Investigations of aluminophosphate molecular sieves applying noble gas and methane NMR spectroscopy
Koskela, Tuomas (2004-11-08)
In this thesis, nuclear magnetic resonance (NMR) spectroscopy of noble gas atoms and methane molecule is applied to the structure studies of solid microporous materials (molecular sieves). This is an indirect method, where changes to the spectral parameters (i.e. isotropic and anisotropic nuclear shielding tensors) of the adsorbate induced by the investigated environment are measured at variable temperatures and adsorbate loadings. The data are analyzed concentrating on lineshape fitting methods and the correlations with the sieve framework parameters are examined. The studies in this thesis involve resonance signals mostly from 129Xe and 13C nuclei.
Molecular sieves, such as zeolites, are typically composed of oxygen, silicon, and aluminium. They are found in the nature, but in most cases they are manufactured synthetically. These materials have a wide variety of industrial applications, such as dehydration, odour and pollutant removal, ion exchange, water softening, etc. The size and shape of molecular-size intracrystallite cavities and channels vary considerably. This thesis focuses on aluminophosphate molecular sieves and their silicon-containing counterparts.
In molecular sieve systems, the effects of intra- and intercrystallite motions and exchange of adsorbates on their NMR spectra is discussed. Spectroscopic methods of magic angle spinning and 2D exchange spectroscopy are applied with good results. A technique employing pulsed field gradients to reveal shielding anisotropy component of methane from overlapping resonances from less restricted methane is used. The effects of cations located in molecular sieve frameworks are studied and discussed. From the fast exchange effects on spectral data the intercrystallite cavity sizes are estimated. Spectral differences between circular and elliptical cross-sections of framework channels are also detected. The roles of adsorbate-wall and mutual adsorbate interactions on spectral characteristics are identified and discussed.
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