Case Study of Quantification of Aromatic Ring Structures in Lignite Using Sequential Oxidation

The quantification of aromatic ring structures in lignite was studied. By the sequential oxidation of a sample of lignite, the filtrates and residues were analyzed and quantified by high-performance liquid chromatography, gel permeation chromatography, and ultimate analysis. The types and amount of aromatic ring structures in lignite were determined. The results indicated the presence of three types of aromatic ring structures based on the oxidation characteristics of lignite: (1) type I structure, which can be oxidized into benzene poly(carboxylic acid)s, (2) type II structure, in which the aromatic ring is appended by containing O/N/S groups, such as benzofuran, indol, benzothiophene, phenol, and anisole, and which can be overoxidized into small-molecule fatty acids, and (3) type III structure, in which the aromatic organic matter cannot be oxidized. The concentration of type I aromatic ring structures is 0.71 mmol/g of lignite (on a dry and ash-free basis). For type II structures, the concentrations of aromatic heterocycles containing nitrogen and sulfur are 0.07 and 0.14 mmol/g, respectively, assuming that these aromatic heterocycles contain an average of one nitrogen or sulfur atom per heterocycle. For type III structures, the concentration of tetrahydrofuran-soluble substances, which are primarily 2,5-cyclohexadiene-1,4-dione, 2,6-bis(1,1-dimethylethyl), dibutyl phthalate, di-n-octyl phthalate, and butylated hydroxytoluene, is 0.035 mmol/g. The results of this study enhance our quantitative understanding of the aromatic structures of lignite.