Synthesis and hydrogen-storage behavior of metal organic framework MOF-5

international journal of hydrogen energy 34 (2009) 1377 1382 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he Synthesis and hydrogen-storage behavior of metal organic framework MOF-5 Jinping Li a, Shaojuan Cheng b, Qiang Zhao a, Peipei Long a, Jinxiang Dong a, * a Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan 030024, Shanxi, PR China b Department of Environmental and Chemical Engineering, Luoyang Institute of Science and Technology, Luoyang, Henan 471023, PR China article info Article history: Received 1 December 2007 Received in revised form 15 July 2008 Accepted 14 November 2008 Available online 27 December 2008 Keywords: Metal organic framework MOF-5 Synthesis Hydrogen storage abstract Metal organic framework MOF-5 (Zn 4 O(BDC) 3 ), a microporous material with a high surface area and large pore volume, was synthesized by three approaches: direct mixing of triethylamine (TEA), slow diffusion of TEA, and solvothermal synthesis. The obtained materials were characterized by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, and nitrogen adsorption, and their hydrogen-storage capacities were measured. The different synthesis methods influenced the pore-structure parameters, morphologies and hydrogen-storage behavior of the obtained MOF-5. MOF-5 synthesized by the solvothermal approach showed a higher surface area and larger pore volume than the samples prepared by the other two approaches. Measurements of the hydrogen-storage behavior showed that the hydrogen-storage capacity was correlated with the specific surface area and pore volume of MOF-5. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. 1. Introduction Hydrogen has long been considered to be an ideal alternative to fossil-fuel systems and much work has now been done on its storage [1,2]. There are four main methods of hydrogen storage: as a liquid; as compressed hydrogen; in the form of metal hydrides; and by physisorption [3]. Numerous materials are being investigated for storing hydrogen by physisorption, including various forms of carbon [4], zeolites [5,6] and metal organic frameworks (MOFs) [7,8]. Recently, metal organic frameworks have attracted great attention for hydrogen storage because they are amenable to being designed [9,10], have extraordinary permanent porosity [11,12], and a large pore volume [13,14]. A series of MOF-n materials with various structures were reported in 2000 by Yaghi and co-workers [15]. One member of the MOF-n family, MOF-5, has a zeolite-like framework in which inorganic [Zn 4 O] 6þ groups are joined to an octahedral array of [O 2 C C 6 H 4 CO 2 ] 2 (1,4-benzendicarboxylate, BDC) groups to form a robust and highly porous cubic framework of space group (Fm 3m) with a ¼ 25.6690(3) Å and V ¼ 16,913.2(3) Å 3.Intheir first paper, they reported that MOF-5 showed high hydrogenstorage capacities, 4.5 wt% at 77 K and 0.8 bar and 1 wt% at room temperature and 20 bar [16]. Later, this group reported that the maximum hydrogen uptake of MOF-5 could reach 1.32 wt% at 1 bar and 77 K [17]. Panella and Hirscher [18] reported a hydrogen-storage value of 1.6 wt% for MOF-5 at 77 K and pressures above 10 bar. Nevertheless, the adsorption capacity of MOF-5 is very low at room temperature: less than 0.2 wt% at pressures up to 67 bar. More recently, Panella et al. [19], showed that the hydrogen uptake of MOF-5 can reach a saturation value of 5.1 wt% at 77 K and over 80 bar due to the synthesis of single-crystal MOF-5. Moreover, there are considerable differences in the hydrogen-storage capacities of MOF-5 * Corresponding author. Tel.: þ86 351 6010550x8; fax: þ86 351 6111178. E-mail address: dongjinxiangwork@hotmail.com (J. Dong). 0360-3199/$ see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.11.048

1378 international journal of hydrogen energy 34 (2009) 1377 1382 Table 1 Summary of previous research on MOF-5. Year BET SA Langmuir SA Micropore SA Micropore volume (cm 3 /g) H 2 uptake at 77 K (wt%) Reference 1999 2900 0.61d0.54 4.5, 0.8 bar [16] 2003 666 722 535 0.21 [20] 650 700 526 0.2 [20] 2004 3362 1.32, at 1 bar [17] 2005 572 1014 519 0.28 1.6, at 10 bar [18] 2006 2296 3840 5.1, >80 bar [19] synthesized by different approaches, and even by the same approach. With reference to different reports of the physical characteristics (Table 1) and hydrogen-storage values for MOF-5, our great interest lies in the synthesis of samples by three different methods direct mixing of triethylamine (TEA), slow diffusion of TEA, and solvothermal synthesis, and investigation of their hydrogen-storage capacities. In this paper, the synthesis conditions of MOF-5 were studied. Typical samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric (TG) analysis, and nitrogen adsorption. The hydrogen-storage capacity was measured at 77 K by a P C T apparatus. 