The roles of ZnFe2O4 and α-Fe2O3 in the biphasic catalyst for the oxidative dehydrogenation of n-butene

Journal of Catalysis 381 (2020) 70–77

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The roles of ZnFe2O4 and a-Fe2O3 in the biphasic catalyst for the oxidative dehydrogenation of n-butene Benqun Yang a, Li Liu a, Guojun Zou a,⇑, Xu Luo a, Hailin Zhu a,b, Shan Xu a,⇑ a b

State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e

i n f o

Article history: Received 5 June 2019 Revised 9 October 2019 Accepted 10 October 2019

Keywords: Oxidative dehydrogenation n-Butene Biphasic catalyst Apparent pre-exponential factor Interface reaction

a b s t r a c t Iron-based catalysts were one of the most effective catalysts in the oxidative dehydrogenation of nbutene, however, the roles of spinel and a-Fe2O3 in the biphasic catalyst have remained controversial. Herein, we found that biphasic catalyst exhibited much superior catalytic performance compared to each individual component. According to the temperature programmed desorption, XPS and kinetics studies results, it was speculated that ZnFe2O4 was primarily responsible for activating n-butene and a-Fe2O3 was used to activate O2. Moreover, the key factor for the catalytic activity of the biphasic catalyst were investigated and kinetic studies showed that reaction rates were well correlated with the apparent pre-exponential factor. Therefore, the efficient contact between ZnFe2O4 and a-Fe2O3 may lead to the improvement of the activity. In addition, the reaction mechanism over a-Fe2O3/ZnFe2O4 was proposed to be the interface reaction based on the catalytic evaluation results of substituting ZnFe2O4 by the redox-inactive ZnAl2O4. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction 1,3-Butadiene (BD) is an important raw material and one of the most important monomers for production of polymer composite materials, various synthetic rubber, synthetic resins and etc [1,2]. With the rapid development of automobile industry and more tire production, the global demand for BD has increased significantly in the last decades. Traditionally, BD is mainly obtained to be extracted from a mixed C4 hydrocarbons byproduct of ethylene cracking reactor. However, due to the lightweight development of the ethylene industry, the amount of BD obtained through extraction is decreasing [3–5]. The oxidative dehydrogenation (ODH) of n-butene to BD can not only break the dependence on oil resources but also use C4 fraction efficiently. Hence, the onpurpose production of BD by ODH of n-butene has regained increasing attention. Iron-based catalysts were considered as one of the most effective catalysts in the ODH of n-butene. Simple iron oxide, in the form of a-Fe2O3 (hematite), has been used as a major component in this reaction [6]. It was later found that c-Fe2O3 (maghemite) with spine1 structure was more selective than a-Fe2O3 of corundum structure [7–10]. However, c-Fe2O3 has poor thermal stabil-

⇑ Corresponding authors. E-mail address: [email protected] (S. Xu). https://doi.org/10.1016/j.jcat.2019.10.016 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

ity, and it converted to the a-form at about 350 °C. It was also reported that the addition of ZnO, MgO, CoO and CuO to Fe2O3 to form the spine1 structure (A+2B+3 2 O4) increased the activity and selectivity at the high temperature of 375–450 °C [11–18]. Following studies found that catalyst containing two phases (one being spinel and the other a-Fe2O3), seemed to exhibit superior activity [19,20]. Although many attempts have been made to find the major factors determining the catalytic performance, the roles of spinel and a-Fe2O3 in the biphasic catalyst have remained controversial and the reaction mechanism has not been clearly elucidated. In the typical Mars-van Krevelen mechanism, spinel was thought to be the active site and lattice oxygen from the spinel was consumed to form the products. Subsequently, lattice oxygen was regenerated by migration from a-Fe2O3 [20,21]. Hightower et al. have reported that spinel can be deactivated irreversibly in parallel with the disappearance of a-Fe2O3 [19]. Therefore, they believed that the presence of a-Fe2O3 seemed to able protect the spinel from deactivation. However, Oh et al. suggested that pure ZnFe2O4 exhibited superior catalytic activity, the presence of aFe2O3 could reduce both the conversion of n-butene and the BD selectivity [16]. As the roles of spinel and a-Fe2O3 in the biphasic catalyst and the reaction mechanism were not explained satisfactorily, further insight into the ODH of n-butene is still an interesting topic.

