VOLUME 21, NUMBER 5
SEPTEMBER/OCTOBER 2007
© Copyright 2007 American Chemical Society
Articles FT-Raman Spectroscopy Quantification of Biodiesel in a Progressive Soybean Oil Transesterification Reaction and Its Correlation with 1H NMR Spectroscopy Methods Grace Ferreira Ghesti,* Julio Lemos de Macedo, Ineˆs Sabioni Resck, Jose´ Alves Dias, and Sı´lvia Cla´udia Loureiro Dias* Laborato´ rio de Cata´ lise, Instituto de Quı´mica, UniVersidade de Brası´lia, caixa postal 4478, Brası´lia-DF, 70904-970, Brazil ReceiVed December 29, 2006. ReVised Manuscript ReceiVed 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 or animal fats. Several studies have focused on the development and improvement of analytical methods for monitoring biodiesel production and determining the fuel quality. Analytical procedures reported in the literature include chromatographic methods (e.g., gas chromatography, high-performance liquid chromatography, gel permeation chromatography, etc.) and spectroscopic methods [e.g., 1H and 13C NMR, near infrared, Fourier transform infrared spectroscopy, and recently, Fourier transform (FT)-Raman]. The study presented in this paper 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 transesterification reaction process involving soybean oil and ethanol to produce fatty acid ethyl esters (biodiesel) over 22 h catalyzed by a heterogeneous Lewis acid catalyst. The results were successfully correlated with two 1H NMR spectroscopic methods reported in the literature and a new 1H NMR method proposed in this work that can be easily extended to other vegetable oils. The correlation coefficients (R2) obtained from the linear fit between FT-Raman measurements and the above 1H NMR methods were 0.9974, 0.9847, and 0.9972, respectively.
1. Introduction Biodiesel is defined by the American Society for Testing and Materials (ASTM) as a fuel comprised of monoalkyl esters of long-chain fatty acids derived from vegetable oils or animal fats meeting the requirements of ASTM D 6751.1,2 Biodiesel has distinct advantages when compared to petroleum-derived diesel fuel (petrodiesel). It is derived from renewable resources; is biodegradable; is nontoxic; has low emission profiles, a higher
flash point, and excellent lubricity; and can be used either pure or blended with petrodiesel fuel.1,3,4 The use of vegetable oils as fuel has been known since the Paris Exposition in 1900.4 However, due to its higher molecular mass and kinematic viscosity, its direct use in diesel engines resulted 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, four possible solutions were investigated in literature: transesteri-
* Corresponding authors. Phone: 55-(61)-3307-2162. Fax: 55-(61)-33686901. E-mail:
[email protected] (S.C.L.D.) and
[email protected] (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, 609614.
(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 ReV. 2006, 10, 248-268.
10.1021/ef060657r CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007
2476 Energy & Fuels, Vol. 21, No. 5, 2007
Figure 1. Scheme of a generic transesterification reaction between one triacylglyceride (TAG) molecule and three ethanol (EtOH) molecules (stoichiometric relation) to form three molecules of ethyl esters (FAEE or biodiesel) and one glycerol (GLY) molecule.
