Metal–organic framework

Metal–organic frameworks are a class of porous polymers consisting of metal clusters, also known as secondary building units, coordinated to organic ligands to form one-, two-, or three-dimensional structures. The organic ligands included are sometimes referred to as "struts" or "linkers", one example being 1,4-benzenedicarboxylic acid. MOFs are classified as reticular materials.
More formally, a metal–organic framework is a potentially porous extended structure made from metal ions and organic linkers. An extended structure is a structure whose sub-units occur in a constant ratio and are arranged in a repeating pattern. MOFs are a subclass of coordination networks, which 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. Coordination networks including MOFs further belong to coordination polymers, which is a coordination compound with repeating coordination entities extending in one, two, or three dimensions. Most of the MOFs reported in the literature are crystalline compounds, but there are also amorphous MOFs, and other disordered phases.
In most cases for MOFs, the pores are stable during the elimination of the guest molecules 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 water remediation, in catalysis, as conducting solids and as supercapacitors.
The synthesis and properties of MOFs constitute the primary focus of reticular chemistry. In contrast to MOFs, covalent organic frameworks are made entirely from light elements with extended structures.
Susumu Kitagawa, Richard Robson and Omar Yaghi were awarded the Nobel Prize in Chemistry in 2025 for their work on MOFs.

History and the first MOF

Following Richard Robson's discovery of "finite structures", in 1995, Omar M. Yaghi demonstrated the crystallization of metal-organic structures using carboxylate-based linkers, a breakthrough that paved the way for creating stable and crystalline porous materials. He further advanced the field in 1998 by introducing the concept of secondary building units — metal-carboxylate clusters that serve as rigid building blocks for constructing frameworks with permanent porosity. This innovation allowed for precise structural design and improved mechanical stability, enabling MOFs to retain their porosity under industrial conditions. Yaghi measured gas adsorption isotherms of these materials, proving their potential for gas storage and separation applications.
A breakthrough came in 1999 with the development of MOF-5, the first MOF to exhibit ultra-high porosity. MOF-5, constructed from zinc oxide clusters and terephthalate linkers, illustrated unique properties such as high surface area, structural robustness, and versatility, and established MOFs as a platform technology with applications ranging from gas storage and separation to catalysis and sensing. With his significant pioneering work on MOFs, today Omar M. Yaghi is widely recognized as the founder of reticular chemistry. This invention had a dramatic impact on the development and applications of Metal-Organic Frameworks, as shown by the inflection point at the year 2000 in the figure on the right.

Structure

MOFs are composed of two main components: an inorganic metal cluster and an organic molecule called a linker. For this reason, the materials are often referred to as hybrid organic-inorganic materials. 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.
To describe and organize the structures of MOFs, a system of nomenclature has been developed. Subunits of a MOF, called secondary building units, 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.
Attached to the SBUs are bridging ligands. For MOFs, typical bridging ligands are di- and tricarboxylic acids. These ligands typically have rigid backbones. Examples are benzene-1,4-dicarboxylic acid, biphenyl-4,4-dicarboxylic acid, and the tricarboxylic acid trimesic acid.
A fundamental aspect in the development of MOFs is that their crystal structures can be determined by X-ray crystallographic techniques as often, many MOFs have good crystallinity allowing their 3D structures to be determined precisely. This has allowed to study reactions taking place within the MOF's channels, revealing the structures of reaction intermediates.

Synthesis

General synthesis

The study of MOFs has roots in coordination chemistry and solid-state inorganic chemistry, but it developed into a new field. In addition, 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 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.
Four developments were particularly important in advancing the chemistry of MOFs. The geometric principle of construction where metal-containing units were kept in rigid shapes. Early MOFs contained single atoms linked to ditopic coordinating linkers. The approach not only led to the identification of a small number of preferred topologies that could be targeted in designed synthesis, but was the central point to achieve a permanent porosity. The use of the isoreticular principle where the size and the nature of a structure changes without changing its topology led to MOFs with ultrahigh porosity and unusually large pore openings. Post- synthetic modification of MOFs increased their functionality by reacting organic units and metal-organic complexes with linkers. Multifunctional MOFs incorporated multiple functionalities in a single framework.
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.
Some MOFs, such as the mesoporous MIL-100, can be obtained under mild conditions at room temperature and in green solvents through scalable synthesis 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. Cu₃₂ can be quickly synthesised in this way in quantitative yield. In the case of Cu₃₂ 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. It has been shown that the addition of small quantities of ethanol for the mechanochemical synthesis of Cu₃₂ significantly reduces the amounts of structural defects in the obtained material.
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 consists 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. This process was successfully scaled up to an integrated cleanroom process, conforming to industrial microfabrication standards.
Numerous methods have been reported for the growth of MOFs as oriented thin films. However, these methods are suitable only for the synthesis of a small number of MOF topologies. One such example being the vapor-assisted conversion which can be used for the thin film synthesis of several UiO-type MOFs.

High-throughput synthesis

High-throughput methods are a part of combinatorial chemistry and a tool for increasing efficiency. There are two synthetic strategies within the HT-methods: In the combinatorial approach, all reactions take place in one vessel, which leads to product mixtures. In the parallel synthesis, the reactions take place in different vessels. Furthermore, a distinction is made between thin films and solvent-based methods.
Solvothermal synthesis can be carried out conventionally in a teflon reactor in a convection oven or in glass reactors in a microwave oven. The use of a microwave oven changes, in part dramatically, the reaction parameters.
In addition to solvothermal synthesis, there have been advances in using supercritical fluid as a solvent in a continuous flow reactor. Supercritical water was first used in 2012 to synthesize copper and nickel-based MOFs in just seconds. In 2020, supercritical carbon dioxide was used in a continuous flow reactor along the same time scale as the supercritical water-based method, but the lower critical point of carbon dioxide allowed for the synthesis of the zirconium-based MOF UiO-66.