Effect of Strontium Addition to Platinum Catalyst for Propane Dehydrogenation

Catalyst Research China Petroleum Processing and Petrochemical Technology 2012,Vol. 14, No. 3, pp 75-82 September 30, 2012 Effect of Strontium Addition to Platinum Catalyst for Propane Dehydrogenation Huang Li 1,2 ; Zhou Shijian 1 ; Zhou Yuming 1 ; Zhang Yiwei 1 ; Zhang Shaobo 1 ; Xu Jun 2 (1. School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189; 2. Nanjing Wolaide Energy Technology Co., LTD, Nanjing 210000) Abstract: PtSnSr/HZSM-5 catalysts with different amounts of strontium were prepared by sequential impregnation method, and characterized by BET analysis, TEM, NH 3 -TPD, H 2 -TPR, TPO and TG techniques. The results showed that the addition of strontium could modify the characteristics and properties of both acid function and metal function of Pt-Sn-based catalysts. In this case, PtSnSr/HZSM-5 catalyst with an appropriate amount of Sr (1.2%) showed higher catalytic activity and lower amount of coke deposits than PtSn/HZSM-5 catalyst. However, excessive loading of Sr could facilitate the reduction of Sn, which was unfavorable to the reaction. Afterwards, 1.0 m% of Na was added into the PtSnSr(1.2%)/HZSM-5 catalyst to improve the catalytic performance in propane dehydrogenation, and this catalyst displayed the best catalytic performance during our experiments. After having been subjected to reaction for 5 h, the PtSnNa(1.0%)Sr(1.2%)/HZSM-5 catalyst had achieved a higher than 95% selectivity towards propene along with a corresponding propane conversion rate of 32.2%. Key words: strontium; propane dehydrogenation; propene; HZSM-5. 1 Introduction Propene is an important raw material for the production of polypropylene, acrolein, acrylic acid and polyacrylonitrile. In the past decade, the catalytic dehydrogenation of propane has been gaining great attention due to the growing demand for propene. Yet dehydrogenation of propane is a highly endothermic and equilibrium limited reaction that requires relatively high temperatures and low pressures to achieve high yield of propene [1-4], however, the high temperature favors thermal cracking reactions to form coke and light alkanes which lead to decreased product yield and increased catalyst deactivation. Hence, catalysts possessing high activity, high stability and high selectivity should be developed [5-6]. The supported Pt-Sn-based catalysts are considered to be a suitable kind of catalysts for propane dehydrogenation and have been studied in depth. Barias, et al. studied the propane dehydrogenation reaction over Pt and Pt-Sn catalysts supported on γ-al 2 O 3 and SiO [7] 2. The results showed that the addition of Sn could lead to increased Pt dispersion, and the Sn-supported catalysts showed superior activity maintenance. The Pt-Sn catalyst supported on alumina-modified SBA-15 was reported by Huang, et al [8]. Fattahi, et al. found that the addition of two types of oxygenate modifiers to the feed during dehydrogenation of propane over PtSn/γ-Al 2 O 3 catalyst showed beneficial effect on catalytic performance [9]. The oxygenate modifier increases the catalyst lifetime by reducing coke formation on the catalyst. However, the catalysts still have some defects on the activity and stability. Nawaz, et al. prepared a series of PtSn/SAPO-34 catalysts and found out that the sample, which was prepared at a calcination temperature of 500 showed the best catalytic performance for propane dehydrogenation [10]. Due to the specific framework structure and the low aluminum content, ZSM-5 zeolite has unique properties and is used as an efficient support for platinum catalyst in the propane dehydrogenation reaction. In comparison with the traditional γ-al 2 O 3 support, ZSM-5 zeolite has larger surface area and particular character of channels, so that catalysts using ZSM-5 zeolite as the support showed much better capacity to accommodate the coke. Grasselli, et al. used Pt-Sn/ZSM-5 catalysts in catalytic dehydrogenation of light paraffins combined with se- Corresponding Author: Zhou Yuming, Tel/Fax: +86-25- 52090617; E-mail: ymzhou@seu.edu.cn. 75

