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Determination of antioxidants and corresponding degradation products in fresh and used engine oils Georg Kreisberger, Christian W Klampfl, and Wolfgang W. Buchberger Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01435 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 10, 2016

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Determination of antioxidants and corresponding degradation products in fresh and used engine oils Georg Kreisberger*, Christian W. Klampfl, Wolfgang W. Buchberger Institute of Analytical Chemistry, Johannes Kepler University, Altenberger Strasse 69, 4040 Linz, Austria

KEYWORDS Engine oil stabilization, HPLC, mass spectrometry, antioxidants

ABSTRACT

At elevated temperatures mineral oil based lubricants are prone to oxidation. Thus, antioxidative stabilization of engine oils is an important issue in lubrication engineering. Although there are various tests in order to assess the oxidative stability of a lubricant, only very little is known about the depletion process of antioxidants on a molecular level. The current study presents a solid phase extraction method capable of isolating antioxidants and corresponding degradation products from engine oils as well as a high performance liquid chromatography (HPLC) method for their subsequent separation. For detection the HPLC system was coupled to a UV detector

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and a high resolution quadrupole time-of-flight mass spectrometer (QTOF-MS). These methods enable not only the quantitation of antioxidants employed in lubricants, but can also be used for investigations of their corresponding degradation products in used engine oils. By means of HPLC-QTOF-MS and MS/MS experiments it was possible to detect numerous reaction products formed from antioxidants during their service life time in engine oils. This allowed a deeper insight in the mode of action of the investigated stabilizers.

1 Introduction Protecting engine oils and industrial lubricants from oxidation is an important and challenging issue in lubrication engineering. Since hydrocarbons are prone to oxidation at high temperatures, antioxidants are added to avoid or at least to retard oxidation processes which do not only lead to the formation of harmful species (acids, oil-insoluble sludge, varnish), but also to a deterioration of the lubricant, compromising the lubricating effect. Up to now numerous different classes of antioxidants applied in the field of lubrication are commercially available or have been described in the literature.1 However, there is still much ongoing research in order to develop more effective, efficient and eco-friendly systems. Two of the most relevant classes of antioxidants are sterically hindered phenolic compounds and amine derivatives.1 In the case of engine oils, the propionate-type phenols and alkylated diphenylamines are commonly employed (see Figure 1). Often both classes are part of the lubricant formulation since they exhibit a significant homosynergism.1,2


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Figure 1: Structure of propionate type phenolic antioxidants (A) and alkylated diphenylamines (B) The development of new lubricant formulations and particularly of new antioxidants requires information on a formulation’s oxidation behavior and therefore reliable testing tools. In order to examine the oxidation stability of a lubricant formulation, several oxidation bench tests are available. Some of these tests have been internationally standardized by standardization organizations such as the Co-ordinating European Council (CEC) or the American Society for Testing and Materials (ASTM) and are nowadays widely used in the industry. Most of these tests simulate the oxidation behavior of the oil using harsh oxidative conditions as for instance elevated temperatures combined with high initial oxygen pressures. In some methods, also metals catalysts such as iron, copper or lead are employed. The most commonly used bench tests include pressurized differential scanning calorimetry (PDSC)3–7, thermal-oxidative engine oil simulation tests (TEOST)8–10, the Institute of Petroleum 48 method (IP 48)11,12, or oxygen uptake tests, for example the rotating pressure vessel oxidation test (RPVOT)13. All these routines rely on different parameters, as for instance the oxidation induction time (OIT), the total acid number (TAN), the viscosity, residual carbon, or the formation of deposits. Due to the different measurement principles it is very difficult to correlate results from one method with those from another. However, all of them provide valuable results to assess a formulation’s oxidation stability. Still, these tests do not provide any molecular information on the consumption and degradation of the employed antioxidants during the service lifetime of the lubricant.