2. Experimental 2.1. Materials Zn(NO 3 ) 2 $6H 2 O, 1,4-benzenedicarboxylic acid (H 2 BDC), N,Ndimethylformamide (DMF), and TEA were of analytical reagent grade from commercial sources and were used without further purification. 2.2. Synthesis The procedure for synthesizing MOF-5 given in the literature [18 20] was modified, and three different synthesis methods were developed. The synthesis methods are described below. 2.2.1. Direct mixing of TEA Zn(NO 3 ) 2 $6H 2 O and H 2 BDC were dissolved in DMF with mild stirring, then TEA was added to the solution under agitation. The system was sealed and stirred at room temperature for 2 h. The white products were collected by centrifugation, washed with DMF and dried at 373 K for 3 4 h. 2.2.2. Slow diffusion of TEA The synthesis solution containing Zn (NO 3 ) 2 $6H 2 O, H 2 BDC, and DMF was placed in a large beaker. TEA was placed in a smaller beaker, which was placed in the center of the larger beaker. The system was sealed with film and kept under static conditions for two days at room temperature. The white products were collected by centrifugation and washed with DMF. Finally, they were dried at 373 K for 3 4 h. 2.2.3. Solvothermal synthesis Zn(NO 3 ) 2 $6H 2 O and H 2 BDC were dissolved in DMF. Then the solution was transferred into a Teflon-lined autoclave, which was then heated at 373 K for 24 h. The reaction products were cooled to room temperature, and the solids were collected by centrifugation, washed with DMF, and dried at room temperature. 2.3. Characterization Powder XRD patterns of the dried samples were recorded on a Rigaku D/Max 2500 powder X-ray diffraction apparatus, using Cu Ka1 radiation and operating at 40 kv and 100 ma. The relative crystallinity was calculated by comparing the diffraction intensities of the four major peaks at 2q ¼ 6.8, 9.6, 13.7, and 15.4. The morphologies of the materials were Table 3 Effect of TEA on MOF-5 synthesis in the slowdiffusion method. Sample TEA/H 2 BDC Phase Relative crystallinity (%) 1 0 Amorphous 2 4.94/1 MOF-5 38 3 7.91/1 MOF-5 46 4 10.9/1 MOF-5 36 Table 2 Effect of TEA on MOF-5 synthesis in the directmixing method. Sample TEA/H 2 BDC Phase Relative crystallinity (%) 1 0 Amorphous 2 4.94/1 MOF-5 44 3 7.91/1 MOF-5 52 4 10.9/1 MOF-5 43 Table 4 Effect of DMF on MOF-5 synthesis by the solvothermal method. Sample DMF/H 2 BDC Phase Relative crystallinity (%) 1 81.7 MOF-5 88 2 129.2 MOF-5 100 3 226.1 MOF-5 99 4 290.7 MOF-5 82

international journal of hydrogen energy 34 (2009) 1377 1382 1379 Intensity c b hydrogen storage by metal alloys. P C T desorption isotherms were measured at 77 K. To ensure the accuracy of the measurements, blank measurements were made using inert materials such as sand and cullet. The samples were heated up to 523 K and kept at this temperature for 12 h under vacuum prior to the measurements. 3. Results and discussion 5 10 15 20 25 30 35 Two theta (degree) Fig. 1 XRD patterns of MOF-5 samples produced by: (a) direct mixing of TEA; (b) slow diffusion of TEA; (c) solvothermal synthesis. observed on a JSM-6700F scanning electron microscope. TG analysis was carried out in air with a heating rate of 10 K/min using a Netzsch STA409C balance. N 2 adsorption was measured on an ASAP 2000 M gas-adsorption apparatus at liquid-nitrogen temperature. 2.4. Hydrogen-storage measurement Hydrogen-storage measurements were performed using a volumetric setup that had previously been tested for a 3.1. Synthesis conditions The effects of the synthesis conditions on the formation of MOF-5 were investigated in the direct-mixing method, and the synthesis conditions were optimized. It was found that MOF-5 with the highest relative crystallinity was synthesized by direct-mixing with Zn(NO 3 ) 2 $6H 2 O/H 2 BDC ¼ 2/1, TEA/ H 2 BDC ¼ 7.91/1, DMF/H 2 BDC ¼ 258.5/1 and a reaction time of 2h.Table 2 shows that TEA was essential for the synthesis of MOF-5. In the slow-diffusion method, the effect of TEA on MOF-5 synthesis was also investigated at Zn(NO 3 ) 2 $6H 2 O/H 2 BDC ¼ 2/ 1, DMF/H 2 BDC ¼ 258.5/1 and with a reaction time of 48 h, as shown in Table 3. This also indicates that the presence of TEA was essential for the synthesis of MOF-5. TEA was not used in the solvothermal method, and a much higher crystallinity was achieved in its absence. However, it was found that the DMF/H 2 BDC ratio had a significant influence on the crystallinity of MOF-5. Table 4 shows that MOF-5 with the highest relative crystallinity was synthesized at a DMF/H 2 BDC ratio of 58.5 at 373 K for a reaction time of 24 h. Fig. 2 SEM images of MOF-5 samples produced by: (a) direct mixing of TEA; (b) slow diffusion of TEA; (c) solvothermal synthesis.

