Low-Temperature Oxidative Dehydrogenation of n-Butene over Oleate-Mediated ZnFe₂O₄ Catalysts

Abstract

Traditional oxidative dehydrogenation of n-butene has typically required relatively high operating temperatures (400–500 °C), which has driven increasing interest in the development of catalysts capable of delivering high activity at lower temperatures. In this study, zinc ferrite (ZnFe₂O₄-ST) was successfully synthesized via hydrothermal hydrolysis of Zn–Fe oleate and demonstrated remarkable catalytic performance for the oxidative dehydrogenation of n-butene under mild conditions. At 300 °C, ZnFe₂O₄-ST achieved a conversion of 72.9% with 92.1% selectivity toward 1,3-butadiene, a result that, to the best of our knowledge, ranks among the best reported in the literature. By contrast, ZnFe₂O₄ prepared by conventional coprecipitation (17.2% conversion with 91.3% selectivity) and sol-gel (10.1% conversion with 86.4% selectivity) methods showed much lower activities, highlighting the critical influence of synthesis strategy on catalytic performance. To better understand the origin of these differences, a detailed structural and physicochemical characterization was undertaken using X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), N₂ adsorption–desorption, X-ray photoelectron spectroscopy (XPS), H₂-temperature-programmed reduction (H₂-TPR), temperature-programmed re-oxidation (TPRO), and NH₃-temperature-programmed desorption (NH₃-TPD). These analyses revealed that the as-synthesized ZnFe₂O₄-ST possessed a significantly smaller average particle size, a larger specific surface area, and superior reducibility compared with the other samples. These properties are believed to be the key factors underpinning its outstanding catalytic behavior and provide important insights into the design of efficient low-temperature catalysts for selective oxidative dehydrogenation.

1. Introduction

1,3-Butadiene (BD) has attracted much attention in the past decades for its wide applications in the production of poly-butadiene rubber, styrene butadiene rubber, and acrylonitrile–butadiene–styrene [1,2]. Traditionally, BD is produced as a byproduct of ethylene production from steam cracking. As the lightweight development of the ethylene industry, BD obtained through steam cracking is obviously decreased [3,4]. Consequently, oxidative dehydrogenation (ODH) of n-butene for BD has regained much attention.

Many efforts have been made to investigate the efficient catalysts for ODH of n-butene. The best catalysts are based on V-containing catalysts [5], manganese oxide molecular sieve [6], Cu-Mo-based catalysts [7], Bi-Mo-based catalysts [8,9,10,11], Fe-based catalysts [12,13,14,15,16,17], Pd-based catalysts [1,18], and Pt-based catalysts [19]. Fe-based spinel has typically been used in practical industry since the 1970s due to its high activity, easy scale-up and low cost [20,21]. However, traditional ODH of n-butene reaction on Fe-based spinel prepared by the coprecipitation method is operated at relatively high temperatures (400–500 °C) [22]. As ODH of n-butene is an exothermic process (Equation (1)), the reaction is favored at a low working temperature. In addition, higher BD selectivity could be achieved at low temperature. ODH of n-butene on metal ferrite spinel catalyst proceeds via the Mars–van Krevelen mechanism [23,24,25], which involves a redox cycle of lattice oxygen in the catalyst. Therefore, the effective activation of molecular O₂ and the high reactivity of surface lattice oxygen are crucial for enhancing catalytic performance.

$$\mathrm{n}\text{-}{\mathrm{C}}_4{\mathrm{H}}_8+1/2 {\mathrm{O}}_2\to {\mathrm{C}}_4{\mathrm{H}}_6+{\mathrm{H}}_2\mathrm{O} \Delta H_{298.15}=-132 \mathrm{kJ} {\mathrm{mol}}^{-1}$$

(1)

Nanoparticles have been utilized for decades as catalysts in petroleum processing, pollutant removal and energy conversion [26,27,28,29,30]; the majority of the industrial catalysts are high-surface-area solids onto which an active component is dispersed in the form of small particles. Compared with the bulk phase, it can be seen that the catalytic performance is sensitive to the particle size because the surface structure and electronic properties can change greatly in this size range. In the Fischer–Tropsch reaction for fuels from synthesis gas, the adsorption heat of CO and the activation energy for CO dissociation both change with the increasing size of Ni particles [31]. Nanoparticles of vanadia supported on oxides are active catalysts for the ODH of alkanes to olefins. It has been shown that the activity per atom of V increases with the increasing size of the vanadia particles. The investigation on electronic properties of dispersed vanadia indicates that this trend is related to the reduction in the ease of O-to-V electron transfer, and hence to the ease of reduction of the vanadia by adsorbed alkane molecules, which is the critical first step in the ODH. Therefore, it is desirable to develop a facile method with narrower nanoparticle size distribution. The promising method in this direction is the hydrothermal synthesis utilizing fatty acids [32,33].

