Experimental and Theoretical Approach to Determine the Average Asphaltene Structure of a Crude Oil from the Golden Lane (Faja de Oro) of Mexico

The structural parameters and average molecular structures of the asphaltenes obtained from the Aguacate oilfield, located in the Golden Lane of Mexico, have been investigated combining experimental analysis and molecular simulation. The average molecular structural parameters of the polycyclic aromatic hydrocarbon (PAH) region, average number of fused aromatic rings (nFAR), average structural isomers in the polydispersity of the PAH core, average architecture, average molecular weight, and substituents in the PAH core have been determined by means of 13C single-pulse excitation (SPE) nuclear magnetic resonance (NMR) in combination with 13C distortionless enhancement by polarization transfer (DEPT)-135° experiments, 1H NMR, X-ray photoelectron spectroscopy, fluorescence emission, and matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry. The total energy of the PAH isomers and their fluorescence emission are calculated with density functional theory and ZINDO/S, respectively. The PAH region structural parameters determined are (a) Y-carbons (CY) or internal triple bridgehead aromatic carbons, (b) external peripheral aromatic carbon atoms at the junction of two fused rings (CAP3), (c) aromatic carbon atoms bonded to hydrogen atoms (CAH), (d) aromatic carbon atoms bonded to heteroatom (CAX), (e) aromatic carbon atoms bonded to hydrogen at the β position with respect to the heteroatom (CAHβX), and (f) substituted aromatic carbon atoms (CA-Caliph). The 13C NMR chemical shift ranges used for the different structural carbon atoms in the PAH core were obtained from our previous study [Ruiz-Morales, Y.; Miranda-Olvera, A. D.; Portales-Martĺnez, B.; Domĺnguez, J. M. Determination of 13C NMR Chemical Shift Structural Ranges for Polycyclic Aromatic Hydrocarbons (PAHs) and PAHs in Asphaltenes: An Experimental and Theoretical Density Functional Theory Study. Energy Fuels 2019, 33 (9), 7950-7970, DOI: 10.1021/acs.energyfuels.9b00182]. There is overlapping of the 13C SPE NMR signals of CAH, CY, and CAP3; therefore, quantitative 13C DEPT-135° NMR is necessary to determine the average CAH, and this value is subtracted from the quantitative 13C SPE NMR spectrum, where the signals overlap, to obtain the average number of bridgehead carbons (CY and CAP3). It is concluded that island is the predominant architecture of the Aguacate asphaltene, in accordance with the Yen-Mullins model. The PAH region contains nine fused aromatic rings (9FAR), where one of the rings is a thiophenic ring.