CO2 capture and sequestration (CCS) has received much attention in recent decades, due to the inexorable rise of carbon dioxide level in the atmosphere, which is the largest contributor to global warming . Prior to zero CO2 emission, the postcombustion capture of CO2 is an excellent mid-term solution for the increasing demands of fossil fuel energy while developing the renewable energy technologies remain in line with global targets . A related solution is using clean energy such as natural gas, but requires pre-combustion upgrades to remove impurities. Because natural gas (mainly CH4) often contains an unacceptable level of CO2 and N2 impurities (over 40%) and is only efficient at low concentration of CO2 to prevent the pipeline corrosion . Among several gas separation technologies, physical sorption using porous materials such as metal-organic frameworks (MOFs, also known as porous coordination polymers) is high energy-efficiency .
Microporous MOFs have emerged as new porous materials that can offer promising solutions for many persistent challenges relating to energy and environmental sustainability . Such materials can be directly produced by self-assembly of suitable metal ions/clusters with multidentate organic ligand(s) through coordination bonds . In particular, MOFs have been widely investigated in a variety of applications including gas storage , separation , luminescent sensors , heterogeneous catalysis , and biomedicine , due to their characteristic advantages of the tunability, designability, highly-ordered structure, rigidity/-flexibility as well as very high surface area. Regarding the diversity of well-defined MOFs based on rational selections of both metal ions and designable organic ligands, efforts have been made to find suitable materials to meet the industrial requirements for gas storage and separation.
Recently, significant progress demonstrates the importance of pore chemistry and size controlling in small gas molecules separation . Nowadays several approaches have been realized to optimize the performance of the MOFs in both gas storage and selective gas separation (e.g. CO2 and C2H2) at ambient conditions. One is by making use of open metal sites (OMSs) within the microporous surfaces, such as HKUST-1 , MIL-100 , MOF-74-M (M = Mg2+, Fe2+, Co2+) [4a], and UTSA-74 [4b]. Likewise, immobilizing functional groups (-NH2 , -OH , etc.) within the porous surfaces is also another very practical approach to change the polarity of pore surface and therefore optimizing gas separation selectivity .
To the best of our knowledge, there is no reported example of MOF only based on 3-aminoisonicotinic acid (Haina, Scheme 1) with the immobilized amino group. Herein, using this rare ligand, we obtained a layered MOF, [Cu(aina)2(DMF)]·DMF (1), which is composed of square grid-like coordination layers with permanent porosity, showing high C2H2/CH4 and CO2/CH4 separation selectivity at ambient conditions.
2. Results and discussion 2.1. Synthesis and structure
The green block-shaped crystals of 1 can be well obtained when a small amount of hydrochloric acid (4 mol/L) was added to the solvothermal reaction of 3-aminoisonicotinic acid (Haina) with Cu (NO3)2·2.5H2O in a DMF solvent at 85 ℃ for 48 h. The high-quality crystals of 1 are suitable for structure determination. Single-crystal X-ray diffraction study revealed that 1 crystallizes in the monoclinic system with the space group P21/c (Table 1). In the asymmetric unit of 1, there are one Cu2+ ion, two aina- ligands, and two DMF molecules, one of which coordinate to Cu(Ⅱ) (Fig. 1a). The Cu atom is coordinated by two N atoms of pyridyl groups and two O atoms of carboxylate groups from four different aina-ligands, followed by one O atom from DMF molecule to accomplish a square pyramidal geometry [Cu-O 1.949(2)-2.328(2) Å; Cu-N 2.010(2)-2.026(2) Å] (Table S1 in Supporting information). The isonicotinate ligand adopts the linear coordination mode using only one N and one O atoms for coordination, which leaves one carboxylate O atom uncoordinated. The coordinated DMF molecule occupies the only axial site of the square pyramidal Cu atom which prevents further coordinated extension. Regarding the Cu atoms as 4-connected nodes and the ligands as linkers, the coordination layer can be simplified as sql topology (Fig. 1b). These coordination layers further stack together via multiple interlayer hydrogen bonding interactions between half of amino groups and all uncoordinated carboxylate O atoms [N…O 2.954(3)-2.986(3) Å; N-H…O 125(2)-178(2)°, (Fig. 1c and Table S2 in Supporting information)]. Such coordination and hydrogen bond hybrid framework contain 1D channels (void: 46.5%, aperture size: 4.7 Å × 4.8 Å) along  direction (Fig. 1d), which are occupied by DMF molecules. Except for the amino groups that are forming interlayer hydrogenbonding, the other half amino groups are exposed to the pore surface, which might benefits selective adsorption of polar gas molecules. The thermogravimetric analysis (TGA) shows that the as-synthesized 1 lost all DMF molecules at about 245 ℃. This is a two-step weight loss (25 wt%) corresponding to the guest and coordinated DMF molecules, respectively, followed by the decomposition of 1 (Fig S1 in Supporting information). The phase purity of 1 was independently confirmed by powder X-ray diffraction (PXRD) analysis. The activated 1a (Fig. S2 in Supporting information) was otherwise demonstrated a considerable degree of crystallinity with only small shifts, indicating certain flexibility and contraction of the framework compare to that of synthesized 1.
