Multifunctional Behavior of Sulfonate-Based Hydrolytically Stable

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 39049−39055

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Multifunctional Behavior of Sulfonate-Based Hydrolytically Stable Microporous Metal−Organic Frameworks Aamod V. Desai,† Biplab Joarder,† Arkendu Roy,† Partha Samanta,† Ravichandar Babarao,‡,§ and Sujit K. Ghosh*,†,∥ †

Indian Institute of Science Education and Research (IISER), Dr. Homi Bhabha Road, Pashan, Pune 411 008, India School of Science, RMIT University, Melbourne, Melbourne 3001, Australia § Commonwealth Scientific and Industrial Research Organisation (CSIRO) Manufacturing, Clayton, Victoria 3169, Australia ∥ Centre for Energy Science, IISER Pune, Pune 411 008, India

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S Supporting Information *

ABSTRACT: An isostructural pair of extremely rare, permanently microporous sulfonate-based metal−organic frameworks (MOFs) having a novel topology has been reported here by integration of rationally chosen building units. The compounds bear polar sites in the pore surfaces and exhibit selective adsorption of CO2, which features among the highest reported uptakes in the domain of organosulfonate-based MOFs. The compounds also exhibit multifunctionality for C6-cyclic hydrocarbon separation and selective detection of neurotransmitter nitric oxide. Such multifunctional behavior on the basis of permanent porosity has been rarely observed for sulfonate-based MOFs. The efficacy of the synthesis approach is further highlighted by the resistance over a wide pH range and promising feasibility of reticular chemistry in porous organosulfonate-based systems. KEYWORDS: metal−organic frameworks, hydrolytic stability, sulfonate-based MOF, multifunctionality, permanent microporosity

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INTRODUCTION Metal−organic frameworks (MOFs) are crystalline, extended coordination polymers built from bi-/multidentate organic linkers and connected by metal ions/clusters in a periodic manner.1,2 These compounds have commanded remarkable attention over the last few years owing to their wide array of potential applications.3−13 The ability to tune physical/ chemical properties and modulate architectures by regulating choice of building units has further propelled research interest in these materials. Despite several advances in this domain, development of synthesis strategies with regard to stability under operating conditions continues to be an important aspect for MOFs to realize their potential toward practical implementation.14−16 Recently, much emphasis has been bestowed toward understanding structure−stability correlation in MOFs, which typically involves evaluation of the specific building blocks.17−20 This has led to increasing reports of MOFs exhibiting stability that are suitable for different applications.21−25 While MOFs offer noteworthy feature of reticular chemistry in porous solids and much research has been devoted to its development,26,27 significantly less attention has been paid to the investigation of different donor groups of the organic ligands. The literature of MOFs signifies the predominance of carboxylate-based linkers for the synthesis of MOFs.28,29 In comparison to carboxylate-terminal linkers, frameworks based on sulfonate and phosphonate © 2018 American Chemical Society

ligands, which can potentially offer competing and in some cases even better properties, are infrequently employed.30−42 In particular, the superior features of sulfonate/phosphonatebased linkers include versatile binding modes on the account of nonplanar binding, affordance of extra polar sites in pore surfaces, structural diversity, and superior stability in contrast to majority carboxylate-based MOFs.42 Although sulfonatebased linkers afford distinct advantages, such donating groups are generally not explored on the account of relatively weaker ligation with hard transition metals and inability to create and retain permanent porosity. Although the feasibility of formation of sulfonate-based coordination networks was proposed long back,43,44 only a limited number of compounds based on such linkers have been reported thus far. Notably, this number is remarkably inferior relative to systems built from carboxylate-based linkers, which rose to prominence around the same period. In particular, the lack of systematic strategies to construct and examine porous frameworks built from sulfonate/phosphonate-donor ligands has made development of such frameworks a subject of interest and relevance toward developing MOFs as stable materials. Received: August 22, 2018 Accepted: October 23, 2018 Published: October 23, 2018 39049

