English

• Epoxides are important intermediates in the synthesis of various kinds of fine chemicals, pharmaceuticals, perfumes, and polymers [1–4]. For example, the cycloaddition of epoxides with CO₂ produces cyclic carbonates [5,6], which are widely adopted as solvent in lithium batteries and monomer for the preparation of polycarbonates [7]. Epoxidation of olefins is a straightforward method for epoxide synthesis [8–11]. In view of the key role of epoxides in the chemical industry, it is highly desired to develop efficient and green epoxidation processes.

  In this respect, Au [12–14], Ag [15–17], Co [18,19], Fe [20], Mo [21,22], Ce [23] and Mn [24] based catalysts have been employed in the epoxidation of alkenes [25,26]. Heterogeneous catalysts, such as Ag/α-Al₂O₃ [27] and gold nanoparticles supported on layered double hydroxide were also reported to be efficient catalysts for the epoxidation of styrene [28]. In transition metal-based catalysts, the heme-like structures constitute an important part, with the metal-N4 site as the catalytic center, since the understanding of cytochrome P450 function in 2000 [29]. However, it was found that metal-N4 complexes (e.g., Fe-Porphyrin, Mn-Phthalocyanine) [30,31] went through dimerization and deactivation under oxidative reaction conditions. This issue limited their further applications.

  In recent decades, various heterogeneous catalysts containing heme-like structures, adopting immobilization [32,33] or crosslinking strategy [34,35] were reported. These heterogenization methods improved the stability of catalysts. On the other hand, the metal-N4 site could be constructed by the coordination of metal on the N-contained supports. Typically, single-atom catalysts could be obtained by pyrolysis of nitrogenous precursors in the presence of metal precursors at elevated temperatures to construct well-dispersed metal centers on N-contained support, e.g., C₃N₄ [36]. In principle, these systems could maximize the catalytic performance. In this term, the Ag-C₃N₄ [26], as well as the Fe₂-C₃N₄ [37] were developed with good epoxidation activity. However, other transition metals were rarely investigated in the target epoxidation as well as the subsequent cyclic addition with CO₂ to cyclocarbonates.

  Herein, the well-dispersed Mn catalyst supported on nitrogen-rich g-C₃N₄ (Mn/g-C₃N₄) was firstly adopted in the epoxidation and subsequent CO₂ cycloaddition. The Mn/g-C₃N₄ exhibits excellent performance in the epoxidation of aromatic olefins (up to 91% conversion, 93% selectivity), and the cycloaddition of styrene epoxide and CO₂ to carbonates (88% conversion, 89% selectivity in the presence of ammonium salts). Moreover, it was inferred that the Mn-N site catalyzes the epoxidation by oxygen transferring from Mn-O species to the olefin, formed via the first tBuOOH addition to the Mn-N site and followed by the heterolytic cleavage of the tBuOO-Mn-N site. Furthermore, the catalyst was applied to promote the sequent cycloaddition of the epoxides with CO₂. This thus provided a new and high-yield heterogeneous system to synthesize cyclic carbonates from olefins.

  The highly-dispersed Mn supported on pyrrolic N-rich graphitic carbon nitride (Mn/g-C₃N₄) was synthesized via an impregnation-calcination method (Fig. 1a). First, melamine (M), cyanuric acid (CA), and adenine (A) were self-assembled in a homogeneous system via hydrogen bonding. Then Mn²⁺ coordinated with the self-assembled polymer. Finally, the well-dispersed Mn/g-C₃N₄ was obtained by calcination of Mn²⁺ loaded polymer in the N₂ atmosphere (see details in the Experimental Section in Supporting information).

  Figure 1

  

  Figure 1.  (a) Schematic illustration of the preparation of well-dispersed Mn/g-C₃N₄. (b) TEM image, (c) HRTEM image, (d) HAADF-STEM image, and corresponding EDX elemental mapping of Mn (e), C (f), N (g) of Mn/g-C₃N₄(3).

