CHINESE JOURNAL OF CATALYSIS Volume 27, Issue 5, May 2006 Online English edition of the Chinese language journal Cite this article as: Chin J Catal, 2006, 27(5): 391–396.
RESEARCH PAPER
Preparation of Biodiesel Catalyzed by Solid Super Base of Calcium Oxide and Its Refining Process ZHU Huaping1, WU Zongbin2,*, CHEN Yuanxiong1, ZHANG Ping1, DUAN Shijie2, LIU Xiaohua1, MAO Zongqiang2,# 1
Faculty of Material Science and Chemical Engineering, China University of Geosciences, Wuhan 430074, Hubei, China
2
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
Abstract: Biodiesel produced by the transesterification of vegetable oils is promising alternative fuel to diesel because of the limited resources of fossil fuel and environmental concerns. An environmentally benign process was developed for the production of biodiesel from jatropha curcas oil using a heterogeneous solid super base catalyst, calcium oxide. The results showed that the base strength of calcium oxide was more than 26.5 after dipping in an ammonium carbonate solution followed by calcination. A study for optimizing the reaction conditions for the transesterification of jatropha curcas oil was performed. Under the optimum conditions of catalyst calcination temperature of 900oC, reaction temperature of 70oC, reaction time of 2.5 h, catalyst dosage of 1.5%, and methanol/oil molar ratio of 9:1, the oil conversion was 93%. The purification of the as-synthesized biodiesel with decalcifying agents to eliminate the remaining calcium was investigated. Citric acid showed good performance for the decalcification. The properties of the refined biodiesel accorded with the domestic and foreign standards. Key Words: solid super base; calcium oxide; heterogeneous catalysis; jatropha curcas oil; biodiesel; refining
Biodiesel, monoalkyl esters of fatty acids derived from vegetable oils or animal fats, is known as a clean and renewable fuel. Biodiesel is usually produced by the transesterification of vegetable oils or animal fats with methanol or ethanol. The reaction is commonly carried out in the presence of basic catalysts, including hydroxides of sodium or potassium, their carbonates or alkoxides, and so on. These catalysts dissolve in the polar reactants and facilitate the transesterification, thereby acting as homogeneous catalysts. The removal of these catalysts is technically difficult, thus increasing the operation cost. Therefore, the development of heterogeneous catalysts that can eliminate the additional running costs associated with the homogeneous catalysts is highly desired [1,2]. Being a heterogeneous basic catalyst, guanidine has the advantage of not producing soaps and thus allowing for an easy phase separation. A series of guanidine-containing catalysts have been studied by Schuchardt et al. [3,4]. The results showed that 1,5,7-triazabicyclo[4.4.0]dec-5-ene had a good performance in the transesterification of rapeseed oil. Because
of leaching, the catalyst could be used only nine times. Na/NaOH/γ-Al2O3 showed almost the same catalytic activity as NaOH under the optimum reaction conditions [5]. The Esterfip-HTM process involving a heterogeneous solid catalyst consisted of a mixed oxide of zinc and aluminium with a spinel structure, which was developed by the petroleum Institute of France [6]. The Esterfip-HTM process did not involve the neutralization and purification processes; therefore, the polluted water and the running costs were considerably reduced. At the same time, glycerol with the purity of over 98% was obtained directly. When commercial CaO was used as a catalyst, the conversion rate of vegetable oil achieved was up to 95.5% after 300 min of reaction [7]. KF/CaO showed a higher activity in transesterification of vegetable oil with methanol as compared with CaO. X-ray diffraction (XRD) and thermogravimetric/differential thermal analysis indicated that a new phase was formed after high-temperature calcination, which improved the performance of the catalyst [8]. Other solid basic
Received date: 2005-10-08. * Corresponding author. Tel: +86-10-89796077; E-mail:
[email protected] # Corresponding author. Tel: +86-10-62780537; E-mail:
[email protected] Foundation item: Suppoted by the Opening Fund of State Key Laboratory of Automotive Safety and Energy, Tsinghua University (KF2005-012). Copyright © 2006, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
ZHU Huaping et al. / Chinese Journal of Catalysis, 2006, 27(5): 391–396
catalysts such as MgO, ZnO, CeO2, and La2O3 were also studied, and the results confirmed that stronger basicity of the catalysts resulted in their higher activity for transesterification [9]. On the basis of the reported results, it can be concluded that the stronger basicity and the presence of more active sites improve the performance of the catalysts used for transesterification. After being dipped in an ammonium carbonate solution and calcination, CaO becomes a solid super base. In this article, the treated CaO whose basicity is more than 26.5 is adopted in the transesterification of jatropha curcas oil (JCO) with methanol. The catalyst functioned well for the target reaction.
