Ir/C and Brφnsted acid functionalized ionic liquids an efficient catalytic system for hydrogenation of nitrobenzene to: P -aminophenol

Article abstract of DOI:10.1039/c7ra04722c

In this study, we found that the phenylhydroxylamine intermediate could desorb more easily from an Ir surface than from a Pt surface, which is beneficial for inhibiting the over-hydrogenation of phenylhydroxylamine to aniline. On the other hand, the Brφnsted acid functionalized ionic liquids with sulfonic acid and bisulfate anions were acidic enough to catalyze the Bamberger rearrangement to form p-aminophenol from phenylhydroxylamine. On this basis, a new catalytic system constructed by Ir/C and Brφnsted acid functionalized ionic liquid was applied, for the first time, to the one-pot hydrogenation of nitrobenzene to p-aminophenol. Our results indicate that the PAP selectivity of Ir/C and [SO₃H-bmim][HSO₄] Brφnsted functionalized ionic liquid was far more than that of the traditional Pt/C and sulfuric acid catalyst system. Furthermore, the dually functionalized ionic liquid ([HSO₃-b-N-Bu₃][HSO₄]) can be used simultaneously as an acid catalyst and also as a surfactant, due to its higher lipophilicity. Therefore, our new catalytic system has unique advantages in the hydrogenation of nitrobenzene to p-aminophenol.

Full text of DOI:10.1039/c7ra04722c

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PAPER
Ir/C and Br4nsted acid functionalized ionic liquids:
an efficient catalytic system for hydrogenation of
nitrobenzene to p-aminophenol
Cite this: RSC Adv., 2017, 7, 31663
*
Hong Wang, Taotao Jiang, Lei Ma,
Hanbing Wang, Xiaoliang Xu, Chunshan Lu
*
and Xiaonian Li
In this study, we found that the phenylhydroxylamine intermediate could desorb more easily from an Ir
surface than from Pt surface, which is beneficial for inhibiting the over-hydrogenation of
a
phenylhydroxylamine to aniline. On the other hand, the Br4nsted acid functionalized ionic liquids with
sulfonic acid and bisulfate anions were acidic enough to catalyze the Bamberger rearrangement to form
p-aminophenol from phenylhydroxylamine. On this basis, a new catalytic system constructed by Ir/C and
Br4nsted acid functionalized ionic liquid was applied, for the first time, to the one-pot hydrogenation of
nitrobenzene to p-aminophenol. Our results indicate that the PAP selectivity of Ir/C and [SO3H-bmim]
[HSO4] Br4nsted functionalized ionic liquid was far more than that of the traditional Pt/C and sulfuric
acid catalyst system. Furthermore, the dually functionalized ionic liquid ([HSO3-b-N-Bu3][HSO4]) can be
used simultaneously as an acid catalyst and also as a surfactant, due to its higher lipophilicity. Therefore,
our new catalytic system has unique advantages in the hydrogenation of nitrobenzene to p-aminophenol.
Received 27th April 2017
Accepted 7th June 2017
DOI: 10.1039/c7ra04722c
rsc.li/rsc-advances
different from those on Pd; Ir was more benecial to the
formation of PHA intermediates than Pd.13 Since PHA is the
Introduction
most important intermediate for the hydrogenation of NB to
PAP, we supposed that the special adsorption properties of Ir
might be favourable for the formation of PAP.
p-Aminophenol (PAP) is an important commercial chemical raw
material, which is widely used in the eld of pharmaceuticals,
dyestuffs, polyurethanes and photographic chemicals.1–3 PAP is
usually synthesized by the reduction of p-nitrochlorobenzene or
p-nitrophenol,4–6 which has several drawbacks including
multistep processes, scarcity of raw materials and environ-
mental problems. Compared to the above synthetic methods,
the one-pot synthesis of PAP from nitrobenzene (NB) is regar-
ded as the most convenient, economical and challenging
method at present. This process involves two steps: the rst step
is the hydrogenation of NB to an intermediate phenyl-
hydroxylamine (PHA), and the second step is the conversion of
PHA to PAP through the acid-catalysed Bamberger rearrange-
ment.7 At the same time, the major by-product aniline (AN) can
also be generated for the over hydrogenation of PHA in this
transformation (Scheme 1).
