Comparative Study on Temperature-Dependent CO2 Sorption

Jan 8, 2018 - The asymmetric unit consists of a half Cu(II), a half bpdado ligand, a half bpa/bpe ligand, and three lattice water molecules. ..... (18...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Comparative Study on Temperature-Dependent CO2 Sorption Behaviors of Two Isostructural N‑Oxide-Functionalized 3D Dynamic Microporous MOFs Wen-Qian Zhang,†,∥ Rui-Dong Wang,†,∥ Zi-Bo Wu,† Yi-Fan Kang,‡ Yan-Ping Fan,† Xi-Qiang Liang,§ Ping Liu,*,† and Yao-Yu Wang† †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, People’s Republic of China ‡ College of Chemistry & Chemical Engineering, Shaanxi University of Science and Technology, Xi’an 710021, People’s Republic of China § Department of Laboratory Medicine, Xi’an Children Hospital, Xi’an, Shaanxi 710003, People’s Republic of China S Supporting Information *

ABSTRACT: By functionalization of the achiral carboxylate-based pyridine-N ligand 2,2′-bipyridine-3,3′-dicarboxylate (H2bpda) with N-oxide groups, the axially chiral ligand 2,2′-bipyridine-3,3′-dicarboxylate 1,1′-dioxide (H2bpdado) has been obtained. On the basis of H2bpdado and auxiliary N-donor ligands, two isostructural 3D dynamic porous Cu(II) metal−organic frameworks (MOFs), {[Cu0.5(bpdado)0.5(L)0.5]·3H2O}n (L = 1,2-bis(4-pyridyl)ethane (bpa), trans-1,2bis(4-pyridyl)ethene (bpe) for 1 and 2, respectively), have been synthesized, which contain N-oxide “open donor sites” (ODSs) and carboxyl sites on the pore surfaces. The modification of pyridine-N into the N-oxide group not only transforms the nonporous structure into a porous framework but also endows the N-oxide group with unique charge-separated plus electron-rich character, which may provide an enhanced affinity toward CO2 molecules. Interestingly, both 1 and 2 present reversible structural transformation upon dehydration and rehydration. The adsorption properties of 1 and 2 have been investigated by N2, H2, CH4, and CO2 gases, and they reveal evident adsorption for CO2 and CH4. Both MOFs have high CO2 uptake, CO2 sorption affinity, and sorption selectivities of CO2 over CH4 and N2. Remarkably, 1′ and 2′ exhibit intriguingly comparable temperature-dependent CO2 sorption behaviors that can probably be attributed to the difference in bpa and bpe. First, at 195 K, 1′ and 2′ exhibit stepwise adsorption and hysteretic desorption behavior for CO2, but in the second step, the isotherms of 2′ display a starting pressure greater than that of 1′. Then, at 298 K, their CO2 isotherms all show nonclassical type I adsorption, while peculiarly, at 273 K, the CO2 isotherm of 1′ still exhibits uncommon stepwise adsorption but that of 2′ does not. Thus, these temperaturedependent CO2 sorption behaviors indicate that there exist different threshold temperatures and pressures of channel expansion for 1′ and 2′.

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narrow-pore and large-pore phases upon CO2 adsorption.3 For the large-pore MOFs, which could effectively attain high capacity of CO2 usually at higher pressures, they may not be such effective at lower pressures, and they also would show less CO2 selectivity upon pore size alone.4 Thus, the exploitation of CO2-triggered dynamic porous MOFs with high CO2 selectivity and CO2 adsorption capacity at ambient temperature and lower pressure would be a significant and effective strategy to capture CO2 molecules.5 In addition, another strategy to develop porous MOFs with better CO2 adsorption is the modification of the pore surface by introduction of functional groups (such as −CF3, −NH2,

INTRODUCTION The efficient capture and separation of CO2 from air under atmospheric pressure has become a significant issue for both industry and science due to its industrial relevance and scientific challenge. In this regard, porous metal−organic frameworks (MOFs), as a kind of newly developing materials,1 have exhibited great promise for CO2 capture and separation because of their designable constructions and facile pore functionalization, in comparison to other conventional zeolites and activated carbon.2 More recently, flexible or dynamic porous MOFs have attracted special attention for CO2 adsorption due to their uncommon properties.2c,d In particular, when this exclusive class of materials displays CO2-triggered “breathing effect” or “gate opening” behaviors, they can selectively respond to CO2 molecules and present a reversible structural transition between © XXXX American Chemical Society

