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RETRACTED: Efficient preparation of biodiesel from rapeseed oil over modified CaO
Applied Energy 88 (2011) 2735–2739
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Efficient preparation of biodiesel from rapeseed oil over modified CaO Ying Tang a,⇑, Mei Meng a, Jie Zhang a, Yong Lu b a b
College of Chemistry and Chemical Engineering, Xi’an shiyou University, Xi’an Shannxi, China Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China
a r t i c l e
i n f o
Article history: Received 19 February 2010 Received in revised form 20 February 2011 Accepted 22 February 2011
Keywords: Biodiesel Base catalyst CaO Modification
a b s t r a c t In this study, the catalytic performance of commercial CaO modified by trimethylchlorosilane (TMCS) for transesterification of rapeseed oil and methanol to biodiesel production was investigated. It was found that the fatty acid methyl esters (FAME) yield of the modified CaO was greatly enhanced from 85.4% to 94.6%. The possible reason lies on promoting the absorption of grease to CaO surface. Good results of repeated experiments showed that the modified catalyst has the capacity of water resistance and can be reused for several runs without significant deactivation, which can be confirmed by the humidity test in the vapor-saturated atmosphere. Both the characterizations of the catalyst and the effects of various factors such as mass ratio of catalyst to oil, reaction temperature and molar ratio of methanol to oil were investigated. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel can be considered as a renewable and economic fuel with regard to considerably low toxic emission and high cetane number. The fatty acid methyl esters of seed oils or fats of biodiesel have already been found suitable for use as fuel in diesel engine with good lubricant properties that extends the engine life which makes it very attractive as an alternative fuel [1–4]. The conventional method for biodiesel production is the transesterification of vegetable oils or animal fats with short-chain alcohols (generally methanol) in the presence of catalysts [5]. As suggested by Balat [6], alkaline-catalyzed transesterification process can be carried out at low temperatures and pressure with low catalyst concentration. Alkaline metal hydroxides are among the most active catalysts for their high biodiesel yield in short time [7]. However, compared to conventional heterogeneous strong base for transesterification, homogeneous base catalysts offer several process disadvantages including product purity and waste water, which make biodiesel production hard to be operated in a continuous process. Because of these disadvantages, many types of solid bases, such as alkali earth oxides and hydroxides, have been reported in the preparation of biodiesel [8,9]. Among the heterogeneous basic catalysts, calcium oxide (CaO) is a potential one for its low cost, high basic strength (H_ = 26.5) and low methanol solubility [10,11]. However, the single contact between reagents and catalysts causes the catalyst less active than homogeneous ⇑ Corresponding author. E-mail address: [email protected] (Y. Tang). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.02.033
strong base, such as sodium hydroxide. Many attempts have been made to promote the activity of the alkali earth oxides by increasing the number of basic sites of the catalyst or enlarging the surface area of the catalysts. Watkins et al. promote the basicity of CaO by doping CaO with lithium [12]. However, they are quite expensive or complicated to prepare, which limited their industrial application. Reddy et al. produced biodiesel using nanocrystalline CaO under room temperature [13], and the results showed that the first three cycles provided >99% conversions, decreasing conversions were obtained in the fourth and fifth cycles, respectively. Recently, the research of accelerating catalytic activity of CaO for biodiesel production has been aimed at activation of CaO by pretreatment with methanol [14,15]. However, the activation mechanism was still controversial. Furthermore, CaO is very easy to hydrate with water and/or react with CO2 which causes the stability of this catalyst decrease dramatically. In this research, we attempt to develop an efficient way for the transesterification between rapeseed oil and methanol by using an activated CaO, which can promote the diffusion of grease to the catalyst surface. The activation condition and various reaction conditions for transesterification were also been investigated. 2. Materials and methods 2.1. Materials Rapeseed oil was purchased from a coal market (Xi’an, China). Before the reaction, the oil was treated by sodium hydroxide and bentonite in which the acid value lowers than 1 mg KOH/g and
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Y. Tang et al. / Applied Energy 88 (2011) 2735–2739
water concentration below 1 mg/g. All the chemicals were of an analytical reagent grade, that is, trimethylchlorosilane TMCS and methanol and were obtained from Sinopharm Chemical Regents Co., Ltd. (Beijing, China). 2.2. Catalysts preparation The surface modification procedure was as follows. Appropriate amount of commercial CaO particles were added into trimethylchlorosilane (TMCS) solution in hexane under stirring at room temperature for activation. After 24 h, the mixture was separated and washed with hexane in order to remove the excess silane, and then the modified CaO was gotten after a drying vacuum process. 2.3. Humidity test of the modified CaO To evaluate the sensitivity of catalysts to water, the humidity test were carried out as follows: amount of commercial CaO and modified CaO particles were kept in a vapor-saturated container at room temperature for several days to allow the absorption of water on the surface. The samples were weighed at regular intervals of time. The absorbing moisture rate (w%) of the samples was evaluated by the following equation, using the Dm as increase weight, as well as the m0 as the initial weight
w% ¼
56Dm 100% 18m0
2.4. Catalytic testing The catalytic activities for the transesterification of rapeseed oil with methanol were measured by typical procedure [10]: a given amount of modified CaO and methanol were placed in a three necked round bottomed flask equipped with a reflux condenser and a thermometer. Then, the rapeseed oil was added into the mixture and heated at a certain temperature for a certain time. After reaction finished, the excess methanol was distilled off under vacuum. The products were analyzed by the GC (HP-6890) using inner standard. The yield was defined as a ratio of the weight of fatty acid methyl esters, determined by GC, to the weight of fatty acid methyl esters that the oil used in the reaction, assuming only traces of esters was transferred to the polar phase and that only the extraction of methanol and glycerin takes place as suggested by López [16].
