Electro-Fenton degradation of cellulose using graphite/PTFE electrodes modified by 2-ethylanthraquinone

Article abstract of DOI:10.1016/j.carbpol.2011.07.021

This paper reports an investigation on the degradation of cellulose via electro-Fenton method. The electrodes chosen were graphite/PTFE electrodes modified by 2-ethylanthraquinone (EAQ). The presence of EAQ greatly improved the rate of hydrogen peroxide formation on air-fed graphite/PTFE electrodes, which was determined by a UV spectroscopic method using molybdate as the color agent. Hydrogen peroxide can react with Fe²⁺ to form hydroxyl radicals in an acidic medium, which accelerates cellulose degradation by cleaving the glycosidic bonds between glucose units. The degradation products of the cellulose contain soluble sugar and 5-HMF, which were fully characterized by ¹H NMR and ¹³C NMR.

Full text of DOI:10.1016/j.carbpol.2011.07.021

Carbohydrate Polymers 86 (2011) 1807–1813
Contents lists available at ScienceDirect
Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol
Electro-Fenton degradation of cellulose using graphite/PTFE electrodes modified
by 2-ethylanthraquinone
∗
Zhong-Xu Wang, Gang Li , Fang Yang, Yu-Lu Chen, Peng Gao
School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper reports an investigation on the degradation of cellulose via electro-Fenton method. The elec-
trodes chosen were graphite/PTFE electrodes modified by 2-ethylanthraquinone (EAQ). The presence
of EAQ greatly improved the rate of hydrogen peroxide formation on air-fed graphite/PTFE electrodes,
which was determined by a UV spectroscopic method using molybdate as the color agent. Hydrogen
Received 12 January 2011
Received in revised form 15 May 2011
Accepted 12 July 2011
Available online 23 July 2011
2
+
peroxide can react with Fe to form hydroxyl radicals in an acidic medium, which accelerates cellulose
degradation by cleaving the glycosidic bonds between glucose units. The degradation products of the
Keywords:
1
13
cellulose contain soluble sugar and 5-HMF, which were fully characterized by H NMR and C NMR.
2011 Elsevier Ltd. All rights reserved.
Cellulose degradation
Hydrogen peroxide
Electro-Fenton
©
2
-Ethylanthraquinone modified carbon
electrode
1
. Introduction
up to +2.8 mV, making the reactions between hydroxyl radicals and
organic compounds take place much more easily.
•
Cellulose is the most abundant and renewable organic substance
In order to produce hydroxyl radicals ( OH), hydrogen per-
in nature. To date, cellulose has mainly been utilized as a building
material, or processed directly into paper or fiber. If cellulose can be
efficiently converted to monomeric sugars, it can better compete
with starch and play an important role to meet future energy needs.
However the degradation of cellulose is limited because its chains
have a strong tendency to aggregate to highly ordered structural
entities due to their chemical constitution and spatial conforma-
tion (Zhao et al., 2007). It is well known that the degradation
methods include acid degradation (Lin, Chang, & Hsu, 2009), ther-
mal degradation (Kim, Eom, & Wada, 2010), biodegradation (Tilki,
Yavuz, Karabacak, C¸ abuk, & Ulutürk, 2010), photocatalytic degra-
dation (Malesic et al., 2005) and oxidative degradation (Lojewska,
Miskowiec, Lojewski, & Proniewicz, 2005).
