Recent advances in oxidative valorization of lignin

Catalysis Today xxx (xxxx) xxx–xxx

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Recent advances in oxidative valorization of lignin Ruoshui Maa, Mond Guoa, Xiao Zhanga,b, a b

⁎

Washington State University, Voiland School of Chemical Engineering & Bioengineering, 2710 Crimson Way, Richland, WA, 99354, USA Pacific Northwest National Laboratory, 2710 Crimson Way, Richland, WA, 99354, USA

A R T I C L E I N F O

A B S T R A C T

Keywords: Biomass Lignin Oxidation Catalysis Selectivity

Lignin, an aromatic macromolecule synthesized by all higher plants, is one of the most intriguing natural materials for utilization across a wide range of applications. Depolymerization and fragmentation of lignin into small chemicals constituents which can either replace current market products or be used building blocks for new material synthesis is a focus of current lignin valorization strategies. Among the variety of lignin degradation chemistries, catalytic oxidation of lignin presents an energy efficient means of lignin depolymerization and generating selective reaction products. This review provides a summary of the recent advancements in oxidative lignin valorization couched in a discussion on how these chemistries may contribute to the degradation of the lignin macromolecule through three major approaches: 1) inter-unit linkages cleavage; 2) propanyl sidechain oxidative modification; and 3) oxidation of the aromatic ring and ring cleavage reactions.

1. Introduction Lignin, a polymer with a molecular weight (MW) in the hundreds of thousands is assembled by phenylpropane units with an average MW of ∼200 [1,2]. Guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) are the most common phenylpropane units participating in lignin construction in the majority of plants. The heterogeneous and complex structural characteristics of the lignin polymer make it difficult to develop lignin for high value applications [3–6]. Breaking the lignin macromolecule down to its monomeric constituents provides an opportunity to generate products with controllable and consistent quality. However, selectively depolymerizing lignin to valuable compounds has been a well-recognized challenge [3,5]. One significant factor hindering effective lignin depolymerization is perhaps its compact structure integrity. While ether (CeOeC) and carbon–carbon (CeC) linkages instigate the interlocking of phenylpropane units, the electron dense nature of these units also contribute significantly to the structural compactness and stability of the macromolecule [7–12]. The mechanism and role of electronic interactions contributing to lignin structural integrity has been an intriguing subject of research [10–15]. One plausible electronic interaction is the π-π interaction between lignin aromatic nucleis. Two common types of π-π interaction models, H- and J- aggregation, have long been recognized in polymer assemblies. [16] J-aggregate behavior is typically instigated by the intramolecular interactions, such as aromatic ring interaction within a single polymer molecule, whereas H-aggregate behavior is a

phenomena generated by inter-chain Coulombic interactions [17]. Molecular orientation is the key factor governing the formation of these types of aggregation. There are three major types of molecular orientations proposed for phenylpropane unit interactions: head-to-head, head-to-tail, and edge-on [10,18–20]. (Fig. 1) Head-to-tail orientation generally results in J-aggregation, whereas a side-by-side orientation favors H-aggregation. A number of previous studies have shown that disturbing these interactions can have a significant influence on lignin stability and cause depolymerization, biodegradation and other chemical modifications of lignin [4,10,21–23]. The strong attractive force formed between π-π systems is a known phenomenon [24–27]. An example of how inter-unit π-π interactions maintain structural integrity is well represented by the transformation between graphite and graphene oxide. Graphite is a carbon allotrope found in nature composed of two dimensional graphene sheets stacked into a three dimensional lattice structure, linked by strong π-π interactions between each layer [28–30]. This strong π-π interaction tightly binds the graphene layers and makes it extremely difficult to obtain stable single layers of graphene [31,32]. Synthesized single layer or multi-layer graphene have an overwhelming tendency to re-stack into a graphene-assembly when they are reduced [33–35]. The most efficient processes to delaminate graphite are chemical oxidation exfoliation processes where graphite is treated with oxidant (e.g., H2O2). Oxidation inserts oxygen functional groups on the graphene surface and forms hydroxyl and carboxyl groups. The addition of oxygen weakens the stacking force exerted by π-π interactions, leading

⁎ Corresponding author at: Washington State University, Voiland School of Chemical Engineering & Bioengineering, 2710 Crimson Way, Richland, WA, 99354, USA. Pacific Northwest National Laboratory, 2710 Crimson Way, Richland, WA, 99354, USA. E-mail address: [email protected] (X. Zhang).

http://dx.doi.org/10.1016/j.cattod.2017.05.101 Received 21 December 2016; Received in revised form 25 March 2017; Accepted 29 May 2017 0920-5861/ © 2017 Elsevier B.V. All rights reserved.

Please cite this article as: Ma, R., Catalysis Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.05.101

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Fig. 1. Representative lignin monomer π-π interaction and hydrogen bond models which lead to molecular aggregation.

Fig. 2. A) Chemical exfoliation to overcome graphite stacking energy to produce graphene. B). Proposed lignin oxidative strategy to depolymerize lignin to overcome molecular aggregation.

will provide a summary of the recent advancements in oxidative lignin valorization couched in a discussion on how these chemistries may contribute to the degradation of the lignin macromolecule through three major approaches: 1) inter-unit linkages cleavage; 2) propanyl side-chain oxidative modification; and 3) oxidation of the aromatic ring and ring cleavage.

to the expansion of graphene layers. Lignin oxidation can bring one or multiple oxygen to the lignin side-chain and/or aromatic ring which may instigate a similar mechanism to reduce the inter-unit force. (Fig. 2) Oxidation has been the predominant lignin depolymerization and degradation chemistry employed in modern commercial pulping and bleaching processes [4,36,37]. Detailed reviews of the oxidative chemistries utilized for wood pulping and bleaching have been well documented and presented elsewhere [23]. There is a significant amount of interest in developing and applying new oxidative chemistries for lignin conversion [4]. The subsequent sections of this review

2. Recent advances in catalytic oxidation of biorefinery lignin Depolymerization and fragmentation are the predominant strategy for transforming lignin for chemicals and fuel production. Three 2

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Fig. 3. Summary of representative catalysts developed for lignin oxidative conversion [51].

linkages. Bruijnincx and Weckhuysen have evaluated metal-organic frameworks (MOF), a highly porous material, for their capability to oxidize lignin-like monomeric phenolics to depolymerization products [44,78]. LMWPC are not the only products generated from oxidative lignin depolymerization. At proper conditions, the lignin phenyl ring can be oxidized to quinones and/or cleaved, which has been observed during pulp bleaching. As mentioned earlier, Co(Salen) was reported to oxidize phenolic lignin units to benzoquinone structures. Polyoxometalate has similarly been demonstrated to depolymerize lignin and convert phenyl rings to benzoquinone structures [79,80]. Deng has applied this chemistry to make photocatalytic biomass-to-electricity hybrid fuel cells [42]. Typically, these benzoquinone structures are not stable and can be further oxidized to yield aromatic ring cleavage products, such as dicarboxylic acids (DCAs). Selectively converting lignin to DCAs and other open chain organic acids by chemical and biological means has now become a major research interest [43,81–85]. These recent advances in lignin oxidation chemistry have greatly energized the field of lignin valorization. A varitey of new lignin degradation chemistries are emerging and the understanding of their effectiveness and underlying mechanisms is evolving. Table 1 summarized the reaction condition, sources of lignin and model compound, products selectivity and yields of these reviewed studies. This review intends to synthesize the current understanding of the major lignin oxidative chemistries and interpret their potential efficacy towards three categories of lignin structural degradation: 1) oxidative cleavage of inter-units linkages; 2) oxidative modification of lignin side-chain; and 3) oxidation of aromatic ring and ring cleavage, with consideration to their impact on disrupting the lignin electron network.

