Production of syringic acid by direct CO2 insertion into syringol via a Kolbe–Schmitt type reaction

Abstract

Syringic acid (4-hydroxy-3,5-dimethoxybenzoic acid) is a valuable hydroxybenzoic acid with applications in pharmaceuticals, food additives, and polymeric materials such as poly(phenylene oxide) (PPO). Despite its industrial relevance, current synthesis methods are reported only in patents, relying on expensive precursors such as syringaldehyde and multi-step chemical reactions, leading to high production costs and chemical waste. In contrast, syringol, a biomass-derived phenolic compound, is commercially available at half the cost of syringaldehyde, making it a more economical alternative. This study presents, for the first time, a one-pot method for the direct carboxylation of syringol to produce syringic acid. Initial experiments using sodium syringolate (SyONa) and the conventional Kolbe–Schmitt reaction yielded only 0.53% syringic acid. However, at 225 °C and pCO₂ between 5 and 50 bar over 2–6 hours, the addition of guaiacol or potassium carbonate (K₂CO₃) significantly promoted the yield of syringic acid. Guaiacol enhanced conversion but led to substantial by-product formation, whereas K₂CO₃ improved selectivity to syringic acid (up to 90%) and increased the yield to 39.2% after 6 h. A mechanistic analysis indicates that K₂CO₃ activates the para-C–H bond of syringolate more effectively than guaiacol, enabling a previously inaccessible carboxylation pathway. By leveraging CO₂ as a reactant and biomass-derived feedstock, this work aligns with Green Chemistry principles, reducing reliance on costly reagents and minimising chemical waste. These findings not only offer a sustainable route for syringic acid synthesis but also open new possibilities for the large-scale production of green PPO, advancing the development of bio-based polymers for a sustainable future.