Synthesis of Highly Porous Coordination Polymers with Open Metal

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Synthesis of Highly Porous Coordination Polymers with Open Metal Sites for Enhanced Gas Uptake and Separation Kyung Seob Song, Daeok Kim, Kyriaki Polychronopoulou, and Ali Coskun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09156 • Publication Date (Web): 21 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016

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ACS Applied Materials & Interfaces

Synthesis of Highly Porous Coordination Polymers with Open Metal Sites for Enhanced Gas Uptake and Separation †

†

§

Kyung Seob Song , Daeok Kim , Kyriaki Polychronopoulou and Ali Coskun*

†,‡

†

Graduate School of Energy, Environment, Water and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea. § Mechanical Engineering, Khalifa University, Abu Dhabi-127788, United Arab Emirates ‡

Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea.

KEYWORDS: microporous polymers, natural gas purification, open metal site, H2 storage, CO2/CH4 separation

ABSTRACT: Metal-containing amorphous microporous polymers are emerging class of functional porous materials, in which the surface properties and functions of polymers dictated by the nature of metal ions incorporated into the framework. In an effort to introduce coordinatively unsaturated metal sites into the porous polymers, herein, we demonstrate an aqueous phase synthesis of porous coordination polymers incorporating bis-(odiiminobenzosemiquinonato)-Cu(II) or -Ni(II) bridges by simply reacting hexaminotriptycene with CuSO4·5H2O, Cu(II)PCP, or NiCl2·6H2O, Ni(II)-PCP, in H2O. The resulting polymers showed surface areas up to 489 m2 g-1 along with a narrow pore size distribution. The presence of open metal sites significantly improved the gas affinity of these frameworks, leading to an exceptional isosteric heats of adsorption value of 10.3 kJ mol-1 for H2 at zero coverage. High affinity of Cu(II)and Ni(II)-PCPs towards CO2 prompted us to investigate removal of CO2 from natural and landfill gas conditions, higher affinity of Cu(II)-PCP compared to that of Ni(II)-PCP not only allowed us to tune the affinity of CO2 molecules towards the sorbent, but it also led to an exceptional CO2/CH4 selectivity of 35.1 for landfill gas and 20.7 for natural gas at 298 K. These high selectivities further verified by breakthrough measurements under the simulated natural and landfill gas conditions, in which both Cu(II)- and Ni(II)-PCPs showed complete removal of CO2. These results clearly demonstrate the promising aspect of metal-containing porous polymers for gas storage and separation applications.

■ INTRODUCTION Porous polymers have become extremely popular in recent years for gas capture, storage and separation,1-2 heterogeneous catalysis, sensing and energy storage applications. 3-15 Importantly, their textural properties and functionalities can be controlled by judicious choice of monomeric units and polymerization routes.16 As these polymers mostly obtained under kinetically controlled reactions conditions, the resulting polymers showed high thermal and water stabilities, thus rendering these materials suitable for CO2 capture under humid conditions at elevated temperatures. Considering the serious environmental impact of CO2 emissions from large point of sources, the development of new porous polymers is of high interest. While purely organic porous polymers showed promising gas sorption properties, in recent years, post-synthetic modification of these polymers via metal nanoparticles, 17-20 metal-doping21 and/or metal coordination through well-defined chelating ligands22-29 have been shown to enhance gas uptake properties. Importantly, the choice of metal dictates the properties and catalytic functions of these polymers. 28, 30-31

Scheme 1. The synthetic strategy for the preparation of porous coordination

polymers

incorporating

different metal ions, i.e., Ni(II), Cu(II).

ACS Paragon Plus Environment

hexaaminotriptycene

and


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Figure 1. XPS spectra of Cu(II)- and Ni(II)-PCPs. (a) XPS survey, (b) Cu 2p, (c) N 1s spectra of Cu(II)-PCP. (d) XPS survey, (e) N 2p and (f) N 1s spectra of Ni(II)-PCP.

