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Al composite oxide-based alkaline catalyst for biodiesel production
Bioresource Technology 128 (2013) 305–309
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effect of calcination temperature on the activity of solid Ca/Al composite oxide-based alkaline catalyst for biodiesel production Yong-Lu Meng a,b, Bo-Yang Wang a,c, Shu-Fen Li a,⇑, Song-Jiang Tian a, Min-Hua Zhang b,⇑ a
Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Key Laboratory for Green Chemical Technology of State Education Ministry, Tianjin University, R&D Center for Petrochemical Technology, Tianjin 300072, China c College of Pharmacy and Tianjin Key Laboratory of Molecular Drug Research, Nankai University, Tianjin 300071, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" A solid base catalyst based on
The Ca/Al composite oxide which contains Ca12Al14O33 and CaO was prepared successfully by a simple method and used as an environmentally benign solid catalyst in the transesterification of rapeseed oil with methanol. The effect of the calcination temperature ranging from 120 °C to 1000 °C on the catalytic activity was investigated. The catalyst calcined at 600 °C showed the highest activity with >94% yield of biodiesel. The activity of the catalyst was closely related to its specific surface area and crystalline structure. In particular, the generation of crystalline Ca12Al14O33 vastly improved the catalytic activity due to the synergistic effect between Ca12Al14O33 and CaO.
"
"
" "
Ca12Al14O33 and CaO was developed. The catalyst showed excellent catalytic activity with biodiesel yields >94%. The effect of calcination temperature on the catalytic activity was investigated and optimal temperature was determined. Structure and properties of the catalyst were studied. The synergistic mechanism between crystalline Ca12Al14O33 and CaO of the catalyst was revealed.
a r t i c l e
i n f o
Article history: Received 31 May 2012 Received in revised form 17 October 2012 Accepted 29 October 2012 Available online 7 November 2012 Keywords: Biodiesel Methyl ester Transesterification Heterogeneous catalyst Ca/Al composite oxide
a b s t r a c t A solid Ca/Al composite oxide-based alkaline catalyst containing Ca12Al14O33 and CaO was prepared by chemical synthesis and thermal activation from sodium aluminate solution and calcium hydroxide emulsion. The effect of calcination temperatures ranging from 120 °C to 1000 °C on activity of the catalyst was investigated. The catalyst calcined at 600 °C showed the highest activity with >94% yield of fatty acid methyl esters (i.e. biodiesel) when applied to the transesterification of rapeseed oil at a methanol:oil molar ratio of 15:1 at 65 °C for 3 h. Structure and properties of the catalyst were studied and the characterizations with XRD, TGA, FTIR, BET, and SEM demonstrated that the performance of the catalyst was closely related to its specific surface area and crystalline structure. In particular, the generation of crystalline Ca12Al14O33 improved the catalytic activity due its synergistic effect with CaO. Ó 2012 Elsevier Ltd. All rights reserved.
⇑ Corresponding authors. Tel./fax: +86 22 87894252. E-mail address: shfl[email protected] (S.-F. Li). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.10.152
306
Y.-L. Meng et al. / Bioresource Technology 128 (2013) 305–309
1. Introduction Biodiesel is commonly produced by the transesterification of triglycerides with methanol using homogeneous alkaline catalysts such as NaOH and KOH (Ma and Hanna, 1999; Pinto et al., 2005; Lotero et al., 2005.); however, production costs are rather high as not only a number of washing and purification steps need to be performed to meet quality requirements but also due to the equipment corrosion caused by acid and alkaline conditions (Canakci, 2007; Granados et al., 2007; Ji et al., 2006; Karmee and Chadha, 2005). Although homogeneous base processes are relatively fast and show high conversions (Semwal et al., 2011), the generation of wastewater is an environmental concern and it is difficult to remove the catalysts from the reaction mixture. As heterogeneously catalyzed processes generally do not generate large amounts of wastewater and are easy to separate from reaction mixtures, they could prove more environmentally friendly than homogeneous catalysts (Semwal et al., 2011; Martino et al., 2008). Particularly, solid base catalysts can allow the production of high quality glycerol as by-production (Bournay et al., 2005). Metal oxides (Yan et al., 2010; Furutaa et al., 2006), zeolites (Brito et al., 2007), and hydrotalcites (Cantrella et al., 2005; Trakarnpruk and Porntangjitlikit 2008) have been explored as heterogeneous catalysts for triglyceride transesterification. CaO has also been investigated and was found to exhibit high activity. For example, Kawashima et al. (2008) found that with calcium oxide as catalyst a 90% of biodiesel yield could be achieved in the transesterification of rapeseed oil with refluxing methanol within 3 h. Reddy et al. (2006) reported that nanocrystalline CaO showed high catalytic activity (99% conversion) in the transesterification reaction of poultry fat and soybean oil at room temperature. Some research has shown that soluble substance leached from CaO during transesterification. Gryglewicz (1999) reported that CaO dissolved slightly in methanol and formed a suspension in the reaction mixture. The separation of CaO from biodiesel after the reaction is very difficult due to gel formation in the lower glycerol layer (Guo et al., 2007; Jiang et al., 2006; Yan et al., 2007). Therefore, it is critical to develop a solid catalyst that not only possesses exceptional catalytic activity and high stability, but is also easily separable from the biodiesel product. Wang et al., 2012 synthesized a solid Ca/Al composite oxidebased catalyst for transesterification of rapeseed oil with methanol and showed that the catalyst calcined by 1000 °C reused for at least seven cycles with yields of the FAME maintained above 87%. The catalyst was easily separated from the reaction mixture, as it is insoluble in methanol and methyl esters. The present study focuses on the effect of calcination temperature on the catalytic activity and structure of this catalyst and compares its performance with that of NaOH and CaO.
