Highly selective hydrogenation of halonitroaromatics to aromatic haloamines by ligand modified Ni-based catalysts

Article abstract of DOI:10.1016/j.cclet.2012.03.017

Ligand modification of Ni-based catalysts by coordination of dicyandiamide to Ni metal leads to enhanced selectivity for the selective hydrogenation of halonitroaromatics. The selectivity of above 99.9% to aromatic haloamines can be achieved at the conversion of 100%. The results of H₂-TPD and FT-IR experiments show that Ni^-H⁺ species possessing the properties of Lewis acid site on the surface of Raney Ni could be responsible for the hydrodehalogenation. When Raney Ni was treated by dicyandiamide, Ni ^-H⁺ species interacted with N atom from the dicyandiamide. This interaction was stable even at reaction temperature, which reduced the possibility to form the intermediate state of ArCl?H⁺Ni ^-. And then CCl bond could not be polarized and activated. The hydrodechlorination process was suppressed effectively.

Full text of DOI:10.1016/j.cclet.2012.03.017

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 545–548
www.elsevier.com/locate/cclet
Highly selective hydrogenation of halonitroaromatics to aromatic
haloamines by ligand modified Ni-based catalysts
*
Chun Shan Lu, Jing Hui Lv, Lei Ma, Qun Feng Zhang, Feng Feng, Xiao Nian Li
Industrial Catalysis Institute of Zhejiang University of Technology, State Key Laboratory Breeding Base of
Green Chemistry Synthesis Technology, Zhejiang University of Technology, Hangzhou 310032, China
Received 5 December 2011
Available online 18 April 2012
Abstract
Ligand modification of Ni-based catalysts by coordination of dicyandiamide to Ni metal leads to enhanced selectivity for the
selective hydrogenation of halonitroaromatics. The selectivity of above 99.9% to aromatic haloamines can be achieved at the
À
+
conversion of 100%. The results of H –TPD and FT-IR experiments show that Ni H species possessing the properties of Lewis
2
acid site on the surface of Raney Ni could be responsible for the hydrodehalogenation. When Raney Ni was treated by
À
+
dicyandiamide, Ni H species interacted with N atom from the dicyandiamide. This interaction was stable even at reaction
+
À
temperature, which reduced the possibility to form the intermediate state of Ar–ClÁ Á ÁH Ni . And then C–Cl bond could not be
polarized and activated. The hydrodechlorination process was suppressed effectively.
#
2012 Published by Elsevier B.V. on behalf of Chinese Chemical Society.
Keywords: Halonitroaromatics; Hydrogenation; Ni catalyst; Hydrodehalogenation
Aromatic haloamines are important intermediates in the synthesis of herbicides, dyes, drugs, pesticides and so on
1]. At present the best choice to synthesize them seems to be the selective hydrogenation of the corresponding
[
halonitroaromatics owing to lower impact on the environment. But, it is generally known that a dehalogenation
reaction is particularly prone to occur. To inhibit dehalogenation, several strategies have been developed, such as
forming alloy with other metals [2,3], modulating the interaction between the metal and the oxide supports [4–6], or
adding special inhibitors into reaction system [7]. However it still is a challenge to achieve high selectivity in the
hydrogenation of halonitroaromatics. It is therefore desired to develop an efficient heterogeneous catalyst with high
activity and selectivity for the hydrogenation of halonitroaromatics to aromatic haloamines.
Ligand modification can be convenient to modulate steric or electronic characteristics of metal catalysts by the use of
the ligands [8]. Some particular modifiers have been used successfully in previous work, for example, diphenyl sulfide has
been shown to enhance selective hydrogenation of alkenes, acetylenes and so on over Pd/C, and triphenyl phosphine has
been used to increase activity and selectivity of supported rhodium in olefin hydroformylation [9,10]. However, very little
research has been done on the ligand modification of metal catalysts in the selective hydrogenation of halonitroaromatics.
Herein, we reported the catalytic performance of Ni catalyst modified with minute quantity of organic amine compounds
in the selective hydrogenation of halonitroaromatics to aromatic haloamines. The selectivity of above 99.9% to aromatic
haloamines can be achieved at the conversion of 100% over ligand modified Ni-based catalysts.
*
Corresponding author.
E-mail address: xnli@zjut.edu.cn (X.N. Li).
1001-8417/$ – see front matter # 2012 Published by Elsevier B.V. on behalf of Chinese Chemical Society.
doi:10.1016/j.cclet.2012.03.017

