Role of Vanadylporphyrins in the Flocculation and ... - ACS Publications

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Role of Vanadylporphyrins in the Flocculation and Sedimentation of Asphaltenes of Heavy Oils with High Vanadium Content Yulia Yurevna Borisova,* Elvira Gabidullovna Tazeeva, Nikolay Alexandrovich Mironov, Dmitry Nikolaevich Borisov, Svetlana Gabidullinovna Yakubova, Guzalia Rashidovna Abilova, Kirill Olegovich Sinyashin, and Makhmut Renatovich Yakubov A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences, Arbuzov Street 8, Kazan 420088, Russian Federation S Supporting Information *

ABSTRACT: Extraction by N,N-dimethylformamide with further column chromatography allowed obtaining concentrate of vanadylporphyrins from asphaltenes of heavy sulfurous oils with high vanadium content. The prevailing types of vanadylporphyrins, their ratio, and molecular mass distribution were determined. The influence of obtained vanadylporphyrin concentrates on the stability of asphaltenes in the system “solvent/precipitator” was investigated. Kinetic studies using UV−vis spectroscopy have revealed that an increase in the content of vanadylporphyrins in asphaltenes leads to acceleration of their deposition from solution and destabilization of colloidal systems.

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INTRODUCTION In recent decades, heavy crude oils and natural bitumen have been intensely developed.1 Heavy oils (HOs) have a high content of asphaltene-resinous components, heteroatoms (S, N, and O), and metals (V and Ni).2 The development of such hydrocarbons requires more expensive technologies at all stages of production, transportation, and processing compared to conventional oils. One important issue is the formation of deposits in the reservoir and technological equipment. In most cases, asphaltenes play the key role in the formation of deposits.3−6 The asphaltenes in heavy oils with a high content of vanadium and nickel contain up to 1% of these elements; the proportion of vanadium often exceeds 80%.7−9 The processing of heavy crude oil (HO) using catalytic processes has significant restrictions on the content of vanadium and nickel that irreversibly inactivate catalysts. Therefore, the processing and refining of HOs mainly requires thermodestructive and solventadsorption processes (deasphalting), as well as a combination thereof.10−13 Vanadium and nickel mainly exist in oils in the form of porphyrin complexes, which may have a great structural diversity.14−16 Vanadylporphyrins (Figure 1) are one of the most common forms of oil metalloporphyrins and are represented by several series of homologues, which differ in the number and position of alkyl, cycloalkane, and aromatic groups. For the asphaltenes with high vanadium content (0.2−1.0%) the concentration of vanadyl complexes, including vanadyl porphyrins, can be predicted to be 2−10%. Vanadylporphyrins, with their flat aromatic structure, heteroatoms, paramagnetic vanadyl-ion, as well as alkyl substituents in the ring periphery, can significantly influence the process of aggregation and deposition of asphaltenes.17−19 Earlier we have demonstrated using the example of heavy oils from various fields that the increase of vanadyl complexes in © 2017 American Chemical Society

resins increases their ability to inhibit the deposition of asphaltenes.19 The authors17,18 suggested that vanadylporphyrin complexes can actively participate in the aggregation of asphaltenes and play a bridging role in this process. The present work is devoted to the proof of concept how additions of vanadyl porphyrins influence the aggregation and sedimentation process of asphaltenes. The simple approach based of the analysis of UV−vis spectroscopy data was used to monitor the stability of asphaltenes in the solvent/precipitator system. The experiments were performed with the asphaltenes isolated from the two heavy high-sulfur crude oils having high vanadium content. Native vanadyl porphyrins and synthetic vanadyl etioporphyrin were used as additives.

