Studying Rotational Mobility of V O Complexes in Atmospheric

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Studying Rotational Mobility of VO Complexes in Atmospheric Residues and Their Resins and Asphaltenes by Electron Spin Resonance Qingyan Cui,† Koji Nakabayashi,† Xiaoliang Ma,‡ Jin Miyawaki,† Keiko Ideta,† Yoshika Tennichi,§ Morio Ueda,§ Adel Al-Mutairi,‡ Abdulazim M. J. Marafi,‡ Joo-Il Park,‡ Seong-Ho Yoon,† and Isao Mochida*,§ †

Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Petroleum Research Center, Kuwait Institute for Scientific Research, Safat 13109, Kuwait § Kyushu Environmental Evaluation Association, Fukuoka 813-0004, Japan ‡

ABSTRACT: Behaviors of VO complexes in atmospheric residues (ARs) from two Kuwait crude oils and their resins and asphaltenes were studied using the electron spin resonance (ESR) to examine the effects of the surrounding matrix, concentration in solvent, and temperature on VO rotational mobility. The results show that the surrounding molecules in the petroleum fractions constrain the VO rotational mobility significantly. The constraint on the VO complexes by the surrounding matrix in different environments increases in the order of resin < AR < asphaltene. Less constraint by the AR than by the asphaltene can be ascribed to the solvent role of the lighter components in the AR. The higher measurement temperature of 100 °C significantly decreases the constraint on VO complexes by the matrix, while a higher sample concentration in the solvent shows stronger constraints. However, the constraint on the VO complexes in the asphaltene is hardly moderated, even when dissolved in toluene at 100 °C. Additionally, Kuwait Export Crude atmospheric residue (KEC-AR) and its resin dissolved in toluene show weaker constraints on the VO complexes than Lower Fars atmospheric residue (LF-AR) and its resin, possibly as a result of lower aggregation of KEC-AR and its resin. and naphthenic alkali earth metal salts also exist in petroleum.14 The structures of the metal porphyrins have been determined.14,15 Recently, high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with positive- and negative-ion electrospray ionization16−18 and atmospheric pressure photoionization19,20 procedures detected VO and Ni porphyrin species in crude oil and residues, which may provide helpful information regarding the molecular structure of the metal species. High-temperature gas chromatography with atomic emission detection (GC− AED)21 has also provided the molecular information on the metal species. Both analyses provided information on isolated metal species. This is a strong basis for understanding metal species at a molecular level. However, V and Ni species co-exist with organic components in petroleum, where there are interactions between the metal species and surrounding organic components through non-covalent, hydrogen-bond, and aromatic π−π interactions. Thus, it is also important to study the environment of the metal species and their relationships with surrounding organic components. The structure of metal species in petroleum can be described as a four-tiered structure, as follows: (1) a tetradentate core ligand, where some core ligands can contain atoms other than nitrogen, (2) pendants on the core ligand, where some of the pendants are bound covalently to the tetradentate core ligand,

1. INTRODUCTION Metal species in petroleum have been targeted extensively for removal in the refining process because they may otherwise be deposited on catalysts, transfer tubes, and filters in the downstream steps of petroleum refining.1 Metal species are usually removed through hydrodemetallization (HDM) over a Mo or NiMo alumina catalyst, where formed V2S6 and NiS deposit on the surface of catalysts, which ultimately results in deactivation of the catalyst.2−5 A deeper extent and larger capacity for metal removal have been explored for longer term continuous operations. Fully understanding the structure and state of the metal complexes as well as their relationships with surrounding molecules is important in designing a better HDM catalyst and a more efficient HDM process. Metal species are present mainly in the heavy fractions of petroleum, such as resin and especially asphaltene, which consists of condensed polynuclear aromatics and contains a high concentration of heteroatoms. The analysis of asphaltene has been reported through fractionation with a binary solvent.6−8 Additionally, asphaltene has been studied widely using nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and transmission electron microscopy (TEM) to demonstrate its structure and properties.9−13 It is important to understand the state of the metal complexes in asphaltene and petroleum, which influences the deposition of the metal species together with carbonaceous substances on the catalyst during the HDM process. V and Ni porphyrins are common metal species in petroleum, although V and Ni non-porphyrin, iron sulfide, © 2017 American Chemical Society

