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Ind. Eng. Chem. Res. 2011, 50, 1580–1587
Hydrogenation of p-Chloronitrobenzene on Mo-Doped NiB Cluster Catalysts Jenn-Fang Su, Bin Zhao, and Yu-Wen Chen* Department of Chemical and Materials Engineering, Nanocatalysis Research Center, National Central UniVersity, Chung-Li 320, Taiwan
The objective of this study was to investigate the effect of Mo content on the catalytic properties of Mo-NiB nanoclusters in the hydrogenation of p-chloronitrobenzene. A series of Mo-doped NiB nanoalloy catalysts with various Mo contents were prepared by chemical reduction method. An excess amount of NaBH4 was used (B/Ni atomic ratio ) 3) to fully reduce Ni. Even adding a small amount of Mo on NiB had a significant effect on the activity and selectivity to p-chloroaniline. The catalyst with the atomic ratio of Mo/Ni ) 0.4 had the highest activity and selectivity to p-chloroaniline. The particle size of Mo-NiB decreased with an increase of Mo content, indicating that the molybdenum species suppressed the growth of the crystalline structure of NiB and helped the NiB nanocluster maintain its amorphous state. Mo-NiB formed a nanoalloy, and no discrete phase was found. High-resolution transmission electron microscopy (HRTEM) images confirmed that the shape of Mo-NiB was spherical and the particle size was in the range of 3-5 nm. The effect of molybdenum on the catalytic performance of the amorphous NiB catalysts was 2-fold. Molybdenum not only acted as a spacer but also donated partial electron to Ni. Since the -NO2 was more electronegative than -Cl, -NO2 was supposed to occupy the active site on the Ni catalytic surface at the start of the reaction. -NO2 adsorbed on the catalyst surface is hydrogenated to form p-ceric ammonium nitrate (p-CAN), which was further desorbed. Therefore, the selectivity of p-CAN would increase as the Mo contents increased. It should be noticed the overdose of Mo would cover the active sites and decreased the activity. In addition, oxygen was even more electronegative, and alloying B could engage the oxygen to activate the polar -NO2 group of p-CNB. The -NH2 of p-CAN might adsorb on the surface, alloying B and coordinating with each other. Hence, it would improve the selectivity of p-CAN by depressing the dehalogenation reaction. Introduction Aromatic haloamines are important in industry for the production of many fine chemicals, such as dyes, herbicides, pesticides, preservatives, plant growth regulators medicines, and light-sensitive or nonlinear optical materials. Because of the environmental impact associated with the use of hydrochloric acid in Bechamp’s reaction, selective hydrogenation of halonitrobenzene to haloaniline over metal catalysts is preferred. Noble metals including platinum, palladium, and rhodium are studied extensively for this reaction, but the application for noble metals is limited due to its low selectivity to haloaniline and high cost. Raney nickel is commonly used in the hydrogenation reaction. However, the hydrodehalogenation of halonitrobenzene and aromatic haloamines often occurs on Raney nickel, and it suffers the disadvantage of self-igniting. Nanosized Ni catalyst modified with boron has been reported to be a good catalyst for the hydrogenation of nitrobenzene and furfural.1 The catalytic properties were highly dependent upon the preparation method.2,3 The nanocatalysts have more surface atoms and a higher concentration of highly coordinated unsaturated sites. Studied on amorphous nanoalloy particles have attracted much attention because of their interesting intrinsic properties, for example, short-range order, long-range disorder, and high dispersion, as well as their potential applications, for example, in powder metallurgy, magnetic materials, catalysts, and ferrofluids. The samples obtained by chemical reduction are highly dispersed and can be compacted to several different purposes. The amorphous nanoalloy particles combine the features of amorphous and ultrafine powder and have properties that are of interest in catalysis: (i) a large number of surface coordinating * To whom correspondence should be addressed. Tel.: (886) 3-4227151 ext. 34203. Fax: (886) 3-4252296. E-mail: ywchen@ cc.ncu.edu.tw.
