Direct catalytic synthesis of ε-caprolactam from cyclohexanol using [n-C16H33N (CH3)3]H2PW12O40 as a catalyst

Article abstract of DOI:10.1016/j.catcom.2015.07.006

ε-Caprolactam was synthesized directly from cyclohexanol via a tandem catalytic process using [n-C₁₆H₃₃N(CH₃)₃]H₂PW₁₂O₄₀ as a catalyst. The highly efficient performance of the catalysts is due to the phase-transfer function of cation, improved coordination with peroxotungsten during oxidation and stabilization function of heteropoly anion on the intermediate formed during Beckmann rearrangement. A ε-caprolactam yield of 73.9% was obtained with a cyclohexanol conversion of 97.1% under optimized conditions.

Full text of DOI:10.1016/j.catcom.2015.07.006

Catalysis Communications 70 (2015) 6–11
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Direct catalytic synthesis of ε-caprolactam from cyclohexanol using
[
n-C H N (CH ) ]H PW O as a catalyst
16
33
3 3
2
12 40
Hefang Wang, Rongbin Hu, Yongfang Yang, Meidan Gao, Yanji Wang ⁎
School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China
a r t i c l e i n f o
a b s t r a c t
Article history:
ε-Caprolactam was synthesized directly from cyclohexanol via a tandem catalytic process using [n-C16H33N
Received 14 May 2015
Received in revised form 2 July 2015
Accepted 3 July 2015
3 3 2
(CH ) ]H PW12O40 as a catalyst. The highly efficient performance of the catalysts is due to the phase-transfer
function of cation, improved coordination with peroxotungsten during oxidation and stabilization function of
heteropoly anion on the intermediate formed during Beckmann rearrangement. A ε-caprolactam yield of 73.9%
was obtained with a cyclohexanol conversion of 97.1% under optimized conditions.
Available online 16 July 2015
©
2015 Published by Elsevier B.V.
Keywords:
ε-Caprolactam
Cyclohexanol
Direct synthesis
Polyoxometalates
1
. Introduction
Increase in awareness of environmental protection has led to the
development of eco-friendly and economical procedure for one-pot syn-
thesis of CL. Several efforts have been made to synthesize CL in the pres-
ence of nitrosyl sulfuric acid from cyclohexane [4–7], nitrocyclohexane
[8], or cyclohexanone [9,10]. The yield of CL in those processes was
about 10%. Our previously work reported the utilization of the novel
Tandem processes that involve multiple chemical transformations in
a single-pot with minimal work-up and less waste generation have
revolutionized synthetic chemistry in recent years. Tandem catalytic
processes can replace multi-step syntheses with efficient catalytic reac-
tions that can have significant impact on the manufacture of fine
chemicals and pharmaceutical intermediates [1].
(NH
anone with the 91% yield of CL, but using ZnCl
yielding one-pot oximation–Beckmann rearrangement of ketones to am-
ides using trifluoroacetic acid or FeCl ·6H O as a catalyst was reported
by Aricò and Mahajan respectively [12,13]. More recently, Lee reported
a multifunctional Pd/Sc(OTf) /ionic liquid catalyst for the tandem one-
pot conversion of phenol to CL, in 67% overall yield, using 300 mol%
1-butyl-methylimidazolium hexafluorophosphate ([bmim][PF ]) as an
2
OH)
2
-IL in the one-step, solvent-free synthesis of CL from cyclohex-
2
as a catalyst [11]. High
ε-Caprolactam(CL) is an important precursor of nylon-6 and plastics
and the global market is forecasted to reach 5 million metric tonnes by
the year of 2015 [2]. CL is currently manufactured using cyclohexanone
as starting reactant, which is converted into cyclohexanone oxime and
finally rearranged to give CL in a three step process. The yield of
Beckmann rearrangement of cyclohexanone oxime to CL is high, using
fuming sulfuric acid as both solvent and catalyst. However, this process
has several disadvantages, such as formation of large amount of ammo-
nium sulfate as a by-product due to the neutralization of the hazardous
sulfuric acid with ammonia, and corrosion of the reactors and environ-
mental pollution caused by the use of fuming sulfuric acid. Though
cyclohexanone can be obtained by hydrogenation of phenol, the pre-
ferred industrial route involves the direct oxidation of cyclohexane.
