Lignin-derived platform molecules through TEMPO catalytic oxidation strategies

Progress in Energy and Combustion Science 72 (2019) 59–89

Contents lists available at ScienceDirect

Progress in Energy and Combustion Science journal homepage: www.elsevier.com/locate/pecs

Lignin-derived platform molecules through TEMPO catalytic oxidation strategies Samira Gharehkhani, Yiqian Zhang, Pedram Fatehi∗ Chemical Engineering Department and Green Processes Research Centre, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada

a r t i c l e

i n f o

Article history: Received 23 June 2018 Accepted 25 January 2019

Keywords: Lignin Oxidation TEMPO Green chemicals Energy Depolymerization Biorefining

a b s t r a c t Lignin is currently an under-used material of the pulping and cellulosic ethanol production plants. The first and foremost objective in dealing with lignin is to augment the use of this aromatic compound and financial profitability of lignin-based processes. Of particular interests are lignin oxidation and depolymerization methods that harvest either the aromatic subunits of polymers or monomeric/oligomeric aromatic products used as platform chemicals. In this context, many studies have focused on the catalytic processes in which a nitroxyl catalyst, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), was employed to oxidize hydroxyl groups in lignin structure and selectively depolymerize lignin via cleavage of the ether and C–C bonds. In discussing the advanced studies especially over the last decade on the oxidation of lignin models with TEMPO catalyst systems, this review provides a description of promising methods that can be employed for authentic lignin conversion. In the present review, a particular emphasis is dedicated to the proposed mechanisms for the oxidation of lignin and lignin models. © 2019 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3. 4.

5.

6.

7. 8.

∗

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lignin structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Dilignols, monolignols and linkages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Bond distances and dissociation energy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Industrial lignins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lignin valorization strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Hydroprocessing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TEMPO reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. TEMPO structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. TEMPO/Solvent interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TEMPO-catalyzed oxidation and depolymerization of lignin models . . . . . . . . . . . . . . . 6.1. Oxidation of lignin models without bond cleavage . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1. TEMPO catalytic systems containing transition-metal-free components . 6.1.2. TEMPO catalytic systems containing transition metal components. . . . . 6.2. TEMPO oxidation of lignin models followed by bond cleavage . . . . . . . . . . . . . . 6.2.1. Bond cleavage initiated by oxidized ketone . . . . . . . . . . . . . . . . . . . . . . . 6.2.2. Bond cleavage initiated by oxidized aldehyde . . . . . . . . . . . . . . . . . . . . . 6.3. TEMPO oxidation of lignin models with bond cleavage . . . . . . . . . . . . . . . . . . . . TEMPO oxidation strategies for technical lignin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Novel potential routes for TEMPO oxidation of lignin compounds. . . . . . . . . . . . . . . . .

Corresponding author. E-mail address: [email protected] (P. Fatehi).

https://doi.org/10.1016/j.pecs.2019.01.002 0360-1285/© 2019 Elsevier Ltd. All rights reserved.

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

60 60 60 60 61 63 63 64 64 64 64 64 64 64 65 72 73 73 74 74 75 77

60

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

8.1. Implementation of ionic liquids . . . 8.2. TEMPO immobilization . . . . . . . . . . 8.3. Electrocatalytic oxidation . . . . . . . . 9. Challenges and future prospects . . . . . . . . 9.1. Diversity in lignin structure. . . . . . . 9.2. Chemoselectivity . . . . . . . . . . . . . . . 9.2.1. Cellulose oxidation . . . . . . . 9.3. Separation. . . . . . . . . . . . . . . . . . . . . 9.4. Implementation of novel methods . 9.5. Yield and price . . . . . . . . . . . . . . . . . 10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . Conflict of interest . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . Supplementary material . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

1. Introduction Lignocellulosic biomass, an important resource for the production of fuels, materials and chemicals, comes from wood and nonwood species and consists of three main components: cellulose, hemicelluloses and lignin (Fig. 1). Lignin is a three-dimensional hydrophobic polymer that acts as glue, binding different layers of cell wall of biomass [1,2]. It is one of the important sources of aromatic compounds in nature and a by-product of the pulping industry, representing an excellent alternative feedstock for sustainable product development [3,4]. Annually, more than 50 million tons of lignin are produced in the pulping industry [5]. The current utilization of lignin is limited in that only about 2–5% of lignin is commercially used for producing fillers [6], binders [7] and dispersing agents [8]. The rest is either burned for energy or treated in wastewater [9,10]. Recent economic studies have reported that effective routes for lignin valorization would result in 5 times more value compared with burning it for energy production [11,12]. Lignin has an amorphous, complex and robust structure. The adverse effect of such complexity is apparent in the low chemical reactivity of lignin, which hampers its conversion to value added products [13]. Various pathways, such as pyrolysis, reduction and oxidation, have been employed for lignin valorization, yielding value added chemicals [14–23]. Among these methods, the oxidation of lignin has attracted considerable interest as a promising method to form polyfunctional aromatic compounds [24–27]. Oxidation can be performed with a variety of oxidants, such as bleaching agents (e.g. chlorine, chlorine dioxide, hypochlorite), H2 O2 and O2 [30–32]. Molecular O2 is highly desirable because it is inexpensive and no toxic byproducts are generated from the oxidizing agent itself [33,34]. However, it is kinetically unyielding. Utilizing an economical and effective method that can be employed under mild condition to produce desirable products is one of the greatest challenges in oxidation processes. In this circumstance, most of studies were dedicated to the selective catalytic oxidation of lignin. Recently, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) catalyzed oxidation processes generated promising interesting lignin oxidation results. However, the available review papers on lignin oxidation do not provide detailed studies on TEMPO oxidation of lignin [23,35–39], which is the focus of the present work. Here, we aim to offer a detailed analysis regarding the TEMPO oxidation of lignin, which covers the modification of technical lignin and lignin model compounds. The progress achieved on oxidation and depolymerization of lignin into valuable chemicals and fuels is covered in this review article. It is notable that a summary of lignin structure, technical lignin isolation methods and TEMPO reactivity is provided in the beginning of this review paper. The

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

79 81 82 84 84 84 84 85 85 85 85 86 86 86 86

main section then discusses the challenges and prospective aspects of lignin functionalization and depolymerization. 2. Lignin structure 2.1. Dilignols, monolignols and linkages The present section provides an overview of lignin structures and models. Complexity of lignin structure comes from the linkages formed between the building units of lignin. From the chemistry standpoint, lignin is produced through the enzymemediated dehydrogenative polymerization of the phenylpropanolic units namely, synapyl (3,5-dimethoxy 4-hydroxycinnamyl) alcohol, coniferyl (3-methoxy 4-hydroxycinnamyl) alcohol and p-coumaryl (4-hydroxycinnamyl) alcohol which form the syringyl (S), guaiacyl (G), and hydroxyphenyl (H) lignin subunits, respectively (Fig. 2(a)) [40–45]. Softwood lignin is predominately G type (90−95%), while hardwood lignin is mostly G and S types [46–50]. These subunits can be manipulated to alter the lignin degradation [51]. The subunits are linked by ether and C–C bonds where the common interunit linkages are generally seen at the β -position of the monolignol species, arylglycerol-β -aryl ethers (β –O–4), phenylcoumarans (β –5), pinoresinols (β –β ), and diphenylethane dimers (β –1) [52]. Dilignols and higher oligomers preferentially couple at the 4 and 5 positions, yielding diaryl ethers (4–O–5) and biphenyls (5–5) (see Fig. 2(b)) [11,37,53]. The 4–O–5 linkages bear ether functionalities while the β –5, β –β , 5–5 and β –1 linkages connect two monolignols via a C–C bond. Of the lignin linkages, the most abundant is the β –O–4 [54] which corresponds to a bond formed between the β carbon of the aliphatic side chain and the oxygen atom attached to the C4 position of the aromatic moiety [26]. The occurrence values (%) for a range of lignin linkages are presented in Fig. 2(b) [46,55]. 2.2. Bond distances and dissociation energy Understanding the properties, such as bond distances and bond dissociation energies, of various linkages in lignin is essential for estimation of lignin resistance toward depolymerization. Density functional theory (DFT), a quantum chemical approach, can be used to predict the bond dissociation energies and reactivity trends [57]. Table 1 presents the bond distances and bond dissociation energies for lignin model compound linkages. In general, the ether bond linkages are shorter than the C–C bond linkages, and 4–O–5 is the shortest one. Calculations reveal that the β –5 and 5–5 linkages are among the strongest bonds. The presence of substituents on the arene rings tends to make the ether linkages weaker. The bond dissociation energy (BDE) calculations suggests that the presence of carbonyl group at the Cα position and ortho-methoxy

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

61

Fig. 1. Hierarchal structure of wood. (a) Yellow pine tree. (b) SEM image of the tissue structure of softwood (yellow pine). (c) Contact resonance force microscopy of cell wall (Reprinted from Ref. [28] with permission of Royal Society of Chemistry). (d) Structure of lignocellulose. (e) Schematic of lignin and cellulose decorated with hemicellulose (Adapted from Ref. [29] with permission of American Chemical Society). (f) Chemical composition of different species [2].

substitutions in the arene ring results in a lower BDE for the β – O–4 linkages, but a negligible effect on the BDEs of C–C bond linkages [58]. Moreover, oxidation of either the primary or secondary alcohol in the β –O–4 lignin model decreases the average BDEs. Lignin models with ketone forms have lower BDE than do the aldehyde forms, which is attributed to the delocalization of the resulting radicals into the carbonyl and the aromatic ring for the ketone [59]. 3. Industrial lignins The properties of lignin, which are affected by the pulping conditions and wood species, alter their targeted applications [60]. In addition, the composition, solubility and molecular weight of lignin differ depending on the isolation process of lignin from pulping spent liquors. Lignin with a high molecular weight is desirable to

be used as an adhesive and binder, whereas low molecular weight lignin is attractive for phenol or aromatic chemical production [33]. Most abundant industrial lignin is currently produced in kraft pulping process and is generally recovered by acidification of black liquor [61]. Recently, LignoBoost, which is commercialized by Domtar in North Carolina, and LignoForce, which is commercialized by WestFraser in Alberta, were designed according to the acidification concept to isolate kraft lignin from black liquor [13,62–64]. Acidification of the alkali black liquor protonates the phenoxy groups, affording phenolic, water insoluble kraft lignin [65,66]. Main characteristics and impurity contents of kraft lignin along with other industrial lignins are presented in Table 2. Currently, the majority of kraft lignin is burned in the recovery boiler to produce energy for the process [67]. Major non-fuel utilization of kraft lignin includes its modification to produce lignin based dispersants, lignin based thermoset plastic, resin, and carbon

62

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

Fig. 2. Structure of (a) typical lignin (Reprinted from Ref. [56] with permission of Nature Publishing Group). (b) Common linkages between primary units of lignin (Adapted from Ref. [57] with permission of American Chemical Society).

Table 1 Bond distances and dissociation energies for the ether and the C–C linkages of lignin model compounds [34]. Linkage models C–O bond linkages

Bond distance (˚A)

Bond dissociation energy (kcal/mol)

β –O–4

1.43 1.38

63.79 80.08

4–O–5

Linkage models C–C bond linkages

Bond distance (˚A)

Bond dissociation energy (kcal/mol)

5–5

1.48 1.46 1.54

116.8 126.4 67.68

β –5 β –1

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

63

Table 2 Main characteristics of different industrial lignins. Lignin

Mw (g/mol)

Production conditions

Polydispersity index

Carbohydrate content (wt%)

Ash content (wt%)

Sulfur content (wt%)

KL

∼150–20 0,0 0 0

5.9–7.3

2.3

3

1.50–3.0

∼0.4

LS

10 0 0–150,0 0 0

∼7

–

∼10

4–8

SL

690 0–850 0

3

2–3

1

OL

10 0 0

2.4–6.4

1–2

HL

190 0–20 0 0

Temp; 170 0 C Time; 3.5 h Temp; 120 0 C Time; 0.67 h Temp; 20–140 0 C, Time; 1.5–24 h Temp; 170 0 C Time; 1 h Temp; 150–200 0 C

3

15–23

fibers [68]. These products are mainly produced via sulfomethylation or amination following the Mannich type reaction. Today, WestRock and Domtar are the main producers of lignin derivatives and kraft lignin worldwide [69]. Their products are mainly known by commercial names of Indulin series, Polyfon series, kraftsperse series and Reax series, and the BioChoice lignin [24,70]. Lignosulfonates (LS) are produced at the rate of 1 million tons annually [71]. LS are amphiphilic and water-soluble lignins in forms of metal or ammonium salts. The introduction of sulfonic acids and cleavage of bonds at propane chains in sulfite pulping process degrade lignin into smaller fractions and make it water soluble [72]. However, Cα and C6 positions in lignin structures increase the molecular weight of lignosulfonate under acid sulfite pulping conditions [73]. LS has a relatively high molecular weight (10 0 0–150,0 0 0 g/mol) and high ash content (Table 2), and is used as a surfactant, dispersant and binder at industrial scales [74,75]. LS can be oxidized to produce vanillin, or chemically modified by carboxylation, amination, and graft copolymerization to produce miscellaneous products [11,76]. The largest producer of LS is Borregaard Ligno Tech. Tembec, La Rochette Venizel, Nippon Paper Chemicals, Cartiere Burgo and Domsjo Fabriker AB are also important lignosulfonate producers [57,77]. Similar to kraft lignin, soda lignin has a hydrophobic characteristic. Soda lignin is produced via soda or soda anthraquinone pulping processes. Unlike kraft lignin, soda lignin is sulfur free. Total hydroxyl groups content in soda lignin is reported to be around 0.8–0.9 per phenyl propane unit, equally split between aliphatic and phenolic hydroxyl groups (Table 2). Soda lignin is suitable for animal feed and nutrition applications as well as binding application. High purity soda lignins are mainly produced by GreenValue SA company [78]. Unpurified soda lignins are offered by Northway Lignin Chemical company in 50 wt% liquid form, which are used as a low end binder [79]. A pulping medium containing a mixture of organic solvent, such as ethanol, methanol, or acetone, with water is used to produce the Organosolv lignin (OL) [80]. Lignin retains much of its original native structure in the form of β –O–4 inter-unit linkages, but it contains some condensed structures [81,82]. OL products are usually pure with low carbohydrate contents due to the hydrolysis of ether bonds that links lignin to hemicelluloses [83]. Compared to kraft pulping process, the organosolv method has lower environmental impacts due to the absence of sulfur in the pulping process [84]. Being highly hydrophobic, OL is insoluble in water, but soluble in dilute aqueous alkaline and many polar organic solvents such as methanol and ethanol. The lower molecular weight and high chemical purity (sulfur free compound) make OL attractive as a source of low molecular weight compounds for phenol or aromatic chemical production [81,85]. The high cost of organosolv process [86], which is due to the complexity of solvent recovery process [57], hampers the expansion of the process at

Aliphatic OH

Phenolic OH

Ref

COOH

OCH3

0.8–1.0

0.2–0.3

∼0.6

[14,89,90]

–

0.2–0.3

0.2–0.3

∼0.7

[8,77,91]

∼0

∼0.4

0.4–0.5

0.2–0.3

1.0–1.2

[8,89,92]

−2

∼0

∼0.4

∼0.4

∼0.1

0.9–1.0

[10,66]

