Ethylenediamine promoted the hydrogenative coupling of nitroarenes over Ni/Ccatalyst

Youdi Yng,Shopeng Li,Cho Xie,Hngyu Liu,Ynyn Wng,Qingqing Mei,Huizhen Liu,*,Buxing Hn,*

a Beijing National Laboratory for Molecular Sciences,CASKey Laboratory of Colloid and Interface and Thermodynamics,CASResearch/Education Center for Excellence in Molecular Sciences,Institute of Chemistry,Chinese Academy of Sciences,Beijing 100190,China

b School of Chemistry and Chemical Engineering,University of Chinese Academy of Sciences,Beijing 100049,China

Key words:Selective hydrogenation Non-noble metal catalyst Electronic state Azobenzene Organic modi fi ers

ABSTRACT Azobenzene and its derivatives are key raw materials and it is an environmentally friendly method for the preparation of azobenzene by hydrogenative coupling of nitrobenzene.The development of nickel based catalyst for organic transformationsisof importance because of its relatively low cost and toxicity.In this work,we found that ethylenediamine can enrich the electron state of Niand make the azobenzene easily desorb from the surface of the catalyst,which inhibits the hydrogenation of azobenzene to aniline.The selectivity of azobenzene is greatly improved.When the ratio of Niand ethylenediamine is 1:10,the yield of the azobenzene can reach 95.5%.

The development of transition-metal-catalyzed organic transformationsbased on the fi rst-row transition metalssuch as Co,Niand Cu is of importance because of their relatively low cost and toxicity relative to precious metals.Carbon is an ideal catalyst support for the follow ing reasons:(1)It can beprepared from biomass.(2)Theprice 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 show n 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 modi fi ers(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 modi fi ers[1–5].Ethylenediamine-coated ultrathin platinum nanow ires exhibited excellent performance for the selective hydrogenation of nitroaromatics to N-hydroxylanilines[1].Polyvinyl pyrrolidone has been used as electronic and geometric modi fi er 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-Al2O3catalysts[7].The catalytic performance of Pd/Al2O3was also modified by N-heterocyclic carbenes[8].However,to the best of our know ledge,the application of organic ligand to modify cheap metal or carbonsupported heterogeneous catalysts is limited.

Azo compounds are key raw materials and are w idely 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%w hen the ratio of ethylenediamine and Ni was 10:1.

The materialsused 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 Scienti fi c 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 puri fi cation.

The impregnation method was employed w hen fabricating the N-modified Ni/Ccatalysts.Certain amounts of ligands(ethylenediamine,1,3-propanediamine,1,10-phenanthroline monohydrate and 2,2'-bipyridine)were added into 5 m Lethanol 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 pow der was ground and calcined under a H2atmosphere at 500?C for 2 h.The temperature was linearly raised from 50?Cto 500?Cat 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.Lwas 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 k V,10.00 m A.Before measurement,the catalysts were suspended in ethanol after ultrasonic dispersion.The obtained dispersions were dropped onto copper-grid-supported carbon fi lms.Pow der X-ray diffraction(XRD)patterns were recorded on a Rigaku D/max-2500 X-ray diffractometer using Cu K a radiation(l=0.15406 nm).The tube voltage was 40 k V and the current was 200 m A.The X-ray photoelectron spectroscopy(XPS)data were obtained with an ESCA Lab 220i-XL electron spectrometer from VG Scienti fi c using 300 W Al K a radiation as the excitation source(h n=1486.6 eV)and operated at 15 k V and 20 m A.The base pressure was about 3?10?9mbar.The binding energies were referenced to the C 1[25_TD DIFF]s line at 284.8 eVfrom adventitious carbon.The contents of different elements in the Ni/Ccatalysts were analyzed by ICP-AES(PROFILE.SPEC,Leeman).

