One-pot synthesis of aluminum oxyhydroxide matrix-entrapped Pt nanoparticles as an excellent catalyst for the hydrogenation of nitrobenzene

Article abstract of DOI:10.1039/c3ra46776g

Aluminum oxyhydroxide matrix-entrapped Pt nanoparticles (Pt/AlO(OH)) were synthesized via a one-pot procedure, by the reduction of Pt⁴⁺ followed by the hydrolysis of Al(O-sec-Bu)₃. Small and well-dispersed Pt nanoparticles were entrapped into an aluminum oxyhydroxide matrix and confirmed by TEM characterization. FTIR analysis indicated that the Pt/AlO(OH) catalyst had a large amount of surface hydroxyl groups, which potentially improves its dispersibilty in aqueous solution. The as-prepared catalyst was used for the hydrogenation of nitrobenzene to aniline at 30 °C and atmospheric hydrogen pressure. Compared with other alcohol-water media, the hydrogenation reaction in a methanol-water medium exhibited a maximum turnover frequency (TOF) of 3620 h^-1. A complete conversion of nitrobenzene with a selectivity of 99.0% was obtained with an increase of time to 150 min.

Full text of DOI:10.1039/c3ra46776g

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One-pot synthesis of aluminum oxyhydroxide
matrix-entrapped Pt nanoparticles as an excellent
catalyst for the hydrogenation of nitrobenzene
Cite this: RSC Adv., 2014, 4, 10997
Guangyin Fan,*ab Yinhu Wang and Chenyu Wangb
a
Aluminum oxyhydroxide matrix-entrapped Pt nanoparticles (Pt/AlO(OH)) were synthesized via a one-pot
4+
procedure, by the reduction of Pt
3
followed by the hydrolysis of Al(O-sec-Bu) . Small and well-
dispersed Pt nanoparticles were entrapped into an aluminum oxyhydroxide matrix and confirmed by
TEM characterization. FTIR analysis indicated that the Pt/AlO(OH) catalyst had a large amount of surface
hydroxyl groups, which potentially improves its dispersibilty in aqueous solution. The as-prepared
Received 18th November 2013
Accepted 3rd January 2014
ꢀ
catalyst was used for the hydrogenation of nitrobenzene to aniline at 30 C and atmospheric hydrogen
pressure. Compared with other alcohol–water media, the hydrogenation reaction in a methanol–water
DOI: 10.1039/c3ra46776g
ꢁ1
medium exhibited a maximum turnover frequency (TOF) of 3620 h . A complete conversion of
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nitrobenzene with a selectivity of 99.0% was obtained with an increase of time to 150 min.
catalytic activity and selectivity for the hydrogenation of hal-
oaromatic nitro-compounds. This was attributed to the forma-
Introduction
Aniline has been recognized as an attractive intermediate for tion of hydrogen bonds between the substrate and the hydroxyl
4
the production of polyurethanes, dyes, pharmaceuticals, groups, to activate the N]O bond in the nitro group. It was
1
explosives and other ne chemicals. In comparison with the proposed that the carriers of catalysts with abundant hydroxyl
other techniques that have been employed for the production of groups would exhibit a higher activity and selectivity in the
aniline, the catalytic method is gaining increasing attention presence of water.
since it is considered to be the “green” pathway, with the
combined advantages of environmental friendliness and atom supported noble and non-noble transition metal NPs (e.g.
Among the catalysts that have been employed, a number of
2,5–7
1,8–13
14–17
18–22
economy. Especially, the catalytic hydrogenation of nitroben- Ru,
Pt,
Pd
and Ni
) have been synthesized for the
zene (NB) to aniline in an aqueous solution using transition or production of aniline from the hydrogenation of nitrobenzene,
noble metals as catalysts, is the subject of the most extensive among which Pt-based catalysts are highly valued. Apart from
studies. Water, the most benign, abundant and inexpensive the commonly used supports, a variety of carriers such as gum
2
3
12
media, can be used as a solvent or additive for a wide range of acacia, reduced graphene oxide (RGO), and carbon nano-
1
chemical transformations, particularly hydrogenation.
