B211239f Doc..b211239f Chapter .. Page224
Highly selective and green aqueous–ionic liquid biphasic
hydroxylation of benzene to phenol with hydrogen peroxide†
Jiajian Peng, Feng Shi, Yanlong Gu and Youquan Deng*
Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences, Lanzhou, 730000, China. E-mail: ydeng@ns.lzb.ac.cn
Received 13th November 2002
First published as an Advance Article on the web 20th March 2003
With equal molar ratio of benzene and hydrogen peroxide and without any additional volatile organic solvent, a
green aqueous–ionic liquid biphasic hydroxylation of benzene to phenol with hydrogen peroxide as oxidant and
metal dodecanesulfonate salts such as ferric tri(dodecanesulfonate) as catalyst was conducted with excellent
selectivity and enhanced conversion.
Introduction
advantages of catalyst or product. Apart from hydroformyla-
tion,12 hydrogenation,13 Friedel–Craft reactions,14 etc., ionic
Direct hydroxylation of benzene with hydrogen peroxide to
liquids as reaction media were also successfully used in the
form phenol has attracted much attention and been extensively
selective catalytic oxidation reaction.15
investigated.1–6 The oxidation of benzene and its derivatives by
Herein, an attempt was made to establish an aqueous–ionic
Fenton’s reagent (Fe2+–H
liquid biphasic catalytic reaction system for direct oxidation of
2O2) has been known for a long time.7
However, its selectivity is rather poor since phenol is more
benzene to phenol with hydrogen peroxide as oxidant and metal
reactive toward oxidation than benzene itself, and classical
dodecanesulfonate salts as catalysts in order to compare or
Fenton chemistry requires large quantities of iron(II) salts,
replace traditional aqueous–organic solvent biphasic catalytic
which are consumed stoichiometrically during the reaction.8
reaction systems. In this aqueous–ionic liquid biphasic process,
Although much effort has been devoted to new processes that
both the catalyst and benzene were dissolved in the ionic liquid,
produce phenol directly with high yield and selectivity,
while the oxidant, i.e. H2O2 was mainly dissolved in the
nevertheless, relatively few catalytic processes have success-
aqueous phase but much less dissolved in the ionic liquid since
fully been developed. Very recently, Bianchi et al9 reported that
the water solubility, for example, in the 1-octyl-3-methylimida-
water–acetonitrile (1 + 1) biphasic reaction medium, in which
zolium hexafluorophosphates could be as low as 0.2 g/100 ml16
the resulting phenol was extracted into the organic phase and the
and H2O2 could be well dissolved in the water. The phenol
catalyst was soluble in the aqueous phase, dramatically
produced could be extracted into the aqueous phase, thus
enhanced the selectivity of the benzene hydroxylation by
possible over-oxidation of the resulting phenol could be
reducing the contact between phenol and the catalyst.
minimized. The aqueous–ionic liquid biphasic reaction system
Recently, considerable interest has manifested in the use of
is schematically shown in Fig. 1. In comparison with the
room temperature ionic liquids, which have negligible vapor
previously reported methods of benzene hydroxylation to
pressure, excellent thermal stability and special physicochem-
phenol, the following advantages were achieved: (1) without
ical characteristics in comparison with conventional organic
any additional volatile organic solvent, (2) low molar ratios of
and inorganic solvents, as environmentally benign media for
benzene/hydrogen peroxide and catalyst/benzene, (3) highly
catalytic reaction processes or chemical extractions.10,11 In
selective for desired product, and (4) reusable catalyst system.
particular, the ionic liquid based biphasic hydroformylation or
These make such a process not only more environmentally
hydrogenation reactions resulted in the separation and recovery
acceptable but also more economically attractive.
Youquan Deng, born in 1957 in
Green Context
China, studied Physical Chem-
istry at Lanzhou University
One of the major difficulties in oxidation chemistry is
(China) and received his PhD
achieving high selectivity including avoiding over oxidation.
in 1996 at the University of
In the direct hydroxylation of benzene to phenol, for
Portsmouth (UK). In 1997, he
example – a very important green chemistry target – the
began work as professor in the
product is more reactive under typical hydrogen peroxide
Lanzhou Institute of Chemical
conditions, so that over oxidation is commonly observed.