Received: November 8, 2017

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DOI: 10.1021/acs.inorgchem.7b02844 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) Coordination environment of the Cu(II) ion in 1. Symmetry codes: (A) −x + 1/2, −y + 1/2, −z; (B) −x, y, −z − 1/2. Color codes: gray, C; red, O; blue, N; cyan, Cu. (b) 1D Cu-(R,S)-bpdado chain. (c) 3D porous structure with 1D diamond channels. (d) Schematic view of the 3D 2,4-connected network topology. The yellow nodes represent the Cu(II) ions, and the plum nodes are bpdado. The hydrogen atoms have been omitted for clarity.

−OH, −COOH) or generation of exposed framework atoms (such as exposed N sites and “open metal sites”).2a,6 It has been proven to allow for polarity and acidity tuning of the porous environment and thus offer higher affinity toward CO2 to improve CO2 adsorption capacity and selectivity. Among these strategies, one such efficient approach is to create so-called “open donor sites” (ODSs). Jiang and Su7 have realized ODSs through the introduction of an N-oxide group into a T-shaped pyridine-N ligand. The resulting O donor has one more lone pair of electrons in comparison to the pyridine-N donor and the O and N atoms of the N-oxide group have opposite charges, which could interact with the electrophilic atom of a guest molecule. Such particularly charge separated and electron rich features may promote the affinity toward CO2 to match its distinct C and O electrophilicity (Figure S1 in the Supporting Information). Hence, the N-oxide donor might provide an attractive type of coordination site to capture CO2. Given that, in this work, we propose an effective cooperative method of functionalization and flexibility for porous MOFs to promote CO2 capture and separation process. First, we introduce a N-oxide group into the prepared achiral 2,2′bipyridine-3,3′-dicarboxylate (H2bpda) ligand to form an axially chiral 2,2′-bipyridine-3,3′-dicarboxylate 1,1′-dioxide (H2bpdado) ligand (Figure S2 in the Supporting Information).8 Then, given the axially chiral H2bpdado ligand with hard rotation between two pyridine rings, we attempt to employ some flexible auxiliary ligands to endow frameworks with dynamic characteristics. Ultimately, we have successfully obtained two isostructural N-oxide-functionalized 3D micro-

porous frameworks, {[Cu0.5(bpdado)0.5(L)0.5]·3H2O}n (L = 1,2-bis(4-pyridyl)ethane (bpa), trans-1,2-bis(4-pyridyl)ethene (bpe) ligands for 1 and 2, respectively), from the cooperative assembly of N-oxide-functionalized H2bpdado and flexible bpa/ bpe ligands, which have ODSs and uncoordinated carboxylic acid sites on the pore surfaces. Interestingly, both 1 and 2 present reversible structural transformation upon dehydration and rehydration. The sorption properties of 1 and 2 have been investigated by N2, H2, CH4, and CO2 gases, and they reveal adsorption for CO2 and CH4. Both MOFs have high CO2 uptake, CO2 sorption affinity, and sorption selectivities of CO2 over CH4 and N2. Remarkably, though the isostructural 1 and 2 have the same topology net, they exhibit comparable temperature-dependent CO2 sorption behaviors. Given the rigid axially chiral H2bpdado ligand, the distinct CO2 sorption behaviors of 1 and 2 probably can be attributed to the structural difference of bpa and bpe ligands. At 195 K, 1′ and 2′ exhibit stepped adsorption and hysteretic desorption behavior for CO2, while the starting pressure of the second step in 1′ is much smaller than that of 2′, though the first steps of 1′ and 2′ are the same type I isotherms. In addition, at 298 K, both CO2 isotherms of 1′ and 2′ show nonclassical type I adsorption isotherms, while particularly, at 273 K, the isotherm of 1′ still exhibits uncommon stepwise adsorption, but that of 2′ does not. Thus, the temperature-dependent CO2 sorption behaviors indicate the different threshold temperatures and pressures of channel expansion for 1′ and 2′. B

