Frameworks fit for purpose: The design and application of metal-organic frameworks

In my (admittedly biased) opinion, metal-organic frameworks or MOFs represent a fascinating, ever-expanding corner of the chemistry world. Which is great news considering I’ve currently signed 3+ years of my life to their development. In this issue I will attempt to convince, persuade and inform you dear reader of all the things I’ve learned about MOFs so far without all the hassle of deadlines and failed experiments.
For me and many other researchers, there are two primary attractions of this concept:

1. The tunability and range of different MOF structures that can be designed for purpose and tailored to specific applications.

2. The vast plethora of areas, both academic and industrial, to which MOFs are relevant. These applications encompass a large variety of functionality and every-day challenges.

Firstly then, I’ll begin with a brief introduction to metal-organic frameworks and some of the key concepts surrounding this research covering the what, the how and the why.

❓The what?

MOFs are a developing class of ‘porous crystalline solids’ based on metal centres linked by organic ligands, think something like a sea sponge. Much of the interest in these materials is related to the channels, cavities and pores within them.
Many MOFs have very high and tuneable porosities with internal surface areas upwards of 6,000 m²/g having been reported—that is approximately the same surface area of an entire football field in a single gram.

Other similar compounds such as zeolites have long been known and utilised, but due to the intrinsic MOF properties such as diverse structures, ready modification and the easy structural determination exhibited by these highly crystalline materials, MOF chemistry is continually gaining increased interest from research groups around the world⁴.

This rapid expansion of said interest also extends well into industry, with the global market reports for metal-organic frameworks estimated at $9.8 billion USD in 2024, with values anticipated to rise to a staggering $29.2 billion USD by the year 2034. This projected CAGR of 13.1% has been attributed to an increase in demand for efficient gas storage, carbon capture and catalysis techniques among many other industries.

⚙️The how?

In most methods of synthesising MOFs, an initial reaction between a simple metal salt and a pre-designed ligand gives a non-active material that has a 3D framework but where the all-important pores are filled with solvent or other unwanted small molecules. The solvent is removed from the framework to provide large, empty pores preferably with a large internal surface area. These vast nothing-filled spaces allow for the incorporation of significant amounts of guest molecules.

The concept of “reticular synthesis” introduced by O.M. Yaghi et al. in 2013 remains a central point in the development of new MOF materials. This seminal research proposed the use of metal-clusters known as “secondary building units” (SBUs) with ligands to design MOFs by carefully selecting SBUs and ligands. Imagine building a Lego with longer names and more numbers involved.

Using this approach, once the structure of the pre-designed SBU has been confirmed, it can be used to dictate the synthesis of structured frameworks with anticipated chemical properties.

This method of synthesis has led to considerable progress in the incorporation of application-directed SBU design into MOF synthesis and hence frameworks fit for purpose.

💡The why?

Now that we are up to speed on what metal-organic frameworks actually are and how they are designed, let’s move on to the most interesting question. Why have researchers across the globe dedicated such a significant amount of time, effort and patience to the study of these particular materials? Perhaps more importantly, who the hell cares?

There are a multitude of examples throughout recent literature of applications for MOFs in industry such as gas-adsorption, proton-transfer, sensors, catalysis, and drug delivery. The variability of metal junctions and organic linkers make MOFs widely applicable to very different areas of application.

In a previous article, the feasibility of hydrogen as a fuel source was discussed. To make it feasible, it is important to develop technologies with better hydrogen-storage capabilities. This hydrogen adsorption potential of MOFs has already been implemented in technology such as the hydrogen fuel tank used in the Mercedes-Benz F125 fuel cell-powered demonstration model. Many car manufacturers have taken particular interest in the hydrogen storage capabilities of frameworks such as “MOF-177” and “Z377”.

Another industrial application of MOFs which is of relevance not just to petrol (or hydrogen) heads but to everyone here on Earth is CO₂ capture. Continuously increasing levels of CO₂ in the Earth’s atmosphere are a large threat to Earth ecosystems, as higher levels of atmospheric CO₂ lead directly to rising global temperatures.

A particularly successful study on capture in 2010 by Hiroyasu Furukawa et al. demonstrated excellent carbon dioxide uptake of a highly porous framework named “MOF-200”.
This study illustrates an example of a material which could be used to store 17 times as much carbon dioxide as similar storage systems without the addition of a MOF.

Final thoughts

The continual development of metal-organic frameworks, enhancement of their structure, and research into their application represents an ever-expanding class of new materials which have the potential to be used in an extensive variety of industrial and academic settings. The interchangeability of each of the components used to construct these frameworks is the key concept that fascinates me most about MOFs and what makes them readily customisable to the task at hand, whatever that may be. From cars to climate change, I think that metal-organic frameworks will be a key tool in the arsenal of world-changing research for years to come. But then again, I’m a MOF chemist—I would say that.

Thank you for reading, goodbye for now.
Dylan Joseph Shaw