Fabrication of Metal–Organic Frameworks inside Silica Nanopores

Mar 14, 2018 - Because of their diverse structure, high porosity, and tunable functionality, metal–organic frameworks (MOFs) are of great interest f...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 12051−12059

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Fabrication of Metal−Organic Frameworks inside Silica Nanopores with Significantly Enhanced Hydrostability and Catalytic Activity Jiahui Kou†,‡ and Lin-Bing Sun*,†,§ †

State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing 210009, China ‡ College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China § College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, China S Supporting Information *

ABSTRACT: Because of their diverse structure, high porosity, and tunable functionality, metal−organic frameworks (MOFs) are of great interest for diverse applications, including catalysis. However, the poor hydrostability of most reported MOFs hinders their catalytic applications seriously. In addition, the development of an effective method to improve the catalytic activity of MOFs is another challenge. Here, we report for the first time the development of a double-solvent strategy to fabricate MOFs inside silica nanopores. A typical MOF (MOF-5) and a mesoporous silica with two-dimensional hexagonal pore regularity (SBA-15) were first attempted. The double-solvent strategy is based on a hydrophobic solvent and a hydrophilic solution containing MOF precursors with a volume equal to or less than the pore volume of the support so that the MOF can be formed selectively in the channels of support. Our results show that upon confinement in silica nanopores the hydrostability of MOF-5 is apparently improved. The framework of MOF-5 is destroyed obviously in a humid environment for 15 min, but that confined in SBA-15 is well preserved after 8 h. Moreover, the catalytic activity of the composite MOF5@SBA-15 is superior to that of pure MOF-5 regarding activity and reaction rate. Under the catalysis of MOF-5@SBA-15, the conversion of benzyl bromide in the Friedel−Crafts alkylation reaction can reach 100% at 80 °C for 3 h, which is much higher than that of pure MOF-5 (61%) and SBA-15 (0%). We also demonstrate that the double-solvent strategy can be successfully extended to other MOFs, such as HKUST-1 and ZIF-8. Our work might open up an avenue for the improvement of stability and performance of MOFs, which is highly expected for a variety of applications. KEYWORDS: MOFs, mesoporous silica, hydrostability, confinement, double-solvent strategy

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INTRODUCTION Metal−organic frameworks (MOFs) are a class of crystalline materials construed by bridging organic ligands between their metallic moieties.1−6 As exciting materials with ultrahigh surface and diverse structure,7−9 MOFs are promising in a wide range of applications varied from gas separation10−13 and storage14−16 to drug delivery17,18 and catalysis,19−24 among others. However, many of them have been shown to be unstable upon exposure to moisture. It is known that the presence of metal−ligand coordination bonds makes MOFs more susceptible to hydrolysis than their inorganic analogues, such as zeolites.25−29 Very tiny amount of moisture is able to attack the coordination bonds in MOFs, which results in the degradation of bonds as well as the damage of framework. MOF-5 is a typical example, whose framework begins to degrade in 10 min upon exposure in humid environment.20 Therefore, the improvement of hydrostability is highly expected for the practical applications of MOFs. Since the discovery of MOFs, they have been employed as catalysts for diverse reactions because metal ions or clusters possessing coordinative vacancies can act as active sites. It is © 2018 American Chemical Society

known that MOF crystals with a smaller size tend to show high catalytic activities because of better exposure of active sites. However, preparation of small-sized MOFs is a challenging task because traditional synthetic methods prefer to form large crystals. If the crystal size of MOFs can be reduced, it is expected to significantly improve the catalytic activities. As for practical applications, it is extremely expected to create effective methods to enhance the hydrostability and catalytic activity of MOFs. Another group of porous materials that have attracted significant attention are mesoporous silicas.30,31 These silica materials with ordered nanopores in the range of 2−50 nm can be selected as supports for including a variety of guests inside the channels, such as metals, oxide nanoparticles, and drug macromolecules.32−36 Taking into account that MOFs have poor hydrostability, if they are incorporated into the nanopores of silica, the special microenvironment in nanopores is expected Received: January 29, 2018 Accepted: March 14, 2018 Published: March 14, 2018 12051

