examined to carry out the epoxidation of olefins with the amphiphilic catalytic dodecyltrimethylammonium polyoxometalate nanoparticles [C12]3[PW12O40] in comparison with [H]3[PW12O40] and [Na]3[PW12O40]. Surprisingly, the screening of solvents with cyclooctene has revealed that the [C12]3[PW12O40] catalyst is much more active with initial turn-over frequencies, TOF0, increasing up to a factor of 10. Moreover, the reaction occurs at competitive rates in four relevant solvents, i.e. cyclopentyl methyl ether, 2-methyl tetrahydrofuran, methyl acetate and glycerol triacetate, for which TOF0 values are higher than 260 h−1. The recyclability of the systems is demonstrated and the scope of substrates has been successfully extended to cyclohexene, 1-octene, limonene, 3-carene, α-pinene, β-pinene and neryl acetate with good epoxide selectivity. The catalytic performances in the “green” solvent are assigned to the formation of stable [C12]3[PW12O40] nanoparticle dispersions which have been characterized by transmission electron microscopy and dynamic and multiple light scattering experiments. Finally, the Kamlet–Taft parameters were measured in order to correlate the physicochemical properties of the solvents and the catalytic activity.

Thermoregulated microemulsions by cyclodextrin sequestration: a new approach to efficient catalyst recovery.

The combination of imidazolium surfactants with alpha-cyclodextrins (CDs, in green) was used as a control element in the thermoregulated aqueous olefin hydroformylation. The self-assembly of the imidazolium surfactants (red) favors the micellization process at high temperatures, whereas at lower temperatures the complexation of the surfactant monomers into the alpha-CDs is favored.

Versatile Eco‐friendly Pickering Emulsions Based on Substrate/Native Cyclodextrin Complexes: A Winning Approach for Solvent‐Free Oxidations

Solvent-less Pickering emulsions were developed and applied to catalytic oxidation. These systems are stabilized by inclusion complexes between cyclodextrins and substrates, forming a 3D network among the dispersed phase. In the presence of hydrogen peroxide as a green oxidant and [Na]3 [PW12 O40 ] as a catalyst, they provide particularly efficient reaction media for the oxidation of olefins, organosulfurs, and alcohols. The reactions proceed at competitive rates (up to 400 h(-1) ) with straightforward separation of the phases by centrifugation or heating. Moreover, these new eco-friendly systems work at a preparative scale (up to 2.5 M) and are recycled without loss of activity.

Structure–activity relationship of cyclodextrin/biocidal double-tailed ammonium surfactant host–guest complexes: Towards a delivery molecular mechanism?

In aqueous solution, the biocidal double-tailed cationic surfactant, di-n-decyl-dimethyl-ammonium chloride can form inclusion complexes with various cyclodextrins (alpha-CD, beta-CD, gamma-CD, HP-alpha-CD, HP-beta-CD and CM-beta-CD). A physicochemical study has been performed to investigate the association parameters of these host-guest complexes by combining the use of ammonium and chloride selective electrodes, NMR spectroscopy and molecular modeling. stoichiometries, equilibrium constants and geometries were determined by resorting to a specific algorithm. The antimicrobial activity of the encapsulated ammonium surfactant was compared with that of the free ammonium showing three different behaviors depending on the cyclodextrin. The close relationship between the complex structure and the biocidal activity is used to propose a delivery molecular mechanism.

Encapsulation of biocides by cyclodextrins: toward synergistic effects against pathogens

Summary Host–guest chemistry is useful for the construction of nanosized objects. Some of the widely used hosts are probably the cyclodextrins (CDs). CDs can form water-soluble complexes with numerous hydrophobic compounds. They have been widespread used in medicine, drug delivery and are of interest for the biocides encapsulation. Indeed, this enables the development of more or less complex systems that release antimicrobial agents with time. In this paper, the general features of CDs and their applications in the field of biocides have been reviewed. As the key point is the formation of biocide–CD inclusion complexes, this review deals with this in depth and the advantages of biocide encapsulation are highlighted throughout several examples from the literature. Finally, some future directions of investigation have been proposed. We hope that scientists studying biocide applications receive inspiration from this review to exploit the opportunities offered by CDs in their respective research areas.

Aqueous mixtures of di-n-decyldimethylammonium chloride/polyoxyethylene alkyl ether: dramatic influence of tail/tail and head/head interactions on co-micellization and biocidal activity.

Mixed aggregate formation and synergistic interactions of binary surfactant mixtures of di-n-decyldimethylammonium chloride, [DiC(10)][Cl], with polyoxyethylene alkyl ethers, C(i)E(j) (i=10, 12, j=4, 6, 8), have been investigated for various [DiC(10)][Cl]/C(i)E(j) ratios. The critical aggregation concentration of the binary mixtures has been determined by tensiometry, and the aggregate characteristics (i.e., size and composition, free ammonium concentration) have been estimated using the pulsed field gradient NMR spectroscopy and a [DiC(10)]-selective electrode. Diffusion coefficient measurements of micelles confirmed the synergistic interaction between the surfactants. It is thus shown that the formation of surface monolayers and mixed aggregates from [DiC(10)][Cl]/C(10)E(j) mixtures is driven by both tail/tail and head/head interactions, whereas [DiC(10)][Cl]/C(12)E(j) co-aggregation is mainly driven by tail/tail interactions. As a consequence, the co-aggregation phenomenon notably influences the biocidal activity of [DiC(10)][Cl] on the Candida albicans fungi. In the presence of C(12)E(j), the biocidal activity of the ammonium salt is inhibited due to the trapping of the cationic surfactants in the mixed aggregates, whereas in the presence of C(10)E(j), the biocidal activity of the surfactant mixture is maintained. The mode of action is also confirmed by a faster increase in the zeta potential of a C. albicans suspension in the presence of [DiC(10)][Cl]/C(10)E(8) than in the presence of [DiC(10)][Cl]/C(12)E(8). Therefore, a judicious adjustment of the alkyl (i) and polyoxyethylene (j) chain lengths of C(i)E(j) avoids its antagonistic effect on the biocidal activity of [DiC(10)][Cl].

Pickering emulsions based on cyclodextrins: A smart solution for antifungal azole derivatives topical delivery.

Surfactants are usually used for the preparation of emulsions. Potential drawbacks on the human body or on the environment can be observed for some of them(e.g. skin irritation, hemolysis, protein denaturation, etc.). However, it is possible to use biocompatible emulsifiers such as native cyclodextrins (CDs). The mixture of oil (paraffin oil or isopropyl myristate), water and native CDs results in the formation of Pickering emulsions. The emulsion properties were investigated by ternary phase diagrams elaboration, multiple light scattering, optical and transmission microscopies. The results prove that these Pickering emulsions were very stable against coalescence due to the dense film format the oil/water interface. The rheological behavior has shown that these emulsions remain compatible for topical applications. This kind of emulsions (biocompatibility, stability and surfactant free) has been used to obtain sustainable formulations for antifungal econazole derivatives delivery. Our results prove that these new formulations are at least as active as commercially available formulations.