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Comparative Study of Resins and Asphaltenes of Heavy Oils as Sources for Obtaining Pure Vanadyl Porphyrins by Sulfocationite-based Chromatographic Method Nikolay Alexandrovich Mironov, Guzalia Rashidovna Abilova, Yulia Yurevna Borisova, Elvira Gabidullovna Tazeeva, Dmitry Valerevich Milordov, Svetlana Gabidullinovna Yakubova, and Makhmut Renatovich Yakubov Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03411 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 2018

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Comparative Study of Resins and Asphaltenes of Heavy Oils as Sources for Obtaining Pure Vanadyl Porphyrins by Sulfocationite-based Chromatographic Method Nikolay Alexandrovich Mironov*, Guzalia Rashidovna Abilova, Yulia Yurevna Borisova, Elvira Gabidullovna Tazeeva, Dmitry Valerevich Milordov, Svetlana Gabidullinovna Yakubova, 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

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 sulfocationite-based chromatographic method developed by us recently. This method consists of removing the low polar non-porphyrin 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 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 non-porphyrin compounds capable of co-eluting with vanadyl porphyrins through the sulfocationite column. According to matrix-assisted laser desorption/ionization time-of-flight 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 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 1 ACS Paragon Plus Environment

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porphyrins, respectively. However, this change in the composition of vanadyl porphyrins is not accompanied by a notable change of their average molecular weight despite 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 non-porphyrin components of the oil. 1. INTRODUCTION Petroporphyrins are the metal-containing components of petroleum and considered as biomarkers confirming its biological origin1,2. 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 latters 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 C-H bond,22-25 oxygenation of sulfides,24,26,27 dimerization of sulfides,28 etc.,23,24 while their 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 non-porphyrin impurities.

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The absence of simple purification methods forces the researchers to study only partially enriched petroporphyrins. Usually, the enrichment procedure consists of the 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 petroporphyrinenriched fractions.2,3,6-8,30-32 Despite this approach does not allow for achieving deep purification (UV-Vis analysis always shows excess of non-porphyrin impurities in the enriched fractions),2,3,6 it proves to be sufficient for further study of the petroporphyrin composition by modern ultra-high 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 more simple approach based on application of sulfocationites as adsorbents for isolation of spectrally pure VPs from primary concentrates derived in one stage on SiO2 column from heavy oil resins and DMF extracts of asphaltenes. Our approach is a further development of the method of Barwise and Whitehead36 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 elution behavior of VPs and non-porphyrin impurities in 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 3 ACS Paragon Plus Environment

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makes sulfocationite-based method more suitable for preparative-scale purification of petroporphyrins. We continue our work on the development of time- and cost-effective chromatographic approach to sulfocationite-based 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 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? 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 PE5400UV 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 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 4 ACS Paragon Plus Environment

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toluene and the same volume of ~1 % solution of the sample in chloroform were sequentially spotted on MTP AnchorChipTM 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 UVVis 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 previously described method42. 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 and 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 high-temperature 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 a 20-fold volume of n-hexane was added 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 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


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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 deposition extraction method described by us previously.39,41,44 Briefly, the asphaltenes were redissolved in a minimum quantity of benzene and 10-fold volume of DMF was added. The mixture was refluxed for 10 minutes 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 were 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 UV-Vis spectrophotometer. 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 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/i-PrOH (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 6 ACS Paragon Plus Environment

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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 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 the most cases. Using the approximation equation, the intensity of background absorption, Ab, (which is determined by concentration of non-porphyrin impurities) at the Soret band wavelength was calculated for each spectrum. Subtracting 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). AVP value is a convenient tool for quantification of VPs distribution between isolated fractions, while 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 7 ACS Paragon Plus Environment

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rhodo-etioporphyrins, Rhodo-Etio, m/z 453+14n; vanadyl rhodo-deoxophylloerythroetioporphyrins, 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 molecularmass 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.

