Structure and catalytic properties of the Zn-modified ZSM-5 supported platinum catalyst for propane dehydrogenation

Structure and catalytic properties of the Zn-modified ZSM-5 supported platinum catalyst for propane dehydrogenation

Chemical Engineering Journal 270 (2015) 352–361 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevie...

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Chemical Engineering Journal 270 (2015) 352–361

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Structure and catalytic properties of the Zn-modified ZSM-5 supported platinum catalyst for propane dehydrogenation Yiwei Zhang, Yuming Zhou ⇑, Li Huang, Shijian Zhou, Xiaoli Sheng, Qianli Wang, Chao Zhang School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Zn-ZSM-5 zeolite with different

a r t i c l e

i n f o

Article history: Received 9 August 2014 Received in revised form 18 December 2014 Accepted 5 January 2015 Available online 13 January 2015 Keywords: Zinc ZSM-5 Supported Pt catalyst Propane dehydrogenation Catalytic activity

41 40 Propane conversion (%)

content of Zn are synthesized and then used as supports.  Zinc species can be incorporated into the framework of the zeolite.  The introduction of zinc at the synthesis leads to the formation of the agglomerated crystal rods.  The use of Zn-ZSM-5 affects the Ptsupport interaction and the platinum dispersion effectively.  The catalyst when the content of Zn is 1.0% exhibits the highest reaction activity and stability.

Formation of the agglomerated crystal rods

39 38 37 36

Homogeneous distribution of metallic particles

PtNa/HZSM-5 PtNa/Zn(0.5%)-ZSM-5 PtNa/Zn(1.0%)-ZSM-5 PtNa/Zn(1.5%)-ZSM-5

35 34 0

2

4

6 8 Time on stream (h)

10

a b s t r a c t Zinc containing ZSM-5 zeolite was hydrothermally synthesized and then was used as support for platinum catalyst in propane dehydrogenation. To investigate the location and the influence of Zn concentration on the catalyst structure and the reaction performance, the prepared samples were studied by several techniques, including XRD, nitrogen adsorption, SEM, NH3-TPD, TEM, hydrogen chemisorption and H2-TPR. It was found that some parts of zinc species could be incorporated into the framework of ZSM-5 zeolite and the introduction of zinc at the synthesis resulted in the formation of the agglomerated crystal rods, which in consequence increased the specific surface area. Additionally, the presence of zinc poisoned the strong acidity of the zeolite evidently. Compared with the Zn-free support, the substitution of Zn in the support strengthened the interaction of platinum with support and increased the platinum dispersion effectively. In this case, relatively homogeneous distribution of metallic particles was found due to the ‘‘geometric effect’’ of Zn. Unlike the impregnation method, the substitution of Zn was more beneficial to reflect the modification effect of the promoter to the metal phase and support acidity. In our experiments, the PtNa/Zn(1.0%)-ZSM-5 catalyst exhibited the highest reaction activity and stability. Nevertheless, with the continuous increase of Zn amount, the metal character had been changed. The formation of PtZn alloy resulted in the loss of catalytic activity and stability, while had a promoting effect for the reaction selectivity. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction

⇑ Corresponding author. Tel./fax: +86 (25)52090617. E-mail address: [email protected] (Y. Zhou). http://dx.doi.org/10.1016/j.cej.2015.01.008 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

The catalytic dehydrogenation of propane is of increasing importance because of the growing demand for propene [1–3]. Indeed, propene is an important raw material for the production of polypropene, acrolein, polyacrylonitrile and acrylic acid.

