Tunable and Functional Deep Eutectic Solvents for the Highly Selective Production of Methoxy-Free Lignin Monomers via Fast Pyrolysis

The highly selective conversion of technical lignin to low-oxygen-content compounds is still challenging due to the presence of methoxy and ether bonds in natural lignin. Herein, tunable ternary deep eutectic solvents (DESs) containing choline chloride, oxalic acid, and another bifunctional reagent with a hydrogen-bond donor and nucleophilic function were used for the β-aryl ether bond cleavage or Cα-etherification of lignin followed by pyrolysis to improve the yield of methoxy-free lignin monomers. The relationship between the lignin structure and pyrolysis behavior was investigated by 2D-NMR, FTIR, GPC, XPS, TGA, TG-FTIR, EPR, etc. The DES pretreatment changed the pathway of lignin pyrolysis toward a higher yield of bio-oil and methoxy-free products. Compared with technical lignin, the yield of phenolic products after Cα-etherification with ethanol and ethylene glycol as nucleophilic reagents increased from 0.73 × 1011 to 4.10 × 1011 and 5.56 × 1011 area/mg, respectively. Besides, the introduction of ethanol and ethylene glycol is more conducive to the demethoxylation of pyrolysis products and the formation of methoxy-free phenols during pyrolysis. This work realizes the conversion of lignin to high-value phenolic products with low-oxygen-content compounds without adding catalysts by a pyrolysis strategy.