Methanolysis of sunflower oil catalyzed by acidic Ta2O5 supported on SBA-15

Applied Catalysis A: General 405 (2011) 93–100

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Methanolysis of sunflower oil catalyzed by acidic Ta2 O5 supported on SBA-15 I. Jiménez-Morales, J. Santamaría-González, P. Maireles-Torres, A. Jiménez-López ∗ Departamento de Química Inorgánica, Cristalografía y Mineralogía (Unidad Asociada al ICP-CSIC), Facultad de Ciencias, Universidad de Málaga, Campus de Teatinos, 29071 Málaga, Spain

a r t i c l e

i n f o

Article history: Received 21 June 2011 Received in revised form 19 July 2011 Accepted 27 July 2011 Available online 5 August 2011 Keywords: Tantalum oxide SBA-15 Transesterification Sunflower oil

a b s t r a c t Tantalum penta-ethoxide has been used as precursor for the preparation, after calcination at 575 ◦ C, of a series of catalysts based on tantalum oxy-hydrate supported on SBA-15 silica. The Ta2 O5 loading ranges between 5 and 25 wt%, and all of them exhibit acid properties, as determined by NH3 -TPD. These catalysts have been assayed in the methanolysis of sunflower oil at 200 ◦ C, being the catalyst with a 15 wt% of Ta2 O5 the most active, giving 92.5% of biodiesel yield with solely 6.7% of catalyst with respect to the oil weight. Moreover, no leaching of tantalum was detected in the liquid medium, and these catalysts are able to simultaneously produce the esterification of free fatty acids (FFAs) and the transesterification of triglycerides, even in the presence of 9% of FFAs. The catalytic activity is well maintained in the presence of 5 wt% of water and after three cycles without any treatment. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Biodiesel is defined as a mixture of long-chain alkyl esters derived from fatty acids obtained from renewable sources, such as vegetable oils and animal fats. Most biodiesel is currently produced by transesterification of triglycerides of edible oils with methanol, in the presence of homogeneous basic catalysts. Basic catalysis offers some advantages, such as higher reaction rates and the requirement of low temperatures to obtain high biodiesel yields in a short period of time. Although biodiesel has some advantageous features over diesel derived from petroleum, its main drawback is the high production cost, which could be lowered by employing cooking oils as sources of triglycerides, and by using heterogeneous catalysis for allowing the easy separation of the solid catalyst from the reaction mixture formed by FAME, the excess of methanol and glycerol, as by product which can be obtained quite pure by a cheap refining process [1]. However, used fried oils cannot be employed under basic conditions, because they contain free fatty acids (FFAs) which neutralize a fraction of the basic sites. Recently, several authors have employed solid acid catalysts for methanolysis of vegetable oil, being compiled these works by Lotero et al. [2], Santacesaria et al. [3] and Melero et al. [4]. Among the most studied acidic solids, we can find resins [5], tungstated zirconia [6–9], sulfated zirconia [10,11], zirconium sulphate [12,13] and heropolyacids [14–16]. On the other hand, the elements of 5th group of the Periodic Table have been quite studied in this field.

∗ Corresponding author. Tel.: +34 952131876; fax: +34 952131870. E-mail address: [email protected] (A. Jiménez-López). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.07.037

Thus, bulk VOPO4 ·2H2 O, after calcination at 400–500 ◦ C, has good activity in the transesterification of soybean oil with methanol at low temperatures and in a short reaction time; although no leaching of the active phase is observed, the catalyst suffers deactivation due to a progressive reduction of V(V) to V(IV) and V(III), but the catalytic activity can be restored by calcination in air [17]. On the other hand, Nb2 O5 ·nH2 O treated with sulphuric acid or phosphoric acid has been also tested in both the esterification of oleic acid and the transesterification of soybean oil with methanol. The most active catalyst was obtained after treatment with sulphuric acid, being the maxima conversion, after 5 h, 78% in the former reaction and 40% in the last one. Although conversions are not very high, the activity of the catalyst was well maintained even after 5 recycling experiments [18]. Ta2 O5 has been used as support of polyoxometalates in order to improve the stability of these acid catalysts due to the high solubility of Keggin unit in polar solvents. Polymetalates/Ta2 O5 composite catalysts are quite active in the esterification reaction of fatty acids [19,20]. The catalytic performance of this type of catalysts can be enhanced by the incorporation of hydrophobic alkyl groups terminally bonded on the Ta2 O5 framework via Ta–O–Si–C bonds, the resulting organic–inorganic hybrid catalysts exhibit excellent catalytic activity in the transesterification of soybean oil and in the esterification of myristic acid at temperatures as low as 65 ◦ C, although at longer reaction time (24 h) [21,22]. The aim of the present work was the preparation of more simple acidic catalysts based on tantalum oxy-hydrate (Ta2 O5 ·nH2 O) by the impregnation of mesoporous SBA-15 silica with tantalum penta-ethoxide, and their application in biodiesel synthesis from transesterification of sunflower oil with methanol. As regards