In this work, stoichiometric zinc ferrite was synthesized via a facile hydrothermal method (from zinc-iron oleate by using simple solvent (ethanol and water) with no other promoters or modifiers, ZnFe₂O₄-ST), sol-gel (ZnFe₂O₄-SG) and co-precipitation (ZnFe₂O₄-CP) methods. The low-temperature catalytic activity of the as-prepared ZnFe₂O₄ samples was evaluated through ODH of n-butene by using a fixed-bed system. Various characterizations such as X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), N₂ adsorption–desorption, X-ray photoelectron spectroscopy (XPS), H₂-temperature-programmed reduction (H₂-TPR), temperature-programmed re-oxidation (TPRO), and NH₃-temperature-programmed desorption (NH₃-TPD) were employed to discuss the relationship between the catalytic activity and the structure of the catalysts.

2. Results and Discussion

2.1. Synthesis and Characterization of ZnFe₂O₄-Oleic Acid-Coated Nanoparticles

As reported in the Experimental section, nearly monodisperse ZnFe₂O₄-oleic acid-coated nanoparticles were obtained by hydrothermal hydrolysis of zinc-iron oleates (Figure 1). Figure 2a shows the XRD pattern of the ZnFe₂O₄-oleic acid-coated nanoparticles, and only the diffraction patterns arising from a cubic ZnFe₂O₄ spinel phase (Powder Diffraction File (PDF): 22–1012) was observed. The average crystallite size was calculated using the Scherrer equation (Equation (2)) [34], and it was estimated to be 9.1 nm.

$$\tau ={\displaystyle \frac{K\lambda }{\beta \cos \theta }}$$

(2)

where τ is the mean size of crystal particles, K is dimensionless shape factor, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity, and θ is the Bragg angle.

TEM image of the ZnFe₂O₄-oleic acid-coated nanoparticles at scale of 20 nm is depicted in Figure 2c. As can be seen, the nanoparticles were of well-separated spheroidal morphology. The mean particle size obtained from TEM image was approximately 7.5 nm, which was similar to their crystallite size obtained from the XRD result. It indicated that the nanoparticles were of high crystallinity. Thermal gravimetric analysis of the ZnFe₂O₄-oleic acid-coated particles was shown in Figure 2b. It was notable that oleic acid was decomposed below 450 °C, as expected. Hence, the nanoparticles were calcined at 450 °C for 3 h in static air to remove the coated oleic acid, and ZnFe₂O₄-ST particles were obtained.

2.2. Catalytic Performance of ZnFe₂O₄-ST, ZnF_e2O₄-CP, and ZnFe₂O₄-SG

2.2.1. Effect of Catalyst Preparation Methods

Catalytic activity evaluation for ODH of n-butene over ZnFe₂O₄ prepared by solvothermal (ZnFe₂O₄-ST), co-precipitation (ZnFe₂O₄-CP), and sol-gel (ZnFe₂O₄-SG) methods was carried out at a low temperature of 300 °C. Figure 3a showed the steady-state catalytic performance of ZnFe₂O₄-X (X = HT, SG, and CP) after 1 h reaction. Two features were evident in the activity evaluations. Firstly, ZnFe₂O₄-ST exhibited much higher n-butene conversion (Equation (5)) at 72.9% than ZnFe₂O₄ synthesized by co-precipitation (17.2%) and sol-gel (10.1%) methods. To the best of our knowledge, this result of ZnFe₂O₄-ST ranked among the best reported in the literature. Secondly, the selectivity of BD (Equation (6)) was also decreased in the order of ZnFe₂O₄-ST (92.1%) > ZnFe₂O₄-CP (91.3%) > ZnFe₂O₄-SG (86.4%). Hence, ZnFe₂O₄-ST exhibited much superior low-temperature catalytic performance.

