Chromatographic Isolation of Petroleum Vanadyl Porphyrins Using

Dec 4, 2017 - Of course, such quantification does not take into consideration the probability of an uneven distribution of VPs differing in extinction...
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Chromatographic Isolation of Petroleum Vanadyl Porphyrins Using Sulfocationites as Sorbents Nikolay Alexandrovich Mironov,* Guzalia Rashidovna Abilova, Kirill Olegovich Sinyashin, Pavel Ivanovich Gryaznov, Yulia Yurevna Borisova, Dmitry Valerevich Milordov, Elvira Gabidullovna Tazeeva, Svetlana Gabidullinovna Yakubova, Dmitry Nikolaevich Borisov, 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: A comparative analysis of the composition of vanadyl porphyrins isolated from heavy oil using two different sulfocationites has been carried out. As a source of vanadyl porphyrins, heavy oil of Volga-Ural basin characterized by a high vanadium content was used. The N,N-dimethylformamide extract of asphaltenes was derived from this oil and subjected for isolation of primary vanadyl porphyrin concentrate on a SiO2 column, which was then chromatographically purified with sulfocationite by our improved method. Strongly acidic cation-exchange resin and asphaltene sulfocationite recently developed in our laboratory were used as sulfocationites. According to ultraviolet−visible spectroscopy, both sulfocationites showed excellent applicability for purification, providing isolation of a broad (>50%) fraction of vanadyl porphyrins with higher spectral purity compared to results of conventional methods. Results of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis showed that composition of isolated vanadyl porphyrins depends upon the chemical nature of sulfocationite. Despite the same range of vanadyl porphyrin homologues (C26−C40, with maximum falling on C32) isolated by both sulfocationites, purification with asphaltene sulfocationite resulted in a 1.4-fold decrease in the content of the most abundant DPEP type of vanadyl porphyrins, with a corresponding 1.1−1.9-fold increase in the content of the rest of the types. It was also established that, when purification is accomplished, a significant part of the same vanadyl porphyrins still remains in the column, which can be explained by their associations with non-porphyrin components of the oil.

1. INTRODUCTION

An important step in this direction was made by Barwise and Whitehead in the study of vanadyl porphyrins (VPs) isolated from Boscan distillate.20 Considering the high content of nitrogen-containing compounds in the resins, they used sulfocationite (silica gel modified with propylsulfonic acid) as a sorbent for the final stage of chromatographic purification of the VP concentrate. In this method, purification occurs because a strong acid cation exchanger retains the impurities containing basic nitrogen and does not interact with VPs whose nitrogen atoms are protected by chelated vanadium. This approach provides almost quantitative (93%) isolation of VPs with purity sufficient for detailed mass spectrometric investigations. Despite the promise of this method, neither its author nor other researchers who used this approach later21−23 have compared the features of the elution of VPs and concomitant impurities. However, this information may be crucial for determining the optimized chromatographic conditions, allowing for the improvement of the quality of VP purification. Another disadvantage of the proposed method is the high cost of the used sulfocationite, which makes it undesirable for purification of preparative amounts of VPs.

The development of more effective methods of isolation and purification of petroporphyrins extends the capabilities of their analysis and identification1 and can stimulate investigation of their practical properties. The current interest to petroporphyrins is caused by their ability to act as geochemical biomarkers2−4 and chelating agents toward vanadium and nickel1 that act as catalyst poisons for many petroleum refinery processes. In addition, in the past decade, the development of ultrahigh-resolution mass spectrometric methods has intensified the search for new types of petroporphyrins.5−11 These studies have attracted the interest for development of more effective methods of purification of petroleum metalloporphyrins. Since Treibs discovered porphyrins in oils and bitumen in 1934, a large number of methods have been proposed to obtain concentrates enriched with petroporphyrins in both their native form (extraction by polar solvents, chromatography, and sublimation)3,5,8,9,11−15 and modified form (acidic demetallization and derivatization).3,16−19 However, as a result of the extreme complexity of the composition of the asphaltene−resin fractions of oils where the metalloporphyrins accumulate, the production of pure metalloporphyrins still remains a significant challenge. The solution of this problem requires the development of multistage approaches that allow for the maximization of the purity of isolated metalloporphyrins. © XXXX American Chemical Society

