Ligand Rigidification for Enhancing the Stability of Metal–Organic

Jun 10, 2019 - It is expected that more MOFs with excellent stability could be .... (a) 8-Connected Zr6 clusters; (b) different ligands LB1, LB2, and ...
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Article Cite This: J. Am. Chem. Soc. 2019, 141, 10283−10293

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Ligand Rigidification for Enhancing the Stability of Metal−Organic Frameworks Xiu-Liang Lv,† Shuai Yuan,‡ Lin-Hua Xie,† Hannah F. Darke,‡ Ya Chen,† Tao He,† Chen Dong,† Bin Wang,† Yong-Zheng Zhang,† Jian-Rong Li,*,† and Hong-Cai Zhou*,‡ †

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Beijing Key Laboratory for Green Catalysis and Separation and Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, People’s Republic of China ‡ Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) have been developing at an unexpected rate over the last two decades. However, the unsatisfactory chemical stability of most MOFs hinders some of the fundamental studies in this field and the implementation of these materials for practical applications. The stability in a MOF framework is mostly believed to rely upon the robustness of the M−L (M = metal ion, L = ligand) coordination bonds. However, the role of organic linkers as agents of stability to the framework, particularly the linker rigidity/ flexibility, has been mostly overlooked. In this work, we demonstrate that a ligand-rigidification strategy can enhance the stability of MOFs. Three series of ligand rotamers with the same connectivity but different flexibility were prepared. Thirteen Zr-based MOFs were constructed with the Zr6O4(OH4)(−CO2)n units (n = 8 or 12) and corresponding ligands. These MOFs allow us to evaluate the influence of ligand rigidity, connectivities, and structure on the stability of the resulting materials. It was found that the rigidity of the ligands in the framework strongly contributes to the stability of corresponding MOFs. Furthermore, water adsorption was performed on some chemically stable MOFs, showing excellent performance. It is expected that more MOFs with excellent stability could be designed and constructed by utilizing this strategy, ultimately promoting the development of MOFs with higher stability for synthetic chemistry and practical applications.

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INTRODUCTION Metal−organic frameworks (MOFs) are a class of highly ordered porous materials constructed from various metal ions/ clusters and organic ligands. These materials have attracted intense attention over the last two decades.1−3 Compared with conventional porous materials such as zeolites and other carbon-based materials, MOFs promise a wider range of applications such as in luminescence, sensing, separation, catalysis, and gas storage. The diverse array of possible applications originates in the programmable structures, customizable chemical functionalities, versatile architectures, and relatively mild synthetic conditions that are hallmarks of the MOF material.4−9 However, despite the advantages of MOF materials, there has yet to be widespread utilization of this material in real-world applications. Stability to the conditions of utilization is the most important prerequisite to any envisaged applications.10−12 Particularly, the low stability toward water has considerably limited MOFs’ practical application and commercialization. Water or moisture is usually present in most industrial processes, including carbon dioxide capture from flue gas,13,14 water desalination,15 proton conduction,16−18 moderate-temperature heat storage,19,20 and many catalytic processes.21−24 Unless a particular MOF is stable in these © 2019 American Chemical Society

conditions, it cannot be utilized. For instance, one of the most well-studied MOFs, MOF-5,25 decomposes gradually in the presence of water vapor, restricting its applications under moisture-rich conditions. MOFs with high stability have been always pursued among the scientific community, but they have been difficult to realize in practicality. New, more stable examples of MOFs are produced every year with the aid of improved understanding of MOF stability. Reducing the contact between water molecules and MOF host frameworks, particularly the coordination bonds, was believed to be an effective strategy to enhance a MOF’s moisture/water resistance.26−31 For instance, through introducing hydrophobic pore surfaces or blocked metal ions, water molecules are prevented from approaching the framework. Therefore, some MOFs have been specifically functionalized by introducing nonpolar alkyl functional, fluorinated, or methyl groups to sustain the framework robustness in the presence of an aqueous medium.26−29,31 Moreover, covering hydrophobic polymers such as polydimethysiloxane (PDMS) on the surface of MOF particles can also effectively enhance the water resistance.30 In addition to enhancing the hydrophobicity of Received: March 18, 2019 Published: June 10, 2019 10283

DOI: 10.1021/jacs.9b02947 J. Am. Chem. Soc. 2019, 141, 10283−10293

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Journal of the American Chemical Society

Figure 1. Construction of a series of Zr-MOFs by using naphthoate-based tetracarboxylate ligands. (a) 12-connected Zr6 clusters; (b) 8-connected Zr6 clusters; (c) different ligands LA1, LA2, and LA3; (d) configuration of ligands; different topologies (e) and structures (f) of LA2-Zr68-csq (5), LA2Zr612-shp (4), LA1-Zr612-shp (3), LA1-Zr68-csq (2), LA1-Zr68-f lu (1), and LA3-Zr68-flu (6) MOFs, respectively.

