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Catalysis Communications 7 (2006) 860–866

Propane dehydrogenation on PtSn/ZSM-5 catalyst:Effect of tin as a promoter

Yiwei Zhang a, Yuming Zhou a,*, Anding Qiu b,c, Yu Wang b,c, Yi Xu b, Peicheng Wu b

a Department of Chemistry and Chemical Engineering, Southeast University, 2# Si Pai Lou, Nanjing 210096, PR Chinab Office of Catalytic Enterprise, Nanjing Linear Alkyl Benzene Plant, Nanjing 210046, PR Chinac School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China

Received 14 November 2005; received in revised form 26 February 2006; accepted 24 March 2006Available online 25 April 2006

Abstract

PtSn/ZSM-5 catalyst with different amounts of Sn was prepared for propane dehydrogenation. It was found that the addition of Snnot only had ‘‘geometric effect’’, thus decreasing the size of the surface Pt ensembles, but also changed the interfacial character betweenmetal and support. The presence of Sn could facilitate the transfer of the carbon deposits from the active sites to the carrier, which inconsequence improved the catalytic stability. Suitable concentration of Sn on PtSn/ZSM-5 catalyst was preferable for the reaction. Withthe continuous addition of Sn, more amounts of Sn0 species could be formed, which was disadvantageous to the reaction. Compared toPtSn/c-Al2O3, the capacity of the catalyst that supported on ZSM-5 zeolite to accommodate the coke was much better. The possiblereason may attribute to the larger surface area and the particular character of the channels of ZSM-5 zeolite.� 2006 Elsevier B.V. All rights reserved.

Keywords: ZSM-5; Sn; Pt; Propane dehydrogenation; Propene

1. Introduction

The study on the propane dehydrogenation process hasbeen receiving significant attention due to the growingdemand for propene. Indeed, propene is an indispensableraw material for numerous products such as polypropene,acrolein, polyacrylonitrile, and acrylic acid. However, thereaction of propane dehydrogenation is an endothermicprocess that requires a relatively high temperature toobtain a high yield of propene. The high reaction tempera-ture favors thermal cracking reactions to coke and lightalkanes that lead to a decrease in product yield and anincrease in catalyst deactivation. Many efforts have beenmade to enhance the propene yield by developing new cat-alysts with high-activity, high-stability and high-selectivity.Among these, chromium-based catalysts and platinum-supported catalysts have been reported widely [1]. How-

1566-7367/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.catcom.2006.03.016

* Corresponding author. Tel.: +86 25 83791274; fax: +86 25 83793171.E-mail address: fchem@seu.edu.cn (Y. Zhou).

ever, both of them have relatively poor reaction stabilitydue to the carbon deposits.

Recently, the use of zeolite as a catalyst in petrochemicalprocesses has been extensively investigated both experi-mentally [2,3] and theoretically [4]. These works includethe study on the reaction of aromatization [5], hydroiso-merization [6] and disproportionation [7]. Zeolite ZSM-5possesses a well-defined 10-membered ring channel systemthat can hinder the formation of large hydrocarbon mole-cules, thus preventing the so-called coke deposits andimproving the catalytic stability. On the other hand, thereis growing interest in developing platinum-impregnatedHZSM-5 zeolites for the catalytic dehydrogenation of pro-pane. For example, Grasselli and his co-workers [8,9] used0.7 wt% Pt–Sn–ZSM-5 as a propane dehydrogenation cat-alyst in DH! SHC! DH micro-reactor process andachieved a near-equilibrium propene yield of 25% at550 �C. But this catalyst is not the main research subjectin this paper and the effect of Sn has not been discussed.In previous studies, the promoter of Sn was known to

Table 1Results from XRF analysis

Catalysts Pt content (%, w/w) Sn content (%, w/w)

Pt/ZSM-5 0.42 –Sn/ZSM-5 – 0.68PtSn(01)/ZSM-5 0.38 0.83PtSn(02)/ZSM 0.43 1.41PtSn(03)/ZSM 0.41 2.36PtSn(04)/ZSM 0.44 3.42

