Chemical Characterization of Lube Oils - Energy & Fuels (ACS

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Article pubs.acs.org/EF

Chemical Characterization of Lube Oils Antonina Kupareva,† Paï vi Mak̈ i-Arvela,† Henrik Grénman,† Kari Eran̈ en,† Rainer Sjöholm,‡ Markku Reunanen,§ and Dmitry Yu. Murzin*,† †

Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, ‡Laboratory of Organic Chemistry, Laboratory of Wood and Paper Chemistry, Process Chemistry Centre, Åbo Akademi University, FI-20500 Turku/Åbo, Finland

§

ABSTRACT: In this paper, estimation of the chemical composition of used oils collected from several European locations was performed on the basis of a comparative analysis of chemical composition of commercially available fresh and used motor oils. Although the motor oil undergoes a range of chemical and physical transformations during routine engine operations, information about the structure of hydrocarbons in the fresh oil allows for an estimation of the approximate ratio of different types of hydrocarbons in the same oil after its use. As an example, a particular type of fresh oil was used in the engine and then reanalyzed by the same analytical techniques. Gas chromatography−mass spectrometry (GC−MS), Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and elemental analysis (CHNS/O analysis) were used to characterize the chemical composition of the oil samples. A comparison of the obtained results showed correlations between chemical properties of the fresh oil and the collected used oil. Both oil FTIR spectra exhibited the bands that are related to the presence of carbonyl groups and amine-containing compounds, respectively. Opposite from the fresh oil, phenols were not found in the used oil. According to the obtained 1H NMR spectra, the paraffinic hydrocarbons of the fresh oil are more linear and have longer chains than those in the used oil.

1. INTRODUCTION Nowadays, globally different automotive sources generate large amounts of used oils. To achieve maximum energy conservation and environmental benefits, it is generally preferable to re-refine used oils into regenerated base oils. The regeneration industry is an important part of European independent lubricant production, which represents one-third of the European market for lubricant volume (1.5−2 million tons of lubricants). On the basis of data published by the European Re-refining Industry Section of UEIL, Groupement Européen de l’Industrie de la Régénération (GEIR),1 today, the European waste oil recycling industry comprises 28 plants. A total of 17 of these plants produce base oils. The industry has a total nameplate capacity of 1.3 million tons/year and a total lube oil production of 400 000 tons/year and produces 500 000 tons/year of other products including fuels, asphalt, gas oil, flux oil, etc. An approximate total turnover is between € 200 million and 250 million/year.1 Composition of new motor oils and new oil additives is changing because of continuous engine modification. The variability of lubricating fluids has increased in recent years to meet the demands of new engines having more stringent requirements because of their operation under more severe conditions or in challenging environments. Significant growth in markets of synthetic and semi-synthetic oils has been observed. Novel oil compositions are certainly known to oil manufacturers, while this information is unavailable to rerefineries processing used oil. For optimal and qualitative oil, re-refining data on the used oil chemical structure is therefore needed. The dependence of the chemical composition of crude motor oils upon the oil refining processes was described.2 It has been demonstrated that various process parameters and severities of operation for lubricant refining can be optimized in terms of structural data to obtain base oils with improved © 2012 American Chemical Society

performance properties. Likewise, the correlation between chemical characteristics of the fresh and used motor oils can be used for optimizing and updating technology of re-refining the waste oils. Motor oil analysis, being of importance for engine manufacturers, usually includes oxidation and nitration measurements, estimation of viscosity, total acid number, total base number, and other general properties of motor oil, which are useful for automobile users. There is, however, very little information on the chemical composition of modern fresh oils and hydrocarbon transformations during their use. A number of literature sources provided detailed information on the chemical and physical properties and performance characteristics of the lube oil.3−16 Infrared spectroscopy plays an important role in lubricant analysis to characterize various constituents qualitatively. Fourier transform infrared (FTIR) spectroscopy has been used for the determination of the moisture content in a wide range of lubricants,7 viscosity indexes, and base numbers of motor oil,8 for studies of chemical changes occurring at the lubricant additive interface during heating and sliding at high temperatures,9 and for estimation of differentiation between used motor oils,10,11 fresh motor oils,12 virgin and recycled engine oils.13 Comparative physical and chemical analyses of fresh and used gasoline engine oils with mineral base have been performed by inductively coupled plasma−optical emission spectroscopy (ICP−OES).14 Correlations between electrical, mechanical, and chemical properties of fresh and used aircraft engine oils have been estimated by measuring their resistivity, permittivity, and viscosity as a function of the temperature.15 The major part of studies Received: June 27, 2012 Revised: December 13, 2012 Published: December 13, 2012 27

