Supported bimetallic catalyst Pt-Pb/SiO2for selective conversion of nitrobenzene to p-aminophenol in pressurized CO2/H2O system

Article abstract of DOI:10.1016/j.cclet.2016.07.029

Various supported Pt-Pb bimetallic catalysts were prepared and applied for the catalytic conversion of nitrobenzene to p-aminophenol in the environmentally benign pressurized CO₂/H₂O system. Among the bimetallic catalysts prepared, Pt-Pb/SiO₂is the best and nitrobenzene could be converted to p-aminophenol with a selectivity as high as 82% when the reaction was carried out using this catalyst at 110 °C under 5 MPa CO₂and 0.2 MPa H₂.

Full text of DOI:10.1016/j.cclet.2016.07.029

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Chinese Chemical Letters xxx (2016) xxx–xxx
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Chinese Chemical Letters
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Original article
Supported bimetallic catalyst Pt-Pb/SiO2 for selective conversion
of nitrobenzene to p-aminophenol in pressurized CO2/H2O system
*
Ting-Ting Zhang, Jing-Yang Jiang , Yan-Hua Wang
State Key Laboratory of Fine Chemicals, Faculty of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology,
Dalian 116024, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Various supported Pt-Pb bimetallic catalysts were prepared and applied for the catalytic conversion of
nitrobenzene to p-aminophenol in the environmentally benign pressurized CO2/H2O system. Among the
bimetallic catalysts prepared, Pt-Pb/SiO2 is the best and nitrobenzene could be converted to p-
aminophenol with a selectivity as high as 82% when the reaction was carried out using this catalyst at
110 8C under 5 MPa CO2 and 0.2 MPa H2.
Received 12 June 2016
Received in revised form 13 July 2016
Accepted 16 July 2016
Available online xxx
ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
Nitrobenzene
p-Aminophenol
Pt-Pb bimetallic catalysts
CO2/H2O system
Environmentally benign
1. Introduction
selectivity of 85% was obtained over Pt-Sn catalyst. In contrast to
the conventional permanent mineral acid, the self-neutralizable
p-Aminophenol (PAP) is an important intermediate for the
production of drugs, pesticides, dyestuffs, and photographic
chemicals [1]. The preparation of PAP by catalytic hydrogenation
of nitrobenzene (NB) is an important process owing to its simple
technology and low cost of raw material. This process involves
hydrogenation of NB to the intermediate N-phenylhydroxyla-
mine (PHA) and PHA’s conversion through an acid-catalyzed
rearrangement to PAP [2]. An industrialized process has been
realized by Mallinckrodt Inc. (USA) several decades ago
[3]. Sulfuric acid, which is used in the process to catalyze the
in situ rearrangement of PHA to PAP, brings considerable burden
to the environment by generating effluents because of the
inevitable neutralization step to separate PAP from the reaction
mixture. Many researchers have applied solid acids such as
S2O82ꢀ/ZrO2 [4], H-ZSM-5 [5] and MgAPO-5 [6] to replace sulfuric
acid, aiming to solve the effluents problem, but coke formation on
the acid sites [5] restricted their performance and lifetime. In our
previous work, the pressurized CO2/H2O system has been applied
in the hydrogenation of NB to PAP [7] and an optimized PAP’s
CO2/H2O system is totally rid of the effluents problem [8,9]. Nev-
ertheless highly selective catalyst for the conversion of NB to PAP
in the new system is very important. It is known that various
catalysts [10–14], including PtO2, Pt, Pd, Mo, Ni-Pt, Au, Ni-Si, etc.,
have been tested for the hydrogenation of NB to PAP, and Pt
catalysts showed to be the best for obtaining high selectivity of
PAP. Pt supported on different supports [15–19] have also been
reported and the selectivity of PAP could be as high as 88%. Some
additives such as dimethylsulfoxide [20], dimethylalkylamine
oxide [21] and the surfactant dodecyltrimethyl ammonium
bromide [22] were also explored in the reaction to modify Pt
catalyst for higher selectivity of PAP, but most of the additives
were not environmentally friendly. It should be noticed that the
application of bimetallic catalysts, using one metal component to
modify another active catalyst metal, has been an effective
method to improve the catalyst’s performance in selective
hydrogenation reactions. Lindlar catalyst [23,24] and Pt-Sn,
which are typical examples, have been applied in many selective
transformations [25–28]. Moreover, the bimetallic Pt-Pb cata-
lysts were also applied in some reactions [29,30]. On continuing
our research for the hydrogenation of NB to PAP in the
pressurized CO2/H2O system [7], various supported Pt-Pb
bimetallic catalysts were prepared and examined in the reaction.
*
Corresponding author.
E-mail address: jyjiang@dlut.edu.cn (J.-Y. Jiang).
http://dx.doi.org/10.1016/j.cclet.2016.07.029
1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: T.-T. Zhang, et al., Supported bimetallic catalyst Pt-Pb/SiO2 for selective conversion of nitrobenzene to
p-aminophenol in pressurized CO2/H2O system, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.07.029

