CaO supported on mesoporous silicas as basic catalysts for transesterification reactions

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Applied Catalysis A: General 334 (2008) 35–43 www.elsevier.com/locate/apcata

CaO supported on mesoporous silicas as basic catalysts for transesterification reactions Moˆnica C.G. Albuquerque a, Inmaculada Jime´nez-Urbistondo b, Jose´ Santamarı´a-Gonza´lez b, Josefa M. Me´rida-Robles b, Ramo´n Moreno-Tost b, Enrique Rodrı´guez-Castello´n b, Antonio Jime´nez-Lo´pez b, Diana C.S. Azevedo a, Ce´lio L. Cavalcante Jr.a, Pedro Maireles-Torres b,* a

Grupo de Pesquisa em Separac¸o˜es por Adsorc¸a˜o (GPSA), Departamento de Eng. Quı´mica, Universidade Federal do Ceara´–UFC, Campus do Pici, Blc. 709, 60455-760 Fortaleza, CE, Brazil b Departamento de Quı´mica Inorga´nica, Cristalografı´a y Mineralogı´a (Unidad Asociada al ICP-CSIC), Facultad de Ciencias, Universidad de Ma´laga, Campus de Teatinos, 29071 Ma´laga, Spain Received 2 July 2007; received in revised form 18 September 2007; accepted 19 September 2007 Available online 25 September 2007

Abstract A new group of basic catalysts supported on mesoporous solids has been prepared with the aim of being used as heterogeneous catalysis in biodiesel production. These catalysts based on calcium oxide supported on porous silica (SBA-15, MCM-41 and fumed silica) have been characterized and evaluated in transesterification processes. They were characterized by DRX, XPS, SEM, FT-IR, CO2-TPD and N2 adsorption. The catalytic activity was evaluated in the transesterification of ethyl butyrate with methanol, and different reaction parameters were optimized by a factorial design response surface methodology. Thus, a sample containing 14 wt.% of CaO supported on SBA-15 was the most active, and, unlike commercial CaO, no lixiviation of the active phase was detected in the reaction medium. The transesterification activity of vegetable oils confirms the results obtained in the reaction of ethyl butyrate with methanol, reaching conversion as high as 95% with sunflower oil (after 5 h of reaction) and 65% (after 1 h) for castor oil. # 2007 Elsevier B.V. All rights reserved. Keywords: Transesterification; SBA-15; CaO; Biodiesel; Heterogeneous catalysis

1. Introduction Alkaline alkoxides and organic amines are usually employed in base-catalyzed organic syntheses by homogeneous catalysis. These processes have serious drawbacks associated to separation, regeneration, and treatment of liquid effluents. Hence, much attention is being focused on the development of novel solid catalysts, mainly as alternatives for liquid bases and acids in many industrially important reactions [1]. Moreover, very few references are available dealing with the application of solid base catalysts for industrial catalysis in contrast to the extensively studied solid acid catalysts [2]. Nevertheless, alkaline and alkaline earth metal oxide and hydroxides have

* Corresponding author. Tel.: +34 952131873; fax: +34 952137534. E-mail addresses: [email protected] (C.L. Cavalcante Jr.), [email protected] (P. Maireles-Torres). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.09.028

been used as base catalysts in a great variety of organic reactions, as recently reviewed by Corma and Iborra [3]. On the other hand, as the supply of fossil fuels is limited while energy demand continues to rise, research directed towards alternative renewable fuels is receiving increasing attention. Biodiesel is a group of monoalkyl esters of long chain fatty acids derived from renewable feedstock, and it has been increasingly used as a substitute of conventional diesel fuel. Diesel engines operated on biodiesel have lower emission of carbon monoxide, unburned hydrocarbons, particulate matter due to the absence of polyaromatic hydrocarbons, and air toxics, in particular SO2, thus, avoiding the acid rain formation when petroleum-based diesel fuels are employed. Other advantages of biodiesel are its good lubricant properties that extend the engine life, its high cetane number, its high flash point and its acceptable cold filter plugging point [4]. It may be produced by a transesterification reaction in which vegetable oil or animal fat feedstocks are reacted with a

