Green synthesis of benzonitrile using ionic liquid with multiple roles as the recycling agent

Article abstract of DOI:10.1039/c9ra02296a

Preparation of benzonitrile from benzaldehyde and hydroxylamine hydrochloride is one of the most advantageous approaches. Nevertheless, it suffers from various constraints such as longer reaction time, corrosion and recovery of hydrochloric acid, the use of metal salt catalysts and their separation. For these reasons, a novel green benzonitrile synthetic route was proposed with ionic liquid as the recycling agent in this study. The results indicated that hydroxylamine 1-sulfobutyl pyridine hydrosulfate salt ((NH₂OH)₂·[HSO₃-b-Py]·HSO₄) was an expert alternative to hydroxylamine hydrochloride. Meanwhile, the ionic liquid [HSO₃-b-Py]·HSO₄ exhibited the multiple roles of co-solvent, catalysis and phase separation, thus the use of metal salt catalyst was eliminated, and no additional catalyst was needed. Hence, the separation process was greatly simplified. When the molar ratio of benzaldehyde to (NH₂OH)₂·[HSO₃-b-Py]·HSO₄ was 1:1.5, the volume ratio of paraxylene to [HSO₃-b-Py]·HSO₄ was 2:1, the benzaldehyde conversion and benzonitrile yield were both 100% at 120 °C in 2 h. Even better, the ionic liquid could be recovered easily by phase separation, and recycled directly after reaction. Additionally, this novel route is applicable to the green synthesis of a variety of aromatic, heteroaromatic and aliphatic nitriles with excellent yields.

Full text of DOI:10.1039/c9ra02296a

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PAPER
Green synthesis of benzonitrile using ionic liquid
with multiple roles as the recycling agent†
Cite this: RSC Adv., 2019, 9, 17631
Zhihui Li,ab Tingting Wang,a Xudong Qi,b Qiusheng Yang,a Liya Gao,a
a
a
Dongsheng Zhang, Xinqiang Zhaoa and Yanji Wang
*
*
Preparation of benzonitrile from benzaldehyde and hydroxylamine hydrochloride is one of the most
advantageous approaches. Nevertheless, it suffers from various constraints such as longer reaction time,
corrosion and recovery of hydrochloric acid, the use of metal salt catalysts and their separation. For
these reasons, a novel green benzonitrile synthetic route was proposed with ionic liquid as the recycling
agent in this study. The results indicated that hydroxylamine 1-sulfobutyl pyridine hydrosulfate salt
((NH2OH)2$[HSO3-b-Py]$HSO4) was an expert alternative to hydroxylamine hydrochloride. Meanwhile,
the ionic liquid [HSO3-b-Py]$HSO4 exhibited the multiple roles of co-solvent, catalysis and phase
separation, thus the use of metal salt catalyst was eliminated, and no additional catalyst was needed.
Hence, the separation process was greatly simplified. When the molar ratio of benzaldehyde to
(NH2OH)2$[HSO3-b-Py]$HSO4 was 1 : 1.5, the volume ratio of paraxylene to [HSO3-b-Py]$HSO4 was 2 : 1,
the benzaldehyde conversion and benzonitrile yield were both 100% at 120 ꢀC in 2 h. Even better, the
ionic liquid could be recovered easily by phase separation, and recycled directly after reaction.
Additionally, this novel route is applicable to the green synthesis of a variety of aromatic, heteroaromatic
and aliphatic nitriles with excellent yields.
Received 26th March 2019
Accepted 18th May 2019
DOI: 10.1039/c9ra02296a
rsc.li/rsc-advances
dehydrated to benzonitrile.15,16 Both of the two steps can be
catalyzed by acid.17,18 As for the one-pot route, it includes oxi-
Introduction
mation of benzaldehyde to benzaldoxime, followed by the
dehydration of in situ generated benzaldoxime to benzonitrile.19
Obviously, the one-pot reaction helps to reduce the energy
consumption, solvent waste and reaction time, which in turn,
substantially avoids the isolation of benzaldoxime. It is a valu-
able strategic way in terms of economic and environmental
aspects.14 Owing to these advantageous features, several studies
for the one-pot synthesis of benzonitrile have been conducted in
recent years.
