Rational Construction of Breathing Metal–Organic Frameworks

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Rational Construction of Breathing Metal−Organic Frameworks through Synergy of a Stretchy Ligand and Highly Variable π−π Interaction Ying Zhang,† Xiao-Qing Meng,‡,§ Hao-Jing Ding,† Xi Wang,‡,§ Mei-Hui Yu,‡,§ Shu-Ming Zhang,*,† Ze Chang,*,‡,§ and Xian-He Bu*,‡,§,∥ †

School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China School of Materials Science and Engineering, National Institute for Advanced Materials, Tianjin Key Laboratory of Metal and Molecule-Based Material Chemistry, Nankai University, Tianjin 300350, China § Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China ∥ State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China

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ABSTRACT: The synergy of a stretchy ligand and highly variable π−π interaction has been proposed as a rational strategy for the construction of breathing metal−organic frameworks (MOFs). Based on this strategy, a breathing MOF, {[Cd2(AzDC)2(TPT)2](DMF)3}n, was successfully constructed with stretchy 4,4′-diazene-1,2-diyldibenzoate acid (H2AzDC) and 2,4,6-tris(4-pyridyl)triazine (TPT) as a source of the π−π interaction. The MOF features structure transformation upon stimulation with solvent guests and varied temperatures, which is straightforwardly characterized by single-crystal structures. Moreover, the solvent-free framework shows breathing behaviors in response to light hydrocarbon (C2H4, C2H6, C3H6, and C3H8) sorption, which was verified by stepwise sorption isotherms and in situ powder X-ray diffraction. Additional investigation of the sorption selectivity of C3/C2 systems indicated that the selectivity can be regulated by the modulation of the dynamic breathing behaviors, which can be used for the selective separation of C3/C2 light hydrocarbons. KEYWORDS: breathing behavior, flexible ligand, π−π interaction, structural transformation, light hydrocarbons separation

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INTRODUCTION As a class of crystalline materials, metal−organic frameworks (MOFs) have a highly designable and tunable nature because of their organic−inorganic hybrid components.1−5 This property leads to their distinct pore shape/size and surface functionality,6−9 which promote their potential applications in adsorption/separation, catalysis, sensing, luminescence, and so forth.10−15 More specifically, the selective separation of light hydrocarbons by MOFs is a hot research topic during recent years for its potential practical applications in national production, for example, C2H4 and C3H6 are the important raw materials for producing polyethylene and polypropylene in petrochemical field. Among the various unique features of MOFs, the stimuliresponsive flexible structure and thus-initiated breathing behavior have attracted much attention, for these characteristics could facilitate the construction of sensors and responsive materials.16,17 Through systematic research, it has been found that the breathing behavior of MOFs can be induced by the structure transformation of secondary building units (SBUs),18 configuration conversion of organic ligands,19 displacement of the interpenetrated subnets,20 changing of the framework topology,21 and so forth. However, it should be noted that © XXXX American Chemical Society

although numerous efforts have been made to investigate breathing MOFs, the number of validated breathing MOFs is very limited. The current state-of-the-art in this field is due to the lack of effective principles and strategies for the targeted construction of breathing MOFs.22,23 The acquirement of breathing behavior in MOFs involves two critical factors, namely, the components and interactions. On the one hand, the structural diversity feature of the components is the foundation of framework transformation. On the other hand, the acquirement of breathing behaviors can be induced by the responses of the framework to the various interactions. In terms of composition, similar performances of specific attributes of inorganic SBUs and organic ligands can be conferred to MOFs to a certain extent.24−26 Therefore, organic motifs showing variable configurations can ideally be introduced to build a breathing framework.27 For example, Barbour et al. have investigated two MOFs based on the conformational isomerism of glutarate, which undergoes spontaneous phase transition behavior.28 With regard to Received: March 17, 2019 Accepted: May 21, 2019 Published: May 22, 2019 A

