New Lignin Streams Derived from Heteropoly Acids Enhanced Neutral Deep Eutectic Solvent Fractionation: Toward Structural Elucidation and Antioxidant Performance

To improve the fractionation efficiency of lignins from Miscanthus × giganteus in the neutral DES (choline chloride/glycerol), three heteropoly acids with environmental origin and economic viability (phosphotungstic, phosphomolybdic, and silicotungstic acids) were utilized as catalysts. It was found that the silicotungstic acid–choline chloride/glycerol system resulted in a high lignin yield of 78.2% under mild conditions (120 °C, 3 h). This was attributed to silicotungstic acid acting as tetrahydric acid, which donated more active protons to selectively cleave ether bonds and further facilitated lignin removal. The following lignin profiling indicated that the recovered lignin had a high purity (92.7%), low molecular weight (Mw 1520–2470 g/mol), low polydispersity (PDI < 1.7), excellent thermal stability, and good solubility in dimethyl sulfoxide and dimethylacetamide. Additionally, phosphomolybdic acid (PMA) lignin exhibited a typical structure of HGS with a relatively higher O/C ratio due to more nativelike C–O-bonded structures (high β-aryl ether content) after treatment. Interestingly, nanoscaled lignin was defined in the DES fractionation procedure based on the SEM and TEM analyses. To further evaluate the potential application of the isolated lignin streams, the antioxidant activity was performed by in vitro 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging ability. The antioxidant performance (IC50 = 0.042 mg/mL) was better than that of the commercial antioxidant butylated hydroxytoluene (IC50 = 0.056 mg/mL). The above results demonstrated that the heteropoly acids–neutral DES system was significantly efficient for lignin extraction and provided a new source of lignin with good antioxidant activity.