Mechanistic Insights with Homogeneous Pd and Pt Complexes for the Oxidative Coupling of Methane.

As a result of the increasing production of natural gas in the U.S., there is a demand to utilize its main component, methane, as a C1 source for value-added products, such as gasoline or diesel fuels. The industrial conversion of methane to liquid fuels uses multi-step processes with heterogeneous catalysts, which require high energy input and result in poor selectivity for the desired products. An attractive alternative to this method employs homogeneous catalysts to perform the oxidative coupling of methane. This approach could directly access higher alkanes at lower temperatures to achieve a more mild and selective process. Additionally, mechanistic insight gained from well-established solution-phase techniques could help improve the yield and selectivity in such homogeneous reactions. The proposed mechanism to form the initial product ethane from methane involves: (i) C‒H activation of methane to generate a M-CH3 intermediate, (ii) oxidatively-induced disproportionation between M‒CH3 intermediates to form a M‒(CH3)2 species, and (iii) reductive elimination from M‒(CH3)2 to liberate ethane. Chapter 2 describes the stoichiometric oxidation of homogeneous mono-methyl PdII complexes to selectively generate ethane. Experimental and computational studies support the formation of a dimethyl PdIV intermediate from methyl transfer. These studies demonstrate steps ii and iii of our proposed catalytic cycle. In Chapter 3, step i of the cycle, C‒H activation, is examined using a model system. PdII species and AgOPiv are employed in the C-H activation of pentafluorobenzene, through the formation of a Ag-C6F5 intermediate. Experimental evidence supports a Ag-mediated activation of pentafluorobenzene to form Ag-C6F5, followed by transmetalation to Pd, generating the PdII product. Chapter 4 explores the conversion of methane to ethane based on mechanistic information gained in the aforementioned stoichiometric studies of steps i, ii, and iii. High throughput experimentation demonstrates the formation of ethane from several reactions containing Pd or Pt catalysts, which are further pursued upon the development of a method to quantify the gaseous products. These studies reveal that a significant decomposition pathway to form ethane arises from ligand decomposition, and that ligand modification suppresses this side reaction. Further studies with these modifications are being pursued in order to achieve methane coupling.PhDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113556/1/lotzm_1.pd