Synthesis of Metal Salen@MOFs and Their Catalytic Performance for

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Kinetics, Catalysis, and Reaction Engineering

Synthesis of metal salen@MOFs and their catalytic performance for styrene oxidation Kai Huang, Lin Lin Guo, and Dongfang Wu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05007 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 10, 2019

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Industrial & Engineering Chemistry Research

Synthesis of metal salen@MOFs and their catalytic performance for styrene oxidation Kai Huang‡*, Lin Lin Guo‡, Dong Fang Wu‡. School of Chemistry and Chemical Engineering, Southeast University, Jiangning District, Nanjing 211189, China. KEYWORDS heterogeneous, Cu or Ni salen, NH2-MIL-101(Cr), styrene oxidation.

ABSTRACT A series of effective heterogeneous catalysts, Cu or Ni salen with various loadings encapsulated in amino functionalized MOFs, were synthesized successfully for the first time via a one-pot method and denoted as metal salen@NH2-MIL-101(Cr). Based on detailed characterization of the catalysts, the varying coordination abilities between the Cu or Ni salen and NH2-MIL-101(Cr) provided the as-prepared Cu or Ni salen@NH2-MIL-101(Cr) with distinct activities toward styrene oxidation. The catalytic results showed that the introduction of Cu or Ni salen can significantly promote the selectivity to epoxide. However, Cu salen@NH2-MIL-101(Cr) showed preferable activity for styrene oxidation, and phenylacetaldehyde was generated with increasing Cu salen content. The optimal conditions for the reaction were also discussed, including the types of oxidants present, molar ratios of styrene and TBHP, and temperature. In particular,

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the mechanism of the styrene oxidation in 70% TBHP over Cu salen@NH2-MIL-101(Cr) was proposed in this work.

1 Introduction Styrene oxidation has generated considerable concern in pharmaceutical and organic synthesis for several decades due to the production of styrene oxide and benzaldehyde.1,2 Over the last few decades, considerable effort has been made to synthesize efficient catalysts for styrene oxidation, which play important roles in the reaction. The main catalysts for the styrene oxidation reaction include heteropoly acid, metal oxides, hetero molecular sieves, and metal organic complexes.3-6 Among these catalysts, metal-salen complexes, which are metal organic complexes, exhibit high activities. Metal-salen complexes denote a class of organic complexes in which the metal ions are connected to tetradentate [O, N, N, O] bis-Schiff base ligands.7,8 These ligands exhibit strong coordination abilities and are easily synthesized, and thereby, they are considered to be the most effective catalysts for selective oxidation. High selectivities of epoxides and conversion of alkenes can be obtained with transition metal (Fe, Co, Mn and W) salen catalysts. However, conventional salen complexes normally possess several limitations, such as the difficulty of separating the catalyst from the reaction mixture due to the high catalyst solubility and the poor catalyst recycling abilities. To overcome these problems, solid supports, such as zeolites (MCM-41 and SBA-15), polymers, and metal nanoparticles, are widely used in homogeneous systems.9-14 Although immobilizing a catalyst on a solid support can convert a homogeneous catalyst to a heterogeneous catalyst, complicated preparation methods, non-uniform catalytic centers, and poor thermal stabilities are remaining issues that must be addressed.


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Metal-organic frameworks (MOFs), composed of organic and inorganic materials, have received considerable attention in the past decade due to their high porosities and large surface areas. In addition, their controllable morphologies and pore sizes make them suitable for gas adsorption and separation, drug delivery, sensing, and catalysis.15-19 Because of their structures, which are related to zeolites, and excellent hydro-thermal stabilities, MOFs and amino functionalized MOFs are a group of emerging materials to be used as catalytic supports.20-22 For example, Cu@UIO-66-NH2 was reported to be an efficient heterogeneous catalyst for the oxidation of cyclohexene capable of 97% conversion, but it exhibited a low selectivity to cyclohexen-1-one.23 Qin et al. reported that molecular iron/citric acid complexes were successfully anchored to NH2-MIL-101(Cr) for photocatalytic hydrogen peroxide splitting.24 Li et al synthesized TiO2@Salicylaldehyde-NH2MIL-101(Cr) for the first time and tested its activity for the photodegradation of methylene blue.25 In addition to being a carrier for catalytic reactions, MOFs are often considered to be good containers for encapsulating metals. Li et al. introduced Co(II) into MIL-101(Cr) by using a “ship in a bottle” method for the electrocatalytic reduction of oxygen.26 In this study, we selected NH2-MIL-101(Cr) as a base material. Cu or Ni salen@NH2-MIL101(Cr) catalysts with novel morphologies were prepared for the first time using a solution impregnation method.27-29 Moreover, the catalytic activities for the styrene oxidation of flat sheet Cu salen@NH2-MIL-101(Cr) and curly sheet Ni salen@NH2-MIL-101(Cr) catalysts were explored under mild conditions. Both catalysts promoted the oxidation of styrene. We also proposed a mechanism to account for the formation of phenylacetaldehyde and epoxide through analysis of the experimental data and XPS characterization of used and fresh Cu salen@NH2-MIL101(Cr) catalyst. The salen ligands of these complexes were synthesized easily from the readily available material, NH2-MIL-101(Cr). Furthermore, the combination of the Cu salen and peroxide


