With increasing demand for effective separation of small-molecule gases – think of carbon caption and storage – there has been a lot of research recently into strategies and materials suitable for those applications. The traditional way to separate gases like nitrogen, oxygen or carbon dioxide is to freeze them out one by one, which is a very energy-consuming and in the case of oxygen occasionally even dangerous thing to do (famous last words: “Should my solvent turn blue in the cryo trap?”). Also, you can’t realistically sequester carbon dioxide from power plants this way.
The most promising way towards effective gas separation are nanoporous materials that don’t just select by size, as molecular sieves do, but also by chemistry. Porous coordination polymers have attracted considerable attention as well as metal-organic frameworks, because of the selectivity with which the metal centers interact with different molecules. A porous crystal from a pincer Iridium complex recently published in nature takes this principle even one step further by selectively catalyzing the hydrogenation of ethylene.
The material is a molecule crystal with nanometer-sized channels that are lined with metal centers, which absorb several small gases like oxygen, ammonia, hydrogen or ethylene. Everything larger than the pore diameter is excluded, for example propylene. A gas that enters the structure coordinates the iridium centers, changing the overall color: Red for ammonia, dark green for oxygen. As transition metals do, the Iridium also works as a catalyst: Treating the crystal with ethylene and hydrogen at 75 °C yields ethane at 99% conversion.
The researchers proved that the reaction happens within the crystal by trying the same with a mixture of ethylene and propylene, the latter being excluded from the pores and the former being hydrogenated with a selectivity of 25:1. That’s not bad for a start.
Of course, setting out by separating gases by chemical means promises even more selectivity than mere molecule size. That’s where things get interesting, and mathching the Nature paper there was a publication in Nature Chemistry that fits the bill. It is also about a metal-organic crystal with nanometer-sized channels, albeit in this case it’s a polymer, and the metal ions seem to have only a structural function. The magic is happening in the ligands.
The monomer that forms the material, [Zn(TCNQ)4bpy2], has a flexible three-dimensional structure constructed from Zn(II) centres and the organic ligands 4,4’-bipyridyl (bpy) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ). Polymerisation happens via dimerisation of TCNQ-units, interconnecting the monomers and setting the stage for what happens next.
According to the researchers it’s this dimer that causes the selectivity. The material ignores all small molecules, independent of size, except O2 and the isoelectronic NO. Both are good electron acceptors and form reversible charge-transfer-complexes with the TCNQ dimer in the pore wall. The electronic configuration of the dimers that form the walls of the nanometer-sized channels enables the material to capture larger molecules as well: aromatic hydrocarbons that are caught by π-stacking.
Shimomura, S., Higuchi, M., Matsuda, R., Yoneda, K., Hijikata, Y., Kubota, Y., Mita, Y., Kim, J., Takata, M., & Kitagawa, S. (2010). Selective sorption of oxygen and nitric oxide by an electron-donating flexible porous coordination polymer Nature Chemistry DOI: 10.1038/nchem.684
Huang, Z., White, P., & Brookhart, M. (2010). Ligand exchanges and selective catalytic hydrogenation in molecular single crystals Nature, 465 (7298), 598-601 DOI: 10.1038/nature09085