Lei Ma,Jianguo Wang*,Hanbing Wang,Qunfeng Zhang,Chunshan Lu,Xiaobo He,Xiaonian Li*

Industrial Catalysis Institute of Zhejiang University of Technology,State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology,Hangzhou 310032,China

1.Introduction

Aromatic haloamines are important organic intermediates in the chemical synthesis of medicines,organic dyes,perfumes,herbicides,pesticides,and preservatives.At present,the reduction of the corresponding nitrocompounds is the main method to synthesize haloamines.Given the environmental impact associated with the use of hydrochloric acid,the hydrogenation route has been developed in the past decades.However,controlling the selectivity is a critical problem when reducing halonitrobenzenes with metal catalyst.Extensive hydrogenolysis of carbon–halogen bond to aromatic amines during hydrogenation can occur.Kosak[1]reported that the susceptibility to hydrogenolysis for the halogen position is in the following order:ortho＞para＞meta.A number of metal catalysts,such as Co[2],Ni[3–5],Ru[6–8],Pd[9–11],Pt[12–15],and Au[16]have been investigated.In addition,much effort has been given to improve the selectivity ofthe reaction,including selective poisoning of a catalyst with compounds containing sulfur[17,18],preparing bimetallic or multi-metallic catalysts[19–25],and tuning the particle sizes of the catalysts[23–29].

The particle size of active metal has very significant effect on several catalytic reactions.Controlling the particle size of Pt or Pd can effectively inhibit the occurrence of hydrogenolysis side reaction.Copet al.[26]found that the large particle size of the Pt catalyst had an inhibitory effect on hydrogenolysis side reaction.Yanet al.[27]and Liet al.[28,29]also found the same rules on Pd catalysts.Liet al.[29]further revealed the cause of the large particle size of Pd to inhibit the hydrogenolysis side reaction through density functional theory(DFT)calculations.

Supported iridium catalysts are outstanding candidates for various catalytic reactions because of their stability,activity,and selectivity under reaction conditions.For example,supported Ir catalysts have been successfully employed for selective hydrogenation of unsaturated aldehydes to corresponding unsaturated alcohols[30–34].In addition,supported Ir catalysts possess unique characteristics for hydrogenolysis ofn-hexane[35]and selective catalytic reduction of NOx[36–38].At the same time,ZrO2or Al2O3supported Ir catalysts have also been found that show high selectivity in selective hydrogenation of halonitrobenzenes to phenylhydroxylamine[39,40]or haloamines[40–42].However,there is no further study of the reasons for the high selectivity of supported Ir catalysts.

In the current study,we observed that the activated carbonsupported Ir catalyst(Ir/C)is highly selective for hydrogenation of chloronitrobenzenes(CNBs)into chloroanilines(CANs),which is particularly notable because hydrogenolysis side reaction does not occur completely.The influence of the Ir nanoparticle in the hydrogenation of CNBs is investigated through DFT calculations.The possible mechanistic reasons for the selectivity of the Ir catalyst are also discussed.

2.Experimental and Theoretical Methods

2.1.Catalyst preparation

The activated carbon was provided by Fujian Xinsen Carbon Co.,Ltd.(China).The commercial activated carbon with Brunauer–Emmett–Teller(BET)surface area of1662 m2·g?1was made from coconut shells.The activated carbon was out gassed in a vacuum at 383 K overnight.A desired volume of H2IrCl6or H2PdCl4(Sino-Platinum Metals Co.,Ltd.)aqueous solution(0.035 g·L?1),with nominal Ir loading of 1 wt%,was added into an aqueous suspension of the activated carbon.The aqueous suspension of the activated carbon was subsequently dried at 353 K for 6 h.The formed catalyst was then reduced by hydrogen with 3 MPa pressure in distilled water at 363 K for 9 h.Afterward,the Ir/C or Pd/C catalyst was filtered,rinsed in distilled water until neutral,and degassed in a vacuum at 383 K overnight.The nominal Pd loading was 1 wt%as same as that of the Ir.ICP experiments showed that the actual loadings of Ir and Pd were 0.9985%and 0.9992%,respectively.

2.2.Catalyst characterization

The particle size of Ir or Pd on the active carbon surface was determined by transmission electron microscopy(TEM)using a Tecnai G2F30S-Twin microscope(Philips-FEI Co.).At least 200 individual Ir particles were counted for each catalyst.The Ir or Pd particle size of the catalysts,ds,was calculated using the following equation:ds=Σnidi3/Σnidi2,where visible particle sizedion the micrographs was measured by a computerized system.

