Enhanced catalytic performance of PtSn catalysts for propane dehydrogenation by a Zn-modified Mg(Al)O support

Fuel Processing Technology 198 (2020) 106222

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Research article

Enhanced catalytic performance of PtSn catalysts for propane dehydrogenation by a Zn-modified Mg(Al)O support

T

Xiaoping Wu, Qiao Zhang, Lungang Chen, Qiying Liu, Xinghua Zhang, Qi Zhang, Longlong Ma, ⁎ Chenguang Wang Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, 510640 Guangzhou, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: PtSn Zn-modified Mg(Al)O Propane dehydrogenation Propene HRTEM

The development of thermally stable Pt-based catalysts with superior performance for propane dehydrogenation reactions remains a daunting challenge. In this study, a series of PtSn-Mg(Zn)AlO catalysts with different Zn and Pt contents were synthesized by the anion-exchange method for the propane dehydrogenation reaction. To investigate the influence of Zn on the reaction performance and the catalyst structure, the prepared catalysts were characterized with several analytical techniques, including XRD, TEM, HRTEM, H2-TPD and CO pulse chemisorption. A good propane conversion (initially higher than 55% and averaging 45.2% over a period of 2 h) and propene selectivity (higher than 99%) were obtained experimentally with PtSn-Mg(3Zn)AlO at 550 °C. The results demonstrated that Pt provided the active sites and that Zn was a promoter. The HRTEM and CO pulse chemisorption results indicated that the addition of Zn to the PtSn-MgAlO catalyst can stabilize the dispersion of metal particles and thus inhibit the sintering of Pt particles. Furthermore, the activities of the used catalysts were almost recovered through reaction-regeneration cycles with the burning-off of coke during five cycles.

1. Introduction Propene is an important chemical used in the petrochemical industry for the production of various chemicals such as polymers, oxygenates and important intermediates, and the catalytic conversion of propane to propene is of great significance in both academic and industrial fields [1–4]. The dehydrogenation of propane to propene represents an economical and environmentally friendly route compared to the traditional thermal or catalytic cracking processes of crude-oilderived naphtha, which represents the increasing contradiction between the supply and demand situations of propene [2,4–7]. For the alkane dehydrogenation reaction, a catalyst support is crucial to attain a stable catalyst performance since the support not only governs the catalytic properties but also has a profound impact on the catalyst activity and selectivity for alkanes due to the interaction with the metal active phase [8–10]. High surface area alumina with a high thermal stability, mechanical strength and retention of dispersed platinum (Pt) nanoparticles is a classic support used in Pt-based dehydrogenation catalysts [1,11,12]. Pt is selectively anchored to the corresponding unsaturated aluminium ions present on the surface of γAl2O3, resulting in a large impact on maintaining the high stability [13–16]. Sattler et al. [16] reported a superior catalyst material for the

⁎

dehydrogenation of propane to propene based on Pt-Ga/Al2O3 and proposed a bifunctional active phase, in which gallium (Ga) is the active dehydrogenation element and where Pt functions as a promoter. Moreover, Pt-based zeolite-supported catalysts have also been studied for the alkane dehydrogenation reaction [6,17,18]. Since zeolites may impose a shape selectivity, zeolites are superior to the formation of dehydrogenation over oligomeric products such as coke [19,20]. Nawaz et al. [17] have synthesized a PtSn-based catalyst using Al2O3-SAPO-34 as a support for propane dehydrogenation and found that the addition of a certain amount of Al2O3 significantly integrates the metal functions of PtSn-based catalysts, thereby ultimately improving the catalytic performance. Shi et al. [21] reported a Pt–Sn–K/θ-Al2O3 catalyst for propane dehydrogenation with an average conversion of 38.6% and an average selectivity of 95.2% over a reaction time of 25 h at 600 °C. Recently, using layered double hydroxides (LDHs) as catalyst supports has become a popular topic for the alkane dehydrogenation reaction since after calcination, the compounds will lose interlayer anions and water molecules, and then, a homogeneous mixture of oxides will form; the latter has a large surface area and excellent thermal stability [22–24]. The most active catalyst for propane dehydrogenation is the Ptbased catalyst owing to its superior ability to activate paraffinic CeH

Corresponding author. E-mail address: [email protected] (C. Wang).

https://doi.org/10.1016/j.fuproc.2019.106222 Received 8 May 2019; Received in revised form 16 September 2019; Accepted 18 September 2019 0378-3820/ © 2019 Published by Elsevier B.V.