oxide must be present. Graphite flat plates (Leng et al., 2006;
Qiang, Chang, & Huang, 2002), carbon felt electrodes (Ozcan, Sahin,
Koparal, & Oturan, 2008; Pimentel, Oturan, Dezotti, & Oturan, 2008;
Wei et al., 2007), three-dimensional electrodes made from retic-
ulated vitreous carbon (RVC) (Alvarez-Gallegos & Pletcher, 1998;
Gonzalez-Garcia, Banks, Sljukic, & Compton, 2007; Martinez &
Bahena, 2009), and gas diffusion electrodes (GDE) (Brillas, Alcaide,
& Cabot, 2002; Giomo et al., 2008; Panizza & Cerisola, 2008; Saha,
Denggerile, Nishiki, Furuta, & Ohsaka, 2003) have all been used to
reduce oxygen to hydrogen peroxide. Traditionally H O2 is manu-
2
factured by the reduction of O2 with H . The reaction is mediated
2
by anthraquinone and demands high availability of hydrogen. Tak-
ing the traditional H O2 manufacturing process as an example, we
2
Among the new oxidation methods or advanced oxidation pro-
cesses, electrochemical technologies are environment-friendly and
effective for the direct production of hydroxyl radicals ( OH) via
adopted a catalyzed H O2 electro-generation process using modi-
2
fied graphite/polytetrafluoroethylene (PTFE) electrode, one of GDE,
in which the catalyst had been incorporated into the graphitic
mass. The organic catalyst chosen for the modification was 2-
ethylanthraquinone (EAQ) (Forti, Rocha, Lanza, & Bertazzoli, 2007).
•
anodic oxidation or indirect generation via the electro-Fenton
process. In recent years, the electro-Fenton methods have been
studied widely to treat various organics in wastewater (Zhang,
Yang, Gao, Fang, & Liu, 2008). This method is greatly effective
The H O2 electrogeneration rate that was suitable for cellulose
2
degradation was optimized relative to potential and catalyst con-
centration.
•
because the oxidative potential of OH formed in the medium reach
In this work, we adopted a UV spectroscopic method to deter-
mine if hydrogen peroxide was present using molybdate as the
coloring agent. The present method is simple, rapid, accurate, and
can be used for on-line process monitoring (Chai, Hou, Luo, & Zhu,
∗ Corresponding author. Tel.: +86 022 60202443; fax: +86 022 26582443.
E-mail address: ligang@hebut.edu.cn (G. Li).
2
004).
0
144-8617/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbpol.2011.07.021

1
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Z.-X. Wang et al. / Carbohydrate Polymers 86 (2011) 1807–1813
2
. Materials and methods
2.1. Materials
The following materials purchased from Tianjin Kermel
Chemical Reagents Co. Ltd. were of AR grade: conductive
graphite powder (99% purity), 60% polytetrafluoroethylene disper-
sion(PTFE), ethanol (99% purity), (NH ) Mo7O ·4H O (99% purity),
4
6
24
2
3
3
% hydrogen peroxide solution, 98% concentrated sulfuric acid,
7% chlorhydric acid, N,N-dimethylformamide (99% purity), CuSO4
(
99% purity), ethylenediamine (99% purity). 2-Ethylanthraquinone
was supplied by Jilin Senxiang Chemical Products Distribution Co.
Ltd. Cotton cellulose (Combed Cotton DP = 1100) was supplied by
Hubei Xiangtai Co. Ltd.
Fig. 1. Scheme of the electrochemical cell used in the voltammetric and electrolysis
experiments.
2.2. MGDE preparation and behavior
The MGDE electrodes that were prepared with different con-
centrations of EAQ (0%, 0.5%, 1%, 3%, 5%, 7% and 10%) were selected
as work electrodes. The reference was SEC and a graphite elec-
trode was the counter electrode. The supporting electrolyte was
Preliminary cyclic voltammetry experiments were performed
to identify 2-ethylanthraquinone (EAQ) redox reactions in aprotic
medium containing dimethylformamide (DMF) plus dried NaNO3
0
.1 M (pH 7.0). For this, three electrode arrangement was used in
1
mol/L HCl and its pH value was about 1. The linear voltammetry
the electrochemical cell with platinum foil serving as the working
electrode, graphite electrode used as the counter electrode and a
saturated calomel electrode (SCE) as the reference electrode. An
electrochemical cell shown in Fig. 1 was used for the voltammetric
and electrolysis experiments. Having been purged for 30 min by N2
gas, the cyclic voltammogram was recorded in the aprotic medium
−
1
was recorded from 0.2 V to 0.9 V vs. SEC at 10 mV s
.
