Environmentally Friendly, Co-catalyst-Free Chemical Fixation of CO2

13 hours ago - Synopsis. Rational construction of highly porous, Co(II)/Ni(II) 3D MOFs, composed of dual-walled cages functionalized with high density...
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Environmentally Friendly, Co-catalyst-Free Chemical Fixation of CO2 at Mild Conditions Using Dual-Walled Nitrogen-Rich ThreeDimensional Porous Metal−Organic Frameworks Bharat Ugale, Sandeep Kumar, T. J. Dhilip Kumar, and C. M. Nagaraja* Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India

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S Supporting Information *

ABSTRACT: Highly porous, polyhedral metal−organic frameworks (MOFs) of Co(II)/Ni(II), {[M6(TATAB)4(DABCO)3(H2O)3]·12DMF· 9H2O}n (where M = Co(II) (1)/Ni(II) (2), H3TATAB = 4,4′,4″-striazine-1,3,5-triyl-tri-p-aminobenzoic acid, and DABCO = 1,4diazabicyclo[2.2.2]octane) have been synthesized solvothermally. Both MOFs 1 and 2 show a 2-fold interpenetrated 3D framework structure composed of dual-walled cages of dimension ∼ 30 Å functionalized with a high density of Lewis acidic Co(II)/Ni(II) metal sites and basic -NHgroups. Interestingly, MOF 1 shows selective adsorption of CO2 with high heat of adsorption (Qst) value of 39.7 kJ/mol that is further supported by theoretical studies with computed binding energy (BE) of 41.17 kJ/mol. The presence of the high density of both Lewis acidic and basic sites make MOFs 1/2 ideal candidate materials to carry out co-catalyst-free cycloaddition of CO2 to epoxides. Consequently, MOFs 1/2 act as excellent recyclable catalysts for cycloaddition of CO2 to epoxides for high-yield synthesis of cyclic carbonates under co-catalystfree mild conditions of 1 bar of CO2. Further, MOF 1 was recycled for five successive cycles without substantial loss in catalytic activity. Herein, rational design of rare examples of 3D polyhedral MOFs composed of Lewis acidic and basic sites exhibiting efficient co-catalyst-free conversion of CO2 has been demonstrated.

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carbonates.7 Often, the catalytic reactions require high temperature and/or high pressure of CO2 to obtain cyclic carbonates in high yield. On the other hand, porous metal− organic frameworks (MOFs) have attracted significant attention as prominent candidate materials for selective CO2 capture applications.8 Several MOFs including the polyhedral ones with confined nanospace have been utilized as efficient catalysts for utilization of CO2 to generate cyclic carbonates.9−12 However, most of these catalysts, require additional co-catalyst such as tetrabutylammonium bromide (TBAB) to achieve high yields of cyclic carbonates.13 One way to overcome the requirement of additional nucleophilic cocatalyst is by designing catalysts incorporating a high density of Lewis acidic and basic sites by utilizing rationally designed organic ligands/linkers and/or postsynthetic modification (PSM) of the MOFs.14 Thus, for an efficient co-catalyst-free cycloaddition of CO2, the MOF should contain a high density of Lewis acidic and basic sites along with high affinity for CO2 capture. Keeping these points in mind, herein we report the construction of two new isostructural, porous, polyhedral MOFs of Co(II)/Ni(II), {[M6(TATAB)4(DABCO)3(H2O)3]· 12DMF·9H2O}n (where M = Co (1)/Ni (2), (H3TATAB =

INTRODUCTION Carbon dioxide (CO2) has gained significant interest as a greenhouse gas and also as an abundant C1 building block for the synthesis of various value-added chemicals due to its easy availability, renewability, and nontoxicity.1 Extensive research efforts are being carried out for selective carbon dioxide capture and its subsequent utilization (CCU) to synthesize fine chemicals.2 It has been known, though, that CO2 could be effectively adsorbed by traditional adsorbents such as aqueous ammonia but the regeneration of the adsorbent is rather energy-intensive and uneconomical. Therefore, the development of cost-effective and environmentally friendly catalytic systems for efficient CCU at mild conditions has attracted significant interest.3 In this context, cycloaddition of epoxides to CO2 to yield cyclic carbonates is an atom economic process to obtain cyclic carbonates with high yield.4 Further, cyclic carbonates find widespread applications as electrolytes in lithium-ion battery and as intermediates in the synthesis of fine chemicals.5 However, the kinetic inertness and stability of CO2 pose a major limitation for its utilization as C1 source at mild conditions. To achieve this, several homogeneous catalysts have been developed which show good catalytic activity for cycloaddition of CO2 with epoxides.6 However, to overcome the limitations of product separation and catalyst recycling associated with these reactions, various heterogeneous catalysts have been utilized for the conversion of CO2 to cyclic © XXXX American Chemical Society