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groups in 70% TBHP enhanced the selectivity to epoxide.30 To the best of our knowledge, there have been no reports of Cu or Ni salen@NH2-MIL-101(Cr) catalysts for the catalysis of styrene oxidation.

2. Materials and methods 2.1 Materials Analytical reagent grade chromium(III) nitrate (Cr(NO3)3·9H2O), 2-aminiterephthalic acid (NH2-BDC), sodium hydroxide, deionized water, N, N-dimethylformamide (DMF), anhydrous ethanol (C2H5OH), cupric acetate monohydrate (Cu(CH3OO)2·H2O), nickel acetate tetrahydrate (C4H6NiO4·4H2O), and salicylaldehyde were purchased from Aladdin Chemistry Co. China and were used without further purification. 2.2 Synthetic methods The preparation mechanism of metal salen@NH2-MIL-101(Cr) is shown in Scheme 1. Cr3+ Cr3+ O O

Cr3+ Cr3+ O O O NH2 O Cr3+

O

H

Cr3+

O Cr3+ Cr

3+O

Cr3+

M(OAC)n M:Cu n=2 M:Ni n=1

O Cr3+

O

OH

O O

NH2

H

O 3+ 3+O Cr Cr O O 3+ Cr3+ Cr

NH2-MIL-101(Cr)

Cr3+ O

O

NH2

O

O

O

NH2

O

N

Cr3+

O N

M O

O

O 3+ Cr O 3+ Cr

Metal salen@NH2 -MIL-101(Cr)

Scheme 1. Preparation of metal salen@NH2-MIL-101(Cr) 2.2.1 Synthesis of NH2-MIL-101(Cr) NH2-MIL-101(Cr) was synthesized using a method similar to that described by Chen et al. First, 3.2 g of chromic nitrate hydrate, 1.44 g of 2-aminiterephthalic acid (NH2-BDC), and 0.8 g of


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sodium hydroxide were dispersed into 60 ml of deionized water and stirred for 15 min.31 The resulting homogeneous solution was transferred to a 100 ml reactor and kept at 150°C for 12 h without stirring. After cooling naturally to the ambient temperature, the small number of white needle crystals present, which were unreacted 2-aminoterephthalic, were removed by centrifugation at 8000 rpm for 6 min to acquire a green precipitate, which was treated with DMF at 70°C for 2 h and immersed in anhydrous ethanol at 100°C overnight. Finally, the solid was activated by drying under vacuum conditions for 24 h. 2.2.2 Synthesis of Cu salen@NH2-MIL-101(Cr) For the preparation of Cu salen@NH2-MIL-101(Cr), various dosages of Cu(CH3OO)2·H2O (10, 20, and 30 wt%) and NH2-MIL-101(Cr) (0.5 g) were dispersed in 35 ml of anhydrous ethanol. After ultrasonic dispersion for 5 min at room temperature, salicylaldehyde was added quickly to the beaker and magnetically stirred to form a homogeneous solution for 10 h at 80°C. After, a yellow-green solid was obtained by centrifuging and washing the solution with anhydrous alcohol several times and finally drying it under a vacuum. The final product was denoted Cu (wt%) salen@NH2-MIL-101(Cr). Ni (wt%) salen@NH2-MIL-101(Cr) was prepared using the same methods. 2.3 Methods of material characterization Powder X-ray diffraction (XRD) patterns were obtained to identify the structures of the prepared catalysts using an Ultima IV X-ray diffractometer with Cu Kα radiation (λ=0.15406 nm). The surface morphologies of the catalysts were observed using scanning electron microscopy (SEM, TESCAN 5136 MM). Transmission electron microscopy (TEM) profiles were obtained using a FEI Tecnai G2 F20 electron microscope with an accelerating voltage of 200 kV. The BET surface area and total pore volume were measured using a Micromeritics ASAP 2020 Plus 1.03 apparatus.


