Comparative Study of Resins and Asphaltenes of Heavy Oils as

Nov 30, 2018 - A.E. Arbuzov Institute of Organic and Physical Chemistry - Subdivision of the Federal State Budgetary Institution of Science ″Kazan S...
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Comparative Study of Resins and Asphaltenes of Heavy Oils as Sources for Obtaining Pure Vanadyl Porphyrins by the Sulfocationite-Based Chromatographic Method

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Nikolay Alexandrovich Mironov,* Guzalia Rashidovna Abilova, Yulia Yurevna Borisova, Elvira Gabidullovna Tazeeva, Dmitry Valerevich Milordov, Svetlana Gabidullinovna Yakubova, and Makhmut Renatovich Yakubov A.E. Arbuzov Institute of Organic and Physical Chemistry - Subdivision of the Federal State Budgetary Institution of Science ″Kazan Scientific Center of Russian Academy of Sciences″, Arbuzov street 8, Kazan 420088, Russian Federation S Supporting Information *

ABSTRACT: The resins and asphaltenes of three heavy oils differing in origin (Permian and Carboniferous) and vanadium content (0.025−0.165 wt %) have been studied as sources for isolation of spectrally pure vanadyl porphyrins by sulfocationitebased chromatographic method developed by us recently. This method consists of removing the low polar nonporphyrin components from the resins or DMF extract of asphaltenes on SiO2-column followed by chromatographic isolation of vanadyl porphyrins on the sulfocationite. The asphaltenes were revealed to be a more promising source of vanadyl porphyrins because they possess ≥5 times higher content of vanadium, provide better accumulation of vanadyl porphyrins during deposition extraction by N,N-dimethylformamide, and are less contaminated by low polar nonporphyrin compounds capable of coeluting with vanadyl porphyrins through the sulfocationite column. According to matrix-assisted laser desorption/ionization time-offlight mass spectroscopy, DPEP vanadyl porphyrins were found to be the most abundant type for all studied samples (34.1− 54.5%). Rhodo vanadyl porphyrins belong to minor components (3.3−8.7% for each subtype) while Etio and Di-DPEP types take the intermediate position (9.8−28.7%). The resins and asphaltenes of the same oil showed significant difference in the group composition of purified vanadyl porphyrins. For the resins, a ∼1.3-fold decreased content of DPEP vanadyl porphyrins was found, which was compensated by a ∼1.5- and/or ∼1.8-fold increase in the content of Etio and Rhodo vanadyl porphyrins, respectively. However, this change in the composition of vanadyl porphyrins is not accompanied by a notable change of their average molecular weight despite the fact that increased concentration of more substituted (i.e., more hydrophobic) vanadyl porphyrins could be expected for less polar resins. This fact was interpreted in favor of association of vanadyl porphyrins with nonporphyrin components of the oil.

1. INTRODUCTION Petroporphyrins are the metal-containing components of petroleum and considered as biomarkers confirming its biological origin.1,2 In the past decade, there has been a rapid progress in the study of petroporphyrin composition by ultrahigh-resolution mass-spectrometric methods which help to discover new types of petroporphyrins3−12 contributing to better understanding the processes of oil genesis. EPR (electron paramagnetic resonance) and ENDOR (electron− nuclear double resonance) spectroscopic methods sensitive to electronic and structural environments of paramagnetic vanadyl ions provided additional insight into the structure and properties of petroporphyrins13−16 and revealed the applicability of the latter in the study of asphaltene aggregation processes.16−18 New HPLC-based approaches directed to solving a problem of quantitative analysis of petroporphyrins in petroleum objects were proposed.19,20 However, the practical aspects of petroporphyrin application still remain unstudied. At the same time, synthetic metalloporphyrins have been tested in a large number of oxidative catalytic processes having great practical importance: epoxidation of olefins,21−24 functionalization of the C−H bond,22−25 oxygenation of sulfides,24,26,27 dimerization of sulfides,28 etc.,23,24 while their © 2018 American Chemical Society

metal-free counterparts exhibited high efficacy as photoactive components of solar cells.29 The lack of notable progress in applied research of petroleum porphyrins is caused by the problems with their isolation and purification. Heavy petroleum fractions (resins and asphaltenes), where petroporphyrins accumulate, have an extremely complex composition, which greatly hinders their complete separation from nonporphyrin impurities. The absence of simple purification methods forces the researchers to study only partially enriched petroporphyrins. Usually, the enrichment procedure consists of two key steps: extraction of petroporphyrins from crude oil or its fractions with polar solvent (e.g., methanol or acetonitrile) and chromatographic fractionation of isolated extract for separation of petroporphyrin-enriched fractions.2,3,6−8,30−32 Despite the fact that this approach does not allow for achieving deep purification (UV−vis analysis always shows excess of nonporphyrin impurities in the enriched fractions),2,3,6 it proves to be sufficient for further study of the petroporphyrin Received: September 27, 2018 Revised: November 28, 2018 Published: November 30, 2018 12435

