A Doubly Interpenetrated Metal–Organic Framework with Open Metal

Mar 21, 2013 - Department of Chemistry, University of Texas at San Antonio, One ... College of Chemistry and Life Sciences, Zhejiang Normal University...
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A Doubly Interpenetrated Metal-Organic Framework with Open Metal Sites and Suitable Pore Sizes for Highly Selective Separation of Small Hydrocarbons at Room Temperature Jianfeng Cai, Jiancan Yu, Hui Xu, Yabing He, Xing Duan, Yuanjing Cui, Chuan-De Wu, Banglin Chen, and Guodong Qian Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg400164m • Publication Date (Web): 21 Mar 2013 Downloaded from http://pubs.acs.org on March 24, 2013

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A Doubly Interpenetrated Metal-Organic Framework with Open Metal Sites and Suitable Pore Sizes for Highly Selective Separation of Small Hydrocarbons at Room Temperature Jianfeng Caia, Jiancan Yua, Hui Xua, Yabing He,d Xing Duana, Yuanjing Cuia, Chuande Wuc, Banglin Chen*,ab and Guodong Qian*,a a

State Key Laboratory of Silicon Materials, Cyrus Tang Center for SensorMaterials and Applications,

Department of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China; b

Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas

78249-0698, USA; c Department of Chemistry, Zhejiang University, Hangzhou 310027, China; d

College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China

ABSTRACT: To immobilize open Cu2+ sites and tune suitable pore spaces, we have realized a doubly interpenetrated metal-organic framework (MOF) [Cu2L(H2O)2]•(DMF)6•H2O (ZJU-30, ZJU=Zhejiang University;

H4L

=

biphenyl-3,3’,5,5’-tetra-(phenyl-4-carboxylic)

acid;

DMF

=

N,N-

dimethylformamide) for the highly selective separation of C2 hydrocarbons over C1 methane with the Henry law’s selectivities of 9.58 to 29.8 in the temperature range of 273 to 298 K.

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Introduction Microporous metal-organic frameworks (MOFs) have been emerging as very promising materials for separation of small hydrocarbons.1 This is mainly because of the unique porosity within MOFs in which the pore sizes can be tuned by the interplay of metal ions/metal-containing clusters and organic linkers to enhance their size-selective separation while the pore surfaces can be functionalized by the immobilization of specific sites, particularly open metal sites, to differentiate their different interactions with hydrocarbons.2-9 For example, we have realized a series of microporous MOFs for highly selective separation of C2H2/C2H4 through deliberate control of pore sizes;7 while Long has targeted the first porous MOF for efficient separation of C2H4/C2H6 by the immobilization of open Fe2+ sites on the pore surfaces of Fe-MOF-74.8 Compared with these two challenging separation, separation of C2 hydrocarbons (C2s) from C1 methane is easier because the C2 hydrocarbons are much larger than C1 methane, so both size selective effects and different interactions among the hydrocarbon substrates and porous MOF host materials can be utilized to improve the separation. We did realize a few porous MOFs for separation of C2/C1 hydrocarbons over the past several years, but mainly focused on how the pore sizes can tune their selectivities.9 It has been foreseen that if the pore sizes can be optimized for size selective effect while the open metal sites can be incorporated simultaneously to differentiate their interactions with C2 and C1 hydrocarbons, more promising porous MOFs should be realized to exhibit higher separation selectivities, though have not been fully explored. The open Cu2+ sites within paddle-wheel Cu2(CO2)4 clusters have been widely utilized for gas storage.10-11 Such open metal sites can be straightforwardly generated through the incorporation of Cu2(CO2)4(solvent) clusters into the MOFs by the self-assembly of copper salt with organic-carboxylic acids, followed by the in situ thermal and/or vacuum activation. It has been well established that when the large pore spaces have been constructed, the pore spaces tend to become smaller by the framework interpenetration, catenation and/or interweaving because “Nature abhors a vacuum”. Although such tendency is not beneficial to construct highly porous MOFs, it can help us to tune the small pores, and thus to enhance gas separation selectivity.12 We use a large tetracarboxylic acid H4L (Scheme 1, H4L = biphenyl-3,3’,5,5’-tetra-(phenyl-4-carboxylic) acid) to construct its copper MOF. The as-synthesized MOF [Cu2L(H2O)2]•(DMF)6•H2O (which we termed as ZJU-30, ZJU=Zhejiang University; DMF=N,Ndimethylformamide) not only has the paddle-wheel Cu2(CO2)4 clusters for the in situ generation of open Cu2+ sites but also has suitable pores of about 4.0 Å to 5.6 Å fulfilled through double framework interpenetration, enabling the activated MOF ZJU-30 as a promising porous material for separating C2

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hydrocarbons from C1 methane with the Henry’s law separation selectivities from 9.58 to 29.8 in the temperature range of 273K to 298K.

