Polyoxomolybdic Cobalt Encapsulated within Zr-Based MetalâOrganic

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Polyoxomolybdic Cobalt Encapsulated within Zr-Based Metal-Organic Frameworks as Efficient Heterogeneous Catalysts for Olefins Epoxidation Xiaojing Song, Dianwen Hu, Xiaotong Yang, Hao Zhang, Wenxiang Zhang, Jiyang Li, Mingjun Jia, and Jihong Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

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ACS Sustainable Chemistry & Engineering

Polyoxomolybdic Cobalt Encapsulated within Zr-Based Metal-Organic Frameworks as Efficient Heterogeneous Catalysts for Olefins Epoxidation

Xiaojing Song, †, § Dianwen Hu, § Xiaotong Yang, § Hao Zhang, § Wenxiang Zhang, § Jiyang Li,† Mingjun Jia,*, ‡, and Jihong Yu*,†‡

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of

†

Chemistry, and ‡International Center of Future Science, Jilin University, 2699 Qianjin Street, Changchun 130012, P. R. China §

Key Laboratory of Surface and Interface Chemistry of Jilin Province, College of

Chemistry, Jilin University, 2699 Qianjin Street, Changchun, 130012, P. R. China * Corresponding authors: Tel: (+86)431-85155390, 85168608; Fax: (+86)431-85168420. Email addresses: [email protected] (M. Jia); [email protected] (J. Yu)

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ABSTRACT The encapsulation of polyoxomolybdic cobalt (CoPMA) and polyoxomolybdic acid (PMA) within the Zr-based metal-organic frameworks (Zr-MOFs) of UiO-bpy (connected by 2,2'-bipyridine-5,5'-dicarboxylic acid linkers) and UiO-67 (connected by 4,4'-biphenyldicarboxylic acid linkers) has been achieved by direct solvothermal synthesis. Relatively high content of polyoxometalate (POM) clusters (ranging from 12 to 15 wt% loading) could be introduced to the cages of Zr-MOFs, to form uniform hybrid composites of POM@Zr-MOFs. The catalytic properties of these composites were investigated for the olefins epoxidation with H2O2 or molecular O2 as oxidant. Among them, the catalyst CoPMA@UiO-bpy performed the highest catalytic activity and stability for cyclooctene epoxidation with H2O2 as oxidant, and could also act as efficient heterogeneous catalyst for the oxidation of styrene and 1-octene with O2 as oxidant and tert-butyl hydroperoxide (t-BuOOH) as initiator. The excellent catalytic performance of the hybrid composite CoPMA@UiO-bpy should be mainly attributed to the uniform distribution of POM clusters within the size-matched cages of Zr-MOFs, as well as the multiple interactions between the CoPMA clusters and the functional groups (bipyridine and Zr-OH) located in the framework of UiO-bpy. KEYWORDS: Zr-MOF, Polyoxomolybdic cobalt, Solvothermal synthesis, Olefin, Epoxidation

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INTRODUCTION The catalytic epoxidation of olefins is of considerable importance because epoxides are widely used as intermediates in fine chemical industry and biotransformation.1-3 As a representative catalyst system, polyoxometalates (POMs)-based heterogeneous catalysts have attracted great interest owing to their notable catalytic activity, selectivity and easy separation. However, most of the resultant catalysts suffer from leaching of POMs species mainly due to the strong complexing capability of solvent and oxidants (like H2O2), thus limiting their practical applications.4-7 Therefore, many recent efforts have been made to improve the stability of the POM-based heterogeneous catalysts on the premise that high activity and selectivity are kept for olefins epoxidation with green oxidant H2O2 or O2. Several strategies have been adopted for preparing more efficient POMs-based heterogeneous epoxidation catalysts, including immobilizing POMs on surface functionalized porous supports by impregnation method, incorporating POMs in SiO2 or ZrO2 matrices via surfactant template-assisted sol-gel route, and self-assembling POMs with organic ligands (or complexes) to form supramolecular structures by solvothermal method.8-16 For instance, Kasai et al. reported that POM anions [H2SiV2W10O40]4- can be stabilized on the surface of dihydroimidazolium-cation modified SiO2 through electrostatic interaction.10 Armatas et al. prepared a series of mesoporous ZrO2-based 12-phosphomolybdic acid (PMo12/ZrO2) catalysts by a surfactant-assisted sol-gel copolymerization route, which exhibited exceptional stability and catalytic activity in olefins epoxidation with H2O2.11 Our recent work 3



