STRUCTURAL CHARACTERIZATION OF ASPHALTENES OF DIFFERENT ORIGINS

Asphaltenes obtained by precipitation with n-heptane from crude oils o f different geological origin has been analyzed by a great number of s pectroscopic techniques in order to show the main molecular features w hich differentiate them. The most important differences found are (1) aromatic carbon and heteroatom content, (2) alkyl side-chain length, a nd (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 throug h the multiplet selective NMR techniques (GASPE) to obtain individual CHn (n = 0-3) quantitative subspectra: no evidence of quaternary aliph atic carbon signal was found while the aliphatic CH3/CH ratio seems to be correlated with aromaticity. The degree of condensation and/or sub stitution 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.