Improvement in Crystallinity and Porosity of Poorly Crystalline Metal

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Improvement in Crystallinity and Porosity of Poorly Crystalline Metal−Organic Frameworks (MOFs) through Their Induced Growth on a Well-Crystalline MOF Template Hoyeon Ji,‡ Sujeong Lee,‡ Jeehyun Park, Taeho Kim, Sora Choi, and Moonhyun Oh* Department of Chemistry, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea

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S Supporting Information *

ABSTRACT: Porous metal−organic frameworks (MOFs) are interesting materials owing to their interesting structural features and their many useful properties and applications. In particular, the structural features are greatly important to optimize the MOFs’ porosities and so properties. Indeed, the MOFs’ well-developed micropore and high surface area are the most important structural features, and as such, many practical applications of MOFs originate from these structural features. We herein demonstrate a strategy for improving the crystallinity of MOFs, and so increasing the porosity and surface area of poorly crystalline MOFs by making them in core−shell-type hybrids through the induced growth on the well-crystalline template. Although poorly crystalline versions of MOFs generate naturally in the absence of the well-crystalline template, well-crystalline versions of MOFs produce inductively in the presence of the well-crystalline template. In addition, the crystallinity enhancement of MOFs brings together the improvement in their porosities and surface areas. The surface areas and pore volumes of the wellcrystalline versions of MOFs produced through the induced growth on the template are calculated based on this study, indicating that MOF surface areas increase by up to 7 times compared to the poorly crystalline versions.

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INTRODUCTION Porous coordination polymers including MOFs are greatly interesting materials owing to their fascinating structural features and a range of applications in gas storage,1−6 sensing,7−10 separation,11−15 recognition,16−18 and catalysis.19−24 During the construction of MOFs, the control of component and functionality is critical for their optimal properties and applications.25−29 Within this aspect, the integration of two different kinds of MOFs to yield a hybrid MOF is an important strategy in the MOF development. In particular, the construction of core−shell-type hybrid MOFs is considered as an excellent approach for complementing the weakness of a pure MOF or for producing multifunctional MOF materials.30−37 And a MOF growth on MOF is a superb method to produce the hybrid MOFs including core−shell MOFs.38−43 For instance, Matzger and co-workers have reported the construction of MOF-5@IRMOF-3 through a seeded growth technique.39 Zhou and co-workers have reported a fascinating one-pot production of core−shell MOFs with mismatched cell parameters.41 In addition, we have reported the interesting MOF growths in isotropic and anisotropic ways on the MIL templates (MIL stands of Materials of Institute Lavoisier).42,43 Along with the MOFs’component, the MOFs’ structural feature is also important to optimize their properties. Welldeveloped micropores and the resulting high surface areas are keys to the useful applications of MOFs. However, the © XXXX American Chemical Society

controlled construction of MOFs with desired structural features is somewhat complicated, as small alterations during the MOF formation frequently cause large variations in their structures and properties. For example, the highly porous three-dimensional (3D) hexagonal MIL-68 structure can be constructed from 1,4-benzenedicarboxylic acid (H2BDC) and trivalent metal ions such as In3+ or Ga3+.44,45 In contrast, the MIL-88B structure bearing small open pores and poor surface area is produced by replacement of the metal cations with Fe3+ or Cr3+.46−49 Furthermore, a well-known 3D cubic structure of MOF-5 can be constructed from Zn2+ and H2BDC; however, the reactions with similar, but relatively long, organic linkers (e.g., 4,4′-biphenyldicarboxylic acid and terphenyl-4,4″dicarboxylic acid) frequently result in interpenetrated structures with reduced porosities and surface areas.50 In addition, the crystallinity of MOFs is vital in defining their porosity and surface area; typically, poorly crystalline characteristics in MOFs yield poor porosity and surface area.51−53 Therefore, improving crystallinity is an important issue for the development of the highly porous MOFs and for the production of the valuable MOFs. Herein, we demonstrate a strategy for improving the crystallinity of poorly crystalline MOFs, and so increasing their porosities and surface areas by making them in core−shell-type hybrids through the induced Received: April 17, 2018

