UCLA

This work focuses on exploring the site identity of ion exchanged Zn sites in BEA zeolites and their role in the conversion of biomass-derived oxygenates. Careful preparation of ion exchanged Zn in BEA (Zn-BEA) allowed for the generation of different catalytic sites within the pores of the zeolite. Depending on the preparation method, Zn species have been suggested to exist as extra-pore large ZnO clusters, isolated Zn2+ ions at the cation-exchange site, Zn(OH)+ ions exchanged to one aluminum sites, binuclear [Zn-O-Zn]2+ clusters formed from the coupling of neighboring Zn(OH)+ species, and Zn2+ exchanged to two aluminum sites. In order to probe the oxidation state and local structure of the Zn, in situ Zn K-edge x-ray absorption spectroscopy was conducted at SLAC National Accelerator Laboratory. Spectra were taken of Zn-BEA before and after the temperature treatment process. The increase in Zn absorption edge energies in these catalysts compared to Zn standards suggests increased oxidation of Zn sites. The features at the absorption edge change during the temperature treatment, splitting from a single peak into two peaks. The lasting changes at the white line suggests the coordination sphere of Zn sites changes significantly during the activation process. Analysis of the EXAFS region was conducted to examine the local structure of Zn in the catalysts. Fitting was carried out simultaneously through the samples, effectively constraining fits with shared parameters to allow for more reliable models. There was a lack of Zn-Zn and Zn-Si contributions in the first and second coordination shell, which indicates that Zn atoms were exchanged into isolated exchange positions without the formation of a significant amount of Zn clusters. In situ thermal treatment of Zn-BEA caused the number of oxygen neighbors to decrease in the first coordination shell. This supports the theory that as-prepared samples of Zn-BEA have water coordinated to Zn sites within the pores of the zeolite and the temperature treatment process dehydrates the catalyst. There was a strong correlation between oxygen bond distance and Zn concentration, suggesting that Zn has an increasingly strong interaction with framework oxygen as the degree of ion exchange decreases. Changes in bond distance as more Zn is introduced to the pores can be explained shifts in the preferential ion exchange positions for Zn. As Zn replaces Br�nsted acid sites during the ion exchange process, the lowest energy exchange locations may shift between the four-, five-, and six-member rings. Br�nsted and Lewis acid site density was measured by in situ titration during acid catalyzed isopropanol dehydration using probe molecules of pyridine and 2,6-di-tert-butylpyridine. By measuring the acid site densities, Zn-BEA has been shown to lose Br�nsted acid sites during the temperature treatment process. Changes in the calculated Lewis acid site concentrations can suggest the presence of shifts in preferred site structures as Zn concentration increases. The chemical compositions of the catalysts were determined by inductively coupled plasma – optical emission spectrometry. This catalyst characterization was combined with the activity of Zn-BEA for methanol dehydration to determine the reactivity of different Zn sites. The conversion of butyraldehyde over a range of Zn loadings on Zn-BEA was also conducted to probe how the local structure of Zn sites influence the rates and selectivity of the various pathways in reactions of butyraldehyde.