Palladium supported catalysts for nitrocyclohexane hydrogenation to cyclohexanone oxime with high selectivity

Article abstract of DOI:10.1002/cctc.201300377

Different kinds of activated carbon- and carbon nanotube-supported palladium catalysts were investigated in the selective hydrogenation of nitrocyclohexane to cyclohexanone oxime under mild conditions. Carbon nanotube-supported palladium catalysts demonstrate better catalytic performance than activated carbon-supported palladium catalysts in general because of their mesoporous structures, which are favorable supports for the accessibility of the reactants to the active sites and the product desorption from the catalyst. Hydrogen chemisorption, transmission electron microscopy and X-ray photoelectron spectroscopy indicate that higher composition of Pd⁺ on the catalyst surface, larger palladium surface area, and better palladium dispersion contribute to an increase in the activity and selectivity toward cyclohexanone oxime. In addition, single-wall carbon nanotube-supported palladium catalysts give the best result of 97.7% conversion of nitrocyclohexane and 97.4% selectivity toward cyclohexanone oxime. On the basis of the results of GC-MS and the designed experiments, a possible reaction scheme was proposed. Brilliance in the performance: Carbon nanotube-supported palladium catalysts demonstrate better catalytic performance than activated carbon-supported palladium catalysts in general because of their mesoporous structures, which are favorable supports for the accessibility of the reactants to the active sites and the product desorption from the catalyst.

Full text of DOI:10.1002/cctc.201300377

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DOI: 10.1002/cctc.201300377
Palladium Supported Catalysts for Nitrocyclohexane
Hydrogenation to Cyclohexanone Oxime with High
Selectivity
Ping-Le Liu,* Hai-Ke Zhang, Si-Hua Liu, Zheng-Jie Yao, Fang Hao,* Hong-Guang Liao, Kui-
Yi You, and He-An Luo[a]
Different kinds of activated carbon- and carbon nanotube-sup-
ported palladium catalysts were investigated in the selective
hydrogenation of nitrocyclohexane to cyclohexanone oxime
under mild conditions. Carbon nanotube-supported palladium
catalysts demonstrate better catalytic performance than acti-
vated carbon-supported palladium catalysts in general because
of their mesoporous structures, which are favorable supports
for the accessibility of the reactants to the active sites and the
product desorption from the catalyst. Hydrogen chemisorp-
tion, transmission electron microscopy and X-ray photoelec-
tron spectroscopy indicate that higher composition of Pd+ on
the catalyst surface, larger palladium surface area, and better
palladium dispersion contribute to an increase in the activity
and selectivity toward cyclohexanone oxime. In addition,
single-wall carbon nanotube-supported palladium catalysts
give the best result of 97.7% conversion of nitrocyclohexane
and 97.4% selectivity toward cyclohexanone oxime. On the
basis of the results of GC–MS and the designed experiments,
a possible reaction scheme was proposed.
Introduction
Cyclohexanone oxime (CHO) can be used to produce e-capro-
lactam, which is an important precursor for the manufacture of
nylon-6 and plastics. The current commercial production of cy-
clohexanone oxime is mainly based on cyclohexane oxidation
and cyclohexanone (CHE) ammoximation, as demonstrated
previously.[1,2] Of these synthetic processes, cyclohexane oxida-
tion is a less efficient process, with per pass yield of 4–5%
without a catalyst and 8–10% with cobalt catalysts.[3] More-
over, this commercial process for the production of cyclohexa-
none oxime could produce a large amount of ammonium sul-
fate as a byproduct.[4–7] Therefore, many researchers have tried
to develop new methods for the production of cyclohexanone
oxime. Raja et al. have developed a process for the production
of cyclohexanone oxime and e-caprolactam from cyclohexa-
none with use of bifunctional heterogeneous nanoporous alu-
minophosphate catalysts.[8] However, the starting material is
still cyclohexanone, which is obtained from cyclohexane oxida-
