An assay on alkyl aromatic hydrocarbons: Unexpected group-type

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

An Assay on Alkyl Aromatic Hydrocarbons: Unexpected Group-Type Separation of Diaromatic Hydrocarbons in Cretaceous Crude Oils from Brazilian Marginal Basin Arkellau K. S. Moura,*,† Danilo O. Ribeiro,† Iolanda S. do Carmo,† Bruno Q. Araújo,*,†,‡ Vinícius B. Pereira,‡ Débora A. Azevedo,‡ and Antônia M. G. L. Citó†

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†

Campus Universitário Ministro Petrônio Portela, CCN, Laboratório de Geoquímica Orgânica, Universidade Federal do Piauí, Teresina, Piauí 64049-550, Brazil ‡ Instituto de Química, LAGOALADETEC, Universidade Federal do Rio de Janeiro, Ilha do Fundão, Rio de Janeiro, RJ, 21941-598, Brazil S Supporting Information *

ABSTRACT: Numerous aromatic biomarkers such as aromatic carotenoids and aryl isoprenoids have been reported in Brazilian Cretaceous oils. The n-alkyl aromatics such as n-alkyl benzenes (ϕ-Cn), toluenes (ϕT-Cn), xylenes, and mesitylenes have also been detected in geological samples and can be valuable tools in geochemical and environmental investigations. In this work, comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC−TOFMS) and GC−tandem MS (GC−MS/MS) were applied to identify the alkyl aromatic series in two crude oils from the SergipeAlagoas Basin, Brazil. Geochemical characteristics indicate a different depositional paleoenvironment for each sample, marine and lacustrine end-members. Aromatic compounds were identified using full mass spectrum information, diagnostic ions (i.e., m/z 92 for alkylbenzenes and m/z 106 for alkyltoluenes), selected reaction monitoring transitions, and coinjection with an authentic C21 n-pentadecylbenzene standard. The Sergipe-Alagoas oils presented a series of n-alkylbenzenes and n-alkyltoluenes ranging from ϕ-C7 to ϕ-C33 and ϕT-C7 to ϕT-C30, respectively. In addition, phytanyl arenes and a new series of α,ωbisphenylalkanes were identified for the first time in Sergipe-Alagoas oils. The n-alkylbenzene profile for marine crude oil resembles that of n-alkanes and n-alkylcyclohexanes, which suggests that they may have the same precursors. The investigation of aromatic fractions and detection of n-alkyl aromatic hydrocarbons in Brazilian Cretaceous oils provide new assignments to the contribution of organic matter and its geochemical significance.

1. INTRODUCTION

(i.e., coelutions) obstructed its detection during the chromatographic separation. GC−flame ionization detection (GC−FID) and gas chromatography−mass spectrometry (GC−MS) are traditional chromatographic techniques employed to geochemical investigations. However, the traditional one-dimensional GC (1DGC) has many limitations due to the high complexity of petroleum samples and chromatographic coelutions.10 The effect of coeluting compounds in the geochemical evaluation is observed by the unresolved complex mixtures, interferences in the mass spectrum identification, and the alteration in biomarker parameters.11,12 Numerous studies applying GC−tandem MS (GC−MS/ MS) in molecular geochemical investigations13 as well as comprehensive two-dimensional GC coupled to time-of-flight MS (GC×GC−TOFMS)11,12,14,15 have been reported. Furthermore, the integrated combination of GC−MS/MS and GC×GC techniques is relevant for identification of unusual hydrocarbons and geochemical studies from high complexity petroleum samples.12,16,17 In the present work, the alkyl aromatic compounds of Sergipe-Alagoas crude oils were

