Influence of the Competitive Adsorbates on the Catalytic Properties of

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Influence of the Competitive Adsorbates on the Catalytic Properties of PtSnNaMg/ZSM-5 Catalysts for Propane Dehydrogenation Linyang Bai, Yuming Zhou,* Yiwei Zhang, Hui Liu, Xiaoli Sheng, and Mengwei Xue School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, P.R. China ABSTRACT: This paper is aimed at studying the influence of the competitive adsorbates (HCl, lactic acid, and citric acid) on the structural and catalytic properties of PtSnNaMg/ZSM-5 catalysts for propane dehydrogenation. N2 adsorption, X-ray diffraction, inductively coupled plasma, NH3 temperature-programmed desorption, H2 temperature-programmed reduction and transmission electron microscopy were applied for characterization of the prepared catalysts. The catalytic performance was tested in a microreactor. The obtained results indicate that the catalyst prepared in HCl medium exhibited the best catalytic activity and stability due to relatively better metal distribution on the support and/or the strong interactions between Pt and Sn as well as between them and the support.

1. INTRODUCTION The dehydrogenation of propane to propylene has received considerable attention in recent years because of its importance in the growing demand for propylene. Propylene is widely used for the production of polypropylene, polyacrylonitrile acrolein, glycerine, isopropanol, and refinery feedstock to improve the octane rating of gasoline and other chemicals. Propane dehydrogenation has been industrialized using platinum or chromium catalysts. The operation conditions best suited for the dehydrogenation reaction are high temperatures, low pressures, and low hydrogen concentrations, which is usually required to overcome thermodynamic constraints and obtain a high yield of propylene. Unfortunately, under the stringent reaction conditions, the deep side reactions are accelerated. The side reactions to lighter hydrocarbons and coke deposition are favored. Coke formation leads to catalyst deactivation. Cracking and hydrogenolysis reactions decrease the propylene selectivity. For these reasons, significant efforts have been made worldwide to develop catalysts with high catalytic performance. Bimetallic platinum-tin supported catalysts have been intensively studied because of their commercial applications in the hydrocarbon dehydrogenation and naphtha reforming processes.1-5 However, the catalytic performance depends on the type of platinum precursor,6,7 the nature of supports,8,9 the pretreatment conditions of the samples,10,11and the preparation method.12,13 All these factors can influence the properties of the final metallic phase. It is well-known that the superior catalytic performance may be the result of a high dispersion of Pt.14,15 Solar and his co-workers16 reported the polarity of the solvent, the pH of the impregnating solution, and the isoelectric point of the support determine the extent of the metal precursor and support interaction, which would be responsible for the metal dispersion in the catalyst. Carvalho et al.17 investigated the effect of water, acetone, and ethanol addition on the properties of Pt/Al2O3 and Pt-Sn/Al2O3 catalysts. They found that the catalysts prepared in the solvents have different metallic dispersion, which is attributed to the different boiling point and polarity of the solvents. Aboul-Gheit et al.18 found that HCl-doped Pt/ZSM-5 catalyst was more active in the hydroconversion of cyclohexene as compared with the undoped r 2011 American Chemical Society

