hydrothermal study of pt–sn-based sapo-34 supported novel catalyst used for selective propane...

Journal of Industrial and Engineering Chemistry 16 (2010) 774–784

Hydrothermal study of Pt–Sn-based SAPO-34 supported novel catalyst used forselective propane dehydrogenation to propylene

Zeeshan Nawaz *, Fei Wei

Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology (FLOTU), Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

A R T I C L E I N F O

Article history:

Received 24 November 2009

Accepted 15 April 2010

Keywords:

Pt–Sn/SAPO-34

Propylene

Dehydrogenation

Hydrothermal treatment

Sintering

A B S T R A C T

Catalyst’s regeneration is unavoidable part during dehydrogenation. The hydrothermal treatment

influence on the performance of Pt–Sn-based SAPO-34 supported novel catalyst, used for propane

dehydrogenation to propylene is investigated in this study. The catalyst shows excellent stability for

mild steaming (nitrogen mixed steam), during four consecutive runs (reaction–regeneration mode). On

the other hand, Pt–Sn/ZSM-5 was largely affected on mild steaming due to severe dealumination. In

order to get into mechanistic understanding, severe hydrothermal treatment was carried our using pure

steam. The substation loss in catalyst activity was noted. Both fresh and severe hydrotreated catalysts

were characterized by XRD, XRF, O2-pulse analysis of coke, NH3-TPD, IR spectrum of adsorbed ammonia,

H2-TPR, HR-TEM and XPS, to explore reasons for change in catalytic properties. The texture/topology is

found stable. Changes in surface ensembles occur due to the loss of Sn, Al, formation of SnOx species and

in particular Pt sintering. This leads Pt active sites (zeolite–SnO–Pt) to inactive sites (zeolite–Pt, zeolite–

PtO–Sn, Pt–Sn alloy, etc.) formation and reduced catalyst activity. TEM micrographs and H2-

chemisorption analysis confirms Pt particles agglomeration and/or sintering. About 98% catalyst

activity is recovered by re-dispersed Pt using chlorination technique and credit goes to hydrothermally

stable support (SAPO-34).

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1. Introduction

The high demand for olefins, especially propylene, boost upresearch activities in both catalysis and reactor design, to developon-purpose propylene technologies. In order to meet the growingmarket demand of propylene, emphasize has been given particu-larly to light alkane dehydrogenation [1–19]. The propane and/orbutane dehydrogenation is the most economical route to propyl-ene, while it is very complex in thermodynamical and engineeringconstraints [1–19]. Therefore, the numbers of catalysts for bothoxidative and direct propane dehydrogenation routes wereproposed in last two decades.

Both platinum and chromium based catalysts were largelyreported for the alkane dehydrogenation. The chromium oxide-based catalysts have been used in the absence of hydrogen as a co-feed gas, and platinum-based catalysts were used in the presenceof hydrogen [10–22]. While, platinum-tin-based catalysts werereported as the most promising catalytic formulation for propanedehydrogenation [10–26]. Therefore, attractive results has been

* Corresponding author. Fax: +86 10 62772051.

E-mail address: [email protected] (Z. Nawaz).

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doi:10.1016/j.jiec.2010.07.002

reported by using Pt–Sn-based catalyst supported on Al2O3

[21,22,27–29], SiO2 [22] and ZSM-5 zeolite [7,9,22]. The Pt–Sn-based SAPO-34 supported novel catalyst for propane and butaneselective dehydrogenation was recently proposed with promisingresults [10,11]. Moreover, in Pt–Sn-based catalysts the presencesof Sn facilitate the transfer of coke from metal to support and Ptdispersion. Relatively weak acidity and larger surface areaprovided by SAPO-34 than ZSM-5 not only provide superiordehydrogenation rate, but also helps in reducing coke formation[10,11]. Moreover, many attempts have been made to improve theperformance of these catalysts using alkali and alkaline earthmetals [30–33]. The influence of tin (as promoter) on theperformance of Pt–Sn-based catalysts has also been extensivelyinvestigated and concluded that Sn modifies the electronic densityof Pt, decreases size of platinum ensembles, reduces cokeformation, etc. [11,21,22,26–39]. Recently it has been reportedthat the Sn forms coordinative unsaturated surface with Pt sitesand preferentially covering of these high energetic Pt sites by Snenhance catalytic performance for propane dehydrogenation [21].In our work it is observed that the intensification of Pt–Sn-basedcatalyst by adopting suitable support can enhance the catalyst’sactivity for alkane dehydrogenation [11]. However, an unavoidableproblems associated with these catalysts are the coke formation,

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Z. Nawaz, F. Wei / Journal of Industrial and Engineering Chemistry 16 (2010) 774–784 775

deactivation and activity recovery [7,26,40], those lead towardsoperational complexities.

Minachev’s school float an idea, using X-ray spectroscopytechniques that the metal clusters on the surface of acid oxides areelectron-deficient species, and this deficiency increases with anincrease in acidity [41]. It was further explained, that the increasein the positive charge on a metallic nanoparticle increases with thelocal concentration of Bronsted acid sites. Therefore Bronsted acidsites act as electron-acceptor centers of the support, and theseinteractions ultimately leads to a decrease in the electron densityon metal nanoparticles. And the increase in electron deficiency ofmetal particles enhances catalytic activity of bifunctional catalyst[42]. It is well known that the dehydrogenation and hydrogenationsteps occur on the metal particles, and isomerization and carbon–carbon bond dissociation occur on the acid site. The stereo-chemistry of the light alkane dehydrogenation and thermody-namics is tide up with each other, and this problem is resolved byselecting suitable support SAPO-34 [10,11,43]. Mees et al.experimentally prove that the transfer of charge from SAPO-34framework further enhance its hydrothermal stability [44].

