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Article

Analysis of Distribution and Structures of Heteroatom Compounds in Asphaltene of Medium/Low Temperature Coal Tar by Negative Anion Mode ESI FT-ICR MS

by
Xiaoyong Fan
1,2,*,
Dong Li
2,
Louwei Cui
3,
Ruitian Shao
2,
Chunran Chang
1,4,
Long Yan
1 and
Bo Yang
1
1
Shaanxi Key Laboratory of Low Metamorphic Coal Clean Utilization, School of Chemistry and Chemical Engineering, Yulin University, Yulin 719000, China
2
School of Chemical Engineering, Northwest University, Xi’an 710069, China
3
The Northwest Research Institute of Chemical Industry, Xi’an 710069, China
4
Shaanxi Key Laboratory of Energy Chemical Process Intensification, School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(23), 15497; https://doi.org/10.3390/su142315497
Submission received: 26 September 2022 / Revised: 16 November 2022 / Accepted: 18 November 2022 / Published: 22 November 2022
Browse Figures
Versions Notes

Abstract

:
The existence of heteroatomic compounds with complex structure and different polarity in the asphaltene of medium and low temperature coal tar (M/LTCT) limits its processing and utilization. Combined with negative ion electrospray ionization source (ESI), Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to characterize the molecular composition of O, N, and S heteroatom compounds in M/LTCT asphaltenes. Acidic oxygen-containing compounds (OCCs) and non-basic nitrogen-containing compounds (NCCs) in asphaltenes were identified, except for sulfur-containing compounds (SCCs). The mass spectra showed that the heteroatom compounds in asphaltene mainly existed as NX, N1OX, N2OX, N3OX, N4OX, N5OX, N6OX, and OX class species (where x = 1–6). The M/LTCT asphaltenes were enriched with O4, N4, and N1O1 class species. The core structure of O4 class species were likely to be composed of 1–7 aromatic rings with 4 phenolic hydroxyl groups, the core structure of N4 class species were likely to be comprised of 4–7 aromatic rings with a piperazine ring and a pyrazole ring, and the core structure of N1O1 was mainly 3–6 aromatic rings with a phenolic hydroxyl group and a pyrrole ring. These results suggest that more condensed NCCs and OCCs with short, substituted alky side chains are presented, which are more easily to undergo condensation to generate fused molecules, making it too difficult to be removed by hydrogenation. Through the analysis of the molecular structures of OCCs and non-basic NCCs in M/LTCT asphaltenes, important information about the molecular composition can be obtained, which can provide basic data for the hydrogenation of deasphaltene.

