CHEM. RES. CHINESE UNIVERSITIES 2011, 27(3), 503—507
Hydroxylation of Benzene to Phenol via Hydrogen Peroxide in
Hydrophilic Triethylammonium Acetate Ionic Liquid
HU Xiao-ke, ZHU Liang-fang, GUO Bin, LIU Qiu-yuan, LI Gui-ying and HU Chang-wei∗
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University,
Chengdu 610064, P. R. China
Abstract A new Fenton-like system in a medium of hydrophilic triethylammonium type of ionic liquid(IL) was
used for the hydroxylation of benzene to phenol. The triethylammonium acetate([Et3NH][CH3COO]) IL exhibited retardation performance for the decomposition of H2O2 and protection performance for the further oxidation of phenol,
thus the yield and selectivity to phenol were promoted greatly. The acidity of the system was proved to be an important factor for the selectivity to phenol. The utilization of H2O2 and the selectivity to phenol, as well as the Turnover
number(TON) of the catalyst were effectively enhanced by a benzene-[Et3NH][CH3COO] bi-phase system. The catalyst with [Et3NH][CH3COO] IL was recycled with stable catalytic performance.
Keywords Hydrophilic; Ionic liquid; Fenton-like; Hydroxylation
Article ID 1005-9040(2011)-03-503-05
1
Introduction
As a one-step and environmentally friendly process, the
direct hydroxylation of benzene to phenol with hydrogen peroxide has been extensively studied. The investigation of the
solvent effect is interesting and has been performed by several
groups. For example, water is the solvent in traditional aqueous
Fenton system[1], acetonitrile and acetic acid are used as the
solvents for most of the catalyzed hydroxylation reactions[2―9].
Sulfolane is believed to form complexes with phenolic compounds inducing increased selectivity to phenol[10]. A bi-phase
system involving water and acetonitrile was developed to decrease the over-oxidation of phenol[11]. Ionic liquid(IL), as a
promising “green solvent”[12,13] has been, however, rarely reported as the medium for the hydroxylation of benzene with
hydrogen peroxide. Peng et al.[14] reported an
aqueous-[Cnmim][X](n=4,8,10; X=PF6, BF4) IL system for this
reaction. In this bi-phase system, both the catalyst and benzene
were dissolved in IL while H2O2 was mainly dissolved in
aqueous phase. The produced phenol was extracted into water
phase, minimizing the over-oxidation. However, the high cost
and difficulty in purification for those hydrophobic ILs have
limited their application. It is supposed that a hydrophilic IL
would also function as the medium for the hydroxylation of
benzene. Compared to those of products formed in the common
solvents, the selectivity of the product is expected to increase
by adjusting the cations and anions of ILs[15]. The bi-phase
system, which could remarkably improve the over-oxidation in
hydroxylation by separating the product and the catalyst, could
also be constructed by the excessive benzene and the hydrophilic IL containing water. In such a system, benzene acts as
not only the recycled reactant but also the extractant. In com-
parison with the aqueous-hydrophobic IL biphasic system, the
separation of phenol from H2O2(an oxidant could also oxidize
phenol), as well as the catalyst was achieved in benzene-H2O2
system.
In this work, several hydrophilic ILs were synthesized and
used as the media for a Fenton-like reaction system for the
hydroxylation of benzene to phenol with the aim of exploring
the role of IL in the reaction. A simple hydrophilic IL, triethylammonium acetate([Et3NH][CH3COO]) was reported as a
proper medium for the hydroxylation of benzene to phenol. The
effect of the nature of the cations and anions of ILs, the acidity
of the system, the utilization of H2O2, as well as the recycle of
the catalyst with the IL were investigated in detail.
2
Experimental
2.1 Synthesis of Ionic Liquids
[Et3NH][CH3COO] was synthesized according to the literature[16]. In a 250 mL round-bottom flask equipped with a
reflux condenser and a magnetic stirrer, glacial acetic acid was
dropped slowly to a stoichiometric amount of triethylamine in
the presence of water with stirring at 80 ºC within 2 h. The
water and a little amount of unreacted substrates were separated
by rotary evaporator, and then the product was dried at 80 ºC in
vacuum(666.61 Pa) until the residue remained constant. The
yield of [Et3NH][CH3COO] was 96%. 1H MNR(DMSO-d6), δ:
1.16(t, 9H), 1.87(s, 3H), 3.02(m, 6H), 8.42(s, 1H). Other triethylammonium ILs, with NO3–, HSO4–, ClO4–, H2PO4–,
C2H5COO– or C3H7COO– as the anions, were synthesized via
the same process as that for [Et3NH][CH3COO] with the corresponding acids as the source of the anions.