Nevertheless, the effect of steam on supported metal catalystswas reported in few reports [27,45], while Pt–Sn-based SAPO-34supported novel catalyst was not studied yet. This work evaluatesthe influence of hydrothermal treatment (mild and extensive) onthe performance of Pt–Sn/SAPO-34 catalyst. The performance ofthe catalysts was experimentally determined using fixed bedmicro-reactor and compared with ZSM-5 supported catalysts [7,9–11]. The possibilities of structural modifications, loss of metalliccontent, acidity, etc. were analyzed using number of physio-chemical techniques. The bi-metallic catalyst is completelycharacterized after extensive steaming and activity loss wassophisticatedly explained. Activity loss is recovered by chlorina-tion technique. Therefore, the present work helps in understandinginfluence of severe steaming on catalytic properties of SAPO-34supported catalyst during dehydrogenation and their superiorityin operation.

2. Experimental

2.1. Catalyst preparation

The bimetallic Pt–Sn based SAPO-34 zeolite supported catalystwas prepared by sequential impregnation method [10–14], withpowdered H-SAPO-34, calcined at 550 8C. First the SAPO-34zeolite was co-impregnated with 0.16 M solution of SnCl2�2H2O at80 8C to ensure 1 wt.% Sn promoter in the final catalyst. Thecatalyst was dried at 100 8C for 3 h and then calcined at 500 8C inmuffle furnace for 4 h. The prepared Sn/SAPO-34 was again co-impregnated with 0.03 M solution of H2PtCl6 at 75 8C to ensure0.5 wt.% Pt in the final catalyst. Afterwards, the same procedurewas repeated, that the finally prepared bi-metallic was dried at100 8C for 3 h and then calcined at 500 8C for 4 h. The prepared bi-metallic catalyst was crushed to fine powder. Pt–Sn/ZSM-5 wasalso prepared in similar manner. Before the reaction, catalystswere passing through hydrothermal treatment and then reducedin hydrogen for 8 h.

2.2. Hydrothermal treatment

In mild hydrothermal treatment, the catalyst was regeneratedat 600 8C for 4 h with nitrogen mixed steam at 40 ml/min, andreused for second run. The steam partial pressure was 20 kPa. Afterregeneration, the catalysts were dried in flowing nitrogen for20 min, and then reduced in hydrogen at 500 8C before the nextreaction run. The similar procedure was adopted before eachexperimental run.

In severe hydrothermal treatment, the catalyst samples wereexposed to pure steam at different temperatures 500, 600 and700 8C, at 2 ml/g flow rates for 4 h. Later two samples weresteamed at 500 and 600 8C with the flow rate of 2 ml/g for 8 h.

2.3. Reaction test

The propane dehydrogenation over mild and severe hydrother-mally treated Pt–Sn/SAPO-34 samples was conducted in a plug-flow micro-reactor at atmospheric pressure. The 99.5% purepropane feed was used in current experimentation (provided byZhong Ke Hui Jie, Beijing, China). The measured amount of catalystwas loaded to the reactor, in order to maintain desired WHSV (i.e.5.6 h�1), and reduced under 10 ml/min flowing H2 at 500 8C, for 8 hprior to each reaction test. The reaction mixture was composed ofH2/C3H8 molar ratios 0.25 and product composition was analyzedusing on line gas chromatography (50-m PLOT Al2O3 capillarycolumn) equipped with FID detector. The process was operated in acontinuous reaction–regeneration–reduction mode for threeconsecutive runs for the performance of mild hydrothermallytreated Pt–Sn-based SAPO-34 and ZSM-5 catalysts; reaction timewas 8 h for each run. Both, mild and severe hydrothermally treatedsamples were tested under identical operating conditions. Theresults were calculated and presented in weight percentages.

2.4. Re-dispersion of Pt by chlorination

The chlorination process is carried out in a conventional setup.Measured amount of catalyst (0.2 g) is heated to 500 8C in a flow ofN2 (15 ml/min), with temperature rising rate 5 8C/min. The C2Cl2H4

solution flask is immersed in the water bath of 0 8C. The N2 flowrate was adjusted to take C2Cl2H4 solution at 2.3 ml/min to thereactor for 1 h, and exhaust gases are passed through concentratedNaOH solution. The dispersed samples were again dechlorinatedand then reduced before reaction tests.

2.5. Catalyst characterization

2.5.1. X-ray diffraction (XRD)

The XRD patterns of pure and hydrothermally treated sampleswere obtained on a powder X-ray diffractometer (Rigaku-2500)having copper anode tube. The X-ray tube was operated at 40 kV/40 mA and Ka radiation (0.1541 nm) was selected with amonochromator. The spectra were scanned at a rate of 58/minfor an angular range 2u from 58 to 458.

2.5.2. X-ray fluorescence (XRF)

The metallic contents of the catalyst samples were obtained byXRF measurements on a Shimadzu XRF 1700 fluorimeter, beforeand after hydrothermal treatment.

2.5.3. BET surface area measurement

The BET surface areas of original and hydrothermally treatedsamples were measured using N2 adsorption/desorption isothermsdetermined through an automatic analyzer Autosrb-1-C. Thesamples were out gassed under vacuum at 300 8C prior toadsorption for 1 h. The surface areas of samples were calculatedusing the BET equation.

