Study on the cleavage of alkyl-: O -aryl bonds by in situ generated hydroxyl radicals on an ORR cathode

Article abstract of DOI:10.1039/c7ra11236j

Hydroxyl radicals (OH) generated by oxygen reduction reaction (ORR) were believed to be responsible for the electrochemical depolymerization of lignin in our previous study. However, the mechanism research was hard to carry out due to the recalcitrant nat

Full text of DOI:10.1039/c7ra11236j

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Study on the cleavage of alkyl-O-aryl bonds by in
situ generated hydroxyl radicals on an ORR
cathode
Cite this: RSC Adv., 2017, 7, 51419
ab
ab
ab
ab
*
Lei Wang,
Linan Wang,
Yongmei Chen,
Shuangyan Liu,
Haomin Jiang,
Yanzhi Sunab and Pingyu Wan
*
ab
ab
Hydroxyl radicals (cOH) generated by oxygen reduction reaction (ORR) were believed to be responsible for
the electrochemical depolymerization of lignin in our previous study. However, the mechanism research
was hard to carry out due to the recalcitrant nature of lignin. In this paper, 4-benzyloxyl phenol (PBP) and
benzyl phenyl ether (BPE) were employed as lignin model compounds (LMCs). Based on the qualitative
and quantitative analysis of the degradation products, a mechanism was put forward, which is that the in
situ generated cOH selectively attacked the active site opposite to the phenolic hydroxyl group and
induced the cleavage of the alkyl-O-aryl ether bond. The proposed mechanism was further verified by the
electrochemical degradation of PBP under controlled conditions, which showed a positive correlation
between the degradation efficiency of PBP and the concentration of in situ generated cOH radicals.
Received 12th October 2017
Accepted 31st October 2017
DOI: 10.1039/c7ra11236j
rsc.li/rsc-advances
breaking of benzene ring from its side chain.25 ROS could be
continuously produced through the oxygen reduction reaction
Introduction
(ORR) on cathode thus avoiding extra expensive reagents.
Previous study of our group has shown that 59.2% of lignin was
depolymerized into LMW aromatic compounds for 1 h elec-
trolysis under atmospheric pressure at 80 ꢁC, but more than 20
kinds of products were found.26 During the research, we also
found that the hydroxyl radicals might have played an impor-
tant role in the cleavage of ether bonds in lignin molecules.
However, systematic mechanism study based on product anal-
ysis is difficult to carry out because of the complexity of lignin
degradation products, which hinders the improvement of the
yield and reaction selectivity through further optimization of
reaction conditions.
In this paper, two kinds of compounds containing alkyl-O-
aryl bond, PBP (benzyl-O-phenolic) and BPE (benzyl-O-phenyl),
were employed as lignin model compounds (LMCs). The
cleavage mechanism of alkyl-O-aryl bond by cOH was proposed
based on the qualitative and quantitative analysis of the
degradation products, and the investigation of degradation rate
under various cOH concentrations was conducted to verify the
mechanism. Finally, this study will lay the foundation for large-
scale application of the electrochemical depolymerization of
lignin.
Lignin is the second most abundant biomass in nature, and it is
the only renewable resource which possesses aromatic struc-
tures.1 It is considered to be the most promising alternative for
fossil fuels to provide platform chemicals for industrial appli-
cation.2 However, less than 5% of lignin has been reported to be
utilized nowadays due to its large molecular weight and irreg-
ular structure.3 In order to achieve low molecular weight (LMW)
aromatic chemicals, researchers are trying to breakdown the
linkages between the C9 units in lignin molecules. Fortunately,
more than 2/3 of these linkages are ether bonds,4 which could
be broken through various methods including pyrolysis,
hydrogenolysis and chemical oxidation.5–10 Particularly, the
oxidation process usually operates under mild temperature and
pressure, which is favorable for large-scale industrial produc-
tion. Reagents with mild oxidation capability, such as Co, V, Mn
oxides and their complexes catalyzed O2 or organic oxidants
(DDQ, TEMPO, ABTS and so on), have been used to depoly-
merize lignin.11–21 However, the recovery of these catalysts and
the high cost of organic oxidants hinder the large-scale
application.22–24
ꢀ
Reactive oxygen species (ROS), mainly H2O2, cOH, and cO2
,
could attack the active site on the benzene ring, leading to the
Experimental section
Materials and reagents
aNational Fundamental Research Laboratory of New Hazardous Chemicals Assessment
& Accident Analysis, Beijing University of Chemical Technology, Chaoyang District, No.
