Active Pd/Fe(OH)x catalyst preparation for nitrobenzene hydrogenation by tracing aqueous phase chlorine concentrations in the washing step of catalyst precursors

Article abstract of DOI:10.1016/j.tet.2014.02.057

By tracing the chlorine concentrations in the aqueous solutions containing the catalyst precursors, Pd/Fe(OH)_x catalysts with different activities can be controllably prepared. For the hydrogenation of nitrobenzene, the active Pd/Fe(OH)_x catalysts were obtained from aqueous solutions with chlorine concentrations below 18 ppm and above 8 ppm.

Full text of DOI:10.1016/j.tet.2014.02.057

Tetrahedron xxx (2014) 1e5
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Active Pd/Fe(OH) catalyst preparation for nitrobenzene
x
hydrogenation by tracing aqueous phase chlorine concentrations in
the washing step of catalyst precursors
a
,
b
a
a
a,
*
Chengming Zhang , Xinjiang Cui , Youquan Deng , Feng Shi
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, No. 18, Tianshui Middle Road, Lanzhou 730000, China
University of Chinese Academy of Sciences, Beijing 100049, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
By tracing the chlorine concentrations in the aqueous solutions containing the catalyst precursors, Pd/
Received 18 November 2013
Received in revised form 13 February 2014
Accepted 20 February 2014
Available online xxx
Fe(OH)
benzene, the active Pd/Fe(OH)
trations below 18 ppm and above 8 ppm.
x
catalysts with different activities can be controllably prepared. For the hydrogenation of nitro-
x
catalysts were obtained from aqueous solutions with chlorine concen-
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Chlorine
Heterogeneous catalysis
Catalyst preparation
Nitrobenzene
Hydrogenation
1
. Introduction
halogen group.9 For the styrene epoxidation on Au(111), the in-
troduction of chlorine significantly enhanced the selectivity of
styrene oxide, and the rate of styrene oxide formation was in-
It is widely accepted that the presence of chlorine in noble metal
catalysts, especially nano-gold catalysts, normally results in ag-
gregation of nano-scale particles, and the chlorine is also possibly
adsorbed on the active sites of the catalysts and then leads to
10
creased, too. During the exploration of Au/Fe
ration with co-precipitation method, we have found that the
activity of Au/Fe catalyst for CO oxidation varied with the
chlorine concentration of the aqueous solution containing the
2 3
O catalyst prepa-
2 3
O
1
e4
dramatic loss of catalytic activity or complete deactivation.
11
Therefore, in order to gain highly active catalysts, the operation of
total removal of chlorine from the catalyst is often preferentially
considered.2 However, the presence of chlorine in the catalysts is
not always detrimental to obtain active catalyst. As an example, if
the chlorine was maintained in the catalyst precursor without
catalyst precursors. By tracing the chlorine concentration, we
found that the catalytic activity for CO oxidation reached the
maximum at an appropriate chlorine concentration. Consequently,
we conjecture that this experience may also be applicable to other
catalyst systems.
e6
washing in the preparation of Au/FeO
x
with co-precipitation
As an important intermediate in chemical industry, the syn-
theses of aromatic amines have been studied for long time. Except
the traditional route using iron as reducing agent, a series of
method, the catalyst can also be active in CO oxidation reaction if
ꢀ
12
it was dried at low temperature (<200 C) and no further calcina-
7
tion at relatively high temperature. Furthermore, it was also re-
heterogeneous catalysts, such as supported Pd and Pt have been
developed and nice results were obtained in the catalytic hydro-
genation of nitrobenzene in the presence of molecular hydro-
ported that for some reactions, the presence of chlorine in the
catalysts was favorable to improve the activity. For instance, the
1
3e19
powdered Fe
2
O
3
treated with chloride ion exhibits appreciable high
gen.
polymer anchored Pd(II) complexes as catalyst was also reported.
