Au/CaO as a highly active heterogeneous catalyst for aerobic oxidation of aliphatic alcohols in water-based mild conditions

The oxidation of alcohols to carboxylic acids is an important reaction in chemistry, which finds use in the production of pharmaceuticals, polymers, surfactants, cosmetics and textiles, to name a few. As an example, oxidation of 1,4-butanediol into succinic acid opens the path towards synthesis of bio-based Nylon 4-4, γ-butyrolactone, tetrahydrofuran and maleic anhydride. Here, we present a 3 wt.% Au/CaO catalyst capable of quantitatively converting aliphatic alcohols, starting with butanol as a model substrate, to the corresponding acids with total selectivity at 60 °C in pure water under ambient atmosphere. The substrate's scope was extended to longer aliphatic chains, up to 8 carbons, with excellent final yield and total selectivity for acid formation. Dicarboxylic acid formation was also tested with 1,4-butanediol leading to succinic acid in over 90 % yield at 5 mM. A catalyst recycling study was conducted, showing consistent yields over 2 cycles, however significant deactivation was observed from the 3rd cycle onwards. Finally, a screening of saline buffers was carried out to consider the future combination of the catalyst with an enzymatic reaction in hybrid systems. Only MOPS maintained catalytic activity, but the use of neutral pH buffers resulted in faster catalyst deactivation.