Recovery and Utilization of Lignin Monomers as Part of the Biorefinery Approach

Kirsten Davis, Marjorie Rover, Robert Brown, Xianglan Bai, Zhiyou Wen, Laura Jarboe
2016 Energies  
Lignin is a substantial component of lignocellulosic biomass but is under-utilized relative to the cellulose and hemicellulose components. Historically, lignin has been burned as a source of process heat, but this heat is usually in excess of the process energy demands. Current models indicate that development of an economically competitive biorefinery system requires adding value to lignin beyond process heat. This addition of value, also known as lignin valorization, requires economically
more » ... es economically viable processes for separating the lignin from the other biomass components, depolymerizing the lignin into monomeric subunits, and then upgrading these monomers to a value-added product. The fact that lignin's biological role is to provide biomass with structural integrity means that this heteropolymer can be difficult to depolymerize. However, there are chemical and biological routes to upgrade lignin from its native form to compounds of industrial value. Here we review the historical background and current technology of (thermo) chemical depolymerization of lignin; the natural ability of microbial enzymes and pathways to utilize lignin, the current prospecting work to find novel microbial routes to lignin degradation, and some applications of these microbial enzymes and pathways; and the current chemical and biological technologies to upgrade ligninderived monomers. Abstract: Lignin is a substantial component of lignocellulosic biomass but is under-utilized relative to the cellulose and hemicellulose components. Historically, lignin has been burned as a source of process heat, but this heat is usually in excess of the process energy demands. Current models indicate that development of an economically competitive biorefinery system requires adding value to lignin beyond process heat. This addition of value, also known as lignin valorization, requires economically viable processes for separating the lignin from the other biomass components, depolymerizing the lignin into monomeric subunits, and then upgrading these monomers to a value-added product. The fact that lignin's biological role is to provide biomass with structural integrity means that this heteropolymer can be difficult to depolymerize. However, there are chemical and biological routes to upgrade lignin from its native form to compounds of industrial value. Here we review the historical background and current technology of (thermo) chemical depolymerization of lignin; the natural ability of microbial enzymes and pathways to utilize lignin, the current prospecting work to find novel microbial routes to lignin degradation, and some applications of these microbial enzymes and pathways; and the current chemical and biological technologies to upgrade lignin-derived monomers. Energies 2016, 9, 808 2 of 28 such as dispersants or binders [6] . It follows that lignin has also been combusted as an energy source in the conversion of biomass to ethanol [7] . There is a vast collection of literature on lignin processing, including improving the recovery of lignin from biomass, depolymerization of lignin into monomers by chemical and/or biological means, and upgrading of the depolymerized lignin monomers to industrially relevant chemicals, which have been described in several other recent reviews (Figure 1) [2, 5, 8, 9] . The purpose of this review is to summarize strategies from each of these processing steps and to briefly describe their economic relevance. Energies 2016, 9, 808 2 of 27 Traditionally, only 1%-2% of lignin was isolated from pulping liquors and used for specialty products, such as dispersants or binders [6] . It follows that lignin has also been combusted as an energy source in the conversion of biomass to ethanol [7] . There is a vast collection of literature on lignin processing, including improving the recovery of lignin from biomass, depolymerization of lignin into monomers by chemical and/or biological means, and upgrading of the depolymerized lignin monomers to industrially relevant chemicals, which have been described in several other recent reviews (Figure 1) [2, 5, 8, 9] . The purpose of this review is to summarize strategies from each of these processing steps and to briefly describe their economic relevance.
doi:10.3390/en9100808 fatcat:2xlk6ibasrdepj56yktye5aayq