Y. Zhang et al. / Chemical Engineering Journal 270 (2015) 352–361

However, the reaction of propane dehydrogenation is an endothermic process that requires a relatively high temperature to obtain high yield of propene. Therefore, it is crucial to develop the catalyst possessing high-activity, high-stability and high-selectivity since the deactivation of the catalyst due to coke formation is inevitable. Supported platinum catalyst has been widely used in reforming or dehydrogenation processes due to its high reaction activity [4– 6]. Unfortunately, the reaction stability of this catalyst is still not satisfactory. As a result, great efforts have been made to improve the reaction performance of the supported Pt catalysts by using the new support with distinctive structure and property [7–9]. For instance, ZSM-5 zeolite has some unique physical properties, such as the special three-dimensional channel and adjustable SiAl ratio, which makes the ZSM-5 as an interesting material for applications as a catalyst carrier in the reaction of propane dehydrogenation [2]. Grasselli and his co-workers [10,11] prepared the ZSM-5 supported PtSn catalyst for propane dehydrogenation and achieved a near-equilibrium propene yield of 25% at 550 °C. Kumar et al. [12] systematically investigated the influence of the pore geometry of the catalyst supports (ZSM-5, Beta and SBA-15) on the reaction performances in the dehydrogenation of propane and pointed out that the three dimensional microporous materials (ZSM-5) were better catalytic supports than the mesoporous SBA15. Besides, it has been reported that the capacity of the catalyst that supported on ZSM-5 zeolite to accommodate the coke is much better than the c-Al2O3 supported one [13]. Possibly, the relatively large surface area and the particular channel character of the ZSM5 zeolite should be responsible for this behavior. On the other hand, the introduction of the metal promoter has also been confirmed as an effective method [14,15]. It is believed that the presence of the metal can improve the reaction selectivity by neutralizing the acidity of the support. In particular, the way in which the promoter is added can influence the final properties, not only of the support but also of the metallic phases. Among the several reported literature [16–18], Zn is a common ingredient on noble metal catalysts to control the activity and selectivity in a wide range of reactions. It has been suggested that the presence of Zn either as a promoter or acting as a support (as ZnO) can improve the catalytic properties of Pt catalysts in the dehydrogenation of light alkanes significantly [19,20]. Based on the results of Silvestre-Albero et al. [21], the presence of zinc on the supported platinum catalyst could greatly improve the selectivity to dehydrogenated product (iso-butene), which could reach a value of as high as 100%. Furthermore, the promoting effect of Zn can be mainly attributed to the strong interactions between Zn and Pt, which result in the formation of PtZn alloy and the change in electronic property of Pt atom (e.g., the formation of Ptd–Znd+ entities). Nevertheless, worthy mention is that over these Zn-doped catalysts, the deposition of Zn is mainly carried out by the impregnating with aqueous solution of the Zn precursor [19,22,23]. As a result, the pure support material does not exhibit the desired interactions with the active sites and the promoters. To create the new active centers, the incorporation of metal species into the material matrix is an effective way [24]. It can be expected that the catalytic behavior is different from those with the traditional preparation method. However, up to now, few reports exist on the incorporation of zinc into the structure of the microporous molecular sieve (ZSM-5). In the present work, Zn-ZSM-5 zeolite with different content of zinc was hydrothermally synthesized and then was used as a support for platinum catalyst in propane dehydrogenation. Particular emphasis was focused on the changes of catalyst acidity and character of Pt active sites with the increasing amount of zinc. Besides, to further discuss the effect of Zn-containing support, the location of zinc and the comparative study of the different loading method of zinc had also been investigated. This can provide us with impor-

353

tant information to understand the role of zinc in the support on metallic catalyst. 2. Experimental 2.1. Synthesis of the zeolite The ZSM-5 zeolite was synthesized by hydrothermal method, described in detail elsewhere [24]. The proton form of ZSM-5 (HZSM-5) was obtained by calcining the ammonium form of ZSM-5 at 550 °C for 8 h. Zn-ZSM-5 was synthesized by adding Zn(NO3)26H2O solid into the silicasol, sodium meta-aluminate, NaOH, hexanediamine and distilled water, followed by stirring. After 1 h of settlement, the mixture was transferred into the autoclave and heated at 170 °C for 24 h for the crystallization to complete. After crystallization, the product was filtered, washed with distilled water, dried at 80 °C and calcined at 550 °C. The samples in acid form were obtained through treatment similar to that mentioned for the preparation of HZSM-5. The Zn(NO3)26H2O solid input was determined by the weight percentage of Zn in the zeolite based on the silicasol. 2.2. Preparation of the Catalysts PtNa/Zn-ZSM-5 and PtNa/HZSM-5 catalysts were prepared by sequential impregnation method. First, the powder Zn-ZSM-5 or HZSM-5 was impregnated in an aqueous solution of 0.427 M NaCl. Subsequently, this sample was impregnated in a solution of 0.033 M H2PtCl6, followed by drying. The nominal compositions of the catalyst samples were 0.5 wt% for Pt and 1.0 wt% for Na. PtNaZn/HZSM-5 catalyst was prepared by successive steps of: (i) impregnation of HZSM-5 with an aqueous solution of 0.146 M Zn(NO3)2; (ii) impregnation with aqueous solution of the Na precursor; (iii) impregnation with aqueous solution of the Pt precursor. The loading of Zn was 1.0 wt.%, and the metal contents of Pt and Na were the same as those of the PtNa/Zn-ZSM-5 and PtNa/ HZSM-5 ones. To obtain larger and more resistant particles, all the prepared samples were fully agglomerated with binder during the process of pelletization [25]. The spaghetti shaped extrudates had a diameter of 1.5 mm and a length of 3–8 mm. After totally dried, the catalysts were calcined at 500 °C for 4 h and dechlorinated at 500 °C for 4 h in air containing steam. Zn-ZSM-5 zeolite supported catalysts used during this investigation were abbreviated as PtNa/ Zn(x)ZSM-5, where x represented the amount of Zn added in the support (wt%). 2.3. Catalyst characterization The metallic contents of Zn in the supports were obtained by Xray fluorescence (XRF) measurements on a SWITZERLAND ARL9800 XRF. Table 1 shows the results of the actual concentration of Zn in the different sample. X-ray diffraction (XRD) patterns of the different samples were obtained on a XD-3A X-ray powder diffractometer coupled to a copper anode tube. The Ka radiation was selected with a diffracted beam monochromator. An angular range 2h from 5° to 50° was recorded using step scanning and long counting times to determine the positions of the ZSM-5 peaks. The crystal morphological of the different support was investigated by SEM using S3400 and UV–vis absorption spectra analysis was performed on a Shimadzu UV 3600 spectrometer. TEM studies were analyzed using a JEM-2010 microscope operated at 200 kV. Reduced catalyst samples were prepared by grind-

