Daniel J. O'Donnell

Characterization of high-boiling petroleum distillate fractions by proton and 13C nuclear magnetic resonance spectrometry

High-boiling (535–675 °C) distillate fractions of Wilmington (Calif. USA) and Gach Saran (Iran) crude oils were separated into saturate, monoaromatic, diaromatic and polyaromatic-polar fractions by passage through a silica-gel—alumina dual packed chromatography column. These fractions were further separated on the basis of molecular volume by gel-permeation chromatography (g.p.c.). Select g.p.c. fractions were then analyzed by 1H and 13C n.m.r. spectroscopy. The fraction of aromatic carbons (Ar-C) of the total carbon content of a given series of g.p.c. fractions, obtained directly from the 13C n.m.r. spectra, showed a significant range of values (e.g. 13.2% to 29.2%) within each series. Furthermore, the values of %Ar-C within a series of g.p.c. fractions showed a maximum in each case. No such maximum was observed in the 1H n.m.r. spectra for the fraction of aromatic proton (%Ar-H) content compared to the total proton content of any of the g.p.c. fraction series. Signals observed in the 13C n.m.r. spectra confirmed the presence of aliphatic chains attached to aromatic structures in the monoaromatic, diaromatic and polyaromatic-polar g.p.c. series of each distillate fraction. Well-defined signals, not attributable to straight-chain aliphatic material, were also observed in the 13C n.m.r. spectra. Comparison of the chemical shifts of these 13C n.m.r. signals with those spectra of model compounds obtained experimentally, and with appropriate systems in the literature, strongly suggested the presence of saturated terpenoid-like structures as well as the presence of methyl groups in sterically hindered positions on an aromatic ring system. Gated decoupling techniques and the use of a relaxation agent were used to overcome the deleterious effects of slow relaxation times and Nuclear Overhauser Enhancement (NOE) on the analytical quality of the 13C n.m.r. spectra.