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monohydric alcohol (methanol or ethanol) in the presence of a catalyst. The resulting biodiesel exhibits similar properties as conventional diesel fuel [5,6]. Typical biodiesel production involves an alkali-catalyzed process, but this brings undesired effects for lower cost high free fatty acid feedstocks, such as to soap formation. Hence, the required purification steps are costly and technically difficult [7,8]. The use of heterogeneous catalysts in methanolysis has been proposed by different research groups worldwide [9–16]. However, the majority of the heterogeneous catalyst developed is quite expensive, or complicated to prepare, which limits their industrial application. Heterogeneous base catalysts, able to catalyze the transesterification of alkyl esters, could solve several drawbacks of the homogeneous process: they can be easily separated from the reaction mixture without requiring the use of a solvent, they are easy regenerated, and have a less corrosive character, leading to safer, cheaper and more environment-friendly operations [17]. The transesterification reaction conditions generally involve a trade-off between molar ratio, reaction time and temperature, in order to get higher activity, selectivity and longer life-time of the catalyst [4]. However, most studies concerning the use of solid base catalysts for biodiesel production lack information about the lixiviation of the catalyst in the reaction medium, which could demand of additional steps to remove metal ions from biodiesel. Transesterification processes can also be used to prepare fine chemicals, being one of the first examples the reaction of acetoacetates reported by Choudary et al. [18]. Concerning catalysts, zeolites are largely used as supports of many different active species, although their limited pore sizes have led to the development of different families of mesoporous materials [19–22]. The excellent textural properties of mesoporous solids (high specific surface area and pore volume, narrow and tuned pore size distributions centered in the mesoporous range) have allowed the development of new catalysts with well-dispersed active phases. Thus, basic mesoporous catalysts have been mainly obtained by incorporation of alkaline and alkaline-earth oxides and by anchoring basic organic molecules on the surface of mesoporous materials [21,1,23–27]. A significant progress in the field of mesoporous materials was the synthesis of SBA-15 [28], which recently is receiving increasing attention mainly due to its higher pore diameter and thermal stability associated to thicker pore walls, which allow its use in catalytic processes demanding high temperatures for catalyst activation or reaction [28–33]. On the other hand, calcium oxide has been used as catalyst for many different reactions, such as the synthesis of 1,3dialkylurea from ethylene carbonate and amine [34], propylene carbonate from urea and 1,2-propanediol [35] and inhibition of gas-phase oxidation of ethylene in the oxidative conversion of methane and ethane [36]. In this paper, we describe the results concerning the evaluation of the transesterification properties of CaO supported on different siliceous supports: fumed silica and two mesoporous silica-based materials, MCM-41 and SBA-15,

