Active palladium catalyst preparation for hydrogenation reactions of nitrobenzene, olefin and aldehyde derivatives

Article abstract of DOI:10.1016/j.molcata.2014.08.026

A series of Pd/FeO_x catalysts were prepared using co-precipitation method via controlling the washing operation after co-precipitation of the catalyst precursors. The prepared catalysts were tested by the hydrogenation reactions of nitrobenzene, benzaldehyde and styrene, respectively. The highest activity could be obtained when the catalyst precursors were washed for ten times containing ～2.2 ppm chlorine concentration in the aqueous phase. XPS characterization suggested that a suitable Pd⁰/Pd^δ+ ratio might be important for the high activity. TEM characterization showed that smaller nano-Pd particles can be generated and TPR characterization suggested the formation of more easily reducible Pd species when the catalyst precursors were washed for 10 times. Less or over-washing of the catalyst precursors result in coagulation of the nano-Pd particles. The washing operation, chlorine concentration, catalyst structure and catalytic performance can be related reasonably. The current results should promote the controllable preparation of active Pd-based catalysts.

Full text of DOI:10.1016/j.molcata.2014.08.026

Journal of Molecular Catalysis A: Chemical 395 (2014) 195–201
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Active palladium catalyst preparation for hydrogenation reactions of
nitrobenzene, olefin and aldehyde derivatives
Yanan Wanga,b, Youquan Deng , Feng Shi
a
a,∗
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:
Received 21 June 2014
Received in revised form 13 August 2014
Accepted 16 August 2014
Available online 26 August 2014
A series of Pd/FeOx catalysts were prepared using co-precipitation method via controlling the washing
operation after co-precipitation of the catalyst precursors. The prepared catalysts were tested by the
hydrogenation reactions of nitrobenzene, benzaldehyde and styrene, respectively. The highest activity
could be obtained when the catalyst precursors were washed for ten times containing ∼2.2 ppm chlorine
0
ı+
concentration in the aqueous phase. XPS characterization suggested that a suitable Pd /Pd ratio might
be important for the high activity. TEM characterization showed that smaller nano-Pd particles can be
generated and TPR characterization suggested the formation of more easily reducible Pd species when
the catalyst precursors were washed for 10 times. Less or over-washing of the catalyst precursors result
in coagulation of the nano-Pd particles. The washing operation, chlorine concentration, catalyst structure
and catalytic performance can be related reasonably. The current results should promote the controllable
preparation of active Pd-based catalysts.
Keywords:
Nano-Pd catalyst
Heterogeneous catalyst
Controllable preparation
Hydrogenation
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
in the styrene epoxidation reaction on Au (1 1 1), the introduction
of chlorine significantly improved the selectivity of styrene oxide
by inhibiting over-oxidation, and the rate of styrene oxide forma-
tion was increased, too [16]. Also, the addition of chlorine could
optimize the redox characteristic of surface molybdenum oxide
species, i.e. depression of the reduction potential of Mo6+, which
leads to increased selectivity of ethylene on Mo/Si:Ti catalysts in
the oxidative dehydrogenation of ethane [17].
It is well accepted that the presence of chlorine in noble metal
catalysts normally results in aggregation of nanoparticles, and the
chlorine might be adsorbed on the active sites of the catalyst,
which often causes deactivation of the catalyst. Therefore, it is well
adopted that the removal of chlorine from the catalyst is essential
to gain active catalysts [1–9]. However, there is no direct evidence
to prove that the low activity is caused by the presence of chlo-
rine. It cannot be concluded definitely that it is the essential step
to obtain active catalysts by removing the chlorine completely.
