Photocatalytic Partial Oxidation of 5‑Hydroxymethylfurfural to 2,5-Diformylfuran Using Exfoliated g‑C3N4/Pd Nanoarchitectures

Graphitic carbon nitride (g-C3N4) can trigger the photocatalytic partial oxidation of HMF to produce 2,5-diformylfuran (DFF), yet limitations still exist in improving its performance. Herein, Pd nanoparticles were deposited over thermally exfoliated g-C3N4 (Ex-g-C3N4) to boost its activity. Graphitic sheets with a thickness of ∼1.2–1.6 nm were obtained and then decorated with spherical Pd nanoparticles with a size of 2.0–2.5 nm. A maximum concentration of DFF (148.5 ± 4.9 μM) was obtained over the Ex-g-C3N4/Pd with 6% (w/w) deposited Pd with a product selectivity toward DFF of 42.9 ± 0.6%. Our results revealed that the extent of DFF production is directly proportional to the Pd content until 6% (w/w), and beyond that it declines. Trapping experiments and EPR analysis showed that singlet oxygen is the dominant species responsible for the selective oxidation of HMF to DFF. Our mechanistic study suggests that triplet excitons are generated via intersystem crossing, which then react with molecular oxygen to produce singlet oxygen. Furthermore, intrinsic reaction coordinate and energy barrier calculations indicated that the reaction is thermodynamically allowable. Our study reveals that singlet oxygen is capable of abstracting hydrogen atoms from the α carbon and hydroxyl moiety of HMF to eventually produce DFF. This work provides deep insight into the benefits of exploiting Pd nanoparticles to enhance production of singlet oxygen over g-C3N4, which in turn enhances the production of DFF.