New Heterogeneous Catalytic Transesterification of Vegetable and

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Ind. Eng. Chem. Res. 2010, 49, 9068–9076

New Heterogeneous Catalytic Transesterification of Vegetable and Used Frying Oil Maria del Remedio Hernańdez, Juan A. Reyes-Labarta,* and Francisco J. Valde´s Department of Chemical Engineering, UniVersity of Alicante, P.O. Box 99, Alicante, Spain

In the present work, a new heterogeneous catalyst for oil transesterification with methanol has been developed. The catalyst is based on the introduction of sodium in calcined hydrotalcite. Two different raw materials have been used to determine the usefulness of this new catalyst: sunflower oil and used frying oil from a university restaurant. In this way, the efficiency of the new heterogeneous catalyst by using an urban residue was tested. Different parameters that can modify the extension of the transesterification reaction have been evaluated. Results obtained at 60 °C show that hydrotalcite modified with sodium presents adequate characteristics for this kind of reactions with both raw materials evaluated. An important factor observed in the present study is that no soap was formed in any of the reactions carried out, even in the case of used oil with high acid index. The iodine index of biodiesel generated with waste cooking oil presents values within the limits established by general biodiesel specifications. 1. Introduction The necessity to find alternatives to petroleum products and energy sources with low cost and with a reduced environmental impact is a reality that has been considered for many years. Vegetable oils can be used for this goal. The first time that vegetable oils had been used as fuel was in 1895, when Rudolph Diesel used peanut oil to run an ignition-compression motor. This alternative renewable energy source presents advantages such as no increase of the global CO2 amount, no presence of sulfur or aromatic compounds in its structure and higher flash point that reduces the fire hazard. But, vegetable oils present several disadvantages that make them difficult to use as fuels such as worse combustion that produces carbon deposits, a lower calorific power than diesel-fuel, and a viscosity ten times higher than diesel-fuel.1-4 By the transesterification process, a lot of these disadvantages can be solved. In this process (that presents three sequential reactions in the presence of an acidic or basic catalyst and heat), three molecules of an alcohol react with a molecule of triglyceride present in the vegetable oil, to produce three molecules of mono alkyl esters known as biodiesel, which can be used as substitute for petroleum fuel, plus glycerine (Figure 1). The most common method to carry out oil transesterification is by homogeneous basic catalysis. The most used basic catalysts are NaOH, KOH, and sodium methoxide.5-7 Homogeneous catalyst presents several disadvantages such as they cannot be recovered at the end of the reaction and the presence of water in the reactants or a high acid index of the oil reduces the effectiveness of the process. Additionally, the use of a homogeneous catalyst requires at the industrial scale a high amount of water to clean the methyl esters (biodiesel) obtained. These disadvantages are especially present by using waste cooking or used frying oil in the transesterification due to its high acid index.5-10 This raw material is a promising raw material for generating biodiesel due to its higher production and its reduced price. In the literature, several studies exist that present different processes to use waste cooking oil to generate biodiesel generally through homogeneous catalysis. In these papers, it is shown that transesterification of waste cooking oil needs a previous acid treatment to eliminate free fatty acids, higher amounts of catalyst, and higher reaction temperatures than vegetable oil.7-13 * Corresponding author. E-mail: [email protected].

Heterogeneous catalysis appears as an interesting alternative to avoid inherent difficulties of homogeneous ones. Solid catalysts are less corrosive, easier to handle and can be separated at the end of the reaction (by simple filtration) and reused. These catalysts generate a low amount of wastewater during the cleaning process. In the literature are examples of heterogeneous catalysts with different composition used in the transesterification process of vegetable oils. Xie et al.14 used a solid base catalyst of potassium loaded on Al2O3 for the transesterification of soybean oil, leading to conversions of 87%. Kim et al.15 developed a heterogeneous catalyst of sodium (Na/NaOH/γ-Al2O3) and used it in the reaction of soybean oil, showing almost the same activity under optimized conditions as the conventional homogeneous NaOH catalyst. Catalysts with different metals have also been tested to generate biodiesel. Zirconia compounds,16,17 vanadyl phosphate,18 a mixture of oxide of zinc and aluminum,19 and different calcium compounds20 have been used with this end. Another type of heterogeneous catalyst used in transesterification reactions are hydrotalcites. The structure of hydrotalcite resembles that of brucite (Mg(OH)2) where the magnesium cations are octahedrally coordinated by hydroxyl ions, resulting in stacks of edge-shared layers of the octahedral. In the hydrotalcite structure, part of the Mg2+ ions are replaced by

Figure 1. Transesterification reaction.

