Electrochemical depolymerization of lignin into renewable aromatic compounds in a non-diaphragm electrolytic cell

Abstract

Electrochemical depolymerization of lignin for production of renewable aromatic compounds is presented. In the designed non-diaphragm electrolytic cell, lignin in alkaline electrolyte was directly electro-oxidized on the anode and chemically oxidized by the electro-generated H₂O₂ formed on the cathode simultaneously. The linkages among C9 units in lignin were broken down and more than 20 kinds of low-molecular-weight (LMW) aromatic compounds containing hydroxyl, aldehyde, carbonyl and carboxyl groups were generated and identified by GC-MS and ESI-MS/MS measurements. The effects of electrolysis conditions on the concentration of H₂O₂, the decomposition rate of H₂O₂ into reactive oxygen species (ROS) and the yields of LMW products were investigated in detail. Results show that H₂O₂ and ROS play very important roles in lignin depolymerization. The electrolysis conditions for producing higher concentrations of H₂O₂ and ROS are in favor of giving higher yields of LMW products. 59.2% of lignin was depolymerized into LMW products after 1 hour-electrolysis at 80 °C under a current density of 8 mA cm⁻² with extra O₂ supplement.

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1. Introduction

Lignin is the second most abundant renewable biopolymer on Earth after cellulose. It is a polyphenolic macromolecule consisting of substantial phenylpropyl (C9) units crosslinked by C–C and C–O–C bonds, so it is considered to be a potential source for sustainable production of fuels and basic industrial aromatic chemicals such as phenols, aldehydes, ketones and acids, which are currently produced from fossil-based feedstocks.^1–4 Plenty of lignin is produced as waste in the paper-making industry and biomass ethanol industry, and discharged in black liquor or burned as a low-caloric-value fuel, resulting in enormous resource waste and serious environmental pollution.^5–7 Thus, lignin depolymerization into value-added aromatic products under mild conditions would greatly benefit both the value-added fine chemical industry and green/renewable chemistry.

Lignin depolymerization is a process of breaking the linkages between C9 units to produce low-molecular-weight (LMW) chemicals. However, lignin is difficult to break down due to its huge molecular weight and robust spatial structure. Reported depolymerization methods can principally be divided into hydrolysis reactions, catalytic reductions and catalytic oxidation reactions. Hydrolysis of C–O–C linkages in lignin in alkaline aqueous solution (hydroxides or carbonates)^8,9 can generate phenol derivatives but the yield is as low as <10%. Catalytic reductions disrupt the structure and remove chemical functionalities from lignin to produce simpler phenols, while catalytic oxidation reactions tend to form aromatic compounds with additional functionality, which could serve as platform chemicals. Several methods for oxidation of lignin via nitrobenzenes¹⁰ and H₂O₂,¹¹ or via catalytic aerobic oxidation by Pd/Al₂O₃ and Co/Mn/Zr/Br catalysts^12,13 are either conducted in severe conditions (such as high temperatures and/or high pressures) or in need of additives, causing energy or environmental problems. Electrochemical method might be a promising alternative for lignin depolymerization, because of its controllability and environmental friendliness.^14–17 Nevertheless, the limited accessibility of lignin macromolecules to the electrode and the anode side reaction of oxygen evolution in aqueous electrolyte reduce the efficiency greatly. Some researchers have tried to avoid O₂ evolution by using nonaqueous electrolytes, e.g. ionic liquid.¹⁸ Undoubtedly, that would increase the cost as compared with using aqueous electrolyte.

In the present study, a non-diaphragm cylindrical electrolytic cell with a cylindrical graphite felt cathode inside and a RuO₂–IrO₂/Ti mesh anode outside was designed for lignin depolymerization in alkaline aqueous solution. In such an electrolytic cell, the by-product O₂ on the anode could be efficiently reduced to H₂O₂ on the cathode in situ. Subsequently, the electro-generated H₂O₂ in alkaline electrolyte could further decomposes into a series of reactive oxygen species (ROS) (HO˙, O₂⁻˙, HOO˙, etc.) to enhance the depolymerization of lignin. As a result, lignin in the electrolyte could be directly electro-oxidized on the anode and chemically oxidized by the electro-generated H₂O₂ formed on the cathode simultaneously. To probe into the essential of the lignin depolymerization by electrolysis, a series of studies, including the effects of electrolysis conditions, the decomposition rate of H₂O₂ into ROS and the yields of LMW products, were intensively carried out.

