Influence of Different Compound Classes on the Formation of

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Influence of Different Compound Classes on the Formation of Sediments in Fossil Fuels During Aging Ruben Epping, Stefanie Kerkering, and Jan T. Andersson* Institute of Inorganic and Analytical Chemistry, University of Muenster, Corrensstrasse 30, 48149 Münster, Germany S Supporting Information *

ABSTRACT: The formation of sediments is a serious instability problem in the storage of fossil fuels. Reactions that lead to sediment formation can be linked to the oxidation of certain fuel components that contain oxygen, nitrogen, or sulfur. To study the oxidation reactions that occur during aging of fuels, we doped a model fuel with several representatives of such compound types. The compounds used were 2,6-dimethylphenol, 2-naphthol, 2,5-dimethylpyrrole, 2-methylindole, dibenzothiophene, and pentamethylene sulfide. After an artificial aging of the samples according to the DGMK-714 protocol, the formed sediments were analyzed by electrospray ionization mass spectrometry (Orbitrap, ESI-MS), elemental analysis, infrared measurements, and mass analysis. Mass spectrometry indicated monomeric and dimeric oxidation products with two to nine oxygen atoms as well as products with different hydrocarbon structures (different C/H ratios) from 2,6-dimethylphenol. 2-Naphthol led to oligomers consisting of up to six monomer units and showing different degrees of oxidation. The first ever recorded cross-coupling between 2,6-dimethylphenol and 2-methylindole and between 2-naphthol and 2,5-dimethylpyrrole is also shown. In general, the tested nitrogen compounds and especially the phenols tended to form oxidized oligomers, whereas the sulfur compounds led to sulfoxides and sulfones. between themselves during storage.6 The oxidation leads, in several steps, to polar and often higher molecular weight products which show a low solubility in the nonpolar matrix and therefore can precipitate.7 Autoxidation and polymerization reactions of unsaturated hydrocarbons and other reactive compounds containing sulfur, nitrogen, or oxygen are thought to be the main routes to gum or sediment formation. The deposit formation depends on several factors like hydrocarbon composition, type of crude oil, type and severity of the refining process, temperature, pressure, oxygen content, and storage conditions.1 With respect to hydrocarbon composition, the stability decreases in the order of paraffins, naphthenes, aromatics, olefins, and diolefins.1 Although gum and deposit formation is associated with oxidation by oxygen,8 the rate of oxidation is not a measure of the rate of gum formation. Two main mechanisms of gum formationone involving chain termination and the other one coupling of fuel molecules by peroxideshave been invoked. There is consensus that of the oxygen compounds, phenols show the largest contribution to sediment formation.3,9 Thus, extraction of the phenols from a fluid catalytic cracking product led to a more stable fuel that produced less insolubles.10 Oxidative coupling of phenols has been proposed as an aging process.11 Phenols occur naturally in fossil fuels,12,13 but they are often also used as stabilizers in fuels. Phenolic stabilizers are primary antioxidants and react with peroxy radicals as hydrogen atom donors to form hydroperoxides. Thus, the abstraction of a hydrogen atom from fuel compounds, leading to a radical that might react with oxygen to oxygenated products, is prevented.14

1. INTRODUCTION Stability is an important property of a fuel because instability impairs its proper use. Fuel stability implies the general resistance of a fuel to change.1 This can mean stability toward oxidation, gum formation, increased viscosity, color change, thermal stability, or corrosivity. A major drawback of instability is the formation of insoluble gum or sediment. These gums can cause undesirable effects such as tank corrosion, clogged filters, fuel lines, and blockage of engine components.2 It is clear that fuel instability can diminish its reliability and usability and that a fuel needs to be sufficiently stable from the moment of its production, during transportation and storage until its final use.3 Fuel deterioration may become a more severe problem in the future because heavier grades of crude are increasingly being used with a larger amount of heteroatoms. These heteroatomic compounds are known to promote particularly strongly the formation of sediments and to decrease the stability toward oxidation.3 Cracking and reforming processes are increasingly being used in refineries and can, especially through olefin formation, lead to instability.4 Also fuels from other sources (e.g., biodiesel, synfuel, or blends of these) are used in considerable amounts to stretch the fossil reserves. These fuels have a high content of unsaturated compounds with an accompanying lowering of their stability. Middle distillate fuels like diesel and heating oils are a complex mixture of paraffinic, naphthenic, and aromatic compounds boiling between approximately 170 and 390 °C with a carbon number in the range of C9−C22. Small amounts of oxygen, nitrogen, and sulfur are also present.5 Gums, sludges, deposits, precipitates and sediments are terms used for the insoluble fraction that can be formed in the reaction of fuel components with atmospheric oxygen and © 2014 American Chemical Society

