2652r 2010 American Chemical Society pubs.acs.org/EF

Energy Fuels 2010, 24, 2652–2656 : DOI:10.1021/ef901172tPublished on Web 03/19/2010

Oxidation Stability of Palm Methyl Ester: Effect of Metal Contaminants

and Antioxidants

Amit Sarin,*,† Rajneesh Arora,‡ N.P. Singh,‡ Rakesh Sarin,§ and R.K. Malhotra§

†Department of Applied Sciences, Amritsar College of Engineering and Technology, Amritsar-143001, India, ‡Punjab TechnicalUniversity, Jalandhar-144011, India, and §Indian oil Corporation Ltd., R&D Centre, Sector-13, Faridabad-121007, India

Received October 14, 2009. Revised Manuscript Received March 10, 2010

The European biodiesel standard EN-14214 calls for determining the oxidation stability (OS) at 110 �Cwith a minimum induction time of 6 h by the Rancimat method (EN-14112). The ASTM standard D-6751has recently introduced aminimum induction period of 3 h. Palmmethyl ester (PME) has been successfullyevaluated as a diesel substitute in summer and with an additive in winter due to its poor cold-flowproperties. Neat PME exhibited an OS of 9.24 h; thus, it was highly stable. Research was conducted toinvestigate the effect of the presence of transition metals, likely to be present in the metallurgy of storagetanks and barrels, on the highly stable PME. It was found that the influence of metal was detrimental andcatalytic even for stable PME. Small concentrations of metal contaminants showed nearly the sameinfluence on OS as large amounts. Copper showed the strongest detrimental and catalytic effect.Antioxidants, namely, tert-butylated hydroxytoluene (TBHT), tert-butylated phenol derivative (TBP),octylated butylated diphenyl amine (OBPA), and tert-butylhydroxquinone (TBHQ) were doped toimprove the OS of metal-contaminated PME. It was found that the antioxidant TBHQwas most effectiveamong all of the antioxidants used.

1. Introduction

Anumber of researchers have investigated alternate renew-able fuel sources and concluded that vegetable oil-based fuelscan be used as alternative fuels.1-6 Biodiesel is commerciallyproduced through the transesterification of vegetable oils,residual frying oils, or animal fats with alcohol and alkalinecatalysts. Soybean oil and rapeseed are common feedstocksused for biodiesel production in USA and Europe. However,Southeast Asian countries such as Indonesia have surpluspalm crops.

The quality of biodiesel is designated by several standardssuch as EN-14214 andASTMD-6751, and oxidation stability(OS) is among the monitored parameters as the EN-14214calls for determining oxidative stability at 110 �C with aminimum induction period (IP) of 6 h by the Rancimatmethod (EN-14112), and the ASTM standard D-6751has recently introduced a minimum IP of 3 h by the same

method.7-9 Indian specification IS-15607 also requires aminimum induction time of 6 h.10,11

The oxidation process is reported in the literature. Relativerates of oxidation are 1 for oleates, 41 for linoleats, and 98 forlinolenates.12,13 The oxidation chain reaction is usually in-itiated at the allylic to double bonds. Therefore, fatty acidswith methylene-interrupted double bonds, for example, lino-leic acid [(9Z,12Z)-octadecadienoic acid], are more suscepti-ble to oxidation because they contain methylene groups thatare allylic to two double bonds. Fatty acids with two methyl-ene groups, for example, linolenic acid [(9Z,12Z,15Z)-octa-decatrienoic acid], are even more susceptible to degradation.

There are several publications on the storage, OS of bio-diesel, and effect of antioxidants on the stability of biodiesel.Dunn has studied the oxidative stability of soybean oil fattyacid methyl esters by oil stability index (OSI).14 Polavka et al.studied the OS of methyl esters derived from rapeseed oil andwaste frying oil, both distilled and undistilled, by differentialthermal analysis and Rancimat.15 Ferrari et al. compared theoxidative stability of neutralized, refined, and frying oil wastesoybean oil fatty acid ethyl ester.16 Mittelbach et al. investi-gated the influence of different synthetic and natural antiox-idants on the OS of biodiesel produced from rapeseed oil,sunflower oil, used frying oil, and beef tallow, both distilledand undistilled.17 Dunn has also studied the effect of different

