Effects of Bi and Ni on the properties of a vanadium phosphorus oxide catalyst

ChineseJournalofCatalysis35(2014)270–276

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Article

EffectsofBiandNionthepropertiesofavanadiumphosphorus oxidecatalyst

YunHinTaufiq‐Yapa,b,*,ChoonSeonYuena,b,Nawi@MohamedNurulSuzianaa,b, RamliIrmawatia,baCatalysisScienceandTechnologyResearchCentre,FacultyofScience,UniversityPutraMalaysia,43400UPMSerdang,Selangor,MalaysiabDepartmentofChemistry,FacultyofScience,UniversitiPutraMalaysia,43400UPMSerdang,Selangor,Malaysia

A R T I C L E I N F O

A B S T R A C T

Articlehistory:Received5August2013Accepted14November2013Published20February2014

Vanadium phosphorus oxide (VPO) catalysts were synthesized by the dihydrate method whichinvolvedthetwostepsforthepreparationofthedihydrate(VOPO42H2O)andtheprecursorhemi‐hydrate(VOHPO40.5H2O).BiandNisaltwereaddedintothemixtureofVOPO42H2Oandisobuta‐nol,andtheobtainedprecursorswerecalcinedinaflowofan‐butane/airmixturetoproducethepromotedVPOcatalysts.ThecatalystswerecharacterizedbyX‐raydiffraction (XRD),N2 adsorp‐tion‐desorption, inductivelycoupledplasma‐atomicemissionspectroscopy, scanningelectronmi‐croscopy (SEM), andH2 temperature‐programmed reduction (H2‐TPR).Their catalytic propertieswere testedusinga fixed‐bedmicroreactor.All thecatalystsgavemainXRDpeaksat2θ=22.9°,28.5°, and 30.0°, attributing to the (020), (204), and (221) planes of the pyrophosphate phase(VO)2P2O7, respectively. The promoted catalysts have smaller crystallite size and higher specificsurface areas. SEMmicrographs revealed the formationofmoreprominent plate‐like crystallitesthat were arranged as rosette clusters. H2‐TPR results showed that the promoted catalysts hadlower reduction peak temperatures and possessed higher amounts of V5+‐O2– and V4+‐O– pairs,whichgavehigherselectivityandactivityintheselectiveoxidationofn‐butanetomaleicanhydride.

©2014,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.PublishedbyElsevierB.V.Allrightsreserved.

Keywords:VanadiumphosphorusoxidecatalystPromotern‐ButaneoxidationMaleicanhydrideselectivity

1. Introduction

Maleic anhydride (MA) is an important petrochemical andpolymerfeedstock[1].Itisproducedbytheselectiveoxidationofn‐butaneovervanadiumphosphorusoxide(VPO)catalysts,whichisbetterthantheuseofbenzenebecausebutaneislesstoxic,lessexpensiveandnocarbonislostintheoxidation[2].VPOcatalystiswellknownastheactivephaseandiscommer‐ciallyused[3],whichhasbeenextensivelystudied.Todateitistheonlyindustrialcommercialcatalystfortheselectiveoxida‐tionofalkane[4].

TheintroductionofametalpromoterintotheVPOcatalyst

isaneffectivemethodforimprovingitscatalyticproperties[5].Thepromoterplaysanimportantroleintheadsorptionofox‐ygen and its diffusion in the lattice by which a nonselectiveroute of n‐butane oxidation is suppressed [6]. According toCenti et al. [7], the effects of promoters can be divided intothree categories as follows.Category1 is the additionof ionsthatinteractwithfreephosphoricacidasameanstofine‐tunethe optimum surface P/V ratio and acidity. Category 2 is theadditionofionsthatsubstituteforPintheprecursor.Thepar‐tial or total elimination of these ions from the vanadyl pyro‐phosphatestructurebycalcination influences themorphologyand leads to defects in the (VO)2P2O7 structure.Category3 is

*Correspondingauthor.Tel:+60‐3‐89466809;Fax:+60‐3‐89466758;E‐mail:[email protected],TechnologyandInnovationofMalaysia.DOI:10.1016/S1872‐2067(12)60749‐9|http://www.sciencedirect.com/science/journal/18722067|Chin.J.Catal.,Vol.35,No.2,February2014

YunHinTaufiq‐Yapetal./ChineseJournalofCatalysis35(2014)270–276

theadditionofelementsthatsubstituteforVandactasmodifi‐ersoftheactivitybyformingstablesolidsolutions.