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the nature of the support, ordered mesoporous silica presents significant advantages with respect to conventional silica, owing to its high specific surface area, large pore size and high thermal stability [23–25]. The influence of experimental parameters, such as reaction time, methanol/oil molar ratio, percentage of catalyst, presence of free fatty acids and water and reutilization of the catalyst, on the catalytic behaviour has been evaluated in order to optimize the experimental conditions for biodiesel preparation.

2. Experimental 2.1. Catalyst preparation The synthesis and characterization of the mesoporous silica (SBA-15), used as support, has been previously described [26]. The precursor of Ta2 O5 was tantalum(V) penta-ethoxide, which was incorporated onto the support by incipient wetness impregnation using anhydrous ethanol solutions of this salt (99.98 wt%, Sigma–Aldrich), under inert atmosphere. The concentration of the precursor solution was adjusted to give rise to catalysts with Ta2 O5 percentages ranging between 5 and 25 wt%. After impregnation, all materials were dried at 60 ◦ C and later activated at 575 ◦ C during 6 h, in order to remove the alcoxide groups and obtain the supported Ta2 O5 ·nH2 O, tantalum oxy-hydrate, with acidic properties [27]. The catalysts were labelled as Ta-x-MCM, where x is the weight percentage of Ta2 O5 in the catalysts.

2.2. Characterization techniques X-ray diffraction (XRD) patterns of catalysts were collected on a PAN analytical X’Pert Pro automated diffractometer. Powder patterns were recorded in Bragg-Brentano reflection configuration by using a Ge (1 1 1) primary monochromator (Cu K␣1 ) and the X’Celerator detector with a step size of 0.017◦ (2Â), between 10 and 70◦ in 2Â with an equivalent counting time of 712 s/step. Thermodiffractometric study of precursor Ta-20-SBA catalyst was carried out using an Anton Paar HTK1200 Camera under static air. Flux of gases was not employed to avoid sample dehydration prior to the diffraction experiment. Data were collected at different temperature intervals ranging between 30 and 900 ◦ C with a heating rate of 10 ◦ C min−1 and maintaining 15 min at each desired temperature to ensure thermal stabilization. The data acquisition range was 8–70◦ (2Â) with a step size of 0.017◦ . Thermogravimetric (TG) and differential thermal analysis (DTA) of catalyst precursors were performed from room temperature until 850 ◦ C on a SDT Q600 apparatus from TA Instruments; calcined alumina as a reference and a heating rate of 10 ◦ C min−1 were employed. X-ray photoelectron spectra were collected using a Physical Electronics PHI 5700 spectrometer with non-monochromatic Al K␣ radiation (300 W, 15 kV, 1486.6 eV) with a multi-channel detector. Spectra of samples were recorded in the constant pass energy mode at 29.35 eV, using a 720 ␮m diameter analysis area. Charge referencing was measured against adventitious carbon (C 1s at 284.8 eV). A PHI ACCESS ESCA-V6.0 F software package was used for acquisition and data analysis. A Shirley-type background was subtracted from the signals. All recorded spectra were always fitted using Gaussian–Lorentzian curves to more accurately determine the binding energy of the different element core levels. The textural parameters of the catalysts were evaluated from nitrogen adsorption–desorption isotherms at −196 ◦ C, as determined by an automatic ASAP 2020 system from Micromeritics. The accumulated pore and micropore volumes were determined by BJH and t plot methods, respectively.