Subsequently, controlled potential electrolysis was used for
the optimization of H O2 electrogeneration rate relative to the
2
applied potential in the range of −2 V < E < 0.4 V vs. SEC. Exper-
iments were performed with mechanical agitation. The air was
insufflated into the cell through a pipe placed directly under the
MGDE. The electrolyte was sampled after 30 min. Hydrogen perox-
ide concentration was determined by a UV–Vis spectrophotometer
−
1
with 20 mmol/L 2-ethylanthraquinone (EAQ) at 100 mV s poten-
tial scan rate. The scan range of potential was from −1.8 V to 0.5 V.
All measurements were controlled by a TD-3691 potentiostat.
The modified gas diffusion electrodes (MGDE) precursor mass
was prepared from conductive graphitic pigment, and a 60% poly-
tetrafluoroethylene dispersion was used as hydrophobic binder.
The weight ratio of conductive graphitic pigment to PTFE was 3/5.
The mixture was homogenized in ethanol. Different contents of 2-
ethylanthraquinone, from 0 to 10% (wt.) relative to the graphitic
carbon mass, were respectively incorporated into the MGDE pre-
(TU-1810), recording the spectra from 200 to 500 nm. A solution
of 2.4 mmol/L (NH ) Mo7O ·4H O in 0.5 mol/L H SO was added
4
6
24
2
2
4
to the samples, which resulted in a yellow color. The absorbance
was determined at 350 nm wavelength. Calibration plots based on
Beer–Lambert’s law were established relating absorbance to con-
centration (Chai et al., 2004).
◦
cursor mass. They were heated at 80 C until the precursor mass
2.3. Degradation of cellulose via electro-Fenton method
became ointment. They were then made into the cylinder (diame-
◦
ter 2 cm, thickness 0.5 cm) and dried at 110 C for 24 h. MGDE was
Potentiostatic electrolysis (−1.2 V vs. SEC) was conducted in an
◦
finally obtained after 1 h at 310 C for calcination and solidification.
undivided cell shown in Fig. 1. The electrolyte was 1 mol/L HCl
Fig. 2. Overall reaction pathway for the oxygen reduction reaction mediated by EAQ redox catalyst and electro-Fenton reaction.

Z.-X. Wang et al. / Carbohydrate Polymers 86 (2011) 1807–1813
1809
Fig. 4. XRD patterns of cellulose before (A) and after (B) degradation with electro-
Fenton method.
Fig. 3. Currents recorded for a line voltammetric potential scan on the MGDE sur-
faces with different EAQ concentrations.
reduction and oxidation peaks were corresponding to 500 mV and
1
00 mL plus 10 g cotton cellulose. In a new series of experiments,
7
80 mV, making the potential of this reaction 640 mV. It is known
different weights of FeCl2 (0 g, 0.1 g, 0.2 g, 0.3 g, 0.4 g and 0.5 g)
were respectively added into this solution. The solution was then
vigorously stirred with a magnetic bar. In order to reach a highly
that the standard potential of oxygen direct reduction to hydro-
gen peroxide in the acidic solution is 0.68 V. Therefore, the redox
peak of Fig. 3 must have been caused by oxygen direct reduction to
hydrogen peroxide.
dissolved oxygen concentration for high H O2 production, air was
2
insufflated into the cell through a pipe placed right under the work
electrode. The experiments were controlled by a potentiostat. The
MGDE with 5% of EAQ was used as the work electrode. When the
reaction was over, the residual cellulose and supernatant liquid
were separated via filtration.
Two different electrodes with 0% and 5% EAQ were contrastively
used for the synthesis of hydrogen peroxide under the same exper-
imental conditions mentioned above. The largest amount of H O2
2
production was 23 mg/L when using 5% MGDE with the potential
being −1.2 V vs. SEC, although the produced hydrogen peroxide
concentrations with the two electrodes gave similar rising profiles
from 0.4 V to −1.2 V vs. SEC. More negative potential values would
stimulate water production in a four-electron reaction.