primary types of products have attracted the bulk of the research in lignin oxidation: 1) low molecular weight phenolic compounds (LMWPC); 2) benzoquinones (BQs); and 3) dicarboxylic acids (DCA). A brief summary of the major oxidative lignin conversion chemistries investigated in recent decades regarding these products is shown in Fig. 3 [21,38–51]. Due to the aromatic skeleton of lignin, converting biorefinery lignin to low molecular weight phenolic compounds (LMWPC) is a sensible research topic. The history behind utilizing lignin as an LMWPC source can be dated back to the mid-twentieth century, as part of the paper industry’s search for a new revenue outlet for lignin [3,5,52]. Most of the early work on lignin oxidation was conducted with oxidant alone or with simple transition metal ions such as Cu2+, Mn3+, Co2+, and Zr4+ [53–56]. Co, Fe, Cu, Mn based metal oxides (e.g., CuO, MnO2) and composite metal oxides were subsequently tested to enhance oxygen catalyzed depolymerization of lignin [41,57–65]. The mechanisms of composite metal oxide catalysts and metal ions share some similarity, as shown by DiCosimo [66]. Besides inorganic metal catalysts, several homogeneous organic structures were also discovered to facilitate lignin oxidation. Stahl et al. demonstrated the use of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) to mediate lignin-like primary alcohol oxidation. Organometallics, a combination of metal and organic ligand which resemble biological catalysts such as laccase and lignin peroxidases, have attracted increasing attention for lignin conversion. Dolphin et al. investigated the use of metalloporphyrins as a biomimetic lignin peroxidase for lignin depolymerization in the early 1990s [47,48,67–73]. Monomeric and dimeric model compounds study indicated the potential of metalloporphyrins to catalyze lignin depolymerization to produce LMWPC as the primary degradation products. Concurrently, Bozell demonstrated the use of the Schiff base Salen salt as a new type of organometallic catalyst for oxidative depolymerization based on results from similar model compounds study [38]; this was developed further by Kervinen [39,74–76] in the early 2000s. In addition to the degradation of lignin to LMWPC, the oxidization of phenolic units to benzoquinone structures was also observed. More recently, Crestini studied the methyltrioxorhenium(VII) (MTO) catalyzed H2O2 oxidation of lignin model compounds and demonstrated the cleavage of ether linkages while leaving CeC linkages intact [49]. Hanson investigated a vanadium-based complex that was shown to cleave lignin ether linkages through a unique aerobic oxidation pathway [46,77]. The vanadium complex can also catalyze the partial depolymerization of pinacol structure CeC

3. Oxidative cleavage of inter-units linkages Phenylpropanoid units in lignin are connected by ether and CeC linkages, with ether linkages–predominantly α-O-4 and β-O-4 linkages–being more abundant in most plant lignin [4–6,20,86,87]. Oxidative cleavage of these linkages will introduce more oxygen-containing functional groups, such as aldehydes, ketones and carboxyls on the resulting lignin fragment [3,4]. An increase in oxygen-rich functional groups on the lignin fragment may also increase the distance between π-π stacked rings and thus weaken the inter-unit forces holding lignin together, facilitating the dissociation of the lignin macromolecule. Due to the abundance of β-O-4 interunit linkage in many types of 3

Oxidant

4 DMSO-d6 pyr-d5

CHCl3

O2

air

CH3COOOH

VO(acac)2/DABCO

(dipic)VV(O)(OiPr)

V(oxo) complexes Nb2O5

Oxidative Modification of Lignin Side-chain 1 Co(salen) O2

H2O2

[bmim]PF6

t-butanol HAc

MeOH/H2O CH3CN

pH = 3–5 Buffer H2O

sodium acetate buffer

NaOH/H2O

NaOH/H2O oxalate buffer

6

4

O2

O2

O2 O2/UV

5

Pt/TiO2 MnO2/Oxalate Pd/Al2O3 Mixed Metal Oxides: LaMnO3 LaCoO3

Metal Oxides: Cu(OH)2/CuO TiO2

alkaline HAc CH3CN/H2O Dioxane/H2O CH3OH/H2O CH3COOH

Solvent

LaFe1-xCuxO3 LaCo1-xCuxO3 Na5(+1.9)[SiV1(0.1)MoW10(+0.1)] Na5[PV2Mo10O40] [AlMnIII(OH2) W11O39]6− [SiMnIII(OH2)W11O39]5− [AlVVW11O40]6– Na5[SiVW11O40] H3PMo12O40 HPA-5-Mn(II) MTO

3

2

Oxidative Cleavage of Inter-units Linkages 1 HomogeneousMetal Ions: O2 CuSO4 Mn(OAc)3 Co(OAc)2 Zr(OAc)4/HBr

Catalyst

Table 1 Summary of recent advances of catalysts used for lignin oxidation.

RT-363

353–373

RT

RT-443

393–493

RT-443

RT-433

T(K)

0.1-atm

atm

atm

0.5–10

0.2–0.5

0.2–1.52

1–1.38

P (MPa)

Reaction Conditions

0.5–28

3–8

1/6

1/3–12

0–3

0.5–20

1/3–10

t(h)

Monomeric phenolic alcohol

Β-O-4 Dimeric Model Compound Diluted acid corn stover lignin Steam explode spruce lignin

Monomeric phenolic alcohol 5-5‘ Dimeric Model Compound β-O-4 Dimeric Model Compound Hydrolytic sugar cane lignin Red spruce kraft lignin Hard wood organosolvent lignin Monomeric phenolic alcohol

unbleached softwood kraft pulp β-O-4 Dimeric Model compounds kraft lignin

Monomeric phenolic alcohol

Monomeric phenolic alcohols β-O-4 Model Compounds softwood lignosulfonate yellow poplar wood chips eucalyptus wood pulp sugar cane bagasse pulp organocell softwood mixture organosolv hardwood lignin lignosulfonate hydrolyzed lignin Monomeric phenolic Alcohols sulfide pulping wood untreated plant/geochemical samples hardwood kraft lignin cellulosic pulps commercial lignin spruce wood sawdust Alakaline lignin enzymatic hydrolyzed steamexplosive cornstalk

Substrate

phenolic aldehydes,

phenolic ketone,

phenolic aldehydes, phenolic acids,

DCA mixed phenolics,

phenolic aldehydes phenolic acids, quinone,

quinone

phenolic acid, phenolic ketone

phenolic aldehyde,

phenolic aldehyde

phenolic aldehyde, phenolic acids, phenolic ketone

phenolic aldehyde phenolic ketone phenolic aldehyde polyphenol phenolic aldehyde phenolic acids phenolics

Products

∼88%

∼94%

∼88%

Conversion

∼76%

∼4

24

∼88%

Yield%

N.P.

∼93%

N.P.

N.P.

N.P.

N.P.

N.P.

Selectivity (up to)%

(continued on next page)

[129,148,162,191–193]

[21,77,137,190]

[167,188,189]

[120,121,156,183–187]

[41,57,181,182]

[58,63,113,114,117,177–180]

[53–56,174–176]

References

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5

4

Na8H[PW9O34] · 19H2O MTO

H2O2

O2

[AlVVW11O40]6– H3PMo12O40

3

t-butanol HAc

buffer H2O

RT

RT-443

333

Hac/NaAc Buffer (pH = 3–5)

H2O2

318–353

CuFeS2

CH2Cl2 CH3CN:H2O = 19:1

333–363

2

Air O2

NaOH/H2O

CH3CN/CH2Cl2 buffer imidazole [BMIM][PF6]

RT-408

573

TEMPO/Br2/NaNO2 AcNH-TEMPO/HNO3/ HCl

O2

NaClO PhIO KHSO5 air O2 magnesium monoperoxyphthalate

CoTSPc FeTSPc Fe(TF5PP)Cl Fe(TF5PS4P)Cl Mn(TSPP)Cl Mn(TPPS)

Fe(TPPS) Fe(TPPS4) Fe(TCl8PPS4) Pt/MOF-5 Co-ZIF-9

4′-methoxyacetonphenone t-BuOOH H2O2

Mn(TSPc)Cl Fe(TSPc)Cl Rh(TSPP)

Microwave

T(K)

atm

0.5–10

atm

Atm-17

0.5–1

0.5–1

0.1-atm

P (MPa)

Reaction Conditions

Oxidation of the Aromatic Ring and Ring Cleavage Reactions 1 Fe3+ H2O2 Alkali buffer

4

3

2

toluene H2O MeOH CH2Cl2 phosphate CH3CN NaOH/H2O

NaOH/H2O CH3CN

H2O2

Co(salen)/SBA-15 Co-sulphosalen

Co(salen) [(pyr)] Co(salen) [Co(N-Me salpr)]

Solvent

Oxidant

Catalyst

Table 1 (continued)

1/6

1/3–12

0–5

0–1/2

1–36

0.5–4

0.5–12

t(h)

Monomeric phenolic alcohol 5-5‘ Dimeric Model Compound β-O-4 Dimeric Model Compound Hydrolytic sugar cane lignin Red spruce kraft lignin Hard wood organosolvent

Dimeric lignin model compounds Diluted acid corn stover lignin Steam explosion spruce lignin Monomeric phenolic alcohol β-O-4 Dimeric Model compounds

Monomeric phenolics Dimeric lignin model compounds Diluted acid corn stover lignin Steam explosion spruce lignin Organosolv hardwood lignin Monomeric phenolics

Vanillyl alcohol phthalan veratryl alcohol cinnamyl alcohol benzyl alcohol 2-(2-Methoxyphenoxy)-1(3,4,5-trimethoxyphenyl)-1,3propanediol

Monomeric phenolic alcohol Monomeric phenolic aldehyde β-O-4 Dimeric Model Compounds

Monomeric phenolic aldehyde β-O-4 Dimeric Model Compounds

Substrate

quinone, DCA

quinone

Dicarboxylic acids

phenolic acids,

Dicarboxylic acids phenolic aldehydes

phenolic aldehyde phenolic ketone

phenolic aldehydes phenolic acids, epoxide

phenolic aldehyde, phenolic ketone

quinone, simple fatty acids

phenolic ketone, phenolic acids,

Products

∼19.5%

∼48

∼14

∼41

∼98

∼89

∼89.7

Yield%

N.P.