The total acidity of catalysts was measured by temperatureprogrammed desorption of ammonia (NH3 -TPD), previously adsorbed at 100 ◦ C. Catalysts (80 mg) were pre-treated at atmospheric pressure by flowing helium (35 mL min−1 ) from room temperature to 550 ◦ C, with a heating rate of 10 ◦ C min−1 , and maintaining the sample at 550 ◦ C for 1 h. Then, catalysts were cooling until 100 ◦ C under a helium flow and ammonia was adsorbed at this temperature. The desorption of ammonia was carried out from 100 to 550 ◦ C, with a heating rate of 10 ◦ C min−1 and maintaining the sample at 550 ◦ C for 15 min. The evolved ammonia was analyzed by on-line gas chromatograph (Shimadzu GC-14A) provided with a TCD. IR spectra were recorded on a Shimadzu Fourier Transform Instrument (FTIR-8300) using KBr pressed powder discs. Raman spectra were obtained with a Raman Senterra (Bruker) microspectrometer equipped with a thermoelectrically cooled CCD detector. Excitation radiation at 1064 nm was used as supplied from a Praseodimium laser. Raman spectra were performed from powder samples without any previous treatment. 2.3. Catalytic test The methanolysis of edible sunflower oil was performed at 200 ◦ C by using a Parr high pressure reactor with 100 mL capacity and a stirring rate of 600 rpm. Before reaction, catalysts were activated in air at 575 ◦ C during 6 h. In a typical experiment, 15 g of oil was incorporated to the reactor together with the methanol and 0.6 g of catalyst. The methanol/oil molar ratio was 12. After 6 h of reaction, the system was cooled and then an aliquot (2 mL) was taken and treated with 1 mL of distilled water and shaking for few minutes. Later, 1 mL of dichloromethane was added, and this mixture was again agitated and set aside to develop two phases: the non-polar phase containing dichloromethane, mono-, di- and triglycerides and methyl esters of fatty acids (FAME) (and traces of methanol and glycerol) and the polar phase containing water, glycerol and methanol (and traces of esters). The dichloromethane was then removed from the organic phase by evaporation at 90 ◦ C. The resulting solution was analyzed by high performance liquid chromatography (HPLC) using a JASCO liquid chromatograph equipped with quaternary gradient pump (PU-2089), multiwavelength detector (MD-2015), autosampler (AS-2055), column oven (co-2065) using a PHENOMENEX LUNA C18 reversed-phase column (250 mm × 4.6 mm, 5 ␮m). The solvents were filtered through a 0.45 ␮m filter prior use and degassed with helium. A linear gradient from 100% methanol to 50% methanol + 50% 2-propanol/hexane (5:4 v/v) in 35 min was employed. Injection volumes of 15 ␮L and a flow of rate of 1 mL min−1 were used. The column temperature was held constant at 40 ◦ C. All samples were dissolved in 2-propanolhexane (5:4, v/v). The weight content in FAME determined by HPLC was considered to represent the FAME yield (in wt%) of the catalytic process, assuming that, during the neutralization and the washing process of the ester phase, only traces of esters were transferred to the polar phase and that only the extraction of methanol and glycerol take place. The degree of leaching of tantalum was measured by using a ICP-MS ELAN DRC equipment (Perkin–Elmer) and employing the following parameters: RF power = 1100 W, argon plasma gas flow = 15.0 L min−1 , auxiliary gas flow = 0.9 L min−1 , sample uptake rate = 0.9 mL min−1 , measured mass number = 73. 3. Results and discussion 3.1. Catalyst characterization The DTA curve of the Ta-20-SBA precursor (Fig. 1) presents two endothermal effects at low temperatures associated to the loss of

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Fig. 1. TG and DTA curves of the Ta-20-SBA precursor.

alcohol molecules and water, and two exothermal effects at 360 and 575 ◦ C corresponding to the combustion of the organic matter. The exothermal peak observed at 750 ◦ C could correspond to the crystallization of Ta2 O5 . On the other hand, the TG curve indicates a continuous weight loss from room temperature till 850 ◦ C, with a total weight loss of 11.5%, lower than the theoretically expected value (14.3%). By taking into account that the residual organic matter in the calcined sample is very low (C = 0.27 wt%), this difference could be ascribed to the existence of some OH groups, pointing out that the formation of crystalline Ta2 O5 was not completely accomplished [28]. A thermodiffractometric study of the as-prepared Ta-25-SBA sample was also run between 30 ◦ C and 900 ◦ C (Fig. 2). All the diffractograms obtained at temperatures lower than 600 ◦ C do not exhibit diffraction peaks indicating the absence of any crystalline phases. Only a broad band centred at 26◦ is observed, a characteristic feature of the amorphous silica walls. At temperatures higher than 700 ◦ C, well defined diffraction peaks at 22.8◦ , 28.5◦ and 36.8◦ , corresponding to orthorhombic Ta2 O5 appear. These results indicate that, once the organic matter is eliminated, according to the thermogravimetric analysis, at temperatures close to 575 ◦ C, the formation of amorphous Ta2 O5 ·nH2 O takes place [27,29]. Although

Fig. 3. O 1s core level spectrum of Ta-20-SBA catalyst activated at 575 ◦ C.