Received: September 19, 2017 Revised: November 22, 2017 Published: December 4, 2017 A

DOI: 10.1021/acs.energyfuels.7b02816 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels In this work, we present an improved method for chromatographic isolation of heavy oil VPs using sulfocationite. The changes in the composition and distribution of isolated VPs depending upon the type of sulfocationite used were studied by ultraviolet−visible (UV−vis), high-performance liquid chromatography (HPLC), and matrix-assisted laser desorption/ionization time-of-flight (MALDI−TOF) methods. We selected two compounds as sulfocationites: cation-exchange resin CU-2-8 (domestic analogue of Amberlite IR-120), a sulfonated copolymer of styrene and divinylbenzene, whose applicability in purification of VPs has been demonstrated in the work,23 and a new asphaltene sulfocationite (sulfonated asphaltenes) developed by us recently.24 A much lower cost of these sulfocationites in comparison to silica gel modified with propylsulfonic acid makes them much more promising sorbents for preparative-scale purification of VPs. The study also reveals advantages of our approach compared to conventional column chromatography on silica gel or alumina, which became the main method of isolation of VPs in the recent works.5,8,11,12,15

Scheme 1. Isolation of the Primary VP Concentrates from the Heavy Oil and Subsequent Fractionation by a Gradient Elution Method through the Column with Sulfocationite

2. MATERIALS AND METHODS 2.1. Materials. The solvents used for oil fractionation and column chromatography with silica gel and sulfocationites were of reagentgrade purity and were applied without further purification (chloroform contained 0.5% ethanol as a stabilizer). HPLC-grade methanol and acetonitrile were used for HPLC measurements. Activated macroporous silica gel was dried prior to the experiment for 8 h at 150 °C. The cation-exchange resin CU-2-8 and asphaltene sulfocationite (n-C6 asphaltenes of Ashalchinskoe oilfield successively modified with an excess of 94% sulfuric acid and 40% oleum, as described in our previous paper24) were ground up using an A11 basic analytical mill (IKA, Germany) and porcelain mortar and pestle into a fine powder and were sieved through the 0.1 mm sieve to delete the more coarse fraction. Considering that asphaltene sulfocationite can contain residual amounts of hydrocarbons and resins, it was subjected to further purification. It was placed in a chromatographic column, and 10-fold volumes of chloroform and 10% solution of isopropanol in chloroform were sequentially passed through it. Then, it was dried for 6 h in a drying oven at 100 °C. As a raw material for the isolation of VPs, heavy oil of Kalmayurskoe oilfield (Volga-Ural basin, Russia) having a high vanadium content was used. This oil is characterized by the following parameters:25,26 density of 0.9531 g/cm3, resins of 37.2 wt %, asphaltenes of 20.0 wt %, and V and Ni contents of 0.165 and 0.014 wt %, respectively. 2.2. Equipment. UV−vis spectra were recorded on a PE-5400UV spectrophotometer (Ecroskhim, Russia) using a 10 mm path length quartz cuvette and chloroform as the solvent. Mass spectra were registered on an UltraFlex III MALDI−TOF/TOF mass spectrometer (Bruker, Germany) with 1,8,9-trihydroxyanthracene as the matrix. Reversed-phase HPLC was performed on a Chromatec-Crystal HPLC 2014 system (Chromatec, Russia) equipped with a 3 μm Hypersil ODS column (4.6 × 25 mm). Elution was carried out with 45:45:10 CH3CN/MeOH/H2O at 1 mL/min. The eluent was monitored with an UV−vis detector set at 410 and 250 nm for detection of VPs and impurities, respectively. The concentration of metals was measured on an AAS-1N atomic absorption spectrophotometer according to a previously described method.27 2.3. Recovery of Asphaltenes and Resins. Asphaltenes and resins were isolated from the oil according to the known method used by us earlier.28 Briefly, asphaltenes were precipitated from petroleum by treatment with 20-fold volume excess of n-hexane (Scheme 1) and left for 24 h. The precipitate was filtered and washed with boiling nhexane in a Soxhlet apparatus up to decoloration of flowing solvent to remove as much maltenes as possible. The obtained asphaltenes were concentrated up to a constant weight using a rotary evaporator. Resins were isolated using a silica gel column by sequential elution of