2. Methods
Table 1 Physical and chemical properties of the rapeseed oil. Property
Unit
Value
Palmitic acid Stearic acid Oleic acid Linoleic acid Linolenic acid Erucic acid Saponification value Acid value Water content
wt.% wt.% wt.% wt.% wt.% wt.% mg KOH/g mg KOH/g wt.%
12 4 16 20 5 43 190.98 0.12 0.12
2.2. Catalyst preparation An aqueous solution of sodium aluminate (NaAlO2) was prepared with 6 g of aluminum foil and 45 mL of 5 mol/mL NaOH. A Ca(OH)2 emulsion was prepared by 7 g of calcium oxide (CaO), prepared by calcination of CaCO3, in 50 mL of distilled water. The NaAlO2 solution and the Ca(OH)2 emulsion were mixed at a Ca:Al molar ratio of 3:2 under 300 r/min stirring rate for 3 h at 80 °C. After the mixture was cooled to room temperature, the solid precipitate was collected through an air pump filtration. The solid material was washed with deionized water until the pH of the washwater was about 7–8. The solid was dried under airflow at 120 °C, followed by calcination in a muffle furnace at temperature ranging from 400 °C to 1000 °C for 8 h. 2.3. Catalyst characterization Powder X-ray diffraction (XRD) patterns of the samples were recorded on a PANAlytical X’Pert diffractometer using Cu–Ka radiation (40 kV and 100 mA). The diffractograms were recorded in the 2h ranges of 10–110° with a 2 h step size of 0.02°. The phases present in the samples were identified according to the Powder Diffraction (PDF) database (JCPDS-International Centre for Diffraction Data (ICDD), 2000). The specific surface area of Ca/Al composite oxides catalysts was determined by N2 adsorption at 77 K using a Micrometrics Tristar 3000 automated system. Prior to analyses, the catalysts samples were degassed at 473 K and 106 mmHg for 5 h. Specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) model. Fourier transform infrared (FTIR) spectroscopy was done on a Nicolet 560 spectrophotometer. The FTIR spectroscopy measurements were performed by mixing samples into KBr pellets (32 scans with 5 cm1 resolution) in the range of 4000–400 cm1. Thermogravimetry differential thermal analysis (TG-DTA) was recorded using a HCT-2 analyzer. The dried samples (prior to calcination) were heated to 1173 K from at a heating rate of 5 K min1 in an air stream. Scanning electron microscopy (SEM) was carried out using a XL30 SEM (Philips, Eindhoven, North Brabant, Netherlands). The samples were coated with gold using a sputter coater.
2.1. Materials 2.4. Experimental procedures Rapeseed oil was purchased from Xingwang Oil Corporation (Guanghan, Sichuan, China) and its physical and chemical properties are listed in Table 1. Standard analytical reagents, such as methyl undecanoate, methyl palmitate, methyl oleate, methyl stearate, methyl linoleate, and methyl linolenate were purchased from Sigma Chemical Corporation, USA. Calcium carbonate and aluminum foil were of guarantee reagent grade (GR) and sodium hydroxide and other chemicals were of analytical grade (AR), and were obtained from Kewei Reagent Corporation, Tianjin, China.
