A novel hydroxylamine ionic liquid salt resulting from the stabilization of NH2OH by a SO3H-functionalized ionic liquid

Article abstract of DOI:10.1039/c4cc08273g

A SO₃H-functionalized ionic liquid was used as an alternative to conventional inorganic acids in hydroxylamine stabilization, leading to the formation of a novel hydroxylamine ionic liquid salt that exhibits improved thermal stability and reactivity in the one-step, solvent-free synthesis of caprolactam in comparison with hydroxylamine hydrochloride and hydroxylamine sulfate.

Full text of DOI:10.1039/c4cc08273g

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A novel hydroxylamine ionic liquid salt resulting from
the stabilization of NH2OH by a SO3H-functionalized ionic
liquid
Cite this: DOI:
10.1039/x0xx00000x
Zhihui Lia,b, Qiusheng Yanga, Xudong Qib, Yuanyuan Xua, Dongsheng Zhanga*,
Yanji Wanga* and Xinqiang Zhaoa
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
SO3H
+
SO3- NH3OH
A SO3H-functionalized ionic liquid was used as an
alternative to conventional inorganic acids in
hydroxylamine stabilization, leading to the
formation of a novel hydroxylamine ionic liquid salt
that exhibits improved thermal stability and
reactivity in the one-step, solvent-free synthesis of
caprolactam in comparison with hydroxylamine
hydrochloride and hydroxylamine sulfate.
CH3
N
N
CH3
N
N
+
2NH2OH
HSO4
-
(HSO
)
)
(
NH2OH
.
4
IL
(NH2OH)2
IL
Scheme 1 Proposed reaction between a SO3H-functionalized IL and
NH2OH
best of our knowledge, this is the first report on stabilizing
hydroxylamine with ILs.
Hydroxylamine (NH2OH) has important industrial uses, especially in
the production of cyclohexanone oxime (COX) for caprolactam
(CPL) synthesis.1 However, handling hydroxylamine free base is a
challenge because it is unstable and decomposes at room
temperature.2 Inorganic acids are normally used to stabilize
hydroxylamine to form hydroxylamine salts, such as hydroxylamine
sulfate (HAS) and hydroxylamine hydrochloride (HAL).2a However,
severe problems, such as equipment corrosion and environmental
pollution, are inevitably encountered when these salts are used in
CPL production because they release strong acids. Hence,
conventional inorganic acids are not the best choice to stabilize
hydroxylamine, and it still remains challengeable to search for green
and eco-friendly substances for hydroxylamine stabilization.
Ionic liquids (ILs) have been widely researched as green reaction
media due to their favorable physico-chemical properties.3 Among
them, SO3H-functionalized ILs are highly attractive4 because they
have the advantages of both liquid acids and solid acids, such as
uniform acid sites, well fluidity, non-volatility and easy recovery,
making them ideal medias or/and catalysts for Beckmann
rearrangement.5 As a result, an idea of whether the green SO3H-
functionalized ILs, instead of the traditional inorganic acids, could
be used for stabilizing hydroxylamine was conceptualized. Herein,
we report the stabilization of hydroxylamine with a SO3H-
functionalized IL, which results in the formation of a novel
hydroxylamine IL salt that shows bettered reactivity in the one-step,
solvent-free synthesis of CPL compared with HAL and HAS. To the
1-Sulfobutyl-3-methyl imidazole hydrosulfate (IL) was used to
stabilize the aqueous solution of hydroxylamine, and the
hydroxylamine IL salt (denoted as (NH2OH)2·IL) was formed as
depicted in Scheme 1. Based on the naming methods of inorganic
acid hydroxylamine salts, the new synthesized complex was
hydroxylamine 1-sulfobutyl-3-methyl imidazole hydrosulfate salt
((NH2OH)2·C 8H16O7N2S2, 16.6 g, 94.1%), mp 149.1-149.8 ℃ .
Found: C, 25.37; H, 5.97; N, 14.71; S, 16.55. Calc. for
(NH2OH)2·C 8H16O7N2S2: C, 25.13; H, 5.76; N, 14.66; S, 16.75.
