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A new solid base catalyst for the transesterification of rapeseed oil to biodiesel with methanol
Fuel 104 (2013) 698–703
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A new solid base catalyst for the transesterification of rapeseed oil to biodiesel with methanol Boyang Wang a,b, Shufen Li a,⇑, Songjiang Tian a, Rihua Feng a, Yonglu Meng a a b
Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China College of Pharmacy and Tianjin Key Laboratory of Molecular Drug Research, Nankai University, Tianjin 300071, China
h i g h l i g h t s " A new solid catalyst containing Ca12Al14O33 and CaO was synthetically prepared. " The catalyst has good activity with ME (methyl ester) content reached 90%. " The catalyst can be used for at least 7 cycles with the ME content over 87%. " The catalyst can be separated from reaction system effectively and easily. " This study may provide benefits for developing a continuous biodiesel process.
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Article history: Received 31 December 2011 Received in revised form 9 August 2012 Accepted 15 August 2012 Available online 13 September 2012 Keywords: Solid base catalyst Biodiesel Rapeseed oil Transesterification
a b s t r a c t A new solid base catalyst containing Ca12Al14O33 and CaO was prepared by chemical synthesis method and was used in the transesterification of rapeseed oil with methanol to produce biodiesel. The catalyst was characterized by Hammett indicators, X-ray diffraction (XRD), infrared spectroscopy (IR), Brunauer– Emmett–Teller (BET) and scanning electron microscopy (SEM), separately. The effect of various factors was investigated to optimize the reaction conditions. The results showed that the ME (methyl ester) content reached 90% after reacting for 3 h at 65 °C, with a methanol/oil molar ratio of 15:1, the amount of catalyst of 6 wt.% and the stirring rate of 270 rpm. Moreover, the catalyst could be used repeatedly for at least 7 cycles with the ME content over 87%, due to its high stability. In particular, this solid base catalyst can be separated from reaction system effectively and easily, as it is insoluble in both methanol and methyl esters, which may provide significant benefits for developing an environmentally benign and continuous process for synthesizing biodiesel. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The research on application of alternative renewable fuels has attracted tremendous attention in recent years due to the fear for the imbalance between diminishing fossil fuel reserves and soaring global energy demands. Many strategies and methods have been developed, in which the transesterification of vegetable oils or animal fats with short carbon-chain alcohols (methanol or ethanol) to produce a mixture of fatty acids esters (known as biodiesel) has great importance because biodiesel possesses many advantages over conventional fossil fuels, including proper lubricity, good biodegradability, excellent combustion efficiency and low toxicity [1–5]. Therefore, biodiesel can be employed for a clean substitute for fossil fuel without any modification to diesel engines, boilers or other combustion equipments [6–8].
⇑ Corresponding author. Tel./fax: +86 22 87894252. E-mail address: shfl[email protected] (S. Li). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.08.034
Two kinds of catalysts, homogeneous and heterogeneous catalysts, have been used in the transesterification reaction [9,10]. The conventional process of biodiesel is the transesterification of oils with methanol or ethanol in the presence of homogeneous catalysts, such as NaOH and KOH, which are the catalyst precursors. The real catalysts may be Na or K ethoxydes. However, homogeneous base catalytic systems have some basic technological problems, such as the production of wastewater to a serious threat for environment and the difficulty for removal of the base catalysts after reaction [11–14], although homogeneous base processes are relatively fast and show high conversions. Recently, solid base catalysts have been increasing attention as substitutes for the highly pollutant liquid homogeneous catalysts to produce biodiesel, as they generally do not generate large amount of wastewater. For example, KF/c-Al2O3 and Na/NaOH/ c-Al2O3 are highly active in the transesterification of vegetable oils [14,15]. However, the active ingredients of most of these supported alkali catalysts are sensitive to moisture and easily corroded by short carbon-chain alcohols, resulting in reducing lifetime of the
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catalysts and therefore limiting their industrial applications [16,17]. Another solid base catalyst of CaO has also been investigated and found that it has high activity and relative long lifetime [18–21]. However, the separation of CaO catalyst from biodiesel product after the reaction is very difficult attributed to gel phenomenon in the lower glycerol layer [22–24]. Therefore, it is critical to develop a novel solid catalyst that not only possesses exceptional catalytic activity and high stability, but also is easily separable from the transesterification product of biodiesel. In this study, a novel solid base catalyst was prepared. The physical and chemical characterizations of the catalyst with some instrumental methods were investigated. It was used to catalyze transesterification of rapeseed oil with methanol for the synthesis of biodiesel, and the effects of various factors were studied to optimize the reaction conditions. 2. Experimental 2.1. Materials Rapeseed oil was purchased from Xingwang Oil Corporation (Guanghan, Sichuan, China). Analytical reagents using as standards for gas chromatograph (GC) were purchased from Sigma Chemical Corporation, USA. Other chemicals were analytical reagents (AR) and purchased from Kewei Reagent Corporation, Tianjin, China. 2.2. Preparation of catalyst The solid base catalyst was prepared by chemical synthesis methods as follows. (1) Preparation of (Ca3Al2O66H2O).