A
tubes have been explored in recent years, to deposit Pt as an
number of groups have demonstrated that the addition of water effective catalyst in the hydrogenation of NB. Despite the
into the reaction system can signicantly improve the catalytic advantage of convenience, most of the Pt-supported catalysts
performance of heterogeneous catalysts. For example, Pie- prepared by the impregnation method lack adhesion between
2
trowski et al. found that the addition of water in methanol can the Pt particles and the support, which severely affects their
signicantly increase the activity of a Ru/MgF
2
catalyst. Very interaction and inhibits further enhancement of their catalytic
3
recently, Zhao and co-workers studied the effect of water on the properties. Thus, improving the adhesion between the Pt
hydrogenation of o-chloronitrobenzene over Pt/C and Pd/C particles and the support is strongly related to the further
catalysts in ethanol, n-heptane and compressed carbon dioxide exploration of the catalytic potentials of Pt catalysts for the
ꢀ
4
at 35 C and 4.0 MPa H
hydrous zirconia-supported Ir NPs exhibited an excellent remained a big challenge.
Bearing this in mind, we propose our work, which involves
2
. In a previous study, we also found that hydrogenation of nitrobenzene. Up to now, this has still
the preparation and characterization of AlO(OH)-supported Pt
Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, NPs with a smaller particle size, as well as the study of their
a
College of Chemistry and Chemical Industry, China West Normal University,
Nanchong 637002, China. E-mail: fanguangyin@cwnu.edu.cn; Fax: +86 817-
catalytic properties. The Pt/AlO(OH) catalyst can be easily
obtained through the entrapped Pt nanoparticles in the
supporting AlO(OH) matrix, which was generated from
2
568081; Tel: +86 817-2568081
b
Department of Chemistry, State University of New York at Binghamton, Binghamton,
New York 13902, USA
3
the hydrolysis of Al(O-sec-Bu) . The catalytic performance of the
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Pt/AlO(OH) was investigated for the hydrogenation of NB, and
ꢀ
was found to be excellent at 30 C with atmospheric hydrogen
pressure in a methanol–water medium.
The morphology of the as-prepared Pt/AlO(OH) was charac-
terized by TEM and the results are illustrated in Fig. 1. The TEM
image of the Pt/AlO(OH) exhibits a nanober prole which is
characteristic for the aluminum oxyhydroxide matrix. High
resolution transmission electron microscopy (HRTEM) was
applied to detect the details of the Pt NPs. It can be seen that the
Pt NPs are well dispersed on the aluminum oxyhydroxide matrix
with a mean particle size of 2.0 nm. In the EDX analysis, the
signals of Pt and Al were distinctly detected, and the loading of
Pt was estimated to be 2.6 wt%, in agreement with the ICP
results.
The XRD pattern of the Pt/AlO(OH) was also investigated. As
can be seen from Fig. 2, the diffraction peaks appear at 28.2, Fig. 2 XRD pattern of the Pt/AlO(OH) catalyst.
ꢀ
3
8.4, 40.5, 45.8, 49.3, and 65.2 which correspond to (021), (130),
(
(
111), (131), (002), and (200) Bragg diffractions of AlO(OH)
JCPDS no. 74-1895), respectively. Interestingly, no obvious agreement with the literature. Moreover, the high yield of the
23
diffraction signals have been detected for the Pt phase, possibly desired product conrmed that most of the Pt NPs could be
due to the small size and well-dispersing condition of the Pt reduced during the hydrogenation process.
particles.
The surface functional groups of the Pt/AlO(OH) were
The XPS elemental survey scans of the surface of the Pt/ detected and characterized by infrared spectroscopy. The
AlO(OH) show that the peaks corresponding to oxygen, carbon, results shown in Fig. 4 exhibit two strong peaks at 3439 and
ꢁ
1
platinum and aluminum are distinctly detected. High resolu- 1648 cm
tion XPS was carried out to determine the electronic state of the bending modes of the adsorbed water, indicating that the
Pt in AlO(OH). As shown in Fig. 3, in spite of the partial overlap catalyst prepared by the hydrolysis of Al(O-sec-Bu) contained a
which can be attributed to the stretching and
24
3
ꢁ
1
between the Al2p and Pt4f peaks, the resolution of the XPS large amount of hydroxyl groups. The peak at 1067 cm was
measurement is high enough to distinguish the electronic state attributed to the asymmetric stretching vibration of (OH)–Al]
2
4,25
ꢁ1
of Pt. The binding energies of the Pt4f5/2 and Pt4f3/2 levels in the O,
and the peaks at 762 and 632 cm were assigned to the
25
catalyst are 74.0 and 77.7 eV respectively, which was attributed bending of (AlO)–O–H and (OH)–Al]O, respectively, indi-
to the lower charge density of the Pt NPs. As it is well-known that cating that the formation of the Pt/AlO(OH) was concomitant
4
+
NaBH
can reduce Pt ions readily, the presence of a lower with the XRD results.