fication (also called alcoholysis), pyrolysis, dilution with conventional petroleum-derived diesel fuel, and microemulsification.1,4-12 The transesterification reaction is the only method that leads to products defined as biodiesel (i.e., alkyl esters) and is by far the most usual method to make biodiesel.1,4 In transesterification, the triacylglyceride (TAG) molecules found in animal fats or vegetable oils (e.g., soybean,12-14 peanut,12 rapeseed,1 etc.) reacted with an alcohol (e.g., methanol,12 ethanol,12-14 etc.) in the presence of a catalyst (e.g., NaOH,12-14 H2SO4,12 lipase,11 etc.) to form esters and glycerol (Figure 1).1,3-5,10,12 In addition to the advantages previously mentioned, biodiesel fuel produced by transesterification also has similar properties to petroleumderived diesel fuel (e.g., cetane number, viscosity, molecular mass, density, etc.),15-18 and no diesel engine modifications are required.19 In Brazil, ethanol is a less expensive alcohol than methanol due to its high production volume from biomass sugarcane. In this sense, the biodiesel produced from ethanol (fatty acid ethyl esters or FAEE) can be seen as a truly renewable fuel. In 2003, the Brazil Federal Government started its own biodiesel programs (PROBIODIESEL and PNPB), which intend to stimulate the scientific and technologic development of biodiesel and to implement the sustainable production and use of biodiesel, respectively, through the gradual increase of renewable resources in the Brazilian energetic matrix.20 The development and optimization methods for biodiesel production in Brazil and in other countries have motivated a great number of publications and patents.21 The analytical procedures reported in the literature for the determination of fuel quality and the monitoring of (6) Schwab, A. W.; Bagby, M. O.; Freedman, B. Fuel 1987, 66, 13721378. (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 Diesel Fuels: Sources and Properties; ASAE Paper No. 87-1583; American Society 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, 353356. (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 ConVers. Manage. 2002, 43, 2349-2356. (16) Fukuda, H.; Kondo, A.; Noda, H. J. Biosci. Bioeng. 2001, 92, 405416. (17) Pryde, E. H. J. Am. Oil Chem. Soc. 1984, 61, 1609-1610. (18) Barnwal, B. K.; Sharma, M. P. Renew. Sustain. Energy ReV. 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.
Ghesti et al.
biodiesel production22 include chromatographic methods [e.g., gas chromatography (GC),23 high-performance liquid chromatography,24 gel permeation chromatography (GPC),22,25 etc.] and spectroscopic methods [e.g., nuclear magnetic resonance (NMR),14,26,27 near-infrared (NIR),3,27 Fourier transform infrared spectroscopy (FTIR),13,25 and recently, FT-Raman spectroscopy28]. Spectroscopic techniques are fast, easily adapted in routine process analysis, and allow nondestructive measurements of the samples22,29 versus time-consuming chromatographic methods. NMR spectroscopy has become one of the most powerful techniques to investigate and identify the structure of chemical compounds and dynamics of molecular systems in almost all branches of chemistry.30,31 Although NMR spectroscopy does not present the detection limit accuracy for the quantification of minor components as chromatographic methods do, NMR is a suitable method to monitor a chemical reaction, since small amounts are required to obtain a quantitative spectrum with significant information related to the substances of interest in the reaction media.32 Alternatively, the employment of vibrational spectroscopic techniques (NIR, FTIR, and Raman) in quality-control monitoring has been growing quickly due to several qualities (e.g., fast measurements, easy handling, accuracy, reliability, possibility of on-line monitoring with fiber-optic probes, etc.).22,33 For example, NIR spectroscopy is being used to analyze free fatty acid contents in oils,34,35 and Raman spectroscopy has been widely used in the pharmaceutical36 and polymer industries.37 Uni- and multivariate regression analyses have been widely used to develop calibration models based on vibrational spectroscopic data.38-40 Actually, all reports in the literature until now describing vibrational spectroscopy methods to monitor and quantify biodiesel fuel production were based on regression analysis.3,13,27 Recently, our group reported the advantages of FT-Raman spectroscopy to quantify the concentration of ethyl esters in (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.; Saunders College 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: New York, 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, 35153521. (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, 18171821.