1. Introduction

Medium and low temperature coal tar (M/LTCT) is a liquid by-product obtained from coal gasification, low-cohesive coal (lignite, bituminous coal, long flame coal, etc.) and dry distillation at a temperature below 600 °C. The daily output of M/LTCT has reached thousands of tons. As a suitable raw material for producing traditional liquid fuels (such as gasoline and diesel) with ultra-low heteroatom content through M/LTCT hydrogenation, it has received extensive attention in China [1]. Currently, two main types of M/LTCT HDT processing methods, i.e., the whole fraction and distillate fraction (<360 °C) fixed bed HDT processes, are used to produce gasoline and diesel. The world’s first whole fraction M/LTCT hydrogenation technology to produce clean fuel oil has been successfully applied in Shenmu Fuyou Co., Ltd. in Yulin, China [2], which means that a large amount of asphaltenes have no choice but to enter the reactor to be processed.
As we all know, asphaltenes refer to carbonaceous resources that are insoluble in normal alkanes, but soluble in aromatics [3,4,5]. Asphaltenes are the most complex and refractory components in M/LTCT, which account for 15–30 wt% [6]. The structure of M/LTCT asphaltenes is believed to be some fused aromatic ring, with several naphthenic and alkyl side chains, and it also contains amounts of sulfur, nitrogen, oxygen, and various metal heteroatom compounds [7]. The heteroatom compounds in asphaltenes are closely related to the operational problems in upstream and downstream M/LTCT processing. For instance, heteroatom compounds are relatively enriched in asphaltene. Aggregation are likely driven by polar heteroatom interactions, resulting in the instability of the M/LTCT processing system. Moreover, these heteroatom compounds will cause serious deactivation of the catalyst during the hydrogenation, and the remaining trace heteroatom compounds will reduce the stability of the products [8,9]. Therefore, the problem caused by asphaltenes in the hydrogenation of M/LTCT has attracted the attention of many researchers, who hope to reveal the structure and properties of asphaltenes through various characterization methods.
The characterization of asphaltenes is necessary for the formation of M/LTCT processing strategies. Recently, various analytical methods, such as nuclear magnetic resonance (NMR) [10], Fourier transform infrared (FTIR) spectroscopy [11,12], and X-ray photoelectron spectroscopy (XPS) [13], have been applied to successfully identify the heteroatom compounds from a macroscopic perspective. Compared with petroleum asphaltenes, M/LTCT asphaltenes were found to have the following characteristics: (1) A smaller relative molecular mass and higher aromaticity. (2) The chemical structure is mainly composed of fused ring aromatic hydrocarbons with a shorter alkyl side chain and rich in heteroatom functional groups. (3) The heteroatoms are mainly oxygen atoms, with fewer nitrogen and sulfur atoms. The oxygen-containing functional groups are mainly phenolic hydroxyl groups and ether oxygen bonds groups, the nitrogen-containing functional groups are mainly pyridine, pyrrole, and amines, and the sulfur-containing functional groups are mainly thiophene and sulfoxide. Recently, researchers also have been attempting to correlate chemical properties and behavior to the molecular composition of asphaltenes. Combined with various ionization techniques, such as electrospray ionization (ESI) [14], field desorption/ionization (FD/FI) [15,16], electron ionization (EI) [17], and atmospheric pressure photoionization (APPI) [18], Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been proven to efficiently reveal the composition of heteroatom-containing compounds in petroleum at the molecular level [19,20]. Among these ionization sources, electrospray ionization (ESI) is more convenient in use, and can highly selectively ionize N, S, and O heteroatom compounds with polarity [21]. Simultaneously, negative-iron ESI coupled with FT-ICR MS can also precisely determine the detailed molecular composition and distribution of acidic OCCs [22,23] and non-basic NCCs [24,25,26] in crude oils and sand oils asphaltenes.
The acidic OCCs in asphaltenes can not only cause corrosion [27] but also lead to fuel instability and reduction of the combustion performance [2]. Wang et al. [21] indicated that the acidic OCCs, such as O2, O2S, O2S2, O3, and O4, show high relative abundance (RA) in the asphaltenes of Venezuelan crude oil, of which the most abundant O2 class species are mainly naphthenic acid compounds by negative-iron FT-ICR MS analysis. Zhang et al. [10] characterized the C5 asphaltenes of LTCT in Yulin, Shaanxi, by negative-iron FT-ICR MS and revealed that the predominant species was the O2 class species, which mainly distributed the DBE (double bond equivalents) of 7–26 and carbon numbers of 14–36. The core structure of the O2 class species was mainly dihydric phenolic OCCs with 2–8 aromatic rings. It showed that, in negative ion mode, the type and distribution of acidic OCCs in different asphaltenes are various.
Non-basic NCCs are more difficult to remove than basic NCCs. In the process of hydrodenitrogenation, non-basic NCCs are first converted to basic NCCs and then can be removed [28]. In addition, NCCs can promote the formation of deposits in fuel oil and increase its instability during storage [29,30]. Xu et al. [31] found that the relative abundance of the non-basic N1 class species was relatively high, which may have been carbazole compounds, mainly distributed with the DBE of 14–24 and the carbon numbers of 21–49 in Sudan vacuum residue C7 asphaltenes by negative ESI FT-ICR MS mode. Shi et al. [32] showed that non-basic NCCs in asphaltenes of crude oil were mainly present as N1, N1O1, and N1O2 class species, and among them, the most abundant N1 class species were mainly distributed at the DBE values of 9–23 and carbon numbers of 15–50. The core structure was mainly pyrrole derivatives with 2–8 aromatic rings. Wang et al. [33] believed that the non-basic NCCs in Canadian oil sands asphaltenes mainly existed as N1 species, and these compounds were mainly distributed with DBE of 10–19 and carbon numbers of 16–50, and the core structure was mainly pyrrole compounds with low condensation. It shows that, in the negative ion mode, the non-basic nitrogen class species in petroleum and oil sand asphaltenes mainly existed as an N1 class species, and these compounds are considered to be pyrrole and carbazole species.
As mentioned above [28,29,30], the acidic OCCs and non-basic NCCs in M/LTCT asphaltenes will cause a large number of operation and transportation problems, which are also detrimental to the quality of the products. Recently, the characterization of petroleum asphaltenes from different regions shows that the OCCs, NCCs, and SCCs are also quite different. In addition, previous studies have shown that M/LTCT has relatively higher aromaticity and content of polar heteroatom compounds, which is conductive to condensation. Recently, the research on the molecular composition and distribution of OCCs, NCCs, and SCCs in M/LTCT asphaltenes has not been received as much attention, which will hinder the further understanding of the inherent characteristics for M/LTCT asphaltenes.
In this work, negative ion ESI FT-ICR MS was used to explore the occurrence and molecular composition of OCCs, NCCs, and SCCs in M/LTCT asphaltenes, including the RA distribution of heteroatom, DBE, and carbon number distributions. This work will be helpful and necessary for further research on the conversion mechanisms of S, N, and O heteroatom compounds in the process of M/LTCT hydrogenation.