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MATERIALS AND METHODS

Isolation of Asphaltenes, Maltenes, and Resins. Asphaltenes were precipitated from the heavy oil of the Kalmayurskoye (Oil-1) and Ashalchinskoye (Oil-2) fields by 20-fold volume excess of n-heptane. After 24 h, the obtained precipitate was filtered and washed with boiling n-heptane in a Soxhlet apparatus up to decolorization of flowing solvent to remove as much maltenes as possible. The solvent from the maltene solution was removed up to a constant weight using a rotary evaporator (60 mmHg/30 °C). Maltenes were separated by elution column chromatography on silica gel.20 n-Heptane was used to elute saturated and aromatics hydrocarbons, the blend of isopropanol/ benzene (1:1, v/v), for the extraction of resins. Isolation of Vanadylporphyrins from Asphaltenes. Extraction of metalloporphyrins from asphaltenes was conducted by N,Ndimethylformamide (DMF)21 after preliminary dissolving of asphaltenes in a minimum quantity of benzene. The mixture was boiled with reflux for 10 min. After cooling, the extract was separated and the solvent was distilled under vacuum. The resulting extract was subjected to further chromatographic separation.14,22,23 The extract was placed Received: August 30, 2017 Revised: October 17, 2017 Published: November 7, 2017 13382

DOI: 10.1021/acs.energyfuels.7b02544 Energy Fuels 2017, 31, 13382−13391

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Energy & Fuels

Figure 1. Common vanadylporphyrins in heavy oils. in a column (1 × 60 cm) filled with silica gel (approximately 40 cm) and eluated with solvents with an increasing polarity: benzene, benzene/chloroform (50:50), chloroform, and chloroform/isopropanol (98:2). Fractions containing high concentrations of vanadylporphyrins were combined and distilled under vacuum. Also, synthetic vanadium(IV) etioporphyrin III oxide (Alfa Aesar, 39586; abbreviated EtioVOP) has been used. Determination of the Content of Vanadylporphyrins, Vanadium, and Nickel. UV−vis spectroscopy and mass spectrometry are common analytical methods used to identify and quantify porphyrins.14,16,23 The content of vanadylporphyrins in the fractions was spectrophotometrically analyzed according to the intensity of absorption band at 575 nm.21,23 Spectra were recorded in the range 200−800 nm using a PE-5400 UV spectrophotometer (“Ekros”, Russia). Matrix-assisted laser desorption/ionization (MALDI) mass spectra were obtained by UltraFlex III TOF/TOF mass-spectrometer (Bruker Daltonik GmbH, Bremen, Germany) in a linear mode with Nd:YAG laser (λ = 266 nm). Spectra were obtained with 25 kV of accelerating voltage and 30 ns of acceleration delay. The data were processed by using FlexAnalysis 3.0 software (Bruker Daltonik GmbH, Bremen, Germany). 1,8,9-Trihydroxyanthracene was used as a matrix. The metal target MTP AnchorChipTM was used. 0.5 μL of 1% solutions of the matrix and the sample in toluene were successively spotted on the target and evaporated. Spectra were collected for all spots using 50 shots per spectrum. Positively charged ions were analyzed. Polyethylene glycol was used as the standard for calibration of MS. The content of vanadium and nickel in the samples was determined by direct flame atomic absorption spectrometry using an AAS-1N spectrophotometer according to the method described previously.24 The solution of the sample was sprayed into the acetylene-air or acetylene-nitrous oxide flame for determination of nickel and vanadium, respectively. The sample/solvent ratio was varied from 1:4 to 1:20 depending on the sample viscosity and the concentration of the element. The mixture of o-xylene, acetone, and ethanol (80:10:10, v/v) was used as a solvent. The calibration curve method was used for quantitative analysis. As standards, dibutyldithiocarbamates of V(II) and Ni(II) dissolved in the same solvent were used.24 Infrared spectra were recorded using an FT-IR spectrometer JFS183 V from Bruker Company in the range 4000−400 cm−1. Samples

