In-situ Micro-Spectroscopy on Coke Formation Processes in Zeolites

Abstract

Zeolite catalysts are used in a large variety of (petro-)chemical conversions. The acid sites in these materials are responsible for the chemical transformation, while their well-defined crystallographic architecture offers unique molecular size and shape selectivity. The finite availability of crude oil demands for new technologies that utilize alternative feedstocks (like biomass, natural gas or coal) for the production of chemicals. In these novel conversions zeolite catalyst can play an important role. In hydrocarbon processing, the formation of carbonaceous deposits is usually inevitable. These undesired by-products are the result of secondary reactions and influence the performance of catalyst materials with possible consequences for their industrial application. In order to develop new and improved catalytic crystals, a thorough understanding of their working principles is essential. The use of spectroscopic characterization techniques has thereby become an important tool. The research described in this PhD thesis examines coke formation processes in zeolite catalysts with in-situ micro-spectroscopic techniques. These tools are applied on large micron-sized zeolite crystals that reveal spatial and temporal heterogeneities in the coke growth process at the individual catalyst particle level. UV-Vis micro-spectroscopy identifies the chemical nature of the formed coke species, while confocal fluorescence microscopy allows studying the migration of these compounds within an individual zeolite crystal. The first part of the thesis deals with the Methanol-to-Olefin (MTO) conversion. A comparative study between H-SAPO-34 and H-ZSM-5 zeolite crystals illustrates the influence of the zeolite framework topology on the coke formation process. Low reaction temperatures generate mild coke formation, whereas more severe reaction conditions allow the growth of larger carbonaceous deposits. This effect is further enhanced with increasing Brønsted acid site density. Bulk analytical methods show that the model H-ZSM-5 zeolite crystals remain active after a number of regeneration treatments and despite their large crystal dimensions display conversion characteristics that are close to the conversion activities of H-ZSM-5 catalysts containing smaller crystal size. These zeolites are subsequently used to investigate the coke growth process during the aromatization of light naphtha (LNA) derivatives. Olefins and paraffins are thereby compared. Increasing reactant chain length generates more severe coke deposition, while methyl branching of the hydrocarbons results in coke deposits of more confined size. A catalytically inactive silicalite-1 shell, covering the H-ZSM-5 zeolite catalyst, prevents the formation of large carbonaceous species at the external surface of the crystal. Finally, fluorescence microscopy illustrates how the physicochemical interplay between reactant and zeolite affects the location of coke formation during the etherification of biomass-based alcohols with long linear alkenes in H-Beta zeolite crystals.