DOI: 10.1021/acsami.8b14420 ACS Appl. Mater. Interfaces 2018, 10, 39049−39055

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ACS Applied Materials & Interfaces

using WinGX, which revealed the atom valencies for Cd01, S1, and S2 to be +2, +6, and +6, respectively. Synthesis of Compound IPM-302. This compound was synthesized similarly as above, except for using sodium 4-amino-1,5naphthalenedisulfonate (9.75 mg) instead of 1,5-naphthalenedisulfonic acid tetrahydrate. The formula for the guest-free phase was estimated to be [{Cd2(NH2-1,5-NDS)(L)2(SO4)}]n (NH2-1,5-NDS refers to 4-amino-1,5-naphthalene disulfonate). Anal. Calcd (guest free phase): C/N, 3.66; N/S, 2.18; C/S, 7.99. Found: C/N, 3.70; N/ S, 2.23; C/S, 8.28. Water Stability Test. The activated compound IPM-301 and IPM-302 (∼55 mg) was dipped in 5 mL of deionized water and kept at room temperature. Subsequently, the compound was filtered off and dried in air for further characterization. The supernatant was collected and its inductively coupled plasma−atomic emission spectroscopy analysis was recorded. pH Stability Test. The activated compound IPM-301 and IPM302 (∼20 mg) was dipped in 5 mL of separate pH solution (pH = 4, 10.01) and stirred at room temperature for 24 h. The respective supernatant solutions were collected, and the residue was washed with deionized water multiple times, followed by washing with methanol. The recovered solid phases were then degassed by heating under reduced pressure for further characterization. For gas adsorption studies, the compounds were pretreated at 110 °C under vacuum before measurement. Fluorescence Measurements. To a dispersed aqueous solution of IPM-302, NO was bubbled for 15 min. The crystals were recovered and washed with deionized water and dried under vacuum for further characterization. Other analytes were prepared according to a reported protocol.73 The respective solutions (200 μL) were added to 2 mL aqueous solution of IPM-302 and equilibrated for 2 min before recording the emission spectra. Physical Measurements. Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å) in 5°−40° 2θ range with a scan speed of 1.2° min−1. The IR spectra were acquired by using a Nicolet 6700 Fourier-transform infrared (FT-IR) spectrophotometer using KBr pellets in the 400−4000 cm−1 range. UV spectra were recorded on a Shimadzu UV 2600 spectrophotometer having stirring attachment. The scanning electron microscopy (SEM) images & energy-dispersive X-ray data were obtained using an FEI Quanta 3D dual beam ESEM system. Thermogravimetric analysis (TGA) profiles were recorded on a PerkinElmer STA6000 TGA analyzer under an N2 atmosphere with a heating rate of 10 °C/min. Gas adsorption measurements were performed using the BELSORP-Max instrument (BEL Japan). Solvent adsorption measurements were performed on the Bel-Aqua instrument (BEL Japan). Prior to adsorption measurements, the activated samples were heated at 110 °C under vacuum for 6 h using BelPrepvacII. 1H and 13C NMR spectra were recorded on a JEOL 400 MHz or a Bruker 400 MHz spectrometer. X-ray Structural Studies. Single-crystal X-ray data of compounds IPM-301 and IPM-302 were collected at 100 K on a Bruker D8 Venture Duo X-ray diffractometer equipped with a Micro Focus X-ray source (operated at 50 W; 50 kV/1 mA), graded multilayer optics for monochromatic Mo Kα radiation (λ = 0.71073 Å) focused X-ray beam and a Photon 100 CMOS chip-based detector system. The crystal was mounted on nylon CryoLoops (Hampton Research) with Paraton-N (Hampton Research). The data integration and reduction were processed with SAINT software.56 A multiscan absorption correction was applied to the collected reflections.57 The structure was solved by the direct method using SHELXTL58,59 and was refined on F2 by the full-matrix least-squares technique using the SHELXL2014/760 program package within the WinGX61 programme. All nonhydrogen atoms were refined anisotropically. All hydrogen atoms were located in successive difference Fourier maps and they were treated as riding atoms using SHELXL default parameters. The structures were examined using the Adsym subroutine of PLATON to assure that no additional symmetry could be applied to the models. The SQUEEZE option62 was used to eliminate the contribution of disordered guest molecules. The bridging SO42− anions are highly

