Cadmium Compounds as Catalysts for Biodiesel Production

Jul 8, 2010 - However, their main drawback is their high free-FA content, which compromises their use as feedstock in the traditional technology for b...
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Cadmium Compounds as Catalysts for Biodiesel Production Melquizedeque B. Alves, Fernando C. M. Medeiros, and Paulo A. Z. Suarez* Laborato´rio de Materiais e Combustı´Veis, Instituto de Quı´mica da UniVersidade de Brası´lia, C.P. 04478, CEP 70904-970, Brası´lia, DF, Brazil

Divalent cadmium compounds, particularly CdO, were used as Lewis-acid catalysts in the hydrolysis, esterification, and transesterification of pure triacylglycerides or fatty acids obtained from neutralized soybean oil, as well as mixtures of these substrates. In more than one step, reaction yields of up to 98% in fatty acids or fatty acid methyl esters were achieved. To study the use of these reactions for the development of technologies for biodiesel production from low-grade starting materials, a study was also carried out using a real sample with a high content of free fatty acids from the Brazilian savanna palm-tree oil called Macauba (Acrocomia sclerocarpa M.). Using multistep, sequential, or combined (one-pot) reaction processes, it was possible to obtain an overall reaction yield of up to 98% in methyl esters. 1. Introduction To address the increasing demand for energy and growing ecological awareness, biofuels have emerged in the past decade as elegant alternatives to fossil fuels.1 Indeed, countries such as Brazil have started national programs to promote the partial or total substitution of gasoline with ethanol2 and diesel with biodiesel.3 However, the use of biofuels has become controversial because of a trilemma: balancing food, energy, and environment (for recent and interesting discussions of this issue, see refs 4 and 5). It is becoming clear to many scientists that the first approach of producing biofuels from high-quality and valuable feedstocks, such as sugar or starch for ethanol and refined fats and oils for biodiesel, is not sustainable and new technologies must be developed to produce fuels from different residues and wastes from the agribusiness sector.6 Regarding the production of biodiesel, it is possible to obtain large amounts of fatty materials from different sources: acid stocks produced during physical neutralization of fats and oils; domestic or industrial sludge; poultry, porcine, or cattle slaughterhouse wastes; algae bioreactors; and nonedible palm-tree oils. For several reasons, such as low price and intensive production, as well as the fact that they are inedible or do not compete with food production, these fatty-acid- (FA-) based materials have great potential as raw materials for biodiesel production. However, their main drawback is their high free-FA content, which compromises their use as feedstock in the traditional technology for biodiesel production. Indeed, since 1937, when the first patent regarding biodiesel production was granted,7 the widespread technology is alkaline transesterification, which is not suitable for raw materials containing free FAs at levels of more than 0.5 mass %8 for several reasons, such as soap formation, which leads to stable emulsions and difficulties in purifying biodiesel, in addition to requiring catalyst consumption. One alternative is the use of Bro¨nsted-acid catalysts, such as sulfuric or hydrochloric acids, that do not react with free FA. However, such catalysts have low activity for transesterification, taking longer to convert raw materials with high contents of free FAs into biodiesel with high reaction yields. It is important to highlight that Bro¨nsted-acid catalysis (usually used for esterification) has been demonstrated to be 4000 times less active in the transesterification of triacylglycerides (TAGs).9 * To whom correspondence should be addressed. Tel.: +55 61 33072167. Fax: +55 61 32734149. E-mail: [email protected].