Y. Zhang et al. / Catalysis Communications 7 (2006) 860–866 861

increase the lifetime of Pt-based catalysts due to thereduced deactivation by coking [10,11]. Larsson et al. [12]investigated the effect of reaction conditions and time onstream on the coke formed for Pt/Al2O3 and Pt–Sn/Al2O3 catalysts and pointed out that the bimetallic Pt–Sncatalyst was more resistant to deactivation by coke forma-tion, than the monometallic Pt. Barias et al. [13] systemat-ically studied Pt and Pt–Sn catalysts supported on c-Al2O3

and SiO2 and found that on c-Al2O3 tin interacted with thesupport and was stabilized in an oxidation state, while onSiO2 the Sn was more readily reduced and alloy formationwas possible. Therefore, the bimetallic Pt–Sn catalyst sup-ported on different carrier can result in the variant interac-tions between Pt and Sn, which may affect on the catalyticperformance significantly.

In this present work, Pt/ZSM-5 and PtSn/ZSM-5 cata-lysts were prepared by impregnation of powder HZSM-5zeolite and used for propane dehydrogenation process,respectively. The catalytic properties were intensively stud-ied, including the interactions between Pt and Sn. In a con-trast, traditional PtSn/c-Al2O3 catalyst was also studied.This can provide us with important information to under-stand the effect of Sn on catalytic properties of ZSM-5 sup-ported Pt–Sn catalyst and the roles of ZSM-5 channels andacid sites during the reaction.

2. Experimental

2.1. Catalysts preparation

The monometallic catalysts were prepared at 80 �C byimpregnation of the powder HZSM-5 zeolite (Si/Al = 120and specific surface area 351.2 m2/g) with aqueous solu-tions of 0.033 M H2PtCl6 or 0.153 SnCl4. Afterward, theprepared samples were dried at 80 �C for 3 h. The bimetal-lic PtSn/ZSM-5 catalyst was prepared using co-impregna-tion method, followed by drying. To obtain larger andmore resistant particles, the prepared catalysts were fullyagglomerated with 5.0 wt% alumina during the process ofpelletization. After being completely dried, the catalystswere calcined at 500 �C for 4 h, then dechlorinated at500 �C for 4 h in air containing steam. Prior to the reactiontest, all of the catalysts were reduced with pure H2 at500 �C for 8 h.

PtSn/c-Al2O3 catalyst was also prepared by co-impreg-nation of the support (BET surface area 182 m2/g). Therewas no the process of pelletization and the following proce-dures were the same as those of the above mentioned cata-lysts. The metal content of Pt and Sn was the same asPtSn(01)/ZSM-5 catalyst.

2.2. Catalysts characterization

The metallic contents were obtained by X-ray fluores-cence (XRF) measurements on a SWITZERLANDARL9800 XRF. The corresponding designation of the dif-ferent catalysts is shown in Table 1.

Catalyst BET surface area was measured using aMicromeritics ASAP 2000 adsorptive and desorptive appa-ratus. The samples were evacuated under a vacuum of5 · 10�3 Torr at 350 �C for 15 h. Specific total surface areaswere calculated using the BET equation.

The platinum dispersion was determined from chemi-sorption measurements. This experiment was carried outusing the dynamic-pulse technique with an argon(99.99%) flow of 50 mL/min and pulses of hydrogen. Theexperiment process was the same as reported by Doradoet al. [6], except that the sample reduction temperaturewas 500 �C and the temperature of the argon gas forremoving the hydrogen was 40 �C higher than the reduc-tion temperature.

X-ray diffraction (XRD) patterns of the prepared cata-lysts were obtained on a XD-3A X-ray powder diffractom-eter coupled to a copper anode tube. The Ka radiation wasselected with a diffracted beam monochromator. An angu-lar range 2h from 5� to 40� was recorded using step scan-ning and long counting times to determine the positionsof the ZSM-5 peaks.

Hydrogen TPD of the different samples was performedon a conventional TPD apparatus for 0.15 g of the sampleplaced in a quartz reactor. The catalyst was reduced inflowing H2 at 500 �C for 1 h and cooled to room tempera-ture in H2. After the base line became stable by flowingnitrogen gas for 30 min, desorptions were programmed at10 �C/min to 700 �C in flowing N2, the high temperature(HT)–TPD profile was obtained. After this experiment,the catalyst was cooled to room temperature in N2,exposed to H2 for 0.5 h, and then the sample was purgedby flowing N2 for 0.5 h. After the base line became stable,desorptions were programmed at 10 �C/min to 600 �C inflowing N2 to obtain the low temperature (LT)–TPDprofile.