one. This behavior was explained in terms of the increased Pt dispersion. Hydrochloric acid and carboxylic acids such as acetic acid and citric acid are frequently used as competitive adsorbates during catalysts preparation,19-22 as they have proved to be very effective in governing active metal dispersion on supports. However, the reports about adsorbates on the catalytic behavior of the dehydrogenation catalysts have not been obtained. The aim of this paper is to study the effects of the different competitive adsorbates on the catalytic behavior of PtSnNaMg/ ZSM-5 catalysts. Catalysts were characterized by X-ray diffraction (XRD), inductively coupled plasma (ICP), N2 adsorption, temperature-programmed reduction (TPR), and transmission electron microscopy (TEM). In addition, the catalytic properties for propane dehydrogenation on the catalysts were tested in a microreactor.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The PtSnNaMg/ZSM-5 catalysts were prepared by sequential impregnation technique. The powder H-ZSM-5 was impregnated in an aqueous solution of 0.587 M Mg(NO3)2 at 80 °C for 4 h and then dried at 80 °C for 3 h. Afterward, the sample was divided into three portions. Each of them was impregnated in a solutions mixture of 0.033 M H2PtCl6, 0.153 M SnCl4, and 0.427 M NaCl aqueous solution with 0.15 M hydorchloric acid, 0.15 M latic acid, and 0.15 M citric acid, Finally, the prepared samples were dried at 80 °C for 3 h. In all cases, the loadings of Pt, Sn, Na, and Mg were 0.5, 1.0, 1.0, and 0.5 wt %, respectively. Afterward, the prepared samples were fully agglomerated with 5.0 wt % alumina during the process of pelletization. After they were totally dried, the catalysts were calcined at 500 °C in air for 4 h, then dechlorinated at 500 °C for 4 h in air containing steam, and finally reduced in H2 at 500 °C for 8 h. The different catalysts were denoted as Cat-A, Cat-B, and Cat-C, respectively. Received: September 5, 2010 Revised: December 10, 2010 Accepted: February 18, 2011 Published: March 08, 2011 4345

dx.doi.org/10.1021/ie1018639 | Ind. Eng. Chem. Res. 2011, 50, 4345–4350

Industrial & Engineering Chemistry Research 2.2. Catalyst Characterization. BET surface areas of the catalysts were measured by N2 adsorption-desorption at 77 K using a Micromeritics ASAP 2020 instrument. Before measurements, samples were degassed under vacuum at 350 °C for 4 h. Inductively coupled plasma (ICP) optical emission spectrometry was used for the determination of the metal content in each sample.The measurements were performed with a JA1100 inductively coupled plasma quantometer, and the sample was dissolved in a mixture of HF and HNO3 acids before the measurement. XRD patterns of the different samples were obtained on a XD-3A X-ray powder diffractometer coupled to a copper anode tube. The KR radiation was selected with a diffracted beam monochromator. An angular range 2θ from 5° to 40° was recorded using step scanning and long counting times to determine the positions of the ZSM-5 peaks. Surface acidity was measured by NH3-TPD in a TP-5000 apparatus at ambient pressure. The sample (150 mg) was preheated at 500 °C for 1 h and then cooled to room temperature in flowing He. At this temperature, sufficient pulses of NH3 were injected until adsorption saturation. TPD was carried out from 100 to 600 °C with a heating rate of 10 °C/min and with helium (30 mL/min) as the carrier gas. TPR was measured in TP-5000 apparatus. Prior to the TPR experiments, 0.15 g catalyst was dried in flowing N2 at 100 °C for 1 h. 5% H2/N2 was used as the reducing gas at a flow rate of 40 mL/min. The rate of temperature rise was 10 °C/min up to 800 °C. Transmission electron microscopy (TEM) studies were conducted using JEOL-2010 instrument working at 200 kV. Prior to the analysis, samples were prepared by grinding, suspending, and sonicating them in ethanol and placing a drop of the suspension on a copper grid with a perforated carbon film. 2.3. Catalytic Tests. Propane dehydrogenation was carried out in a fixed-bed quartz microreactor at atmospheric pressure and 590 °C. About 2.0 g of catalyst was placed in the middle of the reactor with quartz beads support at each end. The weight hourly space velocity (WHSV) during propane dehydrogenation was 3.0 h-1 based on propane and the mole ratio of H2/C3H8 was 0.25. The reaction products were analyzed with a gas chromatograph equipped with a thermal conductivity detector (TCD). Propane conversion, propylene selectivity, and propylene yield were calculated as follows:

C3 H8 conversion ¼ ¼

C3 H8in - C3 H8out 100% C3 H8in

C3 H6 selectivity ¼ ¼

C3 H6out 100% C3 H8in - C3 H8out

C3 H6 yield ¼ ¼

C3 H6out 100% C3 H8in

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The XRD spectra of different samples are displayed in Figure 1. It can be observed that the XRD spectra of the catalysts having sharp intensity peaks are very similar to that observed for ZSM-5, implying that ZSM-5 zeolite structure is well preserved during the catalysts preparation. No characteristic peaks for Pt crystallites and promoters were

