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FT-Raman Spectroscopy Quantification of Biodiesel in a Progressive

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   Articles FT-Raman Spectroscopy Quantification of Biodiesel in a ProgressiveSoybean Oil Transesterification Reaction and Its Correlation with 1 H NMR Spectroscopy Methods Grace Ferreira Ghesti,* Julio Lemos de Macedo, Ineˆs Sabioni Resck, Jose´ Alves Dias, andSı´lvia Cla´udia Loureiro Dias*  Laborato ´ rio de Cata ´ lise, Instituto de Quı ´ mica, Uni V  ersidade de Brası ´ lia, caixa postal 4478, Brası ´ lia-DF, 70904-970, Brazil Recei V  ed December 29, 2006. Re V  ised Manuscript Recei V  ed May 29, 2007  Biodiesel fuel (fatty acid esters) has become more and more attractive due to its environmental benefits.Transesterification is the most common and important method for making biodiesel from vegetable oils oranimal fats. Several studies have focused on the development and improvement of analytical methods formonitoring biodiesel production and determining the fuel quality. Analytical procedures reported in the literatureinclude chromatographic methods (e.g., gas chromatography, high-performance liquid chromatography, gelpermeation chromatography, etc.) and spectroscopic methods [e.g.,  1 H and  13 C NMR, near infrared, Fouriertransform infrared spectroscopy, and recently, Fourier transform (FT)-Raman]. The study presented in thispaper expands our previous research, in which FT-Raman spectroscopy combined with partial least squares(PLS) multivariate analysis was successfully applied to the quantification of soybean oil/ethyl ester mixtures.The FT-Raman/PLS methods developed by our group were used to monitor and quantify a transesterificationreaction process involving soybean oil and ethanol to produce fatty acid ethyl esters (biodiesel) over 22 hcatalyzed by a heterogeneous Lewis acid catalyst. The results were successfully correlated with two  1 H NMRspectroscopic methods reported in the literature and a new  1 H NMR method proposed in this work that can beeasily extended to other vegetable oils. The correlation coefficients (  R 2 ) obtained from the linear fit betweenFT-Raman measurements and the above  1 H NMR methods were 0.9974, 0.9847, and 0.9972, respectively. 1. Introduction Biodiesel is defined by the American Society for Testing andMaterials (ASTM) as a fuel comprised of monoalkyl esters of long-chain fatty acids derived from vegetable oils or animalfats meeting the requirements of ASTM D 6751. 1,2 Biodieselhas distinct advantages when compared to petroleum-deriveddiesel fuel (petrodiesel). It is derived from renewable resources;is biodegradable; is nontoxic; has low emission profiles, a higherflash point, and excellent lubricity; and can be used either pureor blended with petrodiesel fuel. 1,3,4 The use of vegetable oils as fuel has been known since theParis Exposition in 1900. 4 However, due to its higher molecularmass and kinematic viscosity, its direct use in diesel enginesresulted in several operational problems (e.g., poor atomization,carbon deposits due to incomplete combustion, oil ring sticking,lubricating problems, etc.). 1,4,5 To solve these problems, fourpossible solutions were investigated in literature: transesteri- * Corresponding authors. Phone: 55-(61)-3307-2162. Fax: 55-(61)-3368-6901. E-mail: scdias@unb.br (S.C.L.D.) and grace@unb.br (G.F.G.).