showed that porous organic polymers containing functional groups (e.g., triphenylphosphine and triphenylamine) could be used as suitable supports to immobilize phosphomolybdic acid (PMA) or cobalt phosphomolybdate (CoPMA) to obtain relatively stable heterogeneous catalysts.17, 18 These progresses suggest that the stability of POM-based heterogeneous catalysts might be improved by building appropriated interactions between POMs clusters and the solid porous supports. Recently, the rapid development of metal-organic frameworks (MOFs) has provided a versatile platform for fabricating hybrid functional materials.19-21 By incorporating POM clusters into the cavities of MOFs, a variety of POM@MOFs hybrid composites with excellent catalytic oxidation properties have been prepared.22-24 However, the relative low structure stability of MOFs turns to be a main drawback of these novel hybrid catalyst systems. Notably, the appearance of thermally and chemically stable Zrbased MOFs (build up from ZrIV6O4(OH)4 oxocluster nodes and organic linkers) bring significant opportunity for designing more active and stable POM@Zr-MOFs hybrid heterogeneous catalysts.25-33 For instance, Farha and coworkers incorporated H3PW12O40 into a mesoporous Zr-MOF named NU-1000 (connected with 1,3,6,8tetrakis(p-benzoate)pyrene) via an impregnation method, and the resulting hybrid material demonstrated improved catalytic activity and stability for the sulfide oxidation by H2O2.30 Salomon et al. reported the successful encapsulation of three POMs within the pores of Zr(IV) biphenyldicarboxylate (UiO-67) through a direct solvothermal method, and expected that this hybrid material may have potential application in catalysis.27 Lately, Li and the co-authors reported that phosphotungstic acid 4


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encapsulated into the UiO-67, synthesized by the solvothermal method, performed remarkably high catalytic activity and stability in oxidative desulfurization reactions with H2O2 as oxidant.31 By using this direct solvothermal synthesis strategy, POM leaching maybe significantly inhibited due to the space confinement of the sizematched MOF cages, as well as the existence of specific interactions between POM clusters and the UiO framework.27,31 Considering the structure and composition diversities of the Zr-MOF and the POMs, there would be great opportunities for researchers to develop more efficient and stable Zr-MOF supported POM heterogeneous catalysts for the application in the olefins epoxidation. In this work, we incorporated PMA or CoPMA clusters into the Zr-MOFs of UiObpy and UiO-67 by the direct solvothermal method. The resultant composites were characterized and used as the catalysts for the olefins epoxidation with H2O2 or molecular O2 as oxidant. It was found that CoPMA@UiO-bpy catalyst showed excellent activity and stability (recyclability) for the H2O2-based olefin epoxidation reactions. Moreover, the catalysts were also highly efficient for olefins epoxidation with molecular O2 as oxidant and tert-butylhydroperoxide as initiator under solvent-free conditions. EXPERIMENTAL SECTION Materials

ZrCl4,

2,2'-bipyridine-5,5'-dicarboxylic

acid

(H2bpydc)

and

4,4'-

biphenyldicarboxylic acid (H2bpdc) were purchased from Sigma-Aldrich; 30% wt H2O2 aqueous water, phosphomolybdic acid (PMA), hydrochloric acid 37wt%, CoSO4·7H2O and BaSO4 were brought from Beijing chemical factory; Styrene, 5