Transesterification reactions were carried out in a 150 mL fourneck glass flask equipped with a stirrer, a reflux methanol condenser, and an electric jacket with a thermocouple. Firstly, 45 g of rapeseed oil and 24.5 g of methanol (molar ratio of 15:1) were placed into the reactor, and catalyst at 6 wt.% of the rapeseed oil was added. The reaction was carried out under a stirring rate of 270 r/min at 65 °C for 3 h. The catalyst was removed from the mixture by filtration through an air pump filtration, and washed by
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methanol. The catalyst was dried and stored in a desiccator. The filtrate was subjected to batch distillation to evaporate the excess methanol. A standing and layering process was carried out to obtain the FAME (the lower layer) and glycerol. 2.5. Product analysis The fatty acid methyl esters (FAME) content was determined with a SP-2100 gas chromatograph equipped with a flame ionization detector and a H.J PEG-20 M capillary column with a film thickness of 0.5 lm, L I.D. 30 m 0.32 mm. Nitrogen was used as carrier gas, hydrogen and air were used as combustion gas. Methyl salicylate was used as internal standard and ethyl acetate was the solvent. The yield of the FAME (i.e. biodiesel) was calculated by the following equation:
Yield ¼ mmactual 100% ¼ academic
¼
mproduct W FAMEs moil
mproduct W FAMEs moil M FAMEs 3 Moil
882:18 100% ¼ 1:0046 878:15
100%
mproduct W FAMEs moil
100%
where WFAME is the mass concentration of methyl esters in the products, and was determined by GC. mproduct is the mass of the biodiesel obtained by separation. moil is the mass of rapeseed oil used in the reaction. Moil is the average mole mass of triglycerides in oils, where its value is 882.18 g/mol determined from the acid and saponification values. MFAMEs is the average values of the mole mass of FAME in the product, based on the stoichiometric proportion of the transesterification, MFAMEs ¼ Moil 4MH ¼ 878:15 g=mol. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. Structure of the Ca/Al composite oxide catalyst Fig. 1 depicts the XRD patterns of the samples after thermoactivation at 120, 400, 600, 800, and 1000 °C, respectively. The results indicated that cubic calcium aluminum hydroxide (Ca3(Al(OH)6)2, JCPDS 78–1532, 2h diffraction peaks at 20.069°, 23.214°, 30.871°, 33.062°, 37.098°, 45.861°, 52.016°, and 64.324°) was formed from calcium hydroxide and sodium aluminate. When the calcination temperature reached 400 °C, the crystal structure of Ca12Al14O33 (JCPDS 70–2144, 2h diffraction peaks at 21.061°, 34.728°, 38.983°, 42.879°, and 48.241°) emerged. Its diffraction peak intensity increased with the calcination temperature. The crystal phase of Ca3(Al(OH)6)2 vanished at 400 °C. This findings indicated that Ca3(Al(OH)6)2 was converted into amorphous Ca12Al14O33 and then crystallized gradually under higher temperature. Due to the stoichiometric proportion of the decomposition reaction of Ca3 (Al(OH)6)2, 9 mol Ca(OH)2 (2h = 21.010°, 39.814°, 55.341°, and 59.746°) generates with 1 mol Ca12Al14O33 (Soro et al., 2006). A further increase in the calcination temperature above 600 °C led to the formation of CaO species with characteristic 2h diffraction peaks at 37.597°, 63.689°, 63.500°, 59.746°, 76.201°, and 80.253°, where CaO was transformed from Ca(OH)2 by dehydration. Consequently, the catalysts were composed of Ca12Al14O33 and CaO at higher calcination temperatures (600–800 °C). Thermogravimetric analysis of the uncalcined Ca/Al precursor in air showed two noticeable weight losses of 17.5% and 7.92% (Fig. 2). The main weight loss at 310 °C resulted from loss of water due to the thermo-decomposition of Ca3(Al(OH)6)2, while the loss at about 455 °C is assigned to the dehydration of Ca(OH)2. These results are in agreement with those reported by Ukrainczyk et al. (2007). The thermal transitions below 200 °C was assigned to the removal of adsorbed and chemisorbed water and the transition at 615–900 °C probably corresponded to the decomposition of
Fig. 1. Powder X-ray diffraction patterns of the Ca/Al composite oxide catalysts after calcination at different temperatures.
Fig. 2. Thermogravimetry differential thermal analysis curves of the Ca/Al composite oxide catalyst precursor.
CaCO3 formed by CaO with CO2 in the atmosphere. These observations are consistent with these from the powder XRD experiments as structure transformations between 120–400 °C and 400–600 °C were noted. 310 C
7Ca3 ðAlðOHÞ6 Þ2 ! Ca12 Al14 O33 þ 9CaðOHÞ2 þ 33H2 O 455 C
CaðOHÞ2 ! CaO þ H2 O FTIR spectroscopy was used to identify the structural changes of the catalysts at molecular level during calcinating. The spectra (Fig. 3) exhibit a broad band near 3400 cm1 due to the c (O–H) of the free and hydrogen-bonded hydroxyl groups, and another
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Fig. 3. Fourier transform infrared spectroscopy patterns of the Ca/Al composite oxide catalysts calcined at different temperatures.
typical absorption peak at 1630 cm1 is assigned to the deformative vibration of water molecules, absorbed during compaction of the dry powder specimens with KBr (Zawrah, 2004; Dvoranova et al., 2002). The spectra of all the samples showed the same peak at 1448 cm1, which is assigned to Al–O stretching vibration (Chroma et al., 2005), and the shape of these peaks became sharper with higher calcination temperature. The broad absorption region from 750 to 930 cm1 can be assigned to AlO4 stretching vibration (Ouahdi et al., 2005). The two sharp bands at 560 and 678 cm1 correspond to the AlO6 groups. (Ouahdi et al., 2005). 3.1.2. Surface property and topography of the Ca/Al composite oxide catalyst The catalyst exhibited ball-shaped particles surrounded by a flaky film after calcination. Due to the irregular accumulation of the flaky film, numerous large-sized pores were formed. At a calcination temperature above 600 °C, agglomeration of the catalyst particles occurred and became very pronounced at 1000 °C because of high sintering. The specific surface areas of the catalysts after calcination at 120, 400, 600, 800, and 1000 °C was 2.33, 5.14, 27.36, 9.8, and 3.27 m2/g, respectively. The increase from 5.14 m2/g to 27.36 m2/g between calcination at 400 °C and 600 °C may be attributed to the loss of structural water caused by crystalline phase transformation (Behrens, 2009). Overall, the changes in catalyst specific surface area with the calcination temperature were determined by the changes in phase structure, particle size and surface morphology. 3.2. Effect of calcination temperature on catalytic activity The catalytic activity of the Ca/Al composite oxide was compared with CaO and NaOH under the same condition that methanol:rapeseed oil molar ratio of 15:1 at 65 °C for 3 h with the catalyst dosage 6 wt.%. The results are shown in Fig. 4(a), where the values show the average FAME yields of parallel experiment, and the bars donate standard deviation. It indicates that the catalytic activities significantly depended on the calcination temperature, and the optimum calcination temperature was 600 °C. The trend in calcination temperature-dependent activity parallels that
Fig. 4. Catalytic activity of different catalysts in transesterification of rapeseed oil with methanol to produce biodiesel. (a) FAME yields achieved with Ca/Al composite oxide catalysts calcined at different temperature; (b) FAME yields achieved with various catalysts. (The CaO catalyst was prepared by Ca(OH)2 calcination at 800 °C and the Ca/Al composite oxide catalyst was calcined at 600 °C.)