Received: December 10, 2016 Revised: February 27, 2017 Published: April 5, 2017 4748

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performed as follows. AR was dissolved in n-heptane at a ratio of 1:50 (g/g) with stirring at 60 °C for 5 h, and the mixture was then filtered. The insoluble fraction was extracted with toluene in a Soxhlet apparatus and dried to obtain asphaltene. The n-heptane solution fraction (maltene) was eluted consecutively with n-heptane, toluene, and toluene/methanol (9:1, v/v) at a ratio of solvent/maltene of 250 mL/1 g with a glass column packed with activated neutral alumina to yield the saturate, aromatic, and resin fractions, respectively. Their properties and compositions are summarized in Table 1.

(3) axial ligands to the complex, where the ligands can be present in surrounding molecules, which carry the coordinative functions to central metal ions in an axial direction, and (4) physical bonding of the complex with surrounding molecules or molecular groups, to form micelles or stack into the large aromatic sheets.22,23 High-performance liquid chromatography (HPLC)24−26 and gel permeation chromatography (size-exclusion chromatography) inductively coupled plasma GPC (SEC)−ICP27−30 have been used to detect metal complexes in the fractions, such as atmospheric residue (AR), resin, and asphaltene, where the metal complexes are bound to matrix molecules or dissolved in the matrix. It is not yet fully understood how the metal complexes exist in petroleum or how the surrounding matrix interacts with them to change their mobility, although such an understanding is important for improving the metal removal performance of the HDM process. The electron spin resonance (ESR) technique has been used to analyze vanadium(II) porphyrin (VO) complexes in petroleum. Dickson et al.31 proposed using ESR to identify the environment of VO complexes in petroleum and to examine g values and hyperfine coupling constants (A) for several square pyramidal environments around vanadyl complexes. Additionally, vanadyl porphyrin concentrations have been estimated using ESR.21,32 However, there are few reports examining the effects of ligands and the surrounding matrix on VO rotational mobility in petroleum. Campbell and Freed33 measured a series of ESR spectra of VO complexes dissolved in solvents at different temperatures and simulated them on the basis of the tumbling rates and τ values of the complexes at a given temperature. Wong and Yen34 studied the mobility of VO complexes in asphaltene by ESR through distinguishing anisotropic and isotropic spectra and illustrated the mobility through liberating the VO complexes to be mobile in the asphaltene dissolved in some solvents in ESR measurement. These studies indicate that ESR can be useful for studying the mobility of VO complexes in different environments. In the present study, the effects of solvents [toluene and tetrahydrofuran (THF)], measurement temperature, concentration of the samples in solution, and surrounding organic matrixes on the mobility of VO complexes were studied by distinguishing the ESR anisotropic and isotropic spectra, extending the approaches of Campbell and Freed33 and Wong and Yen.34 ESR spectra of Etio and tetraphenyl porphyrin VO complexes dissolved in toluene were determined at a series of temperatures as a frame of reference to classify the profiles of ESR spectra of the petroleum fractions according to the tumbling rate, as reported by Campbell and Freed.33 Such mobility information may provide insight into the interaction between VO complexes and their surrounding matrixes, which may be significant in improving the metal removal performance of the HDM process.

