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Applied Catalysis B: Environmental 105 (2011) 199–205

Contents lists available at ScienceDirect

Applied Catalysis B: Environmental

journa l homepage: www.e lsev ier .com/ locate /apcatb

luminum doped SBA-15 silica as acid catalyst for the methanolysisf sunflower oil

. Jiménez-Morales, J. Santamaría-González, P. Maireles-Torres, A. Jiménez-López ∗

epartamento de Química Inorgánica, Cristalografía y Mineralogía (Unidad Asociada al ICP-CSIC), Facultad de Ciencias, Universidad de Málaga,ampus de Teatinos, 29071 Málaga, Spain

r t i c l e i n f o

rticle history:eceived 21 February 2011eceived in revised form 8 April 2011ccepted 13 April 2011vailable online 20 April 2011

a b s t r a c t

Post-synthesis alumination of a mesoporous SBA-15 silica, with different Si/Al molar ratios, has beensuccessfully carried out and the resulting acid solids were employed as catalysts in the methanolysis ofsunflower oil. The acid properties of these catalysts depend upon the aluminum content and the activationtemperature (350 or 550 ◦C). Although all the catalysts thermally treated at 550 ◦C were very active, thehighest catalytic performance was found with the catalyst containing a Si/Al molar ratio of 20. With only

◦

eywords:ransesterificationethanolysis

BA-15cid catalysts

10 wt% of this catalyst in relation to the oil, a 96 wt% of biodiesel formation was attained at 200 C andafter only 4 h of reaction. The catalysts are stable and no leaching of aluminum ions to the liquid mediumwas found. Moreover, these catalysts are able to simultaneously catalyze the esterification of free fattyacids (FFAs) and the transesterification of triglycerides, even in the presence of 9% of FFAs. The activity ofthis catalyst is well maintained after three cycles of catalysis without any treatment and in the presence

of a

esoporous solids of 5 wt% of water. The use

. Introduction

The continuous increasing in energy consumption and therowing environmental concerns have made renewable fuels anxceptionally attractive alternative. Therefore, it is necessary tose carbon-neutral biofuels to ensure the access of energy at anffordable price and to prevent environment damage [1–3]. Poten-ial feedstocks for the preparation of biodiesel are vegetable oils,uch as palm oil and sunflower oil, but of special interest is the use oflgae or used cooking oils for the preparation of second-generationiofuels, which production is more sustainable. The overall cost ofiodiesel production from these feedstocks could be reduced in areat extent. However, the main drawback is the presence of highercentages of free fatty acids (FFAs), making them unsuitable forhe industrial process based on homogeneous alkaline catalysis,ue to the formation of soaps. In this sense, the use of solid acid cat-lysts is preferable because they have the ability to simultaneouslyatalyze the transesterification of triglycerides and the esterifica-ion of FFAs. In recent years, several solid acid catalysts have beeneported for methanolysis of vegetable oil, being compiled theseorks by Lotero et al. [4], Melero et al. [5] and Di Serio et al. [6].

mong them, it can be found resins [7], tungstated zirconia [8–10],ulphated zirconia [11–13] and heropolyacids [14–16].

∗ Corresponding author. Tel.: +34 952131876; fax: +34 952131870.E-mail address: ajimenezl@uma.es (A. Jiménez-López).

926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apcatb.2011.04.015

co-solvent hardly affects the biodiesel formation.© 2011 Elsevier B.V. All rights reserved.

