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Pulsed field gradient nuclear magnetic resonance and applications in yttrium type zeolites
Abstract
Molecular self-diffusion measurements by Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR) spectroscopy can be applied to numerous fields. PFG NMR spectroscopy usually requires no special labeling for measuring hydrocarbon self-diffusion in a variety of samples. This is a significant advantage over using radioactive isotopes or photolabeled molecules since no special sample preparation or handling is required. A single set of experiments can yield diffusion coefficients and often can be performed in a few hours. The range of diffusion coefficients (10$\sp{-4}$ cm$\sp2$/s-10$\sp{-10}$ cm$\sp2$/s) which can be determined by PFG NMR covers most ranges of molecular diffusion. This work describes the design of a PFG NMR spectrometer for measuring hydrocarbon self-diffusion in zeolites. The principles of PFG NMR spectroscopy are illustrated. A pulsed field gradient 60 MHz NMR spectrometer was constructed. Diffusion data were acquired by PFG NMR for standard samples of water, ammonia and glycerol and are in good agreement with those reported in the literature. Following verification of spectrometer performance, the self-diffusion coefficients of isobutane in cation exchanged Y type zeolites were determined. The results show that the mobility of molecules in zeolites depends on the nature of the cations. For small crystallite zeolites, intercrystalline and intracrystalline diffusion has been observed. The effective diffusion coefficients strongly depend on the concentration of adsorbate as well as the packing method. Large discrepancies between diffusion coefficients in zeolites measured by PFG NMR and by adsorption rate experiments have been reported. Surface area, crystallite size, percentage of water in the zeolite, percentage of cation exchanged into the zeolite and other physical chemical properties effect hydrocarbon diffusivity. Detailed methods for determining these properties are discussed. As an extended study of zeolite catalysts, the adsorption of ethylene on cocation-exchanged Ru-Y zeolites has also been studied by $\sp{13}$C solid-state NMR. Coadsorption of hydrogen and ethylene leads to an increase in the rate of hydrogenation with the initial rate being first order.
Subject Area
Chemistry|Electromagnetism|Chemistry
Recommended Citation
Wu, Shaoxiong, "Pulsed field gradient nuclear magnetic resonance and applications in yttrium type zeolites" (1991). ETD collection for University of Nebraska-Lincoln. AAI9219394.
https://digitalcommons.unl.edu/dissertations/AAI9219394