Highly selective one-step hydrogenation of nitrobenzene to cyclohexylamine over the supported 10% Ni/carbon catalysts doped with 3‰ Rh

Article abstract of DOI:10.1039/c5ra27202e

The carbon supported 10% Ni catalysts doped with 3‰ Rh have been prepared by an impregnation method. These catalysts have been used to catalyze the one-step hydrogenation of nitrobenzene to cyclohexylamine. The results show that the 3‰ Rh-10% Ni/CSC (biocarbon) catalyst exhibits an excellent performance to achieve 100 mol% conversion of nitrobenzene and 91.6% selectivity of cyclohexylamine under reaction conditions of 3.5 MPa and 140 °C. The recycling tests reveal recyclable stability of 3‰ Rh-10% Ni/CSC. This catalyst is active for the hydrogenation of a series of electron-deficient nitrobenzenes. Some factors such as the type of carriers, the content of Ni and Rh, the type of metals and additives play important roles in controlling the selective hydrogenation.

Full text of DOI:10.1039/c5ra27202e

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Highly selective one-step hydrogenation of
nitrobenzene to cyclohexylamine over the
supported 10% Ni/carbon catalysts doped with 3&
Rh†
Cite this: RSC Adv., 2016, 6, 15354
abc
Yang chen, Zhenshuang Zhao, Hao Deng, Dan Zhou,*ac
ac
ac
ac
Xinhuan Lu,
Changcheng Wei, Renfeng Nie and Qinghua Xia*
ac
ac
ac
The carbon supported 10% Ni catalysts doped with 3& Rh have been prepared by an impregnation method.
These catalysts have been used to catalyze the one-step hydrogenation of nitrobenzene to
cyclohexylamine. The results show that the 3& Rh–10% Ni/CSC (biocarbon) catalyst exhibits an
excellent performance to achieve 100 mol% conversion of nitrobenzene and 91.6% selectivity of
Received 26th December 2015
Accepted 27th January 2016
ꢀ
cyclohexylamine under reaction conditions of 3.5 MPa and 140 C. The recycling tests reveal recyclable
stability of 3& Rh–10% Ni/CSC. This catalyst is active for the hydrogenation of a series of electron-
deficient nitrobenzenes. Some factors such as the type of carriers, the content of Ni and Rh, the type of
metals and additives play important roles in controlling the selective hydrogenation.
DOI: 10.1039/c5ra27202e
www.rsc.org/advances
Cyclohexylamine can be used in the synthesis of articial
sweeteners (sodium or calcium cyclamate), metal corrosion
1
. Introduction
Cycloaliphatic amines are one kind of important organic inhibitors, rubber vulcanizing additives, dyestuff, plasticizers
11–13
chemical and ne chemical intermediate, and also an impor- and extracting agents for natural products.
Industrially,
tant raw material for the synthesis of isocyanate. The hydro- CHA is produced via the following two steps, (i) the reduction of
1
4–20
genation of aromatic amines is the most commonly used nitrobenzene (NB) to aniline,
(ii) the ring hydrogenation of
1
–3
21–26
method for the synthesis of cycloaliphatic amines.
The aniline to CHA,
which is a complicated process. More
catalytic hydrogenation of nitroaromatic compounds to the importantly, the selectivity of cyclohexylamine is low only in the
corresponding anilines is an important process for the range of 30–50%. Therefore, the development of a facile ‘one-
production of many ne chemicals, such as dyes, drugs, step’ synthetic pathway from nitroarenes to alicyclic amines is
4,5
herbicides, cosmetics, and pesticides. However, it is not easy of great importance.
6,7
to avoid the occurrence of dehalogenation during reactions.
Transition-metal nanoparticles as catalysts have attracted
The hydrogenation of aniline over transition metal catalysts is much attention due to the fact that if controlled, their surface
accompanied by numerous side reactions. In addition to structures oen give rise to high chemo-, regio-, stereo- and
27–33
cyclohexylamine (CHA), the products always include dicyclo- enantio- selectivities.
As to the industrial process to manu-
hexylamine (DCHA) and ammonia. At elevated temperature, facture aniline through the hydrogenation of nitrobenzene, it is
ꢀ
the formation of diphenylamine, phenyl-cyclohexylamine, generally run above 240 C with the copper catalyst in two-stage
8
34
cyclohexane and benzene is also reported. Moreover, in xed-bed reactor. In the hydrogenation of aniline in the vapor
aqueous media, hydrolytic products like cyclohexanone and phase or in the liquid phase, the activity of a variety of metals
9,10
35–39
cyclohexanol could also be detected.
such as Ni, Co, Rh, Ru, Pd or Pt has been studied as well.
It is
well known that Pt usually catalyzes a fast reduction of nitro
group but with a low full-hydrogenation selectivity, and Ru is
cheaper and more selective than other noble metals for this
a
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials,
Hubei University, Wuhan 430062, P. R. China. E-mail: d.zhou@hubu.edu.cn;
xiaqh518@aliyun.com; Fax: +86-27-88663043; Tel: +86-27-88663043
40,41
reaction but with a low catalytic activity.
