Applied Catalysis A: General 261 (2004) 211219n-Pentane isomerization over platinum-promoted W/Zr mixedoxides supported on mesoporous silicaTao Li, She-Tin Wong, Man-Chien Chao, Hong-Ping Lin, Chung-Yuan Mou, Soon ChengDepartment of Chemistry, National Taiwan University, Taipei 106, TaiwanReceived 20 May 2003; received in revised form 9 October 2003; accepted 8 November 2003AbstractW/Zr mixed oxides supported on various porous silica materials, namely SBA-15, MCM-41 and silica gel, were prepared. Their aciditiesandcatalyticactivitiesinn-pentaneisomerizationwereexamined. Forthemesoporoussilica, SBA-15wasfoundtoretaintheorderedmeso-structure better than MCM-41 after loading with W/Zr mixed oxide. TEM photographs showed that tungstated zirconia was dispersedinside the mesoporous channels of SBA-15, and that the surface area and pore volume decreased with the loading. The SBA-15 supportedWO3/ZrO2 materials promoted with Pt were highly catalytic efcient in n-pentane isomerization with a very high iso-pentane selectivity. Incontrast, MCM-41 and silica gel supported WO3/ZrO2 showed lower catalytic activities. The optimal activity was observed on SBA-15 with1%Pt/20%WO3/40%ZrO2. The mesoporous silica supports played an important role in stabilizing the mixed oxide catalyst since the catalyticactivity decayed less markedly with time-on-stream on the supported catalysts. 2003 Elsevier B.V. All rights reserved.Keywords: Tungstated zirconia; Mixed oxide; Catalyst; Acid; Mesoporous silica; MCM-41; SBA-15; Isomerization of n-pentane; Platinum1. IntroductionSincethediscoveryofmesoporousmolecularsievesofM41S family by a Mobil research group [1], many meso-porous solids with controlled pore sizes and morphologieshavebeensynthesized[2]. Theyareconsideredtobepo-tential catalysts or catalyst supports due to their high ther-mal stability (up to 800C), uniformly arranged mesoporesandveryhighsurfaceareas(ca. 1000 m2/g). However, inspite of having larger pore dimensions, the acidity of meso-porous materials like Al-MCM-41is muchweaker thanthat of microporous zeolites [3]. In order to overcome thisdrawback, agreat effort hasbeenfocusedonintroducingstrong acid sites on the mesoporous materials. Some hybridinorganicorganic mesoporous materials with alkylsulfonicacid groups were synthesized. They were reported to behaveasstrongacidcatalystsinthecondensationreactionsandalso in the esterication reactions [48]. The hybrid mate-rials, however, cannot be applied to systems where reactionCorresponding author. Tel.: +886-2-2363-8017;fax: +886-2-2363-6359.E-mailaddress: chem1031@ccms.ntu.edu.tw (S. Cheng).temperature is higher than ca. 250C due to the low thermalstability of the organic component.Intheefforttodevelopenvironmentallyfriendlystrongsolid acids to replace the hazardous mineral acids, such asHF and H2SO4 commonly employed in todays petrochemi-cal industry, sulfated zirconia (SZ) has attracted great atten-tion in the past decade because it demonstrated high catalyticactivitiesinskeletal isomerizationofalkanesat relativelylow temperatures [911]. Over the past few years, severalways of supportingSZonMCM-41havebeenreported[1216]. The resultant materials were found to be very ac-tive in MTBE synthesis and skeletal isomerization of alkane.However, serious concerns remain about the long-term sta-bility of SZ in reducing and oxidizing environments typicallyencountered in hydrocarbon reactions and catalyst regenera-tion due to the gradual loss of sulfur. Hino and Arata [17,18]found that zirconia loaded with tungsten oxide (abbreviatedasWZ)ormolybdenumoxide(abbreviatedasMoZ)alsoshowedhighcatalyticactivities instrongacidcatalyzedreactions. Very recently, we successfully supported Mo/ZrmixedoxideonMCM-41. Suchmaterialswereexcellentincatalyzingliquidphasecondensationof 2-methylfuranwithacetonetoform2,2-bis(5-methylfuryl)propane[19].Although