(1) Ma, F.; Hanna, M. A.  Bioresour. Technol.  1999 ,  70 , 1 - 15.(2) ASTM D 6751-03a.  Annu. Book ASTM Stand.  2005 ,  05.04 , 609 - 614.(3) Knothe, G.  J. Am. Oil Chem. Soc.  1999 ,  76  , 795 - 800.(4)  The Biodiesel Handbook  ; Knothe, G., Gerpen, J. V., Krahl, J., Eds.;American Oil Chemists’ Society Press: Champaign, IL, 2005.(5) Meher, L. C.; Sagar, D. V.; Naik, S. N.  Renew. Sustain. Energy Re V  . 2006 ,  10 , 248 - 268. VOLUME 21, NUMBER 5 SEPTEMBER/OCTOBER 2007 © Copyright 2007 American Chemical Society 10.1021/ef060657r CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 07/17/2007  fication (also called alcoholysis), pyrolysis, dilution with con-ventional petroleum-derived diesel fuel, and microemulsifi-cation. 1,4 - 12 The transesterification reaction is the only method that leadsto products defined as biodiesel (i.e., alkyl esters) and is by farthe most usual method to make biodiesel. 1,4 In transesterification,the triacylglyceride (TAG) molecules found in animal fats orvegetable oils (e.g., soybean, 12 - 14 peanut, 12 rapeseed, 1 etc.)reacted with an alcohol (e.g., methanol, 12 ethanol, 12 - 14 etc.) inthe presence of a catalyst (e.g., NaOH, 12 - 14 H 2 SO 4 , 12 lipase, 11 etc.) to form esters and glycerol (Figure 1). 1,3 - 5,10,12 In additionto the advantages previously mentioned, biodiesel fuel producedby transesterification also has similar properties to petroleum-derived diesel fuel (e.g., cetane number, viscosity, molecularmass, density, etc.), 15 - 18 and no diesel engine modifications arerequired. 19 In Brazil, ethanol is a less expensive alcohol than methanoldue to its high production volume from biomass sugarcane. Inthis sense, the biodiesel produced from ethanol (fatty acid ethylesters or FAEE) can be seen as a truly renewable fuel. In 2003,the Brazil Federal Government started its own biodiesel pro-grams (PROBIODIESEL and PNPB), which intend to stimulatethe scientific and technologic development of biodiesel and toimplement the sustainable production and use of biodiesel,respectively, through the gradual increase of renewable resourcesin the Brazilian energetic matrix. 20 The development andoptimization methods for biodiesel production in Brazil and inother countries have motivated a great number of publicationsand patents. 21 The analytical procedures reported in the literaturefor the determination of fuel quality and the monitoring of biodiesel production 22 include chromatographic methods [e.g.,gas chromatography (GC), 23 high-performance liquid chroma-tography, 24 gel permeation chromatography (GPC), 22,25 etc.] andspectroscopic methods [e.g., nuclear magnetic resonance(NMR), 14,26,27 near-infrared (NIR), 3,27 Fourier transform infraredspectroscopy (FTIR), 13,25 and recently, FT-Raman spectros-copy 28 ].Spectroscopic techniques are fast, easily adapted in routineprocess analysis, and allow nondestructive measurements of thesamples 22,29 versus time-consuming chromatographic methods.NMR spectroscopy has become one of the most powerfultechniques to investigate and identify the structure of chemicalcompounds and dynamics of molecular systems in almost allbranches of chemistry. 30,31 Although NMR spectroscopy doesnot present the detection limit accuracy for the quantificationof minor components as chromatographic methods do, NMR isa suitable method to monitor a chemical reaction, since smallamounts are required to obtain a quantitative spectrum withsignificant information related to the substances of interest inthe reaction media. 