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available in the literature is devoted to the estimation of the metal content in motor oils9,16,17 and their physical properties, while the chemical composition of engine oils defines the key characteristics of these oils. In view of the complex structure of the used oils, acquiring full information on the oil composition from single analysis is impossible. According to our knowledge, there is only one publication reporting the application of different methods for comparative analysis of fresh and used oil chemical structures. Results of several analyses of fresh, used, and weathered motor oils by gas chromatography−mass spectrometry (GC−MS), nuclear magnetic resonance (NMR) spectroscopy, and FTIR spectroscopy techniques were correlated.18 These results showed the presence of aromatic hydrocarbons, including polycyclic aromatic hydrocarbons in the used oil, while they are absent in the fresh oil. Furthermore, it was demonstrated18 that various analytical techniques can be applied for the analysis of hydrocarbon composition of motor oils as well as various functional groups and compounds present in the used oil. Information dealing with the general structure of the fresh oil is helpful and, for the most part, adequate to predict the fate of the oil after its use in the re-refining process. The objective of the present study is to estimate the chemical composition of used oil, which is a current feedstock for rerefineries. The estimation was based on analytical results of commercially available fresh motor oil, the same type of commercially available motor oil after being used, and the used oil from a re-refinery. The oil samples were analyzed by GC− MS, FTIR spectroscopy, NMR spectroscopy, and elemental analysis (CHNS/O analysis). The results were correlated to obtain detailed information regarding the composition of the used oil.

Figure 1. Distillation curves of analyzed oil samples. suggests some gasoline contaminations in the former oils. The temperature profile of the spent oil rapidly approaches the temperature profile of the fresh oil after this initial low-temperature point. The distillation temperatures of the collected used oil are lower than the fresh and spent oils, with the average difference being 35 °C. This indicates that the used oil contains more volatile compounds compared to the fresh oil, with some amount of residual fuel that distills early. 2.1. GC−MS. One drop of lube oil was diluted with hexane [highperformance liquid chromatography (HPLC) grade] to 2 mL and analyzed by a Hewlett-Packard 6890/5973 gas chromatograph coupled to a mass selective spectrometer detector. The gas chromatograph was equipped with an Agilent 19091J-002 capillary column, with a 25 m length, 0.20 mm internal diameter, and 0.11 μm film thickness. Helium was the carrier gas. The temperature program consisted of a heating rate of 8 °C/min from 80 to 340 °C with a hold time of 6 min. 2.2. FTIR Spectroscopy. Potassium bromide (KBr) discs were used to analyze oil samples by FTIR spectroscopy. Background spectra were obtained by scanning two clean discs in the instrument. One drop of the analyzed oil was placed on one KBr-polished disc and was covered with a second KBr disc. Both discs were placed in a Bruker IFS 66v/S instrument. Scans were carried out in the 4000−500 cm−1 range. 2.3. 1H NMR Spectroscopy. 1H NMR experiments were carried out on all oil samples. Deuterated chloroform (CDCl3) was applied as a solvent. The spectra were obtained with a Bruker AV600 NMR instrument. NMR data and spectra were processed by Bruker’s TopSpin NMR software. 2.4. Elemental Analysis. Carbon, hydrogen, nitrogen, sulfur, and oxygen contents were estimated using a FlashEA 112 organic elemental analyzer according to the standard test procedures. The technique used for the determination of CHNS/O was based on the quantitative “dynamic flash combustion” method.

