Low-temperature alkali lignin depolymerization to functional chemicals

Abstract

Lignin—an underutilized component of lignocellulosic biomass and an aromatic polymer has an immense potential for the production of valuable chemicals. The ultimate goal of our research is to understand the lignin depolymerization at a lower temperature to obtain valuable chemicals such as vanillin, phenol-2-methoxy in the absence of a solid catalyst. Alkali lignin was depolymerized using polar protic solvents such as methanol, ethanol, and water and with co-solvents methanol-water, ethanol-water, and methanol-ethanol system at 120 °C under nitrogen (N2) and hydrogen (H2) and reaction pressure (1, 5, 10, 15, and 20 bar). The results show that efficient depolymerization can be achieved in an ethanol-water (50/50, W/W) co-solvent system under an N2 environment. The use of H2 enhanced lignin depolymerization and maximum bio-oil yield (51.6 wt.%) was observed under 10 bar H2 pressure with an ethanol-water solvent system. The bio-oil was analyzed using GC-MS, FT-IR, and 1H-NMR. The bio-oil compounds identified from GC-MS analysis were classified into G, H, and other type compounds. Interestingly, higher selectivity (90.77%) of G-type compounds was found in lignin depolymerized bio-oil with a high yield of vanillin. The reaction mechanisms showed that functional compounds are produced from the cleavage of ether C-O (β-O-4) type and C-C (5-5) type bonds.