An evaluation of the aromaticity of asphaltenes using atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry – APPI(±)FT-ICR MS

Abstract

Asphaltenes can be considered like the ‘bad guys‘ when heavy petroleum are transported or upgraded. These compounds are not classified by the their chemical structures but by the solubility class, that is defined as solubles in aromatic solvents, such as toluene, and insolubles in n-alkanes, such as n-heptane. Here, we analyze three crude oil samples (classified as extra-heavy, heavy and asphaltic having API degree of 17.3, 22.0 and 13.5, respectively) and their asphaltenes by atmospheric pressure photoionization (APPI) coupled to Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in positive and negative ionization modes, APPI(±)-FT-ICR MS. We apply the DBE (double bond equivalent) versus carbon number (CN) plots as the aim of to predict the aromaticity of crude oil and their asphaltenes. Additionally, average molecular weight, Mw, heteroatomic-containing compounds profile and van Krevelen diagrams were constructed to visualize and interpret the MS data. MS data were compared and discussed with aromaticity parameters obtained from elemental analyses and 1H NMR measurements. For three crude oils analyzed with different magnitude of the viscosities (III>II>I), the amount of the asphaltene extracted follower the behavior analogous (7.63±0.65wt%; 2.23±0.25wt%; and 0.41±0.01wt%, respectively). The aromatic hydrogen content (Har, in molar%) was determined from 1H NMR, where the heavier crude oil and its asphaltene (Har=4.90 and 27.3M%, respectively) were slightly more aromatic than other. An inverse correlation was observed between Har values and the N and O concentrations determined from elemental analyses. For APPI(±)FT-ICR MS data, the values of Mw observed for asphaltenes (m/z 200–650) were significantly distinct, being correlated with the physico-chemical characteristics of their parent crude oil. Using the concept of planar limited, the line generated by connecting maximum DBE values at given CN in the DBE versus carbon number plots, it was possible to extract the aromaticity degree from APPI FT-ICR MS data. The slopes of these lines determined by DBE/carbon number ratio were calculated by linear regression for protonated hydrocarbons (HC[H]) and basic nitrogen (N[H]) compounds classes. We observed an increase in slopes in function of an increasing of the Har values.