2. Experimental Section

2.1. Precipitation of Asphaltenes

The raw material (whole fraction M/LTCT), provided by a coking enterprise in northern Shaanxi, was produced by pyrolysis of low-rank coal in an internal heating vertical furnace (the dry distillation temperature 400~600 °C). The specific steps for the precipitation of asphaltenes are as follows [8]: A certain amount of M/LTCT and n-heptane (50 mL per gram of coal tar sample) were mixed in a flat-bottomed flask, heated to reflux for 0.5 h, kept standing for 1 h in the dark, and then filtered with a filter paper to obtain the insoluble materials, which remained in the filtrate at the reflux condition for about 1 h, until the reflux turned colorless, and then dissolved with toluene in an Erlenmeyer flask. At last, the asphaltenes were obtained after the solvents were driven out with rotary evaporator and vacuum oven (temperature 110 °C, vacuum degree 93 KPa, 1 h) and cooled to ambient temperature. The toluene and n-heptane used were analytical reagent grade solvents. Table 1 lists the properties of M/LTCT and precipitated asphaltenes. The data shows that the contents of N, S, and O in M/LTCT asphaltenes was significantly higher than that of M/LTCT, illustrating that heteroatom-containing compounds tended to be enriched in precipitated asphaltenes.

2.2. Analytical Methods

2.2.1. Elemental Analysis

The contents of C, H, S, and N in asphaltene samples was analyzed by using a Vario MicroCube elemental analyzer from Elementar, Germany, and the content of O was calculated by differential subtraction. The combustion tube temperature was 1150 °C, and complete decomposition by high temperature combustion was used with a deviation of less than 0.1% and a repeat measurement error of less than 1%.

2.2.2. Relative Molecular Mass Analysis

The average relative molecular mass of the actual asphaltenes was determined using a high temperature gel permeation chromatograph (PL-GPC 220) from Agilent, Santa Clara, CA, USA. Tetrahydrofuran (THF) was used as the solvent, with a flow rate of 0.6 mL/min, an injection volume of 10 Iµ, and a column temperature of 40 °C.

2.2.3. ESI FT-ICR MS

The ESI source worked in negative ion mode. A certain amount of M/LTCT asphaltenes were dissolved with toluene into a 10 mg/mL solution, then diluted to 0.2 mg/mL with toluene/methanol (1:3) mixture, and 15 μL of NH4OH (20~30%) was added to promote the deprotonation of the compound. Finally, the prepared solution was injected into the ESI source at a rate of 150 μL/h by using a syringe pump. The asphaltenes were analyzed using Apex-ultra mass spectrometer FT-ICR MS (Bruker Company, Billerica, MA, USA), which was equipped with a 9.4T superconducting magnet (operating at 9.0 T) [32]. The operating procedure for negative-iron ESI FT-ICRMS has been described elsewhere [21,32,34]. The methods of FT-ICR MS quality calibration and data processing procedure have been described elsewhere [35,36].

2.3. Data Analysis Procedure

Since the information obtained by FT ICR MS was quite abundant, it was necessary to properly process and classify the data. In order to identify the homologous series (compounds with the same heteroatom composition and the same number of rings and double bonds, but a different number of CH2 groups) in the sample, the measured mass was converted from International Union of Pure and Applied Chemistry (IUPAC) mass to Kendrick mass (KM), thereby the Kendrick mass differences (KMDs) were derived from the following formulas [37,38].
KM = M × (14.0000/14.0156)
KMD = (INT(KM) − KM) × 1000
M represents IUPAC mass, INT(KM) is the integer closest to KM.
Homologous series have the same KMD and same core structural, but have different CH2 numbers, or rather to say, the same type of compounds with different substituents have the same KMD value. According to the value of KMD, the types of compounds can be quickly and accurately distinguished, which was conducive to programmatic processing of data [39].
DBE was the number of double bonds in each molecular structure plus the number of rings, indicating the degree of condensation of the molecule. The DBE value can be directly derived from the general formula CcHhNnOoSs, DBE = c-h/2 + n/2 + 1, where c, h, and n were the number of carbon, hydrogen, and nitrogen atoms [40,41,42]. Carbon number(c) played a role in the characterization of degree of alkyl substitution.

3. Results and Discussions

3.1. Negative Ion ESI FT-ICR Mass Spectra

The mass spectrum of FT-ICR MS contains abundant molecular composition information. In the negative-iron ESI mode, the broadband mass spectrum of M/LTCT asphaltenes is shown in Figure 1. The value of m/z on the abscissa can directly reflect the relative molecular mass of asphaltenes, and the intensity value on the ordinate represents the relative mass fraction. In Figure 1, it appears that the molecular weight distribution of M/LTCT asphaltenes was relatively wide, mainly concentrated between 200 and 750 Da. After the partial enlargement, it can be seen that there were multiple mass spectrum peaks at m/z = 332 Da. The identified compounds with relatively high mass spectrometry peaks of mass spectra at m/z = 332 are listed in Table 2.

