Structure and evolution features of cutinite with different coal rank from stacking and arrangement of aromatic fringes in HRTEM

Abstract

• The aggregated structure in cutinite was mainly benzene and naphthalene. • The stacking of aromatic systems were closely related to the structural evolution. • The orientation of aromatic fringes were closely related to the length and coal-rank. The structure and evolutionary features of cutinites isolated from the two samples of different ranks were evaluated by image analysis of aromatic fringes in high resolution transmission electron microscopy (HRTEM) combined with Photoshop and ArcGIS software. The distributions of aromatic fringes (length, orientation, curvature, segmented angle and cumulative angle) were quantified to establish the differences in stacking and spatial arrangement of the aromatic systems in the two cutinites. The bonding states of organic carbon in cutinite, derived from XPS spectra, mainly existed in the form of –C C–, followed by –C–O bonds, whereas the content of π–π bonds was the lowest. The content of oxygen-containing groups decreased and the amount of π–π bonds increased with rank. HRTEM aromatic fringe image analysis showed that short (<0.59 nm) and middle-sized fringes (<1.49 nm) in cutinite were dominant, and that the aggregated structure was mainly 1–2-ring aromatics. The preferred orientations and distribution of aromatic fringes were closely related to the stacking configurations of aromatic systems and to the rank. The larger polyaromatic clusters were observed with higher rank, and the aromatic fringes became more regular. The aromatic systems of cutinite-P (cutinite isolated from Panzhihua area) contained abundant carbon structural layers with screw dislocations, and the contents of Y-type and T-shape arrangements were lower than those of cutinite-L (cutinite isolated from Luquan area). T-shape and Y-type structures were widely distributed in the short fringes (<0.59 nm). The stacking geometry of screw dislocations was observed extensively in medium and long fringes, and the local molecular-oriented domain was isolated by short fringes. Further, the evolution of chemical structure in cutinites in terms of the distribution of orientation, curvature, and cumulative angle of aromatic fringes showed different trends. This present paper is one part of a study that seeks to establish new perspectives on the relationships between coal structure and reactivity.