Importance of surface peroxo species in the epoxidation of cyclohexene by Mo-doped TS-1 and O₂ under solvent-free conditions

Abstract

The selective oxidation of cyclohexene (Cy) to cyclohexene oxide (Cy-ep) using O₂ remains challenging due to low epoxidation selectivity. In this work, a series of Mo-doped TS-1 (Mo-TS-1) catalysts were successfully synthesized for the epoxidation of Cy under solvent- and initiator-free conditions with O₂ as the oxidant. Among them, 5Mo-TS-1 exhibited high catalytic performance, achieving 43.5% Cy conversion and 50.6% selectivity toward Cy-ep. Additionally, valuable by-products such as 2-cyclohexen-1-ol (Cy-ol) and 2-cyclohexen-1-one (Cy-one) were obtained with yields of 28.0% and 21.4%, respectively. Since both Cy-ol and Cy-one are valuable intermediates in fragrance synthesis, over 43.5% of Cy was effectively converted into high-value products. Quenching experiments and Raman spectroscopy revealed that surface oxygen vacancies (O_v) facilitate the activation of O₂ to form ≡O_v-superoxo species, which abstract hydrogen from the allylic C–H bond of Cy to generate 3-cyclohexenyl radicals (Cy·). These radicals subsequently react with O₂ to form Cy–OO·, followed by hydrogen abstraction from another Cy molecule to yield 2-cyclohexene-1-hydroperoxide (Cy–OOH). A positive correlation between Cy–OOH and Cy-ep formation underscores the critical role of Cy–OOH in the epoxidation process. Furthermore, Raman spectroscopy confirmed the presence of ≡Mo-(η²-O₂) peroxo species on the catalyst surface, which preferentially attack the C=C bond of Cy to form Cy-ep. DFT calculations elucidated two distinct O₂ activation pathways: in pathway I, O₂ is activated at O_v sites to form ≡O_v-superoxo, which subsequently reacts with Cy to generate ≡O_v-peroxo, Cy–OOH, and Cy·. In pathway II, Mo(V/VI) sites either directly activate O₂ or react with peroxo intermediates (≡O_v-peroxo or Cy–OOH) to form ≡Mo-(η²-O₂). This species selectively epoxidizes the alkene bond in Cy to Cy-ep. Notably, the direct activation of O₂ at Mo(V/VI) sites bypasses the allylic oxidation route, thereby enhancing the epoxidation selectivity beyond the theoretical limit of 50.0%. This study provides new insight on the importance of surface superoxo and peroxo mediated by O_v and Mo(V/VI) in the epoxidation processes.

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1. Introduction

Cyclohexene oxide (Cy-ep) is an important intermediate, which has been extensively used for the production of epoxy resins, surfactants, pesticides, and dyestuffs.¹ However, the selective epoxidation of cyclohexene (Cy) to Cy-ep is challenging in that the simultaneous occurrence of allylic oxidation and epoxidation inevitably brings about a broad distribution of products [e.g. Cy-ep, 1,2-cyclohexanediol (Cy-diol), 2-cyclohexen-1-one (Cy-one), 2-cyclohexen-1-ol (Cy-ol), etc.].^2–4 Conventionally, peroxides such as H₂O₂, tert-butyl hydroperoxide (TBHP), etc. are usually used to epoxidize Cy to achieve high selectivity towards epoxide (Cy-ep + Cy-diol >90%).⁵ However, excess oxidants always increase the operation costs, and the presence of Cy-diol (b.p. 118–120 °C, 10 mm Hg) also increases the difficulties in subsequent separation from Cy-ep.^6–9 In this regard, developing an alternative catalyst for selective oxidation of Cy to Cy-ep by O₂ is essential.

Till now, various by-products such as Cy-diol, Cy-one, Cy-ol and 2-cyclohexene-1-hydroperoxide (Cy–OOH) have been suggested to coexist with Cy-ep during the epoxidation of Cy by O₂.^4,10,11 Given that the dissociation energy of the allylic C–H bond (83.9 kcal mol⁻¹) is significantly lower than that of the vinylic C–H (109.4 kcal mol⁻¹),¹² H in the allylic C–H bond of Cy is more readily abstracted by O₂, thus facilitating the formation of 3-cyclohexenyl radicals (Cy·). After that, O₂ reacts with allylic Cy· to form 3-cyclohexenylperoxy radicals (Cy–OO·), and then Cy–OO· abstracts H from the allylic C–H bond of another Cy to form Cy–OOH peroxo and Cy·.² The Cy–OOH peroxo and another Cy further react to produce Cy-ep and Cy-ol/Cy-one,¹³ thus inevitably resulting in the theoretical selectivity towards Cy-ep less than 50.0%.¹⁴ In this regard, constructing highly efficient oxidizing species to bypass the pathway of allylic oxidation of Cy is an effective method to enhance the selectivity higher than 50% in the epoxidation of Cy by O₂. Although the additives of ethylbenzene, isobutyraldehyde, etc. can effectively improve the oxidation selectivity towards Cy-ep (59%–80%) by O₂via the oxygen atom transfer (OAT) pathway, considerable low-cost by-products (e.g. acetophenone and isobutyric acid) always bring about problems in subsequent separations.^15,16 Searching for more efficient catalysts to epoxidize Cy to Cy-ep by O₂ without adding additives is therefore more desired.

Titanium silicalite-1 (TS-1) is a promising catalyst in the epoxidation reactions given its high reactivity and stability.^17,18 For instance, the isolated Ti(IV) readily reacts with 30 wt% H₂O₂ to generate ≡Ti–OOH peroxo, which then selectively epoxidizes olefins via the OAT pathway (selectivity >90%).¹⁷ However, the rapid decomposition of H₂O₂ to H₂O and O₂ by the crystalline non-framework Ti(IV) in TS-1 inevitably reduces the oxidation efficiencies of H₂O₂,¹⁷ which then results in the low conversion (<5%) of Cy by H₂O₂.^19,20 In addition, the hydrolysis of Cy-ep to Cy-diol by residual H₂O also reduces the yield of Cy-ep in the H₂O₂ system.^6–9 Although the in situ produced H₂O₂ on Au(0)/TS-1 in the presence of H₂ and O₂ is able to epoxidize propene with high selectivity (>90%),²¹ the low conversion (<6%) of propene also limits its applications. Till now, direct activation of O₂ on TS-1 has not been mentioned yet. Given the high efficiency of metal peroxo in the epoxidation of Cy,²² constructing active metal sites on TS-1 to generate peroxo species in the presence of O₂ should be an appropriate method to improve the epoxidation efficiencies of Cy.