Physics, Chinese Academy of
One method for overcoming such problems is via a biphasic
Sciences. His research interests
are centered in Green Cataly-
system whereby the desired product is partitioned away
sis. He has authored around 70
from the catalyst or oxidant. Here we see this achieved
scientific publications and 15
through novel aqueous–ionic liquid biphasic systems
patents.
whereby the catalyst and benzene substrate are in the ionic
liquid while the oxidant and the desired phenol are
† This work was presented at the Green Solvents for Catalysis Meeting held
concentrated in the water.
JHC
in Bruchsal, Germany 13–16th October 2002.
224
Green Chemistry, 2003, 5, 224–226
DOI: 10.1039/b211239f
This journal is © The Royal Society of Chemistry 2003
Table 1
Results of aqueous–ionic liquid biphasic catalytic oxidation of
benzene to phenol
Conver-
sion of
Conver-
Selectivity
Catalyst
benzene sion of
of H2O2
Entry Ionic liquid (ml) (mmol)
(%)
H2O2 (%) (%)a
1
OMImPF6 (1.0) Fe(DS)3 (0.05) 54
60
90
2
OMImPF6 (1.0) Fe(DS)2 (0.05) 49
56
88
3
OMImPF6 (1.0) Co(DS)2 (0.05) 35
41
85
4
OMImPF6 (1.0) Cu(DS)2 (0.05) 38
43
88
5
OMImPF6 (1.0) Ni(DS)2 (0.05) 38
44
86
6
OMImPF6 (2.0) Fe(DS)3 (0.05) 51
56
91
7
OMImPF6 (1.0) Fe(DS)3 (0.04) 52
58
90
8
OMImPF6 (1.0) Fe(DS)3 (0.02) 43
48
90
9
BMImPF6 (1.0)
Fe(DS)3 (0.05) 39
46
85
Fig. 1
A schematic representation of the aqueous–ionic liquid biphasic
10
DMImPF6 (1.0) Fe(DS)3 (0.05) 51
57
89
catalytic reaction system for benzene hydroxylation to phenol with H2O2.
11
DMImBF4 (1.0) Fe(DS)3 (0.05) 40
46
87
Step I charging; Step II reaction; Step III still; Step IV recovery of phenol
12
OMImBF4 (1.0) Fe(DS)3 (0.05) 44
49
90
via extraction; Step V the ionic liquid and catalyst are reused for another
13
No ionic liquid
Fe(DS)3 (0.05) 16b
21
76
reaction cycle.
14c
OMImPF6 (1.0) Fe(DS)3 (0.05) 45
52
87
a (moles of converted benzene/moles of converted H
Experimental
2O2) 3 100. b Selectiv-
ity of phenol based on benzene was 71%. c Reused at the 4th times.
1-n-Butyl-3-methylimidazolium hexafluorophosphate
Higher activity was observed with the catalyst Fe(DS)
(BMImPF
3, and the
6), 1-n-octyl-3-methylimidazolium hexafluorophos-
results of entries 1 and 2 (Table 1) suggested that the chemical
phate (OMImPF6), 1-n-octyl-3-methylimidazolium tetrafluor-
state of the iron cation had some impact on the catalytic
oborate (OMImBF4), 1-n-decyl-3-methylimidazolium hexa-
performance.
fluorophosphate (DMImPF6) and
The influence of the amounts of ionic liquid and catalyst used
1-n-decyl-3-methylimidazolium tetrafluoroborate (DMImBF4),
on the reaction was then tested, entries 6–8. When the volume
which were insoluble with water, were respectively synthesized
of the ionic liquid used was increased from 1.0 to 2.0 ml, the
according to the procedures reported in previous litera-
conversions of benzene and H
ture.11,17
2O2 decreased from 54 and 60%
to 51 and 56%, respectively, and the selectivity of H
Dodecanesulfonate salts with various metal cations, i.e. ferric
2O2 was
almost unchanged. This may be attributed to different concen-
tri(dodecanesulfonate) Fe(DS)3, ferrous bis(dodecanesulfonate)
trations of benzene or catalysts between the water and ionic
Fe(DS)2, cobalt bis(dodecanesulfonate) Co(DS)2, copper bis-
liquid, which is dependent upon the volume ratio of water and
(dodecanesulfonate) Cu(DS)2, and nickel bis(dodecanesulfo-
ionic liquid. If the amount of ionic liquid used was excessive,
nate) Ni(DS)2, were respectively prepared according to the
the benzene or catalyst concentration at the interface of the
procedures reported in previous literature.18
water and ionic liquid may be lower, thus resulting in a
Aqueous–ionic liquid biphasic hydroxylation of benzene was
decreased reaction rate between benzene and H
carried out in a 100 ml round-bottomed flask equipped with a
2O2 over the
catalyst. As expected, the reaction rate was decreased with
magnetic stirrer and thermometer. Catalyst (0.05 mmol) was
decreasing the amount of catalyst added.