Aromatic compounds have been investigated in geological samples from different ages. Aromatic biomarkers and hydrocarbons (i.e., phenanthrenes, naphthalenes, aromatic steroids, dibenzothiophenes, and aryl isoprenoids) are related to geological and environmental processes such as thermal maturation, depositional paleoenvironment, and/or biodegradation of oils and rocks.1−3 n-Alkyl aromatics may also be used as geochemical indicators of paleoenvironment4,5 and organic matter source.6 The distribution of n-alkyl aromatics is reported in geological samples from Precambrian to Cenozoic period, but there have been few studies of these aromatic compounds in Brazilian oils and derivatives.7,8 Vanini et al.8 observed toluene derivatives and short-chain n-alkyl benzenes in the molecular characterization of Brazilian petroleum. The diaromatic series of acyclic isoprenoids with two trimethylaryl end-groups or two dimethylaryl end-groups, including two biphenyl end-groups or one biphenyl and one phenyl end-group have been reported in geological samples.3 However, the complete series of alkanes with two aryl endgroups, named bisphenylalkanes is not detected in rocks and oils; although, Gorchs et al.9 have reported only three bisphenylalkanes (C24−C26) and three phenyltoluylalkanes (C25−C27) in sulfur-rich coal. It is possible that interferences © XXXX American Chemical Society

Received: September 17, 2018 Revised: December 13, 2018 Published: January 16, 2019 A

DOI: 10.1021/acs.energyfuels.8b03268 Energy Fuels XXXX, XXX, XXX−XXX

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peak find and spectral deconvolution. Individual peaks were automatically detected on the basis of a 10:1 signal-to-noise ratio. The identification of aromatic hydrocarbons was performed by examination of extracted ion chromatograms (EIC m/z 92 and 106), full mass spectrum examination and comparison of literature mass spectra,9,20,21 first-dimension retention time (1tR), second-dimension retention time (2tR), and the elution order. 2.4. Gas Chromatography−Tandem Mass Spectrometry. The saturated and aromatic fractions were analyzed using a GC−MS/ MS system, GC Trace Ultra/TSQ Quantum XLS (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an autosampler (AS 3000) and the column Elite-1ht (PerkinElmer, 100% dimethyl polysiloxane, 30 m, 0.25 m i.d., 0.1 μm df). An aliquot of 1 μL of the sample was injected in the PTV split (10:1), temperature of the injector was at 290 °C, temperature of the interface at 300 °C, and the split flow was 10 mL/min. The oven temperature program was 60 °C (4 min) to 280 °C (5 min) at 6 °C/min and then 310 °C (10 min) at 1 °C/min. Helium (99.9999%, White Martins) was the carrier gas used at a constant flow of 1 mL/min. The mass spectrometer operated with ion-source temperature at 230 °C, EI at 70 eV, emission current at 50 mA, and solvent delay time of 5 min. Two mass