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Scheme 1. Schematic illustration of PtSn catalysts supported on Zn-modified Mg(Al)O for propane dehydrogenation.

2. Experiment

bonds and its low activity in CeC bond cleavage [2,3]. However, since Pt nanoparticles tend to be unstable on commonly used catalyst support materials, only the Pt particles loaded on the supports suffer from low olefin selectivity and rapid coke deposition levels, thereby resulting in a rapid irreversible deactivation of the catalyst [25]. More importantly, the use of Pt-based catalysts with other promoter elements (e.g., Sn [26,27], In [28,29], Ir [30], Ga [5,31,32], zinc (Zn) [33,34], and Ca [3]), represents an important strategy for manipulating their compositions and structures, which has been the subject of research to enhance the activity and stability of the alkane dehydrogenation reaction. The presence of Sn can suppress metal sintering and promote the diffusion of coke species from the metal surface to the support, thus improving the stability of the catalyst [1,26,27,35]. Kaylor et al. [27] have explored the influences of Sn loadings on the performances of Sn-promoted Pt nanoparticles for propane dehydrogenation to propene at 773 K. Searles et al. [5] have reported a Ga–Pt bimetallic catalyst prepared via sequential grafting of a Pt precursor onto silica possessing site-isolated Ga sites followed by H2 that displays high activity, selectivity, and stability in propane dehydrogenation. In addition to conventional catalysts, some novel methods have also been used for alkane dehydrogenation processes such as thermally coupled reactors [36] and plasma reactors [37]. This work reports a PtSn catalyst that is supported on Zn-modified MgAlO layered double oxides (LDOs), which has better ability to catalyse the propane dehydrogenation reaction than PtSn catalyst without Zn. Based on preliminary work in the laboratory, we designed and synthesized a series of catalysts [PtSn-Mg(Zn)AlO with different Zn and Pt contents] and evaluated their performances. Moreover, the catalysts were characterized with a suite of analytical techniques, such as X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), hydrogen temperature-programmed desorption (H2-TPD) and CO pulse chemisorption. We demonstrated that the addition of Zn modulated the electronic properties of Pt, increased the dispersion of Pt particles, and ultimately improved the propane conversion and propene yield. Moreover, the regeneration stability was further enhanced after Zn modification.

2.1. Preparation of Mg(x-Zn)AlO supports LDHs were synthesized by the co-precipitation method as previously reported [24,31]. In summary, appropriate amounts of Mg(NO3)2·6H2O (10.0 g, Macklin Co., Shanghai, China), Al(NO3)3·9H2O (7.76 g, Macklin Co., Shanghai, China), and Zn(NO3)2·6H2O (the loadings of Zn were 0, 1, 2, 3, 4 and 5 wt%) were dissolved in 50 mL deionized water. Na2CO3 (4.38 g Sinopharm Chemical Reagent Co., AR., Shanghai, China) and NaOH (3.82 g Sinopharm Chemical Reagent Co., AR., Shanghai, China) were dissolved in 50 mL deionized water to form a mixed base solution. Then, the two mixed solutions were mixed into a third beaker with 100 mL deionized water by dropwise addition, during which the pH of the solution was adjusted and maintained at 10. As the Mg-Zn-Al solution was added to the beaker, the solution was stirred vigorously at room temperature. Then, the mixture was aged at 80 °C for 12 h to obtain a suspension. After the suspension was cooled to room temperature, it was filtered, washed with deionized water to neutrality, and then dried overnight at 100 °C. The dried products were denoted Mg(xZn)AlO, where x represented the mass percentage of Zn in the catalysts. 2.2. Preparation of x-PtSn/Mg(x-Zn)AlO catalysts The x-PtSn/Mg(x-Zn)AlO catalysts were prepared by the anion-exchange method [24,31]. In a typical procedure, appropriate amounts (the amounts of Pt were 0, 0.1, 0.3, and 0.5 wt%, along with Sn amounts of 0 and 0.2 wt%) of K2PtCl6 (Aladdin Biochemical Technology Co., Shanghai, China) and Na2SnO3 (Macklin Co., Shanghai, China) were dissolved in 100 mL deionized water. Next, 1.0 g of dried Mg(x-Zn)AlO catalyst was added to the mixed solution. Then, the mixed solution was heated at 80 °C for 24 h under vigorous stirring to allow PtCl62− and SnO32− to sufficiently exchange with CO32−. Finally, the resulting suspension was filtered and washed with deionized water and then dried at 100 °C overnight and reduced by hydrogen at 600 °C for 3 h. The resulting series of catalysts were denoted x-PtSn-Mg(x-Zn)AlO, where 0.5PtSn-Mg(x-Zn)AlO was denoted PtSn-Mg(x-Zn)AlO. 2.3. Catalytic evaluation The propane dehydrogenation reactions were carried out in a fixed2

