Insight into structure features of coals with different metamorphic degrees via step-wise oxidation

Abstract

A step-wise oxidation method, combining H2O2 oxidation and alkali-oxygen oxidation (AOO), was applied to study the structure features of coals with different metamorphic degrees. The results show that the conversion of Baiyinhua coal, Hongshaquan coal, Pingshuo coal and Hongliulin coal in 30 % H2O2 solution for 10 h at 60 °C are 89.8 wt%, 79.0 wt%, 29.5 wt%, and 14.8 wt%, respectively. The oxygen containing species retrieved from water-soluble product by ethyl acetate extraction are mainly concentrated in O4-O8 class species, including aliphatic carboxylic acids with a carboxylic range of 1–5 and carbon numbers from 3 to 11, and phenols or aromatic carboxylic acids of 1–3 ring. The residues from H2O2 oxidation of four coals can be completely depolymerized through AOO. The maximum benzene carboxylic acid (BCA) yield of RBYH is 7.66 wt%, while that of RHSQ, RPS, and RHLL are 18.24 wt%, 19.52 wt%, and 19.91 wt%, respectively. The AOO products of four residues show the similar BCA distributions, and mellitic acid, benzenepentacarboxylic acid, pyromellitic acid, trimellitic acid, hemimelitic acid, and phthalic acid are the main BCAs. Therefore, step-wise oxidation can be used as an effective method to understand the structural features of coal, including the small molecule structures obtained after breaking weak bonds and the aromatic substitution information in the residue.