Received: May 30, 2014 Revised: July 29, 2014 Published: August 4, 2014 5649

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It is obvious that peroxides increase instability.15 Oxygencontaining species like some acids, esters and ketones were considered moderately harmful but ethers and alcohols seem to have no effect on fuel stability.3 Nitrogen components are also known to contribute to the instability and promote sediment formation. Typical representatives that occur naturally in fossil fuels are amines, anilines, indoles, carbazoles, and quinolines.16,17 The largest effect is shown by the alkylated heterocyclic components of the pyrrole type with alkylation on carbon atoms adjacent to the nitrogen.18 These are neutral nitrogen compounds and have a larger influence on the stability than basic components of the pyridine type. The effect of 2,5-dimethylpyrrole has been extensively studied.19−21 It seems to have a major impact on the formation of sediments but it does not occur naturally in fuels or only in small amounts. A study of other components that are known to occur in fuels like indoles or carbazoles might be more insightful. Among the sulfur compounds thiophenols have also been implicated in fuel instability.22 It was also reported that aromatic thiols and organic sulfonic acids contribute to the sediment formation.23 Hydrodesulfurization as used routinely today is likely to remove such sulfur species efficiently so that their contribution can be neglected for commercial fuels. Small amounts of aromatic sulfur compounds may remain, though.24 The aim of this work is to study the behavior of such heteroatom containing compounds and investigate their products to see which type of compounds or which functional group has a large potential for sediment formation. Because much of the work in this area was done decades ago, modern analytical methods were expected to provide new insights. We selected compounds containing oxygen, nitrogen, and sulfur of kinds that typically occur in fossil fuels and added them to a fuel that models a fossil heating oil. The sediments are analyzed to obtain information on the degree of oxidation, their oligomeric nature, and if the heteroatom-containing dopants react with hydrocarbons from the matrix. To simplify the testing, only one compound was added to the model fuel at a time, so that the products of the dopants are better defined. It has been suggested that cross-reactions between different heteroatom-containing compounds occur (e.g., between phenols and nitrogen heterocycles).25 Therefore, in two additional experiments, two components, one a phenol and one a nitrogen compound, were added together to the model fuel to allow the possibility of an interaction between the added components as would be the case in a fuel. Several methods are used to test the oxidation stability and the formation of sediments of a fuel. Under normal storage conditions, sediments are formed only after a long period of time (months to years), and accelerated tests that allow a reduction of this time are preferred. This is achieved by applying higher pressure and temperature or pure oxygen instead of air. Here we used the so-called DGMK-714 method to artificially age the samples26 and chose a concentration of heteroatoms that mimics that typically present in heating oils. This method is basically a more severe aging method than the conventional PetroOxy method. The product sediments were studied with several methods like electrospray ionization mass spectrometry (ESI-MS), infrared measurements (IR), elemental analysis, and mass analysis of the sediments. We will report separately on the influence of different heterocyclic compounds on oxidation stability as measured by the induction time in the PetroOxy test.

2. EXPERIMENTAL SECTION 2.1. Materials. Composition of the Model Fuel. To facilitate analysis, a simplified model fuel was mixed to simulate a fossil middle distillate that itself does not contain the heteroatom compounds. Its composition is shown in Table 1.

