Highly selective oxidation of furfural to maleic acid with a Brønsted–Lewis dual-acid system

Abstract

BACKGROUND

Maleic acid (MAc) is a high value-added oxygenic hydrocarbon derivative monomer. The synthesis of MAc from biomass-based furfural is a green process and has important academic research value. Herein, selective catalytic oxidation of renewable furfural to MAc was investigated.

RESULTS

The effect of using different heteropoly acids as catalyst and different organic acids/H₂O as solvent was evaluated first, and the ‘phosphomolybdic acid + acetic acid/H₂O’ composite system with good performance was screened out. Acetate anion acted as free radical acceptor in the reaction to stabilize furfural radical and inhibit the polymerization reaction. Under optimal reaction conditions, the conversion of furfural was 97.8%, and the MAc yield and selectivity were 58.3% and 59.6%, respectively. To further improve the selectivity of MAc, different kinds of Lewis acids were introduced to construct a Brønsted–Lewis dual-acid catalytic system. When Cu(OAc)₂ was as Lewis acid, the MAc selectivity was significantly improved to 72.5% with a furfural conversion of 72.3%. The catalytic mechanism of Brønsted–Lewis dual-acid system was finally clarified.

CONCLUSION

In this complex system, the synergy of Cu and Mo is responsible for the outstanding catalytic efficiency, where the Mo^VI/Mo^V redox can be facilitated by the redox of Cu^II/Cu^I with the assistance of O₂; acetate anion can interact with furfural radicals, thereby delaying the chain reaction of free radicals. This process makes the MAc generation reaction more dominant than the polymerization reaction, thus improving MAc selectivity. © 2025 Society of Chemical Industry (SCI).