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temperature. The sample was heated to 573 K in a H2 flow, reduced at this temperature for 1 h and then cooled to room temperature in a helium flow. The CO pulses were injected at that temperature until the adsorption reached saturation. The concentration of the CO pulse was determined by a TCD. The outlet gas was also monitored by a mass spectrometer. The amount of CO adsorption was calculated as the difference between the total amount of CO injected and the amount measured at the outlet in the sample. The metal dispersion was calculated by assuming a CO to surface metal atom ratio of 1:1 [38,39]. The Pt particle size was estimated using the cubic Pt particle model with the calculated Pt metal dispersion value. TPD experiments were carried out in an automatic chemisorption analyser (ChemBET Pulsar TPR/TPD, Quantachrome Instruments Company Limited, Boynton Beach, FL). Prior to the TPD experiments, 200 mg of the catalysts was reduced in flowing H2 (10% H2/Ar 30 mL/ min) at 600 °C at 10 °C/min for 1 h and cooled to room temperature under the Ar atmosphere (30 mL/min). Then, the samples were exposed to H2 (10% H2/Ar, 30 mL/min) until surface saturation was reached. The weakly adsorbed H2 was removed by flushing with Ar (30 mL/min) for 0.5 h. After the baseline was stabilized in the gas flow, the reactor was heated to 700 °C with a heating rate of 10 °C/min, and the amount of H2 desorption was recorded by a TCD detector.

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Time on stream (min) Fig. 1. Propane conversion of the propane dehydrogenation reaction over PtSnMg(3Zn)AlO, Pt-Mg(3Zn)AlO, PtSn-MgAlO, Pt-MgAlO, Sn-Mg(3Zn)AlO and SnMgAlO. Reaction conditions: T = 550 °C, atmospheric pressure, H2/C3H8/ N2 = 0.15:1:4 and WHSV = 18.9 h−1.

bed quartz reactor with an inner diameter of 8 mm under atmospheric pressure; the catalyst (0.1 g, approximately 0.05 g after calcination) was diluted with 0.5 g of quartz particles that were 0.25–0.5 mm in size (Scheme 1). The mixed catalysts were put in the centre of the reactor and then reduced by hydrogen at 600 °C for 3 h. The propane dehydrogenation reaction was carried out at 550–600 °C in a 20% C3H8/N2 atmosphere with a certain amount of hydrogen. For the regeneration operation, the combustion of the coke deposited on the catalyst was alternately carried out in air at 550 °C for 4 h. The reaction products were analysed by gas chromatography (GC) (Agilent, 7890 B, Agilent Technologies, USA) equipped with two thermal conductivity detectors (TCDs) and a flame ionization detector (FID). The FID was used to measure the concentrations of all organic compounds, including CH4, C2H6, C2H4, C3H8 and C3H6. One of the TCDs was used to measure the concentrations of N2, and the other was used to quantify the H2 concentrations. The propane conversion and propane selectivity were defined as follows:

C3 H8 conversion = C3 H6 selectivity =

C3 H8 in − C3 H8 out × 100% C3 H8 in C3 H6 out × 100% C3 H8 in − C3 H8 out

3. Results and discussion 3.1. Effect of the Zn loading Fig. 1 shows the propane conversion in the propane dehydrogenation reaction over PtSn-Mg(3Zn)AlO, Pt-Mg(3Zn)AlO, PtSn-MgAlO, PtMgAlO, Sn-Mg(3Zn)AlO and Sn-MgAlO. The propane conversion rate was almost zero when Pt was not loaded on the supports, implying that Pt was the active centre for the propane dehydrogenation reaction. For the MgAlO system, when monometallic Pt was used, the initial propane conversion activity was 9.1% and decreased to 6.6% after 120 min. For the Mg(3Zn)AlO system, when Pt alone was used, the propane conversion activity was 35.1%, which decreased to 22.7% after 120 min. These results indicated that Zn as a promoter was beneficial for the propane dehydrogenation reaction. However, the Pt-Mg(3Zn)AlO catalysts became deactivated rapidly due to the volatilization of Zn, and the sintering and coking of the Pt-Zn metal phase [40]. The addition of Sn can suppress the coking and side reactions as well as decrease the sintering of metal [6]. Therefore, compared to the other catalysts, PtSnMg(3Zn)AlO exhibited a very high propane dehydrogenation activity. The average conversion of propane in 2 h was almost as high as 45.2%. Nevertheless, the equilibrium conversion of propane based on ASPEN PLUS was about 40% under the same conditions, which indicates that there must have been other side reactions such as coking, cracking, etc. (see Fig. A.1) [1]. Essentially, the activity comparison was not based on rigorous kinetic measurements and such comparison was thereby not quantitative. The propane conversion and propene selectivity over the PtSn-Mg (x-Zn)AlO catalysts are shown in Fig. 2. For PtSn-MgAlO, the initial propane conversion was 34.3%. As the reaction time was prolonged, the catalytic activity could be maintained in a relatively stable stage, and the average propane conversion in 2 h was 29.6%. Compared to PtSnMgAlO, the PtSn-Mg(1Zn)AlO and PtSn-Mg(2Zn)AlO samples exhibited better catalytic activities, thereby demonstrating that the addition of Zn was advantageous to the propane dehydrogenation reaction. With the increase in Zn content, when the Zn content was 3%, the initial conversion of propane reached a maximum of 55.2%, and the average conversion increased to 45.2%. The results indicated that the presence of Zn had a clear impact on the propane dehydrogenation performances of the PtSn-Mg(x-Zn)AlO catalysts. However, with the further increase in Zn loading, the catalytic activity evidently decreased to 42.9% and 37.6% for PtSn-Mg(4Zn)AlO and PtSn-Mg(5Zn)AlO, respectively. This finding may be explained by the fact that an overhigh Zn loading

(1)

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where C3H8 in and C3H8 out are the ethane content in the feed and exit gases, respectively, and C3H6 out is the ethylene content in the exit gas. The weight hourly space velocity (WHSV) was calculated according to the equation:

WHSV (h−1) = 12νM/22.4m

(3)

where ν is the flow rate of 20% C3H8/N2, M is the molecular weight of propane and m is the mass of the catalyst (approximately 0.05 g after calcination). 2.4. Catalyst characterizations The XRD analysis of all of the supports and catalysts was performed on an X'Pert Pro MPD diffractometer (PW 3040/60, PANalytical, Almelo, Netherlands) with Cu Kα radiation (λ = 0.15 nm) operating at 40 kV and an applied current of 100 mA. The TEM images and HRTEM images of the catalysts were taken with a JEM-20100F microscope (JEOL Corp., Tokyo, Japan) operated at 120 kV. CO pulse chemisorption measurements were performed using a Micromeritics 2920 instrument modified for CO adsorption at room 3

