Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Molecular Characterization of Ketones in a Petroleum Source Rock Ping Wang,† Yahe Zhang,† Chunming Xu,*,† Weilai Zhang,† Guangyou Zhu,‡ Zongyue Li,§ Hancheng Ji,§ and Quan Shi*,† †
State Key Laboratory of Heavy Oil Processing and §College of Geoscience, China University of Petroleum, Beijing 102249, People’s Republic of China ‡ Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, People’s Republic of China
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S Supporting Information *
ABSTRACT: A comprehensive investigation of oil composition is important for the petroleum industry as well as the understanding of natural science. Ketones commonly exist in petroleum; however, as a result of the low concentrations and limitations inherent in the analytical technique, the molecular composition of these compounds is rarely studied. In this study, ketones in a source rock were characterized by gas chromatography−mass spectrometry, Fourier transform ion cyclotron resonance mass spectrometry, and Orbitrap mass spectrometry. Ketones were derivatized with the Girard T reagent under weakly acidic conditions to enhance their detectability by electrospray ionization mass spectrometry (ESI MS) analysis. Atmospheric pressure photoionization mass spectrometry (APPI MS) was also used for the molecular characterization of ketones without chemical derivatization. The characterization techniques were complementary to each other, and the results were consistent in general. The chemical derivatization followed by positive-ion ESI MS analysis is suitable for the analysis of ketones in a trace amount, but it had discrimination on high double bond equivalent (DBE) species (DBE ≥ 9). Direct analysis with positive-ion APPI MS showed comparable ionization efficiency throughout the DBE range and could distinguish aliphatic ketones from aromatic ketones according to different ionization pathways. matography.17 Some ketones have been identified using gas chromatography (GC) and/or gas chromatography−mass spectrometry (GC−MS).8,10−12,18−22 However, these methods do not sufficiently isolate ketones from hydrocarbons, nitriles, phenols, and other compounds to enable a definitive identification. Recently, high-resolution mass spectrometry, such as Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS)7,23 and Orbitrap mass spectrometry (Orbitrap MS), coupled with electrospray ionization (ESI) have been used for fossil fuel analysis,24,25 which provided molecular composition of complex mixtures. ESI selectively ionizes polar species by generation of protonated or deprotonated ions ([M + H]+ or [M − H]−).26,27 Nonpolar molecules that are not ionic in solution or not readily ionized by Brønsted or Lewis acid/base chemistry are not accessed by ESI.28,29 Ketones as weakly polar oxygen-containing compounds showed enhanced detection at high ranges of m/z by adding acid at a sufficient concentration,30 and the Girard T reagent can be used to selectively create a positively charged quaternary ammonium moiety on carbonyls that significantly lower the limit of detection in the positive mode.1,31 As an alternative, atmospheric pressure photoionization (APPI) can ionize nonpolar species.32 In this study, GC−MS, FT-ICR MS for Girard T derivatives, and APPI Orbitrap MS were used for molecular characterization of ketones from a petroleum source rock.
1. INTRODUCTION Oxygen-containing compounds are exist in small quantities in crude oils and organic matter of rocks.1 Most available information on the distribution of individual oxygen compounds are polar species, such as carboxylic acids and phenols.2,3 Ketones are other minor oxygen-containing compounds found in fossil fuels.1,4 Like many other heteroatoms in petroleum, ketones are considered to have specific geochemical significance and have been known corresponding to the stability of fuels.5 However, as a result of the low concentrations and limitations inherent in the analytical technique, the molecular composition of ketones have not been well investigated. Long alkyl ketones have been found in the Cretaceous black shale deposits of the Fardes Formation,6 immature coals,7 hydrothermal petroleums and sediment extracts from Guaymas Basin,8 and bitumen of a Proterozoic dolerite sill from McArthur Basin.9 They are common constituents of fossil fuels and show no or only weak odd-over-even predominances. In addition to 2-alkanones, 3-, 4-, 5-, 6-, and 7-alkanones (and higher) are also present in these geological materials.8,9 Isoprenoid ketones have been seen in Tasmanian tasmanite bitumen10 and Green River oil shale.11 Reports on aromatic ketones are less common than those on aliphatic ketones. Fluoren-9-ones and their alkylated derivatives have been identified in Athabasca oil sand bitumen12 and Posidonia shale bitumen.13 Costa Neto et al.14 identified acetonaphthones and 1-indanones in Brazilian oil shale. For molecular characterization, ketones from fossil fuels were generally separated by solid-phase extraction (SPE),14 extrography,15 liquid chromatography,16 and thin-layer chro© XXXX American Chemical Society
Received: May 17, 2018 Revised: September 28, 2018 Published: October 16, 2018 A
DOI: 10.1021/acs.energyfuels.8b01731 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. GC−MS total ion chromatograms of the saturates and aromatics of the extract. Pr, pristine, Ph, phytane, P, phenanthrene, and MP, methyphenanthrene.