3.2. Heteroatom-Containing Compounds Distribution

In order to describe the distinct in molecular composition, all identified mass spectrum peaks were classified according to the heteroatom type, and the relative abundance (RA) of each class was defined as the magnitude of each peak divided by the sum of the magnitudes of all identified peaks (excluding the isotopic peaks) in the mass spectrum. In the negative ESI mode, the RA map of various acidic and non-basic species in the M/LTCT asphaltenes identified were shown in Figure 2. It can be seen that the heteroatom compounds in asphaltenes mainly existed as OX, NX, N1OX, N2OX, N3OX, N4OX, N5OX, and N6OX class species (x = 1–6).
Figure 2 shows that OX class species were the predominant species, due to the high oxygen content of M/LTCT, which was consistent with the results found in other studies [8,38]. In addition, the NX class species have lower ionization efficiency, due to their weak acidity [43]. The OX compounds existed mainly in the O2, O3, and O4 class species. Of these, the O4 class species accounted for 11.19%, which seems much higher than that of any other acidic and non-basic class species, indicating that they have the strongest response to ion sources. NX compounds in asphaltenes mainly existed in the form of N1, N2, N3, and N4 class species. Relative to the O4 class species, the RA of N4 class species was much lower, but it was still the most abundant of all non-basic nitrogen species. Of these multi-heteroatom (N1OX, N2OX, N3OX, N4OX, N5OX, and N6OX), N1O1 was the predominant class species, followed by N2O1, N1O2, N5O2, and N6O3, which have an extremely lower RA, accounting for about 2.20%, 2.06%, 1.85%, and 1.49%, respectively. The RA of the N1O1 species was approximately equal to that of the N1 species.
Compared to the dominant O2 and N1 class species in oil sand asphaltenes [31], O4 and N4 species are the most abundant species in M/LTCT asphaltenes. No sulfur-containing heteroatom species, (such as the N1S1, N1O1S1, O2S1, and O2S2 class species) were identified in the M/LTCT asphaltenes. There are supposed to be two reasons. For one, the concentration of sulfur compounds in M/LTCT asphaltenes is much lower than that of oil sand. For another reason, the negative ion ESI cannot directly ionize the sulfur-containing functional groups, and the non-basic nitrogen or acidic oxygen-containing compounds that can be ionized in the asphaltenes do not contain sulfur heteroatom. This conclusion is similar to the results reported in the literature [31].

3.3. The Molecular Structure of Heteroato-Containing Compounds

Although the accurate molecular mass value allows for unique elemental formulas to be assigned to each peak in the mass spectrum, it does not provide the molecular structures of each species. Combined with the molecular element composition determined by FT ICR MS, the iso-abundance point coding map was constructed, and the DBE and carbon number distribution of various species in asphaltenes were investigated. Based on the DBE value and carbon number, the molecular structure of the compounds can be inferred.

3.3.1. OX(x = 2–4) Class Species in M/LTCT Asphaltenes

Previous studies [44,45,46] believed that the oxygen atoms in the M/LTCT asphaltenes mainly existed in the form of single-bonded oxygen (such as C-OH, Ar-OH, and C-O-C) and double-bonded (ketones and carboxylic acid) oxygen by XPS and FT-IR analysis. Of these, acidic OCCs are phenols and carboxylic compounds. Therefore, in the negative ion mode, the identified OCCs should be phenol hydroxyl and carboxylic acid compounds.

O2 Class Species

As described in Figure 3b, O2 species in asphaltenes have a relatively narrow distribution range centered at DBE values of 6–14 and carbon numbers of 17–25. The O2 species with a DBE value of 8 might be a naphthalene combined with a naphthenic ring, two phenolic hydroxyl groups or acenaphthene with two phenolic hydroxyl groups, or one naphthalene with a carboxyl group. The species with a DBE value of 12 were suspected of pyrene with two phenolic hydroxyl groups, fluoranthene with two phenolic hydroxyl groups, or benzofluorene with two phenolic hydroxyl groups were also suspected of phenanthrene plus a naphthenic ring with a carboxyl group or anthracene plus a naphthenic ring with a carboxyl group. Therefore, the O2 species might be highly condensed aromatic acids phenols. The possible core structure of the acidic O2 species are 0–6 aromatic rings with a carboxylic group or 0–6 aromatic rings with two phenolic hydroxyl groups.

