Energy & Fuels 2008, 22, 1353–1357
1353
Low Boiling Point Organic Amine-Catalyzed Transesterification for Biodiesel Production Lixiong Zhang,*,† Weijun Guo,‡ Dan Liu,† Jianfeng Yao,† Lei Ji,† Nanping Xu,† and Enze Min‡ State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry and Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, P. R. China, and Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, P. R. China ReceiVed October 26, 2007. ReVised Manuscript ReceiVed December 6, 2007
The transesterification of triacetin to methyl acetate catalyzed by 11 kinds of low boiling point amines was examined at subcritical methanol temperature as model reactions to simulate the biodiesel production. It was found that the branched amines showed higher yield of methyl acetate than normal amines. For normal amines, the yield of methyl acetate decreased with the increase of the carbon number of the amines. Three kinds of amines, i.e., isopropylamine, tert-butylamine, and triethylamine with boiling points lower than 90 °C and high activities in the above model reaction were, respectively, applied to the transesterification of rapeseed oil or soybean oil with methanol to produce biodiesel. The process conditions, such as molar ratio of methanol to oil, reaction temperature, catalyst usage, and water content were investigated. HPLC, GC, GC-MS, IR and 1H NMR were used to analyze the products. The results indicated that the yield of methyl ester increased with the increase of the molar ratio of methanol to oil from 4:1 to 20:1, the reaction temperature from 60 to 230 °C, and the catalyst content from 0 to 6 wt %. The increase of the water content in the oil had a negative effect on the yield of methyl ester. The yield of biodiesel could reach over 90% when the reaction carried out with a molar ratio of methanol to rapeseed oil of 20:1, a catalyst dosage of 3 wt %, and at the reaction temperature of 230 °C for 6 to 8 h when using the three kinds of amines as catalysts. The possible reaction between the amines with oil was also examined. It was found that small amounts of isopropylamine and tert-butylamine and trace amounts of triethylamine could react with oil under the reaction conditions studied in this paper.
1. Introduction Biodiesel, which is usually prepared by transesterification of vegetable oil with methanol, is a renewable and environmentally friendly energy. Application of this energy not only can significantly reduce the pollution generated from petroleumbased diesel oil but also can lessen the dependence on petroleum. China is the largest country in terms of population over the world and the demand for energy is fast increasing with its rapid development of economy. In order to solve its energy problem, the Chinese government issued regulations to stimulate the production and application of alternative fuels, such as biodiesel. Production of biodiesel is usually carried out by using alkaline catalysts,1,2 such as NaOH and KOH, due to their wide availability and low cost. However, the formation of soap is inevitable, which gives rise to high viscosity gels and high separation cost.3 Inorganic acids, such as sulfuric acids, sulfonic acid, and hydrochloric acid, can act as the alternative catalysts for the transesterification. However, they show low catalytic efficiency and high corrosibility.4,5 Solid base and acid catalysts, such as ZnAl hydrotalcite,6 KF/ZnO,7 hydrous zirconia sup* Corresponding author. Tel.: +86-25-83587186. Fax.: +86-25-83365813. E-mail:
[email protected]. † Nanjing University of Technology. ‡ Research Institute of Petroleum Processing. (1) Georgogianni, K. G.; Kontominas, M. G.; Tegou, E.; Avlonitis, D.; Gergis, V. Energy Fuels 2007, 21, 3023–3027. (2) Reddy, C.; Reddy, V.; Oshel, R.; Verkade, J. G. Energy Fuels 2006, 20, 1310–1314. (3) Lotero, E.; Liu, Y. J.; Lopez, D. E.; Suwannakarn, K.; Bruce, D. A.; Goodwin, J. G. Ind. Eng. Chem. Res. 2005, 44, 5353–5363.
