Effective CO2 and CO Separation Using [M2(DOBDC)](M=Mg, Co, Ni

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Energy, Environmental, and Catalysis Applications 2

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Effective CO and CO Separation Using [M(DOBDC)](M=Mg, Co, Ni) with Unsaturated Metal Sites and Excavation of Their Adsorption Sites Hyunuk Kim, Muhammad Sohail, Kanghoon Yim, Young Cheol Park, Dong Hyuk Chun, Hak Joo Kim, Seong Ok Han, and Jong-Ho Moon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20450 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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ACS Applied Materials & Interfaces

Effective CO2 and CO Separation Using [M2(DOBDC)](M=Mg, Co, Ni) with Unsaturated Metal Sites and Excavation of Their Adsorption Sites Hyunuk Kim,*,†,§ Muhammad Sohail,†,§ Kanghoon Yim, Young Cheol Park,‡ Dong Hyuk Chun,‖ HakJoo Kim,‡ Seong Ok Han,† Jong-Ho Moon*,‡ †Energy

Materials Laboratory, ‡Greenhouse Gas Laboratory, Platform Research Laboratory and ‖Clean

Fuel Laboratory, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea. §Advanced

Energy and System Technology, University of Science and Technology, Daejeon 305-350, Republic of Korea. [email protected] and [email protected]

Keywords: CO2/CO Separation, Metal-Organic Frameworks, Hexagonal OH Clusters, Unsaturated Metal Sites, CO2 anmd CO Adsorption Sites

ABSTRACT: Isostructural [M2(DOBDC)(EG)2] (M=Mg, Co, Ni) frameworks are first synthesized by controlling the pH* in the reaction medium. Coordinated ethylene glycols form a hexagonal OH cluster, which works as a template to grow single-crystals with high crystallinity. After the liberation of solvated molecules, [M2(DOBDC)] shows notably higher surface areas than the reported values and completely different CO2 and CO separation properties depending on the kinds of unsaturated metal. Therefore, breakthrough experiments using a CO2/CO mixed gas show that Mg-MOF has a longer breakthrough time for CO2 than for CO whereas Co/Ni-MOFs have longer breakthrough times for CO ACS Paragon Plus Environment

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than for CO2. Apart from CO2 and CO, other gases such as CH4, H2 and N2 were almost not adsorbed at all in these materials at 298 K. To reveal the role of unsaturated metal sites, CO2 and CO adsorption sites are unequivocally determined by single-crystal X-ray diffraction analysis. One of very interesting discoveries is that there are two CO2 and CO adsorption positions (site A and site B) in the hexagonal channels. Site A is the unsaturated metal center working as Lewis acidic sites, and site B is the secondary adsorption site located between two A sites. A close inspection of crystal structures reveals that unsaturated Co(II) and Ni(II) sites adsorb both CO2 and CO whereas the unsaturated Mg(II) sites strongly captures only CO2, not CO. Density functional theory calculations elucidate the discrepancy in CO affinity: Co(II) and Ni(II) form strong π-back-donating bonds with CO via electron transfer from the d orbitals of the transition metals to the antibonding molecular orbitals of CO, while Mg(II) does not participate in electron transfer or orbital overlap with CO. This observation provides new insight into the synthesis of novel functional materials with high CO2/CO separation performance.

1. INTRODUCTION CO2 and CO separation and recovery play important roles not only in reducing the emission of greenhouse gas but also in synthesizing value-added chemicals in various industrial refinery processes.1-11 In refineries for the Fischer-Tropsch gas-to-liquid process, selective CO2 and CO separation is essential for refining and recycling syngas synthesized from hydrocarbons.12-14 In conventional applications, pressure swing adsorption technology based on adsorbents such as zeolites has been used for CO2/CO capture.15-16 In addition, CO2 and CO separation from the flue gas in steel mills can help reduce the use of cokes and cope with environmental issues related to global warming.1719

