Validation of the Yen–Mullins Model of Athabasca Oil-Sands Asphaltenes using Solution-State ¹H NMR Relaxation and 2D HSQC Spectroscopy

Abstract

The hierarchical Yen–Mullins model of Athabasca oil-sands asphaltene is strongly supported by solution-state 1H NMR relaxation measurements and 2D HSQC-NMR spectroscopy. For the first time, the 1H T1 and T2 relaxation behaviors of specific sites in asphaltene molecules have been studied, and it is shown that the relaxation behavior is in agreement with the hierarchical molecule-nanoaggregate-cluster model proposed by Mullins (Energy Fuels 2012, 26, 3986). The size of the asphaltene nanoaggregate clusters was determined from the biexponential T2 relaxation behavior of the 1H nuclei. Assignment of overlapping 1D NMR signals was made possible via HSQC methods, where elusive hallmark long-range aromatic–aliphatic heteronuclear correlations were observed. Alicyclic structures were shown to be more closely associated with the aromatic core than what has been proposed for most archipelago-type structures. The NMR parameters obtained from the deconvolution analysis indicate an average of 6–7 pericondensed aromatic rings present in each molecule, in agreement with the island model. The average molecular weight was determined at ∼720 g/mol, which would be typical for island model structures, hence lending strong support for the Yen–Mullins model.