Highly selective oxidation of cyclohexene to 2-cyclohexene-1-one over polyoxometalate/metal–organic framework hybrids with greatly improved performances

Abstract

H_3+xPMo_12−xV_xO₄₀@MIL-100 (Fe) (x = 0, 1, 2) hybrids were prepared by encapsulation of polyoxometalates (POMs) within a metal–organic framework using a direct hydrothermal method. The as-prepared samples were well characterized by X-ray diffraction (XRD), Fourier transform infrared spectrophotometry (FT-IR), transmission electron microscopy (TEM), N₂ adsorption–desorption, UV-vis diffused reflectance spectra and inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis. The catalytic performances of the samples were tested in the oxidation of cyclohexene using H₂O₂ as green oxidant. The results have shown that both H₄PMo₁₁VO₄₀@MIL-100 (Fe) and H₅PMo₁₀V₂O₄₀@MIL-100 (Fe) can effectively catalyze the allylic oxidation of cyclohexene to give 2-cyclohexen-1-one as the main product. In particular, when H₄PMo₁₁VO₄₀@MIL-100 (Fe) was employed, 85% cyclohexene conversion, 91% selectivity for 2-cyclohexene-1-one and 715 h⁻¹ of turnover frequency were obtained under optimized conditions. The catalyst can be reused at least five times without obvious loss of activity.

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1. Introduction

Catalytic oxidation of alkenes into value-added oxygenated derivatives is a fundamental reaction in organic chemistry since the products obtained are valuable and resourceful commercial intermediates and undergo further reactions.^1–4 In particular, allylic oxidation of cyclic olefins, represented by cyclohexene, produces a variety of oxygen-containing derivatives; for example, 2-cyclohexene-1-ol and 2-cyclohexene-1-one are important intermediates in the spice industry and in organic synthesis.⁵ Upon oxidation of cyclohexene, it undergoes accompanying olefinic oxidation at the double bond in addition to allylic oxidation. This leads to great challenges to obtain high selectivity for an allylic oxidation product.⁶ Recently, a variety of catalysts for the oxidation of cyclohexene have been reported in the literature, such as expanded graphite supported copper catalyst,⁷ Fe–Co doped graphitic carbon nitride,⁸ supported metal complex catalyst,^9–11etc. Although there have been some achievements, the search for more stable and efficient catalysts is never-ending.

Alternatively, polyoxometalates (POMs) have received much attention in oxidation chemistry¹² based on their redox properties. These materials are discrete anionic metaloxygen clusters which exhibit diverse structures and tunable properties.¹³ In the past decades, numerous catalytic oxidation reactions by POMs have been discovered and mostly with alkenes as substrates.^14–20 Although POMs have been widely used in oxidation catalysis,²¹ their applications are limited by their low specific surface area, low stability under catalytic conditions and difficult separation from most solvents. One of the important strategies to improve the properties of POMs is encapsulating the POMs in porous solid matrixes,¹⁵ such as molecular sieves^22,23 and porous silica.^13,24,25 However, the use of these materials is limited by several drawbacks, such as low POM loading, POM leaching, conglomeration of POM particles, and nonuniform active sites.²⁶ A suitable support which can overcome the above drawbacks is desirable for immobilization of POMs.

Metal–organic frameworks, MOFs, a new class of crystalline porous materials with open frameworks, act as unique and outstanding candidates for carriers due to their high surface areas and uniform but tunable cavities.²⁶ POM-based MOF catalysts have gained the attention of researchers in recent years.²⁷ For example, Ni-containing polyoxometalate encapsulated in MOF 1 and 2 were used as highly efficient catalysts for visible-light-driven hydrogen evolution reaction;²⁸ sandwich-type Eu-polyoxometalate supported on Al(III) and Cr(III) MIL-type MOFs were used as heterogeneous catalysts in desulfurization processes;²⁹ hybrid materials of MIL-101/phosphotungstic acid were used as catalysts for the Baeyer condensation of benzaldehyde and 2-naphthol;³⁰ hybrids of 12-tungstophosphoric heteropolyacid@MIL-100 (Fe) were used as catalysts for esterification and acetalization reactions;³¹ a direct hydrothermal synthesis was developed to prepare stable H₃PMo₁₂O₄₀ insertion within MIL-100 (Fe);³² and H₃PMo₁₂O₄₀@MIL-100 (Fe) hybrids were used for the photocatalytic selective oxidation of benzylic alcohols and the reduction of Cr(VI) under visible light irradiation.³³ In all the cases, highly improved performances have been obtained from the hybrids compared with the corresponding POMs. All the exciting reports inspire us to develop new POM-MOF catalysts with much higher stability and activity.

