effect of hydrothermal treatment on catalytic properties of ptsnna/zsm-5 catalyst for propane...

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Microporous and Mesoporous Materials 96 (2006) 245–254

Effect of hydrothermal treatment on catalytic properties ofPtSnNa/ZSM-5 catalyst for propane dehydrogenation

Yiwei Zhang a, Yuming Zhou a,*, Kangzhen Yang a,Yian Li a, Yu Wang b, Yi Xu b, Peicheng Wu b

a College 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 China

Received 20 April 2006; received in revised form 26 June 2006; accepted 5 July 2006Available online 22 August 2006

Abstract

The effect of hydrothermal treatment on PtSnNa/ZSM-5 catalyst was analyzed during the dehydrogenation of propane with the pres-ence of hydrogen. Fresh and hydrotreated catalysts were characterized by XRD, 27Al MAS NMR, nitrogen adsorption, NH3-TPD, IRspectrum of adsorbed pyridine, TEM, TPR and hydrogen chemisorption, to determine the effect of hydrothermal treatment on catalyticacidity and metallic character. It was found that under the mild treatment, the pore volume and the average pore diameter of the catalystincreased. However, the opposite effect was observed with the increase of hydrothermal temperature and time. Acidity characterizationshowed that the hydrothermal treatment decreased the acid amount and weakened the acid intensity. Moreover, the intensity of Lewisacid sites decreased slightly from 400 �C to 550 �C, and important loss of acidity took place at 650 �C. Transmission electron micro-graphs and results of hydrogen chemisorption experiment indicated that the hydrothermal treatment had obvious impact on the disper-sive status of Pt particles on the external surface of the catalyst. Sample hydrotreated at 650 �C experienced Sn species loss and Ptsintering strongly, which caused the activity decrease and selectivity modifications during the reaction. It was suggested that the dealu-mination of ZSM-5 was the original reason of the catalytic deactivation and the subsequent loss of Sn species was just the main reason ofthe crystallitic Pt sintering. Finally, a model was proposed for the influence of hydrothermal treatment on catalytic properties of PtSnNa/ZSM-5 catalyst for propane dehydrogenation.� 2006 Elsevier Inc. All rights reserved.

Keywords: ZSM-5; Pt; Hydrothermal treatment; Propene; Propane dehydrogenation

1. Introduction

The catalytic dehydrogenation of propane is of increas-ing importance because of the growing demand for pro-pene. Indeed, propene is an indispensable raw material fornumerous products such as polypropene, acrolein, poly-acrylonitrile, and acrylic acid. However, the reaction ofpropane dehydrogenation is an endothermic process thatrequires a relatively high temperature to obtain a high yieldof propene. Therefore, the deactivation of the catalyst due

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doi:10.1016/j.micromeso.2006.07.003

* Corresponding author. Tel.: +86 25 83791274; fax: +86 25 83793171.E-mail address: [email protected] (Y. Zhou).

to coke formation is inevitable. Many efforts have beenmade to enhance the propene yield by developing newcatalysts with high-activity, high-stability and high-selectiv-ity. Among these, chromium-based catalysts and platinum-supported catalysts have been reported widely [1]. However,both of them have relatively poor reaction stability due tothe carbon deposits.

Recently, the use of zeolite as a catalyst in severalpetro-chemical process has been extensively investigatedboth experimentally [2,3] and theoretically [4]. ZeoliteH-ZSM-5 possesses the well-defined 10-membered ringchannel system, which can hinder the formation of largehydrocarbon molecules, thus preventing the so-called coke

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246 Y. Zhang et al. / Microporous and Mesoporous Materials 96 (2006) 245–254

deposits and improving the catalytic stability. Much workhas been done on the catalysts supported on ZSM-5 zeo-lite for the reaction of aromatization [5], hydroisomeriza-tion [6] and disproportionation [7]. On the other hand,there is growing interest in developing platinum-impreg-nated H-ZSM-5 zeolite for the catalytic dehydrogenationof propane. 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. Our previous works [10–12] discussed the effectof promoter and binder on catalytic performance ofPtSn/ZSM-5 catalyst for propane dehydrogenation andpointed out that PtSnNa/ZSM-5 catalyst with suitableamount of Na showed relatively high propane conversionand propene selectivity.

In order to decrease the coke formation during theprocess of dehydrogenation, reducing the catalytic acidityis very important. It is known that the acidity of zeolitedepends on the framework Si/Al ratio and the acidcatalytic properties of zeolite can be controlled by manip-ulating the Si/Al ratio either during synthesis or by post-treatment dealumination methods [13]. To date, steamingis the most common dealumination technique used to pre-pare industrial catalysts. A number of works have beendone upon this issue and the main focus is concentratedon the influence of steaming on acidic and structuralproperties of H-ZSM-5 zeolite. Triantafillidis et al. [14]systematically investigated the effect of the degree andtype of the dealumination method on the structural, com-positional and acidic characteristics of H-ZSM-5 zeoliteand obtained a lot of important results. Kumar et al.[13] studied the influence of mild dealumination on phys-icochemical, acidic and catalytic properties of H-ZSM-5and pointed out that the extra-framework Al species wereresponsible for the larger activity of the dealuminatedsamples.

