Abstract:

Lignin-based polyphenol(DMOL) was prepared by the tetrahydrofurfuryl alcohol/water(THFA/H₂O) alcoholysis to obtain alcoholysis lignin(OL), in combination with the demethylation of iodocyclohexane(ICH) using beech as feedstocks. The structure of DMOL was characterized by Fourier transform infrared spectrometer(FT-IR), two-dimensional nuclear magnetic resonance(2D NMR), gel permeation chromatography(GPC) and folinol reagent(FC) method. The antioxidant activity of DMOL was evaluated by 1, 1-diphenyl-2-trinitrophenylhydrazine free radical(DPPH·) scavenging. The results showed that lignin based polyphenols DMOL-1, DMOL-2, and DMOL-3 were prepared under the conditions of 40 g beech wood powder, 400 mL of THFA/H₂O(volume ratio 6∶4) solution, 200 min of alcoholysis at 190 ℃, the ratio of the dosage of OL and ICH 1∶7(g∶mL) and 1, 12, and 24 h of demethylation at 130 ℃. The phenolic hydroxyl contents of the three samples were 4.3, 5.8, and 6.8 mmol/g, respectively. The separation yield of DMOL-3 was 8.6%, the weight average relative molecular weight(M_w) was 2 267, the dispersion coefficient was 1.55, and the phenolic hydroxyl(Ar—OH) content was 6.8 mmol/g, which was 5.8 times higher than that of OL. DMOL-1, DMOL-2 and DMOL-3 showed half inhibitory mass concentration(IC₅₀) values of 9.2, 8.5 and 6.4 mg/L for DPPH· scavenging, respectively. The DPPH· scavenging capacity of DMOL-3 was 3.9 times that of OL, 1.8 times that of BHT and 1.5 times that of ferulic acid. The study demonstrated that after ICH demethylation treatment, the Ar—O—CH₃ bonds in the lignin structure were cleaved to form Ar—OH and CH₃I, and the C—O—C bonds in the connecting structural units such as β-O-4, β-5 and β-β were broken to form Ar—OH and Ar—CH₂—. In addition, as the demethylation reaction time increased, the Ar—OH content increased, the molecular mass decreased and the antioxidant activity increased.

Key words: lignin, demethylation, antioxidant, biological activity mechanism