Hydrogenation of nitrocyclohexane to cyclohexanone oxime over Pd/CNT catalyst under mild conditions

Article abstract of DOI:10.1016/j.catcom.2011.12.027

The Pd/C, Pt/C, Ni/CNT and Pd/CNT catalysts were prepared by impregnation method and characterized by BET, XRD, TEM and H₂ chemisorption. These catalysts were tested in the hydrogenation of nitrocyclohexane to cyclohexanone oxime. The results show that 5% Pd/CNT catalyst exhibits good performance, it gives nitrocyclohexane conversion of 97.6% and cyclohexanone oxime selectivity of 85.9% under mild conditions of 0.2 MPa and 323 K. The products include cyclohexanone oxime, cyclohexylamine, cyclohexanol and N- cyclohexylhydroxylamine. It has been found that higher temperature is in favor of the formation of cyclohexylamine, while the amount of cyclohexanol decreases with the increment of reaction temperature.

Full text of DOI:10.1016/j.catcom.2011.12.027

Catalysis Communications 19 (2012) 80–84
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Hydrogenation of nitrocyclohexane to cyclohexanone oxime over Pd/CNT catalyst
under mild conditions
a
b
a,
Hong-Guang Liao a, Yan-Juan Xiao a, Hai-Ke Zhang a, Ping-Le Liu a, , Kui-Yi You , Chao Wei , He'an Luo
⁎
⁎
a
College of Chemical Engineering, Xiangtan University, Xiangtan, 411105, China
b
College of Chemical and Biological Engineering, Changsha University of Science & Technology, Changsha, 410004, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 October 2011
Received in revised form 12 December 2011
Accepted 20 December 2011
Available online 29 December 2011
The Pd/C, Pt/C, Ni/CNT and Pd/CNT catalysts were prepared by impregnation method and characterized by
BET, XRD, TEM and H2 chemisorption. These catalysts were tested in the hydrogenation of nitrocyclohexane
to cyclohexanone oxime. The results show that 5% Pd/CNT catalyst exhibits good performance, it gives nitro-
cyclohexane conversion of 97.6% and cyclohexanone oxime selectivity of 85.9% under mild conditions of
0.2 MPa and 323 K. The products include cyclohexanone oxime, cyclohexylamine, cyclohexanol and
N-cyclohexylhydroxylamine. It has been found that higher temperature is in favor of the formation of cyclo-
hexylamine, while the amount of cyclohexanol decreases with the increment of reaction temperature.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Nitrocyclohexane
Hydrogenation
Pd/CNT catalyst
Cyclohexanone oxime
1. Introduction
have prepared a decorated Pt/TiO2 catalyst doped with sodium cation,
which could reduce nitrocyclohexane to cyclohexanone oxime with
ε-Caprolactam is an important petrochemical primarily used to syn-
thesize nylon-6 fibers and resins. The current commercial production of
ε-caprolactam is mainly based on the traditional route, cyclohexane is
oxidized to cyclohexanone, and then cyclohexanone can be further con-
verted to cyclohexanone oxime and lactam via ammoximation and
Beckmann Rearrangement [1]. The processes for the production of cyclo-
hexanone oxime mainly include cyclohexanone–hydroxylamine and
ammoximation of cyclohexanone [2–4]. It should be noted that cyclo-
hexanone is obtained from cyclohexane oxidation, which is a very low
efficient process with per pass yield of 4–5% without catalyst and
8–10% catalyzed by cobalt catalyst [5,6]. Moreover, the processes for
the production of cyclohexanone oxime involve multi-step reactions
and result in the formation of a large amount of ammonium sulfate
and wastes. To overcome the above-mentioned problems, there have
been many attempts to find novel and environmental friendly methods
for the production of cyclohexanone oxime. One step hydrogenation of
nitrocyclohexane to cyclohexanone oxime is an alternative method.
However, previous studies show that the process catalyzed by a sup-
ported Pd catalyst in the presence of PbO salts or salts of Cu (II) and Ag
(I) homogeneous catalyst has drawbacks such as high pressure and the
formation of environmental unfriendly inorganic wastes [7–9]. Recent
progresses in the nitration of cyclohexane to nitrocyclohexane with
high selectivity urge the research interest on one step hydrogenation
of nitrocyclohexane to cyclohexanone oxime [10,11]. Serna et al. [12]
a selectivity of 84.5% at 95% conversion under 4.0 MPa and 383 K.
