Oxidative Stability of the Plain and Additized Mineral Base Oil

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Article pubs.acs.org/EF

Oxidative Stability of the Plain and Additized Mineral Base Oil Samples Monitored through Gas Chromatography−Mass Spectrometry Imtiaz Ahmad,*,† Jan Ullah,† M. Ishaq,† Hizbullah Khan,‡ Razia Khan,† Waqas Ahmad,† and Kashif Gul† †

Institute of Chemical Sciences, and ‡Department of Environmental Sciences, University of Peshawar, 25120 Peshawar, Khyber Pakhtunkhwa, Pakistan ABSTRACT: Gas chromatography−mass spectrometry (GC−MS) was used to monitor the thermo-oxidative degradation of the plain and additized mineral base oil (MBO) samples. Antioxidants used were MeOH extracts of the rice husk (RHE) and saw dust (SDE). The oxidation was performed for 6 h at 200 °C according to the modified Institute of Petroleum (IP) method. The results indicate that the additized MBO samples inhibited thermo-oxidation compared to the plain sample. Among the two antioxidants, the RHE showed excellent antioxidant potential compared to the SDE.

1. INTRODUCTION Lubricating oil in service suffers from degradation after some time (3000−7500 miles) and needs to be changed with new oil to avoid any possible damage to the engine. This frequent change is a poor choice both economically and environmentally.1 A number of factors are responsible for engine oil degradation, which include oxidation, presence of contaminants, depletion of additives, operating conditions of the engine, etc. Among these, oxidation at an increased temperature is considered to be the major factor that leads to the formation of peroxides, aldehydes, ketones, carboxylic acids, alcohols, and esters.2,3 These products, mostly carboxylic acids, have antagonistic effects on additives and decline their performance/serviceability.1 Further, the presence of contaminants, such as fuel, soot, water, and ethylene glycol, can accelerate oil degradation and lead to the formation of highmolecular-weight sludge and varnish, which bear undesirable effects on some of the oil-desired properties.4 To maintain the integrity of the lubricating oil, the influence of the above-mentioned factors can be neglected or minimized by avoiding its oxidation, minimizing exposure to contaminants, and using additives that can withstand the hostile conditions in the engine. Ample studies are underway worldwide on the use of antioxidants to avoid oil degradation;5−8 however, most of the antioxidants developed to date are alkaline in nature and cannot retain their integrity in the presence of acidic oxidized products.1 Moreover, they are costly and environmentally unfriendly.9,10 Therefore, concerted efforts are needed to develop new, efficient, cheap, and environmentally friendly antioxidants to be effective enough to withstand thermooxidative degradation of oil and to increase its service life. Among the antioxidants, phenolics and aminics have been reported to be effective in increasing the oxidative stability of petroleum and petroleum-like products.11−13 Biomass materials, including saw dust (SD) and rice husk (RH), have been studied as potential natural sources.14−17 In the current study, the antioxidant potentials of MeOH extracts, i.e., RHE and SDE, from non-woody (rice hull) and woody (pine saw dust) biomass materials were evaluated in the oxidative stability of the © 2015 American Chemical Society

