Controlling defects for smarter metal organic frameworks

A new method facilitates fine-tuning defects in a special class of crystalline materials called metal organic frameworks (MOFs). The approach could lead to the development of more effective compounds for industrial processes like catalysis, sensing, and gas storage.

MOFs are formed from metal ion clusters bridged by organic linkers. Sometimes, they acquire structural defects during synthesis, which have the potential for improving MOF properties if scientists could control and fine-tune their engineering. 

Researchers in Saudi Arabia, China, and the UK used formic acid to induce structural defects in a zirconium-based catalytic MOF called UiO-66. They used the latest imaging tools to locate the nanosized defects, called missing cluster defects when metal ions are absent, and missing linker defects when organic linkers are gone.

They demonstrated that formic acid not only created both types of defects but also fixed them, allowing the researchers to tweak the MOF’s properties. 

The relative proportion of defects depended on the amount of time it took for the MOF to crystallize: with prolonged crystallization, missing cluster defects tended to vanish, but missing linker ones remained. 

Also, one-day-old MOFs had higher catalytic efficiency than three-day-old ones, because they had more missing cluster defects than their older counterparts.    

Formic acid made it possible to fine-tune the defects to precisely control the catalytic properties of the MOFs, says functional materials design researcher Mohamed Eddaoudi of King Abdullah University of Science and Technology (KAUST). 

Next, the team plans to utilize their understandings of defects to fine-tune the size of pore openings in MOFs, for applications in gas separation and adsorption systems.

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