Homogeneous catalytic oxidation of coconut shells to high-yield aromatic monomers and holocellulose in mild TMAH-imidazole.

An efficient homogeneous catalytic oxidation (HCO) process based on tetramethylammonium hydroxide derived deep eutectic solvent (TMAH-DES) was designed to enable "lignin-first" biorefining. Compared to the other TMAH-DES, the TMAH-imidazole exhibited superior HCO performance. An aromatic monomer yield of 19.5% (71% of the theoretical value based on nitrobenzene oxidation) with a considerable cellulose (86.3%) and hemicellulose recovery (54.4%) was achieved under oxidation at a mild condition of 100℃ for 2 h using the TMAH-imidazole DES with a molar ratio of 3:7. The kinetic simulation of results indicated that both diffusion and surface chemical reactions played a crucial role in lignin dissolution, with surface chemical reactions being the dominant mechanism. An in-depth investigation of lignin structural change and the model compound study revealed that multiple LCC bonds was cleaved to separate lignin from biomass in TMAH-imidazole, contributing to good carbohydrate retention. Meanwhile, a high aromatic monomer yield can be obtained through the efficient cleavage of the β-O-4 bond. Therefore, an enhanced catalytic oxidation process enabling efficient recovery of carbohydrates was obtained in a low-energy homogeneous system, further advancing lignin-first biorefinery operations.