Enthaler et al.20 presented a direct conversion of benzalde-
hyde to benzonitrile catalyzed by Zn(CH3COO)2 (5 mol%). When
benzaldehyde was 0.72 mmol, n(benzaldehyde) : n(NH2-
OH$HCl) ¼ 1 : 1.2, toluene was 2 mL, reaction temperature was
100 ꢀC, reaction time was 24 h, the benzonitrile yield was 79.0%.
Zhuang et al.21 studied SnCl2$2H2O (10 mol%) catalyzed trans-
formation of benzaldehyde to benzonitrile. When benzaldehyde
Benzonitrile is a versatile chemical that plays key roles in the
synthesis of benzoic acid, benzylamine, benzamide, pesticides
and dyes.1 It is also a prominent intermediate for the produc-
tion of benzoguanamine,2 which is a widely used advanced
coating. Classical methods for benzonitrile synthesis are the
cyanation of benzene halides,3,4 toluene halides and ammonia
reaction,5 ammoxidation of toluene, ammonia and air,6–8 ben-
zoic acid and urea reaction,9 and the cyanidation of benzalde-
hyde.10–13 Among them, preparation of benzonitrile from
benzaldehyde and hydroxylamine hydrochloride (NH2OH$HCl)
is one of the most interesting and advantageous approaches
due to the constant carbon number in the reactant and nal
product, mild reaction conditions, low production cost and
prospect for industrial-scale application.14
The transformation of benzaldehyde and NH2OH$HCl to
benzonitrile can be grouped into two-step and one-pot
methods.15 For the two-step reaction, it is carried out to form
benzaldoxime, and then the isolated benzaldoxime is
was
1
mmol, n(benzaldehyde) : n(NH2OH$HCl)
¼
1 : 1.1,
CH3CN was 0.5 mol, NaHCO3 was 1 mmol, the benzonitrile yield
was 80.0% at 80 C within 24 h. Leggio et al.22 investigated the
ꢀ
catalytic properties of TiCl4. When benzaldehyde was 1 mmol,
n(benzaldehyde) : n(NH2OH$HCl) ¼ 1 : 2, pyridine was 2 mL,
TiCl4 was 2.5 mmol, the benzonitrile yield was 85.0% at 40 ꢀC in
3 h. Wang et al.19 reviewed the synthesis of benzonitrile cata-
lyzed by NaHSO4$H2O. Under the optimized conditions of
molar ratio of n(benzaldehyde) : n(NH2OH$HCl) ¼ 1 : 1.4, and
aHebei Provincial Key Lab of Green Chemical Technology and High Efficient Energy
Saving, Hebei University of Technology, Tianjin 300130, China. E-mail: yjwang@
hebut.edu.cn; Tel: +86-22-60200445
bSchool of Energy and Environmental Engineering, Hebei University of Technology,
Tianjin 300401, China
† Electronic supplementary information (ESI) available: Experimental and the
characterization data. See DOI: 10.1039/c9ra02296a
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reaction at 135 ꢀC for 4.5 h in the presence of tetrabuty- (NH2OH)2$ILs. Thus, the recovery of the ionic liquid with multiple
lammonium bromide as phase transfer catalyst and DMF as roles was fully realized. Moreover, the one-pot clean synthesis of
solvent, benzonitrile was obtained in excellent yield of 92.8%. In benzonitrile was achieved.
addition to homogeneous catalysts, heterogeneous catalysts
were also reported. Fe3O4-CTAB NPs (1.8 mol%) was applied as
Experimental
Materials and methods
a competent catalyst for one-pot synthesis of benzonitrile.14 The
combination of benzaldehyde (0.5 mmol), NH2OH$HCl (0.75
mmol), Fe3O4-CTAB NPs (1.8 mol%), and DMF (5 mL) at 80–
90 ꢀC for 1 h were found to be the optimized reaction conditions
for the desired transformation, with the benzonitrile yield being
97.0%. A green chitosan supported magnetic ionic liquid
(CSMIL) was prepared for direct conversion of benzaldehyde to
benzonitrile.23 When benzaldehyde was 1 mmol, NH2OH$HCl
was 1.2 mmol, dry-CSMIL was 15 mg, MeSO2Cl was 1.2 mmol,
All the chemicals (AR grade) were commercially available and
used without further purication. FTIR spectra were recorded
on a Bruker Vector 22 FTIR spectrometer in the 4000–400 cmꢁ1
range using liquid lm or KBr tablet. The 1H NMR and 13C NMR
spectra were recorded on a Bruker AMX FT 400 MHz NMR
spectrometer using D2O as solvent, and the chemical shis were
expressed in ppm.
