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Cracking the Code for Zeolite Formation Could Lead to More-Energy Efficient Refining and Hydrogen Storage

Silicon-oxygen nanoparticles aggregate to form zeolites, capturing other atoms and molecules in the process. The resulting minerals have regularly-shaped, intricate pore and channel systems throughout their structures. Credit: Michael Tsapatsis, University of Minnesota

Zeolites are porous, sieve-like minerals that have been used for decades in purifiers, filters and other devices. Despite their utility, no one has yet been able to figure out exactly how the crystals form, and creating and refining a new type of zeolite is a matter of sophisticated trial-and-error.

Now, however, researchers have uncovered new details for the step-by-step creation of zeolites that could lead to targeted methods to produce zeolites with precisely the crystal sizes and shapes demanded by specific applications such as hydrogen purification and petroleum refining separations as well as other industrial applications.

Having the ability to produce cost-effective, targeted zeolites on an industrial scale could replace energy-inefficient separations (such as in fuel refining) with membrane separations. In other words, instead of heating the feedstock liquid and distilling the desired chemicals, the new membrane sieves could achieve the same goal when the fluid simply passes through.

University of Minnesota chemical engineer Michael Tsapatsis, graduate student and lead author Tracy Davis, and their colleagues report their findings online in Nature Materials. The research was supported by several National Science Foundation (NSF) grants from across three Divisions.

In an effort to improve the understanding of nucleation and growth processes that control formation of zeolites, Tsapatsis and his colleagues spent more than a year monitoring the growth of zeolites. They used advanced tools that included a state-of-the-art High-Resolution Transmission Electron Microscope purchased with NSF support, and a Small Angle X-Ray Scattering System on loan from Anton Paar GmbH to observe changes on the scale of single nanometers.

These are complex structures containing hundreds of atoms per unit cell and their formation is determined largely by kinetics. Our approach is to slow down the kinetics and exhaustively study the evolution by all techniques available to us.

—Michael Tsapatsis

Ultimately, the researchers hope to develop, validate and improve quantitative mathematical models that describe the complex systems.

The study showed that the zeolites form in a step-by-step, hierarchical fashion, with silicon-oxygen nanoparticles forming first. Those particles then aggregate into larger, more complex structures, incorporating other atoms and molecules while still leaving substantial pores and tunnels. Based on their findings, the researchers developed a set of mathematical equations that describe the nucleation and growth process.

There are essentially unlimited opportunities for these crystals if we can control their pore structure and crystal shape, tailoring designs to specific applications ranging from catalysts to bio-implants.

Membranes made by our current process will cost over $1,000 per square meter—too expensive for widespread use in applications like hydrogen purification and hydrocarbon separations that need thousands of square meters of membrane. With the mechanistic knowledge we now have we are designing one-step film formation processes that could cost one tenth that amount.

—Michael Tsapatsis



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