of the surface area. The increase in FAME yield of the catalyst calcined from 120 °C to 400 °C is possibly attributable to the development of the Ca12Al14O33 phase. 3.3. Transesterification with different catalysts NaOH had the highest catalytic activity with a FAME yield about 96% (Fig. 4b). The Ca/Al composite oxide- and CaO-catalyzed reactions produced yields of 94.34% and 92.59%, respectively. It is possible that Ca12Al14O33 and CaO act synergistically. Since the Ca/Al oxide composite is easier to separate from the reaction mixture and apparently stable, it could be a better choice from an environmental and potentially also an economic point of view than NaOH and CaO for industrial-scale biodiesel production. 4. Conclusions A convenient and economic method for preparation of a Ca/Al composite oxide heterogeneous catalyst for biodiesel production was developed. The catalyst calcined at 600 °C which was composed of Ca12Al14O33 and CaO showed the highest activity with
Y.-L. Meng et al. / Bioresource Technology 128 (2013) 305–309
the FAME yield >94%. There was a positive correlation between the performance and the BET surface area of the catalysts calcined at different temperature. The Ca/Al composite oxide has a great potential for developing green industrial-scaled biodiesel production. Acknowledgements The project is financially supported by the National Natural Science Foundation of China (No.20776107 and No.21104035). References Behrens, M., 2009. Meso- and nano-structuring of industrial Cu/ZnO/(Al2O3) catalysts. J. Catal. 26, 24–29. Bournay, L., Casanave, D., Delfort, B., Hillion, G., Chodorge, J.A., 2005. New heterogeneous process for biodiesel production: a way to improve the quality and the value of the crude glycerin produced by biodiesel plants. Catal. Today 106, 190–192. Brito, A., Borges, M.E., Otero, N., 2007. Zeolite Y as a heterogeneous catalyst in biodiesel fuel production from used vegetable oil. Energ. Fuel. 21, 3280–3283. Canakci, M., 2007. The potential of restaurant waste lipids as biodiesel feedstocks. Bioresour. Technol. 98, 183–190. Cantrella, D.G., Gillieb, L.J., Leea, A.F., 2005. Structure-reactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis. Appl. Catal. A-Gen. 287, 183–190. Chroma, M., Pinkas, J., Pakutinskiene, I., et al., 2005. Processing and characterization of sol–gel fabricated mixed metal aluminates. Ceram. Int. 31, 1123–1130. Dvoranova, D., Nrezova, V., Mazur, M., et al., 2002. Investigations of metal-doped titanium dioxide photo catalysts. Appl. Catal. B: Environ. 37, 91–105. Furutaa, S., Matsuhashib, H., Arata, K., 2006. Biodiesel fuel production with solid amorphous-zirconia catalysis in fixed bed reactor. Biomass Bioenerg. 30, 870– 873. Guo, D.F., Li, W.M., Pan, J.B., Shi, H.Y., Chen, H., 2007. Synthesis of biodiesel with solid base CaO/MgO as catalyst. Chinese J. Appl. Chem. 24, 1149–1152. Granados, M.L., Poves, M.D.Z., Alonso, D.M., et al., 2007. Biodiesel from sunflower oil by using activated calcium oxide. Appl. Catal. B: Environ. 73, 317–326. Gryglewicz, S., 1999. Rapeseed oil methyl esters preparation using heterogeneous catalysts. Bioresour. Technol. 70, 249–253. JCPDS-International Centre for Diffraction Data (ICDD), 2000. Powder Diffraction File (PDF-2 Database). Newtown Square, Pennsylvania.