DOI: 10.1021/acs.inorgchem.7b02844 Inorg. Chem. XXXX, XXX, XXX−XXX

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EXPERIMENTAL SECTION

The Cu1−O1 (1.969(5) Å for 1, 1.973(5) Å for 2) and Cu1− N2 (1.977(5) Å for 1, 1.992(5) Å for 2) bond distances are similar to those in previously reported works.11 The bpdado ligands, showing R and S configurations (Figure S3 in the Supporting Information), take a bis-monodentate mode to link the Cu(II) atoms, forming 1D mesomeric chains (Figure 1b). Then, through the bpa/bpe ligands, each chain can connect four adjacent chains in different directions to generate the 3D porous networks which possess open 1D diamond channels (Figure 1c and Figures S4 and S5 in the Supporting Information) with dimensions of 4.61 × 4.61 Å2 and 4.85 × 4.85 Å2 for 1 and 2, respectively (excluding van der Waals radii of the atoms). PLATON analyses12 reveal that 1 and 2 possess solvent-accessible volumes of 21.7% and 25.9% per unit cell volume, respectively, after omitting solvent molecules. Topologically, the Cu center could be taken as a 4-connected node and the bpdado ligand can be simplified as a 2-connected node, which can be reduced to a binodal 2,4-connected 3D structure with the point symbol {105.12}{10} (Figure 1d). Effect of N-Oxide Groups. Through the functionalization of an achiral pyridine-N-dicarboxylate ligand (H2bpda) with Noxide groups, we have obtained a C2 axially chiral pyridine Noxide dicarboxylate ligand (H2bpdado) and synthesized two isostructural 3D porous MOFs. In order to shed more light on the effect of the functionalization from pyridine-N to pyridine N-oxide groups, another analogous 3D Cu(II) MOF, [Cu0.5(bpda)0.5(4,4′-bipy)0.5]n (3), with the same topological type as 1 and 2, has been synthesized on the basis of H2bpda by hydro-/solvothermal methods and conditions similar to those of 1 and 2.13 Similar to the coordination environment and mode of 1 and 2, the asymmetric unit in 3 consists of a half Cu(II), a half bpda ligand, and a half 4,4′-bipy ligand. Each Cu(II) center has a square-planar geometry with two pyridine N atoms from two different 4,4′-bipy and two carboxylate O atoms from two different bpda ligands; bpda ligands act as bismonodentate ligands to link the Cu(II) atoms into 1D mesomeric chains which are bridged by the N-donor 4,4′bipy ligands in different directions to generate the 3D framework. In 1 (2) and 3, though the bridging N-donor auxiliary ligand and the square-planar geometry of the Cu(II) center are similar, the structural differences between MOFs 1 (2) and 3 are also very evident. (1) In contrast to the dihedral angle between the two pyridine rings of bpda in 3 being 77°, that of axially chiral bpdado is 89° in 1 (2), almost perpendicular with each other (Figure 2a,c). (2) The dihedral angle between carboxylate groups and the linked pyridine rings is 31° in 3, much larger than that of 10° for 1 (2) (Figure 2a,c). (3) The Cu···Cu distance within the 1D chains is 5.713 Å for 3, much smaller than that of 7.243 Å for 1 and 7.119 Å for 2, respectively. (4) Except for the bridging N-donor auxiliary ligands, the interchain linkages in 1 and 2 are also assisted by π−π stacking between the parallel pyridine rings (the centroid distances are 3.719 Å for 1 and 3.849 Å for 2, and the vertical distances are 3.373 Å for 1 and 3.415 Å for 2) and Cpyridine−H···ON‑oxide‑group hydrogen bonds (3.105 Å and 128° for 1 and 3.214 Å and 129° for 2) (Figure S6 in the Supporting Information). The N-oxide group has the capability of hydrogen bonding, which could stabilize the full framework and enrich supramolecular interactions.14 However, for 3, without an N-oxide group, there only exist Cpyridine−H···Cpyridine (3.499 Å and 129°) hydrogen bond interactions between the parallel pyridine rings (the vertical distance is 1.399 Å) and the uncoordinated

