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MATERIALS FOR HETEROGENEOUS CATALYSIS
(Person in charge: Pr. Anne Ponchel)
Our research focuses on the development of new heterogeneous catalysts prepared by innovative cyclodextrin-assisted methods, paving the way to the design of more sustainable chemical processes. Thus, a number of different approaches have been explored for preparing metal and metal oxide supported catalysts as well as nanostructured catalysts via the use colloidal deposition, impregnation, nanocasting or sol-gel methods. A condensed overview of our results is given below.
1- Carbon-supported metal nanoparticles by colloidal deposition
A new type of carbon-supported ruthenium catalysts was elaborated by colloidal deposition including the pre-stabilization of Ru(0) nanoparticles protected by cyclodextrins (&alfa;, β and γ) and their subsequent deposition by liquid-phase adsorption on activated carbons. Tested in the in continuous gas-phase hydrogenation of xylenes using mild reaction conditions (T =85°C and ambient pressure) without further need of reductive treatment, these nano-supported catalysts appeared to be much more efficient than control Ru/C impregnated catalysts, both in terms of activity and stereoselectivity. These works demonstrated that cyclodextrins could be readily used as multi-functional molecules, allowing to stabilize and disperse ruthenium NPs on carbon supports but also to promote the reactivity through host-guest interactions with the substrate during the reaction.
2- Nanostructured carbons based on cyclodextrins precursors
Cyclodextrins could be successfully exploited for the synthesis of carbons and carbon-metal composites with mesoporous architectures by a hard-templating method. Thus, an ordered mesoporous carbon decorated by uniformly distributed Ru NPs (1 to 2 nm) was synthesized by directly carbonizing a host-guest complex formed between a RaMe-β-CD and adamantane carboxylic acid in the presence of RuCl3. The adamantane derivative was selected as a compound capable of interacting with both the cavity of β-CD and metal cation through the terminal carboxylic group. After carbonization and silica removal, the resulting Ru@C replica was shown to be highly active and stable for the hydrogenation of unsaturated fatty acid methyl esters.
3- Nanostructured inorganic materials
This research topic deals with the development of new synthetic approaches for the preparation of nanostructured inorganic materials with multi-scale porous structures that mimic the architecture of cyclodextrin-based supramolecular assemblies. These approaches rely on a tight coupling of the concepts of sol-gel process and the tools of supramolecular chemistry. Our goal is to elucidate the role played by the cyclodextrin in the structure, porosity and architecture of these solids, before evaluating their performance in heterogeneous catalysis (hydrogenation reactions) and photocatalysis (photodegradation of pesticides). We focus mainly on silica materials, transition metal oxides, as well as TiO2-based nanocomposites prepared using three different approaches: (i) nanocasting, (ii) template-directed colloidal self-assembly and (iii) conventional sol-gel process.
4- Supported metal catalysts by wet impregnation
A further type of non-covalent interaction, i.e. the binding of metal salts through the hydroxyl groups of cyclodextrins could be exploited to improve the dispersion and redox properties of metal active phases of supported catalysts prepared by wet impregnation. Supported cobalt catalysts provided illustrative examples. We showed that the impregnation of the support with aqueous solutions of Co(NO3)2 containing β-CD led to higher metal dispersions (lower particle size) and metal reducibility and spectacularly enhanced the reaction rate in Fischer–Tropsch synthesis and VOC oxidation. This was associated with the ability of cyclodextrins to be scaffold for the cobalt ions by forming stable β-CD-Co(II) complexes, delaying the formation, growth and aggregation of cobalt oxide particles during the thermal activation of the catalyst.
5- Carbon materials for aqueous organometallic catalysis
Our research activities are not exclusively limited to the development of metal-based catalytic materials derived from cyclodextrins and supramolecular assemblies, but are concerned with the improvement of clean catalytic processes using water as solvent. Although appropriate for water-soluble molecules, the aqueous-phase processes do not allow the transformation of highly hydrophobic substrates due to mass transfer limitation between the aqueous and the organic phase. To circumvent this, we reported that carbon materials could be employed as efficient mass transfer additives in the Pd-catalyzed allylic cleavage of long-alkyl chain carbonates (Tsuji-Trost reaction). It was shown that the effect of carbon materials was strikingly connected to their textural and surface chemistry properties. With the use of hydrophobic mesoporous carbons, the substrates and the Pd catalyst could be confined within the pores, resulting in improved contacts between both partners and a subsequent increase in the reactivity. Such carbon systems appeared also to be a suitable solution for the aqueous rhodium-catalyzed hydroformylation of hydrophobic bio-sourced substrates, such as unsaturated fatty acid methyl esters (FAMEs).