attributed to its larger cavity and/or the solvent-framework intermolecular forces. It should be pointed out that to date MOFs with high stability typically rely upon employing a robust M−L coordination bond in the framework. However, a divergent approach, the role of organic ligands in stability, particularly their rigidity/flexibility, is rarely studied. In this work, the investigation of the role of the organic linkers in the framework on stability is addressed. A series of three ligand rotamers with the conserved connectivity but different rigidity were rationally designed and synthesized. Constructed from Zr6O4(OH4)(CO2)n (n = 8 or 12) secondary building units (hereafter referred to as Zr6), 13 Zr-MOFs were obtained with the linkers in our study. The resulting stabilities of the structures were intensively studied and compared. Finally, it is demonstrated that ligand rigidification enhances the stability of MOFs toward water/ moisture. First, by using the LA type ligands (LA1, LA2, and LA3, four peripheral naphthoate arms with a varying core), as depicted in Figure 1, we successfully constructed six different Zr-MOFs, referred to as LA1-Zr68-f lu (1), LA1-Zr68-csq (2), LA1-Zr612-shp

the MOF structure, another common strategy is to increase the M−L bond strength. In recent years, many stable MOFs have been constructed with specific nitrogen-based ligands or by the combination of highly charged metal ions with carboxylatebased ligands. This approach can be rationalized by the hard/ soft acid/base (HSAB) principle.32−42 Examples of highly stable MOFs include zeolitic imidazolate frameworks (ZIFs),37 Materials Institute Lavoisier (MILs),36 pyrazolate-based frameworks,33,37,41 and Zr6-based carboxylate MOFs.32,39,40,45 A representative example of a Zr6-based carboxylate MOF is UiO-66. UiO-66 is constructed from Zr6O8 clusters and 1,4benzenedicarboxylate (BDC2−) ligands.15,32 It is well-known for its excellent thermal, mechanical, and chemical stability in water as well as strongly acidic and weakly basic aqueous solutions. The stability of UiO-66 can be attributed to the strong interaction between the ZrIV metal centers and the chelating carboxylate O donors. However, not all of the ZrMOFs demonstrated the same stability. The water stability of some Zr-MOFs has been challenged. For example, UiO67,32,43 an isoreticular version of UiO-66, has been found unstable toward water.44,46 The lack of stability was mainly 10284

DOI: 10.1021/jacs.9b02947 J. Am. Chem. Soc. 2019, 141, 10283−10293

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Journal of the American Chemical Society

Figure 2. Construction of Zr-MOFs by using benzoate-based tetracarboxylate ligands. (a) 8-Connected Zr6 clusters; (b) different ligands LB1, LB2, and LB3; (c) configuration of ligands; different topologies (d) and structures (e) of LB2-Zr68-csq (8), LB2-Zr68-scu (9), LB1-Zr68-f lu (7), and LB3-Zr68flu (10) MOFs, respectively.

(3), LA2-Zr612-shp (4), LA2-Zr68-csq (5, also known as NU100347), and LA3-Zr68-f lu (6), respectively. In this naming scheme, LA1, LA2, or LA3 are the name of the ligands, the Zr68 and Zr612 stand for how many ligands are coordinated to the Zr6 cluster, and the last three letters are the topology of the framework structures. By replacing naphthyl groups in LA-type ligands with phenyl groups, corresponding benzoate-based ligands LB1, LB2, and LB3 have also been synthesized (Figure 2). LB1-Zr68-flu (7), LB2-Zr68-csq (8, also known as NU-100048), LB2-Zr68-scu (9, also known as NU-9014949), and LB3-Zr68-f lu (10) were then constructed with the ligands and 8-connected Zr6 clusters, respectively. Each of these MOFs were tested and compared for differences in connectivity, structure, and ligand rigidity/flexibility. Control experiments were conducted to assess the influence of each variation in the ligand on the stability of the corresponding MOFs. It was found that the rigidity of the ligands was the most important of the characteristics tested for enhancing the stability of the MOFs. In this work, it was determined that topology and porosity are not determinative factors for stability in the system. Finally, the validity of this strategy was further verified by the examination of three Zr-MOFs, LC1-Zr612-fcu (11, also

known as UiO-67), LC2-Zr612-fcu (12), and LC3-Zr68-bcu (13, also known as PCN-700-Me 450 50). These MOFs were constructed from LC-type ditopic ligands (Figure 4). Again, the stability of LC2-Zr612-fcu (12) and LC3-Zr68-bcu (13) structures with rigidified ligands was dramatically enhanced compared to that of LC1-Zr612-fcu (11), which had more flexible biphenyl-4,4′-dicarboxylate ligands in the structure. These systematic studies suggest that ligand rigidification is an efficient and general approach for enhancing the stability of ZrMOFs.

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RESULTS AND DISCUSSION Ligands Regulated Structure Variation. Nine ligands, namely, LA1, LA2, LA3, LB1, LB2, LB3, LC1, LC2, and LC3 have been used to construct a series of Zr-MOFs. LAn and LBn (n = 1, 2, and 3) are tetracarboxylate ligands, and LCn (n = 1, 2, and 3) are dicarboxylate ligands. Each of the ligands consists of a central core and several peripheral arms. The peripheral arms for LAn and LBn are four naphthoates and four benzoates, respectively, and those of LCn are two carboxylates. The cores for LX1 (X = A, B, and C) are biphenyl groups. The core component for the LX2 ligands is pyrene, and those for LX3 are 10285

DOI: 10.1021/jacs.9b02947 J. Am. Chem. Soc. 2019, 141, 10283−10293

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Figure 4. Construction of Zr-MOFs by using dicarboxylate ligands. (a) Structures of bidentate ligands, LC1, LC2, and LC3; (b) 12connected and 8-connected Zr6 clusters; (c) structures of LC2-Zr612fcu (12), LC1-Zr612-fcu (11), and LC3-Zr68-bcu (13) MOFs.