zirconia loaded with tungsten oxide was also0926-860X/$ see front matter 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.apcata.2003.11.006212 T.Lietal. / Applied Catalysis A: General 261 (2004) 211219reported to be a strong acid catalyst [17], up to now, no re-ports have described mesoporous materials supported withthese mixed oxides. The aims of this study are to introduceacid function onto the mesoporous materials by supportingW/Zr mixed oxide on them and to examine the catalytic ac-tivities of the resultant materials in n-pentane isomerization.2. Experimental section2.1. Catalyst preparationPuresiliceous MCM-41was synthesizedaccordingtothe method reported by Das et al. [20]. The molar ratio ofthesynthesisgel compositionisSiO2:0.48CTMA+:0.33TPA+:0.39 Na2O:0.29 H2SO4:110 H2O. Pure siliceousSBA-15 was synthesized according to the method reportedbyKaoetal. [21]. Thegelchemicalcompositioninmo-lar ratio was P123:60 TEOS:1506 HCl:9706 H2O. Theas-synthesized mesoporous samples were calcined in air at560C for 6 h to remove the organic surfactant templates.The W/Zr mixed oxides were supported on various silicamaterials by co-impregnation of zirconium(IV) acetylaceto-nate and ammonium metatungstate hydrate. The silica mate-rials such as MCM-41, SBA-15 and SiO2 gel (Aldrich), inpowder form were dispersed in a methanol solution of zirco-nium(IV) acetylacetonate (Aldrich) under vigorous stirring.Then aqueous ammonium metatungstate hydrate (Aldrich)was added and the stirring was continued for about 1 h. Af-terthesolventwasremovedwitharotaryevaporator, thesolid was dried in air overnight at 110C. Most of the driedsamples were calcined in air at 800C for 3 h except thosementioned separately. The resultant calcined samples wereimpregnated with platinum tetrachloride (Janssen) solutionanddriedovernightat110C. Thisprocesswasfollowedby calcination in air at 500C for 3 h. W/Zr mixed oxideswithout support were also prepared by mixing a methanolsolution of zirconium(IV) acetylacetonate and aqueous am-monium metatungstate hydrate under vigorous stirring; thenthe same treatments as for supported samples were followed.2.2. Catalyst characterizationLow-angle powder X-ray diffraction (XRD) patternsweretakenontheWiggler-Abeamline( =0.1326 nm)of the National SynchrotronRadiationResearchCenter,Hsinchu, Taiwan. Mid-angle patterns were recorded with aScintag X1 diffractometer using Cu K radiation. Nitrogenadsorption-desorption isotherms were obtained at liquidnitrogen temperature with a Micromeritics ASAP 2000apparatus. Transmission electron microscopy (HRTEM)wasperformedonaPhilipsTecnaiFEG-TEMinstrumentoperated at 200 kV.The temperature-programmed desorption of ammonia(NH3-TPD) was carried out on a Micromeritics AutoChem2910instrument.TPDprolesofammoniawereobtainedfrom120to800Cat aheatingrateof 10C/min. Thedesorptionprocess was monitoredbya quadruple massspectrometer (ThermoONIX, ProLab) connectedon-linethrough a heated capillary interface. The mass number 16wasfollowedtoobtainTPDprolesofNH3becausethismass intensity is relatively strong and the interference fromH2O is negligible.DiffusereectanceinfraredFouriertransform(DRIFT)spectra of the samples that adsorb pyridine were recordedusing a BOMEM MB155 FT-IR/Raman spectrometer. Theequipment was furnished with an in situ sample cell (Har-rick). Thesamplewaspre-heatedat300Cfor3 hunder106mbar vacuum before pyridine vapor was introduced atroom temperature; this process was followed by evacuationfor30 min.Spectrawereacquiredfromroomtemperatureto 500C under vacuum.The temperature-programmed oxidation (TPO) was car-ried out on a Du Pont 951 thermogravimetric analyzer. Theheating rate was 10C/min and the carrier gas was dried airwith a 60 ml/min ow rate.2.3. Catalytic studyThe catalytic reaction of n-pentane isomerization was car-ried out in a xed-bed