To address issues of using raw materials with high free-FA contents in the biodiesel industry, different approaches have been described in the literature, as summarized in Figure 1. In the classic route,10 the production of fatty acid methyl esters (FAMEs) is performed in two steps: (a) the esterification of free FAs using esterification-active Bro¨nsted-acid catalysts (reaction i in Figure 1) and (b) alcoholysis of TAGs using transesterification-active alkaline catalysts (reaction ii in Figure 1). Note that the bulk reaction mixture must be neutralized and dried between these two steps and that the reaction medium is highly corrosive at the specified reaction conditions (usually temperatures above 273 K). These facts create some limitations to the technological applications of this process. More recently, a process to produce biodiesel was developed using the acid stock from the physical neutralization of palm-tree oil as raw material, containing 80% free FAs and 20% TAGs (reaction iii in Figure 1).11 In this technology, after a high conversion rate of FAs is achieved, the biodiesel is recovered by flash distillation, and a mixture of unreacted TAGs and free FAs remains as the byproduct. More recently, supercritical methanol12 or catalysts have been used for both esterification and transesterification in combined one-pot13-16 or two-step biodiesel production.17-20 Another alternative for the conversion of substrates that are rich in FAs is to promote the complete hydrolysis of TAGs (step iv in Figure 1), in order to convert the raw material into a mixture of fatty acids, and then perform the esterification (step v in Figure 1). One great advantage of this process is the possibility of obtaining high-grade glycerin as a coproduct and high conversion rates of the FA material into biodiesel. Traditionally, the hydrolysis of TAGs is carried out within the ranges 373-533 K and 1-70 bar using a 0.4-1.5% (w/w) initial water-to-oil ratio with or without catalysts.21 The widespread procedures in industry are discontinuous autoclaving, the Twitchell process, and the continuous countercurrent (ColgateEmery) process.22 New ways to improve the hydrolysis processes have also been studied, some of which involve the use of enzymes,23-25 new Lewis-acid catalysts,26 sub- or supercritical systems,27-29 and application of ultrasound.30,31 The aim of this work was to perform an exploratory study to identify new catalytic precursors with potential application in the preparation of biodiesel using raw materials with high freeFA contents. After a preliminary screening, during which the activities of different Lewis-acid metal compounds in the

10.1021/ie100172u  2010 American Chemical Society Published on Web 07/08/2010

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Figure 1. Different routes to the production of biodiesel using starting materials with high free-FA contents: (i) esterification of free FA, (ii) transesterification of TAGs, (iii) direct alcoholysis of FAs/TAGs, (iv) hydrolysis of TAGs, and (v) esterification of FAs.

hydrolysis of soybean oils were examined, a complete study was done with the most active compounds in the hydrolysis, esterification, and transesterification of soybean oil and soybean FAs and, finally, with the crude acid oil from the Macauba palmtree (Acrocomia sclerocarpa M.). 2. Experimental Section 2.1. General. All reagents used to prepare the catalysts were obtained from commercial sources and were used without further purification. Refined soybean oil was obtained from commercial sources (Soya) and used as received. Macauba oil (Acrocomia sclerocarpa M.) was obtained by a traditional procedure that involves pulp fruit pressing. Distilled water was used in the hydrolysis experiments, and methanol (Cromoline, 99.8%) was used in the esterification and transesterification assays after prior drying with magnesium sulfate. Hydrated niobium oxide (Nb2O5 · H2O) was obtained from the Brazilian Metallurgy and Mining Company (CBMM) and used without further purification. The chemicals CdO, CdCl2, CdS, and CdSO4 were obtained from Merck and used as received. 2.2. Catalyst Synthesis. The catalysts were prepared following the procedures described in the literature. The oxides based on aluminum and acidic or basic metals (Sn, Zn, Mn, Zr,

Ti, Sr, and Ba) were prepared by coprecipitation followed by calcination at 773 K for 4 h of the obtained material, as described elsewhere.19 The inorganic acids HNO3, H2SO4, and H3PO4 were chemically adsorbed in Nb2O5 · H2O for 48 h and then subjected to temperature activation at 283 K for an additional 48 h, as described elsewhere.32 The synthesis of metal complexes involved the deprotonation of the ligands pyrone, quinone, or acetylacetone in aqueous NaOH, followed by coordination to the metal cations Pb2+, Hg2+, Sn2+, or Cd2+, according to previously reported procedures.33,34 Tin oxide (SnO) was obtained by calcination at 773 K for 24 h of tin acetylacetonate, as reported elsewhere.35 Cd(C2H3O2)2 was synthesized by treatment of CdO with acetic acid, followed by evaporation of excess acid. 2.3. Catalytic Experiments. The catalytic experiments were conducted in a 100-mL Parr 4843 steel reactor with mechanic stirring and variable temperature. The reaction mixture was kept under stirring (800 rpm) during all experiments. All reagents were added simultaneously to the reactor, but they were not mixed until the desired reaction temperature was reached in the entire system. At this point, the reaction times began to be recorded. Different reaction parameters (substrate molar ratio, temperature, time, and catalyst amount) were varied according