15, North 3rd Ring Road East, 100029 Beijing, P. R. China. E-mail: chenym@mail.buct.
edu.cn; pywan@mail.buct.edu.cn; Tel: +86-13683294397; +86-13601227797
bInstitute of Applied Electrochemistry, Beijing University of Chemical Technology,
Chaoyang District, No. 15, North 3rd Ring Road East, 100029 Beijing, P. R. China
PBP ($99%) was purchased from J&K Scientic (Beijing, China)
and BPE ($97%) was from Alfa Aesar a Johnson Matthey
Company (Tianjin, China). The structures of the two LMCs were
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product w2. The bottom aqueous phase was treated by rotary-
evaporation, followed by the addition of ethanol to dissolve
the products and centrifugation to remove the salt. Then the
upper supernatant was dried in vacuum, and the white powders
(aqueous phase products) were produced and weighed as w3.
The total weight of the three parts was equal to the initial weight
of LMC (the margin of error was 3%). The degradation rate and
the yield of LMW products were calculated using eqn (1)–(3) as
follows:
Fig. 1 Structures of the two LMCs. (a) PBP, (b) BPE.
given in Fig. 1. Sodium bromocresol green (BG) was provided
by Shanghai Macklin Biochemical Technology Co., Ltd
(Shanghai, China). Water used in this study was deionized water
(18 MU cm) and other chemicals were of analytical grade and
commercially available, including diethyl ether, sodium
hydroxide, hydrochloric acid (HCl, 37.5%) and isopropyl
alcohol (IPA).
w0 ꢀ w1
Degradation rate : D ¼
ꢂ 100%
(1)
(2)
(3)
w0
w2
Yield of ether phase : Yield1 ¼
ꢂ 100%
w0
w3
Yield of aqueous phase : Yield2 ¼
ꢂ 100%
w0
Electrolysis and electrochemical test
The electrolysis was conducted on CT2001B LANHE electro-
chemical testing system (Wuhan, China). A plate-frame divided
electrolytic cell was used for electrolysis, which possessed
a RuO2–IrO2/Ti mesh (25 mm ꢂ 35 mm) anode provided by
Northwest Institute For Non-ferrous Metals Research (Xi'an,
China), and a self-made porous C-PTFE gas diffusion oxygen
cathode (25 mm ꢂ 35 mm) according to literature.27 A piece of
cation exchange membrane (25 mm ꢂ 35 mm) was placed in
the middle to separate the anode chamber and cathode
chamber. Electrolyte in the cathode chamber was 1 mol Lꢀ1
NaOH solution (40 mL), and the concentration of LMC was
0.0125 mol Lꢀ1, while electrolyte in the anode chamber was
40 mL 1 mol Lꢀ1 NaOH solution with no addition of LMC. The
uniformity of solution and stability of temperatures were
guaranteed by external circulation of the electrolyte, and the
The ether phase products were re-dissolved in 10 mL diethyl
ether, and 1 mL was used for GC-MS analysis. The component of
the ether phase products were determined by GC-MS (Shimadzu
QP 2010 Plus) equipped with a DB-5 MS capillary column (30 m
ꢂ 0.25 mm ꢂ 0.25 mm). 1 mL of the sample was injected and
helium was used as carrier gas at a column ow rate of
1.74 mL minꢀ1. The temperature program was conducted as
follows: initial temperature waꢀs1set at 45 ꢁC for 5 min, then
increased to 110 ꢁC at 12 ꢁC min , and maintained at 110 ꢁC for
1 min. Aer that,ꢀt1he temperature was increased to 280 ꢁC at the
ꢁ
rate of 8 C min and maintained for 1 min. Mass spectra in
the range of 35–700 (m/z) was recorded in full-scan mode with
electron ionization (EI). The detected compounds were identi-
ed based on NIST database and the relative content of the
products was quantied through relative peak areas.