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Fourier transform infrared (FT-IR) profiles of the prepared catalysts were recorded using a Thermo Fisher Nicolet 5700 spectrophotometer in the range of 400–2000 cm-1. Raman spectra were obtained using LabRam ARAMIS to confirm the successful formation of C=N bonds and the coordination of these bonds with metal ions. XPS was carried out on a Thermo Fisher ESCALAB 250Xi to gain deeper insight into the compositions and chemical states of the catalyst surfaces. The leaching rate of Cu in the cyclic reaction was measured using Inductively Coupling Plasma spectroscopy (ICP). 2.4 Catalytic performance test The styrene oxidation reaction was performed in a 50-mL three-necked flask equipped with a condenser. First, 1 ml of acetonitrile, 10 mmol of styrene, and 50 mg of catalyst were added to 50ml three-necked flask sequentially, after which the flask was immersed in a water bath with magnetic stirring. As the reaction temperature was approached, a 70% TBHP was added quickly to the solution, and the solution was stirred for 6 h. Subsequently, the reactor temperature was slowly reduced to the ambient temperature. The products were centrifuged from the catalyst and analyzed by 1H NMR (Figure S1) and an INESA GC-126 system equipped with a chromatographic column and a FID detector (Figure S2).

3 Results and discussion 3.1 Catalyst characterization


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Figure 1. XRD profiles of (a) Cu salen@NH2-MIL-101(Cr) and (b) Ni salen@NH2-MIL-101(Cr). (c) XRD profiles of the prepared and the simulated frameworks for NH2-MIL-101(Cr). By comparing the XRD data of the functionalized NH2-MIL-101(Cr) and Ni or Cu salen@NH2MIL-101(Cr) shown in Figure 1, the following conclusions were drawn. From Figure 1c, the XRD diffraction peaks of the NH2-MIL-101(Cr) prepared in this work were consistent with the standard pattern of NH2-MIL-101(Cr), indicating that NH2-MIL-101(Cr) was prepared successfully. As apparent from Cu salen@NH2-MIL-101(Cr) (Figure 1a), all the samples exhibited better crystallinities. After the functionalization, the XRD patterns changed from those of the pure supports. Some of the peak intensities increased, and new peaks formed. The main diffraction peaks located at 5.2°, 8.5°, 9.1°, 10.3°, 11.4° can be clearly seen in the prepared catalysts, corresponding to the characteristic peaks of NH2-MIL-101(Cr). Additionally, diffraction peaks appeared between 2θ=20° and 40°, corresponding to the XRD peaks reported previously.32 Therefore, Cu-salen@NH2-MIL-101(Cr) was successfully prepared, which was further confirmed by subsequent characterization. However, the main peaks of the support were not altered, indicating that the structural integrity of NH2-MIL-101(Cr) was unaffected.33 Unlike Figure 1a, the XRD pattern of Ni-salen@NH2-MIL-101(Cr) (Figure 1b) varied significantly upon the


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introduction of 10, 20, and 30 wt% Ni-salen. When the content of Ni-salen was only 10 wt%, the partial peak intensity increased, which indicates that the successful introduction of Ni-salen caused the sample to grow along a specific crystal plane.34 When the content of Ni-salen increased to 20 wt%, the XRD pattern was consistent with that of the pure support and was slightly enhanced. This was ascribed to the even dispersion of Ni-salen, which was proven by SEM and TEM. When the content of Ni-salen was between 20 and 30 wt%, although the crystallinities of the samples were poor, the main diffraction peaks located at around 8.9° were still observed. This also illustrates that the ability of Ni-salen to bind to the support was weaker than that of Cu-salen. The results of the activity tests also confirmed these binding abilities.

Figure 2. SEM images of catalysts: (a) NH2-MIL-101(Cr), (b) reused 10%Cu salen@NH2-MIL101(Cr), (c) fresh 10%Cu salen@NH2-MIL-101(Cr), (d) 20%Cu salen@NH2-MIL-101(Cr), (e) 30%Cu salen@NH2-MIL-101(Cr), (f) 10%Ni salen@NH2-MIL-101(Cr), (g) 20%Ni salen@NH2MIL-101(Cr), and (h) 30%Ni salen@NH2-MIL-101(Cr).