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destroy the structural integrity of the hydrotalcite support. Since the PtSn-Mg(3Zn)AlO catalyst showed the best performance, the PtSn-MgAlO and PtSn-Mg(3Zn)AlO catalysts were subjected to TEM to observe the distribution of the supported metal on the catalyst surface and particle size, the results are shown in Fig. 4. From Fig. 4a and d, the metal particles were dispersed uniformly on the surface of the supports, and there was no significant aggregation of clusters. Compared to PtSn-MgAlO, the results of HRTEM showed that smaller metal particles were evenly distributed on the supports when doped with a small amount of zinc (Fig. 4b and d). According to the statistical data, the mean diameters of the metal particles of the PtSn-MgAlO and PtSnMg(3Zn)AlO catalyst were 1.91 ± 0.23 nm and 1.42 ± 0.36 nm, respectively, which also indicated that the addition of a small amount of Zn resulted in an improved metallic particle size in this range (Fig. 4c and f). To confirm the effect of the Zn doping of the PtSn-MgAlO catalyst on the average metal particle size and the metal dispersion, CO pulse chemisorption measurements of typical catalysts were carried out at room temperature. The active particle diameter and metal dispersion of the typical catalysts determined by CO pulse chemisorption are shown in Table 1. The PtSn-Mg(3Zn)AlO catalyst exhibited the smallest average particle size of 1.82 nm, accompanied by the best metal dispersion of 62.2%, while the metal dispersion and active particle diameter of PtSn-MgAlO were 26.5% and 4.27 nm, respectively, which are in agreement with the optimal reactivity of the PtSn-Mg(3Zn)AlO catalyst. It was interesting that a low or excessive proportion of Zn addition could not result in a satisfactory particle distribution and a smaller Pt particle size. In summary, the metal dispersion decreased in the following order: PtSn-Mg(3Zn)AlO (62.2%) > PtSn-Mg(2Zn)AlO > (53.6%) > PtSn-Mg(4Zn)AlO > (42.8%) > PtSn-Mg(5Zn)AlO (34.0%) > PtSn-Mg(1Zn)AlO (30.4%) > PtSn-MgAlO (26.5%). The results confirmed that doping zinc in the PtSn-MgAlO catalyst can reduce the size of the metal clusters, thereby improving the metal dispersion. Conspicuously, the active particle diameter measured by CO chemisorption is so different from those measured by HRTEM. This may be due to the structural collapse of the layered hydrotalcite during calcination, with some of the platinum metal deposited sub-surface, and as a consequence never exposed to the reactive environment [41]. Furthermore, to explain the relationship between Zn and the supports, H2-TPD profiles of the reduced catalysts are shown in Fig. 5. All of the catalysts showed broad peaks between 300 °C and 600 °C. Typically, low temperature desorption peaks are attributed to hydrogen on metallic Pt and high temperature desorption peaks are designated as spillover hydrogen, strong chemisorbed hydrogen and hydrogen in the subsurface layers of Pt [42]. It can be seen that, the amounts of

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Time on stream (min) Fig. 2. Effect of the Zn loading on the catalytic performances of the PtSn-Mg(xZn)AlO catalysts in the propane dehydrogenation reaction: (a) propane conversion; (b) propene selectivity. Reaction conditions: T = 550 °C, atmospheric pressure, H2:C3H8:N2 = 0.15:1:4 and WHSV = 18.9 h−1.

suppressed the propane dehydrogenation reaction. It was verified that a moderate Zn loading was advantageous for the catalytic activity of the propane dehydrogenation reaction. Moreover, all catalysts had a gradual process in the initial stage of propane dehydrogenation (Fig. 2a) and exhibited outstanding selectivity levels towards propene that were higher than 99% during the entire dehydrogenation process (Fig. 2b), which indicated that the presence of Zn had no significant effect on the propene selectivity. To investigate the role of Zn in the propane dehydrogenation system, a series of characterization analyses of Pt-Mg(x-Zn)AlO and PtSn-Mg(x-Zn)AlO were conducted. The XRD patterns of the PtSn-Mg(x-Zn)AlO catalysts, which had been prepared by introducing PtSn into LDH supports via the anionexchange method and then reduced in a hydrogen atmosphere at 600 °C for 3 h, are illustrated in Fig. 3. Only characteristic diffraction peaks corresponding to MgO are observed, which means that the calcination products of Mg-Al hydrotalcite have a MgO-like structure [31]. Moreover, the diffraction peaks of metallic Pt, metallic Sn, metallic Zn and their oxides are not detected. The structures of the hydrotalcite-like supports are confirmed by XRD (Fig. A.2). All six samples show peaks identifying the hydrotalcite materials at 2θ values of approximately 11.7° (003), 23.6° (006), 35.0° (012), 39.7° (015), 47.1° (018), 60.9° (110) and 62.3° (113) (Fig. A.2). All of the diffraction peaks are very narrow and sharp, which indicates that the prepared hydrotalcite supports have high purity levels, single crystal phases and high structural regularities. The results implied that the presence of Zn does not 4

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Fig. 4. (a) TEM image, (b) HRTEM image and (c) size distribution of the metal particles of the PtSn-MgAlO catalyst; (d) TEM image, (e) HRTEM image and (f) size distribution of the metal particles of the PtSn-Mg(3Zn)AlO catalyst. The size distribution of platinum particles was calculated according to HRTEM data. Table 1 Active particle diameter and metal dispersion values of the typical catalysts determined by CO pulse chemisorption. Active particle diameter (nm)

