Energy-saving gas separation with boron nitride nanosheets

The petroleum industry currently uses energy-intensive cryogenic distillation processes to separate hydrocarbon gas mixtures. But the high-power demands of the refrigeration units account for a staggering 15% of global energy consumption.

Now researchers from Deakin University, Queensland University of Technology, and RMIT University in Australia have developed an energy-saving, green alternative separation process using boron nitride (BN) powers and ball milling [Mateti et al., Materials Today (2022), https://doi.org/10.1016/j.mattod.2022.06.004].

“We [have] found a highly efficient and entirely novel way to separate, purify, store, and transport huge amounts of gas safely,” says Ying Ian Chen of Deakin University, who led the work. “It will help overcome the key challenge of hydrogen storage by allowing us to safely store and transport green hydrogen as a solid at a fraction of the energy cost, as well as save substantial energy [in the] petroleum industry’s gas purification process.”

In the process, BN powder is simply placed into a ball mill – a grinder containing stainless steel balls – at room temperature with a gas to be stored or a mixture of gases to be separated. As the chamber spins at faster and faster, collisions between the steel balls, hexagonal BN powder, and gases trigger mechanochemical reactions, which result in the absorption of gas molecules onto the surface of the BN. For example, when a mixture of an alkyne (C₂H₂) or an olefin (C₂H₄) and paraffin (CH₄ or C₂H₆) is ball milled with BN, the alkyne or olefin is absorbed onto the BN surface and the paraffin remains in the chamber, from where it can be easily extracted. The absorbed gases can be extracted using a heating process.

“When a mixture gas is introduced into the milling chamber, one type of gas is absorbed more quickly onto BN, separating it out from the others, and allowing it to be easily removed from the mill. We can repeat this process over several stages to separate out gases, one by one,” explains Chen.

Rather than a simple surface area adsorption process, chemical reactions induced by ball milling create chemical bonds between the gas molecules in the chamber and the BN, transforming the BN particles into nanosheets with surfaces covered with chemically bonded gas molecules. The exceptionally large surface area of the nanosheets enables unprecedentedly high gas storage.

The new approach can be scaled up, run continuously, and does not require high temperatures or pressures, promising great potential for energy-efficient industrial gas separation and storage or CO₂ capture.

The authors thank the Australian Research Council under the ARC Research Hub and Discovery grants for financial support.