2. MATERIALS AND METHODS The used lube oil investigated in the present work contained about 85−95% used motor oil, which was collected from several automobile crankcases, with the rest (5−15%) being industrial oil. The combined oil was dried by adding calcium chloride pellets (CaCl2) for carrying out FTIR spectroscopy analysis. A fresh commercially available synthetic motor oil was purchased. The fresh motor oil was added to a 1.5 L engine and run for 7000 km, and then it was drained for analysis. Typical physical properties of the fresh, spent, and used samples are shown in Table 1, while oil distillation curves are depicted

Table 1. Physical Properties of Fresh and Spent Motor Oils and Used Motor Oil from Re-refinery Storage physical property

fresh oil

spent oil

used oil

viscosity at 100 °C (cSt) viscosity at 40 °C (cSt) viscosity index density at 15 °C (mg/L)

10.99 61.76 172 0.854

9.87 57.52 158 0.860

8.49 50.07 146 0.871

3. RESULTS AND DISCUSSION 3.1. GC−MS. GC−MS spectra of oil samples revealed bellshaped curves at 18−32 min (Figure 2). The noise observed in the chromatogram of the used lube oil hindered identification of the observed peaks. The used oil chromatogram showed the peaks at 6−18 min, which indicated the peaks characteristic of gasoline components, including short hydrocarbons, cyclic paraffins, and aromatics. The most abundant peaks were located between 18 and 27 min. Approximate analysis of GC−MS data demonstrated that the used lube oil contains a broad range of aromatic and aliphatic hydrocarbons, with chain lengths ranging

in Figure 1. In the text below, the term “used oil” corresponds to oil delivered to a particular re-refinery from multiple European used oil collection centers, “fresh oil” corresponds to a commercially available motor oil, and “spent oil” denotes the same commercially available motor oil after being used in the engine, specified above. The fresh oil has higher viscosity and distillation temperatures. This trend is not surprising, because higher viscosity oil generally has heavier, less volatile components. The flatter shape of the curve for the fresh oil indicates that the oil contains compounds with similar boiling points. Figure 1 displays the distillation curves of the spent and used lube oils. The first drops of the spent and used oils began to distill at significantly lower temperatures than that of the fresh oil. This 28

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Figure 2. GC−MS spectrum of fresh motor oil, spent motor oil, and used oil: 1, napthalene; 2, butyl-diphenylamine; 3, octyl-diphenylamine; 4, butyl-octyl-diphenylamine; 5, 4,4′- methylenebis(2,6-di-tert-butylphenol); 6, p,p′-dioctyldiphenylamine; 7, (E)-N,N′-di-tert-butyl-2,2,7,7tetramethyloctane-1,8-diimine; 8, N-[1-(1-naphthyl)ethyl]-2-naphthamide; 9, 3-hexenoic acid,5-hydroxy-2-methylmethyl ester; and 10, [1,1′bicyclohexyl]-1,1′-diol.

esters of linear unsaturated acids provide synthetic oils with thermal and oxidative properties.19 Alkylated diphenylamines observed in the spectra of the fresh and spent motor oils are being used as antioxidant additives for engine oils. Opposite from the presence of 4,4′-methylenebis(2,6-di-tert-butylphenol), which is the hindered phenolic antioxidant, in the fresh motor oil, this compound was not found in the spent motor oil. In addition, more volatile compounds, such as naphthalene, were detected in the spent oil sample. The presence of a light fraction was also confirmed by a lower initial boiling point of the spent oil than of the fresh oil. Thus, complexity of the used oil composition allows for estimation of the carbon number range in different compounds being present in the used oil. However, additional studies are

from C16 to C32. Hydrocarbons were also detected in the fresh and spent motor oils. The most abundant peaks were located between 22 and 28 min, which showing the presence of hydrocarbons with longer chains than those in the used oil. Moreover, the chromatogram of the analyzed samples revealed numerous intense peaks, which were identified as various diphenylamine derivates and oxygen-containing organic compounds. The peaks of (E)-N,N′-di-tert-butyl-2,2,7,7-tetramethyloctane-1,8-diimine, N-[1-(1-naphthyl)ethyl]-2-naphthamide, and 3-hexenoic acid,5-hydroxy-2-methylmethyl ester were visible in all analyzed samples. The intensity of these peaks declined for the spent motor oil. Imide and amide derivates can be present in motor oil as dispersants.5 Methyl 29

