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Chemical Engineering Journal 181– 182 (2012) 530– 537

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

jo u r n al hom epage: www.elsev ier .com/ locate /ce j

ffect of La calcination temperature on catalytic performance of PtSnNaLa/ZSM-5atalyst for propane dehydrogenation

iwei Zhang, Yuming Zhou ∗, Menghan Tang, Xuan Liu, Yongzheng Duanchool of Chemistry and Chemical Engineering, Southeast University, Jiang ning District, Nanjing 211189, PR China

r t i c l e i n f o

rticle history:eceived 25 December 2010eceived in revised form4 November 2011ccepted 16 November 2011

eywords:alcination temperature

a b s t r a c t

The effect of La calcination temperature on catalytic performance of PtSnNaLa/ZSM-5 catalyst for propanedehydrogenation was studied. XRD, 27Al MAS NMR, NH3-TPD, TEM, H2-TPR and hydrogen chemisorptionwere used to characterize the catalysts. In contrast to the catalyst without the calcination of La, the treat-ments in the range of 500–650 ◦C resulted in the strong interaction between La species and the support,thus suppressing the dealumination of the support to some extent. Furthermore, in these cases the inter-actions between Sn species and the support as well as Pt and Sn oxides became strengthened, which wereadvantageous to the reaction. In our experiments, the catalyst when the calcination temperature of La

◦

SM-5ropane dehydrogenationt-Sn catalyst

was 650 C showed the best reaction activity and stability. The average yield of propene was about 34.3%over 82 h for the reaction of propane dehydrogenation at 590 ◦C. However, after the thermal treatmentof La at 700 ◦C, sharply decreased of the catalyst acidity was inevitable due to the severe dealumination.Meanwhile, sintering of Pt particles and the formation of Sn0 species were also found, which seriouslyresulted in the deactivation of catalyst. Finally, a model for the effect of La calcined treatment was alsoproposed.

. Introduction

The catalytic dehydrogenation of propane is of increasingmportance because of the growing demand for propene [1–3].ndeed, propene is an important raw material for the productionf polypropene, acrolein, polyacrylonitrile and acrylic acid. How-ver, the reaction of propane dehydrogenation is an endothermicrocess that requires a relatively high temperature to obtain highield of propene. Therefore, it is the key to develop the catalyst pos-essing high-activity, high-stability and high-selectivity since theeactivation of the catalyst due to coke formation is inevitable.

Supported platinum-tin catalysts have been studied in depth,hey show high activity and selectivity in the process [4,5]. How-ver, the reaction stability of these catalysts is still relatively low.s a result of this, the use of new carrier for propane dehydro-enation has received a great deal of attention due to its distinctivetructure and property [6,7]. Recently, the use of ZSM-5 zeolite as

support for propane dehydrogenation is being the subject of an

ntense research work. Kumar et al. [8] investigated the influencef the pore geometry of the catalyst supports on catalytic prop-rties for propane dehydrogenation and pointed out that three

∗ Corresponding author. Tel.: +86 25 52090617.E-mail address: ymzhou@seu.edu.cn (Y. Zhou).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.11.055

© 2011 Elsevier B.V. All rights reserved.

dimensional microporous materials (ZSM-5) were better catalyticsupports than the mesoporous SBA-15 because of their intrinsicnature. Our previous work [9–12] discussed the effects of promoterand reaction condition on catalytic performances of PtSnNa/ZSM-5catalyst for propane dehydrogenation and found that the suitableaddition of La resulted in the relatively higher propane conversionand propene selectivity. However, worthy mention is that on thisreported PtSnNaLa/ZSM-5 catalyst [12], the promoter of lanthanumwas added directly by impregnating the support with a La(NO3)3aqueous solution, not undergoing the succedent process of calcina-tion. That is to say, relatively weak interactions between lanthanumspecies and the support may exist, which can affect the catalyticproperties significantly.

