A “lignin-first” biorefinery towards efficient aromatic monomer conversion from coconut shells using mild TMAH-based alkaline deep eutectic solvents

Abstract

To achieve “lignin-first” dissolution and efficient aromatic monomer conversion from coconut shells, tailored alkaline deep eutectic solvents (DESs) were screened and designed by conductor-like screening model for real solvents (COSMO-RS) calculations. The results indicated that tetramethylammonium hydroxide (TMAH)-based DESs (TMAH-urea, TMAH-ethanolamine, TMAH-lysine and TMAH-imidazole) provide high solubility for lignocellulose and exhibit an excellent fractionation effect for real lignocellulose under mild conditions. This may be attributed to their higher excess enthalpy and strong hydrogen bonding forces. A high delignification rate of 63.33%–67.37% with a good hemicellulose retention of more than 90% could be achieved in 30% TMAH-based DESs at 50 °C for 3 h. An in-depth insight into the evolution of the lignin structure demonstrated that the cleavage of the lignin–carbohydrate ester bond was the main route for lignin extraction, rather than the β-O-4 ether bonds. Therefore, the extracted lignin fragments retained more than 84% of β-O-4 ether bonds, which was conducive to subsequent depolymerization to produce aromatic monomers. The strong oxidation effect of TMAH-imidazole contributed to its superior catalytic oxidation performance and resulted in a high aromatic monomer yield of 74.54%. Overall, this study designed and screened a solvent with strong hydrogen bonding and oxidation abilities, which enabled a feasible “lignin-first” strategy for efficient aromatic monomer production under milder conditions. A “win–win” situation of low energy consumption and high yield was achieved, highlighting a sustainable energy future through the advanced valorization of lignin.