ported 12-tungstonphosphoric acid,8 and sulfated zirconia,9 were studied. They possess shortcomings including high cost, strict reaction conditions, and easy deactivation. A supercritical methanol process could avoid the formation of soap, but the use of a catalyst, long reaction time, high molar ratio of methanol to oil, and high reaction pressure (>45 MPa) and temperature (>300 °C) have to be applied.10 Compared with inorganic and solid bases, organic bases have not attracted enough attention as the catalyst for the production of biodiesel. Schuchardt et al.11 employed alkylguanidines to catalyze rapeseed oil to prepare biodiesel. These catalysts showed good catalytic efficiency, and a high biodiesel yield could be acquired under lower reaction temperature. For example, 90% yield of methyl esters could be reached when 1 mol % of 1,5,7-triazabicyclo[4,4,0]dec-5-ene was used in the transesterification of rapeseed oil with methanol at 70 °C for 1 h. However, the alkylguanidines are quite expensive and they are not easily recovered. Liquid organic amines, such as N,N(4) De Filippis, P.; Borgianni, C.; Paolucci, M. Energy Fuels 2005, 19, 2225–2228. (5) Ataya, F.; Dube, M. A.; Ternan, M. Energy Fuels 2007, 21, 2450– 2459. (6) Bournay, L.; Casanave, D.; Delfort, B.; Hillion, G.; Chodorge, J. A. Catal. Today 2005, 106, 190–192. (7) Xie, W. L.; Huang, X. M. Catal. Lett. 2006, 107, 53–59. (8) Kulkarni, M. G.; Gopinath, R.; Meher, L. C.; Dalai, A. K. Green Chem. 2006, 8, 1056–1062. (9) Kiss, A. A.; Dimian, A. C.; Rothenberg, G. AdV. Synth. Catal. 2006, 348, 75–81. (10) Saka, S.; Kusdiana, D. Fuel 2001, 80, 225–231. (11) Schuchardt, U.; Sercheli, R.; Vargas, R. M. J. Braz. Chem. Soc. 1998, 9, 199–210.
10.1021/ef700636u CCC: $40.75 2008 American Chemical Society Published on Web 02/08/2008
1354 Energy & Fuels, Vol. 22, No. 2, 2008
Zhang et el.
Table 1. Molecular Formulas, Boiling Points, and pH Values of the Amines and the Yield of Methyl Acetate with the Amine As the Catalyst amine n-dipropylamine tri-n-butylamine triethylamine di-n-butylamine n-pentylamine n-butylamine diisopropylamine diethylamine tert-butylamine n-propylamine isopropylamine
molecular formula boiling point (°C) C6H15N C12H27N C6H15N C8H19N C5H13N C4H11N C6H15N C4H11N C4H11N C3H9N C3H9N
108.0 ∼ 110.0 216.0 88.9 ∼ 89.4 159.0 104.5 78.0 83.1 55.5 44.5 49.0 32.0
pH
yield of methyl acetate (%)
10.48 10.75 10.8 11.26 11.27 11.37 11.4 11.4 11.46 11.69 11.81
25.3 4.5 37.7 29.2 36.5 46.1 42.1 44.0 49.6 48.5 50.7
dimethyltrimethylenediamine, 4-methylpiperidine, diethylamine, dimethylethanolamine, tetramethyl diaminoethane, and tetramethylammonium hydroxide solution in methanol were also tested as the catalysts for the production of biodiesel.12,13 The highest conversion of over 80% was achieved with 3 mol % of tetramethylammonium hydroxide as the catalyst at 65 °C for 90 min with a molar ratio of methanol to oil of 8.7:1. However, the recovery process of the catalyst was complex.12 If the above organic liquid amines are changed to low boiling point amines, the recovery process can be simplified, which can be combined with the recovery process of methanol by distillation. Very recently, Wang et al.14 examined that catalytic activities of ethylenediamine (EDA), diethylamine (DEA), and triethylamine (TEA) in the transesterification of crude rapeseed oil in supercritical methanol and found that the order of the catalytic activity was EDA > DEA > TEA. In this paper, we applied 11 kinds low boiling point amines to the transesterification with methanol. First, triacetin was used as the raw material instead of vegetable oils to simplify the analysis work and to accelerate the screening speed for suitable amine catalysts. Afterward, several amines, which showed good catalytic activities in the model reaction, were applied to the transesterification of rapeseed oil and soybean oil with methanol. In addition, our work was carried out mostly at subcritical rather than supercritical methanol temperatures. 