Insight into the aromatic ring structures of a low-rank coal by step-wise oxidation degradation

Aromatic skeleton structure of Naomaohu (NMH) coal was investigated by a step-wise oxidation degradation and XRD, Raman and XPS characterization. Coal was first oxidized with H₂O₂ to result in breakage of only bridged bonds and side chains, then the oxidation residue was further degraded through controlling alkali-oxygen (A-O) oxidation to avoid ring-open of polyaromatics and the conversion of coal sample exceeded 97.4 wt%. Synchronous fluorescence spectrometry (SFS) analysis of the oxidation products inferred that the aromatic structures in NMH coal mainly involve 1–4 rings compounds, among which single-ring structures are dominant accounting for 54.4 mol%. The results of XRD, Raman and XPS showed that NMH coal has a turbostratic structure, an intermediate state between the amorphous state and graphite, comprised by amorphous carbons and orderly aromatic clusters. The heteroatoms mainly exist in the forms of carbonyl, hydroxyl, pyrrolic nitrogen, mercaptan or thiophenol, thiophene, sulfoxide and sulfone. A simplified model of coal skeleton structure was proposed based on the complementary information from various techniques. Moreover, SFS analysis of A-O oxidation products clearly exhibits the gradual cracking process of polyaromatics, which provides an efficient method for controllable production of benzene carboxylic acids by preventing excessive or incomplete oxidation.