Fuel Processing Technology 135 (2015) 157–161
Contents lists available at ScienceDirect
Fuel Processing Technology
journal homepage: www.elsevier.com/locate/fuproc
A FeCl3-based ionic liquid for the oxidation of anthracene
to anthraquinone
Yu-Gao Wang, Xian-Yong Wei ⁎, Sheng-Kang Wang, Rui-Lun Xie, Peng Li, Fang-Jing Liu, Zhi-Min Zong
Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education, China University of Mining & Technology, Xuzhou 221116, Jiangsu, China
a r t i c l e
i n f o
Article history:
Received 28 June 2014
Received in revised form 27 November 2014
Accepted 14 December 2014
Available online 5 January 2015
Keywords:
FeCl3-based ionic liquid
Oxidation
Anthraquinone
Hydrogen peroxide
a b s t r a c t
N-butylpyridinium bromide ferric trichloride (NBPBFTC), a FeCl3-based ionic liquid, was prepared by a two-step
method. NBPBFTC significantly catalyzed the oxidation of anthracene to anthraquinone (AQ) using aqueous hydrogen peroxide (AHPO) as the oxidant. The optimal conditions were determined to be 50 °C, 45 min, 100 mg
NBPBFTC, and 1 mL AHPO for the oxidation of 50 mg anthracene. Under the conditions, AQ was obtained in a
yield of 99.5%. NBPBFTC can be reused at least 3 times without substantial decrease in activity. With the intense
π–π interaction between anthracene and pyridine-based cation in NBPBFTC, anthracene is soluble in the reaction
system. Meanwhile, the iron-based anion and AHPO form a Fenton-like reagent to produce HOO· and HO·, which
attack 9-position in anthracene to induce anthracene oxidation to AQ.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Anthraquinone (AQ) is one of important fine chemicals and chemical intermediates. It is often used to produce the anthraquinone dye
[1] and hydrogen peroxide [2] and to enhance the Kraft process for
paper production [3]. Anthracene, as a main component in coal tar,
is usually subject to oxidation to yield AQ in industry [3–5]. In general,
gas-phase oxidation method involves high temperatures and heavy
metal-based catalysts, while liquid-phase one requires strong acids,
e.g., concentrated sulfuric acid and dichromic acid [4,5].
Ionic liquids (ILs) have been paid increasing attention due to their
negligible vapor pressure, thermal stability, excellent solubility, and design ability by appropriate modifications of cationic or anionic
structures [6–9]. The FeCl3-based ILs not only have the above unique
properties, but also exhibit a strong response to an additional magnetic
field [10–13]. These properties contribute a lot to the potential application of the iron-based ILs in catalytic reactions [14], solvent effects [15],
and separation processes [16]. Three kinds of FeCl3-based ILs were used
to dissolve coal direct liquefaction residues to obtain asphaltene fractions [17]. Among them, an IL with pyridine-based cation is the most effective, implying that the IL can effectively dissolve aromatics. Besides,
FeCl3-based ILs have been employed as an effective medium and catalyst in benzene alkylation [18], isobutene oligomerization [19], and
Friedel–Crafts sulfonylation of some aromatics [20]. More interestingly,
FeCl3-based ILs along with aqueous hydrogen peroxide (AHPO) could
greatly facilitate oxidative desulfurization of liquid fuels [21–24] due
to their good dissolubility and high catalytic activity for oxidation.
⁎ Corresponding author. Tel.: +86 516 83885951.
E-mail address: [email protected] (X.-Y. Wei).
http://dx.doi.org/10.1016/j.fuproc.2014.12.022
0378-3820/© 2014 Elsevier B.V. All rights reserved.
In the present investigation, N-butylpyridinium bromide ferric
trichloride (NBPBFTC) was prepared as a FeCl3-based IL and used in
the oxidation of anthracene to AQ using AHPO as the oxidant.
2. Experimental
2.1. NBPBFTC synthesis
NBPBFTC was synthesized by a two-step method according to
Scheme 1 [25]. The intermediate IL N-butylpyridinium bromide
(NBPB) was prepared by the quaternization reaction of pyridine with
1-butyl bromide at 70 °C in cyclohexane in a round flask with a reflux
condenser and magnetic stirrer for 48 h. The resulting crude product
was purified by repetitious recrystallization and dried in a vacuum
oven. Then NBPBFTC was synthesized by mixing equimolar amounts
of the purified NBPB and FeCl3·6H2O at 50 °C in a dry flask with a mechanical stirrer for 24 h. The coarse product was washed several times
with diethyl ether and deionized water, and then dried in a vacuum
oven to obtain a dark red solid, i.e., NBPBFTC, at room temperature.
