Exceptionally Stable and 20-Connected Lanthanide MetalâOrganic

Article pubs.acs.org/crystal

Cite This: Cryst. Growth Des. 2018, 18, 2432−2440

Exceptionally Stable and 20-Connected Lanthanide Metal−Organic Frameworks for Selective CO2 Capture and Conversion at Atmospheric Pressure Bharat Ugale, Sandeep Singh Dhankhar, and C. M. Nagaraja* Department of Chemistry, Indian Institute of Technology Ropar, Nangal Road, Rupnagar 140001, Punjab, India S Supporting Information *

ABSTRACT: Highly thermal and chemically stable, 20connected lanthanide metal−organic frameworks (MOFs) [{Ln(BTB)(H2O)}·H2O]n (where Ln = Sm (MOF1)/Gd (MOF2), BTB = 1,3,5-tris (4-carboxy phenyl) benzene) have been synthesized solvothermally and characterized by singlecrystal X-ray diffraction analysis and other physicochemical methods. MOF1 and 2 are isostructural and feature threedimensional honeycomb-like structure with large one-dimensional hexagonal channels of dimension ∼10.20 × 10.11 Å2. Gas uptake studies of the samples revealed selective adsorption properties of MOF1 for CO2 over other (N2, Ar, and H2) gases. The activated samples of the MOF1/2 act as efficient recyclable catalysts for heterogeneous cycloaddition of CO2 with styrene oxide, resulting in cyclic carbonate with high yield and selectivity. Interestingly, the pore size-dependent catalytic conversion of epoxides has been observed, suggesting the potential utility of MOF1 as a promising heterogeneous catalyst for cycloaddition of carbon dioxide. Furthermore, the MOF1 catalyst can be easily recycled for several cycles without significant loss of catalytic activity as well as structural rigidity. MOF1 and 2 represent rare examples of 20-c lanthanide MOFs exhibiting selective capture and efficient cycloaddition of CO2 with epoxides at mild conditions.

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INTRODUCTION Carbon dioxide (CO2) has attracted considerable attention not only as a greenhouse gas but also as an abundant C1 building block for organic transformations due to its free availability, nontoxicity, and renewability.1 Therefore, intensive research efforts are being made by researchers worldwide for its efficient conversion to value-added chemicals.2−5 However, the thermodynamic stability and kinetic inertness of CO2 are the major limitations for its conversion at mild conditions. Hence, development of efficient catalytic systems for selective capture and conversion of CO2 into useful chemicals has gained significant importance. In this regard, cycloaddition of CO2 to epoxides represents one of the most promising methods for generation of cyclic carbonates.6−8 Further, cyclic carbonates offer widespread applications, and they are industrially manufactured using highly toxic phosgene. Thus, it is highly desirable to develop alternative green routes for the synthesis of cyclic carbonates. Various homogeneous catalysts such as transition-metal complexes,9−15 ionic liquids,16 organocatalysts,17,18 and quaternary ammonium or phosphonium salts,19−22 etc. have been employed for catalytic cycloaddition of CO2 with epoxides to generate cyclic carbonates. However, these systems suffer from the limitations of catalyst recycling and separation of the product, whereas heterogeneous catalysts, such as metal oxides,23 zeolites,24 and functional polymers,25−30 have also been developed for the catalytic conversion of CO2. However, these reactions often require high pressure of CO2 and/or high activation temperatures. © 2018 American Chemical Society

On other hand, porous metal−organic frameworks or coordination polymers have emerged as promising candidate materials for selective capture of CO2.31−41 Further, metal− organic frameworks (MOFs) are also known to act as heterogeneous catalysts for conversion of CO2 to cyclic carbonates.42−51 Moreover, few MOFs are known to exhibit both selective capture and conversion of CO2.52−64 However, a limited number of MOFs are known for efficient conversion of CO2 at mild conditions of atmospheric pressure and low loading of cocatalyst.65−69 Furthermore, MOFs based on polynuclear metal clusters with low-connected networks such as 3-, 4-, and 6-connected topologies have been extensively studied, whereas the number of examples of highly connected networks is relatively less.70 In this regard, MOFs based on lanthanides have gained special interest due to their rich coordination geometry, high stability, ability to form highly connected networks, and interesting properties.71−76 Recently, the construction of Ln-MOFs with 12-,77 15-,78 and 18-c79 metal clusters has been reported. However, the examples of highly connected (>18-c) Ln-MOFs based on the uninodal cluster are rare. Herein, we report synthesis of two new isostructural, 20connected lanthanide MOFs, [{Ln(BTB)(H 2 O)}·H 2 O] n (where, Ln = Sm(MOF1)/Gd(MOF2)) solvothermally at Received: January 12, 2018 Revised: March 5, 2018 Published: March 14, 2018 2432