*To whom correspondence should be addressed. E-mail: amit.sarin@yahoo.com.(1) Ali, Y.; Hanna, M. A. Bioresour. Technol. 1994, 50, 153–163.(2) Chien,Y.-C.; Lu,M.; Chai,M.; Boreo, F. J.EnergyFuels 2009, 23,

202–206.(3) Qian, J.; Yun, Z. Energy Fuels 2009, 23, 507–512.(4) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15.(5) Yuan, H.; Yang, B. L.; Zhu, G. L.Energy Fuels 2009, 23, 548–552.(6) May, C. Y.; Liang, Y. C.; Foon, C. S.; Ngan,M. A.; Hook, C. C.;

Basiron, Y. Fuel 2005, 84, 1717–1720.(7) Dunn, R. O. J. Am. Oil Chem. Soc. 2002, 79, 915–920.(8) Knothe, G. Energy Fuels 2008, 22, 1358–1364.(9) Sarin, A.; Arora, R.; Singh, N. P.; Sarin, R.; Sharma, M.;

Malhotra, R. K. J. Am. Oil Chem. Soc. [Online early access]. DOI:10.1007/s11746-009-1530-0. Published Online: Dec 29, 2009.(10) Sarin, A.; Arora, R.; Singh, N. P.; Sarin, R.; Malhotra, R. K.;

Kundu, K. Energy 2009, 34, 2016–2021.(11) Sarin, A.; Arora, R.; Singh, N. P.; Sarin, R.; Malhotra, R. K.;

Sarin, S. Energy Fuels [Online early access]. DOI: 10.1021/ef901131m.Published Online: Feb 16, 2010.

(12) Frankel, E. N. Lipid Oxidation; The Oily Press: Dundee, Scotland,1998; p 19.

(13) Hui, Y. H., Ed. Bailey’s Industrial Oil and Fat Products, 5th ed.;John Wiley & Sons, Inc.: New York, 1996; Vol. 4, pp 411-415.

(14) Dunn, R. O. J. Am. Oil Chem. Soc. 2005, 82, 381–387.(15) Polavka, J.; Paligova, J.; Cvengros, J.; Simon, P. J. Am. Oil

Chem. Soc. 2005, 82, 519–524.(16) Ferrari, R. A.; Oliveira, V. D.; Scabio, A. Sci. Agric. 2005, 62,

291–295.(17) Mittelbach, M.; Schober, S. J. Am. Oil Chem. Soc. 2003, 80,

817–823.

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antioxidants on the OS of biodiesel from soybean oil.18

Thermal and oxidative degradation of castor oil biodieselwas also investigated.19 Researchers also studied the effects ofoxidation during long-term storage on the fuel properties ofpalmoil-based biodiesel.20Recently, surrogatemolecules, i.e.,methyl, ethyl, isopropyl, and butyl esters of β-branched fattyacid, were synthesized and had substantially better OS, lowtemperature flow properties, and cetane number.21 Fromthese literature reports and quality survey reports,22-24 itcan be concluded that it will not be possible to use biodieselwithout antioxidants.

Recently, Sarin et al. studied the influence of the presenceof five metals, iron, nickel, manganese, cobalt, and copper,commonly found in the metallurgy of storage tanks andbarrels, on the OS of biodiesel synthesized from nonedibleoil seeds from Jatropha curcas and Pongamia pinnata withthe Rancimat test method.9,25 Loh et al. investigated theoxidative stability and storage behavior of fatty acid methylesters derived from used palm oil,26 and Liang et al. studiedthe effect of natural and synthetic antioxidants on theoxidative stability of palm diesel.27 However, no paper is

available on the influence of the presence of metals on theOS of biodiesel from palm. In the present study, we haveundertaken studies on the stability of biodiesel synthesizedfrom palm. The first objective of this study is to investigatethe influence of the presence of metals on the OS of palmmethyl ester (PME) and then to compare the effects ofvarious metals on OS with results reported in the litera-ture.25 The second objective is to improve the OS of PME bydoping with various antioxidants and to compare theireffectiveness with results reported in the literature.25 Severaltransition metals, iron, nickel, manganese, cobalt, andcopper, commonly found in the metallurgy of storage tanksand barrels, were blended with varying concentrations inPME samples.

