SLAC, Utrecht Univ. team visualize poisoning of FCC catalysts used in gasoline production; seeing changes in pore network materials
Merging two powerful 3-D X-ray techniques, a team of researchers from the Department of Energy’s SLAC National Accelerator Laboratory and Utrecht University in the Netherlands revealed new details of the metal poisoning process that clogs the pores of fluid catalytic cracking (FCC) catalyst particles used in gasoline production, causing them to lose effectiveness.
The team combined their data to produce a video that shows the chemistry of this aging process and takes the viewer on a virtual flight through the pores of a catalyst particle. More broadly, the approach is generally applicable and provides an unprecedented view of dynamic changes in a material’s pore space—an essential factor in the rational design of functional porous materials including those use for batteries and fuel cells. The results were published in an open access paper in Nature Communications.
The FCC catalyst particles are used in oil refineries to “crack” large molecules that are left after distillation of crude oil into smaller molecules, such as gasoline.
The FCC catalyst is designed as a multi-component, hierarchically porous particle of 50–100 μm diameters and consists of catalytically highly active phases (zeolites) embedded in a matrix consisting of an active component (alumina) and a non-active part made from silica and clay.
A highly interconnected hierarchical pore-network in the catalyst with pore sizes ranging from micro-pores (< 2 nm pore diameter) in the zeolite phase, to meso- (<50 nm) and macro-pores( ) in the active alumina phase and matrix provide access for feedstock molecules and also enable the cracking products to leave the catalyst. The pore space and its interconnectivity is thus of key importance to the efficiency of the conversion process.
Although the catalyst material is not consumed in the reaction and in theory could be recycled indefinitely, the pores clog up and the particles slowly lose effectiveness. Worldwide, about 400 reactor systems refine oil into gasoline, accounting for about 40 - 50% of today’s gasoline production, and each system requires 10 - 40 tons of fresh FCC catalysts daily.
Finding new clues about how FCCs age out could be key to improving gasoline production. But the new technique also has potential for understanding the workings of materials for powering cars of the future, according to Yijin Liu, a lead author on the paper and staff scientist at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), a DOE Office of Science User Facility.
The model we created by combining these two imaging methods can readily be applied to studies of rapid changes in the pore networks of similarly structured materials, such as batteries, fuel cells and underground geological formations.—Yijin Liu
|This visualization of the experimental data shows how scientists mapped the distribution of chemical elements in a single fluid catalytic cracking (FCC) particle and merged it with structural information about the pore networks. Because of the high resolution at which they mapped the catalyst, they were able to look deep into the pores and learn more about the metal poisoning reaction. The changing colors of the “fog” inside the pores reflect the changing chemistry. (SLAC National Accelerator Laboratory) Click to enlarge.|
In a previous study at SSRL, the team took a series of two-dimensional images of catalyst particles at various angles and used software they developed to combine them into three-dimensional images of whole particles showing the distribution of elements in catalysts at various ages.
For the new study, the researchers examined an FCC particle recovered from a refinery using two different 3-D X-ray imaging techniques at two experimental stations, or beamlines, at SSRL.
One technique, called X-ray fluorescence, provided a detailed profile of the particle’s chemical elements. The other, X-ray transmission microscopy, captured the nanoscale structure of the particle, including fine details about the porous network where metal poisoning can best be observed.
The high-resolution microscopy data provided a map of the pores, and the high sensitivity of X-ray fluorescence showed us where metals in the refining fluids were poisoning the catalyst, which appeared as a colored fog in our visualization.—Yijin Liu
The results of the study highlight the importance of having multiple techniques to study a single sample at a facility like SSRL.
Liu, who heads up one of the two beamlines used in the research, noted that there was a lot of development on the beamlines to make it possible to register the data in 3-D at this very fine scale.
Going beyond the observation of the experimental data visualized in the video, the scientists developed a model explaining how the accumulation of metals poisons the efficiency of the catalyst.
We used an analogy between electrical resistance and the degree of pore blockage, between two points in the particle using the new combined data. We then applied formulas well-known in electrical engineering to explain accessibility through the pore network, but also how it changes when metals are blocking pores.—co-lead researcher Florian Meirer, assistant professor of inorganic chemistry and catalysis at Utrecht University
The resulting model simulates the aging of the catalyst, allows scientists to quantify this virtual aging, and helps them predict the collapse of its transportation network.
The model explains for the first time how this happens in a connective manner, which is a big step toward improving the design of such catalysts. Furthermore, this novel approach can be applied to a broad range of other materials that involve the transport of fluids or gases, such as battery electrodes.—Bert Weckhuysen, professor of inorganic chemistry and catalysis at Utrecht University
Other researchers who contributed to this work were SSRL’s Courtney Krest and Samuel Webb. This work was supported by the NWO Gravitation program, Netherlands Center for Multiscale Catalytic Energy Conversion, and a European Research Council Advanced Grant.
Yijin Liu, Florian Meirer, Courtney M. Krest Samuel Webb & Bert M. Weckhuysen (2016) “Relating structure and composition with accessibility of a single catalyst particle using correlative 3-dimensional micro-spectroscopy” Nature Communications 7, Article number: 12634 doi: 10.1038/ncomms12634