Catalytic oxidation of lignin to valuable biomass-based platform chemicals: A review

Fuel Processing Technology 191 (2019) 181–201

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Catalytic oxidation of lignin to valuable biomass-based platform chemicals: A review Chao Liu, Shiliang Wu, Huiyan Zhang, Rui Xiao

T

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Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, PR China

ARTICLE INFO

ABSTRACT

Keywords: Biomass Lignin Oxidation Depolymerization Catalysis Platform chemicals

Lignin is the only sustainable aromatic resource in nature, its utilization has attracted much attention worldwide. Oxidation is one of the promising strategies that can convert lignin to a range of value-added platform chemicals. Lignin oxidation is generally carried out in a liquid phase with employed catalytic systems and oxidants, and this process is largely influenced by operation conditions, catalyst types, presence of oxidant, and solvent species. The inter-unit linkages in lignin can be selectively oxidized with the assistance of designed catalytic systems, facilitating the formation of aromatics (phenolic aldehydes, ketones, and acids), benzoquinones, and aliphatic (di)carboxylic acids. This work aims to provide a comprehensive review on traditional and advanced lignin oxidation concerning various catalytic systems for different terminal products. Deficiency in current lignin oxidation is also mentioned which indicates the direction of further lignin oxidative valorization.

1. Introduction Biomass refers to any organic matters produced by living organisms and their metabolites. In the context of energy, this is often used to mean lignocellulosic materials (such as agricultural residues and forestry wastes) and energy crops (for example sugarcane, sweet sorghum, and microalgae). The annual production of biomass all over the world is about 170 billion tons, and it contributes about 10% of the global primary energy [1], which makes biomass the fourth major source of energy after oil, coal, and natural gas [2,3]. Furthermore, among various renewable resources (e.g., solar energy, wind energy, tidal energy, and so on), biomass is the only renewable organic carbon resource in nature, which endows it with unique advantages in producing valueadded products [2]. Lignocellulosic biomass consists of cellulose, lignin, hemicellulose, and other minor components (pectin, pigment, tannin, etc.). In different plant species, the average contents of the three major components vary greatly, as presented in Fig. 1(b) [4]. Cellulose is made up of pyranoglucoses linked by β-(1 → 4)-glycosidic bonds, whereas hemicellulose is an amorphous polymer with branched chains composed of pyranoses and furanoses connected by various glycosidic bonds. Their conversion to glycosyl platform compounds, such as sorbitol [5,6], furfural [7–9], levulinic acid [10–12], γ-valerolactone [13,14], and their derivatives [15–17], has been thoroughly investigated. However, lignin, which is constructed of aromatic rings, contains amounts of

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carbon-carbon and aryl ether bonds with high bond energy, which makes it difficult to be converted. Therefore, a lot of work has been carried out to effectively depolymerize lignin to achieve its valuable utilization. In the past decades, a series of thermochemical transformation strategies have been developed to convert lignin into valuable products, including combustion [18,19], gasification [20–22], liquefaction [21,23,24], pyrolysis [25–28], and carbonization [29,30], in which, liquefaction can be further divided into acidolysis [31,32], base-catalyzed depolymerization [33–35], hydrogenolysis [36–40], and oxidation [4,41–44], as summarized in Fig. 2. Heat is the main product from lignin combustion, and it can be further used for electricity generation. Gaseous products are produced from all of the five strategies, in which gasification aims at the production of syngas. With further catalytic conversion, these gaseous products can be converted to fuels and chemicals [45,46]. Whereas, gaseous products can be also used to produce electricity through further combustion. The same as gas, solid char is also generated from all of the five strategies, and it can be used for further electricity generation. Compared with combustion, gasification, and carbonization, liquefaction and pyrolysis can mainly convert lignin to bio-crude oil. Pyrolysis can rapidly convert lignin into small molecular fragments, but the coupling condensation, as well as coke and char formation are prominent. Main compounds in pyrolysis bio-oil are hundreds of types of phenolics [25–28], and they do not have the value without further separation and purification. On the other hand, the

Corresponding author. E-mail address: [email protected] (R. Xiao).

https://doi.org/10.1016/j.fuproc.2019.04.007 Received 11 January 2019; Received in revised form 1 April 2019; Accepted 3 April 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic illustration of (a) lignocellulosic biomass cell wall and (b) average distribution of the three biopolymers (lignin, cellulose, and hemicellulose) in several representative biomass feedstocks (wt%).

Fig. 2. Schematic diagram of lignin thermal conversion, potential products, and further use.

oxygen content of phenolic mixtures is high, therefore further hydrodeoxygenation need to be carried out to convert pyrolysis bio-oil to liquid fuels [47–50], mainly biomass-based gasoline and diesel fractions. Liquefaction can achieve the oriented depolymerization of lignin to a large extent. Hydrogenolysis can efficiently cleave CeO and/or CeC bonds within lignin, but the harsh reaction conditions (high temperature and pressure) will additionally saturate the aromatic rings [38,40]. Considering from the positioning of aromatic products from lignin, hydrogenolysis is not so satisfactory owing to the uncontrolled hydrogenation of aromatic rings. Selective oxidation under mild conditions can cleave inter-unit linkages, preserve aromatic rings, and transform lignin into phenolic aldehydes, ketones, and acids [4,41–44]. These are all value-added platform chemicals, and can be used as artificial flavors, pharmaceutical intermediates, organic synthetic precursors, and so on. In this work, the structural characterization approaches, isolation methods, and corresponding properties of lignin macromolecules are first introduced together with commonly used lignin model compounds.

Then, traditional lignin oxidations, mainly including pulp bleaching with oxidative deligninfication, structural identification with alkaline nitrobenzene oxidation, and (catalytic) wet air oxidation for aromatic aldehyde production, are comprehensively discussed, followed by advanced catalytic oxidation of lignin to value-added platform chemicals, including phenolic aldehydes, ketones, and acids, benzoquinones, and aliphatic (di)carboxylic acids. Finally, selective oxidation of α-OH group within lignin and lignin model compounds is presented, as well as the further transformation of oxidized lignin dimers and lignin fragments. 2. Lignin and lignin model compounds 2.1. Lignin structure, isolation, and property In plant cell wall, lignin is biosynthesized through coupling reactions between three primary precursors, i.e., p-coumaryl (4-hydroxycinnamyl), coniferyl (3-methoxy-4-hydroxycinnamyl), and sinapyl 182

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Scheme 1. Structures of three primary monolignols and the aromatic residues in lignin.

Table 1 Abundance of the primary lignin units in different types of plants [2]. Monolignol

Softwood

Hardwood

Nonwood

Sinapyl alcohol (S) Coniferyl alcohol (G) p-Coumaryl alcohol (H)

0–1% 90–95% 0.5–3.4%

50–75% 25–50% Trace

25–50% 25–50% 10–25%

as illustrated in Fig. 4(a). Additionally, in grass lignin, some structural units are linked with ester bonds, mainly ferulic acid ester and p-coumaric acid ester [56, 58]. Though we have mastered the basic polymerization units of lignin, as well as its various linkage types, the exact molecular structure of native lignin is still difficult to be characterized. Apart from the influence of different lignocellulosic biomass feedstocks, lignin structure is also determined by the plant organs, the plant ages, and even the plant origins. Isolation method is another factor that would largely alter the structure of lignin fragments. Based on the various preparation methods, isolated lignin fragments can be divided into Klason lignin [59], milled wood lignin (MWL) [25,60], enzymatic hydrolysis lignin (EHL) [61,62], enzymatic/mild acidolysis lignin (EMAL) [63–65], acid lignin [60,66], alkali lignin [26,67,68], Kraft lignin [69,70], organosolv lignin [26,71–74], lignosulfonate [75], and so on [76–82]. Table 2 lists the reported lignin fragments with various preparation conditions and the corresponding molecular weights, β-O-4 linkage contents, as well as S/G ratios. Different preparation methods employ different solvents and chemicals. These solvents and chemicals display a series of lignin reactions and dissolution mechanisms from the plant cell wall, and these reactions and mechanism are difficult to be defined at the molecular level. All of these contribute to the vastly different structure of lignin fragments we obtained. The content of the β-O-4 linkage is one of the main indicators for evaluating lignin structure. However, due to the different isolation processes, the contents of β-O-4 linkage in technical lignin vary greatly. In addition, the significance of depolymerization research on lignin macromolecule is undoubted, however many of the conclusions we draw are based on macroscopic phenomena. For example, β-O-4 aryl ether bonds are easier to be cleaved than other linkages, especially CeC bonds, because of the low bond energy [26], the G units would cause heavier condensation than S units [83], and so on.

