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Improving the Stability and Gas Adsorption Performance of Acylamide Functionalized Zinc MOFs through Coordination Group Optimization Yan-Yuan Jia, Xiaoting Liu, Rui Feng, Shi-Yu Zhang, Ping Zhang, Yabing He, Ying-Hui Zhang, and Xian-He Bu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00119 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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Crystal Growth & Design

Improving the Stability and Gas Adsorption Performance of Acylamide Group Functionalized Zinc MOFs through Coordination Group Optimization Yan-Yuan Jia,† Xiao-Ting Liu,‡ Rui Feng,† Shi-Yu Zhang,† Ping Zhang,‡ Ya-Bing He,‡ Ying-Hui Zhang*‡ and Xian-He Bu* †,‡ †

State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China.

‡

School of Materials Science and Engineering, National Institute for Advanced Materials, Tianjin Key Laboratory of Metal and Molecule-Based Material Chemistry, Nankai University, Tianjin 300350, China.

Supporting Information Placeholder ABSTRACT: In order to elucidate the effect of the structure on stability and gas adsorption performance, three porous Zn(II) metal−organic frameworks, NKU-106, NKU-107 and NKU-108, have been constructed. NKU-106 and NKU-107 assembled from acylamide functionalized tetracarboxylate ligand exhibit low stability and thereby poor gas adsorption ability. However, when one isophthalate moieties of the tetracarboxylate ligand is replaced with pyrazole group, the resulting NKU-108 shows improved thermal and chemical stability as well as good adsorption capacities with respect to CO2 and C2 hydrocarbons. This result demonstrates that coordination group optimization is an effective strategy to improve the stability and gas adsorption properties of MOFs, and thus provides very valuable information for future design and synthesis of porous MOFs for practical application.

Over the past few decades, the ever-growing anthropogenic activities and the combustion of fossil fuel result in increasing CO2 emission, which is widely considered as the main driver to climate change and other environmental issues.1-2 Therefore, how to control and reduce the emission of CO2 has been becoming a growing concern worldwide, and tremendous effort has been made to develop effective strategy and technologies to deal with it.3-4 Besides seeking for renewable and environmentally friendly energy sources, the capture and conversion of redundant CO2 into useful chemicals is also promising, depending mostly on the development of effective porous materials.5-6 Of various porous materials for CO2 and other dangerous gases, metal-organic frameworks (MOFs) are promising alternative due to high specific surface area, large pore volume and tunable structure, and rapidly turn into a current hotspot in many fields such as adsorption and separation, catalysis and energy conversion.7-9 Metal-organic frameworks (MOFs) are a class of inorganic-organic hybrid crystalline materials, featuring ordered network structure encompassing metal ions (or clusters) as nodes and coordinated organic ligands as linkers. 10-14 In light of literatures, commensurate pore size with that of CO2 molecule,15 open metal sites16-17 and high surface area have been identified as key characteristics for the porous MOFs with high CO2 adsorption capacities. In addition, modifying the framework with some heteroatoms and organic

functional groups such as fluorine, methyl and acylamide might create local microenvironment of high dipole moment that should favorably interact with CO2 molecules and thus generate high CO2 adsorption capacities.18-19 However, it is difficult to demonstrate the positive effect of functional group on the adsorption of CO2 sometimes owing to the structural disadvantages of most of MOFs, for example, the weak stability usually encountered with MOFs constructed based on the coordination interactions between carboxylate groups and divalent metal ions.20 As mentioned in some earlier works, the stability of MOFs can be enhanced by improving the match level of the hardness-softness nature between coordinating groups and metal ions. In other words, if the coordinating groups and metal ions are well in line with hard and soft of acids and bases (HSAB) reaction rule, MOFs can be much more stable.21-23 Careful analyses of previous literatures revealed that pyrazole group (soft Lewis base), showing similar electric charges and coordination modes to that of carboxylic group, possesses relatively stronger coordinative interation with soft Lewis acid metal ions, and thus generates MOFs of improved stability.24-25 Therefore, intergating pyrazole and carboxylate coordination groups in one ligand, combined with ligand functionalization with acylamide group, may produce MOFs with improved stability and CO2 adsorption capacity .

Figure 1. The combining capacity of H3Pycia and H4Tdada to Zn2+.