2.3.Reaction and product analysis

Liquid phase catalytic hydrogenation of nitrocompounds to corresponding amines was conducted in this study.Brie fly,25 ml pf ethanol,0.02 mol CNB,and 0.05 g of Ir/C or Pd/C catalyst were mixed in a 75 ml stainless steel stirred reactor(5000 multiple reactor system,Parr Instrument Company).The reactor was initially filled with nitrogen three times,followed by hydrogen to replace nitrogen three times,and then heated slowly until the desired reaction temperature.When pressurized with hydrogen until 1.0 MPa,the reaction was started at a stirring rate of 1200 r·min?1.After the complete conversion of the reactant,the catalyst was filtered from the liquid product.

The hydrogen consumed in the reaction was provided by a 300 ml gas tank with pressure sensor.The pressure changes in the reaction process were recorded by computer.And the amount of hydrogen consumption in the reaction process was calculated.When the product distribution was studied,the stirring was first closed in the setting time,and then the hydrogen was replaced with nitrogen to terminate the reaction.

The liquid reactants,intermediates,products,and byproducts were analyzed by high-pressure liquid chromatography on an Agilent HPLC 1100 system with a UV detector(model 610).Analysis was carried out on a Hedera ODS-2 C18 column(4.6 mm i.d.×250 mm)at a flow rate of 1.0 ml·min?1,with 70:30 mobile phase of acetonitrile and distilled water.

2.4.Computational details

All calculations were performed using the Viennaab initiosimulation package(VASP)[43–45],a periodic density functional theory(DFT)code with projector augmented wave(PAW)potentials.The vdw interactions in the VASP code were implemented through a selfconsistent vdw-DFT functional[46].In this study,the vdw-DF functional with PBE exchange was used,and successfully applied to polycyclic aromatic hydrocarbons and chlorobenzene on Au and Pt surfaces.

The flat Ir(111)surfaces,stepped Ir(211)surfaces,and icosahedral Ir55,Ir13 were used as substrates,which represent the terrace,step,and corner sites of Ir nanoparticles.Four layers(4×4)Ir(111)and five layers(1×4)Ir(211)were used,in which the two bottom layers were fixed during the optimizations and the vacuum layer of Ir(111)and Ir(211)was 1.5 nm.The Ir55 and Ir13 clusters in a 20×20×20 unit cell were fully relaxed.The Brillouin zone integration was performed using the Monkhorst–Pack scheme with 4 × 4 × 1 and 4×2×1 mesh for the flat Ir(111)and stepped Ir(211)surfaces;the gamma points were used for Ir55 and Ir13 clusters.All structures were optimized with a convergence criterion of100 meV·nm?1for the forces and 0.01 meV for the energy.Transition states were searched using the climbing image nudged elastic band(CI-NEB)method.

3.Results and Discussion

3.1.Reaction characteristics of Ir catalyst

The reaction pathways for the hydrogenation of CNB to CANviathe different intermediates are shown in Fig.1[1].There exist two reaction pathways for the hydrogenation of CNB to CAN.A direct pathway is through nitrosobenzene and phenylhydroxylamine intermediates.However,the two intermediates can undergo condensation side reaction to form azoxybenzene intermediates.Another indirect pathway for the hydrogenation of CNB to CAN is through azoxybenzene,azobenzene and dihalohydrazobenzene intermediates.

Fig.1.Reaction pathways for the hydrogenation of CNB to CAN.

The TEM micrograph and particle size distribution of the Ir/C and Pd/C catalysts are shown in Fig.2.The surface metal particles of the two catalysts showed small and uniform state.The average particle size of the Ir/C catalyst is very close to that of the Pd/C catalyst,indicating that the two catalysts should have the similar amount of active sites under the same metal load.In this study,we had investigated the effect of the particle size of the Pd/C catalyst on the catalytic hydrogenation of CNBs[28,29].We found that the catalytic hydrogenation of CNBs was sensitive to Pd particle size.The hydrogenolysis side reaction was more likely to occur on the surface of the small particle size of Pd.Thus,we can infer that the Ir and Pd particles are so small that the Ir/C and Pd/C catalysts may have similar reaction properties.

However,the reaction results of the Ir/C catalyst were completely beyond our expectations(Table 1).The two catalysts exhibited different catalytic properties.The Pd/C catalyst showed a “typical”catalytic performance,and the hydrogenolysis side reaction was dominant.As a comparison,the catalytic performance of the Ir/C catalyst was “abnormal”.Although the reactant iso-CNB,no hydrogenolysis side reaction can be observed.Notably,the selectivity of azobenzene byproducts of the Ir/C catalyst is significantly higher than that of the Pd/C catalyst.The small particle size of Ir can completely inhibit the hydrogenolysis side reaction;this is a very interesting result to conduct a thorough study.

