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Catalysis Communications 8 (2007) 1009–1016

Effect of calcination temperature on catalytic properties ofPtSnNa/ZSM-5 catalyst for propane dehydrogenation

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

a School of Chemistry and Chemical Engineering, Southeast University, 2# Si Pai Lou, Nanjing 210096, PR Chinab Office of Research and Exploitation, Nanjing Linear Alkyl Benzene Plant, Nanjing 210046, PR China

c College of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China

Received 3 June 2006; received in revised form 28 September 2006; accepted 4 October 2006Available online 14 October 2006

Abstract

The effect of calcination temperature on catalytic performance of PtSnNa/ZSM-5 catalysts for propane dehydrogenation was studied.It was found that when the calcination temperature was in the range of 400–500 �C, structure and acidity of the catalyst did not changeobviously. In contrast to this, with the increase of calcination temperature, the specific surface area and pore volume dramaticallydecreased, while the mean pore diameter increased. Under these conditions, more framework aluminum atoms were removed from tet-rahedral positions, which weakened the interactions between Sn species and carrier. Meantime, the degree of Pt sintering and the destruc-tion of Pt active sites with ‘‘sandwich structure’’ were aggravated, which was disadvantageous to the reaction. When the catalyst wascalcined at 500 �C, the interactions between Pt and Sn were strengthened, thus improving the catalytic stability and reaction selectivityevidently. Moreover, it should be remarked that the phenomenon of slow Pt sintering was not clearly observed even though the reactiontemperature of it was higher than the calcination one.� 2006 Elsevier B.V. All rights reserved.

Keywords: Platinum; ZSM-5; Calcination temperature; Propane dehydrogenation; Propene; Catalyst

1. Introduction

The catalytic dehydrogenation of propane is of increas-ing importance because of the growing demand for pro-pene. However, the reaction of propane dehydrogenationis an endothermic process that requires a relatively hightemperature to obtain a high yield of propene. Therefore,it is the key to develop the catalyst possessing high-activity,high-stability and high-selectivity since the deactivation ofthe catalyst due to coke formation is inevitable. Mean-while, two types of catalysts have been reported for pro-pane dehydrogenation: chromium-based catalysts andplatinum-supported catalysts [1], whereas the reaction sta-bility of these catalysts are relatively low.

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

doi:10.1016/j.catcom.2006.10.004

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

Recently, the use of zeolite as a catalyst in several petro-chemical process has been extensively investigated bothexperimentally [2,3] and theoretically [4]. Zeolite H-ZSM-5 possesses the well-defined ten-membered ring channelsystem, which can hinder the formation of large hydrocar-bon molecules, thus preventing the so-called coke deposi-tion and improving the catalytic stability. Intensive effortshave been made to develop the catalysts supported onZSM-5 zeolite for the reaction of aromatization [5], hydro-isomerization [6] and disproportionation [7]. On the otherhand, there is growing interest in developing platinum-impregnated HZSM-5 zeolite for the catalytic dehydroge-nation of propane. Our previous work [8–11] discussedthe influence of alumina binder and reaction conditionson catalytic performance of PtSnNa/ZSM-5 catalyst forpropane dehydrogenation and pointed out that thePtSnNa/ZSM-5 catalyst with suitable amount of Nashowed relatively high propane conversion and propene

1010 Y. Zhang et al. / Catalysis Communications 8 (2007) 1009–1016

selectivity. Nevertheless, it should be noted that thecatalytic performance would be mainly depended on thecatalyst preparation methods [12,13]. Among these, calci-nation is an important factor to influence the catalyticproperties [14,15]. Yang et al. [16] systematically studiedthe effect of calcination temperature on catalytic perfor-mance of Cu–Zn–Al catalysts and pointed out that thelower temperature could not facilitate the precursor tocomplete decomposition and higher calcination tempera-ture would decline the yield for desired products. Junget al. [17] found that the catalytic activities and propertiesof CuO–CeO2 catalyst for selective oxidation were stronglyinfluenced by the calcination temperature and when thecatalyst was calcined at 700 �C, the best catalytic activitywas obtained due to the most stable state of Cu–Ce–O solidsolution.

