ZnAl2O4 Catalysts: Effect of

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Kinetics, Catalysis, and Reaction Engineering

Isobutane dehydrogenation over InPtSn/ ZnAlO catalysts: Effect of indium promoter 2

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Jianfeng Liu, Wei Zhou, Dongyu Jiang, Wenhai Wu, Changxi Miao, Yue Wang, and Xinbin Ma Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01728 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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Industrial & Engineering Chemistry Research

Isobutane dehydrogenation over InPtSn/ZnAl2O4 catalysts: Effect of indium promoter

Jianfeng Liu,†,‡ Wei Zhou,† Dongyu Jiang,*,‡ Wenhai Wu,‡ Changxi Miao,‡ Yue Wang,† and Xinbin Ma*,†

†

Key Laboratory for Green Chemical Technology of Ministry of Education,

Collaborative Innovation Centre of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ‡

State Key Laboratory of Green Chemical Engineering and Industrial Catalysis,

Sinopec Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China

*

Corresponding author: Prof. Xinbin Ma, Dr. Dongyu Jiang

E-mail: [email protected]; [email protected] Fax: +86-22-87401818 Tel: +86-22-27409248

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Dehydrogenation of saturated isobutane provides an alternative process to produce isobutene from nonpetroleum resources. InPtSn/ZnAl2O4 catalysts with various In contents were prepared by sequential impregnation for the isobutane dehydrogenation. The catalysts were investigated by several characterizations such as XRD, N2 adsorption, H2 chemisorption, NH3-TPD, Py-FTIR, XPS, TG and TPO. We reveal that with the suitable addition of In component in the PtSn/ZnAl2O4 catalysts, the Pt dispersion increases and the surface acidity decreases. The presence of In improves the stability of the reaction greatly, because it is beneficial to stabilize the oxidation states of SnOx. The excessive addition of In will lead to decrease of the Pt dispersion and increase of the surface acidity, which cause the decline in the catalytic activity. The addition of 0.4 wt.% In on PtSn/ZnAl2O4 catalyst shows the optimized catalytic performance. The isobutane conversion decreases from 54.7% to 40.5% within 8 h and the isobutene selectivity is above 95% even after 20 reaction-regeneration cycles. Keywords: Isobutane dehydrogenation; Indium; Isobutene; InPtSn/ZnAl2O4

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1. Introduction Isobutene is a very important raw material to produce polybutene, methyl tert-butyl ether and ethyl tert-butyl ether, etc.,1 but the traditional methods for isobutene production such as steam cracking and fluid catalytic cracking units can not fulfill the fast growing requests now. Alternatively, isobutane dehydrogenation provides an economical route to obtain isobutene from low-cost saturated and poorly reactive hydrocarbons.2 However, the dehydrogenation process requires low pressure and high temperature to achieve high yield of isobutene, as it is highly endothermic and equilibrium-limited. The tough reaction conditions also result in more by-products and coke formation.3 Therefore, it is important to develop novel dehydrogenation catalysts with high performance for isobutane. Alumina-supported bimetallic Pt-Sn cataltsts were widely researched and applied in the dehydrogenation reaction of isobutane.4-6 It is noteworthy that the use of MgAl2O4

7-9

and ZnAl2O4 10-11 supports for dehydrogenation has gained more and

more attention in recent years because of its low acidity and high thermal stability. These features can reduce the feedstock cracking and minimize coke formation on catalyst, which is benefit to the selectivity.7,11 Besides, in order to decrease coking rates, the co-feeding of H2 was usually applied.

12-19

Compared with dehydrogenation

in H2, the use of steam as the reaction medium has more advantages. The decrease of feedstock partial pressure is thermodynamically beneficial to the product formation. And the water can promote product desorption and improve the selectivity. In 3



addition, steam is also a heat carrier to prevent the reaction temperature from dropping rapidly.

20

Nevertheless, the hydrothermal stability of catalyst is very

important in this system. Some studies have been carried out to investigate the effect of many metal promoters for the Pt-Sn/Al2O3 catalysts such as alkaline 12, alkali-earth 13

and rare-earth metal ions

14

. Indium promoter attracts much attention for the

dehydrogenation catalysts. Guo’s group found that the PtSn/1.5In-Al2O3 catalyst showed good catalytic performance in the dehydrogenation of propane15 and it could be promoted by calcium (Ca)16 and yttrium (Y).