Recently, Farha et al.24 demonstrated that the metalation of catechol functionalized porous organic polymers (POPs), which enabled the introduction of coordinately unsaturated metal sites, led to a significant enhancement in H2 uptake. More recently, El-Kaderi et al.25 elegantly demonstrated the post-metallation of bis(imino)pyridinelinked polymers with well-defined chelating sites using Cu(BF4)2 for enhanced CO2 uptake and separation from various sources. In addition to these reports, several porphyrins and phthalocyanine-based porous polymers have been introduced lately to demonstrate the impact of metal ions on the gas sorption and catalytic properties of these porous networks.30, 32-33 In this sense, the development of porous polymers with coordinatively unsaturated Lewis acidic metal sites is a promising direction due to the enhanced interactions between gas molecules and metal sites as commonly observed in metal organic frameworks (MOFs) such as MOF-74 and many others.34 As such, the complexes of transition metals to form bis-(o-diiminobenzosemiquinonato)metal(II), M(isq)2, complexes could be a good approach to create coordinatively unsaturated metal sites within porous networks. Dinca et al.35 utilized this approach to create electrically conductive Cu-containing 2D MOF. However, 2D network structure of this MOF limited the accessibility of open metal sites for their interactions with gas molecules. Introduction of triptycene core as a monomeric unit could overcome this problem due to its high internal free volume and symmetry. Evidently, these unique properties of triptycene made it monomer of choice for the construction of polymers of intrinsic microporosity (PIM)36, porous polymers,37 organic cages38 and MOFs. 39, 40. Herein, we report (Scheme 1) on the facile synthesis of

porous coordination polymers possessing coordinatively unsaturated metal sites (M(II)-PCPs) from the reaction of hexaaminotriptycene (HATT) and metal salts (CuSO4 and NiCl2) to in-situ form square planar, charge neutral Ni(isq)2, Ni(II)-PCP, and Cu(isq)2, Cu(II)-PCP, complexes. Both Cu(II)- and Ni(II)-PCPs showed reasonably high surface areas of 448 and 489 m2 g-1, respectively. Moreover, the presence of open metal sites was further verified by the enhanced affinity towards both H2 and CO2 with the isosteric heats of adsorption (Qst) values up to 10.3 and 45.7 kJ mol-1 at zero coverage, respectively. High CO2 affinity and good uptake capacity of M(II)-PCPs also utilized in the context of CO2/CH4 separation under simulated natural (5:95) and landfill gas (50:50) conditions with exceptional IAST selectivities, and breakthrough separation. ■ RESULT AND DISCUSSION The syntheses of Cu(II)- and Ni(II)-PCPs were achieved by reacting HATT.6HCl with CuSO4 and NiCl2 in a basic aqueous solution at 100oC, respectively. While we observed the formation of black precipitate within 2 h in the case of Cu(II)-PCP, it took nearly 24 h for Ni(II)-PCP, thus pointing to the relatively slow kinetics for its formation. The resulting black powders were suspended in H2O and washed three times, and finally in acetone prior to their activation under vacuum at 40°C for 24 h. The formation of M(isq)2 bridges between the triptycene units was verified (Figure 1) by X-ray photoelectron spectroscopy (XPS) analysis for both Cu(II)- and Ni(II)PCPs. XPS survey spectrum of Cu(II)-PCP revealed the presence of Cu, N, C and O peaks. We attribute the O peak to the trapped moisture within the pores of Cu(II)-


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Figure 2. Argon adsorption−desorption isotherms of Cu(II)- and Ni(II)-PCPs measured at 87 K. Inset: NLDFT pore-size distributions of Cu(II)- and Ni(II)-PCPs from Ar adsorption isotherms. For surface area calculations, we selected data points in the pressure range of 0.01 to 0.08 according to the Rouquerol plots.

