Reticular materials

Reticular materials are artificial materials that have been engineered at the molecular level to achieve certain properties, in particular a high porosity and high surface area. Reticular materials can be grouped into three major classes, in the order of their discovery: metal-organic frameworks or MOFs, covalent organic frameworks or COFs, and hydrogen-bonded organic frameworks or HOFs, each with respective sub-classes. An example for a sub-class of MOFs are zeolitic Imidazolate Frameworks (ZIFs). MOFs are the most explored sub-class of reticular materials (around 77% of the approximately 48,000 publications on reticular materials between the 1990s to 2023 were on MOFs). MOFs are a type (or subclass) of coordination polymers. Around 99 thousand synthetic variants of MOF are known. The structures and functionality of reticular materials, for example the pore size, are adjustable (or “tailorable”) during the synthesis process. This makes them in principle adjustable to many different technical requirements. Depending on the choice of materials and synthesis methods, reticular materials, because of the strong bonds, can be designed to withstand extreme temperatures and harsh chemical environments over extended periods of time. This field was pioneered by Susumu Kitagawa, Richard Robson, and Omar M. Yaghi starting in the 1990s. These three scientists received the Nobel Prize in Chemistry in 2025 “for the development of metal–organic frameworks”. There are many potential applications for reticular materials, although only very few applications have reached a commercial level to date. The initial application of reticular materials was mostly for gas adsorption and storage, followed later by other fields such as catalysis, biomedicine, and membrane technologies. Examples for gas adsorption and storage technologies with reticular materials include carbon capture and storage, direct air capture, atmospheric water harvesting and hydrogen storage. These are all applications that several start-up or commercial companies are now pursuing. Other applications that are being researched, including for biomedicine and catalysis, are still in their infancy and emerging states. Research is ongoing around the world to take the most promising reticular materials into the full-scale commercialization process for real-world applications. Hindering factors include scalability, high costs of reticular materials, reproducibility and poor technology readiness level. The company BASF produces raw MOFs at commercial quantities.

Definition and overview Reticular materials are materials that are constructed based on the concept of reticular chemistry. The term reticular is derived from the Latin term reticulum meaning net-like. The first-discovered and largest class of reticular materials are metal–organic frameworks (MOFs) but there are other classes, too, as well as many sub-classes. The two main other classes of reticular materials are covalent organic framework (COFs) and hydrogen-bonded organic framework (HOFs). Reticular chemistry involves the “connection of individual building units such as molecules and clusters using robust bonding to form extensive and coherent architectures with highly ordered arrangement in a designed manner”. In other words, reticular chemistry is “concerned with linking molecular building units by strong bonds to make crystalline large and extended structures”. The term reticular synthesis, or reticular chemistry was introduced in 2002–2003 by the research team at UC Berkeley led by Omar M. Yaghi. This emerging field of reticular chemistry has at its core inorganic and organic chemistry but it also adds the new chemistry of using the strong metal-ion-based bonds and covalent bonds to link molecules into extended structures, features unattainable prior to reticular chemistry. Prior to the discovery of MOFs, zeolites and activated carbons were the main porous materials used in industry for applications where a high surface area was required. However, the surface area of MOFs is much higher than that of zeolites and activated carbons, i.e. in the range of 1000–10,000 m2/g. This makes them an interesting material for industry. In addition, the structure of the cavities and the size of the pore in MOFs can be tailored as a function of metal ions, organic ligands, and synthesis conditions, and hence are thus amenable to engineering for targeted applications.

History

In the 1990s, the research activities of Omar M. Yaghi first at the Arizona State University and then at University of Michigan in the United States involved synthesizing ordered molecular structures. In 1999 he and co-authors published an article in Nature called “Design and synthesis of an exceptionally stable and highly porous metal-organic framework” where they described MOF-5, the first MOF to exhibit ultra-high porosity. This marked the beginning of a new research field. The new term metal–organic framework was coined by that research group.

The evolution of porous materials with diverse topologies has a rich history, traced back to the assembly of inorganic clusters into extended frameworks. This was followed by the synthesis of metal–organic frameworks (MOFs) in the 1990s, which involved linking organic molecules and metal ions. Subsequently, covalent organic frameworks (COFs) emerged after first being synthesized by Omar M. Yaghi and his group in 2005. COFs are a type of reticular material that links organic molecules together. This was later followed by hydrogen-bonded organic frameworks (HOFs) (since around 2011), representing the latest generation of reticular materials. A publication in 2011 by Banglin Chen (University of Texas at San Antonio) introduced the term HOF to the scientific community.Almost 48 000 publications on reticular materials have been published in the three decades between the 1990s and 2023. Of these publications, most of them (around 77%) are on MOFs. They are the most explored sub-class of reticular materials. Reticular materials are now (as of 2022) the “fastest-growing class of materials in chemistry and most-productive research hotspot among global communities of chemistry, engineering, and materials science.” The scientists Omar M. Yaghi, Susumu Kitagawa and Richard Robson received the Nobel Prize in Chemistry in 2025 “for the development of metal–organic frameworks”. They have pioneered the field of reticular materials and chemistry.

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