Effect of Storage and Hydrodesulfurization on the Ketones in Fossil

Jan 20, 2015 - The usefulness of this QAO reagent in fossil fuel analysis was .... are carried out as previously described using alumina as the statio...
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Effect of Storage and Hydrodesulfurization on the Ketones in Fossil Fuels Ahmad Alhassan and Jan T. Andersson* Institute of Inorganic and Analytical Chemistry, University of Muenster, Corrensstrasse 30, 48149 Muenster, Germany S Supporting Information *

ABSTRACT: Stability of fuels on storage is a topic of major concern. Oxidation prone compounds are known to react with air oxygen to form polar oxygenated products that ultimately can precipitate as a gum. Ketones are such reactive compounds, and here they are analyzed in several materials. They are recorded in a diesel as a function of the degree of hydrodesulfurization (HDS). A ketone selective reagent is used to derivatize only ketones, thus avoiding confusion with other one oxygen-atom containing compounds. The analysis was done using ultrahigh resolution Orbitrap mass spectrometry. The alkanones that were present in the original material are removed through HDS. Plenty of mainly aromatic ketones are formed when the fuel is stored in contact with air. A heating oil was stored under near-ambient conditions and analyzed after 12 and 18 months. Aromatic ketones arose throughout the storage, but their pattern did not change with time. A phenol selective derivatization reagent was used to show that many ketones containing a total of two oxygen atoms are in fact hydroxyphenyl alkyl ketones. A sediment contained ketones possessing a total of one to five oxygen atoms. The advantage of using selective derivatization reagents to differentiate between isomers based on their functional groups is demonstrated.

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dolerite sill from the McArthur Basin,8 and in hydrothermal oils and sediment extracts from the Guaymas Basin.9 Since these ketones show no or only weak odd-over-even carbon number predominances and they are common constituents of oil shale pyrolysates, they have been assumed to originate from kerogens as products of pyrolytic processes.9 n-Alkan-2-ones with oddcarbon number homologues predominating have been found in recent marine and lacustrine sediments.10−12 The source of these ketones has been attributed to a microbial production from either oxidation of n-alkanes or from n-fatty acids by βoxidation and subsequent decarboxylation.10,11 Reports on aromatic ketones in fossil materials are even rarer than those on aliphatic ketones. Aromatic ketones such as alkyl phenyl ketones, alkyl naphthyl ketones, 1-indanones, and 1-tetralones have been reported in Posidonia Shale bitumens.13 These ketones have been proposed to be geochemical products and possibly derived either from inter- and/or intramolecular acylation reactions or the oxidation of benzylic methylene groups of aromatic hydrocarbons. Aromatic ketones like fluorenones, benzofluorenones, dibenzo- and/or naphthofluorenones have been identified in Western Siberian Jurassic Oils.14 The authors reported that an intramolecular Friedel−Crafts acylation reaction is the most likely chemical pathway for the formation of fluoren-9-ones. Also, the origin of fluoren-9-ones found in Posidonia Shale15 has been attributed to cyclization reactions of suitable precursors, since Posidonia Shale had been deposited under anoxic conditions and fluorenes, as possible precursors, were present in small amounts even in samples of low maturity. Nevertheless, oxidation of pre-existing aromatic hydrocarbons remains the most likely possible mechanism for

INTRODUCTION Instability of fuels is frequently observed. When fuels age, oxidation processes occur which may lead to sediment formation.1 Such sediments can cause problems like clogging of filters and nozzles. The mechanism through which sediments are formed is not exactly known, but hydroperoxides are suspected to be an initial oxidation product.2 To form hydroperoxides, a compound must possess sites that are reactive enough to form a radical that reacts with a molecule of oxygen. The most likely positions to be oxidized are bisallylic and benzylic ones because of their lower C−H bond energy. Hydroperoxides are unstable molecules and tend to react further, for instance through coupling with each other to larger entities,3 or decomposition to radicals that in turn can abstract a hydrogen atom and lead to a variety of additional radicals. Further oxidation and polymerization of these products can lead to the formation of insoluble sediments. Peroxides can also break down and form aldehydes, ketones, and short chain acids as oxidation products.2,4 The presence of oxygenates has been linked to the tendency of a crude to form solid deposits.1 Ketones are one of several oxygen containing compound classes in fossil materials.5 The carbonyl functionality makes ketones reactive compounds toward acid and base catalyzed condensation reactions as well as to many addition reactions with the carbonyl carbon atom acting as an electrophile. The bond dissociation energy of the α-C−H bond in 3-pentanone has been calculated to be 90.9 kcal/mol, showing that radical reactions should also be facilitated for ketones.6 Thus, chemical considerations dictate that the reactivity of ketones may have an influence on the stability of fuel during storage. Ketones are known to occur naturally in fossil materials, but their origin in these materials is not always clear. Aliphatic ketones have been found in the Green River Formation oil shale,7 bitumen of a Proterozoic © XXXX American Chemical Society

