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

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Highlights

Oxidation of four lignin monomers and lignin by Fenton and UV-Fenton processes were investigated.

Various aromatic/non-aromatic carboxylic acids with mono/di carboxyl groups were determined.

Depending on the oxidation stages, formation and concentration of these compounds varied.

Oxidation products from the lignin fraction had carboxylic acid products in low percentages.

Lignin should be depolymerized with an effective method prior to oxidation with Fenton processes.

Abstract

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 Products in samples withdrawn from the reactor mixture at different oxidation stages were determined by GC-MS.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.

Keywords

Lignin

Carboxylic acids

Oxidation

AOPs

Fenton

UV-Fenton

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