Table 1. Composition of the Model Fuel heating oil component

structure

amount [%] (m/m)

Shellsol D100 Shellsol T100 Shellsol A100 Shellsol A150 toluene 1-methylnaphthalene

n-, iso-, cycloalkane (C13− C17) 98% alkane, 2% cycloalkane aromatic hydrocarbons (C9− C10) aromatic hydrocarbons (C9−C10) monoaromatic compound: C6H5CH3 diaromatic compound: C11H10

34.75 34.75 12.99 12.99 0.03 4.50

Chemicals. The purity of the chemicals 2,6-dimethylphenol, 2naphthol, 2,5-dimethylpyrrole, 2-methylindole, dibenzothiophene, and pentamethylene sulfide was >97%. All chemicals were purchased from Sigma-Aldrich (Munich, Germany) and used as received. Dichloromethane was purchased from Acros Organics (Nidderrau, Germany). 2.2. Methods. DGMK-714 Method. For each model component, 10 mL of model fuel was spiked at 0.1 mol/L of the component. This corresponds to a concentration of 0.17% w/w for nitrogen, 0.19% w/w for oxygen, and 0.38% w/w for sulfur. The samples were charged to a 500 mL steel autoclave and pressurized with air to 4.5 bar. The autoclave was immersed in an oil bath that was heated to 120 °C and maintained at this temperature for 24 h. At the end of the experiment, the autoclave was allowed to cool to room temperature. The liquid remaining and the sediment were separated, and the sediment was weighed. All measurements were performed as a single determination. ESI-MS. The electrospray ionization mass spectrometry (ESI-MS) experiments were carried out on a Thermo Fisher Orbitrap LTQ XL after dissolving 3 μg of the sediments in 100 μL of dichloromethane/ methanol (1:1) and injecting the solution directly. IR Analysis. For the IR analyses, 3 μg of sediment was dissolved in dichloromethane. An IR differential measurement was carried out on a Bruker Vector 22 system. The spectrum of pure dichloromethane was subtracted from the sediment spectrum. Elemental Analysis. The elements carbon, hydrogen, and nitrogen were determined in-house with a VARIO EL III (Elementar Analysensysteme GmbH, Hanau, Germany). Sulfur was determined from a solution in dichloromethane/methanol (1:1) by X-ray fluorescence spectrometry (ASG Analytik-Service, Neusäss, Germany). The oxygen data were obtained by subtracting the measured values from 100%. This is a good approximation because no other element except C, H, N, S, and O should be present in the samples. Weighing. The weighing was done on a Mettler Toledo MT5 balance.

3. RESULTS AND DISCUSSION A model fuel that was intended to mimic a heating oil but having a controlled composition was doped with different components containing either a nitrogen, an oxygen, or a sulfur atom and then aged using the DGMK-714 method. The chosen concentration of the hetero elements was on the order of 0.2− 0.4%, thus agreeing with values that are common in fuels (except sulfur in desulfurized fuels). We chose 2,6-dimethylphenol and 2-naphthol as representatives of the phenols,27,28 and 2,5-dimethylpyrrole and 2-methylindole were selected as nitrogen-containing compounds that have been found in gasoline and diesel.29 2,5-Dimethylpyrrole is a type of compound known to influence the sediment formation and 2-methylindole is a nitrogen heterocycle that occurs naturally in fossil fuels. For the sulfur compounds, we used dibenzothio5650

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phene as an aromatic component,30,24 as well as pentamethylene sulfide30 as an aliphatic sulfur compound, both known to appear in fossil fuel sources. The unaged and aged fuels as well as the formed sediments were analyzed to gain information on the reactions during the aging and the fate of the added components whose structures are shown in Figure 1.

Table 2. Elemental Composition of the Sediments Formed in the DGMK-714 Experiments model component

C [%]

H [%]

N [%]

O [%]

S [%]

no component added 2,6-dimethylphenol 2-naphthol 2,5-dimethylpyrrole 2-methylindole dibenzothiophene pentamethylene sulfide