Scheme 1. The organic linker H4L used to construct ZJU-30

Experimental Section Materials and Measurements. All the chemicals were commercially available and used without further purification. Elemental analyses for C, H, and N were performed on an EA1112 microelemental analyser. Powder X-ray diffraction (PXRD) patterns were collected in the 2θ=3-60o range on an X’Pert PRO diffractometer with Cu Kα adiation (λ= 1.542Å) at room temperature. Thermogravimetric analyses (TGA) were conducted on a Netszch TGA 209 F3 thermogravimeter with a heating rate of 10/min in an N2 atmosphere. Gas Sorption Measurements. A Micromeritics ASAP 2020 surface area analyser was used to measure gas adsorption. In order to remove guest solvent molecules in the framework, the fresh sample of ZJU-30 was exchanged with acetone for 10 times and then activated at 333K under high vacuum for 12 h until the outgas rate was < 5 mHg/min prior to measurements. The sorption measurement was maintained at 77K with liquid nitrogen and 273K with ice-water bath (slush), respectively. As the center-controlled air condition was set up at 25.0℃, a water bath of 25.0℃ was used for adsorption isotherms at 298.0K. Isotherm data were analysed using the virial equation: ln(n/p) = A0+A1n+A2n2+…….

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where the p is pressure, n is the amount adsorbed, and A0, A1, etc., are virial coefficients. The Henry’s Law constant (KH) is equal to exp(A0), and the selectivity can be obtained from the constant KH. Synthesis of ZJU-30. H4L was synthesized according to the reference procedure.13 A mixture of H4L (5.00mg, 0.0079mmol) and CuCl2•2H2O (10.00mg, 0.0587mmol) was dissolved in DMF/H2O (2.4mL, 5:1, v/v) in a screw-capped vial. After concentrated hydrochloric acid (10µL) (37.5%, aq.) were added to the mixture, the vial was capped and placed in an oven at 60℃ for 48 h. The resulting green block single crystals were washed with DMF several times to give ZJU-30. Elemental analysis: Calcd. for [Cu2L(H2O)2]• (DMF)6•H2O (C58H70N6O17Cu2): C, 55.72; H, 5.64; N,6.72; Found: C, 55.83; H, 5.63; N: 6.86. Ray Collection and Structure Determination. Crystallographic measurements for ZJU-30 were taken on an Oxford Xcalibur Gemini Ultra diffractometer with an Atlas detector using graphitemonochromatic Mo Kα radiation (λ = 0.71073 Å) at 293 K. The determinations of the unit cells and data collections for the crystals of ZJU-30 were performed with CrysAlisPro. The data sets were corrected by empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. All structures were determined by direct methods and refined by the fullmatrix least-squares method with the SHELX-97 program package. All non-hydrogen atoms, including solvent molecules, were located successfully from Fourier maps and were refined anisotropically. H atoms on C atoms were generated geometrically. The H atoms of the water molecules were clearly visible in difference maps and were handled in the subsequent refinement with fixed isotropic displacement parameters. Crystallographic data are summarized in Table S1 (CCDC 928743).

Results and Discussion Self-assembly of CuCl2 with H4L in DMF/H2O readily forms ZJU-30. Its crystal structure has been established by single crystal structure determination, while the phase purity has been confirmed by powder x-ray diffraction, elemental analysis. Single crystal structure study indicates that the framework is indeed constructed from paddle-wheel Cu2(CO2)4(H2O) clusters with organic carboxylate L. Because the organic linker L is quite large, the resulting ZJU-30 is a doubly interpenetrated framework. There exist three-dimensional intersecting channels of about 4.0 Å×4.0 Å, 5.6 Å×5.6 Å and 5.0 Å×5.0 Å in the structure of ZJU-30 (Figure 1). PLATON calculations showed that the void space accounts approximately 63.6% of the whole crystal volume (9167.0 Å3 out of the14422.3 Å3 per unit cell

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volume).14 Topologically, each paddle wheel Cu2(CO2)4 cluster is linked to four L connectors to form a rarely observed lvt-type network of 42•84 topology (Figure S1). TG analysis indicates that ZJU-30 loses solvent molecules in the temperature range from room temperature to 200 ℃ (Figure S2). PXRD studies shows that the activated ZJU-30a keeps crystalline feature (Figure S3), although the peaks of the activated ZJU-30a are different from those of the assynthesized ZJU-30 because of the flexible nature of such interpenetrated framework. The assynthesized ZJU-30 was exchanged by acetone for 10 times followed by the thermal/vacuum activation at 333 K to generate activated ZJU-30a. The N2 sorption isotherm at 77K showed that ZJU-30a displays type-I sorption behaviour with the Brunauer-Emmett-Teller (BET) surface area of 228 m2•g-1 (Figure S4).

