Lignin-coordinated niobium-based catalyst for the efficient conversion of industrial lignin in choline chloride-lactic acid integrated with ethanol deep eutectic solvent

Catalytic depolymerization is a favorable option for the valorization of industrial lignin. In this study, a new strategy was demonstrated for the efficient reductive depolymerization of industrial lignin based on a complex solvent of choline chloride-lactic acid (ChCl-LA) DES integrated with ethanol and a C-supported N-doped niobium-based catalyst with industrial lignin as carbon source (NBC@N-LC). It was found that the introduction of ethanol significantly improved the conversion of industrial lignin in ChCl-LA. More delightfully, depolymerization of industrial lignin in the complex solvent of DES-ethanol was further promoted by using the fabricated NBC@N-LC, exhibiting the best lignin dissolution performance, the lowest undepolymerized lignin yield of 21.9 % and the highest monomers yield of 31.8 %. 4-Ethylphenol was observed to dominate the monomers with 53.6 % selectivity, revealing the directional reactivity of the catalytic depolymerization process. Reaction pathway analysis showed that the synergistic effect of NBC@N-LC and DES-ethanol led to the efficient depolymerization of lignin, in which DES acted as an excellent solvent for lignin and depolymerized intermediates, ethanol acted as an in-situ hydrogen donor, and NBC@N-LC acted as a provider of reaction sites and acid active centers. In conclusion, this work offers a good alternative compared to the available lignin depolymerization method in terms of efficiency, selectivity, and cost aspects.