Synergistic catalysis of graphitic carbon nitride and dichromate for augmented oxidative polymerization of hydroquinone leading to enhanced humic-like substance formation

Background: Humic substances (HS) originate from organic matter decomposition, primarily through polyphenol polymerization (known as the browning reaction). The slow reaction kinetics have posed challenges in studying the catalytic oxidation of polyphenols by metal oxides. A substantial knowledge gap persists regarding the role of metal oxides and other catalysts in polyphenol polymerization. Method: Herein, we focused on the oxidative polymerization of hydroquinone (HQ) using potassium dichromate (Cr₂O₇²⁻) to accelerate reaction rates and gain insights into HS formation. Moreover, graphitic carbon nitride (g-C₃N₄) was introduced as an N-doping compound with a substantial surface area and Lewis base properties, enriching the N source and promoting π-π interactions. Significant findings: The addition of g-C₃N₄ to the metal oxide systems tripled reaction rates, enhanced browning reactions, and complex stability. Furthermore, it improved HS surface morphology and particle size, with g-C₃N₄'s electron transfer capabilities facilitating the in-situ HQ oxidation. Empirical data and theoretical calculations demonstrated that the g-C₃N₄…HQ…Cr₂O₇²⁻ complex exhibits greater binding energy than HQ… Cr₂O₇²⁻, explaining the accelerated reaction kinetics. Post-reaction ¹³C NMR spectra confirmed the predicted HS spectra. This work highlights g-C₃N₄'s catalytic role in HQ oxidative polymerization and HS generation, shedding light on abiotic humification and environmental implications.