were prepared in tablets with KBr (Acros Organics 206391000). The intensities of the characteristic bands were calculated above the baseline. The following characteristic absorption bands were used: 1650−1820 cm−1 for the C = O group, 1377 cm−1 for the CH3 groups, 1030 cm−1 for the SO groups, and 720 cm−1 for the CH2 groups with n ≥ 4. The reference line was 1600 cm−1 for the aromatic CC bonds. The obtained values of intensities allowed spectral ratios to be calculated.25 Aromaticity was determined from the ratio of the intensities of absorption bands 1600 cm−1 of CC bonds in aromatic fragments to the total intensities of absorption bands at 720 and 1380 cm−1 of C−H bonds in aliphatic structures. Aliphaticity was determined by the ratio of the total intensity of the methyl and methylene groups (CH3 + CH2) to the intensity of absorption bands 1600 cm−1 of CC bonds in aromatic fragments. The ratio of intensities of methyl and methylene groups (CH3/CH2) was used for determination of branching of paraffin structures. Elemental composition was determined using a CHNS-O Euro EA3028-HT-OM analyzer (EuroVector). Preparation of Samples to Characterize the Stability of Asphaltenes in the System of Solvent/Precipitator. Initially, the toluene solutions of asphaltene (1.7 g/L) were prepared for each sample (Oil-1 and Oil-2). Immediately before the experiment an aliquot of each solution was first mixed with additives and then with precipitator (n-heptane) taken in amount allowing to achieve the heptane/toluene ratio equal to 1.5, 1.7, or 1.9. For the control cell, solutions having the same ratios (1.5, 1.7, and 1.9) of solvent (toluene) and precipitator (n-heptane) were prepared. Both the sample preparation procedure and the start of UV−vis measurements at 610 nm were strictly time-controlled, so that the zero time point was the same for all experiments.

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RESULTS AND DISCUSSION The properties and component composition of heavy oils are given in Table 1. Oil (Oil-1) from the Kalmayursky field (Samara region) is recovered from the deposits of the Carboniferous period using the traditional downhole method. Oil (Oil-2) from Ashalchinsky field (Tatarstan) is recovered from the Permian deposits using steam assisted gravity drainage (SAGD).26 13383


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Energy & Fuels Table 1. Properties and Component Composition of Heavy Oils Oil-1

Oil-2

density (at 20 °C), g/cm3 content of sulfur, wt % resins/asphaltenes ratio

0.9531 4.23 1.9

0.9550 4.67 3.3

Contents, wt % fraction boiling at bp 200 °C saturates and aromatics hydrocarbons resins asphaltenes

6.1 36.7 37.2 20.0

2.8 70.4 20.6 6.2

Table 2. Content of Vanadium and Nickel in the Samples contents, wt % sample

Properties

Oil-1 oil asphaltenes DMF extract (yield, wt % = 25.5) DMF residue (yield, wt % = 7.5) Oil-2 oil asphaltenes DMF extract (yield, wt % = 15.9) DMF residue (yield, wt % = 84.1)

The molecular weight of asphaltenes from HOs defined through the maximum of molecular mass distribution of positively charged ions (MALDI TOF) differs slightly, 1600 and 1700 m/z. The main distinguishing feature of asphaltenes is the content of vanadium and nickel that causes different contents of metalloporphyrins in asphaltene extracts (Figure 2). Oil-1 asphaltenes contain a double amount of vanadium and nickel (Table 2). The ratio of the basic characteristic absorption bands in the FTIR spectra revealed the features of the structural-group composition of asphaltenes. According to the values of spectral coefficients, the content of CO and SO groups is noticeably higher in Oil-2 asphaltenes compared to Oil-1 asphaltenes (Table 3), while the content of methyl and methylene groups differs slightly. Table 4 shows the elemental composition of asphaltenes, DMF Extract, DMF Residue and VOP Concentrate. According to the elemental analysis, nitrogen content in Oil-1 asphaltenes is twice as high as in Oil-2 asphaltenes, while the content of other elements differs slightly. Extraction of vanadylporphyrins and oxygen-containing compounds occurs more efficiently for Oil-1. Use of UV−vis Spectroscopy to Characterize the Stability of Asphaltenes in the System of Solvent/ Precipitator. Different approaches can be used to predict the stability of asphaltenes in crude oils. These are the methods, in

