Lignin bioconversion into valuable products: fractionation, depolymerization, aromatic compound conversion, and bioprodu
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REVIEW
Lignin bioconversion into valuable products: fractionation, depolymerization, aromatic compound conversion, and bioproduct formation Mengxing Li1,2 · Mark Wilkins1,3,4 Received: 17 March 2020 / Revised: 16 September 2020 / Accepted: 18 September 2020 © Jiangnan University 2020
Abstract Lignin fractionation and depolymerization generates heterogeneous streams of aromatic compounds and conversion of aromatic compounds into valuable products, but it is not efficient. Many microbes in nature have evolved metabolic pathways to convert complex lignin polymers into aromatic compounds and transform these aromatic compounds into central intermediates for bioproduct synthesis. The objective of this paper is to review the recent process development of lignin bioconversion into aromatic compounds and bioproducts. Lignin structural and molecular changes during fractionation and depolymerization are presented. Subsequent lignin conversion into aromatic compounds by upper pathways and further converted into central metabolites and bioproducts via lower pathways are emphasized. In particular, enzymes and mediator systems to enhance lignin conversion and key intermediates in lignin catabolic pathways are discussed. Strategies to enhance bioproduct formation through lignin valorization are summarized. Keywords Lignin · Aromatic compounds · Metabolic pathways · Bioproducts · Strategies
Introduction Lignin is an energy-dense, heterogeneous polymer comprising 15–30% of lignocellulosic biomass, and it is the second most abundant biopolymer on Earth after cellulose [1]. Large quantities of lignin are produced from paper and pulping industry, and lignocellulosic biorefineries. Currently, these industries leave lignin approximately 140 million tons per year that were simply burned for heat and power supply [2–6], which places a low value on lignin. For lignocellulosic biorefineries, research on production of valuable * Mark Wilkins [email protected] 1
Department of Biological Systems Engineering, The University of Nebraska-Lincoln, 211 Chase Hall, 3605 Fair St, Lincoln 68583, USA
2
Department of Statistics, The University of Nebraska-Lincoln, Lincoln 68583, USA
3
Department of Food Science and Technology, University of Nebraska-Lincoln, Lincoln 68588, USA
4
Industrial Agricultural Products Center, University of Nebraska-Lincoln, Lincoln 68583, USA
products is active for the last two decades. However, research mainly focused on renewable fuels, chemicals, and polymers production from polymeric sugars of lignocellulosic biomass [7–18]. In contrast, the research on lignin conversion into valuable bioproducts is still limited. Processes to convert lignin to valuable bioproducts could accommodate extra costs for recovery of lignin by capital intensive pretreatments [19, 20]. Until today, lignins have remained the most underutilized renewable feedstock [5, 21]. A scheme for the process, including fractionation, depolymerization, upper and lower pathways, and bioproduct formation, is shown in Fig. 1. Representative pr
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