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Energy & Fuels 2006, 20, 1350-1353
Determination of the Kinetics of Biodiesel Production Using Proton Nuclear Magnetic Resonance Spectroscopy (1H NMR) Mark Morgenstern,* Jessica Cline, Sally Meyer, and Simon Cataldo Chemistry Department, Colorado College, Colorado Springs, Colorado 80903 ReceiVed NoVember 14, 2005. ReVised Manuscript ReceiVed May 26, 2006
Methyl esters were produced at several temperatures (10, 35, and 45 °C) by transesterification batch reactions of soybean oil with methanol utilizing KOH and NaOH catalysts. The reactions were monitored by aliquot removal and subsequent proton nuclear magnetic resonance spectroscopy (1H NMR) analysis. 1H NMR analysis allowed for the calculation of the average degree of fatty acid unsaturation (DU ) 1.52) in oil and methyl ester. 1H NMR analysis also provided initial rates of methyl ester formation and an activation energy of 27.2 kJ/mol. The time-dependent concentration data revealed substantial reaction progress toward equilibrium after only 120 s at a reduced temperature of 10 °C. Understanding the resonance shifts in the 1H NMR spectra of starting materials and products allows for quantitation of reaction progress that is in good agreement with results obtained using other analytical methods.
Introduction Rudolf Diesel first envisioned his diesel engine invention as a powerful engine that could operate on agricultural products such as vegetable oils to counter the dependency on petroleum fuels that was prevalent as early as 1905.1 His engine was a great success, but his desire to use agricultural-derived fuels was not realized because of the inexpensive petroleum production of diesel fuels and the need to chemically modify plant oils prior to use in the diesel engine. However, global events and rising petroleum costs have recently revitalized interest in Rudolf Diesel’s dream. The diesel engine can indeed be run on straight vegetable oil, but many problems result ranging from starting failure and engine wear to polymerization of the lubricating oils.2 Most of the problems associated with the use of straight vegetable oil can be avoided by chemical modification through transesterification of the vegetable oil triglyceride molecules to smaller alkyl esters of the fatty acids commonly referred to as biodiesel.2 This transesterification is normally accomplished using methanol as the alcohol and either NaOH or KOH as a base catalyst (see Figure 1). Reviews of biodiesel production have been published, and the transesterification reaction has primarily been monitored using gas chromatography (GC).3,15 The use of proton nuclear magnetic resonance spectroscopy (1H NMR) has recently been employed to monitor the kinetics and product distributions in transesterification reactions (alcoholysis) between vegetable oils and alcohols.4-6 The use of 1H NMR is convenient and fast when monitoring a reaction, because a small aliquot can be extracted from the batch reaction at any given time and the 1H NMR spectrum analysis provides * To whom correspondence should be addressed. E-mail: mmorgenstern@ coloradocollege.edu. (1) Krawzcyk, T. Inform 1996, 7, 801-815. (2) Harwood, H. J. J. Am. Oil Chem. Soc. 1984, 61, 315-324. (3) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1-15. (4) Suppes, G. J.; Bockwinkel, K.; Lucas, S.; Botts, J. B.; Mason, M. H.; Heppert, J. A. J. Am. Oil Chem. Soc. 2001, 78, 139-145. (5) Knothe, G. J. Am. Oil Chem. Soc. 2000, 77, 489-493. (6) Gelbard, G.; Bres, O.; Vargas, R. M.; Vielfavre, F.; Schuchardt, U. F. J. Am. Oil Chem. Soc. 1995, 72, 1239-1241.