(3,5-dimethoxy-4-hydroxycinnamyl) alcohols [51–53]. Scheme 1 presents the structures of these precursors, as well as the corresponding structural units of lignin after biosynthesis, termed as syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units, respectively [54]. These aromatic units make lignin as the only renewable aromatic resource in nature. The contents of these primary monolignols vary in different types of plants, therefore aromatic units in lignin structure vary in different biomass feedstocks, as summarized in Table 1. Softwood lignin attributes to the S lignin, hardwood lignin belongs to the SG lignin, and grass lignin is the typical GHS lignin [55]. Apart from the three core aromatic units, some lignin species also contain abundant noncanonical aromatic units. For instance, poplar lignin contains the p-hydroxybenzoate (PB) unit, and grass lignin includes ferulate (FA) and pcoumarate (pCA) units, as revealed by 2D HSQC NMR [56] (Fig. 3). Linkages within lignin macromolecules are mainly CeO bonds (β-O4, α-O-4, α-O-γ, 4-O-5) and CeC bonds (β-5, β-β, β-1, 5-5) [21]. Of these linkages, β-O-4 linkage has the highest content, accounting for approximately 43% to 65% in native lignin [2,57]. Substructures in lignin fragments are generated by parts of these linkages, and mainly include β-aryl ether linkage (linked by β-O-4), phenylcoumaran (crosslinked by α-O-4 and β-5), resinol (crosslinked by α-O-γ and β-β), dibenzodioxocin (crosslinked by α-O-4, β-O-4, and 5–5), and other substructures with trace amount, such as spirodienone (linked by β-1), 183

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Fig. 3. 2D NMR spectra revealing lignin unit compositions. Partial short-range 13Ce1H (HSQC) correlation spectra (aromatic regions only) of cell wall gels in DMSOd6/pyridine-d5 (4:1, v/v) from (a) 2-year-old greenhouse-grown poplar wood, (b) mature pine wood, (c) senesced corn stalks, and (d) senesced Arabidopsis inflorescence stems. Adapted from Mansfield and co-workers [56].

Fig. 4. (a) Schematic representation of a softwood lignin structure and (b) various representative lignin model compounds.

184

185

Organosolv lignin Organosolv lignin Organosolv lignin

Deep eutectic solvent (DES) lignin Deep eutectic solvent (DES) lignin Ionic liquid lignin

12 13 14

15

f

e

d

c

b

a

Red oak Lauan

Eucommia Switchgrass

A. donax

Eucalyptus camaldulensis

Willow

Maple Broussonetia papyrifera Cotton stalk

Birch Wheat straw Bamboo Poplar

Eucalyptus urophylla

Eucalyptus grandis × Eucalyptus urophylla

Poplar

Palm kernel shell Wheat straw Corn stalk Poplar

Biomass feedstock

160 °C; 100 °C

110 °C

120 °C

180 °C 160 °C 170 °C

107–110 °C 130 °C 50 °C 160 °C

40 °C; 87 °C

40 °C; 87 °C

50 °C

20 °C Room temperature Reflux extraction NGe

Temperature

1.5 h; 1h

6h

12 h

2h 10 min 1h

3h 20 min 3h 50 min

48 h; 2h

48 h; 2h

48 h

2.5 h 24 h 6h 72 h

Time

Operated on a fluidized bed pyrolyzer at 500 °C with a biomass feed rate of 6 kg/h Operated on a fluidized bed pyrolyzer at 500–550 °C with a biomass feed rate of 5 kg/h

Steam explosion treatment at 2 MPa for 10 min Water and ammonia explosion treatment (loading of 0.8 and 5.0) at 90 °C for 30 min

[C4mim]Cl and solid acid Amberlyst 35DRY

Choline chloride/lactic acid (molar ratio 1:10)

Cellic CTec 2 cellulase and HTec 2 hemicellulase (1:1, mass ratio) in acetate buffer solution Carboxyl methyl cellulase in acetate buffer solution; dioxane/acidified deionized water solutions (85:15, v/v, 0.01 mol/L HCl) Carboxyl methyl cellulase in acetate buffer solution; dioxane/acidified deionized water solutions (85:15, v/v, 0.01 mol/L HCl) Formic acid/acetic acid/water (30:50:20, v/v/v) Formic acid (62 wt%) Sodium hydroxide solution 1 M (solid to liquid ratio, 1:20) Sodium sulfide solution (4:1 liquor/wood ratio, 16% active alkali, and 20% sulfidity) Ethanol/water (1:1, v/v) Ethanol/water solution (1:1, v/v) and 0.9% sulfuric acid (wt%) Gammavalactone/H2O (80:20, v/v) with a trace amount of H2SO4 (10 mM, ~0.1%) Choline chloride/lactic acid (molar ratio 1:10)

Sulfuric acid/water solution (72 wt% of sulfuric acid) Dioxane/water (96:4, v/v) Dioxane/acidified water (9:1, v/v, c(HCl) = 0.01 mol/L) Cellulast and Novozyme 188 in acetate buffer solution

Regents

Reaction conditions

Results expressed per 100 Ar based on quantitative 2D HSQC NMR spectra. Weight-average molecular weight (Mw) tested by GPC. S/G ratio obtained by the: S/G ratio = 0.5I (S2,6) / I (G2). Klason lignin was pretreated by acetylation before NMR analysis. NG: Not given. Tr: Trace.

20 21

18 19

17

Steam explosion Lignin Ammonia steam explosion Lignin Pyrolytic lignin Pyrolytic lignin

Acid lignin Acid lignin Alkali lignin Kraft lignin

8 9 10 11

16

Enzymatic/mild acidolysis lignin (EMAL)

Klason lignin Milled wood lignin (MWL) Milled wood lignin (MWL) Enzymatic hydrolysis lignin (EHL) Enzymatic hydrolysis lignin (EHL) Enzymatic/mild acidolysis lignin (EMAL)

d

Lignin fragments

7

6

5

1 2 3 4

Entry

Table 2 Summaries of preparation conditions, molecular weights, β-O-4 linkage contents, and S/G ratios of various lignin fragments.

0 0

10.88 NG

50.2

831 833

2070 5112

6020

1790

1261

Trf 11.84

NG 9229 870

NG 2666 2840 5730

1980

6600

11,850

4205 2464 13,050 10,602

Molecular weight/ Dab

23 51.1 58.2

36 32.4 46.2 3.4

60.1

70.3

61.1

6.3 39.8 52.22 69.4

β-O-4 linkage content/100 Ara

3.45 0.89

3.78 0.85

1.04

1.28

NG

2.70 0.8 0.44

6.14 0.71 2.06 3.9

2.85

1.34

1.6

0.83 0.44 0.96 1.39

S/G ratioc

[78] [79]

[81] [80]

[82]

[77]

[76]

[66] [73] [74]

[66] [60] [67] [70]

[63]

[64]

[62]

[59] [60] [25] [61]

Ref.

