Trapping of metallic porphyrins by asphaltene aggregates: A size exclusion microchromatography with high-resolution inductively coupled plasma mass spectrometric detection study
Résumé
A combined liquid chromatography coupled to a mass spectrometer with an ICP detector (μSEC-HR ICP MS; μSEC ICP for brevity) technique was used to analyze the metals in four asphaltenes and their corresponding A1 (toluene insoluble), A2 (toluene soluble), and trapped compound (TC, heptane soluble) fractions. For three of the asphaltene samples, the normalized μSEC ICP profiles for both nickel and sulfur were very similar, showing that nickel porphyrins were distributed in almost all types of asphaltene aggregates. Extensive overlapping with sulfur profiles was observed for all vanadium and nickel profiles at retention times below the maximum bands. This suggests that large amounts of nickel and other organometallic or metal-porphyrin-type (MP) compounds are interlocked with asphaltene molecules, forming aggregates in solution. The separation of MP compounds using common separation techniques is very difficult as extraction would require dissociation into several molecules. The presence of TCs (e.g., compounds other than asphaltenes that are soluble in n-heptane) in asphaltene aggregates was related to the fractal structure of asphaltene aggregates in which voids are filled with components coming from the surrounding media. Apparently, complete trapping of TCs is achieved by performing aggregate rearrangement after penetration, leading to an aggregate structure in which the TCs remain trapped. A similar trapping mechanism is proposed herein for the MP compounds. Accordingly, no covalent bonds or specific interactions appear to be required to account for the presence of MPs within asphaltene aggregates.