Structural characterization by X-ray absorption spectroscopy (EXAFS/XANES) of the vanadium chemical environment in Boscan asphaltenes

Abstract

X-Ray absorption spectroscopies, EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near-edge structure), were used for probing the structural and chemical environment of vanadium in two different asphaltenic fractions of a Boscan crude oil. Although the level of porphyrins detectable by u.v.–visible spectrophotometry was substantially reduced (i.e. <15% from the 4 000 p.p.m. of metal) due to successive solvent extractions, the EXAFS spectra of these fractions were still largely dominated by the typical pattern of vanadyl porphyrins, whereas the intense prepeak observed in the XANES spectra also clearly supported the presence of quite large amounts of porphyrins in these fractions. We thus suggest that the u.v.–visible spectrophotometric analyses tend systematically to underestimate the porphyrinic content in these heavy fractions, probably due to some hardly detectable microheterogeneity of the test solutions. E.s.r. spectra are produced which clearly support these various conclusions. Much more elaborate separation procedures are required in order to isolate and identify any ‘non-porphyrinic vanadium fraction’ from Boscan crudes.