1

∼0

∼0.5

0.2–0.3

0.1

0.6–0.7

[88,93]

per phenyl propane unit

a commercial scale. The most common organosolv processes are Alcell process, using ethanol/water as a solvent, and Acetosolv process, which uses acetic acid combined with a small amount of mineral acid, e.g., HCl, as solvent [79,80]. Acid and enzymatic hydrolysis are two methods to produce the hydrolysis lignin (HL). Diluted or concentrated acids are used in acid hydrolysis process, where both of them have drawbacks, e.g., high temperature and pressure requirements in the hydrolysis process and low recovery yield of lignin in the dilute acid hydrolysis process and acid recovery process in the concentrated acid hydrolysis method. The abovementioned obstacles have limited the use of acid hydrolysis (AH) processes. Unlike the AH process, enzyme hydrolysis (EH) process has attracted more interest. EH lignin has high chemical reactivity, which makes it a favorable additive in polymer preparations [87]. The glass transition temperature of HL is lower than that of other lignins. This could be attributed to the presence of high amounts of carbohydrates in HL (Table 2), which in turn increases the amount of retained water molecules in the lignin structure [88]. The hydrolysis lignin has been produced in both pilot and industrial scales. HL is insoluble in polar organic solvents and alkaline solutions due to the strong condensation/polymerization occurring in the hydrolysis route [84]. 4. Lignin valorization strategies As mentioned earlier, different strategies, e.g., pyrolysis, oxidation and hydroprocessing, are performed for catalytic transformation of lignin into value added products. Although the scope of the present review is limited to oxidation processes, a brief explanation of the methods is presented in this section to provide the reader with insights into these key processes. 4.1. Pyrolysis Pyrolysis, a thermochemical process, is a rapid heating of feedstock at high temperature (450–600 °C) often under oxygen free environment to produce small fragments [26,94]. Various products can be generated in primary stage (20 0–40 0 °C) and secondary stage (40 0–70 0 °C) of pyrolysis. A mixture of gases (e.g., H2 , CH4 , C2 H6 , and CO2 ), volatile liquids (e.g., methanol, acetone, and acetaldehyde), monomers (e.g., phenol, guaiacol, syringol) and char are formed in the pyrolysis of lignin [95]. The compositions of products are strongly affected by lignin type. For example, hardwood lignin having more methoxy group in syringyl units affords a higher yield of CO2 , CH4 and char compared to softwood lignin [50]. Presence of a catalyst during the pyrolysis reaction is favorable with respect to the yield and distribution of products. Catalysts not only facilitate the conversion of depolymerized intermediates into valuable compounds, but also favor less char production by preventing the repolymerization and condensation

64

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

of intermediate products. Ma et al. [96] studied the role of pore structure and acid sites of zeolites in fast pyrolysis of lignin. They showed that the larger molecules can be stabilized through adsorption on large pores of the catalyst and further converted to desired products [97]. Although pyrolysis seems a simple method for lignin depolymerization, several drawbacks hinder its application. For example, formation of char having limited functional groups is an obstacle for further utilization of char in various applications, e.g., pollutant adsorption and energy storage devices [95]. 4.2. Hydroprocessing Lignin depolymerization through hydroprocessing has been vastly studied over the last few years [97–101]. There are various hydroprocessing reactions such as, hydrogenolysis, hydrodeoxygenation and hydrogenation. Hydrogenolysis involves cleavage of a single bond and is an important reaction for lignin depolymerization, particularly in cleaving of C–O bonds [23]. Hydrogenation adds H2 across an unsaturated C=C or C=O bond, and hydrodeoxgenation often involves alcohol dehydration followed by hydrogenation. One of the important issues in lignin upgrading through reduction is hydrogenation of benzene rings yielding fully saturated products, which do not readily undergo further hydrogenolysis [98]. Moreover, hydrogen consumption management should be considered as one of the main parameters to expand the reductive cleavage methods [26]. 4.3. Oxidation In opposition to reduction catalytic reactions that generate chemicals with reduced functionality, oxidation methods produce platform chemicals with enhanced functionality. Owing to the presence of various functional groups in lignin structure, functionalization through oxidation is one of the promising approaches for lignin upgrading. Oxidation methods favor the formation of various monomeric and fine chemicals including aldehydes, ketones and carboxylic acids. For example, production of vanillin and syringaldehyde as platform chemicals have been reported [102,103]. A wide range of catalysts including metal-free, organometallics and nitroxide compounds are used for lignin oxidation [57]. Of nitroxides, TEMPO is the most frequently used compound for alcohol oxidation processes because of its stability and availability [104]. The following sections are organized based on the type of lignin modifications via TEMPO oxidation process. Prior to discussion on the mechanism of the reactions, general aspects of TEMPO are presented. 5. TEMPO reactions 5.1. TEMPO structure TEMPO, a red-orange solid with low melting point (36–38 °C), is an inexpensive stable nitroxyl radical, which is used for the conversion of alcohols to the corresponding carbonyl compounds, e.g., aldehydes or ketones [105,106]. TEMPO was first introduced in 1960 by Lebedev and Kazarnovskii [107]. TEMPO can be synthesized through a reductive step of triacetoneamine (a frequently used material as a light stabilizer for plastic) to 2,2,6,6tetramethylpiperidine followed by an oxidation step to TEMPO (Fig. 3(a)) [108,109]. The structure of TEMPO and examples of its derivatives are shown in Fig. 3(b). Intensive studies into the electronic and chemical structures of TEMPO have been performed in order to understand the parameters associated with its reactivity [104,110–113]. It is stated that the NO bond bears “partial double bond” character (bond order of 1.5 and the bond length of 1.25 0 A) attributed to

the delocalization of the free electron between the nitrogen and oxygen atom [104]. The delocalization energy estimated for the free electron is approximately 120 kJ/mol [113]. The two resonance structures of TEMPO as the indications of the electron delocalization are shown in Fig. 3(c). Generally, the spin density distribution is toward the oxygen, hence the spin density at oxygen is larger than at nitrogen [112]. The pair of bulky gem-dimethyl groups in the molecular structure of TEMPO increase its stability by inhibiting dimerization via O–O, N–O or N–N bonds [33]. 5.2. TEMPO/Solvent interactions In lignin oxidation and depolymerization reactions, TEMPO is generally accompanied by a solvent to enhance the yield and conversation rates in reactions. The catalytic compounds need to be dissolved in solvent to minimize the mass transfer limitations [116,117]. Several important factors including polarity and hydrogen bond donating/accepting influence the solvent choice for TEMPO reactions [118]. One of the parameters affecting the reactivity of the catalyst is the electronic distribution in a radical obtained from the dipole moment [110], which is involved in the solvatochromism. The solvatochromism is considered when a solute dissolved in solvents of varying polarity manifests a pronounced change in position, intensity, and shape of an absorption band [119]. TEMPO having a dipole moment of 3D corresponds to a polar molecule. The results presented by Lalevee et al. [110] for dipole moment, spin and electron densities of the TEMPO in different solvents are shown in Table 3. 6. TEMPO-catalyzed oxidation and depolymerization of lignin models As mentioned earlier, lignin structure consists of a variety of alcohols linked by different bonds. Benzylic alcohols and allylic alcohols can be considered as the representatives for monomeric lignin model compounds. Therefore, most of the mechanisms developed for alcohol oxidation can be applied for lignin oxidation. Lignin oxidation investigations can be conducted either on monomeric or dimeric lignin models. Owing to lignin’s complex structure, research on TEMPO oxidation of lignin mostly use dimeric lignin model compounds with specific linkages and functional groups similar to native lignin [120]. Taking synthesized lignin model compounds instead of native lignin for oxidation studies provides the possibility for detailed analysis of reactions, resulting in a better understanding of oxidation and depolymerization processes. Fig. 4 presents examples of lignin models utilized for TEMPO oxidation and depolymerization studies. The phenolic and aliphatic hydroxyl groups in lignin provide reactive sites for producing quinones [94,135], aldehydes or ketones [131,136] by TEMPO catalyzed oxidation. Fig. 5 depicts examples of products from TEMPO oxidation and depolymerization of lignin with and without C–C and C–O bond cleavage. 6.1. Oxidation of lignin models without bond cleavage Lignin oxidation without bond cleavage can be interpreted as lignin functionalization where the linkage remains intact. As described, although lignin structure has many functionalities, they are often sterically hindered by the highly crosslinked lignin macromolecule [57]. Depending on the lignin’s structure and reaction conditions, different products are formed from the oxidation of lignin. Examples of oxidized products without bond cleavage are presented in Fig. 5(a). It has been speculated that TEMPO oxidation can weaken C–C/C–O bonds adjacent to the Cα hydroxyl group, which may lead to lignin depolymerization (see Sections 6.2 and 6.3).

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

65

Fig. 3. (a) TEMPO synthesis (Adapted from Ref. [108] with permissions of Springer, and Adapted from Ref. [114] with permission of Royal Society of Chemistry). (b) The structure of TEMPO and examples of its derivatives (Adapted from Ref. [115] with permission of Elsevier). (c) Resonance structure of TEMPO (Reprinted from Ref. [33] with permission of Thieme Publishing Group). Table 3 TEMPO dipole moment, spin and electronic densities dependences on solvent [110]. Solvent

Dipole moment (D)

Spin N

Spin O

Charge nitrogen

Charge oxygen

No-solvent/vacuum Cyclohexane Tetrahydrofuran Ether Acetone Dimethyl sulfoxide Acetonitrile Ethylacetate Propylene carbonate

2.94 3.19 3.55 3.52 3.67 3.70 3.70 3.78 4.14

0.447 0.453 0.462 0.463 0.466 0.462 0.466 0.473 0.482

0.523 0.515 0.507 0.506 0.504 0.500 0.503 0.497 0.487

−0.048 −0.044 −0.041 −0.042 −0.041 −0.043 −0.040 −0.041 −0.040

−0.396 −0.405 −0.418 −0.419 −0.422 −0.0430 −0.423 −0.431 −0.444

6.1.1. TEMPO catalytic systems containing transition-metal-free components The transition-metal-free TEMPO catalytic oxidation process often goes through N-oxoammonium salt pathway. N-oxoammonium salt (Fig. 6) is produced via one-electron transfer reaction during the TEMPO oxidation and can be isolated as an oxidant [141]. In alcohol oxidation mediated by oxoammonium salt, a two-electron

reduction of the oxoammonium species results in a hydroxylamine (TEMPOH). Various stoichiometric oxidants, such as bleach, hypohalides, or combination of O2 with NOx can be used in combination with TEMPO to conduct the alcohol oxidation process. For example, hypochlorite (NaOCl) has been mentioned in Anelli-Montanari protocol (Fig. 7) where the reaction takes place via the oxoammonium salt [142–144]. In this method, potassium

66

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

Fig. 4. Lignin model compounds used in studying TEMPO oxidation reactions. See Ref. [134].

bromide is used as cocatalyst. It is noted that this is an alcohol oxidation protocol, which is applied for lignin oxidation as well. Depending on the pH, the mechanisms for oxidations are different (Fig. 8). In basic conditions, the reaction complex formed between oxoammonium species and alkoxide could be generated by the attack of an alkoxide on the oxygen or nitrogen atom of the cation. A study affirmed the favourability of the pathway involving the nitrogen atom [145]. Under basic conditions, in the presence

of both primary and secondary alcohols, the steric effects in the formation of an alkoxide adduct with the oxoammonium species result in chemoselective oxidation of primary alcohols attributed to the faster oxidation of primary alcohols in comparison with the more hindered secondary ones [146]. As the primary alcohols are somewhat more acidic than secondary alcohols in solution, there will be a higher concentration of a primary alkoxide than a secondary one in solution [145]. In acid conditions, the selectivity

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

67

Fig. 5. Products mimicking substances produced from TEMPO oxidation and depolymerization of lignin compounds, (a) O-formylated afforded from no bond cleavage of lignin, and (b) fragments afforded from lignin bond cleavage. See Ref. [138].

68

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

Fig. 5. Continued

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

Fig. 5. Continued

69

70

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

Fig. 5. Continued

Fig. 6. TEMPO oxidation/protonation states (Adapted from Ref. [142] with permision of WILEY-VCH).

Fig. 7. Anelli–Montanari protocol for alcohol oxidation (Adapted from Ref. [143] with permission of Royal Society of Chemistry).

changes and the mechanism involves a bimolecular hydride transfer, which favors more-electron-rich substrates, and the adduct is more likely to be a reactive intermediate [108,112,146]. Notably, the higher the pH, the higher the oxidation rate for both primary and secondary alcohols [145]. Aerobic oxidation is a frequently used method for alcohol and lignin oxidation [147]. From the sustainability perspective, O2 as a terminal oxidant is favorable and safe [148]. The number of studies on aerobic TEMPO oxidation systems is increasing vastly. Rahimi

et al. [120] investigated various catalytic systems, e.g., metal and metal-free containing catalytic systems, for aerobic oxidation of the secondary benzylic alcohol within a lignin model compound corresponding to β –O–4 linkage (Table 4, entry 1–8). Most of the catalysts resulted in a high conversion (66−100%) and acceptable selectivity in oxidation of lignin model 1S. Only bleach/ TEMPO catalyst system at pH 9 exhibited acceptable selectivity for primary alcohol oxidation (Table 4, entry 1). Acetonitrile (MeCN)/water as a solvent afforded higher conversion (100%) in oxidation of 1S than MeCN

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

71

Fig. 8. Proposed mechanisms for oxoammonium-mediated oxidation of alcohols under alkaline and acidic conditions (Reprinted from Ref. [146] with permission of American Chemical Society).

Table 4 TEMPO catalytic systems used for oxidation of lignin model compounds without bond cleavage. Entry

Source

Catalyst

Experimental condition Temp, time, oxidant pressure solvent

1

1S

TEMPO+ bleach pH = 9

2

1S

TEMPO/NaNO2 /HCl

3

1S

TEMPO/HNO3 /HCl

O2

45 °C, 20 h

4

1S

air

5

1S

4-acetamido-TEMPO (AcNH-TEMPO)/HNO3 /HCl AcNH-TEMPO/HNO3 /HCl

O2

Room temp., 24 h 45 °C, 24 h

6

1S

AcNH-TEMPO/HNO3 /HCl

O2

7

2S

AcNH-TEMPO/HNO3 /HCl

8

3S

9

Conversion product (%) (Yield, %)

CH3 CN (MeCN) MeCN:H2O

66

2P(∼50) 1P(<5) 1P(64)

98

1P(90)

MeCN:H2O

74

1P(68)

MeCN

89

1P(83)

45 °C, 24 h

MeCN:H2O

100

1P(95)

O2

45 °C, 24 h

MeCN:H2O

–

3P(87)

AcNH-TEMPO/HNO3 /HCl

O2

45 °C, 24 h

MeCN:H2O

–

4P(91)

1S

TEMPO/NaNO2

O2

19 h

DCM

–

1P(53)

10

10S

TEMPO/NaNO2

O2

19 h

DCM

–

8P(96)

11

7S

TEMPO/NaNO2 /NaCl

O2

19 h

–

–

5P5P(97)

12

8S

TEMPO/NaNO2 /NaCl

O2

19 h

–

–

6P(75)

13

9S

TEMPO/NaNO2 /NaCl

O2

19 h

–

–

7P(84)

14

1S

TEMPO/DAIB

MeCN

>99

2P(78)

15

10S 11S 12S 13S 14S 15S 16S 17S

0.1 mmol VOSO4 0.1 mmol TEMPO

O2

Room temp., 7h 100 °C 12 h 0.4 Mpa

MeCN

CuCl/TEMPO 2,6-lutidine

O2

100, 40 h

Toluene

CuCl/TEMPO 2,6-lutidine

O2

60 °C, 16h

Ref.

