Unlike traditional polymers, this structure allows MOFs to have open internal spaces of well-defined sizes, which can allow some molecules to pass through while filtering out others. In addition, the presence of metals allows for interesting chemistry. Metals can serve as catalysts or bind preferentially to a molecule in a mixture.
Knowing what we know now, it seems obvious that it would work. But when Robson began his work at the University of Melbourne, the few people who thought about the question expected that the molecules he was building would be unstable and collapse.
The first MOF built by Robson used copper as the metal of choice. It was linked to an organic molecule that retained its rigid structure thanks to the presence of a benzene ring that does not bend. The organic molecule and copper could form four different bonds, allowing the structure to grow into the rough equivalent of stacking a pile of three-sided pyramids – a conscious choice by Robson.
The world’s first MOF, synthesized by Robson and colleagues.
Credit: Johan Jarnestad/Royal Swedish Academy of Sciences
In this case, however, the internal cavities remain filled by the solvent in which the MOF was formed. But the solvent could move freely through the material. Yet, based on this example, Robson predicted many of the properties that have since been incorporated into various MOFs: the ability to retain their structure even after solvents are removed, the presence of catalytic sites, and the ability of MOFs to act as filters.
Expand the concept
This all may seem like a very optimistic view of someone’s first effort. But the measure of Robson’s success is that he convinced other chemists of the potential. One of them was Susumu Kitagawa of Kyoto University. Kitagawa and his collaborators built a MOF with large internal channels running the length of the material. Made in an aqueous solution, the MOF could be dried and allow gas to pass through it, with the structure retaining molecules like oxygen, nitrogen and methane.