3. Results and discussion 3.1. Characterization of the catalyst Fig. 1 shows the IR spectra of the modified CaO. The spectra display bands at 867 and 1477 cm1, which would correspond to vibration modes of mono and bidentate carbonates. The asymmetric OASi stretching vibration modes appear as a broad band between 990 and 1358 cm1, and the band at 469 cm1 is assigned to the SiAO bending mode as suggested by Albuquerque [17], which provides great evidence for the chemical combination of modification regent to CaO surface. The bands at 1621 and 3460 cm1 are associated with adsorbed water. The important features of the modified CaO appear in the CAH stretching (2800–3000 cm1) and CAH (alkane) bending (1440 cm1). It is also evidenced from the characteristic absorb of C@O between 2000 and 1500 cm1 that the presence of calcium carbonate is formed in the catalyst, as evidenced by the two characteristic broad diffraction lines of CaO in the diffraction pattern (Fig. 2). It indicates that the modified CaO still has strong basic property because it can adsorb CO2 at room temperature. Fig. 2 shows the XRD pattern of commercial CaO and the modified CaO. A series of reflections at 32.1, 37.3 and 53.9 are consistent with X-ray diffractograms of CaO. Minor reflections at 17.9, 28.6, 34.1, 46.9 and 50.7 are attributed to Ca (OH)2 phases, which indicate the hydration of fresh CaO can not avoided during the catalyst activation [16]. The peak at 29.2 and 38.9 are assigned to the reflection of CaCO3 due to the expose of fresh CaO in air. Comparison the main peak area, it can be seen that the diffraction peaks corresponding to the CaO phases of commercial CaO particles are slightly less intense than those at modified CaO. Amorphous CaCl2 formed by combination of TMCS and CaO was not detected by XRD in the sample. The SEM micrographs of commercial CaO particles and modified CaO sample were shown in Fig. 3. Great difference can be found between these two samples. It can be seen that the surface of commercial CaO particle is built of aggregates, while modified CaO presents a more exfoliated morphology, which could be associated with its higher catalytic actives. 3.2. Humidity test of the modified CaO To evaluate the impact of the amount of modifier on the moisture resistance, humidity tests were carried out for the various
2.5. Catalyst stability a b
Transmission
The lifetime of the commercial CaO and modified CaO were tested by repeating transesterification several times with used catalysts under the optimum reaction condition. Used catalysts were separated from the previous reaction mixture by centrifugation, washed with hexane, and then dried at 60 °C. The product was subjected to GC analysis for the FAME determination, as described earlier [5]. 2.6. Catalyst characterization X-ray diffraction (XRD) patterns were recorded on a D/Max-3C X-ray powder diffractometer (Rigalcu Co., Japan); using a CuAK a source fitted with an Inel CPS 120 hemispherical detector. The Fourier transform-infrared (FT–IR) spectrophoto - meters were used to identify the surface group over the catalyst. Scanning electron microscope (SEM) photographs were taken by Quanta 200 scanning electron microscope equipped with an energy dispersive spectrometer (Philips-FEI Co., the Netherlands).
4000
3500
3000
2500
2000
1500
1000
500
-1
Frequency (cm ) Fig. 1. FTIR analysis of commercial CaO and modified CaO. (a) Commercial CaO; (b) modified CaO with amount of 0.1% TMCS.
