Electrocatalytic degradation of aspen lignin over Pb/PbO2 electrode in alkali solution

Article abstract of DOI:10.1016/j.catcom.2015.03.033

Abstract A novel procedure about electrochemical catalysis degradation of aspen lignin with Pb/PbO₂ anode in the three-dimensional electrode (TDE) reactor was investigated. SEM, XRD and cyclic voltammogram tests were employed to study the surface morphology, composition and the electrochemical redox performance of the fabricated Pb/PbO₂ electrode. The lignin was cracked by OH and hydrogenated by [H] atom generated from alkaline water electrolysis, leading to the production of 4-methylanisole and other products. Raw material lignin concentration, current density, temperature and time were optimized. The pathway of electrocatalytic degradation and hydrogenation process of lignin in alkaline solution was also discussed.

Full text of DOI:10.1016/j.catcom.2015.03.033

Catalysis Communications 67 (2015) 49–53
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Electrocatalytic degradation of aspen lignin over Pb/PbO2 electrode in
alkali solution
⁎
Yong-sheng Wang, Fang Yang, Zhi-hua Liu, Lu Yuan, Gang Li
Hebei Provincial Key Lab of Green Chemical Technology & High Efficient Energy Saving, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel procedure about electrochemical catalysis degradation of aspen lignin with Pb/PbO2 anode in the three-
dimensional electrode (TDE) reactor was investigated. SEM, XRD and cyclic voltammogram tests were employed
to study the surface morphology, composition and the electrochemical redox performance of the fabricated Pb/
PbO2 electrode. The lignin was cracked by •OH and hydrogenated by [H] atom generated from alkaline water elec-
trolysis, leading to the production of 4-methylanisole and other products. Raw material lignin concentration, cur-
rent density, temperature and time were optimized. The pathway of electrocatalytic degradation and
hydrogenation process of lignin in alkaline solution was also discussed.
Received 5 January 2015
Received in revised form 7 March 2015
Accepted 24 March 2015
Available online 28 March 2015
Keywords:
Aspen lignin
Electrocatalytic degradation
4-Methylanisole
© 2015 Elsevier B.V. All rights reserved.
Pb/PbO2 electrodes
Alkaline water electrolysis
1. Introduction
into the TDE reactor to form charged microelectrodes and increase its
large specific surface areas in connection with the reactant. The reactiv-
As a kind of renewable resource of organic carbon, lignin accounts
for about a quarter in woody plants. Lignin is considered to be, after cel-
lulose, the second most abundant natural polymer. However, lignin has
been regarded as a waste byproduct of the lignocellulosic biomass con-
version processes despite that it is composed of valuable aromatic
monomers [1]. For exploring the specific structure of lignin and getting
valuable refine chemical, many efforts have been paid to degrade lignin
such as fast pyrolysis, hydrothermal degradation, photodegradation and
electrochemical degradation.
In recent years, owing to its ease of operation and environmental
friendliness, electrocatalytic oxidation has been employed for the
degradation of lignin [2]. The variety and yields of products obtained
from lignin oxidation varied according to the nature of the anode.
B. Mahdavi [3] studied the degradation of lignin model compounds in
aqueous ethanol at Raney nickel electrodes, showing the efficiency of
hydrogenolysis of the C–O bond can be optimized to 100% at proper
condition.
ity of electrochemical oxidation in TDE reactor can be improved. By con-
trast with 2D electrode, the addition of granular activated carbon may
enhance the conductivity and mass transfer or the adsorption of reac-
tant [5], 3D electrode has greater advantage in degrading organics [6,7].
4-Methylanisole, also known as 1-methoxy-4-methyl benzene, be-
longs to methoxybenzene derivatives. It mainly applies in food flavor,
inedible essence, and pharmaceutical intermediate. 4-Methylanisole is
an edible spice (as seen in FDA104-93-8), especially in the preparation
of walnut, hazelnut and other nut essence. It had been achieved by
upgrading of bio-oil in a catalytic pulsed dielectric barrier discharge
plasma reactor [8].