Mo(VI) exhibits high reactivity in the epoxidation of olefins by alkyl hydroperoxides in that the formed ≡Mo-(η²-O₂) peroxo is favorable for attacking the olefinic C=C bonds to form the desired epoxides.^14,16,23 Given the similarity of the ionic radii of Mo(VI) (0.620 Å) to Ti(IV) (0.605 Å), Mo is an appropriate element to be doped in TS-1.^24,25 Actually, Mo(VI) will be transformed to low-valence Mo(IV/V) after being doped in TS-1 or Ti/SiO₂, and then exists in the form of four coordinated tetrahedral rather than octahedral coordination.^26–28 Given that the low-valence Mo(IV/V) sites can also effectively activate O₂ to form ≡Mo-peroxo species,^16,29,30 which then epoxidizes Cy via the OAT pathway,¹⁶ we thus hypothesize that the low-valence Mo(IV/V) in TS-1 can also directly activate O₂ to form ≡Mo-peroxo species, which then effectively epoxidizes Cy. Apart from Mo(IV/V), oxygen vacancies (O_v) can also be produced on the surface of catalysts during the Mo-doping processes,^31,32 which are also favorable for the adsorption and activation of O₂.^33–35 For instance, O₂ is readily activated to surface superoxo at O_v, which then nonselectively oxidizes the allylic C–H bond of Cy to Cy·.^16,36,37 On the other hand, Mo can also mediate peroxo to ≡Mo-(η²-O₂), which then epoxidizes Cy via the OAT pathway.³⁸

In this study, the Mo-doped TS-1 (Mo-TS-1) will be constructed via the hydrothermal method to epoxidize Cy under solvent and initiator-free conditions using O₂ as the oxidant. This catalyst exhibits high catalytic reactivity in Cy conversion (43.5%) and epoxidation selectivity (50.6%), indicating the successful formation of efficient oxidizing species to enhance the selectivity higher than the theoretical selectivity of 50%. Meanwhile, the corresponding yields of high-value by-products of Cy-ol and Cy-one reach up to 28.0% and 21.4%. The objectives of this study are therefore to (i) investigate the catalytic reactivity of Mo-TS-1 in epoxidation of Cy to Cy-ep by O₂, and then (ii) explore the corresponding oxidizing species on the surface of Mo-TS-1 and reveal the corresponding mechanisms in the selective oxidation of Cy.

2. Experimental section

2.1. Materials and chemicals

Tetraethyl orthosilicate (TEOS), tetrabutyl titanate (TBOT), tetrapropylammonium hydroxide (TPAOH), cetyltrimethylammonium bromide (CTAB), isopropyl alcohol (IPA), sodium molybdate dihydrate (Na₂MoO₄·2H₂O), cyclohexene (99%), 1,2-epoxycyclohexane (98%), 1,2-cyclohexanediol (98%), 2-cyclohexen-1-one (98%), 2-cyclohexen-1-ol (95%), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), acetonitrile (99%), dichloromethane (99%), N,N-dimethylformamide (99%), 1,4-dioxane (98%), tert-butyl alcohol (TBA, 99%), nitrotetrazolium blue chloride (NBT, 98%), hydroquinone (HQ, 99%), chloroform (CF, 99.8%), triphenylphosphine (PPh₃, 95%), butylated hydroxytoluene (BHT, 99%), tert-butyl hydroperoxide (TBHP, 70%), tert-butyl alcohol (TBA, 99%), CBrCl₃ (98%), benzoquinone (BQ, 97%), 2,4-hexadiene (2,4-HD, 95%), 1-bromo-2-cyclohexene (95%), 2,4-dimethylquinoline (2,4-DQ) and quinoline (Q) were purchased from Aladdin Reagent Co., Ltd. All reagents were used without further purification.

2.2. Catalyst preparation

TS-1 was synthesized according to the hydrothermal method described previously with some modifications.³⁹ 0.175 mL of TBOT was dissolved in 5 mL of IPA. Then, 2 mL of 2.0 M TPAOH was added dropwise to this TBOT/IPA mixture under vigorous magnetic stirring (800 rpm) and the resulting mixture was stirred for 0.5 h. Concurrently, a second solution was prepared by gradually adding 4.4 mL of 2.0 M TPAOH and 8.184 g of CTAB to 5 mL of TEOS under vigorous magnetic stirring (800 rpm), and this mixture was stirred for 0.5 h. Subsequently, the two mixtures were combined and stirred at 800 rpm for another 5 h. The combined gel was then heated at 353 K for 30 min to evaporate the IPA completely. The resulting mixture was transferred into a Teflon-lined autoclave and subjected to hydrothermal treatment at 443 K for 48 h. The solid product was recovered by centrifugation, washed thoroughly with ethanol, dried overnight at 353 K, and finally calcined at 823 K for 6 h.

In preparation of Mo-TS-1, a series of TS-1 supported Mo catalysts were prepared using the hydrothermal method.⁴⁰ Mo-TS-1 catalysts with nominal Mo loadings of 1.0, 3.0, 5.0, and 7.0 wt% were prepared via a post-synthetic hydrothermal modification method. In a typical procedure for preparing the 5Mo-TS-1 catalyst, a precisely weighed quantity of 0.052 g of Na₂MoO₄·2H₂O was first dissolved in 10 mL of deionized water under stirring. Subsequently, 1.0 g of the pre-synthesized TS-1 support was added to this solution. The resulting suspension was continuously stirred at room temperature for 2 hours to ensure initial impregnation. The mixture was then transferred into a 50 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and maintained at 443 K for 12 h in a forced-air oven to facilitate the hydrothermal insertion of Mo species. After cooling naturally to room temperature, the solid product was recovered by centrifugation, washed thoroughly with absolute ethanol and deionized water until the supernatant was neutral, and dried overnight at 353 K in a laboratory oven. The final step involved calcination in a muffle furnace: the dried powder was heated to 823 K at a ramp rate of 5 K min⁻¹ and held at this temperature for 6 h in static air to remove any residual organic species and stabilize the Mo sites. Based on the nominal weight percentage of Mo added, the resulting catalysts were denoted as xMo-TS-1 (where x = 1, 3, 5, 7).

2.3. Selective oxidation of cyclohexene

Selective oxidation of Cy by O₂ was carried out in a 50 mL reactor. Typically, 5.0 mL of Cy and 50 mg of catalyst were sequentially added into the reactor. The reaction mixture was then heated at 353 K for 6 h in the presence of 1.0 MPa O₂ with magnetic stirring. After reactions, the reaction mixture was filtrated by a 0.22 μm membrane, and the filtrate was used for the analysis of Cy, Cy-ep, Cy-one, and Cy-ol. The oxidation products were analyzed by gas chromatography (GC). In GC analysis, two samples (0.5 mL) were taken out after filtration and added to 5 mL of acetonitrile. One sample was then analyzed by GC, whereas the other was treated with PPh₃ to decompose Cy–OOH to Cy-ol.⁴¹ The amount of Cy–OOH is calculated by comparing the amount of Cy-ol in the two samples.

2.4. Chemical analyses and characterization

The quantitative analysis of the reaction products, including Cy, Cy-ol, Cy-one, and Cy-ep, was performed using an Agilent 8860 GC system equipped with a flame ionization detector (FID) and an HP-5 ms capillary column (30 m × 0.32 mm × 0.25 μm). The detailed GC operating conditions were as follows: the injector and detector temperatures were maintained at 250 °C and 280 °C, respectively; the carrier gas (N₂) flow rate was set at 1.0 mL min⁻¹ with a split ratio of 30 : 1. The oven temperature program was initiated at 60 °C (held for 0 min), ramped at 10 °C min⁻¹ to 110 °C (held for 2 min), and then increased at 25 °C min⁻¹ to a final temperature of 250 °C.

