polymers

Review Catalytic Oxidation of in Solvent Systems for Production of Renewable Chemicals: A Review

Chongbo Cheng 1, Jinzhi Wang 1, Dekui Shen 1,*, Jiangtao Xue 2, Sipian Guan 2, Sai Gu 3 and Kai Hong Luo 4

1 Key Lab of Thermal Energy Conversion and Control of MoE, Southeast University, Nanjing 210096, China; [email protected] (C.C.); [email protected] (J.W.) 2 Jiangsu Frontier Electric Power Technology Co., Ltd., Nanjing 211102, China; [email protected] (J.X.); [email protected] (S.G.) 3 Department of Chemical and Process Engineering, Faculty of Engineering and Physical Sciences, University of Surrey, Surrey GU2 7XH, UK; [email protected] 4 Department of Mechanical Engineering, University College London, London WC1E 7JE, UK; [email protected] * Correspondence: [email protected]; Tel.: +86-138-5170-6572

Academic Editor: Wolfgang Gindl-Altmutter Received: 5 May 2017; Accepted: 16 June 2017; Published: 21 June 2017

Abstract: Lignin as the most abundant source of aromatic chemicals in nature has attracted a great deal of attention in both academia and industry. Solvolysis is one of the promising methods to convert lignin to a number of petroleum-based aromatic chemicals. The process involving the depolymerization of the lignin macromolecule and repolymerization of fragments is complicated influenced by heating methods, reaction conditions, presence of a catalyst and solvent systems. Recently, numerous investigations attempted unveiling the inherent mechanism of this process in order to promote the production of valuable aromatics. Oxidative solvolysis of lignin can produce a number of the functionalized monomeric or oligomeric chemicals. A number of research groups should be greatly appreciated with regard to their contributions on the following two concerns: (1) the cracking mechanism of inter-unit linkages during the oxidative solvolysis of lignin; and (2) the development of novel catalysts for oxidative solvolysis of lignin and their performance. Investigations on lignin oxidative solvolysis are extensively overviewed in this work, concerning the above issues and the way-forward for lignin refinery.

Keywords: biomass; lignin depolymerization; oxidative solvolysis; catalysis

1. Introduction Biomass is an abundant renewable feedstock for the production of fuels, chemicals, and energy. Nowadays, about 10% of the world’s primary energy is from biomass [1]. Biomass is primarily used to generate heat and power, and it stands as the fourth largest source of energy in the word (after oil, coal, and natural gas) [2]. Fuels produced from biomass-refinery processes are replacing the fossil-based fuels in the daily life [3]. However, the use of biomass components and derivatives as the raw feedstocks for the is still in the fledging stages [4–8]. With the pressure of the price of fossil fuels and the high cost of biorefinery processes of biomass, more and more attention has been paid on the production of value-added chemicals from biomass. Biomass consists of three valuable fractions: 38–50% composed of semi-crystalline polysaccharide, 23–32% hemicellulose composed of amorphous multicomponent polysaccharide, and 15–25% lignin composed of amorphous polymer [9,10]. Hemicellulose has important applications for biofuel production and for the generation of valuable chemical intermediates (e.g., furfural) [11]; cellulose can be decomposed into valuable products such as biofuels and platform

Polymers 2017, 9, 240; doi:10.3390/polym9060240 www.mdpi.com/journal/polymers Polymers 2017, 9, 240 2 of 25 chemicals (including levulinic and formic acids, gammavalerolactone and derived products) [12]; and lignin is the most underutilized fraction, only approximately 2% of the lignin available from the and paper industry is commercially used while the remainder is burned as a low value fuel [13]. There is a general lack of efficient processes for the utilization of lignin regarded as the most important renewable source of aromatics on the planet [14]. It becomes highly desirable to develop efficient technologies for the conversion of lignin to aromatic chemicals [15–17]. In the past decades, several strategies have been employed for lignin valorization, such as pyrolysis and gasification [18–23]. Lignin valorization in solvent systems for the production of renewable aromatic chemicals has attracted ever-increasing attention during recent years. The methodology for lignin depolymerization (LDP) in solvent systems can be categorized as hydrolysis, hydrogenolysis, oxidation and a two-step LDP. Lange et al. [3] and Chatel et al. [24,25] have published reviews on catalytic oxidation for lignin valorization into value-added chemicals. Some relevant reviews on bond cleavage mechanisms during lignin depolymerization have been published [26] and the separation and purification processes in lignin depolymerization have been reviewed recently [27]. Catalysis is regarded as a key factor to fulfill the promise of lignin depolymerization to specific products. Catalytic (homogeneous/heterogeneous) conversion of lignin for production of aromatic chemicals has also been reviewed in recent years [1,28–31]. Lignin oxidative solvolysis has been widely applied in the bleaching process for pulping industry due to its economic feasibility [32,33]. Phenolic products with low molecular weight were detected as the primary products from the oxidative solvolysis of lignin [33]. Organometallic catalyzed oxidation, biomimetic oxidation and -based oxidation have been studied [3,34,35], aiming at the cleavage of the designated inter-unit linkages [27]. Fundamental research on understanding and controlling the oxidative process is essential for promoting the production of specific valuable compounds [33]. The chemical structure and morphological properties of lignin are briefly introduced in this work, regarding the typical inter-unit linkages and lignin-related model compounds. Catalytic oxidation of lignin and lignin model compounds in solvent systems are then comprehensively overviewed, concentrating on the cracking of the inter-unit linkages and the effect of catalyst and solvent on the production of renewable aromatic chemicals.

2. Chemical Structure of Lignin The roles of lignin in plants are to afford structural strength, protect the plants from degradation and enable the water conduction system connecting roots with leaves. Lignin is composed of p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units linked by C–O or C–C bonds, which exhibit a complex three-dimensional amorphous structure. The content and chemical structure of lignin are primarily influenced by its sources and isolation methods. The chemical structure for lignin from different sources is different in terms of proportion of p-coumaryl-alcohol, coniferyl-alcohol, and sinapyl-alcohol (C9 unit) (Figure1a). More than 95% of guaiacyl units (that derived from coniferyl alcohol) are estimated to be in softwood lignin, while the proportions of coniferyl-alcohols and sinapyl-alcohols are approximately equal in hardwood lignin. Herbaceous biomass (e.g., straw) lignin and woody lignin are of different chemical structures because of the extra content of p-hydroxyphenyl units and significant amounts of (mainly p-coumaric acid). The proportions of p-coumaryl-, coniferyl- and sinapyl-alcohols in lignin depend on sources of lignin. Lignin units are linked by either C–O or C–C bonds. More than two-thirds of the total linkages are C–O bonds while the others are C–C bonds in native lignin [1,36]. The atoms in aromatic moieties are numbered as 1–6 and those in the aliphatic side chains of the units are labeled as α, β, and γ to distinguish various types of linkages between two structural units. The β-position is generally favored by coupling other units, leading to β-O-4, β-β, β-1 and β-5 becoming main linkages among the units of lignin [28,37–39]. The most common linkage in lignin is the β-O-4 bond, accounting for 45 to 48% in native lignin [40,41]. Other linkages include α-O-4, 4-O-5, 5-5, etc. The linkages in Polymers 2017, 9, 240 3 of 25

Polymers 2017, 9, 240 3 of 25 lignin are illustrated in Figure1b, while the proportion of these linkages and the functional groups The linkages in lignin are illustrated in Figure 1b, while the proportion of these linkages and the dependsfunctional on the ligningroups sourcesdepends andon the extraction lignin sources methods and extraction [28,42,43 methods]. [28,42,43].

Figure 1. (a) Schematic of the structure of monomer C9 units; and (b) schematic of the proposed Figure 1. (a) Schematic of the structure of monomer C9 units; and (b) schematic of the proposed lignin lignin structure with several linkages (B1: β-O-4, B2: α-O-4, B3: β-5, B4: β-1, B5: 5-5, B6: β-β, B7: β α β β β β structure4-O-5) with [44], several reproduced linkages with (B1: permission-O-4, from B2: Springer.-O-4, B3: -5, B4: -1, B5: 5-5, B6: - , B7: 4-O-5) [44], reproduced with permission from Springer. Lignin-related model compounds are widely employed for better understanding of the Lignin-relatedcracking mechanism model of compounds the specific bonds are widely in lignin. employed The most for widely-reported better understanding model compounds of the cracking mechanismand linkages of the specificcontaining bonds aromatic in lignin. or aliphatic The mostethers widely-reported are shown in Figure model 2. The compounds synthetic methods and linkages for making β-O-4 model compounds [45], β-5 model compounds [46], β-1 model compounds [47,48] β containingand β aromatic-β model orcompounds aliphatic [49] have are been shown reported in Figure in the2 .literature. The synthetic However, methods due to for the making lack of -O-4 modelcomplexity compounds of these [45], modelsβ-5 model compared compounds with lignin [46 itself,], β-1 the model application compounds of model [compound47,48] and inβ the-β model compoundsdevelopment [49] have of selective been reported LDP has been in the restricted literature. [46]. However,Unlike a wide due variety to the of lacktrimer, of tetramer, complexity of these modelshexamer compared and polymer with lignin lignin mode itself,l compounds the application containing of only model β-O-4 compound linkage, Ouyang in the et developmental. [50] of selectivesynthesized LDP hasa lignin been model restricted compound [46 ].composed Unlike of a three wide guaiacyl variety structural of trimer, units tetramer, linked by hexamer α-O-4 and polymerand lignin β-O-4 model ether bonds, compounds via a three-step containing sequence only βmethod-O-4 linkage, assisted Ouyangwith microwave et al. [ 50irradiation.] synthesized a Sheldrake et al. [46] also reported a flexible, efficient and convergent preparation method for α β lignin modelsynthesizing compound model lignin composed hexamers of three and octamers guaiacyl linked structural by β-O-4, units 5-5′ linked and β-5. by These-O-4 lignin and model-O-4 ether bonds,compounds via a three-step can reflect sequence some natural method information assisted withof lignin, microwave giving a irradiation.better understanding Sheldrake of the et al. [46] also reportedinherent a mechanism flexible, efficient of lignin anddepolymerization convergent proc preparationesses. Other method novel approaches for synthesizing for synthesizing model lignin hexamersthe model and octamers compounds linked that are by moreβ-O-4, accurate 5-50 inand mimickingβ-5. These characteristics lignin model of lignin compounds are still required. can reflect some naturalSolvolysis information is a promising of lignin, method giving for a converting better understanding lignin to various of thevaluable inherent aromatic mechanism chemicals. of lignin depolymerizationThe lignin macromolecule processes. Other depolymerization novel approaches and fragments for synthesizing repolymerization the model are involved compounds in this that are process, which is conducted by using different catalysts, solvent systems, experimental conditions more accurate in mimicking characteristics of lignin are still required. and even heating methods. The depolymerization of lignin to renewable aromatic chemicals is Solvolysisdifficult due is a promisingto the thermally method stable, for convertingamorphous ligninand complex to various structure valuable of lignin. aromatic Lignin chemicals. The lignindepolymerization macromolecule often depolymerizationnecessitates harsh conditions and fragments to break the repolymerization stable structure of are lignin. involved Under in this process,such which conditions, is conducted the undesired by using repolymerization different catalysts, of fragments solvent is systems,favored, resulting experimental in a decrease conditions of and even heatingproduct methods.yield. The Thetarget depolymerization linkages of lignin might of lignin be hard to renewable to access by aromatic heterogeneous chemicals catalysts is difficult due tobecause the thermally of the steric stable, hindrance amorphous of the lignin and complexamorphous structure backbone. of Although lignin. the Lignin target depolymerization linkages of lignin could access by homogeneous catalysts, it is difficult to separate homogeneous catalysts from often necessitates harsh conditions to break the stable structure of lignin. Under such conditions, the the liquid products. The variability of a large number of substructural units in lignin would result in undesireddifferent repolymerization catalytic reaction of rates fragments at different is favored, sites. A number resulting of inlignin a decrease sources from of product different yield. isolation The target linkagesmethods of lignin should might be beexamined hard to concerning access by the heterogeneous selection of feedstock catalysts for becauseLDP. of the steric hindrance of the lignin amorphous backbone. Although the target linkages of lignin could access by homogeneous catalysts, it is difficult to separate homogeneous catalysts from the liquid products. The variability of a large number of substructural units in lignin would result in different catalytic reaction rates at different sites. A number of lignin sources from different isolation methods should be examined concerning the selection of feedstock for LDP. Polymers 2017, 9, 240 4 of 25 Polymers 2017, 9, 240 4 of 25

Figure 2. Lignin-related model compounds with different inter-unit linkages and functional groups. Figure 2. Lignin-related model compounds with different inter-unit linkages and functional groups.