mineral base oil (MBO). Many analytical methods have been reported in the literature for monitoring oil degradation in the absence and presence of antioxidants. These include Fourier transform infrared spectroscopy (FTIR),18 pressure differential scanning calorimetry (PDSC),19 gel permeation chromatography (GPC),20 flow injection analysis (FIA)−visible spectrometry,21 X-ray absorption near-edge structure (XANES),22 nuclear magnetic resonance (NMR),23 and gas chromatography−mass spectrometry (GC−MS).24 Among these methods, GC−MS has been reported to be an effective method for analysis of residual products, including organic acids, which are commonly formed in lube oils via chemical oxidation.25 In the present work, GC−MS was used to monitor changes in the chemical composition of the MBO during oxidation in the presence and absence of the antioxidants under study.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. The MBO was collected from the Hydrocarbon Development Institute of Pakistan, Peshawar Operation, in a plastic can, and its physicochemical properties, such as density (ASTM D2598), American Petroleum Institute (API) gravity (ASTM D1298), ash (ASTM D482-80), viscosity (ASTM D445-74), viscosity ratio (ASTM D445), viscosity index (ASTM D2270-82), pour point (ASTM D97), Conradson carbon residue (ASTM D189-88), acidity (ASTM D974), and iodine number (ASTM D1959), were determined. Biomass samples, i.e., RH and SD, were collected from a rice mill/saw machine in the local market, ground, sieved to a particle size of about 0.15−0.45 mm, and soaked separately in methanol (Merck) in a vat for several days. The supernatant solution phase was then decanted and concentrated through a rotary evaporator to collect the crude extract. 2,2-Diphenyle-2-picrylhydrazyl (DPPH, Merck) was used as a reagent in the antioxidant activity study. 2.2. Antioxidant Activities of RHE and SDE. The antioxidant activities of the RHE and SDE were determined by the DPPH assay.26 In a typical procedure, DPPH solution was prepared by dissolving 0.0025 g in 100 mL of spectroscopic-grade methanol. Then, the solutions of the crude extract in methanol were prepared in different Received: July 24, 2015 Revised: September 5, 2015 Published: September 10, 2015 6522

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Energy & Fuels concentrations, i.e., 20, 40, 60, 80, and 100 μg/L. An aliquot (0.5 mL) of MeOH extract was taken in a spectroscopic cuvette (quartz), to which 3.9 mL of DPPH solution was added, mixed well, and left to stand at room temperature until stabilization. The absorbance of the resultant solution (upon stabilization) was measured at 515 nm using a double-beam ultraviolet (UV)−vis spectrophotometer (Schimadzu160) against a blank (MeOH extract without DPPH). The percent inhibition of the DPPH radical was calculated using the following equation:

method in terms of percent inhibition. The results are given in Table 2. Of the two samples tested, the RHE showed the Table 2. Antioxidant Activities (Percent Inhibition) of MeOH Crude Extracts as a Function of Their Concentration RHE

percent inhibition/radical scavenging activity (RSA) ⎛ A 0 − A sample ⎞ = 100 × ⎜ ⎟ A0 ⎝ ⎠ where A0 is the absorbance measured in the case of the blank (control) and Asample is the absorbance measured in the case of the test sample. 2.3. Oxidation Tests. The oxidation experiments in the presence and absence of the antioxidants under study were performed in a laboratory oxidation apparatus according to the modified IP 48 method.27 2.4. GC−MS Analysis. The GC−MS analysis of the RHE, SDE, original MBO, and variously additized and oxidized MBO samples was carried out by a gas chromatograph coupled with a MS analyzer (model GCMS-QP2010, Schimadzu, Japan). The gas chromatograph was equipped with a DB-5MS (25 m × 0.25 mm inner diameter, 0.25 μm) column and an auto injector (ADC-201). Helium was used as the carrier gas with the gas flow rate of 1.3 mL/min and the split ratio of 50. The injector temperature was kept as 300 °C, and the initial oven temperature was 35 °C. During the analysis, the initial oven temperature was increased to 100 °C at a heating rate of 5 °C min−1 and the hold time of 5 min, which was further increased to 150 °C with a ramp of 10 °C min−1 and hold time of 10 min. Finally, the temperature was increased to 290 °C at a rate of 2.5 °C min−1 with an isothermal hold of 10 min. The product peaks in the chromatogram were identified using the National Institute of Standards and Technology (NIST) MS library data.

Table 1. Physicochemical Properties of Base Oil property

test method IP IP IP IP

160/87/ASTM D2598 160/87/ASTM D1298 4/81/ASTM D482-80 71/80/ASTM D445-74

level 0.89 35.50 0.13 110

(ASTM D445)

15.50

(ASTM D2270-82) (ASTM D445/IP 71) (ASTM D974, D644)

150.23 1.00 0.40

IP 13/82/ASTM D189-88 ASTM D1959 IP 15/67/ASTM D97 IP 15/67/ASTM D97

0.20 17 244 −25

20 40 60 80 100

SDE percent inhibition (%)

extract concentration (μg/L)

percent inhibition (%)