In addition to industrial purposes, NASA has also been investigating the CO2/CO separation process

for the generation of O2 on Mars. The Mars in situ resource utilization program is related to the conversion of CO2, which is abundant in the atmosphere of Mars, to CO and O2 using a solid oxide electrolysis cell.20 Therefore, selective CO capture from the product of such a fuel cell is an essential process for obtaining pure O2 in Mars. The separation of these gases, however, is quite difficult because

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of their similar properties. To address these issues, novel adsorbents for CO2 and CO separation should be developed. Metal-organic frameworks (MOFs) assembled by metal ions and organic ligands have exhibited extraordinary gas adsorption and separation properties.3-5, 9-11, 21-26 Upon the removal of guest molecules coordinated to metal sites, unsaturated metal sites serve as Lewis acids for CO2 and CO. Isostructural [M2(DOBDC)] (MOF-74/CPO-27) frameworks with unsaturated metal sites have shown interesting CO2 and CO adsorption properties. The role of their unsaturated metal sites for CO2 and CO adsorption have been well documented in the literature.27-37 However, there is no systematic study about CO2/CO separation depending on the kinds of unsaturated metal sites. Here, we report the synthesis of isostructural [M2(DOBDC)(EG)2] (M=Mg, Co, Ni) series with hexagonal OH clusters, different CO2/CO separation properties on the kinds of unsaturated metal, and characterization of their adsorption sites by single-crystal X-ray crystallography and density functional theory (DFT) calculations.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Crystal Structure Determination. Metal salts (Mg(II), Co(II) or Ni(II)) were dissolved in a small amount of N,N-diethylformamide (DEF), and then ethylene glycol (EG) was added to the solutions. The apparent pH (pH*) values of the Mg(II), Co(II) and Ni(II) solutions were 3.01, 1.27 and 2.93, respectively. Finally, melamine was added to the solutions to adjust the pH*. The addition of melamine to the Mg(II), Co(II) and Ni(II) solutions increased the pH* of the reaction medium (to 4.09, 2.93 and 3.44, respectively), leading to deprotonation of H4DOBDC.33 Solvothermal reactions performed in glass tubes at 120 °C for 1 day produced single crystals of [M2(DOBDC)(EG)2]EG0.33H2O (M = Mg(1_EG), Co(2_EG), Ni(3_EG)) (Figure S1). Single-crystal X-ray diffraction analysis revealed that 1_EG, 2_EG and 3_EG have an identical skeleton like MOF-74/CPO-27, but their channels are fully occupied with three crystallographically independent EG molecules: two coordinated EGs and one solvated free EG. A close inspection of crystal structures revealed that 6 coordinated EGs form hydrophilic OH clusters connected by hydrogen ACS Paragon Plus Environment

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bonds in the hexagonal channels (Figure 1). The aperture size of the OH cluster is 2.3 Å. The nearestneighbor O-O distances for the hydrogen bonds between EGs are 2.74 Å, which is comparable to those in ice Ih, indicating the strong interactions between these molecules (Figure S2).34 These hydrophilic OH clusters work as a template to grow single crystals with high crystallinity. The powder XRD profiles of the as-synthesized crystals matched well with the one simulated based on the single-crystal X-ray structure (Figure S3). Considering the formula units of 1_EG, 2_EG and 3_EG, EGs occupy 44.2 %, 38.1 % and 38.2 % of the total weight percentage, respectively. TGA data of these materials revealed that the EGs were not completely removed up to 250 °C because of the strong interactions with each other and coordination to the metal sites through a MOH bond (Figure S4).