Moreover, other supported nano-Pd catalysts stabilized by surfac-
The catalytic hydrogenation of nitrobenzene using
2
0
activity for the photo-adsorption of NO, whereas the pure iron
8
oxide does not. Based on the silver catalyzed partial oxidation of
21
22
23,24
25
ethylene to ethylene epoxide (EO), periodic density functional
theory calculations proved that chlorine was the best promoter in
tants, dendrimers, ionic liquids,
and polyvinylpyridine
were used in various fields and exhibits excellent performance, too.
In this work, the catalytic hydrogenation of nitrobenzene was
used as a model reaction to study whether the activity of supported
*
Corresponding author. E-mail address: fshi@licp.cas.cn (F. Shi).
0
040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.02.057
Please cite this article in press as: Zhang, C.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.02.057

2
C. Zhang et al. / Tetrahedron xxx (2014) 1e5
nano-Pd catalyst can be varied by tracing the chlorine concentrations
of the aqueous solutions during the washing of the catalyst pre-
BET. The results were given in Table 1. The Pd loadings of these
samples are 1.4%, 1.5%, and 1.6%, respectively. Fe(OH) -0 has the
highest BET surface area, i.e., 263 m /g. When Pd was loaded and
chlorine was introduced, the BET surface area decreased slightly.
x
2
cursors without any other stabilizer. A series of Pd/Fe(OH)
x
catalysts
were prepared by tracing the chlorine concentrations of the aqueous
solutions containing the catalyst precursors with reduction-
The BET surface areas of Pd/Fe(OH)
x
-4.2, Pd/Fe(OH)
x
-9, and Pd/
2
precipitation method. To our delight, active Pd/Fe(OH)
x
catalysts
Fe(OH) -48 are 258, 246, and 239 m /g, respectively. These results
x
2
6
for nitrobenzene hydrogenation can be obtained when the chlorine
concentrations are below 18 ppm and above 8 ppm while less active
catalysts will be produced when it deviates from this range.
are in agreement with the reports by Liu and co-eworkers. Fur-
thermore, a remarkable difference was observed from the chemical
state of Pd species according to the XPS spectra as shown in Fig. 2.
0
2þ
The Pd species of Pd/Fe(OH)
x
-4.2 were a mixture of Pd and Pd
0
with a ratio of about 1.2:1. Whereas, the amount of Pd decreased in
2
2
. Results and discussion
samples Pd/Fe(OH)
x x
-9 and Pd/Fe(OH) -48 and the corresponding
0
ratios were about 0.9:1 and 0.7:1. After reaction, the ratio of Pd and
.1. Catalytic activity measurement
2
þ
Pd of Pd/Fe(OH)
x
-4.2 and Pd/Fe(OH)
x
-48 increased to 3.7:1 and
-9 only increased
0
2þ
4.9:1. But the ratio of Pd and Pd in Pd/Fe(OH)
x
3
x
8 samples of Pd/Fe(OH) catalyst were prepared from aqueous
to 1.6:1. These results suggested that chlorine concentration of the
aqueous solution or the different washing operation can mediate
the properties of the catalysts and then obtain the active catalyst
with specific palladium species. If considering the results of cata-
lytic reaction together with the characterization results, we can
solutions with chlorine concentrations in the range of 0e250 ppm
with several batches. The activity of the catalyst samples were
tested by the hydrogenation reaction of nitrobenzene and the re-
sults were shown in Fig. 1. Clearly, there is a peak area in the cat-
alytic hydrogenation activity, i.e., catalyst samples prepared from 8
to 18 ppm chlorine concentrations possessed the best catalytic
performance, which normally generated >90% yields to aniline.
Furthermore, there are two boundaries related to the decrease of
catalytic activity. One side which slope is more sharply is 6e8 ppm,
and the other side which slope is more gently is 18e50 ppm. Also,
when the chlorine concentration is higher than 80 ppm, the hy-
0
conclude partially that suitable BET surface area and suitable Pd /
2
þ
Pd ratio might be important for the high activity and the active
catalyst can be obtained by controlling the chlorine concentrations
of the aqueous solutions in the range of 8e18 ppm.