were carried out in a flow of dry nitrogen gas. 1.3 Evaluation of catalytic activity of CaO catalyst 1.3.1 Average molecular weight of JCO The habitat and extracting methods of JCO alter its composition. As a key parameter, the average molecular weight must be measured. It can be calculated from the acid value (National Standard of China, GB 5530-85) and saponification value (GB 5530-85) of JCO [11]. The average molecular weight of JCO is calculated by M = 56.1 × 1000 × 3/(SV – AV), where AV is the acid value (mKOH/moil, mg/g) and SV is the saponification value (mKOH/moil, mg/g).
1 Experimental 1.3.2 Transesterification 1.1 Preparation of CaO catalyst A commercial calcium oxide (12 g) was dipped into a 0.12 g/ml ammonium carbonate solution. After being stirred for 30 min and after filtration, the solid was collected by centrifugation and desiccated at 110oC. Subsequently, the solid was milled to 60 mesh and calcined at high temperature for 1.5 h. After cooling to 250oC, the as-synthesized catalyst was kept in a desiccator. 1.2 Characterization of CaO catalyst Powder XRD patterns were recorded using a Bruker D8 ADVANCE X-ray diffractometer with Cu Kα radiation (λ = 0.15064 nm) operating at 40 kV and 30 mA. Scanning electron microscopy (SEM) images were obtained using HITACHI S-4500 microscope. BET specific surface area was measured by N2 adsorption at –196oC on a Micromeritics ASAP 2010 apparatus. Prior to the measurements, all the samples were outgassed at 300oC for 3 h. The basicity and distribution of basic sites were determined as follows [10]. Catalysts of 0.15 g were placed in the middle of a small glass tube (Φ 6 mm × 40 mm). Twelve such small tubes were placed into a large tube (Φ 40 mm × 200 mm) and evacuated at 450oC for 24 h. Subsequently, the large tube was cooled to ambient temperature in a flow of dry nitrogen gas. Cyclohexane was dropped slowly until all the small tubes were submerged. Then the small tubes were transferred into conical flasks quickly, and 0.1 mol/L benzoic acid was added using a burette in various dosages. The conical flasks were sealed and shaken at ambient temperature for 4 h. After neutralization, each sample was collected separately in five conical flasks in equal proportions. Two to three drops of indicators were added to each flask, and the flasks were kept at ambient temperature for 12 h. When the basic color disappeared, the volume of benzoic acid was recorded. All vessels were baked in vacuum prior to use and all the transfer processes
The catalyst and 40 ml of JCO were added into a 100 ml flask and stirred for 20 min, and then the temperature was raised to the desired reaction temperature. Subsequently, methanol was added through the constant press dropper. After the reaction, the solid catalyst was separated by centrifugation. The liquid was put into a separating funnel and was kept at ambient temperature for 4 h, after which two liquid phases appeared. The upper layer was biodiesel and the lower layer was glycerol. The conversion of JCO was calculated from the quantities of free glycerol and total glycerol in the product. The glycerol content was analyzed according to the literature method [12]. The transesterification is represented by the equation TG + 3CH3OH = 3ME + GL. In the reaction, 1 mol triglyceride (TG) is transformed into 3 mol methyl ester of fatty acid (ME). The molecular weight of 1 mol TG matches that of 3 mol ME. That is to say, the gross weight of JCO (m0) matches the total weight of methyl esters (m1). The weight of the unreacted TG (m2) can be calculated as m2 = M x / 92, where M is the average molecular weight of JCO, and x is the weight of glycerol (GL) corresponding to the unreacted TG. The conversion of JCO (X) is then calculated using the equation X = 1 – M x / 92 m1. 