To assess the stability of the catalysts, the long-term stability tests of the ZnFe₂O₄-ST, ZnFe₂O₄-CP, and ZnFe₂O₄-SG catalysts were carried out in the ODH of n-butene at 300 °C. As can be seen in Figure 3b,c, the conversion of n-butene and the selectivity to BD were all almost constant, indicating the catalysts exhibited very stable catalytic performance.

2.2.2. Effect of Reaction Temperature

To investigate the effect of reaction temperature, ODH of n-butene on ZnFe₂O₄-ST ZnFe₂O₄-CP, and ZnFe₂O₄-SG were conducted at temperatures ranging from 280 to 330 °C. The n-butene conversion was all increased with the increase in temperature for the three ZnFe₂O₄ catalysts (Figure 4a), while the BD selectivity only had a slightly changes (Figure 4b). For ZnFe₂O₄-ST, when the reaction temperature reached 330 °C, a higher conversion of n-butene (78.2%) and BD selectivity (92.6%) results were obtained. Even at the lower temperature of 280 °C, ZnFe₂O₄-ST exhibited a conversion of 55.5% with BD selectivity of 90.5%, indicating excellent low-temperature catalytic activity.

To obtain insights into the structural origins of the observed excellent catalytic performance on ZnFe₂O₄-ST, we characterized the catalysts by several methods.

2.3. Characterizations of the ZnFe₂O₄-ST, ZnF_e2O₄-CP, and ZnFe₂O₄-SG

TEM images of the catalysts at scale of 50 nm were shown in Figure 2d–f. A nanoparticle aggregation structure of ZnFe₂O₄-ST, ZnFe₂O₄-CP, and ZnFe₂O₄-SG were observed in the TEM images.

For heterogeneous catalysis, the reaction usually occurs on the surface of the catalyst; consequently, the amount of catalytic surface area is vital for the availability of the catalytic sites. As a result, the use of a larger specific surface area catalyst is advanced to improve the catalytic activity [35,36]. The nitrogen adsorption–desorption isotherms were performed to demonstrate the pore structure and pore size distribution of the catalysts (Figure 5). The values of specific surface area, pore volumes and average pore diameter of ZnFe₂O₄-ST, ZnFe₂O₄-SG and ZnFe₂O₄-CP are shown in

Table 1. BET surface morphology results of ZnFe₂O₄ with different preparation methods.

Catalysts	BET Surface Area (m2 g−1)	Pore Volumes (cm3 g−1)	Average Pore Diameter (nm)
ZnFe2O4-ST	42	0.09	6.8
ZnFe2O4-CP	33	0.13	11.2
ZnFe2O4-SG	25	0.14	16.7

. As can be seen, the type IV hysteresis loop was displayed in the N₂ sorption isotherm of all the ZnFe₂O₄ catalysts prepared by different methods (Figure 5a), which corresponded to the mesoporous structure. ZnFe₂O₄-ST possessed BET surface areas of 42 m² g⁻¹, which was larger than ZnFe₂O₄-CP (33 m² g⁻¹) and ZnFe₂O₄-SG (25 m² g⁻¹) (Table 1). The BJH average pore sizes of ZnFe₂O₄-ST, ZnFe₂O₄-CP and ZnFe₂O₄-SG were 6.8, 11.2 and 16.7 nm (Figure 5b), respectively.

Figure 6a showed XRD patterns of the prepared catalysts. It was noted that only cubic zinc ferrite spinel phase (PDF: 22–1012) was observed in ZnFe₂O₄-ST, ZnFe₂O₄-CP and ZnFe₂O₄-SG, and no visible peaks corresponding to FeO_x and ZnO were detected. The average crystallite size of ZnFe₂O₄-ST was about 10.8 nm, which was slightly larger than oleic acid-coated ZnFe₂O₄ (9.1 nm), indicating that during the coated oleic acid decomposition process the crystallite size does not vary significantly. The crystallite sizes of ZnFe₂O₄-CP and ZnFe₂O₄-SG obtained from the XRD profiles were approximately 13.5 and 17.6 nm, respectively, which were larger than ZnFe₂O₄-ST.

Based on the analysis of BET and XRD results, we reasoned that the size of ZnFe₂O₄ nanoparticles was pivotal to their catalytic properties, and the catalytic activity was declined with the increase in particle size (Figure 6b). To further understand the size-dependent catalytic activity, XPS and H₂-TPR analysis were conducted.

