Ca−Mg−Al Hydrotalcite Base

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Energy Fuels 2010, 24, 646–651 Published on Web 11/17/2009

: DOI:10.1021/ef900800d

Biodiesel Synthesis Catalyzed by the KF/Ca-Mg-Al Hydrotalcite Base Catalyst Lijing Gao, Guangyuan Teng, Jianhua Lv, and Guomin Xiao* School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, People’s Republic of China Received July 28, 2009. Revised Manuscript Received October 29, 2009

A series of solid base catalysts, KF/Ca-Mg-Al hydrotalcite (KF mass ratio of 100%) with different cation ratios, were prepared and studied in this research. Effects of the cation ratio of the Ca-Mg-Al hydrotalcite and the methanol/oil molar ratios on the fatty acid methanol ester (FAME) yield of the transesterification of palm oil with methanol were investigated. The results of the transesterification reactions showed that all of these kinds of catalysts had a very high activity for the biodiesel yields, obtaining 90% or more with 10 min of reaction under the conditions of 338 K, 12:1 methanol/oil molar ratio, and 5% (wt/wt oil) catalyst amount. In addition, the FAME yield could reach 99.6% in 10 min catalyzed by the optimal catalyst KF/Ca-Mg-Al hydrotalcite (2.2:0.8:1 Ca/Mg/Al; KF mass ratio of 100%), under the same reaction conditions as above.

CaO, or BaO,10-14 modified metal oxides, such as Li-doped CaO15 and La-modified CaO,16 anionic ion-exchange resin,17 and supported ones, such as Na/NaOH/γ-Al2O318 and KF/ γ-Al2O3.19,20 In particular, layered double hydroxides (LDHs) or so-called hydrotalcite (HT) could also be used as catalysts for the transesterification reaction directly or as catalyst supports. LDH, whose chemical composition can be represented by the general formula [M(1-x)2þMx3þ(OH)2]xþ(An-)x/n 3 yH2O, had also been reported as a catalyst or catalyst carrier for the transesterification reaction. Corma et al.21,22 had reported that calcined Li-Al and Mg-Al LDHs were able to catalyze the glycerolysis of fatty acid methyl esters to monoglycerides (the reverse of biodiesel synthesis). Shumaker et al.23,24 used calcined Li-Al LDHs to catalyze soybean oil with methanol. Liu et al.25 used calcined Mg-Al HT to catalyze poultry fat with methanol. Broito et al.26 showed that Mg-Al LDH could catalyze the transesterification reaction of waste oil with methanol. All of these studies showed that LDHs, except the Li-Al LDHs, performed low activities at lower temperatures and, when the reactions were performed in an autoclave at

1. Introduction Biodiesel, whose main content is fatty acid methanol esters (FAMEs), is one of the most potential substituted energy resources1 and is mainly produced by the transesterification reaction of vegetable oil or fat with methanol catalyzed by base, acid, and enzyme. To solve the problems caused by the homogeneous catalyst, which was widely used in industrial production, such as in a huge amount of waster water, product-separating problems, and limits of raw materials, heterogeneous catalysts, such as solid base, acid,2-5 and immobilized enzyme,6-8 were studied to replace the homogeneous catalyst. Among them, solid base was given more attention. For acid, even super-acid required a greater methanol/oil molar ratio9 and more reaction time, and the enzyme was much more expensive. Recently, studies had reported some solid bases as biodiesel synthesis catalysts, for example, metal oxides, such as MgO, *To whom correspondence should be addressed. Telephone: þ86-2552090612. Fax: þ86-25-52090612. E-mail: [email protected]. (1) Lin, C. Y.; Lin, H. A.; Hung, L. B. Fuel 2006, 85, 1743–1749. (2) Chen, X. R.; Ju, Y. H.; Mou, C. Y. J. Phys. Chem. C 2007, 11, 18731–18737. (3) Jacobson, K.; Gopinath, R.; Meher, L. C.; Dalai, A. K. Appl. Catal., B 2008, 85, 86–91. (4) Suwannakarn, K.; Lotero, E.; Ngaosuwan, K.; Goodwin, J. G. Ind. Eng. Chem. Res. 2009, 48, 2810–2818. (5) Kansedo, J.; Lee, K. T.; Bhatia, S. Biomass Bioenergy 2009, 2, 271– 276. (6) Moreira, A. B. R.; Perez, V. H.; Zanin, G. M.; de Castro, H. F. Energy Fuels 2007, 21, 3689–3694. (7) Ranganathan, S. V.; Narasimhan, S. L.; Muthukumar, K. Bioresour. Technol. 2008, 99, 3975–3981. (8) Xie, W. L.; Ma, N. Energy Fuels 2009, 23, 1347–1353. (9) Sunita, G.; Devassy, B. M.; Sawant, D. P.; Balasubramanian, V. V.; Halligudi, S. B. Catal. Commun. 2007, 8, 1107–1111. (10) Liu, X.; He, H. Y.; Wang, Y. J.; Zhu, S. L.; Piao, X. L. Fuel 2008, 87, 216–221. (11) Patil, P. D.; Deng, S. Energy Fuels 2009, 23, 4619–4624. (12) Wang, L.; Yang, J. Fuel. 2007, 86, 328–333. (13) Granados, M. L.; Poves, M. D. Z.; Alonso, D. M.; Mariscal, R.; Galisteo, F. C.; Moreno-Tost, R.; Santamarı´ a, J.; Fierro, J. L. G. Appl. Catal., B 2007, 73, 317–326. (14) Singh, A. K.; Fernando, S. D. Energy Fuels 2008, 22, 2067–2069. (15) Watkins, R. S.; Lee, A. F.; Wilson, K. Green Chem. 2004, 6, 335– 340. r 2009 American Chemical Society

