Lignin Degradation by ROS

Lignin is a unique renewable aromatic resource in nature. In the past decades, researchers have attempted to breakdown the linkage bonds in lignin to provide aromatic platform chemicals that used to come from the petrochemical industry. In recent years, electrochemical lignin degradation under mild conditions has drawn much attention from the scientific community owing to its potential to scale up and its environmental friendliness. Sustainable electrochemical degradation of lignin consumes less energy and usually requires mild conditions, but low degradation efficiency and insufficient product selectivity are still significant challenges. The method for lignin degradation by reactive oxygen species (ROS) generated through the water oxidation reaction (WOR) at the anode and oxygen reduction reaction (ORR) at the cathode are more attractive for sustainable electrochemical degradation. The present contribution aims to review advancements in electrochemical degradation of lignin in aqueous or non-aqueous supporting electrolytes, focusing on the regulation of ROS in situ generated on the electrode.

lignin degradation,;electrochemical method,;reactive oxygen species (ROS),;water oxidation reaction (WOR),;oxygen reduction reaction (ORR)

1. Introduction

At present, most of the carbon-containing bulk chemicals are transformed from non-renewable fossil resources, such as petroleum and coal, through a complicated conversion process, which brings a series of problems such as the greenhouse effect, climate change, and environmental pollution. According to the European Commission’s report “World energy, technology and climate policy outlook 2030 (WETO)”, the total world energy consumption would increase from 1.21 billion tons of oil equivalent (2010) to 1.45 billion tons of oil equivalent (2020), and up to 1.71 billion tons of oil equivalent by 2030. It is estimated that global carbon dioxide emissions will increase from 29.3 × 109 tons (2010) to 44.5 × 109  tons (2030), so the development and utilization of sustainable and renewable energy are imminent for contemporary people [1]. Lignin is the second most abundant component of lignocellulosic biomass after cellulose, and is the largest source of renewable aromatic compounds [2,3][2][3]. Therefore, a sustainable chemical industry based on lignin degradation to obtain aromatic platform chemicals has been proposed. Considering the complexity of the lignin molecular structure, the cleavage of the linkage bonds among the phenylpropane units during degradation is essential to improve product selectivity, which might simplify the subsequent separation process of the desired products [4,5,6][4][5][6]. Researchers have tried various methods, including pyrolysis, enzymatic processes, and chemical catalysis methods, to breakdown the linkage bonds in lignin [7,8[7][8][9],9], in which the degradation method with mild oxidant reagents (such as O2, H2O2) demonstrates good reaction selectivity in the cleavage of C–C/C–O bonds [10,11][10][11].

Electrochemical methods could be conducted under moderate conditions, such as relatively mild temperature, non-extreme pressure, and minimal needs for additional reagents. Lignin degradation by the electrochemical method has developed rapidly because of its controllable reaction and environmental friendliness. Researchers have tried to convert lignin to useful chemicals through oxidation on anode, reduction on the cathode or the combination of oxidation and reduction [12,13,14][12][13][14]. Several strategies have been proposed for the electrooxidative degradation of lignin: (1) the direct pathway that lignin is heterogeneously oxidized by an anode (for example, Ni [15[15][16],16], PbO2 [17 [17][18],18], or RuO2 [19] [19]) as or a catalyst immobilized on the surface of the electrode; (2) the indirect pathway that lignin is homogeneously oxidized by anode-generated hydroxyl radicals (*OH) through water oxidation reaction (WOR) at the anode or electrochemically recycled Red-Ox mediators (e.g., TEMPO [20[20][21],21], or NHPI [22,23][22][23]); and (3) the electrochemical–chemical combination pathway that lignin is oxidized by reactive oxygen species (ROS) generated at the electrode. Several ROSs, including hydroxyl radicals (*OH), superoxide radical anions (*O2), and hydrogen peroxide/hydrogen peroxide anion (H2O2/OOH), could be generated through WOR at the anode or ORR at the cathode under manipulated electrolysis conditions [24,25,26][24][25][26].