2.1. Kraft Lignin 2.1. Kraft Lignin Kraft pulping is the predominant chemical pulping process around the world. This process is directedKraft pulpingat temperatures is the predominant in a range of chemical 150–180 pulping°C for around process two around hours theand world.at high ThispH with process the is ◦ directedpresence at temperaturesof considerable in amounts a range of aqueous 150–180 sulfide,C for aroundsulfhydryl two and hours polysulfide and at high ions. pH Thus, with the the presencenative oflignin considerable will undergo amounts dramatic of aqueous chemical sulfide, and structural sulfhydryl changes and polysulfide due to these ions. severe Thus, conditions, the native ligninwhich will makes undergo it difficult dramatic to chemical be depolymerized and structural to fragments changes due in tothe these subsequent severe conditions, processes. whichThe makespercentage it difficult of lignin to be in depolymerized the black liquor to generated fragments by inkraft the pulping subsequent ranges processes. from 29 to The 45% percentage for cook of of ligninpaper in grade the black and liquor8 to 16% generated for cook byof liner kraft grade pulping [51] ranges and the from percentage 29 to 45% of kraft for cookblack of liquor paper lignin grade andextracted 8 to 16% from for cook hardwood of liner is gradegenerally [51] lower and the than percentage that of softwood. of kraft black liquor lignin extracted from hardwood is generally lower than that of softwood. 2.2. Sulfite Lignin 2.2. Sulfite Lignin As kraft pulping becomes the dominant pulping process all around world, just about 10% of the pulpAs kraftis produced pulping by becomes sulfite pulping the dominant process pulpingnow [52]. process Sulfite allpulping around is world,conducted just under about pH 10% 2–12, of the pulpdepending is produced on the by cationic sulfite composition pulping process of the now pulping [52]. liquor. Sulfite Nearly pulping all issulfite conducted pulping under processes pH are 2–12, dependingacidic, and on the cationic or magnesium composition (Ca of2+/Mg the2+ pulping) are used liquor. as the Nearly counterions, all sulfite while, pulping in higher processes pH areenvironments, acidic, and calcium sodium oror ammonium magnesium are (Ca usually2+/Mg used2+) are instead used of as Ca/Mg. the counterions, Due to the sulfite while, presence, in higher pHlignin environments, is rendered sodium soluble or throughout ammonium the are full usually range of used pH; instead it cannot of Ca/Mg.simply be Due isolated to the by sulfitepH

Polymers 2017, 9, 240 5 of 25 presence, lignin is rendered soluble throughout the full range of pH; it cannot simply be isolated by pH adjustment. As mixtures with only 70–75% actually lignin, commercial sulfite lignin shows wide molecular weight profiles and variable grades of sulfonation, making it difficult to be applied in lignin depolymerization. The average molecular weight of sulfite lignin from softwood is notably higher that of the sulfite lignin from hardwood.

2.3. Organosolv Lignin Organosolv lignin, derived from organosolv pulping process, is designed to dissolve lignin from plant cell walls by using a high proportion of an organic solvent (e.g., dioxane) in the cooking liquor [53]. Compared to kraft and sulfite pulping, organosolv pulping is a more environmentally benign pathway. Organosolv lignin reserves a large extent of its original structure in the form of original inter-unit linkages (e.g., β-O-4 linkage), which can be used as an appropriate lignin sample for subsequent conversion and valorization.

2.4. Pyrolytic Lignin Pyrolytic lignin has unusual structural properties featured by its C8 basic unit skeleton and lower average molecular weight [54,55]. The average molecular weight of the pyrolytic lignin was reported to be within a range of 650–1300 Da [55]. Pyrolytic lignin is composed of some thermally ejected oligomers and the side chains in pyrolytic lignin are primarily in the form of inter-unit C–C linkages, leading to the difficulty of its application in LDP.

2.5. Dilute Acid Lignin

Diluted acids (e.g., H2SO4 and HCl) pulping have been applied in all kinds of lignocellulosic materials. Diluted acid pulping is generally conducted at a temperature of >160 ◦C with the presence of mineral acids as a catalyst. The C–O linkages as well as ester linkages are partially cracked during the diluted acid pulping process. This process may also undermine the three-dimensional amorphous structure in lignin and produce small lignin fragments with the increased proportion of hydroxyl groups [56,57]. Thus, lignin should be removed under benign conditions if it is considered as a feedstock for further chemical production.

2.6. Steam Explosion Lignin The steam explosion pulping process is a fast processing method for lignocellulose, which releases single biomass components via steam impregnation under pressure and then the fast release of pressure [51]. For example, the disposal of wood or bagasse is conducted under high-pressure steam at a short contact time. More than 90% of lignin in hardwood can be recovered from the steam explosion pulping process. The properties of steam explosion lignin are similar to that of organosolv lignin, showing both a higher solubility and a lower molecular weight in an organic solvent. It is also composed of phenolic oligomers with 3 to 12 benzene rings. As a result, steam explosion lignin has potential for producing renewable chemicals through LDP.

3. Oxidative Solvolysis of Lignin Oxidative solvolysis of lignin is a widely used and efficient strategy for producing multiple aromatic monomers. The oxidative solvolysis of lignin reaction includes the cracking of C–O bonds, C–C bonds in lignin. Aromatic and carboxylic acids are the main products from oxidative solvolysis of lignin with different kinds of homogeneous catalysts. O2,H2O2, metal oxides and nitrobenzene have been considered as effective oxidation agents. In this section, the effect of catalyst and solvent on the production of valuable aromatic chemicals in the catalytic oxidation of lignin process will be discussed. Research on oxidative solvolysis of raw lignin (Kraft lignin, Organosolv lignin, etc.) to the aromatic products is summarized in Table1. Polymers 2017, 9, 240 6 of 25

Table 1. Summary of oxidative depolymerization of the raw lignin material.

Entry Feedstock Conditions Solvent Catalyst Products Yield Ref. Acetonitrile-THF or ethyl Vanadium complexes bearing Schiff Monophenolic compounds (vanillin, 0.78 wt %, 0.67 wt %, 0.59 wt % 1 Organosolv lignin (from M. giganteus) 80 ◦C, 24 h, air [58] acetate-THF base ligands syringic acid, ) (Catalyst = Complex 3) Vanadium complexes and other 2 Organosolv lignin 100 ◦C, 18 h, 0.8 MPa synthetic air Ethyl acetate Bio-oil Mw = 575 Da (Catalyst = Complex 2) [59] organometallic catalysts ◦ 3 Organosolv lignin and Kraft lignin 100 C, 8 h, H2O2 DMSO and acetic acid {Fe-DABCO} Bio-oil / [60] Co(salen) supported on 4 Organosolv lignin 80 ◦C, 24 h, air Acetonitrile-THF Vanillin (main) 3067 g [61] graphene oxide V(acac) and Cu(NO ) ·3H O or 5 Organosolv lignin and Kraft lignin 135 ◦C, 40 h, 1.0 MPa O Pyridine 3 3 2 2 Bio-oil Mw = 300 Da [62] 2 HTc-Cu-V Hydrolytic sugar cane lignin and red MTO or poly(4-vinylpyridine)/MTO 6 25 ◦C, 24 h, H O Acetic acid Bio-oil / [63] spruce kraft lignin 2 2 or polystyrene/MTO H O, tert-butyl Nitrogen-containing graphene 7 Organosolv ligin (from birch wood) 140 ◦C, 24 h, 0.1 MPa O 2 Bio-oil 45.8 wt % [64] 2 hydroperoxide (TBHP) material (LCN) Formic acid, acetic acid, succinic acid, 8 Alkali lignin 175–225 ◦C, 0–1 h, 0.5–1.5 MPa O Water NaOH 44.0 wt % [65] 2 oxalic acid, glutaconic acid Water/methanol/1,4-dioxane/ 9 Wheat alkali lignin 150 ◦C, 1 h, H O CuO, Fe (SO ) and NaOH Monophenolic compounds 17.92 wt % (in methanol-water) [66] 2 2 tetrahydrofuran/ethanol 2 4 3 Copper-phenanthroline complex 10 Wood lignin from Loblolly 80 ◦C, 24 h, 0.27/1.24 MPa O Methanol , vanillin 3.5 wt %, 12.6 wt % [67] 2 and NaOH ◦ 11 Kraft lignin 170 C, 0.3 h, 0.5 MPa O2 Methanol-water and H2SO4 H3PMo12O40 Vanillin, methyl vanillate 5.2 wt % [68] ◦ 12 Kraft lignin 170 C, 0.3 h, 1.0 MPa O2 Methanol-water and H2SO4 H3PMo12O40 Vanillin, methyl vanillate 4.6 wt %, 4.2 wt % [69]

CuSO4; FeCl3; ◦ 13 Kraft lignin 170 C, 1 h, 1.0 MPa O2 Methanol-water and H2SO4 Vanillin, methyl vanillate 6.3 wt % [70] CuCl2; CoCl2 ◦ 14 Kraft lignin 45 C, 1 h, H2O2 Acetone-water Metal salt catalysts Vanillin-based monomers 0.51 wt % [71] Vanillin, vanillin acid, 0.99 wt %, 2.91 wt %, 2.52 wt %, 15 Organosolv lignin 180 ◦C, 2 h, 13.8 MPa air Acetic Acid-water Co/Mn/Zr/Br mixture [72] syringaldehyde, syringic acid 4.51 wt %

16 Organosolv beech wood lignin 200W, 5–30 min, H2O2 NaOH solution La/SBA-15 Vanillin, syringaldehyde 9.94 wt %, 15.66 wt %. [73] Vanillin, , 17 Organosolv lignin 185 ◦C, 24 h, 0.1 MPa O Methanol Pd/CeO 5.2 wt %, 0.87 wt %, 2.4 wt % [74] 2 2 4-hydroxybenzaldehyde Vanillin, p-hydroxybenzyl 18 Enzymatic hydrolysis lignin 120 ◦C, 0–3 h, 0.5 MPa O NaOH solution LaMnO 4.32 wt %, 2.03 wt %, 9.33 wt % [75] 2 3 , syringaldehyde Vanillin, p-hydroxybenzyl 19 Enzymatic hydrolysis lignin 120 ◦C, 0–3 h, 0.5 MPa O NaOH solution LaCoO 4.55 wt %, 2.23 wt %, 9.99 wt % [76] 2 3 aldehyde, syringaldehyde Polymers 2017, 9, x FOR PEER REVIEW 7 of 25 Polymers 2017, 9, 240 7 of 25

3.1. Effects of Catalysts 3.1. Effects of Catalysts Catalysts are involved in most cases of the oxidative depolymerization of lignin to produce particularCatalysts products are involvedand even inperform most caseschemoselective of the oxidative oxidation depolymerization of a particular linkage of lignin in lignin to produce [77,78]. particularThe catalysts products widely and used even for perform the oxidative chemoselective solvolysis oxidation of lignin of a can particular be divided linkage into in ligninfive groups: [77,78]. Theorganometallic catalysts widely catalysts, used metal-free-organic for the oxidative catalyst solvolysiss, acid/base of lignin catalysts, can be dividedmetal salts into catalysts, five groups: and zeoliteorganometallic catalysts catalysts,(heterogeneous metal-free-organic catalyst). catalysts, acid/base catalysts, metal salts catalysts, and zeolite catalysts (heterogeneous catalyst). 3.1.1. Organometallic Catalysts 3.1.1. Organometallic Catalysts Several studies have reported on the oxidative solvolysis of raw lignin by using an organometallicSeveral studies catalyst. have Chan reported et al. on [58] the demonstrated oxidative solvolysis that a ofseries raw ligninof vanadium by using complexes an organometallic bearing Schiffcatalyst. base Chan ligands et al. (e.g., [58] demonstratedComplex 3 in thatFigure a series 3) could of vanadium be used complexesto degrade bearing organosolv Schiff lignin base (extractedligands (e.g., from Complex Miscanthus3 in Figuregiganteus3) could using be dioxane, used to acetone, degrade and organosolv ethanol) lignininto phenolic (extracted products from inMiscanthus acetonitrile–THF giganteus orusing ethyl dioxane,acetate–THF. acetone, After and 24 ethanol) h of reaction into phenolic at 80 °Cproducts under air, in vanillin, acetonitrile–THF syringic ◦ acidor ethyl and syringaldehyde acetate–THF. After were 24the hmost of reactioncommon atdegradation 80 C under products, air, vanillin, resulting syringic from the acid guaiacyl and andsyringaldehyde syringyl units were of thelignin, most respectively common degradation(Table 1, Entry products, 1). Pretreatment resulting fromand isolation the guaiacyl methods and couldsyringyl remarkably units of lignin, alter respectivelythe structure (Table of lignin1, Entry and 1). thus Pretreatment affect the and reactivity isolation of methods lignin toward could catalysts.remarkably Then, alter vanadium the structure Complexes of lignin 1, 2 and andthus 4 (Figure affect 3) the and reactivity other orga ofnometallic lignin toward catalysts catalysts. such Then,as 4-acetamido-TEMPO/acid vanadium Complexes (TEMPO1, 2 and = 42,2,6,6-tetramethylpiperidine-1-oxyl)(Figure3) and other organometallic and catalysts (salen)cobalt(II) such as were4-acetamido-TEMPO/acid also used in the catalytic (TEMPO oxidation = 2,2,6,6-tetramethylpiperidine-1-oxyl) of organosolv lignin [59]. Though and most (salen)cobalt(II) catalysts oxidized were thealso lignin used inextracts, the catalytic bis(8-oxy-quinoline) oxidation of organosolvoxovanadium lignin and [ 59bis(phenolate)pyridine]. Though most catalysts oxovanadium oxidized catalyst,the lignin Complex extracts, 2 bis(8-oxy-quinoline)and Complex 5 (Figure oxovanadium 3), respectively, and bis(phenolate)pyridine exhibited the best performance oxovanadium for depolymerization/oxidationcatalyst, Complex 2 and Complex of organosolv5 (Figure lignin.3), respectively, After 18 h exhibitedof reaction the at best100 °C performance in 0.8 MPa for of ◦ syntheticdepolymerization/oxidation air (8% O2 in Ar), the of organosolvbest depolymerization lignin. After performance 18 h of reaction was exhibited at 100 C by in Complex 0.8 MPa of2, whichsynthetic gave air (8%the Olargest2 in Ar), molecular the best depolymerization weight (MW) shift performance and a considerable was exhibited increase by Complex in the2, whichlower moleculargave the largest weight molecular products weight (Table (M 1,W Entry) shift 2). and Iron a considerable complexes could increase also in be the suitable lower molecular for oxidation weight of productslignin. {Fe-DABCO} (Table1, Entry (DABCO 2). Iron complexes= 1,4-diazabicyclo[2.2.2]-octane) could also be suitable for catalyst oxidation was of lignin.presented {Fe-DABCO} for the (DABCOcleavage of = 1,4-diazabicyclo[2.2.2]-octane)β-O-4 linkages in lignin in the catalyst presence was of presentedH2O2 [60]. for After the cleavage8 h of reaction of β-O-4 at 100 linkages °C, the in ◦ masslignin maximum in the presence in the oflignin H2O sample2 [60]. After had shifted 8 h of reaction from 3200 at 100Da toC, around the mass 2500 maximum Da (Table in 1, the Entry lignin 3). GPCsample analysis had shifted showed from that 3200 the Da mass to around maximum 2500 Dahad (Table shifted1, Entryto lower 3). GPCmasses analysis in all showed lignin samples, that the providingmass maximum evidence had for shifted the formation to lower masses of low in mole all lignincular samples,weight products. providing In evidence addition, for all the of formationthe β-O-4 linkagesof low molecular and resinol weight structures products. were In degraded addition, independent all of the β-O-4 of the linkages lignin andpretreatment resinol structures and isolation were conditions.degraded independent of the lignin pretreatment and isolation conditions.