Figure 1. General reaction for the transesterifaction of a triglyceride using an alcohol.
quantitative information pertaining to the chemical species present in the reaction. In this process, concentrations of reactant oil and product methyl ester can be determined at early times during a batch reaction and the initial rates of the reaction can be obtained. The rate of oil conversion to methyl ester has been reported using aliquot sampling and 1H NMR analysis.5 The work presented in this paper expands the previous study in three significant ways: (i) a detailed interpretation of the 1H NMR spectrum of unsaturated vegetable oil is presented; (ii) initial rates and activation energy for the reaction are obtained; and (iii) the transesterification (alcoholysis) reaction is studied using a stoichiometric 3:1 molar ratio of alcohol/soybean oil as opposed to a 6:1 or larger ratio used in most previous studies.3-5,7,8 Recent work has found that the optimum molar ratio is approximately 3.5:1 and increased quantities of methanol over this ratio result in separation problems during subsequent steps in the process.9 In previous 1H NMR studies of vegetable oil alcoholysis reactions, quantitation is achieved by comparing the integration of the resonance signal from the glyceridic protons in the oil to the methyl ester protons in the product to determine the extent of the reaction at each sampling time.5 Additionally, the olefinic proton signals in the product are compared to the methyl ester signal in the product to determine the average degree of unsaturation in the fatty acid chains.4 However, previous work (7) Darnoko, D.; Cheryan, M. J. Am. Oil Chem. Soc. 2000, 77, 12631267. (8) Freedman, B.; Butterfield, R. O.; Pryde, E. H. J. Am. Oil Chem. Soc. 1986, 63, 1375-1380. (9) Dorado, M. P.; Ballesteros, E.; Mittlebach, M.; Lopez, F. J. Energy Fuels 2004, 18, 1457-1462.
10.1021/ef0503764 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/04/2006
Kinetics of Biodiesel Production Using 1H NMR
incorrectly assigned the glyceridic proton signal at 4.2 ppm as a five-proton signal, while it is actually only a four-proton dimethylene signal, with the remaining methine proton showing up further downfield as part of the olefinic proton signal at 5.25 ppm.5 The shift of the glyceridic methine proton can easily be established by obtaining the 1H NMR spectrum of a saturated triglyceride, such as tributyrin or trimyristin, where there are no olefin protons and the methine glyceridic proton clearly shows up as a multiplet downfield from the four methylene glyceridic protons. Therefore, when using the 1H NMR integration values in unsaturated oils, the olefinic proton integrations must be corrected for the underlying glyceridic methine proton and the glyceridic methylene resonance should be treated as a four-proton signal. The time interval used in aliquot removal from a vegetable oil alcoholysis reaction using methanol must be rapid when the reaction is performed at room temperature or higher. Over 50% conversion to the product within the initial 5 min has been reported for the reaction of soybean oil with methanol at 45 °C using the KOH catalyst.5 This fast reaction would require several aliquots to be removed and analyzed within the first minute to obtain the initial rates for the disappearance of soybean oil and the appearance of the methyl ester product. In the present work, the kinetics of a 3:1 stoichiometric ratio of methanol/oil is studied in contrast to previous papers that utilize a 6:1 ratio of methanol/oil. It has been reported that performing the transesterification as a two-stage reaction results in a more complete reaction.16 In the two-stage transesterification process, the first stage is allowed to approach equilibrium using a 3:1 stoichiometric ratio of methanol/oil before the glycerol is removed. The second stage consists of the addition of an additional equivalent of methanol and stirring to drive the equilibrium further to products and therefore a second separation of the glycerol byproduct. Therefore, this work is partially initiated to determine the time required for the first stage to approach equilibrium when a 3:1 stoichiometric ratio of reactants is used. 1H NMR can be used to monitor the extent of the reaction as an alternative to GC. Experimental Section The progress of the transesterification reaction between soybean oil and methanol was monitored by the extraction of aliquots from a batch reaction at given time intervals. The extent of the reaction was determined by 1H NMR analysis of the aliquots. Batch reactions were carried out at different temperatures and with different catalysts. Batch Reactions. The batch reactions were performed with a stoichiometric molar ratio (3:1) of methanol/soybean oil. A 200 mL two-necked round-bottom flask was charged with 2.50 