Chinese Chemical Letters  2017, Vol. 28 Issue (1): 55-59   PDF    
Impact of the flexibility of pillar linkers on the structure and CO2 adsorption property of "pillar-layered" MOFs
Yue-Ling Xua,b, Qiang Gaoa,b, Meng Zhaoa,b, Huan-Jun Zhanga,b, Ying-Hui Zhanga,b, Ze Changa,b     
a School of Materials Science and Engineering, National Institute for Advanced Materials, TKL of Metal and Molecule-Based Material Chemistry, Nankai University, Tianjin 300350, China;
b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China
Abstract: A new metal-organic framework {[Zn2(bpta)(bpy-ee)(H2O)2]·x solve}n (1) (H4bpta=biphenyl-2, 2', 6, 6'-tetracarboxylic acid and bpy-ee=1, 2-bis (4-pyridyl) ethylene) has been obtained under hydrothermal condition, and structurally characterized by single-crystal X-ray diffraction. Complex 1 reveals a threedimensional (3D) "pillar-layered" framework with non-flexible linker, in which some different structure characters can be found compared to that of some related other "pillar-layered" MOFs based on flexible pillar linkers. It demonstrates the impact of the flexibility of pillar linker on the final structure in this system. In addition, the selective CO2 adsorption performance of 1 was also investigated.
Key words: Metal-organic framework     Pillar-layer structure     Flexibility     CO2 Adsorption     Selectivity    
1. Introduction

Recently, as a class of versatile porous materials, metal-organic frameworks (MOFs) have received much attention owing to their various potential applications in storage [1, 2], separation [3, 4], catalysis [5-7], magnetism [8, 9] and light-emitting [10, 11], which should be attributed to their multiple properties depending on their tailorable components and divers structures. Utilizing the well-developed assembly strategies [12-21], targeted construction of MOFs with desired structures and specified properties could be achieved based on the rational selection of metal centers and organic linkers [22-27]. Among the various strategies reported, the "pillar-layer" method has been proved to be an effective one in the perspective of targeted construction of MOFs since the pore structure as well as the performances of the product could be readily modulated in a straightforward way by tuning the configuration and the attached functional group of the "pillar" linker [28, 29]. Therefore, divers "pillar-layer" MOFs systems aiming at distinct properties have been developed and well investigated [30, 31].

With continuing efforts toward functional MOFs, we have developed a robust "pillar-layer" MOFs system in which layer structure is composed of 1, 1'-biphenyl-2, 2', 6, 6'-tetracarboxylic acid (H4bpta) (Scheme 1) and Zn (Ⅱ) ions while the bipyridine ligands serve as pillars [32]. In this system, the tuning of pore structure could be realized by the rational utilization of pillar linkers with different length. Furthermore, enhanced CO2/CH4 selectivity also could be achieved by tuning the polarity and flexibility of the pillar linkers [33], which makes this system promising for performances targeted constructions.

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Scheme 1. The structures of H4bpta, bpy-ee.

It is worth noticing that, in contrast to the mentioned above "pillar-layer" MOFs, the connection mode between the layers as well as the corresponding structure were observed to be affected by the bipyridine linkers with different flexibility: when 1, 2-bis (4-pyridyl) ethane (bpy-ea) with quite flexible backbone was used as pillar linker, a "pillar-layer" structure showing subtle differences similar configurations (e.g. N-(4-pyridyl) isonicotinamide (4-pna) and 4, 4'-azopyridine (azpy)) (Fig. S1 in Supporting information). Considering the structure of the pillar linkers, this might be attributed to their different flexibility. Based on this possibility, MOFs with distinct pillar-layer connection and pore structures could be constructed by non-flexible linkers, which motivated us for further investigation. In this context, a new "pillar-layer" MOF with non-flexible 1, 2-bis (4-pyridyl) ethylene (bpy-ee) (Scheme 1) linker was synthesized, namely {[Zn2(bpta)(bpy-ee)(H2O)2]·x solve}n(solve=solvent molecules) (1). As we expected, this MOF reveals a new pillar-layer connection manner differing from that obtained with semi-flexible or flexible linkers, which further proved the critical role of the flexibility of pillar linker on the structure modulation in this system. Furthermore, this MOF reveals remarkable CO2 isoteric heat of adsorption (Qst) and CO2/CH4 selectivity owing to its unique structure, which is desired for CO2 capture applications.

2. Experimental

All solvents and reagents for synthesis were obtained commercially and used as received. H4bpta was synthesized according to the reported methods [34]. {[Zn2(bpta)(bpy-ee)(H2O)2]·x solve}n was obtained by the reaction of H4bpta with bpy-ee and Zn (NO3)2·6H2O under hydrothermal condition.

