The influences of preparation methods on the structure and catalytic performance of single-wall carbon nanotubes supported palladium catalysts in nitrocyclohexane hydrogenation

Article abstract of DOI:10.1039/c5ra01318f

Single-wall carbon nanotubes (CNTs) supported palladium catalysts were prepared by different methods. The influences of different preparation methods on the structure and catalytic performance in nitrocyclohexane hydrogenation were investigated. The catalysts were characterized by nitrogen adsorption-desorption, XRD, TEM, hydrogen chemisorption and X-ray photoelectron spectroscopy. The results show that Pd/SWCNTs prepared by a water impregnation method provide a smaller particle size of palladium. The reduction conditions have great influence on the valence state of palladium on the support. A catalyst with smaller particle size, better dispersion and higher content of monovalent palladium exhibits better catalytic performance in nitrocyclohexane hydrogenation to cyclohexanone oxime. Pd/SWCNTs-2 prepared by a water impregnation method and reduced at 723 K has 96.4% selectivity when synthesizing cyclohexanone oxime with a 96.0% conversion from nitrocyclohexane under mild conditions of 0.3 MPa and 323 K.

Full text of DOI:10.1039/c5ra01318f

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PAPER
The influences of preparation methods on the
structure and catalytic performance of single-wall
carbon nanotubes supported palladium catalysts in
nitrocyclohexane hydrogenation
Cite this: RSC Adv., 2015, 5, 22863
Sihua Liu, Fang Hao, Pingle Liu* and He'an Luo
Single-wall carbon nanotubes (CNTs) supported palladium catalysts were prepared by different methods.
The influences of different preparation methods on the structure and catalytic performance in
nitrocyclohexane hydrogenation were investigated. The catalysts were characterized by nitrogen
adsorption–desorption, XRD, TEM, hydrogen chemisorption and X-ray photoelectron spectroscopy. The
results show that Pd/SWCNTs prepared by a water impregnation method provide a smaller particle size
of palladium. The reduction conditions have great influence on the valence state of palladium on the
support. A catalyst with smaller particle size, better dispersion and higher content of monovalent
palladium exhibits better catalytic performance in nitrocyclohexane hydrogenation to cyclohexanone
oxime. Pd/SWCNTs-2 prepared by a water impregnation method and reduced at 723 K has 96.4%
selectivity when synthesizing cyclohexanone oxime with a 96.0% conversion from nitrocyclohexane
under mild conditions of 0.3 MPa and 323 K.
Received 22nd January 2015
Accepted 18th February 2015
DOI: 10.1039/c5ra01318f
www.rsc.org/advances
12
under 0.6 MPa and 373 K. Wang et al. reported gas phase
hydrogenation of nitrocyclohexane over oxide supported gold
1
Introduction
The precursor of Nylon-6 is 3-caprolactam and it is primarily catalysts. Activated carbon and carbon nanotubes supported
1,2
manufactured by a cyclohexanone hydroxylamine route.
palladium catalysts were used in liquid phase hydrogenation of
However, cyclohexanone is obtained from a very low efficiency nitrocyclohexane to cyclohexanone oxime, and the single-wall
cyclohexane oxidation process. Moreover, the cyclohexanone carbon nanotubes supported palladium catalyst prepared by
hydroxylamine route results in the formation of a large amount impregnation method and reduced under 523 K showed a
3,4
of ammonium sulfate and wastes. Researchers have been catalytic performance of 98.9% conversion and 89.5% selectivity
1
3–15
trying to nd novel and environmental friendly methods for to cyclohexanone oxime.
Preparation methods can signi-
5
producing cyclohexane oxime and 3-caprolactam. The route of cantly inuence the physical structure and chemical property of
cyclohexane nitration and reduction to cyclohexanone oxime heterogeneous supported catalysts.
16–19
In this paper, single-
and caprolactam might be a good choice. The reduction process wall carbon nanotubes supported palladium catalysts were
of nitrocyclohexane (NC) is shown in Fig. 1.
