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Hydrogenation of p-Chloronitrobenzene on Lanthanum-Promoted NiB Nanometal Catalysts Yu-Chang Liu and Yu-Wen Chen* Department of Chemical Engineering, Nanocatalysis Research Center, National Central UniVersity, Chung-Li 32054 Taiwan
Amorphous nanosized NiB catalysts have been reported to be a good catalyst for the liquid-phase hydrogenation of chloronitrobenzene. This study was conducted to investigate the effect of lanthanum on the catalytic properties of the NiB catalyst in the liquid-phase hydrogenation of p-chloronitrobenzene (p-CNB). The lanthanumpromoted NiB catalyst was prepared via the liquid-phase reduction of nickel cations by borohydride. The atomic ratio of boron to nickel in the mother solution was B/Ni ) 3:1. The catalysts were characterized by N2 sorption, powder X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. Liquid-phase hydrogenation of p-CNB was performed in a well-stirred, high-pressure batch reactor system. The catalysts prepared in this study had nanosized particles. La-NiB was much more active than NiB and the Raney nickel catalyst. The selectivity of the main product (p-chloroaniline, p-CAN) was >99% on La-NiB catalysts. However, excess amounts of lanthanum caused a decrease in the activity. The effect of the lanthanum promoter can be attributed to the electronic modification of nickel by lanthanum. Introduction The hydrogenation of halonitroaromatics to the corresponding haloanilines over precious metals has been studied extensively. Aromatic haloamines are important intermediates in the chemistry of herbicides, dyes, drugs, and pesticides. The traditional synthesis routes are usually harmful for the environment. It seems to be a best choice by selective hydrogenation of aromatic halonitro compounds to the corresponding haloamine; however, the process is difficult because of extensive dehalogenation.1 The reaction routes involved in the hydrogenation of pchloronitrobenzene (p-CNB) is shown below.2
With a variety of catalysts (e.g., platinum, palladium, nickel, rhodium), the hydrogenation of halonitroaromatics to the corresponding haloanilines is always accompanied by some dehalogenation reaction.3 Depending on the halogen and its position relative to nitro group in the aromatic system, dehalogenation can vary from negligible to 100%. In the process, it is desirable to achieve selective hydrogenation to p-chloroaniline (p-CAN) without dehalogenation, and its selectivity is dependent on the catalyst and the reaction conditions. It is generally known that a dehalogenation reaction as well as a reduction of the nitrogroup occurs. Furthermore, hydrogen chloride (HCl) produced from the dehalogenation contributes to the corrosion of the reactor. To achieve high yields of haloanilines, many approaches have been developed either by dedicated preparation of the * To whom correspondence should be addressed. E-mail: ywchen@ cc.ncu.edu.tw.
catalysts (alloying, controlling the metal particle dispersion and metal-support interaction, etc.) or the use of specific additives (promoters, inhibitors).4-7 Figueras et al.3 investigated the catalytic hydrogenation of p-CNB to p-CAN over a series of ruthenium catalysts of widely varying dispersions and on supported bimetallics RuM (where M ) Sn, Pb, Ge). Liu et al.8 discussed the metal complex effect on the selective hydrogenation of m- and p-CNB over polyvinyl pyrrolidone (PVP)-stabilized platinum colloidal catalysts. Zheng et al.9 have studied the effect of rare earths on the hydrogenation properties of p-CNB over polymer-anchored platinum catalysts. They also have reported the influence of rare earths (cerium, samarium, neodymium, lanthanum, and praseodymium) on the hydrogenation properties of chloronitrobenzene over a Pt/ZrO2 catalyst.10 However, the precious metals are expensive. The effects of a rare-earth promoter, such as lanthanum, on the nanosized NiB catalyst have never been reported. Nanoparticles exhibit distinctive electronic and catalytic properties that differ from those in the bulk. The nanocatalysts have more surface atoms and a higher concentration of highly coordinated unsaturated sites. Their unique isotropic structural and chemical properties have