2.2. Characterizations of NBPB and NBPBFTC
Fourier transformed infrared (FTIR) spectra of the two ILs were recorded on a Nicolet Magna IR-560 FTIR spectrometer by collecting 50
scans at a resolution of 4 cm−1 in reflectance mode with a measuring region of 4000–400 cm−1. Raman analysis was carried out using a Bruker
Senterra Raman spectrometer with a 532 nm laser source at a resolution
of 1.5 cm− 1 with a measuring region of 200–450 cm− 1. 1H-nuclear
magnetic resonance (NMR) spectrum was recorded on a Bruker AV400 NMR spectrometer with tetramethylsilane as the external standard.
158
Y.-G. Wang et al. / Fuel Processing Technology 135 (2015) 157–161
Br-
N
o
Br
+
N
70 C, 48 h
in cyclohexane
[FeBrCl3 ]
FeCl3. 6H2O
-
+
N
50 oC, 24 h
NBPBFTC
Scheme 1. Procedure for NBPBFTC preparation.
Ultimate analysis was performed using a Leco CHN-2000 elemental determinator. The magnetic measurements were conducted on a Quantum Design MPMS-XL-7 magnetometer at 27 °C. The melting point
was measured with a Beijing Keyi XT5 melting point determinator.
2.3. Anthracene oxidation to AQ
About 50 mg anthracene, 5 mL CH3CN, and prescribed amounts
of NBPBFTC and AHPO were put into a glass tube reactor. Then the mixture was stirred at a designed temperature in a KEM PRS Start-up Kit
parallel synthesizer. After reaction for an indicated period of time (15–
75 min), the reaction mixture was extracted with dichloromethane
(DCM). The extract was analyzed with an Agilent 7890/5975 gas chromatograph/mass spectrometer (GC/MS) equipped with a capillary column coated with HP-5 (cross-link 5% PH ME siloxane, 60 m length,
0.25 mm inner diameter, 0.25 μm film thickness) and a quadrupole analyzer and operated in electron impact (70 eV) mode. Pure anthracene
and AQ were adopted as external standards for quantitative analysis.
The yield of AQ was calculated by dividing the molar mass of AQ by
the total molar mass of anthracene (including converted and residual
one), while the selectivity of AQ was determined by dividing the
molar mass of AQ by the converted molar mass of anthracene.
3. Results and discussion
3.1. Characterizations of NBPB and NBPBFTC
3.2. Effects of reaction conditions onanthracene oxidation to AQ
In the presence of NBPBFTC, the decomposition of H2O2 in AHPO is
too rapid to control the oxidation. So, CH3CN was added to the reaction
system as a dispersant to control the oxidation. As shown in Table 1,
under the same conditions except catalyst, AQ selectivity is 100% over
either NBPBFTC or FeCl3·6H2O or without catalyst, but AQ yields
are quite different over NBPBFTC (99.3%), FeCl3·6H2O (21.3%), and
without catalyst (1.8%). Both the yield and selectivity of AQ from
anthracene oxidation over FeBr3 decrease compared to those
over NBPBFTC due to the formation of the by-product 9,10dibromoanthracene over FeBr3.The best result was obtained over
[(CH3CH2CH2CH2)4NH4]6[BW11Mn(H2O)O39] in previous publications
[28–31], but compared to the expensive catalyst, higher temperature
(80 °C), and much longer time (24 h), NBPBFTC should be more appropriate for catalyzing anthracene oxidation to AQ using AHPO as the
oxidant.
0.9
0.6
xg (emu . g-1)
The contents of carbon, hydrogen, and nitrogen in NBPB are 50.84%,
8.90%, and 6.61%, respectively, which are approximately equal to the
theoretical elemental analysis of the IL. The high purity of NBPB synthesized was further evidenced by FTIR (Fig. S1) and 1H NMR (Fig. S2) analyses. FTIR spectrum of NBPBFTC is similar to that of NBPB except for
lacking the strong absorbance of –OH caused by the strong hygroscopicity of NBPB, as shown in Fig. S1. The fact indicates that the cation of
NBPB does not participate in the complexation reaction during the formation of NBPBFTC. In the Raman spectrum of NBPBFTC (Fig. S3), the
bands around 225, 246, 268, 333, and 351 cm− 1 correspond to the
stretch vibration of [FeClBr3]−, [FeBrCl3]−, [FeBr2Cl2]−, [FeCl4]−, and
[Fe2Br2Cl5]−, respectively [25]. The result suggests that free Fe3+, Br−,
and Cl− could be combined to generate different anions in the production of NBPBFTC.
NBPBFTC exhibits the reversible melting and solidifying behaviors.
At room temperature, NBPBFTC is a dark red crystal, while it begins to
melt around 44 °C and become a fluid with a high viscosity at 50 °C, as
displayed in Fig. 1. It turns into a solid when cooling the reaction system
to room temperature. Such a temperature-responsive behavior may facilitate NBPBFTC recycle after the reaction by spontaneous phase
separation.
