Parametric Characterization and Influence of Tin on the Performance

Dec 10, 2009 - Parametric Characterization and Influence of Tin on the Performance of Pt-Sn/. SAPO-34 Catalyst for Selective Propane Dehydrogenation t...
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Parametric Characterization and Influence of Tin on the Performance of Pt-Sn/ SAPO-34 Catalyst for Selective Propane Dehydrogenation to Propylene Zeeshan Nawaz, Xiaoping Tang, Yao Wang, and Fei Wei* Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, P. R. China

The selective propane dehydrogenation to propylene was studied to enhance Pt-Sn/SAPO-34 catalyst performance. The objective was to achieve higher propylene selectivity; therefore, the reaction was parametrically characterized to obtain necessary information to integrate the process operating conditions. The optimum operating conditions were found to be temperature 585 °C, weight hour space velocity 5.6 h-1 or lower, H2/C3H8 molar ratio 0.25, and conversion range 14-25%. Further intensification and the role of Sn on the performance of Pt-Sn/SAPO-34 catalyst were investigated, as stereochemistry and thermodynamics are inextricably tied up with each other. The catalyst was characterized by a number of physiochemical techniques: X-ray fluorescence, Brunauer-Emmett-Teller surface area measurement, X-ray diffraction, temperature-programmed desorption of NH3, temperature-programmed reduction of H2, infrared spectra, temperature-programmed oxidation, and O2-pulse coke analysis. The catalytic performance was largely improved with the presence of Sn up to a certain limit; after that, it caused diminution in the reaction rate. The Sn loading modifies surface Pt ensembles; those helped Pt to be well dispersed by changing the interfacial character between the metal and support. Moreover, Sn facilitates the transfer of carbon deposit from the metal sites to the support. In general, it is noted that the increase in Sn content from 1 wt % affects catalyst performance adversely. Higher propylene selectivity (94%) and total olefin selectivity (97.2%) were obtained using [Pt(0.5 wt %) - Sn(1 wt %)]/SAPO-34. 1. Introduction Light olefins are indispensable raw materials for numerous products. The propylene market demand reached the average annual growth rate of 5-6%.14 However, propane and/or butane is cheap and easily available, as produced by the number of petrochemical processes. Moreover, direct propane dehydrogenation technology does not require a large investment because existing production facilities can be inexpensively retrofitted. It is the era of highly selective catalysts for olefin’s production in the petrochemical industries. Recently, a highly selective catalyst Pt-Sn/SAPO-34 gives a new technological trend in light olefin’s production via direct dehydrogenation route.5-8 Pt-Snbased catalysts supported on amorphous (Al2O3, SiO2, etc.) and zeolites (ZSM-5, SAPO-34, etc.) supports were discussed in many studies.7-17 The dehydrogenation performance of Pt-Snbased catalysts depends largely on the interactions between the Pt, Sn, and also the support. The deactivation of Pt-based catalyst during the propane dehydrogenation process is particularly due to aggregation of Pt particles.18 Therefore, Sn helps Pt to disperse during catalyst manufacturing and in this way it significantly improves catalytic performance. Many studies justified the selection of Sn as a promoter in terms of geometric and/or electronic effects, although it is still controversial.18,19 It is observed that the support has a very important role in stabilizing the activity and performance of the catalyst. The number of drawbacks also observed is due to the supports; those affect the catalyst performance in a distinct manner. Al2O3-supported catalyst has a very short lifetime (quickly deactivated).20-24 Pt-Sn/ZSM-5 zeolite catalyst has been investigated for catalytic dehydrogenation that demonstrates higher conversion and lower propylene selectivity.5-8,23-27 A number of attempts have been made to improve * To whom correspondence should be addressed. Fax: +86-1062772051. E-mail: [email protected].