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2. Experimental
2.1. Catalyst preparation and characterization
The Pt-Pb/SiO2 catalyst was prepared according to the known
method [6] with slightly modification: Silica (Aerosil 300, Macklin,
2 g) was impregnated with 20 mL aqueous solution of H2PtCl6
(Alfa, 82 mg, 0.2 mmol Pt) and (CH3COO)2Pb (Alfa, 65 mg,
0.2 mmol Pb) for 24 h. After the impregnation, the content in
the vessel was dried at 120 8C for 3 h and calcined at 500 8C for 4 h.
Afterward, the dried solid was reduced in hydrogen atmosphere at
300 8C for 4 h forming the catalyst. The catalysts with other
PbPt
Pt
♦
♦
♦
•
♦
♦
•
•
♦
♦
(Pt-Pb/SiO2)
supports, including carbon black (Vulcan XC-72, Macklin),
(basic type, >60 mesh, Aladdin) and ZrO2 (99.99%, 0.2–0.4
g
-Al2O3
m,
m
Aladdin), were also prepared according to the method mentioned
above.
(SiO2)
60
The Pt loading of the Pt-Pb/SiO2 catalysts were measured
using a Optima 2000DV ICP spectrometer. The sample was firstly
dispersed in aqua regia in order to dissolve all Pt, and then sent
for analysis. XRD patterns of the Pt-Pb/SiO2 catalyst and the SiO2
support were recorded on a D/max-2400 diffractometer with Cu
10
20
30
40
θ
(degree)
50
2
Fig. 1. XRD patterns of SiO2 and Pt-Pb/SiO2.
K
a radiation (l
= 0.1541 nm). The scanning rate was 4o/min in
the range of 10–65o. TEM image of the Pt-Pb/SiO2 catalyst was
taken by a Tecnai F30 electron microscope operating at 300 kV.
Sample was mounted on a copper grid-supported carbon film by
placing a few droplets of an ultrasonically dispersed suspension
of samples in ethanol, and followed by drying at ambient
conditions.
3.2. Effect of different catalysts on the hydrogenation of NB to PAP
Different supported catalysts were applied in the reaction, and
the selectivity of PAP when the reaction was catalyzed by Pt-Pb
catalysts were 12.9–19.2% higher than the selectivity when the
reaction was catalyzed by Pt catalyst (entries 1–8, Table 1). PAP’s
selectivity of 65.1% was obtained when the reaction was catalyzed
by Pt-Pb supported on the SiO2, and it was the best result of the
examined catalysts. Afterward, Pt-Pb/SiO2 catalysts with different
Pt loadings 1 wt%, 2 wt%, 5 wt% and 10 wt% were used to catalyze
the reaction. It is found that the conversion of NB increased from
12.2% to 42.6%, and the selectivity of PAP decreased from 67.2%
to 52.8% while Pt loading increased from 1 wt% to 10 wt% (entries
8–11). With lower Pt loading, PHA could desorb from the catalyst
easily and migrate into acidic phase rearranging into PAP, the
results were in agreement with those of previously reported
[6]. Pt-Pb/SiO2 (Pt: 2%) catalyst used in entry 8 was chosen for the
further studies.
2.2. Typical catalysis experiment
A 100 mL autoclave (Parr 4842) was applied to carry out the
reaction of NB to PAP. The catalyst, NB and water were
introduced into the autoclave which was then purged three
times with 0.2 MPa CO2, then the autoclave was gradually
charged with designated pressure of CO2. H2 was charged when
the autoclave reached the designated temperature under
constant stirring. The total operating pressure was maintained
constant by continuous recruitment of H2 during the reaction
process. After the reaction, the gas was released slowly. The
suspension was filtered and the solid was washed with
methanol, then the filtrate was dissolved in methanol to obtain
a homogeneous solution and analyzed by high performance
liquid chromatography (HPLC). The following method was used:
3.3. Reaction results with different nNB/nPt (molar ratio of NB to Pt)
Keeping the amount of catalyst constant, the hydrogenation of
NB to PAP was studied with different nNB/nPt from 2500 to
30000 and the results were shown in Fig. 3. The yield of PAP and the
yield of AN decreased with the increase of nNB/nPt. The selectivity of
PAP increased from 35% to 65% when the nNB/nPt increased from
2500 to 20000, and it was almost constant with nNB/nPt’s further
Agilent TC-C18 column (5
m
m, 4.6 mm ꢁ 250 mm); column
temperature: 30 8C; UV detector: 254 nm; Mobile phase: (A)
70 mmol/L aqueous solution of ammonium acetate; (B) metha-
nol. Gradient: t = 0 min 70% A 30% B, t = 20 min 0% A 100% B;
flow rate: 1.0 mL/min.
3. Results and discussion
3.1. Characterization of the Pt-Pb/SiO2 catalysts
The prepared Pt-Pb/SiO2 (2%) catalyst was characterized via
ICP-OES, XRD and TEM. The exact Pt loading was 1.890 wt%
according to the result determined by ICP-OES. XRD measurements
of the Pt-Pb/SiO2 catalyst and SiO2 support were performed and the
results were shown in Fig. 1. The alloy between Pb and Pt were
confirmed by the diffraction peak of PtPb (101) plane at 29.3o
(JCPDS–ICDD, Card No. 06-374), the result was in agreement with
that of Huang’s work [31]. TEM image of the Pt-Pb/SiO2 catalyst
was shown in Fig. 2, and it can be seen that the majority of metal
particles were dispersed uniformly on the SiO2 support within the
diameter range of 1–4 nm.
Fig. 2. TEM image of Pt-Pb/SiO2 catalyst.
Please cite this article in press as: T.-T. Zhang, et al., Supported bimetallic catalyst Pt-Pb/SiO2 for selective conversion of nitrobenzene to
p-aminophenol in pressurized CO2/H2O system, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.07.029