tion. An alternative promising method for the production of
cyclohexanone oxime is the nitrocyclohexane (NCH) hydroge-
nation process developed by DuPont.[5,9] However, this process
catalyzed by a supported palladium catalyst in the presence of
lead(II) oxide with approximately 70% yield of cyclohexanone
oxime requires high pressure and environmentally unfriendly
lead(II) oxide salts. Recent progresses in nitration technology
have made it possible to obtain nitrocyclohexane with high
selectivity and yield, which promotes the research interest in
the hydrogenation of nitrocyclohexane to cyclohexanone
oxime.[10,11] Serna et al. have developed a sodium-doped Pt/
TiO2 catalyst for the hydrogenation of nitrocyclohexane, with
a cyclohexanone oxime yield of 80.3% at a selectivity of 84.5%
and a nitrocyclohexane conversion of 95% at 4.0 MPa and
383 K.[12] Au/Al2O3 has been investigated by Shimizu et al. in
the selective hydrogenation of nitrocyclohexane, and the cata-
lyst with smaller size gold particles demonstrates a high cyclo-
hexanone oxime yield (86%) at 0.6 MPa and 373 K.[13]
Carbon nanotubes (CNTs) have been widely used as sup-
ports for heterogeneous catalytic reactions since their discov-
ery in 1991.[14–17] They are suitable for both gas phase and
liquid phase reactions and could be easily separated from the
products; moreover, they can efficiently suppress the occur-
rence of side reactions.[18–20] It has been proposed that CNTs
could be favorable supports for palladium in liquid phase reac-
tions after a suitable treatment, which is due to their efficient
improvement in critical mass transfer on their large external
surface area.[21,22] Furthermore, they can boost the selectivity of
the desired reactions.[23] Our research group used Pd/CNTs as
catalysts for the hydrogenation of nitrocyclohexane to cyclo-
hexanone oxime, which gives a nitrocyclohexane conversion of
97.6% and a cyclohexanone oxime selectivity of 85.9% under
mild conditions of 0.2 MPa and 323 K[24]; however, this is still
unsatisfactory. Herein, we report the preparation and charac-
terization of several different types of CNT- and activated
[a] Prof. P.-L. Liu, H.-K. Zhang, S.-H. Liu, Z.-J. Yao, Dr. F. Hao, H.-G. Liao,
Dr. K.-Y. You, Prof. H.-A. Luo
College of Chemical Engineering
Xiangtan University
Xiangtan, 411105 (P.R. China)
Fax: (+86)731-58298267
E-mail: liupingle@xtu.edu.cn
haofang.happy@163.com
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300377.
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carbon (AC)-supported palladium catalysts and study the ef-
fects of different supports on the physical properties of the
catalysts and their catalytic performance in the selective hydro-
genation of nitrocyclohexane to cyclohexanone oxime.
Results and Discussion
Characterization of the catalysts
The palladium loading dose of Pd/MWCNTs-3, Pd/SWCNTs, and
Pd/LAC catalysts (for details on these catalysts, see the Experi-
mental Section) determined by using atomic absorption spec-
trometry (AAS) was 4.82, 4.80, and 4.78%, respectively. After
the Pd/SWCNT catalyst was used four times, its palladium load-
ing dose was 4.24%.
Figure 1. N2 adsorption–desorption isotherms and BJH pore size distribu-
~
&
*
tions of a) Pd/MWCNTs-3 ( ), b) Pd/SWCNTs ( ), and c) Pd/LAC ( ).
The BET surface area, pore diameter, and pore volume of dif-
ferent types of AC- and CNT-supported palladium catalysts are
summarized in Table 1. It can be observed that the pore diam-
supports have a great effect on their catalytic performance.
The surface channels of the AC are easy to block because of
their microporous pore size, which leads to the low content of
active sites on the surface. However, the surface channels of
CNTs could not be blocked easily because of their mesoporous
pore size.[25,26] The catalytic tests reveal that all CNT-supported
palladium catalysts demonstrate better catalytic performance
than AC-supported palladium catalysts. Thus, the support with
larger pore diameter and pore volume may have a positive
effect on the hydrogenation of nitrocyclohexane to cyclohexa-
none oxime.
Table 1. Textural properties of different catalysts.