4
charge density of Pt NPs can be ascribed to the heating treat-
The catalytic properties of Pt/AlO(OH) were investigated by
ment of the as-prepared catalyst in the air, which is in the hydrogenation of NB in a methanol–water mixture, at 30 C
and balloon hydrogen pressure. A blank test, or just an AlO(OH)
ꢀ
catalyst, showed no activity toward NB hydrogenation, indi-
cating that the Pt species provide the active sites for the
reduction of NB. Fig. 5 shows the continuous conversion of NB
with increasing reaction time. The conversion of NB reached
Fig. 1 (a) TEM image of Pt/AlO(OH), (b) HRTEM image of Pt/AlO(OH)
(
inset is the size distribution of the Pt nanoparticles) and (c) EDX of Pt/
AlO(OH).
Fig. 3 XPS pattern of the Pt/AlO(OH) catalyst.
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of 150 min. Apart from NSB no other by-products were detected,
which contributed to a high yield of AN. Compared with the
reported Pt-based catalytic systems, which are highly selective
to the hydrogenation of nitrobenzene, the catalytic performance
of the Pt/AlO(OH) in methanol–water medium is the best one.
23
Sreedhar et al. reported the synthesis of a Pt/GA catalyst for the
hydrogenation of NB to AN in water; they could obtain a 88%
ꢁ1
yield of AN with a TOF of 52 h , at room temperature under
1
atmospheric pressure. Liu et al. reported the solvent-free
hydrogenation of NB to AN using Pt/MWCNTs as a catalyst. They
could achieve a 100% conversion of NB and 100% selectivity to
ꢁ1
AN with the highest TOF of 66 900 h . However, this reaction
ꢀ
should be performed at 4.0 MPa H
2
and 60 C. Later they
2
reported the catalytic hydrogenation of NB over CeO nanowire-
Fig. 4 FTIR pattern of the Pt/AlO(OH) catalyst.
supported Pt nanoparticles, which yielded a 92.0% conversion
ꢀ
of NB and a 99.7% selectivity to aniline at 60 C and 2 MPa H
2
.
However, the expensive equipment for the synthesis of the CeO
2
nanowires in supercritical CO -expanded ethanol hinders its
2
10
application. By using Pt/AlO(OH) as a catalyst, we can obtain a
ꢁ
1
ꢀ
9
9% yield of AN with a TOF of 724 h at 30 C under balloon
hydrogen pressure.
Solvents strongly affect the catalytic properties of heteroge-
neous catalysts, and low boiling point alcohols such as meth-
anol, ethanol and i-propanol etc. were commonly employed in
the hydrogenation of nitro-compounds. Thereby, we investi-
gated the catalytic performances of Pt/AlO(OH) in different
media, including nonprotonic and protonic solvents. The results
are illustrated in Table 1. Generally, when protonic alcohols were
used as solvents, the Pt/AlO(OH) catalyst showed a higher activity
and selectivity, whereas the activity and selectivity obtained in
the nonprotonic solvents were not satisfactory. Particularly, the
turnover frequency (TOF) achieved in the methanol solution was
Fig. 5 Effect of reaction time on the hydrogenation of NB over Pt/
AlO(OH).
ꢁ
1
up to 3260 h , which was much higher than that of other
solvents. Thus, further investigations were focused on catalytic
reactions carried out in the methanol medium.