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Table 1. Yields of All Catalysts Tested in the Hydrolysis Reaction of Soybean Oila

Table 2. Hydrolysis Reaction of Soybean Oil at Different CdO Quantitiesa

entry

catalyst

yield (% fatty acid)

entry

CdO (g)

yield (% FA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

SnO (Al2O3)8(SnO)(ZnO) (Al2O3)8(SnO)2 (Al2O3)8(MnO)2 (Al2O3)8(ZrO2)2 (Al2O3)8(TiO2)2 (Al2O3)8(ZnO)2 (Al2O3)8(SrO)2 (Al2O3)8(BaO)2 (Al2O3)8(MgO)2 (Al2O3)8(CdO)2 Nb2O5 (Nb2O5).n(H2SO4) (Nb2O5).n(H3PO4) (Nb2O5).n(HNO3) Hg(C10H5O3)2 Cd(C6H5O3)2 Cd(CH3COO)2 Cd(C2H7O2)2 Sn(C2H7O2)2 CdO CdCl2 CdSO4 CdS

1 2 2 3 4 2 2 1 3 1 1 5 1 2 1 2 7 21 13 10 1 22 1 4 4

26 27 28 29

0.05 0.1 0.2 0.4

13 25 40 49

a Conditions: 5 g of water and 10 g of soybean oil at 433 K and 4.7 bar for 1 h.

Table 3. Hydrolysis Reaction of Soybean Oil at Different Temperaturesa

to the reaction protocol. Note that the reactors were not pressurized. However, the pressure of the experiments varied from 1.2 to 18.6 bar depending on the temperature of the reaction and was considered the equilibrium pressure due to the vapor pressure of the system. Some representative experiments of each reaction (hydrolysis, esterification, and transesterification) were performed in triplicate, and for these cases, standard deviations are represented by error bars in figures. 2.4. Reaction Analysis. The products of the hydrolysis and transesterification reactions were analyzed by high-performance liquid chromatography (HPLC) in a Shimadzu CTO-20A chromatograph (UV-vis detection at λ ) 205 nm) equipped with a Shim-Pack VP-ODS column (C-18, 250 mm, 4.6 mm i.d.). An injection volume of 10 µL and a flow rate of 1 mL min-1 were used in all experiments. The column temperature was held constant at 313 K. All samples were dissolved in 2-propanol/hexane (5:4, v/v). An 18.5-min binary gradient with one linear gradient step was employed: 100% methanol at 0 min, 50% methanol and 50% 2-propanol/hexane (5:4, v/v) in 10 min, followed by isocratic elution with the same composition for the last 8.5 min.36 All solvents were filtered through a 0.45µm Millipore filter prior to use. The esterification reaction was analyzed by titration according to the standard American Oil Chemists’ Society (AOCS) Cd3d63 method.37 The reaction yield Y (%) was determined from the relationship between the acidity value of the products (Ap) and reagents (Ar) according to the equation

(

)

Ap × 100 Ar

temperature (K)

pressure (bar)

yield (% FA)

30 31 32 33 34

393 413 433 453 473

1.8 2.3 4.7 12.2 13.6

13 16 25 50 70

a Conditions: 5 g of water and 10 g of soybean oil for 1 h, 0.1 g of CdO.

a Conditions: 5 g of water, 10 g of soybean oil, and 0.1 g of catalyst at 433 K and 4.7 bar for 1 h.