Catalyst
Surface area
Pore diameter
[nm]
Pore volume
[cm3 gÀ1
]
[m2 gÀ1
]
Pd/LAC
Pd/CAC
Pd/CSAC
Pd/MWCNTs-1
Pd/MWCNTs-2
Pd/MWCNTs-3
Pd/SWCNTs
Pd/DWCNTs
240.3
363.8
187.9
102.7
81.8
71.9
277.6
257.3
0.96
0.95
1.09
2.44
2.71
2.43
2.73
2.73
0.21
0.30
0.11
0.56
0.31
0.73
0.60
0.55
The XRD patterns of Pd/LAC and five types of CNT-support-
ed palladium catalysts are shown in Figure 2. The crystalline
diffraction peak appearing at 2q=268 in the XRD patterns is
eters of the three AC-supported palladium catalysts are be-
tween 0.95 and 1.09 nm, which is the microporous structure
range. Of these three catalysts, the Pd/CAC catalyst has the
largest surface area of 363.8 m2 gÀ1 and pore volume of
0.3 cm3 gÀ1. CNT-supported palladium catalysts belong to the
mesoporous structure range and have larger pore volume
(0.31–0.73 cm3 gÀ1) and pore diameter (2.43–2.73 nm) than AC-
supported palladium catalysts.
This can also be confirmed through Barrett–Joyner–Halenda
(BJH) pore size distributions and nitrogen adsorption–desorp-
tion isotherms (Figure 1) of Pd/MWCNTs-3, Pd/SWCNT, and Pd/
LAC catalysts. The Pd/LAC catalyst (Figure 1c) shows an ad-
sorption isotherm of type I, which indicates the capillary con-
densation of nitrogen within the uniform microporous struc-
ture. The isotherms of Pd/MWCNTs-3 (Figure 1a) and Pd/
SWCNTs (Figure 1b) are of type III, which indicates that nitro-
gen covers the entire surface and no microholes exist in the
CNT supports; moreover, the interaction between the supports
and palladium plays the dominant role. Of the five types of
CNT-supported palladium catalysts, Pd/SMCNTs and Pd/
DWCNTs have much large surface areas (277.6 and
257.3 m2 gÀ1) than those of multiwall CNT-supported palladium
catalysts. Furthermore, the surface areas of all multiwall CNT-
supported palladium catalysts are smaller than those of AC-
supported palladium catalysts. The textural properties of the
Figure 2. XRD patterns of a) Pd/LAC, b) Pd/MWCNTs-1, c) Pd/MWCNTs-2,
~
&
*
d) Pd/MWCNTs-3, e) Pd/SWCNTs, and f) Pd/DWCNTs: C ( ), Na ( ), Pd ( ).
ascribed to carbon; the peaks at 2q=32, 45, and 578 are as-
cribed to sodium; and the peaks at 2q=40 and 478 are as-
cribed to palladium. The diffraction peak of carbon at 2q=268
(Figure 2a) is unclear in Pd/LAC, and there are distinct diffrac-
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tion peaks of carbon in Pd/MWCNTs-1 (Figure 2b), Pd/
MWCNTs-2 (Figure 2c), and Pd/MWCNTs-3 (Figure 2d). Howev-
er, the diffraction peaks of carbon at 2q=268 are weak in Pd/
SWCNTs (Figure 2e) and Pd/DWCNTs (Figure 2 f), which indi-
cates that the carbon crystal particles are well dispersed. The
characteristic diffraction peaks of palladium are different in Pd/
LAC and CNT-supported palladium catalysts. There are no clear
diffraction peaks of palladium in the Pd/LAC catalyst, which in-
dicates the existence of well-dispersed palladium phases. Two
relatively clear diffraction peaks of palladium at 2q=40 and
478 appear in Pd/SWCNTs and Pd/DWCNTs, and only one dif-
fraction peak of palladium at 2q=408 is detected in three mul-
tiwall CNT-supported palladium catalysts. Thus, the different
types of supports have effects on the morphology, structure,
and dispersion of crystalline palladium, which further affects
their catalytic performance in the hydrogenation of nitrocyclo-
hexane.
compositions of Pd/SWCNTs, Pd/DWCNTs, and Pd/MWCNTs-3
are given in Table 3. It can be observed that Pd/SWCNTs have
the highest content of Pd+ on the catalyst surface.
The TEM images of Pd/LAC and different types of CNT-sup-
ported palladium catalysts are shown in Figure 3. The surface
structures of the prepared catalysts have diverse forms owing
to their different supports. Pd/MWCNTs-1 (Figure 3b), Pd/
MWCNTs-2 (Figure 3c1 and c2), and Pd/MWCNTs-3 (Figure 3d1
and d2) catalysts consist of graphite planes in a fishbone stack-
ing arrangement. The parallel graphite layer can be observed
in Pd/SWCNTs (Figure 3e) and Pd/DWCNTs (Figure 3 f). The
structures of the supports have a great effect on the size, mor-
phology, and crystal of the active component. Under the same
preparation conditions of the catalysts, the statistical analysis
data reveal that the palladium particle size is approximately
2.9–5.1 nm in Pd/LAC; approximately 2.2–4.4 nm in Pd/
MWCNTs-1, Pd/MWCNTs-2, and Pd/MWCNTs-3; and approxi-
mately 2.0–3.0 nm in Pd/SWCNTs and Pd/DWCNTs; the last one
is smaller than others. The palladium particles are in the shape
of circle or regular polygon and are embedded within the
supports.