In deed, calcination is an important factor to influence the cat-alytic structure and properties [13], especially for the La-dopedcatalysts. Rane et al. [14] systematically studied the catalystpreparation method on the catalytic properties of Mo-V-Sb-La-Ox catalysts in the oxidation of propane and pointed out that thedifferent calcination time had obvious influence on the catalyticperformance. When the calcination time was relatively low, theactive sites being responsible for the formation of oxygenated prod-ucts had not been formed, which resulted in the variation in the

reaction conversion and selectivity. Hung [15] studied the calci-nation temperature on the activity of Cu-La-Ce composite metalcatalyst for the catalytic wet oxidation of ammonia solution andreported that the catalyst calcined at 773 K showed the best activity.

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tle decrease in the four-coordinated aluminum and increase in thesix-coordinated aluminum are observed, suggesting that the calci-nation of La suppresses the dealumination of the support. Clearly,this behavior caused by the introduction of La is very different with

Z700

Z650

Z600

Z550

Z500

Z0

HZSM-5

Y. Zhang et al. / Chemical Enginee

ince the calcination treatment can affect the catalytic performancef the La containing catalyst significantly, the investigating of thisffect is becoming necessary.

In the present work, a series of PtSnNaLa/ZSM-5 catalysts wererepared by changing the calcination temperature of La. Particu-

ar emphasis was focused on the La–support interactions and thehanges of catalyst acidity and Pt characters. All the results wereorrelated with a corresponding catalytic reaction of propane dehy-rogenation. This can provide us with important information tonderstand the effect of calcination treatment of La on supportedetallic catalyst.

. Experimental

.1. Catalyst preparation

PtSnNaLa/ZSM-5 catalysts were prepared by sequential impreg-ation method. The powder H-ZSM-5 was impregnated in anqueous solution of 0.123 M La(NO3)3 at 80 ◦C at first, then driedt 80 ◦C for 3 h, calcined in an air flow at 500 ◦C, 550 ◦C, 600 ◦C,50 ◦C and 700 ◦C for 4 h, respectively. Afterwards, the samplesere impregnated in solutions mixture of 0.033 M H2PtCl6, 0.153 M

nCl4 and 0.427 M NaCl for 4 h.Then, the prepared catalysts were agglomerated with 5.0 wt%

lumina fully during the process of pelletization. After totallyried, the catalysts were re-calcined in air at 500 ◦C for 4 h. Theominal compositions of the each sample were 0.7 wt% for La,.4 wt% for Pt, 0.9 wt% for Sn, and 1.0 wt% for Na. Catalysts useduring this investigation were abbreviated as Zx, where x repre-ented the calcination temperature (◦C) after the impregnation ofanthanum.

.2. Catalyst characterization

X-ray diffraction (XRD) patterns of the prepared catalysts werebtained on a XD-3A X-ray powder diffractometer coupled to a cop-er anode tube. The K� radiation was selected with a diffractedeam monochromator. An angular ranging from 5◦ to 40◦ during

h was recorded using step scanning and long counting times toetermine the positions of the ZSM-5 peaks.

Solid-state 27Al MAS NMR spectra were collected in a BruckerSX-400 spectrometer. The 27Al NMR spectra were obtained at0.0 KHz, using 15◦ pulses and 4 s delay, a total of 2000 pulses beingccumulated.

Ammonia temperature-programmed desorption (NH3-TPD)xperiment was measured with a conventional TPD apparatus.bout 0.15 g of sample was placed in a quartz reactor and satu-ated with ammonia at room temperature. TPD was carried outrom 100 ◦C to 550 ◦C with a heating rate ̌ of 10 ◦C/min and withelium (30 ml/min) as the carrier gas.

Transmission electron microscopy (TEM) studies were con-ucted using JEOL (Japan) instruments. Before being placed on theupport, the samples were dispersed in excess ethanol by sonicat-ng for 30 min.

Temperature-programmed reduction (TPR) was measured withhe same apparatus as that of NH3-TPD. Prior to the TPR experi-

ents, the catalysts were dried in flowing N2 at 400 ◦C for 1 h. H2/N25%) was used as the reducing gas at a flow rate of 40 ml/min. Theate of temperature rise in the TPR experiment was 10 ◦C/min upo 700 ◦C.

