Selective Homogeneous Monohydrogenation of Muconic Acid and Muconates via Ru-Catalyzed Transfer Hydrogenation

Abstract

The hydrogenation of muconic acid (MA), a biobased platform molecule, offers a sustainable pathway to adipic acid (AA), a key industrial dicarboxylic acid. In this study, we explore the catalytic transfer hydrogenation (CTH) of muconic acid and muconates using ionic ruthenium complexes. Unlike previous approaches, our method aims to selectively hydrogenate MA toward the monounsaturated compound hexenedioic acid (HDA) or its ester. Alcohols (methanol and ethanol) are employed as a hydrogen donor, providing a safer and more moderate alternative to H₂ gas. Using hydrated RuCl₃ as the catalyst, the reaction successfully produces HDA, with no over-hydrogenation toward AA observed. The resulting product mixture comprises up to four different HDA isomers, which were all identified and distinguished by GC, GC-MS-FID, and ¹H-NMR methods. The trans-2 isomer was the most abundant, which was supported by mechanistic investigation using isotopically labeled experiments. A 2,5-hydrogenation mechanism following a monohydride reaction cycle could be suggested. Furthermore, a kinetic model is presented to provide a deeper understanding of the various reaction pathways.