546
C.S. Lu et al. / Chinese Chemical Letters 23 (2012) 545–548
1
. Experimental
The Ni catalyst with the size of 33–74 mm (Zhejiang Jiali Metal Technology Co., Ltd.) was modified as follows:
.0 g of Raney Ni catalyst was immerged into 5 mL of dicyandiamide aqueous solution with various concentrations at
23 K with stirring for 30 min, and then was separated and washed by the use of the centrifugation until no treated
1
3
reagent can be detected in the filter liquor. The modified Raney Ni catalysts were referred to as RND-n.
The adsorption types of hydrogen on the surface of catalysts were determined by TPD technology. 100 mg catalyst
filled in the quartz tube reactor with inner diameter 6 mm, was heated to 378 K, maintained at that temperature for 2 h,
and then heated to 800 K by 5 K/min in a helium flow of 25 mL/min. The H desorption species (m/z = 2) were
2
monitored by on line mass spectrometry (Omni star TM). FT-IR spectroscopy study of spent catalyst was carried out in
the NEXUS (Nicolet). The spent RND-4 catalyst was placed in in situ cell linked to a heating system, then heated up to
the desired temperature (373 K or 423 K) and held at that temperature for 0–80 min in an Ar stream. Contrast samples
Raney Ni and fresh RND-4 catalysts were maintained at 378 K for 5 h to remove the water before the measurement.
The catalytic hydrogenation of halonitroaromatics was carried out in a 500 mL stainless steel autoclave, which was
charged with 150 mL of methanol, 75 g of halonitroaromatics and 0.75 g of catalyst, under the conditions of hydrogen
pressure 1.0 MPa and temperature 353 K with stirring at 900 rpm. The products were identified by GC–MS (Agilent
5
973N) and analyzed by gas chromatography (Shimadzu, GC-2014), equipped with a FID detector and a capillary
column AT.XE 60 (30 m  0.20 mm  0.25 mm). The quantitative analysis was conducted by area normalization
method.
2
. Results and discussion
First, the catalytic performance of RND catalysts for the selective hydrogenation of 6-chloro-2-nitrotoluene to 3-
chloro-2-methylaniline as a model reaction was investigated. Comparison with the Raney Ni catalyst, as shown in
Table 1, the catalytic activity and selectivity for the 6-chloro-2-nitrotoluene hydrogenation were improved greatly over
RND catalysts. Especially, RND-4 catalyst showed the selectivity of 99.97% to 3-chloro-2-methylaniline when the 6-
chloro-2-nitrotoluene hydrogenation was fully accomplished within 64 min. Further, the hydrogenation of various
halonitroaromatics over RND catalysts was investigated (listed in Table 2). Although the substrates contain different
substituent groups, excellent selectivity to the corresponding aromatic haloamines was achieved. These results show
that the RND catalysts possesses the excellent performance of suppression hydrodehalogenation.
In order to make clear the suppression mechanism, H –TPD experiments of Raney Ni and RND-4 catalysts, and FT-
2
IR measurements of spent RND-4 catalyst were investigated. The H –TPD curves of Raney Ni, as shown in Fig. 1,
2
appeared four desorption peaks at temperatures of 463 K, 543 K, 658 K and 750 K, respectively. Comparison with
Raney Ni catalyst, RND-4 catalyst had only three H desorption peaks and the peak at the temperature of 543 K
2
disappeared. According to the previous work [11–14], H species adsorbed on the Ni catalyst surface can be divided
into three typical categories: weakly adsorbed reversible H species in the linear form of Ni–H, strongly absorbed
À
+
irreversible H species in the bridge form of nNi–H, and Ni H species resulting from the electron unoccupied orbit of
Raney Ni. Most of them are present in the linear form, which are responsible for the hydrogenation reaction. But the
irreversible H species are of no use in the hydrogenation reaction, only contributing to avoiding Ni metal on the
Table 1
Catalytic performance of modified Ni catalysts for the selective hydrogenation of 6-chloro-2-nitrotoluene.
Catalyst
Concentration of
Reaction
Conversion
(%)
Selectivity (%)
-Chloro-2-methylaniline
treated regents (mol/L)
time (min)
3
2-Methylaniline
Others
Raney Ni
RND-1
RND-2
RND-3
RND-4
RND-5
0.00
0.02
0.05
0.10
0.15
0.20
214
98
77
72
64
66
100.00
100.00
100.00
100.00
100.00
100.00
95.73
99.34
99.52
99.92
99.97
99.96
2.43
0.66
0.48
0.08
0.03
0.04
1.84
0.00
0.00
0.00
0.00
0.00
Reaction conditions: 6-chloro-2-nitrotoluene, 75 g; catalyst, 0.75 g; methanol, 150 mL; 900 r/min; 353 K; 1.0 MPa.