(16) Yan, S. L.; Kim, M.; Salley, S. O.; Ng, K. Y. S. Appl. Catal., A 2009, 360, 163–170. (17) Shibasaki-Kitakawa, N.; Honda, H.; Kuribayashi, H.; Toda, T.; Fukumura, T.; Yonemoto, T. Bioresour. Technol. 2007, 98, 416–421. (18) Kim, H. J.; Kang, B. S.; Kim, M. J.; Park, Y. M.; Kim, D. K.; Lee, J. S.; Lee, K. Y. Catal. Today 2004, 93-95, 315–320. (19) Cui, L. F.; Xiao, G. M.; Xu, B.; Teng, G. Y. Energy Fuels 2007, 21, 3740–3743. (20) Xu, B.; Xiao, G. M.; Cui, L. F.; Wei, R. P.; Gao, L. J. Energy Fuels 2007, 21, 3109–3112. (21) Corma, A.; Hamid, S. B. A.; Iborra, S.; Velty, A. J. Catal. 2005, 234, 340–347. (22) Corma, A.; Iborra, S.; Miquel, S.; Primo, J. J. Catal. 1998, 173, 315–321. (23) Shumaker, J. L.; Crofcheck, C.; Tackett, S. A.; Santillan-Jimenez, E.; Crocker, M. Catal. Lett. 2007, 115, 56–61. (24) Shumaker, J. L.; Crofcheck, C.; Tackett, S. A.; Santillan-Jimenez, E.; Morgan, T.; Ji, Y.; Crocker, M.; Toops, T. J. Appl. Catal., B 2008, 82, 120–130. (25) Liu, Y. J.; Lotero, E.; Goodwin, J. G., Jr.; Mo, X. H. Appl. Catal., A 2007, 331, 138–148. (26) Brito, A.; Borges, M. E.; Garı´ n, M.; Hernandez, A. Energy Fuels 2009, 23, 2952–2958.

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Energy Fuels 2010, 24, 646–651

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high temperatures, the conversion of soybean oil or acid cottonseed oil could reach 90%.26-28 While on the basis of the HT properties, loading an active substance could improve the activity of the HT. Sun et al.29 had reported that KNO3/ HT showed stronger alkality and more activity in the methylation of cyclopentadiene than KNO3/Al2O3. Trakarnpruk et al.30 loaded CH3COOK on calcined HT and synthesized biodiesel at the condition of a 30:1 methanol/oil molar ratio at 373 K for 6 h and 7 wt % catalyst, with the fatty acid methyl ester content becoming 96.9%. Palm oil (PO) is one of the four leading vegetable oils traded on the world market. Cheaper than canola, rapeseed, or soybean oil, the use of PO would reduce the overhead cost of biodiesel production and generate a steady supply of diesel fuel substitute. In our early work, KF/Mg-Al HT31 had been obtained and was successfully used in the transesterification of PO and the catalyst showed its activity in this reaction with a FAME yield of 85% under optimum conditions. In this paper, Ca was added to Mg-Al HT to form a three-metal mixed HT, Ca-Mg-Al HT. On the basis of Ca-Mg-Al HT, a kind of solid alkyl catalyst was synthesized and used for biodiesel production with the method of transesterification of PO with methanol.

