B₂O₃ supported La_0.8Sr_0.2FeO₃ for direct ethane oxidation into ethylene and syngas for hydroformylation synthesis

Abstract

Hydroformylation is an important reaction for aldehyde synthesis and requires ethylene and syngas (CO + H₂) with the feedstocks. While these feedstocks are typically synthesized separately, it would be highly desirable to directly convert abundant ethane into ethylene and syngas in a single step. In this work, we loaded B₂O₃ onto perovskite La_0.8Sr_0.2FeO₃ (LSF) and formed a core–shell structured catalyst, with B₂O₃ and Sr₃B₂O₆ being the shell and LSF being the core. The shell structure can substantially inhibit deep oxidation of ethane into CO₂ and the optimized La_0.8Sr_0.2FeO₃@20B₂O₃ can achieve 69.2% ethane conversion and 88.8% ethylene + syngas selectivity at 700 °C with good stability, with a ratio of CO/H₂ close to 1 : 1. The effects of reaction temperature, space velocity, B₂O₃ loading, and La/Sr were also investigated. This catalyst marks a promising progress for the direct oxidation of ethane and has good potential for industrial applications.

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1. Introduction

The hydroformylation of olefins is considered one of the pivotal technologies in the petrochemical industry, serving as an important method for the industrial synthesis of aldehydes and alcohols.¹ The products of this reaction, along with their derivatives, are extensively utilized in the manufacture of plasticizers, fabric additives, surfactants, solvents, pharmaceutical intermediates, and fragrances.² As a representative, ethylene hydroformylation reaction (C₂H₄ + CO + H₂ = CH₃CH₂CHO) requires two feedstocks, namely ethylene and syngas (CO + H₂). Typically, these two raw materials are prepared separately. Syngas is a promising substitute for fossil fuels and is produced mainly from coal gasification and natural gas conversion.^3,4 The primary methods for ethylene production currently include the steam cracking of naphtha and ethane and the ODHE reaction.⁵

Recent studies reported that as compared to the conventional steam cracking process, ODHE presents several advantages including a lower reaction temperature and reduced coke formation.⁶ As an exothermic reaction, ODHE was reported to exhibit potential to reduce the energy consumption per ton of ethylene by 35% as compared to steam cracking.⁷ ODHE can also eliminate the thermodynamic obstacles of ethane cracking and enhance the overall conversion of ethane. Given the advantages of ethylene production with ODHE, current catalysts are summarized in the following paragraph.

Over the past two decades, numerous catalysts have been demonstrated to be active for ODHE, including NiO-based catalysts,^8,9 Cr-based catalysts,¹⁰ Fe-based catalysts¹¹ and M1 phase-based catalysts with a common formula of (TeO)_x(Mo, V, Nb)₅O₁₄ (ref. 12). The majority of ODHE reactions are conducted at temperatures ranging from 400 to 700 °C in the presence of gaseous oxygen over these catalysts. Among those, the M1-based catalysts were reported to be more promising for ODHE due to their lower reaction temperature and high ethylene yield. It was reported that the ethylene selectivity was 90% at an ethane conversion of 78% at a relatively low temperature of 400 °C and a short contact time of 5.5 s. It was reported that the VMoNbTe mixed oxide transformed into active catalytic phases under reaction atmospheres and conditions.¹² However, it is noted that one of the main challenges of ODHE is still the excessive oxidation of ethylene, which results in a significant production of CO and CO₂.^13,14