neighboring aromatic rings (θ1 to θ4) in these structures is shown in Figure 3. The values of θ1 for biphenyl ranges from 0 to 90°, although only 17 structures out of 6427 reported structures have θ1 angles between 80° and 90°. The wide distribution of θ1 indicates that the two phenyl rings in a biphenyl entity readily rotate along the C−C bond between them, and the energy barrier for the rotation of one phenyl ring with respect to the other is low. In contrast, the values of θ2 in 34 structures of the reported structures containing 2,2′,6,6′tetramethylbiphenyl are in the range of 73° to 90°. Over half of the structures reported with this ligand core show θ1 angles higher than 85°. It is suggested that the spatial orientation of the two aromatic rings in 2,2′,6,6′-tetramethylbiphenyl is strongly constrained due to the steric hindrances of the neighboring methyl groups. The rotation of one ring, with respect to the other, to deviate from the ideal configuration of θ2 = 90° is energetically unfavorable. The values of θ3 (57− 90°) and θ4 (41−85°) imply that the rotation of one aromatic ring with respect to the other in 2,6-dimethylbiphenyl or 1phenylpyrene is also constrained, but not as strongly constrained as in 2,2′,6,6′-tetramethylbiphenyl. These statistics clearly indicate that an ordering for the flexibility of those structural motifs should be biphenyl > 1-phenylpyrene > 2,6dimethylbiphenyl > 2,2′,6,6′-tetramethylbiphenyl. It can be consequently expected that the flexibility of the ligands used in this work follows the same ordering as their backbone cores: LX1 > LX2 > LX3. A series of Zr-MOFs were then constructed by these ligands. Although the coordination bonds between Zr4+ and carboxylate O atom are strong, many reported Zr-MOFs have been shown to be unstable,43,46 implying that strong metal−ligand coordination bonds in MOFs are a necessary condition for stability, but not a sufficient condition for high stability. Thus, the investigation of the stability of Zr-MOFs built from these ligands with quite different rigidity/flexibility may offer insight into understanding the determinative factors

Figure 3. Statistic (number of crystal structures) of dihedral angles between two neighboring aromatic rings of biphenyl, 2,2′,6,6′tetramethylbiphenyl, 2,6-dimethylbiphenyl, and 1-phenylpyrene fragments in single-crystal structures deposited in the Cambridge Structural Database (CSD, update of Feb 2018).

methylate biphenyl (2,2′,4,4′,6,6′-hexamethylbiphenyl for LA3 and LB3 and 2,2′,6,6′-tetramethylbiphenyl for LC3). Due to these differences, it is expected that these ligands would generate structures with very different structural rigidity/ flexibility characteristics. A survey of known single-crystal structures in the Cambridge Structural Database (CSD, update of Feb 2018) containing biphenyl, 2,2′,6,6′-tetramethylbiphenyl, 2,6-dimethylbiphenyl, or 1-phenylpyrene fragment was performed. The statistic of dihedral angle between two 10286

DOI: 10.1021/jacs.9b02947 J. Am. Chem. Soc. 2019, 141, 10283−10293

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influence of each functionality on the stability of corresponding MOFs. With the same ligand and connectivity, but different structures, two sets of MOFs, LA1-Zr68-f lu (1) and LA1-Zr68csq (2), and LB2-Zr68-csq (8) and LB2-Zr68-scu (9), were studied and compared. As depicted in Table 1 and Figure S22, unlike

of their stability and further to guide the design of highly stable MOFs. Five Zr-MOFs were obtained from the LX1-type ligands. The MOFs LA1-Zr612-shp (3), LA1-Zr68-csq (2), and LA1-Zr68-f lu (1) were isolated from the solvothermal reactions between H4LA1 and ZrCl4 with different reaction conditions (see Experimental Section for details). The as-synthesized MOFs have a (4, 12)connected framework with the shp topology constructed from 12-connected Zr6 clusters and square-planar tetratopic LA1 ligands. Similarly, a (4, 8)-connected framework with the csq topology constructed from 8-connected Zr6 clusters and square-planar LA1 ligands and a (4, 8)-connected framework with the f lu topology constructed by 8-connected Zr6 clusters and tetrahedral LA1 ligands were also synthesized. LB1-Zr68-f lu (7) was synthesized from the H4LB1 ligand, which is isoreticular to LA1-Zr68-f lu (1) with a slightly smaller unit cell. The framework of LC1-Zr612-fcu (11) is a 12-connected framework with the fcu topology. This system is constructed from 12-connected Zr6 clusters and linear LC1 ligands. Another five Zr-MOFs were obtained from the LX2-type ligands. LA2-Zr68-csq (5) and LA2-Zr612-shp (4) were synthesized from H4LA2. The resulting structures are isoreticular to LA1-Zr68-csq (2) and LA1-Zr612-shp (3), respectively. LB2-Zr68csq (8) and LB2-Zr68-scu (9) were synthesized from H4LB2. The LB2-Zr68-csq (8) is isoreticular to LA1-Zr68-csq (2) and LA2-Zr68csq (5). The framework of LB2-Zr68-scu (9) is constructed by 8connected Zr6 clusters and square-planar LB2 ligands. This structure can be regarded as a (4, 8)-connected framework with the scu topology. LC2-Zr612-fcu (12) was synthesized from ligands H2LC2, and it is isoreticular to LC1-Zr612-fcu (11). Lastly, three Zr-MOFs were obtained from the LX3-type ligands. LA3-Zr68-f lu (6) and LB3-Zr68-flu (10), synthesized from H4LA3 and H4LB3, respectively, are both isoreticular to LA1-Zr68-f lu (1). LC3-Zr68-bcu (13) was synthesized from H2LC3 and has a (4, 8)-connected network with the bcu topology. This MOF is constructed from linear LC3 ligands and 8-connected Zr6 clusters. Additional structural details and structure descriptions for the newly synthesized Zr-MOFs can be found in Section S7 of the Supporting Information. Permanent Porosity. A series of N2 sorption measurements was used to confirm the permanent porosity of the 13 Zr-MOFs. After the solvent molecules (DMF and water) were removed through acetone solvent exchange and subsequent evacuation under vacuum, N2 adsorption isotherms were recorded at 77 K for LA1-Zr68-f lu (1), LA1-Zr68-csq (2), LA1Zr612-shp (3), LA2-Zr612-shp (4), LA2-Zr68-csq (5), LA3-Zr68-f lu (6), LB1-Zr68-f lu (7), LB2-Zr68-csq (8), LB2-Zr68-scu (9), LB3Zr68-f lu (10), LC1-Zr612-fcu (11), LC2-Zr612-fcu (12), and LC3Zr68-bcu (13) (Section 9 in the Supporting Information). Saturated N2 uptakes of 850, 1410, 605, 1290, 575, 590, 585, 910, 620, 580, 595, 395, and 350 cm3 g−1 (STP) were achieved, respectively. The adsorption/desorption isotherms of the 10 MOFs with the flu, shp, scu, fcu, and bcu topologies show type I isotherms, indicating the presence of micropores in their structures. Meanwhile, stepwise type IV N 2 adsorption/desorption isotherms were observed for LA1-Zr68csq (2), LA2-Zr68-csq (5), and LB2-Zr68-csq (8), which implies a mesoporous structure. The N2 adsorption results were used to confirm the permanent porosities of the 13 MOFs after the removal of guest molecules. Chemical Stability. The chemical stability of the Zr-MOFs with tetratopic ligands under a range of conditions was tested. Herein, control experiments were conducted to assess the