micro-reactor at atmospheric pressure.A 0.5 g portion of the catalyst was packed into the reactorand then pretreated in dried air at 450C for 3 h. The reac-tor temperature was then lowered to the reaction tempera-ture of 250C under a ow of N2. The reaction feed was amixture of n-pentane, hydrogen and nitrogen with the owratesof0.62,2,and10 ml/min,respectively.Thereactionfeed owed through the catalyst bed at an n-pentane weighthourlyspacevelocity(WHSV) of 0.24 h1. Thereactionproducts were analyzed on-line with a Shimadzu GC-14Agas chromatograph, equipped with a FID and capillary col-umn (RTX-1, 60 m). Regeneration of the used catalyst wascarried out in situ under owing air at 450C for 3 h.3. Results and discussion3.1. Characterization of catalystsFig. 1AshowstheXRDpatternsof pureSBA-15andthe 1%Pt/WO3/ZrO2/SBA-15 samples with the WO3/ZrO2weight ratio kept constant at 0.50 while the ZrO2/SBA-15weight ratios varied. In the small-angle region, the three dis-tinct diffraction peaks correspond to the 100, 110, 200 re-ections of SBA-15. The well resolved hexagonal structureof SBA-15 was retained even when the ZrO2/SBA-15 weightratio was as high as 0.54. When the ZrO2/SBA-15 weightratio is further increased, the 110 and 200 reections disap-pear, and the intensity of the 100 diffraction peak decreasesdrastically. Thismight beduetothedecreaseindiffrac-tion contrast after lling the mesoporous channels with themixed oxides.T.Lietal. / Applied Catalysis A: General 261 (2004) 211219 213Fig. 1. XRDpatternsof(a)SBA-15andthe1%Pt/WO3/ZrO2/SBA-15sampleswith(A)0.50weight ratioofWO3/ZrO2anddifferent ZrO2/SBA-15weight ratios of (b) 0.10; (c) 0.25; (d) 0.54; (e) 1.0; and (f) 1.86. (B) 1.0 weight ratio of ZrO2/SBA-15 and different WO3/ZrO2weight ratios (b) 0.20;(c) 0.35; (d) 0.50; and (e) 0.65.No diffraction peaks corresponding to either ZrO2 or WO3crystallites were observed in the wide-angle region for sam-ples with WO3/ZrO2 weight ratio of 0.50 and ZrO2/SBA-15weight ratio lower than 0.25. Therefore, ZrO2 and WO3 be-low these loadings should be well dispersed on the surfaceof SBA-15. As the ZrO2/SBA-15 weight ratio is over 0.54,tetragonalZrO2phasewasobserved. Furthermore, mono-clinicZrO2phasealsoappearedwhentheZrO2/SBA-15ratioswerehigherthan1.0. Ontheotherhand, nopeakscorrespondingtoWO3crystallitescouldbeseenuntiltheZrO2/SBA-15ratiowas raisedto1.0. Incomparisonofthe two mesoporous silica samples of hexagonal structure,SBA-15 seemed to keep its ordered porous structure betterthanMCM-41aftersupportingW/Zrmixedoxides, sinceMCM-41 lost its hexagonal diffraction peaks completely af-ter supporting similar amounts of W/Zr mixed oxide. There-fore, most studieshereafterwerefocusedontheSBA-15supported samples.Fig. 1BshowstheXRDpatternsof 1%Pt/WO3/ZrO2/SBA-15 samples with constant ZrO2/SBA-15 weight ratio of1.0 and different WO3/ZrO2 weight ratios. In the small-angleregion, the diffraction peaks of SBA-15 were relatively lowfor samplessupportedwithW/Zr mixedoxides, becauseof the high loading of ZrO2. In the wide-angle region thesituation is similar to that in Fig. 1(a). When the WO3/ZrO2weight ratio increased to 0.50, small peaks corresponding toWO3 phase began to appear. At the same time, monoclinicZrO2phaseappearedinadditiontothetetragonal phase.The latter was the only phase observed when the WO3/ZrO2weight ratio was lower than 0.50.The physical properties measured from nitrogen adsorp-tion-desorption isotherms of various porous silica materialsand those supported with W/Zr mixed oxide are tabulated inTable 1. The parent SBA-15 sample calcined at 560C has aBET surface area of 636 m2/g, a pore volume of 0.86 cm3/gand a pore size distribution centered at 8.4 nm. After sup-porting tungstated zirconia on it, the BET surface area andporevolumewerefoundtodecreasewiththeincreaseinoxideloading.SincealltheSBA-15supportedmixedox-ide