Zeolite-Catalyzed Surface Chemistry in Alkane Isomerization and Methanol-to-Hydrocarbons Catalysis

Abstract

Zeolite catalysts are eminently vital to production of liquid transportation fuels and commodity chemicals. In each particular application, the surface reactions catalyzed by the zeolite determine its productivity. It is a fundamental understanding of these surface reactions that are the main focus of this thesis.

In the case of butane isomerization, the zeolite H-mordenite has been at the center of much controversy. Heavily debated are the relative contributions of intramolecular and intermolecular skeletal isomerization reaction pathways. In this thesis, it is shown that butenes facilitate both pathways, but that intermolecular pathways have a greater dependence on the butene partial pressure. Thus, when the butene partial pressure is kept low an intramolecular skeletal isomerization pathway dominates. When the butene partial pressure is high, either through in situ formation of butenes or through addition of butenes to the feed, an intermolecular skeletal isomerization pathway dominates.

Solid acids, in general, are plagued by a tendency to form carbonaceous deposits during alkane conversion. Platinum and H2 are effective at preventing formation of the deposits, but the deposits themselves are usually not well characterized. In this thesis, diffuse reflectance UV-vis-NIR (Ultraviolet-visible-near infrared) spectroscopy and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) performed in situ are used to show that alkyl-substituted cyclopentenyl cations are formed during conversion of n-butane or n-pentane at industrially relevant reaction temperatures (453-563 K). The corresponding olefins are shown to be precursors of the carbonaceous deposits. At higher reaction temperatures (563-653 K), methyl-substituted acenes are

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formed. An effective strategy combining in situ spectroscopy and extractions of spent catalysts to identify the deposits is demonstrated.

Finally, diffuse reflectance UV-vis-NIR spectroscopy performed in situ is used to characterize the "hydrocarbon pool" on working methanol-to-hydrocarbons (MTH) catalysts. Alkyl-substituted cyclopentenyl cations are again detected, but this time are found to be important reaction intermediates. While the hydrocarbon pool on zeolites such as H-ZSM-5 and H-beta is generally thought of as being aromatic in nature, our analysis indicates that cyclopentenyl species also play a critical role in the reaction chemistry.