Conversion of lignin into renewable carboxylic acid compounds by advanced oxidation processes

The present study was designed to investigate formation of possible carboxylic acids from oxidation of lignin by Fenton and UV-Fenton advanced oxidation processes (AOPs). The study was performed with model lignin monomers (syringaldehyde (SA), 4-hydroxybenzaldehyde (4-HBA), 2-methoxyphenol (2-MP) and 2,6-dimethoxyphenol (2,6-DMP)) and also with lignin isolated from eastern red cedar. Products in samples withdrawn from the reactor mixture at different oxidation stages were determined by GC-MS. Oxidation of one of lignin monomer (2-MP) was also performed with a different AOPs (UV/TiO₂) for comparison of oxidation products. Depending on the oxidation stages, various carboxylic acids having aromatic and non-aromatic structures with mono- or di-carboxyl functional groups were determined. Performing oxidation of 2-MP with UV/TiO₂ led to formation of 2-hydroxypropanoic acid as a low-carbon chain monocarboxylic acid which was not formed in Fenton and UV-Fenton. Oxidation products from the lignin fraction had mono- and di-carboxylic acid products in low percentages.