69.5 75.8 81.6 83.7 81.2 69.0 63.7

7.6 10.4 10.2 13.7 12.2 6.5 6.3

0.0 0.2 0.1 0.6 1.1 0.2 0.6

22.7 13.4 8.0 1.9 5.5 21.5 28.8

0.0 0.0 0.0 0.0 0.0 2.7 0.4

phenol sediments may be unexpected because phenols contain oxygen, but obviously their presence partially prevented the model fuel from oxidation and condensation reactions. This illustrates that the phenols (as well as the nitrogen components) operate as antioxidants. The nitrogen content of the 2-methylindole sediment seems low at 1.1% but correlated with the sediment masses. No less than about 15% of the introduced nitrogen was recovered in the sediment. Because there is no other source of nitrogen available, the nitrogen heterocycle led to a major part of the sediment. For the sulfur-containing samples, the oxygen content of the sediments was much higher. At 22%, oxygen in the dibenzothiophene sediment was in the range of the pure model fuel sediment, and the pentamethylene sulfide sediment is even higher at 29%. The DBT sediment also contained considerable sulfur at 2.7%, corresponding to about 20% of the introduced sulfur. In the pentamethylene sulfide sediment, the sulfur content was much lower at 0.4%, meaning that only 1% of the introduced sulfur could be recovered in the sediment. ESI-MS. Oxygen Compounds. To gain insights into the molecular composition, we analyzed the sediments with electrospray ionization Orbitrap mass spectrometry. The high mass accuracy of this instrument coupled with the knowledge of the compounds present makes it possible to calculate the elemental composition of the ions recorded. It must be kept in mind that this technique does not deliver quantitative information and that signal height is not necessarily correlated with concentration. The most abundant oxidation products are shown in Table 3. Many signals were observed from the model fuel itself, but only those that originated in the added components and their oxidation products are listed here. The 10 most abundant parent ions from oxidation products were selected with the understanding that they may not be the products of the highest concentration. In the sediments derived from the sulfur compounds, sodium ion adducts were found, and these products were analyzed in the positive mode. In the other spectra, the ions were formed through proton loss and were analyzed in the negative mode. The exception was the cross-coupling experiments mentioned below, in which product ions were detected in the positive mode as sodium adducts. For the 2,6-dimethylphenol sediment, we found mostly monomeric and dimeric oxidation products with different numbers (2−9) of oxygen atoms as well as products with a different hydrocarbon structure (different C/H ratio) (Figure S1 in the Supporting Information). In Figure 3, some structural formulas are suggested for these oxidation products; the compounds illustrated are well-known products of 2,6dimethylphenol oxidation, but no compound was rigorously identified here. These structures are based on known reactions and oxidation products of phenols.32−34 A C−C or a C−O bond formation is

Figure 1. Components used in this work.

Sediment Masses. The amount of sediment formed in the DGMK-714 aging is an important factor to describe the stability of a fuel. Figure 2 compares the amount of sediment that was produced from the model fuel itself and the same fuel with different components as additives.

Figure 2. Masses of the sediments in the DGMK-714 experiments.

In the model fuel with added 2,6-dimethylphenol and 2naphthol, the amount of sediment was similar to that of the pure model fuel. 2-Methylindole and dibenzothiophene produced more sediment than the pure fuel, and 2,5dimethylpyrrole and pentamethylene sulfide produced less. 2,5-Dimethylpyrrole should be a reactive component, and the small amount of sediment was unexpected; this was possibly the result of thermal decomposition31 (see below). Elemental Analyses. The results of the elemental analysis of the sediments are presented in Table 2. The fuel itself showed a composition of 86.7% C and 13.5% H, and the sediment derived from this (“reference sediment”) contained 22.7% oxygen. The sediments of the phenol experiments showed much less oxygen, 13 and 8%, than the reference sediment, and the ones derived from the nitrogen compounds contained even less (5% for 2-methylindole), as expected from dopants that do not contain oxygen themselves. The low oxygen content in the 5651

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Table 3. Most Abundant Signals in the ESI-MS Spectra of the Sedimentsa added model component

elemental formula

exact mass [M − H]−(calculated)

mass [M − H]−(measured)

deviation from actual mass [ppm]

