Understanding Supported Copper Oxide Catalysts for Cyclohexane Oxidative Dehydrogenation

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Supported metal oxides are an important class of heterogeneous catalysts active for many selective oxidation reactions including alkane oxidative dehydrogenation. Attempts to develop fundamental structure-function relationships for supported metal oxide catalysts for alkane oxidative dehydrogenation have been challenging, with many conflicting reports in literature. This has been in part due to the use of propane oxidation which, though industrially relevant, poses several challenges for gaining fundamental insights of oxidation pathways. This dissertation seeks to develop a more fundamental understanding of how structural and electronic properties of supported copper oxide catalysts impact oxidation kinetics by oxidative dehydrogenation (ODH) and non-ODH pathways by utilizing cyclohexane as a model alkane for study. The first part of this thesis introduces supported copper oxide as an active alkane ODH catalyst and explores the sensitivity of cyclohexane reactivity to supported copper oxide catalyst design variables such as copper precursor, copper surface density, and the presence of alkali. Characterization by UV-visible and x-ray absorption spectroscopies shows that copper oxide structure is a strong function of copper surface density and a weaker function of copper precursor. The copper oxide structure has a modest impact on cyclohexane ODH activity, but it has no impact on the relative rates of aliphatic and allylic C—H bond activation. By contrast, changing the electronic structure of the active metal oxide phase by addition of alkali or using vanadium instead of copper has a large impact on cyclohexane ODH selectivity. Furthermore, this work identifies a previously unreported spectroscopic feature of Cu(I)/SiO2 catalysts at low copper loadings which is correlated with high catalytic activity. The remainder of this thesis focuses on developing fundamental relationships between copper oxide structure, electronic properties, and the resulting cyclohexane reactivity. Spectroscopic characterization of CuOx/SiO2 catalysts varying in average copper oxide domain size shows that intrinsic Cu(II)/Cu(I) redox activity increases as copper oxide domain size decreases which results in an increase in the intrinsic activity of copper oxide sites for cyclohexane oxidative dehydrogenation. Moreover, a distinct, highly active copper site is identified and fully characterized by a combination of in-situ UV-visible, x-ray absorption, and resonant Raman studies combined with modeling by density functional theory. All constraints imposed by these techniques identify this unique site as a mono(μ-oxo) Cu(II) dimer with copper situated in highly strained siloxane defect sites present in dehydroxylated silica which has implications for reactions beyond alkane oxidative dehydrogenation. Next, the underlying cause of support effects in alkane oxidative dehydrogenation is explored. We provide evidence that charge transfer between the support and copper oxide impacts the rates of C—H bond abstraction and COx formation pathways in the oxidative dehydrogenation of cyclohexane over supported copper oxide catalysts. We experimentally relate the support surface Lewis acid strength, determined by alizarin dye adsorption studies, to changes in copper oxide valence electron density for nine supported copper oxide catalysts. Model cyclohexane ODH reaction studies then systematically relate these changes in copper electronic structure with changes in the relative rates of aliphatic C—H bond abstraction, allylic C—H bond abstraction, and combustion by routes in which C—H bond activation is not kinetically relevant. The structure-function relationships determined for supported copper oxide catalysts have important implications for rational design of supported metal oxide catalysts for alkane oxidative dehydrogenation.

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  • 11/24/2019
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