Ta2 O5 ·nH2 O prepared from the hydrolysis of TaCl5 shows better acidic properties when is calcined at moderate temperatures, 200–400 ◦ C, the use of tantalum penta-ethoxide as precursor for the preparation of Ta2 O5 ·nH2 O makes necessary calcination temperatures higher than 500 ◦ C in order to remove all the alkoxide groups. Calcining at 500–600 ◦ C, the hydrated tantalum pentoxide still retains relevant acidic properties [27]. All catalysts activated at 575 ◦ C maintain the mesoporous structure of the support, since XRD patterns at low angles show a broad and intense peak centred at 2Â = 1.1◦ , similar to the pristine SBA-15 support. In order to get insights into the surface composition of the catalysts, X-ray photoelectron spectroscopy was employed for the characterization of catalysts. The binding energy (B.E) values observed for Si 2p are close to 103.1 eV, in agreement with the data reported for Si 2p in mesoporous silica. The O 1s signal appears as a doublet at 532.9 and at 530.7 eV (Fig. 3), which indicates two different oxide environments: Si–O–Si (532.9 eV) in the support and Ta–O–Ta (530.7 eV) in the TaOx species coming from the thermal

Fig. 2. Thermodiffractometric patterns of the as-prepared Ta-25-SBA sample.

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Fig. 5. FTIR spectra of bulk Ta2 O5 , SBA-15 silica and Ta-20-SBA catalyst.

Fig. 4. Ta 4f core level spectrum of Ta-20-SBA catalyst activated at 575 ◦ C.

decomposition of tantalum penta-ethoxide. The higher intensity of the last peak indicates that the silica surface is masked by the well dispersed Ta2 O5 ·nH2 O. In the core level region of tantalum, two peaks corresponding to Ta 4f7/2 and Ta 4f5/2 are observed (Fig. 4) at 26.3 and 28.2 eV, respectively [30,31]. The binding energies separation close to 1.9 eV and the area ratio of 1.36 confirm that Ta is fully oxidized as Ta(V). The superficial composition of catalysts, as deduced by XPS analysis (Table 1), reveals that the Ta/Si atomic ratios are close to one for samples with Ta2 O5 loading lower than 15 wt%, whereas higher loading lead to higher values, which can be explained by the presence of an important fraction of Ta located on the external surface, perhaps forming large clusters. Thus, the break point of these values would indicate the transition from well dispersed tantalum oxy-hydrate (Ta2 O5 ·nH2 O) to large clusters of Ta2 O5 ·nH2 O, although they are not detected by XRD due to their amorphous character. The skeletal FTIR spectrum of SBA-15 silica (Fig. 5) shows a large band ranging from 800 till 1250 cm−1 due to the Si–O–Si asymmetric and Si–OH terminal silanol stretching vibrations. The weaker band at 794 cm−1 is due to Si–O–Si symmetric/bending modes, whereas in the hydroxyl range the two intense peaks at 3400 cm−1 and 3740 cm−1 can be assigned to silanol groups; the first one being shifted to lower wavenumber and enlarged due to strong interaction with residual water located in the channels and the second one is attributed to non-interacting silanol groups [32]. Fourier transform infrared analysis was also carried out to investigate the chemical nature of bulk tantalum oxy-hydrate prepared by calcination at 575 ◦ C of the product obtained by hydrolysis in acidic medium of tantalum penta-ethoxide, as well as a sup-