cyclooctene, 1-octene, dimethylformamide (DMF), acetonitrile, ethanol, 70% (wt) tertbutylhydroperoxide (t-BuOOH) in water were purchased from Aldrich. Synthesis of PMA or CoPMA immobilized Zr-MOFs catalysts Zr-MOFs of UiObpy and UiO-67 were synthesized following the procedures reported in literature,27 with a yield of 85% for UiO-bpy and 78% for UiO-67. CoPMA with formula of Co1HPMo12O40 was prepared according to the related literature.34 The hybrid composites of CoPMA or PMA encapsulated within Zr-based MOFs were synthesized following the same procedure as the two Zr-MOFs, with the addition of CoPMA (or PMA) in the synthesis media. Typically, ZrCl4 (116 mg), H2bpydc (123 mg) or H2bpdc (121 mg), benzoic acid (1.83 g), hydrochloric acid 37 wt% (83 μL), CoPMA (39.3 mg) or PMA (45.5 mg) were dissolved in DMF (50 ml). The mixture was then transferred to a Teflon-lined stainless-steel autoclave and heated in an oven at 120 oC for 24 h. After cooling down to room temperature, the blue precipitates were filtered, washed repeatedly with DMF and dry acetone, and dried under vacuum at 90 oC overnight. The hybrid composites are named as CoPMA@UiO-bpy, PMA@UiO-bpy, CoPMA@UiO67 and PMA@UiO-67, respectively. Characterization of Catalysts Infrared spectra were recorded with a Nicolet AVATAR 370 DTGS spectrometer using the KBr pellet technique. Thermogravimetry (TG) measurements were performed on a Mettler Toledo with an air flow and heating rate of 10°/min up to 800 °C. X-ray photoelectron spectra (XPS) was measured using an ESCALAB 250 spectrometer. The crystallinity and phase purity of the samples were characterized by powder X-ray diffraction (XRD) on a Shimadzu XRD-6000 6


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diffractometer (40 kV, 30 mA), using Ni-filtered Cu Kα radiation. N2 adsorption/desorption isotherms were carried out on a Micromeritics ASAP 2010 N analyzer at 77.3 K after treating the samples at 150 °C. Specific surface areas were calculated using BET model. Inductively coupling plasma spectrometer analysis was measured with Perkin-Elmer Optima 3300 DV ICP instrument. Solid 31P MAS NMR spectra were recorded on a 400 MHz Bruker spectrometer. Catalytic Epoxidation of Cyclooctene with H2O2 as oxidant The catalytic epoxidation of cyclooctene with H2O2 was carried out as follows: 10 mg catalyst, cyclooctene (1.0 mmol), CH3CN (2.0 ml) were added to a 5 ml flask. The reactions were initiated through adding 30 wt% H2O2 (2 mmol) aqueous water into the above mixtures under the desired temperature. All the reagents and products were analyzed by Shimadzu GC-14C gas chromatograph contained HP-5 capillary column. All the oxidation products were identified by comparison with the standard samples and finally by gas chromatograph mass spectroscopy derived from GC-MS-QP 2010 plus. Catalytic epoxidation of olefins with O2/t-BuOOH The catalytic oxidation of olefins with O2 was carried out in a 50 ml three-necked bottle equipped with a gas supply and reflux condenser, the device was immersed in a thermostatted oil bath. Typically, 20 mg catalyst, 50 mmol styrene, and 1.0 mmol tBuOOH were added into the reaction bottle respectively, the reactions were initiated by inletting O2 at a flow rate of 10 ml/min. After reaction, the liquid products were diluted with dichloroethane and quantified by Shimadzu GC-14C gas chromatograph with HP-5 capillary column. The side product of benzoic acid was identified and 7



quantified by GC-MS-QP 2010 plus. RESULT AND DISSCUSSION Hybrid composites of POM@UiO-bpy and POM@UiO-67 were synthesized through the in situ self-assembly of POM clusters (PMA or CoPMA), ZrCl4, and the corresponding organic linker H2bpydc or H2bpdc under solvothermal conditions. Taking the CoPMA@UiO-bpy as an example, the general synthesis process is shown in Scheme 1. According to the coordination modulation mechanism of Zr-based MOFs synthesis, multiple factors such as the amount and species of modulator, the amount of POM clusters and solvent DMF could affect the formation of crystalline product of POM@Zr-MOF. Based on the results of parallel experiments for synthesizing UiO-bpy, the optimized synthesis conditions for synthesizing POM@UiO-bpy, including synthesis temperature, the amount of ZrCl4, organic linkers, modulator benzoic acid and hydrochloric acid, were determined. In this case, the amount of solvent DMF turned to be a key factor in influencing the quality and yield of the crystalline products. The finally optimized parameters and conditions for synthesizing POM@UiO-bpy and POM@UiO-67 are listed in Table S1.

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Scheme 1. The self-assembly process of CoPMA@UiO-bpy composite.