hydrated
tricalcium
aluminate
Sodium aluminate (NaAlO2) aqueous solution and calcium hydroxide (Ca(OH)2) solution were mixed in a 1000 ml beaker. The mixture was stirred vigorously at 80 °C for 3 h. After the mixture was cooled to room temperature, the solid precipitate was collected by filtration. The solid was washed with deionized water until the pH of the washing solution maintained between 7 and 8 to give product hydrated tricalcium aluminate (Ca3Al2O66H2O), in which the reaction is as the following equation:
3CaðOHÞ2 þ 2NaAlO2 þ 4H2 O ! Ca3 Al2 O6 6H2 O þ 2NaOH
recorded at room temperature in the range of 400–3500 cm1, with 300 scans and 4 cm1 resolution. The KBr pellet technique was applied for determining IR spectra of the samples. The BET surface area of the catalysts was measured by the nitrogen adsorption method at 196 °C using a Tristar 3000 instrument (Micromeritics, Norcross, GA, USA). The surface morphology of the catalyst was investigated by a XL30 SEM (Philips, Eindhoven, North Brabant, Netherlands). SEM was operated at 20.0 kV of an accelerating voltage. 2.4. Reaction procedures The transesterification reactions were carried out in a 150 ml four-neck glass flask equipped with a stirrer, a water cooling condenser and an electric jacket with thermocouple. Firstly, 45 g of rapeseed oil was poured into the reactor with an amount of methanol according to desired molar ratio of methanol to rapeseed oil. The molar ratio of methanol to rapeseed oil (Nmethanol:oil) and average relative molar mass of rapeseed oil ðM oil Þ were calculated [27] as follows:
Nmethanol:oil ¼
Moil ¼
mmethanol Moil moil M methanol
MKOH 1000 3 SV AV
ð2Þ
ð3Þ
where mmethanol is the mass of methanol, g; moil is the mass of rapeseed oil, g; Mmethanol is molar mass of methanol, g mol1, SV is saponification value of rapeseed oil, mg KOH g1; AV is acid value of rapeseed oil, mg KOH g1; MKOH is molar mass of KOH, g mol1. After the mixture was stirred and heated to setting temperature (30–65 °C), the catalyst (2–8 wt.% relative to rapeseed oil) was added to the reaction system. The reaction was carried out under a stirring rate of 60–360 rpm. Very small amount of samples (0.5 ml) were collected by drip tube each time from the reaction mixture at the interval of 1 h for analysis to calculate the ME content. After the reaction was completed, the catalyst was removed from the mixture by filtration, and then it was washed by methanol. The catalyst was dried and then was kept in a desiccator for future reuse. The filtrated solution was further separated by a batch distillation to steaming out the excessive methanol. Next, a standard layering process for the residual liquid was required to obtain product of biodiesel and by-production of glycerol.
ð1Þ
(2) Generation of the solid base catalyst. The hydrated tricalcium aluminate was then calcined at 1000 °C for 8 h in a muffle furnace. The catalyst was cooled down to 400 °C, then it was extracted from the muffle furnace and kept in a desiccator for future use. 2.3. Catalyst characterization Hammett indicators, XRD, IR, BET, and SEM techniques were separately used to characterize the prepared catalyst. Base strength of catalysts (H_) and base total amount of catalysts were measured by using Hammett indicators [25,26], including bromthymol (H_ = 7.2), phenolphthalein (H_ = 9.8), 2, 4-dinitroaniline (H_ = 15.0) and 4-nitroaniline (H_ = 18.4). X-ray diffraction (XRD) measurements were performed with a PANAlytical X’Pert diffractomer using Co Ka radiation (k = 0.179 nm) at 40 kV and 40 mA. Data were collected over a 2h range of 10–110° with a step size of 0.02° at a scanning speed of 5 min1. IR measurements were performed with a Nicolet 560 spectrometer (Nicolet, Madison, Wisconsin, USA). The infrared spectra were
2.5. Analysis As the products in the methyl ester phase consist of methyl esters, monoglycerides, diglycerides and unreacted triglycerides, the amount of methyl esters (i.e. biodiesel) produced was calculated based on an internal standard using methyl salicylate as a reference standard. The methyl ester (ME) content then is calculated by the following expression which is similar to Ref. [28]:
ME content ðwt:%Þ ¼
Calculated weight of methyl esters Weight of methyl ester phase
100% P fester Aester mreference 100% Areference mesters
ð4Þ
where mreference is the mass of internal standard, g; mesters is the mass of methyl esters, g; Areference is the area of internal standard; Aester is the area of methyl ester; fester is correction factor of methyl ester. The methyl esters samples were analyzed in an Sp-2100 gas chromatograph equipped with a flame ionization detector and a capillary column H.J. PEG-20 M (30 m 0.32 mm 0.5 lm).
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Approximate 0.06 g (three dripping from a glass pipette) of each sample and 0.02 g of internal standard were dissolved in 0.3 g ethyl acetate for GC analysis. Samples (0.4 lL) were injected by a sampler at an oven temperature of 130 °C. After an isothermal period of 2 min, the GC oven was heated at 10 °C/min to 240 °C, and held for 7 min. Nitrogen was used as carrier gas. The injector temperature and detector temperatures were 180 °C and 280 °C, respectively. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. Base strength and base total amount of the catalyst Different catalysts were tested in the transesterification reaction under the same reaction conditions. The results (shown in Table 1) indicated that the activities of the catalysts corresponded to base strength and base total amount of the catalysts. The ME content is low for catalysts with base strength (H_) in the range of 9.8–15.0, such as K2CO3/c-Al2O3 and KNO3/c-Al2O3. In addition, for catalysts with the similar base strength, such as the new catalyst prepared in this paper and NaAlO2, the one with higher base total amount showed stronger catalytic activity, although the base amount has not linear relationship with the catalytic activity. However, the base strength and base total amount could help us to select the suitable solid base catalyst. Among the solid base catalysts tested, the new solid base catalyst (containing Ca12Al14O33 and CaO from Fig. 2) had the strongest base strength and largest base total amount and in turn gave the highest ME content. Therefore, the new solid base catalyst containing Ca12Al14O33 and CaO was selected and further studied as the catalyst for transterification of rapeseed oil to biodiesel. 3.1.2. Infrared analysis of the catalyst The investigation of the new solid base catalyst sample was conducted via IR spectroscopy, as shown in Fig. 1. A small and broad absorption peak of O–H stretching vibration at approximately 3430 cm1 and a very minor absorption peak of O–H bending vibration at about 1630 cm1 were assigned to H2O molecule absorbed from air [29]. Additionally, two bands at 1050 and 577 cm1 were probably attributed to absorption peak of Al–O vibration [30]. An intense absorption peak at around 459 cm1 attributed to the absorption peak of Ca–O stretching vibration was observed [31]. Meanwhile, the intense and broad absorption peak at 839 cm1 was assigned to stretching vibration of the Al–O bond in the tetrahedron (AlO4) [32]. It was clear that Ca and Al oxides formed during the calcination procedure of hydrated tricalcium aluminate at the temperature of 1000 °C in the muffle furnace. 3.1.3. XRD analysis of the catalyst X-ray diffraction pattern of the new solid base catalyst was given in Fig. 2. The several diffraction peaks corresponding to
