A new polyoxometalate-based Mo/V coordinated crystalline hybrid and its catalytic activity in aerobic hydroxylation of benzene

Abstract

A novel self-assembled polyoxometalate-based Mo/V coordinated crystalline organic–inorganic hybrid compound [Mo₂V₂O₈(2,2′-bpy)₆][HPMo₁₀V₂O₄₀]·2H₂O (bpy = bipyridyl) was hydrothermally synthesized by introducing polyoxometalate into a metal–organic unit containing metal nodes and 2,2′-bipyridyl ligands. The unit of the crystal consists of one Keggin-structure heteropolyoxoanion [HPMo₁₀V₂O₄₀]⁴⁻, one tetranuclear organic–metal coordination cation [Mo₂V₂O₈(2,2′-bpy)₆]⁴⁺ and two water molecules. The crystalline hybrid presents heterogeneous catalytic performance and superior activity towards benzene hydroxylation with molecular oxygen as the oxidant and ascorbic acid as the sacrificial reducing agent, giving a phenol yield of 12.5% and a high TON (turnover number) of 41. The result indicates that the polyoxometalate-based crystalline organic–inorganic hybrid compound provides an efficient heterogeneous catalyst for the liquid-phase aerobic hydroxylation of benzene to phenol.

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Introduction

Crystalline organic–inorganic hybrids are always composed of inorganic clusters, metal nodes and organic moieties in coordination with them. Such hybrid materials offer a new route for combining multiple functional groups in a framework with explicit structure at the molecular level and show numerous potential applications in absorption, optics, electronics and catalysis.^1–3 Polyoxometalates (POMs) are made up of multiple early transition metal–oxygen clusters, which have attracted numerous attention for applications in biochemistry, photochemistry, magnetochemistry, electrochemistry, as well as catalytic chemistry, due to their structural diversity and various chemical properties.⁴ It is noteworthy that the combination of POMs and metal–organic units in crystalline hybrids will give these compounds “added-values”, which can invoke synergistic effects between organic and inorganic moieties for catalytic applications.⁵ Many studies have been performed on the synthesis of crystalline POM-based organic–inorganic hybrids,^6–9 and the focus and difficulty is how to design and synthesize suitable crystals for catalytic reactions specifically. Hydroxylation of benzene to phenol, one of the most crucial commodity chemicals, is one promising alternative for the conventional energy consuming three-step cumene process.¹⁰ For decades, various catalytic systems with the oxidants of O₂,^11–16 H₂O₂ ^17–19 or N₂O ²⁰ have been introduced into this direct oxidation process, including high-temperature oxidation,²¹ photoinduced oxidation²² and membrane-based oxidation.²³ Among these oxidants, molecular oxygen is most attractive because it is cheap and environmentally benign.²⁴ Normally, it is very difficult to directly perform the aerobic oxidation of benzene with oxygen, and thus reducing agents like ascorbic acid are usually added together with a main catalyst in the reaction.^13–16 Particularly, Keggin-structured molybdovanadophosphoric acids PMoV_n, have been proved to be effective for catalyzing benzene hydroxylation with O₂. Recently, a variety of organic or inorganic compounds have been used to modify PMoV_n for hydroxylation of benzene, but most of them are homogeneous catalysts.^25–28 Tsuruya et al. reported the ion-exchange of the protons of PMoV_n with inorganic Cs⁺ cations and the resultant Cs salt was used as the heterogeneous catalyst for the reaction.¹³ Gao et al. supported POMs onto metal–organic framework (MOF) materials and obtained the heterogeneous catalysts PMoV@HKUST-1.²⁹ Our group used 4,4′-bipyridine to modify PMoV₁ and investigated the catalytic performance in heterogeneous benzene hydroxylation.³⁰ However, these catalysts still suffer from low phenol yields. From green chemistry prospective, it is highly desirable to explore more efficient heterogeneous catalysts in this area.