Table 1
Physicochemical properties of catalyst samples
x
drogenation activity of Pd/Fe(OH) remains stable. In this range, the
effect of chlorine or washing operation might reach the maximum
and therefore the catalytic activity didn’t vary.
Entry
Catalysts
Fe(OH) -0
Pd/Fe(OH)
Pd/Fe(OH)
Pd/Fe(OH)
Pd/wt %a
Pd /Pd
0
2þ b
S
BET/m /g
2
[Cl ]/%b
ꢁ
1
2
3
4
x
0
d
263
258
246
239
0
x
x
x
-4.2
-9
-48
1.4
1.5
1.6
1.2(3.7)
0.9(1.6)
0.7(4.9)
0.94
1.08
1.48
a
Determined by ICP-AES.
b
Determined by XPS. The numbers in the brackets are obtained from the used
catalysts.
Fig. 1. The yield to aniline versus the chlorine concentration of the aqueous solution
during catalyst preparation.
It is easy to understand the decrease of catalytic activity when
the chlorine concentration is in the range of 18e50 ppm because it
is a common sense that the presence of chlorine might poison the
active site. The remarkable dropping in catalytic activity is un-
common if further remove the chlorine when its concentration in
aqueous solution is <8 ppm. As it was shown in the experimental
section, the only difference in these samples was that they were
prepared from different amount of aqueous solutions with different
chlorine concentrations. So the different activity of these catalysts
might be derived from the different chlorine concentrations or the
washing operation.
x
Fig. 2. XPS spectra of Pd/Fe(OH) -Y before (a) and after (b) reaction.
2
.2. Characterization of the catalysts
Following, the detailed results of nitrogen adsorption-
desorption study of Pd/Fe(OH) -4.2, Pd/Fe(OH) -9, and Pd/
Fe(OH) -48 were given in Fig. 3. All samples showed well-
pronounced H type hysteresis loops above P/P
¼0.8. The hyster-
esis loop of Pd/Fe(OH) -9 is distinctly different from the other
In order to explore the influence of washing step on catalyst
x
x
structure, three catalyst samples including Pd/Fe(OH)
Pd/Fe(OH) -9, and Pd/Fe(OH) -48, with which the yields to aniline
were 39%, 99%, and 50%, were characterized by ICP-AES, XPS, and
Please cite this article in press as: Zhang, C.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.02.057
x
-4.2,
x
x
x
1
0
x

C. Zhang et al. / Tetrahedron xxx (2014) 1e5
3
ꢀ
ꢀ
observed at 35 and 63 , which indicate that the Fe(OH)x supports
during the reaction.2
7e29
were partially reduced to Fe
As shown in Fig. 5, samples Pd/Fe(OH)
Pd/Fe(OH) -48 were characterized by TEM and HR-TEM. Clearly,
3 4
O
x
-4.2, Pd/Fe(OH) -9, and
x
x
catalysts prepared from aqueous solutions with different chlorine
concentrations possess quite different micro-structures. The par-
ticle sizes of Pd were about 5 nm and the crystal lattices of Pd(111)
can be observed. In addition, although it was not labeled, the lat-
tices with a distance of 0.27 nm, which might assigned to goethite
(
301), and the lattices with a distance of 0.22 nm, which might be
3
0
assigned to goethite (211), were observable, too.
Fig. 3. N
2 x
adsorptionedesorption isotherms of Pd/Fe(OH) -Y (a) and the pore size
distributions.
samples. In addition, large pores with size of 21 nm were observed
in Pd/Fe(OH) -9 and Pd/Fe(OH) -48. So the changing of chlorine
concentration in aqueous solution or the different washing opera-
tion would influence the structure of Pd/Fe(OH) -Y remarkably, and
then affect the catalytic activity of the catalysts. The XRD diffraction
patterns of catalysts Pd/Fe(OH) -4.2, Pd/Fe(OH) -9, and Pd/Fe(OH)
8 are presented in Fig. 4. All XRD diffraction patterns are similar
x
x
x
x
x
x
-
4
and no obvious diffraction peak is observable in all the samples in
Fig. 4a, which indicates that all the samples are not well-
crystallized. Meanwhile, it suggested that the palladium species
were highly dispersed on the support. As shown in Fig. 4b, the
diffraction patterns of all samples after reaction are similar, too, and
3 4
two distinct peaks for (311) and (440) lattices of Fe O were
Fig. 5. TEM (left) and HR-TEM (right) images of Pd/Fe(OH)
e), Pd/Fe(OH) -48 (c, f).
