Carbon black-polydopamine-ruthenium composite as a recyclable boomerang catalyst for the oxidative cleavage of oleic acid

Abstract

A new composite heterogeneous catalytic system is proposed for the transformation of cheap unsaturated fatty acids into value-added molecules through their oxidative cleavage. The valorization of oleic acid to azelaic and pelargonic ones (which are demanded in pharmacology) is specifically studied. The reaction is efficiently performed with a novel composite catalyst inspired from metal recovery technology, namely Ru species supported on a carbon black that was previously oxidized and then functionalized with polydopamine. Being a polymer with hydroxyl and amine groups polydopamine provides moieties acting as tweezers able to complex Ru species. A representation of this novel composite catalyst is built from its characterization by FTIR, TGA, N2 physisorption TEM and XPS. Using NaIO4 as oxidizing agent in a biphasic system at room temperature the conversion of oleic acid, and selectivity towards pelargonic and azelaic acids, reached around 95% after 3 h. The reaction mechanism is explored by UV–Vis spectrophotometry, ICP elementary analysis and NMR spectroscopy, together with recyclability and hot centrifugation tests. The occurrence of a boomerang mechanism is demonstrated. The solid catalyst is indeed recyclable at least 5 times without activity loss, but at the same time Ru species are shown performing the reaction in homogeneous phase namely in a dissolved state. Actually, polydopamine is confirmed to recover Ru species once the reaction ends, guarantying the recyclability of the catalyst. The composite catalyst therefore combines the advantages of homogeneous and heterogeneous catalysis in the investigated reaction.