Although POM-loaded MOFs have become more and more interesting, MIL-100 (Fe), a MOF derived from an iron salt and benzenetricarboxylic acid, is rarely used for catalytic studies because of its reportedly unknown crystal structure and poor local characterization.³⁴ However, this is not a problem when the material is to be used as a support only. In the current work, hybrid materials of H_3+xPMo_12−xV_xO₄₀@MIL-100 (Fe) have been prepared by a one-pot hydrothermal route and used as catalysts for the allylic oxidation of cyclohexene. The samples have exhibited prominent catalytic activity and high reusability in selectively allylic oxidation of cyclohexene to produce 2-cyclohexene-1-one.

2. Experimental

2.1. Materials and methods

All reagents and solvents are of analytical grade and were used without further purification. The Fourier transform infrared (FT-IR) spectra of the samples were recorded on a ThennoNicolet NEXUS FT-IR instrument (KBr discs) in the 4000–400 cm⁻¹ region. Solid UV-vis diffuse reflection spectra (UV-vis DRS) of the samples were measured with a Shimadzu UV-3100 spectrometer, and BaSO₄ was used as an internal standard. X-ray diffraction (XRD) patterns were collected on a Shimadzu XRD-6000 diffractometer using a Cu Kα radiation source at 40 kV and 150 mA. TEM micrographs were obtained using a Hitachi H-600 microscope. BET surface area measurements were performed on a Micromeritics ASAP 2010 instrument at liquid nitrogen temperature. Metal content was determined by inductively coupled plasma spectrometry on a Perkin-Elmer ICP/6500 atomic emission spectrometer.

2.2. Preparation of catalyst

2.2.1. Preparation of POMs. The samples of V-substituted phosphomolybdic acid H_3+xPMo_12−xV_xO₄₀ (x = 0, 1, 2) were prepared according to the procedure reported in the literature.²⁶ As for the sample of H₄PMo₁₁VO₄₀, 15.8 g (0.11 mol) of MoO₃ and 0.91 g (0.005 mol) of V₂O₅ were added to 250 mL of deionized water and the suspension was then heated to 100 °C under stirring. Afterward, 1.15 g of H₃PO₄ (85 wt% aqueous solution) was added into the above suspension. After the suspension became clear and transparent, it was cooled to room temperature. Finally, the water in the solution was evaporated and an orange solid was collected, dried and ground. The resulting orange fine powder was then dissolved in deionized water and subjected to re-crystallization for further purification. The other two samples were also synthesized according to the above method except for a different vanadium content.

2.2.2. Preparation of MIL-100 (Fe) and POMs@MIL-100 (Fe). The hybrids POMs@MIL-100 (Fe) were prepared by a one-pot method as described in the literature³¹ except for replacing 12-tungstophosphoric heteropolyacid with H_3+xPMo_12−xV_xO₄₀ (x = 0, 1, 2). The bulk MIL-100 (Fe) was also synthesized by the same method but without adding POMs. The as-prepared sample was dried in vacuum at 150 °C for 6 h prior to a further analysis or use.

2.3. Oxidation of cyclohexene

The liquid phase catalytic oxidation of cyclohexene was carried out in a round-bottom flask (50 mL) connected to a reflux condenser. In a typical reaction, cyclohexene (20 mmol) and catalyst (15 mg) were added to the glass flask. The reaction was initiated by adding 30 wt% H₂O₂ solution with vigorous stirring. The typical reaction temperature and time were 70 °C and 9 h, respectively. After reaction, the mixture was centrifuged to remove the catalyst, and the organic phase was analyzed using an HP 6890/5973 GC/MS instrument and quantified with an Agilent 6890 gas chromatograph using toluene as inner standard.

3. Results and discussion

3.1. Catalyst characterization

Fig. 1 and 2 show the FT-IR spectra of H_3+xPMo_12−xV_xO₄₀, MIL-100 (Fe) and H_3+xPMo_12−xV_xO₄₀@MIL-100 (Fe). The POMs (curves 1a–1c) exhibit four infrared bands characteristic of the Keggin structure: 780–800 cm⁻¹ (Mo–O_c–Mo), 860–880 cm⁻¹ (Mo–O_b–Mo), 960–990 cm⁻¹ (Mo=O_d) and 1060–1080 cm⁻¹ (P–O_a). The vibrational bands of MIL-100 (Fe) (curve 2a) around 1628, 1451, 1371, 761, and 709 cm⁻¹ and the peaks of H_3+xPMo_12−xV_xO₄₀ located at 1062, 959, 864, and 812 cm⁻¹ are all observed in the FT-IR spectra of H_3+xPMo_12−xV_xO₄₀@MIL-100 (Fe) (curves 2b–2d), which confirm the presence of MIL-100 (Fe) and H_3+xPMo_12−xV_xO₄₀ in the hybrid H_3+xPMo_12−xV_xO₄₀@MIL-100 (Fe). Moreover, for H_3+xPMo_12−xV_xO₄₀@MIL-100 (Fe), the ν_as(P–O_a) and ν_as(Mo–O_c) vibration bands (959 and 812 cm⁻¹, respectively) were slightly shifted in comparison to those of free POM (967 and 782 cm⁻¹, respectively). This shift discloses the confinement effect of POM inside the porous solid.

