Improving Hydrothermal Stability and Catalytic Activity of Metal

Article pubs.acs.org/JPCC

Improving Hydrothermal Stability and Catalytic Activity of Metal− Organic Frameworks by Graphite Oxide Incorporation Dan-Dan Zu, Lei Lu, Xiao-Qin Liu,* Dong-Yuan Zhang, and Lin-Bing Sun* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China ABSTRACT: This paper aims to improve the hydrothermal stability and catalytic activity of metal−organic frameworks (MOFs), which are believed to play an important role in the practical applications of MOFs in catalysis. Our strategy is to incorporate graphite oxide into a typical MOF, namely HKUST-1 (Cu3(BTC)2, BTC = 1,3,5-benzenetricarboxylic acid; HKUST = Hong Kong University of Science and Technology), and the properties of MOF are successfully tailored by use of this strategy. The obtained MOF/graphite oxide composites show enhanced porosity with high surface areas and some meso/macropores. For the composite containing 8.7 wt % of graphite oxide, the surface area reaches 1257 m2 g−1, which is obviously higher than pure HKUST-1 (841 m2 g−1). The pore structure of composites favors the access of reactant molecules to active sites and accelerates mass transfer in channels. Furthermore, the incorporation of graphite oxide creates a more hydrophobic environment surrounding metallic sites, which prevents the coordination bonds from attacking by water molecules and presents better affinity of active sites to organic reactants. Hence, the hydrothermal stability of MOF as well as the catalytic performance with regard to both activity and reaction rate are greatly improved. For the composite incorporating 8.7 wt % of graphite oxide, the conversion of styrene oxide in the ring-opening reaction can reach 74.1% after reaction for only 20 min, which is much higher than pure HKUST-1 (10.7%). More importantly, the catalytic activity can be well recovered without any loss even after six cycles. The excellent hydrothermal stability, catalytic activity, and reusability make our materials highly promising for use as heterogeneous catalysts in practical applications.

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INTRODUCTION Due to their large surface area, diverse structure, and tunable functionality, metal−organic frameworks (MOFs) have received extensive attention in the past decades.1−5 They are porous crystalline materials composed of inorganic metal ions or clusters and organic ligands which are connected through metal−ligand coordination.6,7 By judicious choice of building blocks, the structure of MOFs can be tailored with almost endless geometrical and chemical variations. Therefore, MOFs show high potential for applications in gas storage8,9 and separation,10,11 catalysis,12−15 as well as sensing.16−19 However, the reports concerning the practical use of MOFs in any processes are quite scarce, if any. One of the biggest problems that hinders the practical applications of MOFs is the poor hydrothermal stability. A case in point is MOF-5, the structure of MOF-5 starts to collapse irreversibly within 10 min when exposed to humid air, even under atmospheric conditions.20 It is known that the hydrophilic property of some MOFs gives rise to strong interaction of water molecules with MOFs.21 The coordination bonds of MOFs are thus easily attacked by even a trace of water, leading to the cleavage of coordination bonds and subsequently the destruction of frameworks. Since the discovery of MOFs, they have been used to try to catalyze a broad range of organic reactions owing to the presence of metal ions or metal-containing clusters with coordinative vacancies in them. Nevertheless, the hydrophilic property of MOFs is unfavorable to the access of hydrophobic © XXXX American Chemical Society

organic substrates to the active sites, which compromises the catalytic activity in some reactions.22,23 Moreover, for the catalytic reactions which can be obstructed by the water poisoning effects originating from humid air or from the water formed during reactions, the catalytic activity can be further influenced by the hydrophilic property of MOFs.24,25 There is no doubt that both hydrothermal stability and catalytic activity are of great significance for practical applications of MOFs, while both are directly correlated to the hydrophilic property of MOFs. It is thus extremely desirable to develop an efficient method to modulate the hydrophilicity/hydrophobicity of MOFs, aiming to improve the hydrothermal stability and catalytic activity. In the present study, we report a facile strategy to modulate the hydrophilicity/hydrophobicity of MOFs by incorporating graphite oxide into MOFs, taking into account that incorporation is an effective way to adjust the properties of MOFs.26−30 Pioneering work has reported the incorporation of graphite oxide into various materials, such as MOFs,31 zeolitic imidazolite frameworks (ZIFs),32 and zeolites.33 The graphite oxide does note influence the preparation and can integrate well with the parent materials. Besides, the addition of graphite oxide reduces the surface area of most composites, but Received: June 25, 2014 Revised: July 30, 2014