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The effect of Sn content on

InPtSn/MgAl2O4 catalyst was studied in n-butane dehydrogenation by Bocanegra et al.18 They reported that Ga can improve the n-butane conversion, the selectivity to butene and the stability. The improvements are attributed to geometric and electronic modifications of the metallic Pt.19 In the present study, ZnAl2O4 was used as support for Pt-Sn catalyst，which has a typical spinel structure and possesses high hydrothermal stability. A series of ZnAl2O4 supported Pt-Sn catalysts were prepared with different loadings of indium for isobutane dehydrogenation under steam atmosphere. The modifications of catalyst surface properties and the effect on Pt dispersion after doped by indium will be investigated systematically. The catalysts were characterized by X-ray diffraction (XRD), N2 adsorption, H2 chemisorption, X-ray photoelectron spectroscopy (XPS), NH3-TPD, and temperature programmed oxidation (TPO). The purpose of this work is to understand the role of additional promoters and optimize the composition to obtain catalysts suitable for industrial isobutane dehydrogenation process. 4


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2. Experimental 2.1 Catalyst preparation Sequential impregnation method was used for the preparation of InPtSn/ZnAl2O4 catalysts. ZnAl2O4 support was pretreated at 650 °C for 8 h. Then, it was impregnated in aqueous solution of In(NO3)3 at 30°C. After dried above 120 °C, these samples were co-impregnated with 0.0106 g·ml-1 H2PtCl6 and 0.019 g·ml-1 SnCl2 solutions, followed by another drying step. Next, the samples were calcined at 650 °C for 8 h. Catalysts synthesized were named as xInPtSn/ZA, where mass fraction of In was expressed as x. In our study, the amount of Pt and Sn were controlled at 0.4 wt.% and 1.0 wt.%, respectively. 2.2 Catalyst characterization BET surface area of each catalyst was characterized by a Micromeritics ASAP 2020 equipment (multi-point method).The Barrett-Joyner-Halenda (BJH) method (multilayer films by Halsey equation) was used to calculate the pore structure data. Before adsorption, the samples must be evacuated at 300 °C for 4 h. The X-ray diffraction (XRD) measurements were performed on a Bruker D8 Advanced Diffractometer equipped with Cu Kα radiation (λ=0.154 nm) at 40 kV and 40 mA within a 2θ range of 5°-80°. The Pt dispersion was characterized by H2-pulse chemisorption using an Autochem 2920 in pure argon flow of 50 ml/min. The sample was reduced in a H2 flow at 550 °C for 2 h, while the physical adsorbed hydrogen on the catalyst surface was removed by Ar gas at 580 °C for 1 h. In the end, the sample was cooled to 45 °C. 5



H2 pulse was introduced after the baseline became stable. An adsorption stoichiometry of Pt/H=1 was assumed for the calculation. Pt dispersion was calculated as dividing the number of exposed surface Pt atoms, which was determined by H2 chemisorption, by the total atoms of Pt on the catalyst. After reduction, the surface electronic states of the catalyst components were studied by XPS (Perkin–Elmer PHI 5000C ESCA, Al Kα radiation). All the samples need to be dried in pure Ar gas before the test. Binding energies were referenced to the C1s peak (284.8 eV). The acidity of the prepared samples was evaluated by NH3-TPD. Catalyst sample (150mg) was calcined at 600 °C for 1 h, and then the temperature dropped to 25 °C in flowing He. Next, the samples were treated by NH3 at 100 °C and purged under dry Ar of 30 mL/min at 100 °C for 1.5 h before measurement. Subsequently, the sample was heated up to 600 °C at a rate of 10 °C /min. The NH3-desorption signal was detected with the TCD. Py-FTIR (Pyridine-adsorbed Fourier transform infrared spectroscopy; Nicolet 380 FTIR Spectrometer) was used for determining the amount of Brønsted acid sites and Lewis acid sites of the catalysts. A regular wafer (R =1.3 cm) was prepared using 20 mg of sample powder, loaded in an infrared cell which was heated up to 400 °C and kept for 2 h under vacuum. Then it was cooled to 200 °C to record the background spectra. The sample was treated by pyridine for 30 min at 30 °C. The pyridine adsorbed sample was outgassed at 200 °C and characterized by FTIR. Coke on the used catalyst was analyzed by temperature programmed oxidation 6