unsaturated sites, (ii) the lack of crystal defects, and (iii) the isotropic, single-phase nature of the materials. Nanosized Ni catalyst modified with boron has been reported to be a good catalyst for hydrogenation of nitrobenzene and furfural.1-5 Boron could donate partial electrons to Ni, resulting in the electron-enriched Ni, and that indeed promotes the activity and thermal stability at a high amount of B content.6 Many efforts have been made to improve the catalytic performance of metalbased catalysts for this kind of hydrogenation reaction. Alloying a second metal was proven to be a good strategy. The nanoalloy particles constituted an overlapping area of amorphous alloys and nanophase materials. Special properties of the particles derived from the combination of their long-range disordered structure and nanosize are of great interest in some catalysis. Their interesting catalytic properties, especially for their unique selectivity in some of the hydrogenation reactions, give them promising potential applications in catalysis.7 Adding a specific modifier during the preparation was another efficient strategy to improve the catalytic behaviors of the catalysts. Researchers have reported that the high activity on Mo-doped alloy catalysts could be attributed to both the structure effect of surface area and the electronic effect of MoO3 favoring adsorption ability.7 The modifier existed in various states, such as elemental state, oxide (with various valents) state, or hydroxide state.8 The content of each state was closely related with the preparation method. For the Mo-modified skeletal Ni, the catalytic activity and selectivity would increase and the lifetime would also improve.9 The objective of this study was to develop a catalyst with a high activity and high selectivity for liquid-phase hydrogenation of p-CNB to p-CAN. From the literature,10 transition metals, such as Ni, Co, Mo, Fe, La, W, etc., were reported to be of low cost and high efficiency in the catalytic hydrogenation process.
10.1021/ie1016865 2011 American Chemical Society Published on Web 12/28/2010
Ind. Eng. Chem. Res., Vol. 50, No. 3, 2011
In this study, Mo was chosen as the promoter of NiB nanocatalysts for hydrogenation of p-CNB. NiB nanocluster has been reported to be a good catalyst for the liquid-phase hydrogenation reaction. However, the stability is not good due to sintering in the reaction. Mo was chosen to be tested as the structural promoter in this study to retard sintering of NiB. Mo may also have electronic interaction with Ni. The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), energydispersive spectrometer (EDS), and X-ray photoelectron spectroscopy (XPS). Experimental Section Catalyst Preparation. A series of Mo-doped NiB nanoclusters were prepared by the chemical reduction method. The synthesis procedures were as follows. Mo(0.1)-NiB was used as an example; the other catalysts were synthesized with a similar method using different amounts of Mo precursor. Nickel acetate (0.4974 g (2 mmol)) and 0.2472 g (0.2 mmol) ammonium heptamolybdate were dissolved in 20 mL methanol at room temperature under vigorous stirring and using nitrogen stream to remove air. The solution of reduction agent, 0.2270 g (6 mmol) sodium borohydride dissolved in 6 mL methanol, was added into the solution in a dropwise manner with a microtubing pump under nitrogen stream. The atomic ratio of Ni/B was fixed at 1:3. An excess amount of sodium borohydride was used to ensure the complete reducing of the metal cations. Mo/Ni atomic ratios were between 0 and 1 for investigation. To remove the impurities, i.e., Na+ ions and boride species, the as-prepared product was centrifuged and washed with hot deionized water three times and with absolute methanol twice. Finally, it was kept in methanol. The sample was denoted as Mo(x)-NiB where x means Mo/Ni atomic ratio (x ) 0, 0.2, 0.4, 0.6, 0.8, 0.9, 1.0). Catalyst Characterization. The catalysts were characterized by XRD, TEM, HRTEM, EDS, and XPS. The crystalline structure of the catalysts was characterized by XRD using a Siemens D500 powder X-ray diffractometer. The XRD patterns were collected by using Cu KR1 radiation (1.5405 Å). The tube voltage and current were 40 kV and 30 mA, respectively. The scanning rate was 0.05° s-1. The morphologies of the catalysts were determined by TEM (Jeol JEM-2000 FX II) and HRTEM equipped with EDS (Jeol JEM-2010 and Oxford’s energy-dispersive spectrometer system). TEM was operated at 160 kV, and the magnification was 200 K. The magnification of HR-TEM was 500 K. A small amount of sample was put into the sample tube filled with a 95% ethanol solution. After agitating under an ultrasonic environment for 90 min, one drop of the dispersed slurry was dipped onto a carbon-coated copper mesh (300#) (Ted Pella Inc., CA, U.S.A.) and dried at room temperature in vacuum overnight. The compositions and the electronic states of each element on the surface of the catalysts were studied by X-ray photoelectron spectroscopy (XPS) on a Thermo VG Scientific Sigma Probe spectrometer. Al KR radiation was used as the excitation source (hν ) 1486.6) (20 kV, 30 mA). The sample was pressed as a self-supported plate and was mounted on the sample cell. It was degassed in the pretreatment chamber at 343 K for 2 h; then it was transferred into the analyzing chamber where the background pressure was 0.2, this peak even disappeared. No other crystalline phases (including molybdenum-related compounds, elemental Ni and B, and the corresponding oxides and hydroxides) were observed. Thus, NiB and Mo-NiB possessed amorphous structure in this study. Furthermore, Mo-NiB was more amorphous
Figure 2. TEM images of (a) NiB, (b) Mo(0.1)-NiB, (c) Mo(0.2)-NiB, (d) Mo(0.4)-NiB, (e) Mo(0.6)-NiB, and (f) Mo(1.0)-NiB.
