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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 [1]. 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-heterocyclic carbenes has been used as ligands for supported
heterogeneous Ru/K-Al2O3 catalysts [7]. The catalytic performance of Pd/Al2O3 was also modified by N-heterocyclic carbenes [8].
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 [9].
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%) were 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 of ligands
(ethylenediamine, 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 °C overnight. The obtained
black powder was ground and calcined under a H2 atmosphere at 500 °C for 2 h. The temperature was linearly raised from 50 °C to
500 °C at a heating rate of 5 °C/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-
phenanthroline monohydrate and 2,2'-bipyridine.
The catalysts were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray
diffraction (XRD) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) techniques. TEM images were measured
on a JEOL-1011 electron microscope operating at 100.0 kV, 10.00 μA. Before measurement, the catalysts were suspended in ethanol
after ultrasonic dispersion. The obtained dispersions were dropped onto copper-grid-supported carbon films. Powder X-ray diffraction
(XRD) patterns were recorded on a Rigaku D/max-2500 X-ray diffractometer using Cu Kα radiation (λ = 0.15406 nm). 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 Kα radiation as the excitation source (hν = 1486.6ꢀeV) and operated
at 15ꢀkV and 20ꢀmA. The base pressure was about 3×10−9 mbar. 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 [13]. 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 ꢀr.p.m. The pressure was determined by a
pressure transducer (FOXBORO/ICT, Model 93), which could be accurate to ±0.025ꢀMPa. Upon reaction completion, the reactor was
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 (0.25ꢀmm in diameter, 30ꢀm in length). Identification of the products and reactant was
done using a GC–MS (Agilent 7890B 5977A, HP-5MS capillary column (0.25ꢀmm in diameter, 30ꢀm 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.