Molecular Structure of Heavy Oil Revealed - 4: Molecular Structure of Heavy Oil Revealed with Non-Contact Atomic Force Microscopy

The structure of heavy oil remains a challenge of significant commercial interest to the petroleum industry and an active area of research. Although techniques such as NMR, XPS, and FTICR-MS can yield detailed chemical information (aromaticity, heteroatom local bonding environment, and exact molecular formulae, respectively), the breathtaking heterogeneity of petroleum samples precludes mapping these analytical results onto unique structures for each molecule.

Schuler et al. (J. Am. Chem. Soc., 2015, 137, 9870-9876) recently imaged individual coal and petroleum asphaltene molecules using non-contact atomic force microscopy (NC-AFM) and found single-core ‘island’ motifs to predominate over multi-core ‘archipelago’ motifs. Recent studies with bispyrenyl model compounds (B. Schuler, Y. Zhang and co-workers, Chem. Sci., 2017, 8, 2315-2320) demonstrated that aromatic cores linked by alkyl spacers can be deposited and imaged successfully. This work extends our study with additional heavy oil petroleum samples:

• A highly aromatic steam cracker tar asphaltene anticipated to have a low degree of methylation and
• A de-asphalted vacuum residue fraction anticipated to have naphthenic rings and alkyl chains

Image analysis of several dozen steam cracker tar asphaltene molecules reveals many unanticipated structures. Specific classes of structural motifs were observed that provide clues for elucidating thermal chemistry pathways. Bulky pendant alkyl groups and non-planar naphthenic rings in the de-asphalted oil compromised their image clarity; further analysis of these images is underway.