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2.2. Lignin model compounds

phenolic residual lignin is much slower, and the subsequent degradation is also realized by a series of free radicals. Reactions between ClO2 and conjugated side chains are electrophilic addition, which destroys chromogenic structures and realizes pulp bleaching. In addition, during the process of ClO2 bleaching, HClO3, ClO3−, HClO2, ClO2−, HClO, Cl2, and Cl− are also generated in the bleaching solution, so there do exist other bleaching reactions. TCF bleaching uses greener H2O2, O2, and O3 as bleaching reagents. H2O2 bleaching is usually carried out in alkaline conditions. That is because active hydroperoxide anions (HOO−) would be produced from the reaction between H2O2 and OH−. HOO− is a strong nucleophile reagent, and its nucleophilic reactions with residual lignin are the main reactions during H2O2 bleaching. Furthermore, with the catalysis of transition metal ions, H2O2 can decompose into hydroxyl radical (HO%), hydroperoxide radical (HOO%), and super-oxide anion radical (O2−%). Among these radicals, the super-oxide anion radicals (O2−%) have been proved to play a very important role in pulp bleaching. The purpose of H2O2 bleaching mainly aims at the destruction of chromogenic groups (quinone structure and conjugated side chains), the specific lignin oxidation mechanisms with H2O2 can be found elsewhere [112]. O2 bleaching is also performed under alkaline conditions with MgCO3 or MgSO4 as the protective agent, so this process is generally called oxygen-caustic bleaching. During oxygen-caustic bleaching, intermediate H2O2 is produced from the reduction of molecular oxygen through single-electron or double-electron transfer. Therefore, many reactions of residual lignin during oxygen-caustic bleaching process are the same as those in H2O2 bleaching. In addition, molecular oxygen can also generate anions (HO−, HOO−, O2−%) and radicals (HO% and HOO %), increasing the complexity of oxygen-caustic bleaching reactions. Ma et al. [44] summarized the main reaction mechanisms of residual lignin during oxygen-caustic bleaching. In addition, during bleaching processes using hydrogen peroxide and molecular oxygen, the oxidant can also attack and break the unconjugated side chains, and further dissolve out the lignin fragments. By the way, this is the main reaction in the production of value-added platform compounds from lignin oxidation with hydrogen peroxide, which will be covered in the following relevant sections. Different from H2O2 and oxygen-caustic bleaching, O3 is easy to decompose under the action of OH−. Therefore, the pH of ozone bleaching solution needs to be adjusted to 1.5–2 to decrease the concentration of OH− and inhibit the decomposition of O3. The high activity of O3 enables it to react with both phenolic and non-phenolic lignin units. Through the five-membered oxonium ring pathways, the benzene rings rupture into carboxylic acids with or without the o-quinone intermediates. O3 can also indiscriminately oxidize double bonds in side chains, alcohol hydroxyl groups, ether bonds, aldehyde groups and other groups into carboxyl groups. However, owing to the high cost, high consumption, and the decline of pulp strength after bleaching, ozone bleaching is not widely used in industrial pulp bleaching. In addition, peroxy acids, such as peracetic acid, are also investigated as reagents for pulp bleaching. The oxidative degradation of lignin during peroxy acid bleaching mainly starts from the hydroxylation of aromatic rings with hydroxonium ions (HO+), which are strong electrophilic reagents generated from the heterolytic cleavage of peroxide bonds. As described by Gierer, six possible types of reactions exist in peroxic acid bleaching, including: (1) ring hydroxylation, (2) oxidative aromatic ring cleavage, (3) side-chain substitution, (4) demethylation on aromatic ring, (5) cleavage of ether bond, and (6) epoxidation of olefin in ring-conjugated structures [113]. Except for the lab-scale research, peracetic acid bleaching was implemented in one of Södra Cell's pulp mills in Sweden.

To better understand the reaction chemistry during lignin conversion, simplified lignin model compounds are widely used to investigate the mechanisms. The simplest lignin model compounds are lignin monomers. Generally, these monomers are employed to explore the influence of functional groups on lignin conversion [84–87]. Lignin dimeric model compounds include β-O-4 dimers [88–95], α-O-4 dimers [96–99], 4-O-5 dimers [98,100,101], β-1 dimers [102–104], β-5 dimers [104,105], and so on. Among these lignin dimers, β-O-4 dimers are the most commonly used because of the most abundant β-O-4 linkages in lignin fragments. As reflected in these abbreviations of lignin dimers, they consist of two aromatic units and are linked with specific linkages, i.e., β-O-4, α-O-4, 4-O-5, β-1, or other bonds. Though, research on lignin dimers can reveal linkage cleavage and subsequent reactions at the molecular level, the simplified linkages have their shortcomings, i.e., they cannot well represent the real internal bonds within lignin fragments. That is to say the conversion rules obtained from lignin dimers cannot be totally applied for lignin macromolecules. Therefore, some researchers synthesized lignin trimers [106,107], linear β-O-4 oligomers [108] and polymers [72,109–111] to better simulate lignin macromolecules to investigate the depolymerization properties. Overall, all these works on lignin model compounds contribute to the deeper understanding of lignin chemistry. 3. Traditional lignin oxidation 3.1. Oxidative delignification for pulp bleaching Pulp bleaching is one form of lignin oxidation. It mainly includes two oxidation pathways: (1) oxidative fragmentation and dissolution of residual lignin in pulp; (2) oxidation of the chromogenic groups in residual lignin, including quinone structures and conjugated side chains. Chlorine (Cl2) is a highly efficient lignin oxidant for pulp bleaching. During lignin chlorination, active chloronium ions (Cl+) are produced from the heterolysis of Cl2, then electrophilic substitution reactions occur on the partial electron-rich negative sites, i.e., C1, C3, and C5 positions in benzene rings, with the attack of Cl+. When the Cl+ attacks the C1 position, it can promote the linkage cleavage of aliphatic side chains and makes the residual lignin fragmented and dissolved. Whereas, when the active Cl+ attacks the C3 or C5 position, the methoxy group is first released, then dechlorination reaction occurs transferring the benzenes into o-quinones, which would be further ringopening oxidized and eventually converted to substituented muconic acid, as the mechanisms illustrated by Ma et al. [44]. The effect of chlorine bleaching is indisputable. However, during the process of chlorine bleaching, large amounts of organohalogens (AOX) are generated. They dissolve in the bleaching effluent with low concentrations and are difficult to be dealt with, placing an enormous burden on the environment. Therefore, elemental chlorine free (ECF) and total chlorine free (TCF) bleaching have been developed to replace chlorine bleaching. As the name implies, ECF bleaching does not use Cl2 but uses hypochlorite (ClO−) or chloride dioxide (ClO2) as the bleaching reagent. Commonly used hypochlorites are sodium hypochlorite (NaClO) and calcium hypochloride (Ca(ClO)2). Hypochlorite is a nucleophilic reagent, it mainly oxidizes the chromogenic groups of residual lignin, i.e. quinone structure and side-chain conjugated structure, and eventually makes it degraded into dicarboxylic acids to achieve the purpose of pulp whiteness improvement [112]. As an oxidant, ClO2 is 2.63 times as reactive as an equivalent mass of Cl2, theoretically [44]. It can easily attack the free phenolic hydroxyl groups in phenolic residual lignin, producing phenoxy and phenyl radicals. These radicals will be further oxidized by ClO2, leading to the ring-opening of aromatic units and further formation of dicarboxylic acids and their derivatives. Compared with phenolic structures, the reaction rate between ClO2 and non-

3.2. Alkaline nitrobenzene oxidation for lignin structural identification Alkaline nitrobenzene oxidation (NBO) is usually carried out under alkaline condition (NaOH solution) using nitrobenzene as the oxidant to 186

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break down the non-condensed side-chain linkages within lignin fragments, transferring the S, G, and H units into syringaldehyde (S), syringic acid (SA), vanillin (V), vanillic acid (VA), 4-hydroxybenzaldehyde (H), and 4-hydroxybenzoic acid (HA) [114]. Using gas chromatography (GC) or liquid chromatography (LC), the yields of oxidation products can be quantitatively determined. To a certain extent, the obtained S:V:H ratio can represent the value of S:G:H ratio in lignin feedstock to reflect the composition of structural units. Prozil et al. compared the S:G:H ratios of native lignin in grape stalk material, dioxane lignins free of extractives and proteins, and free of extractives, proteins, and tannins with the NBO method, and proposed that during alkali pre-extraction of grape stalks, before the isolation of dioxane lignin, a small fraction of lignin rich in H and G structural units was removed from the grape stalk material [115]. Dou et al. also employed the NBO method to identify the contents of three structural units in the bark, inner bark, and wood fractions of the four-year-old tree of willow hybrid ‘Karin’. The predominant unit in bark lignin was G over S and H. On the contrary, S was significantly more abundant in wood lignin. The S:G:H ratio increased progressively from bark to inner bark to wood, and explained why the bark lignin was more condensed than the lignin in other tissues of willow [116]. Species and yields of NBO products largely depend on lignin structural properties. When using softwood lignin as the feedstock, the NBO products are mainly vanillin (V) and vanillic acid (VA); whereas when using hardwood lignin as the feedstock, the NBO products consist of syringaldehyde (S), syringic acid (SA), vanillin (V), and vanillic acid (VA). The content of β-O-4 linkages is another factor that extremely impacts the yields of NBO products. To illustrate the relationship between the contents of β-O-4 linkages and yields of NBO products, Wang et al. prepared five lignin samples from Eucalyptus, did NBO depolymerization, and presented that more β-O-4 linkages led to more aromatic aldehyde yields [117]. In addition, reaction conditions, especially oxidation time, can favor the production of aromatic acids because of the excessive oxidation of products. Therefore, the production of aromatic acids can be inhibited by reaction time controlling, making aromatic aldehydes as the dominant NBO products. Though, NBO is a highly effective way to oxidize lignin into aromatic aldehydes and acids, its fatal weakness of toxicity limits its application. Based on this, in the area of lignin structural identification, more environmentally friendly reductive depolymerization method is developed. In reduction conditions, not only the ether linkages within lignin can be cleaved completely, but also the alkyl side-chains can be well retained, which can present more structural information than oxidative depolymerization does. With the development of advanced techniques, rapid and repeatable analytical fast pyrolysis (Py-GC/MS) [118], nuclear magnetic resonance spectroscopy (NMR) [61,119,120], and their combination [25,121–124] are developed and widely employed for lignin structural characterization.