DOI: 10.1021/acs.cgd.8b00065 Cryst. Growth Des. 2018, 18, 2432−2440

Crystal Growth & Design

Article

100 °C using 1,3,5-tris(4-carboxyphenyl)benzoic acid (BTB) as a triangular tricarboxylate ligand. The X-ray structural analysis revealed that MOF1 and 2 exhibit an interesting 3D honeycomb-like structure with 20-c, uninodal {3^78.4^106.5^6}-net topology. Further, the MOFs exhibit exceptionally high thermal and chemical stability and possess a high surface area. Interestingly, MOF1/2 act as efficient recyclable catalysts for heterogeneous cycloaddition of CO2 with epoxides resulting in cyclic carbonates with high yield and selectivity. Therefore, MOF1 and 2 represent rare examples of 20-c lanthanide MOFs exhibiting selective capture and conversion of CO2 at mild conditions.

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(Figure S2). For the sake of comparison of catalytic activity, an isostructural Ce-MOF was synthesized following procedure similar to that of MOF1 and characterized (Supporting Information, Figure S4). X-ray Crystallography. Single crystal X-ray structural data of MOF1 and 2 were collected on a CMOS based Bruker D8 Venture PHOTON 100 diffractometer equipped with an INCOATEC microfocus source with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) operating at 50 kV and 30 mA. The SAINT80 program was used for integration of diffraction profiles and absorption correction was made with SADABS program.81 The structures were initially solved by SIR 9282 and refined by full matrix least-square method using SHELXL-201383 and WinGX system, Ver 2013.3.84 The nonhydrogen atoms in both the structures were located from the difference Fourier map and refined anisotropically. All the hydrogen atoms were fixed by HFIX and placed in ideal positions and included in the refinement process using riding model with isotropic thermal parameters. The disordered guest solvent molecules in MOFs1 and 2 were treated with SQUEEZE option of PLATON85 multipurpose crystallographic software. The molecular formula of MOF1 and 2 was determined based on the elemental analyses and TGA. The potential solvent accessible void space was calculated using the PLATON85 software. All the crystallographic and structure refinement data of the MOFs1−2 are summarized in Table 1. Selected bond lengths, angles,

