Scanning electron microscope image of the seed inside a MOF crystal
Metal–organic frameworks (MOFs) are compounds consisting of metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. They are a subclass of coordination polymers, with the special feature that they are often porous. The organic ligands included are sometimes referred to as "struts", one example being 1,4-benzenedicarboxylic acid (BDC).
More formally, a metal–organic framework is a coordination network with organic ligands
containing potential voids. A coordination network is a coordination
compound extending, through repeating coordination entities, in one
dimension, but with cross-links between two or more individual chains,
loops, or spiro-links, or a coordination compound extending through
repeating coordination entities in two or three dimensions; and finally a
coordination polymer is a coordination compound with repeating coordination entities extending in one, two, or three dimensions.
In some cases, the pores are stable during elimination of the
guest molecules (often solvents) and could be refilled with other
compounds. Because of this property, MOFs are of interest for the
storage of gases such as hydrogen and carbon dioxide. Other possible applications of MOFs are in gas purification, in gas separation, in catalysis, as conducting solids and as supercapacitors.
The synthesis and properties of MOFs constitute the primary focus of the discipline called reticular chemistry (from Latin reticulum, "small net"). In contrast to MOFs, covalent organic framework (COFs) are made entirely from light elements (H, B, C, N, and O) with extended structures.
Structure
MOFs
are composed of two major components: a metal ion or cluster of metal
ions and an organic molecule called a linker. For this reason, the
materials are often referred to as hybrid organic–inorganic materials;
however, this terminology has recently been explicitly discouraged. The organic units are typically mono-, di-, tri-, or tetravalent ligands. The choice of metal and linker dictates the structure and hence 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.
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|
Dimensionality of Inorganic | ||||
|---|---|---|---|---|---|
| 0 | 1 | 2 | 3 | ||
| 0 | Molecular Complexes | Hybrid Inorganic Chains | Hybrid Inorganic Layers | 3-D Inorganic Hybrids | |
| 1 | Chain Coordination Polymers | Mixed Inorganic-Organic Layers | Mixed Inorganic-Organic 3-D Framework |
| |
| 2 | Layered Coordination Polymer | Mixed Inorganic-Organic 3-D Framework |
| ||
| 3 | 3-D Coordination Polymers |
| |||
To describe and organize the structures of MOFs, a system of
nomenclature has been developed. Subunits 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.
Attached to the SBUs are bridging ligands.
For MOF's, typical bridging ligands are di- and tricarboxylic acids.
These ligands typically have rigid backbones. Examples are
benzene-1,4-dicarboxylic acid (BDC or teraphthalic acid,
biphenyl-4,4'-dicarboxylic acid (BPDC), and the tricarboxylic acid trimesic acid.
SBU's are often derived from basic zinc acetate structure, the acetates being replaced by rigid di- and tricarboxylates.
Synthesis
General Synthesis
The study of MOFs developed from the study of zeolite. Except for the use of preformed ligands, MOFs and zeolites are produced almost exclusively by hydrothermal
or solvothermal techniques, where crystals are slowly grown from a hot
solution. In contrast with zeolites, MOFs are constructed from bridging
organic ligands that remain intact throughout the synthesis.
Zeolite synthesis often makes use of a "template". Templates are ions
that influence the structure of the growing inorganic framework.
Typical templating ions are quaternary ammonium cations, which are
removed later. In MOFs, the framework is templated by the SBU (secondary
building unit) and the organic ligands.
A 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
evacuated, allowing hydrogen to bind at these sites.
Since ligands in MOFs typically bind reversibly, the slow growth
of crystals often 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.
A solvent-free synthesis of a range of crystalline MOFs has been described. Usually the metal acetate and the organic proligand are mixed and ground up with a ball mill. Cu3(BTC)2 can be quickly synthesised in this way in quantitative yield. In the case of Cu3(BTC)2
the morphology of the solvent free synthesised product was the same as
the industrially made Basolite C300. It is thought that localised
melting of the components due to the high collision energy in the ball
mill may assist the reaction. The formation of acetic acid as a
by-product in the reactions in the ball mill may also help in the
reaction having a solvent effect in the ball mill.
A recent advancement in the solvent-free preparation of MOF films and composites is their synthesis by chemical vapor deposition. This process, MOF-CVD,
was first demonstrated for ZIF-8 and consist of two steps. In a first
step, metal oxide precursor layers are deposited. In the second step,
these precursor layers are exposed to sublimed
ligand molecules, that induce a phase transformation to the MOF crystal
lattice. Formation of water during this reaction plays a crucial role
in directing the transformation.
Post-synthetic Modification
Although
the three-dimensional structure and internal environment of the pores
can be in theory controlled through proper selection of nodes and
organic linking groups, the direct synthesis of such materials with the
desired functionalities can be difficult due to the high sensitivity of
MOF systems. Thermal and chemical sensitivity, as well as high
reactivity of reaction materials, can make forming desired products
challenging to achieve. The exchange of guest molecules and counter-ions
and the removal of solvents allow for some additional functionality but
are still limited to the integral parts of the framework.
The post-synthetic exchange of organic linkers and metal ions is an
expanding area of the field and opens up possibilities for more complex
structures, increased functionality, and greater system control.
Ligand Exchange
Post-synthetic
modification techniques can be used to exchange an existing organic
linking group in a prefabricated MOF with a new linker by ligand exchange or partial ligand exchange.
This exchange allows for the pores and, in some cases the overall
framework of MOFs, to be tailored for specific purposes. Some of these
uses include fine-tuning the material for selective adsorption, gas
storage, and catalysis.
To perform ligand exchange prefabricated MOF crystals are washed with
solvent and then soaked in a solution of the new linker. The exchange
often requires heat and occurs on the time scale of a few days. Post-synthetic ligand exchange also enables the incorporation of functional groups
into MOFs that otherwise would not survive MOF synthesis, due to
temperature, pH, or other reaction conditions, or hinder the synthesis
itself by competition with donor groups on the loaning ligand.
Metal Exchange
Post-synthetic
modification techniques can also be used to exchange an existing metal
ion in a prefabricated MOF with a new metal ion by metal ion exchange .
The complete metal metathesis from an integral part of the framework has
been achieved without altering the framework or pore structure of the
MOF. Similarly to post-synthetic ligand exchange post-synthetic metal
exchange is performed by washing prefabricated MOF crystals with solvent
and then soaking the crystal in a solution of the new metal.