attracted extensive interest in recent years. However, only limited endeavors have been devoted to the synthesis of nanosized NiB particles.11-20 A nanosized nickel catalyst modified with boron has been reported to be a good catalyst for the hydrogenation of nitrobenzene and furfural.21-25 The catalytic properties are highly dependent on the preparation method.26,27 Many systematic studies have been made on the catalytic properties for Ni-B catalysts.28-34 Since the 1980s, metal catalysts that contain boron species (generally called metal borides) have been studied extensively in some reactions.35-38 Metal boride catalysts are easily prepared by the reduction of metal salt with borohydride, and they have excellent activity and selectivity for many hydrogenation reactions.39 These amorphous NiB particles were characterized by extended X-ray absorption fine structure (EXAFS) analysis as a structure with long-range disorder and short-range ordering, and by transmission electron microscopy (TEM) with a size range of 10-50 nm. It has been reported that, with the addition
10.1021/ie0509847 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/28/2006
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of a small amount of lanthanum as a promoters of catalysts, the catalytic activity and thermal stability was improved.40 The activity and selectivity of metal borides can be altered significantly by varying the amount of metal-associated boron.27,41 Of the two types of boron species (boride and borate) that are present in colloids prepared via NaBH4 reduction, it has been verified that only borate has a significant effect on the hydrogenation of o-CNB, whereas boride performed as a spectator.47 However, the effect of a lanthanum promoter on the catalytic properties of NiB catalyst in the hydrogenation of p-CNB has not been studied. The objective of this study was to elucidate the effect of the lanthanum additive on the catalytic properties of nanosized NiB catalyst in the hydrogenation of p-CNB. Experimental Section Materials. Nickel acetate, lanthanium chloride, and p-CNB were obtained from Acros. Sodium borohydride (NaBH4, >98%) was obtained from Lancaster. High-purity hydrogen gas (>99.99%, from Air Products) was used without further purification. Raney nickel catalyst was obtained from Merck. Methanol was obtained from Showa. All other solvents and reagents were analytical-grade quality, purchased commercially, and used without any further purification. Water was doubledistilled and demineralized. Catalysts Preparation. La-NiB nanometal catalysts were prepared via a chemical reduction method, using NaBH4 as the reducing agent. Nickel acetate was used as the precursor. LaCl3 was added into the aqueous solution of nickel acetate. The Ni/ La ratios were 10 wt % and 40 wt %, respectively. The solution of NaBH4 with a concentration of 1 M was used as the reducing agent. This solution was added drop by drop to the solution of nickel salt (0.1 M) at 25 °C with vigorous stirring. To ensure full reduction of nickel, excess amount of NaBH4 (in a B/Ni atomic ratio of 3) was used. The solvent was methanol in water, with a volume ratio of 1/1. High-purity nitrogen (99.9999%) was flowed through the reactor. Nitrogen was used as the sheltering gas in this experiment, to prevent oxidation by the dissolved oxygen in water. A black precipitate was formed immediately. The precipitate was washed thoroughly, first with deionized water three times, to remove the soluble B species and K+ or Na+ ions, which may act as a poison for the nickel activity,48 and then with a 99.5% methanolic solution at least two times. The resulting La-NiB samples were soaked in absolute methanol before characterization. The catalyst was designated as La-NiB(x) where x is the weight ratio (%) of La/ Ni. For example, La-NiB(10) means the La/Ni weight ratio is 10%. Characterization. X-ray diffraction (XRD) measurements were obtained using a Siemens model D500 powder diffractometer with Cu KR radiation (40 kV, 30 mA). The sample was scanned over the range of 20°-60° 2θ at a rate of 0.05°/s to identify the crystalline structure. A sample for XRD analysis was prepared as a thin layer on a sample holder. Brunauer-Emmett-Teller (BET) surface areas were obtained by the physisorption of nitrogen at -197 °C, using a Micromeritics 2010 instrument. Prior to measurement, the samples were degassed to 0.1 Pa at 120 °C. The surface areas were calculated in a relative pressure range of 0.05 < p/p0 < 0.2, assuming a cross-sectional area of 0.162 nm2 for a N2 molecule. The morphologies and particle sizes of the samples were determined by transmission electron microscopy (TEM) performed on a JEOL model JEM-1200 EX II electron microscope that was operated at 160 kV. The powder specimen was
