1H-AND 13C-NMR STRUCTURAL CHARACTERIZATION OF ASPHALTENES FROM VACUUM RESIDUA MODIFIED BY THERMAL CRACKING

H-1- and C-13-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 H-1- and C-13-NMR spectra that showed the main changes: p.e. methyl hydrogens -in gamma position or more- to aromatic units H-CH3(gamma); methyl hydrogens -in beta positions- to aromatics units H-CH3(beta); methylene hydrogens -in beta position- to aromatics H-CH2(beta); hydrogens -in naphthenic units beta- to aromatics H-N(beta); hydrogens -in paraffinic and alpha position naphthenic structures-in - to aromatics H-P,N(alpha); hydrogens in monoaromatic units H-Ar(ma); hydrogens in polyaromatics H-AR(pa); carbons in methyl groups C-CH3; carbons in methyl groups -in alpha position- to aromatics C-CH3(Ar); protonated aromatic carbons C-Ar(H); pericondensed aromatic carbons C-AAA; aromatic carbons linked to methyl groups; C-Ar(CH3) catacoWndensed aromatic carbons C-AA and aromatic carbons linked to paraffinic or naphthenic structures C-Ar(S).