Fig. 1 FT-IR spectra of H₃PMo₁₂O₄₀ (1a), H₄PMo₁₁VO₄₀ (1b) and H₅PMo₁₀V₂O₄₀ (1c).

Fig. 2 FT-IR spectra of MIL-100 (Fe) (2a), H₃PMo₁₂O₄₀@MIL-100 (Fe) (2b), H₄PMo₁₁VO₄₀@MIL-100 (Fe) (2c) and H₅PMo₁₀V₂O₄₀@MIL-100 (Fe) (2d).

The UV-vis diffuse reflectance spectra of the samples are presented in Fig. 3 and 4. All samples have shown absorption bands in the region of 200–400 nm. The absorption bands around 206 nm are associated with O → P transition and the wide energy bands appearing around 307 nm correspond to the ligand to metal charge transfer O²⁻ → Mo⁶⁺ in the Keggin units (Fig. 3). It also can be seen from Fig. 4 that MIL-100 (Fe) has a peak near 340 nm (curve 4d). Compared with the corresponding POM and MIL-100 (Fe), the characteristic peaks of the hybrid material (curves 4a–4c) red shifted, indicating interactions between the two moieties.

Fig. 3 UV-vis DRS of H₃PMo₁₂O₄₀ (3a), H₄PMo₁₁VO₄₀ (3b), and H₅PMo₁₀V₂O₄₀ (3c).

Fig. 4 UV-vis DRS of H₃PMo₁₂O₄₀@MIL-100 (Fe) (4a), H₄PMo₁₁VO₄₀@MIL-100 (Fe) (4b), H₅PMo₁₀V₂O₄₀@MIL-100 (Fe) (4c), and MIL-100 (Fe) (4d).

The XRD patterns of the as-synthesized POMs are shown in Fig. 5. All the samples present diffraction peaks typically at 2θ around 8° and 27°. The XRD patterns for the MIL-100 (Fe) and H_3+xPMo_12−xV_xO₄₀@MIL-100 (Fe) hybrids are illustrated in Fig. 6 and all samples present diffraction peaks typically at 2θ of 10.7°. However, no diffraction peaks for the original H_3+xPMo_12−xV_xO₄₀ can be observed in the H_3+xPMo_12−xV_xO₄₀@MIL-100 (Fe) hybrids. This is probably due to the following two reasons: (1) the size of H_3+xPMo_12−xV_xO₄₀ clusters is relatively smaller; (2) the H_3+xPMo_12−xV_xO₄₀ clusters have been encapsulated in the cavities of MIL-100 (Fe). A similar phenomenon was also observed in previous literature.³⁵

Fig. 5 XRD of the samples: H₃PMo₁₂O₄₀ (5a), H₄PMo₁₁VO₄₀ (5b), and H₅PMo₁₀V₂O₄₀ (5c).

Fig. 6 XRD of the samples: MIL-100 (Fe) (6a), H₃PMo₁₂O₄₀@MIL-100 (Fe) (6b), H₄PMo₁₁VO₄₀@MIL-100 (Fe) (6c), and H₅PMo₁₀V₂O₄₀@MIL-100 (Fe) (6d).

The morphologies and elemental compositions of MIL-100 (Fe) and the hybrids H_3+xPMo_12−xV_xO₄₀@MIL-100 (Fe) were characterized by TEM/EDS. A representative image of MIL-100 (Fe) is shown in Fig. 7a and the images and EDS result of the sample H₄PMo₁₁VO₄₀/MIL-100 (Fe) are shown in Fig. 7b–d. It is clear that the MIL-100 (Fe) displays a polyhedral morphology and the morphology and porous network of the MIL-101 (Fe) host remained intact after encapsulation of the POMs and no conglomeration of POM particles was observed. The EDS elemental analysis pattern of the sample H_3+xPMo_12−xV_xO₄₀@MIL-100 (Fe) (Fig. 7d) reveals the presence of Fe, P, Mo, and V elements in the H_3+xPMo_12−xV_xO₄₀@MIL-100 (Fe) hybrids.

