Scientists create microporous MOF traps to reduce harmful gases.

Nitrogen dioxide (NO2) and sulfur dioxide (SO2) are poisonous gases that are damaging to both the environment and human health. They can fly hundreds of kilometers through the atmosphere, contaminating the air and generating acid rain, which affects buildings, trees, and agriculture. Toxic gas exposure can also cause respiratory infections, asthma, and chronic lung illness.

For these reasons, acid gases are high on the list of pollutants targeted by the Clean Air Act, which compels the Environmental Protection Agency to regulate and set limits on NO2 and SO2 emissions in order to improve air quality and prevent widespread disease.

Scientists are working on materials that can detect and trap acid gases, which is one of the most inventive approaches to reducing air pollution and combating climate change. The strategy comprises a variety of technological technologies meant to filter the air by catching or trapping hazardous gases emitted by vehicles. Captured molecules can also be preserved and reused in some situations; for example, carbon dioxide can be reused in specific applications to stimulate photosynthesis and plant development.

Metal organic frameworks, or MOFs, could take acid-gas sequestration to the next level, making it a more viable, practical strategy to improve global air quality. MOFs are essentially small matrices of metal atoms linked together by organic molecules to form a pattern of tiny, interconnecting metal cages. They function similarly to sponges, allowing molecules to attach to or be absorbed by their surface. In fact, MOFs are so porous that the amount that would fit within someone’s pocket would cover the surface of an entire football field if stretched out.

“Metal organic frameworks are really novel in their flexibility, their chemistry, and how you can tailor their structure. If you swap out organic molecules, you can tune the structure to target different gases. Acid gases typically come from combustion processes, so this research could be useful in developing devices to help limit emissions from large-scale industrial facilities like oil refineries and fossil fuel-based power plants.”

said Sandia National Laboratory’s Susan Henkelis, the study’s lead author

Researchers looking for potential materials to remediate NO2 and SO2 studied a variety of MOFs that can be synthesized from the full family of rare-earth metals in a new work published in the journal ACS Applied Materials and Interfaces. To establish the best conditions for synthesis, they employed computer simulations and a mixture of neutron and X-ray scattering studies. They also discovered vital details about an intriguing flaw that arises in MOFs, which they think could be beneficial in developing devices for absorbing emissions or sensing harmful quantities of toxic gases.

Metal organic frameworks are quite innovative in terms of their flexibility, chemistry, and ability to be tailored. You can modify the structure to target different gases by swapping out organic molecules, “Susan Henkelis of Sandia National Laboratory, the study’s lead author, explained. “Acid gases are often produced by combustion processes, so our research could aid in the development of devices to help restrict emissions from large-scale industrial facilities such as oil refineries and fossil-fuel-based power plants.”

Researchers from the Department of Energy’s (DOE) Sandia and Oak Ridge national laboratories (ORNL), as well as the University of Tennessee, Knoxville, are part of the team (UTK). The researchers are part of the UNCAGE-ME Center for Understanding and Control of Acid Gas-Induced Evolution of Materials, a program designed to better understand the interactions between acid gases and solid materials. UNCAGE-ME is part of a larger research effort supported by the DOE’s Energy Frontier Research Center (EFRC) program, which brings together the research capabilities of universities and national laboratories to provide atomic-scale insights into addressing some of the world’s most pressing energy challenges, which can only be accomplished through large collaborations.

The fundamental scientific goal of this work was to understand how the chemistry and synthesis process creates these defects because we want to know how the defects can be controlled and what their effect is on acid gas adsorption. “Peter Metz, a postdoctoral researcher at UTK who worked in Neutron Sciences at ORNL during the study, In order to do so, we must first understand how the atomic bonds in MOFs arise and how the atoms are structured.

The cages inside each synthesized MOF should ideally form a cube. Each corner of the cube includes a cluster of six rare-earth metal ions, with another cluster in the center. A single link, or linker molecule, binds each pair of metal ions in the cluster to another pair in another cluster.

However, occasionally a defect occurs, particularly in MOFs constructed of europium ions, in which the linker kinks and exposes the rare-earth ion, increasing the possibility that a pollutant molecule may become trapped within the structure.

To figure out why this occurs, the researchers utilized a mix of neutron and X-ray scattering studies to map the atomic structures of the materials.

They discovered the heavy metal elements using X-rays, which offered an outline of the entire structure. They also bombarded the materials with neutrons using the POWGEN instrument at ORNL’s Spallation Neutron Source (SNS) to better understand how the organic molecules are arranged. This allowed them to track the positions of the hydrogen, carbon, and oxygen atoms that form the molecular bonds between the metal ion clusters.

The team was able to discover from the experiments that the materials with the faults formed faster than their defect-free counterparts. They also discovered that the faults could be intentionally created by varying the temperatures and times required to grow the crystalline materials.

The scientists then utilized the experimental structural data to run computer simulations to see how each of the materials, with and without the flaws, reacted with the hazardous gases NO2 and SO2.

While these new insights are on the basic research side of things, they could have a major influence down the road, said Tina Nenoff, corresponding author of the paper at Sandia. “We discovered new information about how these materials form, which we may utilize to better precisely control and create MOFs. Furthermore, we created a thorough approach to analyzing a vast series of MOFs, which will aid in the speed with which novel candidate materials are discovered and developed into usable technologies. “

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