1H- and 13C-NMR structural characterization of asphaltenes from Vacuum residua modified by thermal cracking

Abstract

1H- and 13C-NMR data were used to characterize asphaltenes and to follow their chemical changes when they were exposed to thermal cracking under different thermal conditions (673, 693 and 713 K), and treatment times (10, 20 and 30 minutes). Samples of asphaltenes were obtained from a Vacuum Residue (VR) of a mixture of Colombian crude oils. Samples were previously characterized by elemental analysis. Since the characterization of the aromatic substructure is important for this work, special attention was given to those regions in the 1H- and 13C-NMR spectra that showed the main changes: p.e. methyl hydrogens -in gamma position or more- to aromatic units HγCH3; methyl hydrogens -in beta positions- to aromatics units HβCH3; methylene hydrogens -in beta position- to aromatics HβCH2; hydrogens -in naphthenic units beta- to aromatics HβN; hydrogens -in paraffinic and alpha position naphthenic structures-in - to aromatics HαP,N; hydrogens in monoaromatic units HmaAr; hydrogens in polyaromatics HpaAr; carbons in methyl groups CCH3; carbons in methyl groups -in alpha position- to aromatics CArCH3; protonated aromatic carbons CHAr; pericondensed aromatic carbons CAAA; aromatic carbons linked to methyl groups; CArCH3 catacoWndensed aromatic carbons CAA and aromatic carbons linked to paraffinic or naphthenic structures C5Ar.