[120] [120] [120] [120] [120] [120] [120] [120] [124] [124] [105] [105] [105]

17

18

10S

100, 40 h

[121]

Toluene

40

67

8P(82) 9P(98) 10P(80) 11P(82) 12P(85) 13P(90) 14P(82) 17P(12) 20P(9) 19P(3) 18P(<2) 20P(18) 21P(14)

[128]

[130]

[130]

72

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

Fig. 9. Aerobic oxidation mechanism for alcohol using TEMPO/HCl/NaNO2 catalytic system (Adapted from Ref. [149] with permission of WILEY-VCH).

alone (Table 4, entries 5 and 6). Presence of the NOx sources, such as NaNO2 and HNO3 as the recyclable dioxygen activators, appears to be favorable for an aerobic metal-free oxidation of lignin. The mechanism of alcohol oxidation via TEMPO/HCl/NaNO2 was proposed by Wang et al. and shown in Fig. 9 [149]. It was found that hydrochloric acid, an inexpensive and readily available inorganic acid, cooperated exquisitely with NaNO2 /TEMPO for facilitating the aerobic oxidation of the substrate to the corresponding aldehyde under mild conditions. According to the suggested hypothesis, the acidic reaction conditions and the existence of chlorine or bromine, whether positive, negative or neutral, is crucial for obtaining the overall catalytic activity of the TEMPO/NaNO2 catalyst. The catalyst system consisting of 4-acetamido-TEMPO (5 mol%; AcNH-TEMPO) in combination with HNO3 and HCl (10 mol% each) (Table 4, entry 6) showed very good selectivity for secondary alcohol oxidation. Comparison of the reactions of benzylic alcohols showed that electron-rich substrates reacted more rapidly and led to higher yields of ketones [120]. Stahl and co-workers [120] also reported excellent yields of ketone for the oxidation of the S-type models with or without phenols (Table 4, entries 7 and 8). It was shown that the presence of acidic O–H group in phenolic lignin structure could lead to catalyst inhibition [150]. Moderate to excellent yields of ketones 1P (53%) and 8P (96%) were obtained from the oxidation of lignin models 1S and 10S using TEMPO/NaNO2 /O2 in the presence of dichloromethane (DCM) (Table 4, entries 9 and 10) [124]. Encouraging results were reported for the aerobic oxidation of lignin models using TEMPO/NaNO2 /NaCl (Table 4, entries 11–13), where the corresponding ketones 5P,6P and 7P were produced in 97%,75% and 84% isolated yield, respectively. However, this method was not successful for the oxidation of components containing free phenolic hydroxyl groups where they converted to insoluble products [105]. The proposed method may not be efficient on native lignin owing to the presence of unprotected phenolic hydroxyls in lignin. Moreover, bromination of the highly electron-rich aromatic ring using TEMPO/Br2 /NaNO2 and TEMPO/1,3-dibromo-5,5dimethylhydantoin/NaNO2 systems was reported [105]. A transition metal free catalytic system containing TEMPO and (diacetoxy)iodobenzene (DAIB) was recently reported for oxidation of primary hydroxyl groups over secondary ones (Table 4, entry 14) [121]. No product resulting from the secondary alcohol oxidation was observed confirming the high chemoselectivity of oxidation.

6.1.2. TEMPO catalytic systems containing transition metal components TEMPO catalytic systems containing transition metals, e.g., Cu, Fe and V, have emerged as effective systems for alcohol oxidation [136,151–153]. Wang and co-workers [128] used β –O–4 lignin models with Cβ –OH and Cγ –OH bonds to test the performance of vanadyl sulphate (VOSO4 )/TEMPO catalyst system (Table 4, entries 15 and 16), which were introduced for alcohol oxidation [154]. VOSO4 exhibited a higher oxidation activity than other vanadium compounds, e.g., V2 O5 , VOHPO4 and NaVO3 . It was suggested that the oxidation process probably occurred through a non-radical pathway. V(IV) in the presence of TEMPO and acetonitrile (CH3 CN) was oxidized by dioxygen and a V(V) complex was formed, which resulted in alcohol oxidation [154]. Cu/TEMPO catalysts are one of the practical systems for alcohol oxidation. As one of the earliest works, the application of CuCl/TEMPO for the oxidation of primary benzylic and allylic alcohols was reported by Semmelhack and co-workers [155]. Later Stahl and co-worker [150] used a Cu(I) salt with a noncoordinating anion (CuOTf, where OTf is trifluoromethanesulfonate). The catalyst containing (bpy)Cu(I)/TEMPO (where bpy is 2,2ʹ-bipyridine) was effective in the oxidation of alcohols (Fig. 10). A two-step mechanism including catalyst oxidation and substrate oxidation has been proposed. The oxidations of Cu(I) and TEMPOH by O2 taking place in the first step and alcohol oxidation mediated by TEMPO radical and Cu(II) was processed in the second step. This system applies for the oxidation of a broad range of primary alcohols, including allylic, benzylic, and aliphatic derivatives to generate aldehydes. The aforementioned system has some advantages: (i) the reaction can be carried out at room temperature with ambient air as the oxidant, (ii) system is compatible with a wide range of functional groups, (iii) the catalytic system affords a high selectivity for primary alcohols, and (iv) the reactions show excellent selectivity for primary over secondary alcohols without overoxidation of aldehyde to carboxylic acid [156]. The catalyst containing CuCl/TEMPO was applied for the oxidation of lignin models bearing primary and secondary alcohols (Table 4, entries 17 and 18) [131]. As seen in Table 4 (entries 1– 3 and 8), the lignin model compounds are mostly oxidized at the secondary position and clearly the number of reports devoted to the oxidation of primary hydroxyl group over the secondary one is limited.

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

73

Fig. 10. Aerobic oxidation mechanism for alcohol using (bpy)Cu(I)/TEMPO catalytic system (Adapted from Ref. [156] with permission of American Chemical Society).

6.2. TEMPO oxidation of lignin models followed by bond cleavage Reactions could occur in series for producing lignin-based value-added materials. In this case, an intermediate lignin based product, e.g., ketone, could be converted to generate valuable materials, such as acids [157–159]. In these conversions, a bond cleavage reaction of lignin is needed. A selective oxidation of β –O–4 lignin model into more labile ketone or aldehyde that could be further oxidized into low molecular weight aromatic products was proposed in a two-step oxidative depolymerization of lignin in Fig. 11 [122,123,160]. A functionalized lignin compound is produced as an intermediate product, which is followed by cleavage of the bonds to form fragments. In such a process, TEMPO is often used in the first step to convert the alcohol to ketone or aldehyde.

6.2.1. Bond cleavage initiated by oxidized ketone Such a strategy was used by Wang et al. [128] to produce aromatic acids from a model lignin containing β –O–4 linkage (Table 5, entry 1). In the proposed protocol, the oxidation of lignin model using VOSO4 /TEMPO catalyst afforded β –O–4 ketone (Table 4, entries 17 and 18). In the second step, ketone oxidation using a Cu(OAc)2 /1,10-phenanthroline complex results in C–C bond cleavage. The proposed reaction mechanism is shown in Fig. 12. The copper-oxo-bridged dimer generated from the reaction of Cu(OAc)2 /1,10-phenanthroline with oxygen was the catalytically active site for hydrogen-abstraction from Cß –H bond, which was the rate-determining step for the C–C bond cleavage [128]. A wide range of β –O–4 ketones, e.g., ketones with and without methoxy groups and ketones with Cγ –OH, which were formed in the first step, was successfully transformed to acids with yields of 80–95%. The yield of acids and reaction conditions are affected by the β –O–4 ketone’s structure. For example, Cγ –OH bond of the ke-

tone makes the Cα –Cβ bond more resistant and therefore a higher temperature is needed for the reaction. Phenolic β –O–4 ketone also causes a lower yield of acid. Notably, the proposed catalyst was not efficient for bond cleavage of β –1 lignin and the major products resulted from C–H bond oxidation [32,128]. Metal-free aerobic alcohol oxidation using TEMPO in combination with an inorganic nitrogen oxide, such as nitric acid and sodium nitrite, could also be used for the aforementioned purpose [94,120]. Targeting the bond cleavage of the benzylic ketone (1P) (see Table 4, entries 1a–1h), H2 O2 under basic conditions can be used in the second step where veratric acid (34P) and guaiacol (35P) are the final products (Table 5, entry 2) [122]. The treatment performed with excess sodium formate in aqueous formic acid (85–90 wt%) was also effective for C–O bond cleavage (Table 5, entry 3). Considering the difference in the raw materials, it can be concluded that the presence of Cα ketone is important for sufficient bond cleavage [122]. The mechanism of lignin depolymerization using formic acid was explained by Wang [161] in which formic acid plays two roles; first as a substrate, where the hydroxyl group of oxidized lignin is converted to a better leaving formate group and second its formate anion and its proton are used for Hβ abstraction and protonation of Cβ =Cγ , respectively. Photochemical reactions gained attention in the subject of lignin conversion [123,162,163]. In this approach, the excited electron–hole pairs are generated upon light irradiation of a wide-band gap semiconductor, which can be applied in chemical processes to create/degrade specific compounds [164]. There are several reports regarding the bond cleavage using photochemical reactions [163,165,166]. In one report, the chemoselective benzylic oxidation utilizing [4-AcNH-TEMPO]BF4 and silica was performed in the first step, and the fragmentation products were generated in high yield from C–O bond cleavage via the photocatalyst [Ir(ppy)2 (dtbbpy)]PF6 (Table 5, entries 4–6) and in the subsequent step [125].

74

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

Fig. 11. Two-step TEMPO oxidative depolymerization of β –O–4 lignin into aromatic molecules.

6.3. TEMPO oxidation of lignin models with bond cleavage

Fig. 12. Reaction mechanism for two-step catalytic conversion of lignin in Wang’s study (Reprinted from Ref. [128] with permission of American Chemical Society).

6.2.2. Bond cleavage initiated by oxidized aldehyde A retro-aldol reaction was reported for bond cleavage of product 2P generated in situ from a lignin model compound (Table 5, entries 7–11). Product 2P was obtained from the oxidation of primary hydroxyl groups using TEMPO/DBID catalytic system (see Table 4, entry 14). Dl-proline as an organocatalyst was used to effect the bond cleavage of the product afforded in the first step. The benzaldehyde derivatives, methoxyphenols and 2aryloxyacetaldehydes were obtained from bond cleavage step.

Efficient depolymerization methods must be able to cleave the strong C–O and/or C–C bonds and tolerate a large variability of lignin structures derived from different types of biomass [167,168]. Metal complex catalysts are commonly used for this purpose [142,169,170]. A copper(II)/TEMPO catalyst was developed by Gamez et al. [170] for the oxidation of primary alcohols to aldehydes. Benzyl alcohol was converted to benzaldehyde (100% conversion) after 2.5 h of reaction using a CuBr2 (Bipy)-TEMPO catalytic system. It was stated that the presence of ligand in the copper catalytic system was an important factor affecting the conversion rate of substrates [136,137]. For example, only 6% of benzaldehyde (53P) was obtained from benzyl alcohol in a CuBr2 -TEMPO catalytic system (no ligand) in 1.5 h of reaction. The poor solubility of the copper catalyst was proposed as a main reason for this low conversion. Notably, the conversion rate increased to 83% in the presence of 2,2-bipyridine as a ligand. In this type of reaction, TEMPO is proposed to serve as a hydrogen acceptor. A possible pathway for the reaction involves coordination of TEMPO to copper, followed by a hydrogen atom abstraction [129]. Contrarily, no reaction occurred for secondary alcohols in the same catalytic system due to steric hindrance resulting from presence of methyl groups in secondary alcohol structure [136,170]. Progress has been made in alcohol conversions using catalysts containing copper for lignin conversion. Bond cleavage was observed in treatment of 1,2-diphenyl-2-methoxyethanol (19S), benzoin methyl ether (24P) and 1-(3,5-dimethoxyphenyl)-2-(2methoxyphenoxy) propane-1,3-diol (6S) by CuCl/TEMPO in pyridine system (Table 6, entries 1 to 4)). It has been proposed that a single electron transfer pathway operates in the copper/TEMPO system [129,171]. Direct C–C bond cleavage of lignin model 19S

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

75

Table 5 Examples of processes for TEMPO oxidation of lignin followed by bond cleavage. Entry

Raw materials

oxidation (first step) conditions

1

10S

VOSO4 /TEMPO (see 8P Table 4, entry 15)

2

1S

(see Table 4, entries 1–6)

1P

3

1S

1P

4

14S

(see Table 4, entries 1–6) [4-AcNH-Tempo]+ BF4 − , silica, DCM, room temp, 15 h

5

18S

[4-AcNH-Tempo]+ BF4 − , silica, DCM, room temperature, 18 h

22P

6

1S

[4-AcNH-Tempo]+ BF4 − , silica, DCM, room temperature, 15 h

1P

7

1S

(see Table 4, entry 14)

-

8

1S

2P

9

2S; erythro: threo = 1.89: 1 20S erythro: threo = 1.6: 1

2P

5S

2P

10

11

Intermediate Product

12P

2P

Bond cleavage (second step) conditions

Final product / Conversion (Yield, %) (%)

Copper(II) acetate/ 10-Phenanthroline, MeOH, O2, at 0.4 MPa, 3 h, 80 °C NaOH/H2 O2 , THF:MeOH(1:1), 10 h., 50 °C HCO2 H:H2 O, HCO2 Na,12–15 h. 110 °C Photocatalyst[Ir(ppy)2 (dtbbpy)]PF6 , DIPEA, HCO2 H, visible light, MeCN, room temp, 16 h

32P(95) 33P(85)

94

34P(88) 35P(42)

100

Photocatalyst[Ir(ppy)2 (dtbbpy)]PF6 , DIPEA, HCO2 H, visible light, MeCN, room temperature, 20 h Photocatalyst[Ir(ppy)2 (dtbbpy)]PF6 , DIPEA, HCO2 H, visible light, MeCN, room temperature, 14 h dl-proline, 5 h, room temp.

36P(96) 34P(87) 45P(85) 35P(83)

46P(81) 35P(80)

–

47P(84) 35P(93)

–

48P(70) 35P(26) 49P(5) pyrrolidine 23P(58) dl-proline, 5 h, room temp. 48P(25) 34P(10) dl-proline, 10 h, room 50P(68) temp. 35P(28) dl-proline, room temp. 48P(60) 51P(16) 52P(32) 41P(15) 35P(5) 49P(6)

was reported without generation of ketone intermediate [172]. Notably, pure oxygen atmosphere and high catalyst loading are vital to achieve a high conversion rate in the aforementioned system. However, the instability of the active copper catalytic species in these systems necessitates the requirement for multiple additions of CuCl/TEMPO during reaction. The oxidation of 8P with CuCl2 /TEMPO results in the C–C bond cleavage (Table 6, entry 5). An increase in the amount of TEMPO in the system decreased the conversion and yield of by-products, suggesting that the reaction was governed by a free-radical mechanism [137,173]. Reactivity of phenolic and non-phenolic β –O–4 lignin models with [Cu(OTf)]/ 2,6-lutidine/TEMPO was investigated by Baker [126]. The selective bond cleavage of the Cα -Caryl bond was reported for phenolic β –O–4 lignin model (Table 6, entries 6 and 7). Oxidation of non-phenolic lignin model in 2,6-lutidine mediated by stoichiometric CuOTf/TEMPO afforded a high yield of aldehyde (Table 6, entries 8 and 9). However, the formation of undefined by-products limits its utility in reactions. Hanson and coworkers [127] used β –1 linked diols as a substrate to further investigate the potential of aerobic oxidation of lignin with copper/TEMPO catalyst systems. Nonphenolic and phenolic models were chosen to simulate β –1 lignin structure [127]. Under the treatment with CuOTf/2,6-lutidine/TEMPO in toluene (Table 6, entry 10), the nonphenolic lignin model compounds generated products resulting from C–C bond cleavage. The oxidation of phenolic lignin using the stoichiometric copper leads to fragments (Table 6, entry 11) while utilizing catalytic copper (Table 6, entry 12) afforded ketone (30P) as a major product. As previously described, the copper cat-

98

Remark

Ref.

Not efficient for bond cleavage of

[128]

Commercial acids as the final products

[120]

–

[122]

Safe process conducted at room temperature. Need for superstoichiometric additives is a drawback of method. Moreover, second step includes the use of expensive Ir-based photosensitizers (photo)catalysts.

[123]

A one-pot two-step reaction to bond cleavage of lignin model compounds with β –O–4 linkages.