In addition to stability, multifunctionality has been an intensely pursued objective in the domain of MOFs as it can integrate multiple physical/chemical features in a single system.45,46 Typically, a mixed-linker strategy has been employed as an effective route to afford synergistic advantages of individual linkers. The use of neutral N-donor multitopic molecules in conjunction with carboxylate ligands has been the commonly employed strategy to enhance the dimensionality/ porosity and permit access to the features of individual linkers. In general, neutral N-donor ligands are employed for the creation of cationic MOFs and in the preparation of flexible MOFs owing to the softness of the resulting metal−ligand bond.47−53 Recent reports have highlighted the value of fivemembered heterocyclic ring donating linkers, such as imidazole or triazole, both in terms of increasing the density of independent linkers around metal nodes and strengthening the bond strength on the account of higher pKa values of donor groups.49,51−53 Standalone sulfonate-based MOFs are generally nonporous and difficult to crystallize, and hence, the presence of neutral N-donor molecules as a support to sulfonate-based ligands is potentially a favorable combination, but still remains a largely unexplored domain. To examine this hypothesis, we herein report the synthesis of an unusual, robust, permanently microporous Cd(II)centered MOF viz. {[Cd2(1,5-NDS)(L)2(SO4)]·xG}n [1,5NDS: 1,5-naphthalene disulfonate; L: tridentate neutral Ndonor ligand; Gguest solvent molecules; hereafter referred to as IPM-301, where IPM stands for IISER Pune Materials], constructed from a disulfonate linker and extended by a neutral, imidazole terminal tripodal N-donor ligand. The MOF bears accessible polar sites and exhibits selective adsorption of CO2, which is among the highest in the domain of organosulfonate-MOFs. To demonstrate the possibility of reticular chemistry using this synthesis approach, an isostructural MOF built from an amine-pendant disulfonate linker is reported. The pore characteristics and luminescence features in the MOFs bestowed by the building units afford integration of multiple functions [sensing of neurotransmitter nitric oxide (NO) and separation of C6-cyclic hydrocarbons] in the same system, which is rarely observed for organosulfonate-based porous compounds.

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EXPERIMENTAL SECTION

Materials. All solvents and reagents were commercially available and used without further purification. Standard pH buffers (pH = 4.01, 10.01) were procured from Eutech Instruments. The ligand [Ltris(4-(1H-imidazol-1-yl)phenyl)amine] was synthesized according to a reported protocol.49 Synthesis of Compound IPM-301. A mixture of ligand (8.86 mg, 0.02 mmol), 3CdSO4·8H2O (15.4 mg, 0.02 mmol), 1,5naphthalenedisulfonic acid tetrahydrate (9.00 mg, 0.025 mmol), N,N-dimethylformamide (1 mL), and water (2 mL), was placed in a glass vial and heated at 90 °C for 48 h, followed by slow cooling to room temperature. The compound was filtered and washed with water and methanol several times. Single crystals of compound IPM301 viz. [{Cd2(1,5-NDS)(L)2(SO4)}·xG]n were isolated in ∼45% yield. These crystals were dipped in MeOH solution for exchange and heated under vacuum at 75 °C to obtain the guest free phase. We were unable to locate the highly disordered guest solvent molecules in the structure crystallographically. The formula for the guest-free phase was estimated to be [{Cd2(1,5-NDS)(L)2(SO4)}]n (1,5-NDS refers to 1,5-naphthalene disulfonate). Anal. Calcd (guest free phase): C/N, 3.92; N/S, 2.04; C/S, 7.99. Found: C/N, 3.98; N/S, 2.06; C/S, 8.24. Bond valence sum calculations54,55 for IPM-301 were carried out 39050