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In this paper, the catalytic performance of the individual and biphasic catalysts in the ODH of n-butene were investigated. Temperature programmed desorption, XPS and kinetics studies were employed to illustrate the roles of ZnFe2O4 and a-Fe2O3 in the biphasic catalyst. Moreover, the crucial factors for the superior catalytic activity of the biphasic catalyst were investigated in detail. In order to further explore the reaction mechanism, ZnFe2O4 was substituted by the redox-inactive ZnAl2O4 and the catalytic performance of the individual and biphasic catalysts were evaluated. 2. Experimental 2.1. Catalyst preparation Chemicals used in this study were AR grade and utilized without further purification. 2.1.1. Preparation of ZnFe2O4 ZnFe2O4 was prepared by sol-gel method. Zn(NO3)2
6H2O, Fe (NO3)3
9H2O and citric acid measured in a 1:2:3 M ratio were dissolved in deionized water. Then, excess water was evaporated off at 80 °C until the homogeneous solution became viscous and gellike. The final gel was first calcined in air at 250 °C for 1 h. Subsequently, the sample was calcined up to 700 °C with the heating rates of 2 °C min1 and maintained for 6 h. 2.1.2. Preparation of a-Fe2O3 a-Fe2O3 was prepared by precipitation method. A certain amount of Fe(NO3)3
9H2O were first dissolved in deionized water. Then, an ammonia solution (15 vol%) was added dropwise to the solution to reach a pH value of ca. 8.5. After the solution was stirred and aged at 80 °C for 2 h, the resulting precipitate was filtrated and washed several times with distilled water. Subsequently, the resulted brown powder was dried at 80 °C overnight and calcined at 600 °C for 5 h in static air. 2.1.3. Preparation of a-Fe2O3(5–40 wt%)/ZnFe2O4-BM a-Fe2O3/ZnFe2O4-BM was prepared by high-energy mechanical milling method. a-Fe2O3 (5–40 wt%) and ZnFe2O4 powder mixtures were first blended in a Turbula T2C mixer for 20 min, and then the mixture was mechanically milled using ZrC balls (carried on Pulverisette 7, FRITSCH) for 8.5 h. The rotational speed was 300 rpm min1, at a ball-to-powder weight ratio of 10:1. Finally, the sample was calcined at 450 °C for 3 h in static air to exclude any reduction of a-Fe2O3 during the mechanical milling and denoted as a-Fe2O3(5–40 wt%)/ZnFe2O4-BM. 2.1.4. Preparation of a-Fe2O3(2 wt%)/ZnFe2O4-IM a-Fe2O3/ZnFe2O4-IM was prepared by the incipient wetness impregnation method. A certain amount of Fe(NO3)3
9H2O was dissolved in deionized water (0.7 mL). After adding the ZnFe2O4 (2.0 g), the mixture was sonicated for 20 min at room temperature and then dried at 80 °C overnight. Finally, the acquired catalyst precursor was calcined at 600 °C for 5 h in static air and denoted as a-Fe2O3(2 wt%)/ZnFe2O4-IM. 2.1.5. Preparation of the a-Fe2O3(5 wt%)/ZnFe2O4-HN a-Fe2O3/ZnFe2O4-HN was prepared by two steps. In the first step, 0.485 g Fe(NO3)3
9H2O, 0.096 g PVP (K30, Mr = 10000) were dissolved in 80 mL deionized water. Then, this mixture was heated to reflux for 8 h under stirring conditions to get the Fe2O3
nH2O colloid. In the second step, 2.251 g Zn(NO3)2
6H2O, 6.113 g Fe (NO3)3
9H2O and 5.008 g citric acid were dissolved in 75 mL deionized water. The ammonia solution (15 vol%) was used to adjust the pH to ca. 3. Finally, the Fe2O3
nH2O colloid solution prepared in the