maltenes with n-hexane (separation of hydrocarbons) and 50 vol % solution of isopropanol in benzene (Scheme 1). 2.4. Isolation of the N,N-Dimethylformamide (DMF) Extract from Asphaltenes. The deposition extraction method developed by us previously26,29 was employed for preliminary sample enrichment by VPs. DMF was chosen as a solvent for extraction because, among the solvents tested (acetone, acetonitrile, DMF, and isopropanol), it showed the best extraction ability toward asphaltenic VPs.26 Asphaltenes were dissolved in a minimum quantity of benzene. A 10-fold volume of DMF was added to this solution, and the mixture was refluxed for 10 min. After cooling, the DMF extract was separated from the precipitate by filtration. The solvent was removed on a rotary evaporator, and the resulting residue was dried to a constant weight at 60 °C. 2.5. Isolation of Primary VP Concentrates. The glass chromatographic column with 15 mm inner diameter was filled with silica gel suspended in benzene to the height of 40 cm. A total of 100 mg of resins or DMF extract of asphaltenes was dissolved in 5 mL of benzene and introduced into the column. Nickel porphyrins (NiPs) were eluted by benzene, and VPs were eluted by chloroform (Scheme 1). Consumption of benzene and chloroform was ∼200 and ∼400 mL, respectively (if the VP front moves too slowly, 1−2% isopropanol may be added to chloroform), and the volume of collected fractions was 10 mL. The obtained fractions were analyzed by an UV−vis spectrophotometer. The fractions for which the bands characteristic for VPs were detected (the Soret band at ∼410 nm as well as the α and β bands at ∼570 and ∼530 nm, correspondingly) were combined. When fractionation was accomplished, a black residue was found at the top part of the column (the most polar asphaltenes, obviously). Its amount was estimated to be 32% of the weight of the sample introduced into the column. We were able to partially desorb this residue by extraction with an isopropanol/chloroform mixture (1:1) and then with DMF. However, only trace amounts of VPs were found in the extract. 2.6. Purification of VPs by Sulfocationite. The PureFlash cartridges (Starlab Scientific, China) with an inner diameter of 12.8 mm were used for purification of VPs by sulfocationites. The cartridge was filled with 4 cm3 of sulfocationite, which was then conditioned by a 10-fold volume (with respect to the volume of the stationary phase) of the first eluent. Then, a filter paper disc saturated with 1.5 mg of the primary VP concentrate was fixed at the top of the stationary phase. Gradient elution of the VP concentrate was started using hexane and chloroform with a stepwise increase of the chloroform fraction from 0 to 100% (Scheme 1). The volume of eluent at each step was 15 mL, B

