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Ind. Eng. Chem. Res. 2010, 49, 4664–4669
Hydrogenation of Nitrobenzene to Aniline over Silica Gel Supported Nickel Catalysts Junhua Wang, Zhenle Yuan, Renfeng Nie, Zhaoyin Hou,* and Xiaoming Zheng Institute of Catalysis, Department of Chemistry, Key Lab of Applied Chemistry of Zhejiang ProVince, Zhejiang UniVersity, Hangzhou, 310028, China
Nanosized silica gel supported Ni catalysts that have average Ni particle size of 3.7, 11.4, and 11.8 nm were prepared by facile impregnation of different Ni precursors and used for hydrogenation of nitrobenzene (NB). These catalysts were characterized by N2 adsorption, X-ray diffraction, and transmission electron microscopy. It was found that Ni dispersed highly on the surface of silica gel in the catalyst prepared via Ni(en)3(NO3)2, and the detected average Ni particle size was 3.7 nm. On the 3.7 nm sized Ni catalyst, the selectivity of aniline (AN) reached 99% with a 100% conversion of NB in 5.5 h at 90 °C, 1.0 MPa, and NB:Ni ) 305 (mole ratio). But, the activities of 11.8 nm sized Ni catalysts and commercial Raney Ni catalyst are quite lower. The reaction network and mechanism of NB hydrogenation on 3.7 nm sized Ni catalyst were discussed on the basis of products distributions at different temperatures and pressures. The calculated activation energy of NB hydrogenation on Ni-5/SiO2-EN catalyst is 54.5 kJ/mol in 70-90 °C. Scheme 1. Reaction Pathways for the Hydrogenation of Nitrobenzene1-3 a
1. Introduction Aniline (AN) is one of the most important chemicals and intermediates in the production of pharmaceuticals, dyes, pigments, and pesticides.1-3 Nowadays, AN is produced mainly via hydrogenation of nitrobenzene (NB) over Raney Ni catalyst, and this hydrogenation process proceeds consecutively in several steps. As shown in Scheme 1, a series of intermediates, such as nitrosobenzene (NSB), N-phenylhydroxylamine (PHA), azoxybenzene (AOB), azobenzene (AB), and hydrazobenzene (HAB) formed in sequence during the reaction process.1-3 Noble metals (Pt, Pd),4,5 transition metals (Cu, Ni),6 and even nonmetal catalysts (C60)7 were reported for this reaction. NB hydrogenation proceeds easily on noble metal catalysts,8 but its industrial application is limited due to the high cost and scarce resources. Raney Ni, Ni-B amorphous alloys, and Ni nanoparticles were widely reported for their higher activity and lower price.9-14 Meanwhile this reaction must be carried out at higher temperature, higher H2 pressure, and longer time in order to reach a satisfied selectivity of AN.10,11 On a commercial silica supported nickel catalyst, Relvas et al. found that the NB hydrogenation process is highly sensitive to the temperature, pressure, and initial state of the catalyst.15 But all the experimental data presented in this paper are relative values, and the structure of the supported Ni catalyst is not disclosed due to confidentiality. In other related published papers on supported Ni catalysts for liquid phase hydrogenation, Pina et al. found that direct hydrogenation of phenol was insensitive to Ni particle size, and there was a discernible decrease in specific phenol consumption over average Ni diameters less than 3 nm.16 And supported metal particles with diameters from less than 1 nm up to 10 nm can display quite distinct catalytic activities/selectivities associated with a switch from atomic to bulk metal character.17 Ermakova et al. found that the activity of nickel for benzene hydrogenation is shown to increase with increasing dispersion.18 On a series of Al2O3, SiO2, TiO2, and CeO2 supported Ni catalysts, Saadi et al. found that the order * To whom correspondence should be addressed. Tel.: +86-57188273272. Fax: +86-571-88273283. E-mail:
[email protected].
a NB: nitrobenzene. NSB: nitrosobenzene. PHA: N-phenylhydroxylamine. AN: aniline. AOB: azoxybenzene. AB: azobenzene. HAB: hydrazobenzene.