N.P.

95

N.P.

N.P.

91

N.P.

Selectivity (up to)%

(continued on next page)

[167,188,189]

[157,196,197]

[43]

[43,83,85]

[45,195]

[44,147]

[48,149–152,164,194]

References

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β-O-4 Dimeric Model Compounds

0.1-atm

0.5–12

Monomeric phenolic alcohol Monomeric phenolic aldehyde

quinone dicarboxylic acids and derivatives

∼12%

N.P.

[199]

lignin, significant effort has been aimed at developing an efficient method to cleave the β-O-4 linkage in lignin and model compounds. The ether linkages in lignin are more labile than CeC bonds. The cleavage of a significant portion of ether linkages occurs during biomass pretreatments and lignin isolation processes [88–94]. Many of the lignin depolymerization processes are also conducted under either acidic or basic conditions which can facilitate ether linkage cleavage. However, selectively cleaving CeC linkages has always been a challenge. High temperature and high loading of expensive catalysts (e.g., ruthenium, palladium, platinum) are often required to instigate the cleavage of CeC linkages [95–100]. At these severe conditions, condensation of lignin fragments becomes prevalent [36,101–103]. It is known that treating lignin with oxidative reagents including oxygen, hydrogen peroxide, ozone, and peroxy acid can break down the majority of ether linkages as well as a portion of the CeC linkages, through different mechanisms [21,43,46,49,77]. Supplementing oxidation with transition metal ions such as Cu(II), Fe(III), Mn(II, III), Co (II), and Zr(IV) has been shown to enhance the oxygen reactivity and subsequently facilitate the cleavage of β-O-4 and pinacol type CeC linkages cleavage [53,54]. Recent studies have also investigated the use of metal oxides for lignin oxidation [104–109]. Hedges et. al. adapted cupric oxide (CuO) treatment, a lignin analytical method [104–109], to a process that produces monomeric phenolics from lignin isolated from Amazon wood. Kurek et al. tested MnO2 oxalate for oxidizing lignin, based on the mechanism of lignin degradation by wood rot fungi. However, the MnO2/oxalate system showed limited reactivity toward cleaving lignin inter-unit linkages [58–60,110,111]. Augugliaro et al. reported the use of photocatalysts such as TiO2, ZnO to degrade lignin to organic compounds. [112,113]. Due to the high reduction potential of these photocatalysts, they are capable of degrading at a fast rate. It appears that both ether and CeC linkages are susceptible to photochemical oxidation. Higher lignin degradation rates were observed in acidic environments compared to alkaline environments. A mixture of aldehyde compounds was produced from treatment of lignin-rich black liquor with the TiO2/UV photocatalytic system in alkaline solution [114]. Encouraging results from the application of composite metal oxides toward lignin conversion was also demonstrated by several groups [41,57,115,116]. Compared to metal ions, composite metal oxides have shown a pronounced effect in improving the cleavage of β-O-4 ether linkages as well as pinacol CeC bonding. (Fig. 4) Sun, Deng, Ouyang, and several other groups investigated oxidative conversion of lignin to phenolic aldehydes (e.g., vanillaldehyde, syringaldehyde, and p-hydrobenzaldehyde) using composite metal oxides [41,57,115,117–119]. Deng et al. tested several LaBO3 preparations, showing that the tuning the ratio of B in the composite can change lignin oxidation depolymerization rate; however, the reaction products yield was not altered significantly. Another group of potential catalysts are polyoxometalates (POMs), which are polyatomic ions–usually anions–that consist of three or more transition metal oxyanions linked together by sharing oxygen atoms to form closed 3D frameworks. For examples, W(VI), Mo(VI), V(V) and Nb (V) together with oxygen anions can be arranged into MO6 octahedral units [4,6,79]. Weinstock and Evtuguin reported the use of POMs as an efficient catalyst to activate molecular oxygen and oxidize lignin [4,120,121]. The reaction mechanisms for this group of more structurally complex metal oxides were investigated based on dimeric model compounds study. It was found that β-O-4 cleavage is the primary reaction during POMs treatment, resulting in phenolic aldehydes as the main depolymerization products. However, the reactivity of POMs toward CeC linkages cleavage was not discussed. It was also shown that the etherified and non-etherified phenolic groups have different activities toward POMs. A high reactivity between POMs and phenolic-type lignin model compounds was observed at room temperature, while the non-phenolic type of lignin model compounds demanded more severe conditions (∼438 K) with the same catalysts. HPA is a subgroup of

*a: Unknow, RT: Room Temperature. N.P.: Not presented.

CH3CN/CH2Cl2 buffer imidazole [BMIM][PF6]

NaOH/H2O

RT-408 FeTsPc 6

H2O2 O2

toluene H2O MeOH CH2Cl2 phosphate CH3CN

0.5–28 0.1-atm

P (MPa) T(K)

RT-363 Microwave CHCl3 NaOH/H2O CH3CN O2 H2O2 Co(salen) 5

Catalyst

Table 1 (continued)

Oxidant

Solvent

Reaction Conditions

t(h)

Substrate

lignin Monomeric phenolic alcohol Monomeric phenolic aldehyde β-O-4 Dimeric Model Compounds

Products

quinone, simple fatty acids

∼79

Yield%

N.P.

Selectivity (up to)%

[38,198]

References

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Fig. 4. Oxidation cleavage of representative lignin inter-unit ether and CeC linkages. (Structure in dashed circle represents pinacol structures in lignin.).

was applied on neolignan, a β- β dimeric model compounds, only sidechain insertion of hydroxyl groups was observed without CeC bond cleavage. This confirms similar results from previous studies where biphenyl structure lignin model compounds were found to resist the degradation of MTO catalysis. Vanadium group (V, Nb, Ta) catalysts have been examined in the work done by both Hanson and Toste [46,77]. Hanson et al. reported the use of Vanadium-based organometallic complexes to catalyze ether and CeC bonds cleavage of lignin model compounds that contain pinacol structures [46,50,77,134–137]. In pinacol structures, the CeH bond adjacent to the alcohol moiety can break and oxidize to yield the corresponding alcohol and aldehyde. Son and Toste has reported the use of Schiff base vanadium complexes to oxidize dimeric pinacol model compounds 2-phenoxyethanol, 1phenyl-2-phenoxyethanol, and 1,2-diphenyl-2-methoxyethanol, to produce alkenes and 2-methoxyphenol [50]. Jiang et al. studied oxidation of lignin related primary alcohols into acids or aldehydes in ionic liquids [136,137]. The overall reaction is redox-neutral, so additional oxygen is not essential to initiate the reaction. However, the presence of oxygen was observed to increase the reaction rate. The reactions were typically conducted at relatively mild temperatures (∼373 K) under ambient pressure. It was shown that the type of solvents used in the catalytic system plays a vital role in determining not only the reaction rate, but also the products profile. For example, the cleavage of CeH and CeC bonds and release of phenolic alcohols and aldehydes during treatment of pinacol model compounds by vanadium catalyst occurred at a much slower rate in DMSO than in pyridine. The oxidation products profile from dimeric model compounds appeared was also strongly influenced by the solvent. Benzaldehyde and methanol were found to be the major products from 1,2-

PMOs with a general formula of [XxMmOy]q−, where X is a heteroatom (X = P, Si, B, etc.) and M is an addenda atom (M= W(VI), Mo(VI), V (V), etc.). Evtuguin and co-workers tested HPA-5′s reactivity toward the oxidation of different phenolic lignin structures, and suggested this relative relationship in reactivity: hydroxybenzyl > benzylether > alpha-carbonyl [40,122,123]. Cleavage of both β-O-4 ether linkages and CeC bond side-chain linkages were observed during the treatment by HPA/O2. It has been demonstrated that Vanadium-based HPA have enhanced reactivity toward depolymerizing non-phenolic lignin model compounds [124]. The biomimetic catalysts metallosalen and metalloporphyrins have been shown to be effective toward cleaving ether linkages and subsequently releasing phenolic aldehydes/ketones [4,125–127]. The mechanisms of metallosalen and metalloporphyrin mediated oxygen oxidation of lignin have been recently reviewed and proposed [4,6,49]. Metallosalen has been proposed to catalyze lignin degradation through the formation of either superoxo or peroxo complexes depending on reaction temperature and oxygen pressure [128,129]. These complexes can then oxidize lignin side chain and aromatic ring to yield aldehyde [39,74,75,129–131] or quinone [132,133] products. However, these catalysts are apparently not capable of cleaving CeC linkages. Organorhenium has shown reactivity similar to metallosalen in breaking down ether linkages, but is similarly ineffective toward cleaving CeC linkages. Crestini found that the selective cleavage of βO-4 in 1-(4-hydroxy-3-methoxyphenyl)-2-(2,6-dimethoxyphenoxy)propane-1,3-diol (β-O-4 dimeric model) compounds by methyltrioxorhenium(VII) (MTO) can yield a mixture of hydroxyketones and 2,6dimethoxyphenols and vanillic acids resulting from oxidation of the resulting pinacol structure. However, when the same catalytic system