Recently, much effort was focused on this field and the role of
chlorine in active catalyst fabrication was explored. For example,
the presence of chlorine had little impact on the activity of the
unwashed and un-calcined supported nano-Au catalysts [10]. Dur-
ing the preparation of nano-Au-based catalysts, active Au/FeOx
catalysts for CO oxidation could be prepared by co-precipitation
method even the chlorine was maintained in the catalyst precur-
sor if the catalyst was dried at low temperature. Moreover, the
presence of chlorine could be an effective promoter to enhance the
performance of catalysts for some reactions [11–15]. For example,
In the last years, the controllable preparation of active supported
nano-Au or Pd catalysts was explored in our group by controlling
an optimum chlorine concentration in the aqueous solutions con-
taining the catalyst precursors. During the preparation of Au/Fe O3
2
catalyst with co-precipitation method, we have found that the
activity of Au/Fe O for CO oxidation and for reductive N-alkylation
2
3
of nitrobenzene were varied with the chlorine concentration in
the aqueous solutions containing the catalyst precursors, and the
catalytic activity of Au/Fe O3 reached the maximum at an appro-
2
priate chlorine concentration, i.e. 1–3 ppm [18]. We also prepared
a series of Au/NiOx catalysts with co-precipitation method by trac-
ing the chlorine concentration in the washing step and tested by
the total oxidation of CO. The results indicated that Au/NiOx cata-
lysts prepared from aqueous solutions with 4–8 ppm chlorine can
be active catalysts and the temperature for CO total oxidation was
0 C. The catalytic activity dropped sharply if the chlorine concen-
◦
<
∗ Corresponding author. Tel.: +86 931 4968142.
tration was >8 ppm or <4 ppm [19]. The Pd/Fe(OH)x catalysts with
E-mail address: fshi@licp.cas.cn (F. Shi).
http://dx.doi.org/10.1016/j.molcata.2014.08.026
1
381-1169/© 2014 Elsevier B.V. All rights reserved.

1
96
Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 195–201
◦
different activities for the hydrogenation of nitrobenzene can be
controllably prepared by tracing the chlorine concentration in the
aqueous solutions containing the catalyst precursors, and the cat-
alytic activity dropped dramatically if the chlorine concentration
was >18 ppm or <8 ppm [20]. Catalyst characterizations suggested
that catalysts prepared from aqueous solutions containing different
chlorine concentration possess different structure. Typically, much
bigger nano-Au or Pd particles were formed if the catalyst precur-
sors were over or less washed, in which the catalysts were prepared
from solutions with lower or higher chlorine concentrations.
In this work, a series of nano-Pd/FeOx catalysts were pre-
pared with co-precipitation method by controlling the washing
operations of the catalyst precursors. The variation of chlorine
concentration during the washing operation was also studied. We
chose the selective hydrogenation of nitrobenzene, benzaldehyde
and styrene as model reactions to investigate whether the activity
of Pd/FeOx can be tuned by the washing operation or the chlorine
concentration of catalyst precursor solutions.
precursors was tested at 25.0 C through potentiostatic scanning
−
on a CHI660D electrochemical workstation. A Cl ion electrode was
used as a working electrode and a platinum wire and an Ag/AgCl
electrode were used as counter and reference electrodes, respec-
tively. Before analysis, a series of Cl ion standard solutions includ-
ing 2.0 ppm, 4.0 ppm, 8.0 ppm, 40.0 ppm, 200.0 ppm and 400.0 ppm
−
−
were prepared using NaCl as the Cl ion source. The potentiostatic
scanning was carried out at interval of 0.1 s for 1 min in the standard
solutions, and the corresponding average potential value (mV) was
obtained, which were repeated three times. The electrode system
was calibrated with a 5.0 ppm standard solution and the chlorine
determination could be carried out through potentiostatic scan-
ning when it is in the range of 5.0 ± 0.5 ppm. The test limit about
this method is ∼1 ppm. For the temperature-programmed reduc-
tion (TPR) measurement, 13 mg of catalyst balanced with 37 mg
◦
SiO2 was placed in a quartz tube, and it was heated up to 350 C
under 5 mL/min O2 flow (10 C/min) and maintained 100 min. The
sample was then cooled down to 25 C under 50 mL/min Ar flow
◦
◦
and maintained for 1 h at room temperature. Then the catalyst
◦
was heated up to 350 C under 20 mL/min H (10%, balanced with
2
2
. Experimental
◦
N ) at a rate of 10 C/min. The amount of hydrogen uptake was
2
monitored on-line by a TCD detector and recorded as a function of
temperature.
