Recovery and Utilization of Lignin Monomers As Part of the Biorefinery
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energies Review Recovery and Utilization of Lignin Monomers as Part of the Biorefinery Approach Kirsten M. Davis 1, Marjorie Rover 2, Robert C. Brown 3, Xianglan Bai 3, Zhiyou Wen 4 and Laura R. Jarboe 1,* 1 Chemical and Biological Engineering, Iowa State University, Ames, IA 50014, USA; [email protected] 2 Bioeconomy Institute, Iowa State University, Ames, IA 50014, USA; [email protected] 3 Mechanical Engineering, Iowa State University, Ames, IA 50014, USA; [email protected] (R.C.B.); [email protected] (X.B.) 4 Food Science & Human Nutrition, Iowa State University, Ames, IA 50014, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-515-294-2319 Academic Editor: Dimitris S. Argyropoulos Received: 15 June 2016; Accepted: 22 September 2016; Published: 10 October 2016 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. Keywords: valorization; depolymerization; laccase; aromatic; pyrolysis; organosolv 1. Introduction Lignocellulosic biomass includes a wide variety of plant material, such as crops, agricultural residue, and wood. Humankind has utilized biomass throughout history to produce: heat for warmth and cooking; biochemicals, such as the ethanol and lactic acid produced by fermentation; and biofibers, such as those used in clothing and other textiles [1]. Present-day utilization of lignocellulosic biomass instead of petroleum in the production of chemicals and fibers could contribute to the improvement of environmental quality, national security, and rural economic development [1]. One component of lignocellulosic biomass, lignin, has long been viewed as a low-value or waste product in the wood pulping industry. The most common pulping process is the Kraft process, where lignin is dissolved in hot sodium hydroxide and sodium sulfide [2]. The top three pulping processes are the Kraft process, the sulfite process, and the soda lignin process. These three processes produce 60–100 Ktonnes of Kraft lignin, 1 Mtonne of lignosulfonates, and 5–10 Ktonnes of Sulfur-free soda lignin per year, respectively [3]. Typically, lignin is used as a fuel to fire pulping boilers [4]. However, the energy produced through lignin combustion is about sixty percent greater than the demand [5]. Traditionally, only 1%–2% of lignin was isolated from pulping liquors and used for specialty products, Energies 2016, 9, 808; doi:10.3390/en9100808 www.mdpi.com/journal/energies Energies 2016, 9, 808 2 of 28 Energies 2016, 9, 808 2 of 27 suchTraditionally, as dispersants only or1% binders–2% of [ 6lignin]. It follows was isolated that lignin from has pulping also been liquors combusted and asused an energyfor specialty source inproducts the conversion, such as ofdispersants biomass to or ethanol binders [7]. [6]. It follows that lignin has also been combusted as an energyThere source is a in vast the collectionconversion of of literature biomass onto ethanol lignin processing, [7]. including improving the recovery of ligninThere from is a biomass,vast collection depolymerization of literature on of lignin lignin processing into monomers, including by chemical improving and/or the recovery biological of means,lignin from and biomass upgrading, depolymerization of the depolymerized of lignin lignin into monomers monomers by to chemical industrially and/o relevantr biological chemicals, means, whichand upgrading have been of describedthe depolymerized in several lignin other monomers recent reviews to industrially (Figure1 )[relevant2,5,8,9 ].chemicals, The purpose which of have this reviewbeen described is to summarize in several strategies other recent from reviews each of (Figure these processing 1) [2,5,8,9]. steps The andpurpose to briefly of this describe review theiris to economicsummarize relevance. strategies from each of these processing steps and to briefly describe their economic relevance. Figure 1. The lignin polymer can be processed via combustion, chemical processing, thermochemical processing, biological processing or a combinationcombination of thesethese routes.routes. This review covers chemical, thermochemical, and and biological biological processing processing of of depolymerized lignin to produce industrially relevant chemicals. 