2. Experimental Section

2.1. Materials.Methanol used in the synthesis of PMEwas of99.8%purity andwas purchased fromRanbaxy Fine ChemicalsLtd. (New Delhi, India). N-Hexane and MeOH/KOH wereof analytical grade and were procured from Merck SpecialtiesPvt. Ltd. (New Delhi, India) and Sigma-Aldrich Chemical Co.(New Delhi, India), respectively. Antioxidants, namely, tert-butylated hydroxytoluene (TBHT), tert-butylated phenol deri-vative (TBP), octylated butylated diphenyl amine (OBPA), andtert-butylhydroxquinone (TBHQ), were of analytical grade andwere purchased from Sigma-Aldrich Chemical Co. (New Delhi,India). Cobalt, manganese, iron, copper, and nickel naphthe-nates were procured fromM/s Notional Chemicals & Dyes Co.(Varanasi, India).

2.2. Methods. PME was synthesized from refined palm oil inthe laboratory according to the methodology described in the

Table 1. Physico-Chemical Properties of Palm Methyl Ester in Accordance with ASTM D-6751, EN-14214, and IS-15607 Standards10,11

property (units)

ASTM D6751

test methodASTM D 6751

limitsEN 14214 test

methodEN 14214limits

IS 15607 testmethod

IS 15607limits

meanPME

standarddeviation

flash point (�C) D-93 min.130 EN ISO 3679 min. 120 IS 1448 P:21 min. 120 138 1.51viscosity at 40 �C (cSt) D-445 1.9-6.0 EN ISO 3104 3.5-5.0 IS 1448 P:25 2.5-6.0 4.50 0.014sulfated ash (% mass) D-874 max. 0.02 EN ISO 3987 Max. 0.02 IS 1448 P:4 Max. 0.02 0.002 0.0sulfur (% mass) D-5453/

D-4294max. 0.0015(S 15)

EN ISO20846/20884

max. 0.0010 ASTM D 5453 max. 0.005 0.003 0.0012

max. 0.05(S 500)

copper corrosion D-130 max. 3 EN ISO 2160 max. 1 IS 1448 P:15 max. 1 1 0.0cetane number D-613 min. 47 EN ISO 5165 min. 51 IS 1448 P:9 min. 51 55.3 0.10water and sediment(vol. %)

D-2709 max. 0.05 D-2709 max. 0.05 0.01 0.0056

conradson carbon residue(CCR) 100% (% mass)

D-4530 max. 0.05 EN ISO 10370 max. 0.3 D-4530 max. 0.05 0.032 0.0055

neutralization value(mg, KOH/g)

D-664 max. 0.50 EN ISO 14104 max. 0.5 IS 1448 P:1/Sec.1

max. 0.50 0.26 0.025

free glycerin (% mass) D-6584 max. 0.02 EN ISO14105/14106

max. 0.02 D-6584 max. 0.02 0.01 0.0

total glycerin (% mass) D-6584 max. 0.24 EN ISO 14105 max. 0.25 D-6584 max. 0.25 0.015 0.0057phosphorus (% mass) D-4951 max. 0.001 EN 14107 max. 0.0010 D-4951 max. 0.001 <0.001distillation temperature D-1160 90% at 360 �C not under spec. min 90% >90%oxidation stability at110 �C (h)

EN 14112 min. 3 h EN ISO 14112 min. 6 h EN 14112 min. 6 h 9.24 0.013

CFPP (�C) D 6371 EN 116 variable IS 1448 P:10 14 0.57

Table 2. Fatty Acid Methyl Ester Composition of Palm

Methyl Ester10,11

fatty acid methyl ester palm methyl ester (wt %)

palmitic (C16:0) 40.3stearic (C18:0) 4.1oleic (C18:1) 43.4linoleic (C18:2) 12.2saturated 44.4unsaturated 55.6

(18) Dunn, R. O. Fuel Process. Technol. 2005, 86, 1071–1085.(19) Conceicao, M. M.; Fernandes, V. J., Jr.; Aranjo, A. S.; Farias,

M. F.; Santos, L.M.G.; Souza, A.G.Energy Fuels 2007, 21, 1522–1527.(20) Lin, C.-Y.; Chiu, C.-C. Energy Fuels 2009, 23, 3285–3289.(21) Sarin, R.; Kumar, R.; Srivastav, B.; Puri, S. K.; Tuli, D. K.;

Malhotra, R. K.; Kumar, A. Bioresour. Technol. 2009, 100, 3022–3028.(22) McCormick, R. L.; Alleman, T. L.; Ratcliff, M.; Moens, L.