32 Alternatively, the employment of vibrational spectroscopictechniques (NIR, FTIR, and Raman) in quality-control monitor-ing has been growing quickly due to several qualities (e.g., fastmeasurements, easy handling, accuracy, reliability, possibilityof on-line monitoring with fiber-optic probes, etc.). 22,33 Forexample, NIR spectroscopy is being used to analyze free fattyacid contents in oils, 34,35 and Raman spectroscopy has beenwidely used in the pharmaceutical 36 and polymer industries. 37 Uni- and multivariate regression analyses have been widely usedto develop calibration models based on vibrational spectroscopicdata. 38 - 40 Actually, all reports in the literature until nowdescribing vibrational spectroscopy methods to monitor andquantify biodiesel fuel production were based on regressionanalysis. 3,13,27 Recently, our group reported the advantages of FT-Ramanspectroscopy to quantify the concentration of ethyl esters in (6) Schwab, A. W.; Bagby, M. O.; Freedman, B.  Fuel  1987 ,  66  , 1372 - 1378.(7) Dasari, M. A.; Goff, M. J.; Suppes, G.  J. Am. Oil Chem. Soc.  2003 , 80 , 189 - 192.(8) Schwab, A. W.; Dykstra, G. J.; Selke, E.; Sorenson, S. C.; Pryde, E.H.  J. Am. Oil Chem. Soc.  1988 ,  65 , 1781 - 1786.(9) Bagby, M. O.; Freedman, B.; Schwab, A. W.  Seed Oils for DieselFuels: Sources and Properties ; ASAE Paper No. 87-1583; AmericanSociety of Agricultural Engineers: St. Joseph, MI, Dec 1987.(10) Schuchardt, U.; Sercheli, R.; Vargas, R. M.  J. Braz. Chem. Soc. 1998 ,  9 , 199 - 210.(11) Jackson, M. A.; King, J. W.  J. Am. Oil Chem. Soc.  1996 ,  73 , 353 - 356.(12) Freedman, B.; Pryde, E. H.; Mounts, T. L.  J. Am. Oil Chem. Soc. 1984 ,  61 , 1638 - 1643.(13) Zagonel, G. F.; Peralta-Zamora, P.; Ramos, L. P.  Talanta  2004 , 63 , 1021 - 1025.(14) Neto, P. R. C.; Caro, M. S. B.; Mazzuco, L. M.; Nascimento, M.G.  J. Am. Oil Chem. Soc.  2004 ,  81 , 1111 - 1114.(15) Demirbas ¸ , A.  Energy Con V  ers. Manage.  2002 ,  43 , 2349 - 2356.(16) Fukuda, H.; Kondo, A.; Noda, H.  J. Biosci. Bioeng.  2001 ,  92 , 405 - 416.(17) Pryde, E. H. J.  Am. Oil Chem. Soc.  1984 ,  61 , 1609 - 1610.(18) Barnwal, B. K.; Sharma, M. P.  Renew. Sustain. Energy Re V  .  2005 , 9 , 363 - 378.(19) Saka, S.; Kusdiana, D.  Fuel  2001 ,  80 , 225 - 231.(20) Programa Nacional de Produc ¸ a˜o e Uso de Biodiesel Home Page.http://www.biodiesel.gov.br (accessed May 2007).(21) Pinto, A. C.; Guarieiro, L. L. N.; Rezende, M. J. C.; Ribeiro, N.M.; Torres, E. A.; Lopes, W. A.; Pereira, P. A. P.; Andrade, J. B.  J. Braz.Chem. Soc.  2005 ,  16  , 1313 - 1330.(22) Knothe, G.  Trans. ASAE   2001 ,  44 , 193 - 200.(23) Plank, C.; Lorbeer, E.  J. Chromatogr., A  1995 ,  697  , 461 - 468.(24) Holcˇapek, M.; Jandera, P.; Fischer, J.; Prokesˇ, B.  J. Chromatogr., A.  1999 ,  858 , 13 - 31.(25) Dube, M. A.; Zheng, S.; McLean, D. D.; Kates, M.  J. Am. Oil Chem.Soc.  2004 ,  81 , 599 - 603.(26) Geldard, G.; Bre´s, O.; Vargas, R. M.; Vielfaure, F.; Schuchardt, U.F.  J. Am. Oil Chem. Soc.  1995 ,  72 , 1239 - 1241.(27) Knothe, G.  J. Am. Oil Chem. Soc.  2000 ,  77  , 489 - 493.(28) Ghesti, G. F.; Macedo, J. L.; Braga, V. S.; Souza, A. T. C. P.;Parente, V. C. I.; Figuereˆdo, E. S.; Resck, I. S.; Dias, J. A.; Dias, S. C. L.  J. Am. Oil Chem. Soc.  2006 ,  83 , 597 - 601.