Qunfeng Zhang,Chang Su,Jie Cen,Feng Feng,Lei Ma,Chunshan Lu,Xiaonian Li*

Catalysis,Kinetics and Reaction Engineering

The Modif i cation of Diphenyl Sulf i de to Pd/C Catalyst and Its Application in Selective Hydrogenation of p-Chloronitrobenzene☆

Qunfeng Zhang,Chang Su,Jie Cen,Feng Feng,Lei Ma,Chunshan Lu,Xiaonian Li*

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

A R T I C L EI N F O

Article history:

Diphenyl sulf i de

Modif i cation

p-Chloronitrobenzene

Pd/C

Selective hydrogenation

Stability

In this study,diphenyl sulf i de(Ph2S)was employed to prepare a series of Ph2S-modif i ed Pd/C catalysts (Pd-Ph2S/C).CatalystcharacterizationcarriedoutbyBrunner-Emmet-Teller(BET),energydispersivespectrometer(EDS),X-ray diffraction(XRD),X-rayphotoelectron spectroscopy(XPS)and CO chemisorption uptake measurements suggested a chemical interaction between Ph2S and Pd.The ligand was preferably absorbed on the active site of Pd metal but after increasing the amount of Ph2S,the adsorption of Ph2S on Pd metal tended to be saturated and the excess of Ph2S partially adsorbed on the activated carbon.A part of Pd atoms without adsorbing any Ph2S still existed,even for the saturated Pd-Ph2S/C catalyst.The Pd-Ph2S/C catalysts exhibited a good selectivity of p-chloroaniline(p-CAN)in the hydrogenation of p-chloronitrobenzene(p-CNB).However, thechemisorption betweenPh2SandPdwasnotsostrongthatpartofPh2SwasleachedfromPd-Ph2S/Ccatalyst during thehydrogenation,whichcausedthedecline of theselectivityofp-CAN overthe usedPd-Ph2S/C catalyst. Resulf i dation of the used Pd-Ph2S/C catalyst was effective toresume its stability,and the regeneratedPd-Ph2S/C catalyst could be reused for at least ten runs with a stable catalytic performance.

?2014TheChemicalIndustry andEngineeringSocietyofChina,andChemicalIndustryPress.Allrightsreserved.

1.Introduction

Supported noble metal catalysts are widely used in the hydrogenation reactions because of their high catalytic activity,easy separation and reusability[1-4].Unfortunately,in liquid-phase selective hydrogenation of organic compounds with various reducible functionalities,the noble metal catalysts often suffered from over-hydrogenation which limits their applications.In order to improve the selectivity of the supported noble metal catalysts,many studies have been carried out.The useofcarbonmonoxideandsulf i desaspoisonsfornoblemetalcatalysts has been recognized as a reasonable method to enhance the selectivity of the catalysts[5-17].Supported palladium catalysts modif i ed by CO showed a good performance on the selective removal of acetylene from ethylene rich feeds[5,6].However,the use of CO is far from ideal asCOistootoxictocontrolitssuitableconcentrationagainstabackdrop of continuing catalyst deactivation[7].

There are a great variety of sulf i des with adjustable poison,and sulf i de modif i ed noble metal catalysts have attracted much attention due totheircontrollable activities[7-18].Forexample,sulf i demodif i ed noble metal catalysts have been applied to the hydrogenation of halogen-containing aromatic nitro compounds to amines[9,10],selective hydrogenation of acetylene to ethylene[7,8],reductive alkylation ofaromaticaminesandketones[11-14],selectivehydrogenationofunsaturated aldehydes/ketones to saturated aldehydes/ketones[15-17], and selective hydrogenation of aromatic nitro compounds to aromatic hydroxylamine[18].

Pt/C catalysts modif i ed by H2S has been applied to alkylation of p-amino diphenylamine(PADPA)and methylisobutyl ketone(MIBK)to afford N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine(DBPPD) inindustry[12].However,theapplicationofnoblemetalsulf i decatalysts through the sulf i dation with H2S is limited due to their low activity, leading to harsh reaction conditions and poor conversion during the hydrogenation reactions[13,19].Recently,Sajiki's group[14-16]produced a series of organic sulf i de modif i ed Pd/C catalysts,which moderately depressed the catalyst activity of Pd/C to acquire distinguishing chemoselectivity on hydrogenation.Several studies have revealed that differentorganicsulf i deswereabletoenhancetheselectivityofsupported noble metal catalysts in the hydrogenation reactions[7-9,15-18]. However,there is still little information about the effect of organic sulf i des on the structure,the adsorption feature and the properties of noble metal catalysts.Meanwhile,researchers found that the catalytic performance of the catalysts after recycling for the hydrogenation reactions was notably worsen and recycling the catalysts was diff i cult[16].Therefore,further research needs to focus on studying the stability of metal catalysts modif i ed by organic sulf i des and their regeneration.

