Resins and Asphaltenes of Light and Heavy Oils: their Composition


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Resins and Asphaltenes of Light and Heavy Oils: their Composition and Structure Tatyana Cheshkova, Valery P Sergun, Elena Yu Kovalenko, Natalya N. Gerasimova, Tatiana A. Sagachenko, and Raisa S. Min Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00285 • Publication Date (Web): 11 Jun 2019 Downloaded from http://pubs.acs.org on June 11, 2019

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Resins and Asphaltenes of Light and Heavy Oils: their Composition and Structure Tatiana V. Cheshkova, Valery P. Sergun, Elena Yu. Kovalenko, Natalya N. Gerasimova, Tatiana A. Sagachenko, and Raisa S. Min

Institute of Petroleum Chemistry of Siberian Branch of the Russian Academy of Sciences, 4, Akademichesky Ave. Tomsk, 634055, Russian Federation The complex of physicochemical methods of analysis (elemental analysis, cryoscopy in benzene, IR and 1H NMR spectroscopy, structural group analysis, gas chromatographymass spectrometry, and selective chemical cleavage of sulfide and ether bonds) is used to comparatively characterize resins and asphaltenes of light and heavy oils. Attention is paid to the study of their structural-group composition and the composition of moieties bound in molecules of resin-asphaltene substances (RAS) through ether and sulfide bridges, as well as the composition of compounds occluded by asphaltene molecules and nitrogen bases of resins. It is found out that resins and asphaltenes of the heavy oil are characterized by higher average molecular masses and large overall sizes of mean molecules, due to the increased content of aromatic cycles in the naphthenic-aromatic system. The similar sets of linear and branched alkanes, alkylcyclopentanes, alkylcyclohexanes, steranes, mono- and disubstituted alkilbenzenes, and dibenzothiophenes identified in occluded compounds and products of chemolysis of resins and asphaltenes under study suggest the presence of most of these compounds as structural fragments in RAS molecules of light and heavy crude oils under study. Alkyl substituted quinolines and benzoquinolines are identified in nitrogen bases of resins. The feature of the light oil is the presence of ‘sulfur-bound’ alkenes and polycycloalkenes in the structure of its asphaltenes. The findings expand our understanding of the structure of petroleum resins and asphaltenes. They can be used to simulate their structure in developing new controlled methods for processing hydrocarbon feedstock.

1.

INTRODUCTION

In recent years, the global trend towards a reduction in the world light oil reserves has clearly emerged. On the contrary, the share of heavy oils in the total balance of extracted and refined hydrocarbon raw materials steadily grows. The distinctive feature of heavy crude oils is the high content in high molecular weight heteroatomic components – resins and asphaltenes.1–7 This special feature makes it difficult to refine heavy oils using existing technologies, which are ACS Paragon Plus Environment

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developed mainly on the basis of conventional hydrocarbon feedstock. In this connection, studies aimed at obtaining comparative data on the chemical nature of resin-asphaltene components of light and heavy oils become relevant. These data are of fundamental and applied importance for the development of basic technical solutions for the rational use of heavy hydrocarbon raw materials. Asphaltenes are a mixture of various compounds soluble in aromatic hydrocarbons (benzene or toluene), but insoluble in lower alkanes (C5–C7).8,

9

Resins are the components of

deasphaltizate, which cannot be desorbed from the surface of adsorbents with light hydrocarbons but after removal of the oil components can be eluted with solvents having a high elution capacity, for example alcohol-benzene.10 The skeleton of molecules of resins and asphaltenes is formed by naphthenic, aromatic, and heteroaromatic cycles with lateral alkyl substituents.3–5, 10–21 According to modern concepts, asphaltenes contain molecules of the ‘island’ and ‘archipelago’ type.17,

22–24

Unlike molecules of the ‘archipelago’ type, molecules of the ‘island’ type are

characterized by higher molecular masses, a more condensed aromatic ring and a relatively poor alkyl environment. The ratio of these molecules depends on the nature of the oil.21 Resin-asphaltene substances (RAS) are characterized by a fairly high content of heteroatoms, while up to 90 % of the hetero-elements present in the oil are concentrated in resins.25 The functions of individual heteroatoms in RAS of the oils are quite diverse. Nitrogen atoms are present in their molecules in basic (Nbas. – pyridine and its benzo derivatives) and neutral (pyrrole and its benzo derivatives) fragments. Oxygen is represented in petroleum resins and asphaltenes by hydroxyl (alcohol and phenolic), carboxyl, carbonyl, ether and ester functions. RAS molecules contain sulfur atoms in their heterocyclic fragments (thiophene, thiacyclanes), and thiol, sulfide, and sulfoxy (S=O) groups.13,

14, 26–29

The functional groups of ethers and/or

esters and aliphatic sulfides can act as bridges crosslinking individual fragments of molecules of resins and asphaltenes between themselves or with the polycondensed core of their molecules.30, 31

The presence of the bridge bond was established in the molecules of Athabasca asphaltenes,30,

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32

asphaltenes and resins of heavy oils from the Usinskoye and Ashalchinskoye oilfields,4, 5, 33 in

those of light oil from the Krapivinskoye oilfield,34, 35 and in oil components of Ashalchinsky bitumen.36 Main representatives of trace elements in resins and asphaltenes are nickel and vanadium, most of which are concentrated in the metal porphyrin complexes.37, 38 A high concentration of hetero- and microelements in RAS makes the running of catalytic processes during the crude oil refining quite difficult and exert an adverse effect on the quality of commercial oil products, the environment and human health.21, 39–46 Hence, studies that make it possible to evaluate the functional state of heteroatoms (S, N, and O) in resins and asphaltenes become relevant. To describe the chemical structure of RAS molecules, elemental and trace element analyzes9, X-ray diffraction47, 48, nuclear magnetic resonance (NMR)18, 47, 49, infrared spectroscopy (IR)42 and various versions of high-resolution mass spectrometry are widely used.13, 14, 21, 29, 37, 40, 50, 51 The use of these methods allows us to determine the molecular mass of resins and asphaltenes and to evaluate the content of elements in RAS molecules, as well as the degree of their aromaticity, the number of aromatic rings and the average number of carbon atoms in the aliphatic substituents. They also provide valuable insights into the architecture of complex molecules of resins and asphaltenes and the presence of Sx, Nx, Ox, SxNy, SxOy, NxOy, NxSyOz heteroatomic structures in them. However, despite the importance of information obtained, analytical methods provide an incomplete picture of the structure of RAS molecules. The most detailed information about the structure of molecules of resins and asphaltenes could be obtained using the methods of directed cleavage of their macromolecules into identifiable moieties that however can store information about their original structure and in some cases about the form of connectivity. These methods include thermal destruction of RAS within a wide temperature range with a detailed analysis of the products obtained.52–61 However, under high temperature conditions, processes may occur that result in the formation of secondary