which exhibit high specific surface areas. These materials were tested as catalysts in the transesterification of ethyl butyrate with methanol. Prepared catalysts were previously characterized by X-ray diffraction, N2 adsorption at 77 K, scanning electron microscopy, X-ray photoelectron and FT-IR spectroscopies and CO2 temperature-programmed desorption (CO2-TPD). The factorial design and response surface methodology were applied to obtain the relationship between the methyl ester (ME) concentration, expressed by wt.%, and some operating conditions of the transesterification process, such as temperature, catalyst concentration and the methanol:ethyl butyrate molar ratio. This is a powerful tool that involves some general advantages: (a) more information per experiment than unplanned approaches, (b) a reduction in the number and cost of experiments, (c) the possibility of calculating the interactions among experimental variables within the range studied, leading to a better knowledge of the process and (d) the determination of the operating conditions necessary for process scale-up [37]. One of the first applications of experimental factorial design to heterogeneous catalysis has been given by Cativiela et al. [38] on Diels Alder reactions. This methodology has also been extensively used to develop and optimize different ester synthesis process [39–43]. 2. Experimental 2.1. Catalyst preparation Chemicals were supplied by Aldrich and used as received. For the synthesis of SBA-15, 5 g of EO20-PO70-EO20 (Pluronic 123 from Aldrich) polymer was dissolved in 200 mL of a 0.4 M H2SO4 aqueous solution and stirred at room temperature for 1 day. Then, 0.2 g of NaOH and 197.3 mL of a sodium silicate aqueous solution were added, under vigorous stirring. The resulting solution was aged at room temperature for 5 days. The final product was filtered, washed with water and dried at 333 K. The solid was calcined at 823 K for 6 h [28]. In order to compare the effect of the support, a MCM-41 silica synthesized according to Fuentes-Perujo et al. [44] and a fumed silica from Aldrich were also employed. The different supports were impregnated by using the incipient wetness method with aqueous solutions of calcium acetate. The amount of calcium oxide incorporated to the supports, after drying in air at 333 K and calcination at 873 K for 4 h, was ranged between 4 and 20 wt.%. Catalysts were labeled as SBA (MCM or SiO2)-nCaO, where n is the weight percentage of supported calcium oxide, as determined by ICP-AES. 2.2. Catalyst characterization Powder XRD measurements were performed on a Siemens D5000 automated diffractometer, over a 2u range with Bragg– Brentano geometry using the Cu Ka radiation and a graphite monochromator. The FT-IR spectra of the samples were recorded as KBr disks in the wavenumber region of 4000–400 cm1 with a

M.C.G. Albuquerque et al. / Applied Catalysis A: General 334 (2008) 35–43

Shimadzu model 8300 FTIR spectrometer. Scanning electron micrographs (SEM) were obtained on a JEOL SM 840 to observe the morphology of the particles. X-ray photoelectron spectroscopy (XPS) studies were performed with a Physical Electronics PHI 5700 spectrometer equipped with a hemispherical electron analyzer (model 80365B) and a Mg Ka (1253.6 eV) X-ray source. High-resolution spectra were recorded at 458 take-off-angle by a concentric hemispherical analyzer operating in the constant pass energy mode at 29.35 eV, using a 720 mm diameter analysis area. Charge referencing was done against adventitious carbon (C 1s 284.8 eV). The pressure in the analysis chamber was kept lower than 5  106 Pa. PHI ACCESS ESCA-V6.0 F software package was used for data acquisition and analysis. A Shirley-type background was subtracted from the signals. Recorded spectra were always fitted using Gauss–Lorentz curves in order to determine more accurately the binding energy of the different element core levels. N2 adsorption–desorption isotherms at 77 K of catalysts calcined at 1073 K were obtained using an ASAP 2020 model of gas adsorption analyzer from Micromeritics, Inc. Prior to N2 adsorption, the samples were evacuated at 473 K and 1  102 Pa, overnight. Pore size distributions were calculated with the Cranston and Inkley method for cylindrical pores [45]. The basicity of catalysts was studied by temperatureprogrammed desorption using CO2 as probe molecule. Catalysts (100 mg) were pretreated under a helium stream at 1073 K for 1 h (20 K min1, 100 mL min1). Then, temperature was decreased down to 373 K, and a flow of pure CO2 (50 mL min1) was subsequently introduced into the reactor during 1 h. The TPD of CO2 was carried out between 373 and 1073 K under a helium flow (10 K min1, 30 mL min1), and CO2 was detected by an on-line gas chromatograph (Shimadzu GC-14A) provided with a TCD, after passing by an ice-NaCl trap to eliminate any trace of water. 2.3. Catalyst activity The catalytic activity was evaluated in the transesterification of ethyl butyrate (Aldrich) with methanol (ultra pure, Alfa Aesar), and several reaction parameters were optimized by a factorial design response surface methodology. The transesterification reaction was performed in a glass batch reactor with a water-cooled condenser, controlled temperature (333 K) and inert atmosphere (N2). Before the reaction, the catalysts were activated at 1073 K for 1 h (heating rate, 10 K min1) under a He flow. After cooling, the catalyst was quickly added to the reaction mixture. The reaction was stopped by submerging the reactor in an ice bath. The catalyst was separated by filtration, and the reaction products were analyzed in a gas chromatograph (Shimadzu GC model 14A) equipped with FID and a capillary silica fused SPB1 column. The analysis of the reaction course was followed by measuring the conversion in each catalytic run from the ethyl butyrate:methyl butyrate areas ratio. The calibration was carried out by representation of 100/conversion (%) as a function of the ethyl butyrate:methyl butyrate areas ratio, thus, minimizing the