2. Experimental section

2.1 Materials and reagents

The crude lignin, derived from an enzymatic process for ethanol production from corn straw, was provided by Songyuan Chemical Co. Ltd (China). Before using, lignin was purified according to the procedures that were described in detail in our previous study.¹⁷ All chemicals were of analytical grade and commercially available, including diethyl ether, sodium hydroxide and hydrochloric acid (HCl, 37.5%). Water used in the study was deionized water (18 MΩ cm).

2.2 Depolymerization of lignin by electrolysis

An elaborately designed cylindrical electrolytic cell was described in our previous study,¹⁷ which possesses an outer RuO₂–IrO₂/Ti mesh (45 mm in diameter) anode and an inner porous graphite felt (40 mm in diameter) cathode with an interelectrode distance of 2.5 mm. A nylon mesh with flow channels was sandwiched between the two electrodes in case of short circuit. The purified lignin was dissolved in 1 mol L⁻¹ NaOH to prepare 2 wt% lignin alkaline solution used as electrolyte. Electrolysis was conducted in constant current model.

2.3 Separation and quantitative analysis of lignin depolymerization products

The reaction liquid after electrolysis was firstly acidified to pH = 2.0 with 10% hydrochloric acid to transform the phenolates in lignin depolymerization products into phenols. Subsequently, double volume of diethyl ether was added and then a three-phase-system was formed. The upper layer was diethyl ether phase, the middle was lignin solid phase and the bottom was aqueous phase.

The extraction was repeated several times until the diethyl ether phase was colorless. All the collected diethyl ether extraction solutions were mixed together and washed with deionized water and then dewatered by anhydrous Na₂SO₄. Then the diethyl ether phase was concentrated by a stream of nitrogen gas and dried in vacuum, generating certain amount of yellow powders. The solid phase in the middle was separated from the bottom aqueous phase by filtration and a light brown solid product was obtained. The bottom aqueous phase was rotary-evaporated to remove water, and then ethanol was added to dissolve the products, followed by centrifugation to remove the salt. The supernatant was then dried in vacuum, and the dark brown powders (aqueous phase products) were produced. The sum of the weight of the three parts was equal to the weight of initial lignin (the margin of error was 2.5%). The yield of LMW products was calculated using eqn (1)–(3):

Total yield = yield₁ + yield₂ (3)

In addition to the above gravimetric method, the yield of aqueous phase products was further confirmed by UV spectrophotometry. Reaction liquids obtained in the experiments for current density, temperature and electrolysis time effects were sampled, acidified to pH = 2.0, extracted and filtrated. Then the filtrates were diluted with deionized water and then tested by the UV.

2.4. Characterization of lignin depolymerization products

2.4.1 Identification of the products in ether phase by GC-MS. The diethyl ether extractive was re-dissolved in diethyl ether to make a concentration of 10.00 mg L⁻¹. The components and their relative contents were determined using a GC-MS instrument (Shimadzu QP 2010 Plus) equipped with an Rxi-5 MS capillary column (30 m × 0.25 mm × 0.25 μm). The injection volume was 1 μL and helium was used as a carrier gas with column flow rate of 1.74 mL min⁻¹. The temperature program was carried out as follows: initial temperature 45 °C for 5 min, then to 280 °C at 10 °C min⁻¹, and maintained at 280 °C for 10 min. The transfer line temperature was set at 250 °C. The electron ionization (EI) mass spectra in the range of 35–700 (m/z) were recorded in the full-scan mode. The detected compounds were identified based on NIST database. The peak areas for each substance in the total ion chromatogram were obtained automatically through the software and the products were quantified through relative peak areas.

