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Metal-Organic Frameworks for Hydrogen Storage

Background
Given the imminent depletion of petroleum reserves, there is considerable interest in the development of non-petroleum energy carriers for use in transportation. Hydrogen has the potential to be an attractive option because it has a high energy content (120 MJ/kg compared to 44 MJ/kg for gasoline), produces clean exhaust product (water vapor without CO2 or NOx), and can be derived from a variety of primary energy sources. However, the specific energy of uncompressed hydrogen gas is very low, and considerable attention must be given to denser storage methods if hydrogen is to emerge as a serious option for energy storage.

Proposed forms of reversible hydrogen storage include: compressed gas, cryogenic liquid, adsorption to high surface-area materials, chemical storage as metal hydrides, and various reactions of liquid fuels high in hydrogen content (whose products must be collected and recycled after use). Of these, compressed and liquid hydrogen are the most mature technologies and are the most suitable for immediate deployment. The United States Department of Energy (USDOE) projects that with further technological development, adsorptive or chemical storage may prove most effective for storage.

Metal Organic Frameworks (MOFs) attract attention as materials for adsorptive hydrogen storage because of their exceptionally high specific surface areas and chemically-tunable structures. MOFs can be thought of as a three-dimensional grid in which the vertices are metal ions or clusters of metal ions that are connected to each other by organic molecules called linkers. Hydrogen molecules are stored in a MOF by adsorbing to its surface. Compared to an empty gas cylinder, a MOF-filled gas cylinder can store more gas because of adsorption that takes place on the surface of MOFs. (Note that molecular hydrogen adsorbs to the surface, not atomic hydrogen.) Furthermore, MOFs are free of dead-volume, so there is almost no loss of storage capacity as a result of space-blocking by non-accessible volume. Also, MOFs have a fully reversible uptake-and-release behavior: since the storage mechanism is based primarily on physisorption, there are no large activation barriers to be overcome when liberating the adsorbed hydrogen. The storage capacity of a MOF is limited by the liquid-phase density of hydrogen because the benefits provided by MOFs can be realized only if the hydrogen is in its gaseous state.

In order to realize the benefits provided, such as adsorption, by MOFs hydrogen cannot be stored in them at densities greater than its liquid-phase density. The extent to which a gas can adsorb to a MOF's surface depends on the temperature and pressure of the gas. In general, adsorption increases with decreasing temperature and increasing pressure. pressure (until a maximum is reached, typically 20-30 bar, after which the adsorption capacity decreases).,  However, MOFs to be used for hydrogen storage in automotive fuel cells need to operate efficiently at ambient temperature and pressures between 1 and 100 bar, as these are the values that are deemed safe for automotive applications.

US Department of Energy Hydrogen Storage Guidelines
Despite the fact that the USDOE Secretary has declared that MOFs for hydrogen storage are economically non-viable, it remains a highly competitive area of research today. It is a race to develop the MOF that can meet all of the targets set by the USDOE. The USDOE 2010 targets for a hydrogen storage system are: 1) a capacity of 45 g H2 per L, 2) a refueling time of 10 min or less, 3) a lifetime of 1000 refueling cycles, and 4) an ability to operate within the temperature range 30 to 50 °C. Note that these targets are for the entire storage system; therefore, the performance of a storage material must be even higher in order to account for the storage container and, if necessary, a temperature regulating apparatus. MOF-177 currently boasts the hydrogen absorption record, with a surface area of 4526 m2/g and an excess hydrogen uptake of 1.23 wt% and 32.1 g/L at 1 bar and 77 K.

Structure
A MOF is composed of two major components: a metal ion or cluster of metal ions and an organic molecule called a linker. The organic units are typically mono-, di-, tri-, or tetravalent ligands. The choice of metal and linker has significant affects on the structure and properties of the MOF. For example, the metal's coordination preference influences the size and shape of pores by dictating how many ligands can bind to the metal and in which orientation.