Table 1. Properties of LF-AR and KEC-AR boiling point (°C) density (g/mL) C (wt %) H (wt %) S (wt %) N (wt %) V (ppm) Ni (ppm) saturate (wt %) aromatic (wt %) resin (wt %) asphaltene (wt %)

LF-AR

KEC-AR

>360 1.0081 82.57 10.06 3.44 0.33 152.33 23.79 16.66 51.94 19.46 11.94

>360 0.9745 83.84 10.95 3.19 0.29 70.57 13.83 25.69 48.75 18.38 7.18

2.2. ESR Analysis. The ESR analysis was carried out using a JESFA200 ESR spectrometer with a X-band bridge (JEOL, Ltd., Tokyo, Japan) with standard 100 kHz field modulation. The sample measurement was performed at a microwave frequency of 9.0 GHz and power of 1.0 mW using a cylindrical transverse electric (TE) cavity, where the quartz tube (5 mm outer diameter and 4 mm inner diameter) for holding the sample was inserted along the cylindrical axis of the cavity. The magnetic field was calibrated with an ESR marker (Mn2+ powder). A temperature accessory (DVT controller, JEOL) was used to control the measurement temperature of the samples in the cavity, and liquid nitrogen gas was used to cool the sample when it was measured at a temperature below 0 °C. The V etioporphyrin (Etio, Sigma-Aldrich, St. Louis, MO, U.S.A.) and V 5,10,15,20-tetraphenylporphyrin (TPP, Wako Chemicals, Tokyo, Japan) samples were dissolved in toluene at 200 ppm. The ARs, resin, and asphaltene were diluted with toluene (Wako Chemicals, Tokyo, Japan) and THF (Wako Chemicals, Tokyo, Japan) at concentrations from 0.1 to 80 wt % to study the VO tumbling stage. A typical V isotropic spectrum obtained with a TPP VO complex in toluene measured at 20 °C shows eight perpendicular lines, as shown in Figure 1a, indicating that the V complex can tumble rapidly, without constraint. A typical V anisotropic spectrum obtained by the TPP VO complex in toluene measured at −120 °C consists of 16 partially overlapped components of the hyperfine structure in Figure 1b, eight lines in parallel and eight lines perpendicular because of I = 7/2, suggesting that the V complex is strongly constrained and in a rigid state.35−37 The B parameter, which has been reported as a sensitive indicator of the tetragonal distortion that occurs with a change in the VO bond length and distance of four nitrogen ligands in the basal plane,38 can be derived as follows:

2. EXPERIMENTAL SECTION

B = Δg /Δg⊥ = 4α 2ΔExz /γ 2ΔE x 2 − y2

2.1. Sample Preparation. Two kinds of Kuwait atmospheric residues were from Lower Fars (LF) crude and Kuwait Export Crude (KEC). The carbon (C), hydrogen (H), and nitrogen (N) contents were obtained using an elemental analyzer (model EA-1110, CE Instruments, Milan, Italy). The sulfur (S) content was measured by Xray fluorescence (XRF, XGT-1700WR, Horiba, Kyoto, Japan). The vanadium (V) and nickel (Ni) contents were determined by inductively coupled plasma mass spectrometry (ICP−MS, Agilent 7700 series, Agilent Technologies, Santa Clara, CA, U.S.A.). The separation of saturate, aromatic, resin, and asphaltene fractions was

Δg = g − ge = − 8λα 2δ 2/ΔE x 2 − y2 Δg⊥ = g⊥ − ge = − 2λγ 2δ 2/ΔExz where g∥ and g⊥ are the magnetic field values and A∥ and A⊥ are the hyperfine coupling tensors. The g and A values are related to the electronic structure of the vanadyl species. g⊥ > g∥ in the electronic ground state suggests tetragonal distortion. g∥ and g⊥ are used as 4749

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Figure 1. ESR spectra of the VO complex: (a) isotropic spectrum and (b) anisotropic spectrum.

Figure 2. ESR spectra of VO complexes in Etio and TPP porphyrins dissolved in toluene.

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Figure 3. Structure of the Etio and TPP porphyrins.23 parameters for the vanadyl complexes to clarify the vanadium electronic structure. ge is the free electron g value of 2.0023, and λ is the spin−orbit coupling constant of the free ion. The method to obtain these parameter values from an ESR analysis has been described.38,39 The parameter values of A∥, A⊥, g∥, and g⊥ are obtained using anisotropic simulation software, and the deviations of A∥, A⊥, g∥, g⊥, and B are 0.03, 0.01, 0.0003, 0.0001, and 0.05, respectively.