Figure 1. X-ray crystal structures of 1_EG, 2_EG and 3_EG along the c axis

2.2. Guest Exchange in a Single Crystal. Single crystals of 1_EG, 2_EG and 3_EG were soaked in anhydrous methanol and heated up to 200 °C in a Teflon-lined bomb reactor. Single-crystal X-ray analysis showed that coordinated EG and free EG molecules were completely replaced with MeOH without any notable structural change except a reduction in the lattice parameter of the c axis (Figure 2). Two crystallographically independent methanol molecules were observed in the crystal structures: one is coordinated to the metal center, and the other is located at the corner of the hexagonal channels ACS Paragon Plus Environment

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(Figure S5). TGA data showed that methanol was completely removed up to 250 °C, indicating the complete liberation of solvated molecules to generate unsaturated metal sites (Figure S7-S9). 1H-NMR analysis of crystals digested in DCl/D2O/DMSO confirmed the removal of methanol after evacuation at 250 °C under vacuum (Figure S10).

Figure 2. Exchange of coordinated guest molecules in a single crystal

2.3. CO2 and CO Adsorption Properties. The nitrogen adsorption isotherms for guest-free 1, 2 and 3 at 77 K showed typical BET type I behavior for a microporous material (Figure S11). The surface areas of these materials are notably higher than the reported values in the literature (Table 1).29,30,38 Such high surface areas are attributed to the fact that as-synthesized [M2(DOBDC)(EG)2] have high crystallinity by a template of OH hexagonal clusters and maintain their integrity after the evacuation of solvated molecules. Table 1 summarizes the surface area, and CO2 and CO adsorption properties of [M2(DOBDC)] (M=Mg, Co, Ni). According to the CO2 and CO adsorption isotherms, the unsaturated metal sites in 1, 2 and 3 showed different affinities for CO2 and CO depending on the metal characteristics. The adsorption affinity of CO2 to unsaturated metal sites is highest in Mg2+, followed by Ni2+, and Co2+ (Mg2+ > Ni2+ > Co2+). Owing to this difference in affinity for CO2, 1 has a higher CO2 capacity (1.67 mol/mol) than 2 and 3 (0.81 and 1.30 mol/mol, respectively) at 0.1 bar (Figure 3a). To estimate the binding affinity, the enthalpies of CO2 adsorption were calculated by the virial equation (Figure S15-S17). As shown in Figure S18, the enthalpies of CO2 adsorption of 1, 2 and 3 are 44.0, 35.0 and 39.8 kJ/mol at zero coverage, respectively, which is consistent with the order of steep CO2 uptake ACS Paragon Plus Environment

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at low pressure. Unlike CO2 uptake, however, 1 showed much lower CO gas uptake (0.35 mol/mol and 4.01 mol/mol at P/P0 = 0.1 and 1, respectively) than 2 and 3 because of its low affinity for CO (Figure 3b). 2 and 3 showed much faster and higher CO uptake at low pressure, indicating their strong affinity for CO gas. The enthalpies of CO adsorption for 1, 2 and 3 are 38.3, 55.5 and 59.7 kJ/mol, respectively, indicating that the order of CO affinity is Ni2+> Co2+ >> Mg2+ (Figure S22). Apart from CO2 and CO, other gases such as CH4, H2 and N2 were almost not adsorbed at all in these materials at 298 K (Figure S24-26). To further investigate the characteristic CO2/CO separation properties of the unsaturated Mg(II), Co(II) and Ni(II) sites, dynamic column breakthrough experiments were performed at 30 °C and 1 atm using a CO2/CO gas mixture (Figure 3c). 1 showed a longer breakthrough time for CO2 than for CO, whereas 2 and 3 exhibited longer breakthrough times for CO than for CO2, indicating the different affinities of the metals for CO2 and CO. This observation indicates that the Mg-based MOF has a stronger affinity for CO2 than CO and that the Co and Ni-based samples have a stronger affinity for CO than CO2. In particular, the extremely short CO breakthrough time of 1 compared with the other MOFs is noteworthy, as it indicates that Mg(II) does not have effective adsorption sites for CO. Consequently, the difference in breakthrough times between CO2 and CO is in the order of Mg2+ > Co2+ > Ni2+.