samples were calcined at 800C, a parent SBA-15 sam-plecalcinedatsuchahightemperaturewasalsostudied.The surface area, pore volume and pore diameter decreaseto452 m2/g,0.62 cm3/gand7.5 nm,respectively.Relativeto this high temperature calcined parent sample, the surfaceareaandporevolumeoftheSBA-15sampleloadedwithWO3/ZrO2did not decrease so signicantly, implying thatthesupportedoxideshouldbedispersedontotheinternalsurfaces of the mesopores of SBA-15. The HRTEM photo-graph of 20%WO3/40%ZrO2/SBA-15 shown in Fig. 2 con-rmed that the hexagonally arranged mesopores of SBA-15were retained and that WZ was mainly dispersed inside thepores.A comparison of the three different silica materials sup-porting with the same amount of WZ (20%WO3/40%ZrO2)showedthat thesurfaceareas wereclose: 137, 162and141 m2/g for SBA-15, MCM-41 and silica gel, respectively.However, the differences in pore volume were marked: 0.21,0.11 and 0.17 cm3/g, respectively. The relatively lower porevolume on MCM-41 supported sample implies that WZ par-ticles may seriously block its mesopores, but such blocking214 T.Lietal. / Applied Catalysis A: General 261 (2004) 211219Table 1Physical properties of porous silica-supported mixed oxide catalystsSample WO3/ZrO2(weight ratio)ZrO2/support(weight ratio)Calcinationtemperature (C)BET area(m2/g)BJH porevolume (cm3/g)Pore diameter(nm)SBA-15 560 636 0.86 8.4SBA-15 800 452 0.62 7.54.3%WO3/8.6%ZrO2/SBA-15 0.50 0.10 800 411 0.60 8.09.1%WO3/18%ZrO2/SBA-15 0.50 0.25 800 301 0.42 7.015%WO3/30%ZrO2/SBA-15 0.50 0.54 800 201 0.30 6.820%WO3/40%ZrO2/SBA-15 0.50 1.0 800 137 0.21 6.224%WO3/49%ZrO2/SBA-15 0.50 1.86 800 92 0.14 6.29.1%WO3/45%ZrO2/SBA-15 0.20 1.0 800 216 0.25 6.315%WO3/43%ZrO2/SBA-15 0.35 1.0 800 166 0.22 5.824%WO3/38%ZrO2/SBA-15 0.65 1.0 800 125 0.20 6.9SiO2gel 560 660 0.67 20%WO3/40%ZrO2/SiO20.50 1.0 800 141 0.17 17%WO3/ZrO20.20 800 24 0.03 MCM-41 560 1085 1.0 2.720%WO3/40%ZrO2/MCM-41 0.50 1.0 800 162 0.11 2.1S-50%ZrO2/MCM-41 1.0 680 476 0.41 2.5is less signicant on SBA-15 support of relatively larger porediameter. Ontheotherhand, thesupportedcatalystshavemuch larger surface areas than the unsupported WO3/ZrO2(24 m2/g).The temperature-programmed desorption of ammonia(NH3-TPD) wasperformedtodeterminetheamount andstrengthofacidsitesonthecatalysts. ThepresenceofPtpromoter on the samples did not change the TPD proles.Therefore, only the proles of the samples with Pt promoterare shown. Fig. 3A compares the NH3-TPD proles of WZsamples supported on various silica supports with those ofthe unsupported WZ and of the sulfated ZrO2 supported onMCM-41. The WZ loading in the three supported samplesremainedthesame: 20%WO3and40%ZrO2. Fortheun-supported WZ, the 17%WO3/ZrO2 was the one showing theFig. 2. HRTEM photograph of 1%Pt/20%WO3/40%ZrO2/SBA-15 sample.highest catalytic activity in n-pentane isomerization. All thesamples gave a broad desorption band covering 150500Cand centered at ca. 250C, implying that the acid strengthofthesesampleswassimilarandinthemediumstrengthregion. Based on the peak area, it can be concluded that WZsupported on either SBA-15 and MCM-41 contained moreacid sites than that supported on silica gel. Furthermore, thesupported WZ samples contained many more acid sites thanthe unsupported WZ. These results in union with the BETsurfaceareastudiessuggest that WZonthemeso-poroussilicasupportsshouldbehighlydispersedsothat alargenumber of acidsites was generated. Ontheother hand,MCM-41 supported sulfated ZrO2 contained even more andstronger acid sites, which was revealed by the presence ofa shoulder at ca. 320C on the NH3-TPD prole.Fig. 3B compares the NH3-TPD proles of SBA-15 sup-ported WZ samples with different WO3 and ZrO2 loadings.No direct correlation was found between the acid amount andthe oxide loading. However, samples with relatively lowerWO3 and ZrO2 contents (proles a, b and