99.0% with a 90.6% selectivity for AN within 60 min. The
Recently, several groups have reported that the addition of
selectivity of AN increased to 99.0% as a result of the further water into the lower boiling point alcohols can signicantly
hydrogenation of the intermediate NSB to AN, in a reaction time enhance the catalytic performance of the catalysts. To this end,
a
Table 1 Hydrogenation of NB to AN over a Pt/AlO(OH) catalyst with different solvents
Selectivity (%)
b
ꢁ1
Solvents
Conversion (%)
AN
NSB
Others
TOF (h
)
Methanol
Ethanol
n-Propanol
i-Propanol
Ethyleneglycol
THF
72.6
48.3
11.2
21.8
6.3
7.7
6.0
49.6
28.3
78.0
18.9
29.0
37.5
41.2
0
38.0
91.0
1.2
3.1
0
0
1.9
0
0
0
0
0
3620
2412
396
1512
324
360
324
2448
1404
71.0
62.5
56.9
100
62.0
9.0
1
,4-Dioxane
Cyclohexane
Hexane
98.8
95.9
4.1
a
ꢀ
Reaction conditions: 0.07 mol% of platinum; methanol–water solvent, 5 mL (volume ratio ¼ 3 : 2); temperature, 30 C; hydrogen pressure, 1 atm;
b
time, 30 min. Turnover frequency was calculated by the overall rate of nitrobenzene conversion normalized by the number of active sites over the
specied time. The total active site number originated from the metal dispersion and all of the supported metal atoms. The Pt dispersion (D) was
3
estimated by the equation: D ¼ 6(a
m
/v
m
)/d, where the value of a
m
/v
m
was equal to 1.87 Angstrom, and d was the average diameter of Pt estimated
from TEM.
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a
2
Table 2 Hydrogenation of NB to AN over a Pt/AlO(OH) catalyst in a mixed solvent of H O and methanol
Selectivity (%)
AN
Water content
volume %)
(
Conversion (%)
NSB
Others
0
2
4
6
8
1
72.6
76.5
83.3
78.9
66.3
37.7
78.0
76.3
78.1
76.4
90.0
92.5
18.9
20.3
21.9
14.8
10.0
7.5
3.1
3.2
0
8.8
0
0
0
0
0
00
0
a
ꢀ
Reaction conditions: 0.07 mol% of platinum; methanol–water solvent, 5 mL (volume ratio ¼ 3 : 2); temperature, 30 C; hydrogen pressure, 1 atm;
time, 30 min.
and active carbon (AC)-supported heterogeneous catalysts in
3
alcohol–water mixtures. Our results further conrmed that the
addition of adequate water can improve the catalytic properties
of the catalysts. The promotional effect of the water may be
considered to arise from the interaction of the reactants
through the hydrogen bonds and an enhanced solubility of
hydrogen in the reaction medium. It was proposed that the
formation of hydrogen bonds, such as OH/O and OH/N
bonding between water and nitrobenzene, would weaken the N–
O bond on nitrobenzene, hence facilitating the hydrogenation
2
of the NO group. Meanwhile, it is worth pointing out that the
competition between water and methanol for surface adsorp-
tion may also contribute to the resulting high activity. Such
competition may reduce the adsorption of methanol on the
surface of the catalyst as the formation of a water lm occurs on
the surface of the hydrophilic catalyst. Further increasing the
content of water in the reaction media, however, leads to a
decrease of catalytic activity. This can probably be attributed to
the reduced solubility of the hydrogen. With the decreasing
solubility of the hydrogen, the rate determining step was con-
strained by the mass transfer of hydrogen, which resulted in a
lower reaction rate.
It is well-known that the catalytic performance of the active
species can be signicantly inuenced by the support in catalytic
hydrogenation reactions. As a comparison, the catalytic proper-
Fig. 6 Effect of different supports on the hydrogenation of NB.
2 2 3
ties of TiO , Al O , MWCNTs, MCM-41 and AC-supported plat-
inum catalysts (these catalysts were prepared by an
impregnation method with the same loading of Pt) were also
the effect of water content on the hydrogenation of NB was also investigated for the hydrogenation of NB, and the results are
studied, with the results shown in Table 2. As can be observed, shown in Fig. 6. It can be seen that different carriers show varied
the hydrogenation of NB in pure methanol exhibited a conver- catalytic behaviour for the hydrogenation of NB. In comparison
sion of 72.6% and a selectivity of 78.0%, with a large amount of with other supported catalysts, our synthesized Pt/AlO(OH)
intermediate nitroso-benzene (NSB) and other by-products. The nanocomposites still hold rst position in terms of both activity
conversion and selectivity for NB hydrogenation both increased and selectivity. The hydrogenation rates increase in the order: Pt/
2 2 3 2
when 20% of the methanol was replaced by H O. An even higher AlO(OH) >Pt/MWCNTs >Pt/Al O > Pt/AC> Pt/TiO > Pt/MCM-41.
conversion of NB and selectivity to AN (83.3% and 78.1%) can be Concerning the selectivity, Pt/AlO(OH) and Pt/MWCNTs exhibi-
achieved in the 40% water and 60% methanol mixture. ted a high selectivity for producing aniline in a short reaction
However, a negative effect on the catalytic activity of the Pt/ time relative to the other counterparts. Typically, the selectivity
AlO(OH) was detected when the water content was elevated to in a reaction time of 90 min increases in the order: Pt/AlO(OH) >
6
0%. The conversion of NB was reduced to 37.7% when pure MWCNTs > Pt/MCM-41 > Pt/AC > Pt/TiO > Pt/Al O .