3. RESULTS 3.1. ESR Spectra of Etio and TPP VO Complexes Dissolved in Toluene. Figure 2 shows a series of ESR spectra of Etio and TPP VO complexes dissolved in toluene measured at a temperature range from 20 °C to liquid nitrogen temperature (−196 °C). Both VO complexes presented a typical isotropic spectrum at 20 °C, indicating that they both were in a high-tumbling and free stage. Furthermore, both V O complexes gave a typical anisotropic spectrum at −120 °C or below, suggesting that they were in a very low tumbling and constrained state under such conditions.33,36,37 The spectra changed from isotropic to anisotropic through a series of transition spectra from 20 to −120 °C, indicating that the V O tumbling rate decreased gradually, as reported in the literature.33 There is a significant change in the ESR spectra from isotropic to anisotropic when the temperature decreases from −90 to −100 °C for both complexes, because the melting point of toluene is −95 °C, at which the solidification of the solution constrains the mobility of the complexes, resulting in changes in the ESR spectrum (from isotropic to anisotropic in Figure 2). When the ESR spectra of Etio and TPP VO complexes are compared in detail, it is further found that the change in the ESR spectra occurs at a slightly higher temperature for the TPP VO complex than for the Etio VO complex to give a similar spectrum at a temperature range from −40 to −70 °C. This may be due to the larger molecular size of the former compared to the latter, as shown in Figure 3, indicating that the ESR spectrum is sensitive to the structure and environment of the VO complexes. Consequently, the series of transitional ESR spectra were used as references to estimate the mobility state of the VO complexes in AR and its resin and asphaltene fractions. 3.2. ESR Spectra of VO Complexes in LF-AR and Its Resin and Asphaltene Fractions without Solvent. As shown in Figure 5, the VO complexes in LF-AR and its resin and asphaltene fractions at 20 °C presented similar anisotropic spectra with 16 lines, differing from the TPP VO complex in toluene measured at the same temperature, indicating that

Figure 4. Model of V complexes in petroleum or asphaltene: (a) associate at the edge, (b) intercalated, and (c) at the center among a group of micelles.36

Figure 5. ESR spectra of VO complexes in LF-AR and its resin and asphaltene measured at 20 °C.

these VO complexes in LF-AR, resin, and asphaltene could not be freely mobile, possibly as a result of constraints imposed by the surrounding matrixes, as illustrated in the model of the metal complex in petroleum or asphaltene in Figure 4, which corresponds to the spectra of the TPP or Etio VO complex dissolved in toluene at −120 °C. Some solvent or higher 4751

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Energy & Fuels measurement temperature is required to moderate constraints on VO complexes by the surrounding matrixes and, thus, to liberate the VO complexes from them to give an isotropic spectrum. 3.3. Comparing ESR Spectra of VO Complexes in LF-AR Dissolved in THF and Toluene. Figure 6 illustrates a

Table 2. Highest Concentrations of AR and Resin Dissolved in Toluene Giving an Isotropic Spectrum at 20 and 100 °C LF temperature (°C)

AR (%)

resin (%)

20 100

4

5 30

KEC asphaltene (%)

AR (%)

resin (%)

8

5 40

asphaltene (%)