10.1021/ie100978m  2010 American Chemical Society Published on Web 09/08/2010

Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010 3+

Al ions forming positively charged layers. Charge-balancing anions (usually CO3-2) and water molecules are situated in the interlayers between the stacked brucite-like cation layers. Its general molecular formula is MgxAly(OH)2(x+y)(CO3)y/2 · mH2O, and by a thermal treatment it is possible to decompose it into an interactive, high surface area and well dispersed mixed Mg-Al oxides, which represent basic sites that are able to catalyze a variety of base-catalyzed reactions.21-40 An interesting possibility to modify the properties of solids to increase their catalytic properties in the transesterification reaction is by impregnation or exchange with a specific metal. Potassium, cessium, and lanthanum have been introduced in zeolites by incipient wetness impregnation or ion exchange,41-44 increasing the catalytic activity. The main objective of the present work is to synthesize an active heterogeneous catalyst from a commercial hydrotalcite that could work at lower temperatures and present a low influence on the acid index of the oil used as raw material, in the vegetable oil transesterification reaction and therefore to produce biodiesel. Specifically, a modification with sodium was employed to increase the initial reactivity of the commercial hydrotalcite in the transesterification of waste cooking oil from a restaurant. The transesterification of refined sunflower oil with this modified hydrotalcite has also been studied to compare the behavior of the new heterogeneous catalyst in both cases. Several parameters that have a significant influence in the transesterification reaction have been studied to establish the best experimental conditions to improve the final yield of the process. 2. Equipment and Experimental Procedure 2.1. Materials. The raw materials used were refined sunflower oil purchased from a local food store and waste cooking oil supplied from a university restaurant. The water content in this oil was measured by a Karl Fischer titrator, and the values obtained were 0.063% and 0.235%, respectively. The acid index of the two oils used in the present work was determined by a standard titration procedure based on EN14104.30 A volume of each type of oil was dissolved into 20 mL of ethanol and 20 mL of diethyl ether. A 50 mL amount of sunflower oil was used, but in the case of waste cooking oil, amounts higher than 5 mL are not advisible due to the intense color of this oil that makes the identify of the titration end point difficult. A phenolphthalein indicator was added to the dilution of oil to identify the pH change. The titration was performed with a 0.1 M KOH water solution. In this way, the acidity of the refined sunflower oil is 0.08 mg KOH/g oil and the value for the frying oil is 1.9 mg of KOH/g oil, showing the high differences between the two raw materials evaluated in the present work. Commercial hydrotalcite, sodium acetate, tetrahydrofuran, and standard FAME mix (AOCS No. 3) employed in the chromatography determination were supplied by Sigma-Aldrich S.A. (Madrid, Spain). 2.2. Catalyst Preparation. The following procedure describes the method to introduce sodium in the hydrotalcite, using sodium acetate. This modification of the hydrotalcite should be carried out over the calcined hydrotalcite, because the sodium has to be present in the Mg-Al mixed oxides that make up the true catalyst of the transesterification reaction. To obtain the corresponding Mg-Al mixed oxide, the commercial hydrotalcite was calcined in air at 773 K for 8 h in a muffle heron series-74 12-R/300 (Barcelona, Spain). The necessary amount of sodium acetate (depending on the final