of activity for hydrogenation of benzaldehyde was attributed to crystallite size and degree of dispersion of nickel on the supports.19 Until now, the reaction network and mechanism of NB hydrogenation on supported catalysts and its dependence on the particle size of Ni were seldom reported. In this paper, a highly dispersed Ni catalyst with an average particle size of 3.7 nm was prepared via a simple impregnation of an aqueous Ni(en)3(NO3)2 solution on a nanosized silica gel (ca. 20-100 nm). And, this catalyst was utilized for the hydrogenation of NB to AN under milder reaction conditions (90 °C and 1.0 MPa). The performances of different size Ni particles on this nanosized silica gel and commercial Raney Ni for NB hydrogenation were compared. The reaction network and mechanism of NB hydrogenation on 3.7 nm sized Ni catalyst were discussed on the basis of product distributions at different temperatures and pressures.
10.1021/ie1002069 2010 American Chemical Society Published on Web 04/21/2010
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2. Experimental Section 2.1. Catalyst Preparation. A nanosized silica gel was purchased from Qingdao Haiyang Chemical Co., Ltd. (China) and directly used without further treatment. This silica gel was first impregnated into an equal volume aqueous solution of Ni(en)3(NO3)2 (en, ethylenediamine), and then, the precursor was pretreated in an ultrasounic bath for 20 min. Twelve hours latter, it was dried at 110 °C in air for 24 h and finally calcined at 500 °C for 4 h (Ni(en)3(NO3)2 was prepared by addition of ethylenediamine to an aqueous solution of nickel nitrate (3 en per Ni)).20-23 In this procedure, the catalysts were denoted as Ni-x/SiO2-EN, where x represents the loading amount of Ni. According to above procedures, another two Ni catalysts with the Ni loading amount of 5 wt % of silica gel were prepared, in which nickel nitrate (Ni(NO3)2, Sinopharm Chemical Reagent Co., Ltd.) and nickel acetate (Ni(OAc)2, Sinopharm Chemical Reagent Co., Ltd.) were used as precursors instead of Ni(en)3(NO3)2. These catalysts were denoted as Ni-5/SiO2-NI and Ni-5/SiO2-AC, respectively. At the same time, one commercial Raney Ni catalyst (product number A-7F63) that was kindly sponsored by Johnson Matthey Catalysts Co. (UK) was also tested as a reference. 2.2. Catalyst Characterization. The porous structure of the nanosized silica gel and the fresh supported Ni catalysts were detected by N2 physisorption at -196 °C using an autoadsorption analyzer (OMNISORP, 100CX). The sample was degassed first under high vacuum ( 1:10) is prerequisite in industrial production in order to accelerate the formation of AN. Therefore, supported Ni catalysts with smaller Ni particles can overcome this obstacle. In this contribution, the initial mole ratio of NB: Ni in the feed is 305 and the turning point of rapid formation of AN started as the NB:Ni ratio becomes less than 30:1. This enhanced activity of supported Ni catalyst could be ascribed to the exposed surface area (156.0 m2/g-Ni, Table 2) and the fact that the accessibility of Ni particles is higher than that of Raney Ni (32.0 m2/g-Ni). The product distributions in NB hydrogenation over Ni-5/ SiO2-EN catalyst at 70, 80, and 90 °C were summarized in Table 4 and used to calculate the activation energy of reaction. According to the product distributions, the calculated activation energy of NB hydrogenation over Ni-5/SiO2-EN catalyst in the temperature range of 70-90 °C is 54.5 kJ/mol, which is similar with that reported in another work (34.5 kJ/mol).15 4. Conclusions In this work, an efficient Ni/SiO2 catalyst for the hydrogenation of NB to AN is prepared via impregnation of Ni(en)3(NO3)2 on a silica gel. The 3.7 nm sized Ni particle is more active than Raney Ni. Product distributions at different reaction temperatures and different times disclosed that the rapid formation of AN
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ReceiVed for reView January 28, 2010 ReVised manuscript receiVed April 3, 2010 Accepted April 12, 2010 IE1002069