China Petroleum Processing and Petrochemical Technology 2012,14(3):75-82 lective hydrogen combustion process to achieve good selectivity [11-12]. So far, the investigation of PtSn/ZSM- 5 catalysts deserved more and more attention from both theoretical and practical points of view. In our previous work [13-16], we studied the binder, the reaction conditions, and the promoter and the preparation methods for evaluating the catalytic performance of Pt-Sn-based ZSM-5 catalysts. The addition of alkali metals and alkali earth metals to the dehydrogenation catalysts was a common practice to improve PtSn/ZSM-5 catalysts. Bai, et al. reported that calcium loading on the PtSn/ZSM-5 catalyst could obviously enhance the catalytic activity of the catalyst, and reduce the coke deposition during the dehydrogenation reaction [17]. Zhang, et al. found out that a suitable amount of K species not only could decrease the acidic sites on the surface, but also improve the interaction of the metallic Pt and Sn with the support, therefore the catalyst could exhibit high activity and stability [18]. In the present work, a series of PtSnSr/HZSM-5 catalysts were prepared by changing the loading of Sr species in the catalyst. The catalysts were investigated by several methods, including BET analysis, TEM, NH 3 -TPD, H 2 - TPR, TPO and TG techniques. The modifying effect of Sr addition to PtSn/HZSM-5 catalyst on its physicochemical characteristics and catalytic behavior in the propane dehydrogenation reaction was studied. 2 Experimental 2.1 Catalyst preparation Sr/HZSM-5 catalyst, Pt/HZSM-5 catalyst, PtSr/HZSM-5 catalyst, PtSnSr/HZSM-5 catalyst with different contents of Sr species and PtSnNaSr/HZSM-5 catalyst were prepared by sequential impregnation method in our laboratory according to a procedure referred to in our previous work [17, 19]. The nominal metal loading in each sample was 0.5 m% of Pt, and 1.0 m% of Sn, respectively. After having been totally dried, the catalyst samples were fully agglomerated with 5 m% of alumina in the course of agglomeration. After drying, the catalyst samples were calcined at 500 for 5 h, and finally reduced under H 2 stream at 500 for 8 h. 2.2 Catalysts characterization Nitrogen adsorption studies were used to examine the porous nature of each sample. The measurements were carried out on a Micromeritics ASAP 2000 adsorption and desorption apparatus, and all samples were pretreated in vacuum at 350 for 15 h before the measurements. The specific surface area was obtained using the BET method. The microporous volume was calculated by means of the t-plot method. The metal dispersion was measured by the ASAP 2020 chemisorption software; the data was investigated to dig into the influence of Pt particles on the catalyst performance. About 0.8 g of sample was measured; H 2 was used as the adsorption gas. All samples were pretreated in vacuum at first at 250 for 4 h, then purged with a flow of H 2 at 500 for 120 min, and cooled down before they were analyzed at 25. Transmission electron microscopy studies were conducted using a JEM-2010 instrument (made in Japan). The samples were prepared by grinding and suspending in excess ethanol, followed by dropping a small amount of this solution onto a carbon coated copper grid and drying before loading the sample into the electron microscope. The surface acidity of catalyst samples was measured by NH 3 -TPD in a Micromeritics ASAP 2750 instrument. The sample (0.15 g) was preheated at 500 for 1 h, and then cooled down to room temperature in flowing helium gas. At this temperature, sufficient pulses of NH 3 were injected until adsorption saturation. TPD was carried out in a temperature range from 100 to 550 with a heating rate of 10 /min using helium (at a flow rate of 30 ml/min) as the carrier gas. Temperature-programmed reduction (TPR) was conducted with the same apparatus as that used for NH 3 -TPD measurements. Prior to the TPR experiments, the catalyst samples were dried in flowing N 2 stream at 100 for 1 h. A gas mixture composed of 5% H 2 in N 2 was used as the reducing gas at a flow rate of 35 ml/min. The rate of temperature rise in the TPR experiment was 10 /min up to 800. Temperature-programmed oxidation (TPO) measurements were carried out with the same apparatus as that used for NH 3 -TPD measurements. About 0.05 g of sample was 76

