Heterogeneous hydroformylation of internal alkenes over a defect-laden hexagonal BN supported RhCo alloy: reaction performance modulated by N vacancies

Abstract

Rhodium-alloyed catalysts with both high activity and stability hold great promise for the hydroformylation of alkenes. Here, we report a rhodium–cobalt alloy assembled on hexagonal boron nitride nanosheets with abundant N vacancies through a simple one-pot impregnated approach (RhCo/dh-BN), in which vacancies promote metal dispersion and alloy formation, and improve the performance of internal heterogeneous hydroformylation reaction. According to FTIR, XRD, BET, TEM and EPR characterization suggest that N vacancies are constructed on boron nitride and RhCo alloy anchors, while XPS and STEM are used to characterize the structural and electronic properties as well as the morphology of the RhCo alloy. RhCo/dh-BN exhibits good catalytic activity over a wide substrate scope for various aliphatic and aromatic alkenes including internal and terminal ones. As an example, for the 2-octene hydroformylation reaction, the nonanal yield is 97% with a TOF of 923 h⁻¹. In addition, the catalyst could be reused up to five times under the same reaction conditions without loss of activity.

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1. Introduction

Hydroformylation is a typical atom-economical reaction,^1–3 and the product aldehydes are valuable corresponding products and intermediates in the synthesis of bulk chemicals such as alcohols, esters, and amines, and can be used in the manufacture of plasticizers, surfactants, soaps and detergents.^4–7 Currently, the hydroformylation reaction has become the most widely used complex catalytic process in the chemical industry, with the production of chemicals based on this reaction exceeding 10 million tons per year.⁸ It is well known that terminal olefins react much faster than internal ones. The reaction of branched olefins requires significantly more severe reaction conditions or alternatively more active catalysts. Rh is the most active metal for the hydroformylation reaction, and although homogeneous Rh-based catalysts can achieve favourable catalytic activity and selectivity, they are not easy to separate and recover.^9–12 Immobilization of Rh-based active species onto solid support has proved to be an optimal strategy for integrating the advantages of homogeneous and heterogeneous catalysis.^13–15 However, the main problem of all these catalysts is the leaching of active metal species from the support in the presence of CO; the leaching of active metals during the hydroformylation reaction seriously hinders the development of heterogeneous catalysts. Therefore, it is important to improve the stability of active metal species.

Strategies to improve the stability of loaded metal catalysts include coating on oxides,^16–18 recoating on loaded metal catalysts,^19,20 creating metal–support interactions,^21,22 embedding metal particles,²³ alloying catalysts,²⁴ reactor design and process optimization, etc.²⁵ Among them, alloying can not only improve the stability of catalysts but also change the catalytic properties of active metals.^26–28 Alloys are substances with metallic properties synthesized by fusing two or more elements (at least one metal), and they are formed through the process of one component entering the structure of the base component (metal or metal compound).²⁹ Alloying precious metals (Pt, Rh, Ru, Au, etc.) with non-precious metals (Co, Ni, Fe, Cu, etc.), can greatly reduce the use of precious metals while maintaining high catalytic activity.³⁰ Therefore, alloy catalysts are expected to serve as ideal catalysts for hydroformylation reactions, combining high activity and stability.

Boron nitride (BN) is one of the most promising inorganic materials due to their high mechanical rigidity and high thermal and chemical stability, providing ample opportunities to explore a variety of diverse applications for BN materials.³¹ Hexagonal boron nitride (h-BN), also called white graphite, consists of atomically flat layers of alternating hexagonal B and N atoms, and the adjacent B atom and N atom form a σ covalent bond through sp² hybridization.³² The presence of weak van der Waals forces between N and B atoms in neighbouring layers further enhances the anisotropic features. The effect of introducing defects in h-BN to create a new metal-free heterogeneous catalytic candidate taking the form of defect-laden hexagonal boron nitride (dh-BN) has been confirmed, in which dh-BN can facilitate olefin bond activation.^33,34 Meanwhile, Zhu et al. reported Pt NPs assembled on dh-BN to embrace an interfacial electronic effect on Pt induced by 2-dimensional (2D) dh-BN with N-vacancies.³⁵

Herein, we report a RhCo alloy catalyst support on the vacancy-abundant dh-BN (RhCo/dh-BN) exhibiting remarkable catalytic activity and chemoselectivity for internal and terminal olefin hydroformylation reactions. The dispersion of the RhCo alloy is increased by changing the amounts of vacancies in dh-BN to improve the alloy utilization. In the hydroformylation reaction of 2-octene, RhCo/dh-BN shows the highest turnover frequency (TOF) of 923 h⁻¹ and 100% selectivity for nonanal. In addition, after six rounds of reaction, RhCo/dh-BN retains 97.9% of the original reactivity, and the selectivity for nonanal is still maintained, indicating its remarkable stability.

