Abstract
Natural gas, which consists primarily of methane, ethane and propane, is a plentiful domestic resource that accounts for nearly a quarter of global energy production. However, its transportation and utilization is complicated by its gaseous state under ambient conditions. Current methods for the conversion of natural gas to high value liquid products require high temperatures and pressures and are capital-intensive. As a result, sources of natural gas that are located in remote regions are left “stranded.” Because methane is a potent greenhouse gas, with high global warming potential relative to CO2, this stranded natural gas is often unproductively flared to produce CO2. Globally, this flaring has been estimated to add 300 million tons of CO2 to the atmosphere each year and represents a loss of approximately $2 billion per year. Thus, there is significant motivation to develop a more efficient method to convert methane and other light alkanes from natural gas to easily transportable liquid products at the wellhead. This liquid (e.g., methanol) could then be used directly as a fuel or as a precursor to high-value chemicals.
The primary focus of this Dissertation is the development of a selective method for the direct partial oxidation of light alkanes. Iodine oxides and chloride salts have been demonstrated to be highly efficient for the oxy-esterification (OxE) of methane, ethane and propane to generate the corresponding esters in trifluoroacetic acid at temperatures < 200 °C and pressures ranging from 35-1000 psi. Ethylene is also functionalized under similar reaction conditions to generate a derivative of ethylene glycol.
A combined experimental and computational mechanistic study found that light alkane functionalization likely occurs through a radical-based pathway where chlorine radical or IO2 radical are predicted to abstract an H-atom from the starting alkane to generate alkyl radical. The alkyl radical is trapped by iodine which is generated in situ from the reduction of the iodine oxide oxidant to produce alkyl iodide which undergoes solvolysis in trifluoroacetic acid to produce alkyl trifluoroacetate. Although the C–H bond of MeTFA is weaker than that of methane, the alkyl ester was demonstrated to be stable under the highly oxidizing conditions with KCl/NH4IO3 in HTFA at 140 °C. The high selectivity for the alkyl ester product is proposed to be a manifestation of the polar effect, where the electron- withdrawing trifluoroacetate moiety of the alkyl ester prevents additional, undesired oxidation in the polar solvent by decreasing the polarity of the transition state.
Extension of the selective alkane OxE process to oxidants other than iodate was examined with the goal of identifying an oxidant which could generate species capable of H-atom abstraction and be regenerated easily using air. Nitrates and manganese-based oxidants have been demonstrated to be the most effective of those studied. Methane functionalization with these oxidants is stoichiometric rather than catalytic, but efforts to recycle the oxidants in situ are ongoing.
The iodine oxide and chloride system was also examined under photochemical conditions. The functionalization of methane, ethane and propane is highly efficient, producing functionalized products with percent yields of ~50%, 80% and ~40%, respectively. MeTFA is also highly stable under photochemical reaction conditions, likely enabling the high yield of the mono-functionalized product. The conversion of methane to alkyl esters has also been observed using other oxidant systems and the photocatalyst tetrabutylammonium decatungstate.
Additionally, the conversion of toluene and 1-pentene to pentenyltoluenes using the Rh catalyst precursor [Rh(μ-OAc)(η2-C2H4)2]2 has been studied. Rhodium-based catalysis enables high ratios of linear:branched pentenyltoluenes products (up to 11:1). These linear pentenyltoluenes, which cannot be produced using current acid-based industrial methods for arene alkylation, are precursors for the plastic polyethylene naphthalate (PEN), a derivative of polyethylene terephthalate (PET).