26.5 30.4 53.6 62.2 42.8 34.0

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hydrogen on the metallic Pt in Pt-MgAlO are less than those in Pt-Mg (3Zn)AlO. On the PtSn-Mg(3Zn)AlO, the amounts of hydrogen on the metallic Pt are significantly higher than those in Pt-Mg(3Zn)AlO. That is, the PtSn-Mg(3Zn)AlO catalyst exhibited larger peak areas in comparison with those of the other catalysts, which revealed that the addition of Zn to PtSn-MgAlO catalyst can greatly promote its ability to adsorb hydrogen. In fact, the most important cause of the decrease in the hydrogen content in the Pt metal is the sintering of Pt particles [43]. Therefore, the presence of Zn changed the structure of the catalyst to a certain extent and suppressed the formation of coke precursors, thereby resulting in an improved distribution of Pt particles and thus improving the catalyst reaction performance, which was similar to the effect of Sn as a promoter [1,35].

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3.2. Effect of the H2/C3H6 ratio The introduction of hydrogen into the reaction feed gas can greatly improve the stability of the dehydrogenation reaction of propane. This is because the addition of H2 can effectively inhibit the formation of carbon deposits and reduce the deactivation rate of the catalyst, thus improving the dehydrogenation performance of the reaction [44]. By changing the molar ratio of H2 and C3H8 in the feed gas, the detailed 5

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the conversion of propane gradually decreases. When the molar ratios of H2/C3H8 are 0.2:1, 0.25:1, and 0.5:1, the initial conversions of propane are 50.6%, 49.2%, and 45.2%, respectively (Fig. 6a). As the molar ratio of H2/C3H8 increases, the stability of the reaction continuously improves because the introduction of hydrogen inhibits the process of alkane cracking to form carbon deposits. However, with the gradual increase in the hydrogen content, the propane conversion rate first increases and then decreases. The latter is because the hydrogen is used as a product of dehydrogenation of propane. When the content is too high, the reaction proceeds to the positive reaction direction, and the hydrogen suppresses the Pt activities on the catalyst sites, which is not conducive to the progress of the reaction. When hydrogen is not introduced, the propene selectivity tends to decrease slightly with the progress of the reaction, and the selectivity of propylene after maintaining hydrogen is maintained at approximately 99.5% (Fig. 6c).

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Fig. 7. Effect of the temperature on the catalytic performances of the PtSn-Mg (3Zn)AlO catalysts in the propane dehydrogenation reaction. Reaction conditions: T = 550–600 °C, atmospheric pressure, H2:C3H8:N2 = 0.15:1:4 and WHSV = 18.9 h−1.

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3.3. Effect of the reaction temperature Fig. 7 shows the propane conversion and propene selectivity values of the PtSn-Mg(3Zn)AlO catalyst in the propane dehydrogenation reaction at different temperatures. Noticeably, propane dehydrogenation process is an endothermic reaction, and the reaction temperature has a large influence on the propane dehydrogenation reaction. At reaction temperatures of 550 °C, 580 °C and 600 °C, the initial propane conversions were 55.2%, 61.8% and 66.5%, respectively, and decreased to 39.6%, 36.6% and 27.7% after 2 h, respectively. These results indicate that raising the temperature can increase the initial propane conversion because the propane dehydrogenation reaction is an endothermic reaction process. However, increasing the temperature also caused alkane thermal cracking, which formed a larger amount of carbon deposition on the catalyst surface; the latter led to a rapid deactivation of the catalyst, and the activity and stability declined quickly. Despite the highest initial conversion of propane at 600 °C, the average propane conversion after 2 h was only 38.5%, while the propane conversion was 45.2% at 550 °C. Apparently, all of the reactions exhibited relatively high propene selectivities during the entire dehydrogenation process at different temperatures. However, the selectivity values of C3H6 at high temperatures were clearly lower than those at low temperatures because propane cracking produced larger amounts of small molecular olefins and alkanes, such as CH4, C2H6, and C2H4. Considering a high propane conversion with a stable propene selectivity, the reaction temperature of 550 °C was the desired selection.