In this article, a series of PtSnNa/ZSM-5 catalysts wereprepared and undergone the calcination at different tem-perature, respectively. Considering that the 27Al MASNMR spectra of alumina binder showed a peak at64.8 ppm and a broad peak at 6.8 ppm [9], which mighthave interferential influence to distinguish the removedAl species from framework of ZSM-5 zeolite during theprocess of calcination, no binder was used in our investiga-tion. The aim of the present contribution is to describe theinfluence of calcination temperature on physical structuresand catalytic performances of PtSnNa/ZSM-5 catalysts forpropane dehydrogenation.

2. Experimental

2.1. Preparation of the catalysts

PtSnNa/ZSM-5 catalysts were prepared by sequentialimpregnation method in our laboratory according to a pro-cedure in our previous work [9–11]. The nominal composi-tions of the catalyst were 0.5 wt% for Pt, 1.0 wt% for Sn,and 1.0 wt% for Na, respectively.

Then, the prepared samples were pelletized and crushedto 10–12 mesh size. After totally dried, the catalysts werecalcined in air flow at different temperature (400 �C,500 �C, 600 �C, 700 �C, 800 �C) for 4 h, then dechlorinatedat 500 �C in air containing steam for 4 h and finallyreduced under H2 at 500 �C for 8 h. They were abbreviatedas PtSnNa/ZSM-5(400), PtSnNa/ZSM-5(500), PtSnNa/ZSM-5(600), PtSnNa/ZSM-5(700) and PtSnNa/ZSM-5(800).

2.2. Characterization

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 5–40� was recorded using step scanningand long counting times to determine the positions of theZSM-5 peaks. Solid-state 27Al MAS NMR spectra were

collected in a Brucker DSX-400 spectrometer. The 27AlNMR spectra were obtained at 10.0 kHz, using 15� pulsesand 4 s delay, a total of 2000 pulses being accumulated. N2-adsorption studies were used to examine the porous prop-erties of each sample. The measurements were carried outon Micromeritics ASAP 2010 adsorptive and desorptiveapparatus after the samples were pretreated in vacuum at350 �C for 15 h. NH3-TPD experiment was measured witha conventional TPD apparatus. About 0.15 g of samplewas placed in a quartz reactor and saturated with ammoniaat room temperature. TPD was carried out from 100 �C to550 �C with a heating rate b of 10 �C/min and with helium(30 ml/min) as the carrier gas. Transmission electronmicroscopy studies were conducted using JJEOL (Japan)instruments. Before being placed on the support, the sam-ples were dispersed in excess ethanol by sonicating for30 min. H2 chemisorption experiment was performed onthe apparatus that described for NH3-TPD. The sampleswere reduced in flowing H2 at 500 �C for 2 h, then exposedto Ar at 550 �C for 2 h to remove the hydrogen. Details ofthe experimental procedure have been described elsewhere[18] except that the high temperature for H2 chemisorptionis 300 �C and 450 �C. Temperature-programmed reduction(TPR) was measured with the same apparatus as that of H2

chemisorption. Prior to the TPR experiments, the catalystswere dried in flowing N2 at 400 �C for 1 h. H2/N2 (5%) wasused as the reducing gas at a flow rate of 40 ml/min. Therate of temperature rise in the TPR experiment was10 �C/min up to 700 �C. The changes in catalyst weightwere measured in air flow (30 ml/min) with a LCT thermo-gravimetric analyzer (Beijing optical instrument factory,P.R. China) from room temperature to 700 �C at a rateof 20 �C/min. 0.02 g of catalyst was set in the analyzer.

2.3. Catalytic evaluation

Propane dehydrogenation was carried out in a conven-tional quartz tubular micro-reactor. The catalyst (mass2.0 g) was placed into the center of the reactor. Reactionconditions were as follows: 590 �C for reaction tempera-ture, 0.1 MPa pressure, H2/C3 = 0.25 (molar ratio) andthe propane weight hourly space velocity (WHSV) is3.0 h�1. The reaction products were analyzed with anonline GC-14C gaschromatography.