spectrometer modes, full-scan and selected reactionmonitoring (SRM), were applied to the investigation of petroleum hydrocarbons. The GC−MS/MS was initially operated in the full-scan mode with scanning mass range from m/z 50 to 600. The selective GC−MS/MS analysis was performed by the SRM mode with acquisition of chromatograms from precursor ion (molecular ion, M+•) → product ion transitions. The transition lists and selection of segments was performed based on the elution order. MS/MS conditions in the SRM mode were: scan time 0.2 s, mass range of m/z 0.02, peak width in quadrupole 1 (Q1, full width at halfmaximum) of 0.7, collision energy of 5 V, and collision gas pressure (Ar) of 1.2 mTorr. Biomarker parameters were calculated from the chromatogram peak area of the full-scan and SRM analysis. Identification of the petroleum hydrocarbons was carried out based on mass spectra, EIC, and/or SRM chromatograms: m/z 85 (n-alkanes), m/z 82 (nalkylcyclohexanes), m/z 125 (β-carotane), m/z 191 (Tr, tricyclic terpanes), m/z 217 (Dia, diasteranes, steranes), and m/z 259 (TPP, tetracyclic polyprenoids) for the saturated fraction; and EIC and/or SRM chromatograms: m/z 92 (alkylbenzenes), m/z 106 (alkyltoluenes), m/z 178 (phenanthrene), and m/z 192 (methylphenanthrenes) for the aromatic fraction. 2.5. Synthesis of C21 n-Pentadecylbenzene and Identification by GC−MS/MS. C21 n-pentadecylbenzene was prepared in three steps. The cardanol mixture (perhydro, monoene Δ,8 diene Δ,8,11 and triene Δ8,11,14) isolated from cashew nut shell liquid was hydrogenated using H2/Pd/C. The n-pentadecylphenol was converted into the toluene-p-sulfonate derivative,22 and then the elimination of Otosyl group was realized using sodium borohydride-nickel chloride.23 The C21 n-pentadecylbenzene was characterized by 1H and 13C NMR (Varian Inova model 400), FTIR (Agilent Technologies Cary 630), and EIMS (Shimadzu GCMS-QP2010 SE Ultra) analyses. 1H NMR (400 MHz, CDCl3): δ 7.30−7.10 (m, H-2 to H-6), 2.6 (t, H-7), 1.61 (quint, H-8), 1.30 (m, H-9/H-10), 1.27 (br s, H-11 to H-20), and 0.87 (t, H-21); 13C NMR (100 MHz, CDCl3): δ 143.0 (C-1), 128.4 (C-3/C-5), 128.2 (C-2/C-6), 125.5 (C-4), 36.0 (C-7), 32.1 (C-8), 31.4 (C-9), 29.8−29.3 (C-10), 22.7 (C-20), and 14.1 (C-21); FTIR (νmax, cm−1): 2920 (sp3C−H), 2853 (sp3C−H), 1494 (CC), 1458 (CC), 747, and 698 (aromatic monosubstituted); EIMS [m/z (%)]: 92 (100, C7H8+•), 91 (55, C7H7+), 43 (13), 288 (12, C21H36, M+•). The alkylbenzene series was investigated by the GC−MS/MS analysis of the spiked aromatic fraction with the C 21 npentadecylbenzene standard. The GC−MS/MS coinjection analysis was performed based on sampling of an aliquot of 0.5 μL of the aromatic fraction (10 mg/mL in dichloromethane) and 0.5 μL of the C21 n-pentadecylbenzene standard (0.58 ng/μL in dichloromethane). The GC−MS/MS analysis in the SRM mode were performed as described above (Section 2.4).