Ca12Al14O33 (from Powder Diffraction File, 00-009-0413) phase remained with the presence of few peaks related to CaO when the calcination was performed at 1000 °C. Therefore, a synergetic effect between Ca12Al14O33 particles and CaO particles may occur in the solid base catalyzed transesterification. 3.1.4. SEM analysis of prepared catalyst Scanning electron microscopy images of the new solid base catalyst were shown in Fig. 3. The catalyst is comprised from the union of many crystals. There are many cracks (Fig. 3a) with sizes of micrometers and relatively large pores (Fig. 3b) among these agglomerates. Therefore, triglyceride and methanol may fully contact with the surface of the catalyst to synthesize biodiesel, resulting in better activity of the catalyst. From Fig. 3a, these agglomerates congregate to form large particle of the catalyst within range of 5–50 lm. As the size of the catalyst particle is big enough and is insoluble in both methanol and methyl esters, it can be effectively and easily separated from the products by filtration and centrifugation after the reaction. Many researchers only employed CaO as the catalyst for the transesterification to synthesize biodiesel [18–21]. However, it is common that gel phenomenon in the lower layer glycerol appears after the reaction, resulting in increasing the difficulty of separation of CaO catalyst [22–24]. But the separation effect of the new solid base catalyst containing Ca12Al14O33 and CaO from reaction system is obviously better than that of CaO when they are used as catalysts for synthesizing biodiesel. It is very possible that the great improvement of the separation effect can be attributed to the existence of the Ca12Al14O33 phase, which may promote the separation of the new solid base catalyst from reaction system. 3.1.5. BET analysis of the catalyst The BET analysis of the prepared catalyst reveals that the specific surface area of the new solid base catalyst is 5.78 m2/g. And the base total amount of the new solid base catalyst is 23.59 mmol g1.As Guo et al. indicated, the low surface area and high density of base site suggest that most base sites are in the interior of solid base catalyst [33]. It can be found from Fig. 3b that the surface of the catalyst has many pores, the size of which is among 0.5 lm and 1.8 lm. Hence, it is possible that the reactants diffused into the interior of the catalyst pores to make triglyceride (rapeseed oil) and methanol contacting with more base sites where they undergo transesterification reaction. 3.2. Optimization of reaction conditions In this study, the new prepared solid base catalyst was employed to catalyze the transesterification of rapeseed oil with methanol to produce biodiesel. The variables affecting the transesterification, such as methanol-to-oil molar ratio (9:1–18:1), catalyst amount (2.0–8.0 wt.% of oil), reaction temperature (30–65 °C), stirring rate (60–360 rpm) and reaction time (1–6 h), were investigated.
Table 1 Base strength and base total amount of the catalysts as well as the ME content when they were used in the transesterification of rapeseed oil with methanol to produce biodiesel.
a–e c–e
Catalyst
Base strength (H_)
Base total amount (mmol g)
ME content (%)
New solid base catalyst containing Ca12Al14O33 and CaOa NaAlO2b K2CO3/c-Al2O3c KNO3/c-Al2O3d KCl/c-Al2O3e
15.0 < H_ < 18.4 15.0 < H_ < 18.4 9.8 < H_ < 15.0 9.8 < H_ < 15.0 H_ < 7.2
23.59 11.40 0.46 0.05 0
84.15 80.32 53.77 <10 0
Reaction conditions: methanol/oil molar ratio, 15:1; catalyst amount, 2 wt.%; temperature, 65 °C; stirring rate, 270 rpm; reaction time, 6 h. K2CO3/c-Al2O3, KNO3/c-Al2O3, KCl/c-Al2O3 were prepared by the immersion method.
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Transmittance (a. u.)
B. Wang et al. / Fuel 104 (2013) 698–703
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1) Fig. 1. IR spectra of the new prepared solid base catalyst samples.
3.2.1. Effect of mole ratio of methanol/oil From the reaction formula, the stoichiometric molar ratio of methanol to rapeseed oil is 3:1. However, the transesterification is reversible; therefore excess methanol is required to force the equilibrium towards completion to produce more methyl esters. The results of the effect of mole ratio of methanol/oil on the ME content are shown in Fig. 4. It was found that, with increasing molar ratio of methanol to rapeseed oil, resulting in an increase in the reaction rate of transesterification of rapeseed oil with methanol, the high ME content was observed. All ME content exceed 86.0% at the reaction time of 3 h when the molar ratios become higher than 9:1. When the molar ratio reaches 15:1, the ME content reaches 90% after 3 h reaction. Further increase in molar ratio has no effect on the ME content, but increases the energy consumption for recovering un-reacted methanol. In addition, excessive amount of un-reacted methanol could dissolve in by-product glycerin, making the separation more difficult. Therefore, the appropriate molar ratio of methanol/oil should be 15:1. 3.2.2. Effect of catalyst amount The effect of catalyst amount on the ME content in the transesterification of rapeseed oil at 65 °C with 15:1 M ratio of methanol/oil and 270 rpm stirring rate was studied, shown in Fig. 5. The results showed that the ME content increased along with the increase of catalyst amount from 2.0 to 6.0 wt.%. However, the increase of catalyst amount from 6.0 wt.% to 8.0 wt.% had little im-
Fig. 3. SEM images of the new prepared solid base catalyst.
pact on the ME content, which indicated that the equilibrium of the transesterification reaction had reached with 6.0 wt.% of catalyst under the conditions studied. Therefore, the optimum amount of catalyst is 6.0% by weight of oil. 3.2.3. Effect of reaction temperature As shown in Fig. 6, the ME content is only 5.25% at 30 °C after 3 h reaction, while it reaches the maximum value at 65 °C. Obvi-
Fig. 2. XRD patterns of the new prepared solid base catalyst samples.
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100
80
Mole ratio of methanl/oil 9: 1 12: 1 15: 1 18: 1
60
40
Methyl ester content (%)
Methyl ester content (%)
100
80
40 20 0
0
1
2
3
4
5
6
Stirring rate 0 rpm 60 rpm 180 rpm 270 rpm 360 rpm
60
7
0
1
2
Fig. 4. Effect of methanol/oil molar ratio on the ME content at 65 °C, with 6 wt.% catalyst and 270 rpm stirring rate for 6 h.