Huang Li, et al. Effect of Strontium Addition to Platinum Catalyst for ropane Dehydrogenation placed in a quartz reactor at room temperature, and then was heated up to 700 at a heating rate of 10 /min in a mixture consisting of 5% O 2 in He gas (at a flow rate of 30 ml/min). The amount of coke deposited on the catalyst was measured by using a LCT thermogravimetric and differential thermoanalysis (TG-DTA) apparatus. The catalyst samples, ca. 15 mg, were placed in a Pt cell and heated from room temperature to 800 at a heating rate of 10 /min with a gas (air) feed rate of 50 ml/min. 2.3 Catalytic evaluation Propane dehydrogenation was carried out in a conventional quartz tubular micro-reactor. The catalyst (2.0 g) was placed into the center of the reactor. The dehydrogenation reaction was conducted at: a reaction temperature of 590, a reaction pressure of 0.1 MPa, a H 2 /C 3 H 8 molar ratio of 0.25, and a propane weight hourly space velocity (WHSV) of 3.0 h -1. The reaction products were analyzed with an online GC-14C gas chromatograph equipped with an activated alumina packed column and a flame ionization detector (FID). The conversion of propane is defined as the percentage of propane converted to all different products. The selectivity to propene is defined as the amount of propene obtained thereby divided by the amount of reactant converted to all products. 3 Results and Discussion 3.1 Characterization of catalysts Table 1 shows the porous properties of different samples obtained from N 2 adsorption. It can be seen from data listed in Table 1 that the surface area and the pore volume of HZSM-5 zeolite decreased after impregnation with Pt and Sn salts. According to the previous results [19-20], this phenomenon occurred because the platinum species were well dispersed on the external surface of the zeolite and a part of Sn species could enter the main channels of HZSM-5 zeolite. With the increase of Sr loading, the surface area and the pore volume of PtSnSr/HZSM-5 catalysts decreased continuously. Taking into account that the average diameter of Sr 2+ (0.118 nm) is less than that of the average pore mouth of ZSM-5 zeolite, it is reasonable to consider that a part of Sr 2+ species may also enter the main channels of HZSM-5 zeolite during the impregnation process, while the other Sr species are mostly located on the external surface of ZSM-5 zeolite. Table 1 Textural properties of different catalyst samples Surface area, Pore volume, Pt metal Sample m 2 /g cm 3 /g dispersion,% HZSM-5 351.2 0.245 Pt/HZSM-5 345.6 0.240 18.2 PtSn/HZSM-5 342.4 0.218 22.3 PtSr(1.2%)/HZSM-5 343.6 0.209 20.4 PtSnSr(1.2%)/HZSM-5 330.1 0.206 27.6 PtSnSr(1.8%)/HZSM-5 316.6 0.200 27.9 PtSnSr(2.4%)/HZSM-5 313.9 0.197 20.4 Furthermore, Table 1 lists the Pt metal dispersion values of different catalysts. In comparison with monometallic Pt catalyst, it can be found out that both of the added Sr and Sn species could obviously influence the metallic Pt dispersion. With respect to PtSnSr/HZSM-5 catalysts, when the content of Sr was 1.2% and 1.8%, the Pt metal dispersion of the catalysts reached a higher value than other catalysts, indicating that a suitable Sr content was advantageous to the Pt dispersion in PtSnSr/HZSM-5 catalyst. However, when the Sr amount was excessive, the Pt metal dispersion value declined. Owing to the fact that Pt particles are the only active species of the catalyst, the change in the dispersion of Pt affected by the addition of Sr would have some impacts on the activity of the catalyst. Figure 1 TEM micrographs of different catalysts (1) PtSn/HZSM-5 catalyst; (2) PtSnSr(1.2%)/HZSM-5 catalyst In order to better verify the influence of Sr promoter on the dispersion of Pt particles, the TEM micrographs of the 77