3. Results and discussion

3.1. Characterization of the catalysts

Fig. 1 shows the XRD patterns of the different samples.The XRD of the ZSM-5 has the representative peaks at 8–9� and 22–24�, corresponding to that given in Ref. [19] forpure ZSM-5. As for the prepared catalysts that calcined atdifferent temperature, the structure of them is similar asthat of the pure ZSM-5 zeolite, suggesting that the struc-ture of zeolite is not destroyed during the process of cata-lyst preparation. According to our previous results [9–11],

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Fig. 1. XRD patterns of the different samples. (a) ZSM-5, (b) PtSnNa/ZSM-5(400), (c) PtSnNa/ZSM-5(500), (d) PtSnNa/ZSM-5(600), (e)PtSnNa/ZSM-5(700), (f) PtSnNa/ZSM-5(800).

Y. Zhang et al. / Catalysis Communications 8 (2007) 1009–1016 1011

some parts of Na+ and Sn4+ could enter the zeolite mainchannels, while Pt particles were located mainly on theexternal surface of the zeolite. After thermal treatmentfrom 400 �C to 500 �C, the change of the correspondingpeaks is not observed clearly, implying that in theseinstances, the zeolite possesses the favorable thermal stabil-ity. However, with the increase of calcination temperature,the intensities of the corresponding peaks of angle 2h = 22–24� decrease gradually, indicating that the crystallinity ofthe catalyst is degraded [20].

Solid-state 27Al MAS NMR is a sensitive tool com-monly used for determining the coordination of Al. Asshown in Fig. 2, ZSM-5 zeolite exhibits a strong signal atabout 54 ppm, which is attributed to four-coordinated alu-minum, and a weak peak assigned to six-coordinated extra-

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Fig. 2. 27Al MAS NMR spectra of different samples. (a) ZSM-5, (b)PtSnNa/ZSM-5(400), (c) PtSnNa/ZSM-5(500), (d) PtSnNa/ZSM-5(600),(e) PtSnNa/ZSM-5(700), (f) PtSnNa/ZSM-5(800).

framework Al (EFAL) at about 0 ppm [21]. After the cat-alysts are calcined at 400 �C and 500 �C, the changes of thecorresponding peaks at about 54 ppm and 0 ppm can beconsidered negligible, suggesting that in these cases, thedegree of dealumination of the catalyst does not increaseobviously. However, with the increase of calcination tem-perature, more decrease in four-coordinated aluminumand increase in six-coordinated aluminum are observed,meaning that more Al species have migrated from frame-work tetrahedral positions and existed as nonstructuralAl species in the catalyst. Moreover, the extent of the dea-lumination is more significantly at higher calcination tem-perature. Therefore, it is concluded that the degree ofdealuminaton would mainly depend on the thermal treat-ment conditions and this is the possible reason for thedegraded crystallinity of the catalysts in Fig. 1. Neverthe-less, after the 27Al MAS NMR spectra of hydrotreated[11] and calcined samples are compared, it becomes evidentthat no shoulder at about 30 ppm is visible after the ther-mal treatment, which can be assigned to pentacoordinatedaluminum atoms [22] or distorted tetrahedrally coordi-nated aluminum atoms in extra-framework species [23].This implies that the degree of dealumination of the cata-lyst by the calcination is inferior to that by the hydrother-mal treatment.

The porous properties obtained from N2 adsorption aregiven in Table 1. When the calcination temperatureincreases from 400 �C to 500 �C, little changes of the struc-tural parameters are found, revealing that the calcinationhas little impact on the physical structure of the catalysts.However, with the increase of calcination temperature,the specific surface area and the pore volume of the cata-lysts decrease obviously, whereas the mean pore diameterincreases. In addition, this tendency becomes more appar-ent under the severe conditions. This is because the calcina-tion at high temperature can promote the conglomerationof the crystallite on the surface of the catalyst, thusdecreasing the BET surface area and increasing the meanpore diameter [24]. Meanwhile, as indicated above, moreamounts of non-framework aluminum exist in the channelsof the zeolite, which in consequence decreases the pore vol-ume of the catalyst.