PCP. The Cu 2p spectrum of Cu(II)-PCP exhibited two main peaks arising from Cu 2p3/2 and Cu2p1/2 along with the satellite peaks. While the main peaks are assigned to the full electron transfer from ligand p orbitals to 3d shell of Cu (3d10), the satellite peaks were attributed to the weak electronic coupling between ligand and metal (3d9 character)25, 41. This spectrum also indicates the presence of Cu centers with mixed oxidations states (Cu2+ and Cu+).25 Moreover, N1s spectrum of Cu(II)-PCP revealed a single peak at 399.8 eV, thus verifying the formation of Cu(isq)2 complex. In a similar manner, we have also observed the corresponding peaks for Ni, N, C and O in the XPS survey spectrum of Ni(II)-PCP. The presence of single N 1s peak at 399.1 eV also verifies successful formation of Ni(isq)2 complex. It is important to note that these observations are in good agreement with those of 2D MOF incorporating Cu(isq)2 moiety35 and porous polymers with Cu-porphyrins31. Importantly, lack of corresponding peaks for counteranions, i.e., Cl-, SO42-, indicate clearly the formation of charge neutral metal complexes. We have also carried out Fourier transform infrared spectroscopy (FT-IR) analysis on Cu(II)- and Ni(II)-PCPs. Both samples showed (Figures S1 and S2) broad peaks in the range of 3700 ~ 3000 cm-1, which was attributed to the presence of trapped water molecules. We have also carried out FT-IR analysis following thermal activation of M(II)-PCPs, which showed (Figure S2) the presence of N-H stretching at 3400 cm-1 and removal of water molecules. In addition, we also observed C=C and C-N stretching bands at 1475 and 1077 cm-1, respectively. The presence of Cu-N (452 cm-1) and Ni-N (459 cm-1) peaks also further supports for the formation of M(isq)2 complexes. Thermogravimetric analysis (TGA) was performed in order to assess thermal stability of M(II)PCPs (Figure S3). While Cu(II)-PCP was found to be stable up to 250oC, Ni(II)-PCP showed much higher stability up to 350oC. This difference in their thermal stability was attributed to the higher thermodynamic stability of Ni(isq)2 compared to Cu(isq)2 moiety. The

weight loss (~10 wt% for Cu(II)-PCP, ~14 wt% for Ni(II)PCP) below 200oC was attributed to the loss of trapped water molecules. We have also observed (Tables S1 and S2) significant amount water in the elemental analysis (EA) of both Cu(II)- and Ni(II)-PCPs (Table S1 and S2). The EA analysis was further supported by ICP-MS measurements to precisely determine the metal content in each polymer. We observed Cu and Ni contents of 15.48 (2.44 mmol g1 )and 16.79 (2.86 mmol g-1) wt%, respectively. The deviation in the metal c0ntent compared to the calculated values could be explained by the presence of terminal amines and possibly defect sites. In an effort to investigate bulk-scale morphology of M(II)-PCPs, we have carried out scanning electron microscopy (SEM) analysis (Figure S4). Both polymers formed spherical particles with average particle diameters of 52 and 115 nm for Cu(II)- and Ni(II)-PCP, respectively. Formation of smaller particles in the case of Cu(II)-PCP was attributed to the faster reaction kinetics, which showed the formation of polymers within 2 h as oppose to 24 h for Ni(II)-PCP. Transmission Electron Microscopy (TEM) analysis of these samples also verified (Figure S5) the absence of any metal oxide particles, which could form during the synthesis of polymers. The crystallinity of M(II)-PCPs was analyzed (Figure S6) using powder X-ray diffraction (PXRD) analysis. Both Cu(II)- and Ni(II)-PCPs were found to be completely amorphous. We speculate that the coordination bonds are not strong enough to overcome wagging and distortion of the metal-complexes to obtain ideal honeycomb structure. 42

Figure 3. H2 adsorption-desorption isotherms of (a) Cu(II)- and (b) Ni(II)-PCPs recorded at 77 and 87 K. Filled and empty symbols re present adsorption and desorption, respectively. Inset: Isosteric he ats of adsorption (Qst) plots for H2 .