Y (%) ) 1 -

entry

(1)

3. Results and Discussion 3.1. Hydrolysis, Esterification, and Transesterification of Refined Soybean Oil and Soybean Fatty Acids. Initially, we performed a screening of several catalysts for the hydrolysis of soybean oil in order to compare their catalytic activities. The results are summarized in Table 1, where it can be observed

that the oxygenated cadmium compounds showed the highest catalytic activities in promoting the hydrolysis of soybean oil. However, there were differences in their activities according to the nature of the catalyst. Indeed, the best results were achieved for the complexes cadmium acetate (entry 19, Table 1), cadmium pyronate (entry 18, Table 1), cadmium acetylacetonate (entry 20, Table 1), and cadmium oxide (entry 22, Table 1). For all other metal compounds examined, including cadmium chloride, sulfate, and sulfite, low activities were found. Because the best results were obtained with CdO, we decided to perform further studies using this catalyst. Our first concern was to assess the effect of the amount of CdO on the hydrolysis of soybean oil, which can be demonstrated in Table 2 (entries 26-29). Note that all other reaction parameters (quantities of soybean oil and water, reaction time, and temperature) were kept constant. As can be seen, the reaction yields increased when the amount of CdO was increased. As expected, the relationship between the reaction yield and the amount of catalyst is not linear. This phenomenon is easily explained in terms of parameters such as mass diffusion phenomena, which probably control reaction rates in reactive systems that have high concentrations of catalytic sites. Some reactions were carried with CdO as the catalyst under the same conditions to verify the effect of temperature. Some preliminary results of the temperature study are also summarized in Table 3. At low temperatures and pressures, oils and fats are poorly soluble in water, so hydrolysis is very slow. When water is near its critical point, it is able to dissolve most organic compounds, and thus, the reaction occurs more quickly.28 Despite still being under subcritical conditions, within this range of temperature, a wide variation in reaction yields was observed. It is important to emphasize that, in the absence of catalyst, only 6% of FAs was detected in the reaction conducted at 473 K for 1 h. It is well-established that the hydrolysis reaction occurs in the organic phase by three consecutive and reversible steps when intermediates such as diacylglycerides (DAGs) and monoacylglycerides (MAG) are formed, resulting in a FA molecule in each step.38 The water/oil ratio is important to push the equilibrium toward the product side and also to remove the glycerol generated from the organic phase.39 However, there is a limit for the excess water, after which this effect is no longer enhanced because the kinetics assumes a pseudo-zeroth order for water and also because the glycerol formed is completely

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Figure 2. Yield of the hydrolysis reaction as a function of time at (a) 433 K (4.7 bar) and (b) 473 K (13.6 bar). Molar ratios of soybean oil/water/CdO used: (leftward-pointing triangle) 1:50:0.14, (9) 1:25:0.14, ([) 1:12.5:0.14, (b) 1:50:0 (no catalyst), (×) 1:50:0 (no catalyst) in the presence of 1% w/w FA with respect to soybean oil, (+) soybean oil only (for this condition, the final pressure was 1.2 bar).

Figure 3. Yields of (a) esterification and (b) transesterification reactions as a function of time at different temperatures. Conditions: 30 g of FAs or TAGs, 12 g of methanol, and 0.6 g of CdO (molar ratio ≈ 1:32:0.14): ([) 353 K (1.3 bar), (leftward-pointing triangle) 393 K (3.1 bar), (rightward-pointing triangle) 433 K (12.2 bar), (b) 473 K (18.6 bar).

