Phosphotungstic acid encapsulated in metal-organic framework UiO-66: An effective catalyst for the selective oxidation of cyclopentene to glutaraldehyde

Microporous and Mesoporous Materials 211 (2015) 73e81

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Phosphotungstic acid encapsulated in metal-organic framework UiO-66: An effective catalyst for the selective oxidation of cyclopentene to glutaraldehyde Xin-Li Yang a, *, Li-Ming Qiao a, Wei-Lin Dai b, * a b

School of Chemistry & Chemical Engineering, Henan University of Technology, Henan 450001, PR China Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 December 2014 Received in revised form 16 February 2015 Accepted 18 February 2015 Available online 26 February 2015

A heterogenous Zr-based metal organic framework (UiO-66) encapsulating phosphotungstic acid (HPWs) catalyst (HPWs@UiO-66), was prepared by a simple direct hydrothermal reaction of ZrCl4, terephthalic acid, and HPWs in DMF. The as-prepared novel material was very active as the catalyst for the selective oxidation of cyclopentene (CPE) to glutaraldehyde (GA) with environmentally benign hydrogen peroxide as the oxidant. The crystal structure and morphology of UiO-66 were well preserved after the incorporation of HPWs, as confirmed by X-ray diffraction (XRD), SEM, and TEM. Moreover, the XRD, N2 adsorption, and FT-IR analyses reveal that HPW components could stably exist in the nanocages of UiO-66. FT-IR-CO adsorption experiments indicated that additional Lewis acid sites were present in the HPWs@UiO-66 sample, which were essential to catalyze the selective oxidation of CPE to GA. A proper amount of HPWs and their high dispersion accounted for high catalytic activity. Almost complete conversion of CPE (~94.8%) and high yield of GA (~78.3%) were obtained using the 35 wt% HPWs@UiO-66 catalyst. Furthermore, HPW components hardly leached in the reaction solution, enabling the catalyst to be used for three reaction cycles without obvious deactivation. © 2015 Elsevier Inc. All rights reserved.

Keywords: Metal-organic frameworks UiO-66 Phosphotungstic acids Cyclopentene Glutaraldehyde

1. Introduction In recent years, Metal Organic Frameworks (MOFs) have received an increased attention as they are considered as the newest generation porous materials [1e3]. This type of crystalline materials is self-assembled by linking organic ligands with metal ions or metal clusters to form infinite network structures. The porosity, structure, and functionality of MOFs can be adjusted by varying the bridging ligands and/or the metal centers. In addition to their “classic” applications in gas storage [4,5] and separation [6], MOFs have attracted the interest of researchers in the field of catalysis owing to their unique properties such as high surface area, crystalline open structures, tunable pore size, and functionality [7]. Hitherto, mainly three different types of catalytic activities are reported for the catalysts based on MOFs. First, the catalytic activity observed for these materials is directly related to their metallic

* Corresponding authors. Fax: þ86 371 67756718. E-mail addresses: [email protected] (X.-L. Yang), (W.-L. Dai). http://dx.doi.org/10.1016/j.micromeso.2015.02.035 1387-1811/© 2015 Elsevier Inc. All rights reserved.

[email protected]

components either as isolated metal centers or clusters; second, MOFs structure can contain catalytically active centers at the organic linker molecules of the framework; third, none of the components of the MOFs are directly involved in the catalysis. MOFs simply play the role of a support material, and their porous system provides the physical space for catalysis or functions as a cage where the catalytic centers are encapsulated [8]. Although MOFs have been studied as the catalysts in the hydrogenation, oxidation, enantioselective, photocatalytic, carbonyl cyanosilylation, hydrodesulfurization, and esterification reactions, their catalytic applications are still quite limited because of their low thermal and hydrolytic stability apart from often completely blocked metal sites by the organic linker or solvent, leaving no free positions available for substrate chemisorption. Recently, new synthesis strategies have been proposed to overcome the drawbacks including post-synthetic modification [9], the use of specific synthesis modulators [10e13], and ultrasonication and microwaveassisted methods [14,15]. The zirconium (IV) terephthalate UiO-66(Zr), (UiO ¼ University of Oslo), first reported by Lillerud and coworkers, has a rigid 3D cubic close packed structure based on the hexamers of eight-coordinated ZrO6(OH)2 polyhedra and 12

