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Characterization of naphthenic acids in thermally degraded petroleum by ESI(-)-FT-ICR MS and 1H NMR after solid phase extraction (SPE) and liquid/liquid extraction Eliane V. Barros, Heloisa P Dias, Fernanda E Pinto, Alexandre de Oliveira Gomes, Robson R Moura, Alvaro Cunha Neto, Jair C. C. Freitas, Gloria M. F. V. Aquije, Boniek Gontijo Vaz, and Wanderson Romão Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03099 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018
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Graphical Abstract
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Characterization of naphthenic acids in thermally degraded petroleum by
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ESI(-)-FT-ICR MS and 1H NMR after solid phase extraction (SPE) and
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liquid/liquid extraction
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Eliane V. Barros,a,b Heloisa P. Dias,a Fernanda E. Pinto,a Alexandre O. Gomes,c Robson R.
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Moura,c Alvaro C. Neto,a Jair C. C. Freitas,d Gloria M. F. V. Aquije,e Boniek G. Vaz,f Wanderson
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Romão,a,e†
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a
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Santo, 29075-910 Vitória, ES, Brazil.
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b
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ES, Brazil.
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c
CENPES, Petrobras, 21941-598 Rio de Janeiro, RJ, Brazil.
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d
Laboratory of Carbon and Ceramic Materials, Department of Physics, Federal University of
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Espírito Santo, 29075-910, Vitória, ES, Brazil.
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e
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Velha, ES, Brazil.
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f
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Correspondent author: †
[email protected] / Phone: + 55-27-3149-0833
Petroleomic and Forensic Laboratory, Chemistry Department, Federal University of Espírito
Federal Institute of Education, Science and Technology of Espírito Santo, 29056-255 Vitória,
Federal Institute of Education, Science, and Technology of Espírito Santo, 29106-010 Vila
Chemistry Institute Federal University of Goias, 74690-900, Goiânia, GO, Brazil.
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ABSTRACT. Naphthenic acids (NAs), present in a typical Brazilian acid crude oil and its
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thermal degradation products, were investigated using two separation methodologies: solid phase
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extraction (SPE) and liquid-liquid extraction (LLE). Fractions produced were characterized by
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proton nuclear magnetic resonance spectroscopy (1H NMR) and negative-ion mode electrospray
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ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI(-)-FT-ICR MS).
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Among the NAs extraction methods studied, the SPE was more efficient about LLE. Besides, the
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ESI(-)FT-ICR MS results showed that the SPE method with eluent phase variation covered the
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detection of a larger amplitude of NAs compounds (m/z 200-1200), reducing the occurrence of
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ion suppression on the NAs of higher average molecular weight distribution (Mw). It was possible
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to note that the aromaticity or double bond equivalent (DBE) of these produced collective
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fractions as well as their Mw values increased as a function of the polarity of the extraction
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system (DCM → DCM:MeOH:FA). Also, 1H NMR analysis revealed the alkyl predominance
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evidenced by the presence of high Hβ content in fractions, suggesting that the NAs compounds
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have long and unbranched chains. The behavior of NAs species during the thermal degradation
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process was also evaluated, and the results showed the presence of them in only five SPE extracts
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out of six, containing different Mw values (Mw = 366, 417, 531, 662, and 836 Da). This suggests
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that in the last SPE fraction (named SF6: m/z 700-1150; carbon number of C52-C72 and DBE = 0-
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15; detected only in virgin crude oil), the NAs were selectively cracked during the thermal
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degradation process.
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Keywords: petroleomics, naphthenic acids, extraction methods, SPE, ESI(-)FT-ICR MS, 1H
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NMR.