c) contained moreacidsitesthanthosewithhigherWO3andZrO2contents(prolesdande), althoughtheacidstrengthwassimilar.Since the higher loading samples have lower surface area,these results imply that good dispersion of mixed oxide onSBA-15 is necessary in order to generate a large number ofacid sites.The DRIFT spectra of pyridine adsorbed on WO3/ZrO2/SBA-15weretakentostudythenatureoftheacidsites.Fig. 4 shows the DRIFT spectra of 1%Pt/20%WO3/40%ZrO2/SBA-15 sample after adsorption of pyridine anddesorption at various temperatures. It can be seen that bothLewis (characteristicat 1450, 1487and1609 cm1) andBrnsted(characteristicatca. 1550 cm1)acidicsitesarepresent on the sample [14,22]. In addition, two peaks cor-responding to the H-bonded pyridine appeared at 1446 and1597 cm1; thesedisappearedafterheatingthesampleatT.Lietal. / Applied Catalysis A: General 261 (2004) 211219 215200 300 400 500 600 700 800 200 300 400 500 600 700 800(A)Intensity (a.u.)(e)(d)(c)(b)(a)Temperature (oC)(B)(e)(d)(c)(b)(a)Fig. 3. NH3-TPD proles of (A): (a) 1%Pt/50%S-ZrO2/MCM-41 and 1%Pt/20%WO3/40%ZrO2 on various supports; (b) SBA-15; (c) MCM-41; (d) silicagel; and (e) 1%Pt/17%WO3/ZrO2. (B) 1%Pt/WO3/ZrO2/SBA-15 with different WO3and ZrO2contents: (a) 9.1 and 18%; (b) 9.1 and 45%; (c) 20 and40%; (d) 24 and 49%; and (e) 24 and 38%.200C. Judging from the area ratio, one can conclude thatthe number of Lewis acidic sites was greater than that ofBrnsted ones. With the increase in desorption temperature,the intensities of the characteristic peaks of both Lewis andBrnsted acid sites decrease. The strength of Lewis acidic1400 1500 1600 1700 1800BSLS(f)(e)(d)(c)(b)(a) Absorbance (a.u.)Wavenumber (cm-1)Fig. 4. InsituDRIFTspectraofpyridineadsorbedon1%Pt/20%WO3/40%ZrO2/SBA-15. Spectrarecordedafter evacuatingat (a) 25C; (b)100C; (c) 200C; (d) 300C; (e) 400C and (f) 500C.sites was stronger than that of Brnsted ones since the for-mer peak still retained some intensity up to 500C whereasthat of the latter was completely removed. The preservationof the pyridine peaks up to 500C implies that the strengthof theLewisacidsitesonthissupportedWZsampleisrelatively strong.3.2. Catalytic studiesThe catalytic properties of SBA-15 supported WO3/ZrO2were investigated in the isomerization of n-pentane toiso-pentane. Introducingasmall amount ofPt (0.5 wt.%)onto the WZ catalyst was found to cause a great increasein both the conversion of n-pentane and selectivity toiso-pentane. Table 2shows the effect of Pt loadingonn-pentaneisomerizationover20%WO3/40%ZrO2/SBA-15catalyst. On the Pt-free sample, only trace activity was ob-served. The catalytic activity, however, increases markedlywiththeloadingof0.5%Pt. AsthePtloadingincreasedTable 2Isomerizationofn-pentaneoverPt/20%WO3/40%ZrO2/SBA-15catalystwith different Pt loadingsPt loading(wt.%)n-Pentaneconversion (%)iso-Pentaneselectivity (%)0 0.8 850.5 54 991 58 992 61 97Data obtained after 6 h on feed.216 T.Lietal. / Applied Catalysis A: General 261 (2004) 211219700 750 800 850203040506070Conversion of n-pentane (%)Calcination temperature (oC)Fig. 5. Catalyticactivitiesof1%Pt/20%WO3/40%ZrO2/SBA-15catalystwith the supported mixed oxide calcined at various temperatures.from 0.5 to 2%, the n-pentane conversion increased from 54to 61% while theiso-pentane selectivity remained around9799%. Themainby-products werefromthecrackingreaction, including methane, ethane, propane, n-butane andiso-butane. Because the change in catalytic activities for thecatalysts with Pt loading higher than 1 wt.