Among themost investigatedpromoters forVPOcatalysts,Bi has been used for the past two decades [8–11], and highconversion(71%)andselectivity(68%)havebeenreportedfor1%Bipromotedcatalysts[12].Niincreasesthemobilityofthelattice oxygen and lowers the temperature of the reductionpeaks[13,14].Highconversion(87.2%)andselectivity(57.5%)havebeenobtainedfor1%Nipromotedcatalysts[5].TheroleofBi‐Nibimetallicpromotersonthephysicochemicalandcata‐lytic properties of VPO catalysts for n‐butane oxidation wasstudiedhere.

2. Experimental

2.1. Catalystpreparation

In first step, the dihydrate, VOPO42H2O,was prepared byreacting V2O5 (60.0 g, Fluka)with aqueouso‐H3PO4 (577.5 g,85%,J.T.Baker)indistilledwater(24mL/gsolid).Themixturewasthenrefluxedwithcontinuousstirringfor24h.Theyellowsolidproductwasrecoveredbycentrifuging,washedwithace‐toneanddistilledwater,anddriedat373Kfor24hinanoven.

In the secondstep,1%or2%(molar faction)ofBi andNisalts (Bi(NO3)35H2O and NiSO46H2O)were first dissolved in160ml isobutanol(99.9%,FisherChemical)beforetheywereaddedintoareactionmixturewith8gVOPO42H2Otopreparethe promoted precursor hemihydrate, VOHPO40.5H2O. Themixture was rapidly stirred and refluxed for 21 h. The bluesolid productwas removedby centrifuging,washedwithdis‐tilledwaterandacetone,anddriedat373Kfor24hinanoven.

Finally, the unpromoted and promoted precursors,VOHPO40.5H2O, were calcined at 733 K in a flow of 0.75%n‐butane/airmixture for 18 h. The unpromoted catalyst wasdenotedasCatUnpromoted;thepromotedcatalystswithBi/Ni=0.5,1,and2weredenotedasCatBi1Ni1,CatBi1Ni2,andCat‐Bi2Ni1,respectively.

2.2. Catalystscharacterization

X‐raydiffraction(XRD)patternsofthecatalystsatambienttemperature were obtained by a Shimadzu Diffractometer(Model XRD‐6000) employing Cu Kα radiation. The BET(Brunauer‐Emmett‐Teller) method was used to measure thetotalspecificsurfaceareasoftheVPOcatalystsusingaThermoFinnigan Sorptomatic 1990 nitrogen adsorption‐desorptionanalyzerat77K.TheelementalcompositionsoftheVPOcata‐lystsweredeterminedbyaPerkinElmeremissionspectrome‐ter (Model Plasma1000) and sequential scanning inductivelycoupled plasma‐atomic emission spectroscopy (ICP‐AES).Scanning electron microscopy (SEM) was performed using aLeo 1455 VP electron microscope. The samples were coatedwith gold using a Sputter Coater. Temperature‐programmedreduction (H2‐TPR) analysis was performed using a ThermoFinniganTPDRO1110 apparatuswith a thermal conductivitydetector.TheprogramusedtodeterminetheamountofoxygenatomswasTPD/R/O1100version2.3.

2.3. Catalytictest

Astandardmassofcatalyst(0.25g)wasusedfortheoxida‐tion of n‐butane toMA in a fixed‐bedmicroreactor at 673 Kwithagashourlyspacevelocity(GHSV)of2400h–1.n‐Butaneandairwerefedintothereactorbycalibratedmassflowcon‐trollerstogiveafeedstockcompositionof1%n‐butaneinair.Theproductswere fedbyheated lines to anonline gas chro‐matography for product analysis. The reactor comprised astainless steel tubewith thecatalystheld inplacebyplugsofquartzwool.A thermocouplewas located in thecentreof thecatalystbedandthetemperaturecontrolwastypically5K.