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Table 1 Characteristics of the different samples. Sample

CEXP (wt.%)

C (wt.%)

BET surface area (m2/g)

Total pore volume (cm3/g)

HZSM-5 Zn(0.5%)-ZSM-5 Zn(1.0%)-ZSM-5 Zn(1.5%)-ZSM-5

0 0.5 1.0 1.5

0 0.21 0.64 1.15

299 315 351 354

0.186 0.182 0.190 0.183

CEXP, the expected contents of Zn; C, the contents of Zn measured by XRF.

ing and suspending the catalysts in ethanol, followed by dropping then a small amount of this solution onto a carbon coated copper grid and drying before loading the sample in the TEM. The particle size distribution was obtained by measuring 200 individual metallic particles. FT-IR spectra were recorded on a Nicolet Magna-IR 750 (USA) spectrometer. The samples were dried, and then mulled with KBr pellets. Surface acidity of the different sample was measured by NH3TPD in TP-5000 apparatus at ambient pressure. About 0.15 g of sample was placed in a quartz reactor and saturated with ammonia at room temperature. TPD was carried out from 100 °C to 600 °C with a heating rate of 10 °C/min and with helium (30 mL/min) as the carrier gas. The platinum dispersion was determined from chemisorption measurements. This experiment was carried out using the dynamic-pulse technique with an argon (99.99%) flow of 50 mL/min and pulses of hydrogen. The experiment process was the same as reported by Dorado et al. [26] except that the sample reduction temperature was 500 °C and the temperature of the argon gas for removing the hydrogen was 40 °C higher than the reduction temperature. Temperature-programmed reduction (TPR) was measured with the same apparatus as that of NH3-TPD. Prior to the TPR experiments, the catalysts were dried in flowing N2 at 400 °C for 1 h. A 5% (v/v) H2 in N2 mixture was used as the reducing gas at a flow rate of 40 mL/min. Subsequently, the reactor was heated in a temperature-programmed furnace from room temperature to 700 °C (10 °C/min). Coke was analyzed by thermogravimetric (TG) test. This analysis was measured in air flow (30 mL/min) with a LCT thermogravimetric analyzer (Beijing optical instrument factory, PR China) from room temperature to 700 °C at the rate of 20 °C/min. 0.02 g of the catalyst was set in the analyzer. 2.4. Catalytic reaction

where i represents propene, ethene, ethane, and methane; Ni and NC3 are number of carbon atoms in the product and propane, respectively. 3. Results and discussion 3.1. Physicochemical properties of the zeolites Fig. 1 exhibits the XRD patterns of the different samples. As can be seen, the representative peaks at 7–9° and 23–24° are observed for the HZSM-5 sample, which corresponds to that given in ref [24] for pure ZSM-5. By comparing with this, as for the zinc system, the product stays crystalline after the substitution and is identified as a ZSM-5 type material. Considering that the Zn-ZSM-5 samples are prepared by incorporation of different amounts of zinc into the zeolite synthesis gels, it is reasonable that some zinc species are zinc substituted in ZSM-5 framework. In addition, from Fig. 1, the as-synthesized sample shows no diffraction peaks of Zn species, probably due to the low level of substitution or the high dispersion of Zn on the support [2]. The porous properties of the different samples obtained from N2 adsorption are illustrated in Table 1. Although the values of total pore volume between the zinc systems and the parent sample are very close, the increase in the surface area is found with various zinc loadings. From Table 1, the surface area of the Zn(1.5%)-ZSM-5 sample increases about 18.4% when comparing with that of the Znfree one. The possible reason for this behavior may be attributed to the incorporation of zinc into the framework of ZSM-5 zeolite, thus influencing the physical structure of the zeolite to some extent, which will de discussed in the following section. Fig. 2 represents the scanning electron micrographs of HZSM-5 and the Zn-ZSM-5 samples with different content of zinc. With respect to the HZSM-5 zeolite sample, it exhibits a typical MFI structure. As reported previously [24], the crystal is hexagonalshaped crystal rods, quite uniform with crystal size of 6–8 lm in

Propane dehydrogenation was carried out in a conventional quartz tubular micro-reactor and all the catalysts were reduced in H2 at 500 °C for 8 h before catalytic evaluation. The catalyst (mass 2.0 g) was placed into the center of the reactor. Reaction conditions were as follows: 590 °C for reaction temperature, 0.1 MPa pressure, H2/C3 = 0.25 (molar ratio) and the propane weight hourly space velocity (WHSV) was 3.0 h1. The reaction products were analyzed with an online GC-14C gas chromatography equipped with an activated alumina packed column and a flame ionization detector (FID). The propane conversion (X) and reaction product selectivity (S) were calculated by the following formulae:

Propane conversion ðX%Þ ¼

Selectivity ðSi %Þ ¼

nðC3 H8 Þin  nðC3 H8 Þout  100%; nðC3 H8 Þin

moles of product i formed Ni  100%; moles of C3 reacted NC3

(4)

(3)

(2)

(1) 10

20

30

40

50

2 Theta (degree) Fig. 1. XRD patters of different samples: (1) HZSM-5; (2) Zn(0.5%)-ZSM-5; (3) Zn(1.0%)-ZSM-5; (4) Zn(1.5%)-ZSM-5.