Distinguishing Coordination Polymers from MOFs
While coordination polymers and MOFs are both long-range organometallic complexes, there is a distinct difference between the two. Coordination polymers, like coordination compounds, contain weak bonds between metals and ligands, with zero formal bond valence. Metal-organic frameworks, however, possess formal bond valences. MOF-5, the “archetypical and iconic MOF,” has a formal bond valence of 1/2 in the Zn-O bonds.

In addition to bond valence, several other characteristics typify MOFs when compared with coordination polymers. The inorganic secondary building unit (SBU) of a MOF is usually polyatomic, the framework is neutral, and MOFs generally have empty pores. Coordination polymers, on the other hand, typically have single atoms as their inorganic components, charged frameworks, and pores containing counter-ions. There exist intermediate materials with characteristics of both MOFs and coordination polymers, but a general distinction can be made between the two.

Secondary Building Units
Describing and organizing the complex structures of MOFs could be a difficult and confusing task without a logical, unambiguous set of classifications. Recently, a system of nomenclature has been developed to fill this need. Inorganic sections of a MOF, called secondary building units (SBU), can be described by topologies common to several structures. Each topology, also called a net, is assigned a symbol, consisting of three lower-case letters in bold. MOF-5, for example, has a pcu net. The database of net structures can be found at the Reticular Chemistry Structure Resource.

MOFs for Hydgrogen Storage
The most important challenge for creating hydrogen adsorbents that operate at room temperature is increasing the hydrogen binding energy. Several classes of MOFs have been explored, including carboxylate-based MOFs, heterocyclic azolate-based MOFs, metal-cyanide MOFs, and covalent organic frameworks. Carboxylate-based MOFs have by far received the most attention in the literature because
 * 1) they are either commercially available or easily synthesized,
 * 2) they have high acidity (pKa ≈ 4) allowing for facile in situ deprotonation,
 * 3) the metal-carboxylate bond formation is reversible, facilitating the formation of well-ordered crystalline MOFs, and
 * 4) the bridging bidentate coordination ability of carboxylate groups favors the high degree of framework connectivity and strong metal-ligand bonds necessary to maintain MOF architecture under the conditions required to evacuate the solvent from the pores.

The most common transition metals employed in carboxylate-based frameworks are Cu2+ or Zn2+. Lighter main group metal ions have also been explored. Be4O(BDC)3, the first successfully synthesized and structurally-characterized MOF consisting of a light main group metal ion, shows high hydrogen storage capacity, but it is too toxic to be employed practically. There is considerable effort being put forth in developing MOFs composed of other light main group metal ions, such as magnesium in Mg4(BDC)3.

The following is a list several MOFs that are considered to have the best properties for hydrogen storage as of November 2009 (in order of decreasing hydrogen storage capacity). While each MOF described has its advantages, none of these MOFs reach all of the standards set by the USDOE. Therefore, it is not yet known whether materials with high surface areas, small pores, or di- or trivalent metal clusters produce the most favorable MOFs for hydrogen storage.

Zn4O(BTB)2, where BTB3- = 1,3,5-benzenetribenzoate (MOF-177)

Structure: Tetrahedral [Zn4O]6+ units are linked by large, triangular tricarboxylate ligands. Six diamond-shaped channels (upper) with diameter of 10.8 Å surround a pore containing eclipsed BTB3- moieties (lower).

Hydrogen storage capacity: 7.1 wt% at 77 K and 40 bar; 11.4 wt% at 78 bar and 77 K.

MOF-177 has larger pores, so hydrogen is compressed within holes rather than adsorbed to the surface. This leads to higher total gravimetric uptake but lower volumetric storage density compared to MOF-5.

Zn4O(BDC)3, where BDC3- = 1,4-benzenedicarboxylate (MOF-5)

Structure: Square openings are either 13.8 or 9.2 Å depending on the orientation of the aromatic rings.

Hydrogen storage capacity: 7.1 wt% at 77 K and 40 bar ; 10 wt% at 100 bar; volumetric storage density of 66 g/L.