2.1. Catalyst preparation
The Pd/Fe O catalysts were prepared by co-precipitation
2
3
method. Typically, 26 mmol (10.5 g) Fe(NO ) ·9H O and 1.5 mL
2.3. Catalytic activity test
3
3
2
H PdCl (2.8 M) were dissolved in 4 mL water and were drop-
2
4
wise added into 150 mL Na CO3 solution (0.47 M) in ∼0.5 h under
Typically, 1.0 mmol nitrobenzene, styrene or benzaldehyde,
10 mg catalyst and 2 mL EtOH were added into a glass tube (50 mL),
respectively. Then, it was exchanged with H2 and the reaction was
carried out in the presence of H2 at atmospheric pressure (H2 bal-
loon) at the given temperature. After reaction, 154 mg biphenyl and
10 mL EtOH were added for quantitative analysis by GC-FID (Agilent
7890A).
2
vigorous stirring. After it was further stirred for 1 h, the reac-
tion mixture was centrifuged and the recovered precipitates were
re-dispersed into 300 mL distilled water and were further ultra-
sonically washed for 1 h. Then, the reaction mixture was separated
into 6 pieces and all of them were ultrasonically washed again
by deionized water respectively (50 mL × 4, 50 mL × 6, 50 mL × 8,
5
0 mL × 10, 50 mL × 12 and 50 mL × 14). All these samples were
◦
centrifuged and the resulting slurries were dried at 100 C for 6 h,
and were further calcined at 400 C in static air. The temperature
was ramped from room temperature at a rate of 15 C/min to the
3. Results and discussion
◦
◦
3.1. Catalytic activity study
final temperature and maintained for 5 h. In the end, 6 pieces of
dark-brown Pd/Fe O catalyst samples (∼0.3 g × 6) were obtained
2
3
The influence of washing times on the catalytic performance of
and denoted as cat-1 to 6.
Pd/Fe O3 for the hydrogenation of nitrobenzene is examined first,
2
The Pd/Fe O catalysts were obtained by treating the cat-1 to
3
4
and the results were shown in Fig. 1. It could be seen that the con-
version of nitrobenzene increased with the increasing of washing
times until the washing times was up to 10, and then the conversion
of nitrobenzene decreased with more washing times.
Meanwhile, taking into account that other factor that might
affect the catalytic performance, we examined the chlorine con-
centrations of the aqueous solutions from which the catalyst
samples were prepared. It can be found that the catalytic activity
6
under hydrogen flow. The temperature was ramped from room
◦
◦
temperature at a rate of 15 C/min to 200 C and maintained for
h under hydrogen flow (20 mL/min). In the end, 6 pieces of black
Pd/Fe O catalyst samples were obtained and denoted as cat-1-r to
1
3
4
6
-r.
2.2. Catalysts characterization
of Pd/Fe O3 in nitrobenzene hydrogenation can be related to the
2
The TEM analysis was carried out on a FEI-TF20 field emission
chlorine concentration reasonably. The catalyst had the best cat-
alytic performance when the chlorine concentration in the aqueous
solution was ∼2.2 ppm. The conversion of nitrobenzene increased
with the increasing of chlorine concentrations until that was up to
∼2.2 ppm. When the chlorine concentration exceeded ∼2.2 ppm,
the catalytic activity of Pd/Fe O decreased with the increasing of
chlorine concentration. This observation was in agreement with
our former results about nano-Au catalyst preparation for CO oxi-
dation [18,19]. In order to verify the generality of the above rule,
transmission electron microscope. The catalyst samples were dis-
persed in ethanol, and the solution was mixed ultrasonically at
room temperature. A part of solution was dropped on the grid
for the measurement of TEM images. XRD measurements are con-
ducted by an X’Pert PRO (PANalytical) diffractometer. The XRD
2
3
◦
diffraction patterns were scanned in the 2ꢀ range of 10–80 . X-ray
photoelectron spectroscopy (XPS) analysis was measured using a K-
Alpha-surface analysis instrument with Al K˛ radiation (1361 eV).
