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Location of guest species in zeolites by solid-state NMR

University of British Columbia

Zeolites and related microporous materials are exploited in many commercial applications because of the size and shape selectivity conferred by the molecular dimensions of their channel and cage systems. However, at present there is little direct information regarding the exact location of guest species within these materials; it is difficult to determine the structures of the host-guest complexes by X-ray diffraction techniques due to the limited availability of suitable crystals, and theoretical calculations are not always in agreement with one another. This thesis describes the application and evaluation of cross polarization (CP), rotational echo double resonance (REDOR), and transferred echo double resonance (TEDOR) solid-state N M R experiments for determining the complete three-dimensional structures of zeolite frameworks containing guests such as ions or sorbed organic molecules. In principle, these techniques permit accurate internuclear distances between spin-1/2 nuclei to be determined from measurements of the heteronuclear dipolar coupling. The viability and reliability of ¹⁹F-²⁹Si CP, REDOR and TEDOR N M R experiments was demonstrated by locating the fluoride ions within the octadecasil framework. The experimental methods were optimized and appropriate mathematical data analysis strategies developed. A comprehensive description of the mathematical methods is included. Accurate Si-F distances were determined which were in excellent agreement with those known from the single crystal structure. A series of calculations provided insight into the likelihood of being able to obtain reliable distances from REDOR and TEDOR data in cases where information regarding the number of spins, and the geometrical arrangement of these, is not known a priori. Preliminary investigations to locatep-difluorobenzene molecules within the channels of purely siliceous ZSM-5 were undertaken using these techniques. Apparatus which permitted the extended low temperature operation was developed. Powder X-ray diffraction experiments confirmed the sorbate- and temperature-induced phase changes observed for this system. Using a specially designed piece of equipment, a single crystal of ZSM-5 was loaded with a precisely controlled amount of /^-xylene and the structure successfully refined from X-ray diffraction data collected at 180 K. This represents the most accurate determination of the ZSM-5 framework to date.

Thesis/Dissertation

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2009-06-22

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http://hdl.handle.net/2429/9527

Chemistry

University of British Columbia

UBCV

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