Advancing CO2 utilisation via suspension-based carboxylation of single and mixed biomass-derived phenolics to produce high-value hydroxybenzoic acids

Production of organic chemicals from CO2 and biomass-derived feedstocks can combine the twin advantages of reducing carbon emissions and promote sustainable bioeconomy. This study explores a suspension-based Kolbe–Schmitt reaction for transforming CO2 into valuable hydroxybenzoic acids (HBAs). Using sodium salts of biomass-derived phenolic compounds (phenol, 2-cresol, guaiacol, syringol, and catechol) four carboxylation scenarios at 225 °C for 2 h under pCO2 = 30  bar were investigated. The reaction mixture and products were characterised in detail by high-performance liquid chromatography (HPLC) and nuclear magnetic resonance (NMR), revealing previously unreported species, which helped to elucidate the mechanisms of aromatic C-H activation for CO2 insertion. Mechanistic insights were validated by introducing precursor phenolic compounds, which dramatically enhanced the yields of salicylic acid (97.9 %), 2-cresotic acid (89.2 %), and guaiacol (89.9 %), while enhancing the purity of these main products. Notably, adding precursor phenolic compounds in carboxylation of catechol boosted the yield of 2,3-dihydroxybenzoic acid by up to 96 % and improved selectivity by 52.5 %, in 2 h of reaction. Furthermore, the study demonstrates that phenolic salts can act as carboxylating agents via sodium-proton substitution to facilitate new carboxylation possibilities. For example, reacting a mixture of the five phenolics favoured formation of dicarboxylation products, including industrially relevant 2,3-dihydroxyterephthalic acid and 2-hydroxyisophthalic acid. The results of this work underline the promise of integrating advanced reaction engineering with CO2 valorisation, as a sustainable circular economy pathway for carbon capture utilisation and storage (CCUS). Efficient production of different HBAs can drive their demand, ensuring rapid process development for enhanced CO2 utilisation.