Haibin Zhua,
Yongmei Chen*a,
Tefu Qinb,
Lei Wanga,
Yang Tanga,
Yanzhi Suna and
Pingyu Wan*a
aNational Fundamental Research Laboratory of New Hazardous Chemicals, Beijing University of Chemical Technology, 100029 Beijing, China. E-mail: chenym@mail.buct.edu.cn; pywan@mail.buct.edu.cn
bKey Laboratory of Wood Science and Technology of SFA, Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 100091, China
First published on 20th December 2013
Lignin is a natural aromatic macromolecule in huge quantity and might serve as sustainable resources for the chemical industry after being depolymerized. An electrochemical approach combining anode oxidation and electro-generated H2O2 oxidation has been developed for converting lignin into value-added aromatic chemicals in this study. Lignin in alkali solution was electrolyzed in an undivided electrolytic cell with a cylindrical graphite felt cathode and a RuO2–IrO2/Ti mesh anode, in which the by-product O2 on the anode could be efficiently reduced to H2O2 on the cathode in situ. Results display that the depolymerization productivity via the integrated approach obviously surpassed the sum of that by separate H2O2 oxidation and anode oxidation. Moreover, the analysis results of GC-MS, GPC, and C9 expanded formula confirmed that C–C bonds and C–O–C bonds in lignin were cleaved synergistically by direct anodic oxidation and indirect H2O2 oxidation, and the macromolecules are gradually depolymerized into final products of monomers and dimers.
Lignin has been reported to be depolymerized by various chemical methods, including nitrobenzene,9 hydrogen peroxide10,11 and ClO2 oxidation,12 catalytic hydrogenation13–15 and catalytic aerobic oxidation.16–18 Usually severe conditions (such as high temperatures, high pressures or strong reagents) are needed in most of methods, causing energy or environmental problems. Hydrogen peroxide (H2O2) oxidation has proved to be environmentally friendly and practically feasible10,11 among these methods, while the efficiency for lignin depolymerization is still unsatisfactory because of the low oxidation capability of H2O2.
Electrochemical anode oxidation might be a promising alternative for lignin depolymerization, because of its powerful oxidation capability and environmental friendliness.19,20 However, the limited accessibility of lignin macromolecules to the electrode and the side reaction of oxygen evolution on anode in aqueous solvent reduce the efficiency greatly. Some researchers have tried to avoid O2 evolution by using non-aqueous electrolytes, e.g. ionic liquid,21 which undoubtedly increased the cost as compared with aqueous electrolyte.
In the present study, a novel electrochemical method for lignin depolymerization is proposed. An undivided cylindrical electrolytic cell with a cylindrical graphite felt cathode inside and a RuO2–IrO2/Ti mesh anode outside was designed, in which O2 generated on anode could be introduced to graphite felt cathode and subsequently reduced into H2O2 as well as a series of strong oxidants, such as ˙OH and ˙O2−, etc. That is to say, the combination of anode oxidation and electro-generated H2O2 oxidation could be achieved simultaneously. For a better understanding of the above integrated approach, comparative studies on lignin depolymerization by separate H2O2 oxidation and anode oxidation were also conducted.
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 distilled water and then dewatered over anhydrous Na2SO4. Then the diethyl ether phase was concentrated under a stream of nitrogen gas and dried in vacuum, yielding certain amount of yellow powder. The mixture of the middle and the bottom phase was dialysis desalinated and rotary-evaporated to remove water, and then dried in vacuum. Finally, light brown powders were obtained.
The GC-MS ion chromatograms of the three extractives are presented in Fig. 2, the identification of the compounds and their relative contents are listed in Table 1. A total of 29 kinds of monomeric (no. 1–16) and dimeric (no. 17–29) aromatic compounds were detected in the ether extractive. It was found that the obtained aromatic compounds resembled the phenylpropane building blocks (C9 units) of lignin but missed one or two carbons from the propyl side chain. It is reasonable to explain that these compounds were caused by the fracture of C–O–C linkages among the C9 units and/or C–C bonds in propyl side-chains. The C–O–C bond is readily cleaved and the fragmentation of C–O–C linkages under alkaline conditions usually results in the formation of phenolic hydroxyl groups,1,22–24 which is the reason that most of the detected depolymerization products have 4-phenolic hydroxyl groups. The C–C bonds in propyl side-chains are more likely to be cleaved by powerful oxidation (e.g. anodic oxidation), which is accompanied by the oxidation of functional groups leading to the formation of aldehydes (e.g., no. 8),25,26 ketones (e.g., no. 9) and quinones (no. 13).
