Compositional Analysis of High-TAN Sudanese Crude Oil Using High-Resolution Mass Spectrometry and Study of the Effect of Temperature and Catalyst on Acidic Composition

Sudanese Fula crude oil, from the western region, is considered highly viscous and acidic and contains high amounts of heteroatoms (N and O) but a low sulfur content. This work presents an original and comprehensive analysis of its molecular composition in addition to an investigation of the effect of temperature and catalyst on the treatment of the acid fraction. The analysis was performed using a high-resolution Fourier transform mass spectrometer and Orbitrap-Elite with different ionization methods. The results reveal that the Fula crude oil contains a high abundance of nitrogen composition homologue classes N[H], NO2[H], and NO[H]. Their hydrocarbon composition includes low to high aromatic hydrocarbons. The number of oxygen classes varies from acids containing monocarboxylic acids of O2 to acids of multiple carboxylic and phenolic group (CxHyO3 to CxHyO15) classes, which indicate a high content of acidic moiety of 0.765%. In addition to oxygen classes, the acidic fraction that is present as a NOx series indicates the presence of carboxylic carbazole acidic fraction. Low-temperature crude oil treatment at 200 °C decreases the intensity of acids. No significant reduction to low masses was observed; however, there was a clear reduction to high masses. At a high temperature of 350 °C, the carboxylic acid intensity increases (O2 classes), and thus, heating crude oil to 350 °C is unfavorable as it increases the amount of monocarboxylic acids, which are primarily responsible for corrosion in refinery units. Predicted TAN values of residual samples show a reduction in TAN of 62% using thermal treatment at 200 °C, whereas there is an increase in TAN of 5% at 350 °C. A great reduction in acidity results from catalytic treatment with a transition metal catalyst of cobalt and iridium complex. A reduction in all acidic oils is observed; however, the greater reduction is found in mono- and dicarboxylic acids. Catalytic treatment is shown to result in an 85% reduction in predicted TAN values.


INTRODUCTION
The depletion of highly valuable crude oil, with low hydrocarbon molecular weight, low viscosity, and low amounts of heteroatoms (e.g., nitrogen, oxygen, and sulfur) and metals, is driving the global consideration of heavy oil with lower quality. 1 Sudanese crude oil is considered as one of the heaviest crude oils in the world, with a high acid content of 3.68 mg KOH. 2 This value is considerably high as a total acid number (TAN) of greater than 0.5 mg KOH/g is considered as highly acidic and thus more corrosive. 3,4 High-TAN crude oil results in an increased rate of corrosion of refinery units where highly acidic oil is treated at high temperatures to produce valuable hydrocarbons. High acidity constitutes one of the main challenges faced by the oil and gas industry. 5 Sudanese Fula crude oil samples, from the western region, were used. The source of the crude oil is originally from Muglad basin. It is formed from inland fluvial deposition. Reservoirs in Muglad basin are considered as conventional reservoirs. They are thin sandstone layer intersects with shale in depths varying from 700 to 3000 m. The crude oil produced from Fula oilfields is paraffinic oil in nature. The acids are present in different structures, varying from mono-and dicarboxylic acids to cyclic and aromatic acids. It is clear that revealing the structure of those acids helps in their treatment. 6,7 The reported acidic distribution of western Sudanese crude oil 8 describes the distribution of O 2 class only. As it is a highly acidic oil, one may expect to see more acidic fractions in this type of oil. Li et al. 9 characterized acids in Muglad basin, Fula sub-basin, Sudan, using FT-IR, FT-ICR-MS, and GC-MS. They revealed a correlation of TAN and bulk and molecular composition with reservoir depth. Acids from O-O4 were determined with a high abundance of O 2 class.
