Direct Carboxylation Reactions of Lignin-Derived Biobased Aromatics or Lignin with Carbon Dioxide (CO 2 )

Advancing CO 2 utilization and lignin valorization is essential for developing sustainable routes to aromatic chemicals while reducing the dependence on fossil carbon.This study systematically investigates the carboxylation reactivity of 14 ligninderived hydroxybenzenes using Kolbe-Schmitt and Marasse methodologies to convert CO 2 into value-added aromatic acids.FTIR evaluation of metal phenoxide formation showed that the Kolbe-Schmitt precursor step is highly substrate-dependent and often ineffective for several methoxy-and alkyl-substituted phenols, whereas the Marasse single-pot method with K 2 CO 3 provides a more reliable route for all 14 substrates.Comprehensive screening of phenol, catechol, resorcinol, hydroquinone, o-, m-, and p-cresol, anisole, guaiacol, 4-methoxyphenol, syringol, 4-propylphenol, eugenol, and 2-methoxy-4-propylphenol under 150-200 °C, 30 bar of CO 2 , and controlled reaction times (1, 2, and 3 h) revealed the broad applicability of the Marasse method.Numerous monocarboxylated products and several dicarboxylated derivatives were obtained, including the first chemical evidence of double carboxylation for resorcinol and extended reactivity for catechol.Methoxyphenols required elevated temperatures, and anisole remained unreactive, confirming the necessity of a free OH group for electrophilic CO 2 insertion.Direct carboxylation of organosolv lignin was investigated, with 31 P NMR, 2D HSQC, and SEC analyses demonstrating initial -COOH incorporation at 150 °C.However, this functionalization was accompanied by β-O-4 cleavage followed by dominant condensation and repolymerization reactions at prolonged times or higher temperatures, limiting the overall effectiveness of direct lignin carboxylation under the investigated conditions.Overall, this work delivers the first unified reactivity map for 14 lignin-derived aromatics across varied temperature and time regimes, establishing a foundation for optimizing selective CO 2 -based carboxylation toward industrially relevant aromatic acids.