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terephthalate linkers [16]. UiO-66(Zr) solid with tetrahedral and octahedral cages of 8 and 11 Å, respectively, is accessible through microporous triangular windows in the range 5e7 Å, leading to a high porosity combined with high thermal, chemical, and mechanical stability. Furthermore, the textural and physicochemical properties of UiO-66(Zr) solid can be easily tuned by functionalized terephthalate linkers (NH2, Br, NO2, etc.) or extended organic ligands [17e19]. Owing to these interesting properties, UiO-66(Zr) solid appears to be a good candidate in catalysis. De Vos et al. [20] reported the catalytic performances of UiO-66 and UiO-66-NH2 for the synthesis of jasminaldehyde via the cross-aldol condensation of benzaldehyde and heptanal. UiO-66 was less active in comparison to UiO-66-NH2 which acted as a bi-functional catalyst and suppressed the formation of byproduct. They also studied the effect of functionalized terephthalate linkers on the catalytic properties of UiO-66 in the cyclization of citronellal [21]. A combination of catalytic and computational molecular modeling indicated that the type of functional groups present in the linker units could alter the Lewis acidic properties and induce additional stabilizing/destabilizing effects on the reactants depending on their electronic properties. Recently, they reported that the use of specific synthesis modulators (trifluoroacetic acid and HCl) allowed a high level of control on the number and nature of the defect sites in the well-studied zirconium terephthalate, UiO-66, and thereby influenced the catalytic activity of the material [11]. The study of Ahn et al. indicated that the catalytic activities of UiO-66 and UiO-66-NH2 correlated to their Lewis acidebase properties in the cycloaddition of CO2 to styrene oxide [22]. Jhung et al. investigated the effects of linker substitutions on the catalytic properties of porous zirconium terephthalate, UiO-66, in the acetalization of benzaldehyde with methanol. The results showed that the insertion of electron-donating NH2-groups into the linker ligand increased the strength of basic sites in contrast to electronwithdrawing NO2-groups. The strength of Lewis acid sites decreased in the order of UiO-66-NO2 > UiO-66 > UiO-66-NH2, thus their catalytic activities for the acetalization of benzaldehyde with methanol decreased in the same order [23]. Heteropolyacids (HPAs), as a unique class of anionic metaloxygen clusters of early transition metals, have many properties that make them suitable candidates for applications in catalysis. They have been extensively used as acidic and oxidation catalysts in many reactions because their acidebase and redox properties can be tuned easily by changing polyanion chemical composition [24]. Phosphotungstic acid (HPW) with the strongest Brønsted acidity in the HPAs series is a promising heterogenous catalyst for many organic reactions such as esterification, alkylation, hydrolysis, and oxidation [24,25]. The industrial applications of pure HPAs have been hindered notably because of their low surface area (1e10 m2/g). Considerable efforts have been directed to their heterogenization onto various solid supports such as silica [26,27], activated carbon [28,29], ion-exchange resin [30], and mesoporous molecular sieves [31e33]. However, the supported HPAs catalysts usually show low activities because of low HPA loading, HPA leaching, the conglomeration and nonuniform distribution of HPAs particles, and the deactivation of acid sites by water. Therefore, the immobilization of HPAs in a suitable solid matrix, which can overcome these drawbacks, is a step toward the challenging goal of catalysis. The hydrothermal, chemical, and mechanical stabilities, together with its high porosity, make UiO-66 an ideal candidate for host matrices to encapsulate HPAs. The heterogenization of HPAs in the host matrices of a MOF offers many advantages such as the isolation of the Keggin units improving molecular accessibility, simple recovery of catalysts by filtration, and convenient reuse. Therefore, the encapsulation of HPAs in MOFs such as MIL-101 and

HKUST-1 prepared by the impregnation or direct synthesis method, has attracted considerable attentions [34e43]. To the best of our knowledge, there are no reports on the synthesis of HPA incorporated into UiO-66. Herein, for the first time, we report the encapsulation of active HPW species in UiO-66 material via the direct synthesis method and the investigation of its catalytic performance for the selective oxidation of cyclopentene (CPE) to glutaraldehyde (GA) with environmentally benign aqueous H2O2 as the green oxidant. GA is extensively used in the fields of disinfection and sterilization. An important method to produce GA is the selective oxidation of CPE, because a significant quantity of CPE could be easily obtained by the selective hydrogenation of cyclopentadiene, which in turn is easily obtained from the decomposition of dicyclopentadiene, a main byproduct from the C-5 fraction in the petrochemical or coke industry [44,45]. We found that HPWs were highly dispersed into the matrices of the support UiO-66, and the as-prepared heterogeneous catalysts showed much higher activity and selectivity for this oxidation reaction. The recyclability and reusability of the catalysts were also investigated. 2. Experimental 2.1. Catalyst preparation All the chemicals were obtained commercially and used without further purification. UiO-66 was synthesized according to the hydrothermal method described in the literature [16,46]. The encapsulation of HPWs in UiO-66 denoted as HPWs@UiO66 was prepared following the same procedure as for the pure UiO-66, besides HPWs were being added to the mixture during the synthesis. In a typical preparation procedure, 1.45 g zirconium chloride (ZrCl4), 1.06 g terephthalic acid (H2BDC), 0.5 mL condensed HCl, and the required amount of HPWs were added in DMF (40 mL). The resulting reaction mixture was ultrasonicated for ~5 min at room temperature. Then, the obtained mixtures were sealed in a Teflon-lined autoclave and kept in an oven at 393 K for 24 h. The crystallization was carried out under static conditions. Then, the autoclave used for the synthesis was cooled to room temperature in air. The resulting white product was filtered off, washed with DMF to remove excess H2BDC, repeatedly washed with methanol, and finally dried at room temperature. The as-prepared catalyst was dried at 453 K for 10 h before catalytic test. 2.2. Characterizations The powdered X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-rB spectrometer with Cu Ka radiation, which was operated at 60 mA and 40 kV. The FT-IR measurements were carried out with an IR Prestige-21 spectrometer (SHIMADZU) using KBr pellet technique. The in situ FT-IR spectra using CO as a probe molecule were collected on a BRUKER (Tensor 27) spectrophotometer equipped with a DTGS detector at 4 cm1 resolution on a thin self-supported wafer. The sample was activated in a homemade cell under high vacuum (residual pressure 104 mbar) at 473 K for 2 h. CO gas (Peq ¼ 50 torr) was dosed on the sample for 20 min at RT. Nitrogen adsorption and desorption at 77 K were measured by using a Micromeritics ASAP 2020 instrument after the samples were degassed (1.33  102 Pa) at 423 K overnight. The specific surface area was calculated using the BET method. The total pore volume was calculated from the amount of nitrogen adsorbed at a relative pressure of 0.99. Scanning electron micrographs were obtained using JSM-6510LV scanning electron microscope (JEOL). The samples were deposited on a sample holder with a piece of adhesive carbon tape and were then sputtered with a thin film of