2. Lignin Lignin Structure Structure and and Abundance Abundance Lignin is is a stable a stable aromatic aromatic heteropolymer heteropolymer that accounts that accounts for 10–35 for wt% 10–35 of lignocellulosicwt% of lignocellulosic biomass [8]. biomassTable 1 details [8]. Table the variation1 details of the lignin variation content of in lignin various content lignocellulosic in various biomass lignocellulosic types. Lignin biomass is the second types. Ligninmost abundant is the second terrestrial most polymer abundant after terrestrial cellulose, polymer and it afteris the cellulose, only large and-volume it is the renewable only large-volume source of renewablearomatics [10,11] source. In of nature, aromatics lignin [10 ,functions11]. In nature, as a matrix lignin that functions holds asthe a plant matrix together that holds and theprovides plant togetherprotection and from provides environmental protection factors. from environmentalThe properties factors.of lignin The tha propertiest benefit the of ligninplant are that also benefit the theproperties plant arethat alsomake the lignin properties difficult to that access make and lignin convert difficult to industrially to access relevant and convert products. to Although industrially the relevantstructure and products. composition Although of lignin the vary structure from plant and to composition plant, during of lignin lignin production, vary from the plant three to primary plant, duringlignin mo ligninnomers production, coniferyl alcohol, the three sinapyl primary alcohol, lignin and p monomers-coumaryl alcohol coniferyl are alcohol,subject to sinapyl polymerization alcohol, andso thatp-coumaryl the resulting alcohol lignin are polymer subject is to comprised polymerization of three so phenylpropanoid that the resulting monomeric lignin polymer units isguaiacyl comprised (G), ofsyringyl three (S), phenylpropanoid and p-hydroxyphenyl monomeric (H) (Figure units 2) guaiacyl [12,13]. (G), syringyl (S), and p-hydroxyphenyl (H) (Figure2)[12,13]. Table 1. Lignin content in lignocellulosic crops. Biomass CategoryTable 1. Lignin contentBiomass in Type lignocellulosic crops.Lignin Content (wt%) Softwood Pine 28 [14] Poplar 21–27 [15] BiomassHardwood Category Biomass Type Lignin Content (wt%) Eucalyptus 29–32 [16] SoftwoodMiscanthus Pine 28 [149]–13 [17] SwitchgrassPoplar 21–2717 [15–18] [18] HerbaceousHardwood EucalyptusCorn Stover 29–32 [1618] [19] MiscanthusBagasse 9–13 [1720] [20] Switchgrass 17–18 [18] Herbaceous Corn Stover 18 [19] Bagasse 20 [20] Energies 2016, 9, 808 3 of 28 Energies 2016, 9, 808 3 of 27 FigureFigure 2. Primary 2. Primary lignin lignin monomers monomers are are hydroxycinnamyl hydroxycinnamyl alcohols alcohols which which are are known known as monolignols as monolignols.. These primary lignin monomers are polymerized. The corresponding phenylpropanoid monomeric These primary lignin monomers are polymerized. The corresponding phenylpropanoid monomeric units in the lignin polymer are guiacyl units (G), syringyl units (S), and p-hydroxyphenyl units (H), units in the lignin polymer are guiacyl units (G), syringyl units (S), and p-hydroxyphenyl units (H), respectively, which can be polymerized at any of the wavy bond positions [12,13]. respectively, which can be polymerized at any of the wavy bond positions [12,13]. 3. Challenges and Progress in Lignin Recovery 3. Challenges and Progress in Lignin Recovery Lignin is recalcitrant and has a heterogeneous structure. In addition, the separation of lignin Ligninfrom biomass is recalcitrant can be energy and intensive has a heterogeneous and sometimes structure. requires harsh In addition, chemicals. the The separation lignin isolation of lignin frommethods biomass in can Table be energy 2 use combinations intensive and of sometimes acid/base chemistry, requires harshhigh temperatures chemicals. The and lignin pressures, isolation methodssolvents, in Table and2 catalysts. use combinations of acid/base chemistry, high temperatures and pressures, solvents, and catalysts.