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Robustness, Selective Gas Separation, and Nitrobenzene Sensing on Two Isomers of Cadmium Metal−Organic Frameworks Containing Various Metal−O−Metal Chains Lizhen Liu,† Zizhu Yao,† Yingxiang Ye,† Liangji Chen,† Quanjie Lin,† Yisi Yang,† Zhangjing Zhang,*,†,§ and Shengchang Xiang*,†,§

Inorg. Chem. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/21/18. For personal use only.

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Fujian Provincial Key Laboratory of Polymer Materials, College of Chemistry and Materials Science, Fujian Normal University, 32 Shangsan Road, Fuzhou 350007, PR China § State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China S Supporting Information *

ABSTRACT: Poor stability has been one of the major difficulties affecting to the practical application of metal−organic frameworks (MOFs). In this work, we obtained two 3D structurally isomeric Cd-MOFs, {[Cd6(NH2Me2)2(PTB)4(HCOO)2(H2O)]·(DMF)13·(H2O)4}n (FJU-35) and {[Cd6(NH2Me2)2(PTB)4(HCOO)2]·(DMF)6·(H2O)2}n (FJU-36) (H3PTB = pyridine-2,4, 6-tribenzoic acid) containing different CdII−O−CdII chains by varying the addition agents. FJU-35 with coordinated solvent and formate in asymmetric μ3-η1:η2 coordination mode within the CdII−O−CdII chains is vulnerable to external attacks and is apt to collapse after activation, while FJU-36 with no coordinated solvent in the CdII−O−CdII chains but fully protected by the carboxylates from the ligands and the symmetric formate in the coordination mode μ3-η2:η2 is stable, and its activated sample shows efficient separation of C2H2/CH4 and C2H2/CO2 mixtures. Conversely, FJU-35 with more vulnerability is more sensitive to the detection of nitrobenzene than FJU-36.

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interactions.29 Nevertheless, it is still a challenge to construct stable MOF materials. In this work, we report an example to adjust the stability, gas adsorption, and sensing properties of MOFs by tuning metal− O−metal (M−O−M) chains. Two 3D structurally isomeric Cd-MOFs, {[Cd 6 (NH 2 Me 2 ) 2 (PTB) 4 (HCOO) 2 (H 2 O)]· (DMF)13·(H2O)4}n (FJU-35) and {[Cd6(NH2Me2)2(PTB)4(HCOO)2]·(DMF)6·(H2O)2}n (FJU-36) containing different infinite CdII−O−CdII chains were synthesized (H3PTB = pyridine-2,4,6-tribenzoic acid, Scheme 1). FJU-35 with the coordination solvent and formate in asymmetric μ3-η1:η2 coordination mode within the CdII−O−CdII chains is unstable. In contrast, FJU-36 exhibits high rigidity, as its CdII−O−CdII chains with no solvent coordinated are fully protected by the carboxylates from the ligands and the symmetric formate in the coordination mode μ3-η2:η2. Gas adsorption and dynamic breakthrough experiments demonstrate that the activated FJU-36 can selectively separate C2H2/CO2 and C2H2/CH4 mixtures. Both FJU-35 and FJU-36 show strong photoluminescence due to the existence of CdII−O−CdII chains, which have significant quenching effect on nitrobenzene.

INTRODUCTION

Metal−organic frameworks (MOFs) have attracted widespread attention due to their high porosity, structural tailorability, and suitable host−guest interactions, which make them ideal materials for gas adsorption and separation,1−7 sensors,8−11 catalysis,12,13 etc.14,15 Generally, permanent porosity and stability are prerequisites for various applications of the MOF materials. Therefore, how to improve the stability and to retain the permanent porosity of MOF materials has always been the goal pursued by researchers.16 As we know, the presence of coordinated solvent molecules in the metal centers usually affects the stability of MOFs because they are susceptible to competitive attack of species.17 Seeking a strategy to avoid coordination solvents in the metal center might provide an efficient way to circumvent such drawbacks of MOFs. Several methods have been proposed to protect the metal center of MOFs, including introduction of molecular building blocks (MBBs),18−20 insertion of coordination sites,21 using high coordination number metal species,22,23 and incorporating hydrophobic groups.24,25 Recently, our group constructed some robust MOFs with remarkable gas adsorption and separation capacities by adjusting the helical chain secondary building units (SBUs),26 host−guest interaction, 27,28 and intramolecular hydrogen-bonding © XXXX American Chemical Society