Survey of Quality and Stability of Biodiesel and Biodiesel Blends inthe United States in 2004. In National Renewable Energy LaboratoryTechnical Report No. NREL/TP-540-38836, 2005.(23) Alleman, T. L.; McCormick, R. L. Results of the 2007 B100

Quality Survey. In National Renewable Energy Laboratory TechnicalReport No. NREL/TP-540-42787, 2008.(24) Tang, H.; Abunasser, N.; Wang, A.; Clark, B. R.; Wadumes-

thrige, K.; Zeng, S.; Kim, M.; Salley, S. O.; Hirschlieb, G.; Wilson, J.;Ng, K. Y. S. Fuel 2008, 87, 2951–2955.(25) Sarin, A.; Arora, R.; Singh, N. P.; Sharma, M.; Malhotra, R. K.

Energy 2009, 34, 1271–1275.(26) Loh, S.-K.; Chew, S. M.; Choo, Y.-M. J. Am. Oil Chem. Soc.

2006, 83, 947–952.(27) Liang, Y. C.;May, C. Y.; Foon, C. S.; Ngan,M.A.; Hock, C. C.;

Basiron, Y. Fuel 2006, 85, 867–870.

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literature.28,29 Biodiesel from palm oil was prepared by atransesterification process, involving the reaction of oil withmethanol under reflux conditions. A series of experiments weredesigned to determine the optimal reaction conditions to maxi-mum conversion. Methanol (8:1 molar ratio, alcohol/oil) wasadded to the reactor followed by the slow addition of catalyst(0.6 wt % of oil) with stirring. The stirring was continued untilthe complete dissolution of catalyst (15 min). Thus, palm oilwas added, and the reaction temperature was set at 65 �C for theexperiment. After the completion of the reaction, the materialwas transferred to a separating funnel, and both phases wereseparated. The upper phase was methyl ester (biodiesel), andthe lower part was glycerin. Alcohol from both phases wasdistilled off under vacuum. The glycerin phase was neutralizedwith acid and stored as crude glycerin. The methyl ester waswashed with water twice to remove the traces of glycerin,unreacted catalyst, and soap formed during transesterification.The residual product was kept under vacuum to get rid ofresidual moisture. The product obtained (>98%) was suffi-ciently pure for testing. The synthesized PME was tested forphysicochemical properties according to ASTM D-6751, EN-14214, and Indian IS-15607 specifications (Table 1).10,11 It isclear from the data that PME met all of the specifications buthad poor flow properties.

The fatty acid methyl ester composition of PME was deter-mined by gas chromatography on a gas chromatograph (GC)(PerkinElmer, Clarus 500, New Delhi, India, located at IOC, R& D Centre, Faridabad), using nitrogen as a carrier gas and adi(ethylene glycol) succinate column (DEGS) by preparing thecorresponding fatty acid esters and comparing them with stan-dard fatty acid ester samples. TheGCwas equippedwith a flameionization detector (FID) and a glass column 3.1 m�2.1 mmi.d. with a temperature program of 150-250 �C (6 �C/min, holdfor 20 min). The oven temperature was kept at 200 �C; theinjector temperatures were 230 and 250 �C. Detailed fatty acidmethyl ester composition (wt %) is given in Table 2.10,11

Metal naphthenates were selected, being highly soluble inbiodiesel. The metal concentration in metal naphthenates waschecked by the ASTM D4951 test method, using inductivelycoupled plasma atomic emission spectroscopy. The concentra-tions of cobalt, manganese, iron, copper, and nickel in theirnaphthenates were 5.21, 5.20, 3.91, 6.80, and 4.99%, respec-tively. The samples were further diluted in PME to the desiredconcentration. The concentration of carboxylic acid in metalnaphthenates was practically none (<1%); therefore, theirsignificance in biodiesel will be insignificant as naphthenateswere blended at the ppm level.