In this work, a new POM-based single crystal is designed and applied as the heterogeneous catalyst for hydroxylation of benzene with O₂ to phenol. POM-based crystalline organic–inorganic hybrids have been widely used in oxidation of organic substrates,^31–33 but rarely on oxygen-based hydroxylation of benzene.³⁴ Through finely controlling the hydrothermal conditions, the POM-derived single crystal is prepared by self-assembly of the molybdovanadophosphoric acid H₅PMo₁₀V₂O₄₀ (PMoV₂), metal oxide chain and 2,2′-bipyridyl ligand. We choose 2,2′-bipyridyl to coordinate with metal nodes because the bidentate chelate ligand possesses stronger coordination ability and will be suitable for designing desirable structures.¹⁰ The obtained hybrid is fully characterized and its catalytic performance is assessed in the hydroxylation of benzene to phenol using molecular oxygen as the oxidant and ascorbic acid as the reductant. The crystalline hybrid acts as an efficient heterogeneous catalyst for hydroxylation of benzene in this reaction system due to the improved amounts of active centers by introducing metals V/Mo into the crystalline cation.³⁵

Experimental

Materials and characterization

All chemicals (solvents and reagents) were analytic grade and used without any purification. The CHN elemental analysis was performed on an elemental analyzer Vario EL cube. FT-IR spectra were collected using KBr pellets on a Nicolet iS10 FT-IR instrument. X-ray diffraction (XRD) measurements were measured with a SmartLab diffractometer (Rigaku Corporation) equipped with a 9 kW rotating anode Cu source at 40 kV and 100 mA, from 5 to 50° with a scan rate of 0.2° s⁻¹. TG analysis was carried out with a STA409 instrument in dry air from 40 to 700 °C with a heating rate of 10 °C min⁻¹. UV-vis diffuse reflectance (UV-vis DR) spectra were collected on the SHIMADZU UV-2600 in the region of 220–850 nm and BaSO₄ was used as an internal standard. Electron spin resonance (ESR) spectra were recorded on a Bruker EMX-10/12 spectrometer at the X-band. The measurements were done at −110 °C in a frozen solution provided by a liquid/gas nitrogen temperature regulation system controlled by a thermocouple located at the bottom of the microwave cavity within a Dewar insert. SEM images were performed on a HITACHI S-4800 field-emission scanning electron microscope and environmental scanning electron microscope (Quanta 200, FEI).

Synthesis of catalysts. The di-vanadium-containing POM, H₅PMo₁₀V₂O₄₀, was synthesized according to the ref. 36. Typically, MoO₃ (16.57 g, 0.115 mol) and V₂O₅ (2.09 g, 0.0115 mol) were mixed in 250 ml deionized water and then heated up to reflux temperature under vigorously stirring with a water-cooled condenser. After stirring for a period of time, the concentrated H₃PO₄ (85 wt% 1.33 g, 0.0115 mmol) was added drop-wise into the above reaction mixture. With a further stirring for 24 h, the solution became clear and orange-red. After cooling to room temperature, the orange-red powder was obtained by vacuum drying of the solution at 50 °C for 24 h. The final product H₅PMo₁₀V₂O₄₀ was purified by re-crystallization of the powder in deionized water. The POM-based crystalline hybrid, [Mo₂V₂O₈(2,2′-bpy)₆][HPMo₁₀V₂O₄₀]·2H₂O ([MoVbpy]HPMoV₂), was prepared in a hydrothermal system. In a typical procedure, MoO₃ (0.072 g, 0.5 mmol), V₂O₅ (0.0182 g, 0.1 mmol), H₅PMo₁₀V₂O₄₀ (0.1738 g, 0.1 mmol), 2,2′-bipyridyl (bpy, 0.078 g, 0.5 mmol) and deionized water (10 ml) were mixed in a glass beaker (25 ml, cylindrical), then the pH of the mixture was adjusted to 6.0 with 1 M NaOH under vigorously stirring. Afterwards, the mixture was transferred and sealed in a 25 ml Teflon-lined autoclave and kept in 160 °C for 96 h. After cooling to room temperature, the crimson crystal was separated by filtration, washed with ethanol and dried overnight in a vacuum oven (0.1225 g, yield 39.2% based on P). The sample was denoted as [MoVbpy]HPMoV₂. The elemental analysis of the hybrid: found C 23.34%, H 1.64%, and N 5.27% (calc. C 23.03%, H 1.69%, and N 5.37%). During the synthesis, the initial pH value was varied, and the samples, MoVbpy-PMoV-x (x indexes the pH value), were prepared similarly as [MoVbpy]HPMoV₂ except that the reaction pH was changed to be 5.9 and 6.1. For comparison, three POM based organic–inorganic hybrids, named as [Dmim]_2.5PMo₁₀V₂, [Dpy]_2.5PMo₁₀V₂ and [bpy] _2.5PMo₁₀V₂ (Scheme S1†), were prepared through the pairing H₅PMoV₂O₄₀ with 1,1′-(butane-1,4-diyl)-bis(3-methylimidazolium)di(bromide), denoted as ([Dmim]Br₂), 1,1′-(butane-1,4-diyl)-bis-pyridine di(bromide) shorten as [Dpy]Br₂, and 2,2′-bipyridyl in aqueous solution respectively.³⁷ The ionic liquid precursors, [Dmim]Br₂ and [Dpy]Br₂ were prepared according to previous report.³⁶ Elemental analysis of the three hybrids: [Dpy]_2.5PMo₁₀V₂ Calcd: C, 18.50 wt%; N, 3.08 wt%; H, 1.98 wt%. Found: C, 18.00 wt%; N, 2.86 wt%; H, 2.06 wt%. [Dmim]_2.5PMo₁₀V₂ Cacld: C, 15.78 wt%; N, 6.13 wt%; H, 2.20 wt%. Found: C, 15.57 wt%; N, 6.16 wt%; H, 2.25 wt%. [bpy]_2.5PMo₁₀V₂ Cacld: C, 14.11 wt%; N, 3.29 wt%; H, 1.18 wt%. Found: 13.98 wt%; N, 3.21 wt%; H, 1.23 wt%.

