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Development of heterogeneous base catalysts for biodiesel production
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Bioresource Technology 99 (2008) 3439–3443
Development of heterogeneous base catalysts for biodiesel production Ayato Kawashima *, Koh Matsubara, Katsuhisa Honda Environmental Science for Industry, Faculty of Agriculture, Ehime University, 3-5-7 Tarumi, Matsuyama-shi, Ehime 790-8566, Japan Received 18 April 2007; received in revised form 6 August 2007; accepted 6 August 2007 Available online 19 September 2007
Abstract Investigations were conducted on heterogeneous base catalysts for the transesterification of oil aimed at effective production of biodiesel. Thirteen different kinds of metal oxides containing calcium, barium, magnesium, or lanthanum were prepared as catalysts. Their catalytic activities were tested for transesterification at 60 C with a 6:1 molar ratio of methanol to oil and a reaction time of 10 h. The calcium-containing catalysts – CaTiO3, CaMnO3, Ca2Fe2O5, CaZrO3, and CaO–CeO2 – showed high activities and approximately 90% yields of methyl ester. Furthermore, catalytic durability tests were performed by repeating the transesterification reaction several times with the calcium-containing catalysts recovered from the previous reaction mixture. It was found that CaZrO3 and CaO–CeO2 show high durability and have the potential to be used in biodiesel production processes as heterogeneous base catalysts. 2007 Elsevier Ltd. All rights reserved. Keywords: Biodiesel; Heterogeneous catalyst; Transesterification; Metal oxide; Methyl ester
1. Introduction In the recent years, there has been increased focus on global warming and the depletion of resources caused by the heavy consumption of fossil resources. In order to resolve these problems, biomass is increasingly gaining international attention as a source of renewable energy. Biodiesel fuel produced by the transesterification of vegetable oils and animal fats is expected to be one of the biomass-based alternatives to fossil resources due to its characteristics – renewability, lack of aromatic compounds, high biodegradability, and low SOx and particulate matter content (Schuchardt et al., 1998; Michael and McCormick, 1998; Ma and Hanna, 1999). The typical production method for biodiesel fuel is a base-catalyzed process with a homogeneous catalyst such as KOH or NaOH. This process can produce methyl ester with high yield under mild conditions: atmospheric pressure, temperature of 60 C, and a reaction time of about 1 h (Vicente et al., 2004; Meher et al., 2006). However, *
Corresponding author. Tel.: +81 89 946 9970; fax: +81 89 946 9980. E-mail address: [email protected] (A. Kawashima).
0960-8524/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2007.08.009
for satisfactory use of the product as diesel fuel, the base catalyst must be removed from it, requiring a cumbersome purification process; moreover, the process wastewater is environmentally toxic because of its high basicity, which is caused by the homogeneous base catalyst. Furthermore, the total cost of the biodiesel fuel production based on this process is not sufficiently competitive as compared to the cost of production of diesel oil from petroleum. Addressing these issues, a number of studies have been conducted on biodiesel processes, such as acid-catalyzed process (Freedman et al., 1984; Edgar et al., 2005), supercritical process (Minami and Saka, 2006; Demirbas, 2006), enzymatic process (Shimada et al., 2002; Nie et al., 2006), and heterogeneous catalyst process. In particular, the heterogeneous catalyst process is expected to be an effective biodiesel production process with low cost and minimal environmental impact because of the possibility of simplifying the production and purification processes under mild conditions. Therefore, many heterogeneous catalysts for the transesterification of oils have been developed. For example, the transesterification reaction of soybean oil with ETS-10 zeolite has been studied; conversion in excess of 90% was achieved at a temperature of
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100 C (Suppes et al., 2004). It has also been reported that the conversion to methyl ester reaches 87% with the potassium-loaded alumina catalyst, when a mixture with a molar ratio of methanol to oil of 15:1 is refluxed for a reaction time 7 h (Xie et al., 2006). Besides these, there have been several other reports on heterogeneous catalysts (Gryglewicz, 1999; Suppes et al., 2001; Kim et al., 2004; Cantrell et al., 2005; Jitputti et al., 2006). However, the catalytic activities of most of them are not much greater than that of a homogeneous catalyst such as KOH. Furthermore, there is little information regarding their catalytic durability. In this study, we intended to examine heterogeneous base catalysts in order to develop an effective biodiesel process catalyst with high activity and durability. We prepared A-B-O type metal oxides, where A is an alkaline-earth metal, alkaline metal, or rare earth metal and B is a transition metal. Their catalytic activity and durability for the transesterification of oil to fatty acid methyl ester were investigated. 