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CATALYSIS AND SUPRAMOLECULAR CHEMISTRY
(Person in charge: Dr. Bastien Léger)
In order to better respond to environmental standards, the development of metal nanoparticles (NPs) using green approaches has exponentially grown for the last decade. Cyclodextrins, which are cyclic oligosaccharides composed of 6(α), 7(β) or 8(γ) glucopyranose units, have appeared to be interesting candidates for the synthesis of metal nanoparticles. Indeed, cyclodextrins can be successfully employed to stabilize size-controlled zerovalent metallic nanoparticles active for hydrogenation reactions of petrosourced and biosourced substrates carried out in aqueous media. In combination with metal nanoparticles dispersed in liquid phase, cyclodextrins could be used in various forms and environments: (i) in free form, (ii) in complexed form with appropriate guest molecules, (iii) in combination with polymer matrices. Cyclodextrins can be seen as multi-task agents for nanocatalysis.
1- NPs and cyclodextrins in free form
Zerovalent ruthenium(0) nanoparticles in the size range of 2.5 nm were easily prepared by chemical reduction of ruthenium salt with an excess amount of sodium borohydride and were efficiently stabilized by methylated cyclodextrins. The optimization of the catalytic system has been carried out in terms of stability and catalytic activity, considering the hydrogenation of olefinic compounds under biphasic liquid–liquid conditions. Efficient and controlled chemoselectivities were obtained in the hydrogenation of arene derivatives by the relevant choice of cavity and methylation degree of the cyclodextrins. The hydrogenation of α- and β-pinenes leads to the major formation of cis-pinanes, interesting synthons for fine chemistry, with high diastereoisomeric excesses.
Colloidal suspensions of ruthenium nanoparticles (Ru(0) NPs) stabilized by randomly methylated β-cyclodextrins can be easily prepared in aqueous solution by two comparative reductive approaches. The first method relies on a one-step hydrogen reduction of ruthenium trichloride as a metal precursor in the presence of cyclodextrins (one-pot method). The second method consists of the chemical reduction of metal salts with NaBH4, followed by the stabilization of ruthenium NPs by post-addition of the methylated β-cyclodextrins (cascade method). The modified β-cyclodextrin-capped ruthenium NPs obtained were characterized using TEM, DLS and NMR techniques. The highly dispersed colloidal suspensions contain very small particles in the size range of 1–1.4 nm. Their catalytic performances were investigated in terms of activity and selectivity of their biphasic hydrogenation of various substrates (olefins, ketones and disubstituted arenes) under mild conditions.
2- NPs and cyclodextrins in complexed form
Catalytically active metal NPs can be stabilized in water by a 1:1 inclusion complexes formed between methylated cyclodextrins and an ammonium salt bearing a long alkyl chain. These NPs appear more active than the NPs stabilized by classical surfactants. The origin of this better efficiency is probably in connection with a dynamic organization of the protective agents around the NPs.
3- NPs and cyclodextrins in combination with polymer in physical-mixture
Cyclodextrins acted as growth controlling agent in the synthesis of PVP-stabilized Ru(0) nanoparticles, leading to enhancement of the catalytic activity in the hydrogenation of furfural. The most impressive effects were obtained with β- and γ-methylated CDs, allowing the stabilization of catalytically active NPs with a narrow size-distribution centered at around 2.3 nm, while retaining an excellent stability under reaction conditions.
4- NPs and cyclodextrins based-linear polymer
Rhodium(0) nanoparticles stabilized by a polymer containing carboxylate and β-cyclodextrin moieties have high stability and catalytic activity for aqueous hydrogenation reactions of olefins and aromatic substrates such as xylenes or FAMEs. This catalytic system can be recycled and reused without loss of activity. These high catalytic performances can be attributed to conjugated electrostatic interactions (carboxylate groups) and steric interactions (polymer structure and β-cyclodextrin moiety).
5- NPs and cyclodextrins based-3D polymer
The confinement of catalytically active metallic nanoparticles within discrete and robust microenvironments was successfully achieved by using a water-compatible three-dimensional β-cyclodextrin-based polymer. The strategy was examined using ruthenium through an aqueous colloidal approach involving the chemical reduction of ruthenium nitrosyl nitrate by sodium borohydride in the presence of a water-soluble β-CD polymer crosslinked with citric acid (poly(CTR-β-CD)). The advantage of this polymer for nanoparticle synthesis is that (i) additional stabilizing effects are exerted through steric interactions (crosslinked chains and β-cyclodextrin entities) and electrostatic interactions (ionisable–COOH groups) and (ii) accessible nanopockets are provided between the stable junctions of the polymer skeleton. The results highlighted that, in contrast with a series of control colloidal ruthenium catalysts, the specific use of poly(CTR-β-CD) allowed not only the stabilization of smaller size-controlled ruthenium nanoparticles (approximately 1.8 nm) but also their confinement in individual superstructures having sizes mostly in the range of 50 to 100 nm. These polymer-encapsulated ruthenium nanoparticles were applied as catalysts for the aqueous phase hydrogenation of biomass-derived 2-furaldehyde and 3-(2-furyl)acrolein under mild reaction conditions, i.e. 303 K and 1 MPa. The high reactivity was related to the presence of individual globular objects acting as catalytic “microreactors”, in which the consecutive hydrogenation reactions and product/substrate diffusional exchanges can occur efficiently in the confined spaces. The robustness of the system was demonstrated through recycling experiments.