Recently, Langer
et al. reported a low-pressure process for the hydrogenation of
b
aniline to CHA with a high selectivity over rhodium (Rh) cata-
Hubei Key Laboratory for Processing and Application of Catalytic Materials,
42
lysts. This approach might provide a green pathway from the
Huanggang Normal University, Huanggang 438000, P. R. China
c
Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic view point of efficiency.
Functional Molecules, Hubei University, Wuhan 430062, P. R. China
Very recently, our group reported the ‘one-step’ catalytic
hydrogenation from 1,5-dinitronaphthalene to 1,5-diamino
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra27202e
1
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43
decahydronaphthalene over the supported Ni catalysts, which
can completely skip the synthesis of aromatic amine interme-
diates. To date, only one report has approached the ‘one-step’
catalytic hydrogenation from nitrobenzene to cyclohexylamine
44
over noble metal catalyst. The present work describes a direct
synthesis of alicyclic amines through the hydrogenation of
Scheme 1 The hydrogenated products of nitrobenzene.
2
nitroarenes with H over carbon supported 10% Ni catalysts
promoted with 3& Rh. The study is mainly focused on highly
selective one-step hydrogenation from nitrobenzene to cyclo-
hexylamine, for which the best data are 100 mol% conversion of
nitrobenzene and 91.6% selectivity of cyclohexylamine under
mild conditions.
2
Autosorb-1 was used to determine N adsorption–desorption
properties of the samples. The Brunauer–Emmett–Teller (BET)
specic surface area was calculated using the BET equation in
the relative pressure between 0.05 and 0.25. The morphology
and size of crystals were imaged with a JEOL JSM-6510A scan-
ning electron microscope (SEM). Transmission electron
microscope (TEM) images were obtained using an accelerating
voltage of 200 kV on a JEOL-135 2010F transmission electron
microscope. X-ray photoelectron spectra (XPS) were recorded on
a Perkin-Elmer PHI ESCA system. X-ray source was standard Mg
anode (1253.6 eV) at 12 kV and 300 W. The temperature pro-
2. Experimental
2.1. Materials
Raney Ni (99.5%), Ni(NO ) $6H O (98%), RhCl $3H O
3
2
2
3
2
(Rh 40%), PdCl2 (Pd 59–60%), coconut shell charcoal (CSC)
(98%), activated carbon (AC) (97%) and graphite (G) (97%) were
grammed H
analyzer, which was carried out in a N
2
reduction (TPR) on a TP-5076 dynamic adsorption
purchased from Shanghai Aladdin Chemical Co. Note that CSC
is made from high-quality coconut shell biomaterial, which has
porous structure, large specic surface area, strong adsorption
ability and easy regeneration. The main reagents used in the
hydrogenations were nitrobenzene (99.0%), 2-nitrotoluene
ꢂ1
2
ow of 50 mL min (10
ꢀ
vol% H ), from room temperature to 600 C at a heating rate
2
ꢀ
ꢂ1
of 10 C min
.
2.4. Catalytic tests
(
(
(
98.0%), 3-methylnitrobenzene (98.0%), 4-methylnitrobenzene
99.0%), 2-chloronitrobenzene (98.0%), 3-chloronitrobenzene The catalytic hydrogenations of nitrobenzene (NB) to
98.0%) and 4-chloronitrobenzene (98.0%), methanol (99.0%), cyclohexylamine (CHA) were carried out in a 100 mL stainless
dioxane (99.0%), ethyl acetate (99.0%), tetrahydrofuran (THF) autoclave. In a typical procedure, a certain amount of catalyst
99.0%) and cyclohexane (99.0%), which were purchased from was dispersed in 9.0 g solvent, and then added with 1.0 g
Shanghai Aladdin Chemical Co. and directly used as received. substrate. The autoclave was sealed, purged and pressurized
(
The basic additives included LiOH (99.0%), NaNH
CO (99.0%) and NaHCO (99.0%).
2
(99.5%), with hydrogen, and then heated to desired temperature under
magnetic stirring at a rate of 1000 rpm. Aer the completion of
the reaction, the mixture was separated by centrifugation in
order to recover the solid catalyst. Then, the solid catalyst was
washed with acetone and dried in vacuum oven for the next use.