Figure 3. Schematics of the catalyst: vanadium Complexes 1–5. Figure 3. Schematics of the catalyst: vanadium Complexes 1–5.

Polymers 2017, 9, 240 8 of 25

Co(salen)/oxidant has been proven to be an efficient catalytic system for oxidation of phenolic and nonphenolic β-O-4 aryl ether lignin model compounds [79,80]. To improve complexes’ stability, selectivity, and recyclability, Co salen complexes were immobilized on graphene oxide (GO) (Co–GO) for the oxidation of lignin [61]. After 24 h of reaction at 80 ◦C in air, organosolv lignin was converted to vanillin (the main product), vanillic acid, 1-(4-hydroxy-3-methoxyphenyl) ethanol, and other monomeric phenolic compounds (Table1, Entry 4). The oxidation of aliphatic groups resulted in the formation of carbonyl compounds, cleavage of β-O-4 bonds, demethylation of methoxy groups and formation of phenolic OH groups. Recently, the first transition metal system using O2 as an oxidant for cleaving the resinol structure was reported [62]: An oxidative solvolysis of lignin with inexpensive transition-metal-containing hydrotalcites (HTc–Cu–V) or combinations of V(acac)3 (acac = acetylacetonate) and Cu(NO3)2·3H2O as catalysts. As the homogeneous copper and vanadium species from V(acac)3/Cu(NO3)2·3H2O and the leached HTc–Cu–V were most probably very similar in nature, after 40 h of reaction at 135 ◦C, the oxidative solvolysis of lignin afforded products of a mass of approximately 300 Da by using copper-vanadium double-layered hydrotalcites or combined V(acac)3 (acac = acetylacetonate) and Cu(NO3)2·3H2O as catalysts (Table1, Entry 5). Methyltrioxo rhenium (MTO) acts as an active and efficient catalyst for the oxidation of phenolic and non-phenolic lignin model compounds representing the main bond patterns in native lignin resulting in the side-chain oxidation and aromatic ring cracking reaction [81]. Crestini et al. [63] developed immobilized MTO catalysts such as poly(4-vinylpyridine)/MTO and polystyrene/MTO as an easy recovery, reuse and low environmental impact catalyst, for the oxidation of lignin (Table1, Entry 6). Different reaction pathways occurred between soluble MTO and immobilized MTO catalysts in the oxidative solvolysis of lignin: compared to homogeneous MTO, immobilized MTO catalysts with lower Lewis acidity might direct their reactivity toward aliphatic C–H insertion and Dakin-like reactions instead of oxidizing the aromatic ring. Consequently, these differences gave rise to the final product with a higher amount of free phenolic guaiacyl groups in the presence of immobilized MTO catalysts.

3.1.2. Metal-Free-Organic Catalysts Some metal-free-organic catalysts have been identified as effective catalysts for the oxidation of β-O-4 and α-O-4 types of lignin model compounds. Gao et al. [64] used nitrogen-containing graphene material (LCN) to convert birch wood lignin into depolymerized products in water and tert-butyl hydroperoxide solvent. LCN exhibited a similar catalytic behavior in the oxidation of lignin model compounds and birch wood lignin. The catalytic oxidation of benzylic C–H and C–OH bonds into carbonyl groups broke the Cα–Cβ and Cα–O bonds, and further catalyzed the decomposition of some unstable aromatic compounds. Finally, 45.8 wt % bio-oil could be obtained from birch wood lignin ◦ after 24 h of reaction at 140 C in the presence of 0.1 MPa O2 (Table1, Entry 7).

3.1.3. Acid/Base Catalysts NaOH and KOH are utilized most frequently in the alkaline oxidation of lignin. Ouyang et al. [82] reported the oxidative degradation of soda lignin assisted by microwave on NaOH catalyst (pH = 11). Similarly, Wu et al. [83] reported the oxidative degradation of different lignin samples in KOH with H2O2 or K2S2O8 to methanol, formate, carbonate, and oxalate. Even at near-ambient temperatures (60 ◦C), >90% consumption of lignin was observed after 24 h. The microwave irradiation efficiently helped the degradation of lignin to produce high molecular weight compounds. The origin of the lignin, reaction temperature, oxidant concentration, and pH value also had impacts on the conversion efficiency. For example, higher reaction temperature and O2 concentration facilitated the yield of the final products but also gave rise to the repolymerization of lignin fragments. Maziero et al. [84] also reported that sugarcane bagasse lignin oxidized with 9.1% H2O2 (m/v) at pH = 13.3 had the highest fragmentation, oxidation degree and stability. The oxidation of alkali lignin was performed on Polymers 2017, 9, 240 9 of 25

NaOH catalyst (pH = 11) in the presence of O2 [65]. After 20 min of reaction, the maximum total yield of products (formic acid, acetic acid, succinic acid, oxalic acid, and glutaconic acid) of 44 wt % was ◦ obtained at 225 C and 1 MPa O2 (Table1, Entry 8). Ouyang et al. [66] reported the oxidative solvolysis of lignin by using H2O2 as the oxidant and 2+ combination of CuO, Fe2(SO4)3 and NaOH as catalysts. Cu could result in the removal of side chains from basic units of lignin forming some phenolic compounds while with OOH radicals generated by 3+ dissociation of H2O2, Fe could form new reactive intermediates [85], facilitating oxidative solvolysis of lignin. The yield of monophenolic compounds with a higher amount of syringyl unit compounds (syringaldehyde, syringic acid, acetosyringone) reached 17.92 wt % in methanol/water (1:1 (v/v)) solvent at 150 ◦C after 1 h reaction (Table1, Entry 9). Wood lignin from loblolly pine was oxidized by 1,10-phenanthroline and copper (II) sulfate pentahydrate and NaOH catalysts in methanol solvent [67]. Finally, 3.5 wt % vanillic acid and 12.6 wt % vanillin could be obtained after 24 h of reaction at 80 ◦C in the presence of 0.27 MPa O2 (Table1, Entry 10).

3.1.4. Metal Salt Catalysts Polyoxometalates (POMs) consists of a number of metal cluster anions. They are soluble in both water and organic solvents, and its redox potential is high enough to oxidize lignin basic units yet low enough to be reoxidized by O2. A series of POMs are capable of selectively degrading the residual lignin and, in turn, fully converting residual lignin into CO2 and H2O by wet oxidation [86]. The POM treatment of lignin leads to a sharp reduction in the content of α-OH/β-O-4 inter-unit linkages, following demethylation. A high increase in carbonyl groups implies the loss of aromaticity in lignin, probably due to the conversion of aromatic rings to quinone moieties [86,87]. Voitl and Rohr [68] conducted the oxidative solvolysis of lignin by using aqueous POMs in the presence of alcohols for converting kraft lignin into chemicals. After 20 min of reaction, a total yield of products (vanillin and ◦ methyl vanillate) of 5.2 wt % could be obtained at 170 C and 0.5 MPa O2 (Table1, Entry 11). In a stirred batch reactor, vanillin (4.6 wt %) and methyl vanillate (4.2 wt %) could be obtained at 170 ◦C and 1.0 MPa O2 (Table1, Entry 12) [69]. Transition metal salts are another kind of homogeneous catalysts for lignin oxidation, which have a wide range of cation redox potential. Werhan et al. [70] reported the acidic oxidation of kraft lignin with different transition metal salts (CuSO4, FeCl3, CuCl2, CoCl2). The maximum yield of vanillin gained was invariably higher for the transition metal salts than for the POMs. CoCl2 showed the best performance among the investigated catalysts with a maximum yield of 6.3 wt % (vanillin and ◦ methyl vanillate) at 170 C and 1.0 MPa O2 (Table1, Entry 13). In addition, the H 2O2 mediated oxidative solvolysis of kraft lignin was also investigated in the presence of different metal salt catalysts (Fe2(SO4)3, Fe(NO3)·9H2O, Mn(SO4)·H2O, Bi2(SO4)3, Bi(NO3)3·5H2O, H2WO4, and Na2WO4·2H2O) in acetone/water with the use of ultrasound irradiation [71]. The Na2WO4·2H2O catalyst was the most efficient regarding the total yield of vanillin-based monomers (vanillin, acetovanillone, vanillic acid, and guaiacol) with a maximum yield of 0.51 wt % at 45 ◦C (Table1, Entry 14). In addition, the use of ultrasound irradiation in this experiments results in high oxidative coupling of phenoxy radicals generated from LDP. The metal/bromide catalyzed aerobic oxidation of alkylaromatic compounds in acetic acid solvent have shown to be a well-established method to produce aromatic carboxylic acids [88]. Thus, Partenheimer [72] investigated the aerobic oxidative solvolysis of lignin via Co/Mn/Zr/Br catalysts in acetic acid-water solvent. After 2 h of reaction at 180 ◦C in the presence of 13.8 MPa air, the highest yields of aromatic compounds with a total of 10.9 wt % could be obtained from organosolv lignin (Table1, Entry 15). In summary, most of these homogeneous systems lack selectivity for lignin, resulting in a relatively low yield of the target products. The separation and recyclability of homogeneous catalysts is still challenging for the catalytic oxidative solvolysis of lignin. Current research is focused on developing Polymers 2017, 9, 240 10 of 25 novel homogeneous catalysts and another kind of catalysts—heterogeneous catalysts—for the catalytic oxidative solvolysis of lignin.