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 concentration of these metals (Table 1). The two of them, Carboniferous oils from Smorodinskoe and Kalmayurskoe oil fields, are characterized by the highest content of V and Ni among the oils tested. The third sample, Permian oil from Ashalchinskoe oil field, contains relatively low concentration of these metals. Thus, we could compare the influence of both oil origin and metal concentration on recoverability and composition of metalloporphyrins. Before the purification by sulfocationite, the pre-enrichment of the sample with petroporphyrins is required (preparation of so called primary concentrate). This procedure was carried out in accordance with our previous works34,35,39 as presented in Scheme 1. At the each enrichment stage, the content of V and Ni was monitored to reveal the distribution of metals and metalloporphyrins in the samples. According to Table 1, both asphaltenes and resins isolated from the chosen oils demonstrated higher concentration of V and Ni compared to the initial oils. The only exception is the resins isolated from the oil of the Kalmayurskoe oil field for which a slightly decreased concentration of both metals relative to the initial oil was detected. Regarding a distribution of V and Ni between the resins and asphaltenes, at least 5- and 3-fold increase in the 8 ACS Paragon Plus Environment

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contents of V and Ni, respectively, was observed in favor of asphaltenes, Table 1, what agrees with previous results.48 Thus, a higher concentration of metalloporphyrins can be expected for the asphaltenes, albeit some authors noted the absence of direct correlation between concentration of V and VPs in the oil subfractions.3 The next step towards isolation of metalloporphyrins (NPs and VPs) from the resins and asphaltenes was their fractionation on SiO2 column with eluents of increasing polarity. By analogy with ref. 36, we employed a simplified two-step chromatographic method consisting of sequential elution with benzene for isolation of less polar NP concentrate and with chloroform for isolation of VP concentrate. However, crude asphaltenes are less suitable for this method because petroporphyrins are capable of strong association and aggregation with asphaltenes49-51 retained in the column. This may decrease the yield of metalloporphyrins. To overcome this issue, the DMF extracts of asphaltenes were prepared by deposition extraction method39,41,44 prior to chromatographic separation, Scheme 1. As the resins are free of this limitation, they were subjected for column chromatography as is in attempt to maintain the native diversity of metalloporphyrins. The yields of DMF extracts of asphaltenes and their metal contents are given in Table 1. If to compare the metal contents in the initial asphaltenes with those in the DMF extracts, then a 1.1–1.3fold increase in V content and almost the same or 1.6-fold decreased Ni content can be seen for the DMF extracts, Table 1. Considering relatively low yields of DMF extracts (15.9 and 25.5 %), we can conclude that the main part of the metals (64 – 82 % of V and 75 – 90 % of Ni) is tightly bound to asphaltenes. The obtained resins and DMF extracts of asphaltenes were subjected for column chromatography on macroporous SiO2 in order to isolate primary NP and VP concentrates. Orange and wine-red colored fractions were collected upon elution with benzene and chloroform, respectively. The first should correspond for less polar NPs, the second for more polar VPs.36 Disappointingly, we found only negligible amounts of NPs at the UV-Vis analysis of the orange fractions (detection by the Soret band at ~390 nm, data not shown). This may be explained by both 9 ACS Paragon Plus Environment

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low total Ni content in the samples (Table 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 Kalmayurskoe oil field. We combined the fractions most enriched with NPs. The UV-Vis spectrum of NP concentrate thus obtained is depicted on Fig. 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 10 ACS Paragon Plus Environment

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the sulfo-groups of adsorbent.36 Petroporphyrins do not participate in this interaction because their nitrogen atoms are 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 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 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 11 ACS Paragon Plus Environment

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one representative example, KAE, concentrated up to dryness and redissolved in chloroform is shown on Figure S2. It is evident that pink color detected for some fractions relates to VPs because it corresponds to that of 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 non-porphyrin 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, 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 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 Table 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, F6 in the case of asphaltenic samples (AAE, KAE, SAE), and F1, 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 12 ACS Paragon Plus Environment