Materials and Methods. All solvents and starting materials for synthesis were purchased commercially and used without further purification. The method of H2bpdado preparation from H2bpda was according to the literature.8 The Fourier transform infrared (FT-IR) spectra (4000−400 cm−1) were performed with KBr pellets on a Bruker EQUINOX-55 FTIR spectrometer. Elemental analyses (carbon, hydrogen, and nitrogen) were taken on a PerkinElmer 2400C elemental analyzer. Thermogravimetric analyses (TGA), using Netzsch TG209F3 equipment, were performed under a nitrogen stream with a heating rate of 10 °C min−1. The product phase purity was examined by powder X-ray diffraction (PXRD), using a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, 1.5418 Å). Gas sorption studies were conducted by using Micromeritics ASAP 2020 M adsorption equipment. Synthesis of {[Cu0.5(bpdado)0.5(bpa)0.5]·3H2O}n (1). A mixture of Cu(OAc)2·H2O (0.05 mmol, 9.9 mg), bpa (0.05 mmol, 9.2 mg), H2bpdado (0.05 mmol, 13.8 mg), H2O (8 mL), and methanol (2 mL) was stirred in an air atmosphere for 20 min. The solution was placed in a Teflon-lined reactor (25 mL), and then the reactor was sealed and heated at 130 °C for 72 h. After slow cooling to room temperature, blue block crystals of 1 were obtained. Yield: 70% based on Cu. Anal. Calcd for C12H15Cu0.5N2O6: C, 45.75; N, 8.89; H, 4.80. Found: C, 45.91; N, 8.94; H, 4.65. IR (KBr, cm−1): 3423 (s), 2929 (w), 1612 (s), 1512 (w), 1429 (m), 1379 (s), 1240 (m), 1068 (w), 1032 (w), 976 (w), 818 (w), 769 (m), 696 (w), 553 (m). Synthesis of {[Cu0.5(bpdado)0.5(bpe)0.5]·3H2O}n (2). By a procedure similar to the preparation of 1 except that the bpa was replaced by bpe, the resulting blue block crystals of 2 were collected. Yield: 68% based on Cu. Anal. Calcd for C12H14Cu0.5N2O6: C, 45.90; N, 8.92; H, 4.49. Found: C, 46.13; N, 9.06; H, 4.61. IR (KBr, cm−1): 3424 (s), 2925 (w), 1612 (s), 1514 (w), 1429 (m), 1381 (s), 1240 (m), 1066 (w), 1034 (m), 976 (w), 818 (w), 769 (m), 692 (w), 553 (m). X-ray Crystallography. Single-crystal X-ray diffraction measurements were performed on a Bruker SMART APEX II CCD diffractometer equipped with graphite-monochromated Mo Κα radiation (λ = 0.71073 Å) by using the ϕ/ω scan technique at 296(2) K. The reflection data were corrected for Lorentz and polarization effects as well as for empirical absorption on the basis of multiscan. The structures of 1 and 2 were solved by direct methods and refined anisotropically on F2 by the full-matrix least-squares technique using the SHELXTL program.9 The data were corrected for absorption by using the program SADABS. Anisotropic thermal parameters were applied to all non-hydrogen atoms. All hydrogen atoms from the organic ligand were calculated at idealized positions and refined with the riding models. The contribution of the electron density by the remaining lattice water molecules in 1 and 2 were removed by the SQUEEZE routine in PLATON10 owing to the weak diffraction peaks. In addition, the final chemical formulas of 1 and 2 were estimated from the SQUEEZE results combined with the TGA and EA results. Crystallographic data of 1 and 2 are given in Table S1 in the Supporting Information. Selected bond lengths and bond angles are given in Table S2 in the Supporting Information. CCDC: 1567616 and 1567617 for 1 and 2.

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RESULTS AND DISCUSSION Crystal Structures of 1 and 2. MOFs 1 and 2 are isostructural 3D porous frameworks with different bpa/bpe Ndonor ancillary ligands. X-ray diffraction data show that 1 and 2 crystallize in the monoclinic crystal system of the C2/c space group. The asymmetric unit consists of a half Cu(II), a half bpdado ligand, a half bpa/bpe ligand, and three lattice water molecules. The Cu1 ion is four-coordinated with two carboxylic oxygen atoms (O1, O1A) from two separate bpdado ligands and two N atoms (N2, N2A) from two bpa/bpe ligands (Figure 1a). The uncoordinated O atoms (O2, O2A) of the two carboxylate groups are involved in the weaker Cu− Ocarboxylate (2.614(3) Å for 1, 2.646(3) Å for 2) interactions. C

DOI: 10.1021/acs.inorgchem.7b02844 Inorg. Chem. XXXX, XXX, XXX−XXX

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under vacuum for 6 h. Interestingly, the color changes from blue to purple during the desolvation process, while once the desolvated samples are exposed to moist air for a few minutes, they can be completely reversibly restored with the color reverting from purple to blue (Figure 3); the recovery speed of

Figure 3. Reversible color change of 1 (a) and 1′ (b).