Figure 1. Single X-ray crystal structure of ZJU-30 indicating the pores viewed along (a) the a axis and (b) the c axis. Establishment of the permanent porosity of ZJU-30a encourages us to examine its potential application for the industrially important C2/C1 separation. As shown in Figure 2, ZJU-30a exhibits different adsorption capacities to CH4, C2H6, C2H4 and C2H2 at both 273 and 298 K. As expected, ZJU30a systematically adsorbs much more C2 hydrocarbons than C1 methane. At 298 K, ZJU-30a can take up a moderate amount of C2H6 (47.7 cm3g-1), C2H4 (44.3 cm3g-1) and C2H2 (52.6 cm3g-1), but basically

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much smaller amount of CH4 (13.9 cm3g-1) at 1 atm, indicating that ZJU-30a is a promising material for highly selective adsorptive separation of C2 hydrocarbons from CH4 at room temperature. We do the virial analyses studies on the temperature dependent sorption isotherms of these hydrocarbons to figure out their sorption enthalpies and their separation selectivities. Some virial parameters are summarized in Table 1. The enthalpies at zero surface coverage, calculated from A0 virial parameters, are 29.7, 28.1, 31.3, and 18.2 kJ mol-1, respectively, for C2H6, C2H4, C2H2, and CH4. It needs to be mentioned that the adsorption enthalpies for C2H2 of 31.3 kJ/mol in ZJU-30 at zero coverage is even higher than that of MOF-505 of 24.7 kJ/mol with high density of open Cu2+ sites, indicating that the suitable pore sizes in ZJU-30 further play the important roles for the stronger interactions between C2H2 and pore surfaces on ZJU-30. The Henry’s law selectivities for C2H6, C2H4 and C2H2 over CH4, calculated based on the equation Sij=KH(i)/KH(CH4), are 29.8, 16.5 and 15.5 at 273 K, 19.5, 11.5 and 9.58 at 298 K, respectively, which are high and comparable to those found in the microporous UTSA-36a of smaller pores.9a

60

(a)

(b)

70

40

3

3

Gas Uptake (cm /g)

50

Gas Uptake (cm /g)

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30 20 10 0

0

200

400

600

60 50 40 30 20 10 0

800

0

200

P (mmHg)

400

600

800

P (mmHg)

Figure 2. CH4 (black), C2H6 (red) C2H4 (green) and C2H2 (blue) sorption isotherms of ZJU-30 at 298K (a) and 273K (b) (Solid symbols: adsorption, open symbols: desorption). Table 1 Virial graph analyses data for ZJU-30a and its C2H6/CH4, C2H4/CH4 and C2H2/CH4 separation selectivities Adsorbate

CH4 C2H6

T/K

273 298 273

A0

A1

(In(molg-1 Pa-1)

(g mol-1)

-18.089 -18.761 -14.693

-255.324 -284.553 -572.987

R2

KH

Si/CH4a

(mol g-1 Pa-1) 0.995 0.997 0.992

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Qst, n=0 (kJ mol-1) 18.2

29.8

29.7 6

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298 -15.790 -556.680 0.999 1.388×10-7 19.5 -7 273 -15.285 -477.354 0.999 2.300×10 16.5 C2H4 28.1 -8 298 -16.322 -533.587 0.999 8.155×10 11.5 273 -15.346 -604.051 0.995 2.164×10-7 15.5 C2H2 31.3 -8 298 -16.501 504.660 0.999 6.819×10 9.58 a, The Henry’s law selectivity for gas component i over CH4 at the speculated temperature is calculated based on the equation Sij=KH(i)/KH(j)

Conclusions In summary, we have synthesized a porous metal-organic framework ZJU-30 with open metal sites and suitable pore sizes for the highly selective separation of C2 hydrocarbons over C1 methane. Both open metal sites and suitable pore sizes play the important roles to enforce the high separation selectivity through their different interactions with hydrocarbons and by their size selective effects. The effects of some functional groups such as –NH2, -OH and –F on the hydrocarbon separation are still not clear yet. The strategy to immobilize open metal sites and to tune suitable pores within porous MOFs will be very promising to realize highly selective adsorbents for efficient separation of small hydrocarbons, and will certainly be extensively explored to target some industrially useful porous MOF materials for the separation of small hydrocarbons at ambient temperature in the near future. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grants 51272231, 51229201, and 51010002) and Grant AX-1730 from the Welch Foundation (BC). Supporting Information Available: Framework topology, TGA, PXRD, N2 sorption isotherm at 77 K and IR spectrum, single x-ray crystallographic data (CCDC 928743) is given in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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TOC Figure

A doubly interpenetrated metal-organic framework with open metal sites and suitable pore sizes has been realized for highly selective separation of small hydrocarbons at room temperature.

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