The hydrogen chemisorption data of different catalysts are
summarized in Table 2. It can be observed that CNT-supported
palladium catalysts demonstrate better dispersion, larger hy-
drogen uptake quantity, and larger metallic surface areas than
AC-supported palladium catalysts. Of the different CNT-sup-
ported palladium catalysts, Pd/SWCNTs demonstrate the best
dispersion, the largest hydrogen uptake quantity, and the larg-
est metallic surface area, which may favor the nitrocyclohexane
hydrogenation process.
The chemical compositions of Pd/MWCNTs-3, Pd/DWCNTs,
and Pd/SWCNTs were characterized by using X-ray photoelec-
tron spectroscopy (XPS). The XPS core level spectra of Pd3d of
the above catalysts are shown in Figure 4. The Pd3d spectra
are composed of two peaks: the right peak around 335 eV is
attributed to the Pd3d5/2 spectral peak, and the left peak
around 341 eV belongs to the Pd3d3/2 spectral peak. The peaks
around 335 and 337.6 eV are assigned to Pd0 and Pd2+, respec-
tively. The binding energy peak of the core level and a narrow-
ing of the valence band belonging to the bulk metal would be
shifted.[27–29] In highly dispersed samples, the shift in the core
level binding energies can be 1.0–1.3 eV. Thus, another peak
around 335.8 eV is presumed to belong to Pd+. The surface
Figure 3. TEM images of a) Pd/LAC, b) Pd/MWCNTs-1, c) Pd/MWCNTs-2,
d) Pd/MWCNTs-3, e) Pd/SWCNTs, and f) Pd/DWCNTs.
Table 2. Hydrogen chemisorption data for different catalysts.
Catalyst
H2 uptake[a]
Metallic SA[b]
[m2 gÀ1
]
Dispersion
[%]
[mLgÀ1
]
Pd/SWCNTs
Pd/MWCNTs-3
Pd/DWCNTs
Pd/LAC
63.6
54.5
49.3
42.8
120.7
103.5
93.5
27.1
23.2
20.9
18.2
81.2
[a] H2 uptake/cumulative quantity. [b] Metallic surface area.
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Table 4. Nitrocyclohexane hydrogenation catalyzed by different support-
ed catalysts.[a]
Catalyst
Conv. [%]
NCH
Selectivity [%]
CHO
CHA
CHL
CHE
Pd/LAC
Pd/CAC
Pd/CSAC
98.9
99.2
91.0
99.8
99.6
99.9
98.1
97.3
97.3
97.8
95.5
55.0
60.5
38.9
64.2
78.5
87.9
83.5
94.6
94.6
91.4
84.7
5.0
5.6
4.6
11.5
1.0
3.4
0.2
0.3
0.2
0.7
2.6
4.6
3.2
2.9
2.4
1.1
4.6
4.5
2.9
4.9
3.7
4.7
0.1
0.1
0.1
Pd/MWCNTs-1
Pd/MWCNTs-2
Pd/MWCNTs-3
Pd/DWCNTs
Pd/SWCNTs
Pd/SWCNTs[b]
Pd/SWCNTs[c]
Pd/SWCNTs[d]
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
0.1
[a] Reaction conditions: nitrocyclohexane 1.3 g; temperature 323 K; time
6 h; pressure 0.3 MPa; ethylenediamine 10 mL; NCH=nitrocyclohexane;
CHO=cyclohexanone oxime; CHL=cyclohexanol; CHE=cyclohexanone;
CHA=cyclohexylamine; [b] Pd/SWCNTs were recycled once; [c] Pd/
SWCNTs were recycled twice; [d] Pd/SWCNTs were recycled thrice.