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dx.doi.org/10.1021/jp506335x | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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enhances the adsorption capacity on some adsorbates such as hydrogen,34,35 ammonia,36 hydrogen sulfide,37 and nitrogen dioxide.38 However, most reports focus on the adsorption capacity while less attention is given to the catalytic performance induced by incorporation. In our work, the microenvironment of a typical MOF, namely HKUST-1 (Cu3(BTC)2, BTC = 1,3,5-benzenetricarboxylic acid; HKUST = Hong Kong University of Science and Technology), is tailored from hydrophilic to hydrophobic, which results in an obvious improvement of hydrothermal stability with contrast to the parent MOF. Moreover, the surface area is obviously enhanced after incorporating an appropriate amount of graphite oxide. The hydrophobic microenvironment, in combination with the large surface areas, greatly improves the activity and reaction rate in the ring-opening reactions of styrene oxide. More importantly, the catalytic activity can be completely recovered without any loss even after six cycles.

Table 1. Graphite Oxide Content and Textual Parameters of Different Samples sample

graphite oxide content (wt %)

SBET (m2 g−1)

Vp (cm3 g−1)

HKUST-1 MG-1 MG-2 MG-3 MG-4 MG-5 graphite oxide

0 4.0 8.7 17.2 27.2 34.3 100

841 996 1257 1138 929 819 38

0.433 0.527 0.552 0.502 0.477 0.371 0.036

using ASAP 2020 at 77 K. The samples were degassed at 423 K for 4 h prior to analysis. The Brunauer−Emmett−Teller (BET) surface area was calculated by using adsorption data in a relative pressure range from 0.04 to 0.20. The total pore volume was determined from the amount adsorbed at a relative pressure of about 0.99. Fourier transform infrared (IR) spectra of the samples diluted with KBr were carried out on a Nicolet Nexus 470 spectrometer with KBr wafer. Scanning electronic microscopy (SEM) was performed on a Hitachi S4800 electron microscope operating at 20 kV. Thermogravimetric (TG) curves and their derivatives (DTG) were obtained with use of a thermobalance (STA-499C, NETZSCH). The sample was heated from room temperature to 1000 °C with the heating rate 10 deg min−1 under a flow of nitrogen (30 mL min−1). The amounts of copper in the composites were measured by ICPAES. A PerkinElmer sequential ICP spectrometer (Optima 2000 DV) was used for measurements. Samples were dissolved in hydrofluoric acid before measurement. The contents of HKUST-1 and graphite oxide can be calculated from the amounts of copper in the composites. The hydrothermal stability of samples was evaluated as follows. The samples (0.02 g) were put in an open vial and kept inside the autoclave containing 2 mL of deionized water without direct contact between the solid and water. The autoclave was then heated to 363 K and kept at that temperature for 12 h; the resulting solids were characterized by XRD. The change in position and intensity of diffraction lines was used to assess the hydrothermal stability of samples. The water and benzene vapor adsorption−desorption isotherms were determined by using the intelligent gravimetric analyzer (IGA-100, Hiden) at 298 K. Catalytic Reaction. The ring-opening reactions of styrene oxide with methanol were performed in sealed glassware. The solid catalysts were vacuumed beforehand to remove solvents. The whole reaction process was conducted under a nitrogen atmosphere. For a typical reaction, styrene oxide (2.5 mmol) and a solid catalyst (0.1 g) were stirred in methanol (15 mL) at 60 °C. The reaction mixtures were periodically withdrawn at the intervals of 20, 40, 80, and 160 min throughout the reaction. The obtained upper liquid was analyzed by using an Agilent Technologies 7890A gas chromatograph equipped with a flame ionization detector (FID) and an HP-5 capillary column.