DOI: 10.1021/acsami.9b04759 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Tensor-27 FTIR spectrometer with KBr pellets in the 4000−400 cm−1 range. The simulated PXRD patterns were calculated based on the crystal data using the Mercury (Hg) program. The elemental analyses were carried out on a vario EL cube elemental analyzer. Synthesis of {[Cd2(AzDC)2(TPT)2](DMF)3}n (1a). Cd(NO3)2· 6H2O (17.2 mg, 0.05 mmol), H2AzDC (13.5 mg, 0.05 mmol), and TPT (15.6 mg, 0.05 mmol) were dispersed with N,N-dimethylformamide (DMF, 3 mL) in a 10 mL glass vial. The vial was then sealed and incubated at 120 °C for 1 day. Red block crystals were obtained and collected by filtration (56% yield based on the TPT). IR (KBr, cm−1): 2990 (w), 1673 (m), 1591 (m), 1518 (s), 1391 (s), 1373 (s), 1254 (m), 1091 (w), 1061 (w), 1012 (w), 865 (m), 798 (m), 733 (w), 667 (w), 546 (w), 510 (w). Crystal Structure Determination. The crystal structure diffraction data were collected at 120 or 298 K on a Rigaku XtaLAB PRO MM007 DW diffractometer with Cu Kα radiation (λ = 1.54178 Å). The crystal structures were solved and refined using the OLEX2 program suite. Direct methods yielded all nonhydrogen atoms, which were refined with anisotropic thermal parameters. All hydrogen atom positions were calculated geometrically and were riding on their respective atoms. The guest molecules in the framework were highly disordered, and thus, their contribution to the diffraction was removed by the SQUEEZE command in the PLATON software. Gas Sorption Measurements. The gas sorption isotherms were measured with a Micrometrics ASAP 2020 M volumetric gas adsorption analyzer. The as-synthesized sample was degassed in the sample tube for 3 h at 200 °C, and over 100 mg of the desolvated sample was used for the sorption measurements. Adsorption measurements were performed with N2 at 77.4 K and CO2 at 195 K, respectively. The isotherms of C2H4, C2H6, C3H6, and C3H8 were measured at 273 and 298 K, respectively.

interaction, coordination and supramolecular interactions (hydrogen bonding, π−π interaction, etc.), which are highly responsive to the environment of the components, have been shown to play a dominant role in the breathing behavior of MOFs.29−33 Considering the two points mentioned above, the rational design and combination of proper components and interactions could be a promising method for the targeted construction of breathing MOFs. Based on the above-mentioned assumption, we have made continuous efforts to investigate the proper ligands and interactions for the construction of breathing MOFs. Here, we report the construction and breathing behaviors of a MOF, namely, {[Cd2(AzDC)2(TPT)2](DMF)3}n (1a) (H2AzDC = 4,4′-diazene-1,2-diyldibenzoate acid, TPT = 2,4,6-tris(4pyridyl)triazine), obtained based on the combination of flexible ligand and π−π interaction (Scheme 1). In 1a, the Scheme 1. Construction of Breathing MOF Based on the Combination and Synergy of Stretchy Ligand and π−π Interaction

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RESULTS AND DISCUSSION Construction and Structure of the MOF. As mentioned in the introduction section, we have been trying to achieve and modulate the breathing behaviors of MOF through the combination of components with variable configurations and stimulus−response interactions. As a versatile moiety, azobenzene has caught our attention because of its variable configurations. In addition, because of its optically induced structural transformation properties, azobenzene-based ligands have been widely used for the construction of light-responsive MOFs.36−38 Besides the ability to induce the dynamic behavior, the variable distance between the substitution sites on the benzene rings during the cis−trans transformation of the moiety could also adapt the deformation of the framework. Accordingly, H2AzDC was selected for the construction of flexible MOF. Also, with the aim of inducing dynamic behavior, our study has been focused on triazine-based π−π interaction. The π−π interaction between aromatic moieties is widespread and highly variable from a structural perspective. The relative position and distance between the interacting moieties could vary in a wide range in response to the environment, which make them very promising for the construction of flexible MOFs. Specific to the triazine, its highly symmetrical N-heterocyclic structure makes it a typical electron-deficient π system,39,40 which could further promote the stacking assembly of the interactive moieties.41−43 On the basis of our previous investigations, we have found that TPT is an ideal ligand for the introduction of the π−π interaction into MOFs,44−48 and the resulting MOF exhibits a flexible supramolecular framework based on the highly variable π−π interaction. Therefore, in this study, TPT is introduced with the aim of achieving the breathing behavior. When H2AzDC and TPT were allowed to assemble with Cd(II) ions under solvothermal conditions, compound 1a was

stretchy backbone of AzDC serves as a “spring” holder to modify the steric hindrance in the free space, while the π−π interactions between the stacking TPT vary synergistically to acquire the breathing behaviors in response to the uptake/ removal of the guests of the two-fold interpenetrated porous framework.34 Accordingly, 1a displays versatile reversible structural transformations under the stimulation with different temperatures and guest molecules, which is straightforwardly characterized and verified by single-crystal X-ray diffraction (SCXRD) and in situ powder X-ray diffraction (PXRD) analysis. In addition to the successful construction of 1a, its performances in the sorption and selective separation of light hydrocarbons were also evaluated. The results showed that the breathing behaviors of 1a and resulting structural transformations could be triggered by the sorption of light alkanes, which have a significant impact on the gas sorption selectivity of the MOF for potential separation applications.

