Qingyan Cuia,
Koji Nakabayashia,
Xiaoliang Mab,
Keiko Idetaa,
Jin Miyawakia,
Abdulazim M. J. Marafib,
Adel Al-Mutairib,
Joo-Il Parkb,
Seong-Ho Yoona and
Isao Mochida*c
aInstitute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka, Japan
bPetroleum Research Center, Kuwait Institute for Scientific Research, Safat, Kuwait
cKyushu Environmental Evaluation Association, Fukuoka, Japan. E-mail: mochida@keea.or.jp; Tel: +81-092-662-0410
First published on 2nd August 2017
The VO complexes in atmospheric residues (ARs) and their maltene, resin and asphaltene fractions have been investigated using ESR to examine the effects of the surrounding matrices, measurement temperature, pre-heat-treatment of AR as well as addition of toluene on the electron structure and mobility of the VO ion. The B parameter calculated on the basis of the ESR spectrum has been found to be a good index to reflect the molecular entanglement between the VO complexes and their matrixes in ARs and their fractions, and to indicate the effects of the measurement temperature, pre-heat-treatment as well as the solvent on such interactions. The B parameter value for the VO complexes decreases in the order of asphaltenes > AR ≈ resins > maltenes, implying that the constraint on the mobility of the VO complexes in the samples decreases in the same trend. Increasing temperature, pre-heat-treatment and the addition of toluene reduce the B parameter value, thus, favoring the mobility of the VO complexes in the ARs and their fractions. It can be ascribed to the change in the peripheral environment of the VO complexes surrounded by the matrix molecules. A comprehensive understanding of such molecular entanglement between the VO complexes and their matrixes in ARs may give some important hints to improve the hydrodemetallization performance of AR.
It is very significant to fully understand the molecular structure and physicochemical properties of the metal complexes, as well as their relationships with surrounding molecules in petroleum for designing a better HDM catalyst and a more efficient HDM process. Vanadyl (VO) and nickel porphyrins are the common and major metal species in petroleum, although some non-porphyrin V and Ni species were also found in petroleum.5,6 The metal complexes in petroleum have been widely studied.7–9 The form of the VO complexes in the different fractions of Mayan asphaltenes has been studied by X-ray absorption spectroscopy.10 The peripheral substitution of the porphyrin macrocycle can be examined by using the ultraviolet/visible absorption spectroscopy.11 Shi et al. clarified the structures of the VO porphyrins in the crude oil by FT-ICR-MS.12,13 Biktagirov et al. studied the structure of vanadyl porphyrin in crude oil by electron–nuclear double resonance (ENDOR).14 However, there is few report about how these VO porphyrins are associated with their surrounding organic matrix in petroleum, which may affect the approach of the porphyrins to the active phase on HDM catalyst surface, and thus has an impact on their HDM activity.
The major metal complexes exist in the polar fractions (resins and asphaltenes), especially in asphaltenes.15,16 Yen and his group17–19 proposed a trapping model of the metal complexes in the resins and asphaltenes through physical and covalent bonding, indicating that the metal complexes may behave together with their matrix molecules and aggregation. Ideally, the metal complexes are released from their matrixes to reach freely to the surface of the HDM catalyst. In GC-AED20 and FT-ICR-MS,21,22 the metal complexes are vaporized and ionized, where the metal complexes can be released from their matrixes through thermal or photo-electronic effect. Thus it is very important to clarify the molecular entanglement between the metal complexes and their matrixes or their aggregates in AR.
Electron Spin Resonance (ESR) has been applied to analyse VO complexes in petroleum to extract information on their structure and mobility.23–28 Wong and Yen proposed that the mobility of the VO complexes can be defined by using ESR to distinguish anisotropic/isotropic spectrum of the VO complexes.29 It is assumed that the anisotropic spectrum reflects the constraint of VO complexes in petroleum fractions. The differently constrained extent gives different type of ESR spectrum.29 Campbell and Freed30 concluded that the rotational mobility of the VO complex gives the anisotropic or isotropic spectrum according to its rotational rate. They reproduced a series of spectra by simulation based on the perturbation theory applying rotational correlation time (τR) in Brownian diffusion model and the residual line width contribution.