2. Experimental 2.1. Chemicals and catalyst preparation Commercial edible-grade rapeseed oil was purchased from the market and used without further purification. Other chemicals such as high-purity metal oxides, fatty acid methyl esters used as the standards, and solvents were obtained from Wako Pure Chemical Industries (Osaka, Japan), Kanto Chemical Co., Inc., (Tokyo, Japan), and Sigma–Aldrich Corporation (Missouri, USA). All the purchased metal oxides (up to 99.9% purity) were dried in an oven at 300 C for 4 h before use. The catalyst samples were prepared by the conventional solid-state reaction, which involves mixing the metal oxides in the desired proportions followed by calcination. For the synthesis of CaTiO3, an equimolar mixture of TiO2 and CaCO3 was milled in an agate mortar. The mixed powder was calcined in air to 500 C at a rate of 2 C/min and then at 1050 C for 2 h in an alumina crucible. For preparing Ca2Fe2O5, a mixture containing a 1:2 molar ratio of Fe2O3 and CaCO3 was milled and calcined in air to 900 C at a rate of 2 C/min and then at 1050 C for 4 h. The other catalysts were prepared in the same manner. The obtained substances were stored in dry boxes and examined for further experiments. 2.2. Catalytic activity test The catalytic activities for the transesterification of oil were examined with a batch-type reaction. Ten grams of rapeseed oil and 2.6 g of methanol were put in a 50 ml two-necked flask equipped with a reflux condenser. Then, 1.0 g of catalyst was added to the mixture and it was stirred with a magnetic stirrer. The mixture was heated at 60 C in an oil bath for 10 h. Quantitative determinations of the
yield of fatty acid methyl esters were performed as follows. The collected reaction mixtures were evaporated in vacuum to remove the excess methanol, and then the oil phase was separated from the reaction mixture by centrifugation at 9170 g for 5 min. Methyl heptadecanoate (100 mg) was added to the oil phase sample as an internal standard, and the mixture was diluted to 5 ml with isooctane. Two microlitre of the resulting solution was subjected to highperformance liquid chromatography (HPLC) analysis for a quantitative determination of the fatty acid methyl ester. The HPLC system consisted of a JASCO PU-2089 pump, a JASCO AS-2057 auto injector, and a JASCO RI-2031 differential refractive index (RI) detector (JASCO Corp., Tokyo, Japan). Data collection and analysis was performed with the JASCO BORWIN software. The mobile phase was methanol at a flow rate of 1.0 ml/min. The columns were 250 mm · 4.6 mm Intersil ODS-3 (GL Science Inc. Japan, Tokyo, Japan). The columns were protected with an Intersil ODS-3 10 mm · 4.0 mm guard column. The sample injection volume was 2 ll, and the RI analyses were carried out at 35 C. The yield was defined as the ratio of the weight of the methyl esters determined by HPLC to the weight of the oil phase. Catalyst durability tests were performed by repeating the transesterification reaction several times with used catalysts. Catalysts were separated from the previous reaction mixture by centrifugation at 9170g for 5 min, washed with hexane, and then dried at 60 C. A 300 mg reaction mixture sample was collected every hour and was subjected to HPLC analysis for the fatty acid methyl ester determination, as described earlier. 2.3. Catalyst characterization The specific surface areas of the prepared catalysts were measured using the Brunauer-Emmett-Teller (BET) method with a surface area measuring instrument – SURFACE AREA HPP.SA-100 (SIBATA, Tokyo, Japan). X-ray powder diffraction (XRD) analysis was performed with a MiniFlex X-ray diffractometer equipped with monochromatic Cu Ka radiation (Rigaku Corp., Tokyo, Japan). Data were collected over a 2h range of 3–90, with a step size of 0.02 at a scanning speed of 4/min. Basic strengths of the catalysts (H ) were determined by using Hammett indicators. About 25 mg of the catalyst sample was shaken with 5.0 ml of a solution of Hammett indicators diluted with methanol, and left to equilibrate for 2 h. After the equilibration, the color on the catalyst was noted. The following Hammett indicators were used: neutral red (H = 6.8), bromthymol blue (H = 7.2), phenolphthalein (H = 9.3), 2,4-dinitroaniline (H = 15.0), and 4-nitroaniline (H = 18.4). 3. Results and discussion Table 1 summarizes the surface areas, base strengths, and catalytic activities of the prepared metal-oxide cata-
A. Kawashima et al. / Bioresource Technology 99 (2008) 3439–3443
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Table 1 Surface areas, base strengths, and catalytic activities of metal-oxide catalysts Sample
Surface area (m2/g)
Basic strength (H )
Methyl ester yield (%)a
CaTiO3 CaMnO3 Ca2Fe2O5 CaZrO3 CaCeO3 BaTiO3 BaZrO3 BaCeO3 MgTiO3 MgZrO3 MgCeO3 LaZrO3 LaCeO3
4.9 1.5 0.71 1.8 2.9 N.A. 3.3 2.8 N.A. 7.4 7.7 N.A. N.A.
6.8 < H < 7.2 7.2 < H < 9.3 7.2 < H < 9.3 7.2 < H < 9.3 7.2 < H < 9.3 H < 6.8 H < 6.8 H < 6.8 H < 6.8 H < 6.8 H < 6.8 H < 6.8 H < 6.8
79 92 92 88 89 0.0 0.4 0.0 0.0 0.5 0.4 0.0 0.0
N.A.: not analyzed. a Methyl ester yield after 10-h transesterification reaction at 60 C.