ꢀ
the benzonitrile yield was 93.0% (70 C, 1.5 h). It can be seen
that benzonitrile could be efficiently synthesized from benzal-
dehyde and NH2OH$HCl. However, it suffers from various
constrains such as longer reaction time, use of metal salt
catalysts and inconvenient post treatment.24,25 Furthermore, the
using of NH2OH$HCl will inevitably lead to problems of
equipment corrosion and environmental pollution because it
releases HCl.
Preparation of the (NH2OH)2$ILs
The (NH2OH)2$ILs were prepared according to the method
described in the literature.26,27 The synthesis of hydroxylamine
1-sulfobutyl pyridine hydrosulfate salt is given here as an
example. 1-Sulfobutyl pyridine hydrosulfate ([HSO3-b-Py]$HSO4,
32.0 mmol) was added into a 250 mL three-necked ask
equipped with a dropping funnel and a stirrer. The ask was
then placed in a low-temperature reaction bath, and the
temperature was continuously kept under 2 ꢀC. The aqueous
solution of hydroxylamine (80.0 mmol) was added dropwise to
the three-necked ask that contained the [HSO3-b-Py]HSO4
while stirring. When the neutralization process was completed,
a clear solution was obtained. The solution was then evaporated
to dryness under reduced pressure to obtain the white hydrox-
ylamine 1-sulfobutyl pyridine hydrosulfate salt ((NH2OH)2-
$[HSO3-b-Py]$HSO4). The preparation method of hydroxylamine
N,N,N-trimethyl-N-sulfobutyl hydrosulfate salt ((NH2OH)2-
$[HSO3-b-N(CH3)3]$HSO4) and hydroxylamine 1-sulfobutyl-3-
methyl imidazole hydrosulfate salt ((NH2OH)2$[HSO3-b-mim]$
HSO4) were similar to that of the (NH2OH)2$[HSO3-b-Py]$HSO4,
the only difference was to replace the [HSO3-b-Py]$HSO4 with
[HSO3-b-N(CH3)3]$HSO4 or [HSO3-b-mim]$HSO4.27,28
Recently, a series of ionic liquids (ILs) were used as alternatives
to conventional inorganic acids in hydroxylamine stabilization,
leading to the formation of several eco-friendly hydroxylamine
ionic liquid salts ((NH2OH)2$ILs) that exhibited better reactivity in
one-step clean synthesis of caprolactam, phenol and aniline in
comparison with NH2OH$HCl and (NH2OH)2$H2SO4.26–28 In
continuation of our work for application of these (NH2OH)2$ILs,
a green and high efficient reaction route was developed for direct
synthesis of benzonitrile from benzaldehyde and (NH2OH)2$ILs
with ILs as the co-solvent, catalyst, phase separation and recycling
agent, as shown in Scheme 1. Compared with the existing method
with NH2OH$HCl as raw material, this novel route has the
following advantages: (1) it is eco-friendly since no metal salt
catalyst was added and no inorganic acid was released. (2) When
the reaction was completed, the reaction mass was separated into
organic phase and aqueous phase automatically. The organic
phase could be analyzed by gas chromatography and the ionic
liquid in aqueous phase could be easily recovered. (3) The recov-
ered ionic liquid not only can be recycled as the co-solvent and
One-pot synthesis of benzonitrile from benzaldehyde and
(NH2OH)2$ILs
catalyst, but also as the hydroxylamine stabilizer for preparation of The reaction was performed in a 100 mL three-necked ask
equipped with a stirrer and a reux condenser. Typically,
benzaldehyde (14.4 mmol), (NH2OH)2$[HSO3-b-Py]$HSO4 (21.6
mmol), [HSO3-b-Py]$HSO4 (16 mL) and the organic solvent (32
mL) were charged into the ask, and the reaction mixture was
ꢀ
heated in an oil bath at 90–120 C and kept for 0.5–2 h. Along
with the reaction, the organic components (1 mL) were taken
every ve minutes and analyzed with an Agilent 7890B gas
chromatograph connected with a DB-WAX capillary column and
a ame ionisation detector (FID). The analysisꢀconditions were
as follows: N2 as carrier, FID temperature 320 C, the injection
ꢀ
port temperature 250 C, and the program-controlled column
Scheme 1 Green synthesis of benzonitrile using ionic liquid with
multiple roles as the recycling agent. The roles of ionic liquid:
hydroxylamine stabilizer; co-solvent and catalyst; phase separation;
recycled as the co-solvent and catalyst; recycled as the hydroxylamine
stabilizer.