Table 1. Chemical Stability Tests of 13 Zr-MOFs Identified by PXRD Patterns materials LA1-Zr68flu LA1-Zr68csq LA1-Zr612shp LA2-Zr612shp LA2-Zr68csq LA3-Zr68flu LB1-Zr68flu LB2-Zr68csq LB2-Zr68scu LB3-Zr68flu LC1-Zr612fcu LC2-Zr612fcu LC3-Zr68bcu

water stability

acid stability in aqueous HCl (M)

base stability in aqueous NaOH (mM)

unstable unstable unstable stable

1

1

stable

1

1

stable

2

1

stable

0.1

1

stable

0.1

1

stable

2

1

stable

0.0001

0.01

stable

6

1

stable

12

10

unstable

most reported Zr-MOFs, it was found that LA1-Zr68-f lu (1) is unstable in water. The intensity and position of the powder Xray diffraction (PXRD) peaks have obviously changed, indicating a significant change in the crystallinity of the bulk phase. Also collaborating this finding, the N2 sorption measurements have been almost completely lost after the water treatment. Combined, this would indicate the collapse of the framework. A similar situation demonstrating a low stability toward water for LA1-Zr68-csq (2) was observed and confirmed by PXRD and N2 sorption measurements (Figures S23 and S36). Transversely, both LB2-Zr68-csq (8) and LB2Zr68-scu (9) display high chemical stability.48,49 As shown in Table 1 and Figures S23 and S36, they remain intact after immersion in water, acidic solution (pH = 1), or weakly basic (pH = 11) aqueous solutions, respectively. In our MOF system, the Zr-MOFs built from the same ligand but with different structures could show similar stability, suggesting that the flexibility of ligand is the main impact factor that dictates the stability of these Zr-MOFs.50 Second, the chemical stability of Zr-MOFs constructed from the same ligand, but with different connectivities around the Zr6 cluster, was tested. The framework of LA1-Zr612-shp (3) can be viewed as a system that is separated by the hexagonal channels in the LA1-Zr68-csq (2) framework by six additional LA1 ligands and an extra Zr6 cluster (Figure S7i). The framework of LA1-Zr612-shp (3) can be regarded as a derived net of LA1-Zr68-csq (2), where the hexagonal channels of the LA1-Zr68-csq (2) framework are partitioned by six additional 10287

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Journal of the American Chemical Society LA1 ligands and an extra Zr6 cluster (Figure S7i). Even with the increased connectivities and reduced pore sizes, the PXRD pattern of LA1-Zr612-shp (3) was found to have lost the quality of the crystalline PXRD pattern, and the N2 isotherm is greatly reduced after treating with water. These results are consistent with the results observed for LA1-Zr68-csq (2) (Table 1 and Figures S23 and S36). However, LA2-Zr68-csq (5) and LA2Zr612-shp (4) both demonstrate superior chemical stability as indicated by PXRD and N2 sorption measurements (Figures S26 and S39). These results imply that the connectivities also do not have an obvious effect on the stability of these ZrMOFs, as long as the ligand and clusters within the system remain unchanged. Finally, the stability of the Zr-MOFs constructed from different ligands but sharing the same topology, such as LA1Zr68-f lu (1) and LA3-Zr68-f lu (6), LA1-Zr612-shp (3) and LA2Zr612-shp (4), LA1-Zr68-csq (2) and LA2-Zr68-csq (5), or LB1Zr68-f lu (7) and LB3-Zr68-f lu (10), were compared against their sister topology frameworks, respectively. In each set, the two ligands under study contained different amounts of rigidities/ flexibilities, yielding differences in the stability of the resulting framework. The LA1-Zr68-f lu (1) framework was found to be unstable when the sample was exposed to water, while the LA3Zr68-f lu (6) framework remained intact after a variety of treatments in aqueous solution. After the LA3-Zr68-flu (6) sample was soaked in different solutions from 2 M HCl to pH = 11 NaOH aqueous solution, the PXRD pattern showed no loss in crystallinity and the N2 sorption isotherms were unaltered (Figures S27 and S40). These results suggest that LA3-Zr68-f lu (6) is highly chemically stable. Similar stability differences were also found in another three sets of MOFs, LA1Zr612-shp (3) and LA2-Zr612-shp (4), LA1-Zr68-csq (2) and LA2Zr68-csq (5), and LB1-Zr68-f lu (7) and LB3-Zr68-f lu (10). As discussed previously, the frameworks of LA1-Zr612-shp (3) and LA1-Zr68-csq (2) collapsed after water treatment, while the samples of LA2-Zr612-shp (4) and LA2-Zr68-csq (5) retained their crystallinity after immersion in water, HCl (pH = 0), and NaOH solutions (pH = 11) for 24 h, respectively (Figures S18 and S19). The LB3-Zr68-f lu (10) framework also has obvious differences from the LB1-Zr68-flu (7) framework in terms of chemical stability. A comparison of PXRD patterns for both LB1-Zr68-flu (7) and LB3-Zr68-flu (10) after immersion in water supported that the LB3-Zr68-f lu (10) framework is more stable than the LB1-Zr68-f lu (7) framework (Figures S28, S31, S41, and S44). Meanwhile, the N2 sorption measurements for the LB3-Zr68-flu (10) sample were almost the same as that of the original sample after chemical stability testing (from pH = 11 NaOH to 2 M HCl aqueous solution). These systematic results demonstrate that the rigidification of the ligands through restricting the rotation of the core entities has remarkable implications for the stability of the corresponding MOFs. As discussed above, it has been demonstrated that the ligand-rigidification strategy can be used to stabilize the ZrMOFs constructed with tetratopic carboxylate ligands. ZrMOFs constructed from ditopic carboxylate ligands were also investigated to demonstrate the universality of the ligandrigidification strategy. The chemical stability of LC1-Zr612-fcu (11), a ditopic carboxylate ligand based Zr-MOF, has always been challenged.43−45 After immersion of LC1-Zr612-fcu (11) in water for 24 h it was found that the PXRD patterns and N2 sorption measurements of the framework remained almost entirely

