Catalytic applications of hierarchically nanostructured zeolites

Abstract

The work presented in this thesis addressed the effect of hierarchically structured zeolites on the catalytic performances, i.e., activity, product selectivity and lifetime, in correlation with the mesoporosity in zeolites. The effect of mesoporosity on catalyst longevity of methanol-to-hydrocarbon (MTH) reactions have been investigated using a number of mesoporous MFI zeolites, which were obtained via the post-synthetic desilication, dry-gel conversion in nanocarbon templates, and hydrothermal synthesis with the addition of organosilane surfactants. The MTH catalytic lifetime could be increased by more than three times due to the generation of mesopores. The cause for catalyst longevity was investigated by probing the location of coke that formed during the reaction using argon adsorption measurements. The result showed that the coke formed mainly on mesopore walls in the case of mesoporous zeolite. On the other hand, the coke was more heavily deposited inside micropores in the case of solely microporous zeolites. The short diffusion path lengths and hence facile diffusion of coke precursors were most likely the cause of the improvement. In addition, the effect of mesoporosity on enhanced isomer production in hydroisomerization of n-heptanes has been studied with a series of Pt-supported hierarchical MFI zeolites with crystallite sizes ranging from micron-sized crystals to single-unit-cell nanosheets. The catalytic results showed that the selectivity to isomers versus cracked products highly depended on the mesoporosity (or crystal thickness) in MFI zeolites, while all catalysts exhibited similar activity and long-term stability. The maximum isomer yield was gradually increased to the level of the catalytic activity of Pt loaded large-pore zeolite $\beta$, with increasing the mesoporosity. Such improved isomer production could be attributed to the facile outward diffusion of isomer products due to thin zeolitic frameworks and large external surface areas....