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100
CaO Ca(OH)2
Moisture absorption rate (%)
Intensity (counts)
CaCO3
a
80
60 commercial CaO modified CaO with 0.1% TMCS
40
20
b
10
20
30
40
50
60
2θ Fig. 2. XRD patterns of commercial CaO and modified CaO. (a) Commercial CaO; (b) modified CaO with amount of 0.1% TMCS.
coated products with modifier amount in the range from 0.01 to 0.05%. For comparison, the moisture absorption of unmodified samples was also included. The tested samples were kept in a container at nearly 100% humidity at room temperature to allow the hydration. The weight of each samples were measured at regular intervals of time, and the results for samples modified with various amount of TMCS were shown in Fig. 4. It was noted that the moisture absorption continues rising for all samples along with the time. To the unmodified CaO particles, the moisture absorption rate reached to nearly 100% within 50 h, while for the modified CaO, very low weight incensement was found at the same time. 3.3. Catalytic activities test 3.3.1. Influence of amount of modification reagent To determine the influence of the amount of modifier on the activity of the modified CaO catalyst, the commercial CaO particles were treated with different amount of TMCS from 0.005 to 2.0 wt% and then tested for transesterification of rapeseed oil to produce biodiesel. These results (Table 1) indicated that as the amount of TMCS increased, the conversion of triglycerides into FAME also increased. The yield reached a max value, 94.6% yield of FAME, as the amount of TMCS reached 0.1 wt%, and further increase in amount of TMCS made the catalytic activity decrease. The reason maybe lies on the occupation of active sites by modifier on CaO surface (see Table 2).
0
0
20
40
60
80
100
Time (h) Fig. 4. Moisture absorption rate over commercial CaO and modified CaO with amount of 0.1% TMCS.
3.3.2. Influence of methanol/oil ratio The alcohol to oil molar ratio is one of the important factors affecting the conversion efficiency of transesterification. The results (Table 1) showed the yield increased with the ratio of methanol to oil below 15:1. With further increase of the methanol, little effect on the FAME yield can be detected. This is due to the dilution effect by too much alcohol, and moreover high alcohol amount slow down the separation of the glycerin phase and the methyl ester phase [18]. Therefore, the optimum quantity of methanol was 15:1. 3.3.3. Influence of reaction time The yield of methyl esters of transesterification commonly increased with the reaction time. It can be seen from Table 1 that the yields of methyl esters arrived at the maximum value at the reaction time around 3 h and then there was slightly decrease from the 4th h. This is because too long reaction time caused the hydrolysis of esters and more fatty acids to form soap [19]. 3.3.4. Influence of catalyst concentration The effect of modified CaO concentrations on the transesterification was investigated with concentration varying from 1 wt% to 15 wt% (weight to oil). Because low concentration of catalyst was insufficient to catalyze the reaction for completion, initially increase the amount of catalyst resulted in great change of FAME yield. No further enhancement of FAME yield can be found when
Fig. 3. SEM photos of commercial CaO and modified CaO. (a) Commercial CaO; (b) modified CaO with amount of 0.1% TMCS.
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Y. Tang et al. / Applied Energy 88 (2011) 2735–2739 Table 1 The influence of reaction parameters in the FAME yield. Entry
The amount of TMCS (%)
Methanol/ oil ratio
Reaction time (h)
Amount of catalyst (wt%)
Reaction temperature (°C)
Yield of FAME (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
0.005 0.01 0.1 1.0 2.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
15:1 15:1 15:1 15:1 15:1 3:1 5:1 10:1 15:1 20:1 30:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1
3 3 3 3 3 3 3 3 3 3 3 0.5 1 3 4 5 3 3 3 3 3 3 3 3 3
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 1 2 5 10 15 5 5 5 5
65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 50 55 60 65
83.5 90.7 94.6 84.3 76.8 55.3 73.2 87.6 94.6 95.2 96.1 75.1 82.3 94.6 93.4 92.8 80.2 90.4 94.6 93.5 92.8 56.7 76.8 91.4 94.6
100
Table 2 Water content test for biodiesel production over modified CaO. FAME yield (%)
0.5 1.0 2.0 4.0
95.1 95.5 92.7 91.8
The reaction conditions: modified CaO of 5% to oil, methanol/oil molar ratio of 15:1, 3 h, 65 °C.
excess catalysts were introduced. The reason for this decreasing trend was due to the formation of soap in presence of high amount of catalysts, which increased the viscosity of the reactants and lowered the yield of ester as suggested by Yang [18]. Therefore the 5 wt% modified CaO was optimal in the reactions of this study.
80
FAME yield %
Water content (%)
60 modified CaO commercial CaO
40
20
0 0
2
4
6
8
10
12
14
16
Number of repetitions 3.3.5. Influence of reaction temperature Alkaline alcoholysis of vegetable oil is normally performed at temperature range between 45 °C and 65 °C [20]. To study the effect of reaction temperature on FAME the yield, the investigation was carried out under the temperatures of 50 °C, 55 °C, 60 °C and 65 °C with 5.0% modified CaO as catalyst and a methanol/oil molar ratio of 15:1. The results (Table 1) indicated that higher temperature leads to higher yield, and the yield of FAME reaches to 94.6% at 65 °C. However, the rate of transesterification will decrease as the reaction temperature exceeds the boiling point of methanol for the formation of much bubbles of methanol vaporization. 3.3.6. Repeated experiments This study investigated the stability when commercial CaO and modified CaO were representative heterogeneous solid base catalysts for the transesterification of rapeseed oil with methanol to biodiesel under above optimum reaction condition: 65 °C, 15:1 M ratios of methanol/oil, 5 wt% catalyst (weight to oil). Fig. 5 indicated that the yield of FAME over the modified catalyst was enhanced to near to 95% when the reaction time reached to 3 h. It maintained sustained activity even after being used for 15 cycles
Fig. 5. Effect of repeated use on FAME yield over commercial CaO and modified CaO: 65 °C temperature, 15:1molar ratios of methanol to oil, 3 h and 5 wt% catalyst amount.