In the present study,diphenyl sulf i de(Ph2S)was employed to prepare a series of Pd-Ph2S/C catalysts,which were modif i ed by different amounts of Ph2S.The catalysts were characterized by Brunner-Emmet-Teller(BET),energy dispersive spectrometer(EDS), X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and CO chemisorption uptake to explore the adsorption of Ph2S on Pd/C. Then,Pd-Ph2S/C catalysts were applied to selective hydrogenation of p-chloronitrobenzene(p-CNB)to p-chloroaniline(p-CAN).It was foundthat Ph2S wasleached from thePd-Ph2S/Ccatalyst duringhydrogenation,which was the reason why the modif i ed catalysts could not remain stable.A single method was also developed to regenerate the used Pd-Ph2S/C catalyst by resulf i dizing the used Pd-Ph2S/C catalyst with diphenyl sulf i de.

2.Experimental

2.1.Catalyst preparation

3%(by mass)Pd/C was prepared by incipient wetness impregnation using H2PdCl4as a Pd precursor.The activated carbon support was pretreated by 2.5%nitric acid solution at 366 K for 6 h.It was then washed with distilled water until pH value reached about 7 and dried at 383 K for 7 h.A desired volume of 0.05 g·ml?1H2PdCl4was added into the continuously stirred carbon suspension using a dropping funnel,and the pH value of the sample was adjusted to 10-11 by adding 10%NaOH aqueous solution.The precipitated Pd(OH)2was reduced by hydrazine hydrate,and then washed and dried following the same procedure as that of the carbon support.

The Pd-Ph2S/C catalysts were produced based on the as-prepared 3%Pd/C catalyst.Pd/C was added to a Ph2S in methanol solution at the appropriate concentration(the molar ratio of Ph2S:Pd varied between 0.1 and 100),the suspension of 3%Pd/C and Ph2S was stirred at 309 K for 6 h,and then the resulting catalyst(black powder)was collected by f i ltration.After being washed excessively with methanol,the sample was f i nally dried in vacuum for 4 h.Modif i ed samples were denoted as Pd-Ph2S(x)/C,where x referred to the molar ratio of Ph2S:Pd.

2.2.Catalyst characterization

The BET surface areas of the catalysts were determined by nitrogen physical adsorption at 77 K under vacuum condition by using a NOVA 1000e(Quantachrome Instruments Corp)surface area analyzer.

XRD patternsweremeasured byananalytical diffractometer(X'pert Pro)with a Cu Kαradiation source(l=0.15406nm)operating at 40 kV and40mA.Diffractionpatternswerecollectedwithascanningrateof2° per minute and a step of 0.02°.Foil samples were mounted on zero background quartz slides for analysis.

XPS measurements were performed on a Kratos AXIS Ultra DLD system using a monochromatic Al Kαradiation source(1486.6 eV, 15 kV and 20 mA)with pass energy 160 eV for survey scans and 80 eV for narrow scans.All binding energy(BE)values were calibrated by using the value of contaminant carbon(C1s,284.8 eV)as a reference.

CO chemisorption uptake was measured by pulse chemisorption with a mass spectrometer(Omnistar?)at 25°C.

2.3.Catalytic activity test

Liquid-phase selective hydrogenation of p-chloronitrobenzene was used to investigate the performance of the as-synthesized Pd-Ph2S(x)/C catalysts.It was carried out at 348 K and 1.0 MPa H2in a 75 ml stainless steel autoclave(Parr Instrument Company)or a 500 ml stainless steel autoclave(Weihai Zikong Autoclave Co.Ltd.).The ratio of catalyst to p-chloronitrobenzene was 1:100.The reactor was f i rstly purged with N2f i ve times to eliminate the air and then purged again with H2three times before heating.When the desired temperature was reached,the hydrogenation was started immediately by stirringthe reaction mixture vigorously.The stirring rate was kept at 1000 r·min?1to eliminate the external diffusion effects.The reaction products were analyzed by gas chromatography(Agilent 7890A GC)from which the pchloronitrobenzene conversion and the yield of p-chloroaniline were obtained.The capillary column DB-1 with an inner diameter(i.d.)of 0.32 mm and a length of 30 m equipped with a f l ame ionization detector (FID)was used.

