The Secrets Such Structures May Hold: Zeolites and Metal-Organic Frameworks
Laura E. Fernandez and Emily Sahadeo
Just like many people need coffee to get moving in the morning, some chemical reactions need help getting started as well. Catalysts are like coffee for chemical reactions; they are the aid that helps a chemical reaction get going. Catalysts can vary, as can the supports to which they are bound. Several Energy Frontier Research Centers are developing new catalysts and using nanoporous materials. We focus on recent work at two centers: Inorganometallic Catalyst Design Center (ICDC) and Catalysis Center for Energy Innovation (CCEI).
Nanoporous materials are frameworks with pore or cavity sizes of 100 nanometers or smaller. Zeolites and metal-organic frameworks (MOFs) are two common nanoporous materials, with pores less than and larger than 1 nanometer, respectively. In these pores, molecules can move and reactions can take place. Zeolites can be catalysts, while MOFs make ideal support structures for catalysis or catalysts themselves.
Good Vibrations: Insight into zeolite characterization and capabilities. Creating zeolites is like baking – in many desserts you have the same basic ingredients, like flour, eggs, and sugar, but how you mix them and the baking temperature can determine the final product. The basic ingredients of zeolites are silicon, oxygen, and aluminum atoms; when combined in different ways they can make more than 200 frameworks. Engineers in the CCEI have conducted experiments to investigate this class of materials and its applications.
Characterization and reactivity are important aspects of zeolite research – scientists need to have as much information as possible about these intricate materials and how they react with molecules in their pores to better determine catalytic properties and applications of particular zeolite structures.
One structure being investigated at the CCEI is zeolite H-BEA, or BEA. It has all of the basic zeolite ingredients and was used as the catalyst in an experiment probing different reaction pathways for the formation of toluene from two interesting chemicals: 2-methylfuran and ethylene. Toluene is important because it is used in the production of paints, adhesives, rubber, and other consumer products.
This study investigated side reactions, which make compounds other than the desired product, as well as reaction kinetics, which is how fast reactions occur. They discovered that toluene formed at two different rates depending on the concentration of acid sites and which reaction controlled the rate of the process. They also found that the formation of the side products impacted the selectivity – only 46 percent of the product was the desired compound, toluene; the rest of the products were other molecules. By learning this information about toluene formation, researchers can begin to improve catalysts for better selectivity and increase the amount of toluene produced.
Engineers also changed one of the ingredients from aluminum to tin to alter the zeolite properties. Therefore, it is important to determine whether tin is incorporated into its structure. This zeolite, Sn-BEA, has many potential applications, such as altering the structure of sugar. This step is one in a series of reactions for the conversion of cellulose in plant-based biomass; the cellulose can be made into sugar, which can be made into furans, and furans can then be used to make compounds such as toluene.
In the past, Fourier-transform infrared spectroscopy (FTIR) has been used to assess the incorporation of tin in the zeolite framework. FTIR is a tool used to determine different elements and groups in a molecule or material’s structure based on how the atoms in a solid or molecule vibrate when exposed to infrared radiation; different peaks correspond to groups with unique vibrations in the structures. A 960-cm-1 peak has been considered as indicative of tin incorporation into the BEA zeolite, but the team at CCEI discovered otherwise.
These engineers compared fresh samples of the tin zeolite to water-treated tin zeolites and other BEA zeolites using multiple tests to determine the origin of the 960-cm-1 peak. They discovered the peak was not caused by tin incorporation in the structure, but instead was a result of silanol, a hydroxyl (OH) attached to a silicon atom! The silanol groups formed during the water treatment, which allowed them to be studied and identified more clearly during characterization. Because the 960-cm-1 FTIR peak does not indicate tin in the zeolite framework, new methods are needed for tin zeolite characterization.
