Average molecular structures of asphaltenes from vacuum residues of Venezuelan crudes: Theoretical study of thermodynamic properties

From experimental data of five residues of Venezuelan crudes, standard criteria of molecular characterization, molecular modeling and quantum semi-empirical calculations, a set of average molecular structures are proposed for their corresponding asphaltenes. The geometries of the proposed molecules were compared with structures of different asphaltene origins reported in the literature, where significant similarities and differences are observed for some cases. However, the molecular weights, H/C ratio, the number of aromatic and aliphatic carbons and the number of-CH3,-CH2-, CH- aliphatic groups of the average structures of the hypothetical asphaltenes, proposed in this work, agree satisfactorily with experimental values of very well know models. To rationalize the similarities and differences between the proposed asphaltenes and those of the literature, a theoretical study at semi-empiric level of the structures, energetic and thermodynamic properties was performed using the PM3 and PM6 parameterizations as implemented in MOPAC 09 software. As criteria for the stability of the asphaltenes, calculations were carried out for the heats of formation and the associated thermodynamic properties within 200 to 900 K temperature range. At 298.15 K, the results show that for the extended asphaltene structures the formation heat are underestimated at PM3 level, while the PM6 parametrization lead to values of this property that are consistent with the expected values according to the size, molecular composition and chemical nature of the studied structures. By other hand, it was determined that there are exist significant linear correlations between calculated thermodynamic parameters and variables that depend on the intrinsic structure of asphaltenes, such as molecular weight, total number of carbon atoms and total number of aromatic rings.