Ionic liquid 1-n-butyl-3-methylimidazolium acetate
———————————
*Corresponding author. E-mail: [email protected]; [email protected]
Received April 12, 2010; accepted September 14, 2010.
Supported by the National Natural Science Foundation of China(Nos.20502017, 20872102).
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CHEM. RES. CHINESE UNIVERSITIES
round-bottom flask equipped with a reflux condenser and a
magnetic stirrer in open air. In a typical reaction, 0.08 mmol of
FeSO4 was dissolved in 5 mL of solvent consisting of
[Et3NH][CH3COO] and glacial acetic acid with a volume ratio
of 1:1(pH=3.4), then 1 mL(11.25 mmol) benzene and 1.2
mL(11.25 mmol) H2O2(30%, mass fraction) were added to the
solution, and the solution was stirred at 60 ºC for 4 h under
reflux.
After the reaction, the resulting mixture was extracted
with four portions of ether(20 mL). The extracts were dried,
and analyzed by Waters HPLC(ODS, 250 mm×4.5 mm, UV
detector) with o-cresol as the internal standard. The mobile
phase was the acetonitrile/water with a volume ratio of 1:1, and
gradient elution was performed at a rate of 0.5 mL/min in the
first 3 min, then at 1.2 mL/min in the next 3 min, it was kept a
constant afterwards. Phenol, catechol, and hydroquinone were
detected at λ=225 nm, and benzoquinone was detected at
λ=254 nm. The yield and selectivity of phenol based on benzene, the yield of phenol based on H2O2, as well as the turnover
number(TON) based on Fe were determined as follows:
([BMIM]·[CH3COO]) was prepared via the metathesis of
[BMIM]Cl and Ag+CH3COO–[17]. In a 50% aqueous MeOH
solution, [BMIM]Cl and Ag+CH3COO– at a 1.1:1 molar ratio
was mixed. After stirring at room temperature for 48 h, the
precipitate was filtered off, and the water and MeOH were
removed by rotary evaporation. The crude IL was dissolved in
CH2Cl2, and washed by cold water thoroughly until no Cl– was
detected by AgNO3 in water phase. The water phase was separated by a separatory funnel, and the organic phase was evaporated under a vacuum to remove CH2Cl2. The product was
dried at 80 ºC in high vacuum(666.61 Pa) for 2 h. The yield of
[BMIM][CH3COO] was 75%.
1-n-Butyl-3-methylimidazolium nitrate([BMIM]NO3) was
synthesized via the metathesis of [BMIM]Cl and NH4NO3 in
acetone according to a published procedure[18]. The preparation
and purification procedures were the same as those of
[BMIM][CH3COO]. The yield of [BMIM][NO3] was 70%.