China Petroleum Processing and Petrochemical Technology 2012,14(3):75-82 corresponding catalysts were investigated. It can be seen from Figure 1 that in contrast with PtSn/HZSM-5 catalyst, PtSnSr(1.2%)/HZSM-5 catalyst possessed smaller Pt particles with higher Pt dispersion. These results are in agreement with the analytical data on Pt metal dispersion. The NH 3 -TPD profiles of different samples are presented in Figure 2. The TPD curve for pure HZSM-5 zeolite exhibited two desorption peaks in the profile: the first peak was located at 240 and the other was seen at 450, which represented a weak acid site and a strong acid site on the surface of zeolite, respectively [21-22]. After the impregnation of Pt and Sn, the acidity of the zeolite decreased. This occurred as the result of a slight reduction of the acidity on the support by tin [23]. As for the PtSnSr(1.2%)/HZSM-5 catalyst, a decreased area of both two peaks was identified, implying that the presence of Sr could partly neutralize the acidity of HZSM-5 zeolite. With an increasing Sr concentration, the acid amount of the catalysts decreased continuously. Similar effect was also observed by Xu, et al. [24] On the other hand, because of the weak basicity of Sr, the strong acid sites of PtSnSr/ HZSM-5 catalysts remained constant, even if the amount of Sr reached 2.4%. This phenomenon may have some adverse effect on the catalytic reaction. Temperature-programmed reduction (TPR) measurements were carried out to characterize the reducibility of the Pt and Sn species. Figure 3 shows the TPR profiles of different catalysts. The TPR profile of Sr/HZSM-5 catalyst showed no reduction peaks, indicating that Sr species were not reduced in the reduction process. The monometallic Pt/HZSM-5 catalyst showed two reduction peaks: one had a maximum temperature at 260 and the other -- at 450, which were attributed to the different ratios of Pt species interacting with the zeolite. The PtSn/ HZSM-5 catalyst presented three peaks: the first one had a maximum temperature at 280, which could be attributed to the reduction of Pt oxidative species. The peak around 440 was related to the reduction of Sn 4+ to Sn 2+, whereas the high-temperature peak around 550 was assigned to the reduction of Sn 4+ to Sn 2+ and Sn 2+ to Sn 0[25], respectively. Otherwise the reduction temperature of Pt particles would move towards the higher temperature region, which could occur due to interactions between Sn and Pt. Figure 2 NH 3 -TPD profiles of different catalysts (1) HZSM-5 catalyst; (2) PtSn/HZSM-5 catalyst; (3) PtSnSr(1.2%)/HZSM-5 catalyst; (4) PtSnSr(1.8%)/HZSM-5 catalyst; (5) PtSnSr(2.4%)/HZSM-5 catalyst Figure 3 H 2 -TPR profiles of different catalysts (1) Sr/HZSM-5; (2) Pt/HZSM-5; (3) PtSn/HZSM-5 catalyst; (4) PtSnSr(1.2%)/HZSM-5 catalyst; (5) PtSnSr(1.8%)/HZSM-5 catalyst; (6) PtSnSr(2.4%)/HZSM-5 catalyst In comparison to the TPR curve of PtSn/HZSM-5 catalyst, it can be seen that the reduction peak of Pt shifted gradually to the lower temperature region with an increasing Sr content, indicating that the addition of Sr could modify the interaction between Pt particles and the support. The presence of Sr would promote the reduction of Pt oxide species. When the Sr content was 1.2%, no distinct changes in the Sn reduction peaks could be found. With a continuous addition of Sr (1.8%), the reduction peak of Sn species shifted towards the lower temperature range, and the reduction peak area of Sn species at high temperature increased. With regard to the PtSnSr(2.4%)/ HZSM-5 catalyst, this changing tendency became more apparent, which indicated that excessive addition of Sr (1.8%, or 2.4%) could weaken the interaction between Sn species and HZSM-5, so that Sn species were more 78