MOF-5 has received much attention from theorists because of the partial charges on the MOF surface, which provide a means of strengthening the binding hydrogen through dipole-induced intermolecular interactions; however, MOF-5 has poor performance at room temperature (9.1 g/L at 100 bar).

Mn3[(Mn4Cl)3(BTT)8]2, where H3BTT = benzene-1,3,5-tris(1H-tetrazole)

Structure: Consists of truncated octahedral cages that share square faces, leading to pores of about 10 Å in diameter. Contains open Mn2+ coordination sites.

Hydrogen storage capacity: 60 g/L at 77 K and 90 bar; 12.1 g/L at 90 bar and 298 K.

This MOF is the first demonstration of open metal coordination sites increasing strength of hydrogen adsorption, which results in improved performance at 298 K. It has relatively strong metal-hydrogen interactions, attributed to a spin state change upon binding or to a classical Coulombic attraction.

Cu3(BTC)2(H2O)3, where H3BTC = 1,3,5-benzenetricarboxylic acid

Structure: Consists of octahedral cages that share paddlewheel units to define pores of about 9.8 Å in diameter.

High hydrogen uptake is attributed to overlapping attractive potentials from multiple copper paddle-wheel units: each Cu(II) center can potentially lose a terminal solvent ligand bound in the axial position, providing an open coordination site for hydrogen binding.

Structural Impacts on Hydrogen Storage Capacity
To date, hydrogen storage in MOFs at room temperature is a battle between maximizing storage capacity and maintaining reasonable desorption rates, while conserving the integrity of the adsorbent framework (e.g. completely evacuating pores, preserving the MOF structure, etc.) over many cycles. There are two major strategies governing the design of MOFs for hydrogen storage:
 * 1) to increase the theoretical storage capacity of the material, and
 * 2) to bring the operating conditions closer to ambient temperature and pressure.

Rowsell and Yaghi have identified several directions to these ends in some of the early papers.

Surface Area
The general trend in MOFs used for hydrogen storage is that the greater the surface area, the more hydrogen the MOF can store. This is because high surface area materials tend to exhibit increased micropore volume and inherently low bulk density, allowing for more hydrogen adsorption to occur.

Hydrogen Adsorption Enthalpy
High hydrogen adsorption enthalpy is also important. Theoretical studies have shown that 22-25 kJ/mol interactions are ideal for hydrogen storage at room temperature, as they are strong enough to adsorb H2, but weak enough to allow for quick desorption. The interaction between hydrogen and uncharged organic linkers is not this strong, and so a considerable amount of work has gone into synthesis of MOFs with exposed metal sites, to which hydrogen adsorbs with an enthalpy of 5-10 kJ/mol. Synthetically, this may be achieved by using ligands whose geometries prevent the metal from being fully coordinated, by removing volatile metal-bound solvent molecules over the course of synthesis, and by post-synthetic impregnation with additional metal cations. (C5H5)V(CO)3(H2) and Mo(CO)5(H2) are great examples of increased binding energy due to open metal coordination sites; however, their high metal-hydrogen bond dissociation energies result in a tremendous release of heat upon loading with hydrogen, which is not favorable for fuel cells. MOFs, therefore, should avoid orbital interactions that lead to such strong metal-hydrogen bonds and employ simple charge-induced dipole interactions, as demonstrated in Mn3[(Mn4Cl)3(BTT)8]2.

An association energy of 22-25 kJ/mol is typical of charge-induced dipole interactions, and so there is interest in the use of charged linkers and metals. The metal–hydrogen bond strength is diminished in MOFs, probably due to charge diffusion, so 2+ and 3+ metal ions are being studied to strengthen this interaction even further. A problem with this approach is that MOFs with exposed metal surfaces have lower concentrations of linkers; this makes them difficult to synthesize, as they are prone to framework collapse. This may diminish their useful lifetimes as well.