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ACS Applied Materials & Interfaces disordered in both the structures. In case of IPM-302, the pendant −NH2 group is disordered over two positions. CCDC 1831060− 1831061, for IPM-301 and IPM-302, respectively, contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Binding Energy Calculations. Static binding energies for CO2 in IPM-301 and IPM-302 were calculated using density functional theory (DFT) as implemented in the software package VASP.63 It is well-known that standard DFT methods based on generalized gradient approximation do not fully account for the long-range dispersion interactions between the framework and the weakly bound gaseous adsorbates. To accurately estimate static binding energies for weakly bound guest molecules with IPM-301 and IPM-302 framework, we implemented dispersion corrections using the DFTD3 method.64 Electron exchange and correlation were described using the generalized gradient approximation Perdew, Burke, and Ernzerhof65 form, and the projector-augmented wave potentials were used to treat core and valence electrons.66 In all cases, we used a plane-wave kinetic energy cutoff of 600 eV and a Γ-point mesh for sampling the Brillouin zone. The ionic coordinates were relaxed until the Hellman−Feynman ionic forces were less than 0.01 eV/Å. The initial location of the guest molecule in the primitive cell of IPM-301 and IPM-302 was obtained from the classical simulated annealing technique using classical force field as implemented in sorption module in Materials Studio.67 In the simulated annealing method, the temperature was lowered stepwise, allowing the gas molecule to reach a desirable configuration based on different moves such as rotation, translation, and repositioning with preset probabilities of occurrence. This process of heating and cooling the system was repeated in several heating cycles to find the local minima. Forty heating cycles were performed, where the maximum temperature and the final temperature were 105 and 100 K, respectively. Static binding energies (ΔE) at 0 K were calculated using the following expression:

salt of Cd(II) was chosen as it is known to bind strongly with metal nodes, and use of such inorganic anions in the domain of MOFs has recently found considerable interest.68,69 Cambridge Structural Database screening approach suggests that the structure of IPM-301 is unprecedented in terms of both organosulfonate anion and sulfate anions binding to the same metal center. The sulfonate groups are coordinated to the metal center through one O-atom, while the other two Oatoms are held by strong noncovalent interactions with imidazole moiety of the adjacent N-donor linkers (Figure S1b). The tripodal N-donor ligand extends the structure in three dimensions (Figures S2−S4) and further stabilizes the framework as higher dentate linkers are known to afford kinetic stability.19 The three-dimensional structure of IPM-301 has a porous one-dimensional channel along the crystallographic aaxis with the naphthalene moieties aligned along the pore walls (Figure 1a). The uniqueness of the structure is further highlighted in the new topology of the structure, estimated using TOPOS (Table S4),70 having a binodal 3,5-c net and the overall structure comprising two interpenetrated nets (Figure S6). The net has a point symbol of (4·62)(43·66·8) and a long vertex symbol [4·4·4·6·6·6·6·62·62·83], [4·62·62]. Similarly, an isostructural MOF viz. {[Cd2(NH2-1,5-NDS)(L)2(SO4)]·xG}n [NH2-1,5-NDS: 4-amino 1,5-naphthalene disulfonate; hereafter referred to as IPM-302] bearing a primary amine pendant group was synthesized (Scheme S1). The basic characterization of the compounds was carried out to assess their nature. PXRD patterns of the MOFs validated the bulk-phase purity of the compounds (Figures S7 and S25). TGA patterns reflected loss of guest molecules occluded in the pores during the synthesis (Figures S8 and S26) and variable-temperature PXRD substantiated thermal stability (Figure S9). TGA profiles suggested initial loss of guest solvent molecules occluded in the pores during the synthesis, and decomposition of the framework was initiated ∼400 °C. Field-emission SEM (FESEM) images displayed the formation of neat crystals (Figures S10 and S27). FT-IR spectra confirmed the presence of the sulfate groups with peaks corresponding to the S−O stretching frequency (Figure S11). The guest-free phases were obtained upon exchange with methanol followed by heating under vacuum, and the activation was validated using TGA profiles (Figure S8b). The crystallinity was found to be retained after this protocol making it a rare example of sulfonate-based MOF retaining permanent porosity. MOFs have commanded remarkable attention as sorbents for the pressing demands to capture and sequester greenhouse gas CO2.3,4,12 The polar pore surface of IPM-301 propelled us to investigate its ability to adsorb CO2. Low-temperature CO2 adsorption (195 K) performed on IPM-301 yielded an uptake of ∼106 mL g−1 (Figure S12). The compound did not show any significant uptake for N2 (77 K) (Figure S15). Temperature-dependent CO2 isotherms were recorded at 273, 298, and 310 K which displayed uptake amounts of 46.8, ∼37, and ∼33 mL g−1, respectively (Figures 2a, S12, and S13). Notably, these values are among the highest reported for an organosulfonate-based MOF (Table S5).33,37 The zero-coverage isosteric heat of adsorption as estimated from variable-temperature adsorption was found to be 31.1 kJ mol−1, suggesting the high affinity of CO2 toward the MOF (Figure S14). IPM-301 exhibited negligible uptake for related small gases at 298 K [kinetic diameters: N23.64 Å and O2 3.46 Å] (Figure S16). Importantly, the compound was found to retain its integrity after the adsorption measurements

