Vanadyl Tetraphenylporphyrin Research Articles

English

A nickel vanadyl molecularly imprinted nanofiber (NVMIN) was developed by the electrospinning technique, employing polyethylene terephthalate (PET) and linear polyethylenimine (L-PEI) as the matrix polymers, nickel tetraphenylporphyrin (NTPP) and vanadyl tetraphenylporphyrin (VTPP) as the mixed-template molecules. The fabricated NVMIN was evaluated as a sorbent for the removal of NTPP and VTPP from organic media. The effects of trifluoroacetic acid (TFA), dichloromethane (DCM), dimethyl sulphoxide (DMSO), pentane (PEN) on electro-spinnability of the PET/L-PEI solutions and the morphological appearance of the PET/L-PEI fibers were investigated qualitatively by means of a scanning electron microscope (SEM). Electrospinning of 14 to 30% w/v PET/L-PEI solutions in TFA produced beaded and smooth fibers, depending on the concentration range. The addition of DCM and PEN as modifiers helped improve the electrospinnability, with PEN being the best, while the addition of DMSO made the solutions not spinnable. The results indicated that the NVMIN showed higher affinity (99% higher) for NTPP than the non-imprinted nanofiber (NIN) indicating that the NVMIN contained a lot of specific binding sites. It was determined that the NVMIN exhibited the same selectivity specialism for both NTPP and VTPP. However, the interaction strength was strongly dependant on the type of solvent, where the strongest interaction was achieved in chloroform. In addition, the thermodynamic parameters calculated from the adsorption data suggested that the adsorption of NTPP onto NVMIN was a spontaneous and exothermic process. Key words: Electrospinning, mixed-templates, adsorption, demetallation, solvents.

Development of a Styrene Based Molecularly Imprinted Polymer and Its Molecular Recognition Properties of Vanadyl Tetraphenylporphyrin in Organic Media

A highly selective molecularly imprinted polymer (MIP) was synthesized and employed as an adsorbent material for the removal of vanadyl tetraphenylporphyrin (VTPP) from organic media. The maximum adsorption capacity of MIP (0.600 mg/g) toward VTPP was higher than that of the nonimprinted polymer (NIP; 0.281 mg/g). The accuracy of the method was validated by analyzing the certified reference material (CRM) NIST SRM 1634c, Trace Metals in Residual Fuel Oil, and the concentration of VTPP in the CRM was found to be 28.49 mg/kg, which was significantly comparable with the certified value of 28.19 mg/kg.

Electrochemically assisted demetallisation of model metalloporphyrins and crude oil porphyrinic extracts in emulsified media, by using active permeated atomic hydrogen

Electrochemically assisted metalloporphyrins demetallisation can be performed employing a unique palladium membrane without surface deactivation; in other words, palladium foils can be used several times. Vanadyl tetraphenylporphyrin (VO-TPP) or nickel tetraphenylporphyrin (Ni-TPP) dissolved in CH3Cl (nonemulsified media) are converted, producing hydrogenated porphyrins and/or free metal (Ni or V), by active permeated atomic hydrogen generated by applying −1.92mAcm−2 at 25°C. VO-TPP and Ni-TPP conversions are dramatically increased when reactions are carried out in the following Oil/Water (O/W) emulsified media (about 30% in emulsified media vs. 5% in nonemulsified medium): (i) CHCl3/H2O; (ii) CHCl3/1×10−5M HCl (pH=5); (iii) CHCl3/1×10−5M HCl (pH=5)+4×10−4M EDTA and (iv) CHCl3/4×10−4M EDTA. Moreover, VO-TPP and Ni-TPP demetallisation percentages depend on the water phase nature, the highest demetallisation percentage (71%) is achieved using HCl+EDTA as water phase. Crude oil porphyrinic extracts seem to be less reactive than model metalloporphyrins.