FT-Raman Spectroscopy Quantification of Biodiesel
known standard mixtures containing soybean oil.28 The present paper extends that work, and FT-Raman/partial-least-squares (PLS) calibration models were used to monitor and quantify a progressive transesterification reaction with soybean oil and ethanol. The results obtained were correlated with three 1H NMR spectroscopic methods, two of them already reported by other authors14,41 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 dried over 3A molecular sieves (Aldrich) for at least 24 h before the experiments, and MgSO4‚7H2O (Vetec, 98.0%) was dried at 300 °C for 4 h. 2.2. Preparation of the Heterogeneous Catalyst. Cerium trisdodecylsulfate was used as a heterogeneous Lewis acid catalyst for the transesterification reaction. The synthesis, characterization, and modification have been described elsewhere.42,43 For all reaction procedures, the catalyst was activated in a muffle furnace at 100 °C for 4 h. 2.3. Preparation of Biodiesel and Soybean Oil/Biodiesel Standard Mixtures. Ethyl esters were prepared by transesterification according to conditions suggested in the literature12 for basecatalyzed reactions. The reaction was performed in a 50 mL glass round-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 °C under reflux for 90 min. Then, the product was cooled to room temperature, washed several times with consecutive aqueous solutions of HCl (0.5 wt %) and NaCl (5 wt %), and dried over anhydrous magnesium sulfate and residual alcohol was removed in a rotary evaporator at 70 °C. Molecular weights of biodiesel and soybean oil were calculated using the composition of fatty acids obtained from the literature.44 The quality of the product was verified by NMR (section 2.6) and GC (GC-17A Shimadzu chromatograph with a flame ionization detector and polydimethylsiloxane column, CBPI PONA-M50-042). Since no significant contaminants 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 the feedstock (soybean oil) and the ethyl ester standards produced (biodiesel). 2.4. Transesterification Reaction. The heterogeneous transesterifications were carried out in 50 mL glass round-bottom flasks kept under stirring at 100 °C under reflux conditions. A total of six reactions (2, 6, 10, 14, 18, and 22 h) were made under identical conditions in order to reproduce a continuous 22 h reaction. For each 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 cooled to room temperature, centrifuged to remove the catalyst, washed three times with a 5 wt % NaCl solution, and dried over anhydrous magnesium sulfate; residual alcohol was removed in a rotary evaporator at 70 °C; and then the system was kept in a refrigerator (41) Silva, C. L. M. Obtenc¸ a˜o de EÄ steres Etı´licos a Partir da Transesterificac¸ 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 Lewis Acid Catalysts for Biodiesel Production by Transesterification. Brazilian Patent 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.
Energy & Fuels, Vol. 21, No. 5, 2007 2477 for FT-Raman and NMR analyses. The reaction conditions used above were determined to be ideal for this catalyst according to Ghesti.45 2.5. FT-Raman/PLS Analysis. FT-Raman spectra were recorded on a Bruker FRA 106/S module attached to a Bruker Equinox 55 spectrometer using a 1 cm quartz cuvette with a mirrored surface toward the scattering direction (128 scans and 4 cm-1 resolution). The laser excitation (Nd:YAG) and laser power were 1064 nm and 250 mW, respectively, and the signal was detected by a liquid N2 cooled Ge detector. All spectra were recorded at room temperature. The complete procedure of the method developed and theoretical explanations were described in a previous publication.28 For multivariate analysis, PLS-1 (OPUS-NT Quant software, from Bruker) methods were used, also as described previously.28 2.6. 1H NMR Measurements. NMR experiments were performed at 7.05 T using a Varian Mercury Plus NMR spectrometer equipped with 5 mm Varian probes (ATB and SW) using CDCl3 as solvent. 1H (300 MHz) spectra were recorded with a pulse duration of 45°, a recycle delay of 1.36 s and 16 scans. The spectra were referenced to tetramethylsilane (δ ) 0.0 ppm). 13C (75.46 MHz) spectra were recorded with a pulse duration of 45 °, a recycle delay of 0.28 s and 300 scans. The spectra were referenced to CDCl3 (δ ) 77.0 ppm). The two-dimensional experiment heteronuclear multiple quantum correlation (HMQC) was obtained with the field gradient mode, and the attached proton test (APT) pulse sequence was used to distinguish 13C NMR multiplicities.