K
2
3
3
2.2. Preparation of catalysts
An aqueous solution of Ni(NO ) $6H O or precious metal
3 2 2
The ltrate was analyzed by gas-chromatogram (GC, GC9720)
with a 30 m capillary column (Rtx@-5) using a ame ionization
detector (FID). The standard samples of cyclohexylamine and
dicyclohexylamine were used as the reference of the GC reten-
tion time of hydrogenated products. As illustrated in Scheme 1,
the hydrogenated products were mainly consisted of aniline
chlorides was impregnated onto various carbon supports like
CSC, AC and G. The supported catalysts were prepared through
the following procedure. Typically, 2.0 g of carbon material was
dispersed into the solution consisted of Ni(NO ) $6H O
3
ꢂ3
2
2
ꢂ3
(
0.098–1.480 g), RhCl $3H O (1.02 ꢁ 10 to 10.2 ꢁ 10 g) and
3
2
distilled water (30 g). Then, the resulting mixture was heated
(AN), cyclohexylamine (CHA), dicyclohexylamine (DCHA), and
ꢀ
to 80 C while stirring until water was completely evaporated
off. The solid was dried at 100 C for 12 h, then calcined at
00–400 C for 3 h in the ow of air, and reduced by hydrogen
others (some cracked byproducts), GC-MS of which were
shown in Fig. S1.†
ꢀ
ꢀ
3
ꢀ
at 300–400 C for 3 h in a tubular reactor. The resultant catalyst
was designated as x% Ni/CSC, 3& Rh–x% Ni/CSC, x& Rh–10%
Ni/CSC, 10% Ni/AC, 10% Ni/G, in which x% corresponds to
3
. Results and discussion
3.1. Structural characteristics of catalysts
1
–15% (mass percentage). Other carbon supported metal
Fig. 1 presents the XRD patterns of the samples with different Ni
catalysts were prepared with a similar method, which was
designated as, 10% Fe/CSC, 10% Cu/CSC, 10% Co/CSC and
contents, in which weak peaks due to 1% Ni can be seen. The
ꢀ
characteristic peaks of CSC (2q ¼ 23.3 ) and Ni metal (2q ¼ 44.4
10% Mo/CSC.
ꢀ
and 51.6 ) are observed for all the Ni/CSC catalysts with the Ni
ꢀ
ꢀ
loading from 3–15 wt%, in which two peaks at 44.4 and 51.6
are attributed to [111] and [200] diffraction peaks of NiO. This
2.3. Characterization of catalysts
Powder X-ray diffraction (XRD) was performed on a Bruker means that the crystalline structure of CSC was not yet
ꢀ
D8A25 diffractometer with CuKa radiation (l ¼ 1.54184 A) destroyed and the Ni species were reduced to NiO metal in all
ꢀ
operating at 30 kV and 25 mA in the range of 5–65 2q. the catalysts during the preparation. Moreover, with increasing
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Very clearly, the support G and the catalyst 10% Ni/G has the
smallest BET surface areas and pore volumes. Both values of the
catalyst 10% Ni/AC (activated carbon as the support) are the
largest. The increase of Ni content has led to a small drop of
2
ꢂ1
3
BET surface area and pore volume from 684.6 m g (0.32 cm
g
ꢂ1
2
ꢂ1
3
ꢂ1
) of CSC to 673.8 m g (0.29 cm g ) of 15% Ni/CSC. The
BET surface area and pore volume of the catalyst 3% Rh/CSC are
2
ꢂ1
3
ꢂ1
6
78.9 m g and 0.31 cm g
Fig. S3† shows SEM images of CSC supported Ni or Rh
catalysts, inclusive of CSC, 1% Ni/CSC, 3% Ni/CSC, 10% Ni/CSC,
5% Ni/CSC, 0.5% Rh/CSC, 3% Rh/CSC and 3& Rh–10% Ni/
.
1
CSC. Apparently, SEM images of the latter six samples exhibit
irregular spherical morphologies, different from ne particles
of CSC, Ni and Rh components dispersed highly on the support.
The sharp boundaries of the particles demonstrate crystalline
characteristics of these samples, in which the particle shape
and size of different catalysts are decided by the morphology of
different carriers, further affecting catalytic activities. Fig. S4†
shows the SEM images of CSC supported other metal catalysts,
which exhibit irregular spherical morphologies.
Fig. 1 XRD patterns of various Ni/CSC catalysts with different Ni
contents.
Fig. 3 shows TEM images of CSC supported Ni or Rh cata-
lysts. The TEM image of unreduced precursor NiO/CSC of 10%
Ni/CSC shows very vague particle interface. Once reduced, the
10% Ni/CSC catalyst contains nanosized Ni particles, normally
in the order of 5.1–33.1 nm, in which the average Ni particle size
Fig. 2 XRD patterns of CSC supported Rh catalysts.
the Ni content, the intensity of [111] and [200] diffraction peaks
is gradually increased. Fig. 2 shows the XRD patterns of CSC
supported Rh catalysts. The Rh peak of 0.5% Rh/CSC is not
obvious, showing that Rh particles are uniformly dispersed on
the CSC carrier. There are two weak peaks visible at 2q ¼ 43.2
ꢀ
and 50.9 for 3% Rh/CSC, probably due to the formation of
small Rh crystals on CSC. The Rh peak of 3& Rh–10% Ni/CSC is
also not obvious, possibly because of low Rh content and the
concealment of strong Ni peak.