1′ is slightly faster than that of 2′. The solvent removal and readsorption can all be verified by PXRD patterns (Figure S7) and TGA (Figure S8). The PXRD patterns and TGA curves of 1′ and 2′ have been tested at once, and their changes indicate the removal of guest molecules and framework shifts. However, due to the moisture in the air, the desolvated samples will reabsorb water molecules gradually, and thus, the TGA curves of desolvated 1 and 2 display a slight weight loss. In addition, because the readsorption speed of 1′ is slightly faster than that of 2′, purple 1′ has partially turned blue after an immediate PXRD measurement, while 2′ is still almost purple, and thus, the PXRD pattern of 1′ shows more semblance to that of the as-synthesized species in comparison to that of 2′. Ultimately, the TGA curves and PXRD patterns of resolvated 1 and 2 are almost the same as those of as-synthesized 1 and 2, indicating complete resolvation. In addition, 1 and 2 show good chemical stability. Their stability toward water is one of the main challenges in applying porous complexes as gas adsorbents,16 and the experiment results demonstrate that the framework integrity of 1 and 2 could be retained in water at room temperature at least for 2 weeks (Figure S9 in the Supporting Information). Moreover, 1 and 2 are stable in a natural air environment and common organic solvents at least for 2 weeks (Figure S9). In addition, they are also stable at pHs ranging from 5.0 to 9.0 at least for 2 weeks (Figure S10 in the Supporting Information). Sorption Properties. To estimate the porosities of 1 and 2, gas adsorption measurements (N2, H2, CO2, and CH4) were performed. The Langmuir (BET) surface areas calculated according to the sorption of CO2 at 195 K from the first step adsorption isotherms are 592 (410) and 629 (441) m2 g−1 for 1′ and 2′, respectively. The curve analyses reveal that 1′ and 2′ hardly adsorb N2 and H2 (Figures S11 and S12 in the Supporting Information) but adsorb CO2 and CH4, and the uptake amount of CO2 is much more than that of CH4. In particular, 1′ and 2′ exhibit intriguingly comparable temperature-dependent CO2 sorption behaviors. As shown in Figure 4, at 195 K, both MOFs show uncommon stepped adsorption of CO2, but the isotherms display some differences. For 1′, the first step shows a typical type I isotherm and the adsorption saturation occurs at 0.021 atm, where the uptake amount reaches 41.7 cm3 g−1 (8.2 wt %), and then the second step starts at 0.034 atm with the uptake amount reaching 192.8 cm3 g−1 (37.9 wt %) at 1 bar. However, for 2′, though it has the same type I first stepped isotherm, the adsorption saturation occurs at 0.10 atm where the uptake amount reaches 50.2 cm3

Figure 2. (a) Coordination environment of the Cu center in 1. (b) 3D porous structure of 1 with N-oxide groups pointing toward the channel. (c) Coordination environment of the Cu center in 3. (d) 3D close-packed framework of 3. The hydrogen atoms and solvent molecules have been omitted for clarity.

carboxylate O atoms form C4,4′‑bipy−H···OCOOH hydrogen bonds with C atoms of 4,4′-bipy (3.200 Å and 137°) (Figure S6). (5) Due to a lack of N-oxide groups with steric hindrance in 3, the bpda ligands can be arranged back to back, leading to a close-packed framework, which is distinct from the microporous construction of 1 and 2 (Figure 2b,d). (6) From an electronic aspect, in comparison to the pyridine-N atom of bpda with a negative charge in 3, the functionalization of H2bpda into H2bpdado endows the N-oxide group in 1 and 2 with charge variation in which the N atom carries a positive charge and the O atom carries a negative charge.7,15 (7) In addition, different from the one lone pair of electrons of the pyridine-N atom in bpda, the resulting O donor of the N-oxide ligand has two lone pairs of electrons. Thus, it is noted that the above structural discriminations are primarily caused by the modification of pyridine-N into the N-oxide functional group, which transforms the nonporous structure into a porous framework. In addition, the N-oxide functionalized (R,S)H2bpdado ligand with unique charge-separated plus electronrich character would improve the functionality of MOFs and motivate their potential applications in various fields. PXRD, TGA, and Chemical Stability. The experimental PXRD patterns of 1 and 2 match the simulated patterns from the respective single-crystal structures (Figure S7 in the Supporting Information), confirming the phase purity. The differences in intensity may be due to the preferred orientation of the powder samples. To evaluate the thermal stabilities of 1 and 2, TGA experiments were conducted from 30 to 700 °C under a nitrogen atmosphere (Figure S8 in the Supporting Information). The TGA curve of 1 indicates a weight loss of 16.93% in the temperature range of 30−122 °C, and for 2, a weight loss of 16.94% between 30 and 120 °C was observed, which is consistent with the release of three lattice H2O molecules per formula unit (calculated 17.14% for 1 and 17.19% for 2). The desolvated samples 1′ and 2′ were prepared by heating the as-synthesized samples of 1 and 2 at 110 °C D