Besides M1-based ODHE catalysts, perovskite-based metal oxide catalysts (ABO₃) are also considered as promising due to their adjustable ethylene selectivity granted by different A/B element doping and surface modifications. ABO₃ perovskite catalysts are typically composed of lanthanides or alkaline earth metals such as La and Sr in the A sites and transition metals such as Fe and Mn in the B sites. It has been well acknowledged that the unique structure and oxygen vacancies of perovskite make it an effective catalyst for redox reactions.¹⁵ Gao et al.¹⁶ reported that molten Li₂CO₃ covered LSF is beneficial for the ODHE. The molten layer facilitates the transport of reactive peroxide O₂²⁻ species formed on the LSF while blocking the non-selective site. Dai et al.¹⁷ reported that halogen doping can further reduce the deep oxidation of C₂H₄ and thus enhance C₂H₄ selectivity and the resulting La_1−xSr_xFeO_3−δX_δ (X = F, Cl) can achieve an ethane conversion of 76.8% and an ethylene selectivity of 62.1% at 660 °C. However, the stability of these redox catalysts is questionable as halogens may be replaced by oxygen and eventually disappear. These studies demonstrated that the ODHE performances of perovskite catalysts can be improved by either doping or surface promotions.

In addition to conventional metal oxides mentioned above, boron-based non-metallic materials have also gained considerable attention as a novel catalytic material for oxidative dehydrogenation reactions. Studies have indicated that non-metallic B-based materials, such as BN,^18–22 SiB₆,²³ and B₂O₃,^21,24–26 are capable of catalyzing the oxidative dehydrogenation of C₂–C₄ alkanes into alkenes, with a notably low selectivity for CO₂. Tian²⁶ reported that B₂O₃-based catalysts are also selective in the direct conversion of methane to HCHO and CO with surface tri-coordinated BO₃ units being the active sites for methane oxidation. Noting that the melting point of boron oxide is only 450 °C, it has been generally reported that a molten layer of B₂O₃ is present under working conditions (at 500–700 °C) for the enhanced ODHE selectivities. Metal oxides such Al₂O₃,²⁷ ZrO₂,²⁸ TiO₂,²⁹ SiO₂,²⁴ and MgO have also been employed to support the molten B₂O₃. It was reported that the construction of a boron oxide-supported catalyst could accommodate the surface BO_x species and contributes to the stability and dispersion of surface BO_x.

Although ODHE and the effects of BO_x in catalytic oxidation reactions have been well studied, there are very few studies that report the direct oxidative conversion of ethane into hydroformylation-ready products, namely ethylene and syngas with H₂/CO = 1/1. We note that the direct conversion of ethane into ethylene + syngas would be very beneficial for the hydroformylation reaction with reduced product separation cost and process complexities. In this study, a core–shell structure catalyst, with B₂O₃ supported on the surface of La_0.8Sr_0.2FeO₃ (LSF) was synthesized. B₂O₃ loading alters the oxygen species on the catalyst and inhibits the overoxidation of ethane into CO₂, leading to the co-production of ethylene and syngas with H₂/CO = 1/1. The catalyst structure of B₂O₃-loaded LSF was further explored using characterization methods such as TEM and XRD. The catalytic effects of different loadings of B₂O₃ in LSF and different ratios of La to Sr elements in LSF were also investigated and studied.

2. Experimental

2.1 Preparation of catalysts

All the chemical reagents, including La(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, Sr(NO₃)₂, H₂BO₃, citric acid, and polyethylene glycol, were analytical grade and purchased from Shanghai Aladdin Chemical Reagents Co. China. LSF was prepared by the sol–gel method. La(NO₃)₃·6H₂O, Sr(NO₃)₂, Fe(NO₃)₃·9H₂O, citric acid and ethylene glycol were used for the synthesis. La(NO₃)₃·6H₂O, Sr(NO₃)₂ and Fe(NO₃)₃·9H₂O with the required La/Sr/Fe molar ratio were dissolved in deionized water under continuous stirring to acquire a homogeneous solution. Then, citric acid was added to the solution, heated at 50 °C, and stirred for 30 min. Then, ethylene glycol was added into the solution, heated, and stirred at 80 °C until gel formation. The molar ratio of citric acid to metal ions (La³⁺ + Sr²⁺ + Fe³⁺) was kept at 3 : 1. And the molar ratio of ethylene glycol to citric acid was kept at 2 : 1. After drying at 105 °C for 24 h, the prepared sample was annealed at 950 °C for 12 h in air to obtain LSF.

