Nanosheets improve fabrication of gas separation membranes

Zeolites are crystalline solids, generally made from varying combinations of silicon, aluminium and oxygen, with a variety of other atoms incorporated. They are highly porous, so they can selectively block, retain or allow free passage to targeted gaseous molecules. Adjusting the components, and therefore the structure, of specific zeolite-like ('zeolitic') materials gives scientists control over these properties.

“In our case there is no aluminium; just silicon, oxygen and hydrogen,” says Kumar Varoon Agrawal, head of the laboratory of Advanced Separations at the Swiss Federal Institute of Technology in Lausanne.

Zeolitic membranes are normally grown by inducing crystals to form in a chemically complex, high temperature water-based gel, but the process is difficult to control and reproduce. This often leads to problematic cracks and defects.

"We show that high-performance zeolitic membranes can be made by a LEGO-like assembly of chemical building blocks," Agrawal says. This simpler procedure involves heating ultra-thin zeolitic nanosheets in air so they fuse to make the desired membranes. The nanosheet building blocks are sheared off from a precursor material called sodalite.

This breakthrough required some persistence, as early efforts were plagued by cracks developing in the membranes. "We struggled with this for a year," Agrawal reports, before the team eventually found that the cracks could be healed by adding a second nanosheet layer.

"This is important progress," says chemical engineer, Michael Tsapatsis, at Johns Hopkins University in Baltimore, USA, who was not involved in the work. "It reduces fabrication cost and improves scalability, which are essential requirements for industrial use."

Agrawal says one promising application will be in purifying hydrogen gas. The membrane would allow hydrogen to pass while capturing carbon dioxide, which is also produced during the most common method for making hydrogen. This membrane-based separation should be cheaper and more efficient compared to the current procedure that uses chemicals to capture carbon dioxide.

As a next step towards commercialisation the team is scaling up to fabricate membranes with larger areas. Other gas separation possibilities can also be explored.

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