DOI: 10.1021/acs.inorgchem.7b02844 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. CO2 sorption isotherms of 1′ (a) and 2′ (b) at 195 K. Filled and open symbols represent adsorption and desorption curves, respectively. Insets: log profiles showing the double-step adsorption at 195 K.

Figure 5. Sorption isotherms of 1′ (a) and 2′ (b): CO2 273 K (I), 298 K (II) and CH4 273 K (III), 298 K (IV). Filled and open symbols represent adsorption and desorption curves, respectively.

g−1 (9.9 wt %), and the second step starts at 0.15 atm with the adsorption amount attaining a value of 195.5 cm3 g−1 (38.4 wt %). It is noted that their desorption curves also exhibit steps at the corresponding inflection points of the adsorption curve but do not retrace the adsorption pathways with a large hysteresis, revealing the strong affinity of the host framework for CO2.3,5c The stepped CO2 sorption isotherms at low temperature for 1′ and 2′ may be related to their structural characters. Generally, sorption isotherm steps could be ascribed to sorbate−sorbent interactions when guests adsorb along the pore surfaces and sorbate−sorbate interactions when guests fill the channel.3,5a,c Therefore, the probable reason for the firststep sorption of 1′ and 2′ should be attributed to N-oxide ODSs and uncoordinated carboxyl O sites, and the second step can be attributed to the expansion of the frameworks. The strong framework−CO2 and CO2−CO2 interactions open up the channels, which cause the sudden steps in isotherms upon going to the low-temperature region. From the results, the stepwise adsorptions of CO2 at 195 K indicate that the frameworks of 1′ and 2′ are altered depending on the CO2 pressure. In addition, given the axially chiral H2bpdado ligand with hard rotation between two pyridine rings and the same noncovalent bond interaction of 1 and 2, the dynamic behaviors of 1′ and 2′ should be attributed to the N-donor bridging ligands, and the discrimination of stepwise isotherms between 1′ and 2′ are probably related to the structural differences of bpa and bpe. Though the N-donor dipyridyl

ligands bpa and bpe are isostructural, there are some differences between them: (1) the length of bpa is slightly shorter than that of bpe (Nbpa−Nbpa = 9.282 Å in 1, Nbpe−Nbpe = 9.338 Å in 2), because the geometrical structure of bpa is not coplanar while that of bpe is nearly coplanar; (2) bpa is more flexible than bpe, because bpa with a −CH2CH2− group has extra rotational freedom in comparison to bpe with a −CHCH− group; (3) due to its conformational flexibility, bpa has the two conformations trans and gauche while bpe just has a trans conformation;17 (4) carbon−carbon double bonds of a porous framework could play an important role in small gas adsorption and separation applications to some extent.18 Thus, at low temperature, the starting pressure of the second step of 1′ is smaller than that of 2′ probably because the more flexible bpa makes the framework of 1′ easier to change at lower pressure while the less flexible bpe with an ethene double bond has a stronger bond energy, which makes the framework of 2′ more stable and requires greater pressure to achieve a phase transformation when the gas enters the cavity of the framework. When the measured temperature changed to 273 and 298 K, further interesting results of CO2 isotherms were observed (Figure 5). At 273 K, the CO2 adsorption for 1′ still shows a stepwise curve, which stores up to 41.6 cm3 g−1 (8.2 wt %) at P = 400 Torr, and at P > 400 Torr exhibits a small increase with the adsorption amount attaining 58.2 cm3 g−1 (11.4 wt %) at 1 atm. The CO2 desorption curve also shows steps at the corresponding inflection points of the adsorption curve but E

DOI: 10.1021/acs.inorgchem.7b02844 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. CH4 sorption isotherms of 1′ (a) and 2′ (b) at 195 K. Filled and open symbols represent adsorption and desorption curves, respectively.