The B₂O₃-covered catalyst was prepared using the impregnation method, with LSF@B₂O₃ as an illustrative example. A quantitative amount of H₃BO₃ was dissolved in deionized water (10 mL), and the resulting aqueous solution was added dropwise to pure LSF (0.5 g). After vigorous stirring for 1 hour, the impregnated samples underwent multiple dips and were dried at 105 °C for 8 hours. The prepared product was then calcined at 650 °C in air for 5 hours. The B₂O₃-based LSF catalysts will be referred to as LSF@5B₂O₃, LSF@10B₂O₃ and LSF@20B₂O₃ based on different loadings of B₂O₃. LSF@20B₂O₃ indicates that LSF is loaded with B₂O₃ with a mass fraction of 20%.

2.2 Catalytic testing

The ODHE experiment was carried out under ambient pressure in a micro-vertical fixed-bed quartz U-tube reactor. The reactor system for catalyst testing is presented in Fig. 1. The reaction gases, namely C₂H₆, O₂, and Ar, were mixed in the reactor and the flow rate was regulated through a flow meter, and then the gases were introduced from the top of the fixed-bed quartz tube reactor. The gas generated by the reaction was detected using a gas chromatograph/mass spectrometer. A redox catalyst weighing 0.7 g was loaded into a U-shaped tube with dimensions of 300 mm in length, 10 mm in outer diameter, and 3 mm in inner diameter. Quartz wool was loaded on the bottom of the catalyst bed to prevent catalyst particles from entering the gas chromatograph. In the beginning of the reaction, a 100% argon purge gas was introduced for 30 min with a flow rate of 50 mL min⁻¹. The ratio of ethane to oxygen was determined to be 2 : 1 by the previous literature and pre-experimentation. Then, a mixed gas of 22.2% ethane, 11.1% oxygen, and 66.7% argon were fed into the reactor with a flow rate of 33.75 to 73.75 mL min⁻¹ depending on different space velocities. The reactor temperature was then raised to initiate the reaction. The optimal reaction conditions were studied using different reaction temperatures (500 °C, 550 °C, 600 °C, 650 °C, 700 °C) and different mass space velocities. The reaction gas products were passed directly into the gas chromatograph (GC) with a model of Clarus 690 GC (PerkinElmer) for measurements. The GC has two thermal conductivity detector (TCD) channels and a flame ionization detector (FID) for hydrocarbon analysis.

Fig. 1 A schematic drawing for the fixed bed reaction test system.

Conversion and selectivity were calculated as follows. Conversion is defined as the moles of carbon converted divided by the total moles of carbon in the feed. The carbon balance is defined as the total moles of carbon at the reaction outlet divided by the total moles of carbon in the feed gas. The specific calculation formulas for the main reactant conversion rate and product selectivity are as follows:

Here, F_in and F_out represent the volume contents (%) of the components at the inlet and outlet of the reactor.

2.3 Materials characterization

The composition and phase of the catalyst were characterized by X-ray powder diffraction (XRD). The XRD analysis was carried out on a Rigaku Smartlab diffractometer using a Co Kα radiation source at 40 kV and 40 mA. The 2θ value range was set between 10 and 80 degrees, with a scanning speed of 6 degrees per minute and a scanning step of 0.02 degrees. The crystallite size is acquired by the Scherrer equation. The study investigated the effects of active metal and B₂O₃ doping on the transformation of the catalyst's crystal form. X-ray photoelectron spectroscopy (XPS) was utilized to identify the elements in the catalyst and their contents (with a sensitivity of 0.1 at%), and the chemical state of the elements was further determined by the shift in photoelectron kinetic energy. XPS spectra were recorded using monochromatic Al X-rays on a Thermo Scientific K-Alpha ESCALAB 250XI instrument, and the binding energy scale was calibrated with C 1s (284.8 eV). Temperature-programmed hydrogen reduction (H₂-TPR) was performed on a PX100A chemical absorption analyzer equipped with a U-shaped fixed bed reactor and a thermal conductivity detector. Typically, 50 mg of catalyst is calcined at 105 °C in a N₂ atmosphere for 1 hour, then the reactor is cooled to 50 °C, and then the reactor is heated to 1000 °C at a heating rate of 10 °C min⁻¹. The total gas flow rate of H₂/N₂ is 50 mL min⁻¹. H₂-TPR is used to detect the distribution and content of metal-loaded species in active metal oxide catalysts.