Temperature-programmed reduction (TPR) was mea-sured with the same apparatus as that of H2–TPD. Priorto the TPR experiments, the catalysts were dried in flowingN2 at 400 �C for 1 h. Five percentage of H2/N2 was used asthe reducing gas at a flow rate of 40 mL/min. The rate oftemperature rise in the TPR experiment was 10 �C/minup to 650 �C.

Temperature-programmed oxidation (TPO) was mea-sured with the same apparatus as used for TPR. About0.05 g of sample was placed in a quartz reactor and thenheated up to 700 �C at a rate of 10 �C/min in a 5% O2/He mixture (30 mL/min).

5 10 15 20 25 30 35 40

PtSn(04)/ZSM-5

PtSn(03)/ZSM-5

PtSn(02)/ZSM-5

PtSn(01)/ZSM-5

Pt/ZSM-5

ZSM-5

2 Theta (deg)

Fig. 1. XRD patterns of ZSM-5 and different catalysts.

862 Y. Zhang et al. / Catalysis Communications 7 (2006) 860–866

The changes in catalyst weight were measured in air flow(30 mL/min) with a LCT thermogravimetric analyzer (Bei-jing optical instrument factory, PR China) from room tem-perature to 700 �C at a rate of 20 �C /min. 0.02 g of catalystwas set in the analyzer.

2.3. Reaction test

Propane dehydrogenation was carried out in a quartztubular micro-reactor. Reaction conditions for propanedehydrogenation were 590 �C for reaction temperature,0.1 Mpa pressure, 0.25 H2/C3 molar ratio and the weighthourly space velocity (WHSV) of propane of 3.0 h�1. Thereaction products were analyzed with an online GC–14Cgas chromatograph (Shimadzu, Japan).

3. Results and discussion

3.1. Characterization of catalysts

Table 2 lists the basic characterization data of the differ-ent catalysts. The surface area tends to decrease with theincrease of Sn loading. It is proposed that in this case, moreamounts of Sn can be located on the external surface andthe vicinity of micro-pore mouths of ZSM-5 zeolite. Inaddition, the values of platinum dispersion (DH2

) increaseeffectively with the suitable addition of Sn, while the oppo-site effect is observed when the Sn loading is excess, sug-gesting that the amount of Sn can have an obviousinfluence on the surface character of platinum particles.

X-ray diffraction patterns of ZSM-5 zeolite and the dif-ferent catalysts are shown in Fig. 1. The XRD of the zeolitedisplays the same peaks as pure ZSM-5 [14], indicating thatthe impregnation does not destroy the original structure ofZSM-5 zeolite. It is known that the average diameter of Ptmetal particles is about 1.4–2.0 nm [15], which is too largeto enter into the zeolite main channels, so we suggest thatthe platinum particles are located mainly on the externalsurface of the zeolite. Furthermore, as shown in Fig. 1,no new PtO2 species has been detected in the different cat-alysts, indicating that the platinum species are well dis-persed on the external surface of the catalyst. It isinteresting to note that the intensities of correspondingpeaks of angle 2h = 23�–24� decrease slowly with theincrease of Sn loading, indicating that the addition of Sndegrades the crystallinity of ZSM-5 zeolite [16].

Table 2Characterization of data for the different catalysts

Catalysts SBET (m2/g) DH2(%)a Carbon amountb (%)

Pt/ZSM-5 342.6 31.7 5.3PtSn(01)/ZSM-5 339.8 44.6 7.6PtSn(02)/ZSM-5 334.1 48.3 5.4PtSn(03)/ZSM-5 332.7 46.1 4.7PtSn(04)/ZSM-5 332.4 36.4 3.8

a Calculated from hydrogen chemisorption (HC) experiment.b Experimental value calculated from thermogravimetric (TG) analysis.