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percentage of sodium in the catalyst) was dissolved in decarbonized water. This solution and the calcined hydrotalcite were placed in a glass vessel and kept under stirring (700 rpm) in a nitrogen atmosphere for 24 h. The solid product was then dried at 120 °C to a constant weight. After this, the material was grinded and heated in a muffle at 773 K for 2.5 h.42,45,46 Finally, a catalyst with Mg-Al-Na mixed oxides is obtained. The different evaluated catalysts are designated as follows: calcined hydrotalcite (HTc), hydrotalcite impregnated with x % of sodium (HT-Na-x). 2.3. Catalyst Characterization. X-ray fluorescence (XRF) was carried out in a Philips Magix Pro sequential X-ray spectrometer (Almelo, Netherlands) to determine the true amount of sodium (measured as NaO) introduced in each catalyst prepared. The pore size distribution and BET (Brunauer, Emmett, and Teller) surface area were determined by adsorption and desorption data acquired on an adsorption Autosorb-6 and desorption Autosorb degasser Quantachrome equipment. The scanning electron microscope (SEM) was carried out in a JEOL JSM-840 microscope (Freehold, NJ). X-ray diffraction (XRD) was carried out in a Bruker D-8Advance with high temperature camera and an X-ray generator Kristalloflex K 760-80 F (power, 3000 W; voltage, 20-60 kV; current, 5-80 mA) (Karlsruhe, Germany). Catalyst basicity was measured by CO2 temperature programmed desorption (TPD). The experiments were carried out in a Netzsch Thermobalance, model TG209 (Burlington, VT) controlled by a PC under the Windows operating system. The hydrotalcites were first purged at 25 °C for 20 min in a nitrogen atmosphere. The tests were performed in a nitrogen environment with a flow rate of 45 STP mL/min. Samples of 5-8 mg were heated at 10 °C/min from 25 to 450 °C and held at this temperature for 30 min. The sample was cooled to 50 °C under an inert atmosphere. Then the adsorption of CO2 is produced at 50 °C for 30 min. The nitrogen atmosphere is again applied for 60 min to eliminate the physisorbed CO2. The desorption stage is produced by ramping at 10 °C/min under an inert atmosphere to 500 °C and holding at this temperature for 15 min. 2.4. Transesterification Reaction (Biodiesel Production). The transesterification process was carried out in 250 mL spherical reactors provided with a thermostat, a condensation system, and mechanical stirring. The reactor has a heat exchanger chamber through which hot water is circulated. This water is heated by resistance and works as a constant-temperature bath. Different parameters that influence the extension of the reaction have been evaluated in the present work: methanol:oil molar ratio, amount of solid catalyst, and percentage of sodium introduced in the hydrotalcite. The rest of the experimental conditions were as follows: reaction time, 8 h; stirring rate, 740 rpm; reaction temperature, 60 °C. The reactor was initially charged with the raw material used (sunflower or waste cooking oil). In the case of the used frying oil, the sample was filtered to eliminate any solids in suspension. The amount of catalyst was weighed and mixed with the methanol prior to being added over the oil into the reactor. After the reaction time, the mixture was filtered to separate the solid catalyst, and methanol was eliminated by evaporation. Then, the phase separation was carried out in a decantation vessel for 24 h. Both phases are weighed. The upper phase corresponds mainly to the methyl esters (biodiesel), and the lower one is glycerine. The oil conversion is obtained as follows:

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oil conversion (%) )

upper phase weight (g) × 100 weight of oil used (g)

(1) Thermogravimetric analysis (TGA) using a TGA/SDTA851 Mettler Toledo (Barcelona, Spain) with a heating rate of 10 °C/min until 800 °C, was also carried out to qualitatively characterize the extension of the different transesterification reactions. Biodiesel (methyl esters) and vegetable oil present different decomposition points, and this technique can be used to determine in a direct way the biodiesel generated after each experiment. For example, Figure 2 shows the TGA derivative curves for three samples. In future works, mechanistic pseudokinetic models could be suggested to model these curves and determine the corresponding kinetic parameters. The experiments were replicated to determine their reproducibility, showing very good results with a maximum deviation between the repeated runs of about 2%. 2.5. Biodiesel Characterization. The composition of the biodiesel in methyl esters was determined by using a gas chromatograph connect to a mass spectrometer Agilent GC-MS (GC 6890N-MD 5973N) (Barcelona, Spain) with a DB-23 column (30 m, 0.25 mm i.d.). To determine the yield of methyl esters, a certain volume of the upper phase is diluted into a mixture of 2-propanol/hexane 50:50 (v/v) and injected into the GC-MS equipment. The identification of the methyl esters present in the different samples was carried out by library Wiley 275 and for the quantification the standard FAME mix (AOCS No. 3) was used. The column program is as follows: Tinitial: 120 °C, Tfinal: 245 °C; Heating rate: 3 °C/min, Timefinal: 15 min; Timetotal: 56.67 min; Injector temperature: 250 °C; Carrier gas: He 1 mL/min; Average velocity: 37 cm/s; Solvent delay: 3 min; Split ratio: 75:1; Additionally, the yield of methyl esters is calculated by using the following expression: methy esters yield (%) )

weight of esters (g) × 100 upper phase weight (g)

(2) The iodine index of the biodiesel was obtained by its composition of methyl esters as indicated in the European Standard UNE-EN 14214.47 In this method, the iodine index is calculated as the sum of the percentage of each methyl ester (such as saturated, C16:1, C18:1, C18:2, etc) multiplied by its corresponding factor as indicated in the following equation.47 g of iodine/100 g ) contribution factor × methyl ester percentage (%) (3)

∑

3. Results and Discussion In this section, the synthesis of a new catalyst based on the modification of commercial hydrotalcite with different amounts of sodium, and its use in the transesterification of vegetable and used frying oils are discussed. 3.1. Physical Properties of Catalysts. Table 1 shows the XRF results obtained for the different catalysts. As can be seen, the real value of sodium present in all catalysts is very close to the nominal one, a fact that corroborates the impregnation method used in the present work.