and the FAME yield only showed slightly decrease, while, 85.4% FAME yield was obtained for commercial CaO under the same reaction condition. The hydrophobic layer over modified CaO keeps off the diffusion of water and CO2 to catalyst surface resulting in high activity after several repeated uses, furthermore, the increased cost of about 0.3% for the modified CaO is negligible based on reduced demanding of the catalyst for its good stability and high activity, which can be provide greatly economic potential for its industry application. The slightly decrease of FAME yield in present of commercial CaO during the repeated experiments should be due to the sensitivity of the catalyst to water and/or CO2 in the reaction system. 3.3.7. Investigation of water tolerance over modified CaO The effect of water content in methanol on biodiesel yield over the modified CaO was studied under optimum reaction condition:
Y. Tang et al. / Applied Energy 88 (2011) 2735–2739
65 °C, 15:1 M ratios of methanol/oil, 5 wt% catalyst of catalyst (weight to oil). From this study, it can be seen that a small amount of water increased the reaction rates for the generation of more methoxide anions suggested by Liu [14] and the modified CaO maintained 91.8% yield of biodiesel even 4% water contented in the reaction system, which is much higher than the reported water tolerance limit of 2.8%. The possible reason may be contributed to the role of the ethyl groups, which formed a hydrophobic layer over CaO surface. 4. Conclusion In this study, an efficient solid basic catalyst for biodiesel production for transesterification of rapeseed oil with methanol was obtained by the modification on commercial CaO particles using trimethylchlorosilane (TMCS). The modifier over CaO greatly improved the diffusion of grease to catalyst surface and causes the yield reaching to 94.6%. On the other hand, the humidity test of the modified CaO in water-saturated container demonstrated that the hydrophobic modifier can prevent the deterioration of the catalytic performance by contact with a small quantity of water in the reaction system. The modified CaO catalyst can be reused up to 15 times with maintaining the FAME content >90%. From this view, this novel catalyst has the greatly potential for particles biodiesel production and other liquid–solid heterogeneous catalytic reactions. Acknowledgements This work was financially supported by grants from National Science Foundation of China (No. 50874092), Scientific Research Plan Projects of Shaanxi and the Open Founds of the Shanghai Key Laboratory of Green Chemistry and Chemical Process. References [1] Fangrui M, Milford AH. Biodiesel production: a review. Bioresource Technol 1999;70:1–15.
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[2] Meher LC, Vidya Sagar D, Naik SN. Technical aspects of biodiesel production by transesterification - a review. Renew sust energ rev 2006;10:248–68. [3] Dennis YC, Leung XW, Leung MKH. A review on biodiesel production using catalyzed transesterification. Appl Energ 2010;87:1083–95. [4] Farooq A, Umer R, Muhammad A, Muhammad N. Okra (Hibiscus esculentus) seed oil for biodiesel production. Appl Energ 2010;87:779–85. [5] Kawashima A, Matsubara K, Honda K. Development of heterogeneous base catalysts for biodiesel production. Bioresource Technol 2008;99:3443–99. [6] Balat M, Balat H. Progress in biodiesel processing. Appl Energ 2010;87: 1815–35. [7] Dorado MP, Ballesteros E, Lopez JF, Mittelbach M. Optimization of alkalicatalyzed transesterification of brassica carinata oil for biodiesel production. Energ Fuel 2004;18:77–83. [8] Kouzu M, Kasuno T, Tajika M, Sugimoto Y, Yamanaka S, Hidaka J. Calcium oxide as a solid base catalyst for transesterification of soybean oil and its application to biodiesel production. Fuel 2008;87:2798–806. [9] Kouzu M, Hidaka, Konichi J, Nakano YH, Yamamoto M. A process to transesterify vegetable oil with methanol in the presence of quick lime bit functioning as solid base catalyst. Fuel 2009;88:1983–90. [10] Liu XJ, He HY, Wang YJ, Zhu SL, Piao XL. Transesterification of soybean oil to biodiesel using CaO as solid base catalyst. Fuel 2007;87:216–21. [11] Zhu H, Wu Z, Chen Y, Zhang P, Duan S, Liu X, et al. Preparation of biodiesel catalyzed by solid super base of calcium oxide and its refining process. Chin J Catal 2006;27:391–6. [12] Watkins RS, Lee AF, Wilson K. Li-CaO catalysed triglyceride transesterfication for biodiesel applications. Green Chem 2004;6:335–40. [13] Chinta R, Venkat R, Reed O, John GV. Room-temperature conversion of soybean oil andpoultry fat to biodiesel catalyzed by nanocrystalline calcium oxides. Energy Fuel 2006;20:1310–4. [14] Liu XJ, Piao XL, Wang YJ, Zhu SL, He HY. Calcium methoxide as a solid base catalyst for the transesterification of soybean oil to biodiesel with methanol. Fuel 2008;87:1076–82. [15] Kawashima A, Matsubara K, Katsuhisa H. Acceleration of catalytic activity of calcium oxide for biodiesel production. Bioresour Technol 2009;100:696–700. [16] López GM, Zafra Poves MD, Martín Alonso D, Mariscal R, Cabello Galisteo F, Moreno-Tost R, et al. Biodiesel from sunflower oil by using activated calcium oxide. Appl Catal B: Environ 2007;73:317–26. [17] Albuquerque Mônica CG, Jiménez-Urbistondo I, Santamaría-González José. CaO supported on mesoporous silicas as basic catalysts for transesterification reactions. Appl Catal A: Gen 2008;334:35–43. [18] Yang FX, Su YQ, Li XH, Zhang Q, Sun RC. Preparation of biodiesel from Idesia polycarpa var Vestita fruit oil. Ind Crop Prod 2009;29:622–8. [19] Eevera T, Rajendran K, Saradha S. Biodiesel production process optimization and characterization to assess the suitability of the product for varied environmental conditions. Renew Energ 2009;34:762–5. [20] Sharma YC, Singh B, Upadhyay SN. Advancements in development and characterization of biodiesel: a review. Fuel 2008;87:2355–73.