V

Ni

0.165 0.61 0.85 0.52

0.014 0.053 0.051 0.052

0.025 0.27 0.30 0.25

0.003 0.027 0.017 0.029

which the oil stability is determined from the ratio of its components, such as colloidal instability index, colloidal stability index, and qualitative-quantitative analysis.27 So, the authors27−29 have noted that the deposition of asphaltenes depends on the ratio of resins and asphaltenes in crude oils. According to this ratio, Oil-2 is more stable than Oil1 (Table 1). There are also methods, in which the process of asphaltene deposition is studied through gravimetry,30 microscopy,31 viscometry,32 refractive index,33 attenuated total reflection Fourier transform infrared spectroscopy (ATR−FTIR),34 nuclear magnetic resonance imaging (NMR imaging),35 small angle X-ray scattering (SAXS), scanning tunneling microscopy,36 and electron spin resonance (ESR).37 We visualized the deposition of asphaltenes using UV−vis spectroscopy: we added a precipitator (n-heptane) to the asphaltenes dissolved in toluene and recorded the changing of light absorption by solutions at 610 nm in a kinetic mode (from the start of destabilization of the system resulting from asphaltene flocculation until equilibrium state after the deposition of asphaltenes).38 The dynamics of light absorption (Figure 3) by solutions of asphaltenes in toluene with initial concentration (1.7 g/L) after adding the same amount of the precipitant n-heptane (n-

Figure 2. UV−vis spectra of DMF extract and VOP concentrate isolated from asphaltenes of Oil-1 (CVOP(DMF extract) = 3.2 g/100g, CVOP(VOP concentrate) = 10.4 g/100g) and Oil-2 (CVOP(DMF extract) = 1.0 g/100g, CVOP(VOP concentrate) = 6.7 g/100g). 13384


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Energy & Fuels Table 3. Structural-Group Composition of Samples According to FTIR Data amount of structural groups, rel. unitsa

a

sample

aromaticity CC/C−Hal

asphaltenes DMF extract DMF residue VOP concentrate

0.44 0.58 0.51 0.26


0.34 0.49 0.39 0.15

branching CH3/CH2 Oil-1 3.13 3.61 4.55 1.70 Oil-2 2.65 4.66 4.96 2.80

CH3 + CH2

SO

CO

2.26 1.73 1.96 3.78

0.73 0.69 0.48 0.88

0.41 0.83 0.35 1.36

2.95 2.05 2.56 6.55

1.14 1.03 0.92 1.83

0.71 0.74 0.22 0.90

Relative to aromatic CC bonds.

was observed after completion of flocculation, as can be seen from the decreased absorption of asphaltene solutions. A more dramatic increase in absorption and its subsequent decrease for the solution of Oil-1 asphaltenes indicates more rapid flocculation and deposition of asphaltenes from the solution compared to Oil-2 asphaltenes (Figure 3). Depending on the quantity and nature of precipitant, there is a limit to stability of colloids in a medium. Colloids begin to flocculate above this limit.40,41 They can be stabilized with special inhibitors (dispersants).42 Figure 4 shows how the stability of asphaltene solutions changes in the system “solvent/ precipitator” after addition of different volumes of n-heptane and the known dispersant 4-nonylphenol (NP). At the dispersant concentration of 4 g/L, the absorption of the solution of Oil-2 asphaltenes slightly increases; then the plateau appears followed by the restoration of the original value of the absorption parameter. This means that there is an aggregation− disaggregation of the most unstable asphaltenes, but they do not deposit. Therefore, the molecules of NP with a concentration of 4 g/L demonstrate an inhibiting ability. In the case of Oil-1 asphaltenes, the addition of 4 g/L dispersant is insufficient for inhibiting the deposition of asphaltenes, as can be seen from the changing absorption maximum and its shift to a later time. The stabilization of Oil-1 asphaltene solution probably requires a higher concentration of dispersant. Kinetic studies using UV−vis spectroscopy revealed that Oil2 asphaltenes are more stable; they flocculate and deposit from toluene solution after the addition of n-heptane more slowly in comparison with Oil-1 asphaltenes with twice the vanadium content. Influence of Vanadylporphyrin Additives on the Stability of HO Asphaltenes in the “Solvent/Precipitator” System. There are various hypotheses about the nature of the association of VP complexes with asphaltenes. On the one hand, the low intensity and widening of the Soret band in the UV−vis absorption spectrum suggests the presence of the axial coordination of the metal center or annelation of aromatic rings on the π-system of porphyrin.43 On the other hand, the lack of detected association of vanadium and nickel octaethylporphyrin with model condensed and bridging aromatic compounds through UV and fluorescence spectroscopy suggests that the association of the most petroporphyrins in crude oil with asphaltenes can be connected with other functional groups that are attached to the porphyrin ring, rather than with the π−π interactions of aromatic rings with the porphyrin core.44 It is believed that vanadylporphyrins in asphaltenes exist in two states: in the form of “loosely” bound molecules