Received: December 27, 2018

A

DOI: 10.1021/acs.inorgchem.8b03612 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(C128H158O41N30Co6): C, 49.17; H, 5.09; N, 13.44. Found: C, 48.92; H, 5.21; N, 13.61. Activation: The solvent-exchanged MOF 1 was activated by heating to 150 °C under vacuum for 20 h, and the phase purity and stability were analyzed from PXRD analysis (Figure S 1 ). Elem. Ana l. Ca lcd (%) for activated MO F 1 , {[Co6(TATAB)4(DABCO)3]}n (C114H96O24N30Co6): C, 52.19; H, 3.69; N, 16.02. Found: C, 51.83; H, 3.62; N, 15.73. FT-IR (KBr, cm−1): 3289, 2918, 1643, 1601, 1457, 1398, 1291, 1097, 949, 858, 789, 754, 661, 471 (Figure S2). Synthesis of {[Ni6(TATAB)4(DABCO)3(H2O)3]·12DMF·9H2O}n (MOF 2). MOF 2 was synthesized using the procedure similar to that of 1 except that Ni(NO3)2·6H2O (0.058 g, 0.2 mmol) was used in the place of Co(NO3)2·6H2O. Dark-green cubic crystals of MOF 2 were obtained with 78% yield. The phase purity of MOF 2 was examined by PXRD analysis (Figure S3). Elem. Anal. Calcd (%) for MOF 2 (C150H204O48N42Ni6): C, 48.49; H, 5.53; N, 15.83. Found: C, 48.91; H, 5.67; N, 15.58. FT-IR (KBr, cm−1): 3450−3010 (b), 1583, 1478, 1370, 1342, 1232, 1176, 1057, 1007, 853, 773, 689, 488 (Figure S2). As-prepared MOF 2 was subjected to solvent exchange with MeOH to obtain the solvent-exchanged MOF 2, {[Ni6(TATAB)4(DABCO)3(H2O)3]·14MeOH}n. Elem. Anal. Calcd (%) for C128H158O41N30Ni6: C, 49.20; H, 5.10; N, 13.45. Found: C, 49.32; H, 4.83; N, 13.57. Activation: The solvent-exchanged MOF was activated at 150 °C under vacuum for 16 h, and the stability and the phase purity were confirmed from PXRD analysis (Figure S3). Elem. Anal. Calcd (%) for activated MOF 2, {[Ni6(TATAB)4(DABCO)3]}n (C114H96O24N30Ni6): C, 52.21; H, 3.69; N, 16.02. Found: C, 52.05; H, 3.71; N, 16.01. FT-IR (KBr, cm−1): 3298, 2925, 1649, 1598, 1460, 1402, 1286, 1100, 956, 861, 793, 760, 665, 469 (Figure S2). Gas Adsorption Measurements. N2 adsorption studies were performed at 77 and 273 K, while CO2 adsorption studies were performed at 195, 273, and 298 K on a Quadrasorb-SI instrument. H2 and Ar adsorption measurements were conducted at 273 K for gas selectivity studies. Ultrapure (99.995%) gases were employed for the adsorption studies. The MOF samples (∼150 mg) were activated at 423 K under vacuum (20 mTorr) for 16 h on a Flovac degasser and followed by introducing N2 gas upon cooling. The Brunauer− Emmett−Teller (BET) surface areas of MOFs 1 and 2 were determined from N2 and CO2 adsorption studies. Catalytic Cycloaddition Reactions. A 100 mL high-pressure glass reactor was used for the cycloaddition reactions of various epoxides. The activated MOF sample was transferred to the reactor, the epoxides were added at room temperature (RT), the reactor was flushed with CO2 twice, then the desired pressure was introduced (1− 8 bar), and the mixture was stirred at 1000 rpm at 80 °C for the required time. After catalytic reactions, the reactor was allowed to cool to RT, then the CO2 gas was vented, the MOF was separated by filtration, and conversions were determined by recording 1H NMR spectra of the filtrate using 1,1′,2,2′-tetrachloroethane as an internal standard. The catalyst recovered was washed with a water:methanol mixture three times and then again with pure MeOH, dried at RT, and reused for successive cycles after activation. Theoretical Calculations. Density functional theory (DFT) calculations were undertaken to investigate the selective CO2 adsorption property of MOF 1. The generalized gradient approximation (GGA) with Perdew−Burke−Ernzerhof (PBE) exchange− correlation functional combined with double numeric polarized (DNP) basis set was employed to calculate the CO2 adsorption properties of MOF 1 by using the DMOL3 program.16 Grimme’s dispersion corrected DFT-D term was employed to investigate the weak van der Waals’ interactions. The TATAB3− ligand coordinated to a [Co(OOC)2(H2O)2] SBU is considered as a model system and optimized, and three CO2 molecules and epoxides were introduced and optimized. The average binding energy (BE) of a CO2 molecule interacting with the Lewis basic -NH- group of TATAB3− ligand was calculated using the following relation.17