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EXPERIMENTAL SECTION Chemicals. Styrene oxide (>98%) was purchased from Aladdin Chemical Reagent Co., Ltd. Graphite (>99.5%), Cu(NO3)2·3H2O (>99.5%), KMnO4 (>99.5%), 1,3,5-benzenetricarboxylic acid (H3BTC, >98%), H2O2 (30%), hydrofluoric acid (40%), and sulfuric acid (95−98%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Methane (>99.5%), ethanol (>99.7%), N,N-dimethylformamide (DMF) (>99.5%), and dichloromethane (>99.5%) were purchased from Wuxi City Yasheng Chemical Co., Ltd. All chemicals were used directly without further treatment. Deionized water was used for all of the experiments. Materials Synthesis. Graphite oxide was synthesized by oxidation of graphite by using the reported method.39 Briefly, graphite powder (10.0 g) was stirred with concentrated sulfuric acid (230.0 mL) in an ice bath. Then potassium permanganate (30.0 g) was slowly added to the suspension. The mixture was stirred at 35 °C for 30 min. Deionized water (1.4 L) was added to dilute the suspension before adding hydrogen peroxide (100.0 mL). The mixture was filtrated, washed with deionized water, and dried. The MOF/graphite oxide composites were prepared as follows. Cu(NO3)·3H2O (1.0 g) and H3BTC (0.5 g) were dissolved in the mixed solvent consisting of DMF (8.5 mL), ethanol (8.5 mL), and deionized water (8.5 mL). A prescribed amount of graphite oxide was then dispersed in the above solution. The mixture was heated at 85 °C for 21 h with stirring. The obtained solids were filtered, washed by DMF and methanol, and exchanged with dichloromethane. Dichloromethane was changed twice in 3 days. For MOF/graphite oxide composites with different compositions, graphite oxide with different amounts was added in the synthetic process, leading to the formation of products denoted as MG-1, MG-2, MG-3, MG-4, and MG-5, respectively. The contents of graphite oxide in resultant composites vary from 4.0 to 34.3 wt % as shown in Table 1. The results of graphite oxide contents were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) as will be described later. For comparison, the pure MOF, HKUST-1, was synthesized subjected to a similar procedure as for the composites except that no graphite oxide was added. Characterization. X-ray diffraction (XRD) patterns of the materials were recorded with a Bruker D8 Advance diffractometer with Cu Kα radiation at 40 kV and 40 mA. The N2 adsorption−desorption isotherms were measured by

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RESULTS AND DISCUSSION Characterization of MOF/Graphite Oxide Composites. Figure 1 shows the XRD patterns of graphite oxide, HKUST-1, and HKUST-1/graphite oxide composites. Graphite oxide exhibits a single diffraction peak at about 2θ of 11°. It is known that graphite oxide has a multilayer system with the B



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HKUST-1 matches well with that reported in the literature.42 Various bands are observed in the 600−1300 cm−1 region, and they are attributed to the out-of-plane vibrations of BTC. The bands at 1645 and 1590 cm−1 as well as the bands at 1450 and 1370 cm−1 originate from the asymmetric and symmetric stretching vibrations of the carboxylate groups in BTC, respectively.43 The IR spectra of composites are also similar to that of parent HKUST-1. The characteristic bands become weak progressively with the reduction of MOF contents in the composites, which are in agreement with the results of XRD. Figure 3 gives N2 adsorption−desorption isotherms of different samples. Only a tiny amount of N2 is adsorbed on

Figure 1. XRD patterns of graphite oxide, HKUST-1, and MOF/ graphite oxide composites.

interlayer distance varying from 6 to 12 Å according to the level of hydration. The interlayer distance can be reflected by the position of the XRD diffraction peak. The diffraction peak at 2θ of 11° thus indicates that the interlayer distance of the present graphite oxide is 8.1 Å. For HKUST-1, the diffraction pattern is in accordance with that reported in the literature.40 After incorporating graphite oxide, the XRD patterns are rather similar to that of HKUST-1. This indicates that the presence of graphite oxide does not hinder the formation of the MOF structure. With the increase of graphite oxide content, the intensity of diffraction peaks decreases gradually. This can be ascribed to the progressively decreased amount of HKUST-1 in the composites. It should be noted that the diffraction peak for graphite oxide disappears in the composites. The relatively low intensity of graphite oxide as compared with HKUST-1 should be responsible for this phenomenon. Also, the reason could be the exfoliation of graphite oxide in polar solvents during synthesis and subsequent dispersion of layers in resultant composites.41 Further information on the parent materials and MOF/ graphite oxide composites is provided by IR spectra presented in Figure 2. For graphite oxide, the vibration of CO is

Figure 3. Nitrogen adsorption−desorption isotherms at 77 K of HKUST-1 and MOF/graphite oxide composites.