1.3.3 Decalcification of the as-synthesized biodiesel Twenty milliliters of as-synthesized biodiesel followed by the complexing agent were added into a 50 ml conical flask, and the mixture was stirred for 15 min. The product was centrifuged at 3000 r/min for 10 min. The upper clear liquid obtained was purified biodiesel. The quantity of calcium ions that remained in the biodiesel was analyzed using the spectrophotometric method with the Arsenazo(Ⅲ) indicator on a spectrophotometer [13,14]. The performance of the complexing agents was evaluated by determining the change in concentration of the calcium ions in
ZHU Huaping et al. / Chinese Journal of Catalysis, 2006, 27(5): 391–396
biodiesel before and after the decalcification. The decalcification ratio and biodiesel yield were calculated using the equations: decalcification ratio = (1 – remaining calcium ions / total calcium ions) × 100%, and biodiesel yield = (volume of the refined biodiesel / volume of the as-synthesized biodiesel) × 100%.
After calcination at 600, 850, 900, and 1000oC, the BET surface areas of the samples were all about 6 m2/g, whereas those calcined at 1100oC were 2 m2/g.
2 Results and discussion 2.1 Characterization results Commercial CaO contains calcite (Fig. 1). After the commercial CaO was immersed into an ammonium carbonate solution, the content of calcium carbonate increased. Calcination of the treated CaO at high temperatures resulted in the decrease of calcium carbonate content and it vanished completely when the calcination temperature was above 900oC. XRD patterns of CaO calcined at low temperatures were weak and broad, suggesting the existence of defects in the crystal structures. Calcination at higher temperatures led to intensified and narrowed XRD patterns of CaO, indicating the decrease of crystal defects in the treated CaO.
Fig. 2 SEM images of the purchased CaO (a) and CaO calcined at 1100 oC (b)
Indicators 4-chloraniline, aniline, triphenylmethane, and diphenylmethane showed their basic color on the solid CaO catalyst, which confirmed that the basicity of the catalyst was over 26.5. Therefore, the treated CaO was a solid super base. The distribution of the basic sites on the catalyst is listed in Table 1. Table 1 Basic strength distribution of the CaO catalyst Basic strength
Basicity（mmol/g）
17.2–18.4
0.010
18.4–26.5
0.020
26.5–33.0
0.025
Over 33.0
0.020
2.2 Effect of preparation parameters on JCO conversion 2.2.1 Average molecular weight of JCO
Fig. 1 XRD patterns of CaO catalysts calcined at different temperatures (1) Purchased CaO, (2) 600oC, (3) 850 oC, (4) 900 oC, (5) 1000 oC, (6) 1100 oC
The commercial CaO presented an irregular shape (Fig. 2(a)). After treatment, the shape was still irregular, but the morphologies became similar to each other, and the grains were sintered (Fig. 2(b)). The samples obtained at different calcination temperatures had a similar specific surface area.
On the basis of the measured acid value of 0.42 and the saponification value of 188.75, the average molecular weight of JCO was calculated to be 893.6. In this study, the average molecular weight of JCO was taken as 900. The acid and saponification values of JCO were small. The acid value was less than 1.0, which suggested that the concentration of free fatty acids in JCO was low. Accordingly, the free fatty acids in JCO had little influence on the yield and quality of biodiesel. The low saponification value confirmed the high concentration of triglycerides in JCO. Therefore, JCO is a good raw material for the preparation of biodiesel. 2.2.2 Orthogonal test The L16(45) orthogonal table was designed to investigate the influence of preparation parameters, namely the reaction temperature, reaction time, dosage of the catalyst, and molar ratio of methanol to oil, on the conversion of JCO. The results are listed in Table 2.