Figure 7. IAST adsorption selectivities of 1′ (a) and 2′ (b) for equimolar mixtures of CO2 and CH4.

energy and decrease the pressure required by the phase transition.7 The selectivities of CO2/CH4 and CO2/N2 for 1′ and 2′ were further characterized. It is found that, at 298 K, 1′ and 2′ are nonadsorptive for N2 (the adsorption amount is too low to be detected by our instrument) and exhibit low CH4 uptakes (Figure 5, 7.9 cm3 g−1 for 1′ and 8.5 cm3 g−1 for 2′, respectively) but that there are remarkable CO2 loadings of 36.8 cm3 g−1 for 1′ and 40.4 cm3 g−1 for 2′, indicating the significant gas adsorption selectivities for CO2 over N2 and CH4. To predict CO2/CH4 selectivity in 1′ and 2′ for a CO2/ CH4 binary mixture, the ideal adsorbed solution theory (IAST)24 was employed on the basis of the adsorption curves of CO2 and CH4 at 298 K (Figure S13 in the Supporting Information). As shown in Figure 7, for CO2/CH4 mixtures with general feed compositions of landfill gas (CO2/CH4 = 50/ 50), 1′ exhibits a high initial CO2/CH4 selectivity of 44.7, and then the selectivity decreases with increasing pressure to 18.7 at 1 atm, while that of 2′ increases from 12.8 to 15.5. The selectivity of 1′ is greater than that of 2′ probably due to the structural discrimination, in which the framework of 1′ is more flexible than that of 2′. In comparison to most of the known MOFs which possess good CO2/CH4 selectivity under similar conditions (Table S3 in the Supporting Information), the values are even higher. The excellent CO2 sorption selectivity over CH4 and N2 in 1′ and 2′ may be caused by a few factors. First, the smaller kinetic diameter of CO2 (3.3 Å) in comparison to CH4 (3.8 Å) and N2 (3.64 Å)25 makes it easy to enter the pore. Then, the different electrostatic interactions

does not retrace it with a hysteresis loop. This stepwise sorption behavior of 1′ at 273 K may be caused by the slight deformation of the flexible bpa ligand with increasing CO2 pressure. However, for 2′ at 273 K, the CO2 isotherms show no steps with an adsorption amount of 52.8 cm3 g−1 (10.4 wt %). Once the temperature is increased to 298 K, the CO2 isotherm for 1′ also does not show steps in the profiles, the same as the nonstepped CO2 isotherms for 2′ at 298 K, which indicates that the thermal energy of the frameworks and CO2 molecules reduced their interactions.19 The CO2 uptakes for 1′ (36.8 cm3 g−1, 7.2 wt %) and 2′ (40.4 cm3 g−1, 7.9 wt %) at 298 K are moderately high and are similar to those of MIL-47 (8.1 wt %),20 IRMOF-9 (7.8 wt %),21 and ZIF-81 (7.2 wt %)22 and outperform those of most MOFs,23 such as MOF-177 (3.4 wt %),23a SNU-30 (4.9 wt %),23b SNU-70 (3.5 wt %),23c UCY-1 (4.1 wt %),23d IRMOF-3 (4.7 wt %),20 and MOF-205-OBn (4.1 wt %).23e Thus, overall, 1′ and 2′ display distinct CO2 sorption behaviors dependent on the temperatures (195, 273, and 298 K), which indicate their different threshold temperatures and pressures of channel expansion for 1′ and 2′. Furthermore, the CH4 sorption properties at 195 K for 1′ and 2′ were studied with the amounts of 43.2 cm3 g−1 (3.09 wt %) and 47.1 cm3 g−1 (3.36 wt %) at STP (Figure 6), respectively, much lower than that of CO2 under the same conditions. The multistep adsorption was not observed. The probable reason for the different adsorption behaviors between CO2 and CH4 at 195 K could be attributed to the stronger interaction of CO2 with N-oxide ODSs and vacant carboxyl O sites in the pores, which may enlarge the phase transformation F

