Catalyst Design for Methane to Methanol Transformation via C-H Bond Activation using Transition Metal Oxide Dications
Type of DegreePhD Dissertation
Chemistry and Biochemistry
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The push towards greener energy solutions has been a focus for society in the last several decades, due to the need for cleaner industrial processes. One elusive process that draws the focus of several groups is methane to methanol transformation, which despite the continued search for a proper method providing both selectivity and efficiency, no ideal solution has been found. Here transition metal oxide dications are highlighted as potential catalytic candidates for methane to methanol transformation, through the use of high-level electronic structure calculations. Transition metal oxides are popular catalytic candidates due to their natural abundance and industrial applications. With the removal of two electrons for the formation of the dication the character of the metal oxide can be manipulated to favor CH bond selectivity or reaction efficiency. This dissertation illustrates the importance of these complexes and how to best apply them for the transformation of methane to methanol. Investigation began with a neutral transition metal oxide, niobium oxide, which indicated the importance of spin on the efficiency of the reaction mechanism, showing the higher spin states follow an efficient but non-selective pathway, and the lower spins following a selective yet less efficient and more energy intensive path. To understand the metal oxide character of the first-row transition metal oxide dications an extensive electronic structure analysis was performed on these complexes to characterize their behavior and was found that the early transition metals possessed oxo ground states and oxyl excited states, middle transition metals had oxyl ground states and oxo excited states, and the late transition metals having only oxyl states. Using the spectrochemical series allows for the manipulation of oxo or oxyl character depending on the ligand introduced to the system. Due to the oxo metal oxides’ preference for selectivity of the CH bond in methane, a strong field ligand was placed on the metal oxides to understand how the oxo state could be stabilized over the oxyl, even in the late transition metal case of cobalt oxide where a small oxo component at a small distance was stabilized over some oxyl components. Metal oxide dication systems with a fully saturated coordination sphere (five ammonia ligands) were explored to understand the effects of multiple ligands on the electronic structure and potential further stabilization of certain metal oxide character. Reacting a fully coordinated metal oxide dication with methane further emphasized the importance of excited states in these reactions in that they can influence ground state behavior and affect the outcome of the reaction. With this in mind the ground and excited states of the five-ammonia ligated metal oxide dications were analyzed at multiple different DFT functionals and multireference methods to determine a method to best systematically study these systems moving forward.