Structural Changes of Alcohol-Solubilized Yallourn Coal in the Hydrogenation over a Ru/Al2O3 Catalyst

Abstract

Yallourn coal was oxidized in aqueous H2O2 in the presence of 1-propanol at 70 °C for 6 h under atmospheric pressure. The coal was partially decomposed by the OH radicals formed from H2O2, and carboxy and methylene groups originating from H2O2 and 1-propanol were added to the coal structure. As a result, the solubility of the coal in ethanol was increased to 80 kg per 100 kg of the raw coal. The H/C ratio, which was 0.95 for the raw coal, was increased to 1.07 by the oxidization in the presence of 1-propanol. The ethanol-soluble fraction of the oxidized coal was then hydrogenated using a Ru/Al2O3 catalyst in a mixed solvent of ethanol and acetic acid at 120 °C for 12−72 h at a hydrogen pressure of 10 MPa. After the hydrogenation, a yellowish white solid (hereafter referred to as hydrogenated white coal) was obtained. Major gaseous products were CO and CO2, and the yield of these gases produced by the hydrogenation in the presence of acetic acid was less than 3 kg per 100 kg of raw coal. The H/C ratio of the hydrogenated coal increased with increasing hydrogenation time and amount of acetic acid and was 1.23 after a 72 h reaction. The hydrogenation also decreased the total nitrogen content in the raw coal. The denitrogenation increased with increasing hydrogenation time and reached 60% after 72 h. It is noteworthy that this conversion was attained by a reaction at 120 °C. The IR peaks at 3000 and 2900 cm-1, assigned to methylene group, were increased by the hydrogenation. Further structural analyses of the hydrogenated white coal indicated that aromatic rings were changed to saturated rings, i.e., that the sp2 bonding structure of the coal was transformed to the sp3 bonding structure. In order to better understand the hydrogenation mechanism, benzyl alcohol, benzoic acid, phenol, and toluene were hydrogenated as model compounds, representing the structure of the alcohol-soluble fraction of the oxidized coal. The addition of carboxylic acids greatly enhanced the hydrogenation of aromatic rings in the model compounds over the Ru/Al2O3 catalyst. The molecular orbital calculation, based on the WinMOPAC program, suggested that dipole moments and charges on the oxygen atoms of carboxylic acids played important roles in the above transformation of the coal structure.