Integrated Theoretical and Transient Kinetic Investigation of Catalytic Materials for the Selective Synthesis of Hydrogen and Aromatic Hydrocarbons



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Metal-exchanged zeolites are emerging as catalytic materials containing isolated metal centers that perform chemical transformation with high activity and selectivity to desired products. While there exist very successfully commercialized examples, for instance Cu-SSZ-13 for automotive emissions applications, much is unknown about metal sites in zeolites. Thus, statistical analyses of energetic quantities obtained from density functional theory (DFT) simulations of various zeolite models were performed for the activation of methane as probe hydrocarbon species. A comprehensive error analysis overwhelmingly shows that the identity and type of metal species in the zeolite determines the reactivity rather than the zeolite framework properties. Statistical distributions were used to identify and validate other metals that would theoretically function as catalysts for methane activation.

Insights gained from the investigation of a diverse set of metal-exchanged zeolites were applied in understanding the mechanisms by which Ag-ZSM-5 enhances the selective formation of aromatic hydrocarbons from ethylene. Silver sites in the zeolite were experimentally and computationally characterized as Ag cations. Transient pulsing experiments using the temporal analysis of products (TAP) reactor and exploration of reaction mechanisms using DFT reveal that silver sites catalyze dehydrogenation and hydrogen-transfer pathways and accelerate the accumulation of aromatic hydrocarbon forming precursors. The cause for the enhancement of these chemistries arises from the synergistic Brønsted–Lewis acid character of the zeolite.

The TAP technique was used to understand the finer aspects of the methylation of toluene to xylene, both important aromatic hydrocarbons. Pulse responses of reactants and products provide insights on competitive adsorption inside the zeolite, with methanol and methanol-based intermediates of finite lifetime identified as being crucial to the process of methylation. Typically in industry, hydrocarbon conversion processes using zeolites include co-feeds of hydrogen that are usually obtained from methane steam-reforming, a highly capital and energy intensive process. The search for hydrogen production avenues that do not rely on fossil resources has led to a surge of research devoted to electrocatalytic processes. This dissertation also explores the surface science of electrocatalytic hydrogen production over novel metal-phosphide surfaces. Density functional theory calculations reveal that metal-rich surface sites catalyze hydrogen evolution, as opposed to accepted notions that phosphorous-rich sites are the active sites.



Hydrocarbons, Aromatics, Transient kinetics, Density functional theory, Zeolites


Portions of this document appear in: Hsieh, M. F.; Zhou, Y.; Thirumalai, H.; Grabow, L. C.; Rimer, J. D. Silver-Promoted Dehydroaromatization of Ethylene over ZSM-5 Catalysts.Chem-CatChem2017,9, 1675–1682. And in: Thirumalai, H.; Rimer, J. D.; Grabow, L. C. Quantification and Statistical Analysis of Errors Related to the Approximate Description of Active Site Models in Metal-Exchanged Zeolites.ChemCatChem2019,11, 5055–5067. And in: Zhao, Z.; Schipper, D. E.; Leitner, A. P.; Thirumalai, H.; Chen, J.-H.; Xie, L.;Qin, F.; Alam, M. K.; Grabow, L. C.; Chen, S.; Wang, D.; Ren, Z.; Wang, Z.;Whitmire, K. H.; Bao, J. Bifunctional metal phosphide FeMnP films from single source metal organic chemical vapor deposition for efficient overall water splitting.Nano Energy2017,39, 444–453. And in: Schipper, D. E.; Zhao, Z.; Thirumalai, H.; Leitner, A. P.; Donaldson, S. L.; Kumar, A.; Qin, F.; Wang, Z.; Grabow, L. C.; Bao, J.; Whitmire, K. H. Effects of Catalyst Phase on the Hydrogen Evolution Reaction of Water Splitting:Preparation of Phase-Pure Films of FeP, Fe2P, and Fe3P and Their Relative Catalytic Activities. Chemistry of Materials 2018, 30, 3588–3598.