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(TPO) and thermogravimetric (TG) test. About 50 mg of sample was placed in a quartz reactor for the TPO experiment. The samples were placed and heated to 700 °C (10 °C/min) in the mixture of O2/He (40 mL/min). TG and TPO analysis was performed by a TA-STDQ600 (TA Instruments Inc., USA), in air flow of 30 mL/min from 25 °C to 700 °C at a rate of 20 °C/min, where 20mg catalyst was used. 2.3 Catalytic reaction test The catalyst performance for isobutane dehydrogenation was tested in an i.d.8mm quartz tubular reactor. 2 ml of catalyst was loaded into the reactor, and then it was reduced by H2 at 550 °C for 2h. Reaction conditions were 550 °C for reaction temperature, atmospheric pressure, mass space velocity was 4.0 h-1 and H2O/iC4 was 2.0. The catalysts were regenerated with a flow of pure air at 550 °C for 2h and then re-reduced in pure H2 at 550 °C for 2 h. The products were analyzed by an Agilent GC-7890 (activated alumina packed column; FID). The conversion of isobutane (Xisobutane) and the selectivity to isobutene (Sisobutene) was calculated as equation (1) and (2).

X isobutane =

S isobutene =

moles of isobutane reacted ×100% moles of isobutane fed total C moles of product isobutene

(1) ×100%

(2)

∑ total C moles of isobutane reacted

3. Results and discussion 3.1. Structural and textural properties Figure 1 shows the XRD patterns of different samples. As can be seen, all samples show typical peaks of ZnAl2O4 at 31.2°, 36.8°, 44.8°, 49.1°, 55.6°, 59.3° and 7



65.2° which confirms that the original structure of ZnAl2O4 is preserved after the addition of platinum, tin, indium species. Moreover, the peaks associated with these metal species cannot be found. This means that the metal species are dispersed well or poorly crystallized on the ZnAl2O4 support, which is similar to other supported catalysts.21 The textural properties of catalysts were characterized (see Table S1 in the Supporting Information). It can be seen that SBET, VP and Dpore of PtSn/ZA catalysts are 84.9 m2 /g, 0.14 cm3/g and 6.9 nm respectively. And these properties keep unchanged after In modification. Similar phenomenon has been observed by Zhang et al. 22 In conclusion, the In addition has little impact on the structural and textural properties of PtSn/ZA.

Figure 1 XRD patterns of xInPtSn/ ZnAl2O4 catalysts.

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3.2 H2 chemisorption The Pt dispersion on catalysts was measured by H2 chemisorption. The results are illustrated in Table 1. The lowest Pt dispersion of PtSn/ZA was observed. It is noteworthy that both Pt dispersion and Pt surface area of PtSn/ZA increase when the loading of In was 0.2wt.% and 0.4wt.%, while the opposite trend is found when the In loading is excessive (0.6 wt.%, 0.8 wt.%, and 1.0 wt.%). A conclusion can be drawn that proper loading of base-metal oxide additives can inhibit the agglomeration of dispersed Pt,23 thus increase its dispersion and surface area. However, when the loading of In on catalyst surface is excessive, the surface of Pt particles may be covered with In and the exposure of Pt is reduced.14 This implies that the amount of In has a significant effect on the surface properties of Pt particles, and further affects the isobutane dehydrogenation activity. The average Pt particle size of the PtSn/ZA catalysts is under 3 nm, which indicates that the preparation method is suitable to improve Pt efficiency.