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first step was added dropwise to the above Zn-Fe-citric acid solution under stirring. The following process was the same as that for ZnFe2O4. The prepared hybrid nanostructure (HN) catalysts was denoted as a-Fe2O3(5 wt%)/ZnFe2O4-HN. ZnAl2O4 and aFe2O3 (5 wt%)/ZnAl2O4-HN were prepared with the similar procedure. 2.2. Catalyst characterization XRD measurements for the structure determination were carried out on a ShimadzuXD-3D X-ray diffractometer with monochromatized Cu Ka radiation. Diffraction patterns were collected over a 2h range of 10 to 90° for all the samples. Raman spectra were measured with a 532 nm edge by using a LabRAM HR Evolution (HORIBAJobin Yvon S.A.S.). Nitrogen adsorptiondesorption isotherms were measured with a Micromeritics ASAP2020 analyzer. Before measurements, the samples were degassed in vacuum at 150 °C for at least 6 h. The specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method. Surface analysis of the catalysts was performed by X-ray photoelectron spectroscopy (XPS) on VG ESCALAB210 with a mono-chromatic X-ray source of Mg Ka (hm = 1253.6 eV), calibrated internally by carbon deposit C (1s) binding energy (BE) at 284.6 eV. Temperature programmed reduction (H2-TPR) was performed on a chemisorption analyzer equipped with a thermal conductivity detector (TCD). Sample was first pretreated at 600 °C in air flow (30 mL min1) for 1 h and then cooled to room temperature. Subsequently, the flowing 10 vol%H2/Ar (30 mL min1) was introduced with a ramp rate of 10 °C min1 from room temperature to 900 °C. Temperature programmed desorption of oxygen (O2-TPD) was studied in the same system. 100 mg sample was loaded in a quartz reactor and heated under a O2 flow (30 mL min1) up to 600 °C and remained at this temperature for 1 h. After cooling to room temperature, the flowing gas was then switched to He (30 mL min1) for 1 h to eliminate the physically adsorbed oxygen. The sample was then heated to 900 °C with a constant rate of 10 °C min1 from room temperature under He flow (30 mL min1). Oxygen in the reactor outlet was monitored by a thermal conductivity detector. In the temperature programmed re-oxidation (TPRO), 100 mg sample was pretreated at 340 °C for 2 h in flowing n-butene (30 mL min1) and then cooled to room temperature in He (30 mL min1). Subsequent temperature programmed oxidation was performed in flowing 2 vol% O2/He (30 mL min1) from room temperature to 800 °C at 10 °C min1. In the temperature programmed desorption of n-butene (nbutene-TPD), 100 mg sample was loaded in a quartz reactor and heated under a O2 flow (30 mL min1) up to 600 °C and remained at this temperature for 1 h. Then the catalyst was cooled in O2 to 300 °C and the flow was switched from O2 to He (30 mL min1). After 0.5 h at 300 °C, the sample was cooled to room temperature in He. Subsequently, the sample was exposed to a stream of nbutene (30 mL min1) at room temperature for 1 h and flushed again with He (30 mL min1) for 1 h. Next, the desorption profile was recorded at a heating rate of 10 °C min1 from room temperature to 500 °C. 2.3. ODH of n-butene 2.3.1. Catalytic evaluation Catalytic activity measurements were carried out in a fixed-bed reactor in the presence of air and steam. The catalyst (0.6 mL, 35– 60 mesh) and quartz sand (1.4 mL, 35–60 mesh) were loaded in the reactor. Prior to reaction, each catalyst was pretreated at 400 °C for 1 h with an air stream (12.38 mL min1). Water was sufficiently vaporized by passing through a pre-heating zone and continuously fed into the reactor together with C4-raffinate gas mixture (com-

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posed of trans-2-butene, 56.7%; cis-2-butene, 36.1%; 1-butene, 3.8%; n-butane, 3%; 1,3-butadiene, 0.4%) and air. The feed composition was fixed at a ratio of n-butene: oxygen: steam = 1: 0.65: 12 with total gas flow rate of 66.2 mL min1 and the GHSV (gas hourly space velocity) was 400 h1 on the basis of n-butene. The reaction products were analyzed with an online gas chromatograph equipped with a hydrogen flame ionization detector and a thermal 1 conductivity detector. Reaction rates (in mol g1 ) were calcucat s lated using the equation ri = FiXi/W, where Fi was the molar flow rate of n-butene, Xi was the conversion of n-butene to product i and W was the catalyst weight.

2.3.2. Kinetics tests The kinetics studies were conducted in the same fixed-bed reactor as mentioned in catalytic activity measurement. Before the experiments, the heat and mass transfer limitations were eliminated (Fig. S1), thus, the rates reported were in the kinetic regime. In the intrinsic activity determination, 0.1 mL catalyst and 1.9 mL quartz sand were used. The feed composition was fixed at the ratio of n-butene: oxygen: steam = 1: 0.65: 12 with the GHSV on the basis of n-butene at 2400 h1. To determine the reaction orders for n-butene and O2, the tests were conducted with 0.2 mL catalyst and 1.8 mL quartz sand at 340 °C, which the n-butene conversion was less than 20%. Partial pressure dependencies of n-butene and O2 upon the reaction rates were measured by adjusting the corresponding gas concentrations, while keeping the total flow rate constant (GHSV = 45000 h1). The reaction gas compositions were n-butene (20–120 kPa), O2 (13– 80 kPa) and H2O (700 kPa) in N2. To determine the reaction order for H2O, the tests were conducted with 0.2 mL catalyst and 1.8 mL quartz sand at 340 °C, which the n-butene conversion was less than 20%. Partial pressure dependencies of H2O upon the reaction rates were measured by adjusting the corresponding gas concentrations, while keeping the total flow rate constant (GHSV = 25300 h1). The reaction gas compositions were n-butene (46 kPa), O2 (30 kPa) and H2O (180– 740 kPa) in N2. In the apparent activation energies determination, 0.1 mL catalyst and 1.9 mL quartz sand were used and the feed composition was fixed at the ratio of n-butene: oxygen: steam = 1: 0.65: 12 with the GHSV on the basis of n-butene at 2400 h1. The temperature was varied in the range of 260–330 °C, at which the n-butene conversion was less than 10%.

