Quantitative NMR spin-counting of half-integer quadrupoles in solid materials

An analytical solid-state NMR protocol for absolute quantitation of half-integer-spin quadrupolar nuclei was developed. In Chapter 2, quantitative NMR measurements were compared to bull: analysis methods including 29Si NMR, energy dispersive microanalysis, and wavelength dispersive spectroscopy. By controlling instrumental parameters, calibrating pulse lengths, compensating for reduced excitation and signal reception efficiency for lossy samples, and correcting for differences in the fractions of excited spins across samples, absolute quantitation of quadrupolar nuclei was achieved.; NMR allows for absolute quantitation of nuclei in chemically-distinct sites, an advantage over the bulk methods listed above. Our quantitative quadrupolar NMR protocol has been applied to industrially important metallo-silicate catalytic materials called zeolites. In Chapter 3, the effect of sample treatment on the acid sites in zeolite B-IFR (SSZ-24, ITQ-4) was quantitatively investigated. From zeolite synthesis to activation and washing with deionized water, 61% of the boron was lost from the framework. Comparing quantitative 11 B results with 29Si NMR spectra, formation of excess SiOH was found to interfere with the 29Si NMR analysis, making this an unreliable method for measuring the Si/B ratio. Further studies revealed a completely reversible change in 19% of the framework boron from tetrahedral to trigonal coordination upon saturation of an acid-form sample of SSZ-42 with liquid water in a closed system. The flexibility of boron in the framework is likely due to greater stability of the atom in a trigonal vs. tetrahedral coordination.; In Chapter 4, Lewis and Bronsted acid sites in zeolite Y (faujasite, FAU) were characterized by direct quantitative analysis (mmol/g) and by measurement of the quadrupolar frequency of each site. The catalytic performance and stability of this important industrial zeolite depends on the ratio of Lewis/Bronsted acid sites. The quantitatively reversible transformation of aluminum in the framework from octahedral to tetrahedral coordination upon ammonium exchange and heating was demonstrated. Thus the octahedral aluminum is still connected to the framework, and its assignment as extra-framework aluminum in prior literature is erroneous. Measurements of the quadrupolar frequencies support the argument that the broad peak previously assigned as penta-coordinate aluminum is instead aluminum in a distorted tetrahedral coordination.