EXPERIMENTAL SECTION

Materials and Methods. All the reagents used in this work were commercially available and used as received without any further purification. Sm(NO3)3·6H2O, Gd(NO3)3·6H2O and Ce(NO3)3· 6H2O were purchased from Sigma-Aldrich chemical Co. 1,3,5-Tris (4-carboxyphenyl) benzene (H3BTB) was purchased from Alfa Aesar Co. All the reagents and the internal standard used for catalytic reactions were purchased from TCI chemicals and used without further purification. Thermogravimetric analysis (TGA) of the MOFs was carried out using Metler Toledo thermogravimetric analyzer in N2 atmosphere with a flow rate of 30 mL/min in the temperature range of 30−1000 °C and a heating rate of 10 °C/min. Powder XRD measurements were carried out on PANalytical’s X’PERT PRO diffractometer with Cu Kα radiation (λ = 1.54184 Å; 40 kV, 20 mA). Elemental analysis of the samples was carried out on Thermo Fischer Flash 2000 Elemental Analyzer. Fourier transform infrared (FTIR) spectra of the samples were recorded from 650 to 4000 cm−1 on a PerkinElmer ATR-FTIR spectrometer. The products of catalytic reactions were identified, and the catalytic conversions were determined using 1H NMR spectra recorded in CDCl3 on JEOL JNM-ECS-400 spectrometer operating at a frequency of 400 MHz. Synthesis of [{Sm(BTB)(H2O)}·H2O]n (MOF1). MOF1 was prepared by employing solvothermal conditions at temperature of 100 °C. To a 4 mL DMF solution of H3BTB (0.044 g, 0.10 mmol) taken in a glass vial (30 mL), an aqueous solution (2 mL) of Sm(NO3)3·6H2O (0.045 g, 0.1 mmol) was added dropwise with constant stirring. The contents were stirred for 30 min and then transferred to a 23 mL Teflon lined stainless steel autoclave and heated at 100 °C for 72 h and cooled naturally. The colorless needle-shaped crystals of [{Sm(BTB)(H2O)}·H2O]n (1) were obtained. Yield: 88%. The phase purity of the as-synthesized MOF1 was confirmed from the powder X-ray diffraction (XRD) pattern (Figure S1). Anal. Calcd For C27H19O8Sm: C, 52.15; H, 3.08. Found: C, 51.83; H, 3.22. IR (cm−1): 3374, 3012, 2980, 1580, 1507, 1408, 1187, 1016, 849, 780, 701 (Figure S2). As-prepared MOF1 was activated at 220 °C under a vacuum for 20 h prior to gas adsorption and catalytic studies. The phase purity of the activated sample was confirmed from powder XRD pattern (Figure S1). Anal. Calcd for C27H15O6Sm: C, 55.36; H, 2.58. Found: C, 54.93; H, 2.63. IR (cm−1): 2980, 2910, 1582, 1508, 1408, 1186, 1017, 849, 816, 781, 702 (Figure S2). Synthesis of [{Gd(BTB)(H2O)}·H2O]n (MOF2). MOF2 was prepared following the procedure similar to that of MOF1. Here, Gd(NO3)3·6H2O (0.046 g, 0.1 mmol) was used in place of Sm(NO3)3· 6H2O. The colorless needle-shaped crystals of MOF2 was isolated. Yield: 87%. The phase purity of the as-synthesized sample was confirmed by powder XRD pattern (Figure S3) Anal. Calcd for C27H19O8Gd: C, 51.58; H, 3.05. Found: C, 52.05; H, 3.21. IR (cm−1): 3359, 3014, 2980, 1655, 1579, 1511, 1407, 1276, 1184, 1105, 1016, 854, 815, 779, 703, 672 (Figure S2). As-prepared MOF2 was activated at 220 °C under a vacuum for 20 h prior to gas adsorption and catalytic studies. The phase purity of the sample was confirmed from the powder XRD pattern (Figure S3). Anal. Calcd for C27H15O6Gd: C, 54.72; H, 2.55. Found: C, 54.86; H, 2.47. IR (cm−1): 3007, 2970, 1609, 1582, 1530, 1505, 1381, 1275, 1181, 847, 815, 778, 764, 750, 671

Table 1. Crystal Data and Structure Refinement Parameters for MOF1 and 2

a

parameters

MOF1

MOF2

empirical formula formula mass crystal system space group a /Å b /Å c /Å α (deg) β (deg) γ (deg) V (Å3) Z ρ (g cm−3) μ (mm−1) F(000) T (K) λ (Mo Kα) (Å) Θmin (deg) Θmax (deg) total data unique data Rint data [I > 2σ(I)] a R1 b wR2 S CCDC numbers Flank parameter

C27H17O7Sm 603.76 trigonal R32 (No. 155) 28.561(5) 28.561(5) 12.317(5) 90.00 90.00 120.00 8701(4) 9 1.040 1.546 2745 298 0.71073 2.3 28.4 56272 4844 0.092 4428 0.0643 0.1627 1.17 1555305 0.025(11)

C27H17O7Gd 610.66 trigonal R32 (No. 155) 28.550(5) 28.550(5) 12.212(5) 90.00 90.00 120.00 8620(4) 9 1.060 1.759 2691 298 0.71073 2.3 28.4 64365 4797 0.061 4573 0.0267 0.0573 1.11 1555306 0.028(7)

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = [∑[w(Fo2 − Fc2)2]/∑w(Fo2)2]1/2.

and hydrogen bond details are summarized in Tables S1−S2, respectively. The crystallographic information files are deposited with the CCDC numbers 1555305 and 1555306 for MOFs 1 and 2, respectively. Gas Adsorption Measurements. N2 adsorption−desorption measurements were carried out at 77 and 273 K, whereas CO2 adsorption−desorption measurements were carried out at 273, 298, and 195 K using QUANTACHROME Quadrasorb-SI automated 2433



