Energy & Fuels 2009, 23, 2259–2263
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Transesterification of Triglycerides by CaO: Increase of the Reaction Rate by Biodiesel Addition M. Lo´pez Granados,*,† D. Martı´n Alonso,† A. C. Alba-Rubio,† R. Mariscal,† M. Ojeda,† and P. Brettes‡ Instituto de Cata´lisis y Petroleoquı´mica, Consejo Superior de InVestigaciones Cientı´ficas (CSIC), C/Marie Curie 2, Campus de Cantoblanco, Madrid 28049, Spain, and Centro Tecnológico GAIKER, Parque Tecnológico Edificio 202, Zamudio, Bizkaia 48170, Spain ReceiVed NoVember 11, 2008. ReVised Manuscript ReceiVed January 30, 2009
A simple procedure has been found that significantly promotes the transesterification reaction rate catalyzed by CaO and moreover prevents the catalyst poisoning by ambient CO2 and H2O. The presence of a small amount of biodiesel in the initial methanol-triglyceride mixture (3 wt % referred to oil) results in a significant increase of the rate of triglyceride methanolysis carried out in a batch reactor. The biodiesel must be previously mixed with the activated CaO, forming a paste. The formation of such a paste also results in a remarkable protection of the activated CaO against the poisoning by ambient CO2 and H2O that may occur during handling of the activated solid.
1. Introduction Biodiesel is a mixture of fatty acid methyl esters (FAMEs) obtained by transesterification of triglycerides with methanol. Biodiesel is mainly produced from vegetable oils and, hence, is a renewable fuel, beneficial to decrease greenhouse gas emissions and to avoid the current dependence upon petroleumderived fuels.1-5 The transesterification reaction is usually performed using basic homogeneous catalysts (NaOH or KOH dissolved in methanol) at mild conditions (∼333 K and atmospheric pressure). This process requires, however, expensive and non-enviromentally friendly purification and separation steps to remove the homogeneous catalyst from the glycerine and the produced biodiesel.1-3 An alternative approach to avoid these problems is heterogeneous catalysis because solid catalysts can be separated from the reaction mixture by filtration and do not involve the consumption of large amounts of water to eliminate the alkali hydroxide. Many solid catalysts, basic or acidic, have been developed for biodiesel production.6,7 Basic catalysts show higher reaction rates, and different types of these catalysts have been studied, including basic zeolites, alkali and alkali earth oxides and carbonates, hydrotalcites, etc.8-11 Among * To whom correspondence should be addressed. Telephone: +34915854937. Fax: +34-915854760. E-mail:
[email protected]. † Consejo Superior de Investigaciones Cientı´ficas (CSIC). ‡ Centro Tecnológico GAIKER. (1) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (2) Srivastava, A.; Prasad, R. Renewable Sustainable Energy ReV. 2000, 4, 111–133. (3) Barnwal, B. K.; Sharma, M. P. Renewable Sustainable Energy ReV. 2005, 9, 363–378. (4) Frascari, D.; Zuccaro, M.; Pinelli, D.; Paglianti, A. Energy Fuels 2008, 22, 1493–1501. (5) Yori, J. C.; D’Amato, M. A.; Grau, J. M.; Pieck, C. L.; Vera, C. R. Energy Fuels 2006, 20, 2721–2726. (6) Lotero, E.; Liu, Y. J.; Lopez, D. E.; Suwannakarn, K.; Bruce, D. A.; Goodwin, J. G. Ind. Eng. Chem. Res. 2005, 44, 5353–5363. (7) Di Serio, M.; Tesser, R.; Pengmei, L.; Santacesaria, E. Energy Fuels 2008, 22, 207–217. (8) Suppes, G. J.; Dasari, M. A.; Doskocil, E. J.; Mankidy, P. J.; Goff, M. J. Appl. Catal., A 2004, 257, 213–223.