The calcination at different temperature can have obvi-ous influence on the catalyst acidity. NH3 temperature-pro-grammed desorption (TPD) is an effective technique, whichcan provide information on the amount and strength of

Table 1Textural properties of the different catalysts

Catalyst SBET (m2g�1) V (ml g�1) D (nm)

PtSnNa/ZSM-5(400) 308.0 0.2502 3.25PtSnNa/ZSM-5(500) 307.7 0.2492 3.24PtSnNa/ZSM-5(600) 291.8 0.2451 3.36PtSnNa/ZSM-5(700) 273.7 0.2374 3.47PtSnNa/ZSM-5(800) 253.0 0.2271 3.59

SBET, BET surface area; V, pore volume; D, mean pore diameter.

1012 Y. Zhang et al. / Catalysis Communications 8 (2007) 1009–1016

acid sites. Fig. 3 shows the NH3-TPD profiles of the differ-ent samples. It is clear that there exists two ammoniadesorption peaks for ZSM-5 zeolite: the first desorptionpeak at about 240 �C is assigned to the weak acid sites,whereas the second peak at about 450 �C corresponds tostrong acid sites, in good agreement with observations byFu et al. [5]. In a comparison, for the PtSnNa/ZSM-5 cat-alyst that calcined at 400 �C, a distinct decrease of ammo-nia desorption at low temperature, and almost noammonia desorption at high temperature are observed,

100 200 300 400 500 600

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Temperature (˚C)

Fig. 3. NH3-TPD profiles of the different samples. (a)ZSM-5, (b) PtSnNa/ZSM-5(400), (c) PtSnNa/ZSM-5(500), (d) PtSnNa/ZSM-5(600), (e)PtSnNa/ZSM-5(700), (f) PtSnNa/ZSM-5(800).

Fig. 4. TEM micrographs of the different catalysts. (a) PtSnNa/ZSM-5(400), (b(e) SADP of the agglomerated particles.

demonstrating that the addition of sodium can neutralizethe strong acid sites preferentially [10,11]. When the cata-lyst is calcined at 500 �C, it exhibits the similar TPD pro-file, meaning that the acidic property of the catalyst isalmost changeless. However, with increasing calcinationtemperature, the amount of the weak acid sites decreasescontinuously, and the acidic intensity becomes weakmonotonously, evidenced from the lower ammonia desorp-tion temperature compared to other samples. These can beassigned to the fact that the degree of dealumiantion ismore severe at higher calcination temperature.

The morphology of the various catalysts is characterizedby transmission electron microscopy (Fig. 4). It can be seenthat the catalyst calcined at 400 �C, no agglomerated parti-cles are found on the external surface of ZSM-5 zeolite(Fig. 4a). After the calcination at 500 �C, there is no clearchange in the morphology of the catalyst (Fig. 4b). How-ever, some agglomerated particles appear after the thermaltreatment at 600 �C (Fig. 4c). Worthy of mention is that anobvious increase in the population of the large particlesbecause of their sintering by migration and coalescenceare found after the calcination at 800 �C (Fig. 4d). In orderto clarify the possibility of Pt sintering, selected area elec-tron diffraction pattern (SADP) taken from the agglomer-ated particles is shown in Fig. 4e. By measuring threedifferent groups of ring radius, the diffraction rings canbe indexed as (111), (200) and (220) crystal faces of Pt.Therefore, these particles can be identified as Pt phase bysuch diffraction analysis. These findings suggest that thethermal treatment at different temperature have obviousimpact on the dispersive status of Pt particles on the

) PtSnNa/ZSM-5(500), (c) PtSnNa/ZSM-5(600), (d) PtSnNa/ZSM-5(800),

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c

Y. Zhang et al. / Catalysis Communications 8 (2007) 1009–1016 1013

external surface of the catalyst and the calcination at hightemperature promotes the sintering of Pt particles.