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Figure 3. FTIR spectra of oils: I, used oil; II, spent oil; and III, fresh oil. Fresh oil spectra: (1) O−H vibrations in alcohols and phenols. Used oil spectra: (3) Si−H vibrations and (6) NO vibrations in nitrosamines. Fresh/used oil spectra: (2) C−H vibrations in hydrocarbons, (4) CO vibrations, (5) aromatic rings, (6) carbonates, (7 and 9) sulfonates, (9) methacrylates, (8) C−N and N−H vibrations in amines, (10) P−O−C vibrations in ZDDPs, and (11) PS vibrations in ZDDPs.

additive for lube oil and typically exhibits the bands at 1701 and 1154 cm−1.12,13 The intensity of polymethacrylate peaks in the spectrum of the fresh oil is significantly higher than in the spectra of the used and spent motor oils. On the basis of the study by Al-Ghouti et al.,8 which demonstrated that the band at 1701 cm−1 is an informative wavenumber for viscosity index (VI) estimation of lube oils, the fresh oil had a higher viscosity index than the other oils. Several metal-containing compounds are added to oils to improve lubricant characteristics. Some of the characteristic infrared absorption bands of “single” additives were recognized in the spectra of the fresh and used oils. Zinc dialkyl dithiophosphates (ZDDPs) are organometallic compounds, which are one of the most effective antioxidants and, therefore, included as a key component in many oxidation inhibitor packages for engine oils.3 Bands associated with the P−O−C bonds of ZDDPs are located at 1050−920 cm−1.10 The characteristic frequency of the PS bond is around 1040−950 cm−1.20 Sulfonate, phenate, and carboxylate are the common polar groups present in detergent additives of motor oil. The spectra of the analyzed oils showed the peaks at around 1376 and 1154 cm−1, which are associated with the presence of sulfonate salts in the oils. Carbonates of overbased sulfonates absorb at 1490−1410 cm−1.21 Amines are also present in the lube oils as multifunctional additives; moreover, their presence was demonstrated by GC− MS analysis. Different types of amine-containing compounds have been found to provide appreciable antioxidative proper-

needed to further elucidate the chemical structure of unidentified compounds. 3.2. FTIR Spectroscopy. The chemical composition of the fresh, spent, and used motor oils was investigated by comparing their FTIR spectra. The hydrocarbon composition of fresh or used automotive lubricating oils consists primarily of saturated compounds, such as linear and branched chain paraffins. The analyzed samples possessed multiple bands at the 2954−2856 cm−1 region (Figure 3), an intense band at 1463 cm−1, and a less intense band at about 1376 cm−1, which correspond to the presence of a mixture of hydrocarbon compounds with short carbon chain lengths and C−H branching vibrations containing −CH− groups in the sample according to Nakanishi and Solomon.20 Lube oils are made by introducing proper additives, which are used to enhance the natural properties of the oils and to prevent some undesirable properties. Oxygen-containing functional groups can be present in fresh oils as a part of friction modifiers or lubricity additives, which are generally polar molecules, composed of a polar functional group (ketone, ester, and carboxylic acid) and a nonpolar hydrocarbon tail.5 The spectra comprise the bands at 1747 and 1701 cm−1, which are related to compounds with carbonyl groups from esters, ketones, or acids. In more detail, the band at 1747 cm−1 indicated the presence of five- and seven-membered cyclic ketones. The band at 1701 cm−1 can also be referred to as polymethacrylate in the oil samples. Polymethacrylate is being used as a viscosity modifier and a pour-point depressant 30