3. Resultsanddiscussion

3.1. XRDresults

Figure 1 shows the XRD patterns of the unpromoted andpromoted precursors, which perfectly matched the standardreference (JCPDS File No. 37‐0269) of the hemihydrate,VOHPO40.5H2O. The main peaks at 2 = 15.5, 19.6, 24.1,27.0,and30.3correspondedtothe(001),(101),(021),(121),and (130) planes, respectively. These confirmed that all theprecursorshaveVOHPO40.5H2Ophase.Thepromotedprecur‐sorsdidnotshowanyextrapeakintheXRDpattern.Theyhaddecreasedcrystallinityascomparedtotheunpromotedhemi‐hydrates, which was reflected by the lower intensity of thepeaks. According to Haber et al. [15], the (001) plane ofVOHPO40.5H2Ophaseisresponsibleforthecatalyticactivityinthe selective oxidation ofn‐butane and it is transformed intothetopotacticallysimilar(020)planeof(VO)2P2O7.

InFig.2,theXRDpatternsofallthepromotedVPOcatalystsshowedthesamecharacteristicpeaksforthe(VO)2P2O7phaseastheunpromotedone(JCPDSFileNo.34‐1381).Thecharac‐teristic peaks of these VPO catalysts appeared at 2 = 22.9,28.5,and30.0,whichcorrespondedtothe(020),(204),and(221)planes,respectively.

The crystallite size of the unpromoted and promotedVPOcatalysts was calculated using the Debye‐Scherrer equation[16]andthewidthofthepeaksofthe(020)and(204)planes,

10 20 30 40 50 60

HemiUnpromoted

HemiBi1Ni1

HemiBi1Ni2

Inte

nsit

y

2/( o )

HemiBi2Ni1

Fig.1.XRDpatternsoftheprecursorhemihydrate,VOHPO40.5H2O.


and the results are summarised in Table 1. All the promotedVPO catalysts have smaller size as compared with the un‐promoted catalyst,whichwas134.73Å from the (020)planeand276.57Åfromthe(204)plane.AmongthepromotedVPOcatalysts, CatBi1Ni1was the smallest in size, 83.54 197.02 Åfrom the (020) and (204)planes, respectively. The crystallitesizes of CatBi1Ni2 and CatBi2Ni1 from the (020) planewere84.62and85.81Å,respectively,whilethecrystallitesizeofthesameseriesofcatalystsfromthe(204)planewere197.33and208.36 Å, respectively. It can be thus concluded that eventhoughtherewasnophasechangedetected,theadditionofBiandNipromotershadaffected themicrostructureof the syn‐thesized VPO catalyst. The increase of the full width at halfmaximum(FWHM)ofthe(020)planediffractionindicatedthatthecrystallitesizeinthe(020)planedirectionhaddecreased.Thecrystallitesizeat the(204)plane isonly indicativeof themean “length”, while that at the (020) plane is more repre‐sentativeoftheactualthickness[17].Thesurfacewhichisse‐lectivefortheformationofMAisthe(020)plane,andsidefac‐essuchasthe(001)planeareactivefornonselectiveoxidation[18].

TheoriginalVPOcatalystconsistsmainlyofthevanadylpy‐rophosphatephase,(VO)2P2O7.Inthisstudy,BiandNipromot‐erswereaddedinverysmallamounts(1%or2%).AccordingtodeFariasetal.[19],theamountofpromoteraddedwastoolow to have a structural effect,whichwas in agreementwiththeXRDresults.Theauthorsproposedthatthecationorpro‐moter was incorporated into the vanadyl pyrophosphate toformsolidsolutionofthetype((VO)1‐xMx)2P2O7whereMrep‐resentsapromotercation.Thismayformdefectsinthestruc‐ture, which can act as new active sites or, alternatively, themetalcationscanaffecttheredoxpropertiesofthesurface.

Yeetal.[20]reportedthatthe(020)planeof(VO)2P2O7wasactivefortheselectiveoxidationofn‐butane,whichleadstotheexpectationthatahighexposureofthe(020)planewillresultin higher activity and selectivity, but that this is not the solefactor governing catalytic performance. The authors foundbroadening of the (020) peak and disorder along the (020)plane aftermodificationby theadditionofpromoters.On the(020)planeof(VO)2P2O7, therearefivekindsofoxygensites:O...V,V2‐O‐P,V‐O‐P,P‐O‐P,andV=O(V5+=O).TheV=Ospeciesisexpected to act as active lattice oxygen, although several re‐searchersclaimedthatadsorbedoxygenspecieswererespon‐sibleforthereaction.SincesurfaceV=Ospeciesarepresentonthe (020)plane,onewould expect that the surfacedensityofV=O species would increase with the exposure of the (020)planeanddisorderalongthe(020)plane.

