Molecular Characteristics of Jimusaer Shale Oil from Xinjiang, China

Increasing attention is currently obtained by the exploitation and utilization of unconventional energy sources globally. Jimusaer shale oil (JSO) was prepared by dry distillation from oil shale in Jimusaer, Xinjiang, China. Using n-heptane and toluene as solvents, saturate (SA), aromatic (AR), resin (RE), and asphaltene (AS) samples were produced from JSO. Samples were subsequently analyzed by elemental analysis (EA), thermogravimetric analysis (TG-DTG), infrared analysis (FT-IR), high-performance gel chromatography (GPC), and nuclear magnetic resonance (1H-NMR and 13C-NMR). In terms of basic properties, element content, classification of combustible minerals, and refining performance, JSO, which has a high H/C value, low carbon residue yield, low metal content, and excellent refining-processing performance, is considered a high-quality shale oil compared with the shale oil produced in other areas. The refining performance of JSO is even comparable with petroleum. According to column chromatography, the contents of SA, AR, RE, and AS in JSO are 54.32, 18.86, 25.81, and 1.01%, respectively. The results of FT-IR and NMR (1H-NMR and 13C-NMR) demonstrated that the chain alkane or aromatic cycloalkyl substituents of SA, AR, and RE decrease sequentially, while the number of aromatic rings and cycloalkane rings and the degree of condensation increase sequentially. These results indicate that the chain alkanes with a small number of cycloalkanes are the main component of SA. The AR and RE contain more thick-ring aromatic hydrocarbons. According to GPC, the molecular weight (Mn) of JSO is 845 g·mol–1, and those of SA, AR, and RE are 702, 1107, and 2218 g·mol–1, respectively. The estimated molecular formulas (Maf) of JSO, SA, AR, and RE, which were calculated based on the combined results of GPC and EA, are C57.91H115.60O1.38N0.79S0.04, C48.02H101.79O0.69N0.85S0.03, C76.96H137.16O1.08N1.87S0.09, and C156.24H247.75O1.46N4.42S0.32.


INTRODUCTION
Shale oil is a liquid fuel coming from the thermal decomposition of organic matter of oil shale. Although the hydrocarbon ratio of shale oil is similar to crude oil, it contains more unsaturated hydrocarbons and organic compounds including nitrogen, sulfur, and oxygen. 1,2 Shale oil in China is mainly distributed in the Ordos Basin, Songliao Basin, Bohai Bay Basin, and Junggar Basin. 3 The recoverable reserves of shale oil in Jimusaer of the Juggar Basin are estimated as 4.4 × 10 9 tons. 4 Given the global oil shortage and the exploitation, research, and utilization of shale oil, an unconventional energy substitution of oil is urgently needed.
Unconventional energy sources are attracting extensive attention in the past two decades. Therefore, research on the chemical composition and molecular structure of shale oil has been widely reported. Jarvis et al. 5 compared the compositional characteristics of shale oil, petroleum, and biomass oil by FT-IR-MS and found that the total hydrocarbon content of shale oil was close to that of petroleum but much higher than that of biomass oil. According to the FT-IR and NMR ( 1 H-NMR and 13 C-NMR) analysis, the saturated hydrocarbon of soluble matter of Jordanian shale oil dissolved by tetrahydrofuran and methylene chloride was about 85 and 11%. 6 Li 7 analyzed shale oil samples from Fushun and Nongan by GC−MS and found that the chemical compositions of both samples, which contained about 70% of linear and branched saturated alkane and 30% of arenes and oxygenate, were similar to each other. Cui et al. 8 analyzed shale oil samples collected from Huadian by GC−MS as well. According to their report, hydrocarbons accounted for 93% of the total composition, 60% of which was paraffin and the rest were olefins and monoaromatic hydrocarbons. Additionally, the sample also contained less than 5% of heteroatomic compounds such as sulfur, nitrogen, and oxygen. Mukhamatdinov et al. 9 studied the impact of aromatic solvents for enhancing heavy oil recovery from the Ashalcha field and found that when toluene:benzene = 1:1, the macromolecular structure inside the resin could be effectively destroyed, resulting in a significant reduction in the gum content. Furthermore, Sitnov et al. 10 found that iron oxide nanoparticles intensify the cracking and hydrogenolysis reactions through studied effects of iron oxide nanoparticles on the aquathermolysis of oil-saturated sandstones by GC−MS and TG-DSC because the content of saturates increased and the content of resins and asphaltenes decreased significantly. Khelkhal et al. 11 studied the influence of Mn@Cu tallates on the oxidation process of heavy oil by GC−MS and DSC, and the results showed that Mn@Cu tallates could promote the cracking reaction of asphaltenes, resins, and other highmolecular-weight compounds to form lighter hydrocarbons. Therefore, the contents of metal ions not only affect the refining performance of oil but also have a greater impact on its utilization as chemical raw materials.