ESR has also been used for identifying vanadium environment in petroleum by examining g values and hyperfine coupling constants for several square pyramidal environments on the vanadyl complex.31 Espinosa et al. provided a deeper analysis of the electron structure of the vanadyl complex in a series of natural porphyrins in the heavy crude oil through the ESR measurement.32 They estimated relative strength of the V–N bonds in the ligand and the VO bond through the delocalization of the unpaired electrons on V4+ to the coordinating nitrogen. In our previous paper,33 behaviors of the VO complexes in atmospheric residues, their resins and asphaltenes in the presence of the solvents (toluene or tetrahydrofuran) were studied by ESR in detail at 20 to 100 °C to examine the effects of surrounding matrix, concentration in solvent and temperature on the VO rotational mobility. However, the direct examination of the molecular entanglement between the VO complexes and their matrixes in the real ARs and their fractions in the absence of the solvent, which has more application value, has not been involved yet.
In the present study, the electron structure and mobility of VO in three atmospheric residues and their fractions in the absence of the solvent were studied by ESR through the comparison of the ESR parameter B, which is a sensitive indicator to the electron structure of VO32 and the effect of measurement temperature, pre-heat-treatment as well as the solvent of toluene on the VO electron structure were examined to clarify the molecular entanglement between the VO complexes and their surrounding molecules.
LF-AR | KEC-AR | LF/KEC-AR | |
---|---|---|---|
Boiling point (°C) | >360 | >360 | >360 |
Density (g ml−1) | 1.0081 | 0.9745 | 0.9873 |
C (wt%) | 82.6 | 83.8 | 83.4 |
H (wt%) | 10.1 | 11.0 | 10.5 |
S (wt%) | 3.44 | 3.19 | 3.34 |
N (wt%) | 0.33 | 0.29 | 0.32 |
V (ppm) | 152 | 71 | 108 |
Ni (ppm) | 24 | 14 | 20 |
Saturate (wt%) | 16.7 | 25.7 | 20.7 |
Aromatics (wt%) | 51.9 | 48.8 | 50.2 |
Resin (wt%) | 19.5 | 18.4 | 19.2 |
Asphaltene (wt%) | 11.9 | 7.2 | 9.9 |
The maltene, resin and asphaltene fractions from the ARs were obtained by the fractionation shown below: AR was dissolved in n-heptane at a weight ratio of 1/50 (g g−1) with stirring at 60 °C for 5 h, and then the mixture was filtered. The insoluble fraction was extracted in the Soxhlet apparatus with toluene until the extraction solvent became no color, and the solution was rotary-evaporated to recover the asphaltene fraction, which is in the fine powder form. The maltene fraction was obtained by removing n-heptane from the n-heptane solution in a rotary evaporator. The obtained maltene fraction was further fractionated by chromatographic separation in a glass column packed with activated neutral alumina. The maltene fraction with a little amount of n-hexane was poured into the top of the glass column, and then was eluted consecutively with n-heptane, toluene and toluene/methanol mixture (9:1, v/v) with a ratio of solvent to maltene of 250 ml to 1 g for each solvent. The received solutions were rotary-evaporated to obtain the saturate, aromatic and resin fractions, respectively. The resin fraction is in the solid form.
The B parameter has been used as a sensitive indicator of the tetragonal distortion to clarify a change in the VO bond length and distance of four nitrogen ligands in the basal plane, as reported in the literature,32 which can be derived as follows:
B = Δg∥/Δg⊥ |
Δg∥ = g∥ − ge |
Δg⊥ = g⊥ − ge |
The method for studying the vanadyl complexes through the parameter values from the ESR analysis has been described in detail in the literatures.25,32 The relative parameter values are obtained using anisotropic simulation software (JEOL Ltd., Tokyo, Japan), and the deviation for g∥, g⊥ and B is 0.0003, 0.0001 and 0.05, respectively.