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Efficient preparation of biodiesel from rapeseed oil over modified CaO Ying Tang a,⇑, Mei Meng a, Jie Zhang a, Yong Lu b a b
College of Chemistry and Chemical Engineering, Xi’an shiyou University, Xi’an Shannxi, China Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China
a r t i c l e
i n f o
Article history: Received 19 February 2010 Received in revised form 20 February 2011 Accepted 22 February 2011
Keywords: Biodiesel Base catalyst CaO Modification
a b s t r a c t In this study, the catalytic performance of commercial CaO modified by trimethylchlorosilane (TMCS) for transesterification of rapeseed oil and methanol to biodiesel production was investigated. It was found that the fatty acid methyl esters (FAME) yield of the modified CaO was greatly enhanced from 85.4% to 94.6%. The possible reason lies on promoting the absorption of grease to CaO surface. Good results of repeated experiments showed that the modified catalyst has the capacity of water resistance and can be reused for several runs without significant deactivation, which can be confirmed by the humidity test in the vapor-saturated atmosphere. Both the characterizations of the catalyst and the effects of various factors such as mass ratio of catalyst to oil, reaction temperature and molar ratio of methanol to oil were investigated. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel can be considered as a renewable and economic fuel with regard to considerably low toxic emission and high cetane number. The fatty acid methyl esters of seed oils or fats of biodiesel have already been found suitable for use as fuel in diesel engine with good lubricant properties that extends the engine life which makes it very attractive as an alternative fuel [1–4]. The conventional method for biodiesel production is the transesterification of vegetable oils or animal fats with short-chain alcohols (generally methanol) in the presence of catalysts [5]. As suggested by Balat [6], alkaline-catalyzed transesterification process can be carried out at low temperatures and pressure with low catalyst concentration. Alkaline metal hydroxides are among the most active catalysts for their high biodiesel yield in short time [7]. However, compared to conventional heterogeneous strong base for transesterification, homogeneous base catalysts offer several process disadvantages including product purity and waste water, which make biodiesel production hard to be operated in a continuous process. Because of these disadvantages, many types of solid bases, such as alkali earth oxides and hydroxides, have been reported in the preparation of biodiesel [8,9]. Among the heterogeneous basic catalysts, calcium oxide (CaO) is a potential one for its low cost, high basic strength (H_ = 26.5) and low methanol solubility [10,11]. However, the single contact between reagents and catalysts causes the catalyst less active than homogeneous ⇑ Corresponding author. E-mail address: [email protected] (Y. Tang). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.02.033
strong base, such as sodium hydroxide. Many attempts have been made to promote the activity of the alkali earth oxides by increasing the number of basic sites of the catalyst or enlarging the surface area of the catalysts. Watkins et al. promote the basicity of CaO by doping CaO with lithium [12]. However, they are quite expensive or complicated to prepare, which limited their industrial application. Reddy et al. produced biodiesel using nanocrystalline CaO under room temperature [13], and the results showed that the first three cycles provided >99% conversions, decreasing conversions were obtained in the fourth and fifth cycles, respectively. Recently, the research of accelerating catalytic activity of CaO for biodiesel production has been aimed at activation of CaO by pretreatment with methanol [14,15]. However, the activation mechanism was still controversial. Furthermore, CaO is very easy to hydrate with water and/or react with CO2 which causes the stability of this catalyst decrease dramatically. In this research, we attempt to develop an efficient way for the transesterification between rapeseed oil and methanol by using an activated CaO, which can promote the diffusion of grease to the catalyst surface. The activation condition and various reaction conditions for transesterification were also been investigated. 2. Materials and methods 2.1. Materials Rapeseed oil was purchased from a coal market (Xi’an, China). Before the reaction, the oil was treated by sodium hydroxide and bentonite in which the acid value lowers than 1 mg KOH/g and
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Y. Tang et al. / Applied Energy 88 (2011) 2735–2739