THF and toluene was observed on the basis of the B parameter values. 3.4. ESR Spectra of VO Complexes in LF-AR, Resin, and Asphaltene Dissolved in Toluene. Figure 7 shows a series of ESR spectra of the VO complexes in LF-AR dissolved in toluene at low concentrations, from 0.5 to 5.0 wt % measured at 20 and 100 °C, respectively. At a measurement temperature of 20 °C, the VO complexes in LF-AR dissolved in toluene always displayed anisotropic spectra, even at a concentration as low as 2.0 wt %, indicating a slow tumbling rate (nearly rigid state), regardless of the concentrations. Such profiles were similar to that of the TPP VO complex dissolved in toluene measured at −105 °C. The VO complexes in LF-AR dissolved in toluene measured at 100 °C show isotropic spectra up to a concentration of 4.0 wt %, corresponding to that of the TPP VO complex dissolved in toluene measured at 20 °C, indicating that VO tumbled rapidly, overcoming the constraints of the surrounding matrixes at the low concentration. High temperature is favorable to liberating VO complexes from the surrounding matrix. Figure 8 shows the ESR spectra of the VO complexes in LF-AR resin dissolved in toluene at concentrations from 2 to 20 wt % measured at 20 °C and from 30 to 40 wt % measured at 100 °C. It is clear that the ESR spectra of VO complexes in the resin dissolved in toluene at concentrations below 5 wt % at 20 °C were isotropic, whereas the spectra started to change to anisotropic spectra at a concentration above 8 wt %. The spectra at concentrations of 5, 8, and 20 wt % correspond to those of the TPP VO complex dissolved in toluene at 20, −40, and −50 °C, respectively. The VO complexes in LF-AR resin dissolved in toluene measured at 100 °C with concentrations up to 30 wt % still showed normal isotropic spectra. Increasing the concentration to 35 and 40 wt % reduced the VO tumbling rate, with spectra corresponding to those of the TPP VO complex dissolved in toluene at −20 and −50 °C, respectively. The concentration of resin in toluene influenced the VO mobility, similar to AR. Figure 9 shows ESR spectra of the VO complexes in LFAR asphaltene dissolved in toluene at concentrations from 0.1 to 0.5 wt % measured at 20 and 100 °C, respectively. There was almost no ESR signal with LF-AR asphaltene in toluene at a concentration of 0.1 wt % measured at 20 °C, possibly as a result of the too low V concentration to be detected. The ESR spectrum became definite as the concentration increased. LFAR asphaltene in toluene at 0.5 wt % showed a spectrum of VO complexes with a very slow tumbling rate, corresponding to that at −100 °C of the TPP VO complex dissolved in toluene. Meanwhile, the VO complexes in the same sample also gave a spectrum showing a slow tumbling rate, even at 100 °C, corresponding to that of the TPP VO complex dissolved in toluene at −40 °C, indicating strong constraints on the V O complexes in LF-AR asphaltene. 3.5. ESR Spectra of VO Complexes in KEC-AR and Its Resin and Asphaltene Dissolved in Toluene. The ESR

Figure 6. ESR spectra of VO complexes in LF-AR dissolved in (a) THF and (b) toluene.

series of ESR spectra of the VO complexes in LF-AR dissolved in THF and toluene measured at 20 and 50 °C at concentrations from 40 to 80 wt %. The spectra profiles were modified slightly by THF, when the concentration was 40 wt %, corresponding to the spectrum measured at −110 °C for TPP VO dissolved in toluene. There is no obvious difference in the ESR spectra with and without solvent at a sample concentration of 80 wt %. The ESR spectra of the VO complexes in LF-AR dissolved in THF were similar to those in toluene, indicating that the moderating effects of THF and toluene on the constraint on the VO complexes by the surrounding molecules were similar under such conditions. ESR parameter values of the VO complexes in LF-AR dissolved in THF and toluene measured at 20 and 50 °C were obtained, and the results are summarized in Table 3. Both THF and toluene reduced the B parameter value in comparison to the values without any solvent. The B parameter value decreased at lower concentrations of AR and increasing measurement temperature, indicating that both the addition of the solvents and an increase in the temperature increased the mobility of the VO complexes in the matrix. However, no obvious difference in the mobility of the VO complexes in LF-AR dissolved in 4752

