Article pubs.acs.org/EF
Characterization and Comparison of Nitrogen Compounds in Hydrotreated and Untreated Shale Oil by Electrospray Ionization (ESI) Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) Xiaobo Chen,*,†,‡ Benxian Shen,† Jinpeng Sun,‡ Chengxiu Wang,‡ Honghong Shan,‡ Chaohe Yang,‡ and Chunyi Li‡ †
School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555, People’s Republic of China
‡
ABSTRACT: Shale oil has attracted more attention, as a very important substitutable fuel resource. In the present research, the classes and structures of nitrogen species in hydrotreated and untreated Fushun shale oil (FSO) are characterized by electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Experimental results have demonstrated that most of the nitrogen compounds in FSO are removed effectively during the hydrotreatment. N1 and N2 classes are dominant in FSO, and their structures are deduced in terms of the double bond equivalent (DBE) values and the Fourier transform infrared (FTIR) spectra. N1 class species in FSO are probably pyridines, indoles, carbazoles, benzocarbazoles, and their derivatives. After hydrotreating, the N1 class species in hydrotreated Fushun shale oil (HFSO) extend over a wider range of DBE values and carbon numbers than in the original FSO. It can be concluded that the N1 class species in HFSO are generated from compounds containing two or more heteroatoms, such as N2, N1O1, N1O2, N1O1S1, N1S1, and N2S1 class species.
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INTRODUCTION The price of crude oil has been soaring because of the limited supply and the increasing demand.1,2 Therefore, the unconventional resources, such as oil sands bitumen, extra-heavy oil, and oil shale, have attracted more and more interest in the world.3 Oil shale, a sedimentary rock embedded with significant amounts of kerogen, is essentially a geologically immature form of petroleum.4 Because of its huge reserves, oil shale is expected to be a very important substitutable fuel resource. It is conservatively estimated that there are at least 8 trillion barrels of oil shale resources around the world, which is much more than the total known conventional oil resources (∼2.4 trillion barrels).4−6 Oil shale can be converted to fuel feedstock called shale oil through the process of retorting. In comparison to conventional crude oil, shale oil is richer in heteroatomic compounds, especially nitrogen species, which may cause many troubles, such as fuel instability during its transportation or storage, catalyst poisoning, fouling, and air pollution. In addition, direct combustion of the shale oil will result in the emissions of NOx and SOx.7−9 Consequently, the identification of chemical compositions in shale oil and the development of upgrading processes to remove heteroatoms from shale oil have become important areas of studies. At present, the ways of removing heteroatoms from shale oil involved hydrotreating, acid neutralization, solvent extraction, complexation, and adsorption, among which hydrotreating is considered as the most efficient way to remove those S-, N-, and O-containing species.10−13 Many papers13−21 showed that severe process conditions were needed during hydrotreating shale oils. Denitrogenation was more difficult than desulfurization for shale oil. As a result, the characterization of nitrogen © 2012 American Chemical Society
compounds in shale oil must be very important for the development of shale-oil-refining processes. In general, nitrogen compounds in crude oil and its distillates were isolated by solvent extraction and/or liquid chromatography, followed by gas chromatography−mass spectrometry (GC−MS) for characterization.22 However, GC−MS is not capable of analyzing samples that contain higher molecularmass nitrogen compounds because of their low volatility. Liu et al.23 reported that it was hard to identify all of the nitrogen compounds accurately when the molecular weight was more than 300 Da. In recent years, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been widely applied to the petroleomics field, crude oil and its distillates,24−29 oil sands bitumen,30−34 and shale oils.35 FTICR MS could provide a method to identify the molecular structures of bulk molecules with high-molecular-weight and double bond equivalent (DBE) values. Electrospray ionization (ESI) can be used to selectively analyze polar compounds. The high selectivity of ESI improves the analysis for identifying trace polar compounds found in petroleum systems. Therefore, the application of ESI combined with FT-ICR MS can improve the accuracy of MS analysis to characterize the polar compounds.36 Therefore, it becomes possible to determine the trace nitrogen compounds in shale oil both before and after hydrotreatment in terms of their individual chemical nature. ESI FT-ICR MS is a powerful tool that has been used with great success for the application of petroleomics, although it has not yet been frequently applied to the analysis of nitrogen Received: October 2, 2011 Revised: January 12, 2012 Published: February 9, 2012 1707