The structure of (NH2OH)2·IL was fully characterized by FTIR,
Raman spectra, and NMR. The FTIR spectra of HAS and IL were
also presented to aid in the analysis of the (NH2OH)2·IL spectra. The
peaks at 3038 and 2725 cm-1 in the FTIR spectra of (NH2OH)2·IL
+
(Fig. 1a) were assigned to the NH3+ stretching mode from the NH3
group in the positive ion of (NH2OH)2·IL, which agree with those of
HAS.6 The minor peak at approximately 2040 cm-1 and those at 1614
and 1520 cm-1 in the FTIR spectra of (NH2OH)2·ILs were attributed
+
to the characteristic peaks of primary ammonium salt and the NH3
deformation frequencies, which also in agreement with those of
+
HAS.6 The above data indicate that the NH3 group from NH2OH
indeed existed in (NH2OH)2·IL. Furthermore, the Raman spectra of
(NH2OH)2·IL displays exactly the same vibration bands that
correspond to the NH3OH+ species (Fig. 1b, Table S1, ESI†).7 This
finding provides another strong evidence for the presence of
NH3OH+ in (NH2OH)2·IL. In addition, the absorption of SO3, CH3,
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observed at the chemical shifts of 8-9 ppm in the spectra of
+ 9
(NH2OH)2·IL, which was assigned to the NH3 .
a
IL
Density functional theory (DFT) calculations (Computational
details, ESI†) were further performed to verify the structure.10 The
optimal structure and the distance between each atom and its NBO
charge distribution are depicted in Fig. 2. The distances between the
N atom from NH2OH and the H atom from the corresponding acid in
IL were 1.0710 Å (N1'-H1-4) and 1.4290 Å (N2'-H2-4), respectively.
The former bond length was similar to that of a normal N-H bond
(N1'-H1-1, 1.0327 Å). The bonded result was also confirmed by
NBO analysis. Mayer bond orders of the former was 0.25 higher
than that of the latter. The latter distance of N2'-H2-4 was shorter
than the distance between H2-3 and O4 (1.6390 Å). This result
shows that N2' and H2-4 has strong H-bond effect. A large network
of H bonds or bonded forms was formed among the H, O and N
atoms in (NH2OH)2·IL, which stabilized the two hydroxylamines.
The results are consistent with the 1H NMR.
C=N
C=C
CH3
SO3
2040
.
(NH2OH)2·IL
2725
3038
HAS
1520
1614
3750
3000
2250
1500
750
Wavenumber(cm-1)
The thermal stability of (NH2OH)2·IL, compared with those of IL,
HAS and HAL, was investigated in a temperature range between 25
and 1000 °C. The results are displayed in Fig. 3. The TG curve
shows that the mass loss process of (NH2OH)2·IL was divided into
four stages. The mass losses were 7.9% at 192 °C , 13.7% at 281 °C ,
87.6% at 401 °C and 99.1% at 600 °C . According to the results of
the DFT computations, one NH2OH was combined with the H+
released by -SO3H in the anion of IL to form NH3OH+. Another
NH2OH was stabilized by strong H-bond effect with both HSO4- and
-SO3H. The former was more stable than the latter. Therefore, the
first weight loss (with theoretical value of 8.6%) corresponded to the
NH2OH stabilized by the H-bond effect, and the second (with
theoretical value of 17.3%) corresponded to the NH2OH stabilized
by the anion. After 281 °C , the TG curve of (NH2OH)2·IL was
much the same as that of the IL. Thus, the mass loss between 281
NH3
b
N-OH
OH / NH3
OH
NH3
NH3
3000
2250
1500
750
0
Raman Shift (cm-1)
Fig. 1 (a) FTIR spectra of the (NH2OH)2·IL, IL and HAS; (b) Raman and 600 °C resulted from the decomposition of the SO3H-
spectra of the (NH2OH)2·IL
functionalized IL that was used to stabilize the two hydroxylamines.
Final decomposition products were all gases with 99.1% mass loss.
The TG curves of HAL and HAS (Fig. 3) showed two stages of
C=C and C=N groups6, 8 in the FTIR spectra of (NH2OH)2·IL (Fig.
1a) confirmed that IL also existed in (NH21OH)2·IL. The structure of mass loss that led to the complete transformation of hydroxylamine
(NH2OH)2·IL was also confirmed by the H and 13C NMR spectra salt into gaseous products, which are in agreement with the
(Fig. S1 a-d, ESI†). The 1H NMR spectra of (NH2OH)2·IL were literature.2a,11 The TG curves of HAS and HAL were much steeper
generally consistent with that of IL when D2O was used as a solvent, than that of (NH2OH)2·IL, indicating that HAS and HAL may
but all of the chemical shifts were slightly larger. This can be explode easily on heating, and this may lead to explosive exothermic
attributed to the decreased electron cloud density at
(NH2OH)2·IL due to hydrogen bond formation. Most notably, while temperature of (NH2OH)2·IL (600 °C) was much higher than those
H
in reaction in a confined space.12 Moreover, the final decomposition
the solvent D2O was replaced by DMSO-d6, a new resonance was
of HAL (270 °C) and HAS (400 °C). In summary, (NH2OH)2·IL has
a gentler weight loss curve and a much higher final decomposition
temperature, indicating that it has bettered thermal stability than
HAL and HAS.
