Progress toward Lignin Valorization via Selective Catalytic

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Recent Progress toward Lignin Valorization via Selective Catalytic Technologies and the Tailoring of its Biosynthetic Pathways Rebecca E. Key, and Joseph John Bozell ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01319 • Publication Date (Web): 28 Aug 2016 Downloaded from http://pubs.acs.org on September 2, 2016

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ACS Sustainable Chemistry & Engineering

Recent Progress toward Lignin Valorization via Selective Catalytic Technologies and the Tailoring of its Biosynthetic Pathways Rebecca E. Key and Joseph J. Bozell* Center for Renewable Carbon, Center for Direct Catalytic Conversion of Biomass to Biofuels (C3Bio) 2506 Jacob Drive University of Tennessee, Knoxville, TN 37996 Corresponding author: [email protected] ABSTRACT Biomass has received significant attention in the 21st century regarding its ability to become an alternative source of carbon for the production of strategic chemicals and fuels. While multiple approaches for the selective valorization of biorefinery carbohydrates are known, processes of similar selectivity for the valorization of lignin are scarce, which can be attributed to its heterogeneity arising from its biosynthesis, as well as methods used for its isolation within the biorefinery. This perspective highlights advances within the last six years toward lignin valorization in the areas of thermochemical transformations, oxidations, reductions, and the tailoring of the lignin biosynthetic pathway in an effort to produce high-value chemicals and fuels. Should these current hurdles be addressed, significant progress can be made toward the realization of biomass replacing petroleum in meeting the world’s energy and chemical needs. Keywords: Renewable carbon, lignocellulosic biomass, pyrolysis, oxidation, reduction, “lignin-first”, plant science, upgrading INTRODUCTION Lignin exists simultaneously as one of the most promising opportunities and greatest challenges for the lignocellulosic biorefinery. Access to this highly abundant renewable carbon source is growing, as pretreatment methodologies targeting cellulosic ethanol production affords lignin as a separate coproduct and provides a foundation for operating the biorefinery in a method analogous to the petrochemical refinery. The success of the petrochemical industry1 comes from its simultaneous production of inexpensive high volume fuels and lesser amounts of more profitable chemicals from nonrenewable carbon sources. Underpinning this success is the industry’s fundamental understanding of the conversion technologies capable of transforming hydrocarbons into

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chemicals and fuels in high yield and efficiency.2 The impact of chemical production is striking, as converting a mere 7-8% of crude oil into chemicals drives the overall economics of the industry by providing nearly 50% of the profits.3-4 This is an ideal operational model for the biorefinery, as chemical production from alternative and sustainable sources of carbon is justified and necessary to support the production of higher volume but less profitable biofuels. The US has increased its energy usage by 35% in the last 30 years, and more than half of all new global energy demand through 2035 is coming from industrialization and the emergence of a strong middle class in India and China. Although the industry has recently realized significant new supplies of natural gas and tight oil with the advent of hydraulic fracturing (“fracking”), significant concerns remain over the long-term productivity of this technology,5 the loss of large amounts of methane from these new wells to the atmosphere6 (releasing a greenhouse gas, whose impact is 20x that of CO2),7 and the unknown impacts of drilling chemicals on groundwater and geological substructures.8-9 When these issues are coupled with ongoing questions about climate change from the release of enormous amounts of CO2 from nonrenewable sources10-12 including the heavy use of coal for emerging third-world industrialization, a clear need to understand and evaluate new sources of carbon as starting materials for strategic chemicals and fuels emerges.13-19 Lignocellulosic biomass is a critical alternative renewable carbon source for the production of energy, chemicals, and fuels20 and consists mainly of three biopolymers - cellulose, hemicellulose, and lignin.20-21 Due to its facile accessibility, biomass has ample potential to become a replacement for the current nonrenewable resources predominantly used in the chemical industry.19 More than 1.3 billion tons of biomass are projected to be available for biorefining in the US, which is sufficient to supply the nation’s need for high value chemicals.2223

There have been two major approaches noted in the literature regarding the greater incorporation of biomass.

One approach, the “drop-in strategy,” utilizes previously established technologies from the petroleum industry in a biorefinery infrastructure. The “emerging strategy,” on the other hand, exploits the high degree of functionality seen in biopolymers to produce new materials.19,24 These new pathways should give rise to processes involving fewer steps, resulting in greater efficiency and a decrease in waste accumulation, thus providing greener methodologies.19,24-26 However, to effectively valorize biomass feedstocks into useful chemicals and fuels, new processes are needed that accommodate the highly oxygenated and complex biopolymeric structures of biomass components that will be deconstructed and converted in downstream operations.13,27-28 The need for new technology is particularly acute when considering lignin as a biorefinery raw material. “Lignin valorization” is terminology in high current use with respect to biorefinery development, and multiple reviews have appeared on this topic.20,23,29-38 Projections suggest that 2nd generation biorefineries may generate over 65 million tons of lignin or as much as 60% more than is needed to satisfy internal energy requirements.39 However, truly selective lignin transformations remain elusive. It is this recalcitrance that continues to relegate a promising source of renewable carbon to use as a low-value fuel, in contrast to the plethora of literature


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describing methods for valorizing biorefinery carbohydrates.17,40-41 It is particularly striking that the types of targets still commonly suggested as lignin derivatives have changed relatively little over the last 30 years.42 Reducing lignin valorization to practice is still fraught with the now well-recognized challenges of multiple feedstock sources, structures that change depending on both the source of the lignin and the methods used to isolate it from a lignocellulosic matrix, and a lack of simple, selective transformations tailored for its unique structure. Ultimately, the lignocellulosic biorefinery will emulate the petrochemical industry, by fully consuming each of its primary process streams (carbohydrates and lignin) for use as energy sources through the production large volumes of low value fuel, and as profit sources through the production of smaller amounts of higher value chemicals. Lignin will play a key role in this evolution, but lignin’s utility as a source of chemicals is poorly developed. While a portion of biorefinery lignin will likely be used as fuel in biorefining operations, this review discusses ongoing research that is starting to reveal new means for its conversion to chemical products. Catalytic processes are emerging as a means to upgrade biorefinery lignin, and the excellent review from Zakzeski et al. provides a comprehensive summary of work up to 2010.20 In this perspective, we overview recent catalytic processes used in pyrolytic, oxidative, and reductive transformations that target lignin valorization and also illustrate how selectivity remains a critical challenge to effective lignin conversion. Two emerging approaches that may offer solutions to this issue are also presented: 1) the potential beneficial transformation of lignin early in the biorefinery process43-45 and 2) the promising convergence of plant sciences with lignin valorization for the in planta generation of designer lignin, tailored for more effective downstream conversion. The focus of this perspective is on transformations of lignin itself, and accordingly, efforts in lignin model conversions, while extensive, are mentioned only if they provide pertinent background information. OVERVIEW OF LIGNIN Lignin in Nature - “Supramolecular Self-Assembled Chaos”46 The basic principles of lignin structure (Figure 1) and biosynthesis are now well documented,47-48 but briefly, lignin as found in nature is an amorphous, three-dimensional polymer comprising methoxylated phenylpropane moieties (Figure 2).49 Though the absolute structure of native lignin remains unknown, its biosynthesis occurs via the polymerization of three principal monolignols: coniferyl, sinapyl, and p-coumaryl alcohols, leading to lignin’s guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) units, respectively (Figure 2).49 Depending on the source of the plant, the amount, and the molecular weight, the substructural composition of lignin will vary, although five major linkage patterns dominate the lignin matrix. These include the β-O-4 (aryl ether), β-β (pinoresinol), β-5 (phenylcoumaran), β-1 (diphenylmethane), and 4-O-5 (diphenyl ether) linkages (Figure 3).14


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OMe

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O

HO

OH

HO

OMe

MeO

OMe O

OH

MeO

OH

OH

MeO

O OMe

O

OH

OH

OH

OMe

OH

O

HO

OMe

O OMe

OH

OH

OH

OH

MeO OMe

O

O

OMe

OMe

O O

O

OMe MeO

OMe OMe

HO

MeO

OH O

OH

HO

HO

COOR

MeO

OMe

OMe

HO

OMe

O

O OH

HO

OMe

HO

OH

O MeO O

OH

HO

O

O

OMe

O

OMe

OMe

MeO

HO

O

HO

O

O HO

MeO

OH

MeO

OH

O

MeO

O

OMe OH

HO HO

OMe

Figure 1. Representation of native poplar lignin. HO

HO

HO

OMe

MeO

OMe

OH

OH

OH

Coniferyl Alcohol (G unit)

Sinapyl Alcohol (S unit)

p-Coumaryl Alcohol (H unit)

Figure 2. The principal monolignol subunits commonly found in lignin.49 HO HO

O

O

HO

O

MeO OMe OH

O MeO

OMe β-β' Pinoresinol Linkage

β-O-4 Ether Linkage

MeO

OMe OH

HO

O

O

O

OH

β-1' Diphenyl Methane Linkage

OMe β-5 Phenylcoumaran Linkage

OMe

OMe OH

Diphenyl Ether Linkage 4-O-5'

Figure 3. Common linkages observed in the biopolymeric matrix of native lignin.14 Lignin in the Biorefinery - Nature’s Handiwork Undone The literature reveals a surprising tendency to use illustrations similar to Figure 1 to describe lignin’s structure within the biorefinery, especially in more recent literature examining new catalytic methodology for its conversion. Further, the biorefining industry has not settled on an optimum pretreatment/fractionation process, leading to investigation of both a wide array of lignins as starting materials and multiple conversion processes.


