A heterometallic microporous MOF exhibiting high hydrogen uptake

Microporous and Mesoporous Materials 165 (2013) 20–26

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A heterometallic microporous MOF exhibiting high hydrogen uptake Wei Wei a,1, Zhengqiang Xia a,1, Qing Wei a,⇑, Gang Xie a, Sanping Chen a,⇑, Chengfang Qiao b, Guochun Zhang b, Chunsheng Zhou b a Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710069, China b Department of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Comprehensive Utilization of Tailing Resources, Shangluo University, Shangluo 726000, China

a r t i c l e

i n f o

Article history: Received 7 May 2012 Received in revised form 1 July 2012 Accepted 10 July 2012 Available online 3 August 2012 Keywords: Hydrogen storage Microporous materials Heterometallic compound Metal–organic framework Heat of adsorption

a b s t r a c t An unusual heterometallic microporous metal–organic framework Cu3Cr(TSTC)2(H2O)4xG (1) (H4TSTC = trans-stilbene-3,30 ,5,50 -tetracarboxylic acid; G = guest molecules) was solvothermally synthesized and characterized. The structural analysis reveals that compound 1 features a 3D distorted NbOtype network with high solvent-accessible volume of 68.5% assembled by two kinds of distinct cages. The adsorption measurements (77 K) results show that compound 1 has a BET surface area of 2485 m2/g, exhibits the great heat of hydrogen adsorption in the range of 7.0–8.4 kJ/mol and significantly higher H2 hydrogen storage of 2.70 wt.% under 1 bar than those of IRMOF-1 (1.32 wt.%), MOF-177 (1.25 wt.%), UMCM-2 (1.40 wt.%), UMCM-150 (2.10 wt.%) and HKUST-1 (2.18 wt.%). Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Metal–organic frameworks (MOFs) have been developed explosively over the past decade, owing to their potential applications in gas storage, molecular separation, and heterogeneous catalysis [1– 7]. In particular, microporous MOF materials possess exceptionally high surface areas, large free volumes and have low weight densities, which are promising candidates for commercial applications. Especially for the hydrogen storage, microporous MOF materials have been recognized as the potential candidates with a non-dissociative form [8–11]. With regard to hydrogen storage for mobile applications, gravimetric and volumetric storage densities approaching those prescribed by the US Department of Energy [12] have been observed for the highest surface area metal–organic frameworks at cryogenic temperatures [13–15]. Nevertheless, adsorption studies on MOFs such as microporous manganese formate and other microporous metal organic materials, have indicated that there was no direct linear correlation between surface area and hydrogen uptake [16]. Actually, many frameworks with high surface area had relatively low hydrogen uptake [17,18]. Compared with those at cryogenic temperatures, the H2 storage ⇑ Corresponding authors. Tel.: +86 029 88302604; fax: +86 029 88303798. 1

E-mail addresses: [email protected] (Q. Wei), [email protected] (S. Chen). Both authors contributed equally to this work.

1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.07.036

capacity of these materials significantly reduced at ambient temperature, which was assumed that these frameworks showed poor affinity for H2. Indeed, the low-coverage isosteric heat of H2 adsorption within these materials typically lied in the range of 5 to 7 kJ/mol between 1.5 and 30 bar at 298 K [19]. It is thus believed that an optional strategy on improving the hydrogen uptake of microporous MOFs is to increase the heat of H2 adsorption. One method to increase heat of adsorption is employing different coordination centers in microporous MOFs, which could provide a direct way to increase binding sites and stronger electrostatic interactions than the dispersion-type interactions for physisorbed molecules [20–24]. Compared with the widely investigated homometallic microporous MOFs, the heterometallic microporous MOFs for H2 storage were relatively few reported. Of special interest, the heterometallic microporous MOFs with two or more different metal activation centers may yield unexpected outcoming for H2 storage. In this contribution, through the reaction of Cr(II) ion, Cu(II) ion and a flexible linker, trans-stilbene-3,30 ,5,50 -tetracarboxylic acid ligand (H4TSTC), a heterometallic microporous MOF Cu3Cr(TSTC)2 (H2O)4xG (1) (G = guest molecules) was solvothermally synthesized and characterized by single-crystal X-ray diffraction, ICP, TGA and PXRD. The surface area and hydrogen adsorption capacity of compound 1 were also determined by nitrogen and hydrogen adsorption measurements, respectively.

