Efficient aerobic oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran on manganese oxide catalysts

• A 2,5-diformylfuran yield of 97.2% is achieved on OMS-2 for aerobic oxidation of 5-hydroxymethylfurfural (HMF).
• OMS-2 is superior to other MnO2 catalysts with different morphologies.
• A systematic study is carried out to identify the structural requirements for high HMF oxidation activity.
• Reaction mechanism and reaction elementary steps are proposed and verified.
• C–H bond cleavage and reoxidation of Mn3+ to Mn4+ are kinetically-relevant steps.

A cryptomelane-type manganese oxide octahedral molecular sieve with a (2 × 2, 4.6 Å × 4.6 Å) tunnel size (OMS-2) efficiently catalyzed aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) with a high yield of 97.2% at 383 K and 0.5 MPa O2 in N,N-dimethylformamide. OMS-2 was superior to other MnO2 catalysts with different morphologies, including OMS-1, OMS-6, and OMS-7 with various tunnel sizes, amorphous MnO2 and birnessite-type MnO2, apparently due to its (2 × 2) tunnel structure and consequently high reducibility and oxidizability. Kinetic and isotopic studies on OMS-2 showed near half-order dependence of the activities on HMF and O2 concentrations and marked kinetic isotope effects for deuterated HMF at its methylene group. These results, together with the similar initial rates under aerobic and anaerobic conditions, suggest that HMF oxidation to DFF on OMS-2 proceeds via a redox mechanism involving kinetically-relevant steps of C–H bond cleavage in adsorbed alcoholate intermediate, derived from quasi-equilibrated dissociation of HMF, using lattice oxygen and reoxidation of Mn3+ to Mn4+ by dissociative chemisorption of O2.

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