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When considering new methodology for lignin valorization, it is critical to realize that any process used in the biorefinery to isolate lignin converts native material into technical lignin and inevitably induces structural changes in the native material through loss of some substructural units and introduction of new interunit linkages. For example, quantitative

13

C analysis reveals that the level of b-O-4 units in lignin isolated during organosolv

fractionation of switchgrass drops significantly as the severity of the fractionation increases.50 Dilute acid pretreatment was shown to reduce b-O-4 units by 36%,51 and steam explosion nearly eliminates them under proper conditions.52 Thus, the use of lignin as a chemical feedstock requires identifying transformational processes able to accommodate functional groups present after lignin isolation, not only those introduced during biosynthesis. LIGNIN VALORIZATION Lignin Valorization Began with the Pulp and Paper Industry Access to lignin as a separate process stream is a recent benefit of lignocellulosic biorefinery development, as for decades, the great bulk of the world’s lignin supply was sequestered within the confines of the pulp and paper industry - still the largest current practitioner of lignocellulosic biorefining. That industry’s focus on cellulose pulp (~130 million tons/yr worldwide, mostly by the kraft process),53 requires that lignin be removed as completely as possible from the biopolymeric matrix. To achieve this transformation, wood is treated with a mixture of aqueous NaOH and Na2S at temperatures between 170 - 180oC. This process manufactures cellulose by solubilizing most of the lignin present in the starting feedstock and generating black liquor, a mixture of spent pulping chemicals and dissolved lignin. The great majority of black liquor is concentrated and fed to a complex recovery boiler that burns the entrained lignin as a biofuel, generating more than 50% of the industry’s internal energy needs directly on-site54 and simultaneously regenerating the chemicals used in the next pulping run. Lignin is clearly not a waste in this process (in contrast to common characterizations in the literature), but rather, is crucial to the operation and energy balance of the mill. This process, while “selective” (i. e., black liquor combustion generates two primary products: CO2 and H2O), is the least effective means of lignin valorization, as it only recovers lignin’s fuel value. Even the small amounts of residual lignin remaining in the cellulose pulp suffer a fate analogous to black liquor combustion, as the pulp is subjected to a sequence of bleaching steps whose sole purpose is to carry out a nonselective deconstruction of the remaining lignin into small fragments that are easily washed from the cellulose. The pulp and paper industry is not devoid of lignin valorization processes. The kraft industry isolates a small portion of the lignin contained in black liquor by precipitation with CO2 for commercial use. In parallel, the much smaller sulfite pulping industry generates lignosulfonates sold as commercial products (about 1 million tons/yr).55 These lignosulfonates can also be used as a source of vanillin via low yield oxidation processes.56 However, these


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transformations represent only a small portion of the huge amount of lignin generated and burned on an annual basis, more than 70 million tons/yr.55 It is estimated that only about 2% of the annual production of lignin from the pulp and paper industry is used for manufacture of high value products.39,57 Catalytic Reactions - High Selectivity Remains a Challenge As lignin production and availability expands beyond the pulp and paper industry to include lignocellulosic biorefining, successful lignin valorization must incorporate accurate information regarding lignin after isolation, along with design of catalysts able to accommodate its structure. Much of the ongoing work in catalysis for lignin conversion has targeted improving nonselective lignin pyrolysis, oxidation, and reduction. While high selectivity in these processes has not yet been realized, and the potential for such processes to be economically viable remains unclear, the ability to favor production of classes of compounds (aromatic, phenols, etc.) is improving. The following sections provide some representative examples of recent research. Pyrolysis and Catalytic Upgrading A crucial step toward lignin valorization is the ability to disrupt the structurally complex lignin into smaller, simpler subunits. Pyrolysis, a low cost process of wide applicability,58 depolymerizes lignin under anaerobic conditions via rapid heating of the feedstock to form monolignols, monophenols, volatile compounds, gaseous compounds, polysubstituted phenols, and biochar.14 More specific ranges of products can be obtained by the choice of an appropriate heterogeneous catalyst to upgrade the oil following pyrolysis. The zeolite H-ZSM-5 is frequently used for catalytic cracking, as described by Sharma and Bakhshi,59,60 Chantal et al.,61 and Gayubo and Valle et al.,62-63 as it deoxygenates the initial building blocks to produce monomeric aromatic hydrocarbons, such as benzene, toluene, and xylene (BTX), as well as naphthalenes and other polycyclic aromatic hydrocarbons. MCM-41 is a mesoporous material that is arranged hexagonally, has moderate acidity, high surface areas, and a mean pore-size distribution.64-66 As a result, many large molecules, such as lignin-derived oligomers, can diffuse into the pores without blocking them. Thus, Chen et al. utilized tungsten carbide (W2C), a selective deoxygenation catalyst,67 supported on MCM-41 for the catalytic upgrading of lignin pyrolysis vapors and produced arenes in 20% yield, with an 85% selectivity toward monocyclic arenes.58 Alumina-supported Pt catalysts have been utilized to convert kraft, organosolv, and sugarcane bagasse lignin into aromatic monomers with low oxygen content via a two-step process. In an initial liquid phase reforming step, lignin in an alkaline mixture of ethanol and water was depolymerized over 1 wt. % Pt/g-Al2O3, producing guaiacol, syringol, and phenols. In the second step, the lignin oil extracted from the mixture was subjected to


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hydrodeoxygenation over both Mo2C/CNF and CoMo/Al2O3 catalysts to produce BTX and alkyl benzenes. Mo2C/CNF-catalyzed reactions gave total monomer yields for kraft, sugarcane bagasse, and organosolv lignin of 7, 6, and 9%, respectively, with oxygen-free products comprising 15, 16, and 36% of the material. CoMo/Al2O3catalyzed reactions gave total monomer yields for the same lignins of 6, 5, and 6%, respectively, with oxygenfree compounds composing 15, 20, and 25% of the total products observed.68 Titanium dioxide (TiO2) is an effective photocatalyst for the degradation of lignin in wastewater.69-71 Mante et al. evaluated anatase TiO2 to determine its potential for the oxygen removal from monomeric phenols produced during lignin pyrolysis and observed phenol defunctionalization via dehydroxylation, demethoxylation, decarbonylation, and decarboxylation at greater than 97% lignin conversion.72 Olcese et al. sought out to enhance the properties of pyrolysis vapors by targeting a distillable mixture rich in benzene and phenols. Lignin was pyrolyzed at 673 K, followed by upgrading with H2 in the presence of iron supported on either silica or activated carbon (AC). The pyrolysis procedure produced 5.7-6.1% oil, with the 10% Fe/AC catalyst producing the greatest amounts of phenols and alkylphenols.73 Lignin Oxidation using Metal-Organic and Metal-Free Catalysts Catalytic homogeneous lignin oxidation processes are offering new opportunities for the synthesis of chemicals from lignin although process yields still generally remain low. Co-Schiff base complexes have emerged as potential lignin oxidation catalysts.74-76 Specifically, novel asymmetric Co-salen catalysts were developed that incorporated a bulky, aliphatic, nitrogeneous base (Bn-piperazine) onto one of the salicylaldehyde moieties (Figure 4). After determining that the catalyst effectively oxidized S and G lignin models in high yield, it was then evaluated for oxidative activity toward organosolv lignin obtained from tulip poplar (Figure 4).77 The Cosalen catalyst oxidized the lignin in low yield, which was attributed to the lignin having only a low number of free phenolic groups after its isolation.78 Additional studies have used supported metal-Schiff base oxidation catalysts. Gao et al. synthesized Co-salen catalysts supported on a network of ceramic fiber (forming a catalyst “paper”) and examined their activity for kraft lignin oxidation using H2O2.79 The Co-salen paper catalyst gave a 23% yield of oil (relative to the Co-salen catalyst) comprising mostly syringaldehyde, vanillin, and 2,6-dimethoxyphenol. This increase in activity and selectivity of the catalyst paper was attributed to the porosity of the paper, which led to improved catalytic accessibility to the substrate. Zhou et al. supported both Cu-salen and Cu[H4]-salen complexes on NaY zeolite (Figure 5)80 and evaluated their activity for the oxidation of kraft lignin into phenolic monomers with peracetic acid. NaY-supported Cu[H4]-salen was the most active and selective (~80 mol %) for the production of guaiacol.80 There have also been recent studies involving lignin oxidation using homogeneous vanadium complexes.81 Diaz-Urrutia et al. evaluated homogeneous Co, Cu, and oxovanadium complexes for the depolymerization and


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oxidation of organosolv lignin and determined that the oxovanadium(V) complexes [VO(OiPr)Ln], (Figure 6, Ln = dipicolinate [dipic] or 8-oxyquinoline [HQ])82 provided the highest activities. The most efficient oxidation was observed using the oxovanadium(V) catalyst BPAMP(VV)(O)(OiPr) [H2PAMP = N,N-bis-(3,5-di-tert-butyl-2phenol)-N-(methylpyridine)amine] that includes a tethered pyridine (Figure 6).82Focusing on the selective oxidation of the β-O-4 linkage, Lancefield et al. utilized a DDQ/tBuONO/O2 system for a one-pot oxidation of organosolv birch lignin, followed by a zinc-catalyzed cleavage reaction of the C-OAr bond. Through the aforementioned oxidation/cleavage reactions, phenolic monomers were formed in a total yield of 6% and then converted to sinapyl alcohol.83

O Poplar Lignin

O

10 mol % Co(Salen) O2 (50 psi) MeOH/DMSO RT, 72h 3.5% overall mass isolated

MeO

OMe

CHO OMe

+ MeO O :

Ratio: 13

:

1

Co O

OMe

OMe

OH 10

OH :

5

N

N tBu

+

MeO

O

CHO

+

tBu

O tBu

N N Bn

Figure 4. Co-salen-catalyzed oxidative cleavage of organosolv lignin obtained from tulip poplar.77

O O Si O

O N

N

O Si O

O O Si O

Cu O

O

Cu-Salen-NaY

O N N H Cu H O O

O Si O

Cu[H 4]-Salen-NaY

Figure 5. NaY-supported Cu- and Cu[H4]-salen complexes used for the oxidation of kraft lignin in the presence of peracetic acid.80


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ACS Sustainable Chemistry & Engineering O O

O V

N O

OiPr

N

i

O

V

O

OiPr

O

tBu

tBu

VO(OiPr)(HQ)

tBu

N

O

O

VO(O Pr)(dipic)

tBu

O V

N

OiPr N

O

BPAMP(V V)(O)(OiPr)