Nevertheless, little investigators have examined theeffect of steam alone on supported metal catalysts. Ruc-kenstein et al. [15] reported that the presence of waterwas usually considered to cause deactivation, for itenhanced sintering or promoted phase transformationsof the crystallites. In the present study, the influence ofhydrotreatment temperature and time on catalytic proper-ties of PtSnNa/ZSM-5 catalyst was discussed, includingthe changes of catalytic acidity and active sites. For pur-poses of comparison, this paper also presented the resultsof fresh catalyst. Considering that the 27Al MAS NMRspectra of alumina binder showed a peak at 64.8 ppmand a broad peak at 6.8 ppm [10], which might have inter-ferential influence to distinguish the removed Al speciesfrom framework of ZSM-5 zeolite during the process ofsteaming, no binder was used in our investigation. Inaddition, a model for the influence of hydrothermal treat-ment was proposed. This can provide us with importantinformation to understand the effect of steam on sup-ported metallic catalyst.

2. Experimental

2.1. Catalyst preparation

PtSnNa/ZSM-5 catalyst was prepared by sequentialimpregnation method. The powder H-ZSM-5 (Si/Al =120) was impregnated in aqueous solution of 0.427 MNaCl at 80 �C for 4 h at first, then dried at 80 �C for 3 hand followed by impregnation in 0.033 M H2PtCl6 and0.153 M SnCl4 solutions mixture, finally the prepared cat-alyst was dried at 80 �C for 3 h under air flow. The nominalcompositions of the sample 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. Afterwards, the catalysts were calcinedat 500 �C for 4 h, reduced under H2 at 500 �C for 8 h.

2.2. Hydrothermal treatment

Hydrothermal treatment of the catalysts was per-formed in a reactor that we constructed. The sampleswere heated up to a specified temperature (400 �C,550 �C and 650 �C), under a flow of a nitrogen-steam mix-ture at a flow rate of 40 ml min�1 and a steam partialpressure of 19.9 kPa. The samples were kept at this tem-perature for a specified time period (4 or 8 h). Finally,the samples were cooled to room temperature in a nitro-gen stream before the reaction.

2.3. Catalyst characterization

X-ray diffraction (XRD) patterns of the differentsamples were obtained on a XD-3 A X-ray powder diffrac-tometer coupled to a copper anode tube. The Ka radiationwas selected with a diffracted beam monochromator. Anangular range 2h from 5� to 40� was recorded using stepscanning and long counting times to determine the posi-tions of the ZSM-5 peaks.

N2 adsorption studies were used to examine the porousproperties of each sample. The measurements were carriedout on Micromeritics ASAP 2000 adsorptive and desorp-tive apparatus, and all the samples were pretreated invacuum at 350 �C for 15 h before the measurements. Thespecific surface area was obtained using the BET method.The micropore volume and pore diameter were determinedfrom the t-plot method [16].

Solid-state 27Al MAS NMR spectra were collected in aBrucker DSX-400 spectrometer. The 27Al NMR spectrawere obtained at 10.0 kHz, using 15� pulses and 4 s delay,a total of 2000 pulses being accumulated.

NH3-TPD experiment was measured with a conven-tional TPD apparatus. About 0.15 g of sample was placedin a quartz reactor and saturated with ammonia at roomtemperature. TPD was carried out from 100 �C to 550 �Cwith a heating rate b of 10 �C min�1 and with helium(30 ml min�1) as the carrier gas.

Y. Zhang et al. / Microporous and Mesoporous Materials 96 (2006) 245–254 247

IR spectra of adsorbed pyridine were recorded using aNicolet-510P apparatus. The samples were pressed intothin wafers and placed in a Pyrex glass cell equipped withCaF2 windows. The samples were pretreated in situ at300 �C for 1 h under vacuum (10�6 Torr) and then cooledto room temperature. Afterward, the pyridine was passedover the sample for 30 min and the pyridine adsorptionspectra were recorded after desorption at 150 �C for1 h.

Hydrogen 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 N2 at 550 �C for 1 h to remove the hydrogen. Details ofthe experimental procedure have been described elsewhere[17] except that the high temperatures for chemisorptionare 300 �C and 450 �C.

Inductively coupled plasma (ICP) optical emissionspectrometry was used for the determination of the metalcontent in each sample after the hydrothermal treatment.The measurements were performed with a JA1100 induc-tively coupled plasma quantometer, and the sample wasdissolved in a mixture of HF and HNO3 acids before themeasurement.

Transmission electron microscopy studies were con-ducted using JEM-2010 (Japan) instruments after eachtreatment. Before being placed on the support, the sam-ples were dispersed in excess ethanol by sonicating for30 min.

Temperature-programmed reduction (TPR) was mea-sured with the same apparatus as that of the hydrogenchemisorption. Prior to the TPR experiments, the catalystswere dried in flowing N2 at 400 �C for 1 h. 5% H2/N2 wasused as the reducing gas at a flow rate of 40 ml min�1. Therate of temperature rise in the TPR experiment was10 �C min�1 up to 650 �C.

