Potential Large-Scale CO2 Utilisation for Salicylic Acid Production via a Suspension-Based Kolbe–Schmitt Reaction in Toluene

Abstract

Conversion of CO2 into organic chemicals offers a promising route for advancing the circularity of carbon capture, utilisation, and storage in line with the international 2050 Net Zero agenda. The widely known commercialised chemical fixation of CO2 into organic chemicals is the century-old Kolbe–Schmitt reaction, which carboxylates phenol (via sodium phenoxide) into salicylic acid. The carboxylation reaction is normally carried out between the gas–solid phases in a batch reactor. The mass and heat transfer limitations of such systems require rather long reaction times and a high pressure of CO2 and are often characterised by the low formation of undesirable side products. To address these drawbacks, a novel suspension-based carboxylation method has been designed and carried out in this present study, where sodium phenoxide is dispersed in toluene to react with CO2. Importantly, the addition of phenol played a critical role in promoting the stoichiometric conversion of phenoxide to salicylic acid. Under the optimal conditions of a phenol/phenoxide molar ratio of 2:1 in toluene, a reaction temperature of 225 °C, a CO2 pressure of 30 bar, a reaction time of 2 h, and stirring at 1000 rpm, an impressive salicylic acid molar yield of 92.68% has been achieved. The reaction mechanism behind this has been discussed. This development provides us with the potential to achieve a carboxylation reaction of phenoxide with CO2 more effectively in a continuous reactor. It can also facilitate the large-scale fixing of CO2 into hydroxy aromatic carboxylic acids, which can be used as green organic chemical feedstocks for making various products, including long-lived polymeric materials.