|Fig. 1. X-ray crystal structure of 1 indicating (a) asymmetric unit; (b) square grid-like layer structure composed of Cu2+ ions and ania- ligands; (c) side view of the interlayer hydrogen-bonding between different layers; (d) packing diagram along the  axis, showing the pore surface of 1D channels highlighted as yellow/gray (inner/outer) curved plane with pore window (4.7 Å × 4.8 Å).|
2.2. Gas sorption and separation
The potential porosity of 1 motivated us to further investigate its gas separation performance. A fresh sample of 1 was firstly desolvated at 150 ℃, and then at 180 ℃ for full activation until the outgassing rate reached 6 μmHg/min, giving activated 1a for further gas sorption. To check its porosity, CO2 adsorption isotherm was carried out at 195 K using a Micromeritics ASAP 2020 surface area analyzer. As shown in Fig. 2, CO2 sorption isotherm at 195 K exhibits a type Ⅰ character with an uptake of 0.91 mmol/g. The Brunauer-Emmett-Teller and Langmuir surface area were calculated to be 62 and 94 m2/g, respectively (Fig. S3 in Supporting information). The pore volume was calculated as 0.04 cm3/g, which is far smaller than the ideal value of 0.43 cm3/g. This phenomenon might be attributed to the flexible nature of layered structures, which is hard to restore its original open state by only using gas molecules under mild conditions .
|Fig. 2. CO2 sorption isotherm of 1a at 195 K (solid: adsorption, open: desorption).|
The small pore channels along with the structural flexibility as well as the exposure of amino groups in 1a endorsed us to investigate its potential application in gas separation. Regarding the suitable pore size and functional groups that could have collaborative effect for specific gas molecules, carbon dioxide and acetylene adsorption over methane and nitrogen studies were carried out. Consequently, the single component of C2H2, CO2, CH4, and N2 gases were used to study the performance of 1a in the selective gas separation at 296 and 273 K, respectively. At 1 atm and 296 K, 1a showed different capacities for C2H2 (0.54 mmol/g), CO2 (0.37 mmol/g), CH4 (0.06 mmol/g), and N2 (0.02 mmol/g) (Fig. 3). At 273 K, the adsorbed amounts of C2H2, CO2, CH4, and N2 are 0.69, 0.57, 0.10, and 0.05 mmol/g, respectively. The adsorption amounts of C2H2 and CO2 at both temperatures and 1 atm are ~7 and ~6 times, respectively, higher than that of CH4, and ~10 times greater than the N2, which can be attributed to the collaborative effect between small pore size and the presence of amino functional groups on the microporous surface. These results encouraged us to investigate its selectivity of the important C2H2/ CH4 and CO2/CH4 separations. To further understand the behavior of the gas sorption, the coverage-dependent adsorption enthalpies of 1a on C2H2, CO2, CH4, and N2 were calculated by using the virial method as shown in (Fig. S4 in Supporting information) The isosteric enthalpies of adsorption at zero coverage of 1a are 22.4, 20.4 kJ/mol for CO2 and C2H2, respectively, which is even comparable with other microporous MOFs that contain open metal sites, such as HKUST-1 (25.3 and 30.4 kJ/mol) . In contrast, the enthalpy of CH4 at zero coverage is found to be 14.6 kJ/ mol, which is understandable considering the less polar nature of CH4, giving a weaker interaction with the pore surface. These results confirm that 1a showed different affinities toward these gas molecules, which indicates the potential selective separation of CO2/CH4, C2H2/CH4, and CO2/N2 for 1a. To predict gas selectivities of 1a, the popular ideal adsorbed solution theory (IAST) calculation  is further used for mixed C2H2/CH4 (50%:50%), CO2/CH4 (50%:50%), and CO2/N2 (15%:85%) adsorption at different pressures and temperatures. Prior to IAST calculation, single-component C2H2, CO2, and CH4 adsorption isotherms were fitted based on the single-site Langmuir-Freundlich model (Table S3 in Supporting information) . The obtained fitting parameters were further used to predict the separation of different gas mixtures. As shown in (Fig. 4 and Table 2), the pressure independent isotherm and selectivity profiles indicate high C2H2/CH4 and CO2/CH4 selectivities at 296 K and 1 atm. The predicted selectivities of C2H2/CH4, CO2/CH4, and CO2/N2 were calculated to be 98, 20, and 69, respectively, at 296 K and 1 atm. At 273 K and 1 atm, the C2H2/CH4, CO2/CH4, and CO2/N2 selectivities are 48 and 17, and 45, respectively (Fig. S5 in Supporting information). Though the sorption capacity is somewhat small, it is noteworthy that the C2H2/CH4 selectivity of 98 at 296 K and 1 atm are significantly higher than those of UTSA-50a  and UTSA-85a . For CO2/N2, its selectivity are higher than those of some MOFs with open metal sites, such as Ni-MOF-74 [8c] and PCN-88 , while that of CO2/ CH4 is lower than those of HKUST-1  and UTSA-16 . These results highlight the potential application of this material in the practical acetylene separation and carbon dioxide capture.