2.4. Catalytic reaction

After the treatment, propane dehydrogenation was car-ried out in a conventional quartz tubular micro-reactor.The catalyst (mass 0.2 g) was placed into the center ofthe reactor. Reaction conditions were as follows: 590 �Cfor reaction temperature, 0.1 Mpa pressure, H2/C3 = 0.25(molar ratio) and the propane weight hourly space velocity(WHSV) is 3.0 h�1. The reaction products were analyzedwith an online GC-14C gas chromatography and the con-version of propane and selectivity of alkenes or alkaneswere defined as follows:

Conversion of propane ð%Þ ¼ content of propane in the feedstock

content of propan

Selectivity of alkenes ðalkanesÞ ð%Þ ¼ content of alkenes ðalkanes

content of propan

3. Results and discussion

3.1. Catalyst characterization

Fig. 1 shows the XRD patterns of the different samples.The XRD of the ZSM-5 has the representative peaks at8–9� and 22–24�, corresponding to that given in Ref. [18]for pure ZSM-5. After impregnation, the structure of thefresh catalyst (Fig. 1b) is similar as that of the pureZSM-5 zeolite, suggesting that the structure of zeolite isnot destroyed during the process of catalyst preparation.As reported previously [10,11], part of Na+ and Sn4+ couldenter the main channels of ZSM-5 because their averagediameters were less than the ZSM-5 pore mouth, whereasthe platinum particles were located mainly on the externalsurface of the zeolite due to their relatively large averagediameters. Furthermore, as shown in Fig. 1, no new PtO2

species has been detected in the different catalysts, demon-strating that the platinum species are well dispersed on theexternal surface of the catalyst. After hydrothermal treat-ment at 400 �C for 4 h (Fig. 1c), the intensities of the cor-responding peaks of angle 2h = 22–24� decrease a little incomparison with those of the fresh sample, indicating thatthe crystallinity of the catalyst is degraded [19]. Moreover,the extent of the crystallinity loss is more evident with theincrease of hydrothermal temperature (Fig. 1d) and hydro-thermal time (Fig. 1e). Therefore, it is concluded that thepercentages of crystallinity loss would mainly depend onthe hydrothermal treatment conditions. Similar effect wasalso observed by Sombatchaisak et al. [20]. It is worth not-ing that the lowest percent crystallinity is observed for thehydrotreated sample at 650 �C for 4 h (Fig. 1f). As indi-cated by several authors [21,22], the loss in crystallinitywas mainly related to dealumination of the zeolite duringthe process of steaming.

27Al MAS NMR spectra for all of the samples in thisstudy are collected. As shown in Fig. 2, the fresh sampleexhibits a strong signal at about 54 ppm, which is attrib-uted to four-coordinated aluminum, and another weakpeak assigned to six-coordinated extra-framework Al(EFAL) at 0 ppm [23] that is related to the production ofsome non-framework Al species during the process ofcalcination. For a comparison, after the hydrothermaltreatment at 400 �C for 4 h (Fig. 2b), a slight decrease insix-coordinated aluminum is observed, indicating thatthe amount of extra-framework Al species decreases. Thereason of this behavior is that some amounts of non-framework Al species in the main channels of ZSM-5 canbe cleaned under mild hydrotreatment, thus smoothing

� content of propane in product

e in the feedstock� 100

Þ in product� content of alkenes ðalkanesÞ in feedstock

e in feedstock� content of propane in product� 100

150 100 50 0 -50 -100

(e)

(d)

(c)

(b)

(a)

Chemical shift (ppm)

Fig. 2. 27Al MAS NMR spectra of (a) PtSnNa/ZSM-5 (fresh sample), (b)hydrotreated at 400 �C for 4 h, (c) hydrotreated at 550 �C for 4 h, (d)hydrotreated at 550 �C for 8 h, (e) hydrotreated at 650 �C for 4 h.

Table 1Porous properties of different samples

Sample SBET

(m2 g�1)VP

(cm3 g�1)D

(nm)

PtSnNa/ZSM-5 (fresh sample) 335.4 0.1953 3.6Hydrotreated at 400 �C for 4 h 326.1 0.2001 3.8Hydrotreated at 550 �C for 4 h 317.2 0.1788 3.5Hydrotreated at 550 �C for 8 h 301.6 0.1605 3.3Hydrotreated at 650 �C 4 h 278.7 0.1349 3.0

SBET, surface area derived from BET-method; VP, pore volume obtainedat single point at P/P0 � 0.97; D, average pore diameter.

5 10 15 20 25 30 35 40

Int

ensi

ty (

a.u.

)

(f)

(e)

(d)

(c)

(b)

(a)

2 Theta (degree)

Fig. 1. XRD patterns of (a) ZSM-5 zeolite, (b) PtSnNa/ZSM-5 (freshsample), (c) hydrotreated at 400 �C for 4 h, (d) hydrotreated at 550 �C for4 h, (e) hydrotreated at 550 �C for 8 h, (f) hydrotreated at 650 �C for 4 h.


the channels of the zeolite. However, increasing the hydro-thermal temperature and the time, more decrease in four-coordinated aluminum and increase in six-coordinatedaluminum are observed. This means that in these cases moreAl species have migrated from framework tetrahedral posi-tions and existed as nonstructural Al species in the catalysts.Moreover, under severe conditions, a small shoulder atabout 30 ppm is visible, which can be assigned to pentaco-ordinated aluminum atoms [24] or distorted tetrahedrallycoordinated aluminum atoms in extra-framework species[25,26], implying that the strong dealumination hasoccurred.