A 10 m LTe fl on-lined stainless-steel autoclave equipped with a magnetic stirrer was used w hen implementing the reaction,which was same as the equipment we used previously[13].In a typical experiment,2 mmol(246 mg)of nitrobenzene,2 m Lof 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 accurate to?0.025 MPa.Upon reaction completion,the reactor was immediately quenched in an ice-w ater bath and the gas was released.The liquid reaction mixture in the reactor wastransferred 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 fl ame ionization detector(FID)and a HP-5MS capillary column(0.25 mm in diameter,30 m in length).Identi fi cation 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 GCtraces.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 wasonly 3.1%and no azobenzene wasdetected 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,w hen 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 w hen the ratio of ethylenediamine and Ni was 10:1.However,w hen the ratio of ethylenediamine and Ni was 20:1,the product was aniline and no azobenzene was obtained.

The particle size distribution of the catalysts was characterized by TEM,which show s that the particle size of Niis related with the amount of ethylenediamine([23_TD DIFF]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 affectsthe conversion and the selectivity.The mean particle size was 8.4 nm and 8.2 nm w hen 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 carbonnitrogen complex carbonized from ethylenediamine occupies the active sites and decreases the activity of the catalyst.

Fig.1.(a)The effect of EDAon 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)12:1 and(g)20:1.Insert:The corresponding size analysis of the Ninanoparticles.Reaction conditions:nitrobenzene(2 mmol),catalyst(10 mg),NaOH(0.1 mmol),PH2(2 MPa),solvent(methanol 2 mL),reaction temperature(120?C),reaction time(4 h),and stirring speed(800 rpm).

Table 1The particle size of the catalysts modified by different amounts of ethylenediamine.

Table 2 The effect of different modi fi ers.

Fig.2.(a)The UVspectrum of N(acac)2 and modified by ligands.(b-e)The TEM images of catalystsmodified 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.

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), PH 2(2 MPa),solvent(methanol 2 m L)and stirring speed(800 rpm).

Schem e 1.The reaction path of the hydrogenative coupling of nitroarenes.

Fig.4.The XPSspectra of the catalysts modified by different amount of ethylenediamine.(a)Ni 2p,0:1,(b)Ni 2p,10:1,(c)N 1s,10:1.

The other modi fi ers containing nitrogen were also checked and the results were show n in Table 2.The formation of coordination bonds between the precursor Ni(acac)2and N-containing ligands(such as ethylenediamine,1,3-propanediamine,2,2'-bipyridine and 1,10-phenanthroline)improved the activity and 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 fi nd out the effect of N-containing ligands among Ni,we took into account that shift of UV absorption caused by the ligand-to-metal bonding.Asshow n in Fig.2a,the spectrum of Ni(acac)2displayed 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 w hen injected 1,3-propanediamine.Coordinating with 2,2'-bipyridine and 1,10-phenanthroline results in appearance of absorption maxima at 294 nm and 268 nm.Based on the spectroscopic data,we con fi rmed 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 modi fi er 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 modi fi ers presented in Table 2.

In addition,the effect of the particle size on the performance of the catalyst was also obvious,w hen the different modi fi er 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 hydrogenative coupling of nitroarenes is consecutive reaction and the reaction path was show n in Scheme 1.

The influence of reaction temperature on the hydrogenative coupling of nitrobenzene was stud ied and the results are show n 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%w hen the tem perature increased from 90?C to 100?C.Then the conversion and the yield of the desired p rod uct increased slow ly with the increase of temperature from 100?C to 120?C.The highest conversion of nitrobenzene and the yield of azobenzene were got at 120?C.The conversion of nitrobenzene was 100%at 130?C,while the yield of azobenzene decreased from 95.5%to 73.3%w hen the temperature increased from 120?C to 130?C.At higher tem perature,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,Nisignal in Ni/Cshowed three peaks.The peak at 852.7 eV wasassigned to Ni0.The peaks at 853.7 eVand 856.6 eVsuggest the presence of NiO and Ni2O3.The binding energy of Ni0moved 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.

Also,the N 1s spectra of Ni-EDA(10:1)/Cwas 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 m etal-supp ort 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 bene fi t 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 wasthe best modi fi er.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%.

2IF]Acknow ledgm ents

The authors thank the National Natural Science Foundation of China(No.21603235)and the Recruitment Program of Global Youth Experts of China.