2.5.4. O2-pulse coke analysis

The amount of coke formed deposited on catalysts during thedehydrogenation reaction was determined by O2-pulse experi-ments using gas chromatograph flow system equipped with a TCD.The experiments were carried out at 750 8C by injecting pulses ofpure oxygen (99.99%). The CO2 formed was continuously measuredby a TCD detector and amount of coke was calculated. The pulses of

Z. Nawaz, F. Wei / Journal of Industrial and Engineering Chemistry 16 (2010) 774–784776

pure O2 were continued until deposited coke was fully convertedto CO2.

2.5.5. Temperature-programmed desorption of ammonia (NH3-TPD)

The acid properties of Pt–Sn doped SAPO-34 catalyst, before andafter hydrothermal treatment at different temperatures weredetermined by NH3-TPD on conventional apparatus using Auto-chem-II 2920 analyzer. About 0.2 g of each catalyst sample wasplaced into a quartz reactor and treated by dry argon (99.99%) at500 8C for 1 h prior to adsorption of ammonia. The samples werelater saturated with ammonia at room temperature and 1 mbar.Afterwards, the physically adsorbed ammonia was allowed todesorb at 100 8C and 10�3 mbar. Subsequently, the temperaturewas raised to 600 8C at a heating rate of 10 8C/min and NH3

desorption profile was obtained using mass spectrometer Auto-chem-II 2920.

2.5.6. Temperature-programmed reduction of hydrogen (H2-TPR)

Temperature-programmed reduction experiments of pure andhydrothermally treated samples were carried out on a conven-tional setup (same apparatus as used for NH3-TPD) attached toAutochem-II 2920 analyzer. Samples of 0.2 g were loaded into thequartz reactor and dried by flowing N2 at 500 8C for 1 h. The reactorwas heated gradually to 800 8C with a heating rate 10 8C/min. 5%H2/N2 mixture was used as reducing gas. The hydrogen consump-tion as a function of reduction temperature was recorded.

2.5.7. IR spectroscopy

The IR spectra of adsorbed ammonia on hydrothermally treatedsamples were recorded using a NEXUS apparatus (Nicolet, USA).Then the samples were degassed (<10–6 mbar) at 300 8C for30 min and cooled to room temperature. Afterwards, ammoniawas passed over the samples and the ammonia adsorption spectrawere recorded at 100 8C. The concentration of Bronsted and Lewisacid sites was estimated from the areas of the bands, using themolar absorption coefficients of the bands of adsorbed ammonia,set equal to those determined for zeolites.

2.5.8. XPS analysis

The XPS analysis of the samples were obtained both after severehydrothermal treatment and reaction tests at PerkinElmer PHI5000C spectrometer (USA) working in the constant analyzerenergy mode with Al-Ka radiation as the excitation source. Thebinding energies (BEs) were calibrated using carbonaceous C 1sline (284.6 eV) as reference. The elemental analysis was performedusing ion-coupled plasma (ICP) atomic emission spectroscopyequipped with Thermo Electron IRIS Intrepid II XSP spectrometer.

Table 1The performance of Pt–Sn-based catalysts supported on SAPO-34 and ZSM-5, in a contin

were conducted at 585 8C, WHSV 5.6 h�1, H2/C3H8 molar ratio 0.25 and regenerated at

Reaction runs Time-on-

stream (h)

Pt–Sn/SAPO-34

Propane

conv. (%)

Propylene

selectivity (%)

Fresh catalyst I 0 34.8 67.2

2 21.9 88.5

8 11.6 92.7

II 0 26.3 76.9

2 19.2 95.3

8 10.4 93.4

III 0 21.2 77.2

2 16.1 93.3

8 10.1 96.2

IV 0 16.9 78.6

2 12.7 93.7

8 9.3 94.5

2.5.9. TEM analysis

The Pt metal dispersion on support was verified using a JEM2010 high-resolution transmission electron microscope (TEM)equipped with energy dispersive spectroscopy (operated at120.0 kV), both after severe hydrothermal treatment and afterthe reaction. The sample for TEM observation was prepared using acommon sonication method, while thick samples were dispersedin ethanol.

2.5.10. Pulse chemisorptions of hydrogen

The ability to adsorb hydrogen, of fresh and hydrothermallytreated catalysts was determined by pulse chemisorptions ofhydrogen experiments on a conventional setup using GC [46,47].Before pulse chemisorptions experiments, samples (0.2 g ofcatalyst) were reduced under flowing pure H2 (5 ml/min) at400 8C for 2 h, then purged in N2 at 500 8C for 1 h, and successivelycooled to room temperature. The temperature was controlled to�2 8C by a temperature controlled setup. The samples were saturatedby a hydrogen pulse at 25, 300 and 500 8C temperature. Thechemisorptions of hydrogen were measured by using a gaschromatograph (7890-II) with a thermal conductivity detector(TCD). The pulse size was 5 ml/min of 5% (v/v) H2 in N2 mixture,and the time between pulses was 3 min, until no further gas uptake bythe catalyst was observed as indicated by constant peak areas of thelast few injections. The total amount of adsorption was calculated byadding the gas uptake observed in the series of gas injections untilsaturation was reached. The total amount of H2 uptake (volume atroom temperature) equals the sum of H2 uptake at differenttemperatures.