Figure 2. Total ion and mass chromatograms of the aromatics by GC−MS. enriched), and resins were separated by column chromatography on silica gel over neutral alumina. Separation was achieved by elution with hexane, dichloromethane (DCM)/hexane (2:1, v/v), and DCM/ methanol (MeOH) (93:7, v/v), separately. The extract amounted to 0.3 wt % of the rock, and the yields of SARA fractions were 58.3, 16.7, 19.2, and 1.2 wt % for saturates, aromatics, resins, and asphaltenes, respectively. Girard T reagent, cation ion-exchange resin (Amber ICR-50), and empty glass columns with polytetrafluoroethylene (PTFE) frits were purchased from J&K Chemical, Ltd. Hexane,
2. MATERIALS AND METHODS 2.1. Samples and Materials. A black shale source rock was collected from an outcropping Cretaceous mudstone bed, on the margin of Gong He Basin, Western China. A total of 50 g of whole rock sample was powdered to 100 mesh after surface cleaning and subsequently extracted with 150 mL of chloroform and methanol (93:7, v/v) for 72 h in a Soxhlet extractor. The extracted organic matter was concentrated by evaporation and deasphaltened by the addition of excess hexane. Saturates, aromatics (in which ketones were B
DOI: 10.1021/acs.energyfuels.8b01731 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels DCM, MeOH, and toluene were analytical-grade commercial products (Beijing Chemical Reagents Company, Beijing, China), which were purified by distillation before use. 2.2. Derivatization of Aromatic Fractions with the Girard T Reagent. About 10 mg of aromatics was dissolved in 1 mL of a DCM and MeOH (1:1, v/v) mixture in a 2 mL sealed vial. A total of 20 mg of Girard T reagent and 6 mg of acid catalysts (Amberlite ICR-50 ionexchange resin) were added. The mixture was magnetically stirred at 40 °C for 12 h. The solvent was evaporated in a gentle stream of nitrogen at 60 °C until dry. A total of 1 mL of DCM was added to the residue, and the mixture was ultrasonicated for 5−10 min in a tightly closed vial to dissolve the oil sample but leave excess Girard T reagent undissolved. Girard T reagent was then removed by filtration on a PTFE filter, which was subsequently washed with DCM until the filtrate was colorless, and the filtrate was subjected to +ESI FT-ICR MS analysis.1,31 2.3. GC−MS Analysis. Agilent 7890-5975C GC−MS equipped with a HP-5 MS capillary column (60 m × 0.25 mm × 0.25 μm) was used to analyze saturates and aromatics. Electron impact (EI) ionization was operated at 70 eV. Helium was used as the carrier gas with a flow of 1 mL/min. The temperatures of the ion source and transfer line were held at 230 and 280 °C, respectively. The injector was maintained at 300 °C in splitless mode. The GC oven for saturate analysis was held at 50 °C for 1 min, programmed to 120 °C at a rate of 20 °C/min, increased to 250 °C at 4 °C/min, then increased to 310 °C at 3 °C/min, and held constant for 30 min. GC−MS was run in select ion monitoring (SIM, m/z 82, 85, 91, 97, 123, 137, 177, 191, 205, 217, 231, and 412) mode. For aromatic analysis, the oven was held at 50 °C for 1 min, programmed to 120 °C at a rate of 15 °C/ min, then increased to 300 °C at 3 °C/min, and held constant for 35 min. Full scan was acquired in a period of 1 s with a mass range of m/ z 35−600. 2.4. +ESI FT-ICR MS Analyses. The Girard T derivatives were dissolved in DCM/MeOH (1:1, v/v) to a solution of 10 mg/mL. A total of 20 μL of the sample solution was further diluted with 1 mL of toluene/MeOH (1:1, v/v) for MS analysis. A Bruker Apex Ultra FTICR mass spectrometer equipped with a 9.4 T superconducting magnet were used for molecular composition analysis. Sample solutions were introduced via an Apollo II electrospray source at a rate of 200 μL/h with a syringe pump. Typical operation conditions for positive-ion ESI were as follows: −4.5 kV spray shield voltage, −5.0 kV capillary column front end voltage, 280 V capillary column end voltage, 0.9 ms time-of-flight window, and 0.6 s collision cell accumulated time. The mass range was set at m/z 200−700. A total of 64 scans with 4M words were accumulated. Methodologies for mass calibration, data acquisition, and processing were reported elsewhere.33 2.5. +APPI Orbitrap MS Analysis. A Thermo Scientific Orbitrap Fusion mass spectrometer was used to analyze the aromatics. The aromatics was dissolved in toluene with a concentration of 0.4 mg/mL and then directly injected into the APPI source with a syringe pump at a flow rate of 40 μL/min. The positive-ion APPI operating conditions were as follows: sheath, auxiliary, and sweep gas flow rates were 20, 10, and 2 (arbitrary units), respectively. The ion transfer tube temperature and vaporizer temperature were 300 °C. The mass range was from m/z 200 to 800 with a total of 1.5 min ion cyclotron time, in which a mass resolution of 500 000 at m/z 200 was achieved.