soluble in the aqueous phase. In this sense, a study was performed to determine the catalytic activity at two different temperatures using different soybean oil/water/CdO molar ratios. The results are presented in Figure 2, which shows that the temperature strongly affects the activity of the catalyst. Indeed, by comparing the results obtained for all oil/water/CdO molar ratios, it is possible to conclude that, under similar conditions, the reaction yields are always higher for a given period of time at 473 K than at 433 K. It is interesting to note that, without catalyst, FAs formed under the conditions studied in the reaction at 473 K. As observed by other authors studying noncatalytic hydrolysis of TAGs,40 the rate of FA formation is low at the beginning, gradually becoming higher and then achieving a plateau. This phenomenon suggests that, after FAs are produced, they can act as Bro¨nsted-acid catalysts. According to Minami and Saka,40 the FA molecule is dissociated to eliminate a proton, which causes protonation of the carbonyl oxygen in the TAG, DAG, and MAG molecules, thereby favoring a nucleophilic attack by water, followed by deprotonation of the obtained carboxylic acids. Even though it is well-known that, at high temperatures, TAGs can decompose, leading to FAs by an intramolecular mechanism in the absence of water (see Figure 2b),41 no reaction

was observed even at 473 K, strongly suggesting that there is not sufficient energy at this temperature for thermal decomposition of the ester and that water is responsible for the hydrolysis. In a study of the cracking of canola oil in the presence of steam at higher temperatures (up to 623 K), it has already been observed that the presence of steam facilitates the hydrolysis of TAG molecules.42 However, at 433 K, no reaction was observed in the absence of CdO, which probably means that the energy supplied was not sufficient energy to promote the hydrolysis reaction. The catalytic performance of cadmium oxide was also evaluated for soybean FA esterification and soybean oil transesterification, and the best results are summarized in parts a and b, respectively, of Figure 3. Note that high-quality FAs and refined soybean oil were used as substrate models. As can be deduced from Figure 3, cadmium oxide is active for both esterification and transesterification reactions under all studied reaction conditions, and its catalytic activity is strongly influenced by the reaction temperature. 3.2. Investigation of CdO in Combined Hydrolysis, Esterification, and Transesterification Reactions. To evaluate the potential of CdO in hydro-(trans)esterification technology,

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Figure 4. Two-step hydrolysis reaction at 473 K (13.6 bar), carried out with a soybean oil/water/CdO molar ratio of 1:50:0.14. Note that, between the two steps, the water/glycerin phase was removed, and a new freshwater charge was introduced.

mentioned in the Introduction (see Figure 1), we continued the investigation by combining the three reactions. As pointed out in the discussion of Figure 2, the optimal hydrolysis conditions are 473 K with a molar ratio of soybean oil, water, and CdO of 1:50:0.14, respectively, for which the reaction yield in FAs was up to 92% in just 2 h. However, it was possible to achieve higher reaction yields in FAs by performing the hydrolysis in two steps (see Figure 4) using these optimized conditions. In the first step, the reagents and the catalyst were placed in the reactor, and the reaction was conducted for 1.5 h, reaching a reaction yield as high as 80% in FAs. Then, the upper organic phase (with FAs) was separated from the lower aqueous phase (with glycerol) in a separation funnel and washed with distilled water to remove the remaining glycerin. In the second step, only the organic phase obtained in the first stage was again placed in the reactor together with the amount of water needed to maintain the same molar ratio in the two steps of the experiment. Thus, conversion rates as high as 97% in FAs were obtained within 3 h of the hydrolysis reaction, reaching up to 98% in 4 h. Then, the material obtained after the two-step hydrolysis experiment, containing about 98% FA, was used as the raw material for a subsequent esterification step. Initially, this material was washed with distilled water to ensure that no glycerin remained from the previous assay, and then the water was removed under reduced pressure, because it is undesirable in the esterification reactions. After this treatment, an esterification step was conducted at 433 K with an approximately 1:32 molar ratio of FAs/methanol. After 2 h, 82.2% FAMEs was obtained. Note that no more catalyst was added for this reaction,