2. Degradation of Lignin and Lignin Model Compounds by ROS Generated in Situ in Aqueous Electrolytes

Electrochemical oxidation has been described as an effective alternative in wastewater treatment due to its mild treatment conditions and environmental friendliness [50][27]. Organics can be oxidized by the hydroxyl radicals (*OH) and other active oxygen species produced during the electrochemical reaction [51,52][28][29]. In the early studies, the direct electrooxidation method was employed to degrade lignin in aqueous electrolytes, mainly in a strong alkaline medium, in which the *OH generated near the anode was the oxidizing agent [53,54][30][31]. Typical linkage bonds in lignin, such as C–C bonds and C–O bonds, were reported to be cleaved with appropriate potential.

Many studies employed dimensionally stable anodes (DSA, e.g., IrO2 [55] [32], PbO2 [56] [33], SnO2 [57] [34]) to oxidize lignin through *OH generated on the anode surface. IrO2-based electrodes (Ti/SnO2-IrO2, Ti/RuO2-IrO2, Ti/Ta2O5-IrO2, and Ti/TiO2-IrO2) were fabricated by overlaying the binary metal oxides on pretreated Ti substrates, and the Ti/RuO2-IrO2 electrode showed the highest stability and the most increased activity for lignin degradation under 500 mA/cm2, which was a large current that can be provided in the process of electrochemical degradation of lignin [12]. Additionally, PbO2-based electrodes were also applied in electrochemical degradation of lignin due to their high oxygen evolution potential (+2.0 V vs. SCE). Moreover, electro-hydrogenation, as well as electrooxidation, were designed in the electrochemical reaction system, in which lignin was firstly oxidized and cleaved into intermediates through the oxidation of *OH formed by β-PbO2 crystal on the electrode surface. Afterwards, these intermediates went through the electro-hydrogenation process at the cathodes (such as Ni, Cu, Cu/Ni-Mo-Co, alloyed Steel, Ti/Cu/Sn). In the above system, vanillin, syringaldehyde, acetosyringone, 3-hydroxy-4-methoxyphenyl-ethanone, and 4-methoxy-3-methyl-phenol were obtained as the degradation products [18,53,58,59,60,61][18][30][35][36][37][38]. Recently, Movil-Cabrera et al. [14] [14] prepared a Co core/Pt partial shell nanoparticle alloy anode coupled with a Pt ring cathode to electrochemically degrade lignin, suggesting that the *OH generated from water electrolysis. A suggested mechanism on the Pb/PbO2 electrodes as follows:

Co core/Pt partial shell anode: H

2

O − e

 → *OH + H

+                                                       

Pt ring cathode: H

                        Pt ring cathode: H

2

O + e

 → H

ads

 + OH

                                                   

Lignin degradation: R + *OH + H

    Lignin degradation: R + *OH + H

ads-anode → ROH + R′H

→ ROH + R′H                                   

 

It was reported that vanillin could be obtained by using Ni as anode under mild conditions [17]. The researchers investigated different shapes of Ni electrodes and found the apparent mass transfer coefficient was different. As shown in Figure 1, they found that the use of nickel electrodes with high surface area greatly enhanced the degradation percent of lignin under the same electrolysis conditions. Waldvogel et al. prepared a layer of electrochemical active NiO(OH) on Ni anode and used it to directly electrochemical oxidize lignin [62][39]. To increase the products selectivity on the Ni anode, a simple undivided high-temperature electrolytic cell for the electrooxidation of lignin at 160 °C has been designed recently. Under the optimized electrolysis conditions, the vanillin can be obtained with a yield of 4.2 wt%, which in high selectivity with 67% efficiency compared to the common nitrobenzene oxidation [63][40].

Figure 1. Nickel electrode morphologies in an electrochemical flow-through reactor. Materials are: (A) plate, (B) wire, (C) felt, and (D) foam. (E) The relative molecular weight of Kraft lignin after 90 min of electrochemical cleavage using different electrode materials and currents [17]. Copyright 2016, Royal Society of Chemistry.

