Ex Situ and in Situ Thermal Transformations of M-50 Pitch Revealed by Non-contact Atomic Force Microscopy

Petroleum pitch M-50 (or A-240) has been well-known in making valuable carbon materials through thermal treatments. How these molecules react to produce carbon materials and the mechanisms of thermal polymerization and molecular weight growth under thermal conditions are of great significance and yet still unclear. Structures produced by thermal reactions of M-50 pitch were characterized with non-contact atomic force microscopy and compared to the structures in M-50 pitch previously characterized (Chen, P.; Metz, J. N.; Mennito, A. S.; Merchant, S.; Smith, S. E.; Siskin, M.; Rucker, S. P.; Dankworth, D. C.; Kushnerick, J. D.; Yao, N.; Zhang, Y. Petroleum pitch: Exploring a 50-year structure puzzle with real-space molecular imaging. Carbon 2020, 161, 456-465, DOI: 10.1016/j.carbon.2020.01.062). Reaction products were generated from M-50 pitch by two different approaches: an ex situ approach via thermal treatment at 400 °C under N2 and an in situ approach via reaction directly on a Cu(111) surface. Polycyclic aromatic hydrocarbons (PAHs) from the ex situ reaction are larger than those in the starting M-50 pitch and with fewer methyl groups. Both types of five membered rings, conjugated and non-conjugated, are observed. Very large PAHs are formed under the in situ surface conditions as a result of reactions catalyzed by the Cu surface, with five-membered rings preserved as planar moieties in the product. The data suggest that methyl groups play important roles in initiating the polymerization and molecular weight growth of M-50 pitch molecules, but the reactivities of five-membered rings remain unclear.