A Series of Metal–Organic Frameworks Built of Triazolate-Trinuclear

Oct 27, 2015 - Then, by the solid-solution framework approach, four group samples are prepared, which are MAC-4-AB-x, MAC-4-AC-x, MAC-4-AD-x, and MAC-...
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A Series of Metal−Organic Frameworks Built of Triazolate-Trinuclear and Paddlewheel Units: Solid-Solution Framework Approach for Optimizing CO2 Adsorption and Separation Mingli Deng,† Feilong Yang,† Pan Yang,‡ Zhouxun Li,† Jinyu Sun,‡ Yongtai Yang,† Zhenxia Chen,† Linhong Weng,† Yun Ling,*,† and Yaming Zhou† †

Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai, 200433, China ‡ Department of Chemistry, Liaoning University, Shenyang, 110036, China S Supporting Information *

ABSTRACT: Metal−organic frameworks with tunable functional groups like solid-solution have received considerable interest. In this paper, a crs-type structure built of [Zn3(OH)(dmtrz)3] triazolate-trinuclear and [Zn2(COO)4] paddlewheel units, linked by 3,5-dimethyl-1,2,4-triazolate (dmtrz) and isophthalate (ipa) ligands (named as MAC-4-A), has been used as a prototype framework. Four types of functional groups (B: 5-hydroxyisophthalate, C: 5aminoisophthalate, D: 5-ethoxyisophthalate, and E: 5-acetamidoisophthalate) have been integrated into the framework, giving isostructures of MAC-4-B to E, respectively. Then, by the solid-solution framework approach, four group samples are prepared, which are MAC-4-AB-x, MAC-4-AC-x, MAC-4-AD-x, and MAC-4-AE-x (x denoted as the ratio of functional ligands in the framework, x = 0.3, 0.5, 0.7), and their CO2 and N2 adsorptions have been studied. Our results revealed that the CO2 capacity is enhanced with the increase of functional groups and then decreases, showing a maximum uptake amount on MAC-4-AB-0.5, MAC-4-AC-0.5, MAC-4-AD-0.7, and MAC-4-AE0.5 at 298 K and 1 bar, respectively. On the other hand, the calculated selectivity of CO2 over N2 gradually increases, giving the highest selectivity after the pore surface is completely functionalized.

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INTRODUCTION Metal−organic frameworks (MOFs, also called porous coordination polymers, PCPs) are porous species featuring extended architectures, in which inorganic subunits and organic struts are periodically linked via coordination bonds.1−5 These promising crystalline materials not only have shown potential application in gas storage and/or separation,6−11 heterogeneous catalysis,12−15 drug delivery,16−19 and so on20−24 but also can serve as a research platform for systematically exploring the related structure−property relationship.24 Taking the greenhouse gas CO2, for example, adsorption and separation using porous materials are regarded to be one of energetically efficient, environmentally friendly, and economically competitive approaches. In this sense, a comprehensive understanding of its behavior in porous structures and optimizing the performance are of great importance. By the strategy of pre/ postsynthetic modification,25−30 postexchange modification,31−35 functional sites for the enhancement of CO2 have been revealed. Even the exact location of CO2 as well as weak contribution of nonclassical active site has been studied.36−39 Among those reported literatures, it is interesting to note that the pore surface seems to be 100% dressed up by functional groups (such as amino and hydroxyl groups). It makes the rise of questions that (i) whether a MOF structure is predestined in © XXXX American Chemical Society

its best performance for CO2 capacity and separation after the pore surface completely functionalized, and (ii) whether CO2 capacity and separation are enhanced synchronously with the increase of functional groups. In other words, more studies are further required. To explore the above issues, multivariate metal−organic frameworks (MTV-MOFs) or solid-solution frameworks (SSFs) have shown advantages since the functional groups can be integrated and the ratio can be well tailored without changing the framework. For example, the functional groups can be well tuned in a single crystal structure of MOF-5 and MOF-177.40,41 Similarly, by tuning the ratio of 5-nitroisophthalate and 5-methoxyisophthalate, the equilibrium and kinetic behaviors of gas adsorption and separation have been studied in a two-dimensional flexible MOF;42,43 tailing the gateopening pressure for water vapor adsorption has also been studied by integrating triazolate ligands in a SOD-type framework.44 Because of the importance of the CO2 issue, more research works are greatly desired to systematically tailor the integrated functional groups for exploring the relationship Received: August 12, 2015 Revised: October 19, 2015

