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Solid-state NMR, X-ray diffraction and neutron diffraction studies on organic guest/zeolite ZSM-5 complexes Lee, Jang Seob

Abstract

Zeolites are microcrystalline aluminosilicate materials that have intricate channel systems, into which molecules can be adsorbed. Important as industrial materials, they are used in gas separations, sorption and catalysis where the property of 'molecular sieving' is exhibited toward guest species. Thus, understanding the interactions between guest species and the host zeolite frameworks is central to predicting the syntheses and applications of zeolites, and this requires detailed structural information about the guest/zeolite complexes. For most zeolites, structure determination by single crystal XRD is precluded due to the microcrystalline nature of the materials. As an exception, ZSM-5, which was used exclusively for this thesis, is available in large crystals (> 100 μm). In the quest for a universal method to probe the structures of various types of guest/zeolite complexes, we have developed an alternative approach using solid-state NMR, which is not limited by crystal size, unlike single crystal XRD. The main focus of this thesis is exploring the potential of the NMR method by using it to determine various guest/zeolite complexes followed by verification by diffraction techniques where possible. The NMR method uses the dipolar coupling interactions between nuclei on the organic guest molecules and the silicon nuclei in the zeolite framework, which contain information regarding relative proximities of the protons in the guest to the silicon atoms in the rigid framework. The complexes tested were: o-xylene/ZSM-5, p-dicyanobenzene/ZSM-5, p -dinitrobenzene/ZSM-5 and benzene/ p -xylene/ZSM-5 (the mixture of the two organics as guest molecules in the structure determination of the complex for the first time). Precise structures for all the complexes listed have been determined successfully by the NMR method. Other relevant NMR studies, including connectivity, dynamics and spin diffusion, are also presented to support a better understanding of the interactions between the organic guests and the zeolite frameworks. Verification of the first three complex structures determined by NMR has been done successfully by diffraction methods. For o-xylene/ZSM-5, attempts to apply single crystal XRD were unsuccessful due to the very low diffusivity of the organic. Instead, powder neutron diffraction was used.

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