Liquid-Phase Hydrogenation of Chloronitrobenzene to Chloroaniline over Ni-Co-B Amorphous Alloy Catalyst

CHINESE JOURNAL OF CATALYSIS Volume 27, Issue 2, February 2006 Online English edition of the Chinese language journal

RESEARCH PAPER

Cite this article as: Chin J Catal, 2006, 27(2): 119–123.

Liquid-Phase Hydrogenation of Chloronitrobenzene to Chloroaniline over Ni-Co-B Amorphous Alloy Catalyst YAN Xinhuan*, SUN Junqing, XU Yinghua, YANG Jianfeng State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology, Hangzhou 310032, Zhejiang, China

Abstract: The Ni-Co-B bimetallic amorphous alloy with Co/(Co+Ni) molar ratio varying from 0 to 1 was prepared by chemical reduction of Ni(NO3)2 and Co(NO3)2 with KBH4 in solution. Its amorphous structure was verified by X-ray powder diffraction and selected area electron diffraction. The thermostability of the amorphous alloy was characterized by thermogravimetry-differential thermal analysis. Scanning electron microscopy and transmission electron microscopy were used to determine its morphology and particle size. The Ni-Co-B amorphous catalyst with a Co/(Co+Ni) molar ratio of 0.5 exhibits a much higher activity and selectivity than the Ni-B and Co-B catalysts in the liquid-phase hydrogenation of chloronitrobenzene to chloroaniline. The maximum conversion of both o-chloronitrobenzene and 3,4-dichloronitrobenzene reaches 99.9%, whereas the dechlorinations of chloroaniline are 1.12% and 0.42%, respectively, showing a good potential for industrial applications. The higher activity and selectivity of the Ni-Co-B catalyst can be attributed to the electron donation from the alloying B and metallic Co to the metallic Ni. By following this, the B and Co atoms become electron-deficient whereas the Ni atom becomes electron-rich, which can activate the N=O bond, inhibit the hydrodechlorination of chloronitrobenzene, and increase the thermostability of the amorphous alloy. Key Words: nickel; cobalt; boron; amorphous alloy; o-chloronitrobenzene; 3,4-dichloronitrobenzene; catalytic hydrogenation; chloroaniline

Chloroanilines (CAN) are important intermediates in the chemical industry of dyes, drugs, herbicides, and pesticides. The main route to prepare CAN is through the reduction of the corresponding chloronitrobenzes (CNB), either with sulfides and iron or by catalytic hydrogenation using a noble metal catalyst. Catalytic hydrogenation has attracted much attention for its atomic economy and lower impact on the environment. However, it has been found that it is difficult to apply the process of catalytic hydrogenation to the production of CAN because of extensive dehalogenation. In order to solve the problem of the catalytic hydrodechlorination of CAN, the catalyst preparation methods are modified (alloying, metal/support interaction, and so on), and specific inhibitors are used in the reaction system. But none of these methods have been completely satisfactory, because each has its own particular limitations. Since Molnar et al. [1] reported the amorphous alloy cata-

lysts, metal-metalloid amorphous alloy catalysts have been extensively studied owing to their higher catalytic activity, better selectivity, and stronger sulfur resistance in many hydrogenation reactions [2–4]. Up to now, a great number of systematic studies have been made on Ni-W-B amorphous alloy catalysts [5,6], but no work has been done on the hydrogenation of CNB. In our recent work, the excellent activity and selectivity of the Ni-B and Pt-Sn-B/CNTs (carbon nanotubes) amorphous catalysts for the CNB hydrogenation to CAN have been found [7,8]. On the basis of these interesting results, we report in this paper a novel Ni-Co-B amorphous alloy catalyst with different Co contents for the CNB hydrogenation with the aim of obtaining both high activity and selectivity for the corresponding CAN.

1 Experimental

Received date: 2005-05-13. * Corresponding author. Tel/Fax: +86-571-88320791; E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (20276071) and the Special Natural Science Foundation of Zhejing Province (ZE0102). Copyright © 2006, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.

YAN Xinhuan et al. / Chinese Journal of Catalysis, 2006, 27(2): 119–123

1.1 Catalyst preparation The Ni-Co-B amorphous alloy catalysts were prepared by the following procedure. An aqueous solution of 2.0 mol/L KBH4 (AR) containing 2.0 mol/L NaOH was added dropwise into an aqueous solution containing 0.1 mol/L Ni(NO3)2 (CP) and 0.1 mol/L Co(NO3)2 (CP). The solution was kept in an ice water bath and stirred vigorously until there was no gas released from the solution. The resultant black precipitate was filtered, washed with distilled water until pH=7, and subsequently washed twice with 99.9% ethanol. The final product was kept in ethanol. 1.2 Catalyst characterization Transmission electron microscopy (TEM, JEOL JEM200C) and scanning electron microscopy (SEM, Hitachi S-4700 Ⅱ) were used to determine the surface morphology and the particle size of the as-prepared Ni-Co-B catalyst. Its amorphous structure was confirmed by X-ray diffraction (XRD, Thermol ARL X’Tra with Cu Kα radiation, 45 kV, 40 mA) and selected area electron diffraction (SAED, JEOL JEM-200C). The thermal stability of the amorphous alloy catalyst was studied by thermogravimetry-differential thermal analysis (TG-DTA, Pyris Diamond, heating rate 10oC/min, from 40oC to 1000oC) in pure N2 (120 ml/min). The catalyst compositions were analyzed by X-ray energy dispersive spectroscopy (EDS, Thermo Noran Vantage ESI X, accelerating voltage 25 kV, take off angle 32.419o, live time 100 s, dead time 41.578 s) with a matched Hitachi S-4700 Ⅱ scanning electron microscope.