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  The crystalline structure of the obtained g-C₃N₄ and Mn/g-C₃N₄ materials were first examined by X-ray diffraction (XRD). As shown in Fig. S1 (Supporting information), the patterns of g-C₃N₄ and Mn/g-C₃N₄(3), showed a high degree of resemblance, both characterized by a dominant peak at 2θ = 27.4° corresponding to the (002) interlayer reflection of a graphitic structure, as well as a weak diffraction peak at about 13°, attributed to the in-plane repeating units of heptazine [38].

  Notably, typical diffraction peaks of Mn species (either metallic or oxides) were not observed, further eliminating the existence of Mn-containing crystalline species and indicating the high dispersion of Mn in the g-C₃N₄ framework. In addition, the Fourier transform infrared (FT-IR) spectra of g-C₃N₄, and Mn/g-C₃N₄(3) (Fig. S2 in Supporting information) evidences the condensed aromatic C—N—C networks [39]. The band at 808 cm⁻¹ together with the bands in the range of 1100–1600 cm⁻¹ were present, attributed to the breathing mode of tri-s-triazine units and the stretching modes of C—N heterocycles.

  The high-resolution transmission electron microscope (HRTEM) was further employed to determine the morphology of the prepared catalyst. As shown in Figs. 1b and c, and Fig. S4 (Supporting information), g-C₃N₄ and Mn/g-C₃N₄(3) displayed a curved and flake-like structure. No nanoparticles were observed in the transmission electron microscopy (TEM) as well as high-resolution TEM (HR-TEM) images, supporting the high degree of Mn dispersion. The corresponding energy-dispersive X-ray spectroscopy (EDX) mapping images (Figs. 1e-g) reveal that Mn, C, and N elements are homogeneously distributed in the entire g-C₃N₄ framework. Furthermore, the quantitative measurement by inductively coupled plasma mass spectrometry (ICP-MS) indicates ~2.56 wt% manganese was implanted in the catalyst.

  The chemical compositions and electronic configurations of the as-synthesized Mn/g-C₃N₄(3) catalyst were characterized by X-ray photoelectron spectroscopy (XPS). For C 1s spectra in g-C₃N₄ and Mn/g-C₃N₄(3) (Fig. 2a), two distinct peaks at 284.8 eV and 288.0 eV are observed, attributed to sp² C—C bonds and sp² hybridized carbon in N-containing aromatic ring (N—C═N) [40,41]. The observed N 1s spectra of Mn/g-C₃N₄(3) (Fig. 2b) was deconvoluted into four fitted peaks located at around 398.2, 399.9, 400.4, and 403.7 eV, respectively. These peaks are attributed to pyridinic N, pyrrolic N, graphitic N, and π-excitations, respectively [42,43]. Compared with pure g-C₃N₄, the binding energy of pyrrolic N in Mn/g-C₃N₄(3) shifts up by 0.4 eV to 399.9 eV, suggesting that the pyrrolic N is most likely to provide coordination sites for Mn atoms to form Mn-Nx moieties. The Mn 2p spectra of Mn/g-C₃N₄(3) was displayed in Fig. 2c, with one strong peak at 641.2 eV as well as a satellite peak at 645.8 eV detected. The characteristic feature of Mn 2p_3/2 peak for Mn/g-C₃N₄(3) is close to that of Mn^Ⅱ, suggesting that the valence state of the Mn species in Mn/g-C₃N₄(3) is +2 [36].

  Figure 2

  

  Figure 2.  High-resolution XPS spectra of (a) C 1s, (b) N 1s, (c) Mn 2p regions for Mn/g-C₃N₄(3).