Fig. 7 TEM images and EDS pattern of MIL-100 (Fe) (a) and H₄PMo₁₁VO₄₀/MIL-100 (Fe) (b–d).

The N₂ adsorption–desorption isotherms of MIL-100 (Fe) and the hybrid samples are presented in Fig. 8 (inset is the pore size distribution plot). The pore size distribution curves of the as-prepared MIL-100 (Fe) and the hybrids display two different pore sizes centered at about 1.4 and 2.3 nm, demonstrating the presence of the two types of mesoporous cages in these samples. The BET surface areas and pore volumes of the samples are listed in Table 1. The BET surface area and pore volume for the prepared MIL-100 (Fe) are 1689 m² g⁻¹ and 1.02 cm³ g⁻¹, respectively, close to the values reported in ref. 31. On the other hand, both specific surface areas and pore volumes of the hybrids lowered remarkably to ∼900 m² g⁻¹ and ∼0.5 cm³ g⁻¹, respectively, after encapsulation of the POMs (Table 1).

Fig. 8 N₂ adsorption–desorption isotherms and pore size distribution plots of MIL-100 (Fe) (a), H₃PMo₁₂O₄₀/MIL-100 (Fe) (b), H₄PMo₁₁VO₄₀/MIL-100 (Fe) (c) and H₅PMo₁₀V₂O₄₀/MIL-100 (Fe) (d).

Table 1 BET surface area, pore volume and POM loading of the samples

Sample	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	POM loading (×10^−4) (mmol mg^−1)
MIL-100 (Fe)	1689	1.02	—
H3PMo12O40@MIL-100 (Fe)	908	0.35	1.92
H4PMo11VO40@MIL-100 (Fe)	931	0.42	1.76
H5PMo10V2O40@MIL-100 (Fe)	965	0.58	1.64

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The POM loadings of the hybrid based on Mo and V determined by ICP-AES are also listed in Table 1. It can be seen that a higher POM loading leads to more decrease in specific surface area and pore volume. This confirms that the POM molecules were encapsulated in the pores of MIL-101 (Fe).

3.2. Catalytic performance of catalysts

The catalytic performances of the as-prepared catalysts in the oxidation of cyclohexene were investigated using H₂O₂ as oxidant and acetonitrile as solvent. The detectable products are epoxide, 2-cyclohexen-1-ol (denoted as -ol), 2-cyclohexen-1-one (denoted as -one) and 1,2-cyclohexanediol (denoted as -diol) (Scheme 1).

Scheme 1 Oxidation of cyclohexene.

The results of cyclohexene oxidation over different catalysts are summarized in Table 2. The result in the absence of catalyst is also listed in entry 1 for comparison. It was found that only 11% cyclohexene conversion was obtained in the absence of catalyst. Although the POMs and MIL-100 (Fe) have shown considerable catalytic activities, not more than 37% cyclohexene conversion was obtained (entries 2–5) while a greatly improved cyclohexene conversion of 72–85% was obtained in the cases of the POM/MIL-100 (Fe) hybrids (entries 6–8). The main reason for the greatly improved activity for the POM/MIL-100 (Fe) hybrids is that the porous structure of the hybrids can enhance the accessibility of the reactant molecules to active centers and the fast diffusion of products.⁵

Table 2 Catalytic performances of various POM-based MOF catalysts in the oxidation of cyclohexene^a

Entry	Catalyst	Conversion (%)	TOF^b (h^−1)	Selectivity^c (%)
				Epoxide	-ol	-one	-diol
a Reaction conditions: cyclohexene 20 mmol, H2O2 (30 wt%) 40 mmol, catalyst 15 mg, acetonitrile 5 mL, 70 °C, 9 h. b TOF = mol of cyclohexene converted per mol of loaded POM per hour. c Selectivity = mol ratio of the given product to the total products. d From ref. 36. Reaction condition: 10 mL solution of n-heptane–TBHP (nTBHP = 10% ncyclohexene), 6 mmol cyclohexene, 50 mg catalyst, 333 K. e From ref. 37. Reaction conditions: 0.2 M cyclohexene, 0.4 M H2O2, 20 mg catalyst, 1.5 mL MeCN, 70 °C.
1	None	11	—	99	—	—	1
2	H3PMo12O40	31	84	68	—	—	32
3	H4PMo11VO40	33	87	—	—	17	83
4	H5PMo10V2O40	33	85	—	—	29	71
5	MIL-100 (Fe)	37	47	—	13	51	36
6	H3PMo12O40@MIL-100 (Fe)	72	556	—	—	46	54
7	H4PMo11VO40@MIL-100 (Fe)	85	715	—	9	91	—
8	H5PMo10V2O40@MIL-100 (Fe)	75	677	—	16	84	—
9^d	PdCl2-ILs/CuBTC	24		53.2 (-one)
10^e	Ti-POM/MIL-101	39		32 (-ol + -one)