Pt-Sn/ZSM-5 performance by incorporating more metallic promoter like Na, Zn, La, Ca, etc. and/or by increasing the Si/Al ratio.25-30 However, the performance of ZSM-5 zeolitesupported catalysts was affected by frequent regeneration with steam and somewhat by how it takes part in cracking.29,31 The advantages of novel Pt-Sn/SAPO-34 and its better stereochemistry control over propane and butane dehydrogenation to propylene was well explained in previous studies.5-8 However, SAPO-34-supported catalyst is inherently free from the above-discussed drawbacks. Moreover, SAPO-34 has weak surface acidity, as most of the acid sites exist in side pores (make it inert toward dehydrogenation) and have excellent shapeselective opportunities for propylene.3-8,32 The Bro¨nsted acid sites of SAPO-34 were found to be responsible for their activities and based on two distinct hydroxyls (OH bridges to Al and Si) on the structural framework; therefore, the mechanism of propane dehydrogenation was proposed on the basis of propoxy species.8 However, no study to date has focused on detailed characterization of Pt-Sn/SAPO-34 with varying Pt and Sn content and their parametric characterization. This study also examined the performances of Pt-Sn/SAPO-34 integrated catalyst in a microreactor to optimize the operating parameters for maximizing propylene production. 2. Experimental Section 2.1. Catalyst Preparation. The support (SAPO-34 catalyst) was prepared by mixing Al2O3:P2O5:SiO2:TEA:H2O in the molar ratio of 1:1:0.5:2:100.2,29 The bimetallic Pt-Sn/SAPO-34 catalysts of different metallic contents were prepared by sequential impregnation method with calcined SAPO-34 [Brunauer-Emmett-Teller (BET) surface area 444 m2/g].5-8 The support was first impregnated with an aqueous solution of SnCl2 · 2H2O at 80 °C, to ensure 0.8, 1, and 1.2 wt % Sn in the final catalyst. After the impregnation, the samples were dried

10.1021/ie901465s  2010 American Chemical Society Published on Web 12/10/2009

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at 110 °C for 2.5 h and calcined at 500 °C in a muffle furnace for 4 h. Later, this prepared Sn/SAPO-34 catalyst was coimpregnated again with an aqueous solution of H2PtCl6 at 70 °C to give a 0.3 and 0.5 wt % Pt in the final catalyst. The samples were crushed and used in pure form. 2.2. Catalytic Activity Evaluation. The performance of prepared Pt-Sn/SAPO-34 catalyst samples were investigated for direct propane dehydrogenation in a continuous flow microreactor at ambient pressure. The 99.8% pure propane was used as feed, i.e., provided by Zhong Ke Hui Jie (HJAT), Beijing, China. In each experiment, a measured amount of catalyst samples was loaded into the reactor to maintain the desired weight hour space velocity (WHSV) (i.e., 1.59, 2.9, 5.6, and 9.3). At first, the catalysts were dechlorinated at 500 °C for 4 h with N2 mixed steam. Then reduced under flowing H2 (8 mL/min) at 500 °C, for 8 h. The reaction mixture composed of H2 and C3H8 was charged into the microreactor. To determine a suitable reaction mixture, 0.05, 0.15, 0.25, 0.35, and 0.45 H2/ C3H8 molar ratios were injected and the catalyst performance was analyzed. The product distribution was analyzed by an online gas chromatography system having an Al2O3 capillary column equipped with an FID detector. All the values were calculated in weight percentages and calculated using the following simplified relationships, as we use pure propane as feedstock. conversion of propane (%) ) propane in feedstock (wt %) - propane in product (wt %) × 100 propane in feedstock (wt %) propylene selectivity (%) ) propylene in products (wt %) - propylene in products (wt %) × 100 propane in feedstock (wt %) - propane in feedstock (wt %)

2.3. Catalyst Characterization. 2.3.1. X-ray Diffraction (XRD) Analysis. The X-ray diffraction patterns of prepared Pt-Sn/SAPO-34 catalyst samples were obtained on a powder X-ray diffractometer (Rigaku-2500), equipped with a copper anode tube operated at 40 kV. The KR radiation was selected with a monochromator and spectra were scanned at a rate of 5°/min, from 5° to 40° (an angular range 2θ). 2.3.2. Metallic Content Analysis. The metallic content of prepared samples of Pt-Sn/SAPO-34 with varying Pt and Sn were obtained by X-ray fluorescence (XRF) measurements on a Shimadzu XRF 1700 fluorimeter. 2.3.3. BET Surface Area Measurement. The BET surface area was measured by using N2 adsorption/desorption isotherms on an automatic analyzer (Autosrb-1-C). The samples of Pt-Sn/ SAPO-34 were outgassed for 1 h under vacuum prior to adsorption, at 350 °C and the BET surface area of prepared samples were calculated from obtained results. 2.3.4. Temperature-Programmed Desorption of Ammonia (NH3-TPD). The acid properties of Pt-Sn/SAPO-34 catalysts were determined by temperature-programmed desorption of NH3 using Autochem-II 2920 analyzer. About 0.2 g of catalyst sample was used for analysis in a U-type quartz reactor. The samples were pretreated with 99.99 dry argon 500 °C for 1 h prior to ammonia adsorption. Later the samples were saturated with ammonia at room temperature. Before each run the baseline was stabilized by flowing argon with the flow rate of 30 mL/ min. Subsequently, the temperature was raised to 600 °C at a heating rate of 10 °C /min to desorb physically adsorbed ammonia. The NH3 desorption profile was obtained from a thermal conductivity detector. 2.3.5. Temperature-Programmed Reduction of Hydrogen (H2TPR). Temperature-programmed reduction experiments were