The crystalline structure and morphology of the synthesized catalysts were characterized by X-ray diffraction (XRD, D8-Focus, Germany) at a scanning rate of 5° min⁻¹ and scanning electron microscopy (SEM, Hitachi S4800), respectively. The Brunauer–Emmett–Teller (BET) surface area and porosity of the catalysts were measured by N₂ adsorption on a Micromeritics ASAP 2020 system. Raman spectroscopy (Renishaw inVia Reflex) was carried out to identify chemical bonds on the surface of Mo-TS-1 at an excitation wavelength of 532 nm. The oxidation states of Ti, O, and Mo on the surface of catalysts were determined by X-ray photoelectron spectroscopy (XPS, Pe PHI-1600ESCA) with monochromatic Al Ka radiation (1486.6 eV) and a base pressure of 1 × 10⁻⁸ Torr in the analytical chamber. The binding energies were calibrated according to the standard peak (C 1s) at 284.8 eV, and all experimental data were fitted using the software XPS peak 4.1. The elemental contents were determined by inductively coupled plasma (ICP) analysis on a Thermo iCAP 7400 instrument. The H₂ temperature programmed reduction (H₂-TPR) curve was determined using an automatic chemisorption instrument (AutoChem II 2920, American Mike Instrument). 100 mg of sample in Ar (30 mL min⁻¹) was purged at 300 °C for 30 minutes, and then cooled to room temperature. Reducing gas containing 5% H₂/95% Ar was used; the flow rate is 30 mL min⁻¹ and the heating rate is 10 °C min⁻¹, and the room temperature rises to 700 °C. A TCD detector was used to monitor the consumption of H₂. NH₃ temperature programmed desorption (NH₃-TPD) was performed on a chemisorption instrument. 100 mg of the dried catalyst was put in the sample test tube, purged for 2 h in an inert atmosphere of 300 °C, and cooled to 50 °C. In an NH₃/Ar atmosphere, it was adsorbed for 2 h to saturation, and then purged for 1 h in an Ar atmosphere of 50 mL min⁻¹, and then the temperature was increased from 50 °C to 700 °C at a heating rate of 10 °C min⁻¹ in an Ar atmosphere of 50 mL min⁻¹.

2.5. DFT calculations

DFT calculations were conducted to investigate the pathways involved in O₂ activation and H-abstraction during the oxidation of Cy. The CASTEP program in Materials Studio 7.0 was used, and the generalized gradient approximation proposed by Perdew–Burke–Ernzerhof (PBE) was selected for the exchange-correlation potential. The long-range van der Waals interaction was considered by a DFT-D correction. The cutoff energy for plane-wave expansion was set at 340 eV, and the forces were converged to be better than |0.01| eV Å⁻¹, and the van der Waals interactions were considered by performing dispersion-corrected DFT calculations. A 96 T cluster model (Si₉₅TiO₁₉₂) was selected as a standard structure of zeolite, which was extracted from the crystalline structure data in Materials Studio 7.0.

The calculation procedures were similar to those described previously,^14,42 and the adsorption energy of E_ads was defined as eqn (1):

E_ads = E_total − E_cat − E_molecules (1)

where E_total, E_cat, and E_molecules are assigned to the energies of adsorbates with the catalyst, pure catalyst, and free molecules, respectively.

The complete linear synchronous transit/quadratic synchronous transit (LST/QST) method was used to locate the transition states (TS) of the reactions. The energy barrier (E_barrier) of the reaction can be calculated according to the following equation eqn (2):

E_barrier = E_{transition state} − E_intermediate (2)

where E_transition state and E_intermediate represent the total energy of the TS and the total energy of the intermediate, respectively.

3. Results and discussion

3.1. Characterization of the synthesized catalysts

In XRD analysis, all the samples showed typical MFI structure characteristic peaks, without anatase-TiO₂ or other metal oxide peaks appearing (Fig. 1a),^39,43 indicating that these catalysts had high crystallinity and Mo doping has no effects on the MFI structure. SEM images show the morphologies of various TS-1 samples before and after being doped with different contents of Na₂MoO₄. TS-1 exhibits a coffin-like morphology with particle sizes of ca. 300 × 100 × 100 nm (Fig. 2a). Various Mo-TS-1 samples (1Mo-TS-1, 3Mo-TS-1, 5Mo-TS-1 and 7Mo-TS-1) exhibit an aggregated flat-plate morphology with the particle length in the range of 300–1000 nm (Fig. 2b–e). The elemental mapping of 5Mo-TS-1 indicates the good dispersion of O, Si, Ti and Mo on the surface of 5Mo-TS-1 (Fig. 2f–j). In addition, the BET surface areas of TS-1 and 5Mo-TS-1 are 363 and 60 m² g⁻¹, respectively (Table S1), indicating that the doped Mo apparently reduces the surface areas of TS-1. The N₂ adsorption–desorption isotherms of TS-1 and 5Mo-TS-1 indicate that the mesoporous structure of TS-1 has been changed from Type IV (5.16 nm) to Type II (17.00 nm) after being modified by Mo (Fig. S1).

Fig. 1 (a) XRD of synthesized TS-1, 1Mo-TS-1, 3Mo-TS-1, 5Mo-TS-1 and 7Mo-TS-1; XPS analyses of synthesized TS-1 and 5Mo-TS-1: (b) O 1s, (c) Mo 3d, and (d) Ti 2p.

Fig. 2 SEM images of (a) TS-1, (b) 1Mo-TS-1, (c) 3Mo-TS-1, (d) 5Mo-TS-1, and (e) 7Mo-TS-1; and (f–j) elemental mapping of O, Si, Ti and Mo on the surface of 5Mo-TS-1.

In Raman analysis, the characteristic bands at 145, 382, 515, 636 and 970 cm⁻¹ are attributed to the typical characteristic bands of TS-1 (Fig. 3a).⁴⁴ After being modified by Mo(VI), the characteristic bands of TS-1 shift to 135, 375, 505 and 630 cm⁻¹ with the disappearance of the 970 cm⁻¹ band (Fig. 3a), indicating that Mo species incorporate into the TS-1, primarily by substituting for Ti⁴⁺ sites, which introduces Mo–O–Ti bonds and creates significant lattice strain.²⁸ Furthermore, within the composite structure, interactions between the dispersed Mo species and the surrounding SiO₂ matrix, potentially forming Mo–O–Si linkages, cannot be ruled out and may contribute to the observed spectral changes.²⁸ Meanwhile, the corresponding characteristic bands of Mo–O in Na₂MoO₄ at 330, 837 and 896 cm⁻¹ shift to 298, 801 and 883 cm⁻¹ after Mo doping in TS-1, confirming that TS-1 has also changed the Mo–O bonding, and Mo–O–Ti is the primary bond of Mo–O (Fig. 3a).^32,45,46 In XPS analysis, the peaks at 530.2 and 533.3 eV are assigned to Ti–O and Si–O bonds of TS-1 (Fig. 1b).⁴⁷ The presence of the 531.2 eV peak indicates O_v on the surface of 5Mo-TS-1 (Fig. 1b).⁴⁸ The relative concentration of oxygen vacancies increases with Mo loading up to 5 wt%, after which it plateaus or even decreases slightly. This non-linear relationship strongly suggests that at lower loadings, Mo incorporates predominantly as isolated, framework-associated species that effectively generate and stabilize oxygen vacancies. The saturation effect at 5 wt% indicates that this is the approximate threshold for optimal, highly-dispersed Mo incorporation, beyond which the formation of less active Mo–O–Mo oligomers or MoO₃ clusters may occur (Fig. 4a),⁴⁸ confirming that the doped Mo apparently produces O_v in Mo-TS-1. The peaks at 232.6 and 231.6 eV represent the corresponding Mo 3d_5/2 signals of Mo(VI) and Mo(V), respectively (Fig. 1c),^46,49 and the formation of Mo(V) is primarily due to the introduced O_v on 5Mo-TS-1. Likewise, the low-valence Mo(IV/V) is produced in the presence of O_v on MoO₃.^50,51 By contrast, the peaks at 459.0 and 464.7 eV represent Ti 2p_3/2 and Ti 2p_1/2 of Ti(IV), respectively (Fig. 1d),⁵² indicating the presence of Ti(IV) on TS-1. Negligible variations of Ti(IV) on 5Mo-TS-1 indicate that the produced O_v does not change the electronic cloud densities around Ti(IV) (Fig. 1d).

