Zeolite nanosheets for better separation

Zeolites are crystalline minerals largely composed of aluminium, silicon and oxygen (aluminosilicates). These basic components can be arranged in many different geometric orientations, including in both the naturally occurring and synthetic forms of the minerals. Zeolites are microporous materials already widely used to selectively adsorb or separate specific chemicals and to catalyse a variety of chemical reactions. Researchers are constantly looking for ways to improve their effectiveness and to apply them to new chemical processes. Gaining better control over the size and structure of the pores is key to adapting them for specific uses.

The most effective way to achieve the necessary control is by direct synthesis: building the desired zeolite structures from the bottom up. This is in contrast to common alternative methods that shear larger structures into membranes, offering less uniformity and control.

“We discovered an improved method to synthesize high aspect ratio zeolite nanosheets,” says Michael Tsapatsis at Johns Hopkins University, USA. The high aspect ratio refers to the ability to make nanosheets that are just a few nanometres thick but extend far in the other two dimensions, as required for separating specific molecules from gaseous mixtures.

The team used tiny seed crystals of a desired zeolite, which would then grow in size by promoting further crystallization at their edges. Most of the nanosheets had a largely rectangular shape, allowing them to combine to form thin membranes. These membranes proved highly effective at separating different structural forms (isomers) of the compound xylene when they encountered the membranes at high pressures and temperatures. Tsapatsis says that these xylene isomers are important precursors of various polymers made in large quantities by industry. This initial proof of concept application could be followed by other useful separations.

“The direct bottom-up and scale-up-friendly preparation of zeolite nanosheets marks a large step in the direction of zeolite membrane commercialization,” says zeolite expert Emeritus Professor Jürgen Caro of Leibniz University Hannover, Germany, who was not involved in the research.

Some challenges do remain, such as making membranes sufficiently robust for large-scale commercial applications. “We are currently working on creating thinner membranes that we will test at even higher pressures approaching those encountered in industrial use,” Tsapatsis says.

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