[121]

β –1 lignin

alyst is inhibited by the phenolic functional groups. Therefore, the requirement of stoichiometric amount of copper to facilitate C–C bond cleavage can be considered as one drawback of this catalyst system [127]. Treatment of 1:1 mixture of β –1 and β –O–4 lignin model compounds with catalytic amount of copper (Table 6, entry 13) resulted in aldehyde as the major product along with ketone in low yield. The analysis revealed that the β –1 lignin model reacts slightly faster than the β –O–4 lignin model with the copper catalyst. 7. TEMPO oxidation strategies for technical lignin Most TEMPO oxidation strategies have been employed for the oxidation of lignin models. Currently, limited information is available for TEMPO oxidation of technical lignins, which is discussed in this section. In a study performed by Diaz-Urrutia et al. [15], the Organosolv lignin was subjected to different catalyst systems such as CuOTf/TEMPO and 4-acetamido-TEMPO/HNO3 /HCl, and the performance of the catalytic systems was reported according to the quantitative heteronuclear single quantum coherence (q-HSQC) and gel permeation chromatography data (Fig. 13 and Table 7). The catalyst breaks the major units of A, B, C and D, oxidizing S2/6 to Sʹ and Sʺ in which (Sʹ+Sʺ)/S2/6 increased from 0.26 (in the control sample with no catalyst) to 0.73. The results provided in Table 7 reveal that CuOTf/TEMPO/2,6lutidine produces the ketones with a lower molecular weight (413 g/mol). Comparison between entries 2 and 4 (Table) suggests

76

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

Table 6 TEMPO catalytic systems used for oxidation of lignin model compounds with C–C bond cleavage. No

substrate

Catalyst system compounds

Experimental condition

Conversion (%)

Product (Yield, %)

1

19S

100 °C, 48 h

92

53P(84) 54P(88)

[129]

2

24P

CuCl (20 mol%) and TEMPO (30 mol%) Solvent: Pyridine Oxidant: O2 10 mol% CuCl/ TEMPO Solvent: Pyridine Oxidant: O2

100 °C, 18 h

87

[129]

3

6S

Stoichiometric reaction of CuCl/TEMPO Solvent: Pyridine Oxidant: O2

100 °C, 18 h

70

4

6S

stoichiometric reaction of CuCl/TEMPO Solvent: Pyridine Oxidant: O2

100 °C, 40 h

89

5

8P

24 h

47.9

6

3S

100 °C, 18 h

100

7

3S

CuCl2 /TEMPO pyridine as a ligand and BF3 •Et2O as an additive Solvent: Methanol Oxidant: Open air Stoichiometric CuOTf/TEMPO 2,6-lutidine Solvent: Toluene Oxidant: O2 10 mol%CuOTf /TEMPO/ 2,6-lutidine Solvent: Toluene Oxidant: O2

54P(85) 32P(76) 53P(5) Methanol/5 55P(36) 56P(9) 35P(5) 25P(2) 26P(1) 55P(43) 56P(13) 35P(7) 25P(2) 26P(7) 54P(22) 57P(6) 33P(16)

100 °C, 18 h

98

8

6S

CuOTf/TEMPO 2,6-lutidine Solvent: Toluene Oxidant: O2

18 h

99

9

6S

CuOTf/TEMPO 2,6-lutidine Solvent: Toluene Oxidant: O2

40 h

95

10

21S

100 °C, 48 h

100

11

22S

100 °C, 18 h

100

12

21S

10 mol%CuOTf /10 mol% TEMPO/ 2,6-lutidine Solvent: Toluene Oxidant: O2 Stoichiometric CuOTf/TEMPO 2,6-lutidine Solvent: Toluene Oxidant: O2 10 mol%CuOTf /10 mol% TEMPO/ 2,6-lutidine Solvent: Toluene Oxidant: O2

100 °C, 18 h

100

13

21S+6S

20 mol% CuOTf /20 mol% TEMPO/ 2,6-lutidine Solvent: Toluene Oxidant: O2

100 °C, 24 h

80% for β –1 model And 56% for β -O-4 model

that radical processes, not mediated by the metal, are contributing to the conversion process. Attempts for oxidation of organosolv Alcell lignin, which was rich in syringl units and a high proportion of β –O–4 linkages, resulted in only 36% of lignin oxidation [128]. Dabral et al. [121] reported one-pot two-step method for degradation of organosolv beechwood lignin (OS-BWL) (Table 8, entry 1). The catalyst system containing TEMPO/DAIB in CH3 CN (a similar catalyst system reported for lignin models, see Table 5, entries 7–11) was used for chemoselective oxidation followed by degradation using DLproline. The organic fraction (15 wt% of the starting lignin) was analyzed by gas chromatography-mass spectrometry (GC-MS) in

59P(46) 60P(21) 30P(5) 58P(9) 59P(22) 60P(12) 30P(44) 29P(7) 55P(62) 35P(6) 28P(10) 25P(3) 26P(5) 27P(16) 55P(54) 35P(10) 28P(5) 25PS(7) 26P(5) 27P(18) 28P(2) 55P(81) 61P(69) 56P(1) 59P(82) 62P(9) 39P(6) 61P(25) 30P(78) 59P(8) 62P(5) 39P(2) 61P(4) 55P(75) 61P(32) 25P(8)

Ref.

[129]

[129]

[137]

[126]

[126]

[126]

[126]

[127]

[127]

[127]

[127]

which various monomeric aromatics, e.g., 0.3 wt% guaiacol (35P), 2.5 wt% syringaldehyde (39P) and 1.1 wt% vanillin (40P) were detected. Very recently, the same group studied the structural changes in OS-BWL subjected to the mechanochemical oxidative transformation in the presence of HO–TEMPO (0.2 equiv), KBr (0.2 equiv), and Oxone (1.5 equiv) [139]. The two-dimensional (HSQC) NMR spectra (see Fig. 14) from the untreated and mechanochemical treated OS-BWL samples presented the substantial changes in the aliphatic regions, where integrals values corresponding to the β – O–4 aryl ether linkages and phenylcoumaran substructures were diminished. The treatment after 90 min resulted in 40% benzylic

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

77

Table 7 Gel permeation chromatography data for different catalyst systems [15]. Entry

Experiment

Mw (Da)

Mn (Da)

PDI

1 2

Organosolv lignin CuOTf/TEMPO/ 2,6-lutidine Solvent: DMF Temperature: 100 0 C Reaction time: 18 h Control in DMF (no catalyst) for Entry 2 Control TEMPO and 2,6-lutidine (no Cu) for Entry 2 4-acetamido-TEMPO/HNO3 /HCl Solvent: 19: 1 (v/v) CH3CN/H2O Temperature:65 0 C Reaction time: 24 h Control (no 4-acetamido-TEMPO) for 5

2526 413

850 229

2.97 1.80

817 493

347 239

2.36 2.06

502

330

1.52

606

452

1.34

3 4 5

6

oxidation with increased integrals for the cross peaks corresponding to oxidized syringyl S and guaiacyl G units. The degree of oxidation also increased to 84% after 180 min milling. The analysis of the organic-soluble phase was presented in Table 8, entry 2. These monomers resulted from the oxidation of terminal phenolic groups present in the lignin sample. It is notable that, by using a vibrating disk mill, a wider range of monomeric products was obtained in less time (30 min) (Table 8, entry 3) [139]. A method for the oxidation of Cα alcohols to ketones in technical lignin using AcNH- TEMPO/HNO3 /HCl catalyst system resulted in a 90% conversion to oxidized lignin [120]. Following this progress in lignin oxidation, a two-step protocol was examined for technicallignin degradation [122]. The method was first examined on Poplar lignin isolated from an enzymatic pretreatment method (Table 8, entry 4). The oxidized lignin afforded 51.2% yield of low molecular-weight aromatics, while the yield decreased to approximately 10 wt% for non-oxidized lignin [122]. Recently, the same research group applied the above-mentioned method on divers lignin samples derived from different plant sources (poplar, maize, and maple) and pretreatment methods (see Table 8, entries 5– 12) [140]. The pretreatment methods included a mild acidolysis pretreatment employing HCl/dioxane (acidolysis), extraction with γ -valerolactone containing dilute sulfuric acid (GVL), an extractive ammonia process (EA), and a copper-alkaline hydrogen peroxide (Cu-AHP). Comparing to the conventional processes, such

as kraft which extensively altered lignin structures, these GVL, EA and Cu-AHP pretreatment methods have been known to generate Native-like lignins. A variety of low-molecular-weight aromatics with various yields ranging from 3–42%, depending on the lignin source and pretreatment method, was generated after the reaction [140]. These compounds can be used as the primary components of phenol-resins, including resorcinol-formaldehyde-resins. Production of vanillin (41P) is of great interest in the proposed method, as it is the only lignin-derived building block used to create bio-based amines from lignin [174]. Nowadays, such macromolecules are used in different fields, such as biofuel and energy storage [175]. The observations revealed that the loss of the β -ether subunit resulted from pretreatment process or presence of components (for example, hydroxycinnamate components with alkenes), which could lead to the inhibition of radical NOx -based cocatalysts, affected the catalytic cleavage of lignin, resulting in a lower yield of aromatics.

8. Novel potential routes for TEMPO oxidation of lignin compounds As stated earlier, alcohol oxidation is a key reaction in lignin oxidation. In view of recent developments in alcohol oxidation using novel catalysts, it could be favorable to combine these with

Table 8 TEMPO catalytic systems used for oxidation of technical lignin. Entry

Lignin type

Experimental conditions

Final product (Yield, %)

Remark

1

Organosolv beech wood lignin

First step: TEMPO/DAIB, Second step: dl-proline, 5 h, room temp

A one-pot two-step reaction (see Table 4, entry 14)

[121]

2

Organosolv beech wood lignin

HO–TEMPO KBr, and Oxone. 180 min milling using a mixer mill

35P (0.3) 39P (2.5) 41P (1.1) 59P(2.5) 58P(0.5)

Detection of organic-soluble fraction by gas chromatography with a flame ionization detector (GC-FID). Yield of products is with respect to the starting lignin.

[139]

3

Organosolv beech wood lignin

HO–TEMPO KBr, and Oxone. 30 min milling using a vibrating disk mill

4

Aspen lignin

First step: AcNH-TEMPO/HNO3 /HCl Second step: HCO2 H:H2 O, HCO2 Na, 12–15 h. 110 °C

35P(0.3) 58P(0.9) 41P(3.2) 59P(2.6) 39P(6.5) 63P(2.9) 37P(13.1) 38P(6.7) 39P(8.5) 40P(7.9) 41P(3.5) 42P(2.9) 43P(5.2) 44P(4.0)

Ref.

[139]

Two-step protocol (see Table 4, entry 6 and Table 5, entry 3)

[122]

(continued on next page)

78

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

Table 8 (continued) Entry

Lignin type

Experimental conditions

Final product (Yield, %)

5

Poplar lignin (mild acidolysis)

First step: AcNH-TEMPO/HNO3 /HCl Second step: HCO2 H:H2 O, HCO2 Na, 24 h. 110 °C

6

Poplar lignin (Cu-AHP)

First step: AcNH-TEMPO/HNO3 /HCl Second step: HCO2 H:H2 O, HCO2 Na, 24 h. 110 °C

7

Poplar lignin (GVL-extracted)

First step: AcNH-TEMPO/HNO3 /HCl Second step: HCO2 H:H2 O, HCO2 Na, 24 h. 110 °C

8

Maple lignin (mild acidolysis)

First step: AcNH-TEMPO/HNO3 /HCl Second step: HCO2 H:H2 O, HCO2 Na, 24 h. 110 °C

9

Maple lignin (GVL-extracted)

First step: AcNH-TEMPO/HNO3 /HCl Second step: HCO2 H:H2 O, HCO2 Na, 24 h. 110 °C

10

Maize lignin (mild acidolysis)

First step: AcNH-TEMPO/HNO3 /HCl Second step: HCO2 H:H2 O, HCO2 Na, 24 h. 110 °C

11

Maize lignin (GVL-extracted)

First step: AcNH-TEMPO/HNO3 /HCl Second step: HCO2 H:H2 O, HCO2 Na, 24 h. 110 °C

12

Maize lignin (EA)

First step: AcNH-TEMPO/HNO3 /HCl Second step: HCO2 H:H2 O, HCO2 Na, 24 h. 110 °C

37P(10.2) 39P(6) 40P(6.3) 38P(6.6) 41P(2.8) 42P(2.5) 44P(5.7) 64P(1.5) 43P(1.6) 37P(7.9) 39P(4.8) 40P(5.1) 38P(5.1) 41P(2) 42P(2) 44P(2.3) 64P(1.5) 37P(4.5) 39P(3.8) 40P(3.2) 38P(2.7) 41P(1.3) 42P(1.1) 44P(4) 64P(1.7) 32P(0.2) 43P(1.1) 37P(2.1) 39P(1.1) 40P(0.7) 38P(1.5) 41P(1.2) 42P(0.7) 43P(0.8) 37P(1.4) 39P(0.6) 40P(0.5) 38P(1) 41P(0.9) 42P(0.3) 43P(0.5) 37P(2) 39P(1.7) 40P(2.2) 38P(1.9) 41P(2.2) 42P(2.4) 65P(1) 32P(0.2) 43P(1.3) 66P(2) 37P(2.9) 39P(2.4) 40P(1.6) 38P(1.6) 41P(1.7) 42P(2) 65P(0.7) 32P(0.2) 43P(0.4) 66P(1.6) 37P(0.7) 39P(0.7) 40P(0.4) 38P(0.4) 41P(0.8) 42P(0.3) 44P(0.3) 65P(1.4) 43P(0.9)

Remark

Ref.

[140]

[140]

[140]

[140]

[140]

[140]

[140]

[140]

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

79

Fig. 13. q-HSQC NMR spectra (500 MHz; DMSO-d6) of organosolv lignin for (a) control experiment (no catalyst, TEMPO or base) and (b) catalytic oxidation with 10 wt% CuOTf, 10 wt% TEMPO and 100 wt% 2,6-lutidine. For all runs: solvent = DMF; temperature = 100 °C; pressure of synthetic air = 8.2 atm; reaction time = 18 h. Some of the correlations may be affected by paramagnetism of copper(II) traces (Reprinted from Ref. [15] with permission of Royal Society of Chemistry).

conventional TEMPO catalytic systems. This section aims to draw the researchers’ attention to these new strategies. 8.1. Implementation of ionic liquids To simplify product isolation and catalyst recovery, the use of binary supported catalysts or ionic liquids (ILs) offers an alternative [176,177]. Recently, various studies on the alcohol oxidation

have been directed toward the use of IL-supported TEMPO catalysts [178–180]. ILs are organic salts with low melting points and very low vapor pressures [181,182]. Most ILs are viscous liquids, which sometimes hampers mass transfer in reaction systems [183]. However, the possibility to tune its polarity makes the IL a favourable medium for synthesis [176,184,185]. The acetamido-TEMPO/Cu(ClO4 )2 /DMAP (where DMAP is 4Dimethylaminopyridine) catalyst system in IL solvent was applied

80

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

Fig. 14. Analysis of the mechanochemical oxidation reaction of OS-BWL by 2D-HSQC NMR spectroscopy: (left) aliphatic region; (right) aromatic region in DMSO-d6. (a) Untreated OS-BWL; (b) residual oxidized OS-BWL after 90 min of milling at 30 Hz in the presence of the oxidant (HO–TEMPO/KBr/Oxone). (A) β –O–4 aryl ether linkages with a free –OH at the α -carbon; (B) resinol substructures formed by β –β  -, α –O–γ  -, and γ –O–α  - linkages; (C) phenyl coumaran substructures formed by β –5 - and α –O–4 - linkages; (S) syringyl units; (S ) syringyl unit with oxidized benzylic position; (G) guaiacyl units; (G ) guaiacyl unit with oxidized benzylic position (Adapted from Ref. [139] with permission of American Chemical Society).

for oxidation of primary alcohols to aldehydes [186]. High yields (>70%) of ketones and aldehydes could be obtained through the aerobic oxidation of primary and secondary alcohols using the TEMPO/CuCl catalytic system in 1-Butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), one of the most widely used ILs), in which no trace of overoxidation to carboxylic acids was observed [187]. Following this reaction pathway, benzylic and allylic alcohols were oxidized with excellent conversions (>90%), whereas aliphatic alcohols were slow and less reactive [187]. Ox-

idation of lignin model compounds (1S, 10S, 25S and 26S) using TEMPO/NaNO2 /HCl/O2 catalytic system in ILs was also reported [124]. Notably, comparing to the conventional solvents (see Table 4, entries 4, 11 and 12), the oxidation rates were lower in the IL solvents [124]. One of the problems associated with using ILs in lignin oxidation includes difficulty in the separation of aromatic products derived from lignin. It was stated that the aromatic products can often be soluble in the ionic liquid due to the π -π interactions between the ionic liquid and aromatic moieties, which hampers the

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

81

Fig. 15. TEMPO-derived task-specific IL implemented for alcohol oxidation (Adapted from Ref. [183] with permission of Elsevier).

