Hydrogen Storage in Metal-Organic Frameworks

Hydrogen Storage in Metal-Organic Frameworks

(Parte 1 de 5)

Hydrogen Storage in Metal–Organic Framework By Yun Hang Hu* and Lei Zhang

An effective hydrogen-storage technology that provides a high storage capacity and fast kinetics is a critical factor in the development of a hydrogen fuel for transportation. Hydrogen can be stored in three ways: liquefaction, compressed hydrogen, and storage in a solid material.[1–7] The large amount of energy consumed during liquefaction and the continuous boil-off of hydrogen limit the possible use of liquid-hydrogen storage technology.[1] Compressing hydrogen is also an energyconsuming process and requires a very high pressure to obtain enough hydrogen fuel for a reasonable driving cycle of 300 miles, which in turn leads to safety issues related to tank rupture in case of accidents.[2] Therefore, current attention is focused on solid storage materials.[8,9] The storage of hydrogen in solid materials can be achieved by one of three processes: i) chemical reactions, in which hydrogen reacts with the solid material to form new compounds, i) adsorption, in which hydrogen is adsorbed onto the solid material, and i) cage occupancy, in which hydrogen is captured by cages. Materials for the storage of hydrogen through chemical reactions include metals, complex hydrides, and nitrides.[3–6b] These materials with relatively high hydrogenstorage capacities usually have a hydrogen-releasing temperature over 373K (most even higher than 473K) as a consequence of the high energy required to break chemical bonds. Such a high releasing temperature places a challenge on their applications for on-board hydrogen storage, for which the US Department of Energy (DOE) 2010 targets are hydrogen capacities of 6wt% and between 243and 353K. Hydrogenand H2O can form clathrate hydrogen hydrates, in which hydrogen molecules are captured in

H2O cages.[10,1] The maximum hydrogen capacity is 5.3wt% with current clathrate hydrates.[10] Furthermore, the introduction of tetrahydrofuran (THF) guest molecules can extremely reduce the pressure of hydrogen hydrate formation from 20 to

50bar.[1] However, to achieve a H2 capacity of 6wt% or higher, new structures of hydrogen hydrates are needed. Porous materials with high surface areas, such as activated carbon, nanotubes, and zeolites, have been widely investigated for hydrogen storage by adsorption.[12–14] The temperature of hydrogen release is usually low with these porous materials. However, they have low hydrogen capacities.

Over the last 30 years, the science of porous materials has become one of the most challenging issues for chemists and physicists.[15] Along with the progress of inorganic porous solids, the innovation for the synthesis of hybrid porous materials emerged at the beginning of the 1990s with the self-assembly of inorganic metal cations with organic linkers to form a network in the appropriate topology.[16–21] Such metal–organic frameworks (MOFs) attracted much attention because of their unusual structure and properties as well as their potential applications.[20,2] In 1997, Kitagawa et al. first reported gas adsorption on MOFs.[23] In 2003, Yaghi et al. first explored MOFs as H2 storage materials.[24] Since that time, MOFs have become one of the most promising hydrogen-storage materials. Furthermore, MOFs are proving successful for hydrogen adsorption at 77K. Currently, an exciting challenge for MOFs is to reach a high hydrogen capacity at ambient temperature with an acceptable pressure. There are several excellent review articles about MOFs for hydrogen storage,[25–30] which analyzed the effects of metal cluster and organic ligand structures, evaluated relationships between hydrogen capacities and surface areas (and pore sizes), and discussed strategies to improve hydrogen-storage capacity. Here, we review the recent progress in this rapidly developing field, with emphasis on the efforts and challenges to reach high hydrogen uptake in MOFs at ambient temperature and the interaction mechanism between hydrogen and MOFs.