The mineral deposit of Jimusaer oil shale has been discovered and developed in recent years. Several reports on the geological exploration of the reservoir structure, formation, and sedimentation of Jimusaer oil shale have been published. However, the physical and chemical properties, chemical composition, molecular structure characteristics, and refiningprocessing properties of JSO are poorly understood. Therefore, this work aimed to reveal the chemical composition and molecular structure characteristics of both JSO and its group components at the microscopic level using elemental analysis (EA), thermogravimetric analysis (TG-DTG), gel chromatography (GPC), and nuclear magnetic resonance (NMR). Based on the above analysis, the semi-theoretical and semi-empirical model named structure (η)−chemical index (δ) was used to evaluate the classification of combustible minerals and processing performance in shale oil. As shown in Table 1, for shale oil samples, the density of JSO is similar to those of HD and FS, about 900 kg/m 3 . The ash content of JSO is 0.32%, which is in the middle, but still belongs to low ash oil. The moisture content of JSO is only 0.15%, which is the lowest among them, and the salt content is as high as 2.74 mg/L. Those are obviously closely related to the arid and semi-arid inland region of Xinjiang, China. The residual carbon yield of JSO is 3.27%, which is about twice higher than HD and FS, but about twice lower than LK, indicating that coking is not easy during processing JSO. The freezing point of JSO is 15°C, which is between LK and FS, indicating that the content of paraffin in JSO is also between the two. In JSO, the content of Ni and V is much lower than those in LK, HD, and FS, and the content of Fe is also very low. The content of Ca is as high as 15.54 μg/g, which exceeds those of TuHaC and TaHeR, consistent with the previous result of salt content. Obviously, the main salt in JSO is calcium salt, and desalination pretreatment may be required before refining and processing JSO. Even so, JSO is a superior quality shale oil compared to HD, FS, and LK.

RESULTS AND
Moreover, compared with petroleum samples such as TuHaC, KVR, and TaHeR, the density of JSO is between those of TuHaC and KVR, the carbon residue rate is lower than that of TuHaC, and the total content of Ni, V, Fe, and Ca is equivalent to that of TuHaC. Table 2 shows the results of elemental analysis of JSO and its comparison with other samples. For shale oil samples, the H/C value of JSO is 1.99, which is the highest among them.
The O/C value of JSO is 0.024, which is slightly higher than those of HD and FS, but much lower than that of LK. Further, JSO has the highest N content of 1.31%. This is unfavorable since N compounds affect the oxidative stability of the oil. In contrast, the S content is only 0.16%, the lowest, which is a positive factor. Therefore, the results of elemental analysis also show that the quality of JSO is superior.
