The present study has been undertaken to clarify the mechanism of an aqueous-phase oxidative degradation of a brown coal and to examine the applicability of lattice statistics for a quantitative explanation of the degradation. In this paper, first, changes in carbon type distribution as well as those in contents of oxygen functionalities, caused by the oxidation, are analyzed, and the results demonstrate the validity of the previously proposed mechanism of the oxidation: (1) The oxidation depolymerizes the coal by eliminating aromatic clusters; (2) When a cluster is eliminated, the inter-cluster bridges that connected the cluster are converted into peripherals of the neighboring clusters, and carboxyl groups are formed at the ends of the peripheral chains; (3) The elimination of the cluster also accompanies the deactivation of the neighboring clusters due to the formed carboxyls. The relationships among the loss of clusters, that of bridges, and amount of carboxyls formed are quantitatively explained by general lattice statistics assuming that each cluster is bonded to 2.2 bridges on average. Second, a structural model of the coal is developed in order to explain the increase in the solvent-extractable material by the degradation. The model describes the macromolecular structure of the coal as a mixture of Bethe lattice with a coordination number Z = 2.2 (the site occupation probability p0 = 1) and solvent extractable material. The model is found to predict the observed increase in the mass fraction of DMF extract quantitatively as a function of a corrected site-occupation probability, and also to present the molecular mass distribution of the material consistent with that observed by a laser desorption-ionization mass spectrometry.
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