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Figure 4. CO2 adsorption isotherms of (a) Cu(II)-PCP, (b) Ni(II)-PCP at 273, 298 and 323 K, (c) isosteric heat of adsorption plots for CO2 . CH4 adsorption isotherms of (d) Cu(II)-PCP, (e) Ni(II)-PCP at 273, 298 and 323 K, (f) isosteric heat of adsorption plots for CH4 .

The textural properties of Cu(II)- and Ni(II)-PCPs were investigated (Figure 2, Table S3) by Ar sorption measurements 87 K. Both PCPs showed characteristic type I adsorption, thus indicating the presence of welldeveloped micropores. We have also observed H4 hysteresis loop upon desorption presumably due to the pore network effects and interaction of gas molecules with micropore surfaces43. Cu(II)- and Ni(II)-PCPs exhibited Brunauer, Emmett, Teller (BET) surface areas of 448 and 489 m2 g-1, respectively. The pore size distributions of M(II)-PCPs were calculated from Ar isotherms according to nonlocal density functional theory (NLDFT), both PCPs showed pores located in the micropore range (< 2 nm) with pore width maxima located at 0.8 nm. It should be noted that similar pore size distribution of Cu(II)- and Ni(II)-PCPs points to the fact that both polymers possess similar network structures and the properties of resulting polymers can be simply tuned by the nature of metal complex. The presence of open metal sites along with relatively high surface area of M(II)-PCPs prompted (Figure 3) us to investigate their affinity towards H2 gas. Physisorption is a completely reversible process, which facilitates efficient adsorption and release of H2 gas and leads to very fast kinetics. Owing to the low polarizability H2 molecules, it

is, however, rather challenging to obtain high Qst values, which lead to the relatively low H2 uptakes at room temperature. In this sense, the presence of coordinatively unsaturated metal sites could be one promising direction to increase Qst values for H2 as already observed for MOFs34, 44 and post-functionalized POPs24, 45. We have measured (Figure 3) H2 uptake isotherms of Cu(II)- and Ni(II)-PCPs up to 1 bar at 77 and 87 K. Cu(II)-PCP showed H2 uptake capacities of 1.00 and 0.76 wt% at 77 and 87 K, 1 bar, respectively, along with an exceptional Qst value of 10.3 kj mol-1 at zero coverage. Similarly, Ni(II)-PCP showed H2 uptake capacities of 0.97 and 0.74 wt% at 77 and 87 K, respectively, with a Qst value of 10.2 kJ∙mol-1 at zero coverage (Figure S9, Table S4). These high Qst values demonstrate clearly the presence and accessibility of open metal sites to the H2 molecules. As expected, increasing loading amounts led to decreasing Qst values due to the saturation of open metal sites. It is also important to note that these Qst values are among the highest values reported for porous polymers and comparable to those of MOFs with open metal sites. 34,44 We have also investigated (Figure 4, Table 1) CO2 and CH4 adsorption behavior of M(II)-PCPs and tested their potential application for CO2 removal from the simulated

Table 1. CO2, CH4 and H2 uptake capacities of Cu(II)- and Ni(II)-PCPs along with the corresponding isosteric heats of adsorption (Qst) values of CO2, CH4, and H2. CO uptake (mmol g-1) 2

CH 4

uptake (mmol g-1)

H uptake (wt%) 2

T (K) =

273

298

323

Q a st (kj mol-1)

273

298

323

Q a st (kj mol-1)

Cu(II)-PCP

2.80

1.97

1.15

45.7

0.61

0.34

0.16

35.2

1.00

0.76

10.3

Ni(II)-PCP

2.49

1.75

1.04

42.1

0.64

0.35

0.20

33.8

0.97

0.74

10.2

77

87

Q a st (kj mol-1)

a Isosteric heats of adsorption (Q ) values calculated using the gas adsorption data at 273, 298 and 323 K for CO2 and CH4 and at 77 and 87 K for H2 using the Clausius-Clapeyron st equation. The Q values were reported at zero coverage. st