In addition, compared with the petroleum samples, the H/C value of JSO is higher than those of TuHaC, KVR, and TaHeR, the O/C value and N content of JSO are similar to those of TuHaC and much higher than those of KVR and TaHeR, and  Figure 1 shows the TG-DTG curves of JSO (a) and TuHaC (b). As shown in Figure 1a, the mass loss process of JSO may be divided into four stages approximately, namely, temperature change ranges below 135°C , 135−427°C, 427−530°C, and above 530°C. In the first stage, the loss time is about 12.5 min, the maximum loss mass peak's temperature is about 110°C, the loss mass rate is 4.15%/min, and the loss mass is 10.75%, which is mainly the slow escape of water and small molecules in SA. In the second stage, the loss time is about 28.4 min, the maximum loss-mass peak's temperature is about 316°C, the loss mass rate is 14.44%/min, and the loss mass is 65.80%. Obviously, at this moment, the rest of SA and a part of AR pyrolize rapidly. In the third stage, the loss time is about 10.2 min, the maximum loss mass peak's temperature is about 508°C, the loss mass rate is 22.54%/min, and the loss mass is 18.10%, in which the rest of AR and a small part of RE pyrolyze rapidly and the other part of RE is polycondensed. 17 In the fourth stage, the loss mass is only 2.08%, in which the rest of RE and all of AS undergo coking. The final residual carbon yield of JSO is 3.27%, which indicates that JSO has characteristics of a low condensation degree at high temperatures. 18 As shown in Figure 1b, the mass loss process of TuHaC may be divided into three stages approximately, that is, temperature change ranges below 397°C, 397−500°C, and above 500°C. In the first stage, the mass loss is as high as 78.56%, which is equivalent to the sum of the mass loss in the first and second stages of JSO. In the second stage, the mass loss is 17.99%, which is equivalent to that in the third stage of JSO. In the third stage, the mass loss is almost zero, which is equivalent to that in fourth stage of JSO. The final carbon residue yield is 3.45%, almost the same as that of JSO.
In summary, JSO and TuHaC have similar compositions and qualities because of having similar mass loss processes. Figure 2 shows the FT-IR curve of JSO. As shown in Figure 2, its absorption peaks mainly appear in three regions, that is, 3100−2700, 1800−1200, and 900− 600 cm −1 . Absorption peaks in the first region, which has the strongest absorption intensity among the three, belong to the characteristic absorption peak of aliphatic functional groups. 19 So, the composition of JSO is dominated by aliphatic compounds and compounds with aliphatic side chains. Absorption peaks in the second region belongs to the characteristic absorption peak of aromatic ring -C�C-. The middle-strong strong absorption intensity in this region indicates that JSO also contains a certain amount of aromatic compounds. Among them, the intensity of the characteristic peak at 1600 cm −1 indicates the degree of condensation of aromatic compounds, 20 and its middle-strong strong absorption intensity indicates that the content of aromatic compounds in JSO is not high and the molecular condensation degree is low. Absorption peaks in the third region are the characteristic absorption peaks of aromatic substitution. Among them, a weak absorption peak appeared at 723 cm −1 , indicating the presence of a small number of aromatic substituents in JSO. 21 2.1.4. GPC Analysis. Figure 3 shows the GPC curve of JSO with its number average molecular weight (M n ) and mass average molecular weight (M w ). According to Figure 3, the M n and M w of JSO are 845 and 2735 g·mol −1 , respectively, that is,     Figure 4 shows the contents of SA, AR, RE, and AS in JSO and its comparison with other samples. As shown in Figure 4, for shale oil samples, contents of SA, AR, RE, and AS in JSO are 54.32, 18.86, 25.81, and 1.01%, respectively, which are similar to those in FS. Obviously, SA is dominant, that is, there are more saturated hydrocarbons, which is consistent with the characteristics of a high H/C value. Also, the content of AS in JSO is only 1%, indicating that the content of thick-ring macromolecules and the association degree are both low and it is easy to process. AS will not be discussed in the follow-up study because of the very low content of AS.

FT-IR Analysis.