Huang Li, et al. Effect of Strontium Addition to Platinum Catalyst for ropane Dehydrogenation easily to be reduced and much more amount of Sn 0 species could be produced. According to Bacaud R, et al. [26], the oxidative Sn species are favorable to the bimetallic Pt-Sn catalyst. When Sn species exist in a metallic state, intermetallic Pt-Sn alloy would form after the reduction, which will lead to an irreversible deactivation of the catalyst [17, 27]. Coke deposition is the main cause leading to the deactivation of Pt-Sn bimetallic catalysts. Figure 4 shows the TPO profiles of the different catalysts. Generally speaking, there are two successive peaks representing two different carbon deposits on the surface of the catalyst. The first peak at low temperature has been assigned to coke deposited on metallic centers, which are more reactive to oxygen, while the second peak at high temperature represents ones that are located on the external surface of the support [27]. According to Barbier J, et al. [28], the poorly polymerized carbon deposits are located on the metallic surface, and the highly polymerized ones are located on the support. It can be seen from Figure 4 that most of carbon deposits covered the active metal and only a small proportion of carbon deposits covered the external surface of the support. By comparing these profiles, it is recognized that both two peaks moved towards the low temperature range upon a continuous addition of Sr species. Furthermore, the area of the first peak decreased along with an increasing Sr content in the catalyst. As a result it is reasonable to conclude that Sr could weaken the carbon buildup intensity on the catalyst during the reaction, and the addition of Sr species could effectively suppress the formation of carbon on the surface of Pt particles. According to Afonso, et al. [29], olefins are primary precursors in the mechanism of coke formation. The intrinsic acidity of the support could promote undesirable reactions such as cracking/isomerization, thus increasing the carbon deposits. The change of metallic character and catalyst acidity could influence the coke formation obviously. As it has been mentioned before, a suitable addition of Sr is favorable for the dispersion of Pt, so that the homogeneously distributed smaller particles of platinum can decrease the amount of carbon precursors during the process of dehydrogenation. Furthermore, the presence of Sr could decrease the acidity of the HZSM-5 zeolite, which could in consequence decrease the amount and intensity of coke buildup. 3.2 Influence of Sr on properties of catalyst Figure 4 TPO profiles of different catalysts (1) PtSn/HZSM-5 catalyst; (2) PtSnSr(1.2%)/HZSM-5 catalyst; (3) PtSnSr(1.8%)/HZSM-5 catalyst; (4) PtSnSr(2.4%)/HZSM-5 catalyst The catalytic performance of different catalysts is summarized in Table 2. In all runs, the catalyst activity declined with the time on stream as coke was accumulated on the catalyst surface, and the selectivity for propene increased as coke deactivated the active sites. It can be seen from Table 2 Catalytic properties of different catalysts after conducting reaction for 5 h Sample Propane conversion,% Propene selectivity,% Propene yield,% Coke amount,% Deactivation parameter,% 0 h 5 h 0 h 5 h 5 h 5 h Pt/HZSM-5 30.3 18.6 38.9 45.5 8.5 38.6 15.0 PtSn/HZSM-5 34.5 22.9 40.6 49.1 11.2 33.6 11.8 PtSr(1.2%)/HZSM-5 32.0 24.5 40.4 50.8 12.4 23.4 9.0 PtSnSr(1.2%)/HZSM-5 33.6 27.5 42.2 51.5 14.2 18.2 8.3 PtSnSr(1.8%)/HZSM-5 33.8 25.1 45.9 55.9 14.0 25.7 5.7 PtSnSr(2.4%)/HZSM-5 32.3 23.4 48.8 58.9 13.8 27.6 4.1 Note: Deactivation parameter=(x 0 -X f )/X 0 100, in which X 0 is the initial conversion, and X f is the final conversion. 79