3. Results and discussion 3.1. Description of crystal structure

Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the orthorhombic space group Aba2. And further structure analysis demonstrates that there are two types of crystallographically independent Zn (Ⅱ) ions, one bpta4-, one bpyee and two H2O molecules in the asymmetric unit. The Zn1 ion is four-coordinated to two oxygen atoms from two carboxylate groups of two bpta4-, one N atom of bpy-ee and one H2O molecule. [Zn-O=1.943(3)-2.003(3) Å, Zn-N=2.010(3) Å], while Zn2 is s ix-coordinated to four oxygen atoms from two carboxylate groups of two bpta4-, one N atom of bpy-ee and one H2O molecule [Zn-O=2.012(4)-2.326(2) Å, Zn-N=2.090(3) Å] (Fig. 1a). Each bpta4- ligand bounds with four Zn (Ⅱ) through four carboxylate groups, two in chelating mode and two in monodentate mode, resulting in a two-dimensional (2D) layer expanded in the ac plane. Furthermore, the parallel layers were interconnected by bpy-ee linkers to construct a 3D "pillar-layered" framework (Fig. 1c). In the framework, the supporting of pillar linker between the layers results in pseudo-triangle channels with the diameter about 7.0 Å along the c direction (Fig. 1b). After removal of solvent molecules filled in the pore space, the accessible volume is 34.0%, which is estimated using the SOLV function of PLATON [35]. The attempts of determining the species and the amount of solvent molecules based on SQUEEZE results combined with elemental analysis and TGA data were unsuccessful because of the volatility and complexity of solvents used for synthesis.

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Figure 1. (a) Coordination environment of Zn (Ⅱ) ions and the bpta4- ligands in complex 1; (b) Views in parallel with the channels along the a direction (the yellow represents the channels along the c direction); (c) The layer-pillar framework constructed from the 2D layer assembled by Zn (Ⅱ) ions with bpta4- ligands and the pillar (the pink).

Though the framework of 1 reveals "pillar-layer" structure similar to that reported before (Fig. S1 in Supporting information), some obvious differences could be found in the layer structures: in reported before, all the layers in the related complexes including flexible linker (bpy-ea) and semi-flexible linker (4-pna and azpy) are planar, in which the bpta4- shows indiscriminate coordination modes to connect five Zn (Ⅱ) centers. While in this title, each bpta4- coordinates to four Zn (Ⅱ) centers to result in wavy layers. As a result of the altered layer structure, the arrangement of available coordination sites for bpy-ee linkers has also changed, which impacts their connection methods correspondingly. Moreover, the presence of double bond defined the conjugated backbone and planar configuration of bpy-ee, which is quite different from that of bpy-ea, 4-pna, and azpy. In the framework of 1, the closely arranged coordination sites of pillar linkers on the layers determined the relatively short distance between the bpy-ee linkers, and the conjugated backbone of them benefit the π-π stacking interaction between the neighboring linkers to defined a dimer (interplane distance ca. 3.3 Å) (Fig. 1c). Furthermore, the unparallel orientation of the dimers originated from the wavy layer results in the pseudo-triangle channels between the layers.

In conclusion, the connections between the layers as well as the pore structure are different from that reported before (Fig. S1). Considering the similarity of the synthesis condition and the identical reactants except for the pillar linker, the distinctions of the framework structure of 1 from the reported ones should be mainly attributed to the presence of non-flexible bpy-ee linker which may affect the assembly of layers rather than just adapt the connections. This conclusion could be further proved by the result of control experiments with simultaneous introduction of bpy-ee and 4-pna as linkers. In the condition, we could not obtain the solid solution of two kinds of pillar-layer MOFs based on bpy-ee and 4-pna but the mixture of them. These results indicate that in this system, two linkers with obviously different configurations are not compatible in one lattice, and the structures of the MOFs will be determined by the character of pillar linker.

3.2. Adsorption studies

In addition to the determination of the impact of pillar linker on the framework structure, the porous framework of 1 also motivated us for the investigation of its gas adsorption performances. Before the gas adsorption experiments, thermalgravimetric analysis (TGA) and powder X-ray diffraction (PXRD) were performed to study the stability of 1. The TGA curve (Fig. S2 in Supporting information) of the as-synthesized 1 reveals obvious loss of weight with the rising of temperature before 110 ℃, which should be attributed to the departure of water, ethanol, and DMF guest molecules from the framework. More detailed assignment could not be carried out since the amount of the guest molecules could not be determined. The relatively flat TG section between 110 ℃ and the decomposition point at 350 ℃ indicates that the porous framework could be retained after the removal of solvent guests. The results of variable-temperature powder X-ray diffraction studies (Fig. S3 in Supporting information) demonstrate that the framework of 1 was preserved well under 50 ℃, while obvious reduction of diffraction intensity could be observed above 100 ℃. This result indicates the reduction of crystallinity of 1 under high temperature. Anyway, the main peaks observed under high temperature suggest the retainment of porous framework at high temperature.