X-ray crystallography. The single crystal was isolated by visual examination under the microscope, and then glued at the top of a thin glass fiber with epoxy glue in air for data collection. The crystallographic data was collected on a Bruker ApexIICCD with Mo-Kα radiation (λ = 0.71073 Å) at 296 K using ω−2θ scan method. The crystal structure was solved by direct method and refined on F² by full-matrix least-squares methods using the SHELX97 program package. The crystal data is presented in Table 1. The bond distances and angles for [MoVbpy]HPMoV₂ are listed in Table S1.†

Table 1 Crystal data and structure refinement for [MoVbpy]HPMoV₂

Identification code	[MoVbpy]HPMoV2
Empirical formula	C60H53Mo12N12O50PV4
Formula weight	3128.15
Temperature (K)	296(2)
Wavelength (Å)	0.71073
Crystal system, space group	Triclinic, p(1̄)
a (Å)	a = 11.4527(13)
b (Å)	b = 14.6366(17)
c (Å)	c = 14.6690(17)
α (°)	104.877(2)
β (°)	90.706(2)
γ (°)	109.7960(10)
Volume (Å^3)	2222.7(4)
Z	1
Calculated density (mg m^−3)	2.337
Absorption coefficient (mm^−1)	2.150
F (000)	1508
Crystal size (mm^3)	0.15 × 0.13 × 0.12
Theta range for data collection (°)	1.44 to 26.80
Limiting indices	−14 ≤ h ≤ 14
	−18 ≤ k ≤ 17
	−18 ≤ l ≤ 8
Reflections collected/unique	16 393
Independent reflection	8370 [R (int) = 0.0448]
Completeness to theta = 26.80	87.80%
Max. and min. transmission	0.7824 and 0.7386
Refinement method	Full-matrix least-squares on F^2
Data/restraints/parameters	8370/36/655
Goodness-of-fit on F^2	1.024
Final R indices [I > 2sigma (I)]	R1 = 0.0462, wR2 = 0.1073
R Indices (all data)	R1 = 0.0873, wR2 = 0.1267