4,4′,4″-s-triazine-1,3,5-triyltri-p-aminobenzoic acid, and DABCO = 1,4-diazabicyclo[2.2.2]octane) by solvothermal route. Single crystal X-ray structures of MOFs 1 and 2 show the presence of a 3D polyhedral structure composed of dualwalled cages of dimension ∼ 30 Å, which are decorated with a high density of Lewis acidic Co(II)/Ni(II) metal sites and basic -NH- groups. Further, MOF 1 shows highly selective adsorption of CO2 with high Qst value of 39.7 kJ/mol, which is supported by theoretical studies with computed BE of 41.17 kJ/mol. Further, MOFs 1/2 show excellent catalytic activity for conversion of CO2 to cyclic carbonates under environmentally friendly, co-catalyst-free mild conditions. Moreover, the MOF 1 catalyst was recycled for five successive cycles with negligible loss of activity. Interestingly, MOF 1 also catalyzes cycloaddition of CO2 to epichlorohydrin from simulated dry fluegas, CO2:N2 (13:87 (%)), to yield the corresponding cyclic carbonate with 63% conversion suggesting the high affinity of MOF1 for CO2 even at low concentration (13%) of CO2. The observed high catalytic performance of MOF 1 for co-catalystfree conversion of CO2 at 1 bar of CO2 can be attributed to synergistic cooperation between the Lewis acidic Co(II) ions and the basic -NH- groups decorated in the open cages of MOF 1.

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EXPERIMENTAL SECTION

Materials. The reagents were purchased from commercial suppliers and used as received. Co(NO3)2·6H2O and Ni(NO3)2· 6H2O were procured from Sigma-Aldrich Chemical Co. DABCO was purchased from SD Fine Chemicals. All the epoxides were purchased from TCI Chemicals Co. 4,4′,4″-s-Triazine-1,3,5-triyltri-p-aminobenzoic acid (H3TATAB) ligand was synthesized using the previously reported procedure.15 Physical Measurements. Thermogravimetric analysis (TGA) of the samples was performed under N2 atmosphere in the temperature range of 50−900 °C at a heating rate of 10 °C/min on a Metler Toledo thermogravimetric analyzer. Powder X-ray diffraction (PXRD) patterns of the samples were collected on PANalytical’s X’PERT PRO diffractometer with Cu Kα radiation (k = 1.542 Å; 40 kV, 20 mA). Fourier transform infrared (FT-IR) spectra were recorded on a PerkinElmer ATR-FTIR spectrometer (Synthesis Monitoring System) fitted with ATR accessory and analyzed using Spectrum software (version 3.02.01). Elemental analysis for C, H, and N were performed on a Thermo Fischer Flash 2000 elemental analyzer. The catalytic reactions were analyzed by recording 1H NMR spectra of the products on a JEOL JNM-ECS-400 spectrometer operating at 400 MHz using 1,1′,2,2′-tetrachloroethane as an internal standard. Synthesis of {[Co6(TATAB)4(DABCO)3(H2O)3]·12DMF·9H2O}n (MOF 1). To a mixture of Co(NO3)2·6H2O (0.058 g, 0.2 mmol), H3TATAB (0.073 g, 0.15 mmol) and DABCO (0.012 g, 0.1 mmol) in a 23 mL Teflon lined stainless steel autoclave, 5 mL of N,N′dimethylformamide (DMF) was added with continuous stirring. Then the autoclave was sealed and heated to 160 °C for 4 days, and afterward the autoclave was cooled slowly to room temperature. Purple-colored cubic crystals of 1 were collected, washed with DMF and methanol, and dried in air (yield: 76% based on Co(NO3)2· 6H2O). The phase purity of the sample was examined by PXRD analysis (Supporting Information Figure S1). Elem. Anal. Calcd (%) for MOF 1 (C150H204O48N42Co6): C, 48.47; H, 5.53; N, 15.83. Found: C, 49.04; H, 5.71; N, 16.14. FT-IR (KBr, cm−1): 3460−2954 (b), 1581, 1483, 1372, 1338, 1239, 1176, 1052, 1010, 857, 778, 695, 492 (Figure S2). Solvent exchange: as-synthesized MOF 1 was taken in a clean 30 mL glass vial, 10 mL of MeOH was added to it, the vial was allowed to stand for 12 h, and afterward the solvent was replaced by a fresh batch of MeOH, and this process was repeated for every 12 h for 3 days to obtain solvent-exchanged MOF 1. Elem. Anal. Calcd (%) for {[Co 6 (TATAB) 4 (DABCO) 3 (H 2 O) 3 ]·14MeOH} n