DOI: 10.1021/acsami.8b01652 ACS Appl. Mater. Interfaces 2018, 10, 12051−12059

Research Article

ACS Applied Materials & Interfaces

atmosphere, the confined MOFs well preserve their structure, which is quite different from bulk MOFs, whose structure is seriously destroyed. Furthermore, MOFs confined in nanopores with reduced crystal sizes, along with accelerated mass transfer of silica nanopores, make them vastly active for Friedel−Crafts alkylation. We also demonstrate that the double-solvent strategy can be extended to fabrication of a series of MOFs (e.g., MOF-5, HKUST-1, and ZIF-8) in silica nanopores.

to enhance the hydrostability of MOFs. Moreover, the crystal size can be well controlled due to the confined space, which is of high potential for the formation of highly active catalysts. In addition, the mesoporous channels could make the active sites greatly accessible to the substrates and accelerate diffusion of both substrates and products. To date, the composites of MOFs with mesoporous materials like silica37−39 and carbon nanotubes (CNTs)40−42 have been reported. The composites can be promising in various aspects like enhanced mechanical/ chemical stability and novel gas adsorption behavior. Nevertheless, solvothermal synthesis is frequently used in the literature, and this method is difficult (or not efficient) for the introduction of MOFs inside the nanopores of support. Here, we report the fabrication of MOFs in silica nanopores by employing a double-solvent strategy for the first time (Scheme 1). By use of the double-solvent strategy, the high

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EXPERIMENTAL SECTION

Chemicals. Zinc nitrate hexahydrate (Xilong, 99.5%), 1,4dicarboxybenzene (H2BDC, Aladdin, 99%), cupric nitrate trihydrate (Sinopharm, >99.5%), 1,3,5-benzene-tricarboxylic acid (H3BTC, Sinopharm, >98%), 2-methylimidazole (Adamas, 98%), N,N-dimethylformamide (DMF, Sinopharm, >99.5%), n-octane (Lingfeng, >95%), EO20PO70EO20 (P123, Sigma-Aldrich, >99%), tetraethylorthosilicate (TEOS, Sigma-Aldrich, 98%), hydrochloric acid (Lingfeng, >99.5%), benzyl bromide (Sinopharm, >97.5%), dichloromethane (Sinopharm, >99.5%), and toluene (Lingfeng, >99.5%) were utilized directly without further treatment. Deionized water was produced by a Milli-Q integral pure and ultrapure water system and utilized in all of the experiments. Materials Synthesis. MOF-5 was prepared by dissolving Zn(NO3)2·6H2O (0.78 g, 3 mmol) and H2BDC (0.165 g, 1 mmol) in DMF (10 mL). The mixture was transferred to a flask (50 mL) and heated to 120 °C. This temperature was maintained for 24 h. Then, the flask was cooled to room temperature and the crystals located at the bottom of the flask were recovered and washed with the solvent DMF, followed by immersing in CH2Cl2. CH2Cl2 was exchanged twice in the following 2 days. Finally, the obtained crystals were stored in fresh CH2Cl2. Mesoporous silica SBA-15 was synthesized according to the reported method.43 Triblock copolymer Pluronic P123 (3.0 g) was dissolved in the mixed solution of 2 M HCl (90 g) and deionized water (22.5 g). After the solution was stirred for 1 h at room temperature, TEOS (6.4 g) was added dropwise to the solution with constant stirring at 40 °C and then the resulting solution was continuously stirred at 40 °C for 24 h. Afterward, the milky reaction mixture was hydrothermally treated at 100 °C for 24 h. The resultant precipitate was filtered off, washed with water, and air-dried at room temperature. The composite MOF-5@SBA-15 was prepared via a double-solvent strategy. Briefly, Zn(NO3)2·6H2O (0.78 g, 3 mmol) and 1,4-BDC (0.165 g, 1 mmol) were dissolved in DMF (2 mL) as the precursor for MOF-5. Subsequently, SBA-15 (0.1 g) was suspended in dry n-octane (20 mL) as the hydrophobic solvent and the mixture was stirred for 30 min. The hydrophilic precursor solution (0.1 mL) was added dropwise over a period of 30 min with vigorous stirring. The resulting solution was continuously stirred at 120 °C for 24 h. After cooling to room temperature, the supernate was removed and the solids deposited at the bottom of the flask were collected, washed with DMF, and immersed in fresh CH2Cl2. CH2Cl2 was exchanged twice during 2 days. Ultimately, the resulting material was stored in fresh CH2Cl2. The content of MOF-5 in the composite was detected by inductively coupled plasma (ICP), and the results show that the content of MOF5 is 10.3 wt %. Characterization. X-ray diffraction (XRD) analysis of samples was conducted using a Bruker D8 Avance diffractometer. Nitrogen adsorption isotherms were collected by use of ASAP2020 at −196 °C. The materials were degassed at 100 °C for about 4 h before measurement. The Brunauer−Emmett−Teller surface areas of samples were estimated by utilizing the adsorption results with relative pressure varied from 0.04 to 0.20. The pore size distributions were estimated by the Barrett−Joyner−Halenda method based on the adsorption branches. The total pore volumes were obtained from the uptake at the pressure (p/p0) of around 0.99. A Nicolet Nexus 470 spectrometer was employed for the measurement of Fourier transform IR spectra.