dispersed in absolute methanol and sonicated for 20 min. One drop of the suspension was deposited on a specimen grid coated with a holey carbon film (300#) (Ted Pella, Inc., CA) and dried in a vacuum overnight. The average size and size distribution of each specimen were evaluated from ∼300 randomly selected particles. The surface composition and oxidation state of the surface atoms were examined by X-ray photoelectron spectroscopy (XPS), using a Perkin-Elmer PHI-1600 instrument. Mg KR radiation was used to excite photoelectrons, which was analyzed with the analyzer, operated at a pass energy of 150 eV. The nanoparticles were first pressed into the form of a 10 mm × 10 mm disk in absolute ethanol to avoid contamination by air. The sample was fixed on a special sample holder, which was then placed in a vacuum chamber. The base pressure in the analyzing chamber was maintained on the order of 10-7 mbar. The spectrometer was operated at a pass energy of 23.5 eV. The binding energy of XPS was corrected by argon (Ar2p 3/2 ) 242.0 eV) to facilitate the comparisons of the values among the catalysts and the standard compounds. The surface composition of the samples was determined from the peak areas of the corresponding lines, using a Shirley-type background and empirical cross-section factor for XPS. Hydrogenation Reaction. The catalysts were tested for the liquid-phase hydrogenation of p-CNB to p-CAN. A fresh, asprepared catalyst was used in each reaction run, All the experiments were conducted in a cylindrical stirred-tank reactor (Parr Instruments, Model 4842) with a capacity of 160 mL. A four-bladed pitched impeller was inserted for effective agitation, and the agitator was connected to an electric motor with variable speed (up to 700 rpm). A pressure transmitter and an automatic temperature controller were also provided. The gases were supplied from cylinders and introduced to the base of the reactor; another tube was served as the sampling tube for the liquid phase. The concentration of p-CNB was 0.2 M, and the hydrogen pressure was 1.2 MPa. Methanol was used as the reaction medium in this study. In a previous paper,27 the authors have reported that methanol is a better reaction medium than ethanol. The reactor was charged with 2 mmol of nickel catalyst and 2.52 g of p-CNB in a reaction medium (methanol) of 80 mL. Air was flushed out of the reactor with hydrogen at room temperature and hydrogen was then fed into the reactor. When the designated temperature was reached, hydrogen was fed to the predetermined pressure (time zero) that was maintained throughout the reaction, the stirring speed was fixed at 500 rpm. During the run, samples were withdrawn periodically (10 min) and analyzed via gas chromatography (GC). A gas chromatograph that was equipped with a thermal conductivity detector and a 3-m-long × 1/8-in.-diameter (1 in. ) 2.54 cm) stainless steel column packed with 5% OV-101 on Chromsorb WAWDMSC (80-100 mesh) was used for sample analysis. The experiments have been repeated twice at least. The reproducibility was within 98%. Results and Discussion X-ray Diffraction (XRD). The XRD patterns of the La-NiB samples, shown in Figure 1, give only a broad peak at ∼45° 2θ. This was assigned to the amorphous state of a nickelmetalloid alloy.14 Line broadening is so pronounced that the 111 and 200 reflections are not resolved. Particle sizes as determined by XRD showed an average crystallite size of 2.0 nm. Compared to the XRD pattern of the NiB catalyst, the intensity of XRD patterns of La-NiB is slightly intense but still
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Figure 1. X-ray diffraction (XRD) patterns of the La-NiB samples.
