Hydrogen Storage in Mesoporous Coordination Frameworks

Publication Date (Web): August 6, 2009. Copyright © 2009 American Chemical ... Clearly, this material is a promising adsorbent for hydrogen storage. ...
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2009, 113, 15106–15109 Published on Web 08/06/2009

Hydrogen Storage in Mesoporous Coordination Frameworks: Experiment and Molecular Simulation Zhonghua Xiang,† Jianhui Lan,† Dapeng Cao,*,† Xiaohong Shao,† Wenchuan Wang,† and Darren P. Broom*,‡ DiVision of Molecular and Materials Simulation, Key Lab for Nanomaterials, Ministry of Education, College of Chemical Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, P.R. China, and Hiden Isochema Ltd., 231 Europa BouleVard, Warrington WA5 7TN, U.K. ReceiVed: July 07, 2009; ReVised Manuscript ReceiVed: July 30, 2009

A porous coordination framework material, Zn4O(BDC)(BTB)4/3 (labeled 1 for simplification), has been prepared and applied to hydrogen storage. Its storage capacity has been measured, and the absolute and excess H2 adsorption uptakes reach 6.23 and 5.43 wt % at 77 K and 20 bar, respectively. Moreover, a multiscale simulation method, which combines first-principles calculation and grand canonical Monte Carlo simulation, has been used to evaluate H2 adsorption in this material. The simulation results successfully reproduce the experimentally determined hydrogen uptake in 1 at T ) 77 K and in the pressure range 0-20 bar. Furthermore, the simulation predicts that the hydrogen capacity in 1 reaches 9.5 wt % at 77 K and 100 bar, which is among the highest reported in the literature to date. Clearly, this material is a promising adsorbent for hydrogen storage. Hydrogen storage in porous materials is an important and hot topic in hydrogen economics. However, developing a high capacity hydrogen storage material still remains a major challenge. It has been proven that the carbon-based adsorbents cannot meet the 2010 hydrogen storage system target of 6 wt % (45 g L-1) set by the US DOE for hydrogen fuel cell vehicles.1 Recent studies indicate that porous coordination frameworks (PCFs) have shown a great potential for H2 storage2-4 because of their porous nature and high specific surface area (SSA). On one hand, the SSA of PCFs shows a positive relationship with hydrogen capacity.5 On the other hand, H2 is adsorbed by dispersion or van der Waals forces. Accordingly, the small pores of a size comparable with the diameter of H2, which have a stronger interaction with hydrogen due to the overlap of the potential fields from opposing pore walls, display a higher hydrogen storage capacity due to the greater affinity toward hydrogen. Thus, porous materials with large SSAs and appropriate pore sizes are expected to be good adsorbents for hydrogen storage. Combining terephthalic acid (H2BDC) and 1,3,5-tris(4carboxyphenyl)benzene (H3BTB) in the presence of zinc nitrate, Matzger and co-workers first synthesized 16 (Zn4O(BDC)(BTB)4/ 3, named as UMCM-1 in their work), a mesoporous coordination polymer with high microporosity and a large SSA, in which six 1.4 nm × 1.7 nm microporous cage-like structures form a 1D hexagonal channel of 2.7 nm × 3.2 nm (see Figure 1a). The structure of 1 suggests that it may be a promising hydrogen storage material. * To whom correspondence should be addressed. E-mail: caodp@mail. buct.edu.cn or [email protected] (D.C.); [email protected] (D.P.B.). † Beijing University of Chemical Technology. ‡ Hiden Isochema Ltd.

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Figure 1. (a) Structure of Zn4O(BDC)(BTB)4/3, in which six BDC linkers, five BTB linkers, and nine Zn4O clusters form microporous cages with an internal dimension of approximately 1.4 nm × 1.7 nm (the smaller yellow ball) and six microporous cage-like structures form a 1D hexagonal channel of 2.7 nm × 3.2 nm (the bigger yellow ball). (b) SEM image of porous Zn4O(BDC)(BTB)4/3. (c) Enlarged SEM image of one needle-shaped crystal of Zn4O(BDC)(BTB)4/3.

