Alkyl carbon number limits the ring-number separation of polycyclic aromatic compounds in petroleum analysis

Abstract

There have been many attempts to achieve a clear-cut separation of alkylated aromatic compounds based on the number of aromatic rings employing normal-phase liquid chromatography. However, each method has its own limitations to address this issue. Therefore, we aimed to elucidate the involved mechanisms for this limitation. For this purpose, stationary phases viz. amino- and cyano-bonded silica gel were considered. The phases were evaluated with the distilled cuts of a crude oil based on resolution and selectivity, and the optimum phase was selected for the separation of a crude oil. It was observed that there was a significant overlap of compounds with the same ring-number in different peaks. The cause of overlap was linked to the number of alkyl carbon atoms attached to the aromatic rings. Aromatic compounds substituted with higher number of alkyl carbon atoms (total) elute earlier than those with lower number of alkyl carbon atoms. This trend was maintained for hydrocarbons, sulfur-containing compounds (S1), and di-sulfur containing compounds (S2). It was further corroborated using molecular modeling calculations that (i) the increase in the number of alkyl carbon atoms increases the molecular volume, and (ii) the volume of the molecules remains similar irrespective of the number and position of alkyl substituents provided that the total number of alkyl carbon atoms remains the same. Finally, it was revealed that the counteractive role of the size exclusion mechanism limits the application of normal-phase chromatography for a clear-cut separation of alkylated aromatic compounds according to the number of aromatic rings.