Post-synthetic metal exchange allows for a simple route to the
formation of MOFs with the same framework yet different metal ions.
Stratified Synthesis
In
addition to modifying the functionality of the ligands and metals
themselves, post-synthetic modification can be used to expand upon the
structure of the MOF. Using post-synthetic modification MOFs can be
converted from a highly ordered crystalline material toward a
heterogeneous porous material.
Using post-synthetic techniques, it is possible for the controlled
installation of domains within a MOF crystal which exhibit unique
structural and functional characteristics. Core-shell MOFs and other
layered MOFs have been prepared where layers of have unique
functionalization but in most cases are crystallographically compatible from layer to layer.
Composite materials
Another approach to increasing adsorption in MOFs is to alter the system in such a way that chemisorption becomes possible. This functionality has been introduced 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
innovation produced a threefold 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 may prove relevant to
hydrogen storage.
Hydrogen storage
Molecular hydrogen has the highest specific energy
of any fuel. However unless the hydrogen gas is compressed, its
volumetric energy density is very low, so the transportation and storage
of hydrogen require energy-intensive compression and liquefaction
processes.
Therefore, development of new hydrogen storage methods which decrease
the concomitant pressure required for practical volumetric energy
density is an active area of research. MOFs attract attention as materials for adsorptive hydrogen storage because of their high specific surface areas and surface to volume ratios, as well as their chemically tunable structures. Compared to an empty gas cylinder, a MOF-filled gas cylinder can store more hydrogen at a given pressure because hydrogen molecules adsorb
to the surface of MOFs. 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, because the hydrogen uptake is based primarily on physisorption, many MOFs have a fully reversible uptake-and-release behavior. No large activation barriers are required 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.
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 (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.
MOF-177
The U.S. Department of Energy
(DOE) has published a list of yearly technical system targets for
on-board hydrogen storage for light-duty fuel cell vehicles which guide
researchers in the field (5.5 wt %/40 g L−1 by 2017; 7.5 wt %/70 g L−1 ultimate).
Materials with high porosity and high surface area such as MOFs have
been designed and synthesized in an effort to meet these targets. These
adsorptive materials generally work via physical adsorption rather than
chemisorption due to the large HOMO-LUMO
gap and low HOMO energy level of molecular hydrogen. A benchmark
material to this end is MOF-177 which was found to store hydrogen at 7.5
wt % with a volumetric capacity of 32 g L−1 at 77 K and 70 bar. MOF-177 consists of [Zn4O]6+ clusters interconnected by 1,3,5-benzebetribenzoate organic linkers and has a measured BET surface area of 4630 m2 g−1. Another exemplary material is PCN-61 which exhibits a hydrogen uptake of 6.24 wt % and 42.5 g L−1 at 35 bar and 77 K and 2.25 wt % at atmospheric pressure. PCN-61 consists of [Cu2]4+
paddle-wheel units connected through
5,5′,5′′-benzene-1,3,5-triyltris(1-ethynyl-2-isophthalate) organic
linkers and has a measured BET surface area of 3000 m2 g−1.
Despite these promising MOF examples, the classes of synthetic porous
materials with the highest performance for practical hydrogen storage
are activated carbon and covalent organic frameworks (COFs).
Design principles
Practical
applications of MOFs for hydrogen storage are met with several
challenges. For hydrogen adsorption near room temperature, the hydrogen binding energy would need to be increased considerably. 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 because
- they are either commercially available or easily synthesized,
- they have high acidity (pKa ˜ 4) allowing for facile in situ deprotonation,
- the metal-carboxylate bond formation is reversible, facilitating the formation of well-ordered crystalline MOFs, and
- 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+ and Zn2+. Lighter main group metal ions have also been explored. Be12(OH)12(BTB)4,
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 of several MOFs that are considered to
have the best properties for hydrogen storage as of May 2012 (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 U.S. DOE. 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(BTE)(BPDC), where BTE3−=4,4′,4″-[benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)]tribenzoate and BPDC2−=biphenyl-4,4′-dicarboxylate (MOF-210)
Hydrogen storage capacity (at 77 K): 8.6 excess wt% (17.6 total wt%) at 77 K and 80 bar. 44 total g H2/L at 80 bar and 77 K.
Hydrogen storage capacity (at 298 K): 2.90 delivery wt% (1-100 bar) at 298 K and 100 bar. - Zn4O(BBC)2, where BBC3−=4,4′,4″-[benzene-1,3,5-triyl-tris(benzene-4,1-diyl)]tribenzoate (MOF-200) Hydrogen storage capacity (at 77 K): 7.4 excess wt% (16.3 total wt%) at 77 K and 80 bar. 36 total g H2/L at 80 bar and 77 K. Hydrogen storage capacity (at 298 K): 3.24 delivery wt% (1-100 bar) at 298 K and 100 bar.
- 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 BDC2−=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. 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 in 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. Most common strategies to increase this binding energy for MOFs and molecular hydrogen have been reviewed.
Sensitivity to airborne moisture
MOFs
are frequently sensitive to moisture in the air. In particular, IRMOF-1
degrades in the presence of small amounts of water at room temperature.
Studies on metal analogues have unraveled the ability of metals other
than Zn to stand higher water concentrations at high temperatures.
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:
- 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.
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.
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.
A complete explanation of the H2 sorption mechanism in
MOFs was achieved by statistical averaging in the grand canonical
ensemble, exploring a wide range of pressures and temperatures. Later work revealed a pore filling mechanism as important in both COFs and MOFs.
Determining hydrogen storage capacity
Two hydrogen-uptake measurement methods are used for the characterization of MOFs as hydrogen storage materials: 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:
Gravimetric method
The increased mass of the MOF due to the stored hydrogen is directly calculated by a highly sensitive microbalance. Due to buoyancy,
the detected mass of adsorbed hydrogen decreases again when a
sufficiently high pressure is applied to the system because the density
of the surrounding gaseous hydrogen becomes more and more important at
higher pressures. Thus, this "weight loss" has to be corrected using 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):
| Storage method | ρm (mass%) | ρv (kg H2/m3) | T (°C) | P (bar) | Remarks |
|---|---|---|---|---|---|
| High-pressure gas cylinders | 13 | <40 font=""> | 25 | 800 | Compressed H2 gas in lightweight composite cylinder |
| Liquid hydrogen in cryogenic tanks | size-dependent | 70.8 | −252 | 1 | Liquid H2; continuous loss of a few percent of H2 per day at 25 °C |
| Adsorbed hydrogen | ~2 | 20 | −80 | 100 | Physisorption of H2 on materials |
| Adsorbed on interstitial sites in a host metal | ~2 | 150 | 25 | 1 | Atomic hydrogen reversibly adsorbs in host metals |
| Complex compounds | <18 font=""> | 150 | >100 | 1 | Complex compounds ([AlH4]− or [BH4]−); desorption at elevated temperature, adsorption at high pressures |
| Metal and complexes together with water | <40 font=""> | >150 | 25 | 1 | Chemical oxidation of metals with water and liberation of H2 |
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 temperature required for storing hydrogen liquid. The other methods are all being studied and developed extensively.
