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Applications of Pulsed Electron Paramagnetic Resonance Spectroscopy to the Identification of Vanadyl Complexes in Asphaltene Molecules. Part 1: Influence of the Origin of the Feed Karima Ben Tayeb,*,† Olivier Delpoux,*,‡ Jérémie Barbier,‡ Joao Marques,‡ Jan Verstraete,‡ and Hervé Vezin† †

LASIR UMR CNRS 8516, Université Lille 1 Sciences et Technologies, 59655 Villeneuve d’Ascq, France IFP Energies nouvelles, Rond-point de l’échangeur de Solaize, BP 3, 69360 Solaize, France

‡

ABSTRACT: The most abundant metals in heavy feedstocks, vanadium and nickel, are mainly concentrated in the asphaltene fraction, a petroleum fraction that precipitates in the presence of paraffinic solvents. Characterization of vanadium and nickel complexes is therefore important to the development of demetalation and conversion strategies used to process heavy crude oils. The dependence of vanadyl structures upon 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 electron paramagnetic resonance (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 us to distinguish several types of environments of vanadium species, among which are porphyrinic ligands, even in very complex samples, such as C7 asphaltenes, from heavy feedstocks.

1. INTRODUCTION The increasing demand for petroleum products will require the production and upgrading of heavy and extra-heavy oils.1 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 hydroprocessing units through hydrodesulfuration, hydrodenitrogenation, and hydrodemetalation reactions before these oil fractions can be used.1,2 The most abundant metals in heavy feedstocks, vanadium (V) and nickel (Ni), are mainly concentrated in the asphaltene fraction, a petroleum fraction that precipitates in the presence of paraffinic solvents.3 Characterization of vanadium and nickel complexes is therefore important to the development of demetalation and conversion strategies used to process heavy crude oils.4 Indeed, metals are problematic for refineries because they affect upgrading and conversion processes.5 Even at a low concentration ( 2|νI|. Two-dimensional (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 14N and 13 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.

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3. RESULTS AND DISCUSSION 3.1. Elemental Analysis. Table 2 gathers the elemental analysis and the atomic ratios of asphaltenes, and Table 3 presents the 13C 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 levels as observed by the relatively

where [X]feed stands for the weight fraction of X in the feedstock, [X]product stands for the weight fraction of X in the products, mfeed stands for the mass of the feed, and mproduct stands for the mass of the product. 2.3. Elemental Analysis and 13C Nuclear Magnetic Resonance (NMR) Characterization. The elemental analysis in terms of C−H−N was obtained by the thermal conductivity method according to ASTM D5291 (standard test method for instrumental determi4609

DOI: 10.1021/acs.energyfuels.5b00733 Energy Fuels 2015, 29, 4608−4615

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Energy & Fuels Table 2. Elemental Analysis and Atomic Molar Ratios for C7 Asphaltenes from Different Geological Origins and Hydroconversion Effluents C7 asphaltenes from SR feedstocks feedstock origin

Boscan

Cerro Negro

Ural

17.1

16.9

4.7

81.2 7.8 1.8 1.3 6.9

83.1 7.2 1.4 0.9 6.7

84.0 7.4 1.5 1.6 3.9

1.15 0.03 0.01 0.02

1.04 0.03 0.01 0.01

396 4188 4584

C7 asphaltenes from hydroconversion effluents VOTPP

Boscan

Cerro Negro

Ural

asphaltene content (wt %) elemental analysis (wt %) carbon hydrogen nitrogen oxygen sulfur atomic molar ratios H/C S/C O/C N/C elemental analysis (ppm) nickel vanadium Ni + V atomic molar ratio N/V Ni/V CW-EPR (ppm) vanadium vanadium relative deviation (%)

7.9

7.7

1.2

77.8 4.2 8.2 2.4 0.0

84.8 6.7 2.1 1.5 4

86.6 6.4 1.5 1.0 3.8

89.5 6.1 1.2 1.3 1.0

1.11 0.02 0.01 0.0

0.65 0 0.02 0.09

0.94 0.02 0.01 0.02

0.89 0.02 0.01 0.02

0.82 0.01 0.01 0.01

319 1591 1910

303 991 1294

0 74950 74950

473 2412 2885

351 881 1232

89 123 212

15.6 0.08

32.0 0.17

55.0 0.27

4.0 0

31.7 0.17

62.0 0.34

355.4 0.63

7230 72.6

2830 77.9

5070 411.6

74950

4220 75.0

NC NC

NC NC

Table 3. 13C NMR Characterization of Asphaltenes from Feedstocks feedstock origin C C C C C C C C C

aromatic (wt %) aromatic condensed (wt %) aromatic substituted (wt %) aromatic H (wt %) aliphatic (wt %) aliphatic quaternary (wt %) aliphatic H (wt %) aliphatic H2 (wt %) aliphatic H3 (wt %)

Boscan

Cerro Negro

Ural

51 31 16 4 49 nq 7 37 5

49 17 22 10 51 nq nq 42 9

52 21 16 15 48 nq 5 32 11

similar atomic ratios and the carbon aromatic contents. Looking at the effluents, the unconverted asphaltenes from hydroconversion present much lower heteroatom contents than asphaltenes from feedstock, especially for sulfur and vanadium 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 aromatic hydrogenation and/or also explained by a slower kinetics of conversion of such asphaltenic aromatic species.18 3.2. CW-EPR. 3.2.1. 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 species 51V4+ because of coupling of the electron spin (S = 1/2) with the nuclear spin of vanadium (I = 7/2) (Figure 1), as observed in the literature.19,20 EPR spectra of V4+ species shows subsignals at g∥ and g⊥, each of which is split by the interaction with the vanadium nucleus (I = 7/2; 100% natural abundance)