started immediately by stirring the reaction mixture vigorously. The stirring rate was kept at 500 r/min. After reaction for 2 h, the products were analyzed using a gas chromatograph (Fuli 9790) equipped with a flame ionization detector and SE-54 capillary column (0.32 mm × 30 m), from which the CNB conversion and CAN yield were obtained.

2 Results and discussion 2.1 XRD result The XRD patterns of the fresh Ni-Co-B samples, as shown in Fig. 1, revealed that Ni-B, Ni-Co-B-3, and Co-B were all present in the amorphous structure, as only one broad peak around 2θ = 45o appeared, which is the characteristic peak of the Ni(Co)-B amorphous alloy. The amorphous structure of the fresh Ni-Co-B sample was further confirmed by the SAED pattern. As shown in Fig. 2, the fresh samples displayed a series of diffraction cycles, indicating a typical amorphous structure.

1.3 Catalyst activity test Liquid-phase hydrogenation of CNB was conducted as follows. Samples of 0.5 g catalyst, 5.0 g CNB, and 100 ml ethanol were mixed in a 500 ml steel autoclave equipped with a mechanical stirrer and an electric heating system. The reactor was filled with H2 eight times in succession to exclude the air. It was then filled with H2 up to 1.0 MPa, followed by heating slowly to 110oC. Upon reaching 110oC, the hydrogenation was

Fig. 1 XRD patterns of different samples (1) Ni-B, (2) Ni-Co-B-3, (3) Co-B (Ni-Co-B-3—Ni23.8Co23.9B52.3 amorphous alloy.)

2.2 SEM and TEM results The TEM images in Fig. 2 confirmed that the fresh Ni-B, Ni-Co-B-3, and Co-B samples were all well dispersed and

Fig. 2 TEM and SAED images of Ni-B (a), Ni-Co-B-3 (b), and Co-B (c)

YAN Xinhuan et al. / Chinese Journal of Catalysis, 2006, 27(2): 119–123

present in the form of spherical particles. Their average sizes were 50 nm (Ni-B), 70 nm (Ni-Co-B-3), and 110 nm (Co-B), respectively. The SEM image in Fig. 3 showed that the spherical Ni-Co-B-3 particles were covered with cotton-like Ni-B alloy clusters. This is a character of the amorphous sample prepared by the chemical reduction method [9].

Fig. 3 SEM image of Ni-Co-B-3

2.3 EDS result The bulk composition of the Ni-Co-B-3 catalyst determined by EDS analysis is shown in Fig. 4. From this, we can conclude that the molar ratio of Co/(Co + Ni) was 0.5.

Fig. 5 DTA (a) and TG (b) profiles of different samples (1) Ni-B, (2) Ni-Co-B-3, (3) Co-B

in the nitrogen. The weight for Ni-Co-B-3 did not increase, indicating that the Co dopant could increase the stability of the Ni-B amorphous alloy catalyst. 2.5 Selective hydrogenation of CNB

Fig. 4 X-ray energy dispersive spectrum of Ni-Co-B-3

2.4 DTA and TG results The crystallization process of Ni-Co-B amorphous alloys was recorded by the DTA and TG profiles. As shown in Fig. 5(a), the crystallization temperature of the Ni-B and Co-B samples was 915oC and 614oC, respectively. However, the crystallization temperature of the Ni-Co-B-3 sample was 938oC, which was nearly 23oC higher than that of Ni-B and 324oC higher than that of Co-B. The DTA curves clearly demonstrated the stabilizing effect of the Co dopant on the Ni-B amorphous alloy structure. As shown in Fig. 5(b), the TG profiles for Ni-B and Co-B samples exhibited an increase in weight, which could be attributed to the reaction between the samples and trace oxygen

The liquid-phase hydrogenation of CNB is a complicated process, and many by-products are involved. Our purpose is to achieve high selectivity for CAN without the hydrogenolysis of the C–Cl bond. The effect of different Co amounts on the hydrogenation of CNB over the Ni-Co-B amorphous alloy catalyst was investigated. The results are listed in Table 1. It can be seen clearly that the Ni-Co-B amorphous catalyst exhibited higher activity and selectivity for the hydrogenation of CNB than Ni-B and Co-B catalysts when the molar ratio of Co/(Co+Ni) was 0.5. The conversion of o-CNB and 3,4-DCNB could all reach 99.9%, whereas the dechlorination of chloroaniline was less than 1.12% and 0.42%, respectively. As a part of electrons can be transferred from B to Ni and Co in Ni-B and Co-B amorphous alloys, the metallic Ni and Co become electron-rich and the alloying B is electron-deficient. This structure can greatly enhance the catalyst activity, selectivity, and sulfur resistance [5,10]. The nitrogen atom of CNB can also transfer electrons to the oxygen atom. So the relatively high electron density on the Ni and Co active sites may strengthen their adsorption for CNB molecules by adsorbing the electron-deficient nitrogen atom in the nitro-