Received: December 15, 2014 Revised: January 16, 2015

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(Fontenay-sous-Bois, France). 1-(4-Fluoronaphthyl)hexan-1-one (FNH) and 1-(4-fluoronaphthyl)octadecan-1-one (FNO) were synthesized in our laboratories.5 The QAO reagent (CAS# 140269463-6) was purchased as Amplifex Keto reagent from AB SCIEX (Framingham, MA, U.S.A.). The QAO reagent working solution was prepared according to the method described in the sheet delivered with the reagent. 2-Fluoro-1-methylpyridinium p-toluenesulfonate (FMP reagent) was provided by TCI Europe N. V. Alumina (50− 200 μm, 60 Å, neutral) was purchased from Acros Organics and activated overnight at 180 °C before use. Empty glass columns equipped with PTFE frits were delivered by Merck (Darmstadt, Germany). Molecular sieves (3 Å) came from Applichem (Darmstadt, Germany) and were activated overnight at 280 °C. Syringe filters (0.2 μm PTFE membrane) were purchased from VWR International (USA). DCM, EtOH absolute, and acetonitrile used as mobile phases in the chromatographic separation or as solvents for the derivatization reactions were stored over activated molecular sieves for at least 24 h before use. The ratio of the molecular sieves to solvent was 10% m/v. Samples. The four diesel samples were supplied by a European refinery. The first sample, containing 18600 ppm sulfur, was a blend of 30% light cycle oil, 27% light atmospheric gas oil, 25% light coker gas oil, and 18% heavy atmospheric gas oil. The other three samples were obtained after successive HDS of this nondesulfurized diesel and contained 550, 145, and 26 ppm (μg/g) S, respectively. The samples were stored for 14 years in closed dark bottles at room temperature before this analysis. The sulfur determination was performed by the refinery. The heating oil sample was filled into glass bottles that were kept open to the atmosphere at a temperature of 40 °C for various lengths of time in the dark. One portion was stored in a closed bottle in the refrigerator and analyzed together with the aged oils as the 0 months sample. Table 1 shows the storage conditions and storage times

the formation of several aromatic ketones. The oxidation processes may also take place during diagenesis since oxidants like free oxygen, in the early stages, and sulfate might be present. In addition, the oxidation can occur during oil production, product storage as well as in the course of an analysis.1 Hydrodesulfurization (HDS) is a major process in oil refining designed to reduce the sulfur content to below the legal limits through a catalytic reduction with hydrogen to hydrogen sulfide. Due to the harsh conditions applied during HDS, e.g. elevated temperatures, catalyst, and highly reducing conditions, oxygen containing compounds would be expected to be removed in side reactions. In fact, it is known that HDS leads to a reduction of the content of phenols,16 especially of alkylphenols with longer side chains.17 It seems that no information is available in the literature on the influence of HDS on ketones. Given the possible involvement of this group of compounds in sediment formation, it was of obvious interest to obtain knowledge on the effect of HDS on the ketone content and pattern in fuels. Petroleum fractions constitute supercomplex mixtures,18 and therefore some kind of analyte separation is needed before the analysis can be done. A method for the trace analysis of ketones in fossil materials has been developed5 and involves two main steps. The first one is a derivatization of the ketones with a commercial quarternary aminoxy (QAO) reagent. The reagent selectively reacts with ketones and forms a quarternary ammonium ion which is well detectable in mass spectrometry with electrospray ionization (ESI). The second step is necessitated by the presence of basic nitrogen heterocycles in the fuel that tend completely to suppress the detection of the ammonium ions of the derivatized ketones. Therefore, they are isolated through a quick chromatographic separation on an alumina column. The usefulness of this QAO reagent in fossil fuel analysis was demonstrated by analyzing ketones in Wilmington crude and a coal tar.5 The developed method could overcome drawbacks of other methods for the analysis of ketones in fossil materials. Gas chromatography needs the separation of a ketone-enriched fraction8 and is only applicable to the analysis of volatile compounds. Other techniques like FT-IR of the SARA fractions have shown a clear carbonyl band in the resins and asphaltene fractions of Mexican vacuum residua,19 but it was not determined if it orginated from ketones. The primary goal of this work was to gain detailed information about the composition of the mixture of ketone species remaining in fossil materials after HDS using ultrahigh resolution mass spectrometry. A diesel sample that was available before and after hydrodesulfurization to several levels of lower sulfur contents was investigated. After establishing this, the second goal was to investigate whether such ketones, in whole or in part, are formed on storage. Such a process would involve air oxidation of components of the fossil material in a reaction that presumably involves reactive intermediates that may also be involved in instability reactions of the fuel. A heating oil was stored for 18 months to serve as testing material for this investigation.