The porous properties obtained from N2 adsorption aregiven in Table 1. It is obvious that the fresh sample pos-sesses the largest BET surface area; instead, a decrease inthe surface area is observed after the hydrothermal treat-

ment. Furthermore, this tendency becomes more apparentunder the severe conditions. In the sample hydrotreated at650 �C for 4 h, the surface area decreases about 16.9%when comparing with that of the fresh sample. Previousstudy showed that the BET surface area of Y zeolitedecreased upon the hydrothermal treatment because ofthe collapse of the zeolite framework [27]. However, in thisstudy, the possibility about the collapse of ZSM-5 frame-work is little as it is revealed from their XRD patterns,and the changes in BET surface areas are more related tothe modifications of physicochemical properties of the zeo-lite due to the effect of dealumination. With respect to porevolume and average pore diameter of the different samples,the change tendency of them is similar. Both of them reachthe maximum as for the mild steaming sample, whereas theopposite effect is observed under the severe conditions. Ascommented before, in the former case, some non-frame-work Al species in the channels of the zeolite experiencecleaning, which results in an increase of pore volume andaverage pore diameter. However, increasing the hydrother-mal temperature and time, the amount of Al atoms thatremove from the framework positions increases progres-sively and silicon atoms can replace the preserved posi-tions. Therefore, the decrease of the pore volume of thecatalyst is inevitable because more amounts of non-frame-work Al species exist in the channels of zeolite. In addition,considering that the bond length of Si–O (0.165 nm) isshorter than that of Al–O (0.175 nm), which makes theframework of zeolite shrink, decreasing the average porediameter of the catalyst.

3.2. Characterization of acidic sites

The dealumination of the zeolite during the process ofsteaming can affect the catalyst acidity obviously. NH3

temperature-programmed desorption (TPD) is an effectivetechnique, which can provide information on the amountand strength of acid sites. Generally, the peak area of aTPD profile represents the amount of desorbed NH3,whereas the peak position corresponds to the strength ofacidity. Fig. 3 shows the NH3-TPD profiles of the differentsamples. There displays two ammonia desorption peaks forZSM-5 zeolite: the first desorption peak at about 240 �C isassigned to the weak acid sites, whereas the second peak atabout 450 �C corresponds to strong acid sites, which is ingood agreement with the reported literature data [28]. As

100 200 300 400 500 600

(f)

(e)

(d)

(c)

(b)

(a)

Temperature (°C)

Fig. 3. NH3-TPD profiles of (a) ZSM-5, (b) PtSnNa/ZSM-5 (freshsample), (c) hydrotreated at 400 �C for 4 h, (d) hydrotreated at 550 �C for4 h, (e) hydrotreated at 550 �C for 8 h, (f) hydrotreated at 650 �C for 4 h.

1600 1550 1500 1450 1400

(f)

(e)

(d)

(c)

(b)

(a)

Wavenumber (cm-1)

Fig. 4. IR spectra, evacuated at 150 �C, of different samples: (a) ZSM-5,(b) PtSnNa/ZSM-5 (fresh sample), (c) hydrotreated at 400 �C for 4 h, (d)hydrotreated at 550 �C for 4 h, (e) hydrotreated at 550 �C for 8 h, (f)hydrotreated at 650 �C for 4 h.


for the untreated catalyst, a distinct decrease of ammoniadesorption at low temperature, and almost no ammoniadesorption at high temperature are observed, suggestingthat the addition of sodium can neutralize the strong acidsites preferentially. It is proposed that exchange of protonsof Bronsted acid sites by sodium ions may exist since Bron-sted acid sites are the main strong acid sites [29]. After thehydrothermal treatment, all samples exhibit the similarTPD profiles. However, with increasing hydrothermal tem-perature and time, the amount of the weak acid sitesdecreases continuously and the acidic intensity becomesweak gradually, evidenced from the lower ammoniadesorption temperature compared to other samples.

Pyridine adsorption IR spectroscopy is another power-ful technique for measuring and distinguishing the acidicsites on a zeolite surface. Fig. 4 shows the pyridine adsorp-tion behaviors of the different samples. Generally speaking,the two bands at about 1450 and 1540 cm�1 are assigned topyridine molecules adsorbed on Lewis acid sites and Bron-sted acid sites, respectively. Moreover, the absorption bandat 1488 cm�1 is assigned to the absorption of pyridine onthe both acid sites [11,30]. It is clear that ZSM-5 zeolite pre-sents three pyridine absorption bands at 1450, 1488 and1540 cm�1, showing that Lewis acid sites and Bronsted acidsites are all presented. In the case of the fresh sample, theintensity of the Lewis acid sites at 1450 cm�1 decreasesand relatively no band at 1540 cm�1 is observed. Theresults from FTIR spectroscopy are in accordance withthe NH3-TPD results that the promoter of sodium is inpreference to neutralize the Bronsted acid sites. It is foundthat the intensity of Lewis acid sites decreases relativelyslightly after the hydrothermal treatment from 400 �C to550 �C, whereas at 650 �C the loss of these acid sites isimportant. This means that under the conditions of severedealumination, the number of Lewis acid sites that are irre-