3.2. Specificsurfaceareameasurements

The BET surface areas of the unpromoted and promotedVPO catalysts are summarized in Table 2. It showed that theBETsurfaceareaoftheunpromotedVPOcatalystwas23m2/g.CatBi1Ni1andCatBi1Ni2sampleshavehigherspecificsurfaceareas,whichwere31and28m2/g,respectively.ThisindicatedthattheadditionofBiandNifacilitatedtheformationofcrystalphases with a higher surface area. These results were con‐sistentwiththeXRDresults(Table1),whichshowedthatthepromotedVPOcatalystshave smallerparticle sizes comparedwiththeunpromotedone.

Generally, the addition of Bi and Ni into the VPO catalystmatrixgaveremarkablyhighersurfaceareas [11,12]. Inaddi‐tion,Taufiq‐Yapetal.[12]foundthatpromotionbyBiintotheVPO catalyst altered the development of the basal (100)(VO)2P2O7 phase, which is an interesting feature of the highsurfaceareaofthecatalysts.

Yeetal.[20]suggestedthattheincreaseofthesurfaceareawasaccompaniedbyanincreaseinthe(020)planeexposureof(VO)2P2O7.Theyalsosuggestedthatthepromotermetalcationswere incorporated into the interlayer spacing to reduce theweakHbondingofphosphateandwaterinVOHPO4·0.5H2O.Asaresult,cleavagealongthe(001)planeoftheVOHPO4·0.5H2Owaseasier,whichwouldleadtotheincreaseofboththesurfaceareaandexposureof the(020)planeof(VO)2P2O7.AccordingtoHutchings[21],aparticularadvantageofthehighersurfaceareaoftheVPOcatalystsisthattheyperformedwellat lowertemperatures,whichledtoenhancedselectivityandhencehighyieldsofproductobtained.

WesuggestthattheaddedBiandNipromoterswereincor‐porated into the interlayer spacing of VOHPO4·0.5H2O toweaken the H bonding. Consequently, the (001) plane of theVOHPO4·0.5H2Obecameeasiertocleaveandledtotheincrease

10 20 30 40 50 60

CatUnpromoted

CatBi1Ni1

CatBi1Ni2

Inte

nsit

y

2/( o )

CatBi2Ni1

Fig.2.XRDpatternsofunpromotedandpromotedVPOcatalysts.

Table1XRDdataoftheVPOcatalysts.

CatalystLinewidth(°) Crystallitesize(Å)

(020) (204) (020) (204)CatUnpromoted 0.5955 0.2933 134.73 276.57CatBi1Ni1 0.9600 0.4117 83.54 197.02CatBi1Ni2 0.9480 0.4111 84.62 197.33CatBi2Ni1 0.9350 0.3893 85.81 208.36

Table2BETsurfaceareasofunpromotedandpromotedVPOcatalysts.

Catalyst Surfacearea(m2/g)CatUnpromoted 23CatBi1Ni1 31CatBi1Ni2 28CatBi2Ni1 27


ofboththesurfaceareaandtheexposureofthe(020)planeof(VO)2P2O7.

3.3. Elementalanalysis

TheICP‐AESresultsareshowninTable3fortheP/Vratiosof the promoted VPO catalysts. These P/V values fall in therange of 1.0–1.2 required to produce the so‐called active andselective(VO)2P2O7phase[22–24].Thisisgoodagreementwiththe literature that theV4+ species responsible for the catalystactivityisstabilisedbyahigherP/Vatomicratio[25].

The analysis also confirmed the presence of theBi andNipromoterintheVPOcatalystswithBi/VandNi/Vatomicratiosbetween0.010and0.020.Itcanbeinterpretedthattheintro‐ductionofdifferenttypesofmetalpromotersatdifferentM/V(metal promoter/vanadium) ratios into (VO)2P2O7 have aneffectontheformationofdifferentcrystalplateswithdifferentsurfacemorphology. Therefore, we suggest that the differentmetal cations play different roles in the bulk system of the(VO)2P2O7catalysts, which can significantly affect the surfacearea(Table2)ofthecatalysts.

3.4. SEMresults

Figure 3 shows SEM images of the unpromoted and pro‐moted VPO catalysts. The principle structure of the catalystswas the same, and consisted of rosette‐shape plate‐like crys‐tals.Thesecrystalsaremadeupofagglomeratesof(VO)2P2O7platelets that preferentially expose the (100) crystal planes[26].