The OS of methyl ester in the presence of metal contaminantsand their blends with different dosages of different antioxidantswas measured using Rancimat equipment model 743 (Metrohm,Switzerland) in accordance with EN-14112.30 In the Rancimatmethod, oxidation is induced by passing a stream of air at therate of 10L/h through the biodiesel sample (3 g), kept at constanttemperature 110 �C. The vapors released during the oxidationprocess, together with air, are passed into the flask containing50mLof demineralizedwater and an electrode formeasuring theconductivity. The electrode is connected to a measuring andrecording device. It indicates the end of the IP when theconductivity begins to increase rapidly. This accelerated increase

Figure 1. Effect of metal contamination (ppm) on the oxidation stability (h) of PME.

Figure 2. Chemical structures of antioxidants.

(28) Sarin, R.; Sharma, M.; Khan, A. A. Bioresour. Technol. 2009,100, 4187–4192.(29) Vicente,G.;Martinez,M.;Aracil, J.Bioresour. Technol. 2007, 98,

1724–1733.

(30) European Committee for Standardization (CEN). Fat and OilDerivatives. Fatty Acid Methyl Esters (FAME). Determination of Oxi-dative Stability (Accelerated Oxidation Test), EN 14112: 2003; EuropeanCommittee for Standardization (CEN): Brussels, Belgium, 2003.

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is caused by the dissociation of volatile carboxylic acids pro-duced during the oxidation process and absorbed inthe water. When the conductivity of this solution is recordedcontinuously, an oxidation curve is obtained whose point ofinflection is known as the IP. Data for all analytical results arebased on triplicate measurements. Subsequent analysis showedno statistically significant difference between the measurements.Standard deviations of these measurements are shown inTable 1.

3. Results and Discussion

3.1. Analyses of PME. The fatty acid methyl ester compo-sitions of the PME samples (Table 2) showed that the PMEmainly consisted of oleic fatty acidmethyl esters and palmiticacid methyl esters. The saturated fatty acid methyl esters inPME were 44.4%, and the unsaturated fatty acid methylesters were 55.6%.

3.2. OS Study. 3.2.1. OS of Neat PME. Neat PMEshowed an IP of 9.24 h, which met the minimum limit of 3h IP in accordance with recent ASTM D-6751 and theminimum limit of 6 h IP, as required by EN-14112/IS-15607.

3.2.1. Effect of Metal Contaminants on the OS. Differenttransition metals, iron, nickel, manganese, cobalt, and cop-per, commonly found in metal containers, were blended, asmetal naphthenates, with varying concentrations (at the ppmlevel) with PME samples. Figure 1 shows that the presenceof these metals depressed the OS of highly stable PME,as measured by the IP. The presence of metals in biodieselresulted in the acceleration of free radical oxidation due to ametal-mediated initiation reaction.

Copper had the strongest catalytic effect as the IP of PMEdecreased drastically even with small concentrations (at the

ppm level) of copper contamination. Even 2 ppm of copperreduced the IP of PME to below the ASTM D-6751 speci-fication of 3 h. Other metals, namely, iron, nickel, manga-nese, and cobalt, also had a strong negative influence on theOS. The presence of copper and cobalt decreased the IP ofPME to even below the ASTMD-6751 limits of 3 h. Figure 1shows that for all of the metal contaminants, IP valuesbecame almost constant as the concentration of metal in-creased. This demonstrates that the influence of the metalswas catalytic, but small concentrations of metals have great-er negative effect on the OS. These results confirm the effectof various transition metals, commonly found in the metal-lurgy of storage tanks and barrels, on biodiesel stability asreported in the literature.25

3.2.2. Improvement of the OS of Metal-ContaminatedPME. In the present study, antioxidants, namely, TBHT,TBP, OBPA, and TBHQ, were used (Figure 2). As metallicimpurities have a catalytic effect, a 2-ppm metal concentra-tion was selected for antioxidant dose optimization. All ofthe antioxidants were doped into the PME samples withvarying concentrations (at the ppm level), and the corre-sponding induction periods were measured with the Ranci-mat test method to observe the effectiveness of differentantioxidants.