2. Results and discussion

2.1. Catalyst characterization

dh-BN with a high surface area and abundant vacancies is prepared via planetary ball mill treatment of commercial hexagonal boron nitride powders. The amounts of defects in dh-BN are controlled by the grinding time because of the chemical bond breaking and comminution by grinding. Different dh-BN samples with different ball-milling times (0, 20, 40, 120, 240, and 480 min) are prepared and named BN, dh-20BN, dh-40BN, dh-120BN, dh-240BN, and dh-480BN, respectively. In the FTIR spectra of all the samples (Fig. 1a), the peaks near 806 and 1391 cm⁻¹ are attributed to the out-of-plane bending vibration and in-plane stretching vibration of the B–N bond, which are the two intrinsic infrared absorption peaks of BN.³¹ The peak intensities of the OH bands at 3439 cm⁻¹ in dh-20BN, dh-40BN, and dh-120BN are higher than those of the peaks of BN (ref. 36) indicating a decrease in interlayer van der Waals forces and more active sites attached to the hydroxyl groups. With the increase of ball milling time, the OH peaks in dh-240BN and dh-480BN decrease significantly, which may be due to the damage to the BN structure by increasing the milling time. The X-ray diffraction (XRD) pattern further confirms the gradual increase in disorder with increasing milling time, as evidenced by the weakening and broadening of the dh-BN(002) peak (Fig. 1b). Longer grinding times lead to a decrease in crystalline grain size and overall particle size, which is caused by the shear forces during the grinding process.³⁷ As a result, more surfaces and edges are exposed with longer grinding time, which significantly increases the surface area from 44 m² g⁻¹ to 64 m² g⁻¹ (Fig. 1c). Furthermore, the result of N₂ adsorption–desorption analysis (Fig. S1 and Table S1†) shows that all BN samples exhibit mesoporosity. High-resolution transmission electron microscopy (HR-TEM) reveals defects in atomic scale (Fig. 1d–i), BN exhibits a regular hexagonal structure and defective structures can be observed on the HR-TEM images of dh-BN, and most of them are point vacancies, present at the edges of dh-BN. An electron paramagnetic resonance (EPR) measurement is employed to detect unpaired electrons in the samples (Fig. 2a). Compared with BN and defective dh-BN, a distinct signal is noticed at a g value of 2.003, suggesting the formation of nitrogen defects in the prepared samples.³⁸ XPS shows that the N/B atomic ratio in dh-BN is 0.796, indicating the presence of N vacancies along with the B-rich basic nature of the support. Accordingly, dh-BN containing N vacancies is successfully prepared by ball milling.

Fig. 1 Characterization of supports. (a) FT-IR spectra. (b) XRD spectra. (c) BET surface area and total pore volume values. I: BN, II: dh-20BN, III: dh-40BN, IV: dh-120BN, V: dh-240BN, and VI: dh-480BN. (d–f) HR-TEM images of BN. (g–i) HR-TEM images of dh-BN.

Fig. 2 Characterization of RhCo/dh-BN. (a) EPR results of BN, dh-BN, and RhCo/dh-BN. (b) FTIR spectra and (c) XRD spectra of dh-BN and RhCo/dh-BN. XPS spectra of B 1s (d), Rh 3d (e), and Co 2p (f).