As the hydrogen storage capabilities of 1 have not yet been reported, in this work, we synthesize this material and measure its hydrogen uptake. Moreover, we use a multiscale theoretical method7-10 to explain the hydrogen adsorption mechanism in 1 and predict the hydrogen uptake at high pressures. To prepare this material, we employed Matzger’s method with slight modifications to synthesize the mesoporous 1. In the procedure, an approximate 4.5:1.3:1 mixture of Zn(NO3)2 · 6H2O, H2BDC, and H3BTB was dissolved in N,N-diethylformamide (DEF) and heated at 85 °C for 3 days. The needle-shaped crystals were exchanged with CHCl3 for 3 days, and the exchanged guest solvent was removed under vacuum at 200 °C. The resulting sample was found to consist of microsized  2009 American Chemical Society

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Figure 2. Powder XRD patterns using Cu KR radiation of Zn4O(BDC) (BTB)4/3. The black solid line (a) shows a simulated pattern, the red solid line (b) shows the experimental pattern of as-synthesized Zn4O(BDC)(BTB)4/3 activated by CHCl3, and the purple solid line (c) shows the PX RD pattern of Zn4O(BDC)(BTB)4/3 after adsorption of H 2.

needle crystals, which can be seen in the SEM images shown in Figure 1b and c. The structure of 1 was further characterized by powder X-ray diffraction (PXRD), as shown in Figure 2. Note that the crystals show a very similar structure to the one reported previously.6 Thermo-gravimetric analysis (TGA) shows a slight weight loss up to 400 °C (see Figure S1 in the Supporting Information). On the basis of the TGA result, 300 °C was applied for degassing prior to gas adsorption. Figure 2c shows that no significant changes were observed in the PXRD pattern after heating to 300 °C for 24 h for hydrogen adsorption. These measurements suggest that no dramatic structural changes took place and the structure of 1 remains unperturbed during this process, confirming its high thermal stability. To study the porosity of 1, we measured the nitrogen adsorption/desorption isotherms at T ) 77 K. The isotherms exhibit an abrupt secondary uptake increase (see Figure S2 in the Supporting Information) near P/P0 ) 0.2, indicating the presence of mesopores, in agreement with the result of Matzger et al.6 By analyzing the nitrogen isotherms, it was determined that 1 possesses a pore volume of 1.47 cm3 g-1, and a BrunauerEmmet-Teller (BET) SSA of 2932 m2 g-1, as well as a Langmuir SSA of 3950 m2 g-1. Gravimetric measurements of H2 adsorption in 1 were performed at Hiden Isochema Ltd., U.K., using an IGA-003 analyzer. Figure 3a shows the experimental excess and absolute adsorption/desorption isotherms of hydrogen for 1 at 77 K, where the absolute adsorption is obtained using the equation from Lin et al.4 on the basis of the experimentally measured excess isotherm. In the pressure range 0-20 bar, it can be seen that the hydrogen adsorption shows good reversibility without hysteresis and is not saturated, which may be due to the presence of mesopores. The mesopores with high microporosity result in a gradual capture of hydrogen in the pores and on the surface with the increase in pressure. At 77 K and 20 bar, the excess hydrogen adsorption reaches 5.43 wt %, which is only slightly lower than the recently reported high capacity hydrogen storage materials, such as the NOTT series,4 MOF-177,3 MOF-5,2 COF102,11 and COF-103,11 under the same conditions (see Table 1 for details). However, it is greater than the adsorption capacity of nearly 170 other PCF materials listed in the literature.12 The absolute hydrogen uptake by 1, calculated from the experimental excess data presented above, is 6.23 wt % at 77 K and P ) 20 bar. To explore the mechanism of hydrogen adsorption in 1 and to help confirm the experimental results shown above, we have used a multiscale theoretical method, which combines firstprinciples calculation and grand canonical Monte Carlo (GCMC) simulation, to evaluate the hydrogen adsorption capacity. The

