Effective Hydrogenation of Haloaromatic Nitro Compounds Catalysed by Iridium Nanoparticles Deposited on Multiwall Carbon Nanotubes

Article abstract of DOI:10.1080/15533174.2015.1137013

Iridium nanoparticles immobilized on multiwall carbon nanotubes composites were synthesized using anhydrous iridium trichloride as a precursor. The as-prepared composites were applied as catalysts for the hydrogenation of haloaromatic nitro compounds at room temperature and balloon hydrogen pressure in a methanol/water mixture, showing a selectivity of 99.9% to p-chloroaniline at complete conversion of p-chloronitrobenzene. Moreover, the catalysts also exhibited excellent catalytic properties for the reactants with electron-accepting, electron-donating, and heterocycle groups. The high activity and selectivity of the catalyst was probably attributed to the interaction such as competitive adsorption and hydrogen bonding between solvent water and reactants.

Full text of DOI:10.1080/15533174.2015.1137013

Synthesis and Reactivity in Inorganic, Metal-Organic, and
Nano-Metal Chemistry
ISSN: 1553-3174 (Print) 1553-3182 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt20
Effective hydrogenation of haloaromatic nitro
compounds catalysed by iridium nanoparticles
deposited on Multi-Wall Carbon Nanotubes
Hua-Bin Li, Lu Liu & Xiao-Yan Ma
To cite this article: Hua-Bin Li, Lu Liu & Xiao-Yan Ma (2016): Effective hydrogenation of
haloaromatic nitro compounds catalysed by iridium nanoparticles deposited on Multi-Wall
Carbon Nanotubes, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal
Chemistry, DOI: 10.1080/15533174.2015.1137013
To link to this article: http://dx.doi.org/10.1080/15533174.2015.1137013
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Date: 22 February 2016, At: 09:38

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Effective hydrogenation of haloaromatic nitro compounds catalysed by iridium
nanoparticles deposited on Multi-Wall Carbon Nanotubes
Hua-Bin Lia, Lu Liua, Xiao-Yan Mab,*
aState Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu
University of Technology, Chengdu 610064, Sichuan, P. R. China
bCollege of Material and Chemistry Engineering, Chengdu University of Technology,
Chengdu 610059, P. R. China
*Corresponding author.. E-mail: scufgy@163.com
Abstract
Iridium nanoparticles immobilized on Multi-Wall Carbon Nanotubes composites
(Ir/MWCNTs) were synthesized using anhydrous iridium trichloride as a precursor. The as-
prepared composites were applied as catalysts for the hydrogenation of haloaromatic nitro
compounds at room temperature and balloon hydrogen pressure in a methanol/water mixture,
showing a selectivity of 99.9% to p-chloroaniline at complete conversion of p-
chloronitrobenzene. Moreover, the catalysts also exhibited excellent catalytic properties for
the reactants with electron-accepting, electron-donating and heterocycle groups. The high
activity and selectivity of the catalyst was probably attributed to the interaction such as
competitive adsorption and hydrogen bonding between solvent water and reactants.
Keywords
Selective hydrogenation, iridium, haloaromatic nitro compounds, Multi-Wall Carbon
Nanotubes
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1. Introduction
Synthesis of haloaromatic amines from hydrogenation of haloaromatic nitro-
compounds is of great concern due to their useful applications in manufacture of dyes,
drugs, herbicides, pesticides, and fine chemicals[1]. Catalytic hydrogenation is an energy-
saving and high efficiency technology that has been widely applied for liquid phase
hydrogenation of haloaromatic nitro-compounds using supported metals such as Ru[2-6],
Pt[5, 7-11], Ir[12, 13] and Pd[14-17] as catalysts and molecular hydrogen as a reducing
agent. Among these catalysts, Ru is proved to be highly selective to haloaromatic amines
with relatively low activity, whereas Pd possesses the highest catalytic activity with the
lowest selectivity due to the severely side reaction of hydrogenolysis via the scissoring
the carbon-halogen bonds. However, the catalysts based in iridium are rarely investigated
even though they exhibit relatively high activities in the hydrogenation of haloaromatic
nitro-compounds because of the severely hydrogenolysis of carbon-halogen bonds during
the hydrogenation process resulting in a low selectivity to the corresponding amines. In
order to suppress the side reaction of hydrogenolysis, large amount of modifiers and
promoters should be introduced into the reaction systems to modify the catalytic
properties of iridium catalysts. For example, Xu et al. reported the synthesis of bimetallic
Ru-Ir/Al2O3 catalyst for the selective hydrogenation of p-chloronitrobenzene (p-CNB) to
p-chloroaniline (p-CAN)[18]. Promoters such as ethylenediamine are required to improve
the catalytic property of the catalyst, therefore extra work should be done to isolate and
purify the resulting amines. Campos and co-workers prepared commercial ZrO2 supported
iridium catalyst for the hydrogenation of substituted aromatic nitrobenzenes without any
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additives, in which the catalytic performance of iridium catalyst was attributed to the
good stability of the support to active phase even the catalyst was prepared by
impregnation[12]. Fan et al. synthesized Ir/ZrO2•xH2O catalyst by co-precipitation
method, showing an excellent activity and selectivity for the hydrogenation of
haloaromatic nitro compounds[13].