% was negligible,1%Pt loadingwasusedfor most of thecatalyticstudieshereafter.Fig. 5 shows the dependence of n-pentane conversion onthe calcination temperature of 20%WO3/40%ZrO2/SBA-15sample. Anoptimalactivitywasobservedonthecatalystcalcined at 800C. This result is consistent with those re-ported by other researchers on the unsupported WZ catalystsin isomerization of light parafn (C4C6) [2327]. On theother hand, the iso-pentane selectivity is independent of thecalcination temperature and always has a value over 95%.Table 3 demonstrates the effects of WZ mixed oxide andthe SBA-15 support. It can be seen that both unsupportedandSBA-15supportedWZcatalysts without Pt showednearly no catalytic activity in n-pentane isomerization. Byintroducing1 wt.%Pt, veryhighcatalyticactivitieswereobtained on both samples. Obviously, Pt is one of the maincomponents contributing to the catalytic activity. However,if only Pt is loaded onto SBA-15 without W/Zr mixed ox-Table 3Isomerization ofn-pentane over various catalystsCatalyst n-Pentaneconversion (%)iso-Pentaneselectivity (%)17%WO3/ZrO21.5 831%Pt/17%WO3/ZrO252 9820%WO3/40%ZrO2/SBA-15 0.8 851%Pt/20%WO3/40%ZrO2/SBA-15 60 981%Pt/SBA-15 1.4 412%Pt/SBA-15 + 17%WO3/ZrO247 98Data obtained after 6 h on feed.ide, n-pentane conversion was extremely low and crackingproductswerethemainproducts. Therefore, W/Zrmixedoxide is also needed in order to obtain high catalytic activityin n-pentane isomerization. In order to understand the con-tribution of Pt and WZ in acidity, a sample of 2%Pt/SBA-15and 17% WO3/ZrO2 was mechanically mixed in 1:1 ratio,which contains the same amount of Pt and ZrO2 as that ofsample1%Pt/20%WO3/40%ZrO2/SBA-15; this was usedas the catalyst. This sample gave similar catalytic activity tothat of the unsupported 1%Pt/WZ, but was less active thanthe SBA-15 supported catalyst. This means that Pt need nothave close contact with WZ in order to catalyze n-pentaneisomerization. Tomishigeetal. [28]studiedn-butaneiso-merizationoverPt-promotedsulfatedzirconia. Theyalsofoundthat theproximityof Pt andtheacidsiteis notimportant.Noble metals are considered very efcient in activation ofhydrogen and suppressing the coke formation in hydrocar-bon treatment processes over dual function catalysts [29,30].Thenoblemetalsareproposedtoactivatehydrogenandspillover hydrogen to the acidic sites. Without noble metals,the catalyst usually loses its activity due to severe coking onthe acidic sites. Under our reaction conditions, all the usedcatalysts were light grey in color. The TPO(temperature pro-grammed oxidation) results showed that less than 0.5 wt.%coke was formed on all the catalysts after n-pentane isomer-ization reaction for 20 h. Moreover, the Pt-free samples didnot contain much more coke. Therefore, the very low activ-ity on Pt-free catalysts was not due to coke formation andfouling on the active sites.ManystudiesonalkaneisomerizationcatalyzedbyPt-promoted SZ and WZ have suggested a reaction mechanisminvolvingspillover hydrogen[31,32]. Thespilt-over Hspecies are supplied to the carbenium reaction intermediate,which then desorbs as alkane product, and the spilt-over H+species generate Brnsted acidic sites [33]. The hydrogenspillover mechanism is different from the dehydrogenation-hydrogenation mechanism in that carbenium ion instead ofalkene is the intermediate [31]. Since coke, which is eas-ily formed from alkene, is negligible and the proximity ofPt andtheacidsiteisnot important, thespillovermech-anismis moreapplicabletoour system. TheNH3-TPDresults suggestedthat introducingPt ontothesupportedand unsupported WZ samples did not increase the numberT.Lietal. / Applied Catalysis A: General 261 (2004) 211219 2170 5 10 15 2001020304050607080(e)(d)(c)(b)(a)TOS (h)Conversion of n-pentane (%)Fig. 6. Conversion of n-pentane versus time-on-stream over different ca-talysts promoted with 1%Pt: (a) 20%WO3/40%ZrO2/SBA-15; (b)17%WO3/ZrO2; (c) 50%S-ZrO2/MCM-41; (d) 20%WO3/40%ZrO2/MCM-41 and (e) 20%WO3/40%ZrO2/SiO2.of acid sites nor the acid strength. When the Pt-promotedWZsampleswerepretreatedwithH2owat areactiontemperature of 250C for 2 h before the NH3-TPD experi-ment, the TPD prole showed no noticeable changes