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Catalytic test. The catalytic performance was assessed in the hydroxylation of benzene with molecular oxygen using ascorbic acid as the reductant. The reaction was carried out in a customer-designed temperature controllable pressured titanium reactor (25 ml) under magnetic stirring. In a typical experiment, 0.1 g catalyst, 0.3 g ascorbic acid, 0.87 g benzene and 5 ml acetic acid solution (3 ml acetic acid and 2 ml deionized water) were added into the reactor. Afterwards, 2.0 Mpa oxygen was injected into the reactor. The reaction was conducted at 373 K for 10 h. After reaction, 1,4-dioxane was added into the mixture as an internal standard for product analysis. Gas chromatographic (GC) measurement was performed on a SP-6890A equipped with a FID detector and a capillary column (SE-54; 30 m × 0.32 mm × 0.25 μm). Phenol yield % = mmol phenol/mmol initial benzene × 100%. The leaching test was conducted in two steps, for the first step, 0.1 g catalyst, 0.25 g ascorbic acid, 0.87 g benzene and 5 ml acetic acid solution (3 ml acetic acid and 2 ml deionized water) were added into a flask and stirred at 100 °C for 2 h. After stirring, the solid was separated by centrifugation. For the second step, the resulting filtrate was put into the customer-designed temperature controllable pressured titanium reactor and 1.5 Mpa oxygen was injected into the reactor. Afterwards, the mixture went on to react for 8 h at 100 °C.

Results and discussion

Materials preparation and characterizations

The V-containing POM-based single crystal sample is synthesized by hydrothermal reaction of MoO₃, V₂O₅, PMoV₂ and 2,2′-bipyridyl (bpy) in aqueous solution. The hydrothermal technique has been proved to be a useful method for the synthesis of crystalline compounds, and it is well known that various synthetic parameters such as temperature, time and pH value significantly influence the crystal growth process. After optimization, the suitable molar composition for the initial synthetic solution is 5MoO₃/1V₂O₅/1PMoV₂/5bpy/5555H₂O, and the hydrothermal conditions are 433 K and 96 h. The pH value of initial solution before hydrothermal crystallization is found to be the most sensitive parameter for the formation of crystalline structure. When the pH is adjusted to be 6.0, the sample is black and regular particle. However, when pH value turns to be higher (6.1) or lower (5.9), the obtained solids are white and blue powder respectively.

The microscopic analysis indicates that the sample synthesized at pH = 6.0 is single crystal, which is denoted as [MoVbpy]HPMoV₂. Single-crystal X-ray diffraction analysis reveals that [MoVbpy]HPMoV₂ crystallizes in the low-symmetry triclinic space group P1̄. As shown in Fig. 1, the asymmetric unit of [MoVbpy]HPMo₁₀V₂ is composed of half of an cluster anion [HPMo₁₀V₂O₄₀]⁴⁻, three 2,2′-bpy ligands, one Mo center, one V center, four oxygen atoms, as well as two lattice water molecules. The basic unit is shown in Fig. 2. The heteropolyanion [HPMo₁₀V₂O₄₀]⁴⁻ consists of twelve distorted {MoO₆} or {VO₆} octahedral and one central {PO₄} tetrahedron. In other words, the Keggin structure is stable during the synthesis reaction. The V center is coordinated by four N atoms from two 2,2′-bpy ligands and two oxygen atoms and possesses the octahedral coordination geometries. The bond distances around V center are 2.087(6)–2.310(6) Å for V–N, 1.588(5)–1.948(5) Å for V–O, respectively. According to the charge balance calculation and bond valence sum, application of S = exp[(R₀−R)/b] (R₀ = 1.879 b = 0.305), the two V atoms are in +4 reduction state (Table S1†). Moreover, the Mo center exhibits similar octahedral coordination structure and is coordinated by two N atoms from one ligand and four oxygen atoms, with Mo–O bond distance of 1.683(5)–2.130(5) Å, Mo–N bond distance of 2.236(6)–2.328(6) Å. For the [Mo₂V₂O₈(2,2′-bpy)₆]⁴⁺ cation, two VN₄O₂ octahedrons and two MoN₄O₂ octahedrons are connected by sharing angles and edges to form an interesting tetranuclear metal–organic structure. The distance between two Mo atoms is 3.0745 Å (Table S2†). In the structure of [MoVbpy]HPMoV₂, the adjacent [HPMo₁₀V₂O₄₀]⁴⁻ anions and [Mo₂V₂O₈(2,2′-bpy)₆]⁴⁺ cations are connected through H-bonds and electrostatic interactions to generate a 3D supermolecular framework (Fig. 3).

Fig. 1 The asymmetric unit of [MoVbpy]HPMo₁₀V₂.