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In the literature, a number of different approaches for the qualitative and quantitative analysis of additives in lubricants can be found, including TLC14,15, GC-MS16–20, HPLC18,21 and MS methods22–24. Nevertheless, hardly any study describes the depletion process of antioxidants on a molecular level. There are several studies focusing on the identification of degradation products formed from antioxidants used for the stabilization of polyolefins25–30. Even though lubricants made from mineral oil as well as polyolefins represent hydrocarbon matrices, they exhibit significant differences. Besides different diffusion properties (liquid-solid) also the practical conditions of use are considerably different. Standard-usage engine oils are exposed to temperatures up to 300 °C in combination with catalytically active metal parts and a reactive atmosphere consisting of nitric acid, nitrogen oxides, oxygen and water.31 In contrast to that, components made of polyolefines are typically designed for long-time usage at standard conditions. Furthermore, in the case of engine oils, besides bulk oxidation also thin film oxidation (oil film thickness around 5 · 10-7 m) plays an important role, especially in the upper part of the piston and the cylinder liner.31 The present work is a detailed study on the chemical behavior of commonly used antioxidants in engine oils, in order to provide a deeper insight in the protection mechanism of these stabilizers in lubricant formulations. Therefore, intact antioxidants and their corresponding reaction products were extracted from new engine oils as well as from the same oils after a certain mileage by means of solid phase extraction (SPE). Subsequent HPLC analysis combined with MS and UV detection as well as tandem mass spectrometry (MS²) was selected as appropriate analytical technique. This enables structural elucidation of the most relevant reaction products formed during usage and therefore makes it possible to gain insight into the antioxidative mechanism of the investigated stabilizers.


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2 Materials and Methods 2.1 Chemicals and Materials Tert-butylphenol and Di-tert-butylphenol were purchased from Sigma-Aldrich (Steinheim, Germany), diphenylamine from Merck (Darmstadt, Germany). 3,5-di-tert-Butyl-4-hydroxyoctadecyl-hydrocinnamate

(Irganox

L107),

hexane-1,6-bis[3-(3,5-di-tert-butyl-4-hydroxy-

phenyl)propionate] (Irganox L109), 2,5-di-tert-butyl-4-hydroxy-(C12-C14)-isoalkyl-thioacetate (Irganox L118) and 3,5-di-tert-butyl-4-hydroxy-(C7-C9)-hydrocinnamate (Irganox L135) were provided

by

BASF

(Ludwigshafen,

Germany).

Tetrakismethylene-(3,5-di-tert-butyl-4-

hydroxyhydrocinnamate)methane (Irganox L101), alpha-tocopherol (Irganox E201) and 2,2′thiodiethylen-bis(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (Irganox L115) were obtained from Ciba (Basel, Switzerland); 4,4′-bis(α,α-dimethylbenzyl)-diphenylamine (Naugard 445) from

Chemtura

(Philadelphia,

PA,

USA)

and

tris[(4-tert-butyl-3-hydroxy-2,6-

dimethylphenyl)methyl]-1,3,5-triazinane-2,4,6-trione (Cyanox 1790) from Cytec Industries Inc (Woodland Park, NJ, USA). Acetonitrile, n-hexane, ethyl acetate, methanol and acetone, all analytical grade, were supplied by VWR (Vienna, Austria). Several mineral oil based model oils used for method development were provided by Fuchs Europe Schmierstoffe GmbH (Mannheim, Germany). Referring to the classification of the American Petroleum Institute (API)32, they included group I and group III base oils (group I: saturates < 90 % and/or sulfur > 0.03 %, viscosity index of 80 to 120; group III: saturates ≥ 90 % and sulfur ≤ 0.03 %, viscosity index ≥ 120) containing poly methyl methacrylates as viscosity modifiers, color, extreme-pressure/anti-wear additives and a defoamer.