43.3 53.33 91.16 92.03 92.51

20 40 60 80 100

35.11 46.77 70.12 80.86 81.41

percent inhibition of 43.3, 53.33, 91.16, 92.16, 92.03, and 92.51% at 20, 40, 60, 80, and 100 μg/L, respectively, while the SDE showed 35.11, 46.77, 70.12, 80.86, and 81.41% at 20, 40, 60, 80, and 100 μg/L, respectively. The higher percent inhibition [percent radical scavenging activity (RSA)] indicates a higher antioxidant activity. Among the different concentrations of the extracts used, 100 μg/L gave the highest percent RSA in the case of both the RHE and SDE. Among the two crude extracts, RHA showed better antioxidant behavior compared to SDE. The variation can correspond to the different chemical compositions in terms of phenolics and amines. 3.3. GC−MS Analysis. To decide about the changes in the spectroscopic properties of the original and variously oxidized MBO samples, the GC−MS analysis was carried out to ascertain the changes in the individual compounds, the hydrocarbon range products, and the hydrocarbon group types. The results are presented in the proceeding sections. 3.3.1. Analysis of the RHE and SDE. The GC−MS spectra of the RHE and SDE are provided in panels a and b of Figure 1. Different organic compounds belonging to various classes were identified in the case of both MeOH extracts. Among the oxygenates, alcohols found in RHE were propanol and pentanol and alcohols found in the SDE were 2,3-butanediol and 10,12hexadecadienol. The aldehydes identified were 2-butenal and 4methylbenzaldehyde in the RHE and 2-methylpentanal, 2,4dimethylbenzaldehyde, chlorobenzaldehyde, 5-methyl-4-furancarboxyldehyde, and 2-hydroxy-5-methylbenzaldehyde in the SDE. Among the ketones, cyclohexanone, 3-hydroxycyclohexanone, 2,3-dihydorxycylcohexanone, 3-methoxyacetophenone, 1,2-dihydroxyethyl-3,4-dihydroxyfuran-2-one, naphthoquinone, dibenzylideneacetone, and 2,4-dicholo-3,6-dihydroxy-1,4-benzoquinone were found in the RHE and hydroxyacetone, 2cyclopenten-1-one, 4-hydroxy-2-pentanone, 3-methyl-2-cyclopentennone, 3-methyl-2,4-pentanedione, 2-cyclohexanone, 2methyl-4-hydroxyl-1-cyclohexanone, 4-methylacetophenone, 5(1-hydroxyethyl)-3,4-dihydrofuran-2-one, 1,2-dihydroxyethyl3,4-dihydroxyfuran-2-one, and 1,4-naphthoquinone in the SDE. The carboxylic acid members recognized in the RHE were acetic acid, propanoic acid, o-hydroxybenzoic acid, and octadecanoic acid, and the carboxylic acid members recognized in the SDE were acetic acid, propanoic acid, 4-hydroxybenzoic acid, 4-methyl-2-hydroxybenzoic acid, 4-methoxy-2-hydroxybenzoic acid, 2-propyl-4-hydroxybenzoic acid, 4-ethoxy-2,6dihydroxybenzoic acid, 4-aminobenzenesulfonic acid, and octadecanoic acid.

3. RESULTS AND DISCUSSION 3.1. Physicochemical Properties of MBO. The MBO was characterized by determining its physicochemical characteristics using the well-established ASTM methods. The results are compiled in Table 1, which show that the density, API gravity, viscosity, viscosity ratio, viscosity index, pour point, Conradson carbon residue, ash, acidity, and iodine number are comparable to the standard ASTM specifications (ASTM D4304-13). 3.2. Antioxidant Activities. The antioxidant activities of the RHE and SDE were determined by the DPPH radical assay

density at 15 °C (g/cm3) API gravity at 60 °F (deg) ash (wt %) kinematic viscosity at 40 °C (mm2/s) kinematic viscosity at 100 °C (mm2/s) viscosity index viscosity ratio total acidity number (TAN) (mg of KOH/g) carbon residue (wt %) iodine number (mg/g) flash point (°C) pour point (°C)

extract concentration (μg/L)

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Figure 1. GC−MS chromatograms of the antioxidants, original MBO, and MBO samples oxidized at 200 °C: (a) RHE, (b) DE, (c) original MBO, (d) unadditized MBO, (e) RHE-additized MBO, and (f) SDE-additized MBO.