The FT-IR spectra of PMA, CoPMA, UiO-67, UiO-bpy, and the hybrid composites are shown in Figure 1. For the Zr-MOFs and the hybrid composites, the observed vibration bands between 1300 and 1750 cm-1 are mainly attributed to the carboxylate vibration of the organic linkers.29 As for the spectra of CoPMA and PMA, the characteristic vibration bands appeared at 1080 (P=O), 970 (Mo=O), 870 (Mo-Ob-Mo) and 790 cm-1 (Mo-Oc-Mo) are attributed to the Keggin structure of POM clusters. Notably, the characteristic bands of POM in the spectra of the hybrid composites could not be observed very clearly, which might be due mainly to the superimposition with the vibration bands of the organic linkers in the region 700-1200 cm-1.35

Figure 1. The FT-IR spectra of (A) PMA, CoPMA, UiO-bpy, PMA@UiO-bpy and CoPMA@UiO9



bpy; (B) PMA, CoPMA, UiO-67, PMA@UiO-67 and CoPMA@UiO-67.

The XRD patterns of UiO-bpy, UiO-67, and the hybrid composites are shown in Figure 2. The diffraction patterns of UiO-bpy and UiO-67 match well with the results of literature,27, 36 confirming the phase purity of the as-synthesized Zr-MOFs. For the hybrid composites of POM@UiO-bpy and POM@UiO-67, the XRD patterns are quite consistent with those of bare Zr-MOFs, indicating that the crystallinity of the Zr-MOF host is maintained after the incorporation of the POM clusters. The XRD patterns of POM clusters could not be detected in the XRD patterns of the hybrid composites, indicating the uniform dispersion of PMA clusters in POM@Zr-MOFs.

Figure 2. The XRD patterns of (A) UiO-bpy, PMA@UiO-bpy and CoPMA@UiO-bpy; (B) UiO67, PMA@UiO-67 and CoPMA@UiO-67.

The SEM images of UiO-bpy and CoPMA@UiO-bpy are shown in Figure 3a and 3b. UiO-bpy is composed of relatively uniform spherical particles, while the shape and the particle size of the hybrid composite CoPMA@UiO-bpy turn to be a little bit disorder after the introduction of POM clusters.36 Besides, the SEM image of UiO-67 displays the characteristic of octahedral morphology,37 while the crystals of CoPMA@UiO-67 turn to be smaller and irregular compared with the bare UiO-67. The EDS mapping was conducted for CoPMA@UiO-bpy, revealing the uniform distribution of the Zr and Mo elements in the hybrid composite of CoPMA@UiO-bpy, 10


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this suggests that the CoPMA clusters should be uniformly distributed in the cage of UiO-bpy.

Figure 3 The SEM images of (a) UiO-bpy, (b) CoPMA@UiO-bpy, (c) UiO-67 and (d) CoPMA@UiO-67, (e) and (f) the EDS mapping of Zr and Mo signals for CoPMA@UiO-bpy.

The TGA measurements for UiO-bpy, PMA@UiO-bpy and CoPMA@UiO-bpy are shown in Figure S1. For all the samples, a slow weight loss started from 100 oC to 250 oC

could be assigned to the removal of water and solvent adsorbed on the composites.

Then a rapid weight loss for UiO-bpy and POM@UiO-bpy appears at around 500 oC, which could be attributed to the linker decomposition, and formation of inorganic oxides. These results suggest that both the Zr-MOF and the hybrid POM@Zr-MOF have excellent thermal stability, which is a beneficial factor for the catalytic application.27, 36 The XPS spectra of N 1s and Mo 3d are shown in Figure 4. For CoPMA, the appearance of binding energies at 236.4 and 233.3 eV are attributed to the Mo(VI) 3d1/2 and Mo(VI) 3d5/2, respectively.38 As for the hybrid composites of CoPMA@UiO-bpy and PMA@UiO-bpy, the binding energies of Mo 3d shifts negatively, reflecting the 11



presence of interaction between POM clusters and the UiO-bpy frameworks. Compared with UiO-bpy support, the binding energies of N1s in the hybrid composite shift positively (from 398.8 eV to 399.1/399.2 eV), suggesting that relatively strong coordinative interaction is present between the POM clusters and the bipyridine ligands in UiO-bpy.29, 39

Figure 4. The XPS spectra for the binding energies of (A) Mo 3d in CoPMA, CoPMA@UiO-bpy and PMA@UiO-bpy; (B) N 1s in UiO-bpy, CoPMA@UiO-bpy and PMA@UiO-bpy.

