Lignin depolymerization via an integrated approach of anode oxidation and electro-generated H₂O₂ oxidation

Abstract

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.

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1. Introduction

Lignin is the second most abundant biopolymer on Earth after cellulose. It is a highly branched macromolecule consisting of p-hydroxybenzene, guaiacyl (4-alkyl-2-methoxyphenol), and syringyl (4-alkyl-2,5-dimethoxyphenol) phenylpropane units (C9 units) crosslinked by C–C bonds (β–1, β–5, β–β, 5–5) and C–O–C bonds (4–O–5, α–O–4, β–O–4), of which β–O–4 bonds account for more than 50% of the linkages.^1–3 Lignin has received great attention as a sustainable precursor for basic aromatic building blocks which are currently obtained from fossil-based feedstocks.^4,5 However, lignin is resistant to normal chemical or biochemical processes for breaking down linkages among those units, due to its huge molecular weight and robust spatial structure. At present, plenty of lignin is discharged into paper-making black liquor or burned as a low-caloric-value fuel, resulting in enormous resource waste and serious environmental pollution.^6–8

Lignin has been reported to be depolymerized by various chemical methods, including nitrobenzene,⁹ hydrogen peroxide^10,11 and ClO₂ oxidation,¹² catalytic hydrogenation^13–15 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 feasible^10,11 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.

2. Results and discussion

2.1 The separation of the depolymerized products of lignin

The alkaline reaction liquid after electrolysis was firstly adjusted to pH = 2, and then double volume of diethyl ether was added leading to the formation of a three-phase-system (as shown in Fig. 1). It is confirmed that the depolymerization products of lignin with low molecular weight were extracted into the upper diethyl ether phase (this part of lignin depolymerized products is called as “ether extractive”), while the undecomposed lignin molecules or depolymerization products with large molecular weight remained in middle turbid layer and the bottom aqueous phase (called as “ether extraction residue”).

Fig. 1 The appearance of lignin during depolymerization by the integrated approach: (a) original lignin after being purified; (b) the alkali solution after electrolysis; (c) the depolymerized compounds separated in post-treatment, top: diethyl ether phase; middle: turbid phase; bottom: aqueous phase; (d) the lignin depolymerized compounds in diethyl ether phase (called as “ether extractive”) after volatilizing the solvent; (e) the lignin depolymerized compounds in the middle and the bottom phases (called as “ether extraction residue”) after drying.

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.

2.2 The diethyl ether extractive of the lignin depolymerization products

The yield of the ether extractive (i.e. mass fraction of diethyl ether extractive to original lignin) by the integrated approach was 10.1%. The reaction liquid in contrast experiments of the separate anode oxidation and H₂O₂ oxidation were also post-treated as above, and the yield of the ether extractive was 4.1% and 3.4%, respectively.

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).

Fig. 2 The chromatograms of ether extractives obtained from H₂O₂ oxidation (A), integrated approach (B) and anode oxidation (C).

Table 1 Compounds and their relative contents in the ether extractives after lignin depolymerization by the three methods

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

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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., C_α–C_β) is one of the most important roles by electrochemical method in lignin depolymerization.

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

2.3 The ether extraction residues of the lignin depolymerization products

The residues of depolymerized lignin after diethyl ether extraction were analyzed by GPC, elemental analysis and functional groups analysis (Table S1 in ESI†). The results of weight-average molecular weight (M_w), number-average molecular weight (M_w), the polydispersity (M_w/M_n), C9 expanded formulae and the numbers of C9 units (M_w/M_r) of the deploymerization residues and the original lignin were listed in Table 2.

Table 2 (a) Average molecular weight distributions and (b) the calculated C9 expanded molecular formulae of original lignin and the extraction residues of the three methods

(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

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(b)	C9 Expanded formulae	Mr^d	Number of C9 (Mw/Mr)
	C9Hx′Oy′(OCH3)z(OHalip^a)p(OHphen^b)q(OCO^c)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

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M_w of original lignin is 3021, which is calculated to contain sixteen C9 units, and the polydispersity (M_w/M_n) is 3.520, indicating that original lignin has a wide distribution of molecular weight. M_w 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. M_w 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:

C₉H_x′O_y′(OCH₃)_z(OH_alip)_p(OH_phen)_q(O_CO)_m(OOH_COOH)_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 Evtuguin^31,23 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.

3. Experiments

3.1 Materials and reagents

The crude lignin, derived from an enzymatic process of corn straw in ethanol production, was provided by Songyuan Chemical Co. Ltd. (China). Before using, lignin was purified through following steps to remove the remaining cellulose, hemicellulose and sugars.

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).

3.2 Deploymerization of lignin

A cylindrical electrolytic cell was designed (Fig. 4), which possess an outer RuO₂–IrO₂/Ti mesh (45 mm in diameter) anode and 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. Graphite felt was purchased from Beijing Carbonsci Tech Co., Ltd. (Beijing, China), and RuO₂–IrO₂/Ti mesh (mole ratio of Ru–Ir = 1.54 : 1) was provided by Northwest Institute For Non-ferrous Metals Research (Xian, China).

Fig. 4 The schematic diagram of experimental apparatus: (1) power supply; (2) magnetic stirrer; (3) magnetic stirring apparatus; (4) cylindrical electrolytic cell; (5) porous graphite felt cathode; (6) RuO₂–IrO₂/Ti mesh anode; (7) mesh support.

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.

3.3 Analysis of the depolymerization products

The diethyl ether extractive was re-dissolved in diethyl ether to give a concentration of 10.00 mg L⁻¹, and 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.

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

4. Conclusions

In the present report, lignin in alkali electrolyte was depolymerized in the designed cylindrical electrolytic cell consisting of graphite felt cathode inside and RuO₂–IrO₂/Ti mesh anode outside. A combination of anode oxidation and H₂O₂ oxidation for lignin depolymerization was achieved by making full use of the byproduct O₂ to produce H₂O₂ on cathode in situ. Analysis result shows that the depolymerization efficiency by integrated approach, with respect to disruption of C–C and C–O–C bonds and the yield of low-molecular-weight products (monomers and dimers), is obviously higher than the sum of that by the separate anode oxidation and H₂O₂ oxidation. The reaction process in the integrated approach is hypothesized that lignin macromolecules are mainly fractured into several medium fragments through the cleavage of C–O–C linkages, and the fragments have less steric hindrance and more active functional groups which obtain more chance to be oxidized into smaller fragments on anode surface due to the breakage of C–C linkages. Finally, more and more monomeric and dimeric benzene derivatives are formed through such a gradual depolymerization process. This novel integrated approach would be important for valorization of the lignin stream for environmentally friendly production of renewable chemicals in a biorefinery.

Acknowledgements

The authors would like to thank the financial support of National High Technology Research and Development Program of China (2010AA101703). Thanks are also extended to Chinese academy of forestry for supplying the lignin. Thank Prof. Xiaoguang Liu for the important discussions and modification of manuscript.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47516f

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