2. Materials and experimental methods
2.1. Materials
Aspen lignin was supplied by Luohe Huadong Lignin Company,
Henan Province, China. Elemental analysis (C, H, N) of aspen lignin
was conducted using a FlashEA 1112 Series element (Thermo Electron
Corporation). The contents of carbon and hydrogen in aspen lignin
were 59.7% and 5.94%, respectively. Lead chips and stainless steel wire
mesh (purity ≥ 99.99%) were purchased from Tianjin Guangfu Fine
Chemical Research Institute, China. The granular activated carbon with
particle sizes in range of 1.40–3.35 mm provided by Tianjin Fengchuan
Chemical Reagent Technologies Co., Ltd, China. All of the solutions were
prepared using deionized water.
We had previously reported the degradation of lignin to BHT by elec-
trocatalytic reaction in 2D electrode system [4], however there was a
problem of the electrochemical impedance as the case for the strong ad-
sorption of hydroxyl group in lignin. The TDE reactor based on electro-
chemical catalysis has been employed in this research work. Because
there are a large number of the particle electrode materials placed
⁎
Corresponding author.
E-mail address: ligang@hebut.edu.cn (G. Li).
http://dx.doi.org/10.1016/j.catcom.2015.03.033
1566-7367/© 2015 Elsevier B.V. All rights reserved.

50
Y. Wang et al. / Catalysis Communications 67 (2015) 49–53
Fig. 1. SEM images of the Pb/PbO2 electrode before (a) and after (b) electrolysis in 1 mol/L NaOH solution.
2.2. Preparation of the Pb/PbO2 electrode by electrodeposition
precipitate unreacted lignin residua simultaneously. The mixed liquor
of organic solvent and water phase was sufficiently stirred until the lig-
nin residue flocculation. The extract liquor and unreacted lignin residue
were separated by the vacuum filter. The filter cake was dried to calcu-
late the lignin degradation rate and the filtrate was separated by
separatory funnel into water and extractive phase.
The Pb/PbO2 electrode was prepared by electrodeposition [9]. The
pretreatment of lead sheet included following steps: burnish the surface
of lead sheet with sandpaper (P 120), impregnate into acetone, clean by
mixture solution (Na2CO3 20 g/L, Na3PO4 20 g/L, NaOH 50 g/L), acidic
clean by mixed acid solution (HNO3 400 g/L, HF 5 g/L) for 2 min and
boil for 5 min in oxalic acid solution (100 g/L). The pure lead sheet,
about 12 (2 × 6) cm2, was sited in an electrolytic cell which composed
of anode, copper sheet of the same size used as cathode and 0.86 mol/L
H2SO4 electrolyte solution. The electrodeposition was performed with
50 mA/cm2 controlled by a DC regulated power supply (ss1792c,
Shijiazhuang KeHeng Electronic Company, China) at room temperature
for duration 1 h. Then the Pb/PbO2 electrode was obtained.
The crude products of lignin degradation were got by reduced pres-
sure distillation to evaporate and recover the solvent. The separation
and purification of products depended on thin layer chromatography
and column chromatography separation (silica 200–300 mesh, eluent,
n-hexane: ethyl acetate = 6:1, v/v). The main product was 4-
methylanisole and its chemical structure was confirmed by GC–MS,
NMR spectra. 1H NMR (CDCl3, 400 MHz): δ 6.789 (s, 1H, CH), δ 6.81 (s,
1H, CH), δ 7.069 (s, 1H, CH), δ 7.090 (s, 1H, CH), δ 2.97 (s, 3H, CH3), δ
3.769 (s, 3H, CH3); 13C NMR (CDCl3, 400 MHz): δ 157.45, 129.877,
129.808, 113.672, 55.249, 20.442; MS: 123(7), 121(46), 107(30),
79(20), 77(34), 51(9), 50(4). The other degradation products such as
acetovanillone, toluene, vanillin, syringaldehyde, 2,6-dimethoxyphenol
and styrene were confirmed by GC with internal standard methods.
The products were qualitatively and quantitatively measured by in-
ternal standard method on the gas chromatography (GC) instrument
(SHIMADIU, GC-2010) equipped with a FID detector and an Agilent
J&W GC column (DB-FFAP, length 30 m, diameter 0.32 mm, film
0.25 μm). The temperature of injection inlet, column and detector was
240 °C, programming 50 °C–220 °C and 280 °C respectively. The flow
rate of carrier gas, high purity nitrogen (purity of 99.99%), was con-
trolled with 38.7 mL·min−1 and injecting volume of sample solution
was 1 μL for split sampling program of split ratio 20.0.
