Electrochemical conversion of lignin to short-chain carboxylic acids
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.
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