Supported Gold Nanoparticles for Alcohols Oxidation in Continuous-Flow Heterogeneous Systems

Gold nanoparticles (AuNPs) were anchored on alkynylcarbamate-functionalized support materials having the suitable featuresfor application as catalysts in continuous-flow packed bed reactors. Thefunctionalization step was carried out by grafting with the difunctionalorganosilane [3-(2-propynylcarbamate)propyl]triethoxysilane(PPTEOS) three commercial micrometer-sized oxide supports, i.e. silica,alumina, and titania. The alkynyl-carbamate moieties were capable ofstraightforwardly reducing the gold precursor HAuCl4 yielding thesupported AuNPs systems Au/SiO2@Yne, Au/Al2O3@Yne, and Au/TiO2@Yne. A comparison among the three materials revealed that silicaallowed the highest organic functionalization (12 wt %) as well as thehighest gold loading (3.7 wt %). Moreover, a TEM investigation showedonly for Au/SiO2@Yne the presence of homogeneously distributed spherically shaped AuNPs (average diameter 15 nm). Au/SiO2@Yne is an efficient catalyst, both in batch and flow conditions, in the oxidation of a large variety of alcohols using H2O2 asthe oxidizing agent at a temperature of 90 °C. Furthermore, under flow conditions, the catalyst worked for over 50 h without anysignificant decrease in the catalytic activity. The catalytic activity of the three catalysts was evaluated and compared in theoxidation of 1-phenylethanol as a model substrate. We found that the flow approach plays a strategic role in preserving thephysical and chemical integrity of the solid catalysts during its use, with remarkable consequences for the reaction conversion(from 2% in batch to 80% in flow) in the case of Au/TiO2@Yne.