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Figure 1. Asymmetric unit of MOF1 (a) and MOF2 (b) (insets: coordination environment of Sm3+/Gd3+ in MOF1/2, the hydrogen atoms and solvent molecules are omitted for clarity). (c) [SmO9] SBUs in MOF1 forming a 1D chain along the crystallographic c-axis. surface area and pore size analyzer. For selectivity study, adsorption of H2 and Ar was carried out at 273 K. Ultrapure (99.995%) N2, He, H2, Ar, and CO2 gases were used for the adsorption−desorption measurements. Prior to adsorption measurements, the samples (∼100 mg) were evacuated at 493 K under a vacuum (20 mTorr) for 20 h using QUANTACHROME Flovac degasser and further purged with ultrapure N2 (99.995%) gas on cooling. Then the degassed sample was introduced to the instrument, and all the further operations were computer controlled. The Brunauer−Emmett−Teller (BET) surface area was estimated from N2 sorption isotherms carried out at 77 K. The temperature, 77 K was achieved using liquid N2 and 273 and 298 K were achieved using a chiller with an ethylene glycol− water mixture (1:1) as a coolant. The gas selectivity studies were carried out at 273 K. Dead volume of the sample cell was measured using helium gas (99.995%). Catalytic Cycloaddition Reactions of CO2. The catalytic cycloaddition reactions of CO2 with epoxides were carried out in a 50 mL stainless steel reactor at 1 atm pressure of CO2. Prior to catalytic reactions, the MOFs were activated at 493 K for 20 h under a vacuum to remove the guest and metal-coordinated water molecules. All the reactants/reagents were added at room temperature, and the reactor was pressurized with CO2 and flushed twice, and then the required pressure (1 bar) was attained. The reagent 1,2-epoxypropane was handled at low temperature due to its low boiling point. The reaction mixture was stirred at 1000 rpm for 15 h. After the catalytic reactions, the pressure was released slowly, the catalyst was separated by filtration, and the catalytic conversion was determined by 1H NMR spectroscopy. The recovered catalyst after the catalytic reaction was washed with water and methanol three times, dried at r.t. and then at 493 K for 20 h under a vacuum, and reused for successive cycles.

solvent water molecule of crystallization (Figure 1a,b). The Sm1/Gd1 ions are in a tricapped trigonal prismatic geometry with SmO9/GdO9 chromophore satisfied by eight carboxylate oxygen atoms from three BTB3− ligands and an oxygen from coordinated water molecule (Figure 1). These [LnO9] secondary building units (SBUs) are extended as a 1D chain along the crystallographic c-axis (Figure 1c). Further, the [LnO9] SBUs are further connected by three tritopic BTB3− ligands resulting in a 20-c Sm1/Gd1 node (Figure 2a−b and Figure S5) which are extended in three dimensions to generate a 3D honeycomb-like framework with 1D hexagonal channels of dimension ∼10.20 × 10.11 Å2 (Figure 2c). Topological analysis by TOPOS89 suggests that the Sm/Gd center acts as a 20-c node and the overall structure has {3^78.4^106.5^6}-net topology (Figure 2d). The 3D

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RESULTS AND DISCUSSION Synthesis and Crystal Structure. The reaction of Ln(NO3)3·6H2O (Ln = Sm/Gd) with H3BTB in a mixture of DMF/H2O in a 2:1 ratio at 100 °C afforded the MOFs, [{Ln(BTB)(H2O)}·H2O]n (Ln = Sm (MOF1)/Gd(MOF2)). Single crystal X-ray structural determination revealed that MOF1 and 2 are isostructural to the analogues Ln-MOFs (Ln = La, Tb, and Eu) reported by the research groups of Ferey and Kitagawa.86−88 Both MOF1/2 crystallize in the trigonal system with non-centrosymmetric R32 space group (Table 1). The asymmetric unit contains a Sm3+/Gd3+ ion in MOF 1/2, a BTB3− ligand and a coordinated water molecule along with a

Figure 2. (a) View of the 20-connected Sm3+/Gd3+ node (central polyhedron shown in magenta color). (b) A simplified view of 20-c, Sm/Gd center. (c) View of 3D honeycomb-like framework with 1D channels. (d) Topological representation of the 3D framework in MOF1 and 2. 2434



