Compositional analysis of heteroatom compounds in Huadian shale

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Compositional Analysis of Heteroatom Compounds in Huadian Shale Oil Using Various Analytical Techniques Da Cui,† Qing Wang,*,† Zhi Chao Wang,† Qi Liu,† Shuo Pan,† Jingru Bai,† and Bin Liu‡ †

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Engineering Research Centre of Oil Shale Comprehensive Utilization, Ministry of Education, Northeast Electric Power University, Jilin City, Jilin 132012, China ‡ Jishun Oil Shale Development Co. Ltd., Jilin City, Jilin 132013, China ABSTRACT: Various analytical techniques were used to study the molecular composition and structure of heteroatom compounds in Huadian (HD) shale oil. On the basis of the results obtained from gas chromatography−nitrogen chemiluminescence detection and gas chromatography−sulfur chemiluminescence detection, the composition of nitrogen- and sulfur-containing compounds was estimated. Molecular composition and structural information of heteroatom compounds were characterized on the basis of their type (double-bond equivalence (DBE)), class (number of oxygen, nitrogen, and sulfur heteroatoms), and carbon number distribution using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). In the positive-ion ESI FT-ICR MS mode, a total of 14 different basic species were detected for basic nitrogen compounds. N1 class compounds dominated followed by N1O1 and N2 class compounds. The DBE value of N1OX compounds having high relative abundance values increased with the increase in the number of oxygen atoms. In the negative-ion ESI FT-ICR MS mode, a total of 12 classes of compounds were detected for neutral nitrogen compounds and acidic compounds of HD shale oil. O2 class species were dominant, followed by N1 and O1 class compounds. N1 class species with DBE values of 9, 12, and 15 were carbazoles, benzocarbazoles, and dibenzocarbazoles, respectively.

1. INTRODUCTION Oil shale is a kind of sedimentary rock, which has solid organic matter in its mineral skeleton. The organic matter of oil shale is mainly composed of kerogen.1−3 Shale oil can be produced through the pyrolysis of kerogen, which is very similar to crude oil. Therefore, oil shale is usually called a supplementary energy resource for crude oil.4−7 China has abundant oil shale resources (fourth largest reserve in the world) with an output of around 476 million tons. Therefore, oil shale resources are of great value for exploitation and utilization in China.8,9 The composition of shale oil is similar to that of crude oil.10 Retorting of oil shale to produce shale oil can make up for the shortage of crude oil. However, the content of heteroatom compounds in shale oil is relatively high.11 Direct combustion of shale oil without pretreatment will cause air pollution and other environmental problems. In addition, the heteroatom compounds, such as pyridines and thiophenes, affect the catalytic activity during the process of hydrofining and hydroupgrading of shale oil.12−17 Therefore, the study of the composition and structure of heteroatom compounds in shale oil is of great significance for the processing and utilization of shale oil. Gas chromatography (GC) has a high separation efficiency for complex mixtures and has become one of the most commonly used techniques in the field of oil analysis.18−25 The most common method for studying the composition of shale oil is gas chromatography−mass spectrometry (GC-MS). However, because of the sensitivity and selectivity of the detectors, GC-MS is not ideal for characterizing the heteroatom compounds in shale oil, especially the nitrogencontaining and sulfur-containing compounds. At present, a sulfur chemiluminescence detector (SCD) and a nitrogen © XXXX American Chemical Society

chemiluminescence detector (NCD) are the most commonly used special detectors for detecting nitrogen-containing and sulfur-containing compounds in the petrochemical industry. Their detection principle is to characterize the nitrogencontaining and sulfur-containing compounds by detecting the intensity of the burning luminescence, which is very different from an MS detector.26,27 In addition, due to the limitation of the temperature of the chromatographic column, high-boilingpoint compounds cannot be gasified, due to which the chromatographic column will be contaminated because of the presence of abundant heteroatom compounds, especially in glials and asphaltenes of shale oil. NCD and SCD are more suitable for detecting heteroatom compounds in oil shale than the MS detector. In recent years, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been used to analyze the composition of heteroatom compounds in liquid fuel oils, and has made breakthrough progress in understanding the molecular composition of crude oil,28−38 coal tar,39−42 and other liquid fuel oils.43−45 The technique has helped in promoting the formation and development of “petroleomics”.46,47 On the basis of the detailed characterization of petroleum molecular composition, “petroleomics” aims at studying the relationship among molecular composition, physical properties, and conversion performance of various constituents. Furthermore, FT-ICR MS is a mass spectrometer having high quality, accuracy, and resolution, and can completely separate complex mass spectral peaks of petroleum Received: November 7, 2018 Revised: January 22, 2019 Published: January 23, 2019 A

DOI: 10.1021/acs.energyfuels.8b03889 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels samples. In addition, electrospray ionization (ESI) is a soft ionization technique and plays an important role in the application of FT-ICR MS in petroleum analysis.48−50 At present, ESI FT-ICR MS technology is used to determine the molecular composition of crude oil, bitumen, petroleum products, and coal tar.30 However, ESI FT-ICR MS has not been widely used in the study of molecular structure and composition of shale oil. In this study, the basic composition of nitrogen-containing and sulfur-containing compounds in shale oil was characterized using gas chromatography−nitrogen chemiluminescence detection (GC-NCD) and gas chromatography−sulfur chemiluminescence detection (GC-SCD) techniques. The detailed composition of heteroatom compounds was analyzed using positive-ion and negative-ion ESI FT-ICR MS. After that, the composition and the structure of heteroatom compounds in shale oil were studied at the molecular level. Shale oil is the principal product of oil shale pyrolysis. This study can provide a reference for the construction of an average molecular model of kerogen.

