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1. Applied Catalysis B: Environmental 105 (2011) 199–205
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Applied Catalysis B: Environmental
journal homepage: www.elsevier.com/locate/apcatb
Aluminum doped SBA-15 silica as acid catalyst for the methanolysis
of sunflower oil
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 February 2011
Received in revised form 8 April 2011
Accepted 13 April 2011
Available online 20 April 2011
Keywords:
Transesterification
Methanolysis
SBA-15
Acid catalysts
Mesoporous solids
a b s t r a c t
Post-synthesis alumination of a mesoporous SBA-15 silica, with different Si/Al molar ratios, has been
successfully carried out and the resulting acid solids were employed as catalysts in the methanolysis of
sunflower oil. The acid properties of these catalysts depend upon the aluminum content and the activation
temperature (350 or 550 ◦
C). Although all the catalysts thermally treated at 550 ◦
C were very active, the
highest catalytic performance was found with the catalyst containing a Si/Al molar ratio of 20. With only
10 wt% of this catalyst in relation to the oil, a 96 wt% of biodiesel formation was attained at 200 ◦
C and
after only 4 h of reaction. The catalysts are stable and no leaching of aluminum ions to the liquid medium
was found. Moreover, these catalysts are able to simultaneously catalyze the esterification of free fatty
acids (FFAs) and the transesterification of triglycerides, even in the presence of 9% of FFAs. The activity of
this catalyst is well maintained after three cycles of catalysis without any treatment and in the presence
of 5 wt% of water. The use of a co-solvent hardly affects the biodiesel formation.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
The continuous increasing in energy consumption and the
growing environmental concerns have made renewable fuels an
exceptionally attractive alternative. Therefore, it is necessary to
use carbon-neutral biofuels to ensure the access of energy at an
affordable price and to prevent environment damage [1–3]. Poten-
tial feedstocks for the preparation of biodiesel are vegetable oils,
such as palm oil and sunflower oil, but of special interest is the use of
algae or used cooking oils for the preparation of second-generation
biofuels, which production is more sustainable. The overall cost of
biodiesel production from these feedstocks could be reduced in a
great extent. However, the main drawback is the presence of high
percentages of free fatty acids (FFAs), making them unsuitable for
the industrial process based on homogeneous alkaline catalysis,
due to the formation of soaps. In this sense, the use of solid acid cat-
alysts is preferable because they have the ability to simultaneously
catalyze the transesterification of triglycerides and the esterifica-
tion of FFAs. In recent years, several solid acid catalysts have been
reported for methanolysis of vegetable oil, being compiled these
works by Lotero et al. [4], Melero et al. [5] and Di Serio et al. [6].
Among them, it can be found resins [7], tungstated zirconia [8–10],
sulphated zirconia [11–13] and heropolyacids [14–16].
∗ Corresponding author. Tel.: +34 952131876; fax: +34 952131870.
E-mail address: ajimenezl@uma.es (A. Jiménez-López).
On the other hand, since the discovery of the M41S fam-
ily of mesoporous materials in 1992 [17], several new types of
mesoporous solids (MCM-48, MSU, SBA-15, HMS, TUD) have been
prepared to provide new catalysts or sorbents for a large variety of
bulky molecules which cannot access to the small pores of micro-
porous materials, such as zeolites. Moreover, the low acidity of
mesoporous silica can be enhanced by isomorphous substitutions
of Al, Ti(IV), Sn(IV), or Zr(IV) for Si [18,19] which mainly create
Lewis acid sites. Among mesoporous silica, SBA-15 has high specific
surface 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 thicknesses
from 3.1 to 6.4 nm. The incorporation of aluminum to the siliceous
framework gives rise to mesoporous solids with a great potential
in 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 moieties
under strong acidic conditions. In other words, the incorpora-
tion of Al in tetrahedral positions requires a very careful control
of the synthesis conditions. For this reason, the post-synthesis
alumination method is more recommended [22,23] because the
materials thus obtained present a more stable structure and high
acidity than those solids doped with this element during the syn-
thesis step [24,25]. In this sense, we have recently prepared a
series of Al-containing SBA-15 materials with different Si/Al molar
ratios, via post-synthesis incorporation of Al into a low-cost silica
SBA-15 prepared using sodium silicate as silica source [26]. The
acidic properties of these solids depend on the Al content, and
using FTIR spectroscopy of adsorbed acetonitrile it was detected
0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2011.04.015

2. 200 I. Jiménez-Morales et al. / Applied Catalysis B: Environmental 105 (2011) 199–205
the presence of stronger acid sites on the samples with low Al
loading.