2. Hydrogen Storage in MOFs at a Low Temperature of 7 K

The critical factors that determine hydrogen adsorption and desorption are the surface area of the adsorbents and the w.MaterialsViews.com w.advmat.de

[*] Prof. Y. H. Hu, L. Zhang

Department of Materials Science and Engineering Michigan Technological University Houghton, MI 49931-1295 (USA) E-mail: yunhangh@mtu.edu

DOI: 10.1002/adma.200902096

Metal–organic frameworks (MOFs) are highly attractive materials because of their ultra-high surface areas, simple preparation approaches, designable structures, and potential applications. In the past several years, MOFs have attracted worldwide attention in the area of hydrogen energy, particularly for hydrogen storage. In this review, the recent progress of hydrogen storage in MOFs is presented. The relationships between hydrogen capacities and structures of MOFs are evaluated, with emphasis on the roles of surface area and pore size. The interaction mechanism between H2 and MOFs is discussed. The challenges to obtain a high hydrogen capacity at ambient temperature are explored.

Adv. Mater. 2010, 2, E1–E14 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim E1 Final page numbers not assigned w.advmat.de w.MaterialsViews.com interaction between hydrogen and the adsorbents. At a low temperature of 77K, hydrogen uptake on MOFs mainly depends on their total surface areas, particularly at high pressure. So far, various inorganic metal cations and organic linkers have been explored to tune the structure, pore size, and surface area of MOFs for hydrogen adsorption (Table 1).

2.1. Zn-Based MOFs

In 2003, Yaghi et al. reported an interesting MOF material (MOF-5, Fig. 1a) with hydrogen-sorption capacities.[24] MOF-5 has a crystal structure where inorganic [Zn4O]6þ groups are joined to an octahedral array of benzene-1,4-dicarboxylate (BDC) groups to form a porous cubic Zn4O(BDC)3 framework. Such a special structure is ideal for gas absorption because of its isolated linkers, which are accessible from all sides to the sorbate gas molecules. The scaffolding-like nature of MOF-5 and its derivatives led to extraordinarily high apparent surface areas (above 2000m2 g 1). At 77K and 0.7bar, 4.5wt% hydrogen absorption was obtained by using MOF-5.[24] This work prompted numerous investigations into storing hydrogen in MOF materials.[25,31–42] Although the 4.5wt% hydrogen capacity of MOF-5 at 0.7bar and 77K was revised and attributed to the adsorption of some impurity gases,[37] its maximum hydrogen capacity of 4.5–5.2wt% has been confirmed at 77K and about 50bar by three independent groups.[35,39,40] Furthermore, by evaluating MOF-5 derivatives composed of the same inorganic

[Zn4O]6þ groups and different organic linkers,[35] Yaghi et al.

found that the maximum H2 uptakes in MOFs correlate well with surface areas. Among these MOFs, MOF-177 (Fig. 1c) with the highest apparent surface area (Brunauer–Emmett–Teller (BET) surface area: 4746m2 g 1) had the highest hydrogen uptake of 7.5wt% at 77K and 70bar (Fig. 2).[35]

The preparation approaches for MOFs can have effects on their hydrogen adsorption capacities. Yaghi et al. reported that the hydrogen-storage capacity of MOF-5 was variable with synthesis and handling conditions. The maximum H2 uptakes of MOF-5 samples prepared with and without exposure to air were 5.1 and

7.1wt%, respectively.[36] The N2 adsorption measurements showed that the exposure in air led to a reduction in its BET surface area from 3800 to 3100m2 g 1, confirming the deleterious effects of air exposure. This discrepancy was attributed to the decomposition of Zn4O(BDC)3 in humid air. They found that exposure of a pulverized and desolvated sample of Zn4O(BDC)3 to air for 10min resulted in the appearance of a new peak at 2u¼8.98 in the powder X-ray diffraction (XRD) pattern, suggesting the partial conversion in a second phase (Fig. 3).[36] Furthermore, when the sample was further exposed to air, they observed an increase in the relative intensity of this XRD peak and the appearance of two additional peaks at 2u¼15.8 and 17.8, indicating the formation of a compound isostructural to