As depicted in Fig. 2, the magnetization of NBPBFTC shows a nearly
linear field dependence over the applied magnetic field range of
− 20–20 kOe, which is in accordance with a typical paramagnetic behavior. From the slopes of the fitted line, the magnetic susceptibility of
NBPBFTC is determined to be 41.7 memu· g–1, which is similar to a reported value for similar Fe(III)-based ILs [26]. Despite the fact that the
strong response to an additional magnetic field would be helpful for
the recycle of the Fe(III)-based ILs, how to conveniently separate
Fe(III)-based ILs from the reaction system needs further investigation
[27].
0.3
0.0
-0.3
-0.6
-0.9
room temperature
o
50 C1
Fig. 1. Reversible melting and solidifying behaviors of NBPBFTC.
-20
-15
-10
-5
0
5
10
15
20
H (kOe)
Fig. 2. Magnetization of NBPBFTC as a function of applied magnetic field at 27 °C.
Y.-G. Wang et al. / Fuel Processing Technology 135 (2015) 157–161
159
Table 1
Anthracene oxidation to AQ under different conditions.
Entry
Anthracene (mmol)
AHPOa (mL)
Catalyst (μmol)
Organic solvent (mL)
RT (°C)
Time (h)
Yield (%)
Selectivity (%)
Reference
1
2
3
4
5b
6
7
8
9
0.28
0.28
0.28
0.28
1.5
0.28
0.5
0.5
0.25
1
1
1
1
1.2
3.5
0.5
0.5
1.5
NBPBFTC (260)
FeBr3 (260)
FeCl3·6H2O (260)
None
H5BW12O40 (25)
RuCl3 (0.196)
Complex A (0.75)
Complex B (0.75)
HTSB BrCu(NCCH3) (10)
CH3CN (5)
CH3CN (5)
CH3CN (5)
CH3CN (5)
CH3CN (20)
CH3COOH (20)
CH3CN (3)
CH3CN (3)
CH2Cl2 (3) & CH3CN (3)
50
50
50
50
reflux
100
80
80
80
1
1
1
1
0.25
0.5
24
24
2
99.3
94.7
21.3
1.8
85.0
98.0
100.0
93.0
N93.0
100.0
95.1
100.0
100.0
This work
This work
This work
This work
[28]
[29]
[30]
[30]
[31]
N98.0
100.0
100.0
N98.0
RT denotes reaction temperature; complexes A and B are [(CH3CH2CH2CH2)4NH4]6[BW11Mn(H2O)O39] and [(CH3CH2CH2CH2)4NH4]6[BW11Fe(H2O)O39], respectively; HTSB denotes
hydrotrispyrazolylborate.
a
50% H2O2 for entry 6 and 30% H2O2 for other entries.
b
Operated under microwave irradiation.
AQ yield (mol%)
97
91
85
79
100 mg NBPBFTC, 1 mL AHPO, 1 h
73
40
45
50
55
60
Temperature (oC)
Fig. 3. Effect of temperature on anthracene oxidation to AQ.
As Fig. 3 shows, over NBPBFTC (100 mg), AQ yield rapidly increases
with raising temperature from 40 to 45 °C, then slowly reaches the peak
at 50 °C, but obviously decreases at higher temperatures. The high temperatures up to 50 °C facilitate the rapid dispersion of NBPBFTC in the
solution, as exhibited in Fig. S4, which may be responsible for the best
yield of AQ at 50 °C. However, at temperatures higher than 50 °C, the decomposition of H2O2 in AHPO to H2O and O2 [32] could significantly proceed, leading to the decrease in the concentrations of H2O2 and its
resulting active radicals and thus resulting in the decrease in AQ yield.
Thereby, 50 °C is the appropriate temperature for anthracene oxidation
to AQ over NBPBFTC.
The amount of NBPBFTC plays an important role in anthracene oxidation to AQ, as depicted in Fig. 4. Under the conditions of 1 mL AHPO,
50 °C, and 1 h, AQ yield is only 46.9% over 5 mg NBPBFTC and increases
to 86.5% over 25 mg NBPBFTC. With further increasing the amount up to
100 mg, AQ yield gradually increases up to 99.3%. However, further increasing the amount over 100 mg does not further increase AQ yield.
Therefore, 100 mg NBPBFTC appears to be suitable for the oxidation of
50 mg anthracene.
As demonstrated in Fig. 5, under the conditions of 100 mg NBPBFTC,
50 °C, and 1 h, AQ yield gradually increases with increasing AHPO
amount up to 1 mL, but decreases with further increasing the amount
due to the formation of 1-hydroxyanthracene-9,10-dione, suggesting
that the optimal amount of AHPO is 1 mL for the oxidation of 50 mg
anthracene.