Figure 2. Thermogravimetric analysis (TGA) to qualitatively characterize the extension of the transesterification reaction (derivative thermogravimetry curves). Table 1. Sodium Content: XRF Results Obtained for the Different Catalysts sample

% Na2O

HTc HT-Na-4 HT-Na-5 HT-Na-7 HT-Na-10

0 3.921 4.881 7.009 10.647

The results of pore size distribution and BET surface area are shown in Figures 3 and 4 to confirm the effect of the modification process. As can be seen, HTc and the HT-Na-4 are the catalysts with the smaller pore size (the maximum of the size pore distribution is around 30 and 50 Å, respectively), whereas the pore of the rest of the catalysts (HT-Na-5, -7, and -10) are more similar between them (maximum pore size distribution around 100 Å). The BET surface area undergoes a notable increase with the calcination process and decreases when the percentage of sodium introduced (into the modified hydrotalcite) is increased. Figure 4 presents the results obtained for the basicity of the different catalysts measured by CO2 temperature programmed desorption (TPD) for the different catalysts studied. As can be seen, the variation of the basicity value shows a maximum at the catalyst with a 5% of sodium (HT-Na-5). This fact could be explained by considering the BET surface area commented on previously. As was shown, at the higher percentage of sodium in the hydrotalcite structure, a reduction in the BET surface area is also observed. With these low values of the BET surface area, the CO2 adsorption is reduced due to the fact that the sodium is less accessible, leading to a catalyst basicity reduction. Figure 5 shows the TPD profiles of all catalysts evaluated in the present paper. As can be seen, all modified hydrotalcites present a CO2 desoprtion peak at around 130 °C, showing very similar basic strength. Examples of the images obtained with a scanning electron microscope (SEM) are shown in Figure 6. As can be observed, the impregnation process slightly modifies the morphology of the catalyst particle. In general, the commercial calcined hydrotalcite presents spherical particles, whereas impregnated catalysts present more planed particles with a higher size. Figure 7 shows the comparison between the XRD results for the different catalysts evaluated in the present work. In the case of commercial hydrotalcite, peaks at (2θ) 11.6, 23.3, 34.9, 39.5, 47, 60.8, 62.1, and 66.7° are detected. These peaks are typical of a crystalline structure. In the case of the calcined hydrotalcite, the crystalline structure is lost and only peaks at (2θ) 43.5 and 63.3° are observed. These peaks can be assigned to the MgO phase or Mg-Al oxides phase.26 In the case of the modified


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Figure 3. Results of pore size distribution of the different hydrotalcites employed in this work.

Figure 4. BET surface area and measures of the basicity by CO2 temperature programmed desorption for the catalyst studied with 0, 4, 5, 7, and 10% of sodium.

hydrotalcites the same two peaks detected for the calcined hydrotalcite are observed. The only difference with these impregnated catalysts is the peak located at 43.5° that is displaced to lower 2θ measures (43.1°). This displacement to lower 2θ values when an alkali metal is incorporated into the structure of the hydrotalcite has also been detected in the case of K-loaded hydrotalcite.44 3.2. Use of Calcined Hydrotalcite in the Transesterification of Sunflower Oil. Effect of Using THF as Codissolvent. To determine the initial reactivity of the commercial hydrotalcite, a sequence of experiments were carried out with refined sunflower oil avoiding the presence of impurities that could make the transesterification reaction difficult. As commented on before, a thermal activation of this catalyst has been carried out, by calcination of the commercial hydrotalcite at 500 °C for 8 h. These experiments show that the initial activity of this heterogeneous catalyst is not very high with methyl ester yield values lower than 10%. A possibility for increasing the activity of the catalysts in a heterogeneous reaction consists of increasing the contact between the reactants. This can be reached by adding a codissolvent to the reaction mixture. In the present work, thetrahydrofuran initially in a ratio of 10 wt % to the initial oil was used for this purpose. In Table 2 results related to the effect of the methanol:oil ratio and the amount of catalyst used are shown. As can be seen, in any case evaluated, the yield of methyl esters reaches