Catalysis
MOFs have potential as heterogeneous catalysts, although applications have not been commercialized.
Their high surface area, tunable porosity, diversity in metal and
functional groups make them especially attractive for use as catalysts.
Zeolites are extraordinarily useful in catalysis.
Zeolites are limited by the fixed tetrahedral coordination of the Si/Al
connecting points and the two-coordinated oxide linkers. Fewer than
200 zeolites are known. In contrast with this limited scope, MOFs
exhibit more diverse coordination geometries, polytopic linkers, and ancillary ligands (F−, OH−, H2O
among others). It is also difficult to obtain zeolites with pore sizes
larger than 1 nm, which limits the catalytic applications of zeolites to
relatively small organic molecules (typically no larger than xylenes).
Furthermore, mild synthetic conditions typically employed for MOF
synthesis allow direct incorporation of delicate functionalities into
the framework structures. Such a process would not be possible with
zeolites or other microporous crystalline oxide-based materials because
of the harsh conditions typically used for their synthesis (e.g., calcination at high temperatures to remove organic templates).
Zeolites still cannot be obtained in enantiopure form, which precludes their applications in catalytic asymmetric synthesis,
e.g., for the pharmaceutical, agrochemical, and fragrance industries.
Enantiopure chiral ligands or their metal complexes have been
incorporated into MOFs to lead to efficient asymmetric catalysts. Even
some MOF materials may bridge the gap between zeolites and enzymes when they combine isolated polynuclear sites, dynamic host–guest responses, and a hydrophobic cavity environment. MOFs might be useful for making semi-conductors. Theoretical calculations show that MOFs are semiconductors or insulators with band gaps between 1.0 and 5.5 eV which can be altered by changing the degree of conjugation in the ligands indicating its possibility for being photocatalysts.
Design
Example of MOF-5
Like other heterogeneous catalysts, MOFs may allow for easier post-reaction separation and recyclability than homogeneous catalysts.
In some cases, they also give a highly enhanced catalyst stability.
Additionally, they typically offer substrate-size selectivity.
Nevertheless, while clearly important for reactions in living systems,
selectivity on the basis of substrate size is of limited value in
abiotic catalysis, as reasonably pure feedstocks are generally
available.
Metal ions or metal clusters
Example of zeolite catalyst
Among the earliest reports of MOF-based catalysis was the cyanosilylation of aldehydes by a 2D MOF (layered square grids) of formula Cd(4,4’-bpy)2(NO3)2.
This investigation centered mainly on size- and shape-selective
clathration. A second set of examples was based on a two-dimensional,
square-grid MOF containing single Pd(II) ions as nodes and 2-hydroxypyrimidinolates as struts. Despite initial coordinative saturation, the palladium centers in this MOF catalyze alcohol oxidation, olefin
hydrogenation, and Suzuki C–C coupling. At a minimum, these reactions
necessarily entail redox oscillations of the metal nodes between Pd(II) and Pd(0)
intermediates accompanying by drastic changes in coordination number,
which would certainly lead to destabilization and potential destruction
of the original framework if all the Pd
centers are catalytically active. The observation of substrate shape-
and size-selectivity implies that the catalytic reactions are
heterogeneous and are indeed occurring within the MOF. Nevertheless, at
least for hydrogenation, it is difficult to rule out the possibility
that catalysis is occurring at the surface of MOF-encapsulated palladium
clusters/nanoparticles (i.e., partial decomposition sites) or defect
sites, rather than at transiently labile, but otherwise intact,
single-atom MOF nodes. "Opportunistic" MOF-based catalysis has been
described for the cubic compound, MOF-5. This material comprises coordinatively saturated Zn4O
nodes and a fully complexed BDC struts (see above for abbreviation);
yet it apparently catalyzes the Friedel–Crafts tert-butylation of both toluene and biphenyl. Furthermore, para alkylation is strongly favored over ortho alkylation, a behavior thought to reflect the encapsulation of reactants by the MOF.
Functional struts
The porous-framework material [Cu3(btc)2(H2O)3], also known as HKUST-1,
contains large cavities having windows of diameter ~6 Å. The
coordinated water molecules are easily removed, leaving open Cu(II)
sites. Kaskel and co-workers showed that these Lewis acid sites could catalyze the cyanosilylation of benzaldehyde or acetone. The anhydrous version of HKUST-1 is an acid catalyst. Compared to Brønsted vs. Lewis acid-catalyzed
pathways, the product selectivity are distinctive for three reactions:
isomerization of a-pinene oxide, cyclization of citronellal, and
rearrangement of a-bromoacetals, indicating that indeed [Cu3(btc)2] functions primarily as a Lewis acid catalyst.
MIL-101, a large-cavity MOF having the formula [Cr3F(H2O)2O(BDC)3], is a cyanosilylation catalyst.
The coordinated water molecules in MIL-101 are easily removed to expose
Cr(III) sites. As one might expect, given the greater Lewis acidity of
Cr(III) vs. Cu(II), MIL-101 is much more active than HKUST-1 as a
catalyst for the cyanosilylation of aldehydes.
Additionally, the Kaskel group observed that the catalytic sites of
MIL-101, in contrast to those of HKUST-1, are immune to unwanted
reduction by benzaldehyde. The Lewis-acid-catalyzed cyanosilylation of aromatic aldehydes has also been carried out by Long and co-workers using a MOF of the formula Mn3[(Mn4Cl)3BTT8(CH3OH)10].
This material contains a three-dimensional pore structure, with the
pore diameter equaling 10 Å. In principle, either of the two types of Mn(II) sites could function as a catalyst.