On the other hand, since the discovery of the M41S fam-ily of mesoporous materials in 1992 [17], several new types ofmesoporous solids (MCM-48, MSU, SBA-15, HMS, TUD) have beenprepared to provide new catalysts or sorbents for a large variety ofbulky molecules which cannot access to the small pores of micro-porous materials, such as zeolites. Moreover, the low acidity ofmesoporous silica can be enhanced by isomorphous substitutionsof Al, Ti(IV), Sn(IV), or Zr(IV) for Si [18,19] which mainly createLewis acid sites. Among mesoporous silica, SBA-15 has high specificsurface areas (600–1100 m2 g−1), large pore sizes (4.6–30.0 nm)and pore volumes (up to 2.5 cm3 g−1) with silica wall thicknessesfrom 3.1 to 6.4 nm. The incorporation of aluminum to the siliceousframework gives rise to mesoporous solids with a great potentialin acid catalyzed reaction for large molecules [20,21]. However,it is very difficult to introduce the Al ions during the synthe-sis step due to its easy dissociation from the Al–O–Si moietiesunder strong acidic conditions. In other words, the incorpora-tion of Al in tetrahedral positions requires a very careful controlof the synthesis conditions. For this reason, the post-synthesisalumination method is more recommended [22,23] because thematerials thus obtained present a more stable structure and highacidity than those solids doped with this element during the syn-thesis step [24,25]. In this sense, we have recently prepared aseries of Al-containing SBA-15 materials with different Si/Al molar

ratios, via post-synthesis incorporation of Al into a low-cost silicaSBA-15 prepared using sodium silicate as silica source [26]. Theacidic properties of these solids depend on the Al content, andusing FTIR spectroscopy of adsorbed acetonitrile it was detected

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he presence of stronger acid sites on the samples with low Aloading.

In this paper, the results obtained employing these mesoporouscid solids with high Si/Al molar ratios as heterogeneous catalystsn the methanolysis of sunflower oil at 200 ◦C to prepare biodieselre reported. The influence of different experimental parameters,uch as reaction time, percentage of catalyst, methanol/oil molaratio, presence of free fatty acids and water, and reutilization of theatalyst, on the catalytic behaviour has been evaluated in order toptimize experimental conditions for biodiesel production.

. Experimental

.1. Catalyst preparation

A SBA-15 silica was prepared according to the method describedlsewhere [26]. The alumination process was carried out by puttingn contact 1.5 g of this SBA-15 silica with a acid solution (pH 3.5–4)repared by mixing different volumes of a 1.2 M aqueous solu-ion of AlCl3·6H2O and 25 mL of tetramethylammonium hydroxideTMAOH, 5.5 wt% solution in water). The resulting suspension waseated at 80 ◦C during 4 h. The volume of AlCl3·6H2O aqueous solu-ion was varied to achieve Si/Al molar ratios of 30, 20 and 10 in theynthesis gel. The [TMAOH]/[Al] molar ratio was 2.5 in all cases23]. The samples were labelled as x-Al-SBA-T, where x representshe Si/Al molar ratio of the initial mixtures and T is the activationemperature in degrees Celsius.

.2. Characterisation techniques

Laboratory X-ray powder diffraction (XRPD) patterns were col-ected on a PAN analytical X’Pert Pro automated diffractometer, inragg–Brentano reflection configuration using a Ge (1 1 1) primaryonochromator (Cu K�1) and the X’Celerator detector with a step

ize of 0.017◦ (2�). The powder patterns were recorded between 8◦

nd 70◦ in 2� with an equivalent counting time of 712 s/step. Thehemical composition of the catalysts was determined by EDXRFsing a Horiba XGT-5000 equipment.

X-ray photoelectron spectra were collected using a Physicallectronics PHI 5700 spectrometer with non-monochromatic Al� radiation (300 W, 15 kV, 1486.6 eV) with a multi-channel detec-

or. Spectra of samples were recorded in the constant pass energyode at 29.35 eV, using a 720 �m diameter analysis area. Charge

eferencing was measured against adventitious carbon (C 1s at84.8 eV). A PHI ACCESS ESCA-V6.0 F software package was used forcquisition and data analysis. A Shirley-type background was sub-racted from the signals. All recorded spectra were always fittedsing Gaussian–Lorentzian curves to more accurately determinehe binding energy of the different element core levels. The modi-ed Auger parameter of Al (˛′) was calculated using the followingquation [27]: ˛′ = 1253.6 + KE(AlKLL) − KE(Al 2p), where KE(AlKLL)s the kinetic energy of the Auger electron of AlKLL and KE(Al 2p)he kinetic energy of the photoelectron Al 2p.

