Insights into the photoelectrocatalytic lignin oxidation to value-added products using a niobium-doped titanium dioxide photoanode

Abstract

Pulp and paper industry generates high quantities of black liquor (BL), a lignin-rich waste stream with the potential to be converted into value-added products, such as low-molecular-weight phenolic products (LMPPs). This study investigates the use of photoelectrocatalysis (PEC) as a promising, yet unexplored, process for lignin oxidation to produce LMPPs. For the first time, a photoanode based on niobium-doped titanium dioxide (TiO2:Nb) and a photoelectrochemical flow cell were applied. Lignin was isolated from Kraft BL (KBL) and used to prepare alkaline Kraft lignin (KL) solutions. The lignin and TiO2:Nb photoanode were thoroughly characterized before and after the PEC process. Six LMPPs, including aldehydes, aromatic acids, and ketones, were identified and quantified during lignin photoelectrocatalytic oxidation. Applying a constant cell potential of ~0.8 V transformed the lignin structure, yielding a maximum of 26±1 mg kg−1 of LMPPs after 7 h of reaction and a consumed charge of 5 C gLignin−1. The photoelectrocatalytic oxidation of KL predominantly yielded aromatic acids.