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Figure 3. TEM images of (a) NH2-MIL-101(Cr) and (b) 10%Cu salen@NH2-MIL-101(Cr).

Figure 4. TEM images of (a) 10%Ni salen@NH2-MIL-101(Cr), (b) 20%Ni salen@NH2-MIL101(Cr), and (c) 30%Ni salen@NH2-MIL-101(Cr). The morphologies of the catalysts were examined by SEM, as shown in Figure 2, and TEM, as shown in Figure 3 and Figure 4. As shown in Figure 2a, the NH2-MIL-101(Cr) morphology consisted of polyhedra of uniform sizes. As shown in Figure 2c–e and Figure 3b, 10%Cu salen had a compact and interconnected layered structure with porosity, and the particle size decreased, which is of great benefit to styrene oxidation. With the increase of Cu salen content, particles agglomerated into larger aggregates but still exhibited a homogeneous distribution, hence, a less compact sheet morphology can be seen in Figure 2d-e. In addition, the morphology of the reused


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10%Cu salen@NH2-MIL-101(Cr) remained unchanged in Figure 2b, indicating that 10%Cu salen@NH2-MIL-101(Cr) is a stable catalyst for styrene oxidation. However, the morphology of 10%Ni salen@NH2-MIL-101(Cr) was similar to that of NH2-MIL-101(Cr), as shown in Figure 2f and 2a. Partial curly sheet structures were observed in Figure 2g–h, and the three-dimensional network structure was observed clearly in the TEM images (Figure 4b–c). The change in the morphology can be attributed to the steric effect caused by the addition of salicylaldehyde in the synthesis progress.35,36 Due to the presence of curly nanosheet structures without holes for Ni salen@NH2-MIL-101(Cr), the catalytic activity was theoretically weaker than that of 10%Cu salen@NH2-MIL-101(Cr).

Figure 5. N2 adsorption/desorption isotherms of (a) Cu salen@NH2-MIL-101(Cr) and (b) Ni salen@NH2-MIL-101(Cr). Table 1. The BET surface areas and total pore volumes of samples. Samplesa

Metal Loading, wt%

SBET (m2/g)

Total pore volume (cm3/g)

Ni salen@MOF

30

447.2

0.33

Ni salen@MOF

20

574.3

0.45

Ni salen@MOF

10

708.8

0.5

Cu salen@MOF

30

711.8

0.57

Cu salen@MOF

20

862.9

0.66

Cu salen@MOF

10

1041.3

0.78


10

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MOF

—

1092.5

0.98

a MOF=NH -MIL-101(Cr). 2

To further study the effect of the metal salen content on the pore structure, the N2 adsorption/desorption isotherms of the prepared catalysts were obtained, as shown in Figure 5, and the BET surface areas and total pore volumes are listed in Table 1. As shown in Figure 5, all the samples exhibited typeⅠadsorption isotherms, indicating that the micropore structures were well preserved in the metal salen@NH2-MIL-101(Cr). With the addition of metal salen, the BET surface area and total pore volume decreased in the following order: 10%Cu or Ni salen@NH2MIL-101(Cr) > 20%Cu or Ni salen@NH2-MIL-101(Cr) > 30%Cu or Ni salen@NH2-MIL-101(Cr). The highest BET surface area (1041.3 m2/g) and total pore volume (0.78 cm3/g) were achieved by the 10%Cu salen@NH2-MIL-101(Cr), which was consistent with the conclusions drawn from the SEM analysis. Compared with Cu salen@NH2-MIL-101(Cr), the decreased BET surface area (from 708.8 m2/g to 447.2 m2/g) and total pore volume (from 0.5 cm3/g to 0.33 cm3/g) of Ni salen@NH2-MIL-101(Cr) was attributed to the curly sheet morphology generated with the increase of the Ni salen content, and the agglomeration of particles affected the BET surface area and total pore volume correspondingly.