C.S. Lu et al. / Chinese Chemical Letters 23 (2012) 545–548
547
Table 2
Catalytic performance of RND-4 catalyst for the selective hydrogenation of other halonitroaromatics.
Halonitroaromatics
Selectivity (%)
Main products
By-products
3
3
, 4-Dichloronitrobenzene
, 5-Dichloronitrobenzene
99.96
99.91
99.94
99.97
99.95
99.94
0.04
0.09
0.06
0.03
0.05
0.06
o-Fluoronitrobenzene
m-Fluoronitrobenzene
p-Fluoronitrobenzene
2
, 4-Difluoronitrobenzene
Reaction conditions: halonitroaromatics, 75 g; RND-4, 0.75 g; methanol, 150 mL; 900 r/min; 353 K; 1.0 MPa.
catalyst surface oxidation [12]. Therefore, the first peak at the temperature of about 460 K should be ascribed to the
weakly adsorbed reversible H species and the peaks after 600 K to the strongly absorbed irreversible H species. In
addition, associating with high electronegativity of the nitrogen atom in organic amine molecule, it seems to be
À
+
reasonable to infer that the peak at about 540 K should be Ni H species possessing the properties of Lewis acid site.
When Raney Ni was treated by dicyandiamide, it is possible to generate a strong interaction between catalyst and
ligand, and then this H active site was deactivated.
Fig. 2 shows the FT-IR spectra of spent RND-4 catalyst in the hydrogenation of 6-chloro-2-nitrotoluene. The results
À1
À1
show that there were five kinds of bands corresponding to O–H bond (1050 cm ), benzene (1450, 1670 cm ), C N
À1
À1
(
So it could be suggested that the sharp band at 1050 cm was attributed to the methanol. The bands at 1450 cm
1660 cm ) and C–H bond (2970 cm ), respectively. In the present study, ÀOH only roots in the methanol solvent.
À1 À1
,
670 cm and 2970 cm were the characteristic bands of the product 3-chloro-2-methylaniline. These peaks
À1 À1
1
disappeared when catalysts were treated for 60 min at 373 K, which indicated 3-chloro-2-methylaniline was weakly
À1
adsorbed on the RND-4 catalyst. At the same time, the relative intensity of band at 1660 cm , assigned to the C–N
atom from the dicyandiamide, kept invariable even after treatment at 423 K for 40 min. Combined with the results
À
+
above, it can be found that the N atom from the dicyandiamide interact with Ni H species on the surface of Raney Ni,
and this interaction is stable even at reaction temperature.
For the mechanism in the hydrodehalogenation reaction of aromatic halides, most researchers agreed that there was
an electrophilic attack of cleaved hydrogen on the adsorbed aromatic halides. The chloride coordinated to a Lewis acid
+
site, would be converted to hydrogen chloride in the reaction with H spilled over from Ni or noble mental catalyst
surface to the supports [14–16]. This interpretation was supported by the experimental results from Eun-Jae Shin and
Claudia Menini that the hydrodechlorination rate would increase in an order from the strongly electron withdrawing
chlorine substituent to the electron donating –CH and –OH groups [15,17].
3
À
+
Based on the above, we could infer that Ni H acts as Lewis acid site to interact with –Cl with high
+
À
electronegativity, and then form an intermediate state of Ar–ClÁ Á ÁH Ni . Subsequent cleavage of the Ar–Cl bond
463
6
58
750
5
43
Raney Ni
RND-4
400
500
600
700
800
Temperature / K
Fig. 1. H
2
–TPD curves of Raney Ni and RND-4 catalysts.

548
C.S. Lu et al. / Chinese Chemical Letters 23 (2012) 545–548
1660
1
050
Raney Ni
2790
1450
fresh RND-4
heated at 373K for 0min
heated at 373K for 20min
heated at 373K for 40min
heated at 373K for 60min
heated at 373K for 80min
heated at 423K for 20min
heated at 423K for 40min
1000
1500
2000
2500
3000
3500
Wavenumber / cm-1
Fig. 2. FT-IR spectra of spent RND-4 catalyst.
results in the production of HCl, which can inhibit the absorption of hydrogen on the surface of Ni catalyst [18]. And
À
+
then the rate of hydrogenation reaction becomes slower and slower. However, over RND catalyst, Ni H was
overlapped by N atom from the dicyandiamide molecule. The possibility to form the intermediate state of Ar–
+
À
ClÁ Á ÁH Ni would be cut off, and then Ar–Cl bond would not be polarized and activated. The hydrogenolysis of Ar–Cl
bond was suppressed. The excellent catalytic activation and selectivity for the selective hydrogenation were obtained.
3
. Conclusions
Ligand modification of Ni-based catalysts by coordination of dicyandiamide to Ni metal leads to high catalytic
activity and excellent selectivity for the selective hydrogenation of halonitroaromatics to aromatic haloamines. There
is a strong interaction between Raney Ni catalyst and the nitrogen atom in dicyandiamide molecule with high
À
+
electronegativity, which could cover the electron-deficient Lewis acid site Ni H . The possibility to form the
+
À
intermediate state of Ar–ClÁ Á ÁH Ni would be cut off, and then Ar–Cl bond would not be polarized and activated. The
hydrogenolysis of Ar–Cl bond was suppressed.
Acknowledgments
Funding for the present study from the National Basic Research Program of China (973 Program) (No.
011CB710800) and the National Natural Science Foundation of China (No. 20976164) are gratefully acknowledged.
2
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