A Noninterpenetrated Metal–Organic Framework Built from an

Article pubs.acs.org/crystal

A Noninterpenetrated Metal−Organic Framework Built from an Enlarged Tetracarboxylic Acid for Small Hydrocarbon Separation Jianfeng Cai,† Jiancan Yu,† Hailong Wang,‡ Xing Duan,† Qi Zhang,† Chuande Wu,§ Yuanjing Cui,† Yang Yu,† Zhiyu Wang,† Banglin Chen,*,†,‡ and Guodong Qian*,† †

State Key Laboratory of Silicon Materials, Cyrus Tang Center for SensorMaterials and Applications, College of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China ‡ Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0698, United States § Department of Chemistry, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: A novel noninterpenetrated metal−organic framework [Cu2L(H2O)2]f·(DMA)18·(H2O)19 (ZJU-31, ZJU = ZheJiang University; H4L = 5′,5⁗(2,5-dimethoxy-1,4-phenylene)bis[1,1′:3′,1″-terphenyl]-4,4″-dicarboxylic acid; DMA = N,N-dimethylacetamide) has been synthesized and structurally characterized from an expanded organic linker. Interestingly, although the new linker is larger than the developed one for the construction of ZJU-30 with a doubly interpenetrated structure, the incorporation of a 1,4-dimethoxyphenyl group into the new linker enforces the formation of noninterpenetrated ZJU-31. Accordingly, the activated ZJU-31a shows a much higher porosity with a BET surface area of 1115 m2/g than ZJU-30a of 228 m2/g. ZJU-31a exhibits highly selective separation of C2 hydrocarbons over C1 methane at room temperature.

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m2/g than ZJU-30 of 228 m2/g. ZJU-31a exhibits highly selective separation of C2s over C1 methane from 273 to 298 K with the Henry’s law constants systematically higher than those in ZJU-30a. The IAST adsorption selectivities of ZJU-31a for C2H6/CH4, C2H4/CH4, and C2H2/CH4 are 23.4, 22.0, and 22.5, respectively, at 298 K.

INTRODUCTION Microporous metal−organic frameworks (MOFs) have great potential as very promising useful multifunctional materials. This new class of porous materials can be self-assembled by the metal ions and/or metal containing clusters with organic bridging linkers by the coordination bonds.1−3 The strategy of using large/longer ligands for highly porous framework sometimes has been quite challenging because of the framework interpenetration and/or catenation, leading to less or nonporous frameworks.4−6 Quite a lot of efforts have been pursued to construct noninterpenetrated frameworks over the past decade, and some strategies have been realized, including very dilute solutions, addition of templates, and decoration of organic linkers.7−9 Recently, we have reported a doubly interpenetrated MOF ZJU-30 constructed from a large tetracarboxylic acid.10 It is understandable that ZJU-30 is an interpenetrated framework given the fact that this linker is quite large. Interpenetration apparently limits its porosity as demonstrated by its low BET surface area of 228 m2/g and thus its applications on gas separations. With the goal to target higher porous MOFs for light hydrocarbon separations, we developed a spatially even larger organic linker H4L (Scheme 1) but with a decorated 1,4dimethoxyphenyl groups. As expected, such a decoration does favor the formation of a noninterpenetrated MOF: [Cu2L(H2O)2]·(DMA)18·(H2O)19, (ZJU-31, ZJU = Zhejiang University; DMA = N,N-dimethylacetamide). The resulting ZJU31 thus has a higher porosity with a BET surface area of 1115 © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Gas Sorption Measurements. Gas sorption isotherms was measured by a Micromeritics ASAP 2020 surface area analyzer. The as-synthesized ZJU-31 was treated with dry acetone for 3 days and then activated at 373 K under hard vacuum for 12 h to remove guest solvent molecules in the framework until the outgas rate reached 5 μmHg/min before measurements. Henry’s Law Selectivity. Sorption isotherm data were fitted with the well-known virial equation

ln(n/p) = A 0 + A1n + A 2 n2 + ...

(1)

where the p is pressure, n is the adsorbing capacity, and A0, A1, etc., are relevant virial coefficients. The Henry’s Law constant (KH) is defined as exp(A0), and the selectivity can be calculated from the constant KH. The Henry’s law constants of ZJU-31a are summarized in Table S2 (Supporting Information). Adsorption Selectivities. Isotherm data were analyzed according to the well-defined Dual Site Langmuir−Freundlich (DSLF) model Received: May 15, 2015 Revised: June 23, 2015

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DOI: 10.1021/acs.cgd.5b00675 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Scheme 1. Enlarged Tetracarboxylate Organic Linker