production from ligninsulfonate is conducted under alkaline condition at 160–170 °C with 1.0–1.2 MPa of air/O2, mainly including the hydrolysis of ligninsulfonate, oxidative cleavage of lignin linkages, and further isolation and purification of produced vanillin [127]. In a long period of time, the WAO method only shows its good effect on lignosulfonate degradation for the production of vanillin. However, a large proportion of pulp mills all over the word use alkaline pulping process, producing large amounts of black liquor which contains alkali lignin as the main organic component. In order to achieve the utilization of alkali lignin by WAO, some researchers treated black liquor with sodium hydrogen sulfite firstly and converted alkali lignin into ligninsulfonate, then used the abovementioned WAO conditions for vanillin production [128,129]. Converting alkali lignin into ligninsulfonate is an option, but the extra sulfonation is not economical enough. Later, the catalytic wet air oxidation (CWAO) is developed using homogeneous metal ions or heterogeneous metal oxides or supported precious metals as the catalysts [130]. Compared to conventional WAO, CWAO has lower energy requirements, better product yields and selectivity, shorter reaction times, and other advantages. Most importantly, this improvement breaks through the limitation that only ligninsulfonate can be used as the raw material for vanillin production, and makes it possible to prepare vanillin by direct catalytic oxidative depolymerization of alkali lignin and other kinds of lignin fragments. Up to now, the accepted mechanism of vanillin formation from the wet air oxidation of lignin is shown in Scheme 2. Vanillin is finally formed through a retro-aldol condensation and the process involves several unsaturated intermediates. It can be generally concluded that the vanillin yield crucially depends on pH as well as oxygen concentration [131]. Copper-based catalysts, including homogeneous copper ion (Cu2+, generally using copper sulfate) [132–134] and heterogeneous copper oxide [134], have good catalytic performance for the production of aromatic aldehydes from lignin in CWAO process. Xiang and Lee did the CWAO conversion of dilute acid hydrolyzed lignin from Liriodendron tulipifera using Cu2+ and Fe3+ as the co-catalyst, and obtained a ~15% yield of vanillin and syringaldehyde [132]. Bhargava et al. further confirmed the best catalytic activity of copper by the comparison of nine homogeneous catalysts using ferulic acid as the lignin model compound, and the order of homogeneous catalytic activity observed was Cu2+ > Fe2+ > Mn2+ > Ce2+ > Bi2+ > Co2+ > Zn2+ > Mg2+ > Ni2+ [135]. Heterogeneous perovskite-type oxide catalysts can also catalyze the wet air oxidation of lignin to prepare aromatic aldehydes. Reported perovskite-type oxide catalysts are basically synthesized using the sol-gel method or the solution combustion synthesis technique, and mainly include LaCoO3 [136], LaMnO3 [137], LaFeO3 [138], CeFeO3 [138], LaCo1−xCuxO3 (x = 0, 0.1, 0.2) [139], LaFe1−xCuxO3 (x = 0, 0.1, 0.2) [140], LaFe1−xMnxO3 (x = 0, 0.25, 0.5, 0.75, 1) [141], and La0.9Sr0.1MnO3 [141].

3.3. (Catalytic) wet air oxidation for aromatic aldehyde production

4. Advanced lignin oxidation for valuable platform chemicals

Wet air oxidation (WAO) refers to the oxidative degradation of organic or inorganic substances by oxygen or air in aqueous solution or suspension at elevated temperatures and pressures. Typical operation conditions of temperatures, pressures, and residence times range from 180 °C to 315 °C, from 2 MPa to 15 MPa, and from 15 min to 120 min, respectively [125]. WAO is commonly studied for wastewater treatment, and it works well in treating spent pulp mill liquor. Through WAO, lignin can be depolymerized to aliphatic carboxylic acids and aromatic aldehydes, e.g., vanillin, syringaldehyde, and p-hydroxybenzaldehyde. These aromatic aldehydes are high value-added, especial vanillin. It is the most widely used spice in food and cosmetic industries, as well as the important pharmaceutical intermediate [126]. Among the three aromatic aldehydes, only vanillin is industrially manufactured by WAO. Borregaard (Norway) is the only manufacturer of vanillin from lignin in the world. Typical procedure for vanillin

Lignin catalytic oxidative depolymerization means, as the name suggests, converting lignin macromolecules to value-added platform chemicals catalyzed by selected catalysts under oxidative conditions. With the effort of different catalysts or catalytic systems, functional groups and chemical bonds would be selectively oxidized, respectively, such as side-chain linkages, phenolic hydroxyl groups, aromatic rings, and so on, producing various platform chemicals with special functionalities. When the selective oxidative cleavage occurs in the sidechain linkages, the aromatics units would be retained, and lignin macromolecules can be transferred to phenolic platform chemicals. Owing to a series of oxidative mechanisms caused by different catalysts and functional groups on side-chains, lignin fragments can be oxidized to phenolic aldehydes, ketones, and acids, respectively. Whereas, when the phenolic hydroxyl groups or the C1-Cα bonds within lignin fragments are oxidatively cleaved, the lignin macromolecules would be 187

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Scheme 2. Proposed reaction mechanism for vanillin formation during alkaline oxidation of lignin. Adapted from Sun and co-workers [131].

Fig. 5. Schematic diagram of lignin oxidative depolymerization to value-added platform chemicals (benzoquinones, aromatic aldehydes, ketones, and acids, and aliphatic (di)carboxylic acids) and their further use.

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converted to benzoquinone-related platform compounds, mainly including o-benzoquinone, p-benzoquinone, 2-methoxy-1,4-benzoquionone, and 2,6-dimethoxy-1,4-benzoquinone. If the aromatic rings are excessively oxidized to ring opening, aliphatic (di)carboxylic acids would be produced as the final platform chemicals. Fig. 5 presents the platform chemicals (benzoquinones, aromatics, and organic acids) from lignin oxidative depolymerization and their further applications.

the hydrogen atom accompanied by the formation of phenoxy radical. Subsequently, under the action of dioxygen, the new formed vanadiumoxo complex will go on attacking the C1 position in the model compound and induce the break of C1-Cα bond, producing substituted pbenzoquinone products (pro1 in Scheme 4). Whereas, when using oxovanadium complex V2 as the catalyst, the active vanadium site first connects to the oxygen atom on Cα position through ligand exchange. Then the α-C=O bond is generated through hydrogen shift followed by a MECP, which induces the cleavage of β-ether bonds. Finally, the vanadium catalyst removes with final product syringyl acrylketone produced (pro4 in Scheme 4). Therefore, different ligands of oxovanadium complexes have great influence on the selectivity of products, and oriented transformation of lignin can be realized by proper ligand selection. Thereafter, Hanson et al. [94,145,146] and other researchers [88,92] employed various oxovanadium complexes to catalyze the oxidative conversion of a series of lignin model compounds with pinacol structures. The corresponding catalytic oxidation processes are achieved by ligand exchange. As Ma et al. pointed out, these reactions are actually redox-natural, not requiring additional oxygen to initiate the reaction, but the presence of oxygen can accelerate the reaction rate [42]. In order to further confirm the catalytic effects of oxovanadium complexes, Sedai, Diaz-Urrutia, and co-workers respectively compared the activity of (HQ)2VV(O)(OiPr) (HQ = 8-oxyquinolinate) and CuCl/ TEMPO/2,6-lutidine on lignin model compound 2-phenoxyethanol, the activity of (dipic)VV(O)(OiPr) (dipic = dipicolinate) and CuCl/TEMPO on β-O-4 lignin dimers, and the activity of (HQ)2VV(O)(OiPr) and CuOTf/TEMPO/2,6-lutidine (OTf = trifluoromethanesulfonate) on β-1 lignin dimers, and demonstrated the different product selectivity [102,147,148]. After scanning metal-doped hydrotalcites (HTc-Cu and HTc-V), Mottweiler et al. prepared bimetallic HTc-Cu-V catalyst and achieved its good catalytic oxidation effect. At the same time, they also investigated the catalytic activity of homogeneous VO(acac)2/Cu(NO3)2 catalytic system [90]. Later, Rinesch et al. employed both the homogeneous and heterogeneous vanadium-copper catalytic systems to explore the selectivity of produced vanillin and vanillic acid, as well as their mutual transformation during the evolution process [143]. As mentioned before, Cu-based catalysts have the good catalytic activity for lignin oxidation. In CWAO, copper ion and copper oxide can catalyze the depolymerization of lignin to prepare aromatic aldehydes, which has been discussed in Section 3.3 [132–134]. Copper complexes and copper-loaded catalysts used for lignin oxidation are widely reported. Halma et al. employed Cu/1,10-phenanthroline as the catalyst and hydrogen peroxide as the oxidant to convert β-O-4 lignin dimers. They scanned four different lignin model compounds and found that the