Methyl ester content (%)
5
6
7
Fig. 7. Effect of stirring rate on the ME content at 65 °C, with 15:1 M ratio and 6 wt.% catalyst for 6 h.
ter the ME content. Hence, a stirring rate of 270 rpm is sufficient for the transesterification reaction and thus recommended.
100 80
Catalyst amount (relative to oil weight) 0 wt% 2 wt% 4 wt% 6 wt% 8 wt%
60 40 20
0
1
2
3
4
5
6
3.2.5. Effect of reaction time The data from Figs. 4 and 5 indicate that the ME content increases steadily between the reaction time of 1 h and 3 h, and thereafter remains nearly constant as a result of reaching the equilibrium. Thus, the suitable reaction time is 3 h under optimized reaction conditions. 3.3. Reusability of the catalyst
7
Reaction time (h) Fig. 5. Effect of the amount of catalyst on the ME content at 65 °C, with 15:1 M ratio and 270 rpm stirring rate for 6 h.
ously, the reaction rate is faster at high temperature than at low temperature. Therefore, the suitable reaction temperature for the transesterification of rapeseed oil to biodiesel is 65 °C, the boiling point of methanol under atmosphere pressure. 3.2.4. Effect of stirring rate Due to the presence of the new solid base catalyst, the reaction system forms three-phase (oil–methanol–catalyst). The reaction may become diffusion-controlled when there is no agitation or the stirring rate is not high enough [34]. The ME content versus reaction time at different stirring rates are shown in Fig. 7. The results indicate that the transesterification reaction is incomplete with a stirring rate at 60 rpm for the given reaction time. The ME content at 270 rpm reaches 90% after 3 h reaction, while further increasing of the stirring rate to 360 rpm does not significantly al-
Since reusability is one of the most important features of a heterogeneous catalyst for its commercialization, we reused the catalysts to test the lifetime and stability of the catalyst. The used catalyst was pretreated as described in Section 2.4. After the filtration of the catalyst, the recovery was not of 100%. It was re-weighed and then added to the reaction system according to investigated optimal amount. Fig. 8 summarizes the experimental results, which indicate that the all ME content with reused catalysts all exceeds 87% at 65 °C, with a 15:1 M ratio of methanol to rapeseed oil, 6 wt.% of catalyst, 270 rpm of stirring rate and 3 h reaction time. The new solid base catalyst sustains its activity even after being used for 7 cycles with no decrease in the ME content observed, indicating that the catalyst has good stability. 4. Conclusions In this study, a novel solid base catalyst which contains Ca12Al14O33 and CaO is prepared in simple steps and is inexpensive. The physical and chemical characterizations of the prepared
100
100
80 60
Reaction temperature
40 20 0 0
1
2
3
4
5
6
7
Reaction time (h) Fig. 6. Effect of reaction temperature on the ME content with 15:1 M ratio, 6 wt.% catalyst and 270 rpm stirring rate for 6 h.
Methyl ester content (%)
Methyl ester content (%)
4
Reaction time (h)
Reaction time (h)
0
3
80 60 40 20 0 0
1
2
3
4
5
6
7
Number of repeated times Fig. 8. Effect of repeated use of the new prepared solid base catalyst on the ME content at 65 °C, with 15:1 M ratio, 6 wt.% catalyst and 270 rpm stirring rate for 3 h.
B. Wang et al. / Fuel 104 (2013) 698–703
catalyst with some instrumental methods were investigated. The experimental results show that this solid base catalyst has excellent catalytic activity and outstanding stability in the transesterification of rapeseed oil with methanol to produce biodiesel. Meantime, this solid base catalyst, which is insoluble in both methanol and methyl esters, can be separated from reaction system effectively and easily. Therefore, the use of the solid base catalyst prepared in this paper in the transesterification reaction provides significant benefits for developing an environmentally benign and continuous process for synthesizing biodiesel. The optimal transesterification conditions are obtained as follows: methanol/oil molar ratio 15:1, the amount of catalyst 6 wt.%, reaction temperature 65 °C, the stirring rate of 270 rpm and reaction time of 3 h. The ME content is 90% under the optimal conditions. Moreover, the catalyst is used repeatedly for at least 7 cycles with sustained activity and without decreasing the ME content, which sufficiently shows its good stability. Acknowledgment The work has been supported by the National Natural Science Foundation of China (Nos. 20776107, 21104035 and 21176186). References [1] Gerpen JV. Biodiesel processing and production. Fuel Process Technol 2005;86:1097–107. [2] Holser RA, O’Kuru RH. Transesterified milkweed (Asclepias) seed oil as a biodiesel fuel. Fuel 2006;85:2106–10. [3] Ramadhas AS, Jayaraj S, Muraleedharan C. Biodiesel production from high FFA rubber seed oil. Fuel 2005;84:335–40. [4] Dorado MP, Ballesteros E, Arnal JM, Gomez J, Lopez FJ. Exhaust emissions from a diesel engine fueled with transesterified waste olive oil. Fuel 2003;82:1311–5. [5] Schuchardt U, Sercheli R, Vargas RM. Transesterification of vegetable oils: a review. J Brazil Chem Soc 1998;9:199–210. [6] Canakci M. The potential of restaurant waste lipids as biodiesel feed stocks. Bioresource Technol 2007;98:183–90. [7] Ma F, Hanna MA. Biodiesel production: a review. Bioresource Technol 1999;70:1–15. [8] Meher LC, Vidya SD, Naik SN. Technical aspects of biodiesel production by transesterification – a review. Renew Sustain Energy Rev 2006;10:248–68. [9] Dossin TF, Reyniers MF, Marin GB. Kinetics of heterogeneously MgO-catalyzed transesterification. Appl Catal B: Environ 2006;61:35–45. [10] Lopez DE, Goodwin JG, Bruce DA, Lotero E. Transesterification oftriacetin with methanol on solid acid and base catalysts. Appl Catal A–Gen 2005;295:97–105. [11] Freedman B, Pryde EH, Mounts TL. Variables affecting the yields of fatty esters from transesterified vegetable oils. J Am Oil Chem Soc 1984;61:1638–43. [12] Zhang Y, Dube MA, MacLean DD, Kates M. Biodiesel production from waste cooking oil: 1. Process design and technological assessment. Bioresour Technol 2003;89:1–16.