2.4.2 Identification of the products in aqueous phase by ESI-MS/MS. Electrospray ionization is an extremely soft ionization process and therefore provides unfragmented ions for which not only the absolute molecular weight (ESI/MS) but also unequivocal structural information (MS/MS) can be obtained. In our study, mass spectra were obtained using a Waters Quattro Premier XE tandem quadrupole mass spectrometer equipped with an ESI source operating in negative mode. For ESI, the conditions were as follows: capillary voltage, 3.5 kV; RF Lens voltage, 0.3 V; source temperature, 100 °C; desolvation temperature, 350 °C; desolvation gas flow rate, 600 L h⁻¹; cone gas flow rate, 45 L h⁻¹; collision gas (Ar) flow rate, 0.13 mL min⁻¹. The mass spectra of the deprotonated molecule [M − H]⁻ (Q1 scan mode) were acquired over the range m/z 50–1000 in negative ion mode, and the product ion spectra (product ion scan mode) were obtained from the corresponding precursor ions, respectively. The products were assigned according to the respective m/z values of their precursor and product ions, and then compared to the mass spectra of standard compounds in the ref. 19–21.

2.4.3 Analysis of the products in the middle phase by GPC. The molecular weight distribution of the lignin depolymerization residue was determined by a Waters GPC 515-2410 system gel chromatography instrument with tetrahydrofuran as eluent at a flow rate of 1 mL min⁻¹, and system calibration was performed with polystyrene standards.

2.5. Determination of hydrogen peroxide and hydroxyl radicals

The concentration of the electro-generated H₂O₂ in blank electrolyte was determined by a modified spectrophotometry of titanium sulphate¹⁷ (the correlation coefficient: R = 0.99988; the detection limit is 0.10 mg L⁻¹).

The hydroxyl radicals generated from H₂O₂ decomposition were determined by bromocresol green (BG) oxidative assay.²² 5 mL 1.0 × 10⁻⁴ mol L⁻¹ BG solution and 10 mL 150.2 mg L⁻¹ electro-generated H₂O₂ alkaline solution (1 mol L⁻¹ NaOH) were added to four 25 mL volumetric flasks respectively. Then the four volumetric flasks were kept in four water bath pots at different temperatures (20 °C, 40 °C, 60 °C and 80 °C) respectively for 10 min. After that, sulphuric acid (H₂SO₄, 10 mol L⁻¹) was added to the solution dropwise until the color changed into yellow followed by adding 5 mL NaAc–HAc buffer solution (pH = 3.6) and diluted to the calibration mark with deionized water. The absorbance of above solutions (A_i) and the blank solution free of H₂O₂ (A₀) at 442 nm was measured by UV-spectrophotometry. The oxidation potentials of H₂O₂ and O₂⁻˙ in solution are much lower than that of hydroxyl radicals, HO˙ radicals could degrade BG through destroying π–π* conjugated system and attacking electronegative group Br in its triphenylmethane structure.^23–25 So the absorbance of BG was decreased and the difference of absorbance (ΔA = A₀–A_i) was in direct proportion to the concentration of hydroxyl radicals in solution.

3. Results and discussion

3.1 The separation and identification of lignin depolymerization products

Fig. 1c shows the three-phase-system of the reaction liquid after electrolysis, acidification and extraction by diethyl ether, which was mentioned in item 2.3 of Experimental section. It was confirmed that the LMW products with weak polarity (such as aldehydes and ketones) entered the ether phase, those with strong polarity (such as phenols and acids) entered the aqueous phase (shown in Fig. 1d), and the lignin fragments with large-molecular-weight precipitated in the middle phase.

Fig. 1 (a) Schematic representation of the lignin molecular structure; (b) the schematic diagram of electrolytic cell; (c) the three-phase-system formed after adding diethyl ether to the acidified electrolyte; (d) the typical aromatic chemicals obtained after lignin depolymerization.