the alkali and alkali earth oxides, CaO has been previously reported to be one of the most active catalysts.12-18 There are two relevant key limitations for the use of basic solid catalysts from an industrial point of view: (i) heterogeneous catalysts show lower reaction rates compared to the homogeneous counterparts, and (ii) the surface active sites are rapidly poisoned upon exposing the catalyst to ambient air because of the chemisorption of carbon dioxide and water on the surface sites to form carbonates and hydroxyl groups, respectively.12 Therefore, it is critical to avoid the catalyst direct exposure to room air while handling and transferring the catalyst from the activation unit to the reactor. The transesterification of triglycerides with methanol using an activated basic heterogeneous catalyst (e.g., CaO) is usually accomplished in a batch reactor by adding the solid to the oil-methanol mixture previously formed in the stirred tank reactor. We have found here a simple procedure that significantly promotes the transesterification reaction rate and moreover prevents the catalyst poisoning by ambient CO2 and H2O. The (9) MacLeod, C. S.; Harvey, A. P.; Lee, A. F.; Wilson, K. Chem. Eng. J. 2008, 135, 63–70. (10) Bordawekar, S. V.; Doskocil, E. J.; Davis, R. J. Catal. Lett. 1997, 44, 193–199. (11) Xie, W.; Peng, H.; Chen, L. J. Mol. Catal. A: Chem. 2006, 246, 24–32. (12) Lo´pez Granados, M.; Poves, M. D. Z.; Martin Alonso, D.; Mariscal, R.; Cabello Galisteo, F.; Moreno-Tost, R.; Santamaria, J.; Fierro, J. L. G. Appl. Catal., B 2007, 73, 317–326. (13) Albuquerque, M. C. G.; Jimenez-Urbistondo, I.; SantamariaGonzalez, J.; Merida-Robles, J. M.; Moreno-Tost, R.; Rodriguez-Castellon, E.; Jimenez-Lopez, A.; Azevedo, D. C. S.; Cavalcante, C. L.; MairelesTorres, P. Appl. Catal., A 2008, 334, 35–43. (14) Zhu, H. P.; Wu, Z. B.; Chen, Y. X.; Zhang, P.; Duan, S. J.; Liu, X. H.; Mao, Z. Q. Chin. J. Catal. 2006, 27, 391–396. (15) Liu, X. J.; He, H. Y.; Wang, Y. J.; Zhu, S. L.; Piao, X. L. Fuel 2008, 87, 216–221. (16) Liu, X.; Piao, X.; Wang, Y.; Zhu, S.; He, H. Fuel 2008, 87, 1076– 1082. (17) Kouzu, M.; Kasuno, T.; Tajika, M.; Sugimoto, Y.; Yamanaka, S.; Hidaka, J. Fuel 2007, 87, 2798–2806. (18) Kouzu, M.; Kasuno, T.; Tajika, M.; Yamanaka, S.; Hidaka, J. Appl. Catal., A 2008, 334, 357–365.
10.1021/ef800983m CCC: $40.75 2009 American Chemical Society Published on Web 03/09/2009
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exact procedure reported here consists of mixing the activated catalyst with a small amount of biodiesel after the catalyst activation step, thus forming a paste. This paste is then added to the reactants mixture following the procedure indicated in the Experimental Section. The own biodiesel used to form the paste prevents the deactivation by poisoning through contact with CO2 and H2O present in ambient air. This leads to important benefits when considering industrial applications of CaO as a solid catalyst and the possibility of storing and handling the activated catalyst without taking special actions from preventing the contact with ambient air.19 2. Experimental Section The catalytic activity of the catalyst was measured out in a batch tank reactor with the solid suspended in the reaction mixture. The reactor was a 100 mL jacketed three-necked glass reactor connected to a condenser and a dropping funnel. The third neck was used to introduce the gas connections and the thermocouple. Activated CaO catalyst was obtained by treating the precursor CaCO3 (178 mg, Sigma-Aldrich, >99.95%) in a U-shaped quartz reactor at 1073 K for 1 h (rate of 10 K min-1) under 40 mL min-1 of 20 vol % O2/Ar flow (Alphagaz, O2 purity > 99.995%, Ar purity > 99.999%). The obtained CaO (100 mg) was cooled under inert flow and then rapidly poured over the protecting agent (oil, methanol, or biodiesel) in a flask. The paste thus formed was poured over anhydrous methanol (Riedel de Hae¨n, Reag. Ph. Eur., >99.8% GC, H2O < 0.005%) already placed at the reactor at 323 K. Methanol was also used to