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error in the injection process since this ratio is independent of the injection volume. The experimental design was 23 factorial, which application requires an adequate selection of responses, factors and levels [10]. The selected response was the ethyl butyrate conversion in methyl butyrate (Y), and the experimental factors chosen were reaction temperature (T), catalyst concentration (C), and methanol to ethyl butyrate molar ratio (MR). The upper temperature level, 333 K, was determined by considering the boiling point of methanol, whereas the lower level was 298 K, since lower temperatures would require a cooling system for the reactor; which would increase the cost of the process. Catalyst concentration levels were 0.4 and 1.6 wt.%. The methanol:ethyl butyrate stoichiometry is 1:1, but in order to favor the transesterification reaction the levels were varied between 2:1 and 12:1. In all the experiments, the magnetic stirring rate was set at 1250 rpm and the activation temperature of catalysts was 1073 K. The most active catalyst was tested in the biodiesel production from castor and sunflower oils, by using a methanol:oil molar ratio of 12, a reaction temperature of 333 K and a catalyst concentration of 1 wt.%. 3. Results and discussion 3.1. Catalysts characterization The X-ray diffraction patterns of the SBA-15 calcined at 823 and 1073 K (Fig. 1a) show an intense reflection at low angle together with several low intensity peaks. Although the SBA-15 has been synthesized at room temperature in acid medium, without any subsequent hydrothermal treatment, a long-range order is evidenced by the peaks at 2u 2–38, corresponding to the (1 0 0) and (2 0 0) planes. However, the value of pffiffithe ffi unit cell parameter ao, 8.8 nm (calculated as ao ¼ 2d100 = 3), is lower than that obtained under hydrothermal conditions, usually higher than 10 nm [44]. The SBA-15 support is thermally stable, since after thermal treatment at 1073 K under a helium flow for 1 h, the main reflection centred at low angle is still observed, although a small structural shrinkage was observed since the d1 0 0 peak decreases from 7.6 to 6.9 nm. This low angle diffraction peak is maintained after the incorporation of calcium oxide, indicating that the mesoscopic order and the characteristic hexagonal features of the SBA-15 support are maintained after impregnation and thermal activation. However, the MCM-41 support, after impregnation and thermal treatment at 873 K, does not show the typical diffraction signal at low angle, pointing to the structural collapse of the mesoporous framework (Fig. 1b). Therefore, the use of SBA-15, with both thick walls and a long-range ordered structure, stabilizes calcium oxide species on its surface and avoids drastic structural transformations. Powder XRD patterns, in the high angle region, of the catalysts calcined at 1073 K (Fig. 2) only exhibit the typical reflections of cubic CaO. This high temperature is necessary to transform the calcite phase, CaCO3, and the calcium hydroxide into the calcium oxide. The narrow peaks observed in the XRD

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Fig. 2. Powder X-ray diffraction patterns of CaO-supported SBA-15 catalysts calcined at 1073 K: (a) SBA-4CaO, (b) SBA-6CaO, (c) SBA-8CaO, (d) SBA14CaO and (e) SBA-20CaO (inset: SBA-14CaO calcined at 823 K (bottom) and 1073 K (top)).