The X-ray diffraction patterns of the samples of CSC sup-
ported other metals are shown in Fig. S2.† The peak located at
ꢀ
ꢀ
2
3
3.3 is characteristic of typical CSC material. The peaks at 2q ¼
ꢀ
7.6 and 52.7 are ascribed to molybdenum oxide. The XRD
ꢀ
peak of Fe metal appears at 2q ¼ 45.2 , and the peaks at 2q ¼
ꢀ ꢀ ꢀ ꢀ ꢀ
0.8 , 42.6 , 53.0 , 56.6 and 63.5 are indexed to Fe O . The
2 3
3
ꢀ
above results show that the reduction temperature of 400 C is
insufficient for the preparation of 10% Mo/CSC and 10% Fe/
CSC catalysts. As can be seen from Fig. S2,† the peaks at
ꢀ ꢀ ꢀ
4
4
1.0 , 43.9 and 46.8 are attributed to Co metal, and ones at
ꢀ
ꢀ
2.8 and 51.1 correspond the crystalline phase of Cu on CSC,
ꢀ
showing that Co or Cu oxides can be totally reduced at 400 C.
Table S1† presents the BET surface areas and total pore
Fig. 3 TEM images of the supported catalysts and size distribution of
volumes of various catalysts determined by the BET analysis. Ni or Rh particles on 10% Ni/CSC and 3% Rh/CSC.
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Fig. 4 Ni2p XPS spectra of NiO/CSC (precursor of 10% Ni/CSC) and
1
0% Ni/CSC.
2
Fig. 6 Effect of Ni content on H -TPR profiles of NiO/CSC.
ꢀ
of 200–400 C. The nickel species reduced at low temperature is
43
attributed to NiO dispersed on the support; however, those
reduced at high temperature can be assigned to NiO interacted
strongly with the support, which will form a few surface layers
4
7
2+
of the support-NiO complexes. The presence of Ni in the
sample can be excluded by the absence of the reduction peak at
ꢀ
48
around 200 C, in agreement with the XPS result. Thus, in the
TPR prole of 10% Ni/CSC four TPR signals at 238, 285, 315 and
ꢀ
3
30 C are ascribable to the reduction of NiO particles to metal
0
Ni without interaction with the support (at 238, 285 and
ꢀ
2+
3
(
15 C), and to the reduction of Ni with medium interaction
ꢀ
at 330 C). The H consumption peak for the formation of Ni/
2
CSC with different Ni loadings in the range of 3–10% is very
Fig. 5 Rh3d XPS spectra of RhO/CSC (precursor of 3% Rh/CSC) and similar. There is a new TPR reduction peak for the resulting 1%
ꢀ
3
% Rh/CSC.
Ni/CSC at high temperature of 380 C, with the appearance of
ꢀ
other two peaks at 285 and 346 C, attributable to the reduction
of Ni with strong interaction. The H -TPR prole of 15% Ni/
2
2
+
is 10.7 nm. In addition, Rh nanoparticles on the CSC support CSC shows that the reduction of nickel species on CSC occurs
are quite small with the mean particle size of 3.2 nm. The TEM in ve stages with maximal signals at 221, 245, 295, 324 and
ꢀ
image of 3& Rh–10% Ni/CSC composite catalyst looks similar 345 C, in which the rst reduction peak appears at lower
to that of 10% Ni/CSC, in which Ni particles and Rh particles temperature than the former ve samples.
are visible. Fig. 4 shows the Ni2p XPS spectra of 10% Ni/CSC
2
Similarly, the H -TPR tests are conducted to monitor the
and the precursor NiO/CSC (unreduced). The peaks emerge at reduction behaviors of other metal oxides on CSC (calcined at
ꢀ
ca. 855 and 874.2 eV for NiO/CSC, ascribable to the Ni2p 400 C). As shown in Fig. S5,† there is no obvious H2
2
+
binding energy of Ni ; however, aer being reduced by H
2
,
consumption peak for 10% Mo/CSC at the temperature lower
those peaks shi to 856.4 and 874.1 eV, characteristic of Ni2p3/2 than 440 C. Even if at 450 C, molybdenum in the catalyst still
ꢀ
ꢀ
43,45
and Ni2p1/2 binding energies of metallic nickel.
Fig. 5 shows exists in the form of oxides. This means the difficult reduction
the Rh3d XPS spectra of the sample 3% Rh/CSC and the of MoO at low temperature. Three weak peaks at 173, 242 and
x
ꢀ
precursor RhO /CSC (unreduced). The peaks emerges at ca. 355 C can be observed in the TPR prole of the 10% Fe/CSC
x
3
09.7 and 314.6 eV for RhO
x
2 3 3 4
/CSC, ascribable to Rh3d5/2 and sample, attributable to the conversion of Fe O to Fe O . For
3
+
Rh3d3/2 binding energies of Rh ; aer being reduced by H
two peaks shi to 308.8 and 313.7 eV, assigned to two spin peaks from 283 to 377 C belong to the reduction of CoO to Co
splitting Rh3d5/2 and Rh3d3/2 peaks of Rh metal.
2
,
x
CoO /CSC, there are three reduction peaks, in which the TPR
ꢀ
0
x
0
ꢀ
metal, and those from 229 to 283 C correspond to the reduction
2
+
Ranade reported that there were tetrahedrally and octahe- of Cu having weak interaction with the support.