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Table 1. Heating Oils Investigated and Their Storage Time and Storage Conditions heating oil storage at 40 °C for storage at 40 °C with copper for

0 months 12 months 18 months 9 months (the liquid phase and the sediments formed were investigated)

applied for all investigated samples, including the formed sediments. The aging of the heating oil has been described in detail (heating oil A in ref 20). High Resolution Mass Spectrometry. Ultrahigh resolution mass spectra were obtained using a Thermo Scientific Exactive Orbitrap Mass Spectrometer (Thermo, Dreieich, Germany). Sample solutions were introduced at a flow rate of 5 μL/min under nitrogen as the sheath gas. All measurements were carried out with an ion spray voltage of 1.31 kV, capillary temperature of 200 °C, and tube lens voltage of 60 V. The resolving power was 170 000 (m/z = 200) for all samples investigated. The mass accuracy and the associated standard deviation were measured for the ketones CnHxO1−5 from the sediment and are listed in Table S1. Kendrick Plots. The elemental composition and the double bond equivalent DBE (the sum of the number of double bonds and rings in a molecule) were calculated for each signal. Kendrick plots were generated by plotting DBE versus the number of carbon atoms in the analyte (after subtraction of the derivatization unit). Deasphalted Wilmington Oil. About 250 mg of Wilmington crude oil was treated with 10 mL of n-heptane, and the resulting mixture was stirred in the dark overnight at room temperature. The asphaltenes were then removed through a first filtration (paper) and then on a PTFE syringe filter. The solvent was evaporated gently under vacuum. The resulting deasphalted Wilmington oil was stored in the refrigerator until use. Derivatization of Ketones in Diesel Samples. About 25 mg of the diesel was filled into a 1.5 mL vial. A total of 75 μL of the QAO reagent working solution was added. The vial was then tightly closed,

EXPERIMENTAL SECTION

Materials and Chemicals. Dichloromethane (DCM) and acetic acid were provided by Sigma-Aldrich (Steinheim, Germany). Absolute ethanol (EtOH) was from Fisher Scientific (UK). Acetonitrile and triethylamine were purchased from VWR International SAS B

