Optimal extraction, sequential fractionation and structural characterization of soda lignin

In this research, soda lignin was extracted from Bambusa bambos using a soda pulping process, and interaction effects of pulping variables were statistically analyzed by the central composite design methodology. A statistical model predicted that the maximum recovery of soda lignin would be 104.6 mg/g of biomass in the following optimized process conditions: sodium hydroxide concentration (1.3% w/v), biomass concentration (10% w/v) and 150 min of pulping. The precipitated soda lignin was sequentially fractionated with organic solvents (chloroform, dichloromethane and n-butanol). The extracted soda lignin fractions, chloroform fraction (F1), dichloromethane (F2) and n-butanol (F3) were subsequently characterized by UV, FTIR, thermogravimetric analysis and 1H NMR spectral analysis. According to analytical characterization of lignin fractions, the molecular weight of lignin fractions increased gradually from fraction 1 to 3 and their polydispersity decreased dramatically compared with the unfractionated lignin. TG/DTG showed that the low molecular weight fractions generally have lower thermal stability. Functional group analysis results indicated that the guaiacyl unit content, molecular weight and thermal stability increased from F1 to F3, whereas the phenolic acid content significantly decreased. Solvent fractionation method to separate well defined structure of lignin molecules, which are likely having the extreme potential applications.