Scheme 1. Fabrication of MOFs in Mesoporous Silica SBA15 Using the Double-Solvent Strategya

a (a) Dispersion of SBA-15 in hydrophobic n-octane; (b) addition of the hydrophilic N,N-dimethylformamide (DMF) solution containing MOF precursors (metal ions and ligands); (c) formation of MOFs at 120 °C for 24 h.

interfacial tension between hydrophobic solvent and hydrophilic solution (containing MOF precursors) drives the precursors to enter the hydrophilic nanopores. This leads to the construction of MOFs in the nanopores of SBA-15, whereas limits the formation of MOFs outside the pores. In contrast, in the conventional single-solvent process, a large amount of solution containing MOF precursors is used, which results in the deposition of bulk MOFs on the outer surface of support. Our results show that both the symmetry of silica support and the structure of MOFs are well maintained in the composite. It is fascinating that the composite shows superior hydrostability in comparison to pure MOFs. After exposure to humid 12052

DOI: 10.1021/acsami.8b01652 ACS Appl. Mater. Interfaces 2018, 10, 12051−12059

Research Article

ACS Applied Materials & Interfaces The spectra were recorded by diluting the samples with KBr. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out on a Hitachi S4800 electron microscope. Thermogravimetric (TG) analysis was conducted on a thermobalance (STA-499C, NETZSCH), and the derivatives (DTG) can be obtained by software analysis. Typically, the materials were heated to 800 °C with a heating rate of 10 °C·min−1 in nitrogen atmosphere. For ICP measurement, HCl was used to dissolve the materials. The contents of MOF-5 can be calculated from the amount of zinc in the materials. The hydrostability of samples was determined as follows. The materials were exposed to humid atmosphere with a relative humidity (RH) of 60% up to 8 h. Then, the treated materials were detected by XRD. The hydrostability of materials was assessed by comparing the XRD patterns before and after treatment. Catalytic Reaction. The Friedel−Crafts alkylation reactions between benzyl bromide and toluene were run in a flask. Typically, benzyl bromide (1.5 mL, 12.4 mmol), toluene (4.0 mL, 37.3 mmol), and a prescribed amount of catalyst were added. To quantitatively compare the catalytic performance, 100 mg of MOF-5@SBA-15 (containing 10 mg of MOF-5), 10 mg of MOF-5, or 90 mg of SBA-15 were used for the reaction. The reaction was conducted at 80 °C for 3 h. After the reaction was finished, the reaction mixture was centrifuged. The obtained liquid was detected by gas chromatography. The analysis was carried out using Agilent GC 7890A equipped with a flame ionization detector and a HP-5 column.