4.1. Aromatic aldehydes, ketones, and acids During catalytic strategies of lignin oxidation, employed substrates, catalysts, solvents, and oxidants (O2, air, H2O2, and so on) vary from system to system, resulting in their different depolymerization mechanisms and aromatic products. For example, the catalytic oxidative depolymerization occurs on primary hydroxy (γ-OH) or secondary hydroxy (α-OH) in the side chains of lignin macromolecules would lead to different subsequent transformation mechanisms and quite different species of final aromatic monomeric products. As Rahimi et al. revealed [142], when the γ-OH group was firstly selectively oxidized, the formed γ-CHO end group would induce the cleavage of Cα-Cβ bond via retroaldol reaction, producing aromatic aldehyde and guaiacol. Whereas, if the initial oxidation took place in the α-OH group, the oxidized Cα = O group would stimulate the decomposition of Cα-Cβ bond, producing aromatic acid and guaiacol as the final products, as illustrated in Scheme 3. Besides, the generated aromatic aldehydes would be further oxidized to aromatic acids in some specific systems [143]. Therefore, in this section, we review the catalytic oxidative depolymerization of lignin and lignin models for the production of aromatic aldehydes, ketones, and acids based on different catalytic depolymerization mechanisms). Currently, vanadium-based catalysts, mainly the oxovanadium complexes, are extensively investigated for lignin oxidation. The good catalytic ability of oxovanadium complexes was first found by Son and Toste in 2010 [93]. However, they employed these organovanadium catalysts for the non-oxidative CeO bond cleavage using lignin dimeric model compounds as the reactant. Thereafter, Hanson et al. applied a series of oxovanadium complexes for the catalytic oxidative conversion of lignin model compound syringyl glycerol-β-guaiacyl ether [91]. They investigated the influence of oxovanadium complexes with different ligands on product selectivity, and then employed the 13C-labled substrate and [D5]-pyridine solvent to explore the catalytic mechanisms of oxovanadium complexes with different ligands. Jiang et al. verified corresponding mechanisms with the quantum computation method (DFT) [144]. As shown in Scheme 4, when oxovanadium complex V1 is employed, it will first attack the phenolic hydroxyl group, taking away

Scheme 3. Chemoselective alcohol oxidation strategies for lignin and lignin model compounds. Adapted from Rahimi and co-workers [142]. 189

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Scheme 4. Products (a) and proposed mechanisms (b) of oxovanadium complexes-catalyzed oxidation of lignin model compound syringyl glycerol-β-guaiacyl ether with different ligands. Adapted and reproduced from Jiang and co-workers [144].

Scheme 5. Proposed mechanism of Cu/1,10-phenanthroline (a) and Cu(OAc)2/BF3·OEt2 (b) catalyzed CeC bond oxidative cleavage in lignin model compound. Adapted from Wang and co-workers [89,151].

first step was the oxidation of α-OH group to a α-ketone group catalyzed by Cu/1,10-phenanthroline, and then the cleavage of β-O-4 linkages was realized with the attack of nucleophilic peroxide anion (HOO−), producing aromatic aldehydes [149]. Wang et al. also used Cu/1,10-phenanthroline as the catalyst to oxidize lignin model compounds in methanol under dioxygen [89]. As presented in Scheme 5(a), a copper-oxo-bridged dimer promoted the oxidative cleavage of Cα-Cβ bond, producing aromatic acids as the final products, which was quite different from those in Halma's system. CuCl2/polybenzoxazine is another copper complex that can oxidize a series of lignin model compounds, including β-O-4, α-O-4, and 4-O-5 lignin dimers, with the assistance of hydrogen peroxide [150]. Wang et al. reported the good catalytic activity of Cu(OAc)2/BF3·OEt2 on the oxidative cleavage of CαCβ bond within lignin model compounds [151]. As illustrated in

Scheme 5(b), in the presence of Cu(OAc)2/BF3·OEt2, dioxygen is grafted to the Cβ position, then produced oxygen radicals attack the Cα, leading to the cleavage of Cα-Cβ bond. Compared with other oxidative catalytic systems, Cu(OAc)2/BF3·OEt2/O2 can unselectively break the Cα-Cβ bond in both β-O-4 and β-1 lignin dimers. In addition, Cu-N-heterocyclic carbene (Cu-NHC) is also developed for the aerobic cleavage of β-1 lignin dimers with > 99% conversions and satisfactory yields of the corresponding aromatic aldehydes [103]. In addition, metalloporphyrins are another catalysts that can be used for lignin oxidation conversion. They are biomimetic systems representing for lignin peroxidase (LiP) and manganese dependent peroxidase enzymes (MnP) [2,41,152] which can oxidize lignin under mild conditions. Structures and abbreviated names of representative synthetic metalloporphyrins are illustrated in Scheme 6(a), and the 190

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Scheme 6. (a) Structures and abbreviated names of representative synthetic metalloporphyrins; (b) mechanism of metallo-oxo complex formation; (c) mechanisms of metalloporphyrin-catalyzed oxidation of lignin units through side chain oxidation and aromatic nuclei oxidation.

commonly employed metalloporphyrins are Fe, Co, Mn, and Ru complexes [99]. Metalloporphyrins can form highly oxidized metallo-oxo complexes under the trigger of exogenous oxidant species, such as H2O2, tBuOOH, KHSO5, and magnesium monoperoxyphthalate (MMPP) [41,99,153], and the formation of metallo-oxo complex formation is presented in Scheme 6(b). The produced metallo-oxo complexes would oxidize the substrates through an electron-transfer mechanism. The activated metallo-oxo complexes can abstract an electron from either the aryl position or the side-chain position, leading to the formation of final products, such as phenolic aldehydes, ketones, quinones, and muconic acids [154], as shown in Scheme 6(c). The activity, selectivity, and other properties of metalloporphyrin complexes can be modified by changing the R1 and R2 substituents [44,154]. This indicates the substituent selection of metalloporphyrins is important for the selective preparation of aromatic aldehydes and ketones, combined with other factors. For example, Zhu and Ford achieved the oxidation of veratryl alcohol to veratryl aldehyde by using cobalt phthalocyaninetetra

(sodium sulfonate) (CoPcTS) and FePcTs [155]. They found that CoPcTs worked well on veratryl alcohol oxidation under alkali condition with dioxygen as the oxidant, whereas FePcTs exhibited its catalytic effect on veratryl alcohol oxidation in acidic solution with the help of hydrogen peroxide. In addition, Li et al. realized the production of aromatic aldehydes from the heavy fraction of bio-oil with the help of Co(TPPS4) and hydrogen peroxide [156]. The total yields (4.57 wt% vanillin and 1.58 wt% syringaldehyde) were low, but it was acceptable considering the feedstock. Furthermore, in order to overcome the problem of easy loss and difficult recovery of homogeneous catalysts, porphyrin-based catalyst systems are immobilized on solid supports, such as silica gels and clays, which all showed promising results [157–159]. Except for aforementioned oxovanadium complexes, copper-based catalysts, and biomimetic metalloporphyrins, some other catalysts also display their high catalytic activities on the oxidative depolymerization of lignin to produce aromatic aldehydes, ketones, ans acids. Pan et al. carried out a systematic work on the microwave-assisted oxidative 191

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Scheme 7. (a) Representative structures of several Co(salen) catalysts; (b) formation of monomeric Co(salen)-superoxo complexes and dimeric-peroxo-bridged Co (salen)-peroxo complexes; (c) proposed oxidation mechanisms of guaiacol alcohol and veratryl alcohol catalyzed by Co(salen) and dioxygen; (d) proposed oxidation mechanisms of β-O-4 lignin model compound syringyl glycerin-β-guaiacyl ether catalyzed by Co(salen) and dioxygen.