2. Experimental Section 2.1. Materials. Eleven kinds of amines were used in the model reaction of triacetin (99.7%. Sigma-aldrich) with methanol (99.8%, Shanghai Chemical Reagent Company). They were n-dipropylamine, tri-n-butylamine, triethylamine, di-n-butylamine, n-pentylamine, n-butylamine, diisopropylamine, diethylamine, tert-butylamine, n-propylamine, and isopropylamine with analytical purity and were purchased from Shanghai Chemical Reagent Company. Table 1 lists molecular formulas and the boiling points of the amines. Unrefined rapeseed oil and unrefined soybean oil were purchased from local farm, with an acid value (AV) of 0.6 and 2.3 mg KOH/g, a water content of 0.001% and 0.0014%, and a saponification value of 179 and 192 g KOH/g, respectively. 2.2. Transesterification Reaction. The model reactions were carried out in a stainless tubular reactor 4 mm in diameter and 180 mm in length. Both ends were sealed with Swagelok unions. The volume of this reactor was about 2.3 mL. After filling triacetin, methanol, and the amine in the reactor, it was tightly sealed and (12) Cerce, T.; Peter, S.; Weidner, E. Ind. Eng. Chem. Res. 2005, 44, 9535–9541. (13) Schuchardt, U.; Vargas, R. M.; Gelbard, G. J. Mol. Catal. A 1995, 99, 65–70. (14) Wang, L. Y.; Tang, Z. Y; Xu, W. H.; Yang, J. C. Catal. Commun. 2007, 8, 1511–1515.
Figure 1. Effect of temperature on the yield of methyl acetate as a function of reaction time in transesterification of triacetin with methanol. The methanol to triacetin molar ratio was 14:1.
put in a furnace with a preset temperature. After reaction, the reactor was taken out and cooled with tap water. The products were analyzed by high performance liquid chromatography (HPLC). Transesterification of oil with methanol was carried out in a homemade stainless steel reactor. A ø30 mm stainless steel rod was drilled to a reactor 4 mm in diameter and 170 mm in depth. The mouth of the hole was sealed by a screw cap equipped with a copper disk. The volume of the reactor was about 2.1 mL. After charging certain amount of oil, methanol, and the amine into the reactor, with a total volume of 78% of the reactor, it was tightly sealed and then put into a furnace with a preset temperature. The reactor was taken out and cooled with tap water after reaction. The products were analyzed by a gas chromatograph (GC). 2.3. Analytical Methods. Methyl acetate, triacetin, and the intermediate products were analyzed by HPLC (Agilent 2000) with a C18 XDB column and diode array detector. The mobile phase was methanol–water (50:50) and was tested at 205 nm with a flow rate of 1.0 mL/min and a column temperature of 35 °C. The products of transesterification of oil with methanol were analyzed by a GC (Agilent 6890N) equipped with a cold column injection system and an Ultra-ALLOY-DX30 capillary column (5 m × 0.53 mm × 0.15 µm, Frontier Laboratories Ltd.) with a flame ionization detector (FID). The carrier gas was helium with a flow rate of 20 mL/min.15 GC-MS (TRACE GC-TRACE DSQ) and FT-IR (NEXUS, Thermo Nicolet Corporation) were used to analyze the products. 1H NMR spectra were recorded on JEOL EX 90A spectrometer using acetone as the solvent.
3. Results and Discussion 3.1. Transesterification of Triacetin with Methanol. 3.1.1. Transesterification at Supercritical Methanol Temperature without a Catalyst. The critical temperature of methanol is close to 239 °C. Transesterification of triacetin with methanol was examined at subcritical or supercritical methanol temperatures (200–400 °C) for different reaction times (Figure 1). We can see that the yield of methyl acetate increased with the increase of the reaction temperature for the same reaction time. When the temperature was over 300 °C, the yield of methyl acetate could reach 100% for less than 35 min. The high temperature could shorten the time to reach the highest yield of methyl acetate. However, when the temperature was 250 °C, which was just above the critical temperature of methanol, it took about 90 min to reach a yield of methyl acetate of 90%. When the temperature (200 °C) was below the critical temperature of methanol, the highest yield of methyl acetate was only 24.9%. The above results suggested that a catalyst should be used to (15) Li, C. X.; Tang, Z.; Yang, H. Y. J. Instrum. Anal. (Chin.) 2004, 24, 66–68.
