Thermal decomposition behaviour and structural characteristics of asphaltenes

The thermal behaviour of 7 characterised n-C7 asphaltenes has been investigated by thermogravimetry and pyrolysis/gas chromatography/mass spectrometry (Py/GC/MS). Analytical data and results of structural analysis reveal substantial differences among the n-C7 asphaltene samples. The aromatic carbon fraction ranged between 0.69 and 0.48 and the sulphur content was between 2.9 and 10.8%. A substantial fraction of peripheral carbon in hydroaromatic/aromatic sheets (0.2-0.3) was substituted with aliphatic chains and a significant fraction of the aliphatic moieties (15-48%) were found to be present as naphthenic structures. Results of TG pyrolysis and (CNMR)-C-13 data showed that final yields of pyrolysis depended linearly on the aliphatic carbon fraction of the asphaltene. A linear relationship was also found between the fraction of sulphur evolved and final weight losses. Analysis of volatile matter by FT-ir and the Py/GC/MS technique indicated that its composition was predominantly aliphatic (similar to 80%) while the remainder was formed by alkyl-substituted aromatic compounds. The main classes of compounds identified were n-alkanes, 1-n alkenes, isoalkanes, cycloalkane and alkyl-substituted benzene. The amount of methane formed during pyrolysis ranged between 5.8 and 3.7% of carbon present in the initial sample. Its formation occurs mainly at high degrees of conversion, apparently through two reaction pathways. A non isothermal technique using different heating rates was applied to determine the kinetic parameters bf asphaltene pyrolysis. In each case, the data showed an increase of E-act during reaction, particularly at high conversion degrees. (C) 1997 Elsevier Science B.V.