x x
-4.2 (a, d), Pd/Fe(OH) -9 (b,
x
2.3. Scope and limitations
x
Next, the generality of Pd/Fe(OH) -9 catalyst was tested by the
hydrogenation of nitrobenzene derivatives. Almost all the nitroben-
zene derivatives can be converted into the corresponding anilines
with excellent yields, Table 2. First, nitrobenzene derivatives with
methyl groups on different positions of the aromatic rings can be
quantitatively converted into the corresponding anilines in 1 h (en-
tries 1e3). These results suggested that the position of the functional
groups is well tolerant with this catalyst system. Interestingly, the
hydrogenation of 2,6-di-methyl-nitrobenzene can be realized at low
temperature, too (entry 4), which indicated that the steric hindrance
did not affect the reactivity of nitro group. Nitrobenzene derivatives
Fig. 4. XRD diffraction patterns of catalysts before (a) and after (b) reaction.
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4
C. Zhang et al. / Tetrahedron xxx (2014) 1e5
Table 2
hydrogenation of 4-chloro-nitrobenzene and 3-chloro-nitrobenzene,
a
Catalytic hydrogenation of nitrobenzene derivatives
dechlorination reactions occurred and the yields to the desired
products were decreased to 92% and 91% (entries 11e12). The in-
troduction of electro-withdrawing groups, such as eCHO, eCOOH
suppresses the hydrogenation reaction remarkably and longer re-
action time is needed in order to gain good enough yields (entries
ꢀ
Entry
1
Substrate
T/ C
t/h
Yield/%
13e15). Importantly, the reducible aldehyde group was tolerant. As to
the hydrogenation of 1-nitro-naphthalene, low conversion was ob-
tained even it was reacted for 5 h (entry 16).
60
60
60
40
1
>99
x
Finally, the reusability of Pd/Fe(OH) -9 was tested under the typ-
ical reaction conditions using nitrobenzene hydrogenation as model
reaction. The catalyst was separated by centrifugation, washed with
2
3
4
1
1
1
>99
>99
>99
ꢀ
distilled water and ethanol, and followed by drying in air at 80 C.
However, the yield of aniline decreased to 81% in the second run.
According to the ICP-AES measurement, the Pd loading of the catalyst
after use was 1.31%, which indicated that the decrease of catalytic
activity was not caused by Pd leaching. If taking the results of XRD
characterizationintoconsideration, wecanimaginethatthechanging
of catalyst structure might be the reason for the activity losing.
3. Conclusions
5
60
1
>99
In summary, the influence of chlorine or the washing operation
on the catalytic activity of Pd catalyst in nitrobenzene hydrogenation
reaction was discussed. By tracing the chlorine concentrations in the
aqueous solutions containing the catalyst precursors, the catalytic
activity of the final Pd catalysts can be controlled. For the hydroge-
nation of nitrobenzene, the active Pd/Fe(OH)x catalysts were ob-
tained with chlorine concentrations below 18 ppm and above 8 ppm.
By applying the Pd/(HO)x catalyst, nitrobenzenes with different
substituting groups can be hydrogenated to the corresponding ani-
lines with excellent yields under the optimized reaction conditions.
6
7
60
40
1
1
>99
>99
8
9
60
2
>99
60
60
2
5
>99
>99
4
4
. Experimental
.1. Typical procedure for support preparation
1
1
0
1
The Fe(OH)
lution of Fe(NO
into 100 mL Na
x
support was prepared as follows. An aqueous so-
$9H O (1.3 mol/L, 50 mL) was added drop-wise
CO (0.7 mol/L) solution under vigorous stirring.
