ARTICLE pubs.acs.org/EF
Monitoring the Transesterification Reaction Used in Biodiesel Production, with a Low Cost Unilateral Nuclear Magnetic Resonance Sensor Luis F. Cabec- a,† Lucineia V. Marconcini,† Giovanni P. Mambrini,† Rodrigo B. V. Azeredo,‡ and Luiz A. Colnago*,† † ‡
Embrapa Instrumentac-~ao, Rua XV de Novembro, 1452, S~ao Carlos, SP, Brazil, 13560-970 Instituto de Química, Universidade Federal Fluminense, Campus do Valonguinho, 24020-150 Niteroi, Rio de Janeiro, Brazil ABSTRACT: The transesterification reaction used to produce biodiesel was monitored with 1H high-resolution nuclear magnetic resonance (HRNMR), conventional low-resolution NMR (LRNMR), and unilateral NMR (UNMR) spectroscopy. HRNMR was used as a standard method to compare with the methods of LRNMR and UNMR. A CarrPurcellMeiboomGill (CPMG) pulse sequence was used in both LRNMR and UNMR experiments. In LRNMR, the CPMG signal was used to measure the transverse relaxation time (T2), which depended on sample viscosity; it showed a good correlation (R = 0.994) for the concentration of biodiesel in the oil/biodiesel mixture. These measurements could only be used when the samples contained less than 1% of methanol. In UNMR, the CPMG decay of the biodiesel/oil mixture strongly depended on molecular diffusion because of the intrinsic high magnet field gradient (G) associated with the magnetic. The T2eff had a good and negative correlation (R = 0.997) with the biodiesel concentration in the biodiesel/oil mixture. The UNMR measurements were insensitive to the methanol contamination up to 50%, which was contrary to the LRNMR method. As methanol diffused rapidly, its magnetization lost coherency quickly in the presence of strong G and it was not fully refocused by the CPMG sequence. Therefore, the use of the fast, portable, and low cost UNMR sensor to monitor transesterification reactions was demonstrated ex situ. Now, the UNMR sensor has been adapted to monitor the transesterification reaction in situ, in a biodiesel pilot plant.
1. INTRODUCTION Biodiesel is composed of monoalkyl esters of long-chain fatty acids derived from vegetable oils or animal fats.13 Biodiesel is an environment-friendly alternative to petroleum-derived diesel fuel because it is renewable, biodegradable, nontoxic, has low emission profiles (sulfur content, particulates, green house gases), a higher flash point, and excellent lubricity; it can also be used in either its pure form or it can be blended with petrodiesel fuel.46 Biodiesel is obtained by a transesterification reaction7 between triacylglyceride (TAG) molecules and a monohydric alcohol (methanol,8 ethanol9) in the presence of a catalyst (KOH, NaOH, H2SO4, lipase), producing a fatty acid mono ester (biodiesel) and glycerin (as a coproduct). This reaction has been performed to reduce the viscosity of the triglycerides, producing a fuel with physical chemical properties similar to petrodiesel.4,6 The transesterification reaction is affected by the molar ratio of glycerides to alcohol, alcohol-type compounds, catalysts, reaction temperature, reaction time, and free fatty acids as well as the water content of oils or fats.1 Therefore, it is essential to monitor the transesterification reaction to define the end of the process to ensure the quality of the biodiesel and that it meets international or national standards (ASTM, DIN, etc.).10,11 Several analytical methods have been used to monitor the transesterification reaction and biodiesel quality,1,12 such as gas chromatography (GC),13 high-performance liquid chromatography (HPLC),14 gel permeation chromatography (GPC),15 and wet chemistry techniques, which include potentiometric and iodometric titrations; these techniques are destructive, laborious, and time-consuming r 2011 American Chemical Society
methods. Spectroscopic methods, such as nuclear magnetic resonance (NMR),16,17 near-infrared (NIR),17 Fourier transform infrared spectroscopy (FTIR),15 and FT-Raman spectroscopy18 have gained importance in recent years because they are nondestructive, need minimal sample preparation, and are much faster than chromatographic and wet chemistry methods. High-resolution 1H NMR spectroscopy has been used to monitor the transesterification reaction, based on the areas of the methoxy signal at 3.7 ppm, the R-carbonyl methylene signal at 2.3 ppm,19 and the areas of the glycerides signal from 4.04 to 4.25.12 High-resolution NMR is fast, easily adapted in routine process analysis, and allows nondestructive measurements of the samples,20 but it requires the use of expensive and not commonly available equipment when compared to chromatographic or wet chemical methods. The less expensive benchtop, time domain, low-resolution NMR (LRNMR) equipment has been widely used in the analysis TAG, for the oil content and quality in seeds,21,22 but to our knowledge, it was never used to monitor the transesterification reaction. LRNMR uses differences in transverse (T2) and longitudinal (T1) relaxation times to analyze the quality of TAG samples, providing information about the viscosity, cetane number, iodine value, etc.22 Recently, a compact and low cost version of LRNMR equipment based on single sided magnets has Received: February 24, 2011 Revised: May 9, 2011 Published: May 09, 2011 2696
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been introduced, increasing the in situ and industrial applicability of LRNMR.23,24 However, a major difference associated with this system compared to conventional LRNMR is the stronger magnetic field gradient (G = 10 T/m). This strong G reduces the detection of the NMR signal to a thin sample volume, lowering the experiment sensitivity; conversely, it increases the sensitivity of the NMR signal to the diffusion coefficient of the analyte. Therefore, the aim of this study was to evaluate the use of Unilateral NMR to monitor the transesterification reaction. The performance of Unilateral NMR was compared to standard HRNMR methods and to the conventional benchtop LRNMR system.