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Figure 3. (a) CO2 and (b) CO adsorption (open symbols) and desorption (closed symbols) isotherms of 1 (blue circles), 2 (black squares) and 3 (brown triangles), (c) Breakthrough curves of a CO2 and CO mixture in He (25:25:50 vol%) at 323 K for 1 (top), 2 (middle) and 3 (bottom) (brown circles for CO2 and blue squares for CO).


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Table 1. Comparison of surface area, CO2 and CO adsorption data of [M2(DOBDC)] (M=Mg, Co, Ni)

[M2(DOBDC)]

Mg(II)

SA, m2/g

CO2 capacity, mol/mol (wt.%) at 298 K

Enthalpy of CO2 adsorption, kJ/mol

0.35 (4.01)

1.22 (14.09)

Ref.

38.3

7.56

This wor k

1775a/2149b

1.67 (30.27)

2.29 (41.51)

44.0

1495a/1905b

(23.6)

(35.2)

47

29

1957b

1.6

2.2

43.5

30 0.26

1.0

35.4

1.47 (13.18)

1.82 (16.30)

55.5

38 This wor k

1382a/1782b

0.81 (12.01)

2.12 (30.19)

34.4

1080a

(11.7)

(30.6)

37

29

1438b

0.8

2.2

33.6

30 1.4

1.8

48.8

1.61 (14.51)

1.80 (16.22)

59.7

0.95

38 This wor k

1350a/1634b

1.30 (18.32)

2.22 (31.37)

38.8

1070a

(11.6)

(25.6)

41

29

1574b

1.2

2.2

38.6

30

1574b a BET

1.0 bar

CO2/CO selectivit y

1.0 bar

1433b

Ni(II)

0.1 bar

Enthalpy of CO adsorption, kJ/mol

0.1 bar

1957b

Co(II)

CO capacity, mol/mol (wt.%) at 298 K

1.6

1.8

52.7

0.14

38

surface area, b Langmuir surface area

2.4. CO2 and CO Adsorption Sites. To gain insight into the CO2 and CO adsorption sites on the atomic scale, we determined the crystal structures of CO2/CO-adsorbed 1, 2 and 3 by synchrotron radiation single-crystal X-ray diffraction experiments.35-37 Single crystals of activated 1, 2 and 3 were sealed with CO2 and CO in a glass capillary, and then X-ray diffraction data were collected using highflux synchrotron radiation. The crystal structures of CO2-adsorbed 1, 2 and 3 show two CO2 adsorption positions (site A and site B) in the hexagonal channels along the c axis (Figure 4). CO2 molecules at site A are coordinated to Lewis acidic sites of metal(II), indicating monolayer adsorption of CO2 on the surface of the adsorbents. The coordination bond distances of M2+O (Mg2+O2C, 2.134(2) Å;


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Co2+O2C, 2.270(2) Å; and Ni2+O2C, 2.079(6) Å) are much shorter than the sum of the van der Waals radii, indicating strong interactions between the unsaturated metal sites and CO2 molecules. The CO2 molecules at site A adopt the angular geometry of metalOCO complexes (MgOC = 123.2(8), (CoOC = 141.2(5), (NiOC = 133.4(1.5)°), which was also confirmed by periodic DFT calculations.15 The occupancy of CO2 molecules at site A was 77 %, 86 % and 100 % for 1_CO2, 2_CO2 and 3_CO2, respectively. According to the formula unit [M2(DOBDC)] (M = Mg, Co, Ni), therefore, the estimated amounts of CO2 adsorbed at site A are 1.54, 1.72 and 2.00 CO2 per the formula unit, respectively, which are slightly lower than those calculated from the adsorption isotherms at 1 bar, 298 K. Site B is the secondary adsorption site located between two A sites. The number of CO2 molecules adsorbed at site B is 1.66, 1.62 and 1.68 per the formula unit, respectively. In the case of 1_CO2 and 2_CO2, each oxygen atom of the CO2 molecule in site B interacts with the carbon atom of the CO2 molecule at site A through quadrupole-quadrupole interactions in a T-shaped arrangement (O2CO2C distances: 2.195 and 2.944 Å for 1_CO2, 3.075 and 3.108 Å for 2_CO2) (Figure 4a and 4b). In the case of 3_CO2, however, the two CO2 molecules at sites A and B interact with each other in a slipped-parallel geometry (O2CO2C distances, 2.202 Å and 2.569 Å) (Figure 4c). As a result, all the adsorbed CO2 molecules interacted with each other through quadrupole-quadrupole interactions along the hexagonal channels lying on the c axis.