3
. Results and discussion
•
3.1. MGDE behavior
In electro-Fenton methods, OH radicals are produced from Fen-
ton’s reaction between hydrogen peroxide (H O ) generated from
2
2
2+
A cyclic voltammogram was recorded on platinum foil for the
two-electron reduction of oxygen at the cathode and Fe present
in the medium. We will research the effect of hydroxyl radicals on
the degradation of the cellulose.
aprotic solution containing 20 mmol/L of EAQ, in a potential range
from 0.5 V to −1.8 V vs. SEC. Two pairs of reversible peaks are clearly
observed. These peaks are relative to the EAQ reduction in two steps
separated by almost 0.5 V. Such a potential separation may result in
an intermediate compound, a quinone radical, as already hypoth-
esised in the literature (Tammeveski, Kontturi, Nichols, Potter, &
Schiffrin, 2001). Since a short-lived quinone radical is highly reac-
tive, the EAQ redox cycle involved in the oxygen reduction may
include the radical compound instead of the EAQ itself (Forti et al.,
3.2. Degradation of the cellulose via electro-Fenton method
The MGDE with 5% EAQ was used for degradation of the cel-
lulose. The profiles of the FTIR spectra were very similar before
and after degradation of the cellulose, whereas the intensity of
the peaks differed slightly. The crystalline allomorphs of cellulose
were determined by the resolution of wide-angle X-ray diffrac-
tion measurement (Philips PW3040/00 X’Pert MPD instrument, the
diffracted intensity of Cu K␣ radiation k = 0.1542 nm; 50 kV and
40 mA). The X-ray diffractograms in Fig. 4 shows the crystallinity
of cotton cellulose sample (A) and the partly depolymerized cotton
cellulose (B). Compared Fig. 4A, Fig. 4B is indicative of a low crys-
2
007) as shown in Fig. 2.
The MGDE electrodes, prepared with different concentrations
of EAQ, were used for the synthesis of hydrogen peroxide in the
aqueous medium (1 mol/L HCl, pH 1). Currents of EAQ electrode
recorded in these experiments were proportional to EAQ concen-
tration. Fig. 3 shows a linear voltammetric potential scan for the
MGDE. Current values recorded in these experiments were likely a
result of two simultaneous processes: direct reduction of oxygen
on the graphite surface, and reduction of EAQ. When the catalyst
EAQ) concentration was increased, an increasing current response
was observed as a consequence of reduction of the overpotential
for the formation of hydrogen peroxide.
As can be seen from Fig. 3, when increasing the amount of
-ethylanthraquinone from 0 to 5%, the reduction peak potential
decreased and the reduction peak current increased gradually. As
the dosage of 2-ethylanthraquinone increased to 7%, no significant
variation in the peak potential and current was observed. For the
MGDE containing 5% EAQ, the maximum potential values of the
◦
talline, low intensity peaks at 2ꢀ = 22.8 . In addition, the total degree
◦
of crystallinity is 22,452 in Fig. 4A and 15,272 in Fig. 4B at 2ꢀ = 22.8 ,
according to the Gusev method (Gusev, 1978). This can be seen as
a dramatic decrease in crystallinity of the partly depolymerized
cotton cellulose with electrocatalytic depolymerization.
