Zeolite Membranes Promise New Era in Separation Technology

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Professor Rich Noble holds up a model of a zeolite membrane. His research John Falconer will help petroleum companies efficiently remove carbond dioxide impurities from natural gas.
John Falconer and Richard Noble's collaboration has led to the first new membrane material used for gas separation in 30 years.
John Falconer and Richard Noble's collaboration has led to the first new membrane material used for gas separation in 30 years.

Scene: Two chemical engineering professors, one from Colorado and the other from Wyoming, meet at a teaching seminar in Aspen in the late 1970s. One is interested in catalysis, the other in membranes. A friendship is formed as they talk about their common interest in running.

It would be decades before professors John Falconer and Richard Noble would collaborate in research. Even though Noble later joined Falconer in the chemical engineering department at CU-Boulder, their research interests were quite a bit different. That is, until the concept of a catalytic membrane reactor—a device that combines reaction with separation to increase the conversion of molecules—emerged in the early 1990s.

Both Falconer and Noble thought that zeolites—aluminosilicate minerals with a microporous molecular structure that can withstand high temperatures without breaking down—would be a promising material for such a reactor. Synthesizing a workable zeolite membrane was easier said than done, however.

They embarked on a process of trial and error, and had a lot of unrepeatable results over more than a decade. Among the problems was that the sequencing of chemicals used to create a zeolite membrane had a significant effect on the crystal's structure. Many students, ranging from undergraduates to post-docs, participated in the research over the years.

Eventually, perseverance and support from the National Science Foundation and Shell Global Solutions, as well as other sponsors, paid off. The research group has synthesized a zeolite membrane, called SAPO-34, with a molecular structure that can remove carbon dioxide impurities from natural gas reserves without losing methane in the process.

Applied in the laboratory as a several-micron thin film on the interior of alumina and stainless steel porous tubes, the membrane is moving toward commercialization because it will save money for the oil and gas industry and allow recovery of more energy resources, according to the researchers.

"This will be the first new membrane material used for gas separation in 30 years," says Noble, who has been engineering membranes for nearly that long, first at the National Institute of Standards and Technology, and since 1987 at CU-Boulder. Noble co-directs the National Science Foundation Center for Membrane Applied Science and Technology.

"You wouldn't believe how many people told me it wouldn't work," he says. The difference? "We kept at it. We have probably made more kinds of zeolite membranes than any other group in the world."

The CU-Boulder laboratory has produced approximately a dozen types of zeolite membranes, each with a different molecular structure. The crystals must be both selective and permeable to be effective for a designated task.

"A membrane is defined by what it does, not what it is," Noble explains. "A membrane provides a selective barrier that allows separations due to the varying speed of the molecules or particles moving through it."

Polymer membranes have been used to separate gases since the 1970s. Food packaging is a common example that restricts the oxygen flow to the product to preserve freshness.

Polymers also have been used in the energy industry to remove carbon dioxide from natural gas, although with less than perfect results. Zeolite membranes are proving better than polymers because they can withstand the high pressure and chemical environment, Falconer and Noble say.

Now that the researchers have achieved the results they were looking for, they are scaling up their test modules to handle greater volumes of material. And they're investigating whether zeolite membranes could also remove carbon dioxide from coal-fired power plants or remove contaminants from water.

In the lab, an assortment of undergraduate and graduate students and post-doctoral fellows continues to study liquid, gas, and vapor separations and evaluate the performance of the team's membranes for various applications. Numerous custom-built lab setups hint at the extent of their research and accomplishments.

Noble was named CU-Boulder Inventor of the Year for 2008, while Falconer received the Hazel Barnes Prize for groundbreaking research and exceptional teaching. Both hold endowed professorships and have many teaching awards to their names.

Which brings us back to the beginning of the story: Maybe it's not a coincidence that Noble and Falconer were both long-distance runners. They certainly went the distance on this track.

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