H2O2 generated through ORR on cathode in a protic ionic liquid and its utilization in lignin valorization

Abstract

The quantitative study of lignin valorization by H 2 O 2 generated through ORR process on the cathode in [Et 3 NH][HSO 4 ] was performed, that is approximately 20 molecules of H 2 O 2 were needed to produce one molecule of vanillin from lignin depolymerization. • H 2 O 2 generated through ORR process on cathode in electrolyte of [Et 3 NH][HSO 4 ]. • Application of H 2 O 2 electro-generated in situ to lignin valorization. • About 2 molecules of H 2 O 2 needed for breaking one α-O-4 ether bond in PBP molecule. • About 19.4 molecules of H 2 O 2 required for producing one molecule of vanillin from lignin. Lignin valorization to obtain small aromatic compounds requires selective oxidation to break the linkage bonds among the C9 units in lignin molecules. It has been reported that hydrogen peroxide (H 2 O 2 ) generated through the oxygen reduction reaction (ORR) on the cathode could be an appropriate oxidizing reagent to depolymerize lignin, but the current efficiency was still unsatisfactory. In this study, a protic ionic liquid [Et 3 NH][HSO 4 ] is used as electrolyte instead of aqueous solution, and the generation rate of H 2 O 2 is determined as 0.0563 mmol cm −2 min −1 and the current efficiency of 79.7 % by the Ti-TPyP colorimetric method. Then using p -benzylethoxyphenol (PBP) as a lignin model compound, it is figured out that approximately-two molecules of H 2 O 2 are consumed for breaking one α-O-4 ether bond. Finally, in the electrolysis of an actual lignin sample, it is evaluated that about 19.4 molecules of H 2 O 2 are needed for producing one molecule of vanillin when taking vanillin as a representative degradation product of lignin.