Figure 6. FT-IR spectra of (A) Cu salen@NH2-MIL-101(Cr) and (B) Ni salen@NH2-MIL101(Cr). The labels (a), (b), (c), and (d) represent NH2-MIL-101(Cr), 30%Cu or Ni salen@NH2-


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MIL-101(Cr), 20%Cu or Ni salen@NH2-MIL-101(Cr), and 10%Cu or Ni salen@NH2-MIL101(Cr), respectively. The FT-IR spectra of all the functionalized samples are shown in Figure 6, along with that of the support NH2-MIL-101(Cr) for comparison. There were no peaks corresponding to aldehyde groups in the 1700–1800cm-1 range. A strong C=N stretching vibration peak near 1610 cm-1 was detected in all the compounds, indicating that the structure of a Schiff base was formed by condensation of the NH2 and -CHO groups, resulting in the loss of water. Generally, the vibrations of C=N in the Schiff base appeared in the 1620–1640 cm-1 range, while the C=N bonds shifted to lower wavenumbers, which indicates that metal complexes formed.37 The signal at 1339 cm-1, assigned to the C-N stretching vibrations of NH2-BDC, decreased, demonstrating that there were fewer NH2 groups formed during the functionalization. In addition, the new absorption peaks appeared at 1525 cm-1 (Figure 6A), 1528 cm-1 (Figure 6B), and 1150 cm-1 (Figure 6A and Figure 6B) were attributed to the vibrations of H-C=N and the metal salen framework, respectively,38 and no peak at 1528 cm-1 was observed for 10%Ni-salen@NH2-MIL-101(Cr) because of its low concentration. Furthermore, peaks corresponding to a new stretching vibration appeared in the 400–600 cm-1 range due to coordination bond formed by Ni or Cu ions and N atoms. The intensities of the peaks for all the Ni-salen@NH2-MIL-101(Cr) samples were much weaker than those of the Cusalen@NH2-MIL-101(Cr) samples, indicating a weaker binding ability of Ni salen to the support, which was consistent with the XRD results. The FTIR spectra showed evidence of imine formation. To further confirm the formation of metal salen@NH2-MIL-101(Cr), Raman spectroscopy was carried out.


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Figure 7. Raman spectrum of (a) Cu salen@NH2-MIL-101(Cr) and (b) Ni salen@NH2-MIl101(Cr). MOF denotes NH2-MIL-101(Cr). In Figure 7, the signal at approximately 845 cm-1 was obtained and assigned to the C—H bending vibration of NH2BDC in NH2-MIL-101(Cr). A sharp peak at 1536 cm-1 attributed to the stretching vibration of C=N appeared for Cu salen@NH2-MIL-101(Cr), but the intensity attenuated for Ni salen@NH2-MIL-101(Cr). The Raman spectra agreed with the FTIR and XRD analyses, that is, the coordination ability of Ni salen with the carrier was inferior to that of Cu salen.

Figure 8. XPS spectra of (a) 30%Cu salen@NH2-MIL-101(Cr), (b) 20%Cu salen@NH2-MIL101(Cr), and (c) 10%Cu salen@NH2-MIL-101(Cr).


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Figure 9. XPS spectra of (a) 30%Cu salen@NH2-MIL-101(Cr), (b) 20%Cu salen@NH2-MIL101(Cr), and (c) 10%Cu salen@NH2-MIL-101(Cr). To further confirm the successful synthesis of Cu salen@NH2-MIL-101(Cr) and Ni salen@NH2MIL-101(Cr), the surface chemical compositions of the catalysts were investigated, and the corresponding spectra are shown in Figure 8 and Figure 9. As illustrated by Figure 8a3, b3, c3 and Figure 9a3, b3, c3, all the samples contained C, N, O, Cr, Cu, or Ni. The binding energies of Cr 2p1/2 and Cr 2p3/2 appeared at around 587 and 577.24 eV, respectively, which are the typical binding energies of the chromium trimer.39 Other chemical states of chromium were not observed, indicating that the structure of the chromium trimer did not change during the functionalization process. The N 1s spectrum can be decomposed into three peaks for all the catalysts. From the right to left, the first peaks were assigned to M-N groups at 399.1, 398.7, and 398.6 eV for Cu salen@NH2MIL-101(Cr) and at 398.5, 398.6, and 398.4 eV for Ni salen@NH2-MIL-101(Cr). The second set of N1s peaks, located at 399.9, 399.5, and 399.6 eV for Cu salen@NH2-MIL-101(Cr) and at 399.4, 399.8, and 399.3 eV for Ni salen@NH2-MIl-101(Cr), were attributed to -C=N- groups due to the