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Figure 1. Propylene selectivity and propane conversion as a function of time on stream at different Pt and Sn loadings.

also carried out on a conventional setup used for NH3-TPD, equipped with temperature-programmed furnace attached to an Autochem-II 2920 analyzer. Samples of 0.2 g were loaded into the quartz reactor and dried by flowing N2 at 500 °C for 1 h. The reactor was gradually heated from room temperature to 750 °C with the rate 10 °C/min. The 5% mixture of H2/N2 was used as reducing gas. Hydrogen consumption as a function of reduction temperature was obtained by TCD and recorded. 2.3.6. Temperature-Programmed Oxidation (TPO) for Coke Analysis. In the temperature-programmed oxidation for coke analysis the sample of 0.2 g after the reaction test was used. The samples were purged in a N2 at the flow rate 40 mL/ min at 500 °C for 30 min. Later the temperature was slowly decreased to 30 °C. Then the mixture of 2% oxygen and 98% helium by volume was passed through the samples at the flow rate of 40 mL/min. The temperature was ramped up at the rate 10 °C/min and the CO2 was measured by an online gas analyzer QIC-20 (HIDEN, England). 2.3.7. IR Spectroscopy Analysis. IR spectra of adsorbed ammonia on different Pt-Sn/SAPO-34 samples were obtained using a NEXUS apparatus (Nicolet, USA). The samples were dried in UV light and placed in a Pyrex glass cell equipped with a CaF2 window. The samples were degassed by heating at 400 °C for 30 min and then cooled to room temperature. Afterward, the ammonia was passed for 30 min and then the ammonia adsorption spectra were recorded after desorption at 100 °C. 2.3.8. O2-Pulse Analysis of Coke. The coke formed during propane dehydrogenation reaction was determined by O2-pulse analysis using a gas chromatograph equipped with a TCD. The experiments were carried out at 750 °C by injecting 99.99% oxygen pulses on 8-h reacted catalyst (0.02 g). The pulses of oxygen were continued until the coke was completely removed. The CO2 formed was recorded and amount of coke deposited was calculated. 3. Results and Discussion 3.1. Catalyst Activity Evaluation. 3.1.1. Effect of Sn Addition to Pt-Sn/SAPO-34. The influence of Sn content on the catalytic performance of Pt-Sn/SAPO-34 was measured at 600 °C. The results are shown in Figure 1. It is observed that the Sn content has a large influence upon catalyst stability and the dehydrogenation activity. With increase in the Sn loading, the gradual increase in selectivity toward propylene

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Table 1. Deactivation as a Function of Different Pt-Sn Metallic Content Loaded on SAPO-3a Pt-Sn/SAPO-34 catalysts Pt (wt %)

Sn (wt %)

X0 (%)

Xf (%)

Dr (%)

cokeb (wt %)

Sf (%)

0.3 0.3 0.5 0.5

0.8 1 1 1.2

8 10 35 20

5 8 16 12

3 2 19 8

0.45 0.41 0.40 0.39

70.1 86.7 93.6 91.2

a The deactivation parameter was defined as Dr ) [(X0 - Xf)/X0 × 100], where X0 is the initial conversion at 1 min, Xf is the final propane conversion, and Sf is the final propylene selectivity at 8 h. The reaction temperature was 600 °C. b O2-pulse coke analysis.

was noted up to 1 wt %, and later declined, as shown in Figure 1. At Sn content 1.2 wt %, both propylene selectivity and propane conversion are lower than Sn content 1 wt %. Therefore, we can say that the excessive Sn loading adversely affected both selectivity and conversion. This may be because Sn decreased the surface Pt ensembles and form Sn0 species; this fact is discussed in section 3.2. Moreover, the optimum Sn loading was found to be about 1 wt %, which gave selectivity to propylene of 93.6% at a time on stream (TOS) of 8 h. It was interesting to find that the reaction stability and activity of Pt(0.5 wt %)-Sn(1 wt %)/SAPO-34 is superior to other Pt and Sn combinations. Therefore, this optimum combination was experimentally studied in further parametric analysis. The deactivation trends with different Sn/Pt content ratios and the amount of coke were measured by O2-pulse analysis. The results are shown in Table 1. It is generally believed that the coke on the platinum responsible for the deactivation of the catalyst. Nevertheless, deactivation and/or activity loss of the catalyst is not only due to coke deposition. The deactivation rate increased to as high as 19% with a high initial conversion. Moreover, this deactivation is somehow related to catalytic activity. 3.1.2. Influences of Temperature. Propylene selectivity sharply increased with the temperature initially and then starts decreasing after 585 °C, as shown in Figure 2. The decrease in