2,6-dimethylphenol

C16H18O5 C16H16O5 C16H16O2 C16H14O4 C16H16O4 C16H14O5 C16H18O3 C8H8O3 C8H10O C16H16O6 C20H12O4 C20H12O3 C30H20O3 C10H8O C10H8O3 C20H10O5 C40H26O4 C30H20O4 C20H14O5 C20H14O2 C9H9NO3 C18H16N2O4 C18H16N2O3 C18H16N2O C9H9N C9H7NO C9H9NO4 C18H14N2O4 C18H14N2O5 C27H23N3O4 elemental formula

289.10748 287.09183 239.10709 269.08127 271.09692 285.07618 257.11765 151.03940 121.06522 303.08675 315.06562 299.07070 427.13330 143.04957 175.03949 329.04488 569.17517 443.12822 333.07618 285.09144 178.05030 323.10307 307.10815 275.11832 130.06556 144.15206 194.04522 321.08732 337.08233 452.16092 exact mass [M + Na]+ (calculated)

289.10805 287.09241 239.11547 269.08187 271.09752 285.07676 257.11825 151.04021 121.06610 303.08733 315.06615 299.07128 427.13363 143.05042 175.04017 329.04544 569.17543 443.12862 333.07675 285.09204 178.05980 323.10358 307.10827 275.11889 130.06642 144.04565 194.04595 321.08798 337.08288 452.16134 mass [M + Na]+ (measured)

−0.34 −0.41 −0.57 −0.42 −0.23 −0.45 −0.41 0.94 1.75 −0.24 −0.42 −0.29 −0.79 1.27 0.59 −0.32 −0.85 −0.59 −0.29 −0.22 1.517 −0.465 −0.311 −0.351 0.076 −1.509 0.355 −0.312 −0.341 −0.53 deviation from actual mass [ppm]

C12H8SO C12H8SO2 C5H10SO C5H10SO2

223.01936 239.01427 141.03501 157.02992

223.01850 239.01341 141.03421 157.02907

−1.38 −1.30 −1.75 −1.92

2-naphthol

2-methylindole

added model component dibenzothiophene pentamethylene sulfide a

Signals of sediments which emerged from the added components. The 10 most abundant ions in descending order. Negative mode, for the sulfur compounds, positive mode (see Figures S1−5 in the Supporting Information).

carbonyl group selective quarternary aminoxy reagent (QAO)36 and analyzed the ionic products with (+)-ESI-MS (Figure S6, Supporting Information). We could confirm that the carbonyl structures suggested in Figure 3 were functionalized with this selective reagent and thus contain the carbonyl group. Small amounts of trimers were found in the 2,6dimethylphenol sediment with elemental compositions of C24H26Ox and C24H24Ox (x = 4−8). All in all, as many as 29 unique masses were found for oxidation products of 2,6dimethylphenol in the sediment, but taken together, they still represented only a small amount of the total sediment as measured by ion abundance. It must be recognized that ESI can lead to very large selectivities so that ion abundance is not a reliable indication of concentration. As shown in Table 3, similar products were recorded in the 2-naphthol sediment. Here no less than 61 unique masses for the oxidation products could be identified, and the 10 most abundant ones are listed in the table. Besides masses for dimers and trimers, those of tetramers were found in a relatively high amount. Whereas in the 2,6-dimethylphenol sediment many signals originating from the fuel matrix and only small signals of the phenol oxidation products were recorded, in the 2-naphthol

Figure 3. Putative structures for some of the 2,6-dimethylphenol oxidation products.

possible as are peroxide bonds. The most probable mechanism leading to the formation of these products is an oxidative coupling.35 Some experiments were carried out to support the assumed structures. To exclude the possibility of forming Coulomb dimers in the MS, in which two molecules are held together by a cation like H+ or Na+, we increased the collision induced dissociation energy to see whether the signal intensities would decrease, but that was not the case, supporting the assignment as covalent dimers. To confirm the presence of carbonyl groups, we derivatized this functional group with a 5652

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Nitrogen-Containing Compounds. In the indole series, 2-methylindole is known to lead to more sediment formation than either indole or 3-methylindole.38 In the 2-methylindole sediment, several oxidation products were indicated by (−)-ESI-MS, and some likely structures are presented in Figure 5, including 2,2′-dimethyl-[2,3′-bi-1H-indol]-3(2H)-one (CAS 23740-95-6).39,40

sediment, the oxidation products of 2-naphthol dominated, as measured by ion abundance. In Figure 4, some putative

Figure 5. Putative structures for some of the 2-methylindole oxidation products.