Shimizu et al. [13] have developed a heterogeneous Au/Al2O3 catalyst
on selective hydrogenation of nitrocyclohexane, which exhibits high cy-
clohexanone oxime yield under 0.6 MPa and 373 K.
Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have
received increasing attention since they were discovered in 1991
[14]. The outstanding properties of CNTs and CNFs make them
become one of the most promising catalyst support materials for het-
erogeneous catalytic reactions, especially for liquid-phase reactions
[15–17]. The first study that CNTs and CNFs were used as catalyst sup-
ports started from 1994 [18]. CNTs and CNFs supported components
such as Pd, Pt, Ru, Rh, Fe or Ni are mainly used in catalytic hydrogena-
tion processes [16–28]. Geus et al. have reported that CNTs and CNFs
materials undergone suitable treatment can be good candidates as
the supports for Pd in liquid phase medium [20]. However, there
are no reports on the study of partial hydrogenation of nitrocyclohex-
ane to cyclohexanone oxime catalyzed by Pd/CNTs. In this paper, Pd/
CNTs were prepared and characterized, and their catalytic properties
in nitrocyclohexane hydrogenation under mild conditions of lower
temperature and pressure are performed.
2. Experimental
2.1. Materials
Nitrocyclohexane (97 wt.%) was purchased from Tokyo Chemical
Industry Corporation Limited. CNTs were purchased from Shenzhen
Nanotech Port Corporation Limited and active carbon was purchased
⁎
Corresponding authors. Tel.: +86 73158293545; fax: +86 73158298267.
E-mail addresses: liupingle@xtu.edu.cn (P.-L. Liu), heanluo@126.com (H. Luo).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.12.027

H.-G. Liao et al. / Catalysis Communications 19 (2012) 80–84
81
from Baoji Rock New Materials Corporation Limited. PdCl2 and ethy-
lenediamine were analytical grade and purchased from Sinopharm
Chemical Reagent Corporation Limited. H2 (99.99%) was provided
by Zhuzhou Diamond Gas Company.
2.2. Catalyst preparation
The CNTs and active carbon were pretreated in concentrated
nitric acid (68 wt.%) overnight, recovered by vacuum filtration,
washed with distilled water, and dried in vacuum at 383 K for
12 h.
Pd/CNT catalyst was prepared by incipient impregnation method.
PdCl2 was used as the precursor of the active component. 0.2 g PdCl2
was dissolved into a solution of 1.2 g concentrated hydrochloric acid
(38 wt.%) and 10 g distilled water under ultrasonication for 30 min.
Then the sodium hydroxide solution (10 wt.%) was dropped into
the above solution to regulate pH to 5. Afterwards, the pretreated
CNTs were impregnated in the above prepared H2PdCl4 solution for
10 h at 303 K under stirring. Finally, the mixture was dried at 383 K
for 12 h under vacuum and then was calcinated at 473 K for 4 h
under 40 mL/min of nitrogen flow and reduced at 523 K for 3 h
under 40 mL/min of hydrogen. Pd/C catalyst was prepared by the
similar method except that the support was replaced by active car-
bon. Pt/C catalyst was prepared by using H2PtCl6·6H2O as precursor
and Ni/CNT catalyst was prepared by using nickel acetate as
precursor.
Fig. 1. N2 adsorption–desorption isotherms and BJH pore size distributions (inset) of
5% Pd/C and 5% Pd/CNTs.
3. Results and discussions
3.1. Characterization of catalysts
Fig. 1 shows the nitrogen adsorption–desorption curves of 5% Pd/
CNTs and 5% Pd/C. Pd/C exhibits a much higher nitrogen adsorption
capacity than Pd/CNTs. The isotherm of Pd/C shows an adsorption iso-
therm of type I according to the IUPAC classification, representing
capillary condensation of nitrogen within the uniform microporous
structure. Pd/CNTs show an adsorption isotherm of type III, which in-
dicates weak interaction between Pd/CNTs and nitrogen. The hyster-
esis loop of Pd/C is of type H4 which is associated with slit-shaped
pores and Pd/CNTs are of type H1 which is attributed to cylindrical
pores with both sides open. According to the geometrical effect and
Kelvin equation, bigger area of the hysteresis loop indicates more
pores, which is in good agreement with the pore volume in Table 1.