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3
Table 1
Effect of different catalysts on the hydrogenation of NB to PAPa
Entry
Catalyst (Pt loading: wt%)
Pt/C (2%)
Conv. of NBb (%)
Yield of PAPb (%)
Yield of ANb,c (%)
Sel. of PAP (%)
1
31.0
30.2
23.8
33.9
22.5
19.2
18.2
23.8
12.2
32.1
42.6
12.2
11.8
9.2
18.8
18.4
14.6
17.5
10.2
8.0
39.3
39.1
38.7
48.4
54.7
58.3
51.6
65.1
67.2
58.2
52.8
2
Pt/g-Al2O3 (2%)
3
Pt/ZrO2 (2%)
Pt/SiO2 (2%)
Pt-Pb/C (2%)
4
16.4
12.3
11.2
9.4
5d
6d
7d
8d
9d
10d
11d
Pt-Pb/g-Al2O3 (2%)
Pt-Pb/ZrO2 (2%)
Pt-Pb/SiO2 (2%)
Pt-Pb/SiO2 (1%)
Pt-Pb/SiO2 (5%)
Pt-Pb/SiO2 (10%)
8.8
15.5
8.2
8.3
4.0
18.7
22.5
13.4
20.1
a
Conditions: NB = 20 mmol, catalyst: 10 mg, H2O = 60 mL, PCO2 = 5.5 MPa (3 MPa initially at room temperature, at reaction temperature), PH2 = 0.2 MPa, T =120 8C, stir
rate = 1250 r/min, t = 3 h.
b
Determined by HPLC.
c
AN: aniline.
d
Molar ratio of Pb to Pt = 1.
increase. According to the commonly accepted mechanism of NB’s
conversion to PAP [12], the intermediate PHA’s remaining
absorbed on the catalyst tends to generate more byproduct AN
by further hydrogenation, while PHA’s desorption from the
catalyst and migration into the acidic phase would favor its
rearrangement into PAP. It is likely that when the nNB/nPt was low,
more PHA tended to remain absorbed on the catalyst being further
hydrogenated to AN, thus resulting in lower PAP selectivity. When
the nNB/nPt increased, more NB’s competitive adsorption would
facilitate PHA’s desorption from the catalyst, thus leading to higher
PAP selectivity. Apparently, this competitive adsorption’s effect did
not make a great difference with further increase of nNB/nPt from
20000 to 30000.
3.5. Effect of temperature on the hydrogenation of NB
The reaction was studied within a range of temperatures 90–
140 8C and the results were shown in Fig. 5. The selectivity of PAP
increased from 65% to nearly 75% when the temperature increased
from 90 8C to 110 8C, and then decreased from 75% to 55% with the
temperature’s further increase from 110 8C to 140 8C. This may
because that the yield of AN continued increasing with tempera-
ture’s increase but the yield of PAP was almost constant when the
temperature increased from 110 8C to 140 8C. Meanwhile, the color
of the reaction liquid changes gradually from pale yellow to dark
brown when the temperature was higher than 110 8C, as a result of
PAP’s conversion to other byproducts such as 4, 40-diaminodiphe-
nyl ether [32]. Therefore, 110 8C was chosen for the following
studies.
3.4. The hydrogenation of NB under different CO2 pressures
Fig. 4 showed that there was hardly any PAP produced with no
CO2 in the system, but the selectivity of PAP increased dramatically
from 0 to 70% when the initial CO2 pressure increased from 0 to
5 MPa. Apparently, the increase of CO2 pressure increased the
acidity of the system and favored the formation of PAP, the result
was in agreement with that reported in our previous work [7].
3.6. The hydrogenation of NB under different H2 pressures
Different H2 pressure from 0.1 MPa to 2 MPa were applied in
the reaction and the results were shown in Fig. 6. The yield of PAP
and the yield of AN increased with the increase of the H2 pressure,
while the selectivity of PAP gradually decreased from 82% to 62%.
The formation rate of the intermediate PHA, which would convert
to PAP or AN, was accelerated by the increase of H2 pressure, so the
100
selectivity of PAP
conversion of NB
yield of AN
80
selectivity of PAP
conversion of NB
70
80
yield of PAP
yield of PAP
yield of AN
60
60
40
20
0
50
40
30
20
10
0
0
5000
10000
15000
NB/n
20000
25000
30000
0
1
2
3
4
5
n
Pt
Pco2 (MPa)
Fig. 3. Reaction results with different nNB/nPt
. Conditions: Pt-Pb/SiO2 (Pt:
2 wt%,1 mol Pt), H2O = 60 mL, PCO2 = 5.5 MPa (at reaction temperature, 3 MPa
m
Fig. 4. The hydrogenation of NB under different CO2 pressures. Conditions:
NB = 20 mmol, Pt-Pb/SiO2 (Pt: 2 wt%, mol Pt), H2O = 60 mL, PH2 = 0.2 MPa,
T = 120 8C, stir rate = 1250 r/min, t = 3 h. PCO2: initial pressure at room temperature.
initially at room temperature), PH2 = 0.2 MPa, T = 120 8C, stir rate = 1250 r/min,
t = 3 h.
1 m
Please cite this article in press as: T.-T. Zhang, et al., Supported bimetallic catalyst Pt-Pb/SiO2 for selective conversion of nitrobenzene to
p-aminophenol in pressurized CO2/H2O system, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.07.029