2. MATERIALS AND METHODS 2.1. Reaction Preparation. The transesterification reaction, catalyzed by NaOH, was prepared according to Murugesan.1 The reaction was performed in a 20-mL beaker containing 2 g of soybean oil, 2 g of anhydrous methanol, and 0.02 g of NaOH (1% w/w of oil). The system was stirred during the reaction at room temperature and stirring was stopped to collect the samples for NMR measurements. 2.2. High-Resolution NMR Analysis. The reaction was conducted using the same experimental procedure described above. At time intervals between 10 and 90 min, 10 μL aliquots of the reaction medium were collected and dissolved in 500 μL of deuterated chloroform (CDCl3) to stop the reaction. Tetramethylsilane (TMS) was used as an internal chemical shift standard. The 1H NMR spectra were recorded using a Varian INOVA 400 spectrometer, with a 9.4T-superconducting magnet, which was equivalent to a proton resonance of 400 MHz. Each spectrum was recorded with 16 scans, π/2 pulse width of 10.5 μs, and an acquisition time of 5 and 15 s of recycle delay. From the 1 H NMR spectra, the conversion from oil to ester was calculated by using the following equation, presented by Morgenstern and co-workers:19
CME ¼
2I1 100% 2I1 þ 9I2
ð1Þ
where CME is the conversion degree from oil to ester, I1 is the area of signal between 3.63 and 3.69 ppm (methyl ester signal; 9H) and I2 is the area between 4.25 and 4.35 ppm (oil signal; 2H). 2.3. Bench Top LRNMR Analysis. The LRNMR measurements were performed at 25 ( 0.5 °C on an SLK-SG-100 (Spin Lock Magnetic Resonance Solutions, Argentina), 0.23 T (9 MHz for 1H) benchtop spectrometer, using a 32 mm diameter probe and controlled by CONDOR IDE software (Spin Lock Magnetic Resonance Solutions, Cordoba, Argentina). The analysis was performed with a CPMG pulse sequence using π/2 pulse width of 6.5 μs, time between echo (τ) of 1.2 ms, acquisition time of 10.6 μs at midway between the refocusing pulses, 2.000 echoes, a recycle delay of 1.5 and 4 scans. A volume of 0.5 mL of solution was used in each measurement. 2.4. Unilateral NMR Analysis. The magnet of the unilateral NMR (UNMR) sensor was constructed in a classical “U-shaped” geometry, as described by Bl€umich and co-workers.23 Each magnetic pole was built with three pieces of NdFeB alloy (2.54 2.54 1.27 cm) and was axially magnetized. The magnets were mounted on an iron yoke with antiparallel polarization and separated by 1.25 cm. The probe was constructed with a single flat radio frequency coil etched from a
Figure 1. 1H NMR spectra of aliquots removed from the transesterification reaction at different time points.
standard printed circuit board and tuned at 16.5 MHz. The Unilateral sensor was driven by a CAT-100 Tecmag console, a power amplifier 3205 AMT, and a preamplifier of Miteq AU 1114. The analyses were performed with a CPMG pulse sequence using pulse widths of 2 μs, τ = 200 μs, acquisition time of 64 μs at midway between the refocusing pulses, 1000 echoes, a recycle delay of 0.5 and 700 scans. All Unilateral NMR measurements were performed at room temperature (approximately 25 °C). The transesterification reaction was monitored every 10 min, using 1.0 mL of the reaction mixture transferred to a thin microscope glass plate and placed on the surface of the unilateral NMR sensor. 2.5. Measurements of the Biodiesel/MeOH Mixture using LRNMR. The effects of methanol contamination on biodiesel measurements using the LRNMR (benchtop and UNMR) were performed with a CPMG pulse sequence using the same parameters described in Sections 2.3 and 2.4. Mixtures containing 0.1, 0.2, 0.4, 0.6, and 1.0 g of MeOH, equivalent to 5, 10, 20 30, and 50% MeOH in biodiesel were prepared. To calculate methanol influence on the intensity of the CPMG signals of the biodiesel/ methanol mixtures, average values of intensities for the first one hundred echoes were used.
