A one-dimensional polyoxomolybdate polymer as a catalyst for the epoxidation of olefins

Abstract

A one-dimensional coordination polymer, {(NH₄)₄Mo₈O₂₆}_n (1), was synthesized based on polyoxomolybdate nanoclusters under hydrothermal conditions. The crystal structure of the hybrid was determined by single crystal X-ray diffraction analysis. The 1-D coordination polymer 1 was also characterized by FT-IR, powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA) and inductive coupled plasma optical emission spectroscopy (ICP-OES). The catalytic properties of the prepared compound were evaluated in the epoxidation of some olefins using tert-butyl hydroperoxide (TBHP) or H₂O₂ as the oxidant. The 1-D polyoxomolybdate polymer exhibited good activity and stability in the catalytic epoxidation of olefins.

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Introduction

Catalytic epoxidation of olefins is of considerable importance in laboratory research as well as in the chemical industry. Epoxides are valuable intermediates for the production of many chemicals including paints, surfactants, resins and pharmaceuticals.^1,2

Compounds containing certain transition metals such as Mo, W, Ti and V are considered to be active catalysts for the liquid-phase epoxidation of olefins.^3,4 Among these compounds, polyoxometalates (POMs), as a class of nanosized metal-oxygen clusters with great potential applications, have received particular attention owing to their high and tunable catalytic reactivity in the oxidation reactions.^5–9 However, the catalytic applications of POMs have been limited due to their solubility under catalytic conditions.¹⁰ The low stability of these compounds and the difficulty of their separations from the reaction mixture pose a challenge to researchers. To overcome this obstacle, a number of approaches have been developed to prepare heterogeneous POM catalysts, including microemulsion formation,¹¹ immobilization on carbon,¹² metal–organic framework,¹³ polymer,¹⁴ zeolite¹⁵ or silica matrix.¹⁶

Preparation of inorganic–organic hybrid materials and coordination polymers is revealed to be a successful approach for the heterogenization of epoxidation catalysts.¹⁷ For instance, inorganic–organic hybrids such as [H(atrz)]₄[(atrz)₂(γ-Mo₈O₂₆)]·2H₂O (atrz = 3-amino-1,2,4-triazole),¹⁸ [{Co^IICl₃}(tptz){Co^IICl(H₂O)}]·0.25H₂O (tptz = 2,4,6-tris(2-pyridyl)-1,3,5-triazine),¹⁹ [Cu(H₂btec)(bipy)]_∞ (H₄btec = 1,2,4,5-benzenetetracarboxylic acid),^20,21 and [MoO₂Cl₂(H₂O)₂]·(H₂dipy-pra)Cl₂ (dipy-pra = 1,3-bis(4-pyridyl)propane)²² have been prepared and applied as efficient and recyclable catalysts for the epoxidation of olefins. We have reported polyoxomolybdate-based hybrids, which turned out to be efficient catalysts for the epoxidation of olefins and allylic alcohols.^23,24

In our continuing efforts herein, we report the synthesis of an inorganic–organic hybrid material based on polyoxomolybdate and assessed its catalytic activity in the liquid-phase epoxidation of olefins in different solvents using either TBHP or H₂O₂ as oxidant.

Experimental

Materials and methods

All chemicals were of reagent grade and used without further purification. Fourier transform infrared (FT-IR) spectra were obtained on Bruker Equinox 55 spectrometer equipped with a single reflection diamond ATR system. Powder X-ray diffraction (PXRD) patterns were acquired using a Philips PW1800 diffractometer with Cu Kα radiation (λ = 1.5406 Å). Thermogravimetric analysis (TGA) of the sample was conducted with a TGA Q 50 instrument at the heating rate of 20 °C min⁻¹ under argon atmosphere. The metal content of the catalyst was determined by inductive coupled plasma optical emission spectroscopy (ICP-OES) analysis on Optima 8000. The catalytic results were analysed using a gas chromatograph (HP, Agilent 5890) equipped with a capillary column (HP-1) and a flame ionization detector (FID). Gas chromatography-mass spectrometry (GC-MS) was recorded using a Shimadzu-14A fitted with a capillary column (CBP5-M25).