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Energy & Fuels and elution was carried out under an elevated pressure ranging from 0.2 to 1.5 atm, providing a ∼1 mL/min elution rate (the higher the eluent polarity, the higher the required pressure). Samples with residues not eluted were washed away from the column by 10% solution of isopropanol in chloroform. To record UV−vis spectra, the obtained fractions were dried and redissolved in 10 mL of chloroform. 2.7. UV−Vis and MALDI−TOF Spectra Processing. To quantify the light absorption by VPs and background impurities at the wavelength corresponding to the absorption maximum of VPs (410− 412 nm, depending upon the fraction), trend lines were calculated in the MS Excel 2010 software using experimental points in the range of 340−470 nm, excluding the range corresponding to the Soret band (from ∼370 to ∼450 nm). Approximation was made using the exponential function of the formula y = ae−bx, and the approximation accuracy R2 varied from 0.9914 to 0.9996, with the average R2 value being 0.9967. The intensities of the light absorption at the Soret band maxima above and below these trend lines calculated for each spectrum were used as a measure of VP and impurity contents, respectively. VPs were identified in MALDI−TOF mass spectra by their molecular ions [M]+. Molecular masses of homologues belonging to the six main types of VPs (Etio, DPEP, Di-DPEP, Rhodo-Etio, RhodoDPEP, and Rhodo-Di-DPEP)1 were calculated using ChemOffice software. A quantitative distribution of various types of VPs within isolated fractions was assessed by relative intensities of corresponding [M]+ ions, taking total intensity of all mass spectrometry (MS) peaks attributed to molecular ions of VPs equal to 100%. The isotopic contribution of [M − 1]+ ions into the intensities of [M]+ peaks was taken into consideration.

Figure 1. UV−vis spectra of the primary VP concentrates (3 × 10−5 wt % in CHCl3) isolated from the resins (bottom line) and the DMF extract of asphaltenes (top line).

fractions F3−F5 is consistent with the color of the diluted solution of pure synthetic vanadyl C32-etiolporphyrin (Figure S2 of the Supporting Information). In the case of asphaltene sulfocationite, we started the elution from a more polar eluent, 40% chloroform in hexane. Five fractions were obtained: F6 and F7, which were pink, F8 and F9, which were yellow, and F10, which was brown. The necessity to change the chromatographic conditions is explained by a higher polarity of the asphaltene matrix compared to the hydrocarbon matrix of the cation-exchange resin. A higher polarity is associated with both the presence of heteroatoms in the composition of asphaltenes32 and the formation of additional hydrophilic oxygen-containing groups (hydroxyl, carbonyl, and carboxyl) on the surface of asphaltenes during their sulfonation.24 UV−vis spectra were recorded to establish the features of VP distribution between fractions and to estimate the content of impurities in them (Figure 2). The presence of the Soret band at ∼410 nm indicates that all colored fractions contain VPs. In general, for pink fractions (left diagrams in Figure 2), the less light absorption in the short-wave part of the spectrum is observed. It allows for the association of the appearance of yellow and brown colors of fractions with components of a non-porphyrin nature (impurities) whose presence leads to an increase in background absorption (right diagrams in Figure 2). In the UV−vis spectra of F1 and F2 fractions obtained using cation-exchange resin, no bands characteristic to VPs were found (data not shown). To establish the distribution of VPs among the fractions, we used parameter AVP calculated as the difference between the height of the Soret band, AS, and background absorption at the same wavelength, Ab, which depends upon the amount of impurities (AVP = AS − Ab). Using AVP as a measure of the amount of VPs in each fraction, the quantitative distribution of VPs between fractions was calculated (Table 1). Of course, such quantification does not take into consideration the probability of an uneven distribution of VPs differing in extinction coefficients between the fractions. However, the maximums of the Soret bands detected in the spectra of the fractions F3−F10 (Figure 2) lie in the range of 410−412 nm. This range is significantly less than that of the known petroleum VPs (406−414 nm).1,33 This may indirectly indicate that various types of VPs are relatively evenly distributed between the fractions. As seen from Table 1, in the case of cation-exchange resin, almost half (49%) of isolated VPs accumulate in pink fractions F3−F5, whereas in the case of asphaltene sulfocationite, only 36.9% of VPs account for pink fractions F6 and F7. Obviously,