The major phenolics identified were phenol, 4-methoxyphenol, o-xylene, 4-methoxystyrene, o-aminophenol, o-nitrophenol, p-cresol, o-bromophenol, 4-ethylphenol, 4-methyl-1,2-dihydroxybenzene, 3,4,5-trihydroxybenzene, 2.6-xylenol, 8-hydroxyquinoline, 1,1-naphthalenediol, 1-nitroso-2-naphthol, p-anisol, xylenol, benzeneazo-2-b-naphthol, 4-methyl-7-hyroxycoumarine, α-tocopherol, and 2-nitro-1-naphthol in RHE. Similarly, in the SDE, major phenolics found were phenol, 4methoxyphenol, 2-ethoxyphenol, 2-methyl-4-methoxyphenol, 2-methoxy-4-vinylphenol, 4-ethylphenol, 2,5-dimethylphenol, 2,6-dimethylphenol, 4-ethyl-2-methylphenol, 1,4-dihydroxybenzene, 3,4,5-trihydroxybenzene, 6-methyl-1,4-dihydroxybenzene, 2-methoxy-1,4-dihydroxybenzene, 6-ethoxy-1,4-dihydroxybenzene, 6-methoxyxylenol, 8-hydroxyquinoline, 1,1-naphthalene, 2,2-diol, nitroso-2-naphthol, 4-methyl-7-hyroxycoumarine, αocopherol, nitronaphthol, and benzeneazo-2-b-naphthol. Some other compounds with functional groups in the RHE were found to be amyl acetate and methyl methacrylate

(esters), 2-methoxytoluene, 4-ethoxytolune, 2,4-dimethoxytoluene, 4-ethoxy-2-methoxytolune, and 2-ethyl-4-methoxytolune (alkoxylated aromatics), o-phenylenediamine, m-phenyldiamine, and benzoylphenylhydroxylamine (aromatic amines), methylphenyl acetamide (aromatic amide), and n-hexane, nhepane, eicosane, and docosane (paraffins). Similarly, in SDE, some compounds identified with other aromatic functional groups were found to be 3,4-dihydorxycylcohexane, 4-ethyl-1hydroxycyclohexane, 2,4-dimethyl-1-hydroxycylcohexane, and 1-methoxy-1,3-cyclohexadiene (hydroxylated naphthenes), 2,4dimethoxy-6-methylbenzene and 6-ethoxy-1,2-dimethylbenzene (alkoxylated aromatics), o-benzoylphenylhydroxylamine, and aminopyridine (aromatic amines), and methylphenyl acetamide (aromatic amide). The results are in consonance with the earlier studies.28,29 The concentrations of different classes of compounds were also calculated. The summary of the various classes of compounds identified in the original RHE and SDE is given 6524

