Adjusting effect of additives on decatungstate-photocatalyzed HMF oxidation with molecular oxygen under visible light illumination

Abstract This paper discloses that decatungstate (DT) can efficiently catalyze the visible light-triggered oxidation of 5-hydroxymethylfurfural (HMF) with O2 in MeCN under normal temperature and pressure, affording 2,5-diformylfuran (DFF) and 2,5-furandicarboxylicacid (FDCA) as the oxygenated products. The photo-catalytic efficiency of DT anion depends on its counter cations-regulated synthetic quality and is adjusted significantly by the additives. Among the additives examined, NaBr, DMSO and [Bimi]Cl may obviously improve the carbon balance yield (CBY) of this photo-catalytic oxidation owing to their restraining effect on the light-induced HMF polymerization. Water and especially some strong acid solutions play a positive adjusting effect on this photo-catalytic oxidation and the best additive HBr can achieve 67.1% DFF and 5.8% FDCA yields as well as 87.7% CBY. A series of characterizations indicate that the donor-acceptor (D-A) interaction of DT with HMF or HBr is also regulated by its counter cations and contributes to photo-catalytic oxidation by improving the photo-physical and photo-chemical properties of DT. Notably, HBr has a stronger D-A interaction with DT than HMF and can be preferentially oxidized by the photo-excited DT to form a Br atom free radical, the latter thus plays a leading role in improving DFF yield, as supported by a series of control and quenching tests.