Higher relative pressure at the closure point of the hysteresis loops
indicates bigger pore diameter, which is also consistent with the
pore diameter distribution in Fig. 1. A broad pore size distribution is
observed in Pd/CNTs, and its average pore size calculated from the de-
sorption branch by the BJH model is 17.8 nm. However, Pd/C shows a
narrow pore size distribution with the average pore size of 1.7 nm. It
can be seen from Table 1 that the BET surface area of Pd/C
(907 m2 g−1) is much larger than Pd/CNTs (72 m2 g−1). The TEM im-
ages of Pd/CNTs and Pd/C are shown in Fig. 2. The TEM micrographs of
Pd/CNTs and Pd/C show that palladium particles are well dispersed
on the surface of the supports. Statistical results demonstrate that
palladium particle size in Pd/C ranges from 2.9 to 5.1 nm, and palladi-
um particle in Pd/CNTs is slightly smaller than that in Pd/C ranging
from 2.2 to 4.4 nm. Fig. 3 presents the XRD patterns of Pd/CNTs and
Pd/C. Both of the samples exhibit significant feature diffraction peak
around 2θ=25° corresponding to (002) crystalline plane of charcoal.
2.3. Catalyst characterization
Specific surface area, pore volume and pore size distribution of
the samples were obtained from the nitrogen adsorption–desorption
on a Quantachrome NOVA-2200e automated gas sorption system.
Powder X-ray diffraction (XRD) patterns were determined
under an Aolong Y-2000 diffractometer using Cu Kα radiation
(λ=1.542 Å). The tube voltage was 40 kV, the current was
30 mA, and the scan range was 2θ=5–90° with a scanning rate of
1°min− 1
.
The microstructure of the Pd/CNTs was observed by transmission
electron microscopy (TEM) on a Tecnai G2 20 ST electron microscope
working under 200 kV. The instrumental magnification ranged from
2×104 to 10×106. The samples were deposited on a copper grid
and coated with a holey carbon film.
Hydrogen chemisorption was measured on a TP-5080 automated
sorption system. The samples had been previously reduced under
the same conditions of the catalyst preparation and the hydrogen
chemisorption was performed at 323 K.
2.4. Procedures for the catalytic test
Nitrocyclohexane hydrogenation was carried out in a 50 mL
Teflon-lined stainless-steel autoclave equipped with temperature
detector and pressure control. Typically, 1.29 g nitrocyclohexane
(97 wt.%), 10 mL ethylenediamine solvent and 0.2 g catalyst were
added into the autoclave. The reactor was sealed, purged with H2 to
replace the air, and then it was heated to the required temperature
and pressurized to 0.2 MPa with H2 under stirring. The catalysts
were removed from the solution by filtration when the reaction was
finished. The products were identified by GC–MS (SHIMADZU,
QP2010 PLUS) and by comparison with commercially pure products.
The contents of the raw material and the products were determined
by GC (GC-14C, SHIMADZU) with a flame ionization detector (FID)
and a 30 m DB-1701 capillary column using naphthalene as an inter-
nal standard.
Table 1
Textural properties of different catalysts.
Catalysts
BET surface areas
Pore diameter
(nm)
Pore volume
(m2 g−1
)
(cm3 g−1
)
5% Pd/C
5% Pt/C
20% Ni/CNTs
5% Pd/CNTs
907
1058
65
1.7
1.4
18.0
17.8
0.41
0.63
0.29
0.31
72

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H.-G. Liao et al. / Catalysis Communications 19 (2012) 80–84
Fig. 2. TEM images of 5% Pd/C (a, b) and 5% Pd/CNTs (c, d).
There are very weak peaks of Pd presenting on the pattern of Pd/
CNTs, which indicates that the Pd particles are well dispersed on
CNTs. In the XRD pattern of Pd/CNTs, there are seven strong and
spiculate peaks which are attributed to sodium chloride formed
during the procedure of catalyst preparation. XRD peaks of Pd in the
Pd/CNTs are broader than those in Pd/C, which further confirms the
fact of smaller average particle size of Pd on CNT support. Table 2
shows the hydrogen chemisorption measurements of 5% Pd/C and
5% Pd/CNTs. The results also indicate that the dispersion of palladium
particles on CNTs is higher than that on carbon, which are in accord
with the results of XRD patterns and TEM images.
