Proton solid-state nuclear magnetic resonance studies of zeolite catalysts

Abstract

Zeolites are inorganic solid acids which arc important in many industrial heterogeneous catalysis processes involving chemical transformations of hydrocarbons. The Bronsted acidity of a particular zeolite determines its activity for various chemical transformations such as isomerization, cracking, and gasoline synthesis. The goal of the research presented in this Dissertation was to develop and successfully demonstrate variable-temperature 1H magic-angle spinning (MAS) nuclear magnetic resonance (NMR) methods for the study of zeolites, both in the absence and presence of catalytically important adsorbates. This approach allowed simultaneous spectroscopic observation of both the catalyst, including the important Bronsted acid site, and adsorbates, a unique advantage over previous NM R studies. Reliable sample preparation techniques were developed which allowed routine characterization by NMR methods over a wide temperature range while preserving sample integrity. In addition, other developments associated with this project including high-temperature NMR investigations of cracking chemistry, methods for sensitivity enhancement, and variable-temperature 1H combine rotation and multiple-pulse NMR spectroscopy are described. The structure and dynamics of hydrogen-bonded adsorption complexes between various adsorbates, such as acetylene, ethylene, benzene, and carbon monoxide, and the Bronsted acid site in zeolite H-ZSM5 were determined using variable-temperature 1H MAS NMR. The chemical shift of the Bronsted acid site was found to be extremely sensitive to the amount and type of molecule adsorbed. NMR experiments previously developed in our laboratory revealed a specific hydrogen-bonding interaction between the adsorbate and the strong acid site only. Quantitative treatment of the observed changes in the Bronsted chemical shift upon introduction of adsorbates resulted in the determination of equilibrium constants for the formation of the complex as well as hydrogen-bond distances...