Applications of Pulsed Electron Paramagnetic Resonance


Publication Date (Web): June 16, 2015. Copyright © 2015 American ... S. Orlinskii. IOP Conference Series: Earth and Environmental Science 2018 155, 0...
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Applications of pulsed EPR spectroscopy to the identification of vanadyl complexes in asphaltene molecules. Part I: Influence of the origin of the feed. Karima Ben Tayeb, Olivier Delpoux, Jérémie Barbier, Joao Marques, Jan J. Verstraete, and Hervé Vezin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b00733 • Publication Date (Web): 16 Jun 2015 Downloaded from http://pubs.acs.org on June 17, 2015

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Applications of pulsed EPR spectroscopy to the identification of vanadyl complexes in asphaltene molecules. Part I: Influence of the origin of the feed. Karima Ben Tayeb*a, Olivier Delpoux*b Jérémie Barbierb, Joao Marquesb, Jan Verstraeteb and Hervé Vezina a

LASIR UMR CNRS 8516, Université Lille 1 Sciences et Technologies, 59655 Villeneuve

d'Ascq, France b

IFP Energies nouvelles, Rond-point de l'échangeur de Solaize, BP 3, 69360 Solaize, France

(* corresponding authors: [email protected] ; [email protected])

Abstract: The most abundant metals in heavy feedstocks, vanadium and nickel, are mainly concentrated in the asphaltenes fraction, a petroleum fraction which precipitates in presence of paraffinic solvents. Characterization of vanadium and nickel complexes is therefore important to the development of demetallation and conversion strategies used to process heavy crudes. The dependence of vanadyl structures on the geographic origin of feedstocks and their evolution during hydroprocessing in an ebullated-bed pilot unit were studied. The aim of this contribution is to assess the possibilities of the EPR spectroscopy to provide information on the structure of the vanadyl species. This work shows that pulsed EPR spectroscopy is a powerful technique that allows to distinguish several types of environments of vanadium species, amongst which are porphyrinic ligands, even in very complex samples such as C7 asphaltenes from heavy feedstocks.

Keywords: Asphaltene, EPR spectroscopy, Vanadyl structures.

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1. Introduction The increasing demand for petroleum products will require the production and the upgrading of heavy and extra-heavy oils1.Hence, the petroleum industry is confronted with many issues in production, transportation, and refining. Indeed, petroleum residues are complex mixtures of high molecular weight compounds containing high amounts of impurities such as sulfur, nitrogen and metallic species. These elements need to be removed in hydro-processing units through hydrodesulfuration, hydro-denitrogenation and hydro-demetallation reactions before these oil fractions can be used1,2. The most abundant metals in heavy feedstocks, vanadium (V) and nickel (Ni), are mainly concentrated in the asphaltenes fraction, a petroleum fraction which precipitates in presence of paraffinic solvents3. Characterization of vanadium and nickel complexes is therefore important to the development of demetallation and conversion strategies used to process heavy crudes4. Indeed, metals are problematic for refineries because they affect upgrading and conversion processes5. Even at low concentration ( 2|νI|. 2D-HYSCORE experiments are recorded for all samples at 3427 G corresponding to the two overlapped mI = 1/2 transitions. The lengths of the π/2 and π pulses were 16 and 32 ns, respectively, and a delay of 200 ns between the first two π/2 pulses gave the best sensitivity and resolution for the detection of

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N and

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C peaks and avoids the blind spot effect. The use of a

selective pulse of 16 ns (62.5 MHz) and a bandwidth of 50 MHz permit a unique excitation of VO2+ complexes.

3. Results and Discussion Elemental analysis: Table 2 gathers the elemental analysis and the atomic ratios of asphaltenes and the table 3 presents the

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C NMR characterization of asphaltenes from feedstock. Although

the geological origin is not the same (Venezuela for Boscan and Cerro Negro and Russia for Ural), asphlatenes from feedstocks have relatively similar heteroatom contents and aromaticity level as observed by the relatively similar atomic ratios and the carbon aromatic contents. Looking at the effluents, the unconverted asphaltenes from hydroconversion present a much lower heteroatom contents than asphaltenes from feedstock, especially for sulfur and vanadium

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contents. Besides, the H/C ratios of asphaltenes have significantly decreased during hydroconversion showing that the remaining asphaltenes from hydroconversion are more aromatic than those from the feedstock. This can be explained by fast dealkylation reactions in comparison to aromatics hydrogenation and/or also explained by a slower kinetics of conversion of such asphaltenic aromatic species18.

