Structural Characterization of Acid DES-Modified Alkaline Lignin and Evaluation of Antioxidant Properties

Lignin is an abundant and environmentally friendly biopolymer that contains a large number of phenolic hydroxyl functional group. In this paper, alkaline lignin was modified using different acidic DES (choline chloride/p-toluenesulfonic acid and choline chloride/lactic acid) at 130 celcius (TC-lignin and LC-lignin) and the conformational relationship between the modified products and the antioxidant activity was investigated. Lignin was characterized by P-31 NMR, gel permeation chromatography (GPC) and Fourier transform infrared spectroscopy (FT-IR), and its antioxidant activity was evaluated. The results showed that the alkaline lignin products modified by acidic DES formed relatively homogenous dispersions and were characterized by a relatively low molecular weight and a high content of phenolic hydroxyl groups (e.g., TC-lignin, aliphatic-OH: 3.52 mmol/g, G-OH: 4.18 mmol/g, M-w: 3726, M-n: 2053, PDI: 1.81). The antioxidant activity (free radical scavenging rate, 90.35%) of TC-lignin was somewhat higher than that of LC-lignin (free radical scavenging rate, 89.12%) and both were higher than that of the commercially available antioxidant BHT (free radical scavenging rate, 88.79%). More specifically, we discussed the possible mechanisms of antioxidant reactions of lignin model substances in DPPH solutions. In addition, LC-lignin has an excellent UV-blocking capacity due to the specific phenolic hydroxyl and phenyl propane structure. A simple method is proposed for the modification of industrial lignin to make it suitable for use as an antioxidant and UV-resistant product.