BE = B

1 [E(MOF 1 + nCO2 ) − E(MOF 1) − E(nCO2 )] n DOI: 10.1021/acs.inorgchem.8b03612 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry where n = 3, the number of adsorbed CO2 molecules, and E(MOF 1 + 3CO2), E(MOF 1), and E(3CO2) correspond to the energy of MOF 1 plus three CO2, the energy of MOF 1, and the energy of three CO2, respectively.

Co2/Ni2 ions are in a square pyramidal geometry satisfied by four carboxylate oxygens of TATAB3− ligand forming the basal plane, and a nitrogen atom of DABCO occupies the apical position at Co1/Ni1, while a water molecule at Co2/Ni2. Therefore, Co1/Ni1 and Co2/Ni2 are bridged by four carboxylate groups from four different TATAB3− ligands resulting in [M2(O2C)4] (M = Co(II)/Ni(II) paddlewheel (PW) secondary building unit (SBU) with Co−Co/Ni−Ni distances of 2.680/2.632 Å (insets of Figure 1a,b). Interestingly, the [M2(OOC)4] paddlewheel SBUs occupy the 12 vertexes of an icosahedron with TATAB3− ligands occupying the trigonal planes forming a polyhedral cage constituted by 12 SBUs (24 metal sites; denoted as cage A; Figure 2a). Due to the presence of a large void space in cage A, another cage denoted as cage B is generated in the void space of cage A. Cage B is similar to that of A, except that, out of 12 {M2(OOC)4} PW SBUs, the six adjacent ones are further connected by DABCO as shown in Figure 2b. Further, the two cages (cage A and cage B) are alternatively arranged to form a 3D double-walled framework in which each cage of one type is surrounded by eight cages of the other type and vice versa (Figures 2c and S4).18 Therefore, owing to the double-walled cages, the pore walls are decorated with 48 Lewis acidic metal ions and 48 Lewis basic -NH- sites (Figure 2c) with pore diameters of ∼28.41 and 28.28 Å in MOFs 1 and 2, respectively. Topological analysis by TOPOS19 suggests that Co/Ni ions act as binodal centers and the overall structure is 2-fold interpenetrated with {53}4{56·84}3-net topology (Figure S4). In addition, the structures show the presence of π−π interactions between the phenyl rings of TATAB3− linkers with a distance of 3.456 Å (centroid-to-centroid; Figure S4e). The void volumes of the 3D frameworks in 1 and 2 are ∼62.4 and ∼61.8% per unit cell, respectively, after removal of the solvent molecules calculated using PLATON.20 The crystallographic data and structure refinement parameters are provided in Table S1, and selected bond lengths and angles of MOFs 1 and 2 are given in Tables S2 and S3. Thermal Stability of MOFs 1 and 2. As-synthesized MOFs 1 and 2 show a weight loss of about 40% around 50− 220 °C due to loss of 12 DMF and 9 water molecules of crystallization (Figure 3a,b). The solvent-exchanged MOFs show weight losses of ∼16% in the range of 50−150 °C due to