Nitrogen adsorption–desorption isotherms were measured at 77 K
using Micromeritics 2010 instrument. The pore-size distribution
was calculated by Barrett, Joyner and Halenda (BJH) method from
desorption isotherm. The Pd content of the catalysts was measured
by inductively coupled plasma-atomic emission spectrometry (ICP-
the catalytic activity of the Pd/Fe O3 catalysts were further inves-
2
tigated in the hydrogenation of styrene and aldehyde and almost
the same observations were got.
Typically, in order to gain high activity, the supported palla-
dium catalysts should be pre-treated by hydrogen especially when
−
AES), using an Iris advantage Thermo Jarrel Ash device. The Cl
they were used in hydrogenation reactions. So the Pd/Fe O3 cat-
2
ion concentration in the aqueous phase containing the catalyst
alysts were reduced under hydrogen flow to see if the hydrogen

Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 195–201
197
Fig. 2. XRD diffraction patterns of catalysts Pd/Fe3O4 with different washing times:
a) cat-1-r (washed 4 times), (b) cat-4-r (washed 10 times) and (c) cat-1-r washed
4 times).
(
1
pre-treatment would influence the catalytic performance signifi-
cantly. As it was discussed above, the obtained Pd/Fe O4 catalysts
3
were tested by the hydrogenation reactions of nitrobenzene,
styrene and benzaldehyde and the results were given in Fig. 1.
Clearly, the reduction of Pd/Fe O3 results in more active Pd/Fe O4
2
3
catalyst. Similar results were obtained even the reaction time was
shortened to 1 h, but the washing times give similar influence on the
catalytic hydrogenation performance of the catalyst as on the un-
reduced catalysts Pd/Fe O . The catalytic activity increased with
2
3
the increasing of washing times when it was 4, 6, 8, or 10, respec-
tively. However, the catalytic activity decreased with the increasing
of washing times when it exceeds 10 times. About the relationship
between chlorine concentration of the aqueous solution contain-
ing the catalyst precursors and the catalytic performance, it can be
seen that similar rule as the un-reduced catalysts was observed,
too. The catalytic activity increased with the increasing of chlorine
concentration when the concentrations were 1.1, 1.5 and 2.2 ppm,
respectively, while the catalytic activity decreased with the increas-
ing of chlorine concentration when the chlorine concentration of
the aqueous phase containing catalyst precursors exceeded 2 ppm.
3
3
.2. Characterizations of Pd/Fe O
3
4
.2.1. BET characterization
Because similar rule between the washing times/chlorine con-
centration and catalytic performance was got with Pd/Fe O3 and
2
Pd/Fe O catalysts, here only Pd/Fe O was used as the model cata-
3
4
3
4
lysts to check the catalyst structure variation when it was prepared
from different conditions. In order to get comparable results, three
typical catalysts including cat-1-r, cat-4-r and cat-6-r were exten-
sively characterized by ICP-AES, BET, XRD, TEM, TPR and XPS.
The basic physicochemical properties of Pd/Fe O catalysts with
3
4
different washing times during the preparation procedure are
shown in Table 1. It can be seen that Pd/Fe O4 being washed 4
3
times has higher BET surface area than the catalyst being washed
for 10 or 14 times, and the pore volume decreased with the increas-
ing of washing times. The results of ICP-AES suggested similar
Table 1
BET surface area, pore volumes and palladium loadings of Pd/Fe3O4 catalysts.
2
Pd content (wt%)a
Catalysts
SBET (m /g)
Total pore volume (mL/g)
Fig. 1. The influence of washing operation on the catalytic performance of Pd/FeOx
cat-1-r
cat-4-r
cat-6-r
48
41
42
0.189
0.155
0.085
1.9
2.3
2.3
catalysts before (Pd/Fe2O3) and after (Pd/Fe3O4) reduction for hydrogenation reac-
−
tions (left) and the relationship between activity and Cl concentrations of the
aqueous solution from which the catalyst samples were prepared (right).
a
Palladium loadings determined by the ICP-AES.