1380 international journal of hydrogen energy 34 (2009) 1377 1382 350 300 Vol Adsorbed (cm 3 /g) 250 200 150 100 50 0 0.0 0.2 0.4 0.6 0.8 1.0 P/Po direct mixing of TEA slow diffusion of TEA solvothermal synthesis TG (wt%) 0.5 0.0-0.5-1.0-1.5-2.0-2.5-3.0-3.5-4.0-4.5-5.0-5.5-6.0 direct mixing of TEA slow diffusion of TEA solvothermal synthesis 300 400 500 600 700 800 Temprature (K) Fig. 3 N 2 adsorption isotherms of MOF-5 samples synthesized by the three different methods. Fig. 4 TG curves of MOF-5 samples synthesized by the three different methods. Single-crystalline MOF-5 was synthesized by the solvothermal method, but the methods involving direct mixing or slow diffusion of TEA produced powder samples. Obviously, the solvothermal synthesis was preferable for the growth of MOF-5 with high crystallinity. 3.2. Characterization XRD patterns of MOF-5 samples synthesized by the three different methods are shown in Fig. 1. Sample was produced by direct mixing of TEA, sample by slow diffusion of TEA and sample by solvothermal synthesis. The peak positions and relative crystallinities of all the samples were identical to those of MOF-5 samples reported in the literature [18], indicating that the synthesized samples consisted of MOF-5 with different degrees of crystallinity. The results indicated that the solvothermal method in the absence of TEA gave a higher relative crystallinity than the direct-mixing or slow-diffusion method in the presence of TEA. The morphologies of representative samples produced by the different methods were observed by means of SEM. As shown in Fig. 2, the samples synthesized by direct mixing and slow diffusion of TEA consist of small irregular particles, whereas the sample produced by solvothermal synthesis consists of larger particles. The pore properties were analyzed based on the nitrogen adsorption isotherms at 77 K (Fig. 3). The BET surface area and pore volume of the samples synthesized by the three different methods were also calculated according to the isotherms, and are given in Table 5. MOF-5 synthesized by the solvothermal method showed a much higher BET surface area and pore volume than samples produced by direct mixing or slow diffusion of TEA. It is surprising to note that the TG curves are similar for all the MOF-5 samples produced by the different synthesis methods (Fig. 4). It is assumed that the weight loss below 523 K arose from the removal of the solvent DMF occluded in the framework. This suggested that DMF was completely removed when the sample was heated to 523 K. At temperatures above 523 K, the TG curve changed only slightly. However, a dramatic mass loss, indicating the collapse of the framework, was observed when samples produced by direct mixing or slow diffusion of TEA were heated to 673 K, and when samples produced solvothermally were heated to 773 K. Thus MOF-5 synthesized by the solvothermal method showed a higher thermal stability. Intensity c b Table 5 Pore parameters calculated according to the N 2 adsorption isotherms at 77 K of MOF-5 synthesized by different methods. Synthesis method BET SA Langmuir SA Pore volume (cm 3 /g) Pore diameter (nm) Direct mixing of TEA 500.8 606.3 0.19 0.52 Slow diffusion of TEA 481.1 589.6 0.19 0.54 Solvothermal synthesis 839.6 1029.1 0.34 0.51 5 10 15 20 25 30 35 Two theta (degree) Fig. 5 XRD patterns of MOF-5 samples treated by solvent extraction with: (a) chloroform; (b) ethanol; (c) acetone. a