which means that the cadmium content in the reaction mixture was the same as that used during the hydrolysis step. In another departure, CdO was used to esterify free FAs in prepared solutions of FAs and refined soybean oil. Figure 5 summarizes the results obtained for FA/soybean oil solutions containing 1, 5, 10, and 25 wt % FAs in esterification at 353 K (Figure 5a) and at 473 K (Figure 5b). Note that, because the reaction yields presented in Figure 5 were calculated from the initial and final acidity indices, the results represent only the esterification of free FAs. As expected, the best results were obtained at 473 K, achieving reaction yields of up to 80%. On the other hand, it is reasonable to assume that, using these reaction conditions, the alcoholysis of triacylglycerides (transesterification) would also take place. Thus, the reaction products at 473 K were analyzed by HPLC, showing levels of FAMEs from 86% (using 1 wt % solution) to 89% (using 25 wt % solution), obtained either by esterification or by transesterification. The reaction of soybean oil (30 g), methanol (12 g), and water (1.5 g) was also carried out using similar conditions. After 2 h, 66.2% of FAMEs was obtained, as determined by HPLC. The acidity index of the product was 70.2 mg of KOH (g of sample)-1; that is, both transesterification and hydrolysis of TAGs occurred, forming FAMEs and FAs, respectively. It is worth mentioning that it would be necessary to remove the water and perform a second esterification step to obtain high levels of FAMEs. Finally, CdO was used as the catalyst to prepare FAMEs using the hydro-(trans)esterification technology from a real sample of a highly acidic Brazilian savanna palm-tree oil called Macauba (Acrocomia sclerocarpa M.). This oil was obtained by crushing the Macauba fruit pulp and purifying the substrate as described in the literature.43 A high free-FAs content (66%) was determined. The fatty acids composition was determined as follows: lauric acid, 3%; myristic acid, 2%; palmitic acid, 22%; palmitoleic acid, 5%; margarinic acid, 4%; stearic acid, 6%; oleic acid, 53%; and linoleic acid, 5%. Table 4 summarizes the results of the hydrolysis followed by esterification (process 1, Table 4), esterification followed by transesterification (process 2, Table 4), or combined one-pot esterification and transesterification (process 3, Table 4). These processes were carried out in several steps in order to obtain high levels of FAMEs. In each process, the Macauba palm oil was used as the starting material in the first step, and then the material obtained was used as the substrate for the subsequent steps. Between each step, the product was washed with distilled water and dried under reduced pressure.

Figure 5. Esterification of FA/soybean solutions (1, 5, 10, and 25 wt % FA) at (a) 353 K (1.3 bar) and (b) 473 K (18.6 bar). Conditions: 30 g of TAGs + FAs/12 g of methanol/0.6 g of CdO, 3 h.

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Table 4. Production of FAMEs from Macauba Oil in Multistep Processes composition of the product (%) process

step

FAs

MAGs/ DAGs/TAGs

FAMEs

0 1

starting Macauba oil first hydrolysisa second hydrolysisa first esterificationb second esterificationb first esterificationc second esterificationc third esterificationc first transesterificationd first transesterificationesterificationd second transesterificationesterificationd

66 83 98 9 1 46 5 1 1 12

34 17 2 1 1 35 34 34 2 17

0 0 0 90 98 19 61 65 97 71

1

1

98

2

3

a

Macauba substrate/water/catalyst molar ratio of 1:50:0.14, at 473 K, 13.6 bar, 2 h. b Macauba substrate/methanol/catalyst molar ratio of 1:32:0.14, at 433 K, 12.2 bar, 2 h. c Macauba substrate/methanol/catalyst molar ratio of 1:32:0.14 at 353 K, 1.3 bar, 3 h. d Macauba substrate/ methanol/catalyst molar ratio of 1:32:0.14 at 473 K,18.6 bar, 2 h.

