Characterization of asphaltenes molecular structure

The features of molecular structure of seven n-C7 asphaltenes isolated from different feedstocks (i.e., vacuum residue, atmospheric residue and crude oil) have been investigated by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), and their average structural parameters have been estimated using a mathematical optimization method. The results of structural analysis pointed out substantial differences among the asphaltenes investigated. The size of aromatic fused ring systems ranged from 5.1 to 14.9, while a substantial fraction of peripheral carbon in hydroaromatic/aromatic sheets (0.28-0.40) is substituted with aliphatic chains whose average length was between 2.8 and 6.3. A significant fraction of aliphatic moiety (0.15-0.48) was found to be present as naphthenic structures. In all cases, the main classes of compounds identified during pyrolysis tests were homologous series of alkanes (up to C25-30), 1-alkenes (up to C25-30), branched paraffins, and alkyl-substituted aromatic compounds. The formation of pyrolysis products was explained on the basis of literature data regarding thermolysis reactions of model compounds mimicking the structural features of asphaltenes. The results of this study are consistent with the view that asphaltenes are a complex polydisperse mixture of molecules made up of polyaromatic and hydroaromatic units joined by aliphatic bridges and substituted with aliphatic chains containing up to 25-30 carbon atoms. However, both NMR data and results of structural analysis indicate that the major part of these chains (more than 80%) is formed by short alkyl groups (C1-C4).