2
+
46
drally coordinated Ni species on the supported Ni samples,
in which the former was less reducible than the latter. Fig. 6
3
.2. Catalytic one-step hydrogenation of NB to CHA
shows the H -TPR proles of the supported Ni catalysts, in
2
3.2.1. Catalytic activity of the supported metal catalysts. As
which the reduction peaks of nickel species appear in the range
shown in Fig. 7, the catalytic activity of several Rh/CSC catalysts
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64.4% (10% Ni) > 62.1% (15% Ni) > 42.1% (5% Ni) > 11.1%
(3% Ni) > 8.1% (1% Ni). The literature has disclosed the activity
dependence of the catalytic hydrogenation of aromatic rings on
52
the particle size of the supported metals. However, in the
present study it is difficult to distinguish the effect of the
particle size from the Ni content, because the high loading has
led to a large quantity of easily reducible NiO species, and
a number of active metal Ni sites on the support, benecial to
the hydrogenation of aromatic rings.
However, once CSC supported Ni and less Rh composite
catalyst is prepared, the catalytic activity and the selectivity of
CHA are notably improved, as listed in Table 2. The catalytic
activity of Rh–Ni/CSC is higher than those of Rh/CSC and Ni/
CSC catalysts. On all the Rh–Ni/CSC composite catalysts, the
conversion of NB has reached 100 mol%, but the selectivity of
CHA shows a descending order of 3& Rh–10% Ni/CSC (91.6%) >
Fig. 7 Catalytic activity of the supported Rh catalysts. (Conditions:
catalyst amount, 100 mg; NB, 1 g; LiOH, 40 mg; solvent, THF 9 g;
ꢀ
pressure, 3.5 MPa H
2
; temperature, 140 C; time, 6 h).
for the catalytic hydrogenation of nitrobenzene (NB) has been
3& Rh–7% Ni/CSC (88.4%) > 3& Rh–5% Ni/CSC (85.9%) > 5&
ꢀ
tested in THF at 140 C. Under mild conditions, different
Rh–10% Ni/CSC (84.8%) > 3& Rh–15% Ni/CSC (82.9%) > 3&
Rh–3% Ni/CSC (80.9%) > 3& Rh–1% Ni/CSC (80.0%) > 2& Rh–
catalysts with different Rh contents show a great difference in
the catalytic synthesis of cyclohexylamine (CHA). When only the
support CSC is added, only 14.1 mol% of NB is converted with
1
1
0% Ni/CSC (73.8%) > 1& Rh–10% Ni/CSC (70.9%) > 0.5& Rh–
0% Ni/CSC (67.4%). For the titled reaction, 3& Rh–10% Ni/
54.4% selectivity of CHA. The use of 0.5& Rh/CSC as the catalyst
CSC is the most active and selective.
increases the conversion of NB to 88.6 mol%, with only 57.4%
selectivity of CHA. Evidently, the catalytic activity of 0.5& Rh/
CSC catalysts is greatly enhanced, signicantly better than
that of CSC. From Fig. 7, we can see that 3& Rh/CSC achieves
As shown in Table S2,† among various Rh-M/CSC composite
catalysts, only 3& Rh–10% Ni/CSC exhibits the best catalytic
performance, possibly attributed to the effectiveness of Ni and
Rh for catalyzing the titled hydrogenation. All other Rh-M/CSC
catalysts can convert 100 mol% NB, but the selectivity of CHA
is lower than 52.9%. Clearly, over 10% Cu/CSC, 10% Co/CSC,
100 mol% conversion of NB and the highest selectivity of
CHA (71.1%) under the same reaction conditions. The catalytic
activity can be arranged in the order of 1& Rh/CSC ¼ 2& Rh/
CSC ¼ 3& Rh/CSC ¼ 5& Rh/CSC > 0.5& Rh/CSC > CSC,
slightly different from the descending order of the CHA selec-
tivity: 3& Rh/CSC > 2& Rh/CSC > 1& Rh/CSC > 5& Rh/CSC >
10% Fe/CSC and 10% Mo/CSC the partially-hydrogenated
product aniline is dominant, while the selectivity of fully-
hydrogenated product CHA is extremely low <2.2%. Over these
Rh-M/CSC catalysts, the selectivity of CHA exhibits a descending
order of 3& Rh–10% Ni/CSC (91.6%) > 3& Rh–10% Cu/CSC
0.5& Rh/CSC > CSC.
The loading amount of nickel has a great impact on the
(
(
52.9%) > 3& Rh–10% Co/CSC (41.1%) > 3& Rh–10% Mo/CSC
34.6%) > 3& Rh–10% Fe/CSC (25.3%).
49–51
performance of the catalyst.
The effect of the Ni loading on
Ni/CSC has been carefully investigated and presented in
Table 1. When 1 wt% Ni is loaded on CSC, the conversion of NB
is only 34.4 mol%. As the Ni content is increased from 3 to
3
.2.2. Effect of various reaction conditions. Based on the
above results, the catalyst 3& Rh–10% Ni/CSC exhibits the best
activity for the full hydrogenation of NB to CHA, so it is used to
study the effect of various reaction conditions. Under identical
7
1
1
wt%, the conversion of NB is largely improved from 56.2 to
00 mol%. With a continuous increase of the Ni loading from
0 to 15 wt%, the conversion of NB is maintained at 100 mol%.