It has been proposed by Lin et al. [17] that two kinds ofactive Pt species exist on the external surface of Pt-basedcatalyst. One dominates the low temperature H2 adsorp-tion, while the other can adsorb more H2 at high temper-ature. So H2–TPD profiles under the different conditionscan be obtained by changing the hydrogen adsorptiontemperature. When the hydrogen adsorption is carriedout at room temperature, very small amount of H2 isadsorbed by the high temperature active Pt sites. How-ever, when this adsorption is carried out from 500 �C tothe room temperature, both of the two active Pt sitescan adsorb the hydrogen so that the complete H2–TPDprofile may be obtained.

As shown in the LT–TPD profiles of the different sam-ples (Fig. 2(a)), there are broad overlapping hydrogendesorption peaks at about 130 �C (Peak 1) and 175 �C(Peak 2), together with some trailing peaks at high temper-ature for Pt/ZSM-5 catalyst. As reported [18], Peak 1 isattributed to reversibly chemisorbed hydrogen while Peak2 can be speculated to be due to interfacial hydrogen,i.e., a layer of chemisorbed hydrogen between the platinumparticles and the support. The addition of Sn decreases thepeak area (Peak 1 and 2) and this tendency becomes moreevident with the increase of Sn loading. In addition, noclear change at the low hydrogen desorption temperatureis observed for all the catalysts, indicating that the bondstrength of Pt–H is almost identical for each sample. Inother words, the addition of Sn has little impact on theintensity of Pt–H bond. Based on the results that obtainedpreviously by Passos et al. [19] and Balakrishnan and Sch-wank [20], we suggest that the above reduced H2 desorp-tion by Sn is most probably attributed to the ‘‘geometriceffect’’, that is to say, the promoter of Sn can divide the sur-face platinum ensembles into small clusters so as to lessenthe size of the surface Pt ensembles and improve the metal-lic Pt dispersion. This is consistent with the results of themetal dispersion measurements (Table 2), similar effect isalso observed by Barias et al. [13] and Hill et al. [21]. As

100 200 300 400 500 600

Peak 2Peak 1

(a) PtSn(04)/ZSM-5

PtSn(03)/ZSM-5

PtSn(02)/ZSM-5

PtSn(01)/ZSM-5

Pt/ZSM-5

Temperature (˚C)100 200 300 400 500 600 700

(b)

Peak 4Peak 3Peak 2Peak 1

PtSn(04)/ZSM-5

PtSn(03)/ZSM-5

PtSn(02)/ZSM-5

PtSn(01)/ZSM-5

Pt/ZSM-5

Temperature (˚C)

Fig. 2. H2-TPD profiles of the different catalysts: (a) LT–TPD spectra and (b) HT–TPD spectra.

100 200 300 400 500 600 700

PtSn(04)/ZSM-5

PtSn(03)/ZSM-5

PtSn(02)/ZSM-5

PtSn(01)/ZSM-5

Sn/ZSM-5

Pt/ZSM-5

ZSM-5

Temperature (˚C)

Fig. 3. TPR profiles of the different samples.

Y. Zhang et al. / Catalysis Communications 7 (2006) 860–866 863

for the reduced metal dispersion when the concentration ofSn is excessive, some other reasons should be existed.

The corresponding HT–TPD profiles of the samples arepresented in Fig. 2(b). Similar to the LT–TPD profile, thereare a hydrogen desorption peak at about 150 �C and somehigh temperature hydrogen desorption peaks (Peak 2, 3and 4, first two peaks are indistinct) for Pt/ZSM-5 catalyst.The hydrogen desorption peak at low temperature of Pt/ZSM-5 catalyst decreases significantly comparing with thatof the LT–TPD, revealing that in this case the capacity ofthis catalyst to adsorb H2 at low temperature becomesmuch weaker. The sequential desorption peaks at high tem-peratures suggest that the surface energy distribution of Pt/ZSM-5 catalyst is not uniform, at least four active sites forhydrogen adsorption exist and every active site has its cor-responding desorptive activation energy [22]. Generallyspeaking, higher hydrogen desorption temperature corre-sponds to higher activation energy and stronger capacityto adsorb H2.