|Fig. 3. CH4 (red), N2 (black), CO2 (blue), and C2H2 (green) sorption isotherms of 1a at (a) 273 K and (b) 296 K.|
|Fig. 4. Mixture adsorption isotherms (a-c) and selectivities (d) predicted by IAST calculation of 1a for C2H2/CH4 (50%:50%), CO2/CH4 (50%:50%), and CO2/N2 (15%:85%) separation at 296 K.|
In summary, by virtue of functional isonicotinate ligand, we successfully obtained a 2D microporous MOF. The collaborative effect between small pore size and the presence of amino functional groups on the microporous surface make this MOF show selective adsorption of C2H2 and CO2, as shown by singlecomponent gas sorption measurements and selectivity calculations. The high selectivities of C2H2 and CO2 over CH4 and N2 indicate the potential application of this material for the practical separation of acetylene and carbon dioxide capture, which might inspire future developing customizable adsorbents for specific gas separations.4. Experimental 4.1. Materials and methods
All solvents and reagents for synthesis were used as received from commercial sources without further purification. To assess the thermal stability, thermogravimetric analysis (TGA) was carried out from room temperature to 700 ℃ using a Shimadzu TGA-50 analyzer under an Argon atmosphere at a heating rate of 10 ℃/min. Powder X-ray diffraction (PXRD) patterns were collected using a Rigaku Ultima Ⅳ diffractometer functioned at 40 kV and 44 mA with a scan rate of 1.0 degree per minute. Fourier transform infrared (FTIR) spectrum was performed on a Bruker Vector 22 infrared spectrometer at room temperature.4.2. Synthesis of [Cu(aina)2(DMF)]·DMF (1)
A mixture of Cu(NO3)2·2.5H2O (48.0 mg, 0.2 mmol) and Haina (13.8 mg, 0.1 mmol), DMF (1.5 mL), and hydrochloric acid (50 μL, 4 mol/L) added to small amount of water (0.2 mL) was sealed in a 23 mL vial, under ultrasonic for 30 min. The vial was then placed in an oven at 70 ℃ for 72 h. Then the reaction mixture was further allowed to cool to room temperature before the oven cut off. Green block-shaped crystals of 1 were collected, washed with DMF and ethanol, and dried in air (yield: 19.3 mg, 40% based on Haina). IR/ cm-1 (KBr): 3416 (m), 3323 (m), 2929 (w), 1647 (s), 1612 (s), 1574 (s), 1531 (w), 1493 (w), 1434 (s), 1372 (s), 1350 (s), 1287 (w), 1244 (s), 1093 (m), 1058 (w), 899 (w), 848 (w), 817 (m), 802 (s), 712 (s), 689 (s), 661 (m), 591 (w), 525 (m).4.3. Crystal structure determination
Single-crystal X-ray diffraction data of 1 was collected at 98(2) K on a Rigaku AFC12/Saturn 724 CCD fitted with Mo Kα radiation (λ = 0.71073 Å). The final structure was solved by direct methods and refined further on F2 by using full-matrix, least squares techniques with the SHELXL program package . Data collection and unit cell refinement were carried out by using Crystal Clear software . Data processing and absorption correction, giving the minimum and maximum transmission factors, were accomplished with Crystal Clear and ABSCOR , respectively. All nonhydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms on carbon atoms were positioned geometrically and processed using a riding model. Selected bond lengths and angles are listed in Table S1-2 in Supporting information. CCDC 1532767 for 1 contains the supplementary crystallographic data. The data can be obtained free of charge from the Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.4.4. Gas sorption measurements
The gas sorption isotherms were measured using automatic volumetric adsorption apparatus Micromeritics ASAP 2020 surface area analyzer. Prior to the sorption measurements, a freshly prepared sample of 1 was exchanged with dry ethanol at least 10 times, then degassed at 150 ℃ for 24 h, and further at 180 ℃ for another 8 h until the outgassing rate was 6 mm Hg/min, giving the activated 1a for gas sorption analyses. The sorption measurements of 1a were kept at 196, 273, and 296 K by using dry ice-acetone bath, ice-water bath, and water bath, respectively.4.5. IAST calculations
An Ideal Absorbed Solution Theory (IAST) proposed by Myers and Prausnitz  was used to calculate the selective sorption performance toward different binary gases mixtures. For 1a, the selectivities of mixed C2H2/CH4 (50%:50%), CO2/CH4 (50%:50%), and CO2/N2 (15%:85%) at 273 and 296 K were calculated. The data were summarized in 4 (Supporting information) and Table 2. The parameters obtained from the fitting of single -component C2H2, CO2, CH4, and N2 adsorption isotherms based on the Langmuir-Freundlich model were used in the IAST calculations (Table S3 in Supporting information) .Acknowledgment
This work was supported by the grant AX-1593 (JCGZ) and AX-1730 (BC) from the Welch Foundation.Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2017.04.025.
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