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Energy & Fuels and the inhomogeneous reaction mixture was magnetically stirred at room temperature for 3 h. Then 50 μL of DCM and 2.5 μL of acetic acid were added to make the reaction mixture homogeneous. The mixture was stirred overnight at ambient temperature. The solvents were removed using nitrogen at 50 °C until all the acetic acid had evaporated. DCM (1.25 mL) was added, and the excess QAO was removed through filtration on a PTFE filter. The filtrate was concentrated to about 0.5 mL and collected for the chromatographic separation (see below). Derivatization of Ketones in Heating Oils. Exactly 21.5 mg of the heating oil was filled into a 1.5 mL vial. The sample was spiked with 8 μL of an equimolar mixture (0.0193 mol/L) of FNH (4.736 μg/μL) and FNO (7.839 μg/μL) in DCM. For each sample, 67 μL of DCM and 3.75 μL of acetic acid followed by 75 μL of the QAO reagent working solution were added. The vial was then tightly closed, and the mixture was stirred overnight at ambient temperature. The solvents were removed gently by a nitrogen stream until the acetic acid had been completely removed. 1.25 mL of DCM was added to dissolve the residue. The excess of QAO reagent was removed through filtration on a PTFE filter. The filtrate was concentrated to about 0.5 mL and collected for the chromatographic separation (see below). In case of sediments, the derivatized amount was 1.86 mg and no internal standards were added. Derivatization of Ketones in the Deasphalted Wilmington Oil. Eighteen mg of the deasphalted Wilmington oil was filled into a 1.5 mL vial. A total of 150 μL of the QAO reagent working solution was added. The vial was tightly closed, and the mixture was stirred at room temperature for 3 h. Then 185 μL of DCM and 9.25 μL of acetic acid were added, and the mixture was worked-up as described above. Derivatization of Phenols in Fuels with the 2-Fluoro-1Methylpyridinium p-Toluenesulfonate Reagent (FMP). 75 μL of the fuel was filled into a 1.5 mL vial. A total of 150 μL DCM was added to dissolve the sample. 6.1 mg of FMP reagent followed by 3 μL of triethylamine were added, and the vial tightly closed. The mixture was stirred at room temperature for 3 h to obtain a homogeneous solution. The excess of the FMP reagent was removed through filtration on a PTFE filter. The filtrate was collected for the chromatographic separation (see below). Separation of QAO and FMP Derivatives. The separations are carried out as previously described using alumina as the stationary phase.5 Exactly 2 g of activated Al2O3 was filled into a glass column (6 × 0.9 cm). The derivatized sample was poured onto the alumina, and 200 mg of activated Al2O3 was placed on top of the column to prevent disturbances of the bed. Two mL of DCM followed by 5.5 mL of 95:5 (v/v) DCM/EtOH (absolute) was passed through the column and discarded. The QAO or FMP derivatives were eluted with 10 mL of 70:20:10 (v/v/v) DCM/EtOH/acetic acid. The solvent of this fraction was removed on a rotary evaporator under reduced pressure. Finally, extremely reduced pressure was applied (10 mbar for 10 min at 55 °C) to remove the last traces of acetic acid. The residue was then dissolved in 100 μL of acetonitrile for QAO derivatives and in 100 μL of DCM for FMP derivatives. The samples were infused into the ESI-MS after diluting with acetonitrile for the QAO derivatives and with a mixture of 50:50 (v/v) of DCM:acetonitrile for the FMP derivatives.

naminium bromide that reacts with ketones and aldehydes according to the equation in Figure 1.

Figure 1. Derivatization scheme for ketones with the QAO reagent.