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It is very interesting that the encapsulation greatly influenced the product distribution. The POMs are in favor of producing 1,2-cyclohexanediol (entries 3–4) except for H₃PMo₁₂O₄₀, over which 68% selectivity for the main product epoxide has been obtained (entry 2). In the case of MIL-100 (Fe), only 51% selectivity for the main product 2-cyclohexen-1-one has been obtained. However, the POM/MIL-100 (Fe) hybrids are in favor of producing 2-cyclohexen-1-one and higher than 84% selectivity has been obtained (entries 7–8) except for H₃PMo₁₂O₄₀@MIL-100 (Fe) over which 54% selectivity for the main product 1,2-cyclohexanediol has been obtained (entry 6). In particular, the hybrid H₄PMo₁₁VO₄₀@MIL-100 (Fe) has obtained the highest value of 85%, 91% and 715 h⁻¹ of cyclohexene conversion, 2-cyclohexene-1-one selectivity and turnover frequency (TOF), respectively. This is a more exciting result than that reported for metal–organic framework CuBTC immobilized PdCl₂ with the aid of ionic liquids (ILs)³⁶ (entry 9) and titanium-monosubstituted Keggin heteropolyanions electrostatically bound to the chromium terephthalate polymer matrix MIL-101 (entry 10).³⁷

According to the literature,^38,39 the oxidation of cyclohexene with H₂O₂ initially forms 2-cyclohexene-1-hydroperoxide as shown in Scheme 2 (step 1). 2-Cyclohexene-1-hydroperoxide is not stable and can form epoxide and 2-cyclohexene-1-ol by epoxidation of cyclohexene (step 2), decompose to 2-cyclohexene-1-ol and 2-cyclohexene-1-one in the presence of catalyst (step 3), or decompose to 2-cyclohexene-1-one and water (step 4). The conversion of cyclohexene is controlled by the rate of step 1 in Scheme 2 and the product distribution is controlled by the rate ratio of step 2 to steps 3 and 4. That is, the encapsulation greatly changed the rate ratio of step 2 to steps 3 and 4, and thus the distribution of the products.

Scheme 2 Proposed process of cyclohexene oxidation.^38,39

Most importantly, the POMs have been heterogenized after being encapsulated in MIL-100 (Fe). That is, the POM/MIL-100 (Fe) hybrids can be recovered and are reusable. In consideration of the high cyclohexene conversion, 2-cyclohexene-1-one selectivity and TOF, H₄PMo₁₁VO₄₀@MIL-100 (Fe) was chosen as the preferable catalyst to optimize the following reaction conditions.

3.2.1. Effect of catalyst amount. The effect of the catalyst amount on the oxidation of cyclohexene was investigated and the results are shown in Table 3. It can be seen that cyclohexene conversion increased greatly from 26% to 85% when the catalyst amount increased from 5 mg to 15 mg, while cyclohexene conversion was improved by only 7% when the catalyst amount increased further from 15 mg to 25 mg. As for the selectivity for the main product 2-cyclohexene-1-one, the H₂O₂ efficiency and TOF, they increased first but then dropped, and the highest values of 91%, 81% and 715 h⁻¹, respectively, were obtained when 15 mg catalyst was employed (entry 3). Based on the above facts, 15 mg catalyst was employed in the following investigations.

Table 3 The effect of catalyst amount on cyclohexene oxidation^a

Entry	Amount of catalyst (mg)	Conversion (%)	TOF^b (h^−1)	H2O2 efficiency (%)	Selectivity^c (%)
					-ol	-one
a Reaction conditions: cyclohexene 20 mmol, H2O2 (30 wt%) 40 mmol, acetonitrile 5 mL, 70 °C, 9 h. b TOF = mol of cyclohexene converted per mol of loaded POM per hour. c Selectivity = mol ratio of the given product to the total products.
1	5	26	656	56	20	80
2	10	53	669	73	16	84
3	15	85	715	81	9	91
4	20	89	561	74	14	86
5	25	92	464	69	18	82

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3.2.2. Effect of the H₂O₂ amount. The effect of the H₂O₂ amount on cyclohexene oxidation was investigated under varying molar ratios of H₂O₂/cyclohexene from 1 : 2 to 2.5 : 1, and the results are shown in Table 4. It is clear that cyclohexene conversion increased with the increase of H₂O₂. As a result, when the molar ratio of H₂O₂/cyclohexene increased from 1 : 2 to 2.5 : 1, cyclohexene conversion increased from 26% to 88%. The selectivity for 2-cyclohexene-1-one increased from 42% to 91% when the molar ratio of H₂O₂/cyclohexene increased from 1 : 2 to 2 : 1, but then decreased to 87% when the molar ratio of H₂O₂/cyclohexene increased further to 2.5 : 1. The second highest values of 81% and 715 h⁻¹ for H₂O₂ efficiency and TOF, respectively, were also obtained when the molar ratio of H₂O₂/cyclohexene was 2 : 1. Based on the above facts, the molar ratio of 2 : 1 was chosen for the following investigations.