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Scheme 1. Schematic View of Presynthetic Ligand Functionalization for MAC-4 Framework

were synthesized in lab. IR spectra were measured on the Nicolet 470 FT-IR spectrometer in the range 4000−400 cm−1 with KBr pellets, and elemental analyses (EA) for C, H, and N were carried out using the Elementar Vario EL III for MAC-4-A to E. Powder X-ray diffraction (PXRD) patterns were measured on the Bruker D8 powder diffractometer with Cu Kα radiation (λ = 1.5406 Å). Thermogravimetric analyses (TGA) were carried out on the Mettler Tolepo TGA/ SDTA851 analyzer under N2 flow with a heating rate of 10 K min−1. 1 H NMR were measured on the JEOL ECA400. Before the 1H NMR measurements, samples were desolvated and deconstructed by D2SO4 and dissolved in CDCl3 solution. Gas adsorption of N2 at 77 K and CO2, N2 at 298 and 273 K were measured on an ASAP 2020 gas adsorption analyzer (Micromeritics). Before adsorption experiments, all samples were exchanged with CH2Cl2 for 3 days and then degassed at 413 K for 10 h. Helium gas was used for the estimation of the dead volume. The saturation pressure P0 was measured throughout the N2 analyses via a dedicated saturation pressure transducer, which was used to monitor the vapor pressure for each data point. A positive fitted C value is satisfied for calculation of the Brunauer−Emmett−Teller (BET) surface area. The pore size was obtained by the Horvath−Kawazoe (H-K) model. Synthesis of MAC-4-A, MAC-4-B, MAC-4-C, MAC-4-D, and MAC-4-E. All samples were solvothermally synthesized by the same procedure except for MAC-4-C. In a typical synthesis procedure for {[Zn5(OH)(dmtrz)3(ipa)3]·DMF·H2O}n (MAC-4-A): Zn(OAc)2· 2H2O (0.45 mmol, 0.089 g) was dissolved in 5 mL of N,N′dimethylformamide (DMF) and added to the solution of isophthalic acid (H2ipa) (0.3 mmol, 0.049 g) and 3,5-dimethyl-1H,1,2,4-triazolate (Hdmtrz) (0.3 mmol, 0.028 g) in 5 mL of DMF. The solution was stirred at room temperature for 10 min, and then the mixture was transferred to a Teflon-lined stainless steel autoclave (15 mL) and heated at 140 °C for 3 days, followed by cooling down to room temperature. Colorless block crystals were collected by filtration (yield: 62% based on Zn(II)). Replacing H2ipa by 5-hydroxyisophthalic acid (H2ipa-OH, B), 5-ethoxyisophthalic acid (H2ipa-eo, D), and 5-acetamidoisophthalic acid (H2ipa-ac, E), respectively, products of {[Zn5(OH)(dmtrz)3(ipa-OH)3]·DMF·5H2O}n (MAC-4-B, yields 46%), {[Zn5(OH)(dmtrz)3(ipa-eo)3]·DMF}n (MAC-4-D, yields 42%), and {[Zn5(OH)(dmtrz)3(ipa-ac)3]·guest}n (MAC-4-E, yields 55%) were obtained accordingly. For the synthesis of {[Zn5(OH)(dmtrz)3(ipa-NH2)3]·6DMF· DMA}n (MAC-4-C, DMA = dimethylamine), beside replacing H2ipa

