Assembly of Four Kinds of Cages into Porous Metal–Organic

Article pubs.acs.org/crystal

Assembly of Four Kinds of Cages into Porous Metal−Organic Framework for Selective Sorption of Light Hydrocarbons Fei Wang, Hong-Ru Fu, Duan-Chuan Hou, and Jian Zhang* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China S Supporting Information *

ABSTRACT: The direct assembly of four kinds of cages by face-sharing and edge-sharing modes with Zn3(μ3OH−)(H2O)(COO−)4 units and organic ligands generates a porous framework material including 3D channels, which shows high adsorption enthalpy of C3H8 (35 kJ/mol) and excellent adsorption selectivity of C3H8 over CH4 under ambient conditions.

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Zn6(μ3-OH)2(H2O)2(btc)8/3(2-mbim)2·x(guest) (1), including four different kinds of cages. The direct assembly of these cages by face-sharing and edge-sharing modes generates 3D channels, the centers of which fall on the nodes of a 2-fold interpenetrated pcu net. The uncoordinated metal sites as well as the unique cage structure of the porous framework make high C3H8 uptake at 0.1 bar (273 K: 60 mL/g; 285 K: 53 mL/ g) and high enthalpy of C3H8 adsorption (35 kJ/mol). Moreover, it has excellent adsorption selectivity of C3H8 over CH4 under ambient conditions.

INTRODUCTION In the past 20 years, metal−organic frameworks (MOFs) have been widely researched due to their potential applications in gas storage, separation, catalysis, and so on.1−7 Recently, MOFs have also been paid much attention for the enrichment and separation of hydrocarbons because of their very important industrial applications.8−17 The famous MOF-74 series materials were employed to separate light hydrocarbons, which show high uptake of light hydrocarbons, mainly because of the interaction between guest and the open metal sites presented in the framework.8−11 The UTSA series materials developed by Chen and co-workers exhibit high selectively separative capacity of light hydrocarbons.15,16 Remarkably, UTSA-35 showed the highest C3H8/CH4 separation selectivity of over 80.17 In both cases, these materials include 1D or 2D channels; high uptake and selectivity are mainly attributed to the open metal sites. In fact, MOFs constructed by coordination cages is also of particular interest,18,19 because the small window size and big inner space of the cages made them one of the most suitable candidates for gas storage and separation. A lot of interesting packing architectures of metal− organic cages (or metal−organic polyhedra) with different sizes and geometries have been constructed.20−26 However, the direct assembly of cages into functional MOFs has been rarely reported.27 Herein, we report the cooperative assembly of 2-methylbenzimidazole (H-2-mbim) and 1,3,5-benzenetricarboxylic acid (H3btc) ligands with Zn2+ centers into a porous MOF material, © XXXX American Chemical Society

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EXPERIMENTAL SECTION

General Procedures. All reagents were purchased commercially and used without further purification. The purity of all gases is 99.999%. All syntheses were carried out in a 20 mL vial under autogenous pressure. All powder X-ray diffraction (PXRD) analyses were recorded on a Rigaku Dmax2500 diffractometer with Cu Kα radiation (λ = 1.54056 Å) with a step size of 0.05°. Gas adsorption measurement was performed in the ASAP (Accelerated Surface Area and Porosimetry) 2020 System. Synthesis of Zn6(μ3-OH)2(H2O)2(btc)8/3(2-mbim)2·x(guest) (1). The mixture of Zn(NO3)2·6H2O (1.0 mmol, 0.301 g), H-2-mbim (1.25 mmol, 0.163 g), H3btc (0.89 mmol, 0.184 g), N(CH3)4·Br (0.42 mmol, 0.065 g) in 6 mL of N,N-dimethylacetamide (DMA) and 2 mL of water was sealed in a 20 mL vial and heated to 120 °C for 2 days, and then cooled to room temperature. The light yellow crystals were Received: September 4, 2014 Revised: October 29, 2014

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obtained, washed with ethanol, and dried at room temperature (Yield: 28%). IR (KBr cm−1): νs (OH): 3400 cm−1; νs (CC): 1620 cm−1; νs (CC): 1570 cm−1; τ(C−H): 710 cm−1; τ(C−H): 760 cm−1; τ(CH3): 710 cm−1; ω(CH3):1370 cm−1. Elemental analysis (activated) C40H24N4O18Zn6 (1240.96): Calcd C, 38.72; H, 1.95; N, 4.39; Found C, 34.95; H, 3.21; N, 4.43.