analyzed by GC×GC−TOFMS and GC−MS/MS. Moreover, phytanyl arenes and bisarylalkanes were also investigated for the first time in the aromatic fraction of Brazilian oils. To the best of our knowledge, this is the first study to evaluate the geochemical significance of n-alkylbenzenes and n-alkyltoluenes in Brazilian oils.

2. EXPERIMENTAL SECTION 2.1. Crude Oils. The crude oils (n = 2, SEAL1 and SEAL2) from the Sergipe-Alagoas Basin were supplied by National Agency of Petroleum, Natural Gas and Biofuels (ANP, Brazil). The SergipeAlagoas Basin is divided into two sub-basins (Sergipe and Alagoas Basins) and encompasses 44 370 km2 of the East Brazilian margin, 12 620 km2 are onshore. The geological characteristics and biomarkers of Sergipe-Alagoas Basin (northeastern Brazil) are well documented.18,19 In general, the Sergipe-Alagoas Basin developed during the Upper JurassicEarly Cretaceous period and its tectonostratigraphic record is represented by four mega-sequences. Biomarkers revealed five depositional paleoenvironments of the organic matter in the SergipeAlagoas Basin (lacustrine freshwater, saline lacustrine, marine evaporitic, marine carbonate, and open marine anoxic).18 2.2. Sample Preparation. The crude oil samples (ca. 100.0 mg) were fractionated by liquid chromatography on activated silica gel (3.0 g, 120 °C/12 h; 0.063−0.200 mm, Merck, Darmstadt, Germany) and eluted with 10 mL n-hexane, 10 mL n-hexane/CH2Cl2 (9:1, v/v), and 10 mL CH2Cl2/MeOH (9:1, v/v) to yield saturated hydrocarbon, aromatic hydrocarbon, and polar compound fractions, respectively. For the GC×GC−TOFMS analysis, perdeuterated pyrene (C16D10) internal standard in dichloromethane (5.0 ng/μL, 98% purity; Cambridge Isotope Laboratories, Andover, MA, USA) was added to the aromatic hydrocarbon fractions. Chromatographic grade solvents were used in the geochemical preparation (Tedia, Rio de Janeiro, RJ, Brazil). 2.3. Two-Dimensional Gas Chromatography−Time-ofFlight Mass Spectrometry. Aromatic hydrocarbon analyses were performed on a Pegasus 4D GC×GC−TOFMS system (Leco Corporation, St. Joseph, MI, USA), composed of a gas chromatograph (7890A, Agilent Technologies, Palo Alto, CA, USA) equipped with a split/splitless injector, a nonmoving quad-jet dual-stage thermal modulator, and a secondary oven connected to a Pegasus HT time-offlight mass spectrometer (Leco Corporation). A nonpolar stationary phase, DB-5 ms column (5% phenyl−95% methylsiloxane, 30 m, 0.25 mm i.d., 0.25 μm df; Agilent Technologies) was used in the firstdimension (1D) and a midpolar stationary phase, BPX-50 column (50% phenyl polysilphenylene-siloxane, 1.5 m, 0.1 mm i.d., 0.1 μm df; SGE Analytical Science, Ringwood, VIC, Australia) was used in the second-dimension (2D) column. The 1D column was connected to the 2D column via SilTite mini union 0.1−0.25 mm i.d. (SGE Analytical Science), and the 2D column was connected to the TOFMS by an uncoated, deactivated, fused-silica capillary (0.5 m × 0.25 mm i.d.) via μ-union and SilTite metal ferrules 0.1−0.25 mm i.d. (SGE Analytical Science). The GC conditions included pulsed splitless mode injection: 40 psi for 1 min and 1 μL of sample at 310 °C with a purge time of 3 min and purge flow of 5.0 mL/min. Helium (99.9999%; White Martins, RJ, Brazil) was the carrier gas at a constant flow of 2.0 mL/min. The primary oven temperature was programmed from 130 °C for 0.2 min to 370 °C at 4 °C/min. The temperature offset between the secondary and primary oven was 5 °C. The modulator parameters consisted of a modulation period of 8 s with a 2 s hot jet pulse duration, the hot jet pulse was altered for 2.5 s from 300 °C of the primary oven program, and a temperature offset of modulation 30 °C above the primary oven temperature. The temperature of the transfer line to the mass spectrometer was 280 °C. The mass spectrometer conditions were: ion-source temperature of 230 °C, 70 eV electron ionization (EI) energy, 50−600 Da mass range, 1500 V detector voltage, and 100 spectra/s acquisition rate. GC×GC−TOFMS data acquisition and treatment were performed using ChromaTOF software 4.51 (Leco Corporation) for automated B


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Figure 1. GC−MS/MS (full scan) EIC m/z 85 and 82 for SEAL1 and SEAL2. Cn: n-alkanes, ⬡n: n-alkylcyclohexane.

Table 1. Geochemical Parameters for Sergipe-Alagoas Crude Oils geochemical parameters

SEAL1

SEAL2

n-alkane n-alkane maximum Pr/Pha hopane/steraneb Tr26/Tr25c TPP30/Dia27d β-carotanee dinosteranee gammacerane indexf Tr23/H30g TeT24/H30h TeT24/Tr26i H34/H35j C27/C29 steranesk diasterane indexl Ts/Tm (GC−MS)m Ts/Tm (GC−MS/MS)n Ts/(Ts + Tm)o H32 22S/(22S + 22R)p 29αααS/(29αααS + 29αααR)q 29αββ/(29αββ + 29ααα)r m-ϕT-C15/o-ϕT-C15s methylphenanthrene indext 25NH/H30u

C12−C32 C17 1.48 3.82 0.53 0.84 + + 37.2 1.54 0.02 0.48 1.11 1.38 29.1 0.65 0.82 0.39 0.61 0.53 0.51 0.90 0.67 0.07

C11−C37 C16 and C27 3.03 8.14 1.21 5.66 + − 41.0 0.40 0.18 0.64 2.23 1.35 17.9 0.69 1.95 0.41 0.60 0.42 0.59 1.00 0.64 0.09