20%. The best result, a yield of 16.9% in methyl esters, was obtained by using the more extreme conditions: a methanol:oil molar ratio equal to 15:1 and 15% hydrotalcite. The effect of the percentage of codissolvent introduced has also been evaluated. To do this, a set of experiments were run using different THF/oil ratios. For example, while using 10 and 50 wt % THF the followings results were obtained respectively: 10.6 and 5.7% methyl ester yield and 12.3 and 7.8% oil conversion. In this sense, it is remarkable that the methyl ester yield can decrease around 50% when the THF proportion in the reaction medium is increased. As can be seen in all experiments carried out with this catalyst, yields of methyl esters are very low; thus it can be concluded that commercial hydrotalcite activated by a thermal treatment even using a codissolvent to improve the oil-alcohol contact does not work adequately for the transesterification reaction of vegetable oils. Therefore, it is necessary to make modifications in this catalyst to reach better activities. 3.3. Effect of the Amount of Sodium in the Catalyst on the Reactivity of the Modified Hydrotalcite in the Transesterification of Vegetable Oils. Hydrotalcites with different amounts of sodium have been prepared to evaluate their activity. Table 3 shows the oil conversions and methyl ester yields reached in the transesterification of sunflower and used frying oil employing these different modified hydrotalcites. As can be seen, the highest reactivity is reached by using 5% of sodium with sunflower oil. At lower or higher percentages, the reactivity decreases. In the case of waste cooking oil, the increase of sodium in the catalyst until 10% sodium leads to a slight increase in the methyl ester yields, meanwhile the values reached with 4 or 5 wt % sodium are very similar. This behavior can be explained by considering the different structural properties of the catalyst evaluated. Additionally, the basicity of the different catalysts employed should be considered too. As was commented on previously (section 2.3) when the amount of sodium increases from 0 to 5 wt % in the hydrotalcite, the basicity of the solid also increases, but on the other hand, catalysts with percentages of sodium of 7 or 10 wt % present a slight reduction of the basicity from that observed with 5 wt % sodium (Figure 4). In addition to the above, the BET surface area of catalysts with high percentages of sodium (7 and 10 wt %) is lower than that shown by the catalyst with 5% alkali metal. Therefore, the

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Figure 5. TDP profiles of the catalysts used.

HT-Na-7 and HT-Na-10 catalysts have a lower active surface, less accessible basic sites and less reactivity, leading to a lower extension of the transesterification reaction. On the other hand, by comparing the basicity of the calcined hydrotalcite (HTc, with 0 wt % of sodium) and HTNa-7 and HT-Na-10 (Figure 4), one can observe that these present very similar CO2 adsorption values (around 295 µmol of CO2/g of catalyst), while the reactivity of the calcined hydrotalcite is lower than that reached by HT-Na-7 or HTNa-10. This fact, that initially seems contradictory, can be explained by observing the pore size distribution (Figure 3). Calcined hydrotalcite presents the maximum pore size distribution centered in smaller sizes than observed in HTNa-7 and HT-Na-10 (maximum of the distribution centered at 30 Å versus 90 Å), marking the significance that the pore sizes have on facilitating the reactivity of larger molecules like the triglycerides. Because of this, hydrotalcites with 7 or 10 wt % sodium present higher activity than a hydrotalcite that is only thermally activated. This porous size effect is observed in the other hydrotalcites used in the present study.

Therefore, the basicity due to the presence of an alkali metal in the catalyst is not the only parameter that determines the efficiency of a catalyst in the transesterification reaction and the possible increase of its activity. All structural properties can contribute to increasing the reactivity (and even the selectivity) of the catalyst, and it is necessary to evaluate all characteristics together. 3.4. Effect of the Amount of Catalyst on the Reactivity of Modified Hydrotalcite in the Transesterification of Vegetable Oils. Another variable evaluated in the present work is the amount of catalyst employed in the transesterification process of sunflower oil and waste cooking oil. Different experiments with 1-9 wt % modified hydrotalcite HT-Na-5 have been carried out. Table 4 shows the results related to the influence of this variable. As can be seen, both raw materials present a similar tendency. In both cases, with an amount of catalyst higher that 5 wt %, no significant increase of the oil conversions or methyl ester yields are obtained. In the case of using percentages of modified hydrotalcite lower than 5%, the methyl ester yield


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Figure 6. SEM images: (a) calcined hydrotalcite (HTc); (b) hydrotalcite with 3.9% of sodium (HT-Na-4); (c) hydrotalcite with 4.9% of sodium (HT-Na-5); (d) hydrotalcite with 7.0% of sodium (HT-Na-7).

Figure 7. Results of X-ray diffraction (XRD).

decreases significantly whereas the oil conversion shows a slight decrease. This fact indicates that the three reactions of the transesterification process are not completed, generating as reaction products not only glycerine and methyl esters but also mono- or diglycerides. This fact should be due to an insufficient amount of catalyst, and this can be predicted throughout the experiment, because in these cases the mixture is translucent (not really transparent), indicating the presence of nonreacted glycerides.9

Additionally, a slight increase of the oil conversion and methyl ester yields is observed in the transesterification of frying oil when 9 wt % catalyst is used. Comparing with the case for sunflower oil, this fact can be attributed to the presence of impurities in the used frying oil that can use part of the catalyst in other parallel or competitive reactions, such as free fatty acid neutralization or, even, to the presence of oxidative reactions, hydrolytic reactions, etc.10 that can modify its initial composition.