A

DOI: 10.1021/acs.inorgchem.8b01055 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry growth on the well-crystalline MOF template. We synthesized MIL-68@MIL-68-X (X = NO2 or NH2) hybrid coordination polymer particles (CPPs) through the induced growth of MIL68-X on a well-crystalline MIL-68 template. (Scheme 1; e.g., Scheme 1. Schematic Representation for the Construction of Hybrid CPPs (Hybrid-CPP-ii and Hybrid-CPP-iii) of MIL-68@MIL-68-X (X = NO2 or NH2) through the Induced Growth of MIL-68-X on a Well-Crystalline MIL-68 Template

MIL-68-X was synthesized from the reaction of In3+ and H2BDC-X (X = NO2 or NH2) instead of H2BDC, and it has an MIL-68-like structure. See Figure 1 for the detailed structure of MIL-68.) Although poorly crystalline versions of MIL-68-X were produced spontaneously in the absence of the wellcrystalline MIL-68 template, the well-crystalline versions of MIL-68-X generated inductively in the presence of the MIL-68 template. Eventually, the porosities and surface areas of the shells within MIL-68@MIL-68-X were dramatically increased due to the enhanced crystallinity compared to those of the poorly crystalline MIL-68-X. The surface areas and total pore volumes of the well-crystalline versions of MIL-68-X were then calculated based on their components and their N2 sorption properties.

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Figure 1. (a) Schematic representation for the construction of CPPi−CPP-iii. Three analogous reactions of metal ions and organic building blocks yielded three different CPPs bearing structural similarity. SEM images of (b) CPP-i (MIL-68, hexagonal rods), (c) CPP-ii (MIL-68-NO2, walnuts), and (d) CPP-iii (MIL-68-NH2, octahedra and spheres). Ball-and-stick representations for MIL-68 structure are also displayed.

EXPERIMENTAL SECTION

Preparation of CPP-i of MIL-68. H2BDC (3.0 mg, 0.018 mmol) and In(NO3)3·xH2O (7.8 mg, 0.020 mmol) were mixed in N,Ndimethylformamide (DMF, 0.8 mL).44 The mixture was then heated (100 °C oil bath) for 10 min. After this time, the solution was cooled to room temperature and the generated hexagonal rods were collected through a centrifugation and a washing process with DMF and methanol. Preparation of CPP-ii of MIL-68-NO2. 2-Nitroterephthalic acid (H2BDC-NO2, 3.8 mg, 0.018 mmol) and In(NO3)3·xH2O (7.8 mg, 0.020 mmol) were mixed in DMF (0.8 mL). The mixture was then heated (100 °C oil bath) for 40 min. After this time, the solution was cooled to room temperature and the generated walnut-shaped particles were collected through a centrifugation and a washing process with DMF and methanol. Preparation of CPP-iii of MIL-68-NH2. CPP-iii of MIL-68-NH2 was also prepared from the same method described above for CPP-ii of MIL-68-NO2 using 2-aminoterephthalic acid (H2BDC-NH2, 3.3 mg, 0.018 mmol) instead of using H2BDC-NO2. In addition, CPP-iii was isolated after 20 min instead of 40 min. Preparation of MIL-68 Template. H2BDC (3.0 mg) and In(NO3)3·xH2O (7.8 mg) were mixed in DMF (0.9 mL), and 2 equiv of pyridine was added into the above mixture.44 The final

mixture was then heated (100 °C oil bath) for 10 min. After this time, the solution was cooled to room temperature and the generated hexagonal lumps were collected through a centrifugation and a washing process with DMF and methanol. Preparation of Hybrid-CPP-ii (MIL-68@MIL-68-NO2). H2BDCNO2 (3.8 mg) and In(NO3)3·xH2O (7.8 mg) were mixed in DMF (0.8 mL). The MIL-68 template (2.0 mg) was added into the above mixture, and the final mixture was heated (100 °C oil bath) for 20 min. After this time, the solution was cooled to room temperature and the generated rounded hexagonal rods were collected through a centrifugation and a washing process with DMF and methanol. Preparation of Hybrid-CPP-iii (MIL-68@MIL-68-NH2). Hybrid-CPP-iii (MIL-68@MIL-68-NH2) was also prepared from the same method described above for hybrid-CPP-ii using H2BDC-NH2 (3.3 mg) instead of using H2BDC-NO2.

