Carbon‐Supported Ruthenium Nanoparticles Stabilized by Methylated Cyclodextrins: A New Family of Heterogeneous Catalysts for the Gas‐Phase Hydrogenation of Arenes

Abstract

Carbon-supported metal catalysts are widely used in heterogeneous catalysis.[1] Indeed, the carbonaceous materials satisfy most of the desirable requirements for a catalytic support, such as chemical inertness, stability, high surface area, and easy recovery of the metal phase by burning off the spent catalyst. However, the inert nature of the carbonaceous surfaces sometimes makes it difficult to deposit metals, leading to mediocre dispersions of the metallic phase without narrow size distribution. [2] To overcome these difficulties, carbon-supported nanoparticles prepared by the deposition of stabilized metal colloids onto carbon supports have received considerable attention over the past decade.[3] The stabilization of the colloidal metallic species requires the addition of a protective agent to prevent the aggregation of the colloids into bulk materials. Several articles have offered highly detailed reviews of the specific synthesis techniques for preparing nanoparticles by chemical methods. [4] Concurrently to the stabilization of nanoparticle suspensions, it appears also crucial to consider the affinity of stabilized colloids for the carbon surface to obtain well-defined carbon-supported nanoparticles.As cyclodextrins (CDs) are known to adsorb spontaneously onto carbonaceous supports [5] and to stabilize metal nanoparticles, [6] we envisaged combining these properties to synthesize new selective heterogeneous catalysts for gas-phase hydrogenation reactions. In fact, we speculated that the anchorage of the metallic nanoparticles on the organic support would be easier in the presence of CD and that these CDs could induce new selectivities through molecular recognition processes. The specific size, shape, and hydrophobic environment of the CD cavity could, for example, enforce the substrates to adopt conformations that are either not or less populated in the proximity of the metal surface or stabilize certain reactive intermediates. Among the different cyclodextrins described in the literature, the randomly methylated cyclodextrins (RaMe-CDs; Scheme 1) appear to be the best candidates to validate our concept. Indeed, we have recently demonstrated that these methylated CDs can stabilize efficiently Ru 0 nanoparticles in water and that these CD-capped nanoparticles catalyze the hydrogenation of aromatic compounds in water.[6 g] Furthermore, we have recently discovered that RaMe-CDs can be easily adsorbed on carbon supports (see Supporting Information).[7]The carbon-supported ruthenium nanocatalysts were prepared from CD-stabilized aqueous colloidal suspensions, which were obtained by the chemical reduction of ruthenium chloride, followed by their adsorption onto the charcoal support (Scheme 1). The solids are denoted Ru-x-RaMe-

Ruthenium-containing β-cyclodextrin polymer globules for the catalytic hydrogenation of biomass-derived furanic compounds

“…An effective way to solve this problem is to stabilize the NPs by immobilizing them on a solid support (e.g. inorganic oxides, porous carbons or insoluble polymers) [5][6][7][8] or by using soluble capping agents (e.g. surfactants, ligands or polymers).…”

Cyclodextrins as Multitask Agents in Nanocatalysis

“…Cyclodextrins are efficient protective agents for NPs synthesis in organic or aqueous phase but they are known to adsorb spontaneously onto carbon supports [45]. With all these observations, Monflier and Ponchel prepared carbon-supported RaMe-CD ruthenium NPs via the adsorption of prestabilized aqueous Ru NPs on a commercial activated carbon [46,47]. The successful adsorption of the Ru NPs was confirmed by TEM analyses.…”

Carbon Nanomaterials for Targeted Cancer Therapy Drugs: A Critical Review

“…CDs have been used as complexing agents to enhance aqueous solubility of poorly soluble drugs, and to increase their bioavailability and stability . CDs are known to adsorb easily onto carbonaceous supports ,,. Adsorption of the CDs results in a better dispersion of support used in the aqueous phase .…”