Figure 4. XPS Pd3d spectra of a) Pd/MWCNTs-3, b) Pd/DWCNTs, and c) Pd/
SWCNTs.
a large fraction of micropores in the catalyst can partially block
the accessibility of the reactants to the palladium sites.[26]
During the nitrocyclohexane hydrogenation process, the reac-
tant nitrocyclohexane needs to be adsorbed onto the palladi-
um site and the reaction product cyclohexanone oxime needs
to leave the active sites in time to avoid further conversion to
other products. Because the molecular sizes of these substan-
ces are close to the size of the micropores of AC-supported
palladium catalysts, the pore diameter is unfavorable for their
adsorption and diffusion. However, the pore diameter of CNT-
supported palladium catalysts is three to four times larger than
the molecular size of the reactants and products. This could be
the reason why CNT-supported palladium catalysts demon-
strate better catalytic performance than AC-supported palladi-
um catalysts in general.
Table 3. Surface compositions of three CNT-supported palladium
catalysts.
Catalyst
Pd0 [%]
Pd+ [%]
Pd2+ [%]
Pd/SWCNTs
Pd/MWCNTs-3
Pd/DWCNTs
29.7
47.5
40.0
39.0
31.1
26.6
31.3
21.5
33.3
Catalytic tests
The results of the hydrogenation of nitrocyclohexane to cyclo-
hexanone oxime catalyzed by eight supported catalysts are
summarized in Table 4. In general, CNT-supported palladium
catalysts demonstrate better catalytic performance toward cy-
clohexanone oxime than AC-supported palladium catalysts
under the same preparation conditions of the catalysts and re-
action conditions. On the basis of the pore diameter and pore
volume data listed in Table 1, AC-supported palladium catalysts
belong to the microporous structure range whereas CNT-sup-
ported palladium catalysts belong to the mesoporous structure
range. The molecular sizes were estimated by using the Gaussi-
an package, and all geometrical optimizations for the reac-
tants, intermediates, and products were performed by using
a DFT hybrid exchange-correlation energy density functional
consisting of the Becke three-parameter exchange energy den-
sity functional and the Lee–Yang–Parr correlation function-
al.[30–34] All calculations were performed at the DFT B3LYP/6-
311+G(d) level by using the Gaussian 03 package (version
E02), with tight SCF convergence and ultrafine integration
grids. The results show that the molecular sizes of the reac-
tants of nitrocyclohexane is 0.65 nm and those of the reaction
products of cyclohexanone oxime, cyclohexanol, cyclohexa-
none, cyclohexylamine, and cyclohexylhydroxylamine are 0.66,
0.69, 0.65, 0.68, and 0.68 nm, respectively. The presence of
Of the five CNT-supported palladium catalysts, Pd/SWCNTs
give the best result of nitrocyclohexane conversion of 97.3%,
cyclohexanone oxime selectivity of 94.6%, and cyclohexanone
oxime yield of up to 92.0% whereas Pd/MWCNTs-1 give a cyclo-
hexanone oxime yield of just 64.1%. The catalyst support plays
an important role in catalytic selectivity, and the same type of
support with different morphology and structure could greatly
affect the catalytic performance. On the basis of the characteri-
zation results of the catalysts, Pd/SWCNTs, Pd/DWCNTs, and
Pd/MWCNTs-3 were compared and analyzed in detail. Of these
three CNT-supported palladium catalysts and one of the AC-
supported palladium catalysts (Pd/LAC), Pd/SWCNTs demon-
strate the best catalytic performance, which are followed suc-
cessively by Pd/MWCNTs-3, Pd/DWCNTs, and Pd/LAC. In addi-
tion to the reason of the microporous and mesoporous struc-
tures of the different kinds of catalyst supports, it can be ob-
served from the hydrogen chemisorption data that Pd/SWCNTs
show the largest hydrogen uptake quantity, the largest metal-
lic surface areas, and the best dispersion of the active compo-
nents, which are also followed successively by Pd/MWCNTs-3,
Pd/DWCNTs, and Pd/LAC.
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The adsorption and reactivity of nitro compounds depend
not only on the nature of the metal but also on the crystallite
size and the way they interact with the support.[35–37] The
MWCNTs, DWCNTs, and SWCNTs have different structures
(Figure 3), and thus the supported palladium demonstrates dif-
ferent morphology, size, and metal–support interaction, which
may affect the reduction state of palladium. The XPS results
have confirmed that the surface composition of Pd+ in Pd/
SWCNTs is the highest, followed by that of Pd/MWCNTs and
Pd/DWCNTs under the same reduction conditions. The results
of the hydrogenation of nitrocyclohexane catalyzed by Pd/
DWCNTs, Pd/MWCNTs, and Pd/SWCNTs show that the selectivi-
ty toward cyclohexanone oxime increase from 83.5 to 87.9 and
94.6%, which indicates that the reduction state of palladium
(Pd+) favors the formation of cyclohexanone oxime and de-
creases the complete hydrogenation to other products.