ported Ta2 O5 (Ta-20-SBA) catalyst. The bulk tantalum oxy-hydrate exhibits bands at 694 and 1100 cm−1 corresponding to the stretching O–Ta–O bonds and terminals Ta–O bonds, respectively [29,33]. The large band centred at 3450 cm−1 can be assigned to the stretching mode of OH groups bonded to the tantalum species. The IR spectrum of Ta-20-SBA catalyst is very similar to that of the support, although the band corresponding to free OH groups at 3740 cm−1 is shifted towards low energies indicating that they are interacting with Ta2 O5 ·nH2 O moieties, being only visible a large band centred at 3480 cm−1 assigned to Ta–OH groups and silanol groups interacting with water molecules. On the other hand, the small band at 694 cm−1 due to the O–Ta–O stretching vibrations is only visible in samples with high Ta2 O5 loadings. The Raman spectra of these catalysts confirmed the presence of Ta2 O5 on the SBA-15 surface. The Raman spectrum of bulk tantalum oxy-hydrate, Ta2 O5 ·nH2 O, after calcination at 575 ◦ C during 6 h, is shown in Fig. 6. This spectrum in the range 200–1200 cm−1 presents a major characteristic Raman band at 660 cm−1 due to Ta–O–Ta vibrations. The shoulder at about 940 cm−1 corresponds to the symmetric stretching mode of the terminal Ta O bonds [34]. These features are only observable in the samples with the highest Ta2 O5 loadings. Thus, Ta-20-SBA sample exhibits clearly the band at 660 cm−1 , although the shoulder at 940 cm−1 is hardly visible due to the overlapping of this vibration with that intense of Si–OH at 975 cm−1 . Textural characteristics were evaluated from nitrogen adsorption–desorption isotherms at −196 ◦ C. All the tantalumcontaining materials calcined at 575 ◦ C exhibit reversible type IV isotherms in the IUPAC classification, with a sharp inflexion point at P/Po = 0.35–0.40, characteristic of capillary condensation within uniform mesopores with constant cross section. Moreover, the isotherms present a clear hysteresis loop of H1 type. The BET specific surface areas, micropore and accumulated pore volumes

Table 1 Superficial properties and acidity of SBA-15 and Ta2 O5 impregnated catalysts. Sample

SBET (m2 g−1 )

Vp (cm3 g−1 )

Vmicro (cm3 g−1 )

Total acidity (␮mol NH3 g−1 )

Weak-medium acidity (%)

Ta/Si atomic ratioa

SBA-15 Ta-5-SBA Ta-10-SBA Ta-15-SBA Ta-20-SBA Ta-25-SBA Bulk Ta2 O5

675 551 541 494 466 435 5

0.52 0.43 0.42 0.39 0.37 0.35 0.02

0.17 0.12 0.12 0.11 0.10 0.09 0

64 409 443 485 449 426 56

– 73 82 80 72 72 –

– 1.2 0.9 1.2 2.6 2.9 –

a

From XPS analysis.

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Fig. 6. Raman spectra of bulk Ta2 O5 , SBA-15 silica and Ta-20-SBA catalyst.

(Table 1) generally decrease as the tantalum oxide content rises; thus, the higher reduction of BET area with respect to the pristine support was 35% for Ta-25-SBA. Microporosity was assessed by using the t-plot analysis; the presence of microporosity represents connections between mesopores. In general, all materials retain their mesoporous character and high surface areas after impregnation with Ta2 O5 . Total acidity of the catalysts was determined by NH3 -TPD and the obtained results are gathered in Table 1. The total acidity largely increases with respect to that of the pristine SBA-15 silica owing to the incorporation of Ta2 O5 ·nH2 O moieties. In general, all the catalysts are quite acidic, ranging the amount of desorbed ammonia between 409 and 485 ␮mol NH3 g−1 . The Ta-15-SBA catalyst exhibits the maximum acidity in accordance with its better Ta2 O5 dispersion, as was deduced from XPS data. Loadings of Ta2 O5 higher than 15 wt% give rise to catalysts with lower acidity due to the formation of bigger cluster with poor dispersion. 3.2. Transesterification of sunflower with methanol As has been mentioned above, several Ta-x-SBA catalysts have been prepared and tested for the methanolysis of sunflower oil. Firstly, the Ta-15-SBA catalyst has been employed to determine

Fig. 7. Influence of the reaction time on the oil conversion and the biodiesel formation in the methanolysis of sunflower oil for Ta-15-SBA catalyst (reaction conditions: methanol/oil molar ratio = 12, catalyst = 4 wt%, T = 200 ◦ C).