3.Results and Discussion

3.1.Catalyst characterization

The sizes of Pd particles of Pd/C and Pd-Ph2S(x)/C catalysts are determined by transmission electron microscopy(TEM)as shown in Fig.1.The sizes of Pd particles are all ranged between 2 and 5 nm,suggestingthattheinf l uenceof treatmentwithPh2SonPdparticlesof Pd/C catalyst is slight.

The surface areas of Pd/C and Pd-Ph2S(x)/C catalysts are measured by N2adsorption-desorption measurement and are summarized in Table 1.It is found that the surface areas of four catalysts are almost the same,indicating that there is no signif i cant effect of Ph2S on the structure of activated carbon.

TheXRDpatternsofthe3%Pd/CcatalystsbeforeandafterPh2Smodif i cationareshowninFig.2.Thebroaddiffractionpeakswhichappeared at 2θ=23.6°and 2θ=44.9°are ascribed to the activated carbon.Because of thehigh dispersion of palladium on thesurfaceof theactivated carbon,the peak detected at 2θ=40.2°is believed to be the diffraction characteristic peakof(1,1,1)faceof thePd0.Thepattern ofPd/C catalyst after being modif i ed by 0.1 equiv of Ph2S(Pd-Ph2S(0.1)/C)is shown in Fig.2(c).It can be observed that the peak corresponding to the(1,1,1) faceofthePd(0)ofthis complexisshifted slightlytoahigheranglecomparedtothatofthePd/C.IncreasingthePh2Sadditionto1equiv,thediffraction peak of(1,1,1)face of the Pd0becomes weaker and broader [Fig.2(d)]alongwiththeshiftofthepeakofactivatedcarbon.However, thereis noapparentchange in termsof thePd diffraction peak between Pd-Ph2S(100)/C and Pd-Ph2S(1)/C samples but only the shift of the activated carbon characteristic peak[Fig.2(e)].This result illustrates that thereis achemicalinteractionbetweenPh2SandPd.Whentheaddition of Ph2S decreases(0.1 equiv),the ligands preferentially adsorb on the metal active sites.However,when the Ph2S content increases(1-100 equiv),a part of the ligands would interact with the activated carbons. It is consistent with the f i ndings of Sajiki's group[15]and also confi rmed by EDS test of elemental composition and CO chemisorption uptake(Table 1).S content(by mass)of Pd-Ph2S(0.1)/C rapidly increases from 0.1%to 0.8%with increasing molar ratio of Ph2S:Pd from 0.1 to 1. However,S content is just slowly increased to 1.0%when the amount of Ph2S increases to 100 equiv,which seems to be the reason why it is hard for Ph2S to adsorb on Pd/C catalyst after its amount increases to 1 equiv.The CO chemisorption uptake represents the palladium metal surface area and the uptakes for Pd/C,Pd-Ph2S(0.1)/C,Pd-Ph2S(1)/C and Pd-Ph2S(100)/C are 1.7,1.3,0.2 and 0.2 ml·g?1,respectively(no CO uptake is detected for the activated carbon alone).With increasing amount of Ph2S to modify Pd/C catalyst,the CO chemisorption uptake rapidly declined due to reason that a part of Pd atoms are adsorbed by Ph2S.After the amount of Ph2S increases to 1 mole,the palladium metal surface area does not decrease,which indicates that the adsorption of Ph2S on Pd metal has been saturated. However,there are still a part of Pd0atoms that could still adsorb CO without adsorbing any Ph2S for Pd-Ph2S(1)/C and Pd-Ph2S(100)/C (both 0.2 ml·g?1).The excess of Ph2S is partially adsorbed on the activated carbon.

The f i tted XPS spectra of Pd(3d)of the Pd-Ph2S(x)/C catalysts are presented in Fig.3.All three catalysts contain the peak of Pd(3d2/5)at 335.2-335.5 eV as indicative of Pd0,and the peaks of Pd(3d2/5)at336.3-336.6 eV and 337.4-337.7 eV as indicative of Pd2+and Pd4+, respectively,suggestingthatthechemicalbondhasbeenformedbetween Pd and Ph2S ligand.Pd atoms are partially chemisorbed by Ph2S,and the rest still existed as Pd0.There is no obvious difference in the binding energy's intensity and value between Pd-Ph2S(1)/C and Pd-Ph2S(100)/ C[Fig.3(b),(c)],which further indicates the saturation of the actual adsorption of Ph2S after the amount of Ph2S increased to more than 1 equiv.This result is in good agreement with XRD.S(2p)spectra are also shown in Fig.3(d-f).For all Pd-Ph2S(x)/C catalysts,a S(2p)signal is observed at about 163.7 eV,which can be attributed to Ph2S.

