(44c) The Search for Microporous, Strongly Basic Catalysts: Experiment and Theory in the Synthesis and Characterization of Nitrogen-Doped Zeolite Y

The last sixty years have shown remarkable growth in the use of zeolites as acid catalysts. Their use as base catalysts, however, has become a topic of significant interest only in the last decade or so, especially with the recent surge of interest in biomass energy. Particularly promising are silicate and aluminophosphate materials in which oxygen has been replaced by nitrogen, forming an oxynitride. Nitrogen substituted zeolites and AlPO₄ based materials have been shown to catalyze condensation reactions that require a base catalyst. Synthesis of such materials has proven challenging, however, usually requiring a post-synthetic treatment of the material with high-temperature ammonia. We show that such treatment can easily disrupt the zeolite structure on several length scales through a combination of ²⁹Si NMR spectroscopy, infrared and Raman spectroscopy, X-ray diffraction, and physical adsorption. Combined with an estimate of the extent of reaction (based on a fit of a "theoretical" NMR spectrum obtained from density functional theory calculations to the observed NMR spectrum), we are able to show the effects of reaction conditions on the extent of reaction, degree of crystallinity, and retention of microporosity. In general, we find that the synthesis conditions thus far employed in the literature are inadequate to produce a zeolite with reasonable levels of nitrogen substitution while maintaining microporosity and crystallinity. We also find that any one characterization technique is insufficient to determine whether a "good" nitrogen-doped zeolite catalyst has been produced: a combination of NMR, X-ray, and high-resolution adsorption are required.