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RESULTS AND DISCUSSION The construction of hexagonal MIL-68 rods (CPP-i, Figure 1b) from H2BDC and In(NO3)3 is well-known,44 where MILB

DOI: 10.1021/acs.inorgchem.8b01055 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

shown in Figure S1 (Brunauer−Emmett−Teller (BET) surface area (1479 m2 g−1) and total pore volume (0.66 cm3 g−1)). Meanwhile, CPP-ii and CPP-iii would exhibit the reduced N2 sorption capacities because of the −NO2 or −NH2 moieties in their structures. Compared with the H2BDC-derived MIL-68, the presence of these additional moieties (−NO2 or −NH2) in the MIL-68-X structures reduces their pore volume per gram (cm3 g−1), as decreasing the empty space within a structure and increasing a weight per volume. However, this effect does not fully account for the dramatic decrease in N2 sorption capacities of CPP-ii and CPP-iii, as shown in Figure S1, which may be originated from their poorly crystalline characteristics. Typically, poorly crystalline characteristics in MOFs yield poor gas sorption capacities due to the tendency to minimize the void volume within the structure without the well-developed repeating pores.51−53 Indeed, the N2 sorption isotherms of CPP-ii and CPP-iii (blue and cyan lines in Figure S1, respectively), which have poorly crystalline characteristics, displayed noticeably lower sorption capacities compared to the well-crystalline CPP-i. The BET surface area and total pore volume of CPP-ii were 237 m2 g−1 and 0.22 cm3 g−1, respectively, and those of CPP-iii were 161 m2 g−1 and 0.10 cm3 g−1, respectively (Table S1). We therefore attempted to produce the well-crystalline versions of MIL-68-X through their induced growth on the well-crystalline template to improve their porosities and surface areas (Scheme 1). Initially, the reaction of H2BDC-NO2 with In(NO3)3 was conducted in the presence of the well-crystalline MIL-68 template to induce the aligned growth of MIL-68-NO2 onto a well-crystalline MIL-68 template surface. SEM images of the product (hybrid-CPP-ii) showed a significant length increase and a slight thickness increase (Figure 3). In particular, the transmission electron microscopy (TEM) images (Figure 3d and see Figure S2a for an enlarged image) clearly show the core and shell portions, thus a successful construction of the core−shell-type hybrid. In addition, the isotropic growth of MIL-68-NO2 onto a MIL68 template was confirmed from the elemental mapping images of hybrid-CPP-ii (Figure 3e), as evidenced by the distribution of nitrogen atoms in the entire particle as the components of the BDC-NO2 building blocks of the MIL-68NO2 shell. Furthermore, the EDX spectrum profile scanning measurement (Figure S3) confirmed the core−shell structure. The dominant distribution of nitrogen atoms at the edge of the particle as the components of the shell is a general trend of the core−shell structure. Finally, a relative molar ratio of core and shell was determined by 1H NMR analysis, where cosolvent of DMSO-d6 and acetic acid-d4 was used to digest hybrid-CPP-ii. A molar ratio of H2BDC and H2BDC-NO2, and consequently, a molar ratio (1:1.94) of core (MIL-68) and shell (MIL-68NO2), was then obtained by integrating the signals correlating to H2BDC and H2BDC-NO2 building blocks (Figure 3f). The portion of MIL-68-NO2 shell in the hybrid-CPP-ii can be increased by increasing the amounts of In(NO3)3 and H2BDCNO2 used during the reaction while maintaining the amount of MIL-68 template (Figure S4). Remarkably, the PXRD pattern (Figure 3g) of hybrid-CPPii revealed the creation of a well-crystalline MIL-68-NO2 structure as opposed to the somewhat poorly crystalline MIL-68-NO2 that resulted from the solvothermal reaction of H2BDC-NO2 and In(NO3)3 without the MIL-68 template. This indicates that the well-crystalline MIL-68 template induced the creation of a well-crystalline version of MIL-68-

68 has a 3D hexagonal structure with general formula [In(OH)(BDC)]n (see ball-and-stick representations in Figure 1).45 First, the similar solvothermal reactions were conducted to produce CPPs with analogous MIL-68 structures using 2nitroterephthalic acid (H2BDC-NO2) and 2-aminoterephthalic acid (H2BDC-NH2) building blocks (Figure 1a). The resulting CPPs were initially analyzed by scanning electron microscopy (SEM, Figure 1) and powder X-ray diffraction (PXRD, Figure 2). SEM images of the two CPPs, which were prepared from