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Table 2. Chemical Shift of Different Types of Protons in 1H NMR Spectra of Fresh and Used Oil

ties, dispersion, and anticorrosion effects to engine oils. Succinimides, typical dispersants, are normally difficult to identify through their infrared spectra because of weak absorption bands and a possible overlapping by other components, e.g., carbonyl compounds. Their CO band is normally located as a doublet or triplet between 1750 and 1670 and the C−N and N−H vibration bands at 1310−1210 cm−1.21 The fresh oil spectra showed the doublet band at 1747−1709 cm−1 and also bands at 1305 and 1230 cm−1, which confirmed the presence of succinimides in the fresh oil. In addition to carbonyl compounds and metal-containing groups, amines were also present in the used and spent motor oils. However, the bands indicating C−N and N−H vibrations in these samples of lube oils had less intensity than in the fresh oil. The used oil spectra showed the band at 1457 cm−1, which may be associated with not only the presence of a mixture of hydrocarbon compounds, as mentioned above, but also the formation of carcinogenic nitrosamines because of the chemical transformation of amines from motor oil additives. Another class of commonly used additives in industrial and automotive lubricating oils and greases is antioxidants. The fresh oil sample possessed an absorption band at 3648 cm−1, which indicates O−H stretching vibrations in the monomeric alcohols and phenols. The phenols and especially the sterically hindered phenols are being extensively used as antioxidant additives.5 GC−MS of the fresh oil also demonstrated the peak of 4,4′-methylenebis(2,6-di-tert-butylphenol). Opposite from the presence of phenols in the fresh motor oil, these compounds were not found in the used and spent motor oils. Another difference in the FTIR spectra between analyzed samples was the presence of silicon-containing species, as indicated by absorption bands in the 2294−2102 cm−1 region in the used oil, although no silicon compounds were found in the fresh and spent oils. These compounds can be added to industrial as well as motor lube oils as foam inhibitors.5 Even if during motor oil use the concentration of aromatic hydrocarbons is expected to increase, FTIR spectroscopy analysis did not show this growth. However, the NMR spectra demonstrated the presence of aromatic compounds in both the fresh and used oils (see below). The FTIR spectra of all samples exhibited a low-intensity peak at 1600 (1606) cm−1 from the aromatic ring stretching vibrations. On average, polycyclic aromatic hydrocarbons represent about 4−8% of hydrocarbons in the used motor oils.18 Thus, it can be concluded that the FTIR method alone is not adequate for the determination of aromatic compounds in the oils. 3.3. 1H NMR Spectroscopy. 1H NMR spectra of the analyzed oil samples provided more detailed information regarding the hydrocarbon structure of the oils and also confirmed some data obtained by FTIR spectroscopy (Table 2). The spectra of analyzed oils contained the peaks corresponding to oxygen-containing functional groups at δ 3.4−5.0 ppm (Figure 4). The oxygen-containing compounds of the fresh oil were esters and ethers, while the used oil contained essentially alcohols. The fresh oil spectra demonstrated two high-intensity singlets at the chemical shift of δ ∼ 5.0 and 4.0 ppm and a less intense doublet at δ ∼ 4.8 ppm corresponding to the presence of esters; thus, the oil can be qualified as a synthetic fluid. The spent and used lube oils processed only one singlet with low intensity at δ ∼ 4.00 ppm, corresponding to esters. The content of esters in the fresh oil can also be confirmed by FTIR spectroscopy results, which showed a higher amount of carbonyl groups in the fresh oil than in the

proton CH3−C −C−CH2−C R−CH2−NR−N< R−CH2−COO− −R2−CH−COOH −C−CH2−O−R −C−CH2−O−H −C−CH−O−R −C−CH−O−H −C−CH2−O−CO−R H2O −C−CH−O−CO−R −R2−CH−OCOR− olefinic protons aromatic protons phenols

fresh oil δ (ppm)

spent oil δ (ppm)

used oil δ (ppm)

0.9−0.8 1.34−1.25

0.9−0.8 1.33−1.2

0.9−0.8 1.33−1.2

2.3

2.3

2.3

3.6 3.8 3.9 4.0 4.8 5.0 5.65−5.5 6.95−7.05 7.77

4.0

5.55 6.8−7.1

2.6 3.4 3.6 3.7 3.9 4.0 4.8

5.1−5.3 6.75−7.2

Figure 4. NMR spectroscopy spectra of oils (8.0−2.0 ppm): I, used oil; II, spent oil; and III, fresh oil.

used and spent oils. Moreover, the used oil spectra exhibited a broad peak at δ ∼ 2.6 ppm, corresponding to the presence of organic acids, which are commonly formed in lube oils via chemical oxidation processes. 31