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EXPERIMENTAL SECTION

Material and General Methods. All of the reagents and solvents, except for TPT, were commercially available and used without further purification. TPT was synthesized according to a reported method.35 In situ PXRD analysis of the samples was performed on a Rigaku SmartLab diffractometer with Cu Kα radiation (λ = 1.5418 Å), equipped with a TTK 600 low-temperature chamber. In addition to the combined control unit attached to control the temperature, the chamber was connected with a turbo vacuum pump, gas injection line, and pressure detector to provide a controlled environment for the sample. The thermogravimetric analysis (TGA) was performed in an air atmosphere at a temperature ranging from room temperature to 800 °C with a rate of 10 °C/min on a Rigaku TG8121 thermal analyzer, using an empty Al2O3 crucible as the reference. Fourier transform infrared (FTIR) spectra were recorded using a Bruker B

DOI: 10.1021/acsami.9b04759 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces obtained. The structure of 1a was determined by the SCXRD analysis at room temperature. Compound 1a crystallizes in the P1̅ space group. The asymmetric unit of 1a is composed of one binuclear Cd(II) SBU, two TPT, and two AzDC ligands (Figure 1a). The Cd(II) center exhibits a pentagonal

Figure 2. (a) Two-fold interpenetrated structure of 1a. Structure detail of the relatively variable π−π interactions of the (b) triazine with triazine (T−T) and (c) triazine with pyridine (T−P) modes. (d) Adjustable dihedral angle and length of AzDC ligand.

red box of Figure 1b) with the T−T and T−P modes suggests the potential for variable π−π interaction distances between the motifs. On the other hand, it should be noted that both the a and b distances are affected by the distance between the Cd(II) centers, which is mainly defined by the configuration of the AzDC ligand (Figure 2a). Because of the rotation of the C−N and NN bonds (Figure 2d), the dihedral angles between the benzene rings of AzDC are 33.75° and 45.68°, which result in the lengths of 11.64 and 11.90 Å, respectively. Considering the stretchable configuration of the AzDC ligand in the MOF compared with its trans state, the length of the ligand could be further extended with proper stimulus, which will impact the distance between the Cd(II) centers accordingly. In that situation, the variable π−π stacking distances (a and b) could buffer the impact on the whole framework to produce in multiple stable states with distinct pore structures and porosities. From this perspective, the breathing behavior of 1a could be achieved. Variation of the Breathing Behavior by SCXRD. To straightforwardly verify the breathing behavior of 1a, its crystal structure was determined in different conditions. A single crystal of sufficient quality was selected for SCXRD analysis at room temperature (298 K for 1a) and cryogenic temperature (120 K for 1′a), to determine whether the breathing behavior of 1a could be triggered by different temperatures. In addition, to examine the impact of guest species, which has been evaluated as another critical factor that could induce the breathing behavior, the as-synthesized crystal of 1a was immersed in dichloromethane (DCM) or acetone (CP) for solvent exchange and further structure characterized. Additionally, the crystal structures of the MOFs with DCM and CP as guests were also determined at 298 and 120 K and were denoted as of 1b/1′b and 1c/1′c, respectively. These findings benefit the detailed investigation of the structure and variation of the temperatures and guest-dependent breathing behaviors. The refinement details of all the crystal structures are summarized in Table S1. Generally, the cell parameters reveal obvious temperature and guest-dependent features, which indicate that temperature and guest could be effective triggers for breathing behaviors. In detail, the most significant changes in these structures are exhibited on distorted AzDC ligands (or length regulation) as well as relatively variable π−π

Figure 1. (a) Coordination environment of the Cd(II) ions and ligands in 1a; (b) perspective view of the framework of 1a along a axis that shows 3D framework with 1D channels; (c) two-fold interpenetration topology of 1a. (d) Two-fold interpenetrated substructure in distinct colors (purple and green), the linkage of AzDC is presented as stick, a and b represented the π−π interactions of T−P and T−T modes, respectively. Hydrogen atoms have been omitted for clarity.