3. RESULTS AND DISCUSSION We propose the following three-step way to obtain VPs: isolation of the fraction having the highest content of metalloporphyrins from the oil, recovery of primary VP concentrate from this fraction, and final purification of the obtained VP concentrate using sulfocationite. 3.1. Production of Primary VP Concentrates. Because the majority of vanadium and nickel and, accordingly, metalloporphyrins accumulate in asphaltenes and resins during saturate, aromatic, resin, and asphaltene (SARA) fractionation of crude oils,30,31 we obtained two primary VP concentrates: from resins and from DMF extract of asphaltenes (Scheme 1). The yields of these concentrates were 17.9 and 2.4 wt %, correspondingly (the yield is based on the crude oil). Detailed data on the yield of the products of all enrichment steps together with V and Ni concentrations in these products are summarized in Table S1 of the Supporting Information. As expected, the highest V content (1.589 wt %) was found for primary VP concentrate isolated from the DMF extract of asphaltenes. In the case of the resin primary VP concentrate, an 8-fold less concentration of V was detected (0.199 wt %). The UV−vis analysis revealed that VPs are present in both concentrates, but the concentrate derived from the DMF extract of asphaltenes has almost an order of magnitude higher Soret bands than the resin concentrate (Figure 1). Thus, we chose the primary VP concentrate from the DMF extract of asphaltenes for further purification. 3.2. Gradient Elution of Primary VP Concentrates. Gradient elution of the concentrate through the column with cation-exchange resin was carried out according to Scheme 1. Elution resulted in the fractions F1−F10, from which F1 and F2 were colorless, F3−F5 had a distinguishable pink color, F6− F9 were of different shades of yellow, and F10 was brown. The photograph of these fractions concentrated up to 1/15 of the initial volume is given in Figure S1 of the Supporting Information. It is noteworthy that the pink color of the C

DOI: 10.1021/acs.energyfuels.7b02816 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 2. UV−vis spectra of the fractions obtained by stepwise gradient elution of the primary VP concentrate isolated from the DMF extract of asphaltenes through the columns with various sulfocationites (fraction F10 obtained with asphaltene sulfocationite was measured after 2-fold dilution). Left graphs correspond to pink fractions, and right graphs correspond to yellow and brown fractions.

Table 1. Composition of Eluents, Colors of the Fractions and Primary VP Concentrate, Quantitative Distribution of VPs between the Fractions, and AS/Ab Ratio Characterizing the Enrichment of the Sample with VPs According to UV−Vis Data cation-exchange resin

asphaltene sulfocationite

fraction

CHCl3/n-hexane

color

VP (%)

AS/Ab

color

F1 F2 F3 F4 F5 F6 F7 F8 F9 F10

0:100 2:98 5:95 10:90 20:80 40:60 60:40 80:20 100:0 90:10b primary VP concentrate

colorless colorless pink pink pink yellow yellow yellow yellow brown

nda nd 6.1 14.9 28.0 19.9 6.9 3.7 2.4 18.1

18.2 20.0 18.5 7.4 4.2 3.1 2.0 2.4

pink pink yellow yellow brown

color brown

a

VP (%)

16.3 20.6 20.5 15.6 27.0 AS/Ab

AS/Ab

>20 >20 10.5 5.5 2.1

5.8

b

nd = not detected by UV−vis spectroscopy. A 10% solution of isopropanol in chloroform.

from impurities present in primary VP concentrate showed that this procedure can be significantly simplified through the isolation of the most pure pink VP fractions in one step. For this purpose, we have conducted the elution of primary VP concentrate in the three stages: first, by 20 and 70% solution of chloroform in hexane (for cation-exchange resin and asphaltene sulfocationite, respectively), then by pure chloroform, and finally by 10% solution of isopropanol in chloroform. In the case of asphaltene sulfocationite, an intermediate concentration of chloroform in hexane (70%) between the fractions F7 and F8 was taken because the high value of the AS/Ab ratio observed for F8 (10.5 in Table 1) indicates the presence of VPs, which can still be separated from the impurities. As a result, three fractions F′1−F′3 (pink, yellow, and brown) were