The selectivity for CHA is reduced in the descending order of
a
Table 2 Catalytic activity of Ni–Rh/CSC composite catalysts
Selectivity (%)
a
Table 1 Catalytic activity of the supported Ni catalysts
Catalysts
Conversion (mol%)
AN
CHA
Others
Selectivity (%)
3
3
3
& Rh–1% Ni/CSC
& Rh–3% Ni/CSC
& Rh–5% Ni/CSC
100
100
100
100
100
100
100
100
100
100
10.0
9.5
7.0
3.0
0
7.3
29.6
26.2
22.1
0
80.0
80.9
85.9
88.4
91.6
82.9
67.4
70.9
73.8
84.8
9.1
10.5
7.1
8.6
8.4
6.7
3.0
2.9
4.0
Catalysts
Conversion (mol%)
AN
CHA
Others
1
3
5
7
1
1
% Ni/CSC
% Ni/CSC
% Ni/CSC
% Ni/CSC
0% Ni/CSC
5% Ni/CSC
34.4
56.2
89.3
100
100
100
91.9
86.7
65.1
52.8
29.0
24.5
8.1
11.0
2.7
42.1
64.4
62.1
0
3& Rh–7% Ni/CSC
3& Rh–10% Ni/CSC
3& Rh–15% Ni/CSC
0.5& Rh–10% Ni/CSC
1& Rh–10% Ni/CSC
2& Rh–10% Ni/CSC
2.3
5.2
5.1
6.6
13.4
5& Rh–10% Ni/CSC
15.2
a
Conditions: catalyst amount, 100 mg; NB, 1 g; LiOH, 40 mg; solvent,
ꢀ
a
THF 9 g; pressure, 3.5 MPa H
2
; temperature, 140 C; time, 6 h.
Conditions: catalyst amount, 100 mg; NB, 1 g; LiOH, 40 mg; solvent,
ꢀ
THF 9 g; pressure, 3.5 MPa H
2
; temperature, 140 C; time, 6 h.
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experimental conditions, different carriers show different of LiOH is increased from 10 to 40 mg, the conversion of NB is
catalytic activities for the catalytic hydrogenation of NB to CHA, still kept at 100 mol%, accompanied by the increase of the CHA
especially for the selectivity of CHA (in Table 3). For the CSC selectivity from 78.2 to 91.6%. As the amount of LiOH is added
supported Rh catalysts, the catalyst 3& Rh/CSC exhibits the to 50 mg, the selectivity of CHA is slightly reduced to 91.0%.
highest CHA selectivity of 71.1%. When AC (activated carbon) Apparently, the addition of LiOH reduces the occurrence of side
and G (graphite) are used as the catalyst carriers, 52.3% of the reactions, benecial to the enhancement of the CHA selectivity.
CHA selectivity over 3& Rh/AC is higher than 43.3% over 3& Consequently, one possible reason for the enhanced selectivity
Rh/G. For the supported Rh–Ni composite catalysts, the selec- is that the addition of LiOH suppresses the acidity of the cata-
55
tivity of CHA reduces in a sequence of 91.6% over 3& Rh–10% lyst, limiting the occurrence of condensation reactions.
Ni/CSC > 71.5% over 3& Rh–10% Ni/AC > 70.8% over 3& Rh–
As we have known, the solvent plays a key role in deter-
mining the reaction rate or the distribution of products. Various
solvents have been applied in the hydrogenation reaction of NB
10% Ni/G.
Four kinds of basic additives such as LiOH, NaNH , K CO
3
2
2
3
and NaHCO , have been tested in the process of hydrogenation. to CHA over 3& Rh–10% Ni/CSC catalyst (Table 4). The tested
The basic additives can neutralize the surface acidic sites and solvents include THF, cyclohexane, dioxane, ethyl acetate,
thus prevent amine intermediates from the strong adsorption toluene and cyclohexanol. When these solvents are used, the
53
on the support, leading to an increase in the CHA selectivity.
conversion of NB reaches 100 mol%; however, the selectivity of
Moreover, the addition of additives can suppress the occurrence CHA decrease in the order of THF (91.6%) > cyclohexane
of cracked reactions during the hydrogenation, but decrease the (87.6%) > ethyl acetate (56.8%) > cyclohexanol (47.5%) > toluene
54
catalytic activity as well. As listed in Table S3,† the basic (33.2%) > dioxane (27.7%). It seems that THF is preferred for the
additives promote the selective production of CHA. Especially, full hydrogenation of nitrobenzene to CHA.