Transesterification for Biodiesel Production
obtain a high yield of methyl acetate for the model reaction operated at subcritical methanol temperature. 3.1.2. Transesterification Using Amine Catalysts. Different amines were used for transesterification of triacetin with methanol at 230 °C, with a molar ratio of methanol to triacetin of 14:1 and reaction time of 30 min. To investigate the effect of the basicity of amines on the yield of methyl acetate, 11 kinds of amines were tested under similar reaction conditions. The pH values of various amines were obtained by measuring the pH of 0.1 mol/L amine methanol solutions. These catalysts were added to the transesterification system of triacetin and methanol, with a content of 3 wt % of triacetin. The reaction results are shown in Table 1. We can see that the yield of methyl acetate had an upward trend with the increase of the pH of the amine. Tri-n-butylamine showed a yield of methyl acetate of less than 10%, while isopropylamine, t-butylamine, n-propylamine, n-butylamine, diisopropylamine, and triethylamine could result in a yield of methyl acetate of over 40%. However, it was apparent that under the same reaction condition, the yield of methyl acetate was higher with the amine as a catalyst than that without a catalyst. From Table 1, we can also see that, for the isomers, the branched amines always showed higher yields of methyl acetate than the normal amines. For example, the yields of methyl acetate for n-dipropylamine, n-butylamine, and n-propylamine were 25.3%, 46.1%, and 48.2%, respectively, while those for diisopropylamine, tert-butylamine, and isopropylamine were 42.1%, 49.6%, and 50.7%, respectively. This was probably due to the high pH values of the branched amines than the normal amines. For normal alkylamines (n-propylamine, n-butylamine, and n-pentylamine), the yields of methyl acetate (48.5%, 46.1%, and 36.5%) decreased with the increasing carbon numbers. This probably resulted from the lower relative pH value in the reaction system when the same mass of the normal alkylamine with more carbon numbers added. The pH value of the amine was obtained by measuring the pH of 0.1 mol/L amine methanol solution. The measured pH values of the normal alkylamines were very close (Table 1). Thereby, the pH value of the normal alkylamine with more carbon numbers was lower than that of the same mass of the normal alkylamine with less carbon numbers. The same trend as normal alkylamines was also observed for triethylamine and tributylamine. The yield of methyl acetate sharply decreased from 44.1% for triethylamine to 4.5% for tributylamine, which was due to the same reason as that of normal alkylamines. We also examined the catalytic activities of the 11 kinds of amines at 60 °C in the model reaction under the same reaction conditions mentioned above. The results indicated that the yield of methyl acetate was from 0.1% to 6.7%, suggesting that the activities of these amines were low at low temperature. 3.2. Organic Amine-Catalyzed Transesterification of Oil and Methanol. The above results clearly indicated that low boiling point organic amines could catalyze triacetin with methanol at subcritical methanol temperature. Hereafter, we chose three kinds of amines, i.e., isopropylamine, tert-butylamine, and triethylamine, for the transesterification of rapeseed oil and soybean oil with methanol. 3.2.1. Effect of the Molar Ratio of Methanol to Oil. Figure 2 shows the yields of methyl ester as a function of the molar ratio of methanol to oil. The reactions were conducted at 230 °C for 6 or 8 h, with the molar ratios of 4:1, 10:1, 14:1, 20:1, and 40:1, respectively. The contents of the amines were all 3 wt % of the oil. We can see that, for both rapeseed oil and
Energy & Fuels, Vol. 22, No. 2, 2008 1355
Figure 2. Effect of molar ratio of methanol to oil on the yield of methyl ester using rapeseed oil (A) and soybean oil (B) as the raw material. The reaction temperature was 230 °C; the reaction time was 6 h for isopropylamine and tert-butylamine and 8 h for triethylamine; the content of the catalyst was 3 wt % of the oil.