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Jiang, L.H., Yan, L.H., Liang, B., 2006. Heterogeneous solid base catalysts for biodiesel production. Indus. Catal. 14, 34–38. Ji, J., Wang, J., Li, Y., et al., 2006. Preparation of biodiesel with the help of ultrasonic and hydrodynamic cavitation. Ultrasonics 44, 411–414. Karmee, S.K., Chadha, A., 2005. Preparation of biodiesel from crude oil of Pongamia pinnata. Bioresour. Technol. 96, 1425–1429. Kawashima, A., Matsubara, K., Honda, K., 2008. Development of heterogeneous base catalysts for biodiesel production. Bioresour. Technol. 99, 3439–3443. Lotero, E., Liu, Y., Lopez, D.E., et al., 2005. Synthesis of biodiesel via acid catalysis. Ind. Eng. Chem. Res. 44, 5353–5363. Ma, F., Hanna, M.A., 1999. Biodiesel production: a review. Bioresour. Technol. 70, 1– 15. Martino, D.S., Riccardo, T., Lu, P., et al., 2008. Heterogeneous catalysts for biodiesel production. Energ. Fuel. 22, 207–217. Ouahdi, N., Guillemet, S., Demai, J.J., et al., 2005. Investigation of the reactivity of AlCl3 and CoCl2 toward molten alkali-metal nitrates in order to synthesize CoAl2O4. Mater. Lett. 59, 334–340. Pinto, A.C., Guarieiro, L.L.N., Rezende, M.J.C., et al., 2005. Biodiesel: an overview. J. Braz. Chem. Soc. 16, 1313–1330. Reddy, C.R.V., Oshel, R., Verkade, J.G., 2006. Room-temperature conversion of soybean oil and poultry fat to biodiesel catalyzed by nanocrystalline calcium oxides. Energ. Fuel. 20, 1310–1314. Semwal, S., Arora, A.K., Badoni, R.P., et al., 2011. Biodiesel production using heterogeneous catalysts. Bioresour. Technol. 102, 2151–2161. Soro, J., Smith, A., Gault, C., 2006. Thermomechanical characteristics of calcium aluminate cement and sand tapes prepared by tape casting. J. Eur. Ceram. Soc. 26, 3799–3807. Trakarnpruk, W., Porntangjitlikit, S., 2008. Palm oil biodiesel synthesized with potassium loaded calcined hydrotalcite and effect of biodiesel blend on elastomer properties. Renew. Energ. 33, 1558–1563. Ukrainczyk, N., Matusinovic, T., Kurajica, S., et al., 2007. Dehydration of a layered double hydroxide—C2AH8. Thermochim. Acta 464, 7–15. Wang, B.Y., Li, S.F., Tian, S.T., Feng, R.H., Meng, Y.L., 2012. A new solid base catalyst for the transesterification of rapeseed oil to biodiesel with methanol. Fuel. http://dx.doi.org/10.1016/j.fuel.2012.08.034. Yan, S., Mohan, S., DiMaggio, C., Kim, M., et al., 2010. Long term activity of modified ZnO nanoparticles for transesterification. Fuel 89, 2844–2852. Yan, S.L., Lu, H.F., Jiang, L.H., 2007. Solid base catalysts for transesterification of oil with methanol to produce biodiesel. Chinese J. Chem. Indus. Eng. 58, 2506– 2512. Zawrah, M.F., 2004. Investigation of lattice constant, sintering and properties of nano Mg–Al spinels. Mater. Sci. Eng. A 382, 362–370.
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effect of calcination temperature on the activity of solid Ca/Al composite oxide-based alkaline catalyst for biodiesel production Yong-Lu Meng a,b, Bo-Yang Wang a,c, Shu-Fen Li a,⇑, Song-Jiang Tian a, Min-Hua Zhang b,⇑ a
Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Key Laboratory for Green Chemical Technology of State Education Ministry, Tianjin University, R&D Center for Petrochemical Technology, Tianjin 300072, China c College of Pharmacy and Tianjin Key Laboratory of Molecular Drug Research, Nankai University, Tianjin 300071, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" A solid base catalyst based on
The Ca/Al composite oxide which contains Ca12Al14O33 and CaO was prepared successfully by a simple method and used as an environmentally benign solid catalyst in the transesterification of rapeseed oil with methanol. The effect of the calcination temperature ranging from 120 °C to 1000 °C on the catalytic activity was investigated. The catalyst calcined at 600 °C showed the highest activity with >94% yield of biodiesel. The activity of the catalyst was closely related to its specific surface area and crystalline structure. In particular, the generation of crystalline Ca12Al14O33 vastly improved the catalytic activity due to the synergistic effect between Ca12Al14O33 and CaO.
"
"
" "
Ca12Al14O33 and CaO was developed. The catalyst showed excellent catalytic activity with biodiesel yields >94%. The effect of calcination temperature on the catalytic activity was investigated and optimal temperature was determined. Structure and properties of the catalyst were studied. The synergistic mechanism between crystalline Ca12Al14O33 and CaO of the catalyst was revealed.
a r t i c l e
i n f o
Article history: Received 31 May 2012 Received in revised form 17 October 2012 Accepted 29 October 2012 Available online 7 November 2012 Keywords: Biodiesel Methyl ester Transesterification Heterogeneous catalyst Ca/Al composite oxide
a b s t r a c t A solid Ca/Al composite oxide-based alkaline catalyst containing Ca12Al14O33 and CaO was prepared by chemical synthesis and thermal activation from sodium aluminate solution and calcium hydroxide emulsion. The effect of calcination temperatures ranging from 120 °C to 1000 °C on activity of the catalyst was investigated. The catalyst calcined at 600 °C showed the highest activity with >94% yield of fatty acid methyl esters (i.e. biodiesel) when applied to the transesterification of rapeseed oil at a methanol:oil molar ratio of 15:1 at 65 °C for 3 h. Structure and properties of the catalyst were studied and the characterizations with XRD, TGA, FTIR, BET, and SEM demonstrated that the performance of the catalyst was closely related to its specific surface area and crystalline structure. In particular, the generation of crystalline Ca12Al14O33 improved the catalytic activity due its synergistic effect with CaO. Ó 2012 Elsevier Ltd. All rights reserved.