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Figure 1. XRD patterns of the samples: (1) ZSM-5; (2) Cat-A; (3) CatB; (4) Cat-C.

detected by XRD because of the low metal concentrations. For all catalysts, the relative intensities of the diffraction peaks at 2θ = 7-8° and 23-24° decrease as compared with that of ZSM-5. The former means that incorporation of the promoters induces a decrease in the intensity because of some of the promoters at the inner surface of the ZSM-5 pores, whereas the latter implies that the promoters reduce the crystallinity of ZSM-5. Thus, it can be concluded that different competitive adsorbates have little effect on the structure of ZSM-5. The composition and BET surface areas of PtSnNaMg/ZSM5 catalysts and the support are listed in Table 1. Chemical analysis determined by the ICP technique reveals that the experimental metal concentrations of the samples are lower than the theoretial ones. The total BET surface area of the catalysts decreased after metal loading as compared with ZSM-5, suggesting that some of the promoters may be located insides the pores, leading to partial pore blockage. However, there are small differences among the catalysts studied, which is possibly because of the presence of the adsorbates affecting the distribution of metal species on the ZSM-5 support. This possibility is supported by the TEM images, which will be discussed in the following section. Figure 2 shows the NH3-TPD profiles of the different samples. The profile of ZSM-5 sample as a reference is also included. ZSM-5 presents two well-defined peaks at 241 °C (peak I) and 450 °C (peak II); the first peak at low temperature can be assigned to ammonia adsorbed to weak acid sites; the second peak at high temperature corresponds to ammonia adsorbed to strong acid sites, in agreement with the results in the literature.23 After impregnation, the strong acid sites disappear and the total acidity decreases, implying the strong acid sites of the support are neutralized by Naþ ion preferentially.24 Nevertheless, NH3-TPD profiles of the Cat-A catalyst shows an important difference with respect to those of the Cat-B and CatC catalysts. As can be seen from Table 1, the acidity is slightly lower for Cat-A than for Cat-B and Cat-C catalysts. This behavior can be interpreted in terms of the promoters location in the support, which is due to the poisoning effect of Sn and Na on the acid sites of the support. For Cat-A, the presence of Na 4346

dx.doi.org/10.1021/ie1018639 |Ind. Eng. Chem. Res. 2011, 50, 4345–4350

Industrial & Engineering Chemistry Research

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Table 1. Characteristics of the Different Samples sample

NH3 uptake (mmol/g)

SBET (m2 g-1)

0.48

332

0.25

311

0.43

0.27

300

0.43

0.30

291

Pt (wt %)

Sn (wt %)

Na (wt %)

Mg (wt %)

Cat-A

0.44

0.84

0.78

0.42

Cat-B

0.42

0.83

0.74

Cat-C

0.43

0.86

0.75

ZSM-5

Figure 2. NH3-TPD profiles of the different samples: (1) ZSM-5; (2) Cat-A; (3) Cat-B; (3) Cat-C.

and Sn could easily migrate into the pore channels and neutralize some acid sites within the zeolite channels because HCl has no steric hindrance effect. For Cat- B and Cat-C, there exist hydroxylic and carboxylic functions, which may generate steric hindrances effects.25 For these reasons, these promoters may actually block zeolite channels and prevent access to some acid sites. 3.2. TEM. To observe the particle size and particle morphology on the support, the TEM operation was performed and the TEM images are shown in Figure 3. A relatively homogeneous distribution of metallic particles over Cat-A can be seen. Figure 4 shows the histograms of the metallic particles distribution on Cat-A, Cat-B, and Cat-C, which was obtained by directly measuring the size of 200 randomly chosen particles from the TEM images. It can be noted that Cat-A displays a narrow distribution of metallic particle size, whereas Cat-C exhibits a wide dispersion of metallic particle size. The mean particle sizes over Cat-A, Cat-B, and Cat-C are 9.4, 11.8, and 14.2 nm, respectively. The size distribution of metallic particles over Cat-B and Cat-C are broader than that of Cat-A, indicating a relatively uniform distribution of metallic nanoparticles in Cat-A. This behavior may be attributed to the fact that the presence of HCl in the Cat-A preparation may help the adsorption of platinum chloride anion (PtCl62-) with the support, thereby favoring the distribution of metallic particles.26 On the other hand, the spatial constraint effect which comes from carboxylic and hydroxyl functions on lactic acid and citric acid would hinder the interaction between the metal precursors and the support, thus facilitating the migration of metallic particle. Therefore, the difference of the metallic particles distribution may be aroused from the different adsorbates used in the catalysts preparation, which would be expected to have an impact on catalytic performance.