The LMW depolymerization products of lignin in diethyl ether phase were mainly aldehydes and ketones, which was confirmed by GC-MS analysis. Specifically, vanillin, syringaldehyde, acetosyringone, 4′-hydroxy-acetophenone and 3′-methoxy-4′-hydroxyacetophenone were identified (the details are expressed in Fig. S1 and Table S1 in the ESI†). The depolymerization products in aqueous phase were mainly acids and phenols determined by ESI-MS/MS, such as phenol, phenylacetic acid, p-coumaric acid, gallic acid and syringic acid (the details are described in Fig. S2 and Table S2 in the ESI†). The weight average molecular weight (M_w) of the solid product in the middle layer was about 1000, which was determined by GPC, showing much smaller than that of the original lignin (more than 3000).

Most of the LMW products resembled the three kinds of C9 units (p-hydroxybenzene, guaiacyl, and syringyl phenylpropane units, shown in Fig. 1a) in lignin, demonstrating that the C–O–C and C–C linkages among the units were fractured. The C–O–C bonds (linkages of β-O-4, α-O-4 and 4-O-5) were readily cleaved in alkaline medium, resulting in the formation of phenolic hydroxyl groups.^1,26–28 The phenolic groups (phenoxy anion) of lignin molecule in alkaline medium were easy to oxidize into phenoxyl radicals by anode or by HO˙ radicals through single electron transfer (shown in Fig. 2). Then the resulting phenoxyl radical was converted into benzoquinonyl radical by the disproportionation.^29–33 It was supposed that benzoquinonyl radicals were further oxidized in two ways: one was that they transferred electrons to the anode followed by the cleavage of C–C (e.g., C_α–C_β) bonds on propyl side chains where hydroxyl groups transformed into aldehydes or ketones.^18,33,34 The other way was that the benzoquinonyl radicals were susceptible to O₂⁻˙ or HOO˙ radicals, leading to depolymerization of lignin and introduction of hydrophilic (e.g., carboxyl) groups.^29,30,35,36

Fig. 2 The possible pathways of lignin depolymerization by anode oxidation and electro-generated H₂O₂ oxidation.

3.2 Regulation of the concentration of electro-generated H₂O₂ and ROS by adjusting electrolysis conditions

As shown in Fig. 3, the concentration of electro-generated H₂O₂ increased with increasing current density because more O₂ was produced on the anode and then diffused to cathode to be reduced to H₂O₂ in situ. The concentration of H₂O₂ increased significantly in the first 20 min and then kept constant, meaning equilibrium was achieved between the generation and decomposition rates of H₂O₂. It is reported that a series of reactive oxygen species (ROS) (e.g. HO˙, O₂⁻˙, HOO˙, etc.) are produced during the decomposition of H₂O₂ in alkaline conditions,^29,30,37 which would involve in lignin depolymerization.

Fig. 3 The change in concentration of H₂O₂ in 1 mol L⁻¹ NaOH solution vs. current density (j) and electrolysis time at 20 °C.

The decomposition rate of electro-generated H₂O₂ was evaluated by different initial concentrations (c₀) of H₂O₂ after electrolysis at different current densities and the concentrations of H₂O₂ after stopping electrolysis for 10 min and 20 min (the data were shown in Fig. 4). The linear relationship of Δc/Δt vs. c₀ displays that the decomposition rate of H₂O₂ follows first-order kinetics. And the decomposition rate constant (k) was calculated as 0.0121 min⁻¹ at 20 °C.

Fig. 4 The initial concentrations (c₀) of H₂O₂ obtained by electrolysis at different current densities (1 mol L⁻¹ NaOH, 20 °C, electrolysis for 30 min) and the concentrations of H₂O₂ after sopping electrolysis for 10 min and 20 min.