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All the reagents and solvents were commercially available and used without further purification. Thermogravimetric analysis (TGA) experiment was performed on a NETZSCH TG 209 instrument with a heating rate of 5 °C/min from 30 to 600 °C in a dynamic nitrogen flow. Inductively coupled plasma (ICP) analysis was performed on a Perkin–Elmer Optima 3300 DV ICP spectrometer. The sorption isotherm for N2 was measured by a micromeritics ASAP 2020 instrument at 77 K. The pressure–composition– temperature (P–C–T) curves for hydrogen adsorption were determined by an HTP1-V Hiden Analytical Ltd. The powder X-ray diffraction (PXRD) patterns were measured on a Bruker D8 Advance diffractometer (Cu-Ka, 1.5418 Å).

monochromated Mo-Ka (k = 0.71073 Å) using x and u scan modes. The structure was solved by direct methods and refined with full-matrix least-squares refinements based on F2 using SHELXS-97 and SHELXL-97 [25,26]. All framework metal atoms were located from the E-maps, and other non-hydrogen atoms were located in successive difference Fourier syntheses and refined anisotropically. Because a large region of badly disordered solvent molecules occurred in the cavity of this compound, which were difficult to be modeled as discrete atomic sites, the routine PLATON/ SQUEEZE [27] was employed to calculate and remove diffuse electron density associated with the badly disordered solvent molecules. In the end, a set of solvent-free diffraction intensities was generated. Crystal data and details of the data collection and the structure refinement are given in Table S1. CCDC 836345 for compound 1 contains the Supplementary crystallographic data for this paper [28].

2.2. Synthesis of Cu3Cr(TSTC)2(H2O)4xG (1)

3. Results and discussion

A mixture of H4TSTC (0.0357 g, 0.1 mmol), CrCl2 (0.0123 g, 0.1 mmol), Cu(NO3)23H2O (0.0241 g, 0.1 mmol), NaOH (0.0160 g, 0.1 mmol), water (5 mL) and DMF (10 mL) was sealed in a 25 mL stainless steel reactor with a Teflon-liner and kept heating at 160 °C for 3 days, then cooled to room temperature at a rate of 10 °C/h. The crystalline products were collected upon being washed with DMF and H2O. The crystals of compound 1 were obtained in 38% yield based on Cu(II). ICP analysis of compound 1 gave the contents of Cu and Cr as 11.02 and 2.97 wt.%, respectively, indicating a Cu/Cr ratio of about 3:1. SEM and optional images of the crystalline samples are shown in Fig. S1.

3.1. Structural description of Cu3Cr(TSTC)2(H2O)4xG (1)

2. Experimental 2.1. Materials and physical measurements

2.3. X-ray data collection and structure determination A suitable single crystal with a dimension of 0.17  0.15  0.09 mm for compound 1 was selected for single-crystal X-ray diffraction analysis at 293 ± 2 K. The data were collected on a BRUKER SMART APEX II CCD X-ray diffractometer with graphite

Single crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the rhombohedral crystal system, space group R3, which features a 3D distorted NbO-type network constructed by two distinct cages. The asymmetric unit of compound 1 consists of four crystallographically unique M centers (three Cu(II) ions and one Cr(II) ion), two completely deprotonated trans-stilbene3,30 ,5,50 -tetracarboxylate ligands, four coordinated water and disorder solvent molecules (Fig. 1). Each M center is five-coordinated by four oxygen atoms from four different TSTC4 ligands and one oxygen atom from one coordinated water molecule, resulting in a distorted square-pyramidal geometry. In compound 1, there exist two kinds of similar paddlewheel building blocks [M2C4O8] (which can be treated as [Cu2C4O8] and [CuCrC4O8]) formed by four carboxylates bridging binuclear homometallic copper atoms or heteronuclear copper-chromium atoms (Fig. 2a). Four [Cu2C4O8] and two [CuCrC4O8] SBUs are linked by

Fig. 1. Local coordination environment of compound 1.

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Fig. 2. (a and b) [Cu2C4O8] dimer unit (blue, square SBU); [CuCrC4O8] dimer unit (purple, square SBU); TSTC4 unit (gray, rectangular building block). (c and d) The octahedral cage A in compound 1. (e and f) The cuboctahedral cage B in compound 1 (the yellow sphere represents the largest sphere occupied the cavity, taking into account the van der Waals radii of the atoms). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

twelve ligands to an octahedral cage (A—Fig. 2c and d). While six [Cu2C4O8] and six [CuCrC4O8] SBUs are connected by six ligands into a cuboctahedral cage (B—Fig. 2e and f), in which the six square faces are occupied by the six ligands and the eight triangular faces share with eight neighboring octahedral cages. Viewed from c axis, a cuboctahedral cage is surrounded by six octahedral cages in ab plane (Fig. S2) and two others octahedral cages connect the top and bottom of the cuboctahedral cage along c axis. The two distinct cages are further intertwined each other to form a 3D microporous framework, which provides an accessible interior surface for gas storage (Fig. S3). As seen in Fig. S4a, for further understand the construction, the components of compound 1 could be viewed as 4-connected nodes

