Methane Activation and Oxidation in Sulfuric Acid

摘要

The H/D exchange observed when methane is contacted with D_2SO_4 at 270-330 deg C shows that the alkane behaves as a sigma base and undergoes rapid and reversible protonation at this temperature. DFT studies of the hydrogen exchange between a monomer and a dimer of sulfuric acid and methane show that the transition states involved in the exchange are bifunctional, that is one hydrogen atom is transferred from a hydroxy group in sulfuric acid to methane, while one hydrogen atom is abstracted from methane by a non-hydroxy oxygen atom in sulfuric acid. All the transition states include a CH_5 moiety, which shows similarities to the methanium ion CH_5~+. The calculated potential activation energy of the hydrogen exchange for the monomer is 174 kJ mol~(-1), which is close to the experimental value (176 kJ mol~(-1)). Solvation of the monomer and the transition state of the monomer with an extra sulfuric acid molecule, decrease the potential activation energy by 6kJ mol~(-1). The acid-base process is in competition, however, with an oxidative process involving methane and sulfuric acid which leads to CO_2, SO_2, and water, and thus to a decrease of acidity and loss of reactivity of the medium.