Samples | g∥ | g⊥ | B | |
---|---|---|---|---|
ARs | LF | 1.9615 | 1.9943 | 5.10 |
KEC | 1.9615 | 1.9942 | 5.04 | |
LF/KEC | 1.9618 | 1.9943 | 5.06 | |
Maltenes | LF | 1.9618 | 1.9943 | 5.06 |
KEC | 1.9616 | 1.9941 | 4.96 | |
LF/KEC | 1.9616 | 1.9942 | 5.02 | |
Resins | LF | 1.9619 | 1.9944 | 5.11 |
KEC | 1.9614 | 1.9942 | 5.05 | |
LF/KEC | 1.9616 | 1.9943 | 5.09 | |
Asphaltenes | LF | 1.9617 | 1.9945 | 5.21 |
KEC | 1.9615 | 1.9944 | 5.16 | |
LF/KEC | 1.9615 | 1.9944 | 5.16 |
Fig. 2 displays the ESR spectra of VO complexes in LF-AR and its maltene, resin and asphaltene fractions measured at 20 °C. The ESR spectra of LF-AR, LF-Mal, LF-R and LF-As were similar, showing anisotropic spectra. Similarly, it is hard to observe the difference of VO complexes stage in these samples by comparing the ESR spectra at this condition. However, the B parameter value of the VO in asphaltene fraction was slightly, but definitely, larger than those in the AR and its maltene and resin fractions, regardless of the three ARs, as shown in Table 2, indicating a shorter VO bond and/or a longer distance to the nitrogen ligands, i.e., the higher electron density in the axial bond and more electron delocalization to the V orbital are suggested.32 It implies that a larger degree of constraint to the mobility of VO complexes by the surround molecules (matrixes) was found in asphaltene fraction.
The ESR spectra were obtained at 150 °C for VO complexes in LF-AR, its maltenes, resins and asphaltenes, as shown in Fig. 3, further confirm this constraint by the matrixes. VO complexes in LF-maltenes gave a different spectrum, which corresponds to that of TPP VO complex dissolved in toluene measured at −105 °C, while the spectra of VO complexes in LF-AR, LF-resins and LF-asphaltenes correspond to that of TPP VO complex dissolved in toluene measured at −120 °C, which was reported in the previous paper.33 It indicates that the mobility of VO complexes in LF-maltenes was rapider than those in other samples at this measurement condition.17,23 The B parameter value of VO in asphaltenes was also significantly larger than those in AR and resins measured at 150 °C. It is clearly shown that the constraint of the VO complexes by the surrounding molecules was in the order of maltenes < resins ≈ AR < asphaltenes.
Samples | g∥ | g⊥ | B | |
---|---|---|---|---|
20 °C | LF-AR | 1.9615 | 1.9943 | 5.10 |
LF-R | 1.9619 | 1.9944 | 5.11 | |
LF-As | 1.9617 | 1.9945 | 5.21 | |
50 °C | LF-AR | 1.9615 | 1.9943 | 5.10 |
LF-R | 1.9616 | 1.9941 | 4.96 | |
LF-As | 1.9615 | 1.9944 | 5.16 | |
150 °C | LF-AR | 1.9613 | 1.9924 | 4.14 |
LF-R | 1.9612 | 1.9922 | 4.07 | |
LF-As | 1.9605 | 1.9936 | 4.80 |
Samples | g∥ | g⊥ | B | |
---|---|---|---|---|
50 °C | HT-LF-AR | 1.9615 | 1.9941 | 4.98 |
HT-LF-R | 1.9612 | 1.9938 | 4.84 | |
HT-LF-As | 1.9611 | 1.9942 | 5.09 | |
150 °C | HT-LF-AR | 1.9614 | 1.9922 | 4.05 |
HT-LF-R | 1.9613 | 1.9917 | 3.87 | |
HT-LF-As | 1.9606 | 1.9935 | 4.74 |
Fig. 5 ESR spectra of VO complexes in LF-AR, its resins and asphaltenes dissolved in toluene measured at 20 °C. |
Samples | g∥ | g⊥ | B | |
---|---|---|---|---|
AR | LF | 1.9615 | 1.9943 | 5.10 |
LF-T | 1.9628 | 1.9925 | 4.03 | |
Resins | LF | 1.9619 | 1.9944 | 5.11 |
LF-T | 1.9633 | 1.9924 | 3.94 | |
Asphaltenes | LF | 1.9617 | 1.9945 | 5.21 |
LF-T | 1.9625 | 1.9929 | 4.23 |