The textural parameters of the catalysts were evaluated fromitrogen adsorption–desorption isotherms at −196 ◦C, as deter-ined by an automatic ASAP 2020 system from Micromeritics.

emperature-programmed desorption of ammonia (NH3-TPD),reviously adsorbed at 100 ◦C, was carried out from room temper-ture to 550 ◦C, with a heating rate of 10 ◦C min−1 and maintaininghe sample at 550 ◦C for 15 min. The evolved ammonia was ana-yzed by on-line gas chromatography (Shimadzu GC-14A) provided

ith a TCD. Catalysts were pre-treated at atmospheric pres-ure by flowing helium (35 mL min−1) from room temperatureo 550 ◦C with a heating rate of 10 ◦C min−1 and maintaininghe sample at 550 ◦C for 1 h. Then, samples were cooling until

: Environmental 105 (2011) 199–205

100 ◦C under a helium flow and ammonia was adsorbed at thistemperature.

The catalytic isomerisation reaction of 1-butene was performedin a tubular glass flow microreactor. Samples (70 mg) were pre-treated for 2 h in a helium flow (30 mL min−1) at 400 ◦C and theexperiments were carried out at this temperature. Experimentswere performed at � = 67.2 gcat (g1-but)−1 h and the time on streamwas 120 min. The 1-butene and the reaction products were ana-lyzed on-line in a gas chromatograph (Shimadzu GC-14B) equippedwith a wide-bore KCl/AlCl3 column and provided with a FID detec-tor.

FTIR spectra of adsorbed pyridine were recorded on a Shi-madzu Fourier Transform Infrared Instrument (FTIR-8300). Selfsupported wafers of the samples with a weight/surface radio ofabout 15 mg cm−2 were placed in a vacuum cell greaseless stop-cocks and CaF2 windows. The samples were evacuated at 250 ◦Cand 10−2 Pa overnight, exposed to pyridine vapours at room tem-perature for 15 min and then outgassed at different temperatures.

2.3. Catalytic test

The methanolysis of edible sunflower oil was performed at200 ◦C using a Parr high pressure reactor with 100 mL capacity anda stirring rate of 600 rpm. Before reaction, catalysts were activatedin air at 350 or 550 ◦C during 2 h. In a typical experiment, 15 g ofoil was incorporated to the reactor together with the methanoland 0.75 g of catalyst. The methanol/oil molar ratio was 12. After6 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 forfew minutes. Later, 1 mL of dichloromethane was added, and thismixture was again agitated and set aside to develop two phases:the non-polar phase containing dichloromethane, mono-, di- andtriglycerides and methyl esters of fatty acids (FAME) (and tracesof methanol and glycerol) and the polar phase containing water,glycerol and methanol (and traces of esters). The dichloromethanewas then removed from the organic phase by evaporation at 90 ◦C.The resulting solution was analyzed by high performance liq-uid chromatography (HPLC) using a JASCO liquid chromatographequipped with quaternary gradient pump (PU-2089), multiwave-length detector (MD-2015), autosampler (AS-2055), column oven(co-2065) using a PHENOMENEX LUNA C18 reversed-phase col-umn (250 mm × 4.6 mm, 5 �m). The solvents were filtered througha 0.45 �m filter prior use and degassed with helium. A linear gradi-ent from 100% methanol to 50% methanol + 50% 2-propanol/hexane(5:4 v/v) in 35 min was employed. Injection volumes of 15 �L anda flow of rate of 1 mL min−1 were used. The column tempera-ture was held constant at 40 ◦C. All samples were dissolved in2-propanol–hexane (5:4 v/v). The weight content in FAME deter-mined by HPLC was considered to represent the FAME yield (inwt%) of the catalytic process, assuming that, during the neutraliza-tion and the washing process of the ester phase, only traces of esterswere transferred to the polar phase and that only the extraction ofmethanol and glycerol take place.

The degree of leaching of aluminum was measured using anICP-MS ELAN DRCe equipment (Perkin-Elmer) and employingthe following parameters: RF power = 1100 W, argon plasma gasflow = 15.0 L min−1, auxiliary gas flow = 0.9 L min−1, sample uptakerate = 0.9 mL min−1, measured mass number = 27.