unchanged (Table 1 and Figures S32 and S45). However, the PXRD pattern and N2 sorption measurements for the framework rapidly changed after treatment with a weakly acidic aqueous solution (pH < 4) or a base aqueous solution (pH > 9). These results signify the framework’s chemical stability weaknesses under such conditions, restricting its further application under harsher constraints. Given our results with the tetratopic carboxylate ligands, it was speculated that the ligand-rigidification strategy may be applicable for the enhancement of the chemical stability of LC1Zr612-fcu (11). To test this hypothesis, LC2-Zr612-fcu (12) was constructed by utilizing ligand LC2. In comparison to LC1-Zr612fcu (11), the effective pore and window sizes are reduced in LC2-Zr612-fcu (12) owing to the larger dimension of the extended ligand LC2. Thus, the N2 sorption measurement for LC2-Zr612-fcu (12) were slightly different from that of LC1-Zr612fcu (11) (Figure S46). Noticeably, the chemical stability tests and subsequent PXRD profiles (Figure S33) and N2 sorption measurements (Figure S46) have demonstrated that not only is the LC2-Zr612-fcu (12) framework stable to aqueous solutions, but the framework remains intact in strongly acidic solutions (6 M HCl) as well as weakly basic solutions (pH = 11). These results further confirmed that the incorporation of rigidified ligands into a MOF framework contributes to the stabilization of the MOF. Additionally, the chemical stability of LC3-Zr68-bcu (13) was also enhanced in comparison to LC1-Zr612-fcu (11), although with reduced connectivity (Table 1 and Figures S23 and S24). It was found that the LC3-Zr68-bcu (13) also retains high porosity after treatment with water, strong acid (12 M HCl), and weak base (pH = 11), respectively. Clearly, both LC2-Zr612fcu (12) and LC3-Zr68-bcu (13) show higher stability toward aqueous solutions in a wide pH range than the parent MOF, LC1-Zr612-fcu (11). This higher stability can be attributed not only to the reduced pore size and porosity of the isoreticular structures but also to the restricted rotation of the ligand. It is worth noting that another UiO-67-type MOF with enhanced water stability was reported by Liellurd’s group.46 It was found that UiO-67-Me, constructed by 3,3′-position methylated biphenyl dicarboxylate ligands without the restriction of the flexibility of biphenyl backbone, gave a limited enhanced water stability to the UiO-67-Me framework.46 In contrast, the UiO67-BN framework with naphthyl-functionalized ligands disturbed the free rotation of the biphenyl core. This change in the UiO-67-BN structure in comparison to the parent MOF or UiO-67-Me provided UiO-67-BN a higher chemical stability under the same conditions.46 These results imply that the hydrophobic groups in the ligand can lead to enhanced stability in the structure, but the rigidification of the ligand has a greater influence on the chemical stability of the MOF. Proposed Mechanism for Stability Enhancement. The thermodynamic reason, such as the enhanced bond strength, is believed to mainly dictate the framework or bond stability of MOFs. In this work, however, we propose that it is possible to enhance the stability of a MOF through ligand rigidification. Herein, we defined the flexibility of ligands as the rotational freedom of their neighboring aromatic rings. It may be roughly quantified by the energy barrier of rotating C−C bonds between two neighboring aromatic rings. Rigid linkers have a higher energy barrier and tend to keep their conformation in MOFs (both pristine or decomposed states). The approach of a ligand-rigidification mechanism can mainly be explained from a kinetics perspective. In terms of the kinetics, a successive 10288