The RhCo alloy catalyst is prepared experimentally by impregnating Rh into Co metal precursors with dh-120BN samples (RhCo/dh-BN). The weight loading is 0.18 wt% for both Rh and Co in RhCo/dh-BN, respectively, as determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The RhCo alloy catalyst is characterized by EPR, FTIR, XRD, XPS, TEM, and N₂ adsorption–desorption to investigate the catalyst structure. Fig. 2a compares the change of the EPR signal peak before and after metal loading, and the peak at g = 2.003 after metal loading is significantly reduced compared with that of dh-BN, which proves that the unpaired electrons are reduced, and the interactions between the metal and the unpaired electrons occur on the support. Meanwhile, the structure of RhCo/dh-BN and dh-BN are determined by FTIR and XRD. FTIR shows that the peak intensities at B–N–B (806 cm⁻¹) and O–H (3439 cm⁻¹) are reduced after metal loading (Fig. 2b), which indicates that there is an interaction between dh-BN and the metal. The XRD spectra of dh-BN and RhCo/dh-BN are shown in Fig. 2c and S3.† Both dh-BN and RhCo/dh-BN only have crystal surface diffraction peaks of BN, and no diffraction peaks of Rh and Co metals are observed in the XRD spectra, which proves that the metal is highly dispersed, and the XRD peaks of dh-BN are slightly shifted to the left after metal loading, which shows that the diffraction angle becomes smaller, indicating that the crystal spacing becomes larger by the Bragg diffraction equation. Combination of EPR, FTIR, and XRD suggests the existence of the interaction between the metal and the support.

The surface composition and chemical states of each catalyst are studied by XPS spectroscopy in the B 1s, N 1s, Rh 3d, and Co 2p core levels. One major peak at ∼190.7 eV appears in the B 1s XPS spectra. After deconvolution, two peaks at the binding energies (BEs) of 190.6 and 191.5 eV are assigned to the B–N and B–O species in dh-BN. In addition to the peaks corresponding to B–N and B–O in the order of increasing B 1s binding energy, an additional peak at 191.3 eV corresponding to B–M can be fitted in Fig. 2d.³⁹ Besides, it is found that the binding energies of B–N and B–O shift towards lower binding energies on the B 1s of RhCo/dh-BN. Furthermore, RhCo/dh-BN shows the same N 1s fitting peaks with dh-BN, but the binding energy increases by ∼0.2 eV for the overall peak position (Fig. S4†). On the other hand, the weak Rh and Co peaks can be detected in the Rh 3d and Co 2p spectra by repeat XPS scanning (Fig. 2e and f) because of the low Rh and Co loadings. For RhCo/dh-BN, the XPS spectra of Rh unveil strong peaks that emerge from Rh 3d_3/2 and Rh 3d_5/2. These peaks can be further divided into two doublets, indicating the existence of elemental Rh and Rh oxides.⁴⁰ The percentage of metallic Rh in RhCo/dh-BN nanohybrids is 69.3% by calculating the peak areas, showing that Rh mainly exists as metallic Rh. In addition, compared with Rh/dh-BN, the binding energy of Rh 3d in RhCo/dh-BN has a negative shift, which is undoubtedly due to the electron donor from Co to Rh due to the electronegativity difference (Co : 1.88 vs. Rh : 2.28). Similarly, characteristic signals of Co 2p in RhCo/dh-BN, including Co³⁺ and Co²⁺ and corresponding satellite (sat.) signals, further confirm their well-alloyed metal composition. The comparison of Co 2p (Fig. 2f) XPS data with those of the control samples shows the positive shift of the peak for RhCo/dh-BN to a higher binding energy (BE). The shift of binding energy confirmed the generation of the RhCo alloy.

Fig. 3 shows the transmission electron microscopy (TEM) images of RhCo/dh-BN. Highly monodisperse nanocrystals are uniformly anchored on the dh-BN edge (Fig. 3a). The distribution of Rh, Co, O, N, and B elements is investigated by EDX elemental mapping (Fig. 3b). It is noteworthy that the signals of Rh and Co particles are highly dispersed in the dh-BN edge region. Combined with the elemental maps of Rh and Co, the signals of Rh and Co mostly accompany each other, providing possible evidence for the formation of the RhCo alloy. The HR-TEM study of the morphology of RhCo/dh-BN (Fig. 3c) clearly shows that spherical RhCo nanoparticles adhere well to the surface of dh-BN. The enlarged HR-TEM image shows that the lattice spacing value of RhCo nanocrystals is 0.209 nm (inset in Fig. 3c), which is smaller than the theoretical lattice spacing value of the Rh(111) surface (0.220 nm), indicating that Co atoms have been doped into the Rh lattice to form an alloy phase. The particle size distribution histogram shows that the average RhCo alloy particle size is 1.85 nm (Fig. 3d). RhCo/dh-BN shows a smaller RhCo particle size and higher metal dispersion than RhCo/BN (Fig. S5†), indicating that dh-BN containing abundant vacancies is more favourable for metal dispersion. The above analyses suggest that the RhCo alloy is formed and anchored on the N vacancies of dh-BN.

