Hydrogen peroxide synthesis by direct photoreduction of 2-ethylanthraquinone in aerated solutions

Article abstract of DOI:10.1016/j.jphotochem.2010.10.004

A new synthesis method of hydrogen peroxide was investigated by the photoreduction of 2-ethylanthraquinone (AQ) in water-insoluble organic solvents. Through optimizing the photoreduction condition including solvent, atmosphere and irradiated time, the photolysis system of 1,3,5-trimethylbenzene/trioctyl phosphate (3:1) solvent mixture under oxygen atmosphere was found to give a high yield of hydrogen peroxide. Furthermore, the formation mechanism of hydrogen peroxide was proposed, i.e. photoreduction and subsequent oxidation of AQ. The photoreduction of 2-ethylanthraquinone undergoes the hydrogen abstraction from solvent to form the anthrahydroquinone, which is subsequently oxidized by oxygen to give hydrogen peroxide.

Full text of DOI:10.1016/j.jphotochem.2010.10.004

Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 164–168
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Hydrogen peroxide synthesis by direct photoreduction of 2-ethylanthraquinone
in aerated solutions
Ming-Guang Ren, Mao Mao, Xue-You Duan, Qin-Hua Song∗
Department of Chemistry, Joint Laboratory of Green synthetic Chemistry, University of Science and Technology of China, 96 Jinzhai Road, Hefei 230026, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 May 2010
Received in revised form 29 August 2010
Accepted 3 October 2010
Available online 25 October 2010
A
new synthesis method of hydrogen peroxide was investigated by the photoreduction of 2-
ethylanthraquinone (AQ) in water-insoluble organic solvents. Through optimizing the photoreduction
condition including solvent, atmosphere and irradiated time, the photolysis system of 1,3,5-
trimethylbenzene/trioctyl phosphate (3:1) solvent mixture under oxygen atmosphere was found to give
a high yield of hydrogen peroxide. Furthermore, the formation mechanism of hydrogen peroxide was pro-
posed, i.e. photoreduction and subsequent oxidation of AQ. The photoreduction of 2-ethylanthraquinone
undergoes the hydrogen abstraction from solvent to form the anthrahydroquinone, which is subsequently
oxidized by oxygen to give hydrogen peroxide.
Keywords:
Anthraquinone
Hydrogen peroxide
Photochemistry
Hydrogen abstraction
Green chemistry
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
ring of AH2Q undergoes further hydrogenation to yield 5,6,7,8-
tetrahydroanthrahydroquinone (THAHQ). The solution containing
Hydrogen peroxide (H2O2) is widely used in almost all indus-
trial areas, particularly in the chemical industry and environmental
protection, and its demand is growing significantly in the chemical
synthesis due to its “green” character. The only degradation product
of its use is water, and thus it has played a large role in environmen-
tally friendly methods in the chemical industry. Currently, H2O2 is
produced on an industrial scale by the anthraquinone oxidation
(AO) process. However, this process can hardly be considered as
a green method. It is a multistep method that requires significant
energy input and generates waste, which has a negative effect on
its sustainability and production costs [1,2].
The AO process involves the sequential hydrogenation and oxi-
dation of an alkylanthraquinone precursor dissolved in a mixture
of organic solvents followed by liquid–liquid extraction to recover
H2O2. The commercial production of H2O2 is mainly based on
the four major steps: hydrogenation, oxidation, hydrogen perox-
ide extraction, and treatment of the working solution. The two
main reactions are shown in Scheme 1. A 2-alkylanthraquinone
(usually 2-ethylanthraquinone, AQ) in an appropriate solvent or
mixture of solvents, which the anthraquinone/solvent mixture
is commonly called working solution, is hydrogenated catalyti-
cally to the corresponding anthraquinol or anthrahydroquinone
(AH2Q). One main side reaction is that the unsubstituted aromatic
the AH2Q and THAHQ is separated from the hydrogenation cat-
alyst and then oxidized with air to the corresponding quinones,
AQ and tetrahydroanthraquinone (THAQ), and simultaneously pro-
duce equimolecular amounts of hydrogen peroxide. The hydrogen
peroxide is stripped from the organic working solution by dem-
ineralized water in a counter current column to produce a H2O2
solution, and the anthraquinone/solvent mixture is recycled. A
series of side reactions can occur in this process, which leads to
a net consumption of the anthraquinone.
The AO process can give a very high yield of hydrogen perox-
ide per cycle. However, its major disadvantages come from the
side reactions, which require regeneration of the solution and the
hydrogenation catalyst. Excessive hydrogenations and rapid deac-
tivation of catalyst are two serious drawbacks of hydrogenation
process. In view of this, more economical and environmentally
cleaner routes have been explored for the production of H2O2
[1,2].