3.1.5. Heterogeneous Catalyst Considering the limitations of homogeneous catalytic oxidative solvolysis of lignin, heterogeneous catalytic systems have been developed with regard to their advantages in achieving a relatively high yield of the product, combined with an easy separation and recyclability of the catalyst. Microwave assisted catalytic oxidation of organosolv lignin over modified SBA-15 catalyst was investigated [73]. The catalytic oxidation with H2O2 led to different yields of aldehydes, and the respective acids and aceto derivatives. The highest yield of vanillin was 9.94 wt % and that of syringaldehyde was 15.66 wt % (Table1, Entry 16). Then, the oxidative solvolysis of organosolv lignin ◦ in methanol using Pd/CeO2 catalyst was performed [74]. After 24 h of reaction at 185 C under O2, several aromatic monomers including vanillin, guaiacol and 4-hydroxybenzaldehyde were identified and the yields of these monomers were 5.2, 0.87 and 2.4 wt %, respectively (Table1, Entry 17). After that, Deng et al. [75,76] synthesized the perovskite-type LaMnO3 and LaCoO3 catalysts for catalytic wet aerobic oxidation of lignin. Both the perovskite-type oxides, LaMnO3 and LaCoO3, were efficient and recyclable heterogeneous catalysts for catalytic oxidative solvolysis of lignin. When LaMnO3 was used, ◦ the lignin conversion was 57.0% after 3 h at 120 C in the presence of 0.5 MPa O2, and the maximum yields of vanillin, p-hydroxybenzyl aldehyde and syringaldehyde were 4.32 (30 min), 2.03 (120 min), and 9.33 wt % (30 min) respectively (Table1, Entry 18). When LaCoO 3 was used, the lignin conversion after 3.0 h of reaction increased 46.7%, and the maximum yield of vanillin, p-hydroxybenzyl aldehyde and syringaldehyde were 4.55 (60 min), 2.23 (120 min), and 9.99 wt % (50 min) respectively (Table1, Entry 19). It can be concluded that both homogeneous and heterogeneous catalysts for the oxidative solvolysis of lignin should be further developed due to the low selectivity and production for specific compounds.

3.2. Effects of Solvent System

The oxidative solvolysis of lignin has been conducted with the help of molecular O2 or aqueous H2O2 as oxidants. The oxidation of raw lignin as well as lignin model compounds using molecular O2 or aqueous H2O2 as oxidants is highly promising. It was reported that the removal of O2 in the system of H2O2 as an oxidant had no effect on the oxidation reactions [83]. The in-situ oxygen produced by the decomposition of H2O2 during the reaction might be more reactive than the external oxygen. Solvent molecules could coordinate or dissociate to the vacant metal coordination sites in homogeneous catalysis [89]. Different solvents in the oxidative solvolysis of lignin result in a significant change of the yields of aromatic products. Chan et al. [58] conducted the oxidative solvolysis of organosolv lignin into phenolic products in acetonitrile and ethyl acetate. Both acetonitrile and ethyl acetate were optimal solvents, whereas THF had to be added to contribute to the total miscibility of all the reaction components. Ma et al. [90] reported the selective oxidative C–C bond cleavage of a lignin model compound over a vanadium catalyst in different solvents. In triethylamine, the C–H bond cleavage was preferred over the vanadium-catalyzed oxidation with O2, while C–C bond cleavage was dominant in acetic acid. It was believed that the coordination of the carboxylic group in acetic acid solvent to vanadium (V) catalyst was vital for C–C bond cleavage. The oxidative solvolysis of lignin generates a number of radicals resulting in the repolymerization reaction to limit the production of aromatic compounds. Therefore, some research explored the addition of some radical scavengers in order to quench the reactive lignin fragments before repolymerization took place. The alcohol as a quenching agent could generate the radicals such as CH3O· or ·CH3. The role of those alcohols was investigated in the acidic degradation of lignin [68]. Methanol and ethanol might prevent the condensation through the competitive reaction with intermediate carbonium Polymers 2017, 9, 240 11 of 25

ions. Radical coupling of lignin fragments with CH3O· and ·CH3 produced from methanol occurred via acid-catalyzed formation of dimethyl ether (Equations (1) and (2)).

2CH3OH → CH3OCH3 + H2O (1)

−1 CH3OCH3 → CH3O· + ·CH3 BDE = 84 kcal·mol (2) The effect of varied alcohol solvents on the oxidative solvolysis of lignin is still uncertain. Pan et al. [91] found that acetonitrile and methanol exhibit a positive effect on the oxidative degradation of lignin, but dioxane and ethanol exhibit a negative effect. It was reported that a composite-solvent composed of water and an organic solvent could both promote depolymerization of lignin and prevent repolymerization of lignin fragments (monomers), increasing the yield of the aromatic products [66]. In addition, under specific reaction conditions the solvent will play a different role as opposed to its normal role. Deng et al. [74] proposed that methanol as a solvent in oxidative solvolysis of lignin model compounds, which could afford active H species for the hydrogenolysis of the β-O-4 bond, producing and acetophenone. Similarly, Shilpy et al. [92] investigated the influence of the property of the solvent on the oxidative solvolysis of and vanillin. The highest conversion rate was observed in acetic acid (91%), followed by ethyl alcohol (70%), acetonitrile (67%), N,N-dimethyl (35%), tetrahydrofuran (20%) and acetone (1%). Thus it was proposed that the major role was played by peracetic acid that was in-situ generated by reacting acetic acid with H2O2 over an acid catalyst [93]. Then, peracetic acid accelerated and acted as an oxidant in the oxidation reaction. Most of the solvents in the LDP process act as agents for dissolving lignin, products and homogeneous catalysts. In some cases, simple alcohols could exert an unexpected influence for protecting the produced aromatic fragments during the oxidative solvolysis process of lignin. The multi-function of solvents should be further investigated for the low-cost and high-efficient oxidative solvolysis of lignin.

4. Oxidative Solvolysis of Lignin-Related Model Compounds The complexity and variability of lignin structure promoted the use of lignin-related model compounds for LDP studies, giving rise to a better understanding of the cracking mechanism via specific inter-unit linkage thus promoting the production of aromatic compounds. In this section, oxidative solvolysis of lignin-related model compounds (both aromatic monomers and oligomers) will be overviewed, concentrating on the reforming of aromatic monomers and the cracking of inter-unit linkages in oligomers.

4.1. Oxidative Reforming of Lignin-Derived Monomers To understand the reactivity of the functional groups on lignin, much attention have been paid to the oxidative reforming of aromatic monomers. The representative lignin-related aromatic monomers such as apocynol, veratryl alcohol, 3-methoxy-4-hydroxybenzyl alcohol and 4-hydroxybenzyl alcohol were studied in recent years.

4.1.1. Apocynol Sushanta et al. [94,95] reported an efficient oxidative solvolysis of apocynol (M1) over SBA-15 and Co (salen)/SBA-15 by using H2O2 as an oxidant. The reaction pathway for M1 conversion on SBA-15 and Co (salen)/SBA-15 is shown in Figure4. The first oxidation product was acetovanillone, then vanillin was formed through the side chain cleavage while 2-methoxybenzoquinone as a product of oxidation of the phenolic group together with oxidative degradation of the p-alkyl side chain. Co (salen)/SBA-15 showed a highly efficient performance (nearly 100%) for M1 conversion after 40 min microwave heating and SBA-15 showed an unusual oxidative ability and excellent selectivity for acetovanillone. Badamali et al. [96] also demonstrated the oxidation of M1 by mesoporous MCM-41, Polymers 2017, 9, 240 12 of 25

HMS, SBA-15 and amorphous silica with using H2O2 as an oxidant under microwave irradiation. With the same conversion pathway on SBA-15 or Co (salen)/SBA-15, acetovanillone, vanillin and

2-methoxybenzoquinonePolymers 2017, 9, 240 were the main products (Figure4). Silica as a catalyst led to the12 of highest 25 conversion (94%) of M1 among the studied catalysts with the equal selectivity to acetovanillone and 2-methoxybenzoquinone.selectivity to acetovanillone Itand could 2-methoxybenzoquinone. be concluded thatthe It could surface be hydroxylconcluded groupsthat the andsurface internal porehydroxylPolymers structure 2017 groups, of 9, 240 the and employed internal silicaspore structure play a keyof the role employed in the catalytic silicas play oxidative a key role process. in the catalytic In12 addition,of 25 Hansonoxidative et al. [process.97] reported In addition, that dipicolinate Hanson et al. vanadium [97] reported complexes that dipicolinate oxidized vanadium 2-phenoxyethanol complexes in air. oxidizedselectivity 2-phenoxyethanol to acetovanillone in and air. 2-methoxybenzoquinone. After the reaction at 100 °CIt couldfor one be week, concluded approximately that the surface20% of After the reaction at 100 ◦C for one week, approximately 20% of the 2-phenoxyethanol was consumed. thehydroxyl 2-phenoxyethanol groups and internal was consumed. pore structure Phenol of the(18% employed), formic silicasacid (6%), play aand key several role in otherthe catalytic minor Phenolunidentifiedoxidative (18%), process. formic products acidIn addition,were (6%), detected. and Hanson several et otheral. [97] minor reported unidentified that dipicolinate products vanadium were detected. complexes oxidized 2-phenoxyethanol in air. After the reaction at 100 °C for one week, approximately 20% of the 2-phenoxyethanol was consumed. Phenol (18%), formic acid (6%), and several other minor unidentified products were detected.

Figure 4. Reaction pathway for apocynol (M1) conversion on the catalysts. Figure 4. Reaction pathway for apocynol (M1) conversion on the catalysts. 4.1.2. Veratryl alcohol/3-methoxy-4-hydroxybenzyl alcohol/4-hydroxybenzyl alcohol 4.1.2. Veratryl Alcohol/3-Methoxy-4-HydroxybenzylFigure 4. Reaction pathway for apocynol Alcohol/4-Hydroxybenzyl(M1) conversion on the catalysts. Alcohol Mate et al. [98,99] reported the oxidation of veratryl alcohol over a nano-structured, spinel MateCo3O4 etcatalyst al. [98 ,in99 ]the reported presence the of oxidation O2. After of7 h veratryl of reaction alcohol at 140 over °C ain nano-structured, water, the Co3O4 spinel catalyst Co O 4.1.2. Veratryl alcohol/3-methoxy-4-hydroxybenzyl alcohol/4-hydroxybenzyl alcohol 3 4 ◦ catalystyielded in the the presence highest ofconversion O2. After of 7 85% h of together reaction wi atth 140 a 96%C in selectivity water, the to Co veratryl3O4 catalyst aldehyde. yielded In the highestdifferent conversionMate solvents, et al. of[98,99] the 85% catalytic togetherreported activity the with oxidation aof 96% the Co selectivity of3O ve4 catalystratryl to alcohol veratrylwas proved over aldehyde. ato nano-structured, occur Inin differentthe following spinel solvents, Coorder:3O4 catalysttoluene in> waterthe presence > ethanol of O> 2.methanol. After 7 h Theof reaction highest atconversion 140 °C in (75%)water, wasthe Coobserved3O4 catalyst in a the catalytic activity of the Co3O4 catalyst was proved to occur in the following order: toluene > water > nonpolaryielded the solvent highest such conversion as toluene of together85% together with abovewith a97% 96% selectivity selectivity to to veratryl veratryl aldehyde. aldehyde. The In ethanol > methanol. The highest conversion (75%) was observed in a nonpolar solvent such as toluene proposeddifferent solvents, pathway the for catalyticoxidation activity of veratryl of the alcohol Co3O 4over catalyst Co3O was4 catalyst proved was to presentedoccur in the in Schemefollowing 1: together with above 97% selectivity to veratryl aldehyde. The proposed pathway for oxidation of order:The adsorbed toluene veratryl > water alcohol > ethanol formed > methanol. an intermolecular The highest bond conversionbetween Co 3+(75%) and Owas2 while observed the surface in a veratryloxidenonpolar alcohol ion solventformed over Co suchan3O intermolecular4ascatalyst toluene was together bond presented withwith inabovethe Scheme 97%1 selectivity: The of adsorbedthe toalco veratrylholic veratryl group; aldehyde. alcohol then Thethe formed 3+ an intermolecularabstractionproposed pathway of hydrogen bond for between oxidation by the Co oxideof veratryland ion Olead alcohol2 while to the over the formation surfaceCo3O4 catalyst oxideof Co 3+was ionOH presented formedspecies; anafter inintermolecular Scheme that the 1: bondactivatedThe with adsorbed the Co hydrogen3+ OHveratryl species alcohol of thecould alcoholicformed produce an group;intermolecularanother thenintermolecular the bond abstraction between hydrogen Co of3+ hydrogenandbond O and2 while abstraction by the the surface oxide of ion 3+ 3+ leadoxidean to theelectron ion formation formed resulted an of in Cointermolecular the OHformation species; bondof Co after 2+with, H that 2Othe and thehydrogen the activated main of product, the Co alcoOH veratrylholic species group; aldehyde; could then producethe 3+ anotherabstractionreduced intermolecular Co 2+of species hydrogen hydrogen were by reoxidized the bond oxide andwith ion abstraction Olead2 to toregenerate the offormation an the electron active of Co resultedCo3+OHO− species.species; in the formationafter that the of Co2+, activated Co3+OH species could produce another intermolecular2+ hydrogen bond and abstraction of H2O and the main product, veratryl aldehyde; the reduced Co species were reoxidized with O2 to an electron resulted in the formation of Co2+, H2O and the main product, veratryl aldehyde; the regenerate the active Co3+O− species. reduced Co2+ species were reoxidized with O2 to regenerate the active Co3+O− species.

Scheme 1. Proposed pathways for oxidation of veratryl alcohol over Co3O4 catalyst [99], reproduced with permission from Elsevier.