Following this idea, in this work, a tridentate pyrazolecarboxylate ligand H3Pycia (H3Pycia = 5-(1H- pyrazole4-carboxamido)isophthalic acid) and a tetradentate carboxylate ligand H4Tdada (H4Tdada = 5,5'-((thiophene-2,5-dicarbonyl) bis(azanediyl)) diisophthalic acid) (Figure 1 and Scheme S1) were designed, and their coordination behaviors with zinc(II) ions were compared. The resulting three zinc MOFs, [Zn2(Tdada) (H2O) (DMA)]—solvent (NKU-106), [(CH3)2NH2]2 [Zn(Tdada)]—solvent (NKU-107) and [Zn2(OH)(Pycia) (DMF)]—solvent (NKU-108) (NKU =

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Figure 2. The crystal structure of NKU-106. (a) The coordination environment of zinc ions; (b) the 3D framework with open asymmetrical triangle channel along a axis; (c) the topologic structure of NKU-106.

Nankai University), exhibit distinctly different stabilities and gas adsorption properties. Especially, NKU-108 shows relatively outstanding adsorption capacities toward CO2 and C2 hydrocarbons because of its favorable stability as well as open metal sites and acylamide functional groups. NKU-106 was constructed based on H4Tdada ligand and Zinc(II) salt. Single-crystal X-ray determination shows that NKU-106 crystallizes in the monoclinic space group P21. The asymmetric unit contains two distinct Zn2+ ions (Zn1 and Zn2), one Tdada4-, one coordinated water molecule and one coordinated N,N-dimethylacetamide (DMA). Both two zinc ions are five-coordinated with distorted trigonal bipyramid geometry (Figure 2a). Zn1 ion is bound to five O atoms from four Tdada4- ligands and the adjacent Zn2 ion is surrounded by five O atoms from three Tdada4- ligands, one water molecule and one DMA molecule, with Zn-O bond lengths ranging from 1.937 Å to 2.063 Å. Zn1 and Zn2 are linked through carboxylate groups to form a dinuclear cluster. On the other hand, the Tdada4- ligand adopts a (κ1-κ1)-(κ1-κ1)-(κ1-κ1)-(κ1-κ1)-µ7 coordination mode and each Tdada4- ligand links four dinuclear clusters to generate a three-dimensional (3D) framework. Inspection of this 3D network reveals one asymmetrical triangle channel of approximately 13 Å × 17 Å × 19 Å, extending along a axis (Figure 2b). Topologically, NKU-106 can be viewed as a uninodal (4-c) net with lon topology (Figure 2c) if both zinc binuclear cluster and Tdada4- ligand are regarded as 4-connected nodes with vertex symbols 62.62.62.62.62.62. Unfortunately, the framework of NKU-106 is fragile and apt to collapse upon activation for gas adsorption (Figure S4). To overcome this problem, the solvothermal reaction condition was modified by replacing DMA with N,N-dimethylformamide (DMF), which generated a new MOF compound NKU-107. Although the same Zn2+ ions and Tdada4- ligands were used, NKU-107 presents a 2-fold interpenetrated 3D network structure which is distinctly different with that of NKU-106, presumably

due to different solvents used in solvothermal reactions. NKU-107 crystallizes in the triclinic system with P-1 space group and contains one Zn2+ ion and one Tdada4ligand in the asymmetric unit. The Zn2+ ion is coordinated by six oxygen atoms from four Tdada4ligands to form a distorted octahedral geometry (Figure 3a), while the Tdada4ligands adopt a (κ1-κ1)-(κ1-κ1)-(κ1-κ0)-(κ1-κ0)-µ4 coordination mode to link four Zn2+ ions into a 3D framework. There are one-dimensional rectangle channel of approximately 9.7 Å × 16.6 Å extending along a and b axis (Figure 3b). The potential voids are large enough to be occupied by another identical network, yielding a 2-fold interpenetrated 3D structure. The interpenetration can be classified as type Class Ia, Z = 2 (Zt = 2; Zn = 1) (Figure 3c). The free void volume of NKU-107 is 764.1 Å3 that represents 44.8% per unit cell volume (1705.6 Å3) according to a calculation performed with PLATON. For a better description of structures, NKU-107 can be described as a (4-c) network of dia topology in which both zinc ions and Tdada4- ligands are simplified as 4-connected nodes with vertex symbols 62.62.62.62.62.62 (Figure 3c). It’s widely known that interpenetration might enhance the stability of coordination frameworks. However, it is not strong enough for NKU-107 that is still liable to decomposition upon the loss of the free solvents (Figure S6). So, at last we tried the reaction of a tridentate ligand H3Pycia bearinging pyrazole group with zinc salt, which affords a cage-based framework—NKU-108. Single-crystal X-ray diffraction shows that NKU-108 crystallizes in the trigonal system of R-3 space group and there are two Zn2+ ions, one Pycia3-, one hydroxyl and one coordinated DMF molecule in the asymmetric unit. Zn1 atom links three Pycia3-, one µ3-OH and one DMF in a distorted trigonal bipyramid coordination geometry. Zn2 also centers into a distorted trigonal bipyramid geometry by connecting one hydroxyl, three carboxylate oxygen atoms from three Pycia3- and one nitrogen atom from one Pycia3- (Figure S1). Further linking four Zn2+

