Recent advances in the acid-catalyzed conversion of lignin
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REVIEW ARTICLE
Recent advances in the acid-catalyzed conversion of lignin Zhen Wu 1
&
Lei Hu 1 & Yetao Jiang 1 & Xiaoyu Wang 1 & Jiaxing Xu 1 & Qingfeng Wang 1 & Shenfang Jiang 1
Received: 22 July 2020 / Revised: 18 August 2020 / Accepted: 19 August 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract As the only natural aromatic high molecular compound that was obtained from a renewable resource, lignin is resistant to conversion due to its complex structure and stable linkages. The catalytic conversion of lignin via acid catalysts leading to a high yield of phenolic monomers has gained great interest due to its potential for producing biofuels and chemicals. However, up to now, the acid-catalyzed conversion of lignin has not yet been comprehensively summarized. In order to fill this gap, mechanism analysis and influential factors of the acid catalysis process and the latest studies and advancements on the catalytic conversion of lignin via various types of acid catalysts, such as mineral acids, organic acids, acidic ionic liquids, immobilized liquid acids, metal salts, metal oxides, heteropoly acids, and zeolites, are systematically outlined and discussed. Although many exciting results have been achieved, the sufficient utilization of lignin by this technology still faces enormous challenges. To improve the technical feasibility, intensive investigations on improving the solubility, catalytic activity, stability, reusability, and designability of the acid catalysts are crucially important and imperative. Keywords Catalytic conversion . Lignin . Depolymerization . Monomers . Acid catalysts
1 Introduction With the increasing depletion of petroleum resources, lignocellulosic biomass representing a renewable and carbonneutral substitute for fossil fuel has attracted much attention all over the world [1–3]. Actually, the conversion of lignocellulosic biomass into biofuels and biochemicals is currently one of the focuses in the field of novel energy research. Generally, lignocellulosic biomass contains around 35–50% of cellulose, which is a linear homopolymer of glucose units with a very high degree of polymerization (1000–10,000); 25– 30% of hemicellulose, which is a heterogeneous polymer of sugar monomers with a low degree of polymerization (100– 200); and 15–30% of lignin [4, 5], which is usually considered
* Zhen Wu [email protected] * Lei Hu [email protected] 1
Jiangsu Key Laboratory for Biomass-based Energy and Enzyme Technology, School of Chemistry and Chemical Engineering, Jiangsu Collaborative Innovation Center of Regional Modern Agriculture & Environmental Protection, Huaiyin Normal University, Huai’an 223300, China
as a polyphenolic material having an amorphous structure and arising from an enzyme-initiated dehydrogenative polymerization of three different monolignol monomers: p-coumaryl, coniferyl, and sinapyl alcohols. These lignols are incorporated into lignin aromatic cores in the form of phenyl propanoids, namely p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units,
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