occurs at 798 cm1, while the band at 469 cm1 is assigned to the Si–O–Si bending mode [45]. The bands at 1641 and 3460 cm1 are associated with adsorbed water. The results concerning the surface characterization of the catalysts, performed by XPS analyses, are displayed in Table 1. The spectra of the different CaO-based catalysts are very similar, and the variations of binding energy (BE) values with the CaO loading are very small. The O 1s signal is asymmetrical, and it can be deconvoluted in two components at 530.6 and 532.5 eV, which can be assigned to CaO/CaCO3 (CaO: 529.1–531.3 eV, CaCO3: 530.5–531.5 eV) and the siliceous support (533.0 eV). In the C 1s region, three signals are observed at 284.8, 287.2 and 289.6 eV, due to C–H, C–O and CO32 groups, respectively. The Ca 2p exhibit a doublet

Fig. 1. Powder X-ray diffraction patterns of (a) SBA-15 calcined at (i) 823 K, (ii) 1073 K and (iii) the SBA-14CaO catalyst, and (b) MCM-41 and the MCM14CaO catalyst.

patterns point to the existence of large crystallites on the surface of the mesoporous support. The presence of calcium carbonate, before thermal treatment at 1073 K, is also evidenced in the FT-IR spectra of the catalysts, which are presented in Fig. 3. The spectra display bands at 714, 877 and 1450 cm1, which would correspond to vibration modes of mono and bidentate carbonates. However, the IR spectra of the support and CaObased catalysts are dominated by the asymmetric Si–O–Si stretching vibration modes, which appear as a broad band between 990 and 1358 cm1. The symmetric stretching mode

Fig. 3. FT-IR spectra of the SBA-15 support and CaO supported catalysts.

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Table 1 Binding energies (eV) of the support and catalysts Si 2p

O 1s

Ca 2p

SBA-15 SBA-4CaO SBA-6CaO

103.8 103.6 103.5

533.0 532.9 533.0

– 347.7 347.5

351.2 351.0

284.7 284.9 285.0

286.4 286.4 287.0

289.3 290.0 289.8

SBA-8CaO

103.5 103.2

532.8 532.5

347.4 347.1

350.9 350.6

284.9 284.8

287.1 286.7

289.7 289.5

SBA-14CaO a

103.3 103.5

532.6 532.7

347.3 347.4

350.7 350.9

284.8 284.9

286.7 287.0

289.6 289.7

a

C 1s

The values corresponding to three different regions of this catalyst are given.

with a separation of 3.5 eV, 2p1/2 and 2p3/2 components, whose BE values are 350.6–351.2 and 347.1–347.7 eV, respectively. However, no significant variation of the BE values with the CaO loading can be observed, pointing to a similar interaction between the active phase and the support along this series of catalysts. On the other hand, the XPS technique has been also employed to study the degree of dispersion of the calcium

Fig. 4. (a) Comparison of the surface Si/Ca molar ratio, as determined by XPS, with the bulk values. In same cases, the analysis of different regions of a catalyst is shown. (b) Evolution of the Ca and Si atomic concentrations as a function of the depth, after Ar+ etching.