Fig.2.TEM micrograph and particle size distribution of the Ir/C and Pd/C catalysts.

Table 1Experimental results of the hydrogenation of different CNBs over the Ir/C and Pd/C catalysts

In addition to selectivity,the catalytic hydrogenation reaction rate of the two catalysts was also significantly different.The hydrogen consumption of the reaction process is measured,and the hydrogen consumption rate is calculated.The results are shown in Fig.3.The hydrogen consumption rate of the Ir/C catalyst is much lower than that of the Pd/C catalyst,regardless of the position of the chlorine substituent.At the same time,the hydrogen consumption rate of the Ir/C catalyst is almost unaffected by the position of the chlorine substituent,which is obviously different from that of the Pd/C catalyst.Thus,we speculate that the differences in the selectivity of the two catalysts may be related to the difference in reaction rate.Usually,a low reaction rate is helpful to restrain the generation of side reactions.The difference in reaction rate is related to the metal and the number of metal active sites.Fig.2 shows that the two catalysts have similar number of metal active sites.Therefore,the difference in reaction rate of the two catalysts cannot be related to the number of active sites,but may be related to the properties of the metal.However,we are still unable to determine how the special properties of metal Ir can cause significant difference in the reaction rate and the selectivity between the metal Pd.

The azobenzene byproducts of the Ir/C catalyst in the reaction are significantly higher than those of the Pd/C catalyst.Azobenzene byproducts are composed of the nitrosobenzene intermediate and the phenylhydroxylamine intermediate.Therefore,the product distribution of the hydrogenation process was investigated in this study.The nitrosobenzene and phenylhydroxylamine intermediates are the key object of investigation.Given the lack of suitable standard sample,nitrobenzene was chosen as research object.The results are shown in Fig.4.Although the nitrosobenzene intermediate was not detected,the phenylhydroxylamine intermediate can be detected in the reaction process.Notably,the amount of phenylhydroxylamine intermediate in the reaction system of the Ir/C catalyst is significantly more than that of the Pd/C catalyst.This finding may explain why the selectivity of the azobenzene by-products of the Ir/C catalyst is significantly more than that of the Pd/C catalyst.Thus,we hypothesized that the Ir/C and Pd/C catalysts follow the same reaction mechanism in nitrobenzene catalytic hydrogenation reaction.However,the interaction between the reactants,intermediates,or products and the active metals may have significant differences,which may be the main reason for the difference in reaction rate and product selectivity.Therefore,we also studied the adsorption state of the reactants,intermediates,or products on the surface of Pd and Ir by DFT.

3.2.Adsorption of p-CNB on Ir surfaces and clusters

The adsorption ofp-CNB on the Ir(111),Ir(211)surfaces and the Ir13 cluster are investigated.The selected several stable structures ofp-CNB on Ir(111)are shown in Fig.5 and Table 2.Similar to Pd(111),the benzene rings,being either on the bridge or in the hollow site,exhibited stable con figurations.Different with Pd(111),on the most stable structure,the oxygen ofp-CNB directly bonds with Ir,and the distances between two oxygen and Ir are 0.212 nm and 0.208 nm,respectively.However,the distance between Cl and Ir is slightly longer than that on Pd.Therefore,for the most stable structure,the adsorption ofp-CNB on Ir(111)is?1.51 eV,very similar to that on Pd(111)(?1.57 eV).The adsorption energies of two other structures are?1.30 and?1.21 eV,respectively.On the stepped Ir(211)surfaces,the adsorption ofp-CNB on the most stable structure is much stronger than that on Ir(111),which is?2.62 eV,respectively.The benzene ring is adsorbed on the step edge of Ir(211).The distance between the oxygen and Ir is 0.215 and 0.205 nm;the chlorine is away from the Ir with 0.279 nm.Although only one oxygen bonds with Ir,the adsorption energy ofp-CNB is only?2.28 eV.When only benzene ring bonded with step of Ir(211)and two oxygen bonded with Ir,the adsorption energy is?2.35 eV.The adsorption ofp-CNB on the two different ideal Ir13 clusters is shown in Fig.5c.The adsorbedp-CNB can induce the structure change of the Ir 13 clusters,in which the adsorption energy is extremely large(?5.76 eV).In this con figuration,the oxygen bonds with Ir.When oxygen is away from Ir,the adsorption energy is only?4.13 eV.On icosadeltaheronal Ir clusters,the adsorption ofp-CNB is?3.93,?3.78,and?3.13 eV with two,one,and no oxygen bonding with Ir.