Everyone needs a little support, why not a metal-organic framework? Metal-organic frameworks (MOFs) are networks of metal-based clusters or nodes connected by special linkers. MOFs are great for gas storage, adsorption, chemical sensors, drug delivery, and catalysis. The ICDC is conducting scientific research using zirconium-based MOFs as catalyst support for natural gas conversion. The MOF of choice in the ICDC is NU-1000, designed by a research group led by Joseph Hupp and Omar Farha at Northwestern University, thus the abbreviation NU in the naming of the MOF. NU-1000 is a zirconium-based MOF that contains six zirconium atoms at each node. This network of nodes and linkers produces two large pores and two small pores around each node.
Researchers at the ICDC are working on depositing single-site catalysts via reactive metal precursors on the NU-1000 MOF. These precursors (metal-based reagents that invoke a reaction with the MOF) can be deposited on the node or linker depending on the precursor, the process in which it is added, and the metal being deposited. The team at the ICDC has added iridium precursors, where the precursor is chemically bonded onto one side of the node. The scientists wanted to start with these types of catalysts because they wanted to investigate MOFs as supports.
Bruce Gates’s group at the University of California Davis studied two distinct iridium precursors; one that contained two carbon monoxides and one that contained two ethylenes attached to the iridium. The iridium was deposited by first reacting the iridium precursors and the NU-1000 nodes, removing hydrogens from the node, which can be seen in the FTIR results.
To verify the structure, theorists calculated the FTIR bands and compared them to the experimental results. In addition to FTIR, extended X-ray absorption fine structure spectroscopy (EXAFS) was performed at the Stanford Synchrotron Radiation Lightsource to determine more structural data that could be compared with the calculations. EXAFS is generally used to understand the coordination of specific atoms in the structure of interest, informing scientists of the expected geometric structure. Scientists at the ICDC determined that the iridium diethylene complex was catalytically active for conversion of ethylene with hydrogen, whereas the iridium complex with carbon monoxide was inactive.
At the ICDC, researchers have also investigated the addition of aluminum and indium precursors to the MOF nodes. Experimental chemists determined that there are eight metal precursors deposited (either eight aluminum precursors or eight indium precursors) on the metal node of NU-1000; however, theorists determined how these aluminum and indium precursors sit on the node. Experimentally, the aluminum and indium were attached to the node, using atomic layer deposition.
In the theoretical study, step-by-step mechanisms were established for the binding; these results helped the experimentalists refine the X-ray powder diffraction data. When it came to analyzing the reaction intermediates, the infrared spectroscopy was instrumental in helping the theorists understand which structures to focus on for further study.
The results of these two ICDC projects have encouraged the members of ICDC to investigate new mechanisms, one of which can be seen in the video.
Looking to the future. With the CCEI and ICDC research, the plan is to provide the design guidelines necessary for industry to produce the catalytic material with MOF supports/catalysts and zeolite catalysts in large masses. By doing this with inexpensive metals, the plan is to have more affordable energy solutions with theory leading the charge.
While zeolites and MOFs are different nanoporous materials, they are both versatile. These frameworks can be altered by the inclusion of different catalytic materials. Such alterations allow for the possibility of one framework to be used for many applications depending on the changes made. Just like there are a variety of different types of coffee, there are many different types of catalysts. There are multiple catalytic applications for necessary materials like plastics and biofuels. New applications and better results are possible as long as scientists can come up with new ways to use these supports.
(Courtney et al.) This material is supported as part of the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences.
(Green et al.) Financial support was provided from the Catalysis Center for Energy Innovation, a DOE Energy Frontiers Research Center. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility.
(Kim et al.) This work was supported as part of the Inorganometallic Catalysis Design Center, an Energy Frontier Research Center funded by the DOE, Office of Science, Basic Energy Sciences. Work done at Argonne made use of the Advanced Photon Source, an Office of Science User Facility operated for the DOE/Office of Science by Argonne National Laboratory, and was supported by DOE.
(Yang et al.) This work was supported as part of the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center funded by DOE, Office of Science, Basic Energy Sciences.
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