degradation of lignin model compounds by evaluating the catalytic activity of 14 types of metal salts. They found the acidity of metal salt is in favor of the catalytic activity for the degradation, and microwave irradiation is able to accelerate the degradation rate in a large scale, in which CrCl3 and MnCl2 were the most effective [160]. Furthermore, the catalytic activity of FeCl3-derived iron catalysts for oxidative depolymerization of both lignin and lignin model compounds in DMSO was confirmed, and the key reactive species facilitating this cleavage are methyl radicals generated from H2O2 and DMSO [95]. Metal/bromide catalysts, mainly Co/Mn/Br and Co/Mn/Zr/Br, in acetic acid/water mixtures can also convert lignin and lignin model compounds to aromatic aldehydes and acids [161]. Heterogeneous cerium oxide-supported palladium nanoparticles (Pd/CeO2) can efficiently catalyze the one-pot oxidative conversion of lignin model compound 2-phenoxy-1phenylethanol in methanol in the presence of O2, producing phenol, acetophenone and methyl benzoate as the major products [162].

complexes (Scheme 7b). Low temperature, high oxygen partial pressure, and high-polarity solvents can favor the formation of mononuclear superoxo-complexes, whereas high temperature, low oxygen partial pressure, and nonpolar solvents facilitate the formation of binuclear peroxo-bridged species [44,166]. After summarizing a large number of literatures on the Co(salen)catalyzed aerobic oxidation of lignin model compounds, a general rule is found. Phenolic lignin model compounds are converted into p-quinones, whereas non-phenolic lignin model compounds showed low reactivity and generated polymerization products [86,163,164,166,167]. Bozell and Hames investigated Co(salen)-catalyzed oxidation of parasubstituted phenolics and converted all of them to p-quinones [163]. As Scheme 7(c) shown, the generated superoxo complex first abstracts the hydrogen atom from the phenolic hydroxyl group, changing the substrate to phenoxy radical. Then, the superoxo complex or the dioxygen attacks the C1 position, inducing the removal of substituent groups and producing p-quinones [163]. As a comparison, Kervinen et al. converts veratryl alcohol to veratryl aldehyde with Co(salen) and dioxygen in aqueous media [166]. That is because the phenoxy radical cannot be formed owing to the absence of phenolic hydroxyl group. Hence, the benzyl alcohol combines with the Co(salen), forming an alkoxo intermediate and water. Then, the benzyl hydrogen atom is abstracted and veratryl aldehyde is formed, as shown in Scheme 7(c). In addition, Biannic and Bozell successfully reported the catalytic oxidation system that can concert both S and G lignin model phenols to benzoquinones in high yield for the first time with dioxygen using Co (salen) catalysts bearing a bulky heterocyclic nitrogen base as a substituent [164]. The corresponding oxidation mechanism of syringyl

4.2. Benzoquinones Cobalt-salen complexes are a series of Co-Schiff base catalysts that can convert lignin and lignin model compounds to benzoquinones in the presence of hydrogen peroxide or dioxygen. Representative structures of Co(salen) catalysts are presented in Scheme 7(a) [86,163–165], which can catalyze the aerobic lignin oxidation at room temperature. The catalytic ability of Co(salen) comes from the generated active oxocomplexes, which has been demonstrated by Kervinen in 1967 [165]. Generally, Co(salen) catalysts can form both monomeric Co(salen)-superoxo complexes or dimeric-peroxo-bridged Co(salen)-peroxo 192

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glycerin-β-guaiacyl ether is illustrated in Scheme 7(d). First, Co(salen) is converted to superoxo complex under the action of oxygen. The generated superoxo complex attacks the phenolic hydroxyl group in syringyl glycerin-β-guaiacyl ether transferring it to phenoxy radical, and then adds to the C1 position forming the reaction intermediate. 2,6Dimethoxyquinone and enol are formed with the further cleavage of αaryl and β-ether bonds. Associated guaiacoxy radical can dimerize through 5–5 or 1–1 coupling. Meanwhile, guaiacoxy radical can also be trapped with the superoxo complex through radical coupling in the para position, followed by elimination of water and regeneration of the catalyst to give 2-methoxyquinone [164]. The catalytic system was further used for the direct oxidation of organosolv lignin and yielded aromatic aldehydes and p-quinones in roughly equal proportions, but the absolute monomer yield was low (3.5 wt%) [164].

malonic acid, succinic acid, and malic acid with a total yield of 14% and 11%, respectively [168]. Meanwhile, they investigated the formation mechanisms of these aliphatic dicarboxylic acids by using guaiacol, catechol, and vanillin as the substrates. As Scheme 8 displayed, lignin is first depolymerized to substituted quinones in CuFeS2/H2O2 system in HAc/NaAc buffer solution. Then, the produced p-quinones and o-quinones are further ring-opening oxidized, forming fumaric acid, maleic acid, and some small fragments. In the presence of HAc, fumaric acid is finally converted to succinic acid. The production of malonic acid undergoes the intermediate malic acid followed by decarboxylation. Furthermore, if the oxidation conditions are harsh, the produced dicarboxylic acids would be further fragmented to simple carboxylic acids, mainly formic acid and acetic acid. For example, using the NaVO3/H2SO4 as the catalytic system and dioxygen as the oxidant, Niu et al. obtained 11.7% yield of formic acid and 4.39% yield of acetic acid from organosolv lignin under 160 °C with 60 h [169]. The technical route of preparing aliphatic (di)carboxylic acids from lignin is feasible. However, considering from the property of lignin as the only renewable aromatic resource, oxidatively converting of lignin to aliphatic (di)carboxylic acid is not economical, which can be easily prepared from carbohydrates, e.g., cellulose and hemicellulose. On the other hand, oligomers produced from lignin depolymerization, for

4.3. Aliphatic (di)carboxylic acids Aliphatic (di)carboxylic acids are the end products of lignin oxidative degradation. They come from the ring-opening oxidation of quinones (both p-quinone and o-quinone) and the further conversion. Ma et al. employed chalcopyrite as the catalyst to oxidatively convert dilute-acid corn stover lignin and steam-exploded spruce lignin to

Scheme 8. Proposed mechanisms for (a) lignin depolymerization and aromatic nuclei oxidation, (b) aromatic ring cleavage, and (c) formation of final products. Adopted from Ma and co-workers [168]. 193

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Scheme 9. Schematic diagram of lignin condensation and α-OH oxidation modification in β-O-4 linkages.

example heavy fraction of bio-oil, are basically linked with CeC bonds. They are difficult to be further decomposed into aromatic monomers. Therefore, for these lignin oligomers, they can be converted into aliphatic (di)carboxylic acids by excessive oxidation transformation to achieve the full utilization of carbon sources.

Owing to the valence volatility, vanadium is able to transfer electrons, initiate the aerobic oxidation cycle, and achieve the oxidation of the αOH group. DDQ is another commonly used organocatalyst for the selective oxidation of α-OH group into α-ketone [176,177,179,180]. It can be used directly or in conjunction with NHPI. Scheme 10(b) shows DDQ/NHPI-catalytic aerobic oxidation cycle of α-OH to α-ketone with the help of NaNO2/O2. As Zhang et al. described, DDQ abstracts a hydrogen atom from NHPI to generate a highly reactive PINO (phthalimide-N-oxyl) free radical and 2H-DDQ (hydroquinone), and then the PINO radical obtains a hydrogen atom from the α-OH group in lignin dimer to generate NHPI and α-ketone. Finally, the NO molecule generate from NaNO2 decomposition in the reaction media catalyze the oxidation of 2H-DDQ to DDQ with O2 [176]. In addition to the cooxidation of the α-OH group with DDQ, NHPI can also selectively oxidize the α-OH group in lignin under photocatalytic and electrocatalytic conditions [181,182]. Similar to the catalytic oxidation cycles in DDQ/NHPI/NaNO2/O2 system, NIPO radicals are generated from NHPI with hydrogen atoms released under photocatalysis and electrocatalysis, and then oxidize the α-OH group. Except for the abovementioned metal-free organocatalysts, some metal-based catalytic system can also selectively oxidize α-OH group in lignin and lignin model compounds with α-ketones as the stable final products. For example, Karkas et al. used photocatalytic oxidation to convert various lignin dimer model compounds with α-OH group into α-ketones with the help of [Ir{dF(CF3)ppy}2(dtbbpy)]PF6/Pd(OAc)2/ Na2S2O8 catalytic system and blue LEDs [183]. Mottweiler et al. employed iron-based catalysts to oxidize erythro-1-(3,4-dimethoxyphenyl)2-(2-methoxyphenoxyl)-1,3-propanediol into 1-(3,4-dimethoxyphenyl)2-(2-methoxyphenoxyl)-3-ol-1-propanone with tert-butyl hydroperoxide (TBHP) as the oxidant [95]. Although the selectivity of α-ketone products from these metal-catalyzed oxidation systems are acceptable, and the stable α-ketone products can be also isolated, the yields of these α-ketone products are generally lower compared with those in TEMPO, DDQ, and NHPI oxidation processes. Furthermore, these metal-based catalytic systems can also depolymerize the produced ketone dimers, so it is difficult to accept them as the widely applicable catalytic systems for the pre-oxidation of α-OH group within lignin and lignin model compounds. Even in some metal-based catalytic systems, the generated ketone structures are the intermediates, which would be further oxidized to final stable products. In addition, among the catalytic oxidation of lignin monomers, some catalysts can also selectively oxidize the benzyl alcohol, i.e., the α-OH group, producing stable aromatic aldehydes or ketones. Mesoporous silica catalysts, including MCM-41, HMS, SBA-15, and Silica-5, were proved to be the good catalysts for apocynol oxidizing to acetovanillone with high selectivity [184,185]. Among these mesoporous silica catalysts, SBA-15 exhibited the highest catalytic activity.