Electrolysis of V(IV) and free meso -tetraphenyl porphyrin in two immiscible liquids

The electrochemical decomposition of free and vanadyl tetraphenyl porphyrin in xylene solutions, and in the presence of water, using K2SO4 as electrolyte, was studied. Electrolysis of the two-phase system was carried out at constant current density and potential. It was observed that the electrochemical decomposition of the porphyrins, using a two-phase liquid system, is possible. An inexpensive organic solvent (xylene instead of THF, acetonitrile, methylene chloride etc.) can be very effectively used in such a system. The decomposition occurs through an oxidation process, with the subsequent formation of cationic and dicationic radicals, and a final breakdown of the macrocycle.

Hydrogenation of nickel and vanadyl tetraphenylporphyrin in absence of a catalyst: A kinetic study

The hydrodemetallation (HDM) reactions of nickel-5,10,15,20-tetraphenylporphyrin (Ni-TPP) and vanadyl-5,10,15,20-tetraphenylporphyrin (VO-TPP) were studied in the absence of a catalyst. Ni-TPP as well as VO-TPP is found to demetallate through a reversible sequential reaction mechanism in which similar hydrogenated intermediate species are formed. As compared to catalized HDM, reaction rates are low. Hydrogenation reactions of Ni-TPP are first order with respect to the liquid phase concentration of hydrogen. Hydrogen sulfide promotes the conversion of Ni-TPP. An equilibrium constant for the reversible hydrogenation of Ni-TPP to nickel-5,10,15,20-tetraphenylchlorin (Ni-TPC) has been estimated as a function of temperature. A heat of reaction of −97 kJ/mol was found for the reversible hydrogenation of Ni-TPP to Ni-TPC. Comparison of the reactivities of a metal-free porphyrin (H 2-TPP), Ni-TPP and VO-TPP revealed that the metals have a stabilizing influence towards hydrogenation and ring fragmentation of porphyrin macrocycles.

Structural Determination by XAFS Spectroscopy of Non-Porphyrin Nickel and Vanadium in Maya Residuum, Hydrocracked Residuum, and Toluene-Insoluble Solid

The local structure of non-porphyrin Ni and V in Maya residuum, hydrocracked residuum, and toluene-insoluble solid were determined by XAFS spectroscopy. In residuum, the first coordination shell of non-porphyrin V is similar to that for vanadyl tetraphenylporphyrin with four N at a distance of 2.08 Å and one O at 1.67 Å. Similarly, the first coordination shell of Ni non-porphyrin is similar to that in nickle tetraphenylporphyrin, i.e., four N at a distance of 1.90 Å. The Ni and V coordination geometries are remarkably stable toward thermal cracking, catalytic hydrogen, and H2S. After hydrocracking to about 60% conversion of the residuum, the local coordination of Ni and V in the hydrocracked residuum is little changed. During hydrocracking, approximately 1% of the residuum is converted to toluene-insoluble solid, which contains high levels of Ni and V. Approximately 10% of the Ni and V in the solid is still present in a porphyrin-type coordination geometry, while the remaining Ni and V has an octahedral coordination. In the latter, the V coordination contained one O at 1.64 Å, four N at 2.14 Å, and one N (or O) at 1.96 Å, and the Ni coordination contained six N at 2.09 Å. Ni or V sulfides are not formed in the toluene-insoluble solid.

A Resonance Raman Spectroelectrochemical Study of the Zn(II) Tetraphenylchlorin Anion

Zinc(II) tetraphenylchlorin (ZnTPC) and its π-anion radical have been studied by UV−vis absorption and resonance Raman (RR) spectroscopies. Analyses of the RR spectra were aided by data for the pyrrole-d8, meso-C13, and phenyl-d20 isotopomers. Upon formation of the anion, significant frequency shifts are observed in skeletal vibrational modes, while substituent modes remain essentially unshifted. Both frequency upshifts (ν37, ν10, and ν19) and downshifts (ν2, ν11, and ν3) occur for the CαCm and CβCβ stretching modes in the π-anion. Modes involving significant CαCβ stretching motion, ν4 and ν41, upshift in the anion spectrum. The pattern of vibrational frequency shifts for the anion of ZnTPC is similar to what is observed for the anion of vanadyl tetraphenylporphyrin ((VO)TPP) despite the reduction of a pyrrole ring in TPC. This resemblance is due to the Jahn−Teller distortion in (VO)TPP-, which gives its singly occupied molecular orbital the same bonding pattern as in ZnTPC-.