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gold. Transmission electron micrographs (TEM) were obtained on a Philips Tecnai F20 transmission electron microscope. The samples were supported on carbon-coated copper grids for the experiment. The thermogravimetric analysis was carried out in air atmosphere (50 ml/min) using a Perkin Elmer TGA7/DTA7 instrument. The approximate sample weight was 10 mg and the heating rate in TG experiment was 10 K/min. The HPWs contents were determined by means of Inductively Coupled Plasma Optical Emission Spectroscopy (Thermo ICP-OES 6500). 2.3. Activity test The activity test was performed at 308 K for 24 h with magnetic stirring in a closed 100 ml regular glass reactor using aqueous H2O2 as oxygen-donor and tert-butyl alcohol (t-BuOH) as the solvent. The quantitative analysis of the reaction products were performed by using GC method and the identification of different products in the reaction mixture was determined by means of GCeMS. Details can be found elsewhere [47,48]. 3. Results and discussion 3.1. Catalyst characterizations X-ray powder diffraction (XRD) was used to measure the phases of the synthesized samples. Fig. 1 shows the XRD patterns of the different materials and the standard XRD pattern of UiO-66, generated from the original CIF-file [49]. The pattern of the as-obtained UiO-66 matched perfectly with the simulated pattern. The diffraction peak positions and relative diffraction intensities for UiO-66 are found to be in agreement with the standard data, thus proving that the synthesized sample is UiO-66, further confirming the phase purity of the synthesized UiO-66. The XRD patterns of different samples (Fig. 1) exhibit no difference between the pristine UiO-66 and HPWs encapsulated UiO-66 materials (HPWs@UiO-66), apart from the fact that the intensity of the peak decreased for the HPWs@UiO-66 samples with increasing HPWs content, which is
rey et al. [50]. Although consistent with the results reported by Fe the incorporation of HPWs added electron density to the crystal structure of MOFs, the MOFs themselves were not significantly affected by HPW guest molecules [35e37]. Moreover, the XRD

g f Intensity(a.u.)

e d

75

patterns of the HPWs could not be obtained from the XRD patterns of the HPWs@UiO-66 samples, indicating the even distribution of HPWs. Fig. 1 also shows the XRD pattern of the 35 wt% HPWs@UiO66 sample after the third reaction cycle and was almost the same as that of the fresh one, demonstrating that the crystal structure of UiO-66 is highly stable in the system of the CPE selective oxidation with aqueous H2O2 and t-BuOH as the oxidant and solvent, respectively. The textural properties of the obtained UiO-66 and HPWs@UiO66 samples acquired from the low-temperature N2 adsorption measurements are shown in Table 1 and Fig. 2. In the case of UiO66, the BET surface area is smaller than that reported in the literature (1069 m2/g) [49]. The deviation between the two is 195 m2/g, which may be explained by the choice of the pressure region used for the BET calculation or the presence of trapped recrystallised terephthalic acid synthesis residues within the pores of UiO-66 [38,50,51]. The presence of free terephthalic acid impurities in UiO-66 samples, which was difficult to be thoroughly removed by washing, was evidenced by IR spectra which showed an absorption band at 1720 cm1 (Fig. 4) [52]. The structure of UiO-66 contains two types of cages: an octahedral cage (the large one, ca. 1.1 nm) that is face sharing with 8 tetrahedral cages (the small one, ca. 0.8 nm) and edge sharing with 8 additional octahedral pores. The two types of cages are accessible through microporous triangular windows in the range 5e7 Å [16]. The Keggin polyanion has a relatively large particle size of 1.0e1.3 nm diameters and 2.25 nm3 in volume [50]. Taking into account that the pore size of UiO-66(Zr) is smaller than that of the Keggin polyanion, it is obvious that the encapsulation of such active species into the cages of UiO-66(Zr) could not be worked by the impregnation approach, but it could be achieved by the direct synthesis method (one-pot synthesis method). Moreover, the Keggin polyanion could only be restricted in the larger cage of UiO-66 and each cage of UiO-66 is only loaded with one HPW, which would offer many advantages, like a better dispersion of HPW, preventing HPW leaching from the cage since it is bigger than the windows of the cages. Compared to the parent UiO-66 material, the textural properties of the HPWs@UiO-66 samples including the surface area and pore volume significantly decreased because of the incorporation of HPWs into the larger microporous cages of UiO-66 support. UiO-66 exhibited the type I isotherms in the N2 adsorption isotherms at 77 K with no hysteresis. After the encapsulation of HPWs, the N2 adsorption isotherms maintained the same shape as that of UiO-66; however the volume adsorbed decreased, further confirming that HPWs occupied the pores in UiO-66 material. Fig. 3 shows several representative SEM and TEM images of UiO-66 and the fresh 35 wt% HPWs@UiO-66. Note that it was quite difficult to obtain high quality TEM images for the MOFs materials, because the samples were highly sensitive to the electron beam. And in the usual conditions, the structure collapsed under the electron beam exposure [53,54]. Moreover, it was also difficult to produce a regular crystalline morphology of UiO-66 [55]. According

c Table 1 Physico-chemical parameters of various samples.

b a 5

10

15

20

25

30 35 40 2Theta (degree)

45

50

55

60

Fig. 1. XRD patterns of various samples: (a) UiO-66, generated from the original CIF-file [49]; (b) pure UiO-66; (c) 25 wt%; (d) 30 wt%; (e) 35 wt%; (f) 40 wt% HPWs@UiO-66; (g) 35 wt% after the third reaction cycle.