The corresponding H2 uptake of the different samples(Table 2) allows us to confirm this point with great cer-tainty. It is known that H2 chemisorption has beenemployed as one of the effective methods to determinethe metal surface area and dispersion of metallic catalysts.However, the quantitative determination of the surfacearea by H2 adsorption is interfered by the interaction ofPt with Sn oxide. This neighboring effect can be used as aprobe to characterize the properties of different active sitesvia selective adsorption or temperature programmeddesorption of H2 [11]. Following Lin et al. [25], two kindsof active Pt species may exist on the surface of the catalyst,named Pt1 sites and Pt2 sites. Pt1 sites are the sites in whichPt directly anchors on the carrier surface, while Pt2 sitescorresponding to the sites in which Pt anchors Sn oxidesurface with a ‘‘sandwich structure’’. Generally speaking,Pt1 sites are favorable for low temperature H2 adsorption,and responsible for the hydrogenolysis reaction and carbondeposits, while Pt2 sites can adsorb more H2 at high tem-perature and are the main reaction active sites for the dehy-drogenation of propane. From Table 2, it is clear thatPtSnNa/ZSM-5(400) catalyst exhibits a relatively largetotal amount of H2, indicating that the metallic Pt disper-sion of the catalyst is high. Also, the amount of adsorbedH2 at room temperature is large, implying that the propor-tion of Pt1 sites on the external surface of the catalyst is rel-atively big. After the calcination at 500 �C, an obviousdecrease in the amount of H2 that adsorbed at room tem-perature, and increase of H2 adsorption when the experi-ment is carried out at 300 �C and 450 �C are observed,demonstrating that the calcination process promotes thetransformation of Pt active sites, namely decreasing Pt1

sites and increasing Pt2 sites. The possible reason for thisbehavior may be that the interactions between Pt and Snare strengthened after the calcination, thus changing theadsorbed activation energy of H2 [25]. However, concern-ing the catalyst calcined at 600 �C, the total amount ofabsorbed H2 reduces markedly, suggesting that the charac-ter of Pt active sites has changed effectively. According tothe above TEM analysis, this can be attributed to the sin-tering of Pt particles. At the same time, it is interesting tonote that the amounts of absorbed H2 at high temperaturedecrease sharply. To explain these, it should be noted thatthe dealumination of the catalyst is inevitable, which

Table 2Results of hydrogen chemisorption of the different catalysts

Catalyst H2 uptake/ml g�1

30 �C 300 �C 450 �C Total

PtSnNa/ZSM-5(400) 7.2 17.1 11.2 35.5PtSnNa/ZSM-5(500) 3.1 18.5 12.1 33.7PtSnNa/ZSM-5(600) 2.4 11.6 6.8 20.8PtSnNa/ZSM-5(700) 2.0 8.4 3.9 14.3PtSnNa/ZSM-5(800) 1.8 4.9 2.5 9.2

results in the weak interactions between Sn and carrier,because the existence of aluminum can stabilize the Sn spe-cies [11,25]. Consequently, the structure of Pt2 sites with‘‘sandwich structure’’ would be destroyed to some extent.With the increase of calcination temperature, the destruc-tion of Pt2 sites is aggravated effectively. On the otherhand, it is interesting to note that the proportion of Pt1

sites that located on the external surface of the catalystincreases gradually.