Table 4 The effect of the H₂O₂ amount on cyclohexene oxidation^a

Entry	H2O2/cyclohexene (mol/mol)	Conversion (%)	TOF^b (h^−1)	H2O2 efficiency (%)	Selectivity^c (%)
					-ol	-one	-diol
a Reaction conditions: cyclohexene 20 mmol, H4PMo11VO40@MIL-100 (Fe) 15 mg, acetonitrile 5 mL, 70 °C, 9 h. b TOF = mol of cyclohexene converted per mol of loaded POM per hour. c Selectivity = mol ratio of the given product to the total products.
1	1 : 2	26	218	84	40	42	18
2	1 : 1	42	353	77	39	61	—
3	1.5 : 1	70	589	80	22	78	—
4	2 : 1	85	715	81	9	91	—
5	2.5 : 1	88	740	70	13	87	—

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3.2.3. Effect of reaction time. The influence of reaction time on cyclohexene oxidation at 70 °C is shown in Table 5. It can be seen that the cyclohexene conversion and 2-cyclohexene-1-one selectivity increased from 51% to 88% and 61% to 94%, respectively, upon prolonging the reaction time from 3 h to 11 h while TOF decreased from 1287 h⁻¹ to 606 h⁻¹, respectively. 1,2-Cyclohexanediol was hardly detected when the reaction time was over 9 h. This may be due to 1,2-cyclohexanediol being adsorbed in MIL-100 (Fe) or was deeply oxidized to other undetected products. As for H₂O₂ efficiency, it increased from 72% to 81% when the reaction time was prolonged from 3 h to 9 h, respectively, but then decreased to 73% when the reaction time was further prolonged to 11 h. In pursuit of high selectivity for 2-cyclohexene-1-one and H₂O₂ efficiency, 9 h was considered as the optimum reaction time.

Table 5 The effect of reaction time on cyclohexene oxidation^a

Entry	Reaction time (h)	Conversion (%)	TOF^b (h^−1)	H2O2 efficiency (%)	Selectivity^c (%)
					-ol	-one	-diol
a Reaction conditions: cyclohexene 20 mmol, H2O2 (30 wt%) 40 mmol, H4PMo11VO40@MIL-100 (Fe) 15 mg, acetonitrile 5 mL, 70 °C. b TOF = mol of cyclohexene converted per mol of loaded POM per hour. c Selectivity = mol ratio of the given product to the total products.
1	3	51	1287	72	18	61	21
2	5	56	848	74	22	68	10
3	7	78	844	77	11	75	4
4	9	85	715	81	9	91	—
5	11	88	606	73	6	94	—

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3.2.4. Effect of reaction temperature. The effect of reaction temperature on cyclohexene oxidation was also investigated and the results are shown in Table 6. It is obvious that cyclohexene conversion, H₂O₂ efficiency, 2-cyclohexene-1-one selectivity and TOF all increased with elevating temperature. As a result, the highest values of 85%, 81%, 91% and 715 h⁻¹ were obtained for cyclohexene conversion, H₂O₂ efficiency, 2-cyclohexene-1-one selectivity and TOF, respectively, at 70 °C.

Table 6 The effect of reaction temperature on cyclohexene oxidation^a

Entry	Reaction temperature (°C)	Conversion (%)	TOF^b (h^−1)	H2O2 efficiency (%)	Selectivity^c (%)
					-ol	-one	-diol
a Reaction conditions: cyclohexene 20 mmol, H2O2 (30 wt%) 40 mmol, H4PMo11VO40@MIL-100 (Fe) 15 mg, acetonitrile 5 mL, 9 h. b TOF = mol of cyclohexene converted per mol of loaded POM per hour. c Selectivity = mol ratio of the given product to the total products.
1	40	32	269	60	—	5	95
2	50	53	446	70	—	34	66
3	60	67	563	75	15	62	23
4	70	85	715	81	9	91	—

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3.3. Reusability of the catalyst