3. Results and Discussion 3.1. 1H NMR Methods. The first reports involving biodiesel synthesis by transesterification and 1H NMR analysis were primarily focused on yield determination of progressive methanolysis26,27 or ethanolysis14,41 reactions (i.e., transesterification of a vegetable oil or animal fat with methanol or ethanol, respectively). In an article by Knothe,27 the rate of oil conversion to fatty acid methyl ester (FAME) was also studied by removing aliquots through the reaction and analyzing by 1H NMR. This latter work was expanded by recent papers,32,46 where NMR spectroscopy was used to elucidate aspects related to the kinetics and the mechanism of biodiesel production. The transesterification process to produce biodiesel fuel from TAGs follows a stepwise 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-workers32 calculated, from initial rates of FAME formation, an activation energy of 27.2 kJ mol-1 for the rate-determining step (DAG to MAG) in the multistep mechanism proposed by Freedman and co-workers.47 It has been shown48 that 13C NMR can provide valuable information about the acyl positional distribution of TAG (triacylglyceride) in vegetable oils. Jin and co-workers,46 by using 1H and 13C NMR to identify the positional isomers of DAGs and MAGs formed during the transesterification reaction, reported that the methanolysis pathway occurs preferentially through sn-1,3-DAGs to sn-1-MAG intermediates. These results showed that NMR spectroscopy can be successfully applied in all steps of biodiesel production. (45) Ghesti, G. F. Estudo de Catalisadores para Obtenc¸ a˜o de Biodiesel por Transesterificac¸ a˜o e Determinac¸ a˜o do Rendimento por Espectroscopia Raman. 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.
2478 Energy & Fuels, Vol. 21, No. 5, 2007
Figure 2. Full 1H NMR spectra from (a) pure soybean oil and (b) pure ethyl ester.
Figure 3. 1H NMR spectra at 4.00-4.40 ppm region from (a) pure soybean oil, (b) 40:60 (m/m) soybean oil/ethyl esters, and (c) pure ethyl ester.
As described in the literature,49 the transesterification reaction catalyzed by Lewis acid sites occurs through the coordination of acyl groups of the triacylglyceride molecule to lowest unoccupied molecular orbitals of catalytic active centers. This coordination increases acyl group polarization and forms a carbocation that undergoes alcohol nucleophilic attack. The tetrahedral intermediary so formed eliminates the diacylglyceride molecule and produces an ester. Quantification of biodiesel by NMR can be made by simple equations,26,27,41 by building an analytical curve14 or by using an internal standard (e.g., acetone).32 Ethyl ester quantification by 1H NMR spectroscopy is more complex than methyl ester quantification due to a superimposition of the glyceryl methylenic hydrogens in soybean oil and the -OCH2 from ethyl ester in biodiesel (see Figures 1-3). Figure 3 shows the 1H NMR spectra of pure soybean oil (doublet of doublets), pure ethyl esters (quartet), and a 40:60 (m/m) soybean oil/ethyl ester mixture from 4.00 to 4.40 ppm, where it illustrates the difficulties created from signals overlapping caused by partial conversion. To overcome this problem, two methods have already been proposed by Neto and co-workers,14 who prepared an analytical curve, and Silva,41 who used an equation. In the next paragraphs, we will describe the use of both methods, and a third alternative is suggested in this work. (49) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.; Wiley: New York, 1992; pp 270-275.
Ghesti et al.
Figure 4. 1H NMR spectra at 2.20-2.40 ppm region from (a) pure ethyl ester and (b) product from transesterification reaction after 2 h (* indicates NMR signals from intermediate species).
