STRUCTURAL CHARACTERIZATION OF ASPHALTENES OF DIFFERENT ORIGINS

Asphaltenes obtained by precipitation with n-heptane from crude oils of different geological origin has been analyzed by a great number of spectroscopic techniques in order to show the main molecular features which differentiate them. The most important differences found are (1) aromatic carbon and heteroatom content, (2) alkyl side-chain length, and (3) molecular weight. Smaller molecular weight asphaltenes are more aromatic, contain less heteroatoms, particularly S, and have shorter aliphatic side chains. Asphaltenes were characterized in detail through the multiplet selective NMR techniques (GASPE) to obtain individual CHn (n = 0-3) quantitative subspectra: no evidence of quaternary aliphatic carbon signal was found while the aliphatic CH3/CH ratio seems to be correlated with aromaticity. The degree of condensation and/or substitution of aromatic rings was evaluated by applying solid-state C-13 NMR, FT-IR, and EPR. The results of our studies show that asphaltenes with different aromaticity seem to be similar in their aromatic rings condensation and/or substitution degrees. The estimated value of the average number of condensed aromatic ring; is nearly 7.