Kraft Lignin Electro-Oxidation under Ambient Temperature and Pressure

Abstract

Lignin is the largest natural source of aromatic chemicals. Due to its complex polymeric structure, Kraft lignin is under-utilized and usually combusted for heat generation, thus resulting in CO2 emissions in the Kraft process. To valorize lignin with renewable electricity and to convert it into value-added aromatic chemicals, efficient electrochemical methods need to be discovered, based not only on its apparent yield of building block chemicals but also on its energy efficiency. In this study, the electro-oxidative performance of six different metals was systematically evaluated. The results showed that the synthesized Ni-based catalyst can increase the vanillin and vanillic acid apparent yield by 50–60% compared to when Ni-based catalyst is absent. We also found that the oxygen evolution reaction (OER) is more than a competing reaction since the presence of oxygen synergistically aids oxidation of the lignin to increase aromatic chemical production by 63% compared to the sum of vanillin generation from both voltage-only and oxygen-only scenarios. With the novel proposed notion of charge efficiency, we showed that utilizing a thinner layer of Ni catalyst balances the OER and the oxidative reaction of lignin, thus improving the charge efficiency for vanillin by 22%