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Real samples investigated within this work included oil samples obtained from three heavyduty diesel trucks. The brand of oil was the same for the three trucks, but samples were taken after different runtimes, namely 30855 km, 79235 km, and 125900 km. In addition a fresh oil sample of this brand was analyzed. Furthermore, another oil employed in a passenger car was analyzed prior to use and after a runtime of 10000 km.

2.2 Preparation of oil extracts For SPE, glass cartridges (13 mm i.d.) were packed with 800 mg silica gel for column chromatography, particle size 63-200 µm, provided by Merck. Prior to use they were conditioned by flushing it with 5 mL of acetone and another 5 mL of n-hexane. Afterwards, 200 µL oil were applied onto the cartridge. To isolate the target analytes from the rest of the lubricant, the cartridge was flushed with 5 mL of n-hexane with a flow rate of approximately 1 mL min-1, removing the nonpolar matrix. Subsequently the analytes were eluted with 3 mL ethyl acetate applying an approximate flow rate of 1 mL min-1. Before injecting the eluate into a chromatographic system it was diluted with acetone 1:10. For liquid-liquid extraction (LLE), 3 mL of solvent were added (in this study methanol, acetonitrile, isopropanol and acetone were employed) to 0.2 mL of oil in a 10 ml glass vial. This mixture was shaken vigorously by hand for 10 min. After another 10 min of phase separation 1 mL of solvent was transferred into a centrifuge tube and centrifuged for 8 min at 4000 rpm in order to completely separate solvent and residual oil. Prior to further analysis the clear extract was diluted with acetone 1:10.

2.3 Instrumentation


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The experiments with HPLC-UV were performed on an Agilent 1260 Infinity HPLC system equipped with a vacuum degasser, a quaternary pump, an autosampler and a photodiode array detector (Agilent, Waldbronn, Germany). For separation a Kinetex C18 column (50 × 4.6 mm ID, 2.6 µm particle size; Phenomenex, Aschaffenburg, Germany) thermostated at 37.5 °C was used. The injection volume was 5 µL; a binary acetonitrile-water gradient was used. The employed gradient is displayed in Table 1. For detection, UV-Vis spectra from 200 nm to 640 nm (step: 2 nm) were recorded during the whole run. Chromatograms were obtained by extracting the absorbance signal at 200 nm. Table 1. HPLC mobile phase gradient composition Time / min Water (%) 0 60 6 60 9 40 13.5 40 18 25 22.5 13 27 8 31.5 8 37.5 0 45 0 Flowrate: 0.8 ml min-1

Acetonitrile (%) 40 40 60 60 75 87 92 92 100 100

An Agilent 1100 LC System equipped with a vacuum degasser and a quaternary pump coupled to an Agilent 6510 QTOF mass spectrometer was employed for HPLC-MS and MS/MS analyses. The experiments were carried out in positive ionization mode utilizing electrospray ionization. The following MS parameters were used: drying gas flow 10 L min-1, drying gas temperature 340 °C, nebulizer pressure 50 psi, capillary voltage 3750 V, fragmentor voltage 180 V, scan rate


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1 spectrum s-1, acquisition mass range 50-1300 m/z. In order to enhance ionization efficiency at high amounts of organic solvent (at the end of the HPLC gradient) a 10 mM aqueous ammonium formate make-up flow was added after the separation (between column and mass spectrometer) with a flow rate of 0.1 mL min-1. For MS/MS measurements, the collision energy for fragmentation was set to 15 V and 30 V, respectively. GC-MS experiments were carried out with an Agilent 6890N instrument equipped with an MPS 2L autosampler and a programmed temperature vaporization inlet (Cooled Injection System 4, CIS 4) from Gerstel (Mülheim an der Ruhr, Germany). For detection the GC was coupled to an Agilent 5973 mass spectrometer with electron ionization (EI). A ZB-35HT column (15 m, 0.25 mm ID, 0.10 µm film thickness) from Phenomenex (Aschaffenburg, Germany) was employed for separation. Helium with a flow rate of 1.5 mL min-1 was applied as mobile phase. 1 µL of sample was injected in splitless mode at 40 °C and subsequently the CIS 4 was heated up to 275 °C (2 min) with a ramp of 12 °C s-1. For separation a linear temperature gradient starting at 40 °C and ending at 370 °C (held for 2 min) with a heating-rate of 30 °C min-1 was employed. The temperature of the MS transfer line was held at 300 °C, the temperatures of the MS source and the quadrupole at 230 °C and 150 °C, respectively. The specific parameters of the used method are based on work of Sternbauer et al. 33.