ꢁ
inuence of different temperatures (20–80 C), the amount of
quenchers (0–0.04 g) and electrolysis times (1–4 h) were also
investigated.
Determination of hydroxyl radicals
The concentration of cOH was determined by the BG method.28
The linear sweep voltammetry (LSV) test was carried out on
the CHI660E electrochemical workstation (CH Instruments Ins.
USA). Prior to the test, the electrolyte was purged with O2 or N2
bubble for 30 min. The three-electrode system was consisted of
the above C-PTFE gas diffusion electrode as working electrode,
the RuO2–IrO2/Ti mesh electrode as counter electrode, and
a saturated calomel electrode (SCE, in saturated KCl solution) as
reference electrode. And the test was conducted in 40 mL
The electrolyte in cathode chamber was 40 mL 1 mol Lꢀ1 NaOH
solution and the concentration of BG was 1 ꢂ 10ꢀ4 mol Lꢀ1
.
Aer electrolysis for 1 h, 5 mL of the electrolyte was transferred
to a 25 mL volumetric ask and 10 mL deionized water was
added. And then, sulfuric acid was added gradually until the
solution turned yellow, followed by the addition of 5 mL NaAc-
HAc (pH 3.6) buffer solution. Finally, deionized water was used
to dilute the solution to the calibration mark. The absorbance of
the above solution at 442 nm (Ai) and the blank BG solution
without electrolysis (A0) were determined by UV-vis spectro-
photometer (SHIMADZU UV-2550).
1 mol Lꢀ1 NaOH solution at a sweep rate of 20 mV sꢀ1
.
Analysis of the degradation rate and products
The weight of LMC was accurately measured as w0 before the
electrolysis. 10% hydrochloric acid was used to adjust the
electrolyte to pH 2.0 aer degradation, and precipitate (unde-
graded LMC) was produced. The precipitate was then ltered,
washed with dilute hydrochloric acid (pH 2.0), dried in vacuum
Results and discussion
Hydroxyl radicals generated by ORR cathode
under 60 ꢁC for 24 h, and weighed as the undegraded LMC (w1). Fig. 2 shows the cathodic LSV curve in 1 mol Lꢀ1 NaOH solution
On the other hand, the ltrate was extracted with diethyl ether under O2 atmosphere. Compared to that under N2 atmosphere,
for three times, and the extract liquids were mixed together. a reduction peak appeared around ꢀ0.75 V under O2 atmo-
Then the obtained diethyl ether phase was concentrated by sphere, which is assigned to the 2-electron-reduction process of
nitrogen purging, dried in vacuum, and weighed as ether phase oxygen gas providing H2O2 in situ.29,30 As shown in eqn (4)–(9),
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involve nucleophilic attack of Ca and Cb.4 Based on the chemical
properties of cOH radicals, it is concluded that the cOH radicals
tend to attack the rich-electron sites on benzene ring as elec-
trophilic reagents and act as a kind of strong oxidant as well.
The degradation products of PBP through electrochemical
degradation were analyzed by GC-MS. Five major products were
listed in Table 1, namely 1,4-benzoquinone (P1), benzaldehyde
(P2), benzyl alcohol (P3), benzoic acid (P4), and hydroquinone
(P5).