Further, compared with petroleum samples, for JSO, the content of SA is lower than that for TuHaC, but higher than those for KVR and TaHeR, the content of AR is comparable, and the content of RE is between those for petroleum samples, whereas the content of AS is close to those for TuHaC and KVR, which are much lower than TaHeR. This shows that the contents of group components in JSO are similar to that in TuHaC. Table 3 shows the results of elemental analysis of SA, AR, and RE. As shown in Table 3, the H/C values of SA, AR, and RE are 2.12, 1.78, and 1.57, respectively, decreasing sequentially, indicating that the degree of molecular association increases sequentially. Also, their O/C value decreases in turn, while the N and S contents increase successively, indicating that oxygenated compounds are enriched in SA, while nitrogenous and sulfurous compounds are enriched in AR and RE. In addition, as for the distribution of metal elements, Ni, V, and Fe are mainly enriched in AR and RE, which is similar to the distribution of N and S elements. This also indicates that from SA to RE, its composition complexity increases in turn. Figure 5 shows the TG (a) and DTG (b) curves of SA, AR, and RE, and Table 4 gives their pyrolysis properties. As shown in Figure 5 and Table 4, for SA, the mass loss process may be divided into three stages approximately, that is, temperature change ranges below 110°C, 110−397°C, and 397−500°C. In the first stage, the loss time is about 8.02 min and the loss mass is 6.52%, which is mainly the slow escape of small molecules of alkanes from SA. In the second stage, the loss time is about 28.65 min, the maximum loss mass peak's temperature is about 303°C, the loss mass rate is 4.43%/min, and the loss mass is 86.65%, which is mainly the pyrolysis of long-chain alkanes 25 obviously. In the third stage, the loss time is 10.30 min, the maximum loss mass peak's temperature is about 436°C, the loss mass rate is 1.88%/min, and the loss mass is 6.59%, in which a small amount of thick-ring compounds is pyrolyzed. For AR and RE, both pyrolysis properties are similar. Their mass loss process of may be divided into three stages approximately, namely, temperature change ranges below 221°C

TG-DTG Analysis of Group Components.
, 211−633°C, and above 633°C. In the first stage, the loss time is 19.10 min, and the loss mass values are 1.86 and 6.46%, in which alkanes and some ring aromatic compounds are pyrolyzed. In the second stage, the loss time is 41.30 min, the maximum loss mass peak's temperatures are 464 and 461°C, the loss mass rates are 8.91 and 12.51%/min, and the loss mass values are 88.41 and 77.10%, in which intermediate-low ring aromatic compounds and thick-ring compounds are pyrolyzed. In the last stage, the loss time is 16.80 min, and the loss mass values are only 1.06 and 0.50%, in which the polycondensation of some thick-ring compounds occur. The final residual carbon yield of SA is only 0.24%, whereas those of AR and RE are 8.67 and 15.94%, respectively, indicating that during pyrolysis, SA does not condensate easily and changes into volatile products almost completely while RE condensates easily to undergo coking at high temperatures. 26 2.2.4. FT-IR Analysis of Group Components. Figure 6 shows the FT-IR curves of SA, AR, and RE from JSO. As shown in Figure 6, the absorption peaks of SA, AR, and RE all appeared in three regions, that is, 3100−2700, 1800−1200, and 900−600 cm −1 , with roughly the same location but different intensities, indicating that the functional groups contained in the three are similar in structure but different in content. In the first region, the absorption peaks of SA, AR, and RE are all the strongest, indicating that there are more long-chain alkane compounds in the three. In the second region, the characteristic peaks at 1460 and 1600 cm −1 belong to the vibration of -C�C-in aromatic compounds, and their intensity indicates the degree of molecular condensation. 20,27 For AR and RE, the intensity of the two peaks is obviously higher than that of SA, indicating that the content of aromatic compounds and molecular condensation degree of both are higher than those of SA. In the third region, for RE, the  intensity of the peak at 723 cm −1 is obviously higher than those for SA and AR, indicating that the content of aromatic compounds and molecular condensation degree of both are higher than those for SA. The peak intensity of RE is significantly higher than those of SA and AR, indicating that there are more aromatic substituents in RE. Figure 7 and Table 5 give the GPC curves and M n and M w of SA, AR, and RE, respectively. As shown in Table 5, the M n values of SA, AR, and RE increase successively, which are 702, 1107, and 2218 g/mol, respectively, and compared with SA, the latter two increased by 57.7 and 100%, respectively. In terms of the PDI value, SA is only 1.82, while AR and RE are 3.21 and 3.27, respectively, that is, the former is 0.78 times that of the latter two, indicating that the molecular weight distribution of SA is narrower and the difference in composition structure is smaller.