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Fig. 8. Influence of the activation temperature of the Ta-15-SBA catalyst in the transesterification of sunflower oil with methanol (reaction conditions: methanol/oil molar ratio = 12, catalyst = 4 wt%, T = 200 ◦ C and reaction time = 6 h).

the influence of the reaction time on the catalytic performance. The experimental conditions of the catalytic reaction were: sunflower oil = 15 g, methanol/oil molar ratio = 12, amount of catalyst with respect to the oil weight = 4 wt%, stirring rate = 600 rpm and reaction temperature = 200 ◦ C. From Fig. 7 it can be deduced that reaction rates are relatively low and 6 h are necessary to reach 74.3 wt% of biodiesel yield. However, after this time, the oil conversion was 100 wt%, being the other products found mono- and diglycerides. Then, the influence of the activation temperature of catalysts was evaluated by thermally treating the Ta-15-SBA catalyst at 425, 500 and 575 ◦ C during 4 h and assayed in the methanolysis of sunflower oil under the same experimental conditions, but using 6 h of reaction time. The results (Fig. 8) reveal that the dehydration degree of catalysts clearly influences on the biodiesel yield, being necessary to activate them at temperatures higher than 500 ◦ C to obtain yields above 70 wt%. This fact would demonstrate that the elimination of adsorbed water, or even some Ta–OH groups, during the thermal activation increases the hydrophobic properties of the active phase, making easier the approaching of hydrophobic triglycerides molecules to the catalytic centres. From these results, 6 h of reaction time and 575 ◦ C as activation temperature were selected for the study of other experimental parameters. On the other hand, the evolution of the oil conversion and biodiesel formation (wt%) as a function of the Ta2 O5 loading

Fig. 9. Variation of the FAME yield in the methanolysis of sunflower oil as a function of the Ta2 O5 loading (reaction conditions: methanol/oil molar ratio = 12, catalyst = 4 wt% and T = 200 ◦ C, reaction time = 6 h).

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Fig. 10. Influence of the oil/methanol molar ratio in the transesterification of sunflower oil with methanol over the Ta-15-SBA catalyst (reaction conditions: catalyst = 4 wt%, T = 200 ◦ C and reaction time = 6 h).

(Fig. 9) denotes that oil conversion depends upon the active phase loading of catalysts, reaching conversion values between 85 and 100 wt%, being Ta-15-SBA the most active catalyst. The maximum biodiesel formation is also attained for this catalyst, with a biodiesel yield of 74.3 wt%. The high catalytic performance of the Ta-15-SBA catalyst was expected by taking into account that this sample exhibits the highest dispersion of the Ta2 O5 ·nH2 O phase, as was deduced from the analysis of the superficial chemical composition by XPS (Table 1), whereas for loading higher than 15 wt% the atomic ratio Ta/Si sharply increased. Moreover, this catalyst exhibits the maximum total acidity, as determined from NH3 -TPD (Table 1), and high concentration of acid sites of weak-medium strength. The bulk Ta2 O5 ·nH2 O prepared by acid hydrolysis of the tantalum penta-ethoxide and ulterior calcination at 575 ◦ C during 6 h, as well as the bare support, are completely inactive in the catalytic reaction under the same experimental conditions. This behaviour was expected owing to the low acidity of this solid. However, the uncatalysed thermal reaction under these experimental conditions leads to a 37% of biodiesel formation. Another key point to take into account in the biodiesel production is the evaluation of the lixiviation degree of the active phase, Ta2 O5 ·nH2 O. With this goal, the presence of tantalum in the filtered solution, after the transesterification reaction during 6 h at 200 ◦ C, has been analyzed by ICP. The results reveal that lixiviation is practically negligible since the concentration of tantalum found in the reaction liquids is lower than 0.8 ppm. This result was expected due to the low solubility of the active phase in organic media and confirms that Ta2 O5 ·nH2 O is grafted on the surface of pores, as was inferred from the FTIR studies. The study of the influence of the methanol/oil molar ratio on the conversion (Fig. 10) shows that the FAME formation depends on the methanol concentration, being obtained the highest yield for a methanol/oil molar ratio of 12. From this result, it is deduced that the transesterification reaction needs an excess of alcohol higher than the theoretical 3 moles per mole of oil in order to shift the equilibrium to the right hand side [35,36]. For this reason the molar ratio selected for all the experiments was 12. This excess of methanol not only is necessary to facilitate the access of methanol to the previously adsorbed triglycerides on the actives acid sites where are forming carbocations, but also for extracting the reaction products from the catalyst surface renewing the catalytic sites [37]. The influence of the amount of catalyst in the transesterification reaction has been also evaluated, using values varying between

Fig. 11. Variation of the FAME yield in the methanolysis of sunflower oil as a function of the catalyst amount over the Ta-15-SBA catalyst (reaction conditions: methanol/oil molar ratio = 12, T = 200 ◦ C and reaction time = 6 h).