The decomposition rate of H₂O₂ increased with increasing temperature, and the decomposition rate constants (k) at 40 °C and 60 °C were calculated as 0.0396 min⁻¹ and 0.0965 min⁻¹ respectively, showing remarkable decrease in concentration of H₂O₂ with elevating temperature (as shown in Fig. 5a). In order to verify the generation of ROS during the decomposition of hydrogen peroxide, HO˙ radical, as one of ROS, was analyzed by bromocresol green (BG) oxidative assay, and the results were shown in Fig. 5b. It was revealed that the absorption intensity of BG in the presence of H₂O₂ at 442 nm decreased dramatically with increasing temperature, indicating the dramatic degradation of BG. This implied that the amount of HO˙ generated by H₂O₂ decomposition increased significantly with increasing temperature because BG could only be degraded by hydroxyl radicals (HO˙).^22–25 The above results displayed the presence of ROS in the electrolyte and the concentration of H₂O₂ and ROS could be regulated by adjusting current density and temperature.

Fig. 5 (a) The concentration of H₂O₂ in 1 mol L⁻¹ NaOH solution against temperature under electrolysis for 30 min at a current density of 8 mA cm⁻². (b) UV spectra of bromocresol green (BG) in the presence of 150.2 mg L⁻¹ H₂O₂ at different temperature (the decrease in absorbance is due to the degradation of BG by HO˙ radicals generated from H₂O₂ decomposition; ΔA is the absorbance difference between BG solution and the BG–H₂O₂ solution at different temperatures, which is in proportion to the contents of hydroxyl radicals).

3.3 The changes in the yields of the products under different electrolysis conditions

Yields of products under different electrolysis conditions were determined by the gravimetric method. Meanwhile, it was found that the ratio of yields in aqueous phase for different current density obtained by the gravimetric method was consistent with that of the absorbance at λ_max by UV spectrophotometry. The same phenomena were observed for different temperature and electrolysis time. So the UV method could be regarded as a further confirmation method in addition to gravimetry for evaluating the yields of products (Fig. S3–S5 in the ESI†).

In the designed electrolytic cell, lignin was gradually depolymerized into LMW products. The inserted photographs in Fig. 6a show the change in the appearance of three-phase-system. It is obvious that as electrolysis time increased, the precipitated lignin fragments in the middle phase became less and less; the colors of the upper ether phase and the bottom aqueous phase changed from colorless to yellow and brown respectively, indicating that more and more LMW products with different functional groups had entered ether phase and aqueous phase separately in the course of time. Consequently, the total yields of products gradually increased. Similarly, the total yields of LMW products also increased with increasing current density and temperature (Fig. 6b–c).

Fig. 6 The change in the yield of LMW products and the appearance in the three-phase-systems, (a) versus electrolysis time; the rest conditions: temperature 80 °C, current density j = 8 mA cm⁻²; (b) versus current density; the rest conditions: temperature 20 °C, electrolysis time 1 h; (c) versus temperature; the rest conditions: current density j = 8 mA cm⁻², electrolysis time 1 h.

Comparing the changes in the yields of LMW products in ether phase (aldehydes and ketones) with that in aqueous phase (phenols and acids), it is noticeable that more obvious increase in phenols and acids than that of aldehydes and ketones was observed as temperature increased (Fig. 6c). This was due to the fact that more ROS were produced at higher temperature and they were more inclined to produce products containing hydrophilic functional groups (e.g. phenols and acids). In contrast, the anodic oxidation that usually leads to the production of aldehydes and ketones by cleavage of C–C bonds was enhanced by increasing current density. Therefore, the increase of aldehydes and ketones was more remarkable than that of phenols and acids as current density increased (Fig. 6b).

Moreover, Fig. 6a shows that the yields of aldehydes and ketones almost kept constant while the yields of phenols and acids increased significantly as electrolysis time increased. It was inferred that some of aldehydes and ketones were further oxidized into acids during the electrolysis. Another phenomenon confirmed that the depolymerization products underwent further oxidation: when the depolymerized products were removed from the cell and separated after one-hour-electrolysis and the residue (re-dissolved in alkaline solution) was further electrolyzed for another hour, the sum of total yields of the LMW products was much higher than that for two-hour continuous electrolysis (Fig. 7). The results imply that the timely separation of depolymerization products from electrolytic cell is favorable to avoiding further oxidation of products, thus enhancing the yields of aldehydes and ketones.