RESULTS AND DISCUSSION X-ray single-crystal structural determination reveals that compound 1 crystallizes in the cubic Im3̅ space group. In the structure of 1, there are four independent Zn centers with distinct coordination geometries (Figure 1a). One kind of Zn

form a 3D framework including 3D channels (Figure 1c−e), which are occupied by structurally disordered DMA molecules. The total solvent-accessible volume for 1 is estimated to be 43.2% using the PLATON program.28 It should be noted that the terminally coordinated water molecules toward into the channels are potential uncoordinated metal sites for guest molecules (Figure 1b). From the topological view of point, each trinuclear unit is bound to four btc ligands and another trinuclear unit through one 2-mbim, acting as a five-connected node. Each btc ligand connects to three trinuclear units acting as a three-connected node (Figure s1, Supporting Information). In this way, the structure can be cnsidered as a (3, 3, 5, 5)-connected complicated net with the point symbol of (4.5.6)3(4.53.62.84)3(53).29 The prominent structural feature of 1 is the presence of four different kinds of cages (Figure 2). The B cage is formed by 12

Figure 1. (a) The coordination environment in the structure of 1. (b) View of the 3D framework of 1 including 3D channels along three different axes. Yellow ball represents pores, and green ball represents uncoordinated metal sites.

Figure 2. Four kinds of cages and the corresponding tiles. Orange: Zn3(μ3-OH−)(H2O)(COO −)4 unit; pink: Zn3(μ3-OH−)(H2O)(COO−)4 unit; sky blue and green: btc ligand.

center (Zn1 nad Zn2) is octahedrally coordinated by four carboxylate oxygen atoms from four btc ligands, one μ3-OH−, and one terminally coordinated water molecule. Another kind of Zn center (Zn3 and Zn4) is tetrahedrally coordinated by two carboxylate oxygen atoms from two btc ligands, and one nitrogen atom from the 2-mbim ligand. The 2-mbim ligand coordinates two tetrahedral Zn centers. Each btc ligand acts as a μ6-bridge to link six Zn centers. The octahedral Zn center is located at a special symmetry site, and it is surrounded by two symmetry-related tetrahedral Zn centers, which are further linked by one OH− to generate a Zn3(μ3-OH−)(H2O)(COO−)4 unit with the shortest Zn···Zn distance being 3.238 Å. The trinuclear units are linked by btc and 2-mbim ligands to

trinuclear units, 8 btc, and 12 2-mbim ligands. It has fourmembered, five-membered, and six-membered rings windows. Four aromatic rings of 2-mbim are toward the inner space of the cage, giving a oblate spheroid pore. It has an equatorial diameter of ca. 20 Å and a polar diameter of 3 Å, and it is defined as [44.54.64] with 6 membered ring (MR) faces (in the symbols, [...mn...] means that there are n faces that are m-rings) (Figure 2b). The C cage is a dodecahedron (Figure 2c), which is also formed by 12 trinuclear units, 8 btc and 12 2-mbim ligands. It has an inner diameter of 12 Å pore and is defined as [512] with 5 MR faces. The D cage is formed by 12 trinuclear units and 8 btc ligands (Figure 2d). It has an inner diameter of 14 Å and is defined as [86] with 8 MR faces. There is also a

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B

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smaller [4.52.6.82] A cage that just locates at the interstices between these cages (Figure 2a). It is formed by eight trinuclear SBUs, eight btc and four 2-mbim ligands. To better exhibit the structural feature of 1, the packing of these cages can be seen from the illustration of them as tilings in Figure 2. Each D tile from the D cage links six neighboring ones through six A tiles by sharing 8 MR faces (Figure 3a), the

Figure 4. N2 sorption isotherms of 1 at 77K. Solid represents adsorption; open represents desorption.

Table 1. Uptake of CO2 and Light Hydrocarbons for 1 uptake (mL/g) at 273 K uptake (mL/g) at 285 K Qst (kJ/mol)

CO2

CH4

C2H2

C2H4

C2H6

C3H8

84 65 31

20 17 9

90 79 27

82 70 28

78 69 29

75 69 35

Figure 3. Tiling of 1. (a) The D tiles are linked by A tiles to form a pcu net; (b) B tiles and C tiles link each other to form another pcu net; (c) the whole tiling of 1. (d) The centers of all the tiles fall on the nodes of 2-fold interpenetrated pcu nets (D, in turquoise; B, in brown; C, in dark green).