Figure 6. Oxovanadium(V) complexes used for the oxidation of organosolv lignin.82 Metal-free lignin oxidations have recently appeared. Rahimi et al. determined that 4-acetamido-TEMPO in the presence of HCl and HNO3 effectively catalyzed the chemoselective aerobic oxidation of secondary benzylic alcohols in up to 94% yield.84 They further depolymerized oxidized aspen lignin using formic acid/sodium formate at 110oC and isolated 61% of the original lignin, more than half of which was low molecular weight aromatics, such as vanillin and syringaldehyde. The result was described as one of the highest reported yields of monomers obtained from lignin depolymerization. These results are significant to lignin valorization in that these products have commercial value as a stream of soluble, low molecular weight aromatic feedstocks for further catalytic upgrading.85 Lignin Reduction Lignin reduction processes have been primarily based on heterogeneous systems. Early studies with supported lignin reduction catalysts, such as sulfided Ni-Mo/SiO2-Al2O3,86 sulfided Co-Mo/Al2O3,87 and Co-Mo/Al2O3,88 investigated hydrocracking and hydrogenolysis of simple oxygenated aromatics, such as phenol, guaiacol, syringol, anisole, and o-cresol.86,89 Drawing on these model studies, Toledano et al. screened hydrogenolysis catalysts (Pd, Pt, Ni, Ru) supported on Al-SBA-15 and examined them in the hydrogen-free hydrogenolysis of organosolv lignin using tetralin or formic acid as hydrogen donors. A 10 wt. % Ni catalyst was most effective, giving a 30% yield of bio-oil after 30 minutes of microwave irradiation. Of the bio-oil produced, syringaldehyde and mesitol were identified, but the amount of biochar and residual lignin was high, and lignin repolymerization was observed.90 In contrast, use of this catalyst with H2 for the reduction of poplar organosolv lignin gave 84% conversion, with 45% of the product mixture comprising saturated cycloalkanes.91 Continuous hydrogenolysis of kraft lignin was carried out using a Ti-N/Ni catalyst and afforded a 23 wt% yield of compounds with a molecular weight less than 1000 and a 40 wt% yield of compounds with a molecular weight between 1000-2000. Structural characterization of the complex mixture was not reported.92 Singh et al. developed a Cu-Mo-doped ZSM-5 for a reductive one-pot deposition-precipitation reaction in water/methanol to convert lignin to alkyl phenols with a


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minimum of char formation. At 95.7% lignin conversion, about 12 wt% of monomeric phenols was recovered, with 70.3 wt% of this fraction being 3-methoxy-2,5,6-trimethylphenol, or about 8 wt% yield based on the starting lignin. The hydrogen in the reaction was generated via MeOH reforming and a water-gas shift reaction.93 Homogeneously catalyzed reduction of lignin has been less widely investigated. Nagy et al. utilized Ru(PPh3)3Cl2 for the reduction of organosolv lignin via catalytic hydrogenation.94 Recent work by Wu et al. used Ru(H)2CO(PPh3)(xantphos) as a catalyst for the hydrogenolysis of lignin and observed trace amounts of activity.95 Lignin First - Taking Control of the Process While reduction of isolated lignin generally demonstrates low selectivity, recent work suggests that bypassing isolation of a separate lignin fraction and carrying out reductive processes on whole lignocellulosic biomass may offer a significant new opportunity for converting lignin into smaller numbers of low molecular weight aromatics (Figure 7).43,45,96-103 A key feature of these processes is their ability to decrease the amount of lignin repolymerization via reductive stabilization of reactive intermediates while also eliminating the additional structural complexity that arises as a result of isolating the lignin as a separate process stream. Such reductive transformation of whole biomass is not new. Very early work in this area examined whole maple or spruce samples under harsh, exhaustive hydrogenation conditions (Cu/CrO3 catalyst, 250 - 280oC, 50006000 psi H2, 12-16h) and was carried out for the purpose of understanding lignin’s structure. The authors initially estimated that 60-70% of the Klason lignin in maple and 35-40% of spruce was converted to 4propylcyclohexanol (1) and 4-propyl-1,2-cyclohexanediol (2),104 but a subsequent study revised these numbers down to ~ 36% of the wood and reassigned one of the compounds as 3-(4-hydroxycyclohexyl)-1-propanol (3).105 The aromatic analogs of these monomers were obtained by hydrogenolysis of spruce over Rh/C at 195oC and 500 psi H2 in aqueous dioxane for 5h. Approximately 34% of the total lignin was converted to n-propylguaiacol 4 and dihydroconiferyl alcohol 5.96 Similar reduction of aspen led to these compounds, as well as their syringyl counterparts, and accounted for over 40% of the total lignin in the wood.106 R1

R1

HO R2

R1

HO R3

1 - R1, R 2, R 3 = H 2 - R1 = OH, R 2, R 3 = H 3 - R1, R 2 = H, R 3 = OH

R2

HO R3

4 - R1 = OCH3, R 2, R 3 = H 5 - R1 = OCH3, R 2 = H, R 3 = OH 6 - R1, R 2 = OCH3, R 3 = H 7 - R1, R 2 = OCH3, R 3 = OH

R2 8 - R1 = OCH3, R 2 = H 9 - R1, R 2 = OCH3

Figure 7. Monomers obtained via the “lignin-first” approach.43,45,96-103


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More recent work on such “lignin-first” approaches have improved process selectivity through design of new catalyst systems. A particularly selective reaction treats poplar with 500 psi H2 at 225oC over a Pd nanoparticle catalyst with additional Zn sites dispersed on a carbon support. The process converts 40-54% of the available lignin into a mixture of 4 and propylsyringol 6 as the only products. A similar reaction with a genetically modified high syringyl poplar gave a correspondingly higher proportion of 6 as the only phenolic product.97 In similar work, miscanthus was treated with H2 between 150 - 500 psi at 225oC in MeOH over a Ni/C catalyst. Four primary propylphenol derivatives from the S, G, and H units were isolated and accounted for 68% of the lignin present in the starting feedstock. The process also afforded a solid carbohydrate residue that could serve as a starting material for the production of furfural and levulinic acid.98 Commercial application of this process is under investigation by Spero Energy as a means to convert both the lignin and carbohydrate fractions of biomass into fuels and higher value materials. Inexpensive, earth-abundant Ni catalysts were also used to carry out hydrogenolysis of birch sawdust at 235oC and 870 psi H2 over a Ni-W2C catalyst on activated carbon. Both the carbohydrate and lignin fractions were converted, with the latter giving a 37% yield of phenols, 84% of which were 4, 6, or their immediate precursors.99 Song et al. reported an interesting hydrogen-free reduction of birch sawdust at 200oC in alcoholic solvents using Ni/C as the catalyst. A 54% conversion of lignin was observed, with 89% selectivity toward 4 and 6, accounting for over 90% of the available ether bonds in the starting feedstock. The reaction is postulated to proceed via an initial solvolysis of the lignin by the alcoholic solvent, followed by hydrogenation of the intermediate lignin fragments, with activated hydrogen being supplied to the reaction through the Ni-catalyzed conversion of the MeOH solvent.102 Building on this work, Klein et al. depolymerized wood lignin into methoxypropylphenols using Ni/C in methanol and determined that birch gave the highest yields of monomer relative to eucalyptus and poplar.103 The presence of typical biomass pretreatment catalysts also has an impact on the products observed during lignin-first processing. Treatment of poplar with 290 psi H2 in MeOH at 150oC in the presence of Pd/C and H3PO4 affords a lignin oil, whose composition is up to 35 mol% 5 and dihydrosinapyl alcohol 7. Additional components of the transformation included lignin dimers. However, carrying out the same reaction under alkaline conditions gave only a 25% total yield of lignin monomers.43 Reductive treatment of birch sawdust in MeOH at 450 psi and 250oC over a Ru/C catalyst afforded 90% solubilization of the lignin and production of 52% phenol monomers with a 79% selectivity toward 4 and 6. An additional 16% of the lignin was identified as simple dimers.45 Reductive treatment of birch sawdust with either Ru/C or Pd/C in MeOH at 250o and 450 psi H2 afforded 85-90% delignification, a monomer yield of 50%, and a dimer yield of 15%. Pd/C favored production of 5 and 7, while Ru/C led to the formation of 4 and 6 as the primary monomeric products.100 Recently, catalytic transfer hydrogenation (Pd/C, EtOAc/H2O, 120oC, 36h) has been used in a two-step organosolv/reduction process to convert pine organosolv lignin into 4-propenyl guaiacol 8 in 23% yield and birch


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lignin into 4-propenyl syringol 9 in 49% yield, all based on total lignin. Neither reduction required the addition of H2, but instead used formic acid generated during the pulping process as an internal source of reductant.101 Catalytic transfer hydrogenation in i-PrOH catalyzed by Raney Ni has also been used for the conversion of the lignin in poplar pellets.44 Tailoring of Lignin’s Structure for Valorization As illustrated by different examples in this perspective, the primary barrier to selective lignin valorization is the heterogeneity of the lignin polymer, introduced both during biosynthesis and during isolation processes in the biorefinery. This continuing issue suggests that the problem lies not only with the technology, but with the lignin itself. Extensive research has identified the enzymes and structural intermediates used by lignocellulosic plants for the stepwise construction of the three primary monolignols used during lignification: p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol (Figure 8). This deep understanding of lignin biosynthesis47-48 has enabled modification of monolignol composition and structure (and ultimately, the structure of the lignin polymer itself) by selectively up- or downregulating many of the specific enzymatic steps shown in Figure 8.107-108 The geological processes that form petrochemicals cannot be controlled in an analogous manner, and thus, conversion processes must adapt to the structure of the raw material as isolated. In stark contrast, renewable carbon sources can be adapted to the transformation. Using biotechnology and genetic engineering to control the structure of the raw material before its formation is unique to biobased feedstocks and a critical advantage that renewable carbon sources have over nonrenewable. Such work could have a significant impact on selective lignin transformations by reducing lignin’s heterogeneity and improving its utility as a chemical feedstock.109-111 Demonstration of chemical production from transgenic lignocellulosic material is still in its infancy, as is an understanding of the structural changes that will occur in the transgenic lignin upon its isolation. The following examples illustrate the potential in modifying lignin’s structure prior to its use, and how this approach may offer significant new opportunities for producing lignin based chemicals. Three general approaches have emerged that take advantage of controlling lignin biosynthesis to improve its utility and performance in the biorefinery. Given the continued ethanol-centric focus of the lignocellulosic biorefinery, many of these approaches have been designed to improve access of saccharification enzymes to the lignocellulosic polysaccharides. However, these approaches also offer the opportunity to generate lignin whose properties are synchronized with processes for eventual conversion to high value chemicals. There are certainly overlaps between these general categories, but each serves to demonstrate how engineering the raw material may be critical to the future operation of the biorefining industry.