1
98
Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 195–201
Fig. 3. TEM images of Pd/Fe3O4 with different washing times: (a,d) 4 times (b,e) 10 times (c,f) 14 times.
palladium loadings in these three samples. Therefore the high cat-
alytic activity should not be derived from the surface area and
palladium loading variations.
Card No. 89-4319]. No characteristic diffraction peak ascribed to
Pd species was observable, indicating that these palladium particles
were well dispersed and no crystallized palladium formed [21].
3.2.2. XRD characterization
3.2.3. TEM and HR-TEM characterization
Following, the catalysts were characterized by XRD and the XRD
The TEM and HR-TEM characterizations gave more information,
diffraction patterns are portrayed in Fig. 2. Based on the XRD diffrac-
tion patterns, it can be seen clearly that almost all of the diffraction
peaks can be attributed to the cubic phase of pure Fe O phase, i.e.
according to the images of Pd/Fe O4 catalysts with different wash-
3
ing times during the preparation procedure, Fig. 3. Although similar
TEM images were obtained, completely different micro-structures
were observed based on the HR-TEM images. First, the average
nano-Pd particle size in sample cat-1-r was 2.1 nm, in which the
3
4
(
1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (3 3 3), (4 4 0), (6 2 0),
5 3 3) and (6 2 2), according to the JCPDS standard card [JCPDS
(

Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 195–201
199
Table 2
XPS data of Pd/Fe3O4 catalysts with different washing times.
Catalysts
ꢁ(Pd3d5/2 − Pd3d3/2)
Surface content (%)b
Binding energy (eV)a
Pd /Pd
0
ı+
0
Pd /(Pd0 + Pdı+
)
cat-1-r
cat-4-r
cat-6-r
5.3
5.3
5.4
1.4
1.8
2.1
0.58
0.64
0.68
a
The difference (ꢁ) of binding energy between Pd3d5/2 and Pd3d3/2.
The ratio of Pd /Pd was calculated from “peak I area/peak II area” in Fig. 3.
b
0
ı+
catalyst precursors were washed for 4 times, Fig. 3(a) and (d).
Interestingly, smaller nano-Pd particles formed when the catalyst
precursors were washed for 10 times, in which it was only 1.1 nm,
Fig. 3(b) and (e). Surprisingly, bigger nano-Pd particles, i.e. 2.2 nm,
formed again if the catalyst precursors were further washed for
another 4 times, Fig. 3(c) and (f). In addition, the nano-Pd parti-
cles were well dispersed on the surface of Fe O in catalyst cat-4-r
3
4
while coagulation occurred in catalysts cat-1-r and cat-6-r. There-
fore the smaller nano-Pd particle and its well dispersion on the
support might be one of the reasons for the high activity of cata-
lyst cat-4-r. Finally, if we relate the nano-Pd particle size with the
conversions of nitrobenzene, styrene and benzaldehyde in Fig. 1, it
can be seen similar conversions of nitrobenzene, styrene and benz-
aldehyde were obtained when cat-1-r and cat-6-r were used as the
catalysts, in which the average Pd particle sizes were 2.1 and 2.2 nm.
So the catalytic activity was well related to the particle size of Pd.
3.2.4. XPS characterization
As the washing operation might influence the surface structure
of the catalysts remarkably, the Pd/Fe O catalysts were character-
3
4
ized by XPS. The results were shown in Table 2 and the Pd3d XPS
spectra were shown in Fig. 4. It can be seen that the peaks of Pd3d are
consisted of more than one contribution. Thus, the Pd3d lines were
de-convoluted to two peaks with the least-squares fitting routine
using the Gauss functions.
Fig. 4. XPS spectra of Pd/Fe3O4 with different washing times: (a) 4 times (b) 10
times (c) 14 times.
The Pd3d signals of the catalysts showed a maximum at 335.2 eV,
which agrees with the binding energy of metallic Pd. The small
peak around 337.2 eV is attributed to PdOx, and the binding energy
is higher than the PdO, i.e. ∼336.0 eV, which might be caused by
the highly dispersion of nano-Pd particles on the support [22,23].
hydrogenation reactions. Catalyst with smaller nano-Pd size and
0
ı+
suitable Pd /Pd ratio can be prepared when the catalyst precur-
sors were washed for suitable times.