In recent years, great attention has been focused on the applications of carbon
nanotubes as catalyst supports owing to their special properties such as high tensile
strength, large surface area, high electric and thermal conductivity[11, 19]. In the area of
nitro group hydrogenation, the multi-wall carbon nanotubes supported (MWCNTs) Pt
catalyst exhibited 100% conversion of nitrobenzene and 100% selectivity towards aniline
under a solvent-free condition; however they require a high reaction temperature of 60 C
and hydrogen pressure of 4.0 MPa. Therefore, how to alleviate the reaction conditions
while maintain catalytic activity and selectivity of the heterogeneous catalysts is a
tremendous challenge. Nevertheless, there is a lack of information available on the
application of MWCNTs as a carrier for depositing iridium catalysts for the
hydrogenation of haloaromatic nitro-compounds to the corresponding amines. To this end,
the aim of this work is to explore the MWCNTs as a support to immobilize the iridium
nanoparticles for the selective hydrogenation of haloaromatic nitro-compounds to the
corresponding amines under mild conditions. The Ir/MWCNTs catalyst was easily
prepared with the use of IrCl3 as a precursor and NaBH4 as a reducing agent. The catalytic
property of the pre-synthesized Ir/MWCNTs catalyst was tested in an alcohol/water
medium with the use of p-CNB as a model reactant.
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2. Experiment
2.1 Chemicals
Anhydrous iridium trichloride(IrCl3) was purchased from the Kunming Institute of
Precious Metals, China. MWCNTs (500 m2/g) were purchased from the Chengdu Institute
of Organic Chemistry, Chinese Academy of Sciences. Other chemicals and solvents
(analytical grade reagents) were supplied from the Aladdin Industrial Corporation, China,
and used as received without further purification.
2.2 Catalyst preparation
Ir/MWCNTs was prepared as follows: 30.0 mg of IrCl3 was dissolved in 15 mL water and
transferred into a 50 mL round-bottom-flask containing MWCNTs (1.5 g). Then the
mixture was stirring overnight and reduced by an aqueous of NaBH4 (1.5mol/L, 2 mL).
The resulting black solid was washed with water and ethanol several times, dried at 60
under vacuum for 12 h. The content of Ir was 1.1 wt.% estimated by the ICP.
2.3 Catalyst characterization
C
Transmission electron microscopy (TEM) measurements were carried out on a FEI
Tecnai G20 instrument operated at an accelerating voltage of 200 kV. Powder X-ray
diffraction (XRD) patterns were performed on a Rigaku X-ray diffractometerD/max-
2200/PC equipped with Cu Kα radiation (40 kV, 20 mA) over the range of 10-90°. XPS
was performed on a Thermo ESCALAB 250 Axis Ultra spectrometer using a
monochromatic Al Kα (hv = 1486.6 eV). All binding energy (BE) values were referenced
to C1s peak of contaminant carbon at 284.6 eV. All hydrogenation samples were analyzed
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by gas chromatography (Agilent 7890A) with a FID detector and PEG-20M supelco
column (30 m×0.25 mm, 0.25 um film) and nitrogen was used as a carrier gas.
2.4 Activity test
Typically, hydrogenation of halo-aromatic nitro-compounds was conducted as
following：A desired amount of catalyst (10.0mg), solvent(total volume:7 mL),
substrate(1.0 mmol) was introduced into a 25 mL round-bottom-flask equipped with a
balloon and treated under vacuum to remove the air in the reactor. Then high purity
hydrogen was charged into the reactor. The reaction was conducted under balloon
hydrogen pressure with a stirring rate of 1200 rpm when the reaction temperature reached
25 C. The recyclability of the catalyst was performed as the following procedures. After
the catalytic run was finished, the catalyst was collected by centrifugation and washed
with water and ethanol three times and kept into the reactor for the next run.