soybean oil systems, the yield of methyl ester increased rapidly when the methanol to oil molar ratio increased from 4:1 to 20: 1. When the methanol to oil molar ratio exceeded 20, the increase of the yield of methyl ester was slow, probably due to the dilution of the catalyst by the excessive methanol. 3.2.2. Effect of the Reaction Temperature. Figure 3 shows the effect of the reaction temperature on the yield of methyl ester by using rapeseed oil and soybean oil as the raw material. The reactions were carried out at 60, 100, 180, and 230 °C, respectively, for 6 or 8 h with the molar ratio of methanol to oil of 14:1, and the content of the catalyst of 3 wt % of oil. We can see that the yield of methyl ester increased almost linearly with the increasing temperature for both rapeseed oil and soybean oil as the raw material. When the reaction temperature was 60 °C, the yield of methyl ester was 15%–30% and 5%–15% for the rapeseed oil and soybean oil system, respectively, indicating low activity of the catalysts at low temperature. When the reaction temperature was increased to 230 °C, the yield of methyl ester increased to over 70%. 3.2.3. Effect of the Content of the Catalyst. Figure 4 shows the effect of the catalyst content on the yield of methyl ester using rapeseed oil and soybean oil as the raw material. The reactions were conducted at 230 °C for 6 h, with the methanol to oil molar ratio of 14:1. For the rapeseed oil system (Figure 4A), we can see that the yield of methyl ester was 40.9% when no catalyst was used. When only 1 wt % catalyst was added in the reaction system, the yield of methyl ester rapidly increased to 60%–80%. Further increase of the catalyst content to 3 and 6 wt % resulted in the increase of the yield of methyl ester to 65%–98%. For the soybean oil system (Figure 4B), the yield of methyl ester was just 35.1% without catalyst. When only 1
1356 Energy & Fuels, Vol. 22, No. 2, 2008
Figure 3. Effect of reaction temperature on the yield of methyl ester using rapeseed oil (A) and soybean oil (B) as raw material. The reaction time was 8 h for isopropylamine and 6 h for tert-butylamine and triethylamine; the content of the catalyst was 3 wt % of the oil; the molar ratio of methanol to oil was 14:1.
wt % catalyst was added in the reaction system, the yield of methyl ester increased to 68.9%. Further increase of the catalyst content to 3 and 6 wt % increased the yield of methyl ester to 85.1% and 95.8%, respectively. 3.2.4. Effect of the Water Content. Water in oil plays an important role in the transesterification reaction when using inorganic alkali, such as KOH as a catalyst.16 To investigate the effect of water on the yield of methyl ester using amine catalysts, different amounts of distilled water ranging from 0.5, 1, 3, to 6 wt % of the oil were, respectively, added to the rapeseed oil and soybean oil reaction systems. The reactions were carried out at 230 °C for 6 or 8 h, with the catalyst content of 3 wt % of oil and the molar ratio of methanol to oil of 14:1. Figure 5 shows that the yield of methyl ester had a decrease of less than 5% for rapeseed oil when the water content was 0.5 wt % of oil. However, when the water content increased from 0.5 to 3 wt % of the oil, the yield of methyl ester decreased significantly. However, a further increase of the water content only led to a very slow decrease of the yield of methyl ester. It seems that isopropylamine was more tolerable to water than tert-butylamine and triethylamiane in the rapeseed oil system, as could be seen from Figure 5A. For the soybean oil system, the yield of methyl ester rapidly decreased from 85.1% to 42.8% when the water content increased from 0 to 3 wt % of the oil (Figure 5B). 3.3. Possible Reaction of Amines with Oil. It is possible for the organic amines to react with oil. For example, diethylamine can react with soybean oil to produce N,N-diethylstearamide, which can enhance the ignition property of the petro(16) Kusdiana, D.; Saka, S. Bioresour. Technol. 2004, 91, 289–295.
Zhang et el.