DOI: 10.1021/acs.energyfuels.8b03411 Energy Fuels 2018, 32, 12435−12446

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Table 1. Characteristics and Metal Content of Crude Heavy Oils and DMF Extracts of Asphaltenes Isolated from Them Oil field Density, g/cm3 Sulfur content, wt % Resins, wt % Asphaltenes, wt % V, wt %

Ni, wt %

Yield from asphaltenes, % V, wt % Ni, wt %

in in in in in in

oil resins asphaltenes oil resins asphaltenes

Ashalchinskoea

Kalmayurskoeb

Smorodinskoeb

0.9550c 4.67c 20.6c 6.2c 0.025c 0.040d 0.270c 0.0030c 0.0064d 0.0270c

0.9531c 4.23c 37.2c 20.0c 0.165c,e 0.120 0.61c 0.0145c,e 0.0140 0.0530c DMF extract of asphaltenes 25.5c 0.85c 0.051c

0.9523e 3.46 29.7e 18.4e 0.084e 0.140 0.702f 0.0140e 0.0150 0.0481f

15.9c 0.30c 0.017c

nd nd nd

a e

Permian terrigenous deposits, 100−300 m depth. bCarboniferous terrigenous deposits, 700−1300 m depth. cFrom ref 39. dCalculated values. From ref 40. fFrom ref 41; nd = not determined.

petroleum fraction (resins or asphaltenes) is better suitable for the sulfocationite-based purification of petroporphyrins; (3) is there any difference in the composition of petroporphyrins isolated with the help of the sulfocationite column from the resins and asphaltenes of the same oil?

composition by modern ultrahigh resolution mass spectrometric methods. Moreover, the work in which untreated crude oil and asphaltenes were analyzed by one of these methods is also known.4 For the investigation of the chemical (e.g., catalytic) properties of petroporphyrins, more profound purification is required. To date, column chromatography is considered to be the most effective method of petroporphyrin isolation. Therefore, the most obvious way to obtain highly pure petroporphyrins is to enhance the number of chromatographic purification steps. Using this approach, excellently pure vanadyl porphyrins (VPs) were obtained in the work33 from asphaltenes and vacuum residues by sequential enrichment in 3−4 steps on SiO2 and Al2O3 columns. However, this multistep chromatographic method suffers from high time and eluent consumption. In our recent works,34,35 we proposed a more simple approach based on application of sulfocationites as adsorbents for isolation of spectrally pure VPs from primary concentrates derived in one stage on a SiO2 column from heavy oil resins and DMF extracts of asphaltenes. Our approach is a further development of the method of Barwise and Whitehead,36 who discovered that silica gel modified with propylsulfonic acid can be successfully employed for separation of VPs from polar petroleum components bearing basic nitrogen. Our study of the elution behavior of VPs and nonporphyrin impurities in a sulfocationite column revealed optimal chromatographic conditions providing maximization of the yield and spectral purity of recovered VPs.34,35 Also, we adapted this method for much less expensive sulfocationites such as cation exchange resin QU-2-834,35 (domestic analog of Amberlite IR120) and asphaltenic sulfocationite35 created37 and patented38 by our group. This makes the sulfocationitebased method more suitable for preparative-scale purification of petroporphyrins. We continue our work on the development of a time- and cost-effective chromatographic approach to sulfocationitebased isolation of spectrally pure petroporphyrins. The current stage is devoted to answering a number of questions having important practical significance: (1) is there any influence of the source of petroporphyrins (resins and asphaltenes from various heavy oils differing in metalloporphyrin content) on their elution behavior in the sulfocationite column; (2) what