3. Results and discussion

3.1. Catalyst performance under mild steaming

Table 1 shows the performance of Pt–Sn/SAPO-34 and Pt–Sn/ZSM-5 catalysts for propane dehydrogenation to propylene at585 8C and 5.6 h�1 WHSV, in a continuous operation (reaction–regeneration mode), under mild steaming. It was apparentlyobserved that the conversion of both catalysts was decreased aftereach run. In comparison, Pt–Sn/SAPO-34 conversion drop rate isslower than Pt–Sn/ZSM-5, with far superior propylene and totalolefins selectivity. There was no significant change in propyleneand total olefins selectivity pattern, while increase in selectivity’sat higher TOS was due to lower conversion. This problem was alsoreported in previous studies of ZSM-5 and Al2O3 supportedcatalysts, due to dealumination during regeneration with steamand/or oxygen [38]. This built-in defect was eliminated by using

uous runs (reaction–regeneration–reduction cycle, without chlorination). Reactions

600 8C for 4 h.

Pt–Sn/ZSM-5

Olefins

selectivity (%)

Propane

conv. (%)

Propylene

selectivity (%)

Olefins

selectivity (%)

83.7 34.1 45.2 72.6

95.5 29.9 44.5 80.4

97.1 20.4 46.3 83.9

81.3 26.2 46.7 70.8

97.2 22.1 47.1 81.4

96.1 17.5 47.4 84.7

81.8 19.7 48.1 75.1

95.3 14.1 48.8 83.4

97.8 10.8 49.5 87.7

82.4 14.6 50.6 78.2

95.6 9.4 51.9 85.4

95.7 7.3 52.8 89.8

[(Fig._1)TD$FIG]

Fig. 1. Influence of steam temperature on performance of Pt–Sn/SAPO-34 for propane dehydrogenation to propylene (steaming rate is 2 ml/h for 4 h).


inherently safer novel supports SAPO-34, whose structure/acidsites are Si based [10,11]. Therefore, Pt–Sn/SAPO-34 catalystdemonstrated strong hydrothermal stability due to inherentlyrobust characteristics of SAPO-34 support.

3.2. Severe hydrothermal treatment study

In detailed hydrothermal study steaming temperature and timeinfluence on Pt–Sn/SAPO-34 stability was analyzed. The perform-ance results in comparison with fresh catalyst are shown in Fig. 1. Itwas observed that, both propylene selectivity and propaneconversion was stable initially, but reduces largely later on, byincreasing TOS and steam temperature. Similar trend was obtainedfor total olefins selectivity. This reaction behavior is the majortarget of this extensive investigation, in order to find out the routecauses of catalyst deactivation and how to recover it? Normallylower conversion enhances olefins selectivity (Table 1), but afterhydrothermal treatment the opposite trend was obtained, i.e.lower conversion and lower olefins selectivity (Fig. 1), that

[(Fig._2)TD$FIG]

Fig. 2. Effects of steam temperature during hydrothermal treatment on products distribu

is 2 ml/h for 4 h).

indirectly demonstrated domination of directly anchored Pt sites.After steaming at 700 8C the propylene selectivity was decreased to60% at TOS 5 h, with final conversion of about 7%. It is observed thatthe cracking products were enhanced during dehydrogenationover hydrothermally treated catalysts, as shown in Fig. 2.Therefore, ethylene and methane selectivity were increased, whileethane selectivity was found to be stable with steaming tempera-ture. Table 2 shows the deactivation rate of hydrothermally treatedPt–Sn/SAPO-34 catalyst at different temperatures. It was observedthat with the increase in steaming temperature deactivation rateand coke amount were significantly reduced, but it is due to thedecrease in catalyst activity.

Increase in the steaming rate and time at 500 and 600 8C showssignificant changes in desired product selectivity and feedconversion, and results were presented in Fig. 3. The propyleneselectivity dropping rate was almost the same at both tempera-tures. The best propane conversion and highest propyleneselectivity was achieved on hydrothermally treated samples atminimum temperature and minimum steaming rate.

tion of propane dehydrogenation over Pt–Sn/SAPO-34 catalyst (where steaming rate

Table 2Deactivation of fresh and hydrothermally treated Pt(0.5 wt.%)–Sn(1 wt.%)/SAPO-34,

deactivation was defined as Dr = [(X0�Xf)/X0�100]; where, X0 is the initial

conversion at 1 min, Xf is the final conversion at 5 h (reaction temperature was

585 8C and WHSV 2.8 h�1).

Steaming temperature X0 Xf Dr Cokea

Fresh catalyst 25 10 60 0.41

500 8C 20 7 65 0.36

600 8C 5 3.8 2.4 0.07

700 8C 4 3.7 0.75 0.01

a O2-pulse analysis.


3.3. Catalyst characterization

3.3.1. Basic catalyst characterizations

The composition of metals incorporated, was determined fromXRF analysis, before and after severe hydrothermal treatment. Theresults were tabulated in Table 3. It is confirmed that there is noloss of Pt even after severe steaming, while small loss of Sn wasnoted. The surface area (porous properties) tends to decreasedwith the increase in steaming temperature and time. Fresh catalystpossesses larger surface area (i.e. 412.7 m2/g) than hydrothermallytreated catalysts. The rate of loss of BET surface area will increasedwith treatment severity (see Table 3), due to change in surfacecharacteristics, owing to the loss of framework Al, Sn and Ptsintering [48,49]. Sever loss of framework Al and Sn leads theparticles towards pore blocking, which adversely affect the catalystperformance. Decrease in the pore volume is definite as Si–O bondlength (0.165 nm) is smaller than Al–O bond length (0.175 nm)[40]. The smaller dynamic diameter of Sn4+ may accelerate itsmovement to the channels of SAPO-34 [40], while large dynamicdiameter of Pt confirms its presence over the surface of support[10,11]. Moreover, the loss of Sn is verified form XRF results.