Sulfuric acid is usually used as the acid catalyst for the
conversion of PHA to PAP through the Bamberger rearrange-
ment, which causes some serious problems like considerable
amounts of effluents and equipment corrosion.14,15 In the past
few years, chemists have investigated some new acidic catalysts,
including solid acid16–21 and the CO2/H2O system,22 to replace
sulfuric acid for this transformation, but the results have not
been ideal, due to the rapid deactivation or insufficient acidity
of these acid catalysts.23–25
As a kind of environmentally friendly acid catalyst, Br4nsted
acid functionalized ionic liquids (BFILs) have the advantages of
solid acids in addition to the characteristics of liquid acids.
The one-pot hydrogenation of NB to PAP involves two kinds
of catalysts: the hydrogenation catalyst and acid catalyst. The
supported noble metal (e.g. Pt, Pd or Au) catalysts are usually
used as hydrogenation catalysts for the reduction of the nitro
group.8–12 In our previous study of the selective hydrogenation of
halogenated nitrobenzene, we found that the adsorption prop-
erties of halogenated nitrobenzene on Ir were signicantly
Institute of Industrial Catalysis, Zhejiang University of Technology, State Key
Laboratory Breeding Base of Green Chemistry Synthesis Technology, Hangzhou,
310014, P. R. China. E-mail: malei@zjut.edu.cn; xnli@zjut.edu.cn
Scheme 1 Hydrogenation of nitrobenzene to p-aminophenol.
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BFILs also have unique properties like water-stability and stirred at T ¼ 353 K for 5 hours. Aer the reaction was complete,
structure-adjustability, which make them widely used in the the product was washed with ethyl acetate (20 mL ꢁ 3) and
eld of esterication,26,27 hydrolysis,28,29 alkylation,30,31 the diethyl ether (20 mL ꢁ 3), respectively, then dried for 6 hours in
Biginelli reaction,32 hydrodeoxygenation,33 and Beckmann a vacuum at 353 K. The BFIL SF-A was obtained. SF-B ([Et3–NH]
rearrangement,34 instead of traditional mineral acids. Recently, [HSO4]) and SF-C ([HPy][HSO4]) were prepared according to the
Cui et al. reported their work on the one-pot hydrogenation of procedure described above for SF-A with the reaction of trie-
NB to PAP using BFIL [HSO3-b-N-Me3][HSO4] as the acid catalyst thylamine or pyridine with sulfuric acid (Scheme 2).
and Pt/SiO2 as the hydrogenation catalyst.35
The preparation of type B BFILs DF-A ([SO3H-bmim][HSO4])
Based on our previous research results and the performance was similar to that used in the literature37 and the detailed
of BFILs in the hydrogenation of NB to PAP, we constructed procedure is as follows: N-methylimidazole (16.42 g, 0.2 mol)
a new catalytic system by the combination of Ir/C and BFILs to and equimolar 1,4-butanesultone (27.23 g, 0.2 mol) were mixed
catalyze the one-pot hydrogenation of NB to PAP. The catalytic in a 100 mL single-necked ask. Then, the mixture was stirred at
performance of this system was then studied and compared 333 K under nitrogen until the white solid was completely
with that of the traditional Pt/C and sulfuric acid system. In generated. The solid was washed with ethyl acetate (20 mL ꢁ 3)
addition, considering the structure-adjustability of BFILs, we and dried in vacuum at 353 K for 6 hours. A stoichiometric
also wanted to modify the structure of the BFILs to improve the amount of sulphuric acid was added dropwise to the zwitterion
lipophilicity, and found real dual functionalized ionic liquids and the mixture was stirred at 353 K for 6 hours, resulting the
with the functions of acid catalyst and surfactant. The dual formation of DF-A. The product was washed with ethyl acetate
functionalization of acidic ionic liquids might afford an (20 mL ꢁ 3) and dried under vacuum at 383 K for 6 hours. DF-B1
opportunity for us to carry out the hydrogenation of NB to PAP ([HSO3-b-N-Et3][HSO4]) and DF-C ([HSO3-bPy][HSO4]) were
without adding surfactant.
prepared according to the procedure described above for DF-A,
with triethylamine and pyridine, respectively, instead of N-
methylimidazole.
