Hydrothermal Depolymerization of Kraft Lignins with Green C₁–C₃ Alcohol–Water Mixtures

Abstract

Lignin is a potential renewable raw material for synthesis of aromatic molecules and materials. Particularly, Kraft-type lignins are abundantly available as a byproduct of wood and paper industries. Currently, there are practically no valuable products made using this highly complex polymer. Within this work, we have investigated the straightforward, high-yield depolymerization of two different technical lignins (Indulin AT and alkaline lignin) using only a mixture of green short-chain (C1–C3) alcohols and water as solvents. As shown here, at a temperature of 220 °C, autogenous pressure, and isopropanol/water as a co-solvent medium, it is possible to cleave Kraft lignin without char-forming side reactions. The obtained so-called “light” oil contains guaiacol-based monomeric units of 23 wt % yield together with liquid oligomers of 13 wt % yield combining liquid–liquid and solid–liquid extraction. Two-dimensional nuclear magnetic resonance analysis of lignin residues showed that the isopropanol/water treatment caused a marked breakdown of the intermolecular β–O–4 and β–β bonds; thus, the depolymerization produced monomers and lignin residues with lower molecular weight. The results suggested a synergistic effect between isopropanol and water. No sign of repolymerization reactions could be observed with this process.

Sustainable lignin modifications and processing methods: green chemistry as the way forward

“…The highest oil yield (36.3 wt%) was obtained by i-propanol/water without any char formation. The phenolic compounds (12) 220 obtained from i-propanol/water were extracted by petroleum ether, with 23 wt% containing guaiacol derivates such as 4-methylguaiacol, 4-ethylguaiacol, eugenol, vanillin and homovanillic acid. 220 To reduce the energy demand (time) and increase the lignin loading, Kouris et al performed the mild solvolytic depolymerization of technical lignins (obtained from, e.g.…”

“…221 This trend in yield contradicts the study by Guo et al , which could be explained by the absence of water in the reaction mixture. 220 Indeed, the combination of an alcohol ( e.g. , ethanol) and water has been described as a mixture with synergetic effects for the liquefaction of pine sawdust.…”

“…The phenolic compounds (12) 220 obtained from i-propanol/water were extracted by petroleum ether, with 23 wt% containing guaiacol derivates such as 4-methylguaiacol, 4-ethylguaiacol, eugenol, vanillin and homovanillic acid. 220 To reduce the energy demand (time) and increase the lignin loading, Kouris et al performed the mild solvolytic depolymerization of technical lignins (obtained e.g., miscanthus, wheat straw, and hardwood) (2.66-20 g) in different polar organic solvents (40 mL) at 100 °C-350 °C for 0.5 h-4 h. The alcohol influenced the yield, which decreased in the order of MeOH > EtOH > 1-propanol > 1-butanol > 1-octanol. 221 This trend in yield contradicts the study by Guo et al, which could be explained by the absence of water in the reaction mixture.…”

“…221 This trend in yield contradicts the study by Guo et al, which could be explained by the absence of water in the reaction mixture. 220 Indeed, the combination of an alcohol (e.g., ethanol) and water has been described as a mixture with synergetic effects for the liquefaction of pine sawdust. 222 Further, ethanol-assisted solvolysis was investigated (lignin/ethanol ratio of 1 : 15 w/v) at 200 °C for 4 h, which gave a solubilized lignin yield of 73-75 wt%.…”

Hydrothermal Treatment of Biomass Feedstocks for Sustainable Production of Chemicals, Fuels, and Materials: Progress and Perspectives

“…There are comprehensive review papers on the catalytic HTL of lignin to selected products, in which the catalytic mechanisms and effect of operating conditions were discussed. , Here, we integrate the latest research findings on the catalytic HTL of lignin to value-added products. Guo et al employed short-chain (C 1 –C 3 ) alcohols as green cosolvent medium to cleave kraft lignin without formation of hydrochar at 220 °C of HTL . The isopropanol/water cosolvents caused a remarkable breakdown of β-O-4 and β–β bonds, leading to the depolymerization of lignin for the production of monomers as “light” bio-oils .…”