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Fig. 2. SEM micrographs of the different samples: (1) HZSM-5; (2) Zn(0.21%)-ZSM-5; (3) Zn(0.64%)-ZSM-5; (4) Zn(1.15%)-ZSM-5.

(1) Absorbance

length and about 3.2 lm in width (Fig. 2(1)). For comparison, relatively great changes in the morphology are observed after the substitution of zinc. When the content of Zn is low (0.5%, Fig. 2(2)), the sample shows the evidence of increasing aspect ratios, evidence of twinned crystals, and the appearance of raised faces, which are the features for the isomorphous heteroatom substitution [27]. Moreover, with the increase of zinc concentration, the agglomeration of many small hexagonal-shaped crystal rods are found. As revealed in Fig. 2(3) and (4), the morphology of Zn(1.0%)-ZSM-5 and Zn(1.5%)-ZSM-5 samples appeared as polycrystalline and the agglomerated crystal rods with crystal size of 1.2 lm in length and about 0.5 lm in width forming a larger cluster. Possibly, the appearance of special small crystal structure is the main reason for the increased surface area of the Zn-containing materials [28]. Meanwhile, as for the formation of the agglomerated crystal rods, it may be related with the introduction of Zn2+ species at synthesis. It is known that the presence of zinc species may affect the ZSM-5 crystal nucleus forming process [28]. As a result, more nucleus will be formed. To further investigate the possibility of zinc to incorporate into the framework of ZSM-5 zeolite, the prepared samples were characterized by UV–vis spectra and FI-IR technique. As displayed in Fig. 3, over the ZnO sample, an absorption band at 360 nm arises, which can basically be described as a O2Zn2+ ? OZn+ ligand to metal charge transfer transition (LMCT) [29]. Moreover, the absorption edges of the Zn-containing materials are found to shift slightly towards shorter wavelength regions (about 220 nm) in comparison to ZSM-5 zeolite. Ökte et al. [30] obtained the similar results when they synthesized the photocatalytic material by incorporation of lanthanum ions on TiO2 supported ZSM-5. Generally, the emerging absorption peaks below 230 nm in the UV–vis spectra mainly caused by the incorporation of metal atom into the framework of ZSM-5 zeolite [31]. From this point of view, it can be speculated that some parts of zinc species have been incorporated into the framework of the zeolite. The results of FT-IR

(2) (3) (4) (5) 200

250

300

350

400

450

500

550

600

Wave length (nm) Fig. 3. UV–vis spectra of the different samples: (1) ZnO; (2) ZSM-5; (3) Zn(0.5%)ZSM-5; (4) Zn(1.0%)-ZSM-5; (5) Zn(1.5%)-ZSM-5.

spectra of the different samples allow us to confirm this point with great certainty. As presented in Fig. 4, the zinc systems exhibit broadening of the spectral bands, which is similar to those reported for nickel and cobalt series [27,32]. Meanwhile, the materials show a distortion of the spectra between 1250 and 900 m cm1, especially for the sample when the content of Zn is high. As known [24], this region of the spectrum is assigned to asymmetrical T–O– T stretching and is indicative of heteroatom substitution. Accordingly, analysis of the FTIR spectroscopy supports the hypothesis of heteroatom substitution, which is identical with the results of UV–vis spectra. The substitution of Zn in the support may affect the material acidity obviously. Ammonia temperature-programmed desorption

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Transmittance ( a.u.)

(1)

1400

(2) (3) (4)

1200

1000 800 -1 Wave number (cm )

600

400

Fig. 4. FT-IR profiles of the different samples: (1) HZSM-5; (2) Zn(0.21%)-ZSM-5; (3) Zn(0.64%)-ZSM-5; (4) Zn(1.15%)-ZSM-5.

(TPD) and NH3 uptake of the different samples are displayed in Fig. 5 and summarized in Table 2, respectively. There are two peaks of HZSM-5 sample, one is low (peak I), corresponding to the desorption of the adsorbed NH3 on weak acid sites and the other is at high temperature (peak II), corresponding to the desorption of the adsorbed NH3 on strong acid sites [33]. In a contrast, the presence of zinc decreases the ammonia desorption peak at high temperature evidently, indicating that the substitution of zinc poisons the strong acid sites of the support. Furthermore, this change tendency becomes more apparently with the increasing concentration of Zn. It is well known that the existence of strong acid sites can mainly catalyze the side reactions (cracking, isomerization and polymerization) in dehydrogenation reaction [2]. Thereby, it is reasonable to conclude that the use of this zinc-containing material can minimize the undesirable side reactions, which is favorable for the improvement of the reaction selectivity. Additionally, as listed in Table 2, the decreased amount of weak acid sites are observed after the presence of zinc, meaning that the introduction of zinc salt into the gel at synthesis can also destruct the weak acid centers of the zeolites. Meanwhile, over the Zn-doped materials, it is worthy noting that the low desorption temperature of ammonia shifts slightly towards higher range, revealing that the acid intensities of the remaining weak centers have become relatively stronger. As a result, the acidic centers of the Zn-substituted materials