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products, whose presence in the reaction mixture makes it difficult to obtain reliable information about the chemical nature of RAS.3,

54, 55

Hence, works aimed at the detailed study of the

composition and structure of RAS and focused on the selective chemical destruction of RAS macromolecules are growing in number.4, 30–35, 62 Thus, chemical degradation allows to establish the details of the ‘blocks’ involved in the construction of macromolecules of resins and asphaltenes, in particular, to obtain data on the presence of C-S, C-O bridges in their structure and to determine the qualitative composition of the ‘bound’ moieties. It has been shown that the main ‘sulfur- and ether-bound’ compounds in RAS molecules are saturated and aromatic hydrocarbons (HCs) and sulfur- and oxygen-containing structures.4,

30, 32–35, 62

No ‘bound’

nitrogen-containing compounds have been identified in the building blocks of molecules of resins and asphaltenes. This may be due to the fact that the predominant part of N, identified in RAS, enters as structure elements into the molecules of these high molecular compounds, connecting with them through σ-bonds.3,

21, 35, 45, 63, 64

A minority of N is a component of

relatively low molecular weight compounds, which are either adsorbed on the macromolecular formations of RAS due to donor-acceptor interactions, or captured by hollow cells of their structures. Hence, before to start investigation of nitrogen-containing compounds, they must be preliminary isolated from resins and asphaltenes, concentrated and simultaneously differentiated by molecular mass in the course of isolation. The paper presents the results of a comparative study of the composition and structure of asphaltene and resin components of the light and heavy oils. The data on determination of the composition of structural fragments bound in molecules of their RAS by ether and sulfide bridges, compounds adsorbed/occluded by molecules of asphaltenes, and nitrogen bases (NBs) of resins are also summarized. An unified approach to the structural study of resins and asphaltenes made it possible to reveal structural features of the molecules of the components of oils of different chemical natures and their principal similarities and differences. 2.

EXPERIMENTAL SECTION

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2.1. Materials Solvents, i.e. 99.9 % n-hexane (n-C6), benzene (C6H6), chloroform (CHCL3), tetrahydrofuran (THF), and acetic acid (CH3COOH), sulfuric acid (H2SO4) and perchloric acid (HClO4) were purchased from JSC EKOS-1 (Russia), while methanol (MeOH) and ethanol (EtOH) were purchased from JSC “Vekton” (Russia). Diethylamine ((C2H5)2NH) was purchased from BASF AG (Germany), sodium tetrahydroborate (NaBH4, 98 %) and lithium aluminum hydride (LiAlH4, 97 %) were purchased from ABCR Gmbh Co KG (Germany), while boron tribromide (BBr3, 99 %) was purchased from ABCR ORGANICS (Germany). Nickel chloride (NiCl2.6H2O) was purchased from LLC EKOTEK (Russia). ASC silica gel (0.25-0.50 mm) was purchased from HONG KONG CHEMICAL CORPR (Hong Kong), while silica gel L (100/160) for chromatography was supplied by LACHEMA (Czech Republic). 2.2. Samples We used a light oil sampled at the Krapivinskoye oilfield (I) (Upper Jurassic deposits, West Siberian oil and gas basin, Russia) and a heavy oil sampled at the Usinskoye field (II) (PermoCarboniferous deposits, Timan-Pechora oil and gas basin, Russia). These oils have significant differences in density, content of RAS, and heteroatoms (Table 1). Table 1. Characteristics of the objects of investigation Sample Oil I Resins Asphaltenes Oil II Resins Аsphaltenes

Content in oil, %

Density, kg/m3 867

8.8 2.6 1008 19.1 11.2

S 1.05 3.78 2.34 1.98 2.96 3.42

Content, wt % N Nbas 0.35 0.03 0.56 0.25 0.89 0.62 0.19 1.08 0.55 1.14

O 4.42 5.45 3.86 3.24 6.34 9.66

2.2.1. Asphaltenes. Asphaltenes were isolated from the oils by precipitation with a 40-fold excess of n-C6. The precipitate was removed by filtration and washed with n-C6 until the solvent flowed colorless. The n-C6 was chosen as solvent in order to minimize the coprecipitation of

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resins together with asphaltenes, since the ability of n-alkane to desorb resins from the surface of asphaltene aggregates increases with the increasing number of carbon atoms in the molecule.26, 65 Asphaltenes were separated by the method, described in

30, 33.

It consists in the extraction of

initial asphaltenes with hot acetone in a Soxhlet apparatus to obtain an extract and nonextractable compounds and in the treatment of the extract with n-C6 to obtain soluble and insoluble products. The residue after acetone extraction was attributed to the high molecular weight fraction of asphaltenes (HMA), while the product insoluble in n-C6 was attributed to the low molecular weight fraction of asphaltenes (LMA). The product soluble in n-C6 was classified as the compounds adsorbed/occluded by the molecules of initial asphaltenes, which are further called ‘maltenes’. ‘Maltenes’ were separated via liquid-adsorption column chromatography using silica gel L (100/160) to fractions of low polar and polar compounds, eluted with n-C6 /C6H6 (1:1 by vol.) and CHCl3/CH3OH (4:1 by vol.) mixtures. Asphaltene fractionation scheme is shown in Figure 1.