3 Results and discussion 3.1 Development of an extraction method In order to be able to analyze antioxidants and thereof derived degradation and reaction products in engine oils, they must be isolated from the lubricant matrix first. Subsequently to


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sample preparation they can be separated and further investigated using chromatographic methods coupled to spectroscopic detector as for instance MS- or UV-Vis-detectors. For the development of the sample preparation as well as the HPLC method, nine commercial antioxidants for lubricants and two potential degradation products were used (see Figure 2). In order to make both methods applicable for a broad range of possible antioxidants and related (unknown) degradation products in real samples, antioxidants with significantly distinct properties were selected for method development. This includes relative small compounds as tert-butyl-phenol (MW: 150 g mol-1) and diphenylamine (MW: 169 g mol-1) up to big molecules as for instance Irganox L101 (MW: 1178 g mol-1). Furthermore, these compounds exhibit significant differences in polarity, ranging from polar antioxidants with several carbonyl and hydroxyl moieties to rather nonpolar compounds such as Irganox L107 containing a C18 alkyl group. For the separation of the oil matrix from the target analytes two different approaches were investigated: first, normal-phase SPE, and second, the use of a polar solvent for liquid-liquid extraction. In order to be able to compare both techniques, they were applied to model oils spiked with the stabilizers displayed in Figure 2.


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HO

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HO O

O

O

O

2

Irganox L109

C 4

Irganox L101

HO

HO

O S

O

O

S

O

2

11-13

Irganox L118

Irganox L115

HO

HO O

O

C8H17

O

C18H37

O Irganox L107

Irganox L135

HO O Irganox E201

OH

OH

NH H N 2

Di-tert-butylphenol

tert-butylphenol

Diphenylamine

Naugard 445

Figure 2: Nine commercially available antioxidants used for method development and two potential degradation products, 2-tert-butylphenol and diphenylamine.


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3.1.1 Solid phase extraction For SPE sample preparation, the oil is applied on a conditioned SPE glass cartridge packed with silica gel and washed with n-hexane. This procedure allows to remove the oil matrix, since nonpolar hydrocarbons such as paraffins and cycloalkanes, which are the main constituents of mineral oil based lubricants, elute with the n-hexane. More polar components such as antioxidants and their corresponding degradation products stick to the stationary phase and can be subsequently eluted employing an appropriate solvent. It cannot be fully ruled out that the recovery of some rather non-polar degradation products is not complete under these conditions. However, the aim of this work was a qualitative analysis of degradation products formed from the employed antioxidants so that even partial recovery is acceptable. The first elution experiments were carried out with 1:1 mixtures of n-hexane and acetone. It was observed that the elution strength of this solvent mixture was obviously not sufficient for the elution of all compounds present in the used engine oils, as a slight yellow color remained on the cartridge after the elution step. As a result, several different solvents including methanol, acetonitrile, acetone and ethyl acetate were investigated regarding their suitability as eluent. Upon the use of polar solvents such as methanol or acetone turbid eluates were obtained, indicating an insufficient solubility of some compounds in these solvents. However, using ethyl acetate resulted in clear solutions. We assume that this can be attributed to the rather nonpolar nature of ethyl acetate (normalized empirical solvent polarity parameter according to Reichardt ETN = 0.22834; n-decane: ETN = 0.009, water: ETN = 1) compared to acetone (ETN = 0.355) or methanol (ETN = 0.762).34 In addition, the eluting power of ethyl acetate is comparable to that of acetone, being at a similar position in the eluotropic solvent series. According to Snyder’s