BPE, which has similar structure with PBP but no phenolic
hydroxyl groups, was employed in electrochemical degradation
under identical conditions as contrast. The main degradation
products of BPE were phenol and benzyl alcohol. As shown in
Table 2, the degradation rate of BPE was far less than that of
PBP. BPE was barely degraded at either 20 ꢁC or 80 ꢁC with
a degradation rate of no more than 2%, while the dꢁegradation
ꢁ
rate of PBP was 40.5% and 65.1% at 20 C and 80 C, respec-
Fig. 2 The cathodic LSV curve in 1.0 M NaOH under O2 (the solid line)
tively. This phenomenon implies that the existence of phenolic
hydroxyl groups provide the opportunity for cOH radicals to
electrophilically attack the alkyl-O-aryl ether bonds.
and N2 (the dotted line) atmosphere at a scan rate of 20 mV sꢀ1
.
Based on the analysis of the degradation products, the
mechanism that the in situ generated cOH selectively attacks the
active site on the opposite of phenolic hydroxyl group and
induces the cleavage of alkyl-O-aryl bond was proposed. The
reaction scheme was given in Fig. 3. PBP exists in the formation
of phenol hydroxyl anion due to alkaline medium, and then
being oxidized by cOH radical to form phenoxy radical. Aer
that, the phenoxy radical transforms to their resonant quinone
structure, causing the C atom on the opposite of phenolic
hydroxyl group to become an activated site for electrophilic
attack. Finally, another cOH radical attacks the active site as
electrophilic reagents, which leads to the cleavage of ether bond
and degradation of the compound.
the generated H2O2 could decompose into other ROS (including
cOH) via ionization, reduction and restructuring in electro-
chemical system.31
H2O2 ¼ H+ + HO2
(4)
(5)
(6)
(7)
(8)
(9)
ꢀ
ꢀ
ꢀ
H2O2 + HO2 ¼ cOH + cO2 + H2O
ꢀ
H2O2 + 2OHꢀ ¼ cO2 + 2H2O + eꢀ
ꢀ
cO2 + cOH ¼ O2 + OHꢀ
ꢀ
ꢀ
2cO2 + H2O ¼ 1O2 + HO2 + OHꢀ
1O2 + cO2 ¼ O2 + cO2
ꢀ
ꢀ
Table 2 The comparison of the degradation rate of the two LMCs
after 1 h electrolysis at different temperatures
Degradation
rate of PBP
Degradation
rate of BPE
Speculation of the bond-cleavage mechanism via product
analysis
Temperature
Blanka
3.6%
40.5%
65.1%
0.49%
1.46%
1.87%
Previous studies have shown that ROS and some metal-
catalyzed oxidation reagents usually involve with electrophilic
attack of the active site on benzene ring,25,32 while alkali or acid
catalyzed lignin depolymerization process is more prone to
20 ꢁ
80 ꢁ
C
C
a
Blank solution of the LMCs at 20 ꢁC without electrolysis.
Table 1 The main degradation products of PBP
Product
Name
P1
P2
P3
P4
P5
Hydroquinone
1,4-Benzoquinone
Benzaldehyde
Benzyl alcohol
Benzoic acid
Formula
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Fig. 3 Scheme of PBP oxidative bond-cleavage.
The regulation of cOH concentration
cOH radicals.34 That is, the decrement in the absorbance of BG
at 442 nm (DA) is in direct proportion to the concentration of
cOH radicals. Therefore, the BG method was applied in this
work to measure and regulate the amount of the in situ gener-
ated cOH radicals.
The UV-vis spectrum of BG solution in cathode chamber at
different temperatures for 1 h were given in Fig. 4a. The
decrement in BG absorbance (DA) increased as the temperature
increased, which conrmed the amount of generated cOH
radicals in the cathode chamber increased along with rising
temperature. Fig. 4b shows the UV-vis spectrum of BG solution
in cathode chamber in the presence of different amount of IPA
at 20 ꢁC for 1 h. It was shown that the value of DA decreased as
the amount of IPA increased, and the increasing amount of IPA
would consequently reduce the concentration of cOH if the
amount of IPA was not sufficient to quench the entire cOH
radicals.