3.3.2. Structural analysis

The X-ray diffraction patterns of fresh and severe hydrother-mally treated catalysts are shown in Fig. 4. However, peaks of Ptand Sn are not observed as their concentration is too low.Moreover, XRD pattern did not shows any characteristic change inoriginal structure of the SAPO-34 zeolite support, even afterhydrothermal treatment. This indicates that the structuraltopological resistance of SAPO-34 toward hydrothermal treatmentis much higher. The intensities of the corresponding peaks

[(Fig._3)TD$FIG]

Fig. 3. Effect of hydrothermal duration (with steam flow rate 2 ml/h) on Pt–Sn/

SAPO-34 performance for propane dehydrogenation.

gradually decreased at angle 2u = 8–108 demonstrate slightdecrease in crystallinity with severity of hydrothermal treatment.

3.3.3. Characterization of acidic properties

Hydrothermal treatment has an obvious effect on catalystacidity. While, Pt–Sn-based catalysts supported on SAPO-34 showsbetter stability in acidity in comparison with Pt–Sn/ZSM-5 [40].The major reason for acid sites stability over SAPO-34 is due totheir Si-based acid sites, while in ZSM-5 acid sites are Al based. TheNH3-TPD is an efficient technique to measure amount and acidicstrength. The peak area of a TPD profile represents the amount ofdesorbed NH3, whereas the peak position corresponds to thestrength of catalyst’s acidity. The profiles of fresh and hydrother-mally treated samples were shown in Fig. 5. Two ammoniadesorption peaks were appeared in TPD profile. The first peakbetween 185 and 205 8C was representing weak acid sites and thesecond peak between 330 and 350 8C corresponds to the strongacid sites [11]. Therefore, it is confirmed that the Pt–Sn/SAPO-34has distinctive weak and strong acid sites even after the severehydrothermal treatment. It was observed that the weak acid siteswere significantly decreased, while the strong acid sites werealmost unaffected. A slight increment in Bronsted acid sites afterhydrothermal treatment may be due to some hydroxyl group’sattachment to Pt or Sn. It is clearly observed that the weak aciditywas decreased with hydrothermal treatment intensity, with noshift in peaks positions. In general, no substantial loss of strongacidity was observed after the hydrothermal treatment of SAPO-34supported catalyst as their acid sites were Si based and resistant tohydrothermal environment.

Ammonia adsorption IR spectroscopy is a powerful techniquefor measuring and distinguishing the acid sites of SAPO-34supported catalysts. Fig. 6 shows ammonia adsorption behaviorof fresh and severe hydrothermally treated samples evaluated at100 8C. Both, Lewis and Bronsted acid sites appears at ammoniaabsorption bands between 1620–1640 and 1450–1465 cm�1,respectively. The intensity of acid sites decreased with the increasein hydrothermal treatment temperature. Therefore the IR spectraresults are in accordance with the NH3-TPD results, verified thatthe intensities of the corresponding bands associated withchemisorbed ammonia were decreased after severe hydrothermaltreatment. But generally, during the hydrothermal treatment,Bronsted acid sites decreased as steam affect hydroxyl bridging. Itis believed that higher Si/Al ratio of zeolite has high hydrothermalresistance and Si-based acid sites of SAPO-34 make it more robusttowards hydrothermal treatment [50].

3.3.4. Active sites characterization

The reduction properties of catalysts were measured by H2-TPRand profiles were shown in Fig. 7. However, the H2 adsorption issomehow interfered by the interaction of Pt with Sn oxides, andsupport after hydrothermal treatment. Two reduction peaks wereappears, one at lower temperature (around 100 8C) and the secondwas at higher temperature (between 400 and 700 8C). There isalmost no effect on initial reduction peaks ascribed to reduction ofPtO2 particles. The reduction peaks at higher temperatures indicatereduction of Sn4+ and Sn2+ to Sn0 species [11,51]. Sharpness ofreduction peaks at higher temperatures decreases with theincrease of hydrothermal treatment temperature. However, thereduction peak areas were increased. These higher temperaturepeaks corresponds to the reduction of SnO2 to Sn2+, means thatmost of Sn forms SnO2 after the hydrothermal treatment andadversely affect Pt–Sn interaction, and may form Pt–Sn alloy andblocked Pt active sites. The active Pt could be located on SnOx

surface to become favorable for the dehydrogenation reaction [48].While increase in the area of higher temperature peaks and/orformation of third peak at higher temperatures reveals that the

Table 3Basic catalysts characteristics.

Catalysts Steaming

temperature (8C)

Steam

treatment time (h)

Pt contenta

(%, w/w)

Sn contenta

(%, w/w)

SBETb

(m2/g)

Pt(0.5 wt.%)–Sn(1 wt.%)/SAPO-34 Fresh catalyst 0 0.49 0.97 412.7

500 4 0.47 0.87 364.2

600 4 0.47 0.86 344.6

700 4 0.46 0.83 329.3

500 8 0.46 0.81 321.1

600 8 0.45 0.77 298.7

a Results from XRF analysis.b Calculated from N2 physi-sorption.