(4) (3) (2)

(1) 100

are mainly related with the weak-medium acid sites. Obviously, these findings are not completely consistent with the previous results when the Zn component is deposited by impregnation with its precursor [19,22]. Possibly, in the present work, the substitution of Zn may result in stronger interaction between zinc species and the support, thus changing the effect of this promoter on the acidity.

200

300

400

500

600

o

Temperature ( C) Fig. 5. NH3-TPD profiles of the different samples: (1) HZSM-5; (2) Zn(0.5%)-ZSM-5; (3) Zn(1.0%)-ZSM-5; (4) Zn(1.5%)-ZSM-5.

3.2. Characterization of the catalysts In our experiments, the above mentioned zinc systems were used as the supports, and then the corresponding catalysts were prepared by loading the metals of platinum and sodium. Fig. 6 represents the NH3-TPD profiles of the different samples. As can be seen, no ammonia desorption peaks at high temperature are observed for all the samples, suggesting that the addition of sodium promoter may neutralize the strong acid sites of the catalysts preferentially [33]. Possibly, because of this effect, considerably lower acidity is found over the catalyst than the corresponding support material. In addition to this, with respect to the Zn-containing catalysts, the acid amount decreases gradually with the increase of zinc content. H2-TPR profiles of the different samples are shown in Fig. 7. It can be seen that there are no obvious reduction peaks on the HZSM-5 sample. In comparison, after the loading of Pt and Na components (Fig. 7(2)), the sample presents two reduction peaks: one peak around 250 °C and another broad peak around 480 °C, which are in agreement with literature data for Pt/zeolite systems [34]. Usually, there are two types of Pt oxides on the supported catalyst and the latter peak corresponding to a higher reduction temperature indicates that a proportion of Pt species strongly interact with the support [13]. By contrast, the use of Zn-ZSM-5 support causes an obvious change in the TPR profile. As regards the PtNa/ Zn(0.5%)-ZSM-5 sample, the peak due to the reduction of Pt species around 250 °C is gradually shifted to higher temperature and the amount of hydrogen consumption for the reduction of the Pt-related metal phase increases significantly. One possible explanation is that, over this sample, the formation of small crystal structure of the support (Fig. 2) leads to a stronger interaction of platinum with the support and, thus, to a higher resistance towards reduction of Pt species. Meantime, as for the higher amount of hydrogen consumption, it may be associated with the reduction of Zn species [35]. Thus, it is difficult to ascribe the reduction peak at 470 °C correctly, because some amounts of Zn species can also be reduced to metallic Zn during the reduction process [22]. It is reasonable to assign this peak to the conjunct reaction of Pt and Zn components, which indicates the existed strong interactions between these two components. With the increase content of Zn (PtNa/Zn(1.0%)-ZSM5), the hydrogen consumption for the reduction at about 260 °C increases continuously and two additional hydrogen consumption peaks at 410 and 550 °C appears. Obviously, this change tendency becomes more apparently with the continuous increase of Zn content (Fig. 7(5)). With respect to the reduction peak at 410 °C, it may be related with the metals that are mainly located on the outer zeolite surface. In this case, the introduction of platinum promotes the reduction of Zn species. As a result, platinum has possibly alloyed with zinc and the modification of surface metal character is inevitable. The additional peak at 550 °C indicates the presence of another, less noble species which is probably associated with the zinc. As proposed by Cola et al. [35], these zinc species can be the binuclear (Zn–O–Zn)2+, multinuclear (Zn–(O–Zn)n)2+, as well as the partially reduced ZnO species. TEM images and corresponding particle size distribution of the samples are shown in Fig. 8. As for the catalyst when the parent HZSM-5 zeolite is used as the support, the metallic particles are not well distributed and the average particle diameter is

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Y. Zhang et al. / Chemical Engineering Journal 270 (2015) 352–361 Table 2 NH3-TPD results of the different samples. Sample

Peak I

HZSM-5 Zn(0.5%)-ZSM-5 Zn(1.0%)-ZSM-5 Zn(1.5%)-ZSM-5

Temperature/°C

NH3 uptake/mmol g

Temperature/°C

NH3 uptake/mmol g

242 246 247 247

0.31 0.26 0.24 0.21

448 454 446 445

0.33 0.12 0.10 0.06

(4)

(3)

(2)

(1)

100

200

300

400

500

600

o

Temperature ( C) Fig. 6. NH3-TPD profiles of the different catalysts: (1) PtNa/HZSM-5; (2) PtNa/ Zn(0.5%)-ZSM-5; (3) PtNa/Zn(1.0%)-ZSM-5; (4) PtNa/Zn(1.5%)-ZSM-5.