The concentration of H2O2 produced by ORR could be easily
controlled by different electrolytic conditions including current
density, O2 concentration etc. More interestingly, the concen-
tration of cOH radicals could be further controlled by adjusting
the decomposition rate of H2O2 (for example, changing the
temperature) or adding radical quenchers like isopropyl alcohol
(IPA).
As is known, the oxidation capacity of cOH (Eq ¼ 2.8 V vs.
NHE) is much stronger than those of H2O2 (E ¼ 0.867 V, pH ¼
14) and cO2ꢀ (E ¼ 0.20 V, pH ¼ 14). Parimala L.'s group reported
that the decolorization rate of BG was only 5% aer 24 h when
H2O2 (2 wt%) was present, whereas obvious decolorization was
observed in 6 h when Cu nanoparticles were added as catalyst
for H2O2 decomposition to generate cOH radicals.33 This indi-
cates that cOH is the only one in ROS that is capable to degrade
BG. Further studies have shown that cOH degrade BG by
attacking the p–p conjugate bond, and the amount of the
attacked BG molecules is proportional to the concentration of
The above results conrm that the concentration of in situ
generated cOH radicals could be regulated carefully, which is of
great help for mechanism study.
Fig. 4 The UV-vis spectrum of BG solution degraded under different conditions. (a) Different temperatures, (b) different amount of IPA.
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As shown in Fig. 5a, the degradation rate of PBP in the
cathodic chamber increased as the temperature raised. The
degradation rate at 20 ꢁC was 40.5%, while it reached 65.1% at
The degradation rate and product distribution of PBP with
changing cOH concentrations
Our group has published the results on electrochemical
degradation in undivided cells,26 and this study focuses on the
effect of cathodic reaction. It was conrmed that the cation
exchange membrane could prevent H2O2 or other ROS, LMCs
and the degradation products from diffusing to anodic
chamber, so the disturbance from anodic reaction could be
avoided. In the following study, the effects of elevated temper-
atures and addition of different amount of radical quenchers on
the degradation rate and product distribution of PBP were
investigated.
ꢁ
80 C.
The relative content of the ether phase products at different
temperatures shows steady growth in the content of benzyl
alcohol (P3), since P3 is the main product of PBP degradation
(see Fig. 5b). It is noticeable that the relative content of hydro-
quinone (P5) was much lower than that of P3, which can be
explained that P5 cannot be extracted to ether phase completely
because of its good solubility in water. In addition, the relative
content of benzoic acid (P4) increased at elevated temperatures
(about 0.4% and 2% at 20 ꢁC and 80 ꢁC, respectively), which was
caused by further oxidation of benzyl alcohol since too much
amount of cOH radicals present at the elevated temperature.
Fig. 6a gives the degradation rate and product yields of PBP
in the presence of different amount of IPA. When 0.01 g IPA was
added, the degradation rate decreased by 38% because part of
the generated cOH radicals was quenched by IPA. With the
further addition of IPA, the degradation rate decreased
continually, suggesting that the decrease of cOH concentration
controlled by IPA was the reason for the decreased degradation
rate and cOH was responsible for the ether bond cleavage in PBP
In previous studies, we monitored the effect of temperatures
on the concentration of generated H2O2 in electrolytic cell
under the same electrolytic conditions, and found that the
concentration of H2O2 decreased at the elevated temperature
(80 C) and gave better depolymerization rate of lignin.28 It is
ꢁ
explained that the decomposition of H2O2 to cOH radicals is
accelerated by elevating temperatures. Moreover, the decom-
position of H2O2 was similar to the rst-order kinetic reaction,
ꢁ
ꢁ
and the decomposition rate constants at 40 C and 60 C were
0.0396 minꢀ1 and 0.0965 minꢀ1, respectively.