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Figure 5. (a, d) Ideal adsorbed solution theory (IAST) CO2 /CH4 selectivity of Cu(II) and Ni(II)-PCPs at 298 K. Breakthrough experiments were carried out on Cu(II) and Ni(II)-PCP under the simulated (b, c) landfill (CO2 /CH4 = 0.5/0.5) and (e, f) natural gas (CO2 /CH4 = 0.05/0.95) conditions at 298 K.

landfill and natural gas conditions. CO2 uptake capacities of M(II)-PCPs were investigated up to 1 bar at 273, 298 K and 323 K. Cu(II)- and Ni(II)-PCP showed CO2 uptake capacities of 2.80 and 2.49 mmol g-1 at 273 K and 1 bar, respectively. It is noteworthy to mention that these uptake capacities are higher than previously reported metal-containing porous polymers such as Cu(II) or Ni(II) containing POPs (2.26 - 2.55 mmol g-1)30, Cu/BF4/BIPLP-1 (2.57 mmol g-1) 25 and recently reported MP PONs (1.59-1.7 mmol g-1)33. In addition, while these CO2 uptake values are comparable to some of the MOFs with open metal sites such as Cu-BTTri46 and UiO-6647, they are still moderate compared to the those of HKUST-148-49, MOF7450 and [NH2(CH3)2][Zn3(BTA)(BTC)2(H2O)]51. The Qst values of CO2 for Cu(II)- and Ni-PCP were found (Figures 4 and S10 and Table S5) to be 45.7 and 42.1 kJ mol-1, respectively, at zero coverage, which showed sharp decrease with increasing loading amounts thus indicating clearly the presence of open metal sites for CO2 binding. We believe that presence of highly acidic protons around the open metal sites positively contributes to the high affinity of M(II)-PCPs towards CO2 molecule. Slightly higher affinity of Cu(II)-PCP towards CO2 compared to that of Ni(II)-PCP was attributed to the higher charge density of Cu(II) center. These results demonstrate that the chemical nature of PCPs can be tuned by simply varying metal ions. In order to determine the saturation point of open metal sites, we obtained the first derivative of the Qst plots, which showed that CO2 loadings of 0.55 and 0.40 mmol were required to fully saturate the open metal sites in Cu(II)- and Ni(II)-PCPs, respectively. Importantly, these values are much lower compared to the actual amount of open-metal sites within the M(II)PCPs obtained from ICP-MS analysis. We attribute this result to the limited accessibility of open-metal sites by

CO2 molecules and their partial activation within the highly interpenetrated network structure. Methane uptake capacities of Cu(II)- and Ni(II)-PCPs were measured (Figure 4) up to 1 bar at 273, 298 and 323 K. Cu(II)- and Ni(II)-PCPs showed (Figure 4, Table 1) CH4 uptake capacities of 0.61 and 0.63 mmol g-1 at 273 K and 1 bar, respectively. We observed gradual decrease in uptake capacities with rising temperature. The Qst of methane for Cu(II)- and Ni(II)-PCP, which was calculated (Figure S11 and Table S6) from the adsorption data at 273, 298, and 323 K, were found to be 35.2 and 33.8 kJ mol-1 at zero coverage, respectively. We attribute the high affinity of M(II)-PCPs towards methane to the presence of coordinatively unsaturated metal sites along with the presence of micropores with the pore size of 8 Å nm. This result is agreement with the recent analysis of several MOF structures by Snurr et al, 52 which showed that ideal pore size should be in the range of 4 and 8 Å for high affinity methane storage. After confirming the noticeable difference in both uptake capacity and Qst values for Cu(II)- and Ni(II)-PCPs between CO2 and CH4, we have examined (Figures 5a, d and S12, Table S7) their CO2/CH4 selectivity using ideal adsorbed solution theory (IAST). 53 Cu(II)- and Ni(II)PCPs exhibited exceptional CO2/CH4 selectivities of 20.7 and 17.3 for a natural gas (CO2:CH4 =0.05 : 0.95) mixture at 298 K, respectively. We have also observed high CO2/CH4 selectivities of 35.1 (Cu(II)-PCP) and 17.8 (Ni(II)PCP) under landfill gas (CO2:CH4 =0.5 : 0.5) conditions at 298 K. Notably, these selectivity values are higher than those of the representative porous organic polymers5,15, and MOFs54,55. Superior CO2/CH4 selectivity of Cu(II)-PCP compared to that of Ni(II)-PCP can be explained by high affinity of Cu(II)-PCP towards CO2. We believe that the saturation of open Cu(II) sites by CO2 molecules not only