2. MATERIALS AND METHODS 2.1. Materials. All solvents used for oil fractionation and column chromatography were of reagent grade purity and were applied without further purification (chloroform contained 0.5% ethanol as a stabilizer). Deionized water and HPLC grade methanol and acetonitrile were used for HPLC measurements. Activated macroporous silica gel (average pore size is 90 Å) was dried for 8 h at 150 °C prior to experiment. The cation exchange resin CU-2-8 was ground up in a laboratory grinding mill (A11 basic analytical mill, IKA, Germany) and, additionally, in porcelain mortar, after which it was sieved through the 0.1 mm sieve to delete the more coarse fraction. The heavy oils chosen for the experiments are described in Table 1. 2.2. Equipment. UV−vis spectra were recorded in the range of 250−650 nm on a PE-5400UV spectrophotometer (Ecroskhim, Russia) equipped with 10 mm path length quartz cuvettes. All spectra were measured in chloroform. Mass spectra were recorded on an UltraFlex III MALDI-TOF/ TOF mass spectrometer (Bruker Daltonik GmbH, Germany) with a 266 nm Nd:YAG laser in a linear mode. Accelerating voltage was 25 kV, acceleration delay −30 ns. Polyethylene glycol standard was used for calibration in the working mass range. 0.5 μL of 1% solution of the matrix (1,8,9-trihydroxyanthracene) in toluene and the same volume of ∼1% solution of the sample in chloroform were sequentially spotted on an MTP AnchorChip target. Spectra were acquired at 50 shots per spectrum. Data analysis was performed using FlexAnalysis 3.0 software. Positively charged ions were analyzed. The reversed-phase HPLC was performed on a Chromatec-Crystal HPLC 2014 system (Chromatec, Russia) equipped with a 3 μm Hypersil ODS column (4.6 mm × 25 mm) and an UV−vis detector. Elution was carried out with 45:45:10 CH3CN/MeOH/H2O at 1 mL/min. The eluent was monitored at 410 nm (detection of VPs) and 250 nm (to control the absence of impurities at retention times of VPs). Concentration of metals was measured on an AAS-1N atomic absorption spectrophotometer according to a previously described method.42 Acetylene-air and acetylene-nitrous oxide flames were used for determination of Ni and V, respectively. The samples and standards used for construction of calibration curves (dibutyldithiocarbamates of V(II) and Ni(II)) were dissolved in o-xylene, acetone, 12436

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Energy & Fuels and an ethanol mixture (80:10:10, v/v). For the oil from Ashalchinskoe oil field only the asphaltenes were analyzed. For the resins, the metal content was calculated using experimental values given in Table 1 assuming that V and Ni accumulate in the resins and asphaltenes only. Sulfur content in the initial oils was measured on a hightemperature CHNS-O analyzer EuroEA3028-HT-OM (Eurovector SpA, Italy). 2.3. Recovery of Resins and DMF Extract of Asphaltenes. Asphaltenes and resins were isolated from the oils by a known procedure used by us earlier43 (Scheme 1). To the sample of the oil

The fractions for which the Soret bands characteristic for NPs and VPs were detected (∼390 and ∼410 nm, correspondingly) were combined. In all cases, a black residue was retained at the top part of the column when chromatographic fractionation was completed. It can be partially extracted from a stationary phase by DMF and CHCl3/iPrOH (1:1). However, only trace amounts of petroporphyrins were detected in the obtained extracts by UV−vis analysis. 2.5. Purification of VPs by Sulfocationite. A PureFlash cartridge (Starlab Scientific, China) with an inner diameter of 12.8 mm was filled with ∼4 cm3 (2.5 g) of sulfocationite. After conditioning with a 10-fold volume of the first eluent, a filter paper disc loaded 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 fraction of the latter from 0 to 100% according to Scheme 1. The volume of eluent at each step was 15 mL; elution was carried out under elevated pressure providing ∼1 mL/min elution rate (0.2−1.5 atm, the higher the eluent polarity, the higher the required pressure). Not eluted residues of the samples were removed from the column by isopropanol/chloroform mixture (10:90). For measuring UV−vis spectra, obtained fractions were dried and redissolved in equal volumes of chloroform. 2.6. UV−vis Spectra Processing. To separate the light adsorption of VPs from that of background impurities, each UV− vis spectrum was treated in MS Excel 2010 software. The trend lines were calculated from the experimental points in the range 340−470 nm excluding the range corresponding to the Soret band (from ∼370 to ∼450 nm). Approximation was performed using the exponential function of the formula y = ae−bx. The approximation accuracy R2 of treated spectra varied from 0.9855 to 1 exceeding 0.998 in most cases. Using the approximation equation, the intensity of background absorption, Ab (which is determined by concentration of nonporphyrin impurities) at the Soret band wavelength was calculated for each spectrum. Subtracting the Ab value from the experimentally measured intensity of the Soret band, AS, the intensity of light absorption of VPs, AVP, was obtained (AVP = AS − Ab). The AVP value is a convenient tool for quantification of VPs distribution between isolated fractions, while the AS/Ab ratio can be used for the rough assessment of the fraction enrichment by VPs.34,35 2.7. MALDI-TOF Mass Spectra Processing. Metalloporphyrins were identified in mass spectra by their molecular ions [M]+, whose molecular masses were calculated using ChemDraw software. The tendency of metalloporphyrins to form molecular ions [M]+ upon ionization by MALDI and LDI methods is well-known from previous works.33,45−47 As MALDI-TOF MS belongs to a medium resolution method, only the most abundant and readily detectable types of metalloporphyrins (Etio, DPEP, Di-DPEP, Rhodo-Etio, Rhodo-DPEP and Rhodo-Di-DPEP)31 were identified. For this purpose, the signals of [M]+ ions corresponding to VPs of these types (vanadyl etioporphyrins, Etio, m/z 459 + 14n; vanadyl deoxophylloerythroetioporphyrins, DPEP, m/z 457 + 14n; vanadyl dicyclic-deoxophylloerythroetioporphyrins, Di-DPEP, m/z 455 + 14n; vanadyl rhodoetioporphyrins, Rhodo-Etio, m/z 453 + 14n; vanadyl rhododeoxophylloerythroetioporphyrins, Rhodo-DPEP, m/z 451 + 14n; vanadyl rhodo-dicyclic-deoxophylloerythroetioporphyrins, Rhodo-DiDPEP, m/z 449 + 14n, where n is an integer from 0 to 16) were detected and quantified. A molecular-mass distribution (MMD) of each type of VPs within each isolated fraction was calculated based on relative intensities of corresponding [M]+ ions, taking the intensity of the most abundant [M]+ peak equal to 100%. The isotopic contribution of [M − 1]+ ions into the intensities of [M]+ peaks was taken into account.