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RESULTS AND DISCUSSION Synthesis of MOFs 1 and 2. The reaction of M(NO3)2.6H2O (M = Co/Ni) with a tricarboxylate ligand, H3TATAB and a DABCO linker in DMF at 160 °C afforded cubic crystals of {[M6(TATAB)4(DABCO)3(H2O)3] 12DMF· 9H2O}n (where, M = Co (1)/Ni(2) (Scheme 1). Scheme 1. Synthesis Scheme for MOFs 1 and 2

Crystal Structure. Structural Description of {[M6(TATAB)4(DABCO)3(H2O)3]·12DMF·9H2O}n [M = Co(II)/ Ni(II) (MOFs 1/2)]. Structural determination of MOFs 1 and 2 revealed that both are isostructural and analogues to the ZnMOF reported by Du and co-workers.18 Both MOFs crystallize in cubic space group Im3̅, and the asymmetric unit consists of two-quarters of a metal ion, one-third of a TATAB3− ligand, and one-fourth of a DABCO along with a coordinated water molecule (Figure 1a,b). The structures also contain several lattice solvent molecules of DMF and water which were determined by TGA and CHN analyses. The Co1/Ni1 and

Figure 1. Asymmetric unit of MOFs 1 (a) and 2 (b) (insets, coordination environment at Co(II)/Ni(II) (the hydrogen atoms except -NH- groups of TATAB ligand and guest DMF and water are omitted for clarity)). C

DOI: 10.1021/acs.inorgchem.8b03612 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) View of cage A formed by 12 paddlewheel SBUs of Co(II)/Ni(II). (b) View of cage B formed by 12 paddlewheel units of Co(II)/ Ni(II) having coordinated DABCO. (c) View of the dual-walled cage motif formed by encapsulation of cage B in cage A.

Figure 3. TGA plots for MOFs 1 and 2.

match well with the parent MOF supporting the high chemical stability of MOF 1 (Figure S5). Further, the water stability of the as-synthesized MOF 1 was also examined at various pH conditions by treating the sample with acidic (pH = 2−6) and basic (pH = 8−12) aqueous solutions for 12 h. PXRD patterns of isolated samples match well with the original sample, suggesting retaining of the framework structure (Figure S6) confirming the high water stability of MOF 1. Gas Adsorption Properties. As discussed before, single crystal structural analysis of MOFs 1 and 2 revealed the presence of dual-walled cages with a pore diameter of ∼30 Å that are decorated with 48 Lewis acidic Co(II)/Ni(II) ions and 48 Lewis basic -NH- groups of TATAB3− ligand, which can

loss of 14 guest MeOH molecules and three coordinated water molecules, whereas activated MOFs 1 and 2 do not show any weight loss in the temperature range of 50−120 °C, supporting the absence of guest solvents and the coordinated water molecules, and the frameworks are stable up to 420 °C. Further weight loss of 77% around 420−460 °C corresponds to the loss of the framework structure. The as-synthesized MOFs show high chemical and water stability. To check the chemical stability, MOF 1 was treated with various organic solvents including both polar and nonpolar with constant stirring for 12 h, and then it was separated by filtration, followed by washing with MeOH, and dried under vacuum. PXRD pattern of the recovered samples D

DOI: 10.1021/acs.inorgchem.8b03612 Inorg. Chem. XXXX, XXX, XXX−XXX

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paddlewheel Co(OOC)2(H2O)2 SBU as a model for MOF 1 as shown in Figure S13. Three guest CO2 molecules were introduced with the linker, and the geometry was optimized. As shown in Figure 5, the three CO2 molecules are interacting