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  We investigated the catalytic performance of as-prepared catalysts for epoxidation of the model substrate styrene. The conversion of styrene and the selectivity of products are shown in Fig. 3. Using TBHP (5.0~6.0 mol/L in decane) as the oxidant, the Mn/g-C₃N₄(3) sample shows unique and superior catalytic performance (91% conversion, 93% selectivity) toward epoxidation without any additives, along with trace benzaldehyde. Comparatively, when g-C₃N₄ was adopted as the catalyst, the catalytic activity was quite poor, with only 14% styrene oxide obtained. Thus, the well-dispersed Mn played a non-negligible role in the epoxidation. The content of N in Mn/g-C₃N₄ was optimized by varying the molar ratio of MnCl₂ to adenine in the Mn/CA-A-M precursor. With N content increasing, the yield of styrene oxide showed a volcano shape following this order: Mn/g-C₃N₄(3) (85%) > Mn/g-C₃N₄(1) (56%) > Mn/g-C₃N₄(6) (39%). Furthermore, the prepared Mn NPs/g-C₃N₄(3) and MnCl₂ were applied in the epoxidation of styrene. By contrast, only trace or 14% yields of the epoxide were presented. These combined revealed the importance of well-dispersion of Mn and g-C₃N₄ framework in the epoxidation.

  Figure 3

  

  Figure 3.  Catalytic activities for selective oxidation of styrene. Reaction condition: styrene (0.5 mmol), Cat. (7.0 mg), TBHP (5~6 mol/L in decane) (2 equiv.), solvent (2 mL), 100 ℃, 8 h. Conversion and selectivity were determined by GC analysis using dodecane as the internal standard.

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  In addition, various oxidants, such as TBHP, O₂, and H₂O₂ were screened. The catalytic activities of these oxidants toward styrene epoxidation are illustrated in Supporting information (Table S3, entries 1–3). Compared with TBHP, both O₂ and H₂O₂ showed inferior catalytic activity under same experimental conditions (trace to 4%). Therefore, TBHP was chosen as the oxidant for the epoxidation of styrene catalyzed by Mn/g-C₃N₄(3). Afterwards, the catalytic epoxidation using different solvents was investigated and the results are present in entries 4–6 (Table S3). The yields obtained were in the order DCE > MeCN > Heptane > EtOH, indicating that DCE is the optical solvent.

  The influence of reaction temperature is shown in Fig. 4a. The conversion showed a steep increase from 6% to 91%, along with the increase of selectivity of styrene oxide from 50% to 93%, when the temperature increases from 30 ℃ to 100 ℃. Further increasing the reaction temperature to higher than 100 ℃, the decomposition of TBHP might proceed prior to the target oxidation, which led to a decrease in the styrene conversion. Fig. 4b showed the variation in conversion and selectivity with time. In the range of 2–8 h, the conversion increases rapidly from 22% to 92%. Further extending the reaction time to 12 h results in an obvious decrease in the selectivity from 93% to 73%, owing to the side reaction of styrene oxide.

  Figure 4

  

  Figure 4.  Effect of temperature (a) and reaction time (b) on Mn/g-C₃N₄(3) catalyzed epoxidation of styrene. (c) Recycle test of Mn/g-C₃N₄(3) for epoxidation of styrene. Reaction conditions: styrene (0.5 mmol), Mn/g-C₃N₄(3) (0.65 mol%), TBHP (2 equiv.), DCE (2 mL), 100 ℃ and 8 h.

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  In addition, the effect of catalyst/styrene mole ratios on the styrene epoxidation was investigated (Fig. S7a). The selectivity slightly increased with the increasing dosage of Mn/g-C₃N₄(3), up to 94% obtained with 2.5 mol% of catalyst. The effect of TBHP/styrene ratio on epoxidation was further examined (Fig. S7b). As the molar ratio increased to 2, the conversion increases rapidly from 25% to 91%. Thus, the optimum conditions can be summarized as follows: Mn/g-C₃N₄(3) (0.65 mol%), TBHP (2 equiv.), 100 ℃, 8 h.