Sensitivity to Air
MOFs are frequently air-sensitive. To compensate for this, specially constructed storage containers are required, which can be costly. Strong metal-ligand bonds, such as in metal-imidazolate, -triazolate, and -pyrazolate frameworks, are known to decrease a MOF's sensitivity to air, reducing the expense of storage.

Pore Size
In a microporous material where physisorption and weak van der Waals forces dominate adsorption, the storage density is greatly dependent on the size of the pores. Calculations of idealized homogeneous materials, such as graphitic carbons and carbon nanotubes, predict that a microporous material with 7 Å-wide pores will exhibit maximum hydrogen uptake at room temperature. At this width, exactly two layers of hydrogen molecules adsorb on opposing surfaces with no space left in between. 10 Å-wide pores are also of ideal size because at this width, exactly three layers of hydrogen can exist with no space in between. (A hydrogen molecule has a bond length of 0.74 Å with a van der Waals radius of 1.17 Å for each atom; therefore, its effective van der Waals length is 3.08 Å.)

Structural Defects
Structural defects also play an important role in the performance of MOFs. Room-temperature hydrogen uptake via bridged spillover is mainly governed by structural defects, which can have two effects: Structural defects can also leave metal-containing nodes incompletely coordinated. This enhances the performance of MOFs used for hydrogen storage by increasing the number of accessible metal centers. Finally, structural defects can affect the transport of phonons, which affects the thermal conductivity of the MOF.
 * 1) a partially collapsed framework can block access to pores; thereby reducing hydrogen uptake, and
 * 2) lattice defects can create an intricate array of new pores and channels causing increased hydrogen uptake.

Synthesis of MOFs
The study of MOFs developed from the study of zeolites, with very little change in synthetic technique. MOFs and zeolites alike are produced almost exclusively by hydrothermal or solvothermal techniques, where crystals are slowly grown from a hot solution of metal precursor, such as metal nitrates, and bridging ligands. Zeolite synthesis often makes use of a variety of templates, or structure-directing compounds, and a few examples of templating, particularly by organic anions, are seen in the MOF literature as well. A particular templating approach that is useful for MOFs intended for gas storage is the use of metal-binding solvents such as N,N-diethylformamide and water. In these cases, metal sites are exposed when the solvent is fully evacuated, allowing hydrogen to bind at these sites.

Post-synthetic modification of MOFs opens up another dimension of structural possibilities that might not be achieved by conventional synthesis. A great deal of recent work explores covalent modification of the bridging ligands. Of particular interest to MOFs for hydrogen storage are modifications which expose metal sites. This has been demonstrated with post-synthetic coordination of additional metal ions to sites on the bridging ligands, and addition and removal of metal atoms to the metal site.

Since ligands in MOFs typically bind reversibly, the slow growth of crystals allows defects to be redissolved, resulting in a material with millimeter-scale crystals and a near-equilibrium defect density. Solvothermal synthesis is useful for growing crystals suitable to structure determination, because crystals grow over the course of hours to days. However, the use of MOFs as storage materials for consumer products demands an immense scale-up of their synthesis. Scale-up of MOFs has not been widely studied, though several groups have demonstrated that microwaves can be used to nucleate MOF crystals rapidly from solution. This technique, termed “microwave-assisted solvothermal synthesis”, is widely used in the zeolite literature, and produces micron-scale crystals in a matter of seconds to minutes, in yields similar to the slow growth methods.

Composite MOF materials
Another approach to increasing adsorption in MOFs is to alter the system in such a way that chemisorption becomes possible. This has been achieved by making a composite material, which contains a MOF and a complex of platinum with activated carbon. In an effect known as hydrogen spillover, H2 can bind to the platinum surface through a dissociative mechanism which cleaves the hydrogen molecule into two hydrogen atoms and enables them to travel down the activated carbon onto the surface of the MOF. This produced a three-fold increase in the room-temperature storage capacity of a MOF; however, desorption can take upwards of 12 hours, and reversible desorption is sometimes observed for only two cycles. The relationship between hydrogen spillover and hydrogen storage properties in MOFs is not well understood, but further research in this direction may provide inexpensive boosts in hydrogen storage capacity.