Fig. 16. (a) synthesis of ammonium-tagged TEMPO derivative and (b) its implementation for oxidation of cinnamyl alcohol (Adapted from Ref. [188] with permission of WILEY-VCH).

extraction of aromatic products from the reaction mixture [24]. In addition, separation and recovery of TEMPO must be considered. Difficulties in recycling the TEMPO in presence of the IL originate from TEMPO leaching into the organic solvent extraction medium. In this regard, utilizing TEMPO derivatives containing polar functionalities has been suggested [188]. ILs can also be functionalized for specific applications, known as “task-specific ILs” [189]. TEMPOderived task-specific IL (TEMPO/IL) has been implemented as a catalyst in the oxidation of primary and secondary alcohols [178,183]. Metal-free catalysts NaOCl/KBr/H2 O (at pH 8.6) in the presence of TEMPO/IL (Fig. 15) afforded aldehydes and ketones with 86–96% yield [178,183]. In these reactions, the catalysts and oxidant could often be recovered and recycled [178]. Ammonium-tagged TEMPO (Fig. 16) was used for the oxidation of a variety of alcohols in IL; its synthesis is illustrated in Fig. 16(a). This compound was used for oxidation of cinnamyl alcohol in an IL (24S, primary alcohol represented as a monomer lignin model), affording cinnamaldehyde in 99% yield (Fig. 16(b)). Moreover, further experiments revealed that the TEMPO derivative bearing an ammonium moiety is a more effective mediator for alcohol oxidation than TEMPO itself [188]. In this case, the ammonium tag leads to ease of recycling N-oxy radicals in the IL and increasing the oxidation rate [188]. 8.2. TEMPO immobilization Progress in chemistry provided a new class of TEMPO catalysts produced by immobilization onto various supports, e.g.,

silica and polymers, facilitating recyclability [182]. As an example, poly(ethylene glycol)-supported TEMPO (PEG/TEMPO), in which a TEMPO moiety is attached to the polymeric backbone through a benzylic ether linkage, is one of the most studied TEMPO-polymeric catalyst systems (see Fig. 17(a)) [190]. Such catalysts mostly exhibit high solubility in both aqueous and organic solvents and the dissolved polymer-supported catalysts can be recovered by precipitation with a suitable solvent, such as diethyl ether. A combination of PEG/TEMPO and Co(NO3 )2 and Mn(NO3 )2 as co-catalysts used for alcohol oxidation afforded a conversion of mostly higher than 90% [191]. For example, oxidation of cinnamyl alcohol (24S) with a mixture of 2 mol% Mn(NO3 )2 , 2 mol% Co(NO3 )2 and PEG/TEMPO (10 mol%) in acetic acid at 40 °C for 6 h resulted in excellent conversion and selectivity (>99%) (see Fig. 17(b)) [191]. In order to increase the interaction between TEMPO and the surface support, Beejapur and co-workers suggested the modification of TEMPO with an imidazolium tag [133]. In this context, Bis(imidazolium)-tagged TEMPO catalysts were adsorbed on different silica gels, i.e., silica gel modified with highly crosslinked polymeric imidazolium networks (Fig. 18), and magnetic particles entrapped with highly crosslinked polymeric imidazolium networks. A variety of products were obtained with high yields from the oxidation of primary and secondary aliphatic and benzylic alcohols. The combination of catalyst and support acts as a catalyst reservoir, where solvent removal after reaction completion leads to readsorption of catalyst on the support. In this case, solvent extraction of the product did not dissolve the adsorbed catalyst. After 12

82

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

Fig. 17. (a) Synthesis of PEG/TEMPO and (b) its application in alcohol oxidation (Adapted from Ref. [191] with permission of Elsevier).

Fig. 18. (a) Oxidation of alcohols catalyzed by bis(imidazolium)-tagged TEMPO, (b) SEM image of freshly prepared supported catalyst, and (c) SEM image of supported catalyst after 12 cycles (Adapted from Ref. [133] with permission of WILEY-VCH).

cycles, the morphology of the material appeared to be similar to that of the starting material (Fig. 18(b) and (c)). Notably, one of the substrates in the study corresponded to a lignin model compound mimicking the α –1 linkage. Substrate 23S was oxidized using supported catalysts in the presence of [bis(acetoxy)iodo] benzene (BAIB) in DCM at room temperature. The catalysts afforded an excellent yield (95%) of product 31P [133].

Table 9 The catalytic activity of TEMPO and its derivatives [199]. Catalyst

(ipa /ipc )

Catalyst

(ipa /ipc )

TEMPO TEMPO-oxo TEMPO-OH

13.33 0.1 29.14

TEMPO-COOH 4-Amino-TEMPO 4-Acetamido-TEMPO

3.88 42.37 28.51

8.3. Electrocatalytic oxidation Most of the investigations on electrocatalytic TEMPO oxidation were reported for alcohols and not specifically for lignin. Nonetheless, the information provided in the following paragraphs may shed light on TEMPO electrooxidation of lignin. The electrochemical oxidation method provides an alternative to the chemical oxidation of alcohols [192–197]. However, TEMPO exhibits high reactivity with alcohols, and the need for high elec-

trode potentials to generate the reactive oxoammonium species limits its application in electrochemical oxidation [198]. Table 9 summarizes the catalytic activity of TEMPO and its derivatives by means of the ratio of the peak oxidative catalytic current divided by the peak reductive catalytic current (ipa /ipc )cat , which is approx1 imately equal to E −E . a2 a1 Fig. 19 proposes two electrooxidative steps as a mechanism for the electrocatalytic oxidation of alcohols by TEMPO.

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

83

Fig. 19. (a) Proposed catalytic cycle for alcohol oxidation by TEMPO, and (b) The two external oxidation steps, Ea 1 and Ea 2 ; a representative cyclic voltammogram of a nitroxyl radical. (Adapted from Ref. [199] with permission of American Chemical Society).

TEMPO is electrochemically oxidized from a nitroxyl radical T∗ to the oxoammonium cation T+ , i.e., the catalytically active species corresponding to an oxidation potential of Ea 1 . A TEMPO derived intermediate complex is then formed through an alcohol substrate nucleophilic attack on the nitrogen of T+ . In the next step, the aldehyde and reduced hydroxylamine form of TEMPO (TH) are generated via a two-electron oxidation of the substrate. The active form of the catalyst is then electrochemically regenerated by an overall one proton coupled-two-electron oxidation of TH. The second oxidative step occurs at an oxidation potential defined as Ea 2 . Ea 1 is independent of pH, while Ea 2 decreases with the increase in pH. The catalytic activity of TEMPO is proportional to the inverse of the difference in Ea 1 and Ea 2 [199]. A protocol for the TEMPO-mediated electrooxidation of primary and secondary alcohol using a microfluidic apparatus was developed by Hill-Cousins and co-workers [200] and a high conversion of alcohols to aldehydes and ketones was reported in an aqueous tert-butanol (tBuOH) system at ambient temperature. As an example, a high yield of cinnamaldehyde (77%) was obtained with corresponding lignin model 24S. Bosque et al. [132] showed that TEMPO is too weak to abstract the benzylic hydrogen of lignin (BDEC – H = 84 Kcal/mol), which is attributed to its low BDE (71 Kcal/mol). Initial experiments showed a low conversion (maximum 8%) of lignin source14S to product 12P in the bulk electrolysis experiments using TEMPO (and carbonate buffer (pH = 10)/MeCN at potential 1.2 V vs Ag/AgCl as a media) [132]. Considerable effort in the electrochemical oxidation field has shown that a dual catalyst containing TEMPO, as an electronproton transfer mediator, and transition metals, such as copper, could be used to form TEMPO· at much lower potential compared to that needed for TEMPO+ [198]. Badalyan and Stahl [198] reported the oxidation rates of six para-substituted benzyl alcohols with both (bpy)Cu(I)OTf/TEMPO/triethylamine (Et3 N) (Fig. 20) and TEMPO/N-methylimidazole (NMI), where the former exhibited a higher oxidation rate. The electrochemical oxidation of alcohols using (bpy)Cu/TEMPO catalyst proceeds at the (bpy)Cu(II)/ (bpy)Cu(I) redox potential. In the oxidation reaction, Cu(II) and TEMPO act as a one-electron oxidant and an electron-proton acceptor, respectively. The catalyst system exploits the low-potential, proton-coupled TEMPO/TEMPOH redox process rather than the high-potential TEMPO/TEMPO+ process. An interesting development in the use of TEMPO for electrochemical oxidation of alcohols involves the functionalization of electrodes with TEMPO [194]. The crucial factors in developing electrochemical processes are long-term stability, good reactivity, and a broad range of applicability of electrodes to different alcohols. These factors have encouraged a focus on integrating the polymeric, sol–gel and nanostructured electrodes with TEMPO [194,201]. Following the above-mentioned strategies, a thin layer

Fig. 20. Mechanism of (bpy)Cu/TEMPO-mediated alcohol electrooxidation (Adapted from Ref. [198] with permission of Nature Publishing Group).

of organosilica doped with TEMPO electrodeposited on the surface of an ITO-coated glass was used for alcohol oxidation [195,202]. Fig. 21 shows the synthesis process for silane precursor functionalized with TEMPO moiety named “TEMPO@DE” [202]. In the first step, the addition of 3-aminopropyl(trimethoxy) silane to a 4-oxoTEMPO and NaBH3 CN (95%) in MeOH resulted in an organosilane precursor solution. Then, the coated electrode was obtained through stirring a solution containing organosilane precursor solution, methyltrimethoxysilane, ethanol and a buffer solution at pH 4 in the presence of constant negative potential of 1.1 V vs. Ag/AgCl. The electrode can be fabricated with the favorable surface hydrophilicity-lipophilicity balance (HLB) where its positive effects were illustrated by its impact on the oxidation of alcohols in water and inhibition of overoxidation to acids. The TEMPO@DE electrode exhibited excellent selectivity (99%) and good conversion (87%) for cinnamyl alcohol (24S) oxidation affording cinnamaldehyde as the primary product [195]. Despite the advantages associated with use of TEMPO@DE electrode (in terms of high stability and reusability), the process suffers from prolonged reaction times (e.g., 90 h for cinnamyl alcohol oxidation) caused by the low surface area of electrodes and amorphous nature of the organosilica layer. Development of electrodes that provide high accessibility to the active sites can be achieved using ordered mesoporous silica with the desirable pore size. The electro-assisted self-assembly method [203] was reported to prepare ordered mesoporous silica films consisting of hexagonally packed channels of the

84

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

Fig. 21. Synthesis of the TEMPO@DE electrode precursor, which is then poly-condensed electrochemically (Adapted from Ref. [195] with permission of WILEY-VCH).

lignin model 1S resulted in complete and selective oxidation of the primary hydroxyl groups. 9. Challenges and future prospects 9.1. Diversity in lignin structure Fig. 22. Electrochemical oxidation of primary hyroxyl group of lignin models to carboxylic acid.

mesoporous molecular sieve, MCM-41 [204] (Mobil composition of matter number forty-one), perpendicular to the electrode surface. Desirable pore size and channel orientation are two challenging parameters in this approach. The silica channels functionalized with TEMPO afforded a high conversion for benzylic and allylic substrates [205]. Recently, there has been a surge of interest in utilizing nanoparticles for alcohol oxidation owing to their high surface area resulting in high activity under mild conditions. Swiech and co-workers [206] developed a new electrode using the gold nanoparticles decorated by TEMPO with well-defined coverage and narrow size distribution. The catalytic ability of fabricated electrode was evaluated for oxidation of benzyl alcohol in anhydrous acetonitrile and in the presence of 2,6-lutidine as a base. Compared to the TEMPO monolayer electrode with no nanoparticle participating, the TEMPO-gold coated electrode was more efficient for electrocatalytic oxidation, in which its catalytic current was 20 times greater than that for the TEMPO coated electrode [206]. Recently, Stahl and Rafiee [125] employed an electrochemical catalytic oxidation method using TEMPO to selectively oxidized primary hydroxyl groups on β –O–4 linkage to carboxylic acids while the secondary hydroxyl groups remained unchanged (Fig. 22). The oxidation afforded polycarboxylated lignin in the form of a polymeric β -hydroxy acid. Using this method lignin model 17S (as a monomeric 1° alcohol model) completely converted to acid (67P). Similarly, the oxidation of the dimeric β –O–4

Diversity in lignin structure is the most critical challenge in lignin oxidation. Presence of different linkages and aliphatic alcohols in lignin structure affects the types and yields of products. TEMPO oxidation approaches have mostly been applied on lignin models. Therefore, effective oxidation methods for actual lignin streams still need to be developed. 9.2. Chemoselectivity High levels of chemoselectivity are desired in lignin oxidation, but selective oxidation of one alcohol in lignin structure is a challenge. Targeting aldehydes, the major challenge for the oxidation of primary hydroxyl groups over secondary ones is to avoid overoxidation of aldehydes to carboxylic acids that results in reduced catalyst turnovers and consequently a low conversation of the substrate. Understanding the interaction between TEMPO with either other catalytic components presents in the reaction medium or with lignin is difficult. The overoxidized products have also been reported for other polysaccharide materials such as cellulose [207– 209]. In order to give readers insights into the overoxidation of aldehydes to carboxylic acids, the following explanations provide a brief summary of TEMPO oxidation of cellulose to afford overoxidized products. 9.2.1. Cellulose oxidation The conversion mechanism of primary hydroxyl groups of cellulose to carboxylates via aldehydes using TEMPO/NaBr/NaClO catalyst system is presented in Fig. 23. The oxidation process proceeds

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

85

Fig. 23. Regioselective oxidation of C6 primary hydroxyls of cellulose to C6 carboxylate groups by TEMPO/NaBr/NaClO oxidation in water at pH 10–11 (Adapted from Ref. [207] with permission of Royal Society of Chemistry).