3. RESULTS AND DISCUSSION The transesterification reaction between soybean oil and methanol, catalyzed by NaOH, was monitored by 1H NMR and T2 relaxation times, using high- and low-resolution spectrometers, respectively. Figure 1 illustrates the 1H NMR spectral expansions from 3 to 5 ppm for the samples collected from the transesterification reaction (oil/biodiesel phase) between 5 to 90 min. This figure shows the signals of the four hydrogen atoms of glycerol (glycerol C1 and C3) of the TAGs, between 4.25 and 4.35 ppm, and the three hydrogen atoms of the methoxy group of the fatty acid methyl ester (FAME) or biodiesel at 3.7 ppm. The signal at 3.35 ppm was due to the methanol residue in the oil/ biodiesel phase and did not interfere with the measurements because it was not overlapping with the glyceride and methoxy signals. However, this is not the case for the measurements performed on the benchtop LRNMR as presented in Section 3.1. The spectra in Figure 1 clearly show a decrease of the glyceride 2697
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Figure 2. Biodiesel conversion at different reaction times calculated from the 1H NMR spectra and eq 1.19
Figure 3. 1H CPMG decay for soybean oil and biodiesel using a benchtop LRNMR spectrometer.
Figure 4. Transversal relaxation values (T2) for different biodiesel percentages in soybean oil using the benchtop instrument.
signal and as a function of time, which confirmed the conversion of soybean oil to biodiesel. Figure 2 shows the percentage of the production of FAME as a function of time computed from the areas of the glycerides and methoxy signals using eq 1. This figure shows that more than 80% of the oil was converted to biodiesel in the first 30 min. After 50 min, the reaction was completed. As shown in Figures 1 and 2, high-resolution 1HNMR spectroscopy, a well-established technique in quantitative analysis, can be easily used to analyze the conversion of oil to biodiesel. However, it requires sophisticated instrumentation
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Figure 5. Biodiesel percentage produced by the transesterification reaction from 0 to 90 min and calculated by the CPMG decay and calibration curve of Figure 4.
Figure 6. T2 values for different methanol percentages in biodiesel using a benchtop LRNMR spectrometer.
and expensive deuterated solvents. Herein, it was used as a standard method to compare to the measurements performed with low cost LRNMR measurements. 3.1. Low-Resolution NMR Analysis. The LRNMR experiments were performed on a conventional benchtop and using a unilateral sensor. Once the T2 value exhibited a negative correlation with viscosity and the biodiesel viscosity was approximately 1 order of magnitude lower than the respective TAG,25 the T2 measurements of these pure components and mixture were monitored by benchtop LRNMR. Figure 3 shows the decays of the NMR signals of soybean oil and its biodiesel obtained with the CPMG pulse sequence on a benchtop spectrometer. The oil signal showed a faster decay than the respective biodiesel because of the higher viscosity. For quantitative measurements, it was necessary to use a calibration curve correlating the T2 value and the concentration of biodiesel in the respective oil (Figure 4). Figure 4 shows the correlation curve between the monoexponential T2 values and the biodiesel percentage measured by LRNMR in a mixture with soybean oil, from 0% to 100%. This figure shows a T2 = 125 ms for the pure oil and 575 ms for pure biodiesel, at 25 °C. From 0 to 10% of biodiesel, there were no significant differences in T2 values and it was not useful in this concentration range. However, the T2 response was linear (R = 0.994) for the most important concentrations of biodiesel, from 10% to 100%. Figure 5 shows the biodiesel percentage calculated with the correlation curve of Figure 4 produced by the transesterification 2698
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Figure 7. 1H CPMG decay for soybean oil and biodiesel measured using a UNMR sensor.