ZHU Huaping et al. / Chinese Journal of Catalysis, 2006, 27(5): 391–396
According to the range, the order of influence is catalyst dosage > reaction time > reaction temperature > molar ratio of methanol to JCO. The dosage of the catalyst is the most notable factor, whereas the reaction time and temperature have a similar effect. When the molar ratio of methanol to oil is over 6:1, the amount of methanol has little effect on the conversion of JCO. Table 2 Orthogonal test results of transesterification of jatropha curcas oil (JCO) Catalyst
n(methanol)/
dosage (%)
n(JCO)
1.0
0.5
6:1
82.7
55
1.5
1.0
9:1
83.8
55
2.5
1.5
12:1
91.0
4
55
3.5
2.0
15:1
90.0
5
60
1.0
1.0
12:1
81.9
6
60
1.5
0.5
15:1
87.8
7
60
2.5
2.0
6:1
92.3
8
60
3.5
1.5
9:1
91.6
9
65
1.0
1.5
15:1
89.8
10
65
1.5
2.0
12:1
85.9
11
65
2.5
0.5
9:1
91.8
12
65
3.5
1.0
6:1
85.8
13
70
1.0
2.0
9:1
90.8
14
70
1.5
1.5
6:1
92.3
15
70
2.5
1.0
15:1
87.4
16
70
3.5
0.5
12:1
92.6
Optimum
70
2.5
1.5
9:1
No.
θ/ oC
t/h
1
55
2 3
Ⅰ
347.5
345.2
354.9
353.1
Ⅱ
353.6
349.8
338.9
358.0
Ⅲ
353.3
362.5
364.7
351.4
Ⅳ
363.1
360.0
359.0
355.0
R
15.6
17.3
25.8
6.6
X(JCO)/%
Fig. 3 Influence of reaction temperature (a) and reaction time (b) on the transesterification of JCO (n(methanol):n(JCO) = 9:1, catalyst dosage 1.5%, (a) reaction time 2.5 h; (b) reaction temperature 70oC)
2.2.4 Catalyst dosage The effect of the catalyst dosage on the conversion of JCO was examined. The results showed that the conversion was over 83% with the catalyst dosage from 0.5% to 2.5%. The dosage of 1.5% peaked the conversion. When the dosage of the catalyst was too much, more products were adsorbed, and the yield of biodiesel decreased.
R is the value of the minimum total conversion subtracted from the maximum total conversion.
2.2.3 Reaction temperature and time As shown in Fig. 3(a), raising the reaction temperature favored the transesterification. However, for the reaction system that operated at ambient pressure, it is impossible to raise the reaction temperature further. Therefore, the reaction at 70oC is appropriate, at which methanol keeps boiling. Because it is a heterogeneous reaction and the mass transfer is slow, no reaction was observed within 30 min. The conversion was low in the first 1 h, however, the conversion increased rapidly afterwards and reached about 92% quickly, as can be seen in Fig. 3(b). The reaction reached equilibrium after 2.5 h with a conversion of 93%. Too long reaction time resulted in the appearance of a white gel in the product, which increased the viscosity of the product and affected the purification process.