ꢀ
temperature as follows: initial temperature of 90 C for 1 min,
ꢁ1
ꢀ
ꢀ
increasing to 225 C at a rate of 15 C min and holding for
1 min. The benzaldehyde conversion and benzonitrile yield
were determined by the area normalization method.
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to benzaldoxime, followed by the dehydration of the in situ
generated benzaldoxime to benzonitrile. Being a multistep
process, the outcome of the reaction depends on the reactivity
of the substrate towards both oximation and dehydration step.
As for the rst step, it is believed that benzaldoxime is produced
through the nucleophilic attack of nitrogen electron pairs in the
NH2OH to the C]O carbon in the benzaldehyde.29,30 Free
NH2OH,31 resulted from the decomposition of hydroxylamine
salt, is the nucleophile necessary for conversion of benzalde-
hyde to the corresponding benzaldoxime. It was evident that,
the aromatic hydrocarbon solvent, such as toluene, ethyl-
benzene and paraxylene, demonstrated an obvious promotion
on the decomposition of hydroxylamine salt to form free
NH2OH,32,33 since high benzaldehyde conversions were ob-
tained in these solvents. This solvent promoted decomposition
role was probably due to the formation of hydrogen bond
between the nitrogen electron pairs in the hydroxylamine salts
and hydrogen in the solvent.34 Among them, paraxylene was
a better choice for direct synthesis of benzonitrile.
Besides paraxylene, a certain amount of [HSO3-b-Py]$HSO4
was also added. To clarify their roles, a comparative study on
paraxylene alone, [HSO3-b-Py]$HSO4 alone and different ratio of
paraxylene to [HSO3-b-Py]$HSO4 was made and the results were
listed in Table 1. When paraxylene was added alone, a large
number of white particles, which was the undissolved (NH2-
OH)2$[HSO3-b-Py]$HSO4, was dispersed in the reaction mass. It
indicated that pure paraxylene could not effectively dissolve the
(NH2OH)2$[HSO3-b-Py]$HSO4, which then inhibited the reac-
tion between benzaldehyde and hydroxylamine, resulting in
a low benzaldehyde conversion of 54.8%. Moreover, most of the
product was benzaldoxime, the benzonitrile yield was only
2.12%, indicating that paraxylene only acted as solvent and had
no catalytic effect. When [HSO3-b-Py]$HSO4 was added, the
reaction turned to liquid–liquid two phase. It could be
concluded that the addition of [HSO3-b-Py]$HSO4 was favorable
for the dissolution of (NH2OH)2$[HSO3-b-Py]$HSO4. Moreover,
[HSO3-b-Py]$HSO4 being a Brønsted acid coordinated with the
carbonyl oxygen of benzaldehyde thus increasing its electro-
philicity and facilitating its reaction with hydroxylamine.
Therefore, the [HSO3-b-Py]$HSO4 prompted the benzaldehyde
oximation. The benzaldehyde conversion was greatly increased
to over 95%. Furthermore, when the [HSO3-b-Py]$HSO4 was
Recycling of the ILs
For recycling of the ILs, the one-step synthesis of benzonitrile
from benzaldehyde and (NH2OH)2$[HSO3-b-Py]$HSO4 was
carried out under optimal reaction conditions. Aer the reac-
tion, the resulting mixture was cooled and separated into
organic phase and aqueous phase automatically. The organic
phase was analyzed by gas chromatography, and the leaving
ionic liquid in aqueous phase was easily recovered by distilled
with a rotary evaporator under vacuum to remove the water. The
recovered ILs could be recycled as the co-solvent and catalyst for
one-pot synthesis of benzonitrile. Furthermore, it also could be
used again as the hydroxylamine stabilizer to synthesis the
(NH2OH)2$[HSO3-b-Py]$HSO4.