Type C BFILs DF-B2 ([HSO3-b-N-Pr3][HSO4]) and DF-B3
([HSO3-b-N-Bu3][HSO4]) were prepared using the same proce-
dure describe above, replacing N-methylimidazole with tripro-
pylamine and tributylamine.38,39
Experimental
Materials
N-Methylimidazole, triethylamine, tripropylamine, tributyl-
amine, pyridine, 1,4-butanesultone and hexadecyl trimethyl
ammonium bromide (CTAB) were purchased from Aladdin
Reagent Co. Ltd. Sulfuric acid, nitrobenzene, aniline ethyl
acetate, diethyl ether, acetonitrile, ethanol and isopropanol
were purchased from Shanghai Lingfeng Chemical Reagent Co.
Ltd. All reagents and raw materials were commercially available
and used without any further purication.
Characterization of catalysts
TEM and XPS. X-ray Photoelectron Spectroscopy (XPS) was
conducted on a Thermo ESCALAB 250 Axis Ultra using mono-
chromatic Al Ka radiation (hv ¼ 1486.6 eV). The binding ener-
gies were regulated according to the C 1s peak at 284.8 eV from
the samples.
Preparation of catalysts
The particle size of Ir or Pt on the active carbon surface was
determined by transmission electron microscopy (TEM) using
a Tecnai G2 F30 S-Twin microscope (Philips-FEI Co.). At least
200 individual Ir particles were counted for each catalyst. The Ir
or Pt particle size of the catalysts, ds, was calculated using the
Hydrogenation catalysts. The commercial coconut shell
activated carbon with Brunauer–Emmett–Teller (BET) surface
area of 1662 m2 gꢀ1 was provided by Fujian Xinsen Carbon Co.
Ltd. (China). The activated carbon was outgassed in a vacuum
at 383 K overnight. A desired volume of H2IrCl6 or H2PtCl6
(Sino-Platinum Metals Co., Ltd.) aqueous solution (0.035 g
Lꢀ1), with nominal Ir or Pt loading of 1 wt%, was added to an
aqueous suspension of the activated carbon. The aqueous
suspension was subsequently dried at 353 K for 6 h. The
formed catalyst was then reduced by hydrogen with 3 MPa
pressure in distilled water at 363 K for 9 h. Aerward, the Ir/C
or Pt/C catalyst was ltered, rinsed in distilled water until
neutral and degassed in a vacuum at 383 K for 7 h. ICP
experiments showed that the actual loadings of Ir and Pt were
0.9985% and 0.9994%, respectively.
P
P
following equation: ds
particle size di on the micrographs was measured by a comput-
erized system.
¼
nidi3/ nidi2, where the visible
Functionalized ionic liquids catalysts. Type A BFILs SF-A
([Hmim][HSO4]) was prepared according to the literature36 and
the synthetic procedure is as follows: N-methylimidazole
(16.42 g, 0.2 mol) was placed in a 100 mL single-necked ask
and equi-molar sulfuric acid was added dropwise under
vigorous stirring in an ice-water bath. The mixture was then Scheme 2 Structures of the BFILs.
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1H-NMR and IR spectroscopy of ionic liquids. The structures Solubility of NB in the reaction solution
of BFILs were characterized by 1H-NMR and IR spectroscopy.
The solubility of NB in the reaction solution was checked by
HPLC, which was similar to that used in the literature.44 The
method is as follows: the slight excess of 6 NB was sonicated in
an ultrasonic washer for 30 minutes and then stirred at 353 K
for 5 hours. Aer that, 1 mL of the upper homogeneous was
transferred to the ethanol solution and diluted to a xed
volume, then determined by HPLC.