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

was added a 20-fold volume of n-hexane under stirring, and the resulting mixture was left for 24 h. Precipitated asphaltenes were filtered, washed with n-hexane in a Soxhlet apparatus up to decoloration of flowing solvent, and dried. For obtaining the resins, the filtrate remained after precipitation of asphaltenes with n-hexane and the solution fractions collected during their purification in the Soxhlet apparatus were combined and the solvent was evaporated. The maltenes thus obtained were subjected to column chromatography on silica gel for isolation of hydrocarbon and resin fractions through the sequential elution with n-hexane and 50 vol % solution of isopropanol in benzene, respectively (Scheme 1). The DMF extract of asphaltenes was obtained by a deposition extraction method described by us previously.39,41,44 Briefly, the asphaltenes were redissolved in a minimum quantity of benzene and a 10-fold volume of DMF was added. The mixture was refluxed for 10 min and cooled. Freshly precipitated asphaltenes were removed by filtration. Filtrate obtained after removing of asphaltenes was concentrated on a rotary evaporator and the resulting residue (DMF extract of asphaltenes) was dried to a constant weight at 60 °C. 2.4. Isolation of Primary Metalloporphyrin Concentrates. The 1.5 × 60 cm glass chromatographic column was packed with macroporous silica gel (∼40 cm) suspended in benzene. 100 mg of resins or DMF extract of asphaltenes was dissolved in 5 mL of benzene and introduced into the column. Less polar nickel porphyrins (NPs) were eluted by benzene; VPs were eluted by chloroform (Scheme 1). Consumption of the solvents was ∼200 and ∼400 mL, respectively (if the VP front moves too slowly, 1−2% of isopropanol may be added to chloroform). During elution, ∼10 mL fractions were collected. After drying and redissolving in equal amounts of chloroform, they were analyzed on a UV−vis spectrophotometer.

3. RESULTS AND DISCUSSION 3.1. Isolation of Primary Concentrates of Metalloporphyrins. In our recent work we analyzed more than 50 oil samples in respect of their V and Ni content.40 Based on these data, we selected three samples substantially differing in 12437

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1) and decreased NP stability in contact with silica gel reported in some works.52−54 Surprisingly, much more substantial (but still low) amounts of NPs were observed in the beginning fractions eluted with chloroform. The NP/VP ratios of these fractions estimated by the absorption intensity at the Soret band wavelength of corresponding metalloporphyrins (∼390 and ∼410 nm, respectively) were higher for the resins than for the DMF extracts of asphaltenes (data not shown). Obviously, the low polar NPs are weaker retained by the asphaltenes during the deasphalting process. This finding agrees with results of the work55 in which ΔG° and ΔH° of adsorption of nickel and vanadyl octaethylporphyrins on the surface of graphene, used as a model of asphaltenes, were less favorable in the case of nickel porphyrin. The highest NP content was observed in the resins of the Kalmayurskoe oil field. We combined the fractions most enriched with NPs. The UV−vis spectrum of NP concentrate thus obtained is depicted in Figure S1. The shoulder peak at ∼408 nm corresponds to VPs concomitantly eluted with NPs. Considering very low yield of NP concentrate (40% vs ∼20% in the case of asphaltenes, Table 1). As a result, 2−3 times less effective enrichment of DMF extract with VPs is achieved in the case of the resins. 3.2. Purification of VPs on Sulfocationite Column. The method of chromatographic purification by sulfocationite is based on the ability of polar components of resins and asphaltenes bearing basic nitrogen to be retained by the stationary phase due to the acid−base interaction with the sulfo-groups of adsorbent.36 Petroporphyrins do not participate in this interaction because their nitrogen atoms are 12438

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Figure 2. UV−vis spectra of the fractions obtained on a sulfocationite column during gradient elution of the primary VP concentrates isolated from the DMF extracts of asphaltenes (SAE, top spectra) and resins (SR, bottom spectra) of Smorodinskoe oil field. Fraction F10 obtained from SR was measured after 2-fold dilution.