Fig. 3 Raman spectra of (a) TS-1, 1Mo-TS-1, 3Mo-TS-1, 5Mo-TS-1, 7Mo-TS-1 and Na₂MoO₄; and (b) 5Mo-TS-1, 5Mo-TS-1-O₂, Cy, 5Mo-TS-1-Cy, 5Mo-TS-1-Cy-O₂, 5Mo-TS-1-H₂O-O₂, TBHP and 5Mo-TS-1-TBHP.

Fig. 4 (a) EPR analysis of TS-1, 1Mo-TS-1, 3Mo-TS-1, 5Mo-TS-1 and 7Mo-TS-1; (b) EPR analysis of aqueous O₂˙⁻ or surface superoxo in the presence of DMPO using methanol as the solvent.

3.2. Catalytic reactivity of various catalysts

The catalytic reactivity of various catalysts in selective oxidation of Cy to Cy-ep by O₂ was evaluated in an autoclave reactor. In the absence of a catalyst, 60.1% of Cy is converted with only 5.5% of Cy-ep and 32.6% allylic oxidation products (Cy-ol and Cy-one) being produced (Fig. 5, Table 1), and some Cy has been polymerized during the thermal auto-oxidation.¹⁴ In the presence of TS-1, 58.0% of Cy is converted with only 7.3% of Cy-ep being obtained, indicating the low selectivity of TS-1 towards Cy-ep. In the presence of Na₂MoO₄, although only 10.2% of Cy is converted, the high selectivity of 29.7% towards Cy-ep indicates that Mo(VI) facilitates improving the epoxidation of Cy by O₂ (Fig. 5). In the presence of Na₂MoO₄ and TS-1, 38.6% of Cy is converted with 30.5% of Cy-ep being obtained, indicating that physical mixing between Na₂MoO₄ and TS-1 can slightly improve the epoxidation of Cy by O₂ (Fig. S2). As to various Mo-TS-1 catalysts, the conversions of Cy by 1Mo-TS-1, 3Mo-TS-1, 5Mo-TS-1 and 7Mo-TS-1 are 77.4%, 59.8%, 43.5% and 49.0%, respectively, with the corresponding epoxidation selectivity of 19.2%, 23.3%, 50.6% and 41.3% (Fig. 5, Table 1), indicating that an appropriate content of Mo (e.g. 5Mo-TS-1) is more favorable for improving the epoxidation selectivity. In addition, Na₂MoO₄ was also doped into SBA-15 and ZSM-5 supports to produce Mo-SBA-15 and Mo-ZSM-5, which exhibits low catalytic reactivity in Cy conversion and Cy-ep selectivity (Fig. S2). Furthermore, 5Mo-TS-1 exhibited superior performance under solvent-free conditions compared to reactions conducted in various organic solvents, including acetonitrile, dichloromethane, N,N-dimethylformamide, and 1,4-dioxane (Fig. S3). This enhanced efficacy is attributed to the increased effective concentration of reactants near the active sites in the absence of a solvent, which subsequently accelerates the reaction rate and improves the overall conversion.¹⁴

Fig. 5 Oxidation of Cy by various catalysts and O₂. Reaction conditions: 50 mg of catalyst, 5 mL of Cy, 1.0 MPa O₂, 80 °C and 6 h reaction time.

Table 1 Cyclohexene oxidation in the presence of different catalysts

Entry	Catalyst	Conversion (%)	Selectivity (%)
			Cy-ep	Cy-ol	Cy-one
1	Blank	60.1	5.5	15.2	17.4
2	TS-1	58.0	3.82	16.7	31.7
3	Na2MoO4	10.2	29.7	40.6	29.7
4	1Mo-TS-1	77.4	19.2	12.3	13.4
5	3Mo-TS-1	59.8	23.3	20.2	25.7
6	5Mo-TS-1	43.5	50.6	28.0	21.4
7	7Mo-TS-1	49.0	41.3	38.7	18.4
8	Mo and TS-1	38.6	30.5	17.5	26.1
9	Mo-SBA-15	14.4	19.5	4.85	16.1
10	Mo-ZSM-5	17.1	30.4	11.7	32.7
11	Mo/TS-1	33.4	34.2	16.5	13.2
12	Mo-S-1	15.6	32.4	10.4	20.8

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The effect of temperature on the conversion of Cy was investigated by varying the reaction temperature from 40 to 90 °C (Fig. 6a). With increasing reaction temperature from 40 to 80 °C, the conversion of Cy increases from 1.0% to 43.5% with increased selectivity towards epoxide from 40.5% to 50.6% (Fig. 6a), indicating that high temperature favors the epoxidation of Cy by O₂. On the other hand, further increasing the temperature from 80 to 90 °C results in a decrease in epoxidation selectivity from 50.6% to 39.6% (Fig. 6a), which is attributed to rapid radical polymerization or hydrolysis of the epoxide products.¹⁶ O₂ pressure also affects the conversion and selectivity of Cy. Fig. 6b shows negligible conversion of Cy (<1%) in the absence of O₂, whereas the conversion of Cy increases from 5.5% to 43.5% with the increase of O₂ pressure from 0.1 to 1.0 MPa, implying that high pressure enhances the solubility of O₂ in the reaction system and thus improves the conversion and epoxidation selectivity. In addition, the conversion of Cy and epoxide selectivity at 1.0 MPa O₂ pressure are comparable to those at 1.5 MPa O₂ pressure (Fig. 6b), indicating that 1.0 MPa is an appropriate O₂ pressure. When the reaction time was increased from 2 to 10 h, the conversion of Cy gradually increased from 3.8% to 48.5% (Fig. 6c), whereas the Cy-ep selectivity increased from 31.2% to 50.6% in the first 6 h but decreased to 45.5% after 10 h, which may be attributed to the occurrence of Cy-ep polymerization with the production of Cy oxide polymers.^16,53

Fig. 6 Factors affecting the conversion and selectivity of Cy. Effects of (a) temperature, (b) oxygen pressure, (c) reaction time, and (d) repeated cycles. Reaction conditions: 50 mg of catalyst, 5 mL of Cy.

In addition, a series of kinetic experiments were performed at temperatures of 40, 50, 60, 70 and 80 °C, while maintaining all other reaction parameters constant. The initial reaction rates for both the total consumption of Cy and the formation of Cy-ep were determined from the linear portion of the respective concentration–time profiles during the initial reaction period (Fig. S4a and b). Arrhenius plots of ln (initial rate) versus 1/T were constructed for both processes (Fig. S4c and d). Both datasets exhibited excellent linearity with correlation coefficients (R²) greater than 0.90, confirming the validity of the Arrhenius relationship and allowing for accurate calculation of the apparent activation energies from the slopes of the fitted lines. The apparent E_a values for the total consumption of Cy and Cy-ep were found to be 71.60 and 90.56 kJ mol⁻¹, respectively. The significant difference in activation energies (ΔE_a = 18.96 kJ mol⁻¹) provides important mechanistic insights. The higher activation barrier for Cy-ep formation compared to Cy consumption suggests that the epoxidation step represents the rate-determining step in the overall transformation. This energy difference further indicates that competing pathways, such as allylic oxidation, may occur with lower activation barriers, contributing to the total consumption of Cy without forming the desired epoxide product.