ΔE = EMOF + gas − EMOF − Egas where Ex refers, respectively, to the total energies of the MOF + gas complex, the MOF alone, and gas molecule.

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RESULTS AND DISCUSSION Single crystals of IPM-301 were obtained in a solvothermal reaction of CdSO4, 1,5-naphthalene disulfonic acid, and ligand (L) in a solvent mixture of water and N,N′-dimethylformamide (Scheme S1). The compound crystallized in orthorhombic P2221 space group, with the asymmetric unit comprising one Cd(II) atom, one unit of ligand, half unit NDS linker, and half SO42− anion (Figure S1a). Each Cd(II) metal node is octahedral, with one coordination from the NDS ligand and three from three independent Ndonor linkers (Figure 1c). Two adjacent Cd(II) nodes are bridged by a SO42− anion (Figures 1a,c and S2). The SO42−

Figure 1. Packing diagrams of (a) IPM-301 and (b) IPM-302 along the crystallographic a-axis. (c) Coordination environment in IPM301. (Color code: C, gray; N, blue; S, yellow; O, red; Cd, orange. Hatoms have been omitted for clarity, and pendant N-atoms of −NH2 in IPM-302 have been highlighted in (b). 39051


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Figure 2. (a) Temperature-dependent CO2 adsorption isotherms of IPM-301. (b) CO2 adsorption isotherms for IPM-302 after treatment under different pH conditions. (c) Isosteric heat of adsorption for CO2 in IPM-301 and IPM-302. (d) Separation selectivity at 298 K predicted by IAST for CO2/N2 (15/85) for IPM-301 (blue) and IPM-302 (wine red).

N2 for IPM-302 was calculated to be 40 at 298 K and 1 bar (Figures S44 and 2d). IPM-302 was found to be hydrolytically stable and retained its structure at pH = 4 and 10.01 (Figures 2b, S33−S37). Intriguingly, the two MOFs exhibited similar adsorption performances despite IPM-302 bearing the additional polar site of pendant amine group (Figures S38 and S39). Amine groups have been well known to provide strong binding sites in MOFs for CO2 and other polar adsorptive.4,12 To understand this behavior further in the current case, we performed dispersion-corrected DFT (DFT-D3) calculations to investigate the binding modes. The static binding energies for CO2 at 0 K using DFT calculations were found to be ∼44 and ∼36 kJ mol−1 for IPM-301 and IPM-302, respectively. The calculations suggested that CO2 interacts strongly with the oxygen atom directed toward the pore from the bridging SO42− anions, even in the case of IPM-302 which bears −NH2 pendant sites (Figure 3). Some important reports in this