2.2
Hydroxylation Reaction
The hydroxylation reaction was carried out in a 50 mL
Yield of phenol based on benzene(%, molar fraction)=
Selectivity to phenol based on benzene(%)=
Molar amount of phenol
Molar amount of benzene initially added
Molar amount of phenol
Total molar amounts of phenol, catechol,
hydroquinone, and benzoquinone
Yield of phenol based on H2O2(%)=
Results and Discussion
3.1 Effect of the Nature of Cations and Anions in
ILs on the Hydroxylation
Several hydrophilic ILs consisting of triethylammonium
cation and different anions(CH3COO–, C2H5COO–, C3H7COO–,
NO3–, HSO4–, H2PO4–, ClO4–) were firstly synthesized, and
used as the solvents for a Fenton-like reaction. FeSO4, for its
low cost, was used as the catalyst. The solubilities of benzene
in these hydrophilic ILs were also measured and the results are
shown in Table 1. It was found that benzene was miscible with
the ILs consisted of carboxylate anions, while the solubility
declined in the order of [Et3NH][NO3]>[Et3NH][ClO4]>
[Et3NH][HSO4], [Et3NH][H2PO4]. Correspondingly, the yield
of phenol in these ILs was positively correlated with the solubility of the ILs. Higher phenol yield was obtained in
[Et3NH][CH3COO] and [Et3NH][NO3]. However, there was
nitro-benzene produced in the latter IL, causing the decrease of
selectivity in addition to over-oxidation. Thus, in the present
work, we focused on the use of [Et3NH][CH3COO] as the solvent. The hydroxylation activity decreased in the order of
[Et3NH][CH3COO]>[Et3NH][C2H5COO]>[Et3NH][C3H7COO],
contrary to the electron-donating ability of the anions. The
UV-Vis spectra of the FeSO4-IL systems(Fig.1) reveal that the
×100%
×100%
Molar amount of phenol
Molar amount of H2O2 initially added
Turnover number(TON) based on Fe=
3
Vol.27
×100%
Molar amount of phenol
Molar amount of FeSO4
aliphatic carboxylate anions coordinated to the Fe2+ ion, forming the Fe complexes. Thus a higher electrophilicity of the
Fe-complexes was deemed to be favorable for the interaction of
H2O2 with the Fe center. In comparison, some common organic
solvents, such as ethanol, acetonitrile, and acetone were employed as the solvents under the same reaction conditions. Only
trace amounts of phenol were obtained, while most of the catalyst, FeSO4, remained un-dissolved even after the hydroxylation reaction.
Table 1 Solubility of benzene and hydroxylation
activity in hydrophilic ILsa
Entry
IL
Solubility of
benzeneb /
(g·mL–1)
Yield of
phenol
(%)
TON based
on Fe
1
[Et3NH][CH3COO]
Miscible
9.7
13.6
2
[Et3NH][C2H5COO]
Miscible
4.9
6.8
3
[Et3NH][C3H7COO]
Miscible
4.6
6.4
4
[Et3NH][NO3]c
0.58
10.0
14.0
5
[Et3NH][HSO4]c
0.04
2.1
2.9
6
[Et3NH][H2PO4]c
0.05
1.6
2.2
7
[Et3NH][ClO4]c
0.39
2.4
3.3
8
[BMIM][CH3COO]
0.10
Trace
―
9
[BMIM][NO3]
0.07
Trace
―
a. Reaction conditions: triethylammonium IL, 5 mL; FeSO4, 0.08 mmol;
benzene, 1 mL; H2O2(30%, mass fraction), 1.2 mL; 333 K; 4 h. b. Measured
at 293 K. c. In solid state at room temperature.
No.3
HU Xiao-ke et al.
Fig.1
UV-Vis absorption spectra of FeSO4 in [Et3NH]X
X–: a. CH3COO–; b. C2H5COO–; c. C3H7COO–.
Considering the higher activity of the catalyst in the triethylammonium ILs with acetate and nitrate anions, two corresponding hydrophilic ILs based on 1-n-butyl-3-methylimidazolium cation([BMIM]) were prepared for comparison(Table 1,
Entries 8, 9). However, only trace amounts of phenol were
produced in [BMIM][CH3COO] and [BMIM][NO3]. Substances such as N-butyl-formamide and 1-butyl-3-methyl-2, 4,
5-trioxoimidazolidine, originated from the degradation of
BMIM cation, were detected by GC-MS. The result was consistent with the literature dealing with the effective degradation
of 1-butyl-3-methylimidazolium in a Fe3+/H2O2 system[19]. So a
hydrophilic IL consisted of [Et3NH] cation was more suitable
than commonly used [BMIM] cation as the Fenton-like reaction medium for the target reaction.
3.2 Effect of the Acidity in [Et3NH][CH3COO] IL
on the Selectivity to Phenol
The medium of [Et3NH][CH3COO] was near neutral with
a pH value equal to 6.4. The hydroxylation reaction carried out
in this medium gave a phenol yield of 9.7% based on benzene,
with small amounts of catechol, hydroquinone and benzoquinone formed. Tars were observed according to the color variation of the reaction mixture from orange to black. The medium
was then adjusted to be acidic by mixing [Et3NH][CH3COO]
and acetic acid with different volume ratios. As indicated in
Fig.2, in the pH value range of 3―4 was found most favorable
for the enhancement of the phenol yield up to 12%, but Tars
still formed, with trace amounts of catechol, hydroquinone and
benzoquinone produced(total yield of about 0.8%). When the
Fig.2
Effect of the pH values on the yield of the
products based on benzene
■ Phenol; ● hydroquinone; ▲ catechol; ▼ benzoquinone. Reaction conditions: VIL+VAcOH=5 mL; FeSO4, 0.08 mmol; benzene, 1 mL; H2O2(30%,
mass fraction), 1.2 mL; 333 K; 4 h.