3
)
3
2
2
3
40
1
92
The reaction mixture was further stirred for 1 h and the precipitate
was separated by centrifugation. The precipitate was ultrasonically
ꢀ
washed by water (1000 mLꢂ3), dried at 80 C for 5 h and a brown
1
1
2
3
40
60
1
5
91
x
Fe(OH) sample was obtained.
4.2. Typical procedure for catalyst preparation
>99
1
g Fe(OH) and 46 mg K PdCl (0.185 mmol) were added into
x
2
4
a solution of methanol and water (20:40/mL) and stirred for 3 h.
Then it was ultrasonically treated for 6 h. The obtained solution was
divided into four equal parts, centrifugally separated and ultra-
sonically washed for 0.5 h with different amount of water. The
brown precipitation obtained by centrifugation was dried at 80 C
for 5 h in air and a series of catalyst samples were obtained and
1
1
4
5
6
60
60
60
5
5
5
>99
>99
ꢀ
x
denoted as Pd/Fe(OH) -Y, in which Y represented the chlorine
concentration of the aqueous solution in ppm in the last washing
step. Typically, aqueous solutions containing 4.2, 9, 48, and 86 ppm
chlorine can be obtained with the addition of 800, 450, 80, and
25 mL deionized water for washing.
1
>55
a
1
mmol substrates, 50 mg catalyst, 2 mL toluene, 1 atm (H
2
balloon). The yields
of the products were analyzed by GC-FID (Agilent 7890A) using biphenyl as internal
standard material.
4.3. Catalyst characterization
substituted by methoxyl, hydroxyl, and amine groups were well re-
duced from the nitro compounds to the corresponding amines, too
TEM analysis was carried out with a Tecnai-G2-F30 field emission
transmission electron microscope. XRD measurements were con-
(
entries 5e10). Moreover, the 4-nitro-phenol can be converted into
the corresponding amine with >99% yield (entry 7). In the case of
Please cite this article in press as: Zhang, C.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.02.057
ducted by an X’Pert PRO (PANalytical) diffractometer. The XRD dif-
ꢀ
fraction patterns were scanned in the 2
q
range of 10e80 . The XPS

C. Zhang et al. / Tetrahedron xxx (2014) 1e5
5
measurements were performed with a VG ESCALAB 210 instrument
References and notes
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3
ꢁ
4
00.0 ppm chloride were prepared using NaCl as the Cl ion source.
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.5. Characterization results of typical hydrogenation
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ꢀ
1
p-Toluidine: white powder, mp¼44e45 C; H NMR (400 MHz,
2
3
9. Wu, Z. S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; M u€ llen, K. J. Am. Chem. Soc. 2012,
CDCl
s, 3H); IR:
3
)
d
¼6.96e6.99 (d, 2H), 6.59e6.62 (d, 2H), 3.47 (s, 2H), 2.24
134, 9082e9085.
ꢁ
ꢀ
1
(
n
3420, 3340, 1620, 1513, 1268, and 813 cm
.
0. A referee pointed out that Fig. 4, XRD showed that all catalysts are amorphous.
However, Fig. 5, HR-TEM showed the catalysts are still crystalline. For a matter,
they are either crystalline or amorphous. It seemed not clear about the nature
of the catalysts themselves and this made the interpretation look contradict.
Our response is following. The crystal particles determined by HR-TEM were
normally <5 nm. In XRD characterization, such small crystals normally result in
peak broadening and peak overlap, finally forming amorphous like diffraction
patterns. Therefore our observations from XRD and TEM characterizations
should not be contradict.
1
p-Phenylenediamine: brown powder, mp¼146e147 C; H NMR
(
1
400 MHz, CDCl
3
)
d
¼6.57 (s, 4H), 3.33 (s, 4H); IR:
n 3380, 3302, 3196,
ꢁ
1
630, 1573, 1435, 1263, and 830 cm
.
Acknowledgements
We thank the National Natural Science Foundation of China
(
21073208) and the CAS for financial support.
Please cite this article in press as: Zhang, C.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.02.057