A 5-minute read
Prof. Krista S. Walton and Prof. Yu-Sheng Chen, the winner of the DECTRIS Award 2020.
Rational design of functional materials is driven by tuning their structural properties in order to achieve a specific performance, such as gas separation or catalysis. In this quest, the stability of a material plays an important role. Under particular environmental conditions, materials may become instable, which hinders their functionality or their transfer to industry applications. As the instability of a material is usually demonstrated as a significant change of its crystal lattice, structural investigations of unstable materials provide information used in the design of materials with improved properties.
Prof. Yu-Sheng Chen, a research professor at the NSF’ ChemMatCARS at the University of Chicago , Prof. Krista S. Walton of the Georgia Institute of Technology, and co-workers prove that a detailed analysis of stable materials is equally important. When exposed to non-lab conditions, stable materials may undergo very small structural changes. Investigating these changes in detail can help to understand their stability, paving a way for developing new design strategies for sorbent materials.
“In a time-resolved experiment, fine structural changes of a crystal can go under the radar for many reasons”, says Dr. Dubravka Šišak Jung, application scientist at DECTRIS. “This work presents a smart approach to the experimental challenges, and it involves careful data interpretation. Continuing this project brings a chance to provide a routine in situ analysis of functional materials.”
DECTRIS Award was established with an aim to support researchers to share their findings at a scientific conference of their choice. Prof. Yu-Sheng Chen will use the award to support one of his students. We hope that we will be able to meet both scientists at one of the crystallographic conferences next year.
Below, Prof. Yu-Sheng Chen describes his award-winning work
Metal-organic frameworks (MOFs) are nanoporous materials, formed by linking inorganic nodes with organic molecules. The sheer size of this group of compounds provides a fruitful playground for the synthesis of crystals with various pore sizes, structures and properties. As these nanopores can serve as a host to molecules of specific chemico-physical properties, MOFs can find their applications in gas separation and storage, catalysis, chemical sensing, drug delivery, and more. One of the most relevant obstacles on the way to a successful technology transfer of MOFs is water. Present in industrial streams of gas separation and gas purification systems, water can be adsorbed in the material and cause two problems: (i) it can occupy active sites of the MOF (ii) it can cause degradation or defect formation of the MOF. Consequently, the effect of water on MOFs can cause significant changes of the crystal lattice. However, some MOFs are stable, that is, they are able to host water molecule without suffering major structural changes.
“Detailed structural analysis of stable MOFs during the water adsorption is often not investigated. In our work, we aimed to see these fine structural differences and relate them to the stability of the material”, explains Prof. Chen. “This is particularly interesting in cases where two isostructural compounds exhibit different stabilities.”
Figure 1. Representation of the potential structural changes of the DMOF-TM, induced by the adsorption of water. The structural changes were derived from dynamic in situ SCXRD experiments and are supported by computational data. Figure partially reproduced from Burtch, N.C. et al. (2020) Nature Chemistry 12, 186–192.
The investigated MOF, DMOF-TM, proved to be stable under high relative humidity as opposed to its isostructural analogue that does not feature methyl groups on the terephthalate ligand. As the stability of the DMOF-TM could not be explained with the methylation of the ligand, the sample was investigated using dynamic in situ powder and single-crystal X-ray diffraction (SCXRD), as well as in situ infrared spectroscopy and molecular modeling.
In the dynamic in situ SCXRD experiment, designed/conducted by Dr. Ian M. Walton, the sample was exposed to a continuous range of relative humidity while data was collected. The results were used to examine the structural changes that are responsible for the observed stability throughout the course of the experiment. The rapid data collection was facilitated by a combination of the synchrotron radiation and PILATUS3 X CdTe 1M detector. The experimental setup at the NSF’s ChemMatCARS beamline at the Advanced Photon Source involved modifications to the existing in situ SCXRD system that would allow for the dynamic control of the relative humidity. Gaining adequate time resolution to study the phenomena was critical to the success of the experiment and was not possible without the PILATUS detector. The data collected and the resulting structures were critical in the formulation of the proposed structural changes attributed to the stability of DMOF-TM. “The time resolution afforded by the combination of synchrotron radiation and the PILATUS detector enables us to examine fleeting meta-stable structures and gain a more accurate understanding of the dynamic adsorption process,” explains Dr. Ian Walton.
“The control over the environment around the sample and the impressive time resolution allowed for insights into the dynamic interactions within the crystal lattice”, says Prof. Chen. “This has enabled us to see that during the water absorption even stable compounds can exhibit small structural changes due to guest–host interactions such as water-induced bond rearrangements”, concludes Prof. Chen.
The awarded work has been published earlier this year in the Nature Chemistry journal: Burtch, N.C. et al. (2020) Nature Chemistry 12, 186–192.
The experimental setup and PILATUS3 X CdTe 1M detector used in this work are supported, respectively, by the National Science Foundation (USA) (grant number NSF/CHE-1834750), and Major Research Instrument program (grant number NSF/DMR-1531283).