Results and discussion
Co-solvent and catalysis role of the ionic liquid
The effect of solvent on direct synthesis of benzonitrile was
presented in Fig. 1. It could be seen that solvent strongly
affected the results. When bromobenzene and ortho-dichloro-
benzene were used as solvents, the benzaldehyde conversion
and benzonitrile yield were relatively low. While toluene, eth-
ylbenzene and paraxylene were used, the benzaldehyde
conversion and benzonitrile yield were very high, especially the
latter two solvents. The conversion and yield were both
improved to over 96.0%. The solvent effect of aryl halide was
inferior to that of aromatic hydrocarbon. The probably reason is
that, the present reaction includes oximation of benzaldehyde
Table 1 The amount of paraxylene and [HSO3-b-Py]$HSO4 on direct
synthesis of benzonitrilea
No Paraxylene (mL) [HSO3-b-Py]$HSO4 (mL) XBD (%) YBT (%)
1
2
3
4
12
10
8
6
4
0
12
12
0
2
4
6
8
12
0
54.8
95.0
98.0
98.8
100
100
3.71
3.34
2.12
92.0
97.0
96.9
96.7
96.3
5
6
7b
8c
2.43
6.29
0
Fig. 1 Effect of solvent on direct synthesis of benzonitrile (BT).
Reaction conditions: benzaldehyde (BD) 14.4 mmol, n(BD) : n((NH2-
OH)2$[HSO3-b-Py]$HSO4) ¼ 1 : 1.5, solvent 32 mL, [HSO3-b-Py]$HSO4
16 mL, temperature 100 ꢀC.
a
Reaction conditions: benzaldehyde 3.6 mmol, n(BD) : n(NH2OH) ¼
b
1 : 3, temperature 100 ꢀC, time 90 min.
NH2OH$HCl.
c
(NH2OH)2$H2SO4.
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added alone, the reaction was homogeneous. The benzaldehyde
conversion and benzonitrile yield were 100% and 96.3%
respectively, which were much higher than those in pure para-
xylene. The results demonstrated that [HSO3-b-Py]$HSO4 not
only can be used as solvent to promote the benzaldehyde oxi-
mation, but also as catalyst to promote the benzaldoxime
dehydration.35–37
Considering that when the [HSO3-b-Py]$HSO4 alone was
applied as both the solvent and catalyst, the reaction liquid was
too viscous to separate and recover the ionic liquid. For this
reason, a mixture of paraxylene and [HSO3-b-Py]$HSO4 were
employed as solvent for the synthesis of benzonitrile. It can be
seen that the benzonitrile yield was related to their ratio. With
the increase of the amount of [HSO3-b-Py]$HSO4, the benzoni-
trile yield improved rst, then remained almost unchanged.
The benzonitrile yield reached a maximum value of 97.0% when
the volume ratio of paraxylene and [HSO3-b-Py]$HSO4 was 2 : 1.
With NH2OH$HCl and (NH2OH)2$H2SO4 as raw materials and
paraxylene as a solvent, the benzaldehyde conversion and
benzonitrile yield were extremely low, which veried the co-
solvent and catalytic role of the [HSO3-b-Py]$HSO4.
Inuence of reaction parameters on direct synthesis of
benzonitrile
Fig. 2 Effect of temperature on direct synthesis of benzonitrile.
Reaction conditions: benzaldehyde 14.4 mmol, n(BD) : n((NH2OH)2-
$[HSO3-b-Py]$HSO4) ¼ 1 : 1.5, paraxylene 32 mL, [HSO3-b-Py]$HSO4
16 mL.
(1) Temperature. The inuence of reaction temperature on
direct synthesis of benzonitrile was depicted in Fig. 2. It could
be seen that the benzaldehyde conversion and benzonitrile
yield were relatively low when the temperature was 90 ꢀC.
