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

Solid-state H-1 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 H-1) 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 pi-pi 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 (similar to 7 or more). Transverse relaxation times (T-2) 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. (C) 2016 Elsevier Ltd. All rights reserved.