of CO2 adsorption/separation versus the ratio of functional groups. In addition, previously reported SSF-MOFs are commonly constructed by homo-SBUs, for example, MOF-5, MOF-177 uniformly built of Zn4O clusters. Structures constructed by hetero-SBUs showing tailorable functional groups like solid-solution are rarely known. We have previously reported two isostructures built of triazolate-trinuclear (Zn3(OH)(dmtrz)3) and paddlewheel (Zn2(COO)4) units,45 namely, [Zn5(OH)(dmtrz)3(ipa-x)3] (Hdmtrz = 3,5-dimethyl-1H,1,2,4-triazolate, H2ipa = isophthalic acid for MAC-4-A, and H2ipa-OH = 5-hydroxyisophthalic acid for MAC-4-B), which are three-dimensional porous structures possessing a crs topology and showing 1D zigzag channels along the c axis. In this paper, isoreticular frameworks were synthesized by instead of an H2ipa ligand with 5aminoisophthalic acid (H2ipa-NH2, C), 5-ethoxyisophthalic acid (H2ipa-eo, D), and 5-acetamidoisophthalic acid (H2ipa-ac, E), respectively (Scheme 1). Then, the ratio of integrated functional groups was tailored by the SSF approach, and four group samples are prepared, which are MAC-4-AB-x, MAC-4AC-x, MAC-4-AD-x, and MAC-4-AE-x (x denoted as the ratio of functional ligands in a framework). On the basis of the four group samples, CO2 adsorption and separation are studied, and our results revealed that the uptake amount of CO2 is enhanced with the increase of functional groups and then decreases, showing a maximum uptake amount on MAC-4-AB-0.5, MAC4-AC-0.5, MAC-4-AC-0.7, and MAC-4-AD-0.5, respectively. On the contrary, the calculated selectivity of CO2 over N2 is enhanced with the increase of functional groups in the whole range. We believe that these results will benefit greatly for optimizing the performance of a porous material as well as designing new porous materials for more effective CO2 capture and/or separation.

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EXPERIMENTAL SECTION

Materials and General Characterization. All reagents were purchased from commercial sources and used as received, except for Hdmtrz, H2ipa, H2ipa-NH2, H2ipa-OH, H2ipa-ac, and H2ipa-eo, which B

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Figure 1. (a) 1D chain formed by trinuclear-triazolate [Zn3(OH)(dmtrz)3] SBUs (in dark yellow); (b) 1D zigzag chain structure formed by paddlewheel SBUs and ipa ligands in form-I (ipa-I, in pink) coordination geometry; (c) the above two chain structures interweaved together, resulting in a three-dimensional porous structure; (f) three-dimensional structure of MAC-4 showing the ipa-II ligands (in green) as walls for the channel structure; (e)−(h) the structures of MAC-4-B to MAC-4-E (the functional groups are in stick-and-ball model; hydrogen atoms are omitted for clarity. The structure of MAC-4-E was simulated based on the structure model). by 5-aminoisophthalic acid (H2ipa-NH2, C), an additional 0.02 mL of HNO3 and 0.02 mL of HOAc were added to the mixture solution, and then followed the same procedure as that of MAC-4-A (Yields: 36% based on Zn(II)). Synthesis of MAC-4-AB-x, MAC-4-AC-x, and MAC-4-AD-x, MAC-4-AE-x (x = 0.3, 0.5, 0.7). MAC-4-AB-x, MAC-4-AC-x, MAC4-AD-x, and MAC-4-AE-x were synthesized by mixing isophthalic acid and 5-substituted-isophthalic acid ligands at a defined molar ratio, and then following the procedure described below (x indicates the molar ratio of 5-substituted-isophthalic acid in the mixed acid ligands, taking MAC-4-AB-x for an example: x = 0.3 means that B/(A + B) = 0.3, which means B:A = 3:7). In a typical synthetic procedure for MAC-4AB-0.3: H2ipa (0.21 mmol, 0.035 g) and H2ipa-OH (0.09 mmol, 0.016 g) were dissolved in 5 mL of DMF and stirred at room temperature for 60 min. After that, the solution was added to the 5 mL DMF solution of Zn(OAc)2·2H2O (0.45 mmol, 0.101g) and dmtrz (0.3 mmol, 0.031g). The mixture was then stirred at room temperature for another 10 min and then transferred into the Teflon-lined stainless steel autoclave (15 mL) and heated at 140 °C for 3 days. Yields: 43% based on Zn(II). Varying the molar ratio of H2ipa and H2ipa-OH, MAC-4AB-0.5, and MAC-4-AB-0.7 can be isolated, respectively. Following the same procedure, MAC-4-AC-x, MAC-4-AD-x, and MAC-4-AE-x can be obtained, respectively. Single-Crystal Diffraction Studies. Single-crystal data of MAC4-B, MAC-4-C, MAC-4-D, MAC-4-AB-0.5, MAC-4-AC-0.5, and MAC-4-AD-0.5 were collected at 293 (K) using a Bruker APEX Duo diffractometer (Mo Kα radiation, λ = 0.71073 Å). Data reduction and cell refinement were performed with the SAINT program, and the absorption correction program SADABS was employed to correct the data for absorption effects.46 The structures were solved by direct methods using the SHELXS program and refined with the SHELXL program and final refined by full-matrix least-squares methods with anisotropic thermal parameters for all non-hydrogen atoms on F2.47−49 The hydrogen atoms of the phenyl were theoretically added; the hydrogen atoms of the guest water molecules were found from the