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Fig. 2 The chromatograms of ether extractives obtained from H2O2 oxidation (A), integrated approach (B) and anode oxidation (C). |
No | The compounds | Relative content (%) | ||
---|---|---|---|---|
Integrated approach | Anode oxidation | H2O2 oxidation | ||
1 | Ethylbenzene | 2.33 | 1.01 | — |
2 | o-Dimethyl benzene | 4.34 | 1.18 | — |
3 | m-Dimethyl benzene | 1.90 | 0.52 | — |
4 | 2,3-Dihydro-benzofuran | 6.35 | 10.27 | 5.08 |
5 | 4-Hydroxy-3-methylacetophenone | 2.99 | 4.34 | 3.22 |
6 | 4-Hydroxy-benzaldehyde | 3.21 | 3.79 | — |
7 | 4′-Hydroxypropiophenone | 0.55 | — | — |
8 | Vanillin | 12.36 | 10.17 | 7.07 |
9 | 4′-Hydroxy-acetophenone | 5.02 | 5.75 | 3.24 |
10 | 2,5-Dimethyl-p-anisaldehyde | 5.43 | 9.97 | 5.92 |
11 | 4-Methoxy-phenylacetone | 0.85 | 2.43 | — |
12 | 3′-Methoxy-4′-hydroxyacetophenone | 2.32 | 2.86 | 1.92 |
13 | 2,6-Dimethoxy-1,4-benzoquinone | 0.44 | — | — |
14 | 2,4-Dihydroxy-3,6-dimethylbenzaldehyde | 0.51 | — | — |
15 | Syringaldehyde | 4.89 | 3.27 | 1.85 |
16 | Acetosyringone | 10.89 | 13.18 | 9.32 |
17 | 4-Phenoxyacetophenone | 0.97 | — | — |
18 | 9-Methylene-9H-fluorene | 3.50 | 3.60 | 3.21 |
19 | 4-Methoxydiphenylmethane | 1.49 | 0.50 | 0.71 |
20 | 4,4′-Ethylidenediphenol | 3.94 | 6.23 | 6.56 |
21 | 2,2′-Dihydroxy-4-methoxybenzophenone | — | — | 0.72 |
22 | 4′-(4-Methoxyphenoxy)acetophenone | 9.26 | 13.80 | 10.38 |
23 | 1-(2-Hydroxyphenyl)-3-phenyl-1-propanone | 1.65 | — | 2.06 |
24 | (2S,3S,4E)-3,5-Bis(4-hydroxyphenyl)-4-pentene-1,2-diol | 0.97 | — | 10.40 |
25 | 4-(4-Hydroxy-2,3,5-trimethylphenylmethyl)-2,6-dimethylphenol | 5.86 | 7.15 | 9.37 |
26 | 3-Hydroxy-9-methoxypterocarpan | 0.73 | — | 2.96 |
27 | 3-(3,4-Dimethoxyphenoxy)-4-methoxybenzoic acid | — | — | 2.61 |
28 | 2,3-Dimethoxy-2′,4′-dihydroxychalcone | 5.80 | — | 10.35 |
29 | 5,7-Dihydroxy-2-(4-hydroxyphenyl)-6-methoxy-4H-1-benzopyran-4-one | 1.48 | — | 3.04 |
Based on different structures of the depolymerization products by the three methods (Table 1), the possible reaction paths of those benzene derivatives from lignin macromolecule are shown in Fig. 3. p-Hydroxyphenyl derivatives (no. 5, 9 and 10), guaiacyl derivatives (no. 8 and 12) and syringyl derivatives (no. 15 and 16) all existed in the products of anode oxidation and H2O2 oxidation, suggesting that both two methods could depolymerize lignin into the basic structural units. However, an obvious difference was the formation of xylene and ethylbenzene (no. 1–3) in anode oxidation and integrated approach rather than in H2O2 oxidation. It has been reported that ethylbenzene could be obtained from the cleavage of the β–5 bond (Path 1 in Fig. 3) and xylene-like products are formed by the alkylation reactions involving the migration of CH3 group to the aromatic ring.1,27 The separate H2O2 oxidation may not have enough oxidation capability to cleave such kind of C–C bonds, so these simple hydrocarbons are not found. Moreover, the content of these monomeric compounds (no. 1–3) by integrated approach (8.57%, the sum of contents for the three monomeric compounds) is much higher than that by anode oxidation (2.71%), which demonstrates that the integrated approach has the strongest capability in C–C bonds cleavage, which is consistent with previous reports26 that has proved fracturing of C–C bonds in propyl side chains (e.g., Cα–Cβ) is one of the most important roles by electrochemical method in lignin depolymerization.