As can be deduced from Table 4, in all cases, it was possible to obtain high levels of FAMEs. In process 1, the two-step hydrolysis produced a product with 98% FAs that was then esterified in two steps to generate a product with 98% FAMEs. Process 2 involved three esterification steps to convert the free FAs into FAMEs, and then the MAGs, DAGs, and TAGs were transesterified in another step, leading to an overall reaction yield as high as 97%. Process 3 seemed more appropriate with respect to the amount of time required for the production of FAMEs, considering that only two steps were needed to obtain a product with 98% FAMEs; however, this process requires more drastic conditions. 3.3. Mechanistic Aspects of CdO-Catalyzed Hydrolysis, Esterification, and Transesterification. Pyronate and acetylacetonate metal complexes are active for polyesterification reactions,34,44 and some of these complexes have been extensively studied for the transesterification of vegetable oils.33,35,45 In these works, it was suggested that a typical Lewis-acid mechanism takes place, which means that coordination of the carbonyl of the aclyglyceride on the Lewis-acid site is the main step to activate the ester and then a nucleophilic attack by the oxygen of the alcohols leads to the transesterification product. When cadmium is used as the catalyst for hydrolysis, esterification, and transesterification reactions, it is reasonable to assume that a similar mechanism probably takes place, as illustrated in Figure 6. The carbonyl groups of the ester (Figure 6, reaction i) and carboxylic acid (Figure 6, reaction iv) coordinate at a vacant site in the catalytic active species, increasing the normal polarization of the carbonyl group. Thus, the nucleophilic attack by an alcohol (Figure 6, reaction ii or v) or water (Figure 6, reaction iii) molecule is enhanced. Then, after the breaking and making of C-O and O-H bonds to produce glycerin (or MAGs or DAGs) or water and biodiesel or fatty acids, the dissociation of the carbonyl metal bond completes the cycle. 3.4. Recycling Studies. We tried to recover and reuse the CdO catalyst after transesterification, esterification, and hydrolysis. In all cases, it was observed that it was not possible to recover all of the starting catalyst because of leaching of the Cd to the organic phase. Thus, the catalytic system lost its activity from batch to batch. It is interesting to note that, after four cycles, no activity was observed. This fact is considered

Figure 6. Mechanistic aspects of the hydrolysis, esterification, and transesterification reactions catalyzed by cadmium.

an important technological issue not only from an economic point of view, but also because it brings along environmental concerns, because cadmium compounds are highly toxic and carcinogenic. Thus, it is important to highlight that a further step to remove this metal from biodiesel should be performed before the burning of the biodiesel produced using this protocol. However, we believe that cadmium can be easily separated from the FAMEs by flash distillation. It is worth mentioning that there are some companies producing high-quality biodiesel at the industrial scale using flash distillation as the final purification step. For example, the only FAME producer in Brazil using esterification utilizes this approach (flash distillation) to produce high-purity biodiesel using a low-grade acid stock from palm oil with profit.14 4. Conclusions We have identified the potential application of CdO as a Lewis-acid catalyst and studied its performance in various technologies involved in the production of biodiesel. Indeed, we have shown that this catalytic system has excellent catalytic activity for the hydrolysis, esterification, and transesterification of fatty acids and acylglycerides, either with previously prepared samples obtained by mixing refined soybean oil and soybean FAs or with real low-grade samples as starting materials. This study shows that it is possible to work with a Lewis-acid catalyst that remains active under different reaction conditions involving fatty acids and their derivatives, which can produce biodiesel from low-grade fats and oils in sequential or combined procedures. This is particularly important because a variety of cheap fatty materials can be used as feedstock for biodiesel production, although requiring a specific technology that can be tuned using CdO or other Lewis catalysts. However, an approach to recover and recycle the catalyst must still be determined, and such an effort is currently being made in our research group. Acknowledgment The authors thank FBB, FINATEC, FAPDF, FINEP, CNPq, and CAPES for financial support and CNPq for research fellowships. Literature Cited (1) Lapis, A. A. M.; de Oliveira, L. F.; Neto, B. A. D.; Dupont, J. Ionic Liquid Supported Acid/Base-Catalyzed Production of Biodiesel. ChemSusChem 2008, 1, 759.

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ReceiVed for reView January 25, 2010 ReVised manuscript receiVed June 21, 2010 Accepted June 22, 2010 IE100172U