mmol of catalyst (0.100 g of NaOH or 0.140 g of KOH) and 9.40 mL (0.232 mol) of methanol. A magnetic stir bar was added, and with continuous stirring on a magnetic stir plate, the catalyst was completely dissolved in about 15 min. Once the catalyst was in solution, 80 mL of soybean oil (0.077 mol based on an average molar mass of 895.9 g/mol from percent composition data) was added and the rate of stirring increased to approximate a homogeneous system. The batch reactions were performed in an ice bath (10 °C) and in an oil bath (35 and 46 °C) to obtain the temperaturedependent data for the reaction. The oil and alcohol/catalyst solutions were allowed to equilibrate at the desired temperature prior to mixing and initiation of the reaction. A reflux condenser was employed at the higher temperatures to prevent the alcohol from evaporating. Aliquot Removal. Aliquots (1.0 mL) were removed from the batch reaction using an autopipet with disposable tips. The maximum rate of aliquot extraction was one sample every 10-30 s using this method. The aliquots were immediately transferred into
Energy & Fuels, Vol. 20, No. 4, 2006 1351 Table 1. Reported Percent Fatty Acid Composition of Soybean Oil and Corresponding Degree of Unsaturation per Fatty Acid (FA)10,11,12 fatty acid identity
average ( SD
myristic (14:0) palmitic (16:0) stearic (18:0) oleic (18:1) linoleic (18:2) linolenic (18:3) average degree of unsaturation per FA
0.2 ( 0.1 10.7 ( 1.0 3.0 ( 0.9 25.0 ( 3.5 53.4 ( 2.3 7.3 ( 1.2 1.54 ( 0.02
a test tube containing 2.5 µL of 12 M HCl (0.030 mmol) and placed on ice to quench any further alcoholysis reaction in the test tube. Acetone (0.100 mL, 1.35 mmol) was added to each aliquot to serve as an internal standard for the 1H NMR analysis of concentrations present in a sample. Several 1H NMR analyzes were performed with known concentrations of oil and methyl ester, and back calculations using the acetone standard showed a consistent 10% error. Individual spectra of each aliquot were obtained using a Varian 200 MHz NMR and CDCl3 (99.8%) as the solvent. Reagents. Soybean oil, King Soopers brand, was purchased in 1 gallon jugs from the local King Soopers grocery store (Colorado Springs, CO). Methanol (99.9% purity), KOH (88.1% purity), NaOH (98.0% purity), and concentrated (12 M) HCl were all certified ACS-grade and purchased from Fischer Scientific (Fairlawn, NJ). Acetone (99.8% purity) was certified ACS-grade and purchased from the J. T. Baker Chemical Company (Phillipsburg, NJ). The CDCl3 [99.8% D with 0.03% (v/v) tetramethylsilane (TMS)] used as the NMR solvent was purchased from Aldrich Chemical Co. (Milwaukee, WI). All reagents were used without further purification.
Results and Discussion The general reaction for the transesterification of a triglyceride with an alcohol (alcoholysis) is shown in Figure 1. Note that this reaction is reversible and every triglyceride molecule reacts with 3 equiv of the alcohol to produce glycerol and three fatty acid ester molecules. In base-catalyzed reactions, the catalyst is typically NaOH or KOH. The general fatty acid chains denoted as R1, R2, and R3 vary in identity and percent composition depending upon the triglyceride source (see Table 1 for soybean oil composition). Plant oils generally contain a high percent of unsaturated fatty acid chains, where R1, R2, and R3 would be oleic, linoleic, and linolenic acid chains. The high percent of unsaturated fatty acid chains results in plant-derived triglycerides having a liquid physical state (oil) at moderate temperatures. The alcohols (R′OH) used in the transesterification of triglycerides are normally methanol or ethanol, with methanol being the most common because of the low cost, low reaction temperatures, fast reaction times, and higher quality methyl ester product.9,14 Typical 1H NMR spectra of soybean oil and methyl ester product resulting from transesterification with methanol are presented in Figures 2 and 3. The corresponding 1H resonance assignments are presented in Figure 4. It is interesting to note that the presence of polyunsaturated fatty acids can be readily detected by the appearance of a resonance at 2.72 ppm from the methylene group between two double bonds of a linoleic, linolenic, or higher polyunsaturated fatty acid chain. The splitting pattern for the protons in the glyceride moiety is an ABX pattern because of the chemical-shift inequivalence of the glyceride methylene protons at 4.21 ppm, and the measured coupling constants are Jab ) 12 Hz, Jax ) 6 Hz, and Jbx ) 4 Hz for the system represented by R1OCHaHbC(OR2)HxCHaHbOR3.13 In this system, there is a plane that runs along the carbon-carbon bond and the Ha protons are on the same side of this plane as the -OR2 group, while the Hb protons are on
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Morgenstern et al.