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limits the interaction of CH4 molecules with the binding sites, but it also restricts their diffusion into the pores, thus leading to increasing selectivity with increasing pressure as also previously observed for Li-POPs.45 This behavior was not observed in the case of Ni(II)-PCP, which points to the fact that the binding kinetics of CO2 to the Ni(II) center is much slower compared to the Cu(II) one. While IAST is a powerful tool predict mixed gas selectivities from pure gas isotherms, it is important to verify actual separation performance of sorbents by conducting breakthrough experiments. We have carried out (Figures 5b, c and 5e, f) breakthrough experiments on both Cu(II)- and Ni(II)-PCPs and investigated their CO2 separation of performance from the simulated natural (CO2:CH4 = 0.5:95) and landfill CO2:CH4 = 0.5:0.5) gas conditions at 298 K. Both Cu(II)- and Ni(II)-PCP showed complete removal of CO2 with storage capacities of 0.26 and 0.25 mmol g-1 along with the breakthrough times of 5.4 and 5.4 min, respectively, for a simulated natural gas mixture. For the land fill gas conditions, CO2 storage capacities of 1.06 and 1.03 mmol g-1 along with breakthrough times 2.4 and 2.1 min observed for Cu(II)and Ni(II)-PCP, respectively. These results clearly demonstrate the potential of M(II)-PCPs for the removal of CO2 from natural and land fill gas mixtures, which is an important requirement for the utilization of methane as an energy source. ■ CONCLUSION We have demonstrated that the combination of triptycene with the bis-(odiiminobenzosemiquinonato)metal(II) complexes could be a good direction to introduce coordinatively unsaturated metal sites into the porous polymers for enhanced gas capture and separation. The aqueous phase preparation of these frameworks reduces both the cost and the environmental impact of synthesis. Importantly, the gas uptake properties and functions of these polymers can be simply tuned by varying metal ions for high affinity. The presence of transition metal complexes with open metal sites could also impart new catalytic functions such as CO2 conversion into the resulting porous coordination polymers and facilitate the development of new functional porous polymers. ■ EXPERIMENTAL SECTION Materials and instrumentation: Triptycene, CuSO4.5H2O (≥98.0 %), tin(II) chloride anhydrous (>99 %), sodium hydroxide (≥97.0%, pellets) and fuming HNO3 were purchased from Sigma Aldrich. NH4OH (28~30 %) and NiCl2.6H2O (97.0%) was obtained from SAMCHUN. 2,3,6,7,14,15-Hexaamoniumtriptycene hexachloride (HATT.6HCl) was synthesized following the previously reported literature procedure in two-steps starting from triptycene.56. All chemical reagents and solvents were used without further purification. The X-ray photoelectron spectroscopy (XPS) analysis was carried out in X-ray photoelectron spectroscopy (K-Alpha)