(dimer units and ligands). The whole 3D framework is assembled by these three types of nodes to an extended NbO-type network with two types of oval-shaped windows, one has an open window size of 15.40(11)  14.99(5) Å (purple) and the other is 15.03(3)  14.99(17) Å (blue) in Fig. S4b. The accessible pore volume from the single crystal structure is 68.5% as calculated using the PLATON program. 3.2. TGA and PXRD In order to investigate the thermal stability of the framework, TG experiment was performed on microcrystal samples of compound 1 under a nitrogen atmosphere with a heating rate of

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Fig. 3. TG curve of compound 1.

and activated at 150 °C for 10 h until the outgas rate was <4 mm Hg min1. To check the permanent porosities of the guest-free compound 1, N2 sorption experiment at 77 K was carried out. The isotherm obtained with N2 at 77 K reveals a typical Type-I adsorption curve for compound 1 (Figs. 5 and 6). The nitrogen adsorption abruptly increases at the start of the experiment and reaches a plateau of 698 cm3 (STP)/g at P/P0 = 0.06, indicating a uniform microporous structure for compound 1. The adsorption isotherm data reveal that compound 1 has a BET surface area of 2485 m2/g, which is lower than 2793 m2/g fitted by Langmuir equation while greater than 2034 m2/g calculated using Material Studio 5.0. Based on these data, a total pore volume of 1.086 cm3/g is determined for compound 1. 3.4. Hydrogen adsorption Gravimetric measurement obtained for H2 adsorption on the activated compound 1 is given in H2 adsorption measurements at the high pressures are given by Ntotal = Nexcess + qhydrogenVpore, where Vpore is the pore volume calculated from N2 isotherm at 77 K. The density of H2 (qhydrogen) at 77 K and various pressures is obtained from the Soave modification of the Redlich-Kwong equation of state: Fig. 4. Powder X-ray diffraction patterns of compound 1.

5 °C/min from 30 to 600 °C. As shown in Fig. 3, the first weight loss of 41.9% occurs from the ambient temperature to 120 °C. It is followed that the remaining substance keeps stable up to 210 °C and then undergoes consecutive steps with a mass loss of 39.7% from 210 to 450 °C, corresponding to the decomposition of the organic ligands. As can be seen from Fig. 4, the PXRD patterns for compound 1 reveal that there is no phase change recorded when the sample was treated in the temperature region of 418–448 K. 3.3. Nitrogen isotherm To attain the guest-free sample, referred to the results of the TGA and PXRD, nearly 70 mg crystal sample of compound 1 was exchanged several times with methanol for two days, air-dried

P¼

RT aa  V m  b V m ðV m þ bÞ

a¼

0:42747R2 T 2c 0:08664RT c ; b¼ Pc Pc

2 a ¼ ð1 þ ð0:48508 þ 1:55171x  0:51613x2 Þð1  T 0:5 r ÞÞ ; T r ¼

ð1Þ

T Tc

for H2, a = 1.202exp(0.30288Tr). As shown in Fig. S5, H2 density at 77 K is calculated using the Redlich–Kwong–Soave equation and the density of H2 at 60 bar and 77 K is 0.0158 g/cc.The H2 adsorption data in Fig. 7 show that compound 1 reaches saturation of 50.32 mg/g at 28 bar, and the large pore volume allows the total uptake of compound 1 to reach 60.32 mg/g at 60 bar which is higher than the former experimental value 44.01 mg/g at 60 bar.

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Fig. 5. N2 adsorption isotherm for the activated compound 1 (77 K).

Fig. 6. H2 adsorption isotherm for compound 1 at 77 K under 1 bar.

3.5. Isosteric heat of hydrogen adsorption As shown in Fig. 8, the H2 isotherm is recorded by the gravimetric method at 77 K. The heat of adsorption is between 8.4 and 7.0 kJ/ mol, indicating compound 1 has a good affinity for H2 at low pressures.Because H2 is supercritical under the experimental conditions, the DR Eq. (2) [29,30] is used to analyze hydrogen isotherms.

