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

: 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 iden-tiﬁed, except for sulfur-containing compounds (SCCs). The mass spectra showed that the heteroatom compounds in asphaltene mainly existed as N X , N 1 O X , N 2 O X , N 3 O X , N 4 O X , N 5 O X , N 6 O X , and O X class species (where x = 1–6). The M/LTCT asphaltenes were enriched with O 4 , N 4 , and N 1 O 1 class species. The core structure of O 4 class species were likely to be composed of 1–7 aromatic rings with 4 phenolic hydroxyl groups, the core structure of N 4 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 N 1 O 1 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.


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 of 21-49 in Sudan vacuum residue C 7 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 N 1 , N 1 O 1 , and N 1 O 2 class species, and among them, the most abundant N 1 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 N 1 species, and these compounds were mainly distributed with DBE of 10-19 and carbon numbers of , 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 N 1 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.

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. 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%.

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.

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 NH 4 OH (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].

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 CH 2 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]. Homologous series have the same KMD and same core structural, but have different CH 2 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 C c H h N n O o S s , 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.

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. 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.

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.

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

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 O X , N X , N 1 O X , N 2 O X , N 3 O X , N 4 O X , N 5 O X , and N 6 O X class species (x = 1-6). Figure 2 shows that O X 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 N X class species have lower ionization efficiency, due to their weak acidity [43]. The O X compounds existed mainly in the O 2 , O 3 , and O 4 class species. Of these, the O 4 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. N X compounds in asphaltenes mainly existed in the form of N 1 , N 2 , N 3 , and N 4 class species. Relative to the O 4 class species, the RA of N 4 class species was much lower, but it was still the most abundant of all non-basic nitrogen species. Of these multiheteroatom (N 1 O X , N 2 O X , N 3 O X , N 4 O X , N 5 O X , and N 6 O X ), N 1 O 1 was the predominant class species, followed by N 2 O 1 , N 1 O 2 , N 5 O 2 , and N 6 O 3 , which have an extremely lower RA, accounting for about 2.20%, 2.06%, 1.85%, and 1.49%, respectively. The RA of the N 1 O 1 species was approximately equal to that of the N 1 species. 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].

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.

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) 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 sulfurcontaining 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].

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.

O X (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.

O 2 Class Species
As described in Figure 3b, O 2 species in asphaltenes have a relatively narrow distribution range centered at DBE values of 6-14 and carbon numbers of 17-25. The O 2 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 O 2 species might be highly condensed aromatic acids phenols. The possible core structure of the acidic O 2 species are 0-6 aromatic rings with a carboxylic group or 0-6 aromatic rings with two phenolic hydroxyl groups. thalene 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  Figure 3a), which have a formula of C 16 H 13 O 3 , were suspected of a core structure of phenanthrene with three carboxyl groups or anthracene with three carboxyl groups. The O 3 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 O 3 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 O 3 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 O 3 species was higher than the O 2 species, thus most additional oxygen atoms should have an aromatic group. Therefore, the possible core structure of the acidic O 3 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.

O 4 Class Species
As described in Figure 3c, O 4 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 O 2 class monomers will form O 4 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 O 4 species were monomers. There are few O 4 species with a DBE value of 0, indicating that there were only a small amount of tetrahydric alcohol compounds. The O 4 species with a DBE value of 8 were likely to be acenaphthylene tetraphenols compounds. The O 4 species with a DBE value of 12 were likely to be pyrene tetraphenols or fluoranthene tetraphenols. The O 4 species with a DBE value of 15 were likely to be benzopyrenete traphenols compounds. Therefore, the core structure of the acidic O 4 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 O 4 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 O 2 naphthenic acids species dominated in sand oil asphaltenes with a lower DBE value. Obviously, the O 4 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 O 2 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]. Among all non-basic N 1 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 N 1 species are concentrated at DBE of 15-21 and carbon numbers of 27-37. Among them, the compounds of C 36 H 39 N 1 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 N 1 species with a DBE value of 19 were likely to be dibenzopyrene with a pyrrole ring. The N 1 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 N 1 species in M/LTCT asphaltenes was suspected of a pyrrole ring with higher degree of condensation. However, the N 1 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 N 2 class species. The N 2 species contains double nitrogen atoms. As shown in Figure 4c, the N 2 species in the M/LTCT asphaltenes were concentrated at the DBE of 17-25 and the carbon numbers of 26-35. Compared with the N 1 species, N 2 species have higher DBE values and lower carbon numbers, indicating that molecules of N 2 species have a high degree of condensation. Of them, the compounds of C 34 H 30 N 2 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 N 2 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 N 2 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 N 2 species were suspected of 4-7 aromatic rings with a pyrazole ring or 4-7 aromatic rings with an imidazole ring. N 4 class species. As described in Figure 4d, the N 4 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 C 26 H 16 N 4 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 N 4 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 N 4 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 N 4 species were suspected of 4-7 aromatic rings with a pyrazole ring.
In short, the N 4 species dominated in the N X 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 N 1 species were most abundant in crude asphaltenes, which have the core structure of pyrrole rings or carbazole rings. The DBE values for N 1 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 N 1 species but also the N 4 species in M/LTCT asphaltenes are more difficult to remove by hydrogenation.    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 N 1 O 2 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 N 1 O 2 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 N 1 O 2 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. N 2 O 1 class species. As presented in Figure 5d, the N 2 O 1 species in the asphaltenes were concentrated at the DBE values of 15-20 and carbon numbers of 30-40. Of these, the compound of C 36 H 40 N 2 O 1 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. N 2 O 1 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 N 2 O 1 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 N 1 species, the more abundant N 1 O 1 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 N 1 O 1 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 N 1 O 1 species of hydroxyl-containing carbazole compounds in petroleum asphaltenes, the core structure of N 1 O 1 compounds in M/LTCT asphaltenes may be hydroxyl-containing pyrrole compounds. In addition, the distributions of N 1 O 1 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].

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 O x(x=1-6) class species, which were comprised of a significant proportion of all species. Different from petroleum asphaltenes, the O 4 species were predominant in the M/LTCT asphaltenes. The non-basic NCCs were identified, including N X , N 1 O X , N 2 O X , N 3 O X , N 4 O X , N 5 O X , and N 6 O X (x = 1-6), in which the N 4 species were the most abundant species.
The distributions of DBE values and carbon numbers indicated that the core structure of the O 4 species was suspected to be 0-6 aromatic rings with four phenolic hydroxyl groups and N 4 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 N 1 O 1 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 O 4 and N 4 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,