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RESULTS Structural Characterization. The first studied MOF is MOF-5 due to its poor hydrostability. And a typical mesoporous silica, SBA-15, was employed as the host. Figure 1a shows the low-angle XRD patterns of MOF-5, MOF-5@ SBA-15, and SBA-15. There are no peaks in the pattern of MOF-5. The pattern of SBA-15 shows three well-resolved peaks, which can be indexed as (100), (110), and (200) diffraction peaks associated with p6mm hexagonal symmetry.44−47 Although the diffraction intensity of MOF-5@SBA-15 decreases a little, which is due to the presence of MOF-5 crystals in the SBA-15 channels, the mesoporous hexagonal structure is obviously remained. The wide-angle XRD patterns of the materials are shown in Figure 1b. MOF-5 exhibits welldefined diffraction lines, which is in line with the reported results.48−50 For SBA-15, there is a broad peak at about 23°, which is caused by its amorphous walls. After incorporating MOF-5 crystals into the channels of SBA-15, the pattern has the characteristics of both MOF-5 and SBA-15. This indicates that the crystalline framework of MOF-5 is formed successfully in the channels of SBA-15. Furthermore, the intensity of the XRD peaks is low in the composite, which indicates the small dimension of MOF-5 in the composite. Further information on the individual MOF-5 and SBA-15 as well as the composite MOF-5@SBA-15 is given by IR spectra (Figure 2). For MOF-5, the asymmetric stretching of carboxylate groups (−COO−) of BDC appears around 1625 and 1500 cm−1 and symmetric stretching appears around 1389 cm−1.51,52 The typical vibration mode of SBA-15 is observed. The O−H vibration of the adsorbed water appears at 1630 cm−1, whereas the band between 1300 and 1000 cm−1 is due to the Si−O−Si bond. The band observed at 962 cm−1 is attributed to Si−OH, whereas the band at 462 cm−1 can be ascribed to Si−O vibration. The IR spectrum of the composite has the characteristics of MOF-5 and SBA-15, indicating the successful formation of MOF-5 in SBA-15. Figure 3a gives N2 adsorption−desorption isotherms of MOF-5 and SBA-15 as well as MOF-5@SBA-15. The isotherm for MOF-5 is of type I, indicative of the microporous feature.

Figure 1. (a) Low-angle and (b) wide-angle XRD patterns of MOF-5, MOF-5@SBA-15, and SBA-15.

Figure 2. IR spectra of MOF-5, MOF-5@SBA-15, and SBA-15.

The SBA-15 shows type IV with an H1 hysteresis loop, which is the typical characteristic of a material with cylindrical mesopores.53,54 For MOF-5@SBA-15, the isotherm shows the characteristic of SBA-15. In Figure 3b, a pore size distribution at 8 nm is observed for SBA-15. Due to partial filling of the mesopores, there is a decrease in the pore size of MOF-5@ SBA-15. Further textual parameters are listed in Table S1. Interesting results can be found from microporous and mesoporous volumes. The composite MOF-5@SBA-15 has a mesopore volume of 0.68 cm3·g−1, whereas the mesoporous volume of MOF-5 is negligible. In addition, the composite 12053

DOI: 10.1021/acsami.8b01652 ACS Appl. Mater. Interfaces 2018, 10, 12051−12059

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Figure 4. SEM and TEM images of (a, b) MOF-5, (c, d) MOF-5@ SBA-15, and (e, f) SBA-15.

proved that MOF-5 is successfully grown in mesoporous silica SBA-15.

Figure 3. (a) N2 adsorption−desorption isotherms and (b) pore size distributions of MOF-5, MOF-5@SBA-15, and SBA-15.

MOF-5@SBA-15 shows a microporous volume of 0.20 cm3·g−1, which is higher than that of SBA-15 (0.14 cm3·g−1). For the ratio Vmicro/Vtot, the composite MOF-5@SBA-15 presents a value of 0.23, which is obviously larger than that of SBA-15 (0.12). These results suggest that MOF-5 is successfully incorporated into SBA-15; and in comparison to pure SBA15, more micropores are generated due to the incorporation of microporous material MOF-5. Additionally, abundant mesopores in the composite MOF-5@SBA-15 is beneficial to mass transfer, as discussed below. The morphology and pore structure of materials were studied using SEM and TEM, as shown in Figure 4. MOF-5 presents cubic-like morphology, and the crystal width is about 10 μm (Figure 4a). SBA-15 exhibits the typical wheatlike morphology and consists of aggregates of ropelike particles with relatively uniform size (Figure 4e). The composite MOF5@SBA-15 shows a similar morphology to SBA-15 (Figure 4c). No MOF particles scattered on the outer surface are observed. In the TEM images, the crystal of MOF-5 is damaged (Figure 4b) because it is unstable during irradiation with high-energy Xray. SBA-15 displays well-ordered hexagonal pore channels (Figure 4f). After the introduction of MOF-5, the cylindrical shape of pores and the nearly perfect two-dimensional hexagonal order are maintained (Figure 4d). It is worth mentioning that the MOF-5 particles (dark spots) are visible inside the channels of SBA-15. The size of these particles are much smaller than bulk MOF-5 synthesized by the traditional method. Elemental mappings were also taken and are shown in Figure 5. In addition to Si and O derived from SBA-15, the element Zn stemmed from MOF-5 is detected. This further