1
Some of the BFILs were further characterized by 13C-NMR. H-
NMR and 13C-NMR spectra were recorded on a Bruker
AVANCE III spectrometer in D2O. FT-IR measurements were
performed using KBr disc on a Nicolet iS50 FT-IR infrared
spectrometer. The spectral data were obtained as follows and
were all in agreement with that reported in the corresponding
ref. 27 and 40–42.
1
Spectral data for SF-A ([Hmim][HSO4]): H-NMR (500 MHz,
Evaluation of catalyst
D2O) d 9.06 (s, 1H), 7.70 (t, 1H), 7.65 (t, 1H), 3.88 (s, 3H). 13C-
NMR (125MHZ, D2O) d 35.86, 120.41, 122.95, 135.86. IR
(cmꢀ1): 3149, 2967, 1639, 1586, 1451, 1048, 761.
Spectral data for SF-B ([Et3–NH][HSO4]): 1H-NMR (500 MHz,
D2O) d 3.09 (q, 6H), 1.18 (t, 9H). IR (cmꢀ1): 2991, 2951, 1476,
1399, 1165, 1036.
This reaction was carried out in a 75 mL stainless steel autoclave
(Parr MRS5000). In a typical experiment, NB, catalyst, BFILs or
10% sulphuric acid solution and CTAB were introduced into the
autoclave. Aer the air in the reactor was replaced with nitrogen
and hydrogen, three times, respectively, the reactions were
performed at 0.8 MPa H2, 353 K, the optimal reaction condi-
tions that were previously determined. The nal products were
analysed by HPLC on an Agilent 1260 (column Agilent TC-C18, 5
mm) system with UV detector (l ¼ 254 nm). The column
temperature was 298 K. CH3CN/H2O (70/30, v/v) was used as the
Spectral data for SF-C ([HPy][HSO4]): 1H-NMR (500 MHz,
D2O) d 8.66 (d, 2H), 8.50 (m, 1H), 8.08–7.84 (m, 2H). IR (cmꢀ1):
3071, 1639, 1545, 1488.
1
Spectral data for DF-A ([SO3H-bmim][HSO4]): H-NMR (500
MHz, D2O) d 8.59 (s, 1H), 7.32 (m, 2H), 4.10 (t, 2H), 3.80–3.72 (m,
3H), 2.83–2.62 (m, 2H), 1.88 (m, 2H), 1.76–1.41 (m, 2H). 13C-
NMR (125 MHz, D2O) d 21.50, 28.43, 35.87, 49.28, 51.07,
122.64, 123.55, 136.56. IR (cmꢀ1): 3156, 3115, 2960, 1575, 1169,
1031, 794, 747, 579.
mobile phase and the ow rate was 1 mL minꢀ1
.
Recycling of Ir/C and ionic liquids
The Ir/C catalyst in the system was separated by vacuum ltra-
tion, and the aniline by-product was separated by ordinary
extraction. Aer most of the water was removed by distillation
under reduced pressure, PAP precipitated in the ice-water bath,
due to its low solubility in aqueous solution. The remaining
BFILs could then be reused.35
Spectral data for DF-B1 ([HSO3-b-N-Et3][HSO4]): 1H-NMR (500
MHz, D2O) d 3.08 (q, 6H), 2.99 (dd, 2H), 2.76 (dd, 2H), 1.76–1.52
(m, 4H), 1.06 (t, 9H). 13C-NMR (125 MHz, D2O) d 7.36, 19.98,
21.90, 50.26, 52.28, 56.10. IR (cmꢀ1): 2991, 1488, 1396, 1189,
730, 587.