The photochemical reduction of AQ derivatives (AQs) is the sim-
plest and most extensively studied reaction [3], and it involves
the transfer of an electron or hydrogen atom from an appropri-
ate donor to the oxygen atom of the carbonyl group of AQs in their
excited states, and AQs are reduced to anthrahydroquinones [4].
The photochemical reduction of AQs may be intermolecular and
intramolecular. Water and almost all organic compounds that can
act as hydrogen donors may serve as reducing agents in intermolec-
ular photochemical reduction [3–5]. Based on the photochemical
property of AQs, it may become a synthesis method of hydrogen
∗
Corresponding author. Tel.: +86 551 3607524; fax: +86 551 3601592.
E-mail address: qhsong@ustc.edu.cn (Q.-H. Song).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.10.004

M.-G. Ren et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 164–168
165
Table 1
(1) alkylanthraquinone hydrogenation
The yields of hydrogen peroxide from photolysis under N2/air/O2 atmospherea.
O
OH
Atmosphere
N2
Air
O2
1280
R
R
cat.
H2O2 (mg L−1
)
430
1020
+
H
2
a
100 mM AQ, 1,4-dimethylbenzene/trioctyl phosphate (v/v, 3:1), bubbling the
corresponding gas for 15 min, and irradiation for 1 h.
O
OH
AQ
AH Q
2
dimethylbenzene/trioctyl phosphate (v/v, 3:1) solvent mixture
were irradiated for 1 h under N2, air or O2 atmosphere, respec-
tively. The concentrations of hydrogen peroxide were measured
and listed in Table 1. As showed in Table 1, the yields of hydro-
gen peroxide show big differences for different atmospheres. Under
oxygen atmosphere the concentration of H2O2 is the highest,
1280 mg L−1 among three atmospheres, near to three times than
that under N2 atmosphere. The reaction solution with bubbling air
gave 1020 mg L−1 H2O2, similar to the bubbling O2. AQ is excited
to a singlet state (1AQ*), which undergoes a fast intersystem cross-
ing process to a triplet excited state (3AQ*, Eq. (1)) [3]. Under N2
atmosphere, AQ is photoreduced to AH2Q (Eq. (2)). Besides a lit-
tle of oxygen in the N2-saturated solution of AQ, the formation of
H2O2 may mainly derive from oxidation of AH2Q by oxygen in the
extraction process, which was exposed to air.
There are two possible reactions involving oxygen in the pho-
tolysis of AQ in solution: (1) oxidation of AH2Q generating H2O2
(Scheme 1 (2)); (2) quenching of triplet AQ giving 1O2(ꢀg) (Eq. (3))
[7]. Because the concentration of solvent as a hydrogen donor is
very high, a hydrogen abstraction by triplet AQ should be predom-
inant relative to its quenching by oxygen. Hence, the accelerating
oxidation of AH2Q in a high-concentration-oxygen solution should
be responsible for a high yield of H2O2.
(2) hydrogen peroxide formation
OH
O
R
R
O
2
H O
2 2
+
+
OH
AH Q
O
AQ
2
Scheme 1. Athraquinone autoxidation reactions.
peroxide that the photochemical reduction replaces the hydro-
genation in the AO process.
In this work, we investigated synthesis of hydrogen peroxide by
direct photoinduced reduction of AQ. The concentrations of hydro-
gen peroxide under different photolysis conditions including sol-
vent, concentration of AQ and irradiation time were measured, and
the formation mechanism of the hydrogen peroxide was proposed.
2. Materials and methods
2-Ethylanthraquinone (AQ) and potassium permanganate were
purified by recrystallization. Other materials obtained from com-
mercial suppliers were used as received. Solvents of technical
quality were distilled prior to use. UV–vis absorption spectra
were recorded with a Shimadzu UV-2450 UV–Vis spectropho-
tometer. Fluorescence emission spectra were measured at room
temperature on a Shimadzu RF-5301PC spectrofluorophotomer.
ESR spectrum was recorded on JES-FA200 electron spin resonance
spectroscopy.
hv
ISC
3
∗
∗
1
AQ−→ AQ −→ AQ
(1)
solvent
AQ* (1AQ*, 3AQ*)
AH2Q
(2)
(3)
3AQ∗ + O2 → AQ + 1O2
3.1.2. Selection of solvents
2-Ethylanthraquinone was dissolved in various solvents, and
irradiated for 0.5 h accompanying with bubbling air. The con-
centrations of H2O2 generated were measured, listed in Table 2.