species. Fig. 4a shows the surface Si/Ca atomic ratio, as obtained by XPS, versus the bulk Si/Ca values obtained by ICP. The results reveal that by increasing the amount of CaO, the difference between the surface and bulk Si/Ca values becomes lower, showing the excellent dispersion of the active phase, except for catalysts with a lower CaO content. Moreover, the analysis of different regions of several catalysts gives rise to similar surface Si/Ca atomic ratio, thus, confirming the homogeneous distribution of calcium species on the surface of the SBA-15 support. In order to verify this assumption, a depth profile analysis of samples SBA-8CaO and SBA-14CaO was performed by etching with an Ar+ gun, and observing the evolution of the Si and Ca atomic concentrations as a function of the etching time (Fig. 4b). The data indicate that Ca percentages are barely modified after 10 min of treatment (40 nm of depth), and therefore, it can be stated that the distribution of Ca into the siliceous SBA-15 support is uniform. Scanning electron micrographs (SEM) of SBA-15 and SBA14CaO materials (Fig. 5) show aggregates of flat particles, which sizes range between 0.5 and 3 mm. Calcium oxide/ carbonate particles are clearly visible on the external surface of the support, corroborating the conclusion extracted from the XRD patterns about the existence of calcium oxide/carbonate particles. The textural characteristics of the support and catalysts have been evaluated from the corresponding N2 adsorption– desorption isotherms. There is a drastic reduction of specific surface area and pore volume values after the incorporation of calcium oxide on the SBA-15 support (Table 2). Moreover, the adsorption–desorption isotherm of the SBA-14CaO catalyst does not show the typical features of the mesoporous support, such as the important raise of the amount of adsorbed N2 in a narrow range of relative pressures (Fig. 6). This fact could be explained not only by the partial blocking of the porous network by the presence of large CaO crystallites, already observed by XRD and SEM techniques, but also by the filling of pores as deduced by XPS. However, the destruction of the siliceous framework by both effects, interaction with the calcium species and the thermal treatment at 1073 K, can be ruled out because the supported phase, CaO, can be removed from the catalyst by treatment with HCl aqueous solution. After this acid treatment, the typical XRD peaks of the SBA-15 support and its textural characteristics are observed again. It must also be noted that the pore size distribution of the support

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Fig. 6. N2 adsorption–desorption isotherms at 77 K and pore size distributions, as determined by the Cranston and Inkley method, of SBA-15 and SBA-14CaO.

3.2. Catalyst activity

Fig. 5. SEM micrographs of SBA-15 and SBA-14CaO samples.

exhibit a maximum centered at 4.2 nm, which contrasts with the values reported for SBA-15 materials, usually higher than 1.0 nm [28]. As previously indicated, the absence of the hydrothermal step in the synthesis modifies the structural and textural characteristics of the SBA-15 support. The CO2 temperature-programmed desorption plots are displayed in Fig. 7. In all cases, a broad desorption band is observed between 700 and 900 K, which intensity and temperature depend on the CaO content. Thus, the catalyst with the lowest CaO loading, SBA-4CaO, shows a CO2 desorption extending from 700 until 800 K, with maximum values in the 750–800 K range. By increasing the amount of CaO on the catalysts, the band become more intense and the more clearly defined desorption maximum is shifted towards higher temperatures, thus, confirming the highest basicity of the SBA-14CaO catalyst.

The activity in transesterification reactions of calcium oxide supported on different porous silicas has been evaluated by choosing the reaction of ethyl butyrate with methanol as model reaction. The initial experimental conditions were: reaction temperature 333 K, inert atmosphere (nitrogen), stirring rate of 1250 rpm and 1 h reaction time. Parameters such as activation temperature, amount of catalysts, methanol:ethyl butyrate molar ratio and reaction temperature have been optimized in order to achieve the maximum catalytic activity. It has been previously pointed out that a thermal treatment at 1073 K is necessary to transform the CaCO3 into CaO. For this reason, a study concerning the influence of the activation temperature of the catalysts on the catalytic performance has been carried out and the results are shown in Fig. 8. Without activation, all catalysts are inactive in the transesterification of

Table 2 Textural parameters of the SBA-15 support and catalysts calcined at 1073 K Catalyst

SBET (m2/g)

VP (cm3/g)

dp (av) (nm)

SBA-15 SBA-4CaO SBA-6CaO SBA-8CaO SBA-14CaO

413 34.0 24.2 21.8 7.4

0.370 0.038 0.028 0.042 0.019

4.2 3.0 3.0 4.8 5.4

Fig. 7. Temperature-programmed desorption profiles of CO2 over (a) SBA4CaO, (b) SBA-6CaO, (c) SBA-8CaO, (d) SBA-14CaO and (e) SBA-20CaO catalysts.

