Evaluation of relative hydrogen-donating abilities of different heavy oils during mild thermal conversion by ¹H-NMR

The relative hydrogen-donating abilities of different heavy oils were determined by ¹H-NMR. Three typical heavy residues and the corresponding subfractions were thermally treated with equal weight of chemical accepting probe (i.e. anthracene) at 380℃ for 8 min under the nitrogen pressure of 4MPa; the toluene insoluble yield, gas yield and composition, and the distillation curve of liquid products were determined. The results show that the thermal reaction of samples under the testing conditions is relatively moderate with no apparent structural changes. By comparing the hydrogen distribution of samples with that of residues after reaction, it is proved that the primary reaction during the thermal process can make the hydrogen transfer from oil samples to anthracene. The reduction value of the hydrogen content of H_cβ(chemical shift ranging from 1.4~2.0 in ¹H-NMR spectrum, representing naphthenic hydrogen two positions from the aromatic ring of naphtheno-aromatics) and H_cα (chemical shift ranging from 2.5~4.7 in ¹H-NMR spectrum, representing hydrogen on naphthenic ring adjacent to fused aromatic ring of naphtheno-aromatic) is correlated well to the donated hydrogen content measured by chemical method. Thus it is reasonable to regard H_cβ and H_cα as the main donated hydrogen during mild thermal conversion of heavy oils, and to entatively evaluate the relative hydrogen-donating ability of heavy oil using the two type protons content in ¹H-NMR is acceptable.