centers of which lie on the nodes of a six-coordinated pcu net.30 Each B tile links four neighboring ones and two C tiles, and each C tile links six B tiles by sharing edges, the centers of which lie on the nodes of another six-coordinated pcu net (Figure 3b). Two kinds of pcu nets are interpenetrated into each other (Figure 3c,d). A freshly prepared sample of 1 was soaked in ethanol at room temperature for several days to exchange guest DMA molecules. The TGA results demonstrated that the DMA molecules can be replaced by EtOH completely (Figure s2, Supporting Information). Subsequently, the sample was degassed under a dynamic vacuum at 80 °C for 5 h to activate the sample. To confirm the permanent porosity of this activated sample 1, N2 gas sorption experiments were carried out (Figure 4). It exhibits typical type I sorption isotherms and takes up N2 to 209 cm3/g at 77 K, corresponding to Langmuir and BET surface areas of 830 and 630 m2/g, respectively. A single data point at relative pressure 0.997 gives a maximun pore volume of 0.324 cm3/g by the Horvath−Kawazoe equation. The uncoordinated metal sites as well as unique cage structures with 3D channels suggest that 1 might be a promising candidate for gas separation. The pure component sorption isotherms of 1 for CO2 and various hydrocarbons under ambient conditions were examined, respectively (Table 1). As shown in Figure 5, the CO2 uptake values of 1 are 84 cm3/g (2.5 mmol/g) at 273 K, and 65 cm3/g (2.3 mmol/g) at

Figure 5. Gas sorption isotherms of 1: (a) CH4; (b) C2H2; (c) C2H4; (d) C2H6; (e) C3H8; (f) CO2. Solid represents 273 K; open represents 285 K.

285 K, respectively. The isosteric heat of adsorption (Qst) for 1 was calculated using the adsorption data collected at 273 and 285 K. Virial analysis of the CO2 adsorption isotherms revealed that the Qst of CO2 adsorption of 1 at zero surface coverage is 31 kJ/mol (Figures S4 and S5, Supporting Information). As mentioned before, the uncoordinated metal sites and rich π electrons presumably contribute to high uptake and isosteric heat of adsorption of CO2. The pure component sorption isotherms of 1 for various hydrocarbons under ambient conditions are also shown in Figure 5. Compound 1 exhibits notable adsorption capacities to C3H8 (75.3 cm3 g−1, 147.9 mg g−1), C2H6 (77.8 cm3 g−1, 104.2 mg g−1), C2H4 (81.8 cm3 g−1, 102.2 mg g−1), and C2H2 (90.0 cm3 g−1, 104.5 mg g−1) at 1 bar and 273 K, respectively. These adsorption results are comparable to the reported values of UTSA-35a5 and UTSA-33a.4 The most significant feature is that the C3H8 adsorption isotherms in 1 display quick C

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Notes

saturation at low pressure, showing strong affinity of 1 with C3H8. The calcated isosteric heat of adsorption for C3H8 is 35.1 kJ·mol−1 (Figures S5 and S10, Supporting Information), which is also higher than that of other light hydrocarbons (C2H6: 29.3 kJ·mol−1; C2H4: 28.5 kJ·mol−1; C2H2: 27.3 kJ·mol−1; CH4: 9.4 kJ·mol−1) (Figures S5−S9, Supporting Information). Importantly, the significant differences in the uptake at 273 K and 0.1 bar (C3H8: 60 mL/g; C2H6: 32 mL/g; C2H4: 25 mL/g; C2H2: 30 mL/g; CH4: 2 mL/g) indicate that 1 is a potential candidate for the separation of CH4 from C2 and C3 hydrocarbon mixtures. It is deduced that the unique cage structures with the small pore size induce stronger interaction with larger hydrocarbons than the smaller ones. The adsorption selectivities of different hydrocarbons with respect to CH4 were calculated by the ideal adsorbed solution theory (IAST), which had been shown to be valid to predict multicomponent adsorption behaviors from the experimental single-component gas isotherms. The predicted adsorption selectivity for equimolar C3H8 and C2 over CH4 mixtures in 1 is presented in Figure 6. As expected, the selectivity of C3H8 over CH4 has

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the 973 program (2011CB932504 and 2012CB821705) and NSFC (21221001, 21103189) for the support of this work.

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Figure 6. IAST predicted selectivity of different hydrocarbons with respect to CH4 at 285 K in 1.

an unprecedented value (ca. 70−105) in the region of 1 bar at 285 K, which suggests that 1 may be a good candidate material for the separation of C3H8 over CH4.

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CONCLUSION In summary, the direct assembly of four kinds of cages by facesharing and edge-sharing modes generates a porous framework material including 3D channels. The uncoordinated metal sites and unique cage structure of the framework show high C3H8 adsorption enthalpy and has potential application in selective separation of C3H8 over CH4 under ambient conditions.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

REFERENCES

S Supporting Information *

Additional figures, powder X-ray diffraction patterns, and sorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected] (J.Z.). D

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