The most abundant signal in the spectrum of the 2methylindole sediment is the 3-fold oxidized 2-methylindole with the chemical formula of C9H9NO3. Besides that, as many as 22 oxidation products could be counted. Similar to the phenol cases, dimeric, trimeric, and tetrameric structures were recorded. The oxidation products of 2-methylindole made up the strongest signals in the MS of the sediment. This indicates a considerable involvement of this nitrogen compound in the sediment formation. However, the elemental analysis (Table 2) shows that there is about five times more oxygen than nitrogen in the sediment, and thus, a large part must be derived from products of the model fuel. The IR spectrum (Figure S9, Supporting Information) indicates that the oxidation products mainly carried carbonyl rather than hydroxyl oxygen atoms. In general, these oxidation products seem to be not so highly oxidized as those in the phenol cases and this is also indicated by the lower amount of oxygen in the elemental analysis. Sulfur-Containing Compounds. The ESI-MS spectra of the sulfur containing samples looked very different than the phenol and nitrogen ones, and only signals corresponding to singly and doubly oxidized products of dibenzothiophene and pentamethylene sulfide were recorded (Figure 6). These spectra were recorded in the positive mode to detect the sodium ion and proton adducts of the oxidized sulfur. No signals from the sulfur compounds were found in the negative mode. No other products of the sulfur compounds were found. In the case of dibenzothiophene, the spectra of these oxidized forms showed very high signal intensity, which also explains the large amount of sediment formed. The high sulfur amount found in the elemental analysis could be explained by this as

Figure 4. Putative structures for some of the 2-naphthol oxidation products.

structures for several of the products are illustrated. A bond formation is most likely in the 1, 3, or 7 position of 2naphthol,32,33 with a new C−C or a C−O bond being formed.33 In the case of the dimers C20H12Ox, both a (ringclosed) furan and a quinone structure are possible, but the furan, containing the acidic −OH group, may be more likely because its ionization yield should be higher and therefore result in a higher ion abundance. However, we confirmed separately that quinones (2,6-dimethylbenzoquinone was used, Figure S7, Supporting Information) are detectable in (−)-ESIMS so that they cannot be excluded based on this MS experiment. Further oligomers of 2-naphthol consisting of up to six monomer units and showing different degrees of oxidation could be detected in smaller amounts. This is in agreement with the higher total mass of sediment formed by 2-naphthol. The higher oxygen content in the 2,6-dimethylphenol sediment might be explained by the higher oxygen/carbon ratio in the dimethylphenol structure compared with the naphthol. IR measurements (Figure S8, Supporting Information) confirmed a stronger presence of OH groups in the phenol sediments than in the sediment of the pure model fuel, as recorded by the band in the range of 3300−3600 cm−1.37 The increase in the signals for carbonyl groups at about 1700 cm−1 was also larger for the 2,6-dimethylphenol sediment than for the 2-naphthol sediment, which might be an explanation for the higher amount of oxygen. 5653

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Figure 6. Structures for the dibenzothiophene and pentamethylene sulfide oxidation products.