the catalyst 3& Rh–10% Ni/CSC can obtain the highest selec-
tivity of CHA (91.6%) in the presence of LiOH. However, the NB with H
The effect of reaction temperature on the hydrogenation of
over 3& Rh–10% Ni/CSC catalyst is studied, as
2
addition of NaNH
2
, K
2
CO
3
or NaHCO
3
decreases the selectivity illustrated in Fig. S6.† As the reaction temperature is increased
ꢀ
of CHA. The effect of LiOH additive on the hydrogenation of NB from 110 to 120 C, the conversion of NB is increased from 85.2
is shown in Fig. 8. When no LiOH is added, 100 mol% of NB is to 100 mol%. At reaction temperatures above 120 C, the NB
ꢀ
converted with only 53.3% selectivity of CHA. When the amount conversion of 100 mol% is maintained. The selectivity for CHA
ꢀ
is only 52.4% at 110 C, which is rapidly increased to 86.5% at
ꢀ
ꢀ
1
9
20 C and 90.1% at 130 C, gradually to the maximum of
ꢀ
ꢀ
1.6% at 140 C, and then slightly reduced to 89.2% at 150 C
a
Table 3 Effect of carriers on the hydrogenation
ꢀ
and 84.3% at 170 C. Fig. S7† exhibits the effect of reaction
time on the hydrogenation of NB over 3& Rh–10% Ni/CSC at
Selectivity (%)
ꢀ
1
40 C. Within the reaction time of 1 h, the conversion of NB is
Catalysts
Conversion (mol%)
AN
CHA
Others
about 89.2 mol%, which is rapidly increased to 100 mol% in
h. As the reaction time is prolonged to 4, 6 or 8 h, the
2
3
3
3
3
3
3
& Rh/CSC
& Rh–10% Ni/CSC
& Rh/AC
& Rh–10% Ni/AC
& Rh/G
& Rh–10% Ni/G
100
100
100
100
100
100
0
0
0
20.2
16.1
23.7
71.1
91.6
52.3
71.5
43.3
70.8
28.9
8.4
47.7
8.3
40.6
5.5
conversion of NB has been kept at 100 mol%. The selectivity of
fully-hydrogenated product (CHA) is lower than 61% within
2 h, which is rapidly increased to 80.1% in 4 h, and to the
maximum of 91.6% in 6 h, and then slightly decreased to
8
7.2% in 8 h.
a
Conditions: NB, 1 g; catalyst, 100 mg; LiOH, 40 mg; solvent, THF 9 g;
In the catalytic hydrogenation reaction, the hydrogen
pressure is a very important parameter, as hydrogen is one of
ꢀ
pressure, 3.5 MPa H
2
; temperature, 140 C; time, 6 h.
the reactants. Fig. S8† shows the effect of hydrogen pressure
ꢀ
on the conversion of NB over 3& Rh–10% Ni/CSC at 140 C.
a
Table 4 Effect of solvents on the hydrogenation
Selectivity (%)
Conversion
Catalysts
& Rh–10%
Solvent
(mol%)
AN
CHA
Others
3
THF
Cyclohexane
Dioxane
Ethyl acetate
Toluene
Cyclohexanol
100
100
100
100
100
100
0
0
0
20.2
16.1
23.7
71.1
91.6
52.3
71.5
43.3
70.8
28.9
8.4
47.7
8.3
40.6
5.5
Ni/CSC
Fig. 8 Effect of LiOH amount on the hydrogenation. (Conditions:
a
catalyst, 3& Rh–10% Ni/CSC 100 mg; NB, 1 g; solvent, THF 9 g;
Conditions: catalyst, 3& Rh–10% Ni/CSC 100 mg; NB, 1 g; LiOH, 40
ꢀ
ꢀ
pressure, 3.5 MPa H
2
; temperature, 140 C; time, 6 h).
mg; solvent, 9 g; pressure, 3.5 MPa H
2
; temperature, 140 C; time, 6 h.
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Table
nitrobenzenes
5
The hydrogenation results of other substituted
a
Selectivity (%)
Partial Full
Conversion
(mol%)
Catalysts
Solvent
Others
3
& Rh–10%
100
0
0
91.6 8.4
89.6 10.4
Ni/CSC
100
1
1
00
00
0
94.4 5.6
b
c
3.0
80.0 17.0 (14.8)
1
00
2.3
75.6 22.1 (19.9)
Fig. 9 Recycling results of the catalyst 3& Rh–10% Ni/CSC. (Condi-
ꢀ
tions: solvent, THF 9 g; pressure, 3.5 MPa H
2
; temperature, 140 C;
time, 6 h).
d
e
1
1
00
00
3.4
3.2
76.7 19.9 (17.2)
72.9 23.9 (20.1)
The conversion of NB is 90.2 mol% under 2.0 MPa of H2
pressure, which is increased to 100 mol% at 2.5–4.0 MPa. The
selectivity of CHA is increased from 64.4% (at 2.0 Mpa) to the
maximum of 91.6% (at 3.5 MPa), and then slightly decreased
a
Conditions: catalyst, 3& Rh–10% Ni/CSC 100 mg; substrate, 1 g; LiOH,
ꢀ
4
0 mg; solvent, THF 9 g; pressure, 3.5 MPa H
2
; temperature, 140 C;
b
0
c
time, 6 h. 4,4 -Dimethyl-dicyclohexylamine 14.8%. Cyclohexylamine
1
d
e
9.9%. Cyclohexylamine 17.2%. Cyclohexylamine 20.1%.