Figure 4. (a) HRTEM mapping image of Mo(0.4)-NiB and (b) the same portion HRTEM image of Mo(0.4)-NiB.
and had a lower order in crystallinity than NiB. In other words, Mo-NiB had a shorter-range order than NiB. The smaller particle size was also confirmed at the centrifuge step in catalyst preparation. After adding Mo, the separation of catalyst became harder, with many tiny particles suspended in the solution even after a long centrifugation time. The amorphous nature of these materials makes them own several intrinsic characteristics, including short-range order/long-range disorder, the presence of more surface coordinating unsaturated sites, more crystalline defects, and isotropic structure. Consequently, the amorphous structure is favorable in terms of catalytic activity for the hydrogenation reactions, as will be discussed in a later section. TEM, HRTEM, and EDS. The morphology of materials was analyzed by TEM and HRTEM. Figure 2 shows the TEM images of the modified and unmodified NiB catalysts. Each sample was composed of many small particles, and the size of the aggregates was ∼50 nm. It was difficult to obtain detailed information on the morphology because of the aggregation of the particles by the very strong van der Waals force. However, the primary particles were quite small. One could conclude that the size of the primary particles was much smaller than 50 nm. The addition of molybdenum to NiB changed the structure from curling to flat structure. HRTEM was employed to obtain the details of the Mo-NiB samples. A typical HRTEM image of Mo(0.4)-NiB catalyst is shown in Figure 3. It confirms that the Mo-NiB catalysts were composed of many small spherical particles and the size of the primary particles was Mo(0.8)-NiB > Mo(0.6)-NiB > NiB > Mo(0.9)-NiB > Mo(1.0)-NiB > Mo(0.2)-NiB. Even the molybdenum oxide could improve the activities of the catalysts; an overdose of Mo would cover the active sites and decrease the activity. The proper amount of Mo promoter could enhance the activity and selectivity of p-CAN greatly. The optimum molar ratio of Mo/ Ni is 0.4. Figure 8B shows the selectivity-time curves of Mo-NiB catalysts. Mo-dopant could promote the selectivity to p-CAN. The yield of p-CAN shown in Figure 8C further presented this effect. Molybdenum oxide could decrease the particle size of NiB and made the Ni more unsaturated on the catalyst surface. Thus the catalyst tended to adsorb the nitro group, in favor of hydrogenation of p-CNB. The nitro group owned two different elements, N and O, which were both highly electronegative. With the electron donation from alloying B and Mo to Ni, the Ni on the catalyst surface became more electron-enriched. Higher electronegativity of -NO2 would be adsorbed on the surface easily, and the active sites would activate the NdO bond, which was polarized. The para-substituted nitro group had the higher electron negativity, resulting from the combination of both inductive and resonance effect. Since the -NO2 group was more electronegative than -Cl, -NO2 was supposed to occupy the active site in Ni catalytic surface at the start of the reaction. -NO2 adsorbed on the catalyst surface is hydrogenated to form p-CAN, which is further desorbed. For the molecule of p-CAN, the -Cl atom is an electron-extracted group; the carbon atom connected with -Cl is easier to be attacked by H atoms. The -NH2 group is an electron-repulsive group; the electrons transferred from -NH2 group and the benzene ring to the -Cl group due to the inductive and conjugative effect. Consequently, the polarity of the C-Cl bond increased, which favored the leaving of -Cl group. The intensity of the Lewis base of -NH2 group is much stronger than that of -Cl; there is also a kind of competitive adsorption of -Cl and -NH2 with the Lewis acid sites such as boron oxide and Mo oxides. They could interact with the -NH2 group of p-CAN. This kind of interaction resulted in the decrease of polarity of C-Cl bond, and the nucleophilic attack by the H atoms become difficult. Therefore, the hydrodehalogenation of p-CAN on Mo-NiB catalysts was effectively suppressed. In addition, the interaction of B oxide and Mo oxide with p-CAN molecules could favor their desorption process, which could further suppress the hydrode-
Table 4. Hydrogenation of p-CNB over Mo-NiB Catalysts selectivity (%) sample
reaction time (min) to 50% conversion
reaction time (min) to 50% p-CAN selectivity
reaction time (min) to 100% conversiona
p-CAN
AN
NB
NiB Mo(0.2)-NiB Mo(0.4)-NiB Mo(0.6)-NiB Mo(0.8)-NiB Mo(0.9)-NiB Mo(1.0)-NiB
8 22.2 6.7 7.8 6.7 8.9 12.2
16.4 6.15 6.15 6.15 6.15 6.15 6.15
95.6 57.8 46.8 47.8 57.8 46.7 108.9
96.9 89.1 96.1 96.0 95.6 97.3 98.6
2.5 10.8 3.9 4.0 4.3 2.7 1.4
0 0 0 0 0 0 0
a
Reaction condition: 1.2 MPa hydrogen pressure, 353 K reaction temperature, absolute methanol was medium, 500 rpm stirring speed, 0.2 mmol Ni catalyst. p-CAN, p-chloroaniline; AN, aniline; and NB, nitrobenzene.
