Synergistic effect of water-ethanol-formic acid for the depolymerization of industrial waste (black liquor) lignin to phenolic monomers

In this study, synergistic effect of a mixture solvent of water-ethanol-formic acid was investigated for the depolymerization of concentrated acid hydrolysis lignin (CAHL, isolated from industrial black liquor waste; source: pine wood) to produce phenolic monomers. The effects of solvent ratios, reaction temperature and time on CAHL depolymerization and the distribution of products were also explored. The highest bio-oil yield of 64.24 wt% containing 23.02 wt% monomers was obtained at 330 °C for 2 h with a water-ethanol-formic acid ratio of 1:1:0.4. Ethanol played an important role in increasing the solubility of CAHL and partly suppressing the repolymerization of reactive intermediates, while formic acid enhanced lignin depolymerization through an acid-catalyzed reaction and efficaciously preventing the repolymerization of unstable intermediates generated. Among the different monomers, 4-propylguaiacol (5.51 wt%), 4-methylguaiacol (3.43 wt%), catechol (3.02 wt%), 4-ethylguaiacol (2.73 wt%) and phenol (2.11 wt%) were identified as the major compounds in phenol-rich bio-oils. The MALDI-TOF MS spectra revealed that the oligomers present in the bio-oil were composed of dimers and trace amounts of trimer type compounds. Meanwhile, GPC results showed that CAHL (Mw = ~2156 g/mol) was efficiently depolymerized into lower molecular weight (Mw = 887 g/mol) compounds. The process parameters, such as water/ethanol ratio, FA-to-water/ethanol ratio, reaction temperature and time were also optimized using a central composite rotatable design (CCRD) in response surface methodology (RSM). ANOVA results showed that reaction temperature and time had highly significant effect (ρ < 0.05) on the yield of bio-oil. The depolymerization approach and outcomes of this study suggest that this method is efficient for pulp and paper industry waste management and production of value-added chemicals.