However, the aerobic oxidation of cyclohexane to cyclohexanone limits
the whole efficiency of the three step process owing to the low yield per
pass. Only yields of 8% and 10% were received in the cases of homoge-
neous cobalt complex [2] and heterogeneous Co-HZSM-5 [3] as catalysts
respectively.
3
2
3
6
additive [14]. Though very high phenol conversion and CL selectivity
were achieved, this catalyst system suffered from the problems of diffi-
cult separation of the ionic liquid from the reaction mixture.
Polyoxometalates (POMs) with oxygen clusters of early transition
metals have drawn wide attention because of high Brønsted acidity
and redox properties [15]. A variety of different catalytic systems for
hydrogen peroxide oxidation of alcohols catalyzed by POMs have been
developed [16–21].
3
In recent years, H PW12O40 [22], propane sulfoacid-functionalized
imidazolium salt of phosphotungstate [23], and CsxH
3
–xPW12 40 [24]
O
as catalysts in Beckmann rearrangement have been reported, affording
a new approach for liquid phase Beckmann rearrangement. However,
environmentally undesirable solvents such as benzonitrile and acetoni-
trile were used.
To overcome these abovementioned problems, the synthesis of CL
directly from cyclohexanol, being an unavoidable side-product during
⁎
Corresponding author.
E-mail address: yjwang@hebut.edu.cn (Y. Wang).
http://dx.doi.org/10.1016/j.catcom.2015.07.006
566-7367/© 2015 Published by Elsevier B.V.
1

H. Wang et al. / Catalysis Communications 70 (2015) 6–11
7
oxidation of cyclohexane, by oxidation in liquid phase using H
2
O
2
com-
spectrometer. The products were analyzed by a SP-3420A gas chro-
matograph equipped with a KB-Wax column (30 m, 0.32 mm id,
0.25 μm film thickness) with toluene as internal standard.
bined with simultaneous oximation and Beckmann rearrangement in a
pot type reactor is an alternative route. This route is very interesting
from an industrial point of view, because the oxidation of cyclohexanol
to cyclohexanone, oximation of cyclohexanone to CHO and CHO rear-
rangement to CL will take place in presence of multifunctional catalysts.
In this paper, a more efficient tandem catalytic process to synthesize
Cyclohexanol conversionð%Þ
¼ ½ðmoles of cyclohexanol added–moles of unconverted cyclohexanolÞ
=mole cyclohexanol addedꢀ ꢁ 100
3 3 2
CL directly from cyclohexanol using [n-C16H33N(CH ) ]H PW12O40 as a
catalyst was reported. [n-C16 33N(CH ]H
motes tandem reactions without the requirement of any additional cat-
alyst and the final products are easily separated by simple extraction.
H
3
)
3
2
PW12
O
40 effectively pro-
Product selectivityð%Þ ¼ ½moles of the product
=ðmoles of cyclohexanol added–moles of unconverted cyclohexanolÞꢀ
ꢁ 100
Carbon balance ð%Þ ¼ ½sum of moles of carbon in the identified products
2
. Experimental
=
moles of carbon in converted cycloh−exanolꢀ ꢁ 100
2
.1. Material and methods
H
3
PMo12
Br, n-C14
purchased from Tianjin Guangfu Fine Chemical Research Institute. 30%
aqueous was purchased from Tianjin Bodi Chemical Co. Ltd.