Figure 1. Scheme of asphaltene fractionation. 2.2.2. Resins. Deasphalted oils were separated into oil components and resins by column liquid-adsorption chromatography using silica gel ASC (0.25-0.50 мм). The oil components were eluted with a mixture of n-C6/C6H6 (7:3 by vol.) while resins – with a mixture of EtOH/C6H6 (1:1 by vol.), respectively. 2.2.3. Nitrogen bases. Isolation of NBs from resins was carried out according to the scheme (Figure 2), providing their fractionation by molecular masses in the course of isolation. It is based on the subsequent use of methods of precipitation of high molecular weight NBs and ACS Paragon Plus Environment

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extraction of low molecular weight NBs.35 To isolate high molecular NBs, a solution of resins in n-C6 (5 g in 100 mL) was treated with dry gaseous HCl until the precipitation stopped. Precipitated complex salts were separated from the Raffinate 1, dissolved in chloroform (50 mL), and decomposed with 10 % aqueous KOH to obtain C-1 concentrate. After isolating compounds composing C-1, soluble hydrochloride salts of high molecular NBs were precipitated by adding diethylamine (1.8 and 4.0 mL for Raffinate 1 of the oil sample I and the oil sample II, respectively). The precipitate was separated from the Raffinate 2, dissolved in 50 mL of chloroform, and decomposed with 10 % aqueous KOH to obtain C-2 concentrate. Low molecular NBs were extracted from the Raffinate 2 solution with H2SO4 acetic acid solution with a mass ratio of mineral and organic acids to water equal to 25:60:15. The resulted salts were decomposed with 10 % aqueous KOH to obtain C-3 concentrate.

Figure 2. Scheme of isolation of organic nitrogen bases. 2.3 Chemical destruction of resins and asphaltenes. To carry out reactions, which cleave carbon–sulfur bond in sulfides and carbon–oxygen bond in ethers and esters under mild conditions, nickel boride and boron tribromide were used.32 The destruction of sulfide and ether bonds in molecules of HMA and resins was performed at 60 °C according to the methods described in30. Desulfurization was carried out in THF and MeOH, while the rupture of C–O bonds in chloroform medium. The n-C6-soluble products of chemical decomposition of HMA and resins were separated on silica gel L (100/160) into

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fractions of non-polar and polar compounds, using for desorption a mixture of n-C6/benzene (7:3 by vol.) and benzene/ethanol (1:1 by vol.) respectively. Alkyl bromides of the nonpolar fraction were reduced by LiAlH4. The composition of reduction products and non-polar compounds of the desulfurization products were analyzed by GC/MS. 2.4. Analytical methods Elemental analysis of the samples was performed using a ‘Vario EL Cube’ CHNS analyzer. The absolute error of the analyzer did not exceed ± 0.1 % for each element being determined. The oxygen content was evaluated by the difference between 100 % and the sum of C, H, N, and S elements. Molecular masses (MM) were measured by cryoscopy in benzene. Non-aqueous potentiometric titration66 of samples of resins and NB fractions was carried out in automatic mode using an ATP-02 potentiometric titrator (‘NPO AKVILON’ LLC, Russia), acetic acid as medium and solution of perchloric acid in acetic acid as a titrant. Fourier transform infrared spectra (FTIR spectra) of samples were recorded as a thin film obtained from a solution in chloroform within the range of 4000-400 cm-1 using a NICOLET 5700 spectrometer. 1H

NMR spectra were recorded using a Bruker AVANCE AV 300 NMR-Fourier

spectrometer at 300 MHz in CDCl3 solutions. Tetramethylsilane was used as a standard. The main parameters of distribution of hydrogen atoms in various structural fragments of molecules of resins, asphaltenes and NBs were determined from the corresponding signal areas of 1H NMR spectra:49 8.5…6.6 ppm – the percentage of protons contained in aromatic structures (Har); 4.0…2.2 ppm – the percentage of protons near the carbon atom in the α-position of aliphatic substituents of aromatic structures (Hα); 2.1…1.1 ppm – the percentage of protons in methylene groups (Hβ);

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1.1…0.3 ppm – the percentage of protons in the terminal methyl groups of the alkyl moieties of molecules (Hγ). The SGA method67 is based on data on the elemental composition, average MM, and distribution of protons between different fragments of molecules of the compounds under study. Using the SGA method the information was obtained about the size and structure of the molecules of resins, asphaltenes and high molecular weight NBs. The parameters were calculated using the program registered in Rospatent (Russian Federal Service for Intellectual Property).68 The following parameters were determined as a result of calculations: Ca, Cn, Cp were the numbers of carbon atoms in aromatic, naphthenic and paraffin structures of the mean molecule; fa, fn, fp were the percentages of carbon atoms in aromatic, naphthenic, and paraffinic structural fragments, respectively, %; ma was the number of structural blocks in a mean molecule, which are naphtheno-aromatic formations framed with alkyl substituents. In the molecules of resins and asphaltenes, these blocks can be linked together by carbon, sulfide, and ether bridges. Rt*, Ra*, and Rn* were the total number of rings (cycles) and the number of aromatic and naphthenic cycles in a structural block, C* was the total number of carbon atoms, and Cp* was the number of carbon atoms in the paraffinic fragments of the structural block, Сα* was the number of C atoms in the α-position to the aromatic core, and Сγ* was the number of C atoms in terminal methyl groups not bound to aromatic core. GC-MS analysis of liquid products of chemical destruction of HMA, resins, low polar fractions of ‘maltenes’, and low molecular weight NBs was performed using a Thermo Scientific DFS device. The energy of ionizing electrons was 70 eV, the temperature of ionization chamber – 270 °C, the interface temperature – 270 °C, and the injector temperature – 250 °C. For chromatographic separation, a DB-5MS column 30 m in length, 0.25 mm in diameter and 0.25 μm phase thickness was used. The carrier gas was helium flowing at a constant flow rate of 0.8 ml/min. The thermostat program was as follows: the initial temperature 80 ºС (3 min), rise up to 300 ºС (4 ºС/min), and 30 min holding at the final temperature. Mass spectra were scanned every

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second in the mass range up to 500 a.m.u. The mass chromatograms (molecular weight separation) of compounds of various types were reconstructed using characteristic ions based on chromatograms of the total ion current using the Xcalibur program. To identify individual compounds, literature data and a computer library of mass spectra of the National Institute of Standards and Technology were used. Below is a list of compounds identified in the liquid products of chemical destruction of HMA and resins, low polar fractions of ‘maltenes’ and concentrates of low molecular weight NBs and m/z of characteristic ions. Types of compounds n-alkanes, branched (including isoprenoides) n-alkenes Cycloalkanes Terpanes Terpenes Steranes Alkylbenzenes Naphtalenes Phenantrenes, anthracenes Fluoranthenes Pyrenes Triphenylenes, chrysenes Perylenes/benzofluorenes/benzo[a]pyrenes Benzo[g,h,i]perylenes, dibenzochrysenes Dibenzopyrenes Thiophenes Benzothiophenes Dibenzothiophenes Naphtobenzothiophenes Carbazoles Benzocarbazoles Dibenzocarbazoles Quinolines Benzoquinolines Aliphatic acids n-Alkanoic acid ethyl esters Aliphatic alcohols Fluoren-9-ones Dibenzofuranes 3. RESULTS AND DISCUSSION 3.1. Asphaltenes