lysts. The surface areas of each of the catalysts were small and varies from 7.7 m2/g for MgCeO3 to 0.71 m2/g for Ca2Fe2O5. The reason of this small surface area is that catalysts preparation methods involve a calcination step at high temperature. In this experiment, a 1 g of catalyst was used to 10 g of oil. This is approximately 10 times more than the weight of KOH which is usually used as homogeneous base-catalyst. However, for example, molecular weight of CaZrO3 is 179 and KOH is 56. Therefore, this is three times the number of moles of the KOH catalyst. In this experiment, the amount of catalysts was increased in order to detect the catalyst activity with small surface area. The base strengths of the catalysts were determined by the Hammett indicators. CaTiO3 had a base strength (H ) in the range of 6.8–7.2. CaMnO3, Ca2Fe2O5, CaZrO3, and CaCeO3 had the highest base strengths, in the range from 7.2 to 9.3. However, the Ba, Mg, and La series catalysts showed the weakest base strengths, which were less than 6.8. These results suggest that the Ca series catalysts have high catalytic activity for the transesterification reaction, but the other catalysts have low activity. Subsequently, we conducted the catalytic activity tests for the prepared catalysts at 60 C with a 1:6 molar ratio of the oil to methanol. The tabulated catalytic activities in Table 1 indicate the yield of fatty acid methyl ester after transesterification for 10 h. The Ca series catalysts showed a high yield of methyl ester. CaMnO3, Ca2Fe2O5, CaZrO3, and CaCeO3, whose catalytic basicities were the highest, showed 92%, 92%, 88%, and 89% yields of the ester, respectively. Next, CaTiO3, whose basicity is also high, provided a yield of 79%. However, for the other catalysts with low basicities – BaTiO3, BaZrO3, BaCeO3, MgTiO3, MgZrO3, MgCeO3, LaZrO3, and LaCeO3 – ester yields were 1% or less and little activity as a basic catalyst was observed. Fig. 1 shows the XRD patterns of the Ca series catalysts (without CaCeO3). The prepared CaTiO3 and CaZrO3 samples had XRD patterns identical to those of the perovskite-type structures of CaTiO3 and CaZrO3. As for the
Fig. 1. Powder XRD patterns for metal-oxide catalysts: CaTiO3, CaMnO3, Ca2Fe2O5, and CaZrO3 samples. The labeled peaks: square for CaTiO3, diamond for CaMnO3, and circle for CaZrO3 are perovskite structure; triangle for Ca2Fe2O5 is a kind of oxygen-deficient perovskite structure.
CaMnO3 sample, the characteristic peaks of CaMnO3, Ca2MnO4, and the parent CaO were observed, indicating a mixture of these components. Similarly, for the Ca2Fe2O5 sample, the characteristic peaks of Ca2Fe2O5 and the minor parent CaO were observed. However, the CaCeO3 sample only had the characteristic XRD peaks of the parent CeO2 and CaO, and no new peak was observed. This indicates that the CaCeO3 sample does not contain CaCeO3 but CaO and CeO2, the raw materials. It is considered that CaO would be dispersed on CeO2. The notation of CaCeO3 sample is denoted by CaO–CeO2 hereafter. Next, catalytic durability tests were performed for the CaTiO3, CaMnO3, Ca2Fe2O5, CaZrO3, and CaO–CeO2 samples, from which excellent methyl ester yields were obtained in the catalytic activity tests. In these experiments, 10-h batch-type reactions were repeated with the recycled catalyst from the previous reaction mixture. Fig. 2 shows the results of the durability test (change in methyl ester yield for each operation) for the CaTiO3 sample. In the first operation, the initial reaction rate was very low, but
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100
100 : Operation 1 : Operation 2 : Operation 3 : Operation 4
80
60
Yield (%)
Yield (%)
80
40 20
40 : Operation 1 : Operation 2 : Operation 3 : Operation 4 : Operation 5
20
0
2
6 8 4 Reaction time (h)
10
0
12
Fig. 2. Methyl ester yield profiles for the CaTiO3 catalyst in the transesterification reaction with methanol at 60 C. The catalyst was recovered and used for the following transesterification reaction.
increased gradually, and a 79% ester yield was obtained after 10 h. The initial rate quickened in the second operation and the ester yield reached 85%. In the third and forth operations, however, the catalytic activity was decreased and the obtained ester yields were 68% and 1%, respectively. The durability test for CaMnO3 is similar result. In the first operation, the initial reaction rate was very low, but a 91% ester yield was obtained after 7 h. In the subsequent operation, a methyl ester yield of 88% was obtained and high catalytic activity was observed. However, the catalytic activity during the third and forth operation was decreased and the obtained ester yields were 61% and 0.3%, respectively. A similar behavior was also observed in the case of the Ca2Fe2O5 sample. The catalytic activity was high in the first and second operations, but became very low in the following operations. Thus, the catalytic activities of CaTiO3, CaMnO3, and Ca2Fe2O5 were high in the beginning, but subsequently decreased. One of the reasons of this seems to be the possible obstruction of catalytic activity by glycerin and the adsorption of the fatty acid onto the active sites of the catalysts. Another reason may be the dissolution of the catalytic active species by the glycerin solution of the by-product. As for CaZrO3 and CaO–CeO2, the obtained results were different from those of the former three catalysts. The results for the CaZrO3 sample are shown in Fig. 3. The initial reaction rate was very low, in the same manner, and the methyl ester yield was 88% in the first operation. However, the methyl ester yields in the second and third operations were 89% and 91%, respectively. Furthermore, the reaction maintained methyl ester yields greater than 80% in the subsequent forth and fifth operations. Fig. 4 shows the results for the CaO–CeO2 sample. In the first operation, the initial reaction rate was very low and the methyl ester yield was 90% after 10 h. The methyl ester yields in the second to fifth operations were between 87% and 89%, and yields greater than 80% were observed in the subsequent sixth and seventh operations. The recovered
0
2
6 8 4 Reaction time (h)
10
12
Fig. 3. Methyl ester yield profiles for the CaZrO3 catalyst in the transesterification reaction with methanol at 60 C. The catalyst was recovered and used for the following transesterification reaction.