versibly lost is greater than that under the mild conditions.The possible reason for this behavior is that the dealumina-tion of aluminum is generally accompanied the transfer ofNa+ to balance the charge. It should be noted that the Na+

cations are also weak acid sites [6], which can interfere indensity of these acid sites measurements, since at least someof the acid sites can be attributed to Na+. However, uponthe severe conditions, the rate of dealumination mayexceed the removal rate of Na+ or the amounts of removalNa+ may approach its saturation, which in consequencedecreases the catalyst acidity. But earlier work [31] foundthat the important loss of acidity due to irreversible deacti-vation took place at 500 �C, and this phenomenon wasabsent at 400 �C, incipient at 450 �C. The difference inthese two observations may be the Si/Al ratio of the zeolitethat we used is much higher than that of the reported one(Si/Al = 24). Generally, the zeolite can obtain higherhydrothermal stability with higher Si/Al ratio [32]. Onthe other hand, it has been proven that the treatment withsteam can yield a decrease in the amount of acidic bridginghydroxyl (Si–OH–Al) groups [13], thus decreasing theamount of Bronsted sites. Nevertheless, in our experiments,the amount of Bronsted sites in the untreated catalyst is lit-tle, which induces the change of these acid sites cannot beclearly observed during the process of hydrothermaltreatment.

3.3. Characterization of active sites

The morphology of the different catalysts is character-ized by transmission electron microscopy (Fig. 5). It is evi-dent that for the fresh sample, no species is observed on theexternal surface of ZSM-5 zeolite under a certain magnifi-cation (Fig. 5a). The high-resolution electron microscopy(HREM) image shows that the Pt particles with diameter

Fig. 5. Transmission electron micrographs of the fresh sample (a, b),hydrotreated sample at 400 �C for 4 h (c, d), at 550 �C for 4h (e) and 8 h(f), at 650 �C for 4 h (g), SADP of the agglomerated particles (h).

100 200 300 400 500 600 700

(e)

(d)

(c)

(b)

(a)

Temperature (°C)

Fig. 6. TPR profiles of (a) PtSnNa/ZSM-5 (fresh sample), (b) hydro-treated at 400 �C for 4 h, (c) hydrotreated at 550 �C for 4 h, (d)hydrotreated at 550 �C for 8 h, (e) hydrotreated at 650 �C for 4 h.

Table 2H2 uptake of different samples

Catalyst H2 uptake (ml g�1)

30 �C 300 �C 450 �C Total

Fresh sample 5.9 20.2 11.3 37.4Hydrotreated at 400 �C for 4 h 5.2 19.4 10.7 35.3Hydrotreated at 550 �C for 4 h 3.9 17.6 9.4 30.9Hydrotreated at 550 �C for 8 h 2.8 12.3 7.6 22.7Hydrotreated at 650 �C for 4 h 1.6 3.5 1.8 6.9


of 1.5–2.5 nm are observed and dispersed on the externalsurface of the zeolite (Fig. 5b). Concerning the samplehydrotreated at 400 �C for 4 h, only a slight change inthe particle size and particle population is observed(Fig. 5c and d), suggesting that the mild steaming has littleimpact on the surface morphology of the catalyst. How-ever, some agglomerated particles, about 10 nm in size,are clearly observed after the hydrothermal treatment at550 �C for 4 h (Fig. 5e). It should be noted that the agglom-erated particles are formed in the neighboring regions ofthe initial particles that spread during heating in steam.Consequently, the formation of the new particles is a resultof the conglomeration of the initial particles. Prolonging

the hydrotreatment time, this phenomenon becomes moreapparent (Fig. 5f). Worthy of mention is that an obviousincrease in the population of the large particles becauseof their sintering by migration and coalescence are foundafter the treatment at 650 �C, as well as the average particlesize. In order to clarify the possibility of Pt sintering,selected area electron diffraction analysis is measured, asshown in Fig. 5h, SADP taken from the agglomeratedparticles. By measuring three different groups of ring radius,the diffraction rings can be indexed as (111), (200) and(22 0) crystal faces of Pt. Therefore, these particles can beidentified as Pt phase by such diffraction analysis. Thesefindings suggest that the hydrothermal treatment can affectthe dispersive status of Pt particles on the external surface ofthe catalyst effectively.

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. It can be seen that the fresh catalyst


shows a relatively little amount of H2 that adsorbed atroom temperature, whereas the amount of adsorbed H2 isrelatively high when the experiment is carried out at300 �C and 450 �C, suggesting that some interactions existbetween Pt and SnOx. Following Lin et al. [33], Pt could belocated on SnOx modified carrier surface to form a newactive sites with ‘‘sandwich structure’’, which was favorablefor the reaction. With the increase of hydrothermal temper-ature and time, the amounts of adsorbed H2, not only atroom temperature, but also at high temperature decreaselinearly, revealing that in these circumstances, the interac-tions between Pt and SnOx decrease, and surface characterof active sites changes. On the sample hydrotreated at650 �C for 4 h, sharply decreased amounts of H2 includingat room and high temperature are observed, demonstratingthat severe hydrothermal treatment may significantlychange the interactions between Pt and SnOx, and in thiscase, the proportion of Pt that is dispersed highly on exter-nal surface of the catalyst decreases remarkably. Accordingto the above TEM analysis, this can be attributed to thestrong sintering of Pt particles. The results of the chemi-sorption experiment are consistent with the TEM findings.