The earliest research on nitrocyclohexane (NC) hydrogena-
6,7
tion was reported by DuPont in liquid phase by using a Pd
catalyst to give a 70% yield of cyclohexanone oxime at 413 K and
8,9
3
5 bar. Knion used homogenous Cu (I, II) and Ag (I) catalysts
to reduce nitrocyclohexane with CO to give an 89% oxime yield;
however, the catalyst was difficult to recover and the reaction
10
was carried out under high pressure. Serna et al. reported a Pt/
TiOx catalyst doped with sodium cation which provided 84.5%
selectivity to make cyclohexanone oxime with 95% nitro-
cyclohexane conversion under 4.0 MPa and 383 K. Shimizu
11
2 3
et al. compared an Au/Al O catalyst and the catalyst with
smaller particle sized gold showed better catalytic performance
College of Chemical Engineering, Xiangtan University, Xiangtan, 411105, China.
E-mail: liupingle@xtu.edu.cn; Fax: +86 73158298172; Tel: +86 73158298005
Fig. 1 Nitrocyclohexane hydrogenation process.
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prepared by different methods, and the inuences of different and then dried at 383 K for 12 h under vacuum. The prepared
22
preparation methods on the structure and catalytic perfor- catalyst was labeled as Pd/SWCNTs-6.
mance in nitrocyclohexane hydrogenation were investigated.
2.3 Catalyst characterization
2
.3.1 adsorption–desorption. Specic surface area, pore
N
2
2
Experimental procedures
volume and pore size distribution of the samples were obtained
2.1 Reagents
by nitrogen adsorption–desorption on a Quantachrome NOVA-
2
200e automated gas sorption system. Specic surface areas
Nitrocyclohexane (95 wt%) was purchased from Tokyo Chem-
ical Industry Corporation Limited. Carbon nanotubes were
purchased from Shenzhen Nanotech Port Corporation Limited.
PdCl , KBH , formaldehyde and ethylenediamine were analyt-
ical grade and purchased from Sinopharm Chemical Reagent
Corporation Limited. H2 (99.99%) was provided by Zhuzhou
Diamond Gas Company.
and pore size distributions were calculated by Brunauer–
Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH)
methods.
2
4
2.3.2 X-ray diffraction. Powder X-ray diffraction (XRD)
patterns were determined on a D/max2500 TC diffractometer
˚
using Cu Ka radiation (l ¼ 1.542 A). The tube voltage was 40 kV,
ꢀ
the current was 30 mA, and the scan range was 2q ¼ 5–90 with a
ꢀ
ꢁ1
scanning rate of 1 min
.
2.3.3 Transmission electron microscopy. The microstruc-
2.2 Catalyst preparation
ture of the catalysts was observed by transmission electron
microscopy (TEM) on a TecnaiG2 20 STwin electron microscope
working at less than 200 kV. The instrumental magnication
Single-wall carbon nanotubes were pretreated in concentrated
nitric acid (68 wt%) at 303 K overnight, then ltered and washed
with distilled water until the pH ¼ 7; nally, the carbon nano-
tubes were dried in vacuum at 383 K for 10 h.
4
6
ranged from 2 ꢂ 10 to 10 ꢂ 10 . The samples were deposited
on holey carbon coated Cu grids.
2
.2.1 Methanol pregnation method. The catalyst was
2.3.4 Hydrogen chemisorption. Hydrogen chemisorption
20
prepared by referring to the following procedures. PdCl
dissolved into a solution of concentrated hydrochloric acid (38
wt%) and distilled water to prepare a H PdCl solution. Then, a
2
was
was measured by using Quantachrome's ChemBET 3000
instrument. The samples had been previously reduced in a
hydrogen stream and cooled to ambient temperature under a
nitrogen stream. The hydrogen chemisorption was performed
at 323 K, and the hydrogen pulses (0.02 mL) were injected until
the eluted areas of consecutive pulses became constant.
2
4
sodium hydroxide solution (10 wt%) was dropped into the
above solution to regulate it to pH ¼ 5. Aerwards, the pre-
treated carbon nanotubes and methanol were added and stirred
for 10 h. Finally, the mixture was dried at 383 K for 12 h under
vacuum, calcinated at 473 K for 4 h under nitrogen, and reduced
at 523 K for 3 h under hydrogen; the prepared catalyst was
labeled as Pd/SWCNTs-1.