It was generally known that the ODH of n-butene followed the Mars–van Krevelen mechanism, and the reducibility of the catalyst was reported to be one of the crucial factors to the catalytic performance [22]. In the XPS spectrum of ZnFe₂O₄-ST, ZnFe₂O₄-CP and ZnFe₂O₄-SG (Figure 7a–c), a satellite peak (Fe_sat³⁺ 2p_3/2) around 719.3 eV could be observed, suggesting that all Fe cations of ZnFe₂O₄ catalysts were Fe³⁺ [37]. A peak around 710.9 eV was attributed to the Fe³⁺ cations located at the octahedral site (Fe_O³⁺ 2p_3/2) in the ZnFe₂O₄, while a peak around 714.0 eV was assigned the Fe³⁺ cations located at the tetrahedral site (Fe_T³⁺ 2p_3/2) in the ZnFe₂O₄. To clarify the redox properties of the catalysts with different particle size, XPS and H₂-TPR analysis were performed. XPS experiments were often used to investigate the oxygen mobility of the catalyst [38,39]. Figure 7d–f presents the XPS results of O 1s in different samples. According to the deconvolution of the O 1s spectra, three peaks (denoted as O_I, O_II, and O_III with the increasement of the binding energy) were observed in the catalysts, and the surface oxygen species compositions were calculated and are summarized in

Table 2. Binding energies and the O_II content of O 1s in the ZnFe₂O₄ with different preparation methods determined by XPS.

Catalysts	Binding Energy (eV) of O 1s
	OI	OII	OIII	OII/(OI + OII + OIII) (%)
ZnFe2O4-ST	530.2	532.0	532.9	41
ZnFe2O4-CP	530.1	531.6	532.6	19
ZnFe2O4-SG	529.2	531.4	533.5	11

. As reported in previous studies [40,41], O_I with the binding energy ranging from 529.2 to 530.2 eV was attributed to the oxygen species strongly bonded to the metal component in the catalyst, and O_III with the binding energy of 532.6 to 533.5 eV might be assigned to the surface adsorbed oxygen. But only the O_II (531.4–532.0 eV) reflected the oxygen mobility of zinc ferrite catalyst. The corresponding concentrations of O_II in ZnFe₂O₄-ST was estimated to be 41%, whereas those in ZnFe₂O₄-CP and ZnFe₂O₄-SG were approximately 19% and 11%, respectively. Obviously, the ZnFe₂O₄-ST sample possessed richer Type-II oxygen species on the surface than the other samples.

H₂-TPR and TPRO were effective characterization approaches used to test the reducibility of catalysts [42,43]. Figure 8a displayed the H₂-TPR profiles of the different spinel samples, and two main peaks were observed for the three catalysts. Generally, ZnO could not be reduced at a temperature below 750 °C. The reduction peak at low temperature was attributed to the reduction of ZnFe₂O₄ to Fe₃O₄, and the high temperature peak could be corresponded to the further reduction of Fe₃O₄ to FeO, and finally to metal Fe [44]. It seemed that in the H₂-TPR profiles the second and third peaks were overlapped. As shown in Figure 8a, the low temperature peak with its maximum for ZnFe₂O₄-ST (310 °C) was lower than that for ZnFe₂O₄-CP (431 °C) and ZnFe₂O₄-SG (369 °C). Following the H₂-TPR analysis, the above samples were treated with flowing O₂ to perform the TPRO experiments. Figure 8b presented the results obtained during TPRO runs. As discussed in the previous paper [45], the peak in the low-temperature region (200–350 °C) of the TPRO profile indicated the capacity of oxygen mobility, which was an index for the oxygen mobility of the catalyst. The lower temperature of the low temperature peak suggested higher oxygen mobility. It can be seen that the low temperature peak with its maximum over ZnFe₂O₄-ST (253 °C) was also lower than that of ZnFe₂O₄-CP (315 °C) and ZnFe₂O₄-SG (302 °C). The observation of H₂-TPR and TPRO analysis confirmed that ZnFe₂O₄-ST with smaller particle size could facilitate the better reducibility and oxygen mobility [46,47,48].

Based on the combined XPS, H₂-TPR, and TPRO results, it indicated that ZnFe₂O₄-ST with smaller particle size had better reducibility than ZnFe₂O₄-CP and ZnFe₂O₄-SG. The catalytic activity declined with the decrease in the reducibility of the catalyst.