100
IL
(NH2OH)2·IL
80
60
40
20
HAL
HAS
0
0
200
400
600
800 1000
Temperature (℃)
Fig. 2 Optimized geometry (distance unit: Å, red) and NBO charge
Fig. 3 TG curves of the (NH2OH)2·IL, IL, HAS and HAL
(blue) of (NH2OH)2·IL
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O
structure and composition of which were verified by experimental
and theoretical (DFT computations) methods. The obtained
hydroxylamine IL salt exhibits bettered reactivity in the one-step,
solvent-free synthesis of CPL in comparison with HAL and HAS.
The authors are grateful for the financial support of the National
Natural Science Foundation of China (21236001, 21106029 and
20876033), National Natural Science Foundation of Tianjin
(12JCQNJC03000) and National Natural Science Foundation of
Hebei Province (B2012202043).
N
ZnCl2
130~160 oC, 0.5~2.5 h
.
2
(NH OH) IL
+
O
2
Scheme 2 One-step, solvent-free synthesis of CPL from CYC
The utilization of the novel (NH2OH)2·IL in the one-step, solvent-
free synthesis of CPL from cyclohexanone (CYC) was tested, as
shown in Scheme 2. The advantages of this one-step process13 over
the conventional double-step process14 are its much shorter route, no
generation of ammonium sulfate and environmental friendliness.
Although ZnCl2 based SO3H-functionalized ILs have been used as
green reaction media for Beckmann rearrangement,5 insufficient
information is available about the one-step synthesis of CPL from
CYC with ZnCl2 as catalyst. Table 1 summarizes the results. The
optimum reaction condition was at 150 °C for 2 h, and the best
molar ratio of (NH2OH)2·IL: CYC: ZnCl2 was 1:2:2. Under the
optimal reaction condition, most of the CYC was converted into
Notes and references
a
Hebei Provincial Key Lab of Green Chemical Technology and High
Efficient Energy Saving, Hebei University of Technology, Guang Rong
Dao 8, Hongqiao Distric, Tianjin 300130 (China). Tel/Fax: 86-22-
60200445; E-mail: yjwang@hebut.edu.cn; zds1301@hebut.edu.cn.
b
School of Energy and Environmental Engineering, Hebei University of
Technology, Tianjin 300401 (China).
CPL, corresponding to 91.0% yield and 91.0% selectivity toward Electronic Supplementary Information (ESI) available: [Experimental and
CYC. Moreover, HAS and HAL were used in place of (NH2OH)2·IL
under the optimized conditions. However, the CPL yields were
slightly inferior to that of (NH2OH)2·IL. Therefore, (NH2OH)2·IL
was an excellent substitute for HAL and HAS in CPL synthesis. A
controlled reaction was performed to recover the IL, the FTIR
spectra of the recovered and fresh IL were highly similar (Fig. S2,
ESI†). This result demonstrate that the IL combined in (NH2OH)2·IL
could be recovered. Additionally, an in situ IR study was carried out
to monitor the reaction (In situ IR, ESI†). COX sharply increased at
the beginning of the reaction and then decreased, while CPL
appeared and increased gradually as the reaction proceeded. Further
study was undertaken to understand the reaction mechanism and the
recyclability of the reaction media, as well as other applications of
this new hydroxylamine salts.
the characterization data; The density functional theory (DFT)
calculations using the M062X hybrid functional with the Gaussian 09
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In summary, NH2OH was successfully stabilized by a SO3H-
functionalized IL to form a novel hydroxylamine IL salt, the
Table 1 One-step synthesis of CPL from CYC and (NH2OH)2·IL.a
4
5
Catalyst
(mmol)
T
t
YCPL
(%)
SCPL
(%)
SCOX
(%)
No.
(°C )
150
150
150
150
150
150
150
150
150
150
150
150
150
130
140
160
150
150
(h)
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.5
1
1.
2.
none
6.3
10.6
2.5
70.2
97.5
23.9
24.8
11.9
75.2
22.2
13.8
19.8
37.0
33.4
3.6
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ZnO (2)
2.5
3.
Zn(OAc)2 (2)
ZnSO4 (2)
ZnCl2 (2)
ZnCl2 (1)
ZnCl2 (1.5)
ZnCl2 (2.5)
ZnCl2 (3)
ZnCl2 (2)
ZnCl2 (2)
ZnCl2 (2)
ZnCl2 (2)
ZnCl2 (2)
ZnCl2 (2)
ZnCl2 (2)
ZnCl2 (2)
ZnCl2 (2)
30.0
46.6
75.1
12.8
55.8
73.8
64.4
47.2
52.1
91.0
45.2
16.5
49.8
45.3
88.3
56.1
31.2
46.6
76.1
15.7
55.8
73.8
64.4
47.2
52.1
91.0
45.2
16.9
49.8
48.8
89.8
56.1
4.
5.
6
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a Reaction conditions: CYC (1 mmol), (NH2OH)2·IL (2 mmol);
b HAS (2 mmol); c HAL (4 mmol)
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