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further degradation products were identified. The formation of the αcarbonyl group reduces the electron density on the lignin aromatic ring and increases steric hindrance to the side-chain, thus likely reducing ππ interaction. Co-ZIF-9 also has the unique catalytic activity of oxidizing olefinic structures to epoxides, which was demonstrated by using cinnamyl alcohol as a model compound [44]. In addition to selective modification of the lignin side-chain, many of the catalysts capable of cleaving lignin inter-unit linkage also showed additional effects toward side-chain modification. For a example, Kervinen treated the lignin model compound veratryl alcohol with Co (Salen) catalysts under low O2 pressure (0.1 MPa) in an alkaline aqueous solution at 353 K; the reaction proceeded with the formation of the corresponding veratryl aldehyde as a stable product. No side-chain cleavage products or further ring oxidation products were observed [129]. This is a striking phenomenon compared to a typical Co(Salen) catalyzed lignin monomer oxidation. Similar results were observed when embedding the Co(Salen) catalysts on a silica support (SBA-15), where the apocynol can be oxidized to a phenolic ketone after treating with H2O2 in CH3CN solvent. However, instead of proceeding with a selective side-chain modification, further oxidation of aromatic nuclei was observed [148]. This phenomenon was also observed using Co, Fe, and Mn based metalloporphyrins. At room temperature, most existing studies demonstrated the oxidation of veratryl alcohol to veratryl aldehyde with several different oxidants such as H2O2 and KHSO5 in a variety of solvents and solutions including buffers and ionic liquids [149–152]. However, their capability to cleave inter-unit linkages remains to be proven. It is likely that the reaction is quenched after oxidizing the side-chain hydroxyl group to carbonyl groups under such mild conditions. This might provide a highly selective lignin oxidative modification method and warrants further investigation.

diphenyl-2-methoxyethanol oxidation in DMSO, while benzoic acid and methyl benzoate were formed primarily when the reaction was conducted in pyridine [77]. Ma et al. has recently discovered that niobiumbased catalysts also have a very unique catalytic property for oxidative lignin depolymerization. Nb2O5 was reported to have a pronounced effect on catalyzing peracetic acid treatment of lignin, which can rapidly solubilize and depolymerize lignin. Both ether and CeC linkage can be cleaved efficiently to depolymerize lignin. Nb2O5 catalyzed peracetic acid can effectively remove the lignin side-chain by side-chain replacement or side-chain oxidation to produce a selective group of phenolics, such as hydroxyphenolics and phenolics acids [21]. 4. Oxidative modification of lignin side-chain Most of phenylpropane units in lignin are linked through the side chain. As discussed earlier, one challenge hindering the application of metal and metal oxide catalysts for complete lignin depolymerization is the limited capability to completely cleave all inter-unit linkages. The incorporation of catalysts capable of modifying the side chain electron density with these metal/metal oxide catalysts may help disrupt the lignin macromolecule integrity and can significantly enhance lignin depolymerization efficiency. As a typical consequence of oxidative treatments, new oxygen-containing groups are introduced to lignin propanyl side-chain which can subsequently impair π-π interactions among phenylpropane units. The precise patterns of side chain modification by catalytic oxidation is complicated, and has been largely overlooked. A common observation from the oxidative modification of the lignin side-chain is the formation of ketones as reported in a number of studies (Fig. 5) [44,46,47,50,75,124,138]. This has been shown to occur during lignin degradation by organocatalysts which is a large family of organic compounds typically containing nitrogen, sulfur, or phosphor as active site constituents for promoting chemical reactions. [138,139]. For example, the organocatalyst 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) can oxidize primary and secondary alcohols in lignin structures to aldehydes. Stahl et al. verified this mechanism on the lignin structure by the use of the dimeric model compound 3-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-2-propenol. The resulting oxidized lignin model compounds can be further cleaved following other treatments, such as alkaline treatment to remove ether linkages. A similar reaction was also observed in work done by Rahimi et al., where oxidized lignin ether model compounds were found to be depolymerized to monomeric molecules by formic acid treatment [140]. Organocatalysts certainly present a promising catalyst for lignin conversion. Metal-organic frameworks (MOF) is another group of catalysts have the capability to oxidize lignin. Kustov reported the use of a 5% Pt on [Zn4O(BDC)3] MOF to catalyze the oxidation of vanillyl alcohol to vanillin. [141–147] Similar reactions were also observed for gold supported on MIL-101 MOF, which catalyzed the conversion of primary and secondary benzylic alcohols to aldehydes and ketones, respectively, with high yield and selectivity. Differing from simple transition metal ions or metal oxides, the reactivity and pore structure of the MOF can be tailored by changing the organic linker component to perform specific catalytic functions. In addition, its highly porous structure can provide a larger accessible catalytic surface. Zakzeski et al. investigated Co-ZIF-9 [44] for oxidizing veratryl alcohol and vanillyl alcohol in toluene at 150 °C under 0.5 MPa O2, yielding their respective aldehydes after 4 h. The reaction stops upon formation of the carbonyl group; no

5. Oxidation of the aromatic ring and ring cleavage reactions While depolymerizing lignin to aromatic and phenolic products is a sensible approach, there is an increasing amount of recent research interest toward producing quinones and open chain hydrocarbon compounds from lignin [43,91]. Extensive oxidation of lignin can lead to aromatic ring oxidation to quinones and subsequent ring cleavage products, dicarboxylic acids (DCAs). These reactions have long been observed in plant delignification and paper bleaching [4,23,36,153,154]. However, it was not until very recently that research attention has been directed toward designing a specific approach to convert lignin to quinones and DCAs [42,43,81,83,85,98]. Weinstock has demonstrated the ability of [AlVVW11O40]6– and Na5[SiVW11O40] to oxidize lignin/lignin model compounds under mild conditions in aqueous solutions [155–157]. A considerable amount of para- and ortho-benzoquinone structures were produced [158]. The benzoquinones were recently demonstrated by Huskinson et al. to be a family of molecules with favorable chemical and electrochemical properties for energy storage, such as for battery applications [159–161]. This chemistry has been recently applied to design biomass fuel cells based on biorefinery lignin. As recently reported by Deng, treatment of lignin with H3PMo12O40 (PMo12) as a photocatalyst and charge carrier can generate electricity to operate a solar fuel cell [42]. Zhao and Zhu demonstrated the direct conversion of biorefinery lignin to electricity using the same POM as electron and proton carrier in the anode solution, and platinum catalyzed carbon electrode consuming O2 as an oxidant. The benzoquinone and hydroquinone reduction pair Fig. 5. Oxidation modification of lignin side-chain in β-O-4 dimeric model compounds.