to 90.2% at 4.0 MPa. Therefore, the most appropriate H
2
pressure is about 3.5 MPa, considerably conducive to the
adsorption of reactants on the catalyst surface and thereby the
fast occurrence of full-hydrogenation of NB to CHA. This
implies that fully-hydrogenated products can be merely
Rh–10% Ni/CSC is as well described in Table 5. Compara-
tively, the conversion of these substituted nitrobenzenes
reaches 100 mol%, totally different from the descending order
of the selectivity of fully-hydrogenated products: 3-methyl NB
4
4
formed under a suitable H
2
pressure. This dependence is in
agreement with the observations during the hydrogenation
(
(
94.4%) > NB (91.6%) > 2-nitrotoluene (89.6%) > 4-methyl NB
80.0%) > 3-chloro NB (76.7%) > 2-chloro NB (75.6%) > 4-
5
6,57
of naphthalene.
chloro NB (72.9%). For chloro-substituted nitrobenzene
compounds, the selectivity of fully-hydrogenated products is
lower than that of CHA, possibly relevant to the occurrence of
partially-dechlorinated reaction during the hydrogenation
process, for which some dechlorinated products have been
detected by GC-MS.
3
.3. Recycling studies
The recycling experiments of 3& Rh–10% Ni/CSC catalyst for
the hydrogenation of NB are conducted. As observed from
Fig. 9, the activity of catalyst is still maintained at a high level,
even if 3& Rh–10% Ni/CSC has been reused six times. This
could be ascribed to the strong interaction of the supported
metals with the support as disclosed by the H
2
-TPR proles, 4. Conclusion
further inhibiting the leaching of metallic species during the
reaction. The conversion of NB is always kept at 100 mol%,
while the selectivity of fully-hydrogenated CHA has kept an
inconspicuous uctuation of around 90.6–91.6%. ICP analyses
of the reaction mixture did not detect the leaching of Ni or Rh
from 3& Rh–10% Ni/CSC recycled. The structure of the reused
catalyst has been characterized by XRD, XPS and TEM, and the
corresponding data are shown in the Fig. S9–S11. The XRD
patterns, XPS spectra and TEM images testify that the amor-
phous structure of recovered 3& Rh–10% Ni/CSC is still
retained even if it has been reused 5 times. This study further
reveals the recyclable stability of the active catalyst 3& Rh–
This work reports highly selective ‘one-step’ liquid-phase
hydrogenation of nitrobenzene with nitro group and aromatic
ring to cyclohexylamine over the supported 10% Ni/CSC cata-
lysts doped with 3& Rh. Under optimal conditions, almost 100
mol% conversion of the substrate is obtained over 3& Rh–10%
Ni/CSC catalyst with a high selectivity of cyclohexylamine up to
91.6%, notably higher than 30–50% selectivity in industrial
processes. Some factors affecting the reaction are revealed
through conducting a number of reactions and wide charac-
terizations. The addition of alkaline LiOH as an additive limits
the occurrence of side reactions, benecial to the enhancement
of full hydrogenation. The XPS results suggest that there is a net
transfer of electrons towards Rh–Ni to increase the binding
1
0% Ni/CSC for the hydrogenation reaction of NB.
energies of Ni2p and Rh3d once NiO–RhO
x
oxides are reduced
3
.4. Catalytic hydrogenation of other substituted nitro-
to metallic Rh–Ni with H . The H -TPR temperatures of
2
2
benzenes
different samples are different from one another, which affects
Aside from the catalytic hydrogenation of NB with H
2
as stated the catalytic activity. The recycling tests reveal recyclable
above, that of other six substituted nitrobenzenes over 3& stability of 3& Rh–10% Ni/CSC catalyst.
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2
2
5 H. Greeneld, J. Org. Chem., 1964, 29, 3082.
6 C. F. Winans, Ind. Eng. Chem., 1940, 32, 1215.
Acknowledgements
The authors thank the nancial supports by National Natural 27 G. A. Somorjai and Y. Li, Top. Catal., 2010, 53, 832.
Science Foundation of China (21173073, 21273064, 21503074, 28 C. P. Vinod, Catal. Today, 2010, 154, 113.
2
1571055), by the Natural Science Fund for Creative Research 29 E. Schmidt, W. Kleist, F. Krumeich, T. Mallat and A. Baiker,
Groups of Hubei Province of China (2014CFA015), and by the
Chem.–Eur. J., 2010, 16, 2181.
014 Sci-tech Support Project of the Science and Technology 30 C. A. Witham, W. Huang, C. K. Tsung, J. N. Kuhn,
2
Department of Hubei Province of China (2014BAA098).
G. A. Somorjai and F. D. Toste, Nat. Chem., 2010, 2, 36.
1 S. Mostafa, F. Behafarid, J. R. Croy, L. K. Ono, L. Li,
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