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When, back in 1999, the journal Nature published a letter about a material with a surface area of 2900 square meters in a single gram, many people believed an error had crept in – like the one involving spinach and its supposedly high iron content. In contrast to the cartoon figure Popeye and his magical vegetable, the reported number was not a mistake; rather, it marked the inception of a novel class of materials – metal-organic frameworks, now commonly known under the acronym MOFs.
As the name suggests, metal-organic frameworks consist of two components. Inorganic units (metal ions or clusters) coordinate with organic linkers to form an ordered, porous framework. Here, metals and organic linkers can be chosen freely, and virtually infinite combinations can be achieved with this modularity. Thousands of MOFs have been described in the past two decades, and many MOFs are yet to be discovered – making them the fastest growing class of materials in chemistry today.
The MOFs seem to be becoming a trend. But what does that imply – and what are the advantages of this material class? The modularity allows for fine tuning of the framework so that it can be designed to exhibit precise properties. For instance, one MOF can be very efficient in removing impurities such as carbon dioxide from a gas stream, while another can be used as a drug delivery system, carrying and protecting an active pharmaceutical ingredient in its pores. These nanoporous materials can be compared to sponges, being able to absorb, hold and release molecules from the pores.
Rather than being an accumulation of ordered matter, metal-organic frameworks should be imagined more as a hollow framework of pores: they are “full of empty space”. Chemical species can interact with the walls of the nanometer-sized pores. As a consequence, chemical and physical processes can take place within the pores of the MOF, the properties of which can be tuned very precisely through the modular architecture. This makes MOFs so desirable in academia and industry.
By virtue of their modularity and porosity, MOFs are used for various volume-specific applications, outperforming conventional porous materials (e.g. activated carbon or zeolites). MOFs offer tremendous opportunities in the field of gas sorption, storage, separation and purification, catalysis, drug delivery, heat transformations and energy systems. One commercial demonstration of MOFs has been achieved in the battery sector. Here, MOFs are used to fabricate more efficient, stable cathode materials. MOFs enhance lithium-ion battery performance by coating the cell and improving the transportation and diffusion of electrolytes. As a result, these batteries can be charged much faster – within minutes – and they exhibit higher cycle stabilities than state-of-the-art batteries. Other examples are carbon capture and sequestration (CCS), natural gas upgrading, adsorption-driven heat pumps, water harvesting, hydrogen storage and many more. MOFs can be considered the next technology wave and might impact emerging applications.
The author: Daniel Steitz holds a master’s degree in chemical and bioengineering from the ETH Zurich. He co-founded the PSI spin-off novoMOF AG in February 2017 and has been managing the company as CEO since then. With his team Daniel won the >>venture>> Business Idea track in 2016, was in the finals of the Swiss Technology Award 2016 in the category inventors and has been nominated for the Pionierpreis 2017. The spin-off has been actively supporting the MOF field, sponsoring the international conference on metal-organic frameworks (MOF2016) in September 2016 and the European conference EuroMOF2017 in October 2017.