Spectral data for DF-B2 ([HSO3-b-N-Pr3][HSO4]): 1HNMR (500
MHz, D2O) d 3.19–3.08 (m, 2H), 3.07–2.98 (m, 6H), 2.88–2.74 (m,
2H), 1.77–1.67 (m, 2H), 1.64 (dd, 2H), 1.61–1.46 (m, 6H), 0.80 (t,
9H). IR (cmꢀ1): 2976, 2884, 1487, 1473, 1169, 1054, 720, 579.
Spectral data for DF-B3 ([HSO3-b-N-Bu3][HSO4]): 1H NMR
(500 MHz, D2O) d 3.01–2.89 (m, 6H), 2.89–2.80 (m, 2H), 2.71–
2.60 (m, 2H), 1.60–1.45 (m, 4H), 1.43–1.32 (m, 6H), 1.15–1.02
(m, 6H), 0.66 (q, 9H). IR (cmꢀ1): 2963, 2876, 1472, 1168, 1054,
726, 579.
Results and discussion
Hydrogenation catalyst
In our previous study of the hydrogenation of halogenated nitro-
benzene, we had compared the catalytic performance of the Ir/C
and the Pd/C catalysts.13 DFT calculations showed that adsorp-
tion properties of halogenated nitrobenzene on the Ir surface were
different from that on the Pd surface. The distance between
oxygen and Ir was much shorter than that between oxygen and Pd.
Moreover, in our study of the hydrogenation of NB to AN, we also
found that the amount of PHA intermediate detected in the
solution of the Ir/C catalyst was much larger than that of the Pd/C
catalyst. The adsorption properties of halogenated nitrobenzene
on Ir aroused our interest. As we all know, for the hydrogenation
of NB to PAP, NB was rst hydrogenated to the PHA intermediate
and then the PHA intermediate was desorbed from the surface of
the hydrogenation catalyst and converted to PAP through the acid-
catalysed Bamberger rearrangement. Therefore, the desorption
properties of the PHA intermediate from the hydrogenation
catalyst directly determined the amount of PHA involved in the
next Bamberger rearrangement. Considering the above results, we
conceived of Ir as the hydrogenation catalyst for the one-pot
hydrogenation of NB to PAP. Thus, we rst compared the cata-
lytic performance of Ir with the traditional Pt catalyst.
Spectral data for DF-C ([HSO3-bPy][HSO4]): 1H-NMR (500
MHz, D2O) d 8.73 (d, 2H), 8.42 (m, 1H), 7.95 (t, 2H), 4.53 (t, 2H),
2.95–2.72 (m, 2H), 2.15–1.86 (m, 2H), 1.75–1.42 (m, 2H). 13C-
NMR (125 MHz, D2O) d 20.92, 29.56, 49.92, 61.28, 128.58,
144.38, 146.10. IR (cmꢀ1): 3070, 2949, 1635, 1489, 1456, 1173,
728, 579.
Acidity of ionic liquids
In order to explore the relationship between the catalytic
performance and the acidity of BFILs, the acidity of BFILs was
measured by an acid–base titration method.43 Phenolphthalein
was used as an indicator, and the concentration of NaOH
aqueous solution was 2 mol Lꢀ1
.
nðNaOHÞ
nðBFILsÞ
Acidity ¼
The XPS spectra of Ir/C and Pt/C are shown in Fig. 1. The Ir
4f7/2 could be deconvoluted into two peaks at about 61.1 eV and
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Fig. 1 Ir/C (Pt/C) XPS spectra of the survey scan (a, c), and Ir(Pt) 4f
high-resolution XPS spectra of Ir/C (Pt/C) (b, d).
62.2 eV, which were assigned to metallic Ir(0) and Ir(4+)
respectively.45,46 The Pt 4f7/2 could be deconvoluted into two
peaks at about 71.5 eV and 72.3 eV, which were assigned to
metallic Pt(0) and Pt(2+) respectively.47,48 The presence of Ir(4+)
and Pt(2+) might be related to the oxidation of Ir and Pt particles
in air.46
High resolution transmission electron microscopy (HR-
TEM) images (Fig. 2a, b, e and f) revealed that Ir(Pt) particles
were found on the surface of the activated carbon support.