BiandNipromotedVPOcatalystsshowedamorecompactstructurewithmorelayeredplate‐likecrystalswhichformedatthesurfaceoftheclusters.Therosette‐shapeclustersobservedweresmallerthanthoseoftheunpromotedcounterpart.ThisisconsistentwiththatthepromotedVPOcatalystsshowedhighersurfaceareasascomparedwithunpromotedone.

3.5. H2‐TPRresults

Figure 4 shows the TPR profiles of the unpromoted andpromotedVPOcatalysts,whichshowedthenatureof thesur‐faceandreducibilityofthecatalysts.Allthecatalystshadtworeductionpeaks,whichwere assigned to the reductionofV5+speciesandtheremovaloflatticeoxygenfromV4+species,re‐spectively [27]. The peak attributed to V5+ is associatedwiththeremovalofO2−anionsand thepeak fromV4+ is related totheO−anionoxygenspecies[28].Therearegoodrelationshipsbetween theamountofoxygenspecies removed fromtheV5+andV4+ specieswithMA selectivity andn‐butane conversion,respectively[29].

AsshowninFig.4,CatUnpromotedsamplegaveashoulderfirstreductionpeakandabellshapedsecondreductionpeakat893and1050K,respectively.Aninterestingobservationisthatby introducingBiandNipromoters into thecatalysts, the re‐ductionpatternsweresignificantlychangedwithmoreobviousreductionofV5+. Furthermore, these reductionpeakswere atremarkably lowertemperatures in therangeof820to860K.On theotherhand, the secondpeak,which is associatedwiththe reductionofV4+ species, appeared at lower temperatures(910–940K)forallpromotedVPOcatalysts.Alowerreductiontemperature allows theVPO catalyst to operatewith a lowerenergyconsumption.

The amount of oxygen atoms removed from unpromotedand promoted VPO catalysts were calculated by a suppliedsoftwareprogram,TPD/R/O1100version2.3.Thenumberofoxygen species removed was given in mol/g, and was con‐vertedtoatom/gbymultiplyingwithAvogradoconstant(6.021023).AsshowninTable4,CatUnpromotedsamplehadtheleastamountsofoxygenatomsremoved,andforthetwopeaks,thesewere 0.04 × 1021 and 0.46 × 1021 atom/g, respectively. The amount of oxygen atoms removedwas increased by the

Table3P/V ratio forunpromoted,Bi‐promoted, andNi‐promotedVPOCata‐lysts.

CatalystMolarratio(ICP)

P/V Bi/V Ni/VCatUnpromoted 1.04 — —CatBi1Ni1 1.09 0.009 0.009CatBi1Ni2 1.06 0.008 0.018CatBi2Ni1 1.05 0.018 0.009

1 m 1 m

1 m 1 m

(a) (b)

(c) (d)

Fig.3.SEMmicrographsofunpromotedandpromotedVPOcatalysts.(a)CatUnpromoted;(b)CatBi1Ni1;(c)CatBi1Ni2;(d)CatBi2Ni1.

650 700 750 800 850 900 950 1000 1050 1100

CatUnpromoted

CatBi1Ni1

CatBi1Ni2

Inte

nsit

y

Temperature (K)

CatBi2Ni1 826 912

858940

851 914

893

1050

Fig.4.H2‐TPRprofilesofunpromotedandpromotedVPOcatalysts.


additionoftheBiandNipromoters.CatBi1Ni1sampleshowedthelargestamountofoxygenatomsremoved,whichwas0.44×1021atom/gsampleforthelowpeaktemperature(851K)and0.73×1021atom/gforthehighpeaktemperature(914K).Thetotal amountof oxygen atoms removed frombothpeakswasalso increased for promoted samples as compared to un‐promotedone.DuetothelargeamountofreactiveoxygenfromV5+removedfromCatBi1Ni1,thiscatalystgavethehighestMAselectivity. In addition, the higher surface areamay also con‐tributetothehigherselectivity.

Thus, theadditionofBiandNisuccessfullypromotedVPOcatalysts which consequently possessed more reactive andmore labile oxygen species, which gave better reducibilitycompared to the unpromoted VPO catalyst. According to deFariasetal.[19],elementsaddedinsmallamountstogivelowpromoter/vanadiumatomicratiosactasanelectronicpromot‐er, and thiswas categorised as Type 2 promotion. CatBi1Ni1wasconsideredtobeType2.Itconsistedof1%Biand1%Niandshowedthelargestamountofoxygenatomsreleasedinthetwopeaks.For thecatalystswhichcontain2%ofeitherBiorNi,theamountofoxygenatomsremovedwasslightlylower.Inthis Type 2 promotion, the cation or promoter was incorpo‐ratedintothevanadylpyrophosphatewithV4+insolidsolutionofthetype((VO)1‐xMx)2P2O7whereMisapromotercation.Thismayformdefectsinthestructurewhichcanactasnewactivesites or, alternatively, themetal cationsmay affect the redoxpropertiesofthecatalystsurface.