Table 1. CCD Conditions for the FAME Yield Catalyzed by KF/ Ca-Mg-Al HT run

M1 (Ca/Al molar ratio)

M2 (Mg/Al molar ratio)

1 2 3 4 5 6 7 8 9 10 11 12 13

2.00 3.00 3.41 2.00 0.59 2.00 1.00 3.00 2.00 2.00 2.00 2.00 1.00

2.00 1.00 2.00 2.00 2.00 2.00 1.00 3.00 2.00 2.00 0.59 3.41 3.00

Table 2. FAME Yield over Different Catalysts catalyst Mg-Al HT Ca-Al HT Ca-Mg-Al HT calcined Ca-Mg-Al HT KF KF/Mg-Al HT KF/Ca-Al HT KF/Ca-Mg-Al HT (2:3:1 Ca/Mg/Al)

2. Experimental Section 2.1. Materials. PO and CH3OH were purchased from the Nanjing Runtai market, Nanjing Chemical Reagent Co., Ltd., and KF 3 2H2O, Ca(NO3)2 3 4H2O, Mg(NO3)2 3 6H2O, Al(NO3)3 3 9H2O, NaOH, and Na2CO3 were purchased from Guangdong Guanghua Chemical Factory Co., Ltd. 2.2. Catalyst Preparation. The Ca-Mg-Al hydroxides were prepared by the co-precipitation method. Solution A (200 mL) contained a certain amount Ca(NO3)2 3 4H2O, Mg(NO3)2 3 6H2O, and Al(NO3)3 3 9H2O, with the total cation molar concentration being 1 mol/L, while solution B (200 mL) contained NaOH (0.4 mol) and Na2CO3 (0.1 mol). Both solutions were synchronously dropped into 100 mL of water at 338 K, slowly accompanied with vigorous mechanical stirring, while maintaining the pH between 10 and 11. The resulting mixture was held at 338 K while continuing to stir vigorously for 48 h and then filtered and washed with water until the pH value of the filtrate was near 7. The precipitate was dried at 373-398 K overnight and calcined at 723 K for 5 h. The calcined powder was ground with KF 3 2H2O, with the mass ratio of 100% (wt/wt calcined powder), while dropping some water. Then, the paste was dried at 338 K overnight. 2.3. Catalyst Characterization. The catalyst was characterized by powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). XRD measurements were performed on a Rigaku D/max-A instrument with Cu KR radiation at 50 kV and 30 mA and a scan speed of 0.02°/min. 2.4. Transesterification. Transesterification experiments were carried out in a 100 mL four-necked flask, provided with a thermostat and mechanical stirring systems. A total of 30 g of PO and different amounts of methanol and catalyst were added and heated to a given temperature, accompanied with stirring. The reactions were undertaken with a refluxing temperature.

reaction time 5h 5h 5h 5h 5h 3h 1h 10 min

FAME yield (%) 3.6 4.4 5.6 10.3 probability > F ), which proved that, on the basis of the significance of the effects of factors in the experimental interval and possible interactions, the interpretation was correct. Details of the experimental results on the FAME yield through response surface analysis were shown in Figure 2b. According to the model, an optimal Ca2þ and Mg2þ ratio was obtained as follows: Ca/Mg/Al was 2.2:0.8:1, and at the same reaction conditions the FAME yield predicted was 99.4%. According to the CCD analysis results, the transesterification reaction was undertaken with the following conditions: reaction temperature, 338 K; reaction time, 10 min; methanol/oil molar ratio, 12:1; catalyst, KF/Ca-Mg-Al HT (2.2:0.8:1 Ca/Mg/Al; KF mass ratio of 100%); amount, 5% (wt/wt oil); and FAME yield, 99.6%, which was not different from the predicted value. Therefore, the model obtained by the CCD method had some guiding significance. 3.1.3. Influence of the Methanol/Oil Molar Ratio. Both panels a and b of Figure 3 showed the influence of the methanol/oil molar ratio on FAME yields with different catalysts. Because the transesterification reaction is an equilibrium reaction, an excess of methanol will increase the oil conversion by shifting this equilibrium to produce FAME. From both panels a and b of Figure 3, the FAME yields increased as the methanol/oil molar ratio increased from 6:1 to 12:1. This could be explained by the pushing effect of excess methanol on the reaction balance. Unlike the situation catalyzed by KF/Mg-Al HT;31 however, the methanol/ oil molar ratio did not influence the FAME yield significantly, which might be a result of the fact that the high activity catalysts were provided more basic sites, which made the reaction much faster and gave a relatively higher FAME yield. From panels a and b of Figure 3, a lower methanol/oil molar ratio, such as 9:1, could be used to cut cost without significant biodiesel yield falling. 3.1.3. Recycle Using Catalyst. Table 3 showed the FAME yield catalyzed by calcined and uncalcined fresh catalysts 648

