Searching for Microporous, Strongly Basic Catalysts: Experimental and Calculated 29Si NMR Spectra of Heavily Nitrogen-Doped Y Zeolites
Fulya Dogan†, Karl D. Hammond‡, Geoffrey A. Tompsett‡, Hua Huo†, W. Curtis Conner, Jr.‡, Scott M. Auerbach‡§ and Clare P. Grey*†
Nitrogen substituted zeolites with high crystallinity and microporosity are obtained by nitrogen substitution for oxygen in zeolite Y. The substitution reaction is performed under ammonia flow by varying the temperature and reaction time. We examine the effect of aluminum content and charge-compensating cation (H+/Na+/NH4+) on the degree of nitrogen substitution and on the preference for substitution of Si−O−Al vs Si−O−Si linkages in the FAU zeolite structure. Silicon-29 magic angle spinning (MAS) nuclear magnetic resonance (NMR) and 1H/29Si cross-polarization MAS NMR spectroscopy have been used to probe the different local environments of the nitrogen-substituted zeolites. Experimental data are compared to simulated NMR spectra obtained by constructing a compendium (>100) of zeolite clusters with and without nitrogen, and by performing quantum calculations of chemical shifts for the NMR-active nuclei in each cluster. The simulated NMR spectra, which assume peak intensities predicted by statistical analysis, agree remarkably well with the experimental data. The results show that high levels of nitrogen substitution can be achieved while maintaining porosity, particularly for NaY and low-aluminum HY materials, without significant loss in crystallinity. Experiments performed at lower temperatures (750−800 °C) show a preference for substitution at Si−OH−Al sites. No preference is seen for reactions performed at higher temperatures and longer reaction times (e.g., 850 °C and 48 h).