3. Results and discussion

3.1. Catalyst characterisation

The low-angle powder XRD patterns of the x-Al-SAB catalysts,after calcination at 550 ◦C, exhibit a very intense peak at 2� = 1.1◦

I. Jiménez-Morales et al. / Applied Catalysis B: Environmental 105 (2011) 199–205 201

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ig. 1. XRD patterns in the low angle region of x-Al-SBA-550 catalysts: (a) SBA-15,b) 30-Al-SBA-550, (c) 20-Al-SBA-550 and (d) 10-Al-SBA-550.

orresponding to (1 0 0) reflection, together with three other weakeaks between 2◦ and 3◦, assigned to (1 1 0), (2 0 0) and (2 1 0)lanes in a hexagonal arrangement (Fig. 1). The XRD diffractionata can be indexed to a hexagonal lattice with a d100 spacing rang-

ng between 7.7 and 8.3 nm, corresponding to an average unit cellarameter a0 of 8.9–9.5 nm, according to the formula a0 = 2d100/

√3.

t is noteworthy that after alumination and ulterior calcination at50 ◦C, all the samples retain the hexagonal structure. On the otherand, the XRD patterns at high angles of the catalysts calcined at50 ◦C with different aluminum loadings only show the typicalroad band associated with the amorphous silica walls, withoutny evidence pointing to the presence of crystalline Al2O3.

The textural parameters of catalysts after calcination at 350 and50 ◦C, deduced from the N2 adsorption isotherms at −196 ◦C, areompiled in Table 1. All the N2 adsorption–desorption isothermsre similar and belong to the type IV in the IUPAC classification,ypical of mesoporous solids. The BET surface areas decrease withhe amount of aluminum incorporated, and, for a given catalyst,lightly diminish with the increment of the calcination tempera-ure, possibly due to the structural shrinkage associated with theondensation of Al–OH and Si–OH groups on the silica walls. Theaximum reduction in the BET surface area is found for the 10-l-SBA-550 sample, with a 48% with respect to the pristine SBA-15ilica. The pore volumes follow the same trend, slightly decreasingith the aluminum contents.

The presence of aluminum was confirmed by X-ray photoelec-ron spectroscopy. The binding energy values obtained for Si 2p,

1s and Al 2p are gathered in Table 2 together with the surfacei/Al atomic ratios. The BE of the Si 2p and O 1s are in the ranges02.9–103.4 and 532.5–532.9 eV, respectively, values which areharacteristic of these elements in silicon and aluminum oxides.he BE of Al 2p at 74.8 eV matches well with the value reported byashutski et al. [28] for this element in compounds with composi-ions such as AlxOy, where aluminum is less oxidized than Al2O3. Onhe other hand, the surface Si/Al atomic ratios are very close to thosesed in the synthesis gel. In order to get insight into the chemicalnvironment of aluminum in these compounds, the Auger param-ter (˛′) has been calculated since it depends on the coordinationdopted by this element. Thus, ˛′ values higher than 1461.0 eVre typical of octahedral Al, whereas tetrahedral Al has ˛′ value

ower than 1460.4 eV. All the Al KLL Auger spectra exhibit a peak at384 eV and a shoulder at 1386 eV, whose intensity increases whenhe aluminum content does it (Fig. 2). The ˛′ parameters have beenalculated and the corresponding values are compiled in Table 2. All

Fig. 2. Al KLL spectra of x-Al-SBA-550 catalysts.

the catalysts have two ˛′ values for Al, one on the 1458.7–1459.6 eVinterval typical of Al with tetrahedral environment, and another onthe interval 1460.4–1461.2 eV, which is characteristic of Al withoctahedral coordination. These results point to that aluminum isincorporated in a first step with tetrahedral coordination substi-tuting some Si on the pore walls. By increasing the amount of Al,this adopts an octahedral coordination. The presence of surface alu-minum with low oxygen coordination can confer acidic propertiesto these catalysts.