Samples

BET surface areaa SBET (m2g1)

Pore volume VP(cm3g1)

HPWsb (wt%)

UiO-66 25 wt% 30 wt% 35 wt% 40 wt%

874 806 732 603 480

0.43 0.43 0.42 0.41 0.38

0 24.3 29.6 34.7 39.4

a b

HPWs@UiO-66 HPWs@UiO-66 HPWs@UiO-66 HPWs@UiO-66

BET surface area calculated per grams of sample. Measured by ICP.

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400 350

b

c a

250

d

200

e

3

-1

V (cm (STP) g )

300

150 100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/PO) Fig. 2. Nitrogen sorption isotherms for (a) pure UiO-66; (b) 25% wt%; (c) 30 wt%; (d) 35 wt%; (e) 40 wt% HPWs@UiO-66.

to Fig. 3(a) and (d), both of the synthesized samples were composed of agglomerated small particles, with typical irregular inter-grown microcrystalline poly-octahedra morphologies as shown in the low-magnification TEM images (Fig. 3(b) and (e)). Fig. 3(c) and (f) is the high-resolution TEM images of the two samples. As shown in Fig. 3(c), 10 of lattice planes are 2.32 nm for UiO-66, so the average adjacent lattice planes are 0.232 nm; whereas 10 of lattice planes are 2.75 nm for the 35 wt% HPWs@UiO-66 sample (Fig. 3(f)), and the average adjacent lattice planes are 0.275 nm. It implies that the adjacent lattice planes of UiO-66 are lager due to the introduction of HPWs [56]. Moreover, the SEM and TEM images of 35 wt% HPWs@UiO-66 after the third reaction cycle show the same morphologies as those of UiO-66 and the fresh 35 wt% HPWs@UiO-66 samples (Fig. 1(S)). The comparison of the SEM and TEM images

of the two samples shown in Fig. 3 and Fig. 1(S) do not exhibit a significant change in the morphology, indicating that UiO-66 has a highly stable structure, and HPWs encapsulated in UiO-66 do not alter the structure and morphology of UiO-66. Similar to the result observed in XRD, no HPWs particles are observed on the surface of 35 wt% HPWs@UiO-66 sample, demonstrating that HPWs are well dispersed in the microporous cages of UiO-66 materials. Fig. 4 shows the FT-IR spectra of different materials in the skeletal mode region. It has been reported that HPWs with the Keggin structures present several strong and typical FT-IR bands at ~1084 cm1 (stretching frequency of PeO in the central PO4 tetrahedron), 985 cm1 (terminal bands for W]O in the exterior WO6 octahedron), 889 and 808 cm1 (bands for the WeObeW and WeOceW bridges, respectively) [57,58]. The spectra of HPWs@UiO66 samples not only contain typical infrared bands corresponding to UiO-66, but also contain typical infrared bands belonging to HPW with the Keggin structures, revealing that HPW structure remains intact even when encapsulated in the pores of UiO-66. However, the four characteristic bands of HPWs shifted to the higher wavenumbers, probably because of the confining effect of pores of UiO-66 matrix for the guest HPW molecules. The same phenomenon was also found for MIL-101 material reported by Zhang et al. [37]. For 35 wt% HPWs@UiO-66 sample, the FT-IR spectra remain unchanged after the third catalytic cycle, indicating the HPWs@UiO-66 material has a highly stable structure in the selective oxidation reaction system, which is in the agreement with the XRD result. CO adsorption is followed by in situ infrared spectroscopy to investigate the nature of acid sites on the surface of UiO-66 and HPWs encapsulated UiO-66 materials (35 wt% HPWs@UiO-66). Fig. 5 shows the FT-IR spectra of the samples recorded after outgassing at 473 K for 2 h, after adsorption of CO at room temperature. In both the samples, a broad massif band with a shoulder at 2135 cm1 is present at 2115 cm1 (Fig. 5(A)), which corresponds to CO physisorbed species [52,59]. In parallel at higher frequency, a quite intense and broad band appear at 2170 cm1 for UiO-66, which maybe belong to the heterogeneity of the Lewis acid sites in UiO-66 [59]. After HPWs are incorporated into UiO-66, a rather

Fig. 3. SEM/TEM images of various samples. SEM images of (a) pure UiO-66; (d) 35 wt% HPWs@UiO-66; Low-magnification TEM images of (b) pure UiO-66; (e) 35 wt% HPWs@UiO-66; High-resolution TEM images of (c) pure UiO-66; (f) 35 wt% HPWs@UiO-66.