  The benefit of a heterogeneous catalyst compared to a homogeneous one is its easy separation and recycling. The stability of the catalyst was tested by four-time recycling under the same reaction conditions. As shown in Fig. 4c, the catalyst could be reused without significant loss of reactivity and selectivity. Moreover, the ICP-MS result showed that less than 0.3 ppm Mn ions were detected in the supernatant fluid after centrifugation, indicating good stability of the catalyst. Meanwhile, to probe the near-surface electronic structure of recovered Mn/g-C₃N₄(3), XPS spectra was collected (Fig. S5b). It was observed that the intensities of the Mn 2p3/2 peak at 641.2 eV as well as the satellite peak at 645.8 eV attributed to Mn (Ⅱ) were slightly lower than the pristine sample, demonstrating that the local Mn center on the surface went through limited changes. This might be explained by possible oxidation of Mn^Ⅱ to Mn^Ⅲ in the presence of oxidant.

  To test the substrate scope of Mn/g-C₃N₄(3), a series of aromatic olefins with different substituents were screened under identical conditions. The results are presented in Scheme 1. Various mono-substituted styrene with either electron-donating or electron-withdrawing groups were converted with moderate to good yields (2a-2i). In general, higher selectivity and yields were obtained for electro-deficient substrates, except for substitution by NO₂ group (2g, 43% yield) or othro-substitution (2h-2i, 39%–41% yields). While for the aliphatic alkenes, only trace amounts of desired products were detected (2j and 2k).

  Scheme 1

  

  Scheme 1.  Chemical epoxidation of different alkenes with Mn/g-C₃N₄(3) catalyst. Reaction conditions: Olefin (0.5 mmol), Cat. (0.65 mol%), TBHP (2 equiv.), DCE (2 mL), 100 ℃, 8 h. Isolated yields. ^a NMR yield. Selectivity of 2 determined by NMR analysis using 1,1,2,2-tetrachloroethane as the internal standard.

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  Furthermore, in terms of the development of carbon dioxide as C1 building block in the synthesis of valuable chemicals, synthesis of cyclic carbonates from styrene epoxide and CO₂ has been explored using the same catalyst. Styrene-derived carbonate was afforded with 88% conversion of styrene epoxide and 89% selectivity, as shown in Scheme 2 and Table S4 (Supporting information). The activity might originate from the basicity of C₃N₄ and the Lewis acidic Mn site. To better clarify the role of Mn-C₃N₄(3) in CO₂ cycloaddition, the strength of the basic and acidic sites on the catalyst were assessed by CO₂-TPD (temperature programmed desorption) and NH₃-TPD. As shown in the CO₂-TPD profile (Fig. S8 in Supporting information), the main peak around 207 ℃ indicated the presence of weak basic sites, while the small peaks around 378~460 ℃ correspond to medium basicity of Mn/g-C₃N₄. Besides, the main peak around 211 ℃ in the NH₃-TPD signal suggested weak acidic sites. According to previous studies [44], the catalyst combined with acidity and basicity could effectively promote the activation of both CO₂ and epoxides. These combined elucidated the bifunctional role of Mn/g-C₃N₄(3) in epoxidation as well as CO₂ cycloaddition.

  Scheme 2

  

  Scheme 2.  Two-steps synthesis of styrene carbonate. Reaction conditions: (1) styrene (0.5 mmol), (2) styrene epoxidation (1 mmol). Isolated yields. Selectivity of 2 and 3 determined by GC analysis using dodecane as the internal standard.

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  To further verify the important role of the highly dispersed Mn-N site in epoxidation, the poisoning experiment using KSCN was carried out. Normally, SCN⁻ can strongly bind to metal centers. The strong coordination enables the block of metal sites. As shown in Scheme S1 (Supporting information), after the addition of SCN⁻, the yield of styrene oxide decreases from 85% to trace along with a significantly lower conversion. Therefore, Mn-Nx sites should play a predominant role in the catalytic process.