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1) Reducing the lignin content in the starting feedstock. Lignin forms a physical barrier within the cell wall that inhibits enzymatic saccharification. At the same time, lignin’s hydrophobic nature can adsorb hydrolytic enzymes.112-113 Modification of lignin biosynthesis has therefore been used as a simple means to reduce the amount of lignin in the plant and thus improve access to the fermentable sugars in the lignocellulosic raw material. In several cases, this hypothesis has been reduced to practice. Chen and Dixon reported a direct correlation between the amount of lignin in six different transgenic alfalfa mutants and the level of recalcitrance during pretreatment. The plants with the lowest amount of lignin afforded higher levels of sugar after enzymatic digestion.114 Downregulation of COMT in switchgrass reduced lignin content and led to a 38% increase in ethanol yield when compared to wild type switchgrass.115 Downregulating the C3H and C4H genes in eucalyptus led to a 26-36% reduction in lignin and almost 100% release of sugars after a hot water pretreatment, 15-20% better than the wild type.116 Transgenic poplar with reduced lignin content (up to 50% in some samples) was subjected to both organosolv and steam explosion pretreatments and exhibited greatly improved sugar release (nearly quantitative from low lignin material) after enzymatic hydrolysis.111 When moved from the greenhouse to the field, transgenic poplar downregulated for CCR showed reduction in lignin and up to 161% increase in ethanol yield compared to the wild type.117 A particularly novel approach to lignin reduction recognizes the central role played by phenoxy radical formation in lignin biosynthesis. Arabidopsis was engineered to overexpress monolignol-4-O-methyltransferase, a methyltransferase targeting the 4-OH group of standard monolignols. By converting the 4-OH group of a portion of the plant’s monolignols to the corresponding methyl ether, the ability of the monolignol to form the phenoxy radical required for polymerization was eliminated. Plants with this mutation showed lignin reductions of as much as 24% compared to the control and displayed no significant alteration of the plant phenotype. The mutation did not interfere with normal lignification, as the plant simply formed lesser amounts of a structurally conventional lignin. No evidence was found for incorporation of the methylated monolignols, which apparently became metabolic dead-ends and were detoxified by conjugation to cell wall saccharides. Importantly, precursors to ferulic acid also underwent methylation, which suggests a process for reducing the ferulate-enabled crosslinking of biopolymers within the cell as a further mechanism to improve digestability.118 While these modifications often lead to improved saccharification and fermentation of the contained polysaccharides, there is also a frequent cost in the form of lost overall biomass yield, xylem collapse, or severe dwarfing of the plants. However, by disrupting a transcriptional co-regulatory complex (Mediator) in Arabidopsis, normal growth was recovered, with the resulting lignin consisting almost exclusively of H units. Further, the number of b-O-4 units was significantly reduced, while the number of phenylcoumaran and resinol linkages were increased by two- and threefold, respectively.119 These results indicate the level of control that is being developed over the structure of lignin, which is critical for its downstream conversion.


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ONH 3+ phenylalanine PAL

O

O

O O-

O

C4H

C4H/C3'H

O-

HO

p-coumarate

caffeate

O-

O O

C3'H O

4CL

HO

OH

O

OH

4CL

4CL

caffeoyl shikimate

O HO OH

OCH3 SCoA CCoAOMT HO OCH3

p-coumaraldehyde

H

HO OH

HO

OCH3

OH

COMT

OH caffeoyl alcohol

H

COMT

OCH3 coniferyl alcohol

H OCH3

OCH3

F5H

O

MeO HO

CAD

HO

HO

O

HO

5-OH coniferaldehyde

CAD

OH

p-coumaryl alcohol

CCR

HO

coniferaldehyde

CAD

OH

F5H

HO

caffealdehyde

CAD

CCR

O H

HO

OCH3 sinapoyl CoA

CCR

O

H

SCoA

SCoA

feruoyl CoA

CCR

O

H 3CO COMT HO

5-OH feruloyl CoA

O

caffeoyl CoA

O

SCoA

HO

O

CCR

O

HO

HCT

p-coumaryl CoA

4CL

OH

OH

HCT SCoA

OCH3 sinapate

4CL

HO

HO

OCH3 5-OH ferulate

O

OH

p-coumaryl shikimate

OCH3

O-

HO

O-

O

CSE

COMT

ferulate

O

H 3CO

O-

F5H HO

HO

OH

HO cinnamate

COMT

O

HO

O-

O-

sinapaldehyde CAD

HO

OH COMT

HO

MeO

OH

HO OCH3

OCH3

5-OH coniferyl alcohol

sinapyl alcohol

PAL - phenylalanine lyase; C4H - cinnamyl 4-hydroxylase; 4CL - 4-coumarate CoA ligase; HCT - p-hydroxycinnamoyl CoA transferase; C3'H - p-coumaryl 3-hydroxylase; CSE - caffeoyl shikimate esterase; CCoAOMT - caffeoyl CoA methyltransferase; F5H - ferulate 5-hydroxylase; COMT - 5-hydroxyconiferaldehyde O-methyltransferase; CCR - cinnamoyl CoA reductase; CAD - cinnamyl alcohol dehydrogenase

Figure 8. The phenylpropanoid pathway for lignin biosynthesis (adapted from107-108). An alternative process suggested to reduce the phenotypic shortcomings of some transgenic materials is engineering of the system to produce specific proteins that restrict expression to specific locations within the plant. The selectivity of such proteins can be used to incorporate alternate monolignols within the polymer. For example, Arabidopsis was engineered to convert 3-dehydroshikimate to protocatechuate, reducing the amounts of lignin, coniferyl alcohol, and sinapyl alcohol, among other plant-based aromatics. The resulting plant showed marked changes in its metabolite profile, an increase in the relative amount of H-lignin, and improved reactivity in saccharification.120 Ongoing work has revealed additional approaches for overcoming the growth penalty,119 but more work is needed to fully understand the links between reducing lignin content, conversion efficiency, and plant phenotype.121 2) Biosynthetic modification of lignin composition. Related work recognizes that the structure of the lignin may be as significant as the amount and that biosynthetic control may be employed to design the lignin polymer from the ground up. While lignin dogma posits the construction of a heterogeneous polyaromatic from the three primary (or “canonical”108) monolignols, changes in polymerization conditions or the presence of alternative aromatics can result in lignification proceeding from a different set of initial building blocks. A very recent review


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describes the potential in this approach, which takes advantage of the “plasticity” of lignin.108 Lignification appears to be highly adaptable, allowing construction of a wide variety of non-canonical lignins depending on the intermediates available within the plant at any given time. Recent work has demonstrated the ability of tricin, a flavonoid, to act as a monolignol.122 Indeed, more than 150 different, naturally occurring building blocks have been identified that could conceivably be incorporated during lignification.110 It appears that plants accommodate lignin of widely differing structures as long as the plant function served by the lignin is retained. A large number of lignin modifications has been reported. By regulation of proper enzymes, lignin enriched in, or almost exclusively composed of H, G, or S units can be prepared123 and lead to lignocellulosic materials with significantly altered properties. Transgenic poplar in which expression of F5H was driven by the cinnamate4-hydroxylase promoter (C4H::F5H) afforded a lignin composed almost exclusively of S units and gave a woody feedstock that underwent pulping in half the normal amount of time to give higher quality cellulose than wild type poplar.124 By combining a mutation in the gene encoding COMT with the overexpression of C4H::F5H1 fusion in Arabidopsis, lignification proceeded almost exclusively through 5-hydroxyphenylpropanoids (hydroxyconiferyl and hydroxysinapyl alcohols), generating a lignin unknown in Nature that was almost 92% benzenedioxane units.125 Similarly, “new” lignin was generated by knocking out the CAD gene in Medicago trunactula. Lignification led to a biopolymer constructed almost exclusively (~95%) from coniferaldehyde and sinapaldehyde.126 Aldehyde polymerization rises to almost 99% in Arabidopsis by disrupting the genes encoding for CAD. Coniferaldehyde-derived lignin did not appear to interfere with plant growth, but sinapaldehyde-derived material led to the formation of severely dwarfed plants. The resulting biomass exhibited significantly improved reactivity toward enzymatic hydrolysis when compared to wild type material.107 By downregulating CCR in poplar, lignin production was reduced, but carbon flux during lignin biosynthesis was transferred toward increased ferulic acid production, leading to its incorporation in the lignin polymer.127 By suppressing CCoAOMT in Pinus radiata, lignification led to the incorporation of caffeoyl alcohol as a monolignol.128 Tissue-specific methodology has been used to overproduce hydroxycinnamoyl-CoA hydratase-lyase in Arabidopsis. This enzyme, in short, cleaves the propanoid side chain of the monolignol precursors, leading to formation of hydroxybenzaldehydes. When incorporated into the lignin polymer, they serve as end groups and reduce the overall polymerization degree of the growing polymer.129 3) Design of lignin for deconstruction. The dramatic progress that has been made in understanding lignin biosynthesis offers the opportunity for constructing a lignin polymer that retains function necessary for healthy plant growth while simultaneously containing interunit linkages that afford simpler deconstruction and utility of the resulting intermediates for conversion to high value chemicals. By incorporating a tyrosine-rich peptide sequence into lignifying tissue in poplar, peptide crosslinks were introduced into the lignin that were expected to make lignin susceptible to deconstruction when treated with a protease. Several of these transgenic materials showed improved sugar release during saccharification compared to wild type after protease treatment.130-131 A


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model system study using 2D NMR has identified a linkage mechanism for a cysteine oligopeptide to monolignols, but the tyrosine linkage process is still not understood.132 A particularly elegant example of this approach is the incorporation of ester linkages into the lignin polymer by the site-specific overexpression of feruloyl-CoA monolignol transferase.133 When expressed in transgenic poplar, an estimated 7-23% of the interunit linkages possessed a labile ester linkage. Mild alkaline pretreatment of this lignin led to improved digestibility during conversion to EtOH.133 This “zip” lignin is a leading example of how lignin structural modification can be employed for the purposes of improved chemical production. CONCLUSIONS Lignin is a critical biomass component that has the potential to produce valuable chemicals and fuels, making its valorization central to modern biorefinery development. The last six years have witnessed significant progress regarding catalysis to improve its transformation, as well as multiple breakthroughs in understanding and controlling its biosynthesis. However, the greatest opportunity and value proposition for the biorefinery still lies in developing processes that target single compounds from lignin. Although catalytic upgrading of thermochemical processes for lignin (pyrolysis, hydrodeoxygenation, hydrogenolysis, etc.) has improved its ability to produce specific classes of compounds and retains a prominent role in lignin valorization efforts, the sheer complexity of these mixtures limits their potential value. There is a vast difference between a valuable mixture of chemicals and a mixture of valuable chemicals. Catalytic oxidation and reduction processes are offering new insight into lignin transformation, as they address lignin’s complexity by better targeting of specific functionality present in most, if not all technical lignins - the aromatic rings, increased levels of free phenolics, or oxygenation at the benzylic position. Similarly, “lignin-first” approaches using catalytic reduction of whole biomass are able to use native lignin and have revealed a marked improvement in the selectivity of lignin transformation. By eliminating lignin separation and the structural changes that are introduced by the production of technical lignin, catalysis can effectively target a single structure (b-O-4) that makes up as much as 60% of the linkages in lignin, rather than attempting to consume the wider range of more recalcitrant linkages resulting from lignin isolation. Finally, the control of lignin biosynthesis presents a fundamentally new opportunity to synchronize catalyst design with a tailored lignin structure. While this effort is still new, the potential of developing a more structurally uniform lignin may have a dramatic impact on whole biomass transformations, or the use of isolated lignin within the biorefinery, by improving reactivity, tailoring lignin’s structure for use, and simplifying its selective deconstruction into small numbers of useful renewable building blocks. Such improvements will offer a significant step forward in the valorization of lignin and its utility in the production of high value biorefinery chemicals.