2.3.5 X-ray photoelectron spectroscopy. X-ray photoelec-
tron spectroscopy (XPS) was obtained on a K-Alpha 1063 from
Thermo Fisher Scientic using Al Ka X-rays and a micro gath-
ered monochromator. The tube voltage was 12 kV, the current
2.2.2 Water pregnation method. The preparation process
ꢀ
was 6 mA, and the analyzer was a 180 double gathered hemi-
was similar to the methanol pregnation method; however,
distilled water instead of methanol was added to the carbon
nanotubes mixture and stirred for 10 h. The prepared catalyst
was labeled as Pd/SWCNTs-2.
sphere (energy resolution ¼ 0.5 eV, vacuum degree ¼ 10–9
mBar, spot diameter ¼ 400 mm).
2.4 Procedure for the catalyst test
2
.2.3 Two-step water pregnation method. The preparation
process was similar to the water pregnation method; however, Liquid phase hydrogenation of nitrocyclohexane was conducted
PdCl
was added into the mixture two times. The prepared as follows: ethylenediamine (4.8 g), nitrocyclohexane (0.6 g),
catalyst was labeled as Pd/SWCNTs-3.
and catalyst (0.1 g) were mixed in a 50 mL Teon-lined stainless-
.2.4 Ion exchange method. An ammonia solution (38 steel autoclave equipped with a magnetic stirrer. The reactor
wt%) was dropped into a solution of H PdCl
to form the was lled with nitrogen and then evacuated to exclude air. It was
2
2
2
4
complex. Then, pretreated carbon nanotubes and water were then pressurized to 0.3 MPa with hydrogen, and heated to the
added to undergo ion exchange in the above solution. Finally, setting reaction temperature. Aer the reaction, the reactor was
the mixture was calcinated at 473 K for 4 h under nitrogen, and cooled to ambient temperature and the catalysts were removed
reduced at 523 K for 3 h under hydrogen; the prepared catalyst by ltration. The contents of the products were determined by
21
was labeled as Pd/SWCNTs-4.
.2.5 Methanol reduction method. The preparation and a 30 m DB-1701 capillary column using dimethyl phthalate
GC (GC-14C, SHIMADZU) with a ame ionization detector (FID)
2
process was similar to the water pregnation method; however, (DMP) as an internal standard.
the catalyst was reduced in 37 wt% methanol solution at 333 K
for 4 h, and then dried at 383 K for 12 h under vacuum. The
3
Results and discussions
prepared catalyst was labeled as Pd/SWCNTs-5.
.2.6 KBH4 reduction method. The preparation process
was similar to the water pregnation method; however, the
3.1 Characterization of the catalysts
2
3.1.1 The effects of preparation methods. Fig. 2 shows the
catalyst was reduced in 10 wt% KBH
4
solution in an ice-bath, XRD patterns of the prepared catalysts. All of these catalysts
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materials. All of the catalysts have a mesoporous structure
which means that the preparation methods have no obvious
inuence on the pore structure of the catalysts. It is well-known
that a larger area of the hysteresis loop indicates more pores on
the basis of the geometrical effect and Kelvin equation, which is
in good agreement with the pore volumes listed in Table 1.
The hydrogen chemisorption data of different catalysts
are summarized in Table 2. It can be seen from Table 2 that
Pd/SWCNTs-2 demonstrates better dispersion, larger hydrogen
uptake quantity, and larger metal surface area than other cata-
lysts. This indicates that catalysts made by the water impregna-
tion method and hydrogen reduction are more favorable for the
dispersion of palladium; perhaps this is because diffusion of the
precursor inside the pores during ion exchange is faster than
during impregnation, which may cause a non-uniform distribu-
16
tion of the precursor inside the particle.
Fig. 2 XRD patterns of different catalysts.