The study of the total acidity of the catalysts by NH3-TPD revealsthat this increases with the amount of aluminum (Table 1). Thus,the 30-Al-SBA-550 catalyst with the lowest aluminum loading isthe less acidic. On the other hand, the amount of desorbed ammoniadepends on the activation temperature, in such a way that a highactivation temperature lowers the total acidity. It means that, byheating at 550 ◦C, some Si–OH and Al–OH Brönsted acid groups con-dense evolving water and reducing the number of total acid sites,although some new Lewis centres are formed. The concentrationof both Brönsted and Lewis acid sites have been determined usingadsorption of pyridine coupled to FTIR spectroscopy. The vibrationband at 1550 cm−1 is assigned to the pyridinium ion formed on aBrönsted acid site, whereas that at 1450 cm−1 corresponds to thepyridine coordinated to Lewis acid centres. The concentrations ofboth types of acid sites were estimated for the integrated absorp-tion of both bands and using the extinction coefficients obtained byDakta et al. [29], EB = 0.73 cm mmol−1 and EL = 1.11 cm mmol−1, forBrönsted and Lewis sites, respectively. The data compiled in Table 3clearly reveal an increase of the Lewis acid concentration when theactivation temperature is 550 ◦C. However, these acid sites are ofweak strength because, after evacuation at 200 ◦C, the concentra-tion is largely reduced. In contrast, the concentration of Brönstedacid sites is maintained, even after evacuation at 300 ◦C. The mostacidic sample is 20-Al-SBA activated at 550 ◦C. The total acidity(CL + CB) after evacuation at 100 ◦C is higher for catalysts activatedat 550 ◦C; this is in contrast with the results found from NH3-TPDwhere the most acidic catalysts are those calcined at 350 ◦C. Sincepyridine is as a base weaker than ammonia, this could indicate thatsamples activated at 350 ◦C contain a higher number of weak acidsites which can be neutralized by ammonia but not by pyridine, ascan be deduced by comparison of the amount of ammonia desorbed

◦

at 200 C (considered as weak acid sites) with the total amount ofammonia desorbed in the interval 100–550 ◦C (Table 1). Finally, theacid properties of these catalysts have been tested in the isomerisa-tion reaction of 1-butene. Table 4 display the catalytic results, which

202 I. Jiménez-Morales et al. / Applied Catalysis B: Environmental 105 (2011) 199–205

Table 1Textural parameters and acidity of SBA-15 and aluminum doped catalysts.

Sample SBET (m2g−1) Vp (cm3g−1) Total acidity (�molNH3 g−1)

Weak/totalacidity (%)

SBA-15 675 0.39 64 32.830-Al-SBA-350 500 0.37 397 29.730-Al-SBA-550 481 0.37 285 28.120-Al-SBA-350 443 0.36 459 30.120-Al-SBA-550 414 0.33 411 17.810-Al-SBA-350 373 0.35 996 32.310-Al-SBA-550 353 0.34 585 28.5

Table 2Binding energies (eV), Si/Al atomic ratios and modified Auger parameters (˛) of x-Al-SBA-550 catalysts.

Sample Al 2p Si 2p O1s ˛tetrahedral Al ˛octahedral Al Surface Si/Al Bulka Si/Al

SBA-15 – 103.4 532.9 – – ∞ ∞30-Al-SBA-550 74.8 103.4 532.9 1459.6 1460.4 32 5720-Al-SBA-550 74.7 103.1 532.6 1458.7 1460.8 22 4410-Al-SBA-550 74.9 102.9 532.5 1458.7 1461.2 11 18

a By EDXRF analysis.

Table 3Concentration of Brönsted (CB) and Lewis (CL) acid sites (expressed in �mol g−1 for x-Al-SBA-T catalysts), after evacuation at different temperatures.

Evacuation temperature (◦C) 30-Al-SBA-350 30-Al-SBA-550 20-Al-SBA-350 20-Al-SBA-550 10-Al-SBA-350 10-Al-SBA-550

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eveal that all catalysts are quite active in this reaction with con-ersions ranged between 83.1% and 75.4%, although conversionsecrease when the activation of catalyst took place at 550 ◦C. Bear-

ng in mind that only Brönsted acid sites with medium and strongtrength are active in this reaction [30], this finding confirms thatome strong Brönsted acid sites have been eliminated after thermalreatment at 550 ◦C with the concomitant formation of new Lewiscid sites. For this reason, the iso-butene formation, which dependsn the presence of strongest acid sites, is clearly diminished whenhe activation temperature is 550 ◦C. From the NH3-TPD, adsorptionf pyridine and the isomerisation reaction of 1-butene data, it cane deduced that all the catalysts are acidic solids, confirming thathe post-synthesis alumination of SBA-15 silica is quite effective.

