Characterization of Maya Crude Oil Maltenes and Asphaltenes in Terms of Structural Parameters Calculated from Nuclear Magnetic Resonance (NMR) Spectroscopy and Laser Desorption−Mass Spectroscopy (LD−MS)

Abstract

Fractions of Maya (Mexico) crude oil were examined by quantitative liquid-state 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. The NMR data was combined with molecular-mass estimates determined in a previous work to calculate average structural parameters. The approach clearly showed structural differences between the fractions and allowed for the inference of detailed information on the chemical structures involved. The sample of crude oil was first fractionated into maltenes and asphaltenes, and the asphaltenes were further sub-fractioned into 1-methyl-2-pyrrolidinone (NMP)-soluble (labeled as “MNS”) and NMP-insoluble (labeled as “MNI”) fractions. The aromatic rings per average molecule value derived were 2−5 for the MM sample, 8−10 for the MA sample, ∼5 for the MNS sample, and 11−38 for the MNI sample. Thus, while some differences between maltenes and asphaltenes could be expected from prior general knowledge, some results were unexpected. The MNS fraction turned out to be more aliphatic than the MNI fraction. On average, the MNS sample appeared to contain a greater number of smaller aromatic cores, linked by means of biphenyl-like aromatic−aromatic single bonds and aliphatic/naphthenic bridges, and shorter aliphatic side chains. If the aromatic groups are in external positions, this would account for the solubility of these species in NMP (a more open structure). Meanwhile, results for the MNI sample strongly suggest fewer, larger aromatic cores per average molecule, linked by long alkyl side chains and naphthenic structures. The alkyl substituents would appear to form a barrier between the aromatic core and the solvent. These findings have implications regarding how different structural sub-groups within an asphaltene will respond to ultraviolet (UV) laser desorption−mass spectrometry (LD−MS) and UV-fluorescence spectroscopy.