a stronger retention of VPs by asphaltene sulfocationite is explained by a higher polarity of the asphaltene matrix compared to the hydrocarbon matrix of the cation-exchange resin. To determine which fractions are enriched with VPs relative to the starting primary VP concentrate and which fractions lose them, the AS/Ab ratio was used. A comparison of this parameter for the obtained fractions to that calculated for the primary VP concentrate suggests that concentrating of VPs occurs only in the fractions F3−F5 and F6−F8 isolated with the help of cation-exchange resin and asphaltene sulfocationite, respectively (Table 1). However, the best purification with the yield of pink fractions corresponds to AS/Ab = 18 and higher. 3.3. Optimized Method for Purification of VPs. The detailed investigation of chromatographic separation of VPs D

DOI: 10.1021/acs.energyfuels.7b02816 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. UV−vis spectra of the fractions obtained by sequential elution of the primary VP concentrate with the following eluents: 20 or 70% CHCl3 in hexane (for cation-exchange resin and asphaltene sulfocationite, respectively), pure CHCl3, and 10% solution of i-PrOH in CHCl3.

obtained by both sulfocationites. The UV−vis spectra of these fractions are shown in Figure 3. Despite the change of chromatographic conditions, the pink fraction obtained with cation-exchange resin also contains nearly half of VPs (50.9% for the fraction F′1 versus 49% for the sum of the fractions F3−F5 in Table 1) with AS/Ab > 20. The same high value of the AS/Ab ratio was observed for the F′1 fraction obtained using asphaltene sulfocationite, despite increasing the proportion of chloroform up to 70%. The increased proportion of chloroform in the eluent has led to the increase in the yield of VPs up to 52.4%, whereas the total yield with F6 and F7 fractions was only 36.9% (Table 1). The comparison of the UV−vis spectra of F′1 fractions to literary data5,8,11,13,14 suggests that the spectral purity of VPs obtained by our approach as a wide (>50%) fraction is comparable or better than the best results of other purification methods that allow for the acquisition of only narrow fractions of VPs of similar purity. The F′2 and F′3 fractions contain 33.9 and 15.3% of VPs in the case of cation-exchange resin and 25.7 and 21.9% of VPs in the case of asphaltene sulfocationite, respectively. The shift of the VP distribution from F′2 toward the more polar fraction F′3 observed for asphaltene sulfocationite can also be explained by a higher polarity of its matrix. A similar shift is also visible for impurities: in the range of 300−370 nm, the amount of light absorption of fraction F′2 obtained with the help of asphaltene sulfocationite is less than that for cation-exchange resin (Figure 3). 3.4. Composition of the Fraction F′1. Because cationexchange resin and asphaltene sulfocationite differ in polarity, we decided to find out how this circumstance affects the composition of VPs derived with the F′1 fraction. For this purpose, HPLC and MALDI−TOF MS methods were used. The chromatograms obtained for the fraction F′1 under conditions analogous to those in ref 21 have a poor peak resolution (Figure 4), which indicates the presence of a great diversity of isomers, homologues, and various types of VPs. In general, both chromatograms have a similar configuration, and retention time intervals ranged from 13 to 90 min. However, the distinct changes in the shape and height of some nonresolved peaks can be found, especially in the left part of the chromatogram (Rt < 43 min) that corresponds to more polar VPs. This unequivocally means that the chemical nature of sulfocationite affects the composition of isolated VPs. To gain further insight into the composition of purified VPs, the MALDI−TOF mass spectra were recorded in the range of m/z 300−1500. MS spectra showed the same C26−C40

Figure 4. Reversed-phase HPLC of the F′1 fraction isolated by various sulfocationites.