2.2.5 Molar ratio of methanol to oil Theoretically, raising the molar ratio of methanol to oil favors the reaction. But excessive methanol is not favorable for the purification of the as-synthesized biodiesel. In addition, much energy is needed to recover the large amount of unreacted methanol. Moreover, methanol can increase the dissolution of JCO, intermediates, and biodiesel, resulting in the wastage of the materials. Therefore, the molar ratio of methanol to oil of 9:1 is appropriate. 2.2.6 Calcination temperature of the catalyst To form super basic sites on calcium oxide, the adsorbed water and carbon dioxide must be removed. At the same time, Ca2+ and O2− should be in a certain coordination state on the surface of calcium oxide [15]. After calcination at 850oC, the catalyst showed high activity for the transesterification of
ZHU Huaping et al. / Chinese Journal of Catalysis, 2006, 27(5): 391–396
JCO, as seen in Fig. 4. This result agreed well with those reported previously. Calcium carbonate decomposed at 850oC and produced calcium oxide that had many defects in its crystal structure [16]. The defects favored the formation of calcium methyloxide, which is a surface intermediate in the transesterification. When the calcination temperature was over 900oC, the catalytic activity gradually decreased. The result of nitrogen adsorption showed that the BET surface area of the catalysts obtained at different calcination temperatures did not change significantly. That is to say, the decrease in the catalytic activity did not result from the change of the specific surface area, but from the transformation of the crystal structure, which became more intact with the rise in calcination temperature. Meanwhile, the coordination of Ca2+ and O2− on the surface of calcium oxide changed, which varied the amount and strength of the basic sites, and hence the catalytic activity [15].
2.3 Decalcification of as-synthesized biodiesel When a basic catalyst was used to prepare biodiesel, some cations remained in the products. Usually, an acidic aqueous solution was adopted to remove the cations and other polar compounds from the biodiesel. On analyzing the effect of water washing methods, some conclusions could be drawn as follows. (1) A large amount of water is needed. Usually, the volume ratio of oil to water should be 2:1–5:1, which is inevitable to produce a great deal of polluted water. (2) The operability of the refining process is poor. When the stirring speed is too low, the effect of the refining is not obvious; contrarily, too high stirring speed leads to emulsification, which decreases the yield of the product. (3) A large amount of the product is lost. Usually, the yield of biodiesel is about 70%. In our case, when only water was used to wash away the impurity, the decalcification rate is about 50%, and the quality of the product cannot meet the standard of biodiesel. Therefore, the water washing method is not suitable for our process. Our strategy for purifying the as-synthesized biodiesel is to employ complexing agents to remove calcium ions. Three complexing agents were tested for decalcification under the same experimental conditions. The results are listed in Table 3. Table 3 Decalcification efficiency of different agents for biodiesel
Fig. 4 Influence of calcination temperature of the CaO catalyst on the transesterification of JCO (n(methanol):n(JCO) = 9:1, catalyst dosage 1.5%, reaction temperature 70oC, reaction time 2.5 h)
On the basis of the above results, the optimum reaction conditions are the catalyst calcination temperature of 900oC, reaction temperature of 70oC, catalyst dosage of 1.5%, and molar ratio of methanol to oil of 9:1. Under these conditions, the reaction was repeated three times with the recovered catalyst, and the conversion of JCO was above 92%. This confirmed that the catalyst could be reused without an obvious decrease in catalytic activity. After the reaction was carried out for 1.5 h under the optimum conditions, the reaction temperature was reduced rapidly and the solid catalyst was separated from the mixture of the reaction. Subsequently, the reaction temperature was raised again and kept for 1 h. The conversion did not increase obviously. This elucidated that calcium ions that remained in the biodiesel have no catalytic function. Further research to investigate the heterogeneous behavior of the treated CaO is underway.
Decalcifying
Dosage
Remaining
Decalcification
agent
(g)
Ca2+ (µg/ml)
efficiency (%)
Yield (%)
Water
10.00
500.00
58.33
69.45
Oxalic acid
0.0540
93.83
92.18
90.65
Citric acid
0.1153
45.20
96.23
95.45
EDTA
0.1753
193.75
83.85
92.31
EDTA—Ethylenediaminetetraacetic acid. Reaction conditions: n(agent):n(calcium) = 1:1, V(JCO):V(water) = 10:1, reaction temperature 45oC, reaction time 20 min.