under alkaline condition and primary hydroxyl groups are oxidized to carboxyl groups through C6-aldehydes. In the case of native cellulose crystals, the oxidation resulted in the conversion of primary hydroxyl groups to carboxylate groups while the crystallinity was maintained and no water-soluble products were observed. Compared to native cellulose, regenerated and mercerized celluloses are more reactive toward TEMPO oxidation owing to their lower crystallinity index [210]. Almost all C6 primary alcohol groups presented in regenerated cellulose, i.e., the cellulose II allomorph, are oxidized to carboxyl groups, resulting in water-soluble β -(1,4)-dpolyglucuronic acid, named cellouronic acid [211–213]. Cellouronic acid prepared by TEMPO/NaBr/NaClO at pH 10 often have a very low weight-average degree of polymerization (DPw ) [207]. For example, cellouronic acid with DPw of 40 was obtained from oxidation of Bemliese regenerated cellulose [209]. Applied to the same cellulose, 4-acetamide-TEMPO/NaClO/NaClO2 system at pH 4.8–6.8 afforded cellouronic acid with higher DPw values ranging from 220 to 490 [209]. Low DP of oxidized samples especially under an alkaline condition can be attributed to the depolymerization phenomenon due to over oxidation of glucosyl repeating units. Oxidation at secondary hydroxyl groups increases the chance for β -elimination reactions and results in the cleavage of glycosidic bonds [210,212,214]. 9.3. Separation The separation of products from the reaction mixture is a major concern that needs to be addressed in future. In general, separation in heterogeneous systems is simpler than that in homogeneous catalyst systems [215]. It has been reported that the functionalized TEMPO can probably make the separation step easier [188]. 9.4. Implementation of novel methods In addition to the conventional catalysts for lignin oxidation, the novel methods, such as implementation of IL, immobilization of TEMPO onto different supports (e.g., polymers), and electrocatalyst oxidation deserve adequate consideration for lignin oxidation. However, there is insufficient data on the use of these systems on lignin oxidation, and further studies are emphatically recommended. Use of IL offers many benefits; one of them is the relatively easy recovery of the final products. ILs are good candidates

for immobilization of catalytic compounds. The affinity of such compounds towards IL can be enhanced by tuning the polarity of IL. 9.5. Yield and price Despite the great studies on lignin’s potential for producing valuable materials, concerns regarding yield and price of products obtained from lignin modification and depolymerization hamper the extension of processes to commercial scales. The available studies on the oxidation of lignin models showed that the products can be obtained in reasonable yields, while oxidation of technical lignin affords a low yield of low molecular-weight components (as mentioned in Table 8). Oxidation of technical lignin is an active area of research that shows a path for further studies. As a result, more scientific and technological developments are required for making lignin oxidation industrially worthwhile. 10. Conclusions Lignin is available in a large quantity from numerous pulping and biorefinery industries. In order to functionalize and transform lignin to value-added green products, TEMPO catalytic oxidation is a promising method that converts lignin into small molecules or macro-monomers. An efficient TEMPO catalytic oxidation of lignin results in a high yield of a target chemical. The results from depolymerization of diols with ether and C–C bonds at C(β ) position are proposed to forecast the reactivity of authentic industrial lignin samples, providing an important foundation for future efforts focused on selective cleavage reactions of technical lignins by TEMPO depolymerization. Regardless of the types of lignin structure, TEMPO-copper catalytic systems have shown strong affinity for C–C and C–O bond cleavage of a variety of lignin structures. In contrast, the metal-free systems are often used for lignin functionalization. As described, co-catalysts and ligands play vital roles in enhancement of the conversion rates. Most reactions of TEMPO catalytic systems are conducted at temperatures not higher than 100 °C, which can be counted as an advantage. It has been stated that the chemoselectivity can be tailored by pH adjustment. The oxidation of primary hydroxyl groups over secondary ones or vice versa has also been reported. Electrochemical oxidation is highly desirable, in which the generation of waste can be prevented. Promising results have

86

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

been achieved in the electrocatalytic oxidation of lignin monomer using TEMPO-functionalized electrode. The approaches developed for lignin valorization have been vastly improved over past years, opening new avenues towards the production of platform molecules. Undoubtedly, close collaborations between academic and industry scientists are needed to overcome the present obstacles hindering the practicability of TEMPO oxidation of lignin. Conflict of interest The authors declare no conflict of interest. Acknowledgments We would like to acknowledge NSERC, Canada Research Chairs, Northern Ontario Heritage Fund Corporation-Industrial Research Chair program, Ontario Research Fund, and the Canada Foundation for Innovation for funding this research. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.pecs.2019.01.002. References [1] Serrano-Ruiz JC, Luque R, Sepulveda-Escribano A. Transformations of biomass-derived platform molecules: from high added-value chemicals to fuels via aqueous-phase processing. Chem Soc Rev 2011;40:5266–81. [2] Gharehkhani S, Sadeghinezhad E, Kazi SN, Yarmand H, Badarudin A, Safaei MR, Zubir MNM. Basic effects of pulp refining on fiber properties—a review. Carbohydr Polym 2015;115:785–803. [3] Zhang S, Sun J, Zhang X, Xin J, Miao Q, Wang J. Ionic liquid-based green processes for energy production. Chem Soc Rev 2014;43:7838–69. [4] Laurichesse S, Avérous L. Chemical modification of lignins: towards biobased polymers. Prog Polym Sci 2014;39:1266–90. [5] Numan-Al-Mobin AM, Kolla P, Dixon D, Smirnova A. Effect of water–carbon dioxide ratio on the selectivity of phenolic compounds produced from alkali lignin in sub-and supercritical fluid mixtures. Fuel 2016;185:26–33. [6] Shen J, Fatehi P. A review on the use of lignocellulose-derived chemicals in wet-end application of papermaking. Curr Org Chem 2013;17:1647–54. [7] Cavdar AD, Kalaycioglu H, Hiziroglu S. Some of the properties of oriented strandboard manufactured using kraft lignin phenolic resin. J Mater Process Technol 2008;202:559–63. [8] Norgren M, Edlund H. Lignin: recent advances and emerging applications. Curr Opin Colloid Interface Sci 2014;19:409–16. [9] Kleinert M, Barth T. Phenols from lignin. Chem Eng Technol 2008;31:736–45. [10] Tejado A, Pena C, Labidi J, Echeverria J, Mondragon I. Physico-chemical characterization of lignins from different sources for use in phenol–formaldehyde resin synthesis. Bioresour Technol 2007;98:1655–63. [11] Tuck CO, Pérez E, Horváth IT, Sheldon RA, Poliakoff M. Valorization of biomass: deriving more value from waste. Science 2012;337:695–9. [12] Wu W, Dutta T, Varman AM, Eudes A, Manalansan B, Loqué D, Singh S. Lignin valorization: two hybrid biochemical routes for the conversion of polymeric lignin into value-added chemicals. Sci Rep 2017;7:8420. [13] Fatehi P, Chen J. Extraction of technical lignins from pulping spent liquors, challenges and opportunities. In: Fang Z, Smith Jr R, editors. Production of biofuels and chemicals from lignin. Singapore: Springer; 2016. p. 35–54. [14] Kai D, Tan MJ, Chee PL, Chua YK, Yap YL, Loh XJ. Towards lignin-based functional materials in a sustainable world. Green Chem 2016;18:1175–200. [15] Díaz-Urrutia C, Chen W-C, Crites C-O, Daccache J, Korobkov I, Baker RT. Towards lignin valorisation: comparing homogeneous catalysts for the aerobic oxidation and depolymerisation of organosolv lignin. RSC Adv 2015;5:70502–11. [16] Sergeev AG, Hartwig JF. Selective, nickel-catalyzed hydrogenolysis of aryl ethers. Science 2011;332:439–43. [17] Farag S, Chaouki J. Economics evaluation for on-site pyrolysis of kraft lignin to value-added chemicals. Bioresour Technol 2015;175:254–61. [18] Yang Y, Fan H, Meng Q, Zhang Z, Yang G, Han B. Ionic liquid [OMIm][OAc] directly inducing oxidation cleavage of the β –O–4 bond of lignin model compounds. Chem Commun 2017;53:8850–3. [19] De Lasa H, Salaices E, Mazumder J, Lucky R. Catalytic steam gasification of biomass: catalysts, thermodynamics and kinetics. Chem Rev 2011;111:5404–33.

[20] Morgan TJ, Kandiyoti R. Pyrolysis of coals and biomass: analysis of thermal breakdown and its products. Chem Rev 2013;114:1547–607. [21] Zhou C-H, Xia X, Lin C-X, Tong D-S, Beltramini J. Catalytic conversion of lignocellulosic biomass to fine chemicals and fuels. Chem Soc Rev 2011;40:5588–617. [22] Huang X, Korányi TI, Boot MD, Hensen EJ. Ethanol as capping agent and formaldehyde scavenger for efficient depolymerization of lignin to aromatics. Green Chem 2015;17:4941–50. [23] Li C, Zhao X, Wang A, Huber GW, Zhang T. Catalytic transformation of lignin for the production of chemicals and fuels. Chem Rev 2015;115:11559–624. [24] Zakzeski J, Bruijnincx PC, Jongerius AL, Weckhuysen BM. The catalytic valorization of lignin for the production of renewable chemicals. Chem Rev 2010;110:3552–99. [25] Zakzeski J, Jongerius AL, Weckhuysen BM. Transition metal catalyzed oxidation of Alcell lignin, soda lignin, and lignin model compounds in ionic liquids. Green Chem 2010;12:1225–36. [26] Deng W, Zhang H, Wu X, Li R, Zhang Q, Wang Y. Oxidative conversion of lignin and lignin model compounds catalyzed by CeO2 -supported Pd nanoparticles. Green Chem 2015;17:5009–18. [27] Zhu C, Ding W, Shen T, Tang C, Sun C, Xu S, Chen Y, Wu J, Ying H. Metallo-deuteroporphyrin as a biomimetic catalyst for the catalytic oxidation of lignin to aromatics. ChemSusChem 2015;8:1768–78. [28] Ciesielski PN, Resch MG, Hewetson B, Killgore JP, Curtin A, Anderson N, Chiaramonti AN, Hurley DC, Sanders A, Himmel ME. Engineering plant cell walls: tuning lignin monomer composition for deconstructable biofuel feedstocks or resilient biomaterials. Green Chem 2014;16:2627–35. [29] Zhu H, Luo W, Ciesielski PN, Fang Z, Zhu J, Henriksson G, Himmel ME, Hu L. Wood-derived materials for green electronics, biological devices, and energy applications. Chem Rev 2016;116:9305–74. [30] Gargulak JD, Lebo SE, McNally TJ. Lignin. Kirk-othmer encyclopedia of chemical technology. New York: John Wiley & Sons, Inc.; 2001. [31] Hofrichter M. Review: lignin conversion by manganese peroxidase (MnP). Enzyme Microb Technol 2002;30:454–66. [32] Wang M, Li L, Lu J, Li H, Zhang X, Liu H, Luo N, Wang F. Acid promoted C–C bond oxidative cleavage of β –O–4 and β –1 lignin models to esters over a copper catalyst. Green Chem 2017;19:702–6. [33] Patil ND. A study of lignin depolymerization by selective cleavage of the Cα –Cβ linkages in lignin model compounds via baeyer-villiger oxidation & an investigation of the channeling reaction in nitrogen-doped multiwalled carbon nanotubes (N-MWCNTS). University of Kentucky; 2014. [34] Lupoi JS, Singh S, Parthasarathi R, Simmons BA, Henry RJ. Recent innovations in analytical methods for the qualitative and quantitative assessment of lignin. Renew Sust Energ Rev 2015;49:871–906. [35] Behling R, Valange S, Chatel G. Heterogeneous catalytic oxidation for lignin valorization into valuable chemicals: what results? What limitations? What trends? Green Chem 2016;18:1839–54. [36] Ragauskas AJ, Beckham GT, Biddy MJ, Chandra R, Chen F, Davis MF, Davison BH, Dixon RA, Gilna P, Keller M. Lignin valorization: improving lignin processing in the biorefinery. Science 2014;344:1246843. [37] Lange H, Decina S, Crestini C. Oxidative upgrade of lignin – recent routes reviewed. Eur Polym J 2013;49:1151–73. [38] Pandey MP, Kim CS. Lignin depolymerization and conversion: a review of thermochemical methods. Chem Eng Technol 2011;34:29–41. [39] Ma R, Guo M, Zhang X. Recent advances in oxidative valorization of lignin. Catal Today 2018;302:50–60. [40] Hatakeyama H, Hatakeyama T. Lignin structure, properties, and applications. In: Abe A, Dusek K, Kobayashi S, editors. Biopolymers. Berlin, Heidelberg: Springer; 2009. p. 1–63. [41] Huber GW, Iborra S, Corma A. Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chem Rev 2006;106:4044–98. [42] Liu W-J, Li W-W, Jiang H, Yu H-Q. Fates of chemical elements in biomass during its pyrolysis. Chem Rev 2017;117:6367–98. [43] Welker CM, Balasubramanian VK, Petti C, Rai KM, DeBolt S, Mendu V. Engineering plant biomass lignin content and composition for biofuels and bioproducts. Energies 2015;8:7654–76. [44] Menon V, Rao M. Trends in bioconversion of lignocellulose: biofuels, platform chemicals & biorefinery concept. Prog Energy Combust Sci 2012;38:522–50. [45] Gharehkhani S, Ghavidel N, Fatehi P. Kraft lignin-tannic acid as a green stabilizer for oil/water emulsion. ACS Sustain Chem Eng 2019;7(2):2370–9. [46] Rinaldi R, Jastrzebski R, Clough MT, Ralph J, Kennema M, Bruijnincx PC, Weckhuysen BM. Paving the way for lignin valorisation: recent advances in bioengineering, biorefining and catalysis. Angew Chem Int Ed 2016;55:8164–215. [47] Bruijnincx P, Weckhuysen B, Gruter G-J, Engelen-Smeets E. Lignin valorisation: The importance of a full value chain approach. Netherlands, Utrecht: Utrecht University; 2016. [48] Wang Y-Y, Cai CM, Ragauskas AJ. Recent advances in lignin-based polyurethanes. TAPPI J 2017;16:203–7. [49] Azadi P, Inderwildi OR, Farnood R, King DA. Liquid fuels, hydrogen and chemicals from lignin: a critical review. Renew Sust Energ Rev 2013;21:506–23. [50] Anca-Couce A. Reaction mechanisms and multi-scale modelling of lignocellulosic biomass pyrolysis. Prog Energy Combust Sci 2016;53:41–79.