Fig. 5 shows the H2-TPR profiles of the different cata-lysts. PtSnNa/ZSM-5 catalyst calcined at 400 �C presentsa peak, whose maximum is placed at 240 �C, and alsotwo other peaks at about 410 �C and 550 �C, respectively.The signal at 240 �C is ascribed to reduction of Pt, whereasthe high-temperature peaks represent reduction of Sn4+ toSn2+ and Sn2+ to Sn0, respectively [10,11]. In contrast tothis, after the thermal treatment at 500 �C, the shape ofTPR spectra is similar. However, the reduction tempera-ture of Pt species shifts toward the higher range obviouslyand the area of hydrogen consumption peak decreases syn-chronously, suggesting that the interactions between Pt andcarrier are strengthened and the reduction of Pt speciesbecomes more difficult. In addition, the two reduction tem-peratures of Sn species shift towards the higher tempera-tures, indicating that the interactions between Sn andcarrier are also intensified. Respect to the catalyst that cal-cined at 600 �C, distinct decrease of the reduction peak ofPt species is observed. Moreover, this tendency becomesmore apparent with the increase of calcination tempera-ture. This fact allows us to confirm that the calcinationat high temperature can affect the reduction behavior ofPt species obviously. This is because the sintering of Pt par-ticles is remarkable, thus decreasing the signal of hydrogenconsumption. Besides, it is also noteworthy that the reduc-tion temperature of Pt species moves towards the highervalues. This is most likely due to the strong interactions

200 300 400 500 600 700

b

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Temperature (˚C)

Fig. 5. TPR Profiles of the different catalysts. (a) PtSnNa/ZSM-5(400), (b)PtSnNa/ZSM-5(500), (c) PtSnNa/ZSM-5(600), (d) PtSnNa/ZSM-5(700),(e) PtSnNa/ZSM-5(800).

1014 Y. Zhang et al. / Catalysis Communications 8 (2007) 1009–1016

between the reserved non-sintered Pt particles and the car-rier. On the other hand, after the calcination at 700 �C, thecatalyst exhibits the larger reduction peaks of Sn species,meaning that in this case more amounts of Sn componentsare reduced. This is because that the interactions betweenSn species and the carrier are weakened due to the dealumi-nation of the zeolite. Increasing calcination temperaturemakes this tendency become more distinct.

3.2. Catalytic properties of the different catalysts

Fig. 6 shows the catalytic properties of the different cat-alysts for propane dehydrogenation. Although the initialactivity of PtSnNa/ZSM-5(400) catalyst is high, the reac-tion stability of it is poor. After reaction for 4 h, propaneconversion drops from 43.7% to 28.3%. This is possiblerelated to the high metallic Pt dispersion, which makesthe reactant gas contact with the active sites easily, thusincreasing the initial activity. However, at the same time,the side reactions can also be carried out easily, whichdecreases the selectivity to propene. In this situation, theproduced coke can cover the active sites fully, resulting ina rapid deactivation. After the calcination at 500 �C, themetallic Pt dispersion of the catalyst decreases for thereduced total amount of H2 adsorption (Table 2), whichleads to a decrease of the initial propane conversion. How-ever, as mentioned above, the interactions between Pt andSn are strengthened and the Pt1 sites decreases obviously,thus improving the reaction stability and selectivity. Con-sidering that the PtSnNa/ZSM-5 catalyst is bifunctionaland the two active centers (the metal particles and the acidsites) may work collaboratively [26]. Therefore, the changeof catalyst acidity and character of Pt active sites can influ-ence the catalytic performance effectively. In the catalystthat calcined at 500 �C, the catalytic acidity is almost

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Fig. 6. Propane conversion (1) and propene selectivity (2) vs reaction time of thPtSnNa/ZSM-5(600), (d) PtSnNa/ZSM-5(700), (e) PtSnNa/ZSM-5(800)WHSV = 3.0 h�1, m(cat) = 2.0 g.).

changeless, and the Pt2 sites increase, thus improving thematching between the metallic function and acid function,which is advantageous to the dehydrogenation of propane.With the increase of calcination temperature, the BET sur-face areas of the catalysts decrease obviously and thedegree of Pt sintering and the destruction of Pt2 sites wouldbe aggravated, which results in the permanent deactivationof the catalysts. Meanwhile, when the proportion of Pt1

sites in the catalyst increases, the side reactions can be car-ried out relatively easily, which leads to a decreasing selec-tivity to propene. It is obvious that with the highercalcination temperature, this change trend becomes moreevident. Accordingly, it is concluded that the calcinationat the different temperature has obvious influence on thecatalytic performance of PtSnNa/ZSM-5 catalyst andwhen the calcination temperature is 500 �C, the catalyticproperties maintain the best state.