100 80
Yield (%)
0
60
60 : Operation 1 : Operation 2 : Operation 3 : Operation 4 : Operation 5 : Operation 6 : Operation 7
40 20 0 0
2
4 6 8 Reaction time (h)
10
12
Fig. 4. Methyl ester yield profiles for the CaO–CeO2 catalyst in the transesterification reaction with methanol at 60 C. The catalyst was recovered and used for the following transesterification reaction.
catalyst amounts decreased in each durability test, from the initial 1 to 0.3 g after the final operation, the 5th reaction of CaZrO3 and the 7th reaction of CaO–CeO2, because clear separation of catalyst from reaction mixture, especially from glycerol, was difficult due to high polarity and high viscosity of glycerol; however, an ester yield of 80% or more was maintained for all the operations. This transesterification reaction is three phase reaction: oil phase, methanol phase, and catalyst phase, and contact efficiency of these three phases is one of the important parameters for this reaction rate. Therefore even if the amount of addition of a catalyst is increased, observed increase of reaction rate would reach saturation at a certain level. CaZrO3 and CaCeO3 have the high catalyst activity and therefore, the methyl ester yield at 10 h remained approximately the same with less than 1/3 amount of the initial catalyst. These results show that both CaZrO3 having a perovskite-type structure and CaO–CeO2 composed of dispersed CaO supported on CeO2 have high durability and catalytic activity.
A. Kawashima et al. / Bioresource Technology 99 (2008) 3439–3443
The reason why the durability of the high catalytic activity is seen only in CaZrO3 and CaO–CeO2 is not clear at present. However, as far as perovskite type catalysts: CaTiO3, CaMnO3, Ca2Fe2O5, and CaZrO3, it is considered that CaZrO3 has distortion in the crystal structure since the ion radius of transition metal Zr is larger than Ti, Mn, and Fe, and this may have a certain relation to catalytic activity. The reason of high activity of CaO–CeO2 seems that CaO would be supported on CeO2 and stabilized on the catalyst surface and basic sites are able to have higher contact probability of reaction substance than others. Furthermore, the strength of the affinity or solubility with highly polar materials such as glycerin might have also a large effect on the catalytic activity. In each catalyst, the initial reaction rate of the first operation was very low, but in the following operation, the reaction rate increased. The reason for this may be that the catalyst was activated in some way during the initial part of the reaction of the first operation. We think about the activation of catalyst as follows. Catalysts react initially with methanol and formed calcium methoxide-like compounds which have high catalytic activity for the transesterification reaction due to their higher basicity. In fact, we performed the transesterification reaction as the same manner using CaZrO3 which was initially immersed in methanol. As a result, the increase in the reaction rate in initial stages of a reaction was observed. Catalysts used in this experiment have large grain size and low surface area because catalysts preparation methods involve a calcination step at high temperature. Heterogeneous catalysts can be used repeatedly, however, from a practical viewpoint, it is important to increase reaction efficiency for reducing process cost. Therefore, further examinations of the catalyst synthesis method or immobilization method to which aimed at large surface area are needed. 4. Conclusions In this study, we intended to examine heterogeneous base catalysts in order to develop an effective catalyst for biodiesel process. Thirteen kinds of prepared catalyst of metal oxides were demonstrated to be catalysts for the transesterification of plant oil with methanol. The catalysts with Ca were found to have high basicities and catalytic activities for this reaction. Batch-type reactions were carried out at 60 C with a 1:6 molar ratio of the oil to methanol for a reaction time of 10 h, in which the methyl ester yield reached 79–92%. In particular, a high durability of catalytic activity was found for the catalyst samples of CaZrO3 and CaO–CeO2, which were able to provide
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methyl ester yields greater than 80% 5 times and 7 times, respectively. These results show the possibility for these catalysts being effective heterogeneous base catalysts for the manufacture of biodiesel. References Cantrell, D.G., Gillie, L.J., Lee, A.F., Wilson, K., 2005. Structurereactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis. Appl. Catal. A: Gen. 287, 183–190. Demirbas, A., 2006. Biodiesel production via non-catalytic SCF method and biodiesel fuel characteristics. Energy Convers. Mgmt. 47, 2271– 2282. Edgar, L., Liu, Y., Lopez, D.E., Kaewta, S., Bruce, D.A., Goodwin, J.G., 2005. Synthesis of biodiesel via acid catalysis. Ind. Eng. Chem. Res. 44, 5353–5363. Freedman, B., Pryde, E.H., Mounts, T.L., 1984. Variables affecting the yields of fatty esters from transesterified vegetable oils. J. Am. Oil Chem. Soc. 61, 1638–1643. Gryglewicz, S., 1999. Rapeseed oil methyl esters preparation using heterogeneous catalysts. Bioresour. Technol. 