Synthesis of {(NH₄)₄Mo₈O₂₆}_n (1)

To prepare 1, (NH₄)₆Mo₇O₂₄·4H₂O (0.31 g, 0.25 mmol), ZnCl₂ (0.17 g, 1.25 mmol) and terephthalic acid (0.12 g, 0.72 mmol) were added to the mixture of deionized water (5 mL) and HCl (concentrated, 0.02 mL). The suspension was stirred for 15 min and then placed in Teflon-lined stainless reactor and heated at 140 °C for 4 days. After slow cooling to room temperature, the grey crystals were isolated by filtration and washed with deionized water.

Catalytic epoxidation of olefins using {(NH₄)₄Mo₈O₂₆}_n (1)

The epoxidation reaction was carried out as follows: 14.4 mmol tert-butyl hydroperoxide (TBHP, 70% in H₂O) or hydrogen peroxide (H₂O₂, 30% in water) as oxidant was added to a mixture of olefin (8 mmol) and catalyst (50 mg) in solvent (10 mL). TBHP was dried prior to use according to the procedure described in the literature.²⁵ The mixture was refluxed for an appropriate time and the filtrate was analyzed by gas chromatography.

Single crystal X-ray data collection and refinement

Single crystal X-ray diffraction data for 1 were recorded on a four-circle κ geometry KUMA KM-4 diffractometer equipped with a two-dimensional area CCD detector using graphite monochromatic Mo Kα radiation. The structure was solved by the direct method using SHELXS-97 and refined using SHELXL-2014/7 program.²⁶ All non-hydrogen atoms were refined anisotropically. Visualisation of the structure was made with the Diamond 3.1 program.²⁷ Crystallographic data for the structure have been deposited in the Cambridge Crystallographic Data Center with CCDC number: CCDC 970131. The detailed crystallographic data and structure refinement parameters for 1 are summarized in Table 1. Selected geometrical parameters for 1 are given in Table 2.

Table 1 Crystal and structure refinement data for compound 1

Compound	1
Empirical formula	H8Mo4N2O13
Molecular weight	627.84
Crystal system	Triclinic
Space group	P1̄
Temperature (K)	295 (2)
Wavelength, Mo Kα (Å)	0.71073
a (Å)	8.2674 (4)
b (Å)	8.3564 (4)
c (Å)	10.2593 (5)
α (°)	104.702 (5)
β (°)	106.043 (4)
γ (°)	109.624 (6)
Cell volume (Å^3)	592.71 (7)
Z	2
ρ (g cm^−1)	3.518
μ (mm^−1)	4.207
Absorption correction	Numerical
Tmin/Tmax	0.4665/0.7317
Total reflections	7192
Unique reflections	2945
Observed reflections [F^2 > 2σ(F^2)]	2330
Rint	0.0222
Data/restraints/parameters	2945/20/196
Goodness-of-fit (GOF) on F^2	1.000
R [F^2 > 2σ(F^2)] (R1, wR2)	0.0275, 0.0511
R (all data) (R1, wR2)	0.0421, 0.0556
Δρmax, Δρmin (e Å^−3)	0.848, −0.754

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Table 2 Selected bond lengths (Å) and angles (°) for compound 1^a

Bond lengths (Å):
a Symmetry code: (i) −x, 2−y, −z; (ii) −x, 1−y, −z.
Ot-terminal
Mo1–O1	1.72 (1)	Mo1–O11	1.702 (6)	Mo2–O13	1.693 (5)
Mo2–O3	1.711 (9)	Mo3–O4	1.690 (6)	Mo3–O12	1.709 (10)
Mo4–O2	1.704 (10)
μ2-O
Mo1–O8	1.959 (5)	Mo1–O5	2.353 (11)	Mo2–O8	1.903 (5)
Mo4–O5	1.733 (5)
μ3-O
Mo1–O10	1.891 (6)	Mo2–O7	2.000 (5)	Mo2–O9	2.315 (14)
Mo3–O9	2.020 (5)	Mo3–O7	2.233 (11)	Mo3^i–O7	1.943 (7)
Mo3^ii–O10	2.257 (6)	Mo4–O9	1.958 (6)	Mo4^ii–O10	2.138 (6)
μ4-O
Mo2–O6	2.267 (7)	Mo1–O6	2.303 (6)	Mo4–O6	2.435 (14)
Mo4^ii–O6	1.922 (6)