Ind. Eng. Chem. Res., Vol. 50, No. 3, 2011
halogenation of p-CAN. Consequently, the selectivity of p-CAN would increase as the Mo content increased.20-22 The selectivities of p-CAN on various catalysts as the conversions of p-CNB reach to 50% and 100% are shown in Table 4. The Mo-modified samples exhibited similar selectivities to p-CAN as the unmodified one. The selectivities of all the Mo-doped samples could exceed 90%. For the Mo(0.4)-NiB catalyst, the reaction was completed within 50 min and the selectivity of p-CAN was 96.1%. It could be concluded that Mo-NiB is a good catalyst for the hydrogenation of p-CNB. It is evident that the dispersing effect of molybdenum oxide is very beneficial for the hydrogenation reaction, not only enhancing the activity but also increasing the selectivity of p-CAN. Conclusion A series of Mo-modified NiB nanoalloy catalysts with various Mo contents were prepared by chemical reduction method by NaBH4. The catalysts were characterized by XRD, TEM, HRTEM, EDS, and XPS. The catalytic properties of these catalysts were tested in the hydrogenation of p-CNB at 353 K and 1.2 MPa H2 pressure. The thermal stability of NiB increased upon the addition of Mo. Most of the Mo was in the form of oxide and acted as a spacer to prevent NiB from aggregation/ agglomeration. NiB was amorphous as indicated by the broad peak around 2θ ) 45°. Upon modification with Mo, it maintained NiB in the amorphous structure, decreased the degree of crystallinity, and enhanced the thermal stability. Mo-NiB formed nanoalloy, and no discrete phase was found. With the addition of Mo, the particle size of NiB decreased. HRTEM images confirmed that the shape of Mo-NiB was spherical and the particle size was in the range of 3-5 nm. The higher concentration of molybdenum species would enhance the dispersion of NiB and increased the concentration of highly unsaturated active sites. The sample of Mo(0.4)-NiB owned the highest concentration of Ni0, resulting in the higher activity and selectivity for the hydrogenation of p-CNB to p-CAN. The effect of molybdenum on the catalytic performance of the amorphous NiB catalysts was 2-fold. Molybdenum not only acted as a spacer but also donated partial electron to Ni. The optimum molar ratio of Mo/Ni atomic ratios was 0.4. Since the -NO2 was more electronegative than -Cl, -NO2 was supposed to occupy the active site on Ni catalytic surface at the start of the reaction. -NO2 adsorbed on the catalyst surface is hydrogenated to form p-CAN, which was further desorbed. Therefore, the selectivity of p-CAN would increase as the Mo contents increased. It should be noticed that an overdose of Mo would cover the active sites and decrease the activity. In addition, oxygen was even more electronegative and alloying B could engage the oxygen to activate the polar -NO2 group of p-CNB. The -NH2 of p-CAN might adsorb on the surface, alloying B and coordinating with each other. Hence, it would improve the selectivity of p-CAN by depressing the dehalogenation reaction. Acknowledgment This research was supported by the Ministry of Economic Affairs, Taiwan, Republic of China, under contract No. 99-EC17-A-09-S1-022. Literature Cited (1) Liu, I. H.; Chang, C. Y.; Liu, S. C.; Chang, I. C.; Shih, S. M. Absorption removal of sulfur dioxide by falling water droplets in the presence of inert solid particles. Atmos. EnViron. 1994, 28, 3409–3415.
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ReceiVed for reView August 9, 2010 ReVised manuscript receiVed November 18, 2010 Accepted December 1, 2010 IE1016865