Figure 3. Adsorption isotherms for hydrogen in Zn4O(BDC)(BTB)4/3 at 77 K: (a) experimental excess and converted absolute hydrogen adsorption/desorption isotherms for Zn4O(BDC)(BTB)4/3 at 77 K; (b) experimental and simulated adsorption isotherms at p ) 0-20 bar; (c) absolute hydrogen uptake predicted by GCMC simulation at p ) 0-100 bar, where the absolute adsorption isotherm obtained from the experimental excess uptake at p ) 0-20 bar is also inserted for comparison.

first-principles calculations were performed at the theoretical level of the second-order Møller-Plesset (MP2) method using the cc-PVTZ basis set to obtain the interaction between H2 and 1. The calculated potential energies were then fitted to the force fields (FFs), achieving the FF parameters between H2 and 1 (the fitted parameters are listed in Table S2 in the Supporting Information). Using the fitted force fields, the GCMC simulations were performed to predict the adsorption isotherm of H2 in 1 (further details have been presented in our previous work7,13). The simulation results of H2 adsorption at 0-20 bar are shown in Figure 3b for comparison to the experimental data. It can be seen that both the absolute and excess hydrogen uptakes simulated in 1 at 0-20 bar are in good agreement with the experimental data. The simulated excess adsorbed quantity at 20 bar, 5.37 wt %, is consistent with the experimental result of 5.43 wt %, and the simulated absolute adsorbed quantity at the same pressure, 6.49 wt %, is also comparable to the converted absolute result of 6.23 wt %. This agreement indicates that H2 adsorption in 1 is satisfactorily described by our multiscale theoretical method. Because of the pressure limitation of the IGA-003 analyzer, our experimental data are restricted to the range 0-20 bar. Consequently, in order to study hydrogen storage capacities in 1 at higher pressures, we employed our multiscale simulation method to predict hydrogen adsorption in this regime. The predictions from the simulation at high pressure indicate that the absolute gravimetric hydrogen adsorption capacity of 1 reaches 9.5 wt % at 77 K and P ) 100 bar (see Figure 3c), which is higher than the capacities of the other materials listed in Table 1, with the exception of MOF-5 (11.5 wt %/72 bar),2 MOF-177 (11.9 wt %/180 bar),3 COF-102 (11.0 wt %/86 bar),11 and COF-103 (10.7 wt %/86 bar).11

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TABLE 1: Hydrogen Storage Capabilities of High Capacity PCF Materials at 77 Ka hydrogen storage (wt %) BET SSA (m2 g-1)

materialsb Zn4O(BDC)(BTB)4/3 Zn4O (BTB)2 Zn4O(BDC)3 C25H24B4O8 C24H24B4O8Si C12H6BO2 Zn4O(T2DC)(BTB)4/3 Cu2(qptc) Cu2(tptc) Cu2(bptc)

Cu3(bhtc)2 Zn4O(ttdc)2 Cr3OF(BDC)3 Cu2(abtc) Mn4Cl(BTT)3/8 CuCl(BTT)3/8 Ni3(OH)(pbpc)3 Cu4(TTPM)2 · 0.7CuCl2

1 MOF-177 MOF-5 COF-102 COF-103 COF-10 UMCM-2 MOF-505 NOTT-102 NOTT-103 NOTT-101 UMCM-150 IRMOF-20 MIL-101 PCN-11

2932 4526 3800 3620 3530 1760 5200 2932 2247 2932 2942 2929 2316 2300 3409 5500c 1931 2850 2100 1710 1553 2506

pore size (Å) 14/32 17 11.2 11.520 12.520 16 14-18/24-30d 8.3 7.3 8.3 8.0 7.3 6.5-8/10.5-12/12.5-13d 14 8.6/29/34 10.5 10

pore volume free volume maximum H2 (cm3 g-1) (%) excessi/20 bar uptake (absolutei) 1.47 1.59 1.19 1.55 1.54 0.69 n.a. 1.28 1.08 1.14 1.14 1.14 0.89 1.00 1.53 1.90 0.91 1

h

80.9 68f 5919g

n.a. 75.5 70.4 75.5 71.6 70.4

71.1

5.43 6.4 6.6 6.86 6.58 3.47 6.2 6.07 6.06 6.07 6.0 6.1 5.6 5.5 5.3 5.1 5.05 4.4 4.6 4.1 4.15 4.1