The reusability of 5Mo-TS-1 was also evaluated in the repeated oxidation of Cy. After each cycle of Cy oxidation, the catalyst was recovered from the suspension by centrifugation and washed thoroughly with ethanol. The catalysts were then dried under vacuum at 80 °C overnight before the next cycle. Fig. 6d shows that the conversion of Cy slightly decreases from 43.5% to 41.5% after 7 consecutive experiments with negligible changes in selectivity, indicating the high reusability and stability of 5Mo-TS-1. In addition, 5.0 mL of reaction mixture or 25.0 mg of catalyst was taken out for digestion with concentrated HNO₃ and HCl, and was then used for ICP analysis. There were no detectable Mo and Ti (both below detection limit, <10 ppb), ruling out the possibility of Mo and Ti leaching during the oxidation reaction (Table S2). In addition, comparable percentages of Mo and Ti species in 5Mo-TS-1 before and after the reaction also confirm the high stability of 5Mo-TS-1 during the oxidation reaction (Table S2).

H₂-TPR measurements reveal that Mo incorporation markedly alters the reduction behavior of TS-1 (Fig. S5). While the parent TS-1 exhibits only minor reduction features at low and high temperatures, Mo-modified catalysts display prominent reduction peaks in the medium-to-low temperature range (∼100–300 °C), attributable to the reduction of Mo species. With increasing Mo loading, the intensity and area of these reduction peaks increase, indicating a greater population of reducible Mo sites. Moreover, a slight shift in peak temperature suggests changes in the dispersion and local environment of Mo species, with potential agglomeration occurring at higher loadings. These results demonstrate that the introduction of Mo shifts the reduction process from being dominated by Ti species in TS-1 to being governed by Mo species, thereby modulating the redox properties in a Mo-content-dependent manner.

NH₃-TPD profiles indicate that Mo incorporation does not significantly alter the intrinsic acidic properties of TS-1 (Fig. S6). All catalysts exhibit a consistent low-temperature desorption peak near 130 °C, associated with framework Ti sites. This confirms that Mo doping does not introduce substantial Brønsted acidity. The observed formation of by-products such as cyclohexanol and cyclohexanone at high conversion is attributed to the limited ring-opening activity of the inherent weak acid sites in TS-1, rather than to newly generated acidic centers from Mo. The high epoxide selectivity under optimized conditions underscores that the Lewis acid sites associated with Ti and Mo govern the epoxidation pathway, with minimal interference from Brønsted acid sites.

3.3. Oxidizing species involved in the epoxidation of Cy

Negligible conversion of Cy in the presence of 0.01 M HQ indicates H-abstraction from Cy with the formation of Cy· during the epoxidation of Cy in that HQ is an efficient scavenger for carbon radicals (R·).^14,54 To further confirm the production of Cy· during the epoxidation of Cy, 1.25 mol L⁻¹ CBrCl₃ was added into the reaction system given that the C–Br bond of CBrCl₃ can be readily cleaved by R· with the formation of corresponding bromine-substituted compounds.^55,56 In the absence of 5Mo-TS-1, 45.0% of Cy is converted with a low selectivity of 8.3% towards 1-bromo-2-cyclohexene in the O₂ atmosphere, and in the presence of 5Mo-TS-1, the conversion of Cy decreases to 3.3% with a selectivity of 98.5% in the Ar atmosphere (Fig. 7a), indicating the negligible ability of 5Mo-TS-1 to abstract H from the allylic C–H bond of Cy in the absence of O₂. In the presence of 5Mo-TS-1 and O₂, the conversion of Cy increases to 50.5% with a high selectivity of 99.0% towards 1-bromo-2-cyclohexene (Fig. 7a), confirming that O₂ and 5Mo-TS-1 are indispensable in H-abstraction from allylic C–H of Cy to form Cy·. By contrast, our recent work suggested H-abstraction from allylic C–H of Cy to Cy· on the surface of Mg–Mo bimetallic oxide in the absence of O₂.¹⁴

Fig. 7 (a) Oxidation of Cy in the presence of CBrCl₃. Reaction conditions: 50 mg of catalyst, 5 mL of Cy, 1.0 MPa O₂ or Ar, 80 °C and 6 h reaction time. Blank is the experiment in the absence of 5Mo-TS-1. (b) Quenching experiments using various scavengers in the oxidation of Cy. Reaction conditions: 50 mg of 5Mo-TS-1, 5 mL of Cy, 1.0 MPa O₂, 80 °C and 6 h reaction time. Blank is the experiment in the absence of any scavenger.

To identify the oxidizing species involved in the oxidation of Cy, a series of quenching experiments were carried out. Negligible decreases in the yields of Cy-ep, Cy-ol and Cy-one in the presence of 0.01 M TBA or ethanol rule out the primary contribution of ·OH to Cy oxidation. By contrast, the presence of 0.01 M NBT, CF or BQ significantly decreases the yields of Cy-ep, Cy-ol and Cy-one (Fig. 7b), confirming the importance of superoxide radicals (O₂˙⁻) or surface superoxo in Cy oxidation.⁵⁷ In EPR analysis, given that surface superoxo, instead of O₂˙⁻ is detected in the 5Mo-TS-1-Cy-methanol-O₂ system (Fig. 4b),⁵ we therefore deduce that surface superoxo is the important oxidizing species in the epoxidation of Cy. Considering that 2,4-HD can effectively scavenge the reactivity of ROO·,⁵⁸ the significant decrease in the yield of Cy-ep from 22.01% to 0.22% in the presence of 0.01 M 2,4-HD confirms the importance of Cy–OO· during the epoxidation of Cy by O₂ (Fig. 7b).

Given that Cy–OOH peroxo is suggested as an important intermediate in the epoxidation of Cy,¹³ whose reactivity can be readily quenched after the reaction with PPh₃ to form PPh₃=O and Cy-ol,^11,41,59 the variations of Cy–OOH peroxo during the epoxidation of Cy were investigated. In the presence of PPh₃, the selectivity towards Cy-ol increases from 12.1% to 76.2%, whereas the selectivity towards Cy-ol was only 12.1% in the absence of 0.01 M PPh₃ after a 3.0 h reaction (Fig. 8a), indicating the presence of Cy–OOH peroxo in the reaction system. When the reaction time was prolonged to 6.0 h, Cy–OOH peroxo cannot be detected (Fig. 8a), which is attributed to the complete decomposition of Cy–OOH peroxo after a long reaction time. In addition, the corresponding yields of Cy–OOH peroxo and Cy-ep reach up to 20.1% and 22.1% in the presence/absence of PPh₃=O (Fig. 8b), indicating that Cy–OOH plays an important role in Cy epoxidation. Actually, the produced Cy–OO· intermediate abstracts another H from Cy to form Cy–OOH.

Fig. 8 (a) Oxidation of Cy in the presence of PPh₃ for 3 and 6 h. (b) Yields of Cy–OOH and Cy-ep in the presence of 5Mo-TS-1 and O₂. Reaction conditions: 50 mg of 5Mo-TS-1, 5 mL of Cy, 1.0 MPa O₂ and 80 °C.