Zn3(OH)2(BDC)2 2DEF (DEF¼N,N-diethylformamide) (MOF-69C). After exposure to air for 24h, Zn4O(BDC)3 was converted into a solid of formula C24H22O18Zn4. In contrast, the structure of

Zn4O(BDC)3 was not affected by exposing to dry O2 or anhydrous organic solvents such as methanol, N,N-dimethylformamide

(DMF), and dimethyl sulfoxide (DMSO). Very recently, Hafizovic et al. studied the structural difference between low and high surface area MOF-5 samples, which are dependent on preparation approach.[43] The low surface area MOF-5 had two types of crystals. In the dominant phase, the Zn(OH)2 species, which partly occupied the cavities, makes the hosting cavity and adjacent cavities inaccessible, which leads to a reduction in the pore volume and the effective surface area of the material. Furthermore, the minor phase consisted of doubly interpenetrated MOF-5 networks, which lowers the adsorption capacity.

Thus, the hydrogen adsorption capacity of Zn4O(BDC)3, which is determined by surface area, strongly depends on preparation conditions. This can explain the difference in hydroge- n-adsorption capacities reported for Zn4O(BDC)3 from various groups (Table 1).[19,36–42]

The effect of Pd on hydrogen storage in MOF-5 was examined by Sabo et al.[4] Although the surface area of MOF-5 decreased from 2885 to 958m2 g 1 by supporting Pd on it, its hydrogen-adsorption capacity increased from 1.15 to 1.86wt% at 77K and 1bar. This happened probably because Pd can increase the hydrogen adsorption energy, which determines the hydrogen capacity at a low pressure.

The intergrowth of two or more frameworks can also affect their properties for hydrogen storage. As shown in Figure 4, one can see that, compared with non-interpenetrating MOFs, the

Yun Hang Hu received his Ph.D. in physical chemistry from Xiamen University. He was Assistant Professor and Associate Professor at Xiamen University, Senior Staff Engineer at the ExxonMobil Research and Engineering Company, and Research Full Professor of Chemical and Biological Engineering at the State University of New York at Buffalo. Currently, he is an assistant professor of materials science and engineering at Michigan Technological University. His main research interests range from hydrogen-storage materials, nanostructured materials, CO2 conversion, catalysis, surface science, quantum chemistry, to solar energy.

Lei Zhang is currently pursuing her Ph.D. in materials science and engineering in the group of Prof. Y. H. Hu. Her current research involves the synthesis, characterization, and application of metal-organic frameworks. She received her B.Sc. and M.Sc. in materials science and engineering from China University of Mining and Technology (Beijing).

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Table 1. Summary of hydrogen adsorption on MOFs at 77K.

Heat of adsorption [kJ mol 1]

H2 uptake [i][wt%] Pressure[bar] Ref.

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interwoven IRMOF-1 material showed the greatest hydrogen uptake at 77K and pressure below 800torr (1.07bar).[37] This happened because catenation can reduce the free diameter of pores.[25,45,46]

A series of dinuclear paddlewheel-structured MOFs were explored.[47–49] Dybtsev et al.[47] synthesized a paddlewheel- structured [Zn2(BDC)2(DABCO)] 4DMF 0.5H2O (DABCO¼ 1,4-diazabicyclco[2.2.2]octane). The framework is composed of dinuclear Zn2 units with a paddlewheel structure, which are bridged by BDC dianions to form a distorted 2D square-grid

[Zn2(BDC)2]. The axial sites of the Zn2 paddlewheels are occupied by DABCO acting as pillars to extend the 2D layers into a 3D structure (Fig. 5). Although BDC is generally considered to be a linear and rigid linker, the linker in this structure is bent, leading to severe twisting of the Zn2 paddlewheel from an ideal square grid. Interestingly, after evacuation of guest molecules, the BDC ligands linking the Zn2 paddlewheel units became linear, which resulted in a perfect 2D square grid of [Zn2(BDC)2]. Such a

Materials[a] Apparent surface area [b] [m2 g 1]

Heat of adsorption [kJ mol 1]

H2 uptake [i][wt%] Pressure[bar] Ref.