Meanwhile, the reaction rate was slower since it took longer
time to achieve the same conversion and yield. However, the
benzaldehyde conversion and benzonitrile yield increased
signicantly with rising temperature from 90 to 120 ꢀC. The
reaction rate also increased rapidly, indicating that higher
reaction temperature could enhance the reaction rate, favoring
the synthesis of benzonitrile. The highest conversion and yield
Ionic liquid was reported to be the catalyst for the dehydra-
tion of benzaldehyde oxime.35–37 The dehydration process could
be promoted by the protonation of hydroxyl group in the
benzaldehyde oxime through the H+ provided by the ionic
liquid, converted this moiety to a better leaving group and thus
facilitated the formation of benzonitrile.36 In order to explore
the relationship between the catalytic performance and prop-
erties of the ionic liquids, which were combined in the (NH2-
OH)2$ILs, their acid strengths were measured according to the
method reported previously.38 The results were shown in Table 3
and Fig. S1,† from which we can see that the acid strength order
is as follows: [HSO3-b-N(CH3)3]$HSO4 < [HSO3-b-mim]$HSO4 z
ꢀ
obtained (120 C) were 99.9% and 99.7% respectively.
(2) Time. The inuence of reaction time was also investi-
gated to see whether the benzaldehyde conversion and benzo-
nitrile yield could reach 100% by prolonging the time. As shown
in Fig. 3, the benzaldehyde conversion and benzonitrile yield
depended greatly on time at the early stage of reaction. They
increased rapidly with the extension of time, and reached
a value of 98.6% and 98.4% respectively when the time was 1 h.
Then the benzaldehyde conversion and benzonitrile yield
increased slightly when further prolonging the time. When the
reaction time was 2 h, they both reached a value of 100%.
Reactivity comparison of various hydroxylamine salts
A comparison between the (NH2OH)2$[HSO3-b-Py]$HSO4 and
other (NH2OH)2$ILs, as well as the traditional hydroxylamine
inorganic acid salts was summarized in Table 2. It can be seen
that (NH2OH)2$[HSO3-b-mim]$HSO4 and (NH2OH)2$[HSO3-b-
Py]$HSO4 demonstrated better reactivity than (NH2OH)2$[HSO3-
b-N(CH3)3]$HSO4. The benzonitrile yield was around 99.9% and
100%, respectively. No other by-products were generated.
Fig. 3 Effect of time on direct synthesis of benzonitrile. Reaction
conditions: benzaldehyde 14.4 mmol, n(BD) : n((NH2OH)2$[HSO3-b-
Py]$HSO4) ¼ 1 : 1.5, paraxylene 32 mL, [HSO3-b-Py]$HSO4 16 mL,
temperature 120 ꢀC.
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Table 2 Effect of different hydroxylamine salts on direct synthesis of
mechanism for the present reaction was proposed and depicted
in Scheme 2. Firstly, free NH2OH and [HSO3-b-Py]$HSO4 were
released by the decomposition of (NH2OH)2$[HSO3-b-Py]$HSO4.
benzonitrilea
No
Hydroxylamine salt
XBD (%)
YBT (%)
Then
a nucleophilic attack of NH2OH to benzaldehyde
b
occurred, followed by a dehydration step to form the benzal-
dehyde oxime. At last, the hydroxyl group in the benzaldehyde
oxime was protonated by the H+ offered by the [HSO3-b-Py]$
HSO4 to form the intermediate, which then converted to ben-
zonitrile through a dehydration step, along with the regenera-
tion of [HSO3-b-Py]$HSO4 to complete the catalytic cycle.
1
2
3
4
5
(NH2OH)2$[HSO3-b-N(CH3)3]$HSO4
84.3
99.9
100
99.7
94.3
84.3
99.9
100
99.7
94.3
c
(NH2OH)2[HSO3-b-mim]$HSO4
d
(NH2OH)2$[HSO3-b-Py]$HSO4
NH2OH$HCld
d
(NH2OH)2$H2SO4
a
Reaction conditions: benzaldehyde 3.6 mmol, n(BD) : n(NH2OH) ¼
b
1 : 3, paraxylene 8 mL, IL 4 mL, temperature 120 ꢀC, time 2 h. IL is
[HSO3-b-N(CH3)3]$HSO4. IL is [HSO3-b-mim]$HSO4. IL is [HSO3-b-
Py]$HSO4.