3.2.Catalytic performance of Pd-Ph2S/C catalysts

Fig.1.TEM images of(a)Pd/C,(b)Pd-Ph2S0.1/C,(c)Pd-Ph2S1/C and(d)Pd-Ph2S100/C.

Table 1Physical properties of Pd/C and Pd-Ph2S(x)/C catalysts

Fig.2.XRD patterns of Pd-Ph2S(x)/C catalysts:(a)activated carbon,(b)Pd/C, (c)Pd-Ph2S(0.1)/C,(d)Pd-Ph2S(1)/C and(e)Pd-Ph2S(100)/C.

The prepared Pd-Ph2S(x)/C catalysts were tested in the liquid phase selectivehydrogenationp-chloronitrobenzene(p-CNB)ina75mlstainless steel autoclave.The hydrogenation process of p-CNB went through nitroso and hydroxylamine intermediate stages and aniline was the main side product.Table 2 shows the catalytic performance of both unmodif i ed and modif i ed Pd/C catalysts and it is observed that the addition of Ph2S has signif i cantly altered the catalytic performance of Pd/C. Unmodif i ed Pd/C presents high activity with the complete reaction time to be just 15 min,however,the selectivity of p-CAN is only 41.4%.With the addition of Ph2S,the activity of the modif i ed Pd/C catalyst is inhibited and the selectivity of p-CAN over Pd-Ph2S(1)/C is signif i cantly improved (～99.6%),andthecompletereactiontimeisprolongedto65min.Thecatalytic performance of Pd-Ph2S(100)/C is nearly the same as that of Pd-Ph2S(1)/C,which is consistent with the characterization results.Apossible explanation for the higher selectivity is that the interaction of Pd andStooccupypartofactivesites,namelytheoverhighcatalyticcenters, leadstoreposefulhydrogenationperformancesothattheproperreaction rate for the high catalytic selectivity of desired product is obtained.

Fig.3.Pd 3d and S 2p XPS spectra of Pd-Ph2S(x)/C catalysts.

3.3.Stability of Pd-Ph2S/C catalysts

The reusability test of Pd-Ph2S(1)/C catalyst was carried out in a 500 ml stainless steel autoclave.After each reaction,the used Pd-Ph2S(1)/Ccatalystwasf i ltered,andthenwasappliedtothenexthydrogenation reaction without any treatment.The results are listed in Table 3.The activity of the recycled Pd-Ph2S(1)/C slightly increases with the complete reaction time decreasing from 60 min to 48 min, however,the selectivity of p-CAN is notably decreased(99.6%to 97.3%).It is quite different from ordinary deactivation of supported palladium catalyst during liquid hydrogenation[20-24].

TheusedPd-Ph2S(1)/Ccatalystwasmeasured byBET,XRD,XPSand EDS.The surface area of the Pd-Ph2S(1)/C catalyst just slightly decreased from 1626 m2·g?1to 1429 m2·g?1after six recycles. Moreover,there was no signif i cant decrease in surface area each time the Pd-Ph2S(1)/C is reused.

Table 2 Catalytic performance of Pd/C and Pd-Ph2S(x)/C catalysts in hydrogenation of p-CNB①

Table 3Reusability test of Pd-Ph2S(1)/C in hydrogenation of p-CNB①

The XRD patterns of the fresh and used Pd-Ph2S(1)/C catalysts are illustratedinFig.4.ThereareonlynormaldiffractionpeaksofPdandactivated carbon,and no other peaks are detected for the used catalysts, which indicates that no palladium sulf i de crystals are formed duringthe hydrogenation.And also,there is no signif i cant change in the halfmaximum(FWHM)of the peaks between the fresh and used catalyst.

The morphology of the Pd-Ph2S(1)/C used for six times is detected by TEM as shown in Fig.5.The size of Pd particles of the used Pd-Ph2S(1)/C is almost the same as that of the fresh catalyst.