Figure 3. Quantitative distribution of VPs between the fractions (left graphs) and AS/Ab ratio characterizing the enrichment of the fractions with VPs according to UV−vis data (right graphs).

conventional adsorbents, petroporphyrins elute from the column as middle fractions.2,6,10,32,33 Additionally, the source of petroporphyrins influences the chromatographic conditions at which their elution starts.33 In the present work, we aimed to ascertain that irrespective of the source of VPs and their content in the primary concentrate, they will always be eluted first during chromatographic purification on the sulfocationite. For this purpose, we first studied a gradient elution of primary VP concentrates with eluents of increasing polarity (Scheme 1). As a result of elution, fractions F1−F10 were obtained. As expected, asphaltenic and resinous primary VP concentrates showed different behavior at the chromatographic

protected by chelated vanadium. In our recent works devoted to study of sulfocationite applicability in purification of VPs,34,35 we found out that our two-stage approach (isolation of primary VP concentrate followed by its purification on sulfocationite, Scheme 1) has an important practical advantage over the purification with conventional solid adsorbents (SiO2 and Al2O3). Namely, due to preliminary chromatographic removing of residual low polar components from the resins and DMF extracts of asphaltenes (elution with benzene, Scheme 1), the main bulk of purified VPs leaves the sulfocationite column as a beginning fraction. This seems very convenient from a practical point of view. At the multistep purification by 12439

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Energy & Fuels purification by the sulfocationite. In the case of AR, KR, and SR (see the list of abbreviation), all collected fractions were colored (the color distribution of the fractions is summarized in Table S1): fractions F1−F9 were generally yellow but with noticeable reddish hue for the fractions F1−F3. For KAE and SAE, possessing the highest VP content (Figure 1), the first two fractions were colorless. Fractions F3−F5 were distinctly pink while the color of F6−F9 changed from reddish-yellow to almost pure yellow. AAE, whose VP content is closer to that of SR and KR (Figure 1), showed intermediate results: while the beginning fractions F1−F3 were also colorless, the next fractions (F4−F9) were yellow except F5 and F6, which were yellow-pink and reddish-yellow, respectively. Fractions F10 were dark-brown in all cases. The photograph of the colored fractions of one representative example, KAE, concentrated up to dryness and redissolved in chloroform is shown in Figure S2. It is evident that the pink color detected for some fractions relates to VPs because it corresponds to that of a solution of pure synthetic vanadyl C32-octaethylporphyrin.35 To reveal the quantitative distribution of VPs between the fractions, the UV−vis spectra were recorded (Figures 2, S3 and S4) (data for colorless fractions were omitted due to negligible light absorption). The presence of the Soret band at ∼410 nm indicates that all colored fractions contain VPs. The rise in the absorption at the short-wave part of the spectra corresponds to nonporphyrin impurities because pure VPs weakly absorb in the UV region detectable by UV−vis spectrometers.46,56 For demonstration of VP distribution between the fractions, AVP values of each fraction were derived from their UV−vis spectra as described in section 2.6. In the first approximation, the AVP value is proportional to the concentration of VPs in the fraction. Meanwhile, it is possible that various VPs differing in extinction coefficients unevenly distribute between various fractions. However, for the fractions of each studied sample the Soret band maxima vary within 3 nm (from 410 to 412 in most cases) while for the known petroleum VPs this parameter changes in a broader range from 406 to 414 nm or within 9 nm.57 These facts may be interpreted in favor of rather comparable diversity of various types of VPs in all fractions of the particular sample. Quantitative distribution of VPs between the fractions of particular sample calculated from AVP values is given in Tables S2 and S3. The histograms based on these data are shown in Figure 3, left graphs. The main difference between histograms is the shift of VPs emergence to less polar eluent in the case of resinous samples (AR, KR, SR). In spite of different origin of the oils, all samples showed rather similar VP distribution patterns inside each group (asphaltenic and resinous). The highest amount of VPs is found for the fractions F5 and F6 in the case of asphaltenic samples (AAE, KAE, SAE), and F1 and F2 in the case of resinous ones (AR, KR, SR). Only minor differences in VP distribution can be found between the samples of asphaltenic or resinous groups (Figure 3). Thus, for KR the highest VP content falls on the fraction F1 instead of F2 as in the cases of AR and SR. Also, the shift of VP emergence to the fraction F4 is observed for AAE. It is worthy of note that the sharp slope of the UV−vis curve in the Soret band region observable for the fractions F10 may cause an overestimation of AVP values. On the other hand, the Soret band quenching is possible due to trapping of VPs by nonporphyrin components when they present in a large excess.33,50 As a result, only rough estimation of AVP value is possible for the fractions F10.