To investigate the reusability of the catalyst H₄PMo₁₁VO₄₀@MIL-100 (Fe), the catalyst was separated by centrifugation after the first run, dried at 150 °C under vacuum and then subjected to the second run under the same conditions. After five consecutive cycles under our optimized conditions, the catalytic activity of the sample hardly dropped. The average values for cyclohexene conversion, the selectivity to 2-cyclohexene-1-ol, 2-cyclohexene-1-one and TOF were 83%, 10%, 90% and 698 h⁻¹, respectively, which were nearly the same as those of the fresh one (Fig. 9). To estimate the stability of the catalyst, the loading of POM and the porous structure of the catalyst were investigated again after five recycle runs. The values of POM loading, BET surface area and pore volume are 1.70 mmol mg⁻¹, 854 m² g⁻¹ and 0.37 cm³ g⁻¹, which are 3.04%, 8.20% and 10.78% lower than those of the fresh one, respectively. The porous structure of the five-run reused catalyst hardly changed except for a 0.3 nm decrease in pore diameter (centered at 1.0 nm and 1.9 nm) compared with the fresh one as depicted in Fig. 10. The results confirmed that the catalyst has high stability and reusability.

Fig. 9 Reusability of the catalyst H₄PMo₁₁VO₄₀@MIL-100 (Fe). Reaction conditions: cyclohexene 20 mmol, H₂O₂ (30 wt%) 40 mmol, H₄PMo₁₁VO₄₀@MIL-100 (Fe) 15 mg, acetonitrile 5 mL, 70 °C, 9 h.

Fig. 10 N₂ adsorption–desorption isotherm and pore size distribution plot of H₄PMo₁₁VO₄₀/MIL-100 (Fe) after five runs.

4. Conclusions

Polyoxometalate catalysts have been successfully heterogenized through a simple one-pot hydrothermal method by encapsulation of H_3+xPMo_12−xV_xO₄₀ in the metal–organic framework MIL-100 (Fe). The hybrids have shown greatly improved catalytic activities and stabilities in challenging the allylic oxidation of cyclohexene using H₂O₂ as green oxidant compared with the corresponding moieties. In particular, the hybrid H₄PMo₁₁VO₄₀@MIL-100 (Fe) has obtained the average values of 83% and 90% for cyclohexene conversion and 2-cyclohexene-1-one, respectively, after five cycles. This is a most outstanding example of the preparation of POMs/MOF hybrids and selective allylic oxidation of cyclohexene to produce 2-cyclohexene-1-one with high selectivity.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (51302222, 21363021, 21202133), the Natural Science Foundation of Gansu Province (1308RJYA017), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R56) for financial support.