9.5/100 bar 11.5/72 bar 11.9/180 bar 11.0/86 bar 10.7/86 bar 7.7/86 bar 6.9/46 bare

6.7/60 bar 7.2/60 bar 6.2/60 bar 5.7/45 bare 6.25/77.6 bare 6.1/60 bare 6.0/45 bar 6.76/50 bar 6.9/90 bar 5.7/84 bar

ref this work 3 2 11 11 11 21 22 22 23 4 4 4 24 25 26 27 28 29 30 31 32

a It should be noted that this table only lists the highest capacity PCF materials found currently in the literature. b T2DC ) thieno[3,2-b]thiophene-2,5-dicarboxylate; qptc ) quaterphenyl-3,3′′′,5,5′′′-tetracarboxylate; tptc ) terphenyl-3,3′′,5,5′′-tetracarboxylate; bptc ) 4,4′-biphenyldicarboxylate; bhtc ) biphenyl-3,4-bipyridine; ttdc ) thieno[3,2-b]thiophene-2,5-dicarboxylate; abtc ) azobenzene-3,3′,5,5′-tetracarboxylate; BTT ) 1,3,5-benzenetristerazolate; pbpc ) pyridine-3,5-bis(phenyl-4-carboxylate); TTPM ) tetrakis(4-tetrazolylphenyl)methane. c Langmuir SSA. d The pore size distribution analysis by DFT methods utilizing Ar gas at 87 K. e Excess hydrogen adsorption. f The ratio between the absolute adsorbed amount of H2 and the liquid density of H2 (71 g L-1). g Calculations were performed using the Cerius2 software package, and calculated using a probe radius of 1.45 Å, which corresponds to the kinetic diameter of H2. h The free volume is the accessible volume of H2 within one unit cell. It is accessible if the potential energy of the interaction between H2 and the solid framework is less than 104 K. i Excess adsorption refers to the amount of H2 taken up beyond that which would be contained, under identical conditions, within a free volume equivalent to the volume of the adsorbed phase under those same conditions.12 Absolute uptake corresponds to the amount of hydrogen contained within the adsorbed hydrogen phase.3

As mentioned above, 1 exhibits a slightly lower absolute hydrogen capacity than MOF-5 and MOF-177. This may be because 1 has a larger average pore size than these materials, which leads to less overlap of the potential fields from opposing pore walls and therefore presents a weaker affinity toward hydrogen. This suggests that a promising material for hydrogen storage should possess a combination of high BET SSA and small pores based on physisorption affinity.14 Furthermore, it can also be seen that SSA is not the only controllable factor for high hydrogen storage capacity, because appropriate pore size and pore volume should also be considered in the design of new kinds of PCFs for hydrogen storage.15 Figure 4 shows snapshots of hydrogen adsorbed in 1 at 77 K and four different pressures. It has been found previously16 that the physisorption of H2 in PCFs is mainly due to London dispersion between linkers and connectors with hydrogen. Our simulation results indicate that the site in the center of the three ZnO3 triangular faces exhibits the strongest affinity toward hydrogen, while the site at the top of the single ZnO3 triangle also plays an important role. At low pressures, hydrogen is captured first on the two sites (see Figure 4a). In our firstprinciples calculations, the binding energies of the two sites at the top and side of the linkers are lower than that of the two sites mentioned above. As for further loading, hydrogen is captured by the additional adsorption sites around the linker, which can be seen in Figure 4b-d. This coincides with both the recent neutron diffraction analysis of hydrogen adsorption sites on deuterated MOF-5 at 3.5 K5 and the results of simulation.17,18 Thus, both experiment and simulation results here indicate that the metal oxide clusters are more important for hydrogen storage than the organic linkers in this PCF series.