To clarify the methods to be described, the following notation was used (see Figure 2 and 3 for further clarification): (i) ITAG ) integration of glyceryl methylenic hydrogens at 4.25-4.35 ppm; (ii) ITAG+EE ) integration of glyceryl methylenic hydrogens and -OCH2 of ethoxy hydrogens superimposed at 4.104.20 ppm; and (iii) IRCH2 ) integration of R-acyl methylenic hydrogens in soybean oil and ethyl esters at 2.20-2.40 ppm. The analytical curve was built by plotting known concentrations of soybean oil/ethyl ester mixtures (see experimental section 2.3) versus ITAG/ITAG+EE ratios. Values of 0 and 1 were attributed to pure ethyl esters and pure soybean oil in ITAG/ITAG+EE, respectively. The correlation coefficient (R2) so obtained was 0.9867. The equation proposed by Silva41 and presented below (eq 1) was applied to the same standard mixtures of soybean oil and ethyl ester prepared (see experimental section 2.3) to determine soybean oil conversion into biodiesel (CEE):
%CEE ) 100
(
)
(ITAG+EE - ITAG) IRCH2
(1)
A plot of predicted values versus real values for eq 1 showed an R2 ) 0.9977. However, in our 1H NMR spectral analyses,45 it was observed that intermediary compounds formed during transesterification46 persisted in biodiesel fuel after purification steps, which regularly causes the signals from -OH groups in mono- and diacylglyceride species to overlap with RCH2 signals at 2.20-2.40 ppm (Figure 4).50 Their presence was also observed at 3.50-3.80 ppm.50 To use the integrated values in eq 1, all spectra were treated by deconvolution and subtraction steps to ensure reliable results. To avoid these time-consuming procedures, a new equation is proposed in this work (eq 2), where R-acyl methylenic hydrogens (RCH2) are not taken into account:
%CEE ) 100
(
4(ITAG+EE - ITAG)
4(ITAG+EE - ITAG) + 6(2ITAG)
)
(2)
As one can see, it is obvious that the above equation can be simplified to %CEE ) 100[(ITAG+EE - ITAG)/(ITAG+EE + 2ITAG)]. However, to facilitate the following explanations, it was left in the original form. The numbers 4 and 6 in eq 2 are related to four glyceryl methylenic hydrogens present in TAG molecules
FT-Raman Spectroscopy Quantification of Biodiesel
Energy & Fuels, Vol. 21, No. 5, 2007 2479
Table 1. Biodiesel True Concentration Values and Those Predicted by Eq 2 with and without Assumption of Six Standard Soybean Oil/Ethyl Esters Mixtures true CEE values (wt %)
predicted CEE values (wt %) without assumptiona
predicted CEE values (wt %) with assumptionb
0.00 20.45 40.39 59.85 79.62 100.00
0.00 20.64 42.46 60.62 76.80 99.40
0.63 21.19 42.90 60.94 77.00 99.41
aC EE values calculated from eq 2 without assuming that glyceryl methylenic hydrogens areas at 4.25-4.35 and 4.10-4.20 ppm are identical. bC EE values calculated from eq 2 assuming that glyceryl methylenic hydrogens areas at 4.25-4.35 and 4.10-4.20 ppm are identical.
and to six hydrogens formed in three ethyl ester products (see Figure 1), respectively. The number of glyceridic hydrogens assigned at 4.35-4.10 ppm was obtained from the literature,32,46,48,50 and also from our own 2D heteronuclear correlation in the HMQC experiment (not shown), where four glyceridic hydrogens at 4.35-4.10 ppm were correlated with the CH2Ocarbon signal at 62.0 ppm and one glyceridic hydrogen at 5.205.33 ppm was correlated with the CHO- carbon signal at 68.8 ppm. The APT experiment (not shown) and computational simulation analysis were used to discriminate and correctly assign carbon ressonances. The difference ITAG+EE - ITAG (i.e., integration from the superimposition between glyceryl methylenic hydrogens of TAG molecules and the -OCH2 of ethoxy hydrogens from ethyl ester molecules at 4.10-4.20 ppm minus the integration of glyceryl methylenic hydrogens at 4.25-4.35 ppm) in eq 2 is associated with ethyl ester formation. To simplify eq 2, the areas from glyceryl methylenic groups for soybean oil at 4.25-4.35 and 4.10-4.20 ppm were assumed to be equal (see Figure 3), that is, 2ITAG. Although this is not exactly true, because the ratio between the two areas is 0.98 instead of 1.00, the values obtained from eq 2 on standard soybean oil/ethyl ester mixtures, considering hydrogen areas to be equivalent or not, were very similar (see Table 1). Indeed, a plot of biodiesel concentration predicted by eq 2 versus the true concentration, using both calculations in Table 1, resulted in the same correlation coefficient (R2 ) 0.9983). 