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Figure 4. Single-crystal X-ray crystal structure of CO2-adsorbed (a) 1, (b) 2 and (c) 3. The coordination bonds between CO2 and site A are highlighted in purple. Dotted lines represent quadrupole-quadrupole interactions along the hexagonal channels lying on the c axis The crystal structures of CO-adsorbed 1, 2 and 3 also show two independent CO adsorption sites, A and B, similar to the CO2 adsorption sites (Figure 5). Site A is located at the unsaturated metal center, and site B is the secondary adsorption site between site A. CO molecules at site B do not have notable interactions with the framework and have only van der Waals interactions with CO molecules at site A. In the case of CO-adsorbed 2 and 3, CO molecules at site A were bound to unsaturated metal centers via the OC∙∙∙M2+ adsorption mode. The bond distances of Co2+/Ni2+∙∙∙CO are 2.151(2) Å and 2.147(3) Å, respectively. The occupancies of CO molecules at site A of CO-adsorbed 2 and 3 determined by X-ray crystallography are 95 % and 100 %, respectively. Considering the adsorption strength and occupancy of CO-adsorbed 2 and 3, site A is the predominant position for CO adsorption. The CO molecule at site A in CO-adsorbed 1 is also located near unsaturated Mg2+ center with 82 % occupancy. The Mg2+CO distance, however, is 2.420(2) Å, which is much longer than the Co2+/Ni2+∙∙∙CO distances. In situ FT-IR analysis also showed no binding mode for Mg2+∙∙∙CO, while the vibration modes of Co2+ and Ni2+∙∙∙CO were found at 2160 cm-1 and 2170 cm-1, respectively (Figure S27).38,39

Figure 5. Single-crystal X-ray crystal structure of (a) CO-adsorbed 1, (b) 2 and (c) 3.


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To identify the significant difference in CO adsorption between activated 1 and 2/3, DFT calculations were conducted for CO-adsorbed 1, 2 and 3 (Figure 6). In the considered DFT models, only site A is fully occupied by a CO molecule since site A is energetically more preferable than site B. From the calculated models, a trend that is consistent with the experimental trend is found: the Mg2+∙∙∙CO distance (2.604 Å) is much longer than the Co2+/Ni2+∙∙∙CO distances (2.145 Å and 2.488 Å, respectively). The short bond distances between Co2+/Ni2+ and the CO molecules are due to π-backdonating bonds formed via electron transfer from d orbitals of the transition metals to the antibonding molecular orbitals of CO. The electron transfer from Co2+ and Ni2+ to CO is directly described in the band-decomposed partial charge density (Figure 6c,e), which is obtained from the band that has the largest overlap in the partial density of states (PDOS) of Co2+/Ni2+ and the carbon atom from CO (Figure 6d,f). However, in CO-adsorbed 1, neither electron transfer nor noticeable overlap in the PDOS is found (Figure 6a,b) because no valence electrons exist at the Mg2+ site, while the Co2+ and Ni2+ sites have localized d orbitals. This lack of valence electrons implies that the existence of localized d electrons is the origin of the stronger adsorptions in activated 2 and 3 compared with activated 1. In contrast, in the case of CO2-adsorbed 1, 2 and 3, electron transfer from the d orbital of Co2+/Ni2+ to the CO2 molecule is not observed. (Figure S29) Unlike dipolar CO molecules with lone-pair electrons, the interaction of CO2 molecules with localized d orbitals at metal sites is difficult. Therefore, activated 1, 2 and 3 have similar adsorption abilities for CO2 regardless of whether they have transition metals or not.