In comparison with Pt/ZSM-5, the peak area at hightemperature of PtSn(01)/ZSM-5 catalyst increases obvi-ously and the corresponding temperature (Peak 2, 3 and4) moves towards the higher temperature, demonstratingthat the capacity to adsorb H2 becomes stronger after theaddition of Sn, and this tendency becomes more apparentfrom PtSn(02)/ZSM-5 to PtSn(04)/ZSM-5. It has beenreported that the hydrogen desorbed at high temperaturesof Pt-based catalyst can be attributed to the following fac-tors [23]: (1) spillover hydrogen, (2) strongly adsorbedhydrogen and (3) platinum–hydrogen species, for example,platinum hydride or subsurface hydrogen. However, in ourexperiments, such large amount of the desorbed hydrogenat high temperatures can not be attributed to the stronglyadsorbed hydrogen and platinum–hydrogen species [23].Consequently, the high temperature desorptions arethought to be spillover hydrogen. According to Sermonand Bond [24], the degree of spillover hydrogen was

concerned with many factors and the M/A (M: the metallicactive sites; A: the accepter of spillover hydrogen) interfacewas the most important one to influence this. Therefore, itis concluded that the addition of Sn changes the interfacialcharacter between metal and support.

TPR profiles of the different samples are shown inFig. 3. There are no obvious reduction peaks for ZSM-5zeolite during the reduction process. Monometallic Pt cat-alyst exhibits two reduction peaks: one with maximumtemperature at 250 �C and the other at 440 �C, which arein good agreement with literature data for Pt/zeolite sys-tems [25]. The latter peak corresponding to a higher reduc-tion temperature indicates that a proportion of Pt speciesstrongly interacts with the zeolite. Sn/ZSM-5 catalyst pre-sents two reduction peaks: one peak around 380 �C andanother broad peak around 560 �C. The former may corre-spond to the partial formation of Sn2+ species from the

80

864 Y. Zhang et al. / Catalysis Communications 7 (2006) 860–866

reduction of Sn4+, while the latter can be associated withthe reduction of Sn2+ species to form Sn0 species [10].

Comparing bimetallic catalyst PtSn(01)/ZSM-5 to themonometallic catalyst, it is noteworthy that the reductiontemperature of Pt particles moves towards higher region,indicating that some interactions between platinum andtin exist. However, with the increase of Sn loading, thereduction temperatures of Pt particles moves towardslower values and the peak area corresponding to the reduc-tion of Sn species increases effectively. This becomes moreapparent for PtSn(04)/ZSM-5 catalyst, suggesting that thereduction of Sn species becomes more easily. That is to say,in this case, more amounts of Sn0 species have been pro-duced. These results are in agreement with Cortright andDumesic, who reported that a proportion of the Sn inter-acted with Pt to form Pt/Sn alloy particles in reduced Pt/Sn/K–L catalysts with excess amount of Sn [26]. Generally,keeping the oxidized state of Sn species on bimetallic Pt–Sncatalysts is very important for the reaction [27]. The forma-tion of alloy results in the permanent deactivation of thecatalysts and this is the possible reason for the reducedmetal dispersion with the excessive loading of Sn.

After reaction for 14 h, the TPO profiles of the differentcatalysts are shown in Fig. 4. There exist two successivepeaks that represent two different carbon deposits on thesurface of Pt/ZSM-5 catalyst. According to the previousresults for Pt/Al2O3 and PtSn/Al2O3 [28], we suggest thatonly a small part of the coke is located on the metal, whilea larger part stays close to the metal. To obtain some infor-mation about the location of the carbon deposits, partialoxidation experiments are undertaken with the oxidationtemperature kept to the point corresponding to the topof the first peak. Partial temperature-programmed oxida-tion of Pt/ZSM-5 catalyst (not shown) indicate that,although the carbon deposits corresponding to the secondpeak is intact, the activity of the catalyst is mostly restoredafter partial oxidation, suggesting that the second peak haslittle impact on the catalytic activity. Since the platinum

200 300 400 500 600 700

PtSn(04)/ZSM-5

PtSn(03)/ZSM-5

PtSn(02)/ZSM-5

PtSn(01)/ZSM-5

Pt/ZSM-5

Temperature (˚C)

Fig. 4. TPO profiles of the different catalysts.

particles are located on the external surface of the catalyst,the carbon deposits corresponding to the first peak aremainly the ones that cover the active metal, while the sec-ond peak represents the deposits that are located on theexternal surface of the ZSM-5 zeolite.

