HMF was partially photo-oxidised to FDC in aqueous medium in the presence of g-C3N4. Thermal exfoliation of g-C3N4 enhanced both HMF conversion and selectivity to FDC. O2− was the main reactive species responsible for HMF oxidation to FDC. Exfoliated g-C3N4 showed better activity and selectivity to FDC under sunlight.