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m/z of characteristic ions 71 55 69, 83 191, 177 189, 231 217, 218 91, 92, 105, 106, 119, 120, 133 128+14n 178+14n 202+14n 228+14n 252+14n 276+14n 302+14n 97 134+14n 184+14n 234+14n 167+14n 217+14n 267+14n 129+14n 179+14n 60 88 55 180+14n 218+14n

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3.1.1. General characterization of asphaltenes. The content of asphaltenes in the oil sample II is 4.3 times higher than that in the oil sample I (Table 1). According to the data given in Table 2, they are characterized by higher values of the average MM and larger sizes of mean molecules (number of carbon atoms in a mean molecule, C). This is mainly due to the higher number of carbon atoms (Ca and Cn) in the aromatic (Ra) and naphthenic (Rn) cycles. Table 2. Calculated values of the structural parameters of asphaltene molecules of the oils under study Number in a mean molecule MM, Structural Carbon atoms Structural fragments Sample a.m.u. block ma С Са Сn Сp Rt* Rа* Rn* Cp* Asphaltenes Oil I 940 2.2 65.5 25.7 36.7 3.1 7.9 2.8 5.1 1.5 Oil II 1405 3.0 93.6 38.8 49.7 5.1 8.0 3.3 4.7 1.7 Carbon atoms form three building blocks in mean molecules of asphaltenes of the oil sample II and two building blocks in mean asphaltene molecules of the oil sample I (ma = 3.0 and 2.2, respectively), which are rather similar in total cyclicity (Rt*). At the same time, the percentage of aromatic (Rа*) and aliphatic (Cn*) fragments in the structural block of mean molecules of asphaltenes of the oil sample II is slightly higher, while the structural block of mean molecules of asphaltenes of the oil sample I contains a higher percentage of naphthenic fragments (Rn*). Analysis of the results of fractionation of asphaltenes under study revealed that, whatever the type of oil, they are similar in MM distribution of compounds. The main contribution to the composition of asphaltene components is made by HMA (МM 1009 and 1500 a.m.u. for the oils samples I and II, respectively). The percentages of LMA (MM 550 and 770 a.m.u.) and ‘maltenes’ (MM 500 and 700 a.m.u.) are significantly lower. Taking into account the values of the average MM, it can be assumed that HMA molecules are mainly represented by structures of the ‘island’ type, while LMA molecules – by structures of the ‘archipelago’ type. The oil sample II differs from the oil sample I in a higher content of HMA (92.2 vs. 87.1 % rel.). The unique feature of oil sample I are a higher content of LMA (5.4 vs. 3.7 % rel.) and nearly double content of maltenes (6.7 vs. 3.4 % rel.). In order to comparatively characterize

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asphaltenic components of oils of different nature, the most different in yield fractions of asphaltenes (HMA and ‘maltenes’) were used. 3.1.2. Characterization of high molecular asphaltenes. The use of selective chemical degradation has revealed that moieties linked through sulphide and ether bridges present both in the structures of NMA of light and heavy oils. The yield in desulphurisation products soluble in n-C6 is significantly higher than that of soluble products of ether bond cleavage (Figure 3). 80

Yield, wt %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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HMA

resins

60 40

C-S

20 0

C-O Oil-I OilII

Oil-I OilII

Figure 3. Yield in n-C6-soluble products of C-S and C-O bond cleavage in the molecules of HMA and those of resins. Hence it is 24.1 and 12.8 % for HMA molecules of oil sample I and 33.3 and 9.6 % for HMA molecules of oil sample II, respectively. This may indicate that the percentage of ‘sulfur-bound’ moieties is higher than that of ‘ether-bound’ moieties in the structure of HMA of both oils. In addition, the percentage of ‘sulfur-bound’ moieties is higher in the structure of HMA of the oil sample II, while the ‘ether-bound’ moieties prevail in the structure of HMA of the oil sample I. Saturated and aromatic HCs and hetero-organic compounds (HOC) have been identified among the moieties linked in the molecules of HMA under study both by sulfur- and etherbridges, but the set of identified ‘sulfur-bound’ moieties is much wider than that of ‘ether-bound’ ones. In both cases, normal and branched alkanes, alkylcyclopentanes, alkylcyclohexanes, steranes, terpanes, alkyl benzenes, and alkyltoluenes have been identified in the ‘bound’ moieties. It must be noted that in the structure of HMA of oil sample I the same representatives of saturated and aromatic HCs are characterized by a larger number of carbon atoms in the main chain or in alkyl substituents. As an example, Figure 4 shows the distribution of n-alkanes in the ACS Paragon Plus Environment

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composition of products of sulfide C-S bond cleavage in HMA molecules of the oil samples I and II.

Figure 4. Mass-chromatogram of distribution of n-alkanes as products of cleavage of sulfide bonds in the HMA molecules of oil samples I and II for ion at m/z 71 (CnH2n+2). The number refers to the number of carbon atoms in n-alkanes. Another feature of HMA molecules of the oil sample I is the presence of unsaturated HCs, and a wider set of aromatic hydrocarbons (AHs) in their ‘sulfur-bound’ moieties. The following compounds have been identified among them: 1-alkenes from C16 to C34 with an even number of carbon atoms in a molecule, C30 hopenes, C1-C4 naphthalenes, C0–C1 anthracenes, C0–C4 phenanthrenes, C0–C2 fluoranthenes, pyrenes, C0–C2 triphenylenes, chrysenes, C0–C2 perylenes, benzofluaranthenes, benzo[a]pyrenes, C0–C2 benzo[g,h,i]perylenes, dibenzochrysenes and C0–C1 dibenzopyrenes. As an example, Figure 5 shows the distribution of tetra- (m/z 202, 228), penta- (m/z 252) and hexacyclic (m/z 276, 302) aromatic hydrocarbons in the products of sulfide bond cleavage in HMA molecules of the light oil.