Given that the low-valence Mo(IV/V) sites can effectively activate O₂ to form ≡Mo-peroxo to epoxidize Cy,^16,29,30 Raman analysis is used to identify the Mo-bearing oxidizing species in the presence of O₂ and 5Mo-TS-1. In the presence of Cy, the 561 cm⁻¹ band and the broad band in the range of 824–877 cm⁻¹ are assigned to the Mo–O and O–O stretching of ≡Mo-(η²-O₂), respectively, in the 5Mo-TS-1-Cy-O₂ system (Fig. 3b),^14,38 whereas such bands cannot be detected in the 5Mo-TS-1-O₂ and 5Mo-TS-1-Cy systems, indicating that both O₂ and Cy are indispensable in the formation of ≡Mo-(η²-O₂) on 5Mo-TS-1. When Cy is replaced by H₂O, the bands at 561 cm⁻¹ and 828 cm⁻¹ are also detected in the 5Mo-TS-1-H₂O-O₂ system (Fig. 3b), which also confirms the formation of ≡Mo-(η²-O₂) in the absence of Cy. Actually, the H-donor of Cy/H₂O plays an important role in the formation of ≡Mo-(η²-O₂) peroxo.⁶⁰ Moreover, the bands at 561 and 836 cm⁻¹ in the reaction between 5Mo-TS-1 and TBHP also represent the Mo–O and O–O stretching of ≡Mo-(η²-O₂) (Fig. 3b), indicating that the produced Cy–OOH peroxo intermediate can be further activated by 5Mo-TS-1 to form ≡Mo-(η²-O₂).¹⁴

Although Ti-peroxo is also believed to be an important oxidizing species in the epoxidation of Cy in the TiO₂-H₂O₂ system,^61,62 such a Ti-peroxo band cannot be detected in the 5Mo-TS-1 system (Fig. 3b). In addition, when the Ti sites on 5Mo-TS-1 are poisoned by 2,4-DQ,²⁰ the slight decreases in the conversion of Cy (from 43.5% to 40.2%) and selectivity towards Cy-ep (from 50.6% to 48.5%) indicate that Ti(IV) on 5Mo-TS-1 is not an important site in the epoxidation of Cy (Fig. 9). By contrast, when the Mo sites on 5Mo-TS-1 are poisoned by Q,⁶³ the decreases in the conversion of Cy (from 43.5% to 35.5%) and selectivity towards Cy-ep (from 50.6% to 16.4%) indicate that Mo(V/VI) plays an important role in the epoxidation of Cy (Fig. 9).

Fig. 9 Catalytic oxidation of Cy in the presence of poisoning reagents. Reaction conditions: 50 mg of 5Mo-TS-1, 25 mg of 2,4-DQ or Q, 5 mL of Cy, 1.0 MPa O₂ and 80 °C.

3.4. DFT calculations for epoxidation of Cy by O₂

To further elucidate the mechanisms involved in the activation of O₂ by Mo-TS-1, DFT calculations are used to calculate reaction energy barriers in the activation of O₂.³⁹ The adsorption and activation energy of O₂ on O_v (including the activation barrier) is −1.27 eV (Fig. 10). Likewise, Yang et al. suggested that O₂ was activated upon initial adsorption on O_v with an adsorption and activation energy of −1.75 eV,⁴⁸ indicating the thermodynamically feasible activation of O₂ on O_v. Meanwhile, the O–O length of O₂ is significantly elongated from 1.240 to 1.394 Å at O_v and the respective O¹ and O² obtain 0.36 and 0.47e after O₂ adsorption on O_v with the corresponding spins of O¹ and O² changing from 0 and 0 to 0.53 and 0.50 in the Mulliken charge analysis (Table S3), confirming that O₂ has been activated to ≡O_v-superoxo.⁶⁴ By contrast, the adsorption energy of O₂ at the doped Mo of Mo-TS-1 is −0.32 eV, and its low energy barrier (TS-ΔG = 0.408 eV) also indicates that the activation of O₂ by the doped Mo sites is also thermodynamically feasible (Fig. 10). Meanwhile, the O–O length of O₂ at the Mo sites is significantly elongated from 1.248 Å to 1.461 Å, which is the characteristic length of the O–O bond in ≡Mo-(η²-O₂) (Table S3),⁶⁵ indicating that O₂ has been transformed to ≡Mo-(η²-O₂) peroxo on Mo-TS-1. Meanwhile, the respective O¹ and O² obtain 0.37 and 0.30e after O₂ is activated to ≡Mo-(η²-O₂) peroxo with the corresponding spins of O¹ and O² changing from 0.53 and 0.50 to 0 and 0 in the Mulliken charge analysis (Table S3), confirming that O₂ has been directly activated to ≡Mo-peroxo on the surface of Mo-TS-1.⁶⁵

Fig. 10 Energy profiles involved in the oxidation of Cy by 5Mo-TS-1 and O₂.

In the oxidation of Cy by ≡O_v-superoxo species, the low energy barrier (TS₁-ΔG = 0.346 eV) of H-abstraction from Cy by ≡O_v-superoxo with the allyl C–H being significantly elongated from 1.074 to 1.901 Å indicates that H-abstraction from allylic Cy is thermodynamically feasible (Fig. 10). Meanwhile, 0.61e is transferred from Cy to ≡O_v-superoxo with the spin of Cy changing from 0 to 0.98 in the Mulliken charge analysis (Table S4), indicating that H has been effectively abstracted Cy by ≡O_v-superoxo with the formation of Cy·. In addition, the O–O length of ≡O_v-superoxo is significantly elongated from 1.394 to 1.489 Å, and the respective O¹ and O² obtain 0.30 and 0.43e after O₂ adsorption on O_v with the spin of O¹ and O² changing to 0 in the Mulliken charge analysis (Table S4), confirming that ≡O_v-superoxo has been transferred to ≡O_v-peroxo after H-abstraction from the allyl C–H of Cy. Apart from the direct activation of O₂ to form ≡Mo-(η²-O₂) peroxo, previous studies also suggested that ≡O_v-peroxo and Cy–OOH peroxo during the epoxidation of Cy by O₂ can also react with the Mo(VI) sites to form ≡Mo-(η²-O₂) peroxo,¹⁶ indicating that ≡Mo-(η²-O₂) peroxo is an important intermediate in the selective epoxidation of Cy. In addition, the larger positive charge of the Mo(VI) sites can abstract O atoms from ≡O_v-peroxo and Cy–OOH to form ≡Mo-peroxo via the OAT pathway.¹⁶

In addition, the high energy barriers in the H-abstraction from allyl C–H (TS₂₁-ΔG = 0.898 eV) and vinylic C–H (TS₂₂-ΔG = 0.790 eV) of Cy by ≡Mo-(η²-O₂) peroxo indicate that H-abstraction is not readily mediated by ≡Mo-(η²-O₂) (Fig. 10). By contract, the low energy barrier (TS₂₃-ΔG = 0.078 eV) in the epoxidation of Cy indicates that ≡Mo-(η²-O₂) peroxo can effectively epoxidize the alkene group (Fig. 10). After the reaction, the O¹–O² bond of ≡Mo-(η²-O₂) peroxo increases from 1.461 to 2.658 Å with the bonds of Mo–O¹ and Mo–O² changing from 2.047 and 2.082 Å to 1.771 and 3.082 Å, respectively (Table S5), indicating that O² is more favorable than O¹ to take part in the epoxidation of Cy. Meanwhile, the corresponding bonds of C¹–O² and C²–O² change to 1.496 and 1.495 Å, indicating the occurrence of OAT between ≡Mo-(η²-O₂) peroxo and Cy (Table S5). In the Mulliken charge analysis, given that O² (−0.30e) is more positive than O¹ (−0.37e) (Table S5), O² is more likely to electrophilically attack the alkene group of Cy.¹⁶

The results demonstrate that 5Mo-TS-1 achieves superior combination of high conversion and epoxide selectivity under mild conditions (Table S6). We attribute this performance to the unique reaction pathway enabled by the TS-1 framework. Mechanistic studies reveal that O₂ activation occurs simultaneously through two pathways. The mechanisms involved in the oxidation of Cy by O₂ and Mo-TS-1 under solvent- and initiator-free conditions can therefore be summarized as follows (Scheme 1): (i) the O_v sites on Mo-TS-1 adsorbs and activates O₂ to produce ≡O_v-superoxo; (ii) the ≡O_v-superoxo abstracts the allylic H from Cy to produce ≡O_v-peroxo with the transformation of Cy to Cy·; (iii) Cy· rapidly traps O₂ to give Cy–OO·, which then abstracts H from the allylic C–H of another Cy to form Cy–OOH peroxo and Cy·; (iv) Mo(V/VI) can also activate O₂ and/or peroxo (≡O_v-peroxo and Cy–OOH peroxo) to produce ≡Mo-(η²-O₂), which then electrophilically attacks the alkene bonds of Cy to give Cy-ep.