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empirical eluent strength parameter for hydrophilic adsorbents it even outperforms acetone with a value of ε0 = 0.58 compared to ε0 = 0.56 for acetone.35 The determination of the recovery of the presented SPE method was performed using HPLCUV (see section 2.3). For all analytes depicted in Figure 2 calibration curves with satisfactory values of correlation coefficients were obtained. Furthermore, they exhibit a broad linear range, from their limits of quantitation (depending on the analyte between 50 µg l-1 and 1 mg l-1) up to 100 mg l-1. This corresponds to mass concentrations of 0.0015 % or 0.03 % up to 3 % in the finished formulation and thus perfectly covers typical concentrations of antioxidants in oils, which are between 0.1 % and 0.8 %. To determine the recovery, model oils were spiked with the stabilizers presented in Figure 2 and processed with SPE according to section 2.2. For quantitation,

1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-benzene

(Irganox

1330) was employed as internal standard by adding it to the extract prior to the chromatographic analysis. This compound is a polynuclear phenolic stabilizer, mainly used to protect plastic materials against thermo-oxidative degradation, which is not employed in lubricants. The results of the subsequent HPLC-UV analysis revealed recovery rates between 97.0 % and 105.6 % (n=3). GC-MS measurements were carried out in order to verify a complete matrix removal. Since no matrix related hydrocarbon signals were obtained, it can be assumed that the matrix is completely removed. In order to optimize solvent consumption different amounts of n-hexane were tested. 15 mL, 10 mL and 5 mL were used to remove the matrix of 200 µL of oil (approximate flow rate 1 mL min-1). Subsequent elution and analysis of the obtained eluate by means of GC-MS indicated that already 5 mL n-hexane result in a sufficient removal of the hydrocarbon matrix. Nevertheless, more polar hydrocarbon oxidation products may not be


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removed by elution with n-hexane. However, in this study the focus was on the employed antioxidants, and therefore investigations on the oxidation products of the oil itself were only of minor interest.

3.1.2 Comparison of LLE and SPE Four different solvents were evaluated regarding their suitability for liquid-liquid extraction of engine oils: methanol, isopropanol, acetonitrile and acetone. At first glance all of them seemed to be immiscible with the oil matrix when they were added to a lubricant. However, to ensure that the obtained extracts do not contain any matrix components they were analyzed using GC-MS. Only in the case of methanol and acetonitrile acceptable chromatograms could be obtained, whereas for isopropanol and acetone the chromatograms were dominated by an unresolved hump, revealing the high matrix content in the latter. In order to compare SPE and liquid-liquid extraction, a model oil was spiked with the stabilizers displayed in Figure 2 resulting in stabilizer concentrations of approximately 1500 mg l-1 respectively. 200 µL of the spiked oil were processed four times by SPE (final elution with 3 mL ethyl acetate) and liquid-liquid extractions (3 mL methanol or acetonitrile), respectively, according to section 2.2. Subsequently the extracts were diluted with acetone (1:10) and analyzed by HPLC-UV. The obtained results revealed that for the majority of the investigated compounds SPE exhibits superior extraction efficiency compared to liquid-liquid extraction. Except for one substance (Irganox E201, LLE with methanol, no statistically significant difference between LLE and SPE) all analytes could be detected in statistically significant higher concentrations (n=4, α = 0.95) in the extracts obtained by SPE compared to those from LLE. The average recovery relative to that from SPE was 89.6 % for methanol and 73.0 % for acetonitrile. Furthermore, the results indicate that more non-


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polar substances exhibit recovery rates below the average. For example, the rather non-polar Irganox L107 (containing a C18 moiety) exhibited a recovery of only 81.9 % for methanol and 61.3 % for acetonitrile relative to SPE. Therefore, SPE was preferred over LLE which was thus not included in further experiments.