Noteworthy features of this catalyst are high conversion yields (for
small substrates) and good substrate-size-selectivity, consistent with
channellocalized catalysis.
Encapsulated catalysts
The MOF encapsulation approach invites comparison to earlier studies of oxidative catalysis by zeolite-encapsulated Fe(porphyrin) as well as Mn(porphyrin) systems. The zeolite studies generally employed iodosylbenzene
(PhIO), rather than TPHP as oxidant. The difference is likely
mechanistically significant, thus complicating comparisons. Briefly,
PhIO is a single oxygen atom donor, while TBHP is capable of more
complex behavior. In addition, for the MOF-based system, it is
conceivable that oxidation proceeds via both oxygen transfer from a manganese oxo intermediate as well as a manganese-initiated
radical chain reaction pathway. Regardless of mechanism, the approach
is a promising one for isolating and thereby stabilizing the porphyrins against both oxo-bridged dimer formation and oxidative degradation.
Metal-free organic cavity modifiers
Most examples of MOF-based catalysis make use of metal ions or atoms as active sites. Among the few exceptions are two nickel- and two copper-containing MOFs synthesized by Rosseinsky and co-workers. These compounds employ amino acids(L- or D-aspartate) together with dipyridyls as struts. The coordination chemistry is such that the amine group of the aspartate cannot be protonated by added HCl, but one of the aspartate carboxylates can. Thus, the framework-incorporated amino acid can exist in a form that is not accessible for the free amino acid. While the nickel-based compounds are marginally porous, on account of tiny channel dimensions, the copper versions are clearly porous.
The Rosseinsky group showed that the carboxylic acids
behave as Brønsted acidic catalysts, facilitating (in the copper cases)
the ring-opening methanolysis of a small, cavityaccessible epoxide at up to 65% yield. Superior homogeneous catalysts exist however.
Kitagawa and co-workers have reported the synthesis of a catalytic MOF having the formula [Cd(4-BTAPA)2(NO3)2].
The MOF is three-dimensional, consisting of an identical catenated pair
of networks, yet still featuring pores of molecular dimensions. The
nodes consist of single cadmium ions, octahedrally ligated by pyridyl nitrogens. From a catalysis standpoint, however, the most interesting feature of this material is the presence of guest-accessible amide functionalities. The amides are capable of base-catalyzing the Knoevenagel condensation of benzaldehyde with malononitrile. Reactions with larger nitriles,
however, are only marginally accelerated, implying that catalysis takes
place chiefly within the material's channels rather than on its
exterior. A noteworthy finding is the lack of catalysis by the free
strut in homogeneous solution, evidently due to intermolecular H-bonding
between bptda molecules. Thus, the MOF architecture elicits catalytic
activity not otherwise encountered.
In an interesting alternative approach, Férey and coworkers were able to modify the interior of MIL-101 via Cr(III) coordination of one of the two available nitrogen atoms of each of several ethylenediamine molecules. The free non-coordinated ends of the ethylenediamines were then used as Brønsted basic catalysts, again for Knoevenagel condensation of benzaldehyde with nitriles.
A third approach has been described by Kim Kimoon and coworkers. Using a pyridine-functionalized derivative of tartaric acid and a Zn(II)
source they were able to synthesize a 2D MOF termed POST-1. POST-1
possesses 1D channels whose cross sections are defined by six trinuclear
zinc clusters and six struts. While three of the six pyridines are coordinated by zinc ions,
the remaining three are protonated and directed toward the channel
interior. When neutralized, the noncoordinated pyridyl groups are found
to catalyze transesterification reactions, presumably by facilitating deprotonation of the reactant alcohol. The absence of significant catalysis when large alcohols are employed strongly suggests that the catalysis occurs within the channels of the MOF.
Achiral catalysis
Schematic Diagram for MOF Catalysis
Metals as catalytic sites
The metals in the MOF structure often act as Lewis acids.
The metals in MOFs often coordinate to labile solvent molecules or
counter ions which can be removed after activation of the framework. The
Lewis acidic nature of such unsaturated metal centers can activate the
coordinated organic substrates for subsequent organic transformations.
The use of unsaturated metal centers was demonstrated in the
cyanosilylation of aldehydes and imines by Fujita and coworkers in 2004. They reported MOF of composition {[Cd(4,4′-bpy)2(H2O)2] • (NO3)2 • 4H2O} which was obtained by treating linear bridging ligand 4,4′-bipyridine (bpy) with Cd(NO3)2
. The Cd(II) centers in this MOF possesses a distorted octahedral
geometry having four pyridines in the equatorial positions, and two
water molecules in the axial positions to form a two-dimensional
infinite network. On activation, two water molecules were removed
leaving the metal centers unsaturated and Lewis acidic. The Lewis acidic
character of metal center was tested on cyanosilylation reactions of imine
where the imine gets attached to the Lewis-acidic metal centre
resulting in higher electrophilicity of imines. For the cyanosilylation
of imines, most of the reactions were complete within 1 h affording
aminonitriles in quantitative yield. Kaskel and coworkers
carried out similar cyanosilylation reactions with coordinatively
unsaturated metals in three-dimensional (3D) MOFs as heterogeneous
catalysts. The 3D framework [Cu3(btc)2(H2O)3] (btc: Benzene-1,3,5- tricarboxylate) (HKUST-1) used in this study was first reported by Williams et al. The open framework of [Cu3(btc)2(H2O)3]
is built from dimeric cupric tetracarboxylate units (paddle-wheels)
with aqua molecules coordinating to the axial positions and btc bridging
ligands. The resulting framework after removal of two water molecules
from axial positions possesses porous channel. This activated MOF
catalyzes the trimethylcyanosilylation of benzaldehydes
with a very low conversion (<5 10="" 1="" 24="" 293="" 313="" 57="" 72="" 89="" a="" after="" as="" at="" background="" but="" by="" comparison="" conditions.="" conversion="" cu="" decomposition="" due="" for="" framework="" from="" good="" h.="" h="" href="https://en.wikipedia.org/wiki/Aldehyde" in="" increase="" k.="" k="" less="" like="" mof="" observed="" obtained="" of="" problems="" raised="" reaction="" reduction="" same="" selectivity="" some="" strategy="" suffers="" temperature="" than="" the="" this="" title="Aldehyde" to="" under="" was="" with="" without="">aldehydes
;
2) strong solvent inhibition effect; electron donating solvents such as
THF competed with aldehydes for coordination to the Cu(II) sites, and
no cyanosilylation product was observed in these solvents; 3) the
framework instability in some organic solvents. Several other groups
have also reported the use of metal centres in MOFs as catalysts Again, electron-deficient nature of some metals and metal clusters makes the resulting MOFs efficient oxidation catalysts. Mori and coworkers reported MOFs with Cu2 paddle wheel units as heterogeneous catalysts for the oxidation of alcohols. The catalytic activity of the resulting MOF was examined by carrying out alcohol oxidation with H2O2
as the oxidant. It also catalyzed the oxidation of primary alcohol,
secondary alcohol and benzyl alcohols with high selectivity. Hill et al. have demonstrated the sulfoxidation of thioethers using an MOF based on vanadium-oxo cluster V6O13 building units.