Figure 2. (a) Simulated pattern of MIL-68. PXRD patterns of (b) hexagonal rods of CPP-i (MIL-68), (c) walnuts of CPP-ii (MIL-68NO2), and (d) octahedra and spheres of CPP-iii (MIL-68-NH2). Although CPP-ii and CPP-iii have poorly crystalline characteristics, in principle, all CPP-i−CPP-iii have a 3D hexagonal structure of [In(OH)(L)] n, where L is BDC, BDC-NO2, or BDC-NH 2, respectively.

the reactions of In(NO3)3 with H2BDC-NO2 or H2BDC-NH2, revealed the formation of walnut-like CPPs (CPP-ii) and octahedral CPPs containing spherical particles (CPP-iii), respectively (Figure 1c,d). In general, the structural features of MOFs are reflected on their morphological features;44,48,54−56 however, the structural features of CPP-ii and CPP-iii were not reflected on their morphologies. For this inconvenience, it should be noted that the particles of CPP-ii and CPP-iii should be not the single crystal but the aggregators of small particles. Despite their completely different morphological features compared to CPP-i (hexagonal rods), the PXRD patterns of CPP-ii and CPP-iii (Figure 2c,d) indicated that they are basically composed of a 3D hexagonal MIL-68 structure. However, the slightly broad PXRD patterns of them suggested a somewhat poorly crystalline characteristic compared to the well-crystalline CPP-i. These broad features may result from defects and/or disordered sections within the structure, likely as a result of the steric constraints originating from the bulky −NO2 and −NH2 groups. The N2 sorption isotherms of CPP-i−CPP-iii were measured to achieve their N2 sorption properties (Figure S1). Overall, a good correlation between N2 sorption properties and crystallinity of CPPs was observed. First, the well-crystalline CPP-i44 displays a quite porous property, as C

DOI: 10.1021/acs.inorgchem.8b01055 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. SEM images of (a) the template, (b) the intermediate, and (c) hybrid-CPP-ii of MIL-68@MIL-68-NO2. (d) TEM image, (e) STEM image and elemental mapping images (In, cyan; N, purple), and (f) 1H NMR spectrum of hybrid-CPP-ii. (g) PXRD patterns of CPP-ii (top) and hybrid-CPP-ii (bottom). (h) N2 sorption isotherms of the template (red), CPP-ii (blue), and hybrid-CPP-ii (green).

NO2. Thus, to identify how enhanced MOF crystallinity influences its porosity, N2 sorption isotherms of hybrid-CPP-ii were measured. The sorption isotherms were located between those of pure MIL-68 and pure MIL-68-NO2 (Figure 3h). A 1:1.94 molar ratio corresponds to a 1:2.23 weight ratio of MIL68 and MIL-68-NO2 (Table S3). Therefore, a simple sum of a 1/3.23 contribution of MIL-68 and a 2.23/3.23 contribution of MIL-68-NO2 should result in its sorption isotherms being located close to those of MIL-68-NO2. The estimated values of the BET surface area and total pore volume for MIL-68@MIL68-NO2 calculated from a simple sum based on a weight ratio of MIL-68 and MIL-68-NO2 were 622 m2 g−1 and 0.36 cm3 g−1, respectively (Table S4). However, the measured N2 sorption isotherms of hybrid-CPP-ii were located close to those of MIL-68, and the BET surface area (1191 m2 g−1) and total pore volume (0.53 cm3 g−1) were considerably larger than the estimated values. This improvement in the sorption capacity of hybrid-CPP-ii may originate from the enhanced crystallinity of the MIL-68-NO2 shell within the MIL-68@ MIL-68-NO2 structure compared to the poorly crystalline MIL-68-NO2 structure (CPP-ii). On the basis of the N2 sorption data of MIL-68 and MIL-68@MIL-68-NO2 along with a relative core/shell ratio of MIL-68@MIL-68-NO2, the BET surface area and total pore volume of the well-crystalline version of MIL-68-NO2 were calculated (Table S5). As expected, the calculated BET surface area of the well-crystalline version of MIL-68-NO2 (1062 m2 g−1) was considerably larger than that of the poorly crystalline version of MIL-68-NO2 (237 m2 g−1, Table 1).

Table 1. BET Surface Areas of the Poorly Crystalline Versions (Measured) and the Well-Crystalline Versions (Calculated) of MIL-68-X (X = NO2 or NH2)

MIL-68-NO2 MIL-68-NH2

surface area of the poorly crystalline versions [m2 g−1]

surface area of the wellcrystalline versionsa [m2 g−1]

237 161

1062 1129

a

These values were calculated based on the BET surface areas of pure MIL-68 and hybrid MIL-68@MIL-68-X, and the core/shell ratios in hybrid MIL-68@MIL-68-X.