2.3. Electrocatalytic degradation of aspen lignin to chemicals based on
three-dimensional electrodes
The experiments were carried out in three-dimensional electrode re-
actor [10]. Particle electrode was filled between Pb/PbO2 electrode
(anode) and stainless steel wire mesh (5 ∗ 5) electrode (cathode). Sodium
hydroxide (1 mol/L) solution was used to dissolve lignin and gave the
concentrations of lignin solution of 40 g/L in the electrolytic cell. The DC
regulated power supply provided current 20–90 mA/cm2 for the
electrocatalytic degradation of lignin. The lignin alkali solution was stir-
ring by magnetic stirring for mass transfer uniform. The reaction temper-
ature and time were controlled within 30–90 °C and 0–8 h, respectively.
2.4. The analysis and separation of the degradation products
The degradation liquid of lignin was extracted with the same volume
ethyl acetate and acidified to pH 2–3 by 0.86 mol/L H2SO4 solution to
Fig. 3. CV curve recorded for the Pb/PbO2 electrode without and with lignin in the three-
Fig. 2. XRD patterns of Pb/PbO2 before and after electrolysis in 1 mol/L NaOH solution.
dimensional electrochemical reactor.

Y. Wang et al. / Catalysis Communications 67 (2015) 49–53
51
Meanwhile, the reduction potential value −2.5 V was obviously lower
than equilibrium potential [14] (1.459 V) of Pb/PbO2 electrode, the gen-
eration of hydrogen atoms (as seen in Eq. (2)) on the surface of cathode
could be carried out by alkaline water electrolysis [15,16].
þ
Pb=PbO2 anode : H2O−e−→ OH þ H
ð1Þ
ð2Þ
]
Fe–Cr–Mn–Cucathode : H2O þ e−→½Hꢀ þ OH−
0
0
]
R…R þ OH þ ½Hꢀ=alloy→ROH þ R H
ð3Þ
3.2. Electrochemical degradation of lignin in the three-dimensional elec-
trode reactor
As shown in Fig. 5, two different analysis methods by GC were
applied to analyze the samples. It was found that the main product
4-methylanisole, and coproducts 2,6-dimethoxyphenol, vanillin,
acetovanillone, syringaldehyde, toluene as well as styrene were
confirmed by internal standard method which corresponded with the
retention time at 8.39, 2.07, 3.02, 3.28, 5.67, 2.77 and 5.30 min respec-
tively. Some other unidentified products were also observed.
Fig. 4. CV curves recorded for the Pb/PbO2 electrode before (a) and after electrolysis 4 h
(b) in 1 mol/L NaOH solution.
3. Results and discussion
The electrochemical oxidation of lignin was conducted at 50 mA/cm2.
The residual amount of lignin and the products yield of lignin degrada-
tion were shown in Fig. 6. About 88% lignin had been decomposed within
first 5 h. At the end of degradation 8 h, lignin was almost completely de-
graded, the residual lignin was only 1.5 g/L and the cumulative yield of
main product 4-methylanisole reached 343.3 (g/kg lignin). Other co-
products yield, such as vanillin and syringaldehyde, was shown in the
micro-enlarge figure of Fig. 6.
3.1. Characterization of the prepared Pb/PbO2 electrode
SEM images (seen in Fig. 1) were used to characterize morphology
and coating surface structure of Pb/PbO2 electrode. The surface of the
original Pb/PbO2 electrode was uniform and the crystal size was very
fine. By contrast, after electrolysis 4 h, the electrode was not such a uni-
form crystal size and the electrode surface became rough and had many
“cracks”. This might be attributed to the generation of dioxygen on the
anode surface resulting in the crystal structure change [11] in the pro-
cess of electrochemical reaction.
Fig. 2 showed the XRD pattern of the internal PbO2 layer deposited in
the alkaline solution. Comparison of electrode XRD spectra before and
after electrolytic reaction indicated that α-PbO2 crystal was easier to
transform into β-PbO2, as β-PbO2 had stronger stability, the effective
constituent to the benefit of electrochemical degradation of lignin in
the alkaline solution was β-PbO2 crystal structure.