Fourier map and then refined as a rigid model. Crystallographic data are given in Tables S1 and S2.

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RESULTS AND DISCUSSION Crystal Structure and Framework Stability. Since the structure has been previously described,45 only a short brief introduction of the MAC-4 framework was given (Figure S1a). It is a three-dimensional (3D) porous structure built of triazolate-trinuclear [Zn3(OH)(dmtrz)3] and paddlewheel [Zn2(COO)4] SBUs, showing 1D zigzag channels along the c axis (Figure 1a−d). In the structure, the [Zn3(OH)(dmtrz)3] SBU propagates itself infinitely, resulting into a 1D chain along the a axis, in which there are four kinds of extendable coordination sites, two from Zn sites and two from 4N sites of triazolate. The 4N site coordinates to the Zn(II) of the paddlewheel SBU, making the neighboring 1D [Zn3(OH)(dmtrz)3] chains interweave with the 1D zigzag chains that are built of ipa-I ligands and paddlewheel units, generating a 3D porous structure. Then, the ipa-II ligand further bridges the paddlewheel SBU and [Zn3(OH)(dmtrz)3] SBU together with one side of carboxylate in μ1,2-coordination mode and the other side in η1,2-coordination mode, similar to a previously reported connection mode.50 Considering [Zn3(OH)(dmtrz)3] and paddlewheel [Zn2(COO)4] units as two kinds of 6-connected nodes, respectively, MAC-4 can be described as a (6,6)connected net with a Schläfli symbol of (36.66.73)2 (Figure S2), which is a crs topology. Temperature-dependent PXRD patterns demonstrated that MAC-4-A can retain its crystal phase until to 140 °C; no obvious shift of diffraction peaks has been observed during this process, confirming that it is a rigid framework to some extent. Further increasing the temperature, even to 350 °C, the main C