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A new solid base catalyst for the transesterification of rapeseed oil to biodiesel with methanol Boyang Wang a,b, Shufen Li a,⇑, Songjiang Tian a, Rihua Feng a, Yonglu Meng a a b
Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China College of Pharmacy and Tianjin Key Laboratory of Molecular Drug Research, Nankai University, Tianjin 300071, China
h i g h l i g h t s " A new solid catalyst containing Ca12Al14O33 and CaO was synthetically prepared. " The catalyst has good activity with ME (methyl ester) content reached 90%. " The catalyst can be used for at least 7 cycles with the ME content over 87%. " The catalyst can be separated from reaction system effectively and easily. " This study may provide benefits for developing a continuous biodiesel process.
a r t i c l e
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Article history: Received 31 December 2011 Received in revised form 9 August 2012 Accepted 15 August 2012 Available online 13 September 2012 Keywords: Solid base catalyst Biodiesel Rapeseed oil Transesterification
a b s t r a c t A new solid base catalyst containing Ca12Al14O33 and CaO was prepared by chemical synthesis method and was used in the transesterification of rapeseed oil with methanol to produce biodiesel. The catalyst was characterized by Hammett indicators, X-ray diffraction (XRD), infrared spectroscopy (IR), Brunauer– Emmett–Teller (BET) and scanning electron microscopy (SEM), separately. The effect of various factors was investigated to optimize the reaction conditions. The results showed that the ME (methyl ester) content reached 90% after reacting for 3 h at 65 °C, with a methanol/oil molar ratio of 15:1, the amount of catalyst of 6 wt.% and the stirring rate of 270 rpm. Moreover, the catalyst could be used repeatedly for at least 7 cycles with the ME content over 87%, due to its high stability. In particular, this solid base catalyst can be separated from reaction system effectively and easily, as it is insoluble in both methanol and methyl esters, which may provide significant benefits for developing an environmentally benign and continuous process for synthesizing biodiesel. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The research on application of alternative renewable fuels has attracted tremendous attention in recent years due to the fear for the imbalance between diminishing fossil fuel reserves and soaring global energy demands. Many strategies and methods have been developed, in which the transesterification of vegetable oils or animal fats with short carbon-chain alcohols (methanol or ethanol) to produce a mixture of fatty acids esters (known as biodiesel) has great importance because biodiesel possesses many advantages over conventional fossil fuels, including proper lubricity, good biodegradability, excellent combustion efficiency and low toxicity [1–5]. Therefore, biodiesel can be employed for a clean substitute for fossil fuel without any modification to diesel engines, boilers or other combustion equipments [6–8].
⇑ Corresponding author. Tel./fax: +86 22 87894252. E-mail address: shfl[email protected] (S. Li). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.08.034
Two kinds of catalysts, homogeneous and heterogeneous catalysts, have been used in the transesterification reaction [9,10]. The conventional process of biodiesel is the transesterification of oils with methanol or ethanol in the presence of homogeneous catalysts, such as NaOH and KOH, which are the catalyst precursors. The real catalysts may be Na or K ethoxydes. However, homogeneous base catalytic systems have some basic technological problems, such as the production of wastewater to a serious threat for environment and the difficulty for removal of the base catalysts after reaction [11–14], although homogeneous base processes are relatively fast and show high conversions. Recently, solid base catalysts have been increasing attention as substitutes for the highly pollutant liquid homogeneous catalysts to produce biodiesel, as they generally do not generate large amount of wastewater. For example, KF/c-Al2O3 and Na/NaOH/ c-Al2O3 are highly active in the transesterification of vegetable oils [14,15]. However, the active ingredients of most of these supported alkali catalysts are sensitive to moisture and easily corroded by short carbon-chain alcohols, resulting in reducing lifetime of the
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catalysts and therefore limiting their industrial applications [16,17]. Another solid base catalyst of CaO has also been investigated and found that it has high activity and relative long lifetime [18–21]. However, the separation of CaO catalyst from biodiesel product after the reaction is very difficult attributed to gel phenomenon in the lower glycerol layer [22–24]. Therefore, it is critical to develop a novel solid catalyst that not only possesses exceptional catalytic activity and high stability, but also is easily separable from the transesterification product of biodiesel. In this study, a novel solid base catalyst was prepared. The physical and chemical characterizations of the catalyst with some instrumental methods were investigated. It was used to catalyze transesterification of rapeseed oil with methanol for the synthesis of biodiesel, and the effects of various factors were studied to optimize the reaction conditions. 2. Experimental 2.1. Materials Rapeseed oil was purchased from Xingwang Oil Corporation (Guanghan, Sichuan, China). Analytical reagents using as standards for gas chromatograph (GC) were purchased from Sigma Chemical Corporation, USA. Other chemicals were analytical reagents (AR) and purchased from Kewei Reagent Corporation, Tianjin, China. 2.2. Preparation of catalyst The solid base catalyst was prepared by chemical synthesis methods as follows. (1) Preparation of (Ca3Al2O66H2O).
