Zeolites: The inside story
EMSL users find new ways to probe centuries-old catalysts
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More than 200 years ago, a Swiss mineralogist discovered zeolites. These unusual minerals emitted steam when heated, garnering a name based on the Greek words for "boiling stones." Since then, scientists have found these porous minerals are well suited for catalysis, among other applications. Despite decades of industrial use, the exact transformations occurring within the minerals during catalytic reactions remained poorly understood. Now researchers at EMSL are applying new techniques to track molecular action inside these lattice-like structures. With an atomic-level understanding of these catalysts, scientists can design more efficient zeolite materials that may lead to new resources for energy and the environment.
"We're always pushing our capabilities to get scientifically important information we haven't been able to get before," said Don Baer, EMSL Science Theme lead for Energy Materials and Processes.
Structure is Function
For many years scientists have known zeolites have a crystalline framework composed of tetrahedral (having four triangular faces) patterns made primarily of silicon or aluminum bonded to oxygen. These lattices form sheets, chains or other polyhedral (a solid in three dimensions with flat faces, straight edges and sharp corners) units. Up to 50 percent of these structures are voids, with pores and channels that provide a convenient place - for industrial purposes - to collect contaminants, stow reactants or sieve chemical products after reactions.
The reactive properties of a zeolite vary depending upon the nature and degree to which other ions substitute for silicon. For chemical stability, the zeolite seeks other ions to balance the charge forming a solid acid site.
By tweaking the ratios of aluminum (or other charged molecules or ions) to silicon, as well as pore and channel sizes, scientists can synthesize zeolites that preferentially trap, adsorb or exchange with other molecules or ions to speed up specific reactions.
"If researchers can better understand reaction sites and how to stabilize the zeolites, we can design new catalysts with longer lifetimes and at lower costs," Baer said.
Industrial Strength
Among the 200-plus zeolite structures known today, both naturally occurring and synthesized, only a handful find regular industrial application. That number could increase with deeper understanding of the catalytic performance of zeolites.
"Zeolites have always been very intriguing catalytic materials," said Bert Weckhuysen, professor of inorganic chemistry and catalysis at Utrecht University in the Netherlands. "Philosophically it's important to realize that every year one or two new families of porous materials are discovered. Waves of new porous materials come and go - certainly with promising applications - but the zeolites stick around. One reason is they are robust, solid materials that can survive the harsh conditions often employed in heterogeneous catalysis."
Since the early 1960s, zeolites have been a workhorse for oil refineries. The minerals catalyze reactions that "crack" long chains of hydrocarbons into shorter chains and ring structures. These smaller products can then be converted to gasoline or become feedstock for other petrochemical reactions; an example of the latter being propylene, used for making plastics.
Zeolites have now found steady work in the automobile industry, too. Catalytic converters for gasoline engines typically use precious metal-based catalysts to convert unburned hydrocarbons -- as well as carbon monoxide and nitrogen oxide, or NOx - into carbon dioxide, water, nitrogen and oxygen.
Demand for more fuel-efficient diesel engines has driven the search for new catalysts. In lean-burning engines (more air, less fuel), excess oxygen accumulates in the exhaust. In that chemical environment, the removal of NOx emissions wasn't so favorable using precious metal-based catalysts.
To solve that problem, scientists discovered zeolite-based catalysts could remove NOx - even with the oxygen excess - via selective catalytic reduction. This process introduces ammonia into the exhaust system as a "selective" reductant for converting the NOx into nitrogen. Although plenty of zeolites were quite active for the selective catalytic reduction reaction, a big stumbling block to developing this new approach was that none of those catalysts were stable enough to last the 200,000-mile lifespan of a car, explained Charles Peden, a Laboratory Fellow at Pacific Northwest National Laboratory, or PNNL, and associate director of PNNL's Institute for Integrated Catalysis.
By the late 2000s, scientists identified small pore zeolites that provided the major breakthrough to solve this problem. In particular, Cu-SSZ-13, a zeolite containing ion-exchanged copper ions, had the smaller pores necessary for preventing unburned hydrocarbons from poisoning the catalyst. Furthermore, this zeolite was much more thermally stable in the vehicle exhaust environment than others that had been tried.
"It was an unusual situation," Peden said. "They started commercializing these new emission control catalysts when the community, as a whole, didn't know exactly what the material was."
When scientists added copper ions to zeolite SSZ-13, they surmised where copper was located based on chemical properties and molecular structure; however, they were not sure where the atoms were exchanged in these frameworks. Nor did they know the position of reactive sites. To learn more about Cu-SSZ-13 catalysis, Peden and his colleagues used a combination of EMSL's instrumentation, such as electron paramagnetic resonance, or EPR, X-ray diffraction and computational modeling.
"One of the real surprises was how much the copper seems to move under reaction conditions," Peden said of their findings. "You couldn't learn that by taking just one measurement. But under reaction conditions, copper moves maybe 5-10 ångstroms. That doesn't sound like much, but that's significant for changing the zeolite catalyst's nature."
