Electrochemical conversion of lignin to short-chain carboxylic acids

Abstract

Lignin valorization is a challenge for chemical and materials scientists due to its complex molecular structure and intra- and inter-molecular assembly. Although catalytic conversion of lignin into small aromatic organic molecules has been widely investigated, it faces the problems of low selectivity and complex products. Herein, we report a direct electrochemical conversion method for the selective oxidation of industrial lignin to produce short-chain carboxylic acids (SCCAs) with a wide range of concentrations. In an electrolytic cell, lignin is chemically converted to SCCAs by H₂O₂ with titanium silicate-1 (TS-1) as a catalyst. H₂O₂ is generated in situ by the recombination of H⁺ and HO₂⁻ in a three-compartment cell by separately introducing hydrogen (H₂) and oxygen (O₂) as the sources of protons and HO₂⁻ into the anode and cathode compartments. We have achieved a record-high concentration of SCCAs up to 109.8 mg mL⁻¹ through the catalytic depolymerization of lignin. A high concentration of SCCAs of 80.1 mg mL⁻¹ could also be achieved at a cell voltage of 2.13 V by coupling the 2e⁻-ORR and water oxidation using air and 1 M aqueous H₂SO₄ solution as feedstocks. Our approach could eliminate the additional economic consumption and environmental hazards caused by the transportation and storage of commercially concentrated and stabilized H₂O₂. Aside from the SCCAs produced in this process, the formation of new carboxyl groups in the lignin residue during the oxidation reaction greatly broadens the application potential of industrial lignin.