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Bond angles (°)
O1–Mo1–O6	100.04 (13)	O9–Mo3–O12	107.40 (15)
O1–Mo1–O5	169.91 (13)	O7–Mo3–O12	163.57 (15)
O11–Mo1–O10	105.00 (14)	O4–Mo3–O12	102.94 (17)
O7–Mo2–O8	154.69 (13)	O2–Mo4–O9	103.47 (14)
O13–Mo2–O9	89.60 (13)	O2–Mo4–O5	105.19 (14)
O3–Mo2–O9	162.78 (13)	O2–Mo4–O6	176.75 (13)

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Results and discussion

Structure description of {(NH₄)₄Mo₈O₂₆}_n (1)

The ORTEP structure of 1 is illustrated in Fig. 1. The asymmetric unit of 1 is composed of a tetranuclear unit of [Mo₄O₁₃]²⁻ and two NH₄⁺ cations. The [Mo₄O₁₃]²⁻ anion is built from four edge-sharing {MoO₆} octahedra. The tetranuclear building block consists of four types of Mo–O bonds with terminal (O_t) and bridging μ₂-O, μ₃-O and μ₄-O oxygen atoms. As seen in Table 2, the shortest Mo–O distances are observed for the terminal oxygen atoms (O_t). The Mo–O bond distances and angles in 1 lie in the ranges reported in our previous article.^28,29 The [Mo₄O₁₃]²⁻ units are connected together to form an extended chain displayed in Fig. 2.

Fig. 1 View of the asymmetric unit of 1 with 50% probability ellipsoids.

Fig. 2 View of one-dimensional [Mo₄O₁₃]_n²ⁿ⁻ polymer.

The two-dimensional array is achieved due to hydrogen bonding N–H⋯O interactions between oxygen atoms of [Mo₄O₁₃]²⁻ and ammonium cations (Fig. 3 and 4). These 2-D layers are then connected by slightly weaker hydrogen bonds to make the entire structure. The structure of [Mo₄O₁₃]²⁻ in 1 is different from those found in [(NH₄)₂(Mo₄O₁₃)]_n,³⁰ [{Cu₂(triazolate)₂(H₂O)}Mo₄O₁₃]_n,³¹ [(Hbpa)₂(Mo₄O₁₃)]_n (Hbpa = 4-pyridyl-4′-pyridinium amine)³² and [M(tpytrz)₂Mo₄O₁₃]_n (M = Fe, Co, Ni, Zn; tpytrz = tripyridyltriazine).³³ In [(NH₄)₂(Mo₄O₁₃)]_n and [{Cu₂(triazolate)₂(H₂O)}Mo₄O₁₃]_n, the [Mo₄O₁₃]²⁻ anion is made from three octahedral and one five-coordinated unit, whereas this anion in [M(tpytrz)₂Mo₄O₁₃]_n is composed of a pair of tetrahedral {MoO₄} and five-coordinated {MoO₅N} unit, and in [(Hbpa)₂(Mo₄O₁₃)]_n it consists of a pair of square-pyramidal {MoO₅} and five-coordinated {MoO₅N}.

Fig. 3 View of the structure in 1 showing [Mo₄O₁₃]_n²ⁿ⁻ chains intercalated ammonium ions.

Fig. 4 Hydrogen-bonding interactions for 1.

In the FT-IR spectrum of 1 before the catalytic reaction, displayed in Fig. 5, the broad band at 2800–3181 cm⁻¹ and the peak at 1397 cm⁻¹ are related to the vibration of NH₄⁺. The peaks appeared at 719–929 cm⁻¹ are ascribed to Mo=O and Mo–O–Mo vibrations.^28,30

Fig. 5 FT-IR spectra of (a) fresh and (b) recycled {(NH₄)₄Mo₈O₂₆}_n (1).

The powder X-ray diffraction (PXRD) patterns of 1 are presented in Fig. 6. The similarity of the experimental and calculated PXRD patterns of 1 suggests the phase purity of the bulk material.