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Figure 3. (a) CO2 adsorption−desorption isotherms carried out at 195, 273, and 298 K for MOF1. (b) Adsorption−desorption isotherms of MOF1 for different adsorbates (CO2, H2, Ar, N2) at 273 K showing selective adsorption for CO2.

frameworks possess a void volume of ∼47.5 and ∼49.6% per unit cell for MOF1 and 2 respectively, calculated using PLATON85 after removal of guest water molecules. The selected bond lengths and angles are summarized in Tables S1 and S2. Chemical and Thermal Stability of MOF1/2. Interestingly, the as-synthesized MOFs were found to exhibit high chemical and thermal stability, and such MOFs are beneficial for large-scale industrial applications. To test the chemical stability, MOF1 was soaked in different organic solvents (ranging from highly protic to aprotic) with vigorous stirring for 48 h, and then the solids were recovered by filtration washed thoroughly before recording their powder XRD patterns (Figure S6). The water stability was tested at various pH conditions in which MOF1 was stirred in aqueous acidic (pH = 2−6) and basic (pH = 8−12) solutions for 48 h. The powder XRD patterns of the recovered samples of MOF1 indicate retaining of the original framework structure (Figures S6 and S7) suggesting their high chemical and water stability. This unusual behavior could be originating due to the rigidity of the framework constituted by highly connected Sm3+/Gd3+ ions and the close stacking of aromatic rings of BTB ligand providing a hydrophobic surface. Thermogravimetric analysis of the MOFs revealed loss of coordinated water molecules in the temperature range of 100− 120 °C and the dehydrated frameworks of MOF1 and 2 were found to be stable up to 600 °C, suggesting their high thermal stability with retaining of the original framework structure. Further, to test the regeneration of the MOFs, the activated MOFs were soaked in water for 4 h, and the isolated samples were analyzed by TGA and powder XRD measurements. TGA of the resolvated samples showed weight loss corresponding to coordinated water molecules (Figure S8). In addition, powder XRD patterns of the resolvated samples are in good agreement with the as-synthesized samples, indicating regeneration of the original structure (Figure S9). Interestingly, the TGA of the isostructural MOFs, MIL-103(Ln) (Ln = Ce, Tb) carried out under N2 and O2 gas flow show an interesting difference in thermal stabilities. There is an increase in stability of about 200 °C observed when carried out under N2 atmosphere compared to those carried out under O2 atmosphere. The relatively lower stability of the MOFs under O2 atmosphere has been attributed to the redox character of the MOFs. Gas Adsorption Study. N2 adsorption−desorption measurements of the activated samples of MOF1 and 2 revealed a typical type-I isotherm with a steep uptake at the low-pressure

region, supporting their microporous nature (Figure S10). The estimated value of Brunauer−Emmett−Teller (BET) surface area for MOF1 and 2 were 946.74 and 834.26 m2/g, respectively, which are in accordance with the surface area reported for isostructural MOFs reported by Kitagawa and coworkers.88 Further, N2 adsorption measurements of MOF1 recovered after chemical treatment showed type-I behavior similar to that of pristine MOF suggesting retaining of microporosity (Figure S11). Further, the pore size distribution for MOF1 and 2 calculated from the N2 adsorption isotherms was found to be 10.2 Å (inset of Figure S10) and is consistent with the pore diameter observed from the crystal structure. Furthermore, CO2 adsorption isotherms of MOF1 and 2 carried out at 273 K and 1 atm pressure follow a typical type-I profile with the volumetric uptake of 1.29 and 1.87 mmol/g, respectively (Figures 3a and S12), while at 195 K, MOF1 and 2 show steep uptake of CO2 at the low-pressure range with the volumetric uptake of 9.12 and 8.89 mmol/g respectively, confirming the inherent permanent porosity. The observed CO2 uptake by MOF1 and 2 is comparable to the analogues La-MOF reported by Ferey and co-workers.87 The adsorption branches of the isotherms carried out at 273 and 298 K were fitted to the Freundlich−Langmuir equation90 (Figures S13 and S14) to obtain a precise prediction over the quantity of CO2 adsorbed at the saturation point. The estimated values of isosteric heat of adsorption (Qst) for MOF1 and 2 were found to be 21.6 and 24.5 kJ/mol, respectively (Figures S15 and S16), calculated using the Clausius−Clapeyron equation.91,92 Interestingly, the gas adsorption studies of MOF1 for other gases (N2, H2, and Ar) showed negligible uptake of 0.20, 0.18, and 0.09 mmol/g for Ar, N2, and H2, respectively (Figure 3b) indicating the selective adsorption property of MOF1 for CO2 over other gases. The observed selectivity for CO2 can be attributed due to its significant quadrupole moment (−1.4 × 10−39 C m2) and polarizability over other gases, leading to its stronger interaction with the unsaturated Sm3+ ions lined in the 1D channels of MOF1. Literature survey revealed that the observed CO2 uptake by MOF1 is not significantly high in comparison to the well-known MOFs reported in the literature.31−41 However, the uptake is comparable among the lanthanide MOFs, known for CO2 storage (Table S3). Catalytic Cycloaddition of CO2 with Epoxides. As it can be seen in Figure 2, the hexagonal channels of MOF1 and 2 are decorated with Sm3+ and Gd3+ ions, respectively. Upon activation, the coordinated water molecule on the lanthanide 2435