.2. Transesterification of sunflower oil with methanol

These x-Al-SBA-T catalysts have been tested for the methanol-sis of sunflower oil, after activation at 350 and 550 ◦C, in order tovaluate the influence of the dehydration degree of catalysts on theatalytic performance. The experimental conditions were: amount

f oil = 15 g, methanol/oil molar ratio = 12, amount of catalyst withespect to the oil weight = 5 wt%, stirring rate = 600 rpm, reactionime = 6 h and reaction temperature = 200 ◦C. Under these experi-

ental conditions, both sets of catalysts were very active, leading

able 4-Butene isomerisation data for x-Al-SBA-T catalysts, at 400 ◦C after 120 min of reaction t

Sample Conversion (%) Yield (%)

Butane

SBA-15 4.7 030-Al-SBA-350 80.7 8.430-Al-SBA-550 76.3 4.320-Al-SBA-350 82.2 9.720-Al-SBA-550 75.3 3.310-Al-SBA-350 83.1 11.510-Al-SBA-550 77.9 5.5

22 120 128 92 41 128 705 69 31 58 0 113 312 57 14 40 0 81 33

to 100% of sunflower conversion, with a biodiesel yield higher than70 wt%. The other products found were mono and diglycerides.However, the FAME formation depends upon the aluminum con-tent and the activation temperature, as can be deduced from datadisplayed in Fig. 3. Catalysts activated at 350 ◦C exhibit a decreasein the biodiesel formation for increasing aluminum content, i.e. theactivity decreases when the total acidity is lower. However, whenthe catalysts are activated at 550 ◦C, the FAME formation is alwayshigher than 80 wt%, being the maximum yield found for the 20Al-SBA-550 catalyst (93 wt% of FAME yield). This sample exhibits themaximum acidity, as determined from pyridine adsorption, hav-ing also the highest concentration of Lewis acid sites. Taking intoaccount that after the thermal activation at 550 ◦C some acidic–OH groups are eliminated with the concomitant formation of newLewis acid sites, the increase in the formation of biodiesel couldbe related to the presence of these new acid centres and espe-cially to the higher hydrophobic surface character, thus facilitatingthe approaching of the hydrophobic triglycerides molecules to thecatalytic centres. The 20-Al-SBA-550 catalyst exhibits higher for-mation of FAME, possibly due to the higher surface area and the

presence of these new Lewis acid sites, where it is assumed thatthe triglyceride molecules are chemisorbed. Once the formationof this Lewis complex takes place, it is attacked by methanol in asecond step, producing the transesterification reaction [31].

ime.

Cis-2-butene Trans-2-butene Isobutene

47.1 53.2 023.7 30.7 37.330.7 41.5 23.423.1 30.5 36.634.4 47.5 14.822.4 29.8 36.329.8 39.1 25.3

I. Jiménez-Morales et al. / Applied Catalysis B: Environmental 105 (2011) 199–205 203

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be reused as such or after regeneration. In the present study, thespent 20-Al-SBA-550 catalyst after 4 h of reaction was reused, with-

ig. 3. Evolution of the biodiesel formation in the transesterification of sunfloweril with methanol for x-Al-SBA-T catalysts (reaction conditions: methanol/oil molaratio = 12, catalyst = 5 wt%, T = 200 ◦C and reaction time = 6 h).

The influence of different experimental parameters on themount of biodiesel formation has been accomplished using the0-Al-SBA-550 catalyst, which exhibited the maximum formationf biodiesel amid the set of assayed catalysts. Firstly, the influencef the temperature on the reaction was studied using 0.75 g of cat-lyst at temperatures between 150 and 200 ◦C, and 6 h of reactionime. From Fig. 4, it is deduced the important effect of the reac-ion temperature. At 150 ◦C the formation of biodiesel is almostil, being only detected the formation of a 37 wt% mono and di-lycerides. The maximum biodiesel yield is attained at 200 ◦C, with92 wt% for a full conversion of the sunflower oil. For this rea-

on, 200 ◦C was the temperature chosen for the study of the otherxperimental parameters.