reaction from 0 to 90 min. The T2 values (data not shown) and percentages of biodiesel increased exponentially until approximately 50 min and then remained constant, indicating the end of the transesterification reaction, which corresponded to a T2 ≈ 575 ms. These measurements were performed after the methanol residue was removed by evaporation from the oilbiodiesel mixture because the methanol residue increased the T2 value and caused errors in the oil/biodiesel proportion. This effect is illustrated in Figure 6, which shows that the T2 measurements, using benchtop LRNMR, were very sensitive to the presence of methanol contamination in biodiesel. This sensitivity could be explained by the increase of the methanol in the CPMG signal. This result demonstrated that the T2 measurements, using the benchtop LRNMR spectrometer, could be used to monitor biodiesel reactions when the methanol contamination is kept below 1%. 3.1.1. Unilateral NMR Analysis. The major difference between conventional benchtop LRNMR and Unilateral NMR (UNMR) is the extremely strong magnetic field gradient (G) of the UNMR magnet,26 which restricts the analyzed volume to a very thin slice of the sample. Therefore, for the same number of scans or analysis time, the signal-to-noise in UNMR is much lower (Figure 7) than the one obtained in conventional LRNMR, which uses the full sample volume (Figure 3). Figure 7 shows the CPMG decay using the UNMR sensor for the soybean oil and respective biodiesel. The decay of the CPMG signal of the soybean oil was longer (107 ms) than the one observed for biodiesel (43 ms), which had an inverse result when compared to the conventional LRNMR experiment (Figure 3). A longer CPMG decay value for more viscous samples is a wellknown behavior of NMR signals acquired with UNMR sensors.27 The difference between the CPMG decay observed in conventional LRNMR and UNMR can be explained by eq 2. For LRNMR, the weaker G made the CPMG decay dependent on T2 only. However, for UNMR the second term in eq 2 became relevant for liquids or solutions, even when small τ values in the range of 100 μs were used. ! 2 t 2 2 τ t Mt ðecho at tÞ µ exp γ G D ð2Þ T2 3 The variable, Mt, corresponds to the magnetization at time t, γ is the gyromagnetic ratio, G is the magnetic field gradient, and D is the diffusion coefficient.
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Figure 8. Effective transversal relaxation values (T2eff) for different biodiesel/soybean oil mixtures, obtained by the CPMG pulse sequence using the UNMR sensor.
Figure 9. Biodiesel percentage produced by the transesterification reaction calculated from the UNMR signal and calibration curve obtained from Figure 8.
For samples with low viscosity, diffusion became the dominant factor in the time-constant CPMG signal and could explain why biodiesel had a shorter effective time constant (T2eff) than the respective oil. Therefore, the sensitivity of the CPMG signal to molecular diffusion could be used to monitor the transesterification reaction using a UNMR sensor. Figure 8 shows how T2eff was dependent on the proportion of the soybean oil/biodiesel. This figure exhibited a linear and negative correlation R = 0.997 with the biodiesel concentration in the mixture. With this curve, it was possible to predict the concentration of the biodiesel/TAG during the transesterification reaction. Figure 9 shows the variation of the percentage in the reaction mixture as a function of time, up to 90 min, calculated with the CPMG signal and the calibration curve (Figure 8). In the first 10 min, the reaction was slow and showed a small increase in biodiesel concentration or decrease of T2eff. From 10 to 40 min, the biodiesel concentration increased rapidly. The end of the reaction was indicated by T2eff reaching a minimum (approximately 43 ms), and a concentration equivalent to pure biodiesel was reached. We also evaluated the sensitivity of UNMR measurements to the methanol contamination (up to 50%) in biodiesel. The T 2eff of biodiesel did not vary with methanol contamination. The T 2eff was 45 (3 ms for all methanol concentration studied. 2699
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HRNMR, conventional LRNMR, and UNMR show good correlation in the analysis of biodiesel, even though they were based on different physical properties.
Figure 10. Variation in the average intensity of the first 100 echoes of the CPMG signal for a mixture biodiesel/methanol using the UNMR sensor.