bipyramid coordination geometry, with five oxygen atoms from three carboxylic groups of the three AzDC ligands at the equatorial plane and two N atoms from the two TPT ligands at the axial positions. The Cd−O bonds (2.290−2.438 Å) and Cd−N bonds (2.315−2.338 Å) are all in the normal range. For each AzDC ligand, one of the carboxylic groups adopts the chelating mode to coordinate with one Cd(II) ion, while the other adopts the chelating bridging mode to coordinate with two Cd(II) centers. Through the bridging of carboxylic groups, two Cd(II) centers are bonded into a binuclear SBU, which connects AzDC ligands into two-dimensional (2D) layers. The 2D layers are further interconnected by TPT ligands, as bidentate pillars, into a three-dimensional (3D) network. Two individual networks interpenetrate to form the whole structure of 1a (Figure 1b). Then, the whole structure could be simplified into a two-fold interpenetrated pcu topology (Figure 1c). There are also rectangular channels (17.187 Å × 12.860 Å) defined by the layers and pillars, which are occupied by solvent guests. Furthermore, the structure details of 1a were investigated to evaluate its potential as a breathing MOF. For each TPT ligand, only two of the three pyridine groups coordinate with Cd(II) ions, while the left one stacks with the triazine motif of the neighboring TPT ligands from the same set of network. The distance between the motifs is 3.35 Å (defined as a in Figure 1d), indicating a relatively strong π−π stacking interaction (defined as T−P, Figure 2c). Moreover, the triazine motifs from a distinct set of network also exhibit the π−π stacking interaction (defined as T−T, Figure 2b) with a distance of 3.31 Å (defined as b in Figure 1d). Overall, the formation of the stacking unit module (the part inside the dark C

DOI: 10.1021/acsami.9b04759 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces interactions (Table S2). For the supramolecular π-system interaction, the stacking interactions between triazine units (T−T) have been researched by theoretical calculation and can be evaluated through these parameters of the distance between centroids (R), relative rotation angle (β), and displacement (r).41 Similarly, the triazine−pyridine (T−P) packing mode could be evaluated in the same way. Along with the drop in temperature, one of the dihedral angles of AzDC ligands for 1a and 1′a changed from 33.75° to 35.05°, which directly affected the π-system interaction of a (T−P) following the switch range R = 4.17 Å, d = 3.35 Å, r = 2.48 Å, β = 0° to R = 4.31 Å, d = 3.23 Å, r = 2.85 Å, β = 0°. In the meantime, the other dihedral angles of AzDC ligands and the associated interaction of the b (T−T) synchronously change, which changed from 45.68° and R = 3.51 Å, d = 3.31 Å, r = 1.17 Å, β = 60° to 47.68° and R = 3.51 Å, d = 3.25 Å, r = 1.33 Å, β = 60°. These correlative behavior modes fit our expectation well, and lead to the displacement of the networks and convert the rectangle channels to squarer channels. Together with the structure transformation, the porosity of the framework calculated by PLATON also changed from 32.7 to 31.5%. On the other hand, the distinct cell parameters (Table S1) and π−π interaction modes (Table S2) of 1b and 1c from that of 1a suggest the impact of guests on the framework structure. The porosity of 1b and 1c are 29.3 and 28.6%, which are lower than that of 1a. Compared with that of 1′a, the porosity of 1′b and 1′c shows a reversely increasing trend (29.8% for 1′b and 30.3% for 1′c), which is also induced by the breathing behavior of the framework (Figure 3, Tables S1 and S2). In short, 1a could display highly flexible and reversible dynamic breathing behaviors under the external stimuli of different temperatures and guest solvents. Breathing Behavior in Response to Gas Sorption. The porosity nature of the MOF motivated us to investigate its performances in gas sorption and separation. In general, the guest-responsive breathing behavior of the MOF is expected to affect its gas sorption performance. The TGA results indicated that the solvent guest molecules can be completely removed from 1a when the temperature is above 200 °C (Figure S3). Accordingly, the sample was fully activated under vacuum at 200 °C to obtain the 1′. In situ PXRD analysis of 1′ under vacuum revealed a different pattern compared with that of 1a, which indicated that the removal of guest can also induce the breathing behavior. It should be noted that the pattern somewhat matches that of 1b, demonstrating the similarity of their skeleton structure (Figures S4 and S5). For detailed understanding of the structure of 1′, the low-angle diffraction region between 11° and 16° was chosen for comparison. Comparison of the pattern of 1′ with that of 1b revealed that the peak of (a), corresponding to (0, 2, 2) crystallographic plane and that of (c), identical with (2, 0, 0), had shifted to a low angle, indicating that the layer distance increased along these planes. Therefore, the rectangular aperture was enlarged along two diagonal lines and changed from a narrow pore (np) to a wide pore (wp). The peak of (h), which originated from the (0, 2, 4) plane, shifted to a low angle, indicating the extent of the AzDC and compression of the π−π interaction of a, which in contrast to the compression of the ligand in the mode b is manifested by the shifting of the (j) peak to a high diffraction region. Based on the identification of the breathing behavior under vacuum conditions, gas sorption experiments were performed with N2 at 77.4 K and CO2 at 195 K, respectively (Figures S6