However, the B parameter is sensitive to the electron structure of VO, thus it is used in the present study to describe the change of VO electron structure in AR and its fractions to clarify the molecular entanglement between the VO complexes and their surrounding molecules. The B parameter value for VO in LF-AR is slightly larger than that in KEC-AR. For the samples of AR, maltenes, resins and asphaltenes measured at 20 °C, the B parameter value decreases in the order of asphaltenes > AR ≈ resins > maltenes. The larger B parameter value reflects shorter length of VO bond and/or a longer of the distances to the nitrogen ligands, which is caused by the higher electron density in the axial bond and larger electron delocalization to the V orbital due to the charge transfer from the π system of the porphyrin ring.32 The larger aromatics group bonding with the porphyrin ring in the heavy AR or its fractions may transfer more charge from the extended π system, resulting in the larger electron delocalization, particularly when the aromatic sheets are stacked.26,35 Thus, the larger B parameter value indicates the stronger constraint on the VO complexes by the surrounding matrix.
At 150 °C, the ESR spectrum of VO complexes in maltenes, which consists of saturate, aromatic and resin fractions without asphaltenes, shows a little rapider mobility in comparison with those in AR and its resin and asphaltene fractions. Since asphaltene fraction is free from saturate, aromatic and resin fractions, the mobility of the VO complexes in the asphaltene fraction is constrained strongly by itself (the most heavy fraction), while the aromatic fraction benefits the mobility of VO complexes in the maltenes. Mutual interaction among the fractions is fatal for the mobility of VO complexes present in the real AR (a mixture of the fractions).
The heat-treated AR and its resin and asphaltene fractions provide the smaller B parameter values compared with the non-heat-treated ones. The heat-treatment at 330 °C under a hydrogen pressure of 9 MPa for 3 h shifted the VO complexes from asphaltenes to resins, and decreased the amount of VO complexes in the heavy sub-fraction of asphaltenes, as reported in the previous paper.16 Thus such smaller B parameter value is possibly contributed to a change in interaction between the VO complexes and their surrounding matrix in AR and asphaltenes during the heat-treatment process, which results in the change of the VO electron structure.
When the VO complex was dissolved in solvent, it gave isotropic spectrum, as observed for the standard VO complex dissolved in toluene.33 However, in the present study, LF-AR and its resin and asphaltene fractions dissolved in toluene with the concentration of 20 wt%, although presenting in a kind of soluble form give anisotropic spectra at the same measurement condition. Nevertheless, the spectra of LF-AR and its resin and asphaltene fractions indicate that the solvent of toluene certainly enhances the mobility of VO. In addition, the B parameter values for the VO in LF-AR and its resin and asphaltene fractions dissolved in toluene are obviously smaller than those without toluene. Thus, toluene, which solvates the resins and asphaltenes, loosens the interaction between the VO complexes and the surrounding molecules in the matrix, and causes the change of the VO electron structure. The VO complexes in LF-AR and its resin and asphaltene fractions dissolved in toluene are still trapped by the surrounding molecules, because they still show anisotropic spectra. How to release the VO complexes from their matrix in AR will be a further research topic. A important message got from this study is to reduce the constraint by the surrounding matrix on the VO complexes through some effective way, which will favour the approach of the VO complexes freely to the catalyst surface and/or into the catalyst pore, and thus benefit the metal removal and reduction of the coke formation on the HDM catalyst.
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