5. Advanced selective oxidation of α-OH for lignin further conversion 5.1. Advanced selective oxidation of α-OH Lignin condensation is a common phenomenon in various strategies of lignin conversion. It mainly occurs between the Cα and C2,6 positions. That is because during lignin depolymerization, the α-OH group could be shed. Correspondingly, the Cα would transfer to Cα+ cation, then further couple with the C2− or C6− anion, forming the new CeC bond [61,170,171]. Lignin condensation would largely reduce the product yields and will lead to the further coking and charring. Therefore, it is necessary to modify the α-OH to achieve the purpose of inhibiting lignin condensation. There are many ways for α-OH modification, such as methylation [172], bromination [173,174], acetalization [170,175], as well as pre-oxidation [89,142,176]. Through pre-oxidation, the αOH can be transferred to α-ketone, which can hinder the formation of Cα+ cation, and then further inhibit lignin condensation. Scheme 9 presents the schematic diagram of lignin condensation and condensation inhibition via α-OH oxidized to α-ketone during lignin conversion. Catalysts used for the selective oxidation of the α-OH group are mainly the (metal-free) organocatalysts containing nitrogen, sulfur, or phosphor as reaction site constitutes, such as TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) [89,142,177,178], DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) [176,177,179,180], and NHPI (N-hydroxyphthalimide) [176,181,182]. TEMPO-catalyzed aerobic oxidation cycle of α-OH to α-ketone is commonly realized under the synergistic action of HNO3 and O2, as Scheme 10(a) displayed [142]. However, the selectivity of TEMPO/HNO3/O2 system on α-OH group oxidation is not high. It can oxidize primary alcohols (γ-OH) to aldehydes, or secondary alcohols (α-OH) to ketones in lignin structure at the same time. Therefore, it is necessary to increase the selectivity of TEMPO catalytic system so that it can only oxidize the α-OH group to achieve the purpose of active site shielding within lignin. Rahimi et al. screened a serious of 4-substituted TEMPO derivatives and the corresponding oxidation systems to assess their abilities on selective oxidation of α-OH group by using lignin dimer 3-(3,4-dimethoxyphenyl)-2-(2-methoxyohenoxy)-2-propenol. Among these TEMPO derivatives, 4-acetaminoTEMPO exhibited the best performance with 90% α-OH oxidized but no γ-OH oxidized [142]. Wang et al. also achieved the α-OH group oxidation to α-ketone by using VOSO4/TEMPO composite system [89]. 194

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Scheme 10. TEMPO- and DDQ/NHPI-catalytic aerobic oxidation cycle of α-OH to α-ketone. Adapted from Rahimi et al. [142] and Zhang et al. [176].

Therefore, SBA-15 was selected as the support to load metal active sites. For example, Co(Salen)/SBA-15 [186] and La/SBA-15 [187] can selectively oxidize apocynol to acetovanillone and guaiacol alcohol to vanillin, respectively. In addition, Co-based catalysts were found to selectively oxidize lignin monomeric model compounds with the α-OH group well. Mate et al. reported the differences in reactivity and selectivity from veratryl alcohol to veratryl aldehyde oxidatively catalyzed with Co3O4 in various solvents by using molecular oxygen, and presented the detailed catalytic reaction chemistry [87,188]. Kervinen et al. also employed veratryl alcohol as the feedstock, and observed its oxidation mechanisms catalyzed by Co(Salen) with the assistance of insitu ATR-IR, Raman, and UV/Vis spectrum [166]. Zakzeski et al. found that Co-ZIF-9 can oxidize a serious of small aromatic molecules with αOH group, including guaiacol alcohol and veratryl alcohol [189]. Other catalysts or catalytic systems can also be applied for the oxidation of lignin monomeric model compounds, converting them into aromatic aldehydes or ketones with high selectivity [158,190–194]. The significance of studies on the catalytic oxidation of lignin monomeric model compounds is obvious. However, the chemical properties of reactants themselves have great influence on the monomer conversion, which may limit the application of these catalytic systems for the selective oxidation of lignin dimeric model compounds and lignin fragments. Meanwhile, no literatures reported their applications in lignin and lignin dimeric model compounds. Therefore, the application of selective catalytic oxidation system obtained from lignin monomeric model compounds remains to be further tested.

unstable, and easy to be broken down for the production of monomer products. If further hydrolysis is carried out, corresponding acids and phenols would be the final products [177], whereas if alcoholysis is performed, corresponding esters and phenols would be generated [195]. Cu(OAc)2/BF3-OEt2 is another reported catalyst capable of oxidatively converting both oxidized β-O-4 and β-1 lignin dimers in methanol [151]. Mechanisms for both oxidized β-O-4 and β-1 lignin dimers catalyzed by Cu(OAc)2/BF3-OEt2 were the same, but different internal linkages contributed to different product species. Final stable products from oxidized β-O-4 lignin dimers were methoxyl-substituted methyl benzoates and phenols, whereas those from oxidized β-1 lignin dimers were methoxyl-substituted methyl benzoates and benzaldehydes. By the way, BV oxidation and Cu(OAc)2/BF3-OEt2 catalytic system are the only two reported methods that can further convert oxidized β-1 lignin dimers. In addition, Cu(OAc)2/1,10-phenanthroline can also transfer oxidized β-O-4 lignin dimers into aromatic acids and phenols in methanol with 0.4 MPa O2, and the detailed reaction mechanisms are proposed in Scheme 5a [89]. Notably, only non-phenolic lignin dimers were discussed in BV oxidation and Cu(OAc)2/BF3-OEt2 catalytic system. Wang et al. reported the conversion of phenolic lignin dimer 2-phenoxy-p-hydroxyacetophenone in Cu(OAc)2/1,10-phenanthroline catalytic system. The substrate conversion was up to 99%, and the yields of p-hydroxybenzoic acid and phenol reached 68% and 81%, respectively [89]. Two-step conversion of oxidation-reduction is another strategy to convert oxidized lignin and lignin model compounds into value-added platform compounds, as painted in green in Fig. 6. Lancefield et al. achieved the degradation of β-ether linkages within oxidized lignin dimers and birch lignin with the assistance of Zn/NH4Cl in 2-methoxyethanol/H2O under 80 °C [179]. Wang et al. reported the hydrogenolysis of oxidized β-O-4 lignin dimers catalyzed with a carbonmodified nickel catalyst Ni/MgAlO-C in methanol with the production of aromatic ketones and phenols. They also applied this reductive system to oxidized birch lignin, and obtained 22 wt% yields of phenolic mixtures, mainly ethyl-, propyl-, allyl-, and isoallyl-substituted guaiacols and syringols [111]. NiMo sulfide can also efficiently realize the hydrogenolysis of β-ether linkages with oxidized lignin model

5.2. Further conversion of oxidized lignin and lignin dimers Two-step oxidation, i.e. re-oxidation after per-oxidation modification, is a common strategy for lignin depolymerization, as painted in blue in Fig. 6. Baeyer-Villiger (BV) oxidation is a widely used method for converting ketones into their corresponding esters. It plays the good role in further conversion of oxidized lignin model compounds, including β-O-4 and β-1 lignin dimers [177,195,196]. Due to the asymmetry of oxidized lignin model compounds, two different types of esters would be produced after BV oxidation. The generated esters are 195

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Fig. 6. Subsequent cleavages of oxidized lignin dimers and lignin fragments. 196