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reaction of -CHO with NH2 groups. The third set of N1s peaks, located 400.85, 400.3, and 401 eV in Figure8a1, b1, c1, respectively, and similarly at 400.1, 401.2, 401.4 eV in Figure 8a1, b1, c1, respectively, were observed for all the catalysts, implying that Cu or Ni salen successfully combined with NH2-MIL-101(Cr). Additionally, the peak at around 406 eV was caused by the presence of NO3- groups.40,41 The O 1s spectrum also exhibited three resolved peaks. The first peaks, closely linked with stretching vibrations of metal and oxygen (M-O), appeared at 531.2, 531.3 and 531.5 eV for Cu salen@NH2-MIL-101(Cr) and at 531.6, 531.7, and 531.4 eV for Ni salen@NH2-MIL-101(Cr). The second peaks, located at 532, 532.1, and 532.3 eV for Cu salen@NH2-MIL-101(Cr) and at 532.2, 532.3, and 532.1 eV for Ni salen@NH2-MIL-101(Cr), were associated with the C-O binding energy from the metal-salen complex. The last peak positions, located at 532.6,532.9, and 533 eV for Cu salen@NH2-MIL-101(Cr) and at 532.9, 533.1, and 532.7 eV for Ni salen@NH2-MIL101(Cr), were attributed to the O-H in the excess salicylaldehyde.42-44 Thus, the binding energies once again confirmed the formation of Cu or Ni salen@NH2-MIL-101(Cr). 3.2 Oxidation of styrene O

O

O

Metal salen@NH2-MIL-101(Cr)

+

+

Oxidant,CH3CN a Styrene oxide

b Benzaldehyde

c Phenylacetaldehyde

Scheme 2 Oxidation of styrene using metal salen@NH2-MIL-101(Cr). Table 2. Catalytic activities and selectivities of various catalysts for the oxidation of styrene with TBHPa Catalyst

Metal

Conversion,%

Loading, wt% Ni salen@MOF

30

61.56

Selectivity,% 1a

1b

1c

84.84

15.16

—


15


a

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Ni salen@MOF

20

77.62

80.17

19.83

—

Ni salen@MOF

10

86.69

63.03

36.97

—

Cu salen@MOF

30

99.44

59.87

13.85

26.28

Cu salen@MOF

20

98.86

78.33

12.43

9.24

Cu salen@MOF

10

98.78

89.58

10.42

—

MOF

—

38.45

25.67

76.43

—

Reaction conditions: 50 mg catalysts, 10 mmol styrene, 10 mmol acetonitrile, 30 mmol TBHP,

80°C, 6 h, MOF=NH2-MIL-101(Cr). The catalytic activities of the metal salen@NH2-MIL-101(Cr) catalysts for styrene oxidation were measured. The chemical equation of the reaction is shown in Scheme 2. First, the effects of different metal salen contents on the reaction were studied and are summarized in Table 2. Compared with Ni or Cu salen@NH2-MIL-101(Cr), the reaction was also carried out in the presence of pure support NH2-MIL-101(Cr), but epoxide was barely produced. It could be inferred that the metal salen incorporated into NH2-MIL-101(Cr) was responsible for this chemical transformation (styrene to epoxide). However, various products were obtained as the metal salen content increased. For the Ni salen@NH2-MIL-101(Cr) catalysts, the conversion of styrene decreased abruptly (from 86.69% to 61.56%). However, the selectivity to epoxide increased (from 63.03% to 84.84%), which was completely different from Cu salen@NH2-MIL-101(Cr) catalysts. Furthermore, there was not a significant change in styrene conversion (99%) due to the increased Cu salen content of the Cu salen@NH2-MIL-101(Cr), indicating that Cu salen@NH2-MIL101(Cr) is a more efficient catalyst than Ni salen@NH2-MIL-101(Cr). It is noteworthy that


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phenylacetaldehyde was formed by the Cu (20 and 30 wt%) salen@NH2-MIL-101(Cr) catalyst. This can be explained by the magnitude of the standard electrode potential, which is more positive for Cu2+, with Eɵ(Cu2+/Cu)=+0.34 V, and more negative for Ni2+, with Eɵ(Ni2+/Ni)=-0.25 V, suggesting that Cu2+ is a better oxidant than Ni2+.45 When the content of Cu salen increased, some of the epoxide converted to phenylacetaldehyde in the presence of TBHP. The mechanism for producing epoxide and phenylacetaldehyde will be discussed in detail in Section 3.2.2. According to the experimental data, although part of the phenylacetaldehyde was produced due to the Cu salen@NH2-MIL-101(Cr), the selectivity to epoxide was still above 50%. Thus, the catalytic effect of Cu salen@NH2-MIL-101(Cr) was better than that of Ni salen@NH2-MIL-101(Cr). The results agreed with the conclusion derived from the characterizations results. 3.2.1 Optimization of the reaction conditions Table 3. Oxidation of styrene using Cu salen@NH2-MIL-101(Cr) under various conditionsa. Styrene:oxidant