2.1 and 6.7 wt% of catalyst with respect to the oil weight. The corresponding results (Fig. 11) reveal that the oil conversion is complete using an amount of catalyst higher than 0.6 g; however, the biodiesel formation only increases monotonically achieving a maximum of 92.5% when the loading of catalyst was only 6.7 wt% of catalyst, with a methanol/oil molar ratio of 12. An important factor in the developing of solid catalysis for diesel production is the reusability of catalysts. This aspect was considered by recovering the solid after each catalytic run by filtration and reusing it in a new cycle without any treatment, such as washing or calcination. Fig. 12 shows that the Ta-15-SBA catalyst can be reutilised for three cycles without important loss of catalytic activity because the FAME yield and the triglycerides conversion only decrease about 7% after the second run and they were maintained in an ulterior run. This partial loss of catalytic activity could be due to the blockage of some active sites by adsorbed intermediates or products, more polar than the original reactants [38] and/or carbon deposition [39]. In fact, the carbon content of this spent catalyst, as determined by microanalysis after washing several times with hexane, was 2.3 wt%. An important advantage of solid acid catalysts with respect to basic ones is the possibility of employ, as feedstock for the biodiesel production, used oils such as fried or nonedible oils with high

Fig. 12. Reusability test of the Ta-15-SBA catalyst in the methanolysis of sunflower oil (reaction conditions: methanol/oil molar ratio = 12, catalyst = 4 wt%, T = 200 ◦ C and reaction time = 6 h).

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case the employ of a co-solvent is not recommended, and thus its ulterior separation step is avoided. 4. Conclusions Biodiesel can be efficiently obtained from methanolysis of sunflower oil catalyzed by Ta-x-SBA catalysts activated at 575 ◦ C. The catalysts characterization revealed that the good activity of these catalysts is due to their acidic properties, related with the formation of tantalum oxy-hydrate dispersed on the SBA-15 surface. The most active catalyst was Ta-15-SBA, since, with only 6.7 wt% of catalyst, a 92.5 wt% of FAME is obtained and maintains a high percentage of its activity after three cycles of reaction. Moreover, no leaching of tantalum was detected. On the other hand, this catalyst is able to simultaneous produce the esterification of FFAs and the transesterification of sunflower oil, even with an acidity of 9◦ , at 200 ◦ C, and in the presence of 5 wt% of water. Fig. 13. Dependence on the biodiesel yield by the presence of free fatty acids in the methanolysis reaction (reaction conditions: methanol/oil molar ratio = 12, Ta-15SBA catalyst = 4 wt%, T = 200 ◦ C and reaction time = 6 h).

contents of FFAs. A previous esterification step is necessary in the case of base catalysts, where the presence of FFAs would lead to a partial neutralization of the solid catalyst with the corresponding formation of soaps. To study the ability of the Ta-15-SBA catalyst to simultaneously give the esterification of free fatty acids and the transesterification of triglycerides, three waste oils were simulated by adding several quantities of oleic acid to 15 g of sunflower oil, thus reproducing oils with acidity degrees ranging between 3◦ and 9◦ (defined as g of oleic acid per 100 g of oil). The results obtained reveal that this catalyst is able to fully accomplish both reactions with a biodiesel yield close to 75% (Fig. 13), even for an acidity degree of 9◦ . Finally, the influence of a co-solvent in the process of preparation of biodiesel, such as toluene, was also studied by incorporating 10 and 15 v/v% of this solvent in the reactants. By using the Ta-15SBA catalyst (4 wt%) after 6 h of reaction at 200 ◦ C, the biodiesel formation were in both cases very similar to the value obtained without the use of co-solvent (Fig. 14). Taking into account that toluene is a good solvent for vegetable oils and miscible with methanol, it was expected a little increase in the yield of biodiesel when this co-solvent is added. This increase means to be compensated by the dilution effect on the reactants and as a consequence the biodiesel yield is maintained constant. In conclusion, in this

Fig. 14. Influence of toluene as co-solvent in the methanolysis of sunflower (reaction conditions: methanol/oil molar ratio = 12, Ta-15-SBA catalyst = 4 wt%, T = 200 ◦ C and reaction time = 6 h).

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