Three-level structure change of asphaltenes undergoing conversion in a hydrogen donor solvent

Abstract The three-level structure of asphaltenes; including molecule unit, nanoaggregate, and cluster, undergoing hydrogenation in a hydrogen donor solvent was analyzed using proton nuclear magnetic resonance ( 1 H NMR), X-ray diffraction (XRD), and small-angle X-ray scattering (SAXS). Upon conversion, the asphaltene molecule unit became more compact with shorter alkyl side chains. The nanoaggregate, consisting of an inner cylinder of polycyclic aromatic hydrocarbons (PAH) stacking and an outer cylinder engulfing the pendant alkyl chains, suffered a decrease in the radii of the outer and inner cylinders together with a decrease in the height of the concentric cylinders. The layer distance between the aromatic sheets ( d m ) of the nanoaggregate increased, whereas the average height of the aromatic sheet stack ( L c ) and the number of aromatic sheets per nanoaggregate ( N) decreased. The π-π interactions within the nanoaggregate structure were likely weakened by the hydrogen radicals. The top and/or the bottom aromatic sheets of the stacked structure were stripped upon hydrogenation and the weakening of the van der Waals interactions. Lastly, the cluster size initially decreased, owing to smaller size nanoaggregates and lesser extent of solvent swelling. For longer reaction times, however, the increase in aromaticity enabled higher extent of interaction among the nanoaggregates and resulted in an increase of the cluster size.