BIT’s major progress in demethylation of lignin for polyphenol production promoted by ionic liquid

News Resource: School of Life Science

Editor: News Agency of BIT

Translator: Han Miaomiao, News Agency of BIT

Recently, the green biomanufacturing research group from School of Life Science of Beijing Institute of Technology (BIT) has made important research progress in the preparation of polyphenol by demethylation of lignin. The related research result has been published in the top international journal Chemical Engineering Journal (IF=""13.273) titled “Efficient demethylation of lignin for polyphenol production enabled by low-cost bifunctional protic ionic liquid under mild and halogen-free conditions”. The first author of the paper is Zhao Wanting, a 2019 master graduate student, and the corresponding authors are Professor Liu Xiaomin of Qingdao University and Professor Sun Jian of BIT. BIT is the corresponding unit of the first author.

Lignin is a natural high polymer formed by oxidative polymerization of three monolignols (coumaric alcohol, coniferyl alcohol, and sinapinic alcohol) during plant growth. As the only renewable aromatic polymer in nature, lignin has a polyphenol structure and tannin-like properties, but it has not been effectively and comprehensively utilized. The production of polyphenol from lignin is a mutually beneficial route for the exploration of polyphenol and the application of lignin. Demethylation reaction is an effective way to prepare polyphenol materials from lignin. It has the advantages of relatively mild conditions, and directly obtaining more ortho-phenolic hydroxyl structures without significantly reducing the high molecular weight of lignin. However, the reagents used in the current lignin demethylation methods generally contain halogens, which have problems such as low efficiency, high cost, volatile, strong toxicity, etc., hindering the mass production and industrial application of polyphenol prepared from lignin (Fig. 1).

Fig 1. Comparison of methods for demethylation of lignin

In order to realize the halogen-free improvement of lignin demethylation methods, the research group based on the long-term accumulation of ionic liquid microenvironment and its process intensification mechanism, especially the research basis of protic ionic liquids (PILs) in biomass dissolution and transformation (ACS Sustain. Chem. Eng., 2019, 7, 18593; Green Chem., 2017, 19, 3152), through low-cost, halogen-free multifunctional PILs (Fig. 2A), has developed a more green lignin demethylation strategy for efficient conversion of lignin to polyphenol. Among the eight PILs studied, the halogen-free [EOA][OAc] has exhibited the highest demethylation activity due to its dual function of solubilization and demethylation on lignin (Fig. 2B, the green histogram). The molecular simulation results have showed that the strong interaction between [EOA][OAc] and the lignin building block, and the close nucleophilic ability of its acetate anion and halogen anion are important factors in the smooth progress of the reaction (Fig. 2C).

Fig 2. A) Eight selected PILs; B) Electrical conductivity of PILs and phenolic hydroxyl content of demethylated products; C) Surface electrostatic potential analysis of eight PILs.

In addition, [EOA][OAc] can be easily recovered and reused by adding methanol to the reaction system. The stable recovery efficiency and high lignin conversion rate during the cycle help to further reduce the reaction cost (Fig. 3A and B). Compared with other traditional demethylation reagents, it is found that the demethylation efficiency of enzymatic hydrolysis of lignin in this study is about 73%, which is better than most of the demethylation reagents reported so far (Fig. 3C). In addition to the enzymatic hydrolysis of lignin, other types of lignin (such as sodium lignosulfonate, alkali lignin, etc.) can also be well suited for this method (Fig. 3D). When the amount of starting material is enlarged from 1g to 30g, the reaction efficiency remains basically unchanged (Fig. 3D).

Fig 3. A) The phenolic hydroxyl content in the recycled product; B) The structural stability of the recovered PIL; C) The comparison of the demethylation efficiency of lignin by different methods; D) The demethylation effects of different types of lignin.

The above work not only realizes the efficient demethylation of lignin under halogen-free conditions, but also has the advantages of low cost, relatively mild reaction conditions, and good recovery effect. It is of great significance for the high-value utilization of lignin and the development of polyphenol industry.

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Introduction of the research group attached:

According to the national "double carbon" and sustainable development strategic needs, the green biomanufacturing research group focuses on the interdisciplinary integration of chemistry, biology, chemical industry, materials, etc., and conducts basic and applied research on functionalized ionic liquid, with specific directions including the design of functionalized ionic liquid and microenvironment effects, bio-based chemical and material synthesis, process coupling of light, electricity, heat, biology, etc., as well as bionic functional device construction, etc. The research team leader is Professor Sun Jian, doctoral supervisor, deputy director of the Technical Committee of China Cellulose Industry Association, member of Ionic Liquid Special Committee of the Chemical Industry and Engineering Society of China, and member of Pharmaceutical and Biological Membrane Technical Committee of Membrane Industry Association of China. He has long been engaged in the research of green process based on ionic liquid. He has published more than 70 academic papers in internationally renowned journals such as Chem. Soc. Rev., Energy Environ. Sci., Chem. Eng. J., and authorized 21 Chinese invention patents, 3 US patents and 1 Japanese patent. He has presided and participated in more than 10 projects of the National Natural Science Foundation of China; he is currently serving as the youth editorial board member of journals such as Energy Environ. Mater., Chinese J. Chem. Eng., Green Chem. Eng., etc.