Depolymerization of lower rank coals by low-temperature O₂ oxidation

Four lower rank coals were oxidized in 0.5 N Na₂CO₃ aqueous solution, into which atmospheric oxygen gas was bubbled. The reaction was carried out at 20-85°C, and, after cooling to ambient temperature, the slurry was acidified (pH 1.3) with a 5 N HCl solution. The coals oxidized at 85 °C for 6-24 h were extracted with methanol/tetrahydrofuran mixtures of 25/75 and 50/50 in volume ratio at 30°C under ultrasonic irradiation. The O₂ oxidation dramatically enhanced the extractability of the coals, and extraction yields reached 80-90 wt % daf. Dimethyl sulfoxide and a mixture of benzene and methanol gave similar extraction yields for each oxidized coal. Pyridine showed lower extraction yields in spite of its higher hydrogen-bond-breaking ability. Extractability was, thus, better correlated with the solubility parameter of solvents than with the heat of hydrogen bond complexation. Diffuse reflectance FTIR analysis revealed that the O₂ oxidation introduced carboxylic, phenolic, and alcoholic hydroxyls into the coals with a decrease in alkyl groups and aryl-alkyl ethers. This structural change was due to the oxidation of aliphatic carbon-hydrogen bonds to peroxides and subsequent dissociation by acid-catalyzed hydrolysis. The O₂ oxidation also converted a portion of the coals to water-soluble acids such as oxalic, formic, acetic, and malonic acids and carbon dioxide. The carbon conversion to CO₂ was negligible for the O₂ oxidation at 20-50°C and was 5-9 wt % daf for that at 85°C.