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Figure 2. (a) Normal plot of residuals. (b) Three-dimensional response surface for the FAME yield in relation to the Ca2þ and Mg2þ molar ratio.

Figure 3. (a) Influence of the methanol/oil molar ratio on the FAME yield (0.5:3:1 to 3:3:1 Ca/Mg/Al). (b) Influence of the methanol/oil molar ratio on the FAME yield (3:0.5:1 to 3:3:1 Ca/Mg/Al). Table 3. FAME Yield Catalyzed by Fresh and Reused Catalyst (3:2.5:1 Ca/Mg/Al) used times

fresh (%)

used once (%)

used twice (%)

after loaded, calcined at 873 K, fresh (%)

after use 3 times and then calcined at 873 K (%)

FAME yield

95.93

51.88

40.61

90.35

78.62

and reused and regenerated ones. When the FAME yield catalyzed by the uncalcined and calcined fresh catalysts was compared, the uncalcined ones gave a higher FAME yield. In combination with the XRD patterns and our former research on KF/Mg-Al HT,31 it was believed that the layer structure of the catalyst helped raise the activity of the catalyst. It was clearly seen that the FAME yield decreased apparently when the reaction was catalyzed by the fresh catalyst and the used once one without any disposal and the FAME yields catalyzed by the used once and twice catalysts were not as discrepant as that of the first and second time. In combination with the XRD patterns (Figure 6), the huge decline of the FAME yield might not be caused by the loss of the active crystal (in Figure 6, there are still strong XRD peaks

belonging to them) but by the adsorption of the byproduct, glycerin, which decreased the effective contact surface of the catalyst with the reactants. In Figure 7, the used catalyst showed clear FTIR peaks belonging to the -OH of glycerin. After calcinations, the glycerin adsorbed was burnt and the active site emerged again; therefore, the FAME yield went up again, reaching 78.6%, which was much higher than the results by the used once catalyst. According to the XRD patterns, calcination might also reform the active crystal, which seemed greatly reduced after the reaction 3 times. 3.2. Catalyst Characterization. Figure 4 showed the XRD patterns of different catalysts. In pattern A, peak a was the typical HT peak, which meant that the sample had a layered structure, and peak b belonged to the active crystal (KMgF3) 649

Energy Fuels 2010, 24, 646–651

: DOI:10.1021/ef900800d

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Figure 4. XRD pattern of different catalysts: (A) KF/Mg-Al HT (mass ratio of 100%), (B) KF/Ca-Al HT (mass ratio of 100%), and (C) KF/Ca-Mg-Al HT (2:3:1 Ca/Mg/Al; mass ratio of 100%). Figure 6. XRD patterns of catalysts before and after use.

Figure 5. XRD patterns of KF/Ca-Mg-Al HT with different cation molar ratios (Ca2þ ratio rose).

Figure 7. FTIR profiles of different catalysts: (A) catalyst used once, (B) fresh calcined catalyst, and (C) used calcined catalyst.