GuScheme et 1. Proposedal. [100,101] pathways performed for oxidation the of veratryl oxidation alcohol overof Coa 3Olignin4 catalyst model [99], reproduced compound, Scheme 1. Proposed pathways for oxidation of veratryl alcohol over Co O catalyst [99], reproduced 3-methoxy-4-hydroxybenzylwith permission from Elsevier. alcohol (M2), over a mesoporous La/SBA-153 4 catalyst. Sixty-eight withpercent permission conversion from of Elsevier.M2 to vanillin was obtained after 30 min of reaction under 200 W microwave Gu et al. [100,101] performed the oxidation of a lignin model compound, 3-methoxy-4-hydroxybenzyl alcohol (M2), over a mesoporous La/SBA-15 catalyst. Sixty-eight percent conversion of M2 to vanillin was obtained after 30 min of reaction under 200 W microwave

Polymers 2017, 9, 240 13 of 25

Gu et al. [100,101] performed the oxidation of a lignin model compound, 3-methoxy-4- hydroxybenzyl alcohol (M2), over a mesoporous La/SBA-15 catalyst. Sixty-eight percent conversion Polymers 2017, 9, 240 13 of 25 of M2 to vanillin was obtained after 30 min of reaction under 200 W microwave irradiation with the presence of H O as oxidant. Reaction pathway for M2 conversion on La/SBA-15 was irradiation with the presence2 2 of H2O2 as oxidant. Reaction pathway for M2 conversion on La/SBA-15 shownwas shown in Figure in Figure5. Pan5. Pan et et al. al. [ [91]91] also report reporteded the the microwave-assisted microwave-assisted oxidative oxidative degradation degradation of of3-methoxy-4-hydroxybenzyl 3-methoxy-4-hydroxybenzyl alcohol alcohol (M2) (andM2) 4-hydroxybenzyl and 4-hydroxybenzyl alcohol alcohol (M3) with (M3 14) withtypes 14of metal types of metal salts in the presence of H O . After 10 min of reaction with a CrCl catalyst at salts in the presence of H2O2. After2 102 min of reaction with a CrCl3 catalyst 3at 80 °C in 80 ◦C in H O/acetonitrile (1:1, v/v), M2 was converted to vanillin (59.8%), vanillic acid (9.7%), H2O/acetonitrile2 (1:1, v/v), M2 was converted to vanillin (59.8%), vanillic acid (9.7%), 5-hydroxyl-4-methoxyl-7-ketone-4-heptylic acid acid (17.0%), (17.0%), etc., etc., through through the pathway the pathway depicted depicted in Figure in5. DifferentFigure 5. fromDifferent the conversionfrom the conversion on La/SBA-15, on La/S vanillicBA-15, acid vanillic underwent acid underwent a ring-opening a ring-opening reaction to producereaction 5-hydroxyl-4-methoxyl-7-ketone-4-heptylicto produce 5-hydroxyl-4-methoxyl-7-ketone-4-heptylic acid. Under the sameacid. reactionUnder the conditions, same reactionM3 was convertedconditions, to M3 hydroquinone was converted (33.2%), to 4-hydroxybenzaldehyde hydroquinone (33.2%), (22.2%), 4-hydroxybenzaldehyde 4-hydroxybenzonic acid (22.2%), (7.6%), M3 etc.4-hydroxybenzonic The reaction pathway acid (7.6%), for etc.conversion The reaction on CrCl pathway3 was shown for M3 in conversion Figure6. 4-hydroxybenzaldehyde on CrCl3 was shown in andFigure 4-hydroxybenzonic 6. 4-hydroxybenzaldehyde acid were the and first two4-hydroxybenzonic oxidation products acid from wereM3 .the Then first the decarboxylationtwo oxidation ofproducts 4-hydroxybenzonic from M3. Then acid the resulted decarboxylation in the formation of 4-hydroxyben of phenol andzonic then acid phenol resulted reacted in the with formation hydroxyl of radicalsphenol and formed then byphenol a microwave reacted with irradiation hydroxyl of radicals H2O2, toformed produce by a hydroquinone. microwave irradiation In addition, of H2 theO2, pathwayto produce for hydroquinone.M2 on immobilized In addition, MTO [63 ]the and pathway on CoTiO for3 [ 92M2] was on alsoimmobilized different fromMTO the [63] conversion and on onCoTiO La/SBA-153 [92] was and also on different CrCl3 (Figure from the5). conversion After 6 h ofon reactionLa/SBA-15 with and immobilized on CrCl3 (Figure MTO 5). catalyst After 6 at h M2 roomof reaction temperature with immobilized in CH3COOH/H MTO2 Ocatalyst2, was at convertedroom temperature to vanillin in (14.2%), CH3COOH/H vanillic2O acid2, M2 (18.9%), was muconolactoneconverted to vanillin (5.5%), (14.2%), etc. The vanillic muconolactone acid (18.9%), was muconolactone mostly gained (5.5%), via an excessiveetc. The muconolactone oxidative ring openingwas mostly reaction gained to avia muconic an excessive acid intermediate oxidative ring followed opening by thereaction formation to a ofmuconic a lactone acid moiety, intermediate perhaps catalyzedfollowed by by the MTO. formation For the of CoTiO a lactone3 catalyst, moiety, with perhaps the help catalyzed of H2O by2 inMTO. acetic For acid the andCoTiO isopropanol3 catalyst, ◦ solvent,with the ahelp remarkable of H2O2 selectivityin acetic acid of 99.8%and isopropanol to vanillin solvent, was obtained a remarkable at 85 C. selectivity The major of products99.8% to werevanillin vanillin, was obtained vanillic at acid 85 °C. and The guaiacol. major products Scheme were2 illustrated vanillin, thevanillic proposed acid and mechanism guaiacol. forScheme the oxidation2 illustrated of the vanillyl proposed alcohol mechanism over CoTiO for 3the[92 oxidation]. Initially of thevanillyl catalyst alcohol reacted over with CoTiO H23 O[92].2 to Initially form a hydroperoxylthe catalyst reacted radical, with which H2O acts2 to form as asource a hydroperoxyl of highly activeradical, oxidant, which acts and as then a source attacks of the highly catalyst active to formoxidant, an intermediate.and then attacks When the vanillylcatalyst alcoholto form wasan intermediate. activated on theWhen catalyst vanillyl surface, alcohol the was electron-rich activated hydroperoxylon the catalyst attacks surface, the the partially electron-rich positive hydroper chargedoxyl carbon attacks and the the partially hydrogen positive of the hydroxylcharged carbon group throughand the hydrogen oxygen to formof the an hydroxyl activated group oxygen through species. ox Thisygen facilitated to form an the activated removal ofoxygen the hydroxyl species. group This fromfacilitated the hydroperoxyl the removal andof the produced hydroxyl water group as a fr by-product.om the hydroperoxyl At the same and time, produced the vanilloxy water cation as a releasedby-product. a At the that same was attacked time, the by electron-richvanilloxy cati oxygenon released of the hydroperoxyla proton that group was to attacked produce theby finalelectron-rich product, oxygen vanillic of acid. the hydroperoxyl group to produce the final product, vanillic acid.

Figure 5. Reaction pathway for 3-methoxy-4-hydroxybenzyl alcohol (M2) conversion on the Figure 5. Reaction pathway for 3-methoxy-4-hydroxybenzyl alcohol (M2) conversion on the catalysts. catalysts.

Polymers 2017, 9, 240 14 of 25 Polymers 2017, 9, 240 14 of 25 Polymers 2017, 9, 240 14 of 25

Figure 6. Reaction pathway for 4-hydroxybenzyl alcohol (M3) conversion on the catalyst [91], Figure 6. Reaction pathway for 4-hydroxybenzyl alcohol (M3) conversion on the catalyst [91], Figurereproduced 6. Reaction with permission pathway fromfor 4-hydroxybenzylAmerican Chemical alcohol Society. (M3 ) conversion on the catalyst [91], reproducedreproduced with with permission permission from from American American Chemical Chemical Society.

Scheme 2. The suggested mechanism for formation of the main oxidation product (vanillin and

Schemevanillic acid)2. The from suggested vanillyl mechanismalcohol in the for presence formation of CoTiOof the3 [92],main reproduced oxidation product with permission (vanillin fromand Scheme 2. The suggested mechanism for formation of the main oxidation product (vanillin and vanillic vanillicRoyal Society acid) from of Chemistry. vanillyl alcohol in the presence of CoTiO3 [92], reproduced with permission from acid)Royal from Society vanillyl of alcoholChemistry. in the presence of CoTiO3 [92], reproduced with permission from Royal Society4.2. Oxidative of Chemistry. Cracking of Typical Inter-Unit Linkages in Lignin-Derived Oligomers 4.2. Oxidative Cracking of Typical Inter-Unit Linkages in Lignin-Derived Oligomers 4.2. Oxidative4.2.1. C–C Cracking Linkage of Typical Inter-Unit Linkages in Lignin-Derived Oligomers 4.2.1. C–C Linkage Homogeneous copper and vanadium complexes are well-known to be effective catalysts for 4.2.1. C–C Linkage aerobicHomogeneous oxidation reactions copper ofand diols vanadium with concomitan complexest C–C are bondwell-known cleavage to [102,103]. be effective Recognizing catalysts thatfor Homogeneousaerobiccopper oxidationand vanadium copperreactions complexes and of diols vanadium withcould concomitan potentially complexest C–Cbe areused bond well-known for cleavage the C–C [102,103]. tobond be effectivecleavage Recognizing in catalysts lignin that for aerobiccopperdepolymerization, oxidation and vanadium reactions some complexes scientists of diols couldbegan with concomitantpotentiallyto synthesize be oxovanadium C–Cused bondfor the cleavage C–C(e.g., bond Complex [102 cleavage,103 1 in]. Figurein Recognizing lignin 3) depolymerization, some scientists began to synthesize oxovanadium (e.g., Complex 1 in Figure 3) that copperand copper and catalysts vanadium and complexes explore their could fundamental potentially reactivity. be used Hanson for the et C–C al. [97,104] bond cleavagesynthesized in lignina andseries copper of vanadium catalysts complexes and explore and their homogeneous fundamental coppe reactivity.r catalysts Hanson to explore et al. their[97,104] reactivity synthesized toward a depolymerization, some scientists began to synthesize oxovanadium (e.g., Complex 1 in Figure3) and seriesaerobic of oxidationvanadium of complexes a simple andlignin homogeneous model compound, copper catalysts1,2-diphenyl-2-methoxyethanol to explore their reactivity (M4 toward). The copper catalysts and explore their fundamental reactivity. Hanson et al. [97,104] synthesized a series of aerobicvanadium oxidation complex of (dipic)V a simpleV(O)(O ligniniPr) model (Complex compound, 1) reacted 1,2-diphenyl-2-methoxyethanol with M4 in DMSO with the addition (M4). The of vanadiumvanadiumair at 100 complexes °C,complex producing and (dipic)V homogeneous predominantlyV(O)(OiPr) (Complex copperbenzaldehy catalysts1) reactedde and towith methanol, explore M4 in their butDMSO in reactivity pyridine with the it towardaddition produced aerobicof oxidationairpredominantly at of100 a °C, simple producing benzoic lignin acid predominantly model and compound,methyl benzaldehybenzoate 1,2-diphenyl-2-methoxyethanol (Figurede and 7). methanol, The ketone but benzoin in pyridine (methylM4). it The producedether vanadium was ◦ complexpredominantlyan intermediate (dipic)VV(O)(O benzoic in thisiPr) acid reaction. (Complex and methyl The1) CuCl/2,2 reacted benzoate with,6,6-tetramethylpiperidine-1-oxyl (FigureM4 in7). DMSOThe ketone with benzoin the addition methylradical of ether(TEMPO) air atwas 100 C, producinganmixture intermediate predominantly could alsoin this react benzaldehydereaction. with M4The inCuCl/2,2 andpyridine methanol,,6,6-tetramethylpiperidine-1-oxyl under but O2 inat pyridine 100 °C, itaffording produced radical a predominantlymixture (TEMPO) of benzoicmixturebenzaldehyde acid andcould methyl (84%) also andreact benzoate methyl with benzoate (FigureM4 in 7pyridine (88%)). The (Fig ketone underure 7).benzoin OUnlike2 at 100the methyl vanadium°C, affording ether catalyst was a an mixturesystem, intermediate theof in thisbenzaldehydecopper-catalyzed reaction. The (84%) CuCl/2,2,6,6-tetramethylpiperidine-1-oxyl reactions and methyl were benzoate proposed (88%) for direct(Figure C–C 7). Unlikebond radicalcleavage the vanadium (TEMPO) of M4 catalyst without mixture system, ketone could the or also copper-catalyzedaldehyde intermediates. reactions Recently, were proposed four◦ new for directvanadium(V) C–C bond complexes cleavage ofof M4amino-bis(phenolate) without ketone or react with M4 in pyridine under O2 at 100 C, affording a mixture of (84%) and methyl aldehydeligands (e.g., intermediates. Complex 5 inRecently, Figure 3) four have new been vanadium(V)synthesized, andcomplexes all complexes of amino-bis(phenolate) were employed for benzoate (88%) (Figure7). Unlike the vanadium catalyst system, the copper-catalyzed reactions were ligandsthe catalytic (e.g., aerobicComplex oxidative 5 in Figure C–C 3) bond have cleavage been synthesized, of M4 in air and at all100 complexes °C, where were benzaldehyde employed andfor proposedthemethyl catalytic for benzoate direct aerobic C–C were oxidative bond identified cleavage C–C as bondthe of M4major cleavagewithout products of ketoneM4 [105]. in air or In aldehydeat DMSO, 100 °C, a where intermediates.reaction benzaldehyde with Complex Recently, and 5 four newmethyl vanadium(V)produced benzoate benzaldehyde complexes were identified at 90%, of amino-bis(phenolate) methanolas the major at 46%,products methyl [105]. ligands benzoate In DMSO, (e.g., at 36%, Complexa reaction benzoin 5with methylin FigureComplex ether3) at5 have beenproduced18%, synthesized, and abenzaldehyde small and amount all complexes at of 90%, benzoic methanol wereacid. In at employed pyridine,46%, methyl methyl for benzoate the benzoate catalytic at 36%, (67%), aerobic benzoin benzoin oxidative methyl methyl ether C–C ether at bond cleavage18%,(15%), of and M4and a insmallmethanol air amount at 100 (2%)◦ C,of were benzoic where obtained. benzaldehydeacid. InBenzaldehyde pyridine, and methyl methylwas detectedbenzoate benzoate as(67%), a wereminor benzoin identified product methyl (1%) as ether the and major products(15%),no benzoic [105 and]. Inmethanolacid DMSO, was detected.(2%) a reaction were obtained. with Complex Benzaldehyde5 produced was detected benzaldehyde as a minor at90%, product methanol (1%) and at 46%, methylno benzoatebenzoic acid at 36%,was detected. benzoin methyl ether at 18%, and a small amount of benzoic acid. In pyridine, methyl benzoate (67%), benzoin methyl ether (15%), and methanol (2%) were obtained. Benzaldehyde was detected as a minor product (1%) and no benzoic acid was detected. Polymers 2017, 9, 240 15 of 25 Polymers 2017, 9, 240 15 of 25