M af of Group Components.
Based on the GPC and EA results of SA, AR, and RE in Table 3 and Table 5, M af of SA, AR, and RE have been calculated, and the results are given in Table 6. As shown in Table 6, in the M af of SA, AR, and RE, the number of O, N, and S increases successively. Combined with the EA results in Table 3, oxygen-containing compounds are enriched in SA and AR, nitrogen-containing compounds in AR and RE, and sulfur-containing compounds in RE, whereas the S contents of SA, AR, and RE are all relatively low.

1 H-NMR Analysis of Group Components.
The 1 H-NMR and 13 C-NMR spectra of SA, AR, and RE were assigned by the "cutoff" method 28,29 because the chemical shifts cannot be accurately divided, in which their complex chemical composition and structure are similar to petroleum. Then, the relative contents of different types of hydrogen and carbon were calculated through area integration with the help of MestReNova. Figure 8 and Table 7 show the 1 H-NMR spectra and their integration values of SA, AR, and RE, respectively. As shown in Table 7, in SA, AR, and RE, first of all, the H β contents are 65.40, 56.36, and 61.02%, which are 2.23, 2.72, and 4.27 times that of H γ , respectively, indicating that the contents of alkane or aryl alkyl substituents are decreased successively and the degree of condensation is increased sequentially. Furthermore,      Figure 9 shows the 13 C-NMR spectra of SA, AR, and RE. In order to further explore the differences of structural characteristics of the three, the structural parameters were calculated by the modified Brown−Landner formula 30 based on Tables 7 and 8. Results are given in Table 9.
As shown in respectively. This indicates that methyl carbon in the alkane chain or the chain of the aromatic cycloalkyl group decreases successively, and there are a large number of long-chain methylene groups in them, which is consistent with the results of hydrogen spectra. The f ar C contents of the three are 2.64, 15.27, and 12.50%, respectively, which are much smaller than all of f ali C in SA, AR, and RE, indicating that all of the group components are mainly composed of aliphatic chains. Table 9 shows the calculation results of structural parameters of SA, AR, and RE. As shown in Table 9, the F A and σ of the three are 0.01, 0.45, 0.70 and 0.25, 0.69, 0.59, respectively, increasing in sequence, which are consistent with the increasing trend of C A , R A , and R N . Also, the f P of SA is as high as 0.72, whereas the f P values of AR and RE are only 0.18 and 0.07, indicating that SA basically does not contain aromatic and consists of chain alkanes and a few of cycloalkanes mainly, while AR and RE contain more thickring aromatic hydrocarbons. In addition, the σ values of AR   and RE are all higher than those of SA, which are 0.69 and 0.59, respectively, indicating that the aromatic ring side chains and association degree of group components increase successively. The H AU /C A values of the three are 0.00, 0.68, and 0.64, respectively, indicating that the aromaticity of AR and RE is much higher than that of SA. From the distribution of ring numbers, R A and R N increase significantly with the order of SA, AR, and RE. The N values of SA and AR are 1.0 and 2.05, respectively, while the N of RE is 5.14, which is significantly higher than the former.

Classification and Evaluation of JSO. 2.3.1. Classification of Combustible Minerals.