N = N1max ×

b1p1/ n1 1/ n1

1 + b1p

+ N2 max ×

b2p1/ n2 1 + b2p1/ n2

by matching the powder X-ray diffraction patterns of experimental and simulated (PXRD, Figure S1, Supporting Information). ZJU-31 can be formulated as [Cu2(C48H30O10)(H2O)2]·(DMA)18·(H2O)19 on the basis of single-crystal structure, elemental analyses, and thermogravimetric analyses (TGA, Figure S2, Supporting Information). A mass loss of 68.36% was observed up to 453 K under a nitrogen atmosphere, corresponding to the absence of free solvents in the pore space and terminal water molecules (Calcd 68.53%). Single-crystal X-ray diffraction anayases reveals that ZJU-31 belongs to the tetragonal space I4/mmm with a = b = 39.580(4) Å and c = 27.445(5) Å. ZJU-31 adopts the paddlewheel clusters [Cu2(CO2)4] as secondary building units (SBUs), and the copper sites become coordinatively unsaturated upon removal the axial aqua (Figure 1). Obviously, the

(2)

where p (unit: Kpa) is the pressure of the bulk gas when in equilibrium with the adsorbed phase, N (unit: mol/kg) is the loading amount of adsorbent, N1max and N2max (unit: mol/kg) are the saturation loadings and b1 and b2 (unit: 1/kPa) are the affinity parameters of sites 1 and 2, respectively, while n1 and n2 represent the deviations stemmed from the ideal homogeneous surface model. Herein, the single-component C2H6, C2H4, C2H2, CH4 adsorption isotherms have been fitted for the following calculation of IAST in predicting the performance of ZJU31a under an equimolar mixed gas. The fitting parameters of the DSLF equation are listed in Table S3 (Supporting Information). Adsorption isotherms and selectivities were established by IAST for mixed C2H6/ CH4 (C2H6/CH4 = 50:50), C2H4/CH4 (C2H6/CH4 = 50:50), and C2H2/CH4 (C2H2/CH4 = 50:50) in the ZJU-31a. The adsorption selectivities, Sads, are defined by the following equation Sads =

q1/q2 p1 /p2

(3)

in which pi, the partial pressure of species i, and qi, the component molar adsorped of species i, can be determined using the ideal adsorbed solution theory (IAST) proposed by Myers and Prausnitz.11 Isosteric Heat of Adsorption. The isosteric enthalpy of adsorption, Qst, defined as

⎛ ∂ ln p ⎞ ⎟ Q st = RT 2⎜ ⎝ ∂T ⎠q

(4)

These values were calculated by using the pure component isotherm fittted by the virial method. Synthesis of ZJU-31. A mixture of 5.0 mg of H4L (0.0065 mmol) and 10 mg of Cu(NO3)2·2.5H2O (0.0215 mmol) was dissolved in 2.0 mL of DMA in a screw-capped vial. After 0.30 mL of H2O and 30 μL of HNO3 (65%, aq.) were added into the mixed solution, the vial was sealed and moved to a precise oven at 80 °C for 1 day. The resulting blue rodlike crystals were washed with DMA three times to remove the unreacted organic link and metal salt to give the fresh ZJU-31 sample. Elemental analysis: Calcd for [Cu2(C48H30O10)(H2O)2]· (DMA)18·(H2O)19 (C120H234Cu2N18O31, %): C, 50.36; H, 7.81; N, 9.34; Found: C, 50.75; H, 8.32; N, 8.88. Selected FTIR (neat, cm−1): 1618, 1507, 1400, 1492, 1260, 1195, 1170, 1023, 861, 796, 743, 592.

Figure 1. Comparison of single X-ray crystal structures of ZJU-30 (a) and ZJU-31 (b) viewed along the a axis.

channels of about 18.0 × 18.0 Å2, 6.7 × 6.7 Å2 along the c axis and 6.7 × 13.0 Å2, 6.0 × 6.0 Å2 along the a axis can observed, respectively. PLATON calculations12 indicated that the solventaccessible volume in ZJU-31 is approximately 36861 Å3 out of the unit cell of 42996 Å3, which constitutes 85.7% of the whole volume of the unit. After removing the coordinated water molecules, the framework density of ZJU-31 is 0.269 g/cm3. Its theoretically calculated BET surface area from its X-ray singlecrystal structure is 3954 m2/g. Topology analysis showed that the previously reported MOF ZJU-30 forms a rarely observed lvt-type network of 42·84 topology, while the noninterpenetrated ZJU-31 exhibits a rarely (62·84)(62·8)2-connected stu-type network, indicating that appropriate design of organic building blocks by

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RESULTS AND DISCUSSION ZJU-31 was obtained as green rodlike crystals via a solvothermal reaction of Cu(NO3)2·2.5H2O and organic ligand H4L in DMA/H2O with addition of hydrogen nitrate at 80 °C for 24 h. The structure of the as-synthesized compound was determined by single-crystal X-ray diffraction analyses, and the phase purity of the bulk material was independently confirmed B