In order to discuss the possible reasons of Pt sintering,elemental contents of different samples (by inductively cou-pled plasma spectroscopy) are measured. As listed in Table3, after hydrothermal treatment, the change tendency ofPt and Na contents can be considered negligible, whilethe contents of Sn and Al decrease clearly, suggestingthat the hydrothermal treatment obviously influences theelement contents of the catalysts. As mentioned above,the decreased content of Al can be attributed to the dealu-mination of ZSM-5 zeolite. Some Al species may havemigrated from the framework to form extra-frameworkaluminum species and these species can dissolve easily inwater [34]. As for the change of Sn content, it is proposedthat the loss of Sn species is also related to the dealumina-tion of the zeolite, because the existence of aluminum canstabilize Sn species [33]. Therefore, the degree of Sn losswould mainly depend on the extent of dealumination ofthe zeolite. According to this mechanism, under the severesteaming conditions (650 �C for 4 h), the loss of Sn speciesis stronger, as can be seen from the data in Table 3.

Fig. 6 shows the H2-TPR profiles of the different sam-ples. The fresh catalyst presents a peak, whose maximumis placed at 260 �C, and also two other peaks at about440 �C and 590 �C, respectively. The signal at 260 �C is

Table 3Element contents of different samples

Catalyst Element contents (%)

Pt Sn Na Al

Fresh sample 0.56 0.84 0.76 11.01Hydrotreated at 400 �C for 4 h 0.57 0.81 0.77 10.67Hydrotreated at 550 �C for 4 h 0.58 0.73 0.75 8.56Hydrotreated at 550 �C for 8 h 0.57 0.64 0.74 6.72Hydrotreated at 650 �C for 4 h 0.56 0.31 0.71 2.54

ascribed to reduction of Pt, whereas the high-temperaturepeaks represent reduction of Sn4+ to Sn2+ and Sn2+ toSn0 [11,35]. After the hydrothermal treatment at 400 �Cfor 4 h, sharply decreased reduction peak of Pt species isobserved. Moreover, with the increase of hydrothermaltreatment temperature and time, this tendency becomesmore apparent. This fact reveals that the hydrotreatmenthas obvious impact on reduction behavior of PtO2. Atthe same time, in the sample hydrotreated at 400 �C for4 h, the higher temperature peak corresponding to thereduction of SnO2 to Sn2+ increases considerably, meaningthat most of Sn forms SnO2 after the hydrothermal treat-ment. Morales et al. [36] investigated the oxidation statesof Sn on the surface of PtSn/H[Al]ZSM-5 catalyst byXPS and found that most of tin existed in a form of SnOx

(0 < x < 2). Therefore, the increase in the Sn4+ reductionsignal can be attributed to the oxidative effect of steam.This is because the dealumination of ZSM-5 is inevitableduring the steaming process, which in consequence weak-ens the interactions between Sn species and the carrier.However, when the sample that is hydrotreated at 550 �Cfor 8 h, a decrease in this reduction peak is found. The pos-sible reason for this behavior is that the distinct loss of Snspecies, thus reducing the amount of oxidized Sn species.Moreover, this tendency becomes more apparent byincreasing the hydrotreatment temperature (650 �C). Onthe other hand, it is interesting to note that another high-temperature peak representing the reduction of SnO toSn0 decreases remarkably after the hydrothermal treat-ment, demonstrating that the possibility about the forma-tion of Pt–Sn alloy is little in our experiments.

Concerning the sample hydrotreated at 650 �C, it is sug-gested that the bimetallic Pt–Sn catalyst changes into themonometallic Pt catalyst to some extents due to the strongloss of Sn species, which can influence the catalytic behavioreffectively. It should be noted that Tamman temperature ofPt particles is about 409 �C, upon this temperature theseparticles start to move [17]. Taking into account that someinteractions between Pt and carrier exist, so that the moveof Pt particles must conquer these interactions. Previouswork indicated that the adsorption of water between themetal and support could weaken the interactions betweencrystallite and support [15], thus accelerating the processof sintering. In general, it can be concluded that the dealu-mination of ZSM-5 upon the hydrothermal treatment is theorigin of the sintering, which weakens the stabilized effectfor Sn species and the subsequent loss of Sn species is justthe main reason of the crystallitic sintering.