DOI: 10.1021/jacs.9b02947 J. Am. Chem. Soc. 2019, 141, 10283−10293

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fcu (11) were then taken, and the results were compared to the data from LA1-Zr68-flu (1), LA1-Zr68-csq (2), LA1-Zr612-shp (3), and LB1-Zr68-f lu (7), respectively. The water sorption for LA2Zr68-csq (5) begins at P/P0 = 0.7 and reaches a maximum of 1800 cm3 g−1 (corresponding to a H2O-accessible pore volume of 1.45 cc g−1, where the N2-accessible pore volume is 1.81 cc g−1) in the first cycle (Figure S51). The well-defined water isotherm and corresponding high water sorption indicate the enhanced stability of LA2-Zr68-csq (5) toward water relative to the corresponding data for LA1-Zr68-csq (2). However, less than half of the maximum water sorption (850 cm3 g−1) occurred under identical conditions in the second adsorption cycle for LA2-Zr68-csq (5). The results of water uptake and N2 sorption experiment on the material demonstrate a decrease in the porosity. These results indicate that after the first cycle there was some decomposition of the material from the water adsorption experiment (Figure S49). A similar phenomenon was observed in LA2-Zr612-shp (4), LB2-Zr68-csq (8), LB2-Zr68scu (9), and LC1-Zr612-fcu (11). The maximum amount of water adsorbed of these four materials was 940, 1590, 920, and 440 cm3 g−1, respectively. A significant decrease in the amount of water adsorbed from the first to the second cycle (710, 610, 390, and 210 cm3 g−1) was also observed for these materials (Figure S49). After several cycles, the water adsorption capacities of the four MOFs were successively decreased, suggesting that their respective structures undergo partial decomposition during the water adsorption/desorption process. The instability of these frameworks during water adsorption/desorption cycling could be attributed to the capillary-force-driven pore collapse. This suggests that causatively from the evacuation of water from the framework, rather than a competitive hydrolytic instability based on water, the pores within the framework collapse.45 Finally, we found that compared to the other frameworks in this study, LA3-Zr68-f lu (6), LB3-Zr68-f lu (10), LC2-Zr612-fcu (12), and LC3-Zr68-bcu (13) exhibit superior water uptake performance. The water sorption isotherm for LA3-Zr68-f lu (6) shows obvious steps with a hysteresis loop (Figure S50). The limited water uptake at lower pressure for this material indicates that the affinity of the water molecules to the MOF surface is weak. Therefore, a higher water vapor pressure is required to induce pore filling. Once the pore filling begins, at P/P0 = 0.70, maximum sorption is quickly reached with a total capacity of 920 cm3 g−1 at P/P0 = 0.9. Similarly to the water isotherm of LA3-Zr68-f lu (6), the water adsorption of LB3-Zr68f lu (10) demonstrated an isotherm with a steep adsorption curve that began at P/P0 = 0.45 and reached 1250 cm3 g−1 at P/P0 = 0.80 (Figure S50). The water isotherm of LC2-Zr612-fcu (12) (Figure S51) has a sigmoidal shape with a moderate hysteresis loop at P/P0 = 0.40−0.50. For LC2-Zr612-fcu (12), the maximum water sorption was 550 cm3 g−1 (at 298 K) at P/ P0 = 0.90. For LC3-Zr68-bcu (13), the water sorption began at P/P0 = 0.65 and reached a maximum water sorption of 385 cm3 g−1 at P/P0 = 0.85. Cycle performance for water sorption experiments for LA3-Zr68-f lu (6), LB3-Zr68-f lu (10), LC2-Zr612fcu (12), and LC3-Zr68-bcu (13) was also conducted (Figure S50). The cycling experiments indicated that these four MOFs retain most of the cycling performance integrity between the first and last water sorption cycle. It should be noted that the LC2-Zr612-fcu (12) framework showed a high mechanical stability in the water adsorption/desorption processes. The mechanical stability of this MOF was shared with other MOFs built from methylate cores (LX3). However, the MOFs built

substitution reaction is generally considered to be the main contributor to MOF decomposition in the presence of water. In this process, defects emerge through the replacement of coordination bonds between the ligand and metal cluster with water molecules that become bound to the metal clusters. The backbones of rigid ligands such as LA2, LA3, LB2, LB3, LC2, and LC3 can be viewed as whole pieces, while flexible ligands such as LA1, LB1, and LC1 have higher degrees of rotational freedom due to their rotatable biphenyl cores. When the dissociation through water substitution occurs on one side of a ligand bound to the metal cluster, the ligand must have the rotational freedom to leave the original coordination sites. In the case of rigid ligands, the structure deformation is restricted by other coordination sites; thus the ligand does not move too far from its original position in the framework. Therefore, the lability of the M−L coordination bond at a particular dissociated sites can lead to rapid structural repair. This self-repair effect is more effective for ligands with higher connectivity because these ligands can tolerate the displacements of more coordination sites than a ligand with lower connectivity. This “three-dimensional (3D) chelating effect” has been described in our previous work.51 Overall, when high connectivity is exhibited for the ligand and metal cluster, dissociation due to water treatment does not result in structural collapse. However, for flexible ligands, the dissociation of the bond between a metal cluster and a ligand can happen without disturbing other coordination sites, resulting in a slower and more difficult L−M bond re-formation. As a result, the defect ratio in the system will increase, further leading to the collapse of the framework. Furthermore, as depicted in Figures 1c and 2b, introduction of hexamethyl/tetramethyl into specific positions can latch the tetratopic/ditopic ligands LA3, LB3, and LC3, in which the latching effect is more evident compared with LA2, LB2, and LC2, resulting in MOFs with superior stability. Water Adsorption. The ability to synthesize highly stable Zr-MOFs by utilizing the rigid ligand approach allows for further exploration of potential applications for these materials. Water adsorption/desorption performance in MOFs is an important consideration for the implementation of these materials for practical application.52−58 High water uptake capacity and good cycling performance of the materials are both required for water capture applications. Water adsorption/desorption processes require not only chemical stability but also the mechanical stability of the adsorbents. The rigidified Zr-MOFs in this work that have shown high chemical stability in the presence of water have thus been further examined for their water adsorption/desorption performances (Section S13). For comparison, the water uptake properties of LA1-Zr68-f lu (1), LA1-Zr68-csq (2), LA1-Zr612-shp (3), and LB1-Zr68-f lu (7) were first studied. These MOFs were all synthesized from the biphenyl-based ligands LA1 and LB1, whose water isotherms suggest collapse of structures during the first round of desorption (Figure S48). This was verified by the water sorption experiments in subsequent cycles where the adsorption was greatly reduced. The N2 sorption measurements for the recovered MOF materials also showed signs of MOF decomposition, indicating the collapse of the structure after water adsorption. These results are consistent with the known low chemical stability of these MOFs. The water adsorption properties of LA2-Zr68-csq (5), LA2Zr612-shp (4), LB2-Zr68-csq (8), LB2-Zr68-scu (9), and LC1-Zr61210289