3.2 Analysis of real engine oil samples In the present work most of the investigations were done on heavy-duty diesel engine oil. First, the actual antioxidative stabilization of the fresh oil was determined. For this purpose an oil sample was processed with the described SPE method and the obtained extract was analyzed with HPLC-UV as well as with HPLC-MS and GC-MS. By means of the exact masses and the obtained EI-MS spectra one phenolic and two aminic stabilizers could be identified. For the phenolic compound, a comparison of the obtained EI-MS spectrum with those from standards of Irganox L107 and Irganox L135 indicated a substance with the same constitution, solely differentiating in the length of one alkyl group (see Figure 3). This could be confirmed by the determination of the exact mass, which revealed the molecular formula C21H34O3. For the aminic antioxidants the structures were suggested by an EI spectra library (see Figure 3) and could be confirmed subsequently with the exact masses obtained from QTOF-MS measurements.


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Figure 3: EI-MS spectra of the identified phenolic antioxidant (A) as well as EI-MS spectrum of a standard of the structural similar Irganox L135 (B). Below (C), a comparison of the measured EI-MS spectrum and the database spectrum of 4,4‘-bis(dinonyl)-diphenylamine. In the next step the used oils were analyzed in same way as the fresh one. A significantly larger number of peaks compared to the fresh oil could be detected (see chromatograms, Figure 4). In order to gain more structural information, signals were further analyzed using tandem mass spectrometry. These experiments suggested that the detected compounds can be attributed to sterically hindered phenolic stabilizers such as those found in the fresh oil.


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Especially at higher collision energies typical fragments associated with this class of antioxidants were observed (see Figure 5A).

Figure 4: HPLC-MS chromatogram of extracts from fresh (A) and used (B) engine oil.

Figure 5: MS² spectrum of one of the most intense HPLC-MS peaks (retention time: 27.6 min) from the chromatogram in Figure 4B, parent ion m/z = 677.44, and the spectrum of one of the most intense GC-EI-MS peaks (retention time: 9.78 min (B).


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Analysis of the investigated extracts employing GC-EI-MS provided further information. Besides the fact that also with EI-MS typical fragments of phenolic antioxidants could be detected (see Figure 5B), the software-supported comparison of the acquired spectra with spectra libraries in combination with the results obtained from the QTOF measurements (corresponding ESI-QTOF mass spectra are shown in the supplement) made it possible to elucidate the structures of several smaller phenolic compounds (see Scheme 1, compounds 1-4), e.g. the toxic 4-nitro-2,6-di-tert-butylphenol.

Scheme 1: Identified degradation products: 1-4 small degradation products; 5-6 dimeric reaction products; 7-10 compounds formed from small degradation products


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Most signals which could not be attributed to a certain structure exhibit m/z values significantly higher than those of the employed stabilizers. Together with the observations from the MS/MS and EI-MS experiments, this indicates that these compounds are higher-molecular reaction products formed from the stabilizer. Analyzing the calculated molecular formulas, it is noticeable that some of these exhibit exactly the double number of carbon and oxygen atoms compared to the employed phenolic antioxidant. This indicates that dimerization could be an important reaction pathway in the formation of the detected substances. The chemical protecting mechanism of primary antioxidants can be attributed to their ability to act as a hydrogen donor. This enables them to trap alkyl and peroxy alkyl radicals, formed in the oil due to oxidation under standard usage conditions, and therefore inhibits the oxidative degradation of the oil. This reaction leads to the transformation of the phenolic compound to the corresponding phenoxyl radical.1,31 In the literature it has been reported that under oxidative conditions, phenolic compounds such as methyl-4-hydroxy-3,5-di-tert-butylcinnamate or more precisely the corresponding phenoxyl or mesomeric cyclohexadienonyl radicals can undergo dimerization reactions via a β-β coupling of two radicals.36,37 In a detailed study by Samsonova et al.38 this general phenomenon has been further investigated describing the (photo)oxidation behavior of propionate type phenolic antioxidants (Metilox and Irganox 1076) under model conditions with respect to the aging processes of stabilized polyolefins. In that study it has been found that oxidizing agents such as lead dioxide or potassium ferricyanide lead to the formation of phenolic dimers as well as quinonemethide dimers derived from the investigated phenolic compounds. Based on the results of these model experiments and the calculated molecular formulas, two dimeric (see Scheme 1, compounds 5-6) and analog trimeric structures could be identified