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Table 2. Reactivity of the Calcined Commercial Hydrotalcite (HTc) with the Presence of THF as Co-dissolvent (10 wt %) in the Transesterification of Sunflower Oil; Effect of the Methanol:Oil Molar Ratio and Effect of the Amount of Catalyst Used (Conditions: 8 h, 740 rpm) methanol:oil ratioa

hydrotalcite (%)b

parameter

6:1

9:1

15:1

7

10

15

oil conversion (%) yield of methyl esters (%)

2.7 3.2

10.8 6.2

12.3 10.6

12.3 10.6

12.3 10.8

19.2 16.9

a

7 wt % of catalyst. b 15:1 methanol:oil.

Table 3. Effect of Amount of Sodium in the Catalyst in the Transesterification of Vegetable Oils (Methanol:Oil 9:1, 8 h, 740 rpm, 7 wt % of Catalyst) sunflower oil

used frying oil

sodium (%)

oil conversion (%)

yield of methyl esters (%)

0 4 5 7 10

17.4 83.3 92.5 85.2 84.5

14.8 70.7 83.2 80.9 68.1

oil conversion (%)


88.5 93.0

54.8 54.8

90.4

67.2

Table 4. Effect of Amount of Catalyst in the Transesterification of Vegetable Oils (Methanol:Oil 9:1, 8 h, 740 rpm, HT-Na-5) sunflower oil

used frying oil

hydrotalcite (%)

oil conversion (%)


oil conversion (%)


1 5 7 9

89.4 77.8 92.5 84.3

53.9 77.0 83.2 81.9

81.8 90.5 87.4 91.6

0.1 60.5 54.8 61.2

Table 5. Effect of the Methanol:Oil Molar Ratio in the Transesterification of Vegetable Oils (Conditions: 8 h, 740 rpm, 7 wt % of Catalyst HT-Na-5) sunflower oil

sunflower oil

oil yield of oil yield of conversion methyl esters conversion methyl esters methanol:oil ratio (%) (%) (%) (%) 3:1 6:1 9:1 15:1

77.7 85.6 92.5 86.7

40.6 73.6 83.2 88.1

89.0 87.4 85.1

54.9 54.8 56.8

For these reasons, the oil conversion and methyl ester yields are around 10% lower when waste cooking oil is used as the raw material instead of sunflower oil. 3.5. Effect of the Methanol:Oil Molar Ratio on the Reactivity of the Modified Hydrotalcite in the Transesterification of Vegetable Oils. As commented previously (section 2.2), in the present work the hydrotalcite has been modified with sodium to increase its activity. One of the main parameters that can affect the transesterification reaction is the methanol:oil molar ratio. In this sense, Table 5 shows the results obtained for sunflower and used frying oil using different proportions of methanol:oil and the catalyst HT-Na-5. As can be seen, in the case of the sunflower oil, an increase in the oil conversions and in the methyl ester yields is obtained when the methanol:oil ratio is increased from 3:1 to 9:1. At this latter ratio, the oil conversion reaches a maximum. In the case of used frying oil, the variation with this variable is less significant, showing a practically constant value from relation 6:1 methanol:oil up to