Hydrogen chemisorption experiment was performed on thepparatus that described for NH3-TPD. The samples were reduced

n flowing H2 at 500 ◦C for 2 h, then exposed to N2 at 550 ◦C for 1 h toemove the hydrogen. Details of the experimental procedure haveeen described elsewhere [16] except that the high temperaturesor chemisorption are 300 ◦C and 450 ◦C.

urnal 181– 182 (2012) 530– 537 531

2.3. Catalytic reaction

Propane dehydrogenation was carried out in a conventionalquartz tubular micro-reactor and all the catalysts were reduced inH2 at 500 ◦C for 8 h before catalytic evaluation. The catalyst (mass2.0 g) was placed into the center of the reactor. Reaction conditionswere as follows: 590 ◦C for reaction temperature, 0.1 MPa pressure,H2/C3 = 0.25 (molar ratio) and the propane weight hourly spacevelocity (WHSV) is 3.0 h−1. The reaction products were analyzedwith an online GC-14C gas chromatography.

3. Results and discussion

3.1. Characterization of catalysts

Fig. 1 shows the XRD patterns of the different samples. As canbe seen, the XRD patterns of the support have the representativepeaks at 7–8◦ and 23–24◦, corresponding to that given in Ref. [17]for pure ZSM-5. In the case of Z0 sample, no changes of the charac-terized diffraction peaks are observed, indicating that the catalystpreparation without the calcination of La does not destroy thestructure of ZSM-5 zeolite. Similarly, it is interesting to note thatthe changes of the corresponding peaks are not also found clearlyafter the thermal treatment of La from 500 ◦C to 700 ◦C. However, inthese instances, the intensities of the corresponding peaks of angle2� = 7–8◦ decrease slightly. Generally, the low-angle XRD intensi-ties in the pattern of ZSM-5 are sensitive to the presence of anyspecies inside the channels [18]. Therefore, these phenomena sug-gest that the calcined treatment of La can promote the La species toenter into the ZSM-5 channels since the average diameter of La3+

(0.119 nm) is less than that of ZSM-5 pore mouth. Furthermore,as exhibited in Fig. 1, with the increase of calcination tempera-ture, the intensities of the corresponding peaks of angle 2� = 23–24◦

decrease gradually, especially for Z700 sample, implying that thecrystallinity of the catalyst is degraded after the thermal treatment[19].

27Al MAS NMR spectra for some catalysts in this study are thencollected. As shown in Fig. 2, Z0 sample exhibits a relatively strongsignal at about 54 ppm, which is attributed to four-coordinatedaluminum, and also a weak peak assigned to six-coordinated extra-framework Al (EFAL) at about 0 ppm [20]. When the calcinationtemperature of La increases from 500 ◦C to 600 ◦C, relatively lit-

105 15 20 25 30 35 402 Theta(degree)

Fig. 1. XRD patterns of different samples.

532 Y. Zhang et al. / Chemical Engineering Journal 181– 182 (2012) 530– 537

-30 0 30 60 90

Z0

Z500

Z600

Z700

tbfoiesAaotfnmooctrtwfmba

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100 200 300 400 500 600Temperature (oC)

HZSM-5

Na/ZSM-5

Z0Z500Z550

Z600Z650

Z700

TN

Chemical shift (ppm )

Fig. 2. 27Al MAS NMR spectra of different catalysts.

hat of the PtSnNa/ZSM-5 catalyst [10] in the absence of La. It haseen found that more framework aluminum atoms can be removedrom tetrahedral positions and existed as non-structural Al speciesn the PtSnNa/ZSM-5 catalyst when the calcination temperaturencreases in the same range. Therefore, this observation can bexplained by the existent interactions between La species and theupport, which result in the higher thermal stability of the catalyst.t this moment, the location and species of rare earth ions may playn important role in determining this [21]. According to the previ-us works [22,23], on ZSM-5 zeolites, the most probable species forhe lanthanum cations are La(OH)2

+ and these lanthanum ions areavorably accommodated in the two 6-T rings of the straight chan-els. Through the reduction of La(OH)2

+ when heated, LaO(OH) isost likely formed, which can interact with the zeolite and turns

ut to the structure of Z-La(OH)2+ or Z-LaO(OH) after geometrical

ptimization [24]. Generally speaking, the Z-La(OH)2+ species can

hange into the Z-LaO(OH) as the temperature is raised. In this way,he strong interaction between La3+ and the support can partiallyestrict the breaking of the Si–O–Al bonds of the ZSM-5 zeolite [25],hus preventing the dealumination of the support. In comparisonith these, as for the Z700 sample, more severe dealumination is

ound. These results indicate that the degree of dealumination isainly depended on the calcination temperature of La and this may

e the possible reason for the degraded crystallinity of the catalysts revealed in Fig. 1.