In addition, there were more oxygen species removed atlowerreductiontemperatureforthepromotedVPOcatalystsascomparedwiththeunpromotedonebecausetheformerhadanincreasedsurfacedensityofV=Ospecieson (VO)2P2O7,whichcausedtheV=Obondattheadjacentsitetobecomeweaker.Asaresult,thespecificactivityincreased,asreportedbyTakitaetal.[30].Yeetal.[20]foundthatthesurfacedensityofV=Ospe‐cies on the (020) plane increased the exposure and disorderalongthe(020)plane.TheseV=Ospeciesareexpectedtoactasactive lattice oxygen, although several researchers suggestedthat adsorbed oxygen species were responsible for the reac‐tion.Therefore,ourobservationsuggestedthatBiandNipro‐motedVPOcatalystseffectively improvedthecatalyticperfor‐mancefortheselectiveoxidationofn‐butanetowardsMA.

The illustration of an ideal (VO)2P2O7 plane and themetalpromoter(M)substituted(020)planeisshowninFig.5[30].

3.6. Catalyticoxidationofn‐butane

TheeffectofvariousBi/Niratiosontheselectivityandac‐tivityofthecatalystsareshowninTable5.WhenVPOcatalystswerepromotedwithBiandNi, thecatalyticperformancewasimproved providing n‐butane conversion of 51% and 52%,while theMA selectivitywasbetween36%and39%as com‐pared to thatover theunpromotedone (conversion38%, se‐lectivity 28%). These findings showed that the addition ofpromotersplayedanimportantroleinenhancingtheselectivi‐ty and activity of VPO catalysts. CatBi1Ni1 gave the highestn‐butaneconversion(52%)andMAselectivity(39%).

Figure6showsn‐butaneconversionasafunctionofamountofoxygenspecies removedassociatedwithV4+.Agood linearcorrelationcoefficient(R2=0.84)betweenn‐butaneconversionandamountofoxygenspeciesassociatedwithV4+(section3.5)wasobserved.ThisprovedthatthecatalystactivitywashighlydependentonthepresenceoftheV4+phase.TheseresultsareinagreementwithTaufiq‐Yapetal.[27,29],whoconcludedthatV4+–O– acted as the centre for the activation of n‐butane. AsreportedearlierbyMarsetal. [31],a largeamountofsurface

Table4Amount of oxygen atoms removed fromunpromoted andpromotedVPOcatalystsbyreductioninH2/Ar.

CatalystTmax(K)

Oxygenremoved(1021atom/g)

V5+/V4+ ratio

CatUnpromoted 8931050

0.040.46

0.08

CatBi1Ni1 851 914

0.440.73

0.60

CatBi1Ni2 858 940

0.360.68

0.52

CatBi2Ni1 826 912

0.280.67

0.41

O O

O OV

OO

OM

O O

O OVO O

OV

O Fig.5.Comparisonbetweenidealandmetalsubstituted(020)planesof(VO)2P2O7.

Table5CatalystperformanceofunpromotedandpromotedVPOcatalysts.

Catalystn‐Butane

conversion(%)Productselectivity(%)MA CO2 CO

CatUnpromoted 38 28 71 1CatBi1Ni1 52 39 60 1CatBi1Ni2 52 37 62 1CatBi2Ni1 51 36 63 1Reactionconditions:673K,1%n‐butaneinair,GHSV=2400h–1.

0.45 0.50 0.55 0.60 0.65 0.70 0.750

10

20

30

40

50

60

n-B

utan

e co

nver

sion

(%

)

Amount of oxygen species removed (1021 atom/g)

R2 = 0.84

Fig. 6. n‐Butane conversion as a function of the amount of oxygenspeciesremovedassociatedwithV4+.


lattice oxygen, which act as active species, also contribute toimproving the catalyst activity. The results implied that thisoxygenspecieswashighlyactivefortheselectiveoxidationofn‐butane,andagreedwith theelectricalconductivitydataob‐tainedbyWitkoandco‐workers[32].CatBi1Ni1gavethehigh‐est n‐butane conversion (52%) and removed 0.73 × 1021 at‐om/gofoxygenspeciesassociatedwithV4+.