propylene selectivity was mainly due to the thermal/catalytic cracking of propylene and propane and has no relationship with the loss of catalyst activity. At every TOS value, the continuous increase in conversion was observed with the rise in temperature. The optimum propane dehydrogenation temperature for higher propylene selectivity was observed to be 585 °C. 3.1.3. Influence of WHSV. The variation in WHSV has a significant influence on the performance of Pt-Sn/SAPO-34 catalyst for selective propane dehydrogenation. A decrease in the WHSV initially increases the propylene selectivity at the cost of higher conversion. At higher WHSV’s above 5.6 h-1 both selectivity and conversion dropped as shown in Figure 3. This is particularly because of a low catalyst-to-feed ratio; therefore, the optimum WHSV is 5.6 h-1. 3.1.4. Influence of H2 Atmosphere. The presence of H2 not only prevents the catalyst from coke formation but also continuously reduces the active Pt sites and maintains catalytic activity without affecting secondary propylene formation reaction.8 As discussed above, Pt-Sn supported on the SAPO-34 exhibited good stability and a relatively lower rate for hydrogentransfer reactions during the propane dehydrogenation.8 Therefore, the H2/C3H8 molar ratio during the propane dehydrogenation reaction over Pt-Sn/SAPO-34 was prominent among the operating parameters. The influence can be observed from Figure 4. Deviations from a H2/C3H8 molar ratio of 0.25 decreased both selectivity and conversion. 3.1.5. Detailed Olefin’s Performance Envelope (OPE). The overall picture of selective propane dehydrogenation over Pt(0.5 wt %)-Sn(1 wt %)/SAPO-34 at 600 °C is shown in the OPE plot in Figure 5. The data was obtained at various WHSV and TOS values. The best propane conversion range to have good propylene selectively was between 14% and 23% conversions, where the propylene selectivity was as high as 94%, with high total olefins selectivity. While at higher conversions propylene selectivity dropped sharply with a minor drop in total olefin’s selectivity, and ethane formation was promoted. It was further noted that lower conversion favors cracking, while higher

Figure 2. Effect of temperature on selective propane dehydrogenation over Pt(0.5 wt %)-Sn(1 wt %)/SAPO-34.

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Figure 3. Influence of WHSV on propane dehydrogenation to propylene at 600 °C.

Figure 4. Effect of H2/C3H8 molar ratio on dehydrogenation performance of Pt(0.5 wt %)-Sn(1 wt %)/SAPO-34.

conversions favor both cracking and hydride-transfer reaction with the decrease in dehydrogenation rate. While in the optimum conversion range (13-24 wt %), the dehydrogenation rate is found to be far superior than cracking. Moreover, it is noted that the deactivation of catalyst also leads the reaction toward cracking. 3.2. Characterization of Catalysts. 3.2.1. X-ray Diffraction. XRD patterns of different metallic combinations supported on SAPO-34 were shown in Figure 6. It is observed that, after the metallic incorporation by sequential impregnation method, the original structure/topology of the SAPO-34 zeolite support was not changed. As the average diameter of Pt particles was larger than SAPO-34 pore channels, the platinum particles were located on the surface of SAPO-34.8,16,31-34 The peaks for Pt and Sn were not found owing to their smaller concentration, but it is believed that they are monodispersed initially.27 Meanwhile, the peak height decreased slowly between (angle 2θ) 8° and 10°

with the increase in Sn loading, which may be due to the change in surface characters. 3.2.2. XRF and BET Surface Area Analysis. The successful loading of metals were confirmed from XRF analysis and shown in Table 2. It was noted from BET results that the metallic incorporation modified the geometric effect of the catalyst. As shown in Table 2, that the surface area decreased from 419 to 396 m2/g, with the increase in Sn loading from 0.8 to 1.2 wt %. This surface area loss is due to the pores blocking by Sn and/or Pt.8,16 Therefore, it is observed that the content of Sn has a wide impact on the catalyst geometry and the experimental results are shown in Figure 1. 3.2.3. NH3 Temperature-Programmed Desorption Analysis. The dehydrogenation of light paraffins is strongly dependent on the acidity of the catalyst.6,14 The overall weak acidity of support is the potentials requirement of intermediate conversions to propylene, without taking part in cracking. The

