Methane — the essential component of natural gas and biogas — is never found in its pure form. It needs to be separated from carbon dioxide to be used as a source of energy for heating, while the CO2 can be used as a building block for renewable fuels and chemicals.

The conventional way to accomplish this is through cryogenic distillation, which is both energy-intensive and environmentally unfriendly. A low-cost, low-energy, scalable alternative is the use of polymeric membranes. But those that are commercially available are limited by their permeability and selectivity tradeoff, as well as insufficient thermal and chemical stability.

"An effective membrane only allows the CO2 to pass through, and as much of it as possible,” says University of Leuven (KU Leuven) bioscience engineering professor, Ivo Vankelecom. "Another important problem is the fact that the membranes plasticize 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."

As recently published in the journal Energy & Environmental Science, a collaborative study by KU Leuven and the University of Antwerp, both located in Belgium, focused on making the polymeric separation process much more effective. To that end, researchers introduced a novel type of amorphous mixed-matrix membrane, consisting of a metal-organic framework (MOF) with nanoscale pores.

The study showed that membrane performance improved significantly when heated above 160° C during the production process. “At these temperatures, the structure of the MOF changes, and it becomes more selective," Vankelecom explains. "The high-temperature treatment also improves polymer-filler adhesion — the gas mixture can no longer escape through little holes at the filler-polymer interface."

This gave the new membrane the highest selectivity ever reported.

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

More information on the project can be found on the Membrane Technology Group website.