Figure 4. Snapshots of the structures of Zn4O(BDC)(BTB)4/3 with adsorbed hydrogen at T ) 77 K and P ) 0.1, 1, 5, and 10 MPa (Zn, blue; O, red; C, orange; H, white).

In summary, the mesoporous material 1, Zn4O(BDC)(BTB)4/3, has been prepared and characterized experimentally in this work. The storage capacities of hydrogen in 1 have been measured using an IGA-003 gravimetric analyzer. The gravimetric excess and absolute hydrogen uptake in 1 reaches 5.43 and 6.23 wt %, respectively, at 77 K and 20 bar. In addition, a multiscale theoretical method has been used to successfully reproduce the hydrogen uptake in 1 at T ) 77 K and P ) 0-20 bar. Furthermore, the simulation further predicted that the hydrogen capacity in 1 reaches 9.5 wt % at 77 K and 100 bar, which is among the highest reported in the literature to date. In short, the framework material prepared in this work is a promising adsorbent for hydrogen storage.

Letters Acknowledgment. This work is supported by NSF of China (20776005, 20736002, 20874005), National Basic Research Program of China (2007CB209706), Beijing Novel Program (2006B17), NCET Program (NCET-06-0095) from the MOE and Chemical Grid Program from BUCT. Kathryn Gallimore and Dr. Mark Roper are gratefully acknowledged for their help with the hydrogen adsorption measurement and data analysis. Supporting Information Available: Some details of the experiments and theoretical calculations, including the synthetic procedure and the characterization of Zn4O(BDC)(BTB)4/3, gas adsorption measurements, and the multiscale theoretical method from the first-principles calculations to grand canonical Monte Carlo (GCMC) simulations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kikkinides, E. S.; Yang, R. T.; Cho, S. H. Ind. Eng. Chem. Res. 1993, 32, 2714. Zhang, X. R.; Cao, D. P.; Chen, J. F. J. Phys. Chem. B 2003, 107, 4942. (2) Kaye, S. S.; Dailly, A.; Yaghi, O. M.; Long, J. R. J. Am. Chem. Soc. 2007, 129, 14176. (3) Furukawa, H.; Muller, M. A.; Yaghi, O. M. J. Mater. Chem. 2007, 17, 3197. (4) Lin, X.; Telepeni, I.; Blake, A. J.; Dailly, A.; Brown, C. M.; Simmons, J. M.; Zoppi, M.; Walker, G. S.; Thomas, K. M.; Mays, T. J.; Hubberstey, P.; Champness, N. R.; Schroder, M. J. Am. Chem. Soc. 2009, 131, 2159. (5) Rowsell, J. L. C.; Spencer, E. C.; Eckert, J.; Howard, J. A. K.; Yaghi, O. M. Science 2005, 309, 1350. (6) Koh, K.; Wong-Foy, A. G.; Matzger, A. J. Angew. Chem., Int. Ed. 2008, 47, 677. (7) Cao, D. P.; Lan, J. H.; Wang, W. C.; Smit, B. Angew. Chem., Int. Ed. 2009, 48, 4730. (8) Cao, D. P.; Feng, P. Y.; Wu, J. Z. Nano Lett. 2004, 4, 1489. (9) Klontzas, E.; Mavrandonakis, A.; Froudakis, G. E.; Carissan, Y.; Klopper, W. J. Phys. Chem. C 2007, 111, 13635. (10) Klontzas, E.; Tylianakis, E.; Froudakis, G. E. J. Phys. Chem. C 2008, 112, 9095. (11) Furukawa, H.; Yaghi, O. M. J. Am. Chem. Soc. 2009, 131, 8875.

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