Moreover, the hydrogenation of NB to PAP did not cause any
appreciable changes in the catalysts. At the same time, trans-
mission electron microscopy (TEM) images and particle size
distribution of the Ir(Pt) particles (Fig. 2c and d) demonstrated
that the surface of metal particles were in a small and uniform
state and the average particle size of the Ir/C catalyst was very
close to that of the Pt/C catalyst. All these results indicated that
the Ir and Pt should have similar amounts of active sites under
the same metal load.
The hydrogen consumption rates for the hydrogenation of NB
to AN were then studied and the results are shown in Fig. 3,
where we can easily nd that the hydrogen consumption rate of
the Pt/C catalyst was much higher than that of the Ir/C catalyst.
Obviously, this signicant difference in the hydrogenation rate
was not caused by the number of metal active sites. In order to
nd out the reason, the product distribution of the Pt/C catalyst
for hydrogenation of NB to AN was then investigated and the
results were demonstrated in Fig. 4. Compared to our previous
study results,13 we found that the amount of the PHA interme-
diate distribution of Pt/C was much lower than that of Ir/C (about
21%). This indicated that the PHA intermediate desorbed more
easily from the Ir surface than from the Pt surface. The more
easily PHA was desorbed from the metal surface, the less PHA
was le on the metal surface to be further hydrogenated to AN,
which should be the main reason why the hydrogen consump-
tion rate of Ir/C was much lower than that of Pt/C.
Fig. 2 HR-TEM (a, b, e, f) images, TEM images (c, d) and particle size
distribution (c, d) of fresh and used Ir/C and Pt/C catalysts.
As an intermediate of the hydrogenation of NB to PAP, the
distinct adsorption properties of PHA on different metal cata-
lysts could not only affect the hydrogen consumption rate, but
also inuence the PAP selectivity. Fig. 3 showed the hydrogen
consumption rate of the hydrogenation of NB to PAP. As we had
speculated, the hydrogen consumption rate of Ir and Pt cata-
lysts all decreased sharply, compared to the hydrogenation of
NB to AN, and the hydrogen consumption rate of the Ir/C
catalyst was still much lower than that of the Pt/C. On the
other hand, we further studied the catalytic performance of Ir/C
and Pt/C for the hydrogenation of NB to PAP (Table 1). Our
results indicated that the PAP selectivity of the Ir/C catalyst was
signicantly higher than that of the Pt/C catalyst. We thought
the reason was that the easier desorption of PHA from Ir
allowed more PHA to be involved in the following Bamberger
rearrangement, which led to the lower hydrogen consumption
rate and higher selectivity of PAP. Thus, we concluded that Ir
was more suitable than Pt for the hydrogenation of NB to PAP.
Br4nsted acid functionalized ionic liquids
When the hydrogenation catalyst was determined, we selected
BFILs as acid catalysts and constructed a new catalytic system
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with Ir/C for the hydrogenation of NB to PAP. In this regard,
three types of BFILs were synthesized. The type A BFILs (SF-A,
SF-B and SF-C) were single functionalized ionic liquids, which
were characterized by the bisulfate ion as the anion and N-
methylimidazole, triethylamine or pyridine as the cations,
respectively. Based on the preparation of type A BFILs, type B
BFILs (DF-A, DF-B1 and DF-C) were designed with butyl sulfonic
acid functionalized to the N atom. The purpose of this reform
was to increase the acid strength of the corresponding BFILs. In
order to enhance the lipophilicity of DF-B1, type C BFILs (DF-B2
and DF-B3) were then prepared by replacing the ethyl group in
DF-B1 with n-propyl (DF-B2) or n-butyl (DF-B3) respectively.
Type
C BFILs were more targeted to functionalization,
compared to DF-B1. They were a type of dually functionalized
ionic liquid and were expected to be used as acid catalyst and
surfactant at the same time.
The hydrogen consumption rates in the BFILs were rstly
investigated in the presence of surfactant. As shown in Fig. 5,
the hydrogen consumption rates in type A and type B BFILs were
different. We could easily see that the hydrogen consumption
rate in type A BFILs was much higher than that in type B BFILs.
