C3N4 Impregnated with Porphyrins as Heterogeneous Photocatalysts for the Selective Oxidation of 5-Hydroxymethyl-2-Furfural Under Solar Irradiation

The photocatalytic oxidation of 5-hydroxymethyl-2-furfural (HMF) under natural solar irradiation was carried out in liquid–solid regime by using a batch reactor. Home prepared meso-tetra aryl substituted porphyrins (Pps) whether metal-free (H2Pp) or containing Cu(II) or Zn(II) complexes (CuPp, ZnPp), opportunely functionalized with lipophilic [(3-n-pentadecylphenoxyethoxy)-] chains, were supported on graphitic (g-C3N4) or thermo-exfoliated carbon nitride (C3N4-TE) to obtain a set of heterogeneous photocatalysts. The loading of the Pps on both carbon nitrides was 6 μmol per gram of support. During the experiments all the photocatalysts resulted active for the conversion of HMF, and in all cases its partial oxidation product, 2,5-furandicarboxyaldehyde (FDC), was selectively obtained in the aqueous suspension. Both carbon nitrides played the key role as photocatalysts whereas the presence of porphyrin improved the visible light absorption. The presence of the metal-porphyrin in the composite slightly increased the selectivity to FDC. The best compromise between selectivity to FDC (36%) and conversion of HMF (73%) was obtained in the presence of the ZnPp-C3N4-TE photocatalyst at natural pH. Moreover, at pH 9 the CuPp-C3N4-TE sample showed the best performances, indicating that the pH can affect the photocatalytic activity of the samples towards the partial selective oxidation of HMF to FDC.