Figure 3. Temperature- and guest-responsive breathing behavior of MOF. For each state, the top figures show the 2D layer composed of SBUs and AzDC ligands, and the bottom pictures show the two-fold interpenetrated framework structures. The induction solvents for 1a/ 1′a, 1b/1′b, and 1c/1′c are DMF, DCM, and CP, respectively. All images are viewed along c axis. Hydrogen atoms have been omitted for clarity in all of the figures.

and S7). The saturation adsorption amount of N2 and CO2 was 159 and 150 cm3/g, respectively, and based on the N2 isotherm, the Brunauer−Emmett−Teller and Langmuir surface areas were calculated to be 392 and 433 m2/g, respectively. More than the variation of the porous nature of the MOF, the stepwise isotherm between the sorption/desorption isotherms further confirm the occurrence of the breathing behavior in response to increasing pressure.49 It should be noted that the experiments with N2 and CO2 reveal distinct sorption behaviors, which might be attributed to the different size of the gas molecules and distinct affinity between the gas molecules and porous framework, that triggered the diverse breathing response of the framework. For better understanding of the structure change of MOF during the sorption process, in situ PXRD has been measured during the adsorption− desorption process of CO2 at 195 K (Figure S6). The results show that the patterns mainly shifted to the low-angle direction accompanied with the increased pressure, which implied the framework extension during the adsorption of CO2. In addition, a dramatic change of pattern was observed around 10 and 80 kPa, respectively, indicating phase change around these pressures. Furthermore, the patterns obtained during the desorption process show that the structure transformation is reversible. All these results consistent with the stepwise D

DOI: 10.1021/acsami.9b04759 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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behavior during the sorption processes. Instead, only the isotherms of C3H6 and C3H8 reveal a stepwise profile at 298 K. This can be attributed to the relatively higher affinity of C3 hydrocarbons with the MOF compared with that of C2 hydrocarbons that induce the breathing behavior of the MOF, which is clearly demonstrated by the distinct initial heat of adsorption calculated from the isotherms (initial Qst = 42 kJ/ mol for C3H6, 41 kJ/mol for C3H8, 31 kJ/mol for C2H4, and 34 kJ/mol for C2H6). These results further prove that the breathing behavior of the MOF was induced by hydrocarbons. Moreover, the Qst profiles of C3 species show an increase− decrease trend. The rising profile in the low pressure range indicates a strong interaction between the framework and gas molecules. However, with the loading of the gas, the interaction between the guest molecules begins to work, which should be relatively low than that with the framework, that caused the decreased trend at higher pressure.56,57 (Figures S8 and S9). To characterize the breathing behavior more straightforwardly, in situ PXRD was used to monitor the dynamic process in response to the sorption of hydrocarbons. Herein, three terms were proposed to describe the dynamic behavior, that are breathing behavior, phase transformation, and structural transformation. Breathing behavior is a process in which the framework is gradually changed in response to external stimuli. During the process, a relatively stable phase of the structure could be achieved, and the change from one phase to another is called phase transformation. Both breathing behavior and phase transformation are structural transformation. With the increase of the pressure and corresponding gas loading, the diffraction pattern reveals obvious changes, including the shift, disappearance, and emergence of peaks, because of the breathing behavior and phase transformation. Generally, the structure transforms from the shrunken state to the expanded state along with the loading of gases, while the process details are gas and pressure dependent. For ethylene (Figure 5a,c), the pattern recorded at 273 K and under 20 kPa was almost

adsorption−desorption isotherms well, further proving the structure flexibility of the framework. Light hydrocarbons are important raw materials for industrial and energy applications, whose storage and separation have attracted much attention.50−55 On the basis of the breathing behavior of the MOF in response to distinct guest molecules, we assumed that the breathing behavior can be utilized for the modulation of the C3/C2 separation performances of the MOF. To prove our assumption, adsorption measurements were performed for C2H4, C2H6, C3H6, and C3H8 at 273 and 298 K, respectively (Figure 4a,b).

