Aromatic-Selective Size Exclusion Chromatography (ASSEC) of Petroleum Using Traditional Normal Phase Columns: A...

The comprehensive characterization of petroaromatics, composed of different number of rings and alkylation patterns, is of paramount interest considering their key roles in hydrocarbon exploration and refining processes. The analytical methods based on gas chromatography lack the access to non-volatile part, whereas the methods based on high-resolution mass spectrometry are not suitable for low molecular weight compounds (<200 Da) of a petroleum mixture. Therefore, an analytical method that can characterize the aromatic compounds in low- to high-boiling distillation fractions would be ideal for the purpose. Recently, a new mode in liquid chromatography, namely, aromatic-selective size exclusion chromatography (ASSEC) using a combination of amino-bonded silica and dichloromethane, has been employed to determine the alkyl carbon atom distributions (alkylation patterns) of aromatic compounds in a variety of petroleum samples. In the present work, we evaluated the selectivity of the three commonly used polar-bonded silica phases (amino, diol, and cyano) and a polystyrene-divinylbenzene phase while using dichloromethane as the mobile phase for the separation of aromatic compounds based on the alkylation pattern. Using a set of standard aromatic compounds, it was found that the separation of the compounds on the polystyrene-divinylbenzene phase was more influenced by the adsorption chromatography than the size exclusion chromatography. The separation on the cyano-bonded silica phase was primarily due to the size exclusion effect, whereas the separation on the other two silica phases was selective to the number of alkyl carbon atoms in ASSEC mode. The alkylation patterns of aromatic compounds in the petroleum samples were determined by ASSEC using three polar-bonded silica phases. The determined alkyl carbon atom numbers using the ASSEC method were compared with the ones obtained using GC × GC. The trends, for alkyl carbon atom numbers, obtained from both techniques were similar, confirming the suitability of the ASSEC method for the characterization of aromatics.