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bigger than the size of the substrates (Table S5). On the other hand, the relatively lower catalytic conversion of 1,2epoxydecane (Table 2, entry 4 and Figure S22) can be ascribed to its larger size compared to the pore size of MOF1 (revealed from the optimized geometries of the cyclic carbonates using Gaussian09 at the B3LYP/6-311G (d,p)96 level) (Table S5). Furthermore, the catalytic activity of MOF1 was compared with other lanthanide-based MOFs known for cycloaddition of CO2, and remarkably, the reaction conditions employed in this work are milder than the previously reported examples of Ln-MOFs (Table 3). Furthermore, to rule out the possibility that homogeneous catalysis by the MOF leached into the solution, the reaction was stopped at 6 h, and at that time, the conversion of styrene oxide was found to be 54%; then the MOF catalyst was removed by filtration, and the filtrate was allowed to stir for an additional 6 h. The analysis of the aliquot taken at 12 h revealed a slight increase (∼5%) in the conversion of styrene oxide, which is considerably lower in comparison with the reaction carried out in the presence of MOF catalyst (Figures S23 and S24). The slight increase in the conversion of styrene oxide can be attributed to the presence of cocatalyst, TBAB. Thus, the high catalytic efficiency and size-dependent selectivity for epoxides suggest the potential utility of MOF1 as a promising heterogeneous catalyst for cycloaddition of CO2 at mild conditions. Furthermore, the MOF1 catalyst can be easily separated from the reaction mixture by simple filtration followed by washing with H2O/EtOH and reused for five consecutive cycles without significant loss of catalytic activity (Figure 4). Remarkably, powder XRD analysis of the recycled catalyst after five catalytic cycles matches well with the parent MOF, suggesting retention of the original framework (Figure S25). The plausible mechanism for cycloaddition of CO2 with epoxides catalyzed by MOFs has been reported in the literature. It involves a binary catalytic system composed of a Lewis acid catalytic site and a nucleophilic cocatalyst (TBAB) required for ring opening of epoxide.42−69 The proposed mechanism is shown in Scheme 1; the first step involves coordination of the epoxide to the Lewis acidic Sm/Gd center. The evidence for the coordination of epoxide with the unsaturated metal center has been shown in the literature by means of theoretical calculations and structural determination of an epoxide coordinated to nickel center in MOF.6,7 The next step involves the nucleophilic attack of the Br− anion of TBAB leading to ring opening of the epoxide and formation of metalcoordinated bromo-alkoxide. Subsequent insertion of CO2 to generate metal carbonate species which undergoes an intramolecular ring-closure reaction to form the cyclic carbonate and its elimination from the metal center results in regeneration of the active catalyst.