  Moreover, in Mn-based homogeneous catalytic systems, Mn(Ⅳ)-oxo species have been frequently reported as the active intermediate in the epoxidation of olefins [45,46]. As a heterogeneous model of Mn-Nx in the Mn/g-C₃N₄(3) framework, Mn-alkyl peroxo species could be facially formed after activation of the peroxide by manganese center. The possible mode of MnO—O cleavage was investigated adopting cumyl hydroperoxide (CHP) probe. The resulting products of the probe reaction could help identify the homolytic or heterolytic cleavage pathway of MnO—O bond [47,48]. As demonstrated in Scheme S2 (Supporting information), cumyl alcohol directs to the heterolytic O—O bond cleavage, with acetophenone corresponding to homolytic O—O bond cleavage. Compared to the results without catalyst, cumyl alcohol was found to be the major product in the system catalyzed by Mn/g-C₃N₄, which directed to a heterolytic O—O bond cleavage. Based on the results, a possible reaction mechanism was inferred as Scheme S3 (Supporting information). The Mn-Nx site catalyzes the epoxidation by oxygen transfer between the olefin and Mn-O species, which is formed via the ^tBuOOH addition to the Mn-N site and followed by the heterolytic cleavage of the tBuOO-Mn-N site.

  In summary, inspired by the bio-enzyme (cytochrome P450) catalytic systems for the epoxidation of olefins and the fact that well-dispersed active centers can maximize the catalytic performance, we have synthesized highly-dispersed Mn/g-C₃N₄ materials. The as-prepared Mn/g-C₃N₄ catalyst exhibits good catalytic performance for olefin epoxidation with excellent conversion (up to 91%), high selectivity (up to 93%), broad substrate scope as well as outstanding cyclic stability. Furthermore, the catalysts were utilized to promote the cycloaddition of the epoxides with CO₂, laying foundations for a promising industrial process of environmentally benign chemical fixation of CO₂.

  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.

  Acknowledgments

  We are grateful for the financial supports from the National Natural Science Foundation of China (Nos. 21633013 and 22102197), Jiangsu Province Natural Science Foundation (No. BK20211096) and the Science Fund of Shandong Laboratory of Advanced Materials and Green Manufacturing (Yantai, No. AMGM2021F07).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108269.

  
  
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• 

  Figure 1  (a) Schematic illustration of the preparation of well-dispersed Mn/g-C₃N₄. (b) TEM image, (c) HRTEM image, (d) HAADF-STEM image, and corresponding EDX elemental mapping of Mn (e), C (f), N (g) of Mn/g-C₃N₄(3).

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  Figure 2  High-resolution XPS spectra of (a) C 1s, (b) N 1s, (c) Mn 2p regions for Mn/g-C₃N₄(3).

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  Figure 3  Catalytic activities for selective oxidation of styrene. Reaction condition: styrene (0.5 mmol), Cat. (7.0 mg), TBHP (5~6 mol/L in decane) (2 equiv.), solvent (2 mL), 100 ℃, 8 h. Conversion and selectivity were determined by GC analysis using dodecane as the internal standard.

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  Figure 4  Effect of temperature (a) and reaction time (b) on Mn/g-C₃N₄(3) catalyzed epoxidation of styrene. (c) Recycle test of Mn/g-C₃N₄(3) for epoxidation of styrene. Reaction conditions: styrene (0.5 mmol), Mn/g-C₃N₄(3) (0.65 mol%), TBHP (2 equiv.), DCE (2 mL), 100 ℃ and 8 h.

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  Scheme 1  Chemical epoxidation of different alkenes with Mn/g-C₃N₄(3) catalyst. Reaction conditions: Olefin (0.5 mmol), Cat. (0.65 mol%), TBHP (2 equiv.), DCE (2 mL), 100 ℃, 8 h. Isolated yields. ^a NMR yield. Selectivity of 2 determined by NMR analysis using 1,1,2,2-tetrachloroethane as the internal standard.

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  Scheme 2  Two-steps synthesis of styrene carbonate. Reaction conditions: (1) styrene (0.5 mmol), (2) styrene epoxidation (1 mmol). Isolated yields. Selectivity of 2 and 3 determined by GC analysis using dodecane as the internal standard.

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