In this paper, the results obtained employing these mesoporous
acid solids with high Si/Al molar ratios as heterogeneous catalysts
in the methanolysis of sunflower oil at 200 ◦C to prepare biodiesel
are reported. The influence of different experimental parameters,
such as reaction time, percentage of catalyst, methanol/oil molar
ratio, presence of free fatty acids and water, and reutilization of the
catalyst, on the catalytic behaviour has been evaluated in order to
optimize experimental conditions for biodiesel production.
2. Experimental
2.1. Catalyst preparation
A SBA-15 silica was prepared according to the method described
elsewhere [26]. The alumination process was carried out by putting
in contact 1.5 g of this SBA-15 silica with a acid solution (pH 3.5–4)
prepared by mixing different volumes of a 1.2 M aqueous solu-
tion of AlCl3·6H2O and 25 mL of tetramethylammonium hydroxide
(TMAOH, 5.5 wt% solution in water). The resulting suspension was
heated at 80 ◦C during 4 h. The volume of AlCl3·6H2O aqueous solu-
tion was varied to achieve Si/Al molar ratios of 30, 20 and 10 in the
synthesis gel. The [TMAOH]/[Al] molar ratio was 2.5 in all cases
[23]. The samples were labelled as x-Al-SBA-T, where x represents
the Si/Al molar ratio of the initial mixtures and T is the activation
temperature in degrees Celsius.
2.2. Characterisation techniques
Laboratory X-ray powder diffraction (XRPD) patterns were col-
lected on a PAN analytical X’Pert Pro automated diffractometer, in
Bragg–Brentano reflection configuration using a Ge (1 1 1) primary
monochromator (Cu K␣1) and the X’Celerator detector with a step
size of 0.017◦ (2Â). The powder patterns were recorded between 8◦
and 70◦ in 2Â with an equivalent counting time of 712 s/step. The
chemical composition of the catalysts was determined by EDXRF
using a Horiba XGT-5000 equipment.
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 detec-
tor. 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 sub-
tracted 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 modi-
fied Auger parameter of Al (˛ ) was calculated using the following
equation [27]: ˛ = 1253.6 + KE(AlKLL) − KE(Al 2p), where KE(AlKLL)
is the kinetic energy of the Auger electron of AlKLL and KE(Al 2p)
the kinetic energy of the photoelectron Al 2p.
The textural parameters of the catalysts were evaluated from
nitrogen adsorption–desorption isotherms at −196 ◦C, as deter-
mined by an automatic ASAP 2020 system from Micromeritics.
Temperature-programmed desorption of ammonia (NH3-TPD),
previously adsorbed at 100 ◦C, was carried out from room temper-
ature 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 ana-
lyzed by on-line gas chromatography (Shimadzu GC-14A) provided
with a TCD. Catalysts were pre-treated at atmospheric pres-
sure 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, samples were cooling until
100 ◦C under a helium flow and ammonia was adsorbed at this
temperature.
The catalytic isomerisation reaction of 1-butene was performed
in 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 the
experiments were carried out at this temperature. Experiments
were performed at = 67.2 gcat (g1-but)−1 h and the time on stream
was 120 min. The 1-butene and the reaction products were ana-
lyzed on-line in a gas chromatograph (Shimadzu GC-14B) equipped
with 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). Self
supported wafers of the samples with a weight/surface radio of
about 15 mg cm−2 were placed in a vacuum cell greaseless stop-
cocks and CaF2 windows. The samples were evacuated at 250 ◦C
and 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 at
200 ◦C 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 350 or 550 ◦C during 2 h. In a typical experiment, 15 g of
oil was incorporated to the reactor together with the methanol
and 0.75 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 liq-
uid chromatography (HPLC) using a JASCO liquid chromatograph
equipped 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 through
a 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 and
a flow of rate of 1 mL min−1 were used. The column tempera-
ture was held constant at 40 ◦C. All samples were dissolved in
2-propanol–hexane (5:4 v/v). The weight content in FAME deter-
mined by HPLC was considered to represent the FAME yield (in
wt%) of the catalytic process, assuming that, during the neutraliza-
tion 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 aluminum was measured using an
ICP-MS ELAN DRCe 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 = 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 2Â = 1.1◦

3. I. Jiménez-Morales et al. / Applied Catalysis B: Environmental 105 (2011) 199–205 201
Fig. 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.
corresponding to (1 0 0) reflection, together with three other weak
peaks between 2◦ and 3◦, assigned to (1 1 0), (2 0 0) and (2 1 0)
planes in a hexagonal arrangement (Fig. 1). The XRD diffraction
data can be indexed to a hexagonal lattice with a d100 spacing rang-
ing between 7.7 and 8.3 nm, corresponding to an average unit cell
parameter a0 of 8.9–9.5 nm, according to the formula a0 = 2d100/
√
3.