3. Results and discussion
3.1 Characterization of Ir/MWCNTs
The morphology of Ir nanoparticles (NPs) was characterized by TEM. As shown in
Fig.1, Ir NPs was demonstrated to be well-distributed on the surface of MWCNTs with an
average size of 2.5 nm. EDX characterization exhibited that Ir NPs were successfully
immobilized on the surface of the MWCNTs via reaction of Ir4+ ion using NaBH4 as a
reducing agent. The crystal structure of Ir/MWCNTs was characterized by XRD and the
result was shown in Fig.2a. The characteristic peak situated at 2= 25.8◦ was estimated
to be the (002) plane of graphite. No peaks toward metallic Ir NPs were detected because
of the small particle sizes of Ir, indicating that the Ir NPs were well-dispersed on the
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surface of MWCNTs which is in agreement with the TEM results. The electronic state of
Ir species on MWCNTs was measured by XPS. As shown in Fig.3, Ir4+ ion was partly
reduced to metallic Ir, which was confirmed by the binding energy values measured at
61.7 and 64.9 eV. However, the binding energy was 0.6 eV higher than that of the
standard state of Ir(0), indicating that metallic Ir was at an electron deficient state in
Ir/MWCNTs. In addition, two peaks at 63.5 and 66.8 eV belonged to the iridium oxide
were distinctively observed, which was probably assigned to the oxidation of air and/or
incomplete reduction of iridium oxide. However, the high activity of the catalyst
Ir/MWCNTs indicates that the iridium oxide could be reduced during the hydrogenation
process due to the fact that the iridium oxide was inactive to catalyze the nitro group prior
to reduction.
Catalytic property of Ir/MWCNTs was investigated using p-CNB as a model substrate
at 25 C and balloon hydrogen pressure. The products for the hydrogenation of p-CNB
were p-CAN and p-CNSB detected by the GC-MS. Blank test or using MWCNTs as the
catalyst showed no conversion of p-CNB, indicating Ir NPs were the indispensible active
sites for obtaining a high yield of p-CAN. Regarding the reaction medium, previous
reports have indicated that adding adequate amount of water to replace organic solvent
can significant increase catalytic property of supported catalysts for the hydrogenation of
nitro-groups[6, 20]. To this end, we investigated the effect of water on the catalytic
property of the Ir/MWCNTs and the results are illustrated in Fig.4a. It can be seen that the
catalyst Ir /MWCNTs showed a low activity for the hydrogenation of p-CNB in methanol
without the introduction of water. The conversion of p-CNB achieved 86.9% with a
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selectivity of 77.8% toward p-CAN due to the formation of large amount of intermediates
p-CNSB (20.1%) in a long reaction time of 120 min (Fig.4a). When 28% methanol was
replaced by water, the conversion of p-CNB increased to 100% with a selectivity of 87.5%
to p-CAN and 11.8% to p-CNSB in a short reaction time of 60 min (Fig.4b). The
selectivity of p-CAN could reach 99.5% with the elevation of reaction time to 120 min as
a result of the further hydrogenation of the intermediate p-CNSB to the desired product p-
CAN. The promotional effect of water on the selective hydrogenation of nitro groups was
attributed to the competitive adsorption between water and methanol on the surface of the
support and the formation of hydrogen bonds between the product and water in the
solvent. It was reported that a water film would be formed on the surface of the
hydrophilic support, which reduced the adsorption of methanol on the surface of catalyst
due to the competitive adsorption of water [6, 13]. When the products were generated,
they were easily escaped from the active site of the catalyst and resulted in a high yield
due to the formation of hydrogen bonds between the products and water. The detection of
high selectivity of the intermediate p-CNSB confirmed our suggestion. On the other hand,
water can also inhibit the re-adsorption of p-CAN and avoid further hydrolysis of the
carbon-halorine bonds. As a consequence, much higher catalytic activity in the
hydrogenation of p-CNB was observed in such a methanol/water mixture. Up to now, the
most successful example of Ir catalysts were reported by Campos et al. [12] and Fan et al.
[13], in which a complete conversion of haloaromatic nitro compounds was achieved with
a high selectivity to the corresponding amines(>99%) in the presence of commercial ZrO2
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supported iridium and Ir/ZrO2•xH2O. The Ir/MWCNTs catalyst also showed comparable
catalytic activity and selectivity with the reported results.
We also investigated the catalytic property of Ir/MWCNTs in other alcohol/water
mixture and the results listed in Table 1. As expected, the catalyst showed low activity
toward p-CNB hydrogenation using alcohols such as methanol, ethanol and i-propanol as
sole solvents. The addition of water significantly increased not only the catalytic activity
but also the selectivity to p-CAN. Therefore, it can be concluded that the protic solvents
such as methanol, ethanol, and i-propanol exhibit higher catalytic activity than that of
non-protic solvents like 1,4-dioxide and THF, which was in agreement with the reported
literature[6, 13]. Interestingly, the addition of water not only can enhance the catalytic
activity and selectivity of Ir/MWCNTs toward p-CNB hydrogenation in protic solvents
but also in non-protic solvent. Our results made further support to the previous reports [4,
6, 7, 10, 13].