graphite oxide. The isotherm for HKUST-1 is of type I, which indicates its microporous character. This is expected since the HKUST-1 networks are made of 9 Å square channels. Although the shape of isotherms for composites is quite similar to that of HKUST-1, a small hysteresis loop at high relative pressures can be seen. This means that the incorporation of graphite oxide leads to the formation of some meso/macropores in the composites. Table 1 presents the textual parameters of different samples calculated from isotherms. Almost all of the composites show higher surface areas and pore volumes except for MG-5, the one with the highest graphite oxide content. The enhanced surface area may be caused by the new pores created at the interface between HKUST-1 units and graphene layers, which can be attributed to the coordination of the oxygencontaining groups of graphite oxide with the metallic centers of HKUST-1.44 With increasing content of graphite oxide in the composites, the surface areas and pore volumes first increase and then decrease. The composite MG-2 with a graphite oxide content of 8.7 wt % exhibits a surface area as high as 1257 m2 g−1 and a pore volume of 0.552 cm3 g−1, which is evidently larger than that of pure HKUST-1 with a surface area of 841 m2 g−1 and a pore volume of 0.433 cm3 g−1. With a further increase in the graphite oxide content, the porosity of composites declines. This can be explained by the distortion in structure when elevated amounts of graphite oxide present in the composites. Another possible reason is the number of groups on graphite oxide surpasses the accessible sites on MOF, which causes the agglomeration of distorted graphene layers from graphite oxide. Figure 4a,b presents the TG and DTG data of graphite oxide, HKUST-1, and HKUST-1/graphite oxide composites. In the case of graphite oxide, the initial weight loss centered at about 100 °C corresponds to the removal of physically adsorbed water. The decomposition of epoxy groups occurs at around 200 °C, and the broad hump between 250 and 400 °C is related

Figure 2. IR spectra of graphite oxide, HKUST-1, and MOF/graphite oxide composites.

observed at 1060 cm−1, whereas the band at 1630 cm−1 can be attributed to the OH bond in water and/or oxygen surface groups. The vibration of CO from carbonyl and/or carboxyl appears at 1735 cm−1. The band at 990 cm−1 can be assigned to epoxy/peroxide groups, while the band at 1228 cm−1 is related to the SO asymmetric stretching vibration in sulfonic groups and/or vibration of CO in epoxides.34 The spectrum of C



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Figure 5. SEM images of (a) graphite oxide, (b) HKUST-1, and (c, d) MG-2. Scale bars represent 10 μm.

Modulation of Hydrothermal Stability and Hydrophilicity. Hydrothermal stability is an important issue for many industrial applications since water exists at temperatures ranging from ambient to several hundred degrees. To probe the hydrothermal stability, the samples were exposed to water vapor at 363 K for 12 h. The XRD patterns of samples after treatment were used to evaluate the hydrothermal stability. As presented in Figure 6, the main peak of HKUST-1 at 11.6°

Figure 4. (a) TG and (b) DTG curves of graphite oxide, HKUST-1, and MOF/graphite oxide composites. Curves are plotted offset for clarity.

to the decomposition of carboxyl groups and sulfonic groups.45 For HKUST-1, the weight loss centered at around 100 °C can be attributed to the release of free water inside the pores, whereas the release of crystallization water takes place at about 300 °C.42 The complete collapse of MOF structure can be observed at around 350 °C, which is accompanied by the release of CO2 and formation of copper oxide. For the MOF/ graphite oxide composites, some interesting TG and DTG curves can be observed. After incorporating 4.0 wt % of graphite oxide, the weak DTG peak at about 300 °C derived from the release of crystallization water degrades to a shoulder peak. Moreover, such a peak disappears with the further increase of graphite oxide content. This suggests that the presence of graphite oxide provides a hydrophobic environment for some copper centers in the composites.34 It is interesting to notice that the peak related to the decomposition of the epoxy groups centered at about 200 °C in graphite oxide is absent in the composites. This can be associated with the involvement of these groups in linkages with the metallic sites in HKUST-1. Figure 5a−d shows SEM images of graphite oxide, HKUST1, and the HKUST-1/graphite oxide composite. In the case of graphite oxide (Figure 5a), dense agglomerates of stacked graphene sheets can be observed. HKUST-1 exhibits an octahedra-like shape with a few irregularities (Figure 5b). However, the HKUST-1/graphite oxide composites show quite different images from both HKUST-1 and graphite oxide (Figure 5c,d). They are thin platelets stacked together, which may be related to the improved porosity of the composites. According to these results and previous investigations,46 it is assumed that graphene layers are generated from the exfoliation of graphite oxide during synthesis and that the growth of MOF occurs between the graphene layers and their thin agglomerates. As a result, the layer-spacing structure is formed in the resultant MOF/graphite oxide composites.