c
d
Table 4 Recycling of the [HSO3-b-Py]$HSO4 as co-solvent and
catalysta
[HSO3-b-Py]$HSO4, which was in accordance with their catalytic
activity. Thus it could be deduced that the catalytic performance
of ionic liquids depended greatly on their acid strength. The
ionic liquid with greater acid intensity had the stronger ability
to give proton and higher catalytic activity. Moreover, the reac-
tivity of (NH2OH)2$[HSO3-b-Py]$HSO4 was better than that of
(NH2OH)2$H2SO4 and NH2OH$HCl. Hence, (NH2OH)2$[HSO3-b-
Py]$HSO4 was regarded to be a better alternative to traditional
hydroxylamine salts.
Recycle times
XBD (%)
YBT (%)
1
2
3
4
5
100
100
99.5
98.9
96.7
100
100
99.5
98.9
96.7
a
Reaction
conditions:
benzaldehyde
3.6
mmol,
n(BD) : n((NH2OH)2$[HSO3-b-Py]$HSO4) ¼ 1 : 1.5, paraxylene 8 mL,
[HSO3-b-Py]$HSO4 4 mL, temperature 120 ꢀC, time 2 h.
Plausible mechanism for the present reaction
Based on the reported reaction mechanism for aldehyde and
hydroxylamine to produce aldehyde oxime,30,31 followed by
benzaldoxime dehydration to benzonitrile,35,36 a plausible
Table 3 Calculation and comparison of H0 values of different ionic
liquidsa
No
IL
Amax
I (%)
HI (%)
H0
1
2
3
4
Blank
0.53
0.44
0.38
0.37
100
[HSO3-b-N(CH3)3]$HSO4
[HSO3-b-mim]$HSO4
[HSO3-b-Py]$HSO4
83.0
71.7
69.8
17.0
28.3
30.2
1.68
1.39
1.35
a
Indicator: 4-nitroaniline (5 mg Lꢁ1, pKa ¼ +0.99), ILs (5 mmol Lꢁ1).
Fig. 4 FTIR spectra of the recovered and fresh [HSO3-b-Py]$HSO4.
Scheme 2 Plausible mechanism for synthesis of benzonitrile from Fig. 5 FTIR spectra of the (NH2OH)2$[HSO3-b-Py]$HSO4 obtained
benzaldehyde and (NH2OH)2$[HSO3-b-Py]$HSO4.
from the recovered and fresh [HSO3-b-Py]$HSO4.
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Table 5 Reactivity of different aldehydesa
No
Aldehyde
Nitrile
X (%)
Y (%)
1
80.6
80.6
2
3
86.7
94.4
86.7
94.4
4
5
6
99.7
99.9
77.1
99.7
99.9
77.1
7
8
99.8
91.5
99.8
91.5
9
100
100
100
85.4
41.0
100
10
11
12
13
100
98.7
83.1
38.7
14
15
99.4
100
99.4
98.1
16
99.9
99.9
a
Reaction conditions: aldehyde 3.6 mmol, n(aldehyde) : n((NH2OH)2$[HSO3-b-Py]$HSO4) ¼ 1 : 1.5, paraxylene 8 mL, [HSO3-b-Py]$HSO4 4 mL,
temperature 120 ꢀC, time 2 h.
and (NH2OH)2$[HSO3-b-Py]$HSO4 was carried out under the
above mentioned optimal reaction conditions. Aer completion
of the reaction, the resulting mixture was cooled and separated
into organic phase and aqueous phase automatically. The
organic phase was analyzed by gas chromatography, and the
The recovery and recycling of the ionic liquid
(1) Recycling of the ionic liquid as co-solvent and catalyst. To
investigate the feasibility of the recovery and recycling of the
ionic liquid, direct synthesis of benzonitrile from benzaldehyde
17636 | RSC Adv., 2019, 9, 17631–17638
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leaving ionic liquid in aqueous phase was distilled with a rotary carbonyl group of aromatic aldehydes leading to an increase in
evaporator under vacuum to recover the [HSO3-b-Py]$HSO4. The its electrophilicity thus increasing the yield of nitrile. This effect
recovered [HSO3-b-Py]$HSO4 was further used as co-solvent and is more pronounced for an electron withdrawing substituent in
catalyst for the next round. It can be seen that the benzaldehyde the para position compared to its meta position. Moreover, for
conversion and benzonitrile yield were still as high as 96.7% aliphatic aldehydes, increasing chain length decreases the
aer ve times of recycling (Table 4), indicating that [HSO3-b- electrophilicity of the aldehydic group due to increase in posi-
Py]$HSO4 was stable enough to be recycled.