Table 4. Elemental Composition of Samples contents, wt % sample

C


77.23 70.50 78.08 82.73


79.39 75.35 79.13 82.06

H Oil-1 7.84 7.13 8.15 13.61 Oil-2 7.75 7.24 7.71 12.50

N

S

O, Me

C/H

2.11 3.03 1.44 0.85

7.60 6.54 7.72 0.96

5.22 12.8 4.61 1.85

9.8 9.9 9.6 6.1

0.75 2.08 0.95 0.84

6.94 6.59 6.95 2.11

5.17 8.74 5.26 2.49

10.2 10.4 10.3 6.6

Figure 3. Dynamics of light absorption by solutions of asphaltenes (1.7 g/L) in toluene after addition of n-heptane (n-heptane/toluene = 1.9), λ = 610 nm.

heptane/toluene = 1.9) shows that Oil-2 asphaltenes possess a higher colloidal stability. It is believed that light absorption depends on two factors: the absorption due to excitation of molecules and scattering of light by particles or aggregates.39 Flocculation of asphaltenes was tracked through an increase in absorption and a decrease in the concentration of asphaltenes in solution as a result of separate phase formation, through the reduction of the visible absorption over time. The deposition of asphaltene particles 13385


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Figure 4. Dynamics of light absorption by solutions of asphaltenes (1.7 g/L) in toluene after addition of 1.5-, 1.7-, and 1.9-fold volumes of n-heptane or solution of dispersant in n-heptane (NP, 4 g/L), λ = 610 nm.

Figure 5. MALDI-TOF mass spectra of asphaltenes and DMF extracts.

(connected by noncovalent interactions or adsorbed45 and extracted easily) and “tightly” bound molecules (connected through multiple intermolecular interactions).17,44 In this work, we retrieved “loosely” bound molecules of VPs through the DMF extraction from HO asphaltenes (Table 2). DMF extracts have a more aromatic and branched structure (Table 3), a higher content of vanadium and nitrogen (Tables 2 and 4), and a lower molecular mass (Figure 5) in comparison with the original asphaltenes. At the same time, a lot of vanadium and nitrogen-containing compounds, “tightly” bound VP molecules, remain after extraction (Tables 2 and 4). The mass spectrum of DMF extract from Oil-1 asphaltenes (Figure 5) demonstrates only C27−C35 vanadylporphyrins. The maximum signal in most cases corresponds to the ion [M + H]+. Another feature of this spectrum is that the dominant porphyrin in C27−C29 row is Rhodo-ETIO type, in C30-C33 row is Di-DPEP, and the most intense signal of porphyrins C34 and C35 refers to the DPEP type. The maximum content falls on C31 Di-DPEP vanadylporphyrin.

DMF extract from Oil-2 asphaltenes demonstrates the shift of the maximum on the spectrum toward the homologue C30 of DPEP-porphyrin. The predominant type of vanadylporphyrin is DPEP. The spectrum shows an interval of C 27 −C 36 homologues, signals of ETIO and Di-DPEP vanadylporphyrins. Further concentration of vanadylporphyrins was carried out in the column with silica gel after addition of DMF extract. Fractionation was performed using solvents with increasing polarity: benzene, benzene/chloroform (50:50), chloroform, and chloroform/isopropanol (98:2). Fractions with a high content of vanadylporphyrins and a smaller content of other background substances in UV−vis spectra were combined into the VOP concentrate (Figure 2). The samples of VOP concentrate chromatographically isolated in column with silica gel are characterized by less aromatic structures (Tables 3 and 4) than asphaltenes, from which they were obtained. In addition, according to IRspectrometry, vanadylporphyrins were isolated along with molecules with a large amount of CO and SO groups (Table 3). 13386


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Figure 6. MALDI-TOF mass spectra of VOP concentrate.

Figure 7. Distribution of vanadylporphyrin types in VOP concentrate.

