Petroporphyrins were enriched and purified from atmospheric residues of two typical heavy oils, Canadian oil sand bitumen (OSAR, Ni: 80 ppm, V: 190 ppm, S: 3.97 wt%) and Chinese Liaohe heavy oil (LHAR, Ni: 68.7 ppm, V: 1.81 ppm, S: 0.36 wt%) by silica gel chromatography. The separation and purification were confirmed by atomic absorption spectroscopy (AAS) combined with UV-vis spectroscopy, and the petroporphyrins were characterized by positive-ion electrospray ionization (ESI) Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). Vanadyl and nickel porphyrins in OSAR are simultaneously identified by mass measurement and isotopic fine structure. Vanadyl porphyrins with structures of N 4 VO, N 4 VO 2 and N 4 VOS are all detected as protonated analyte ([M+H] + ). Both molecular ion (M +• ) and analyte ([M+H] + ) as well as their corresponding isotopes are observed for N 4 Ni porphyrins in OSAR and LHAR with an average mass resolving power of over 400 000 (m/∆m 50% ). This is rarely detected by FT-ICR MS using ESI technique previously. Formation of molecular ion can be attributed to the low oxidation potential of nickel porphyrins, effect of oil matrix on the solution conductivity and the relatively low flow rate of solution into the capillary. Three more highly unsaturated types of N 4 VO porphyrins were identified in addition to the six well-documented structures. Compared to N 4 VO porphyrins, N 4 VOS porphyrins present higher DBE ranging from 21 to 27 while N 4 VO 2 porphyrins show lower DBE ranging from 18 to 20 and narrower carbon number distribution, suggesting possible different origins of sulfur (pyrolysis of kerogen) and oxygen (diagenesis of chlorophyills). Ni/V and ratio of relative abundance of ETIO porphyrins to DPEP porphyrins (∑ETIO/∑DPEP) for nickel porphyrins indicate that Liaohe oil and Canadian oil sand bitumen are continental and marine sediment, respectively, and Liaohe oil has a higher maturity.Enrichment by the simple chromatographic method facilitates the mass spectral identification of nickel porphyrins even for heavy residue with low content of nickel and high content of sulfur.
Anthracene was used as a chemical probe to evaluate hydrogen donating abilities (HDAs) of two petroleum vacuum residues and their SARA fractions (i.e., saturates, aromatics, resins, and asphaltenes), and hydrogen donating kinetics of aromatics and resins were then analyzed. Also, 9,10-dihydroanthracene was used as a chemical probe to evaluate hydrogen accepting abilities of the asphaltenes of the residues. Coking propensities of both residues under thermal processing at 400 °C were evaluated by their coke induction periods. Results show that HDA of either residue proceeds to increase at first and then tends to decline under thermal processing, forming a maximum in the middle. The HDA peak value increases progressively with temperature increasing; the two residues may exhibit either different or similar HDAs at certain test conditions. When the four SARA fractions coexist in the form of a residue for further thermal processing, there exhibits synergism in HDA among the four SARA fractions. Hydrogen donating of aromatics and resins can be treated by first-order kinetics, and both rate constant and initial rate in hydrogen donating for resins show much higher values than those for aromatics. Asphaltenes accept substantially more hydrogens than the amounts they donate. A comprehensive analysis of the data thus obtained shows hydrogen transfer among the SARA fractions is essentially related to coking propensity of residue under thermal processing, where asphaltenes accept hydrogens from resins in the immediate neighborhood that are then supplemented by aromatics. Donatable hydrogens in asphaltenes alone appear insufficient to prevent asphaltenic radicals from combining to form coke. A residue whose asphaltenes accept more hydrogens with resins and aromatics releasing fewer hydrogens exhibits a higher coking propensity under thermal processing. Hydrogen donor/acceptor additives may serve to suppress/promote coke formation by influencing the coking rates of asphaltenes in the center by supplementing/depleting donatable hydrogens of the surrounding medium constituents, that is, resins and aromatics.
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