tive inductive effect of the aliphatic chain thus decreasing the
Aer the [HSO3-b-Py]$HSO4 was recycled for ve times, it was overall yield of aliphatic nitriles for higher aliphatic chain
recovered, collected and characterized. The FTIR spectra of the aldehydes.
recovered and fresh [HSO3-b-Py]$HSO4 were similar (Fig. 4). In
addition, the 1H NMR and 13C NMR spectra (Fig. S2–S5†) of the
recovered [HSO3-b-Py]$HSO4 was consistent with that of the
Conclusions
fresh [HSO3-b-Py]$HSO4. These results demonstrated that the In summary, a convenient and facile method for direct
[HSO3-b-Py]$HSO4 was structurally stable even aer recycling conversion of aldehydes into nitriles using ionic liquid as the
ve times.
recycling agent has been established. The optimum reaction
(2) Recycling of the ionic liquid as hydroxylamine stabilizer. condition was at 120 ꢀC for 2 h. The suitable molar ratio of
[HSO3-b-Py]$HSO4 was recovered when it was recycled for ve benzaldehyde to (NH2OH)2$[HSO3-b-Py]$HSO4 was 1 : 1.5, and
times, and then reused as the hydroxylamine stabilizer. The the suitable volume ratio of paraxylene to [HSO3-b-Py]$HSO4
(NH2OH)2$[HSO3-b-Py]$HSO4 was prepared again. The FTIR was 2 : 1. Under the optimal reaction conditions, the benzal-
spectra of (NH2OH)2$[HSO3-b-Py]$HSO4 obtained from the fresh dehyde conversion and benzonitrile yield were both 100%. Ionic
and recovered [HSO3-b-Py]$HSO4 was presented in Fig. 5. It can liquid exhibited the multiple roles of co-solvent, catalysis and
be seen that they were almost identical. The peaks at 3120 and phase separation. It could be recovered easily, and recycled
+
2725 cmꢁ1 were assigned to the NH3 stretching mode. The directly aer reaction. Additionally, this novel route is appli-
peaks at 2041 and 1540 cmꢁ1 were attributed to the character- cable to green synthesis of a variety of aromatic, heteroaromatic
+
istic peaks of primary ammonium salt and the NH3 deforma- and aliphatic nitriles with excellent yields.
tion frequencies.26,27 Moreover, the 1H NMR and 13C NMR
spectra (Fig. S6–S9†) of the (NH2OH)2$[HSO3-b-Py]$HSO4 ob-
tained from the recovered [HSO3-b-Py]$HSO4 was consistent
Conflicts of interest
with the (NH2OH)2$[HSO3-b-Py]$HSO4 obtained from the fresh The authors declare no competing nancial interest.
one, indicating that the recovered [HSO3-b-Py]$HSO4 could be
used again as the hydroxylamine stabilizer even aer it was
recycled as co-solvent and catalyst for ve times.
Acknowledgements
Next, the (NH2OH)2$[HSO3-b-Py]$HSO4 obtained from the The authors are grateful for the nancial support of the
recovered [HSO3-b-Py]$HSO4 was used in direct synthesis of National Natural Science Foundation of China (No. 21236001,
benzonitrile under the optimal reaction conditions. The benz- 21878069, 21576069 and 201802031), National Natural Science
aldehyde conversion and benzonitrile yield were 100% and Foundation of Hebei Province (No. B2016202335, B2015202369
98.2%, respectively. The results demonstrated that the reactivity and B2017202133).
of the (NH2OH)2$[HSO3-b-Py]$HSO4 obtained from the recov-
ered and fresh [HSO3-b-Py]$HSO4 were much the same.
Notes and references
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