>60 (high).18 g(C23 13β(H),14α(H)-tricyclic terpane)/(C30 17α(H),21β(H)-hopane)EIC m/z 191. h(C24 tetracyclic terpane)/ (C30 17α(H),21β(H)-hopane)EIC m/z 191. i(C24 tetracyclic terpane)/(ΣC26 13β(H),14α(H)-tricyclic terpanes)EIC m/z 191. j (C34 22R + 22S 17α(H),21β(H)-tetrakishomohopane, SRM m/z 468 → 191 transition)/(C35 22R + 22S 17α(H),21β(H)-pentakishomohopane, SRM m/z 482 → 191 transition)EIC m/z 191. Low 1. k(C27 20R 5α(H),14α(H),17α(H)-cholestane, SRM m/z 372 → 217 transition)/(C29 20R 5α(H),14β(H),17β(H)-24-ethylcholestane, SRM m/z 400 → 217 transition)SRM m/z 217 transition. l[(ΣC27 20R + 20S 13β(H),17α(H)-diacholestane/ΣC27 20S + 20R 5α(H),14α(H),17α(H)-cholestane) × 100]EIC m/z 217. 100 (high).18 m(C27 18α(H)22,29,30-trisnorneohopane/C27 17α(H)-22,29,30-trisnorhopane) calculated by full-scan analysis (EIC m/z 191). n(C27 18α(H)22,29,30-trisnorneohopane/C27 17α(H)-22,29,30-trisnorhopane) calculated by SRM m/z 370 → 191 transition. o[(C27 18α(H)22,29,30-trisnorneohopane)/(C27 18α(H)-22,29,30-trisnorneohopane + C27 17α(H)-22,29,30-trisnorhopane)]EIC m/z 191. p(C32 22S 17α(H),21β(H)-bishomohopane/C32 22S + 22R 17α(H),21β(H)-bishomohopane)SRM m/z 440 → 191 transition. Thermodynamic equilibrium (0.57−0.62).40 q[(C29 20S 5α(H),14α(H),17α(H)-24-ethylcholestane)/(C29 20S 5α(H),14α(H),17α(H)-24-ethylcholestane + C29 20R 5α(H),14α(H),17α(H)-24ethylcholestane)]SRM m/z 400 → 217 transition. Thermodynamic equilibrium (0.52−0.55).39 r[(C29 20R + 20S 5α(H),14β(H),17β(H)-24-ethylcholestane)/(C29 20R + 20S 5α(H),14β(H),17β(H)-24ethylcholestane) + (C29 20S + 20R 5α(H),14α(H),17α(H)-24ethylcholestane)]SRM m/z 400 → 217 transition. Thermodynamic equilibrium (0.67−0.71).39 s(meta n-pentadecyltoluene/ortho npentadecyltoluene)SRM m/z 302 → 106 transition. tMPI-1 = {[1.89 × (2-MP + 3-MP)]/[P + 1.26 × (1-MP + 9-MP)]} phenanthrene (P) EIC m/z 178; methylphenanthrene (MP) EIC m/z 192. u(C29 17α(H),21β(H)-25-norhopane, SRM m/z 398 → 191 transition/C30 17α(H),21β(H)-hopane, SRM m/z 412 → 191 transition).