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AUTHOR INFORMATION Corresponding Author Prof. Joseph J. Bozell. E-mail: [email protected]. Tel.: 865-974-5991 Notes The authors declare no competing financial interest. Biographies

Joseph J. Bozell is Professor of Biomass Chemistry in the Center for Renewable Carbon at the University of Tennessee and has worked in the development of new methodology for the conversion of renewables to chemicals for more than 25 years. His research interests include new synthetic methodology for conversion of renewable carbon and tailoring of homogeneous catalysts for the transformation of carbohydrates and lignin to high value chemicals. Rebecca Key received her Ph.D. in Organic Chemistry in 2015 at the Georgia Institute of Technology under the direction of Stefan France and Christopher Jones. She is currently a Postdoctoral Research Associate at the University of Tennessee under the direction of Professor Joseph J. Bozell. Her current research focuses on covalently binding Co-salen catalysts to lignin to produce high-value, low molecular weight chemicals and fuels.


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ACKNOWLEDGEMENTS We would like to thank the Center for Direct Catalytic Conversion of Biomass to Biofuels (C3Bio), an Energy Frontier Research Center (EFRC) funded by the United States Department of Energy, Office of Science, Basic Energy Sciences, award DE-SC0000997. We are grateful to Professor Clint Chapple of Purdue University for useful comments on the tailoring of lignin’s structure. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

(12) (13) (14) (15) (16) (17)

Yergin, D. The Prize; Free Press: New York, 1991. Weissermel, K.; Arpe, H.-J. Industrial Organic Chemistry, 4th edition; Wiley-VCH: Weinheim, 2003. Guide to the Business of Chemistry 2006; American Chemistry Council: Arlington, 2006. Marshall, J. Biorefineries: Curing our addiction to oil. New Scientist 2007, 2611 (July 4, 2007), 28-31. Hughes, J. D. Drill, Baby, Drill: Can Unconventional Fuels Usher in a New Era of Energy Abundance?; Post Carbon Institute: Santa Rosa, CA, 2013. Howarth, R. W.; Santoro, R.; Ingraffea, A. Methane and the greenhouse-gas footprint of natural gas from shale formations. Climatic Change 2011, 106, 679-690. Inventory of U. S. Greenhouse Gas Emissions and Sinks: 1990-2011. 2013, U. S. Environmental Protection Agency, EPA 430-R-13-001. Jackson, R. B.; Vengosh, A.; Darrah, T. H.; Warner, N. R.; Down, A.; Poreda, R. J.; Osborn, S. G.; Zhao, K. G.; Karr, J. D. Increased stray gas abundance in a subset of drinking water wells near Marcellus shale gas extraction. Proc. Natl. Acad. Sci. 2013, 110, 11250-11255. Ellsworth, W. L.; Hickman, S. H.; Lleons, A. L.; Mcgarr, A.; Michael, A. J.; Rubinstein, J. L. Are seismicity rate changes in the midcontinent natural or manmade? Seismological Society of America 2012 Annual Meeting, San Diego, CA. Bellard, C.; Bertelsmeier, C.; Leadley, P.; Thuiller, W.; Courchamp, F. Impacts of climate change on the future of biodiversity. Ecol. Lett. 2012, 15, 365-377. Moss, R. H.; Edmonds, J. A.; Hibbard, K. A.; Manning, M. R.; Rose, S. K.; van Vuuren, D. P.; Carter, T. R.; Emori, S.; Kainuma, M.; Kram, T.; Meehl, G. A.; Mitchell, J. F. B.; Nakicenovic, N.; Riahi, K.; Smith, S. J.; Stouffer, R. J.; Thomson, A. M.; Weyant, J. P.; Wilbanks, T. J. The next generation of scenarios for climate change research and assessment. Nature 2010, 463, 747-756. Solomon, S.; Plattner, G. K.; Knutti, R.; Friedlingstein, P. Irreversible climate change due to carbon dioxide emissions. Proc. Natl. Acad. Sci. 2009, 106, 1704-1709. Deuss, P. J.; Barta, K.; de Vries, J. G. Homogeneous catalysis for the conversion of biomass and biomass-derived platform chemicals. Catal. Sci. Technol. 2014, 4, 1174-1196. Liu, W.-J.; Jiang, H.; Yu, H.-Q. Thermochemical conversion of lignin to functional materials: a review and future directions. Green Chem. 2015, 17, 4888-4907. Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. The path forward for biofuels and biomaterials. Science 2006, 311, 484-489. Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chem. Rev. 2006, 106, 4044-4098. Corma, A.; Huber, G. W.; Sauvanaud, L.; O'Connor, P. Processing biomass-derived oxygenates in the oil refinery: Catalytic cracking (FCC) reaction pathways and role of catalyst. J. Catal. 2007, 247, 307327.


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Page 19 of 34

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(18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29)

(30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40)


Azadi, P.; Inderwildi, O. R.; Farnood, R.; King, D. A. Liquid fuels, hydrogen and chemicals from lignin: A critical review. Renew. Sustainable Energy Rev. 2013, 21, 506-523. Vennestrom, P. N. R.; Osmundsen, C. M.; Christensen, C. H.; Taarning, E. Beyond Petrochemicals: The Renewable Chemicals Industry. Angew. Chem. Int. Ed. 2011, 50, 10502-10509. Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Weckhuysen, B. M. The catalytic valorization of lignin for the production of renewable chemicals. Chem. Rev. 2010, 110, 3552-3599. Ritter, S. K. In Chem. Eng. News; Vol. 86, p 59-68. Perlack, R. D.; Stokes, B. J. U. S. Billion-Ton Update: Biomass Supply for a Bioenergy and Bioproducts Industry. 2011, ORNL/TM-2011/224. Bozell, J. J.; Holladay, J. E.; Johnson, D.; White, J. F. Top Value Added Chemicals from Biomass. Volume II – Results of Screening for Potential Candidates from Biorefinery Lignin. 2007, Pacific Northwest National Laboratory, PNNL-16983. Dapsens, P. Y.; Mondelli, C.; Pérez-Ramírez, J. Biobased chemicals from conception toward industrial reality: lessons learned and to be learned. ACS Catal. 2012, 2, 1487-1499. Clark, J. H.; Luque, R.; Matharu, A. S. Green chemistry, biofuels, and biorefinery. Annu. Rev. Chem. Biomol. Eng. 2012, 3, 183-207. Anastas, P. T.; Warner, J. C. Green chemistry: theory and practice; Oxford University Press: New York, 1998. Hicks, J. C. Advances in C–O bond transformations in lignin-derived compounds for biofuels production. J. Phys. Chem. Lett. 2011, 2, 2280-2287. Dutta, S. Deoxygenation of biomass-derived feedstocks: hurdles and opportunities. ChemSusChem 2012, 5, 2125-2127. Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M.; Langan, P.; Naskar, A. K.; Saddler, J. N.; Tschaplinski, T. J.; Tuskan, G. A.; Wyman, C. E. Lignin valorization: improving lignin processing in the biorefinery. Science 2014, 344, 709-+. Lange, H.; Decina, S.; Crestini, C. Oxidative upgrade of lignin - Recent routes reviewed. Eur. Polym. J. 2013, 49, 1151-1173. Azadi, P.; Inderwildi, O. R.; Farnood, R.; King, D. A. Liquid fuels, hydrogen and chemicals from lignin: a critical review. Renew. Sus. Ener. Rev. 2013, 21, 506-523. Gasser, C. A.; Hommes, G.; Schaffer, A.; Corvini, P. F. X. Multi-catalysis reactions: new prospects and challenges of biotechnology to valorize lignin. Appl. Microbiol. Biotechnol. 2012, 95, 1115-1134. Picart, P.; de Maria, P. D.; Schallmey, A. From gene to biorefinery: microbial beta-etherases as promising biocatalysts for lignin valorization. Front. Micro. 2015, 6, 10.3389/fmicb.2015.00916. Laurichesse, S.; Avérous, L. Chemical modification of lignins: Towards biobased polymers. Prog. Polym. Sci. 2014, 39, 1266-1290. Li, C. Z.; Zhao, X. C.; Wang, A. Q.; Huber, G. W.; Zhang, T. Catalytic transformation of lignin for the production of chemicals and fuels. Chem. Rev. 2015, 115, 11559-11624. Chatel, G.; Rogers, R. D. Review: oxidation of lignin using ionic liquids-an innovative strategy to produce renewable chemicals. ACS Sus. Chem. Eng. 2014, 2, 322-339. Bozell, J. J. Approaches to selective catalytic conversion of lignin: a grand challenge for biorefinery development. Top. Curr. Chem. 2014, 353, 229-256. Rinaldi, R.; Jastrzebski, R.; Clough, M. T.; Ralph, J.; Kennema, M.; Bruijnincx, P. C.; Weckhuysen, B. M. Paving the way for lignin valorisation: Recent advanced in bioengineering, biorefining and catalysis. Angew. Chem. Int. Ed. 2016, 55, 2-54. Gordobil, O.; Moriana, R.; Zhang, L. M.; Labidi, J.; Sevastyanova, O. Assesment of technical lignins for uses in biofuels and biomaterials: Structure-related properties, proximate analysis and chemical modification. Ind. Crop. Prod. 2016, 83, 155-165. Mäki‐Arvela, P.; Holmbom, B.; Salmi, T.; Murzin, D. Y. Recent progress in synthesis of fine and specialty chemicals from wood and other biomass by heterogeneous catalytic processes. Catal. Rev. 2007, 49, 197-340. 19 ACS Paragon Plus Environment