[(Fig._5)TD$FIG]


hydrothermal treatment significantly reduced Sn-support interac-tion. This indicates Sn sintering, mobile species and loss. Theincrease in hydrothermal temperature and time, decreases theamounts of adsorbed H2, revealing that the interactions between Ptand SnOx decreased and ultimately change the surface character-istics [45].

The more in sight analysis of the chemical species werecharacterized by XPS analysis. The XPS data give information aboutthe oxidation states of the metal phases (Pt and/or Sn) of Pt–Sn/SAPO-34 catalyst. The Pt4f7/2 (B.E. 70–80 eV) and Sn3d5/2 (B.E.482–502 eV) XPS spectra were obtained, and shown in Figs. 8 and9, respectively. The main signal of each spectrum was deconvo-luted, evidencing a number of signals in each spectrum. Thereferences values were taken from the Handbook of X-rayphotoelectron spectroscopy [52].

In Fig. 8, after the deconvolation of main signal with Gaussianfitting, the binding energy signals at 71.8, 72.2 and 74.1 eV areidentified that correspond to the Pt–Sn alloy, Pt0 and PtOads speciesrespectively, while later signals were assigned to PtOx. The Pt 4f7/2

signal in the XPS spectrum for the Pt–Sn/SAPO-34 catalyst appearsoverlapped with the Al 2p; this undoubtedly makes it difficult totake effective advantage of this spectrum. But there is a clearchange in electronic effect with the hydrothermal treatment. It isobserved that the Pt0 was decrease with the severity ofhydrothermal treatment, while on the other hand PtOx wereincreased. This demonstrates that a part of the platinum (orreduced) would gain electronic density from tin, as reported byStagg et al. [53]. And fundamental reason for the new Pt 4f7/2 signalat lowers binding energies (71.8 eV). According to the chargedensity admission effect, if a displacement of the platinum signaloccurs at lower binding energies due to the formation of the Pt–Sn[(Fig._4)TD$FIG]

Fig. 4. XRD patterns of the Pt–Sn/SAPO-34 supported catalysts (before and after

hydrothermal treatment at steam rate 2 ml/h).

alloy, a similar effect should also take place but in the oppositedirection for tin signal of the Pt–Sn alloy [54,55].

Fig. 9 presents the XPS spectrum for Sn after deconvolation ofmain signal. In Sn 3d5/2 region spectrum, three peaks are observedat 486.3, 487.2, and 488.7 eV, those were attributed to species ofSn0, and oxidized (Sn(II) and/or Sn(IV)). But its quite difficult todifferentiate between Sn(II) and Sn(IV) by means of the XPS studies[56,57]. Therefore, we can only differentiate between the statesthat tin exists, only by saying SnOx. It is believed that the completereduction of Sn is not possible due to a strong interaction betweenSn and structural oxygen and/or between Pt metal and thealuminum in the SAPO-34. It is noted that the signal attributable toSn0 is a little displaced toward binding energy higher than energyas reported by Coloma et al. [56,57]; this displacement suggeststhat Sn may giving up electronic density to platinum, which meansthat they are forming a Pt–Sn alloy. In our detailed XPS analysis, ageneral trend was observed that with the increase in severity of thehydrothermal conditions, Sn0 species concentration decreasedwith the increase in concentration of SnOx.

The Sn(0)/SnTotal and Sn(0)/Pt ratio were shown in Table 4. Thepossibilities of coexistence of different alloys with different Pt/Snratios could not be ruled out. However, the absence or very smallexistence of Pt–Sn alloy in the PtSn/SAPO-34 can be explained interms of the quantity of Sn available to form the alloy. XPS resultsshowed that in fresh catalyst has more Sn atoms available to formthe alloy, which leads to a Sn(0)/Pt ratio of approximately 0.75.But most of the ionic atoms were dispersed and interacted withthe support. Therefore, the small concentration of Sn and Pt,

Fig. 5. NH3-TPD spectra of Pt(0.5 wt.%)–Sn(1 wt.%)/SAPO-34 (steaming rate is 2 ml/

h for 4 h).

[(Fig._6)TD$FIG]

Fig. 6. IR spectra of ammonia adsorbed on fresh and hydrothermally treated Pt–Sn/

SAPO-34 catalysts (steaming rate is 2 ml/h for 4 h).

[(Fig._8)TD$FIG]Z. Nawaz, F. Wei / Journal of Industrial and Engineering Chemistry 16 (2010) 774–784780

discourage the Pt–Sn alloy formation in the presence of supportinteraction [11,55]. Moreover, Pt–Sn alloy is not active duringalkane dehydrogenation [11,55]. The intensity of hydrothermaltreatment largely reduce the Sn(0)/Pt ratio to 0.45, that meanshydrothermal treatment increases chances of Pt–Sn alloy forma-tion, but in actual ionic Sn decreased due to the increase in SnOx.Therefore, the inhibiting effect of tin is observed, even for thecatalysts with low Sn/Pt ratio. The contribution of Sn(0) withrespect to SnTotal decreased with the hydrothermal treatmentintensity. This situation suggests that there is an electron chargetransfer from tin to platinum.

The increase in Sn(II) and Sn(IV) reduction signals can beattributed to the oxidative effect of steam. The possible reason forthis decrease in reduction peaks of hydrotheated samples weremay be due to the distinct loss of Sn species, and reduction inamount of oxidized Sn species or increased Sn sintering. Thistendency directly related to the nature of severity of hydrothermaltreatment. Even at higher hydrothermal treatment temperatures,the peak representing the reduction of SnOx to Sn0 decreasesremarkably, demonstrating that the formation of Pt–Sn alloy is[(Fig._7)TD$FIG]

Fig. 7. H2-TPR profiles of fresh and hydrothermally treated Pt(0.5 wt.%)–Sn(1 wt.%)/

SAPO-34.