O
40, (NH
2
OH)
2
∙H
2
SO
4
, H
3
PW12
O
40, cyclohexanol, n-C16
H
33
N
3. Results and discussion
(
3
CH )
3
H29N(CH
3
3
) Br, and n-C12
H
25N(CH Br, Na WO were
)
3 3
2
4
3.1. Catalytic performances of various POM catalysts during oxidation of
cyclohexanol to cyclohexanone
2 2
H O
2
.2. Preparation of quaternary ammonium decatungstates
Direct synthesis of CL from cyclohexanol involves three steps. The
first step is the selective oxidation of cyclohexanol to cyclohexanone.
Cyclohexanone then reacts with hydroxylamine to form oxime and
oxime rearranges to CL. First, the oxidation of cyclohexanol catalyzed
by quaternary ammonium decatungstate was investigated. The results
are shown in Fig. 1. In the absence of any catalysts, the yield of cyclohex-
anone was only 1.08% (Fig. 1, none). After the addition of the quaternary
ammonium decatungstate, the yield of cyclohexanone increased
dramatically compared with that of the absence of a catalyst. Based on
the results above, a series of POM catalysts with phosphotungstic acid
3 3 4
Hexadecyl trimethyl ammonium decatungstate [n-C16H33N(CH ) ]
W
10
O
32 (CTAW), myristyl trimethyl ammonium decatungstate[n-
29N(CH 32 (TTAW), and dodecyl trimethyl ammonium
decatungstate [n-C12 25N(CH 32 (DTAW) were synthesized
according to previously reported methods respectively [18].
C
14
H
3 3 4 10
) ] W O
H
3 3 4 10
) ] W O
2
.3. Preparation of quaternary ammonium heteropolyacid salts
Hexadecyl trimethyl ammonium phosphomolybdate ([n-C16
H
33
N
(PW:H
as anion were prepared respectively and used for the reaction(Fig. 1).
CTAPW, CTAHPW and CTAH PW showed good catalytic activities. The
conversions of cyclohexanol were extremely low in the cases of quater-
nary ammonium phosphomolybdic acids (Fig. 1, CTAPMo, CTAHPMo,
3 3 40
PW12O40) and 12-phosphomolybdic acid (PMo:H PMo12O )
(
CH
reported methods [25]. [n-C16
n-C16 33N(CH ]H PMo12
PW12 40 (CTAPW), [n-C16 33N(CH
n-C16 33N(CH ]H PW12 40 (CTAH
accordingly by controlling molar ratios of cation and anion.
3
)
3
]
3
PMo12
O
40
)
(CTAPMo) was synthesized according to
33N(CH HPMo12 40 (CTAHPMo),
40 (CTAH PMo), [n-C16 33N(CH
H-PW12 40 (CTAHPW) and
PW) were also prepared
H
O
3
)
3
]
2
O
2
[
H
3
)
3
2
2
H
3
)
3
]
3
O
H
O
3
)
3
]
2
O
[
H
3
)
3
2
2
2
CTAH PMo) as a catalyst. Actually, the different catalytic activities re-
sulted from different heteropoly anions with a similar organic modifier
have been also observed in the oxidation of benzyl alcohol with aqueous
2
.4. Characterization of catalysts
The POMs were characterized by H NMR, FT-IR, UV, TGA and XRD.
2 2
H O previously [26]. In order to determine the role of long-chain
alkylammonium cations in the oxidation of cyclohexanol, same mol of
1
1
31
H and P NMR were obtained on an AVAVCE 400 instrument in
31
3 4
DMSO. Chemical shifts of P NMR were referenced to 85% H PO as ex-
ternal standard. Fourier transform infrared spectroscopy (FT-IR) spectra
were recorded using FT-IR spectrometer (BRUKER TENSOR 27).
Ultraviolet–visible (UV) absorption spectra were obtained using a UV
INESA L5 in acetonitrile. TGA–DTA measurements were conducted
on a DuPont TA 2000 with 10 k/min heating rate under atmosphere.