hydrated
tricalcium
aluminate
Sodium aluminate (NaAlO2) aqueous solution and calcium hydroxide (Ca(OH)2) solution were mixed in a 1000 ml beaker. The mixture was stirred vigorously at 80 °C for 3 h. After the mixture was cooled to room temperature, the solid precipitate was collected by filtration. The solid was washed with deionized water until the pH of the washing solution maintained between 7 and 8 to give product hydrated tricalcium aluminate (Ca3Al2O66H2O), in which the reaction is as the following equation:
3CaðOHÞ2 þ 2NaAlO2 þ 4H2 O ! Ca3 Al2 O6 6H2 O þ 2NaOH
recorded at room temperature in the range of 400–3500 cm1, with 300 scans and 4 cm1 resolution. The KBr pellet technique was applied for determining IR spectra of the samples. The BET surface area of the catalysts was measured by the nitrogen adsorption method at 196 °C using a Tristar 3000 instrument (Micromeritics, Norcross, GA, USA). The surface morphology of the catalyst was investigated by a XL30 SEM (Philips, Eindhoven, North Brabant, Netherlands). SEM was operated at 20.0 kV of an accelerating voltage. 2.4. Reaction procedures The transesterification reactions were carried out in a 150 ml four-neck glass flask equipped with a stirrer, a water cooling condenser and an electric jacket with thermocouple. Firstly, 45 g of rapeseed oil was poured into the reactor with an amount of methanol according to desired molar ratio of methanol to rapeseed oil. The molar ratio of methanol to rapeseed oil (Nmethanol:oil) and average relative molar mass of rapeseed oil ðM oil Þ were calculated [27] as follows:
Nmethanol:oil ¼
Moil ¼
mmethanol Moil moil M methanol
MKOH 1000 3 SV AV
ð2Þ
ð3Þ
where mmethanol is the mass of methanol, g; moil is the mass of rapeseed oil, g; Mmethanol is molar mass of methanol, g mol1, SV is saponification value of rapeseed oil, mg KOH g1; AV is acid value of rapeseed oil, mg KOH g1; MKOH is molar mass of KOH, g mol1. After the mixture was stirred and heated to setting temperature (30–65 °C), the catalyst (2–8 wt.% relative to rapeseed oil) was added to the reaction system. The reaction was carried out under a stirring rate of 60–360 rpm. Very small amount of samples (0.5 ml) were collected by drip tube each time from the reaction mixture at the interval of 1 h for analysis to calculate the ME content. After the reaction was completed, the catalyst was removed from the mixture by filtration, and then it was washed by methanol. The catalyst was dried and then was kept in a desiccator for future reuse. The filtrated solution was further separated by a batch distillation to steaming out the excessive methanol. Next, a standard layering process for the residual liquid was required to obtain product of biodiesel and by-production of glycerol.
ð1Þ
(2) Generation of the solid base catalyst. The hydrated tricalcium aluminate was then calcined at 1000 °C for 8 h in a muffle furnace. The catalyst was cooled down to 400 °C, then it was extracted from the muffle furnace and kept in a desiccator for future use. 2.3. Catalyst characterization Hammett indicators, XRD, IR, BET, and SEM techniques were separately used to characterize the prepared catalyst. Base strength of catalysts (H_) and base total amount of catalysts were measured by using Hammett indicators [25,26], including bromthymol (H_ = 7.2), phenolphthalein (H_ = 9.8), 2, 4-dinitroaniline (H_ = 15.0) and 4-nitroaniline (H_ = 18.4). X-ray diffraction (XRD) measurements were performed with a PANAlytical X’Pert diffractomer using Co Ka radiation (k = 0.179 nm) at 40 kV and 40 mA. Data were collected over a 2h range of 10–110° with a step size of 0.02° at a scanning speed of 5 min1. IR measurements were performed with a Nicolet 560 spectrometer (Nicolet, Madison, Wisconsin, USA). The infrared spectra were
2.5. Analysis As the products in the methyl ester phase consist of methyl esters, monoglycerides, diglycerides and unreacted triglycerides, the amount of methyl esters (i.e. biodiesel) produced was calculated based on an internal standard using methyl salicylate as a reference standard. The methyl ester (ME) content then is calculated by the following expression which is similar to Ref. [28]:
ME content ðwt:%Þ ¼
Calculated weight of methyl esters Weight of methyl ester phase
100% P fester Aester mreference 100% Areference mesters
ð4Þ
where mreference is the mass of internal standard, g; mesters is the mass of methyl esters, g; Areference is the area of internal standard; Aester is the area of methyl ester; fester is correction factor of methyl ester. The methyl esters samples were analyzed in an Sp-2100 gas chromatograph equipped with a flame ionization detector and a capillary column H.J. PEG-20 M (30 m 0.32 mm 0.5 lm).
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Approximate 0.06 g (three dripping from a glass pipette) of each sample and 0.02 g of internal standard were dissolved in 0.3 g ethyl acetate for GC analysis. Samples (0.4 lL) were injected by a sampler at an oven temperature of 130 °C. After an isothermal period of 2 min, the GC oven was heated at 10 °C/min to 240 °C, and held for 7 min. Nitrogen was used as carrier gas. The injector temperature and detector temperatures were 180 °C and 280 °C, respectively. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. Base strength and base total amount of the catalyst Different catalysts were tested in the transesterification reaction under the same reaction conditions. The results (shown in Table 1) indicated that the activities of the catalysts corresponded to base strength and base total amount of the catalysts. The ME content is low for catalysts with base strength (H_) in the range of 9.8–15.0, such as K2CO3/c-Al2O3 and KNO3/c-Al2O3. In addition, for catalysts with the similar base strength, such as the new catalyst prepared in this paper and NaAlO2, the one with higher base total amount showed stronger catalytic activity, although the base amount has not linear relationship with the catalytic activity. However, the base strength and base total amount could help us to select the suitable solid base catalyst. Among the solid base catalysts tested, the new solid base catalyst (containing Ca12Al14O33 and CaO from Fig. 2) had the strongest base strength and largest base total amount and in turn gave the highest ME content. Therefore, the new solid base catalyst containing Ca12Al14O33 and CaO was selected and further studied as the catalyst for transterification of rapeseed oil to biodiesel. 3.1.2. Infrared analysis of the catalyst The investigation of the new solid base catalyst sample was conducted via IR spectroscopy, as shown in Fig. 1. A small and broad absorption peak of O–H stretching vibration at approximately 3430 cm1 and a very minor absorption peak of O–H bending vibration at about 1630 cm1 were assigned to H2O molecule absorbed from air [29]. Additionally, two bands at 1050 and 577 cm1 were probably attributed to absorption peak of Al–O vibration [30]. An intense absorption peak at around 459 cm1 attributed to the absorption peak of Ca–O stretching vibration was observed [31]. Meanwhile, the intense and broad absorption peak at 839 cm1 was assigned to stretching vibration of the Al–O bond in the tetrahedron (AlO4) [32]. It was clear that Ca and Al oxides formed during the calcination procedure of hydrated tricalcium aluminate at the temperature of 1000 °C in the muffle furnace. 3.1.3. XRD analysis of the catalyst X-ray diffraction pattern of the new solid base catalyst was given in Fig. 2. The several diffraction peaks corresponding to