M.C.G. Albuquerque et al. / Applied Catalysis A: General 334 (2008) 35–43

Fig. 8. Conversion in the transesterification of ethyl butyrate with methanol (activation temperature, 1073 K; stirring rate, 1250 rpm; amount of catalyst, 62 mg; methanol:ethyl butyrate molar ratio, 4:1; reaction temperature, 333 K; reaction time, 1 h).

ethyl butyrate with methanol, and the maximum conversion is attained after activation at 1073 K. The nature of the siliceous support has an important influence on the catalytic activity (Fig. 8). Thus, except for the SBA-4CaO catalyst, the use of SBA-15 gives rise to basic catalysts more active than those prepared with MCM-41 and fumed silica. This fact could be explained by taking into account the highest thermal stability of the SBA-15, which allows the activation of these catalysts at high temperatures without suffering important structural modifications. The study of the catalytic performance of SBA-nCaO catalysts as a function of the CaO loading reveals that an increment until 20 wt.% does not increase the transesterification activity (Fig. 8). This fact could be explained by the formation of larger CaO particles, without increasing the surface of the active phase accessible to reactants, as indicated from the data obtained by CO2-TPD, where the highest desorption value is found for SBA-14CaO. After defining the most suitable activation temperature, the study of the influence of the stirring rate has revealed that a value of 1250 rpm is at least necessary to get a suitable contact between the reactants and the solid catalyst. As regards the reaction temperature, as expected, an increase in temperature favors the conversion of ethyl butyrate, and taking into account that the boiling point of methanol is 337.7 K, a temperature slightly lower than this, 333 K, has been chosen to carry out all experiments. It has been previously pointed out that most papers concerning the use of basic catalysts for transesterification reaction, and particularly those addressed for biodiesel production, do not include the evaluation of the resistance of solid catalysts against lixiviation in the reaction medium. This fact has to be considered, since the presence of catalyst in solution might imply contribution of the homogeneous reaction, which requires additional steps of washing and purification of the biodiesel fraction [46]. For this reason, as a

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key aspect of the present work, the degree of lixiviation of the CaO-based catalysts was evaluated. Previous studies have shown that a fraction of bulk calcium oxide [46,47] is dissolved in the methanolic solution, and the objective of using supported CaO catalysts was to stabilize the active phase to avoid it. The experimental procedure employed consisted in putting in contact the catalyst with methanol under the same experimental conditions as used in the transesterification process, except for the presence of ethyl butyrate. After 1 h of reaction, the catalyst was quickly removed by filtration, and methanol was mixed with the necessary volume of ethyl butyrate, and maintained at 333 K for 1 h, under inert atmosphere. If catalysts are lixivied, conversion will be observed due to the homogeneous contribution. The conversion values found for the different supported CaO catalysts were in all cases negligible, thus, confirming the stabilization of the active phase on the SBA-15 siliceous support, which prevents its lixiviation. However, when bulk CaO and CaO–SrO were used, conversions of 8.5 and 23.6% were measured, respectively. On the other hand, the behavior of the most active catalyst, SBA-14CaO, has been compared with NaOH dissolved in methanol, under similar experimental conditions. In Fig. 9, it can be observed that a maximum conversion of 80% is reached after 30 min when a homogeneous process is used, whereas the heterogeneous catalyst reaches a maximum value close to 60% after 2 h of reaction. However, if the catalytic activity is expressed per gram of active phase (CaO, NaOH), the performance of the solid catalyst is much better than that of NaOH. The experimental design applied was 23 factorial design, with one central point. The measured response, Y0, was conversion in methyl ester after 1 h of reaction. The studied factors were catalyst concentration, XC, molar ratio, XMR and reaction temperature, XT (Fig. 10). Stirring rate was fixed at 1250 rpm for all experiments.

Fig. 9. Evolution of the conversion with the time for NaOH dissolved in methanol and SBA-14CaO (stirring rate, 1250 rpm; amount of catalyst, 124 mg; methanol, 30 mL; ethyl butyrate, 25 mL; reaction temperature, 333 K).

