Type of Document Dissertation Author Ma, Xisai Author's Email Address email@example.com URN etd-04052005-115546 Title Transition Metal Incorporated Silicalites for Heterogeneous Oxidative Catalysis Degree Doctor of Philosophy Department Chemistry and Biochemistry, Department of Advisory Committee
Advisor Name Title Albert E. Stiegman Committee Chair Ashby Plant Committee Member John G. Dorsey Committee Member Kennneth A. Goldsby Committee Member Keywords
- Phenol Hydroxylation
- Structure Characterization
- Manganese Silicalite-2
- Chromium Silicalite-2
Date of Defense 2005-01-31 Availability unrestricted AbstractZeolite materials, in which transition metal ions have been substituted into the silicate or aluminosilicate framework, have been of considerable recent interest due to their potential as selective heterogeneous oxidative catalysts. Taking Ti-silicalite-1 as a paradigm, other metal ions possessing similar ionic radii have been substituted into the silicalite framework and explored for catalytic activity. Among transition metals, manganese and chromium are particularly attractive candidates for substitution into silicalite because of their established activity as both homogeneous and heterogeneous catalysts in other ligand environments.
We have successfully synthesized highly crystalline Cr-Silicalite with ZSM-11 structure. The Chromium incorporation into the ZSM-11 framework is proved by a systematic increase in the unit cell volume as a function of Cr loadings. The oxidation states of Cr in pre- and post-calcined silicalite are determined to be +3 and +6, respectively, by using Magnetic Susceptibility, X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopic (XPS), and Electron Paramagnetic Resonance (EPR), UV-visible, and Infrared spectroscopic methods. The band shifts in an 18O-labeled IR experiment enabled us to confirm the existence of mono-oxo groups on each Cr sites in the post-calcined silicalite. The Cr (VI)-silicalite-2 shows little catalytic activity. However, after reduction under CO, it turns into an active catalyst with yields and selectivity for phenol hydroxylation comparable to TS-1. The Cr sites in the silicalite after reduction under CO were determined to be +4. They have proven to be remarkably robust and are not readily reoxidized to Cr (VI) even under oxidative conditions.
An inherent difficulty with transition metal substitution into the silicalite lattice, and one that is acute with manganese, is that the basic conditions typically utilized to produce high quality silicalite causes precipitation of the metal as the hydroxide. While different strategies have been used to overcome this problem none have generated high quality Mn substituted silicalite with levels of metal framework substitution comparable to those attained with Ti. We also report here the use of the manganese cluster complex, Mn12O12 (O2CCH3)16, to deliver manganese ions quantitatively into the silicalite-2 lattice under basic conditions. The materials that result are monophasic, of high crystallographic quality with substitution of manganese in amounts up to 2.5 mole percent. The X-ray diffraction pattern proves the formation of highly crystalline silicalite with ZSM-11 structure whereas the regular increase in unit cell with the degree of substitution further proves the incorporation of manganese into the lattice. The Oxidation states of manganese are determined for pre- and post- calcined sample by using the Magnetic Susceptibility, X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopic (XPS), and Electron Paramagnetic Resonance (EPR) methods.
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