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aromatic ring. However, catalysts with low reduction potential can escape into solution during the reaction and cause side chain oxidations. Increasing the number of electrophilic groups on the porphyrin ligand tends to facilitate nuclei oxidation reactions [164]. Dolphin also reported the supplementation of metalloporphyrins with oxidants with high reduction potential, such as magnesium monoperoxyphthalate [47,48,68]. Aromatic nuclei oxidation can yield either quinone structures (ortho or para) or muconates, which can be directed by changing the porphyrin substitution [164]. Quinones are typically produced when a metallo-oxo complex attacks the para position, forming the hydroquinone molecule that is further oxidized to para-benzoquinone. Attacks on the ortho position of aromatic nuclei by the metallo-oxo complex can lead to the production of muconates via aromatic ring cleavage through the oxygen donation process. This ring opening reaction can be achieved by using different oxidants such as hydrogen peroxide, potassium monopersulfate and magnesium monoperoxyphtalate as the oxidizing agents [165,166]. It has been suggested that these mechanisms may occur differently in non-polar media. Changing the reaction media will also have the impact of affecting the aromatic ring cleavage efficiency [166]. The solubility and reactivity of metalloporphyrins can be tuned by changing substitution groups on the porphyrin rings. Introducing more phenyl groups is a typical practice to enhance porphyrin stability. Organorhenium, generally speaking, can only catalyze limited degradation of lignin, resulting in side-chain modification or ether linkage cleavage. However, when treating β-O-4 ether linked model compounds, a small portion of muconolactones were detected as by-products [167]. The exact mechanism behind muconolactones formation needs further investigation. The metal sulfite group has recently been utilized for selective lignin conversion. The combination of chalcopyrite (CuFeS2) along with hydrogen peroxide in acetate buffer (pH = 2–4) led to the oxidative depolymerization of lignin and the formation of DCAs (succinic acid, malonic acid, and maleic acid) through a mild Fenton reaction mechanism [43]. The results from model compounds oxidation show that both HO% and HO+ were present as the primary reactive species. The products profile changes along with increasing reaction time which suggests a two-stage reaction mechanism: 1) lignin depolymerization to monomeric phenolic compounds, and 2) aromatic ring cleavage of lignin. Monomeric phenolics, benzoquinone and muconic acid derivatives were suggested as the key intermediates. HO+ is proposed to be responsible for lignin depolymerization while HO% is necessary for aromatic ring cleavage. However, there is no direct detection method for benzoquinone and muconic acid derivatives in the reaction mixture. Developing new analytical techniques such as in-situ detection methods will provide a promising avenue toward understanding the reaction mechanisms. DCAs such as adipic acid, muconic acid, muconolactone, maleic acid, succinic acid, and malonic acid are important platform chemicals for the polymer, pharmaceutical, and food industries. [43,81,168–173] Apart from the economic benefit from generating value-added products, attacking aromatic ring to yield benzoquinone and DCAs/carboxylic acids will ultimately reduce and remove the electronic stabilization from the aromatic ring π-π interaction. (Fig. 6) It provides a pathway towards completely unraveling the complex lignin structure into an open chain molecule. This strategy is attracting increasing interest as a pathway toward lignin valorization, and many catalysts likely have the potential to instigate the ring opening reaction [43,81,85,98]. It is worth noting that a number of lignin-to-phenolics/aromatics reactions already include aromatic ring cleavage as a side-reaction [83,167]. There is an opportunity to redirect these existing reactions toward selective ring opening reactions.

Fig. 6. Oxidation of lignin aromatic ring to benzoquinone and ring opened DCAs.

were generated to mediate the conversion. This is a promising route to convert lignin, a solid fuel, into a sustainable form of energy. The occurrence of lignin re-condensation is one of the key challenges identified and requires more work before implementation. These benzoquinone intermediates can also be readily oxidized to ring opened products. Aromatic ring oxidation products, such as benzoquinone, are also formed during Co(Salen) oxidation of lignin model compounds [148,162]. Co(salen)-superoxo complex can activate molecular oxygen to form reactive phenoxy radicals. Phenoxy radicals then dissociate from the complex and are attacked by either O2 or the Co-superoxo complex to yield benzoquinone [128,162,163]. Aromatic ring opening reaction is the most significant step that reverses the electronic densification trend of lignin biosynthesis and converts lignin from an aromatic polymer to open-chain structures. Fig. 6 presents a general mechanistic understanding of aromatic ring cleavage of lignin aromatic nuclei to DCAs. It needs to be emphasized that the nature of ring cleavage chemistries is different from the oxidations of lignin side-chains or cleavage of inter-unit linkages and thus require different types of catalysts and reaction conditions. Cleavage of the lignin aromatic ring is often facilitated by radical reactions. In nature, white-rot fungi are notable for their ability to effectively and selectively degrade lignin through one-electron mechanisms, where phenolic aromatic rings are first attacked and oxidized. Many biomimetic catalysts, such as Schiff base Salen, metalloporphyrins, organorhenium, and vanadium complexes therefore have this functionality. These catalysts have been investigated recently for catalyzing lignin depolymerization. Artaud et al. found that metalloporphyrins can activate oxygen/ peroxide to form the metallo-oxo complex [164]. A radical cation intermediate is generated to attack electron-rich sites on either the lignin aromatic ring or side-chain. If the reduction potential of the catalyst is sufficiently high, the reduced metallo-oxo complex is more likely to remain in the solvent cage with a preference toward attacking the

6. Conclusions and future perspective Nature has purposefully constructed lignin as an electron dense macromolecule in plants, with the likely function of maintaining 9

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[34] Y. Wang, Y. Wu, Y. Huang, F. Zhang, X. Yang, Y. Ma, Y. Chen, J. Phys. Chem. C 115 (2011) 23192–23197. [35] D. Li, R.B. Kaner, Nat. Nanotechnol. 3 (2008) 101. [36] D. Fengel, G. Wegener, Wood: Chemistry, Ultrastructure, Reactions, Walter de Gruyter, 1983. [37] J. Gierer, Wood Sci. Technol. 19 (1985) 289–312. [38] J.J. Bozell, B.R. Hames, D.R. Dimmel, J. Org. Chem. 60 (1995) 2398–2404. [39] K. Kervinen, Studies on Veratryl Alcohol Oxidation Catalyzed by Co (salen) Type Complexes and Molecular Oxygen in Aqueous Solution, Kaisa Kervinen, 2005. [40] D.V. Evtuguin, A.I.D. Daniel, A.J.D. Silvestre, F.M.L. Amado, C.P. Neto, J. Mol. Catal. A- Chem. 154 (2000) 217–224. [41] H.B. Deng, L. Lin, S.J. Liu, Energy Fuels 24 (2010) 4797–4802. [42] W. Liu, W. Mu, M. Liu, X. Zhang, H. Cai, Y. Deng, Nat. Commun. 5 (2014). [43] R. Ma, M. Guo, X. Zhang, ChemSusChem 7 (2014) 412–415. [44] J. Zakzeski, A. Debczak, P.C.A. Bruijnincx, B.M. Weckhuysen, Appl. Catal. A: Gen. 394 (2011) 79–85. [45] A. Rahimi, A. Azarpira, H. Kim, J. Ralph, S.S. Stahl, J. Am. Chem. Soc. 135 (2013) 6415–6418. [46] S.K. Hanson, R.L. Wu, L.A. Silks, Angew. Chem. Int. Ed. 51 (2012) 3410–3413. [47] F. Cui, D. Dolphin, Can. J. Chem.-Rev. Canadienne De Chimie 73 (1995) 2153–2157. [48] F.T. Cui, D. Dolphin, Bioorg. Med. Chem. 3 (1995) 471–477. [49] C. Crestini, M. Crucianelli, M. Orlandi, R. Saladino, Catal. Today 156 (2010) 8–22. [50] S. Son, F.D. Toste, Angew. Chem. Int. Ed. 49 (2010) 3791–3794. [51] R. Ma, Dicarboxylic Acids Platform Chemicals for Valorization of Biorefinery Lignin, Washington State University, 2016. [52] P. Bajpai, Biorefinery in the Pulp and Paper Industry, Academic Press, 2013. [53] R. Dicosimo, H.C. Szabo, J. Org. Chem. 53 (1988) 1673–1679. [54] W. Partenheimer, Adv. Synth. Catal. 351 (2009) 456–466. [55] V.E. Tarabanko, N.A. Fomova, B.N. Kuznetsov, N.M. Ivanchenko, A.V. Kudryashev, React. Kinet. Catal. Lett. 55 (1995) 161–170. [56] Q. Xiang, Y.Y. Lee, Appl. Biochem. Biotechnol. 91-3 (2001) 71–80. [57] H.B. Deng, L. Lin, Y. Sun, C.S. Pang, J.P. Zhuang, P.K. Ouyang, J.J. Li, S.J. Liu, Energy Fuels 23 (2009) 19–24. [58] B. Kurek, F. Gaudard, J. Agric. Food Chem. 48 (2000) 3058–3062. [59] B. Kurek, B. Hames, C. Lequart, K. Ruel, B. Pollet, C. Lapierre, F. Gaudard, B. Monties, Chemistry of lignin degradation by MnO2/oxalate within spruce, poplar, end wheat strew: an overview, Abstracts of Papers of the American Chemical Society, AMER CHEMICAL SOC 1155 16TH ST, NW, WASHINGTON, DC 20036 USA, 2000 (U284-U284). [60] C. Lequart, B. Kurek, P. Debeire, B. Monties, J. Agric. Food Chem. 46 (1998) 3868–3874. [61] J.I. Hedges, R.A. Blanchette, K. Weliky, A.H. Devol, Geochim. Cosmochim. Acta 52 (1988) 2717–2726. [62] A. Otto, M.J. Simpson, Biogeochemistry 80 (2006) 121–142. [63] N. Jiang, A.J. Ragauskas, ChemSusChem 1 (2008) 823–825. [64] N. Jiang, D. Vinci, C.L. Liotta, C.A. Eckert, A.J. Ragauskas, Ind. Eng. Chem. Res. 47 (2008) 627–631. [65] N. Jiang, A.J. Ragauskas, J. Org. Chem. 71 (2006) 7087–7090. [66] R. DiCosimo, H.C. Szabo, J. Org. Chem. 53 (1988) 1673–1679. [67] Y.S. Perng, C.W. Oloman, P.A. Watson, B.R. James, Tappi J. 77 (1994) 119–125. [68] F.T. Cui, T. Wijesekera, D. Dolphin, R. Farrell, P. Skerker, J. Biotechnol. 30 (1993) 15–26. [69] P.A. Watson, L.J. Wright, T.J. Fullerton, J. Wood Chem. Technol. 13 (1993) 391–409. [70] M. Shimada, Mokuzai Gakkaishi 37 (1991) 1103–1114. [71] M. Shimada, T. Habe, T. Higuchi, T. Okamoto, B. Panijpan, Holzforschung 41 (1987) 277–285. [72] M. Shimada, T. Higuchi, Abs. Pap. Am. Chem. Soc. 194 (1987) 27–Cell. [73] T. Habe, M. Shimada, T. Higuchi, Mokuzai Gakkaishi 31 (1985) 54–55. [74] K. Kervinen, H. Korpi, J. Gerbrand Mesu, F. Soulimani, T. Repo, B. Rieger, M. Leskelä, B.M. Weckhuysen, Eur. J. Inorg. Chem. 2005 (2005) 2591–2599. [75] K. Kervinen, H. Korpi, M. Leskelä, T. Repo, J. Mol. Catal. A: Chem. 203 (2003) 9–19. [76] K. Kervinen, P. Lahtinen, T. Repo, M. Svahn, M. Leskelä, Catal. Today 75 (2002) 183–188. [77] S.K. Hanson, R.T. Baker, J.C. Gordon, B.L. Scott, D.L. Thorn, Inorg. Chem. 49 (2010) 5611–5618. [78] J. Zakzeski, A.L. Jongerius, P.C.A. Bruijnincx, B.M. Weckhuysen, Chemsuschem 5 (2012) 1602–1609. [79] Y.S. Kim, H.-m. Chang, J.F. Kadla, Holzforschung 62 (2008) 38–49. [80] A.R. Gaspar, J.A. Gamelas, D.V. Evtuguin, C.P. Neto, Green Chem. 9 (2007) 717–730. [81] D.R. Vardon, M.A. Franden, C.W. Johnson, E.M. Karp, M.T. Guarnieri, J.G. Linger, M.J. Salm, T.J. Strathmann, G.T. Beckham, Energy Environ. Sci. 8 (2015) 617–628. [82] D.R. Vardon, N.A. Rorrer, D. Salvachúa, A.E. Settle, C.W. Johnson, M.J. Menart, N.S. Cleveland, P.N. Ciesielski, K.X. Steirer, J.R. Dorgan, Green Chem. 18 (2016) 3397–3413. [83] J. Zeng, C.G. Yoo, F. Wang, X. Pan, W. Vermerris, Z. Tong, ChemSusChem 8 (2015) 861–871. [84] L. Yang, M. Lübeck, B.K. Ahring, P.S. Lübeck, Appl. Microbiol. Biotechnol. 100 (2016) 1799–1809. [85] G. Yin, F. Jin, G. Yao, Z. Jing, Ind. Eng. Chem. Res. 54 (2015) 68–75. [86] L.B. Davin, A.M. Patten, M. Jourdes, N.G. Lewis, Biomass Recalcitrance Deconstructing the Plant Cell Wall for Bioenergy, (2008), pp. 213–305. [87] W. Boerjan, J. Ralph, M. Baucher, Annu. Rev. Plant Biol. 54 (2003) 519–546. [88] R. Samuel, Y. Pu, B. Raman, A.J. Ragauskas, Appl. Biochem. Biotechnol. 162 (2010) 62–74. [89] E.W. Rutkowska, P. Wollboldt, G. Zuckerstaetter, H.K. Weber, H. Sixta, BioResources 4 (2008) 172–193.