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M.C.G. Albuquerque et al. / Applied Catalysis A: General 334 (2008) 35–43 Table 4 Statistical analysis for the 23 factorial design

Fig. 10. Response surface for the model.

Table 3 shows the experimental matrix for the 2n factorial design (n factors, each run at three levels). Coded levels for the three factors are given in columns 2–4. The last column corresponds to the response, the experimental conversion obtained in this study for each run. A statistical analysis was carried out with these experimental results, and the main effect and interaction effect of the variables was estimated. Table 4 shows the test of statistical significance with a confidence level of 95%, revealing that the main effect and interaction effect, MR was significant in effect and the interaction values were higher than the confidence interval (0.007019). Experimental results were fit to a linear model, the following equation having found as valid within the experimental range studied in this work. Y ¼ 21:71  2:55  X C þ 13:47  X MR  4:1925  X C  X MR

ðr 2 ¼ 0:97Þ

The response surface indicates that, for low catalyst concentration, ME conversion increases with the increase of molar ratio. At high temperatures, a similar effect is observed, with an increment of the conversion to ME with the molar ratio. This indicates that the most significant factor is the methanol:ethyl butyrate molar ratio, and it exerts a positive effect on the transesterification reaction. Table 3 Experimental matrix for the factorial design Run

XC

XMR

XT

Y0 (%)

1 2 3 4 5 6 7 8 9

1 1 1 1 1 1 0 1 1

1 1 1 1 1 1 0 1 1

1 1 1 1 1 1 0 1 1

17.85 2.83 4.39 13.97 9.72 41.79 18.04 48.36 43.97

Response

Conversion

Number of experiments Degrees of freedom Main effects and interaction effect Significance test Confidence level Average of factorial runs Standard deviation, S Student’s t-value Confidence interval Significant effects Significance Response equation

9 8 IMR = 26.94 Student’s t 95% 21.71 1.828378 11.87330 0.007019 IMR Yes Y ¼ 21:71  2:55  X C þ 13:47  X MR  4:1925  X C  X MR

The most active catalyst, SBA-14CaO, has been used for biodiesel production from castor and sunflower oils. In both cases, the experimental conditions were 1 wt.% catalyst, methanol:oil molar ratio of 12 and reaction temperature of 333 K. The biodiesel yields found from castor and sunflower oils, after 1 h of reaction, were 65.7 and 4.1%, respectively. In the latter case, a maximum value of 95% is obtained after 5 h of reaction time. 4. Conclusions A new family of base catalysts has been prepared by impregnation and subsequent thermal treatment of calcium acetate supported on mesoporous SBA-15 silica. This support, as compared to MCM-41 and fumed silica, has proven to be thermally more resistant, and the interaction between CaO and silica is strong enough to prevent the lixiviation of the active phase in methanol. Process optimization was performed by application of the factorial design 23 and response surface methodology. From the statistical analysis, it was found that the methanol:ethyl butirate molar ratio is the main effect for the three studied factors. A linear model has been obtained to predict conversion levels as a function of the factors studied. For the transesterification of castor and sunflower oils with methanol, conversion values of 65.7 and 95% were attained after 1 and 5 h of reaction time, respectively. Acknowledgements The authors are grateful to financial support from Spanish Ministry of Education and Science (ENE2006-15116-C04-02 project), Junta de Andalucı´a (PO6-FQM-01661) and to the Brazilian Ministry of Education by CAPES/MECD 084/05, for the financial support to M.C.G. Albuquerque. References [1] J. Weitkamp, M. Hunger, U. Rymsa, Micropor. Mesopor. Mater. 48 (2001) 255. [2] K. Tanabe, W.F. Ho¨lderich, Appl. Catal. A 181 (1999) 399. [3] A. Corma, S. Iborra, Adv. Catal. 49 (2006) 239.

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