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Energy & Fuels Table 3. ESR Parameters of VO Complexes in LF-AR Dissolved in THF and Toluene Measured at 20 and 50 °C sample

toluene

20 °C 50 °C 20 °C

50 °C

THF

20 °C

50 °C

LF-AR LF-AR LF-AR-T 40% LF-AR-T 60% LF-AR-T 80% LF-AR-T 40% LF-AR-T 60% LF-AR-T 80% LF-AR-THF 40% LF-AR-THF 60% LF-AR-THF 80% LF-AR-THF 40% LF-AR-THF 60% LF-AR-THF 80%

A∥ (mT)

A⊥ (mT)

g∥

g⊥

B

17.14 17.12 16.95 17.02 17.07 16.88 17.01 17.06 17.04 17.05 17.14 16.92 17.01 17.10

6.00 5.99 6.02 6.01 5.98 6.05 6.05 6.00 6.05 6.03 6.01 6.09 6.05 6.01

1.9615 1.9615 1.9621 1.9620 1.9618 1.9622 1.9618 1.9616 1.9622 1.9622 1.9616 1.9624 1.9621 1.9620

1.9943 1.9943 1.9931 1.9936 1.9941 1.9924 1.9931 1.9937 1.9932 1.9937 1.9941 1.9925 1.9931 1.9937

5.10 5.10 4.37 4.63 4.94 4.05 4.40 4.73 4.41 4.66 4.96 4.07 4.37 4.69

Figure 7. ESR spectra of VO complexes in LF-AR dissolved in toluene measured at (a) 20 °C and (b) 100 °C.

Figure 8. ESR spectra of VO complexes in LF-AR resin dissolved in toluene measured at (a) 20 °C and (b) 100 °C.

spectra of the VO complexes in KEC-AR dissolved in toluene at the concentrations from 2 to 10 wt % measured at temperatures of 20 and 100 °C are shown in Figure 10. The VO complexes in KEC-AR at 20 °C showed spectra with a low tumbling rate, regardless of the concentrations from 2 to 10 wt %, corresponding to that of the TPP VO complex dissolved in toluene from −60 to −90 °C. However, KEC-AR at concentrations of 2−8 wt % at 100 °C showed isotropic spectra. The highest concentration of KEC-AR that gave a normal isotropic spectrum was higher than that of LF-AR at the same temperature of 100 °C, as summarized in Table 2.

Figure 11 shows ESR spectra of the VO complexes in KEC-AR resin dissolved in toluene at various concentrations measured at 20 and 100 °C, respectively. The VO complexes in KEC-AR resin dissolved in toluene gave an isotropic spectrum at a concentration of 5 wt % measured at 20 °C; the concentration giving an isotropic spectrum was similar to that of LF-AR resin dissolved in toluene. However, the VO complexes in KEC-AR resin dissolved in toluene showed normal isotropic spectra at 100 °C at concentrations up to 40 wt %, obviously higher than that of LF-AR resin dissolved in toluene. The KEC-AR resin dissolved in toluene at 45 wt % started to transform the spectrum from isotropic to anisotropic, 4753

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Figure 9. ESR spectra of VO complexes in LF-AR asphaltene dissolved in toluene measured at (a) 20 °C and (b) 100 °C.

Figure 10. ESR spectra of VO complexes in KEC-AR dissolved in toluene measured at (a) 20 °C and (b) 100 °C.

corresponding to that of the TPP VO complex dissolved in toluene measured at −20 °C, and a concentration of 50 wt % gave a spectrum with a slow tumbling rate, corresponding to that of the TPP VO complex dissolved in toluene measured at −50 °C. Figure 12 shows the ESR spectra of the VO complexes in KEC-AR asphaltene dissolved in toluene measured at 20 and 100 °C at different concentrations from 0.3 to 1.0 wt %. KECAR asphaltene in toluene showed the ESR spectra with some differences at 20 and 100 °C. The spectra under these two temperatures at a concentration of 1.0 wt % correspond to those of the TPP VO complex dissolved in toluene at −100 and −40 °C, respectively. Like the asphaltene from LF-AR, improvement in the mobility of the VO complexes in asphaltene from KEC-AR by reducing the sample concentration in toluene to give an isotropic spectrum is also difficult, even at a measurement temperature of 100 °C.