enhance the affinity for CO2. Therefore, gas adsorption studies were performed to evaluate the permanent porosity of the framework. Prior to the adsorption studies, the MOFs were activated at 423 K under vacuum (18 mTorr), and the framework stability was confirmed from PXRD analysis (Figures S1 and S3). The N2 adsorption isotherms showed a type-I profile supporting the porous nature of the frameworks (Figure S7). Further, MOF 1 exhibits a typical type-I behavior for CO2 adsorption at 273 and 298 K with uptake of 1.93 and 1.23 mmol/g, respectively (Figure S8). The BET surface areas of MOFs 1/2 were 98.9/94.2 m2/g, estimated based on CO2 adsorption isotherm at 273 K, which is in accordance with the value reported for the isostructural Zn-MOF.18 The hysteresis nature of the CO2 isotherms can be ascribed to stronger interaction with the Lewis basic -NH- groups exposed in the open cages of MOFs 1/2. The CO2 adsorption plots were fitted to the Freundlich−Langmuir equation21 (Figures S9 and S10) for accurate prediction of the amount of adsorbed CO2. The calculated value of Qst for the interaction of CO2 with MOF 1 was found to be 39.7 kJ/mol based on the Clausius− Clapeyron equation (please see the Supporting Information, Figure S11).22 Interestingly, the Qst curve decreases first and then increases with an increase in the loading; this behavior can be attributed to the fact that, at lower loading, CO2 molecules interact with the basic -NH- groups decorated in the open cages of MOF 1 (CO2−MOF interaction), and as the loading increases the CO2−CO2 interaction dominates over CO2−MOF interaction.8j Further, the gas uptake study of MOF 1 carried out at 273 K with other gases (H2, Ar, and N2) showed negligible uptake of 0.07, 0.15, and 0.13 mmol/g of H2, Ar, and N2, respectively (Figure 4). The observed selectivity of MOF 1 for

Figure 5. Optimized structure of TATAB3− ligand coordinated to a paddlewheel Co(OOC)2(H2O)2 SBU with three CO2 molecules (atom colors: Co, green; C, dark gray; O, red; H, white; N, blue).

with the three basic -NH- groups of TATAB3− ligand through a weak (-NH···OCO) hydrogen bonding interaction with -NH···O bond distances ranges from 2.20 to 2.26 Å and the bond angles of 167.4, 164.1 and 169.5° for the interactions labeled as types-I, -II, and -III, respectively (Figure 5). Further, the other oxygen atoms of CO2 molecules are engaged in C− H···OCO interaction with benzene rings of TATAB3− ligand with C−H···O bond distance ranges from 2.57 to 2.66 Å (Figure 5). The calculated average binding energy for CO2 molecules interacting with the ligand is 41.17 kJ/mol, which is in accordance with the experimental value of Qst (39.7 kJ/mol) determined from CO2 adsorption measurements. Moreover, electrostatic potential maps (ESP) of MOF 1 with three CO2 molecules are shown in Figure 6. The color variation in ESP plots indicate the electron density variation upon interaction of CO2 molecules with the -NH- groups of TATAB3− ligand. The red color shows high electron density while blue color represents electron deficiency. As it can be seen from Figure 6, the yellow-red color at oxygen of free CO2 changes to greenish-blue and the blue color at -NH- group of TATAB3− linker changes to bluish-green upon interaction with CO2. This observation indicates partial shifting of electron density between CO2 and -NH- moiety. Further, it was also observed that the bond angle of CO2 molecules deviates from 180 to 179.38° supporting the interaction with -NH- groups of the ligand. Hence, the calculated binding energy of CO2 molecule and ESP plots support the interaction between CO2 and -NH- groups. Catalytic Cycloaddition of CO2 with Epoxides. As discussed before, structures of MOF1 and 2 showed the presence of highly porous dual-walled cages with dimension of ∼30 Å with the pore walls decorated with 48 Lewis acidic Co(II) (1)/Ni(II) (2) metal centers and the pore apertures functionalized with 48 basic -NH- groups resulting in MOFs 1/2, ideal candidate materials to study co-catalyst (TBAB)-free cycloaddition of CO2 to epoxides. The MOFs were activated at

Figure 4. Selective adsorption behavior of MOF 1 for CO2.

CO2 over other gases can be ascribed to the presence of basic -NH- functionalized open cages. Further, the selectivity constants for CO2 over other gases have been calculated following Henry’s law,23 and the estimated values for CO2/H2, CO2/N2, and CO2/Ar were found to be 91, 49, and 42, respectively (Figure S12). The considerably high values of separation constants for CO2 over other gases suggest the potential utility of MOF 1 for separation of CO2 from industrial effluent gases and flue gas. In order to investigate the preferred sites for interaction of CO2 molecules with MOF 1, theoretical calculations were carried out with TATAB 3− ligand coordinated to a E

DOI: 10.1021/acs.inorgchem.8b03612 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. ESP maps indicating total electron density of (a) TATAB3− linker and (b) TATAB3− linker with three CO2 molecules and (c) for a free CO2 (where, for example, 3.00e-2 represents 3.00 × 10−2).

