Electrosynthesis of hydrogen peroxide in room temperature ionic liquids and in situ epoxidation of alkenes

Article abstract of DOI:10.1039/b416837b

Hydrogen peroxide can be electrosynthesized from oxygen in [bmim][BF ₄]-water and used in situ for the epoxidation of alkenes. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b416837b

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Electrosynthesis of hydrogen peroxide in room temperature ionic liquids
and in situ epoxidation of alkenes{
Michael Chi-Yung Tang, Kwok-Yin Wong and Tak Hang Chan*
Received (in Cambridge, UK) 9th November 2004, Accepted 21st December 2004
First published as an Advance Article on the web 19th January 2005
DOI: 10.1039/b416837b
Hydrogen peroxide can be electrosynthesized from oxygen in
bmim][BF ]–water and used in situ for the epoxidation of
Investigations into the optimal operational parameters (optimal
applied potential and volume ratio of [bmim][BF ] to water) were
performed using a H-cell with batch electrolysis (see supporting
[
4
4
alkenes.
10
information{). The anion [BF ] was used in lieu of [PF ] because
4
6
1
1
Even though it is generally recognised that room temperature ionic
liquids (RTILs) have a wide electrochemical window and are
thermally stable, electrochemical applications of RTILs to organic
of the known hydrolytic instability of [PF6]. In addition, the ionic
liquid [bmim][BF ] was known to be stable towards relatively high
4
12
concentrations of hydrogen peroxide.
1
synthesis have not been extensively explored. Some recent
A micro flow cell (Scheme 2) was used for galvanostatic
continuous flow electrolysis to generate hydrogen peroxide for the
epoxidation. Oxygen electroreduction occurred at the reticulated
vitreous carbon (RVC) cathode while the anode was a platinum
examples include reduction of dimethyl maleate and benzalde-
2
3
hyde, polymerization of arenes and preparation of functionalized
4
siloxanes. Hydrogen peroxide has recently been advocated as a
1
green oxidant because it leaves no hazardous residues, only oxygen
and water, after reaction. It has been widely applied to the paper
pulp bleaching process, degradation of hazardous organic
molecules in effluents treatment, and chemical synthesis.
However, the usual method of production of hydrogen peroxide
gauze and each compartment was separated by Nafion . Both
compartments consisted of the optimized volume ratio of
4
[bmim][BF ] to water (8 : 2 v/v). All measurements were carried
out in controlled current (60 mA). Hydrogen peroxide concentra-
tion was determined using standard titration methods with
potassium permanganate. The results are summarized in Table 1.
5
,6
(
the anthraquinone process) involves the use of large amounts of
organic solvents and the consumption of alkylated anthraqui-
nones. Furthermore, because of the potential hazards of neat
hydrogen peroxide, it is generally used as a 30% aqueous solution,
4
The applied potential and the amount of water in [bmim][BF ]
were the significant factors which affected the yield of hydrogen
peroxide. Different potentials from 2600 mV to 2750 mV (vs.
7,8
SCE) were applied and different volume ratios of [bmim][BF
water were investigated. The optimal conditions were found to be
650 mV (vs. SCE) and 8 : 2 (v/v) respectively. Under these
4
] to
which adds to high transportation costs. Recently, Weidner et al.
demonstrated that the stable superoxide ion can be electrogener-
ated from oxygen in the ionic liquid, 3-butyl-1-methylimidazolium
2
operational conditions, the yield of hydrogen peroxide increased
steadily with time, and reached a value of 102 mM after 4 h with
an initial current efficiency of 62%. Electrogeneration of hydrogen
6
hexafluorophosphate, [bmim][PF ]. Since the superoxide ion can
9
be rapidly reduced in water to give hydrogen peroxide, we
reasoned that hydrogen peroxide can be generated by electro-
reduction of oxygen in water-containing ionic liquids. In this
communication, we provide the first report on the electrosynthesis
of hydrogen peroxide from oxygen in the ionic liquid, 3-butyl-1-
peroxide in [bmim][BF ]–water (8 : 2 v/v) mixture was also
4
performed with a slightly alkaline solution (0.04 M NaOH). Under
alkaline conditions, the formation of the hydroperoxide ion is
expected to be favoured because the hydrogen peroxide is a
methylimidazolium tetrafluorborate, [bmim][BF
4
]. We also
] can
1
3
stronger acid than water (pK = 11.64 at 25 uC). This was found
a
demonstrate that the hydrogen peroxide in [bmim][BF
4
to be the case (Table 1) with a yield of 124 mM of hydrogen
peroxide after 4 h and an initial current efficiency of 71%. Table 1
also compares the yields of hydrogen peroxide in RTILs with the
be used in situ for the epoxidation of alkenes (Scheme 1).
Furthermore, the whole cycle can be repeated with high efficiency.
Scheme 1
{ Electronic supplementary information (ESI) available: experimental
procedures for the electrosynthesis of hydrogen peroxide and the
epoxidation of alkenes. See http://www.rsc.org/suppdata/cc/b4/b416837b/
*bcchanth@polyu.edu.hk
Scheme 2
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Table 1 Comparison of hydrogen peroxide concentration and current efficiencies in different electrolytes under continuous flow electrolysis
a
O
a
b
c,
Organic phase + 2 M NaOH (1 : 9 v/v)
b
[bmim][BF
4
] + H
2
[bmim][BF
4
] + 0.04 M NaOH
2 M NaOH
d
d
[
Current efficiency (%)
H
2
O
2
]/mM
102
62
4
124
71
4
50
40
4
150
60
4
Time/h
a
Electrolysis current 0.6 A, projected area 6.7 mA cm
Tributylphosphate and diethylbenzene were added to 2 M NaOH. See ref. 14.
methods with KMnO
22
b
22