(29) Drago, R. S.  Physical Methods for Chemists , 2nd ed.; SaundersCollege Publishing: New York, 1992; pp 162 - 192.(30) Silverstein, R. M.; Bassler, G. C.; Morril, T. C.  Spectrometric Identification of Organic Compounds , 7th ed.; John Wiley & Sons: NewYork, 2005; pp 165 - 202.(31) Engelhardt, G.; Michel, D.  High-Resolution Solid-State NMR of Silicates and Zeolites ; John Wiley & Sons: New York, 1987; p 1.(32) Morgenstern, M.; Cline, J.; Meyer, S.; Cataldo, S.  Energy Fuels 2006 ,  20 , 1350 - 1353.(33) Cooper, J. B.; Wise, K. L.; Jensen, B. J.  Anal. Chem.  1997 ,  69 ,1973 - 1978.(34) Sato, T.  Biosci. Biotechnol. Biochem.  2002 ,  66  , 2543 - 2548.(35) Zhang, H.-Z.; Lee, T.-C.  J. Agric. Food Chem.  1997 ,  45 , 3515 - 3521.(36) Vankeirsbilck, T.; Vercauteren, A.; Baeyens, W.; Van der Weken,G.  TrAC, Trends Anal. Chem.  2002 ,  21 , 869 - 877.(37) Chalmers, J. M.; Everall, N. J.  TrAC, Trends Anal. Chem.  1996 , 15 , 18 - 25.(38) Cooper, J. B.; Wise, K. L.; Groves, J.; Welch, W. T.  Anal. Chem. 1995 ,  67  , 4096 - 4100.(39) Yu, Z.; Ma, C. Y.; Yuen, S. N.; Phillips, D. L.  Food Chem.  2004 , 87  , 477 - 481.(40) Ampiah-Bonney, R. J.; Walmsley, A. D.  Analyst   1999 ,  124 , 1817 - 1821. Figure 1.  Scheme of a generic transesterification reaction betweenone triacylglyceride (TAG) molecule and three ethanol (EtOH)molecules (stoichiometric relation) to form three molecules of ethylesters (FAEE or biodiesel) and one glycerol (GLY) molecule. 2476  Energy & Fuels, Vol. 21, No. 5, 2007 Ghesti et al.  known standard mixtures containing soybean oil. 28 The presentpaper extends that work, and FT-Raman/partial-least-squares(PLS) calibration models were used to monitor and quantify aprogressive transesterification reaction with soybean oil andethanol. The results obtained were correlated with three  1 H NMRspectroscopic methods, two of them already reported by otherauthors 14,41 and a new approach reported here. 2. Experimental Section 2.1. Materials.  Commercial refined soybean oil (Soya), NaOH(Vetec, 99%), NaCl (Vetec, 99.5%), and concentrated HCl (Vetec,38%) were used as received. Ethanol (Vetec, 99.8%) was driedover 3A molecular sieves (Aldrich) for at least 24 h before theexperiments, and MgSO 4 ‚ 7H 2 O (Vetec, 98.0%) was dried at 300 ° C for 4 h. 2.2. Preparation of the Heterogeneous Catalyst.  Cerium tris-dodecylsulfate was used as a heterogeneous Lewis acid catalystfor the transesterification reaction. The synthesis, characterization,and modification have been described elsewhere. 42,43 For all reactionprocedures, the catalyst was activated in a muffle furnace at 100 ° C for 4 h. 2.3. Preparation of Biodiesel and Soybean Oil/BiodieselStandard Mixtures.  Ethyl esters were prepared by transesterifi-cation according to conditions suggested in the literature 12 for base-catalyzed reactions. The reaction was performed in a 50 mL glassround-bottom flask containing 20.00 g of soybean oil, 6.34 g of anhydrous ethanol (1:6 oil-to-alcohol mole ratio), and 0.20 g of NaOH (1% w/w of oil). The system was kept stirring at 80  ° Cunder reflux for 90 min. Then, the product was cooled to roomtemperature, washed several times with consecutive aqueoussolutions of HCl (0.5 wt %) and NaCl (5 wt %), and dried overanhydrous magnesium sulfate and residual alcohol was removedin a rotary evaporator at 70  ° C. Molecular weights of biodieseland soybean oil were calculated using the composition of fatty acidsobtained from the literature. 