The data are presented in the form of Kendrick plots that show the double bond equivalent (DBE) as a function of the number of carbon atoms.21 Three types of ketones were found in the nonhydrodesulfurized sample, namely CnHxO (normal ketones), CnHxO2 (oxygen-containing ketones), and CnHxNO (ketones containing a nitrogen atom). Ketones of the first group were more prominent than the others. On other hand, the hydrodesulfurized samples contained only two types, namely CnHxO and CnHxO2, with all nitrogen-containing ketones (shown in Figure S1) absent. Figure 2 displays ketones CnHxO in the diesel samples before and after HDS. The aliphatic ketones with DBE of 1, 2, and 3 were the most abundant ones before HDS (Figure 2a). Alkanones (DBE 1) are known as major components of the carbonyl containing compounds in fossil material.22−24 These ketones are mainly 2-alkanones with small quantities of 3alkanones.22,24 A DBE of 2 corresponds to cyclic ketones. Cyclopentanones (with zero to ten carbon atoms as substituents, C0−C10) in reasonable quantities, cyclohexanones (C0−C2), and cycloheptanones have been identified in shale oils.22 A DBE of 3 probably corresponds to ketones containing two saturated rings (or one saturated ring and one carbon−carbon double bond). This class of ketones has been found in a shale oil22 and suggested to be cyclopentenones and cyclohexenones. Ketones with a DBE of 5 most likely represent alkyl phenyl ketones that have been reported in coal tar25 and shale oil.22 DBE 6 may represent components such as indanones and/or tetralones that occur in shale oil22 and in Green River oil shale.7 DBE 8 likely represents ketones containing the naphthalene nucleus. Such ketones have been found in the terrestrial organic matter of Late Palaeozoic age as naphthyl ketones.26 A DBE of 10 most likely represents the 9fluorenone series. These ketones are known in Western Siberian Jurassic oils,14 Wilmington oil,27 and shale oil.15 A DBE of 13 is thought to represent compounds like benzofluorenones, also reported in Western Siberian Jurassic oils.14 In the hydrodesulfurized samples, only a few aliphatic ketones CnHxO with DBEs from 1 to 3 were observed (Figure 2b, c, and d). Obviously their abundance was very low despite being prominent in the nonhydrodesulfurized sample. This points to a nearly complete removal of aliphatic ketones under the strongly reducing conditions in the HDS process. Plenty of aromatic ketones (DBE 6−12) were prominent in all the hydrodesulfurized samples despite being hardly detectable in the original diesel. The distribution of ketones found in all samples after HDS was quite similar. The aromatic ketones of DBE of 6, 7, 9, and 10 were the most abundant ones. There is no reason why these ketones should survive the HDS, so we assume that the aromatic ketones were formed as air oxidation products during the long storage of the samples. The oxidation

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RESULTS AND DISCUSSION Being so complex, petroleum-derived samples often need sample preparation for a reliable group-type analysis. The derivatization with an ionic reagent to produce a charged derivative suitable for mass spectrometric analysis can be such a step in the sensitive analysis of ketones in fossil materials. This route was described for a crude oil and a coal tar5 and shown to give excellent results with a minimum of sample preparation. It was necessary to remove the basic nitrogen compounds that otherwise caused complete signal suppression of the ketone signals. The reagent chosen was a commercial quarternary aminoxy compound, 3-(aminooxy)-N,N,N-trimethyl-1-propaC

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Figure 2. Kendrick plots of the ketones CnHxO in diesel samples − (effect of HDS and storage). (a) The sample before HDS and (b, c, d) in the samples after HDS with declining sulfur content from b to d.

Figure 3. Kendrick plots of the ketones CnHxO2 in diesel samples− (effect of HDS and storage). Sample before HDS (a) and samples after HDS (b, c, and d) with declining sulfur content from b to d.

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Figure 4. Kendrick plots of the ketones CnHxO3 in diesel samples after derivatization show the effect of HDS and storage. Sample before HDS (a) and samples after HDS (b, c, and d) with declining sulfur content from b to d.

Figure 5. Kendrick plots of the ketones CnHxO in (a) a nonaged heating oil and after aging at 40 °C for (b) 12 months, (c) 18 months, and (d) after aging on copper at 40 °C for 9 months. The red circles indicate the signals of the fluorinated ketones that were added in an equimolar ratio as internal standards.

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Figure 6. Kendrick plots of the ketones CnHxO2 in (a) a nonaged heating oil and after aging at 40 °C for (b) 12 months and (c) 18 months and (d) after aging on copper at 40 °C for 9 months. The red circles indicate the signals of the fluoroketones that were added in an equimolar ratio as internal standards.

because of their very low abundances that led to variations in their ESI response. To verify the possibility of formation of ketones during storage, the ketones in four heating oil samples stored for different lengths of time were analyzed. The storage conditions and the storage periods are shown in Table 1. The first sample was nonaged heating oil. The second and third ones were the same heating oil but after storage for 12 and 18 months at 40 °C and with exposure to air. The fourth sample was the same heating oil that was stored for nine months together with copper metal as an aging catalyst. Copper is well-known to enhance the aging of heating oil and is frequently used to simulate the metals in equipment used for storage tanks and oil burners.29 For the MS analysis, all samples were spiked at the same level with an equimolar mixture of two fluorinated ketones, FNO and FNH, as internal standards. The fluorine atom imparts a unique mass on these compounds that makes the assignment of the MS signal simple. Based on the MS data, two classes of ketones of the general formulas CnHxO and CnHxO2 could be identified in heating oil samples. The former was always more abundant than the one with an additional oxygen atom. Ketones CnHxO with DBEs of 6 and 10 always dominated (see Figure 5). The high DBE number again indicates an aromatic structure, for instance tetralones and indanones for DBE 6. Weak signals at DBE 5 point at alkyl phenyl ketones. These three kinds of ketones are known to occur in shale oil22 as well as in coal tar where acetonaphthones (DBE 8) also were identified.25 The major signals by far occurred at DBE 10 that we interpret as 9fluorenones. Only weak signals were seen for alkanones at DBE 1. The ketones seen in the nonaged sample have probably been