international journal of hydrogen energy 34 (2009) 1377 1382 1381 Hydrogen Storage (wt%) 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Hydrogen Pressure (MPa) 3.3. Hydrogen-storage measurement heat treat CHCl 3 dispose Fig. 6 Hydrogen desorption isotherms at 77 K of MOF-5 samples after the removal of organic guests. The organic guests contained in an MOF-5 sample will occlude the majority of the voids in the framework. In general, an organic guest can be removed by heating or solvent extraction [16]. In our experiments, both methods were tested. The solvents included chloroform, ethanol, and acetone. The XRD patterns of MOF-5 samples show that extraction by ethanol or acetone partially destroys the framework of MOF-5, while the framework retains its integrity when chloroform is used in the extraction (Fig. 5). After removal of the organic guests, the hydrogen-storage capacities of the MOF-5 samples were measured. The hydrogen uptakes of MOF-5 synthesized by the direct-mixing method are shown as a function of hydrogen pressure in Fig. 6. Little difference is observed between the results of heat treatment at 523 K for 12 h and of chloroform extraction. In our study, the heat-treatment method was used to remove organic guests. The hydrogen-storage capacities of MOF-5 samples synthesized by the three different methods are shown in Fig. 7. The hydrogen-storage values were 2.63 wt% for MOF-5 synthesized by the slow diffusion of TEA, 3.20 wt% for the direct mixing of TEA, and 3.60 wt% for the solvothermal method at 1.74 MPa and 77 K. It is clear that MOF-5 synthesized by the solvothermal method is superior to that from the direct-mixing or slow-diffusion method. This suggests that the surface area and pore volume of the MOF-5 samples are related to their hydrogen-storage capacities. 4. Conclusion MOF-5 samples can be synthesized by the direct mixing of TEA, the slow diffusion of TEA, or by solvothermal synthesis. However, the procedure for synthesizing MOF-5 significantly influences its morphology, pore parameters, and hydrogenstorage capacity. The advantage of the solvothermal method lies in the higher crystallinity of MOF-5. Both heat treatment and solvent extraction can completely remove the organic guests and produce a porous framework. All the MOF-5 samples exhibit high hydrogen-storage capacities: 2.63 wt% for MOF-5 produced by the slow-diffusion method, 3.20 wt% for the direct-mixing method, and 3.60 wt% for the solvothermal method at 1.74 MPa and 77 K. The results show that the surface area and pore volume of MOF-5 materials are correlated with their hydrogen-storage capacities. Acknowledgements The authors gratefully acknowledge financial support from the Special Funds for Major State Basic Research 973 Project (No. 2005CB221202) and the Natural Science Foundation of Shanxi Province in China (No. 2006011021). references 4.0 Hydrogen Storage (wt%) 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Hydrogen Pressure (MPa) direct mixing of TEA slow diffusion of TEA solvothermal synthesis Fig. 7 Hydrogen desorption isotherms at 77 K of the MOF- 5 samples synthesized by the three different methods. [1] Swesia Y, Ronze D, Pitault I, Dittmeyer R, Heurtaux F. Purification process for chemical storage of hydrogen for fuel cell. Int J Hydrogen Energy 2007;32(18):5059 66. [2] Broom DP. The accuracy of hydrogen sorption measurements on potential storage materials. Int J Hydrogen Energy 2007;32(18):4871 88. [3] Park SJ, Kim BJ, Lee YS, Cho MJ. Influence of copper electroplating on high pressure hydrogen-storage behaviors of activated carbon fibers. Int J Hydrogen Energy 2008;33(6): 1706 10. [4] Lamari Darkrim F, Malbrunot P, Tartaglia GP. Review of hydrogen storage by adsorption in carbon nanotubes. Int J Hydrogen Energy 2002;27(2):193 202. [5] Langmi HW, Walton A, Al-Mamouri MM, Johnson SR, Book D, Speight JD, et al. Hydrogen adsorption in zeolites A, X, Y and RHO. J Alloys Compd 2003;356 357:710 5. [6] Dong JX, Wang XY, Xu H, Zhao Q, Li JP. Hydrogen storage in several microporous zeolites. Int J Hydrogen Energy 2007; 32(18):4998 5004.

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