Based on the results of TGA and PXRD, the sample of 1 was solvent exchanged with ethanol and then evacuated at 50 ℃ for 12 h before the gas adsorption investigations. To access the porosity of the sample, N2 adsorption was performed. As shown in Fig. 2, the N2 adsorption of the activated sample of 1 presents a type adsorption behavior, indicating the microporosity of the sample which is consist with the crystal structure. The estimated apparent BET and Langmuir surface areas are 262 and 345 m2 g-1 and the pore volume is calculated to be 0.12 cm3 g-1. It should be noted that the pore volume is much lower than the theoretical value (0.27 cm3 g-1) estimated from the crystal structure. For better understanding of this phenomenon, the PXRD of the activated sample was recorded. The main diffraction peak at 9.12° (indexed as (0, 2, 0)) reveals a significant movement toward higher degree compared with that of the as-synthesized sample (observed at 7.16°) (Fig. S4 in Supporting information). Interesting, the shifted peak could be restored after the activated sample was soaked in DMF again. Since the peak could be assigned to the layer structure in the framework [33], the shift and restore of the peak under different conditions suggest the flexibility nature of the framework, which will affect the porosity of the framework correspondingly. This could well rationalize the relatively lower pore volume obtained from the N2 adsorption results. Considering the non-flexible backbone of the pillar linker in contrast to the previous reports [32, 33], the flexibility of 1 should be originated from the wavy layer, and these results demonstrate the versatility of this system for the construction of MOFs with distinct properties.

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Figure 2. Isothermal adsorption curves of N2 gas for 1.

Furthermore, the selective CO2 adsorption ability of 1 was also investigated. CO2 adsorption isotherms were recorded at 273 and 298 K, respectively (Fig. 3), and the uptake reaches 43.7 m3 g-1 at 273 K/900 mmHg and 26.8 m3 g-1 at 298 K/900 mmHg, respectively. The CO2 adsorption enthalpies (Qst) of 1 calculated using the Virial method [36] (Fig. 4) show a maximum of 30.90 kJ mol-1 at zero loading. This value is lower than that observed in the MOFs with 4-pna as linker (34.90 kJ mol-1), but relatively higher than that with bpy-ea as linker (27.69 kJ mol-1) [33], which should be attributed to the moderate affinity of the ethylene motif compared with that of the polar acylamide groups and the alkyl motif [33]. In addition, this value is relatively higher than many MOFs such as the 28.2 kJ mol-1 in CPF-13 [37], 13.73 kJ mol-1 in IRMOF-1, 12.67 kJ mol-1 in IRMOF-8, 11.96 kJ mol-1 in IRMOF-10, 13.28 kJ mol-1 in IRMOF-14, 10.25 kJ mol-1 in IRMOF-16, 20.86 kJ mol-1 in IRMOF-11, 14.43 kJ mol-1 in MOF-177, and 25.60 kJ mol-1 in Cu-BTC [38].

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Figure 3. CO2 and CH4 adsorptions capacity and the selectivity for carbon dioxide from an mixture of 85% methane and 15% carbon dioxide of complexes 1 at 273 K and 298 K.

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Figure 4. CO2 adsorption enthalpies (Qst) of complexes 1.

In order to estimate the CO2 selectivity over CH4 of 1, the CH4 adsorption capacities were also tested at 273 K and 298 K and the data were used to calculate the CO2/CH4 selectivity of 1 for the mixture consisting of 85% CH4 and 15% CO2 by using the ideal adsorbed solution theory (IAST) method (Fig. 3). As shown in Fig. 3, the initial selectivity at 273 K reaches 14, and a decrease-increase trend was observed upon increasing the pressure with a value of 8.0 achieved at 900 mmHg, while the selectivity at 298 K shows a decreasing trend from seven to six in the range of 0-1 bar. The relatively higher selectivity at lower pressure and temperature indicate the potential of 1 for CO2 capture and separation applications under corresponding conditions. In conclusion, the selective adsorption for CO2 is higher than that for CH4. This result could be explained with respect to the relatively high Qst value and consequently high uptake of CO2, which should be attributed to the large dipole-quadrupole interactions between CO2 molecules and the framework owing to the significant quadrupole moment of CO2 (-1.4 × 10-39 cm2). The relatively low selectivity may indicate the competition of CO2 and CH4 for the identical sorption sites.

4. Conclusion

In conclusion, by introducing pillar linker with non-flexibility bpy-ee, a new "pillar-layered" Zn-MOF based on layers composed of bpta4- and Zn (Ⅱ) ions was obtained. The distinct structure of this MOF compared with that constructed by flexible linkers suggests the critical role of the flexibility of pillar in the structure modulation in this system. In addition, the CO2 capture capacity and CO2/CH4 selectivity adsorption performances of this MOF were also investigated to show that low temperature and low pressure would be preferred conditions for the achievement of better performances.

Acknowledgment

This work was financially supported by the 973 program (Nos. 2014CB845600 and 2012CB821700), NNSF of China (Nos. 21531005, 21421001, and 2129017), and MOE Innovation Team (No. IRT13022) of China.

Appendix A. Supplementary data

Crystallographic data file (CIF), thermalgravimetric analysis (TGA) and powder X-ray diffraction (PXRD) of 1, crystal data and structure refinement parameters and selected bond distances and angles for 1 etc. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2016.06.006.

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