However, the degradation of lignin through electro-generated *OH on the anode is accompanied by a strong oxygen evolution reaction (OER). As a competitive reaction, OER inhibits the charge transfer of lignin on the electrode surface, requires a higher voltage to be applied, and may have a more significant impact on electrode corrosion. Additionally, it is supposed to be attributed to over-oxidation of degradation products to organic acids and CO2. To prevent over-oxidation and achieve the enrichment of the target products, Wessling et al. [64] [41] attempted to design a new type of electrochemical membrane reactor. Combined with the in situ nanopore filtration method, the electrochemical oxidation products of lignin were removed from the oxidizing environment to achieve the effect of sustainable electrolysis (see Figure 2). Separation of low-molecular-weight (LMW) products by a tubular ceramic nanoporous filtration (NF)-membrane was shown to double the number of products isolated from Kraft lignin.

Figure 2. (a) Scheme of the electrochemical membrane reactor with in situ product removal for the electro–oxidative cleavage of lignin. (b) Detailed scheme of the electrode/mixer–unit, dimension are given in mm. (c) Process scheme of the complete experimental setup (AEM: Anion exchange membrane; NFM: Nanofiltration membrane). (d) Cross–section of the module. The inside of the ceramic membrane is used as the reactor, the volume between the ceramic and the ion–exchange membrane is used for the collection of the permeate and the cathodic work is only used to close the electrical circuit [64][41]. Copyright 2015, Elsevier.

In our previous research, ROS generated through the ORR process in an aqueous solution was introduced to degrade lignin [65][42]. A non-membraned cylindrical electrolytic cell with the inner layer of graphite felt cathode and the outer layer of a mesh RuO2-IrO2/Ti anode with lignin in alkali solution as an electrolyte was designed (Figure 3A), in which led to O2 evolution on the anode and O2 reduction to form H2O2 on the cathode. The degradation products were extracted by diethyl ether and the yield was calculated by the weight of the extractant. The yield of lignin degradation products after electrolysis for 1 h in this non-membraned cell was 10.1%, which was more than the sum of the yields in the anodic and cathodic chamber of a membrane cell (4.1% and 3.4%, respectively). The result implied that a synergic effect existed between the anodic and cathodic electrode reaction. It seems that the electrolysis conditions for producing a higher concentration of H2O2 and ROS were in favor of giving higher yields of LMW products. In another study, 59.2% of lignin was degraded into LMW products after 1 h-electrolysis at 80 °C under a current density of 8 mA/cm2 with extra O2. More than 20 kinds of lignin degradation products were identified, which originated from the over-oxidation during the degradation due to the strong oxidation capability of *OH, generated through the decomposition of H2O2 (Figure 3B) [66][43]. On the other hand, the electric quantity in this electrochemical reactor is decreased comparing with the others [53,54,57][30][31][34].

Figure 3. (A) A non-membraned cell for lignin degradation: (1) electrochemical workstation; (2) magneton; (3) magnetic stirrer; (4) electrolytic cell; (5) cylindrical inner layer graphite felt cathode; (6) cylindrical outer layer RuO2-IrO2/Ti mesh anode; (7) electrode support [65][42]. Copyright 2014, Royal Society of Chemistry. (B) The possible pathways of lignin degradation by anode oxidation and electro-generated H2O2  oxidation [66][43]. Copyright 2014, Royal Society of Chemistry. (C) Scheme of PBP oxidative alkyl-O-aryl bond-cleavage [67][44]. Copyright 2017, Royal Society of Chemistry.

Several simple molecules with alkyl-O-aryl bonds, such as PBP (benzyl-O-phenol) and BPE (benzyl-O-phenyl), were used as lignin model compounds to study the mechanism of the bond cleavage during the electrolysis. By comparing the degradation yield and products of PBP and BPE, we deduced that the phenolic hydroxyl group in PBP might be the active site to be selectively attacked by ROS and finally trigger the cleavage of alkyl-O-aryl bond (as shown in Figure 3C). Based on several well-designed experiments, it was confirmed that the main ROS was *OH. The degradation rate and product yield displayed a good positive linear relationship with the concentration of the electro-generated *OH on the ORR cathode [67][44]. This result indirectly proves that, compared with a single electrode, the synergistic effect of anode and cathode greatly increase the degradation efficiency of lignin.