This unexpected result suggested that the alcohol was not influencing the FAME diffusion or contributing to the T2eff. This hypothesis was tested and pure methanol had been analyzed in the UNMR sensor. The CPMG signal of the pure methanol shows less than ten echoes and with 2:1 signal-to-noise ratio for the first echo. As methanol diffuses very fast, its magnetization loses coherency very quickly in the presence of strong G and it is not fully refocused by the CPMG sequence. Therefore, the methanol signal had been attenuated by the diffusion effect. However, the presence of the methanol in the biodiesel using UNMR can be monitored by the reduction of the intensity of the CPMG signal. Figure 10 illustrates the decrease of the CPMG signal (average intensity of the first hundred echoes) with the increase of methanol concentration. Therefore, the results in Figures 9 and 10 demonstrate that methanol diluted the biodiesel, but it was not interfering with T2eff. One of the problems observed when using different UNMR sensors to monitor the transesterification reaction was their larger difference in the G. The magnitude of the gradient is strongly dependent on the size of the magnets, its construction, and the distance of the sensitive volume from the sensor. Therefore, the sensitivity of the sensor to diffusion should always be tested to obtain similar values. The UNMR measurements presented in Figures 710 were performed ex situ using small samples collected from the reaction media. Now, the UNMR sensor has been adapted to monitor the transesterification reaction in situ, in a biodiesel pilot plant. 3.2. Comparison between UNMR Performance with Conventional HR and LRNMR. 1H HRNMR is an efficient technique used to monitor the transesterification reaction between vegetable oil and methanol and can be used to calculate the conversion degree in reactions and estimate the reaction time. Figure 2 shows the conversion of oil into biodiesel as a function of time. The biodiesel concentration reached a plateau at approximately 50 min, which indicated the end of the reaction. Similar results were obtained with LRNMR and UNMR as shown in Figures 5 and 9, respectively, where both LRNMR spectrometers could be used to monitor the transesterification reaction. These two NMR techniques utilized a CPMG pulse sequence, but the LRNMR measurements were based on the dependence on the T2, which was directly correlated to the biodiesel concentration in the mixture. In UNMR, the CPMG decay (T2eff) was governed by molecular self-diffusion and had an inverse correlation with the biodiesel concentration. Therefore, the results obtained with
4. CONCLUSIONS High-resolution, conventional low-resolution, and unilateral NMR are fast, efficient, and nondestructive techniques used to monitor transesterification reactions. The high-resolution technique is the most sensible; however, it uses expensive equipment and solvents that have limited applications in an industrial environment. The conventional and unilateral low-resolution NMR spectrometers are more suitable for monitoring transesterification reactions in industries. The biggest limitation of the conventional LRNMR technique to monitor the reaction was the presence of alcohols in the oil/biodiesel mixtures. This problem could easily be solved by the evaporation of the alcohol. The UNMR was the most suitable conventional NMR technique examined in this study to monitor the transesterification reaction in an industrial environment. It was the cheapest NMR technique, was not greatly affected by methanol contamination in the oil/biodiesel mixture, and may be used to monitor the reaction in situ. ’ AUTHOR INFORMATION Corresponding Author
*Phone: þ55 (16) 2107 2821. E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the FAPESP, CNPq, and FINEP (Brazilian agencies) for financial support. ’ REFERENCES (1) Murugesan, A.; Umarani, C.; Chinnusamy, T. R.; Krishnan, M.; Subramanian, R.; Neduzchezhain, N. Renew. Sustain. Energy Rev. 2009, 13, 825–834. (2) Meher, L. C.; Sagar, D. V.; Naik, S. N. Renew. Sustain. Energy Rev. 2006, 10, 248–268. (3) Pinto, A. C.; Guarieiroa, 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. (4) Demirbas, A. Energy Convers. Manage. 2002, 43, 2349–2356. (5) Knothe, G. J. Am. Oil Chem. Soc. 1999, 76, 795–800. (6) Barnwal, B. K.; Sharma, M. P. R. Renew. Sustain. Energy Rev. 2005, 9, 363–378. (7) Otera, J. Chem. Rev. 1993, 93, 1449–1470. (8) Freedman, B.; Pryde, E. H.; Mounts, T. L. J. Am. Oil Chem. Soc. 1984, 61, 1638–1643. (9) Neto, P. R. C.; Caro, M. S. B.; Mazzuco, L. M.; Nascimento, M. G. J. Am. Oil Chem. Soc. 2004, 81, 1111–1114. (10) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (11) ASTM. ASTM D6751-03a. Annu. Book ASTM Stand. 2005, 05 (04), 609–614. (12) Ghesti, G. F.; Macedo, J. L.; Resck, I. S.; Dias, J. A.; Dias, S. C. L. Energy Fuels 2007, 21, 2475–2480. (13) Plank, C.; Lorbeer, E. J. Chromatogr. A 1995, 697, 461–468. (14) Holcapek, M.; Jandera, P.; Fischer, J.; Prokes, B. J. Chromatogr. A 1999, 858, 13–31. (15) Dube, M. A.; Zheng, S.; McLean, D. D.; Kates, M. J. Am. Oil Chem. Soc. 2004, 81, 599–603. (16) Geldard, G.; Bres, O.; Vargas, R. M.; Vielfaure, F.; Schuchardt, U. F. J. Am. Oil Chem. Soc. 1995, 72, 1239–1241. (17) Knothe, G. J. Am. Oil Chem. Soc. 2000, 77, 489–493. 2700
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