3.2. Catalytic performance
The prepared catalysts were tested in liquid phase hydrogenation
of nitrocyclohexane to cyclohexanone oxime under mild conditions of
0.2 MPa and 323 K in a Teflon-lined stainless-steel autoclave using
ethylenediamine as solvent. The effects of H2 pressure and agitation
rate were investigated and the results show that the selectivity of
cyclohexanone oxime increases with the increments of H2 pressure
from atmosphere pressure to 0.2 MPa and agitation rate from
500 rpm to 1100 rpm. However, the cyclohexanone oxime selectivity
has no significant change with the further increments of pressure and
agitation rate. Therefore, the hydrogenation of nitrocyclohexane was
tested under the conditions of 0.2 MPa and 1100 rpm.
Nitro compound hydrogenation on metal catalysts depends not
only on the nature of metal, but also on the crystallite size and the
reactant interacts with the support [29–31]. It was reported that
Pt/Al2O3, Pt/C or Pd/C was able to catalyze nitrocyclohexane hydroge-
nation to cyclohexanone oxime using ethanol as solvent. However, it
is of very low selectivity to cyclohexanone oxime (less than 25%) and
the corresponding amine is the main product through the complete
reduction of the nitro group [12]. It has been reported that Pt nano-
particles decorated with TiOx and modified with sodium cations im-
proved cyclohexanone oxime selectivity to 84.5% at 95% conversion
under 4.0 MPa and 383 K using ethanol as solvent [12]. Au/Al2O3
with smaller Au particle size (2.5 nm) exhibited high cyclohexanone
oxime yield under 0.6 MPa and 373 K also using ethanol as solvent
[13]. We test this reaction with Ni, Pd, Pt supported catalysts using
ethylenediamine as solvent under mild conditions of 0.2 MPa and
323 K. As shown in Table 3, besides cyclohexanone oxime, the
main by-products include cyclohexylamine, cyclohexanol and N-
cyclohexylhydroxylamine. The N-cyclohexylhydroxylamine is testi-
fied to be an intermediate, which mainly can be further hydrogenated
to cyclohexylamine. Ni/CNT catalyst is able to catalyze nitrocyclohexane
Fig. 3. XRD patterns of 5% Pd/C and 5% Pd/CNTs.

H.-G. Liao et al. / Catalysis Communications 19 (2012) 80–84
83
Table 2
Hydrogen chemisorption datas of 5% Pd/C and 5% Pd/CNTs.
Catalysts
H2 uptake
Metallic surface areas
Dispersion
(%)
(μmol g−1
)
(m2 g−1
)
5% Pd/C
5% Pd/CNTs
16.3
22.1
39.4
53.5
6.9
9.4
Table 3
The results of nitrocyclohexane hydrogenation over different catalysts.
Catalyst
Time
(h)
%Conversion
(NCH)
%Selectivity
CHO
CHA
CHOL
CHOA
5% Pd/C
5% Pt/C
20% Ni/CNTs
5% Pd/CNTs
3
1
6
6
100
55.1
36.4
21.6
85.9
6.2
6.1
28.9
5.0
7.9
10.5
22.8
2.9
30.8
47.0
26.7
6.2
76.3
62.9
97.6
Fig. 5. The effect of reaction temperature on nitrocyclohexane hydrogenation over 5%
Pd/CNTs. Reaction conditions: stirring rate, 1100 rpm; PH2 =0.2 MPa; t=6 h.
NCH—nitrocyclohexane; CHO—cyclohexanone oxime; CHA—cyclohexylamine; CHOL—
cyclohexanol; CHOA—N-cyclohexylhydroxylamine.
Reaction conditions: T=323 K, PH2 =0.2 MPa.
the influence of Pd loading amount as well as reaction time and reaction
temperature. Results in Fig. 4 show that 5% Pd/CNTs give nitrocyclohex-
ane conversion of 100% and cyclohexanone oxime selectivity of 71% at
7.5 h under 0.2 MPa and 323 K. It gives the best result of nitrocyclo-
hexane conversion of 97.6% and cyclohexanone oxime selectivity of
85.9% at 6 h. The results show that the catalytic activity decreases
with the decrement of the Pd loading amount. The nitrocyclohexane
conversion is only 47.2% under the catalysis of 3% Pd/CNTs for 8 h,
and it reaches to 99.4% till 24 h with the cyclohexanone oxime selec-
tivity of 82.8%.