(5) (4) (3) (2) (1)

100

200

Total NH3 uptake/mmol g1

Peak II 1

300 400 500 o Temperature ( C)

600

700

Fig. 7. H2-TPR profiles of the different samples: (1) HZSM-5; (2) PtNa/HZSM-5; (3) PtNa/Zn(0.5%)-ZSM-5; (4) PtNa/Zn(1.0%)-ZSM-5; (5) PtNa/Zn(1.5%)-ZSM-5.

15.30 nm. Comparing with this, relatively smaller particles which are homogeneous distributed on the catalyst are observed on the PtNa/Zn(0.5%)-ZSM-5 sample, demonstrating that the existence of zinc in the support is favorable for the distribution of metallic particles. Platinum dispersion estimates, which were calculated from pulse chemisorption at room temperature using the assumption H:Pt = 1 [19], are listed in Table 3. As can be seen, the presence of zinc increases platinum dispersion effectively, implying again that the substitution of Zn in the support can increase the interactions between platinum and the support, thus suppressing the migration of metallic particles and decrease the size of platinum particles. From Fig. 8, this phenomenon becomes more apparently with the increase content of zinc. In the case of the PtNa/Zn(1.0%)ZSM-5 sample, the average particle diameter decreases to 9.25 nm.

1

0.64 0.38 0.34 0.27

To explain this, it should be noted that the promoter of Zn has the ‘‘geometric effect’’, which implies that zinc can act as spacer to reduce the size of the platinum particles as the role of tin in PtSn catalyst [22]. In this way, the number of contiguous accessible platinum sites on the catalyst surface maybe decreased, which is beneficial to the improvement of metal dispersion. Previous studies have shown that the active site for propane dehydrogenation is the single platinum atoms, while the side reactions need the large platinum ensembles [19]. Therefore, it can be concluded herein that the undesired reactions can be suppressed effectively over the Zn-doped catalysts. Other reason for the improved dispersion of metallic particles may be attributed to the increased surface area of the zinc-containing support (Table 1). In this circumstance, the increased surface area may strengthen the interaction between Pt, Na and Zn three components, which in consequence decreases the degree of Pt sintering during the process of calcination. However, the opposite phenomenon is found when the content of Zn is excessive (1.5 wt.%, Fig. 8(4)). Over this sample, the average particle diameter increases to 13.5 nm and a decrease in platinum dispersion is observed. Clearly, this behavior may be explained in terms of the modifications of the metallic phase. As analyzed before (Fig. 7), some amounts of Zn species can be reduced to metallic Zn when the concentration of Zn is high, which results in the formation of Pt–Zn alloy. Therefore, it can be speculated herein that the Pt particles on the catalyst surface would be presented as mixture of pure Pt and alloyed Pt–Zn alloy, which in consequence changes the distribution of metallic particle size. 3.3. Reaction performance Fig. 9 shows the effect of the different support on the reaction performances of PtNa catalysts at 590 °C in the dehydrogenation of propane. It can be seen that the PtNa/HZSM-5 sample exhibits relatively poor reaction activity and stability. After reaction for 10 h, the conversion decreases from 38.6% to 34.7%. Meanwhile, the selectivity to propene increases progressively. Obviously, the catalyst deactivates quickly due to the coke deposition [36]. In this circumstance, the active sites of the catalyst can be gradually covered by the coke with the reaction time prolonged. Accordingly, it can be speculated that the active centers for the cracking and coking reactions are decreased so that the improvement of the reaction selectivity is inevitable. In contrast to this, the use of the zinc-containing support improves the reaction activity and stability significantly. From Fig. 9(a), the initial conversions of propane catalyzed by PtNa/Zn(0.5%)-ZSM-5, PtNa/Zn(1.0%)-ZSM-5 and PtNa/Zn(1.5%)-ZSM-5 catalysts are 39.67%, 40.60% and 39.51%, respectively. The deactivation parameter D (defined as D = [X0  Xf]  100%/X0, where X0 is the initial propane conversion and Xf is the final propane conversion) for these catalysts are 7.81%, 7.29% and 9.16%, respectively. It is clear that the PtNa/Zn(1.0%)ZSM-5 catalyst exhibits the highest reaction activity and lowest deactivation value, which suggests that the existence of zinc in the support with suitable concentration is favorable for the reaction to be carried out. Coke quantitative analysis (Table 3) shows that the amounts of coke over the Zn-doping catalysts are much lower than that over the Zn-free one (PtNa/HZSM-5). Moreover,

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Fig. 8. TEM micrographs of the different catalysts: (1) PtNa/HZSM-5; (2) PtNa/Zn(0.5%)-ZSM-5; (3) PtNa/Zn(1.0%)-ZSM-5; (4) PtNa/Zn(1.5%)-ZSM-5.