0
ı+
In addition, the ratio of Pd to Pd changed significantly in these
catalyst samples, in which the only difference was that their pre-
3.2.5. TPR characterization
0
cursors had been washed for different times. The ratio of Pd to
From the results shown in Table 2, it can be seen that higher
chlorine concentration seems to enhance the Pd reduction. It will
be interesting to see its concentration effect on the TPR profiles of
the catalyst samples, which might affect the properties of both Pd
ı+
Pd increased slowly with the increasing of washing times. The
0
ı+
ratio of Pd to Pd , i.e. 1.8:1, might be a suitable composition for
high activity. Thus, the washing operation can be used as a fac-
tor for the controllable preparation of active nano-Pd catalysts for
and FeOx, Fig. 5. Clearly, from the TPR profiles of Pd/Fe O3 it can
2
0
100
200
300
0
100
200
300
o
o
Temperature ( C)
Tmeperature ( C)
Fig. 5. TPR profiles of Pd/Fe2O3 with different washing times (left): 4 times (black), 10 times (red) and 14 times (blue) and Pd/Fe3O4 with different washing times (right,
Pd/Fe2O3 was re-tested after TPR characterization): 4 times (black), 10 times (red) and 14 times (blue). (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)

2
00
Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 195–201
be seen that more reducible Pd species formed in catalyst cat-4.
The temperature for Pd reduction is at 86 and 101 C. However,
than cat-1, cat-4 and cat-6. Finally, although the reducibility of Pd
species was different in cat-1, cat-4 and cat-6, more reducible Pd
species were formed in Pd/Fe O with more washing times because
the Pd reduction peaks turned bigger synchronously. It is in good
agreement with the observation of XPS, which suggested the for-
mation of more Pd0 species with more washing times but more
reducible Pd species were formed with suitable washing times.
◦
the corresponding temperatures in samples cat-1 and cat-6 were
2
3
◦
9
2 and 134 C. Similarly, the peak temperature for the reduction of
◦
◦
FeOx in cat-4 was 258 C but it was 270 and 273 C in cat-1 and cat-
. Following, the catalysts cat-1, cat-4 and cat-6 were re-oxidized
6
for the next run of TPR characterization. Clearly, completely dif-
ferent profiles were obtained. The reduction temperature of FeOx
◦
decreased to ∼129 C. Although the peaks for Pd species reduc-
3
3
.3. Catalytic activity study
tion were overlapped, it still can be seen that more reducible Pd
species formed after reduction–oxidation cycle in cat-1-r, cat-4-r
and cat-6-r, which should be responsible for their higher activity
.3.1. Catalytic hydrogenation of nitrobenzene derivatives
Next, the scope and limitations of catalyst Pd/Fe O regarding
3
4
to a range of structurally diverse nitro aromatics were examined.
As the results shown in Table 3, Pd/Fe O4 exhibited high activ-
Table 3
3
a
Selective hydrogenation of nitro aromatics with cat-4-r.