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Table 1 H2 chemisorption results for xInPtSn/ ZnAl2O4 catalysts. Pt dispersion

Pt surface

Average Pt particle

（ %）

area(m2/g)

size (nm)

PtSn/ZA

40.2

99

2.8

0.2InPtSn/ZA

42.7

105

2.7

0.4InPtSn/ZA

47.3

117

2.4

0.6InPtSn/ZA

45.4

112

2.5

0.8InPtSn/ZA

38.9

96

2.9

1.0InPtSn/ZA

37.5

94

3.0

Samples

3.3 Acidity analysis The acidic properties of catalyst surface is characterized by NH3-TPD, Figure S1 shows four NH3-TPD curves of different xInPtSn/ZA catalysts. It can be found that In has a significant impact on the surface acidity of PtSn/ZA samples. A semi-quantitative analysis of the acidity strength distribution is acquired by deconvoluting the peaks using the normal distribution functions. The results listed in Table 2 including total acidity and acidity strength distribution are calculated by desorption peak areas. Three peaks appear on the curves of all the samples. The peak I (190 °C) is assigned to weak acid sites. The peak II (270 °C) is attributed to neutral acid sites. The peak III (350 °C) is a characteristic peak of strong acid sites. When moderate amount of In (0.4 or 0.6 wt. %) is added to the catalyst of PtSn/ZA, the temperature of ammonia desorption shifts toward the lower range. It implies that the acid strength becomes weaker. The total acid sites also decrease obviously, deduced from the peak area in Table 2. It is suggested that a proper amount of In can reduce the catalyst acidity. With an increasing In content in the catalyst (1.0 wt.%), the total acid sites increase, especially the amount of strong acid sites, which may be 10


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introduced by the excess In promoter. Py-FTIR spectra on different samples were acquired to identify the Brønsted acid sites and Lewis acid sites of the catalysts. The results are shown in Figure 2. The curves of all the catalysts have two peaks at 1450 cm-1 and 1610 cm-1 respectively, which are ascribed to Lewis acid sites.24,25 There is no obvious peak at 1540 cm-1 which is assigned to the Brønsted acid sites. From NH3-TPD profiles, it can be concluded that only Lewis acid sites exit on the catalyst surface. Table 2 Results of NH3-TPD of xInPtSn/ ZnAl2O4 catalysts. Peak area fraction (%)

Tm (°C) Samples

Total area (a.u.)

PeakI

PeakII

PeakIII

PeakI

PeakII

PeakIII

PtSn/ZA

197

288

434

28

60

12

5.72

0.4InPtSn/ZA

185

275

425

30

60

10

5.63

0.6InPtSn/ZA

191

269

394

22

57

21

5.21

1.0InPtSn/ZA

200

281

428

23

59

18

6.53

Figure 2 FTIR spectra of pyridine adsorbed on xInPtSn/ ZnAl2O4 catalysts

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3.4 XPS analysis

Figure 3 Sn3d5/2 XPS spectra of xInPtSn/ ZnAl2O4 catalysts. XPS spectra are applied for characterizing the chemical state of the components on the reduced catalyst surface. The detailed data of Sn3d5/2 of four representative xInPtSn/ZA catalysts are illustrated in Figure 3 and Table S2 (see Supporting Information), respectively. The Sn3d5/2 XPS spectra in Figure 3 can be deconvoluted into three peaks at around 485.5 eV, 486.5 eV and 487.4 eV. The peak of low binding energy is attributed to metallic tin (Sn0), while the other two peaks correspond to different types of SnOx. Nevertheless, it is hard to discriminate between Sn2+ and Sn4+ because of their similar binding energies.26 The percentages of metallic tin for PtSn/ZA, 0.4InPtSn/ZA, 0.6InPtSn/ZA, and 1.0InPtSn/ZA, catalysts are 29%, 18%, 12


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23% and 28%, respectively (as shown in Table S2). This suggests that the amount of Sn0 decreases with suitable loading of In. Whereas an excessive In loading (1.0 wt.%) increases the amount of Sn0. These results are well consistent with the study by Zhang et al .27 3.5. Catalytic activity. The catalytic performances of different catalysts are exhibited in Figure 4. The initial and final isobutane conversion of PtSn/ZA is 50.6% and 31.0% respectively (Figure 4a). When 0.2-0.6 wt.% In is loaded to the PtSn/ZA sample, the activity of the catalyst increase obviously. With 0.4 wt.% In loading, the conversion of isobutane achieves the highest value. Because the dehydrogenation reaction of paraffin only occurs on the metallic surface.28, 29 The cases could be ascribed to a better particle distribution which leads to more Pt atoms exposure. Moreover, it can be seen that the stability is improved over In modified catalysts. More Sn species exist as SnOx state from XPS results. SnOx species can transfer deposited coke from Pt surface to support, consequently