3.3. Optimization of operating parameters for electrocatalytic degradation
of lignin
Because the effective content of products was comparatively high in
first hour and second hour, the operating parameters for electrocatalytic
degradation of lignin were therefore optimized in unit hour, and the
maximum yield of seven kinds of products was listed in Table 1.
All experiments were carried out with aspen lignin dissolving in the
concentration 1 mol/L of NaOH solution to give a homogeneous system.
As shown in Table 1, it was found that in unit time the maximum yield
of main product 4-methylanisole (64.9 g/kg lignin) was achieved in run
1 as following optimal operations: the content of aspen lignin dissolving
in alkali liquid 40 g/L, current density 50 mA/cm2, temperature 50 °C, in
2nd hour, the volume ratio of extraction solvent ethyl acetate to lignin
degradation liquid (R) was 1. Similar with this operations, the maxi-
mum yield of coproduct styrene was 1.8 g/kg lignin at 60 mA/cm2 (as
The cyclic voltammogram tests were conducted in a three-electrode
system. In Fig. 3, the cyclic voltammogram tests in 1 mol/L NaOH solu-
tion with 40 g/L lignin and without lignin were carried out. Fig. 4(a) in-
dicated a Pb/PbO2 before electrolysis, the anodic peak corresponding to
the formation of PbO2 and was followed by vigorous oxygen evolution
on the surface of PbO2, the cathode peak might be assigned to the reduc-
tion of PbO2 to Pb [12]. In Fig. 4(b), after electrolysis 4 h, an anodic peak
was observed at E +0.3 V which might be due to the oxidation of lignin
•
by OH generated from water electrolysis (as seen in Eq. (1)) [13].
Fig. 5. Lignin degradation products (black line) and standard substance (red line) gas chromatograms with column temperature constant method (M) and temperature programming
method (N).

52
Y. Wang et al. / Catalysis Communications 67 (2015) 49–53
Fig. 6. Effective content of lignin and products during the lignin degradation.
seen in run 3), and 2,6-dimethoxyphenol was 9.48 g/kg lignin at tem-
perature 50 °C (as seen in run 4). The selection of the extraction solvent
affected the extraction efficiency of lignin degradation products. For ex-
ample, if the extraction solvent was chloroform instead of ethyl acetate
as shown in run 8, not only didn't get the styrene, but also the extraction
efficiency of other products reduced obviously. Other extraction sol-
vents, 1,1,1-trichloroethane and anisole, seemed don't fit for extraction
yet.
Fig. 7(I) showed that the maximum yield of 4-methylanisole ap-
peared in the lignin concentration of 40 g/L, beyond that the yield was
no longer enhanced with the raw material lignin concentration in-
creased. Because lignin was one of the antioxidants [17], excessive lig-
nin had made the antioxidation increase greatly so that the effect of
the raw material lignin concentration on the yield of 4-methylanisole
was not crucial.
The impact of current density on the electrochemical oxidation of
lignin was presented in Fig. 7(II). For 40 g/L lignin mentioned above,
lignin oxidation was accelerated with an increase of the applied current
density from 20 to 50 mA/cm2. The extra current density over
50 mA/cm2 exhibited indistinctive contribution to the oxidation of lig-
nin. According to the theory proposed by Scialdone [18], higher values
of current density resulted in a marked decrease of the faradic efficiency
of the electrolysis process [19]. It was revealed in Fig. 7(III) that the yield
of 4-methylanisole increased with temperature rising from 30 to 50 °C.
Because the •OH radical was one of oxidants, if the temperature contin-
uously increased, oxidants would be decomposed [20,21]. Therefore,
part of the •OH was broken when temperature was higher than 50 °C,
which leaded to the reduction of 4-methylanisole.