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1. Introduction
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The presence of naphthenic acids (NAs) in petroleum implies in a significant impact on its
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productivity and economic value, since under certain conditions it may favor the formation of
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emulsions and the corrosion of materials.1 In addition, NAs can precipitate as metal salts
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(naphthenates), where the most common being calcium naphthenate.1,2 This type of deposit is
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problematic, being formed in oil production systems with high total acid number (TAN).2
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The NAs represent an average of around 2 to 4 wt. % of the crude oils, being composed of
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a complex mixture of monocarboxylic acids of chemical formula defined as CnH2n+zO2, where n
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represents the number of carbon (CN) atoms and z indicates the deficiency of hydrogen elapsed
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from the cyclization of the structure, where z ≤ 0.3 Despite this definition, the term NA is also
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commonly used in a broader sense, encompassing all organic acids (cyclic, acyclic and aromatic)
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found in crude oil. The identification and characterization of NAs in an oil matrix have shown to
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be quite challenging since a single crude oil sample can contain about 1,500 different NAs with a
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range of their molecular weight from 115 to 1500 Da.3,4,5
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The literature reports some concern on the behavior of NAs present in crude oils subjected
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to high temperatures. This is because crude oils with TAN above 0.5 mg of KOH g-1 oil are
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considered unsuitable for processing in traditional refineries. Although the corrosion does not
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present a linear relation with the TAN; in the most cases, the TAN directly influences in
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corrosion rate of petroleum.6 Based on classical and high-resolution analytical techniques, Barros
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et. al.7 concluded that when heavy and acidic crude oils are exposed to long periods of thermal
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degradation (t ≥ 24 h), they have characteristics of "lighter" oils due mainly to the disaggregation
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of resins and asphaltenes and decarboxylation of NAs, thus favoring, the TAN reduction in a 4 ACS Paragon Plus Environment
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level of 68% after 36 h of degradation at 350 ° C. 6, 7 Therefore, at high degradation temperatures
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(T ≥ 280 oC), the crude oil undergoes thermochemical reactions favoring the decarboxylation of
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NAs (CO2 elimination) and the formation of light hydrocarbons (HCs).7 In this context, Fu et al.8
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proposed a possible mechanism of thermal decarboxylation of NAs, which can be explained in
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three steps, as shown in Figure 1a. Initially, NAs are deprotonated to produce carboxylate anions
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(reaction 1), which are stabilized by resonance. Then, a heterolytic cleavage occurs on the CO2
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group forming negatively charged alkyl radicals (reaction 2). Finally, the radicals combine with
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H+ species to form the HCs (reaction 3). Another possible pathway is via [1,4]H rearrangement
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as illustrated in Figure 1b. By this mechanism pathway, carboxylic acids with low CNs are
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formed.
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Figure 1. Mechanism of thermal decarboxylation of NAs, adapted from Fu et al.8 (a) and
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proposed by the present study (b).
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Some studies highlight advances in the use of analytical instrumentation for analysis of
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NAs,9-21 where the main techniques to characterize them are: gas chromatography coupled to 6 ACS Paragon Plus Environment
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mass spectrometry,9,10 Fourier transform infrared spectroscopy,11 nuclear magnetic resonance
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(NMR) spectroscopy12,7 and high-performance liquid chromatography.9 Besides these techniques,
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some modern methods have been applied due to their high selectivity, such as two-dimensional
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gas chromatography associated with time-of-flight mass spectrometry,10 tandem mass
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spectrometry with electrospray ionization source,17 high-performance liquid chromatography
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coupled with time mass spectrometry,18,19 and Fourier transform ion cyclotron resonance mass
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spectrometry (FT-ICR MS).7,20,21
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FT-ICR MS provides an ultra-high mass resolution power and mass accuracy that results
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in high degree of confidence in the molecular weight assignments and, consequently, being
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suitable for the complex mixtures analysis in petroleomics.3
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negative ion-mode electrospray ionization (ESI(-)), is often used for polar compounds analysis,
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generating deprotonated molecules, which are heteroatom species such as NAs, phenols, and
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species with carbazole core structure. Another powerful analytical technique for the analysis of
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petrochemicals is NMR spectroscopy, which is widely used for structure elucidation and
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confirmation of organic compounds.3,5,7,13,14 Its coverage ranges of oil samples, from the
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characterization of lighter fractions (jet fuels and gas oils) to heavier fractions of oil (vacuum
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residues, crude oils, tar pitches, resins, and asphaltenes),13,15,16 allowing a fast, non-destructive
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and quantitative analysis.
FT-ICR MS, associated with
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However, prior to the characterization of petroleum fractions, such as NAs, three
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separation methods have been commonly employed: liquid-liquid extraction (LLE), solid phase
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extraction (SPE), and the preparative liquid chromatography.9,11,23-26 In the LLE, alkaline
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solutions are usually employed to obtain NAs fractions.11,27,28 Colati et al.11 proposed, to isolate
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NAs by LLE, washing with alkaline hydroalcoholic solutions, achieving selectivity and acid
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removal efficiency with reductions in the TAN of around 90% at pH 14. On the other hand, the
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SPE method allows the separation of NAs using a wide range of materials, which, the literature
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highlights: zeolites, clays, aluminosilicates, silica gel, granulated activated carbon, and ion-
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exchange resins.28 Rowland et al.21 employed an SPE methodology to obtain NAs subfractions
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using aminopropyl silica, reaching a separation of NAs by their polarity differences and by
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distinct Mw ranges, being characterized by ESI-FT-ICR MS.21
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In the present work, we investigate NAs species present in an acid crude oil and its
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thermal degradation products obtained by two separation methodologies, which are SPE and
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LLE, where the produced fractions were characterized by 1H NMR and ESI(-)-FT-ICR MS. The
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NAs corresponding to linear, naphthenic and aromatic chain molecules.