Figure 5. STEM image of the composite MOF-5@SBA-15 and elemental mapping images of Si, O, and Zn. The domain in the red box was used for elemental analysis, and the domain in the yellow box was used for correction.

From the aforementioned results, it is clear that the doublesolvent strategy is effective in incorporating MOF-5 into silica nanopores. After the incorporation of MOF-5, the pore regularity of SBA-15 is well preserved. The newly formed composite MOF-5@SBA-15 thus exhibits the characteristic of microporous and mesoporous materials. The MOF crystals confined in nanopores show interesting stability and catalytic performance as demonstrated as follows. Stability Examination. TG technique was employed to examine the thermal stability of samples. MOF-5 shows two weight loss steps (Figure 6a). The first weight loss less than 200 °C is caused by solvent removal. The second main weight loss is due to the degradation of organic ligands, which corresponds to an obvious DTG peak at 530 °C (Figure 6b). It means that MOF-5 has good thermal stability. For SBA-15, the initial 12054

DOI: 10.1021/acsami.8b01652 ACS Appl. Mater. Interfaces 2018, 10, 12051−12059

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Figure 6. (a) TG and (b) DTG curves of MOF-5, MOF-5@SBA-15, and SBA-15.

Figure 7. Wide-angle XRD patterns of (a) MOF-5 and (b) MOF-5@ SBA-15 after being exposed to humid atmosphere (RH = 60%).

weight loss centered at about 100 °C is caused by the removal of physically adsorbed water. The second step between 150 and 300 °C is due to the decomposition of partial organic groups and removal of hydration water. There is a slight weight loss between 300 and 600 °C, which corresponds to the combustion of residual carbon. For the composite, the weight loss trend is similar to MOF-5. However, the predominant weight loss takes place at 480 °C, which is lower than bulk MOF-5. The reason might be that the size of MOF crystals incorporated into mesoporous supports is much smaller than the size of the bulk MOF-5. The hydrostability is quite essential from the viewpoint of practical applications. To examine the hydrostability, the samples were put in a humid environment (RH = 60%) at room temperature for different time. XRD patterns of related samples after treatment are employed to determine the hydrostability. Figure 7a shows that significant loss of the diffraction peaks (e.g., the peak at 9.7°) and the emergence of new diffraction peaks (e.g., the peak at 8.9°) can be observed for treated MOF-5. In fact, upon exposure to humid atmosphere, MOF-5 is converted to MOF-69c and related derivatives.55,56 In the presence of moisture, MOF-69c can be transformed to another structure ZnBDC·xH2O, where x is between 1 and 2. The XRD pattern of ZnBDC·xH2O is reported in the literature, but the exact crystal structure has not been obtained to date.56 This is different from the destruction of crystalline structure that often gives amorphous characteristic. The emergence of new characteristic peaks of ZnBDC· xH2O, together with the loss of structural peaks of MOF-5, can