Ca12Al14O33 (from Powder Diffraction File, 00-009-0413) phase remained with the presence of few peaks related to CaO when the calcination was performed at 1000 °C. Therefore, a synergetic effect between Ca12Al14O33 particles and CaO particles may occur in the solid base catalyzed transesterification. 3.1.4. SEM analysis of prepared catalyst Scanning electron microscopy images of the new solid base catalyst were shown in Fig. 3. The catalyst is comprised from the union of many crystals. There are many cracks (Fig. 3a) with sizes of micrometers and relatively large pores (Fig. 3b) among these agglomerates. Therefore, triglyceride and methanol may fully contact with the surface of the catalyst to synthesize biodiesel, resulting in better activity of the catalyst. From Fig. 3a, these agglomerates congregate to form large particle of the catalyst within range of 5–50 lm. As the size of the catalyst particle is big enough and is insoluble in both methanol and methyl esters, it can be effectively and easily separated from the products by filtration and centrifugation after the reaction. Many researchers only employed CaO as the catalyst for the transesterification to synthesize biodiesel [18–21]. However, it is common that gel phenomenon in the lower layer glycerol appears after the reaction, resulting in increasing the difficulty of separation of CaO catalyst [22–24]. But the separation effect of the new solid base catalyst containing Ca12Al14O33 and CaO from reaction system is obviously better than that of CaO when they are used as catalysts for synthesizing biodiesel. It is very possible that the great improvement of the separation effect can be attributed to the existence of the Ca12Al14O33 phase, which may promote the separation of the new solid base catalyst from reaction system. 3.1.5. BET analysis of the catalyst The BET analysis of the prepared catalyst reveals that the specific surface area of the new solid base catalyst is 5.78 m2/g. And the base total amount of the new solid base catalyst is 23.59 mmol g1.As Guo et al. indicated, the low surface area and high density of base site suggest that most base sites are in the interior of solid base catalyst [33]. It can be found from Fig. 3b that the surface of the catalyst has many pores, the size of which is among 0.5 lm and 1.8 lm. Hence, it is possible that the reactants diffused into the interior of the catalyst pores to make triglyceride (rapeseed oil) and methanol contacting with more base sites where they undergo transesterification reaction. 3.2. Optimization of reaction conditions In this study, the new prepared solid base catalyst was employed to catalyze the transesterification of rapeseed oil with methanol to produce biodiesel. The variables affecting the transesterification, such as methanol-to-oil molar ratio (9:1–18:1), catalyst amount (2.0–8.0 wt.% of oil), reaction temperature (30–65 °C), stirring rate (60–360 rpm) and reaction time (1–6 h), were investigated.
Table 1 Base strength and base total amount of the catalysts as well as the ME content when they were used in the transesterification of rapeseed oil with methanol to produce biodiesel.
a–e c–e
Catalyst
Base strength (H_)
Base total amount (mmol g)
ME content (%)
New solid base catalyst containing Ca12Al14O33 and CaOa NaAlO2b K2CO3/c-Al2O3c KNO3/c-Al2O3d KCl/c-Al2O3e
15.0 < H_ < 18.4 15.0 < H_ < 18.4 9.8 < H_ < 15.0 9.8 < H_ < 15.0 H_ < 7.2
23.59 11.40 0.46 0.05 0
84.15 80.32 53.77 <10 0
Reaction conditions: methanol/oil molar ratio, 15:1; catalyst amount, 2 wt.%; temperature, 65 °C; stirring rate, 270 rpm; reaction time, 6 h. K2CO3/c-Al2O3, KNO3/c-Al2O3, KCl/c-Al2O3 were prepared by the immersion method.
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Transmittance (a. u.)
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4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1) Fig. 1. IR spectra of the new prepared solid base catalyst samples.
3.2.1. Effect of mole ratio of methanol/oil From the reaction formula, the stoichiometric molar ratio of methanol to rapeseed oil is 3:1. However, the transesterification is reversible; therefore excess methanol is required to force the equilibrium towards completion to produce more methyl esters. The results of the effect of mole ratio of methanol/oil on the ME content are shown in Fig. 4. It was found that, with increasing molar ratio of methanol to rapeseed oil, resulting in an increase in the reaction rate of transesterification of rapeseed oil with methanol, the high ME content was observed. All ME content exceed 86.0% at the reaction time of 3 h when the molar ratios become higher than 9:1. When the molar ratio reaches 15:1, the ME content reaches 90% after 3 h reaction. Further increase in molar ratio has no effect on the ME content, but increases the energy consumption for recovering un-reacted methanol. In addition, excessive amount of un-reacted methanol could dissolve in by-product glycerin, making the separation more difficult. Therefore, the appropriate molar ratio of methanol/oil should be 15:1. 3.2.2. Effect of catalyst amount The effect of catalyst amount on the ME content in the transesterification of rapeseed oil at 65 °C with 15:1 M ratio of methanol/oil and 270 rpm stirring rate was studied, shown in Fig. 5. The results showed that the ME content increased along with the increase of catalyst amount from 2.0 to 6.0 wt.%. However, the increase of catalyst amount from 6.0 wt.% to 8.0 wt.% had little im-
Fig. 3. SEM images of the new prepared solid base catalyst.
pact on the ME content, which indicated that the equilibrium of the transesterification reaction had reached with 6.0 wt.% of catalyst under the conditions studied. Therefore, the optimum amount of catalyst is 6.0% by weight of oil. 3.2.3. Effect of reaction temperature As shown in Fig. 6, the ME content is only 5.25% at 30 °C after 3 h reaction, while it reaches the maximum value at 65 °C. Obvi-
Fig. 2. XRD patterns of the new prepared solid base catalyst samples.
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100
80
Mole ratio of methanl/oil 9: 1 12: 1 15: 1 18: 1
60
40
Methyl ester content (%)
Methyl ester content (%)
100
80
40 20 0
0
1
2
3
4
5
6
Stirring rate 0 rpm 60 rpm 180 rpm 270 rpm 360 rpm
60
7
0
1
2
Fig. 4. Effect of methanol/oil molar ratio on the ME content at 65 °C, with 6 wt.% catalyst and 270 rpm stirring rate for 6 h.