The 31P NMR spectra of CoPMA and CoPMA@UiO-bpy are shown in Figure 5. The pure CoPMA exhibited a wider signal centered at -6.14 ppm, agreed well with the previous reports.17 For CoPMA@UiO-bpy, the 31P NMR signal appears at -4.78 ppm, quite near to the signal of pure CoPMA. The down-field shift of 31P NMR signal for the hybrid composite CoPMA@UiO-bpy relative to pure CoPMA cluster reveals the presence of interaction between CoPMA clusters and the framework of UiO-bpy.

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Figure 5. The 31P CP-MAS NMR spectra of CoPMA and CoPMA@UiO-bpy.

The N2 adsorption-desorption isotherms, the pore size distributions and the textural parameters of UiO-bpy and CoPMA@UiO-bpy are shown in Figure S2 and Table S2, respectively. The samples of UiO-bpy and CoPMA@UiO-bpy display a type I isotherm, and the BET specific surface areas are 1946 and 1006 m2/g, respectively. Besides, the pore volume of CoPMA@UiO-bpy is 0.51 cm3/g, much lower than that of UiO-bpy (0.92 cm3/g), which should be mainly attributed to the filling of POM clusters into the MOF cavities. On the basis of the related literature,40 it was known that both UiO-67 and UiO-bpy have two types of cages, named supertetrahedral cages and superoctahedral cages with diameters of 11.5 Å and 18.0 Å, respectively. Besides, the interconnected cages have relatively small opened trianglar windows (about 8 Å). Therefore, one can see that the cages of the Zr-MOF are just big enough for accomodating the size-matched POM clusters (around 10 Å), while the relatively small window of the cages may effectively prevent the loss of POM clusters from the ZrMOF during the catalysis tests. The catalytic properties of PMA supported Zr-based MOFs catalysts were studied in 13



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the cyclooctene oxidation with H2O2 as oxidant. As shown in Table 1, CoPMA@MOFs catalysts exhibited better catalytic activity than the corresponding PMA@MOFs catalysts. Among them, CoPMA@UiO-bpy performed the highest catalytic activity, with 91% of conversion and >99% of selectivity to epoxide after 6 h reaction. Notably, the recycling experiments revealed that CoPMA@UiO-bpy had excellent recyclability and stability against leaching of active POM species, much better than CoPMA@UiO67 (Figure S3). Besides, the bulk amounts of Co and Mo in the used CoPMA@UiObpy are well consistent with the fresh one (0.061 and 0.77 mmol/g Vs. 0.063 and 0.76 mmol/g), further confirming the high stability of the catalyst. From these results, it can be deduced that the presence of bipyridine groups in the framework of UiO-bpy should play a crytical role to enhance the stability of the POM-based catalyst. In addition, CoPMA@UiO-bpy catalyst displayed higher catalytic activity for cyclooctene oxidation with H2O2 than the previously reported triphenylamine-based (or triphenylphosphine) porous organic polymers supported PMA or CoPMA catalysts,17, 18

indicating that UiO-bpy is a better candidate for immobilization of POMs clusters.

Table 1. The compared results of cyclooctene oxidation with H2O2 as oxidant catalyzed by different catalysts Entry

catalyst

time (h)

Con. (%)

Sel. (%)

Related work

1

CoPMA@UiO-bpy

6

91

>99

This work

2

PMA@UiO-bpy

6

80

>99

This work

3

CoPMA@UiO-67

6

82

>99

This work

4

PMA@UiO-67

6

75

>99

This work

5

CoPMA/POP-II

9

79

>99

Ref17

6

PMA/KAP

9

26

>99

Ref18

Reaction conditions: catalyst 10 mg, cyclooctene 1 mmol, H2O2 2 mmol, CH3CN 1 ml, temperature 70 oC.