China Petroleum Processing and Petrochemical Technology 2012,14(3):75-82 the data listed in Table 2 that the monometallic Pt/HZSM-5 catalyst showed a relatively poor reaction activity and stability. After having been subjected to dehydrogenation reaction for 5 h, the propane conversion of this catalyst decreased from 30.3% to 18.6%. The deactivation parameter was 38.6%, which was the highest among all catalysts. The poor stability was related to a high amount of strong acidity sites, making cracking reaction take place easily. In this circumstance, the coke could cover the Pt particles fully, leading to a rapid catalyst deactivation. The addition of Sn species could result in an increase in the catalytic activity and stability. For the case of PtSr(1.2%)/ HZSM-5 catalyst, the catalytic activity was also low, and the deactivation parameter was 23.4%. However, it is worthy to indicate that the PtSnSr(1.2%)/HZSM-5 catalyst showed high catalytic activity and stability, because the addition of Sn had corroborated its geometric effect and electronic effect that could improve the activity and stability of the catalyst [27]. Moreover, the presence of Sr could decrease the acid sites of the zeolite and facilitate the dispersion of Pt. In contrast to this catalyst, the PtSnSr(1.8%)/HZSM-5 catalyst displayed a higher deactivation parameter than PtSnSr(1.2%)/HZSM-5 catalyst. With the addition of 2.4% Sr, this tendency became more obvious and the final propane conversion reached 23.4%. In view of the TPR analysis, this phenomenon could be explained by the formation of Pt-Sn alloy in the reaction. The coke deposited during a successive reaction time of 5 h over different catalysts was quantitatively analyzed by TG-DTA technique, with the data listed in Table 2. It turns out that the amount of carbon deposits decreased with an increasing Sr content. This result is in agreement with the outcome of TPO analysis. It can be seen also from Table 2 that the propene selectivity of the catalyst was enhanced with an increasing Sr loading, which could be attributed to the modification of the acidic property and the metallic phase in the catalyst by Sr addition. In spite of this fact, the maximum propene selectivity was still too low. In this case the propene yield of the PtSnSr(1.2%)/HZSM-5 catalyst after having been subjected to dehydrogenation reaction for 5 h was 14.2%, which was far less than that achieved in our previous work [17-20, 30]. To explain this phenomenon, it should be noted that dehydrogenation of propane is assumed to proceed through carbonium-ion intermediates and more acid sites generally can promote the subsequent cracking reaction of the initially formed C + 3 carbenium ions [31]. As it has been discussed earlier, Sr species could only partly neutralize the acidic sites of HZSM-5 zeolite. Therefore, the propene selectivity of PtSnSr/HZSM-5 catalysts was low. 3.3 Improvement of PtSnSr/HZSM-5 catalyst performance Depending on our previous work [20], the addition of Na (1.0 m%) not only could entirely neutralize the strong acid sites of HZSM-5 zeolite, but also reduce the moderate/strong Lewis acid sites. Therefore we attempted to prepare the PtSnNa(1.0%)Sr(1.2%)/HZSM-5 catalyst to improve the catalytic performance. Figure 5 Propane conversion versus time on stream of different catalysts (1) PtSn/HZSM-5 catalyst; (2) PtSnSr(1.2%)/HZSM-5 catalyst; (3) PtSnNa(1.0%)/HZSM-5 catalyst; (4) PtSnNa(1.0%)Sr(1.2%)/HZSM-5 catalyst Figure 5 shows the propane conversion on different catalysts. The PtSnNa/HZSM-5 catalyst displays a 33.8% initial conversion. After 5 h on stream, the propane conversion on this catalyst decreased to 20.8%. However, the catalytic activity and stability of this catalyst were superior to those of PtSn/HZSM-5 catalyst and PtSnSr(1.2%)/ HZSM-5 catalyst. With regard to the PtSnNa(1.0%) Sr(1.2%)/HZSM-5 catalyst, its catalytic performance was the best (with an initial conversion of 34.1% and a final conversion of 32.2%). Figure 6 shows the change in propene selectivity. It can be clearly seen that the selectivity to propene increased considerably because of the presence of Na species. The average selectivity to propene of PtSn/ 80