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In addition to oxygen-containing compounds, there were a few notable differences between the fresh, spent, and used motor oils found in the NMR spectra. The fresh oil 1H NMR spectrum showed the presence of phenols in the chemical shift range δ ∼ 7.4−7.9 ppm, in line with GC−MS and FTIR spectroscopy analyses. Opposite from it, the presence of phenol was not found in the spent and used oils. Thus, phenolic antioxidants were depleted during use of the motor oil. The used oil spectrum contained a broad signal at δ ∼ 4.8 ppm, showing the presence of water. In fact, water is one of the contaminants in the motor oil, usually appearing in the used oil at the stages of collection, transportation, and storage of the used oil. The spectra of analyzed samples exhibited a strong signal with chemical shifts in the range δ ∼ 2.1−2.4 ppm, which could be associated to α-methylene protons of amines, as well as carboxylic acids and esters.22 These compounds were also identified by FTIR spectroscopy and GC−MS. Moreover, a reduction of the amine peaks intensity was observed in FTIR and GC−MS spectra of the spent and used oils. The peaks observed at the chemical shift of δ ∼ 7.0 ppm corresponded to aromatic hydrocarbons in the samples. The molar percentage of aromatic protons was equal to 0.27 and 0.94 for the fresh and used oil samples, respectively. Aromatic hydrocarbons can be added to fresh motor oils as a part of various additives. Aromatic amines are used as multifunctional antioxidants, antiwear agents, and viscosity index improvers for lubricants, while aromatic sulfides represent oxidation and corrosion inhibitors.5 GC−MS fresh oil spectra also contained several aromatic-containing compounds. In the used oil, aromatics may arise from the formation of polycyclic aromatic hydrocarbons, nitrogen-based compounds, i.e., nitrates, and other substances, i.e., acids, esters, and peroxides, formed during engine operation.13 A comparison of NMR spectra of the fresh and spent oils showed a 2-fold increase of aromatics in the latter motor oil. Estimation of the total concentration of olefinic protons exhibiting a signal at δ ∼ 5.6 ppm in the fresh and spent oils from NMR spectra showed that the amount of olefins decreased with lube oil usage. However, the spectra of the industrially collected used oil demonstrated an olefin content almost 3-fold higher than in the fresh oil. This fact can be explained assuming that the collected used oil was in nature more olefinic. The molar percentage of aliphatic protons was equal to 99.7 for the fresh oil sample, whereas it was 98.9 for the used oil sample. Methyl and methylene proton peaks were observed between 0.95 and 0.8 ppm and in the 1.35−1.2 ppm range, respectively (Figure 5). A comparison of the spectra showed that the paraffinic hydrocarbons in the fresh oil are more linear and have longer chains than those in the used oil. No differences in the concentration of aliphatic hydrocarbons and their structures in the fresh and spent oil samples were observed. Furthermore, the analyzed oil spectra showed a sharp singlet produced by protons of the t-butyl group at the chemical shift of δ ∼ 1.1 ppm. The intensity of the t-butyl group peak diminished with lube oil usage. The ratio between −C−CH2−C and CH3−C groups is equal to 3.1 and 2.1 for the fresh and used oils, respectively. 3.4. Elemental Analysis. This analysis, which was performed for the fresh and used motor oils demonstrated differences in the elemental compositions (Figures 6 and 7). The used oil contained about 4.5 molar percent less hydrogen

Figure 5. NMR spectroscopy spectra of oils (2.05−0.55 ppm): I, used oil; II, spent oil; and III, fresh oil.