Table 1. Optimization of Catalytic Conditions for Cycloaddition of CO2 to Epichlorohydrina

entry no.

catalyst (mol %)

gas/mixed gas

pressure (bar)

time (h)

temp (°C)

conversion (%)b

1 2 3 4 5

none MOF MOF MOF MOF

1 2 1 1

CO2 CO2 CO2 CO2 PCO2/PN2 (2/6)c

1 1 1 8 8

15 15 15 15 15

80 80 80 80 80

5 93 93 100 94

6

MOF 1

PCO2/PN2 (4/4)c

8

15

80

97

7

MOF 1

PCO2/PN2 (6/2)

c

8

15

80

99

8

MOF 1

CO2/N2(13/87 (%))d

4

15

80

63

a

Epichlorohydrin (20 mmol), MOF 1 catalyst (0.2 mol %). bThe conversions were determined from 1H NMR analysis using an internal standard. c The gases were introduced with varying pressure ratios shown in parentheses. dCO2: N2 (13:87 (%)) mixed gas was used.

423 K to generate frameworks with unsaturated Lewis acidic metal ions. Furthermore, the activated samples were tested for co-catalyst-free cycloaddition of CO2 with epoxides under optimized reaction conditions (Table S4). Optimization of the reaction conditions using epichlorohydrin as a model substrate with 0.2 mol % of MOF 1/2 catalysts at 80 °C and 1 bar of CO2 resulted in the cyclic carbonate with 93% conversion within 15 h in the absence of a co-catalyst (Table S4), whereas the catalytic reaction carried out in the absence of MOF catalyst showed only 5% conversion of epichlorohydrin to cyclic carbonate, indicating the requirement of MOFs 1/2 as catalyst for high-yield generation of cyclic carbonate. The 1H NMR analysis of the aliquots taken from the catalytic reactions revealed the formation of the cyclic carbonates with 100% selectivity, and formation of no side products was observed (Figures S14−S18). Motivated by the high catalytic performance of MOF 1, the cycloaddition reaction of epoxides with CO2 was carried out in the presence of N2 with varying pressure of CO2 and N2 (Table 1). When the pressure ratio of CO2/N2 is 2/6, the conversion of epichlorohydrin to cyclic carbonate was found to be 94% (Table 1, entry 5, and Figure S18). Upon increasing the pressure of CO2 to 6 bar (CO2/N2

is 6/2), the conversion of epichlorohydrin was increased to 99% (Table 1, entries 5−7, and Figures S18−S20). Furthermore, the catalytic activity was extended for conversion of CO2 from simulated dry flue gas, CO2:N2 (13:87 (%)) (Table 1, entry 8, and Figure S21). Interestingly, even at a low concentration of CO2 (13%), the cycloaddition of epichlorohydrin was found to be 63%, suggesting the high affinity of MOF 1 for CO2 even in the presence of high concentration of N2. The scope of the catalysis was extended to various substituted epoxides with different functional groups under the optimized conditions using MOF 1 as a catalyst. Interestingly, the catalytic conversion of alkyl epoxides such as 1,2-epoxybutane, 1,2-epoxyhexane, and 1,2-epoxydecane were found to be lower than that of epichlorohydrin, except 1,2-epoxypropane, which can be attributed to their reduced activity owing to the presence of alkyl chain (Table 2, entries 2−5, and Figures S22−S25), whereas relatively higher catalytic conversion of butyl glycidyl ether over allyl glycidyl ether can be ascribed to the electron donating nature of the former compared to that of the latter (Table 2, entries 6 and 7, and Figures S26 and S27). Further, cycloaddition of CO2 with F

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Inorganic Chemistry Table 2. Cycloaddition Reaction of Substituted Epoxides with CO2 Catalyzed by MOF 1.a

Reaction conditions: epoxide, 20 mmol; MOF 1, 0.2 mol %; temperature, 80 °C; time, 15 h. bConversions were determined from 1H NMR spectroscopy. cTON: moles of cyclic carbonate/(moles of MOF 1 used).