Table 2. Yields and Cumulative Weights of Distillate Fractions boiling point (°C)

yield (wt %)

cumulative weight (%)

IBP−100 100−120 120−140 140−160 160−180 180−200 200−220 220−240 240−260 260−280 280−300 300−320 320−340 340−360 360−380 380−400 >400

1.38 0.35 0.48 0.87 1.05 4.96 3.24 3.17 4.38 7.45 11.33 10.64 9.13 8.31 6.45 3.12 21.77

1.38 1.73 2.21 3.08 4.13 9.09 12.33 15.50 19.88 27.33 38.66 49.30 58.43 66.74 73.19 76.31 98.08

2. EXPERIMENTAL SECTION used as the carrier gas, whereas the split ratio was 10:1. The gas flow rate of helium was 1 mL/min. Moreover, the gas flow rates of hydrogen and oxidant for the SCD were 42 and 62 mL/min, respectively. The injector and the SCD temperatures were 280 and 800 °C, respectively. The detector pressure was 4.6 Torr. 2.4. ESI FT-ICR MS Analysis. An apex-Ultra FT-ICR MS equipped with a 9.4 T superconducting magnet was used to analyze the molecular composition of heteroatom compounds in HD shale oil. Both positive- and negative-ion modes of ESI were carried out using an Agilent electrospray ionization source. The data analysis was performed using the custom software.51,52 2.4.1. Positive-Ion ESI FT-ICR MS Analysis. The HD shale oil sample was dissolved in toluene to a concentration of 10 mg/mL, and then, 20 μL of the solution was diluted to 0.2 mg/mL using toluene/ methanol (1:3 (vol/vol), respectively). To facilitate the deprotonation of basic compounds to yield [M + H]+ ions, 10 μL of formic acid was added to the solution after dilution. All reagents used in this study were analytical reagents produced by Liaoning Quanrui Chemical Reagent Co., Ltd. The conditions of positive-ion ESI were as follows. The inlet flow rate was set at 180 μL/h using an Apollo II electrospray source, while the emitter voltage was −4000 V. The capillary column’s entrance and exit voltages were −4500 and −320 V, respectively. The optimized mass for Q1 was 200 Da, in which ions accumulated for 0.3 s. The extraction period for ions from the hexapole to the ICR cell was 1.0 ms. The radio-frequency excitation was attenuated at 15 dB, and the mass range of the excited ions was 100−700 Da. A total of 128 FR-ICR data scans were co-added to improve the signal-to-noise ratio (S/N). 2.4.2. Negative-Ion ESI FT-ICR MS Analysis. For negative-ion ESI FT-ICR MS analysis, the dissolution and dilution of shale oil samples were the same as those in the positive-ion ESI FT-ICR MS mode. However, to facilitate the deprotonation of neutral nitrogen compounds and acid compounds to yield [M − H]− ions, 15 μL of ammonia was added to the solution. The inlet flow rate was set at 180 μL/h, while the emitter voltage was 3500 V. The capillary column’s entrance and exit voltages were 4000 and 320 V, respectively. The optimized mass for Q1 was 200 Da. Nevertheless, the accumulation time for ions was 0.5 s. In addition, the extraction period for ions from the hexapole to the ICR cell was set at 0.9 ms. The radio-frequency excitation was attenuated at 15 dB. Moreover, the mass range of the excited ions was 124−800 Da.

2.1. Materials. The oil shale sample used in this study for the preparation of shale oil was obtained from Huadian (HD), Jilin province, China. The sample was crushed and screened to 300 C fraction from low temperature coal tar. Thermochim. Acta 2012, 538, 48−54. (21) Cui, D.; Wang, Q.; Wang, P.; Chi, M. S.; Pan, S.; Liu, B. A comparison of the compositions of acidic and basic fractions in < 300 °C fractions from six Chinese shale oils. Fuel 2018, 211, 251−260.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 432 64807366. Fax: +86 432 64807281. ORCID

Qing Wang: 0000-0002-9840-6492 Shuo Pan: 0000-0001-5635-6426 Notes

The authors declare no competing financial interest. I

DOI: 10.1021/acs.energyfuels.8b03889 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.8b03889 Energy Fuels XXXX, XXX, XXX−XXX