Fractionation of asphaltenes in n-hexane and on adsorption onto CaCO3 and characterization by ESI(+)FT-ICR MS: part I

Abstract

Two methods of asphaltenes fractionation have been employed to facilitate the characterization of their respective subfractions. The methods are based on step-wise precipitation with different n-hexane/crude oil ratios, and on adsorption onto CaCO3. Three subfractions were produced for each method, being named of 3.5 V, 3.5–6 V, and 6–40 V (for the first method); and non-adsorbed (bulk), adsorbed, and irreversibly adsorbed (for the second method). The fractions were characterized by elementary analysis, nuclear magnetic resonance of proton (1 H NMR) and by positive ion-mode electrospray Fourier transform ion cyclotron resonance mass spectrometry (ESI(+)FT-ICR MS). The elemental analysis, described in previous work, revealed that the C/H ratio for whole asphaltene and its sub-fractions varied between a narrow range (0.83–0.88) which means they present similar aromaticity or unsaturation. Furthermore, the elemental analysis corroborates with the ¹H NMR analysis suggesting that subfraction 6–40 V presented a more aromatic profile than of remaining subfractions, while for the fractionation using CaCO3, this behavior was observed for the adsorbed subfraction. However, a more detailed molecular information was obtained from ESI(+)-FT-ICR MS data, showing that polar compounds species with lower carbon numbers were mainly found for the irreversibly adsorbed subfraction. Besides, the double bond equivalent (DBE) distribution is an important tool to associate the chemical information with solubility parameters, in which, a narrower DBE distribution was observed for irreversibly adsorbed (for fractionation onto CaCO3) and subfraction 3.5 V (fractionation in n-hexane) samples, indicating that they are less soluble in hydrocarbons. Also, solubility parameters (δ) were calculated from ESI(+)FT-ICR MS data, where the results indicate that subfractions produced in n-hexane have a lower tendency to precipitate in hydrocarbons in relation to subfractions produced onto CaCO3.