water concentration below 1 mg/g. All the chemicals were of an analytical reagent grade, that is, trimethylchlorosilane TMCS and methanol and were obtained from Sinopharm Chemical Regents Co., Ltd. (Beijing, China). 2.2. Catalysts preparation The surface modification procedure was as follows. Appropriate amount of commercial CaO particles were added into trimethylchlorosilane (TMCS) solution in hexane under stirring at room temperature for activation. After 24 h, the mixture was separated and washed with hexane in order to remove the excess silane, and then the modified CaO was gotten after a drying vacuum process. 2.3. Humidity test of the modified CaO To evaluate the sensitivity of catalysts to water, the humidity test were carried out as follows: amount of commercial CaO and modified CaO particles were kept in a vapor-saturated container at room temperature for several days to allow the absorption of water on the surface. The samples were weighed at regular intervals of time. The absorbing moisture rate (w%) of the samples was evaluated by the following equation, using the Dm as increase weight, as well as the m0 as the initial weight
w% ¼
56Dm 100% 18m0
2.4. Catalytic testing The catalytic activities for the transesterification of rapeseed oil with methanol were measured by typical procedure [10]: a given amount of modified CaO and methanol were placed in a three necked round bottomed flask equipped with a reflux condenser and a thermometer. Then, the rapeseed oil was added into the mixture and heated at a certain temperature for a certain time. After reaction finished, the excess methanol was distilled off under vacuum. The products were analyzed by the GC (HP-6890) using inner standard. The yield was defined as a ratio of the weight of fatty acid methyl esters, determined by GC, to the weight of fatty acid methyl esters that the oil used in the reaction, assuming only traces of esters was transferred to the polar phase and that only the extraction of methanol and glycerin takes place as suggested by López [16].
3. Results and discussion 3.1. Characterization of the catalyst Fig. 1 shows the IR spectra of the modified CaO. The spectra display bands at 867 and 1477 cm1, which would correspond to vibration modes of mono and bidentate carbonates. The asymmetric OASi stretching vibration modes appear as a broad band between 990 and 1358 cm1, and the band at 469 cm1 is assigned to the SiAO bending mode as suggested by Albuquerque [17], which provides great evidence for the chemical combination of modification regent to CaO surface. The bands at 1621 and 3460 cm1 are associated with adsorbed water. The important features of the modified CaO appear in the CAH stretching (2800–3000 cm1) and CAH (alkane) bending (1440 cm1). It is also evidenced from the characteristic absorb of C@O between 2000 and 1500 cm1 that the presence of calcium carbonate is formed in the catalyst, as evidenced by the two characteristic broad diffraction lines of CaO in the diffraction pattern (Fig. 2). It indicates that the modified CaO still has strong basic property because it can adsorb CO2 at room temperature. Fig. 2 shows the XRD pattern of commercial CaO and the modified CaO. A series of reflections at 32.1, 37.3 and 53.9 are consistent with X-ray diffractograms of CaO. Minor reflections at 17.9, 28.6, 34.1, 46.9 and 50.7 are attributed to Ca (OH)2 phases, which indicate the hydration of fresh CaO can not avoided during the catalyst activation [16]. The peak at 29.2 and 38.9 are assigned to the reflection of CaCO3 due to the expose of fresh CaO in air. Comparison the main peak area, it can be seen that the diffraction peaks corresponding to the CaO phases of commercial CaO particles are slightly less intense than those at modified CaO. Amorphous CaCl2 formed by combination of TMCS and CaO was not detected by XRD in the sample. The SEM micrographs of commercial CaO particles and modified CaO sample were shown in Fig. 3. Great difference can be found between these two samples. It can be seen that the surface of commercial CaO particle is built of aggregates, while modified CaO presents a more exfoliated morphology, which could be associated with its higher catalytic actives. 3.2. Humidity test of the modified CaO To evaluate the impact of the amount of modifier on the moisture resistance, humidity tests were carried out for the various
2.5. Catalyst stability a b
Transmission
The lifetime of the commercial CaO and modified CaO were tested by repeating transesterification several times with used catalysts under the optimum reaction condition. Used catalysts were separated from the previous reaction mixture by centrifugation, washed with hexane, and then dried at 60 °C. The product was subjected to GC analysis for the FAME determination, as described earlier [5]. 2.6. Catalyst characterization X-ray diffraction (XRD) patterns were recorded on a D/Max-3C X-ray powder diffractometer (Rigalcu Co., Japan); using a CuAK a source fitted with an Inel CPS 120 hemispherical detector. The Fourier transform-infrared (FT–IR) spectrophoto - meters were used to identify the surface group over the catalyst. Scanning electron microscope (SEM) photographs were taken by Quanta 200 scanning electron microscope equipped with an energy dispersive spectrometer (Philips-FEI Co., the Netherlands).
