Gold on cerium oxycarboante for allyl alcohol oxidation

The selective oxidation of allyl alcohols to the corresponding unsaturated carbonyl compounds is an important organic transformation, which should apply heterogeneous catalysts and oxygen or air as hydrogen acceptor in order to be considered a sustainable process. Promising heterogeneous catalyst are Pd , Ru and Au-based. Particularly, Au-based heterogeneous catalysts show good chemoselectivity in alcohol oxidation reactions, but exhibit only a sluggish activity in the absence of a Brønsted base, which aids the generation of the alcoholate intermediate. Hence the close approximity of a suitable Brønsted base site and Au-nanoparticles (NPs) is a requisite for a fast alcohol dehydrogenation reaction. We used cerium oxycarbonate monohydrate (Ce2O(CO3)2·H2O) as support for Au-NPs, which were generated by the metal vapor synthesis (MVS) approach (Scheme 1). For comparison reason, Au-NPs on CeO2 (nano-powder from Aldrich) were synthesized. TEM and XPS measurements carried out on both latter catalysts confirmed important differences: (i) The NPs' size of Au@Ce2O(CO3)2·H2O was significantly larger compared to that of Au@CeO2 (i.e. 4.0 vs 2.0 nm); (ii) The oxidation state of Au in Au@Ce2O(CO3)2·H2O was exclusively 0, while Au@CeO2 showed, as reported, oxidized Au species.[4] The application of both catalyst for the aerobic allyl alcohol oxidation to the corresponding carbonyl compounds conducted in toluene gave for Au@Ce2O(CO3)2·H2O a threefold higher catalytic activity compared to Au@CeO2. This beneficial solvent effect is absent, when methanol is used as reaction medium, due to its strong interactions with surface carbonate groups.