of KF/Mg-Al HT.30 In pattern B, according to the early research of our team, peak c belonged to the active crystal (KCaF3, CaAlF5, and KCaCO3F) of KF/Ca-Al HT. In pattern C, it was clearly seen that the KF/Ca-Mg-Al HT synchronously had peaks a, b, and c, which meant that this catalyst had the high active crystals that showed in KF/ Mg-Al HT and KF/Ca-Al HT as well as the layered structure. These two parts, active crystal and layered structure, made the new catalyst show high catalysis activity in the transesterification reaction of oil with methanol. Therefore, the layered structure, KMgF3, KCaF3, CaAlF5, and KCaCO3F were the active components of the KF/Ca-Mg-Al HT catalyst. Figure 5 showed the XRD patterns of KF/Ca-Mg-Al HT with different cation molar ratios. When the Ca2þ molar ratio was increased, the peaks at 20°, 23°, 28.5°, 35°, and 47°, which belonged to the active centers CaAlF5, KCaF3, and KCaCO3F, enhanced. At the same time, the peaks belonging to the layer structure and KMgF3 decreased. This phenomenon might be caused by the following reasons: when the Ca2þ molar ratio increased, more Al3þ combined with it and partly destroyed the layer structure, the Mg2þ relative molar ratio decreased instantaneously, and then less KMgF3 was formed. However, when Ca/Mg/Al reached 3:3:1, the peaks of the active

crystals and layer structure all went down, because too much divalent cations were added and there were not enough trivalent cations to act with them, and then Ca2þ and Mg2þ aggregated respectively and reduced the amount of content that could interact with KF to form the active structure. In Figure 6, the peaks belonging to the active structure of the KF/Ca-Mg-Al HT were not reduced after use once. It was shown that, after use twice, the active centers (peaks at 20°, 23°, 28.5°, 35°, and 47°) nearly disappeared and the layer structure (peaks at 11.6°, 23.4°, 35°, 39.6°, and 47.1°) was destroyed completely. This meant that this catalyst should be regenerated after been used twice. After use once, the peaks belonging to the active centers of the catalysts did not weaken obviously; however, the FAME yields decreased greatly (from 96 to 58%). This could be explained by the following reason: in combination with the active site of the catalyst was the precondition that methanol reacted with oil; however, the byproduct glycerin could also combine with the catalyst, which largely reduced the contact opportunity of the methanol and catalyst, which can be seen from Figure 7. In Figure 7, it can been seen that, in comparison to the calcined fresh catalyst (curve B), the used once catalyst (curve A) showed two clear peaks around 2800 cm-1, which belonged to the -OH of glycerol, and after calcined 650

Energy Fuels 2010, 24, 646–651

: DOI:10.1021/ef900800d

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Ca showed much stronger peaks of CaAlF5, KCaF3, and KCaCO3F. In combination with the FAME yield catalyzed by these two catalysts, the FAME yield was a little higher for the Ca2þ-excess catalyst than the Mg2þ-excess one. Then, it was believed that the active center CaAlF5, KCaF3, and KCaCO3F played a more important role in building the catalyst activity than KMgF3. In addition, the samples with excess Ca2þ showed distinct peaks belonging to CaCO3 (23°, 29.5°, 31.5°, and 36°), which meant that the Ca2þ actually aggregated and formed a non-active center. 4. Conclusion The results of this study confirmed that KF/Ca-Mg-Al HT was a very highly effective catalyst for the transesterification reaction of PO with methanol. Catalyzed by this kind of catalyst, the biodiesel yield could obtain over 90% in 10 min under the following reaction conditions: reaction temperature, 338 K; methanol/oil molar ratio, 12:1; and catalyst amount, 5% (wt/wt oil). The catalyst with an optimal cation ratio (2.2:0.8:1) was obtained by the CCD method, and at the same reaction conditions, the FAME yield could reach 99.6%. This catalyst largely shortened the reaction time of the biodiesel production and showed a hopeful future of the new producing method of biodiesel.

Figure 8. XRD patterns of KF/Ca-Mg-Al HT with different Ca2þ and Mg2þ molar ratios.

(curve C), the two peaks disappeared, which meant that the glycerol has been burnt during the calcinations. Figure 8 showed the XRD patterns of KF/Ca-Mg-Al HT, in which Ca/Mg/Al was 1:3:1 and 3:1:1, respectively. These two samples could delegate the catalyst, which had excess Ca2þ or Mg2þ. From this figure, it was seen that the sample with excess Mg2þ had more obvious HT typical peaks of the layer structure, while the one with excess Ca2þ scarcely had any, which meant that the Ca2þ may affect the formation of the lay structure of HT and too much Ca2þ would destroy the regular layer structure. However, the catalyst with more

Acknowledgment. The authors are grateful to the National High Technology Research and Development Program of China (2009AA03Z222 and 2009AA05Z437) and “Six Talents Pinnacle Program” of Jiangsu Province of China (2008028) for financial support.

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