Fig. 6 PXRD patterns of (a) simulated, (b) fresh and (c) recycled {(NH₄)₄Mo₈O₂₆}_n (1).

Thermogravimetric analysis

Thermal stability of 1 was investigated by the thermogravimetric analysis (TGA). As can be seen in Fig. 7, compound 1 showed no weight loss up to about 220 °C and then decomposed in the temperature range of 220–400 °C. The observed weight loss can be due to the release of ammonium ions and decomposition of the molybdenum cluster. The final residue remained at 600 °C might be related to a molybdenum oxide compound.

Fig. 7 TGA curve of {(NH₄)₄Mo₈O₂₆}_n (1).

Catalytic activity studies

The catalytic activity of {(NH₄)₄Mo₈O₂₆}_n (1) was evaluated in the epoxidation of some olefins. To investigate the optimal reaction conditions, the epoxidation of cyclooctene with TBHP or H₂O₂ as oxygen sources in different solvents was performed. The formation of epoxycyclooctane was confirmed by NMR (Fig. S1†) and GC analyses (Fig. S2†). When TBHP was used as oxidant (Fig. 8), compound 1 exhibited more catalytic activity in CHCl₃ rather than CH₃CN and C₂H₅OH as solvents. This can be explained with the increasing coordination ability of the solvents as C₂H₅OH > CH₃CN > CHCl₃ and their competition with oxygen molecules to occupy the coordination sites of the catalyst.

Fig. 8 Epoxidation of cyclooctene with TBHP in the presence of 1 in different solvents. Reaction conditions: cyclooctene (8 mmol), TBHP (14.4 mmol), solvent (10 mL), catalyst (50 mg).

When H₂O₂ was used as oxidant, the catalytic activity of 1 was improved in solvents with more polarity (Fig. 9). This might be related to better miscibility of the reactants in polar solvents. These observations are in agreement with our previous results obtained for the epoxidation of cyclooctene and 3-methyl-2-butene-1-ol in the presence of polyoxomolybdate catalysts.^23,24

Fig. 9 Epoxidation of cyclooctene with H₂O₂ in the presence of 1 in different solvents. Reaction conditions: cyclooctene (8 mmol), H₂O₂ (14.4 mmol), solvent (10 mL), catalyst (50 mg).

The catalytic activity of the prepared compound was also studied in the epoxidation of some other olefins in the presence of 50 mg of it using TBHP as oxygen source and CHCl₃ as solvent. From Table 3 and Fig. S3–S6,† it is evident that 1 can act as active and selective catalyst in the epoxidation reactions. Inspection of the results reveals that cyclic olefins undergo the epoxidation reactions within shorter period of time compared to that of linear olefins. More epoxidation reactivity of cyclic olefins is due to higher electron donating ability of their double bonds that facilitates the epoxidation. These observations are consistent with the previous reports.^34,35

Table 3 Catalytic epoxidation of some olefins in the presence of {(NH₄)₄Mo₈O₂₆}_n (1)^a

Entry	Substrate	Time (h)	Conversion^b (%)	Selectivity^c (%)
a Reaction conditions: catalyst (50 mg), olefin (8 mmol), TBHP (14.4 mmol), refluxing chloroform (10 mL).b GC yield based on starting substrate.c Selectivity towards formation of epoxide determined by GC-mass or injection of a reference standard.d The conversion obtained in the presence of first and second recovered catalyst.
1	Styrene	8	92	>99
2	Cyclohexene	2	99	>99
3	Cyclooctene	2	98	>99
4^d	Cyclooctene (1st)	2	93	>99
5^d	Cyclooctene (2nd)	2	87	>99
6	1-Hexene	24	99	>99
7	1-Octene	24	92	>99

