Optimisation of Reaction Temperature during Carboxylation of Single and Mixed Model Bio-derived Phenolics as Effective Route for CO2 Utilisation

This study investigates the temperature-dependent carboxylation of single and mixed biomass-derived phenolic sodium salts with CO₂ via the Kolbe–Schmitt reaction. Reactions were performed at T = 175–225 °C, t = 2 h, and pCO₂ = 30 bar. Five model phenolics; phenol, 2-cresol, guaiacol, catechol, and syringol were examined individually and in mixtures. Characterisation via high-performance liquid chromatography (HPLC) and nuclear magnetic resonance (NMR) analysis showed that 2-hydroxybenzoic and dicarboxylic acids were favoured at higher temperatures, while 4-hydroxybenzoic acids dominated at 175 °C. In mixtures, dicarboxylic acid yields increased significantly, reaching 41.9% for 2,3-dihydroxyterephthalic acid and 20.5% for 2-hydroxyisophthalic acid. These dicarboxylic acids possess up to 10-fold higher market value than their monocarboxylic counterparts. Syringic acid synthesis via Kolbe–Schmitt is reported here for the first time, with yields rising to 33.0% in mixtures versus <2.0% molar yield when reacted individually. The study also presents the first detailed mechanistic explanation of Brønsted acid–base interactions and temperature-driven selectivity in phenolic salt carboxylation. While previous research suggested that producing phenolics solely from lignin was not viable, this work demonstrates that CO₂ incorporation not only enhances product value but also narrows product distribution and enables broader industrial applicability - ultimately opening new opportunities for potential large-scale, economically viable CO₂ utilisation.