Fig. 3 Morphology of RhCo/dh-BN. (a) HAADF-STEM image, (b) EDX elemental mappings, (c) HRTEM image and magnified HR-TEM image, and (d) particle size distribution histogram.

2.2. Catalytic performance

Then, a series of alloy-loaded catalysts are employed in the hydroformylation reaction; the catalytic performance is evaluated using 2-octene as a model substrate and compared with monometallic catalysts, and different support catalytic systems are shown in Table 1. The conditions used for the hydroformylation reaction of 2-octene are as follows: toluene as the solvent, and the reaction is carried out in syngas (CO/H₂ = 1 : 1) at 6 MPa for 12 h at an elevated temperature up to 90 °C.

Table 1 Catalyst conditions in the hydroformylation of 2-octene

Entry	Catalysis	Conversion (%)	Selectivity (%)		TOF (h^−1)
			Linear	Branched
Conditions: 2-octene (2 mmol), toluene (2 mL), CO/H2 = 3/3 MPa, catalyst (10 mg), stirring at 90 °C, using hexadecane as the internal standard for GC yield.
1	RhCo/dh-BN	97	0	100	923
2	RhCo/BN	27	28	13	106
3	RhCo/ZnO	42	56	44	401
4	RhCo/CaO	—	—	—	—
5	RhCo/Al2O3	—	—	—	—
6	RhCo/CeO2	—	—	—	—
7	RhCo/TiO2	—	—	—	—
8	Rh/dh-BN	25	0	100	239
9	Co/dh-BN	—	—	—	—
10	RhFe/dh-BN	—	—	—	—
11	RhCu/dh-BN	84	15	85	801
12	RhCe/dh-BN	26	0	100	248
13	RhNi/dh-BN	41	0	100	391

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Monometallic Rh and Co and bimetallic RhCo are loaded on the ball-milled BN support for the hydroformylation reaction, and the results show that Co/dh-BN is not catalytically active and that the RhCo alloy catalyst has a fourfold increase in TOF compared with the Rh monometallic catalyst. To verify the effect of the support on the catalytic activity, the different support-loaded Rh and Co bimetallic catalysts are tested (Table 1, entries 1–7), and the activity is observed only for RhCo/dh-BN, RhCo/BN, and RhCo/ZnO, where RhCo/dh-BN has 100% branched aldehyde selectivity and 97% 2-octene conversion. RhCo/dh-BN has a 9 times higher TOF value than RhCo/BN, and thus it can be found that the dh-BN support is more favourable for the hydroformylation reaction after ball milling. Next, different bimetallic catalysts loaded on dh-BN carriers with Rh as the stationary metal are prepared by the same preparation method (Table 1, entries 10–13), and the effects of different metals on the catalytic performance are investigated, and the results show that Fe plays an inhibitory role, and Cu, Ce, and Ni slightly promote the hydroformylation reaction.

Furthermore, compared to several other reported Rh heterogeneous catalysts applied to terminal alkenes (Table S2,† entries 1–10), the RhCo/dh-BN catalyst demonstrates significant advantages and excellent catalytic performance when applied to internal alkenes under milder conditions. When compared to other catalysts applied to internal alkenes (Table S2,† entries 11 and 12), the RhCo/dh-BN catalyst exhibits superiority over catalysts containing organic ligands in terms of hydroformylation, fully showcasing its superiority.

After determining the RhCo/dh-BN catalyst, the ball milling conditions and reaction conditions are optimized in Fig. 4 and S6–S8,† including the ball milling time, the type of dispersant, the amount of dispersant, the number of balls, the proportion of the RhCo metal, the amount of catalyst, the reaction time, pressure and temperature.

Fig. 4 Reaction optimization in the hydroformylation of 2-octene. (a and b) Changes of the ball mill conditions of ball-milling times and types of ball-milling dispersants; (c) Rh and Co molar ratio of the catalyst; (d–f) reaction time, pressure, and temperature, respectively.