Figure 3. The crystal structure of NKU-107. (a) The coordination environment of Zinc ion; (b) the 3D framework with a rectangle channel; (c) 2-fold interpenetrated net for NKU-107.

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Figure 4. The crystal structure of NKU-108. (a) The tetra-nuclear cluster [Zn4(µ3-OH)2]6+; (b) the ligand of H3Pycia; (c) the microporous octahedral cage Zn24(µ3-OH)12(Pycia)6(DMF)6; (d) the packing of octahedral cages and (e) 1D channel formed by packing octahedron cages along c directions; (f) the polyhedral representation of [Zn4(µ3-OH)2]6+ tetranuclear cluster .

(two Zn1 and two Zn2) by two µ3-OH forms a classical tetra-nuclear cluster [Zn4(µ3-OH)2]6+ (Figure 4a). The completely deprotonated Pycia3- (Figure 4b), each behaving like a three-connected ligand, links three tetra-nuclear cluster [Zn4(µ3-OH)2]6+ (Figure S2), and six Pycia3- assemble through sharing six [Zn4(µ3-OH)2]6+ cluster vertexes to form a microporous octahedral cage Zn24(µ3-OH)12(Pycia3-)6(DMF)6 (Dmax = 18.5 Å) (Figure 4c), in which six faces are occupied by Pycia3- and the other two by DMF molecules. Furthermore, each octahedron is linked with six neighboring octahedrons by sharing [Zn4(µ3-OH)2]6+ vertex to form an extended 3D non-interpenetrated cage-stacking framework (Figure 4d). Viewing along the crystallographic c axis, one 1D channel with coordinated DMF molecules on the wall (Figure 4e) can be observed. The total accessible volume is 8488.8 Å3 per unit cell, which is about 54% of the total crystal volume (15755 Å3 per unit cell) as calculated by PLATON.26 Topologically, regarding [Zn4(µ3-OH)2]6+ tetra-nuclear as 6-connected nodes with vertex symbols 4.4.4.4.62.62.82.82.82.82.84.84.108.108.1016 and Pycia3- as 3-connected nodes with vertex symbols 4.4.62 transforms the framework into a new binodal net of (3,6)-connected topology with a Schläfli symbol of (42.6)2(44.62.86.103) (Figure S3). As expected, NKU-108 exhibits significantly improved thermostability (Figure S5, S7 and S9) and chemical stability relative to NKU-107 and NKU-106. The framework of NKU-108 remained intact after activation or immersion in water for 24 h (Figure S8). This improvement can be easily interpreted in terms of hard and soft acid-base (HSAB) theory. Pyrazole group is softer base than carboxylate group, and therefore bound much stronger with soft acid Zn2+ than the latter.27 Taking account of the high stability and porous structure, NKU-108 was further explored in their gas adsorption performance. The as-synthesized NKU-108 sample exhibits high thermal stability as well as crystal purity as demonstrated by good agreement of experimental powder X-ray diffraction (PXRD) pattern with the simulated one (Figure S4 and S5). The activation of NKU-108 sample for gas adsorption measurement was carried out as follows: The sample was soaked in ethanol for 72 h and then filtered and activated under high vacuum (less than 10-5 Torr) at 110 °C. The porous