The desorption of NH3 is used to characterize the acidic prop-rties of surface and this technique can provide some informationbout the amount and strength of acidic sites. The NH3-TPD profilesf different samples are displayed in Fig. 3 and the results are sum-arized in Table 1. As can be seen, HZSM-5 sample presents twoell-defined peaks at 241 ◦C and 445 ◦C; the first peak at low tem-

erature can be assigned to ammonia adsorbed to weak acid sites,hile the second peak at high temperature corresponds to ammo-ia adsorbed to strong acid sites [26]. In a comparison, after the

oading of Na (Na/ZSM-5), the strong acid sites disappear and the

able 1H3-TPD results of different samples.

Sample Total acidity (mmol NH3/g) Weak acidity (mmol NH3/g)

HZSM-5 0.61 0.30

Na/ZSM-5 0.36 0.36

Z0 0.33 0.33

Z500 0.37 0.37

Z550 0.41 0.41

Z600 0.44 0.44

Z650 0.54 0.54

Z700 0.30 0.30

Fig. 3. NH3-TPD profiles of the different samples.

total acidity decreases effectively, demonstrating that the strongacid sites of the support can be neutralized preferentially by Na+

ion [27]. As regards the PtSnNaLa/ZSM-5 catalyst without the cal-cination of La (Z0), the ammonia desorption peak areas at lowtemperature decrease continuously and the total acidity is only 54%with that of the HZSM-5 support. After the thermal treatment of La(Z500), little increased amount of the weak acid sites and weakeracid intensities are found as evidenced from the ammonia desorp-tion temperature moves towards the lower range. Obviously, thehigher the calcination temperature is, the more apparent of thistendency is. The possible reason for this behavior may be attributedto the acidic properties of lanthanum ions. It is known that the La3+

cations are also weak Lewis acid sites [12], thus the aggregationof these acid sites is inevitable with more amounts of La speciesto enter the main channels of the carrier (Fig. 1). Furthermore, fol-lowing the analysis by density function calculation in Ref. [24], inthe structure of Z-LaO(OH), the La3+ ions can offer the empty tracks(Lewis acid sites), which is used for the adsorption of reactant. Obvi-ously, both of these factors should be responsible for the increasedcatalyst acidity. However, when the calcination temperature of Lais 700 ◦C, more amounts of Al species can migrate from the frame-work tetrahedral positions (Fig. 2), which in consequence reducethe amount of the weak acid centers significantly.

The morphology of the various catalysts is characterized bytransmission electron microscopy (Fig. 4). It can be seen that oncatalysts Z0, Z500, Z550, Z600 and Z650 (Fig. 4(a–e)), no agglom-erated particles are found on the external surface of the support.Nevertheless, more agglomerated particles appear obviously asthe calcination temperature of La increases to 700 ◦C (Fig. 4(f)). In

order to clarify the possibility of Pt sintering, selected area electrondiffraction pattern (SADP) taken from the agglomerated particlesis shown in Fig. 4(g). By measuring three different groups of ringradius, the diffraction rings can be indexed as (1 1 1), (2 0 0) and

Temperature (◦C) Strong acidity (mmol NH3/g) Temperature (◦C)