The ball milling conditions are screened under the same conditions, in which the ball milling time has a greater influence on the catalytic activity, and the activity of the hydroformylation reaction exhibits a volcano curve with the extension of the ball milling time (Fig. 4a). The maximum value is reached when the ball milling time is 120 min. The conversion rate and branched selectivity are decreased by continuing to extend the ball milling time. In addition, the ball material ratio also has a large influence on the ball milling support (Fig. S6†); the experimental results show that with the increase of the amount of balls, the conversion of 2-octene gradually increased, and the selectivity of the branched aldehyde was unchanged. When the ball material ratio is 50 : 1, the branched-chain aldehyde reaches the maximum yield of 97% and continues to increase with the amounts of balls, and 2-octene conversion begins to show a downward trend. The dispersant is a major factor under ball milling conditions. Dispersants can play a variety of roles such as wetting, grinding aid, dilution, stability, etc. Experiments are carried out on different dispersants, including water, ethyl acetate, and a series of alcohols (Fig. 4b). The results show that alcohols are more favourable for the dispersion of BN during ball milling as well as for the breaking of chemical bonds and are more favourable for the hydroformylation reaction as the carbon number of alcohols increases, but the viscosity of the alcohols increases due to the continued growth of the carbon chains, which negatively affects the BN in the hydroformylation reaction. The amount of dispersant is also closely related to the activity of hydroformylation and reaches the optimal value when the ratio of dispersant to BN is 5 : 1 (Fig. S7†); too little dispersant cannot fully exert its wetting and grinding effects, while too much dilution of dispersant BN can prevent the ball milling process from achieving its maximum effectiveness.

The Co/Rh molar ratio in the catalyst also affects the reaction activity (Fig. 4c); with the increase of the Co/Rh molar ratio, the conversion of 2-octene increases, when the ratio of Co/Rh is 4/1, the conversion of 2-octene reaches the maximum, and when the ratio of Co/Rh continues to increase to 10, the conversion of 2-octene begins to show a decreasing trend. The presence of Co species increases the outer electron density of Rh species, which may affect the adsorption of reaction substrates.^41,42 As the Co/Rh ratio continues to increase, the exposed Rh sites on the surface of the alloy catalyst may be covered by a large amount of Co, leading to a reduction in active sites and subsequently decreased activity. The amount of catalyst utilized under reaction conditions plays a pivotal role in the process (Fig. S8†). The conversion rate of 2-octene increases with the increase in catalyst dosage. However, when the catalyst dosage is further increased from 10 to 20 mg, there is no significant intensification in the conversion rate, indicating that the catalyst reaches its maximum atomic utilization efficiency.

Subsequently, the reaction temperature is screened under the same conditions (Fig. 4d), and as can be seen from the figure, the conversion and aldehyde yield of the 2-octene hydroformylation reaction are substantially increased with the initial elevation of the reaction temperature. The elevated reaction temperature can promote the mass transfer and contact between the reaction substrate and the catalytically active sites to enhance the reaction rate. However, when the temperature is further increased, the isomeric aldehyde yield decreases, indicating that the further increase in temperature might increase the side reactions, and the equilibrium of the reaction might be affected by the increase in temperature.

In addition, we screen the reaction time under the same conditions (Fig. 4e), which shows that the conversion and aldehyde yield of the 2-octene hydroformylation reaction increased substantially with the increase in reaction time. At the reaction time of 1 h, the reaction time is short and most of the feedstock is still unreacted, and with the time extended, the conversion and aldehyde yield of the reaction is balanced, indicating that the reaction has reached the kinetic equilibrium. The extension of the reaction time has little effect on the 2-octene hydroformylation reaction, and the selectivity of the reaction has changed a little.

Finally, the effect of reaction pressure on the 2-octene hydroformylation reaction is also investigated under the same conditions (Fig. 4f). With the increase of the gas pressure in the reaction system, the conversion and aldehyde yield of the reaction increase to a certain extent, which is attributed to the fact that the process of the 2-octene hydroformylation reaction is a process of gas pressure decrease. Therefore, as the pressure increases, it helps the reaction to proceed in a positive direction. When the pressure is further increased, the side reaction at high pressure affects the yield and conversion of the reaction and also increases the cost and safety issues in the industry.

2.3. Scope of the substrates

The scope and limitations for the hydroformylation reactions of various alkenes into the corresponding aldehydes are discussed over RhCo/dh-BN. A range of alkenes, including functionalized substrates, as shown in Fig. 5, have good generalizability under optimal reaction conditions, converting the corresponding product aldehydes in good to excellent yields. Initially, we examine the generalizability of internal alkenes. Experimental results show that the catalytic system gave excellent yields (up to 97%) for cycloalkenes and short-chain and large-site-resistance internal alkenes (1–8), such as 2-pentene, 2-hexene, cyclopentene, cyclooctene, and 2,3-dimethyl-2-butene. Subsequently, we also examine the terminal aliphatic alkenes. Notably, excellent yields (up to 92%) are obtained for short-chain, long-chain, and large-site-resistance olefins (9–14), e.g., ethylene, 1-octene, 1-hexadecene, and 3,3-dimethyl-1-butene.