Polymers 2017, 9, 240 15 of 25

Figure 7. The aerobic oxidation of 1,2-diphenyl-2-methoxyethanol (M4) with vanadium and copper Figure 7. The aerobic oxidation of 1,2-diphenyl-2-methoxyethanol (M4) with vanadium and catalysts. copper catalysts. Degradation research indicated that β-1 structures are widespread, especially in hardwood Degradationlignin [106].Figure The 7.research The β-1 aerobic linkage indicated oxidation has not of yet that1,2-diphenyl-2-methoxyethanol beenβ-1 adequately structures addressed are widespread, (M4 by) with reductive vanadium especially cleavage. and copper However, in hardwood catalysts. ligninHanson [106]. Theet al.β [47,104,107]-1 linkage hassuccessfully not yet beenused adequatelytheir CuOTf/2,6-lutidine/TEMPO addressed by reductive catalyst cleavage. system However,and (HQ)2VV(O)(OiPr) (Complex 2) to achieve the aerobic oxidation of nonphenolic and phenolic β-1 Hanson et al.Degradation [47,104,107 research] successfully indicated usedthat β their-1 structures CuOTf/2,6-lutidine/TEMPO are widespread, especially catalyst in hardwood system and lignin models (M5 and M6). It was found that the selectivity of the aerobic oxidation of β-1 lignin (HQ) VligninV(O)(O [106].iPr) The (Complex β-1 linkage2 )has to not achieve yet been the adequately aerobic addressed oxidation by of reductive nonphenolic cleavage. and However, phenolic β-1 model2 compounds depended on both the catalyst and substrate. In the absence of phenolic groups, lignin modelsHanson (etM5 al. and[47,104,107]M6). It successfully was found used that their the CuOTf/2,6-lutidine/TEMPO selectivity of the aerobic catalyst oxidation system of andβ-1 lignin α Complex 2V showedi a high selectivity for C –H bond cleavage, however the copper catalyst system model compounds(HQ)2V (O)(O dependedPr) (Complex on 2) both to achieve the catalyst the aerobic and oxidation substrate. of Innonphenolic the absence and of phenolic phenolic β-1 groups, workedlignin on models Cα–C β(M5 bond and cleavage M6). It was(Figure found 8a). that The the introduction selectivity of of the a phenolic aerobic oxidation functional of groupβ-1 lignin in the Complex 2 showed a high selectivity for Cα–H bond cleavage, however the copper catalyst system substratemodel compoundsenabled the depended cracking onof boththe Ctheα–C catalystaryl bond and with substrate. the help In the of absenceboth vanadium of phenolic and groups, copper workedcatalystsComplex on C (Figureα–C 2 showedβ bond8b). As a cleavage highfor the selectivity copper (Figure catalyst,for 8Ca).α–H Theth bonde reaction introduction cleavage, of C–C however bond of a the phenoliccleavage copper might functionalcatalyst proceed system group via in the substrateeitherworked a one-electron enabled on Cα–Cβ the bond oxidation cracking cleavage ofor (Figure thea selective C α8a).–C Thearyl oxidation introductionbond with of ofthe a phenolicprimary help of bothfunctionalalcohol, vanadium followedgroup in and theby coppera catalystsretro-aldolsubstrate (Figure reaction,enabled8b). As the forwhile, cracking the copperfor theof the catalyst,vanadium Cα–Caryl the bondcatalyst, reaction with thethe of reactionhelp C–C of bond both was cleavagevanadium in accordance might and copper proceedwith a via eithertwo-electron acatalysts one-electron (Figure oxidation oxidation 8b). Aspathway for or the a copper[47,104]. selective catalyst, oxidation the reaction of the of primary C–C bond alcohol, cleavage followed might proceed by a retro-aldolvia reaction,either while, a forone-electron the vanadium oxidation catalyst, or a selective the reaction oxidation was inof accordancethe primary withalcohol, a two-electron followed by oxidationa retro-aldol reaction, while, for the vanadium catalyst, the reaction was in accordance with a pathwaytwo-electron [47,104]. oxidation pathway [47,104].

Figure 8. (a) The aerobic oxidation of non-phenolic β-1 linkage lignin model compound (M5) with

vanadium and copper catalysts; and (b) the aerobic oxidation of phenolic β-1 linkage lignin model Figure 8. (a) The aerobic oxidation of non-phenolic β-1 linkage lignin model compound (M5) with Figurecompound 8. (a) The (M6 aerobic) with oxidationvanadium ofand non-phenolic copper catalystsβ-1 linkage[104], reproduced lignin model with compound permission (fromM5) with vanadium and copper catalysts; and (b) the aerobic oxidation of phenolic β-1 linkage lignin model American Chemical Society. vanadiumcompound and copper (M6) with catalysts; vanadium and and (b) thecopper aerobic catalysts oxidation [104], reproduced of phenolic withβ-1 permission linkage lignin from model

compoundAmerican (M6) withChemical vanadium Society. and copper catalysts [104], reproduced with permission from American Chemical Society. Polymers 2017, 9, 240 16 of 25

Polymers 2017, 9, 240 16 of 25 4.2.2. C–O Linkage 4.2.2. C–O Linkage The benzylic C–H or C–OH bonds in lignin would be easily attacked and transformed to carbonyl groups underThe oxidativebenzylic C–H conditions. or C–OH Next, bonds the in C αlignin–O or would Cβ–O be bond easily can attacked be cracked and to transformed the corresponding to fragmentscarbonyl [3]. groups The oxidation under oxidative of α-O-4 conditions. or β-O-4 linkagesNext, the containing Cα–O or Cβ lignin–O bond model can be compounds cracked to hasthe been extensivelycorresponding investigated. fragments The [3]. key The for oxidation achieving of theα-O-4 production or β-O-4 oflinkages specific containing chemicals lignin from model selective compounds has been extensively investigated. The key for achieving the production of specific oxidation of lignin depends on the advance of catalysts. chemicals from selective oxidation of lignin depends on the advance of catalysts. 4.2.3. α-O-4 Containing Model Compound 4.2.3. α-O-4 Containing Model Compound HaibachHaibach et al. et [al.108 [108]] investigated investigated the the dehydroaryloxylation dehydroaryloxylation of of aryl aryl alkyl alkyl ethers ethers in p in-xylenep-xylene using using ◦ pincerpincer iridium iridium catalysts catalysts at at a moderatea moderate temperature temperature (150 (150 or or 200 200 °C).C). This This system system showed showed a rare a rare atom-economicalatom-economical process process for for C–O C–O ether ether bond bond cleavage,cleavage, producing producing a alimited limited number number of substituted of substituted alkylalkyl aryl ethers.aryl ethers. Recently, Recently, Gao Gao et et al. al. [64 [64]] used used nitrogen-containingnitrogen-containing graphene graphene material material (LCN) (LCN) as an as an effectiveeffective catalyst catalyst for thefor oxidationthe oxidation of αof-O-4 α-O-4 types types of of a a lignin lignin model model compoundcompound ( (M7M7) )in in the the presence presence of tert-butylof tert hydroperoxide-butyl hydroperoxide (TBHP), (TBHP), to produce to produce aromatic aromat aldehydesic aldehydes and acidsand acids in high in high yield. yield. The The reaction pathwayreaction for M7pathwayconversion for M7 conversion on LCN is on shown LCN inis shown Figure in9. Figure A free-radical 9. A free-radical mechanism mechanism was involved, was involved, initiated by a benzylic C–H bond activation, followed by a Cα–O bond cleavage, and initiated by a benzylic C–H bond activation, followed by a Cα–O bond cleavage, and finally completed finally completed by a further oxidation of intermediate aromatics. Similarly, an oxidative C–O by a further oxidation of intermediate aromatics. Similarly, an oxidative C–O bond cleavage of M7 bond cleavage of M7 was demonstrated◦ on Complex 5 at 80 °C, but the conversion was significantly was demonstratedlimited [105]. on Complex 5 at 80 C, but the conversion was significantly limited [105].

Figure 9. Reaction pathway for α-O-4 lignin model dimer (M7) conversion on the catalyst [64], Figure 9. Reaction pathway for α-O-4 lignin model dimer (M7) conversion on the catalyst [64], reproduced with permission from John Wiley and Sons. reproduced with permission from John Wiley and Sons. 4.2.4. β-O-4 Containing Model Compound 4.2.4. β-O-4 Containing Model Compound The oxidative solvolysis of a lignin model compound, 2-phenoxy-1-phenylethanol (M8), was Theperformed oxidative with solvolysisa CrCl3 catalyst of a while lignin using model H2O compound,2 as an oxidant 2-phenoxy-1-phenylethanol at 80 °C [91]. After 10 min of reaction (M8), was in H2O/acetonitrile solvent, M8 was converted to phenol (3.1%), benzoic◦ acid (7.4%), performed with a CrCl3 catalyst while using H2O2 as an oxidant at 80 C[91]. After 10 min 2-hydroxy-1-phenylethanone (17.3%), 2-phenoxy-1-phenylethanone (20.1%), etc. The reaction of reaction in H2O/acetonitrile solvent, M8 was converted to phenol (3.1%), benzoic acid (7.4%), pathway for M8 on CrCl3 is shown in Figure 10. M8 was firstly oxidized to produce 2-hydroxy-1-phenylethanone (17.3%), 2-phenoxy-1-phenylethanone (20.1%), etc. The reaction 2-phenoxy-1-phenylethanone. The β-O-4 bond in 2-phenoxy-1-phenylethanone was cleaved through pathway for M8 on CrCl3 is shown in Figure 10. M8 was firstly oxidized to produce two pathways: (1) between the Cβ–O atoms with 2-hydroxy-1-phenylethanone and phenol as the 2-phenoxy-1-phenylethanone. The β-O-4 bond in 2-phenoxy-1-phenylethanone was cleaved through major product; and (2) between the Cα–Cβ atoms with benzoic acid as the major product. It is two pathways:estimated that (1) the between cleavage theof the C βC–Oβ–O atomsbond was with easier 2-hydroxy-1-phenylethanone than that of the Cα–Cβ bond. Hanson and phenol et al. as the major[109] used product; the CuOTf/2,6-lutidine/TEMPO and (2) between the C αcatalyst–Cβ atoms system with to conduct benzoic the acidaerobic as oxidation the major of product.M8 It isobtaining estimated the that overall the conversion cleavage of of 67% the after Cβ 40–O h bondat 100 °C. was The easier major thanproducts that were of the formylated Cα–Cβ bond. Hansonsubstrate et al. [1092-phenoxy-1-phenylformate,] used the CuOTf/2,6-lutidine/TEMPO and the catalystTEMPO-functionalized system to conduct theketone aerobic oxidation2-phenoxy-1-phenyl-2-(2,2,6,6-tet of M8 obtaining the overallramethylpiperidin-1-yloxy)ethanone, conversion of 67% after 40 h atwhile 100 phenol,◦C. The benzoic major productsacid wereand the formylated2-phenoxyacetophenone substrate 2-phenoxy-1-phenylformate, were produced only in small amounts. and the TEMPO-functionalizedRecently, a new strategy for ketone catalytic oxidation cleavage of aryl ethers in aralkyl aryl ethers involving a hemiacetal-like structure 2-phenoxy-1-phenyl-2-(2,2,6,6-tetramethylpiperidin-1-yloxy)ethanone, while phenol, benzoic acid was proposed: VO(acac)2-catalyzed aerobic oxidation of M8 in the presence of acetic acid [90,110]. and 2-phenoxyacetophenone were produced only in small amounts. Recently, a new strategy Benzaldehyde (5%), benzoic acid (14%), phenyl formate (9%), phenol (54%), for catalytic2-phenoxy-1-phenylethanone oxidation cleavage (2%) of aryland some ethers esterification in aralkyl products aryl ethers (16%) involvingwere obtained a hemiacetal-like after 8 h at structure80 °C. was It was proposed: proposed VO(acac)that a cleavage2-catalyzed of the linkage aerobic (C–C oxidation bond and of C–OM8 bond)in the between presence the oftwo acetic acidaromatic [90,110]. rings Benzaldehyde could occur through (5%), at benzoic least two acid pathways: (14%), route phenyl A and formate a new route (9%), B (Figure phenol 10). (54%), 2-phenoxy-1-phenylethanoneFor route A, 2-phenoxy-1-phenylethanone (2%) and some was esterification the intermediate products that further (16%) cleaved were to obtained benzoic acid, after 8 h ◦ at 80 phenylC. It was formate proposed and phenol. that aFor cleavage route B, ofanthe initial linkage oxidative (C–C activation bond and of the C–O Cβ–H bond) bond between produced the a two aromaticβ-hydroxyl rings couldhemiacetal. occur Then, through through at least a further two clea pathways:vage of the route C–C A bond, and athe new following route Boxidative (Figure 10). For route A, 2-phenoxy-1-phenylethanone was the intermediate that further cleaved to benzoic acid, phenyl formate and phenol. For route B, an initial oxidative activation of the Cβ–H bond produced a Polymers 2017, 9, 240 17 of 25

