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Distribution of Oxygen-containing Compounds and its Significance on Total Organic Acid Content in Crude Oils by ESI negative ion FT-ICR MS Fernando A. Rojas-Ruiz, and Jorge Armando Orrego-Ruiz Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01597 • Publication Date (Web): 14 Sep 2016 Downloaded from http://pubs.acs.org on September 18, 2016
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Distribution of Oxygen-containing Compounds and its Significance on Total Organic Acid Content in Crude Oils by ESI negative ion FT-ICR MS Fernando A. Rojas-Ruiz,† Jorge A. Orrego-Ruiz,*† †
ECOPETROL, Instituto Colombiano del Petróleo, Piedecuesta, Santander 681018, Colombia.
ABSTRACT In the present work the distribution of oxygen compounds on the total organic acid content of ten crude oils was assessed by means of Negative Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry ((-) ESI FT-ICR-MS). At a first attempt the relative abundance of O2 class was related with Total Acid Number (TAN) of samples following the state of art, and none positive correlation was achieved. Therefore, we addressed to perform the selective isolation of acidic compounds via solid phase extraction using Amino-Propyl Silica (APS) finding an acceptable correlation (R2=0.98) between acidic fraction percentage and TAN. Both reliability and performance of the APS method was confirmed using a chosen sample as control. FT-IR spectroscopy was employed to validate the acidic nature of the isolated fraction. In the IR spectrum of the acidic fractions characteristic signals of carboxylic acids such as the sharp band around 1700 cm-1 and the wide band around 2300-3500 cm-1 were identified. Additionally in such fraction oxygenated classes such as O2, NO2, O3, SO2 and O3S were detected through (-) ESI FT-ICR-MS. Nevertheless, it can be said that none of these classes exclusively belong to the acidic fraction since for instance, O2 and NO2 compounds were found
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in either non-acids or acid fractions. In this sense, some O2 compounds may be considered to be alcohols, phenols, ketones or ethers bi-functionalized. Finally, by comparing the contour plots DBE vs. Carbon number of chosen samples, it was possible to infer that the contribution of O2 class over the TAN is structure dependent for samples with TAN lower than 0.5 mg KOH/g. Thus the DBE distribution within the acidic and non-acidic fractions must be carefully considered in order to estimate their relevance over the total acid content. 1.
INTRODUCTION
Cost-effectiveness relationship is the most outstanding factor for oil companies. The quality of crude oils - usually measured by density, viscosity, sulfur content and/or acidity - is decisive to establish that relationship.1-3 In this sense, high acidity crude oils receive special attention, since they represent both an economic opportunity and a technical challenge by being considered to be included in the diet of refineries.4-6 In consequence, substantial efforts must be done by companies to obtain more precise information regarding crude oil quality in order to attain a rational selection of crude oils and processing decision making.7-9 Traditionally, the lightness measured as density (API) and the sweetness measured as sulfur content have been the main quality parameters employed by oil companies and industry consultants.2,5 However, the impact of acidity (measured as the Total Acid Number TAN) on the quality/price differentials must be considered since the volume of high acidic crude oils has steadily increased in recent years.3,6,8 TAN measurement implies a titration where all the acidic compounds are neutralized by a strong base (KOH). Among these, the naphthenic acids (NAPS) are the most relevant acidic compounds. NAPS can cause corrosion in oil refining as well as production deposits and emulsion stabilization.10,11 Molecular level characterization of NAPS can be crucial to afford
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better selection criteria of initial crude oils. Determining the ring type (DBE) and carbon number distributions of NAPS might help to understand the corrosivity of crude oils as this property depends on the size and the structure of their molecules.12-14 The distribution of O2 class in crude oils with different TAN has been measured by laser desorption ionization Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry LDI FT-ICR MS.15 However, NAPS have been particularly studied using ESI (-).16-18 Different authors have reported the characterization of NAPS and other acid-extractable in water samples.19 Qian et. al. reported the identification of almost the total of acidic species present in a south American crude oil using negative mode ESI.20 They found that the acidic species comprised classes O2, O3, O4, and O2S, as well as O3S and O4S, with aromatic structures bearing 1-3 rings, cyclic moieties with 1-6 rings and carbon numbers between C15 and C55. Other authors have described acceptable correlations between total acid number (TAN) and the relative abundance of O2 class detected by ESI (-).21-23 Additionally, different efforts have been done on the selective isolation and characterization of the acidic species present in petroleum.24-28 Solid-phase extractions (SPEs) have been used for acid isolation from crude oils.25,29 Rowland et. al. developed two methods to fractionate NAP acids. Aminopropyl silica (APS) method isolates acids collectively in one fraction, whereas modified APS (MAPS) isolates NAP acids in six fractions corresponding to different molecular weight ranges.29 Encouraged by the facts mentioned above, we performed the isolation and (-) ESI-FT-MS characterization of acids from ten South American crude oils using APS method. Recovery
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percentage, repeatability of the method as well as a correlation between % of acidic fraction and TAN are reported. 2.