To estimate the enrichment of the fractions F1−F10 by VPs, AS/Ab ratios were calculated according to section 2.6 (Tables S2, S3 and Figure 3, right graphs). As expected, the best VP enrichment falls on the first several fractions (Figure 3, right graphs). A particularly excellent result was found for pinkcolored fractions F3−F5 isolated from KAE and SAE samples. It is evident that the higher the VP content in the initial primary concentrate, the better the result that can be achieved during their isolation on the sulfocationite. Using the AS/Ab ratio, we also determined the fractions enriched with VPs relative to initial primary VP concentrates (F0). According to Tables S2 and S3, the higher AS/Ab values than for corresponding primary VP concentrates were obtained for the fractions F3− F6 in the case of all asphaltenic samples and for F1−F4 in the case of resinous samples AR and KR. For SR, the fractions showing better enrichment with VPs are F1−F5. With results of VP distribution and enrichment in hand, we can establish optimized conditions for one-step isolation of the most pure VPs (called as fractions F′1 below) by the isocratic elution method. The isocratic method is more suitable for preparative-scale purification due to reduced time and eluent consumption. In the case of asphaltenic samples AAE, KAE, and SAE, application of 20% CHCl3 in hexane should provide onestep isolation of pink fractions F3−F5 containing 33−52% of total VPs according to Table S2. Unfortunately, pink fractions, which could guide the choice of solvent composition for isocratic elution, were not formed during gradient elution of resinous samples (AR, KR, SR). In these cases, a 10% CHCl3 in hexane was chosen as an eluent for one-step VP purification. This eluent composition covers fractions F1−F4 from Table S3, thus providing isolation of ∼60% VPs (based on data from Table S3) with A S /A b values higher than those of corresponding initial primary VP concentrates (Table S3). In addition, residual VPs were sequentially eluted from the sulfocationite column by pure CHCl3 and 10% i-PrOH in CHCl3 to obtain fractions F′2 and F′3, respectively. These two steps are not obligatory, but we executed them in order to determine the quantitative distribution of VPs among F′1−F′3 fractions. Fractionation of asphaltenic primary VP concentrates in conditions proposed above afforded pink, yellow, and brown fractions F′1, F′2, and F′3, respectively. The same color distribution was found in the case of F′1−F′3 fractions derived from resinous primary VP concentrates, except the first fraction was yellow with a visible pink hue, especially noticeable for KR and SR. The UV−vis spectra of isolated fractions are given in Figures 4 and S5. For all samples, the highest VP content is observed for the first fraction F′1 (Table S4). As followed from Table S4, the VP content in the fractions F′1 is well correlated with the results of the gradient elution method. The only exception is the AAE sample which showed 1.4-fold higher VP content at purification under isocratic conditions. As expected, asphaltenic primary concentrates provide isolation of more pure VPs within F′1 fractions (AS/Ab = 15.1−25.5 vs 4.8−8.2 for resinous primary concentrates). In addition, less pure VPs were isolated from AAE and AR samples, which is undoubtedly associated with low V content in the initial oil (Table 1). Summarizing the above results, we can conclude that primary VP concentrates differing both in oil origin and VP content demonstrate rather similar behavior within each group (resins and asphaltenes) at purification on the sulfocationite column. The yield of pure VPs in the first fraction means a great saving of time and eluents. This is the main advantage of 12440

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Figure 4. UV−vis spectra of the fractions obtained by sequential elution of the primary VP concentrates through the sulfocationite column by 20% (for SAE) or 10% (for SR) CHCl3 in hexane, pure CHCl3, and 10% i-PrOH in CHCl3.

sulfocationite-based purification over conventional multistep purification on silica gel. VPs isolated from asphaltenic primary concentrates AAE, SAE, and KAE possess higher spectral purity compared to their resinous counterparts AR, SR, and KR (Figures 4 and S5). This, in turn, indicates the advisability of obtaining pure petroporphyrins from asphaltenes rather than from a combined asphaltene-resin fraction or from resins alone. 3.3. Composition of the Fractions F′1. Less effective purification of resinous VPs by sulfocationite can be explained by lower polarity of resins compared to asphaltenes.58 As a result, coelution of low polar nonporphyrin components of the resins with VPs occurs. But what is the reason for accelerated elution of resinous VPs (Figure 3, left graph)? In order words, why does 10% CHCl3 in hexane allow for the acquisition of ∼60% of resinous VPs (Table S4) and just less than 25% of asphaltenic ones (Table S2)? The simplest explanation could be the difference in the composition of asphaltenic and resinous VPs with prevailing of more substituted (i.e., more hydrophobic) homologues in the latter. In order to confirm or deny this version, we studied the composition of the F′1 fractions by HPLC and MALDI-TOF MS methods. The HPLC chromatograms of the fractions F′1 obtained under conditions comparable to those in previous works59−61 have a poor peak resolution indicating the presence of a great diversity of VP homologues and isomers (Figures 5 and S6). The samples KAE, KR, SAE, and SR showed very similar chromatographic profiles of their F′1 fractions, which indicates comparable composition of these objects. The similarity in VP composition of different oils is explained by geographic proximity of Smorodinskoe and Kalmayurskoe oil fields (both are located in the Samara region of Russia) and similar geological conditions of their formation (Table 1). Visible