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Figure 6. Electronic structures calculated using DFT. (a) Isosurface of total charge density and (b) partial density of states (PDOS) of CO-adsorbed 1. (c) Isosurface of partial charge density and (d) PDOS of CO-adsorbed 2. (e) Isosurface of partial charge density and (f) PDOS of CO-adsorbed 3. The partial charge densities in (c) and (e) are obtained from the states indicated by the red dashed ellipse. 3. CONCLUSIONS Isostructural [M2(DOBDC)(EG)2] (M=Mg, Co, Ni) frameworks with hexagonal OH clusters were first synthesized by controlling the pH* in the reaction medium. Single-crystal X-ray analysis revealed that six coordinated EGs form hexagonal OH clusters by hydrogen bonds in the channels. Such hexagonal OH clusters work as a template to grow single-crystals with high crystallinity. The solvated EGs in the frameworks were completely exchanged with methanol in a single crystal without structural deformation. After the removal of methanol, [M2(DOBDC)] showed notably higher surface areas than the reported values. Such high surface areas are attributed to the fact that as-synthesized [M2(DOBDC)(EG)2] have high crystallinity by a template of OH hexagonal clusters and maintain their integrity after the evacuation of solvated molecules. More interestingly, [M2(DOBDC)] showed


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completely different CO2 and CO adsorption and separation properties depending on the kinds of unsaturated Mg(II), Co(II), and Ni(II) sites. Considering the heat of CO2 adsorption estimated by the virial equation, the CO2 affinity for the unsaturated metal sites follows the order Mg2+ > Ni2+ > Co2+, while the CO affinity for unsaturated metal sites follows the order Ni2+ > Co2+ >> Mg2+. Owing to such different affinity for unsaturated metal sites, breakthrough experiments using a CO2/CO mixed gas showed that 1 has a longer breakthrough time for CO2 than for CO whereas 2 and 3 have longer breakthrough times for CO than for CO2. Apart from CO2 and CO, other gases such as CH4, H2 and N2 were almost not adsorbed at all in these materials at 298 K. To reveal the role of unsaturated metal sites, CO2 and CO adsorption sites are unequivocally determined by single-crystal X-ray diffraction analysis. One of very interesting discoveries is that there are two CO2 and CO adsorption positions (site A and site B) in the hexagonal channels. Site A is the unsaturated metal center working as Lewis acidic sites, and site B is the secondary adsorption site located between two A sites. A close inspection of CO2 or CO-adsorbed 1, 2 and 3 revealed that the unsaturated Co(II) and Ni(II) sites (site A) of 2 and 3 preferentially adsorb both CO2 and CO gases, whereas the unsaturated Mg(II) sites (site A) strongly capture only CO2, not CO. DFT calculations elucidate the discrepancy in CO affinity for unsaturated metal sites: Co(II) and Ni(II) form strong π-back-donating bonds with CO via electron transfer from d orbitals of the transition metals to the antibonding molecular orbitals of CO, while Mg(II) does not participate in electron transfer or orbital overlap with CO because of the lack of valence electrons at the Mg2+ site. This observation regarding the adsorption of CO2 and CO to unsaturated metal sites provides new insight into the synthesis of novel materials with high CO2/CO separation performance.

4. EXPERIMENTAL SECTION 4.1. Materials and General Methods. All the reagents and solvents were purchased from commercial sources and used without further purification. The single-crystal X-ray diffraction data were collected on an ADSC Quantum 210 CCD diffractometer with synchrotron radiation at Pohang Accelerator Laboratory (PAL, 2D Supramolecular Beamline). The X-ray powder diffraction patterns were recorded from 5 º to 50 º with an interval of 0.02 º on a Rigaku D/max 2500PC equipped with a Cu sealed tube (λ ACS Paragon Plus Environment

