Multinuclear solid-state NMR studies of Bronsted acid sites in zeolites HY, HZSM-5 and HMOR.

摘要

The research presented in this dissertation involves the use of variable temperature solid-state NMR techniques to investigate the local structure and dynamic properties of the Bronsted acid sites in a variety of zeolites, including HY, HZSM-5 and H-MOR.;Bronsted acid sites play a key role in controlling the catalytic performances in acidic catalysts. A determination of the structure of the acid site, in particular, the O-H bond length, is fundamental to the understanding of its acid strength. The O-H distances in zeolite HY and HZSM-5 extracted from 17O-1H CP rotational-echo double resonance (CP-REDOR) NMR data acquired at room temperature are noticeably longer than the results from ab-initio calculations due to the presence of some restricted motions at room temperature, such as zeolite framework vibrations and O-H librational motion. We present here our 17O- 1H CP-REDOR NMR results of zeolite HY and HZSM-5 at the lower temperature of 183 K, where some of these motions are frozen out. By comparing the line shapes of simulation results from the SIMPSON package with the experimental data, an O-H distance of about 0.97 ∼ 0.98 A was obtained for zeolite HY, which is consistent with the previous ab-initio calculation results. The results demonstrate that low temperature REDOR NMR spectroscopy can provide more accurate estimates of the O-H distance, which should prove useful in understanding zeolite structure and acidity.;In order to further illustrate the dynamics of protons in zeolite HZSM-5, low temperature 1H MAS NMR spectroscopy has been carried out on both samples that have been dried using procedures that are standard in the literature, and samples that have been more carefully dehydrated. A significant enhancement of proton mobility is seen for the "standard" dehydrated HZSM-5 sample, in comparison to that seen for the much drier sample. This is ascribed to a vehicle-hopping mechanism involving the residual water that is present in these zeolites. A gradual change of the framework structure is observed on cooling to approx. 213 K, as monitored via the change in 1H chemical shift values of the Bronsted acid resonances and by X-ray diffraction. A more sudden change in structure is seen by differential scanning calorimetry (DSC) and NMR, at approx. 220--230 K, which is associated with changes in both the mobility and the modes of binding of the residual water to the Bronsted acid sites and the zeolite framework.;17O/1H double resonance NMR spectroscopy was used to study the local structure of zeolite H-Mordenite. Different contact times were used in cross polarization magic angle spinning (CPMAS) NMR, CP-REDOR NMR and heteronuclear correlation (HETCOR) NMR spectroscopy to selectively investigate Bronsted acid sites with different O-H distances. Efforts were made to distinguish the locations of different Bronsted acid sites in H-Mordenite (H-MOR) and develop correlations between the NMR parameters and the local/long range structure and Bronsted acidity. 17 O multiple-quantum (MQ) MAS NMR was applied to monitor both the normal framework and Bronsted acid oxygen sites in zeolite HMOR. The results demonstrate that 17O/1H double resonance NMR provides a powerful and sensitive method to investigate the local structure of different Bronsted acid sites and to probe their changes upon adsorption.