From hydroxycetylammonium salts to their chiral counterparts. A library of efficient stabilizers of Rh(0) nanoparticles for catalytic hydrogenation in water

International audienceNowadays, the nanometer-size and shape control of metallic species remains a strategic research area in material development due to their particular physical and chemical properties. Among their various applications, catalysis has emerged as one of the most pertinent due to its large range of potentialities. From now on, noble metal nanoparticles (NPs) are considered as an unavoidable family of catalysts, at the border between heterogeneous bulk materials and molecular complexes. The design of stable noble metal nanoparticles in various solvents such as ionic liquids, fluorous or organic media has become a concern over the past few years. Nevertheless, for economic and ecological purposes, the development of green approaches remains an active research area, as well as the reusability of nanocatalysts. In this context, the stabilisation in colloidal suspension of nanoparticles, thus providing a catalyst recycling thanks to a biphasic approach constitutes an efficient alternative to the heterogeneization of the nanocatalysts. Among the various water-soluble protective agents, easily modulated surfactants proved to be attractive candidates to efficiently stabilize catalytically active nanospecies in aqueous media. Here, we review our recent progresses in the hydrogenation of benzene derivatives by rhodium nanoparticles which were capped by an ammonium salts bearing an achiral or chiral polar head group

Model arenes hydrogenation with silica-supported rhodium nanoparticles:The role of the silica grains and of the solvent on catalytic activities

Silica-supported rhodium-based nanoheterogeneous catalysts were easily prepared by impregnation with a pre-stabilized colloidal suspension. The resulting catalysts contain rhodium nanoparticles well-dispersed in the silica pores with a mean size of 5 nm. Influence of the silica grains size and of the solvent was investigated in arenes hydrogenation. It appeared that the size of the silica grains has a minimal influence on the reaction rate but the supported nanocatalysts displayed higher TOFs in hexane than in water

Rhodium colloidal suspension deposition on porous silica particles by dry impregnation: Study of the influence of the reaction conditions on nanoparticles location and dispersion and catalytic reactivity

Rhodium composite nanomaterials were synthesized by an innovating process called dry impregnation in a fluidized bed. It consists in spraying an aqueous colloidal suspension of rhodium on silica porous particles. The use of this precursor solution containing preformed nanoparticles avoids calcination/activation step. Different composite nanomaterials were prepared displaying various metal loadings. The operating conditions were tuned to modify τs, the solvent vapour saturation rate value, in order to influence the deposit location: either uniform on the whole silica particles or at the particles surface like a coating. τs is defined as the ratio between solvent content in the bed atmosphere and the maximum solvent content. The obtained samples were investigated in catalytic hydrogenation of aromatic compounds under very mild conditions. Their catalytic performances were compared to those of the original colloidal suspension in one hand and of a similar catalyst prepared through wet impregnation in another hand. Interesting activity and selectivity were observed.This illustrates the interest of the dry impregnation method: this way allows an easy control of the metal loading as well as of the metal loading location in the support particles. Moreover, the support particle size and morphology are preserved

Cyclodextrin-based systems for the stabilization of metallic(0) nanoparticles and their versatile applications in catalysis

International audienceIn order to better respond to environmental standards, the development of metal nanoparticles 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, through the ability to form inclusion complexes or supramolecular adducts with organic molecules or metal precursors, cyclodextrins can be successfully employed to stabilize size-controlled zerovalent metallic nanoparticles active for hydrogenation reactions carried out in aqueous or gas-phase media. In this summary of our works, we report that cyclodextrins could be used in various forms and environments: (i) in free form, (ii) in complexed form with appropriate guests molecules, (iii) in combination with polymer matrices, (iv) in thermosensitive hydrogels and (v) immobilized onto porous carbons supports. All these studies highlight the fact that cyclodextrins can be seen as multi-task agents for nanocatalysis

β-Cyclodextrins grafted with chiral amino acids: A promising supramolecular stabilizer of nanoparticles for asymmetric hydrogenation?

International audienceWater-soluble ruthenium nanoparticles stabilized by randomly methylated β-cyclodextrins (RaMeCDs) grafted with chiral amino-acid moieties like l-alanine (Ala) and l-leucine (Leu) were prepared in aqueous solution by two approaches: (i) a one-step hydrogen reduction of ruthenium trichloride as metal source in the presence of appropriate cyclodextrins (one-pot method) or (ii) a NaBH4 reduction of the metal salts, followed by the stabilization of ruthenium hydrosol by the addition of chirally modified RaMeCDs (cascade method). The influence of the ligand's nature and the synthesis methodologies on the size, dispersion and surface properties of the obtained ruthenium colloids were studied by TEM and NMR analyses. The spherical ruthenium suspensions contain very small particles (0.82-1.00 nm) with narrow size distributions. Their catalytic properties were evaluated in biphasic hydrogenation of various prochiral compounds (olefins, ketones and disubstituted arenes) showing promising results in terms of activity and selectivity. Nevertheless, no significant enantiomeric excesses were observed

Ammonium surfactant-​capped Rh(0) nanoparticles for biphasic hydrogenation

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Metal nanoparticles in neat water for catalytic applications

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Noble Metal Nanoparticles Stabilized by Cyclodextrins: A Pertinent Partnership for Catalytic Applications

International audienceFor economic and ecological purposes, the design of stable and reusable noble metal nanoparticles in water has become a concern over the past few years. Among the various water-soluble protective agents, cyclodextrins proved to be attractive candidates to efficiently stabilize catalytically active nanospecies in aqueous media and also to improve the mass transfer in aqueous-organic two-phase systems. Various catalytic reactions such as arenes hydrogenations, olefins reduction or cross-coupling reactions were successfully achieved with good yields and selectivities according to the relevant choice of the cyclodextrins (cavity or functionalization)

Silver-doped rutile TiO2 nanotubes synthesis. First insights into selective photo-oxidation of benzyl alcohol derivatives under visible light.

International audienceThe synergistic effect between rutile TiO2 nanotubes and silver nanoparticles on the surface was studied. For that purpose, rutile TiO2 nanotubes were elaborated by an hydrothermal method, doped with silver by a wetness impregnation approach and fully characterized. The as synthesized material showed an improved photocatalytic activity, compared with bare rutile nanotubes, which was correlated to the changes in the optical, structural, and textural properties associated with the variation of silver amount. The photocatalytic activity of the as-prepared catalysts was evaluated into the oxidation of different benzyl alcohol derivatives under visible light

Unprecedented selective synthesis of imines by photo-oxidative coupling under N2O atmosphere

The synthesis of imines was unprecedently performed using nitrous oxide (N2O) as a highly selective oxidative agent for the photocatalytic homo-coupling of amines. In a similar way, unsymmetrical imines were produced through cross-coupling of amines with alcohols. Promisingly, this greenhouse gas showed excellent performances towards the target reactions when photocatalytically activated at atmospheric pressure, with silver photodeposited on titanium dioxide as catalyst. Relevantly, reactions under N2O atmosphere exhibited high conversion rates and turn-over frequencies, and higher selectivities than the ones carried out under a dioxygen atmosphere. Mechanisms for both reactions were proposed with release of nitrogen and water as benign by-products