Table 3 Pt dispersion and coke amount of the different catalysts. Catalysts

Pt dispersion (%)a

Coke amount (wt.%)b

PtNa/HZSM-5 PtNa/Zn(0.5%)-ZSM-5 PtNa/Zn(1.0%)-ZSM-5 PtNa/Zn(1.5%)-ZSM-5

27 36 41 35

4.8 3.2 2.4 1.9

a

Calculated from hydrogen chemisorption (HC) experiment. Experimental value calculated from thermogravimetric (TG) analysis after the reaction for 10 h. b

the amount of coke over zinc-doped PtNa catalyst is determined by the zinc content. It can be seen that the catalytic capacity to coke tolerance increases significantly with the increasing amount of zinc. Besides, the influences of different Zn content on the catalytic selectivity are also investigated. As presented in Fig. 9(b), the increasing concentration of Zn gives a rapid increase in selectivity towards propene, meaning that the side reactions can hardly take place over the catalyst with high Zn content. To further investigate the possible distribution of side reaction products, the selectivities to alkenes (alkanes) of the different catalysts after reaction for 10 h

41

(b)

(4)

98

39 38 (3) 37 (2) 36

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Fig. 9. (a) Conversion, and (b) selectivity as function of time for the different catalysts: (1) PtNa/HZSM-5; (2) PtNa/Zn(0.5%)-ZSM-5; (3) PtNa/Zn(1.0%)-ZSM-5; (4) PtNa/ Zn(1.5%)-ZSM-5. Reaction conditions: 590 °C, H2/C3 = 0.25 (molar ratio), m(cat) = 2.0 g, WHSV = 3.0 h1.

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are also presented. From Fig. 10, the HZSM-5 supported catalyst exhibits the highest selectivities to methane, ethene and ethane. While, the selectivities to side reaction products decrease significantly when the Zn-substituted materials are used as the supports. Moreover, among these Zn-doped catalysts, it can be noted that the content of Zn has an obvious influence on the distribution of side reaction products. The higher Zn content is, the lower alkenes (alkanes) selectivities can be obtained. To explain these, it should be noted that on supported platinum catalyst, there exists two active centers (the metal particle and the acid site) and these active centers may work collaboratively [37], which plays an important role in directing the catalytic performances. As aforementioned, the role of Zn can be proposed as a spacer to reduce the platinum particles and the most uniform distribution of metallic particles can be obtained over the PtNa/ Zn(1.0%)-ZSM-5 sample. Furthermore, as deduced from the analysis of the NH3-TPD-profiles (Fig. 5), the substitution of Zn in the support decreases the total acidity of the support evidently and the acidic centers of the supports are mainly related with the weak-medium acid sites. In this way, the matching between the Pt active sites and the acid function on the Zn-ZSM-5 supported catalyst can be improved, which is advantageous to the dehydrogenation of propane. Moreover, as discussed in the TPR experiment (Fig. 7), when the content of Zn is suitable, the small crystal structure of the support may strengthen the interaction between platinum and support, thus suppressing migration of Pt particles over the support. Meanwhile, relatively strong interactions between Pt and Zn components also exist. For these reasons, the dehydrogenation reaction can be promoted to be preceded, while the side reactions are inhibited effectively, which is beneficial to increase the reaction activity and to decrease the coke amount during the reaction. Nevertheless, with the continuous increase of Zn content, the character of the metallic phase has been modified. As commented before, over this sample, the formation of PtZn alloy is inevitable, which results in the decreased metal dispersion due to the inactive nature of the alloy. At the same time, the catalyst acidity decreases continuously because of the ‘‘poisoned’’ effect of Zn. Thereby, the initial ratios between the number of active metal sites and acid sites on the catalyst would be destroyed, thus resulting in the loss of activity and stability. As for the changes of reaction selectivity, it is noteworthy to mention that on supported Pt catalyst platinum is the only active metal

and propene is only formed on the metal by dehydrogenation, the main cracking product (ethene) is mainly formed from cracking on the carrier and the ethane is formed by hydrogenolysis of propane and by hydrogenation of ethene, with both reactions taking place on the metal [38]. Moreover, it is known that the dehydrogenation and cracking of propane are assumed to proceed through carbonium-ion intermediates [39]. The higher acid sites generally promote the subsequent cracking reaction of the initially formed C+3carbenium ions. Therefore, the changes of catalytic acidity and active metal character are also responsible for the selectivity to propene. As shown in Fig. 6, when the Zn-free zeolite is used as the support, the amount of weak acidity is still relatively large, which induces more side reactions, especially the cracking one (Fig. 10). In addition, over this sample, the relatively large particle diameter may also account for the low selectivity to propene since the occurrence of side reactions needs the large platinum ensembles. Comparing with this, the substation of Zn in ZSM-5 zeolite reduces the acidity of the support. Meantime, the dilution effect of platinum by zinc improves the distribution of metallic particles. Hence, the undesired reactions can be suppressed, which results in the enhancing selectivity. As regards the PtNa/Zn(1.5%)-ZSM-5 sample, although the average particle diameter increases a little, the catalyst acidity decreases continuously. Furthermore, over this sample, the modification of metal phase (e.g. formation of Pt–Zn alloy) may also have a promoting effect for the reaction selectivity [40]. In this way, the undesired reactions can be suppressed, which is favorable for the formation of desired reaction products. Since the PtNa/Zn(1.0%)-ZSM-5 catalyst shows the highest reaction activity and stability, this catalyst is chosen for the subsequent tests. Fig. 11 illustrates the performance comparison of the PtNa/ Zn(1.0%)-ZSM-5 and PtNaZn(1.0%)/ZSM-5 catalysts by increasing the reaction temperature (605 °C). The BET surface area and the actual content of Zn (measured by XRF analysis) over the PtNaZn(1.0%)/ZSM-5 sample are 291 m2/g and 0.67 wt.%, respectively. It is clear that the propene selectivity decreases with increasing propane conversion. By comparing with the PtNaZn(1.0%)/ZSM-5 catalyst, higher propane conversion and similar propene selectivity can be obtained on the PtNa/Zn(1.0%)-ZSM-5 sample, implying that the introducing method of Zn has an obvious influence on the reaction performance even at higher reaction temperature. As analyzed before, the substitution of zinc results in the appearance of special small crystal structure, which in consequence increases the surface

