Cyclodextrins as multitask agents in nanocatalysis

Abstract

The efficiency of colloidal metallic particles in catalysis is closely related to their stability in the course of reaction. The choice of the capping agent is therefore critical as it controls both the size and shape of the particles, but also the dispersion of the metal core, while providing long-term stability. Among the wide variety of protective agents, cyclodextrins (cyclic oligosaccharides formed of glucopyranose units) have proven to be effective for synthesizing metallic nanoparticles by bottom-up chemical processes. The first examples in the literature emphasize the use of native cyclodextrins as metal nanoparticles (NPs) stabilizers but major improvements concerning the stability and catalytic activity of NPs either in the form of colloidal suspensions or immobilized onto supports were done by using chemically modified cyclodextrins. Indeed, appropriate functionalizations can lead to beneficial effects in the cyclodextrin affinity toward metal species and mass transfer of the substrate during the catalytic processes. It clearly appears that cyclodextrins can act as multitask agents from the formation (growth and nucleation process) of metal nanoparticles to the catalytic activity enhancement via specific interactions with the substrate. They can also be used to form supramolecular adducts with inorganic metal salts leading to heterogeneous catalysts with a homogeneous particles dispersion onto the support.