Effective Chemical Fixation of CO₂ into Phenol Using Conventional Gas-Solid and Novel Suspension-Based Kolbe-Schmitt...

The Kolbe-Schmitt reaction is a classic route for CO₂ utilisation through organic synthesis of industrially relevant chemicals. Despite the reaction's long-standing history, detailed product separation and analysis remain underexplored, which inherently limits an accurate mechanism elucidation. This study introduces a new comprehensive approach for isolating and characterising reaction products using high-performance liquid chromatography (HPLC) and proton nuclear magnetic resonance (¹H-NMR). Comparative experiments at 225 °C and 30 bar CO₂ were carried out using the conventional gas-solid and novel suspension-based methods with varying reaction times. A new two-step reaction mechanism is proposed. In the first step, 2-disodium salicylate and phenol are formed. In the second step, 2-monosodium salicylate is formed, with subsequent regeneration of sodium phenoxide. This mechanism was validated by adding pure (free) phenol to the reaction media in both conventional and suspension-based methods. The presence of added phenol was found to increase the yield of salicylic acid by 25.0 % and 8.5 % after 2 hours of reaction, for each method, respectively, compared to experiments without added phenol. Successful application of these enhanced carboxylation methods to other biomass-derived single-ring phenolic compounds will offer new routes for potential large-scale CO₂ utilisation.