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1 – fluoranthene, 2 – pyrene, 3 – triphenylene, 4 – chrysene, 5 – perylene, 6, 7, 8 – benzofluoranthenes and benzo[a]pyrene, 9 – benzo[g, h, i]perylene, 10 – dibenzochrhizene, 11 – dibenzopyrene.

Figure 5. Distribution of polycyclic aromatic hydrocarbons in the products of sulfide bond cleavage in HMA molecules of the oil sample I when scanning at ions m/z 202, 228, 252, 276, 302. Dibenzothiophene and its C1–C3 alkyl derivatives and C13 and C15 aliphatic alcohols have been identified among HOC of the products of S-C bond cleavage and those of C-O bond cleavage of HMA molecules of the oil sample I. The presence of C14-C22 alkylthiophenes, dibenzothiophene, a number of aliphatic acids from C14 to C18, and C16–C20 n-alkanoic acid ethyl esters has been identified among HOC in the products of cleavage of S-C and C-O bonds of HMA molecules of the oil sample II. The presence of alkylthiophenes in the structure of asphaltenes was established by Strausz, O.P. et al.52 in the products of thermal destruction of Athabasca asphalthenes. As for the identified n-alkanoic acid ethyl esters, they are most likely occluded by HMA, since chemical destruction can result not only in the rupture of covalent bonds, but also in the release of compounds inside asphaltene structures.69 It should be noted that despite of a rather high content of total nitrogen in asphaltenes (Table 1), no organic nitrogen compounds of neutral (carbazole derivatives) or basic character (pyridine derivatives) have been identified in the products of C-S and C-O bond cleavage of HMA under study. Therefore, it can be assumed that the above-mentioned components are not linked with the core of asphaltene molecules by sulfide or ether bridges, but they are located ACS Paragon Plus Environment

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mainly in their condensed polycyclic blocks or bound to each other and to other structural moieties by the bonds of biphenyl type, by methylene bridges or short (2–5 carbon atoms) polymethylene links.33,

52, 54, 64

Thus, using pyrolysis and concentration of NBs from liquid

products by the method of complexation with copper salts, Strausz, O.P. et al.52 revealed the presence of alkyl substituted pyridines and quinolines in a polycondensed system of Athabasca asphaltenes. It is also possible that the percentage of nitrogen-containing compounds in the ‘bound’ moieties is very low, which makes their identification difficult.70 3.1.3. Characterization of maltenes. According to the results of chromatographic separation, the great bulk of ‘maltenes’ in asphaltenes of both light and heavy oils are compounds of polar fractions (60.6 and 59.7 %, respectively). According to the data of high-quality IR spectroscopy, the compounds containing functional groups of acids (3300–3100, 1727, 1709-1700 cm-1), amides (1700-1600 cm-1), and sulfoxides (1040-1010 cm-1) have been identified in the composition of ‘maltenes’. The presence of acids and sulfoxides in the composition of ‘maltenes’ was reported in 71, 72. Using high-resolution mass spectrometry, Chacon-Patino, M. L. et al.51 have established the presence of heteroaromatic structures with a high content of heteroatoms, in particular, compounds containing N, O and S, O atoms in their structure, in the components occluded by asphaltenes. The GC-MS analysis of the compounds of low-polar fractions of ‘maltenes’ in asphaltene components of both oils revealed that they contain alkanes, naphthenes, AHs, and HOC. Among saturated HCs of both samples, homologous series of normal and branched alkanes, alkyl cyclopentanes, alkyl cyclohexanes, steranes, and terpanes have been identified. The AHs contain n-alkylbenzenes, phytanylbenzenes, alkyltoluenes, alkylxylols, naphthalenes, and phenanthrenes, while HOC include benzo-, dibenzothiophenes, dibenzofuranes, and benzocarbazoles (Figure 6). These types of compounds have a similar molecular mass distribution.

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Figure 6. Mass chromatogram of benzocarbazoles (BC) of the low-polar fraction of ‘maltenes’ in asphaltene components of the oil sample I for ions at m/z 217 (C0BC), 231 (C1BC), 245 (C2BC), 259 (C3BC), 273 (C4BC) and 287(C5BC). Chacon-Patino, M. L. et al.51 have found out that in addition to saturated hydrocarbons including hydrocarbon biomarkers, the components occluded by asphaltenes of Colombian oils contain aromatic compounds with a high content of heteroatoms, vanadyl porphyrins, and representatives of high aromatic structures. The latter, in their opinion, should be attributed to low molecular weight asphaltenes, which are converted into n-heptane in the course of extraction of initial asphaltenes. A distinctive feature of the low-polar fraction of ‘maltenes’ of asphaltene components of the oil sample II is the presence in its composition of a wider set of tricyclic terpanes (Figure 7), phenylalkanes with different positions of phenyl substituent in the alkyl chain, tetra- and pentacyclic

AHs,

including

phenyl

and

naphthene

derivatives

of

AHs,

and

benzonaphtothiophenes, carbazoles, fluorenones, and ethyl esters of higher fatty acids containing an even number of carbon atoms. The low-polar fraction of ‘maltenes’ in asphaltene components of the oil sample I is characterized by the presence of dibenzocarbazoles and 1- and 2-alkenes with an even number of carbon atoms in a molecule (Figure 8).

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Figure 7. Mass chromatogram of the low-polar fraction of ‘maltenes’ of the oil sample I (a) and the oil sample II (b) for ions at m/z 191.

Figure 8. Distribution of n-alkenes and n-alkanes in ‘maltenes’ of asphaltene components of the oil sample I. The number refers to the number of carbon atoms in n-alkanes and n-alkenes.

A series of n-alkanoic acid ethyl esters and 1-alkenes with an even number of carbon atoms in the molecule were identified among ‘maltenes’ of asphaltene aggregates of oils from northwestern of China.73 The presence of occluded 1-alkenes with an even number of carbon atoms in the molecule is reported in74, 75. Cheng B. at al.75 reported that the source of these compounds is esters present in the original organic matter (OM). Esters generate olefins in the course of geochemical transformation of OM, which are captured by macromolecules of kerogen and then inherited by asphaltenes formed from kerogen.