Scheme 1 Proposed mechanisms involved in the oxidation of Cy by Mo-TS-1 and O₂.

4. Conclusion

In summary, a series of Mo-doped TS-1 (Mo-TS-1) catalysts have been successfully synthesized to epoxidize Cy under solvent- and initiator-free conditions using O₂ as the oxidant, and 5Mo-TS-1 exhibits high catalytic reactivity in Cy conversion (43.5%) and epoxidation selectivity (50.6%). Meanwhile, the corresponding yields of high-value by-products of Cy-ol and Cy-one reach up to 28.0% and 21.4%. Considering that Cy-ol and Cy-one are also high-value intermediates for fragrance industries, more than 43.5% Cy can be effectively transformed to high-value products. Quenching experiments indicate that ≡O_v-superoxo on the surface of 5Mo-TS-1 plays an important role in H-abstraction from the allylic C–H bond of Cy to form Cy·, which subsequently reacts with O₂ to produce Cy–OO·. After that, Cy–OO· abstracts another H from other Cy to form Cy–OOH. The positive correlations between Cy–OOH and Cy-ep indicate the importance of Cy–OOH in Cy epoxidation. Raman analysis further confirms the presence of ≡Mo-(η²-O₂) peroxo on the surface of 5Mo-TS-1, which is more favorable for electrophilically attacking the C=C bond of Cy to produce Cy-ep. DFT calculations reveal two pathways involved in the activation of O₂. In pathway I, O_v on the surface of Mo-TS-1 activates O₂ to produce ≡O_v-superoxo, which then undergoes a series of reactions with Cy to produce ≡O_v-peroxo, Cy–OOH peroxo and Cy·. In pathway II, Mo(V/VI) can directly activate O₂ or react with the produced peroxo (≡O_v-peroxo and Cy–OOH peroxo) to ≡Mo-(η²-O₂), which then electrophilically attacks the alkene bonds of Cy to give Cy-ep. Given that the direct activation of O₂ by Mo(V/VI) to ≡Mo-(η²-O₂) cleverly bypasses the pathway of allylic oxidation of Cy, the epoxidation selectivity of Cy towards Cy-ep (50.6%) is higher than the theoretical selectivity of 50.0%. This study provides new insight on the importance of oxidizing species in the Mo-TS-1-O₂-Cy system to selectively epoxidize Cy.

Author contributions

Yu-Le Wang: conceptualization; methodology; data curation; formal analysis; visualization. Song-Hai Wu: resources. Yu-Zhen Xu: resources. Yu-Dong Shan: resources. Yong Liu: resources. Xu Han: conceptualization; methodology; investigation; validation; supervision funding acquisition; project administration; resources; writing – original draft; writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request. Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5cy01095k.

Acknowledgements

We greatly acknowledge the financial support from the National Natural Science Foundation of China (No. 51978455).