Fig. 2 The structural unit in [MoVbpy]HPMo₁₀V₂. The H atoms are omitted for clarity.

Fig. 3 A view (from a axis and b axis) of supermolecular structure of [MoVbpy]HPMo₁₀V₂.

The sample [MoVbpy]HPMoV₂ is further characterized by SEM, IR, ESR, UV-vis and TG analyses. Fig. 4 gives the SEM images, showing the morphology of uniform rectangular block crystals. The three-dimensional size of [MoVbpy]HPMoV₂ is 80, 50 and 35 μm for length, width and height, respectively. Fig. 5(A) illustrates the FT-IR spectrum of [MoVbpy]HPMoV₂ and the peaks at 798 and 866 cm⁻¹ are attributed to the vibrations of Mo–O_c–Mo and Mo–O_b–Mo for the Keggin structure. The peaks at 1054 and 1074 cm⁻¹ are attributed to the vibrations of P–O and the peaks at 948 and 976 cm⁻¹ are attributed to the vibrations of Mo–O_d. The bands ranged from 1444 to 1599 cm⁻¹ correspond to the characteristic vibrations of C–C vibration in the 2,2′-bpy ligand, as well as the absorption at 768 cm⁻¹. These characterization peaks are intact after hydrothermal reaction, meaning that the structure of bpy ligand and PMoV₂ anion can be fully kept in the crystal [MoVbpy]HPMoV₂. The ESR spectrum of [MoVbpy]HPMoV₂ (Fig. 5(B)) shows a Mo⁵⁺ signal with g = 1.948, which suggests that Mo⁶⁺ in POM framework has partially been reduced to Mo⁵⁺.³⁸ This feature indicates that the electronic state of Mo ions has been changed due to the strong electronic interaction between cations and anions. The UV-vis DR spectrum of [MoVbpy]HPMoV₂ in Fig. 5(C) displays two absorption bands at 226 and 305 nm that are attributed to the π–π* transition of the pyridine rings.³⁹ In addition, the large broad absorption band at 650–800 nm is assigned to the charge transfer from V⁵⁺ to V⁴⁺.⁴⁰ Meanwhile, this absorption band may also be related to the Mo⁵⁺/Mo⁶⁺ charge transfer,⁴¹ which has been also reflected by the ESR spectrum in Fig. 5(B). In addition, the crystal hybrid [MoVbpy]HPMoV₂ exhibits an intense absorption pattern in the visible range at 390 nm associated with O → Mo charge transfer (LMCT).⁴² The existence of the reduced V⁴⁺ and Mo⁵⁺ species are on account of the synergistic effect between building cation [Mo₂V₂O₈(2,2′-bpy)₆]⁴⁺ and heteropolyanion [HPMo₁₀V₂O₄₀]⁴⁻. The TG analysis is carried out to investigate the thermal stability of the [MoVbpy]HPMoV₂ crystal, as shown in Fig. 5(D). The slight weight loss below 380 °C is due to desorption of adsorbed water, indicating good thermostability up to 380 °C. In the next temperature range 380–410 °C, the weight loss is attributed to the decomposition of the two 2,2′-bpy ligands and the weight loss data 9.8% corresponds to the calculated value 9.9%. The drastic weight loss at 450–500 °C is due to the complete decomposition of the rest 2,2′-bpy ligands and the data 19.0% is also close to the calculated one (19.9%).

Fig. 4 SEM images for [MoVbpy]HPMoV₂.

Fig. 5 FT-IR spectrum (A), ESR spectrum (B), UV-vis DR spectrum (C) and TG curve (D) of [MoVbpy]HPMoV₂.

Catalytic test of hydroxylation of benzene with O₂

The hydroxylation of benzene with O₂ to phenol is carried out to investigate the catalytic activity of [MoVbpy]HPMoV₂. The reaction condition is optimized by investigating various parameters including oxygen pressure, catalyst and ascorbic acid amounts, reaction temperature and time (Table S3†), based on which, a relatively high phenol yield of 12.5% is achieved under the suitable conditions as follows: 0.10 g (0.03 mmol) catalyst, 0.25 g (1.42 mmol) ascorbic acid, 0.87 g (11.15 mmol) benzene, 5 ml aqueous acetic acid solution (60 vol%), 1.5 MPa O₂, 373 K and 10 h.