X.-L. Yang et al. / Microporous and Mesoporous Materials 211 (2015) 73e81

903

1090

1720

defects in encapsulated polyanion forming new Lewis acid sites or (ii) Zr4þ sites with different acidity because of the presence of HPWs [52]. This is in agreement with what has been observed on Cr-MIL-101 encapsulated HPWs, an active nanomaterial for catalyzing the alcoholysis of styrene oxide [52]. The CO probing study reveals that an additional Lewis acid site is noted for the 35 wt% HPWs@UiO-66 sample, which is essential to catalyze the selective oxidation of CPE to GA. The thermal stability of UiO-66 and 35 wt% HPWs@UiO-66 was measured by thermogravimetric method, as shown in Fig. 6. The TG curve of the UiO-66 sample shows two weight loss steps between 323 and 823 K. The first weight loss of ~20% between 323 and 623 K is attributed to the release of guest H2O and DMF molecules. On further heating, a weight loss of ~38% between 773 and 823 K may be ascribed to the release of the organic ligand, which leads to the framework structure decomposition. After the decomposition, ~42% of the starting weight remained, corresponding to the formation of 6ZrO2 obtained from the formula Zr6O4(OH)4(CO2)12 as reported by Lillerud et al. [16]. The analysis of the TG curve of the UiO-66 sample strongly indicated that its structure was high stable, and no further weight loss occurred at <823 K. The TG curve of the 35 wt% HPWs@UiO-66 sample show the similar shape to that of UiO-66. The first weight loss of ~20% is due to the loss of guest H2O and DMF molecules. The second weight loss of 32% maybe corresponds to the release of the organic ligand and the decomposition of the Keggin structure of HPWs into the simple oxides [60,61], therefore ~48% of the starting weight is remained because of the incorporation of HPWs in UiO-66 matrix.

f

Intensity (a.u.)

e d c b a 820 600

800

990 1000

1200

1400

1600

1800

2000

-1

Wavenumber (cm ) Fig. 4. FT-IR spectra of various samples: (a) pure UiO-66; (b) 25 wt%; (c) 30 wt%; (d) 35 wt%; (e) 40 wt% HPWs@UiO-66; (f) 35 wt% HPWs@UiO-66 after the third reaction cycle.

broad band centered at 2170 cm1 is present. In order to find out the difference between the two samples, the curve-fitting procedure is adopted to analysis the broad band centered at 2170 cm1 according to Lewis sites in MIL-100 material reported by Daturi et al. [59]. Peak-fitting results of the FT-IR spectra corresponding to the two samples are summarized in Fig. 5(B) and (C). For UiO-66 sample, two main n(CO) bands are obtained (2170 and 2190 cm1, Fig. 5(C)), which are assigned to CO coordinated on Lewis acid sites, that is, coordinatively Zr sites. However, for the 35 wt% HPWs@UiO66 sample, besides the two 2170 and 2190 cm1 bands in common with UiO-66 sample, it shows a third band centered at 2204 cm1 (Fig. 5(B)). The assignment is not straightforward, but it could be hypothesized the presence of other Lewis acid sites, different from the Zr4þ environment present in UiO-66 sample. The new Lewis acid sites are possibly brought out by two new species: (i) some

(A)

3.2. Catalytic activity tests for the selective oxidation of CPE to GA The results of the selective oxidation of CPE to GA over several HPWs@UiO-66 catalysts are listed in Table 2. For the purpose of comparison, the catalysts used in these experiments contain the same amount of HPWs. As shown in Table 2, the pristine UiO-66 shows little activity for the selective oxidation of CPE to GA, whereas those with well-dispersed HPWs on the UiO-66 support show substantial activity and selectivity to the cleavage reaction. This result indicates that the HPWs incorporated in the 3D framework of the UiO-66 material act as the active centers for the selective oxidation of CPE. Moreover, GA yield is strongly

2170

(B)

2170

77

(C)

2115

Intensity (a.u.)

2135

b a 2204 2250

2200

2150

2100

2250

2200

2190 2150

2100

2250

2200

2150

2100

-1

Wavenumber (cm ) Fig. 5. (A), FT-IR spectra of (a) UiO-66 and (b) 35 wt% HPWs@UiO-66 recorded after outgassing at 473 K for 2 h, after adsorption of CO at room temperature; (B) and (C) peak-fitting FT-IR spectra of 35 wt% HPWs@UiO-66 and UiO-66, respectively.

X.-L. Yang et al. / Microporous and Mesoporous Materials 211 (2015) 73e81

a b

80

Conversion (%)

Weight loss (%)

100

60

100

100

80

80

60

60

40

40

20

20

0

0

Selectivity (%)

78

40

20 200

0

300

400

500

600

700

800

900

1000

Temperature (K) Fig. 6. TGA curves of (a) pure UiO-66 and (b) 35 wt% HPWs@UiO-66.

dependent on the content of HPWs in the HPWs@UiO-66 catalysts; such a result reveals that the optimum catalyst in this reaction is that with the HPWs mass content of 35%. The conversion of CPE reached 83.6%, and the GA yield reached 66.7% using the optimum catalyst. However, if the HPWs content was more than 35 wt%, a much lower yield (49.3%) of GA was obtained. The results are probably attributed to that too much HPWs inevitably led to significant decrease in the SBET of the material (Table 1), then hindering the mass transfer of the reactants to the active centers. The significant differences in the catalytic performance among the HPWs@UiO-66 catalysts indicate that the content of HPWs affects their dispersion and the physical texture of the UiO-66 MOF in the HPWs@UiO-66 catalysts, thus leading to different catalytic performances, and the presence of the highly dispersed HPWs in the MOF matrixes is necessary for the selective oxidation of CPE to GA using aqueous H2O2 as the oxidant. To optimize the product yield and selectivity, the influences of different reaction parameters (i.e., reaction time, reaction temperature, oxidant amount, solvent amount, and catalyst amount) on the performance of the 35 wt% HPWs@UiO-66 catalyst toward the selective oxidation of CPE were investigated. Fig. 7 shows the effect of reaction time on the performance of 35 wt% HPWs@UiO-66 catalyst. With increasing reaction time from 2 to 24 h, both the conversion of CPE and the selectivity of GA increased progressively and reached almost saturation after 24 h. With a further increase in the reaction time, the conversion of CPE slightly increased, whereas

