b School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
The development of transition-metal-catalyzed organic transformations based on the first-row transition metals such as Co, Ni and Cu is of importance because of their relatively low cost and toxicity relative to precious metals. Carbon is an ideal catalyst support for the following reasons: (1)It can be prepared from biomass. (2) The price is low. (3) The metal can be recycled by carbon burning.
It is important to control the particle size and dispersion of the metal particles on the support, since these properties have shown great influence on the catalyst activity, selectivity and lifetime. Impregnation is a common method for preparation of supported catalysts. There are many factors affecting the dispersion of the active component of the catalyst on the supports such as metal precursor, dispersant, stirring rate and temperature. Metal complex can be used as homogeneous catalyst for the reaction and organic modifiers (ligands) on metal-based heterogeneous catalysts also can help to enhance the catalytic selectivity to some extent. Recently, there are many reports on improving the performance of noble metal catalysts by inorganic and organic modifiers [1-5]. Ethylenediamine-coated ultrathin platinum nanowires exhibited excellent performance for the selective hydrogenation of nitroaromatics to N-hydroxylanilines . Polyvinyl pyrrolidone has been used as electronic and geometric modifier of palladium nanoparticles and Ru nanoparticles [2, 6]. Phosphine oxide ligands also affect the performance of gold nanoparticles for the chemoselective hydrogenation of substituted aldehydes [3, 4]. N-hydroxylanilines carbenes has been used as ligands for supported heterogeneous Ru/K-Al2O3 catalysts . The catalytic performance of Pd/Al2O3 was also modified by N-heterocyclic carbenes . However, to the best of our knowledge, the application of organic ligand to modify cheap metal or carbon-supported heterogeneous catalysts is limited.
Azo compounds are key raw materials and are widely used in the synthesis of organic dyes, food additives, indicators, and drugs . It is an environmentally friendly method for the preparation of azobenzene by hydrogenative coupling of nitrobenzene. Pt, Pd and Au catalysts have been used to catalyze this reaction [10-12]. However, the high cost and relatively low abundance limit their large-scale application to a certain degree.
Herein, we found that ethylenediamine can promote the performance of Ni/C catalyst for the hydrogenative coupling of nitroarenes. Ethylenediamine could be aid of the dispersion of the Ni nanoparticles on carbon support and improve the catalytic performance significantly. The yield of azobenzene could reach 95.5% when the ratio of ethylenediamine and Ni was 10:1.
The materials used are listed below. Ethylenediamine (99%), 1, 3-propanediamine (99%), 1, 10-phenanthroline monohydrate (99%), 2, 2'-bipyridine (99%), azobenzene (98%), azoxybenzene (98%), nickel(Ⅱ) acetylacetonate (95%), and dodecane (99%) were purchased from J&K Scientific Ltd. Nitrobenzene (99%) was purchased from Acros Organics. Sodium hydroxide (A.R.), methanol (> 99%) and ethanol (> 99%) were supplied by Beijing Chemical Works. Activated carbon was purchased from Xinsen Carbon Industry Co., Ltd. Hydrogen (> 99.99%) was provided by Beijing Analytic Instrument Company. All chemicals were used without further purification.
The impregnation method was employed when fabricating the N-modified Ni/C catalysts. Certain amounts ofligands (ethylenedi-amine, 1, 3-propanediamine, 1, 10-phenanthroline monohydrate and 2, 2'-bipyridine) were added into 5 mL ethanol solution which dissolved 46.1 mg nickel(Ⅱ) acetylacetonate. After adding 200 mg commercial activated carbon, the black liquid was stirred for 10 h. Then ethanol was removed under reduced pressure and the sample was dried under vacuum at 40 ℃ overnight. The obtained black powder was ground and calcined under a H2 atmosphere at 500 ℃ for 2 h. The temperature was linearly raised from 50 ℃ to 500 ℃ at a heating rate of 5 ℃/min. The ethylenediamine content was adjusted by controlling the molar ratio of ethylenediamine and nickel(Ⅱ) acetylacetonate added. The catalysts were denoted as Ni-L/C. L was ethylenediamine, 1, 3-propanediamine, 1, 10-phenanthro-line monohydrate and 2, 2'-bipyridine.