One of the methods of treatment is decarboxylation with the use of a catalyst. This method has shown wide success, however with some restrictions. The use of a metal oxide catalyst is proven for low acidic crude oil. 10 The use of a Cu/Ce/Al 2 O 3 catalyst calcined at 1000°C 11 is effective in decreasing the high TAN of crude oil to less than 1. However, addition of many dopants, required as metal oxides, and high calcination temperatures are not recommended in large-scale treatment. A proton reduction catalyst has been studied earlier 12,13 in stoichiometric addition of oxidants such as lead tetraacetate 14 to convert a carboxylic compound to olefins. Sun et al. 15 used a dehydrogenative decarboxylative catalyst in nonstoichiometric addition to produce alkenes from carboxylic acids. The main benefit of this method is its applicability in a large scale. Here, we apply this method to our large classes of acids to reduce their presence in the oil. In this study, Sudanese crude oil from Fula basin is studied in detail. The use of ultrahigh-resolution mass spectrometric methods provides a clear advantage over other methods in terms of higher resolving power, mass accuracy, and sensitivity. The Orbitrap-Elite technology enables the discovery of new acid classes, which will facilitate the future treatment of high-TAN crude oil.

MATERIALS AND METHODS
2.1. Identification of Nitrogen-Containing Compounds Using Electrospray Ionization (+) Methods. The Fula crude oil sample was stored under argon. A concentration of 250 μg/mL was prepared for all samples by taking 25 mg of the sample, which was dissolved in toluene (25 mL, HPLC grade, Overlack, Germany, 99%) to obtain a concentration of 1000 ppm. A 250 mL solution was diluted with 750 mL of methanol (HPLC grade, Baker, Germany, 99.8%) + toluene to reach a final concentration of 250 μg/mL. Mass spectrometric analysis was performed on a 7 T linear trap quadrupole (LTQ) FT-ICR mass spectrometer (Thermo Fisher Scientific, Bremen, Germany) equipped with an ESI source with a resolving power of R = 400,000 (FWHM at m/z 400, 1.5 s transient). Electrospray (ESI) in positive ion mode was used as an ionization method. The ionization was performed with a stainless steel needle capillary with a voltage of 4 kV for the crude oil with a molecular weight of 200−1200. Nebulization was assisted by a sheath gas flow of 7 a.u. (arbitrary units), while the auxiliary and sweep gas flow rates were set to 5.0 a.u. for Fula crude oil and the temperature was set at 275°C.
2.2. Identification of Hydrocarbon Compounds Using Atmospheric Pressure Photoionization (APPI) Methods. APPI (+) was analyzed using an Orbitrap-Elite (Thermo Fisher, Bremen, Germany) with a resolving power of R = 480,000 (FWHM at m/z 400, 1.5 s transient). The sample was infused with the flow rate of 20 μL/min for APPI (+) measurements and evaporated at 275°C with a continuous sheath gas flow of 25 a.u. (arbitrary units) and auxiliary gas flow of 10 a.u. In the APPI (+) measurement, an APPI lamp was connected to the Orbitrap with a continuous sheath gas flow of 40 a.u. (arbitrary units) and an auxiliary gas flow of 20 a.u. with a molecular weight of 200− 1200. The shift to the Orbitrap-Elite method was mainly due to the high ppm error of the resulted composition. Consequently, a much higher accuracy was achieved.
2.3. Identification of Oxygen-Containing Compounds Using Electrospray Ionization (+) Methods. 2.3.1. Extraction Methods. Liquid−liquid extraction was conducted to extract water-soluble acid compounds. One milliliter of crude oil was mixed with NH 4 OH at a ratio of (1:9 v/v) to pH = 12 for 1 h and mixed at 10,000 rpm using a magnetic stirrer. Subsequently, the mixture was left standing for 24 h to allow the phases to separate. Re-extraction was performed five times via liquid−liquid   The crude oil sample was heated to 200°C and then extracted prior to injection to the mass spectrometer. To treat acids at 350°C, the acids were condensed under argon for 6 h. The acids were extracted with dichloromethane and then injected to the mass spectrometer after evaporating the solvents with a rotatory evaporator.

Prediction of TAN Values.