Table 2 Catalytic performance in the selective oxidation of CPE over various samples.a Sample

UiO-66 25% wt% 30% wt% 35% wt% 40% wt%

HPWs@UiO-66 HPWs@UiO-66 HPWs@UiO-66 HPWs@UiO-66

Conversion of CPE (%)

GA yield (%)

Selectivity (%) GA

CPLE

CPDL

Othersb

21.6 67.3 78.4 83.6 74.5

3.9 40.4 57.3 66.7 49.3

18.1 60.0 73.1 79.8 66.2

13.7 11.0 6.6 2.1 8.9

12.1 8.1 2.3 2.1 4.3

56.1 20.9 18.0 16.0 20.6

a Reaction condition: The molar ratio of CPE:H2O2:HPWs ¼ 1000:2000:3, the volume ratio of t-BuOH/CPE ¼ 10, reaction time 24 h, reaction temperature 308 K; CPE, cyclopentene; GA, glutaraldehyde; CPLE, 2-t-butyloxy-1-cyclopentanol; CPDL, cyclpentan-1,2-diol. b Others, including cyclopentene oxide, and cyclopentenone.

5

10

15

20

25

30

35

Time (h) Fig. 7. Dependence of conversion of CPE and selectivity to GA, CPO, and others on the reaction time. - ¼ CPE, C ¼ CPO, : ¼ GA, ; ¼ others including of CPLE, CPDL, and small amounts of cyclopentenone.

the selectivity toward GA slightly decreased. No significant increase in the conversion and selectivity was observed when the reaction time was increased to 30 h. Hence, the optimum reaction time was 24 h. According to the GCeMS analysis, along with the main product GA, the main byproducts were cyclopentene-epoxide (CPO), cyclpentan-1,2-diol (CPDL), 2-t-butyloxy-1-cyclopentanol (CPLE), and traces of cyclopentenone. As shown in Fig. 7, the CPO content increases rapidly at the beginning and then decreases progressively with increasing GA content. Thus, it can be proposed that CPO is possibly a main intermediate from which GA is produced via its further oxidative cleavage. Therefore, the mechanism of the CPE oxidation using HPWs@UiO-66 as the catalyst may be similar to that using WO3/SiO2 as the catalyst reported previously by our group [62]. The molar ratio of H2O2 to CPE has a substantial effect on the selective oxidative reaction results; therefore, the effect of H2O2 amount on the catalytic performance of 35 wt% HPWs@UiO-66 catalyst was investigated under the same amount of CPE (Table 3). By varying the ratio of H2O2 to CPE from 1:1 to 2.5:1, the conversion of CPE increased from 36.8% to 94.8%, and no H2O2 was detected in the reaction mixture when the ratio was less than 2.5:1. However, the selectivity of GA slightly decreased when the ratio of H2O2 to CPE increased to 3:1, indicating that the optimum ratio was equal to 2.5:1. The optimum ratio of H2O2 to CPE was equal to 2:1 obtained from the theoretical stoichiometric value for the oxidative cleavage of CPE to GA. Low GA yield would inevitably result from the incomplete conversion of CPE when the ratio of H2O2 to CPE was less than 2:1. However, the GA yield could not reach the maximum value when the ratio was fixed at 2:1, probably because of slight decomposition of H2O2. With further increase in the H2O2 to CPE ratio, the yield of GA increased and reached the highest value of 78.3% at the ratio of 2.5:1. The effect of the reaction temperature on the catalytic performance of the 35 wt% HPWs@UiO-66 catalyst in the range 298e313 K is listed in Table 4. The catalytic performance strongly depends on the reaction temperature. At 298 K, the CPE conversion was only 86.4% after 24 h; however, at 308 K, 94.8% CPE conversion and 78.3% GA yield were obtained. A further increase in the reaction temperature to 318 K slightly increased the CPE conversion; however, the byproduct yield also increased. This is because the selective oxidation of CPE is an exothermic reaction, and thus a high conversion is expected if the reaction is performed at a high

X.-L. Yang et al. / Microporous and Mesoporous Materials 211 (2015) 73e81 Table 3 Influence of the H2O2 amount on the catalytic performance of 35 wt% [email protected] The molar radio of H2O2 to CPE

Conversion of CPE (%)

GA yield (%)

Selectivity (%) GA

CPLE

CPDL

Others

1:1 1.5:1 2:1 2.5:1 3:1

36.8 64.6 83.6 94.8 95.6

7.5 36.5 66.7 78.3 72.3

20.4 56.5 79.8 82.6 75.6

10.8 9.8 4.1 3.1 5.8

8.8 3.4 2.1 3.1 5.8

60.0 30.3 14.0 11.2 12.8

79

Table 5 Effect of catalyst amount on the GA preparation over 35 wt% [email protected] HPWs/CPE (mol.%)