Functional linkers as catalytic sites
Functional linkers can be also utilized as catalytic sites. A 3D MOF {[Cd(4- BTAPA)2(NO3)2] • 6H2O • 2DMF} (4-BTAPA=1,3,5-benzene tricarboxylic acid tris [N-(4-pyridyl)amide], DMF=N,N-dimethylformamide) constructed by tridentate amide linkers and cadmium salt catalyzes the Knoevenagel condensation reaction.
The pyridine groups on the ligand 4-BTAPA act as ligands binding to the
octahedral cadmium centers, while the amide groups can provide the
functionality for interaction with the incoming substrates.
Specifically, the – NH moiety of the amide group can act as electron acceptor whereas the C=O group can act as electron donor to activate organic substrates for subsequent reactions. Ferey et al. reported a robust and highly porous MOF [Cr3(µ3-O)F(H2O)2(BDC)3]
(BDC: Benzene-1,4- dicarboxylate) where instead of directly using the
unsaturated Cr(III) centers as catalytic sites, the authors grafted ethylenediamine
(ED) onto the Cr(III) sites. The uncoordinated ends of ED can act as
base catalytic sites, ED-grafted MOF was investigated for Knoevenagel
condensation reactions. A significant increase in conversion was
observed for ED-grafted MOF compared to untreated framework (98% vs
36%).
Entrapment of catalytically active noble metal nanoparticles
The entrapment of catalytically active noble metals
can be accomplished by grafting on functional groups to the unsaturated
metal site on MOFs. Ethylenediamine (ED) has been shown to be grafted
on the Cr metal sites and can be further modified to encapsulate noble
metals such as Pd. The entraped Pd has similar catalytic activity as Pd/C in the Heck reaction. Ruthenium nanoparticles have catalytic activity in a number of reactions when entrapped in the MOF-5 framework. This Ru-encapsulated MOF catalyzes oxidation of benzyl alcohol to benzyaldehyde, although degradation of the MOF occurs. The same catalyst was used in the hydrogenation of benzene to cyclohexane.
In another example, Pd nanoparticles embedded within defective HKUST-1
framework enable the generation of tunable Lewis basic sites.
Therefore, this multifunctional Pd/MOF composite is able to perform
stepwise benzyl alcohol oxidation and Knoevenagel condensation.
Reaction hosts with size selectivity
MOFs might prove useful for both photochemical and polymerization reactions due to the tuneability of the size and shape of their pores. A 3D MOF {[Co(bpdc)3(bpy)] • 4DMF • H2O} (bpdc: biphenyldicarboxylate, bpy: 4,4′-bipyridine) was synthesized by Li and coworkers. Using this MOF photochemistry of o-methyl dibenzyl ketone (o-MeDBK)
was extensively studied. This molecule was found to have a variety of
photochemical reaction properties including the production of cyclopentanol.
MOFs have been used to study polymerization in the confined space of
MOF channels. Polymerization reactions in confined space might have
different properties than polymerization in open space. Styrene, divinylbenzene, substituted acetylenes, methyl methacrylate, and vinyl acetate have all been studied by Kitagawa and coworkers as possible activated monomers for radical polymerization. Due to the different linker size the MOF channel size could be tunable on the order of roughly 25 and 100 Å2.
The channels were shown to stabilize propagating radicals and suppress
termination reactions when used as radical polymerization sites.
Asymmetric catalysis
Several strategies exist for constructing homochiral MOFs. Crystallization of homochiral MOFs via self-resolution from achiral linker ligands is one of the way to accomplish such a goal. However, the resulting bulk samples contain both enantiomorphs and are racemic. Aoyama and coworkers successfully obtained homochiral MOFs in the bulk from achiral ligands by carefully controlling nucleation in the crystal growth process. Zheng and coworkers
reported the synthesis of homochiral MOFs from achiral ligands by
chemically manipulating the statistical fluctuation of the formation of
enantiomeric pairs of crystals. Growing MOF crystals under chiral
influences is another approach to obtain homochiral MOFs using achiral
linker ligands. Rosseinsky and coworkers have introduced a chiral coligand to direct the formation of homochiral MOFs by controlling the handedness of the helices during the crystal growth. Morris and coworkers utilized ionic liquid
with chiral cations as reaction media for synthesizing MOFs, and
obtained homochiral MOFs. The most straightforward and rational strategy
for synthesizing homochiral MOFs is, however, to use the readily
available chiral linker ligands for their construction.
Homochiral MOFs with interesting functionalities and reagent-accessible channels
Homochiral MOFs have been made by Lin and coworkers using 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (BINAP), 1,1′-bi-2,2′-naphthol (BINOL) as a chiral ligands. These ligands can coordinate with catalytically active metal sites to enhance the enantioselectivity. A variety of linking groups such as pyridine, phosphonic acid, and carboxylic acid
can be selectively introduced to the 3,3′, 4,4′, and the 6,6′ positions
of the 1,1'-binaphthyl moiety. Moreover, by changing the length of the
linker ligands the porosity and framework structure of the MOF can be
selectivily tuned.
Postmodification of homochiral MOFs
Lin
and coworkers have shown that the postmodification of MOFs can be
achieved to produce enantioselective homochiral MOFs for use as
catalysts. The resulting 3D homochiral MOF {[Cd3(L)3Cl6]• 4DMF • 6MeOH • 3H2O}
(L=(R)-6,6'-dichloro-2,2'-dihydroxyl-1,1'-binaphthyl-bipyridine)
synthesized by Lin was shown to have a similar catalytic efficiency for
the diethylzinc addition reaction as compared to the homogeneous
analogue when was pretreated by Ti(OiPr)4 to
generate the grafted Ti- BINOLate species. The catalytic activity of
MOFs can vary depending on the framework structure. Lin and others found
that MOFs synthesized from the same materials could have drastically
different catalytic activities depending on the framework structure
present.