The TAN values of crude oil residues, following thermal and catalytic treatment, were predicted based on the relation proposed by Terra et al., 16      compositions, showing the difference in intensity for more clearance.
The data was collected in a full mass range. Figure 4 shows the spectrum of molecular composition assigned by ESI (+). The spectrum is expanded to show the highest intensity peak of nitrogen compositions C 27 H 38 N and C 52 H 88 N. Figure 5 shows possible chemical structures for nitrogencontaining compounds. Compound (1) (C 15 H 25 N) is a pyridine derivative with DBE = 4 and Mwt = 119.20 commonly found in crude oil. 17 Compound (2) (C 14 H 17 N) with DBE = 7 and Mwt = 199.14 is a quinoline derivative. Compound (3) (C 16 H 17 N) is a carbazole derivative with DBE = 12 and Mwt = 208.127. Due to their polarities, pK a = 6.8 for carbazole 18 and pK a = 4.9 for quinolone 19 are clearly detected by ESI (+). Compound (4) is C 14 H 17 NO.
3.2. Identification of Hydrocarbon-Containing Compounds Using APPI (+) Methods Employing the Orbitrap-Elite. Figure 6 shows [HC] as the dominant class with the highest intensity. Figure 7 shows the distribution of double-bond equivalents of the most dominant nonpolar hydrocarbons versus their carbon number.
Using photoionization techniques such as APPI (1-photon VUV ionization), compounds of nonpolar hydrocarbons, naphthenes, and polyaromatic heterocycles (PAHS and PASHS) such as benzo-and dibenzothiophenes or dibenzofurans with low to medium polarity are effectively ionized by producing ions in radical M +• or protonated [M + H] + form. Figure 8 shows the spectrum of molecular composition assigned by APPI (+), which was expanded to show the highest intensity peak of hydrocarbon compositions C 22 H 12 and C 49 H 84 .
The [O] class is considered as a benzketone group, which can be represented as a radical cation [M +• ] to be determined by APPI (+). This benzketone is a derivative of ketone compound detected by APPI (+) due to the low acidity and basicity of the delocalized π-bond attached to the ketone group, thus decreasing the basic properties.
The [HC] group is a hydrocarbon that appears in the highest intensities. It ranges from aliphatic hydrocarbon with DBE = 1 to highly aromatic compounds with DBE = 50. Figure 9 shows the core structures for polycyclic aromatic sulfur heterocycles     Sulfur compositions ( Figure 6) have a very low intensity in Fula crude oil. Those compositions have a carbon number from 10 to 60, although they have lower molecular weight than the other compounds detected by APPI (+). The DBE values of 3, 6, 9, and 12 are commonly attributed to the thiophene compound (1) (Figure 9), benzothiophene compound (2), dibenzothiophene compound (3), and benzonaphthothiophene compound (4), respectively.
3.3. Identification of Oxygen-Containing Compounds Using the ESI (−) Orbitrap-Elite. Figure 11 shows O 4 [H] as the dominant class with the highest intensity. Figure 12 shows the distribution of double-bond equivalents of the most dominant oxygen classes versus their carbon number. Figure 13 shows the spectrum of molecular composition assigned by ESI (−). The spectrum is expanded to show the highest intensity peak of oxygen compositions C 18 Figure 13). The negative ESI is highly selective for the acidic group, and as a result, the oxygen group is dominant. In Figure 14,    (7) is C 16 H 10 O 4 with DBE = 12, which is known as anthracene dicarboxylic acid. Compounds (8) and (9)  class was detected in low intensities in Fula crude oil by ESI (−), which is considered as a benzketone group with DBE = 10, which is best represented as a radical cation M +• to be detected better by APPI (+), (see Figure 14, compound (9)).