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Figure 5. The OPE of the propane dehydrogenation reaction over Pt-Sn/SAPO-34. Table 3. NH3-TPD Spectra of Catalysts SAPO-34 supported catalysts Pt(0.3)-Sn(0.8) Pt(0.3)-Sn(1.0) Pt(0.5)-Sn(1.0) Pt(0.5)-Sn(1.2)

peak I

peak II

total NH3 temperature NH3 uptake temperature NH3 uptake uptake (°C) (mmol/g) (°C) (mmol/g) (mmol/g) 225 240 220 215

0.29 0.36 0.41 0.27

418 425 415 400

0.51 0.52 0.54 0.50

0.80 0.88 0.95 0.77

Table 4. IR Spectra of Ammonia Desorb at 100 °C of Different Catalysts SAPO-34 supported catalyst Pt(0.3 Pt(0.3 Pt(0.5 Pt(0.5

Figure 6. XRD patterns of the SAPO-34 supported catalysts with different metallic combinations. Table 2. Basic Characteristics of the Pt-Sn/SAPO-34 Catalysts Pt-Sn/SAPO-34 catalysts Pt (wt %)

Sn (wt %)

Pt contenta (% w/w)

Sn contenta (% w/w)

SBETb (m2/g)

0.3 0.3 0.5 0.5

0.8 1.0 1.0 1.2

0.32 0.33 0.51 0.52

0.67 0.86 0.90 1.04

419 411 407 396

a Results from XRF analysis. using BET equation.

b

Calculated from N2 physisorption

influence of Sn on the acidity of the prepared catalysts was determined by NH3-TPD analysis and results are shown in Table 3. Two ammonia desorption peaks that were obtained between 215-240 °C and 400-425 °C demonstrate weak acid sites (Si-O-Si) and strong acid sites (due to framework Al species), respectively.8 The analysis confirmed that after the impregnation of different Sn contents shows that there is almost negligible effect on the acidity of support.

wt wt wt wt

%)-Sn(0.8 wt %) %)-Sn(1 wt %) %)-Sn(1 wt %) %)-Sn(1.2 wt %)

Bro¨nsted acid sites (1430-1450 cm-1)

Lewis acid sites (1620-1630 cm-1)

2.03 2.17 2.29 2.43

6.02 6.31 6.52 6.99

Meanwhile, small shifts in peaks toward lower temperature were observed, i.e., due to slight decrease in acid intensity. Moreover, the highest ammonia uptake was noted at Pt(0.5 wt %)-Sn(1 wt %)/SAPO-34. 3.2.4. IR Spectroscopy Analysis. Ammonia adsorption IR spectroscopy analysis of prepared catalysts is in accordance with the NH3-TPD results. Both Lewis and Bro¨nsted acid sites of catalysts obtained from the ammonia absorption bands at 1620-1630 cm-1 and 1430-1450 cm-1, respectively, together with many trailing peaks after evacuation at 100 °C. The results are shown in Table 4. It has long been known that the Lewis acid sites were consumed by Sn and Bro¨nsted acid sites were consumed by Pt.35,36 The small decrease in peak intensities was noted after the increase in Sn loading. Moreover, this decreased intensity of Bronsted acid sites makes the catalyst least active for cracking of propane and improve stereochemistry control over the reaction toward higher propylene selectivity.8 At higher Sn concentration (1.2 wt %) both L and B acidity was higher, but it blocked Pt active sites; therefore, even at higher acidities the conversion is low. 3.2.5. Temperature-Programmed Reduction with Hydrogen. The influences of the Sn content on the reduction properties of Pt-Sn/SAPO-34 catalyst were analyzed by H2-TPR and results are shown in Figure 7. The reduction peaks at higher

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Figure 7. H2-TPR profiles of different catalysts: (a) Pt(0.3 wt %)-Sn(0.8 wt %)/SAPO-34; (b) Pt(0.3 wt %)-Sn(1 wt %)/SAPO-34; (c) Pt(0.5 wt %)-Sn(1 wt %)/SAPO-34; (d) Pt(0.5 wt %)-Sn(1.2 wt %)/SAPO-34.