In order to investigate the effect of surface acidity on the catalytic performance of ZnFe₂O₄ spinel catalysts, NH₃-TPD analysis were conducted. It was noted that for all the ZnFe₂O₄ catalysts, two main peaks were obtained between 200 and 500 °C (Figure 9). The ZnFe₂O₄ catalysts exhibited a significant difference in acid strength (peak temperature) and a slight difference in total acidity (peak area) with the ZnFe₂O₄ of different preparation methods. The yield for BD was related to the acid strength of ZnFe₂O₄ catalysts. Consequently, it was expected that the ZnFe₂O₄ catalyst with a weak acid strength was favorable for the reaction of the ODH of n-butene. In conclusion, the surface acidity of ZnFe₂O₄ catalysts played a key role in the catalytic performance.

3. Materials and Methods

3.1. Experimental Materials

All chemicals were of analytical grade and were used as received without further purification. Their purity, suppliers and applications are listed in

Table 3. Chemicals used in the present work, their purity, suppliers and applications.

Chemicals	Purity (%)	Supplier	Application
NaOH	≥96	Sinopharm Chemical (Shanghai, China)	Catalyst Preparation
Oleic acid	90%	Sigma Aldrich (St. Louis, MO, USA)	Catalyst Preparation
Ethanol	99.5	Shanghai Aladdin (Shanghai, China)	Catalyst Preparation
Fe(NO3)3·9H2O	98	Sinopharm Chemical (Shanghai, China)	Catalyst Preparation
Zn(NO3)2·6H2O	99	Sinopharm Chemical (Shanghai, China)	Catalyst Preparation
Hexane	≥97	Shanghai Aladdin (Shanghai, China)	Catalyst Preparation
Citric acid	99%	Sigma Aldrich (St. Louis, MO, USA)	Catalyst Preparation
C4-raffinate gas mixture	trans-2-butene (56.7%) + cis-2-butene (36.1%) + 1-butene (3.8%) + n-butane (3%) + BD (0.4%)	Dalian Date Gas (Dalian, China)	ODH of n-butene
Air	99.99%	Dalian Date Gas (Dalian, China)	ODH of n-butene

.

3.2. Catalyst Synthesis

3.2.1. ZnFe₂O₄-ST

The synthesis of ZnFe₂O₄-ST was via hydrothermal method. In a typical synthesis, NaOH (66 mmol) was dissolved in 15 mL of deionized water to obtain solution A. Oleic acid (68 mmol) was dissolved in 20 mL of ethanol to obtain solution B. Then, solution B was added to solution A to create a transparent solution. A total of 16 mmol of Fe(NO₃)₃ and 8 mmol of Zn(NO₃)₂ were dissolved in 10 mL of deionized water, and it was added into the above solution. After stirring, 20 mL of hexane was added, and the solution was separated to two phases: upper black organic phase and lower water phase. This mixture was then boiled under reflux condenser for one hour to complete the following reactions, Equations (3) and (4).

$$3\mathrm{Na}\left(\mathrm{oleate}\right)+\mathrm{Fe}{\left({\mathrm{NO}}_3\right)}_3\to {\mathrm{Fe}\left(\mathrm{oleate}\right)}_3+3{\mathrm{NaNO}}_3,$$

(3)

$$2\mathrm{Na}\left(\mathrm{oleate}\right)+\mathrm{Zn}({\mathrm{NO}}_3{)}_2\to {\mathrm{Zn}\left(\mathrm{oleate}\right)}_2+2{\mathrm{NaNO}}_3,$$

(4)

Subsequently, this mixture was cooled, and the water phase was removed. Deionized water (20 mL), ethanol (5 mL) and hexane (5 mL) were then added to the organic phase, and this mixture was heated to reflux for one hour. After the washing step was done three times, the mixture was heated at 85 °C to evaporate the hexane. The product was moved into a Teflon-lined autoclave. Then, 30 mL of ethanol and 60 mL of deionized water were added, and it was kept at 180 °C for 13 h.

Afterwards, the liquid phase was discarded, and the nanoparticles were collected by magnet. The following purification process was done twice; the nanoparticles were first dispersed in 10 mL of hexane. Ethanol (10 mL) was added to wash and precipitate the nanoparticles. After washing, the nanoparticles were re-dispersed in hexane and denoted as oleic acid-coated ZnFe₂O₄ nanoparticles. Finally, they were precipitated by ethanol, dried at 80 °C overnight and calcined at 450 °C for 3 h, denoted as ZnFe₂O₄-ST.