EXPERIMENTAL DEVELOPMENT
2.1. Materials Ten crude oils were chosen in order to afford the most compositional variability in O2 compounds. API Gravity and Total Acid Number (mg KOH/g) were either determined according to ASTM D287 and ASTM D664 methods, respectively. Analytical grade toluene and methanol were used as solvents for sample preparation, which were distilled twice and kept in glass bottles with ground glass stoppers before usage. 2.2. Acidic compounds isolation using amino propyl-silica (APS) Acidic fraction isolation was performed as follows. Depending on the TAN of the sample, an initial mass ranging from 100 to 500 mg of crude oil was dissolved in 1 mL of dichloromethane (DCM) and 2.0 g (10 mL) amino-propyl silica (APS) cartridge (Agilent technologies part # 14256012) was pre-treated with 5 mL of DCM. Posteriorly, the sample was transferred to the APS cartridge and formerly the first fraction (F1) containing the non-acidic compounds was eluted using 18 mL of a 50:50 methanol:DCM mixture. Subsequently, a second fraction (F2) containing the acidic compounds was eluted and recollected using 18 mL of 50:50 methanol:DCM with 5 % of formic acid. Each fraction (F1 and F2) was collected on different pre-weighted flasks for further mass quantification. Recovery percentages were calculated according to the equation 1. Where Wfx corresponds to the weight in milligrams for the fraction x and Wcrude refers to the initial weight of the oil sample. ܴ݁ܿ % = [ܹfݔ/ܹܿ݀ݑݎe] x 100%
Equation 1
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2.3. FT-ICR MS analysis FT-ICR MS analyses were performed using a 15 T SolariX FT-ICR mass spectrometer from Bruker Daltonics (Billerica, MA). Nitrogen was used as drying and nebulizing gas. Argon was used in the collision cell and the prepared samples were directly injected with a syringe pump (Harvard, Holliston, MA). Stock sample solutions were prepared by dissolving the samples to a concentration of ~10 mg/mL in toluene. For ESI analysis, 0.2 mg/mL (whole crude and F1) and 0.05 mg/mL (F2) solutions in methanol: toluene (50:50) were used and spiked with 1 % (V/V) ammonium hydroxide before sample loading, to improve ion detection in ESI negative ion mode. External calibration was performed using a 0.05 mg/mL sodium trifluoroacetate solution in methanol. It was employed a flow rate of 450 µL/h, drying gas temperature of 200 °C and spray voltage of 4500 V; -60 V for skimmer voltage and a time of flight window of 0.7 ms. The resolving power reached for all mass spectra was higher than 400000 at m/z 400. RMS error of the calibration and RMS error of the molecular formula calculation were below 0.1 ppm and 0.5 ppm, respectively. 2.4. FT-IR measurements. FT-IR spectra were recorded from samples infused as dichloromethane drops after its evaporation. The acquisition was done in an Alpha spectrometer from Bruker with a spectral resolution of 4 cm-1 over the range of 4000-650 cm-1 by the accumulation of 32 scans. Attenuated total reflectance (ATR) diamond cell with single reflection and angle set up to 45° was used.
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2.5. Mass Calibration and Data Analysis Data was acquired in broadband mode using 4 megaword data sets, with each mass spectrum resulting from the sum of 100 scans. Internal spectral calibration was performed using two homologous series -CcHhO2 DBE 3 and CcHhN DBE 12- using Data Analysis version 4.0 (Bruker Daltonics). Peaks with relative abundance higher than 10 times the S/N were exported to excel in .acs format. Compositional assignment was done using Composer software version 1.0.6 (Sierra Analytics, Modesto, CA, USA) with 1 ppm tolerance. 3. RESULTS AND DISCUSSION 3.1. Sample Characterization The Total Acid Number (TAN) and density (°API) for all crude oils are shown in table 1. TAN values are distributed between