DOI: 10.1021/acs.inorgchem.7b02844 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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between gas molecules and frameworks, in which the quadrupole moment of CO2 (−1.4 × 10−39 C m2) is larger than those of CH4 (0 C m2) and N2 (−4.7 × 10−40 C m2),26 can induce the framework to interact with CO2 molecules and thus increase the CO2 binding. Significantly, this can be mainly attributed to the stronger interaction of CO2 with N-oxide functional ODSs and vacant carboxyl O sites in the pores. The remarkable selectivities for CO2 over CH4 and N2 render 1′ and 2′ as promising materials in postcombustion CO2 capture and landfill gas purification. To further study the affinity of 1′ and 2′ for CO2, the isosteric heat of CO2 adsorption (Qst) was calculated on the basis of the virial II equation from the CO2 sorption isotherms at 273 and 298 K (Figure S14 in the Supporting Information). The equation shows that Qst displays high values of 34.8 kJ mol−1 for 1′ and 30.7 kJ mol−1 for 2′ at initial coverage (Figure S15 in the Supporting Information), which are relatively high in comparison with the MOFs containing activity sites (Table S4 in the Supporting Information), reflecting strong framework− CO2 interactions. The high Qst values for CO2 in 1′ and 2′ are ascribed to the following key factors: first, the small pore strengthens the potential field overlap from multiple sides of the channels, which could enhance the framework−CO2 interactions;21,27 second, CO2 guests are close together in small pores, producing CO2−CO2 interactions and offering moderate contributions to Qst; third, the N-oxide ODSs and numerous uncoordinated carboxyl O sites point toward the channels to form a strong affinity for CO2 molecules. These results lead to high sorption affinity and capacity of 1′ and 2′ for CO2 and indicate their potential application in gas separation.

Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02844. Crystallographic data, additional structural figures, PXRD patterns, TGA curves, and additional sorption patterns (PDF) Accession Codes

CCDC 1567616−1567617 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for P.L.: [email protected]. ORCID

Ping Liu: 0000-0001-8778-3199 Yao-Yu Wang: 0000-0002-0800-7093 Author Contributions ∥

W.-Q.Z. and R.-D.W. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF of China (Nos. 21673173, 21572177, 21371142, and 21531007), the NSF of Shaanxi Province of China (Nos. 2016JZ004 and 2015JZ003), the Xi’an City Science and Technology Project (No. 2017085CG/RC048(XBDX004)), the Northwest University Excellent Doctoral Dissertation Cultivation Project (No. YYB17011), the Chinese National Innovation Experiment Program for University Students (G201710697034 and G201710697041), and the “Top-rated Discipline” construction scheme of Shaanxi higher education.

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CONCLUSION In summary, by employment of the N-oxide-functionalized axially chiral H2bpdado ligand and flexible N-donor bpa/bpe ligands, two 3D dynamic porous Cu(II) MOFs have been successfully constructed under solvothermal conditions. Interestingly, 1 and 2 exhibit a reversible structural transformation upon dehydration and rehydration. The wise modification of the pyridine-N into the N-oxide group endows the N-oxide ODSs with charge-separated and electron-rich characters, which play an important role in modulating the structural features and sorption functions. The adsorption properties show that both 1′ and 2′ reveal evident adsorption for CO2 and CH4 and have high CO2 uptake, sorption affinity, and sorption selectivities of CO2 over CH4 and N2. Remarkably, both MOFs display comparable temperature-dependent CO2 sorption behaviors that can probably attributed to the differences in the N-donor ligands bpa and bpe. At 195 K, the isotherms for CO2 of 1′ and 2′ exhibit stepwise adsorption and hysteretic desorption with differences in the channel-expanding pressures. Furthermore, the CO2 isotherms of both 1′ and 2′ show nonclassical type I adsorption isotherms at 298 K, but peculiarly, at 273 K, the CO2 isotherm of 1′ still exhibits uncommon stepwise adsorption while that of 2′ does not. Thus, these temperature-dependent CO2 sorption behaviors indicate that there exist different threshold temperatures and pressures of channel expansion for 1′ and 2′. This work gives a good example for comparison of the structure−property discriminations between similar frameworks.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02844 Inorg. Chem. XXXX, XXX, XXX−XXX