With prolonging time from 15 to 45 min under the conditions of
100 mg NBPBFTC, 1 mL AHPO, and 50 °C, AQ yield increases from
96.6% to 99.5%, but does not increase when the time is longer than
45 min (Fig. 6). Therefore, the appropriate reaction time is 45 min.
3.3. Regeneration and recycling of NBPBFTC
The regeneration and subsequent recycling of an IL are of great importance for practical use of the IL. So, we investigated the possibility
of recycling NBPBFTC in anthracene oxidation to AQ. After extraction
with DCM, the DCM-insoluble portion was distillated under reduced
pressure to remove CH3CN and water. Afterwards, fresh AHPO,
98
AQ yield (mol%)
AQ yield (mol%)
95
85
75
65
90
82
74
55
1 mL AHPO, 50 oC, 1 h
45
0
25
50
75
100
NBPBFTC (mg)
Fig. 4. Effect of NBPBFTC amount on anthracene oxidation to AQ.
125
100 mg NBPBFTC, 50 oC, 1 h
66
0.25
0.50
0.75
1.00
AHPO (mL)
Fig. 5. Effect of AHPO amount on anthracene oxidation to AQ.
1.25
160
Y.-G. Wang et al. / Fuel Processing Technology 135 (2015) 157–161
[33–35], as displayed in Fig. S6. Fenton reagent was widely used to oxidize organic compounds [36–39]. The free Fe3+ can be dissociated from
the anion of NBPBFTC and form a Fenton-like reagent with AHPO. As
strong oxidizing species, HOO· and HO· are effectively produced from
H2O2 [40] in the Fenton-like reagent. A carbon atom in an aromatic
ring with a larger superdelocalizability (Sr) accepts active species, such
as radicals, more readily [41,42]. As displayed in Table S1, C9 and C10
have the largest Sr in anthracene. Therefore, as Scheme 2 illustrates, either HOO· or HO· preferentially attacks C9 and C10 in the soluble anthracene molecules to initiate anthracene oxidation to generate AQ via
subsequent water elimination from 9,10-dihydroperoxy-9,10dihydroanthracene or via subsequent hydrogen abstraction by HO..
4. Conclusions
The characterizations demonstrate that the prepared NBPBFTC can
reversibly melt and solidify with the temperature change and strongly
respond to the additional magnetic field, facilitating its convenient
separation in the organic reaction system. With the good dissolubility
for anthracene and high catalytic activity of the formed Fenton-like
agent, the NBPBFTC–AHPO–CH3CN system could effectively oxidize
anthracene to AQ. Under the optimal conditions, AQ was obtained in a
yield of 99.5% from anthracene oxidation. Furthermore, the reuse of
the oxidation system proved to be feasible and satisfactory. The investigation provides an effective approach for oxidizing anthracene to AQ.
Fig. 6. Time profile of anthracene oxidation to AQ.
anthracene, and CH3CN were added for the next oxidation. As displayed
in Fig. S5, AQ yield did not obviously decrease for the 3 recycles, indicating that NBPBFTC could be recycled at least 3 times without a substantial
decrease in its activity.
3.4. Possible formation pathway of AQ in NBPBFTC–AHPO–CH 3 CN
oxidation system
Acknowledgments
This work was supported by National Basic Research Program of
China (Grant 2011CB201302), National Natural Science Foundation of
China (Grant 21276268), Fund from National Natural Science Foundation of China for Innovative Research Group (Grant 51221462), and
the Research and Innovation Project for College Graduates of Jiangsu
Anthracene oxidation in the system may involve two steps,
i.e., dissolution and subsequent catalytic oxidation, as exhibited in
Scheme 2. With the intense π–π interaction between anthracene and
pyridine-based cation, anthracene is easily dissolved in the system
Step 1. Dissolution of anthracene in the CH3CN-NBPBFTC system
[FeBrCl3]-
Fe 3+ + 3Cl- + Br-
Fe 3+ + H2O2
Fe 2+ + HOO. + H+
Fe2+ + H2O2
H
O
O H
+ HOO.
Fe 3+ + HO. + HO-
O H
O H
+ HOO.
+
2HO.
OH
- H2 O
OH
+
2HO.
O
O
OH
- H2 O
O H O.
H
H
HOO. + H2O
- 2H2 O
O H
O H
H
H
O H O.
H2O2 + HO.
O
+
OH
O.
H
H
O
2HO.
- 2H2 O
O
H
O.
O
H
Step 2. Possible pathways for the formation of HOO. and HO. and anthracene oxidation to AQ over NBPBFTC
Scheme 2. Possible pathway for the formation of AQ from anthracene oxidation in the NBPBFTC–AHPO–CH3CN system.
Y.-G. Wang et al. / Fuel Processing Technology 135 (2015) 157–161
Province (Grant KYLX_1396), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.fuproc.2014.12.022.
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