On the other hand, the study of the influence of the reactionime of reaction (Fig. 5) reveals that a biodiesel yield of 82% can beeached after only 4 h of reaction time, with a 100% of sunfloweronversion. From these data, 4 h of reaction time was chosen forhe successive experiments. The contribution of the uncatalyzedhermal reaction has been measured under these experimentalonditions (4 h of reaction at 200 ◦C), and the value found for theiodiesel formation was 24 wt%. Under these experimental con-itions, a commercial Al2O3–SiO2 (8.8 wt% Al2O3, Grace Davisonhemical, with a specific surface area of 467 m2 g−1) has been used,eaching a lower biodiesel yield (65 wt%).

The optimization of the oil/methanol molar ratio on the FAMEield is important since the recuperation of the excess of methanolsed in the biodiesel production has an important economic cost.ig. 6 shows the results obtained by varying this ratio from 1/4

ig. 4. FAME formation as a function of the reaction temperature for the 20-Al-SBA-50 catalyst (reaction conditions: methanol/oil molar ratio = 12, catalyst = 5 wt% andeaction time = 6 h).

Fig. 5. Influence of the reaction time on the biodiesel formation in the methanolysisof sunflower oil using the 20-Al-SBA-550 catalyst (reaction conditions: methanol/oilmolar ratio = 12, catalyst = 5 wt% and T = 200 ◦C).

until 1/16. It can be observed that the yield slightly increases withincreasing the amount of methanol up to 1/12, giving 82 wt% ofbiodiesel yield, but an ulterior increment maintains the FAME for-mation constant. Therefore, a oil/methanol molar ratio of 1/12was selected for the all the catalytic tests. From this result, it canbe deduced that the transesterification reaction needs an excessof alcohol higher than the theoretical 3 moles per mole of oil inorder to shift the equilibrium to the right hand side. This excessof methanol also extracts the reaction products from the catalystsurface renewing the catalytic sites [32].

The effect of the catalyst loading was investigated using 1.7,5 and 10 wt% of catalyst with respect to the weight of sunfloweroil. The other parameters of the reaction were: reaction time = 4 h,methanol/oil molar ratio = 12 and reaction temperature 200 ◦C.The results obtained (Fig. 7) reveal that the sunflower conversionis almost complete in all the cases and that biodiesel formationincreases with the amount of catalyst, attaining 95.5% with themaximum loading (10 wt%). The evolution of the FAME yields is alineal function of the catalyst weight, clearly indicating the absenceof diffusional limitations in this reaction.

Heterogeneous acid catalysts take advantages of the easy recov-ering from the reaction medium over liquid acids, and thus they can

out any pre-treatment, in two new catalytic cycles. Fig. 8 shows theresults of three successive reactions, and it can be observed that the

Fig. 6. Influence of the oil/methanol molar ratio in the transesterification of sun-flower oil with methanol over the 20-Al-SBA-550 catalyst (reaction conditions:catalyst = 5 wt%, T = 200 ◦C and reaction time = 4 h).

204 I. Jiménez-Morales et al. / Applied Catalysis B

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ig. 7. Variation of the FAME yield in the methanolysis of sunflower oil as a func-ion of the catalyst amount over the 20-Al-SBA-550 catalyst (reaction conditions:

ethanol/oil molar ratio = 12, T = 200 ◦C and reaction time = 4 h).

atalyst maintains its catalytic activity, especially in the two lastycles where the biodiesel yield is 76 wt%. The small loss of activityith respect to the first reaction cycle (7%) could be due to the acid

ite blockage by some adsorbed intermediates or reaction productore polar than the reactants [7] and/or carbon deposits [33,34]. In

act, CNH analysis of this spent catalyst, rinsed several times withexane, indicates 7.2 wt% of carbon. Simultaneously with the reuti-

ization test, it was evaluated the possible leaching of aluminumons into the reaction medium. For this purpose, the catalyst waseparated by filtration and the resulting liquid was analyzed by ICPo determine the presence of aluminum ions. The analytical resulteveals that the aluminum concentration is nil, thus indicating thereat stability of the catalyst under the experimental conditionsf the transesterification reaction and confirming that aluminumons are grafted on the pore surface forming a stable structure withtrong acidic properties.