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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 non-porphyrin 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 pink-colored fractions F3-F5 isolated from KAE and SAE samples. It is evident that the higher VP content in the initial primary concentrate, the better result can be achieved during their isolation on the sulfocationite. Using 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 isocratic elution method. 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 one-step 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 AS/Ab values higher than those of corresponding initial primary VP concentrates (Table S3). In addition, residual VPs were 13 ACS Paragon Plus Environment

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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 visible pink hue, especially noticeable for KR and SR. The UV-Vis spectra of isolated fractions are given on 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, VP content in the fractions F’1 is well correlated with the results of 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 sulfocationite-based purification over conventional multistep purification on silica gel. VPs isolated from asphaltenic primary concentrates AAE, SAE, KAE possess higher spectral purity compared to their resinous counterparts AR, SR, 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, 14 ACS Paragon Plus Environment

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co-elution of low polar non-porphyrin components of the resins with VPs occurs. But what is the reason of 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 co 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 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 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, on Figures 5 and S6 this shift is not visualized. This non-selectivity of VPs was not surprising for us because in our recent paper we have already speculated that petroporphyrins can associate with non-porphyrin oil components.35 This phenomenon has been also 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 it has been shown in previous works,2,30,33-35,39 MALDI and 15 ACS Paragon Plus Environment

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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-Di-DPEP).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 the 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 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 massspectra. To better visualize the results, we constructed the molecular-mass distribution (MMD) diagrams of the identified VPs, Figures 7 and S8. As it 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 all mass range, except for the four lightest homologues of AAE and AR among which VPs of Etio series predominate (Figures S8). The total group composition of VPs in the studied fractions is shown in Table 3. 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%. 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 sharp increase of Etio-VP content in the low-mass region of AR was observed, Figure S8. This change in the composition explains significant difference in the shape of chromatograms of these two samples, Figure S6. Analogously to HPLC, MMDs based on MS data also did not reveal notable shift to more substituted VPs for AR, KR and SR seeming reasonable for resinous samples (Figures 7 and S8). Only the inversion of C31 and C33 peaks 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, 16 ACS Paragon Plus Environment

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which are characterized by increased molecular weights due to the presence of additional aromatic ring. However, average molecular weight of resinous VPs calculated from MS data exceeds that of asphaltenic ones just by 6.6–10.1 a.u. (i.e., by less than one homologic difference or 14 a.u.), Table 3. Much more notable changes occur in the group composition of VPs. Compared to asphaltenic objects, all resinous samples exhibit reduced amount of DPEP VPs compensated by the growth (with some exception) in the content of Etio and Rhodo types of VPs, Table 3. On the 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 exception of KAE/KR pair (Table 3). In general, the MALDI-TOF MS results also confirm our conclusion on the absence of direct relationship between the hydrophobicity of VPs and their selectivity in the distribution between resins and asphaltenes. This non-selectivity can be explained by the tendency of VPs to coordinate with non-porphyrin 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 vanadyl group of VPs. This additional complexation eliminates the contribution of hydrophobic substituents of VPs to their distribution behavior between resins and asphaltenes during oil deasphalting process. 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 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 sulfocationite-based method can form the basis of time- and cost-effective approach to preparative-scale purification of VPs. Comparative study of the resins and DMF extracts of asphaltenes revealed the latters to be a more promising source of petroleum VPs due to the 17 ACS Paragon Plus Environment

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following factors: 1) the tendency of vanadium to accumulate in the asphaltenes more largely than in the resins during 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 non-porphyrin components capable of co-eluting 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 ultra-high 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.