Scheme 1. A proposed reaction mechanism for the liquid phase hydrogena-
tion of nitrocyclohexane.
tivity toward cyclohexanone oxime is only approximately 72%
at such a low temperature; higher temperature is also not
good for the selectivity toward cyclohexanone oxime. The
effect of the reaction pressure also indicates that the yield of
cyclohexanone oxime first increases and then decreases; it has
the maximum value at 0.5 MPa. A comprehensive research on
the reaction time, pressure, and reaction temperature gives
the best result of nitrocyclohexane conversion of 97.7% and
cyclohexanone oxime selectivity of 97.4%, and the yield of cy-
clohexanone oxime increases to 95.2%, which improves the re-
sults reported in the previous literature.
The proposed reaction scheme
On the basis of the results of GC–MS, it can be found that in
addition to the main product cyclohexanone oxime, other
products were also formed, such as cyclohexylamine, cyclohex-
anol, and cyclohexanone, along with an unstable intermediate
cyclohexylhydroxylamine. The distribution of the products
relies heavily on the catalyst and the large amount of the inter-
mediate, and some extremely small amounts of unquantifiable
products are observed with AC-supported palladium catalysts
(Table 4). From a mechanistic point of view, several reaction
steps are required to produce cyclohexylamine, cyclohexanone,
and cyclohexanol from nitrocyclohexane and they were ex-
pected to appear as secondary products unless the intermedi-
ates react before desorbing from the catalyst surface.[12] Cyclo-
hexylamine could be produced through consecutive hydroge-
nation, with cyclohexylhydroxylamine as the intermediate.[38,39]
However, cyclohexanone oxime was used as the raw material
to proceed with the hydrogenation reaction and extremely
small amounts of cyclohexylamine and cyclohexanol were ob-
served. This was also confirmed. It was found that cyclohexa-
none oxime can be reduced with hydrogen in the presence of
a metal-based catalyst, which forms either cyclohexylamine or
cyclohexanone[22,40–42] and cyclohexanol can be obtained
through the hydrogenation of cyclohexanone. Furthermore, cy-
clohexylhydroxylamine was detected by using GC–MS. Taking
into account the above discussion, we can propose a possible
reaction scheme (Scheme 1) that can illustrate the nitrocyclo-
hexane hydrogenation process.
The results of the catalyst recycle reactions are also summar-
ized in Table 4. After each reaction, the Pd/SWCNT catalyst was
separated through filtration and then used for the next cycle.
However, the catalytic performance has little changes in the
second cycle. To explore the leaching of the metal particles
during the recycle reactions, we separated the catalyst from
the reaction solution and the results of AAS analysis demon-
strate that the active component did not get lost in the solu-
tion. The palladium loading dose of the recycled catalyst was
also tested by using AAS. After the Pd/SWCNT catalyst was
used four times, its palladium loading dose was 4.24%. The
reasons why the catalytic performance decreased may be that
part of the active sites were covered with organic materials,
rather than the metal leaching from the support.
Conclusions
Different kinds of activated carbon- and carbon nanotube
(CNT)-supported palladium catalysts were prepared and char-
acterized by using nitrogen adsorption–desorption, XRD, TEM,
hydrogen chemisorption, X-ray photoelectron spectroscopy,
and atomic absorption spectrometry. The effect of the support
was investigated in the selective hydrogenation of nitrocyclo-
hexane to cyclohexanone oxime on these supported palladium
catalysts. The nitrogen adsorption–desorption and the calcula-
tion results indicate that the CNT support with a mesoporous
structure is favorable for the accessibility of the reactants to
the active sites and the product desorption from the catalyst.