3. Results and discussion

3.1 Determination of the core–shell structure

XRD was first conducted to determine the crystal structures of LSF and B₂O₃ supported LSF. Fig. 2 shows the XRD patterns of LSF, LSF@5B₂O₃, LSF@10B₂O₃ and LSF@20B₂O₃. Based on the standard database (ICDD File No. 00-035-1480), it was confirmed that the desired LSF perovskite structure was formed in all the samples. The characteristic peak of the parent LSF phase was gradually widened with the increasing amount of B₂O₃ loadings, indicating a decrease of grain size after supporting with B₂O₃. The grain sizes of LSF with different B₂O₃ loadings were calculated via the Scherrer equation, and the common Scherrer formula is:

D = Kλ/(β cos θ)

where k is the constant, 0.89, λ is the X-ray wavelength, β is the diffraction peak half-height width, and θ is the diffraction angle. The calculated results are shown in Table 1. As can be seen, the grain size of LSF, LSF@5B₂O₃ and LSF@20B₂O₃ are 97.52 nm, 50.69 nm and 5.16 nm, respectively. The reason for this trend could be that molten B₂O₃ is filled into the edges of LSF and limits the growth of its grain size. As will be shown in the following XPS section, this hypothesis is feasible as B₂O₃ almost completely covers the surface of LSF. Besides, some Sr₃B₂O₆ (ICDD File No. 00-020-1189) could be observed after the impregnation of B₂O₃, indicating that B₂O₃ can partially react with surface Sr in LSF and form another phase. However, we note that the intensity of Sr₃B₂O₆ did not increase significantly as the B₂O₃ loading increased from 5 wt% to 20 wt%. This indicated that the reaction between B₂O₃ and LSF is limited.

Fig. 2 XRD patterns of LSF and B₂O₃-loaded LSF.

Table 1 Crystallite size of LSF with different B₂O₃ loadings

Catalysts	LSF	LSF@5B2O3	LSF@10B2O3	LSF@20B2O3
D (crystallite size) (nm)	97.52	50.69	7.32	5.16

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To further determine the surface compositions of LSF and B₂O₃ covered LSF, the XPS spectra of the LSF, LSF@10B₂O₃ and LSF@20B₂O₃ catalysts are collected and illustrated in Fig. 3. The surface atomic concentrations, obtained from XPS (Fig. 3a), were analyzed to determine the surface element composition. The metal element compositions on the surface of LSF are mainly Fe, La and Sr. After the impregnation of B₂O₃, the surface of the LSF@10B₂O₃ and LSF@20B₂O₃ samples is enriched with B while Fe and La are substantially suppressed.²⁷ This indicates a surface coverage of mainly B after the B₂O₃ impregnation. We also note that the content of Sr on the surface remained basically unchanged. As discussed above in the XRD, Sr may react with B₂O₃ and form a minor Sr₃B₂O₆ phase. Thus, the stable presence of Sr may indicate a sub-layer of Sr₃B₂O₆ between the B₂O₃ coverage and the LSF surface. The surface of LSF, LSF@10B₂O₃, and LSF@20B₂O₃ were further analyzed by detailed scanning of O 1s. As shown in Fig. 3b, the O 1s peak could be fitted with three peaks. These include the lattice oxygen peak O_L (O²⁻) at a lower binding energy,³⁰ the adsorption peak O_A (O₂²⁻/O⁻) at a medium binding energy, the higher lattice oxygen peak on the catalyst surface, and hydroxide and/or carbonate oxygen (OH⁻/CO₃²⁻) at the high binding energy peak.³¹ The O_L/O_A in LSF is calculated to be 0.77. After 10 wt% B₂O₃ loading, this ratio decreased substantially to 0.30. With the B₂O₃ loading increased to 20 wt%, only the O_A peak could be observed. This indicates that only surface-absorbed oxygen species are dominant with an increased amount of B₂O₃, and this result is consistent with other studies.^20,23,32,33 Simultaneously, the surface-absorbed oxygen species shifted from 531.1 eV in LSF and LSF@10B₂O₃ to a higher binding energy of 531.9 eV in LSF@20B₂O₃, indicating that surface-absorbed oxygen in LSF@10B₂O₃ is an intermediate state between LSF and LSF@20B₂O₃. Fig. 3c further displays the B 1s spectrum of the reduction catalyst using Al₂O₃@20B₂O₃ as a comparison. As can be seen, Al₂O₃@20B₂O₃ shows a B 1s peak near 192 eV. The peak of B at 192 eV is mainly from B₂O₃. Besides the 192 eV peak, B 1s peaks at 190 eV, 195 eV and 198 eV were also observed for LSF@10B₂O₃ and LSF@20B₂O₃.³⁴ Although there are no other reports in the literature, we believe that these peaks should be assigned to the sublayer minor Sr₃B₂O₆ phase based on the XRD and elemental compositions in Fig. 3(a). As the B₂O₃ loading increased from 10 wt% to 20 wt%, the peaks at 190 eV, 195 eV and 198 eV were all suppressed, indicating the thickening of the B₂O₃ layer. Based on these results, a schematic drawing of B₂O₃ covered LSF is shown in Fig. 3(d), where B₂O₃ covers the outer layer and a sub-layer of Sr₃B₂O₆ exists between B₂O₃ and LSF.