β-hydroxylPolymers hemiacetal. 2017, 9, 240 Then, through a further cleavage of the C–C bond, the following17 of oxidative25 activation of the Cα–H bond, and the eventual cleavage of the C–O bond can generate benzaldehyde, activation of the Cα–H bond, and the eventual cleavage of the C–O bond can generate benzaldehyde, benzoicbenzoic acid,phenyl acid, phenyl formate formate and and phenol. phenol.

Figure 10. Reaction pathway for β-O-4 lignin model dimer (M8) conversion on the catalysts. Figure 10. Reaction pathway for β-O-4 lignin model dimer (M8) conversion on the catalysts. The catalytic oxidative solvolysis of The1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-propane-1,3-diol catalytic oxidative solvolysis of 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)- (M9) and 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-propane-1,3-diol (M10) was performed over a 2:1 propane-1,3-diol (M9) and 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-propane-1,3-diol (M10) mixture of 1,10-phenanthroline and copper (II) sulfate pentahydrate in NaOH solution [67]. This was performedreaction was over conducted a 2:1 mixture in the ofpresence 1,10-phenanthroline of O2 in methanol and at 80 copper °C. Vanillin (II) sulfate was the pentahydrate major identified in NaOH ◦ solutionproduct [67]. This from reaction M9 while was veratric conducted acid was in the the major presence product of from O2 in M10 methanol (Figure 11). at 80 TheC. β-O-4 Vanillin bond was the major identifiedwas cleaved product mainly between from M9 thewhile Cα–Cβ atoms veratric and acid different was mechanisms the major productwere proposed from becauseM10 (Figure of 11). the phenolic hydroxyl group para to the side chain. Blandez et al. [111] found that graphene oxide The β-O-4 bond was cleaved mainly between the Cα–Cβ atoms and different mechanisms were promoted the oxidative degradation of M9 mainly to guaiacol, 2-methoxyquinone, vanillic acid and proposed because of the phenolic hydroxyl group para to the side chain. Blandez et al. [111] found that coniferyl aldehyde. After 24 h of reaction at 140 °C in acetonitrile solvent in the presence of O2, M9 graphenewas oxide converted promoted to coniferyl the aldehyde oxidative (20%), degradation vanillic acid of (2%),M9 vanillinmainly (6%), to guaiacol, 2-methoxyquinone 2-methoxyquinone, (5%) ◦ vanillicand acid guaiacol and coniferyl (96%). Reaction aldehyde. pathway After for 24M9 h on of graphene reaction oxide at 140 wasC shown in acetonitrile in Figure solvent11. in the presence{Fe-DABCO} of O 2oxidative, M9 was cleavage converted of M10 towith coniferyl peroxides aldehyde in DMSO was (20%), also reported vanillic [60]. acid Guaiacol (2%), vanillin (6%), 2-methoxyquinoneand veratraldehyde with (5%) 42% and and guaiacol35% yield (96%).were obtained Reaction as the pathway major products for M9 at 100on °C graphene after 16 oxide was shownh. The inreaction Figure pathway 11. {Fe-DABCO} for M10 on {Fe-DABCO} oxidative is shown cleavage in Figu of reM10 11. Transition-metal-containingwith peroxides in DMSO was hydrotalcites (HTc) and V(acac)3/Cu(NO3)2·3H2O mixtures were employed for the catalytic cleavage also reported [60]. Guaiacol and veratraldehyde with 42% and 35% yield were obtained as the of M10 with O2 as an◦ oxidant [62]. After 16 h of reaction at 135 °C in pyridine solvent in the presence major productsof O2, M10 at was 100 mainlyC after converted 16 h. to The veratraldehyde reaction pathway (38%), veratric for M10 acidon (31%) {Fe-DABCO} and phenol.is The shown in Figure 11reaction. Transition-metal-containing pathway for M10 on HTc is shown hydrotalcites in Figure 11. (HTc) and V(acac)3/Cu(NO3)2·3H2O mixtures were employedA ligand for is the an catalyticion or molecule cleavage (functional of M10 group)with Olinked2 as anto a oxidant central metal [62]. atom After to 16structure h of reaction a at ◦ coordination complex. Compared to other ligands, salen complexes are relatively stable, inexpensive 135 C in pyridine solvent in the presence of O2, M10 was mainly converted to veratraldehyde (38%), veratricand acid can (31%) be readily and phenol.synthesize Thed. A reaction series of pathwayvanadium for(V) M10complexeson HTc (e.g., is shownComplex in 3 Figurein Figure 11 3). bearing a Schiff base ligand for the non-oxidative solvolysis of a lignin model compound (M11) was Ademonstrated ligand is an at ion 80 or°C molecule[112]. Different (functional from the group)conversion linked pathway to a on central copper metal complex, atom the to β-O-4 structure a coordinationbond was complex. cleaved Compared mainly at the to otherCβ–O ligands,bond (Figure salen 11). complexes Thus M11 are was relatively converted stable, to alkene, inexpensive and can2-methoxyphenol be readily synthesized. and ketone after A series 24 h of of reaction vanadium in CD3 (V)CN complexessolvent. Another (e.g., series Complex of dipicolinate3 in Figure 3) bearingvanadium a Schiff base(V) complexes ligand for (e.g., the Complex non-oxidative 2 in Figure solvolysis 3) for of the a ligninoxidative model degradation compound of (M11) was demonstrated1-phenyl-2-phenoxyethanol at 80 ◦C[112 (M8].) was Different studied fromat 100 the °C in conversion air [97]. Differently, pathway M8 on was copper mainly complex, converted to formic acid, benzoic acid, phenol, and 2-phenoxyacetophenone (Figure 10). the β-O-4 bond was cleaved mainly at the Cβ–O bond (Figure 11). Thus M11 was converted to Ruthenium-triphos complexes also exhibited an outstanding catalytic activity and selectivity in the alkene,redox-neutral 2-methoxyphenol C–C bond and cleavage ketone of M10 after [113]. 24 hThe of use reaction of Ru(CO)(Cl)(H)–(triphos) in CD3CN solvent. [114] Another resulted in series of dipicolinatethe products vanadium from the (V) Cα complexes–Cβ bond cleavage (e.g., in Complex 66% yield,2 whereasin Figure the3 products) for the from oxidative the Cβ–O degradation bond of 1-phenyl-2-phenoxyethanolcleavage were formed in 4% yield (M8 )at was160 °C studied after 4 h of at reaction 100 ◦C (Figure in air 11). [ 97It should]. Differently, be noted thatM8 was mainly converted to formic acid, benzoic acid, phenol, and 2-phenoxyacetophenone (Figure 10). Ruthenium-triphos complexes also exhibited an outstanding catalytic activity and selectivity in the redox-neutral C–C bond cleavage of M10 [113]. The use of Ru(CO)(Cl)(H)–(triphos) [114] resulted in the products from the Cα–Cβ bond cleavage in 66% yield, whereas the products from the Cβ–O bond cleavage were formed in 4% yield at 160 ◦C after 4 h of reaction (Figure 11). It should be noted that the Polymers 2017, 9, 240 18 of 25

Polymers 2017, 9, 240 18 of 25 phenolic group played an important role: (1) Cα–OH was required for both the Cα–Cβ and the Cβ–O Polymersthe phenolic 2017, 9, 240 group played an important role: (1) Cα–OH was required for both the Cα–Cβ and the 18Cβ of– 25 bond cleavage; and (2) Cγ–OH was essential for accessing to the Cα–Cβ bond cleavage. O bond cleavage; and (2) Cγ–OH was essential for accessing to the Cα–Cβ bond cleavage.

the phenolic group played an important role: (1) Cα–OH was required for both the Cα–Cβ and the Cβ– O bond cleavage; and (2) Cγ–OH was essential for accessing to the Cα–Cβ bond cleavage.

Figure 11. Reaction pathway for β-O-4 lignin model dimer (M9, M10, and M11) conversion on the Figure 11. Reaction pathway for β-O-4 lignin model dimer (M9, M10, and M11) conversion on catalysts. the catalysts.

To compare the differences in selectivity of products between Complex 2 and Complex 3, the Figure 11. Reaction pathway for β-O-4 lignin model dimer (M9, M10, and M11) conversion on the Toaerobic compare oxidation the differences of a lignin model in selectivity compound of (M13 products) reacting between with Complex Complex 2 and2 Complexand Complex 3 was 3, the investigatedcatalysts. [115]. Complex 3 afforded the production of alkene, 2-methoxyphenol and ketone from aerobic oxidation of a lignin model compound (M13) reacting with Complex 2 and Complex 3 was Cβ–O bond cleavage, while Complex 2 gave the production of 2,6-dimethoxybenzoquinone and To compare the differences in selectivity of products between Complex 2 and Complex 3, the investigatedacrolein [115 derivative,]. Complex and 3ketoneafforded fromthe a C productionα–Caryl bond cleavage of alkene, (Figure 2-methoxyphenol 12). An application and of ketone an from Cβ–Oaerobic bondisotope oxidation cleavage, tracer technique of whilea lignin showed Complex model that compound 2Complexgave the (3M13 broke production) reacting the C–O with bond of 2,6-dimethoxybenzoquinoneComplex in the model 2 and compound Complex 3via was and investigated [115]. Complex 3 afforded the production of alkene, 2-methoxyphenol and ketone from acroleincracking derivative, of the andbenzylic ketone Cα–H from bond, a while Cα–C Complexaryl bond 2 broke cleavage the Cα–C (Figurearyl bond 12 without). An applicationcleavage of of an isotopeCβ–Othe tracer bondbenzylic technique cleavage, Cα–H bond. showedwhile Further Complex that experiments Complex 2 gave showedthe3 broke production that the no C–OC ofα–C 2,6-dimethoxybenzoquinonearyl bond bond in cleavage the model was observed compound and via acroleinin the aerobicderivative, oxidation and ketoneof a non-phenolic from a C modelα–Caryl compoundbond cleavage (M12 )(Figure with Complex 12). An 2. Itapplication was consistent of an cracking of the benzylic Cα–H bond, while Complex 2 broke the Cα–C bond without cleavage of isotopewith thetracer result technique of aerobic showed oxidation that of Complex a β-1 linkage 3 broke lignin the model C–O bondcompound inaryl the on model Complex compound 2 (Figure via the benzylic Cα–H bond. Further experiments showed that no Cα–C bond cleavage was observed cracking8). Mechanisms of the benzylic of Complex Cα–H 2 bond, and Complex while Complex 3 catalyses 2 brokeduring the reactions Cα–Carylaryl were bond proposed without in cleavage a recent of in thethe aerobicpublication benzylic oxidation C α[116].–H bond. Parker of a Further non-phenolic et al. [117]experiments attempted model showed compoundto determine that no ( M12 theCα–C effect)aryl with bond of Complexligand cleavage structure2 .was It was observedon the consistent within the theactivity result aerobic of oxidation aerobicvanadium oxidation ofSchiff-base a non-phenolic of catalysts a β-1 model linkage (e.g., compound Complex lignin model (3M12) towards) compoundwith Complexthe degradation on 2 Complex. It was of consistent β2-O-4(Figure 8). Mechanismswithcontained the result of Complexdimers. of aerobic Ortho2 -bulky,oxidationand Complex electron-donating of a β-13 linkagecatalyses ligand lignin during substi modeltuents reactions compound were determined were on Complex proposed to make 2 (Figure in the a recent publication8). mostMechanisms [active116]. Parkercatalyst, of Complex etwhich al. [ 117 2can and] be attempted Complex enhanced 3 toby catalyses determinethe addition during the of bulky effectreactions aliphatic of ligand were substituents proposed structure in such on a therecent as activity tert-butyl and adamantyl groups. The trityl-substituted complex was not that active due to the of vanadiumpublication Schiff-base [116]. Parker catalysts et al. (e.g.,[117] Complexattempted3 )to towards determine the the degradation effect of ligand of β-O-4 structure contained on the dimers. enhanced steric hindrance, confining the substrate access to the active sites of catalyst [117]. Orthoactivity-bulky, of electron-donating vanadium Schiff-base ligand catalysts substituents (e.g., wereComplex determined 3) towards to makethe degradation the most activeof β-O-4 catalyst, whichcontained can be enhanced dimers. Ortho by the-bulky, addition electron-donating of bulky aliphatic ligandsubstituents substituents were such determined as tert-butyl to and make adamantyl the most active catalyst, which can be enhanced by the addition of bulky aliphatic substituents such as groups. The trityl-substituted complex was not that active due to the enhanced steric hindrance, tert-butyl and adamantyl groups. The trityl-substituted complex was not that active due to the confiningenhanced the substratesteric hindrance, access confin to theing active the substrate sites of catalystaccess to [the117 active]. sites of catalyst [117].