YAN Xinhuan et al. / Chinese Journal of Catalysis, 2006, 27(2): 119–123

Table 1 Catalytic performance of different catalysts in liquid-phase hydrogenation of chloronitrobenzene Catalyst

Composition

Chloronitrobenzene

Conversion (%)

Dehalogenation (%)

Ni-B[7]

Ni47.6B52.4

o-CNB

99.9

3.31

3,4-DCNB

99.8

2.70

o-CNB

30.4

0.00

3,4-DCNB

17.1

0.00 0.00

Ni-Co-B-1 Ni-Co-B-2

Ni35.9Co12.1B52.0 Ni32.0Co15.8B52.2

Ni-Co-B-3

Ni23.8Co23.9B52.3

Ni-Co-B-4

Ni15.6Co31.8B52.6

Ni-Co-B-5 Co-B

o-CNB

71.8

3,4-DCNB

98.2

0.61

o-CNB

99.9

1.12

3,4-DCNB

99.9

0.42

o-CNB

54.2

0.00

3,4-DCNB

99.7

1.52 0.56

Ni11.9Co35.8B52.3

o-CNB

70.9

3,4-DCNB

98.6

0.84

o-CNB

98.4

2.46

3,4-DCNB

99.7

2.51

Co48.3B51.7

Reaction conditions: catalyst 0.5 g, chloronitrobenzene 5 g, ethanol 100 ml, p(H2) = 1.0 MPa, θ = 110℃, t = 120 min. o-CNB—o-chloronitrobenzene; 3,4-DCNB—3,4-dichloronitrobenzene.

gen–oxygen bond. On the other hand, the electron-deficient B can attract the oxygen to activate the polar –NO2 group of CNB, and the B atom can also coordinate with the –NH2 group of the CAN molecule, thereby promoting the CNB hydrogenation and depressing the CAN dehalogenation. The promoting effect of the Co dopant can be attributed to the dispersing effect that resulted in the higher stability and the electron donation that was favorable for hydrogenation. On one hand, according to the adsorption mechanism [11,12], the electronic interaction between the N=O groups in CNB and the metallic active sites is a forward donation of the electrons from the highest occupied molecular orbital (HOMO) of the N=O bonding, i.e., from the πN=O to the dz2 and s orbits of the metallic Ni atom, and a back donation from the dx2-y2 orbit of the metallic Ni atom to the lowest unoccupied molecular orbital (LUMO), i.e., π*N=O. As π*N=O is an antibonding orbit, the increased back electron donation to the π*N=O, which resulted from the high electron density on the Ni active sites, can also activate the N=O bond and promote the hydrogenation. The function of the Co dopant is similar to B, in which a part of electrons are transferred from Co to Ni and make the metallic Ni more electron-rich, and so enhance the activity and selectivity of the catalyst [13,14]. When the molar ratio of Co/(Co+Ni) was 0.5, the electron donation between Ni and Co was the strongest, and the activity of the Ni-Co-B amorphous alloy was the best. On the other hand, it is known that the catalyst activity is related greatly to the Fermi level and the density of state near the Fermi level in heterogeneous catalysis [15]. In the catalytic hydrogenation, the H–H bond is weakened by the electron transfer from HOMO of the catalyst to LUMO of hydrogen. Thus, the closer the two orbital energy levels and the bigger the density of state, the higher the catalytic activity of

the catalyst. When the contents of Co and Ni in the Ni-Co-B amorphous alloy were Co/(Co + Ni) = 0.5 in molar ratio, the HOMO energy of the catalyst is most close to the LUMO of hydrogen, so the Ni-Co-B-3 amorphous alloy catalyst has higher activity and selectivity than Ni-B and Co-B.

3 Conclusions Compared with Ni-B and Co-B, the Ni-Co-B amorphous alloy catalyst with a suitable content of the Co dopant (Co/(Co + Ni) molar ratio of 0.5) has excellent catalytic performance for the liquid-phase hydrogenation of CNB to corresponding CAN. The conversions of o-CNB and 3,4-DCNB were both over 99.9%, and the dechlorination of chloroaniline was less than 1.12% and 0.42%, respectively. The promoting effect of the Co dopant is attributed to a dispersing effect that increases the stability of the Ni-Co-B amorphous alloy. This effect is also due to the electron transfer from Co to Ni, which makes the Ni atoms more electron-rich, thereby promoting the hydrogenation of CNB and depressing the dehalogenation of CAN. The results show that the Ni-Co-B amorphous alloy catalyst is a promising catalyst for industrial application.

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