Figure 4. Adsorption isotherms of C2H4, C2H6, C3H6, and C3H8 at (a) 273 and (b) 298 K; the IAST selectivity of each pair of gases in 50%/50% at (c) 273 and (d) 298 K.

The gas uptakes were normal and moderate, considering the porosity of the MOF (Table S3). As shown in Figure 4a, the isotherms of all four kinds of gases reveal obvious stepwise profile at 273 K, indicating the occurrence of breathing

Figure 5. (b,e) Adsorption and desorption isotherms of C2H4 at 273 and 298 K; (a,c) and (d,f) in situ PXRD patterns collected during the adsorption and desorption process at 273 and 298 K under different pressures: (1) under vacuum, (2) 20, (3) 40, (4) 60, (5) 80, and (6) 100 kPa, respectively. E

DOI: 10.1021/acsami.9b04759 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. (b,e) Adsorption isotherms of C3H8 at 273 and 298 K; (a,c) and (d,f) in situ PXRD of adsorption and desorption at 273 and 298 K under different pressures: (1) under vacuum, (2) 5, (3) 20, (4) 40, (5) 60, (6) 80, and (7) 100 kPa, respectively.

Figure 7. (b,e) Adsorption isotherms of C2H6 at 273 and 298 K; (a,c) and (d,f) in situ PXRD of adsorption and desorption at 273 and 298 K under different pressures: (1) under vacuum, (2) 20, (3) 40, (4) 60, (5) 80, and (6) 100 kPa, respectively.

completely left shifted compared with that of the initial state under vacuum. Structurally, these results indicate the expansion of AzDC ligands and decrease of the π−π interaction distances of a and b. Both responses will result in the expansion of the framework. There might be a coexistence of different phases under 40 kPa because the reflection at 12.50° to 13.50° began to differ to some extent, except for that most shifted to the lower diffraction region. When the pressure increased to 60 kPa, a new reflection appeared at 13.00° compared with that at 40 kPa, and the peak splitting behavior arose for the reflection at 13.75°. These results clearly indicate that the phase transformation occurs over a certain pressure range. In addition, the in situ PXRD patterns recorded under 80 and 100 kPa matched well with that collected under 60 kPa,

indicating that the framework achieved a relatively stable state under 60−100 kPa. In particular, the structural transformation process monitored by in situ PXRD perfectly matches the stepwise adsorption isotherm (Figure 5b), which further proves the occurrence of the breathing behavior and phase transformation in the adsorption process. Moreover, the adsorption and desorption isotherms match well at specific pressure points, indicating the reversibility of the structure transformation. The adsorption and desorption of ethylene at 298 K were also characterized by in situ PXRD analysis (Figure 5d,f). Though no obvious stepwise isotherms were observed (Figure 5e), the in situ PXRD patterns clearly showed the occurrence of breathing behavior. Along with the increase of the gas uptake, gradually, the reflections slightly shifted to F

DOI: 10.1021/acsami.9b04759 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 8. (b,e) Adsorption isotherms of C3H6 at 273 and 298 K; (a,c) and (d,f) in situ PXRD of adsorption and desorption at 273 and 298 K under different pressures: (1) under vacuum, (2) 20, (3) 40, (4) 60, (5) 80, and (6) 100 kPa, respectively.

selectivity at 298 K shows a decrease trend with a relatively high value at high pressure, and a turning point at 20 and 40 kPa for C3H8 and C3H6, respectively. However, it should be noted that the partial pressures of C3 species (10 kPa for C3H8 and 20 kPa for C3H6) at the turning points are quite similar to the pressure required for the phase transformations observed during in situ PXRD experiments. This is reasonable because the C3 species have stronger affinity for the framework, which should be dominant in the triggering of structure transformation. Therefore, the drastic change of selectivity should be attributed to the structure transformation of the framework triggered by the sorption of the C3 species guests. More importantly, these results show that the selectivity of MOFs can be regulated through their stimulus−response dynamic behavior, which could be a promising method for the modulation of their separation performances.