Homochiral MOFs with precatalysts as building blocks
Another
approach to construct catalytically active homochiral MOFs is to
incorporate chiral metal complexes which are either active catalysts or
precatalysts directly into the framework structures. For example, Hupp
and coworkers have combined a chiral ligand and bpdc (bpdc: biphenyldicarboxylate) with Zn(NO3)2
and obtained twofold interpenetrating 3D networks. The orientation of
chiral ligand in the frameworks makes all Mn(III) sites accessible
through the channels. The resulting open frameworks showed catalytic
activity towards asymmetric olefin epoxidation reactions. No significant
decrease of catalyst activity was observed during the reaction and the
catalyst could be recycled and reused several times. Lin and coworkers have reported zirconium phosphonate-derived Ru-BINAP systems. Zirconium phosphonate-based chiral porous hybrid materials containing the Ru(BINAP)(diamine)Cl2 precatalysts showed excellent enantioselectivity (up to 99.2% ee) in the asymmetric hydrogenation of aromatic ketones.
Biomimetic design and photocatalysis
Some MOF materials may resemble enzymes when they combine isolated polynuclear sites, dynamic host–guest responses, and hydrophobic cavity environment which are characteristics of an enzyme. Some well-known examples of cooperative catalysis involving two metal ions in biological systems include: the diiron sites in methane monooxygenase, dicopper in cytochrome c oxidase, and tricopper oxidases which have analogy with polynuclear clusters found in the 0D coordination polymers, such as binuclear Cu2 paddlewheel units found in MOP-1 and [Cu3(btc)2] (btc=benzene-1,3,5-tricarboxylate) in HKUST-1 or trinuclear units such as {Fe3O(CO2)6} in MIL-88, and IRMOP-51. Thus, 0D MOFs have accessible biomimetic
catalytic centers. In enzymatic systems, protein units show "molecular
recognition", high affinity for specific substrates. It seems that
molecular recognition effects are limited in zeolites by the rigid
zeolite structure.
In contrast, dynamic features and guest-shape response make MOFs more
similar to enzymes. Indeed, many hybrid frameworks contain organic parts
that can rotate as a result of stimuli, such as light and heat. The porous channels in MOF structures can be used as photocatalysis
sites. In photocatalysis, the use of mononuclear complexes is usually
limited either because they only undergo single- electron process or
from the need for high-energy irradiation. In this case, binuclear
systems have a number of attractive features for the development of
photocatalysts. For 0D MOF structures, polycationic nodes can act as semiconductor quantum dots which can be activated upon photostimuli with the linkers serving as photon antennae. Theoretical calculations show that MOFs are semiconductors or insulators with band gaps between 1.0 and 5.5 eV which can be altered by changing the degree of conjugation in the ligands. Experimental results show that the band gap of IRMOF-type samples can be tuned by varying the functionality of the linker.
Additional potential applications
Biological imaging and sensing
MOF-76
crystal, where oxygen, carbon, and lanthanide atoms are represented by
maroon, black, and blue spheres, respectively. Includes metal node
connectivity (blue polyhedra), infinite-rod SBU, and 3D representation
of MOF-76.
A potential application for MOFs is biological imaging and sensing
via photoluminescence. A large subset of luminescent MOFs use
lanthanides in the metal clusters. Lanthanide photoluminescence
has many unique properties that make them ideal for imaging
applications, such as characteristically sharp and generally
non-overlapping emission bands in the visible and near-infrared (NIR)
regions of the spectrum, resistance to photobleaching or 'blinking', and
long luminescence lifetimes. However, lanthanide emissions are difficult to sensitize directly because they must undergo LaPorte
forbidden f-f transitions. Indirect sensitization of lanthanide
emission can be accomplished by employing the "antenna effect," where
the organic linkers act as antennae and absorb the excitation energy,
transfer the energy to the excited state of the lanthanide, and yield
lanthanide luminescence upon relaxation. A prime example of the antenna effect is demonstrated by MOF-76, which combines trivalent lanthanide ions and 1,3,5-benzenetricarboxylate (BTC) linkers to form infinite rod SBUs coordinated into a three dimensional lattice.
As demonstrated by multiple research groups, the BTC linker can
effectively sensitize the lanthanide emission, resulting in a MOF with
variable emission wavelengths depending on the lanthanide identity. Additionally, the Yan group has shown that Eu3+- and Tb3+-
MOF-76 can be used for selective detection of acetophenone from other
volatile monoaromatic hydrocarbons. Upon acetophenone uptake, the MOF
shows a very sharp decrease, or quenching, of the luminescence intensity.
For use in biological imaging, however, two main obstacles must be overcome:
- MOFs must be synthesized on the nanoscale so as not to affect the target's normal interactions or behavior
- The absorbance and emission wavelengths must occur in regions with minimal overlap from sample autofluorescence, other absorbing species, and maximum tissue penetration.
Regarding the first point, nanoscale MOF (NMOF) synthesis has been
mentioned in an earlier section. The latter obstacle addresses the
limitation of the antenna effect. Smaller linkers tend to improve MOF
stability, but have higher energy absorptions, predominantly in the
ultraviolet (UV) and high-energy visible regions. A design strategy for
MOFs with redshifted
absorption properties has been accomplished by using large,
chromophoric linkers. These linkers are often composed of polyaromatic
species, leading to large pore sizes and thus decreased stability. To
circumvent the use of large linkers, other methods are required to
redshift the absorbance of the MOF so lower energy excitation sources
can be used. Post-synthetic modification (PSM) is one promising
strategy. Luo et al. introduced a new family of lanthanide MOFs with
functionalized organic linkers. The MOFs, deemed MOF-1114, MOF-1115,
MOF-1130, and MOF-1131, are composed of octahedral SBUs bridged by amino
functionalized dicarboxylate linkers. The amino groups on the linkers
served as sites for covalent PSM reactions with either salicylaldehyde
or 3-hydroxynaphthalene-2-carboxyaldehyde. Both of these reactions
extend the π-conjugation of the linker, causing a redshift in the
absorbance wavelength from 450 nm to 650 nm. The authors also propose
that this technique could be adapted to similar MOF systems and, by
increasing pore volumes with increasing linker lengths, larger
pi-conjugated reactants can be used to further redshift the absorption
wavelengths. Biological imaging using MOFs has been realized by several groups,
namely Foucault-Collet and co-workers. In 2013, they synthesized a
NIR-emitting Yb3+-NMOF using phenylenevinylene dicarboxylate
(PVDC) linkers. They were observed cellular uptake in both HeLa cells
and NIH-3T3 cells using confocal, visible, and NIR spectroscopy.