exhibits an amorphous structure (no long-range order). No lanthanium oxide XRD peaks were observed. The XRD patterns confirmed that these La-NiB catalysts had a structure with shortrange order. The XRD patterns of the La-NiB catalysts calcined at 250 °C for 2 h did not show intense peaks, indicating that these catalysts had good thermal stability. Deng et al.29 confirmed the amorphous structure of NiB using EXAFS; they exhibit long-range disorder and short-range order. Undoubtedly, the NiB and La-NiB catalysts maintained their amorphous structure at a reaction temperature of 90% within 10 min and 100% conversion was observed within 30 min. In contrast, Raney nickel catalyst only reached 80% conversion after 170 min. Figure 5 also demonstrates that the activity of the La-NiB catalysts decreased in the order of La-NiB(10) > La-NiB(40) > NiB. Although lanthanum could promote the reaction rate, excess lanthanum caused a decrease in activity. The timeconversion curves showed that the reaction has first-order kinetics, in regard to p-CNB concentration. The reaction rate was calculated based on the first-order kinetics equation, and the results are listed in Table 3. The activities per weight of the catalysts decreased in the order: La-NiB(10) > La-NiB(40). Suitable concentrations of either of these lanthanum promoters could further enhance the hydrogenation activity of a NiB
Figure 6. Effect of reaction temperature on the conversion of p-CNB; the reaction solvent was methanol, and the catalyst was La-NiB(10).
amorphous alloy, because these promoters, when present in the low-valency state, in contact with the La active sites, could act as Lewis adsorption sites for the O atom of the carbonyl group, which was then polarized, and thus, more easily hydrogenated via a nucleophilic attack on the C atom by the dissociated H atom on surface La active sites. The difference in the catalytic activities can be attributed to the differences in the surface area and the electronic density of the nickel metal. In this study, we did not measure the nickel surface area by hydrogen chemisorption because nickel metal would undergo a large amount of sintering upon high-temperature treatment. One can assume that the specific activity per surface area of the catalyst is proportional to its turnover frequency, which, here, is called the quasi-turnover frequency (QTOF). The activities per unit of surface area (the QTOF values) of the catalysts also decreased in the same order, i.e., La-NiB(10) > La-NiB(40) > NiB. This confirms that the catalytic activity was affected by the electronic structure of the nickel metal. The higher activity on La-NiB than that on NiB can be attributed to the electronic effect of lanthanum donating electrons to Ni, thereby weakening the Ni-H bond strength and activating the H atoms. Figure 6 shows the conversion of p-CNB at different temperatures on the La-NiB(10) catalyst. The reaction at 120 °C was faster than that at 70 °C by 2-fold. The activation energy was calculated to be ∼33.5 kcal/mol. This reconfirmed that the reaction was performed in a kinetics-controlled regime, because the activation energy would be much lower if it were in a masstransfer-controlled regime. The nanosized La-NiB catalyst was significantly more active than NiB and commercial Raney nickel catalysts. The selectivity of p-CAN on La-NiB was >99%, and that on Raney nickel was only ∼92%. Undoubtedly, the La-NiB catalyst is a promising candidate for replacing Raney nickel and NiB in hydrogenation reactions.