3.4. Catalytic behavior of the different samples

Fig. 7 shows catalytic activities of the different samplesin the dehydrogenation of propane with H2 at 590 �C.The fresh sample exhibits the highest initial reaction activ-ity, after reaction for 6 h, propane conversion decreasesfrom 38.57% to 33.93%. This is related to the carbondeposits that produced during the reaction, which covers

0 1 2 3 4 5 610

15

20

25

30

35

40

(e)

(d)

(c)

(b)(a)

Prop

ane

conv

ersi

on (

%)

Time on stream (h)

Fig. 7. Effect of hydrothermal treatment on catalytic activity of (a) freshsample, (b) hydrotreated at 400 �C for 4 h, (c) hydrotreated at 550 �C for4 h, (d) hydrotreated 550 �C for 8 h, (e) hydrotreated at 650 �C for 4 h.Reaction conditions: 590 �C, H2/C3 = 0.25 (molar ratio), m(cat) = 2.0 g,WHSV of propane is 3.0 h�1.

0 1 2 3 4 5 690

92

94

96

98

100

(e)

(d)

(c)

(b)

(a)

Prop

ene

sele

ctiv

ity (

%)

Time on stream (h)

Fig. 8. Propene selectivity of (a) fresh sample; (b) hydrotreated at 400 �Cfor 4 h, (c) hydrotreated at 550 �C for 4 h, (d) hydrotreated 550 �C for 8 h,(e) hydrotreated at 650 �C for 4 h. The reaction conditions are the same asin Fig. 7.


the active metal. After the hydrothermal treatment at400 �C, the catalytic activity decreases, even though themain channels of ZSM-5 zeolite are cleaned to someextents. It is proposed that in our study, the chance ofthe reactant gas to enter the ZSM-5 channels is little,because the active metal is located on the external surfaceof the catalyst and it is easier for reactant gas to escapeto the gas stream than forcing its entrance through the poremouths. As for the sample hydrotreated at 550 �C, the con-version of propane decreases almost linearly, especially theone that is hydrotreated for 8 h. These findings suggest thatthe steam treatment at different temperature can stronglyinfluence the catalytic properties. To explain these, itshould be noted that the catalysts are bifunctional andthe two active centers of PtSnNa/ZSM-5 (the metal particleand the acid site) may work collaboratively [37]. In thisway, an optimum ratio between the number of active sitesand the number of acid sites should exist. As commentedbefore, the catalytic acidity decreases, and the characterof Pt particles changes after the hydrothermal treatment,thus destroying the initial matching between the metallicfunction and acid function in PtSnNa/ZSM-5 catalyst,which results in the decrease of catalytic activity. In thecase of the steamed sample at severe conditions (650 �C),the deactivation is very fast, propane conversion decreasesabout 47.56% after reaction for 6 h. This is because in thiscondition, the sintering of Pt particles is effective, so thatthe metallic function of the catalyst is destroyed strongly,which is disadvantageous to the reaction. Furthermore,the surface area of the catalyst decreases largely, whichcan also significantly affect the catalytic behavior in dehy-drogenation of propane.

The corresponding propene selectivities of the differentsamples are shown in Fig. 8. It is clear that for the fresh

sample, propene selectivity increases gradually with thereaction time. After the hydrothermal treatment at400 �C for 4 h, the increase of propene selectivity slowsdown and reaches 98.67% after reaction for 6 h. The hydro-treatment at 550 �C for 4 h results in the changeless selec-tivity to propene. However, the decreased selectivity topropene is observed by prolonging the hydrothermal treat-ment time. These findings suggest that the mild treatmentwould be favorable for the production of propene. It isworth noting that the selectivity to propene decreases con-siderably when the steaming temperature is 650 �C, itdecreases about 6.34% after reaction for 6 h.

It is known that on PtSnNa/ZSM-5 catalyst platinum isthe only active metal and the propene is only formed on themetal by dehydrogenation; the main cracking product (eth-ene) is mainly formed from cracking on the carrier and theethane is formed by hydrogenolysis of propane and byhydrogenation of ethene, with both reactions taking placeon the metal [38]. Furthermore, on HMFI zeolite thedehydrogenation and cracking of propane is assumed toproceed through carbonium-ion intermediates [39]. Obvi-ously, the higher acid sites generally promote the subse-quent cracking reaction of the initially formed Cþ3carbenium ions. According to these mechanisms, it is pro-posed that the changes of catalytic acidity and active sitesare responsible for the selectivity to propene. Table 4 liststhe reaction data of the different samples after reactionfor 6 h. When the hydrothermal temperature is 400 �C,the selectivities to side reaction products decrease, thusimproving propene selectivity. This is because the catalyticacidity decreases and relatively no Pt sintering is observed.In other words, only the acid function of the catalystchanges, so that the reduced acidity inhibits the side reac-tions to be carried out. However, on the sample hydro-treated at 550 �C, the opposite effect is observed, which is

Table 4Reaction data of different samples after reaction for 6 h

Catalyst Propaneconversion(%)

Selectivity (%) Propene yield (%)

Methane Ethane Ethene Propene

Fresh sample 33.93 0.80 1.08 0.51 97.61 33.12Hydrotreated at 400 �C for 4 h 32.03 0.42 0.56 0.33 98.69 31.61Hydrotreated at 550 �C for 4 h 28.24 0.66 1.01 0.54 97.79 27.62Hydrotreated at 550 �C for 8 h 22.32 1.32 1.74 0.80 96.14 21.46Hydrotreated at 650 �C for 4 h 12.37 3.35 0.93 5.25 90.47 11.19


related to the slight loss of Sn species and small percent ofPt sintering. As known, the promoter of Sn can improvethe catalytic selectivity by inhibiting the side reactions,and the sintering of Pt causes a reduced yield of propeneduring the reaction. Obviously, by extending the hydro-thermal time, this tendency becomes more apparent.Respect to the sample hydrotreated at 650 �C, strong lossof Sn species is found as observed in ICP experiment.Therefore, the side reactions carry out evidently, althoughthe acid amount of the catalyst is little. On the other hand,it is worthy noting that in this case the selectivity to ethaneis much lower than that of the hydrotreated sample at550 �C, which can be attributed to the more percents ofPt sintering, thus decreasing the amount of hydrogenolysisreaction products during the reaction.

