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 H 2 O 2 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.