Powder X-ray diffraction (XRD) data were recorded on a D/max-2500
diffractometer operated at 45 kV and 40 mA, using Nickel filtrated
CuKα radiation with 1.5406 Å between 5° and 40°(2 theta), with a scan-
ning speed of 5°/min.
2
.5. Catalytic test
In a typical experiment, the catalyst (0.25 mmol) was mixed with
cyclohexanol (50 mmol) in a 100 mL three-necked flask equipped
with a reflux condenser, a magnetic stirrer and a thermometer. When
the required temperature reached, 30% aqueous hydrogen peroxide
(
75 mmol) was then added dropwise to the reaction mixture, with vig-
orous stirring for 300 min. Hydroxylamine sulfate (25 mmol) was added
to the flask with vigorous stirring for a certain time. After the comple-
tion of reaction, the reaction mixture was divided into two phases.
Water phase was extracted with dichloromethane and the products
were identified by a Thermo Trace DSQ gas chromatograph–mass
Fig. 1. Catalytic performance of various POM catalysts during oxidation of cyclohexanol.
2 2 2 2 2 4
The reaction conditions: alcohol: H O : (NH OH) ·H SO : catalyst (molar ratio) = 1.00:
1.50:0.500: 0.005, T = 80 °C, oxidation time = 360 min.

8
H. Wang et al. / Catalysis Communications 70 (2015) 6–11
H
3
PW12
O
40 was used as a catalyst under the same reaction condition
role in lowering the activation energy by interacting directly with
the N-protonated oxime and forming a cyclic transition state. Such a sta-
bilizing interaction between the heteropoly anion and the migrating
hydrogen in the transition structure would lower the energy barrier of
1, 2-H-shift. Hence heteropoly anion may accelerate the 1, 2-H-shift of
the N-protonated oxime due to the higher ability to stabilize the transi-
tion structure or to reduce the energy barrier of the transition state.
As shown in Fig. 2, the maximum yield of CL was 73.9% along with
(
Fig. 1). The conversion of cyclohexanol was 34.5%, which was lower
than alkylammonium 12-phosphotungstate. Although the Keggin
heteropoly anion is the active center for oxidation, alkylammonium is
still considered to play a very significant role during reaction. The in-
crease in the yield of cyclohexanone with long-chain alkylammonium
cations of the POM catalyst is attributed to the higher phase-transfer
ability in the oil–water two-phase reaction, which is in agreement
with the results of Zhang [20].
2
7.4% of cyclohexanone oxime interaction with CTAH PW as a catalyst.
For the three quaternary ammonium tungstophosphoric salts
However, the maximum yield of ε-caprolactam was 19.1% with CTAPMo
(
CTAPW, CTAHPW and CTAH
2
PW), the yield of cyclohexanone
as a catalyst. The order of softness of heteropoly anions in aqueous solu-
tion was estimated as follows: PW12O40 N PMo12O40 . The softness of
heteropoly anions is assumed to play an important role in stabilizing
organic intermediates [28]. It is known that the stabilization of organic
intermediates is closely relative to the performance of the catalyst,
which accounts for the lower conversion of cyclohexanol for CTAPMo
3
−
3−
increased from 85.0% to 91.0% with an increase of the content of
C
2
+
16
H
33N(CH
3
)
cations. The same results have been obtained by
using heteropolyacids in conjunction with phase transfer catalysts in
the oxidation of benzyl alcohol to benzaldehyde with hydrogen perox-
ide as the oxidizing agent. The catalytic activity is found to increase
gradually with the decrease of acidity by replacement of protons in
the heteropolyacid with sodium ions [27].
than that for CTAH
For the three quaternary ammonium tungstophosphoric salts
CTAPW, CTAHPW and CTAH PW), the yield of CL increased from
4.5% to 73.9% with a decrease of the content of C16
Fig. 3 shows the XRD pattern of CTAH PW, CTA HPW and CTA PW.
