Pore research = great science

Michael Tsapatsis is an architect of the molecular world

The crystal lattices Michael Tsapatsis builds are architecturally stunning.

And they pack a lot of power, chemically speaking.

Created by the University of Minnesota chemical engineering professor and his colleagues, the lattices contain holes, or pores, that act like a molecular sieve, allowing only molecules below a certain size—or even just certain forms of the same molecule—to pass.

In one example, Tsapatsis and his team are making a nanosheet that can separate ethanol and water. If applied in Minnesota ethanol plants, it could substantially reduce the costs and energy consumption needed to purify ethanol.

The team also has assembled sheets of the lattices into stable 3-D structures with built-in catalysts that can, among other things, turn molecules found in crude oil into gasoline. These structures may revolutionize the synthesis of many industrially important chemicals by giving higher yields.

The research could also lead to slashed energy consumption and costs—along with much greater efficiency—in making plastics, biofuels, and pharmaceuticals, as well as gasoline, diesel, ethylene and propylene. Consumer costs could also drop. The discoveries have both been published in the journal Science.

So how did they do it?

Holey sheet

The star of this story is zeolite, a naturally occurring mineral in which atoms of silicon and oxygen crystallize into intricate lattice structures. The pores vary in size, depending on the particular crystal structure. Tsapatsis and his colleagues discovered a way to make extremely thin "nanosheets" from zeolite lattices in which the pores act like the holes in a sieve to separate small molecules. 

"The smallest nanosheets are about two nanometers thick and can be hundreds of nanometers long," says Tsapatsis. "These are the only nanosheets that can be used as a filter." 

The zeolite nanosheets can be spread over a highly porous disk that would be placed in a tube to let only molecules below a certain size flow past. One example is the separation of different forms of xylene (see diagram).

Nanosheet photo

Pores in nanosheets can be constructed so as to let valuable p-xylene (right) pass while keeping out m-xylene, which has the same constituents but in a different arrangement.

The p-xylene form, which is valuable for making plastic bottles and polyester clothing, has the same chemical constituents as its cousin m-xylene. But only the "p" form can squeeze through the pores in one of Tsapatsis' zeolite nanosheets. If the walls of a chemical reactor tube were porous and spread with layers of this nanosheet, only p-xylene would escape to be collected.

House of cards

In some zeolites, a certain proportion of the silicon atoms are replaced by aluminum and other atoms, which allow the zeolites to function as catalysts to accelerate chemical reactions. Their regular pore structure allows them to favor formation of certain desirable products. But most zeolite catalysts lack an efficient way to guide molecules to the catalytic sites. 

"Traditionally, molecules must travel a long path through a zeolite crystal's pores to get to all the catalytic sites," says Tsapatsis. "Some of the catalytic sites—deep inside the crystal —cannot be reached by the molecules. Moreover, many molecules get stuck in the pores as they try to leave the catalyst."

Michael Tsapatsis research image - 300x225

Model of a zeolite "house of cards" assembling itself.

If you were a molecule, it would be like driving the narrow side streets of a town to find a barbershop (catalytic site), then trying to find your way out again after being shorn (chemically changed).

But Tsapatsis and his colleagues have coaxed catalytic zeolite nanosheets into connecting at right angles, forming a structure like a house of cards.

Now, molecules just have to diffuse through broad spaces between the nanosheets, dart into one of numerous shallow, regularly spaced pores that harbor a catalytic site, then diffuse out again. It's like getting a haircut at a mall where the barbershops are numerous and at regularly distributed.

The result: immensely speeded-up chemical reactions, and time and money saved. 

"The efficiencies of these new catalysts could lower the costs of gasoline and other products for all of us," Tsapatsis says. The technology has been licensed to Minnesota-based Argilex Technologies, which aims to use it to obtain higher yields of desired chemical products.