44 The quality of the product wasverified by NMR (section 2.6) and GC (GC-17A Shimadzuchromatograph with a flame ionization detector and polydimeth-ylsiloxane column, CBPI PONA-M50-042). Since no significantcontaminants were observed, the biodiesel was treated as 100%ethyl ester. Six soybean oil/ethyl ester samples (0:100, 20:80, 40:60, 60:40, 80:20, and 100:0%) were prepared by weighing thefeedstock (soybean oil) and the ethyl ester standards produced(biodiesel). 2.4. Transesterification Reaction.  The heterogeneous transes-terifications were carried out in 50 mL glass round-bottom flaskskept under stirring at 100  ° C under reflux conditions. A total of six reactions (2, 6, 10, 14, 18, and 22 h) were made under identicalconditions in order to reproduce a continuous 22 h reaction. Foreach run, 10.00 g of soybean oil, 15.85 g of anhydrous ethanol(1:30 oil-to-alcohol mole ratio), and 1.00 g of the catalyst (10%w/w of oil) were used. After the reaction, the system was cooledto room temperature, centrifuged to remove the catalyst, washedthree times with a 5 wt % NaCl solution, and dried over anhydrousmagnesium sulfate; residual alcohol was removed in a rotaryevaporator at 70  ° C; and then the system was kept in a refrigeratorfor FT-Raman and NMR analyses. The reaction conditions usedabove were determined to be ideal for this catalyst according toGhesti. 45 2.5. FT-Raman/PLS Analysis.  FT-Raman spectra were recordedon a Bruker FRA 106/S module attached to a Bruker Equinox 55spectrometer using a 1 cm quartz cuvette with a mirrored surfacetoward the scattering direction (128 scans and 4 cm - 1 resolution).The laser excitation (Nd:YAG) and laser power were 1064 nm and250 mW, respectively, and the signal was detected by a liquid N 2 cooled Ge detector. All spectra were recorded at room temperature.The complete procedure of the method developed and theoreticalexplanations were described in a previous publication. 28 Formultivariate analysis, PLS-1 (OPUS-NT Quant software, fromBruker) methods were used, also as described previously. 28 2.6.  1 H NMR Measurements.  NMR experiments were per-formed at 7.05 T using a Varian Mercury Plus NMR spectrometerequipped with 5 mm Varian probes (ATB and SW) using CDCl 3 as solvent.  1 H (300 MHz) spectra were recorded with a pulseduration of 45 ° , a recycle delay of 1.36 s and 16 scans. The spectrawere referenced to tetramethylsilane ( δ  )  0.0 ppm).  13 C (75.46MHz) spectra were recorded with a pulse duration of 45  ° , a recycledelay of 0.28 s and 300 scans. The spectra were referenced to CDCl 3 ( δ  )  77.0 ppm). The two-dimensional experiment heteronuclearmultiple quantum correlation (HMQC) was obtained with the fieldgradient mode, and the attached proton test (APT) pulse sequencewas used to distinguish  13 C NMR multiplicities. 3. Results and Discussion3.1.  1 H NMR Methods.  