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(46) (47) (48) (49) (50) (51) (52) (53) (54) (55) (56) (57) (58) (59) (60) (61) (62) (63)

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Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Liquid-phase catalytic processing of biomass-derived oxygenated hydrocarbons to fuels and chemicals. Angew. Chem. Int. Ed. 2007, 46, 7164-7183. Fengel, D.; Wegener, G. Wood. Chemistry, Ultrastructure Reactions; Walter de Gruyter: Berlin, 1984. Renders, T.; Schutyser, W.; Van den Bosch, S.; Koelewijn, S. F.; Vangeel, T.; Courtin, C. M.; Sels, B. F. Influence of acidic (H3PO4) and alkaline (NaOH) additives on the catalytic reductive fractionation of lignocellulose. ACS Catal. 2016, 6, 2055-2066. Ferrini, P.; Rinaldi, R. Catalytic biorefining of plant biomass to non-pyrolytic lignin bio-oil and carbohydrates through hydrogen transfer reactions. Angew. Chem. Int. Ed. 2014, 53, 8634-8639. Van den Bosch, S.; Schutyser, W.; Vanholme, R.; Driessen, T.; Koelewijn, S. F.; Renders, T.; De Meester, B.; Huijgen, W. J. J.; Dehaen, W.; Courtin, C. M.; Lagrain, B.; Boerjan, W.; Sels, B. F. Reductive lignocellulose fractionation into soluble lignin-derived phenolic monomers and dimers and processable carbohydrate pulps. Energ. Environ. Sci. 2015, 8, 1748-1763. Achyuthan, K. E.; Achyuthan, A. M.; Adams, P. D.; Dirk, S. M.; Harper, J. C.; Simmons, B. A.; Singh, A. K. Supramolecular self-assembled chaos: polyphenolic lignin's barrier to cost-effective lignocellulosic biofuels. Molecules 2010, 15, 8641-8688. Boerjan, W.; Ralph, J.; Baucher, M. Lignin biosynthesis. Ann. Rev. Plant Biol. 2003, 54, 519-546. Vanholme, R.; Demedts, B.; Morreel, K.; Ralph, J.; Boerjan, W. Lignin biosynthesis and structure. Plant Physiol. 2010, 153, 895-905. Chakar, F. S.; Ragauskas, A. J. Review of current and future softwood kraft lignin process chemistry. Ind. Crops Prod. 2004, 20, 131-141. Bozell, J. J.; O'Lenick, C. J.; Warwick, S. Biomass fractionation for the biorefinery: heteronuclear multiple quantum coherence-nuclear magnetic resonance investigation of lignin isolated from solvent fractionation of switchgrass. J. Agric. Food. Chem. 2011, 59, 9232-9242. Samuel, R.; Pu, Y. Q.; Raman, B.; Ragauskas, A. J. Structural characterization and comparison of switchgrass ball-milled lignin before and after dilute acid pretreatment. Appl. Biochem. Biotechnol. 2010, 162, 62-74. Li, J. B.; Gellerstedt, G.; Toven, K. Steam explosion lignins; their extraction, structure and potential as feedstock for biodiesel and chemicals. Bioresour. Technol. 2009, 100, 2556-2561. Tran, H.; Vakkilainnen, E. K. The kraft chemical recovery process. Tappi Kraft Pulping Short Course 2008. Kinstrey, R. B.; White, D. Pulp and Paper Industry - Energy Bandwidth Study: Project Number 16CX8700; Jacobs and the Institute of Paper Science and Technology: Atlanta, 2006. de Oliveira, F.; Ramires, E. C.; Frollini, E.; Belgacem, M. N. Lignopolyurethanic materials based on oxypropylated sodium lignosulfonate and castor oil blends. Ind. Crop. Prod. 2015, 72, 77-86. Fache, M.; Boutevin, B.; Caillol, S. Vanillin production from lignin and its use as a renewable chemical. ACS Sus. Chem. Eng. 2016, 4, 35-46. Lora, J. H. In Monomers, polymers and composites from renewable resources; Belgacem, M. N., Gandini, A., Eds.; Elsevier: United Kingdom, 2008, p 225-242. Chen, Y. X.; Zheng, Y.; Li, M.; Zhu, X. F. Arene production by W2C/MCM-41-catalyzed upgrading of vapors from fast pyrolysis of lignin. Fuel Process. Technol. 2015, 134, 46-51. Sharma, R. K.; Bakhshi, N. N. Upgrading of wood-derived bio-oil over HZSM-5. Bioresour. Technol. 1991, 35, 57-66. Sharma, R. K.; Bakhshi, N. N. Catalytic upgrading of pyrolysis oil. Energy Fuels 1993, 7, 306-314. Chantal, P.; Kaliaguine, S.; Grandmaison, J. L.; Mahay, A. Production of hydrocarbons from aspen poplar pyrolytic oils over H-ZSM5. Appl. Catal. 1984, 10, 317-332. Valle, B.; Gayubo, A. G.; Aguayo, A. T.; Olazar, M.; Bilbao, J. Selective production of aromatics by crude bio-oil valorization with a nickel-modified HZSM-5 zeolite catalyst. Energy Fuels 2010, 24, 2060-2070. Gayubo, A. G.; Valle, B.; Aguayo, A. T.; Olazar, M.; Bilbao, J. Olefin production by catalytic transformation of crude bio-oil in a two-step process. Ind. Eng. Chem. Res. 2010, 49, 123-131. ACS Paragon Plus Environment

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Bhuiyan, T. I.; Arudra, P.; Akhtar, M. N.; Aitani, A. M.; Abudawoud, R. H.; Al-Yami, M. A.; AlKhattaf, S. S. Metathesis of 2-butene to propylene over W-mesoporous molecular sieves: A comparative study between tungsten containing MCM-41 and SBA-15. Appl. Catal. A 2013, 467, 224-234. Lee, H. W.; Kim, T. H.; Park, S. H.; Jeon, J. K.; Suh, D. J.; Park, Y. K. Catalytic fast pyrolysis of lignin over mesoporous Y zeolite using Py-GC/MS. J. Nanosci. Nanotech. 2013, 13, 2640-2646. Topka, P.; Balcar, H.; Rathousky, J.; Zilkova, N.; Verpoort, F.; Cejka, J. Metathesis of 1-octene over MoO3 supported on mesoporous molecular sieves: The influence of the support architecture. Microporous Mesoporous Mater. 2006, 96, 44-54. Ren, H.; Chen, Y.; Huang, Y. L.; Deng, W. H.; Vlachos, D. G.; Chen, J. G. G. Tungsten carbides as selective deoxygenation catalysts: experimental and computational studies of converting C3 oxygenates to propene. Green Chem. 2014, 16, 761-769. Jongerius, A. L.; Bruijnincx, P. C. A.; Weckhuysen, B. M. Liquid-phase reforming and hydrodeoxygenation as a two-step route to aromatics from lignin. Green Chem. 2013, 15, 3049-3056. Kamwilaisak, K.; Wright, P. C. Investigating laccase and titanium dioxide for lignin degradation. Energ. Fuels 2012, 26, 2400-2406. Ma, Y. S.; Chang, C. N.; Chiang, Y. P.; Sung, H. F.; Chao, A. C. Photocatalytic degradation of lignin using Pt/TiO2 as the catalyst. Chemosphere 2008, 71, 998-1004. Machado, A. E. H.; Furuyama, A. M.; Falone, S. Z.; Ruggiero, R.; Perez, D. D.; Castellan, A. Photocatalytic degradation of lignin and lignin models, using titanium dioxide: the role of the hydroxyl radical. Chemosphere 2000, 40, 115-124. Mante, O. D.; Rodriguez, J. A.; Babu, S. P. Selective defunctionalization by TiO2 of monomeric phenolics from lignin pyrolysis into simple phenols. Bioresour. Technol. 2013, 148, 508-516. Olcese, R. N.; Lardier, G.; Bettahar, M.; Ghanbaja, J.; Fontana, S.; Carre, V.; Aubriet, F.; Petitjean, D.; Dufour, A. Aromatic chemicals by iron-catalyzed hydrotreatment of lignin pyrolysis vapor. ChemSusChem 2013, 6, 1490-1499. Bozell, J. J.; Hames, B. R.; Dimmel, D. R. Cobalt-Schiff base complex catalyzed oxidation of parasubstituted phenolics. Preparation of benzoquinones. J. Org. Chem. 1995, 60, 2398-2404. Badamali, S. K.; Luque, R.; Clark, J. H.; Breeden, S. W. Microwave assisted oxidation of a lignin model phenolic monomer using Co(salen)/SBA-15. Catal. Commun. 2009, 10, 1010-1013. Canevali, C.; Orlandi, M.; Pardi, L.; Rindone, B.; Scotti, R.; Sipila, J.; Morazzoni, F. Oxidative degradation of monomeric and dimeric phenylpropanoids: reactivity and mechanistic investigation. Dalton Trans. 2002, 3007-3014. Biannic, B.; Bozell, J. J. Efficient cobalt-catalyzed oxidative conversion of lignin models to benzoquinones. Org. Lett. 2013, 15, 2730-2733. Robert, D. R.; Bardet, M.; Gellerstedt, G.; Lindfors, E. L. Structural changes in lignin during kraft cooking part 3. On the structure of dissolved lignins. J. Wood Chem. Technol. 1984, 4, 239-263. Gao, T.-T.; Zhou, X.-F.; Zhu, Z.-L. Catalytic conversion of kraft lignin using paper-like Co(salen) as an effective catalyst. Drewno 2015, 58, 79-90. Zhou, X. F. Catalytic oxidation and conversion of kraft lignin into phenolic products using zeoliteencapsulated Cu(II) H-4 salen and H-2 salen complexes. Environ. Prog. Sust. Energy 2015, 34, 11201128. Hanson, S. K.; Baker, R. T. Knocking on wood: base metal complexes as catalysts for selective oxidation of lignin models and extracts. Acc. Chem. Res. 2015, 48, 2037-2048. Diaz-Urrutia, C.; Chen, W.-C.; Crites, C.-O.; Daccache, J.; Korobkov, I.; Baker, R. T. Towards lignin valorisation: comparing homogeneous catalysts for the aerobic oxidation and depolymerisation of organosolv lignin. RSC Adv. 2015, 5, 70502-70511. Lancefield, C. S.; Ojo, O. S.; Tran, F.; Westwood, N. J. Isolation of functionalized phenolic monomers through selective oxidation and C-O bond cleavage of the β-O-4 linkages in lignin. Angew. Chem. Int. Ed. 2015, 54, 258-262. Rahimi, A.; Azarpira, A.; Kim, H.; Ralph, J.; Stahl, S. S. Chemoselective metal-free aerobic alcohol oxidation in lignin. J. Am. Chem. Soc. 2013, 135, 6415-6418. 21 ACS Paragon Plus Environment


