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Location, disorder, and dynamics of guest species in zeolite frameworks studied by solid state NMR and X-ray diffraction Brouwer, Darren Henry

Abstract

Zeolites are microcrystalline framework materials with well-defined cavities and channels of molecular dimensions which enable them to act as 'molecular sieves' with size and shape selectivity towards which guest species may occupy positions within or diffuse through the pore and channel systems. Understanding the interactions between guest species and host zeolite frameworks, which are central to the synthesis and applications of zeolites, with a view to reliably modeling these systems, requires detailed structural information for these zeolite host guest complexes. For most zeolites, the application of x-ray diffraction to the structure determination of these complexes is precluded due to the microcrystalline nature of almost all zeolites and the weak scattering of the guest species. Solid state NMR, which is not limited by crystal size, offers an alternative, or at least complementary, method for structure determination of zeolite host/guest complexes. This thesis explores the potential of solid state NMR to provide information about the structure, dynamics, and disorder in zeolite guest/host complexes. A general strategy for locating guest species in zeolites was developed and implemented. This strategy consists of three main steps: (1) assign resonances (e.g. in the ²⁹Si spectrum ) to specific sites of the zeolite framework by performing two-dimensional correlation experiments, (2) experimentally probe the strengths of dipolar couplings (which are related to internuclear distances) between nuclei of the guest species (e.g. ¹H or ¹⁹F) and the nuclei of the zeolite framework (e.g. ²⁹Si), and (3) use this distance information to determine the location of the guest species with respect to the zeolite framework. The location of the fluoride ion in an as-made zeolite synthesized from fluoride-containing medium was determined by first assigning the peaks in the ²⁹Si spectrum from a two-dimensional ²⁹Si INADEQUATE spectrum, after which ¹H/¹⁹F/²⁹Si triple-resonance dipolar recoupling experiments were performed to measure F-Si distances to various Si sites of the zeolite framework, from which the location of the fluoride ion was deduced, along with the nature of the fluoride ion dynamics. The fluoride ions were found to be directly bonded to the zeolite framework, giving five-coordinate silicon sites whose local structure geometry was determined (due to the presence of structural disorder) only by a combination of solid state NMR distance measurements and single crystal XRD. The locations of several small organic sorbate molecules in highly siliceous ZSM-5 zeolite were determined by first assigning the peaks in the ²⁹Si spectrum, followed by relating the experimentally measured rates of ²⁹Si{¹H} cross-polarization to the calculated ¹H/²⁹Si dipolar coupling second moments. This solid state NMR structure determination protocol was shown to be reliable and robust over a range of sorbate loadings and temperatures and relatively insensitive to the exact nature of the zeolite framework used. The guest molecule locations determined by this solid state NMR method are in excellent agreement with the few single crystal XRD structures available.

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