O3 Class Species

According to Figure 3b, the O3 species were concentrated at DBE of 7–27 and carbon numbers of 14–27. Among them, species with DBE of 10 have the highest relative abundance (as shown in Figure 3a), which have a formula of C16H13O3, were suspected of a core structure of phenanthrene with three carboxyl groups or anthracene with three carboxyl groups. The O3 species with a DBE value of 13 were likely to be a pyrene plus a naphthenic ring with three phenolic hydroxyl groups or a fluoranthene plus a naphthenic ring with three phenolic hydroxyl groups or chrysene with three phenolic hydroxyl groups were also likely to be a pyrene with a phenolic hydroxyl group and a carboxyl group or a fluoranthene with a phenolic hydroxyl group and a carboxyl group. The O3 species with a DBE value of 14 were suspected to be a pyrene plus two naphthenic rings with three phenolic hydroxyl groups, a fluoranthene plus two naphthenic ring with three phenolic hydroxyl groups, or chrysene plus one naphthenic ring with three phenolic hydroxyl groups were also likely to be a pyrene plus a naphthenic ring with a phenolic hydroxyl group and a carboxylic acid group, a fluoranthene plus a naphthenic ring with a carboxylic acid group and a phenolic hydroxyl group, or chrysene with a carboxylic acid group and a phenolic hydroxyl group. The O3 species with a DBE value of 15 were suspected of a core structure of benzopyrene with three phenolic hydroxyl groups or triphenylene plus a naphthenic ring with a carboxyl group and a phenolic hydroxyl group. The DBE value of the O3 species was higher than the O2 species, thus most additional oxygen atoms should have an aromatic group. Therefore, the possible core structure of the acidic O3 species were mainly composed of 2–6 aromatic rings with three phenolic hydroxyl groups or 2–6 aromatic rings with a carboxylic group and a phenolic hydroxyl.

O4 Class Species

As described in Figure 3c, O4 species in asphaltenes have the highest RA, mainly centered at a DBE of 4–15 and carbon numbers of 12–27. Previous studies [22] have shown that O2 class monomers will form O4 molecular aggregates with double DBE values and carbon number when the oil sample concentration is too high. However, the carbon number distribution range shows that O4 species were monomers. There are few O4 species with a DBE value of 0, indicating that there were only a small amount of tetrahydric alcohol compounds. The O4 species with a DBE value of 8 were likely to be acenaphthylene tetraphenols compounds. The O4 species with a DBE value of 12 were likely to be pyrene tetraphenols or fluoranthene tetraphenols. The O4 species with a DBE value of 15 were likely to be benzopyrenete traphenols compounds. Therefore, the core structure of the acidic O4 species were suspected of 0–6 aromatic rings with four phenolic hydroxyl groups.
Different from that of the oxygen-containing species in other asphaltenes (as shown in Table 3), the O4 species dominated in the M/LTCT asphaltenes, and they were likely quaternary phenolic OCCs with a relatively higher DBE values and smaller carbon numbers, while the O2 naphthenic acids species dominated in sand oil asphaltenes with a lower DBE value. Obviously, the O4 phenolic OCCs have a high condensed structure with either additional short or multi-substituted alkyl side chains, which were difficult to remove by hydrogenation, in comparison to the O2 naphthenic acid compounds. In addition, the presence of a large amount of phenolic compounds adversely affected high-quality diesel products through M/LTCT hydrogenation [47].

