A comparative analysis
of the composition of vanadyl porphyrins
isolated from heavy oil using two different sulfocationites has been
carried out. As a source of vanadyl porphyrins, heavy oil of Volga-Ural
basin characterized by a high vanadium content was used. The N,N-dimethylformamide extract of asphaltenes
was derived from this oil and subjected for isolation of primary vanadyl
porphyrin concentrate on a SiO2 column, which was then
chromatographically purified with sulfocationite by our improved method.
Strongly acidic cation-exchange resin and asphaltene sulfocationite
recently developed in our laboratory were used as sulfocationites.
According to ultraviolet–visible spectroscopy, both sulfocationites
showed excellent applicability for purification, providing isolation
of a broad (>50%) fraction of vanadyl porphyrins with higher spectral
purity compared to results of conventional methods. Results of matrix-assisted
laser desorption/ionization time-of-flight mass spectrometry analysis
showed that composition of isolated vanadyl porphyrins depends upon
the chemical nature of sulfocationite. Despite the same range of vanadyl
porphyrin homologues (C26–C40, with maximum
falling on C32) isolated by both sulfocationites, purification
with asphaltene sulfocationite resulted in a 1.4-fold decrease in
the content of the most abundant DPEP type of vanadyl porphyrins,
with a corresponding 1.1–1.9-fold increase in the content of
the rest of the types. It was also established that, when purification
is accomplished, a significant part of the same vanadyl porphyrins
still remains in the column, which can be explained by their associations
with non-porphyrin components of the oil.
Comparative analysis of the composition
and properties of heavy
oils from various oil fields has revealed an inverse dependence of
the asphaltene/resin ratio upon the vanadium content in resins. This
indicates the diminishment of asphaltene precipitation ability with
an increase in the content of vanadyl complexes in heavy oil resins.
Experiments have shown that the addition of vanadyl porphyrins to
resins enhances their inhibition activity toward asphaltene precipitation.
For this purpose, vanadyl porphyrins have been concentrated by N,N-dimethylformamide extraction from resins
and additionally purified by column chromatography. The obtained vanadyl
porphyrin concentrate has been added to resins at the ratio of 1–5
wt %, and its influence on asphaltene stabilization has been further
evaluated. Evaluation has been performed by measuring the optical
density of deasphalted oil obtained by dilution of the crude oil with
20-fold excess of n-hexane, where resins with various
contents of vanadyl porphyrins have been initially dissolved. The
change in composition and properties of precipitated asphaltenes has
been analyzed, and an increase in their absorbance, aromaticity, and
degree of ring fusion with the growth of the vanadyl porphyrin content
in resins has been demonstrated. Results have shown the efficiency
of vanadyl porphyrins in resins as inhibitors of asphaltene precipitation.
The
resins and asphaltenes of three heavy oils differing in origin
(Permian and Carboniferous) and vanadium content (0.025–0.165
wt %) have been studied as sources for isolation of spectrally pure
vanadyl porphyrins by sulfocationite-based chromatographic method
developed by us recently. This method consists of removing the low
polar nonporphyrin components from the resins or DMF extract of asphaltenes
on SiO2-column followed by chromatographic isolation of
vanadyl porphyrins on the sulfocationite. The asphaltenes were revealed
to be a more promising source of vanadyl porphyrins because they possess
≥5 times higher content of vanadium, provide better accumulation
of vanadyl porphyrins during deposition extraction by N,N-dimethylformamide, and are less contaminated by low polar nonporphyrin
compounds capable of coeluting with vanadyl porphyrins through the
sulfocationite column. According to matrix-assisted laser desorption/ionization
time-of-flight mass spectroscopy, DPEP vanadyl porphyrins were found
to be the most abundant type for all studied samples (34.1–54.5%).
Rhodo vanadyl porphyrins belong to minor components (3.3–8.7%
for each subtype) while Etio and Di-DPEP types take the intermediate
position (9.8–28.7%). The resins and asphaltenes of the same
oil showed significant difference in the group composition of purified
vanadyl porphyrins. For the resins, a ∼1.3-fold decreased content
of DPEP vanadyl porphyrins was found, which was compensated by a ∼1.5-
and/or ∼1.8-fold increase in the content of Etio and Rhodo
vanadyl porphyrins, respectively. However, this change in the composition
of vanadyl porphyrins is not accompanied by a notable change of their
average molecular weight despite the fact that increased concentration
of more substituted (i.e., more hydrophobic) vanadyl porphyrins could
be expected for less polar resins. This fact was interpreted in favor
of association of vanadyl porphyrins with nonporphyrin components
of the oil.
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