Recombination process via gap-states responsible for photocurrent synergism observed with porphyrin solids

Synergistic effect is observed for photovoltaic cells of vanadyl tetraphenylporphyrin illuminated simultaneously with visible (VIS) and near-infrared (NIR) lights. When the degree of synergism (η) is measured as a function of wavelength of VIS light under illumination with constant NIR light, the η spectrum exhibits a distinct peak at 430 nm where a dip is seen in a photocurrent action spectrum. The finding strongly supports the idea that the photocurrent synergism is closely associated with a recombination process of electron-hole pairs via energy states lying within a bandgap of the molecular solid.

Electron Spin Resonance Linewidth Studies of Vanadyl Tetraphenylporphyrin

The linewidths of the hyperfine components of the ESR spectra of vanadyl tetraphenylporphyrin in liquid toluene have been measured as a function of temperature and analyzed in detail. This has allowed the separate contributions to the observed ESR line widths from spin rotational, g tensor anisotropy, hyperfine anisotropy and unresolved nitrogen hyperfine coupling to be evaluated. These results have been used to estimate the size of the nitrogen isotropic hyperfine coupling constant and thus the extent of delocalization of the unpaired electron onto the nitrogen atoms.

X-ray absorption study of vanadium on regenerated catalytic-cracking catalysts

Vanadium deposited at a level of 8000 ppm on SiO 2 Al 2O 3, rare-earth exchanged Y zeolite in a silica-alumina matrix, NdY, and USY by impregnation with vanadyl tetraphenylporphyrin was studied by extended X-ray absorption fine structure (EXAFS) after calcination and after further steaming. Catalyst preparations were chosen to simulate conditions within a fluid catalytic cracking unit. In all cases, radial structure functions obtained from the EXAFS analysis were very similar and showed only a single peak due to coordination of vanadium to nearest neighbor oxygen. This is interpreted to mean vanadium exists as highly dispersed, perhaps isolated, VO x units where x is probably near four.

Vanadyl tetraphenyl porphyrin adsorption on oxides and on Mo/Al 2O 3 catalysts: An ESR study

The adsorption interaction of vanadyl-tetraphenylporphyrin (VO-TPP) with oxides (MgO, bimodal Al 2O 3, SiO 2 and SiO 2-Al 2O 3) and with Mo/Al 2O 3 catalysts in the oxidized (Mo12Al-ox.), reduced (Mo12Al-red.) and sulfided (Mo12Al-sulf.) state as well as with NiMo/Al 2O 3 in its reduced form (Ni3Mo12Al-red.) was studied by ESR. An air-sensitive and an air-stable vanadium signal was detected for VO-TPP adsorbed on Al 2O 3, on Mo/Al 2O 3 in its oxidized, reduced and sulfided state as well as for Ni3Mo12Al-red. Evidence is presented that the porphyrin adsorption preferentially occurs on the uncovered Al 2O 3 surface on these catalysts. For the VO-TPP/MgO and the VO-TPP/SiO 2-Al 2O 3 systems only one adsorbed VO-TPP species could be monitored in nitrogen corresponding as regards its ESR parameters to the air-stable form of adsorbed VO-TPP obtained for the samples mentioned above. It is suggested that VO-TPP is bound to the catalysts by donor—acceptor interaction via the vanadyl—group on the one hand and through the π-system of the porphyrin macrocycle on the other hand. For the air-stable adsorbed VO-TPP species, interaction of V 4+ with Lewis basic surface sites and simultaneously of the porphyrin macrocycle with surface acceptor sites (Lewis- or Bro˝nsted-acid sites) is postulated. An influence of support acidity on the ESR parameters of adsorbed VO-TPP was observed. This adsorption mechanism was previously suggested by Raman and optical spectra. The air-sensitive ESR signal can be attributed to VO-TPP adsorbed via the vanadyl oxygen with the π-system of the porphyrin macrocycle strongly interacting with Lewis acid surface sites. Pyridine poisoning prevented the formation of the air-sensitive VO-TPP species.