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aromatic amides, ascorbic acid, stearic acid, tocohpherols, etc.29,36 The phenolics, aromatic amines, and their derivatives have been used on commercial scale in blended form with lubricants from several decades in individual forms. 3.3.2. Analysis of the Unoxidized Original Base Oil. The chemical composition of the unoxidized MBO was also studied by GC−MS. The corresponding chromatogram is given in Figure 1c, which indicates the presence of the various classes of compounds, including the paraffins, olefins, naphthenes, and aromatics.37 The paraffins identified were n-heptane, 2,5dimethyloctane, 4-methyloctane, nonane, 3-ethyl-2,4-dimethylpentane, 2,6-dimethyloctane, nonane, 2-dimethylnonane, 3dimethyldecane, 2,3,6-trimethyloctane, undecane, 5-methylundecane, dodecane, 6-methyltridecane, tetradecane, pentadecane, 5-propyldecane, hexadecane, heptadecane, 3-methylhexadecane, octadecane, 2,6,10,14-tetramethylhexadecane, tetratriacontane, nonadecane, eicosane, octadecane, henicosane, docosane, octacosane, tetracosane, and hexacosane. Among the olefins, the major compounds identified were 2methyl-1-hexene, 2,3,4-trimethyl-2-pentene, 2-nonene, 5-methyl-(Z)-2-decene, and Z,E-7,11-hexadecadiene-1-yl acetate. The aromatics identified were toluene, ethylbenzene, 1,3-dimethylbenzene, 1-ethyl-3-methylbenzene, 1,2,4-trimethylbenzene, and 1-ethyl-2,3-dimethylbenzene, while the naphthenes identified were cyclohexene, 1,1,3-trimethylcyclohexane, propylcyclohexane, 1-methyl-3-propylcyclohexane, butylcyclohexane, 2-methyl-3-isopentylcycloheptanone, and cyclododecane. The GC−MS data were also used to determine the distribution of different hydrocarbon range compounds. The relative abundance of hydrocarbon types, i.e., the paraffins, olefins, naphthenes, and aromatics, were calculated. The results are provided in Table 4. Their proportions were 92.719, 2.389,

in Table 3, which shows that these compounds belong to various families and found to be alkanes/naphthenes (0.880 Table 3. Various Classes of Compounds Identified in Original MeOH Crude Extracts concentration (%) compound class

RHE

SDE

alkanes/naphthenes alcohols aldehydes ketones carboxylic acids esters phenolics aromatic amines/amides

0.880 2.308 0.618 4.844 7.619 7.761 67.050 8.919

4.313 1.652 1.837 17.850 4.744 2.208 61.310 6.086

and 4.313%), alcohols (2.308 and 1.652%), aldehydes (0.618 and 1.837%), ketones (4.844 and 17.850%), carboxylic acids (7.619 and 4.744%), esters (7.761 and 4.744%), phenols and their derivatives (67.050 and 61.310%), and aromatic amines/ amides (8.919 and 6.086%), respectively. For compounds having functional groups other than aldehydes and ketones, the concentrations were found to be 14.33 and 14.50% in the case of the RHE and SDE, respectively. As evident from the data, phenols and their derivatives were found to be the most abundant ingredients found in both antioxidants. The order of the various classes of compounds regarding concentration determined in the case of the RHE was phenolics > aromatic amines > esters > carboxylic acids > ketones > alcohols > alkanes > aldehydes

Table 4. Hydrocarbon Group Type Distribution in Original MBO and MBO Samples Oxidized at 200 °C

and in the case of the SDE was phenolics > ketones > aromatic amide > carboxylic acids

distribution (wt %)

> naphthenes > esters > aldehydes > alcohols

As reported by many researchers, phenols, their derivatives, and aromatic amines have been found to be the most effective antioxidants that exist in the RHE and SDE.30−35 Among these, phenolics, aromatic amines, tocopherols, stearic acid, ascorbic acid, and citric acid are reported to be radical scavengers,30 while citric acid acts as a chelating agent.31 These antioxidants are extensively used individually in lubricants to enhance their thermo-oxidative stability. Engine oil increasingly uses phenolic, aminic, and organometallic antioxidants capable of trapping peroxy radicals or decomposing peroxides, thereby acting as synergists with amines (both antioxidant and antiwear). Among these, aromatic amines and polyphenols (having two aromatic rings) are more effective than a simple phenolic antioxidant. It is because of the fact that each phenol molecule scavenges two peroxy radicals and each amine and polyphenol trap four peroxy radicals.32 Phenolic antioxidants are the first class of antioxidants discovered possessing extensive applications in engine oil.33 They are mostly employed in a situation where the temperature does not exceed 180 °C,34 while amines are used at a temperature above 200 °C.35 The antioxidants found in MeOH extracts, i.e., RHE and SDE, were dihydroxybenzene, pyrogallol, alkylated phenols (anisols), aromatic amines, hydroquinone, naphthoquinone, hydroxybenzoic acid, hydroxyquinoline, carboxylated furanone, hydroxybenzaldehyde, alkylated hydroxycoumarine, naphthol,

sample

paraffins

olefins

naphthenes

aromatics

esters

original MBO unadditized MBO RHE-additized MBO SDE-additized MBO

92.719 81.00 89.382 87.581

2.389 7.102 4.203 4.442

2.694 4.121 3.002 3.320

2.198 3.657 3.657 3.004

4.011 1.410 1.653

2.198, and 2.694%, respectively. On the basis of the results, the original oil fractional hydrocarbon distribution followed the order paraffins > naphthenes > olefins > aromatics