well. In this case, about 20% of the DBT was oxidized and became part of the sediment. The oxygen concentration in the dibenzothiophene oxide and especially dibenzothiophene dioxide is rather large, which is a partial explanation for the high amount of oxygen in the sediment. However, the molar O/S ratio is approximately 16, a fact that cannot be accounted for by the dibenzothiophene oxides. Therefore, it is most likely that the sediment also contains other highly oxidized compounds that were not detected with this MS method, similar to the indole case. It is known that in ESI-MS a complete ion suppression can occur so that no trace of an ion of a compound that is present and ionizable is seen.36 In the case of pentamethylene sulfide, both ESI-MS and elemental analysis showed a much smaller amount of oxidation products of the sulfide in the sediment. Also, the mass of the sediment was rather low. The very high proportion of oxygen in the sediment obtained in the elemental analysis must be derived from products of the model fuel. Notable is that in no case could any cross-coupling products from the added components with compounds of the model fuel be observed within the limitations of the mass spectromteric technique used. Thus, it appears that there is no formation of oxidation products that contain both an added hetero compound and a component of the model fuel. Cross-Couplings of N- and O-Containing Compounds. Further experiments were performed under the same conditions as above but with one oxygen-containing and one nitrogen-containing compound to investigate if any products could be found as a result of a cross-coupling of such entities. This kind of reaction has been postulated as resulting in oxidative deposits.25 Because the sulfur compounds used here only resulted in S-oxidized products, they seemed of less interest in cross-coupling studies and were thus omitted. A mixture of 2,6-dimethylphenol and 2-methylindole, each in the same concentration as used above, led to a sediment that showed the same products as formed from 2-methylindole with the exception of an ion obtainable in (+)-ESI-MS of very low abundance of composition C17H17NO2, corresponding to a molecule composed of one unit of each of the reactants plus one oxygen atom. When 2-naphthol and 2,5-dimethylpyrrole were aged together, several MS signals were recorded that are the result of cross-coupling reactions (Figure 7). Dimers containing a total of two, three, and four oxygen atoms were recorded. A reaction mechanism leading to coupled products of electron-rich aromatic compounds with electrophiles generated by autoxidation has been suggested.25 The putative structures for the masses observed here are based on this mechanism (Figure 8). Because a large number of oxidized products from the model fuel were present, IR measurements did not reveal any information on these coupled products.

Figure 7. Part of the (+)-ESI-MS spectrum for the 2-naphthol + 2,5dimethylpyrrole sediment. Cross-coupling products are labeled, and all other signals originate from oxidation of components of the model fuel. The m/z values shown include Na+.

Figure 8. Putative structures for the mixed 2,6-dimethylphenol + 2methylindole (left) and the 2-naphthol + 2,5-dimethylpyrrole (right) oxidation products.

4. CONCLUSION In this work, we demonstrate that different types of added components influence the sediment formation and therefore one aspect of stability of fuels in various ways. The tested sulfur-containing components led to singly or doubly oxidized products in the sediment. They seem to increase the total oxygen content of the sediment and do not seem useful as antioxidative additives. Phenols on the other hand should work well as antioxidants and lead to a lower oxygen content in the sediment. They formed polymeric oxidation products that contained up to six monomers in different degrees of oxidation. 2-Methylindole reacted in a similar way to the phenols and formed polymeric oxidation products during the aging. In the experiments with two added components, including the compound classes phenols and neutral nitrogen-containing heterocycles, we could also obtain cross-coupled products between these two classes. Such compounds have been suggested to form, but the experimental evidence presented here seems to be the first concrete proof of this coupling actually occurring. We could not detect any cross-coupling products between the heterocompounds and the hydrocarbon components of the model fuel. The oxidation products from the added components also had different shares of the sediment formation. 2-Napthol, 2-methylindole, and dibenzothiophene oxidation products formed the main part of their sediment, as judged by MS, whereas the sediments of the 2,6-dimethylphenol and pentamethylene sulfide samples only contained small amounts of oxidation products of these compounds. The strategy of adding a compound to a fuel and analyzing the products, in contrast to aging a complete fuel, has the big 5654

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advantage that individual products are formed in higher amounts. The fact that only one compound containing a certain functional group−and not, say 100, as may be present in the fuel−is present and reacts, limits its reaction partners to itself so that the products are much more easily detected. These results may be used as a foundation for further research involving a correlation of heteroatom containing compounds in fuels and the storage stability of the fuels.

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ASSOCIATED CONTENT

S Supporting Information *

ESI mass spectra of the sediments, as referenced in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +49-251-8336013. Tel.: +49-251-8333102. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Oel-Waerme-Intitut in Herzogenrath, Germany, for providing us with the model fuel and the use of their DGMK-714 instrumentation. We are grateful to Matthias Letzel for the Orbitrap measurements.

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dx.doi.org/10.1021/ef501230c | Energy Fuels 2014, 28, 5649−5656