a (Pristane/phytane)EIC m/z 183. b[(C30 17α(H),21β(H)-hopane, EIC m/z 191)/(ΣC27 20R + 20S 5α(H),14α(H),17α(H)-cholestane, EIC m/z 217)]. (Low 7).18 c(ΣC26 13β(H),14α(H)-tricyclic terpanes)/(C25 13β(H),14α(H)-tricyclic terpane)EIC m/z 191. d[(C30 TPP)/(ΣC27 20R + 20S 13β(H),17α(H)-diacholestane)]EIC m/z 259 1 (lacustrine).16 e(+): detected; (−): not detected. f[(gammacerane, SRM m/z 412 → 191 transition/C30 17α(H),21β(H)-hopane, SRM m/z 412 → 191 transition) × 100]. 1 indicates suboxic and oxic depositional environment for SEAL1 and SEAL2, respectively. The Pr/Ph value may be affected by the formation of pristane originated from catagenetic transformation of methyltrimethyltridecylchromans35 and degradation of other isoprenoids. Pr/Ph, gammacerane index, and β-carotane are additional information for saline source rocks. Furthermore, gammacerane is a specific paleoenvironmental indicator of water column stratification. H34/H35, diasterane index, and tetracyclic ratios are commonly used to distinguish varied lithology.18,34,36 In general, abundant H35 is a selective indicator of highly reducing marine depositional paleoenvironment,37 as observed for SEAL1 (Table 1). Ts/Tm value is a common maturity parameter evaluated by 1D-GC, although this parameter is influenced by organic matter source, as shown in Table 1. The variation on the calculated Ts/Tm using peak area acquired by full-scan (EIC m/z 191) and SRM (m/z 370 → 191 transition) is explained by the coelution of Ts with C30 tetracyclic terpane (TeT30), Tm with C30 tricyclic terpane (Tr30), and/or Tm with C28 tetracyclic polyprenoid (TPP28).16,29 For the hopanoid series, several coelutions may be observed in the 1D-GC.12 The multidimensional techniques (GC−MS/MS and GC×GC− TOFMS) resolve coelution problems of petroleum biomarkers, minimizing and eliminating the interferences when compared to GC−MS. Some changes in the geochemical values were previously observed in the comparative GC−MS studies.38 The sterane maturity ratios were lower than isomerization equilibrium,39 with the exception of 29αααS/(29αααS + 29αααR) value (0.53) for SEAL1, as shown in Table 1. Meanwhile, H32 22S/(22S + 22R) values (0.60−0.61) suggest the endpoint value at the peak of the oil-generative window.40 The greater variation on maturity values from post-salt Brazilian crude oils are well known.19,31,41 In general, the saturated biomarker parameters used to estimate the thermal maturity degree are shown to be contradictory, as previously observed for Brazilian crude oils.29,31 With regard to the distribution of n-alkyltoluenes (m/z 106), the Sergipe-Alagoas oils show similar abundance patterns of meta and ortho isomers based on the m-ϕT-C15/o-ϕT-C15 values (0.90 and 1.00 for SEAL1 and SEAL2, respectively), which indicates that both samples are thermally evolved.21 Albaigés et al.42 evaluated oils from the Tarragona Basin (Spain) and observed that immature crude oils presented high proportion of ortho isomers, whereas both meta and para isomers tend to increase with thermal maturation and thermodynamic stability.21 Moreover, the methylphenanthrene index (MPI-1, 0.64−0.67) for Sergipe-Alagoas oils suggests identical maturity parameters in the oil window. Also, the preserved n-alkane profiles and low 25NH/H30 value ( C28; C27/C29 steranes >1.30; Table 1), and C27 to C29 diasteranes were identified in both samples. Short-chain steranes have been detected in organic matter derived from saline, marine carbonate, or sulfur-rich depositional paleoenvironment.32,33 Concomitantly, 4,23,24trimethylcholestanes (dinosteranes), which are important indicators of marine organisms such as dinoflagellates and diatoms, were detected only in the SEAL1 oil (Table 1), as previously noted for the Sergipe-Alagoas crude oils.19 In addition, the aryl isoprenoids and related compounds were detected in both samples based on EIC m/z 134 and m/z 237 (Figure S2). Aryl isoprenoids, especially C40 isorenieratane, and diagenetic compounds are indicators of organic matter contribution from marine sulfur bacteria (i.e., Chlorobiaceae).3 However, the abundance of aryl isoprenoids may be influenced by the maturation degree and paleo-redox D


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Figure 2. EIC m/z 92 and m/z 106 for alkyl aromatic in marine and lacustrine crude oils from Sergipe-Alagoas Basin. ϕ-Cn: n-alkylbenzene; ϕT-Cn: n-alkyltoluene; ϕ-MeCn: branched-alkylbenzene; ϕT-MeCn: branched-alkyltoluene; ϕ-In: phytanyl arenes; ϕ-Cn‑ϕ: α,ω-bisphenylalkanes; and ϕTCn‑ϕ: α,ω-phenyl-tolylalkanes.

Figure 3. Representative mass spectra of alkyl aromatics.

TOFMS EIC m/z 92 (alkylbenzenes) and m/z 106 (alkyltoluenes) are presented in Figure 2. The distributions from ϕ-C7 to ϕ-C33 and ϕT-C7 to ϕT-C30 were observed for SEAL1 and SEAL2, respectively. The identification of nalkylbenzenes, n-alkyltoluenes, and branched-alkyl aromatics

was supported by the elution order on GC×GC−TOFMS and GC−MS/MS (Figure S3) and mass spectral fragmentation patterns. These compounds were identified by molecular ion (DBE = 4, ϕ/ϕT-CnH2n+1) and fragments m/z 91 and 105 and m/z 92 and 106, corresponding to the β-cleavage of the alkylE