The TEM images of catalysts prepared by different methods
are shown in Fig. 4. The dark dots represent palladium particles
on the surface of carbon nanotubes. The parallel graphite layer
can be seen from the TEM images and the images show that
palladium particles are well dispersed on the surface of the
support. As shown in Fig. 3, the particle size of palladium is
about 4.0 to 9.0 nm in Pd/SWCNTs-1, 3.5 to 6.0 nm in Pd/
SWCNTs-2, and 4.0 to 6.0 nm in Pd/SWCNTs-3. However, the
particle size of palladium in Pd/SWCNTs-4 ranges from 6.5 to
ꢀ
present a diffraction peak of carbon around 2q ¼ 26 , the peaks
ꢀ
ꢀ
ꢀ
at 2q ¼ 32 , 45 , and 57 are ascribed to sodium chloride; and
ꢀ
ꢀ
the peaks at 2q ¼ 40 and 47 are ascribed to the face centered
22
cubic structure of palladium. It can be seen from Fig. 2
that the catalysts prepared by water impregnation exhibit
ꢀ
ꢀ
signicant characteristic diffraction peaks at 2q ¼ 40 and 47
corresponding to palladium. However, there is a very weak
13.5 nm, Pd/SWCNTs-5 ranges from 5.0 to 10.0 nm, and Pd/
ꢀ
ꢀ
diffraction peak around 40 and 47 in the XRD pattern of
SWCNTs-6 ranges from 5.0 to 12.0 nm. The catalysts prepared
by impregnation methods have smaller particle sizes of palla-
dium than the catalysts prepared by the chemical reduction
method and ion exchange method, which is in agreement with
Pd/SWCNTs-1 and, as well, there is no obvious characteristic
ꢀ
ꢀ
diffraction peak around 40 and 47 in the XRD pattern of
Pd/SWCNTs-4.
Fig. 3 shows the nitrogen adsorption-desorption curves of
Pd/SWCNTs. The isotherm of these catalysts shows an adsorp-
tion isotherm of type III according to the IUPAC classication,
representing a weak interaction between Pd/SWCNTs and
nitrogen. The hysteresis loops of these catalysts are of type H1,
which is attributed to cylindrical pores with both sides open,
and indicates a narrow pore size distribution of mesoporous
23–25
the literature.
3.1.2 The effects of preparation conditions. Fig. 5 shows
the XRD patterns of Pd/SWCNTs-2 prepared at different reduc-
tion temperatures. All of the samples show the diffraction peak
Table 1 Textural properties of the catalysts prepared by different
methods
Surface area
Average pore
size (nm)
Pore volume
2
ꢁ1
3
ꢁ1
Catalysts
(m g
)
(cm g
)
Pd/SWCNTs-1
Pd/SWCNTs-2
Pd/SWCNTs-3
Pd/SWCNTs-4
Pd/SWCNTs-5
Pd/SWCNTs-6
275.8
220.6
263.9
362.2
319.3
296.7
3.8
3.8
3.6
3.8
3.8
3.8
1.3
1.0
1.1
1.5
1.5
1.4
Table 2 Hydrogen chemisorption data of different catalysts
H
2
uptake
Metal surface
Dispersion
(%)
ꢁ
1
2
ꢁ1
Catalysts
(mmol g
)
area (m g
)
Pd/SWCNTs-1
Pd/SWCNTs-2
Pd/SWCNTs-3
Pd/SWCNTs-4
32.6
77.7
39.4
19.7
12.5
19.6
61.9
147.4
74.8
37.5
23.7
37.7
13.9
33.0
16.8
8.4
5.3
15.3
Fig. 3
N
2
adsorption–desorption isotherm and BJH pore size distri- Pd/SWCNTs-5
butions of different catalysts.
Pd/SWCNTs-6
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Fig. 6 TEM images of Pd/SWCNTs-2 prepared at different reduction
temperatures: (a) 623 K, and (b) 723 K.
The reason may be that a high reduction temperature makes the
palladium particle more prone to sinter.
The TEM images of Pd/SWCNTs-2 prepared at different
reduction temperatures are shown in Fig. 6. And Pd/SWCNTs-2
reduced at 523 K is shown in Fig. 4(b). The dark dots represent
palladium particles on the surfaces of carbon nanotubes. The
parallel graphite layer can be seen from the TEM images and
they show that palladium particles are well dispersed on the
surfaces of the support. The TEM micrographs show that
palladium particles of the catalyst reduced at 523 K range from
3
6
.5–6.0 nm while palladium particles of the catalyst reduced at
23 K and 723 K range from 4.0–6.5 nm and 4.0–7.0 nm
respectively. These results show that the particle size of palla-
dium increases slightly with a reduction in temperature.