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It should be noted that olefins were also identified in bituminoids of terragenic OM,76 in the Devonian oils of Canada, 77 Paleozoic oils, 78 and the Mezo-Cenozoic oils of Russia.79 The results of the comparative analysis show that some compounds identified in the low-polar fractions of ‘maltenes’ are also included in the structure of HMA molecules. These are mainly nalkanes, naphthenes, and AHs. They have a similar molecular mass distribution but differ in the pattern of the concentration distribution. 3.2. Resins 3.2.1. General characterization of resins. From the data in Table 1 it follows that in large part the heteroelements identified in the oils are concentrated in the resins under study. We have found that the number of sulfur atoms is higher in molecules of resins of the oil sample I, while the molecules of resins of the oil sample II contain a higher number of nitrogen and oxygen atoms. Oscillation bands in the IR spectra allowed us to conclude that heteroatoms in the resin components of both oils are present in the functional groups of acids (3500–2900, 1730–1700 cm-1), amides (1660 cm-1), ethers (1750–1740 cm-1), sulfoxides (1030 cm-1), and pyridine cycles (1580–1560 cm-1). At the same time, structural analysis showed that the resin components of the light and heavy oils differ in structural group characteristics (Table 3). Hence, resins of oil I consist of monoblock molecules (ma = 1.2). Their structural blocks are constructed from two or three saturated rings (Rn* = 2.5) and one or two aromatic rings (Ra* = 1.5). The total number of alkyl carbon atoms in structural blocks of resins in the oil sample I varies from eight to nine (Cp* = 8.5). Table 3. Calculated values of structural parameters of resin molecules of the oil samples under study Number in a mean molecule Sample Resins of oil sample I Resins of oil sample II

MM a.m.u.

Structural block ma

C

Ca

Cn

Сp

Rt* Rа* Rn*

Cp*

Сα* Сγ*

480

1.2

30.3

7.7

12.3

10.3

4.0 1.5 2.5

8.5

3.1

2.0

877

1.8

59.3 17.3

22.7

19.3

5.5 2.4 3.1 10.8 4.6

2.6

Carbon atoms

Structural fragments

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Mean molecules of resins of the oil sample II are larger. As a rule, they consist of two structural blocks (ma = 1.8), which are constructed from two or three arene (Ra* = 2.4) and three saturated (Rn* = 3.1) rings. The alkyl frame of the naphtene-aromatic system of such a structural unit is constituted by ten or eleven carbon atoms (Cp* = 10.8). However, most of paraffinic carbon atoms in structural blocks of the mean molecules of both resins are in long weakly branched alkyl substituents. This is indicated by close values (23.5 and 24.1 %) of the relative content of terminal methyl groups (Сγ*/Сп*·100). 3.2.2. Ni2B Desulfurization and Ether Cleavage. Using C–O and C–S bonds cleavage reactions it was established that ‘ether-bound’ moieties prevail in the structure of resins of the oil sample I, while ‘sulfur-bound’ moieties dominate in resins of the oil sample II. Thus, the yield in n-C6-soluble products of sulfide bond cleavage in molecules of resins in the oil samples I and II is 40.9 and 55.3 %, while the yield of soluble products of ether bond cleavage is 65.0 and 42.6 %, respectively (Figure 3). A similar distribution of ‘sulfur- and ether-bound’ moieties was also established for the HMA molecules of these oils. The results of GC-MS analysis of n-C6 soluble products of ether bond cleavage suggest the presence of normal and methyl-substituted alkanes with different positions of the replacement radical,

isoprenanes

(2,6,10-trimethyl

alkanes,

pristane,

phytane),

alkylcyclopentanes,

alkylcyclohexanes, steranes, hopanes, phytanylbenzenes, mono-, bi-, tri-, and tetraalkylbenzenes. in ‘ether-bound’ fragments of resins of the oil samples I and II.5, 35 Despite the similarity of the qualitative compositions of saturated and aromatic hydrocarbons, there is a difference in the molecular mass distribution of the individual representatives of the moieties connected through the ether bridge. Hence, the ‘bound’ monoaromatic hydrocarbons present in the structure of resins of the oil sample I are characterized by a larger number of C atoms in the acyclic substituent of n-alkyl-, n-alkylmethyl-, n-alkyldimethyl-, and nalkyltetramethylbenzenes.

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The presence of pregnanes, cheilanthanes, C0–C4 bi- and C0–C2 tricyclic AHs, C2 – C5 benzo(Figure 9) and C0 – C4 dibenzothiophenes (Figure 10), as well as C12, C14, C16, C18 saturated monoatomic alcohols in ‘ether-bound’ moieties of oil II resins is a distinctive feature of the resin molecules of this oil.5 No polycyclic AHs and HOC were identified among С-О bound fragments of resin molecules of the oil sample I.

Figure 9. Mass fragmentogram of benzothiophenes (BT) in the products of C-O bond cleavage in resin molecules of the oil sample II for the ion at m/z 162 (C2BT), 176 (C3BT), 190 (C4BT), and 204 (C5BT).

Figure 10. Mass fragmentogram of dibenzothiophenes (DBT) in the products of C-O bond cleavage in resin molecules of the oil sample II for the ion at m/z 184 (C0DBT), 198 (C1DBT), 212 (C2DBT), 226 (C3DBT), and 240 (C4DBT). As in the case of C–O bound structures, C-S bound fragments of resins of the oil sample I are of special mention. ‘Sulfur-bound’ alkanes and monocyclic naphthenes of resins of this oil are

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characterized by a wider molecular mass distribution (Figure 11) and monocyclic AHs

by a

larger number of C atoms in the alkyl substituent.