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H2/C3H8 ratio Fig. 6. Effect of H2/C3H6 ratio of PtSn-Mg(3Zn)AlO catalysts on catalytic performances in propane dehydrogenation reaction: (a) propane conversion; (b) propene selectivity; (c) average conversions of propane and average selectivities of propene at 2 h. Reaction conditions: T = 550 °C, atmospheric pressure and WHSV = 18.9 h−1.

propane conversion, propene selectivity, average conversion of propane and average selectivity of propene of the PtSn-Mg(3Zn)AlO catalysts for different H2/C3H6 ratios at 2 h are shown in Fig. 6. In the absence of H2, i.e., the molar ratio of H2/C3H8 was 0:1, the activity of the catalyst rapidly decreased from 48.2% to 13.4% at 2 h (Fig. 6a). When the molar ratio of H2/C3H8 was 0.1:1, the conversion of propane decreased from 53.4% to 31.8% at 2 h. When the molar ratio of H2/C3H8 is 0.15:1, the catalytic performance of the PtSn-Mg(3Zn)AlO catalyst reaches an optimal level, the initial conversion of propane is 55.2%, and the propane conversion rate is optimized after 2 h of reaction remained at a high level (up to 39.6%); the average conversion of propane at 2 h was 45.2% (Fig. 6c). As the molar ratio of H2/C3H8 continues to increase, 6

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Fig. 8. Effect of the WHSV on the catalytic performances of the PtSn-Mg (3Zn)AlO catalysts in the propane dehydrogenation reaction. Reaction conditions: T = 550 °C, atmospheric pressure, H2:C3H8:N2 = 0.15:1:4 and WHSV = 6.3–18.9 h−1.

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3.4. Effect of the WHSV Fig. 8 displays the effect of the WHSV on the performance of the PtSn-Mg(3Zn)AlO catalyst in propane dehydrogenation. Surely, all of the reactions exhibited relatively high propene selectivity values during the entire dehydrogenation process. The initial propane conversions were 55.3%, 56.5% and 55.2% at a WHSV of 6.3 h−1, 12.6 h−1 and 18.9 h−1, respectively, and the average propane conversions after 2 h were 53.1%, 51.1%, 45.2%, respectively. The average conversion of propane decreased with increasing WHSV, which may be explained by the shortening of the contact time of the reactants on the catalytic bed with the active sites on the catalyst as the WHSV was increased and the reaction did not proceed adequately, thereby resulting in a decrease in the reactant conversion.

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Time on stream (min) Fig. 9. Effect of the Pt loading on the catalytic performances of the x-PtSn-Mg (3-Zn)AlO catalysts in the propane dehydrogenation reaction: (a) propane conversion; (b) propene selectivity. Reaction conditions: T = 550 °C, atmospheric pressure, H2:C3H8:N2 = 0.15:1:4 and WHSV = 18.9 h−1.

3.5. Effect of the Pt loading The noble metal Pt still has significant limitations in terms of its availability, although Pt-based catalysts have shown high activity levels for alkane dehydrogenation reactions. Thus, it is very attractive to reduce the Pt content of the catalyst while maintaining a considerable catalytic performance. Fig. 9 shows the propane conversion and the propene selectivity of the x-PtSn/Mg(3Zn)AlO catalyst in propane dehydrogenation. When Pt had not been loaded on the supports, propane was only slightly pyrolysed at a given temperature to produce very small amounts of methane, ethylene, propene and other products, thus resulting in a low propene selectivity. For the 0.1PtSn/Mg(3Zn)AlO catalysts, after being on stream for 2 h, the propane conversion decreased from 22.0% to 11.1%, along with a relatively high propene selectivity towards propene that was higher than 97% at the beginning of the reaction and higher than 93% after being on stream for 2 h. With the increase in Pt loading, the initial propane conversion was markedly improved for the 0.3PtSn/Mg(3Zn)AlO and 0.5PtSn/Mg(3Zn)AlO samples, attaining values of 48.6% and 55.2%, respectively, along with an outstanding selectivity towards propene that was higher than 99% during the entire dehydrogenation process for both cases. After being on stream for 2 h, the propane conversions decreased from 48.6 to 36.0% and from 55.2% to 39.6%, corresponding to deactivation parameters of 25.9% and 28.2% for the 0.3PtSn/Mg(3Zn)AlO and 0.5PtSn/ Mg(3Zn)AlO catalysts, respectively.

values for the propane dehydrogenation reaction are shown in Fig. 10. After being on stream for 14.2 h, the propane conversions of the PtSnMg(3Zn)AlO and PtSn-MgAlO catalysts decreased from 55.3% to 40.6% and from 56.5% to 23.1%, respectively, which corresponded to deactivation parameters of 26.5% and 59.1%, respectively (Fig. 10a). The addition of electropositive metals, such as Zn, to the Pt catalysts for the dehydrogenation of propane led to an increased activity, improved product selectivity and catalyst lifetime [45]. The propene selectivity for the PtSn-Mg(3Zn)AlO catalyst remained stable above 99% throughout the process, while the selectivity gradually decreased for the PtSn-Mg(3Zn)AlO catalyst (Fig. 10b). This result was mainly due to the electron transfer from Zn to Pt, which increased the electron density of Pt, resulting in a decrease in the activation energy of propene desorption, thus reducing the occurrence of side reactions and increasing the selectivity to propene [35].

