Aromatic-SelectiveSize Exclusion Chromatography (ASSEC)for Alkyl Carbon Distribution in Petroleum: Insights from High-ResolutionMass Spectrometry

Petroleum aromatic compounds play major roles in the hydrocarbon exploration and refining processes. The contemporary analytical techniques, such as gas chromatography and high-resolution mass spectrometry, are not capable of providing a comprehensive characterization of aromatics in complete boiling ranges of petroleum samples. Previous methods report the compositional information (in terms of average molecular weights) of petroleum samples using size exclusion chromatography and particularly for aromatics using aromatic-selective size exclusion chromatography (ASSEC). However, unlike in polymers, the average molecular weight information does not accurately reflect the composition of petroleum products. Especially, petroleum aromatics are composed of different ring sizes (or numbers) and different alkylation patterns (or alkyl chain lengths). For comprehensive characterization of aromatics, it is essential to conduct a two-dimensional separation based on ring size in one dimension and alkylation in another dimension. Ring-size separations have been well reported; however, separations based on alkylation for petroleum aromatics are scarce. Therefore, we developed a molecular-modeling-assisted ASSEC method to determine the alkyl substituent carbon distribution of aromatic compounds. Subsequently, the method was applied to the determination of the alkyl chain length of aromatics in the complete boiling ranges of petroleum samples (crude oil, distillation cuts, and residue). It is found that an increase in the boiling temperature of the petroleum samples is associated with an increase in the number of alkyl carbon substituents of aromatics. Additionally, both <i>C</i>_p (alkyl carbon atom number corresponding to the highest intensity) and <i>C</i>_w (weighted average alkyl carbon atom number) values increase successively discerning the alkylation pattern of aromatics in petroleum samples with the increase in boiling temperature. The developed method can be adopted in one dimension in a two-dimensional liquid chromatographic method for the comprehensive characterization of petroaromatics.