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

Ming Guo,Jussi Kontro,Rui Zhang,Riku Maltari,Timo Repo,Enlu Ma

doi:10.1021/acs.energyfuels.1c01968

Ming Guo, Jussi Kontro + Show 4 more

Open Access

https://doi.org/10.1021/acs.energyfuels.1c01968

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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.

Highlights

Lignin is the most attractive, renewable potential starting material for sustainable synthesis of aromatic chemicals substituting petroleum-derived products.[1]
We placed Kraft lignin (KL) and an alcohol and water mixture into the autoclave, which was sealed to heat to the desired temperature at the highest stirring speed without expelling air
After the reaction solutions were filtered, oligomers and monomers were extracted with organic solvents, such as petroleum ether

Summary

Introduction

Lignin is the most attractive, renewable potential starting material for sustainable synthesis of aromatic chemicals substituting petroleum-derived products.[1]. Others are using alkaline[11] or acidic liquids for pretreatment,[12] and metal-based catalysts are developed to enhance the depolymerization process.[13] KL is more difficult to be degraded than native lignin as a result of its condensed structure, which requires harsh conditions to break the cross-linking C−C bonds. A higher temperature would engender hard coke that deactivates the catalyst. There is still a great need to develop new and straightforward strategies to depolymerize KL, because they are key to transforming lignin into valueadded products in contemporary biorefinery concepts.[14]

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