The average degree of polymerization (DP) of cellulose was
determined via copper ethylenediamine (CED) solution method
(ISO5351/1). Having electrolysed for 1 h in 1 mol/L HCl electrolyte
with MGDE of 5% EAQ, the DP of the cellulose decreased from
1100 to 706 due to the acid degradation. Furthermore, the DP of
(
2
cellulose decreased to 390 when adding 0.2 g FeCl . An amount
2
2+
of hydrogen peroxide synthesized by MGDE reacted with Fe to

1
810
Z.-X. Wang et al. / Carbohydrate Polymers 86 (2011) 1807–1813
Fig. 5. Suggested mechanism of degradation of the cellulose by hydroxyl radical-catalyzed.
produce hydroxyl radical which might accelerate the degradation
of cellulose.
was convincing evidence that hydroxyl radical could accelerate the
degradation of cellulose. There was a suitable potential for the pro-
When HCl concentration was increased from 0 to 1 mol/L, the
average degree of polymerization of cellulose decreased from 986
duction of H O and FeCl concentration for the conversion of H O
to hydroxyl radical (Fenton reaction). The excess of FeCl2 might
2 2 2 2 2
to 390. Although is known HCl to accelerate the acid degradation
promote the decomposition of hydrogen peroxide into H O and
2
+
of cellulose, the increase in H concentration would cause H O2
O2 and make the yield of hydroxyl radical decrease, this was dis-
advantageous for the degradation of cellulose. So the DP of the
cellulose increased obviously when FeCl2 concentration changed
from 0.4 g to 0.5 g. The optimal conditions were established based
on the results of the following experiments of Table 1: the poten-
2
electro-synthesized by MGDE to decompose into O2 and water,
resulting in a decrease of the amount of hydroxyl radical. This is
significant because hydroxyl radicals are another important fac-
tor in promoting cellulose degradation (Henriksson, Johansson, &
Pettersson, 2000). Therefore, excess HCl is unnecessary for the
degradation of the cellulose.
When the catalyst (EAQ) concentration of MGDE was increased,
an increasing current response was observed as a consequence of a
reduction of the overpotential for the formation of hydrogen per-
tial vs. SEC was −1.2 V, the amount of FeCl was 0.4 g, reaction time
2
was 4 h, the corresponding DP of degraded cellulose was 156.
The suggested mechanism of cellulose degradation by hydroxyl
radical is shown in Fig. 5. Initially, a hydrogen atom is extracted
from C4 of the glucose unit by the hydroxyl radical creating a car-
bon radical. This was oxidized with the formation of a superoxide
anion, followed by a cleavage of the glycosidic bond (Henriksson
oxide, which meant that much more H O2 was electro-produced.
2
A decreasing DP of cellulose was simultaneously observed, which
Fig. 6. Soluble sugar yield (A) and 5-HMF yield(B) after cellulose degradation.

Z.-X. Wang et al. / Carbohydrate Polymers 86 (2011) 1807–1813
1811
Fig. 7. (A) The UV spectrum of 5-HMF. (B) The 1H NMR spectrum of 5-HMF. (C) The 13C NMR spectrum of 5-HMF.

1
812
Z.-X. Wang et al. / Carbohydrate Polymers 86 (2011) 1807–1813
Table 1
The optimal experiments for degradation of cellulose with electro-Fenton method.
a
◦
HCl (mol/L)
Electrode
Potential vs. SEC (V)
FeCl2 (g)
T ( C)
Time (h)
DP
1
0
5% MGDEb
5% MGDE
5% MGDE
5% MGDE
5% MGDE
graphite
0% MGDE
1% MGDE
3% MGDE
7% MGDE
10% MGDE
5% MGDE
5% MGDE
5% MGDE
5% MGDE
5% MGDE
5% MGDE
5% MGDE
5% MGDE
5% MGDE
−1.0
−1.0
−1.0
−1.0
−1.0
−1.0
−1.0
−1.0
−1.0
−1.0
−1.0
−1.0
−1.0
−1.0
−1.0
−0.5
−1.5
−1.2
−1.2
−1.2
0
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
1
1
1
1
1
1
1
1
1
1
1
2
4
5
4
4
4
4
4
5
706
986
473
390
455
663
598
507
473
540
516
350
230
238
185
243
297
156
209
157
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.4
0.4
0.4
0.4
0.5
0.4
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
.5
.5
a
1
0 g cellulose was added into electrolyte for all experiments.
b% MGDE means the content of EAQ in MGDE relative to conducting graphite.
