Metal-organic frameworks: Warming up to better crystallography images

To verify the precise structure of metal-organic frameworks (MOFs), scientists have been using an imaging technique at a temperature of 100K (-173C) to obtain the most accurate image of their structure. Researchers from the US, Switzerland and Saudi Arabia, however, have discovered that to achieve high-quality structural images of MOFs, the imaging process temperature needs to be varied according to the subject MOF. 

MOFs are synthetic materials constructed using networks of metal ions connected by organic linkers. Their structures can therefore be manipulated for multiple, diverse applications. The highly-porous MOFs can be used as absorbents to clean up pollutants, for example, to store hydrogen for fuel cells, in purification and separation, in catalysis, and in sensing applications.

To determine the molecular structure of MOFs, scientists use an imaging technique called X-ray crystallography, in which data is collected from X-rays that are diffracted from the MOF surface at different angles depending on its crystalline structure. 

“The process is usually conducted at a temperature of 100K (-173C), to stop thermal vibration of atoms within the MOF from generating [flawed] results,” says Omar Yaghi from the University of California-Berkeley in the US, who led the project team, which included KACST’s Sultan Alshmimri. “However, we found that for certain MOFs, including some well-established designs, X-ray data collected at low temperatures resulted in poor-quality images.”

The researchers believe X-ray diffraction may be disturbed by ‘guest molecules,’ which are gas or solvent molecules which become trapped in the MOF pores. 

Yaghi’s team used both new and established MOFs for their experiments. First, they thoroughly cleaned the new MOFs and soaked them in solvent to charge the pores with guest molecules. They used X-ray diffraction to image these MOFs at different temperatures and compared their structures with clean ‘evacuated’ MOFs that had no guest molecules present. 

“Imagine an array of jars full of marbles, randomly distributed and with a different arrangement of marbles in each jar. This is analogous to the MOF pores full of guest molecules,” says Yaghi. “The wall of the pore is flexible enough to be distorted by the force of the guest molecules. This effect causes differently-shaped walls in each pore, introducing disorder.”

At 100K, the guest molecules interacted more strongly with the MOF walls, disrupting their structure. When the team carried out the same process at room temperature (290K or 16.85C), the guest-framework interactions were weaker, and they achieved higher resolution datasets, providing them with clearer images. The same was true for the two established MOFs. 

The team believe their findings are widely applicable to other porous MOFs and challenge the assumption that X-ray diffraction should always be carried out at low temperatures.  

1. Lee, S., Burgi, H-B., Alshmimri, S.A., & Yaghi, O.M. Impact of disordered guest-framework interactions on the crystallography of metal-organic frameworks. Journal of the American Chemical Society 140 8958-8964 (2018) | article