Continuous-Wave EPR: (1) Determination of magnetic parameters. EPR spectra of all samples (Boscan, Ural and Cerro Negro) contain similar signals with a carbon radical and another with hyperfine structure characteristic to vanadium specie

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V4+ due to coupling of the electron

spin (S=1/2) with the nuclear spin of vanadium (I=7/2) (Figure 1) as observed in the literature19,20. EPR spectra of V4+ species shows subsignals at g|| and g⊥, each of which splits by the interaction with the vanadium nucleus (I = 7/2, 100% natural abundance) into eight partially superimposed hyperfine structure (hfs) lines. Depending on the symmetry of the vanadium paramagnetic site, the g and A tensor are anisotrope. The different solids have an axial symmetry with the principal values gzz = g|| = 1.962 (orientation of the external magnetic field parallel to the magnetic z-axis) and gxx = gyy = g⊥ = 1.995 (orientation of the external magnetic field perpendicular to the magnetic z-axis) where g⊥ > g||. The same is true for the hyperfine tensor: Azz = A|| = 157.10-4 cm-1; Axx = Ayy = A⊥ = 55. 10-4 cm-1 where A⊥ < A||. These experimental EPR values are in good accordance with the values of literature12,21. In contrast, the organic free radical - with the most intense line10 - has isotropic g value close to that of the free electron (g = 2.0023). Due to the multiplicity of molecular structures, an unresolved single line with a width of approximately 7G is observed in agreement with the literature22. Dickson et al.23 explained that both the g value and the hyperfine coupling constant, A, may possibly be used to determine more

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explicitly the nature and environment of the vanadium in petroleum. They showed that VOS4 ligand environment gives a higher g value and a lower A value in comparison with VON4, VON2O2 and VOO4. No signal characteristic of nickel has been observed for the different solids. Several assumptions can explain this observation. Firstly, if the nickel is in tetrahedral configuration, the EPR signal is silent. Secondly, if nickel is in octahedral environment which is strongly axially distorted (case of porphyrinic complexes), the ZFS is much larger than thermal energy kT, so that only Ms=0 state is populated and the complex is EPR silent. Other authors have indicated a possible overlap with the vanadium resonance signal10. (2) Quantification of vanadium. The total amount of vanadium present in asphaltenes cannot be associated with the amount of porphyrins present12. But it has been suggested that almost all vanadium in crude oils is present in its paramagnetic VO2+ form which had been detected for the first time by EPR spectroscopy by Saraceno et al.10. The total vanadium available in the solids analyzed by elemental analysis (Table 1) depends on the origin of the C7 asphaltenes and increases as follows: Boscan > Cerro Negro > Ural (Table 1). Boscan asphaltenes contain much higher concentrations of metals (especially vanadium) in comparison to Cerro Negro and Ural asphaltenes. A quantification of vanadium by CW-EPR spectroscopy has been performed by using a vanadyl-porphyrin compound with similar structural characteristics. For this, a reference of vanadyl tetraphenyl porphyrin (VOTPP) is used where vanadium is present in its V4+ form. All EPR absorption spectra were integrated and were normalized for 1 gram of solid according to the solid mass weighed to record the CW-EPR spectra. Each area of C7 asphaltenes, after suppression of radical contribution, was compared to the reference area which permits to determine vanadium quantification. All data are summarized in the Table 1. Compared to the elemental analysis, the results present some differences that are probably due to the incomplete suppression of radical 9

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area and/or the absence of VOTPP calibration line and/or the difference in vanadium content between the reference and the samples. Few publications focus on the vanadium quantification by EPR except, for instance, Saraceno et al.10 who showed a good agreement between the EPR determinations and the chemical analysis by using vanadyl etioporphyrin (I) as a standard dissolved in heavy oil distillate. Furthermore, Schultz et al.24 used a calibration line which was obtained from EPR spectra of a series of crude oil samples with known amounts of vanadyl complexes. In both cases, the experimental methods are different from our study which probably explains this difference. Pulsed EPR: Interactions with

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N (I=1, ν=1.07 MHz at 3500G) and

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C (I=1/2, ν=3.7 MHz at

3500G, 1.11% natural abundance) nuclei can be measured using the HYSCORE sequence which is more adapted to nuclei with small magnetic moments and small hyperfine interactions. It should be noted that HYSCORE spectra are composed of two quadrants. The first quadrant (−,+) A>2νI (with νI the nuclear frequency of the atom I) corresponds to the nuclei with a strong hyperfine coupling A with the unpaired electron of the vanadium. This means that the nuclei are chelating the vanadium or are very close. The second quadrant (+,+) A