3.3. Coke analysis of the different catalysts

The carbon amount versus the reaction time of the dif-ferent catalysts is shown in Fig. 7. It is obvious that the car-bon amount increases monotonously in the initial reactionstage. Afterwards, the increase of it slows down. Withincreasing of calcination temperature, the carbon amountdecreases gradually, meaning that the calcination at hightemperature favors the improved action of coke. However,when the temperature of thermal treatment is 800 �C, littleincrease of the carbon amount is found. Following Afonsoet al. [27], the coke formation on the catalyst involved sev-eral processes: (1) successive dehydrogenation/cyclizationof alkyl chains; (2) n-alkane oligomerization; (3) Diels-Alder type reactions. Olefins were primary precursors ofthe mechanism of coke formation. According to this mech-anism, it is proposed that the change of Pt active sites and

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e different catalysts. (a) PtSnNa/ZSM-5(400), (b) PtSnNa/ZSM-5(500), (c)(Reaction conditions: 590 �C, H2/C3 = 0.25 (molar ratio), 0.1 Mpa,

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Fig. 7. Carbon amount vs reaction time of the different catalysts. (a)PtSnNa/ZSM-5(400), (b) PtSnNa/ZSM-5(500), (c) PtSnNa/ZSM-5(600),(d) PtSnNa/ZSM-5(700), (e) PtSnNa/ZSM-5(800).

Y. Zhang et al. / Catalysis Communications 8 (2007) 1009–1016 1015

catalyst acidity can influence the coke formation obviously.In general, the decreased metallic Pt dispersion can reducethe amounts of the carbon precursors and the intrinsicacidity of the support can promote the undesirable reac-tions such as cracking/isomerization, thus increasing thecarbon deposits. As commented before, with the increaseof calcination temperature, the catalyst acidity and theplatinum dispersion decrease, which in consequencereduces the carbon amount. However, when the calcinationat 800 �C, the proportion of Pt1 sites in the catalystincreases significantly, which is favorable for the coke for-mation despite the catalyst acidity is little. On the otherhand, in the case of the catalyst that calcined at 500 �C,the BET surface area decreases only from 307.7 m2 g�1 to305.8 m2 g�1 after the reaction. Moreover, the catalyticactivity would be restored completely after the partial oxi-dation experiment. Therefore, it is concluded that the phe-nomenon of slow Pt sintering is not clear, even though thereaction temperature of the catalyst is higher than the cal-cination one.

4. Conclusions

Calcination temperature has obvious impact on catalyticproperties of PtSnNa/ZSM-5 catalyst for propane dehy-drogenation. When the catalyst is calcined at 400 �C and500 �C, no clear change of the physical structure and acid-ity is observed. However, after the calcination at 500 �C,the interactions between Pt and Sn are strengthened, thusmaking the metallic function and acid function keep thebest state, which is advantageous to the reaction. Withthe increasing of calcination temperature, more frameworkaluminum is removed and more non-framework aluminumis generated, which results in the decreased catalyst acidity.Meanwhile, the sintering of Pt particles is found as evi-denced from the results of TEM and H2 chemisorption.

The interactions between Sn and carrier become weakdue to the dealumination of the catalyst, thus the destruc-tion of Pt2 sites with ‘‘sandwich structure’’ is inevitable. Inthis way, the initial matching between the active sites andacid sites is destroyed, which leads to the irreversible deac-tivation of the catalyst and a decreased selectivity to pro-pene. The higher the calcination temperature is, the moreapparent of this tendency is. In our experiments, the cata-lyst which is calcined at 500 �C for 4 h in air flow, showsthe best reaction stability and selectivity. Furthermore, inthis case the phenomenon of slow Pt sintering on the exter-nal surface of the catalyst is not clear, even though thereaction temperature of the catalyst is higher than the cal-cination one.

Acknowledgements

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 (50377005) and Innovation plan topostgraduates in University of Jiangsu Province. Theyare also grateful to Key Lab of Mesoscopic Chemistry ofMOE Nanjing University for their help in the 27Al MASNMR analysis.

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