Fig. 5 Synthetic scope for hydroformylation. [a] Reaction conditions: alkenes (2.0 mmol), catalyst 10 mg, CO/H₂ = 3/3 MPa, toluene = 2 mL, 90 °C for 12 h, using hexadecane as the internal standard for GC yield. [b] Reaction conditions: alkenes (2.0 mmol), catalyst 10 mg, CO/H₂ = 3/3 MPa, toluene = 2 mL, 140 °C for 12 h, using triphenylmethane as the internal standard for NMR yield.

To further extend the utility of the hydroformylation reaction, the generality of various functionalized olefinic substrates is examined. Aromatic endoalkenes represented by vinyl toluene obtain an 84% yield for the corresponding aldehydes (15). Subsequently, the substitution of styrene with different functional groups as substrates can obtain the corresponding product aldehydes in 91–98% yields (16–24). The yield of the product corresponding to α-methyl styrene aldehyde (25) was 92%. With allylbenzenes and isobutyl benzene as the substrates, the hydroformylation reactions afford 91–97% yields (26–27). Alkenes containing biomass-active structures, methyl 5-allyl-3-methoxysalicylate (28), and pentafluorostyrene (29) are also found to be well reactive in yields up to 94 and 93%.

2.4. Stability of the RhCo/dh-BN catalyst

The biggest difference between heterogeneous catalysts and homogeneous catalysts is that they can be separated and reused. Therefore, the reusability of a catalyst is a key indicator for evaluating a heterogeneous catalyst. In this work, we use the hydroformylation of 2-octene as a model reaction and toluene as a solvent to investigate the reusability of the RhCo/dh-BN catalyst (Fig. 6a). The results show that the RhCo/dh-BN catalyst could be recycled at least 5 times without any significant change in the catalytic performance and without any significant decrease in the metal content by ICP-AES (Table S2†). A conversion of 94% is still obtained at the 5th use. In addition, thermal filtration experiments show that the yield of the product is 68% after 6 h. After the catalyst is removed from the reaction mixture by thermal filtration, the filtrate is allowed to continue to react for 6 h without any increase in yield, thus proving the multiphase nature of the catalyst (Fig. 6b).

Fig. 6 (a) The reusability of the RhCo/dh-BN catalyst for the reaction of the hydroformylation of 2-octene; (b) leaching experiment of the hydroformylation of 2-octene.

Conclusions

In summary, we have reported a research strategy for the RhCo alloy anchored on the defect-containing boron nitride by the defect sites, improving the performance of the alkene hydroformylation reaction. The experimental results show that RhCo alloy catalysts (RhCo/dh-BN) are successfully constructed, exhibiting an 8-fold increase in TOF compared with the defect-free BN. Moreover, the catalytic activity and selectivity of the catalyst do not decrease significantly after 5 cycles of recycling under the optimal reaction conditions, and the thermal filtration experiment proves that it is a multiphase reaction. In addition, RhCo/dh-BN can be applied to a wide range of hydroformylation substrates including cycloalkenes, aromatic olefins, halogenated olefins, etc., and provide the desired products with >85% conversion and >99% selectivity. This strategy developed a heterogeneous hydroformylation catalyst that is inexpensive, easily prepared, and can activate internal olefins under mild conditions, with high chemical selectivity and stability.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

F. Shi and X. J. Cui conceived the idea and supervised the project. B. W. Qiu designed and performed the experiments. D. C. He and B. Wang supported the XRD, STEM and BET analysis. S. M. Liu conducted the EPR experiment and analysis. B. W. Qiu, S. J. Liu and K. Zhao prepared the manuscript. All authors discussed the results and assisted during manuscript preparation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Sciences Foundation of China (21925207 and U22A20393), the Young Scientists Fund of the National Natural Science Foundation of China (No. 22102195), the Major Project of Gansu Province, China (23JRRA603), the Special Research Assistant Project of the Chinese Academy of Sciences and the Science and Technology Planning Project of Lanzhou City, Gansu Province, China (2023-1-15).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cy01252f

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