Petroleum pitch: Exploring a 50-year structure puzzle with real-space molecular imaging

Petroleum pitch has played a significant role in carbon science as a key abundant resource for polycyclic aromatic hydrocarbons in making various higher value carbon materials. Despite many detailed studies using advanced characterization techniques over 50 years, the exact nature of the molecular structures of petroleum M-50 pitch and their mesophase products remains unclear, due to the molecular diversity and the low solubility of this material. In this study, we applied real-space single molecule imaging non-contact atomic force microscopy to obtain exact structures of individual molecules, and compared the results from other characterization techniques to validate some of the previously hypothesized average structures. We identified a diverse slate of largely catacondensed polycyclic aromatic hydrocarbons with short alkyl chains, such as methyl and methylene groups. Furthermore, both single core and multi-core structures have been observed, in contrast to previous assertions that only one type would be present. The presence of these structures enables a mechanistic rationalization for their formation and allows potential mechanisms for the thermal conversion of pitch into larger bonding networks to be postulated.