TOFb (h1)

Conversion of CPE (%)

GA yield (%)

Selectivity (%) GA

CPLE

CPDL

Others

0.1 0.2 0.3 0.4 0.5

32.8 17.6 13.2 9.9 7.9

78.6 84.5 94.8 94.7 95.2

54.1 65.4 78.3 78.7 78.9

68.8 77.4 82.6 83.1 82.9

6.8 3.3 3.5 3.6 3.2

4.6 3.7 3.1 2.9 2.8

19.8 15.6 10.8 10.4 11.1

Reaction condition: The molar ratio of CPE:HPWs ¼ 1000: 3, the volume ratio of t-BuOH/CPE ¼ 10, reaction time 24 h, reaction temperature 308 K.

a Reaction condition: The molar ratio of CPE:H2O2 ¼ 1000:2500, the volume ratio of t-BuOH/CPE ¼ 10, reaction time 24 h, reaction temperature 308 K. b TOF ¼ moles of CPE converted per moles of HPWs in the catalyst per hour.

temperature, which would be favorable for the equilibrium conversion to the products. Although a high reaction temperature could increase the conversion, it may simultaneously increase the formation of the byproducts. All these results indicate that an optimum temperature (i.e., 308 K) is necessary for the selective oxidation of CPE to GA, taking into account the balance between the reaction rate and equilibrium. The effect of the varying amounts of 35 wt% HPWs@UiO-66 catalyst was also investigated (Table 5). The CPE conversion increased with increasing amount of the catalyst in the oxidative reaction in a fixed reaction period and the same amount of CPE, indicating that the reaction rate increased with increasing amount of the catalysts. However, the TOF values decreased significantly with increasing catalyst amount. Moreover, the yield of GA first increased significantly when the molar ratio of HPWs/CPE increased from 0.1 to 0.3%, and then slightly increased with further increase in the molar ratio of HPWs/CPE from 0.3 to 0.5%, which may be ascribed to the increased difficulty of mass transfer during the reaction. This result indicates that the optimum molar ratio of HPWs/CPE for the selective oxidation of CPE to GA is 0.3%. Solvent is known to play a very important role in determining the catalytic activity and selectivity in many catalytic oxidations with H2O2 as the oxidant [63]; therefore, the effect of the relative amount of t-BuOH on the catalytic performance of 35 wt% HPWs@UiO-66 catalyst during the CPE oxidation to GA was also investigated. As listed in Table 6, an appropriate amount of t-BuOH was necessary for this reaction. When using small amounts of tBuOH, H2O2 apparently decomposed during the reaction, releasing heat and leading to volatilization of the reagents. More t-BuOH leads to low reagent concentrations and thus decreased the reaction rate, consequently decreased the CPE conversion and GA yield. Based on the experimental data, the optimal volume ratio of t-BuOH to CPE is determined to be 10:1. Since Furukawa et al. reported an interesting one-step route for the synthesis of GA by the selective oxidation of CPE in a nonaqueous H2O2 system in 1987 [44], great efforts have been dedicated to investigate the non-aqueous system in which various solvents were used while the catalysts employed were mainly

homogeneous system based on molybdenum or tungsten heteropoly acids. Unfortunately, the GA yield did not exceed 60% over all mentioned homogeneous catalytic systems (Table 1S). Additionally, our group has made great improvements to this interesting onestep route for the synthesis of GA. A high GA yield of ca. 80% was achieved by aqueous H2O2 oxidation of CPE using homogeneous tungstic acid as an efficient catalyst [45]; however, the homogeneous catalytic system restricts its further application for the difficult separation and reuse of the homogeneous catalyst. Afterwards our group has designed different type of W-containing heterogeneous catalysts in succession and their catalytic performances are delineated in Table 1S. As shown, the homogeneous HPW catalyst showed little activity for this oxidation reaction in the non-aqueous system and only 5.4% of GA yield was obtained (Entry 2); however, the GA yield could reach 70.2% in the aqueous H2O2/t-BuOH system (Entry 1). For the non-aqueous system, the best homogeneous and heterogeneous catalysts in the selective oxidation of CPE to GA were H3PMo10W2O40 (Entry 4) and 20 wt% WO3-SBA-15 (Entry 14), respectively. Meanwhile, for the aqueous H2O2/t-BuOH system, the best heterogeneous catalyst were the 20 wt% WO3-MCF on which the GA yield reached 83.5%, much higher than those on the TiO2 spheroid or amorphous SiO2 and siliceous mesoporous molecular sieves (Entry 7e12). Compared with the homogenenous HPW and the amorphous siliceous mesoporous molecular sieves, the 35 wt% crystalline HPWs@UiO-66 catalyst also exhibited excellent performance for this oxidation reaction and the GA yield could reach 78.3%, a litter lower than that on the 20 wt% WO3-MCF catalyst. Therefore, it could be concluded that the metal organic framework UiO-66 was suitable for encapsulating HPWs, and then overcome those drawbacks of the homogeneous system. The essential properties of solid catalysts are the stability of the active component with respect to leaching and other transformations, and the stability of the porous structure. To investigate the stability and duration of the active HPW components in the 35 wt% HPWs@UiO-66 catalyst, a three-consecutive-cycle experiment in the selective oxidation of CPE was performed, and the HPWs remaining in the catalyst were determined by ICP analyses

a

Table 4 Influence of the reaction temperature on the catalytic performance of 35 wt% [email protected] Reaction temperature(K)

Conversion of CPE (%)

GA yield (%)

Selectivity (%) GA

CPLE

CPDL

Others

298 303 308 313

86.4 90.3 94.8 95.1

60.8 69.8 78.3 74.6

70.4 77.3 82.6 78.4

5.8 5.1 3.5 6.6

4.6 4.6 3.1 4.1

19.2 13.0 10.8 10.9

a

Reaction condition: The molar ratio of CPE:H2O2:HPWs ¼ 1000:2500:3, the volume ratio of t-BuOH/CPE ¼ 10, reaction time 24 h.