et al., 2000). A C carbon radical and an alkoxyl radical were formed
in yields of 30% (Scheirs, Camino, & Tumiatti, 2001). Fortunately, 5-
HMF has been obtained from cellulose through our electro-Fenton
methods. However, the largest molar yield of 5-HMF in electro-
Fenton methods is only 5.6% (see Fig. 6.B), which is far from that of
the typical aqueous acid hydrolysis methods, and there was 10.2%
soluble sugar yield. The oxidation potential of hydroxyl radicals is
so high that 5-HMF might be oxidized and decomposed into other
substances when the reaction time is too long.
1
on the new end groups, respectively. The former reacted with a
hydroxyl radical to produce a hydroxyl group. The latter would
react with a hydroxyl radical to produce an alkyl hydroperoxide
(
Graham Solomons & Fryhle, 2004). The alkyl hydroperoxide was
transformed to a hydroxyl by reacting with two water molecules.
Eventually, cellulose was partly converted to glucose and other
sugars by electro-Fenton radical oxidation.
4. Conclusions
3.3. Production of soluble sugar and 5-HMF from cellulose
The graphite/PTFE electrodes modified by EAQ offer an alter-
With the continuous reducing of DP, cellulose was converted
native process for manufacturing H O . The MGDE with 5% EAQ
to soluble sugar. The suspension was filtered through a filter
membrane, then the filtrate was collected and the soluble sugar
content in the supernatant was determined by the phenol–sulfuric
acid method (Dubois, Gilles, Hamilton, Rebers, & Smith, 1956).
The experimental results indicated that cellulose degradation with
MGDE of 5% EAQ could produce more soluble sugar (molar yield
0.2%) than cellulose only treated in hydrochloric acid (molar yield
.9%) when the reaction time for both was 4 h (see Fig. 6A).
The UV absorption spectrum for the filtrate was recorded,
2
2
was applied in the degradation of cellulose with the electro-
Fenton method. H O2 generated by MGDE can react with Fe2+ to
2
form hydroxyl radicals in an acidic medium, which can promote
degradation of the cellulose. The optimal reaction conditions are
established as follows: electrolysis potential −1.2 V, the reaction
time 4 h and the FeCl2 dosages 0.4 g. The DP of degraded cellulose
was reduced to 156. The soluble sugar and 5-HMF were major prod-
ucts of the cellulose degradation and their total molar yield can
reach up to 15.8%.
1
4
and distilled water was used as the blank. The supernatant liq-
uid absorbed in the UV–vis range with maximum absorptivity at
wavelength 284 nm (see Fig. 7A). This suggested that the prod-
uct contained compounds with aldehyde functional groups. The
supernatant liquid was then extracted with ethyl acetate sev-
eral times. The combined organic layers were removed of solvent
under reduced pressure distillation, and the residue was then puri-
fied via column chromatography on silica gel (eluent:petroleum
ether/ethyl acetate = 20:1–5:1). This resulted in a yellow liquid,
Acknowledgments
This work is financially supported by the National Natural Sci-
ence Foundation of China (No. 20876032), Tianjin Key Research
Program of Application Foundation and Advanced Technology (No.
1
1JCZDJC23600) and the Natural Science Foundation of Hebei
Province (No. 2009000023). We thank Dr. Sam Churukian and Dr.
Jane Zhou, University of Michigan, for valuable discussions and lin-
guistic revision.
1
13
which was further characterized through H NMR and C NMR. The
1
results were as follows: H NMR (CDCl , 400 MHz) ı: 4.054–4.087(t,
3
1
H, CH OH),4.679 (s, 2H, CH OH), 6.510–6.519 (d, 1H, J = 3.6 Hz,
2
2
C
CH), 7.236–7.245 (d, 1H, J = 3.6 Hz, C CH), 9.514 (s, 1H, COH);
1
3
C NMR (CDCl , 400MHz) ı: 57.368, 77.169, 110.057, 152.186,
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