H NMR spectra of some acid fractions were compared to those of the commercial standards of
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2. Materials and methods
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2.1 Chemicals
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All chemicals employed were of analytical grade (purity higher than 99.5%). Potassium
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hydroxide, toluene, diethyl ether, chloridric acid, n-heptane, and dichloromethane were supplied
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by Vetec Fine Chemicals Ltda, Brazil. Petroleum ether, ethanol, and methanol were purchased by
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Dinâmica Química Contemporânea LTDA, Brazil. Formic acid, dichloromethane-d2 (99.5 atom
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% D, contains 0.03 wt%, tetramethylsilane, TMS), ammonium hydroxide, sodium trifluoroacetate
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and NAs standards (decanoic acid, pentadecanoic acid, stearic acid, arachidic acid, cyclopentane 8 ACS Paragon Plus Environment
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carboxylic acid, cyclohexane-1,1-dicarboxylic acid, 3-cyclohexane propionic acid, naphthoic acid
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and naphthenic acetic acid) were purchased from Sigma-Aldrich Chemicals, USA. A mega bond
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elut-NH2 cartridge, 2g x 12 mL, was supplied by Agilent Technologies, USA.
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2.2 Characterization and thermal degradation of petroleum
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In this study, crude oil (A0) was selected from a production field located in a sedimentary
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basin on the Brazilian coast. It was characterized in the facilities of the Petroleum
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Characterization Laboratory of the Federal University of Espírito Santo (Labpetro/UFES),
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following the American Society of Tests and Materials (ASTM). Thus, the API gravity was
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obtained according to ISO 12185 (1996)29 and TAN was measured according to ASTM D664
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(2011) by potentiometric titration (Metrohm 836 automatic titrator).30 The thermal degradation
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assays were performed using an autoclave containing approximately 1000 mL of the oil and
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heated to 320 °C during degradation times of 24 and 72 h, producing two degraded samples
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named A24 and A72.
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2.3 Extraction and characterization of NAs
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The acid fractions were extracted from samples A0, A24, and A72 by the LLE method
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proposed by Colati et al.,11 and by SPE method proposed by Rowland et al.21 Figure 2a-c shows
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an overall scheme of the extractions performed in this study. In the SPE procedure, fractions
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resulting from clean up steps – CU1 and CU2 (Figure 2b); as well as CU-I to CU-IV (Figure 2c)
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– were not included in the characterization analysis. Acid fractions of the NAs were characterized
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Figure 2. Scheme of NA extractions: (a) LLE; (b) SPE and; (c) SPE as a function of Mw. (AE –
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aqueous extract, CU – Clean up, EtOH – Ethanol, MeOH – Methanol, FA – formic acid, SF –
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acids subfractions). (Adapted from refs. 11 and 21)
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2.3.1 1H NMR
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H NMR spectra were acquired on a Varian (VNMRS 400) spectrometer, operating at a
9.4 T magnetic field using a 5mm Broad Band 1H /19F /X probe at 25.0 ºC.
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For the analysis of the nine commercially available NA standards, a mass of 20-30 mg of
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each standard was dissolved in 600 µl of dichloromethane-d2 (CD2Cl2). The analyses were
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performed under the following experimental conditions: 1H NMR frequency of 399.73 MHz;
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6410.3 Hz spectral window; 90º pulse with a duration of 12.2 µs; 3.8339 s acquisition time; 64
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scans; and TMS as the reference standard.
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The crude oil samples and their respective NAs fractions were also dissolved in CD2Cl2,
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according to Barros et al.7 The analyses were carried out under the following experimental
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conditions: frequency of 399.73 MHz; 6410.3 Hz spectral window; 90º pulse with duration of
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9.32 µs; 2.5559 s acquisition time; 512 scans; and TMS as the reference standard.
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2.3.2 ESI(-)-FT-ICR MS
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The analyses were carried out on a Bruker Daltonics® mass spectrometer (model 9.4 T
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Solarix, Bremen, Germany), equipped with a commercially available ESI source, configured to
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operate over a mass range of m/z 200-2000. The FT-ICR mass spectra of samples were acquired
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using the negative ionization mode, ESI(-).
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For the analysis of the samples obtained from LLE, 2 mg of the acid fraction were
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dissolved in 1 mL of toluene. Then, 0.5 mL of this solution was mixed with 0.5 mL of methanol
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(1:1) at 0.1% m/v of ammonium hydroxide (NH4OH), similar to the procedure described by
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Colati et al.11 The acid fractions obtained by SPE were dissolved up to 1 mg mL-1 in a toluene:
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methanol solution (1:1) containing 0.25% m/v of NH4OH.
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The ESI(-) conditions were: nebulizer gas pressure of 1.5 bar, a capillary voltage of 3.9
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kV and transfer capillary temperature of 250 ºC. The ion accumulation time in the hexapole was
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0.1 s, and each spectrum was acquired by accumulating 200 scans of time domain transient
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trapping voltage of the ICR cell was -0.60V. All mass spectra were externally calibrated using
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0.05 mg mL-1 of a sodium trifluoroacetate (NaTFA) solution (m/z from 200 to 1200), followed by
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internal recalibration using a set of more abundant homologous alkylated compounds for each
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sample. The resolving power (m/∆m50% ≈ 550 000, where ∆m50% is the full peak width at half-
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maximum peak height) of m/z = 400 and mass accuracy