Figure 2. Typical 1H NMR spectrum of soybean oil (CDCl3, Varian 200 MHz NMR).
Figure 3. Fatty acid methyl ester product from soybean oil alcoholysis with methanol.
Figure 4. Useful 1H NMR assignments in the CDCl3 solvent for a hypothetical triglyceride of soybean oil, containing the three most prominent fatty acid chains as examples (the assignments are downfieldshifts relative to TMS).
the side of the plane with the Hx proton. Therefore, the Ha protons (4.30 ppm) show up as the downfield doublet of doublets, and the Hb protons (4.15 ppm) are a doublet of doublets further upfield (see the inset of Figure 2). The large coupling constant, Jab ) 12 Hz, is assigned to the geminal coupling interaction between the nonequivalent methylene protons of both methylene groups. The long fatty acid chains R1, R2, and R3 are very similar or even identical in the effect that they have on the environment of the glyceride protons. If the groups were very different, the resulting pattern at 4.21 ppm would be
much more complex because the four glyceride methylene protons would all have different chemical shifts. The complex patterns observed in this region for mono- and diglycerides have been observed and serve as an example.5 The major differences between the 1H NMR spectra of the starting soybean oil (Figure 2) and the resulting fatty acid methyl ester (Figure 3) are the disappearance of the glyceride protons at 4.21 ppm (4H) and the appearance of the methyl ester protons at 3.67 ppm (3H). The average degree of unsaturation (DU) for the fatty acid methyl ester product can be calculated on the basis of a comparison of 1H NMR integration values for the methyl group (3.67 ppm) and the olefin protons (5.35 ppm) in the methyl ester product. The average DU can also be obtained from the starting oil by a comparison of the integration value for the methylene group adjacent to the carbonyl group (2.31 ppm) to that of the olefin protons (adjusted for the underlying methine proton). The DU value from soybean oil obtained in this manner was 1.52, which corresponds with the DU values calculated using percent composition data in Table 1 and also agrees with a DU value of 1.487 reported in previous research using 1H
Kinetics of Biodiesel Production Using 1H NMR
Figure 5. Kinetic plot of the KOH-catalyzed methanolysis of soybean oil at 10 °C.