Temperature

Conversion

Entry

Oxidant

molar ratio

(°C)

(%)

1

H2O2b

1:3

80

2

TBHPc

1:3

3

TBHPc

4

Selectivity (%) 1a

1b

1c

93.85

6.44

93.56 —

80

98.78

89.58 10.42 —

1:1

80

37.36

60.34 39.66 —

TBHPc

1:2

80

86.39

71.44 28.56 —

5

TBHPc

1:4

80

97.78

84.54 15.46 —

6

TBHPc

1:3

60

46.37

9.93

90.07 —

7

TBHPc

1:3

100

—

—

—

—

a

Reaction conditions: 50 mg Cu (10%) salen@NH2-MIL-101(Cr), 10 mmol styrene, 10 mmol acetonitrile, 6 h. b

H2O2=30 wt% in water. c TBHP=70 wt% in water.


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The Cu (10%wt) salen@NH2-MIL-101(Cr) catalyst was determined to be the best catalyst in this work for subsequent studies of styrene oxidation in acetonitrile solution. Several factors influencing the reaction products were also investigated, such as the oxidant species, ratio of styrene to 70% TBHP (tert-butyl hydrogen peroxide), and temperature. Different oxidants, such as H2O2 and TBHP, had different effects on the styrene epoxidation. The results of the experiment are shown in Table 3. When H2O2 and TBHP were employed as oxidants, most of the styrene was converted. The styrene conversions obtained using H2O2 and TBHP were 93.85% and 98.78%, repectively. The difference between the use of these two types of oxidants was that the main product obtained with H2O2 was benzaldehyde, while the main product obtained with TBHP was epoxide. However, it was difficult to separate the products and catalyst in the presence of H2O2. Hence, TBHP was selected as an appropriate oxidant for the styrene oxidation. The effect of different molar ratios of styrene and TBHP on the oxidation of styrene was also explored (Table 3, entries 2–5). The amount of styrene present was kept at 10 mmol. As the quantity of TBHP increased, the conversion of styrene increased dramatically from 37.36% (Table 3, entry 3) to 98.78% (Table 3, entry 2). Meanwhile, the selectivity for styrene oxide increased from 60.34% (Table 3, entry 3) to 89.58% (Table 3, entry 2). Further increasing the molar ratio to 1:4 resulted in a slight decrease in conversion and selectivity, but they still retained high values. Thus, the ratio of 1:3 was confirmed to be the best proportion of styrene to TBHP. We also performed three experiments at temperatures of 60, 80, 100°C to illustrate the importance of the temperature for styrene oxidation. As shown in Table 3 (entries 2, 6, and 7), the conversion of styrene improved considerably from 46.37% at 60°C to 98.78% at 80°C. Furthermore, the selectivity varied as the temperature was increased. Styrene oxide was the main


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product at 80°C. When temperature was reduced to 60°C, benzaldehyde was the mainly product. The products and catalyst were difficult to separate at 100°C, i.e., the reaction system became homogeneous. Thus, 80°C was the optimum reaction temperature for styrene oxidation. 3.2.2 Mechanism investigation

Figure 10. XPS Cu 2p3/2 spectra of fresh and reused 10%Cu salen@ NH2-MIL-101(Cr). To further study the mechanism of epoxide and phenylacetaldehyde formation, the reused and fresh Cu salen@NH2-MIL-101(Cr) catalyst were examined by XPS (Figure 10.). According to the literature,46 the binding energy of Cu 2p3/2 is in the range of 930–940 eV. As shown in Figure 9, the peaks located between 930 and 935 eV corresponded to Cu(II), and the other peaks located between 940 and 945 eV were assigned to the shake-up satellite peaks of Cu 2p3/2. A typical band at 935.1 eV, which was observed for the fresh catalyst, shifted to a lower value of 934.2 eV for the reused catalyst, indicating that the surrounding electron density of the Cu (II) increased. Based on the previous analysis and because the oxidizing power of Cu2+ is greater than that of Ni2+, the Cu salen@NH2-MIL-101(Cr) catalyst can enhance the selectivity to epoxide. This explains why the phenylacetaldehyde formation increased with the increase of Cu salen content. Moreover, the


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shake-up satellite peaks of Cu 2p3/2 also revealed the coordination between the salen ligands and Cu centers.