14


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Compared with H2O2 and other oxidants, the utilization of molecular O2 to oxidize olefins is more attractive in accordance with the principles of green chemistry.41-44 Two recent reports including a work reported by our group and the group of Kang revealed that the activation of molecular O2 for olefin epoxidation may be achieved at relatively mild conditions by utilizing t-BuOOH as the initiator over hybrid catalysts selfassembled from metal complexes and POM clusters.16, 45 Here, the catalytic properties of POM@Zr-MOFs catalysts were also studied for the styrene oxidation with O2 as oxidant and t-BuOOH as the initiator under solvent-free condition. As shown in Table 2, styrene can be converted to benzaldehyde (Bza) and other products even without the addition of catalysts (Entry 6 and 7), indicating that there exist an auto-oxidation process of O2. The conversion of styrene increased significantly when the POM-based catalysts were introduced into the reaction systems, and CoPMA@UiO-bpy performed the highest catalytic activities and epoxide selectivity, with 80% of conversion of styrene and 59% of selectivity to styrene epoxide. Analysis of the reaction results in Table 2 reveals that the existence of Co2+ and bipyridine groups in the hybrid catalyst POM@Zr-MOFs is beneficial for the improvement of the catalytic activities and the epoxide selectivity. Besides, the leaching tests and the recycling experiments further confirmed that CoPMA@UiO-bpy had excellent recyclability and stability under the operated conditions (Figure S4). The XRD analysis shows that the characteristic peaks of the used CoPMA@UiO-bpy are in agreement with those of the fresh one, indicating the excellent structure stability of the hybrid catalyst (Figure S5). Table 2. The catalytic oxidation results of styrene with O2 as oxidant and t-BuOOH as initiator over various POM-based ZrMOF catalystsa 15



Entry

Catalyst

Con (%)

Bza

Page 16 of 26

Sel (%) Sod Phae

Othersf

1

CoPMA@UiO-bpy

80

32

59

4

5

2

PMA@UiO-bpy

50

55

38

2

5

3

CoPMA@UiO-67

72

59

20

17

4

4

PMA@UiO-67

65

57

25

12

6

5

UiO-bpy

11

70

22

6

2

6

Blankb

18

59

27

12

2

7

Blankc

5

61

30

5

4

(a) Reaction conditions: Catalyst 20 mg, styrene 50 mmol, t-BuOOH 1 mmol, reaction temperature 80 oC, time 6 h, O2 10 ml/min; (b) Identical reaction conditions just without the addition of catalyst; (c) Without the addition of catalyst and t-BuOOH; (d) So: styrene epoxide; (e) Pha: phenylacetaldehyde; (f) Other byproducts include benzoic acid and phenyl-1,2-ethanediol.

Additional experiments suggest that CoPMA@UiO-bpy is also catalytically active for olefins oxidation with O2/t-BuOOH (Table S2). For instance, cyclooctene can be converted to corresponding epoxide with very high selectivity after 24 h reaction. The relatively inert 1-octene can also be efficiently converted to epoxide under the same reaction conditions. Notably, the catalytic performance of CoPMA@UiO-bpy is better than the previously reported hybrid assembly derived from copper-triazole and PMA under identical reaction conditions (see Table S2).16 Previous literatures revealed that the catalytic oxidation of styrene with O2/t-BuOOH follows a radical mechanism, which can lead to various products through different reaction paths, highly depending on the catalyst and reaction conditions.46 For the supported POM-based heterogeneous catalysts (e.g., PMo11/ZrO2, PMo11/Al2O3 and CoPW12/ZrO2), it was reported that the unfavorable benzaldehyde was the commonly main product, while the selectivity of epoxide was rather low under their tested conditions (Table S3).47-79 In the present case, the hybrid catalysts of CoPMA@UiO16


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67, PMA@UiO-67 and PMA@UiO-bpy also exhibit very high selectivity to benzaldehyde, while relatively high epoxide selectivity (61%) can only be achieved over the catalyst CoPMA@UiO-bpy. Apparently, the coexistence of Co2+ and bipyridine ligand in the UiO-bpy framework seems play a quite positive synergistic interaction in adjusting the chemical environment of the POM clusters encapsulated within the cages of UiO-bpy, thus resulting in the formation of highly efficient active sites for the epoxidation of styrene. Based on the above characterization results and the related literature,46, 50 it can be deduced that the superior catalytic performance of CoPMA@UiO-bpy should be highly dependent on the composition and structure features of this hybrid catalyst.