Methyl ester content (%)
5
6
7
Fig. 7. Effect of stirring rate on the ME content at 65 °C, with 15:1 M ratio and 6 wt.% catalyst for 6 h.
ter the ME content. Hence, a stirring rate of 270 rpm is sufficient for the transesterification reaction and thus recommended.
100 80
Catalyst amount (relative to oil weight) 0 wt% 2 wt% 4 wt% 6 wt% 8 wt%
60 40 20
0
1
2
3
4
5
6
3.2.5. Effect of reaction time The data from Figs. 4 and 5 indicate that the ME content increases steadily between the reaction time of 1 h and 3 h, and thereafter remains nearly constant as a result of reaching the equilibrium. Thus, the suitable reaction time is 3 h under optimized reaction conditions. 3.3. Reusability of the catalyst
7
Reaction time (h) Fig. 5. Effect of the amount of catalyst on the ME content at 65 °C, with 15:1 M ratio and 270 rpm stirring rate for 6 h.
ously, the reaction rate is faster at high temperature than at low temperature. Therefore, the suitable reaction temperature for the transesterification of rapeseed oil to biodiesel is 65 °C, the boiling point of methanol under atmosphere pressure. 3.2.4. Effect of stirring rate Due to the presence of the new solid base catalyst, the reaction system forms three-phase (oil–methanol–catalyst). The reaction may become diffusion-controlled when there is no agitation or the stirring rate is not high enough [34]. The ME content versus reaction time at different stirring rates are shown in Fig. 7. The results indicate that the transesterification reaction is incomplete with a stirring rate at 60 rpm for the given reaction time. The ME content at 270 rpm reaches 90% after 3 h reaction, while further increasing of the stirring rate to 360 rpm does not significantly al-
Since reusability is one of the most important features of a heterogeneous catalyst for its commercialization, we reused the catalysts to test the lifetime and stability of the catalyst. The used catalyst was pretreated as described in Section 2.4. After the filtration of the catalyst, the recovery was not of 100%. It was re-weighed and then added to the reaction system according to investigated optimal amount. Fig. 8 summarizes the experimental results, which indicate that the all ME content with reused catalysts all exceeds 87% at 65 °C, with a 15:1 M ratio of methanol to rapeseed oil, 6 wt.% of catalyst, 270 rpm of stirring rate and 3 h reaction time. The new solid base catalyst sustains its activity even after being used for 7 cycles with no decrease in the ME content observed, indicating that the catalyst has good stability. 4. Conclusions In this study, a novel solid base catalyst which contains Ca12Al14O33 and CaO is prepared in simple steps and is inexpensive. The physical and chemical characterizations of the prepared
100
100
80 60
Reaction temperature
40 20 0 0
1
2
3
4
5
6
7
Reaction time (h) Fig. 6. Effect of reaction temperature on the ME content with 15:1 M ratio, 6 wt.% catalyst and 270 rpm stirring rate for 6 h.
Methyl ester content (%)
Methyl ester content (%)
4
Reaction time (h)
Reaction time (h)
0
3
80 60 40 20 0 0
1
2
3
4
5
6
7
Number of repeated times Fig. 8. Effect of repeated use of the new prepared solid base catalyst on the ME content at 65 °C, with 15:1 M ratio, 6 wt.% catalyst and 270 rpm stirring rate for 3 h.
B. Wang et al. / Fuel 104 (2013) 698–703
catalyst with some instrumental methods were investigated. The experimental results show that this solid base catalyst has excellent catalytic activity and outstanding stability in the transesterification of rapeseed oil with methanol to produce biodiesel. Meantime, this solid base catalyst, which is insoluble in both methanol and methyl esters, can be separated from reaction system effectively and easily. Therefore, the use of the solid base catalyst prepared in this paper in the transesterification reaction provides significant benefits for developing an environmentally benign and continuous process for synthesizing biodiesel. The optimal transesterification conditions are obtained as follows: methanol/oil molar ratio 15:1, the amount of catalyst 6 wt.%, reaction temperature 65 °C, the stirring rate of 270 rpm and reaction time of 3 h. The ME content is 90% under the optimal conditions. Moreover, the catalyst is used repeatedly for at least 7 cycles with sustained activity and without decreasing the ME content, which sufficiently shows its good stability. Acknowledgment The work has been supported by the National Natural Science Foundation of China (Nos. 20776107, 21104035 and 21176186). References [1] Gerpen JV. Biodiesel processing and production. Fuel Process Technol 2005;86:1097–107. [2] Holser RA, O’Kuru RH. Transesterified milkweed (Asclepias) seed oil as a biodiesel fuel. Fuel 2006;85:2106–10. [3] Ramadhas AS, Jayaraj S, Muraleedharan C. Biodiesel production from high FFA rubber seed oil. Fuel 2005;84:335–40. [4] Dorado MP, Ballesteros E, Arnal JM, Gomez J, Lopez FJ. Exhaust emissions from a diesel engine fueled with transesterified waste olive oil. Fuel 2003;82:1311–5. [5] Schuchardt U, Sercheli R, Vargas RM. Transesterification of vegetable oils: a review. J Brazil Chem Soc 1998;9:199–210. [6] Canakci M. The potential of restaurant waste lipids as biodiesel feed stocks. Bioresource Technol 2007;98:183–90. [7] Ma F, Hanna MA. Biodiesel production: a review. Bioresource Technol 1999;70:1–15. [8] Meher LC, Vidya SD, Naik SN. Technical aspects of biodiesel production by transesterification – a review. Renew Sustain Energy Rev 2006;10:248–68. [9] Dossin TF, Reyniers MF, Marin GB. Kinetics of heterogeneously MgO-catalyzed transesterification. Appl Catal B: Environ 2006;61:35–45. [10] Lopez DE, Goodwin JG, Bruce DA, Lotero E. Transesterification oftriacetin with methanol on solid acid and base catalysts. Appl Catal A–Gen 2005;295:97–105. [11] Freedman B, Pryde EH, Mounts TL. Variables affecting the yields of fatty esters from transesterified vegetable oils. J Am Oil Chem Soc 1984;61:1638–43. [12] Zhang Y, Dube MA, MacLean DD, Kates M. Biodiesel production from waste cooking oil: 1. Process design and technological assessment. Bioresour Technol 2003;89:1–16.
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