3.2. FT-Raman/PLS Methods. Knothe’s work,3 the first report linking vegetable oil transesterification and vibrational spectroscopic analysis, described biodiesel quantification and fuel quality assessment by NIR spectroscopy using a fiber-optic probe. The major spectral differences between soybean oil and their respective methyl esters were distinguished at 4425-4430 and 6005 cm-1, where it was observed that methyl esters displayed bands while soybean oil triacylglyceride molecules exhibited only shoulders. These regions were analyzed for biodiesel and contaminant quantifications (glycerol and methanol) in the spectra obtained from soybean oil/methyl ester and contaminant/methyl ester mixtures by PLS calibration models with excellent results for both spectral regions. In a second NIR paper,27 Knothe successfully applied the calibration models built in the first paper to a transesterification reaction in progress and correlated, with good agreement, with 1H NMR spectroscopy. Although FTIR spectra of vegetable oil and biodiesel are almost identical, Zagonel and co-workers13 developed an analytical procedure, based on CdO stretching vibration displacement from 1746 cm-1 (TAGs) to 1735 cm-1 (ethyl esters), to monitor soybean oil ethanolysis by FTIR multivariate (50) Jie, M. S. F. L. K.; Lam, C. C. Chem. Phys. Lipids 1995, 77, 155171.
Table 2. Predicted CEE Values by Raman/PLS Models with Increased Data Set for Standard Soybean Oil/Ethyl Ester Mixtures of Known Composition Treated as Unknown predicted CEE values (wt %) true CEE values (wt %)a
6 samples data set size (R2 ) 0.9990)
11 samples data set size (R2 ) 0.9991)
21 samples data set size (R2 ) 0.9997)
0.00 20.01 49.82 79.69 100.00
3.73 25.91 51.23 80.52 99.64
3.71 25.72 51.34 80.42 99.18
0.52a 21.88a 50.28a 80.64a 99.31
a
Values obtained from ref 28.
analysis models. The results obtained in a progressive transesterification reaction by the calibration model, built from FTIR spectra of standard mixtures of triolein and ethyl esters analyzed by principal component analysis and PLS regression, were correlated with GPC analyses showing only small differences (R2 ) 0.9837). Raman spectra of soybean oil and their corresponding ethyl esters showed several differences: (i) a peak at 2932 cm-1 (νCH2), whereas the soybean oil spectrum displays only a shoulder; (ii) νCdO displacement from 1748 (soybean oil) to 1739 cm-1 (ethyl esters); and (iii) bands at 861 (assigned as νR-CdO and νC-C) and 372 cm-1 (assigned as δCO-O-C).28 Raman spectra present well-defined bands and less congestion than FTIR and NIR spectra, and as a consequence, there is a smaller possibility of band overlapping, making quantitative measurements simpler.51,52 As mentioned earlier, in our previous work,28 FT-Raman/ PLS methods showed good prediction capabilities when applied to known samples treated as unknown. A data set of 63 spectra from 21 standard soybean oil/ethyl ester mixtures (triplicate measurements) was used, and from the numerous correlations and regression methods tested for analysis, FT-Raman/PLS methods at the CH stretching region (3100-2740 cm-1, mainly the peak at 2932 cm-1) could be distinguished from the others due to their predicted capabilities: (i) R2 ) 0.9994 for the model using 21 samples averaged from 63 measurements as a data set (model 4, notation used in ref 28) and (ii) R2 ) 0.9993 for the model using all 63 measurements as a data set (model 5). A detailed description of both methods was given elsewhere,28 and no further discussion will be mentioned here. The two models were applied to the heterogeneous transesterification reaction, and similar results were obtained. A plot of model 4 predicted values versus model 5 predicted values results in an R2 ) 0.9976. However, when compared to the predicted values found by 1H NMR methods, model 4 was slightly better then model 5. Although multivariate analysis generally requires a large number of samples to build reliable calibration models (at least 20 samples for each component), the articles of Knothe3,27 using NIR/PLS, and Zagonel et al.13 using FTIR/PLS, were successfully applied to progressive transesterification reactions with smaller data sets in their models. In our case, the effects of increased data sets on FT-Raman/PLS models led to increased predicted capabilities and linearity (see Table 2). 3.3. Correlation Between FT-Raman/PLS and 1H NMR Methods. In preceding articles dealing with spectroscopic measurements of biodiesel content in transesterification reac(51) Baran´ska, H.; Łbudzin´ska, A.; Terpin´ski, J. Laser Raman Spectroscopy: Analytical Applications; Ellis Horwood Limited: Chichester, U.K., 1987; pp 151-168. (52) Skoog, D. A.; Holler, F. J.; Nieman, T. A. Principles of Instrumental Analysis; Saunders College Publishing: Philadelphia, PA, 1997; pp 429443.
