| The homogeneously catalyzed, single-pot conversion of methane to acetic acid was investigated. The studied reaction pathways are grouped by the reacting carbon species in the gas phase: (1) CH4 and CO2, (2) CH4 and CO, and (3) CH4 alone. Liquid-phase reaction products were analyzed by NMR, UV-visible spectroscopy, and Raman spectroscopy. Gas-phase products were measured by gas chromatography.;Methane and carbon dioxide were found to react, producing the mixed anhydride of acetic acid and an acid solvent. VO(acac)2 and K2S 2O8 promoted the reaction. Acetic acid could be recovered by hydrolysis of the reaction mixture. The reaction conditions were investigated, and a reaction scheme was proposed. The reaction is initiated by the thermal decomposition of K2S2O8 to produce SO 4·- free radicals that then abstract a hydrogen from methane. Controlled experiments with peracetic acid showed that VO(acac) 2 stabilized an intermediate acetoxy radical.;Oxidative carbonylation of methane to acetic acid was catalyzed by Pt(II) in concentrated sulfuric acid. The effects of reaction conditions were studied and a reaction pathway was postulated. Methyl bisulfate was identified as an intermediate that can be carbonylated to acetic acid by CO in the presence of Pt(II). During the reaction the catalyst is reduced by CO to Pt(0), which is catalytically inactive. The use of N-ligated Pt(II) complexes, undertaken in an effort to stabilize the catalyst, slowed the rate of acetic acid formation. However, addition of a co-catalyst and molecular oxygen increased acetic acid yield, presumably due to an increase in the rate of Pt(0) oxidation to Pt(II).;Lastly, direct oxidation of methane to acetic acid was studied in sulfuric acid. Pd(II) in concentrated sulfuric aid was the only metal found to catalyze the reaction. The reaction mechanism was elucidated using isotopic labeling and varying the reaction conditions. The active catalyst was found to be Pd(II) cations coordinated with bisulfate anions. Methyl bisulfate was a byproduct and a reactive intermediate, the oxidation of which produced CO and reduced Pd(II) to Pd(0). CO was also found to reduce the catalyst. Addition of Cu(II) and O2, or just O2 by itself, increased the rate of Pd(0) oxidation to Pd(II), thereby increasing the rate of acetic acid production. This catalytic system was made heterogeneous by incipient wetness impregnation of a sulfuric acid solution with Pd(II) onto amorphous silica. However, the reaction productivity decreased by ∼50% relative to the homogeneous reaction, due likely to reaction of the bisulfate anions with silica. |