The catalysts were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and inductivelycoupled plasma-atomic emission spectroscopy (ICP-AES) techniques. TEM images were measured on a JEOL-1011 electron microscope operating at 100.0kV, 10.00 mA. Before measurement, the catalysts were suspended in ethanol after ultrasonic dispersion. The obtained dispersions were dropped onto copper-grid-supported carbon films. PowderX-raydiffraction (XRD) patterns were recorded on a Rigaku D/max-2500 X-ray diffractometer using Cu Ka radiation (l = 0.15406nm). The tube voltage was 40 kV and the current was 200 mA. The X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCA Lab 220i-XL electron spectrometer from VG Scientific using 300 W Al Ka radiation as the excitation source (hv = 1486.6 eV) and operated at 15 kV and 20 mA. The base pressure was about 3 × 10—9mbar. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. The contents of different elements in the Ni/C catalysts were analyzed by ICP-AES (PROFILE. SPEC, Leeman).
A 10 mL Teflon-lined stainless-steel autoclave equipped with a magnetic stirrer was used when implementing the reaction, which was same as the equipment we used previously . In a typical experiment, 2 mmol (246 mg) of nitrobenzene, 2 mL of the solvent (methanol) 0.1 mmol of NaOH, and the 10 mg of the catalyst were loaded into the reactor. The reactor was sealed and purged with hydrogen to remove the air at room temperature and then placed in a furnace at desired temperature for a set of time. Hydrogen was charged to the desired pressure and the stirrer was started with a stirring speed of 800 rpm. The pressure was determined by a pressure transducer (FOXBORO/ICT, Model 93), which could be accurateto±0.025MPa.Upon reaction completion, thereactorwas immediately quenched in an ice-water bath and the gas was released. The liquid reaction mixture in the reactor was transferred into a centrifuge tube. The catalyst was separated by centrifugation.
The quantitative analysis of the reaction mixture was conducted using a GC (Agilent 6820) equipped with a flame ionization detector (FID) and a HP-5MS capillary column (0.25 mm in diameter, 30m in length). Identification of the products and reactant was done using a GC-MS (Agilent 7890B 5977A, HP-5MS capillary column (0.25 mm in diameter, 30 m in length)) as well as by comparing the retention time with dodecane which is used as the internal standard in GC traces. The conversion of nitrobenzene and the selectivity of the products were calculated from the GC data.
In order to screen catalysts and optimize reaction conditions, different kinds of the catalysts were prepared by varying the ratio of the ethylenediamine and Ni. The contents of Ni element maintained in 4.5% ± 0.05% based on ICP-AES. Fig. 1a presents the results of hydrogenative coupling of nitrobenzene over the Ni/C catalysts. The ratio of ethylenediamine and Ni affected greatly the conversion and the selectivity of the reaction. The conversion of nitrobenzene was only3.1% and no azobenzene was detected in the absence of ethylenediamine. When the ratio of ethylenediamine and Ni increased to 1:2, 5.7% conversion of nitrobenzene was got and also no azobenzene was detected. However, when the ratio of ethylenediamine and Ni increased to 2:1, the conversion of nitrobenzene and the selectivity of azobenzene increased greatly, and 49.1% conversion of nitrobenzene and 33.6% yield of azobenzene were got. The highest conversion of nitrobenzene and the yield of azobenzene were obtained when the ratio of ethylenediamine and Ni was 10:1. However, when the ratio of ethylenediamine and Ni was 20:1, the product was aniline and no azobenzene was obtained.
|Fig. 1. (a) The effect ofEDA on the performance of the catalyst. (b-g) The TEM images of catalysts modified by different amount of ethylenediamine (b) 0:1, (c) 2:1, (d) 8:1, (e) 10:1, (f)12:l and (g) 20:1. Insert: The correspondingsize analysis of the Ni nanoparticles. Reaction conditions: nitrobenzene (2mmol), catalyst(10mg), NaOH (0.1 mmol), Ph2 (2MPa), solvent (methanol 2mL), reaction temperature (120 ℃), reaction time (4h), and stirring speed (800rpm).|
The particle size distribution of the catalysts was characterized by TEM, which shows that the particle size of Ni is related with the amount of ethylenediamine (Figs. 1b-g). The larger the amount of ethylenediamine, the smaller the particle size (Table 1). When there is no ethylenediamine, the obvious aggregation was observed. The conversion of nitrobenzene and the yield of azobenzene are all very low. It means that the particle size is the important factor that affects the conversion and the selectivity. The mean particle size was 8.4 nm and 8.2 nm when the ratio of ethylenediamine and Ni was 12:1 and 20:1. However, the conversion of nitrobenzene and the yield of azobenzene are all decreased. The possible reason is that the too much carbon-nitrogen complex carbonized from ethylenediamine occupies the active sites and decreases the activity of the catalyst.