Scheme 3. Mechanism of styrene oxidation over Cu salen@NH2-MIL-101(Cr). Based on the experimental results and other literature reports, a reasonable reaction mechanism for styrene oxidation catalyzed by Cu salen@NH2-MIL-101(Cr) is proposed in Scheme 3.47-49 Cu salen plays an important role during the entire reaction progress. The reaction includes two paths. TBHP first interacts with Cu(II) salen to form an active trivalent copper-based peroxide I, which interacts with C=C in the styrene, generating intermediate II. Due to the departure of a tert-butoxy radical (tBuOO·), the remaining molecule forms complex III, and epoxide finally forms. The tertbutoxy radical (tBuO·) and Cu(II) salen participate in the next cycle. Phenylacetaldehyde forms due to the excessive oxidation of styrene oxide under the oxidative Cu(II) salen catalyst. Meanwhile, the formation of benzaldehyde is also underway due to the adsorption of the C=C bond on Cu(II) salen. In summary, the aggregation of the ambient electron density of Cu(II) can activate styrene and promote abundant trivalent copper-based peroxide I, resulting in the formation of more epoxide and phenylacetaldehyde. 3.2.3 Stability test


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Figure 11. Reusability of (a) 10%Cu salen@NH2-MIL-101(Cr) and (b) 10%Ni salen@NH2-MIL101(Cr).

Figure 12. (a) SEM and (b) TEM images of 10%Ni salen@NH2-MIL-101(Cr) after four cycles. Stability tests of the 10%Cu salen@NH2-MIL-101(Cr) and 10%Ni salen@NH2-MIL-101(Cr) catalysts during styrene oxidation with 70% TBHP were conducted. The results are shown in Figure 11. Styrene conversion decreased to 90% after four cycles, and the selectivity to epoxide varied slightly for 10%Cu salen@NH2-MIL-101(Cr), implying that catalytic activity was not affected, which agreed with the SEM results (Figure 2b-c) and mechanism. As shown in Figure 11b, there was a significant reduction in the selectivity to epoxide, and styrene conversion also decreased. To explain this reduction, the SEM and TEM images are shown in Figure 12. The structure and morphology changes of 10%Ni salen@NH2-MIL-101(Cr) can be clearly seen. After four cycles, the 10%Ni salen@NH2-MIL-101(Cr) morphology was slightly stacked compared to that of the catalyst before the reaction (Figure 2f and Figure 4a, respectively), which agreed with results of the stability tests. The contents of Cu and Ni salen in the filtrate after each reaction cycle were determined by ICP-AES, and no Cu(II) was detected, indicating that 10%Cu salen@NH2-


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MIL-101(Cr) was stable for styrene oxidation, while 9.20 mg/L of Ni2+ was detected in filtrate, implying that Ni salen@NH2-MIL-101(Cr) catalyst was not stable.

4 Conclusion This work demonstrated that Ni and Cu salen were embedded on NH2-MIL-101(Cr) by a traditional solution method. The resultant Cu salen@NH2-MIL-101(Cr), with its compact and interconnected pore structure, exhibited an excellent catalytic activity for styrene oxidation in the presence of TBHP. Ni salen@NH2-MIL-101(Cr) showed a lower catalytic activity because of its curly sheet morphology and weaker coordination ability, which were confirmed by various characterization methods. According to the characterization, the better catalytic selectivity to epoxide over Cu salen@NH2-MIL-101(Cr) was due to the concentrated electronic density around Cu(II), which was beneficial for the formation of tBuOOCu(III)–salen, promoting the selectivity to epoxide. Phenylacetaldehyde was formed as the content of Cu salen increased due to the strong oxidation performance of Cu(II). In summary, Cu and Ni salen@NH2-MIL-101(Cr) with novel morphologies are effective catalysts for styrene oxidation.

AUTHOR INFORMATION Corresponding Author Kai Huang‡* Present Addresses School of Chemistry and Chemical Engineering, Southeast University, Jiangning District, Nanjing 211189, China.


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Funding Sources This work was supported by the National Natural Science Foundation of China (No. 21576049) and the Fundamental Research Funds for the Central Universities (No. 2242016K40082). REFERENCES 1.

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Synthesis of Metal Salen@MOFs and Their Catalytic Performance for

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