Efficient lignin depolymerization by continuous flow microreactor-assisted electrochemical advanced oxidation in water/co-solvent system

Abstract

This study presented for the first time a continuous flow microreactor system for efficient dealkaline lignin depolymerization applying an electrochemical advanced oxidation process (EAOP). The insolubility of dealkaline lignin in water poses a challenge for its chemical depolymerization via EAOP. To address this limitation, several co-solvent systems were developed and investigated to achieve efficient dealkaline lignin depolymerization. Five water-miscible organic solvents, namely methanol (MeOH), ethanol (EtOH), acetonitrile (MeCN), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF), were used as co-solvents. With the co-solvent fraction of 1% in water, the solubility of dealkaline lignin followed the order of THF > DMF > MeCN > MeOH > EtOH. The electron spin resonance (EPR) of various co-solvents revealed that different co-solvents affected the concentration of in situ OH radical generation, leading to the difference in dealkaline lignin conversions. For dealkaline lignin in water–MeCN system, 72% conversion was observed within 100 s of residence time under ambient conditions with a 1 mA current. Moreover, the two main lignin model compounds (2-phenoxy-1-phenyl ethanol (PPE) containing β-O-4 and benzyl phenyl ether (BPE) containing α-O-4) were used to understand the kinetic analysis and degradation of dealkaline lignin. High conversions of 75% and 92% were achieved for the β-O-4 (PPE) and α-O-4 (BPE) lignin model compounds, respectively. All the products were confirmed using GC-MS analysis. Additionally, mechanistic insights for lignin depolymerization were provided using lignin model compounds.