3.3.Dechlorination of p-CNB and p-CAN on the Ir surfaces and clusters

In our previous study[28,29],we investigated the reaction mechanism of hydrogenation and dechlorination ofp-CNB on Pd surfaces and clusters.Our results showed that the reaction barriers of the hydrogenation ofp-CNB on Pd(111)and Pd(211)are nearly same.In addition,the reaction barriers of the dechlorination ofp-CNB andp-CAN are dependent on the Pd models.Based on the method describing the size and the catalytic properties developed in our group,the optimum Pd size of the selective hydrogenation ofp-CNB is 30 nm based on the DFT calculations and experiments.Our present study shows that Ir has different catalytic properties with Pd,in which small Ir and Pd nanoparticles have very high and low selectivity for the ideal product.Therefore,only the dechlorination ofp-CNB andp-CAN was investigated on different Ir models,as shown in Fig.6.On Ir(111)and Ir(211),the dechlorination ofp-CNB is endothermic with 0.63 and 0.33 eV,respectively.Meanwhile,on the Ir13 cluster,the dechlorination is slightly exothermic with about?0.38 eV,which is probably caused by the cluster structure fluctuation.Especially,the reaction barriers of dechlorination ofp-CNB are extremely large,which is more than 1 eV.From Ir(111),Ir(211)to Ir13 cluster,the reaction barrier increases from 1.14,1.37,and 1.45 eV,which contradicts that of the Pd models.The optimized structures of the transition states of the dechlorination ofp-CNB on Ir(111),Ir(211)to Ir13 are also shown in Fig.7.For thep-CAN,similar conclusions are identified.The dechlorinations ofp-CAN on Ir(111)and Ir(211)are 0.66 and 0.25 eV.On the Ir13 cluster,the dechlorination is slightly exothermic with about?0.47 eV.The reaction barriers also increases from 1.35,1.74 and 1.81 eV on Ir(111),Ir(211)to Ir13.The reaction barriers of the dechlorination ofp-CNB andp-CAN over different Ir models are much larger than those on Pd ones.Especially,lower coordination of Ir results to larger the barriers of the dechlorination reaction.As discussed in our previous paper,a relationship exists between the percentages of the reaction sites(terraces,steps,and corners)and the nanoparticle size.The percentage of the terrace sites significantly increases with the increase in nanoparticle size.The percentage of the edge sites increases with the decrease in particle size.The edge sites become predominant when the particle size is below～2 nm.Small metal nanoparticle contains large percentage of edge sites,which has very large reaction barriers of the dechlorination reaction.Therefore,small Ir nanoparticles have very high selectivity of hydrogenation of halogenated nitrobenzenes.

Fig.3.Hydrogen consumption rate of the Ir/C catalyst and the Pd/C catalyst.Reaction conditions:0.05 g catalyst;25 ml ethanol;0.02 mol chloronitrobenzene;p H2=1.0 MPa;T=353 K;stirring rate=1200 r·min?1.

Fig.4.Product distribution of catalytic hydrogenation of nitrobenzene on the Ir/C catalyst and the Pd/C catalyst.Reaction conditions:0.05 g catalyst;25 ml ethanol;0.02 mol chloronitrobenzene;p H2=1.0 MPa;T=353 K;stirring rate=1200 r·min?1.

Fig.5.The optimized structures of p-CNB on Ir(111),Ir(211)surfaces and Ir13 clusters.

Table 2The adsorption site,energy and geometry of p-CNBs on p-CNB on Ir(111),Ir(211)surfaces and Ir13 clusters

Fig.6.Reaction energy diagram for the dechlorination of p-CNB(p-CAN)on Ir(111)and Ir(211)surfaces and Ir13 clusters.

Fig.7.The optimized structures for the transition states of dechlorination of p-CAN and p-CNB on Ir(111),Ir(211)surfaces and Ir(13)clusters.

4.Conclusions

In this study,high selectivity for hydrogenation of halogenated nitrobenzene(＞99%)was achieved over small(＜3 nm)Ir nanoparticles,in which the selectivity over Pd with the same size was much lower than those on Ir nanoparticles.Meanwhile,Ir and Pd had different hydrogen consumption and reaction rates.p-CNB showed different adsorption properties on Ir and Pd based on the DFT calculations.The distance between oxygen(cholorine)and Ir are much shorter(longer)than that between oxygen and Pd.The reaction barriers of the dechlorination ofp-CNB andp-CAN over different Ir models are much higher than those on Pd ones.In particular,lower coordination of Ir leads to higher barriers of the dechlorination reaction.These theoretical results explain the difference between Ir and Pd in the hydrogenation of halogenated nitrobenzene.

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