[a] Acronyms: BDC¼benzene-1,4-dicarboxylate; R6-BDC¼1,2-dihydrocyclobutylbenzene-3,6-dicarboxylate, NDC¼naphthalene-2,6-dicarboxylate; HPDC¼4,5,9,10-tetrahydropyrene–2,7-dicarboxylate; TMBDC¼2,3,5,6-tetramethylbenzene-1,4-dicarboxylate; TTDC¼thieno[3,2-b]thiophene-2,5-dicarboxylate; NTB¼4,4 ,4 -nitrilotrisbenzoate; BTB¼benzene-1,3,5-tribenzoate; DEF¼N,N -diethylformamide; DABCO¼1,4-diazabicyclo[2.2.2]octane; TFBDC¼tetrafluoroterephtharephthalate; TED¼triethylenediamine; DMF¼N,N-dimethylformamide; TBIP¼5-tert-butyl isophthalate; BPDC¼biphenyldicarboxylate; BPY¼4,4 -bipyridine; BDT¼ 1,4-benzeneditetrazolate; BPTC¼ biphenyl-3,3 ,5,5 -tetracarboxylate; TPTC¼ terphenyl 3,3 :5,5 -tetracarboxylate; QPTC ¼ quaterphenyl 3,3 ;:5,5 ;-tetracarboxylate;

BTC¼ benzene-1,3,5-tricarboxylate; TATB¼ 4,4 ,4 -s-triazine-2,4,6-triyltribenzoate; PYENH2¼ 5-methyl-4-oxo-1,4-dihydro-pyridine-3-carbaldehyde CYCLAM¼1,4,8,1-tetraazacyclotetradecane; BPYDC ¼2,2 -bipyridyl-5,5 -dicarboxylate; 3-PIC¼3-picoline; PD¼propane-1,2-diol; SIP¼5-sulfoisophthalate. Calculated from

N2 adsorption at 77K using the Langmuir model except where indicated. [c] BET surface area from N2 adsorption at 77K. [d] BET surface area from CO2 adsorption at 195K. [e] BET surface area from O2 adsorption at 77K. [f] Values obtained from adsorption experiments at different temperatures. [g] Value obtained from microcalorimetric experiments at low coverage. [h] Values deduced from simulations. [i] Gravimetric uptake of hydrogen (wt%) except where indicated. [j] Volumetric uptake of hydrogen (g H2 L 1).

Figure 1. MOFs: a) MOF-5, Zn4O(BDC)3, b) IRMOF-8, Zn4O(NDC)3, and c) MOF-177, Zn4O(BTB)2. Reproduced with some modifications from ref. [25].

Figure 2. Hydrogen isotherms for the activated materials measured at 77K in gravimetric units (mg g 1). Filled markers represent adsorption, open markers denote desorption. Reproduced with permission from ref. [35]. Copyright 2006, American Chemical Society.

Table 1. Continued

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paddlewheel-structured MOF, which has a high BETsurface area of 1450m2 g 1, exhibited an adsorption capacity of 2.0wt% at 77K and 1bar. Furthermore, Chun et al. combined various aromatic dicarboxylates,[50] including BDC, tetramethylterephthalate (TMBDC), 1,4-naphthalenedicarboxylate (1,4-NDC), tetrafluoroterephthalate (TFBDC), 2,6-naphthalenedicarboxylate (2,6-NDC), DABCO, and 4,40-dipyridyl (BPY), to form paddlewheel-structured frameworks [Zn2(BDC)2(DABCO)], [Zn2(BDC)(TMBDC)(DABCO)], [Zn2(TMBDC)2(DABCO)], [Zn2

(Parte 1 de 5)