The preparation of MIL-68@MIL-68-NH2 (hybrid-CPP-iii, Figure 4) was then carried out via the similar solvothermal reaction of H2BDC-NH2 and In(NO3)3 with the MIL-68 template. SEM and TEM images (Figure 4c,d) of hybrid-CPPiii showed a substantial length increase but a slight thickness increase. Elemental mapping images of product (Figure 4e) confirmed the presence of nitrogen atoms, which are components of the BDC-NH2 building blocks used to construct the MIL-68-NH2 shell, existed in the entire particle. In addition, scanning TEM (STEM) images (Figure 4e and Figure S2b) of hybrid-CPP-iii clearly show the core and shell portions, thus confirming the creation of the core−shell-type hybrid. In the 1H NMR spectrum (Figure 4f), integration of the signals correlating to H2BDC and H2BDC-NH2 gave a core/shell molar ratio of 1:1.99 (a 1:2.09 weight ratio of core/ shell) within MIL-68@MIL-68-NH2. In particular, the creation of a well-crystalline MIL-68-NH2 shell was confirmed from the D

DOI: 10.1021/acs.inorgchem.8b01055 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. SEM images of (a) the template, (b) the intermediate, and (c) hybrid-CPP-iii of MIL-68@MIL-68-NH2. (d) TEM image, (e) STEM image and elemental mapping images (In, cyan; N, purple), and (f) 1H NMR spectrum of hybrid-CPP-iii. (g) PXRD patterns of CPP-iii (top) and hybrid-CPP-iii (bottom). (h) N2 sorption isotherms of the template (red), CPP-iii (blue), and hybrid-CPP-iii (green).

porosities and surface areas. In particular, the well-crystalline version of MIL-68-NH2 displayed a 7 times enhanced surface area compared to its poorly crystalline version. A MOF-onMOF growth strategy described here could be a significant route for the crystallinity enhancement and so for the improvement of porosity and surface area of the poorly crystalline MOF materials.

PXRD pattern of hybrid-CPP-iii (Figure 4g). Finally, as a result of this enhanced crystallinity of the MIL-68-NH2 shell, the measured BET surface area (1242 m2 g−1) and total pore volume (0.52 cm3 g−1) of hybrid-CPP-iii were significantly higher than the values estimated considering the 1:2.09 weight contributions of CPP-i and CPP-iii (i.e., BET surface area = 588 m2 g−1, total pore volume = 0.28 cm3 g−1, Table S4). The BET surface area and total pore volume of the well-crystalline version of MIL-68-NH2 were obtained to be 1129 m2 g−1 and 0.45 cm3 g−1, respectively (Table S5), based on N2 sorption and 1H NMR data. Lastly, the pore size distributions for two hybrid CPPs calculated from the nonlocal density functional theory method were comparable to that of the crystalline MIL68 template (Figure S5).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01055.

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CONCLUSIONS In conclusion, hybrid CPPs of MIL-68@MIL-68-X (X = NO2 or NH2) were created from the reactions of H2BDC-X (X = NO2 or NH2) and In(NO3)3 in the presence of the wellcrystalline MIL-68 template. A strategy for improving the crystallinity, and so increasing the porosity and surface area of poorly crystalline MOFs, was successfully demonstrated by making them in core−shell-type hybrids via their induced growth on a well-crystalline MOF template. We found that a well-crystalline MIL-68 template efficiently induced the wellcrystalline MIL-68-X shell on the MIL-68 template surface, even when the poorly crystalline versions of the MIL-68-X structures were produced in the absence of the template. In addition, the enhanced crystallinity of the MIL-68-X structure eventually resulted in a dramatic improvement in their

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General methods, N2 sorption isotherms, TEM image, STEM image, EDX spectrum profile scanning data, SEM images, pore size distributions, surface areas, pore volumes (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Moonhyun Oh: 0000-0001-8935-7820 Author Contributions ‡

H. Ji and S. Lee contributed equally to this work.

Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acs.inorgchem.8b01055 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (no. NRF-2017R1A2B3007271).

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DOI: 10.1021/acs.inorgchem.8b01055 Inorg. Chem. XXXX, XXX, XXX−XXX