On the other hand, the addition of Sn increases the arearatio of the carbon deposits on the external surface of thecatalyst to the ones that cover the active metal, whichimplies that the presence of Sn facilitates the transfer ofthe carbon deposits from the metal to the support. Thisis because the promoter of Sn decreases the size of the sur-face Pt ensembles so that the hydrocarbon cannot readilyform multiple carbon–metal bonds, the adsorbed speciesare more mobile and can more easily migrate to the sup-port [21]. As a result, the catalytic stability is improvedobviously, even though the carbon amount is relativelylarge. With the increase of Sn loading, the TPO profilesof the samples present only one type of coke and the car-bon amounts decrease continuously (Table 2). This effectis supported by the change in the ratio of the peak heightsin the TPO spectra with the continuous addition of Sn.

3.2. Catalytic properties of PtSn/ZSM-5 catalysts

In order to compare the catalytic properties as a resultof the different carrier and discuss the possible roles ofZSM-5 channels and acid sites during the process of pro-pane dehydrogenation, the catalytic behavior of PtSn/c-Al2O3 and PtSn(01)/ZSM-5 catalyst is then investigated.Considering that the metal dispersion of PtSn/c-Al2O3

(38.2%) is lower than that of PtSn(01)/ZSM-5 (Table 2)and the definite content of alumina binder exists in theZSM-5 supported catalyst, we should first change theweight of the corresponding samples to ensure the reactionto be carried out at the same space velocity. Fig. 5 shows

22 24 26 28 30 32 34 3660

65

70

75

Prop

ene

sele

ctiv

ity (

%)

Propane conversion (%)

PtSn/ -Alγ2O

3

PtSn(01)/ZSM-5

Fig. 5. Propene selectivity as a function of propane conversion fordifferent catalysts. Reaction conditions: 590 �C, H2/C3 = 0.25 (molarratio), WHSV of propane is 3.0 h�1.

Y. Zhang et al. / Catalysis Communications 7 (2006) 860–866 865

propene selectivity vs. propane conversion for the differentcatalysts. For all samples, the selectivity decreases gradu-ally with increasing propane conversion and the lower pro-pene selectivity is obtained for PtSn(01)/ZSM-5 catalyst.To explain this finding, it should be noted that on bimetal-lic Pt–Sn catalyst platinum is the only active metal and thepropene is only formed on the metal by dehydrogenation;the main cracking product (ethene) is mainly formed fromcracking on the carrier and the ethane is formed by hydrog-enolysis of propane and by hydrogenation of ethene, withboth reactions taking place on the metal [12]. Besides, thedehydrogenation of propane is assumed to proceedthrough carbonium-ion intermediates [29] and the higheracid sites generally promote the subsequent cracking reac-tion of the initially formed Cþ3 carbenium ions [30].According to this mechanism, the higher acidity of ZSM-5 zeolite is the main factor that is responsible for the lowerselectivity to propene. Nevertheless, it is interesting to findthat the reaction stability of PtSn(01)/ZSM-5 is better thanPtSn/c-Al2O3, although the coke amount of the former cat-alyst (5.9%) is much larger than the latter one (4.5%) afterreaction for 8 h. These results demonstrate that the capac-ity of ZSM-5 supported catalyst to accommodate the coke

0 2 4 6 8 10 12 14

20

24

28

32

36

40

Prop

ane

conv

ersi

on (

%)

Time on stream (h)

Pt/ZSM-5PtSn(01)/ZSM-5PtSn(02)/ZSM-5PtSn(03)/ZSM-5PtSn(04)/ZSM-5

Fig. 6. Propane conversion vs. the reaction time over the differentcatalysts. Reaction conditions: m(cat) = 2.0 g, H2/C3 = 0.25 (molar ratio),WHSV of propane is 3.0 h�1.