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Fig. 3 The possible reaction paths of depolymerization of lignin macromolecular into monomers and dimers. |
Some of the carbon–carbon bonds and C–O bonds in lignin constitute the most difficult bonds to break, and many of these linkages tend to survive the depolymerization process.22 In this study, the dimeric aromatic compounds with α–1 linkage (no. 19, 20, 21 and 25), 5–5′ linkage (no. 18), β–5 linkage (no. 26), 4–O–5 linkage (no. 17, 22 and 27) are detected in the products. So the dimeric products with these linkages remain intact probably result from the oxidative cleavage of the C–C or C–O bonds in adjacent structures. Moreover, the dimers linked by C–C bonds (no. 23, 24, 28 and 29) that are not common in lignin structures are only detected in H2O2 oxidation and integrated approach. This may be due to H2O2 oxidation involves mainly free radical reactions,28 resulting in condensation reactions between some newly formed monomer radicals consequently. Similar results were also found in other researches.28–30
(a) | Average molecular weight distribution | ||
---|---|---|---|
Mn | Mw | Polydispersity (Mw/Mn) | |
a Phenolic hydroxyl.b Aliphatic hydroxyl.c Carbonyl group.d The molecular weight of C9 expanded formulae. | |||
Original lignin | 858 | 3021 | 3.520 |
Residue of H2O2 oxidation | 856 | 1858 | 2.170 |
Residue of anode oxidation | 706 | 1217 | 1.724 |
Residue of integrated approach | 642 | 964 | 1.501 |
(b) | C9 Expanded formulae | Mrd | Number of C9 (Mw/Mr) |
---|---|---|---|
C9Hx′Oy′(OCH3)z(OHalipa)p(OHphenb)q(OCOc)m(OOHCOOH)n | |||
Original lignin | C9H7.91O1.31(OCH3)0.68(OHalip)0.58(OHphen)0.39(OCO)0.53(OOHCOOH)0.13 | 187.21 | 16 |
Residue of H2O2 oxidation | C9H7.30O0.97(OCH3)0.62(OHalip)0.67(OHphen)0.44(OCO)0.54(OOHCOOH)0.14 | 182.17 | 10 |
Residue of anode oxidation | C9H6.95O0.86(OCH3)0.50(OHalip)0.65(OHphen)0.43(OCO)0.60(OOHCOOH)0.16 | 177.45 | 7 |
Residue of integrated approach | C9H6.42O0.43(OCH3)0.44(OHalip)0.75(OHphen)0.51(OCO)0.65(OOHCOOH)0.19 | 173.03 | 5 |
Mw of original lignin is 3021, which is calculated to contain sixteen C9 units, and the polydispersity (Mw/Mn) is 3.520, indicating that original lignin has a wide distribution of molecular weight. Mw of the residues were determined as 2015, 1528 and 964 after lignin was electrolyzed by integrated approach for 20, 40 and 60 min, confirming that lignin depolymerization undergoes a gradual process. Mw of the residues obtained by the three methods for 60 min were 1858, 1217 and 964, and the polydispersity decreased to 2.170, 1.724 and 1.501, respectively. This implies that the integrated method has the most powerful depolymerization capability in the three methods.
The C9 expanded formula of lignin is expressed as follows:
C9Hx′Oy′(OCH3)z(OHalip)p(OHphen)q(OCO)m(OOHCOOH)n
Here p and q represent the number of aliphatic hydroxyl and phenolic hydroxyl groups respectively; m and n represent the number of carbonyl (in ketone or aldehyde groups) and carboxyl groups respectively; z is with respect to methoxyl group; y′ is calculated by the total oxygen content, obtained by elemental analysis, subtracting the oxygen content in the methoxyl, hydroxyl, carbonyl and carboxyl groups, so y′ represents the content of ether linkages (C–O–C) among lignin units.
It is noticeable that the least number of C9 units was observed in the residue of the integrated approach, suggesting that the largest quantity of intramolecular linkages were fractured among the three methods.
Judging from the changes in C9 formulae, the contents of ether bonds (the value of y′) in lignin residues decreased and those of aliphatic hydroxyl, phenolic hydroxyl, carbonyl and carboxyl groups (corresponding to the value of p, q, m and n respectively) increased after depolymerization. It can be explained that the ether (C–O–C) linkages of lignin were ruptured during depolymerization, resulting in the introduction of some oxygen-containing functional groups (e.g. phenolic hydroxyl and carbonyl). The value of y′ in C9 formula decreased from 1.31 (original lignin) to 0.43 by the integrated method, indicating that 67.2% of the ether linkages was disrupted, whereas the percentage of disruption by H2O2 oxidation and anode oxidation was calculated as 26.0% and 34.4%, respectively. In addition, the average number of phenolic hydroxyl in C9 formula increased from 0.39 (original lignin) to 0.51 by the integrated approach, indicating that 30.8% of phenolic hydroxyls were newly formed, whereas the percentage of formation by H2O2 oxidation and anode oxidation was 12.8% and 10.3% respectively. These results show that the integrated approach possesses a much higher depolymerization efficiency of lignin than the sum for the two separate methods.
