Evolution of Aromatic Clusters in Vitrinite-Rich Coal during Thermal Maturation by Using High-Resolution Transmission Electron Microscopy and Fourier Transform Infrared Measurements

In this study, we focus on a quantitative characterization of aromatic cluster evolution in vitrinite-rich coal during coalification processes. By employing high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FT-IR), a detailed structural evolution of the aromatic clusters has been characterized and analyzed. Based on the results, a stepwise aromatic structure evolution was observed and can be divided into four stages: 0.5 to ∼1.4%, ∼1.4 to ∼2.0%, ∼2.0 to 3.7%, and 3.7 to 4.2%. In the first stage (Ro,ran =0.5% to ∼1.4%), naphthalene and 2 × 2 aromatic rings are tailored down from the main coal structure due to a cleavage of ether bonds and aliphatic bridge bonds. For Ro,ran from ∼1.4% to ∼2.0%, new naphthalene and 2 × 2 aromatic rings are formed in the second stage through the aromatization of aliphatic structures. When Ro,ran is from ∼2.0 to 3.7%, much more 3 × 3 and 4 × 4 aromatic rings are formed through the condensation of small aromatic rings. In the last stage for anthracite coals (Ro,ran > 3.7%), larger-sized aromatic rings are formed mainly by the condensation of 3 × 3 aromatic rings. During the coalification process, 3 × 3 aromatic rings play important roles in the evolution of aromatic clusters. Small aromatic rings are first enlarged to 3 × 3 aromatic rings, and then these 3 × 3 aromatic rings would form large-scale aromatic clusters. HRTEM can also provide quantitative information of the spatial alignment of aromatic clusters in coal. In anthracite coals with Ro,ran >3.7%, most of these fringes are highly aligned and share the same angle.