3.3.2. Distribution of NX Class Species

N1 class species. Among all non-basic N1 species, pyrrole and its derivatives are typical NCCs that can be effectively ionized in negative ion mode, which has been confirmed in other reports [32,43,48]. In addition, the GC-MS results also showed that indole and carbazole are the main components of non-basic NCCs in coal tar [3,8]. As described in Figure 4b, the N1 species are concentrated at DBE of 15–21 and carbon numbers of 27–37. Among them, the compounds of C36H39N1 with a DBE value of 18 have the highest RA (as shown in Figure 4a). The compounds were suspected of a benzopyrene combined with two naphthenic rings and a pyrrole ring or benzonaphthocarbazoles. The N1 species with a DBE value of 19 were likely to be dibenzopyrene with a pyrrole ring. The N1 species with the DBE values of 20 might have been a dibenzopyrene plus a naphthenic ring and a pyrrole ring. Therefore, the core structure of the N1 species in M/LTCT asphaltenes was suspected of a pyrrole ring with higher degree of condensation. However, the N1 species were also the most abundant in crude oil asphaltenes, with the core structure of carbazoles with 2–7 aromatic rings [25,32,49].
N2 class species. The N2 species contains double nitrogen atoms. As shown in Figure 4c, the N2 species in the M/LTCT asphaltenes were concentrated at the DBE of 17–25 and the carbon numbers of 26–35. Compared with the N1 species, N2 species have higher DBE values and lower carbon numbers, indicating that molecules of N2 species have a high degree of condensation. Of them, the compounds of C34H30N2 with DBE of 21 have the highest RA (as shown in Figure 4a). The compounds were suspected of a benzoperylene plus two naphthenic rings and a pyrazole ring or a dibenzoperylene plus a naphthenic ring and an imidazole ring. The N2 species with DBE of 20 might be a benzoperylene plus a naphthenic ring and a pyrazole ring and were also likely a dibenzoperylene with an imidazole ring. The N2 species of 22 DBE were likely a benzoperylene combined with three naphthenic rings and a pyrazole ring and were also likely a dibenzoperylene combined with two naphthenic rings and an imidazole ring. Therefore, the core structure of N2 species were suspected of 4–7 aromatic rings with a pyrazole ring or 4–7 aromatic rings with an imidazole ring.
N4 class species. As described in Figure 4d, the N4 species in the asphaltenes have the highest RA, mainly centered at the DBE of 16–24 and carbon numbers of 12–23. Of these, the compounds of C26H16N4 with a DBE value of 22 have the highest RA (as shown in Figure 4a). The compounds were suspected to be a naphthopyrene plus a naphthenic ring with a piperazine ring and a pyrazole ring. The N4 species with DBE of 22 might be a naphthopyrene plus two naphthenic rings with a piperazine ring and pyrazole ring and also might be a naphthopyrene plus three naphthenic rings with a piperazine ring and an imidazole ring. The N4 species with a DBE value of 23 might be a naphthopyrene plus three naphthenic rings with a piperazine ring and a pyrazole ring and might be a benzonaphthopyrene plus a naphthenic ring with a piperazine ring and an imidazole ring. Therefore, the core structure of the N4 species were suspected of 4–7 aromatic rings with a pyrazole ring.
In short, the N4 species dominated in the NX species, which have a higher degree of condensation with short and multi-substituted alkyl side chains, and the core structure was likely a pyrazole ring or piperazine ring with an imidazole ring. Compared with the research results of other asphaltenes (as shown in Table 4), the RA of N1 species were most abundant in crude asphaltenes, which have the core structure of pyrrole rings or carbazole rings. The DBE values for N1 species in M/LTCT asphaltenes were distributed in a wide range, indicating that the species have higher aromaticity than that of the crude oil asphaltenes. Thus, not only the N1 species but also the N4 species in M/LTCT asphaltenes are more difficult to remove by hydrogenation.

3.3.3. Distribution of Muti-Heteroatom Class Species

N1O1 class species. The N1O1 species contains two heteroatoms of nitrogen and oxygen and have the most abundant contents in multi-heteroatom containing species. As shown in Figure 5b, the N1O1 species were concentrated at DBE of 12–18 and the carbon numbers of 19–28. Of these, the compounds of C21H19N1O1 with 13 DBE have the highest RA. The compounds might be naphthopyrene plus two naphthenic rings with a phenolic hydroxyl and a pyrazole ring. The N1O1 species of 15 DBE might be a pyrene plus a naphthenic ring with a phenolic hydroxyl and a pyrazole ring. The N1O1 species of 16 DBE might be a pyrene plus two naphthenic rings with a phenolic hydroxyl and a pyrazole ring. The N1O1 species of 17 DBE might be benzopyrene plus naphthenic rings with a phenolic hydroxyl and a pyrazole ring.
N1O2 class species. As described in Figure 5c, the N1O2 species in the asphaltenes were concentrated at DBE of 17–22 and carbon numbers of 29–36. Of these, the compounds of C30H27N1O2 with a DBE value of 18 have the highest RA (as shown in Figure 5a). The compounds were suspected of a benzopyrene with two naphthenic rings, two phenolic hydroxyl, and a pyrazole ring were also likely to be the combination of a pyrene with a naphthenic ring, a carboxyl, and a pyrazole ring. The N1O2 species with a DBE value of 19 were likely to be the combination of a dibenzopyrene with two phenolic hydroxyl and a pyrazole ring and were also likely to be the combination a pyrene with two naphthenic rings, a carboxyl, and a pyrazole ring. The N1O2 species of 20 DBE might be dibenzopyrene with naphthenic ring, two phenolic hydroxyl, and a pyrazole ring and were also likely to be dibenzopyrene with a carboxyl and a pyrazole ring. The N1O2 species with a DBE value of 21 were likely to be the combination of benzopyrene with two naphthenic rings, two phenolic hydroxyl, and a pyrazole ring and were also likely to be the combination of a benzopyrene with a naphthenic ring, a pyrazole ring, and a carboxyl.
N2O1 class species. As presented in Figure 5d, the N2O1 species in the asphaltenes were concentrated at the DBE values of 15–20 and carbon numbers of 30–40. Of these, the compound of C36H40N2O1 with a DBE value of 18 had the highest RA (as shown in Figure 5a). The compounds were most likely to be the combination of benzopyrene with two naphthenic rings, a phenolic hydroxyl, and a pyrazole ring and were also likely to be the combination of benzopyrene with two naphthenic rings, a phenolic hydroxyl, and an imidazole ring. N2O1 species with a DBE value of 17 were likely to be a benzopyrene with a naphthenic ring, a phenolic hydroxyl, and a pyrazole ring. The N2O1 species of 19DBE were likely to be the combination of benzopyrene with three naphthenic rings, a pyrazole ring, and a phenolic hydroxyl.
Compared with the N1 species, the more abundant N1O1 species have higher DBE values and lower carbon numbers, suggesting that they contained more condensed molecular structures. The nitrogen and oxygen atoms existed in the form of pyrrole rings and phenolic hydroxyl, respectively. The oxygen atoms in N1O1 species were considered to be phenolic hydroxyls, rather than furans. There are two reasons for it. Hydroxy compounds are more polar than furan compounds. Highly polar hydroxyl compounds were more likely to exist in the asphaltenes during the separation process. In addition, based on the knowledge of the phenolic compounds that is abundant in coal tar [50], there were a large number of pyrrole compounds containing phenolic hydroxyl groups in M/LTCT. Thus, unlike N1O1 species of hydroxyl-containing carbazole compounds in petroleum asphaltenes, the core structure of N1O1 compounds in M/LTCT asphaltenes may be hydroxyl-containing pyrrole compounds. In addition, the distributions of N1O1 compounds were dispersed in a relatively wide range. Moreover, the nitrogen-containing heteroatom compounds with additional oxygen are easier to remove than pure nitrogen-containing heteroatom compounds through hydrogenation [51].