Figure 3 shows the variation of IP of PME that was con-taminated with 2 ppm of metal with varying concentrationsof the antioxidant TBHT. The OS of metal-contaminatedPMEwas found to increase with an increase in the dosage ofthe antioxidant TBHT. Finally, it was found that aminimum100 ppm dosage of TBHT was needed to improve the IP ofiron- and nickel-contaminated PME and that a minimumdosage of 200 ppm of TBHT in manganese-contaminated

Figure 3. Effect of antioxidant and TBHT concentration (ppm) on the oxidation stability (h) of metal-contaminated (2 ppm) PME.

Figure 4. Effect of antioxidant and TBP concentration (ppm) on the oxidation stability (h) of metal-contaminated (2 ppm) PME.

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Energy Fuels 2010, 24, 2652–2656 : DOI:10.1021/ef901172t Sarin et al.

PME was needed to meet the EN-14112 specification forbiodiesel OS (Figure 3). Figure 3 also shows that for cobalt-and copper-contaminated PME, a minimum dosage of250 ppm was required to meet EN-14112 specifications.

Figure 4 shows the variation of IP of PME that wascontaminated with 2 ppm of metal with varying concentra-tions of the antioxidant TBP. Itwas found that aminimumof150 ppm dosage of TBP was needed to improve the IP ofiron- and nickel-contaminated PME and that a minimumdosage of 200 ppm of TBP in manganese-contaminatedPME was needed to meet the EN-14112 specification forbiodiesel OS (Figure 4). Figure 4 also shows that for cobalt-and copper-contaminated PME, a minimum dosage of300 ppm was required to meet the EN-14112 specifications.The antioxidant OBPA almost showed the same effect asTBP (Figure 5). These results confirmed that TBHT was themost effective antioxidant among the first three antioxidantsused, as reported in the literature.25

Further, tests were done with the antioxidant TBHQ.Figure 6 shows the variation of IP of PME that wascontaminated with 2 ppm of metal with varying concentra-tions of the antioxidant TBHQ. Itwas found that aminimumof 50 ppm dosage of TBHQwas needed to improve the IP ofiron- and nickel-contaminated PME and that a minimumdosage of 100 ppm in manganese-contaminated PME wasneeded to meet the EN-14112 specification for biodiesel OS.Figure 6 shows that for cobalt- and copper-contaminatedPME, a minimum dosage of 150 ppm was required to meetthe EN-14112 specifications. Therefore, it was found that the

antioxidant TBHQ is the most effective among all of theantioxidants used. This can be explained on the basis ofthe molecular structures of the antioxidants. TBHQ pos-sesses more OH groups attached to the aromatic ring thanthe others (Figure 2). Therefore, TBHQ offers more sites forthe formation of complexes between free radicals and anti-oxidant radicals for lipid stabilization.Hence, TBHQ ismoreeffective than other antioxidants at the same stage.

Conclusions

The PME used in this work was found to exhibit an OS of9.24 h in the Rancimat test. The IP of PME decreaseddrastically evenwith small concentrations (ppm level) ofmetalcontaminants. The influence of the metals on IP was found tobe catalytic, but small concentrations of metals had greaternegative effects on the OS. Of the five metals investigated,copper appeared to have the strongest detrimental and cata-lytic effect.

The OS of metal-contaminated PMEwas found to increasewith increases in the dosage of antioxidants. It was found thatthe antioxidant TBHQ was most effective among all of theantioxidants used. A minimum 50 ppm dosage of TBHQwasneeded to improve the IP of iron- and nickel-contaminatedPME, and a minimum dosage of 100 ppm was needed inmanganese-contaminated PME to meet the EN-14112 speci-fication for biodiesel OS. For cobalt- and copper-contami-nated PME, a minimum dosage of 150 ppm was required tomeet EN-14112 specifications.

Figure 5. Effect of antioxidant and OBPA concentration (ppm) on the oxidation stability (h) of metal-contaminated (2 ppm) PME.

Figure 6. Effect of antioxidant and TBHQ concentration (ppm) on the oxidation stability (h) of metal-contaminated (2 ppm) PME.