Photo courtesy of Dawn Adin and John Buchner
Researchers shaped zeolite into house-of-cards configurations to create more efficient catalysts.
Construction of a house of cards may not seem like much of an accomplishment. In reality, however, building a house of cards requires quite a bit of planning, more than a little basic engineering know-how, and an abundance of patience. And from time to time, a house of cards can actually come in handy. For example, who would have thought that such a structure could someday play an important role in decreasing the high energy costs associated with purifying and separating chemicals and fuels critical to industries across the world, ranging from gasoline and plastics to cosmetics?
Since the early 1990s, Michael Tsapatsis has been trying to determine how to do just that. Tsapatsis is a chemical engineering and materials science professor at the University of Minnesota (U of M) and currently a senior researcher with the Catalysis Center for Energy Innovation, a DOE Office of Science Energy Frontier Research Center (EFRC) led by the University of Delaware, in which U of M is a major partner.
Tsapatsis and his research team initially set out to find a way to get rid of the thermodynamically inefficient distillation columns that are used to purify chemicals, such as ethanol, in chemical processing plants. As with making moonshine, tremendous amounts of heat are applied to liquids to separate the ethanol (or other product) from the contaminating chemicals in the solution. More recently, they expanded their activities in efficiently transforming biomass and oil to value-added products. What Tsapatsis ended up focusing on was a class of materials from nature—zeolites.
Originally found as a natural volcanic mineral, zeolites are often compared to sponges because they can preferentially trap liquids as well as gases and filter out impurities by acting as a molecular sieve, separating molecules with only small differences in size and shape.
The difference between zeolites and sponges lies in the way they collect liquids and release them. Sponges absorb liquid, meaning that they "drink it or soak it up," and the liquid becomes part of the sponge structure until squeezing releases the liquid.
Zeolites, however, are composed of a rigid, honeycomb-like crystal structure, and they adsorb liquids or the contaminants passing through, meaning that the liquid, gas, or impurity sticks to the surface of the zeolite. Instead of being squeezed, zeolites release the molecules stuck to them when the zeolites are heated. That's how they got their name from Swedish mineralogist Axel Fredrick Cronstedt in 1756: the Greek words "zeo"—to boil—and "lithos"—stone—combine to mean "the rock that boils."
Every drop of gasoline we use needs a catalyst to change the oil molecules into usable gasoline during the refining process.” Michael Tsapatsis
Professor of Chemical Engineering and Materials Science, University of Minnesota
A defining trait of zeolites is that the building unit of their crystal structure, or framework, resembles a pyramid with a triangle as its base—in other words, a tetrahedron. In the middle of the pyramid is a silicon or aluminum atom bound to four oxygen atoms, which form the bases of the pyramid. Individual tetrahedrons link together at their corners, forming a variety of structures—including cages, cavities, or channels—that certain molecules can enter while others cannot, depending on their size and shape.
While over 200 naturally occurring and synthetically made zeolite frameworks are currently known, much work continues, including that of Tsaptatsis's group, to develop unique new zeolite frameworks synthetically. Indeed, many tons of synthetic zeolites are produced each year for use in a variety of products, including such household staples as laundry detergent and kitty litter. Based on the small, regularly arranged pores, zeolites are ideal filters for applications such as the extraction of nitrogen from air to increase the oxygen content for industrial and medical purposes.
Beyond these uses, industry employs synthetic zeolites as catalysts, substances that increase the rate of a chemical reaction without themselves undergoing any permanent chemical change, allowing the catalyst to be reused numerous times. For example, zeolites can provide an acidic environment for a catalytic reaction to take place, such as those found in the zeolites used by oil refineries to break long hydrocarbon chains in oil into the shorter, volatile hydrocarbons in gasoline.
"Every drop of gasoline we use needs a catalyst to change the oil molecules into usable gasoline during the refining process," Tsapatsis said.
Regardless of the reaction that takes place, the size and shape of the pore system in the particular zeolite crystal affects which molecules can get in and what products can get out. For instance, during the refining process, some of the oil, especially larger molecules, gets stuck or slowed down in the catalytic material used today, increasing chemical processing costs. Tsapatsis compares this to driving.
