Aerobic oxidation of 5-Hydroxymethylfurfural (HMF) to 2,5-Diformylfuran (DFF) using O2 gas represents a sustainable approach for valorization of lignocellulosic compounds. As manganese dioxide (MnO2) is validated as a useful oxidation catalyst and many crystalline forms of MnO2 exist, it is critical to explore how the crystalline structures of MnO2 influence their physical/chemical properties, which, in turn, determine catalytic activities of MnO2 crystals for HMF oxidation to DFF. In particular, six MnO2 crystals, α-MnO2, β-MnO2, γ-MnO2, δ-MnO2, ε-MnO2, and λ-MnO2 are prepared and investigated for their catalytic activities for HMF oxidation to DFF. With different morphologies and crystalline structures, these MnO2 crystals possess very distinct surficial chemistry, redox capabilities, and textural properties, making these MnO2 exhibit different catalytic activities towards HMF conversion. Especially, β-MnO2 can produce much higher DFF per surface area than other MnO2 crystals. β-MnO2 could achieve the highest CHMF = 99% and YDFF = 97%, which are much higher than the reported values in literature, possibly because the surficial reactivity of β-MnO2 appears to be highest in comparison to other MnO2 crystals. Especially, β-MnO2 could exhibit YDFF > 90% over 5 cycles of reusability test, and maintain its crystalline structure, revealing its advantageous feature for aerobic oxidation of HMF to DFF. Through this study, the relationship between morphology, surface chemistry, and catalytic activity of MnO2 with different crystal forms is elucidated for providing scientific insights into design, application and development of MnO2-based materials for aerobic oxidation of bio-derived molecules to value-added products.
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