 

References

  1. Directorate-General for Energy Research; European Commission. Publication EUR 20366: World Energy, Technology and Climate Policy Outlook 2030 (WETO); Luxembourg Office for Official Publications of the European Communities: Luxembourg, 2003.
  2. Upton, B.M.; Kasko, A.M. Strategies for the conversion of lignin to high-value polymeric materials: Review and perspective. Chem. Rev. 2015, 116, 2275–2306.
  3. Schutyser, W.; Renders, T.; Van den Bosch, S.; Koelewijn, S.-F.; Beckham, G.T.; Sels, B.F. Chemicals from lignin: An interplay of lignocellulose fractionation, depolymerisation, and upgrading. Chem. Soc. Rev. 2018, 47, 852–908.
  4. Di Marino, D.; Stöckmann, D.; Kriescher, S.; Stiefel, S.; Wessling, M. Electrochemical depolymerization of lignin in a deep eutectic solvent. Green Chem. 2016, 18, 6021–6028.
  5. Di Marino, D.; Aniko, V.; Stocco, A.; Kriescher, S.; Wessling, M. Emulsion electrooxidation of kraft lignin. Green Chem. 2017, 19, 4778–4784.
  6. Bawareth, B.; Di Marino, D.; Nijhuis, T.A.; Wessling, M. Unravelling Electrochemical Lignin Depolymerization. ACS Sustain. Chem. Eng. 2018, 6, 7565–7573.
  7. Li, C.; Zhao, X.; Wang, A.; Huber, G.W.; Zhang, T. Catalytic Transformation of Lignin for the Production of Chemicals and Fuels. Chem. Rev. 2015, 115, 11559–11624.
  8. Zhang, Z.; Song, J.; Han, B. Catalytic Transformation of Lignocellulose into Chemicals and Fuel Products in Ionic Liquids. Chem. Rev. 2017, 117, 6834–6880.
  9. Sun, Z.; Fridrich, B.; De Santi, A.; Elangovan, S.; Barta, K. Bright Side of Lignin Depolymerization: Toward New Platform Chemicals. Chem. Rev. 2018, 118, 614–678.
  10. Sippola, V.O.; Krause, A.O.I. Oxidation activity and stability of homogeneous cobalt-sulphosalen catalyst: Studies with a phenolic and a non-phenolic lignin model compound in aqueous alkaline medium. J. Mol. Catal. A Chem. 2003, 194, 89–97.
  11. Chen, Y.Y.; Xue, A.; Jiang, H.; Cheng, Y.; Ren, Y.; Sun, Y.; Chen, Y. A Two-phase Reaction System for Selective Oxidative Degradation of Lignin Model Compounds. BioResources 2020, 15, 6526–6538.
  12. Tolba, R.; Tian, M.; Wen, J.; Jiang, Z.; Chen, A. Electrochemical oxidation of lignin at IrO2-based oxide electrodes. J. Electroanal. Chem. 2010, 649, 9–15.
  13. Garedew, M.; Young-Farhat, D.; Bhatia, S.; Hao, P.; Jackson, J.E.; Saffron, C.M. Electrocatalytic Cleavage of Lignin Model Dimers Using Ruthenium Supported on Activated Carbon Cloth. Sustain. Energy Fuels 2020, 4, 1340–1350.
  14. Movil-Cabrera, O.; Rodriguez-Silva, A.; Arroyo-Torres, C.; Staser, J.A. Electrochemical conversion of lignin to useful chemicals. Biomass Bioenergy 2016, 88, 89–96.
  15. Stiefel, S.; Marks, C.; Schmidt, T.; Hanisch, S.; Spalding, G.; Wessling, M. Overcoming lignin heterogeneity: Reliably characterising the cleavage of technical lignin. Green Chem. 2016, 18, 531–540.
  16. Stiefel, S.; Schmitz, A.; Peters, J.; Di Marino, D.; Wessling, M. An integrated electrochemical process to convert lignin to value-added products under mild conditions. Green Chem. 2016, 18, 4999–5007.