4000
3500
3000
2500
2000
1500
1000
500
-1
Frequency (cm ) Fig. 1. FTIR analysis of commercial CaO and modified CaO. (a) Commercial CaO; (b) modified CaO with amount of 0.1% TMCS.
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100
CaO Ca(OH)2
Moisture absorption rate (%)
Intensity (counts)
CaCO3
a
80
60 commercial CaO modified CaO with 0.1% TMCS
40
20
b
10
20
30
40
50
60
2θ Fig. 2. XRD patterns of commercial CaO and modified CaO. (a) Commercial CaO; (b) modified CaO with amount of 0.1% TMCS.
coated products with modifier amount in the range from 0.01 to 0.05%. For comparison, the moisture absorption of unmodified samples was also included. The tested samples were kept in a container at nearly 100% humidity at room temperature to allow the hydration. The weight of each samples were measured at regular intervals of time, and the results for samples modified with various amount of TMCS were shown in Fig. 4. It was noted that the moisture absorption continues rising for all samples along with the time. To the unmodified CaO particles, the moisture absorption rate reached to nearly 100% within 50 h, while for the modified CaO, very low weight incensement was found at the same time. 3.3. Catalytic activities test 3.3.1. Influence of amount of modification reagent To determine the influence of the amount of modifier on the activity of the modified CaO catalyst, the commercial CaO particles were treated with different amount of TMCS from 0.005 to 2.0 wt% and then tested for transesterification of rapeseed oil to produce biodiesel. These results (Table 1) indicated that as the amount of TMCS increased, the conversion of triglycerides into FAME also increased. The yield reached a max value, 94.6% yield of FAME, as the amount of TMCS reached 0.1 wt%, and further increase in amount of TMCS made the catalytic activity decrease. The reason maybe lies on the occupation of active sites by modifier on CaO surface (see Table 2).
0
0
20
40
60
80
100
Time (h) Fig. 4. Moisture absorption rate over commercial CaO and modified CaO with amount of 0.1% TMCS.
3.3.2. Influence of methanol/oil ratio The alcohol to oil molar ratio is one of the important factors affecting the conversion efficiency of transesterification. The results (Table 1) showed the yield increased with the ratio of methanol to oil below 15:1. With further increase of the methanol, little effect on the FAME yield can be detected. This is due to the dilution effect by too much alcohol, and moreover high alcohol amount slow down the separation of the glycerin phase and the methyl ester phase [18]. Therefore, the optimum quantity of methanol was 15:1. 3.3.3. Influence of reaction time The yield of methyl esters of transesterification commonly increased with the reaction time. It can be seen from Table 1 that the yields of methyl esters arrived at the maximum value at the reaction time around 3 h and then there was slightly decrease from the 4th h. This is because too long reaction time caused the hydrolysis of esters and more fatty acids to form soap [19]. 3.3.4. Influence of catalyst concentration The effect of modified CaO concentrations on the transesterification was investigated with concentration varying from 1 wt% to 15 wt% (weight to oil). Because low concentration of catalyst was insufficient to catalyze the reaction for completion, initially increase the amount of catalyst resulted in great change of FAME yield. No further enhancement of FAME yield can be found when
Fig. 3. SEM photos of commercial CaO and modified CaO. (a) Commercial CaO; (b) modified CaO with amount of 0.1% TMCS.