Fig. 8. Pt4f7/2 XPS spectra of (1) fresh Pt–Sn/SAPO-34, and sever hydrothermally

treated catalysts at steam rate 2 ml/h at (2) 500 8C, (3) 600 8C, (4) 700 8C for 4 h

respectively and (5) 600 8C for 8 h.

less. It is also known that the adsorption of water between themetal and support could weaken these interactions [45,58].

3.3.5. Effect of hydrothermal treatment on surface metallic dispersion

The morphology of fresh and hydrothermally treated Pt–Sn/SAPO-34 catalysts are characterized by high-resolution transmis-sion electron microscopy and shown in Fig. 10. It is observed thatthe hydrothermal treatment has large geometric effects, with thedominant electronic effect is as discussed above. Under highmagnification it was clear that Pt particles (1.4–2.5 nm) were onthe surface of SAPO-34 and well dispersed. A slight change in theparticle size and particle population was noted after hydrothermaltreatment. However, some agglomerated particles were observedafter the hydrothermal treatment at different temperatures, while

Table 4XPS analysis of Pt(0.5 wt.%)–Sn(1 wt.%)/SAPO-34 samples before and after the

dehydrogenation reaction.

Sample no. Conditions Sn(0)/SnTotal Sn(0)/Pt

Pre-reaction samples of catalyst

1 Fresh catalyst 0.38 0.75

2 Hydrothermally treated at 500 8Cfor 4 h at 2 ml/h

0.26 0.63


0.24 0.62


0.22 0.54


0.21 0.45

Post-reaction after 5 h at 600 8C and WHSV 5.6/h (reduced with H2 before

reaction)

6 Fresh catalyst 0.40 0.76


0.28 0.64


0.25 0.63


0.24 0.56


0.22 0.44

[(Fig._9)TD$FIG]

Fig. 9. Sn3d5/2 XPS spectra of (1) fresh Pt–Sn/SAPO-34, and sever hydrothermally

treated catalysts at steam rate 2 ml/h at (2) 500 8C, (3) 600 8C, (4) 700 8C for 4 h

respectively and (5) 600 8C for 8 h.


population density largely decreased. This decrease in populationdensity of surface Pt was due to the Sn sintering by forming SnOx

mobile species, loss of Sn and change in Pt–Sn interaction.Consequently, the phenomenon becomes more apparent with theincrease in hydrothermal temperature.

The sintering of Pt particles and/or Pt dispersion, and theinteraction changes between Pt, Sn promoter, and support (SAPO-34) were the most important factors of the catalyst’s activity lossafter hydrothermal treatment. It has also known that, oxygen andsteam atmosphere drastically enhance Pt sintering [45,59,60].Therefore the pulse chemisorptions of hydrogen were commonlyused to estimate the sintering of Pt particles and structural

changes. The results of hydrogen chemisorptions of fresh andhydrothermally treated catalysts are shown in Table 5. Here, thetotal amount of H2 uptake of the samples at low and hightemperatures is the sum of H2 uptake at 500, 300, and 25 8C. Theamount of hydrogen chemisorptions at low temperature (25 8C)can be used to characterize the size of the Pt particles, and at hightemperatures (500 and 300 8C) hydrogen chemisorptions wererelated to the interactions between Pt and Sn promoter and/orsupport (SAPO-34) [60,61]. Directly anchored Pt (with SAPO-34)sites are favorable for low temperature H2 chemisorptions, andresponsible for the hydrogenolysis reaction and carbon deposits.While Pt sites (anchored with SAPO-34 through SnOx) can adsorbmore H2 at high temperatures and are the main reaction activesites for the dehydrogenation of propane. It was further reportedthat, the oxygen or steam adsorption over Pt surface can distinctlyreduce the interactions between metal and support, thuspromoting the sintering of Pt particles and destroy Pt–Sn clusters[58,62]. The high uncertainty in the H2 chemisoption resultssuggests that, it is an effective technique to characterize surfacedispersions of Pt.

It is observed that the fresh catalyst shows a relatively littleamount of H2 that adsorbed at room temperature, in comparisonwith the adsorbed H2 at higher temperatures (300 and 500 8C),evidently suggesting the existence of interactions between Pt andSnOx. That is the active Pt interaction (Pt–SnO–SAPO-34), neededfor propane dehydrogenation. In the hydrothermally treatedsamples, the amount of adsorbed H2 decreases at both roomand higher temperatures, linearly with the intensity of treatment.This reveals that, the favorable interaction for dehydrogenation,between Pt and SnOx decrease. Therefore, the surface character-istics were changed and the Pt active sites concentration decreasedremarkably. The results of the hydrogen chemisorptions analysisare consistent with the TEM analysis. While, the possible reasons ofPt sintering, can be attributed to the loss of Sn species, that is alsorelated to the dealumination, as the existence of aluminum canstabilize Sn species [51,63].

Fig. 11 demonstrates the possible interactions of metals withsupport. Previously, it is believed that two types of Pt and Sn siteswere appeared on ZSM-5 zeolite [42,50]. Pt sites directly anchorson the SAPO-34 surface, Pt anchored with Sn oxide surface, SnOattached to SAPO-34 surface and SnOx sintered on Pt anchoredwith support (see Fig. 11). First Pt site is responsible for sidereactions like hydroisomerization, cracking, etc. while the second

[(Fig._10)TD$FIG]

Fig. 10. HR-TEM micrographs of the fresh and hydrothermally treated Pt–Sn/SAPO-34 catalysts.


