Nano-level resolution determination of aromatic nucleus in coal

Based on the oxidative cutting method of coal-based graphene quantum dots (GQDs), a new idea to study the aromatic nuclei in coal is proposed: the dispersed aromatic nuclei can be obtained by eliminating the weak aliphatic and functional cross-links. The structural characteristics of coal aromatic nuclei were visualized based on transmission electron microscopy with nano-resolution. Distinct differences in structural characteristics exist among aromatic nuclei, which depends on rank. With increasing rank, the number of condensed rings along with the diameter of the singular aromatic nucleus and the aggregates increased significantly. In high-volatile bituminous coal, most of the aromatic carbon rings are curved and poorly orientated, and internal defects could be observed. The carbon rings in low-volatile bituminous coal are densely arranged and exhibit improvement in crystal orientation. Most heterocyclic compounds and amorphous carbon structures in the anthracite disappeared, and the arrangement and orientation of aromatic rings is the strongest, which closely resembles graphite structure. X-ray diffraction (XRD) and Raman spectra of raw coals showed the average crystallite diameter (La) and the minimum aromatic nuclei sizes are in agreement to those observed under HRTEM, the aromatic nuclei had undergone a process of evolution from small to large and from disorder to order. The first direct observation and quantitative analysis of the geometric and the polycondensation characteristics of the aromatic nuclei structure in various rank coals will provide a new perspective for further research on the molecular structure of coal.