Abstract

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 (ConOH) 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 H₂O₂. 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.