Ultrasound‐Enhanced Solvo‐Hydrothermal Synthesis of Metal‐Free Carbon Catalysts for Eco‐Friendly Visible‐Light‐Driven Oxidation

This study examines the optimization of an ultrasound‐assisted solvo‐hydrothermal route for the synthesis of metal‐free carbon materials and evaluates the role of sonication in the selective oxidation of the lignin‐derived monomer coniferyl alcohol, an important precursor to high‐value aromatic compounds. Sonication parameters applied during the preparation of the chitosan/lignin (C/L) system, including frequency, power, and irradiation time, were systematically varied to assess their influence on product yield, selectivity, and the structural and morphological properties of the resulting carbon materials. The as‐prepared materials were characterized by X‐ray photoelectron spectroscopy (XPS), Fourier‐transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Photocatalytic performance was investigated through the visible‐light‐driven selective oxidation of coniferyl alcohol (C on OH) to ferulic acid in acetonitrile. Among the materials examined, the sample synthesized under sonication at 22 kHz for 2 h exhibited the highest photocatalytic activity, affording 22% conversion of coniferyl alcohol and 35% selectivity toward ferulic acid after 4 h of irradiation, together with the formation of 68 µmol of H 2 O 2 . The improved performance is attributed to the beneficial effect of ultrasound during the solvo‐hydrothermal process, where acoustic cavitation is proposed to enhance mass transfer and promote pathways favorable to selective oxidation. In addition, the ultrasound‐assisted method outperformed conventional preparation approaches by reducing processing time and improving product quality. Overall, this work demonstrates that ultrasound is an effective tool for optimizing solvo‐hydrothermal synthesis and provides a sustainable strategy for the preparation of metal‐free carbon photocatalysts for the selective valorization of renewable feedstocks into fuels and other value‐added products.