help the pouring of the paste. The final amount of methanol in the reactor was 27 g. The methanol-solid mixture was agitated for 15 min to equilibrate the temperature at 323 K. This step is required to ensure the reproducibility of the experiments. A longer equilibrium time did not result in better results, and a shorter contact time resulted in the lack of reproducibility of the experiments. The reaction started when 50 g of vegetable oil (methanol/oil molar ratio ca. 14:1, which is in the range frequently used) preheated at 323 K was added to the methanol and catalyst mixture under vigorous agitation (1000 rpm) through the dropping funnel. The reaction was conducted at atmospheric pressure for 3-5 h. The reaction temperature was set at 323 K. Only 0.2 wt % of catalyst was used to show more clearly the differences of the activity tests and to facilitate the timing in the sampling. A higher temperature or larger catalyst loading would only result in much faster kinetics, which is unnecessary for the purpose of this work. For comparison purposes, homogeneous catalytic experiments were also conducted using 0.2 wt % of KOH dissolved in the methanol by using the same reaction conditions as in the heterogeneous case. Aliquots for sampling analysis were extracted with a syringe and neutralized with an slight excess of 0.1 M HCl to stop the reaction, and the resulting solution was washed with dichloromethane. This washing with HCl and dichloromethane was performed twice. The alcohol phase (water, glycerine, methanol, HCl, and CaCl2) was separated from the organic phase (glycerides, dichloromethane, and methyl esters) by decantation, and the residual dichloromethane in the methyl ester phase was finally removed by evaporation at 353 K. Once the methyl esters were purified, quantitative analysis was carried out following the procedure described elsewhere.12 Briefly, the content in FAME was determined in accordance with the European regulated procedure EN 14103 using a gas chromatograph (Agilent 6890GC) connected to a flame ionization detector (FID) equipped with a HP INNOwax capillary column. Different catalytic tests carried out under exactly the same operation conditions showed that the deviation in the FAME yield values is within 5% all over the reaction time range. Three different vegetable oils were used: edible sunflower and refined soybean and cynara oils. The oil and methanol used to (19) Lo´pez Granados, M.; Martı´n Alonso, D.; Mariscal, R.; Brettes, P. Spanish Patent Application ES200801372, 2008.
Figure 1. Yield to FAME (wt %) obtained by sunflower oil transesterification with methanol when the activated CaO catalyst is first contacted with 1 g of sunflower oil (blue [), methanol (red 2), or biodiesel (green b). The yield obtained with a homogeneous catalyst (KOH) is also shown (black 9). The orange 9 represents the yield obtained when the activated catalyst is directly added to the oil-methanol reaction mixture, and the open symbols represent the yield obtained when the activated catalyst was first contacted with oil (blue ]) and methanol (red 4) and then added to the reactant mixture containing 1 g of biodiesel. Reaction conditions: 50 g of sunflower oil, 0.2 wt % catalyst, 323 K, 1000 rpm, and molar methanol/oil ca. 14.
protect the catalyst are those used in activity measurements, whereas the biodiesel was previously obtained by methanolysis of sunflower oil at 323 K for 2 h using 1% of KOH as a catalyst and 14:1 molar MeOH/oil (a higher catalyst loading was used to prepare a large amount of biodiesel in a short reaction time). This biodiesel was washed with HCl and dichloromethane, following the same procedure mentioned above. The activated CaO was characterized by measuring the N2 adsorption-desorption isotherms at 77 K in a Micromeritics ASAP 2000 apparatus after evacuation at 363 K for 20 h. For this study, the activated CaO was obtained by decomposing the precursor CaCO3 at 1073 K for 2 h under outgassing in a vacuum line in the same cell used for recording the isotherm.