It is a general practice that the base oil used in the formulation of a good lubricant will contain C18−C35 range hydrocarbons in the concentration of >50%. The different carbon range hydrocarbons found in MBO under tests were C6−C11, C12−C17, and C18−C35 in concentrations of 31.019, 15.440, and 53.541%, respectively (Table 5). It can be observed that the concentration of the heavy hydrocarbons (C18−C35) that lie within the range of a lubricant specification is quite significant compared to the hydrocarbons that fall in the fuel range, such as gasoline (C5−C11), kerosene (C12−C15), and diesel fuel oil (C16−C18). The fuel range hydrocarbons are usually blended with the heavier hydrocarbons to produce good lubricity for a desired lubricant.38 The distribution of hydrocarbons found in MBO under study followed the order 6525

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The hydrocarbon group type distribution was also calculated from the GC−MS data, which, in turn, was used to investigate the overall change in composition of the various fragments of the oil under tests. The relative abundance of the various hydrocarbon types contained the paraffins, olefins, naphthenes, and aromatics (Table 4). Their proportions were found to be 81.0, 7.102, 3.657, 4.121, and 4.011%, respectively. The various hydrocarbon group types determined in the case of the unadditized base oil after oxidation followed the order:

Table 5. Carbon Number Distribution in Original MBO and MBO Samples Oxidized at 200 °C concentration (wt %) sample

C6−C11

C12−C17

C18−Cn

original MBO unadditized MBO RHE-additized MBO SDE-additized MBO

31.019 52.962 27.489 29.977

15.440 20.009 22.022 20.668

53.541 27.029 50.489 49.365

paraffins > olefins > naphthenes > aromatics > ester

C18−C35 > C6−C11 > C12−C17

The various carbon range compounds were determined, and the ranges found were C6−C11, C12−C17, and C18−C35. Their concentrations were 52.962% (C6−C10), 20.009% (C11−C20), and 27.029% (C21−C35), respectively (Table 5). The various carbon range compounds followed the order of C21−C35 > C6− C10 > C11−C20. It was found that the concentration of C20−C35 (the lubricant range) was quite lower than determined in the case of the original MBO. The results indicate significant degradation of both the light and heavy hydrocarbons. Likewise, ester was confirmed as the oxidized product in the concentration of 4.011%, which is found to be higher than the significant extent, indicating the high level of degradation. The RHE- and SDE-additized MBO samples oxidized under the same set of conditions were also investigated. The respective GC−MS profiles are given in panels e and f of Figure 1. The composition of these samples contained similar aliphatic and aromatic hydrocarbons as determined in the case of the original (unoxidized) and unadditized (oxidized) samples. The compositions of the RHE- and SDE-additized oxidized MBO samples were found between the original and unadditized oxidized MBO samples. The intensities of the peaks in the case of the additized samples were lesser compared to the unadditized oxidized MBO, as evident from the corresponding chromatograms. The results suggest a low concentration of the compounds compared to the original and unadditized oxidized MBO. It is attributed to the oxidation of some of the paraffins that formed the degraded products to some extent in the presence of the RHE and SDE as antioxidants. Most of the individual compounds identified in the case of the RHE- and SDE-additized MBO samples after oxidation appeared to be similar to the original unoxidized MBO. The results further showed that the RHE- and SDE-additized MBO samples maintained the composition of the original base oil to a greater extent, with the exception of the appearance of new peaks for compounds formed as a result of degradation. In the case of the RHE-additized oxidized MBO, the concentrations of the paraffins, olefins, aromatics, naphthenes, and ester were 89.382, 4.203, 2.003, 3.002, and 1.410%, and in the case of the SDE-additized oxidized MBO, the concentrations of the paraffins, olefins, aromatics, naphthenes, and ester were 87.581, 4.442, 3.004, 3.320, and 1.653%, respectively (Table 4). The results indicate a reduction in the concentration of the paraffins with the corresponding increment in the olefins, aromatics, and naphthenes. The order of the hydrocarbon group distribution for the RHE- and SDE-additized MBO samples was