The hydrogen consumption rates in DF-A, DF-B1 and DF-C were
basically within the same range. The above experimental results
could be explained through the mechanism of the hydrogena-
tion of NB to PAP. This reaction involved two competitive
parallel reactions, and the acidity of BFILs affects the trend of
the Bamberger rearrangement reaction. When the acidity of
BFILs was stronger, the Bamberger rearrangement was domi-
nant in the parallel reaction and the over hydrogenation side
reaction was inhibited, which led to the lower hydrogen
consumption rate. The acidity of type B BFILs stemmed not only
from the bisulfate anion, but also the sulfonic acid, which made
them more acidic than type A BFILs and resulted in the lower
hydrogen consumption rate.
Fig. 3 Hydrogen consumption rate of the Ir/C and Pt/C catalysts for
the hydrogenation of NB to ANa and hydrogenation of NB to PAPb.
Reaction conditions: (a) 0.02 mol of NB, 0.1 g of catalyst, 25 mL of
ethanol, PH ¼ 1.0 MPa, T ¼ 353 K, stirring rate ¼ 1200 rpm. (b) 0.02 mol
2
of NB, 0.1 g of catalyst, 25 mL of 10% sulphuric acid solution, 0.025 g of
CTAB, PH ¼ 0.8 MPa, T ¼ 353 K, stirring rate ¼ 1000 rpm.
2
Fig. 4 Product distribution of catalytic hydrogenation of NB to AN on
the Pt/C catalyst. Reaction conditions: 0.05 g of catalyst, 25 mL of
ethanol, 0.02 mol of NB, PH ¼ 1.0 MPa; T ¼ 353 K, stirring rate ¼
2
1200 rpm.
Table 1 Catalytic performance of Pt/C and Ir/Ca
Selectivity (%)
Conversion
Catalyst
(%)
PAP
AN
1% Pt/C
1% Ir/C
100
100
40.5
60.4
59.5
39.5
a
Fig. 5 The hydrogen consumption rate of the reaction in different
BFILs in the presence of CTAB. Reaction conditions: 0.02 mol of NB,
25 mL of H2O, 0.1 g of Ir/C, 0.025 g of CTAB, 353 K, 0.8 MPa, 1000 rpm,
0.05 mol BFILs of type A, 0.025 mol BFILs of type B.
Reaction conditions: 0.1 g of catalyst, 25 mL of 10% sulphuric acid
solution, 0.02 mol of NB, 0.025 g of CTAB, PH ¼ 0.8 MPa, T ¼ 353 K,
2
stirring rate ¼ 1000 rpm.
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Table 3 Catalytic performance of BFILs without surfactanta
Selectivity (%)
Considering the inuence of the acidity on the Bamberger
rearrangement reaction, we thought that it was necessary to
further compare the acidity of the different BFILs quantitatively.
Therefore, the acidity of the BFILs was quantitatively tested by
sodium hydroxide titration. The relationship between the
amount of acid and the catalytic performance of different kinds
of BFILs was studied. The results are presented in Table 2,
where it can be seen that the selectivity of PAP was proportional
to the acidity of BFILs. The strong acidity of BFILs beneted the
Bamberger rearrangement to form PAP and thus increased the
selectivity of PAP; the relative acidity of the SO3H-functionalized
Solubility
Conversion
(%)
BFILs
(g Lꢀ1
)
PAP
AN
H2SO4
Type B
—
—
100
100
100
100
100
100
43.9
32.6
37.8
30.8
44.3
50.8
56.1
67.4
62.2
69.2
55.7
49.2
DF-A
DF-B1
DF-C
DF-B2
DF-B3
3.1
3.4
2.8
7.6
11.5
Type C
a
Reaction conditions: 0.02 mol NB, 0.025 mol BFILs, 25 mL of H2O,
ILs was stronger than that of the anion-functionalized ILs33 and 0.1 g of Ir/C, 353 K, 0.8 MPa, 1000 rpm.