4. Conclusions

A variety of acidic OCCs and non-basic NCCs, except for SCCs, were identified in M/LTCT asphaltenes by negative ESI FT-ICR MS. The distributions of relative molecular weight were mainly concentrated between 200 Da and 750 Da. The acid OCCs identified in M/LTCT asphaltenes include the Ox(x=1–6) class species, which were comprised of a significant proportion of all species. Different from petroleum asphaltenes, the O4 species were predominant in the M/LTCT asphaltenes. The non-basic NCCs were identified, including NX, N1OX, N2OX, N3OX, N4OX, N5OX, and N6OX(x = 1–6), in which the N4 species were the most abundant species.
The distributions of DBE values and carbon numbers indicated that the core structure of the O4 species was suspected to be 0–6 aromatic rings with four phenolic hydroxyl groups and N4 species, probably consisting of a core structure of 4–7 aromatic rings with a piperazine ring and a pyrazole ring. The muti-heteroatom compounds mainly existed as the N1O1 species, which may have a core structure of 3–6 aromatic rings with a pyrrole ring and phenolic hydroxyl group.
Furthermore, compared to the molecular composition of petroleum asphaltenes, the most abundant O4 and N4 species contained in M/LTCT asphaltenes have higher DBE values centered at 4–15 and 16–24 and lower carbon numbers centered at 12–27 and 12–23, indicating that more condensed nitrogen-containing and oxygen-containing heteroatom compounds with short substituted alky side chains existed, which more easily undergo condensation to generate more refractory molecules, making it too difficult to remove by hydrogenation. The detailed description of these compounds can provide guidance for revealing the properties of asphaltenes and the basic data for the hydrogenation of deasphaltene.

Author Contributions

All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by X.F., L.C. and D.L. The first draft of the manuscript was written by X.F. and all authors commented on previous versions of the manuscript. Conceptualization: X.F. and D.L.; methodology: R.S.; formal analysis and investigation: L.C. and B.Y.; writing—original draft preparation: X.F. and D.L.; writing—review and editing: L.Y.; supervision: C.C. All authors have read and agreed to the published version of the manuscript.

Funding

The financial supports of this work were provided by the Doctor Scientific Research Fund of Yulin university (22GK05), Youth Innovation Team Research Program Project of Shaanxi Provincial Department of Education (22JP104), the Innovation Capability Support Program of Shaanxi (2020TD-028), the Technology Innovation Leading Program of Shaanxi (2019CGHJ-11), and the Science and Technology Plan of Yulin Government (CXY-2020-002-07).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

No conflict of interest was declared by the authors.