Fig. 3 (a) Elemental composition of LSF and LSF@20B₂O₃ based on XPS. Detailed XPS spectra of the perovskite catalysts: (b) O 1s; (c) B 1s. (d) A schematic drawing of the core–shell structure of B₂O₃ loaded LSF.

In-depth characterization of the B₂O₃-loaded LSF structure was further conducted via high-resolution TEM to confirm the schematic structure in Fig. 3d. Fig. 4(a–d) depict the TEM and elemental mapping images of the catalysts. Although the B element was observed to be spreading all over the particles and did not show a clear edge, this may be attributed to the less sensitivity of B in EDS. As compared, the edges of the LSF particles are clearly more populated with Sr. This is consistent with the sub-layer of Sr₃B₂O₆ on top of LSF, as suggested by XRD and XPS and illustrated in Fig. 3(d).

Fig. 4 (a–d) TEM and elemental mapping images of LSF@20B₂O₃.

3.2 ODHE performances of B₂O₃ covered LSF

The ODHE performances of LSF, LSF@5B₂O₃, LSF@10B₂O₃ and LSF@20B₂O₃ were further tested. Fig. 5a and b illustrate the change of ethane conversion and the yield of ethylene + syngas with temperature for different B₂O₃ loadings. As can be seen in Fig. 5a, the ethane conversion of standalone LSF was the highest with 17.5% at 500 °C, while all other B₂O₃ loaded samples exhibited minimal ethane conversions below 3%. With the temperature increasing from 500 to 700 °C, the ethane conversion of LSF slightly increased to 29.66%. All B₂O₃ loaded samples exhibited a sharper increase of ethane conversion with respect to temperature. Moreover, the increase of B₂O₃ loading also leads to the increase of ethane conversion, and the ethane conversion of LSF@20B₂O₃ is at 62.33% in 700 °C. The yield of ethylene + syngas followed a similar trend with the ethane conversion as shown in Fig. 5b. As can be seen, the ethylene + syngas yield of LSF@20B₂O₃ sharply increased from 16.65% at 650 °C to 55.37% at 700 °C, which is the highest yield among all samples. The detailed product selectivity distribution obtained at 700 °C is displayed in Fig. 5c. As can be seen, there is a similar product selectivity distribution between standalone LSF, LSF@5B₂O₃ and LSF@10B₂O₃, where there is a significant amount of CO₂ formed with the selectivity between 30 to 40% and the ethylene + syngas selectivity remained at around 56%. As the B₂O₃ loading increased to 20 wt%, the ethylene + syngas selectivity sharply increased to 88.8% and the CO₂ selectivity drops to only 3.03%. Moreover, the H₂/CO ratio obtained with LSF@20B₂O₃ was 0.89/1, very close to the H₂/CO = 1/1 that is required for the hydroformylation reaction, indicating that fewer downstream cost would be needed in the case of direct ethane oxidation for the hydroformylation reaction. The effects of WHSV were also investigated. As shown in Fig. 5(d), with the increase of WHSV, the selectivity to ethylene + syngas only increased slightly, but the conversion rate of ethane decreased significantly. Given that the maximum ethylene + syngas yield occurs at 700 °C and 4.8 h⁻¹, this temperature and WHSV will be used for testing in the subsequent section unless otherwise specified.