Figure 3. TEM images of Cat-A (a), Cat-B (b), and Cat-C (c).

3.3. TPR. The TPR profiles of the PtSnNaMg/ZSM-5 catalysts prepared with different competitive adsorbates are depicted in Figure 5. Three reduction peaks appear in the TPR profile of CatA, which is in accordance with the results reported in the literature.27,28 The low-temperature peak centered at ca. 270 °C is indicative of the reduction of oxidized Pt species, the intermediate band (around 450 °C) corresponds to the reduction of Sn4þ to Sn2þ, and the high-temperature peak at about 600 °C is ascribed to the reduction of Sn2þ to Sn0. The reducibility of Sn in Pt-Sn-based catalysts depends on the extent of interaction between the tin species and support.29 Keeping the oxidized state of Sn species on bimetallic Pt-Sn catalysts is vital for the dehydrogenation reaction. From the TPR results, it appears that the Cat-A behaves differently from the Cat-B and Cat-C. The interactions between active component and promoters or support influence the reducibility of catalysts. The hydrogen consumption corresponding to the reduction of tin species is greater for Cat-B and Cat-C than for Cat-A. In the case of Cat-C, the high reduction temperature peak disppears, while the intermediate temperature peak area increases, which indicates a simultaneous reduction of Sn4þ to Sn2þ and Sn2þ to Sn0 occurs and the reduction of Sn species becomes easier, as was previously reported.24,30 This behavior is likely to reflect the weaker anchoring of tin oxides species on the support during the impregration process. Thus, a large fraction of tin oxides on Cat-C is readily reduced to metallic tin, which is probably alloyed with Pt. As known, lactic acid and citric acid have carboxylic and hydroxyl functions, which may produce the steric hindrances that are encountered in the impregnation step. And this 4347



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Figure 5. H2-TPR profiles of the different catalysts: (1) Cat-A; (2) CatB; (3) Cat-C.

Figure 6. Propane conversion as a function of reaction time for the different catalysts: (1) Cat-A; (2) Cat-B; (3) Cat-C.

Figure 4. Histograms of particle size distribution obtained by TEM: (a) Cat-A; (b) Cat-B; (c) Cat-C.

Table 2. Values of Initial Conversion (X0), Final Conversion (Xf), and the Deactivation Parameter (D) after 9 h of Propane Dehydrogenation Reaction at 590 °C for Different Catalysts catalysts

steric hindrance effect appears to be more pronounced as the number of carboxylic and hydroxyl functions increases, which would hinder the metal-support interaction. Thus, the reduction of tin species is favored significantly. On the other hand, the Cat-A consumes less hydrogen corresponding to the reduction of tin species, which suggests the presence of HCl could establish a strong interaction between tin species and the support, leading to a significant amount of Sn species existing still in oxidized states and thus facilitating the platinum-tin interaction.31 Taking into account the above-mentioned results, it can be concluded that the Cat-A catalyst prepared in HCl medium would have a higher interaction between Pt and Sn as

X0 (%)

Xf (%)

D

Cat-A

34.7

33.5

3

Cat-B

34.0

32.2

5

Cat-C

32.9

29.7

10

well as between them with the support than those prepared in lactic acid and citric acid mediums, which would affect the catalytic properties for propane dehydrogenation. 3.4. Catalytic Tests. Dehydrogenation of propane over different catalysts was carried out at 590 °C.The results are given in Figure 6 and Table 2. The deactivation of the catalysts during the propane dehydrogenation may result from the formation of 4348