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= 1.54178 Å). In situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) data were collected on a Thermo Scientific Nicolet iS50 FT-IR spectrometer. Scanning electron microscopy (SEM) images were acquired on a HITACHI S-4800 SEM. TGA data were obtained on a TA 2050 instrument with a heating rate of 10 °Cmin-1 under a N2 atmosphere. Gas sorption isotherms were volumetrically recorded on a BELSORP-MAX, and all the NMR data were recorded on a Bruker Fourier 300 spectrometer. 4.2. Synthesis of 1_EG, 2_EG and 3_EG. Mg(NO3)2∙6H2O (0.600 g, 2.340 mmol), Co(NO3)2∙6H2O (0.600 g, 2.062 mmol) or Ni(NO3)2∙6H2O (0.600 g, 2.063 mmol) was completely dissolved with H4DOBDC (0.116 g, 0.590 mmol) in 2.0 mL of N,N-diethylformamide, and then 10 mL of EG was added. Finally, melamine (0.100 g, 0.793 mmol) was added to the solution. The melamine should be added last to prevent precipitation. The solution was divided into 1.0 ml aliquots that were added to separate glass tubes and flame-sealed. The sealed tubes were heated at 130 °C in an oven for 20 hrs, and then single-crystals were collected. 4.3. Guest Exchange of 1_EG, 2_EG and 3_EG. To exchange EG molecules with methanol, single crystals of 1_EG, 2_EG and 3_EG were soaked in anhydrous methanol (10 mL) and then heated at 200 °C for 4 days in a 20 mL Teflon-lined bomb reactor to completely replace EG with methanol. 4.4. Single-Crystal X-ray Crystallography. The diffraction data from all single crystals of 1_EG/MeOH, 2_EG/MeOH, and 3_EG/MeOH mounted on a loop were collected at 100 K on an ADSC Quantum 210 CCD diffractometer with synchrotron radiation (λ = 0.70000 Å) at Supramolecular Crystallography Beamline 2D, PAL, Pohang, Korea. To investigate the CO2 and CO adsorption sites in 1, 2 and 3, we also collected diffraction data from CO2- or CO-adsorbed 1, 2 and 3. Single crystals of 1_MeOH, 2_MeOH and 3_MeOH were placed in a thick-wall glass capillary and then evacuated at 200 ºC for 4 days. The guest-free crystals were filled with CO2 and CO gas at 1 bar and then sealed by a torch flame at RT. The diffraction data of CO2 or CO-adsorbed 1, 2 and 3 were collected at 100 K. The raw data were processed and scaled using the HKL3000 program. The structure was solved by direct methods, and refinements were carried out with full-matrix least-squares on F2


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with appropriate software implemented in the SHELXTL program package.40 Details about crystallography with CCDC numbers are included in the supplemental information. 4.5. Breakthrough Experiments. Dynamic column breakthrough experiments were carried out using a home-built setup coupled with a quadrupole mass spectrometer (ThermoStar, Pfeiffer Vacuum, Germany). MeOH-exchanged MOF samples (0.340 g for 1, 0.318 g for 2, 0.322 g for 3) were packed into a stainless-steel column (inner diameter of 0.995 cm). The packed column containing the MOF sample was mounted in an empty GC oven (Agilent 6890, USA) to precisely control and maintain the system temperature. Before the breakthrough experiments, the packed column was heated at 473 K for 24 hrs under constant He flow (100 ccmin-1 at 1 atm) to remove the MeOH included in the MOFs. Then, the system temperature was lowered to 300 K, and CO2/CO/He (25/25/50 vol%) mixture gas at a flow rate of 100 ccmin-1 was introduced through the bypass line by switching a 4-port valve (Valco Instruments Co., USA). The system pressure was maintained at 1 atm by controlling the back pressure regulator (Tescom, USA). 4.6. In situ DRIFTS. 1_MeOH, 2_MeOH and 3_MeOH were placed in a sample cup and activated at 200 °C under He flow for 1 day. After cooling to room temperature, the background of the activated samples was taken, and then in situ DRIFTS data were measured under CO gas flow. 4.7. Electronic structure calculations. The Vienna ab initio simulation package (VASP)41 is used for DFT calculations to obtain the electronic structures of CO-adsorbed 1, 2 and 3. We employ the generalized gradient approximation (GGA) for the exchange-correlation functional between electrons. Since the positions of d levels in 3d-transition metal ions are significantly overestimated in conventional DFT calculations, we adopt GGA+U calculations for Co2+ and Ni2+. For the effective onsite interaction energy U−J, 3.3 eV and 6.4 eV are used for Co2+ and Ni2+, respectively, which show general agreements with the experimental formation enthalpies.42 First, to optimize the CO-adsorbed structures, the primitive cells of 1, 2 and 3 (Mg, Co, Ni) without adsorbed molecules are fully optimized. Then, CO-adsorbed structures (6 CO molecules per unit cell) are optimized with atomic relaxation. The energy cutoff of the plane-wave basis is set to 400 eV, and gamma-only k-point sampling is used for