homological series of VPs for both F′1 fractions (Figure 5) (the higher mass VPs were not analyzed as a result of the noisy spectra at m/z >660). Both spectra exhibited close molecular weight distribution (MWD) of VPs, having a characteristic bellshaped form, with maximum falling on the homologue C32. The most noticeable visual difference between the two spectra is a less convex shape of the spectrum in the case of asphaltene sulfocationite. To gain more detailed information from MALDI−TOF MS spectra, the distribution of the most common types of VPs1 within fractions was examined. Because metalloporphyrins are known to form molecular ions [M]+ upon ionization by MALDI and laser desorption/ionization (LDI) methods,3,16,34,35 quantification of VPs was carried out using relative intensities of corresponding [M]+ ions. The following six types of VPs were analyzed: vanadyl etioporphyrins (Etio), m/z 459 + 14n; vanadyl deoxophylloerythroetioporphyrins (DPEP), m/ z 457 + 14n; vanadyl dicyclic-deoxophylloerythroetioporphyrins (Di-DPEP), m/z 455 + 14n; vanadyl rhodo-etioporphyrins (Rhodo-Etio), m/z 453 + 14n; vanadyl rhodo-deoxophylloerythroetioporphyrins (Rhodo-DPEP), m/z 451 + 14n; and vanadyl rhodo-dicyclic-deoxophylloerythroetioporphyrins (Rhodo-Di-DPEP), m/z 449 + 14n, where n is an integer from 0 to 14. Their [M]+ signals were derived from the spectra and used for construction of histograms describing the E

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while in the case of asphaltene sulfocationite, the Di-DPEP series sometimes competes with DPEP, especially in the area of low and high mass (Figure 6). Further differences in the composition of the F′1 fractions can be seen in Table 2, where the distribution of various types of VPs within each fraction is compared. The DPEP VPs prevail over the rest of the types for both sulfocationites. The Di-DPEP series of VPs follows DPEP in both cases. The total amount of DPEP VPs in the F′1 fraction obtained using asphaltene sulfocationite is 1.4 times less than that obtained using cationexchange resin. This decrease is compensated by the corresponding 1.1−1.9-fold increase in the content of remaining types of VPs (Table 2). The obtained results confirm the significant effect of the sulfocationite matrix on the composition of purified VPs. 3.5. Differences in the Composition of the Fractions F′1 and F′2. It was also interesting to know which VPs remained in the column after separation of the pink fraction. Unfortunately, among the fractions F′2 and F′3, only fraction F′2 obtained with the help of asphaltene sulfocationite was pure enough to provide a good mass spectrum of VPs by the MALDI−TOF method (Figure 5). A more narrow range of homologues (C27−C36), with the total MWD slightly shifted to the lighter VPs, was detected for this fraction (Figures 5 and 6). This shift is reasonable because more polar VPs (i.e., less substituted by hydrophobic alkyl groups) should start to release from the column with increase of the eluent polarity. With the study of the VP distribution in the F′2 fraction, an even more significant shift toward DPEP VPs than in the case of the F′1 fraction was found. The increase in DPEP VP abundance is accompanied by a notable decrease in the DiDPEP VP content. Also, more than a 2-fold decrease in the content of Etio and Rhodo-Etio VPs occurred with simultaneous less significant increase for Rhodo-DPEP and Rhodo-Di-DPEP VPs (Table 2). In addition, all Rhodo-VPs

Figure 5. MALDI−TOF mass spectra of VPs present in fractions F′1 and F′2 isolated by various sulfocationites.

quantitative distribution of various types of VPs within the fraction (Figure 6). According to Figure 6, the most abundant type of VPs found in both F′1 fractions is VPs of the DPEP series. For the VPs purified using cation-exchange resin, the DPEP series prevails in almost the entire identified mass range,

Figure 6. Distribution of various types of VPs in the fractions F′1 and F′2 according to MALDI−TOF MS. F

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Energy & Fuels Table 2. Distribution of Various Types of VPs in Fractions F′1 and F′2 (%) F′1, cation-exchange resin F′1, asphaltene sulfocationite F′2, asphaltene sulfocationite

