Helium is the second most abundant gas in the universe after hydrogen, with a mass fraction of 24-25%. However, concentration of helium in the Earth’s atmosphere is very small – not more than 0.0005%. Therefore, the main source of industrial helium production is natural gas processed with cryogenic, adsorption and membrane methods. The cryogenic method involves gas separation into its components – nitrogen, methane, ethane, propane, butane and helium – at extremely low temperatures. In its turn, the adsorption method involves filtering a helium-containing gas mixture through a column with an adsorbent, as a result, the latter absorbs impurities (including helium), and the output is pure gas. Finally, membrane gas separation uses membranes that either trap impurities (nitrogen and methane) and pass only helium, or pass impurities, leaving helium at the inlet.
Metal–organic coordination polymers (MOCPs) are promising materials for helium production: they are porous and consist of metal clusters connected by organic structural units. The combination of MOCPs produces various crystalline structures, in which the gas is separated in porous space. These combinations are tens of millions. To determine the parameters affecting the efficiency of MOCPs, the scientists from the Institute of Catalysis of the SB RAS have screened ten thousand variants of compounds.
“Based on the screening results, we were able to identify six structural descriptors affecting the efficiency of the material during both adsorption and membrane gas separation of helium-containing mixtures. They are: a limiting pore size, the largest cavity diameter, an available surface area, an available pore volume, the density and the porosity. If our structure falls within this range, we can expect it to be extremely effective for helium extraction,” the Institute of Catalysis SB RAS quotes Ivan Grenev, a researcher at the Department of Materials Science and Functional Materials.
Most international studies have focused on finding helium-selective membrane materials, but the reverse process – nitrogen and methane selectivity – when the membrane allows them to pass through, retaining the desired gas, still remains in the background. The study of the Institute of Catalysis of the SB RAS has demonstrated that organometallic polymers, which are simultaneously methane and nitrogen selective, are more promising than helium selective materials.