4. Conclusion
In this study, we have put forward a novel preparation of 4-
methylanisole from aspen lignin in alkali solution by three-
dimensional electrode reactor. The β-PbO2 crystal on the surface of
Pb/PbO2 electrode played an effective role on providing •OH radical for
lignin oxidation degradation. And the [H] atom was supplied with the
electrolysis of water molecule while the cathode potential became
lower than the equilibrium potential. Lignin was firstly cracked to
intermediates by •OH radical oxidation, and then hydrogenation by [H]
atom made the intermediates molecular bond saturation so that new
compounds were given. The cumulative yield of main product 4-
methylanisole reached 343.3 (g/kg lignin) at the end of degradation
8 h with the optimal conditions as following: aspen lignin content
40 g/L, current density 50 mA/cm2, temperature 50 °C. By the end of 8 h

Y. Wang et al. / Catalysis Communications 67 (2015) 49–53
53
Fig. 7. The effects of reaction factors on yield of 4-methylanisole. (I) Raw material lignin concentration, (II) current density, (III) temperature.
[4] Y.M. Zhang, Y. Peng, X.L. Yin, Zh.H. Liu, G. Li, J. Chem. Technol. Biotechnol. 89 (2014).
[5] L. Wei, S. Guo, G. Yan, C.M. Chen, Electrochim. Acta 55 (2010) 8615.
[6] L. Xu, H.Z. Zhao, S.Y. Shi, Dyes Pigments 77 (2008) 158.
of lignin degradation, most of aspen lignin had been decomposed. Other
coproducts such as vanillin, syringaldehyde, 2,6-dimethoxyphenol,
acetovanillone, toluene, and styrene were also achieved. In summary,
the electrocatalytic degradation of lignin is a promising approach for
the conversion of lignin into valuable products such as 4-methylanisole,
which not only enhances the potential utility of lignin but also produces
fine chemicals instead of the fossil raw materials.
[7] X.B. Wu, X.Q. Yang, D.C. Wu, Chem. Eng. J. 138 (2008) 47.
[8] T. Hmed, K. Mahsa, G. Mohammad, R. Parisa, C.G. Bruce, Energy Fuel 28 (2014) 4545.
[9] X. Li, D. Pletcher, F.C. Walsh, Chem. Soc. Rev. 40 (2011) 3879.
[10] L. Xu, H. Zhao, S. Shi, G.Z. Zhang, J.R. Ni, Dyes Pigments 77 (2008) 158.
[11] Y. Zheng, W. Su, S. Chen, X.Z. Wu, X.M. Chen, Chem. Eng. J. 174 (2011) 304.
[12] P. Veluchamy, M. Sharon, J. Electroanal. Chem. 361 (1993) 261.
[13] M.A. Quiroz, S. Reyna, C.A. Martinez-Huitle, S.A. Ferro, Appl. Catal. B Environ. 59
(2005) 259.
[14] D. Claus, O.B. Jurgen, John Wiley & Sons, Germany, 2011 p.169.
[15] K. Jurewicz, E. Frackowiak, F. Beguin, Appl. Phys. A 78 (2004) 981.
[16] A. Manabe, M. Kashiwase, T. Hashimoto, T. Hayashida, A. Kato, K. Hirao, Electrochim.
Acta 100 (2013) 249.
[17] K. Toh, H. Yokoyama, H. Noda, Y. Yuguchi, J. Food Biochem. 34 (2010) 192.
[18] O. Scialdone, A. Galia, C. Guarisco, S. Randazzo, Electrochim. Acta 53 (2008) 2095.
[19] C. Tang, W. Yan, C. Zheng, Front. Environ. Sci. Eng. China 8 (2014) 337.
[20] F.L. Souza, J.M. Aquino, D.W. Miwa, M.A. Rodrigo, J. Environ. Chem. Eng. 2 (2014)
811.
Acknowledgment
This work was financially supported by the National Natural Science
Foundation of China (Grant No. 21176055).
References
[1] C.R. Lee, J.S. Yoon, Y.W. Suh, J.W. Choi, J.M. Ha, D.J. Suh, Catal. Commun. 17 (2012)
54.
[21] M. Panizza, G. Cerisola, J. Hazard. Mater. 153 (2008) 83.
[2] P. Parpot, A.P. Bettencourt, E.M. Belgsir, J. Appl. Electrochem. 30 (2000) 727.
[3] B. Mahdavi, A. Lafrance, A. Martel, Appl. Electrochem. 27 (1997) 605.