References

1. D. E. De Vos, S. de Wildeman, B. F. Sels, P. J. Grobet and P. A. Jacobs, Angew. Chem., Int. Ed., 1999, 38, 980–983.
2. C. Yin, Z. Yang, B. Li, F. Zhang, J. Wang and E. Ou, Catal. Lett., 2009, 131, 440–443.
3. I. Y. Skobelev, A. B. Sorokin, K. A. Kovalenko, V. P. Fedin and O. A. Kholdeeva, J. Catal., 2013, 298, 61–69.
4. Y. Cao, H. Yu, F. Peng and H. Wang, ACS Catal., 2014, 4, 1617–1625.
5. G. Zou, D. Jing, W. Zhong, F. Zhao, L. Mao, Q. Xu, J. Xiao and D. Yin, RSC Adv., 2016, 6, 3729–3734.
6. D. R. Godhani, H. D. Nakum, D. K. Parmar, J. P. Mehta and N. C. Desai, J. Mol. Catal. A: Chem., 2016, 415, 37–55.
7. R. Zhang and R. Tang, J. Mater. Sci., 2016, 51, 5802–5810.
8. D. Yang, T. Jiang, T. Wu, P. Zhang, H. Han and B. Han, Catal. Sci. Technol., 2016, 6, 193–200.
9. S. Urus, H. Achguzel and M. Incesu, Chem. Eng. J., 2016, 296, 90–101.
10. M. Bazarganipour and M. Salavati-Niasari, Chem. Eng. J., 2016, 286, 259–265.
11. L. Q. Xu, J. C. Chen, S. S. Qian, A. K. Zhang, G. D. Fu, C. M. Li and E.-T. Kang, Macromol. Chem. Phys., 2015, 216, 417–426.
12. W. Adam, M. Herold, C. L. Hill and C. R. Saha-Moller, Eur. J. Org. Chem., 2002, 941–946.
13. J. A. F. Gamelas, F. Oliveira, M. G. Evtyugina, I. Portugal and D. V. Evtuguin, Appl. Catal., A, 2016, 509, 8–16.
14. U. Jameel, M. Q. Zhu, X. Z. Chen and Z. F. Tong, J. Mater. Sci., 2016, 51, 2181–2198.
15. A. Dolbecq, E. Dumas, C. R. Mayer and P. Mialane, Chem. Rev., 2010, 110, 6009–6048.
16. X. J. Song, W. C. Zhu, K. G. Li, J. Wang, H. L. Niu, H. C. Gao, W. X. Gao, W. X. Zhang, J. H. Yu and M. J. Jia, Catal. Today, 2016, 259, 59–65.
17. M. R. Farsani, F. Jalilian, B. Yadollahi and H. A. Rudbari, Appl. Organomet. Chem., 2015, 29, 7–11.
18. F. Bentaleb, O. Makrygenni, D. Brouri, C. C. Diogo, A. Mehdi, A. Proust, F. Launay and R. Villanneau, Inorg. Chem., 2015, 54, 7607–7616.
19. A. Mouret, L. Leclercq, A. Muhlbauer and V. Nardello-Rataj, Green Chem., 2014, 16, 269–278.
20. V. Mirkhani, M. Moghadam, S. Tangestaninejad, I. Mohammadpoor-Baltork, E. Shams and N. Rasouli, Appl. Catal., A, 2008, 334, 106–111.
21. M. Sun, J. Zhang, P. Puta, V. Caps, F. D. R. Lefebvre, J. Pelletier and J.-M. Basset, Chem. Rev., 2014, 114, 981–1019.
22. D. J. Thompson, Y. Zhang and T. Ren, J. Mol. Catal. A: Chem., 2014, 392, 188–193.
23. L. Suo, R. Q. Meng, D. M. Zheng, L. X. Wu and L. H. Bi, Appl. Organomet. Chem., 2014, 28, 845–851.
24. M. Li, M. Zhang, A. Wei, W. Zhu, S. Xun, Y. Li, H. Li and H. Li, J. Mol. Catal. A: Chem., 2015, 406, 23–30.
25. N. Wu, B. Li, W. Ma and C. Han, Microporous Mesoporous Mater., 2014, 186, 155–162.
26. D. Y. Du, J. S. Qin, S. L. Li, Z. M. Su and Y. Q. Lan, Chem. Soc. Rev., 2014, 43, 4615–4632.
27. G. Férey, Chem. Soc. Rev., 2008, 37, 191–214.
28. X.-J. Kong, Z. Lin, Z.-M. Zhang, T. Zhang and W. Lin, Angew. Chem., 2016, 55, 6411–6416.
29. C. M. Granadeiro, L. S. Nogueira, D. Juliao, F. Mirante, D. Ananias, S. S. Balula and L. Cunha-Silva, Catal. Sci. Technol., 2016, 6, 1515–1522.
30. L. Bromberg, Y. Diao, H. M. Wu, S. A. Speakman and T. A. Hatton, Chem. Mater., 2012, 24, 1664–1675.
31. F. M. Zhang, Y. Jin, J. Shi, Y. J. Zhong, W. D. Zhu and M. S. El-Shall, Chem. Eng. J., 2015, 269, 236–244.
32. R. Canioni, C. Roch-Marchal, F. Secheresse, P. Horcajada, C. Serre, M. Hardi-Dan, G. Ferey, J. M. Greneche, F. Lefebvre, J. S. Chang, Y. K. Hwang, O. Lebedev, S. Turner and G. Van Tendeloo, J. Mater. Chem., 2011, 21, 1226–1233.
33. R. Liang, R. Chen, F. Jing, N. Qin and L. Wu, Dalton Trans., 2015, 44, 18227–18236.
34. A. Micek-Ilnicka and B. Gil, Dalton Trans., 2012, 41, 12624–12629.
35. J. S. Li, Y. J. Tang, C. H. Liu, S. L. Li, R. H. Li, L. Z. Dong, Z. H. Dai, J. C. Bao and Y. Q. Lan, J. Mater. Chem. A, 2016, 4, 1202–1207.
36. Q. X. Luo, M. Ji, S. E. Park, C. Hao and Y. Q. Li, RSC Adv., 2016, 6, 33048–33054.
37. N. V. Maksimchuk, M. N. Timofeeva, M. S. Melgunov, A. N. Shmakov, Y. A. Chesalov, D. N. Dybtsev, V. P. Fedin and O. A. Kholdeeva, J. Catal., 2008, 257, 315–323.
38. L. Yanyong, M. Kazuhisa, I. Megumu, N. Hitoshi, K. Masahiko and T. Keizou, Chem. Lett., 2004, 33, 200–201.
39. M. N. Timofeeva, S. H. Jhung, Y. K. Hwang, D. K. Kim, V. N. Panchenko, M. S. MeIgunov, Y. A. Chesalov and J. S. Chang, Appl. Catal., A, 2007, 317, 1–10.

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