241 0.31 445230 – –227 – –194 – –182 – –184 – –171 – –179 – –

Y. Zhang et al. / Chemical Engineering Journal 181– 182 (2012) 530– 537 533

) Z550

(attaiqioLccrtbTbp

pni2to

Fig. 4. TEM micrographs of the different catalysts. (a) Z0, (b) Z500, (c

2 2 0) crystal faces of Pt. Therefore, these particles can be identifieds Pt phase by diffraction analysis. These observations demonstratehat the calcination temperature of La has an obvious influence onhe dispersive status of Pt particles and the high treatment temper-ture (i.e.700 ◦C) leads to the sintering of Pt particles. In addition, its worth noting that the agglomerated mechanism of Pt particles isuite different with that of the previous work [10]. This is because

n our experiments the calcination temperature after the loadingf Pt is the same (500 ◦C), while the one after the impregnation ofa is different. Consequently, as for the agglomeration of Pt parti-les on Z700 sample, the influence of heat treatment of La must beonsidered. As analysis before (Fig. 2), in this case, the carrier expe-iences relatively severe dealumination. Thus, it is reasonable thathe treated catalyst is incapable of stabilizing Sn species effectively,ecause the existence of aluminum can stabilize the tin species [28].hese unstable Sn species result in the weak surface interactionsetween Pt and Sn, which in consequence decrease the ability of Ptarticles to resist agglomeration.

To get more information about the oxidation and reductionroperties of supported noble metals before and after the calci-ation of La, H2-TPR experiments are then investigated. As shown

n Fig. 5, Z0 sample presents a peak, whose maximum is placed at

60 ◦C, and also two other peaks at about 440 ◦C and 560 ◦C, respec-ively. In general, the signal at 260 ◦C can be ascribed to reductionf Pt oxide species, whereas the high-temperature peaks represent

100 200 300 400 500 600 700 800Temperature (ºC)

Z700

Z650

Z600

Z550

Z500Z0

Fig. 5. TPR Profiles of the different catalysts.

, (d) Z600, (e) Z650, (f) Z700, (g) SADP of the agglomerated particles.

reduction of Sn4+ to Sn2+ and Sn2+ to Sn0, respectively [29,30]. Withthe increase of calcination temperature of La, the reduction peakarea of Sn4+ to Sn2+ increases slightly, while the peak area corre-sponding to Sn2+ to Sn0 species is almost changeless. Moreover,this tendency becomes more apparent as for Z650 sample. Suchphenomena indicate that the interactions between Sn species andthe support become strengthened and more amounts of tin canexist in oxidative states after the heat treatment of La. To explainthese, it should be noted that in these cases the degree of dealu-mination of the carrier is relatively gentle and more strengthenedinteractions between La species and the support exist. Thus the roleof lanthanum acting as an anti-reducing agent for Sn species can beelaborated more sufficiently [12]. Nevertheless, when the calcinedtreatment is 700 ◦C, sharply decreased amounts of hydrogen con-sumption for Pt species are observed. This fact allows us to confirmthat the calcination of La at high temperature can affect the reduc-tion behavior of Pt species obviously. According to the results ofTEM (Fig. 4), this can be attributed to the sintering of Pt particles.Besides, as for the Z700 sample, it exhibits the much larger reduc-tion peaks of Sn species at high temperature, suggesting that in thiscase more amounts of Sn0 components have produced. Similarly,this behavior may be linked with the severe dealumination of thecatalyst, which in consequence weakens the interactions betweenSn species and the carrier. Usually, keeping the oxidative state ofSn is very important for bimetallic Pt-Sn catalyst and the deactiva-tion of the catalysts is inevitable due to the formation of Pt–Sn alloy[31,32].

The corresponding H2 uptake of the different samples (Table 2)allows us to confirm this point with great certainty. It is known thatH2 chemisorption has been employed as one of the effective meth-

ods to determine the metal surface area and dispersion of metalliccatalysts. However, the quantitative determination of the surfacearea by H2 adsorption is interfered by the interaction of Pt with Sn

Table 2Results of hydrogen chemisorption of the different catalysts.