(Figure S17). To further examine the selectivity of IPM-301, the mixture selectivity of CO2/N2 (15:85, composition of flue gas) at 1 bar was calculated using ideal adsorbed solution theory (IAST) (Figures S44 and 2d). A selectivity of 35 was obtained at 298 K and 1 bar, which, notably, is highly competitive with some of the well-studied carboxylate-based MOFs.12 The hydrolytic stability of the compound was established by dipping the compound in deionized water. PXRD pattern and CO2 adsorption isotherm of the phase recovered after 7 days confirmed the retention of integrity of the framework (Figures S18 and S19). Also, we checked the water adsorption nature of the MOF after 3 cycles of adsorption−desorption cycles, which displayed almost no change in the patterns and uptake amounts (Figure S20). Remarkably, such stability is very rarely observed for a MOF built from metal nodes having higher coordination spheres such as Cd(II), which further endorses the choice of the building units. The unusual stability can be ascribed to the combination of multiple factors viz. higher denticity of linker for affording kinetic stability,19 higher binding strength of the imidazole groups to enhance coordination bond strength,51 bridging of adjacent Cd(II) by strongly coordinating sulfate anion,68,69 and extended coordination of the sulfonate ligand,31 which is further interacting with the framework backbone through strong noncovalent interactions. Enthused from the robust hydrolytic stability, we further checked the stability of IPM-301 over different pH conditions. The phases obtained at pH = 4 and pH = 10.01 were found to be stable, and porosity was intact (Figures S21 and S22). FESEM images for these phases endorsed the retention of morphology (Figures S23 and S24). Similar adsorption studies were carried out with IPM-302. Like the previous case, we observed selective uptake for CO2, with adsorption amounts of ∼95, 45.9, 36.6, and 32.1 mL g−1 at 195, 273, 298, and 310 K, respectively (Figures S28, S29, S31, and S32). The isosteric heat of adsorption at zero-loading was found to be 28.7 kJ mol−1 (Figure S30). The IAST selectivity for CO2/

Figure 3. DFT optimized locations of CO2 for (a) IPM-301 and (b) IPM-302. (The framework is shown in grayscale and the atoms of interest shown in color. Color code: O, red; C, dark gray; N, blue).

domain have demonstrated the role of polar functional sites in MOFs, such as −F, −NH 2 , and −SO 3 toward CO 2 adsorption.4,33,71 The present reports attempt to shed insight on the adsorption features in the presence of multiple accessible functional moieties. The findings further suggest that the presence of polar sites in the framework backbone is effective, if they are better aligned than the competing polar groups as secondary functional sites. 39052


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Figure 4. (a) Change in fluorescence intensity upon addition of NO to IPM-302 relative to competing reactive species; (b) adsorption isotherms of benzene and cyclohexane at 298 K for IPM-301 (open and closed symbols represent adsorption and desorption, respectively).

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CONCLUSIONS In summary, we have synthesized an extremely rare pair of robust, isostructural, multifunctional, organosulfonate-based MOFs bearing permanent microporosity which are hydrolytically stable. The compounds have unique binding modes and the structure represents a new topology. The MOFs exhibited selective uptake of CO2, which are among the highest known in the domain of organosulfonate MOFs. The compounds are hydrolytically stable and offer resistance over a wide pH range. IPM-301 is found to be a selective sorbent for benzene over its C6-cyclic congener cyclohexane, making it a potentially useful compound for the separation of this challenging industrial mixture. The highly emissive MOF with pendant amine group selectively detects NO in water. The features bestowed by the building units afford integration of multifunctionality, which is rarely observed for MOFs based on organosulfonate-backbone. These findings contribute substantially to the principles guiding the synthesis of sulfonate-MOFs as porous, multifunctional solids. Further exploration in such systems using linkers of varying lengths can lead to realization of potential toward reticular chemistry and complete understanding of stability in sulfonate-based MOFs. We believe that the current work will actuate development of hitherto under-explored potential of organosulfonate-based MOFs and contribute coherently to the evolution of stable MOFs as functional porous solids.

