Insights into asphaltene aggregate structure using ultrafast MAS solid-state 1H NMR spectroscopy

- First ultrafast MAS solid-state 1H NMR study of asphaltenes.
- High spin-rate allows detection of otherwise obscure signals.
- Double Quantum correlation and T2 relaxation experiments help elucidate asphaltene aggregate structure.
- Asphaltenes mostly consist of large single-core PAHs.
- Nanoaggregation occurs through π-π interactions, and clusters consist of a rigid central core and a mobile periphery.

Solid-state 1H NMR spectroscopy under ultrafast magic angle spinning (60 kHz) is used for the first time to investigate the solid phase aggregate structure of a petroleum asphaltene. The fast spinning speed, combined with high-magnetic field (16.4 T, 700 MHz 1H) spectral filtering techniques facilitate the observation of signals that generally remain obscured even in solution-state NMR techniques. Signals shielded by aromatic ring currents were observed in both the aromatic and aliphatic regions of the spectrum, indicating a stacked geometry. A homonuclear double quantum correlation experiment reveals molecular interactions, which suggest that asphaltene molecules aggregate through skewed π-π stacking. These nanoaggregates further cluster among themselves through alkyl-alkyl and alkyl-aromatic interactions. Based on the range of chemical shifts and molecular interactions, it is proposed that asphaltenes predominantly consist of large PAHs (∼7 or more). Transverse relaxation times (T2) and dipolar filter experiments reveal two domains in a solid asphaltene cluster: a rigid central core composed of stacked aggregates and a much smaller fraction of mobile peripheral alkyl sidechains. Additionally, the relaxation results are consistent with the presence of small fraction of small PAHs as substituents attached to large central PAHs.

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