The water washing method can only remove half the calcium ions in the as-synthesized biodiesel containing 1.200 mg/ml of Ca2+, and the yield of the purified biodiesel is low. In contrast, the method with the complexing agent shows higher decalcification efficiency and yield of the purified biodiesel. During the experiments, it was observed that when more water was used, the yield of the purified biodiesel obtained was less. The molar ratio of the complexing agent to the remaining calcium ions had a great influence on the decalcification efficiency. When the ratio was 1:1, the decalcification efficiency of oxalic acid and citric acid was over 90%. Halving the dosage of the complexing agent decreased the decalcification efficiency by half. However, the decalcification efficiency did not change obviously in the case of using the complexing agents when the dosage of water was reduced. Therefore, for high decalcification efficiency, the ratio of the complexing agent to the remaining calcium ions should not be
ZHU Huaping et al. / Chinese Journal of Catalysis, 2006, 27(5): 391–396
decreased, and the water dosage could be reduced further when the complexing agents were used. In addition, water-washing purification formed emulsion, which hindered the separation of biodiesel from the mixture. Oxalic acid disperses in biodiesel easily and is therefore separated with difficulty from the mixture, resulting in easy formation of emulsion after infusion with water. Citric acid does not disperse readily in biodiesel because of agglomeration, but it can be eliminated easily from biodiesel and does not lead to emulsion after infusion with water. EDTA is hard
to dissolve in water, which limits its application. In summary, citric acid is the best decalcifying agent. Several key properties of the purified biodiesel have been characterized, and the results are shown in Table 4. Most of the properties of the purified biodiesel meet the criteria of DIN E 51606 and ASTM PS 121-99, except for the slightly higher viscosity. To improve the practicability of our process, raw JCO was used directly without employing degumming and dehydrating treatment. Therefore, a further decrease in viscosity is desirable.
Table 4 Comparison of the properties of purified biodiesel with international standards Item This study Standard
Density
Viscosity
Flash
Coking
Sulfate
Sulfur content
Cetane
(kg/m3)
(mm2/s)
point (oC)
value (%)
ash (%)
(%)
number
Method
SH/T 0604
GB/T 265
GB/T 261
Result
878.4 (20oC)
7.320 (20oC)
> 170
< 0.05
< 0.005
DIN E 51606 (Germany)
875–900 (15oC)
3.5–5.0 (40oC)
> 110
< 0.05
< 0.03
< 0.01
> 49
ASTM PS 121-99 (USA)
–
1.9–6.0
> 100
< 0.05
< 0.02
< 0.05
> 40
3 Conclusions
GB/T 17144 GB/T 2433
SH/T 253-92
–
0.0036
56.1
109(1): 37 [5] Kim H-J, Kang B-S, Kim M-J, Park Y M, Kim D-K, Lee J-S,
Being treated with an ammonium carbonate solution and calcinated at high temperature, calcium oxide becomes a solid super base, which shows high catalytic activity in transesterification. The optimized reaction conditions have been obtained by the orthogonal test. Under the optimum conditions, the conversion of JCO reaches 93%. Three kinds of complexing agents have been studied for decalcification of the as-synthesized biodiesel and critic acid shows good performance. The biodiesel produced by this process claims to exhibit good properties. The whole process is simple and repeatable, and seems promising for potential application.
Lee K-Y. Catal Today, 2004, 93–95: 315 [6] Bournay L, Casanave D, Delfort B, Hillion G, Chodorge J A. Catal Today, 2005, 106(1–4): 190 [7] Yan J. Machinery Cereals Oil Food Process, 2005, (8): 47 [8] Meng X, Xin Zh. Petrochem Technol, 2005, 34(3): 282 [9] Bancquart S, Vanhove C, Pouilloux Y, Barrault J. Appl Catal A, 2001, 218(1–2): 1 [10] Cai Zh Q, Wu G Y, Lin X P, Zhu Ch, Liu L L. China Oils Fats, 2004, 29(8): 29 [11] Wang X, Cui Ch Y, Tan N D. Jihua Sci Technol, 1997, 5(3): 5 [12] Liu W W, Su Y Y, Zhang W D, Liu Sh Q, Xia Ch F. Renewable Energy, 2005, 121(3): 14
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