thus reflect the poor hydrostability of MOF-5.57,58 Hence, the variation in the XRD patterns demonstrates the destruction of crystalline structure. Surprisingly, the powder XRD pattern of MOF-5@SBA-15 remains unchanged, the original peaks keep constant, and no new peaks emerge (Figure 7b). This implies the enhanced hydrostability of the composite MOF-5@SBA-15. Apparently, the hydrostability of MOF-5 is greatly improved upon confinement in the mesopores of SBA-15. In short, the decomposition temperature of MOF-5 in the composite is somewhat lower than bulk MOF-5 due to the small-sized MOF crystals. The hydrostability of MOF-5 improves obviously upon confinement, and the MOF is stable under humid atmosphere up to 8 h, whereas bulk MOF-5 collapses within 15 min. Catalytic Performance. In chemical industries, Friedel− Crafts alkylation of aromatic compounds is an important reaction.59,60 These liquid-phase reactions have been traditionally catalyzed by Lewis acids, such as AlCl3, ZnCl2, and FeCl3.61−65 However, these catalysts present undesirable economic and environmental issues. Therefore, solid acid catalysts have been investigated for the Friedel−Crafts alkylation. Because MOF-5 has high surface area as well as coordination-unsaturated open metal sites, it was used to catalyze Friedel−Crafts alkylation. The obtained materials were assessed for its catalytic activity in the Friedel−Crafts alkylation by studying reaction of benzyl bromide with toluene. As shown in Figure 8, no benzyl bromide was converted at all even after 3 h on the silica support SBA-15 because there is no active sites. 12055

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distributed in the whole sample. Third, both thermal stability and hydrostability change obviously for MOF-5 confined in SBA-15 (Figures 6 and 7). If the MOF-5 crystals are outside the pores, their stability should not be changed too much, if any. It is notable that the incorporation of MOF-5 into SBA-15 improves the hydrostability significantly. The structure of bulk MOF-5 is damaged seriously after being exposed to a humid atmosphere within 15 min, whereas MOF-5 confined in mesopores is stable up to 8 h. The main reason may be that the silica walls of SBA-15 could provide protection for MOF-5, which prevents the MOF crystals confined in mesopores from being attacked by the water vapor in the atmosphere. Consequently, the composite MOF-5@SBA-15 shows enhanced hydrostability. Our results prove that the composite is highly active in Friedel−Crafts alkylation. The conversion over the composite (100% at 3 h) is obviously higher than that over MOF-5 (only 61% at 3 h). It is well known that the catalytic activity generally increases with the decrease of catalyst size, as smaller MOFs have higher surface areas available for reactants. In our study, we have proved that the size of MOF-5 crystals grown in silica mesopores is much smaller than the bulk ones. Thus, the reactant molecules are more readily accessible to the catalyst active sites. In addition, the mesoporous structure can accelerate the diffusion of reactant/product molecules. These properties make the mesoporous silica-confined MOFs highly promising in various applications. To generalize the incorporation of MOFs into silica nanopores, fabrication of other MOFs by use of the doublesolvent strategy was attempted. In addition to MOF-5, two other typical MOFs (i.e., HKUST-1 and ZIF-8) were examined. The synthetic process for HKUST-1@SBA-15 was recorded as shown in Figure 9. After dropwise adding the blue hydrophilic

Figure 8. Conversion in Friedel−Crafts alkylation of benzyl bromide with toluene over MOF-5, MOF-5@SBA-15, and SBA-15.

On MOF-5, the conversion of benzyl bromide is 61% at 3 h, indicating that there exists active sites for the reaction. The incorporation of MOF-5 into nanopores adjusts the activity apparently. Under the catalysis of MOF-5@SBA-15, the conversion of benzyl bromide reaches 100% under the same reaction conditions, which is much higher than that over MOF5. Remarkably, the reaction rate is rather different in the case of MOF-5 before and after being incorporated into SBA-15. Under the catalysis of MOF-5, the conversion of benzyl bromide is 36% at 1.5 h, which accounts for 59% of the conversion at 3 h (61%). Nevertheless, MOF-5@SBA-15 can convert 80% of benzyl bromide at 1.5 h, which is 80% of the conversion at 3 h (100%). The recyclability of MOF@SBA-15 was studied as well. After three recycle steps, the conversion of benzyl bromide is 85% at 3 h. This activity is kind of lower than that over the fresh catalyst, while most of catalytic activity is remained. On the basis of these results, it is safe to say that the incorporation of MOF-5 into SBA-15 leads to the increase of reaction rate. In short, the catalytic performance of MOF-5 is significantly enhanced upon confinement in silica nanopores regarding catalytic activity as well as reaction rate.