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Figure 5. Water uptake capacity of Zr-MOFs studied in this work.

from pyrene cores, LA2-Zr68-csq (5), LA2-Zr612-shp (4), LB2Zr68-csq (8), and LB2-Zr68-scu (9), did not share the same mechanical stability. The difference in the mechanical stability for these two groups of MOFs is not unexpected, as the LA2 and LB2 ligands are more flexible than the LC2 ligand due to the extra rotational freedom between the peripheral phenyl arms and the central cores in the LA2 and LB2 ligands (θ4 in Figure 3d). Clearly, the high cycling performance in the water adsorption/desorption performance for these Zr-MOFs can be attributed to the rigidity of their corresponding ligands.

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strategy to obtain highly porous and stable (chemically or mechanically) MOFs.

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EXPERIMENTAL SECTION

General Information. Commercially sourced chemicals were used as purchased without further purification. A detailed list of chemical sources can be found in the Supporting Information Section 1. Instrumentations. The PXRD patterns were recorded on a Rigaku Smartlab3 X-ray powder diffractometer equipped with a Cu sealed tube (λ = 1.541 78 Å). Simulations of the PXRD patterns were carried out by the single-crystal data and diffraction-crystal module on Mercury. The Mercury program is available free of charge at https:// www.ccdc.cam.ac.uk/. N2 and water vapor sorption experiments were conducted using a Micrometritics ASAP 2020 system at 77 and 298 K, respectively. All 1H NMR data sets were collected on a Mercury 400 MHz NMR spectrometer. All FT-IR spectra were recorded on an IR Affinity-1 instrument. TGA data were obtained on a TGA-50 (Shimadzu) thermogravimetric analyzer with a heating rate of 10 °C min−1 under a nitrogen atmosphere. Inductively coupled plasma (ICP) were performed by a PerkinElmer Optima 8000 optical emission spectrometer. Elemental analysis was performed by a Vario EL cube Elementar. Synthesis of Ligands. A detailed procedure for the synthesis of H4LA1, H4LA2, H4LA3, H4LB1, H4LB3, and H2LC2 can be found in the Supporting Information in Section 2. Synthesis of Reported MOFs. NU-1103 (LA2-Zr68-csq (5)), NU1000 (LB2-Zr68-csq (8)), NU-901 (LB2-Zr68-scu (9)), UiO-67 (LC1Zr612-fcu (11)), and PCN-700-Me4 (LC3-Zr68-bcu (13)) were prepared according to reported methods. Synthesis of LA1-Zr68-f lu (1, BUT-91). A solution of H4LA1 (25 mg, 0.028 mmol), ZrCl4 (100 mg, 0.56 mmol), and formic acid (0.3 mL, 88%) in 10 mL of dimethylformamide (DMF) was generated and sealed in a 20 mL glass vial. Then the vial was placed into an oven and heated to 120 °C for 48 h followed by cooling to 25 °C at a rate of 0.5 °C/min. The resulting colorless crystals were collected by suction filtration, washed with DMF (50 mL) and acetone (50 mL), in turn, three times at room temperature, and then dried in air (20 mg).

CONCLUSIONS

In summary, 13 3D Zr-MOFs with distinct framework topologies and varied porosities have been constructed. The frameworks consisted of a variety of Zr6 clusters and three classes of ligands, each with different degrees of flexibility, LX1 > LX2 > LX3. The results of the study determined that the five LX1 ligand based Zr-MOFs showed less stability than the other LX2 and LX3 ligand based Zr-MOFs. This was confirmed by a comparison of PXRD patterns and N2 adsorption experiments from before and after water treatment experiments under a dynamic range of pH. Moreover, water cycling adsorption/ desorption experiments revealed that the 13 Zr-MOFs showed a significant difference not only in chemical stability but also in mechanical stability. All of the LX1 ligand-based MOFs lost most of their water sorption performance during the second cycle of water sorption. The water sorption performance of the LX2 ligand-based MOFs gradually decreased in successive water sorption cycles. In contrast to the LX1 and LX2 ligand-based MOFs, the performance of the LX3 ligand-based MOFs was almost completely conserved from cycle to cycle. These results can be correlated to the differences in the flexibility of the ligands and simultaneously suggest a useful ligand-rigidification 10290