The peaks of vanadylporphyrins C26−C38 are identified in the mass spectrum of VOP concentrate (Figure 6) isolated from Oil-1 asphaltenes. The identification of heavier homologues is hindered by the complexity of the mass spectrum in the range of higher masses. The most intensive peak falls on the

homologue C31 of Di-DPEP (525 m/z). This type of vanadylporphyrin is predominant among the porphyrins with the number of carbon atoms C28−C31 and C34 and C36. In other cases (porphyrins with the number of carbon atoms C26, C27, C32, C33, C35, C37, and C38), the homologues of DPEP13387


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Energy & Fuels Table 5. Contents of Vanadylporphyrin Types in VOP Concentrate Oil-1 VOP type (C26−C38) ETIO DBE = 17, m/z = DPEP DBE = 18, m/z = Di-DPEP DBE = 19, m/z = Rhodo-ETIO DBE = 20, m/z = Rhodo-DPEP DBE = 21, m/z = Rhodo-Di-DPEP DBE = 22, m/z =

Oil-2 amount, %

VOP type (C27−C36)

amount, %

7.2

ETIO (C27−C35) DBE = 17, m/z = 459 + 14n DPEP (C27−C35) DBE = 18, m/z = 457 + 14n Di-DPEP (C27−C35) DBE = 19, m/z = 455 + 14n Rhodo-ETIO (C30-C35) DBE = 20, m/z = 453 + 14n Rhodo-DPEP (C31−C35) DBE = 21, m/z = 451 + 14n Rhodo-Di-DPEP (C33−C35) DBE = 22, m/z = 449 + 14n

23.9

459 + 14n 27.4 457 + 14n 31.0 455 + 14n 9.4 453 + 14n 13.8 451 + 14n 11.2 449 + 14n

43.6 14.6 9.4 6.0 2.5

Figure 8. Dynamics of light absorption by solutions of asphaltenes (1.7 g/L) in toluene after addition of 1.5- or 1.9-fold volumes of n-heptane or solutions of VOP concentrate (100 mg/L) or EtioVOP (30 mg/L) in n-heptane, λ = 610 nm.

To study the role of vanadylporphyrins as additives to asphaltenes in the process of flocculation and deposition of HO asphaltenes, we used VOP concentrates that were isolated from the same asphaltenes. The impact of the content of VOP concentrates on the stability of asphaltenes in the system “solvent/precipitator” was investigated in a kinetic study (Figure 8). Synthetic vanadyl etioporphyrin EtioVOP was used as additive in order to perform comparative analysis. Figure 8 shows that solutions of asphaltenes (1.7 g/L) with the additives of VOP concentrate (100 mg/L) and EtioVOP (30 mg/L) are characterized by a sharper increase in absorption and its subsequent decrease, which suggests a more rapid flocculation and deposition from solution compared with the same asphaltenes without additives. In addition, a lower value of absorption of asphaltene solutions after the flocculation and deposition of asphaltene particles is observed in case of a lesser concentration of synthetic vanadyl etioporphyrin. The complexity of asphaltene composition makes it difficult to understand the true nature of aggregation. Thus, Porte et al.48 suggested that aggregation and precipitation are apparently controlled by various intermolecular forces. Aggregation is induced by strong specific interactions (i.e., hydrogen bonds) in good nonpolar solvents, and precipitation is determined by weak nonspecific dispersions in bad nonpolar solvents. Using

vanadylporphyrins are predominant. The signal intensity of homologues of ETIO-vanadylporphyrin is several times lower than the intensity of DPEP-vanadylporphyrins and in most cases even inferior to the signal intensity of other types of vanadylporphyrins (Rhodo-ETIO, Rhodo-DPEP, and RhodoDi-DPEP) identified in the mass spectrum. VOP concentrate (Figures 6 and 7) isolated from Oil-2 asphaltenes is characterized by a smaller number of homologues: C27−C36. The maximum falls on the homologue C32 of DPEP-type vanadylporphyrin (m/z 541). This type prevails among porphyrins with the number of carbon atoms C27, C28, and C31−C36. ETIO-type porphyrins prevail among porphyrins C29 and C30. The porphyrins with DBE ≥ 20 appear on the spectrum later than other types: Rhodo-ETIO, RhodoDPEP, and Rhodo-Di-DPEP vanadylporphyrins become visible only starting from homologues C30, C31, and C33, respectively. Table 5 and Figure 7 contain types of vanadylporphyrins identified on the mass spectrum of VOP concentrate samples, as well as their proportion calculated using the intensity of signals in the mass spectrum. The ratio ΣDPEP/ΣETIO is widely used as an indicator of the oil maturity. It increases along with maturity.46,47 It is 1.8 for Oil-2, which indicates that this oil is at a stage of maturation, while Oil-1 (ΣDPEP /ΣETIO = 3.8) is more mature. 13388