of pre-existing aromatic hydrocarbons is known to be one of the most likely routes for the formation of several aromatic ketones. Aromatic hydrocarbons, especially those containing benzylic methylene groups, are reactive toward oxidation, and the oxidation of such groups in the hydrocarbons by air would lead to formation of the corresponding ketones.28 Ketones CnHxO2 were also found in the diesel samples before and after HDS and are displayed in Figure 3. Similar to the ketones discussed above, HDS led to a clear reduction of the aliphatic oxygen-containing ketones so that very few of them were visible in the hydrodesulfurized samples although, especially those of DBE 3, they were by far the most abundant ones in the nonhydrodesulfurized sample. Like the ketones CnHxO, the ketones CnHxO2 were similarly distributed in all samples after HDS and displayed mainly DBEs of 5−7, i.e. in the aromatic region. This lends support to our hypothesis that the aromatic ketones CnHxO1−2 were mainly formed during the long storage. Some masses that fit ketones that contain a total of three oxygen atoms, CnHxO3, were recorded in all diesel samples, but they were by far not as abundant as the ketones CnHxO1−2. Kendrick plots for the CnHxO3 were generated (Figure 4) and again show that the diesel before HDS contained mainly aliphatic ketones (DBEs of 1−3). These ketones were nearly completely absent in the samples after HDS. As above, this is attributed to the HDS process that led to the reduction of these aliphatic ketones. Like the ketones CnHxO1−2, the ketones CnHxO3 appear mainly in the aromatic region in all sample after HDS which is an additional sign of being formed during the storage. Unlike the ketones CnHxO1−2, the CnHxO3 were differently distributed in the samples after HDS, possibly in part F

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Figure 7. Kendrick plots of the ketones CnHxO3 in the heating oil after aging at 40 °C for (a) 18 months and (b) for 9 months with copper. The red circles indicate the signals of the fluoroketones that were added in an equimolar ratio as internal standards. Due to the low abundance of ketones CnHxO3, the size of the circles for the fluorinated ketones was reduced ten times.

Figure 8. Kendrick plots of the ketones CnHxO1−5 in the sediment formed after aging the heating oil with copper for 9 months at 40 °C.

formed on air oxidation after the HDS process since this would be expected to reduce efficiently such compounds.30 Even if the sample is designated “nonaged”, it had been exposed to air for an unknown period of time until the storage experiment was initiated. The fluoroketones were always added in the same amount as internal standards. We did not intend to use them as quantitative standards since a strong dependence of the ESI signal height on the surface activity of the corresponding ions is known31 but for relative purposes only. This dependence of the signal size is also indicated by the two fluoroketones that are added in an equimolar ratio, but the heavier ketone shows a signal that is approximately 90% higher than that of the lowermolecular weight ketone. A notable increase in the abundance of the ketones CnHxO relative to that of the fluoroketones with time of storage was obvious. In the heating oil that had been stored for 12 months at 40 °C, higher concentrations of ketones compared with the nonaged oil were observed, and this trend continued in the sample after 18 months (without copper). The sample that was stored for 9 months with copper

showed somewhat higher levels of ketones than the 18 months sample without copper. This shows that, during sample storage in the dark, a nonphotochemical oxidation took place. The benzylic carbon in the aromatic compounds can be readily oxidized and forms a keto group in the product ketone. It has been confirmed that the amount of fluorenone goes up strongly if a crude oil is stored over long periods of time.32 Fluorenes are the most likely precursor for the formed fluorenones because the benzylic carbon of fluorenes (C9) can be oxidized under mild conditions to the keto group in a fluorenone.33,34 The ketones with an additional oxygen atom also showed an increase in concentration with time, although their levels were generally lower than that of the simple ketones (Figure 6). Here, too, copper exerted a pronounced catalytic effect (Figure 6d). Some ketones containing a total of three oxygen atoms, CnHxO3, and of a DBE mainly of 6 and 7, were found in low amounts in the sample stored for 18 months (Figure 7a) and in G

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Figure 9. Kendrick plots of the ketones CnHxO in the deasphalted Wilmington crude. (a) The sample had been stored in a dark bottle at room temperature and (b) the sample had been stored in a glass ampule.