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Moreover, the recyclability of 1 was explored in the epoxidation of cyclooctene using TBHP and the results are presented in Table 3 (entries 4 and 5). After each experiment, the catalyst was separated and washed with CHCl₃ and then used for the next run. The recovered catalyst maintained the catalytic activity after two cycles with only a slight decrease in the conversion. The heterogeneity of 1 is also checked in the epoxidation of cyclooctene using H₂O₂ as oxidant and CH₃CN as solvent. The reaction was carried out in the presence of recycled 1 and the conversion was about 45% after 2 h. To measure the possible catalytic contribution of the solubilized molybdenum species in different solvents, the reaction mixture was filtered off after 1 h and the filtrate was left to react for 30 min (Table 4). The increase in the conversion of the filtrate is not considerable suggesting that the catalytic reaction is heterogeneous in nature. Further support for the heterogeneous nature of the catalyst was provided by the low leaching of molybdenum (ca. 4%), which is determined by ICP-OES chemical analysis. Also, the FT-IR spectra (Fig. 5) and the PXRD patterns (Fig. 6) of the catalyst after and before the epoxidation reaction are almost similar suggesting the stability of the catalyst during the catalytic process.

Table 4 Epoxidation of cyclooctene with H₂O₂ in the presence of recycled 1 in different solvents^a

Entry	Solvent	Time^b (h)	Conversion (%)	Selectivity (%)
a Reaction conditions: cyclooctene (8 mmol), H2O2 (14.4 mmol), solvent (10 mL), catalyst (50 mg).b The catalyst filtered off after 1 h and the reaction of the filtrate continued for 0.5 h.
1	CH3CN	1.5	40	>99
2	C2H5OH	1.5	22	>99
3	CHCl3	1.5	13	>99

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Feasible mechanism for the epoxidation of olefins catalyzed by Mo(VI) species has been reported in the literature.^23,36 This involves the coordination of the oxidant to Mo(VI) and the attack of the olefin on the Mo(VI) centre.

The catalytic results obtained by the prepared catalyst were compared with the previously reported polyoxomolybdate-based materials (Table 5). It can be seen that the catalytic activity and stability of 1 is comparable to the other self-supported heterogeneous epoxidation catalysts. Structure analysis has shown that catalyst 1 exhibits [Mo₄O₁₃]_n²ⁿ⁻ chains connected by hydrogen bonds which may be effective in improving the stability of the catalyst. The possible positive role of these interactions in the stability of polyoxomolybdate supramolecular catalyst has been previously reported in the literature.¹⁸

Table 5 Comparison of the results obtained for the epoxidation of cyclooctene catalyzed by polyoxomolybdate-based materials

Entry	Catalyst	Reaction conditions	Time (h)	Conversion (%)	Mo leaching (%)	Reference
1	[Cu^I3Cl(4,4′-bipy)4], [Cu^II(1,10-phen)2Mo8O26] and [CuMoO4(N2C12H8)]·H2O	Cyclooctene (8 mmol), TBHP (14.4 mmol), CHCl3 (10 mL), catalyst (100 mg)	2	96	3	24
2	[4,4′-H2bipy], [Co0.23Mo6O20]·2H2O	Cyclooctene (10 mmol), TBHP (30 mmol), CHCl3 (5 mL), catalyst (70 mg)	2	94	1	23
3	[H(atrz)]4[(atrz)2, (γ-Mo8O26)]·2H2O, (atrz = 3-amino-1,2,4-triazole)	Cyclooctene (1 mmol), TBHP (1 mmol), CHCl3 (2 mL), catalyst (0.005 mmol)	12	92	—	18
4	(NH4)4Mo8O26	Cyclooctene (8 mmol), TBHP (14.4 mmol), CHCl3 (10 mL), catalyst (50 mg)	2	98	4	This work

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Conclusions

Polyoxomolybdate-based one-dimensional coordination polymer 1 has been hydrothermally prepared and structurally characterized. Single crystal X-ray diffraction analysis revealed that the compound is anionic one-dimensional chains formed through the self-assembly of [Mo₄O₁₃]^2- units and involved in the hydrogen bonding interactions with ammonium cations through terminal O atoms. The prepared compound was found to be efficient and stable heterogeneous catalyst in the epoxidation of some olefins with TBHP as oxidant and CHCl₃ as solvent.

Acknowledgements

We gratefully acknowledge University of Tehran and project number 93038370 from Iran National Science Foundation: INSF for the financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 970131. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra02248k

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