Solid-State ¹H and ¹³C Nuclear Magnetic Resonance Spectroscopy of Athabasca Oil Sands Asphaltenes: Evidence for Interlocking π-Stacked Nanoaggregates with Intercalated Alkyl Side Chains

Abstract

Solid-state 1H and 13C nuclear magnetic resonance (NMR) spectroscopic studies on heptane-extracted Athabasca oil sands asphaltenes show strong support for the “island” or “Yen–Mullins” model. Structural calculations on the deconvolved 13C direct-polarization magic-angle spinning (DP-MAS) NMR spectrum reveal the existence of large, peri-condensed polycyclic aromatic hydrocarbon (PAH) systems. Further support for this model was obtained from the variable contact time and dipolar dephasing experiments. Variable contact time experiments demonstrated decreased mobility of the alkyl groups with increasing proximity to the aromatic core. Furthermore, it was shown that all of the aromatic groups are highly rigid, with no evidence for mobile aromatic moieties pendant from the main aromatic core. The cross-polarization discrimination induced by variable amplitude minipulses (CP-DIVAM) nutation curves enable discrimination between signals from mobile and rigid components and show that the alicyclic groups are condensed with the main aromatic core. The dipolar and T2 filter sequences enabled selection of low-level well-resolved signals underlying the mostly broad 1H NMR spectrum for the first time. These signals provide strong experimental support for aliphatic groups occupying spaces between stacked aromatic sheets in the solid state, which are thought to interfere with the π–π stacking interactions, while promoting stacking via π–σ interactions. The T2 relaxation behavior of the aliphatic protons suggests clustering of nanoaggregates, forming interlocking structures. All of these observations are consistent with the hierarchical Yen–Mullins model for asphaltenes and provide direct evidence for the existence of the nanoaggregate cluster architecture in a solvent-free environment.