98

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4

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94 (1) 92 (2)

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90 38

40

42

44

46

48

Propane conversion (%) Fig. 10. Selectivities to alkanes (alkenes) of the different catalysts after reaction for 10 h: (1) PtNa/HZSM-5; (2) PtNa/Zn(0.5%)-ZSM-5; (3) PtNa/Zn(1.0%)-ZSM-5; (4) PtNa/Zn(1.5%)-ZSM-5. Reaction conditions: 590 °C, H2/C3 = 0.25 (molar ratio), m(cat) = 2.0 g, WHSV = 3.0 h1.

Fig. 11. Propene selectivity as a function of propane conversion for the different catalysts: (1) PtNa/Zn(1.0%)-ZSM-5, (2) PtNaZn(1.0%)/ZSM-5. (Reaction conditions: 605 °C, H2/C3 = 0.25 (molar ratio), 0.1 MPa, WHSV = 3.0 h1, m(cat) = 2.0 g.)

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area of the zeolites. By contrast, during impregnation process, the decrease of surface area is inevitable due to the pore blockages of the metal precursors [41]. Furthermore, as for the PtNaZn(1.0%)/ ZSM-5 catalyst, the metal is deposited in the order of Zn ? Na ? Pt. In this case, Zn may firstly block the sites of the support and has less chance to strongly interact with Pt. That is to say, by comparing with the impregnation method (PtNaZn(1.0%)/ZSM-5 catalyst), the substitution of Zn in the support results in the increased surface area and a stronger surface interactions between active sites, promoter, and support. In this way, it is expected that the modification effect of Zn to the metal phase and support acidity may be reflected more effectively. In other words, the dilution of Pt by Zn minimizes the multisite reactions and the poisoning effect by Zn decreases the occurrence possibility of the side reactions, thus favoring the production of olefins. Besides, as for the catalyst that the zinc component is loaded by the impregnation method, the existed competitive adsorption behavior during the process of catalyst preparation may also decrease the metal dispersion. Accordingly, it can be concluded that Zn-containing ZSM-5 zeolite is an attractive support material to stabilize Pt active metals and to derive its optimum catalytic activity for the dehydrogenation of propane. 4. Conclusions The Zn modified ZSM-5 zeolites with different content of Zn have been hydrothermally synthesized and then this kind of zeolite has been used as the support for platinum catalyst in propane dehydrogenation. To further investigate the role of zinc substitution, the comparative study of the PtNa catalysts with different Zn introducing method has also been made. In this zinc-substituted material, UV–vis spectra and FI-IR technique confirms the incorporation of some parts of zinc species into the framework of the ZSM-5 zeolite. The analysis by SEM shows the appearance of the agglomerated crystal rods with the increase content of Zn. Meanwhile, the substitution of Zn reduces the total acidity of the support significantly and the acidic centers of the Zndoping materials are mainly related with the weak-medium acid sites. Interestingly, the use of this zinc containing ZSM-5 results in the strong interactions between Pt and the support, thus leads to a higher resistance towards reduction. Moreover, relatively homogeneous distribution of metallic particles and the improved Pt dispersion are observed over the Zn-doped catalysts. Compared with the traditional impregnation method, the incorporation of Zn into the support makes the modification effect of the promoter to the metal phase and support acidity is reflected more effectively. In our experiments, the catalytic behavior of the PtNa/Zn(1.0%)-ZSM5 sample merits to be highlighted, it exhibits the highest initial activity (40.60%) and the lowest deactivation parameter (7.3%) after the reaction for 10 h. However, with the increasing content of Zn, the metal character has been changed. In this case, the formation of PtZn alloy decreases the platinum dispersion and modifies the metallic phase, which leads to the decreased reaction activity and stability. Acknowledgments The authors are grateful to the financial supports of National Natural Science Foundation of China (Grant No. 21376051, 21106017 and 21306023), Natural Science Foundation of Jiangsu Province of China (Grant No. BK20131288), China Scholarship Council (No. 201308320238), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant No. BA2011086) and Instrumental Analysis Fund of Southeast University.

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