a

aromatic epoxides such as styrene oxide and 4-chlorostyrene oxide showed the conversion of 82 and 81%, respectively, to the corresponding cyclic carbonates (Table 2, entries 8−10, and Figures S28−S30). It is worth mentioning that the reaction conditions utilized in the present work for cycloaddition of CO2 to epoxides are relatively milder than the conditions normally employed in the literature for co-catalyst-free conversion of CO2 catalyzed by various MOFs (Table S5). The high catalytic performance of MOF 1 could be attributed to the presence of the high density of Lewis acidic catalytic sites and the basic -NH- moieties with

synergistic cooperation between the two sites, resulting in enhanced activity for selective CO2 capture and conversion. To rule out the possibility of leaching of MOF 1 catalyst, the reaction was stopped after 6 h and the conversion of epichlorohydrin was found to be ∼59%; then the MOF was separated by filtration, and the filtrate was stirred for an additional 9 h in the presence of 1 bar of CO2 (Figure S31). The analysis of the aliquot showed a slight increase (∼2%) in the conversion of epichlorohydrin which is significantly lower than the conversion observed in the presence of MOF catalyst supporting the absence of leaching of MOF. Further, the G

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subsequent ring-closure reaction by an intramolecular nucleophilic attack of the oxyanion with CO2 leads to the formation of cyclic carbonate. Then, reductive elimination of the cyclic carbonate regenerates the catalyst for incoming molecules of CO2 and epoxides, making the catalytic cycle continuous. Interestingly, the binding of epoxides to the Lewis acidic Co(II) center was supported by theoretical optimization of the epoxides with TATAB3− ligand coordinated to Co(OOC)2(H2O) SBU (Figure S30). Thus, theoretical calculations support the participation of both Lewis acidic metal and basic -NH- sites. Therefore, the high catalytic performance of MOF 1 for co-catalyst-free conversion of CO2 at mild conditions can be attributed to the synergistic cooperation between the Lewis acidic metal site and the basic -NH- groups as proposed in Scheme 2. The MOFs which lack Lewis basic nucleophilic centers require an additional cocatalyst for efficient generation of cyclic carbonates.

MOF1 catalyst recovered from the reaction mixture and reused for subsequent cycles after washing with H2O/MeOH. Remarkably, the catalytic activity of MOF 1 was retained even after five consecutive cycles (Figure 7). Moreover, the

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CONCLUSIONS In summary, rational design of 3D dual-walled, 2-fold interpenetrated, polyhedral MOFs of Co(II)/Ni(II) featuring large open cages composed of the high density of Lewis acid metal ions and basic -NH- groups has been achieved. The MOF 1 shows selective storage of CO2 with the high Qst value of 39.7 kJ/mol supported by theoretically calculated binding energy of 41.17 kJ/mol. Owing to the presence of a high density of metal ions and basic -NH- groups, MOFs 1/2 act as efficient, green, recyclable catalysts for the cycloaddition of epoxides and CO2 at 1 bar of CO2 and 80 °C for efficient synthesis of cyclic carbonates in the absence of a co-catalyst. The MOF was reused for five cycles with negligible loss in catalytic activity. Hence, this work demonstrates rational design of porous MOFs composed of both Lewis acidic and basic sites for selective CO2 storage and co-catalyst-free conversion to cylic carbonates.

Figure 7. Recyclability test of MOF 1 for five consecutive cycles.

PXRD pattern and the FT-IR spectra of the recycled sample matched well with those of the parent MOF 1 (Figures S32 and S33). Furthermore, SEM images of the as-synthesized and the recovered sample of MOF 1 after catalysis support retaining of the original morphology of the framework (Figure S34). The plausible mechanism for the catalytic reaction is shown in Scheme 2. The reaction initiates by epoxide coordination to the Co(II) center followed by polarization of CO2 at the basic -NH- moieties resulting in the formation of carbamate species which act as a nucleophile and assist in ring opening of the epoxide coordinated to the metal center adjacent to it. The

Scheme 2. Proposed Catalytic Mechanism for Co-catalyst-Free Conversion of CO2

H

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03612. PXRD plots of the compounds, FT-IR, additional figures, gas adsorption isotherms, and 1H NMR spectra (PDF) Accession Codes

CCDC 1836674 and 1836675 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]. ORCID

Sandeep Kumar: 0000-0002-4652-9546 T. J. Dhilip Kumar: 0000-0002-1208-4112 C. M. Nagaraja: 0000-0002-4271-6424 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Sandeep Singh Dhankhar for the gas adsorption studies. B.U. is thankful to IIT Ropar for the fellowship, and S.K. acknowledges DST, India for SRF.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b03612 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03612 Inorg. Chem. XXXX, XXX, XXX−XXX