Pt site is the main active site for the propane dehydrogenationreaction. It has long been known that, the formation of highlyactive ‘‘Sn4C–Pt’’ ensemble sites are in the atomic closeness of Pt, isimportant for dehydrogenation. After severe hydrothermal treat-ment some amount of Sn species loosed from second site anddestroyed the sandwich structure and form first directly anchoredPt sites. This increases proportion of direct anchored Pt sites, leads

towards agglomeration/sintering and decrease activity of thecatalyst.

3.4. Recovery of Pt–Sn/SAPO-34 activity

The activity of Pt–Sn/SAPO-34 catalyst treated at differenttemperatures were recovered by re-dispersion of Pt by chlorina-

[(Fig._11)TD$FIG]

Fig. 11. Possible sites/interaction of Pt and Sn with SAPO-34, mechanism of active Pt

site conversion to directly anchored Pt during hydrothermal treatment.

Table 5Influence of calcinations temperature on hydrogen chemisorptions of Pt–Sn/SAPO-

34 catalysts before and after hydrothermal treatment (at constant flow rate of

steam 2 ml/min for 4 h).

Steam H2 uptake at (ml H2/g Pt)

Temperature 25 8C 300 8C 500 8C Total

Fresh catalyst 9.9 21.3 14.7 45.9

500 8C 6.7 14.4 9.6 30.7

600 8C 4.4 7.2 6.8 18.4

700 8C 2.8 3.7 3.1 9.6

Table 7Influence of calcinations temperature on hydrogen chemisorptions of Pt–Sn/SAPO-

34 catalysts before and after hydrothermal treatment (at constant flow rate of

steam 2 ml/min for 4 h).

Steam H2 uptake (ml H2/g Pt) at

Temperature 25 8C 300 8C 500 8C Total

500 8C 8.0 15.7 14.6 38.4

600 8C 8.2 15.6 13.9 37.7

700 8C 8.4 14.9 13.6 36.9

Table 6Reaction results of severe hydrotreated samples under identical reaction conditions

as in Table 1 after re-dispersion of metals by chlorination.

Pt–Sn/SAPO-34 Propane

conversion

(%)

Propylene

selectivity

(%)

Total olefins

selectivity

(%)

Time-on-stream (TOS) 1 h 5 h 1 h 5 h 1 h 5 h

Fresh catalyst 22.3 11.1 91.1 92.2 95.8 96.1

Hydrotreated at 500 8C 21.8 10.4 90.3 92.6 95.1 95.9




tion technique and its reaction performance results for propanedehydrogenation are shown in Table 6. Almost all the activity ofcatalyst is recovered. The reaction results were further verified byH2 chemisorption to confirm the active sites distribution afterrecovery. The results were tabulated in Table 7. This intensiveachievement was made possible due to the SAPO-34, as it is therobust support that not only enhances the Pt–Sn-based catalystperformance, but also maintains its stability in conventionalprocessing.

4. Conclusions

The hydrothermal treatment influence on the performance ofPt–Sn/SAPO-34 novel catalyst for propane dehydrogenation topropylene is investigated and compared with Pt–Sn/ZSM-5. Thecatalyst shows excellent stability after mild steaming (nitrogenmixed steam) for four continuous reaction–regeneration runs(regenerated at 600 8C). The propane conversion, propylene

selectivity and total olefin selectivity after fourth consecutiveruns were 9.3%, 94.5% and 97.3%, respectively at TOS 8 h. Severehydrothermal treatment with pure steam results obtained at 500,600 and 700 8C and varying time durations 4–8 h, showsinteresting results and provide room for their detailed characteri-zation. The physio-chemical analysis of sever hydrothermallytreated samples were helped to find reasons for activity loss duringregeneration with steam. Basic catalytic properties and texture ofPt–Sn/SAPO-34 was found to be stable after hydrothermaltreatment. The small changes in acidity was noted and foundproportional to the severity of hydrothermal conditions. From XPSanalysis, four types of metallic species were found over SAPO-34,with some mobile species. In accordance with the severity ofhydrothermal treatment conditions Pt0 and Sn0 decreased with thesubstational increase in their corresponding oxides. The Pt–Snalloy formation was enhanced, and loss of Al and Sn leads Ptsintering, is the probable reasons for reduction in activity of thehydrothermally treated catalysts. Ultimately, the loss of Sn andframework Al changes the surface characteristics of the catalysttowards lower dehydrogenation activity. HR-TEM micrographshows that Pt particles become agglomerated/sintered and theirpopulation density also decreased with the increase in hydrother-mal treatment temperature and time. The impressive results wereobtained from H2-chemisorption analysis, which demolishes theambiguity in other characterization results. The amazing break-through is the 98% recovery of catalyst activity after severehydrothermal treatment by re-dispersion of Pt and credit goes tothe hydrothermally stable support SAPO-34 due to its Si-basedstable acid cites.

Acknowledgments

This research was jointly supported by Higher EducationCommission, Islamabad, Pakistan (No. 2007PKC013) and NaturalScientific Foundation of China (Nos. 20606020, 20736004,20736007). Authors are very thankful to Dr. Tang Xiaoping andMeng-Qiang Zhao for their help in catalyst characterization.

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