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diffraction peaks can be retained, but a continuous decay of the diffraction intensity is observed. Furthermore, it can retain its crystal phase in air condition at room temperature at least for 1 month, confirming its framework stability (Figure S3). In contrast, MOF structures build of only paddlewheel [Zn2(COO)4)] SBUs are highly sensitive and easily collapse in air.51,52 The enhanced framework stability could be ascribed to the strong coordination ability of azolate ligands and hydrophobic methyl groups in the structure.53,54 Ligand Functionalization and Gas Sorption. Functional ligands, 5-hydroxyisophthalic acid (H2ipa-OH, B), 5-aminoisophthalic acid (H2ipa-NH2, C), and 5-ethoxyisophthalic acid (H2ipa-eo, D), are then, respectively, introduced into the crstype porous framework by instead of isophthalic acid (H2ipa) during the synthetic process (Figure 1e−h). All the compounds were characterized by TG, EA, IR, and PXRD to assess the thermal stability, element ratio, vibration bands, and crystal phase, respectively (Figures S4, S5; Table S3). Single-crystal Xray diffraction studies of MAC-4-C and D confirmed that they have the same framework as MAC-4-A and B (Figures S1 and S6). MAC-4-D is crystallized in the Pnma space group, the same as that of MAC-4-A and B. It should be pointed out that the ethoxyl group on the phenyl ring is in a seriously disordered state over two positions because of an equal probability by symmetry (Figure S1d). For MAC-4-C, solving this structure in the Pnma space group failed because phenyl groups are seriously disordered with high residual peaks (Min./max. resd. dens. +2.6, −1.89, R1 = 0.1999, and wR2 = 0.4486). A chiral P212121 space group is more reasonable. It could be ascribed to DMF and DMA molecules, which have supramolecular interactions with the amino groups, leading to the slightly torsion of phenyl groups (Figure 1f). N2 sorption at 77 K shows a typical type-I sorption isotherm for MAC-4-A, B, C, D, and E, respectively (Figure 2a), confirming their microporous structures. BET surface area decreases gradually, giving a data of 1225, 804, 741, 667, and 622 m2/g for MAC-4-A to E, respectively (Table 1, Table S4 for their theoretical pore volumes), which could be ascribed to the increased bulk size of integrated functional groups. Although BET surface area decreases, the CO2 capacity is enhanced compared with that of MAC-4-A (1.53 mmol/g, Figure 2b), giving the uptake amount of 3.11, 2.71, 2.43, and 2.26 mmol/g for MAC-4-B, C, D, and E at 298 K, 1 bar, respectively. The enhancement of CO2 adsorption could be ascribed to the enhanced binding affinity of the pore surface to CO2 because of integrated functional groups (Figure S7, Table 1), among which MAC-4-B shows the highest adsorption enthalpy (∼31 kJ/mol) at zero coverage (Qst), indicating a stronger interaction between the hydroxyl group and CO2 compared with other organic groups. It is interesting to note the CO2 capacity of MAC-4-B, which is 3.11 mmol/g at 298 K, 1 bar, comparable to that of the widely studied MOFs (Table S5), such as PCN-6 (3.6 mmol/g, 1 bar, 298 K),55 mmen-Cu-BTTri (3.50 mmol/g),56 and Zn(bdc-OH)(dabco)0.5 (2.98 mmol/g),57 but is lower than that of Mg-MOF-74 (5.91 mmol/g)58 and MAF-X25ox (7.1 mmol/ g).59 It should be pointed out that either high surface area or high adsorption enthalpy is observed in those reported MOFs, which will contribute to the high CO2 capacity. For MAC-4-B, the surface area and adsorption enthalpy are lower than those reported ones. Why does MAC-4-B have a high CO2 capacity? Besides the contribution of hydroxyl groups that are decorated on the pore surface, we assume that the unique zigzag channel

Figure 2. (a) N2 sorption isotherm of MAC-4-A to E at 77 K, respectively. (b) CO2 sorption isotherm of MAC-4-A to E at 298 K, respectively.

