Oxidative weathering of Illinois No.6 coal

Abstract

The exposure of freshly-mined Illinois No.6 bituminous coal (Monterey Mine No.1) to atmospheric oxygen at ambient conditions resulted in a slow oxidation reaction which appeared to be complete within two months to produce an oxidized coal product with ≈26% more organically bound oxygen than the fresh coal. An oxygen functional group analysis was performed to determine the carboxylic acid, hydroxyl and ether group content of the coal before, during and after the reaction. Infrared analysis showed no carbonyl production; however, ether functionality was being produced. O-methylation reactions which employed isotopically labelled methyl groups (both C-13 and deuterium) were used to identify and quantify O-H and CO 2H sites in the weathering coal samples. It was found that these acidic groups were not the products of oxidative weathering. Essentially all of the chemically incorporated oxygen eventually formed ether linkages. In a related study on this coal, hydroperoxide was detected as a transient intermediate in the early stages of the oxidation. A mechanism is proposed which involves the simultaneous formation under mild temperatures of a hydroperoxide and a carbon radical that undergoes a radical displacement producing the ether product directly. Solvent swelling studies revealed that these ethers act as cross-linking agents in the coal structure. The more highly cross-linked coal structure that resulted from weathering was responsible for the destruction of the plastic properties of this bituminous coal.