structural integrity. An effective lignin depolymerization method would incorporate a strategy to cope with the electron dense structure of lignin. Oxidative lignin conversion methods provide a suitable chemistry to effectively reduce the electron density by inserting oxygenates into the lignin propanyl side-chain and aromatic ring structure. Three types of reactions are commonly observed during lignin oxidative conversion: 1) inter-unit linkage cleavage; 2) side-chain modification; and 3) oxidation of the aromatic ring and ring cleavage. All three of these reaction types have their own distinct role in the development of a lignin valorization strategy. For example, LMWPCs produced from the cleavage of inter-unit linkages and DCAs from the cleavage of the aromatic ring are both important groups of platform chemicals. Sidechain oxidative modification can introduce active functional groups onto lignin, providing greater opportunities for material preparation. Preliminary work with catalysts have shed light on their potential to facilitate lignin oxidative conversion. However, the conversion selectivity and efficiency remains difficult to simultaneously optimize. There is a critical need to develop robust lignin oxidation catalysts and identify favorable reaction conditions to obtain high products yield and selectivity. Incorporating state-of-the-art catalyst development with existing lignin oxidation chemistries will lead to the creation of new lignin valorization strategies. Acknowledgement Authors are grateful to the financial support from National Science Foundation (Award no: 1454575), Northwest Advance Renewable Alliance (NARA)/United States Department of Agriculture (USDA Grant no. 2011-68005-30416) and Sungrant/United States Department of Transportation (Contract no. T0013G-A-9). References [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]

M.S. Tunc, A.R. van Heiningen, Carbohydr. Polym. 83 (2011) 8–13. A. Rezanowich, D. Goring, J. Colloid Sci. 15 (1960) 452–471. X. Zhang, M. Tu, M.G. Paice, BioEnergy Res. 4 (2011) 246–257. R. Ma, Y. Xu, X. Zhang, ChemSusChem 8 (2015) 24–51. A.J. Ragauskas, G.T. Beckham, M.J. Biddy, R. Chandra, F. Chen, M.F. Davis, B.H. Davison, R.A. Dixon, P. Gilna, M. Keller, Science 344 (2014) 1246843. J. Zakzeski, P.C. Bruijnincx, A.L. Jongerius, B.M. Weckhuysen, Chem. Rev. 110 (2010) 3552–3599. S.a. Contreras, A.R. Gaspar, A. Guerra, L.A. Lucia, D.S. Argyropoulos, Biomacromolecules 9 (2008) 3362–3369. M. Norgren, H. Edlund, L. Wågberg, Langmuir 18 (2002) 2859–2865. J. Benko, Tappi 47 (1964) 508–514. Y.-r. Chen, S. Sarkanen, Phytochemistry 71 (2010) 453–462. Y. Deng, X. Feng, M. Zhou, Y. Qian, H. Yu, X. Qiu, Biomacromolecules 12 (2011) 1116–1125. Y. Deng, X. Feng, D. Yang, C. Yi, X. Qiu, BioResources 7 (2012) 1145–1156. S.Y. Lin, C.W. Dence, Methods in Lignin Chemistry, Springer, 1992. K.V. Sarkanen, C.H. Ludwig, Lignins: Occurrence, Formation, Structure and Reactions, (1971). S. Kubo, J.F. Kadla, Biomacromolecules 6 (2005) 2815–2821. E.E. Jelley, Nature 138 (1936) 1009–1010. F.C. Spano, C. Silva, Annu. Rev. Phys. Chem. 65 (2014) 477–500. D. Wright, S.A. Brown, A. Neish, Can. J. Biochem. Physiolo. 36 (1958) 1037–1045. K. Sarkanen, Lignins, Occurrence, Formation, Structure and Reactions, (1971). N.G. Lewis, S. Sarkanen, Lignin and Lignan Biosynthesis, ACS Publications, 1998. R. Ma, M. Guo, K. t. Lin, V.R. Hebert, J. Zhang, M.P. Wolcott, M. Quintero, K.K. Ramasamy, X. Chen, X. Zhang, Chem.-A Eur. J. 22 (2016) 10884–10891. D.L. Crawford, M.J. Barder, A.L. Pometto III, R.L. Crawford, Arch. Microbiol. 131 (1982) 140–145. J. Gierer, Wood Sci. Technol. 20 (1986) 1–33. C.A. Hunter, J.K. Sanders, J. Am. Chem. Soc. 112 (1990) 5525–5534. F.J. Hoeben, P. Jonkheijm, E. Meijer, A.P. Schenning, Chem. Rev. 105 (2005) 1491–1546. V. Georgakilas, M. Otyepka, A.B. Bourlinos, V. Chandra, N. Kim, K.C. Kemp, P. Hobza, R. Zboril, K.S. Kim, Chem. Rev. 112 (2012) 6156–6214. C.A. Hunter, K.R. Lawson, J. Perkins, C.J. Urch, J. Chem. Soc. Perkin Trans. 2 (2001) 651–669. B. Partoens, F. Peeters, Phys. Rev. B 74 (2006) 075404. A.C. Ferrari, Solid State Commun. 143 (2007) 47–57. E.C.H. Sykes, Nat. Chem. 1 (2009) 175–176. Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Adv. Mater. 22 (2010) 3906–3924. S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558–1565. X. Yang, J. Zhu, L. Qiu, D. Li, Adv. Mater. 23 (2011) 2833–2838.