Under the same conditions, not any products (including phenol) are detected without adding a catalyst and ascorbic acid (Table 2, Entry 1). Even though when ascorbic acid is added, it can only give 3.6% phenol yield (Table 2, Entry 2). When the parent POM H₅PMo₁₀V₂O₄₀ is used as the catalyst with ascorbic acid as co-reductant, 4.2% yield of phenol is detected (Table 2, Entry 3). The results suggest that the [PMo₁₀V₂O₄₀]⁵⁻ anion is active for hydroxylation of benzene, in agreement with the previous literature.²⁶ However, it is noteworthy that H₅PMo₁₀V₂O₄₀ can easily dissolve in this reaction solution, giving a homogeneous catalytic system. By contrast, single crystal [MoVbpy]HPMoV₂ not only generates a heterogeneous system but also gives a high phenol yield of 12.5% (Table 2, Entry 4). The turnover number (TON: mmol phenol/mmol catalyst) on the crystal catalyst is 41. The leaching test is performed to confirm its heterogeneous nature for the hydroxylation reaction. The single crystal [MoVbpy]HPMoV₂ is put in the reaction solution in the absence of O₂ and the mixture is stirred at 100 °C for 2 h. After that, the solid is separated by centrifugation, and the resulting filtrate is injected with oxygen to react for 8 h. The phenol yield is 3.7% (Table 2, Entry 5), similar to the result without adding the catalyst. The above phenomenon indicates that it is the single crystal solid that performs the catalytic active sites in this reaction.

Table 2 Hydroxylation of benzene with O₂ over various catalysts^a

Entry	Catalyst	Phenomenon	Yield^b (%)
a Reaction condition: 0.1 g catalyst, 0.25 g (1.42 mmol) ascorbic acid, 1 ml (0.87 g 11.15 mmol) benzene, 5 ml acetic acid solution (3 ml acetic acid and 2 ml deionized water), 1.5 MPa O2, 373 K, 10 h.b Yield of phenol (%) = mmol phenol/mmol initial benzene.c Leaching test.d Catalysts prepared under different pH values.
1	Without catalyst and ascorbic acid	—	—
2	Without catalyst	—	3.6
3	H5PMo10V2O40	Homogeneous	4.2
4	[MoVbpy]HPMoV2	Heterogeneous	12.5
5^c	Without catalyst	—	3.7
6	MoVbpy-PMoV-5.9^d	Heterogeneous	6.5
7	MoVbpy-PMoV-6.1^d	Heterogeneous	7.2
8	[bpy]2.5PMo10V2	Heterogeneous	7.5
9	[Dmim]2.5PMo10V2	Heterogeneous	8.0
10	[Dpy]2.5PMo10V2	Heterogeneous	7.8

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Table 2 also compares the catalytic performance of control catalysts for hydroxylation of benzene to phenol under the same conditions. The two samples MoVbpy-PMoV-5.9 and MoVbpy-PMoV-6.1 synthesized only at slightly different pH values (though otherwise under the similar conditions) are polycrystalline materials lacking the whole Keggin structure and intense electronic response from the structure characterization (XRD, FT-IR, ESR, UV-vis, TG curves in Fig. S1 and S2†). They also generate a heterogeneous catalytic system for the hydroxylation of benzene, but the phenol yields 6.5% and 7.2% (Table 2, Entries 6 and 7) are lower than 12.5% for [MoVbpy]HPMoV₂. Directly using 2,2′-bpy to attach [PMo₁₀V₂O₄₀]⁵⁻, the obtained solid is non-crystal without a cation chain, also giving rise to a heterogeneous catalytic system. Nevertheless the phenol yield is 7.5% (Table 2, Entry 8). In addition, when [Dmim]Br₂ and [Dpy]Br₂ are respectively used to anion-exchange with H₅PMo₁₀V₂O₄₀, the obtained organic–inorganic hybrids both with cation chains but without metal–organic coordination structure give phenol yields of 8.0% and 7.8% (Table 2, Entries 9 and 10).