Based on a large number of measured data of combustible minerals such as natural gas, oil, shale oil, peat, coal, and graphite from the former Soviet Union, Dr. A.M. Zyulimayev from the Russian National Academy of Sciences proposed a semi-theoretical and semiempirical model named structure (η)−chemical index (δ), shown in Figure 10, to judge the universality of combustible minerals. 31 As shown in Figure 10, the values of δ and η vary from −12.5 to 16.67 and 0 to 5, respectively, and η has a linear relationship with δ. There are four regions, namely, the gas    region, oil−peat region, coal region, and graphite region. Furthermore, with the increase in δ, the order of combustible minerals is natural gas (starting point), petroleum, shale oil, peat, coal, and graphite (end point).
The model is based on the elemental analysis (EA) results of combustible minerals. Taking a 1 g sample as the calculation basis, δ and η are calculated through eqs 1−4.
where N i (i = C, H, O, N, and S) refers to the atom number of the i element in the sample (wt %), N aT refers to the total atom number, N CB refers to the total number of chemical bonds, δ refers to the unsaturation, and η refers to the structure index. According to the values in Table 2, N C , N H , N O , N N , and N S of JSO and other oil samples were calculated respectively, and then, N aT , N CB , δ, and η were calculated through eqs 2−5, respectively. The results are listed in Table 10 and plotted in Figure 10.
As shown in Figure 10, JSO falls into the upper part of the oil−peat region and is close to TuHaC, indicating that JSO has similar composition to petroleum and belongs to a better shale oil. HD and FS fall into the lower part of the oil−peat region, indicating that they are typical shale oil with a better quality than KVR and TaHeR. Meanwhile, LK is close to the peat location, indicating that its quality is lower than those of KVR and TaHeR. Figure 11 shows the empirical classification diagram of petroleum refining-processing performance. 32 As shown in Figure 11, according to the basic properties of the samples in Table 1, the sum of V and Ni contents is 21.57 μg/g, and the carbon residue yield is 3.27% in JSO, which is in the easy processing area, that is, JSO has excellent refining-processing performance, whereas LK, HD, and FS fall into the slightly difficult processing region, indicating that their refining-processing performance is inferior to JSO. Moreover, KVR and TaHeR fall into the not difficult and the difficult processing region, respectively. In conclusion, the refining-processing performance JSO and TuHaC are the best followed by KVR, LK, HD, and FS, and TaHeR is the worst one.

Evaluation of Processing Performance.
The contents of Ni, V, Fe, and Ca in JSO not only affect the refining performance of JSO oil but also have a greater impact on its utilization as chemical raw materials. Therefore, further studies are planned on the occurrence forms of various metal elements in JSO and its group components, especially the existing forms of the Ca element in JSO, so as to provide scientific basis for the removal and utilization of these metal elements.

CONCLUSIONS
This paper demonstrated the first chemical structure and composition and refining performance of shale oil collected from Jimusaer, Xinjiang, China. We focused on the character of JSO as a raw material for the preparation of fine chemicals in addition to an energy resource. Several meaningful conclusions were made as listed in the following: (1) In terms of basic properties, metal content, H/C, O/C, N, and S content, combustible mineral classification and refining-processing performance, JSO is a high-quality shale oil with excellent refining-processing performance compared with shale oil sampled from other areas such as LK, HD, and FS. (2) JSO is also a superior oil sample compared with petroleum since it has eidentical thermogravimetric processes, total metal contents, and refining properties compared with TuHaC. (3) For the contents of group components, JSO with SA as a dominant component is closer to FS in the shale oil group samples. In the petroleum oil group, JSO is closer to TuHaC. (4) The results of 1 H-NMR analysis demonstrated that the substitution sites of alkanes or aryl groups of SA, AR, and RE decrease in turn. However, the degree of condensation of these three group components increases. The length of aromatic side chains and the degree of condensation of aromatic rings of the AR and RE are much higher than SA. Additionally, the 13 C-NMR analysis indicated that SA mainly consists of chain alkanes with a few cycloalkanes, whereas AR and RE contain more content of thick-ring aromatic hydrocarbons. R A and R N increase significantly, which indicated that the association degree of RE is significantly higher than those of SA and RE.   Figure 11. Processing performance of JSO and other samples.