Structural Studies of Vacuum Gas Oil Distillate Fractions of Kuwaiti Crude Oil by Nuclear Magnetic Resonance

Abstract

Nitrogen-free vacuum gas oil (VGO) distillate was separated, using preparative column chromatography, into five hydrocarbon groups: saturates (SAs), monoaromatics (MAs), diaromatics (DAs), polycyclic aromatic hydrocarbons (PCAHs), and polar hydrocarbons (POLHs). The hydrocarbon groups MA, DA, PCAH, and POLH were separated into low- and high-sulfur compounds (LSCs and HSCs, respectively) on silica gel impregnated with PdCl2 (aqueous). SAs and LSCs obtained from MAs, DAs, PACHs, and POLHs were further fractionated into subfractions using gel permeation chromatography (GPC), based on their molecular size in solution. All the GPC subfractions were analyzed for elemental carbon, hydrogen, nitrogen, and sulfur, and for molecular weight. Nuclear magnetic resonance (NMR) spectroscopic techniques also were applied to elucidate the average molecular structure of selected GPC subfractions, POLHs, and HSCs. The SAs had average alkyl chain lengths of n = 13−15 and two naphthenic rings, and they were free of aromatics. The average molecule in the three groups (MAs, DAs, and PCAHs) contained (i) one aromatic ring, two cyclo-paraffinic rings, two alkyl groups on aromatic rings, and no bridged aromatic carbon; (ii) two aromatic rings, two naphthenic rings, five alkyl-substituted groups, and two bridged aromatic C atoms; or (iii) three aromatic rings, one naphthenic ring, four bridged aromatic C atoms, and four alkyl-substituents attached to the aromatic or naphthenic ring(s), with an average chain length of seven C atoms. The average molecule of the POLH group contained one S atom, four aromatic rings, four naphthenic rings, five bridged aromatic C atoms and four alkyl substituents attached to the aromatic or naphthenic ring with an average chain length of seven C atoms.