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 similar to 1.4%, similar to 1.4 to similar to 2.0%, similar to 2.0 to 3.7%, and 3.7 to 4.2%. In the first stage (R-o,R- ran = 0.5% to similar to 1.4%), naphthalene and 2 x 2 aromatic rings are tailored down from the main coal structure due to a cleavage of ether bonds and aliphatic bridge bonds. For R-o,R- ran from similar to 1.4% to similar to 2.0%, new naphthalene and 2 x 2 aromatic rings are formed in the second stage through the aromatization of aliphatic structures. When R-o,R- ran is from similar to 2.0 to 3.7%, much more 3 x 3 and 4 x 4 aromatic rings are formed through the condensation of small aromatic rings. In the last stage for anthracite coals (R-o,R- ran > 3.7%), larger-sized aromatic rings are formed mainly by the condensation of 3 x 3 aromatic rings. During the coalification process, 3 x 3 aromatic rings play important roles in the evolution of aromatic clusters. Small aromatic rings are first enlarged to 3 x 3 aromatic rings, and then these 3 x 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 R-o,R- ran >3.7%, most of these fringes are highly aligned and share the same angle.