  17. Chen, F.; Lu, Z.; Tu, B. Electro-degradation of sodium lignosulfonate. J. Wood Chem. Technol. 2003, 23, 261–277.
  18. Cai, P.; Fan, H.; Cao, S.; Qi, J.; Zhang, S.; Li, G. Electrochemical conversion of corn stover lignin to biomass-based chemicals between Cu/Ni-Mo-Co cathode and Pb/PbO2 anode in alkali solution. Electrochim. Acta 2018, 264, 128–139.
  19. Reichert, E.; Wintringer, R.; Volmer, D.A.; Hempelmann, R. Electro-catalytic oxidative cleavage of lignin in a protic ionic liquid. Phys. Chem. Chem. Phys. 2012, 14, 5214–5221.
  20. Sannami, Y.; Kamitakahara, H.; Takano, T. TEMPO-mediated electrooxidation reactions of non-phenolic beta-O-4-type lignin model compounds. Holzforschung 2017, 71, 109–117.
  21. Rafiee, M.; Alherech, M.; Karlen, S.D.; Stahl, S.S. Electrochemical Aminoxyl-Mediated Oxidation of Primary Alcohols in Lignin to Carboxylic Acids: Polymer Modification and Depolymerization. J. Am. Chem. Soc. 2019, 141, 15266–15276.
  22. Kishioka, S.; Yamada, A. Kinetic study of the catalytic oxidation of benzyl alcohols by phthalimide-N-oxyl radical electro-generated in acetonitrile using rotating disk electrode voltammetry. J. Electroanal. Chem. 2005, 578, 71–77.
  23. Shiraishi, T.; Takano, T.; Kamitakahara, H.; Nakatsubo, F. Studies on electrooxidation of lignin and lignin model compounds. Part 2: N-Hydroxyphthalimide (NHPI)-mediated indirect electrooxidation of non-phenolic lignin model compounds. Holzforschung 2012, 66, 311–315.
  24. Zhang, Y.; Peng, Y.; Yin, X.; Liu, Z.; Li, G. Degradation of Lignin to BHT by Electrochemical Catalysis on Pb/PbO2 Anode in Alkaline Solution. J. Chem. Technol. Biotechnol. 2014, 89, 1954–1960.
  25. Nonni, A.J.; Dence, C.W. The Reactions of Alkaline Hydrogen Peroxide with Lignin Model Dimers Part 3: 1,2-Diaryl-1,3-Propanediols. Holzforschung 1988, 42, 37–46.
  26. Agnemo, R.; Gellerstedt, G.; Leban, J.J.; Björkroth, U.; Rosell, S.; Folkers, K.; Yanaihara, N.; Yanaihara, C. The Reactions of Lignin with Alkaline Hydrogen Peroxide. Part II. Factors Influencing the Decomposition of Phenolic Structures. Acta Chem. Scand. 1979, 33, 337–342.
  27. Chen, G. Electrochemical technologies in wastewater treatment. Sep. Purif. Technol. 2004, 38, 11–41.
  28. Nagpurkar, L.P.; Chaudhari, A.R.; Ekhe, J.D. Formation of industrially important chemicals from thermal and microwave assisted oxidative degradation of industrial waste lignin. Asian J. Chem. 2002, 14, 1387–1392.
  29. Sun, X.; Sun, R.; Tomkinson, J.; Baird, M. Degradation of wheat straw lignin and hemicellulosic polymers by a totally chlorine-free method. Polym. Degrad. Stabil. 2004, 83, 47–57.
  30. Xu, H.; Yuan, Q.; Shao, D.; Yang, H.; Liang, J.; Feng, J.; Yan, W. Fabrication and characterization of PbO2 electrode modified with [Fe(CN)6]3- and its application on electrochemical degradation of alkali lignin. J. Hazard. Mater. 2015, 286, 509–516.
  31. Pan, K.; Tian, M.; Jiang, Z.; Kjartanson, B.; Chen, A. Electrochemical oxidation of lignin at lead dioxide nanoparticles photoelectron-deposited on TiO2 nanotube arrays. Electrochim. Acta 2012, 60, 147–153.