The

uniform distribution of CoPMA clusters within the cages of UiO-bpy could provide a suitable space environment for stabilizing the POM active sites, and the open cage of the Zr-MOF framework makes the reactants easily accessible to the active sites. Therefore, the excellent catalytic activity and stability of CoPMA@UiO-bpy could be mainly attributed to the fact that the size-matched cages of UiO-bpy can well confine the POM cluster inside the cavity. Besides, the existence of multiple interactions between the POM clusters and the functional groups (bipyridine, Zr-OH) in UiO-bpy framework as shown in Scheme S1, including coordination bonds, electrostatic interactions and hydrogen bonds, should also play a critical role in stabilizing the POM clusters against leaching.29 CONCLUSIONS

17



The encapsulation of polyoxomolybdic cobalt and polyoxomolybdic acid within the Zr-based metal-organic frameworks of UiO-bpy and UiO-67 has been achieved successfully by using a direct solvothermal method. The resultant composite of CoPMA@UiO-bpy exhibits excellent catalytic activity and stability for the olefins epoxidation with green oxidants H2O2 and molecule O2. The presence of multiple interactions between the encapsulated CoPMA clusters and the functional groups (bipyridine and Zr-OH) located in the framework of UiO-bpy plays a determinative role in improving the activity and stability of the hybrid catalysts. Further work is currently in progress to investigate the catalytic properties of this hybrid catalyst for other catalytic oxidation reactions. Moreover, additional characterizations of these materials, including DFT calculation, are still required for deep understanding of the nature of active sites and the catalytic reaction mechanism. ASSOCIATED CONTENTS Supporting information Optimized synthetic parameters of catalysts, TGA profiles for UiO-bpy and POM@UiO-bpy, N2 adsorption/desorption isotherms and texture parameters of UiObpy and CoPMA@UiO-bpy, recycle and leaching experimental results of olefins oxidation with H2O2/O2 as oxidant catalyzed by CoPMA@UiO-bpy, XRD of recycled CoPMA@UiO-bpy and schematic representation of the speculated structures of CoPMA@UiO-bpy. AUTHOR INFORMATION *E-mail: [email protected] (M. J.). 18


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*E-mail: [email protected] (J.Y.). NOTES The authors declare no competing financial interest. ACKNOWLEDGEMENTS We thank the National Key Research and Development Program of China (Grant 2016YFB0701100), the National Natural Science Foundation of China (Grant 21835002, 21621001 and 21173100), and the 111 Project (B17020) for supporting this work. REFERENCES (1) Bitterlich, B.; Anilkumar, G.; Gelalcha, F. G.; Spilker, B.; Grotevendt, A.; Jackstell, R.; Tse, M. K.; Beller, M., Development of a general and efficient iron-catalyzed epoxidation with hydrogen peroxide as oxidant. Chem. Asian J. 2007, 2 (4), 521-529, DOI 10.1002/asia.200600407. (2) Chen, L.; Yang, Y.; Guo, Z.; Jiang, D., Highly efficient activation of molecular oxygen with nanoporous metalloporphyrin frameworks in heterogeneous systems. Adv. Mater. 2011, 23 (28), 3149-3154, DOI 10.1002/adma.201100974. (3) Tian, S.; Fu, Q.; Chen, W.; Feng, Q.; Chen, Z.; Zhang, J.; Cheong, W. C.; Yu, R.; Gu, L.; Dong, J.; Luo, J.; Chen, C.; Peng, Q.; Draxl, C.; Wang, D.; Li, Y., Carbon nitride supported Fe2 cluster catalysts with superior performance for alkene epoxidation. Nat. Commun. 2018, 9 (1), 2353-2360, DOI 10.1038/s41467-018-04845-x. (4) Rezaeifard, A.; Haddad, R.; Jafarpour, M.; Hakimi, M., Catalytic epoxidation activity of keplerate polyoxomolybdate nanoball toward aqueous suspension of olefins under mild aerobic conditions. J. Am. Chem. Soc. 2013, 135 (27), 10036-10039, DOI 10.1021/ja405852s. (5) Wang, S. S.; Yang, G. Y., Recent advances in polyoxometalate-catalyzed reactions. Chem. Rev. 2015, 115 (11), 4893-4962, DOI 10.1021/cr500390v. (6) Canioni, R.; Roch-Marchal, C.; Sécheresse, F.; Horcajada, P.; Serre, C.; Hardi-Dan, 19



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25



Graphic abstract

Synopsis: POM@Zr-MOFs were synthesized through the direct solvothermal method and used as catalysts for olefin epoxidation with green oxidants H2O2 or O2.


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Polyoxomolybdic Cobalt Encapsulated within Zr-Based MetalâOrganic

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