either.Theseresultsimplythat theacidityofthecatalyst isnotenhanced by the presence of Pt nor in combination of H2atmosphere. In other words, the NH3-TPD experiment didnot detect any stronger acidic sites generated by the spilt-over H+ species. It is therefore suspected that the carbeniumintermediate, whichisgeneratedduringtheisomerizationreaction and proposed to accept spilt-over Hspecies,should induce the formation of strong Brnsted acidic sitesbyreleasingthespilt-overH+speciesfromtheadsorbedhydrogen.Fig. 6 compares the catalytic activities in n-pentane iso-merization of Pt-promoted WZ mixed oxide supported onSBA-15, MCM-41 and silica gel. SBA-15 support gave thehighest n-pentane conversion. In contrast, MCM-41 and sil-ica gel supports gave relatively low conversions. The BETsurfaceareas of thesethreesupportedcatalysts wereinacloserangeof140160 m2/g, buttheBJHporediame-ter of SBA-15 catalyst was ca. 6 nm and much larger thanthe2 nmofMCM-41. Moreover, theporevolumeoftheMCM-41 supported WZ catalyst was only about half of thatoftheSBA-15catalyst. Therefore, theeasierdiffusionofthe gaseous reactants and products in SBA-15 probably ac-countsforitshigheractivity. Ontheotherhand, thelowcatalytic activity of silica gel-supported catalyst is probablydue to the low content of tetragonal ZrO2 crystalline phase.20 30 40 50 60 70WO3(m)(m)ZrO2(t)(c)(b)(a)Relative Intensity2 theta (degree)Fig. 7. XRD patterns of 1%Pt/20%WO3/40%ZrO2 supported on differentsilica supports: (a) MCM-41; (b) SBA-15 and (c) silica gel.Fig. 7 compares the XRD patterns of WZ catalysts on thethreesilicasupports. SBA-15supportedcatalyst containsprimarily tetragonal ZrO2 phase of highly crystalline form,while silica gel-supported catalyst contains relatively smallamount of crystalline ZrO2 with both tetragonal and mono-clinic phases. These results are consistent with the proposalsin many publications that tetragonal ZrO2 phase is the activephase for acid-catalyzed reactions. The MCM-41 supportedsulfatedzirconiagaveveryhighinitialactivity, butitde-cayed rapidly with time-on stream. Thus the acidic strengthof supported sulfated zirconia is much stronger than that ofW/Zr mixed oxide; this leads to severe coking as the reac-tion prolongs.The unsupported WZ gave conversion similar to that overSBA-15 supported WZ catalyst. However, the unsupportedcatalyst decays withtime-on-streamfaster thanthesup-portedones. Theseresultsimplythat thesupportsplayarole in stabilizing the catalytic active centers. On the otherhand, becauseNH3-TPDexperimentsshowedthatunsup-ported WZ has very low acid amount (Fig. 3A), the Brnstedacidic sites generated by the spilt-over H+species duringthe isomerization reaction should play an important role incatalyzing the reaction.Fig. 8 compares the catalytic performance of 1%Pt/WZ/SBA-15sampleswithdifferent WO3andZrO2contents.It can be seen that the catalysts containing WO3 =9.1%218 T.Lietal. / Applied Catalysis A: General 261 (2004) 2112190 1 2 3 4 5 60102030405060700 1 2 3 4 5 6010203040506070(A)(e)(d)(c)(b)(a)TOS (h)Conversion of n-pentane (%)(B)(d)(c)(b)(a)Fig. 8. Conversionof n-pentaneversustime-on-streamover 1%Pt/WZ/SBA-15with(A) 0.50weight ratioof WO3/ZrO2anddifferent ZrO2/SBA-15weight ratios of (a) 1.0; (b) 1.86; (c) 0.54; (d) 0.25 and (e) 0.10. (B) 1.0 weight ratio of ZrO2/SBA-15 and different WO3/ZrO2weight ratios (a) 0.50;(b) 0.65; (c) 0.35 and (d) 0.20.or ZrO2=18%haveextremelylowcatalyticactivitiesinn-pentaneisomerization(prolesdandeinFig. 8A).Theconversions of n-pentanewerelower than2%, andiso-pentane selectivities were lower than 50%. Similar lowconversion (6.9%) andiso-pentane selectivity (64%) wereseen on the catalyst with 9.1%WO3but higher ZrO2con-tent of45%(proledinFig. 