The reasonable explanation of the “synergistic” effect in the integrated method is that the lignin fragments containing phenolic hydroxyls which are formed by H2O2 oxidation in the electrolyte are benefit for the C–C bonds cleavage in following oxidation on anode. Hanson and Evtuguin31,23 have reported that phenolic β-aryl ether lignin model compound have a conversion rate at least five-times higher than that for the non-phenolic structures, and the phenolic hydroxyl groups are more favorable to C–C bonds and C–O bonds cleavage. Moreover, comparing with the non-phenolic compounds, the phenolic derivatives will be completely ionized in the alkaline medium and consequently easily oxidized into phenoxyl radicals by single-electron transfer only at lower potential on anode,26,32 which would then undergo further breakage of the bonds to form more fragments. That is to say, both of the decrease in molecular weight and the introduction of phenolic hydroxyl groups during the deploymerization by H2O2 oxidation in the electrolyte were helpful to further anode oxidation. As a result, the integrated approach has a high efficiency in depolymerizaton of lignin.
Firstly, lignin was dissolved in NaOH solution (pH = 13), followed by centrifugation to remove the insolubles. The supernatant was then acidified with 10% hydrochloric acid to pH 2.0, after being heated at 80 °C for 10 min under stirring, lignin was precipitated and then filtered out after cooling down to room temperature. Then the solid was washed with dilute hydrochloric acid (pH = 2.0) repeatedly for three times and then rinsed with distilled water until neutral.33 After being dried and ground, it was kept as the preliminary purified lignin. After that, lignin was further purified by organic solvents. The obtained preliminary purified lignin was dissolved in a mixture of 1,4-dioxane–ethanol (8:
2, v/v), and centrifugalized to remove the precipitate. Then, the supernatant was added to the mixed solvent of diethyl ether–petroleum ether (1
:
1, v/v) drop by drop to five volumes under stirring, and purified lignin was precipitated. Separated by centrifugation, the purified lignin was then washed by mixture of diethyl ether–petroleum ether (1
:
1, v/v) for 4–5 times, and then freeze-dried.34
All chemicals except lignin were of analytical grade and commercially available, including diethyl ether, sodium hydroxide, hydrogen peroxide (H2O2, 30%) and hydrochloric acid (HCl, 37.5%). Water used in the study was deionized water (18 Ω cm).
The purified lignin was dissolved in NaOH solution (pH = 12) to make 2% (wt%) lignin alkaline solution, used as electrolyte. Electrolysis was conducted in constant current model at a current density of 6 mA cm−2 for 1 hour at room temperature. The concentration of the generated H2O2 in the electrolyte with lignin absent was determined by modified spectrophotometry of titanium sulfate35 (the standard curve for determination of hydrogen peroxide: y = 0.02127x − 0.00138, correlation coefficient: R = 0.99988; the detection limit of the method is 0.1 mg L−1). It was shown that the concentration of H2O2 increased rapidly in the first 10 min, then kept steady around 85 ± 1 mg L−1 during the following electrolysis process. However, if a stainless steel electrode instead of graphite felt was applied as cathode in the cell, less than 1.5 mg L−1 H2O2 was detected in the electrolyte under the same electrolysis conditions, demonstrating the specific catalytic effect of graphite felt cathode.
For comparison, the depolymerization of lignin by separate anode oxidation and H2O2 oxidation was also conducted. For anode oxidation, the electrolyte and electrolysis conditions were the same as the integrated approach, except that a stainless steel electrode was employed as cathode in the cell. For H2O2 oxidation, H2O2 (30%) solution was added to 2% lignin alkaline solution to obtain a H2O2 concentration of 85 mg L−1, followed by stirring for 1 h without electrolysis.
The residue after diethyl ether extraction as well as the purified original lignin was characterized by molecular weight distribution, elemental composition and the C9 expanded formulae.
The molecular weight distribution was determined by a Waters GPC515-2410 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. Elemental composition (C, H and O) was analyzed by a Vario EL cube Elemental Analyser instrument. The C9 expanded formulae contain complete information about the lignin structure, which were obtained based on elemental analysis and functional groups analysis. The carbonyl groups were determined by the modified oximation potentiometric titration36 on a Metrohm's 877 Titrino plus automatic potentiometric titrator. The phenolic hydroxyl and carboxyl groups were measured by potentiometric titration.37 The determination of total hydroxyl groups in lignin was performed by acetic anhydride acylation and potentiometric titration.37 Alcoholic hydroxyl contents were calculated by the difference between the total hydroxyl groups and the phenolic hydroxyl groups. The methoxyl contents of lignin were determined using proton nuclear magnetic resonance spectroscopy (1H NMR).37
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47516f |
This journal is © The Royal Society of Chemistry 2014 |