Figure 6. XRD patterns of HKUST-1 and MOF/graphite oxide composites after hydrothermal treatment.

almost disappeared after treatment; meanwhile, a new peak at 10.1° emerged. This mirrors a significant loss of structure integrity, and collapse and/or transformation of structure takes place. Apparently, pure HKUST-1 is not stable under the hydrothermal conditions investigated. For the composite MG-1 with a graphite oxide content of 4.0 wt %, a small diffraction peak at 10.1° also appeared after treatment. That means, the structure of MG-1 is only partially destroyed due to the incorporation of graphite oxide. It is worth noting that the composites MG-2, MG-3, and MG-4 after treatment exhibit quite similar XRD patterns to the composites before treatment, and no new diffraction peaks are observable. These results clearly show that the incorporation of a proper amount of graphite oxide (8.7−27.2 wt %) is highly efficient in the improvement of hydrothermal stability. Further increasing the graphite oxide content to 34.3 wt %, however, the hydrothermal D



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completely contrary adsorptive property. The adsorption data of water and benzene, in combination with the aforementioned results, demonstrates that the incorporation of graphite oxide creates a more hydrophobic environment and makes the structure more stable upon hydrothermal treatment. Catalytic Performance of Resultant Composites. The resultant composites were applied to catalyze the synthesis of 2methoxy-2-phenylethanol (MPE) from the ring opening of styrene oxide. MPE is a valuable organic solvent and an intermediate for the pharmaceutical and chemical industries.48 Traditionally, the ring opening of styrene oxide is catalyzed by homogeneous acids or bases, leading to the problem of waste production and the difficulty in separation. The substitution of conventional homogeneous catalysts with heterogeneous ones thus attracts increasing attention. Because of the coordinatively unsaturated metal active sites and large surface areas, MOFs are active as heterogeneous catalysts in a collection of reactions.49−51 The obtained composites were used to catalyze the conversion of ring opening of styrene oxide and the results are shown in Figure 8. No styrene oxide was converted at all even

stability declines and partial destruction of structure can be seen for MG-5. The improved hydrothermal stability may be attributed to the modulation of hydrophilicity/hydrophobicity of MOFs. The incorporation of graphite oxide creates a more hydrophobic environment surrounding some copper centers owing to the presence of carbonaceous component,34 which prevents the coordination bonds from attack by water via blocking the adsorption of water molecules, and thus enhances the hydrothermal stability of the MOF/graphite oxide composites. It should be noted that MG-1 and MG-5 show partial structure collapse after hydrothermal treatment. The low content of graphite oxide can explain the structural damage of MG-1 since the amount of graphite oxide (4.0 wt %) is not sufficient to create a hydrophobic environment. As for MG-5, the possible reason for its partial collapse may be that the presence of a rather high amount of graphite oxide could result in much distortion of the structure as well as the agglomeration of distorted graphene layers, which makes the frameworks easily attacked by water molecules.47 To further verify the modulation of hydrophilicity/hydrophobicity, the adsorption of a typical polar solvent (water) and a typical nonpolar one (benzene) on the MOF before and after graphite oxide incorporation was conducted. As shown in Figure 7a, the adsorption amount of water on pure HKUST-1 is

Figure 8. Conversion of the ring opening of styrene oxide catalyzed by HKUST-1, graphite oxide, and the MOF/graphite oxide composites.

at 160 min on graphite oxide owing to the absence of active sites. In the case of HKUST-1, the conversion of styrene oxide is 26.7%, indicating the existence of catalytically active sites. The incorporation of graphite oxide evidently changes the catalytic activity. With increasing content of graphite oxide, the conversion of styrene oxide first increases and then decreases. It is noticeable that under the catalysis of MG-2, the conversion of styrene oxide reaches 79.2%, which is much higher than that over the MOF before incorporation. For the composite MG-5 with a high graphite oxide content of 34.3%, the conversion at 160 min is somewhat lower as compared with pure HKUST-1. In addition to the difference in conversion of styrene oxide, the reaction rate is also quite different for the MOFs before and after graphite oxide incorporation. Under the catalysis of HKUST-1, the conversion of styrene oxide is 10.7% at 20 min, which accounts for 40% of the conversion at 160 min (26.7%). Nevertheless, the MOF/graphite oxide composite MG-2 can convert 74.1% of styrene oxide at 20 min, which is 94% of the conversion at 160 min (79.2%). In other words, when MOF/ graphite oxide composites are employed as catalysts, most reactions take place at the initial stage (within 20 min). Nonetheless, the conversion of styrene oxide over HKUST-1 increases gradually with reaction time in the whole process. On the basis of these results, it is clear that the reaction rate is enhanced by the incorporation of graphite oxide. Two factors are considered to be responsible for the enhanced catalytic performance due to graphite oxide

Figure 7. Water and benzene vapor adsorption−desorption isotherms of samples (a) HKUST-1 and (b) MG-2.