505
pH value was adjusted to below 3, the yield of phenol was lowered to about 10% with only 0.25% of benzoquinone formed.
In such a case, there were no tars produced. The selectivity of
phenol based on benzene was improved to about 97.5% at a pH
value of 2. The lowest phenol yield of about 1% was obtained
in glacial acetic acid(pH=1.2) with no over-oxidation products.
The results reveal that the selectivity of phenol based on benzene was improved by increasing the acidity of the system.
The effect of the acidity on the selectivity to phenol based
on H2O2 was also investigated by monitoring the amount of O2
evolved during the hydroxylation reaction in the [Et3NH]·
[CH3COO] IL at different pH values. As shown in Table 2,
although the decomposition of H2O2 to O2 increased in more
acidic media, the selectivity to phenol based on H2O2 in the
hydroxylation was enhanced from 14% to 97%. That is, 97% of
the un-decomposed H2O2 was used in the hydroxylation of
benzene to phenol at a pH value of 2. On account that benzene
could be recycled but H2O2 could not, the enhancement of the
selectivity to phenol based on H2O2 was of more importance.
Thus a [Et3NH][CH3COO] IL medium with pH range between
2 and 3 was more favorable for obtaining a higher selectivity to
phenol based on H2O2.
Table 2 Decomposition of H2O2 to dioxygen during
the hydroxylation reaction in [Et 3 NH]·
[CH3COO] at different pH valuesa
Conversion of Yield of phenol based Selectivity to phenol
H2O2b(%)
on H2O2(%)
based on H2O2c(%)
52
7.4
14
49
14.2
30
Entry
pH
1
2
6.4
4.7
3
3.4
39
17.0
45
4
2.7
32
16.5
52
5
2.4
19
15.9
82
6
2.0
15
14.8
97
a. Reaction conditions: VIL+VAcOH =10 mL; FeSO4, 0.08 mmol; benzene, 11.25 mmol; H2O2(30%, mass fraction), 1.76 mmol; 333 K; 4 h. b.
Conversion of H2O2(%) =(initial amount of H2O2 –decomposed amount of
H2O2)/initial amount of H2O2×100%. c. Selectivity to phenol based on
H2O2(%)=amount of phenol/(initial amount of H2O2 – decomposed amount
of H2O2) ×100%.
3.3 Efficiency of H2O2 in [Et3NH][CH3COO] IL
for the Hydroxylation
The efficiency of H2O2 is a major problem for the direct
production of phenol from benzene and hydrogen peroxide.
The low efficiency always generates from the un-productive
decomposition of H2O2 and the further-oxidation of the produced phenol. To partly solve this problem, an excessive
amount of benzene was used. As indicated in Fig.3, as the molar ratio of benzene to H2O2 was elevated from 2:1 to 10:1, the
absolute amount of phenol produced increased and the yield of
phenol based on H2O2 augmented from 15.5% to 21%. The
only by-product was trace amount of benzoquinone, and its
amount successively decreased with increasing the amount of
benzene. Notably, when the molar ratio of benzene to H2O2
exceeded 6, while 5 mL solvent remained constant, a benzene-IL bi-phase system was observed.
Although the
above-mentioned experiment showed remarkable solubility of
benzene in the [Et3NH][CH3COO] IL, the addition of water
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CHEM. RES. CHINESE UNIVERSITIES
contained in the aqueous H2O2(30%, mass fraction) surely decreased the solubility. It was found that the catalyst still existed
in the IL phase, while most of phenol was dissolved in benzene
layer. Because phenol formed was transferred to benzene layer,
the contact of phenol with the catalyst and oxidant was directly
avoided. The selectivity to phenol based on benzene was enhanced to above 99.5% in such a benzene-IL bi-phase system.