Fig. 7 The yield of LMW products for 2 hour continuous electrolysis vs. the sum of the yields for twice electrolysis and separation.

3.4 The influence of extra oxygen on the efficiency of lignin depolymerization

The above results indicated that electro-generated H₂O₂ oxidation played a very important role in lignin depolymerization process, so it is deduced that the efficiency of lignin depolymerization will be enhanced definitely if the electro-generated H₂O₂ oxidation was strengthened. In this electrolysis method, H₂O₂ was generated from the cathodic reduction of anodic by-product O₂. However, the amount of by-product O₂ on the anode is less than that needed by cathode according to eqn (4) and (5), so the concentration of electro-generated H₂O₂ in the electrolytic cell could be greatly increased if extra O₂ (from O₂ cylinder) is supplied into the cell.

Anode: 4OH⁻ − 4e⁻ → O₂ + 2H₂O (4)

Cathode: O₂ + 2H₂O + 2e⁻ → H₂O₂ + 2OH⁻ (5)

At room temperature, the concentration of electro-generated H₂O₂ in the blank electrolyte without extra O₂ supplement was 81.9 mg L⁻¹ after electrolysis for 20 min, while it reached 230.2 mg L⁻¹ with extra O₂ supplement (Fig. 8). The increase in concentration of electro-generated H₂O₂ benefited the depolymerization of lignin. Without extra O₂ supplement, only 29.2% of lignin in alkaline electrolyte was depolymerized into LMW products after 1 hour-electrolysis at 80 °C under a current density of 8 mA cm⁻² (see Table 1). However, 59.2% was depolymerized under the same condition except with extra O₂ supplement, which was close to that for 10 h-electrolysis without extra O₂ supplying, indicating that O₂ played an important role in electro-generation of H₂O₂ and 80% of the required electric quantity was decreased when achieving the same yield of LMW products.

Fig. 8 The concentration of H₂O₂ in 1 mol L⁻¹ NaOH solution with and without extra O₂ supplement for electrolysis at a temperature of 20 °C and a current density of 8 mA cm⁻².

Table 1 The yield of lignin depolymerization products by electrolysis with and without extra O₂ supplement^a

	Extra O2	Current density (mA cm^−2)	Time (h)	Yield (wt%)
				Ether phase	Aqueous phase	Total
a Experimental conditions: 2% lignin alkaline aqueous solution (1 mol L^−1 NaOH), 80 °C.
1	No	8	1	19.7	9.5	29.2
2	No	8	10	18.1	47.6	64.5
3	Yes	8	1	22.6	36.6	59.2

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4. Conclusion

A green and effective electrochemical method for lignin depolymerization was studied, in which the lignin in alkaline electrolyte was oxidized by the electro-generated H₂O₂ oxidation and anodic oxidation. Owing to the effective breakage of the C–C and C–O–C linkages among C9 units, lignin macromolecules were depolymerized into LMW aromatic products with different functional groups, such as aldehydes, ketones, phenols and acids. The electro-generated H₂O₂ and its decomposed products (ROS) played very important roles in lignin depolymerization. The concentration of electro-generated H₂O₂ and ROS could be regulated by adjusting the current density and temperature. The electrolysis conditions for producing higher concentration of H₂O₂ and ROS were in favor of giving higher yields of LMW products. 59.2% of lignin was depolymerized into LMW products after 1 hour-electrolysis at 80 °C under a current density of 8 mA cm⁻² with extra O₂ supplement, meaning 80% electric quantity was decreased as compared with no extra O₂ supplement. Meanwhile, timely separation of the products could avoid further oxidation of products, enhancing the electrolytic yield consequently. This strategy features full utilization of by-product O₂, mild conditions, energy-saving, greenness and sustainability for production of renewable chemicals.

Acknowledgements

The authors greatly appreciate the support from Chinese universities scientific Fund (no. JD1313) and the national natural science foundation of China (no. 21176024, 51374016). The authors thank Prof. Xiaoguang Liu for valuable suggestions in the preparation of this manuscript.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: See DOI: 10.1039/c4ra03793f

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