Figure 6. Carbon and hydrogen contents in the oil samples.

compared to that in the fresh oil, but the carbon content was very similar in both samples. These results may be associated with the higher concentration of olefinic and aromatic hydrocarbons in the used oil than in the fresh oil, what was also identified by NMR analysis. The obtained data showed that the content of nitrogen and oxygen in the lube oil after its use is higher than in the fresh oil. The differences were 0.73 and 0.25 mol % for nitrogen and oxygen contents, respectively. Lube oil oxidation occurs, causing the breakdown of a lubricant because of aging and 32

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Differences in the concentration of aliphatic hydrocarbons and their structures in the modern fresh and used motor oils were elucidated. The used oil showed a higher content of methyl protons than the fresh oil; thus, the used oil is more branched. Dependent upon the chemical structure of the used oil, it is possible to select the type of used oil re-refining processes and to regulate the severity of them. On the basis of the obtained data, the process of recovery could be catalytic hydrotreating for the removal of heteroatoms and saturation of unsaturated hydrocarbons. The content of aromatics and olefins in the used oil allows for the estimation of the severity of hydrogenation and the rate of needed hydrogen for oil re-refinery. The branched aliphatic hydrocarbons present in the used oil should provide a sufficient pour point, and hydroisomerization of this oil is optional. FTIR spectroscopy results demonstrated the reduction of carbonyl groups with lube oil use, which leads to a decrease of the viscosity index of the oil. GC−MS and NMR results showed the depletion of phenol-containing antioxidants. Hence, the re-refined oil should be blended with the new additive packages.

Figure 7. Oxygen, nitrogen, and sulfur contents in the oil samples.

harsh operating conditions. Nitration is primarily a problem in natural gas engines. The reason for the nitration process of the motor oil may be excessive “blow-by” from cylinder walls and compression rings. A higher amount of oxygen in the used oil compared to that in the fresh oil confirmed by NMR results demonstrated a high amount of water and alcohols in the sample of used oil. However, the elemental analysis results regarding nitrogen content in the oils are not in line with the FTIR spectroscopy and NMR results, which both showed more intense peaks of nitrogen-containing compounds in the fresh oil. Likewise, the elemental analysis did not show the presence of sulfur in the fresh oil sample, while FTIR spectroscopy clearly indicated the peaks, which correspond to ZDDPs being used as antioxidant additives for lube oils. Elemental analysis is thus not that precise in the determination of elements contained in the oils, and obtained results should be confirmed by other analytical techniques, such as FTIR spectroscopy, GC−MS, and NMR.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 358-221-549-85. E-mail: dmurzin@abo.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was funded by the National Technology Agency of Finland (Tekes), which is gratefully acknowledged. Ida Rönnlund is gratefully acknowledged for the provided equipment to carry out elemental analysis.

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REFERENCES

(1) European Re-refining Industry Section of UEIL, Groupement Européen de l’Industrie de la Régénération (GEIR). An Environmental Review of Waste Oils Regeneration; GEIR: Brussels, Belgium, 2004; http://www.geir-rerefining.org/GEIR_documents.php (accessed Nov 2004). (2) Sharma, B. K.; Adhvaryu, A.; Perez, J. M.; Erhan, S. Z. Effects of hydroprocessing on structure and properties of base oils using NMR. Fuel Process. Technol. 2008, 89 (1), 984−991. (3) Isa, F. M.; Haji-Sulaiman, M. Z. An investigation of the relationship between used engine oil properties and simulated intake valve deposits. Proc. Inst. Mech. Eng., Part D 1997, 211 (5), 379−389. (4) Mang, T.; Dresel, W. Lubricants and Lubrication, 2nd ed.; WileyVCH: Weinheim, Germany, 2007. (5) Rudnick, L. R. Lubricant Additives: Chemistry and Applications, 2nd ed.; CRC Press: New York, 2009. (6) Rudnick, L. R. Synthetics, Mineral Oils, and Bio-Based Lubricants; CRC Press: New York, 2006. (7) Van de Voort, F. R.; Sedman, J.; Cociardi, R.; Juneau, S. An automated FTIR method for the routine quantitative determination of moisture in lubricants: An alternative to Karl Fisher titration. Talanta 2006, 72 (1), 289−295. (8) Al-Ghouti, M. A.; Al-Degs, Y. S.; Amer, M. Application of chemometrics and FTIR for determination of viscosity index and base number of motor oils. Talanta 2010, 81, 1096−1101. (9) Piras, F. M.; Rossi, A.; Spencer, N. D. Combined in situ (ATR FT-IR) and ex situ (XPS) study of the ZnDTP−iron surface interaction. Tribol. Lett. 2003, 15 (3), 181−191. (10) Zięba-Palus, J.; Kościelniak, P.; Łącki, M. Differentiation between used motor oils on the basis of their IR spectra with