Table 6 Effect of t-BuOH amount on the GA preparation over 35 wt% [email protected] t-BuOH/CPEb

Conversion of CPE (%)

GA yield (%)

Selectivity (%) GA

CPLE

CPDL

Others

6 8 10 12 15

73.8 86.7 94.8 89.8 80.3

51.2 68.3 78.3 73.1 61.1

69.4 78.8 82.6 81.4 76.1

6.7 4.7 3.5 3.4 5.3

6.2 5.3 3.1 3.8 4.6

17.7 11.2 10.8 11.4 14.0

a Reaction condition: The molar ratio of CPE:H2O2:HPWs ¼ 1000:2500:3, reaction time 24 h, reaction temperature 308 K. b The volume ratio of t-BuOH/CPE.

80

X.-L. Yang et al. / Microporous and Mesoporous Materials 211 (2015) 73e81

may account for the high catalytic properties of HPWs@UiO-66. The developed HPWs@UiO-66 may be highly valuable in exploring the full potential of UiO-66 as catalysts.

Table 7 Reusability of 35 wt% [email protected] Recycle entry

Conversion of CPE (%)

GA yield (%)

Selectivity (%) GA

CPLE

CPDL

Others

1 2 3

94.8 92.6 90.7

78.3 75.3 73.1

82.6 81.3 80.6

3.5 3.8 4.3

3.1 4.0 3.4

10.8 10.9 11.7

a

Reaction condition: The molar ratio of CPE:H2O2:HPWs ¼ 1000:2500:3, the volume ratio of t-BuOH/CPE ¼ 10, reaction time 24 h, reaction temperature 308 K.

after three consecutive reaction cycles (Table 7). Approximately 3 wt% HPWs from the catalyst, which was ca. 8.6 wt% accounted for the total amounts of supported HPWs, leached after three consecutive cycles. During the catalyst preparation, a small amount of HPWs may be remained on the external surface and could not enter the microporous cages of UiO-66; therefore the HPWs on the external surface were easily leached into the reaction solution. However, such a loss of HPWs did not result in the appreciable decrease of the CPE conversion. The CPE conversion dropped from 94.8 to 90.7% under the same experimental conditions after three consecutive cycles. We believed that the slightly decrease in the observed catalytic activity was possibly because of the leaching of small amounts of HPW species and the adsorption of oxidized products on the catalyst. Thereby, 35 wt% HPWs@UiO-66 catalyst shows high stability and catalytic activity for the selective oxidation of CPE to GA. Moreover, another experiment was performed to test the heterogeneity of the HPWs@UiO-66 catalyst in the reaction. After the reaction over the 35 wt% HPWs@UiO-66 catalyst was carried out for 6 h, the catalyst was removed by simple filtration, and the reaction solution was stirred for another 24 h. No detectable increase in the GA yield and CPE conversion were observed after the catalysts were removed, indicating that the selective oxidation process is truly heterogeneous and occurs on the catalyst surface rather than in the solution (Fig. 2(S)). Even if very small amounts of HPW species leached from the catalyst during the selective oxidation process, the observed catalytic activity was not because of these species. From the results of catalytic activity tests and characterizations of the catalysts, it is conceivable that the rigid 3D cubic structure of UiO-66 with a high porosity and high surface area was more suitable to encapsulate HPWs, then confine HPWs into its microporous cages and prevent active HPWs from leaching. The occluded HPWs in the microporous cages of UiO-66 generate additional Lewis acid sites inferred to be responsible for the selective oxidation of CPE to GA, which is demonstrated by in situ FT-IR using CO as a probe molecule. Moreover, it could be concluded that the incorporation of HPWs into the microporous cages of UiO-66 can provide catalysts with a large concentration of accessible, highly dispersed, and structurally well confined active sites for the selective cleavage of CPE. Hence, the HPWs@UiO-66 catalyst shows high catalytic performance for the selective oxidation of CPE to GA.

4. Conclusion In this study, a novel MOF UiO-66 encapsulated HPWs material (HPWs@UiO-66) was successfully prepared for the first time by means of a simple direct hydrothermal synthesis method. The resulting material exhibited high activity and excellent reusability in the selective oxidation of CPE to GA, behaving as a truly heterogenous oxidative catalyst without dramatic HPWs leaching during its reuse; the product shows typical structure of UiO-66. The unique characteristics of UiO-66 and the well-dispersed HPWs confined in the microporous cages of UiO-66 matrix with proper HPW contents

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 20903035, 21373054), the Plan of Nature Science Fundamental Research in Henan University of Technology (2013JCYJ09), and the Fundamental Research Funds for the Henan Provincial Colleges and Universities (2014YWQQ13). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2015.02.035. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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