Received: August 5, 2018

A

DOI: 10.1021/acs.inorgchem.8b02212 Inorg. Chem. XXXX, XXX, XXX−XXX

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FTIR (KBr, cm−1): 3076(w), 2921 (w), 1951 (m), 1643 (s), 1577 (s), 1560 (s), 1369(s), 1111 (m), 1008 (m), 852(m) 775 (s), 696 (m), 536(w), 474 (m). Synthesis of FJU-36. H3PTB (0.022 g, 0.05 mmol) and Cd(NO3)2·4H2O (0.0308 g, 0.1 mmol) were dissolved in DMF (3 mL) in a 20 mL vial, then EtOH (2 mL) and HBF4 (0.2 mL) were added. The solution was heated at 100 °C for 36 h to produce yellow lamellate crystals. Elemental Analysis for C128H120N12O36Cd6 (%) Calcd: C 49.97, H 3.93, N 5.46. Found: C 48.90, H 3.91, N 5.53. FTIR (KBr, cm−1): 3072 (w), 2935(w), 1673 (m), 1585 (s), 1383 (s), 1095 (m), 1008 (m), 1015(s); 869 (m), 792 (s), 698 (s), 538 (w).

Scheme 1. Control of Various M−O−M Chains and Structural Stability in Cd-MOF Isomerism

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RESULTS AND DISCUSSION Structural Description. The single crystal X-ray diffraction analysis show FJU-35 and FJU-36 are constructed of infinite 1D CdII−O−CdII chains and PTB3− ligands, and they exhibit different 3D porous structure due to different CdII−O− CdII chains. FJU-35 crystallizes in the triclinic crystal system with space group P1̅. Its asymmetric unit contains five and two half independent Cd2+, two [(CH3)2NH2]+ cations, one coordinated H2O molecule, four PTB3−, and two HCOO− (this HCOO− and [(CH3)2NH2]+ may be derived from the metalinduced hydrolysis of DMF under solvothermal conditions32,33) (Figures 1a, S3, and S4). while, FJU-36 crystallizes in the orthorhombic space group Pbcn, and the asymmetric unit contains one and a half independent Cd2+, one PTB3− ligand, a

While FJU-35 is vulnerable to external attacks, it exhibits more sensitivity for the detection of nitrobenzene than FJU-36.

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EXPERIMENTAL SECTION

General. All reagents and solvents are commercially available and no further purification was required. Pyridine-2,4,6-tribenzoic acid (H3PTB) was synthesized by literature methods.30,31 Synthesis of FJU-35. H3PTB (0.022 g, 0.05 mmol), 1,4-diazabicyclo[2.2.2]octane (DABCO) (0.011 g, 0.1 mmol) and Cd(NO3)2·4H2O (0.0308 g, 0.1 mmol) were dissolved in dimethylformamide (DMF, 3 mL) in a 20 mL vial, then HBF4 (0.2 mL) was added. The solution was heated at 100 °C for 12 h to produce yellow bulk crystals. Elemental Analysis for C149H175N19O46Cd6 (%) Calcd: C 49.13, H 4.84, N 7.31. Found: C 47.65, H 4.68, N 7.46.

Figure 1. Structure of FJU-35 and FJU-36. Coordination environment of the Cd(II) ions in FJU-35 (a) and FJU-36 (b) (Cd, olive; O, red; C, gray; N, blue; H, yellow). 1D infinite CdII−O−CdII inorganic chain in FJU-35 (c) and FJU-36 (d).View of 3D frameworks in which water molecules orient toward the channel direction in the FJU-35 (e). View of c axis containing two 1D open channels and the ligand of the pyridine N decorating the channel in the FJU-36 (f). Guest molecules and H atoms have been omitted for clarity. B

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Figure 2. Powder X-ray diffraction patterns of simulated, as-synthesized, and activated FJU-35 and FJU-36.