3. Results and Discussion It is well-known that the exposure of activated CaO to ambient air leads to a severe catalyst deactivation because of the poisoning by carbonation/hydration of the active sites.12 We have foreseen that this drawback could be prevented by wetting the catalyst with a small amount of vegetable oil, methanol, or even with the same biodiesel, because of the lower polarity and, in some cases, hydrophobic character of these compounds. The filling of the pores of the catalyst with these liquids prevents the rapid diffusion of CO2 and H2O to the surface sites. The paste is then added to the reactant mixture following the procedure indicated in the Experimental Section. This procedure significantly simplifies the special measures that must be taken when handling, storing, or transferring the activated catalyst from the activation equipment to the reactor. Figure 1 depicts the yield to FAME obtained during sunflower oil transesterification with methanol when the activated CaO catalyst is first mixed with either a small amount (1 g) of oil, methanol, or biodiesel (representing 2 wt % of protecting agent with respect to the oil). The yield using a homogeneous process (KOH) is also shown for comparison purposes. The transesterification performance significantly depends upon the identity of the substance (oil, methanol, or biodiesel) initially mixed with the CaO catalyst. At a specific reaction time, the lowest yield to FAME is found when the CaO catalyst is previously mixed with oil. The paste with methanol results in a larger FAME yield
Transesterification of Triglycerides by CaO
Figure 2. Yield to FAME (wt %) obtained during the transesterification of different vegetable oils with methanol using KOH (open symbols) and activated CaO (closed symbols). Reaction conditions: 50 g of oil, 0.2 wt % catalyst, 323 K, 1000 rpm, and molar methanol/oil ca. 14. CaO was previously protected with 1 g of biodiesel. Sunflower oil (black O and b), soybean oil (red 4 and 2), and cynara oil (blue 0 and 9).
but still considerably lower than that shown by the homogeneous catalyst. Figure 1 also includes the results when the catalyst is directly incorporated to the reaction mixture: the activity is similar to that found when the catalyst is protected with oil. When the paste is formed upon wetting the activated solid with biodiesel, a significant improvement of the catalytic performance is found, being much closer to the activity displayed by the homogeneous catalysis. This improvement in the FAME yield is much larger than the 2 wt % initially added. The results represented by the open symbols will be discussed below. The significant positive effect of biodiesel addition to the CaO catalyst on the transesterification rate is also observed with other vegetable oils. Figure 2 compares the yields to FAME obtained in sunflower, soybean, and cynara oil methanolysis using the activated CaO catalyst mixed previously with 1 g of sunflower biodiesel. The differences in reaction rates observed with the three vegetable oils studied here are beyond the error bar of the FAME yield that was determined to be 5%. They are very likely related to some differences in oil composition (such as water and FFA contents and mainly because of the different type of fatty acids chain in triglycerides). It seems also very reasonable to expect that, for the same reasons, the use of different types of biodiesel arising from different type of oils results in variations in the performance of the same order of magnitude as those shown in Figure 2. The amount of biodiesel initially added to the activated CaO catalyst influences the extent of the catalytic activity enhancement. As shown in Figure 3, the FAME yield increases as the amount of the biodiesel added to CaO increases in the range of 0-3 wt % (with respect to the sunflower oil phase). The 0 wt % curve corresponds to the direct addition of the activated catalyst to the reactant mixture. It must be taken into account that, in this figure, the FAME initially added was subtracted to obtain the reaction rate for FAME formation. The biodiesel addition above 3 wt % does not result in an additional increment of the reaction rate. It must also be noticed that the curve representing the no use of biodiesel is sigmoid, which indicates that there is an induction period in the progress of the reaction and as reaction progresses the rate increases. In the case of homogeneous catalysis, the induction period has been associated with the building up of the oil concentration in the methanol
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Figure 3. Effect of the amount of initial biodiesel used to form the catalyst paste on the FAME yield. Reaction conditions: 50 g of sunflower oil, 0.2 wt % catalyst, 323 K, 1000 rpm, molar methanol/oil ca. 14, and percentage of biodiesel expressed with respect to the sunflower oil phase: 0% (black 9), 1% (red b), 2% (green 2), 3% (blue [), and 4% (cyan f).