⇑ Corresponding authors. Tel./fax: +86 22 87894252. E-mail address: shfl[email protected] (S.-F. Li). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.10.152
306
Y.-L. Meng et al. / Bioresource Technology 128 (2013) 305–309
1. Introduction Biodiesel is commonly produced by the transesterification of triglycerides with methanol using homogeneous alkaline catalysts such as NaOH and KOH (Ma and Hanna, 1999; Pinto et al., 2005; Lotero et al., 2005.); however, production costs are rather high as not only a number of washing and purification steps need to be performed to meet quality requirements but also due to the equipment corrosion caused by acid and alkaline conditions (Canakci, 2007; Granados et al., 2007; Ji et al., 2006; Karmee and Chadha, 2005). Although homogeneous base processes are relatively fast and show high conversions (Semwal et al., 2011), the generation of wastewater is an environmental concern and it is difficult to remove the catalysts from the reaction mixture. As heterogeneously catalyzed processes generally do not generate large amounts of wastewater and are easy to separate from reaction mixtures, they could prove more environmentally friendly than homogeneous catalysts (Semwal et al., 2011; Martino et al., 2008). Particularly, solid base catalysts can allow the production of high quality glycerol as by-production (Bournay et al., 2005). Metal oxides (Yan et al., 2010; Furutaa et al., 2006), zeolites (Brito et al., 2007), and hydrotalcites (Cantrella et al., 2005; Trakarnpruk and Porntangjitlikit 2008) have been explored as heterogeneous catalysts for triglyceride transesterification. CaO has also been investigated and was found to exhibit high activity. For example, Kawashima et al. (2008) found that with calcium oxide as catalyst a 90% of biodiesel yield could be achieved in the transesterification of rapeseed oil with refluxing methanol within 3 h. Reddy et al. (2006) reported that nanocrystalline CaO showed high catalytic activity (99% conversion) in the transesterification reaction of poultry fat and soybean oil at room temperature. Some research has shown that soluble substance leached from CaO during transesterification. Gryglewicz (1999) reported that CaO dissolved slightly in methanol and formed a suspension in the reaction mixture. The separation of CaO from biodiesel after the reaction is very difficult due to gel formation in the lower glycerol layer (Guo et al., 2007; Jiang et al., 2006; Yan et al., 2007). Therefore, it is critical to develop a solid catalyst that not only possesses exceptional catalytic activity and high stability, but is also easily separable from the biodiesel product. Wang et al., 2012 synthesized a solid Ca/Al composite oxidebased catalyst for transesterification of rapeseed oil with methanol and showed that the catalyst calcined by 1000 °C reused for at least seven cycles with yields of the FAME maintained above 87%. The catalyst was easily separated from the reaction mixture, as it is insoluble in methanol and methyl esters. The present study focuses on the effect of calcination temperature on the catalytic activity and structure of this catalyst and compares its performance with that of NaOH and CaO.
2. Methods
Table 1 Physical and chemical properties of the rapeseed oil. Property
Unit
Value
Palmitic acid Stearic acid Oleic acid Linoleic acid Linolenic acid Erucic acid Saponification value Acid value Water content
wt.% wt.% wt.% wt.% wt.% wt.% mg KOH/g mg KOH/g wt.%
12 4 16 20 5 43 190.98 0.12 0.12
2.2. Catalyst preparation An aqueous solution of sodium aluminate (NaAlO2) was prepared with 6 g of aluminum foil and 45 mL of 5 mol/mL NaOH. A Ca(OH)2 emulsion was prepared by 7 g of calcium oxide (CaO), prepared by calcination of CaCO3, in 50 mL of distilled water. The NaAlO2 solution and the Ca(OH)2 emulsion were mixed at a Ca:Al molar ratio of 3:2 under 300 r/min stirring rate for 3 h at 80 °C. After the mixture was cooled to room temperature, the solid precipitate was collected through an air pump filtration. The solid material was washed with deionized water until the pH of the washwater was about 7–8. The solid was dried under airflow at 120 °C, followed by calcination in a muffle furnace at temperature ranging from 400 °C to 1000 °C for 8 h. 2.3. Catalyst characterization Powder X-ray diffraction (XRD) patterns of the samples were recorded on a PANAlytical X’Pert diffractometer using Cu–Ka radiation (40 kV and 100 mA). The diffractograms were recorded in the 2h ranges of 10–110° with a 2 h step size of 0.02°. The phases present in the samples were identified according to the Powder Diffraction (PDF) database (JCPDS-International Centre for Diffraction Data (ICDD), 2000). The specific surface area of Ca/Al composite oxides catalysts was determined by N2 adsorption at 77 K using a Micrometrics Tristar 3000 automated system. Prior to analyses, the catalysts samples were degassed at 473 K and 106 mmHg for 5 h. Specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) model. Fourier transform infrared (FTIR) spectroscopy was done on a Nicolet 560 spectrophotometer. The FTIR spectroscopy measurements were performed by mixing samples into KBr pellets (32 scans with 5 cm1 resolution) in the range of 4000–400 cm1. Thermogravimetry differential thermal analysis (TG-DTA) was recorded using a HCT-2 analyzer. The dried samples (prior to calcination) were heated to 1173 K from at a heating rate of 5 K min1 in an air stream. Scanning electron microscopy (SEM) was carried out using a XL30 SEM (Philips, Eindhoven, North Brabant, Netherlands). The samples were coated with gold using a sputter coater.