10.7 mmol g−1, which is 3.6 times higher than the adsorption amount of benzene (3.0 mmol g−1, Figure 7b). Interestingly, the adsorption amount of water decreases sharply to 1.2 mmol g−1 on the HKUST-1/graphite oxide composite, whereas the adsorption amount of benzene increases to 5.5 mmol g−1. In other words, the composite prefers to adsorb the nonpolar solvent benzene rather than the polar solvent water, and the adsorption amount of benzene is 4.6 times higher than that of water. The incorporation of graphite oxide into MOF leads to a E



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incorporation. The first factor is the favorable pore structure caused by the incorporation of graphite oxide. During synthesis, the exfoliation of graphite oxide occurs and leads to the formation of graphene layers. As demonstrated by the IR technique, a collection of oxygen-containing groups (such as carbonyl and sulfonic groups) exist in graphite oxide. The interaction between graphite oxide and HKUST-1 can be attributed to the coordination of the oxygen-containing groups of graphite oxide with the metallic centers of HKUST-1. The nucleation and growth of HKUST-1 thus proceed between the graphene layers or their thin agglomerates. The layer-spacing structure is thus formed in the obtained MOF/graphite oxide composites. In contrast to pure MOF, some new pores are produced at the interface between MOF units and graphene layers. As a result, the surface areas of composites with an appropriate amount of graphite oxide are greatly enhanced, and some meso/macropores are generated. The pore structure of composites makes the active sites highly accessible to the reactants and accelerates mass transfer of reactants/products. Hence, the catalytic performance can be improved with regard to both activity and reaction rate. The second factor is the modulation of hydrophilicity/ hydrophobicity. The incorporation of graphite oxide creates a more hydrophobic environment surrounding some copper centers in MOF as demonstrated by various techniques above. In addition to augmenting hydrothermal stability, the hydrophobic environment presents better affinity of active sites to organic reactant (styrene oxide). The access of reactant molecules to active sites is thus enhanced, which should also be responsible for the improvement of catalytic performance. On the basis of the discussion above, it is thus safe to say that the enhanced catalytic performance can be attributed to the improvement of porosity and the creation of a hydrophobic environment in MOFs caused by the incorporation of graphite oxide. Because reusability is an important factor influencing the practical applications of heterogeneous catalysts, the catalytic activity of recovered MOF/graphite oxide composite is evaluated. As depicted in Figure 9, the conversion of styrene

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CONCLUSIONS Although MOFs are of great interest for various applications, the reports regarding their practical use are sporadic, at best, due to some shortcomings inherent in MOFs. To overcome the shortcomings, we demonstrate the successful modulation of MOFs properties by incorporation of graphite oxide in the present study. The obtained MOF/graphite oxide composites exhibit layer-spacing structure due to the growth of MOF between the graphene layers or their thin agglomerates, giving rise to enhanced porosity with high surface areas and some meso/macropores. This favors the access of reactant molecules to active sites and mass transfer in channels. Moreover, the incorporation of graphite oxide creates a more hydrophobic environment surrounding metallic sites, which shields the coordination bonds from attack by water molecules and presents better affinity of active sites to organic reactants. As a result, the hydrothermal stability of MOFs as well as the catalytic performance with regard to both activity and reaction rate are greatly improved. Furthermore, the MOF/graphite oxide composites show excellent reusability, and the catalytic activity can be well retained even after six cycles. The excellent hydrothermal stability, catalytic activity, and reusability make the present composites highly promising for practical applications in catalysis.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X-Q.L.). *E-mail: [email protected] (L.-B.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We acknowledge financial support of this work by the National High Technology Research and Development Program of China (863 Program, 2013AA032003), the National Basic Research Program of China (973 Program, 2013CB733504), Distinguished Youth Foundation of Jiangsu Province (BK20130045), the Fok Ying-Tong Education Foundation (141069), and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Figure 9. Reusability of the MOF/graphite oxide composite MG-2 as a heterogeneous catalyst for the ring opening of styrene oxide.

oxide remains almost constant in the six consecutive cycles over MG-2. The recovered catalyst still shows good catalytic activity that is comparable to the fresh catalyst. The XRD patterns of recovered catalysts were also recorded and the intense diffraction lines indicate the good preservation of crystallinity (data not shown). This demonstrates the excellent reusability of the composites in the ring-opening reaction of styrene oxide. F



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