NMR analysis of fatty acid methyl esters synthesized from soybean oil.4 Similar 1H NMR analysis of fatty acid methyl esters synthesized from olive oil gave a DU value of 1.06, which also corresponds to the value of 0.94 calculated from percent composition data.10-16 These calculations and comparisons serve to provide confidence in the functional group integration values of the vegetable oil and the methyl ester product to calculate concentrations during kinetic runs. The kinetics of the base-catalyzed transesterification reaction between methanol and soybean oil in a 3:1 molar ratio was studied by 1H NMR at temperatures of 10, 35, and 45 °C. An example kinetic plot for the reaction at 10 °C using the KOH catalyst is presented in Figure 5. It is obvious from the kinetic plot that the reaction has significantly approached equilibrium within 120 s even at a reduced temperature of 10 °C. Therefore, in a two-stage process, the first stage can begin separation after a very short stirring period of only a few minutes. Furthermore, the kinetic plot shows that the methyl ester product has a much faster rate of formation than the rate of total glyceride depletion. This has been observed in previous studies and is expected in this work because the concentration of glycerides is not effected until all three fatty acid chains are cleaved and glycerol is produced.8 Only with the production of glycerol is the 1H NMR signal for the glyceride methylene protons diminished because the mono- and diglyceride intermediates still contribute to the proton integration at 4.25 ppm and are noticeable as complex patterns in addition to the triglyceride pattern discussed earlier. (10) Wade, L. G., Jr. Organic Chemistry, 2nd ed.; Prentice Hall: New Jersey, 1987; p 1150. (11) Goering, C. E.; Schwab, A. W.; Daugherty, M. J.; Pryde, E. H.; Heakin, A. J. Trans. Am. Soc. Agric. Eng. 1982, 25, 1472-1483. (12) Kincs, F. R. J. Am. Oil Chem. Soc. 1985, 62, 815-818. (13) Silverstein, R. M.; Bassler, G.C.; Morrill, T. C. Spectroscopic Identification of Organic Compounds, 4th ed.; John Wiley and Sons: New York, 1981; pp 192-212. (14) Tickell, J. From the Fryer to the Fuel Tank: The Complete Guide to Using Vegetable Oil as an AlternatiVe Fuel, 3rd ed.; Tickell Energy Consultants, 2000. (15) Khan, A. K. Research into Biodiesel Kinetics and Catalyst DeVelopment; University of Queensland: Brisbane, Queensland, Australia, 2002. (16) Encinar, J. M.; Gonzalez, J. F.; Rodriguez-Reinares, A. Ind. Eng. Chem. Res. 2005, 44, 5491-5499.
Energy & Fuels, Vol. 20, No. 4, 2006 1353
Figure 6. Arrhenius plot of data presented in Table 2 to determine the activation energy for methanolysis of soybean oil using the NaOH catalyst. Table 2. Temperature Dependence of Initial Rates of Methyl Ester Formation Obtained by 1H NMR Analysis temperature (°C)
catalyst
methyl ester initial rate (M/s)
10 35 45 10
NaOH NaOH NaOH KOH
6.4 × 10-3 1.6 × 10-2 2.3 × 10-2 2.7 × 10-3
The glycerol product is not observed in the 1H NMR analysis because of insolubility in the system. Analogous kinetic data using NaOH as the catalyst provided the initial rates for methyl ester formation at several temperatures and are presented in Table 2. Figure 6 shows the Arrhenius plot (of the initial rates presented in Table 2) used to obtain the activation energy from the temperature dependence of the initial rates for the methanolysis of soybean oil using the NaOH catalyst. An activation energy (Ea) value of 27.2 kJ/mol was obtained from the slope of the plot according to the equation: ln(rate) ) -Ea/RT + C. In this equation, R is the gas constant (8.31 J mol-1 K-1), T is the absolute temperature (in Kelvin), and C is a constant. The plot provided an excellent linear fit (R2 ) 0.9999), and a calculated Ea of 27.2 kJ/mol is in agreement with the lower values reported from experiments using a 6:1 molar ratio (methanol/oil) and GC analysis.3,7,8 This activation energy would relate to the conversion of diglyceride to monoglyceride as the rate-determining step in the proposed multistep mechanism.8 However, more work with this 1H NMR method is necessary to obtain rate data and an Ea for each step in the proposed mechanism for base-catalyzed methanolysis of soybean oil. Although the 1H NMR spectra of starting oils and product methyl esters are very similar, it is possible to differentiate resonance signals to the extent that quantitation of the reaction progress can be achieved. The results obtained using 1H NMR analyses agree with the results reported from the more traditional GC analysis of the soybean oil transesterification reaction. Acknowledgment. J. C. would like to thank the Colorado College Otis and Margaret Barnes Trust for financial support of undergraduate research. EF0503764
