Air oxidation of the kerogen/asphaltene vanadyl porphyrins: an electron spin resonance study
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The thermal behavior of vanadyl porphyrins was studied by electron spin resonance during heating of kerogens, isolated from the La Luna (Venezuela) anti Serpiano (Switzerland) bituminous rocks, at 250 degrees C for 1 to 20 days in the presence of air. During the thermal treatment of the kerogens, the vanadyl porphyrins resonance signals decrease monotonically and become quite small after 6 days of heating. Concomitantly, new vanadyl signals appear and, at longer heating limes, dominate the spectrum. It is suggested that the secondary vanadyl species must have been formed from vanadyl porphyrins. Similar conversions of vanadyl porphyrins are observed under the same experimental conditions for asphaltenes extracted from the La Luna and Serpiano rocks, and floating asphalt from the Dead Sea (Israel). A comparison of the spin-Hamiltonian parameters for vanadyl porphyrins and the vanadyl compounds obtained during pyrolysis of the kerogens/asphaltenes suggests that the latter are of a non-po...rphyrin type. For comparison a study was conducted on Western Kentucky No. 9 coal enriched with vanadium ( GT 400 ppm) from six mines. AII the coal sam!,les:show only the presence of predominant by non-porphyrin vanadyl compounds, similar to those generated through laboratory heating of the kerogens/asphaltenes in ail: In addition, some samples also contain a minor amount of vanadyl porphyrins.