8B). Thecatalyticactivityincreases drastically with the increases in WO3and ZrO2loadings. However, anoptimal catalyticactivitywasob-servedonthesupportedcatalyst withWO3/ZrO2weightratioof0.50andZrO2/SBA-15weightratioof1.0(sam-ple 20%WO3/40%ZrO2/SBA-15). Further increase in eitherWO3or ZrO2loading caused some decrease inn-pentaneconversion. Since the over-loaded samples have lower sur-face areas than sample 20%WO3/40%ZrO2/SBA-15, the de-crease in catalytic activity is probably due to the blockingof the mesoporous pores of SBA-15 by too large amountsof WZ. In contrast, other samples of lower oxide loadingthan sample 20%WO3/40%ZrO2/SBA-15 have higher sur-face areas. The lower catalytic activities of these catalystsimply that factors other than surface area should play thekeyrole. It is noticedthat thecatalysts inproles b, canddofFig. 8BhavethesameWO3contents(9.1, 15,and24%, respectively)asthoseinprolesb, canddofFig. 8A, but different ZrO2 loadings. In comparison of thecatalysts of the same WO3loading, the catalytic activitiesin Fig. 8B are all higher than those in Fig. 8A. The ZrO2loadingsinthecatalystsinFig. 8Bareinacloserangeof 3845%, while those in Fig. 8A vary from 8.6 to 49%.Based on these results, we conclude that the SBA-15 sup-ported WZ must meet the requirement of specic ZrO2 size(ZrO2/SBA-15about 1)andWO3/ZrO2ratioaround0.5,inordertogenerateefcient catalyticsitesforn-pentaneisomerization.The1%Pt/20%WO3/40%ZrO2/SBA-15catalyst usedinthe n-pentane isomerization was recycled three times in or-der to examine the deactivation and regeneration behavior ofthe catalyst. The results of these experiments are shown inFig. 9. The catalyst after 20 h reaction at 250C was able torecover its activity completely after it was treated in owingair at 450C for 3 h.0 5 10 15 20405060708090100405060708090100Conversion ofn-pentane (%)TOS (h)Selectivity toiso-pentane (%) run 1 run 2 run 3Fig. 9. Activity proles of 1%Pt/20%WO3/40%ZrO2/SBA-15 catalyst forthree repeated runs.T.Lietal. / Applied Catalysis A: General 261 (2004) 211219 2194. ConclusionsW/Zr mixed oxides supported on mesoporous silica weresuccessfully prepared. For the mesoporous silica, SBA-15was found to retain the ordered porous structure better thanMCM-41 after loading with W/Zr mixed oxide. Tungstatedzirconia was mainly dispersed inside the mesoporous chan-nels of SBA-15, and the surface area and pore volume de-creasedwiththeloading. BothLewisandBrnstedacidsites are present on these samples, and the acidity of Lewisacid sites was stronger than that of the Brnsted ones. TheSBA-15supportedandunsupportedWO3/ZrO2materialspromoted with Pt in hydrogen environment were highly ef-cient incatalyzingn-pentaneisomerizationwithaveryhighselectivityof iso-pentane. Becausethecatalyticac-tivitydoesnotdirectlycorrelatetotheacidamount, itisproposedthat theBrnstedacidicsitesgeneratedbythespilt-over H+ species should play an important role in cat-alyzingtheisomerizationreaction. Themesoporoussilicasupports play an important role in stabilizing the WZ catalystsince the supported catalysts decayed slower than the unsup-ported one. Moreover, the catalytic activity of the SBA-15supported WZ could be completely recovered by regenera-tion. The tungstated zirconia needs to meet the requirementsof specic size and W/Zr ratio to become the catalytic ac-tive sites for n-pentane isomerization. The optimal activitywas observed on SBA-15 supported with 1%Pt/20%WO3/40%ZrO2.AcknowledgementsWearegrateful for thenancial supportsfromChinaPetroleum Corporation and the National Science Council ofTaiwan.References[1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Back,Nature 359 (1992) 710.[2] D.-Y. Zhao, J.-L. Feng, Q.-S. Huo, N. Melosh, G.H. Fredrickson,B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548.[3] A. Corma, V. Fornes, M.T. Navarro, J. Perez-Pariente, J. Catal. 148(1994) 569.[4] W. VanRhijn, D. DeVos, W. Bossert, J. Bullen, B. Wouters, P.Grobet, P. 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