2
PW.
(
5
2
+
3 3
H33N(CH ) cations.
3
.2. One-pot conversion of cyclohexanol to CL
2
2
3
The free acid HPW(a) shows the main diffraction peaks, which are
consistent with Joint Committee on Powder Diffraction Standards
The direct synthesis of CL was investigated with POM as catalysts
Fig. 2). In the absence of any catalysts, no CL was produced (none,
(
(JCPDS)50-0656. Compared with HPW, the CTAH PW, CTAHPW and
2
Fig. 2). Compared with the results obtained in the oxidation reaction
above, the conversion of cyclohexanol increased over POM catalysts, es-
CTA PW show the main diffraction peaks in which some differences
occur because of the partial exchange of the H ion with the CTA ion.
3
pecially for CTAPMo, CTAHPMo, and CTAH
2
PMo. In the periods of
The diffraction peaks at 2θ (in degree) = 8–11°,18–22°, 24–30° and
33–40° are characteristic of HPA anions with Keggin structure [28,29].
The average particle size was calculated by X-ray diffraction line broaden-
oximation and arrangement, cyclohexanone obtained during the oxida-
tion process reacted with hydroxylamine in situ to form CHO, which
was favorable for cyclohexanol being oxidized with the removal of
cyclohexanone.
ing using the Debye–Scherrer equation. The particle sizes of CTAH PW,
2
CTA HPW and CTA PW were 22.2 nm, 24.0 nm and 47.2 nm respectively.
2
3
In the previously proposed mechanism for the rearrangement of CHO,
the rearrangement is initiated by the formation of a N-protonated oxime
by the interaction of the proton from a Brønsted acid site with the nitro-
gen atom of oxime, followed by the proton transfer from nitrogen to ox-
ygen atom [24]. The formation of O-protonated oxime by proton transfer
from nitrogen to oxygen is a 1, 2-H-shift reaction with high activation
energy and is considered as the rate-determining step in Beckmann
rearrangement. In this process, heteropoly anion can play an important
Small particle size facilitates the permeation of the organic intermedi-
ates into the bulk of the hybrid. This proposal consists with the well-
established view point of “pseudo-liquid behavior” for the bulk-type
catalysis of a HPA catalyst [26,30].
The fresh catalyst and recovered catalysts in oxidation stage and
Beckmann rearrangement stage were characterized by 31P NMR
(Fig. 4). A single peak at −15.6 ppm (shown in Fig. 4a) was attributed
3
−
to the structure of [PW12O40
]
[19]. The spectrum of the recovered
catalysts in oxidation stage (Fig. 4b) shows two peaks, which suggests
Fig. 2. Catalytic performance of various POM catalysts during direct synthesis of CL. The
reaction conditions: alcohol: H
.50:0.500: 0.005, T = 80 °C, oxidation time = 360 min, oximation and arrangement
time = 300 min.
2 2 2 2 2 4
O : (NH OH) ·H SO : catalyst (molar ratio) = 1.00:
1
3 2 2
Fig. 3. XRD patterns of HPW(a),(CTA) PW(b),(CTA) HPW(c) and (CTA)H PW(d).

H. Wang et al. / Catalysis Communications 70 (2015) 6–11
9
cyclohexanol from 1.2 to 2.1. The selectivity for CL first increased from
3.6% to 76.1%, then decreased from 76.1% to 67.5% with an increase
of the amount of H . In order to determine the residual hydroxyl-
amine in the aqueous phase after reaction, cyclohexanone together
with NH ·H O (25 wt.%) was added to induce the oximation of
6
2 2
O
3
2
cyclohexanone with residual hydroxylamine to produce oxime. Fig. 5
showed the decomposition rate of hydroxylamine with different
amounts of H
lectivity for cyclohexanone was the decomposition rate of hydroxyl-
amine with an increase amount of H . Oximation was promoted
2
O2. The main reason for low selectivity for CL and high se-
2 2
O
with the increase amount of catalyst, increase of rearrangement
temperature, and prolong of rearrangement time. The selectivity for
CL decreased, mostly due to the hydrolysis cyclohexanone oxime.
