Vanadyl porphyrin

PTLC may be also applied to obtain very narrow organic compoimd groups such as petroporphyrins. The most simple approach to isolate nickel and vanadyl porphyrin fractions is to use sihca gel TLC plates and hexane-dichloromethane (1 1 v v) or toluene-chloroform (3 1 v v) as a developer. The Rf values for nickel and vanadyl porphyrins are 0.40 to 0.85 and 0.05 to 0.40, respectively [106-109]. 

Pena, M. E. Manjarrez, A., and Campero, A., Distribution of vanadyl porphyrins in heavy crude oil a Mexican offshore. Fuel Process. Technol., 1996. 46 pp. 171-182. 

EPR and ENDOR spectra very similar to those found in VO(TPP) have also been observed2885 in a sample of a black solid obtained by chloroform extraction of a phosphate mineral from Youssoufia (Morocco). The close correspondence of the spectra suggests that the black solid contains a vanadyl porphyrin derivative. 

In solution, vanadyl porphyrins [VOP] can form adducts [VOPX] in the presence of a large excess of X (X = py or piperidine) but the equilibrium constants are rather small.879-881... 

Hydrodemetallation reactions require the diffusion of multiringed aromatic molecules into the pore structure of the catalyst prior to initiation of the sequential conversion mechanism. The observed diffusion rate may be influenced by adsorption interactions with the surface and a contribution from surface diffusion. Experiments with nickel and vanadyl porphyrins at typical hydroprocessing conditions have shown that the reaction rates are independent of particle diameter only for catalysts on the order of 100 /im and smaller (R < 50/im). Thus the kinetic-controlled regime, that is, where the diffusion rate DeU/R2 is larger than the intrinsic reaction rate k, is limited to small particles. This necessitates an understanding of the molecular diffusion process in porous material to interpret the diffusion-disguised kinetics observed with full-size (i -in.) commercial catalysts. 

Vanadyl porphyrin interaction with the surface is a function of the catalyst. Adsorption through electron acceptor sites dominates on the oxide surface, whereas the sulfided catalyst interacts through electron donor sites (see Section IV,B,5). Heats of adsorption have been estimated to be 8 to 12 kcal/mole. Values in this range are indicative of weak adsorption interactions that are of reduced importance at hydroprocessing conditions. 

Limited experimental data for nickel porphyrins indicate weaker adsorption than their vanadyl analogs, which is consistent with the oxygen ligand on vanadium playing an active role. With this difference it stands to reason that competitive adsorption contributes to the inhibition observed in nickel demetallation rates in the presence of vanadyl porphyrins. Therefore, the higher vanadium reactivity relative to nickel can be rationalized on the... 

Figure 7. Luminescence spectra of vanadyl porphyrin on LaY (A) before and (B) after calcination and (C) after steam aging. Excitation wavelength was set at 310 nm.

Figure 10. Luminescence spectrum of calcined vanadyl porphyrin on LaY at 77K with the excitation wavelength set at 468 nm.

Figure 11. Luminescence spectra of tin loaded LaY impregnated with vanadyl porphyrin (A) calcined and (B) steam aged. The excitation wavelength was set at 468 nm.

AAA-alumina (75% Si02/25% AI2O3 aluminosilicate gel provided by Davison) was used without further pretreatment. Benzene solutions of either vanadyl porphyrin (5, 10, 15, 20-tetraphenyl-21H, 23H-porphine vanadium oxide) or vanadyl naphthenate were impregnated by incipient wetness onto the EuY or the gel to generate a loading of 1.0 wt % vanadium. Vanadyl naphthenate has been widely utilized as a model V-contaminant and metalloporphyrins have been identified as metallic species in crude oil (29-30). The solvent was removed under... 

When calcination was performed at 470°C, the sample turns light yellow, indicating the conversion of intermediates probably into oxides and other species, accompanied by further V migration to the gel (Fig. 11c). Luminescence measurements from our laboratory (33) have shown that calcination at 470°C results in complete disappearance of vanadyl porphyrin peaks and appearance of some new excitation peaks which can not be observed in the corresponding excitation spectrum of the EuYV(p)AAAC sample. These data suggest that intermediates exist after calcination and that these species may play a critical role during V migration. 

The nickel concentration on this particle is extremely low with traces of the nickel impurity at the external surface. This low nickel concentration means that the particle in Figure 2 was in the refinery unit only a short time, otherwise more nickel would be present for the average particle (by bulk chemical analysis) contains 900ppm Ni and 4700 ppm V. The particle in this image contains a disproportionate amount of vanadium relative to nickel. If the nickel on the external skin of the catalyst accumulates by nickel porphyrin cracking at the first surface contacted, then vanadium must arrive not only by cracking vanadyl porphyrins but also by some other means like transfer from older catalyst particles in the FCC unit. The high vanadium concentration relative to nickel on new catalyst particles provides evidence that vanadium has interparticle mobility as well as intraparticle mobility. 

Extended X-ray absorption fine structure (EXAFS) and K edge absorption spectroscopy have been used to study vanadium in coaI7). In contrast to the common belief that vanadyl porphyrins are the major compounds present, these studies demonstrate that in the high vanadium content samples examined, the metal is tetravalent and coordinated to oxygen ligands. Similar investigations have been carried out on the vanadocytes of aci-dians to identify the form of vanadium in tunicate blood cells22 (vide infra). 

Vanadyl porphyrins, VOP, mentioned above [see (5)] have been used as precursors for nonoxo vanadium(rv) porphyrins, [VPX2] (P = OEP, TPP, TMTP, TPTP X = Cl or Br). EXAFS at the Br K-edge has been used to coirfirm the trans configuration of X atoms in the complexes. The bare atom renders the compounds extremely reactive and one theory is that they could act as precursors in the synthesis of complexes formed during coal conversion. ... 

The poisoning of fluid cracking catalysts (FCC) by vanadium is well known (7, 2). In general, vanadium is deposited on the cracking catalyst as coke by vanadyl porphyrins in the feed. During regeneration, the coke is burned off, and vanadium can be oxidized to the V+5 oxidation state. Woolery et al. (3) have shown that the oxidation state of the vanadium can alternate between +4 to +5 in... 

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