3.3.3. Analysis of Oxidized Base Oil Samples. The GC−MS analysis of the various MBO samples oxidized at 200 °C for 6 h was also carried out. The corresponding chromatograms are displayed in panels d−f of Figure 1. The GC−MS chromatogram of the unadditized MBO oxidized at 200 °C for 6 h (Figure 1d) exhibited many weak, some medium, and several intense peaks (1−74 min), which indicates a complex mixture of compounds. The profile further illustrates that the unadditized MBO is comprised of a wide range of aliphatic and aromatic hydrocarbons. The chromatogram contains almost all of the peaks that were observed in the case of the original MBO but with change intensities along with the appearance of some new peaks. It indicates that the unadditized BMO after oxidation retained all of those constituents that were found in the original MBO, except some degraded products. The changed intensity of the peaks implies the different concentrations of these compounds observed in the case of the unadditized sample when oxidized at 200 °C. The intensity of the bands corresponding to the paraffins was reduced, while the bands that appeared for aromatics, naphthenes, olefins, and ester became enhanced, which reveals their presence in a high concentration. A number of new aliphatic and aromatic hydrocarbons were also identified that may have formed as a result of the thermo-oxidative degradation. Among the new paraffins (16 compounds) recognized were 3-methyheptane, 3,6-dimethylheptane, 2,7-dimethyl-3-ethyloctane, 3,7-dimethylnonane, 2-methyl-3-methylene nonane, 3methylundicane, 3,9-dimethylundecane, 4-methyltridecane, tetradecane, 3-methyltridecane, 2,6,11-trimethyldodecane, 2,6,10,15-tetramethylheptadecane, 2-methylheptadecane, 4methylpentadecane, hentriacontane, pentacosane, and tritetracontane. These new paraffins were recognized with a net concentration of 2.388%. Some olefins identified (six new compounds) with overall concentration of 0.362% were 1-heptene, 2,3-dimethyl-1hexene, 2-undecene (E), 2-dodecene (E), 1-tetradecene, and 9-octadecene (E). Six new aromatics recognized with a total concentration of 0.036% were 1, 3,5-trimethylbenzene, methyl3-(1-ethylmethyl)benzene, 1,1-dimethylpropylbenzene, 1,8-dimethylnaphthalene, 2,6-dimethylnaphthalene, and phenanthrene. Three new naphthenes confirmed were cyclododecane, 1,2,3-trimethylcyclohexane, and 2-cyclohexylundacane in the concentration of 0.946%, while the carbonyl compound as an oxidized degraded product found was 10-undecanoic acid, 3methylbutyl ester. Thus, the oxidized degraded composition mainly comprised of the constituents that were found to be mostly ester, alkylated benzene, alkylated polycyclic aromatic hydrocarbons (PAHs) (di- and tricyclic), five- and six-member alkylated naphthenes, and some straight and branched olefins having carbon contents in the range of C6−C16.39

paraffins > olefins > naphthenes > aromatics > ester

The concentrations of different carbon range compounds, i.e., C6−C10, C11−C20, and C21−C35, were found to be 27.489, 22.022, and 50.489%, respectively, in the case of the RHE6526

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RHE- and SDE-additized samples were found to be close to the original MBO, which indicates their thermo-oxidative stability. The results also showed that RHE retained the lube oil desired range to 23.460% and SDE retained the lube oil desired range to 22.336% in comparison to unadditized MBO. The overall results of the carbon number range distribution reflect the excellent antioxidant behavior of RHE and SDE. However, the RHE proved to be associated with more antioxidant character than the SDE in terms of conserving the desired lube oil carbon range from decomposition. The sum of the percent differences (overall percent difference) calculated for all of the constituents of the RHEand SDE-additized samples including the ester content with respect to the control sample was found to be 8.245 and 10.981%, which revealed that the MBO was degraded to 8.245% in the presence of the RHE and 10.981% in the case of the SDE. The overall percent differences in the case of the RHE and SDE were insignificant, which indicates their outstanding antioxidant behavior, while the overall percent difference for all of the components along with the ester content calculated in the case of the unadditized oxidized MBO in comparison to the original sample was significant (19.389%) (Figure 4).

