‘Highest selectivity ever’ recorded in new carbon/methane filter membrane

A new membrane has been developed that makes more effective the process of separating methane from CO₂. This process is crucial in making natural gas and biogas suitable for use

Methane is never found in its pure form. Natural gas, for instance, always contains a large amount of carbon dioxide. Industry generally uses membranes that act as molecular sieves to purify this natural gas (i.e. remove the CO₂). Once filtered, the methane can be used as a source of energy for heating, for the production of chemicals or a fuel, while the CO2 can be reused as a building block for renewable fuels and chemicals.

Researchers at the University of Leuven developed the new membrane to increase the efficiency of the purification process. A study detailing their work has been published in the journal Energy and Environmental Science.

“An effective membrane only allows the CO₂ to pass through, and as much of it as possible,” said Ivo Vankelecom from the KU Leuven Faculty of Bioscience Engineering.

“The commercially available membranes come with a trade-off between selectivity and permeability: they are either highly selective or highly permeable. Another important problem is the fact that the membranes plasticise if the gas mixture contains too much CO₂. This makes them less efficient: almost everything can pass through them, so that the separation of methane and CO₂ fails.”

According to a statement, the best membranes available consist of a polymeric matrix with a filler in it, for example a metal-organic framework (MOF). This MOF filler has nanoscale pores, and the new study has shown that the characteristics of such a membrane improve significantly with a heat treatment above 160°C during the production process.

“You get more crosslinks in the polymeric matrix: the net densifies, so to speak, and that in itself already improves the membrane performance, because it can no longer plasticise. At these temperatures, the structure of the MOF - the filler - changes, and it becomes more selective. Finally, the high-temperature treatment also improves polymer-filler adhesion: the gas mixture can no longer escape through little holes at the filler-polymer interface,” Vankelecom explained.

The heat treatment apparently gives the new membrane the highest selectivity ever reported, and prevents plasticisation when the CO₂ concentration is high.

“If you start with a 50/50 CO₂ mixture, this membrane gives you 164 times more CO₂ than methane after permeation through the membrane,” explained Dr. Lik Hong Wee from KU Leuven. “These are the best results ever reported in scientific literature.”