Synergy of ball milling, microwave irradiation, and deep eutectic solvents for a rapid and selective delignification: walnut shells as model for lignin-enriched recalcitrant biomass

The combination of ball milling (BM), microwave irradiation (MI), and deep eutectic solvents (DES) results synergistic for an efficient, selective, and very rapid (10 min) delignification of materials with high lignin content (ca. 50 wt%) such as walnut shells (WS). Lignin is dissolved in the DES, whereas the polysaccharide fractions remain suspended with limited degradation, due to the rapid pretreatment. After ball milling procedure (3 h), biomass loadings in the range of 100-200 g L-1 are selectively delignified in 10 min at 150 degrees C by using choline chloride:formic acid DES (1:2 molar ratio), rendering lignin yields of 60-80% (ca. similar to 40-60 g lignin L-1). Ball milling, microwave irradiation, and DES systems are much more efficient than ball milling, conventional heating, and DES system. The obtained lignins exhibited similar Fourier-transform infrared spectroscopy (FTIR) profile to that of milled wood lignin (MWL), indicating minimal functional group changes.