increasing the catalytic stability.30 However, the catalytic activity

decreases with excessive introduction of In, which may be ascribed to a worse Pt particle dispersion. According to the Figure 4b, with the reaction time prolonged, the selectivity for PtSn/ZA increases from 93.7% to 95.4% after 8 h. Isobutene selectivity enhanced obviously upon the proper addition of In (0.2-0.8 wt.%). Because the addition of In (0.4 or 0.6 wt.%) can weaken the acid strength and reduce the acid sties of the support, which leads to less byproducts. By contract, when the In content is excessive (1.0 13



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wt.%), the surface acidity increases . Thus more byproduct generates, resulting in the decrease of isobutene selectivity. (a)

(b)

Figure 4 Isobutane conversion(a) andisobutene selectivity (b) versus reaction time

3.6. Coke Analysis The catalyst deactivation during paraffin dehydrogenation process is mainly caused by coke deposition. Coke deposited on the catalysts was measured by TPO. The curves of three deactivated samples are illustrated in Figure 5 and the corresponding amount of coke is shown in Table S3. There are three successive peaks on TPO, which are ascribed to different carbon deposits. Generally, the first peak at about 410 °C indicates that the coke shields the Pt metal surface, while the peak at higher temperature relates to the coke locating on the support surface.31 The peak area can reflect the amount of coke generated during the reaction. It is noted that increasing In loading (0.4 wt.%) prevents the coke formation on catalyst surface. Nevertheless, the negative effect is observed as the loading of In is excessive (1.0 wt.%). Generally, the coke deposition process includes three steps: paraffin chains dehydrogenate or cyclize; n-alkane oligomer and followed by Diels-Alder reactions.32 14


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Coke deposition is usually considered initiating from olefins. The support acidity can result in more olefins cracking/isomerization side-reactions, which increases the carbon formation. According to this mechanism, it is important to modify Pt particle properties and reduce support acidity for less coke deposition.

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As a result, the

loading of In can increase the Pt dispersion, which leads to the modification of the Pt particles. Moreover, the suitable presence of In on catalyst surface can weaken the support acidity, which leads to decrease of coke and improvement of the reaction stability.

Figure 5 TPO profiles of used catalysts

3.7. Stability test The proper metal In loading on PtSn/ZA catalysts is beneficial to catalytic activity and isobutene selectivity, and improves the reaction stability at the same time. The 0.4InPtSn/ZA exhibits the best isobutane conversion (initial: 54.3%; final: 38.5%) and a higher isobutene selectivity (initial: 93.9%; final: 96.8%). Stability tests of the PtSn/ZA and 0.4InPtSn/ZA catalyst were performed. As shown in Figure 6, the initial 15



conversion decreases from 50.5% to 48.5% after 20 cycles for the PtSn/ZA. By contrast, the catalytic activity did not decrease obviously in 20 cycles for the 0.4InPtSn/ZA. The isobutene selectivity is more than 95%, while the initial and final isobutane conversions reach 54.7% and 40.5% respectively. As a result, InPtSn/ZA catalyst exhibits excellent regeneration stability of dehydrogenation in this study.

Figure 6 Stability test of PtSn/ZA (A) and 0.4InPtSn/ZA (B) catalyst in the isobutane dehydrogenation.

4. Conclusions Different concentrations of indium on catalyst surface have great effect on the activity of InPtSn/ZnAl2O4 in isobutane dehydrogenation to isobutene. With the 16


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suitable addition of In, Pt dispersion increases and the surfaces acidity decreases. And the oxidation states of Sn species can be stabilized by the existence of In promoter. Consequently, the performance is improved. Nevertheless, when the addition of In is overloaded, the decrease of Pt dispersion and the strength of surface acidity are observed. The In species have no significant impact on the structural and textural properties of xInPtSn/ZnAl2O4 catalysts. In our work, the best reaction activity and stability are achieved over the catalyst with 0.4 wt.% of In. The initial and final isobutane conversions are 54.7% and 40.5% respectively, while isobutene selectivity keeps over 95% even after 20 regenerations.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21325626). Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

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ZnAl2O4 Catalysts: Effect of

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