lower diffraction angles without the disappearance of the original peaks and emergence of new ones. This implies that pore changed from the original state to an open state during the gradual adsorption process. Such distinct breathing behavior, compared with that observed at 273 K, can be due to the increased temperature that caused the initial state of the framework to be a relatively stable state. As a result, no obvious phase transformation will occur except for the slight breathing of the framework structure in response to the uptake of guest molecules. The in situ PXRD patterns recorded under other hydrocarbons atmosphere (Figures 6−8) reveal similar framework breathing behavior and phase transformation process. For propane (Figure 6), the phase transformation at 273 K took place at 20 kPa, and it showed a similar transformation process at 298 K. For ethane, the phase transformation was observed under 60 kPa at 273 K, whereas only breathing behavior was observed at 298 K with an increased gas uptake (Figure 7). In contrast, the framework was highly sensitive to propylene, and its complete phase transformations were observed under 20 kPa at 273 K and 40 kPa at 298 K, respectively (Figure 8). All these findings suggest the flexibility of the MOF. In addition, the separation properties of the MOF toward C3/C2 hydrocarbons were evaluated using the ideal adsorbed solution theory (IAST) method, with the aim of illustrating the impact of the breathing behavior on the separation performances of the MOF. The selectivity was calculated on the basis of a 50%/50% two-gas components at 273 and 298 K, respectively. Because of the stepwise isotherms, we also adopted piecewise data treatment. As shown in Figure 4, the separation calculation of alkene/alkane shows that the selectivity is general, but the pressure-dependent selectivity for C3/C2 systems are remarkable. At 273 K, the initial selectivity for the C3H8/C2H4, C3H8/C2H6, C3H6/C2H4, and C3H6/C2H6 systems are 14.28, 8.52, 15.67, and 9.48, respectively (Table S4). The selectivity shows a decreasing trend in the low-pressure range before 40 kPa. When the pressure is higher than 40 kPa, the selectivity shows a sharp increase to a relatively stable value. In contrast, the C3/C2

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CONCLUSIONS

In summary, here, we proposed a promising method for the construction of breathing MOFs based on the rational combination of flexible components and variable interactions. Based on this method, the strechy ligand H2AzDC, acting as flexible component, and triazine-based π−π interaction of TPT, serving as variable supramolecular interaction, were introduced into the targeted framework, and a MOF (1a) with breathing behavior was successfully constructed. The structure analysis showed that the π−π interactions of TPT have potential variability and are affected by the configuration of AzDC. Given these dynamic mechanisms, 1a undergoes reversible structural transformations upon stimulation with different temperatures and solvent guest molecules. Additionally, SCXRD straightforwardly determined these transformed structures as 1′a, 1b, 1′b, 1c, and 1′c. In addition, the C3/C2 light hydrocarbon adsorption isotherms and in situ PXRD along the gases loading process of 1′ established that the breathing behavior and resulting structural transformation can be triggered by the sorption of light hydrocarbons, which has a G

DOI: 10.1021/acsami.9b04759 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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significant impact on the gas sorption selectivity of the MOF for potential separation applications.

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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/acsami.9b04759. Full experimental details, including crystal data, crystal structures, TGA profiles, PXRD patterns, FTIR spectra, and adsorption data (PDF) Crystallographic data of 1a (CIF) Crystallographic data of 1′a (CIF) Crystallographic data of 1b (CIF) Crystallographic data of 1′b (CIF) Crystallographic data of 1c (CIF) Crystallographic data of 1′c (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.-M.Z.). *E-mail: [email protected] (Z.C.). *E-mail: [email protected] (X.-H.B.). ORCID

Xi Wang: 0000-0002-2786-6985 Shu-Ming Zhang: 0000-0002-8547-498X Ze Chang: 0000-0002-3865-2165 Xian-He Bu: 0000-0002-2646-7974 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (21531005, 21875115, and 21671112), the Programme of introducing Talents of Discipline to Universities (B18030), and the Fundamental Research Funds for the Central Universities (Nankai University, grant 63191435).

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REFERENCES

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DOI: 10.1021/acsami.9b04759 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.9b04759 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX