(5al) Quantifying Physical and Chemical Defects in Zeolites and Zeolite Membranes

Zeolites are some of the most intriguing materials ever discovered. Their crystal structures contain pores the size of small molecules, giving them a “molecular sieving” property and making them ideal materials for use as adsorbents. Zeolites with over 190 different framework connectivities have been discovered, and applications involving zeolites exist in such diverse fields as landscaping, water treatment, mixture separations, catalysis, and even nuclear waste remediation. Particular interest has surfaced in recent years to develop zeolites and similar materials that do two things: (1) Serve as separation membranes with high throughput and high selectivity, and (2) Catalyze reactions that require strong bases in a selective fashion. The microporous nature of zeolites is what provides the selectivity: some molecules are large enough to fit into the zeolite pores, while others are not. In membranes, high selectivity via such molecular sieving requires that all paths through the membrane pass through a zeolite pore. Predicting the properties of zeolite membranes therefore requires knowledge of the presence and influence of physical defects in the membranes, which facilitate transport via non-zeolitic porosity. In the case of zeolites as strongly basic catalysts, it is chemical defects that are of import: strong base sites do not exist in zeolites, and must be created. Such sites can be created by a post-synthetic treatment which substitutes framework oxygen for nitrogen, but these treatments can also create physical defects and even destroy the microporous structure of the zeolite. This work addresses various challenges associated with the characterization of zeolite membranes and nitrogen-substituted zeolites, using techniques such as X-ray diffraction, NMR spectroscopy, vibrational spectroscopy, and high-resolution physical adsorption to quantify defects and structural changes in zeolites and zeolite membranes. Experimental work is combined with theoretical methods to yield unique insight into these intriguing materials.