Glycol-based Alkaline Deep Eutectic Solvents for “Lignin-First” Dissolution from Coconut Shells

Herein, K2CO3-ethylene glycol (EG) is designed to achieve “lignin-first” dissolution from coconut shells. Previous deep eutectic solvents (DESs) could achieve effective lignin removal but compromise the efficient recovery of hemicellulose. Differently, a high delignification rate of 70.7% with a good hemicellulose retention of 70.2% could be achieved in K2CO3-EG at 130 °C for 1 h. The possible reason is that hemicellulose has a lower excess enthalpy in K2CO3-EG, and lignin has a higher excess enthalpy according to the COSMO-RS calculations. These results demonstrate that K2CO3-EG possesses a huge potential for the preferential extraction of lignin. An in-depth insight into the evolution of the lignin structure indicates that K2CO3-EG extracts lignin mainly by the cleavage of lignin–carbohydrate complex bonds, especially the lignin–carbohydrate ester bond. The regenerated lignin exhibited a relatively complete structure with abundant β–O–4 ether bonds and less condensation structure. When the DES-extracted lignin was subjected to catalytic oxidation, a higher content of aromatic monomers was obtained. Their aromatic monomer content is 116.08% higher than that of alkaline lignin, which indicates the superior depolymerization performance. This study realizes the priority extraction of lignin and the relatively intact lignin structure that could promote its aromatic monomer conversion. The reusable DES can achieve “lignin first” dissolution and efficient depolymerization of lignin, improving the utilization efficiency of carbon and sustainability.