half HCOO− anion, and a half [(CH3)2NH2]+ cation (Figures 1b and S5). Both of their 1D infinite CdII−O−CdII chains are formed by carboxylate groups from PTB3− and HCOO− (Figure 1c,d). The most significant difference between FJU-35 and FJU-36 lies in the HCOO− connection mode and coordination solvent on the CdII−O−CdII chains. In FJU-35, the HCOO− adopt asymmetric coordination mode (μ3-η1:η2) and the coordination H2O molecules on the chains are exposed outside, which may make FJU-35 susceptible to external stimuli, whereas in FJU-36, formic acids employ symmetrical coordination model (μ3-η2:η2), and there are no coordination solvent in the chains. Overall, such differences fundamentally alter the whole connection mode and channel space, leading to the formation different crystal structures. The 1D chain of the two compounds is further connected by PTB3− to generate a 3D network. In FJU-35, the 3D framework contains rhombus shaped channels with dimensions 19.9 × 25.2 Å2, and the water molecules oriented toward the channels (Figure 1e). Remarkably, FJU-36 contains two different channels with dimensions of ca. 9.1 × 13.4 Å2 and 10.2 × 15.4 Å2 and the pyridine N of the ligand as binding sites face into the larger channels (Figure 1f). After elimination of guest molecules, the total accessible volume is 56.04% and 45.43% for FJU-35 and FJU-36, respectively, using the PLATON34 software. FJU-35 and FJU-36 have a large void space, which is expected to be useful for gas storage and separation. Stability. Prior to performance testing, we conducted powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) on FJU-35 and FJU-36 to confirm the purity and thermal stability of the samples. From the PXRD, we can see that the as-synthesized PXRD patterns of FJU-35 are in good agreement with the simulated pattern, but it readily loses framework integrity and porosity when exposed to air for 2 days or after activation (Figures 2a and S1). This is consistent with the structural analysis that conjectures that the coordination H2O molecules and asymmetric formic acids in CdII−O−CdII chains of FJU-35 may cause instability. In comparison, powder X-ray diffraction patterns for simulated, as-synthesized, and activated samples of FJU-36 agree well (Figure 2b), demonstrating that the framework of FJU-36 is stable. The TGA curve shows that FJU-35 starts to lose solvent molecules at room temperature and loses 27.65% of the solvent molecules at 180 °C, corresponding to the loss of solvent molecules DMF and H2O in the channels (calcd 28.57%) (Figure S6). Further weight loss of 5.35% at 180−282 °C should be attributed to [(CH3)2NH2]+ cation, coordination H2O, and HCOOH (calcd 5.44%). After that, the sample is still continuously releasing weight without a noticeable platform. This is also reflected in FJU-35 instability due to

the loss of a large number of solvent molecules. The TGA curve of FJU-36 shows a 15.31% weight loss, which is attributed to the loss of guest molecules DMF and H2O (calcd 15.82%) (Figure S6). After that, a plateau appears at 226−380 °C, indicating a more robust framework with higher thermal stability. To further establish the framework stability of FJU-36, variable-temperature PXRD tests were performed on the as-synthesized sample. The result shows that FJU-36 can be stable to close to 350 °C, indicating that FJU-36 has high thermal stability (Figure S2). Such superior stability in FJU-36 is a beneficial factor in the exploration of MOFs as functional materials for practical applications. Gas Adsorption Properties. In order to examine the permanent porosity of FJU-35 and FJU-36, gas adsorption studies were performed. Prior to the gas adsorption measurements, the FJU-35 and FJU-36 were immersed in the CH3OH to replace the guest molecules, followed by evacuation at 80 °C until the degassed vessel reached 5 μmHg, generatong the fully desolvated samples FJU-35a and FJU-36a. It has been observed that FJU-35a could only adsorb a little N2 (8.0 cm3/g) and CO2 (14.0 cm3/g) at 77 K (Figure 3a) and 273 K (Figure S11), respectively, which showed the collapse or at least partial decomposition of the framework. In contrast, the FJU-36a displays fully reversible type-I sorption behavior with a N2 sorption amount of 131.2 cm3/g (STP) at 77 K, revealing their microporous characters (Figure 3a). According to the N2 adsorption isotherm, the Brunauer−Emmett−Teller (BET) and Langmuir surface areas are estimated to be 409.0 and 554.2 m2/g, respectively. The calculated pore volume of FJU-36a is 0.203 cm3/g. The pore size distribution analysis by nonlocal density functional theory (NLDFT) utilizing N2 adsorption data at 77 K shows that FJU-36a exhibits pore distribution around 5−15 Å, which is close to the sizes of two channels in the framework (Figure 3b). Considering the unique channel structures decorated with Lewis basic nitrogen sites and permanent microporosity of FJU-36a, we decided to investigate its potential application for adsorption of industrially important gases such as C2H2, CO2, and CH4.35−38 As shown in Figure 3c,d, the adsorption isotherms of C2H2, CO2, and CH4 at 273 and 296 K indicate that compound FJU-36a has obviously different adsorption amounts for C2H2, CO2, and CH4, although the kinetic diameters of the three gases are very close (C2H2 3.3 Å, CO2 3.3 Å, CH4 3.8 Å). At 1 atm, the FJU-36a displays a C2H2 uptake of 66.5 cm3/g and 52.2 cm3/g at 273 and 296 K, respectively. The uptake capacity for C2H2 in FJU-36a at room temperature is lower than some famous MOFs with high density open metal sites, for instance, CoMOF-74 (197 cm3/g),39 SNNU-65-Cu-Sc (178.9 cm3/g)40 and UTSA-20 (150 cm3/g).41 C