The effects of reduced temperature on the valence state of
palladium on the support are tested by X-ray photoelectron
spectroscopy and the results are shown in Fig. 7. There are two
peaks of Pd3d spectra; the right peak around 335 eV is attrib-
uted to the Pd3d5/2 spectral peak and the le peak around 341
eV is ascribed to the Pd3d3/2 spectral peak. The binding energy
peaks and the valence peak could be shied; the shi in the
core level binding energies can be 1.0-1.3 eV in the highly
Fig. 4 TEM images of different preparation methods: (a) Pd/SWCNTs-
1
, (b) Pd/SWCNTs-2, (c) Pd/SWCNTs-3, (d) Pd/SWCNTs-4, (e) Pd/
SWCNTs-5, and (f) Pd/SWCNTs-6.
26,27
dispersed samples.
37.6 eV are assigned to Pd , Pd and Pd . The surface
compositions of different valence states of palladium are shown
The peaks around 335.5 eV, 336.1 eV and
0
+
2+
3
2
+
+
in Table 3. It can be observed that the Pd turns into Pd when
the reduction temperature increases, and this means that a
Fig. 5 XRD patterns of Pd/SWCNTs-2 prepared at different reduction
temperatures: (a) 523 K, (b) 623 K, and (c) 723 K.
ꢀ
ꢀ
ꢀ
ꢀ
of carbon around 2q ¼ 26 ; the peaks at 2q ¼ 32 , 45 , and 57
ꢀ
are ascribed to sodium chloride, and the peaks at 2q ¼ 40 and
ꢀ
4
7 are ascribed to the face centered cubic structure of palla-
dium. It can be seen from Fig. 5 that the diffraction peaks at
ꢀ
ꢀ
2
q ¼ 40 and 47 corresponding to palladium get stronger along
with the increase of the reduction temperature, which means
that the higher reduction temperature causes larger particle
Fig. 7 XPS Pd 3d spectra of Pd/SWCNTs-2 prepared at different
sizes of palladium. This is consistent with the results of TEM. reduction temperatures: (a) 523 K, (b) 623 K, and (c) 723 K.
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Table 3 Surface compositions of the valence states of palladium with Table 5 Effect of reduction temperatures with catalysts on nitro-
a
different reduction temperatures
cyclohexane hydrogenation
Reduction
temperature (K)
Selectivity (%)
0
+
2+
Pd (%)
Pd (%)
Pd (%)
Reduction temp.
(K)
Conversion
of NCH (%)
Catalyst
CHO
CHA
5
6
7
23
23
23
30.1
30.4
29.5
33.0
33.8
38.7
37.7
35.8
31.9
Pd/SWCNTs-1
523
623
99.7
99.7
99.1
98.9
98.5
96.0
99.6
99.5
99.8
99.4
99.5
99.8
70.5
69.1
85.5
85.9
93.5
96.4
79.7
86.2
86.3
65.5
65.5
67.7
2.8
5.0
4.2
6.3
4.2
3.2
2.0
1.9
1.6
5.0
4.6
4.7
7
23
523
23
Pd/SWCNTs-2
Pd/SWCNTs-3
Pd/SWCNTs-4
6
reasonable reduction temperature is favorable for the formation
of monovalent palladium.