70, 249–253. Jitputti, J., Kitiyanan, B., Rangsunvigit, P., Bunyakiat, K., Attanatho, L., Jenvanitpanjakul, P., 2006. Transesterification of crude palm kernel oil and crude coconut oil by different solid catalysts. Chem. Eng. J. 116, 61–66. Kim, H.J., Kang, B.S., Kim, M.J., Park, Y.M., Kim, D.K., Lee, J.S., Lee, K.Y., 2004. Transesterification of vegetable oil to biodiesel using heterogeneous base catalyst. Catal. Today 93–95, 315–320. Ma, F., Hanna, M.A., 1999. Biodiesel production: a review. Bioresour. Technol. 70, 1–15. Meher, L.C., Sager, D.V., Naik, S.N., 2006. Technical aspects of biodiesel production by transesterification – a review. Renew. Sustain. Energy Rev. 10, 248–268. Michael, S.G., McCormick, R.L., 1998. Combustion of fat and vegetable oil derived fuels in diesel engines. Prog. Energy Combust. Sci. 24, 125– 164. Minami, E., Saka, S., 2006. Kinetics of hydrolysis and methyl esterification for biodiesel production in two-step supercritical methanol process. Fuel 85, 2479–2483. Nie, K., Xie, F., Wang, F., Tan, T., 2006. Lipase catalyzed methanolysis to produce biodiesel: optimization of the biodiesel production. J. Mol. Catal. B: Enzym. 43, 142–147. Schuchardt, U., Sercheli, R., Vargas, R.M., 1998. Transesterification of vegetable oils: a review. J. Braz. Chem. Soc. 9, 199–210. Shimada, Y., Watanabe, Y., Sugihara, A., Tominaga, Y., 2002. Enzymatic alcoholysis for biodiesel fuel production and application of the reaction to oil processing. J. Mol. Catal. B: Enzym. 17, 133–142. Suppes, G.J., Bockwinkel, K., Lucas, S., Botts, J.B., Mason, M.H., Heppert, J.A., 2001. Calcium carbonate catalyzed alcoholysis of fats and oils. J. Am. Oil Chem. Soc. 78, 139–145. Suppes, G.J., Dasari, M.A., Doskocil, E.J., Mankidy, P.J., Goff, M.J., 2004. Transesterification of soybean oil with zeolite and metal catalysts. Appl. Catal. A: Gen. 257, 213–223. Vicente, G., Martı´nez, M., Aracil, J., 2004. Integrated biodiesel production: a comparison of different homogeneous catalysts systems. Bioresour. Technol. 92, 297–305. Xie, W., Peng, H., Chen, L., 2006. Transesterification of soybean oil catalyzed by potassium loaded on alumina as a solid-base catalyst. Appl. Catal. A: Gen. 300, 67–74.
Bioresource Technology 99 (2008) 3439–3443
Development of heterogeneous base catalysts for biodiesel production Ayato Kawashima *, Koh Matsubara, Katsuhisa Honda Environmental Science for Industry, Faculty of Agriculture, Ehime University, 3-5-7 Tarumi, Matsuyama-shi, Ehime 790-8566, Japan Received 18 April 2007; received in revised form 6 August 2007; accepted 6 August 2007 Available online 19 September 2007
Abstract Investigations were conducted on heterogeneous base catalysts for the transesterification of oil aimed at effective production of biodiesel. Thirteen different kinds of metal oxides containing calcium, barium, magnesium, or lanthanum were prepared as catalysts. Their catalytic activities were tested for transesterification at 60 C with a 6:1 molar ratio of methanol to oil and a reaction time of 10 h. The calcium-containing catalysts – CaTiO3, CaMnO3, Ca2Fe2O5, CaZrO3, and CaO–CeO2 – showed high activities and approximately 90% yields of methyl ester. Furthermore, catalytic durability tests were performed by repeating the transesterification reaction several times with the calcium-containing catalysts recovered from the previous reaction mixture. It was found that CaZrO3 and CaO–CeO2 show high durability and have the potential to be used in biodiesel production processes as heterogeneous base catalysts. 2007 Elsevier Ltd. All rights reserved. Keywords: Biodiesel; Heterogeneous catalyst; Transesterification; Metal oxide; Methyl ester
1. Introduction In the recent years, there has been increased focus on global warming and the depletion of resources caused by the heavy consumption of fossil resources. In order to resolve these problems, biomass is increasingly gaining international attention as a source of renewable energy. Biodiesel fuel produced by the transesterification of vegetable oils and animal fats is expected to be one of the biomass-based alternatives to fossil resources due to its characteristics – renewability, lack of aromatic compounds, high biodegradability, and low SOx and particulate matter content (Schuchardt et al., 1998; Michael and McCormick, 1998; Ma and Hanna, 1999). The typical production method for biodiesel fuel is a base-catalyzed process with a homogeneous catalyst such as KOH or NaOH. This process can produce methyl ester with high yield under mild conditions: atmospheric pressure, temperature of 60 C, and a reaction time of about 1 h (Vicente et al., 2004; Meher et al., 2006). However, *
Corresponding author. Tel.: +81 89 946 9970; fax: +81 89 946 9980. E-mail address: [email protected] (A. Kawashima).