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(corresponding mass spectra are displayed in the supplement). Due to the coupling in the βposition higher conjugated systems compared to the original stabilizer are formed, resulting in a bathochromic shift in the UV-spectra of the dimeric substances (see Figure 6). Furthermore, also five corresponding oxidation products of the dimers containing one or two additional oxygen atoms (C42H60O7 as well as two isomers of C42H62O8 and C42H60O8 each) were observed.

Figure 6: UV-spectra of the peaks of the intact phenolic antioxidant (A), the phenolic dimer (B) and the quinone methide dimer (C).

In a next step, the calculated molecular formulas of the other peaks with unidentified structure were scanned for possible dimeric products formed via a reaction of small degradation products or the aminic antioxidants with the phenolic stabilizer. Signals with molecular formulas which were almost the sum of the phenolic compound and the employed aminic antioxidants were discovered. The only difference observed was in the number of H atoms (four less), which, however, can be attributed to the fact that the phenolic antioxidant had transformed into its corresponding quinone methide and that an additional bond between the reactants had been formed. In addition, also the further oxidized species of the described compound could be detected, characterized via a molecular formula with exactly two hydrogen atoms less.


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Besides the reaction product of the phenolic stabilizer with itself or with one of the aminic antioxidants, also coupling products of the intact stabilizer and smaller degradation products could be detected. As already reported for the dimeric species, also for these reaction products the oxidized quinone species (with the corresponding bathochromic shift) and several oxidation products with additional oxygen atoms could be detected. Moreover, smaller compounds which originate from coupling of two smaller degradation products were found in the used engine oil. Scheme 1 (compounds 7-10) displays the proposed structures of these compounds. The mass

spectra obtained from the high resolution QTOF-MS measurements which were used for the calculation of the molecular formulas are shown in the supplement. In addition to the heavy-duty diesel engine oil, a passenger car diesel engine oil was also investigated. The analysis revealed the presence of Irganox L135 and the same aminic antioxidant shown in Figure 3. Structure elucidation of the degradation products demonstrates the presence of species analogue to those found in the heavy-duty diesel truck engine oil.

4. Conclusion Within the current study a method capable of isolating antioxidative stabilizers and thereof derived reaction products from mineral oil based lubricants was developed. Furthermore, chromatographic methods for the analysis of the obtained extracts were presented. These methods do not only allow a quantitation of stabilizers in engine oils, but also enable qualitative analysis with respect to unresolved antioxidants and their corresponding reaction or degradation products. Employing the developed methods for real oil samples enabled the identification of the most important degradation products of the used antioxidants formed during usage of the stabilized


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lubricant in a combustion engine. It became apparent that dimerization, or more general a radical C-C coupling, is the main reaction pathway of the investigated classes of antioxidants. Further oxidation of these reaction products results in dimeric quinone methides or other oxidized compounds originated from additional oxygen uptake. This reaction behavior can be seen as a probable reason for the high effectiveness of the investigated stabilizers.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], Phone: +43 732 24688721

Notes ABBREVIATIONS HPLC, high performance liquid chromatography; MS, mass spectrometry; QTOF, quadrupole time-of-flight; GC, gas chromatography; EI, electron ionization; TLC, thin layer chromatography; SPE, solid phase extraction; LLE, liquid-liquid extraction.

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Determination of Antioxidants and Corresponding ... - ACS Publications

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