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Y. Tang et al. / Applied Energy 88 (2011) 2735–2739 Table 1 The influence of reaction parameters in the FAME yield. Entry
The amount of TMCS (%)
Methanol/ oil ratio
Reaction time (h)
Amount of catalyst (wt%)
Reaction temperature (°C)
Yield of FAME (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
0.005 0.01 0.1 1.0 2.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
15:1 15:1 15:1 15:1 15:1 3:1 5:1 10:1 15:1 20:1 30:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1 15:1
3 3 3 3 3 3 3 3 3 3 3 0.5 1 3 4 5 3 3 3 3 3 3 3 3 3
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 1 2 5 10 15 5 5 5 5
65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 65 50 55 60 65
83.5 90.7 94.6 84.3 76.8 55.3 73.2 87.6 94.6 95.2 96.1 75.1 82.3 94.6 93.4 92.8 80.2 90.4 94.6 93.5 92.8 56.7 76.8 91.4 94.6
100
Table 2 Water content test for biodiesel production over modified CaO. FAME yield (%)
0.5 1.0 2.0 4.0
95.1 95.5 92.7 91.8
The reaction conditions: modified CaO of 5% to oil, methanol/oil molar ratio of 15:1, 3 h, 65 °C.
excess catalysts were introduced. The reason for this decreasing trend was due to the formation of soap in presence of high amount of catalysts, which increased the viscosity of the reactants and lowered the yield of ester as suggested by Yang [18]. Therefore the 5 wt% modified CaO was optimal in the reactions of this study.
80
FAME yield %
Water content (%)
60 modified CaO commercial CaO
40
20
0 0
2
4
6
8
10
12
14
16
Number of repetitions 3.3.5. Influence of reaction temperature Alkaline alcoholysis of vegetable oil is normally performed at temperature range between 45 °C and 65 °C [20]. To study the effect of reaction temperature on FAME the yield, the investigation was carried out under the temperatures of 50 °C, 55 °C, 60 °C and 65 °C with 5.0% modified CaO as catalyst and a methanol/oil molar ratio of 15:1. The results (Table 1) indicated that higher temperature leads to higher yield, and the yield of FAME reaches to 94.6% at 65 °C. However, the rate of transesterification will decrease as the reaction temperature exceeds the boiling point of methanol for the formation of much bubbles of methanol vaporization. 3.3.6. Repeated experiments This study investigated the stability when commercial CaO and modified CaO were representative heterogeneous solid base catalysts for the transesterification of rapeseed oil with methanol to biodiesel under above optimum reaction condition: 65 °C, 15:1 M ratios of methanol/oil, 5 wt% catalyst (weight to oil). Fig. 5 indicated that the yield of FAME over the modified catalyst was enhanced to near to 95% when the reaction time reached to 3 h. It maintained sustained activity even after being used for 15 cycles
Fig. 5. Effect of repeated use on FAME yield over commercial CaO and modified CaO: 65 °C temperature, 15:1molar ratios of methanol to oil, 3 h and 5 wt% catalyst amount.
and the FAME yield only showed slightly decrease, while, 85.4% FAME yield was obtained for commercial CaO under the same reaction condition. The hydrophobic layer over modified CaO keeps off the diffusion of water and CO2 to catalyst surface resulting in high activity after several repeated uses, furthermore, the increased cost of about 0.3% for the modified CaO is negligible based on reduced demanding of the catalyst for its good stability and high activity, which can be provide greatly economic potential for its industry application. The slightly decrease of FAME yield in present of commercial CaO during the repeated experiments should be due to the sensitivity of the catalyst to water and/or CO2 in the reaction system. 3.3.7. Investigation of water tolerance over modified CaO The effect of water content in methanol on biodiesel yield over the modified CaO was studied under optimum reaction condition:
Y. Tang et al. / Applied Energy 88 (2011) 2735–2739
65 °C, 15:1 M ratios of methanol/oil, 5 wt% catalyst of catalyst (weight to oil). From this study, it can be seen that a small amount of water increased the reaction rates for the generation of more methoxide anions suggested by Liu [14] and the modified CaO maintained 91.8% yield of biodiesel even 4% water contented in the reaction system, which is much higher than the reported water tolerance limit of 2.8%. The possible reason may be contributed to the role of the ethyl groups, which formed a hydrophobic layer over CaO surface. 4. Conclusion In this study, an efficient solid basic catalyst for biodiesel production for transesterification of rapeseed oil with methanol was obtained by the modification on commercial CaO particles using trimethylchlorosilane (TMCS). The modifier over CaO greatly improved the diffusion of grease to catalyst surface and causes the yield reaching to 94.6%. On the other hand, the humidity test of the modified CaO in water-saturated container demonstrated that the hydrophobic modifier can prevent the deterioration of the catalytic performance by contact with a small quantity of water in the reaction system. The modified CaO catalyst can be reused up to 15 times with maintaining the FAME content >90%. From this view, this novel catalyst has the greatly potential for particles biodiesel production and other liquid–solid heterogeneous catalytic reactions. Acknowledgements This work was financially supported by grants from National Science Foundation of China (No. 50874092), Scientific Research Plan Projects of Shaanxi and the Open Founds of the Shanghai Key Laboratory of Green Chemistry and Chemical Process. References [1] Fangrui M, Milford AH. Biodiesel production: a review. Bioresource Technol 1999;70:1–15.
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