Catalyst H2 uptake (ml g−1)

30 ◦C 300 ◦C 450 ◦C Total

Z0 2.5 17.1 9.5 29.1Z500 2.7 16.9 10.3 29.9Z550 2.6 17.2 10.6 30.4Z600 3.1 17.6 10.9 31.6Z650 2.9 18.2 11.4 32.5Z700 1.7 15.4 9.2 26.3

534 Y. Zhang et al. / Chemical Engineering Journal 181– 182 (2012) 530– 537

0 642 10826

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F he difW

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3

fvtictZtcFc

ig. 6. Propane conversion (1) and propene selectivity (2) vs reaction time of tHSV = 3.0 h−1, m(cat) = 2.0 g).

xide. This neighboring effect can be used as a probe to character-ze the properties of different active sites via selective adsorptionr temperature programmed desorption of H2. Following Lin et al.28], two kinds of active Pt species may exist on the surface ofhe catalyst, named Pt1 sites and Pt2 sites. Pt1 sites are the sitesn which Pt directly anchors on the carrier surface, while Pt2 sitesorresponding to the sites in which Pt anchors Sn oxide surface with

“sandwich structure”. Generally speaking, the Pt2 sites can adsorbore H2 at high temperature and are the main reaction active sites

or the dehydrogenation of propane. As listed in Table 2, the Z0ample exhibits the different amounts of adsorbed H2 at the cor-esponding temperature. Also, the amounts of adsorbed H2 at highemperature are relatively large, implying that the main active sitesn the external surface of the catalyst are the Pt2 sites. After the heatreatment from 500 ◦C to 650 ◦C, the increases in the total amountsf H2 and also the amounts of adsorbed H2 at high temperaturere observed, indicating that the calcination of La can promote theormation of Pt2 active sites. As discussed above (Fig. 5), in theseases more amounts of tin exist in oxidative states. In this way, thenteractions between Pt and Sn oxides can be strengthened, thushanging the adsorbed activation energy of H2 [28]. Nevertheless,ith respect to the Z700 sample, the total amount of absorbed H2

educes markedly, meaning again that the character of Pt activeites has changed effectively. Meanwhile, it is interesting to notehat the amounts of absorbed H2 at high temperature decreaseignificantly, demonstrating that the thermal treatment of La atevere condition weakens the existent interactions between Pt andn oxides. In other words, the anchoring of Pt on Sn oxide surfaceecomes relatively difficult, thus the intrinsic structure of Pt2 sitesith “sandwich structure” would be destroyed to some extent.

.2. Catalytic performance

Fig. 6 shows the catalytic performance of the different catalystsor propane dehydrogenation. For all samples, the propane con-ersion decreases, while the selectivity to propene increases withhe elongation of reaction time. Clearly, this phenomenon may benterpreted in terms of the effect of produced coke. In this case, thearbon depositions may act as a promoter to improve the selec-ivity by deactivating the active sites. As depicted in Fig. 6(1), the0 sample displays a 34.8% initial conversion. After 7 h on-stream,

he propane conversion decreases to 30.9%. In contrast to this, thealcination of La results in the higher reaction activity and stability.or example, when the calcination temperature is 550 ◦C, the initialonversion increases to 35.4% and the final conversion is 32.0%. In

ferent catalysts (reaction conditions: 590 ◦C, H2/C3 = 0.25 (molar ratio), 0.1 MPa,

our experiments, the catalyst calcinated at 650 ◦C exhibits the beatcatalytic activity and stability, the maximum initial and final con-versions appear as 37.2 and 34.1%, respectively. However, with thecontinuous increase of calcined temperature, the dehydrogenationactivity decreases sharply. As for the Z700 sample, the final propaneconversion decreases about 15.5% when comparing with that of theZ650 one. On the other hand, there are no obvious changes in theselectivity to propene when the calcination temperature of La is inthe range of 500 ◦C to 650 ◦C (Fig. 6(2)). Despite of the poorest reac-tion activity and stability on Z700 sample, the highest selectivity topropene can be obtained.

To explain these, it should first be noted that the PtSnNaLa/ZSM-5 catalyst is bifunctional and the two active centers (the metalparticle and the acid site) may work collaboratively [33]. Fromthis, it is assumed that an optimum ratio between the number ofactive sites and the number of acid sites over the catalyst shouldexist. As commented before, when the calcined temperature of Lais 650 ◦C, the catalyst acidity is relatively more than others (NH3-TPD results). Meanwhile, the introduction of La produces a strongcombination with the support, which results in the relatively gen-tle dealumination of the catalyst and strong interactions betweenPt and Sn oxides. So that the better catalytic performance mightbe obtained due to the optical matching between the acid functionand the metallic function. Nevertheless, when the treatment tem-perature of La increases to 700 ◦C, more amounts of Al species onthe ZSM-5 zeolite have migrated from the framework tetrahedralpositions and the catalyst acidity has decreased sharply. Simulta-neously, sintering of Pt particles and the formation of Sn0 speciesare also inevitable, which seriously result in the deactivation ofcatalyst.

