Structural characterization of asphaltene fraction from vacuum residue of Arabian crude mixture by spectroscopic and pyrolytic methods

In view of the major problems caused by petroleum asphaltenes either during crude oil recovery or during thermal and/or catalytic processing of petroleum residues, the structural properties of the n-C₅ insoluble asphaltene isolated from the vacuum residue of Arabian crude mixture were investigated by referring to the data obtained from pyrolysis GC/MS, ¹H/¹³C-NMR, gel permeation chromatography (GPC), and matrix assisted laser desorption/ionization time-of-flight MS methods. The average size of aromatic fused ring systems within the asphaltene was 4-5. The separation of aliphatic CH₃, CH₂, CH carbons was discussed by referring it to the distortionless enhancement by polarization transfer experiments, and the percentages of individual functionalities toward total carbon were evaluated. The pyrolysis of asphaltene produced 35.1% of volatile fraction along with 8.1% of tar and 56.8% of coke. It was observed in the pyrogram homologous series of alkanes and corresponding 1-n-alkenes, benzenes, alkyl benzenes, cycloalkanes, isoparaffins, alkylated naphthalenes, and trace amounts of three ring aromatics. The addition of naphthenic and thiophenic rings on the clusters even made the cluster a larger fused system, resulting in the formation of coke materials. The relative abundance of the homologous series of alkane and alkene products agreed reasonably with the average chain size (6.2) of the asphaltene molecule, suggesting that the remarkable part of the chain carbons consists of the constituents of bridge sructures. These results implied that asphaltene is a mixture of complex polydispersed molecules with large variation in molecular sizes.