Hydrogen Adsorption
Adsorption is the process of trapping atoms or molecules that are incident on a surface; therefore the adsorption capacity of a material increases with its surface area. In three dimensions, the maximum surface area will be obtained by a structure which is highly porous, such that atoms and molecules can access internal surfaces. This simple qualitative argument suggests that the highly porous metal-organic frameworks (MOFs) should be excellent candidates for hydrogen storage devices.

Adsorption can be broadly classified as being one of two types: physisorption or chemisorption. Physisorption is characterized by weak van der Waals interactions, and bond enthalpies typically less than 20 kJ/mol. Chemisorption, alternatively, is defined by stronger covalent and ionic bonds, with bond enthalpies between 250 and 500 kJ/mol. In both cases, the adsorbate atoms or molecules (i.e. the particles which adhere to the surface) are attracted to the adsorbent (solid) surface because of the surface energy that results from unoccupied bonding locations at the surface. The degree of orbital overlap then determines if the interactions will be physisorptive or chemisorptive.

Adsorption of molecular hydrogen in MOFs is physisorptive. Since molecular hydrogen only has two electrons, dispersion forces are weak, typically 4-7 kJ/mol, and are only sufficient for adsorption at temperatures below 298 K.

Determining Hydrogen Storage Capacity
For the characterization of MOFs as hydrogen storage materials, there are two hydrogen-uptake measurement methods: gravimetric and volumetric. To obtain the total amount of hydrogen in the MOF, both the amount of hydrogen absorbed on its surface and the amount of hydrogen residing in its pores should be considered. To calculate the absolute absorbed amount (Nabs), the surface excess amount (Nex) is added to the product of the bulk density of hydrogen (ρbulk) and the pore volume of the MOF (Vpore), as shown in the following equation: Nabs=Nex+(ρbulk)(Vpore)

Gravimetric Method
The increased mass of the MOF due to the stored hydrogen is directly calculated by a highly sensitive microbalance. The mass of the adsorbed hydrogen decreases when high pressure is applied to the system due to its buoyancy. This weight loss is calculated by the volume of the MOF’s frame and the density of hydrogen.

Volumetric Method
The changing of amount of hydrogen stored in the MOF is measured by detecting the varied pressure of hydrogen at constant volume. The volume of adsorbed hydrogen in the MOF is then calculated by subtracting the volume of hydrogen in free space from the total volume of dosed hydrogen.

Other Methods of Hydrogen Storage
There are six possible methods that can be used for the reversible storage of hydrogen with a high volumetric and gravimetric density, which are summarized in the following table, (where ρm is the gravimetric density, ρv is the volumetric density, T is the working temperature, and P is the working pressure):

Of these, high-pressure gas cylinders and liquid hydrogen in cryogenic tanks are the least practical ways to store hydrogen for the purpose of fuel due to the extremely high pressure required for storing hydrogen gas or the extremely low pressure required for storing hydrogen liquid. The other methods are all being studied and developed extensively.

Other Applications of MOFs
There are many uses of MOFs other than hydrogen storage, such as gas purification, gas separation, gas storage (other than hydrogen), and heterogeneous catalysis. MOFs are used for gas purification because of strong chemisorption that takes place between electron-rich, odor-generating molecules (such as amines, phosphines, oxygenates, alcohols, water, or sulfur-containing molecules) and the framework, allowing the desired gas to pass through the MOF. Gas separation can be performed with MOFs because they can allow certain molecules to pass through their pores based on size and kinetic diameter. This is particularly important for separating out carbon dioxide. Regarding gas storage, MOFs can store molecules such as carbon dioxide, carbon monoxide, methane, and oxygen due to their high adsorption enthalpies (similar to hydrogen). Finally, MOFs are used for catalysis because of their shape and size selectivity and their accessible bulk volume. Also, because of their very porous architecture, mass transport in the pores is not hindered.

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