Figure 5. Reversed-phase HPLC of the fractions F′1.

similarity in the shape of chromatograms of resinous and asphaltenic samples (KAE vs KR and SAE vs SR) reveals a weak correlation between VP hydrophobicity and their asphaltene/ resin distribution selectivity during the oil deasphalting process. Under RP-HPLC conditions, an increased content of hydrophobic (i.e., more substituted) VPs in resinous fractions F′1 should look like a shift of their chromatograms to the right side59 relative to chromatograms of asphaltenic samples. However, in Figures 5 and S6 this shift is not visualized. This nonselectivity of VPs was not surprising for us because in our recent paper we have already speculated that petroporphyrins can associate with nonporphyrin oil components.35 This phenomenon has also been discussed in some previous works.62−64 If so, this association should affect the distribution behavior of VPs during oil fractionation (isolation of asphaltenes). To gain detailed insight into the composition of purified VPs, the MALDI-TOF mass spectra of all F′1 fractions were measured. As has been shown in previous works,2,30,33−35,39 MALDI and LDI methods can provide valuable information on the content of the main types of petroporphyrins (Etio, DPEP, Di-DPEP, Rhodo-Etio, Rhodo-DPEP, and Rhodo-DiDPEP).31 On the mass spectra of the F′1 fractions, C26−C42 homologues of VPs were identified (Figures 6 and S7) (the residual higher mass VPs were not analyzed because of the noisy spectra at m/z > 700). The C32 homologue of VPs prevails over the rest in all spectra. Because of the low content of NPs in the samples, their signals are not distinguishable in the middle-resolution mass spectra of VPs. The features of NP 12441

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Figure 6. MALDI-TOF mass spectra of VPs present in the fractions F′1.

Figure 7. Distribution of the various types of VPs in the fractions F′1 according to MALDI-TOF MS.

enrichment and sulfocationite-based chromatographic purification will be the subject of our separate work. The signals of the main structural types of VPs (see section 2.7) were found in the mass-spectra. To better visualize the results, we constructed the molecular-mass distribution (MMD) diagrams of the identified VPs (Figures 7 and S8). As can be seen from MMD diagrams, in all cases the most abundant type of VPs is the porphyrins of DPEP series. This

type prevails almost over the entire mass range, except for the four lightest homologues of AAE and AR among which the VPs of Etio series predominate (Figures S8). The total group composition of VPs in the studied fractions is shown in Table 2. According to this table, the DPEP VPs whose content ranges from 34 to 55% are followed by Etio VPs (10−28%). Di-DPEP VPs have a comparable content (13−22%) while the content of each subtype of Rhodo VPs does not exceed 9%. 12442

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produced from the heavy oils differing in origin and vanadium content. The decisive advantage of this method is the emergence of purified VPs in the composition of the first colored fraction. Due to this advantage, our sulfocationitebased method can form the basis of a time- and cost-effective approach to preparative-scale purification of VPs. Comparative study of the resins and DMF extracts of asphaltenes revealed the latter to be a more promising source of petroleum VPs due to the following factors: (1) the tendency of vanadium to accumulate in the asphaltenes more largely than in the resins during the oil deasphalting process; (2) better results of deposition extraction method achievable for the asphaltenes (used for the sample enrichment by VPs); and (3) the lowest content of low polar nonporphyrin components capable of coeluting with VPs through the sulfocationite column. Besides, we also established that VPs obtained with the help of sulfocationite from the DMF extract of asphaltenes and resins of particular oil differ in group composition but not in average molecular weight despite a higher affinity of more substituted (i.e., more hydrophobic) VPs for the resins might be expected. To further increase the productivity of the sulfocationite-based purification method, we plan to develop a more effective strategy for preliminary enrichment of primary concentrates with VPs. The appearance of ultrahigh resolution mass-spectrometric technique has had an extremely positive effect on research activity toward petroporphyrins. Similarly, the development of simple approaches to preparative-scale isolation of pure petroporphyrins may additionally stimulate their study. The cardinal increase in the petroporphyrin content should facilitate the search of new types of porphyrin complexes. On the other hand, an easy access to preparative amounts of pure petroporphyrins may draw the interest of researchers to their applied properties, including catalytic ones. Taking into account the huge reserves of heavy oils, petroleum porphyrins could become a promising alternative to their synthetic and plant analogues for potential industrial application.