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Figure 7. Stability test of Cat-A catalyst in the propane dehydrogenation.

carbonaceous deposits, which block the active sites of metallic Pt. The deactivation parameter (D) along the reaction time is defined as D = 100 (Xf - X0)/X0, where X0 is the initial conversion (at the beginning of reaction time) and Xf is the final conversion (at 9 h of the reaction time). The Cat-A catalyst displays the highest initial activity and lowest deactivation. In other words, Cat-A shows higher activity and stability than Cat-B and Cat-C. This catalytic behavior can be ascribed to the following reasons: (1) The presence of HCl can help to prevent metallic particles from aggregating, resulting in a large number of smaller metallic particles on the surface of the catalyst. Thus, the active components are distributed very well. (2) HCl also produces the stronger interactions between Pt and Sn as well as those with the support, inhibiting the reduction of oxidized tin species to metallic tin. According to the literature, the oxidized states of tin seem to be responsible for the enhancement of the catalytic activity and stability for PtSn-based catalysts during propane dehydrogenation.32,33 On the other hand, the presence of lactic acid and citric aid could not effectively inhibit the reduction of tin oxides to metallic state. The Sn0 could be alloyed with Pt to induce the possible formation of PtSn alloy. Therefore. the relatively high amount of PtSn alloy and/or the important surface enrichment of metallic phases in Sn would not be avoidable. It has been proposed that PtSn alloy in the bimetallic PtSn catalysts is inactive in the dehydrogenation processes.34-36 The proportion of PtSn alloy must be kept relatively low with respect to nonalloyed Pt so as not to affect the catalytic activity. Similar results were previously obtained by Serrano-Ruiz et al.,37 who reported that in Pt-Sn/CeO2/C catalysts with different ratio of Sn/Pt, the reaction activity for isobutane dehydrogenation decreases with the content of alloyed tin. Therefore, Cat-C exhibits low activity and stability, which can be explained in terms of the large amount of metallic tin and the probable PtSn alloy formation. To further assess the catalytic properties of the optimized catalyst, a stability test was also performed. The reaction conditions were as follows: P = 0.1 MPa, WHSV = 3.0 h-1, initial temperature of 610 °C, and final temperature of 625 °C. The propane conversion and the selectivity to propylene as a function of the reaction time are shown in Figure 7. To maintain the

conversion of propane at about 30%, the temperature and the ratio of H2/C3H8 were adjusted. It can be observed that the small deactivation rate is displayed and the selectivity to propylene is higher than 95% during the course of reaction for 970 h.

4. CONCLUSIONS The competitive adsorbates used in the preparation of PtSnNaMg/ZSM-5 catalysts have an important influence on the physicochemical characteristics and catalytic performance in propane dehydrogenation. The competitive adsorbates do more than affect the distribution of metallic particles on the support; they also modify the interaction between the metal and the support. The PtSnNaMg/ZSM-5 catalyst prepared with lactic acid or citric acid displayed low catalytic performance owing to steric effect, and thus hindered the dispersion of metallic partcles. TEM results indicated that the presence of HCl promoted the metallic particles distribution. On the other hand, the PtSnNaMg/ZSM-5 catalyst prepared using HCl adsorbate improved metallic distribution and enhanced the interactions between Pt and Sn as well as between them with the support, resulting in the highest catalytic properties in the dehydrogenation of propane. ’ AUTHOR INFORMATION Corresponding Author

*Telephone/fax: þ86 25 52090617. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by a production and research prospective joint research project of Jiangsu Province of China (BY2009153), the Key Program for the Scientific Research Guiding Found of Basic Scientific Research Operation Expenditure, Southeast University (3207040103); 333 high-level talent training project, Jiangsu Province of China (BRA2010033), and the National Natural Science Foundation of China (50873026). 4349



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Influence of the Competitive Adsorbates on the Catalytic Properties of

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