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compounds. Except for hydrolysis, hydrogenation of α-ketone groups was also detected because of the formation alkyl-substituted aromatics. Meanwhile, the yields of phenolic monomers from pre-oxidized birch powders were up to 32% [176]. Guo et al. employed W2C/AC as the catalyst for the hydrogenolysis of oxidized β-O-4 lignin dimers in methanol, and achieved good results without doubt. However, when they moved this reductive system to oxidized beech lignin, lower monomer yields were determined compared to untreated lignin [197]. So far, this is the only reported case that pre-oxidized lignin decreased product yields, and reasons were proposed as undesired degradation of linkages, condensation, and modification of lignin occurred during lignin preoxidation. In addition, photocatalytic conversion is also investigated for further depolymerization of oxidized lignin model compounds. As painted in brown in Fig. 6, with the help of DIPEA, HCOOH, and light in methanol or acetonitrile, [Ir(ppy)2(dtbbpy)]PF6 [181,198], [Ir (ppy)2(bpy)]PF6 [199], Ir(ppy)2(bpy)-MCFs [199], and CzCP33 [182] can break down the β-O-4 linkages of oxidized lignin dimers, producing α-ketone aromatics and phenols. Compared with other transformation strategies, photocatalytic conversion is more suitable for oxidized phenolic lignin model compounds. Aerobic amination is also adopted for the further conversion of oxidized β-O-4 lignin dimers (as painted in pink in Fig. 6). Zhang et al. investigated a serious of CuX catalysts, solvents, amines, and lignin model compounds, and obtained various amides [200]. Furthermore, distinguished with those reductive depolymerization of oxidized lignin, Rahimi et al. reported acid-induced depolymerization of oxidized aspen lignin using HCOONa/HCOOH system at 110 °C with 24 h (as painted in red in Fig. 6). The produced monomers are mainly p-hydroxyphenyl, guaiacyl, and syringyl aldehydes, ketones, and acids with the total yields of 61.2 wt% [31]. Generally, after pre-oxidation modification, the electron-donating α-OH group is transferred into the electron-withdrawing α-ketone group, which efficiently decreases the electron cloud density on side-chains within lignin model compounds and lignin macromolecules, lowers the bond energies of β-O-4 linkages [89,111,180,201], and makes the substrates easy to be further depolymerized.

not report the yields of produced monomeric products. They only mentioned that the ether bonds in lignin macromolecules are well degraded and the molecular weights are greatly reduced, monitored by HSQC and GPC [89,95,150]. Furthermore, compared with lignin model compounds (mainly lignin dimers), lignin macromolecules have more complex space effects and electron effects because of the rigid construction, as well as the interference of other substituted functional groups (mainly the phenolic OH, OMe, and γ-CH2OH groups). These contributed to the different reaction mechanisms and different final products between lignin macromolecules and lignin model compounds. For example, Deng et al. employed the CeO2-supported Pd nanoparticles to oxidize organosolv lignin and lignin model compound 2phenoxy-1-phenylethanol, and obtained aromatic aldehydes and mixtures of aromatic ketones and esters as the final products [162]. Therefore, developing efficient catalytic oxidation systems directly based on lignin macromolecules is necessary. It is also the requirement of further application and industrialization. In addition, many of the reported catalysts used for the oxidative depolymerization of lignin and lignin model compounds are homogeneous catalysts, such as metal ions [132–134,160], oxovanadium complexes [88,91,92,94,102,145–148], Salen complexes [86,163–167], and metalloporphyrins [99,155,156]. These homogeneous catalysts are well dissolved in reaction mediums, and difficult to be separated, regenerated, and reused. Furthermore, the synthesis processes for organometallic catalysts are fairly complex, which leads to their high cost, together with the expensive precursors and reagents. All of these restrict the utilization of lignin catalytic oxidation in large scale. Therefore, developing inexpensive heterogeneous catalysts for lignin oxidative depolymerization should be one direction of the future work. For example, Mottweiler et al. developed the new heterogeneous catalyst HTc-Cu-V using hydrotalcite as the precursor, and achieved good catalytic oxidation results for both lignin dimer model compounds and lignin fragments [90]. The commonly used reaction mediums for lignin catalytic oxidative depolymerization can be mainly divided into two categories, i.e., alkaline solutions and organic solvents. Using alkaline solution as the reaction medium for lignin oxidative depolymerization is summarized in Section 3.3, and the main product is vanillin (and syringaldehyde). The conventional route to isolate the reaction products is to acidify the reaction medium and extract the products with an organic solvent, thus requiring large amounts of acid and organic solvent, and producing large volumes of saline waste water. On the other hand, organic solvents used for lignin oxidative depolymerization are mainly pyridine, toluene, and acetonitrile. All of these solvents are not green enough. They are toxic and harmful for the environment to a certain extent. Besides, how to isolate oxidation products from these organic solvents and further purification of isolated products are rarely reported. Hence, preparing efficient and recyclable heterogeneous catalysts, screening new environmentally friendly solvent systems, developing sustainable procedures for product isolation and purification, and other aspects are all issues that should be concerned for lignin oxidative depolymerization.

6. Summary and outlook With the continuous development of lignin catalytic oxidation, it has become an indispensable strategy for lignin valorization for the production of valuable platform chemicals, including phenolic aldehydes, ketones, and acids, benzoquinones, and aliphatic (di)carboxylic acids. However, there are still some deficiencies and challenges existed in current lignin catalytic oxidation. First, to assess the effects of employed catalytic oxidation systems and clarify the corresponding depolymerization mechanisms, lignin model compounds are usually employed as the reactants, especially the non-phenolic species. These catalytic oxidation systems well favor the conversion of selected non-phenolic lignin models. However, when applying these catalytic oxidation systems to the phenolic lignin models, the results are commonly not satisfactory. The yields of oxidation products from phenolic lignin models are rather lower than those form non-phenolic models with the same chemical structures. As for lignin macromolecules, except for the non-phenolic substructures, i.e., ether linkages (β-O-4, α-O-4, 4-O-5), there are also abundant free phenolic OH groups (Li et al. summarized the phenolic OH contents in various lignin fragments [2]). Therefore, it is also essential to develop efficient catalytic oxidation systems for phenolic substructures, producing stable final monomer products. In addition, most of the reported catalytic oxidation systems are based on lignin model compounds. Only a few of the catalytic oxidation reactions with aliphatic (di)carboxylic acids as the target products use technical lignin fragments as the raw materials. Though, some researchers applied the catalytic oxidation systems they obtained from lignin model compounds for the production of aromatic aldehydes, ketones, and acid to technical lignin, they did

Conflicts of interest The authors declare no conflict of interest. Acknowledgements This work was supported by the China National Funds for Distinguished Young Scientists (Grant no. 51525601), the Local Innovation and Research Teams Project of Guangdong Pearl River Talents Program (Grant no. 2017BT01N092), the Scientific Research Foundation of Graduate School of Southeast University (Grant no. YBJJ1707), the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant no. KYCX17_0080), and the Fundamental Research Funds for the Central Universities. 197

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Prof. Dr. Huiyan Zhang is a professor of Thermal Engineering at Southeast University. He obtained his Ph.D. degree under the guidance of Prof. Rui Xiao at Southeast University and Prof. George W. Huber at University of Massachusetts-Amherst in 2012. His research focuses on biomass conversion into value-added products including transportation fuels, chemicals and carbon materials. He obtained 4 national and international prizes including the first prize of Natural Science of Ministry of Education, and the special gold medal of Geneva International Inventions Exhibition. At present, he is the (co-)author of 4 international and 40 Chinese patents and over 80 international papers (indexed by SCI) including those published in Science, Energy & Environmental Science, and so on. His work has been cited by SCI index > 2500 times. Prof. Dr. Rui Xiao is currently the dean of the School of Energy and Environment, and the director of key laboratory of Energy Thermal Conversion and Control, Ministry of Education at Southeast University. He is the distinguished professor of the Cheung Kong Scholars Program, the standing director of the Biomass Energy Committee of Renewable Energy Society, and the member of the Chinese Society of Engineering Thermophysics. He received his Ph.D. degree (2005) and M.S. degree (1997) in “Thermal Engineering” at Southeast University. He served as a visiting scientist at University of Kentucky (2007–2008) and University of Cambridge (2014). His research expertise is in the clean utilization of coal and biomass, CO2 capture, and bio-fuels.

Chao Liu received his M.S. degree in 2015 from the Key laboratory of Pulp and Paper Engineering, and the school of Light Industry and Engineering at South China University of Technology. He is currently a Ph.D. candidate in the Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, and the School of Energy and Environment at Southeast University under the supervision of Prof. Rui Xiao. His research focuses on the thermochemical conversion of lignocellulosic biomass into biofuels and biochemicals, and the comprehensive utilization of lignin resources.

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