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Energy & Fuels substituted aromatic ring and the McLafferty rearrangement in the aromatic ring, respectively.9,20,21 The representative alkyl aromatic mass spectra are shown in Figure 3. In addition, EIC m/z 92 and m/z 106 show the presence of phytanyl arenes (Figure 2). These compounds were identified by molecular ions and fragments (e.g., Figure 3C and 3D) and comparison with literature.45,46 The phytanylbenzene (ϕ-I20), phytanyltoluene, and related compounds show a slightly lower 2 tR on the GC×GC−TOFMS chromatograms in relation to the n-alkyl aromatics, as shown in Figure 2. Similarly, Gros et al.47 verified the GC×GC separation of the n-alkanes, methylalkanes, and isoprenoids series using nonpolar and midpolar columns on 1D and 2D, respectively. Using this GC×GC column set, the branched alkanes and acyclic isoprenoids present lower 2D elution time than their n-alkane counterparts. In any case, the increase in the branching of the hydrocarbon chains apparently leads to a lower 2tR,48 as observed for n-alkylbenzenes (ϕ-Cn) and phytanylbenzenes (ϕ-In) (Figure 2). The presence of alkyl substituents on the hydrocarbon chain induces the decrease in the van der Waals interaction with the stationary-phase column in 2D.29 Furthermore, the diaromatic compound series (DBE = 8), that elutes between 3.0 and 3.5 s in the 2D, were observed in GC×GC−TOFMS EIC m/z 92 and 106 (Figure 2). The mass spectra are characterized by molecular ions and fragments m/z 91/92 or 105/106 (Figure 3G and 3H). These diaryl compounds present the same diagnostic ion (m/z 92 or 106) and similar 1 t R for the early series of n-alkyl monoaromatics, that is, this coelution problem can influence the identification of the entire series using traditional 1D-GC. To the best of our knowledge, the identification of a complete series of α,ω-bisphenylalkanes is reported for the first time in crude oils. In previous works, C24−C26 bisphenylalkanes, C25− C27 phenyltolylalkanes, and C26−C28 phenylxylylalkanes were reported in Utrillas sulfur-rich coal (middle Albian, Lower Cretaceous) from the Maestrazgo Basin, NE Spain.9 It is important to highlight that the presence of these diaromatic compounds and aryl isoprenoids is remarkable in marine crude oil from Sergipe-Alagoas (Brazil) when compared to lacustrine oil (Figure 2). Figure 4 shows the SRM chromatographic profile for nalkylbenzenes based on the precursor ion (M+•) and product ion (m/z 92) transitions in SEAL1. GC−MS/MS analysis in the SRM mode was applied to ratify the n-alkylbenzenes in the aromatic fraction because of the interference of diaryl compounds, as shown in Figure 2. Moreover, the coinjection of C21 n-alkylbenzene to the aromatic fraction of SEAL1 is shown in Figure 4. Identification of C21 n-pentadecylbenzene (ϕ-C15) is evidenced by the selective SRM chromatogram m/z 92 and increase in the peak area for the SRM chromatogram m/z 302 → 92 transition. 3.3. Geochemical Importance of Alkyl Aromatics. The presence of aryl isoprenoids, aromatic carotenoids, and related compounds in crude oils, rocks, and sulfur bacteria3,49,50 have been related to anoxic conditions in the euphotic zone. In general, the diaromatic compound carotenoid-like derivatives have been detected in marine geological samples from highly reducing paleoenvironmental conditions, as observed for SEAL1. Although the origin of α,ω-bisarylalkanes is unknown, the presence of these aromatic hydrocarbons may be linked to the contribution of marine organic matter and photosynthetic green sulfur bacteria input.9

Figure 4. SRM chromatograms of n-alkylbenzenes for SEAL1 and coinjection of C21 n-alkylbenzene standard. ϕ-Cn: n-alkylbenzene.