Figure 11. Distribution of cyclohexanes in the products of sulfur bond cleavage in resin molecules of the oil samples I (a) and II (b) when scanning at the ion m/z 83. The number refers to the number of carbon atoms in cyclohexanes. Among ‘sulfur-bound’ moieties of resins of the light and heavy oils, n-alkanes, monomethylsubstituted alkanes with different positions of the substituting radical, isoprenoids (2,6,10trimetilalkanes, pristane, phytane), alkylcyclopenthanes, alkylcyclohexanes, phenylalkanes, monoarenes and monobasic aliphatic acids have been identified. A distinctive feature of resins of the oil sample I is the presence of tetracyclic naphthenes (steranes), tri- and pentacyclic saturated hydrocarbons (cheilanthanes and hopanes), similar in their composition to ‘C-O bound’ analogues, and a wider set of HOC in C-S-bound moieties.35 Aside from saturated monobasic acids, ethyl esters of n-alkanoic acids, acyclic alcohols, and bicyclic terpenoid sulfides have been identified among HOC. 3.2.3. Comparative characteristics of nitrogen bases of resins. Analysis of the results given in Table 4 showed that the amount of NBs isolated from resins and the pattern of their distribution over the concentrates depends upon the oil type. Hence, the total yield of NBs from resins of the oil sample II is significantly higher (40.8 wt %) than that from resins of the oil sample I (26.1 wt %). In both cases, most of the isolated compounds are components of C-1 and C-2 concentrates. However, components of C-1 concentrate predominate in resins of the oil

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sample II, while components of C-2 concentrate predominate in those of the oil sample I. The content of extracted compounds of C-3 concentrate in resins of the oil sample II (2.3 wt %) is nearly the same as the content of these compounds in resins of the oil sample I (1.9 wt %). The relative content of Nbas in compounds isolated from resins of the oil sample I is nearly equal to that of compounds isolated from resins of the oil sample II (84.3 and 85.9 % rel., relatively). However, in the case of resins of the oil sample II, its main amount (79.9 % rel.) falls on components of C-1 concentrate. The components of C-2 and C-3 concentrates account for only 3.6 and 2.4 % rel., respectively. In the case of resins of the oil sample I, the percentage of Nbas accounted for the components of C-1 concentrate is significantly lower (31.8 % rel.), while that of Nbas accounted for the components of C-2 and C-3 concentrates is significantly higher (38.1 and 14.4 % rel., respectively). Table 4. Isolation of high molecular and low molecular organic nitrogen bases from resins Mass fraction, %

MM, a.m.u.

C-1 C-2 C-3

10.93 13.21 1.94

1018 607 383

C-1 C-2 C-3

38.6 2.2 2.3

844 519 365

Samples

Nbas

Percentage of Nbas of resins, %

0.73 0.72 1.86

31.8 38.1 14.4

1.17 0.90 1.22

79.9 3.6 2.4

Mass fraction, % C

H

S

Nt

resins of the oil sample I 82.18 8.55 2.32 1.29 79.68 8.55 2.27 1.73 78.19 9.37 3.66 2.20 resins of the oil sample II 81.08 8.53 3.19 1.36 81.17 9.50 2.68 1.36 80.01 10.81 2.95 1.30

With a general trend of changes in MM in the series of isolated concentrates (C-1> C-2> C3), NBs of resins of the oil sample I differ from NBs of resins in the oil sample II by higher average MM values. These differences may be due to the structural features of mean molecules of NBs in the resins under study. According to the data presented in Table 5, the mean NBs molecules of resins in the oils of both types form systems consisting of aromatic (Ra), naphthenic (Rn) and paraffin (Rp) structural fragments. However, the contribution of aromatic fragments (fa = 29.0–38.3 versus 20.6–34.7 %) is more pronounced in the structure of mean molecules of NBs in resins of the oil sample I, while ACS Paragon Plus Environment

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that of naphthenic fragments (fn = 41.3–56.9 vs. 39.4–42.0 %) is higher in the structure of mean molecules of NBs in resins of the oil sample II. A comparative analysis showed that the mean NBs molecules of the similar concentrates are close in the total number of cycles (Rt*) present in their structural blocks (mа). The values of Rt* in the structural block of NBs of the extracted compounds of both C-3 concentrates are 3.4 and 3.5, Rt* in the structural block of C-2 concentrates – 4.7 and 5.4, and Rt* in the structural block of C-1 concentrates – 5.8 and 6.0, relatively. The NBs molecules of concentrates C-1 and C-2 of resins of both oils differ slightly in the number of paraffinic carbon atoms (Cp*) in the structural blocks. Significant differences are observed for the paraffinic structural blocks of NBs molecules of the C-3 concentrate.

Table 5. Calculated values of structural parameters of molecules of organic nitrogen bases of the resins under study Samples Parameters Resins of the oil sample I Resins of the oil sample II C-1 C-2 C-3 C-1 C-2 C-3 Number of C atoms of various types in a mean molecule Са 26.7 14.3 7.3 19.8 8.8 5.1 Сn 27.6 16.9 9.8 27.7 20.0 10.2 Сp 15.4 9.1 7.9 9.5 6.4 9.4 Distribution of C atoms, % fа 38.3 35.5 29.0 34.7 25.0 20.6 fn 39.6 42.0 39.4 48.6 56.9 41.3 fp 22.1 22.5 31.6 16.7 18.1 38.1 Parameters of average structural blocks mа 2.3 1.6 1.2 1.9 1.3 1.0 Rо* 5.8 4.7 3.4 6.0 5.4 3.5 Rа* 2.8 2.2 1.4 2.5 1.6 1.1 Rn* 3.0 2.5 2.0 3.5 3.8 2.4 С* 30.6 25.0 20.9 29.7 27.6 24.1 Сp* 6.8 5.6 6.6 5.0 5.0 9.2 Сα* 4.7 4.0 3.7 4.3 3.6 3.2 Сγ* 1.7 1.5 1.8 2.4 2.4 2.9 The number of paraffinic carbon atoms in the structural blocks of NBs molecules in resins of the oil sample II is higher (Cn* = 9.2) than that in the structural blocks of NBs molecules in resins of the oil sample I (Cn* = 6.6). The average number of substituents (Cα*) at the aromatic carbon atoms in structural blocks of NBs molecules in resins of the oil sample I ranges from 3.7 ACS Paragon Plus Environment