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Conclusion A series of nanosized lanthanum-promoted NiB (La-NiB) amorphous alloy catalysts with different lanthanum loadings were prepared by a chemical reduction method, using sodium borohydride (NaBH4) as the reducing agent. Nickel acetate was used as the nickel precursor. The reduction was performed in a methanolic solvent and at room temperature with vigorous stirring and using nitrogen as the sheltering gas to prevent the oxidation of nickel. The La-NiB catalysts were characterized by transmission electron microscopy (TEM) as being nanoscale (10-20 nm) particles and by X-ray diffraction (XRD) as having an amorphous structure. The catalyst only showed slightly differences in particle size, relative to unpromoted NiB, and the morphology did not change significantly. The La-NiB catalyst was more stable than NiB and Raney nickel. The LaNiB catalyst had a higher activity for the hydrogenation of p-CNB, compared to the unpromoted NiB and Raney nickel catalyst. However, excess amounts of lanthanum caused a decrease in the activity. The effect of a lanthanum promoter can be attributed to the electronic modification of nickel by lanthanum. The selectivities for p-chloroaniline (p-CAN) were >99% on both La-NiB catalysts. Acknowledgment This research was supported by the Ministry of Economic Affairs, Taiwan, Republic of China (under Contract No. 92EC-17-A-09-S1-022). Literature Cited (1) Baltzly, R.; Phillips, A. P. The Catalytic Hydrogenolysis of Halogen Compounds. J. Am. Chem. Soc. 1946, 68, 261-265. (2) Yu, Z.; Liao, S.; Xu, Y.; Yang, B.; Yu, D. A Remarkable Synergic Effect of Polymer-anchored Bimetallic Palladium-Ruthenium Catalysts in the Selective Hydrogenation of p-Chloronitrobenzene. J. Chem. Soc., Chem. Commun. 1995, 1155-1156. (3) Tijani, A.; Coq, B.; Figue´ras, F. Hydrogenation of para-Chloronitrobenzene over Supported Ruthenium-based Catalysts. Appl. Catal. 1991, 76, 255-266. (4) Coq, B.; Tijani, A.; Figue´ras, F. Pathways and the Role of Solvent in the Hydrogenation of Chloronitrobenzenes. J. Mol. Catal. 1992, 71, 317333. (5) Coq, B.; Tijani, A.; Figue´ras, F. Particle Size Effect on the Kinetics of p-Chloronitrobenzene Hydrogenation over Platinum/Alumina Catalysts. J. Mol. Catal. 1991, 68, 331-345. (6) Coq, B.; Tijani, A.; Dutartre, R.; Figue´ras, F. Influence of Support and Metallic Precursor on the Hydrogenation of p-Chloronitrobenzene over Supported Platinum Catalysts. J. Mol. Catal. 1993, 79, 253-264. (7) Greenfield, H.; Dovell, F. S. Metal Sulfide Catalysts for Hydrogenation of Halonitrobenzenes to Haloanilines. J. Org. Chem. 1967, 32, 36703671. (8) Tu, W.; Liu, H.; Tang, Y. The Metal Complex Effect on the Selective Hydrogenation of m- and p-Chloronitrobenzene over PVP-stabilized Platinum Colloidal Catalysts. J. Mol. Catal. A: Chem. 2000, 159, 115120. (9) Han, X.; Zhou, R.; Zheng, X.; Jiang, H. Effect of Rare Earths on the Hydrogenation Properties of p-Chloronitrobenzene over Polymeranchored Platinum Catalysts. J. Mol. Catal. A: Chem. 2003, 193, 103108. (10) Han, X.; Zhou, R.; Laia, G.; Yue, B.; Zheng, X. Influence of Rare Earth (Ce, Sm, Nd, La, and Pr) on the Hydrogenation Properties of Chloronitrobenzene over Pt/ZrO2 catalyst. Catal. Lett. 2003, 89, 255-259. (11) Schlesinger, H. I.; Brown, H. C.; Finholt, A. E.; Gilbreat, J. R.; Hoeksta, H. R.; Hyde, E. K. Sodium Borohydride, Its Hydrolysis and Its Use as a Reducing Agent and in the Generation of Hydrogen. J. Am. Chem. Soc. 1953, 75, 215-219. (12) Raymond, P.; Buisson, P.; Joseph, N. Catalytic Activity of Nickel Borides. Ind. Eng. Chem. 1952, 44 (5), 1006-1010. (13) Brown, H. C.; Brown, C. A. The Reaction of Sodium Borohydride with Nickel Acetate in Ethanol SolutionsA Highly Selective Nickel Hydrogenation Catalyst. J. Am. Chem. Soc. 1963, 85, 1003-1005.
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ReceiVed for reView August 31, 2005 ReVised manuscript receiVed February 15, 2006 Accepted February 22, 2006 IE0509847