3. Results and discussion 3.1. Catalyst characterization and catalytic activity of biphasic catalysts Iron-based catalysts with individual ZnFe2O4 and biphasic (ZnFe2O4 and a-Fe2O3) which possessed better activity, and the roles of ZnFe2O4 and a-Fe2O3 in the biphasic catalyst have remained controversial. In previous studies, ZnFe2O4 was commonly synthesized by the method of coprecipitation. However, the solid state reaction of ZnO and Fe2O3 perhaps could not form ZnFe2O4 completely, and the small amount of residue a-Fe2O3 cannot be detected by XRD. Consequently, the results of catalytic activity of spinel in the previous researches maybe not reflected the real situation. Herein, ZnFe2O4 was synthesized by sol-gel method which allowed mixing of Zn2+ and Fe3+ in the precursor solutions with atomic level homogenization to get the spinel with high purity. Then the biphasic catalysts were fabricated by physical mixing method, which could minimize the mutual contamination between the ZnFe2O4 and a-Fe2O3. In order to identify the crystal

phases and surface compositions of these catalysts, XRD, Raman spectroscopy and XPS characterization were performed. XRD analysis was performed to characterize the crystal structures of ZnFe2O4, a-Fe2O3 and biphasic catalysts and the results were shown in Fig. 1a. It was noted that only cubic spinel ferrite phase (PDF: 22-1012) was observed in ZnFe2O4, no visible peaks corresponding to iron oxide and zinc oxide were detected. This indicated that it had ideal pure ZnFe2O4 structure. As expected, characteristic diffraction peaks at 2h = 24.1°, 33.2°, 35.6°, 43.5°, 49.5° and 54.1° (PDF: 33-0664) of a-Fe2O3 were observed in the pure a-Fe2O3. In the samples of biphasic catalysts, two phases containing both ZnFe2O4 and a-Fe2O3 were obtained, and no diffraction peaks related to new crystal phases were detected. The average crystallite sizes were estimated by the Scherrer equation from the X-ray peak broadening (full-width at half maximum, FWHM) of the most intense peak. Based on this equation, the crystallite size of ZnFe2O4 and a-Fe2O3 just decreased from 41 and 31 nm to 33 and 30 nm, respectively, after the high-energy ball milling. Raman spectroscopy was also a powerful technique to characterize the structures of catalyst, and Fig. 1b showed the Raman spectra of the individual and biphasic catalysts. In ZnFe2O4, the modes at about 630 cm1 could be considered as Ag symmetry [22,23]. Eg at about 250 cm1 was due to symmetric bending of oxygen with respect to cation in tetrahedral surrounding. F2g (2) at 350 cm1 and F2g (3) at 450 cm1 corresponded to the vibrations of octahedral group: F2g (2) was due to asymmetric stretching and F2g (3) was caused by asymmetric bending of oxygen. F2g (1) at 160 cm1 was due to translational movement of the whole tetrahedron. For pure a-Fe2O3, the Raman bands at 219 and 492 cm1 corresponded to two A1g modes, while the bands at 285, 403 and 605 cm1 corresponded to the Eg modes [24]. Similarly, the biphasic catalyst existed the characteristic peaks of ZnFe2O4 and aFe2O3, and no peaks related to new crystal phases were detected. The Raman results were in a good agreement with the XRD results. The surface compositions of the individual and biphasic catalysts were determined by XPS and the spectra of O 1s region upon different samples were shown in Fig. S2. Two surface oxygen species could be clearly observed from the figures. The peak at 529.8 to 530.1 eV was attributed to the oxygen ions in the crystal lattice (O2–) [25,26] (hereafter denoted as Oa), whereas the binding energy of 531.5 to 532.3 eV could be assigned to the adsorbed oxygen [27,28] (hereafter denoted as Ob). The surface compositions calculated from the XPS spectra of O 1s for the different catalysts were listed in Table 1. The relative percentages of the Ob component in ZnFe2O4 and a-Fe2O3 were approximately 20% and 46%. These results indicated that a-Fe2O3 contained richer surface oxygen species than ZnFe2O4. In addition, Ob in the biphasic catalyst aFe2O3(20 wt%)/ZnFe2O4-BM was nearly the sum of those in the pure ZnFe2O4 and a-Fe2O3, indicating the surface characterization of the biphasic catalyst was almost the same as the individual components. In general, specific surface area was an important factor affecting the catalytic property of a catalyst [29,30]. The values of specific surface area of pure ZnFe2O4, a-Fe2O3 and the biphasic catalysts were summarized in Table 2. As can be seen, the BET surface areas of the biphasic catalyst was almost the sum of those of the two individual components. It indicated that the change of specific surface area during high-energy ball milling was slight and maybe do not have obvious influence on the catalytic performance of the biphasic catalyst. Based on the analysis of XRD, Raman, XPS and BET results, we reasoned that the structural properties of individual ZnFe2O4 and a-Fe2O3 remained almost unchanged during the process of highenergy ball milling, since no new phase and no clear variation were detected.

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Fig. 1. (a) X-ray diffraction patterns and (b) Raman spectra of the individual ZnFe2O4, a-Fe2O3 and biphasic catalysts.

Table 1 Binding energies of O 1s and the relative content of Ob calculated from the XPS spectra of O 1s for different catalysts. Catalysts

ZnFe2O4

a-Fe2O3

Fe2O3(20 wt%)/ZnFe2O4-BM

Binding energy (eV) of O 1s Oa

Ob

Ob/(Oa + Ob)a (%)

529.8 530.0 530.1

531.5 532.3 531.7

20 46 24

a

The relative content of Ob calculated from the XPS spectra of O 1s for different catalysts.