15:1. This fact can be attributed to the presence of free fatty acids that can limit the transesterification reaction of waste cooking oil. By comparing both raw materials, one can observe that transesterification of waste cooking oil produces methyl ester yields around 10% smaller than the transesterification of sunflower oil. The highest oil conversion was 92.5% with sunflower oil and 89% with used frying oil, meanwhile the highest methyl ester yields were 88% and 56.8%, respectively. To summarize, it can be concluded that hydrotalcites modified with sodium can produce an adequate transesterification reaction using sunflower or used frying oil, without the formation of soaps. The highest methyl ester yield reached with sunflower oil is 88% with reaction conditions very similar to those used in homogeneous catalysis: a methanol:oil molar ratio equal to 15:1, 7 wt % catalyst, 5% sodium in the catalyst, and a relatively low temperature, 60 °C. This value is higher than the one shown by using hydrotalcite loaded with potassium: 86.6% value obtained using much more extreme conditions such as a reaction temperature of 100 °C and a methanol:oil ratio 30:1.44 In the same way, values shown in the literature by using hydrotalcite without modification and conditions similar to those used in the present study are around 60-65%, lower than those reached in the present study.50 In the case of the transesterification of used frying oil by using modified hydrotalcite as a heterogeneous catalyst, the highest methyl ester yield reached is 67.2% with the following reaction conditions: methanol:oil ratio equal to 9:1, 7 wt % catalyst, 10% sodium in the catalyst (HT-Na-10), and 60 °C. Although this value seems to be low, it has to be considered that the frying oil was not treated previously to eliminate water or free fatty acids. To analyze the initial potential and efficiency of the proposed modified hydrotalcite and the possible formation of soap (parallel saponification reactions), the used frying oil was only filtered, and therefore, any acid pretreatment, typical when this type of raw material is used, was not used. Instead of this, soap was not formed in any of the experiments carried out, and phase separation was very clear and easy. In any case, and to evaluate the influence of acid pretreatment, a sample of used frying oil was esterified prior to the transesterification reaction using sulfuric acid.51 By this procedure, the acid index of the oil was reduced from 1.9 to 0.8 mg of KOH/g of oil. By using HT-Na-5 as catalyst, and the optimal reaction conditions, the methyl ester yield increased to 75% from 54.8%, obtained when this previous stage was not carried out. In view of the results obtained, as expected, sunflower oil is an efficient raw material for biodiesel synthesis. But the use of used frying oil is more interesting and sustainable because this material is an urban residue and its recycling to produce biocombustibles presents many environmental advantages. In this way, it is necessary to evaluate different alternatives to increase the yield of methyl esters by using this kind of raw material in biodiesel generation. 3.6. Iodine Index of the Biodiesel Generated. In section 2.4 it was commented that the iodine index of the different samples obtained in the transesterification of sunflower and used frying oils was calculated from their composition, that is, mainly methyl oleate and linoleate in the sunflower oil and methyl palmitate, oleate, and linoleate in the used oil. Results obtained are shown in Table 6. As can be seen, the iodine index in the case of used frying oil is lower than the one obtained for the biodiesel obtained from sunflower oil. This aspect has been shown in the literature.48

Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010 Table 6. Iodine Index of the Different Biodiesel Obtained through the Transesterification Experiments Using Modified Hydrotalcite as Heterogeneous Catalyst iodine index (g of I2/ 100 g)

methanol:oil ratio

catalyst (wt %)

Na in catalyst (%)

variable

sunflower oil

3:1 6:1 9:1 15:1 1 5 7 9 4 5 7 10

133.7 134.1 134.3 131.1 135.9 135.3 134.3 134.9 135.1 134.3 134.7 138.4

waste cooking oil 96.6 98.2 96.4 97.7 98.2 96.8 96.5 98.2 96.9

The initial European specifications for biodiesel established that the maximum iodine index was 120 g of I2/100 g. However, a revision of the Spanish normative (RD 61/2006 31st January)49 modified this value and established the maximum value of the iodine index at 140 units. Therefore, the values around 130 g of I2/100 g obtained for the biodiesel generated from sunflower oil obey the Spanish normative but are out of the general biodiesel specifications. Biodiesel generated by using waste cooking oil presents an iodine index between 96 and 99 g of I2/100 g and is within the limits fixed for general normative. 3.7. Catalyst Deactivation in the Heterogeneous Transesterification Reaction. An important factor in the use of catalysts is the possibility of reusing them and its activity decays. In the transesterification reaction, porous catalyst can be occluded with oil, decreasing the activity of this catalyst in the following reactions. To establish the deactivation of the catalyst in the transesterification reaction, the same catalyst (HT-Na-5) was re-employed on three successive transesterification runs with sunflower oil. The reaction conditions were methanol:oil molar ratio 9:1, 7 wt % catalyst, 8 h of reaction, and 740 rpm. After each transesterification reaction, HT-Na-5, which presents brown color, was recovered by filtering and simply dried in an inert atmosphere. The methyl ester yields obtained in these consecutive experiments are 81.7, 36.1, and 4.1%, respectively. As can be seen, HT-Na-5 loses around 55% of its activity in the second run. In the third reaction, the catalyst presents only 5% of its initial activity. To evaluate the possible regeneration of the modified hydrotalcite, a regeneration procedure has been applied to the HT-Na-5 used only once and used three consecutives reactions. The recovered catalyst was washed with methanol, dried at 120 °C until a constant weight and calcined at 500 °C for 2.5 h. This regenerated catalyst was employed in the transesterification reaction of sunflower oil in the same conditions as before. The methyl ester yield in the case of the regenerated HT-Na-5 after only one use was 74.3% while in the case of the regenerated HT-Na-5 after three consecutive uses was 72.0%, values that are around 90% of the initial activity of the catalyst. 4. Conclusions In the present work a new heterogeneous catalyst for triglyceride transesterification has been developed. This catalyst presents interesting results for the production of biodiesel, especially from used frying oil, working at relatively low temperatures, without the disadvantages of an acid pretreatment to eliminate free fatty acids to avoid interference with collateral