The first reports involving biodieselsynthesis by transesterification and  1 H NMR analysis wereprimarily focused on yield determination of progressive metha-nolysis 26,27 or ethanolysis 14,41 reactions (i.e., transesterificationof a vegetable oil or animal fat with methanol or ethanol,respectively). In an article by Knothe, 27 the rate of oil conversionto fatty acid methyl ester (FAME) was also studied by removingaliquots through the reaction and analyzing by  1 H NMR. Thislatter work was expanded by recent papers, 32,46 where NMRspectroscopy was used to elucidate aspects related to the kineticsand the mechanism of biodiesel production. The transesterifi-cation process to produce biodiesel fuel from TAGs follows astepwise mechanism of consecutive reversible reactions: (1)conversion of TAG into diacylglyceride (DAG) and FAME, (2)conversion of DAG into monoacylglyceride (MAG) and FAME,and (3) conversion of MAG into glycerol (GLY) and FAME. 47 Morgenstern and co-workers 32 calculated, from initial rates of FAME formation, an activation energy of 27.2 kJ mol - 1 forthe rate-determining step (DAG to MAG) in the multistepmechanism proposed by Freedman and co-workers. 47 It has beenshown 48 that  13 C NMR can provide valuable information aboutthe acyl positional distribution of TAG (triacylglyceride) invegetable oils. Jin and co-workers, 46 by using  1 H and  13 C NMRto identify the positional isomers of DAGs and MAGs formedduring the transesterification reaction, reported that the metha-nolysis pathway occurs preferentially through  sn -1,3-DAGs to sn -1-MAG intermediates. These results showed that NMRspectroscopy can be successfully applied in all steps of biodieselproduction. (41) Silva, C. L. M. Obtenc ¸ a˜o de E Ä steres Etı´licos a Partir da Transes-terificac ¸ a˜o do O Ä  leo de Andiroba com Etanol. M.S. Thesis, University of Campinas, Campinas, SP, Brazil, 2005.(42) Ghesti, G. F.; Macedo, J. L.; Dias, J. A.; Dias, S. C. L. Green LewisAcid Catalysts for Biodiesel Production by Transesterification. BrazilianPatent Appl. 325, 2007.(43) Ghesti, G. F.; Macedo, J. L.; Parente, V. C. I.; Dias, J. A.; Dias, S.C. L. To be submitted for publication, 2007.(44) Gunstone, F. D.  Fatty Acid and Lipid Chemistry ; Aspen Publishers,Inc.: Gaithersburg, MD, 1999; p 76.(45) Ghesti, G. F. Estudo de Catalisadores para Obtenc ¸ a˜o de Biodieselpor Transesterificac ¸ a˜o e Determinac ¸ a˜o do Rendimento por EspectroscopiaRaman. M.S. Thesis, University of Brası´lia, Brası´lia, DF, Brazil, 2006.(46) Jin, F.; Kawasaki, K.; Kishida, H.; Tohji, K.; Moriya, T.; Enomoto,H.  Fuel  2007 ,  86  , 1201 - 1207.(47) Freedman, B.; Butterfield, R. O.; Pryde, E. H.  J. Am. Oil Chem.Soc.  1986 ,  63 , 1375 - 1380.(48) Mannina, L.; Luchinat, C.; Emanuele, M. C.; Segre, A.  Chem. Phys. Lipids  1999 ,  103 , 47 - 55. FT-Raman Spectroscopy Quantification of Biodiesel Energy & Fuels, Vol. 21, No. 5, 2007   2477  As described in the literature, 49 the transesterification reactioncatalyzed by Lewis acid sites occurs through the coordinationof acyl groups of the triacylglyceride molecule to lowestunoccupied molecular orbitals of catalytic active centers. Thiscoordination increases acyl group polarization and forms acarbocation that undergoes alcohol nucleophilic attack. Thetetrahedral intermediary so formed eliminates the diacylglyceridemolecule and produces an ester.Quantification of biodiesel by NMR can be made by simpleequations, 26,27,41 by building an analytical curve 14 or by usingan internal standard (e.g., acetone). 