3.4. Temperature Treatment for Acids. Figure 15 shows Three different experiments at three different temperatures were scanned with negative electrospray in almost the same conditions, and the data was recorded in the full spectrum ( Figure 18). The results indicate that the high mass is mostly affected by temperature. The acids were studied under two temperatures (200 and 350°C). The main objective was to investigate the possibility of acid decarboxylation under moderate to high temperatures. Extracted acids contain the highest amount of acids with 29.670 compositions of the oxygen group from O1-O15, while heated crude had an oxygenated group from O1-O9 with 20.794 assigned compositions at 200°C and 6625 at 350°C (Figure 18). The spectrum was zoomed out of the full spectrum to identify the differences in intensities of molecular composition at three different temperatures. The peaks were expanded at low and high masses as shown in Figures 19 and 20. 3      the increasing concentration of the monocarboxylic acids. This could be due to evaporation of CO 2 resulting from the group O9-O15 as monocarboxylic acids. With the expanded mass spectrum (Figure 20), we realize that many carboxylic acids of high molecular weight are highlighted in pink color, which disappear when heating the acid to 200 and 350°C.
3.4.2. O 4 Classes. O 4 species are considered as dicarboxylic acids. In the extracted acid, it has a DBE that expands over 1−30. A DBE of 2 is likely to indicate a dicarboxylic saturated fatty acid as it has the highest intensity over heated carboxylic acids. The species with DBEs of 5 and 6 were the second-most abundant compounds in degraded oil and are likely to be dicarboxylic acids with two and three naphthenic rings, respectively. With the expanded mass spectrum (Figure 19), C 28 H 53 O 4 with DBE = 3 exists when heating the crude oil to two different temperatures with almost the same intensity, although the effect of temperature is not effective in low molecular weight.
3.5. Catalytic Dehydrogenative Decarboxylation of Acidic Crude Oil by Using Negative Electrospray Orbitrap-Elite Mass Spectrometry. Figure 21 compares the intensities of catalyzed oxygenated groups with the acid group in the extracted acid. In the carboxylic acid classes of extracted acid, the intensity decreases for the treated acidic sample. Figure 22 compares the distribution of double-bond equivalents of the most dominant oxygen classes for catalyzed and uncatalyzed   acids. Comparing the molecular composition of DBE versus a carbon number, one realizes that the highest intensity ranges from DBE 1 to 15 and carbon number C 12 to C 35 , while for the same range when treated with the catalyst, the intensity is greatly decreased. For dicarboxylic classes (C x H y O 4 ), the highest intensity classes range from DBE 1 to 25 and carbon number C 15 to C 40 , while for the same range when treated with the catalyst, the intensity decreases due to catalytic transformation of the carboxylic group to alkene. By using a catalyst, for extracted samples of Fula crude oil, a great reduction of mono-and dicarboxylic acids (C x H y O 2 ) is achieved. The reduction in the intensity of the acidic class is also observed in other classes. This is due to the ability of cobaloxime Co(dmgH) 2 (4-OMe-py)Cl to reduce a proton from a carboxylic group.
3.6. TAN Values of Thermally and Catalytically Treated Samples. Figure 23 shows the class distribution of the untreated and treated crude oil samples as categorized by Terra et al. 16 The measured and predicted TAN values are shown in Table 1. It is clear that the catalytic treatment results in the most significant reduction in TAN with a value of 85% relative to the predicted TAN value for the untreated sample. Heat treatment of samples at 200°C results in a 62% reduction in TAN value, whereas heat treatment at 350°C results in an increase of 5% in TAN value relative to the predicted value for the untreated sample.

CONCLUSIONS
Sudanese crude oil contains a highly aromatic hydrocarbon composition with the existence of nitrogen composition homologue groups with high intensity. Their oxygenated composition extends from O[H] to O15[H], which indicates the high presence of acidic composition. The low molecular weight of dicarboxylic acid O 4 [H] is mostly responsible for acid corrosion; however, it is unaffected by temperature. A high temperature affects carboxylic acids at high molecular weights. The largest effect of decarboxylation is realized when acids are treated with dehydrogenative decarboxylative catalysts.