Table 2 BET surface area and the intrinsic activity of the individual ZnFe2O4, a-Fe2O3 and biphasic catalysts prepared by high-energy mechanical milling method with a-Fe2O3 range from 5 to 40 wt% in the ODH of n-butene at 340 °C after a 1 h-reaction. Catalysts

BET surface area (m2 g1)

R (mol s1 m2)

ZnFe2O4 Fe2O3 Fe2O3(5 wt%)/ZnFe2O4-BM Fe2O3(10 wt%)/ZnFe2O4-BM Fe2O3(15 wt%)/ZnFe2O4-BM Fe2O3(20 wt%)/ZnFe2O4-BM Fe2O3(30 wt%)/ZnFe2O4-BM Fe2O3(40 wt%)/ZnFe2O4-BM

3.7 14.9 5.4 7.6 7.7 8.3 9.0 10.2

1.27  108 0.38  108 8.26  108 8.22  108 8.89  108 8.96  108 7.28  108 5.54  108

Reaction conditions: 0.1 mL catalyst with 1.9 mL quartz sand, n-butene space velocity was 2400 h1, n-C4H8: O2: steam ratio was 1: 0.65: 12.

Catalytic activity evaluation over individual ZnFe2O4, a-Fe2O3 and biphasic catalysts were carried out at 340 °C. The stoichiometric ZnFe2O4 exhibited extremely low n-butene conversion and medium BD selectivity of 2.7% and 80%, respectively (Fig. 2). This was consistent with the previous study, where ZnFe2O4 was prepared via the decomposition of hydrazinated oxalate complexes of iron and zinc [31]; Pure a-Fe2O3 showed a higher conversion of n-butene at 24%, however, the selectivity of BD was only 68%. The biphasic catalysts showed substantial increase both in nbutene conversion and BD selectivity even the content of a-Fe2O3 was only 5 wt%, and achieving its maximum at 20 wt% a-Fe2O3 (60% conversion with 95% selectivity) with the O2 conversion of 73%. The conversion and selectivity only slightly declined with the further increase of a-Fe2O3. The intrinsic activity of ZnFe2O4 and a-Fe2O3 for BD at 340 °C were 1.27  108 and 0.38  108 mol s1 m2 (Table 2), respectively. The biphasic catalysts possessed higher activity with respect to each component, and which containing 20 wt% a-Fe2O3 exhibited the best activity (8.96  108

Fig. 2. Catalytic performances of individual ZnFe2O4, a-Fe2O3 and biphasic catalysts prepared by high-energy mechanical milling method with a-Fe2O3 range from 5 to 40 wt% in the ODH of n-butene at 340 °C after a 1 h-reaction. Reaction conditions: 0.6 mL catalyst and quartz sand 1.4 mL, n-butene space velocity was 400 h1, nC4H8: O2: steam ratio was 1: 0.65: 12.

mol s1 m2). This intrinsic activity results were in good agreement with the above maximum conversion results. To understand the remarkable increase in the catalytic performance of the biphasic catalysts compared to each individual component, O2-TPD, H2-TPR, TPRO, n-butene-TPD analysis and kinetics studies were conducted. It has been reported that the reducibility of the catalyst was one of the crucial factors to the catalytic performance in the ODH of nbutene [32]. In order to clarify the reducibility of each individual phase, O2-TPD, H2-TPR and TPRO analysis were conducted. Fig. 3a showed the results obtained during O2-TPD runs. As discussed in the previous papers [33,34], the low temperature desorption peak of ZnFe2O4 in the 300–600 °C range could be attributed to desorption of the chemisorbed oxygen species. While the high temperature ones (600–900 °C) depended on the partial reduction of metal ions to lower oxidation state. The first and the second peaks of high temperature region should be related to the desorption of lattice oxygen on the surface and bulk of catalyst, respectively. Similar desorption behavior was also observed for a-Fe2O3 [35]. It can be seen that the desorption temperature of lattice oxygen on the surface of catalyst over a-Fe2O3 (640 °C) was lower than ZnFe2O4 (676 °C), and the amount of desorbed superficial lattice oxygen was also in the order of a-Fe2O3 > ZnFe2O4. Thus, it may be indicated that a-Fe2O3 had better reducibility than ZnFe2O4. Fig. 3b presented the H2-TPR of the ZnFe2O4 and a-Fe2O3, it could be seen that there were two reduction peaks for both ZnFe2-

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Fig. 3. (a) O2-TPD and (b) H2-TPR profiles of ZnFe2O4 and a-Fe2O3.

