archives@tulane.edu / Zeolites are microporous crystalline materials that are commercially used in ion exchange, adsorption, and catalysis applications. ZSM-5, the aluminosilicate with the MFI framework topology, is used industrially in fuel upgrading and xylene production, among other applications. Over the last three decades, zeolite research has produced several d- block metal-exchanged MFI zeolites relevant in problems including NOx abatement and partial oxidation of alkanes. Increased attention has been given to Cu, Ni, Fe, and Zn ions in this system, which have demonstrated the ability to activate methane and unlock the pathway for conversion to methanol.
The direct partial oxidation of methane using molecular oxygen under mild conditions represents a holy grail reaction. Methane availability is currently experiencing a global excess, so high economic incentives are in place for any conversion scheme. Despite downhill energetics, several metal-exchanged zeolites have demonstrated the ability to oxidize methane without taking the products all the way to carbon dioxide. While conversion remains low, continued research has begun to elucidate the mechanisms by which the process is carried out and strategies for further improvements. Given the potential of MFI type materials, a detailed understanding of the reaction pathway and active sites is highly desirable.
The first objective of this dissertation is to describe the chemical composition and state of a suite of exchanged MFI zeolites in a manner that informs the structure of active metal ion sites. This work utilizes the titration of certain metal ions with others to determine relationships between the two, with emphasis placed on stoichiometric exchange. It is concluded that Cu ions are not wholly preferential in the pore structure but in fact establish
proportional uptake and retention ratios based in part on the exchange ion and solution conditions. The data also support the theoretical maximum of one stabilized Cu ion per two Z-sites without excessive exchange parameters. Conversely, Zn appears to be capped at one ion per Z-site, whereas Ni experiences a much lower threshold value. However, all ions reported in this dissertation were found to exchange with each other to some extent.
The second objective of this dissertation is to detail the effects of these metal exchanges on the activity of MFI zeolites in the partial oxidation of methane. Catalytic testing indicates a broad window of tunable metal contents. Preliminary characterization suggests the existence of a –Cu–O–Cu– active site, where the bridging O acts as the principal methane binding site. Methanol STY* for the Cu-MFI zeolites approached 15 mmol molCu–1 hr–1 at loadings of Cu:Al ≥ 0.2. Titration of Cu-MFI using Na tended to extinguish the little remaining Brønsted acidity, which shut off the reaction entirely. Titration with the three d-block metals yielded mixed results; only 50% exchanged Zn/Cu- MFI zeolites performed better than the Cu standard in their own series. Zeolites with nearly equivalent Cu and Zn outperformed the highest reported literature values by approximately 40% on a per mole Cu basis: 9.41±0.33 vs. 5.2±0.05 mmol CH3OH/molCu/hr . / 1 / Zachary T. Gentle
| Identifer | oai:union.ndltd.org:TULANE/oai:http://digitallibrary.tulane.edu/:tulane_120515 |
| Date | January 2020 |
| Contributors | Gentle, Zachary (author), Shantz, Daniel (Thesis advisor), School of Science & Engineering Chemical and Biomolecular Engineering (Degree granting institution) |
| Publisher | Tulane University |
| Source Sets | Tulane University |
| Language | English |
| Detected Language | English |
| Type | Text |
| Format | electronic, pages: 112 |
| Rights | No embargo, Copyright is in accordance with U.S. Copyright law. |
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