Role of the Nitro Group on Coal Solubilization in Aqueous Organic Solvents

Abstract

Studies on the solubilization of two low-rank bituminous coals in aqueous organic solvents were conducted. The nitro group was introduced in the organic matrix of coal via oxidative nitration, using dilute nitric acid. The solubility of raw coals and the coals in which the nitro group was introduced were determined in aqueous acetone and aqueous tetrahydrofuran (THF). The experimental results reveal that introduction of the nitro group substantially enhanced the solubilization of coals in aqueous organic solvents. Heat treatment of the oxynitrated coal at 150 ± 10 °C in air caused the elimination of NO2 (as is experimentally evident) and a reduction of solubility in potent mixed solvents (from 49.5% to 25% in aqueous acetone, and from 56.2% to 29.3% in aqueous THF). The study reveals that the nitro groups present in oxynitrated coals had an important role in solubilizing it in aqueous organic solvents. The decrease in the nitro group was detected using the Fourier transform infrared (FTIR) difference spectra and was confirmed by the material balance data. A hypothetical route of the elimination of the nitro group and a possible mechanism for the reduction of solubility of oxynitrated coal, consequent upon its thermal treatment in the presence of air, have been presented for the first time, and these presentations are substantiated by FTIR spectra.