The study published today in Nature Communications used computer modelling to screen a large material database of thousands of candidates to find a material that can separate xenon and krypton. These volatile radionuclides usually escape in the gasses emitted as byproducts of the chemical process.
We spoke with author Praveen K Thallapally of the Department of Energy's Pacific Northwest National Laboratory about the study.
Praveen Thallapally: We discovered a new material called metal organic framework (SBMOF-1) that can selectively trap Xenon (Xe) from air at room temperature.
Thallapally: The existing methods for this separation in the nuclear reprocessing industry and from air includes the cryogenic distillation. Typically, air is cooled to very low temperatures in a series of distillation columns to separate individual gases. As a result, the process is very laborious and expensive. Our approach includes separation at room temperature using a nanoporous metal organic framework (MOF). This MOF selectively traps Xenon even in the presence of other gases including Krypton, Nitrogen, oxygen, argon and even moisture. In brief, we can pass the air through a column packed with this material at room temperature, which then traps Xenon selectively. The air without Xenon will pass through a second bed that consists of the same material and which selectively traps Krypton and rest of the gases you want to exclude. Separating Xenon selectively from air or in nuclear reprocessing has some economic advantages because Xenon is very expensive and used in a wide range of energy applications such as commercial lighting and building windows. In medicine, Xenon can be used for imaging, anesthesia, and neuroprotection. In Science, Xenon finds use in nuclear magnetic resonance, and as a propellant in ion propulsion engines.
Thallapally: Our collaborators in Berkeley used a molecular modeling tool to screen all the available MOFs in the database (5000 MOFs), based on their prediction. The SBMOF-1 was found to be the best among other MOFs in terms of Xenon loading capacity. This is in fact a rare example of computationally inspired material discovery.
Thallapally: There are quite lot of MOFs (20,000) reported in scientific literature so it would be very difficult to screen all the materials experimentally. Molecular simulation could help to screen quickly and predict which MOF material is the best based on its pore size and functionalities.
Thallapally: Initially we and the Lawrence Berkeley National Laboratory (LBNL) were working on this material independently. However, LBNL was focused on modelling and published a paper indicating this material as the best among the many materials that they have screened. Based on our experimental result, we felt that was incorrect and contacted Maciek and Berend (now collaborators) to share our results and discuss why our modelling and experiments did not match. Typically, when experiments are performed on these materials, chemists activate the material at high temperatures (300 degrees C) to remove any solvent molecules. To test if this activation made any difference, we performed the activation again at lower temperatures (100 degrees C) and repeated the experiments, ultimately reaching the exact same results as our collaborators.
Thallapally: We are in the process of scaling up this material to demonstrate a two-bed approach to remove Xenon in bed 1 and Krypton in bed 2 at room temperature using this material. We are working with our collaborators to predict the best material for Krypton to use in bed 2 in the presence of other competing gases including CO2, N2 and O2.
Reference:
Banerjee D, Simon CM, Plonka AM, Motkuri RK, Liu J, Chen X, Smit B, Parise JB, Haranczyk M, Thallapally PK. Metal-organic framework with optimally selective xenon adsorption and separation. Nature Communications 7:11831. DOI: 10.1038/ncomms11831