3.2.2. ZnFe₂O₄-SG

ZnFe₂O₄-SG was synthesized by sol-gel method. In a typical synthesis, Zn(NO₃)₂·6H₂O (8 mmol), Fe(NO₃)₃·9H₂O (16 mmol) and citric acid (28.8 mmol) were dissolved in deionized water (25 mL), and a brown homogeneous solution was formed. Then it was heated at 80 °C until a viscous and gel-like sample was formed. The combustion reaction was ignited at about 170 °C. After the combustion, the reaction vessel was cooled. Finally, the product powder was calcined in air at 450 °C for 3 h with the heating rates of 2 °C min⁻¹ and denoted as ZnFe₂O₄-SG.

3.2.3. ZnFe₂O₄-CP

ZnFe₂O₄-CP was synthesized by co-precipitation method. In a typical synthesis, Zn(NO₃)₂·6H₂O (8 mmol) and Fe(NO₃)₃·9H₂O (16 mmol) were dissolved in deionized water. Then, the above solution and NaOH solution (3 mol L⁻¹) were added dropwise to deionized water for co-precipitation with stirring, and the pH of the mixed solution was precisely controlled at ca. 9. After the resulting solution was stirred for 2 h, it was aged for 12 h at room temperature. The resulting precipitates were filtrated and washed several times with deionized water and dried at 80 °C overnight. Subsequently, the resulting powders were calcined at 450 °C for 3 h in air and denoted as ZnFe₂O₄-CP.

3.3. Catalyst Characterization

XRD measurements were performed with a ShimadzuXD-3D X-ray diffractometer (Shimadzu Corporation, Kyoto, Japan) with Cu Ka radiation from 10 to 80°. Transmission electron microscopy (TEM) was obtained using a Tecnai-G2-F30 field emission transmission electron microscope (FEI Company, Hillsboro, OR, USA). TGA was conducted using a Zetzsch Sta 449F3 (NETZSCH Group, Selb, Germany) in a continuous flow of air from room temperature to 700 °C. BET specific surface areas were measured on a Micromeritics ASAP2020 analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA) under N₂. Prior to measurement, the catalysts were degassed at 200 °C for 5 h. X-ray photoelectron (XPS) spectroscopy analyses were performed with a VG ESCALAB210 (VG Scientific, St Leonards-on-Sea, UK) instrument with a mono-chromatic X-ray source of Mg Ka (hν = 1253.6 eV). The binding energies were calibrated versus adventitious carbon (C1s reference peak at 284.8 eV).

H₂-TPR, TPRO and NH₃-TPD experiments were carried out on a ChemBET Pulsar (Anton Paar GmbH, Ashland, VA, USA) TPR/temperature-programmed desorption (TPD) automated chemisorption analyzer with a thermal conductivity detector (TCD). In the H₂-TPR analysis, prior to the reduction of the sample, catalyst was first pretreated in air (30 mL min⁻¹) at 450 °C for 1 h. Thereafter, 10 vol% H₂/Ar (30 mL min⁻¹) was introduced as a reducing agent, and the catalyst was analyzed from room temperature to 750 °C with a ramp rate of 10 °C min⁻¹. Then, it was cooled to room temperature in He flow (30 mL min⁻¹). Subsequently, TPRO experiments were performed in flowing 2 vol% O₂/He (30 mL min⁻¹) from room temperature to 500 °C at 10 °C min⁻¹.

In the NH₃-TPD measurements, the catalyst (70 mg) was loaded in a quartz reactor and pretreated in air (30 mL min⁻¹) at 450 °C for 1 h. Then, it was cooled down to 120 °C, followed by He flow (30 mL min⁻¹) to purge the residual gaseous and weakly adsorbed oxygen. Subsequently, the catalyst was exposed to NH₃ at 120 °C for 1 h, followed by placing it under He flow (30 mL min⁻¹) for 1 h to remove the physically adsorbed NH₃. Finally, the sample was heated by a temperature ramp at a rate of 10 °C min⁻¹ in the He (30 mL min⁻¹).