Following the catalytic hydrogenation of p-CNB catalyzed by catalyst Ir/MWCNTs
presented above, we have investigated hydrogenations of other haloaromatic nitro
compounds. The results are presented in Table 2. The catalyst Ir/MWCNTs remained
high efficiency in the investigated substrates containing electron-donating groups or
electron-withdrawing groups. For example, chloronitrobenzenes can be quickly
hydrogenated to the desired product in 2 h. In addition, other reactants with electron-
donating groups, such as --OH, --CH3, and --OCH3 also can be hydrogenated to the
corresponding amines with extremely high yield (>99%). The reactants with electron-
accepting and heterocycle groups also can be hydrogenated to the corresponding amines.
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Reusability of catalyst is of great importance in view of industrial applications. Therefore,
we investigated the reusability of Ir/MWCNTs by performing five consecutive runs under
the same reaction conditions. The catalyst Ir/MWCNTs showed good recyclability as
shown in Fig.5. From the XRD (Fig.2b) and TEM (Fig.6) characterizations of the catalyst
recycled five times, the structure and morphology of the catalyst did not changed
obviously, which was attributed to the good recyclability, exhibiting some promise in use
of practical applications.
4. Conclusion
In summary, the Ir NPs immobilized on MWCNTs was synthesized by impregnation and
applied as catalysts for the hydrogenation of a variety of aromatic nitro compounds at 25

C under balloon hydrogen pressure in methanol/water mixture. The catalyst Ir/MWCNTs
exhibited high efficiency for the clean synthesis of amines due to the completely suppress
the hydrolysis side reaction of hydrodechlorination of carbon-chroline bond in the
substrate and the products. Moreover, the catalyst Ir/MWCNTs was quite stable and can
be recycled at least five times without significant deactivation. Therefore, the present
work provides a kind of more robust and eco-friendly catalytic system in view of the
development of a sustainable catalytic route for the production of aromatic amines.
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Table 1 Effect of solvent on hydrogenation of p-CNBa
Selectivity (%)
p-CAN
77.8
Solvents
Methanol
Ethanol
water
Conversion (%)
p-CNSB
20.1
0.5
No
86.9
Yes
No
100(60)
98.7
99.5
88.8
10.8
2.3
Yes
No
100(80)
23.1
97.3
60.7
37.9
0.4
i-propanol
1,4-dioxide
THF
Yes
No
100(80)
10.5
98.9
54.9
44.7
17.3
64.7
19.7
Yes
No
16.8
82.4
12.7
34.1
Yes
40.3
79.5
aReaction conditions: catalyst,10 mg; p-CNB,1 mmol; temperature, 25 C; pressure, 1atm;
reaction time, 120 min. b reaction time was 80 min.
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Table 2 Hydrogenation of Catalyst for the hydrogenation of different substrates
Entry Nitro-aromatics Product
Time Conversion Selectivity
(h)
2.5
2
(%)
100
100
100
(%)
NO2
NH2
1
99.9
99.5
95.4
NO2
NH2
Cl
Cl
2
3
NO2
NH2
2
Cl
Cl
NO2
NH2
Cl
Cl
4
5
6
7
8
2
99.7
88.7
100
99.4
99.0
99.0
95.9
86.8
NO2
NH2
Br
Br
3
NO2
NH2
H3C
H3C
2.5
2.5
3
NO2
CH3
NH2
CH3
100
NO2
NH2
HO
HO
91.2
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NO2
NH2
H3C
H3C
3
6
95.3
95
95.8
97
O
O
9
NH2
NO2
10
O2N
HOOC
NH2
NO2
11
12
13
14
4
5
2
2
96
98
HOOC
H2N
NO2
NH2
94
96
COOH
COOH
NO2
N
NH2
N
100
100
99.0
99.2
Cl
N
Cl
O2N
H2N
N
Cl
Cl
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Fig.1 TEM image of catalyst Ir/MWCNTs
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(b)
10
20
30
40
50
60
70
80
(a)
10
20
30
40
50
60
70
80
2 theta (degrees)
Fig.2 XRD patterns of fresh Ir/MWCNTs (a) and the catalyst recycled five times (b).
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56
58
60
62
64
66
68
70
72
Binding Energy(eV)
Fig.3 XPS spectrum of Ir/MWCNTs
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100
Selectivity to p-CNSB
(a)
Selectivity to p-CAN
Conversion
80
60
40
20
100
80
60
40
20
0
(b)
20
40
60
80
100 120
Reaction time(min)
Fig.4 Catalytic properties of Ir/MWCNTs in methanol (a) and methanol/water mixture
(volume ratio: 5:2) (b)
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Conversion
Selectivity
100
80
60
40
20
0
1
2
3
4
5
Recycling times
Fig.5 Recyclability of catalyst Ir/MWCNTs
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Fig.6 TEM image of Ir/MWCNTs recycled five times.
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