0960-8524/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2007.08.009
for satisfactory use of the product as diesel fuel, the base catalyst must be removed from it, requiring a cumbersome purification process; moreover, the process wastewater is environmentally toxic because of its high basicity, which is caused by the homogeneous base catalyst. Furthermore, the total cost of the biodiesel fuel production based on this process is not sufficiently competitive as compared to the cost of production of diesel oil from petroleum. Addressing these issues, a number of studies have been conducted on biodiesel processes, such as acid-catalyzed process (Freedman et al., 1984; Edgar et al., 2005), supercritical process (Minami and Saka, 2006; Demirbas, 2006), enzymatic process (Shimada et al., 2002; Nie et al., 2006), and heterogeneous catalyst process. In particular, the heterogeneous catalyst process is expected to be an effective biodiesel production process with low cost and minimal environmental impact because of the possibility of simplifying the production and purification processes under mild conditions. Therefore, many heterogeneous catalysts for the transesterification of oils have been developed. For example, the transesterification reaction of soybean oil with ETS-10 zeolite has been studied; conversion in excess of 90% was achieved at a temperature of
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100 C (Suppes et al., 2004). It has also been reported that the conversion to methyl ester reaches 87% with the potassium-loaded alumina catalyst, when a mixture with a molar ratio of methanol to oil of 15:1 is refluxed for a reaction time 7 h (Xie et al., 2006). Besides these, there have been several other reports on heterogeneous catalysts (Gryglewicz, 1999; Suppes et al., 2001; Kim et al., 2004; Cantrell et al., 2005; Jitputti et al., 2006). However, the catalytic activities of most of them are not much greater than that of a homogeneous catalyst such as KOH. Furthermore, there is little information regarding their catalytic durability. In this study, we intended to examine heterogeneous base catalysts in order to develop an effective biodiesel process catalyst with high activity and durability. We prepared A-B-O type metal oxides, where A is an alkaline-earth metal, alkaline metal, or rare earth metal and B is a transition metal. Their catalytic activity and durability for the transesterification of oil to fatty acid methyl ester were investigated. 2. Experimental 2.1. Chemicals and catalyst preparation Commercial edible-grade rapeseed oil was purchased from the market and used without further purification. Other chemicals such as high-purity metal oxides, fatty acid methyl esters used as the standards, and solvents were obtained from Wako Pure Chemical Industries (Osaka, Japan), Kanto Chemical Co., Inc., (Tokyo, Japan), and Sigma–Aldrich Corporation (Missouri, USA). All the purchased metal oxides (up to 99.9% purity) were dried in an oven at 300 C for 4 h before use. The catalyst samples were prepared by the conventional solid-state reaction, which involves mixing the metal oxides in the desired proportions followed by calcination. For the synthesis of CaTiO3, an equimolar mixture of TiO2 and CaCO3 was milled in an agate mortar. The mixed powder was calcined in air to 500 C at a rate of 2 C/min and then at 1050 C for 2 h in an alumina crucible. For preparing Ca2Fe2O5, a mixture containing a 1:2 molar ratio of Fe2O3 and CaCO3 was milled and calcined in air to 900 C at a rate of 2 C/min and then at 1050 C for 4 h. The other catalysts were prepared in the same manner. The obtained substances were stored in dry boxes and examined for further experiments. 2.2. Catalytic activity test The catalytic activities for the transesterification of oil were examined with a batch-type reaction. Ten grams of rapeseed oil and 2.6 g of methanol were put in a 50 ml two-necked flask equipped with a reflux condenser. Then, 1.0 g of catalyst was added to the mixture and it was stirred with a magnetic stirrer. The mixture was heated at 60 C in an oil bath for 10 h. Quantitative determinations of the
yield of fatty acid methyl esters were performed as follows. The collected reaction mixtures were evaporated in vacuum to remove the excess methanol, and then the oil phase was separated from the reaction mixture by centrifugation at 9170 g for 5 min. Methyl heptadecanoate (100 mg) was added to the oil phase sample as an internal standard, and the mixture was diluted to 5 ml with isooctane. Two microlitre of the resulting solution was subjected to highperformance liquid chromatography (HPLC) analysis for a quantitative determination of the fatty acid methyl ester. The HPLC system consisted of a JASCO PU-2089 pump, a JASCO AS-2057 auto injector, and a JASCO RI-2031 differential refractive index (RI) detector (JASCO Corp., Tokyo, Japan). Data collection and analysis was performed with the JASCO BORWIN software. The mobile phase was methanol at a flow rate of 1.0 ml/min. The columns were 250 mm · 4.6 mm Intersil ODS-3 (GL Science Inc. Japan, Tokyo, Japan). The columns were protected with an Intersil ODS-3 10 mm · 4.0 mm guard column. The sample injection volume was 2 ll, and the RI analyses were carried out at 35 C. The yield was defined as the ratio of the weight of the methyl esters determined by HPLC to the weight of the oil phase. Catalyst durability tests were performed by repeating the transesterification reaction several times with used catalysts. Catalysts were separated from the previous reaction mixture by centrifugation at 9170g for 5 min, washed with hexane, and then dried at 60 C. A 300 mg reaction mixture sample was collected every hour and was subjected to HPLC analysis for the fatty acid methyl ester determination, as described earlier. 2.3. Catalyst characterization The specific surface areas of the prepared catalysts were measured using the Brunauer-Emmett-Teller (BET) method with a surface area measuring instrument – SURFACE AREA HPP.SA-100 (SIBATA, Tokyo, Japan). X-ray powder diffraction (XRD) analysis was performed with a MiniFlex X-ray diffractometer equipped with monochromatic Cu Ka radiation (Rigaku Corp., Tokyo, Japan). Data were collected over a 2h range of 3–90, with a step size of 0.02 at a scanning speed of 4/min. Basic strengths of the catalysts (H ) were determined by using Hammett indicators. About 25 mg of the catalyst sample was shaken with 5.0 ml of a solution of Hammett indicators diluted with methanol, and left to equilibrate for 2 h. After the equilibration, the color on the catalyst was noted. The following Hammett indicators were used: neutral red (H = 6.8), bromthymol blue (H = 7.2), phenolphthalein (H = 9.3), 2,4-dinitroaniline (H = 15.0), and 4-nitroaniline (H = 18.4). 3. Results and discussion Table 1 summarizes the surface areas, base strengths, and catalytic activities of the prepared metal-oxide cata-
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Table 1 Surface areas, base strengths, and catalytic activities of metal-oxide catalysts Sample
Surface area (m2/g)
Basic strength (H )
Methyl ester yield (%)a
CaTiO3 CaMnO3 Ca2Fe2O5 CaZrO3 CaCeO3 BaTiO3 BaZrO3 BaCeO3 MgTiO3 MgZrO3 MgCeO3 LaZrO3 LaCeO3
4.9 1.5 0.71 1.8 2.9 N.A. 3.3 2.8 N.A. 7.4 7.7 N.A. N.A.