4. CONCLUSION The chemical composition of used oil, collected from several European locations, was compared to the chemical composition of the commercially available fresh motor oil. The fresh and used motor oils were investigated via GC−MS, FTIR spectroscopy, and NMR techniques and elemental analysis with an organic elemental analyzer. Combined analysis of the obtained results demonstrated that these techniques could be applied for the determination of the chemical nature of the lube oil and could be a helpful tool for distinguishing between compositions of oil samples of various degrees of use. The obtained data showed that the hydrocarbon structure of the motor oil is changed insignificantly during its operation and the major part of the changes is accounted for with depleted oil additives. The modern fresh oil has the following chemical composition: 99.7% aliphatic hydrocarbons, 0.27% aromatic hydrocarbons, and 0.03% olefinic hydrocarbons. During its application, oxidation processes occur, resulting in a slight increase of aromatic hydrocarbons and sulfur contents in used oil, although these changes can be probably related to the presence of used industrial oil with a higher amount of the latter compounds in the analyzed used oil. The estimation of the hydrocarbon structure of the used oil showed the presence of 98.9% aliphatic hydrocarbons, 0.94% aromatic hydrocarbons, and 0.08% olefinic hydrocarbons. 33

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application of the correlation method. Forensic Sci. Int. 2001, 122, 35− 42. (11) Zięba-Palus, J.; Kościelniak, P.; Łącki, M. Differentiation of used motor oils on the basis of their IR spectra with application of cluster analysis. J. Mol. Struct. 2001, 596, 221−228. (12) Zięba-Palus, J.; Kościelniak, P. Differentiation of motor oils by infrared spectroscopy and elemental analysis for criminalistic purposes. J. Mol. Struct. 1999, 482−483, 533−538. (13) Al-Ghouti, M. A.; Al-Atoum, L. Virgin and recycled engine oil differentiation: A spectroscopic study. J. Environ. Manage. 2009, 90, 187−195. (14) Rahimi, B.; Semnani, A.; Ejhieh, A. N.; Langeroodi, H. S.; Davood, M. H. Application of ICP−OES in the comparative analysis of a used and fresh gasoline motor oil. Global J. Sci. Front. Res. 2012, 12 (2), 9−14. (15) Gajewski, J. B.; Głogowski, M. J.; Paszkowski, M.; CzarnikMatusewicz, B. Correlation between electrical, mechanical and chemical properties of fresh and used aircraft engine oils. J. Phys.: Conf. Ser. 2011, 301, No. 012050. (16) Meltzer, M. Metal-Bearing Waste Streams: Minimizing, Recycling, and Treatment; Noyes Data Corporation: Park Ridge, NJ, 1990. (17) Coyle, T. D.; Siedle, A. R. Metals in oil occurrence and significance for reuse of spent automotive lubrication oil. NBS Spec. Publ. (U. S.) 1979, 556−559. (18) Dominguez-Rosado, E.; Pichtel, J. Chemical characterization of fresh, used and weathered motor oil via GC/MS, NMR and FTIR techniques. Proc. Indiana Acad. Sci. 2003, 112 (2), 109−116. (19) Schlosberg, R. H.; Chu, J. W.; Knudsen, G. A.; Suciu, E. N. High synthetic esters for synthetic lubricant applications. Lubr. Eng. 2001, 56, 21−26. (20) Nakanishi, K.; Solomon, P. H. Infrared Absorption Spectroscopy, 2nd ed.; Holden-Day, Inc.: San Francisco, CA, 1977. (21) Fodor, G. E. Lubricants Quality Analysis System Development; Defense Technical Information Center, Cameron Station: Alexandria, VA, 1994; Vol. 2: Chemical Characterization of Lubricants, Interim Report, BFLRF 293. (22) Hase, T. Tables for organic spectrometry; Otatieto, Helsinki, 2008.

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dx.doi.org/10.1021/ef3016816 | Energy Fuels 2013, 27, 27−34