CO2 Utilisation for Effective Production of Hydroxybenzoic Acids via Suspension-based and Gas-Solid Carboxylation Methods

Abstract

Hydroxybenzoic acids (HBAs) are essential compounds widely used in pharmaceuticals, cosmetics, fragrances, flavouring, and plastics manufacturing. Industrially, they are produced via the conventional Kolbe-Schmitt reaction, where phenolic compounds react with CO₂. However, this process remains time-intensive, inefficient due to significant side-product formation, and limited by batch-mode operation. Furthermore, HBA production is heavily reliant on petroleum-based chemicals, raising sustainability concerns. Despite being in use for over a century, the reaction mechanism remains unclear, and no substantial advancements have been made to improve its efficiency. This thesis addresses these challenges by first overcoming the inherent batch-mode limitations of the Kolbe-Schmitt reaction through the development of a novel suspension-based process for phenolic carboxylation. Experimental results demonstrated that salicylic acid yields of up to 40.5% were achieved within just 2 hours using toluene as a suspension medium. Comparative studies of the conventional and suspension-based Kolbe-Schmitt reaction at various reaction times enabled the proposal of a new reaction mechanism, which was further validated. It was found that the addition of free phenolics enhances selectivity and significantly increases product yield in a much shorter time. For instance, using sodium catecholate, the reaction selectivity improved by 52.5% while reducing reaction time over 20-fold compared to the industrial standard. Additionally, the effect of temperature on the conventional Kolbe-Schmitt reaction was investigated, providing a comprehensive characterisation of all possible carboxylation products for various phenolics (phenol, 2-cresol, catechol, syringol, and guaiacol). Carboxylating mixtures of these phenolics, mimicking bio-oil, revealed the significant formation of dicarboxylic acids, which are crucial for polymer synthesis. Integrating these findings provides critical insights into phenolic carboxylation, enhancing both mechanistic understanding and process efficiency. This further accelerates the plausibility to transition towards obtaining phenolics from bio-oil to produce long-term CO₂ storage materials, contribution to carbon capture, utilisation, and storage (CCUS) strategies in achieving a negative-emission future.