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to 4.7, while it ranges from 3.2 to 4.3 in structural blocks of NBs molecules of resins of the oil sample II. If the value of the Cα* parameter is lower than 4, it may suggest the extreme location of aromatic cores in the polycyclic system. If it is higher than 4, this suggests that aromatic cores occupy a central position within an united naphthenoaromatic system being condensed with two separate naphthenic fragments.67, 80 The analysis of the data given in Table 5 showed that the second type of structures is characteristic of NBs molecules of C-1concentrates, while the first type of structures is characteristic mainly for NBs molecules of C-3 concentrates. Comparison of the values of Cγ* parameter, which reflects the content of terminal methyl groups showed, that the structural blocks in the NBs molecules of oil resins differ in the number of relatively long or branched paraffin chains. In structural blocks of NBs molecules of resins of the oil sample I (Сγ* = 1.5 – 1.8) this number is lower than that in structural blocks of NBs molecules of the oil sample II (Сγ* = 2.4 – 2.9). 3.2.4. Low molecular organic nitrogen bases of resins. The additional information on the low molecular NBs of the resin components of light and heavy oils was obtained using the methods of IR spectroscopy and GC-MS. According to IR spectroscopy data, they are represented by a mixture of strongly and weakly basic compounds. A bending band in the region of 1576 cm-1, characteristic of strong bases such as pyridine benzologues, and a bending band at 1650 cm-1, characteristic of weak bases of the amide type were observed for the vibrational spectra of NBs of the resins under study. Absorption bands at 3211 and 1720 cm-1 suggest the presence of carboxyl-containing compounds in the NBs mixture, which, depending on the position of the carboxyl group in the pyridine ring, can exhibit the properties of strong or weak bases.81 In addition, absorption bands observed in the spectra of C-3 at 3600, 3585 and 1037 cm-1 suggest the possible presence of hydroxypyridines and their benzologues and sulfoxides in the mixture. Nitrogen-containing sulfoxides, which are similar in their properties to weakly basic nitrogen compounds, can be extracted from complex organic mixtures along with them.82

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The use of GC-MS method made it possible the identification of C2-C3 quinolines and C2-C5 benzoquinolines as components of low molecular NBs (Figure 12). The presence of structures containing only methyl substituents was established for them. This conclusion follows from the analysis of the mass spectra of intense peaks in mass chromatograms. Their characteristic feature is the maximum intensity of the molecular ion peak, the low [M-H]+/M+ ion ratio, and the absence of peaks of rearrangement ions.83

Figure 12. Mass-chromatogramm of alkyl benzoquinolines (B) in C-3 fraction for ions at m/z 207 (C2B), 221(C3B), 235 (C4B), and 249 (C5B). Hence, 2,3,4-trimethylquinoline and 2,4,6-trimethylquinoline have been unambiguously identified

among

alkylquinolines,

while

2,4-dimethylbenzo(h)quinoline

and

2,4,6-

trimethylbenzo(h)quinoline have been identified among benzoquinolines. The identified isomers are the most important types of bi-and tricyclic pyridine benzologues present in oils.84

4. CONCLUSIONS A comparative analysis of the composition and structure of resin-asphaltene components of the light oil sampled from the Krapivinskoye oilfield and the heavy oil sampled from the Usinskoye oilfield is carried out. It has been established that resins and asphaltenes of heavy oil have higher values of average molecular masses and larger overall sizes of mean molecules due to the increased content of aromatic cycles in the naphtheno-aromatic system.

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No matter what the chemical nature of the oils, HMA, LMA, and compounds adsorbed and/or occluded by the molecules of asphaltene components (‘maltenes’) are present in their asphaltenes. The main contribution to the composition of asphaltene components is made by HMA. Heavy oil sampled from the Usinskoye oilfield is characterized by a higher HMA content, while the LMA content is higher in the light oil sampled from the Krapivinskoye oilfield. The content of ‘maltenes’ in the light oil sampled from the Krapivinskoye oilfield is almost two times higher. Among the compounds bound in the HMA molecules both by C-O bonds and C-S bonds, nalkanes, alkyl cyclopentanes, alkyl cyclohexanes, terpanes, alkyl benzenes and alkyl toluenes have been identified. The same representatives of sulfur- and ether-bound naphthenes and aromatic hydrocarbons have a similar distribution in the structure of HMA molecules of both oils. A distinctive feature of the light oil is the presence of n-alkenes, unsaturated polycyclic biomarkers, and a wider set of tricyclic terpanes and polycyclic AHs among ‘sulfur-bound’ moieties. Among low-polar ‘maltenes’ the same classes of hydrocarbons have been identified as among ‘bound’ moieties. A distinctive feature of ‘maltenes’ of the heavy oil is the presence of tricyclic terpanes, tetra- and pentacyclic AHs, and phenyl and naphthene derivatives in their composition. ‘Maltenes’ of the light oil are characterized by the presence of n-alkenes and dibenzocarbazoles in their composition. Normal and branched alkanes, phenylalkanes, alkylcyclopentanes, alkylcyclohexanes, mono-, bi-, tri-, and tetraalkylbenzenes have been identified among the compounds bound in the resin molecules of both types of oils by both ether and sulfide bridges. The special feature of resin molecules of heavy oils is the presence of polycyclic AHs and HOC in their ethers-bound moieties, while light-oil resins are characterized by the presence of sulfur-bound moieties of polycycloalkanes, n-alkanoic acid ethyl esters, alcohols, and bicyclic sulphides.

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Resins of the heavy oil are characterized by a higher total yield in NBs. In addition, NBs of light oil resins differ from those of heavy oil resins by higher values of average MM, which are due to the structural features of mean molecules of NBs in resin components of the light oil. The contribution of aromatic and paraffinic fragments to the structure of their mean molecules is higher, while naphtenic fragments prevail in the structure of mean molecules of NBs of the heavy oil resins. Low molecular NBs of the resins under study are represented by alkyl substituted chynolines and benzochynolines. The same set of compounds identified in the composition of studied resins and asphaltenes suggests their presence as structural fragments in the RAS molecules. The revealed differences in the group composition of HCs and HOC most likely reflect the specific structure of the molecules of resins and asphaltenes, which is due to the nature of the original source oil substance, the prehistory and the depth of transformation of the oil system.

The authors thank the staff of the laboratory of physicochemical methods of analysis of the Institute of Petroleum Chemistry, SB RAS, for analyzing samples using 1H NMR and IR spectroscopy. The authors are also grateful to Piotr Kadychagov, senior researcher of the laboratory of natural petroleum transformations of IPC SB RAS for the analysis of samples by the GC-MS method.

The work was performed within the framework of the state assignment of the Government of the Russian Federation (Project No. V.46.2.2).

REFERENCES

(1) Demirbas, A. Asphaltene Yields from Five Types of Fuels via Different Methods. Energy Conver. Manage. 2002, 43, 1091–1097.

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