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instrument with ion gun beam current up to 5.0 uA @ 4kV. Thermogravimetric analysis (TGA) data was obtained by using a NETZSCH-TG 209 F3 instrument and the samples were heated to 800oC at a rate of 10oC min-1 under air atmosphere. Fourier-transform infrared spectroscopy (FT-IR) spectra were recorded on ATR accessory by using IRT racer-100 instrument. The scanning electron microscopy (SEM) images were obtained by using a Hitachi S-4800 FE-SEM at 2.0 - 10 kV. Powder X-ray diffraction (PXRD) patterns of polymers were collected over the 2Ѳ range 5 to 60o on a Rigaku D/MAX-2500 (18 kW) multi-purpose high power X-ray diffractometer. Elemental analysis (C, H, N) was done on a Flash EA 2000 (Series) [C, H, N, S]. Transmission electron microscopy (TEM) analysis was performed using 200kV Field-emission Transmission Electron Microscope (FE-TEM) Synthesis of Cu(II)-PCP: The solution of CuSO4.5H2O (67 mg, 0.27 mmol) in 50 mL of H2O was added to the mixture of 2,3,6,7,14,15-Hexaammoniumtriptycene hexachloride (HATT.6HCl, 100 mg, 0.18 mmol) and NH4OH (28~30 %, 3.78 mL) in H2O (50 mL) at room temperature. The resulting mixture was refluxed for 2 h. During the course of the reaction, immediate formation of black colored precipitate was observed. After cooling the reaction mixture to room temperature, the resulting precipitate was filtered and washed three times with H2O and finally with acetone. The black solid (85 mg) was isolated and dried under vacuum at 40oC for 24 h. Synthesis of Ni(II)-PCP: The solution of NiCl2.6H2O (63 mg, 0.27 mmol) in 50 mL of H2O was added to the mixture of 2,3,6,7,14,15-Hexaammoniumtriptycene hexachloride (HATT.6HCl, 100 mg, 0.18 mmol) and NaOH (49.7 mg, 1.26 mmol) in H2O (50 mL). The resulting mixture was refluxed for 24 h. During the course of the reaction, the formation of a black precipitate was observed. After cooling the reaction mixture to room temperature, it was filtered. The black colored precipitate was washed three times with H2O and finally with acetone. The black solid (121 mg) was then dried under vacuum at 40°C for 24 h. Breakthrough test: Breakthrough experiment was carried out by the system illustrated in the Figure S10. Each adsorbent was packed in the testing column for CO2/CH4 separation. Before the measurements, the column containing adsorbent was flushed with He gas for 10 min and then evacuated at 353 K, by which residual gas molecules in the column was completely removed. After stabilizing the temperature of testing apparatus at 298 K, CO2/CH4 mixtures were injected at the target pressure and flow rate. Effluent from the column was directly injected into GC and its composition was analyzed. Breakthrough time was determined to be when CH4 was detected. The CO2 storage capacity of adsorbents was calculated by using the following equation: 1

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Surface and gas uptake analysis: The adsorption and desorption measurements of Cu(II)- and Ni(II)-PCPs were performed at Micrometrics ASAP 2020 and porosimetry analyzer by measuring Ar isotherms at 87 K. Before the measurements, the samples were degassed under vacuum at 80oC for 24 h. The BET and Langmuir models were used to calculate the surface areas of Cu(II)- and Ni(II)PCPs in the pressure range where the V(1-P/P0) term increases with P/P0 for BET calculation based on Rouquerol plot (Figure S7). Nonlocal Density Functional Theory (NLDFT) was used to obtain the pore size distributions of samples. The low pressure CO2 and CH4 adsorption isotherms of samples were collected at 273, 298 and 323 K up to 1 bar. The temperature was kept constant during the measurements by using a circulator. The low pressure H2 adsorption isotherms of samples were measured at 77 and 87 K up to 1 bar. Isosteric heats absorption, Qst, of CO2 and H2 were calculated by using the standard calculation routines in the data master offline data reduction software (Micrometrics), that is Clausius–Clapeyron equation. Gas Selectivity Calculation: All adsorption isotherms are fitted using either single–site Langmuir model or a dual–site Langmuir model. The CO2/CH4 selectivity for natural and landfill gas mixtures were based on Ideal Adsorbed Solution Theory (IAST) using Mathematica.

ASSOCIATED CONTENT Supporting Information. Additional spectroscopic data and experimental details of breakthrough experiments. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors and all authors have given approval to the final version of the manuscript.

Funding Sources National Research Foundation of Korea and KAISTKUSTAR institute.

ACKNOWLEDGMENT This research was supported by the KUSTAR-KAIST Institute, Korea, under the R&D program supervised by the KAIST. This work was also supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (NRF-2014R1A4A1003712).

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Synthesis of Highly Porous Coordination Polymers with Open Metal

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