Evaluation of macromolecular structure of a brown coal by means of oxidative degradation in aqueous phase

Acid-washed Morwell brown coal was oxidized at 85 degrees C in a 0.5 N Na2CO3 aqueous solution in a closed system, through which oxygen gas was bubbled. The oxidation took 1-12 h and consumed 1.8-5.8 mmol of O-2 per gram of the coal. Analyses of the products revealed that aromatic carbon was selectively oxidized into carboxyls in the residual solid (oxidized coal), water-soluble nonaromatic acids, and carbon dioxide, while aliphatic carbon was minimally involved in the reaction. Considering that the coal consists of two major structural elements, aromatic clusters (mono- or fused aromatic rings with peripheral groups) and intercluster bridges (alkyl groups and ethers connecting clusters), it was concluded that the oxidation converted aromatic carbons bonded to bridges into peripheral carboxyl groups on the neighboring clusters and also converted the other aromatic carbons into nonaromatic acids or carbon dioxide. According to the mechanism of the oxidation, which decomposed the macromolecular network of the coal-eliminating clusters and converting bridges into peripherals, the number of carboxyls formed as peripherals was expected to equal that of bridges which were bonded to eliminated clusters. From the relationship between the decrease in the amount of aromatic carbon and the corresponding increase in the amount of carboxylic carbon, the average number of bridges per eliminated cluster was estimated as 2.9 and 1.3 in the early and later stages of the oxidation, respectively, assuming the average number of aromatic carbon atoms per cluster to be 6. The mass fraction of solvent-extractable material increased as oxidation progressed and reached 0.97 after 12 h of oxidation. The oxidation thus enabled us to determine the quantitative relationship between the decrease in the number of bridges and clusters and the increase in the proportion of low-molecular-mass network fragments.