phase,20,21 where the catalyst is; it takes time for the oil to reach a concentration large enough in the methanol phase for the reaction to be kinetically controlled. In the results of Figure 1, the homogeneous catalysis curve is not sigmoid because it is well-known that the induction period takes only a few minutes.20,21 Similar effects must be observed in the heterogeneous catalysis, but the induction period is longer than in the homogeneous case. Very likely the mesoporous character of the solid catalyst results in a longer induction period; now, the oil phase must diffuse through the mesoporous of the solid to reaching the surface active sites. This induction period is not observed in the curves of the figure representing the biodiesel-promoted experiments, which indicates that the addition of small amounts of biodiesel improves the methanolysis reaction rate from the very beginning of the reaction. We will tentatively propose some possible explanations related to the improvement of the mass transfer of triglyceride molecules to the active surface sites. A first plausible explanation is related to the formation of microemulsions that increases both the solubility of oil in methanol and methanol in oil. Biodiesel (a mixture of FAME) can be viewed as a surfactant,22,23 and therefore, microemulsions oil-in-water (triglycerides in methanol) or water-in-oil (methanol in triglycerides) may be readily formed. As a result, the solubility of oil in the methanol-FAME phase can be increased and that of methanol in the oil-FAME phase as well. The formation of these microemulsions, in which triglycerides (or methanol) are better dispersed in the alcoholic phase (or in the oil phase) would improve the accessibility and the diffusion of the reactant molecules to the surface active sites, provided that the size of the micelles are much smaller than the dimension of the catalyst pores, thus leading to higher reaction rates. Figure 1 displays the yield obtained when the activated CaO was first pasted with 1 g of oil (blue ]) or 1 g of methanol (red 4) and then added to a reaction mixture containing the methanol-oil reactant mixture and 1 g of biodiesel. It is evident that the presence of the biodiesel in the reaction mixture did not improve the catalytic performance; in the oil case, the behavior is slightly worse. (20) Boocock, D. G. B.; Konar, S. K.; Mao, V.; Sidi, H. Biomass Bioenergy 1996, 11, 43–50. (21) Boocock, D. G. B. U.S. Patent 6,712,867 B1, 2004. (22) Johansson, I.; Svensson, M. Curr. Opin. Colloid Interface Sci. 2001, 6, 178–188. (23) Zhou, H.; Lu, H. F.; Liang, B. J. Chem. Eng. Data 2006, 51, 1130– 1135.
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Figure 4. N2 adsorption-desorption isotherms at 77 K of activated CaO. Full and empty circles represent the adsorption and desorption branches, respectively.
Therefore, the promotion via the formation of a microemulsion driven by the presence of biodiesel in the reaction mixture can be discarded. A second hypothesis that explains the positive effect of mixing the catalyst with biodiesel before the reaction could be the filling of the pores of the catalyst with liquid biodiesel instead of oil or methanol. If the catalyst is previously wetted with oil, we must bear in mind that oil is largely insoluble in methanol (a very small amount of oil can be dissolved in methanol). Then, once the catalyst is in the reaction mixture, the methanol cannot extract the oil out of the pores and the practical consequence is that the methanol has no easy access to the surface active sites; it must diffuse through the oil phase that fills the pores. A similar argument can be proposed when the catalyst is first wetted with methanol. Methanol is not soluble in oil, and consequently, it cannot be removed by triglycerides. Therefore, the oil molecules will not have much access to a porous surface covered by a methanol phase. On the contrary, biodiesel is known to fully dissolve both triglycerides and methanol.23 Consequently, even if the pores of the catalyst are initially filled with biodiesel, the access of both triglycerides and methanol molecules is greatly facilitated and this would increase the reaction rate. In an attempt to assess the effect of the mesoporous texture on the catalytic behavior, the N2 adsorption and desorption isotherms of the activated CaO were recorded (Figure 4). The isotherm corresponds to type IV (IUPAC classification) and shows an hysteresis loop (type H4) resulting from the interparticle mesoporosity. The specific surface area of the catalyst was determined by the Brunauer-Emmett-Teller (BET) method, and it was found to be 43 m2 g-1. The mean pore diameter obtained by applying the Barrett-Joine-Halenda (BJH) method on the desorption branch of the isotherm is 24 nm. The catalyst pore volume is ∼0.3 mL gcat-1. The minimum amount of biodiesel added to CaO in our experiments is ∼6 mL gcat-1 (1 wt %, referred to oil), which is more than 1 order of magnitude higher than the quantity required to completely fill the catalyst pores, bearing in mind that we have used 100 mg of CaO. Therefore, in principle, the gradual increment of the FAME yield observed in Figure 3 as the amount of biodiesel is increased (1-3%) could not be explained by the progressive covering of the surface because the catalyst pores should already be filled with the minimum amount of biodiesel added (1%). However, it must be taken into account that biodiesel can be removed out of the pores because the paste is subjected to a washing step with methanol and FAMEs are
Granados et al.