Figure 1. CW-EPR spectra of C7 asphaltenes from different geological origins: black line, Boscan; red line, Ural; and blue line, Cerro Negro.

into eight partially superimposed hyperfine structure (hfs) lines. Dependent upon the symmetry of the vanadium paramagnetic site, the g and A tensor are anisotropic. 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, and Axx = Ayy = A⊥ = 55.10−4 cm−1, where A⊥ < A∥. These experimental EPR values are in good accordance with the values of the literature.12,21 In contrast, the organic free radical, with the most intense line,10 has an 4610


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Figure 2. Comparison of HYSCORE spectra of the first coordination spheres of the vanadium complexes of Boscan, Cerro Negro, and Ural with the VOTPP reference.

isotropic g value close to that of the free electron (g = 2.0023). Because of the multiplicity of molecular structures, an unresolved single line with a width of approximately 7 G is observed in agreement with the literature.22 Dickson et al.23 explained that both the g value and the hyperfine coupling constant, A, may possibly be used to determine more explicitly the nature and environment of vanadium in petroleum. They showed that the VOS4 ligand environment gives a higher g value and a lower A value in comparison to VON4, VON2O2, and VOO4. No signal characteristic of nickel has been observed for the different solids. Several assumptions can explain this observation. First, if nickel is in a tetrahedral configuration, the EPR signal is silent. Second, if nickel is in an octahedral environment, which is strongly axially distorted (in the case of porphyrinic complexes), the zero-field splitting (ZFS) is much larger than thermal energy kT, so that only the ms = 0 state is populated and the complex is EPR-silent. Other authors

have indicated a possible overlap with the vanadium resonance signal.10 3.2.2. Quantification of Vanadium. The total amount of vanadium present in asphaltenes cannot be associated with the amount of porphyrins present.12 However, 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 upon 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 CWEPR spectroscopy has been performed 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. 4611


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Figure 3. Comparison of HYSCORE spectra of the second coordination spheres of the vanadium complexes of Boscan, Cerro Negro, and Ural with the VOTPP reference.

All EPR absorption spectra were integrated and normalized for 1 g of solid according to the solid mass weight to record the CW-EPR spectra. Each area of C7 asphaltenes, after suppression of radical contribution, was compared to the reference area, which permits us to determine vanadium quantification. All data are summarized in Table 1. In comparison to the elemental analysis, the results present some differences that are probably because of the incomplete suppression of the radical area and/or the absence of the VOTPP calibration line and/or the difference in the 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 good agreement between the EPR determinations and the chemical analysis using vanadyl etioporphyrin (I) as a standard dissolved in heavy oil distillate. Furthermore, Schultz et al.24 used a calibration line that 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. 3.3. Pulsed EPR. Interactions with 14N (I = 1 and ν = 1.07 MHz at 3500 G) and 13C (I = 1/2 and ν = 3.7 MHz at 3500 G; 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 being 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 vanadium or are very close. The second quadrant (+, +) A < 2νI corresponds to the nuclei with weaker interactions, which compose the structure of the ligand (except the chelating atoms). All of the 14 N correlation peaks are localized in the (+, −) quadrant (strong coupling), while 13C and 1H correlations are localized in the (+, +) quadrant (weak coupling). 4612


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Figure 4. HYSCORE spectra of the first coordination spheres of the vanadium complexes of the Boscan feed and its hydrotreated effluent.

3.3.1. First Coordination Shell for the Study of Chelating Nuclei. Figure 2 shows the 2D HYSCORE spectra of quadrant (+, −) for the nuclei that strongly interact with the electron of vanadium for the three C7 asphaltenes feed samples (Cerro Negro, Ural, and Boscan). These spectra, recorded at 4 K, are compared to the spectrum of a VOTPP reference. The spectra of the three C7 asphaltene samples were found to be quite similar, indicating that, regardless of the geological origin, the structure of the first coordination shell seems identical. However, the spectrum of VOTPP is different because it is largely dominated by two sharp peaks corresponding to singleand double-quantum transitions of nitrogen, as observed by Gourier et al.13 Two weaker correlation peaks at low frequency are assigned to the single-quantum correlations (sq+, sq−) corresponding to the selection rule ΔmI = 1, while the two larger correlation peaks at high frequency are attributed to the double-quantum correlations (dq+, dq−) corresponding to the selection rule ΔmI = 2. From this spectrum, two experimental parameters are obtained from a direct measurement; the correlation peaks are separated along the diagonal by the quadrupolar interaction 3Pzz and along the anti-diagonal by the hyperfine interaction A. The quadrupolar interaction 3Pzz is equal to 4.1 MHz for double-quantum correlations against 2.1 MHz for single-quantum correlations with a hyperfine value A = 4.7 MHz. In the rest of the paper, the porphyrinic nitrogen will be noted N1. With regard to the C7 asphaltenes, HYSCORE spectra of chelating nitrogen atoms are composed of four peaks corresponding to the {sq, sq} and {dq, dq} transitions, which are symmetric along the diagonal and a fifth less intense signal characteristic of the carbon 13C (the contour intensities are multiplied by 1.5 to observe the ridge). In comparison of the spectrum of VOTPP to those of C7 asphaltenes, it seems that nitrogen ridges are composed of two types of signatures, which are attributed to nitrogen chelating vanadium or are very close; one of them corresponds to nitrogen nuclei that are very close to the signal of the VOTPP nitrogen (noted N1), but the other one has not been attributed (noted N2). As mentioned by Spencer et al.,25 it is known that the vanadyl ion, VO2+, forms

chelates with porphyrins and also with other largely unknown “non-porphyrins”. The contribution of this latter species seems to be minor (