Etio

DPEP

Di-DPEP

Rhodo-Etio

Rhodo-DPEP

Rhodo-Di-DPEP

9.8 11.1 5.5

50.9 36.0 53.3

22.0 25.6 18.1

8.2 12.9 5.4

4.9 9.2 10.9

4.2 5.2 6.8

a simultaneous 1.1−1.9-fold increase in the content of the remaining types of VPs was observed. Despite each elution step being carried out until the disappearance of VPs in the eluate, generally the same homologues of VPs were found in the fractions F′1 and F′2. The low cost of the studied sulfocationites makes them suitable for preparative-scale purification of petroleum VPs. This may both facilitate their analysis by physical methods and stimulate investigation of their chemical (including catalytic) properties.

demonstrate unusual changes in their MWD: in the case of the F′2 fraction, their MWD is shifted to more substituted homologues, while for F′1, an opposite distribution is observed (Figure 6). It is unclear why more substituted (i.e., less polar) Rhodo-VPs are better retained by asphaltene sulfocationite than their less hydrophobic homologues. Although a poor MALDI−TOF spectrum was obtained for the fraction F′2 isolated by cation-exchange resin (data not shown), a number of detectable peaks were found in it. The most abundant peaks belong to C29 −C 33 and C31−C35 homologues of DPEP and Rhodo-DPEP series, respectively. Surprisingly, the peaks corresponding to C38−C43 homologues of Rhodo-Di-DPEP VPs were also obtained. Probably, their appearance in the F′2 fraction has the same reasons as the shift of MWD to higher masses obtained for Rhodo-VPs described above. In general, we can conclude that almost the same VPs remain in the column after isolation of the pink fraction. Honestly, we expected more dramatic changes in VP composition. There was no continuous transition from the pink to yellow fraction during elution. Instead, the pink coloring of the eluate was permanently changed to colorless (what corresponds to the absence of VPs) before the next eluent was applied. At least two explanations can be given for this behavior of VPs. The first is the presence of various adsorption sites differing in their sorption affinity for VPs. VPs eluting with more polar fraction F′2 occupied energetically more favored adsorption sites. The second is an association of VPs with non-porphyrin oil components discussed in previous works.36−38 VPs eluting with fraction F′2 participate in the formation of more stable and/or less mobile associates, whose dissociation or passing through the sulfocationite requires a more polar eluent. The first explanation seems less valid because, in preliminary tests, we fixed the appearance of the pink fraction with the same VP distribution (>50% for the F′1 fraction), even for the primary concentrates with a ∼10-fold less content of VPs.

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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.7b02816. Yields and metal contents of products of each enrichment step (Table S1) and photographs of fractions F3− F10 and synthetic vanadyl C32-etioporphyrin (Figures S1 and S2) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nikolay Alexandrovich Mironov: 0000-0003-1519-6600 Yulia Yurevna Borisova: 0000-0003-1677-3668 Dmitry Valerevich Milordov: 0000-0003-2665-526X Dmitry Nikolaevich Borisov: 0000-0002-3755-7764 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant of the Russian Science Foundation (Project 15-13-00139).

4. CONCLUSION The CU-2-8 cation-exchange resin and asphaltene sulfocationite behave as promising sorbents for chromatographic purification of VPs. Both sulfocationites provide isolation of a wide (>50%) fraction of VPs from the DMF extract of heavy oil asphaltenes. The spectral purity of isolated VPs is comparable to the best results of other purification methods that allow for the acquisition of only narrow fractions of VPs of similar purity. The chemical nature of sulfocationite affects both the composition of isolated VPs and the chromatographic conditions required for their effective isolation. A more polar eluent should be used to isolate the same amount of VPs by asphaltene sulfocationite having a more hydrophilic matrix compared to the hydrocarbon matrix of the cation-exchange resin. Although the same range of VP homologues (C26−C40, with maximum falling on C 32 ) was derived by both sulfocationites, significant changes in the composition of isolated VPs were detected by HPLC and MALDI−TOF MS methods. A 1.4-fold decrease in the content of DPEP VPs with

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DOI: 10.1021/acs.energyfuels.7b02816 Energy Fuels XXXX, XXX, XXX−XXX