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soap productions. The catalyst is based on hydrotalcite, a material with characteristics that can make it useful in this type of reaction. Thus, a commercial hydrotalcite has been modified with sodium to increase its reactivity in the transesterification process. Different parameters that can modify the extension of the transesterification of sunflower and used frying oil have also been evaluated: methanol:oil molar ratio, percentage of catalyst present in the reaction medium, and percentage of sodium introduced in the catalyst. Results obtained show that oil conversion and methyl ester yield present similar tendencies by using both raw materials. In the case of used frying oil, methyl ester yields show values 10% lower than those reached with sunflower oil. The highest methyl ester yield with sunflower oil is 88% (with 15:1 methanol:oil, 7 wt % oil of catalyst, 5% of sodium, 8 h, 740 rpm), meanwhile with frying oil it is 67% (9:1 methanol: oil, 7 wt % oil of catalyst, 10% of sodium, 8 h, 740 rpm). The iodine index of the biodiesel generated with waste cooking oil is lower than 120 g of I2/100 g, meanwhile biodiesel generated with sunflower oil present values around 130 g of I2/100 g. Acknowledgment We gratefully acknowledge financial support from the Vicepresidency of Research, Development, and Innovation of the University of Alicante, Spain (GRE078P). Literature Cited (1) Meher, L. C.; Sagar, D. V.; Naik, S. N. Technical aspects of biodiesel production by transesterification a review. Renewable Sustainable Energ. ReV. 2006, 10, 248. (2) Helwani, Z.; Othman, M. R.; Aziz, N.; Kim, J.; Fernando, W. J. N. Solid heterogeneous catalysts for transesterification of triglycerides with methanol: A review. Appl. Catal. A: Gen. 2009, 363, 1. (3) Yang, Z.; Xie, W. Soybean oil transesterification over zinc oxide modified with alkali earth metals. Fuel Process. Technol. 2007, 88, 631. (4) Antunes, W. M.; Veloso, C.; Henriques, C. A. Transesterification of soybena oil with methanol catalyzed by Basic solids. Catal. Today 2008, 133-135, 548. (5) Antolıń, G.; Tinaut, F. V.; Bricenõ, Y.; Castanõ, V.; Pe´rez, C.; Ramirez, A. I. Optimization of biodiesel production by sunflower oil transesterification. Bioresour. Technol. 2002, 83, 111. (6) Marchetti, J. M.; Miguel, V. U.; Errazu, A. F. Possible methods for biodiesel production. Renewable Sustainable Energ. ReV. 2007, 11, 1300. (7) Van Gerpen, J. Biodiesel processing and production. Fuel Process. Technol. 2005, 86, 1097. (8) Bautista, L. F.; Vicente, G.; Rodrı´guez, R.; Pacheco, M. Optimization of FAME production from waste cooking oil for biodiesel use. Biomass Bioenerg. 2009, 33, 862. (9) Dorado, M. P.; Ballesteros, E.; Mittelbach, M.; Lo´pez, F. J. Kinetic parameters affecting the alkali-catalyzed transesterification process of used olive oil. Energy Fuel 2004, 18, 1457. (10) Kulkarni, M. G.; Dalai, A. K. Waste cooking oil-an economical source for biodiesel: A review. Ind. Eng. Chem. Res. 2006, 45, 2901. (11) Issariyakul, T.; Kulkarni, M. G.; Dalai, A. K.; Bakhshi, N. N. Production of biodiesel from waste fryer grease using mixed methanol/ ethanol system. Fuel Process. Technol. 2007, 88, 429. (12) Wang, Y.; Ou, S.; Liu, P.; Xue, F.; Tang, S. Comparison, of two differnet processes to synthesize biodiesel by waste cooking oil. J. Mol. Catal. A: Chem 2006, 252, 107. (13) Leung, D. Y. C.; Guo, Y. Transesterification of neat and used frying oil: Optimization for biodiesel production. Fuel Process. Technol. 2006, 87, 883. (14) Xie, W.; Peng, H.; Chen, L. Transesterification of soybean oil catalyzed by potassium loaded on alumina as a solid-base catalyst. Appl. Catal. A: Gen. 2006, 300, 67. (15) Kim, H.-J.; Kang, B.-S.; Kim, M.-J.; Park, Y. M.; Kim, D.-K.; Lee, J.-S.; Lee, K.-Y. Transesterification of vegetable oil to biodiesel using heterogeneous base catalyst. Catal. Today 2004, 93-95, 315.

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ReceiVed for reView April 28, 2010 ReVised manuscript receiVed July 27, 2010 Accepted August 13, 2010 IE100978M

New Heterogeneous Catalytic Transesterification of Vegetable and

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