Niobium-based homogeneous and heterogeneous catalysts were prepared by immobilizing triperoxidoniobium, [Nb(O 2 ) 3 ] À species on poly(sodium methacrylate), a linear water-soluble polymer, and glycine functionalized poly(styrene-divinylbenzene), a cross-linked polymer resin, respectively. The compounds were characterized by elemental analysis, NMR ( 1 H, 13 C, and 93 Nb), FTIR, Raman, XRD, XPS, SEM, EDX, UV-Visible, BET, and TGA analysis. The catalysts were successfully applied in the targeted synthesis of 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) via 5-hydroxymethylfurfural (HMF) oxidation with H 2 O 2 as oxidant, under eco-compatible reaction condition. Both the catalysts displayed excellent activity with 100 % conversion of HMF to yield HMFCA with 100 % selectivity. The oxidation protocol provided the highest TON and TOF values of 50 and of 78 h À 1 , respectively. The heterogeneity of the insoluble catalyst was confirmed by a hot filtration test. The heterogeneous catalyst showed impressive recyclability at least up to five consecutive cycles without any significant alteration in its activity or HMFCA selectivity. Overall, the use of water as a solvent, environmentally benign H 2 O 2 as oxidant, reasonably low catalyst: substrate ratio, workable reaction temperature (60 °C), and acceptable reaction time (1.5 h), are the distinct green attributes of the developed catalytic procedure.
