Investigation of the three-dimensional structures of zeolite molecular sieves by high resolution solid state NMR

This thesis work describes the use of solid state NMR experiments to obtain important structural information about zeolite complexes with organic guest molecules. A highly siliceous zeolite ZSM-5 loaded with p-xylene was chosen for the detailed study. A methodology to investigate the zeolite complexes of highly siliceous ZSM-5 with pxylene molecules by locating the organic molecules inside the framework using only NMR techniques was developed. In this way a complete three-dimensional characterisation of these systems was obtained. The application of the double and triple resonance dephasing experiments of Cross- Polarization (CP), Rotational Echo Double Resonance (REDOR), and Transferred Echo Double Resonance (TEDOR), to attain internuclear distances from the dipolar interaction between the guest molecules nuclei and the silicon nuclei in the zeolite framework was investigated. Specifically deuterated and ¹³C labelled p-xylenes were used in these studies. Triple resonance dephasing experiments of REDOR and TEDOR were performed on the complex of the high loaded form of zeolite ZSM-5 with p-xylene [¹³C,CH₃] to obtain structural information through the dipolar interaction between the labelled ¹³C methyl group of the guest molecules and the silicon atoms. The unambiguous assignment of the ¹³C NMR spectra of the guest p-xylene molecules was done using 2D-TEDOR experiments. The acquisition of reliable quantitative data from these techniques was complicated due to the dynamics of the absorbed molecules within the zeolite framework. The motional characteristic of the molecules was investigated by ²H NMR and chemical-exchange spectroscopy. The application of high resolution ¹³C CP/MAS 2D NMR exchange spectroscopy to investigate the molecular motions of guest molecules inside zeolite catalysts and to obtain critical kinetic information on these processes was demonstrated for the first time. By application of the CP experiment and the protocol developed in this thesis a threedimensional structure of the low-loaded complex of p-xylene in ZSM-5 was obtained. This structure was derived without any previous x-ray diffraction information. The NMR results indicated that the p-xylene molecules were located in the intersection of the sinusoidal and straight channels of the zeolite.Science, Faculty ofChemistry, Department ofGraduat