Table 2. Distribution of Various Types of VPs in the Fractions F′1 (in %) and Their Average Molecular Weight According to MALDI-TOF MS Data Sample

Etio

DPEP

DiDPEP

SAE SR KAE KR AAE AR

13.0 18.2 9.8a 16.2 28.7 27.9

54.5 40.2 50.9a 44.8 44.1 34.1

19.5 18.6 22.0a 16.3 14.3 13.4

RhodoEtio

RhodoDPEP

RhodoDiDPEP

M W, g/mol

5.1 8.3 8.2a 7.8 5.4 8.7

4.5 8.6 4.9a 8.3 4.2 8.5

3.5 6.1 4.2a 6.8 3.3 7.3

563.3 572.7 549.8 559.9 559.7 566.3

a

From ref 35.

In accordance with HPLC data, KAE, KR, SAE, and SR samples show rather identical MMD of VPs except slightly narrowed range of detectable homologues for KR and SR (Figures 7 and S8). In contrast to them, for the AAE/AR pair a visual difference in MMD of VPs caused by a sharp increase of Etio-VP content in the low-mass region of AR was observed (Figure S8). This change in the composition explains the significant difference in the shape of the chromatograms of these two samples (Figure S6). Analogously to HPLC, MMDs based on MS data also did not reveal a notable shift to more substituted VPs for AR, KR, and SR, which seems reasonable for resinous samples (Figures 7 and S8). Only the inversion of C31 and C33 peak intensities can be noticed (for AR, KR, and SR, the ratio (abundance of C33)/(abundance of C31) becomes >1) together with raised content of Rhodo-VPs, which is characterized by increased molecular weights due to the presence of an additional aromatic ring. However, the average molecular weight of resinous VPs calculated from MS data exceeds that of asphaltenic ones just by 6.6−10.1 au (i.e., by less than one homologic difference or 14 au) (Table 2). Much more notable changes occur in the group composition of VPs. Compared to asphaltenic objects, all resinous samples exhibit a reduced amount of DPEP VPs compensated by the growth (with some exception) in the content of Etio and Rhodo types of VPs (Table 2). On average, the proportion of DPEP VPs is reduced by ∼1.3 times while the proportions of Etio (excluding AAE/AR pair) and Rhodo (excluding KAE/KR pair for Rhodo-Etio) VPs are increased by ∼1.5 and ∼1.8 times, respectively. The proportion of Di-DPEP VPs remains almost constant with the exception of the KAE/KR pair (Table 2). In general, the MALDI-TOF MS results also confirm our conclusion on the absence of a direct relationship between the hydrophobicity of VPs and their selectivity in the distribution between resins and asphaltenes. This nonselectivity can be explained by the tendency of VPs to coordinate with nonporphyrin petroleum components bearing Lewis base heteroatoms.49 As the elemental composition of resins is rather close to that of asphaltenes,65,66 both of them can possess the same heteroatomic moieties capable of coordination with the vanadyl group of VPs. This additional complexation eliminates the contribution of hydrophobic substituents of VPs to their distribution behavior between resins and asphaltenes during the oil deasphalting process.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b03411. UV−vis spectra of primary NP concentrate (Figure S1), photograph of fractions F3−F10 (Figure S2), color distribution of fractions F1−F10 (Table S1), VP distribution among F1−F10 and F′1−F′3 fractions together with corresponding AS/Ab values (Tables S2− S4), HPLC, MALDI-TOF MS, UV−vis, and MMD data of AAE, AR, KAE, and KR samples (Figures S3−S8) (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

4. CONCLUSION In the present work, we investigated the method of sulfocationite-based chromatographic isolation of spectrally pure VPs from the resins and DMF extracts of asphaltenes

Notes

The authors declare no competing financial interest. 12443

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ACKNOWLEDGMENTS The authors are grateful to the Assigned Spectral-Analytical Center of FRC Kazan Scientific Center of RAS for technical assistance in research.

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NOMENCLATURE AAE, KAE, SAE =primary vanadyl porphyrin concentrates isolated from the DMF extracts of asphaltenes (AE) of the oils from Ashalchinskoe (A), Kalmayurskoe (K), and Smorodinskoe (S) oil fields AR, KR, SR =primary vanadyl porphyrin concentrates isolated from the resins (R) of the oils from Ashalchinskoe (A), Kalmayurskoe (K), and Smorodinskoe (S) oil fields VP =vanadyl porphyrin NP =nickel porphyrin 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 MALDI TOF MS =matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry UV−vis =ultraviolet−visible spectroscopy RP-HPLC =reversed phase high-performance liquid chromatography MMD =molecular-mass distribution DMF =N,N-dimethylformamide

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DOI: 10.1021/acs.energyfuels.8b03411 Energy Fuels 2018, 32, 12435−12446

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DOI: 10.1021/acs.energyfuels.8b03411 Energy Fuels 2018, 32, 12435−12446