3.7. Reaction-regeneration cycles of the PtSn-Mg(3Zn)AlO catalyst The deactivation of the Pt-based catalysts in the alkane dehydrogenation reaction is primarily caused by two main processes [1]. Carbon deposition on the catalyst surface is the primary reason. Moreover, the aggregation or sintering of Pt nanoparticles initiated by the regeneration process or the high temperature of the dehydrogenation reaction causes the concomitant loss of active sites, which also results in catalyst deactivation. To study the cause of the PtSn-Mg

3.6. Catalyst stability test Stability tests were carried out for the PtSn-Mg(3Zn)AlO and PtSnMgAlO catalysts, and the propane conversion and propene selectivity 7

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treatments, the conversion of propane was notably decreased. During the 5 cycles, the average conversions of propane were 45.2%, 45.6%, 45.0%, 42.6% and 38.5%. The activities of the used catalysts were almost recovered by treating the catalysts with oxygen at 550 °C for 3 h during the first three cycles, but the decrease from the fourth cycle on was visible. This result means that the reasons for the loss of catalytic activity were not only the carbon depositions on the catalyst surface but also the sintering of metal particles. The coke can be fully removed by oxidation, but the metal particle sintering process is irreversible. The metal dispersion values after catalyst regeneration cycles were shown in Table A.1. On the whole, the metal dispersion decreases as the number of reaction cycles increases. However, it can be seen that the metal dispersion and the catalyst activity are not completely equivalent. In fact, the catalyst activity is not only related to metal dispersion, but also related to many other factors, such as metal particle size, support interface properties, surface structure, mechanical strength, etc.

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In summary, a series of PtSn catalysts supported on Zn-modified Mg (Al)O supports (synthesized by the co-precipitation method) were synthesized by the anion-exchange method for the propane dehydrogenation reaction. The influences of the H2/C3H6 ratio, Zn loading, reaction temperature, WHSV and Pt loading were investigated. The results demonstrated that Pt provided the active sites and that Zn was a promoter as well as Sn. PtSn-Mg(3Zn)AlO exhibited a better performance of propane dehydrogenation, where the average conversion of propane was 45.2% at 2 h and the average selectivity of propene was higher than 99%. Moreover, the deactivation parameter of PtSn-Mg (3Zn)AlO was only 26.5% after 14.2 h. The activities of the used catalysts were almost recovered by reaction-regeneration cycles with the burning-off of coke. The mean diameters of the metal particles of the PtSn-MgAlO and PtSn-Mg(3Zn)AlO catalysts were 1.91 ± 0.23 nm and 1.42 ± 0.36 nm, respectively, suggesting that the smaller metal particles were evenly distributed on the Mg(Al)O supports when doped with a certain amount of Zn. As indicated by CO pulse chemisorption, the addition of Zn can change the structures of the catalysts to a certain extent, thus increasing the metal dispersion. The results of H2-TPD demonstrated that the presence of Zn changed the interfacial character between the metal particles and supports, thus indicating weaker interactions between the hydrogen and the catalysts, which was beneficial to the dehydrogenation reaction. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Fig. 10. Stability tests of the PtSn-Mg(3Zn)AlO and PtSn-MgAlO catalysts in the propane dehydrogenation reaction: (a) propane conversion; (b) propene selectivity. Reaction conditions: T = 550 °C, atmospheric pressure, H2:C3H8:N2 = 0.15:1:4 and WHSV = 6.3 h−1.

Acknowledgements This work was supported by the CAS Pioneer Hundred Talents Programme and Natural Science Foundation of China (No. 51776206).

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Appendix A. Supplementary data

Catalyst recycle runs

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuproc.2019.106222.

Fig. 11. Reaction-regeneration cycles during propane dehydrogenation over the PtSn-Mg(3Zn)AlO and PtSn-MgAlO catalysts. Reaction conditions: and T = 550 °C, atmospheric pressure, H2:C3H8:N2 = 0.15:1:4 WHSV = 18.9 h−1.

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