  32. Trasatti, S. Electrocatalysis: Understanding the success of DSA. Electrochim. Acta 2000, 45, 2377–2385.
  33. Liu, M.; Wen, Y.; Qi, J.; Zhang, S.; Li, G. Fine Chemicals Prepared by Bamboo Lignin Degradation through Electrocatalytic Redox between Cu Cathode and Pb/PbO2 Anode in Alkali Solution. Chem. Select. 2017, 2, 4956–4962.
  34. Shao, D.; Liang, J.; Cui, X.; Xu, H.; Yan, W. Electrochemical oxidation of lignin by two typical electrodes: Ti/Sb-SnO2 and Ti/PbO2. Chem. Eng. J. 2014, 244, 288–295.
  35. Wang, Y.; Yang, F.; Liu, Z.; Yuan, L.; Li, G. Electrocatalytic degradation of aspen lignin over Pb/PbO2 electrode in alkali solution. Catal. Commun. 2015, 67, 49–53.
  36. Chen, A.; Wen, Y.; Han, X.; Qi, J.; Liu, Z.; Zhang, S.; Li, G. Electrochemical Decomposition of Wheat Straw Lignin into Guaiacyl-, Syringyl-, and Phenol-Type Compounds Using Pb/PbO2 Anode and Alloyed Steel Cathode in Alkaline Solution. Environ. Prog. Sustain. Energy 2018, 38, 13117–13126.
  37. Jia, Y.; Wen, Y.; Han, X.; Qi, J.; Liu, Z.; Zhang, S.; Li, G. Electrocatalytic degradation of rice straw lignin in alkaline solution through oxidation on a Ti/SnO2-Sb2O3/α-PbO2/β-PbO2 anode and reduction on an iron or tin doped titanium cathode. Catal. Sci. Technol. 2018, 8, 4665–4677.
  38. Lan, C.; Fan, H.; Shang, Y.; Shen, D.; Li, G. Electrochemically catalysed conversion of cornstalk lignin to aromatic compounds: An integrated process of anodic oxidation of a Pb/PbO2 electrode and hydrogenation of a nickel cathode in sodium hydroxide solution. Sustain. Energy Fuels 2020, 4, 1828–1836.
  39. Schmitt, D.; Regenbrecht, C.; Hartmer, M.; Stecker, F.; Waldvogel, S.R. Highly selective generation of Vanillin by anodic degradation of lignin: A combined approach of electrochemistry and product isolation by adsorption. Beilstein J. Org. Chem. 2015, 11, 473–480.
  40. Zirbes, M.; Quadri, L.L.; Breiner, M.; Stenglein, A.; Bomm, A.; Schade, W.; Waldvogel, S.R. High-Temperature Electrolysis of Kraft Lignin for Selective Vanillin Formation. ACS Sustain. Chem. Eng. 2020, 8, 7300–7307.
  41. Stiefel, S.; Lölsberg, J.; Kipshagen, L.; Möller-Gulland, R.; Wessling, M. Controlled depolymerization of lignin in an electrochemical membrane reactor. Electrochem. Commun. 2015, 61, 49–52.
  42. Zhu, H.; Chen, Y.; Qin, T.; Wang, L.; Tang, Y.; Sun, Y.; Wan, P. Lignin depolymerization via an integrated approach of anode oxidation and electro-generated H2O2 oxidation. RSC Adv. 2014, 4, 6232–6238.
  43. Zhu, H.; Wang, L.; Chen, Y.; Li, G.; Li, H.; Tang, Y.; Wan, P. Electrochemical depolymerization of lignin into renewable aromatic compounds in a non-diaphragm electrolytic cell. RSC Adv. 2014, 4, 29917–29924.
  44. Wang, L.; Chen, Y.; Liu, S.; Jiang, H.; Wang, L.N.; Sun, Y.; Wan, P. Study on the cleavage of alkyl-O-aryl bonds by in situ generated hydroxyl radicals on an ORR cathode. RSC Adv. 2017, 7, 51419–51425.
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