Table 3The reaction data of the different catalysts after reaction for 10 h

Catalyst Propane conversion(%)

Selectivity (%)

CH4

Pt/ZSM-5 19.15 0.31PtSn(01)/ZSM-5 25.18 0.22PtSn(02)/ZSM-5 29.65 0.19PtSn(03)/ZSM-5 28.27 0.17PtSn(04)/ZSM-5 24.05 0.14

is much better. The possible reason may attribute to thelarger surface area of ZSM-5 zeolite, which is advanta-geous to the dispersive status of metallic Pt, thus reducingthe coverage of catalytically active sites by the cokeformed. Moreover, it is proposed that the possibility aboutthe blockage of the zeolite micro-pore mouths by the car-bon deposits is relatively little, which is related to the shortchannels and more pore mouths of ZSM-5 zeolite, thusconfirms the surface area of the catalyst does not decreasesharply during the reaction.

Fig. 6 shows the effect of Sn loading on catalytic activityof the different catalysts at 590 �C, Pt/ZSM-5 catalyst pre-sents relatively high propane conversion, whereas the accu-mulated coke on the platinum is responsible for thedeactivation of the catalyst. Suitable addition of Sn resultsin an increase in the initial activity of the catalyst and itsreaction stability, whereas the opposite effect is observedwith the excessive of Sn loading. Table 3 lists the reactiondata of the different catalysts after reaction for 10 h. It isclear that the selectivities to various side reaction productsdecrease gradually with the increase of Sn loading. This isbecause the presence of Sn reduces the catalyst acidity [13]and decreases the size of the surface Pt ensembles, therebyinhibiting the side reactions to be carried out and reducingthe formation of highly dehydrogenated surface speciesthat leads to coke and other undesirable products [21].As shown in Table 3, with the increasing of Sn loading, dif-ferent level of propane conversion and propene yield can beobtained and both of them reach the maximum just forPtSn(02)/ZSM-5 catalyst. It should be noted that the Pt/ZSM-5 catalyst is bifunctional and the two active centers(the metal particles and the acid sites) might work collabo-ratively [31]. Given that the promoter of Sn can clearlyinfluence the metallic Pt dispersion and the catalyst acidity,it is reasonable to believe that an optimum ratio betweenthe number of acid sites and the number of metal sites ofPtSn/ZSM-5 catalyst should exist. It is clear that, forPtSn(02)/ZSM-5 catalyst, the matching of acid functionsand metal functions is best. However, when the concentra-tion of Sn continues to increase, more amounts of Sn0 spe-cies can be produced, which may form alloy particles withPt, thus making the catalytic activity drop. In other words,in such circumstances the previous optimal matching isdestroyed, which is disadvantageous to the reaction. Thecatalytic activity of PtSn(04)/ZSM-5 catalyst is even lowerthan that of PtSn(01)/ZSM-5 catalyst.

Propene yield(%)C2H6 C2H4 C3H6

10.33 19.82 69.54 13.328.68 17.21 73.89 18.617.22 14.73 77.86 23.096.61 13.44 79.78 22.555.76 11.97 82.13 19.75

866 Y. Zhang et al. / Catalysis Communications 7 (2006) 860–866

4. Conclusion

Effect of Sn loading on catalytic properties of PtSn/ZSM-5 catalyst for propane dehydrogenation was studied.The addition of Sn not only has ‘‘geometric effect’’, butalso changes the interfacial character between metal andsupport. The role of Sn can facilitate the transfer of the car-bon deposits from metal to the carrier, therefore improvesthe catalytic stability. Suitable addition of Sn is preferablefor the reaction of propane dehydrogenation and the pro-pane conversion and propene yield can jointly reach themaximum for PtSn(02)/ZSM-5 catalyst. With the excessaddition of Sn, more amounts of Sn0 species can be formedand the optimal matching between the acid sites and themetal sites is destroyed, which is disadvantageous to thereaction. In comparison with PtSn/c-Al2O3, PtSn(01)/ZSM-5 catalyst shows lower propene selectivity due to itshigher acidity, however, the capacity to accommodate thecoke of this catalyst is much better. This is related to thelarger surface area and the particular character of channelsof ZSM-5 zeolite.

Acknowledgments

The authors thank the financial support for this studyby Program for New Century Excellent Talents in Univer-sity of China (NCET-04-0482), National Natural ScienceFoundation of China (No. 50377005) and Innovation planto postgraduates in University of Jiangsu Province.

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