The origin of isoprenoid benzenes has been formalized by the diagenetic transformation of isoprenoid quinone precursors or direct biosynthesis by specific archaebacteria.45 Recently, the abundance of phytanyl benzene and phytanyl toluene was reported in mudstones from Permian−Triassic Boundary sections, and its occurrence was associated to marine organic matter deposition in mid-paleolatitudes and microorganisms that could live under drastic paleoenvironmental conditions.46 Several organic matter sources (Gloeocapsamorpha prisca,20 Curtobacterium pusillum,51 Alicyclobacillus spp,52 and Phormidium ectocarpi53) and biomass (i.e., olefin, triacylglycerol, fatty acids, macromolecules, and biopolymers)4,5,19 may contribute to the formation of alkyl aromatics in geological samples. Furthermore, the origin of n-alkylbenzenes has been proposed on the basis of the transformation of alcohols and saturated and unsaturated fatty acids.24,54 Williams et al.55 verified a direct relationship between the homologous series of n-alkylbenzenes and n-alkylcyclohexanes in Michigan oil (United States). The similar distribution for these hydrocarbons suggests a common precursor, a syngenetic origin, or interconvertibility.54,56 Zhang et al.57 observed that the organic matter source and thermal maturation may affect the distribution of n-alkyl aromatics. Additionally, the n-alkylbenzenes series in SEAL1 (marine OM) is similar to its distribution of n-alkanes (Figure 1). Meanwhile, the n-alkylbenzenes profile for SEAL2 (lacustrine OM) is significantly different from its distribution of n-alkanes, but similar to the n-alkylbenzenes fingerprint for SEAL1. This is indicative of the specific source contribution of n-alkyl aromatics in both crude oils. Thus, the n-alkyl aromatics profile for SEAL2 suggests oil mixing or marine organic matter input in lacustrine depositional paleoenvironment. This is supported by the transgressive marine episodes on SergipeAlagoas Basin during the Cretaceous period.58,59 F


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4. CONCLUSIONS The detailed geochemical and molecular investigation allowed the characterization of two distinct source rock depositional paleoenvironments from the Sergipe-Alagoas crude oils. Hopane/sterane, Tr26/Tr25, TPP30/Dia27, and Tr23/H30 values, including the detection of dinosteranes and marine sulfur biomarkers, were useful indicators for crude oil classification derived from marine and lacustrine organic matter. In addition, the preserved aryl isoprenoids and β-carotane associated with low Pr/Ph and H34/H35 ratios revealed anoxic conditions in the Sergipe-Alagoas Basin. The application of GC×GC−TOFMS and GC−MS/MS allowed the identification and assessment of the alkylbenzenes and alkyltoluenes series in crude oils from the Sergipe-Alagoas Basin. GC×GC−TOFMS was useful for geochemical characterization of the aromatic fractions from marine and lacustrine oils because this technique allows better resolution of complex samples. In addition, phytanyl arenes and a new series of α,ωbisarylalkanes were identified in these oils, suggesting contribution of marine organic matter. The coinjection of the synthetized C21 n-pentadecylbenzene allowed its confirmation and assisted in the identification of the homologous series of n-alkylbenzenes by GC−MS/MS. The distribution of alkyl aromatics in two-distinct endmember oils from the Sergipe-Alagoas Basin may be associated to oil mixture, marine organic matter input, or specific organic matter source. The similar profiles for n-alkanes, nalkylcyclohexanes, and n-alkylbenzenes constitutes a potential rule of the formation of alkyl aromatic hydrocarbons in Sergipe-Alagoas crude oils. Thus, the investigation of aromatic fractions from the Brazilian Cretaceous oils raises new assignments to the organic matter contribution and its geochemical significance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b03268. Figure S1: GC−MS/MS SRM m/z 191 and 217 for SEAL1 and SEAL2; Figure S2: GC−MS/MS SRM m/z 134 and m/z 237 for SEAL1 and SEAL2; Figure S3: GC−MS/MS SRM m/z 92 and m/z 106 for SEAL1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +55-86-32372202 (A.K.S.M.). *E-mail: [email protected] (B.Q.A.). ORCID

Bruno Q. Araújo: 0000-0002-6209-2102 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank CNPq, CAPES, and FAPERJ (Brazilian research councils) for scholarships and financial support. Special thanks go to the anonymous reviewers for their comments. The authors are grateful to A. A. de Souza for NMR analysis. G


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An assay on alkyl aromatic hydrocarbons: Unexpected group-type

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