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Figure 3. (a) N2 sorption isotherms for FJU-35a and FJU-36a at 77 K; (b) pore size distributions of FJU-36a; (c,d) C2H2, CO2, and CH4 sorption isotherms of FJU-36a at 273 K (c) and 296 K (d); (e) IAST adsorption selectivity of FJU-36a for C2H2/CH4 (50:50) and C2H2/CO2 (50:50); (f) isosteric heat of C2H2, CO2, and CH4 in FJU-36a.

hydrogen bonding and π−π interactions with framework to further improve the C2H2 uptake. This phenomenon was also observed in other MOFs with uncoordinated nitrogen sites, such as UTSA-50a,46 ZJU-5a,53 and ZJU-40a.54 Breakthrough Separation Experiments. To further evaluate the feasibility of FJU-36a for the purification of C2H2 from C2H2/CO2 and C2H2/CH4 mixtures, we carried out real-time dynamic breakthrough tests under conditions that simulate industrial processes. In a typical BASF acetylene oil quench process, C2H2 (7.5%−8.8%), CO2 (3.2%−3.5%), CH4 (5%), H2 (42.7%−56.5%), CO (25.8%−37.9%), C2H4 (0.3%−0.5%), O2 (0.2%), and C3+ hydrocarbons (0.5%−0.7%) are also present in the cracked gas.55,56 Therefore, a gas mixture comprising 5% C2H2, 5% CO2, and 90% He (5% C2H2, 5% CH4, and 90% He) was used to perform the real-time dynamic breakthrough experiments at 296 K and 1 atm. The He served as an inert carrier gas. The 5:5:90 C2H2/CH4/He (5:5:90 C2H2/CO2/He) gas mixtures were passed over a tubular column packed with activated FJU-36a solid at a flow rate of 5.4 mL/min (3.6 mL/min) at 296 K (Figure S12). The CH4 breaks through the column quickly under the breakthrough curve of FJU-36a in 5:5:90 C2H2/CH4/He gas mixture, whereas the C2H2 breakthrough did not occur until about 40 min (Figure 4a). From the breakthrough curve, the working capacity calculated for C2H2 and CH4 for FJU-36a is 0.549 mmol/g and 0.063 mmol/g, respectively, and the

However, its value is higher than those of ZIF-8 (25 cm3/g)42 and MOF-5 (26 cm3/g)42 with large pores and high surface areas and comparable to those of ZJU-16a (58 cm3/g),43 UTSA-5 (59.8 cm3/g)44 and UTSA-35 (65.0 cm3/g).45 FJU-36a exhibits a small amount of CO2 (35.5 cm3/g) and a minimal amount of CH4 (10.5 cm3/g) uptake at 296 K and 1 atm. Compared to the low capture of CO2 and CH4, such a high amount of C2H2 is probably attributed to C2H2 molecule having an acidic H atom at both ends, optimized channel size, and the Lewis basic pyridyl site ligand decorating the pore surfaces in FJU-36a has a stronger interaction with the C2H2 molecule.46,47 The observed discrepancies between the C2H2, CO2, and CH4 absorption indicated that FJU-36a has the potential for separations under ambient conditions. The adsorption selectivity can be predicted from single component adsorption isotherms of each species by using ideal adsorbed solution theory (IAST).48 The C2H2 selectivity values over CO2 and CH4 for binary gas mixtures (50:50 v/v) are 2.8 and 17.7, respectively, at 296 K and 100 KPa (Figure 3e). The C2H2 selectivity values over CO2 and CH4 are comparable to those of other published MOF materials.49−52 Furthermore, the zero-coverage C2H2, CO2, and CH4 adsorption enthalpies (Qst) of FJU-36a were calculated to be 32.9, 31.1, and 16.9 kJ/mol, respectively (Figure 3f), using the virial method (Figure S8−S10). The higher adsorption enthalpies of C2H2 may be due to H−CC−H···N (uncoordinated nitrogen sites) D