structure of NK-108 was characterized by N2 sorption isotherm measured at 77 K, which demonstrates typical Type-I characteristic (Figure 5a) and gives a predicted Brunauer–Emmett–Teller (BET) and Langmuir surface areas of 982 and 1295 m2/g, respectively. The pore distribution analysis by H-K (Horvath–Kawazoe) method shows a main distribution in the range of 0.8-1.5 nm (Fig. 5a (Inset)). The CO2 and N2 adsorption isotherms of NKU-108 were measured at both 273 K and 298 K (Figure 5b and Fig. S11), with a CO2 uptakes at 1 atm of 104.23 cm3/g at 273 K and 53.85 cm3/g at 298 K. These values are much higher than that of some classical MOFs such as HKUST-1 (78.2 cm3/g, 273 K), MOF-5 (33.6 cm3/g at 273 K and 22.8 cm3/g at 298 K), ZIF-100 (32.6 cm3/g, 273 K),28-29 UIO-67 (22.9 cm3/g, 298 K), and MOF-177 (17.2 cm3/g, 298 K).30-31 The relatively high adsorption capacity of NKU-108 with respect to CO2 may be ascribed to the strong interaction of CO2 with active sites such as the free Zn (II) sites derived from the

Figure 5. The gas adsorption properties of NKU-108. (a) N2 sorption isotherms at 77 K. Inset: Horvath-Kawazoe pore size distribution plot; (b) The CO2 adsorption isotherms at 273 K and 298 K; (c) CO2 adsorption isotherms; (d) adsorption enthalpy of CO2.

removal of coordinated solvents and the polar acylamide groups abound on the pore wall, which was supported by density functional theoretical (DFT) calculation (Figure S12-S13). To evaluate the interactions between CO2 molecules and framework, the CO2 isosteric enthalpy (Qst) of NKU-108 was predicted by using viral method based

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on the adsorption isotherms collected at 273 K and 298 K (Figure 5c). The Qst value of 22.81 kJ mol−1 at zero loading is comparable to that of other complexes showing relatively high CO2 uptakes (Figure 5d).32 Light hydrocarbons are important raw chemicals in modern industry. In recent years, the storage of the small hydrocarbons (CH4, C2H2 C2H4 and C2H6) promises new energy alternatives and has attracted increasing attention. Herein, the potential of NKU-108 for small hydrocarbons adsorption was also investigated. The adsorption isotherms of these small hydrocarbons were measured at 273 K and 298 K under 1 bar (Fig. S14), and the maximum uptakes at 273K and 298 K and 1 atm reach 24.95 and 14.26 cm3/g for CH4, 136.70 and 87.16 cm3/g for C2H2, 112.40 and 76.18 cm3/g for C2H4, and 110.07 and 80.53 cm3/g for C2H6, respectively. The adsorption selectivities for CO2/CH4 and CO2/N2 of NKU-108 have also been evaluated (Fig. S15). Notably, the PXRD pattern of the sample after gas adsorption still agrees well with the simulated one, indicating the high stability of NKU-108 framework sufficient to survive through gas adsorption and desorption processes. In conclusion, two MOFs (NKU-106 and NKU-107) based on tetracarboxylates ligands and zinc ions, whether interpenetration or not, exhibit poor stabilities and gas adsorption properties. However, modulating the ligands with soft-base pyrazole group strengthens the coordination interaction with zinc ions and therefore results in NKU-108 with enhanced stability. NKU-108 shows outstanding capacity of adsorbing CO2 and light hydrocarbons, owing to the favorable stability, suitable pore space and open metal sites as well as acylamide functional groups.

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ASSOCIATED CONTENT Supporting Information. Materials and methods, Crystal Structure Determination, Experimental Section, Structural Figures, Gas Adsorption, Density functional theoretical calculation and CCDC No: 1528197-1528199. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (Y.-H. Zhang) *E-mail: [email protected]. (X.-H. Bu)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the NSFC (21421001, 21531005, and 91622111), and the NSF of Tianjin (15JCZDJC38800).

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Synopsis Improving the Stability and Gas Adsorption Performance of Acylamide Group Functionalized Zinc MOFs through Coordination Group Optimization Yan-Yuan Jia,† Xiao-Ting Liu,‡ Rui Feng,† Shi-Yu Zhang,† Ping Zhang‡ Ya-Bing He‡ Ying-Hui Zhang*‡ and Xian-He Bu* †,‡ In order to elucidate the effect of the structure on stability and gas adsorption performance, three porous Zn(II) metal−organic frameworks, NKU-106, NKU-107 and NKU-108, have been constructed. NKU-106 and NKU-107 assembled from acylamide functionalized tetracarboxylate ligand exhibit low stability and thereby poor gas adsorption ability, whereas NKU-108 constructed based on dicarboxylate/ pyrazole shows improved thermal and chemical stability as well as good adsorption capacities with respect to CO2 and C2 hydrocarbons. Such a strategy provides very valuable information for future design and synthesis of porous MOFs for practical application.

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