This could be the reason why CNT-supported catalysts demon-
strate better catalytic performance than activated carbon-sup-
ported catalysts in general. The results of hydrogen chemisorp-
Effect of the reaction conditions
We have investigated the effect of reaction time, temperature,
and pressure on nitrocyclohexane hydrogenation catalyzed by
Pd/SWCNTs (Figure 5). The effect of reaction time on the distri-
bution of the products indicates that the yield of cyclohexa-
none oxime reaches the maximum value in 6 h and then de-
creases. The reaction temperature has a great effect on the se-
lectivity toward cyclohexanone oxime. The conversion of nitro-
cyclohexane reaches 98% at 303 K, 0.3 MPa, 6 h, and the selec-
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tion, TEM, and X-ray photoelectron spectroscopy reveal that
higher surface composition of Pd+, larger palladium surface
area, and better palladium dispersion contribute to an increase
in the activity and selectivity toward cyclohexanone oxime. In
addition, the optimization results show that the yield of cyclo-
hexanone oxime reaches 95.2% at a nitrocyclohexane conver-
sion of 97.7% and cyclohexanone oxime selectivity of 97.4%
with Pd/SWCNTs. On the basis of the results of GC–MS, the de-
signed experiments, and the literature, a possible reaction
scheme was proposed.
Experimental Section
Preparation of the catalysts
Three kinds of ACs and five kinds of different structure CNTs were
purchased from Shenzhen Nanometer Gang Co., Ltd., and used as
supports. The ACs made from lignin, coal, and coconut shell were
labeled as LAC, CAC, and CSAC, respectively, and the five different
structure CNTs were multiwall CNTs with different diameters and
lengths, double-wall CNTs, and single-wall CNTs, which were la-
beled as MWCNTs-1, MWCNTs-2, and MWCNTs-3; DWCNTs; and
SWCNTs, respectively. The following is an example to illustrate the
catalyst preparation process by using one of these supports.
The CNTs were first treated with concentrated nitric acid (68 wt%)
at 303 K for 12 h and then filtered and washed several times with
distilled water until the pH reached 7; finally, the CNTs were dried
in vacuum at 383 K for 12 h. The catalyst was prepared by using
the incipient impregnation method. Palladium chloride (PdCl2) was
used as the precursor of the active component. First, PdCl2 (0.2 g)
was dissolved into a solution of distilled water (12 g) and concen-
trated hydrochloric acid (1.2 g, 38 wt%) under ultrasonication for
30 min. Then, the sodium hydroxide solution (10 wt%) was added
into the above solution and the pretreated supports were impreg-
nated in the solution at 303 K for 10 h under stirring. Finally, the
mixture was dried in vacuum at 383 K for 12 h and calcined under
flowing nitrogen at 473 K for 4 h. The catalysts were obtained after
reduction under flowing hydrogen at 523 K for 3 h.
Characterization of the catalysts
Specific surface area, pore volume, and pore size distribution of
the samples were determined by using a Quantachrome’s NOVA
2200e automated gas sorption system. Specific surface areas and
pore size distributions were calculated by using BET and BJH meth-
ods. The XRD data were collected by using a Rigaku D/Max 2550
VB+ 18 kW X-ray diffractometer under the conditions of 40 kV,
30 mA, CuKa radiation, with a scanning rate of 18minÀ1 in the
range of 2q=10–608. The TEM measurements were determined by
using a JEOL JEM-3010 microscope operating at 200 kV, which al-
lowed recording of high-resolution electron microscopy images
with a point-to-point resolution of 0.17 nm. The instrumental mag-
nification ranged from 50 to 1500000, which was measured by
using an Oxford Instruments’ energy dispersive microanalysis
system (energy resolution=133 eV). The types and the amount of
the metal state were analyzed by using a Kratos’s AXIS Ultra DLD
instrument (energy resolution=0.48 eV, imaging space resolu-
tion=3 mm, and minimum analysis area=15 mm). Hydrogen chem-
isorption was measured by using a Quantachrome’s ChemBET
3000 instrument. The catalyst was reduced previously at 523 K for
3 h in hydrogen stream, and the adsorbed hydrogen on the palla-
dium surface was removed under the flowing nitrogen stream. The
Figure 5. Effect of reaction time, temperature, and pressure on nitrocyclo-
&
hexane hydrogenation. Conversion of nitrocyclohexane ( ), selectivity
!
~
toward cyclohexanone oxime ( ), selectivity toward cyclohexanol ( ), selec-
*
tivity toward cyclohexylamine ( ).
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ChemCatChem 2013, 5, 2932 – 2938 2937

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Keywords: cyclohexanone
nanostructures · nitrocyclohexane · supported catalysts
oxime
·
hydrogenation
·
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Published online on July 24, 2013
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ChemCatChem 2013, 5, 2932 – 2938 2938