3.4. ODH of n-Butene

The ODH of n-butene was evaluated under quasi-steady-state conditions in a vertical fixed-bed quartz tubular reactor. Each catalyst (0.6 mL, 1.02 g, 40–60 mesh) and quartz powder (1.4 mL, 2.24 g, 40–60 mesh) were installed in the reactor. Before the reaction, the catalyst was pre-treated with air (12.38 mL min⁻¹) and water stream (48.0 mL min⁻¹) at 400 °C for one hour. Following this, the catalyst was cooled under flowing air to 100 °C. The reactant C4-raffinate gas mixture (trans-2-butene (56.7%) + cis-2-butene (36.1%) + 1-butene (3.8%) + n-butane (3%) + BD (0.4%)) was then passed through the reactor with total gas flow rate of 66.2 mL min⁻¹ and the GHSV (gas hourly space velocity) of 400 h⁻¹ on the basis of n-butene. The feed composition was fixed at n-butene:oxygen:steam = 1:0.65:12, and the reaction temperatures were varied in the range of 280–330 °C. Online analyses were performed by gas chromatograph (GC) analysis (one thermal conductivity detector and one flame ionization detector). The catalytic performance was evaluated by the following equations:

$$\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n} ({\mathrm{X}}_{\text{n-butene}})={\displaystyle \frac{{\mathrm{A}}_{\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{a}}\times {\mathrm{f}}_{\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{a}}+{\mathrm{A}}_{{\mathrm{C}\mathrm{O}}_2}\times {\mathrm{f}}_{{\mathrm{C}\mathrm{O}}_2}/4}{{\mathrm{A}}_{\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{a}}\times {\mathrm{f}}_{\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{a}}+{\mathrm{A}}_{\mathrm{i}\mathrm{s}\mathrm{o}\mathrm{m}\mathrm{e}\mathrm{r}\mathrm{s}}\times {\mathrm{f}}_{\mathrm{i}\mathrm{s}\mathrm{o}\mathrm{m}\mathrm{e}\mathrm{r}\mathrm{s}}+{\mathrm{A}}_{{\mathrm{C}\mathrm{O}}_2}\times {\mathrm{f}}_{{\mathrm{C}\mathrm{O}}_2}/4+{\mathrm{A}}_{\text{n-butene}}\times {\mathrm{f}}_{\text{n-butene}}}}\times 100\%$$

(5)

$$\mathrm{s}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y} ({\mathrm{S}}_{\text{1,3-butadiene}})={\displaystyle \frac{{\mathrm{A}}_{\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{a}}\times {\mathrm{f}}_{\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{a}}}{{\mathrm{A}}_{\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{a}}\times {\mathrm{f}}_{\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{a}}+{\mathrm{A}}_{{\mathrm{C}\mathrm{O}}_2}\times {\mathrm{f}}_{{\mathrm{C}\mathrm{O}}_2}/4}}\times 100\%$$

(6)

where A_buta, ${\mathrm{A}}_{\mathrm{i}\mathrm{s}\mathrm{o}\mathrm{m}\mathrm{e}\mathrm{r}\mathrm{s}}$, ${\mathrm{A}}_{{\mathrm{C}\mathrm{O}}_2}$, and ${\mathrm{A}}_{\text{n-butene}}$ represent the chromatographic peak areas of 1,3-butadiene, isomerization products, CO₂, and n-butene, respectively, and $\mathrm{f}$ represents the correction factor for each species.

4. Conclusions and Outlook

In this work, a series of zinc ferrite spinel ODH of n-butene catalysts were synthesized by different strategies. The influence of different preparation methods on the low-temperature ODH of n-butene was carefully investigated. The result showed that the catalyst synthesized by hydrothermal hydrolysis of Zn-Fe oleate strategy exhibited excellent catalytic performance at low temperature, to the best of our knowledge, which ranks among the best reported in the literature. The smaller particle size of ZnFe₂O₄-ST significantly improved reducibility and oxygen mobility, resulting in remarkable catalytic performance. XPS, H₂-TPR and TPRO results also confirmed that the as-synthesized ZnFe₂O₄-ST has excellent redox properties, which was one of the crucial factors to the catalytic performance in the ODH of n-butene. NH₃-TPD analysis indicated that the ZnFe₂O₄ catalyst with a weak acid strength was favorable for the reaction of the ODH of n-butene. Furthermore, this work emphasizes the importance of the particle size for catalytic reactions and provides a research strategy for mixed metal oxide catalysts in n-butene ODH and other similar reactions.