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3.2.6. Temperature-Programmed Oxidation Analysis. Coke formation during propane dehydrogenation is the inherent factors that adversely affect catalyst performance. The amount of coke formed over Pt-Sn/SAPO-34 catalysts of varying Sn content (after 8 h reaction) was analyzed by TPO. The results are shown in Figure 8. The typical TPO profile shows two successive peaks, indicating coke deposited on the metallic sites and support, while the peak height represents the intensity of deposition. The peaks between 300-400 and 550 - 650 °C corresponds to coke deposited on metallic site and support, respectively. It was noted that the presence of Sn facilitated the transfer of carbon deposits on metal sites to support. It is only possible when the metals interacted well with the support. Moreover, Sn modifies Pt ensembles by decreasing surface area and reduces carbon-metal bonds/interaction and allows the intermediates to move easily to support.8 The mechanism of coke transfer from the active metal site to the support is also described in Figure 8. Therefore, the presence of Sn helps in decreasing coke deposition by suppressing the coke formation reaction and ultimately improve catalyst stability. 4. Conclusion

Figure 8. TPO profiles of coke deposits on different catalysts after 8 h propane dehydrogenation and their mechanism of transfer of coke from metal to support.

temperatures (above 450 °C) indicated that the Pt interacted well with the support in the presence of Sn.5,6,14,34 The peak height and width increase with the increase in Sn loading, between 400 and 600 °C, which may be due to joint reduction and valuable interaction between Pt and Sn on the SAPO-34.8 The increased Sn loading starts shifting the higher temperature peaks toward lower reduction temperatures. It was due to the formation of Sn0 species, with the increase in Sn loading.8,37 But the oxidation state Sn2+ in a bimetallic Pt-Sn catalyst was important, while the Pt-Sn alloy formation results in permanent deactivation.7,8,14,38 However, it is concluded from H2-TPR results that the presence of Sn improved the reduction properties of Pt, while higher Sn concentration (from 1 wt %) leads to the formation of Sn0 species. Moreover, it is known that the direct anchored Pt sites with SAPO-34 were not active for dehydrogenation.31 The Pt attached to SnOx is believed to be active for dehydrogenation reaction.

The effect of Sn on the catalytic performance of Pt-Sn/ SAPO-34 was investigated by physiochemical techniques and a series of reaction tests. The integrated content of Sn was found to be 1 wt %, evidently obtained from characterization and experimental data. The operational optimum was also experimentally explored over a range of operating parameters for superior catalytic performance of Pt-Sn/SAPO-34. The role of Sn as a promoter modifies Pt electronic density and retarded coke formation. NH3-TPD and IR analysis suggested that the presence of Sn (in the range of 0.8-1.2 wt %) has almost negligible effect on acidic properties of the catalyst. However, H2-TPR results showed that the presence of Sn improved the reduction properties of Pt, while higher Sn concentration (from 1 wt %) leads to the formation of Sn0 species. It is impressive that the Sn facilitates the transfer of carbon deposits from metallic sites to support, and in this way improves catalyst stability. The optimum operating parameters to produce propylene selectively from propane dehydrogenation are as follows: temperature 585 °C, WHSV 5.6 h-1, TOS 1-5 h, and H2/C3H8 ratio 0.25. The OPE suggested that the best conversion range for direct propane dehydrogenation to propylene is around 23% conversion, where the propylene selectivity was as high as 94 wt %. Acknowledgment This research was financially supported by the Higher Education Commission, Islamabad, Pakistan (No. 2007PK0013) and the Natural Scientific Foundation of China (No. 20736007). The authors are very thankful for Dr. Qiang Zhang’s help in organizing the manuscript. Literature Cited (1) Tang, X. P.; Zhou, H. Q.; Qian, W.; Wang, D. Z.; Jin, Y.; Wei, F. High selectivity production of propylene from n-butene: thermodynamic and experimental study using a shape selective zeolite catalyst. Catal. Lett. 2008, 125, 380. (2) Nawaz, Z.; Tang, X. P.; Zhu, J.; Wei, F.; Naveed, S. Catalytic cracking of 1-hexene to propylene using integrated SAPO-34 catalysts topologies. Chin. J. Catal. 2009, 30, 1049. (3) Zhou, H. Q.; Wang, Y.; Wei, F.; Wang, D. Z.; Wang, Z. Kinetics of the reactions of the light alkenes over SAPO-34. Appl. Catal., A 2008, 348, 135.

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ReceiVed for reView September 17, 2009 ReVised manuscript receiVed November 4, 2009 Accepted November 23, 2009 IE901465S