10

Catalysis Today xxx (xxxx) xxx–xxx

R. Ma et al. [90] [91] [92] [93] [94] [95] [96] [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [116] [117] [118] [119] [120] [121]

[122] [123] [124] [125] [126] [127] [128] [129] [130] [131] [132] [133] [134] [135] [136] [137] [138] [139] [140] [141] [142] [143]

[144] F. Zhang, C. Chen, W.M. Xiao, L. Xu, N. Zhang, Catal. Commun. 26 (2012) 25–29. [145] M. Jahan, Q.L. Bao, K.P. Loh, J. Am. Chem. Soc. 134 (2012) 6707–6713. [146] M.J. Beier, W. Kleist, M.T. Wharmby, R. Kissner, B. Kimmerle, P.A. Wright, J.D. Grunwaldt, A. Baiker, Chem.-a Eur. J. 18 (2012) 887–898. [147] A.L. Tarasov, L.M. Kustov, V.I. Isaeva, A.N. Kalenchuk, I.V. Mishin, G.I. Kapustin, V.I. Bogdan, Kinet. Catal. 52 (2011) 273–276. [148] S.K. Badamali, R. Luque, J.H. Clark, S.W. Breeden, Catal. Commun. 10 (2009) 1010–1013. [149] P. Zucca, G. Mocci, A. Rescigno, E. Sanjust, J. Mol. Catal. A- Chem. 278 (2007) 220–227. [150] G. Labat, B. Meunier, New J. Chem. 13 (1989) 801–804. [151] G. Labat, B. Meunier, Abs. Pap. Am. Chem. Soc. 197 (1989) 450–INOR. [152] A. Kumar, N. Jain, S.M.S. Chauhan, Synlett (2007) 411–414. [153] J. Kolar, B. Lindgren, B. Pettersson, Wood Sci. Technol. 17 (1983) 117–128. [154] E. Sjöström, Wood Chemistry: Fundamentals and Applications, Gulf Professional Publishing, 1993. [155] T. Voitl, M.V. Nagel, P.R. von Rohr, Holzforschung 64 (2010) 13–19. [156] T. Yokoyama, H.M. Chang, R.S. Reiner, R.H. Atalla, I.A. Weinstock, J.F. Kadla, Holzforschung 58 (2004) 116–121. [157] I.A. Weinstock, E.M. Barbuzzi, M.W. Wemple, J.J. Cowan, R.S. Reiner, D.M. Sonnen, R.A. Heintz, J.S. Bond, C.L. Hill, Nature 414 (2001) 191–195. [158] I.A. Weinstock, K.E. Hammel, M.A. Moen, L.L. Landucci, S. Ralph, C.E. Sullivan, R.S. Reiner, Holzforschung 52 (1998) 311–318. [159] B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon, M.J. Aziz, Nature 505 (2014) 195–198. [160] S. Er, C. Suh, M.P. Marshak, A. Aspuru-Guzik, Chem. Sci. 6 (2015) 885–893. [161] D. Vonlanthen, P. Lazarev, K.A. See, F. Wudl, A.J. Heeger, Adv. Mater. 26 (2014) 5095–5100. [162] J.J. Bozell, B.R. Hames, D.R. Dimmel, J. Org. Chem. 60 (1995) 2398–2404. [163] G.T. Musie, M. Wei, B. Subramaniam, D.H. Busch, Inorg. Chem. 40 (2001) 3336–3341. [164] I. Artaud, K. Benaziza, D. Mansuy, J. Org. Chem. 58 (1993) 3373–3380. [165] B. Kurek, I. Artaud, B. Pollet, C. Lapierre, B. Monties, J. Agric. Food Chem. 44 (1996) 1953–1959. [166] C. Fabbri, C. Aurisicchio, O. Lanzalunga, Cent. Eur. J. Chem. 6 (2008) 145–153. [167] C. Crestini, M.C. Caponi, D.S. Argyropoulos, R. Saladino, Bioorg. Med. Chem. 14 (2006) 5292–5302. [168] S.Y. Lee, S.H. Hong, S.H. Lee, S.J. Park, Macromol. Biosci. 4 (2004) 157–164. [169] K. Sato, M. Aoki, R. Noyori, Science 281 (1998) 1646–1647. [170] M. Dugal, G. Sankar, R. Raja, J.M. Thomas, Angew. Chem. Int. Ed. 39 (2000) 2310–2313. [171] K. Nakajima, Y. Baba, R. Noma, M. Kitano, J.N. Kondo, S. Hayashi, M. Hara, J. Am. Chem. Soc. 133 (2011) 4224–4227. [172] E. de Jong, A Report Prepared for IEA Bioenergy-Task 42 (2011). [173] K. Niemelä, J. Chem. Technol. Biotechnol. 48 (1990) 17–28. [174] T. Sugimoto, T. Morishita, Y. Matsumoto, G. Meshitsuka, Effect of Oxygen Pressure on the Oxidation of Syringyl Alcohol Initiated by Manganese(III) Acetate, Holzforschung, 2000, p. 262. [175] S. Hwang, Y.W. Lee, C.H. Lee, I.S. Ahn, J. Polym. Sci. Part a-Polym. Chem. 46 (2008) 6009–6015. [176] A.R. Goncalves, U. Schuchardt, Appl. Biochem. Biotechnol. 77-9 (1999) 127–132. [177] V.E. Tarabanko, D.V. Petukhov, G.E. Selyutin, Kinet. Catal. 45 (2004) 569–577. [178] K. Kaiser, R. Benner, Anal. Chem. 84 (2012) 459–464. [179] J.C. Villar, A. Caperos, F. Garcia-Ochoa, Wood Sci. Technol. 35 (2001) 245–255. [180] D. da Silva Perez, A. Castellan, S. Grelier, M.G. Terrones, A.E. Machado, R. Ruggiero, A.L. Vilarinho, J. Photochem. Photobiol. A: Chem. 115 (1998) 73–80. [181] H.B. Deng, L. Lin, Y. Sun, C.S. Pang, J.P. Zhuang, P.K. Ouyang, Z.J. Li, S.J. Liu, Catal. Lett. 126 (2008) 106–111. [182] J.H. Zhang, H.B. Deng, L. Lin, Molecules 14 (2009) 2747–2757. [183] K. Ruuttunen, T. Vuorinen, Ind. Eng. Chem. Res. 44 (2005) 4284–4291. [184] I.A. Weinstock, E.M.G. Barbuzzi, M.W. Wemple, J.J. Cowan, R.S. Reiner, D.M. Sonnen, R.A. Heintz, J.S. Bond, C.L. Hill, Nature 414 (2001) 191–195. [185] T. Voitl, P.R. von Rohr, Chemsuschem 1 (2008) 763–769. [186] T. Voill, P.R. von Rohr, Ind. Eng. Chem. Res. 49 (2010) 520–525. [187] A.R. Gaspar, D.V. Evtuguin, C.P. Neto, Ind. Eng. Chem. Res. 43 (2004) 7754–7761. [188] W.A. Herrmann, T. Weskamp, J.P. Zoller, R.W. Fischer, J. Mol. Catal. A- Chem. 153 (2000) 49–52. [189] C. Crestini, P. Pro, V. Neri, R. Saladino, Bioorg. Med. Chem. 13 (2005) 2569–2578. [190] B. Sedai, C. Diaz-Urrutia, R.T. Baker, R.L. Wu, L.A. Silks, S.K. Hanson, Abs. Pap. Am. Chem. Soc. 242 (2011). [191] C. Canevali, M. Orlandi, L. Pardi, B. Rindone, R. Scotti, J. Sipila, F. Morazzoni, J. Chem. Soc.-Dalton Trans. (2002) 3007–3014. [192] R.S. Drago, B.B. Corden, C.W. Barnes, J. Am. Chem. Soc. 108 (1986) 2453–2454. [193] V.O. Sippola, A.O.I. Krause, Catal. Today 100 (2005) 237–242. [194] M.J. Robinson, L.J. Wright, I.D. Suckling, J. Wood Chem. Technol. 20 (2000) 357–373. [195] R.H. Liu, X.M. Liang, C.Y. Dong, X.Q. Hu, J. Am. Chem. Soc. 126 (2004) 4112–4113. [196] K. Bergstad, H. Grennberg, J.-E. Bäckvall, Organometallics 17 (1998) 45–50. [197] M.G. Egusquiza, G.P. Romanelli, C.I. Cabello, I.L. Botto, H.J. Thomas, Catal. Commun. 9 (2008) 45–50. [198] J.E. Holladay, J.F. White, J.J. Bozell, D. Johnson, Top Value-Added Chemicals from Biomass-Volume II—Results of Screening for Potential Candidates from Biorefinery Lignin, Pacific Northwest National Laboratory (PNNL), Richland WA (US), 2007. [199] W. Zhu, W.T. Ford, J. Mol. Catal. 78 (1993) 367–378.

