Thermal control of the defunctionalization of supported Au25(glutathione)18 catalysts for liquid phase oxidation reactions
Résumé
Atomically well-defined gold nano-clusters (GNCs), due to their size related properties, are known to be active in oxidation catalysis at relatively low temperature. Supported on metal oxides, Au25(SR)18 clusters are able to catalyze benzyl alcohol and CO oxidation reactions, styrene epoxidation in addition to nitrobenzene reduction. During these catalytic reactions, the gold thiolate clusters have been calcined to totally remove the thiolate molecules, in order to exhibit maximum activity. Nevertheless, recently, Haruta showed that mild temperature calcination, that generates partially defunctionalized Au38(2-phenylethanethiolate)24 supported over activated carbon, can also provide a maximum catalytic activity toward glucose oxidation. In this context, we successfully synthesized new composite materials, Au25(SG)18 (SG – glutathione) clusters deposited on ZrO2 nanoparticles, and calcined them at different temperatures to study the ligand and particle size effects in liquid phase oxidation reactions by using different substrates. Thermally treated at 300 °C, the partially defunctionalized Au25(SG)18@ZrO2 catalyst loses 53 % of its ligands, resulting in well-dispersed 1.7 nm gold particle size. As a heterogeneous catalyst for oxidation reaction, it exhibited complete conversion of benzyl alcohol into benzaldehyde in toluene under mild conditions, with a great TOF = 261 h-1.
To valorize this composite material as a sustainable catalyst for biomass conversion, its catalytic activity was tested for the conversion of furfural in aqueous and different alcoholic media. Thus, furoic acid was selectively formed when the reaction was performed in water, and the corresponding esters were observed in the different alcoholic media. This clusters-based catalyst showed excellent activity in the transformation of a bio-based molecule, furfural, into sustainable alternatives of petroleum-based compounds, even without a base.