Electrolysis current 1.37 A, projected area 22.5 mA cm . See ref. 14.
d
.
c
2 2
H O concentration was determined by standard titration
4
.
traditional electrochemical syntheses. Based on a similar electro-
chemical setup, the yield of hydrogen peroxide in aqueous 2 M
been converted to epoxides with yields ranging from fair to good
17
after isolation by extraction with diethyl ether. The yields of the
14
4
NaOH electrolyte only is half of that in the [bmim][BF ]–water
epoxides compare well with those in the literature using 2 M
16
mixture. This is presumably because of the low oxygen solubility
in aqueous solution. Under the biphasic system of tributyl
4
hydrogen peroxide in [bmim][BF ].
Finally, we demonstrated that it was possible to recycle the
electrolyte for the regeneration of hydrogen peroxide. After diethyl
ether extraction of the epoxide (from 3,5-dimethylcyclohexen-1-
1
4
phosphate–diethylbenzene and aqueous NaOH, the yield of
hydrogen peroxide after 4 h is higher than that of the [bmim][BF ]–
NaOH mixture but with a lower initial current efficiency.
4
one, entry 3 in Table 2) in the first cycle, the [bmim][BF ]–NaOH
4
We demonstrated that the hydrogen peroxide thus generated in
RTIL can be used in situ for oxidation without the need for
extraction or isolation of the hydrogen peroxide. We chose
epoxidation of alkenes for demonstration because of the industrial
significance of epoxides. There have been extensive studies on the
mixture was recovered without loss of the ionic liquid. However,
because of the loss of water in the course of extraction,
compensation of water was necessary in order to retrieve the
optimized volume ratio of [bmim][BF ] to NaOH for the next
4
electrochemical cycle. The correction of the volume ratio was
achieved according to the calibration curve of the density of the
15
epoxidation of alkenes in clean solvents such as water or ionic
12,16
liquids
employing hydrogen peroxide as oxidant. The alkaline
]–NaOH mixture
[bmim][BF ]–NaOH mixture versus the volume ratio of
4
hydrogen peroxide generated in the [bmim][BF
4
4
[bmim][BF ] to NaOH. After measuring the density of the
was used successfully for the epoxidation of a number of
a,b-unsaturated ketones. The results are summarized in Table 2
showing that several different electrophilic alkene substrates have
recovered [bmim][BF ]–NaOH mixture, an appropriate volume
4
of water was added to the mixture to give the optimized 8 : 2 v/v
ratio. The mixture was then reused for the electrochemical
generation of hydrogen peroxide, and used in situ again for the
epoxidation. The results are summarized in Table 3. It can be seen
a
Table 2 Epoxidation of electrophilic alkenes
4
that the [bmim][BF ]–NaOH mixture can be recycled for at least
Entry
1
Substrate
Time/min
1
Yield (%)
92
four cycles with little diminution of the yield of hydrogen peroxide.
th
The slight decline in the yield of epoxide in the 4 cycle was
attributed to the formation of side products as the alkene substrate
was completely converted.
4
In summary, we have shown that [bmim][BF ]–water or NaOH
mixtures can be used as promising electrolytes for the effective
electrogeneration of hydrogen peroxide which is subsequently used
for the epoxidation of alkenes. The whole process can be regarded
as a totally clean system as only oxygen, water and electricity are
required.
2
3
4
5
8
8
91
86
80
We thank the University Grants Committee Area of Excellence
Scheme in Hong Kong (Project No AoE/P-10/01) and the Hong
Kong Polytechnic University Area of Strategic Development Fund
for financial support of this research.
4
Table 3 Recovery and reuse of [bmim][BF ] for the epoxidation of
a
3
,5-dimethylcyclohexen-1-one
5
120
65
Cycle
1
2
3
4
[
H
2
O
2
]/mM
78
2
86
79
1.5
84
79
1.5
83
80
1.5
80
b
Time/h
Epoxide yield (%)
c
a
a
Reaction conditions: 0.37 mmol substrate, 78 mmol of
]–NaOH
mixture, room temperature. Epoxides were extracted by diethyl
A suitable amount of water was added to optimize the volume
b
electrogenerated hydrogen peroxide in 12 mL [bmim][BF
4
4
ratio of [bmim][BF ] to NaOH in cycles 2 to 4. Time for
c
electrolysis under the same conditions as in Table 1. Epoxidation
was carried out under the same conditions as in Table 2. Yields were
determined by GC-MS and the products identified by H NMR
ether. Yields were determined by GC-MS versus an internal standard
H
1
1
and all epoxides were isolated and identified by
spectroscopy.
NMR
spectroscopy.
1
346 | Chem. Commun., 2005, 1345–1347
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Michael Chi-Yung Tang, Kwok-Yin Wong and Tak Hang Chan*
Department of Applied Biology and Chemical Technology and the Open
Laboratory for Chirotechnology, Institute of Molecular Technology for
Drug Discovery and Synthesis, The Hong Kong Polytechnic University,
Hung Hom, Hong Kong SAR, China. E-mail: bcchanth@polyu.edu.hk;
Fax: +852-2364-9932; Tel: +852-2766-5605
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Notes and references
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2
2
3
4
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17 It is presumed that the epoxides can be extracted by supercritical carbon
dioxide in place of diethyl ether. See: O. Bortolini, V. Conte, C. Chiappe,
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5
G. Strukul, Catalytic oxidations with hydrogen peroxide as oxidant,
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6
7
4804. In the event that CO
electrolyte, Na CO instead of NaOH can be used for the electrosyn-
thesis of hydrogen peroxide (see ESI{).
2
would react with the NaOH in the
2
3
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