Figure 7 shows the selectivity to MA as a function of theamount of oxygen species removed associated with V5+. Thelineargraphshowsagoodcorrelationwithahighcorrelationcoefficient (R2=0.97)which indicated that theseoxygenspe‐cies(O2–)amountsweredirectlyproportionaltotheselectivityofMA[28,29,33].IthasbeenshownthattheV5+speciesplayanessentialroleinhydrogenabstractionfromn‐butane[34].Thepresence of V5+ surface specieswould favour selective oxida‐tionbypreventing or limiting the over‐oxidationof adsorbedMAmolecule [35]. All the Bi and Ni promoted VPO catalystsgavelargeramountsoftheactiveoxygenspecies,O2–associat‐edwithV5+,whichcontributedtobetterMAselectivity.Itwasproved that the promotedVPO catalysts showedhigher cata‐lytic performance. CatBi1Ni1 gave the highest MA selectivity(39%) and it removed 0.44 × 1021 atom/g of oxygen speciesassociatedwithV5+.

The principle role of the promoters is to enable the for‐mationof smaller crystallite size in the (020)planedirection.According to Ye et al. [20], a smaller crystallite sizewith thebroadeningof(020)planeanddisorderalongthe(020)planewerepresentaftermodificationbytheadditionofpromoters.Ithasbeenwidelyacceptedthatthe(020)planeisactivefortheselectiveoxidationofn‐butane,whichleadstotheexpectation

that a high exposure of the (020) plane will result in higheractivityandselectivity.

Duringthereactionofn‐butanetoMA,asurfaceredoxcycletakesplacebetweenV5+andV4+,i.e.surfaceV5+isfirstreducedbybutanetoV4+, then it isquicklyoxidizedtoV5+witheitheradsorbed or gas phase oxygen [36]. If over‐oxidation occurs,V4+isreducedtoV3+asshowninFig.8.

4. Conclusions

AmongthethreedifferentBiandNipromotedVPOcatalystsforn‐butane oxidation toMA, CatBi1Ni1was themost activecatalystwithaconversionof52%andMAselectivityof39%.All the Bi and Ni promoted catalysts have smaller crystallitesizeandhigher specific surface areasas compared to theun‐promoted one. The reduction behaviour over the promotedcatalystswereinvestigatedtodiscovertheroleofoxygenspe‐ciesinthisselectiveoxidationreaction.Therewasagoodcor‐relationbetweentheamountofoxygenspeciesassociatedwithV4+ andn‐butane conversion. All the promoted VPO catalystswith a larger amount of active oxygen species, O– associatedwith V4+, gave higher catalytic activity. The excess amount ofoxygenspecies(O2–),whichisassociatedwithV5+,significantlyincreasedMAselectivity.

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0.0 0.1 0.2 0.3 0.4 0.50

5

10

15

20

25

30

35

40

45M

A s

elec

tivity

(%

)

Amount of oxygen species removed (1021 atom/g)

R2 = 0.97

Fig.7.MA selectivity as a function of amount of oxygen species re‐movedassociatedwithV5+.

V5+ V4+

O2- O2

Butane MA

over reductionV3+

Fig.8.Changes of vanadium valence duringn‐butane oxidation overVPOcatalyst.


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GraphicalAbstract

Chin.J.Catal.,2014,35:270–276 doi:10.1016/S1872‐2067(12)60749‐9

EffectsofBiandNionthepropertiesofavanadiumphosphorusoxidecatalyst

YunHinTaufiq‐Yap*,ChoonSeonYuen,Nawi@MohamedNurulSuziana,RamliIrmawatiUniversitiPutraMalaysia,Malaysia

C CHH

H

C

H

H

H

H

C

H

H

H OOO O

O

H HO

n-ButaneOxygen

Maleic Anhydride WaterO O

O OV

OO

OM

Note: Promoter denoted as M

O O

O OVO O

OV

O

+ +Promoted VPO Catalysts

BiandNipromotersinaVPOcatalystimproveditscatalyticpropertiesinselectiveoxidationofn‐butanetomaleicanhydride.

Documents

Effects of Bi and Ni on the properties of a vanadium phosphorus oxide catalyst