The effects of reaction time and reaction temperature over 5%
Pd/CNTs were discussed. Nitrocyclohexane hydrogenation over 5%
Pd/CNTs at different reaction time was carried out at 323 K. As
shown in Fig. 4, as the reaction time proceeds from 1 h to 2 h, the
conversion of nitrocyclohexane increases greatly from 47.2% to
82.9%, and the nitrocyclohexane is completely converted at 7.5 h.
The selectivity to cyclohexanone oxime increases gradually from
71.0% to 85.9% and then declines to 71.8% with the extending of
the reaction time. It is confirmed that the performance of catalyst
is related to the size and the surface area of the active metal nano-
particles and its support, which could regulate the adsorption and
activation of substrates and desorption of products by controlling the
number of corner, edge and face atoms of the catalysts [32–34].
Compared with Pd/C and Pt/C catalyst, the smaller Pd particle size,
higher Pd surface area and CNTs support are helpful to the production
of cyclohexanone oxime.
to cyclohexanone oxime with lower selectivity, but more cyclohexyl-
amine and cyclohexanol are obtained. The nitrocyclohexane conversion
achieves to 76.3% with cyclohexanone oxime selectivity of 36.4% cata-
lyzed by Pt/C for 1 h. The catalytic performance of Pd/C is better than
Pt/C, the nitrocyclohexane is completely converted after 3 h and the se-
lectivity to cyclohexanone oxime increases to 55.1%. Pd/CNT catalyst
gives the best result of nitrocyclohexane conversion of 97.6% and cyclo-
hexanone oxime selectivity of 85.9%, along with a small amount of cy-
clohexylamine and cyclohexanol. The reason may be that smaller Pd
particle size and higher Pd surface area on CNTs present higher activity
and local higher hydrogen concentration caused by higher H2 adsorp-
tion capacity in Pd/CNTs is of benefit to the formation of cyclohexanone
oxime. In our work, ethylenediamine was tested to be a more suitable
solvent than ethanol, acetonitrile and cyclohexylamine under the catal-
ysis of Pd/CNTs and Pd/C, which is different from the report of P. Serna
et al. [12] and K.-i. Shimizu et al. [13] using ethanol as solvent under the
catalysis of Pt/TiO2 and Au/Al2O3. The results show that the cyclohexa-
none is not detected according to the GC–MS and the comparison with
commercially pure product. Therefore, cyclohexyl-cyclohexylidene
amine and 2-cyclohexylidene cyclohexanone generated from cyclohex-
anone do not emerge. Since Pd/CNT catalyst gives better results for
nitrocyclohexane hydrogenation to cyclohexanone oxime, we focus on
The effect of reaction temperature on the nitrocyclohexane hy-
drogenation over 5% Pd/CNTs was investigated in the temperature
range from 313 K to 353 K for 6 h. It can be seen from Fig. 5 that
the nitrocyclohexane conversion increases with the increment of re-
action temperature under the same reaction time and pressure,
however, the selectivity to cyclohexanone oxime achieves to the
maximum value of 85.9% at 323 K and then decreases with the in-
crement of reaction temperature. Results in Fig. 5 show that higher
temperature is in favor of the formation of cyclohexylamine, while
the amount of cyclohexanol decreases with the increment of reac-
tion temperature.
4. Conclusions
The Pd/C, Pt/C, Ni/CNT and Pd/CNT catalysts were prepared by
impregnation method and characterized by BET, XRD, TEM and H2
chemisorption. 5% Pd/CNT catalyst exhibits good performance on
nitrocyclohexane hydrogenation to cyclohexanone oxime, it gives
nitrocyclohexane conversion of 97.6% and cyclohexanone oxime
Fig. 4. The effect of reaction time on nitrocyclohexane hydrogenation over 5% Pd/CNTs.
Reaction conditions: stirring rate, 1100 rpm; PH2 =0.2 MPa; T=323 K.

84
H.-G. Liao et al. / Catalysis Communications 19 (2012) 80–84
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selectivity of 85.9% under mild conditions of 0.2 MPa and 323 K. The
effects of Pd loading amount, reaction time and reaction temperature
are discussed, higher temperature is in favor of the formation of
cyclohexylamine and the amount of cyclohexanol decreases with
the increment of reaction temperature.
Acknowledgments
This work was supported by the NSFC (20976148), program
for New Century Excellent Talents in University (NCET-10-0168),
Scientific Research Fund of Hunan Provincial Education Department
(09CY007, CX2011B264) and Project of Hunan Provincial Science &
Technology Department (2010WK2008, 2010GK3169).
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