It is well known that a more sustainable biodiesel productionan be achieved using either fried oils or non edible oils with a highontent of free fatty acids (FFAs). In this sense, the effect of theresence of these FFAs on the catalytic performance of the 20-Al-BA-550 catalyst has been evaluated using in the transesterificationeaction a sunflower oil to which different amounts of oleic acid3◦, 6◦ and 9◦) were added. The catalytic results indicate that this

atalyst accomplished both the esterification of FFAs and the trans-sterification of sunflower oil in the same extent, i.e. a biodieselormation close to 82 wt% even in the presence of 9◦ of oil acidity.his result was expected because the 20-Al-SBA-550 catalyst with a

ig. 8. Reusability test of the 20-Al-SBA-550 catalyst in the methanolysis ofunflower oil (reaction conditions: methanol/oil molar ratio = 12, catalyst = 5 wt%,= 200 ◦C and reaction time = 4 h).

[

: Environmental 105 (2011) 199–205

strong superficial acidity is stable against a weak acid as is oleic acidand, as a consequence, no leaching of aluminum ions toward thesolution liquids was found. On the other hand, at 200 ◦C the ester-ification of oleic acid with methanol is quite fast [35]; thus, underthese experimental conditions, oleic acid is fully transformed intoFAME. Finally, the influence of the presence of 5 wt% of water inthe oil was investigated, and a reduction of 11% in the biodieselformation was observed. This fact can be explained by consideringthe adsorption of water molecules on the strongest acid sites of thecatalyst, which could decrease the hydrophobic character of thesecentres leading to a slight decrease in the biodiesel formation.

The influence of co-solvent adding was studied by incorporating10 and 15 v/v% of toluene to the reagents. Using the 20-Al-SBA-550catalyst (5 wt%), after 4 h of reaction at 200 ◦C, the biodiesel for-mation was 78 and 77 wt%, respectively, values slightly lower thanthat found in the reaction without co-solvent. Taking into accountthat toluene is a good solvent for vegetable oils and miscible withmethanol, it was expected a little increase in the biodiesel yieldwhen this co-solvent is added. Therefore, the slight inhibitory effectobserved on the reaction due to the presence of toluene can onlybe explained by the dilution effect on the reactants and as a conse-quence on the decrease of the reaction rate, thus after 4 h of reactionthe yield values of biodiesel found are slightly lesser.

4. Conclusions

Post-synthesis alumination of a SAB-15 silica affords a easy andlow cost method to synthesize mesoporous acid solids with differ-ent Si/Al molar ratios, showing interesting acidic properties due tothe incorporation of aluminum ions on the pore walls, where areforming Si–O–Al bonds. The total acidity of these solids dependsupon the aluminum concentration and the thermal treatment. Aftercalcination at 550 ◦C, the remaining acid sites are manly of Lewistype, which are able to produce the transesterification of sunfloweroil with methanol at 200 ◦C, with a low percentage of catalyst afteronly 4 h of reaction time. All the catalysts activated at 550 ◦C giverise to FAME yields higher than 80 wt%, although the best result areachieved with the 20-Al-SBA-550 catalyst. With only 10 wt% of thiscatalyst, 96 wt% of biodiesel yield is attained. This catalyst is stableagainst oils with acidity even of 9◦, and no leaching of aluminumions into the reaction liquids is found; moreover it is able to simul-taneously accomplish the oil transesterification and esterificationof the FFAs. The presence of 5 wt% of water or the use of a co-solventhardly affect to the biodiesel formation.

Acknowledgements

The authors are grateful to financial support from the Span-ish Ministry of Science and Innovation (ENE2009-12743-C04-03Project) and Junta de Andalucía (P09-FQM-5070) and FEDERfounds. I.J.M. would like to thank the Agencia Estatal CSIC for aJAE-Predoctoral Grant.

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