Figure 5. FAME yield (wt %) obtained with 3 wt % biodiesel-protected CaO without contact with room air (black 9) and contacted with ambient air for 1 h (red b) and 24 h (red O). For comparison purposes, the FAME yield obtained with CaO contacted first with ambient air for 1 h and then pasted with biodiesel (3 wt %) is also included (green 2).
fully soluble in methanol. It seems reasonable that a small amount of biodiesel can still remain within the pores, despite the immersion of the paste in methanol. The properties of the liquid molecules when constrained within nanometric pores are different from those in bulk liquids, and this may result in the lack of effectiveness in the extraction by methanol. If this is true, a certain amount of biodiesel can still be left within the pores after the methanol rinsing in the region of biodiesel concentration studied here (1-3 wt % referred to oil). The 3 wt % of biodiesel would then represent the minimum amount of biodiesel required to fully cover the surface after the immersion in methanol. Neither oil nor methanol can be dissolved out of the pores to the reactant mixture; therefore, they become an impervious layer that prevents the access of either methanol or oil, respectively, to the surface sites. With the aim at testing whether the layer of biodiesel protects against the deactivation, the paste formed between the activated CaO and biodiesel (3 wt % with respect to the oil) was set aside in an open flask, allowing the ambient air to contact the paste for 1 and 24 h. Then, as described in the Experimental Section, the paste was poured over methanol and the transesterification reaction was carried out. These results are represented in Figure 4, which also shows for comparison purposes the catalytic behavior of the activated CaO protected with biodiesel but without exposure to the ambient air. The figure additionally includes the results for an activated CaO that was directly exposed to ambient air for 1 h in the open flask without biodiesel protection. After this contact, the resulting solid was mixed with biodiesel (3 wt % with respect to oil), the paste was poured over methanol, and the transesterification reaction was carried out. These experiments clearly show that, if CaO is not protected with biodiesel, it experiences a notable deactivation with only 1 h of contact with room air. On the contrary, if CaO is pasted with the biodiesel, the deterioration was negligible after 1 h of contact with room air and a very small degree of deactivation was observed when contacted for 24 h. Therefore, the results of Figure 5 clearly demonstrate that the addition of biodiesel to the activated CaO forming a paste not only results in a remarkable increase of the reaction rate but is also a proven procedure to protect the solid catalyst from the H2O and CO2 present in the ambient air. This has practical consequences for the handling of the activated CaO. Quite often, the activation of the catalyst must be performed ex situ, and therefore, during transferring the catalyst from the activation unit to the trans-
Transesterification of Triglycerides by CaO
Figure 6. FAME yield (%) obtained with the activated catalyst previously protected with biodiesel (3 wt % of biodiesel with respect to the oil used in the reaction) and after successive runs. Reaction conditions: 200 g of sunflower oil, 0.2 wt % catalyst, 333 K, 1000 rpm, molar methanol/oil ca. 14, and after 300 min of reaction).