Data showed that alkyl substituted benzene solutions gave high
yields of H2O2, and alkyl substituted benzene/ester mixture such
as butyl acetate and a phosphate gave higher yields of H2O2. In con-
trast, photolysis of AQ in three solvents with no good H-donating
ability (tetrachlorocarbon, chloroform or benzene) gave very low
yields of H2O2, and the formation of H2O2 may derive from self-
photoreduction between AQ molecules, such as an intermolecular
hydrogen abstraction. These results show that the yield of H2O2
relates to the hydrogen-donating ability of the solvent [3] and sol-
Working solutions (30 mL) were prepared by dissolving AQ in
different solvents in a 50 mL Pyrex pear-shaped flask, and irradi-
ated with a 500 W Xe lamp (unfocused) for certain time. The flask
was fixed in the same position to absorb similar photons for dif-
ferent samples. The irradiated solution of 20 mL was taken out,
and extracted twice (2× 10 mL) with water. In order to inhibit
the decomposition of hydrogen peroxide, the water containing 1%
phosphorous acid was employed. The concentration of hydrogen
peroxide in the aqueous extracts was then determined by titra-
tion with standard potassium permanganate solution. Meanwhile,
as a comparison, the concentrations of H2O2 were also determined
by iodimetry [6]. The measurement error is within 10%, from the
unstable intensity of incident light, and decomposition of H2O2.
Table 2
The yields of hydrogen peroxide from photolysis in various solventsa.
3. Results and discussion
Solvents
H2O2 (mg L−1
)
To find an appropriate photoreduction condition, AQ solutions
were irradiated with a Xe lamp under various conditions. Hydro-
gen peroxide in irradiation solution was extracted with the same
volume of water, and then measured by titration with standard
potassium permanganate solution.
CCl4
Chloroform
Benzene
Toluene
1,4-Dimethylbenzene
1,3,5-Trimethylbenzene
Diethyl ether
38
31
41
255
344
350
249
270
510
509
515
3.1. Optimization of irradiation conditions
3.1.1. Selection of N2/air/O2 atmosphere
Butyl acetate
1,4-Dimethylbenzene/butyl acetate (3:1)
1,4-Dimethylbenzene/trioctyl phosphate (3:1)
1,3,5-Trimethylbenzene/trioctyl phosphate (3:1)
To decide what atmosphere achieves
a high concentra-
a
100 mM AQ solution, air-bubbling and irradiation for 0.5 h.
tion of hydrogen peroxide, 100 mM AQ solutions of 1,4-

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M.-G. Ren et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 164–168
Table 3
The yields of H2O2 for various concentrations of AQa.
Conc. (mM)
5
10
50
75
100
125
150
200
250
H2O2 (mg L−1
)
990
1080
1000
1220
1470
1070
1120
1110
1200
a
1,4-Dimethylbenzene/trioctyl phosphate (3:1), irradiation accompanying O2 bubbling for 1 h.
Table 4
The yields of H2O2 in the AQ solution irradiated for different timesa.
t (min)
10
30
60
120
300
4110 (6410)b
690
8370 (11,140)b
H2O2 (mg L−1
)
250
750
1470
1960 (2970)b
a
100 mM AQ, 1,4-dimethylbenzene/trioctyl phosphate (3:1),with O2 bubbling.
Values in brackets from extractions per irradiation for 30 min.
b
vent polarity. Namely, a high H-donating ability and a high-polarity
solvent would favor to hydrogen abstraction of the excited AQ.
In other measurements in this work, 1,4-dimethylbenzene/trioctyl
phosphate (v/v, 3:1) solvent mixture was selected as the solvent of
work solution.
than those by only once extraction after a continuous irradiation.
3.2. Recycling number of work solution
To test the cycle number of work solution, three concentrations
of work solutions (10 mM, 100 mM, and 300 mM) were preformed
for ten times. The concentrations of H2O2 formed were listed in
Table 5. For the work solution after 10 cycles, the concentration of
hydrogen peroxide has no significant decrease. This implies that the
non-photoreduction reaction such as a photodegradation is minor.
Comparing data in Table 5 with Table 4, total sum of H2O2 yields
for 10 cycles (11,770 mg/L and 13,450 mg/L for 10 mM and 100 mM
AQ solutions respectively) is higher than that (11,140 mg/L) from
the extraction per 0.5 h during the irradiation for 690 min. Because
the latter is multi-extraction (per 30 min), and one-time measure-
ment, initial extracted aqueous solutions were placed for a long
time, and decomposition of H2O2 may partly occur, leading to a
decrease in the yield.