From above results, it can be found that the crystalline hybrid [MoVbpy]HPMoV₂ presents higher phenol yield than other non-crystalline or polycrystalline hybrids that are also prepared from PMoV₂. The high phenol yield can be attributed to the special V state in the catalyst, considering the previous viewpoint that the reversible redox process via V⁵⁺/V⁴⁺ redox pair was the active sites for hydroxylation of benzene with oxygen.¹⁵ Therefore, more V⁵⁺/V⁴⁺ redox pairs will be in favor of accelerating this hydroxylation reaction. A similar phenomenon was reported on H₂O₂-based hydroxylation of benzene to phenol catalyzed by V-containing Schiff base functionalized ionic liquid with V-polyoxometalate.⁴³ In this work, the crystalline hybrid contains vanadium centers both in tetranuclear cation and Keggin anion, thus displaying an enhanced catalytic activity in aerobic hydroxylation of benzene. As can be seen from the UV-vis spectra, the featured absorption band of V⁵⁺/V⁴⁺ is strong, providing an evidence for above deduction. In addition to this, some unique crystal faces in the single-crystal structure of [MoVbpy]HPMoV₂ may contribute to the high catalytic activity, which however is still short of direct evidence now. Besides, the more V⁵⁺/V⁴⁺ redox pairs that favor the high phenol yield does not mean the inferior activity as per active site. Actually, the molar amount of the utilized [MoVbpy]HPMoV₂ catalyst relative to the substrate is low, offering the TON of 41, much higher than the previous catalysts such as Cs₅PMo₁₀V₂,¹³ Cu/Al₂O₃ ¹⁴ and Bipy₂PMoV₁ ²⁹ with TONs of 10–18. In addition, if the TON is calculated based on the molar amount of V⁵⁺/V⁴⁺ redox pair rather than the catalyst itself, the TON in the V species in [MoVbpy]HPMoV₂ is about 21, still higher than those previous catalysts.

The reusability of [MoVbpy]HPMoV₂ is also investigated in hydroxylation of benzene. After separation by direct filtration, the recovered [MoVbpy]HPMoV₂ is used for the next run, which gives 5.4% phenol yield in the secondary cycle, suggesting that the catalyst can be reused. The main peaks of heteropolyanion and bpy ligand still exist in the FT-IR spectrum of reused catalyst (Fig. S3†), suggesting that the parent structures of the organic and inorganic moieties of the hybrid can be preserved for the isolated catalyst. However, according to the XRD patterns (Fig. S2†) and ESR spectra (Fig. S3†) of reused catalyst, the single-crystal structure has been changed mostly to polycrystalline during the reaction. The deactivation in the catalyst recycling test may thus be mainly due to the change of the single-crystal structure. On the other hand, the unavoidable slight loss of catalyst weight during the recovery operation also could also be responsible for the decrease of catalytic activity. The hydroxylation of benzene with molecular oxygen is a harsh reaction for majority of catalysts. Although a lot of efforts have been devoted to hydroxylation of benzene with oxygen as oxidant, only few of them are heterogeneous systems^{13–15,28,29,33} and the phenol yield still remains to be enhanced. For the reported heterogeneous catalyst, it was hard to balance the high phenol yield and better reusability. In contrast, the crystalline hybrid [MoVbpy]HPMoV₂ can generate a relative superior phenol yield and a high TON and simultaneously keeps a relative high activity in the recycling test, though the reusability is still to be enhanced.

Conclusions

The POM-based organic–inorganic crystalline hybrid [MoVbpy]HPMoV₂ is hydrothermally synthesized by finely adjusting the pH value of initial reaction solution. The obtained single crystal can be used as the heterogeneous catalyst for hydroxylation of benzene with molecular oxygen and displays a high phenol yield of 12.5% and TON of 41, thus provides an efficient heterogeneous catalyst for aerobic hydroxylation of benzene to phenol.

Acknowledgements

The authors thank greatly the National Natural Science Foundation of China (no. 21136005, 21303084 and 21476109), Jiangsu Provincial Science Foundation for Youths (no. BK20130921), and Specialized Research Fund for the Doctoral Program of Higher Education (no. 20133221120002).

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Footnote

† Electronic supplementary information (ESI) available: Supplementary tables and figures, bond lengths and angles and catalysts structure. CCDC 1002947 ([MoVbpy]HPMoV₂). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra06736c

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