Selective oxidation of 5-hydroxymethylfurfural with H2O2 catalyzed by a molybdenum complex

Abstract

Organic solvent free 5-hydroxymethylfurfural (HMF) oxidation into 2,5-furandicarboxylic acid (FDCA) with hydrogen peroxide using quaternary ammonium octamolybdate and quaternary ammonium dectungstate was studied. Tetra-1-ethyl-3-methylimidazolium octamolybdate ([EMIM]₄Mo₈O₂₆), tetra-hexadecyltrimethyl ammonium octamolybdate ([CTAB]₄Mo₈O₂₆) and tetra-ethylpyridinium octamolybdate ([EPy]₄Mo₈O₂₆) displayed high activity for the selective oxidation of HMF to FDCA, and the selectivity of FDCA could reach 100% with a 99.5% conversion of HMF in the presence of [EMIM]₄Mo₈O₂₆. The byproduct formed in competition with FDCA was identified as the intermediate 5-hydroxymethyl-2-furan carboxylic acid (HMFCA) and 5-formyl-2-furan carboxylic acid (FFCA), and neither 2,5-diformyl furan (DFF) nor any other byproducts from the oxidative cleavage of the HMF furan ring were detected during the oxidation process, which indicated that the aldehyde group of HMF oxidizes first, followed by the oxidation of the hydroxymethyl group in this reaction system. Although the quaternary ammonium salts, such as [EMIM]Br, EPyBr and CTAB, prevented FDCA formation from the HMFCA advanced oxidation, they could eliminate the oxidative cleavage of the furan ring and improve the affinity of HMF and catalysts, to make the catalytic active centers readily accessible to HMF molecules. However, tetra-1-ethyl-3-methylimidazolium dectungstate ([EMIM]₄W₁₀O₃₂), tetra-hexadecyltrimethyl ammonium dectungstate ([CTAB]₄W₁₀O₃₂) and tetra-ethylpyridinium dectungstate ([EPy]₄W₁₀O₃₂) were unfavorable for FDCA formation. The great difference in performance of quaternary ammonium octamolybdate and quaternary ammonium dectungstate in HMF oxidation with H₂O₂ was attributed to their different structure.