3.5. Model for the influence of hydrothermal treatment on

catalytic properties of PtSnNa/ZSM-5 catalyst

According to the results that obtained above, a modelfor the influence of hydrothermal treatment is proposed(Fig. 9).

On the fresh sample (Fig. 9a), Pt particles are dispersedhighly on the external surface of the catalyst and someamounts of tin are deposited over the surface of platinum,

PtxSnO

SnO

SnO

SnO

SnOx

Pt1

SnO

O O

O

OPt2 Pt2

Pt1Sn

O

O O

O

OO

Pt2 Pt2 Destroyed

Pt Pt

Pt Pt

Ptxx

Ptx

Pt

Pt

SnO

Fig. 9. Model for the influence of hydrothermal treatment on catalyticproperties of (a) fresh PtSnNa/ZSM-5 sample, (b) hydrotreated at 400 �Cfor 4 h, (c) hydrotreated at 550 �C for 4 h, (d) hydrotreated at 650 �C for4 h.

existing in a form of SnOx (0 < x < 2). After the hydrother-mal treatment at 400 �C for 4 h (Fig. 9b), slight loss of Snspecies is found as shown in Table 3, therefore, the interac-tions between Pt and Sn are almost identical in comparisonwith those of the fresh one. It is known that two kinds ofactive Pt species may exist on the surface of the catalyst,named Pt1 sites and Pt2 sites [33]. Pt1 sites are the sites inwhich Pt directly anchors on the carrier surface, while Pt2

sites corresponding to the sites in which Pt anchors Sn oxidesurface, with a ‘‘sandwich structure’’ composed of PtSnNa/ZSM-5. Generally, Pt1 sites are responsible for the side reac-tions, while Pt2 sites are the main reaction active sites for thedehydrogenation of propane. Concerning the sample hydro-treated at 550 �C for 4 h (Fig. 9c), some amounts of Sn spe-cies experience loss and the initial ‘‘sandwich structure’’ isdestroyed to some extents, making the Pt species that anchorSn oxide previously anchor the carrier surface directly. Thatis to say, in this circumstance, the proportion of Pt1 sitesincreases, while the proportion of Pt2 sites decreases, whichresults in the decrease of selectivity to propene and increaseof selectivities to side reaction products, as evidenced fromthe data in Table 4. Prolonging the hydrothermal treatmenttime, this change direction becomes more remarkably. Asfor the sample hydrotreated at 650 �C for 4 h (Fig. 9d),strong loss of Sn species is occurred and the ‘‘sandwich struc-ture’’ is destroyed almost completely. In other words, in thiscase the bimetallic Pt–Sn catalyst nearly changes into themonometallic Pt catalyst. Consequently, the transport ofPt particles on the surface of the catalyst is inevitable becausethe interactions between the metal and carrier become weakvia the adsorption of water, which in consequence acceler-ates the sintering of Pt particles.

4. Conclusions

Hydrothermal treatment has obvious influence on cata-lytic pore properties of PtSnNa/ZSM-5 catalyst. Under themild treatment, some non-framework Al species in the chan-nels of ZSM-5 zeolite can be cleaned, thus increasing thepore volume and the average pore diameter of the catalyst.However, the opposite effect is observed with the increaseof hydrothermal temperature and time, because the extentof the dealumination of ZSM-5 increases under severesteaming conditions. The acidic characterization techniqueshows that the acid amount and acid intensity are affectedby hydrothermal treatment. The intensity of Lewis acid sites


decreases slightly when the temperature of hydrotreatmentchanges from 400 �C to 550 �C, and important loss of aciditytakes place at 650 �C. Moreover, the hydrothermal treat-ment has evident impact on the dispersive status of Pt parti-cles on the external surface of the catalyst. Results ofcatalytic activity indicate that the hydrotreatment at650 �C induces the irreversible deactivation during the useof this treated catalyst in the dehydrogenation of propane.

This deactivation is assumed to be related to modifica-tions in the behavior of active sites or to changes in theactive sites themselves. According to the analysis byTEM and hydrogen chemisorption, deactivation can berelated to sintering of Pt species. This is because underthe severe conditions, strong loss of Sn species is occurredand the initial ‘‘sandwich structure’’ of active sites isdestroyed almost completely. Therefore, the bimetallicPt–Sn catalyst changes into the monometallic Pt catalystto certain extents, accelerating the process of Pt sintering.It is suggested that the dealumination of the zeolite is theoriginal reason of the catalytic deactivation and the subse-quent loss of Sn species is just the main reason of the crys-tallitic Pt sintering.

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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