Figure 12. Reaction pathway for β-O-4 lignin model dimer (M13) conversion on the catalysts.

Figure 12. Reaction pathway for β-O-4 lignin model dimer (M13) conversion on the catalysts. Figure 12. Reaction pathway for β-O-4 lignin model dimer (M13) conversion on the catalysts.

Polymers 2017, 9, 240 19 of 25

The oxidation of lignin-related model compounds, 2-phenoxy-1-phenylethanol (M8) and

2-(4-methoxyphenoxy)-1-phenylethanolPolymers 2017, 9, 240 (M14), on a Pd/CeO2 catalyst was performed19 in of methanol 25 under O2 [74]. The reaction mechanism for the conversion of M8 over Pd/CeO2 in methanol with The oxidation of lignin-related model compounds, 2-phenoxy-1-phenylethanol (M8) and the presencePolymers 2017 of O, 9,2 240is shown in Scheme3. The C α–OH of 2-phenoxy-1-phenylethanol19 of 25 was first oxidized2-(4-methoxyphenoxy)-1-phenylethanol due to the catalytic function of ( PdM14 nanoparticles,), on a Pd/CeO2 catalyst to form was 2-phenoxy-1-phenylethanone, performed in methanol underThe O2 [74].oxidation The reaction of lignin-related mechanism modelfor the conversioncompounds, of 2-phenoxy-1-phenylethanolM8 over Pd/CeO2 in methanol (M8 with) and the whose β-O-4 bond became weakened due to the presence of Cα=O. The subsequent hydrogenolysis 2-(4-methoxyphenoxy)-1-phenylethanolpresence of O2 is shown in Scheme 3. The (M14 Cα–OH), on ofa Pd/CeO2-phenoxy-1-phenylethanol2 catalyst was performed was first in methanol oxidized of the β-O-4 bond of 2-phenoxy-1-phenylethanone resulted in the form of acetophenone and phenol underdue to Othe2 [74]. catalytic The reactionfunction mechanism of Pd nanopartic for theles, conversion to form 2-phenoxy-1-phenylethanone, of M8 over Pd/CeO2 in methanol whose with β-O-4 the over thepresencebond Pd/CeO became of 2O catalyst.2weakened is shownOn indue Scheme the to the other 3.presence The hand, Cα –OHof the Cα of=O. oxidative 2-phenoxy-1-phenylethanol The subsequent cracking hydrogenolysis of the βwas-O-4 first of bond the oxidized β-O-4 over CeO2 might leadduebond to to of the the 2-phenoxy-1-phenylethano catalytic formation function of phenolof Pd nanoparticne and resulted anles, oxidized into formthe form intermediate2-phenoxy-1-phenylethanone, of acetophenone (e.g., 2-hydroxyacetophenone).and phenol whose over β-O-4 the Meanwhile,bondPd/CeO abecame small2 catalyst. amountweakened On the of dueother acetophenone to hand, the presence the oxidative can of beCα produced=O.cracking The subsequent of duringthe β-O-4 thishy bonddrogenolysis process. over CeO Finally, of2 might the β the -O-4lead oxidized to the formation of phenol and an oxidized intermediate (e.g., 2-hydroxyacetophenone). Meanwhile, intermediatebond of experienced 2-phenoxy-1-phenylethano the C–C bondne cracking, resulted producingin the form benzoicof acetophenone acid and and methyl phenol benzoate. over the Phenol, Pd/CeOa small 2 amountcatalyst. Onof acetophenonethe other hand, can the oxidativebe produced cracking during of thethis β -O-4process. bond Finally,over CeO the2 might oxidized lead acetophenone and methyl benzoate were produced with the yield of 48%, 38% and 14% from the tointermediate the formation experienced of phenol andthe C–Can oxidized bond cracking intermediate, producing (e.g., 2-hydroxyacetophenone). benzoic acid and methyl Meanwhile, benzoate. oxidationaPhenol, small of M8 acetophenoneamount, while of 4-methoxyphenol, acetophenone and methyl benzoate can be acetophenone wereproduced produced during and with this methylthe yieldprocess. benzoateof 48%, Finally, 38% were andthe 14% producedoxidized from with ◦ yields ofintermediatethe 82%, oxidation 38% experiencedandof M8 40%, while from the 4-methoxyphenol, C–C the oxidationbond cracking ofacetophenoneM14, producingat 185 and benzoicC methyl for 24acid benzoate h. and It could methyl were be producedbenzoate. concluded that with yields of 82%, 38% and 40% from the oxidation of M14 at 185 °C for 24 h. It could be concluded ortho-methoxyPhenol, acetophenone substituents and further methyl elevatedbenzoate were the performanceproduced with the of yield the Pd/CeOof 48%, 38%2 catalyst, and 14% from which was consistentthethat withoxidation ortho-methoxy the otherof M8 substituents,report while 4-methoxyphenol, stating further that elevatedortho acetophenone -methoxythe performance substituents and of methyl the Pd/CeO benzoate could2 catalyst, weaken were which produced the was stability of consistent with the other report stating that ortho-methoxy substituents could weaken the stability of the aromaticwith yields ring, of promoting 82%, 38% and its 40% over-oxidation from the oxidation [64]. of M14 at 185 °C for 24 h. It could be concluded thatthe aromatic ortho-methoxy ring, promoting substituents its further over-oxidation elevated [64]. the performance of the Pd/CeO2 catalyst, which was consistent with the other report stating that ortho-methoxy substituents could weaken the stability of the aromatic ring, promoting its over-oxidation [64].

Scheme 3. Proposed pathway for oxidative conversion of 2-phenoxy-1-phenylethanol (M8) over a Scheme 3. Proposed pathway for oxidative conversion of 2-phenoxy-1-phenylethanol (M8) over a Pd/CeO2 catalyst [74], reproduced with permission from Royal Society of Chemistry. Pd/CeO2 catalyst [74], reproduced with permission from Royal Society of Chemistry. Scheme 3. Proposed pathway for oxidative conversion of 2-phenoxy-1-phenylethanol (M8) over a Moreover, Wang et al. [118] reported a two-step strategy for lignin C–C bond conversion via a Pd/CeO2 catalyst [74], reproduced with permission from Royal Society of Chemistry. Moreover,β-O-4 alcohol Wang oxidation et al. [to118 ketone] reported over the a VOSO two-step4/TEMPO strategy [(2,2,6,6-tetramet for ligninhylpiperidin-1-yl)oxyl)] C–C bond conversion via a catalyst,Moreover, followed Wang by aet ketoneal. [118] oxidation reported overa two-step Cu(OAc) strategy2/1,10-phenanthroline for lignin C–C bond catalyst conversion to acids via and a β-O-4 alcohol oxidation to ketone over the VOSO4/TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl)] βphenols.-O-4 alcohol The proposed oxidation reaction to ketone mechanism over the VOSO is shown4/TEMPO in Scheme [(2,2,6,6-tetramet 4. The oxidationhylpiperidin-1-yl)oxyl)] of a Cα–OH alcohol catalyst, followed by a ketone oxidation over Cu(OAc)2/1,10-phenanthroline catalyst to acids and catalyst,to a ketone followed activated by athe ketone Cβ–H oxidation bond. The over Cu(OAc) Cu(OAc)2/1,10-phenanthroline2/1,10-phenanthroline reacted catalyst with to oxygenacids and to . The proposed reaction mechanism is shown in Scheme4. The oxidation of a C –OH alcohol phenols.form a copper-oxo-bridged The proposed reaction dime mechanismr. The activation is shown of the in CSchemeα-Cβ bond 4. The in theoxidation form of of a ahydroxyl Cα–OH αalcohol ketone to a ketonetostructure-like a ketone activated activated intermediate the C theβ–H C significantlyβ–H bond. bond. The The decrease Cu(OAc) Cu(OAc)d its2/1,10-phenanthroline/1,10-phenanthroline bond energy, resulting reacted in reacted the with formation oxygen withoxygen toof to form a copper-oxo-bridgedformbenzoic a copper-oxo-bridged acid and phenyl dimer. formate. dime Ther. Transfer The activation activation of a hydroxyl of of thethe C groupαα-C-Cβ bondβ generatedbond in the in benzoicform theform of aacid, hydroxyl of restoring a hydroxyl ketone the ketone structure-likestructure-likeinitial copper intermediate complex.intermediate significantly Finally, significantly the oxidative decreased decrease deca itsdrboxylation bondits bond energy, energy, of phenyl resulting resulting formate in in the generatedthe formation formation phenol ofof benzoic and CO2. acid andbenzoic phenyl acid formate. and phenyl Transfer formate. of Transfer a hydroxyl of a hydroxyl group group generated generated benzoic benzoic acid, acid, restoring restoring the the initial initial copper complex. Finally, the oxidative decarboxylation of phenyl formate generated phenol copper complex. Finally, the oxidative decarboxylation of phenyl formate generated phenol and CO2. and CO2.

Scheme 4. Proposed reaction mechanism for two-step, catalytic C–C bond oxidative cleavage process [118], reproduced with permission from American Chemical Society. Scheme 4. Proposed reaction mechanism for two-step, catalytic C–C bond oxidative cleavage process Scheme[118], 4. reproducedProposed with reaction permission mechanism from American for two-step, Chemical Society. catalytic C–C bond oxidative cleavage process [118], reproduced with permission from American Chemical Society.

Polymers 2017, 9, 240 20 of 25

The oxidative solvolysis of lignin-related model compounds with homogeneous catalysts was mostly reported for enhancing the production of aromatic aldehydes, aromatic ketones, benzoquinones, carboxylic acids, etc. The oxidative solvolysis of model compounds with heterogeneous catalysts may achieve a high selectivity of specific products. However, the performance of both homogeneous and heterogeneous catalysts in the cleavage of inter-unit linkages is still ambiguous in the oxidative solvent system, confining the understanding of inherent mechanism of whole lignin depolymerization process.

5. Summary and Outlook The catalytic oxidation of lignin in solvent systems is attracting more and more attention for producing renewable aromatic chemicals. A number of functionalized monomeric products were obtained from oxidative depolymerization of lignin, including vanillin, vanillic acid, syringaldehyde and syringic acid. Considering the ambiguities in the inherent mechanism of (catalytic) oxidative solvolysis of lignin process and the limited selectivity and yield of renewable chemicals, several issues should be addressed: (1) identification of the oligomer structure for specifying the inherent oxidative depolymerization mechanism of natural lignin; (2) design of highly efficient catalyst for C–C type inter-unit linkages; (3) strategies for suppressing the repolymerization of the produced fragments; (4) low-cost separation and purification technology of the specific aromatic compounds from an LDP system; and (5) stability of the heterogeneous catalysts during LDP and its reactivation.

Acknowledgments: The authors greatly acknowledge the funding support from the projects supported by National Natural Science Foundation of China (Grant Nos. 51676034 and 51628601) and Natural Science Foundation of Jiangsu Province (project reference: BK20161423). Conflicts of Interest: The authors declare no conflict of interest.

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