dissolved in the ionic liquid (1.0 ml), then benzene (1.0 ml,
It is well-known that the physicochemical properties of ionic
11.25 mmol) and H2O (25.0 ml) containing 50 mM H2SO4 was
liquids can be varied over a wide range through the selection of
further added. The mixture was vigorously stirred for 0.5 h at 50
a suitable cation and anion.10 The performance of different ionic
°C, then an aqueous solution of hydrogen peroxide (30%, 1.2
liquids in the hydroxylation of benzene was also investigated
ml, 11.25 mmol) was added. The resulting biphasic system was
(entries 1, 9–12). Although a detailed mechanism is not clear at
stirred for 6.0 h at 50 °C. At the end of the reaction, the resulting
this stage, the experimental results showed that both the length
products and unreacted substrate were extracted from the
of alkyl chains on the 1-alkyl-3-methylimidazolium cations and
aqueous and ionic liquid phases with ether (5 ml 3 3). The
the anions had some effect on the conversion and selectivity,
extracted liquid mixture was analyzed on a Hewlett-Packard
and the best results were obtained by using OMImPF
6890/5793 GC-MS equipped with a HP 5MS column (30 m
6 as an
ionic liquid phase. OMImPF
long, 0.25 mm i.d., 0.25 µm film thickness). The concentration
6, OMImBF4, DMImPF6
and
DMImBF
of organic reactant and products was directly given by the GC/
4 ionic liquids showed good solubility for the metal
dodecanesulfonate salts and formed one phase with benzene,
MS chemstation system according to the area of each chromato-
however, these metal dodecanesulfonate salts were only slightly
graph peak. The amount of residual hydrogen peroxide in the
soluble in BMImPF
aqueous phase was determined by titration with potassium
6 and existed as suspended particles in
BMImPF
permanganate.
6 and BMImPF6 formed two phases with benzene, thus
resulting in a poor catalytic performance.
If ionic liquid was not used, lower conversion and selectivity
were obtained, entry 13, and some by-products such as
Results and discussion
hydroquinone and biphenyl were produced. This suggested that
the aqueous–ionic liquid biphasic reaction system could not
Firstly, the catalytic activities of the dodecanesulfonate salts
only enhance the selectivity for phenol, but also enhance the
with Fe3+, Fe2+, Cu2+, Co2+ and Ni2+ cations were tested in an
benzene conversion.
aqueous–OMImPF6 biphasic system, and the results are
The possibility of recycling of the aqueous–ionic liquid
summarized in Table 1. For all the catalysts used in this work,
catalyst system was also examined. After the oxidation products
the only product detected by GC-MS after reaction was phenol,
and unreacted substrate were extracted from the aqueous and
so the selectivities for phenol, based on benzene, were almost
ionic liquid phase with ether at the end of each reaction, the used
100%. Beside moderately high conversion of benzene, high
aqueous–ionic liquid catalyst system was recharged with
selectivity of H2O2 for benzene conversion was achieved,
benzene and a certain amount of H2O2 according to the
indicating that such an aqueous–ionic liquid biphasic reaction
consumption of H2O2 during the previous reaction and the
system for benzene hydroxylation was successfully established.
reaction was conducted once again. 45% of benzene conversion
Green Chemistry, 2003, 5, 224–226
225
Table 2
Results of aqueous–ionic liquid biphasic catalytic oxidation of toluene
Substrate (ml)
Ionic liquid (ml)
Catalyst (mmol)
Con. of toluene (%)
Product distribution
Toluene (1.0)
OMImPF6 (1.0)
Fe(DS)3 (0.05)
1
benzaldehyde 100%
Toluene (1.0) + benzene (0.02)
OMImPF6 (1.0)
Fe(DS)3 (0.05)
3
2-hydroxybenzaldehyde 15.7%
o-hydroxytoluene 53.5%,
p-hydroxytoluene 30.8%
was maintained after the aqueous–ionic liquid catalyst system
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Green Chemistry, 2003, 5, 224–226