Figure 10. Kendrick plots of the phenols CnHxO2 in (a) the hydrodesulfurized diesel with 145 ppm sulfur and (b) in the heating oil aged at 40 °C for 18 months.

the one stored in contact with copper for 9 months (Figure 7b). The copper catalyzed sample developed a sediment that was investigated in the same way as above. As shown in Figure 8, the ketones in the sediments display up to four additional oxygen atoms. Those with the formulas of CnHxO4−5 were absent in the aged heating oil in contact with the sediment. That these highly oxidized ketones precipitate can be explained by the high polarities of ketones containing several oxygen atoms and by the low polarity of the solvent. Presumably these polar molecules develop strong interactions between themselves so that aggregation takes place, leading to precipitation. These findings urged us to investigate two Wilmington crude oils of the same origin that had been kept for a long time under different storage conditions. The first one was stored for ca. 15 years in a dark bottle at room temperature before analysis. The second one was stored in a closed glass ampule (SRM 1582 from NIST) with only very limited contact with air. The Kendrick plots of the most abundant ketones (CnHxO) found in the two oils indicated a larger presence of ketones in the sample that had been stored in a bottle (Figure 9), especially those with DBEs from 5 to 16. Also those with DBEs from 17 to 19 were only detectable in that sample. The distribution of ketones was not quite similar in the two samples, despite the same origin. Although the aliphatic ketones (DBEs of 1, 2, 3) were prominent and similarly distributed, some aromatic ones, especially those of DBE of 10 and 13, were dominant but only in the one stored in the bottle. In the one from the ampule they were hardly detectable.

As above, DBEs of 10 and 13 indicate compounds like fluorenones and benzofluorenones, respectively. Their presence in high quantities only in the sample stored in the bottle is a strong indicator that they were formed as air oxidation products during the storage. We have reported the analysis of ketones in a related deasphalted Wilmington crude oil, again of the same origin.5 That sample was also stored in a dark bottle before the analysis but in the refrigerator for a similar length of time (see Figure 8 in ref 5). It had undergone less oxidation than the present one that was stored at room temperature. This also can be explained by the more advanced oxidation process at room temperatures than at refrigerator temperature. Characterization of the Compounds of the Formula CnHxO2. Since the ketones with a DBE of 5 and 6 were the most abundant ones of formula CnHxO2 in both the heating oil and the hydrodesulfurized samples, we were intrigued by the possibility that the extra oxygen atom might be present as a phenol group. Thus, to verify whether these ketones also contain a phenol OH group, the heating oil (stored for 18 months) and the diesel sample after HDS (145 ppm of S) were derivatized with 2-fluoro-1-methylpyridinium p-toluenesulfonate (TOS−), FMP, as the derivatization reagent. This reagent has been used for a selective derivatization of alcohols and phenols in essential oils.35 It is well-known that phenols can be analyzed by using the negative mode of ESI-MS,36 but it would be less useful for the phenols CnHxO2 since ions from carboxylic acids of the same formula also present would be recorded as well and thus no selective information on the functionality would be gained. Therefore, the reagent FMP that is known to be selective for phenols was employed here. Since H