O4 and a-Fe2O3. Because ZnO was hard to be reduced at this experimental condition, the reduction process of ZnFe2O4 was considerably similar to that of a-Fe2O3. The reduction peak at low temperature was related to the reduction of ZnFe2O4 and a-Fe2O3 to Fe3O4, and the high temperature peak could be assigned to the further reduction of Fe3O4 to FeO or metal Fe [36]. Obviously, the low-temperature peak for a-Fe2O3 (379 °C) which was used as an index for the reduction ability of the iron-based catalysts was much lower than that for ZnFe2O4 (538 °C). This was an expected result, as the Fe3+ in the spinel matrix was more stable, thus, resulting that the reduction of ZnFe2O4 became difficult, which was in accordance with the early report [37]. Moreover, the XPS results of O 1s also exhibited that a-Fe2O3 contained more surface oxygen species than ZnFe2O4 (Table 1). In summary, the combined O2-TPD, H2-TPR, TPRO (Fig. S3) and XPS results indicated that a-Fe2O3 had better reducibility than ZnFe2O4. n-Butene-TPD experiments were conducted to investigate the interaction between n-butene and the catalyst. The resulting desorption profiles (Fig. S4) over ZnFe2O4 and a-Fe2O3 both could be separated into three distinct regions. The desorption temperatures of ZnFe2O4 and a-Fe2O3 observed below 200 °C which related to physical adsorption were almost the same. Moreover, the desorption peaks and desorption capacity of a-Fe2O3 around 300 °C were both higher than ZnFe2O4. On the contrary, at the high temperature region about 400 °C, ZnFe2O4 exhibited relatively higher maximum desorption temperature and desorption capacity than a-Fe2O3. Apparently, it indicated stronger adsorption of n-butene on ZnFe2O4. In order to investigate the roles of ZnFe2O4 and a-Fe2O3 in the ODH of n-butene, we performed the reaction orders studies for n-butene and O2 over the individual and biphasic catalysts. The reaction orders for n-butene and O2 over ZnFe2O4 were 0.42 and 1.0, respectively, as shown in Fig. 4, which suggested that the reaction rate strongly depended on the gaseous oxygen partial pressure and the concentration of n-C4H8 had less influence. The reaction orders for n-butene and O2 over a-Fe2O3 were 0.78 and 0.26, respectively. It indicated that the concentration of the gaseous oxygen had little influence on the reaction rate, while which apparently depended on the n-butene partial pressure. The reaction order for O2 over a-Fe2O3(20 wt%)/ZnFe2O4-BM dramatically decreased from 1.0 (ZnFe2O4) to 0.16 owing to the existence of a-Fe2O3, which was possibly attributed to the better ability of aFe2O3 to activate O2, because a-Fe2O3 has better reducibility than ZnFe2O4 (combining the O2-TPD, H2-TPR, TPRO and XPS results); The reaction order for n-butene over a-Fe2O3(20 wt%)/ZnFe2O4-

Fig. 4. Dependence of reaction rates on the partial pressure of the gas compositions upon the ODH of n-butene. The reaction rates were measured at 340 °C and the partial pressures of n-butene and O2 were in the ranges of 20–120 kPa and 13– 80 kPa, respectively.

BM decreased from 0.78 (a-Fe2O3) to 0.62, indicating the activation of n-butene on ZnFe2O4 was stronger than a-Fe2O3 (combining the n-butene-TPD results). These results suggested that a-Fe2O3 was used to activate O2, and ZnFe2O4 was primarily responsible for the activation of n-butene. The addition of a-Fe2O3 to the ZnFe2O4 could overcome the inhibition of the activating O2 in pure ZnFe2O4.

B. Yang et al. / Journal of Catalysis 381 (2020) 70–77

In addition, the reaction order for H2O over a-Fe2O3(20 wt%)/ ZnFe2O4-BM was 0.42 (Fig. S5), indicating H2O was not participated in the reaction. In summary, the individual ZnFe2O4, a-Fe2O3 and biphasic catalysts were all with high purity, and the structural properties of individual ZnFe2O4 and a-Fe2O3 remained almost unchanged during the process of high-energy ball milling. The biphasic catalyst exhibited much superior catalytic performance compared to each individual component. According to the temperature programmed desorption and kinetics studies results, it was speculated that ZnFe2O4 and a-Fe2O3 were responsible for activating n-butene and O2, respectively. 3.2. The key factor for the superior catalytic activity of the biphasic catalyst To further investigate the key factor for the superior catalytic activity of the biphasic catalyst, we prepared another two biphasic catalyst by physical mixing method. The first one was just mixed the a-Fe2O3 (35–60 mesh) and ZnFe2O4 (35–60 mesh) shaking by hand, another was made the two powders with the mass ratio of 1:4 ground in a mortar for 30 min. The two biphasic catalysts were denoted as a-Fe2O3(20 wt%)/ZnFe2O4-SH (the XRD and Raman spectroscopy were in Fig. S6) and a-Fe2O3(20 wt%)/ZnFe2O4-GM, respectively. The intrinsic activity of a-Fe2O3(20 wt%)/ZnFe2O4SH at 340 °C was 1.13  108 mol s1 m2, which was slightly lower than that of ZnFe2O4 (1.27  108 mol s1 m2) (Fig. S7). It indicated that the ODH of n-butene was not successive reaction manipulated by different types of active sites [38]. The intrinsic activity of a-Fe2O3(20 wt%)/ZnFe2O4-GM) was increased to