References

1. A. S. Sharma, V. S. Sharma, H. Kaur and R. S. Varma, Green Chem., 2020, 22, 5902–5936.
2. J. Büker, M. Muhler and B. Peng, ChemCatChem, 2022, 15, e202201216.
3. Y. Cao, H. Yu, F. Peng and H. Wang, ACS Catal., 2014, 4, 1617–1625.
4. B. P. C. Hereijgers, R. F. Parton and B. M. Weckhuysen, ACS Catal., 2011, 1, 1183–1192.
5. Y. Song, F. Xin, L. Zhang and Y. Wang, ChemCatChem, 2017, 9, 4139–4147.
6. Y. Chen, S. Ahn, M. R. Mian, X. Wang, Q. Ma, F. A. Son, L. Yang, K. Ma, X. Zhang, J. M. Notestein and O. K. Farha, J. Am. Chem. Soc., 2022, 144, 3554–3563.
7. S. Ahn, S. L. Nauert, K. E. Hicks, M. A. Ardagh, N. M. Schweitzer, O. K. Farha and J. M. Notestein, ACS Catal., 2020, 10, 2817–2825.
8. H. Noh, Y. Cui, A. W. Peters, D. R. Pahls, M. A. Ortuno, N. A. Vermeulen, C. J. Cramer, L. Gagliardi, J. T. Hupp and O. K. Farha, J. Am. Chem. Soc., 2016, 138, 14720–14726.
9. H. Shima, M. Tanaka, H. Imai, T. Yokoi, T. Tatsumi and J. N. Kondo, J. Phys. Chem. C, 2009, 113, 21693–21699.
10. H. Cao, B. Zhu, Y. Yang, L. Xu, L. Yu and Q. Xu, Chin. J. Catal., 2018, 39, 899–907.
11. I. M. Denekamp, M. Antens, T. K. Slot and G. Rothenberg, ChemCatChem, 2018, 10, 1035–1041.
12. Z. Tian, A. Fattahi, L. Lis and S. R. Kass, J. Am. Chem. Soc., 2006, 128, 17087–17092.
13. J. Büker, X. Huang, J. Bitzer, W. Kleist, M. Muhler and B. Peng, ACS Catal., 2021, 11, 7863–7875.
14. Y. Z. Xu, Y. L. Wang, Y. D. Shan, S. H. Wu, Y. Liu and X. Han, Ind. Eng. Chem. Res., 2024, 63, 14554–14566.
15. S. Su, G. Lv, X. Zou, J. Wang, C. Zhou, Y. Chen, J. Shen, Y. Shen and Z. Liu, Green Chem., 2023, 25, 9262–9271.
16. W. Zhong, M. Liu, J. Dai, J. Yang, L. Mao and D. Yin, Appl. Catal., B, 2018, 225, 180–196.
17. B. Wang, Y. Guo, J. Zhu, J. Ma and Q. Qin, Coord. Chem. Rev., 2023, 476, 214931.
18. M. G. Clerici and P. Ingallina, J. Catal., 1993, 140, 71–83.
19. S. Kwon, N. M. Schweitzer, S. Park, P. C. Stair and R. Q. Snurr, J. Catal., 2015, 326, 107–115.
20. W. Fan, P. Wu and T. Tatsumi, J. Catal., 2008, 256, 62–73.
21. J. W. Arvay, W. Hong, C. Li, W. N. Delgass, F. H. Ribeiro and J. W. Harris, ACS Catal., 2022, 12, 10147–10160.
22. C. Li, N. Pu, K. Huang, C. Xia, X. Peng, M. Lin, B. Zhu and X. Shu, Green Chem., 2022, 24, 6200–6214.
23. E. da Palma Carreiro and A. J. Burke, J. Mol. Catal. A: Chem., 2006, 249, 123–128.
24. L. Xu, H. Huang, L. Xu, D. Zhu, B. Zhang, J. He, H. Li and W. Jiang, Sep. Purif. Technol., 2024, 354, 129385.
25. P. Sun, Q. Lu, J. Zhang, T. Xiao, W. Liu, J. Ma, S. Yin and W. Cao, Chem. Eng. J., 2020, 397, 125444.
26. F. Fan, S. Han, Y. Li, L. Qi, J. Kang, Z. Chen, C. Tian, D. Wang and W. Zhou, Sep. Purif. Technol., 2025, 354, 128702.
27. Y. Zou, C. Xiao, X. Yang, Y. Wang, X. Kong, Z. Liu, C. Wang, A. Duan, C. Xu and X. Wang, J. Catal., 2024, 435, 115576.
28. W. Liang, G. Xu and Y. Fu, Appl. Catal., B, 2024, 340, 123220.
29. Z. Wang, A. Gao, P. Chen, H. Hu, Q. Huang and X. Chen, J. Catal., 2018, 368, 120–133.
30. K. Chen, X. M. Zhang, X. F. Yang, M. G. Jiao, Z. Zhou, M. H. Zhang, D. H. Wang and X. H. Bu, Appl. Catal., B, 2018, 238, 263–273.
31. S. Z. Noby, A. Fakharuddin, S. Schupp, M. Sultan, M. Krumova, M. Drescher, M. Azarkh, K. Boldt and L. Schmidt-Mende, Mater. Adv., 2022, 3, 3571–3581.
32. M. C. Tsai, T. T. Nguyen, N. G. Akalework, C. J. Pan, J. Rick, Y. F. Liao, W. N. Su and B. J. Hwang, ACS Catal., 2016, 6, 6551–6559.
33. K. Yu, L. L. Lou, S. Liu and W. Zhou, Adv. Sci., 2020, 7, 1901970.
34. Y. Li, H. Li, K. Li, R. Wang, R. Zhang and R. Liu, ACS Appl. Nano Mater., 2023, 6, 14214–14227.
35. Y. Mao, P. Wang, L. Li, Z. Chen, H. Wang, Y. Li and S. Zhan, Angew. Chem., Int. Ed., 2020, 59, 3685–3690.
36. A. Ansari, P. Jayapal and G. Rajaraman, Angew. Chem., Int. Ed., 2015, 54, 564–568.
37. A. Ruiz Puigdollers, P. Schlexer, S. Tosoni and G. Pacchioni, ACS Catal., 2017, 7, 6493–6513.
38. E. Z. Ayla, D. S. Potts, D. T. Bregante and D. W. Flaherty, ACS Catal., 2020, 11, 139–154.
39. P. Tao, X. Wang, Q. Zhao, H. Guo, L. Liu, X. Qi and W. Cui, Appl. Catal., B, 2023, 325, 122392.
40. X. Gao, Y. Zhang, Y. Hong, B. Luo, X. Yan and G. Wu, Microporous Mesoporous Mater., 2022, 333, 111731.
41. J. Büker and B. Peng, Mol. Catal., 2022, 525, 112367.
42. Y. D. Shan, S. H. Wu, Y. L. Wang, C. Wang, S. Q. Zhi, Y. Liu and X. Han, Inorg. Chem., 2023, 62, 4872–4882.
43. Y. Shao, H. Wang, X. Liu, P. R. Haydel, T. Li, J. Chen, P. Huang, Q. Xiao, T. Tatsumi and J. Wang, Microporous Mesoporous Mater., 2021, 313, 110828.
44. C. Liu, Q. Wei, Y. Zhou, X. Liu, K. Deng, W. Huang, H. Liu and Z. Yu, Fuel, 2023, 338(126), 922.
45. T. D. Nguyen, A. Worrad, D. Thirulogachandar, F. E. Celik, S. Caratzoulas and G. Tsilomelekis, J. Phys. Chem. C, 2024, 128, 9169–9181.
46. A. Uchagawkar, A. Ramanathan, H. Zhu, L. Chen, Y. Hu, J. Douglas, M. Mais, T. Kobayashi and B. Subramaniam, ACS Catal., 2024, 14, 8317–8329.
47. M. Liu, Z. Xiao, J. Dai, W. Zhong, Q. Xu, L. Mao and D. Yin, Chem. Eng. J., 2017, 313, 1382–1395.
48. J. Yang, S. Hu, Y. Fang, S. Hoang, L. Li, W. Yang, Z. Liang, J. Wu, J. Hu, W. Xiao, C. Pan, Z. Luo, J. Ding, L. Zhang and Y. Guo, ACS Catal., 2019, 9, 9751–9763.
49. M. Xie, F. Dai, H. Guo, P. Du, X. Xu, J. Liu, Z. Zhang and X. Lu, Adv. Energy Mater., 2023, 13, 2203032.
50. M. T. Greiner, L. Chai, M. G. Helander, W. M. Tang and Z. H. Lu, Adv. Funct. Mater., 2012, 23, 215–226.
51. M. Vasilopoulou, A. M. Douvas, D. G. Georgiadou, L. C. Palilis, S. Kennou, L. Sygellou, A. Soultati, I. Kostis, G. Papadimitropoulos, D. Davazoglou and P. Argitis, J. Am. Chem. Soc., 2012, 134, 16178–16187.
52. W. Li, L. Chen, M. Qiu, W. Li, Y. Zhang, Y. Zhu, J. Li and X. Chen, ACS Catal., 2023, 13, 10487–10499.
53. Y. Rusconi, M. C. D'Alterio, C. De Rosa, G. W. Coates and G. Talarico, Green Chem., 2025, 27, 4196–4204.
54. J. Dai, W. Zhong, W. Yi, M. Liu, L. Mao, Q. Xu and D. Yin, Appl. Catal., B, 2016, 192, 325–341.
55. G. Coin, P. Dubourdeaux, F. Avenier, R. Patra, L. Castro, C. Lebrun, P. A. Bayle, J. Pécaut, G. Blondin, P. Maldivi and J. M. Latour, ACS Catal., 2021, 11, 2253–2266.
56. J. Cheng, Y. Shiota, M. Yamasaki, K. Izukawa, Y. Tachi, K. Yoshizawa and H. Shimakoshi, Inorg. Chem002E, 2022, 61, 9710–9724.
57. Y. M. Lee, S. Hong, Y. Morimoto, W. Shin, S. Fukuzumi and W. Nam, J. Am. Chem. Soc., 2010, 132, 10668–10670.
58. M. Xie, C. Zhang, H. Zheng, G. Zhang and S. Zhang, Water Res., 2022, 217, 118424.
59. M. Sasidharan, Y. Kiyozumi, N. K. Mal and F. Mizukami, Adv. Funct. Mater., 2006, 16, 1853–1858.
60. D. Sheng, Q. He, Z. H. Cao, L. B. Chen, S. H. Wu, Y. Liu, H. T. Ren and X. Han, Ind. Eng. Chem. Res., 2024, 63, 9038–9049.
61. D. T. Bregante, J. Z. Tan, R. L. Schultz, E. Z. Ayla, D. S. Potts, C. Torres and D. W. Flaherty, ACS Catal., 2020, 10, 10169–10184.
62. S. Bordiga, A. Damin, F. Bonino, G. Ricchiardi, C. Lamberti and A. Zecchina, Angew. Chem., Int. Ed., 2002, 41, 4734–4737.
63. V. M. Kogan, R. G. Gaziev, S. W. Lee and N. N. Rozhdestvenskaya, Appl. Catal., A, 2003, 251, 187–198.
64. Y. Liu, Y. Peng, M. Naschitzki, S. Gewinner, W. Schollkopf, H. Kuhlenbeck, R. Pentcheva and B. Roldan Cuenya, Angew. Chem., Int. Ed., 2021, 60, 16514–16520.
65. S. Ishikawa, K. Shimoda, T. Kamachi, N. Aoki, T. Hagiwara, A. Urakawa and W. Ueda, ACS Catal., 2023, 13, 15526–15534.

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