ThePd-Ph2S(1)/CusedforsixtimeswasfurtherexaminedbyXPSto determine its structure.The f i tted Pd(3d)and S(2p)XPS spectra of the used Pd-Ph2S(1)/C catalyst are illustrated in Fig.6.Two S(2p)peaks at binding energies of 163.7 eV and 168.3 eV are observed,which are corresponded to S2?and S6+,respectively.S2?should be assigned to Ph2S and S6+should be attributed to the oxidation of a part of Ph2S on the surface of Pd-Ph2S(1)/C by air during the reusability test.The only two peaks of Pd(3d5/2)at 337.5 eV and 335.4 eV are also detected, which are regarded as the Pd-Ph2S phase and Pd0,respectively. However,the intensity of the peak at 337.5 eV is much weaker than that of the fresh Pd-Ph2S(1)/C catalysts[Fig.3(b)],which suggests that the content of Pd-Ph2S phase in the used catalyst is decreased after six recycles.It might be attributed to the leaching of Ph2S from Pd-Ph2S(1)/C catalyst at the atmosphere of H2during the hydrogenation.EDS data of elemental composition in Table 4 also conf i rmed this result.S content(by mass)in Pd-Ph2S(1)/C catalyst decreased from 0.8%to 0.4%after six recycles,which shows that the chemisorption between Ph2S and Pd is not so strong and part of Ph2S has been leached from the catalyst during the process.The loss of Ph2S decreases the inhibitory effect towards Pd/C,which causes the decline of the selectivity of p-CAN.

Reabsorption of Ph2S is necessary for the regeneration of the used Pd-Ph2S/C catalyst.The used Pd-Ph2S(1)/C catalyst was added to 20 ml of methanol solution containing 0.01 g Ph2S,and was f i ltrated after being stirred at 309 K for 6 h.The resulting catalyst was then washed successively with methanol.A reusability test of the regenerated Pd-Ph2S(1)/C catalyst was carried out to assess the eff i ciency of the resulf i dation process.The used catalyst was resulf i ded using the same procedure as above after each reaction and the results are listed in Table 5.It is found that the regenerated Pd-Ph2S(1)/C catalyst can still maintain relative stable catalytic performance even after 10 runs.It proves that resulf i dation of the used Pd-Ph2S(1)/C catalyst is effective to resume its stability during the hydrogenation.

Fig.4.XRD patternsof thefresh and usedPd-Ph2S(1)/C catalysts:(a)fresh Pd-Ph2S(1)/C; (b)Pd-Ph2S(1)/C used once;(c)Pd-Ph2S(1)/C used for six times.

Fig.5.TEM image of Pd-Ph2S(1)/C catalyst after being used for 6 times.

Table 4Physical properties of the fresh and used Pd-Ph2S(x)/C catalysts

Fig.6.XPS spectra of Pd-Ph2S1/C catalyst after being used for 6 times.

Table 5Reusability test of the regenerated Pd-Ph2S(1)/C in hydrogenation of p-CNB①

4.Conclusions

Withthemodi fi cation of Pd/C catalystbyPh2S,therewasa chemical interactionbetweenthePh2SandPd.WhentheamountofthePh2Swas less,theligandwaspreferentiallyadsorbthemetalactivesite.However, after increasing the Ph2S content to 1 equiv,the adsorption of Ph2S on Pd metal tended to be saturated,and the excess of Ph2S partially was adsorbed on activated carbon.Due to Ph2S which was coated on the Pd particles and occupied part of active sites,namely the over high catalytic active sites,the Pd-Ph2S/C catalysts consequently improved the selectivity of p-CAN during hydrogenation of p-CNB.However,a part of Ph2S was leached from Pd-Ph2S/C catalyst during hydrogenation, which decreased the inhibitory effect of Pd/C and caused the decline ofselectivityofp-CANovertheusedofPd-Ph2S/Ccatalyst.Resul fi dation oftheusedPd-Ph2S/Ccatalystwasusefulforitsstability,andtheregenerated Pd-Ph2S/C catalyst could be reused for at least ten runs with stable catalytic performance.

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14 April 2014

☆Supported by National Basic Research Program of China(2011CB710800)and Zhejiang Provincial Natural Science Foundation of China(LY12B03009).

*Corresponding author.

E-mail address:xnli@zjut.edu.cn(X.Li).

http://dx.doi.org/10.1016/j.cjche.2014.08.007

1004-9541/?2014 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.

Received in revised form 19 May 2014

Accepted 6 July 2014

Available online 19 August 2014