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atomic force microscopy (AFM), Wang et al.49 showed that the forces between asphaltenes are very sensitive to the composition of the solvent. In pure toluene, there is a steric long-range repulsion. Steric repulsion in heptol (a mixture of toluene with n-heptane) gradually decreases and changes to a weak attraction with an increase in the heptane fraction. The attraction in pure n-heptane can be explained only by van der Waals forces,50 which are believed to promote aggregation of asphaltenes leading to their precipitation. A key role in the mechanism of aggregation of asphaltenes is attributed by Gawrys et al.51 to interactions of metalloporphyrins with nitrogen-containing fragments of asphaltenes due to hydrogen bonding and axial coordination.43,52

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yulia Yurevna Borisova: 0000-0003-1677-3668 Nikolay Alexandrovich Mironov: 0000-0003-1519-6600 Dmitry Nikolaevich Borisov: 0000-0002-3755-7764 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Russian Science Foundation (Project 15-13-00139).

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CONCLUSIONS A study of the composition and properties of asphaltenes was carried out on the example of two heavy high-sulfur oils with high vanadium content from Volga-Ural oil and gas basin. Oil-1 asphaltenes (Kalmayurskoye field HOs, Carboniferous period) have a higher content of vanadium, nickel, and nitrogen. Oil-2 asphaltenes (Ashalchinskoye field HOs, Permian period) are characterized by a higher proportion of CO and SO groups in an averaged molecule. The C/H ratio and the sulfur content as well as the proportion of methyl and methylene groups in the asphaltenes differ insignificantly for both oils. The content of vanadium is about 1 order of magnitude higher than the content of nickel. Extraction of asphaltenes with N,N-dimethylformamide and subsequent chromatographic purification of the extracts allows to obtain the vanadyl porphyrin concentrates with vanadyl porphyrin content ranged from 6.7 to 10.4 wt %. DPEP and DiDPEP are the predominant forms of porphyrins in obtained concentrates. The maximum on mass spectra fall on the homologue C31 of Di-DPEP vanadylporphyrin (525 m/z) for Oil-1 and the homologue C32 of vanadylporphyrin of DPEPtype (m/z 541) for Oil-2, respectively. Kinetic studies using UV−vis spectroscopy revealed that Oil2 asphaltenes are more stable, because they more slowly flocculate and deposit from the toluene solution after addition of n-heptane. It has been demonstrated that an increase in the content of both native vanadylporphyrins and synthetic vanadyl etioporphyrin leads to a faster flocculation in the system “solvent/precipitators” and accelerated sedimentation of asphaltene aggregates from the solution. Thus, it has been demonstrated that for heavy crude oils with high vanadium content molecules of vanadylporphyrins affect the process of aggregation of asphaltenes and accelerate the deposition of aggregates in solution. Therefore, the content of vanadylporphyrins in asphaltenes should be taken into account along with other factors when predicting the stability of heavy crude oils in different processes of production, transportation and processing.

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NOMENCLATURE HO = heavy oil ETIO = etioporphyrins DPEP = deoxophylloerythroetio porphyrins Di-DPEP = dicyclic-deoxophylloerythroetio porphyrins Rhodo-ETIO = rhodo-etioporphyrins Rhodo-DPEP = rhodo-deoxophylloerythroetio porphyrins Rhodo-Di-DPEP = rhodo-dicyclic-deoxophylloerythroetio porphyrins DBE = double bond equivalence, shows the degree of proton deficiency of the oil components HCs = hydrocarbons DMF = N,N-dimethylformamide R/A = resin/asphaltenes ratio VOP = vanadyl porphyrin VOP concentrate = vanadyl porphyrin concentrate NP = 4-nonylphenol b.p. = boiling point REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02544. FTIR spectra of VOP concentrates for Oil-1 and Oil-2, considered in the paper. (PDF) 13389


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