esterification reactor, attention must be paid to completely prevent the contact of the activated CaO surface with CO2 and H2O. It is demonstrated here that no special measures must be taken with the biodiesel-CaO paste to prevent the contact with room air as long as the transferring preferentially proceeds in less than 24 h. If industrial application is considered, a practical solution can be the storing of the biodiesel-activated CaO paste in closed containers filled with an inert gas, such as N2, to allow to the increase the storing period. Figure 6 informs about the reuse of the activated catalyst previously pasted with biodiesel (3 wt %), clearly showing that the catalyst can be reused for three consecutive cycles. The solid was recovered after each run by filtering and used in a new catalytic test. The addition of biodiesel to the catalyst was only performed for the first run. This operation was not conducted for the second and successive runs because the catalyst was already in contact with biodiesel during the reaction. Our results are consistent with previous reports,24,26,27 although the number of reuse is lower in our case. This is explained by the lower catalyst loading used here. It has been demonstrated that the catalyst is leached into the reaction media, mainly in the alcohol phase.12,25-27 When a sufficient amount of solid catalyst is initially loaded, the leaching does not hinder the reuse for a considerable number of runs.24,26,27 However, the effect of solid (24) Liu, X. J.; He, H. Y.; Wang, Y. J.; Zhu, S. L.; Piao, X. L. Fuel 2008, 87, 216–221. (25) Albuquerque, M. C. G.; Santamarı´a-Gonza´lez, J.; Me´rida-Robles, J. M.; Moreno-Tost, R. M.; Rodrı´guez-Castello´n, E.; Jime´nez-Lo´pez, A.; Azevedo, D. C. S.; Cavalcante, C. L., Jr.; Maireles-Torres, P. Appl. Catal., A 2008, 347, 162–168. (26) Kouzu, M.; Yamanaka, S.; Hidaka, J.; Tsunomori, M. Appl. Catal., A 2009, doi: 10.1016/j.apcata.2008.12.003. (27) Lo´pez Granados, M.; Martı´n Alonso, D.; Sa´daba, I.; Mariscal, R.; Oco´n, P. Appl. Catal., B 2009 (doi:10.1016/j.apcatb.2009.02.014).
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solubilization and concomitant loss of mass is more significant at low catalyst loadings, as in our case (0.2 wt %). According to very recent work,26,27 the extension of the solubilization is 0.5-1 mg of equivalent CaO/mL of the final alcohol mixture (glycerol and methanol). A conservative estimation of the catalyst remained after the third and fourth run (Figure 6) indicates ca. 0.11 and 0.08 wt % of equivalent CaO, respectively. Less conservative estimations would lead to even lower values. In conclusion, the decline in activity found in this work beyond the third run is consistent with the previous observations for CaO. This indicates that biodiesel addition to the catalyst before the reaction does not alter the typical behavior reported for this solid. Notwithstanding that more experiments may be required to confirm the explanation of the biodiesel effect on the reaction rate, it is clear that this procedure results in important benefits for the industrial application of CaO as a catalyst for biodiesel production. First, it remarkably improves the reaction rate; although the heterogeneous process based on CaO is still less active than the KOH homogeneous process, on either a weight or molar basis, this difference can be overcome using larger initial CaO loading. It must be taken into account that CaO can be reused several times,24,26 and therefore, CaO is more productive on a mass and molar basis than KOH homogeneous catalysis. The second benefit is that biodiesel protects the activated CaO from poisoning by atmospheric CO2 and H2O. This simplifies the action to be taken when activated CaO is transferred from the activating unit to the reactor or when storing the activated CaO catalyst before use. The results presented here have other practical implications: they stress the importance of the order of contact between the solid catalysts and the different reactant or products (oil, methanol, or biodiesel), and finally, they can stimulate other researchers to use the same procedure for other acidic or basic solid catalyst. 4. Conclusions We report in this work an unprecedented procedure to increase the reaction rate in the triglyceride methanolysis reaction when CaO was used as catalyst. This significant effect was observed when the catalyst is mixed with biodiesel, forming a paste after its activation. The optimum amount of biodiesel required is very low (3 wt % referred to oil). The formation of such a paste also results in a remarkable protection of the activated CaO against the poisoning by ambient CO2 and H2O that may occur during handling or storing of the activated solid. Acknowledgment. The financial support of the Spanish Ministry of Science and Innovation (ENE2006-15116-C04-01) is gratefully acknowledged. D.M.A. thanks the Regional Government of Madrid for his predoctoral fellowship, and M.O. is indebted to the CSIC for the financial support through the JAE program. EF800983M