Interestingly, the compounds are found to present additional functions. The pore surfaces are decorated with polar sites and have a nonpolar aromatic-rich wall. The naphthalene backbone and the presence of Cd(II) metal nodes afford highly emissive character to both the compounds (Figure S41). The emission profiles for the MOFs were linked to the fluorescence for the sulfonate-linker (Figure S40). MOFs with pendant secondary sites have been found to be selective sensory probes.9 Sensing of neurotransmitters has been an important research subject owing to the need of understanding their mechanisms in biological reactions.9,72,73 Among them, there has been only limited number of reports for MOF-based sensors for the detection of NO, which have been based on carboxylate-based MOFs.72,73 Employing this knowledge, we checked the efficacy of IPM-302 toward recognition of NO, as it can undergo the deamination reaction owing to accessible recognition sites. As anticipated, we observed specific changes in the emission profiles of the compound toward NO, over other reactive species. Also, the emission spectra reverted to the profile corresponding to IPM-301 (Figure S42), suggesting that the fluorescence change was accompanied by the deamination reaction, and the response was selective (Figure 4a), as reported previously. The PXRD patterns confirmed the retention of crystallinity (Figure S43). The sensing ability further endorses the synthetic strategy as a promising route to develop functional luminescent MOFs. Additionally, the features endowed to the compounds by the building blocks were investigated for adsorptive separation of important industrial liquid-mixtures such as benzene−cyclohexane. IPM-301 was found to be a selective adsorbent toward benzene (Bz) over its C6-cyclic analogue cyclohexane (Cy) (Figure 4b). Cyclohexane is produced from hydrogenation of benzene, and their separation in the outlet stream has been a major challenge owing to inability of conventional distillation process to separate the mixtures efficiently.8 Adsorption-based methods have been found to be efficient, and, in particular, MOFs have shown promise to be employed for such applications. We evaluated the separation ability using singlecomponent adsorption isotherms, which resulted in substantial uptake at 298 K for Bz (∼1.5 molecules per NDS unit) and negligible amount for Cy (Figure 4b). The selectivity can be primarily ascribed to the favorable π−π interactions of benzene with the π-rich channel walls of IPM-301.8 To the best of our knowledge, such adsorptive separation of bulky cyclic molecules has not been investigated using sulfonate-MOFs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b14420.

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Crystallographic information file for IPM-301 (CIF) Crystallographic information file for IPM-302 (CIF) Representation of the protocol employed for the synthesis of IPM-301 and IPM-302; PXRD data, TGA plots, FT-IR spectra, FESEM images, gas adsorption profiles, fluorescence spectra of IPM-301 and IPM-302; ICP-AES analysis results; crystal data and structure refinement for IPM-301 and IPM-302; topology analysis using TOPOS; calculation of CO2/N2 selectivity; and list of some representative MOFs based on organosulfonate linkers showing CO2 adsorption (PDF)

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Aamod V. Desai: 0000-0001-7219-3428 Biplab Joarder: 0000-0002-2747-6071 Arkendu Roy: 0000-0002-2425-5003 Partha Samanta: 0000-0002-2329-3729 Sujit K. Ghosh: 0000-0002-1672-4009 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.V.D., B.J., A.R., and P.S. thank IISER-Pune, CSIR, DSTINSPIRE, and UGC, respectively, for research fellowships. R.B. thanks the National Computing Infrastructure (NCI), CSIRO Pearcey cluster, and the Pawsey supercomputing facilities for the computing support and the Australian Research Council for the DECRA fellowship (DE160100987). We thank Dr. Soumya Mukherjee, Dr. Avishek Karmakar, and Dr. Jian-Bin Lin for valuable inputs. S.K.G. thanks SERB (project no. EMR/2016/000410) and DST Nanomission Thematic Unit for funding. DST-FIST (SR/FST/CSII-023/2012; for microfocus SCXRD instrument) is acknowledged for generous support.

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