The other modifiers containing nitrogen were also checked and the results were shown in Table 2. The formation of coordination bonds between the precursor Ni(acac)2 and N-containing ligands (such as ethylenediamine, 1, 3-propanediamine, 2, 2'-bipyridine and 1, 10-phenanthroline) improved the activityand the selectivity of the reaction. More specifically, the N-containing ligands could change the coordination environment of metal centers, which leads the metal atoms with appropriate electronic modification exposed on the particles with small size after reduction. To find out the effect of N-containing ligands among Ni, we took into account that shift ofUV absorption caused bythe ligand-to-metal bonding. As shown in Fig. 2a, the spectrum ofNi(acac)2 displayed maxima at 313 nm in ethanol solution. The combinations of the ligand result in blue-shifted absorption spectra. After adding ethylenediamine, the color of the solution turns from green to purple and displays a single strong band at 302 nm. And the peak shifts to 300 nm when injected 1, 3-propanediamine. Coordinating with 2, 2'-bipyridine and 1, 10-phenanthroline results in appearance of absorption maxima at 294nm and 268nm. Based on the spectroscopic data, we confirmed that the N-containing ligands have changed the coordination environment of metal center and alter the electron state of Ni subsequently. And with appropriate coordination ability, the modifier can promote the activity and the selectivity of Ni/C catalyst for the hydrogenative coupling of nitroarenes. Ethylenediamine was the best among the all the modifiers presented in Table 2.
|Fig. 2. (a) The UV spectrum ofN(acac)2 and modified by ligands. (b-e) The TEM images of catalysts modified by different ligands (a) ethylenediamine, (b) 1, 3-propanediamine, (c) 2, 2'-bipyridine and (d) 1, 10-phenanthroline. Insert: The corresponding size analysis of the Ni nanoparticles.|
In addition, the effect of the particle size on the performance of the catalyst was also obvious, when the different modifier was used (Figs. 2b-e). The particle size of Ni/C catalyst modified by ethylenediamine was the smallest; the catalyst exhibited the best performance.
The influence of reaction time on nitrobenzene conversion and azobenzene yield over the catalyst was also investigated (Fig. 3a). The conversion of nitrobenzene increased with time. The yield of azobenzene increased with time and then decreased. The highest yield of azobenzene was 95.5%. The results indicate that hydro-genative coupling of nitroarenes is consecutive reaction and the reaction path was shown in Scheme 1.
|Fig. 3. Optimization of reaction conditions. (a) The influence of reaction time on nitrobenzene conversion and azobenzene yield over the catalyst. (b) The influence of reaction temperature on the hydrogenative coupling of nitrobenzene. Reaction conditions: nitrobenzene (2 mmol), catalyst (10 mg), NaOH (0.1 mmol), PH2 (2 MPa), solvent (methanol 2 mL) and stirring speed (800 rpm).|
|Scheme 1. The reaction path of the hydrogenative coupling of nitroarenes.|
The influence of reaction temperature on the hydrogenative coupling of nitrobenzene was studied and the results are shown in Fig. 3b. It can be seen that the temperature played an important role for the reaction. The conversion of nitrobenzene increased from 17.6% to 64.7% and the yield of azobenzene increased from 5.4% to 54.5% when the temperature increased from 90 ℃ to 100 ℃. Then the conversion and the yield of the desired product increased slowly with the increase of temperature from 100 ℃ to 120 ℃. The highest conversion of nitrobenzene and the yield of azobenzene were got at 120 ℃. The conversion of nitrobenzene was 100% at 130℃, while the yield of azobenzene decreased from 95.5% to 73.3% when the temperature increased from 120 ℃ to 130 ℃. At higher temperature, the azobenzene was easily to be hydrogenated to aniline.