ACKNOWLEDGMENTS The authors are grateful to the Assigned Spectral-Analytical Center of FRC Kazan Scientific Center of RAS for technical assistance in research. ASSOCIATED CONTENT Supporting Information Available: 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 18 ACS Paragon Plus Environment

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S2-S4), HPLC, MALDI-TOF MS, UV-Vis and MMD data of AAE, AR, KAE and KR samples (Figures S3-S8) (MS Word). This material is available free of charge via the Internet at http://pubs.acs.org. 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 Notes The authors declare no competing financial interest. 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 19 ACS Paragon Plus Environment

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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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(38) Yakubov, M. R.; Gryaznov, P. I.; Yakubova, S. G.; Milordov, D. V.; Borisov, D. N.; Mironov, N. A.; Sinyashin, K. O. Method of preparation of sulfocationites from heavy oil residues. RU patent 2623574 C1, 2016. (39) Borisova, Y. Y; Tazeeva, E.G.; Mironov, N. A.; Borisov, D. N.; Yakubova, S. G.; Abilova, G. R.; Sinyashin, K. O.; Yakubov, M. R. Role of vanadylporphyrins in the flocculation and sedimentation of asphaltenes of heavy oils with high vanadium content. Energy Fuels 2017, 31 (12), 13382–13391. (40) Yakubov, M. R.; Sinyashin, K. O.; Abilova, G. R.; Tazeeva, E. G.; Milordov, D.V.; Yakubova, S. G.; Borisov, D. N.; Gryaznov, P. I.; Mironov, N. A.; Borisova, Yu. Yu. Differentiation of heavy oils according to the vanadium and nickel content in asphaltenes and resins. Pet. Chem. 2017, 57, 849–854. (41) Yakubov, M. R.; Milordov, D. V.; Yakubova, S. G.; Borisov, D. N.; Gryaznov, P. I.; Mironov, N. A.; Abilova, G. R.; Borisova, Y. Y.; Tazeeva, E. G. Features of the composition of vanadyl porphyrins in the crude extract of asphaltenes of heavy oil with high vanadium content. Pet. Sci. Technol. 2016, 34, 177-183. (42) Abdullin, A. F.; Bufatina, M. A.; Ivanov, V. T.; Badretdinov, G. Z.; Budnikov , G. K. Method of preparation of a standard solution for the atomic-absorption determination of vanadium in petroleum and petroleum products. SU patent 1749793, 1992. (43) Yakubov, M. R.; Abilova, G. R.; Sinyashin, K. O.; Milordov, D. V.; Tazeeva, E. G.; Yakubova, S. G.; Borisov, D. N.; Gryaznov, P. I.; Mironov, N. A.; Borisova, Y. Y. Inhibition of asphaltene precipitation by resins with various contents of vanadyl porphyrins. Energy Fuels 2016, 30, 8997-9002. (44) Yakubov, M. R.; Abilova, G. R.; Sinyashin, K. O.; Milordov, D. V.; Tazeeva, E. G.; Yakubova, S. G.; Borisov, D. N.; Gryaznov, P. I.; Mironov N. A.; Borisova Y. Y. Isolation of porphyrins from heavy oil objects. In Phthalocyanines and Some Current Applications; 2017; DOI: 10.5772/intechopen.68436. 25 ACS Paragon Plus Environment

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(45) Srinivasan, N.; Haney, C. A.; Lindsey, J. S.; Zhang, W.; Chait, B. T. Investigation of MALDI-TOF mass spectrometry of diverse synthetic metalloporphyrins, phthalocyanines and multiporphyrin arrays. J. Porphyrins Phthalocyanines 1999, 3, 283-291. (46) Chen, W.; Suenobu, T.; Fukuzumi, S. A vanadium porphyrin with temperature-dependent phase transformation: synthesis, crystal structures, supramolecular motifs and properties. Chem. Asian J. 2011, 6, 1416-1422. (47) Giraldo-Dávila, D.; Chacón-Patiño, M. L.; Ramirez-Pradilla, J. S.; Blanco-Tirado, C.; Combariza, M. Y. Selective ionization by electron-transfer MALDI-MS of vanadyl porphyrins from crude oils. Fuel 2018, 226, 103-111. (48) Liu, H.; Wang, Z.; A. Guo, Z.; Lin, C.; Chen, K. The distribution of Ni and V in resin and asphaltene subfractions and its variation during thermal processes. Pet. Sci. Technol. 2015, 33, 203–210. (49) Dechaine, G. P.; Gray, M. R. Chemistry and association of vanadium compounds in heavy oil and bitumen, and implications for their selective removal. Energy Fuels 2010, 24, 2795– 2808. (50) Castillo, J; Vargas, V. Metal porphyrin occlusion: adsorption during asphaltene aggregation. Pet. Sci. Technol. 2016, 34 (10), 873–879. (51) Acevedo, S.; Guzmán, K.; Labrador, H.; Carrier, H.; Bouyssiere, B.; Lobinski, R. Trapping of metallic porphyrins by asphaltene aggregates: a size exclusion microchromatography with high-resolution inductively coupled plasma mass spectrometric detection study. Energy Fuels 2012, 26, 4968–4977. (52) Faramawy, S.; El-Sabagh, S. M.; Moustafa, Y. M.; El-Naggar, A. Y. Mass spectrometry of metalloporphyrins in Egyptian oil shales from Red Sea area. Pet. Sci. Technol. 2010, 28, 603–617.


