Ruthenium-containing β-cyclodextrin polymer globules for the catalytic hydrogenation of biomass-derived furanic compounds†
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 poly(CTR-β-CD) Ru(0) system was characterized at different stages of the synthesis by combining proton nuclear magnetic resonance spectroscopy, dynamic light scattering and transmission electron microscopy measurements. 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 and TEM characterizations after catalytic tests.