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ffi W0 RT ln P ¼ RT ln P0q  bE0 ln W

ð2Þ

where W0 is the estimated maximum uptake obtained from the Langmuir plot (Fig. S6, W0 = 1/B), P0q is the saturated vapor pressure

of the quasi-vaporized supercritical gas, and bE0 is the adsorption energy. As shown in Fig. S6, the adsorption energy bE0 is the slope of the plot.In order to quantify the strength of interaction between the hydrogen and host framework, the affinity of the framework for H2 is determined by the adsorption energy bE0 and the vaporization heat DHv of solvent. Thus, the isosteric heat of adsorption, qstU = 1/e, at the fractional filling of 1/e can be obtained by using qstU = 1/e = bE0 + DHv, where DHv is the heat of vaporization of hydrogen (DHv = 0.92 kJ/mol) [31]. Compound 1 shows a remarkable affinity for H2 with an isosteric heat of adsorption qstU = 1/e at 77 K of 8.235 kJ/mol, which reveals that the large heat of adsorption of compound 1 could prompt the H2 adsorption to reach saturation at low P/P0.

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Fig. 7. Excess (points) and total (line) hydrogen uptakes for compound 1 at 77 K under 60 bar.

Fig. 8. Heat of H2 adsorption for compound 1 at 77 K as a function of gravimetric uptake. Table 1 Absorption characteristics for selected MOFs.

a b c d e f g

Material

Surface area (m2/g) (BET/calcda)

Porosityb (%)

H2 sorption at 1 bar (wt.%)

The highest H2 sorption (wt.%)

Heat of absorption (kJ/mol)

Compound 1 IRMOF-1c MOF-177d UMCM-2e UMCM-150f HKUST-1g

2485/2034 3800/3110 4746/3654 5200/4732 2300/1980 1507/1403

68.5 77.9 80.0 83.1 69.6 50.0

2.70 1.32 1.25 1.40 2.10 2.18

5.0 4.7 7.5 6.9 5.7 3.6

7.0–8.4 4.8–4.2 – 7.3 4.2–6.4 6.6–5.8

Calculated using Material Studio 5.0. Calculated using PLATON. Data from Refs. [17,18]. Data from Refs. [17,18,32]. Data from Ref. [33]. Data from Ref. [34]. Data from Refs. [35–37].

at at at at at at

28 bar 50 bar 70 bar 46 bar 45 bar 50 bar

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As shown in Table 1, compared with the well-known MOFs, the value of 2.70 wt.% of H2 adsorption for compound 1 (under 1 bar) is higher than 1.32 wt.% of IRMOF-1, 1.25 wt.% of MOF-177, 1.40 wt.% of UMCM-2, 2.10 wt.% of UMCM-150, 2.18 wt.% of HKUST-1. While the highest H2 sorption value of 5.0 wt.% of compound 1 is slightly less than 5.7 wt.% of UMCM-150, 6.9 wt.% of UMCM-2 and 7.5 wt.% of MOF-177. From the heat of adsorption value (7.0–8.4 kJ/mol) listed in Table 1 for compound 1, it would be not difficult to form the impression that the isosteric heat predominate the adsorption capacity for H2 at a low pressure while the pore volume of microporous MOFs becomes determinant at a high pressure. 4. Conclusions In summary, this work reports the synthesis and structure of an unusual heterometallic microporous MOF Cu3Cr(TSTC)2(H2O)4xG (1) (H4TSTC = trans-stilbene-3,30 ,5,50 -tetracarboxylic acid; G = guest molecules) with high solvent-accessible volume of 68.5% built from two kinds of distinct cages. Gas sorption studies (77 K) reveal that compound 1 features a higher heat of hydrogen adsorption coverage of 7.0–8.4 kJ/mol, leading to greater hydrogen uptake of 2.70 wt.% under 1 bar, in contrast to those of some wellknown MOFs. The results endow compound 1 a promising candidate for hydrogen storage, indicating the great isosteric heat and high pore volume are beneficial to H2 uptake. Acknowledgments We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 21173168 and 21127004), and the Nature Science Foundation of Shaanxi Province (Grant Nos. 11JS110, 2010JK882, 2010JQ2007 and 08JK459). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2012. 07.036. References [1] S.Q. Ma, D.F. Sun, J.M. Simmons, C.D. Collier, D.Q. Yuan, H.C. Zhou, J. Am. Chem. Soc. 130 (2008) 1012–10166. [2] R. Matsuda, R. Kitaura, S. Kitagawa, Y. Kubota, R.V. Belosludov, T.C. Kobayashi, Nature 436 (2005) 238–241. [3] R.E. Morris, P.S. Wheatley, Angew. Chem. Int. Ed. 47 (2008) 4966–4981.

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