ity on the hydrogenation of nitrobenzene with full conversion
(Table 3, entry 1). Then, the catalytic hydrogenations of nitroben-
zene derivatives, which contain different electron-donating and
electron-withdrawing groups were tested. Clearly, the involving
of methyl group at different positions affords the corresponding
aniline in excellent yields, i.e. 97–99% (entries 2–4). The intro-
duction of electron-donating groups influences the hydrogenation
reaction less and good yields were also obtained. For example,
◦
Yield/%b
Entry
1
Nitro aromatics
T/ C
t/h
2
25
25
>99
2
2
>99
>
99% yields were obtained if m- or p-MeO-nitrobenzene was
3
4
25
25
2
2
97
used as starting materials (entries 5–6). Also, the hydrogenations
of m- and p-Cl-nitrobenzene were carried out. Disappointingly,
hydrodechlorination reactions occurred although completely con-
versions were obtained. The yields of the aimed products were
>99
7
0% and 73% (entries 7–8). Hydroxyl and amine groups were well
5
6
25
2
>99
tolerated for the hydrogenation of the nitro compounds to the cor-
responding amines catalyzed by Pd/Fe O , and the products were
obtained almost quantitatively in 2 h (entries 9–13). In the case of 1-
nitro-naphthalene and 1,4-dinitrobenzene, Pd/Fe O4 showed high
catalytic activity on the hydrogenation reaction with >99% yields
3
4
25
25
25
2
2
2
>99
70
3
7c
◦
at 25–40 C in 2–3 h (entries 14–15). The hydrogenation of 2,6-di-
Me-nitrobenzene is difficult and it needs 3 h to gain 94% yield (entry
8c
73
1
6). The low activity of 2,6-di-Me-nitrobenzene might be caused by
the steric hindrance of methyl groups.
9
25
2
95
3.3.2. Catalytic hydrogenation of olefins
The catalytic hydrogenation of several olefins was further per-
1
0
1
25
25
2
3
>99
>99
formed using cat-4-r, Table 4. The hydrogenation of styrene can
be finished in 20 min with 98% yield to ethylbenzene (entry 1).
The incorporation of methyl group retards the hydrogenation reac-
tion slightly and 94% p-Me-ethylbenzene was obtained in 30 min
1
(entry 2). The ˛-Me-styrene can be hydrogenated efficiently and
9
9% yield to the desired product can be generated (entry 3). For
1
1
2
3
25
25
3
2
>99
>99
Table 4
a
Hydrogenation of olefins with cat-4-r.
Entry
1
Alkenes
t/min
20
Yield/%b
98
1
1
4
40
2
>98
2
3
4
30
30
94
99
5
25
25
3
3
98
94
1
6
30
30
99
96
a
Reaction conditions: 1.0 mmol substrate, 10 mg Pd/Fe3O4 (cat-4-r, 2 wt% Pd,
5
◦
∼
1 mol% Pd to starting material), 2 mL EtOH, H2 balloon, 25–40 C, 2–3 h.
b
a
GC yield.
Reaction conditions: 1.0 mmol substrate, 10 mg Pd/Fe3O4 (cat-4-r, 2 wt% Pd,
c
◦
Hydrodechlorination reaction occurred during the hydrogenation reactions and
∼1 mol% Pd to starting material), 2 mL EtOH, H2 balloon, 25 C.
b
the sole by-product is aniline.
GC yield.

Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 195–201
201
Table 5
styrene reached the maximum at ∼2.2 ppm chlorine when the cat-
alyst precipitate was washed for 10 times, otherwise the catalytic
activity would drop gradually. However, at this stage, it cannot be
determined is the chlorine or the washing operation responsible for
Hydrogenation of nitro aldehydes with cat-4-r.a
◦
Yield/%b
>99
Entry
1
Aldehydes
T/ C
t/h
1
the catalytic activity variation. By applying the Pd/Fe O4 catalyst,
3
25
nitrobenzene, benzaldehyde and styrene with different substituted
groups can be hydrogenated to the corresponding products with
excellent yields under the optimized reaction conditions.
2
3
25
25
16
16
95
99
Acknowledgements
We thank the National Natural Science Foundation of China
(21303228) and the CAS for financial support.
References
4
40
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98
[
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6
a
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5
[
3.3.3. Catalytic hydrogenation of benzaldehyde derivatives
[
Then the catalytic hydrogenation of benzaldehyde derivatives
1
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◦
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2
5–40 C in 12–16 h (entries 2–4).
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1
4
. Conclusions
[
[
In summary, the active Pd/Fe O3 and Pd/Fe O4 catalysts can be
2
3
controllably prepared with co-precipitation method via controlling
the washing operation after co-precipitation but before drying and
calcination. Also, this procedure can be realized by tracing the chlo-
rine concentrations in the aqueous phase during the washing of the
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2
3
Pd/Fe O for the hydrogenation of nitrobenzene, benzaldehyde and
3
4