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Figure 4. (a, c) Breakthrough curves and (b, d) recycle runs under gas mixture of (a, b) 5:5:90 C2H2/CH4/He and (c, d) 5:5:90 C2H2/CO2/He for FJU-36a at 296 K. He was excluded from the breakthrough curves because He is used as the carrier gas.

separation factor (α) for FJU-36a is 8.7. Therefore, FJU-36a has a great potential for the separation C2H2/CH4. For 5:5:90 C2H2/CO2/He mixed gas flow through FJU-36a, owing to CO2 having a stronger affinity than CH4 for FJU-36a, CO2 did not break through the column until about 25 min. After 50 min, C2H2 appeared on the breakthrough curve (Figure 4c). The adsorption capacity of C2H2 and CO2 in FJU-36a under dynamic conditions was calculated to be 0.463 mmol/g and 0.223 mmol/g, respectively, and the separation factor (α) for FJU-36a is 2.1. Regeneration experiments showed that FJU-36a has a good cyclability (Figures 4b,d and S13). All these results strongly indicate that FJU-36a possesses structural stability and reproducibility and is a promising material for selective separation C2H2/CH4 and C2H2/CO2. Photoluminescence (PL) Behaviors and Sensing Properties. Metal−organic frameworks constructed from Cd ion and π-conjugated organic ligands generally exhibit excellent photoluminescence, and they are therefore potential candidates for fluorescence probes.57−60 FJU-35 and FJU-36 with different CdII−O−CdII chains may bring unexpected fluorescence properties. To detect the fluorescence characteristics of FJU-35 and FJU-36, solid-state photoluminescence spectra of both compounds and their free ligands were tested at room temperature (Figure 5). The photoluminescence spectrum of the free ligand H3PTB exhibits maximum peaks at 516 nm at excitation of 350 nm, which may originate from the ligandcentered electronic π···π* transition.61,62 FJU-35 and FJU-36 display similar fluorescence emission spectra upon excitation at 350 nm with a strong emission peak and a weak emission shoulder peak at 396 and 557 nm, respectively. The strong emission peak could be assigned to metal chain centered emission, which originates from the CdII−O−CdII charge transfer (O → CdII) excited state.63 The weak shoulder peak at 557 nm can be attributed to the ligand-centered π → π* and or n → π* transition with a red shift (41 nm) with respect to free H3PTB, and the weaker emission on FJU-35 may be caused by more sensitive CdII−O−CdII chains.64,65 Such interesting fluorescent features stimulate us to further explore their photoluminescent properties in different solvents,

Figure 5. Solid-state emission spectra (λex = 350 nm) for the H3PTB ligand, FJU-35, and FJU-36 at room temperature.

such as, N,N-dimethylformamide (DMF), acetonitrile, dichloromethane, 1,4-dioxane, methanol (CH3OH), ether, acetone, N, N-dimethylacetamide (DMA), ethyl acetate (EAC), chloroform (CHCl3), and nitrobenzene (NB). As shown in Figure 6a,c, the luminescence intensity largely depends on the type of solvent, in particular, NB molecules, which show a pronounced quenching effect. Such sensitive quenching behavior is of great interest for sensing trace amounts of NB molecules, as evidenced by some pioneering workers.66−68 To better understand the quenching effect of NB, we have investigated the sensing of NB in FJU-35 and FJU-36 in more detail. The FJU-35 and FJU-36 were dispersed in DMF solution with gradually increasing NB content to monitor the emission respond. As expected, as the NB concentration increases from 0 to 12.2 μM, both FJU-35 and FJU-36 show significant quenching (Figure 6b,d). The fluorescence quenching efficiency was analyzed using the Stern−Volmer equation,69,70 I0/I = KSV[M] + 1, where I0 and I are the fluorescence intensities before and after the addition of NB, respectively, [M] is the molar concentration of NB, and KSV is the quenching constant (M−1). As shown in the inset of Figure 6b,d, the Stern−Volmer plots of FJU-35 and FJU-36 exhibit a good linear correlation as the concentration of NB increases. The Ksv values for NB in FJU-35 and FJU-36 are 1.63 × 106 M−1 and 9.13 × 105 M−1, respectively, revealing very high sensitivity E