Although low quantum yields persist in water and Hepes buffer solution,
the luminescence intensity is still strong enough to image cellular
uptake in both the visible and NIR regimes.
Drug delivery systems
The
synthesis, characterization, and drug-related studies of low toxicity,
biocompatible MOFs has shown that they have potential for medical
applications. Many groups have synthesized various low toxicity MOFs and
have studied their uses in loading and releasing various therapeutic
drugs for potential medical applications. A variety of methods exist for
inducing drug release, such as pH-response, magnetic-response,
ion-response, temperature-response, and pressure response.
In 2010 Smaldone et al., an international research group,
synthesized a biocompatible MOF termed CD-MOF-1 from cheap edible
natural products. CD-MOF-1 consists of repeating base units of 6
γ-cyclodextrin rings bound together by potassium ions. γ-cyclodextrin
(γ-CD) is a symmetrical cyclic oligosaccharide that is mass-produced
enzymatically from starch and consists of eight asymmetric α-1,4-linked
D-glucopyranosyl residues.
The molecular structure of these glucose derivatives, which
approximates a truncated cone, bucket, or torus, generates a hydrophilic
exterior surface and a nonpolar interior cavity. Cyclodextrins can
interact with appropriately sized drug molecules to yield an inclusion
complex.
Smaldone's group proposed a cheap and simple synthesis of the CD-MOF-1
from natural products. They dissolved sugar (γ-cyclodextrin) and an
alkali salt (KOH, KCl, potassium benzoate) in distilled bottled water
and allowed 190 proof grain alcohol (Everclear) to vapor diffuse into
the solution for a week. The synthesis resulted in a cubic (γ-CD)6
repeating motif with a pore size of approximately 1 nm. Subsequently,
in 2017 Hartlieb et al. at Northwestern did further research with
CD-MOF-1 involving the encapsulation of ibuprofen. The group studied
different methods of loading the MOF with ibuprofen as well as
performing related bioavailability studies on the ibuprofen-loaded MOF.
They investigated two different methods of loading CD-MOF-1 with
ibuprofen; crystallization using the potassium salt of ibuprofen as the
alkali cation source for production of the MOF, and absorption and
deprotonation of the free-acid of ibuprofen into the MOF. From there the
group performed in vitro and in vivo studies to determine the
applicability of CD-MOF-1 as a viable delivery method for ibuprofen and
other NSAIDs. In vitro studies showed no toxicity or effect on cell
viability up to 100 μM. In vivo studies in mice showed the same rapid
uptake of ibuprofen as the ibuprofen potassium salt control sample with a
peak plasma concentration observed within 20 minutes, and the cocrystal
has the added benefit of double the half-life in blood plasma samples. The increase in half-life is due to CD-MOF-1 increasing the solubility of ibuprofen compared to the pure salt form.
Since these developments many groups have done further research
into drug delivery with water-soluble, biocompatible MOFs involving
common over-the-counter drugs. In March 2018 Sara Rojas and her team at
Paris-Saclay University published their research on drug incorporation
and delivery with various biocompatible MOFs other than CD-MOF-1 through
simulated cutaneous administration. The group studied the loading and
release of ibuprofen (hydrophobic) and aspirin (hydrophilic) in three
biocompatible MOFs (MIL-100(Fe), UiO-66(Zr), and MIL-127(Fe)). Under
simulated cutaneous conditions (aqueous media at 37 °C) the six
different combinations of drug-loaded MOFs fulfilled "the requirements
to be used as topical drug-delivery systems, such as released payload
between 1 and 7 days" and delivering a therapeutic concentration of the
drug of choice without causing unwanted side effects.
The group discovered that the drug uptake is "governed by the
hydrophilic/hydrophobic balance between cargo and matrix" and "the
accessibility of the drug through the framework." The "controlled
release under cutaneous conditions follows different kinetics profiles
depending on: (i) the structure of the framework, with either a fast
delivery from the very open structure MIL-100 or a slower drug release
from the narrow 1D pore system of MIL-127 or (ii) the
hydrophobic/hydrophilic nature of the cargo, with a fast (Aspirin) and
slow (Ibuprofen) release from the UiO-66 matrix."
Recent research involving MOFs as a drug delivery method includes
more than just the encapsulation of everyday drugs like ibuprofen and
aspirin. In early 2018 Chen et al., from Zhejiang University in Hangzhou
China, released an article in ACS's Applied Materials and Interfaces
detailing their work on the use of MOF ZIF-8 (zeolitic imidazolate
framework-8) in antitumor research "to control the release of an
autophagy inhibitor, 3-methyladenine (3-MA), and prevent it from
dissipating in a large quantity before reaching the target."
The group performed in vitro studies and determined that "the
autophagy-related proteins and autophagy flux in HeLa cells treated with
3-MA@ZIF-8 NPs show that the autophagosome formation is significantly
blocked, which reveals that the pH-sensitive dissociation increases the
efficiency of autophagy inhibition at the equivalent concentration of
3-MA." This shows promise for future research and applicability with
MOFs as drug delivery methods in the fight against cancer.
Methane storage
In
October 2011, researchers at the California Institute of Technology
synthesized 14 new Covalent-Organic Frameworks (COFs). Two of these
COFs, COF-103-Eth-trans and COF-102-Ant, were found to exceed the DOE
target of 180 v(STP)/v at 35 bar for methane storage.
They reported that using thin vinyl bridging groups aid performance by
minimizing the interaction between methane and the COF at low pressure.
It is believed that this method could be extended to MOFs due to the
similarity between MOFs and COFs
Semiconductors
In 2014 researchers proved that they can create electrically conductive thin films of MOFs (Cu3(BTC)2
(also known as HKUST-1; BTC, benzene-1,3,5-tricarboxylic acid)
infiltrated with the molecule 7,7,8,8-tetracyanoquinododimethane) that
could be used in applications including photovoltaics, sensors and
electronic materials and a path towards creating semiconductors. The
team demonstrated tunable, air-stable electrical conductivity with
values as high as 7 siemens per meter, comparable to bronze.