Fig. 5 (a) Ethane conversions of LSF with different B₂O₃ loadings with respect to reaction temperature. (b) The yield of ethylene + syngas with respect to reaction temperature. (c) Product selectivities and ethane conversions of LSF with different B₂O₃ loadings at 700 °C. (d) Product selectivities and ethane conversions of LSF@20B₂O₃ at WHSV values of 4.8, 7.8 and 11.8 h⁻¹ at 700 °C.

To validate the influence of B₂O₃ on the reduction performance of the catalyst, the reducibility of LSF and 20% B₂O₃ supported catalysts was examined by H₂-TPR. The results, depicted in Fig. 6, showed three prominent reduction peaks for LSF at 632 °C, 680 °C, and 833 °C, respectively. Redox catalysts employ lattice oxygen to convert hydrocarbons and the first peak could be ascribed to the reduction of Fe⁴⁺ to Fe³⁺, while the latter peak may be associated with the reduction of Fe³⁺ to Fe²⁺.^16,35,36 For the B₂O₃ supported LSF catalyst, The reduction peaks at 632 °C and 680 °C have disappeared, leaving only the reduction peaks at around 833 °C. The introduction of boron moves the reduction peak to a higher temperature and concurrently decreases the number of reduction peaks. The XPS peak of Fe is shown in Fig. S2,† in which an increase in Fe³⁺ can be observed, which also confirms the enhancement of the signal of the reduction peak at high temperatures, which plays a facilitating role in the reaction at high temperatures. Thus, it can be seen that the presence of boron inhibits the reduction of LSF at low temperatures and is consistent with the activity trend as shown in Fig. 5.

Fig. 6 H₂-TPR analysis of (a) LSF and (b) LSF@20B₂O₃.

The stability of the B₂O₃ supported LSF catalyst for direct oxidation of ethane was demonstrated by conducting a 400 min test at 700 °C. The experimental conditions were C₂H₆ : O₂ : Ar = 1 : 0.5 : 3, with a WHSV of 643 mL h⁻¹ g_cat⁻¹. The ethane conversion and product selectivity with respect to reaction time are shown in Fig. 7a. As the reaction time increases from 10 min to 60 min, the C₂H₆ conversion gradually decreases from 60% to 57% and was stabilized after 60 min, while the ethylene + syngas selectivity remains around 90%. It was noted that the ratio of H₂/CO is also almost 1/1. This again indicates that the direct ethane oxidation can produce ethylene + syngas that is beneficial for industrial processes.

Fig. 7 (a) Product distributions and ethane conversions obtained on LSF@20B₂O₃. Reaction time = 400 min; temperature = 700 °C. (b) XPS profiles of O 1s before and after the 400 min catalyzed reaction. (c) XPS profiles of B 1s before and after the 400 min catalyzed reaction. (d) XRD comparison of LSF@20B₂O₃ before and after stability tests.