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Figure 1. Mass spectra of the M/LTCT asphaltenes. The narrow segment of mass spectra at 332 Da.
Figure 1. Mass spectra of the M/LTCT asphaltenes. The narrow segment of mass spectra at 332 Da.
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Figure 2. The RA of heteroatom-containing class species in M/LTCT asphaltenes.
Figure 2. The RA of heteroatom-containing class species in M/LTCT asphaltenes.
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Figure 3. (a) The RA of O2, O3, O4 class species as a function of DBE. (b–d) The plots of DBE versus carbon number for O2, O3, and O4class species. (e) The suggested core structure of compounds with higher RA in O2, O3, and O4 class species.
Figure 3. (a) The RA of O2, O3, O4 class species as a function of DBE. (b–d) The plots of DBE versus carbon number for O2, O3, and O4class species. (e) The suggested core structure of compounds with higher RA in O2, O3, and O4 class species.
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Figure 4. (a) The RA of N1, N2, and N4class species as a function of DBE. (b–d) The plots of DBE versus carbon number for N1, N2, and N4 class species. (e) The suggested core structures of N1, N2, and N4 class species.
Figure 4. (a) The RA of N1, N2, and N4class species as a function of DBE. (b–d) The plots of DBE versus carbon number for N1, N2, and N4 class species. (e) The suggested core structures of N1, N2, and N4 class species.
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Figure 5. (a) The RA of N1O1, N1O2, and N2O1 class species as a function of DBE. (a–d) show the plots of DBE versus carbon number of N1O1, N1O2, and N2O1 class species. (e) The suggested core structures of N1O1, N1O2, and N2O1 class species.
Figure 5. (a) The RA of N1O1, N1O2, and N2O1 class species as a function of DBE. (a–d) show the plots of DBE versus carbon number of N1O1, N1O2, and N2O1 class species. (e) The suggested core structures of N1O1, N1O2, and N2O1 class species.
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Table
Table 1. The properties of M/LTCT and precipitated asphaltenes.
Table 1. The properties of M/LTCT and precipitated asphaltenes.
PropertiesM/LTCTM/LTCT Asphaltenes
Elemental analysis/%C81.4681.72
H8.615.68
N1.262.33
S0.341.86
O8.038.92
H/C atomic ratio1.2680.818
Density (20 °C)/g·cm−31.041-
Mw420663
Table
Table 2. Identified compounds with high mass spectrometry peaks at m/z 332.
Table 2. Identified compounds with high mass spectrometry peaks at m/z 332.
NOFormula [M + H]−Theoretical
Mass (Da)		Measured
Mass (Da)		Error (m/z)Resolving Power
N 1 O 1 C 22 H 36 N 1 O 1 332.1465332.13630.0102132,193
O 4 C 21 H 15 O 4 332.3495332.22290.1266126,027
N 2 O 2 C 20 H 32 N 2 O 2 332.4932332.48410.0091121,912
N 1 O 2 C 21 H 32 N 1 O 2 332.6737332.67150.0022134,457
Table
Table 3. Main oxygen-containing heteroatom species in asphaltenes of different raw material.
Table 3. Main oxygen-containing heteroatom species in asphaltenes of different raw material.
FeedMost Abundant Oxygen SpeciesDBE CenterCarbon Numbers
Center		Possible Core Structure
The present workM/LTCT asphaltenesO44–1512–27Quaternary phenolic hydroxyl
Wang et al. [33] Oil sands bitumen asphaltenes O23–528–35Naphthenic acid
Xu et al. [31]Sudan sand oil asphaltenesO20–825–35Naphthenic acid
Shi et al. [32]Crude Oil asphaltenesO14–627–40Phenolic hydroxyl
Table
Table 4. Main nitrogen-containing heteroatom species in asphaltenes of different raw material.
Table 4. Main nitrogen-containing heteroatom species in asphaltenes of different raw material.
FeedMost Abundant Nitrogen SpeciesDBE CenterC
Center		Possible Core Structure
The present workM/LTCT asphaltenesN416–2512–23A piperazine ring with a pyrazole ring
Wang et al. [33]Oil sands bitumen asphaltenes N114–2422–37Pyrrole ring
Xu et al. [31] Sudan asphaltenesN110–1622–35Carbazole ring
Shi et al. [32]Crude Oil asphaltenesN19–1921–37Carbazole ring
Wang et al. [21]Petroleum asphaltenesN19–2020–46Carbazole ring
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Fan, X.; Li, D.; Cui, L.; Shao, R.; Chang, C.; Yan, L.; Yang, B. Analysis of Distribution and Structures of Heteroatom Compounds in Asphaltene of Medium/Low Temperature Coal Tar by Negative Anion Mode ESI FT-ICR MS. Sustainability 2022, 14, 15497. https://doi.org/10.3390/su142315497

AMA Style

Fan X, Li D, Cui L, Shao R, Chang C, Yan L, Yang B. Analysis of Distribution and Structures of Heteroatom Compounds in Asphaltene of Medium/Low Temperature Coal Tar by Negative Anion Mode ESI FT-ICR MS. Sustainability. 2022; 14(23):15497. https://doi.org/10.3390/su142315497

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Fan, Xiaoyong, Dong Li, Louwei Cui, Ruitian Shao, Chunran Chang, Long Yan, and Bo Yang. 2022. "Analysis of Distribution and Structures of Heteroatom Compounds in Asphaltene of Medium/Low Temperature Coal Tar by Negative Anion Mode ESI FT-ICR MS" Sustainability 14, no. 23: 15497. https://doi.org/10.3390/su142315497

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