2.1. Materials 2.4. Experimental procedures Rapeseed oil was purchased from Xingwang Oil Corporation (Guanghan, Sichuan, China) and its physical and chemical properties are listed in Table 1. Standard analytical reagents, such as methyl undecanoate, methyl palmitate, methyl oleate, methyl stearate, methyl linoleate, and methyl linolenate were purchased from Sigma Chemical Corporation, USA. Calcium carbonate and aluminum foil were of guarantee reagent grade (GR) and sodium hydroxide and other chemicals were of analytical grade (AR), and were obtained from Kewei Reagent Corporation, Tianjin, China.
Transesterification reactions were carried out in a 150 mL fourneck glass flask equipped with a stirrer, a reflux methanol condenser, and an electric jacket with a thermocouple. Firstly, 45 g of rapeseed oil and 24.5 g of methanol (molar ratio of 15:1) were placed into the reactor, and catalyst at 6 wt.% of the rapeseed oil was added. The reaction was carried out under a stirring rate of 270 r/min at 65 °C for 3 h. The catalyst was removed from the mixture by filtration through an air pump filtration, and washed by
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methanol. The catalyst was dried and stored in a desiccator. The filtrate was subjected to batch distillation to evaporate the excess methanol. A standing and layering process was carried out to obtain the FAME (the lower layer) and glycerol. 2.5. Product analysis The fatty acid methyl esters (FAME) content was determined with a SP-2100 gas chromatograph equipped with a flame ionization detector and a H.J PEG-20 M capillary column with a film thickness of 0.5 lm, L I.D. 30 m 0.32 mm. Nitrogen was used as carrier gas, hydrogen and air were used as combustion gas. Methyl salicylate was used as internal standard and ethyl acetate was the solvent. The yield of the FAME (i.e. biodiesel) was calculated by the following equation:
Yield ¼ mmactual 100% ¼ academic
¼
mproduct W FAMEs moil
mproduct W FAMEs moil M FAMEs 3 Moil
882:18 100% ¼ 1:0046 878:15
100%
mproduct W FAMEs moil
100%
where WFAME is the mass concentration of methyl esters in the products, and was determined by GC. mproduct is the mass of the biodiesel obtained by separation. moil is the mass of rapeseed oil used in the reaction. Moil is the average mole mass of triglycerides in oils, where its value is 882.18 g/mol determined from the acid and saponification values. MFAMEs is the average values of the mole mass of FAME in the product, based on the stoichiometric proportion of the transesterification, MFAMEs ¼ Moil 4MH ¼ 878:15 g=mol. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. Structure of the Ca/Al composite oxide catalyst Fig. 1 depicts the XRD patterns of the samples after thermoactivation at 120, 400, 600, 800, and 1000 °C, respectively. The results indicated that cubic calcium aluminum hydroxide (Ca3(Al(OH)6)2, JCPDS 78–1532, 2h diffraction peaks at 20.069°, 23.214°, 30.871°, 33.062°, 37.098°, 45.861°, 52.016°, and 64.324°) was formed from calcium hydroxide and sodium aluminate. When the calcination temperature reached 400 °C, the crystal structure of Ca12Al14O33 (JCPDS 70–2144, 2h diffraction peaks at 21.061°, 34.728°, 38.983°, 42.879°, and 48.241°) emerged. Its diffraction peak intensity increased with the calcination temperature. The crystal phase of Ca3(Al(OH)6)2 vanished at 400 °C. This findings indicated that Ca3(Al(OH)6)2 was converted into amorphous Ca12Al14O33 and then crystallized gradually under higher temperature. Due to the stoichiometric proportion of the decomposition reaction of Ca3 (Al(OH)6)2, 9 mol Ca(OH)2 (2h = 21.010°, 39.814°, 55.341°, and 59.746°) generates with 1 mol Ca12Al14O33 (Soro et al., 2006). A further increase in the calcination temperature above 600 °C led to the formation of CaO species with characteristic 2h diffraction peaks at 37.597°, 63.689°, 63.500°, 59.746°, 76.201°, and 80.253°, where CaO was transformed from Ca(OH)2 by dehydration. Consequently, the catalysts were composed of Ca12Al14O33 and CaO at higher calcination temperatures (600–800 °C). Thermogravimetric analysis of the uncalcined Ca/Al precursor in air showed two noticeable weight losses of 17.5% and 7.92% (Fig. 2). The main weight loss at 310 °C resulted from loss of water due to the thermo-decomposition of Ca3(Al(OH)6)2, while the loss at about 455 °C is assigned to the dehydration of Ca(OH)2. These results are in agreement with those reported by Ukrainczyk et al. (2007). The thermal transitions below 200 °C was assigned to the removal of adsorbed and chemisorbed water and the transition at 615–900 °C probably corresponded to the decomposition of
Fig. 1. Powder X-ray diffraction patterns of the Ca/Al composite oxide catalysts after calcination at different temperatures.