Huang Li, et al. Effect of Strontium Addition to Platinum Catalyst for ropane Dehydrogenation HZSM-5 catalyst, PtSnSr(1.2%)/HZSM-5 catalyst, PtSn- Na(1.0%)/HZSM-5 catalyst and PtSnNa(1.0%)Sr(1.2%)/ HZSM-5 catalyst was 45.3%, 46.7%, 97.9% and 98.4%, respectively. The PtSnNa(1.0%)Sr(1.2%)/HZSM-5 catalyst showed a better catalytic performance as compared to that of the PtSnSr(1.2%)/HZSM-5 catalyst. HZSM-5 catalyst exhibited better catalytic activity than the PtSnSr/HZSM-5 catalyst. However, the side reaction of cracking is also prevailing, resulting in low selectivity to propylene. Afterwards, 1.0 m% of Na was added into the PtSnSr(1.2%)/HZSM-5 catalyst to improve its catalytic performance. The test results implied that the PtSnNa(1.0%)Sr(1.2%)/HZSM-5 catalyst was an ideal catalyst for propane dehydrogenation. Figure 6 Propene selectivity versus time on stream of different catalysts (1) PtSn/HZSM-5 catalyst; (2) PtSnSr(1.2%)/HZSM-5 catalyst; (3) PtSnNa(1.0%)/HZSM-5 catalyst; (4) PtSnNa(1.0%)Sr(1.2%)/HZSM-5 catalyst For the case of the PtSnNa(1.0%)Sr(1.2%)/HZSM-5 catalyst, the presence of Sr could improve the Pt dispersion, while Na and Sr species could decrease the surface acid sites of the support simultaneously. Considering that the PtSn bimetallic catalyst is bifunctional and has two active centers (the metal centers and the acid centers), and the dehydrogenation reaction requires a fine equilibrium between the two centers. It is reasonable to conclude that the PtSnNa(1.0%)Sr(1.2%)/HZSM-5 catalyst is an ideal propane dehydrogenation catalyst with high initial activity to form a good match between its metal function and acid function. 4 Conclusions The present work manifests that the addition of Sr species to the PtSn/HZSM-5 catalyst for propane dehydrogenation could obviously affect its surface chemistry and catalytic behavior. The existence of Sr species could modify the acid function of the support. In these cases, Sr could inhibit the coke formation during the reaction. A suitable addition of Sr (1.2%) could facilitate the dispersion of Pt particles, while excessive Sr (1.8%, 2.4%) will lead to an easy reduction of Sn species, which would be unfavorable to the dehydrogenation reaction. The PtSnSr(1.2%)/ Acknowledgments: The authors are grateful to The Production and Research Prospective Joint Research Project (BY2009153) and The Science and Technology Support Program (BE2008129) of Jiangsu Province of China and the National Nature Science Foundation of China (50873026, 21106017) for financial support. References [1] Zhang Y, Zhou Y, Qiu A, et al. Effect of alumina binder on catalytic performance of PtSnNa/ZSM-5 catalyst for propane dehydrogenation[j]. Ind Eng Chem Res, 2006, 45: 2213-2219 [2] Zhang Y, Zhou Y, Yang K, et al. Effect of hydrothermal treatment on catalytic properties of PtSnNa/ZSM-5 catalyst for propane dehydrogenation[j]. Microporous and Mesoporous Materials, 2006, 96: 245-254 [3] Cola P, Glaser R, Weitkamp J. Non-oxidative propane dehydrogenation over Pt-Zn-containing zeolites[j]. Appl Catal A: Gen, 2006, 306: 85-97 [4] Kogan S B, Schramm H, Herskowitz M. Dehydrogenation of propane on modified Pt/θ-alumina performance in hydrogen and steam environment[j]. Appl Catal A: Gen, 2001, 208:185-191 [5] Yu C, Xu H, Ge Q, et al. Properties of the metallic phase of zinc-doped platinum catalysts for propane dehydrogenation[j]. J Mol Catal A: Chemical, 2007, 266(1/2): 80-87 [6] Kumar. M, Chen. D, Walmsley. J, et al. Dehydrogenation of propane over Pt-SBA-15: Effect of Pt particle size[j]. Catal Commun, 2008, 9: 747-750 [7] Barias O, Holmen A, Blekkan E. Propane dehydrogenation over supported Pt and Pt-Sn catalysts: Catalyst preparation, characterization and activity measurements[j]. J Catal, 1996, 158: 1-12 [8] Huang L, Xu B, Yang L, et al. Propane dehydrogenation 81

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