moderates the directional anisotropy when the VO complex is present in a liquid state at differing degrees of Brownian motion, as observed with the TPP VO complex dissolved in toluene over a wide temperature range from −196 to 20 °C. The VO complex in the liquid state and with low or medium molecular weight tends to give an isotropic spectrum, even at low measurement temperatures. In contrast, a VO complex with a high molecular weight, such as those with large substituents bonded to the tetradentate ligand or with the axial ligands22,23 to the central VO ion (e.g., VO complexes in petroleum), shows an anisotropic spectrum, even in a solvent. Standard Etio and TPP VO complexes dissolved in toluene show a series of spectra from free rotation to a rigid state with variable tumbling rates, where complete dissolution of the V O complex in toluene provides an isotropic spectrum as a result of the free rotational mobility of the VO complex. VO complexes with different pendant ligands, such as Etio and TPP VO complexes, can give different spectra when measured at −40 and −70 °C. However, the difference of the spectra of Etio and TPP VO complexes is smaller when compared to those of VO complexes in petroleum; therefore, the surrounding molecules of VO complexes in petroleum may exert more influence on VO rotational mobility. 4.2. Effect of ARs and Their Fractions on VO Complexes Mobility. The VO complexes in the AR show anisotropic spectra that are obviously different from those of TPP VO complex in toluene under the same measurement conditions. The differences can be ascribed to the greater variety in core ligands, their pendants, and surrounding molecules in the AR matrix. Regardless of the ligands, VO

4. DISCUSSION 4.1. VO Complexes in Solvents. The ESR spectra of VO complexes in the petroleum fractions reflect strong constraints by the surrounding environment, where the rotational mobility of the VO complexes is governed by their structure and peripheral organic components, including the solvent. For the ESR spectra of VO complexes, the isotropic and anisotropic natures of the spectra determined by the VO tumbling rate reflect principally the V rotational mobility.33 Thus, the VO complex in a rigid state shows an anisotropic spectrum, while the spectrum becomes more isotropic with increasing VO rotational mobility, this 4754

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Figure 11. ESR spectra of VO complexes in KEC-AR resin dissolved in toluene measured at (a) 20 °C and (b) 100 °C.

Figure 12. ESR spectra of VO complexes in KEC-AR asphaltene dissolved in toluene measured at (a) 20 °C and (b) 100 °C.

complex in petroleum appear molecularly dispersed in petroleum and in a liquid state. The surrounding molecules of the matrix chemically or physically bound to the VO complexes, as suggested in the Yen model,36 may govern the mobility and solubility of the VO complexes. The different ESR spectra of TPP VO complex in toluene and VO complexes in the AR indicate that the VO complexes in the AR are strongly constrained by the surrounding matrix molecules via the non-covalent bonds or other interactions, resulting in their very slow rotational mobility. A recent FT-ICR MS16 study showed several VO porphyrins in crude oil, and different structures and derivatives of the VO complexes are present in different crude oils. They include complexes with low or medium molecular weight, which are smaller than the TPP VO complex, and they should show an isotropic spectrum when the petroleum fraction becomes liquid or dissolved completely in a solvent, particularly at a low concentration through overcoming the constraints of the surrounding matrix. The matrixes of AR and its resin and asphaltene are known to govern the degree of mobility of VO complexes. Thus, whether the VO complexes in a matrix show an isotropic or anisotropic spectrum depends upon the degree of liberation of VO complexes from the matrix. When ARs or their fractions are dissolved in toluene, whether an isotropic or anisotropic spectrum is observed depends upon the composition and properties of the matrixes (ARs and their resin or asphaltene), the sample concentration in the solvent, and the measurement temperature. The maximum concentration in toluene to give an isotropic

