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.