723
523
6
7
23
23
523
3
.2 Catalytic performance
6
7
23
23
The results of liquid phase hydrogenation of nitrocyclohexane
over Pd/SWCNTs catalysts prepared by different methods are
shown in Table 4. The products include cyclohexanone oxime,
cyclohexylamine, and a small amount of cyclohexanone and by- CHO ¼ cyclohexanone oxime; CHA ¼ cyclohexylamine.
products. It can be seen from Table 4 that Pd/SWCNTs-4 gives
a
Reaction conditions: nitrocyclohexane 0.6 g; temperature 323 K; time 6
h; pressure 0.3 MPa; ethylenediamine 5 mL; NCH ¼ nitrocyclohexane;
the lowest selectivity to cyclohexanone oxime at almost the
same nitrocyclohexane conversion catalyzed by Pd/SWCNTs-1, oxime. Pd/SWCNTs-2 reduced at 723 K gave 96.4% selectivity to
Pd/SWCNTs-2, Pd/SWCNTs-3 and Pd/SWCNTs-6. Pd/SWCNTs- cyclohexanone oxime with nitrocyclohexane conversion at 96.0%;
5
has the lowest conversion of nitrocyclohexane under the the selectivity to cyclohexanone oxime is more than 10% higher
same reaction conditions. It can be seen from the hydrogen than that reduced at 523 K. The different preparation methods
chemisorption data in Table 2 that Pd/SWCNTs-5 has the lowest may have inuences on the interaction between the palladium
hydrogen uptake quantity, metal surface area, and dispersion of and the support, and the reduction conditions can affect the
palladium. Pd/SWCNTs-2 gives the best result of 85.9% selec- valence state of the palladium on the support.1
6–19
It can be seen
tivity to cyclohexanone oxime at a nitrocyclohexane conversion from the results of XPS that Pd/SWCNTs reduced at 723 K has the
+
of 98.9%. Pd/SWCNTs-2 prepared by the water impregnation highest content of Pd . This indicates that mono valent palla-
method and reduced by hydrogen produces the smallest dium favors the formation of cyclohexanone oxime and prevents
particle size of palladium, the largest hydrogen uptake quantity, complete hydrogenation to other byproducts.
metal surface area, and highest dispersion of palladium, which
favor the formation of cyclohexanone oxime.
Table 5 shows the results of the catalysts prepared by different 4. Conclusion
reduction temperatures in the hydrogenation of nitro-
Single-wall carbon nanotubes supported palladium catalysts
cyclohexane to cyclohexanone oxime. The hydrogenation reac-
were prepared by different methods. The catalysts were charac-
terized by nitrogen adsorption–desorption, XRD, TEM, hydrogen
chemisorption and X-ray photoelectron spectroscopy. The results
tion over these catalysts was carried out at 323 K and 0.3 MPa. It
can be seen from Table 5 that higher reduction temperatures can
promote selectivity to cyclohexanone oxime. All the catalysts
show that Pd/SWCNTs-2 prepared by a water impregnation
reduced at 723 K gave the highest selectivity to cyclohexanone
method and reduced by hydrogen, produces smaller particle
sizes of palladium, larger quantities of hydrogen uptake, higher
metal surface area, and greater dispersion of palladium. The
Table
catalysts
4
Nitrocyclohexane hydrogenation catalyzed by different
a
reduction conditions have great inuence on the valence state of
palladium on the support. Results show that a catalyst with
smaller particles, better dispersion and higher content of mono
valent palladium favors the formation of cyclohexanone oxime.
Pd/SWCNTs-2 prepared by water impregnation and reduced by
hydrogen at 723 K gives 96.4% selectivity to cyclohexanone oxime
formation with a nitrocyclohexane conversion of 96.0% under
the mild conditions of 0.3 MPa and 323 K.
Selectivity (%)
Conversion
Catalyst
of NCH (%)
CHO
CHA
Pd/SWCNTs-1
Pd/SWCNTs-2
Pd/SWCNTs-3
Pd/SWCNTs-4
Pd/SWCNTs-5
Pd/SWCNTs-6
99.7
98.9
99.6
99.4
79.7
99.8
70.5
85.9
79.7
65.5
85.4
76.9
2.8
6.3
2.0
5.0
0.2
3.6
a
Acknowledgements
Reaction conditions: nitrocyclohexane 0.6 g; temperature 323 K; time 6
h; pressure 0.3 MPa; ethylenediamine 5 mL; NCH ¼ nitrocyclohexane;
CHO ¼ cyclohexanone oxime; CHA ¼ cyclohexylamine.
This work was supported by the NFSC (21276218), SRFDP
(20124301110007), Scientic Research Fund of Hunan Provincial
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