The laboratory of Shengqian Ma (University of North Texas) has reported the application of a certain metal-organic framework (MOF) in the capture of xenon from exhausts (Niu et al. DOI). MOFS are three-dimensional porous structures based on metal ions and organic ligands. The cavities inside can trap small molecules and MOFS are extensively used in gas separation and gas storage. Ordinary targets are hydrogen and carbon dioxide and xenon is an unusual case but it can be found in abundance (400 ppm) in the gas exhaust of nuclear plants. Because of the radioactivity of the gas it must be removed and as the article explains MOF capture could potentially be an alternative to conventional cryogenic distillation.

The MOF selected for the job has been around for 20 years (DOI) and was made by dissolving 1,3,5,7-adamantanetetracarboxylic acid and copper nitrate hydrate in aqueous sodium hydroxide and heating it to 200 degrees centigrade for 18 hours and further dehydrating the resulting crystals in a high vacuum at 190 degrees for another 12 hours. The reoccurring motif in this type of MOF is a copper carboxylate paddle wheel.

At atmospheric conditions the xenon uptake can be 5 mmol per gram compared to 0.8 mmol per gram for nitrogen gas. The Xe to N2 ratio is 66. The uptake mechanism as described in the article is complex. The MOF has two types of cavities, one with a lot of hydrogen atoms and one with hydrogen and oxygen atoms. Xenon certainly avoids the spaces that contain copper. In the presence of xenon regardless of cavity type, one part shrinks and another part expands leading to 4 different cavity types, each equally capable of holding a xenon molecule. This effect is absent when the MOF is filled with nitrogen gas or oxygen or methane and hence the MOF is said to be self-adjusting. The effect may come across as counter-intuitive because xenon is supposed to be a noble gas and inert to any interactions it may have with the inside of a cavity.

The article radiates a very positive nuclear energy vibe. It mentions in the introduction the energy source has prevented 1.84 million air-pollution related deaths and avoidance of 64 billion tons of emitted carbon dioxide. It also alludes to the promise that that the harvested radioactive xenon can be converted to benign xenon and thus drive down xenon prices but the citations offered to back up this claim (two) are either not relevant (one) or none existing (one). In the month that the European Union has to decide if nuclear is green or not, it is good to know that running a nuclear plant has just become a little bit more complex.