Decoding the Stacking and Arrangement of Aromatic Structures in Liptinite Macerals from Low-Rank Coal Utilizing HRTEM and 13C NMR

The arrangement and differences of aromatic clusters of liptinite macerals in low-rank coal are considered as essential foundations for exploring their thermal conversion behavior and enabling clean utilization. Density gradient centrifugation was used to enrich liptinite macerals (cutinite, sporinite, barkinite, and resinite) in four low-rank coals (Ro,max = 0.52–0.67%), and the purity of liptinite macerals could reach over 90%. The aromatic clusters and carbon skeleton structure of liptinite macerals were evaluated using a high resolution transmission electron microscope (HRTEM) and nuclear magnetic resonance (13C NMR) to reveal chemical structural and aggregated structural differentiation. The results of 13C NMR showed that the relative content of aliphatic carbon was approximately 56.60–77.59%, among which the resinite was the highest and the sporinite was the lowest. The aromatic carbon and oxygen-substituted aromatic carbon showed the opposite trend. The bridgehead carbon index (χb) of the liptinite macerals was between 0 and 0.25, indicating that the aromatic structures were mainly composed of benzene and naphthalene. The HRTEM results showed significant differences in the length, orientation, tortuosity, and stacking pattern of the aromatic layers for the liptinite macerals. The size of the aromatic layers in the sporinite was relatively long with an average of 0.502 nm, good orientation, low curvature, and abundant polycyclic aromatic hydrocarbons. The aromatic layers were mainly composed of screw dislocation geometry and T-shaped structures. The aromatic layers of cutinite had a medium size (0.443 nm) with a mixed feature of local order and overall disorder. The resinite had the shortest aromatic layers (0.380 nm), random orientation, and disordered stacking. Overall, the resinite and cutinite had hydrogen-rich and oxygen-poor characteristics, while sporinite had a relatively developed aromatic ring structure, and barkinite was in between the cutinite and sporinite.