thus, the type B BFILs should possess stronger acidity than that
of the type A BFILs. Due to the acidity of type A BFILs being
much weaker than that of type B, there was an obvious gap in
the PAP selectivity between type A and type B BFILs. We could
also see that the PAP selectivity in type B BFILs were very close to
that in sulfuric acid. Among all the BFILs of type B, DF-A showed
the best catalytic performance and the PAP selectivity of DF-A
was even higher than that of sulfuric acid. We thought that
this reason could be attributed to its weak steric hindrance49
and strong acidity. Our results indicate that BFILs could already
replace sulfuric acid for the hydrogenation of NB to PAP in the
presence of surfactant.
Due to the lower solubility of NB in the reaction solution,
surfactant must be added for the hydrogenation of NB to PAP.
The addition of surfactant led to some problems such as
a complicated system and troublesome operation. Considering
the properties of the BFILs, we further explored the catalytic
performance of different BFILs without the existence of
Fig. 6 Recycling experiments of Ir/C and DF-B3 for hydrogenation of
surfactant. The result is shown in Table 3. As we predicted, the
PAP selectivity greatly decreased both the type B BFILs and
sulfuric acid, implying the non-negligible importance of
surfactant for such a complex reaction system. However, the
catalytic performance of DF-B1 caught our attention. Although
DF-B1 was the weakest acid among all the type B BFILs, it
showed the highest PAP selectivity without the existence of
surfactant. We speculated that the catalytic performance of the
BFILs was also related to lipophilicity in addition to acidity. The
lipophilicity of the BFILs increased the solubility of the reac-
tants, which would make the reaction occur more easily. This
provided us with a good idea for modifying the DF-B1. In order
NB to PAP. Reaction conditions: 0.02 mol of NB, 0.025 mol of DF-B3,
25 mL of H2O, 0.1 g of Ir/C, 353 K, 0.8 MPa, 1000 rpm.
to further enhance the lipophilicity of DF-B1, we increased the
length of the carbon chain connected to the N atom by replacing
the ethyl group in DF-B1 with the n-propyl or n-butyl carbon
chain. As expected, the solubility of NB in type C BFILs
improved signicantly, and the selectivity of PAP increased with
the elongation of the carbon chain. DF-B3 showed the best
catalytic performance with 50.8% selectivity of PAP. Our results
indicated that the DF-B3 dually functionalized ionic liquid
could simultaneously be used as an acid catalyst and surfactant
for the hydrogenation of NB to PAP.
Recyclability is one of the important properties of catalysts.
Herein, we chose the catalytic system composed of Ir/C and DF-
B3 to conduct the recycling experiments. As shown the Fig. 6,
the conversion of NB remained stable and the selectivity of PAP
decreased slightly aer being recycled three times. This indi-
cated that the new catalytic system of Ir/C and DF-B3 had good
stability in the hydrogenation of NB to PAP.
Table 2 Catalytic performance of BFILs with surfactanta
Selectivity (%)
Amount of
acid
Conversion
(%)
BFILs
PAP
AN
Type A
Type B
SF-A
SF-B
SF-C
DF-A
DF-B1
DF-C
1.65
0.83
1.06
1.94
1.73
1.85
100
100
100
100
100
100
51.2
29.3
32.7
62.4
56.4
56.9
48.8
70.7
67.3
37.6
43.6
43.1
Conclusions
a
Reaction conditions: 0.02 mol of NB, 25 mL of H2O, 0.1 g of Ir/C,
0.025 g of CTAB, 353 K, 0.8 MPa, 1000 rpm, 0.05 mol of type A,
0.025 mol of type B.
The desorption properties of the PHA intermediate from the
hydrogenation metal catalyst and the acidity of the acid catalyst
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Acknowledgements
This work is nancially supported by National Natural Science 26 E. Raee and F. Mirnezami, J. Mol. Struct., 2017, 1130, 296–
Foundation of China (Grant No. 21473159, 21476208) and 302.
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