2 2 3
water was used as a solvent, with the highest selectivity to AN, at
We also surveyed the catalytic performance of Pt/AlO(OH) for
92.5%. Similar phenomena have been witnessed and reported hydrogenating other nitroarenes with electron-donating groups
by other groups when studying the catalytic behavior of SiO
2
or electron-withdrawing groups. The results are shown in
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a
Table 3 Hydrogenation of different nitroarenes over Pt/AlO(OH)
b
Entry
Substrates
Product
Time (min)
Selectivity (%)
99
1
2
150
150
c
98
3
4
150
150
98
99
5
6
160
180
99
98
7
400
90
8
9
150
220
240
97
98
99
1
1
0
1
240
99
a
b
c
ꢀ
Reaction conditions: 0.07 mol% of platinum; methanol–water solvent, 5 mL (volume ratio ¼ 3 : 2); temperature, 30 C; hydrogen pressure, 1 atm.
Time needed for obtaining the maximum selectivity of the corresponding amines (all of the investigated substrates achieved 100% conversion).
The Pt/AlO(OH) catalyst was reused ve times.
Table 3. The catalyst shows excellent catalytic properties for the the active sites easy, prohibiting the side reaction between the
complete hydrogenation of the chloronitrobenzenes and o- methanol solvent and the AN. Therefore, a higher selectivity was
4,26
bromonitrobenzene with a high yield of the corresponding observed in the present work. The cycling catalytic test of the
amines of 98%. However, it should be noted that the hydroge- as-prepared Pt/AlO(OH) (Table 3) indicated its ne stability,
nation of m-iodonitrobenzene requires a longer reaction time with no loss of activity in the hydrogenation of the NB for ve
(400 min) for a 90% yield of m-iodoanline, which is probably runs and with negligible leaching of Pt (0.11%, assessed by ICP).
attributed to the deactivation of the Pt nanocatalyst caused by
:
the formation of I anions from the dehalogenation side reac-
Conclusions
tion. Apart from that, the other substituted nitroarenes can be
hydrogenated to the corresponding amines with a high yield In summary, Pt/AlO(OH) was synthesized by a facile one-pot
and efficiency. The excellent catalytic activity of the Pt/AlO(OH) process employing H PtCl and Al(O-sec-Bu) as precursors. It
2
6
3
catalyst can probably be attributed to the following aspects: (1) was applied for the catalytic hydrogenation of nitroarenes in a
the small Pt particle size with a higher dispersion enables more methanol–water medium under mild conditions. It can be seen
active sites to be available for the hydrogenation reaction; (2) that Pt nanoparticles are small and well dispersed on the
the Pt/AlO(OH) catalyst with a large amount of hydroxyl groups, AlO(OH) matrix. The Pt/AlO(OH) catalyst exhibited a much better
together with water as a solvent, facilitates the hydrogenation of catalytic performance than other Pt based catalysts prepared by
2,4
NB through the activation of the N]O bonds in the NB. In impregnation method using common supports, such as
terms of the selectivity, owing to the large amount of hydroxyl MWCNTs, MCM-41, AC, TiO and Al O . In addition, this mate-
2
2 3
groups on the surface of the support, a water lm is formed. rial is quite stable and can be reused ve times without loss of any
This makes desorption of the hydrogenated product AN from activity and selectivity. This advanced composite material
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provides a kind of novel and effective catalyst with great promise (11ZA034), and the Opening Project of Key Laboratory of Green
for the hydrogenation of nitroarenes in practical applications.
Catalysis of Sichuan Institutes of High Education (no. LZJ1205).
Experimental
Notes and references
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1
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Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (21207109), the Scientic 26 J. Ning, J. Xu, J. Liu, H. Miao, H. Ma, C. Chen, X. Li, L. Zhou
Research Fund of Sichuan Provincial Education Department and W. Yu, Catal. Commun., 2007, 8, 1763–1766.
11002 | RSC Adv., 2014, 4, 10997–11002
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