Furthermore, the quantum yield of H2O2 formed in the modi-
fied AO process was measured through preparing in 1,4-dimethyl
benzene/trioctyl phosphate (3:1) solution (3 mL) of AQ in a quartz
cuvette (10 × 10 mm), and irradiated with 350 nm monochromatic
light from a fluorescence spectrometer for 1 h, bubbling with air
for three times (5 min per one time) during the irradiation, then
extracting with 10 mL water for three times. The obtained aqueous
solution of H2O2 was titrated by iodimetry. The intensity of light
beam was determined with ferrioxalate actinometry [8]. Based on
these values, the formation quantum yield of H2O2 was calculated,
˚ = 0.19. This shows that the photochemical process for H2O2 for-
mation is very efficient.
3.1.3. Selection of concentration of AQ
In order to investigate a concentration effect on the yield of
H2O2, various concentrations of AQ ranging from 5 to 250 mM,
were employed to perform the photochemical synthesis, and the
yield was measured after irradiation for 1 h accompanying with O2
bubbling, as shown in Table 3. The results show that the yields of
H2O2 generated increase initially with the concentration of AQ and
reach to a maximum value, 1470 mg L−1, and then decrease with
increasing the concentration. The concentration giving the maxi-
mum yield of H2O2 is 100 mM, under our experimental condition.
In the fact, the yield of H2O2 mainly depends on the intensity of
incident light, which is completely absorbed for a high concentra-
tion of work solution. However, a small irradiation area for a very
high-concentration solution would be unfavorable for the diffusion
of photoproducts to the non-irradiation area. The photoproducts
absorb competitively photons with the reactant AQ, and lead to a
decrease in the yield of H2O2. This result also implies that pho-
toreduction is not the reaction between two AQ molecules, but the
reaction between AQ and a solvent molecule. Otherwise, the yield of
H2O2 would increase with the concentration of AQ from beginning
to end.
3.1.4. Irradiation time
100 mM AQ solution of 1,4-dimethylbenzene/trioctyl phosphate
(3:1) mixture was irradiated for different times, and then H2O2 in
these irradiated solutions were extracted and measured. The yield
of H2O2 increased from 250 mg/L to 8370 mg/L, and its increase rate
became to slow, with increasing irradiation time. There are two
possible reasons for explaining this phenomenon: (1) the forming
AH2Q absorbs competitively photons with AQ; (2) H2O2 degrades
under UV light. To avid the two factors leading to a decrease in
the yield of H2O2, the extraction was performed per irradiation for
30 min, and all aqueous phases were combined, at last, the con-
centration of H2O2 was measured, and the total concentrations of
H2O2 were listed in brackets of Table 4. As shown in Table 4, total
concentrations of H2O2 by the extraction per 0.5 h are much higher
3.3. Mechanism of formation of hydrogen peroxide
The photochemical reduction of AQs may be intermolecular and
intramolecular via electron transfer or direct hydrogen-abstraction.
Almost all organic compounds that can act as hydrogen donors
may serve as reducing agents in intermolecular photochemi-
cal reduction [3,4]. However, there is a large difference in the
hydrogen-donating ability for different solvents [4c, 5c, 9]. Based
on results from solvent effect and concentration effect, the inter-
molecular photoreduction should be a key step in the formation
of hydrogen peroxide. Fig. 1a shows absorption spectra of the N2-
Table 5
The yields of hydrogen peroxide vs the recycling number of work solutiona.
N
1
2
3
4
5
6
7
8
9
10
10 mM
1290
1330
760
1240
1360
621
1170
1180
791
1190
1360
691
1180
1450
851
1180
1520
578
1150
1330
706
1120
1220
515
1120
1360
696
1130
1340
757
100 mM
300 mMb
a
1,4-Dimethylbenzene/butyl acetate (3:1), O2-bubbling and irradiation for 1 h.
Irradiation for 0.5 h.
b

M.-G. Ren et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 164–168
167
327
900
600
300
0
468
a
b
0.4
0.2
0.0
irradiation time
383
irradiation time
480
400
480
Wavelength /nm
560
320
400
560
Wavelength /nm
Fig. 1. Absorption (a) and fluorescence emission spectra (b) of AQ in deaerated 1,4-dimethylbenzene/trioctyl phosphate (3:1) solutions recorded at different irradiation
times (0 → 12 min) under 350 nm light.
saturated AQ solution recorded at different irradiation times. The
peak at 327 nm, which is assigned to absorption of AQ, is decreas-
ing, and the absorption band from 360 nm to 550 nm is arising, with
increasing irradiation time. The changes in the absorption spectra
would derive from the photoreduction of AQ into anthrahydro-
quinone AH2Q, which has a new absorbing region from 360 nm to
550 nm. Similarly, fluorescence spectra recorded after irradiation
for different times show an increase in intensity of fluorescence
emission with irradiation time, and because AQ is nonfluorescent,
the fluorescence spectra should be assigned to the dihydroxyan-
thracene, AH2Q (Fig. 1b). When the irradiated solution was exposed
to air, the absorption spectrum became back the spectrum of the
solution of AQ before irradiation (not shown), that is, the reduction
product AH2Q was oxidized into AQ by O2.