6.8 < H < 7.2 7.2 < H < 9.3 7.2 < H < 9.3 7.2 < H < 9.3 7.2 < H < 9.3 H < 6.8 H < 6.8 H < 6.8 H < 6.8 H < 6.8 H < 6.8 H < 6.8 H < 6.8
79 92 92 88 89 0.0 0.4 0.0 0.0 0.5 0.4 0.0 0.0
N.A.: not analyzed. a Methyl ester yield after 10-h transesterification reaction at 60 C.
lysts. The surface areas of each of the catalysts were small and varies from 7.7 m2/g for MgCeO3 to 0.71 m2/g for Ca2Fe2O5. The reason of this small surface area is that catalysts preparation methods involve a calcination step at high temperature. In this experiment, a 1 g of catalyst was used to 10 g of oil. This is approximately 10 times more than the weight of KOH which is usually used as homogeneous base-catalyst. However, for example, molecular weight of CaZrO3 is 179 and KOH is 56. Therefore, this is three times the number of moles of the KOH catalyst. In this experiment, the amount of catalysts was increased in order to detect the catalyst activity with small surface area. The base strengths of the catalysts were determined by the Hammett indicators. CaTiO3 had a base strength (H ) in the range of 6.8–7.2. CaMnO3, Ca2Fe2O5, CaZrO3, and CaCeO3 had the highest base strengths, in the range from 7.2 to 9.3. However, the Ba, Mg, and La series catalysts showed the weakest base strengths, which were less than 6.8. These results suggest that the Ca series catalysts have high catalytic activity for the transesterification reaction, but the other catalysts have low activity. Subsequently, we conducted the catalytic activity tests for the prepared catalysts at 60 C with a 1:6 molar ratio of the oil to methanol. The tabulated catalytic activities in Table 1 indicate the yield of fatty acid methyl ester after transesterification for 10 h. The Ca series catalysts showed a high yield of methyl ester. CaMnO3, Ca2Fe2O5, CaZrO3, and CaCeO3, whose catalytic basicities were the highest, showed 92%, 92%, 88%, and 89% yields of the ester, respectively. Next, CaTiO3, whose basicity is also high, provided a yield of 79%. However, for the other catalysts with low basicities – BaTiO3, BaZrO3, BaCeO3, MgTiO3, MgZrO3, MgCeO3, LaZrO3, and LaCeO3 – ester yields were 1% or less and little activity as a basic catalyst was observed. Fig. 1 shows the XRD patterns of the Ca series catalysts (without CaCeO3). The prepared CaTiO3 and CaZrO3 samples had XRD patterns identical to those of the perovskite-type structures of CaTiO3 and CaZrO3. As for the
Fig. 1. Powder XRD patterns for metal-oxide catalysts: CaTiO3, CaMnO3, Ca2Fe2O5, and CaZrO3 samples. The labeled peaks: square for CaTiO3, diamond for CaMnO3, and circle for CaZrO3 are perovskite structure; triangle for Ca2Fe2O5 is a kind of oxygen-deficient perovskite structure.
CaMnO3 sample, the characteristic peaks of CaMnO3, Ca2MnO4, and the parent CaO were observed, indicating a mixture of these components. Similarly, for the Ca2Fe2O5 sample, the characteristic peaks of Ca2Fe2O5 and the minor parent CaO were observed. However, the CaCeO3 sample only had the characteristic XRD peaks of the parent CeO2 and CaO, and no new peak was observed. This indicates that the CaCeO3 sample does not contain CaCeO3 but CaO and CeO2, the raw materials. It is considered that CaO would be dispersed on CeO2. The notation of CaCeO3 sample is denoted by CaO–CeO2 hereafter. Next, catalytic durability tests were performed for the CaTiO3, CaMnO3, Ca2Fe2O5, CaZrO3, and CaO–CeO2 samples, from which excellent methyl ester yields were obtained in the catalytic activity tests. In these experiments, 10-h batch-type reactions were repeated with the recycled catalyst from the previous reaction mixture. Fig. 2 shows the results of the durability test (change in methyl ester yield for each operation) for the CaTiO3 sample. In the first operation, the initial reaction rate was very low, but
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100
100 : Operation 1 : Operation 2 : Operation 3 : Operation 4
80
60
Yield (%)
Yield (%)
80
40 20
40 : Operation 1 : Operation 2 : Operation 3 : Operation 4 : Operation 5
20
0
2
6 8 4 Reaction time (h)
10
0
12
Fig. 2. Methyl ester yield profiles for the CaTiO3 catalyst in the transesterification reaction with methanol at 60 C. The catalyst was recovered and used for the following transesterification reaction.