Fig. 2. Thermogravimetry differential thermal analysis curves of the Ca/Al composite oxide catalyst precursor.
CaCO3 formed by CaO with CO2 in the atmosphere. These observations are consistent with these from the powder XRD experiments as structure transformations between 120–400 °C and 400–600 °C were noted. 310 C
7Ca3 ðAlðOHÞ6 Þ2 ! Ca12 Al14 O33 þ 9CaðOHÞ2 þ 33H2 O 455 C
CaðOHÞ2 ! CaO þ H2 O FTIR spectroscopy was used to identify the structural changes of the catalysts at molecular level during calcinating. The spectra (Fig. 3) exhibit a broad band near 3400 cm1 due to the c (O–H) of the free and hydrogen-bonded hydroxyl groups, and another
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Fig. 3. Fourier transform infrared spectroscopy patterns of the Ca/Al composite oxide catalysts calcined at different temperatures.
typical absorption peak at 1630 cm1 is assigned to the deformative vibration of water molecules, absorbed during compaction of the dry powder specimens with KBr (Zawrah, 2004; Dvoranova et al., 2002). The spectra of all the samples showed the same peak at 1448 cm1, which is assigned to Al–O stretching vibration (Chroma et al., 2005), and the shape of these peaks became sharper with higher calcination temperature. The broad absorption region from 750 to 930 cm1 can be assigned to AlO4 stretching vibration (Ouahdi et al., 2005). The two sharp bands at 560 and 678 cm1 correspond to the AlO6 groups. (Ouahdi et al., 2005). 3.1.2. Surface property and topography of the Ca/Al composite oxide catalyst The catalyst exhibited ball-shaped particles surrounded by a flaky film after calcination. Due to the irregular accumulation of the flaky film, numerous large-sized pores were formed. At a calcination temperature above 600 °C, agglomeration of the catalyst particles occurred and became very pronounced at 1000 °C because of high sintering. The specific surface areas of the catalysts after calcination at 120, 400, 600, 800, and 1000 °C was 2.33, 5.14, 27.36, 9.8, and 3.27 m2/g, respectively. The increase from 5.14 m2/g to 27.36 m2/g between calcination at 400 °C and 600 °C may be attributed to the loss of structural water caused by crystalline phase transformation (Behrens, 2009). Overall, the changes in catalyst specific surface area with the calcination temperature were determined by the changes in phase structure, particle size and surface morphology. 3.2. Effect of calcination temperature on catalytic activity The catalytic activity of the Ca/Al composite oxide was compared with CaO and NaOH under the same condition that methanol:rapeseed oil molar ratio of 15:1 at 65 °C for 3 h with the catalyst dosage 6 wt.%. The results are shown in Fig. 4(a), where the values show the average FAME yields of parallel experiment, and the bars donate standard deviation. It indicates that the catalytic activities significantly depended on the calcination temperature, and the optimum calcination temperature was 600 °C. The trend in calcination temperature-dependent activity parallels that
Fig. 4. Catalytic activity of different catalysts in transesterification of rapeseed oil with methanol to produce biodiesel. (a) FAME yields achieved with Ca/Al composite oxide catalysts calcined at different temperature; (b) FAME yields achieved with various catalysts. (The CaO catalyst was prepared by Ca(OH)2 calcination at 800 °C and the Ca/Al composite oxide catalyst was calcined at 600 °C.)
of the surface area. The increase in FAME yield of the catalyst calcined from 120 °C to 400 °C is possibly attributable to the development of the Ca12Al14O33 phase. 3.3. Transesterification with different catalysts NaOH had the highest catalytic activity with a FAME yield about 96% (Fig. 4b). The Ca/Al composite oxide- and CaO-catalyzed reactions produced yields of 94.34% and 92.59%, respectively. It is possible that Ca12Al14O33 and CaO act synergistically. Since the Ca/Al oxide composite is easier to separate from the reaction mixture and apparently stable, it could be a better choice from an environmental and potentially also an economic point of view than NaOH and CaO for industrial-scale biodiesel production. 4. Conclusions A convenient and economic method for preparation of a Ca/Al composite oxide heterogeneous catalyst for biodiesel production was developed. The catalyst calcined at 600 °C which was composed of Ca12Al14O33 and CaO showed the highest activity with
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