additized sample. While in the case of the SDE-additized MBO, their concentrations were found to be 29.977, 20.668, and 49.365%, respectively (Table 5). The results exhibit that the hydrocarbon range compounds, i.e., C6−C11 and C12−C17, were increased in comparison to the unadditized oxidized MBO. The results indicate a reduction in the level of degradation in the case of the RHE- and SDE-additized MBO samples oxidized at 200 °C, which suggests their excellent antioxidant character. The percent differences in concentrations of the paraffin, olefin, aromatic, and naphthene with respect to the unadditized sample were calculated (Figure 2) and found to be 3.337, 1.814, 0.198, and 0.308% in the case of the RHE-additized MBO and 5.138, 2.053, 0.806, 0.626, and 2.358% in the case of the SDEadditized MBO.

Figure 2. Percent difference in hydrocarbon group type distributions in the unadditized and RHE- and SDE-additized base oil samples oxidized at 200 °C.

To evaluate the extent to which the desired lubricant range hydrocarbons were conserved, the percent differences of the various hydrocarbon range products, such as C6−C11, C12−C17, and C18−C35, in the case of the RHE- and SDE-additized oxidized MBO were calculated (Figure 3). In the case of the RHE-additized sample, the percent differences were 3.530, 6.582, and 3.052%, respectively, while in the case of the SDEadditized sample, the percent differences were 1.042, 5.228, and 4.176%, respectively, in comparison to the original oil. The results exhibited that their levels determined in the case of the

Figure 4. Overall percent change in composition of the unadditized and RHE- and SDE-additized oxidized MBO samples oxidized at 200 °C.

Briefly, the results reveal that the overall degradation (change in composition) of 19.389% occurred in the case of the unadditized MBO, which declined to 8.245 and 10.981% in the presence of the RHE and SDE, respectively. Thus, the RHE reduced the degradation up to 11.144%, and the SDE reduced the degradation up to 8.408%. This significant reduction in inhibiting the percent change in composition of the RHE- and SDE-additized MBO samples reveals the outstanding and excellent antioxidant behavior. The degree of degradation prevented by the RHE and SDE can be analyzed from the percent decrease in the concentration of the undesired degraded (oxidized) products, such as ester between the additized and unadditized oxidized MBO samples. The ester concentration determined in the case of the unadditized oxidized sample was found to be 4.011%, and the ester concentration determined in the case of the RHE- and SDE-additized MBO samples was 1.42 and 1.653%, respectively. As clear from the percent differences of degraded

Figure 3. Percent change in the concentration of different carbon range products in the unadditized and RHE- and SDE-additized MBO samples oxidized at 200 °C. 6527

DOI: 10.1021/acs.energyfuels.5b01675 Energy Fuels 2015, 29, 6522−6528

Article

Energy & Fuels

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products, both antioxidants have declined the concentration of the degraded products to a greater extent, which indicates their efficacy as antioxidants.

4. CONCLUSION (1) The GC−MS analysis in terms of the changes in the individual compounds, carbon range, and hydrocarbon group type distributions indicates stabilities of the additized samples compared to the unadditized base oil, which has established the antioxidant activities of the RHE and SDE. (2) Upon comparison of the efficiency of the two antioxidants among themselves, the overall performance of the RHE can be ranked as excellent and the SDE as better at 200 °C.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +92-91-9216652. E-mail: patwar2001@ yahoo.co.in. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the University of Peshawar for allowing for the research facilities and the Pakistan Council of Scientific and Industrial Research (PCSIR) for GC−MS analysis.

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DOI: 10.1021/acs.energyfuels.5b01675 Energy Fuels 2015, 29, 6522−6528