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DISCUSSION Because of their diverse structure, high surface area, and adjustable functionality, MOFs are highly potential in diverse applications. Nevertheless, there are few reports concerning the practical applications of MOFs because of their poor hydrostability. To overcome this shortcoming, we report a doublesolvent strategy to fabricate MOF-5 within the mesopores of SBA-15, a typical mesoporous silica. The double-solvent strategy is based on a hydrophilic solution and a hydrophobic solvent. The former contains the precursor with a volume set equal to or less than the pore volume of the support, which can be absorbed within the hydrophilic support pores, and the latter, in a large amount, plays an important role to suspend the support and facilitate the synthetic process. This leads to the construction of dispersed MOFs in the SBA-15 channels, avoids the deposition of MOFs outside the pores, and produces MOFs with small size. There are some evidences that can prove the introduction of MOF-5 to the pores of SBA-15. First, the results of pore size distributions indicate that the introduction of MOF-5 leads to the decrease of pore size distributions (Figure 3b). If MOF-5 is located outside the pores, the pore size of SBA-15 would keep constant. Second, the elemental mapping images show that Zn derived from MOF-5 is well

Figure 9. (a) Wide-angle XRD patterns of HKUST-1, HKUST-1@ SBA-15, and SBA-15. Photographs for the fabrication of HKUST- 1 in mesoporous silica SBA-15 using the double-solvent strategy: (b) dispersion of SBA-15 in hydrophobic n-octane; (c) addition of the hydrophilic DMF solution containing HKUST-1 precursors (metal ions and ligands); (d) formation of MOFs at 120 °C for 24 h; and (e) washing twice with DMF and methanol. 12056

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ACS Applied Materials & Interfaces solution of MOF precursors, the white mesoporous silica becomes light blue, whereas the supernate is colorless (Figure 9c). This phenomenon clearly proves that the HKUST-1 precursor has been completely adsorbed in the pores of SBA15. The results of ICP show that about 11.2 wt % of HKUST-1 is introduced to mesoporous silica. Figure 9d shows the formation of composite after reacting at 120 °C for 24 h. As can be seen from the picture, the color of the sample is deepened, indicating the successful formation of MOF crystal. After washing, the composite was stored in CH2Cl2 (Figure 9e). Moreover, the wide-angle XRD patterns (Figure 9a), low-angle XRD patterns (Figure S1), and IR spectra (Figure S2) attest the successful synthesis of HKUST-1@SBA-15. Further information is given in TG and DTG curves (Figure S3), i.e., the decomposition temperature of HKUST-1@SBA-15 is slightly lower than that of HKUST-1, which agrees with the results reported in MOF-5@SBA-15. The decreased thermal stability actually indicates the smaller size of crystals in comparison to the bulk ones. By utilizing the double-solvent strategy, the composite ZIF-8@SBA-15 is also synthesized successfully according to the low-angle XRD patterns (Figure S4), wide-angle XRD patterns (Figure S5), IR spectra (Figure S6), and TG and DTG curves (Figure S7). The abovementioned results thus demonstrate that the double-solvent strategy can be extended to the incorporation of different MOFs into mesoporous silica.

ACKNOWLEDGMENTS

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REFERENCES

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CONCLUSIONS A facile and efficient double-solvent strategy has been developed for the construction of MOFs inside the silica nanopores, for the first time. In comparison to bulk MOFs, MOFs confined in silica nanopores have greatly improved hydrostability. Because of the nanometer-sized MOF crystals in nanopores, the active sites in MOFs become easily accessible. Also, the mesoporous channels of support favor the mass transfer during the reactions. As a result, the composite exhibits excellent catalytic performance in Friedel−Crafts alkylation reactions regarding catalytic activity and reaction rate, which is much superior to bulk MOFs. We also demonstrate that the double-solvent strategy can be extended to the fabrication of a series of MOFs inside the nanopores of silica support. The present study may bring light to the construction of functional materials combining MOFs and mesoporous materials, resulting in composites with enhanced stability and activity for a variety of applications. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01652. Textual parameters, XRD patterns, IR spectra, and TG curves of different samples (PDF)

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The authors acknowledge financial support from the National Natural Science Foundation of China (21722606 and 21576137).

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lin-Bing Sun: 0000-0002-6395-312X Notes

The authors declare no competing financial interest. 12057

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ACS Applied Materials & Interfaces

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