32 Ethyl ester quantificationby  1 H NMR spectroscopy is more complex than methyl esterquantification due to a superimposition of the glyceryl meth-ylenic hydrogens in soybean oil and the  - OCH 2  from ethylester in biodiesel (see Figures 1 - 3). Figure 3 shows the  1 HNMR spectra of pure soybean oil (doublet of doublets), pureethyl esters (quartet), and a 40:60 (m/m) soybean oil/ethyl estermixture from 4.00 to 4.40 ppm, where it illustrates thedifficulties created from signals overlapping caused by partialconversion. To overcome this problem, two methods havealready been proposed by Neto and co-workers, 14 who preparedan analytical curve, and Silva, 41 who used an equation. In thenext paragraphs, we will describe the use of both methods, anda third alternative is suggested in this work.To clarify the methods to be described, the following notationwas used (see Figure 2 and 3 for further clarification): (i)  I  TAG )  integration of glyceryl methylenic hydrogens at 4.25 - 4.35ppm; (ii)  I  TAG + EE  )  integration of glyceryl methylenic hydro-gens and  - OCH 2  of ethoxy hydrogens superimposed at 4.10 - 4.20 ppm; and (iii)  I  R CH 2  )  integration of   R -acyl methylenichydrogens in soybean oil and ethyl esters at 2.20 - 2.40 ppm.The analytical curve was built by plotting known concen-trations of soybean oil/ethyl ester mixtures (see experi-mental section 2.3) versus  I  TAG  /   I  TAG + EE  ratios. Values of 0 and1 were attributed to pure ethyl esters and pure soybean oil in  I  TAG  /   I  TAG + EE , respectively. The correlation coefficient (  R 2 ) soobtained was 0.9867.The equation proposed by Silva 41 and presented below (eq1) was applied to the same standard mixtures of soybean oiland ethyl ester prepared (see experimental section 2.3) todetermine soybean oil conversion into biodiesel ( C  EE ):A plot of predicted values versus real values for eq 1 showedan  R 2  )  0.9977. However, in our  1 H NMR spectral analyses, 45 it was observed that intermediary compounds formed duringtransesterification 46 persisted in biodiesel fuel after purificationsteps, which regularly causes the signals from  - OH groups inmono- and diacylglyceride species to overlap with  R CH 2  signalsat 2.20 - 2.40 ppm (Figure 4). 50 Their presence was alsoobserved at 3.50 - 3.80 ppm. 50 To use the integrated values ineq 1, all spectra were treated by deconvolution and subtractionsteps to ensure reliable results. To avoid these time-consumingprocedures, a new equation is proposed in this work (eq 2),where  R -acyl methylenic hydrogens ( R CH 2 ) are not taken intoaccount:As one can see, it is obvious that the above equation can besimplified to  %C  EE  )  100[(  I  TAG + EE  -  I  TAG )/(  I  TAG + EE  +  2  I  TAG )].However, to facilitate the following explanations, it was left inthe srcinal form. The numbers 4 and 6 in eq 2 are related tofour glyceryl methylenic hydrogens present in TAG molecules (49) Parshall, G. W.; Ittel, S. D.  Homogeneous Catalysis , 2nd ed.;Wiley: New York, 1992; pp 270 - 275. Figure 2.  Full  1 H NMR spectra from (a) pure soybean oil and (b)pure ethyl ester. Figure 3.  1 H NMR spectra at 4.00 - 4.40 ppm region from (a) puresoybean oil, (b) 40:60 (m/m) soybean oil/ethyl esters, and (c) pure ethylester. Figure 4.  1 H NMR spectra at 2.20 - 2.40 ppm region from (a) pureethyl ester and (b) product from transesterification reaction after 2 h(* indicates NMR signals from intermediate species). % C  EE  )  100 ( (  I  TAG + EE  -  I  TAG )  I  R CH 2 )  (1)% C  EE  )  100 (  4(  I  TAG + EE  -  I  TAG )4(  I  TAG + EE  -  I  TAG )  +  6(2  I  TAG ) )  (2) 2478  Energy & Fuels, Vol. 21, No. 5, 2007 Ghesti et al.
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