Imidazole-Functionalized Polyoxometalate Catalysts for the Oxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran Using Atmospheric O2

Biomass as a sustainable and abundant carbon source has attracted considerable attention as a potential alternative to petroleum resources. The selective oxidation of 5-hydroxymethylfurfural (HMF), a versatile platform molecule, to value-added 2,5-diformylfuran (DFF) provides an efficient pathway for biomass valorization. Herein, three discrete imidazole-functionalized polyoxometalates (POMs), (HPMo8V4O40)-O-VI((VO)-O-V)(2)[((VO)-O-IV)(IM)(4)] (2)center dot nH(2)O center dot(IM)(m) (IM = 1-methylimidazole, n = 4, m = 8 for 1; IM = 1-ethylimidazole, n = 4, m = 9 for 2; IM = 1-propylimidazole, n = 0, m = 4 for 3), have been successfully synthesized by a facile solvothermal method and thoroughly characterized by routine techniques. Compounds 1-3 contain a bi-capped pseudo-Keggin {HPMo8V4O40(VO)(2)} and two imidazole-functionalized {(VO)(IM)(4)} groups, which, to our knowledge, represent the first examples of organic-functionalized Mo-V clusters. Compounds 1-3 as heterogeneous catalysts can effectively promote the transformation of HMF to DFF using atmospheric O-2 as oxidant. Under minimally optimized conditions, 95% of HMF was converted by 1 with 95% selectivity for DFF and its catalytic activity was basically maintained after five cycles. Moreover, the important roles of the bi-capped pseudo-Keggin cluster and the functionalized V groups in the selective oxidation of HMF have been explored. According to experimental and spectroscopic results, a three-step oxidation mechanism of HMF to DFF has been proposed.

Automated summary

In this study, three imidazole-functionalized polyoxometalates were successfully synthesized as heterogeneous catalysts for the conversion of HMF to DFF. The catalysts showed high efficiency and selectivity, maintaining activity after multiple cycles. The study also proposed a three-step oxidation mechanism for the transformation of HMF to DFF.