"It's faster and more efficient to use freeways to get where we want to go and exit to do our business compared to driving the side streets the entire way," he said. "The catalysts used today are more like driving only on the side streets. Molecules move slowly and get stuck."
To ensure that the molecules make optimal use of the freeway, Tsapatsis and colleagues wanted to improve the efficiency by giving molecules (oil or some other molecule) faster access to the catalyst. To do this they needed to find a way to provide more surfaces on which the catalytic reactions could occur, while preventing the bigger molecules from setting up road blocks.
Tsapatsis and his colleagues had learned previously how to optimize the surface area of zeolites by creating ultrathin nanosheets. They overcame a significant challenge in processing zeolitic materials as ultrathin sheets that remain intact. They developed "carpets" of flaky crystal-type nanosheets that were flat and had the appropriate thickness, only a few nanometers thick.
These flat sheets were shown to be ideal for making thin selective membranes. But for catalytic applications the issue became how to get more molecules to interact with the catalysts, while allowing the larger molecules to pass through. Think of a zeolite nanosheet as a playing card. When the cards are stacked in the deck, there is minimal space in-between the cards. However, if the cards are used to build a house or other structure, more of the cards' surfaces can be seen. By creating a "house-of-cards" with the zeolite nanosheets, more visible cards means more space available for the molecules to access the catalyst.
Image courtesy of the University of Minnesota
Schematic of repetitive branching of zeolite nanosheets creating a hierarchical zeolite catalyst containing micropores within the nanosheet and 10-fold larger pores in-between the nanosheets.
The researchers used a novel and simple synthesis technique called repetitive branching to stack the thin zeolite sheets at right angles generating the "house-of-cards" shaped crystal. By creating zeolite crystals with large-pore "highways," which are about 10 times bigger than the zeolite pores, chemicals and molecules can pass rapidly through the channels to reach the smaller, reactive pores within the crystal. This results in faster, more selective, and more stable catalysts, produced at the same cost as traditional zeolite catalysts.
"This synthesis method significantly improves the effectiveness of traditional zeolites with no unwanted change in functionality and is the first cost-efficient route that can enable large scale commercialization," Tsapatsis states.
Because the technique can control both the pore size and the size of the channels created between the zeolite thin sheets, it could be applied to a variety of industries that use catalysts in processing their products, including gasoline, plastics, biofuels, pharmaceuticals, and other chemicals.
And with a faster catalyst available at no extra cost to the industry producer, manufacturing cost-savings could lead consumer costs to drop.
As stated by Tsapatsis, "The efficiencies of these new catalysts could lower the costs of gasoline and other products for all of us."
This technology was recently licensed by the Minnesota start-up company Argilex Technologies and is a key component to the company's materials-based platform. With the development of the new catalyst now complete, the material is ready for customer testing.
The use of these novel zeolites can lower operating costs and reduce waste streams. Thus, the potential application of this technology could end up having far-reaching impact worldwide.
The work, which was reported in the journal Science¸ was supported by DOE's Office of Science, the Abu Dhabi–Minnesota Institute for Research Excellence, the National Science Foundation, and the University of Minnesota Institute on the Environment—Initiative for Renewable Energy and the Environment.
—Dawn Adin, DOE Office of Science, Dawn.Adin@science.doe.gov
Some material was adapted with permission from the website of University of Minnesota at http://www1.umn.edu/news/news-releases/2012/UR_CONTENT_395514.html.
DOE Office of Science, Office of Basic Energy Sciences
Abu Dhabi-Minnesota Institute for Research Excellence
National Science Foundation
University of Minnesota Institute on the Environment—Initiative for Renewable Energy and the Environment
Zhang, X, et al. "Synthesis of self-pillared zeolite nanosheets by repetitive branching." 2012. Science 336: 1684-1687. DOI: 1-.1126/science.1221111
Varoon, K, Zhang X., et al. "Dispersible exfoliated zeolite nanosheets and their application as a selective membrane." 2011. Science 334: 72-75. DOI: 10.1126/science.1208891
Catalysis Center for Energy Innovation (CCEI)
DOE Energy Frontier Research Centers
University of Minnesota Characterization Facility
Chemical Sciences, Geosciences, & Biosciences Division, Office of Basic Energy Sciences, DOE Office of Science