2480 Energy & Fuels, Vol. 21, No. 5, 2007
Figure 5. Biodiesel yield from a heterogeneous transesterification reaction determined by four methods: 1H NMR analytical curve, eq 1, eq 2, and FT-Raman/PLS model.
Figure 6. Plot from FT-Raman/PLS model predicted reaction values vs 1H NMR analytical curve, eq 1, and eq 2 predicted reaction values (R2 ) 0.9974, 0.9847, and 0.9972, respectively).
tions, the results obtained were correlated with other techniques to validate their values. Knothe27 correlated NIR spectroscopy and two 1H NMR equations with good agreement. Neto and co-workers14 correlated their 1H NMR analytical curves with viscosity measurements, obtaining a linear correlation coefficient
Ghesti et al.
of 0.9981. Zagonel and co-workers13 used GPC to validate their FTIR/PLS methods, obtaining an R2 ) 0.9837 between both techniques. Figure 5 displays FT-Raman/PLS (model 4) and 1H NMR methods applied to a progressive heterogeneous transesterification reaction. The results obtained from all four methods (FT-Raman/PLS and three 1H NMR methods) showed good correlation (Figure 5) with each other, indicating their mutual validation. Indeed, the linear fit between FT-Raman measurements and all NMR methods tested (i.e., analytical curve, eq 1, and eq 2) showed R2 values of 0.9974, 0.9847, and 0.9972, respectively (Figure 6). The difference observed between biodiesel yield values from eq 1 and the other methods after 2 h of reaction is associated with integration difficulties produced by signal superimposition from glyceridic intermediates (MAGs and DAGs) and R-acyl CH2. Although both methods presented in this work do not attend the detection limits required by ASTM to determine low levels of contaminants in biodiesel fuel as chromatographic methods do, spectroscopic techniques allows a faster analysis of the reaction by easily identifying the formation of peaks or bands related to ethyl ester products. These methods, when coupled to multivariate analysis, can be applied to fast reversible reactions, such as transesterification,3,13,14,26,27,32,46 to evaluate a new catalyst potentiality to produce biodiesel. In conclusion, FT-Raman/PLS models are an attractive alternative to rapidly monitor biodiesel synthesis due to their inherent qualities and their reduced costs when compared with other techniques, including NMR. However, 1H NMR methods, especially when involving simple equations, are more easily adapted to other vegetable oils’ methanolysis or ethanolysis reactions. Both procedures described here were successfully correlated with each other and validate by reported methods, evidencing and increasing their potential for biodiesel synthesis and quality monitoring. Acknowledgment. Acknowledgement is given to CNPq, CAPES/ PQI, FINATEC, Finep-CTPetro, Finep-CTInfra 970/01, UnB-IQ, UnB-IG, and FAPDF/SCDT/CNPq. EF060657R