B.C. Saha, L.B. Iten, M.A. Cotta, Y.V. Wu, Process Biochem. 40 (2005) 3693–3700. C. Zhang, J.Y. Zhu, R. Gleisner, J. Sessions, Bioenergy Res. 5 (2012) 978–988. B.B. Hallac, Y. Pu, A.J. Ragauskas, Energy Fuels 24 (2010) 2723–2732. M.R. Sturgeon, S. Kim, K. Lawrence, R.S. Paton, S.C. Chmely, M. Nimlos, T.D. Foust, G.T. Beckham, ACS Sustain. Chem. Eng. 2 (2013) 472–485. C.A. Vasco, R. Ma, M. Quintero, M. Guo, S. Geleynse, K.K. Ramasamy, M. Wolcott, X. Zhang, Green Chemistry, (2016). H. Ohta, H. Kobayashi, K. Hara, A. Fukuoka, Chem. Commun. 47 (2011) 12209–12211. Q. Bu, H. Lei, A.H. Zacher, L. Wang, S. Ren, J. Liang, Y. Wei, Y. Liu, J. Tang, Q. Zhang, Bioresour. Technol. 124 (2012) 470–477. C. Zhao, Y. Kou, A.A. Lemonidou, X. Li, J.A. Lercher, Angew. Chem. 121 (2009) 4047–4050. D.D. Laskar, M.P. Tucker, X. Chen, G.L. Helms, B. Yang, Green Chem. 16 (2014) 897–910. H. Yang, R. Yan, H. Chen, D.H. Lee, C. Zheng, Fuel 86 (2007) 1781–1788. Q. Liu, S. Wang, Y. Zheng, Z. Luo, K. Cen, J. Anal. Appl. Pyrolysis 82 (2008) 170–177. R. Hashaikeh, Z. Fang, I. Butler, J. Hawari, J. Kozinski, Fuel 86 (2007) 1614–1622. H. Goyal, D. Seal, R. Saxena, Renew. Sustain. Energy Rev. 12 (2008) 504–517. M.E. Himmel, S.-Y. Ding, D.K. Johnson, W.S. Adney, M.R. Nimlos, J.W. Brady, T.D. Foust, Science 315 (2007) 804–807. S.J. Feng, B. Yue, Y. Wang, L. Ye, H.Y. He, Acta Phys. Chim. Sin. 27 (2011) 2881–2886. A.M. Hosseini, A. Tungler, V. Bakos, React. Kinet. Mech. Catal. 103 (2011) 251–260. S.V. Sirotin, I.F. Moskovskaya, B.V. Romanovsky, Catal. Sci. Technol. 1 (2011) 971–980. S.Q. Song, S.J. Jiang, R.C. Rao, H.X. Yang, A.M. Zhang, Appl. Catal. A: Gen. 401 (2011) 215–219. L. Wei, X.H. Mao, A. Lin, F.X. Gan, Russ. J. Electrochem. 47 (2011) 1394–1398. X. Chen, W.G. Zhao, F. Wang, J. Xu, J. Nat. Gas Chem. 21 (2012) 481–487. B.R. Hames, B. Kurek, B. Pollet, C. Lapierre, B. Monties, J. Agric. Food Chem. 46 (1998) 5362–5367. V. Meyer-Pinson, K. Ruel, F. Gaudard, G. Valtat, M. Petit-Conil, B. Kurek, C. R. Biol. 327 (2004) 917–925. A.L. Linsebigler, G. Lu, J.T. Yates Jr., Chem. Rev. 95 (1995) 735–758. S. Yurdakal, G. Palmisano, V. Loddo, V. Augugliaro, L. Palmisano, J. Am. Chem. Soc. 130 (2008) 1568–1569. Y.-S. Ma, C.-N. Chang, Y.-P. Chiang, H.-F. Sung, A.C. Chao, Chemosphere 71 (2008) 998–1004. H. Deng, L. Lin, Y. Sun, C. Pang, J. Zhuang, P. Ouyang, Z. Li, S. Liu, Catal. Lett. 126 (2008) 106–111. J. Zhang, H. Deng, L. Lin, Molecules 14 (2009) 2747–2757. F.G. Sales, L.C.A. Maranhao, N.M. Lima, C.A.M. Abreu, Ind. Eng. Chem. Res. 45 (2006) 6627–6631. M.I. Badawy, M.Y. Ghaly, M.F. Zawrah, M.E.M. Ali, F. El Gohary, Afinidad 67 (2010) 386–392. B.R. Yadav, A. Garg, Ind. Eng. Chem. Res. 51 (2012) 15778–15785. D.M. Sonnen, R.S. Reiner, R.H. Atalla, I.A. Weinstock, Ind. Eng. Chem. Res. 36 (1997) 4134–4142. A. Grigoriev Vladimir, L. Hill Craig, A. Weinstock Ira, Polyoxometalate Oxidation of Phenolic Lignin Models, Oxidative Delignification Chemistry, American Chemical Society, 2001, pp. 297–312. D.V. Evtuguin, C.P. Neto, J. Rocha, Holzforschung 54 (2000) 381–389. D.V. Evtuguin, C.P. Neto, Abs. Pap. Am. Chem. Soc 219 (2000) U284–U285. D.V. Evtuguin, C.P. Neto, H. Carapuca, J. Soares, Holzforschung 54 (2000) 511–518. X.-F. Zhou, J.-X. Qin, S.-R. Wang, Drewno. Prace Naukowe. Doniesienia. Komunikaty 54 (2011). S.K. Badamali, R. Luque, J.H. Clark, S.W. Breeden, Catal. Commun. 12 (2011) 993–995. G.M. Keserű, G.T. Balogh, S. Bokotey, G. Árvai, B. Bertók, Tetrahedron 55 (1999) 4457–4466. S. Satish, P.A. Ganeshpure, Proc. Indian Acad. Sci.-Chem. Sci. 96 (1986) 59–65. K. Kervinen, H. Korpi, J.G. Mesu, F. Soulimani, T. Repo, B. Rieger, M. Leskela, B.M. Weckhuysen, Eur. J. Inorg. Chem. (2005) 2591–2599. K. Ambrose, B.B. Hurisso, R.D. Singer, Can. J. Chem. 91 (2013) 1258–1261. S. Velusamy, M. Ahamed, T. Punniyamurthy, Org. Lett. 6 (2004) 4821–4824. D. Cedeno, J.J. Bozell, Tetrahedron Lett. 53 (2012) 2380–2383. B. Biannic, J.J. Bozell, Org. Lett. 15 (2013) 2730–2733. G.Q. Zhang, B.L. Scott, R.L. Wu, L.A. Silks, S.K. Hanson, Inorg. Chem. 51 (2012) 7354–7361. Y.F. Geng, S.H. Zhong, Chin. J. Catal. 22 (2001) 563–566. N. Jiang, A.J. Ragauskas, J. Org. Chem. 72 (2007) 7030–7033. N. Jiang, A.J. Ragauskas, Tetrahedron Lett. 48 (2007) 273–276. P.R. Schreiner, Chem. Soc. Rev. 32 (2003) 289–296. G. Pozzi, M. Cavazzini, S. Quici, M. Benaglia, G. Dell'Anna, Org. Lett. 6 (2004) 441–443. A. Rahimi, A. Ulbrich, J.J. Coon, S.S. Stahl, Nature 515 (2014) 249–252. R.S. Kumar, S.S. Kumar, M.A. Kulandainathan, Microporous Mesoporous Mater. 168 (2013) 57–64. Z.H. Li, L.P. Xue, L. Wang, S.T. Zhang, B.T. Zhao, Inorg. Chem. Commun. 27 (2013) 119–121. F. Carson, S. Agrawal, M. Gustafsson, A. Bartoszewicz, F. Moraga, X.D. Zou, B. Martin-Matute, Chem.-a Eur. J. 18 (2012) 15337–15344.

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