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89 [51] Anderson NA, Tobimatsu Y, Ciesielski PN, Ximenes E, Ralph J, Donohoe BS, Ladisch M, Chapple C. Manipulation of guaiacyl and syringyl monomer biosynthesis in an Arabidopsis cinnamyl alcohol dehydrogenase mutant results in atypical lignin biosynthesis and modified cell wall structure. The Plant Cell 2015;27:2195–209. [52] Wang S, Dai G, Yang H, Luo Z. Lignocellulosic biomass pyrolysis mechanism: a state-of-the-art review. Prog Energy Combust Sci 2017;62:33–86. [53] Liu C, Wang H, Karim AM, Sun J, Wang Y. Catalytic fast pyrolysis of lignocellulosic biomass. Chem Soc Rev 2014;43:7594–623. [54] Silveira RL, Stoyanov SR, Gusarov S, Skaf MS, Kovalenko A. Supramolecular interactions in secondary plant cell walls: effect of lignin chemical composition revealed with the molecular theory of solvation. J Phys Chem Lett 2014;6:206–11. [55] Zhou Z-Z, Liu M, Li C-J. Selective copper-n-heterocyclic carbene (copper-NHC)-catalyzed aerobic cleavage of β –1 lignin models to aldehydes. ACS Catal 2017;7:3344–8. [56] Shao Y, Xia Q, Dong L, Liu X, Han X, Parker SF, Cheng Y, Daemen LL, Ramirez-Cuesta AJ, Yang S. Selective production of arenes via direct lignin upgrading over a niobium-based catalyst. Nat Commun 2017;8:16104. [57] Upton BM, Kasko AM. Strategies for the conversion of lignin to high-value polymeric materials: review and perspective. Chem Rev 2015;116:2275–306. [58] Parthasarathi R, Romero RA, Redondo A, Gnanakaran S. Theoretical study of the remarkably diverse linkages in lignin. J Phys Chem Lett 2011;2:2660–6. [59] Kim S, Chmely SC, Nimlos MR, Bomble YJ, Foust TD, Paton RS, Beckham GT. Computational study of bond dissociation enthalpies for a large range of native and modified lignins. J Phys Chem Lett 2011;2:2846–52. [60] Balakshin MY, Capanema EA, Chang HM. Recent advances in the isolation and analysis of lignins and lignin-carbohydrate complexes. In: Hu TQ, editor. Characterization of lignocellulosic materials. Oxford: Blackwell Publishing Ltd; 2009. p. 148–70. [61] Yuan JS, Li Q, Ragauskas AJ. Lignin carbon fiber: the path for quality. TAPPI J 2017;16:107–8. [62] Aro T, Fatehi P. Tall oil production from black liquor: challenges and opportunities. Sep Purif Technol 2017;175:469–80. [63] Fatehi P, Gao W, Sun Y, Dashtban M. Acidification of prehydrolysis liquor and spent liquor of neutral sulfite semichemical pulping process. Bioresour Technol 2016;218:518–25. [64] Oveissi F. Extracting lignocelluloses from various spent liquors via adsorption. Lakehead University; 2014. [65] Gosselink R, De Jong E, Guran B, Abächerli A. Co-ordination network for lignin—standardisation, production and applications adapted to market requirements (EUROLIGNIN). Ind Crop Prod 2004;20:121–9. [66] Alekhina M, Ershova O, Ebert A, Heikkinen S, Sixta H. Softwood kraft lignin for value-added applications: fractionation and structural characterization. Ind Crop Prod 2015;66:220–8. [67] Ragauskas AJ, Williams CK, Davison BH, Britovsek G, Cairney J, Eckert CA, Frederick WJ, Hallett JP, Leak DJ, Liotta CL. The path forward for biofuels and biomaterials. Science 2006;311:484–9. [68] Baker DA, Hosseinaei O. High glass transition lignins and lignin derivatives for the manufacture of carbon and graphite fibers. US patent, US20140271443A1 2014. [69] Belgacem MN, Gandini A. Monomers, polymers and composites from renewable resources. first ed. Amsterdam: Elsevier; 2011. [70] Le DM, Nielsen AD, Sørensen HR, Meyer AS. Characterisation of authentic lignin biorefinery samples by fourier transform infrared spectroscopy and determination of the chemical formula for lignin. Bioenerg Res 2017;10:1025–35. [71] Mancera C, Ferrando F, Salvadó J, El Mansouri N. Kraft lignin behavior during reaction in an alkaline medium. Biomass Bioenergy 2011;35:2072–9. [72] Telysheva G, Dizhbite T, Paegle E, Shapatin A, Demidov I. Surface-active properties of hydrophobized derivatives of lignosulfonates: effect of structure of organosilicon modifier. J Appl Polym Sci 2001;82:1013–20. [73] Gellerstedt G, Henriksson G. Lignins: Major sources, structure and properties. Amsterdam, the Netherlands: Elsevier; 2008. [74] Rana D, Neale G, Hornof V. Surface tension of mixed surfactant systems: lignosulfonate and sodium dodecyl sulfate. Colloid Polym Sci 2002;280:775–8. [75] Yang D, Qiu X, Zhou M, Lou H. Properties of sodium lignosulfonate as dispersant of coal water slurry. Energy Convers Manage 2007;48:2433–8. [76] Song Q, Wang F, Xu J. Hydrogenolysis of lignosulfonate into phenols over heterogeneous nickel catalysts. Chem Commun 2012;48:7019–21. [77] Vishtal AG, Kraslawski A. Challenges in industrial applications of technical lignins. BioResources 2011;6:3547–68. [78] Agrawal A, Kaushik N, Biswas S. Derivatives and applications of lignin-an insight. SciTech J 2014;1:30–6. [79] Lora J, Naceur B, Gandini A. Industrial commercial lignins: sources, properties and applications. In: Belgacem MN, Gandini A, editors. Monomers, polymers and composites from renewable resources. Amsterdam: Elsevier; 2008. p. 225–41. [80] Xu F, Sun J-X, Sun R, Fowler P, Baird MS. Comparative study of organosolv lignins from wheat straw. Ind Crop Prod 2006;23:180–93. [81] Holladay J, Bozell J, White J, Johnson D. Top value-added chemicals from biomass: Results of screening for potential candidates from biorefinery lignin. United States: Pacific northwest national lab; 2007. [82] Kumar S, Mohanty A, Erickson L, Misra M. Lignin and its applications with polymers. J Biobased Mater Bio 2009;3:1–24.

87

[83] Sarkanen KV, Tillman DA. Progress in biomass conversion. Amsterdam: Elsevier; 2013. [84] Berlin A, Balakshin M. Industrial lignins: analysis, properties and applications. In: Gupta VK, Tuohy MG, Kubicek CP, Saddler J, Xu F, editors. Bioenergy research: Advances and applications. Amsterdam: Elsevier; 2014. p. 315–36. [85] Meister JJ. Modification of lignin. J Macromol Sci Polym Rev 2002;42:235–89. [86] Carvajal JC, Gómez Á, Cardona CA. Comparison of lignin extraction processes: economic and environmental assessment. Bioresour Technol 2016;214:468–76. [87] Jin Y, Cheng X, Zheng Z. Preparation and characterization of phenol-formaldehyde adhesives modified with enzymatic hydrolysis lignin. Bioresour Technol 2010;101:2046–8. [88] Hatakeyama H, Tsujimoto Y, Zarubin MJ, Krutov S, Hatakeyama T. Thermal decomposition and glass transition of industrial hydrolysis lignin. J Therm Anal Calorim 2010;101:289–95. [89] El Mansouri N-E, Salvadó J. Analytical methods for determining functional groups in various technical lignins. Ind Crop Prod 2007;26:116–24. [90] Chakar FS, Ragauskas AJ. Review of current and future softwood kraft lignin process chemistry. Ind Crop Prod 2004;20:131–41. [91] Fredheim GE, Braaten SM, Christensen BE. Molecular weight determination of lignosulfonates by size-exclusion chromatography and multi-angle laser light scattering. J Chromatogr A 2002;942:191–9. [92] Lin SY, Dence CW. Methods in lignin chemistry. Berlin: Springer Science & Business Media; 2012. [93] Rabinovich ML. Lignin by-products of Soviet hydrolysis industry: resources, characteristics, and utilization as a fuel. Cellulose Chem Technol 2014;48:613–31. [94] Bozell JJ. Approaches to the selective catalytic conversion of lignin: a grand challenge for biorefinery development. In: Nicholas K, editor. Selective catalysis for renewable feedstocks and chemicals. New York: Springer; 2014. p. 229–55. [95] Liu W-J, Jiang H, Yu H-Q. Thermochemical conversion of lignin to functional materials: a review and future directions. Green Chem 2015;17:4888–907. [96] Ma Z, Troussard E, van Bokhoven JA. Controlling the selectivity to chemicals from lignin via catalytic fast pyrolysis. Appl Catal A Gen 2012;423:130–6. [97] Zhang J, Asakura H, van Rijn J, Yang J, Duchesne P, Zhang B, Chen X, Zhang P, Saeys M, Yan N. Highly efficient, NiAu-catalyzed hydrogenolysis of lignin into phenolic chemicals. Green Chem 2014;16:2432–7. [98] Konnerth H, Zhang J, Ma D, Prechtl MH, Yan N. Base promoted hydrogenolysis of lignin model compounds and organosolv lignin over metal catalysts in water. Chem Eng Sci 2015;123:155–63. [99] Shu R, Long J, Xu Y, Ma L, Zhang Q, Wang T, Wang C, Yuan Z, Wu Q. Investigation on the structural effect of lignin during the hydrogenolysis process. Bioresour Technol 2016;200:14–22. [100] Bu Q, Lei H, Zacher AH, Wang L, Ren S, Liang J, Wei Y, Liu Y, Tang J, Zhang Q. A review of catalytic hydrodeoxygenation of lignin-derived phenols from biomass pyrolysis. Bioresour Technol 2012;124:470–7. [101] Li H, Fang Z, Smith Jr RL, Yang S. Efficient valorization of biomass to biofuels with bifunctional solid catalytic materials. Prog Energy Combust Sci 2016;55:98–194. [102] Dai J, Patti AF, Saito K. Recent developments in chemical degradation of lignin: catalytic oxidation and ionic liquids. Tetrahedron Lett 2016;57:4945–51. [103] Murphy BM, Xu B. Foundational techniques for catalyst design in the upgrading of biomass-derived multifunctional molecules. Prog Energy Combust Sci 2018;67:1–30. [104] Wertz S, Studer A. Nitroxide-catalyzed transition-metal-free aerobic oxidation processes. Green Chem 2013;15:3116–34. [105] Patil ND, Yao SG, Meier MS, Mobley JK, Crocker M. Selective cleavage of the Cα –Cβ linkage in lignin model compounds via Baeyer–Villiger oxidation. Org Biomol Chem 2015;13:3243–54. [106] Marshall DL, Christian ML, Gryn’ova G, Coote ML, Barker PJ, Blanksby SJ. Oxidation of 4-substituted TEMPO derivatives reveals modifications at the 1-and 4-positions. Org Biomol Chem 2011;9:4936–47. [107] Lebedev O, Kazarnovskii S. Catalytic oxidation of aliphatic amines with hydrogen peroxide. Zh Obshch Khim 1960;30:1631–5. [108] Bragd P, Van Bekkum H, Besemer A. TEMPO-mediated oxidation of polysaccharides: survey of methods and applications. Top Catal 2004;27:49–66. [109] Ciriminna R, Pagliaro M. Industrial oxidations with organocatalyst TEMPO and its derivatives. Org Process Res Dev 2009;14:245–51. [110] Lalevee J, Allonas X, Jacques P. Electronic distribution and solvatochromism investigation of a model radical (2, 2, 6, 6-tetramethylpiperidine N-oxyl: TEMPO) through TD-DFT calculations including PCM solvation. J Mol Struc Theochem 2006;767:143–7. [111] Kubala D, Regeta K, Janecˇ ková R, Fedor J, Grimme S, Hansen A, Nesvadba P, Allan M. The electronic structure of TEMPO, its cation and anion. Mol Phys 2013;111:2033–40. [112] Tebben L, Studer A. Nitroxides: applications in synthesis and in polymer chemistry. Angew Chem Int Ed 2011;50:5034–68. [113] Vogler T, Studer A. Applications of TEMPO in synthesis. Synthesis 20 08;20 08:1979–93. [114] Kovacˇ B, Ljubic´ I, Kivimäki A, Coreno M, Novak I. Characterisation of the electronic structure of some stable nitroxyl radicals using variable energy photoelectron spectroscopy. PCCP 2014;16:10734–42.

88

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89

[115] Megiel E. Surface modification using TEMPO and its derivatives. Adv Colloid Interface Sci 2017;250:158–84. [116] Vazquez-Duhalt R, Westlake DW, Fedorak PM. Lignin peroxidase oxidation of aromatic compounds in systems containing organic solvents. Appl Environ Microbiol 1994;60:459–66. [117] Li C-P, Du M. Role of solvents in coordination supramolecular systems. Chem Commun 2011;47:5958–72. [118] Dyson PJ, Jessop PG. Solvent effects in catalysis: rational improvements of catalysts via manipulation of solvent interactions. Catal Sci Technol 2016;6:3302–16. [119] Buncel E, Rajagopal S. Solvatochromism and solvent polarity scales. Acc Chem Res 1990;23:226–31. [120] Rahimi A, Azarpira A, Kim H, Ralph J, Stahl SS. Chemoselective metal-free aerobic alcohol oxidation in lignin. J Am Chem Soc 2013;135:6415–18. [121] Dabral S, Hernández J, Kamer P, Bolm C. Organocatalytic chemoselective primary alcohol oxidation and subsequent cleavage of lignin model compounds and lignin. ChemSusChem 2017;10:2707–13. [122] Rahimi A, Ulbrich A, Coon JJ, Stahl SS. Formic-acid-induced depolymerization of oxidized lignin to aromatics. Nature 2014;515:249–52. [123] Nguyen JD, Matsuura BS, Stephenson CR. A photochemical strategy for lignin degradation at room temperature. J Am Chem Soc 2014;136:1218–21. [124] Yao SG, Meier MS, Pace III RB, Crocker M. A comparison of the oxidation of lignin model compounds in conventional and ionic liquid solvents and application to the oxidation of lignin. RSC Adv 2016;6:104742–53. [125] Stahl SS, Rafiee M. Nitroxyl-mediated oxidation of lignin and polycarboxylated products. US patent, US9903028B2, 2018. [126] Sedai B, Baker RT. Copper catalysts for selective c-c bond cleavage of β -O-4 lignin model compounds. Adv Synth Catal 2014;356:3563–74. ´ [127] Sedai B, Diaz-Urrutia C, Baker RT, Wu R, Silks LP, Hanson SK. Aerobic oxidation of β -1 lignin model compounds with copper and oxovanadium catalysts. ACS Catal 2013;3:3111–22. [128] Wang M, Lu J, Zhang X, Li L, Li H, Luo N, Wang F. Two-step, catalytic C–C bond oxidative cleavage process converts lignin models and extracts to aromatic acids. ACS Catal 2016;6:6086–90. ´ [129] Sedai B, Diaz-Urrutia C, Baker RT, Wu R, Silks LP, Hanson SK. Comparison of copper and vanadium homogeneous catalysts for aerobic oxidation of lignin models. ACS Catal 2011;1:794–804. [130] Díaz-Urrutia C, Sedai B, Leckett KC, Baker RT, Hanson SK. Aerobic Oxidation of 2-Phenoxyethanol Lignin Model Compounds Using Vanadium and Copper Catalysts. ACS Sustain Chem Eng 2016;4:6244–51. [131] Nguyen T-AD, Wright AM, Page JS, Wu G, Hayton TW. Oxidation of alcohols and activated alkanes with lewis acid-activated TEMPO. Inorg Chem 2014;53:11377–87. [132] Bosque I, Magallanes G, Rigoulet M, Kar¨ kas¨ MD, Stephenson CR. Redox catalysis facilitates lignin depolymerization. ACS Cent Sci 2017;3(6):621–8. [133] Beejapur HA, Giacalone F, Noto R, Franchi P, Lucarini M, Gruttadauria M. Recyclable catalyst reservoir: oxidation of alcohols mediated by noncovalently supported bis (imidazolium)-tagged 2, 2, 6, 6-tetramethylpiperidine 1-oxyl. ChemCatChem 2013;5:2991–9. [134] Rafiee M, Karimi B, Alizadeh S. Mechanistic study of the electrocatalytic oxidation of alcohols by TEMPO and NHPI. ChemElectroChem 2014;1:455–62. [135] Hunter D, Barton D, Motherwell W. Oxoammonium salts as oxidizing agents: 2, 2, 6, 6-tetramethyl-1-oxopiperidinium chloride. Tetrahedron Lett 1984;25:603–6. [136] Gamez P, Arends IW, Sheldon RA, Reedijk J. Room temperature aerobic copper–catalysed selective oxidation of primary alcohols to aldehydes. Adv Synth Catal 2004;346:805–11. [137] Liu X, Xu H, Ma Z, Zhang H, Wu C, Liu Z. Cu-catalyzed aerobic oxygenation of 2-phenoxyacetophenones to alkyloxy acetophenones. RSC Adv 2016;6:27126–9. [138] Hanson SK, Baker RT, Gordon JC, Scott BL, Thorn DL. Aerobic oxidation of lignin models using a base metal vanadium catalyst. Inorg Chem 2010;49:5611–18. [139] Dabral S, Wotruba H, Hernández JG, Bolm C. Mechanochemical oxidation and cleavage of lignin β –O–4 model compounds and lignin. ACS Sustain Chem Eng 2018;6:3242–54. [140] Das A, Rahimi A, Ulbrich A, Alherech M, Motagamwala AH, Bhalla A, Costa Sousa L, Balan V, Dumesic JA, Hegg EL. Lignin conversion to low-molecular-weight aromatics via an aerobic oxidation-hydrolysis sequence: comparison of different lignin sources. ACS Sustain Chem Eng 2018;6:3367–74. [141] Judith Percino M, Cerón M, Soriano-Moro G, Pacheco JA, Eugenia Castro M, Chapela VM, Bonilla-Cruz J, Saldivar-Guerra E. 2,2,6,6-Tetramethyl-1-oxopiperidinetribromide and two forms of 1-hydroxy-2,2,6,6-tetramethylpiperidinium bromide salt: syntheses, crystal structures and theoretical calculations. J Mol Struct 2016;1103:254–64. [142] Ryland BL, Stahl SS. Practical aerobic oxidations of alcohols and amines with homogeneous copper/TEMPO and related catalyst systems. Angew Chem Int Ed 2014;53:8824–38. [143] Cao Q, Dornan LM, Rogan L, Hughes NL, Muldoon MJ. Aerobic oxidation catalysis with stable radicals. Chem Commun 2014;50:4524–43. [144] Fey T, Fischer H, Bachmann S, Albert K, Bolm C. Silica-supported TEMPO catalysts: synthesis and application in the Anelli oxidation of alcohols. J Org Chem 2001;66:8154–9. [145] Bailey WF, Bobbitt JM, Wiberg KB. Mechanism of the oxidation of alcohols by oxoammonium cations. J Org Chem 2007;72:4504–9.