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Table 1. Characteristics and metal content of crude heavy oils and DMF extracts of asphaltenes isolated from them. Oil field Ashalchinskoea

Kalmayurskoeb

Smorodinskoeb

Density, g/cm3

0.9550c

0.9531c

0.9523e

Sulfur content, wt. %

4.67c

4.23c

3.46

Resins, wt. %

20.6c

37.2c

29.7e

Asphaltenes, wt. %

6.2c

20.0c

18.4e

V, wt. %

in oil in resins in asphaltenes

0.025c 0.040d 0.270c

0.165c,e 0.120 0.61c

0.084e 0.140 0.702f

Ni, wt. %

in oil in resins in asphaltenes

0.0030c 0.0064d 0.0270c

0.0145c,e 0.0140 0.0530c

0.0140e 0.0150 0.0481f

DMF extract of asphaltenes Yield from asphaltenes, %

15.9c

25.5c

nd

V, wt. %

0.30c

0.85c

nd

Ni, wt. %

0.017c

0.051c

nd

aPermian

terrigenous deposits, 100-300 m depth; bCarboniferous terrigenous deposits, 700 – 1300 m

depth; cfrom ref. 39; dcalculated values; efrom ref. 40; ffrom ref. 41; nd = not determined.

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.

19.5 18.6

RhodoEtio 5.1 8.3

RhodoDPEP 4.5 8.6

Rhodo-DiDPEP 3.5 6.1

MW, g/mol 563.3 572.7

50.9a 44.8

22.0a 16.3

8.2a 7.8

4.9a 8.3

4.2a 6.8

549.8 559.9

44.1 34.1

14.3 13.4

5.4 8.7

4.2 8.5

3.3 7.3

559.7 566.3

Sample Etio

DPEP

Di-DPEP

SAE SR

13.0 18.2

54.5 40.2

KAE KR

9.8a 16.2

AAE AR

28.7 27.9

afrom

ref. 35


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Scheme 1. Isolation of the primary metalloporphyrin concentrates from the heavy oil and their further fractionation by gradient elution method through the column with sulfocationite. 127x108mm (96 x 96 DPI)

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Figure 1. UV-Vis spectra of the primary VP concentrates isolated from the resins (bottom lines) and DMF extract of asphaltenes (upper lines). A list of abbreviation is given at the end of the text. 80x71mm (96 x 96 DPI)


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Figure 2. UV-Vis spectra of the fractions obtained on 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. 140x116mm (96 x 96 DPI)


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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). 150x107mm (96 x 96 DPI)


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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. 76x119mm (96 x 96 DPI)


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Figure 5. Reversed-phase HPLC of the fractions F'1. 83x134mm (96 x 96 DPI)


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Figure 6. MALDI-TOF mass spectra of VPs present in the fractions F’1. 169x125mm (96 x 96 DPI)


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Figure 7. Distribution of the various types of VPs in the fractions F’1 according to MALDI-TOF MS. 149x126mm (96 x 96 DPI)


Comparative Study of Resins and Asphaltenes of ... - ACS Publications

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