increased gradually, and a 79% ester yield was obtained after 10 h. The initial rate quickened in the second operation and the ester yield reached 85%. In the third and forth operations, however, the catalytic activity was decreased and the obtained ester yields were 68% and 1%, respectively. The durability test for CaMnO3 is similar result. In the first operation, the initial reaction rate was very low, but a 91% ester yield was obtained after 7 h. In the subsequent operation, a methyl ester yield of 88% was obtained and high catalytic activity was observed. However, the catalytic activity during the third and forth operation was decreased and the obtained ester yields were 61% and 0.3%, respectively. A similar behavior was also observed in the case of the Ca2Fe2O5 sample. The catalytic activity was high in the first and second operations, but became very low in the following operations. Thus, the catalytic activities of CaTiO3, CaMnO3, and Ca2Fe2O5 were high in the beginning, but subsequently decreased. One of the reasons of this seems to be the possible obstruction of catalytic activity by glycerin and the adsorption of the fatty acid onto the active sites of the catalysts. Another reason may be the dissolution of the catalytic active species by the glycerin solution of the by-product. As for CaZrO3 and CaO–CeO2, the obtained results were different from those of the former three catalysts. The results for the CaZrO3 sample are shown in Fig. 3. The initial reaction rate was very low, in the same manner, and the methyl ester yield was 88% in the first operation. However, the methyl ester yields in the second and third operations were 89% and 91%, respectively. Furthermore, the reaction maintained methyl ester yields greater than 80% in the subsequent forth and fifth operations. Fig. 4 shows the results for the CaO–CeO2 sample. In the first operation, the initial reaction rate was very low and the methyl ester yield was 90% after 10 h. The methyl ester yields in the second to fifth operations were between 87% and 89%, and yields greater than 80% were observed in the subsequent sixth and seventh operations. The recovered
0
2
6 8 4 Reaction time (h)
10
12
Fig. 3. Methyl ester yield profiles for the CaZrO3 catalyst in the transesterification reaction with methanol at 60 C. The catalyst was recovered and used for the following transesterification reaction.
100 80
Yield (%)
0
60
60 : Operation 1 : Operation 2 : Operation 3 : Operation 4 : Operation 5 : Operation 6 : Operation 7
40 20 0 0
2
4 6 8 Reaction time (h)
10
12
Fig. 4. Methyl ester yield profiles for the CaO–CeO2 catalyst in the transesterification reaction with methanol at 60 C. The catalyst was recovered and used for the following transesterification reaction.
catalyst amounts decreased in each durability test, from the initial 1 to 0.3 g after the final operation, the 5th reaction of CaZrO3 and the 7th reaction of CaO–CeO2, because clear separation of catalyst from reaction mixture, especially from glycerol, was difficult due to high polarity and high viscosity of glycerol; however, an ester yield of 80% or more was maintained for all the operations. This transesterification reaction is three phase reaction: oil phase, methanol phase, and catalyst phase, and contact efficiency of these three phases is one of the important parameters for this reaction rate. Therefore even if the amount of addition of a catalyst is increased, observed increase of reaction rate would reach saturation at a certain level. CaZrO3 and CaCeO3 have the high catalyst activity and therefore, the methyl ester yield at 10 h remained approximately the same with less than 1/3 amount of the initial catalyst. These results show that both CaZrO3 having a perovskite-type structure and CaO–CeO2 composed of dispersed CaO supported on CeO2 have high durability and catalytic activity.
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The reason why the durability of the high catalytic activity is seen only in CaZrO3 and CaO–CeO2 is not clear at present. However, as far as perovskite type catalysts: CaTiO3, CaMnO3, Ca2Fe2O5, and CaZrO3, it is considered that CaZrO3 has distortion in the crystal structure since the ion radius of transition metal Zr is larger than Ti, Mn, and Fe, and this may have a certain relation to catalytic activity. The reason of high activity of CaO–CeO2 seems that CaO would be supported on CeO2 and stabilized on the catalyst surface and basic sites are able to have higher contact probability of reaction substance than others. Furthermore, the strength of the affinity or solubility with highly polar materials such as glycerin might have also a large effect on the catalytic activity. In each catalyst, the initial reaction rate of the first operation was very low, but in the following operation, the reaction rate increased. The reason for this may be that the catalyst was activated in some way during the initial part of the reaction of the first operation. We think about the activation of catalyst as follows. Catalysts react initially with methanol and formed calcium methoxide-like compounds which have high catalytic activity for the transesterification reaction due to their higher basicity. In fact, we performed the transesterification reaction as the same manner using CaZrO3 which was initially immersed in methanol. As a result, the increase in the reaction rate in initial stages of a reaction was observed. Catalysts used in this experiment have large grain size and low surface area because catalysts preparation methods involve a calcination step at high temperature. Heterogeneous catalysts can be used repeatedly, however, from a practical viewpoint, it is important to increase reaction efficiency for reducing process cost. Therefore, further examinations of the catalyst synthesis method or immobilization method to which aimed at large surface area are needed. 4. Conclusions In this study, we intended to examine heterogeneous base catalysts in order to develop an effective catalyst for biodiesel process. Thirteen kinds of prepared catalyst of metal oxides were demonstrated to be catalysts for the transesterification of plant oil with methanol. The catalysts with Ca were found to have high basicities and catalytic activities for this reaction. Batch-type reactions were carried out at 60 C with a 1:6 molar ratio of the oil to methanol for a reaction time of 10 h, in which the methyl ester yield reached 79–92%. In particular, a high durability of catalytic activity was found for the catalyst samples of CaZrO3 and CaO–CeO2, which were able to provide
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