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Rahimi, A.; Ulbrich, A.; Coon, J. J.; Stahl, S. S. Formic-acid-induced depolymerization of oxidized lignin to aromatics. Nature 2014, 515, 249-252. Bredenberg, J. B. s.; Huuska, M.; Räty, J.; Korpio, M. Hydrogenolysis and hydrocracking of the carbonoxygen bond: I. Hydrocracking of some simple aromatic O-compounds. J. Catal. 1982, 77, 242-247. Hurff, S. J.; Klein, M. T. Reaction pathway analysis of thermal and catalytic lignin fragmentation by use of model compounds. Ind. Eng. Chem. Fundam. 1983, 22, 426-430. Kallury, R. K. M. R.; Restivo, W. M.; Tidwell, T. T.; Boocock, D. G. B.; Crimi, A.; Douglas, J. Hydrodeoxygenation of hydroxy, methoxy and methyl phenols with molybdenum oxide/nickel oxide/alumina catalyst. J. Catal. 1985, 96, 535-543. Gevert, B. S.; Otterstedt, J. E.; Massoth, F. E. Kinetics of the HDO of methyl-substituted phenols. Appl. Catal. 1987, 31, 119-131. Toledano, A.; Serrano, L.; Pineda, A.; Romero, A. A.; Luque, R.; Labidi, J. Microwave-assisted depolymerisation of organosolv lignin via mild hydrogen-free hydrogenolysis: Catalyst screening. Appl. Catal., B 2014, 145, 43-55. Wang, X. Y.; Rinaldi, R. Bifunctional Ni catalysts for the one-pot conversion of organosolv lignin into cycloalkanes. Catal. Today 2016, 269, 48-55. Molinari, V.; Clavel, G.; Graglia, M.; Antonietti, M.; Esposito, D. Mild continuous hydrogenolysis of kraft lignin over titanium nitride-nickel catalyst. ACS Catal. 2016, 6, 1663-1670. Singh, S. K.; Ekhe, J. D. Cu-Mo doped zeolite ZSM-5 catalyzed conversion of lignin to alkyl phenols with high selectivity. Catal. Sci. Technol. 2015, 5, 2117-2124. Nagy, M.; Kasi, D.; Britovsek, G. J. P.; Ragauskas, A. J. Catalytic hydrogenolysis of ethanol organosolv lignin. Holzforschung 2009, 63, 513-520. Wu, A.; Patrick, B. O.; James, B. R. Inactive ruthenium(II)-xantphos complexes from attempted catalyzed lignin reactions. Inorg. Chem. Commun. 2012, 24, 11-15. Pepper, J. M.; Lee, Y. W. Lignin and related compounds. I. A comparative study of catalysts for lignin hydrogenolysis. Can. J. Chem. 1969, 47, 723-727. Parsell, T.; Yohe, S.; Degenstein, J.; Jarrell, T.; Klein, I.; Gencer, E.; Hewetson, B.; Hurt, M.; Kim, J. I.; Choudhari, H.; Saha, B.; Meilan, R.; Mosier, N.; Ribeiro, F.; Delgass, W. N.; Chapple, C.; Kenttamaa, H. I.; Agrawal, R.; Abu-Omar, M. M. A synergistic biorefinery based on catalytic conversion of lignin prior to cellulose starting from lignocellulosic biomass. Green Chem. 2015, 17, 1492-1499. Luo, H.; Klein, I. M.; Jiang, Y.; Zhu, H. Y.; Liu, B. Y.; Kenttamaa, H. I.; Abu-Omar, M. M. Total utilization of miscanthus biomass, lignin and carbohydrates, using earth abundant nickel catalyst. ACS Sus. Chem. Eng. 2016, 4, 2316-2322. Li, C. Z.; Zheng, M. Y.; Wang, A. Q.; Zhang, T. One-pot catalytic hydrocracking of raw woody biomass into chemicals over supported carbide catalysts: simultaneous conversion of cellulose, hemicellulose and lignin. Energ. Environ. Sci. 2012, 5, 6383-6390. Van den Bosch, S.; Schutyser, W.; Koelewijn, S. F.; Renders, T.; Courtin, C. M.; Sels, B. F. Tuning the lignin oil OH-content with Ru and Pd catalysts during lignin hydrogenolysis on birch wood. Chem. Commun. 2015, 51, 13158-13161. Galkin, M. V.; Samec, J. S. M. Selective route to 2-propenyl aryls directly from wood by a tandem organosolv and palladium-catalysed transfer hydrogenolysis. ChemSusChem 2014, 7, 2154-2158. Song, Q.; Wang, F.; Cai, J. Y.; Wang, Y. H.; Zhang, J. J.; Yu, W. Q.; Xu, J. Lignin depolymerization (LDP) in alcohol over nickel-based catalysts via a fragmentation-hydrogenolysis process. Energ. Environ. Sci. 2013, 6, 994-1007. Klein, I.; Saha, B.; Abu-Omar, M. M. Lignin depolymerization over Ni/C catalyst in methanol, a continuation: effect of substrate and catalyst loading. Catal. Sci. Technol. 2015, 5, 3242-3245. Godard, H. P.; McCarthy, J. L.; Hibbert, H. Hydrogenation of wood. J. Am. Chem. Soc. 1940, 62, 988988. Godard, H. P.; McCarthy, J. L.; Hibbert, H. Studies on lignin and related compounds. LXII. High pressure hydrogenation of wood using copper chromite catalyst (Part 1). J. Am. Chem. Soc. 1941, 63, 3061-3066. 22 ACS Paragon Plus Environment

Page 23 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(106) Pepper, J. M.; Fleming, R. W. Lignin and related compounds. V. The hydrogenolysis of aspen wood lignin using rhodium-on-charcoal as catalyst. Can. J. Chem. 1978, 56, 896-898. (107) Anderson, N. A., Tobimatsu, Y., Ciesielski, P. N., Ximenes, E., Ralph, J., Donohoe, B. S., Ladisch, M., Chapple, C. Manipulation of guaiacyl and syringyl monomer biosynthesis in an Arabidopsis cinnamyl alcohol dehydrogenase mutant results in atypical lignin biosynthesis and modified cell wall structure Plant Cell 2015, 27, 2195-2209. (108) Mottiar, Y.; Vanholme, R.; Boerjan, W.; Ralph, J.; Mansfield, S. D. Designer lignins: harnessing the plasticity of lignification. Curr. Opin. Biotechnol. 2016, 37, 190-200. (109) Simmons, B. A.; Logue, D.; Ralph, J. Advances in modifying lignin for enhanced biofuel production. Curr. Opin. Plant Biol. 2010, 13, 313-320. (110) Vanholme, R.; Morreel, K.; Darrah, C.; Oyarce, P.; Grabber, J. H.; Ralph, J.; Boerjan, W. Metabolic engineering of novel lignin in biomass crops. New Phytologist 2012, 196, 978-1000. (111) Mansfield, S. D.; Kang, K. Y.; Chapple, C. Designed for deconstruction - poplar trees altered in cell wall lignification improve the efficacy of bioethanol production. New Phytologist 2012, 194, 91-101. (112) Liu, C. J.; Cai, Y. H.; Zhang, X. B.; Gou, M. Y.; Yang, H. J. Tailoring lignin biosynthesis for efficient and sustainable biofuel production. Plant Biotech. J. 2014, 12, 1154-1162. (113) Weng, J. K.; Li, X.; Bonawitz, N. D.; Chapple, C. Emerging strategies of lignin engineering and degradation for cellulosic biofuel production. Curr. Opin. Biotechnol. 2008, 19, 166-172. (114) Chen, F.; Dixon, R. A. Lignin modification improves fermentable sugar yields for biofuel production. Nat. Biotechnol. 2007, 25, 759-761. (115) Fu, C. X.; Mielenz, J. R.; Xiao, X. R.; Ge, Y. X.; Hamilton, C. Y.; Rodriguez, M.; Chen, F.; Foston, M.; Ragauskas, A.; Bouton, J.; Dixon, R. A.; Wang, Z. Y. Genetic manipulation of lignin reduces recalcitrance and improves ethanol production from switchgrass. Proc. Natl. Acad. Sci. 2011, 108, 38033808. (116) Sykes, R. W.; Gjersing, E. L.; Foutz, K.; Rottmann, W. H.; Kuhn, S. A.; Foster, C. E.; Ziebell, A.; Turner, G. B.; Decker, S. R.; Hinchee, M. A. W.; Davis, M. F. Down-regulation of p-coumaroyl quinate/shikimate 3′-hydroxylase (C3′H) and cinnamate 4-hydroxylase (C4H) genes in the lignin biosynthetic pathway of Eucalyptus urophylla × E. grandis leads to improved sugar release. Biotech. Biofuels 2015, 8, 1-10. (117) Van Acker, R.; Leple, J. C.; Aerts, D.; Storme, V.; Goeminne, G.; Ivens, B.; Legee, F.; Lapierre, C.; Piens, K.; Van Montagu, M. C. E.; Santoro, N.; Foster, C. E.; Ralph, J.; Soetaert, W.; Pilate, G.; Boerjan, W. Improved saccharification and ethanol yield from field-grown transgenic poplar deficient in cinnamoyl-CoA reductase. Proc. Natl. Acad. Sci. 2014, 111, 845-850. (118) Zhang, K. W.; Bhuiya, M. W.; Pazo, J. R.; Miao, Y. C.; Kim, H.; Ralph, J.; Liu, C. J. An engineered monolignol 4-O-methyltransferase depresses lignin biosynthesis and confers novel metabolic capability in Arabidopsis. Plant Cell 2012, 24, 3135-3152. (119) Bonawitz, N. D.; Kim, J. I.; Tobimatsu, Y.; Ciesielski, P. N.; Anderson, N. A.; Ximenes, E.; Maeda, J.; Ralph, J.; Donohoe, B. S.; Ladisch, M.; Chapple, C. Disruption of Mediator rescues the stunted growth of a lignin-deficient Arabidopsis mutant. Nature 2014, 509, 376 - 380. (120) Eudes, A.; Sathitsuksanoh, N.; Baidoo, E. E. K.; George, A.; Liang, Y.; Yang, F.; Singh, S.; Keasling, J. D.; Simmons, B. A.; Loqué, D. Expression of a bacterial 3-dehydroshikimate dehydratase reduces lignin content and improves biomass saccharification efficiency. Plant Biotech. J. 2015, 13, 1241-1250. (121) Poovaiah, C. R.; Nageswara-Rao, M.; Soneji, J. R.; Baxter, H. L.; Stewart, C. N. Altered lignin biosynthesis using biotechnology to improve lignocellulosic biofuel feedstocks. Plant Biotech. J. 2014, 12, 1163-1173. (122) Lan, W.; Lu, F. C.; Regner, M.; Zhu, Y. M.; Rencoret, J.; Ralph, S. A.; Zakai, U. I.; Morreel, K.; Boerjan, W.; Ralph, J. Tricin, a flavonoid monomer in monocot lignification. Plant Physiol. 2015, 167, 1284-U1265. (123) Vanholme, R.; Morreel, K.; Ralph, J.; Boerjan, W. Lignin engineering. Curr. Opin. Plant Biol. 2008, 11, 278-285.


