Lignin depolymerization with alkaline ionic liquids and ethylene glycol in a continuous flow reactor

A novel alkaline ionic liquids N-allylpyridinium hydroxide ([APy]OH) was successfully designed and developed as a catalyst and co-solvent with ethylene glycol in the continuous depolymerization of lignin. Various reaction conditions, like lignin concentration, water content, and reaction temperature, were investigated under atmospheric pressure. Ethylene glycol was used as main solvent and repolymerization inhibitor of the aldehydes produced. The combination of [APy]OH and alkali metal salts exhibited a good synergistic effect for lignin depolymerization. Then, the depolymerization of three different lignins, including dealkaline lignin (DA-lignin), alkaline lignin (A-lignin), and sodium lignosulfonate (S-lignin), were also studied. S-lignin exhibited the highest yields of total liquid products (21.3 %) and aromatics (10.7 %). The change in lignin structure after depolymerization was observed by means of FT-IR and DTG-DTA techniques. Lastly, a hypothetical pathway for lignin depolymerization was proposed based on the catalytic results of lignin model compounds (LMCs) with different linkages.