spectrum decreases in the order of resin > AR > asphaltene, and asphaltene dissolved in toluene hardly gives an isotropic spectrum, even at a very low concentration and measurement temperature of 100 °C, suggesting that the constraining intensity of the surrounding molecules in asphaltene is significantly stronger than in the AR and resin. Resin dissolved in toluene at 20 °C can give an isotropic spectrum, and higher temperatures more readily result in an isotropic spectrum. Solubility of the petroleum fraction can reflect their molecular composition. Additionally, compositional distribution should be considered in the presence or absence of toluene. Smaller molecules in the matrix may play the role of a solvent in the same fraction. KEC-AR and its resin give isotropic spectra at higher concentrations than those of LF-AR and its resin, as summarized in Table 2, probably as a result of weaker constraints on the VO complexes in KEC-AR and its resin in comparison to those in LF-AR and its resin. The present findings indicate that higher concentrations of petroleum fractions dissolved in toluene tend to give anisotropic spectra as a result of the very slow tumbling rate of the VO complexes. There may be a series of stages in which the VO complexes in the matrix are liberated to varying extents. Complete dissolution of the matrix in toluene at a low concentration may allow VO free mobility, whereas solvation of the matrix may moderate partially the constraint on the VO complexes, which are still restricted to some extent by the surrounding matrix. The extent of this constraint defines the spectrum profile, as seen in the experimental results. The constraint extent should be influenced by the solvent/matrix ratio and the measurement temperature. It is important to 4755

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Article

Energy & Fuels

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further clarify the solvated state of the matrix and the VO tumbling rate in the matrix. This discussion may suggest that VO complexes in petroleum behave in concert with co-existing molecules in its fraction, even under conditions of a hydrotreatment reaction. The combined behavior of the VO complexes and the matrix should be taken into consideration, particularly for coke formation and catalyst deactivation in the HDM process. Liberation of VO complexes from the matrix, as indicated by their molecular mobility, is important in improving metal removal.

5. CONCLUSION In this study, we have demonstrated that matrixes of VO complexes in the AR, resin, and asphaltene restrict their rotational mobility by comparing their ESR spectra to those of the standard porphyrin complexes (Etio or TPP VO complex) dissolved in toluene at temperatures ranging from −196 to 20 °C. VO complexes in resin dissolved in toluene can give an isotropic spectrum at 20 °C. The highest concentration of resin dissolved in toluene that gives an isotropic spectrum at 100 °C is obviously much higher than those of the AR. The VO complexes in both the AR and the resin dissolved in toluene show isotropic spectra at 100 °C, whereas the VO complexes in the asphaltene dissolved in toluene hardly give an isotropic spectra. This is probably due to the chemical constraints on the VO complexes in asphaltene by the intermolecular interactions. We conclude that the constraint on VO complexes in different fractions increases in the order of resin < AR < asphaltene. The roles of aromatic and resin fractions should be recognized when dissolving VO complexes in the asphaltene fraction present in the AR. KEC-AR and its resin dissolved in toluene at a measurement temperature of 100 °C show isotropic spectra of VO complexes up to concentrations of 8 and 40 wt %, respectively, which are obviously higher than those of LF-AR and its resin, suggesting stronger constraints of LF-AR and its resin on VO complexes than those in KEC-AR and its resin, indicating stronger aggregation in LF-AR.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 081-092-662-0410. E-mail: [email protected]. ORCID

Qingyan Cui: 0000-0002-6497-0255 Xiaoliang Ma: 0000-0003-0450-0662 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Japan Cooperation Center, Petroleum (JCCP), the Kuwait Oil Company (KOC), and the Kuwait Institution for Scientific Research (KISR) for collaboration on this joint project. Acknowledgement is also extended to the Kuwait National Petroleum Company (KNPC) for the in-kind contribution and technical support.

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DOI: 10.1021/acs.energyfuels.6b03279 Energy Fuels 2017, 31, 4748−4757

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DOI: 10.1021/acs.energyfuels.6b03279 Energy Fuels 2017, 31, 4748−4757