From the other point of view, on PtSnNaLa/ZSM-5 catalystplatinum is the only active metal and propene is only formed onthe metal by dehydrogenation, the main cracking product (ethene)is mainly formed from cracking on the carrier and the ethaneis formed by hydrogenolysis of propane and by hydrogenationof ethene, with both reactions taking place on the metal [34].Furthermore, on HMFI zeolite the dehydrogenation and cracking ofpropane are assumed to proceed through carbonium-ion interme-diates [35]. The higher acid sites generally promote the subsequentcracking reaction of the initially formed C3

+carbenium ions. There-fore, the changes of catalytic acidity and Pt active sites should be

responsible for the selectivity to propene. As mentioned above,when the calcination temperature of La is below 700 ◦C, althoughthe amounts of the weak acid sites increase a little, no sintering ofPt particles are observed. That is to say, in these cases, the character

Y. Zhang et al. / Chemical Engineering Journal 181– 182 (2012) 530– 537 535

34 36 38 40 42 44 4694

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FeW

otdvcd

i(FitpttaadZiaZ

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Fp

ig. 7. Propene selectivity as a function of propane conversion for the differ-nt catalysts (reaction conditions: 610 ◦C, H2/C3 = 0.25 (molar ratio), 0.1 MPa,HSV = 3.0 h−1, m(cat) = 2.0 g).

f metallic function does not change obviously. However, afterhe thermal treatment at 700 ◦C, the acid amount of the catalystecreases sharply. Furthermore, the degree of Pt sintering is aggra-ated, which leads to the loss of active sites. The side reactionsan be inhibited to some extent in the meantime of the catalysteactivation. Thus, the increase of propene selectivity is inevitable.

To further assess the performances of Z650 catalyst, the compar-sons with the Z0 catalyst by increasing the reaction temperature610 ◦C) and using a recycle reaction are also made. As shown inig. 7, the propene selectivity decreases gradually with increas-ng propane conversion. Compared with the Z0 catalyst withouthe calcination of La, the higher propane conversion and the lowerropene selectivity can be found on Z650 sample, suggesting thathe thermal treatment of La is favorable for the improvement ofhe catalytic activity even at higher reaction temperature. Besides,s exhibited in Fig. 8, the initial propane conversions increasefter catalyst regeneration. Nevertheless, the propane conversionsecrease with the process time and the deactivation rate of the

0 catalyst is much faster than that of the Z650 catalyst with thencrease in recycle number. These results indicate that the remark-ble stability in the dehydrogenation reaction can be obtained over650 catalyst.

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(1)

ig. 8. The performance comparison of Z0 (1) and Z650 (2) catalysts in the dehydrogeerformed by oxidative treatment using pure air (35 ml/min) for 2 h at 500 ◦C. Then, the c

Fig. 9. Stability test of Z650 catalyst in the propane dehydrogenation at 590 ◦C. (1)Propane conversion (2) propene selectivity.

Since the Z650 catalyst shows the highest reaction activity andstability, this catalyst is chosen for the subsequent tests. Fig. 9shows the stability test of this catalyst. It can be seen that the selec-tivity to propene increases relatively quickly in the initial periodand then becomes smooth, a high value (97.8%) is obtained inthe overall process. Meanwhile, only a small deactivation rate isobserved in the overall process. The mean yield of propene is about34.3% during the course of the reaction for 82 h.

3.3. Model for the effects of La calcined treatment on the catalyticperformances

According to the results that obtained above, a model for theeffects of La calcined treatment is proposed (Fig. 10).