structure may play an important role, in which phenyl rings of ipa-I ligands directly protrude to the pore space in a manner of offset face-to-face arrangement with a distance of ∼10 Å (Figure 3), thus dividing the channel into small segments. Such kind of pore-space partition would help to enhance the CO2 capacity in MAC-4.25,27,60 Solid-Solution Framework Approach and Gas Sorption. To further explore the question of whether the porous structure is in its best performance for CO2 adsorption after the pore surface is completely dressed up by functional groups, and whether CO2 capacity and separation is enhanced synchronously during the process of tuning the functional groups, we then integrate B, C, D, and E, respectively, into the prototype framework of MAC-4-A, and four group structures are obtained, respectively (Scheme 2).41,43 PXRD patterns confirm that all isolated samples have the same crystal phase as that of MAC-4-A (Figure S8). The ratio of functional groups is identified by 1H NMR analyses, respectively (Figure S9), which is consistent with the reaction stoichiometry. Single-crystal Xray diffraction studies of MAC-4-AB-0.5, AC-0.5, and AD-0.5 confirm that they have the same framework as that of MAC-4-A (see CIFs). It should be mentioned that the phenyl ring with and without a functional group has an equal probability to take the place of the linker in the crs-type framework. Therefore, overlap of the position of phenyl groups with and without functional groups can occur. Considering the equal diffraction density around the 5-position of phenyl groups in the structure, the occupancies of functional groups on ipa-I and ipa-II are refined in half, corresponding to 1H NMR results. N2 sorption at 77 K shows a typical type-I sorption isotherm for each case (Figure S10). It could be seen that BET surface area decreases gradually when the ratio of functional groups increases (Table S6). CO2 adsorption shows that the uptake D

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Table 1. Physical Parameters of MAC-4-A to E Calculated by Gas Adsorption surface area (m2/g) sample

BET

Langmuir

pore size (Å)

CO2 @ 298 K (mmol/g)

Qst (kJ/mol)

SCO2/N2

MAC-4-A MAC-4-B MAC-4-C MAC-4-D MAC-4-E

1225 804 741 667 622

1509 878 822 768 684

7.0 6.5 6.3 5.9 6.2

1.53 3.11 2.71 2.43 2.26

21 31 29 26 28

5 34 31 26 23

Selectivity of CO2 over N2 is calculated by the equation of S = (qi/qj)/(Pi/Pj)6 (where S is the selectivity factor, qi represents the quantity adsorbed of component i, and Pi represents the partial pressure of component i. Here, the partial pressures of CO2 and N2 are 0.15 and 0.75 bar, respectively). The selectivity factor S is around 5 for MAC-4-A, and then it is enhanced to ∼20, 18, 14, and 18 for MAC-4-AB-0.5, MAC-4-AC-0.5, MAC4-AD-0.5, and MAC-4-AE-0.5, respectively (Figure 5, Table S6), and the best performance is achieved when it is completely dressed up by functional groups, giving the S = 34, 31, 26, 23 for MAC-4-B, MAC-4-C, MAC-4-D, and MAC-4-E, respectively. Therefore, it could be concluded that selective adsorption of CO2 is enhanced with the increase of integrated functional groups in this structure. This trend is different from previously reported work by Kitagawa, in which the best performance for selective adsorption of CO2 is reached during the process of tuning the ratio of functional groups, rather than the pore surface completely dressed by functional groups. In that case, it is a flexible structure, and the gas adsorption mainly depends on the gate-opening pressure, which are determined by the ratio of integrated functional groups.42 In our case, MAC-4 is a rigid framework, and the selectivity mainly depends on the uptake amount of CO2 and N2 at the specified pressure. A higher uptake amount of CO2 at 0.15 bar and a lower uptake amount of N2 at 0.75 bar will result into a high selectivity. For CO2 capacity, it could be related to (i) the BET surface area and (ii) the pore surface binding affinity. More integrated functional groups will lead to the enhancement of host−guest interaction, leading to the increase of CO2 uptake amount (Table S7, Figures S11−14). However, it will also lead to the decrease of BET surface area, making a decrease of CO2 amount. If we consider the uptake amount of CO2 simply as a function of the binding affinity ( f1) and surface area (f 2), it gives yCO2 = f1(x)·f 2(x) (x: the ratio of functional groups). When these two aspects reach to a balance in the process of varying the ratio of functional groups, an optimized uptake amount for CO2 will be reached. On the other hand, the dipole moment of N2 is zero and its quadrupole moment is only 1.5 × 1026/esu·cm2,61 which is very small, giving a very weak interaction between N2 and the functional groups. Therefore, the uptake amount of N2 in porous materials depends on the surface area significantly: the lower the BET surface area, the lower the uptake amount of N2 will be. Similarly, it gives yN2 = f 2′(x). Accordingly, the S can be reduced to S = 5 × (yCO2)/ (yN2) = 5 × f1(x)·f 2(x)/f 2′(x). Owing to the overall trend of enhancement of CO2 and continuous decrease of N2, the enhancement of S is reasonable. However, detailed explanations for CO2 uptake amount and S versus the ratio changes still remain a great challenge because of the blank of functional domains in a crystal structure, which has great effects on the gas adsorption.62 Nevertheless, the emerging trends for CO2 uptake and/or selectivity vs functional groups are studied in this MAC4 framework.