The catalyst after 400 min of ethane direct oxidation experiment was labeled as used-LSF@20B₂O₃. The XPS scans of O and B elements before and after the reaction are shown in Fig. 7b. It could be seen that the XPS of O on the surface of used-LSF@20B₂O₃ and fresh-LSF@20B₂O₃ is generally consistent. As shown in Fig. 7c, the B peaks with signals of 196.5 eV and 199 eV increased slightly on used-LSF@20B₂O₃. From the previous analysis, it could be seen that the peaks at these two positions correspond to Sr₃B₂O₆. The relative increase of the Sr₃B₂O₆ characteristic peaks suggested that B₂O₃ is gradually vaporized from the catalyst surface during prolonged catalytic reactions, but the catalyst activity is still steady within the time of investigation. Fig. 7d presents the XRD patterns of the samples before and after the reaction. Comparison of these patterns shows only minor changes in the positions and intensities of the characteristic XRD peaks, further demonstrating the robust stability of the catalyst.

To further explore the effect of the La/Sr ratio on the catalytic performance, we have also prepared and tested LSF@20B₂O₃ with La/Sr = 1, 9/1, 8/2, 7/3, and 6/4. The XRD results obtained for these catalysts are illustrated in Fig. 8a. When there is no Sr in the LSF, the B₂O₃ loaded on the catalyst reacts with La and Fe and forms a substantial amount of LaBO₃ and FeBO₃. With Sr doping into the perovskite, B₂O₃ more preferentially reacts with Sr and forms Sr₃B₂O₆. Fig. 8b and c show the trend of the catalytic performance with the La/Sr ratios at 700 °C. As shown in Fig. 8b, increasing the Sr content from 0 to 20% increases the ethylene + syngas selectivity from 76.9% to 88.8% and the ethane conversion from 41.9% to 62.3%. And the highest ethylene + syngas yield of 55.3% could be obtained with La_0.8Sr_0.2FeO₃@20B₂O₃. Further increasing the Sr content from 20% to 40% decreases the ethylene + syngas selectivity from 88.8% to 78.8% and the ethane conversion from 62.3% to 44.5%, and the CO/H₂ ratio also decreases significantly from 1.12 to 0.67.

Fig. 8 (a) XRD patterns of LSF@20B₂O₃ with different La/Sr ratios. (b) Product selectivities and ethane conversions of LSF@20B₂O₃ with different La/Sr ratios at 700 °C. (c) The CO + H₂ + C₂H₄ yields of LSF@20B₂O₃ with different La/Sr ratios at 700 °C.

B₂O₃ preferentially forms Sr₃B₂O₆ with Sr. When the Sr content is greater than 0.2, B₂O₃ starts to form LaBO₃ with La. The formation sequence of Me_xB_yO_z is FeBO₃–Sr₃B₂O₆–LaBO₃. Meanwhile, it can be seen that the catalytic effect of Sr₃B₂O₆ is the most significant. The catalytic effect of Sr₃B₂O₆ is the most significant, which is related to the type of metal. Therefore, a decrease in ethane conversion and ethylene + syngas yield was observed when the Sr content exceeded 0.2.

Conclusion

In this work, we have developed a B₂O₃ loaded La_1−xSr_xFeO₃ catalyst for the direct oxidation of ethane into a hydroformylation-ready gas, namely ethylene and syngas with H₂/CO close to 1/1. As confirmed by XRD and TEM, the catalyst has a core–shell structure with a B₂O₃ and Sr₃B₂O₆ layer encapsulating a solid LSF substrate. The B₂O₃ loading can substantially inhibit the formation of CO₂ as an overoxidized product. At a reaction temperature of 700 °C and a B₂O₃ loading of 20 wt%, the catalyst demonstrated excellent catalytic activity, achieving an ethane conversion rate of 62.3% and an ethylene + syngas selectivity of 88.6% with a H₂/CO ratio close to 1/1. The effects of the B₂O₃ loading and La/Sr ratio on catalyst performance were further investigated and optimized. The optimized La_0.8Sr_0.2FeO₃@20B₂O₃ catalyst showed good stability in terms of catalytic performance and material structure in the long-term test. Overall, the LSF@B₂O₃ catalyst can convert ethane into ethylene and syngas in a single step and is promising in terms of industrial applications.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2022YFA1504701 and 2022YFB4101900), the National Natural Science Foundation of China (22208104), the Shanghai Yangfan Program (22YF1410300) and the Shanghai Chenguang Program (21CGA35).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cy01268b

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