Furthermore, in comparison with the yield of 17% based on
H2O2 obtained in a homogeneous system(pH=3.4, at a molar
ratio of benzene to H2O2 of 6:1 in Table 2, Entry 3), the yield
increased to 20% in this bi-phase system. Thus it is deduced
that the efficiency of H2O2 and selectivity of the hydroxylation
reaction are both improved by the hydrophilic IL-benzene biphasic system.
Fig.3
Vol.27
[1, 20]
the traditional Fenton chemistry
. However, in the present
acidic [Et3NH][CH3COO] system, TON could be greatly enhanced to 25 when the molar ratio of Fe2+/H2O2 was 1:280.
Furthermore, it was proved that the TON was further enhanced
to 120 in the benzene-IL bi-phase system, comparable to that of
the Fe-complexes mimicking cytochromes P450 with N-S donor[21] or the no-heme monoxygenase system[22]. The distinct
improvement of TON in the [Et3NH][CH3COO] system meant
much higher catalytic efficiency was attained.
After the products were extracted by ether, the IL with the
catalyst was reused for the next cycle without any recovery
treatment. Under the identical reaction conditions, the yield of
phenol remained about 10% after four cycles which indicated
that the hydroxylation activity remained relatively stable. The
IR spectra of the initial [Et3NH][CH3COO] and that after four
times of usage are presented in Fig.4. The characteristic bands
for tert-ammonium salts at 2596, 2491 and 1263 cm–1, and the
bands at 1563 and 1399 cm–1 ascribed to C=O stretching vibration for acetate anion still remained after reaction. Further
more, there was no other new band appeared. The IR spectra
indicate that the structure of the IL remained stable during the
hydroxylation.
Effect of molar ratio of benzene to
H2O2 on efficiency of H2O2
■ Selectivity to phenol; ▲ yield of phenol based on benzene; ▼ yield of
phenol based on H2O2. Reaction conditions: VIL+VAcOH=5 mL, pH=3.4;
FeSO4, 0.08 mmol; H2O2(30%, mass fraction), 0.6 mL; 333 K; 4 h.
The efficiency of H2O2 on the formation of phenol in the
[Et3NH][CH3COO] IL was 20%―25% with a relatively larger
molar ratio of H2O2 to Fe(70:1). It is well known that the higher
concentration of H2O2 will induce its fast decomposition to O2,
lowering the efficiency of H2O2 for the hydroxylation. A control experiment on the decomposition of H2O2 catalyzed by
FeSO4 in water or [Et3NH][CH3COO] system in the absence of
benzene was carried out by titrating the residual H2O2 with
aqueous potassium permanganate at 60 ºC. It was found that
H2O2 still remained in [Et3NH] [CH3COO] after 30 min, while
no H2O2 was left in water only within 5 min. The result indicates that the decomposition of H2O2 catalyzed by FeSO4 was
retarded in medium [Et3NH][CH3COO].
In addition, the control experiments on the oxidation of
phenol by H2O2 in water or [Et3NH][CH3COO] system were
then comparatively carried out. Under the identical reaction
conditions, no matter in the presence or absence of the catalyst,
much more amounts of phenol were converted to overoxidation products in water than that in [Et3NH][CH3COO] IL.
As phenol was taken as an intermediate for the over-oxidation
in the hydroxylation of benzene, over-oxidation was restrained
in [Et3NH][CH3COO] IL compared with that in the traditional
aqueous Fenton system.
Fig.4
FTIR spectra of [Et3NH][CH3COO]
a. Before reaction; b. after four times of reactions.
4
Conclusions
When a Fenton-like system in a medium of hydrophilic IL
was applied to the hydroxylation of benzene to phenol, the
solubility of benzene was proved to be an important factor. In
an optimized hydrophilic IL, [Et3NH][CH3COO], the acidity of
the medium was proved to be crucial for the reaction. Compared with that of H2O2 in the aqueous Fenton system at the
similar pH, the decomposition of H2O2 was retarded, and
over-oxidation was effectively reduced in the [Et 3 NH]·
[CH3COO] IL. The selectivity of phenol, the utilization of
H2O2, as well as the TON of the catalyst were enhanced in a
benzene-[Et3NH][CH3COO] bi-phase system. Significantly, the
catalyst together with [Et3NH][CH3COO] IL could be recycled
with stable catalytic performance.
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3.4
Efficiency and Reusability of Catalyst
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