Ni
3(2,3,6,7,10,11-hexaiminotriphenylene)2 was shown to be a metal-organic graphene analogue that has a natural band gap, making it a semiconductor, and is able to self-assemble. It represents a family of similar compounds. Because of the symmetry and geometry in 2,3,6,7,10,11-hexaiminotriphenylene (HITP), the overall organometallic complex has an almost fractal nature that allows it to perfectly self-organize. By contrast, graphene must be doped to give it the properties of a semiconductor. Ni3(HITP)2 pellets had a conductivity of 2 S/cm, a record for a metal-organic compound.
3(2,3,6,7,10,11-hexaiminotriphenylene)2 was shown to be a metal-organic graphene analogue that has a natural band gap, making it a semiconductor, and is able to self-assemble. It represents a family of similar compounds. Because of the symmetry and geometry in 2,3,6,7,10,11-hexaiminotriphenylene (HITP), the overall organometallic complex has an almost fractal nature that allows it to perfectly self-organize. By contrast, graphene must be doped to give it the properties of a semiconductor. Ni3(HITP)2 pellets had a conductivity of 2 S/cm, a record for a metal-organic compound.
Bio-mimetic mineralization
Biomolecules
can be incorporated during the MOF crystallization process.
Biomolecules including proteins, DNA and antibodies could be
encapsulated within ZIF-8. Enzymes encapsulated in this way were stable
and active even after being exposed to harsh conditions (e.g. aggressive
solvents and high temperature). ZIF-8, MIL-88A, HKUST-1, and several
luminescent MOFs containing lanthanide metals were used for the
biomimetic mineralization process.
Carbon capture
Because of their small, tunable pore sizes and high void fractions,
MOFs are a promising potential material for use as an adsorbent to
capture CO2. MOFs could provide a more efficient alternative to traditional amine solvent-based methods in CO2 capture from coal-fired power plants.
MOFs could be employed in each of the main three carbon capture
configurations for coal-fired power plants: pre-combustion,
post-combusiton, and oxy-combustion.
However, since the post-combustion configuration is the only one that
can be retrofitted to existing plants, it garners the most interest and
research. In post-combustion carbon capture, the flue gas from the power
plant would be fed through a MOF in a packed-bed reactor setup. Flue
gas is generally 40 to 60 °C with a partial pressure of CO2 at 0.13 - 0.16 bar. CO2 can bind to the MOF surface through either physisorption, which is caused by Van der Waals interactions, or chemisorption, which is caused by covalent bond formation. Once the MOF is saturated with CO2, the CO2
would be removed from the MOF through either a temperature swing or a
pressure swing. This process is known as regeneration. In a temperature
swing regeneration, the MOF would be heated until CO2
desorbs. To achieve working capacities comparable to the amine process,
the MOF must be heated to around 200 °C. In a pressure swing, the
pressure would be decreased until CO2 desorbs.
In addition to their tunable selectivities for different
molecules, another property of MOFs that makes them a good candidate for
carbon capture is their low heat capacities. Monoethanolamine (MEA)
solutions, the leading method for capturing CO2 from flue
gas, have a heat capacity between 3-4 J/g K since they are mostly water.
This high heat capacity contributes to the energy penalty in the
solvent regeneration step, i.e., when the adsorbed CO2 is removed from the MEA solution. MOF-177, a MOF designed for CO2 capture, has a heat capacity of 0.5 J/g K at ambient temperature.
In a collaborative project sponsored by the U.S. DOE, MOFs were shown to separate 90% of the CO2 from the flue gas stream using a vacuum pressure swing process. The MOF Mg(dobdc) has a 21.7 wt% CO2
loading capacity. Estimations showed that, if a similar system would be
applied to a large scale power plant, the cost of energy would increase
by 65%, while a U.S. NETL baseline amine-based system would cause an increase of 81% (the U.S. DOE goal is 35%). The cost of capturing CO2 would be $57 / ton CO2 captured, while for the amine system the cost is estimated to be $72 / ton CO2
captured. The project estimated that the total capital required to
implement such project in a 580 MW power plant would be $354 million.
Desalination/ion separation
MOF
membranes can mimic substantial ion selectivity. This offers the
potential for use in desalination and water treatment. As of 2018 reverse osmosis
supplied more than half of global desalination capacity, and the last
stage of most water treatment processes. Osmosis does not use dehydration of ions, or selective ion transport
in biological channels and it is not energy efficient. The mining
industry, uses membrane-based processes to reduce water pollution, and
to recover metals. MOFs could be used to extract metals such as lithium
from seawater and waste streams.
MOF membranes such as ZIF-8 and UiO-66 membranes with uniform
subnanometer pores consisting of angstrom-scale windows and
nanometer-scale cavities displayed ultrafast selective transport of
alkali metal ions. The windows acted as ion selectivity filters for
alkali metal ions, while the cavities functioned as pores for transport.
The ZIF-8 and UiO-66 membranes showed a LiCl/RbCl selectivity of ~4.6 and ~1.8, respectively, much higher than the 0.6 to 0.8 selectivity in traditional membranes.
Water vapor capture
A team of researchers at the Massachusetts Institute of Technology and University of California Berkeley
have developed a prototype that captures water vapor from the air, and
then releases it with the application of a smaller amount of heat
compared to existing commercially available technologies.
Ferroelectrics and Multiferroics
Some
MOFs also exhibit spontaneous electric polarization, which occurs due
to the ordering of electric dipoles (polar linkers or guest molecules)
below a certain phase transition temperature. If this long-range dipolar order can be controlled by the external electric field, a MOF is called ferroelectric.
Some ferroelectric MOFs also exhibit magnetic ordering making them
single structural phase multiferroics. This material property is highly
interesting for construction of memory devices with high information
density.
Interaction with Gravitational forces
Gravitational force is considered as a tool for managing the MOFs' crystal size; in which high g‐force led to the formation of small MOF crystals, while low g‐force
(g less than 1) produced rather bigger crystals, caused by the
facet‐oriented crystal fusion. Hence, g‐force would be used to increase
or decrease MOFs' size by changing their convection and sedimentation.