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structural relaxation to ensure total energy and pressure convergence within 10 meV per atom and 10 kbar, respectively. For density of states (DOS) calculations, a 3 ×3 ×3 Monkhorst-pack grid is used for k-point sampling, while the band-decomposed charge density is calculated using the gamma point only.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI DOI: 10.1039/x0xx00000x: Single-crystal and powder XRD data, Crystal images, structural figures, crystallographic information tables, heat of adsorption, TGA data, NMR data, gas adsorption isotherms, and FT-IR data.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS. This work was conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B9-2441) and was also supported by the Center for Advanced Meta-Materials (CAMM) funded by the Ministry of Science and ICT (MSIT) as a Global Frontier Project. We also gratefully acknowledge the National Research Foundation of Korea (NRF) grant (2016R1C1B2010838). X-ray diffraction studies with synchrotron radiation were performed at the Pohang Accelerator Laboratory (Beamline 2D-SMC) supported by MSIT and POSTECH.

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Table of Contents Graphics

CO2 and CO separation depending on the kinds of unsaturated metal of [M2(DOBDC)] (M=Mg, Co, Ni)


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X-ray crystal structures of 1_EG, 2_EG and 3_EG along the c axis 373x214mm (150 x 150 DPI)



Exchange of coordinated guest molecules in a single crystal 297x90mm (150 x 150 DPI)


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(a) CO2 and (b) CO adsorption (open symbols) and desorption (closed symbols) isotherms of activated 1 (blue circles), 2 (black squares) and 3 (brown triangles), (c) Breakthrough curves of a CO2 and CO mixture in He (25:25:50 vol%) at 323 K for activated 1 (top), activated 2 (middle) and activated 3 (bottom) (brown circles for CO2 and blue squares for CO). 377x295mm (150 x 150 DPI)



Single-crystal X-ray crystal structure of CO2-adsorbed (a) 1, (b) 2 and (c) 3. The coordination bonds between CO2 and site A are highlighted in purple. Dotted lines represent quadrupole-quadrupole interactions along the hexagonal channels lying on the c axis 774x324mm (150 x 150 DPI)


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Single-crystal X-ray crystal structure of (a) CO-adsorbed 1, (b) 2 and (c) 3 787x301mm (150 x 150 DPI)



Electronic structures calculated using DFT. (a) Isosurface of total charge density and (b) partial density of states (PDOS) of CO-adsorbed 1. (c) Isosurface of partial charge density and (d) PDOS of CO-adsorbed 2. (e) Isosurface of partial charge density and (f) PDOS of CO-adsorbed 3. The partial charge densities in (c) and (e) are obtained from the states indicated by the red dashed ellipse. 305x258mm (150 x 150 DPI)


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CO2/CO separation depending on the kinds of unsaturated Mg(II), Co(II) and Ni(II) sites 404x469mm (150 x 150 DPI)


Effective CO2 and CO Separation Using [M2(DOBDC)](M=Mg, Co, Ni

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