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(124) Huntley, S. K.; Ellis, D.; Gilbert, M.; Chapple, C.; Mansfield, S. D. Significant increases in pulping efficiency in C4H-F5H-transformed poplars: Improved chemical savings and reduced environmental toxins. J. Agric. Food. Chem. 2003, 51, 6178-6183. (125) Vanholme, R.; Ralph, J.; Akiyama, T.; Lu, F. C.; Pazo, J. R.; Kim, H.; Christensen, J. H.; Van Reusel, B.; Storme, V.; De Rycke, R.; Rohde, A.; Morreel, K.; Boerjan, W. Engineering traditional monolignols out of lignin by concomitant up-regulation of F5H1 and down-regulation of COMT in Arabidopsis. Plant J. 2010, 64, 885-897. (126) Zhao, Q.; Tobimatsu, Y.; Zhou, R.; Pattathil, S.; Gallego-Giraldo, L.; Fu, C.; Jackson, L. A.; Hahn, M. G.; Kim, H.; Chen, F.; Ralph, J.; Dixon, R. A. Loss of function of cinnamyl alcohol dehydrogenase 1 leads to unconventional lignin and a temperature-sensitive growth defect in Medicago truncatula. Proc. Natl. Acad. Sci. 2013, 110, 13660-13665. (127) Leplé, J.-C.; Dauwe, R.; Morreel, K.; Storme, V.; Lapierre, C.; Pollet, B.; Naumann, A.; Kang, K.-Y.; Kim, H.; Ruel, K.; Lefèbvre, A.; Joseleau, J.-P.; Grima-Pettenati, J.; De Rycke, R.; AnderssonGunnerås, S.; Erban, A.; Fehrle, I.; Petit-Conil, M.; Kopka, J.; Polle, A.; Messens, E.; Sundberg, B.; Mansfield, S. D.; Ralph, J.; Pilate, G.; Boerjan, W. Downregulation of cinnamoyl-Coenzyme A reductase in poplar: multiple-level phenotyping reveals effects on cell wall polymer metabolism and structure. Plant Cell 2007, 19, 3669-3691. (128) Wagner, A.; Tobimatsu, Y.; Phillips, L.; Flint, H.; Torr, K.; Donaldson, L.; Pears, L.; Ralph, J. CCoAOMT suppression modifies lignin composition in Pinus radiata. The Plant Journal 2011, 67, 119129. (129) Eudes, A.; George, A.; Mukerjee, P.; Kim, J. S.; Pollet, B.; Benke, P. I.; Yang, F.; Mitra, P.; Sun, L.; Cetinkol, O. P.; Chabout, S.; Mouille, G.; Soubigou-Taconnat, L.; Balzergue, S.; Singh, S.; Holmes, B. M.; Mukhopadhyay, A.; Keasling, J. D.; Simmons, B. A.; Lapierre, C.; Ralph, J.; Loque, D. Biosynthesis and incorporation of side-chain-truncated lignin monomers to reduce lignin polymerization and enhance saccharification. Plant Biotech. J. 2012, 10, 609-620. (130) Xu, Y.; Chen, C. F.; Thomas, T. P.; Azadi, P.; Diehl, B.; Tsai, C. J.; Brown, N.; Carlson, J. E.; Tien, M.; Liang, H. Y. Wood chemistry analysis and expression profiling of a poplar clone expressing a tyrosinerich peptide. Plant Cell Reports 2013, 32, 1827-1841. (131) Liang, H. Y.; Frost, C. J.; Wei, X. P.; Brown, N. R.; Carlson, J. E.; Tien, M. Improved sugar release from lignocellulosic material by introducing a tyrosine-rich cell wall peptide gene in poplar. Clean-Soil Air Water 2008, 36, 662-668. (132) Diehl, B. G.; Brown, N. R. Lignin cross-links with cysteine- and tyrosine-containing peptides under biomimetic conditions. J. Agric. Food. Chem. 2014, 62, 10312-10319. (133) Wilkerson, C. G.; Mansfield, S. D.; Lu, F.; Withers, S.; Park, J. Y.; Karlen, S. D.; Gonzales-Vigil, E.; Padmakshan, D.; Unda, F.; Rencoret, J.; Ralph, J. Monolignol ferulate transferase introduces chemically labile linkages into the lignin backbone. Science 2014, 344, 90-93.


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For Table of Contents Use Only

OH

OH

Lignocellulosic biomass

R

R

O

OH

OMe R

O

O R

Catalyst design Lignin first Genetic engineering

OH

OH

OMe

OMe OMe OH

CHO

HO OH

OMe O

OH OMe

OMe

Lignin (native or technical)

R

OMe O

R3

R3

Searching for improved selectivity in lignin valorization TOC/Abstract Art Recent Progress toward Lignin Valorization via Selective Catalytic Technologies and the Tailoring of its Biosynthetic Pathways Rebecca E. Key and Joseph J. Bozell* SYNOPSIS Biorefinery development is central to sustainability. We highlight recent progress in thermochemical transformations, oxidations, reductions, and biosynthetic modifications that aim to improve lignin valorization.


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OH

OH

Lignocellulosic biomass

R

R

O O

O R

OH

Catalyst design Lignin first Genetic engineering

OH

OMe

OMe OMe OH OMe R

CHO

HO

OH

OH

OMe O

OH OMe

OMe

Lignin (native or technical)

R

OMe O

R3

R3

Searching for improved selectivity in lignin valorization


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1 2 OMe 3 O 4 HO HO 5 MeO OH MeO OMe O 6 OH HO O 7 HO O O OH 8 OMe OMe OMe MeO 9 HO OH HO 10 MeO O OH OH 11 O O MeO O 12 MeO OMe O COOR 13 MeO O O OH HO 14 OH 15 OMe OMe OMe 16 HO HO OH OMe 17 MeO OMe O O O 18 O OH O OMe O OH OH OMe 19 OMe MeO OH OMe HO OMe OH O 20 MeO OH OMe HO O 21 OMe O O 22 MeO OMe HO OH HO 23 MeO O OMe OH OH OH O OMe 24 25 OH HO 26 HO 27 OMe 28 29 30 31 32 33 34 35 36 37 38 39 ACS Paragon Plus Environment 40 41 42


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37x8mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

52x16mm (600 x 600 DPI)


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41x12mm (600 x 600 DPI)


O


O-

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NH 3+ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

phenylalanine PAL

O

O

O O-

O

C4H

C4H/C3'H

O-

OHO

p-coumarate

caffeate

O-

O O HO

OCH3

ferulate

5-OH ferulate

OH

O

OH

4CL

4CL

caffeoyl shikimate

HO OH

OCH3 O

OCH3

HO p-coumaraldehyde

H

HO

p-coumaryl alcohol

CCR

coniferaldehyde

HO

HO OH caffeoyl alcohol

OCH3 coniferyl alcohol

H OCH3

5-OH coniferaldehyde

F5H

O

MeO HO

CAD

OH

COMT

H

COMT

OCH3

CAD

OH

O

HO HO

OCH3

CAD

HO

F5H

HO

OH

OH

CCR

O H

caffealdehyde

CAD

OCH3 sinapoyl CoA

CCR

O

H

SCoA

SCoA

feruoyl CoA

CCR

O

H 3CO COMT HO

5-OH feruloyl CoA

SCoA CCoAOMT HO

caffeoyl CoA

O

SCoA

HO

O

O

CCR

O

HO

HCT

p-coumaryl CoA

OCH3

4CL

OH

OH

HCT SCoA

HO sinapate

4CL

HO

HO

OCH3

O-

O

OH

p-coumaryl shikimate

COMT

O-

O

CSE

C3'H O

4CL

HO

O

H 3CO

O-

F5H HO

OH

HO cinnamate

COMT

O

HO

O-

sinapaldehyde CAD

HO

OH COMT

HO

MeO

OH

HO OCH3

OCH3

5-OH coniferyl alcohol

sinapyl alcohol

ParagonCoA Plus Environment PAL - phenylalanine lyase; C4H - cinnamyl 4-hydroxylase; 4CLACS - 4-coumarate ligase; HCT - p-hydroxycinnamoyl CoA transferase; C3'H - p-coumaryl 3-hydroxylase; CSE - caffeoyl shikimate esterase; CCoAOMT - caffeoyl CoA methyltransferase; F5H - ferulate 5-hydroxylase; COMT - 5-hydroxyconiferaldehyde O-methyltransferase; CCR - cinnamoyl CoA reductase; CAD - cinnamyl alcohol dehydrogenase

Progress toward Lignin Valorization via Selective Catalytic

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