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positively charged “FMP derivatives” are formed (Figure S2), ESI-MS detectability is excellent. The properties of FMP as a derivatization reagent were first monitored through derivatizing an equimolar standard mixture containing phenols, primary and secondary alcohols, and carboxylic acids. The carboxylic acids were included to check whether the reagent can react with these compounds. If so, then it would lead to derivatives of the same elemental composition as phenols with one additional oxygen atom. The ESI-MS analysis of the isolated FMP derivatives (Figure S3) showed that the reagent was selective for phenols and primary alcohols and less reactive toward the secondary alcohols. No FMP derivatives of the carboxylic acids were observed (see the SI). The FMP derivatives had to be separated from the matrix containing triethylamine to avoid the formation of a signal of high intensity at m/z = 375.266 Da (see Figure S4 for the diesel sample with 145 ppm of S) which corresponds to a cluster of two protonated triethylamine molecules and one TOS−. Three types of OH-containing compounds with general formulas of CnHxO (Figure S5), CnHxO2, and CnHxO3 (Figure S6) were found in both the hydrodesulfurized diesel and the heating oil after 18 months of storage. In Figure 10, phenols of the general formula CnHxO2 found in both the hydrodesulfurized diesel and the heating oil after 18 months of storage are compared. The phenols of DBE 5 and 6 were prominent in both studied samples. Despite the very different origins of these two materials, the Kendrick plots show a considerable similarity that indicates that those aromatics that are easily oxidized have reacted in both cases to the same products. A particularly pleasing finding is the similarity that can clearly be observed between the phenols CnHxO2 and the ketones CnHxO2 (compare Figures 10a and 3c and Figures 10b and 6c) where the compounds with DBE of 5 and 6 were dominant. This very strong similarity suggests that these signals are derived from the same compounds. Furthermore, a QAO derivatization of an equimolar mixture of propiophenone and 4hydroxypropiophenone followed by ESI-MS analysis confirmed that the hydroxyl group in the 4-hydroxypropiophenone has no influence on the QAO derivatization (Figure S7). Therefore, a DBE of 5 corresponds to phenols containing a carbonyl group in an alkyl chain, and those of DBE 6 are phenols containing a keto group plus a saturated ring. We suggest that these products are hydroxyphenyl alkyl ketones and hydroxyindanones and/or hydroxytetralinones. The direct analysis in the negative ESI mode (Figures S8−S11) showed that compounds containing a total of two and three oxygen atoms were visible in all heating oil and diesel samples. The two-oxygen containing compounds were generally distributed in three main groups: those with DBEs ≤ 4, those from 5 to 7, and those with DBEs from 8 and up to 18. The first group represents mainly aliphatic carboxylic acids with straight or branched chains and one- and two-saturated ring acids (naphthenic acids). The third group corresponds to carboxylic acids with at least two condensed aromatic rings, e.g. naphthoic acids (DBE 8) and phenanthrene carboxylic acids (DBE 11). The advantage of using selective derivatization reagents to differentiate between isomers is clearly seen for the compounds with DBEs of 5 and 6. (−)-ESIMS analysis cannot differentiate between carboxylic acids and phenols of the general formula C n H x O 2 . A selective derivatization reagent makes it possible to establish that these compounds are not only carboxylic acids, as may be assumed on first sight, but also in part phenols containing a keto group in an alkyl chain.

CONCLUSION Derivatization of ketones to yield positively charged moieties for the ESI-MS analysis with a minimum of sample preparation has proven itself to be an excellent method for the analysis of ketones in complex mixtures. Both the HDS and the long-term storage are shown to affect the ketone content and pattern in fossil fuels. The presence of aliphatic ketones (DBEs of 1−3) as major ketone constituents in the sample before HDS, and being nearly completely absent in all ones after HDS, leads to the conclusion that during HDS the ketones are reduced in a side reaction. On the other hand, high amounts of aromatic ketones that show similar distributions in all hydrodesulfurized materials despite being hardly detectable in the nonhydrodesulfurized sample is evidence that these aromatic ketones are formed from oxidation of fuels components during storage. The oxidation processes during the long storage produced ketones containing several oxygen atoms. For those that possess one extra oxygen, a selective phenol derivatization reagent made it possible to conclude that they are phenols containing a keto group. The present method provides a rapid and simple way of analyzing the reactive class of ketones that has been linked to instability of fuels.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1− S11, text, and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for Matthias Letzel and his co-workers for the Orbitrap measurements. We thank Stephen A. Wise of NIST for donating the Wilmington crude oils.

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DOI: 10.1021/ef5028108 Energy Fuels XXXX, XXX, XXX−XXX