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Figure 6. (a) Luminescence spectra of FJU-35 in various organic pure solvents. The inset shows the emission intensity at 385 nm. (b) Luminescence spectra of FJU-35 at different NB concentrations in DMF. The inset provides the Stern−Volmer plot (excited and monitored at 350 and 385 nm, respectively). (c) Luminescence spectra of FJU-36 in various organic pure solvents. The inset shows the emission intensity at 402 nm. (d) Luminescence spectra of FJU-36 at different NB concentrations in DMF. The inset provides the Stern−Volmer plot (excited and monitored at 350 and 402 nm, respectively).

for sensing NB, superior to most of reported values.68,71,72 The quenching behavior of FJU-35 is more notable than that of FJU-36, which may be originate from larger channels and more sensitive CdII−O−CdII chains. To better understand the mechanism for sensing of NB, UV−vis absorption spectra of different solvents and NB were investigated (Figure S14). The absorption spectrum data display maximum overlap between the absorption band of NB and the excitation wavelength of FJU-35 and FJU-36. Therefore, fluorescence quenching may be due to the charge transfer mechanism being hindered from the π···π interactions in the MOF framework to the electrondeficient NB adsorbed on the surface or channels.58,73,74

Accession Codes

CCDC 1856694−1856695 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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*E-mail: [email protected] (S.X.). *E-mail: [email protected]. (Z.Z.).

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ORCID

CONCLUSIONS In conclusion, we demonstrate that regulating the metal−O− metal chains can enhance stability, gas adsorption, and sensing of MOF materials. Two structurally isomeric Cd-MOF materials, namely, FJU-35 and FJU-36 with different CdII−O−CdII chains were successfully synthesized. Gas adsorption and dynamic breakthrough experiment demonstrate that the stable FJU-36 exhibits a high C2H2 selectivity over CO2 and CH4, while unstable FJU-35 demonstrates almost no gas adsorption capacity. Conversely, FJU-35 with more vulnerability is more sensitive to the detection of nitrobenzene than FJU-36. Our work shows that strengthening the stability of metal−O−metal chains could be a smart strategy for developing highly stable MOF materials.

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AUTHOR INFORMATION

Corresponding Authors

Yingxiang Ye: 0000-0003-3962-8463 Shengchang Xiang: 0000-0001-6016-2587 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by NSFC (Grant Nos. 21573042 and 21673039) and Fujian Science and Technology Department (Grant Nos. 2016J01046, 2014J06003, and 2014H6007). S.X. gratefully acknowledges the support of the Recruitment Program of Global Young Experts.

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REFERENCES

(1) Zhou, H. C.; Long, J. R.; Yaghi, O. M. Introduction to metalorganic frameworks. Chem. Rev. 2012, 112, 673−674. (2) Li, J. R.; Sculley, J. L.; Zhou, H. C. MetalOrganic Frameworks for Separations. Chem. Rev. 2012, 112, 869−932. (3) Zhang, Z. J.; Yao, Z. Z.; Xiang, S. C.; Chen, B. L. Perspective of microporous metal-organic frameworks for CO2 capture and separation. Energy Environ. Sci. 2014, 7, 2868−2899. (4) Wang, Y.; He, M. h.; Gao, X. X.; Li, S. D.; Xiong, S. S.; Krishna, R.; He, Y. B. Exploring the Effect of Ligand-Originated MOF Isomerism and Methoxy Group Functionalization on Selective Acetylene/Methane and Carbon Dioxide/Methane Adsorption

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02212. Experimental methods, crystal data, PXRD, TG, IR spectra, gas adsorption figures, and breakthrough experimental setup and figures (PDF) F

DOI: 10.1021/acs.inorgchem.8b02212 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b02212 Inorg. Chem. XXXX, XXX, XXX−XXX