3
.3. Direct synthesis of various alcohols to amide catalyzed by CTAH
2
PW
Under the optimized reaction conditions, a series of N-substituted
amides was prepared to establish the scope of this method (Table 1). To
our delight a wide range of alcohol substrates including various aromatic
and cyclic alcohols gave desired products in excellent yield compared to
previous results from corresponding ketoximes over the catalysts of
H
3
PW12
phosphotungstate [23], CsxH
Brønsted acidic ionic liquid [32], and heteropolyanion-based ionic liquid
33].
In addition, the catalyst had a good catalytic performance in the con-
O
40 [10], propane sulfoacid-functionalized imidazolium salt of
Fig. 4. 31P NMR spectra of the fresh catalyst of CTAH
stage(b); in Beckmann rearrangement stage(c).
2
2
PW (a); CTAH PW in oxidation
3
–xPW12 40 [24], ε-caprolactam-based
O
[
that the catalyst is a mixture of different polytungstophosphate species.
The main peak at 1.2 ppm can be attributed to active tungsten-peroxo
complex, which is the real active species in oxidation stage. After the
oxidation of cyclohexanol, the peroxo species lost its active oxygen
completely or partly and were converted into large stable compounds
with a Keggin structure by forming intermolecular W–O–W bonds
versation of isopropanol and sec-butyl alcohol, which is in contrast to
the early report that these substrates were inert in the conversion to
amides [34]. The yield of CL in this work is lower than that reported
by Zhang (99%), which converted cyclohexanone to CL directly cata-
lyzed by trifluoroacetic acid with acetonitrile as an additive [35].
2 2
when all H O was consumed [31].
The effect of the amount of H
2
O
2
and catalyst, rearrangement
3.4. Catalyst recycling
temperature, and rearrangement time on the direct synthesis of CL
was shown in Fig. 2. The conversion of cyclohexanol increased drastical-
ly from 90.2% to 97.1% with an increase of the molar ratio of H O to
2 2
After the reaction, the reaction mixture was clearly divided into two
phases. The aqueous phase was extracted with dichloromethane. The
2
Fig. 5. Influence of reaction conditions on direct synthesis of CL catalyzed by CTAH PW.

10
H. Wang et al. / Catalysis Communications 70 (2015) 6–11
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PW.
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82.8
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72.6
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Reaction conditions: alcohol: H
.50:0.500: 0.005, oxidation temperature = 80 °C, oxidation time = 360 min, oximation
2
O
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2 2 2 4
: (NH OH) ·H SO : catalyst (molar ratio) = 1.00:
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a
Oximation and rearrangement temperature = 80 °C.
Oximation and rearrangement temperature = 100 °C.
3
2
b
[
3
solid catalyst could be easily recovered by filtration of aqueous phase
with a high recovery ratio of above 90% and directly reused for the
next run. These procedures were repeated for four cycles. After four
times of recycle, the yield of CL only slightly decreased to 70.1%. The
loss of activity is probably due to the catalyst loss in the recycling pro-
cess, which is proportional to reused-catalyst amount. Furthermore,
due to the catalyst low protonation ability of the catalyst, the resulting
amides can be easily recovered as pure compounds. High carbon bal-
ances with different alcohols were obtained (Table 1).
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4
. Conclusions
2
n-(CH NC16H33]H PW12O40 as a multifunctional catalyst, catalyzes
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This work was financially supported by the National Natural
zSciences Foundation of China (Nos. 2090618 and 21236001) and the
Natural Sciences Foundation of Hebei province. (Nos. B2010000027,
B2015202262). We thank to Prof. J. Zhao for helpful comments.
[
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