Wet partial oxidation of guaiacol into carboxylic acids using hydrogen peroxide

The Wet Partial Oxidation (WPO) of guaiacol, a primary product of hydrothermal decomposition of lignin, was carried out using an injection batch reactor (99 mL). Hydrogen peroxide was used as oxidant and the process was investigated at different temperatures (150 to 300 °C) and reaction times (1 to 25 min), for given oxygen supply (40 %) and pressure (100 bar). At the tested conditions, six C2-C5 dicarboxylic acids (DCA: oxalic, malonic, maleic, succinic, fumaric, and glutaric acid) and three short-chain monocarboxylic acids (MCA: formic, acetic, and lactic acid) were identified and quantitated. Dihydroxybenzenes, aromatic aldehydes, and aromatics acids were also identified at lower temperatures and reaction times, whereas they disappeared at higher reaction severity. The total carbon-based yield of MCA + DCA was in the range of 4 % to 19 %, showing higher values at intermediate reaction severity. Overall, these preliminary results show that MCA and DCA are intermediate products of complex reaction schemes, involving formation of 1-ring oxygenated aromatics, ring-opening reactions to DCA, and decomposition to short-chain MCA. The results suggest that the WPO of lignin can be a source of MCA and DCA in good yields, but fine tuning of reaction conditions (temperature, time, oxygen supply) is necessary.