In situ preresonance raman and opticaL spectra of vanadyl tetraphenylporphyrin adsorbed on Mo/Al 2O 3 catalysts

The adsorption interaction of vanadyl tetraphenylporphyrin (VO-TPP) with a bimodal γ-Al 2O 3 and molybdenum catalysts was studied by in situ preresonance Raman and optical spectroscopy. Mo/Al 2O 3 in the oxidized state (Mo12Al-ox.) and its reduced (Mol2Al-red.) and sulphided (Mo12Al-sulph.) forms as well as NiMo/Al 2O 3 in its reduced form (Ni3Mo12Al-red.) were used as catalysts, and the decomposition reactions of VO-TPP in hydrogen and moist oxygen were monitored. The main conclusions are that VO-TPP is bound to the catalysts via a donor-acceptor interaction through the vanadyl group. V 4+ interacts with donor (Lewis base) sites by charge transfer from the surface to the porphyrin ring. The acceptor (Lewis acid) sites on the surface are responsible for the reduction of excess negative charge in the ring via π-bonding interactions. Under the experimental conditions applied, thermal decomposition of porphyrin adsorbed on Mo/Al 2O 3 catalysts starts at about 470 K. At about 520 K and higher temperatures, coke formation during H 2 treatment is detected by Raman spectroscopy. Regeneration of the catalysts by heating them in a stream of moist oxygen at 770 K could be achieved as evidenced by both Raman and optical spectroscopy.

KINETICS OF TRIPLET OXIDATION OF METALLO‐PORPHYRIN COMPOUNDS TO THEIR CORRESPONDING RADICAL CATIONS

Abstract— The kinetics of photooxidation of triplets of metalloporphyrin compounds to their corresponding radical cations was investigated. Zn‐tetraphenyl porphyrin (ZnTPP) and Mg‐tetraphenylpor‐phyrin (MgTPP) triplets were oxidized by europium salt with rate constants of 4.8 × 105M‐1s‐1 and 2.1 × 106M‐1s‐1, respectively. The high rate constant of oxidation of MgTPP triplet might be related to the ground state oxidation potential, being 0.54 V (SCE) for the Mg complex and 0.71 (SCE) for the Zn complex.The rate constant of oxidation of ZnTPP excited singlet is in the order of diffusion control, i.e. ˜ 1010M ‐1 s‐1. Excitation of ferric, cupric, cobaltic, and vanadyl tetraphenylporphyrin did not result in a long‐lived triplet state that would allow oxidation studies using flash photolysis.

The determination of metals in petroleum samples by atomic absorption spectrometry: Part III. The effect of added alkali metals on the determination of vanadium in organic solutions

The effects of potassium or sodium added as the cyclohexanebutyrate and of potassium added as the Conostan standard, on the absorbance of vanadium present as vanadyl tetraphenylporphyrin, acetylacetonate, or benzoylacetonate, or as Conostan vanadium standard in xylene solutions, or in real petroleum samples are described. The effect of the alkali metal depends both on the kind of vanadium compound and on the type of alkali metal compound added. There is no analogy between the behaviour of vanadium in aqueous and xylene solutions containing large amounts of easily ionizable metal. Addition of potassium (or sodium) for the determination of vanadium in xylene solutions cannot be recommended.