Figure 4. Effect of the content of the catalyst on the yield of methyl ester using rapeseed oil (A) and soybean oil (B) as raw material. The reaction temperature was 230 °C; the reaction time was 6 h; the molar ratio of methanol to oil was 14:1.
chemical diesel fuel.17 Acyl amine appeared in the products of catalytic transesterification of crude rapeseed oil with methanol by EDA or DEA as the catalyst.14 In order to examine the possible reactions between the amines with triacetin or oil, GCMS, IR, and 1H NMR were used to characterize the reaction products. The GC-MS spectrum of the product produced by n-butylamine-catalyzed transesterification of triacetin with methanol at 230 °C for 30 min displayed the presence of acetylamine at a retention time of 10.17 min. (see the Supporting Information, S-Figure 1) The GC-MS spectrum of the product produced by isopropylamine-catalyzed transesterification of rapeseed oil with methanol operated at 230 °C for 6 h displayed the presence of fatty acid amides at retention times of 27.17, 29.32, and 29.6 min. (see the Supporting Information, S-Figure 2) Figure 6 shows the IR spectra of pure methyl ester produced at supercritical methanol temperature with rapeseed oil and the methyl ester produced by transesterification of rapeseed oil with methanol catalyzed by isopropylamine, tert-butylamine, and triethylamine, respectively, at 230 °C for 6 h. There was no obvious difference of the IR spectra between pure methyl ester (Figure 6a) and triethylamine-catalyzed product (Figure 6d). An absorption band of CdO at a wavenumber of 1565 cm-1 for both the isopropylamine- and tert-butylamine-catalyzed products indicated the formational of acyl amine (Figure 6b and c), whose content was roughly estimated to be about 8 mol % of the total CdO in the product by the integrated peak area method.14 In order to further verify if there was any reaction between rapeseed oil and triethylamine, 1H NMR of the triethylamine-catalyzed (17) Alcantara, R.; Amores, J.; Canoira, L.; Fidalgo, E.; Franco, M. J.; Navarro, A. Biomass Bioenergy 2000, 18, 515–527.
Transesterification for Biodiesel Production
Energy & Fuels, Vol. 22, No. 2, 2008 1357
Figure 6. IR spectra of pure methyl ester (a), methyl ester using rapeseed oil catalyzed by isopropylamine (b), tert-butylamine (c), and triethylamine (d). The reaction temperature was 230 °C; the reaction time was 6 h; the molar ratio of methanol to oil was 20:1; the content of the catalyst was 3 wt %.
Figure 5. Effect of water content on the yield of methyl ester using rapeseed oil (A) and soybean oil (B) as raw materials. The reaction temperature was 230 °C; the reaction time was 6 h for isopropylamine and tert-butylamine and 8 h for triethylamine; the molar ratio of methanol to oil was 14:1; the content of the catalyst was 3 wt %.
product was examined and compared with that of pure methyl ester, as shown in Figure 7. It was apparent that two very small peaks appeared at 4.08 and 1.23 ppm in the 1H NMR spectrum of triethylamine-catalyzed product (Figure 7b), indicating that a trace amount of triethylamine reacted with rapeseed oil. 4. Conclusion In this paper, low boiling point organic amines were used as catalysts for the production of biodiesel by transesterification of triacetin and oil, respectively, with methanol under subcritical methanol temperature. The activity of the amines for transesterification of triacetin with methanol generally increased with the increase of the basicity of the amines. Three kinds of organic amines, i.e., isopropylamine, tert-butylamine, and triethylamine, were used to catalyze rapeseed oil and soybean oil with methanol for biodiesel production. The yield of methyl ester rapidly increased when the molar ratio of methanol to oil increased from 4:1 to 20:1, and the reaction temperature was increased from 60 to 230 °C. The suitable content of the catalyst
Figure 7. 1H NMR spectra of pure methyl ester (a) and methyl ester using rapeseed oil catalyzed by triethylamine (b). The reaction temperature was 230 °C; the reaction time was 6 h; the molar ratio of methanol oil was 20:1; the content of the catalyst was 3 wt %.
was 3-6 wt % of the oil, and the water content in the oil should be limited to less than 0.5%. The yield of methyl ester can reach over 98.5% under optimal conditions. However, small amounts of isopropylamine and tert-butylamine and trace amounts of triethylamine could react with oil under the reaction conditions studied. The low boiling point organic amines were confirmed as effective catalysts for biodiesel production. Thereby, a biodiesel production process using this kind of catalyst would be much simpler than the inorganic alkaline-catalyzed one since the recovery of the catalysts can be fulfilled with the process of methanol recovery by distillation. Furthermore, soap formation can be completely avoided. Supporting Information Available: GC-MS spectra. This material is available free of charge via the Internet at http://pubs.acs.org. EF700636U