DOI: 10.1021/acs.cgd.5b00675 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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equation Sij = KH(i)/KH(CH4). The calculated C2H6/CH4, C2H4/CH4, and C2H2/CH4 adsorption selectivities are 86.7, 32.3, and 23.8 at 273 K, and 63.9, 29.5, and 18.8 at 298 K, respectively. Obviously, the Henry’s law selectivities for C2s over C1 methane in ZJU-31a are systematically higher than those in ZJU-30a. The C2H2/CH4 separation selectivity of 18.8 at 298 K is also higher than most MOFs, including UTSA-36a16 (13.8), Zn5(BTA)6(TDA)217 (15.5), and Zn4(OH)2(1,2,4BTC)218 (14.7). IAST calculations for mixd gas uptake from C2H6/CH4 (C2H6/CH4 = 50:50), C2H4/CH4 (C2H4/CH4 = 50:50), and C2H2/CH4 (C2H2/CH4 = 50:50) maintained under isothermal conditions at 273 and 298 K are shown in Figures S6−S8 (Supporting Information). Figure 4 shows the IAST calcu-

incorporating the 1,4-dimethoxyphenyl group can effectively block the framework interpenetration/catenation (Figure S3, Supporting Information). As shown in Figure 2, the N2

Figure 2. Comparison of N2 sorption isotherms of ZJU-30a and ZJU31a at 77 K (solid symbols: adsorption; open symbols: desorption).

sorption of ZJU-31a at 77 K exhibits a reversible type-I isotherm, indicating the microporous nature of the framework. ZJU-31a takes up 259 cm3/g of N2 at 77K, the Brunauer− Emmett−Teller (BET) surface area is 1115 m2/g (Figure S4, Supporting Information), and the corresponding pore volume is 0.4 cm3/g. Because of the noninterpenetrated framework nature, ZJU-31a is much more porous than ZJU-30a with a BET surface area of 228 m2/g, although it still does not meet the theoretically calculated value because of framework deformation. The moderately high surface area, suitable pore size, and open copper sites within the framework of ZJU-31a impelled us to research its potential application for the industrially important C2/C1 separation. As shown in Figure 3, ZJU-31a

Figure 4. IAST adsorption selectivities for equimolar C2/C1 gas mixture at 273 and 298 K.

lations of the adsorption selectivity at 273 and 298 K in ZJU31a. For a wide range of pressures to 100 kPa, the IAST adsorption selectivities for C2H6, C2H4, and C2H2 over CH4 are in excess of 9.5, 9.9, and 7.5 at 273 K and 23.4, 22.0, and 22.5 at 298 K, respectively, which are systematically higher than those evaluated in UTSA-33a.19 The C2H6/CH4 IAST selectivities of 23.4 at 298 K is higher than that of JLU-Liu520 (17.6) with a similar surface area. The isosteric heats of adsorption (Figure S9, Supporting Information) are 27.89, 21.42, 22.04, and 19.52 kJ/mol, respectively, for C2H6, C2H4, C2H2, and CH4.

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CONCLUSIONS

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ASSOCIATED CONTENT

Incorporation of a 1,4-dimethoxyphenyl group into a large tetracarboxylic acid leads to the formation of a noninterpenetrated metal−organic framework of the moderately high porosity. The resulting activated ZJU-31a exhibits highly selective separation of C2 hydrocarbons over C1 methane. This work suggested the feasibility to control framework interpenetration through such a decorated organic linker strategy. Because the properties and applications of MOFs are heavily dependent on their porosities and pore sizes/ curvatures, such a decorated organic linker approach will be further explored to finely tune the pores and thus develop multifunctional MOF materials.

Figure 3. CH4 (black), C2H6 (red), C2H4 (green), and C2H2 (blue) sorption isotherms of ZJU-31 at 273 K (a) and 298 K (b) (solid symbols: adsorption; open symbols: desorption).

systematically takes up much more C2 hydrocarbons than C1 methane. The uptake capacities at 1 atm and 273 K for CH4, C2H6, C2H4, and C2H2 are 23.9, 73.1, 69.0, and 85.1 cm3/g, respectively, whereas those at 1 atm and 298 K are 16.6, 67.4, 65.3, and 71.1 cm3/g, respectively. The uptake capacity of ZJU31a for C2H2 at 298 K is comparable to those of most MOFs, including MIL-5313 (72 cm3/g), UTSA-35a14 (65 cm3/g), and Cu(etz)15 (70 cm3/g), which is moderately high. In order to confirm the adsorption selectivities in ZJU-31a, the widely used Henry’s law selectivity and ideal adsorbed solution theory (IAST) calculation are applied to estimate the C2H2/CH4, C2H4/CH4, and C2H6/CH4 adsorption selectivities. The Henry’s law selectivity is figured up according to the

* Supporting Information S

Synthetic route of H4L, TGA, PXRD, N2 sorption isotherm at 77 K, and single X-ray crystallographic data (CCDC 1015571) is given in CIF format. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00675. C

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.C.). *E-mail: [email protected] (G.Q.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 51272231 and 51229201) and the Program for Innovative Research Team in University of Ministry of Education of China (IRT13R54). The Project was supported by the Natural Science Foundation of Zhejiang pvovince, China (LZ15E020001), Grant AX-1730 from the Welch Foundation (B.C.).

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DOI: 10.1021/acs.cgd.5b00675 Cryst. Growth Des. XXXX, XXX, XXX−XXX