Fig. 5 The degradation rate and product distribution of PBP at Fig. 6 The degradation rate and product distribution of PBP in the
different temperatures. (a) The yield of the degradation products and presence of different amount of IPA. (a) The yield of the degradation
the degradation rate, (b) the relative content of the ether phase products and the degradation rate, (b) the relative content of the ether
products.
phase products.
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molecules. The product distribution in Fig. 6b shows that the some sort of repolymerization of the degradation products
content of benzoic acid (P4) was 0.4% without the addition of occurred. As the electrolysis time was prolonged, over-oxidation
IPA, but it was not found as the amount of IPA increased. This of the products was observed as well, suggesting that cOH could
could be explained that the presence of IPA decreased the only conduct selective attack at relatively low concentration due
amount of cOH radicals and avoided the over-oxidation of the to the cage effect of the solvents. When the concentration of
degradation products.
cOH rised to a high level, the cOH radicals would rip extra
electrons from the benzene ring without selectivity and lead to
ring-open reaction, which was unexpected in our study.
Evaluation and prospect of electrochemical oxidative
degradation
More interestingly, in the above bond-cleavage mechanism,
the phenolic hydroxyl group on the benzene ring plays a key role
in the formation of quinone structures and the formation of
active sites for electrophilic attack. This is very similar to the
process of industrial production of phenol and acetone by
cumene process, in which ROS was used for oxidizing cumene
to form cumene radicals, and obtained a phenolic hydroxyl
group on the benzene ring.35 In view of this, similar methods
can be used in the depolymerization of lignin to introduce
phenolic hydroxyl groups on the benzene ring and further
utilize the introduced phenolic hydroxyl groups to form active
sites, trigger the chain-break of the linkages among the struc-
tural units, and achieve the nal depolymerization of lignin.
As shown in Fig. 7a, the yields of the ether phase products and
the aqueous phase products gradually increased with the
prolongation of electrolysis time, and the total degradation rate
increased steadily as well. As the dynamic balance of cOH
generation and consumption was reached under steady elec-
trolysis, the electrolysis time determined the total amount of
generated cOH, which in turn affected the nal degradation
yield. In the quantitative analysis of degradation products in
Fig. 7b, the relative content of benzoic acid (P4) increased with
prolonged electrolysis time, suggesting the over-oxidation of
benzyl alcohol due to the accumulation of cOH in the system.
The total degradation yield of PBP by electrochemical
method was 40.5% at 1 h, and reached 49.1% at 4 h. The
degradation yield was no longer effectively increased because
Conclusions
The cOH radical involved alkyl-O-aryl ether bond-cleavage
mechanism was proposed by analyzing the degradation prod-
ucts of PBP. The comparative study of BPE degradation indi-
cated that the presence of phenolic hydroxyl groups favored the
formation of active sites for the selective attack of cOH and
eventually triggered the cleavage of alkyl-O-aryl bonds. Besides,
the degradation rate of PBP and the degradation products at
different temperatures and in presence of radical quenchers
were observed. It is found that the concentration of in situ
generated cOH radicals was in positive correlation with the
efficiency of bond cleavage. When the concentration of cOH
maintained at relative low level, it exhibited selective attack of
the active sites, whereas it led to over-oxidation of the products
without selectivity at high concentrations. Finally, it is expected
that the local hydroxylation of the benzene ring is benecial for
the formation of active sites in the electrochemical depoly-
merization of lignin, which further leads to the chain-break of
lignin structural units and realizes the depolymerization of
lignin. This is undoubtedly instructive for the depolymerization
of lignin.
Conflicts of interest
There are no conicts of interest to declare.
Acknowledgements
The authors greatly appreciate the support from the National
Natural Science Foundation of China (No. 51374016, 21706004,
21506010) and the Fundamental Research Funds for the Central
Universities of China (No. JD1701, JD1712).
Fig. 7 The degradation rate and product distribution of PBP with
different electrolysis times. (a) The yield of the degradation products
and the degradation rate, (b) the relative content of the ether phase
products.
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