In our experiments, the thermal treatment is carried out afterthe impregnation of La. In this way, relatively strong interactionsbetween lanthanum species and the support exist. In this contribu-tion, it is suggested that the calcination of La works in the following

three ways. First, it improves the thermal stability of the support.As has been observed in XRD and 27Al MAS NMR spectra, moreamounts of La species can enter into the ZSM-5 channels and thedealumination of the support can be inhibited when the calcination

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Time on stream (h)

FreshFirst cycleSecond cycleThird cycle

nation of propane at 590 ◦C. Regeneration conditions: catalyst regeneration wasatalyst was reduced in flowing pure hydrogen (35 ml/min) for 4 h at 500 ◦C.

536 Y. Zhang et al. / Chemical Engineering Journal 181– 182 (2012) 530– 537

fects o

tlfmadrwZtsttcofspoose[Paaslc“ah

4

oclis5l

Fig. 10. Model for the ef

emperature is less or equal to 650 ◦C. Thus, it is proposed that theocation and species of lanthanum cations should be responsibleor the increase in the thermal stability. As mentioned before, the

ost probable species for the lanthanum cations on ZSM-5 zeolitere La(OH)2

+, and these lanthanum ions are favorably accommo-ated in the two 6-T rings of the straight channels [22]. Through theeduction of La(OH)2

+ when heated, LaO(OH) is most likely formed,hich can interact with the zeolite and turns out to the structure of

-La(OH)2+ or Z-LaO(OH) after geometrical optimization [24]. With

he increase of temperature, the Z-La(OH)2+ species can change

lowly into the Z-LaO(OH) ones, which remains still stable at highemperature. In this way, the introduction of La can partially restricthe breaking of the Si–O–Al bonds of the ZSM-5 zeolite, which inonsequence improves the thermal stability of the support. The sec-nd role of La calcined treatment relates with its stabilized effector Sn species. It is well known that the existence of aluminum cantabilize the Sn species. Combined the TPR results with the catalyticerformances, it is deduced that more amounts of tin can exist inxidative states. This stabilized effect likely reflects the influencef a monotonic decrease in the size of Pt ensembles present at theurface of the supported metal particles, together with electronicffects caused by the presence of tin species at the catalyst surface9]. Third, the calcination of La strengthens the interaction betweent and Sn oxide, which has been testified by the increased totalmounts of adsorbed hydrogen and also the ones at high temper-ture (Table 2). In this circumstance, the anchoring of tin oxidespecies on the support becomes strong, which is favorable for theocation of Pt active sites and the dispersion of Pt particles. In thisase, Pt can anchor Sn oxide surface to form new active sites withsandwich structure” [28]. These new sites are the main reactionctive sites for the dehydrogenation reaction, which result in theigher catalytic properties in the dehydrogenation process.

. Conclusions

In summary, different calcination temperature of La has obvi-us influence on the catalytic performance of PtSnNaLa/ZSM-5atalyst for propane dehydrogenation. Compared with the cata-yst without the calcined treatment of La, the total catalyst acidity

ncreases gradually and the interaction between La species andupport becomes strengthened after the thermal treatment from00 ◦C to 650 ◦C. In these cases, the presence of La inhibits the dea-

umination of the support and also stabilizes the tin species, which

f La calcined treatment.

result in the strong interaction between Pt and Sn oxides. In ourexperiments, the Z650 catalyst that calcined at 650 ◦C possessesthe best reaction activity and stability. The mean yield of propeneis about 34.3% during the course of the reaction for 82 h. Moreover,this treatment is also beneficial to the improvement of the catalyticactivity even at higher reaction temperature (610 ◦C). However,when the calcination temperature of La is 700 ◦C, the catalyst acid-ity decreases sharply due to the relatively severe dealumination.At the same time, the sintering of Pt particles is evidenced by theresults of TEM. In this way, the initial matching between the activesites and acid sites on PtSnNaLa/ZSM-5 catalyst is destroyed, whichleads to the irreversible deactivation of the catalyst.

Acknowledgment

The authors are grateful to the financial supports of NationalNatural Science Foundation of China (21106017 and 50873026),Production and Research Prospective Joint Research Project ofJiangsu Province of China (BY2009153) and Specialized ResearchFund for the Doctoral Program of Higher Education of China(20100092120047).

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