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80
selectivity of PAP
conversion of NB
yield of AN
90
80
70
60
50
40
30
20
10
0
70
yield of PAP
60
50
40
30
20
10
0
selectivity of PAP
conversion of NB
yield of AN
yield of PAP
1
2
3
4
5
6
7
90
100
110
120
130
140
t (h)
T (oC)
Fig. 7. Effect of reaction time on the hydrogenation of NB to PAP. Conditions:
NB = 20 mmol, Pt-Pb/SiO2 (Pt: 2 wt%, 1 mol Pt), H2O = 60 mL, PCO2 = 7 MPa (at
Fig. 5. Effect of temperature on the hydrogenation of NB. Conditions: NB = 20 mmol,
Pt-Pb/SiO2 (Pt: 2 wt%, mol Pt), H2O = 60 mL, PCO2 = 7 MPa (at reaction
m
1
m
reaction temperature, 5 MPa initially at room temperature), PH2 = 0.2 MPa,
T = 110 8C, stir rate = 1250 r/min.
temperature, 5 MPa initially at room temperature), PH2 = 0.2 MPa, stir
rate = 1250 r/min, t = 3 h.
yield of PAP and the yield of AN increased. It is worth mentioning
that the rate of PHA’s desorption from the catalyst was almost
constant with the temperature keeping at 110 8C, so the PHA which
cannot desorb from the catalyst in time were further hydrogenated
to AN, thus leading to the decrease of PAP’s selectivity. 0.2 MPa was
chosen for the following studies.
4. Conclusion
In summary, supported Pt-Pb bimetallic catalysts were proven
to be selective for the hydrogenation of NB to PAP in pressurized
CO2/H2O system. When the reaction was carried out at 110 8C
under 0.2 MPa H2 and 5 MPa CO2 for 2 h by employing Pt-Pb/SiO2
as catalyst, PAP’s selectivity was up to 82%.
3.7. Effect of reaction time on the hydrogenation of NB to PAP
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decrease, and the result was consistent with the results in
Fig. 3. Another probable cause of PAP selectivity’s decrease may
be that the generated PAP in the system might convert to other
byproducts such as 4, 40-diaminodiphenyl ether [32].
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Please cite this article in press as: T.-T. Zhang, et al., Supported bimetallic catalyst Pt-Pb/SiO2 for selective conversion of nitrobenzene to
p-aminophenol in pressurized CO2/H2O system, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.07.029