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Journal of the American Chemical Society Synthesis of LA1-Zr68-csq (2, BUT-92). A solution of H4LA1 (5 mg, 0.0057 mmol), ZrCl4 (3 mg, 0.013 mmol), and formic acid (0.25 mL, 98%) in 2 mL of DMF was generated and sealed in a 5 mL glass vial. Then the vial was placed into an oven and heated to 100 °C for 12 h followed by cooling to 25 °C at a rate of 0.5 °C/min. The resulting colorless crystals were collected by suction filtration, washed with DMF (50 mL) and acetone (50 mL), in turn, three times at room temperature, and then dried in air (3.5 mg). Synthesis of LA1-Zr612-shp (3, BUT-93). A solution of H4LA1 (5 mg, 0.006 mmol), ZrCl4 (3 mg, 0.013 mmol), and formic acid (0.35 mL, 88%) in 2 mL of DMF was generated and sealed in a 5 mL glass vial. Then the vial was placed into an oven and heated to 100 °C for 12 h followed by cooling to 25 °C at a rate of 0.5 °C/min. The resulting colorless crystals were collected by suction filtration, washed with DMF (50 mL) and acetone (50 mL), in turn, three times at room temperature, and then dried in air (4.0 mg). Synthesis of LA2-Zr612-shp (4, BUT-94). A solution of H4LA2 (25 mg, 0.028 mmol), ZrOCl2 (100 mg, 0.56 mmol), and benzoic acid (3.0 g) in 10 mL of DMF was generated and sealed in a 20 mL glass vial. Then the vial was placed into an oven and heated to 120 °C for 48 h followed by cooling to 25 °C at a rate of 0.5 °C/min. The resulting colorless crystals were collected by suction filtration, washed with DMF (50 mL) and acetone (50 mL), in turn, three times at room temperature, and then dried in air (20 mg). Synthesis of LA3-Zr68-flu (6, BUT-95). A solution of H4LA3 (5 mg, 0.03 mmol), ZrOCl2 (20 mg, 0.11 mmol), and benzoic acid (0.30 g) in 2 mL of DMF was generated and sealed in a 5 mL glass vial. Then the vial was placed into an oven and heated to 120 °C for 48 h followed by cooling to 25 °C at a rate of 0.5 °C/min. The resulting colorless crystals were collected by suction filtration, washed with DMF (50 mL) and acetone (50 mL), in turn, three times at room temperature, and then dried in air (4.0 mg). Synthesis of LB1-Zr68-flu (7, BUT-96). A solution of H4LB1 (8 mg, 0.028 mmol), ZrOCl2 (100 mg, 0.56 mmol), and benzoic acid (3.0 g) in 10 mL of DMF was generated and sealed in a 20 mL glass vial. Then the vial was placed into an oven and heated to 120 °C for 48 h followed by cooling to 25 °C at a rate of 0.5 °C/min. The resulting colorless crystals were collected by suction filtration, washed with DMF (50 mL) and acetone (50 mL), in turn, three times at room temperature, and then dried in air (6.0 mg). Synthesis of LB3-Zr68-flu (10, BUT-97). A solution of H4LB3 (5 mg, 0.03 mmol), ZrCl4 (20 mg, 0.11 mmol), and acetic acid (0.3 mL) in 2 mL of DMF was generated and sealed in a 5 mL glass vial. Then the vial was placed into an oven and heated to 120 °C for 48 h followed by cooling to 25 °C at a rate of 0.5 °C/min. The resulting colorless crystals were collected by suction filtration, washed with DMF (50 mL) and acetone (50 mL), in turn, three times at room temperature, and then dried in air (4.0 mg). Synthesis of LC2-Zr612-fcu (12, BUT-98). A solution of H4LC2 (10 mg, 0.034 mmol), ZrCl4 (20 mg, 0.11 mmol), and formic acid (0.10 mL) in 10 mL of DMF/NMP (N-methyl-2-pyrrolidone) (1.5/ 0.5 mL) was generated and sealed in a 20 mL glass vial. Then the vial was placed into an oven and heated to 120 °C for 48 h followed by cooling to 25 °C at a rate of 0.5 °C/min. The resulting colorless crystals were collected by suction filtration, washed with NMP (50 mL) and acetone (50 mL), in turn, three times at room temperature, and then dried in air (8.0 mg). Sample Activation and Gas Sorption. In each case, the assynthesized samples (about 100 mg) obtained through solvothermal synthesis were immersed in fresh DMF (30 mL) at 60 °C for 24 h to remove all unreacted starting materials. After two additional solvent exchanges in acetone (50 mL) for 24 h each, the samples were then collected and activated at 40 °C under vacuum for 12 h. It should be noted that LA1-Zr68-flu (1) and LB1-Zr68-f lu (7) were first activated by supercritical CO2. The N2 sorption experiments were recorded at 77 K. PXRD Measurements. The activated samples (about 10 mg in each) were immersed in about 3 mL of aqueous solutions in a dynamic range of pH utilizing HCl and NaOH. Each of the solutions were prepared with HCl and NaOH to achieve the ideal pH for the

study. Each sample was immersed in each of the aqueous solutions at room temperature for 24 h. The PXRD patterns were then recorded after collection of the samples by filtration. N2 Sorption Stability Tests. Activated samples of the MOFs (about 100 mg) were immersed in 35 mL of aqueous solution for 24 h. These solutions were prepared in a dynamic range of pH utilizing HCl and NaOH. The samples were then immersed in 200 mL of acetone at 40 °C for 12 h. After filtration and subsequent air drying, the samples were activated at 100 °C for 10 h before the N2 sorption experiments were conducted at 77 K.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b02947.

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Crystallographic data (CIF) Structure refinement, general characterizations (PXRD, FT-IR, TG, N2 isotherms), water adsorption, and additional structural figures (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Lin-Hua Xie: 0000-0003-0017-3887 Ya Chen: 0000-0001-8477-1328 Tao He: 0000-0003-3443-6243 Yong-Zheng Zhang: 0000-0002-2077-296X Jian-Rong Li: 0000-0002-8101-8493 Hong-Cai Zhou: 0000-0002-9029-3788 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (nos. 51621003, 21576006, and 21771012).

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REFERENCES

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DOI: 10.1021/jacs.9b02947 J. Am. Chem. Soc. 2019, 141, 10283−10293

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DOI: 10.1021/jacs.9b02947 J. Am. Chem. Soc. 2019, 141, 10283−10293