[146] Rafiee M, Miles KC, Stahl SS. Electrocatalytic alcohol oxidation with TEMPO and bicyclic nitroxyl derivatives: driving force trumps steric effects. J Am Chem Soc 2015;137:14751–7. [147] Zhang G, Scott BL, Wu R, Silks LP, Hanson SK. Aerobic oxidation reactions catalyzed by vanadium complexes of bis (phenolate) ligands. Inorg Chem 2012;51:7354–61. [148] Ben-Daniel R, Alsters P, Neumann R. Selective aerobic oxidation of alcohols with a combination of a polyoxometalate and nitroxyl radical as catalysts. J Org Chem 2001;66:8650–3. [149] Wang X, Liu R, Jin Y, Liang X. TEMPO/HCl/NaNO2 catalyst: a transition-metal-free approach to efficient aerobic oxidation of alcohols to aldehydes and ketones under mild conditions. Chem Eur J 2008;14:2679–85. [150] Hoover JM, Stahl SS. Highly practical copper (I)/TEMPO catalyst system for chemoselective aerobic oxidation of primary alcohols. J Am Chem Soc 2011;133:16901–10. [151] Ragagnin G, Betzemeier B, Quici S, Knochel P. Copper-catalysed aerobic oxidation of alcohols using fluorous biphasic catalysis. Tetrahedron 2002;58:3985–91. [152] Dijksman A, Arends IW, Sheldon RA. Cu (ii)-nitroxyl radicals as catalytic galactose oxidase mimics. Org Biomol Chem 2003;1:3232–7. [153] Kumpulainen ET, Koskinen AM. Catalytic activity dependency on catalyst components in aerobic copper-TEMPO oxidation. Chem Eur J 2009;15:10901–11. [154] Du Z, Ma J, Ma H, Wang M, Huang Y, Xu J. Vanadyl sulfate: a simple catalyst for oxidation of alcohols with molecular oxygen in combination with 2, 2, 6, 6-tetramethyl-piperidyl-1-oxyl. Catal Commun 2010;11:732–5. [155] Semmelhack M, Schmid CR, Cortes DA, Chou CS. Oxidation of alcohols to aldehydes with oxygen and cupric ion, mediated by nitrosonium ion. J Am Chem Soc 1984;106:3374–6. [156] Hoover JM, Ryland BL, Stahl SS. Mechanism of copper (I)/TEMPO-catalyzed aerobic alcohol oxidation. J Am Chem Soc 2013;135:2357–67. [157] Yoshikawa T, Yagi T, Shinohara S, Fukunaga T, Nakasaka Y, Tago T, Masuda T. Production of phenols from lignin via depolymerization and catalytic cracking. Fuel Process Technol 2013;108:69–75. [158] Delidovich I, Hausoul PJ, Deng L, Pfut¨ zenreuter R, Rose M, Palkovits R. Alternative monomers based on lignocellulose and their use for polymer production. Chem Rev 2015;116:1540–99. [159] Toledano A, Serrano L, Labidi J. Organosolv lignin depolymerization with different base catalysts. J Chem Technol Biotechnol 2012;87:1593–9. [160] Bruijnincx PC, Weckhuysen BM. Biomass conversion: lignin up for break– down. Nat Chem 2014;6:1035–6. [161] Qu S, Dang Y, Song C, Guo J, Wang Z-X. Depolymerization of oxidized lignin catalyzed by formic acid exploits an unconventional elimination mechanism involving 3c–4e bonding: a DFT mechanistic study. ACS Catal 2015;5:6386–96. [162] Portjanskaja E, Stepanova K, Klauson D, Preis S. The influence of titanium dioxide modifications on photocatalytic oxidation of lignin and humic acids. Catal Today 2009;144:26–30. [163] Kar¨ kas¨ MD, Bosque I, Matsuura BS, Stephenson CR. Photocatalytic oxidation of lignin model systems by merging visible-light photoredox and palladium catalysis. Org Lett 2016;18:5166–9. [164] Colmenares JC, Luque R. Heterogeneous photocatalytic nanomaterials: prospects and challenges in selective transformations of biomass-derived compounds. Chem Soc Rev 2014;43:765–78. [165] Hasegawa E, Takizawa S, Seida T, Yamaguchi A, Yamaguchi N, Chiba N, Takahashi T, Ikeda H, Akiyama K. Photoinduced electron-transfer systems consisting of electron-donating pyrenes or anthracenes and benzimidazolines for reductive transformation of carbonyl compounds. Tetrahedron 2006;62:6581–8. [166] Tian M, Wen J, MacDonald D, Asmussen RM, Chen A. A novel approach for lignin modification and degradation. Electrochem Commun 2010;12:527–30. [167] Feghali E, Carrot G, Thuéry P, Genre C, Cantat T. Convergent reductive depolymerization of wood lignin to isolated phenol derivatives by metal-free catalytic hydrosilylation. Energ Environ Sci 2015;8:2734–43. [168] Guadix-Montero S, Sankar M. Review on catalytic cleavage of c–c inter-unit linkages in lignin model compounds: towards lignin depolymerisation. Top Catal 2018:1–16. [169] Allen SE, Walvoord RR, Padilla-Salinas R, Kozlowski MC. Aerobic copper-catalyzed organic reactions. Chem Rev 2013;113:6234–458. [170] Gamez P, Arends IW, Reedijk J, Sheldon RA. Copper (II)-catalysed aerobic oxidation of primary alcohols to aldehydes. Chem Commun 2003:2414–15. [171] Cho DW, Parthasarathi R, Pimentel AS, Maestas GD, Park HJ, Yoon UC, Dunaway-Mariano D, Gnanakaran S, Langan P, Mariano PS. Nature and kinetic analysis of carbon−carbon bond fragmentation reactions of cation radicals derived from set-oxidation of lignin model compounds. J Org Chem 2010;75:6549–62. [172] Hanson SK, Baker RT. Knocking on wood: base metal complexes as catalysts for selective oxidation of lignin models and extracts. Acc Chem Res 2015;48:2037–48. [173] Fan H, Yang Y, Song J, Meng Q, Jiang T, Yang G, Han B. Free-radical conversion of a lignin model compound catalyzed by Pd/C. Green Chem 2015;17:4452–8. [174] Froidevaux V, Negrell C, Caillol S, Pascault J-P, Boutevin B. Biobased amines: from synthesis to polymers; present and future. Chem Rev 2016;116:14181–224. [175] Esposito D, Antonietti M. Redefining biorefinery: the search for unconventional building blocks for materials. Chem Soc Rev 2015;44:5821–35. [176] Qian W, Jin E, Bao W, Zhang Y. Clean and selective oxidation of alcohols catalyzed by ion-supported TEMPO in water. Tetrahedron 2006;62:556–62.

S. Gharehkhani, Y. Zhang and P. Fatehi / Progress in Energy and Combustion Science 72 (2019) 59–89 [177] Gunasekaran N. Aerobic oxidation catalysis with air or molecular oxygen and ionic liquids. Adv Synth Catal 2015;357:1990–2010. [178] Wu X-E, Ma L, Ding M-X, Gao L-X. TEMPO-derived task-specific ionic liquids for oxidation of alcohols. Synlett 20 05;20 05:607–10. [179] Suzuki Y, Iinuma M, Moriyama K, Togo H. TEMPO-mediated oxidation of alcohols with ion-supported (Diacetoxyiodo) benzenes. Synlett 2012;2012:1250–6. [180] Mehnert CP, Cook RA, Dispenziere NC, Afeworki M. Supported ionic liquid catalysis − a new concept for homogeneous hydroformylation catalysis. J Am Chem Soc 2002;124:12932–3. [181] Stärk K, Taccardi N, Bösmann A, Wasserscheid P. Oxidative depolymerization of lignin in ionic liquids. ChemSusChem 2010;3:719–23. [182] Giacalone F, Gruttadauria M. Covalently supported ionic liquid phases: an advanced class of recyclable catalytic systems. ChemCatChem 2016;8:664–84. [183] Liu L, Ma J, Xia J, Li L, Li C, Zhang X, Gong J, Tong Z. Confining task-specific ionic liquid in silica-gel matrix by sol-gel technique: a highly efficient catalyst for oxidation of alcohol with molecular oxygen. Catal Commun 2011;12:323–6. [184] Pu Y, Jiang N, Ragauskas AJ. Ionic liquid as a green solvent for lignin. J Wood Chem Technol 2007;27:23–33. [185] Reichert E, Wintringer R, Volmer DA, Hempelmann R. Electro-catalytic oxidative cleavage of lignin in a protic ionic liquid. PCCP 2012;14:5214–21. [186] Jiang N, Ragauskas AJ. Copper (II)-catalyzed aerobic oxidation of primary alcohols to aldehydes in ionic liquid [bmpy] PF6. Org Lett 2005;7:3689–92. [187] Ansari IA, Gree R. TEMPO-catalyzed aerobic oxidation of alcohols to aldehydes and ketones in ionic liquid [bmim][PF6]. Org Lett 2002;4:1507–9. [188] Hirashita T, Nakanishi M, Uchida T, Yamamoto M, Araki S, Arends IW, Sheldon RA. Ionic TEMPO in ionic liquids: specific promotion of the aerobic oxidation of alcohols. ChemCatChem 2016;8:2704–9. [189] Giernoth R. Task-specific ionic liquids. Angew Chem Int Ed 2010;49:2834–9. [190] Pozzi G, Cavazzini M, Quici S, Benaglia M, Dell’Anna G. Poly (ethylene glycol)-supported TEMPO: an efficient, recoverable metal-free catalyst for the selective oxidation of alcohols. Org Lett 2004;6:441–3. [191] Benaglia M, Puglisi A, Holczknecht O, Quici S, Pozzi G. Aerobic oxidation of alcohols to carbonyl compounds mediated by poly (ethylene glycol)-supported TEMPO radicals. Tetrahedron 2005;61:12058–64. [192] Semmelhack M, Chou CS, Cortes DA. Nitroxyl-mediated electrooxidation of alcohols to aldehydes and ketones. J Am Chem Soc 1983;105:4492–4. [193] Weiss CJ, Wiedner ES, Roberts JA, Appel AM. Nickel phosphine catalysts with pendant amines for electrocatalytic oxidation of alcohols. Chem Commun 2015;51:6172–4. [194] Ciriminna R, Palmisano G, Pagliaro M. Electrodes functionalized with the 2, 2, 6, 6-tetramethylpiperidinyloxy radical for the waste-free oxidation of alcohols. ChemCatChem 2015;7:552–8. [195] Palmisano G, Ciriminna R, Pagliaro M. Waste-free electrochemical oxidation of alcohols in water. Adv Synth Catal 2006;348:2033–7. [196] Barbier M, Breton T, Servat K, Grand E, Kokoh B, Kovensky J. Selective TEMPO-catalyzed chemicals vs. Electrochemical oxidation of carbohydrate derivatives. J Carbohydr Chem 2006;25:253–66. [197] Ciriminna R, Ghahremani M, Karimi B, Pagliaro M. Electrochemical alcohol oxidation mediated by TEMPO-like nitroxyl radicals. ChemistryOpen 2017;6(1):5–10. [198] Badalyan A, Stahl SS. Cooperative electrocatalytic alcohol oxidation with electron-proton-transfer mediators. Nature 2016;535:406–11. [199] Hickey DP, Schiedler DA, Matanovic I, Doan PV, Atanassov P, Minteer SD, Sigman MS. Predicting electrocatalytic properties: modeling structure–activity relationships of nitroxyl radicals. J Am Chem Soc 2015;137:16179–86. [200] Hill-Cousins JT, Kuleshova J, Green RA, Birkin PR, Pletcher D, Underwood TJ, Leach SG, Brown RC. TEMPO-mediated electrooxidation of

[201]

[202]

[203] [204]

[205]

[206] [207] [208]

[209]

[210] [211]

[212] [213] [214]

[215]

89

primary and secondary alcohols in a microfluidic electrolytic cell. ChemSusChem 2012;5:326–31. Kishioka S-Y, Ohki S, Ohsaka T, Tokuda K. Reaction mechanism and kinetics of alcohol oxidation at nitroxyl radical modified electrodes. J Electroanal Chem 1998;452:179–86. Palmisano G, Mandler D, Ciriminna R, Pagliaro M. Structural insight on organosilica electrodes for waste-free alcohol oxidations. Catal Lett 2007;114:55–8. Walcarius A, Sibottier E, Etienne M, Ghanbaja J. Electrochemically assisted self-assembly of mesoporous silica thin films. Nat Mater 2007;6:602–8. Ma Y, Chen H, Shi Y, Yuan S. Low cost synthesis of mesoporous molecular sieve MCM-41 from wheat straw ash using CTAB as surfactant. Mater Res Bull 2016;77:258–64. Karimi B, Rafiee M, Alizadeh S, Vali H. Eco-friendly electrocatalytic oxidation of alcohols on a novel electro generated TEMPO-functionalized MCM-41 modified electrode. Green Chem 2015;17:991–10 0 0. Swiech O, Bilewicz R, Megiel E. TEMPO coated Au nanoparticles: synthesis and tethering to gold surfaces. RSC Adv 2013;3:5979–86. Isogai A, Saito T, Fukuzumi H. TEMPO-oxidized cellulose nanofibers. Nanoscale 2011;3:71–85. Mishra SP, Thirree J, Manent A-S, Chabot B, Daneault C. Ultrasound-catalyzed TEMPO-mediated oxidation of native cellulose for the production of nanocellulose: effect of process variables. BioResources 2010;6:121–43. Hirota M, Tamura N, Saito T, Isogai A. Oxidation of regenerated cellulose with NaClO2 catalyzed by TEMPO and NaClO under acid-neutral conditions. Carbohydr Polym 2009;78:330–5. Tahiri C, Vignon MR. TEMPO-oxidation of cellulose: synthesis and characterisation of polyglucuronans. Cellulose 20 0 0;7:177–88. Patel I, Opietnik M, Böhmdorfer S, Becker M, Potthast A, Saito T, Isogai A, Rosenau T. Side reactions of 4-acetamido-TEMPO as the catalyst in cellulose oxidation systems. Holzforschung 2010;64:549–54. Elboutachfaiti R, Delattre C, Petit E, Michaud P. Polyglucuronic acids: structures, functions and degrading enzymes. Carbohydr Polym 2011;84:1–13. Isogai A, Kato Y. Preparation of polyuronic acid from cellulose by TEMPO-mediated oxidation. Cellulose 1998;5:153–64. Habibi Y, Vignon MR. Optimization of cellouronic acid synthesis by TEMPO-mediated oxidation of cellulose III from sugar beet pulp. Cellulose 2008;15:177–85. Xu C, Arancon RAD, Labidi J, Luque R. Lignin depolymerisation strategies: towards valuable chemicals and fuels. Chem Soc Rev 2014;43:7485–500.

Dr. Samira Gharehkhani is a postdoctoral fellow in Chemical Engineering Department of Lakehead University. She has coauthored several articles in the area of biomass utilization including cellulose nanocrystals and lignin. Her main research interest is physicochemical alteration of biomass for generating value-added products. Dr. Yiqian Zhang is also a postdoctoral fellow in Chemical Engineering Department of Lakehead University. She has involved in several research projects on lignin valorization and her primary research focus is on chemical modification of lignin. Professor Pedram Fatehi is a Canada Research Chair and Industrial Research Chair in Green Chemicals and Processes at Lakehead University. He has coauthored more than 150 articles in the area of lignin valorization since 2008. He is also the director of Green Chemicals Research Centre at Lakehead University. He has been the leader of many projects on producing water soluble lignin-based products over the past 7 years.