dx.doi.org/10.1021/ef201500r | Energy Fuels 2012, 26, 1707−1714
Energy & Fuels
Article
methanol used were analytical-reagent-grade solvents that were distilled twice and kept in glass bottles with ground glass stoppers.28,29 ESI FT-ICR MS Analysis. The FSO and HFSO samples were analyzed by Bruker apex-ultra FT-ICR MS equipped with a 9.4 T superconducting magnet at the China University of Petroleum (Beijiing, China). The sample solution was infused via an Apollo II electrospray source at 180 μL/h by a syringe pump. The conditions for positive-ion (or negative-ion) formation were −4.0 kV (or 3.5 kV) emitter voltage, −4.5 kV (or 4.0 kV) capillary column front end voltage, and 320 V (or 320 V) capillary column end voltage. Ions accumulated for 0.1 s in a hexapole with 2.4 V (or −2.4 V) directcurrent voltages and 200 Vp−p radio frequency (RF) amplitudes. The optimized mass for quadrupole 1 (Q1) was 200 Da. Hexapoles of the Qh interface were operated at 5 MHz and 200 Vp−p RF amplitude, in which ions accumulated for 0.2 s (or 0.4 s). The delay was set to 1.1 ms to transfer the ions to an ICR cell by electrostatic focusing of transfer optics. The ICR was operated at 11.75 db (or 11.75 db) attenuation, 200−800 Da (or 150−800 Da) mass range, and 4 M acquired data size. The time domain data sets were co-added from 64 data acquisitions.28,29 Fourier Transform Infrared (FTIR) Analysis. The chemical group presented in FSO was analyzed using a Nexus FTIR spectrometer, which was produced by Thermol Nicolet Co., Ltd. The film of the sample was created by placing a drop of FSO in KBr plates. The FTIR spectrum for the sample was obtained at 4 cm−1 resolution and collected in the 4000−400 cm−1 range. Mass Calibration and Data Analysis. Sodium formate was used to calibrate the mass spectrometer. The mass spectra obtained were calibrated internally according to a known and highly abundant homologous series of N-containing compounds. Peaks with relative abundance greater than 5 times the standard deviation of the baseline noise were exported to an Excel spreadsheet. Data analysis was performed using the software that was described in the literature.39 In general, the data analysis by selecting a two-mass scale-expanded segment in the middle of the spectra was followed by detailed identification of each peak. The peak of at least one of each heteroatom class species was arbitrarily selected as a reference. Species with the same heteroatom class and its isotopes with different DBE values and carbon numbers were searched within a set of ±0.001 Kendrick mass defect (KMD) tolerances.40
compounds in shale oil and its secondary products. In the present study, the classes and structures of nitrogen species in Fushun shale oil (FSO, from Fushun Mining Group, China) and the sample after being hydrotreated were characterized using the 9.4 T ESI FT-ICR MS. The purpose of this analysis was to characterize and compare the nitrogen compounds in hydrotreated Fushun shale oil (HFSO) and untreated FSO and to determine the mechanism of the heteroatomic hydrogenation reaction preliminarily. More comprehensive and detailed characterization of the composition of nitrogen compounds present in shale oil can be useful in the designing and developing of better catalysts and technologies for processing shale oil.
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EXPERIMENTAL SECTION
Shale Oils. The shale oil used in this study is provided by Fushun Mining Group, China. Its properties are listed in Table 1. The
Table 1. Properties of FSO and HFSO FSO
HFSO
conventional oil (Daqing, China)
density at 20 °C (kg/m3) 894.3 CCR (wt %) 1.55 elements (wt %) C 85.08 H 12.04 N 1.27 S 0.35 BNS (wt %) 0.629 group compositions (wt %) saturates 49.64 aromatics 16.7 resins 33.11 asphaltenes 0.55
848.4 0.015
855.4 2.9
85.88 13.70
