Novel materials for the separation of xenon and krypton
Computer simulations establish industrial relevance
Novel materials might one day capture noble gases from air. Professor Rajamani Krishna of the Van 't Hoff Institute for Molecular Sciences (HIMS) contributed to an American study of the characteristics of these so-called metal organic frameworks (MOFs). In the highly regarded Accounts of Chemical Research a potential first application of MOFs is foreseen in the capture of volatile radionuclides emitted at the reprocessing of used nuclear fuel.
Over the last few decades chemists have developed a promising class of microporous materials of which the adsorption characteristics can be tuned by clever molecular design. These metal organic frameworks (MOFs) consist of metal ions combined with organic molecules to form highly structured porous molecular architectures.
Professor Krishna of the Computational Chemistry research group at HIMS has performed numerous simulations of the performance of MOFs in a broad range of potential applications. He now contributed to research at Pacific Northwest National Laboratory (PNNL) in Richland, Washington (United States) on MOF-based technologies for the separation of xenon and krypton from gaseous mixtures.
Currently the separation of xenon and krypton from air requires energy-intensive and therefore expensive cryogenic distillation. The broad range of applications of the two noble gases, particularly in lighting and medical industries, spawns interest in alternate approaches with a potential for lowering the production costs. The adsorption on porous materials is one of the most important alternative pathways.
However, traditional porous adsorbents such as zeolites and activated carbon generally show low capacity and selectivity when tested for noble gas capture. Improving this by fine-tuning of the pore properties of these materials is rather difficult. On the other hand, MOFs can be easily tailor-made for adsorption. Their hybrid nature and synthetic modularity permits the development of highly functional materials with diverse chemical compositions and pore sizes.
The research published at the Accounts of Chemical Research website earlier this month discusses six representative types of MOFs as well as the contribution of computational studies to the selection of adequate MOFs for Xe/Kr separation. HIMS professor Krishna contributed with so-called transient breakthrough simulations of industrial fixed-bed adsorbers. These helped to establish a hierarchy among the six MOFs in which a Ni-DOBDC MOF loaded with silver-nanoparticles showed the best performance.
The power of the transient breakthrough simulations is that they take account of the uptake capacity and diffusion limitations in an industrial setting. This is important since the classification of MOFs solely on the basis of their adsorption selectivity - as is rather common - often leads to misleading conclusions.
Reprocessing of used nuclear fuel
Looking into the future the researchers propose an effective combination approach in which multiple MOF adsorbents will be used to fully separate and capture Xe and Kr from a gas stream. They note that to date no large-scale commercial use of MOF-based adsorbents has been reported and that the production of MOFs on this scale is rare, unlike industrial adsorbents such as zeolites. However the team expects that investments and advances in synthetic methodology may lead to competitive MOF pricing.
Until then a first opportunity for application may exist in the reprocessing of used nuclear fuel. This generates volatile radionuclides including (but not limited to) isotopes of xenon and krypton. Currently, as with the capture of noble gases from air, the energy-intensive cryogenic distillation is the primary means to capture and separate radioactive noble gas isotopes. However, since this is a comparatively small-scale and dedicated process, the cost of the adsorbent material may not have a major impact on the overall process costs. Separation of radioactive noble gas isotopes in the reprocessing of used nuclear fuel might therefore be one of the first viable industrial applications of MOFs.
Debasis Banerjee, Amy J. Cairns, Jian Liu, Radha K. Motkuri, Satish K. Nune, Carlos A. Fernandez, Rajamani Krishna, Denis M. Strachan, and Praveen K. Thallapally: Potential of Metal–Organic Frameworks for Separation of Xenon and Krypton Acc. Chem. Res., Article ASAP, Publication Date (Web): December 5, 2014 DOI: 10.1021/ar5003126