The XPS results of Ni/C and Ni-EDA(10:1)/C provide more intuitive and detailed information on the surface (Fig. 4). In Ni XPS spectra, Ni signal in Ni/C showed three peaks. The peak at 852.7 eV was assigned to Ni0. The peaks at 853.7 eV and 856.6 eV suggest the presence of NiO and Ni2O3. The binding energy of Ni0 moved to 853.0 eV, that indicates the electron transferred from the surface of support to the Ni particles. The results indicate that the existence of the carbon-nitrogen complex carbonized from ethylenediamine changes the electronic state that maybe make the azobenzene easily desorb from Ni surface. So the further hydrogenation of azobenzene to aniline was inhibited.
|Fig. 4. The XPS spectra of the catalysts modified by different amount of ethylenediamine. (a) Ni 2p, 0:1, (b) Ni 2p, 10:1, (c) N 1s, 10:1.|
Also, the N 1s spectra ofNi-EDA(10:1)/C was given in Fig. 4c. The electron-binding energy of 400.9 eV was characteristic of a pyrrolic-N, which is much higher than other types of nitrogen. This type of N is deemed to significantly alter the physicochemical properties of the materials such as thermal stability, textural properties and surface wetness, and it is activated in hydrogenation reaction [14, 15].The pyrrolic-N group, carbonized form ethylenediamine, could have promoted the interactions between Ni nanoparticles and support by forming Ni-N coordination complexes. The improved activity of Ni-EDA(10:1)/C is probably due to the alteration of electronic properties of Ni NPs which is influenced by the stronger metal-support interactions causing by the pyrrolic-N complexes. Such strong interactions also can enhance dispersion of Ni particles and reduce agglomeration and sintering during reactions, which is benefit to promote the reactions.
In conclusion, ethylenediamine, 1, 3-propanediamine, 2, 2'-bipyridine and 1, 10-phenanthroline all promote the activity and the selectivity of Ni/C catalyst for the hydrogenative coupling of nitroarenes. Ethylenediamine was the best modifier. The electronic modification of Ni and the increasement of pyrrolic-N content synergistically promoted the activity of the catalyst. When the ratio of Ni and ethylenediamine was 1:10, the yield of the azobenzene can reach 95.5%.Acknowledgments
The authors thank the National Natural Science Foundation of China (No. 21603235) and the Recruitment Program of Global Youth Experts of China.
G.X. Chen, C.F. Xu, X.Q. Huang, et al., Nat. Mater. 15 (2016) 564-569. DOI:10.1038/nmat4555
S. Jones, J. Qu, K. Tedsree, X.Q. Gong, S.C.E. Tsang, Angew. Chem. Int. Ed. 51 (2012) 11275-11278. DOI:10.1002/anie.201206035
I. Cano, A.M. Chapman, A. Urakawa, P.W.N.M. Van Leeuwen, J. Am. Chem. Soc. 136 (2014) 2520-2528. DOI:10.1021/ja411202h
I. Cano, M.A. Huertos, A.M. Chapman, et al., J. Am. Chem. Soc. 137 (2015) 7718-7727. DOI:10.1021/jacs.5b02802
K.R. Kahsar, D.K. Schwartz, J.W. Medlin, J. Am. Chem. Soc. 136 (2014) 520-526.
H. Liu, Q. Mei, Y. Wang, H. Liu, B. Han, Sci. China Chem. 59 (2016) 1342-1347. DOI:10.1007/s11426-016-0223-0
J.B. Ernst, S. Muratsugu, F. Wang, M. Tada, F. Glorius, J. Am. Chem. Soc. 138 (2016) 10718-10721. DOI:10.1021/jacs.6b03821
J.B. Ernst, C. Schwermann, G.I. Yokota, et al., J. Am. Chem. Soc. 139 (2017) 9144-9147. DOI:10.1021/jacs.7b05112
A.K. Singh, J. Das, N. Majumdar, J. Am. Chem. Soc. 118 (1996) 6185-6191. DOI:10.1021/ja954286x
L. Hu, X. Cao, L. Chen, et al., Chem. Commun. 48 (2012) 3445-3447. DOI:10.1039/c2cc30281k
X. Liu, H.Q. Li, S. Ye, et al., Angew. Chem. Int. Ed. 53 (2014) 7624-7628. DOI:10.1002/anie.201404543
L. Hu, X. Cao, L. Shi, et al., Org. Lett. 13 (2011) 5640-5643. DOI:10.1021/ol202362f
H. Liu, T. Jiang, B. Han, S. Liang, Y. Zhou, Science 326 (2009) 1250-1252. DOI:10.1126/science.1179713
L.M. Ombaka, P.G. Ndungu, V.O. Nyamori, RSC Adv. 5 (2015) 109-122. DOI:10.1039/C4RA12523A
W. Guo, H. Liu, S. Zhang, et al., Green Chem. 18 (2016) 6222-6228. DOI:10.1039/C6GC02630C