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2.01  108 mol s1 m2. However, it was still far below that of the a-Fe2O3(20 wt%)/ZnFe2O4-BM (8.96  108 mol s1 m2). Hence, how the a-Fe2O3 introduced to ZnFe2O4 was important. The activities of the biphasic catalysts prepared by the impregnation method were then evaluated, interestingly, the catalyst with only 2 wt% a-Fe2O3 showed comparable intrinsic activity (7.06  108 mol s1 m2, 340 °C) with a-Fe2O3(20 wt%)/ZnFe2O4BM. It was proposed that the contact efficiency of ZnFe2O4 and aFe2O3 might be the key factors to the catalytic performance. To fabricate more ZnFe2O4/a-Fe2O3 interfaces, monodisperse Fe2O3
nH2O colloid capped by PVP were first synthesized, and ZnFe2O4 was deposited onto the surface of the premade Fe2O3
nH2O colloid, yielding the hybrid nanostructure a-Fe2O3(5 wt%)/ZnFe2O4-HN catalyst. The intrinsic activity of a-Fe2O3(5 wt%)/ZnFe2O4-HN readily achieved 1.15  107 mol s1 m2 at 340 °C. Our speculation was also confirmed by further kinetic studies. The reaction orders with respect to O2 over a-Fe2O3(2 wt%)/ZnFe2O4-IM and a-Fe2O3(5 wt%)/ZnFe2O4-HN were 0.04 and 0.03, respectively, as shown in Fig. 5b and Fig. 5d. These results suggested that the activation of O2 would not hinder the reaction rates in these catalysts with efficient contact of ZnFe2O4 and a-Fe2O3. In addition, the reaction order for n-C4H8 over a-Fe2O3(2 wt%)/ZnFe2O4-IM was 0.64 (Fig. 5a), which was similar to the a-Fe2O3(20 wt %)/ZnFe2O4-BM. And it decreased to 0.46 (Fig. 5c) over aFe2O3(5 wt%)/ZnFe2O4-HN, which possessed more ZnFe2O4/aFe2O3 interfaces. Fig. 6a showed the corresponding Arrhenius plots of the biphasic catalysts for the ODH of n-butene, and reaction order was considered in the calculation of rate constant k. The apparent activation energy (Ea) over a-Fe2O3(2 wt%)/ZnFe2O4-IM and

Fig. 5. Dependence of reaction rates on the partial pressure of the gas compositions upon the ODH of n-butene and O2 over (a), (b) a-Fe2O3(2 wt%)/ZnFe2O4-IM and (c), (d) aFe2O3(5 wt%)/ZnFe2O4-HN. The reaction rates were measured at 340 °C and the partial pressures of n-butene and O2 were in the ranges of 20–120 kPa and 13–80 kPa, respectively.

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Fig. 6. (a) Arrhenius plots of the catalysts upon the ODH of n-butene: Fe2O3(5 wt%)/ZnFe2O4-BM, blue d; Fe2O3(2 wt%)/ZnFe2O4-IM, green ▲; Fe2O3(5 wt%)/ZnFe2O4-HN, red w. (b) Plots of the intrinsic activity in the ODH of n-butene at 340 °C vs ln Aapp over biphasic catalysts prepared by different methods.

Table 3 Activation energies (Ea) and apparent pre-exponential factors (Aapp) of the catalysts obtained from Arrhenius plots in Fig. 6a. Ea (kJ mol1)

Catalysts

a-Fe2O3(20 wt%)/ZnFe2O4-BM a-Fe2O3(2 wt%)/ZnFe2O4-IM a-Fe2O3(5 wt%)/ZnFe2O4-HN

Aapp (mol s1 m2) 4

4.4  10 2.0  104 8.6  104

119 116 121

ln Aapp

Ra (mol s1 m2)

10.7 9.9 11.4

8.96  108 7.06  108 1.15  107

Reaction conditions: 0.1 mL catalyst with 1.9 mL quartz sand, n-butene space velocity was 2400 h1, n-C4H8: O2: steam ratio was 1: 0.65: 12. The 90% confidence interval was reported and the maximal errors of Ea was ± 5 kJ mol1. a Intrinsic activity in the ODH of n-butene at 340 °C.

a-Fe2O3 (5 wt%)/ZnFe2O4-HN were 116 and 121 kJ mol1 (Table 3), respectively, which was almost the same as that of the a-

Fe2O3(20 wt%)/ZnFe2O4-BM (119 kJ mol1). This implied that the reaction pathway for the ODH of n-butene in these cases was almost identical. The apparent pre-exponential factor for aFe2O3(20 wt%)/ZnFe2O4-IM, Fe2O3(2 wt%)/ZnFe2O4-BM and aFe2O3(5 wt%)/ZnFe2O4-HN were 2.0  104, 4.4  104 and 8.6  104 mol s1 m2, respectively. It could be seen from Fig. 6b that the intrinsic activity of the biphasic catalysts was well correlated with the apparent pre-exponential frequency factor (Aapp). The apparent pre-exponential frequency factor in mathematical terms is:

Aapp ¼ A  ð½Activ e Sites lphaÞ a

ð1Þ

As the absolute frequency factor A was almost the same, this indicated that the difference in the reaction rate was due to the variation in the number of active sites. The smaller the particle size was, the more ZnFe2O4/a-Fe2O3 interfaces were fabricated and the efficient contact between ZnFe2O4 and a-Fe2O3 could be achieved. The efficient contact between ZnFe2O4 and a-Fe2O3 increased the number of active sites. Therefore, the higher reaction rate of aFe2O3(5 wt%)/ZnFe2O4-HN was ascribed to the richness of the ZnFe2O4/a-Fe2O3 interfaces, which possessed more ZnFe2O4-aFe2O3 contact sites.

[20,21]. Another explanation was that the presence of a-Fe2O3 could protect the spinel structure to avoid deactivation [19]. In the reported two mechanisms, Fe ion in the spinel both underwent an oxidation-reduction cycle during the reaction. However, according to our above research, we proposed that the oxidationreduction cycle of Fe ion was happened on a-Fe2O3 rather than spinel, and this reaction should be the interface bimolecular activation reaction. In order to verify our speculation, Fe3+ in the spinel was substituted by the redox-inactive Al3+ and the catalytic performances of the individual ZnAl2O4, a-Fe2O3 and biphasic catalysts were investigated. Table 4 showed the intrinsic activity for BD over the ZnAl2O4, aFe2O3 and a-Fe2O3(5 wt%)/ZnAl2O4-HN at 420 °C. The intrinsic activity of ZnAl2O4 was only 0.18  108 mol s1 m2, whereas the corresponding value for a-Fe2O3(5 wt%)/ZnAl2O4-HN increased obviously to 3.71  108 mol s1 m2. This was in accordance with the results of ZnFe2O4 and a-Fe2O3(5 wt%)/ZnFe2O4-HN. Since the lattice oxygen of ZnAl2O4 was difficult to be abstracted, lattice oxygen reacted with n-butene must be originated from a-Fe2O3. In the proposed mechanism (Fig. S8), n-butene was first interacted with ZnFe2O4 to generate the intermediate, subsequently, the intermediate reacted with the lattice oxygen of a-Fe2O3 to form BD and H2O. Finally, the reduction Fe2+ of a-Fe2O3 was reoxidized by the

3.3. Reaction mechanism exploration It was evident that biphasic catalyst exhibited much superior catalytic activity with respect to each individual component, which could be explained in terms of synergistic effect. And the efficient contact between ZnFe2O4 and a-Fe2O3 may lead to the improvement of the activity. Based on this understanding, it was important to further identify the possible reaction mechanism. In previous studies, typical mechanism for the iron-based catalyst in the ODH of n-butene was thought to be Mars-van Krevelen mechanism

Table 4 The intrinsic activity of the different samples at 420 °C after a 1 h-reaction. Catalysts

BET surface area (m2 g1)

R (mol s1 m2)

ZnAl2O4 Fe2O3 Fe2O3(5 wt%)/ZnAl2O4-HN

16.5 14.9 18.5

0.18  108 1.42  108 3.71  108

Reaction conditions: 0.1 mL catalyst with 1.9 mL quartz sand, n-butene space velocity was 2400 h1, n-C4H8: O2: steam ratio was 1: 0.65: 12.

B. Yang et al. / Journal of Catalysis 381 (2020) 70–77

gas-phase oxygen. Further characterizations and theoretical calculation studies to confirm this reaction mechanism is in progress. 4. Conclusions In summary, individual ZnFe2O4, a-Fe2O3 and biphasic catalysts were fabricated precisely in this work. Compared to each individual sample, biphasic catalyst exhibited much superior catalytic performance. It was speculated that ZnFe2O4 was primarily responsible for activating n-butene and a-Fe2O3 was used to activate O2. The methods of introducing a-Fe2O3 to ZnFe2O4 were investigated and the kinetics studies suggested that the reaction rates were well correlated with the apparent pre-exponential factor. The synergistic effect between ZnFe2O4 and a-Fe2O3 was of great significance and the efficient contact between ZnFe2O4 and a-Fe2O3 could lead to the improvement of the activity. When ZnFe2O4 was substituted by the redox-inactive ZnAl2O4, the catalytic activities of the individual and biphasic catalysts were in accordance with the result of ZnFe2O4. Hence, this work experimentally indicated that the ODH of n-butene over a-Fe2O3/ZnFe2O4 should be the interface bimolecular activation 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. Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 21902163) and PetroChina Innovation Foundation (2018D-5007-0506). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.10.016. References [1] S. Furukawa, M. Endo, T. Komatsu, ACS Catal. 4 (2014) 3533–3542.

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