Figure 3. MAC-4 structure viewing along the c axis (a) and the zigzag channel (b) (the blue region shows the accessible surface, and the red region shows the occupied places).

Scheme 2. View of SSF-MOF for Tuning the Ratio of Functional Groups in the crs-Type Framework

amount increases gradually with the increase of functional groups, and reaches the maximum amount of 3.42, 3.47, 2.90, and 2.91 mmol/g on MAC-4-AB-0.5, MAC-4-AC-0.5, MAC-4AD-0.7, and MAC-4-AE-0.5, respectively (Figure 4). For the case of MAC-4-AB-0.5, the uptake amount increases nearly 130% compared with that of MAC-4-A, and 10% compared with that of MAC-4-B, and is obviously higher than that of a physical mixture of MAC-4-A and MAC-4-B (2.32 mmol/g), suggesting that the functional groups are integrated into the framework rather than a mixture.41 Further increasing the ratio of functional groups will lead to the decrease of uptake amount. All four group samples have shown similar trends. These results indicate that an optimized performance for CO2 is achieved during the process of tuning the ratio of functional groups in a single crystal structure rather than the pore surface completely dressed up by functional groups. E

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Figure 4. CO2 and N2 sorption isotherms of MAC-4-AB-x (a), MAC-4-AC-x (b), MAC-4-AD-x (c), and MAC-4-AE-x (d) at 298 K.

Figure 5. (a−d) Bar graphs illustrating the trends of CO2 capacity and selectivity of CO2 over N2 vs the ratio of functional groups in the four group samples of MAC-4-AB-x to MAC-4-AE-x.

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CONCLUSION

changing their prototype structure. On the basis of these structures, CO2 adsorption has been carried out, and the results demonstrate that a maximum loading of CO2 can be reached on the samples of MAC-4-AB-0.5, MAC-4-AC-0.5, MAC-4AD-0.7, and MAC-4-AE-0.5, indicating that an optimized performance for CO2 adsorption could be reached when the

In this paper, we have prepared a series of MOFs built of triazolate-trinuclear and paddlewheel SUBs, in which the functional groups (5-hydroxyisophthalic acid, B; 5-aminoisophthalic acid, C; 5-ethoxyisophthalic acid, D; and 5acetamidoisophthalic acid, E) can be modulated without F

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pore surface is partially, rather than completely, dressed up by functional groups. On the contrary, high CO2/N2 selectivity is achieved only when the pore surface is completely functionalized. Our study results not only introduce an interesting SSFMOF structure built of hetero-SBUs but also present a systematic study on CO2 adsorption, including the capacity and selectivity versus the ratio of functional groups. Although a mechanism explanation to the behaviors of CO2 in this framework still remains a challenge, the emerging trends would help to design new functional porous materials for more effective CO2 capture and/or separation.

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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/acs.cgd.5b01155. Crystallographic data for MAC-4-A, MAC-4-B, MAC-4C, MAC-4-D, MAC-4-AB-0.5, MAC-4-AC-0.5, and MAC-4-AD-0.5 (CIF) Figures including structure information, CO2/N2 adsorption isotherms, calculated Qst, etc. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax and Tel: +86 21 65643925. Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from NSFC (Nos. 21203032, 21201039, and 21471035), the Shanghai Leading Academic Discipline Project (B108), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1117).

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