Görner [5] has studied the photolysis of 9,10-anthraquinone,
2-methyl- and 2,3-dimethyl-9,10-anthraquinone in air-saturated
acetonitrile/water solvent mixture with very poor H-donating abil-
ity in the presence of various donors: formate, ascorbic acid,
alcohols, and amines. On the basis of steady-state and laser flash
photolysis studies, the H2O2 generated mechanism was proposed.
The photoreaction is initiated by H-atom or electron transfer from
the donor to the AQ triplet state. The conversion of oxygen into
hydrogen peroxide occurs via the superoxide radical and its conju-
gate acid [5]. In our photolysis system, hydrogen abstraction via the
AQ triplet state may be a main process, but a singlet pathway cannot
be excluded from the photo-induced hydrogen abstraction because
the hydrogen donor is solvent, with a very high concentration.
The mechanism of photoreduction by solvents can be proposed.
AQ is excited to the excited singlet state (1AQ*), and undergoes a
fast intersystem crossing (ISC) to the excited triplet state (3AQ*).
Because the concentration of H-donating solvent is very high
(>10 M), hydrogen abstraction from the solvent via a singlet excited
state cannot be ruled out. Hence, the excited anthraquinone (AQ*)
abstracts a hydrogen atom from solvent RH to form the semi-
anthraquinone radical (AHQ•) and the solvent radical (R•) (Eq.
(4)). The semianthraquinone radical can further abstract hydrogen
from the solvent RH to produce AH2Q and another solvent rad-
ical (R•), shown in Eq. (5). In the presence of oxygen, the AH2Q
is oxidized back to the original anthraquinone (AQ), meanwhile
giving hydrogen peroxide (Scheme 1, (2)). Alternatively, two semi-
anthraquinone radicals may undergo a disproportionation reaction
to AH2Q and AQ (Eq. (6)).
4. Conclusion
In summary, a new synthesis method of hydrogen peroxide
was investigated through using direct photoinduced reduction of
AQ, and AQ solution of 1,3,5-trimethylbenzene/trioctyl phosphate
(3:1) solvent mixture as a work solution gave the highest yield
of H2O2 among water-insoluble solvents investigated under air
atmosphere. The formation mechanism of hydrogen peroxide was
proposed, i.e. hydrogen peroxide is produced from photoreduction
of AQ undergoing hydrogen abstraction from solvent, and subse-
quent oxidation by oxygen. The modified AO process that direct
photoreduction of 2-ethylanthraquinone replaces the hydrogena-
tion would be more “green” than the normal AO process with
expensive hydrogenation catalyst and accompanying with the side
reactions. Furthermore, if the light source is sun light instead of
Xe lamp light, the modified process will be a more “green” syn-
thesis method of hydrogen peroxide. Of course, the concentration
of hydrogen peroxide per cycle need to be improved by further
modifying the process.
AQ∗ + RH → AHQ• + R•
AHQ• + RH → AH2Q + R•
2AHQ• → AH2Q + AQ
(4)
(5)
(6)
Acknowledgments
The photolysis system under O2 atmosphere gave the highest
yield of H2O2 among photolysis systems under three atmosphere.
In the formation process of H2O2, the reactions involving oxygen
have: (1) oxidation of AH2Q into AQ and H2O2; (2) quenching
triplet AQ to give singlet oxygen 1O2, which was conformed by
EPR spectroscopy for photolysis of AQ solution containing 2,2,6,6-
tetramethyl-4-piperidinol, but this process is minor comparing to
hydrogen abstraction of the AQ triplet state from solvent due to
its very high concentration. In the case of the former, AH2Q is oxi-
dized into AQ to prevent AH2Q absorbing photons competitively,
and achieve to a high yield of H2O2. This is why the O2-saturated
system gave the highest yield of H2O2 among three atmospheres.
This work was supported by the National Natural Science Foun-
dation of China (Grant Nos. 30870581, 20972149) and the Graduate
Innovation Fund of USTC.
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