To further characterize the catalyst's nature, scientists followed the distribution of copper ions in SSZ-13 while the catalytic reaction progressed under different conditions. To do this, Eric Walter, an EPR expert and senior research scientist at EMSL, engineered a special cell to control reactant gas flow conditions in the EPR spectrometer. When varying amounts of copper were added to the catalyst, scientists correlated changes in performance and molecular properties with the changing conditions.
In the future, fuel economy standards will require vehicles with much higher mile-per-gallon numbers. When that happens, scientists will have a new problem to solve: Increasing fuel efficiency means more heat will be captured in the engine, resulting in exhaust temperatures too low for the current crop of catalysts to work well.
"We're looking for new catalyst materials, or something that significantly enhances what we've got now," Peden said.
Researchers are building dossiers on different zeolites to determine which ones might perform best in these new engine environments using sophisticated instruments at EMSL. In the past eight months, Peden and his colleagues have used EMSL's Mössbauer spectroscopy to study zeolite catalysts that contain iron ions rather than copper.
Probing Further
Scientists prefer to synthesize zeolites, because naturally occurring minerals often contain impurities that interfere with intended reactions. But synthesizing these structures with specific geometries, under readily reproducible conditions, presents an ongoing challenge.
One technology that pushed zeolite work forward was the development of bubbling reactor technology at EMSL. This allowed researchers to quickly (within two hours) synthesize large quantities of stable zeolite MFI suspensions, with controllable pore size and silicon-to-aluminum ratios. Among its many uses, zeolites with MFI structure are commonly the catalyst of choice for processes that lead to the formation of paraxylene, the raw material for manufacturing plastics products such as beverage containers.
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Zeolites are also attractive acid catalyst candidates for converting biomass into diesel-range biofuels. Lignin is the main component of abundant, woody biomass. If scientists can design catalysts that efficiently break down this tough, long-chain polymer, then the resulting smaller components can be readily degraded into biofuels.
The slowest step in the overall chemical breakdown sequence for lignin is the alcohol dehydration reaction. To model ways to accelerate that rate-limiting step, Aleksei Vjunov, a research associate at PNNL, used zeolite H-BEA to catalyze the dehydration of cyclohexanol in water, at high temperature and pressure. Using EMSL's high-resolution magic angle spinning nuclear magnetic resonance, or NMR, spectroscopy and isotope labeling of the alcohol molecules, he followed the chemical transformations and the associated redistribution of the 13C labels in the products and reactants during the reaction.
"If you want to improve the reaction, you need to know the exact mechanisms by which it takes place," Vjunov said. This novel approach nailed down the mechanism of this critical step.
To further study the structure of H-BEA catalysts, Vjunov and colleagues analyzed aluminum positioning among the nine tetrahedral sites in the silicate-zeolite structure. Their novel approach to this combined extended X-ray absorption fine-structure spectroscopy, performed at the Swiss Light Source in Switzerland, with NMR spectroscopy that was supported by theoretical calculations using EMSL's NWChem software package. As a result, researchers discovered aluminum atoms preferentially populate the tetrahedral sites located in particular ring-structures of the framework.
"Next we'll look at catalyst stability at high temperatures, and estimate the impact of structural degradation on the zeolite's catalytic performance," Vjunov said.
Another innovative way to follow aluminum atoms inside zeolites is with atom probe tomography, or APT. The technique has been used for more than 50 years to visualize three-dimensional structures of metals at the atomic level. But the tiny, needle-tip sample, and high electrical potential needed to vaporize individual atoms made it a challenge to apply this technology to zeolites. To address the challenge, EMSL scientists developed new ways to work with the porous, highly insulated zeolites and then track aluminum atoms inside a prototypical zeolite structure.
With APT, research collaborators, including Simon R. Bare, a senior scientist at UOP, LLC, as well as Weckhuysen, wanted to know: How are aluminum atoms bonded in the structure? Where do aluminum ions distribute? Furthermore, can aluminum ion distribution be correlated with catalytic properties, so more efficient materials can be designed?
Similar to the work performed using zeolite H-BEA, the APT indicated preferential aluminum clustering at specific sites inside the MFI zeolite particle.
"We're leading the effort using APT in this area," said Daniel Perea, an EMSL senior research scientist. "We can position ourselves to be world leaders in this because of our expertise and very unique tools," he said.
Accelerating Ahead
With a 50-year history on the catalytic scene, zeolites have already made a huge impact, said Baer.
"We now need catalysts that will turn the messy soup of biology into simple products that have the right form and structure to be a fuel or a valuable chemical," he said. "Once we understand these catalysts, we can have a whole new generation of products that can help us make the most of our resources."
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The research highlighted in this article received funding from a variety of sources. Funding for the various projects came from the Department of Energy's Office of Biological and Environmental Research, DOE's Office of Basic Energy Sciences, Pacific Northwest National Laboratory's Laboratory Directed Research and Development fund, the Netherlands Research School Combination-Catalysis, and the Netherlands Research Council.
EMSL, or Environmental Molecular Sciences Laboratory, is a national scientific user facility funded by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory in Richland, Wash.
Elizabeth Devitt is a science journalist and freelance writer.