ion can be reversibly removed to obtain framework composed of unsaturated, Lewis acidic lanthanide centers which can act as potential catalytic sites for acid catalyzed organic transformations. Motivated by the high thermal stability and selective CO2 capture properties of the MOF1 and 2, we envisioned that the activated MOFs can act as efficient heterogeneous catalysts for cycloaddition of CO2. Therefore, the catalytic activity of MOF1 and 2 was investigated for cycloaddition of CO2 and styrene oxide as a model substrate (epoxide) at mild conditions of 1 atm of CO2 and in the presence of 1 mol % TBAB as cocatalyst. Interestingly, the catalytic results showed the formation of the corresponding cyclic carbonate with about 93% conversion in 15 h (Table S4). Further, controlled experiments revealed that both catalyst (MOF1/2) and TBAB are essential for the catalytic activity. It is known that TBAB acts as a nucleophilic cocatalyst and facilitates the ring opening of the epoxides.93−95 For high catalytic activity, the synergistic effect between the catalyst (MOF) and TBAB is essential. Further, the progress of the catalytic reaction can be easily monitored by recording the 1H NMR spectra of the aliquots taken at regular time intervals with reference to an internal standard (1,1′,2,2′-tetrachloroethane). Remarkably, 100% selectivity for cyclic carbonates was observed with no additional byproducts (Figure S17). For the sake of comparison, a Ce-MOF isostructural to MOF1/2 was synthesized, and its catalytic activity was studied at similar conditions mentioned before.86 The catalytic investigation revealed that Ce-MOF catalyzes the cycloaddition of CO2 with styrene oxide to corresponding cyclic carbonate with high yield (98%) relatively higher than those of MOF1 and 2 (Table S4, Figure S18). The scope of the reaction was further extended to cycloaddition of CO2 with alkyl epoxides with increasing alkyl chain length and using MOF1 as the catalyst (Table 2). Interestingly, the catalytic conversion of 1,2-epoxypropane, 1,2-epoxybutane, and 1,2-epoxyhexane were found to be almost similar (Table 2, entries 1−3 and Figures S19−21). This observation can be attributed to the large hexagonal pore channels of MOF1 (10.20 × 10.11 Å2) which are sufficiently Table 2. Cycloaddition of Various Substituted Epoxides Catalyzed by MOF1d

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CONCLUSIONS In conclusion, synthesis of high thermal and chemically stable, 20-c lanthanide MOFs of Sm3+/Gd3+ ions featuring an interesting 3D honeycomb-like structure with large 1D hexagonal channels of dimension ∼10.20 × 10.11 Å2 is reported. Further, the MOFs exhibit selective adsorption properties for CO2 over other (N2, Ar, and H2) gases. More interestingly, the activated samples of MOF1/2 act as efficient recyclable catalysts for heterogeneous cycloaddition of CO2 with epoxides resulting in cyclic carbonates with high yield and selectivity. Further, the pore size-dependent catalytic con-

a Conversion: the catalytic conversions were determined by 1H NMR analysis using 1,1′,2,2′-tetrachloroethane as the internal standard. b TON: moles of cyclic carbonate/mol of catalyst used. cTOF: TON/ reaction time in hours. dReaction conditions: epoxide (20 mmol), catalyst/TBAB (0.5:1.0 mol %), temperature of 80 °C.

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Table 3. Catalytic Conditions Employed for Cycloaddition of CO2 with Epoxides by Various Lanthanide MOFs S. no.

catalyst

TBAB (mol %)

pressure [MPa]

temperature [°C]

conversion (%)

reference

1 2 3 4 5 6

M-BDC/NDC [M = Y, Er, Tb] Gea-MOF1 Eu-BTB-Phen [Tb-M(BPD)2(μ2H2O)Cl] Gd-MOF Sm/Gd-BTB

1.5 0.75 5.0 2.5/5 2.5 1.0

1.0 2.0 0.1 0.1 2.0 0.1

60 120 70 70 80 80

89/87 88 97 77 98.4 100

42 79 32 67 68 this work

Accession Codes

CCDC 1555305−1555306 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 91- 1881-242229. ORCID

C. M. Nagaraja: 0000-0002-4271-6424 Notes

The authors declare no competing financial interest.

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Figure 4. Recycling test of MOF1 for five successive cycles.

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Scheme 1. Plausible Reaction Mechanism for the Cycloaddition of CO2 with Epoxides

version of epoxides has been observed, suggesting the potential utility of MOF1 as a promising heterogeneous catalyst for cycloaddition of carbon dioxide. Remarkably, MOF1 catalyst can be easily recycled for several cycles without significant loss of catalytic activity as well as structural rigidity. Therefore, MOF1 and 2 represent rare examples of 20-c lanthanide MOFs exhibiting selective capture and efficient cycloaddition of CO2 with epoxides at mild conditions.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b00065. PXRD plots for the compounds, structural data, TGA plots, gas adsorption−desorption isotherms, 1NMR spectra, optimized geometries of epoxides (PDF) 2437



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