Figure 3. EI mass spectra of hentriacontan-16-one, tricosan-16-one, and pentatriacontan-18-one.
ring polycyclic aromatic hydrocarbons (PAHs) with or without short alkyl chains.35 An unresolved complex mixture (UCM) formed a large hump in the chromatogram of the aromatics, indicating that the composition of the aromatics is very complex. Long-chain alkyl ketones with high relative abundance were also detected at the high boiling end of the large UCM. As a result of the limit of volatility, compounds with a high boiling point could not be detected by GC−MS. Figure 2 shows the GC−MS TICs and mass chromatograms of the aromatics. According to the matching of mass spectra with the National Institute of Standards and Technology (NIST) library and the references, the major carbonyl compounds detected in aromatics included seven homologous series of alkyl ketones. Mass chromatograms of m/z 58, 72, 86, 100, 114, and 128 show the distribution of aliphatic methyl ketones, 3-alkanones, 4-alkanones, 5-alkanones, 6-alkanones, and 7-alkanones, respectively.8,22 An example of compound
3. RESULTS AND DISCUSSION 3.1. GC−MS Analysis for the Saturates and Aromatics. Total ion chromatograms (TICs) of saturates and aromatics by GC−MS are shown in Figure 1. Normal paraffins with carbon numbers ranging from 13 to 35 were detected in the saturates. The predominance of pritane on phytane suggests oxidation depositional environments.34 The major carbonyl compounds detected in aromatics were linear ketones. The composition of ketones was different from those found in ancient sediments, which have abundant 3−6C
DOI: 10.1021/acs.energyfuels.8b01731 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 4. GC−MS total ion and mass chromatograms of fluorenones in the aromatics. “−Cn−” denotes the carbon number of substitute alkyl(s). F, fluoren-9-one; 1-MF, 1-methylfluoren-9-one; 2-MF, 2-methylfluoren-9-one; 3-MF, 3-methylfluoren-9-one; 4-MF, 4-methylfluoren-9-one; and 1,4DF, 1,4-dimethylfluoren-9-one.
extracts and were interpreted to originate from anthropogenic sources by combustion or exposure to high temperatures.8 C18 isoprenoid methyl ketone (6,10,14-trimethylpentadecan-2one) was also detected in aromatics, and such compounds have been found in immature coals,22 bitumen,10 marine sediments,36 and oil shale.11 C18 isoprenoid ketone was proposed to be derived from phytol and the linear methyl ketones from β-oxidation of the corresponding alkane.10 It should be noted that there were three peaks at the end of TICs with a relatively high abundance (see Figure 1 and the bottom of Figure 2), and the mass spectra of these compounds are shown in Figure 3. According to the monitoring of the mass spectra and the report documented by Netting and Macey37 for nonacosan-15-one, these ketones were identified as hentriacontan-16-one, tricosan-16-one, and pentatriacontan18-one, respectively. Such compounds were rarely detected in geological samples. Boon et al.38 have found a series of very long mid-chain ketones from Walvis Bay diatomaceous ooze. Cranwell39 also reported these compounds. Raven et al.40 and Evershed et al.41 believed that hentriacontan-16-one, tricosan16-one, and pentatriacontan-18-one were derived from fatty acid metal salts, which were the product of fatty acids and metal ions. The detectable long mid-chain ketones in aromatics might be particular biomarkers for the presence of organic acid salts. Figure 4 shows mass chromatograms of m/z 180 + 14n mass series, in which fluorenone and its C1−C4 alkyl homologues were detected. The assignment of 9-fluorenone, 1-, 2-, 3-, and 4-methylfluoren-9-ones, and 1,4-dimethylfluoren-9-one were based on their mass spectra and relative retention time from refs 12, 20, 42, and 43. C3 and C4 alkylfluorenones were
Figure 5. (a) Broadband +ESI FT-ICR mass spectrum of Girard T derivatives of the aromatics and (b) bubble plot of DBE versus carbon number for corresponding O1 class species.
identification of C14 ketone isomers is shown in Figure S1 of the Supporting Information. Aliphatic methyl ketones and 3-, 4-, and 5-alkanones with carbon numbers of 13−29 were abundant in the aromatics, whereas 6- and 7-alkanones with carbon numbers of 13−27 were also detected but with a lower abundance. Aliphatic ketones with a carbon number of