It is noteworthy that after alumination and ulterior calcination at
550 ◦C, all the samples retain the hexagonal structure. On the other
hand, the XRD patterns at high angles of the catalysts calcined at
550 ◦C with different aluminum loadings only show the typical
broad band associated with the amorphous silica walls, without
any evidence pointing to the presence of crystalline Al2O3.
The textural parameters of catalysts after calcination at 350 and
550 ◦C, deduced from the N2 adsorption isotherms at −196 ◦C, are
compiled in Table 1. All the N2 adsorption–desorption isotherms
are similar and belong to the type IV in the IUPAC classification,
typical of mesoporous solids. The BET surface areas decrease with
the amount of aluminum incorporated, and, for a given catalyst,
slightly diminish with the increment of the calcination tempera-
ture, possibly due to the structural shrinkage associated with the
condensation of Al–OH and Si–OH groups on the silica walls. The
maximum reduction in the BET surface area is found for the 10-
Al-SBA-550 sample, with a 48% with respect to the pristine SBA-15
silica. The pore volumes follow the same trend, slightly decreasing
with the aluminum contents.
The presence of aluminum was confirmed by X-ray photoelec-
tron spectroscopy. The binding energy values obtained for Si 2p,
O 1s and Al 2p are gathered in Table 2 together with the surface
Si/Al atomic ratios. The BE of the Si 2p and O 1s are in the ranges
102.9–103.4 and 532.5–532.9 eV, respectively, values which are
characteristic of these elements in silicon and aluminum oxides.
The BE of Al 2p at 74.8 eV matches well with the value reported by
Pashutski et al. [28] for this element in compounds with composi-
tions such as AlxOy, where aluminum is less oxidized than Al2O3. On
the other hand, the surface Si/Al atomic ratios are very close to those
used in the synthesis gel. In order to get insight into the chemical
environment of aluminum in these compounds, the Auger param-
eter (˛ ) has been calculated since it depends on the coordination
adopted by this element. Thus, ˛ values higher than 1461.0 eV
are typical of octahedral Al, whereas tetrahedral Al has ˛ value
lower than 1460.4 eV. All the Al KLL Auger spectra exhibit a peak at
1384 eV and a shoulder at 1386 eV, whose intensity increases when
the aluminum content does it (Fig. 2). The ˛ parameters have been
calculated 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 eV
interval typical of Al with tetrahedral environment, and another on
the interval 1460.4–1461.2 eV, which is characteristic of Al with
octahedral coordination. These results point to that aluminum is
incorporated 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 properties
to these catalysts.
The study of the total acidity of the catalysts by NH3-TPD reveals
that this increases with the amount of aluminum (Table 1). Thus,
the 30-Al-SBA-550 catalyst with the lowest aluminum loading is
the less acidic. On the other hand, the amount of desorbed ammonia
depends on the activation temperature, in such a way that a high
activation temperature lowers the total acidity. It means that, by
heating 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 concentration
of both Brönsted and Lewis acid sites have been determined using
adsorption of pyridine coupled to FTIR spectroscopy. The vibration
band at 1550 cm−1 is assigned to the pyridinium ion formed on a
Brönsted acid site, whereas that at 1450 cm−1 corresponds to the
pyridine coordinated to Lewis acid centres. The concentrations of
both types of acid sites were estimated for the integrated absorp-
tion of both bands and using the extinction coefficients obtained by
Dakta et al. [29], EB = 0.73 cm mmol−1 and EL = 1.11 cm mmol−1, for
Brönsted and Lewis sites, respectively. The data compiled in Table 3
clearly reveal an increase of the Lewis acid concentration when the
activation temperature is 550 ◦C. However, these acid sites are of
weak strength because, after evacuation at 200 ◦C, the concentra-
tion is largely reduced. In contrast, the concentration of Brönsted
acid sites is maintained, even after evacuation at 300 ◦C. The most
acidic sample is 20-Al-SBA activated at 550 ◦C. The total acidity
(CL + CB) after evacuation at 100 ◦C is higher for catalysts activated
at 550 ◦C; this is in contrast with the results found from NH3-TPD
where the most acidic catalysts are those calcined at 350 ◦C. Since
pyridine is as a base weaker than ammonia, this could indicate that
samples activated at 350 ◦C contain a higher number of weak acid
sites which can be neutralized by ammonia but not by pyridine, as
can be deduced by comparison of the amount of ammonia desorbed
at 200 ◦C (considered as weak acid sites) with the total amount of
ammonia desorbed in the interval 100–550 ◦C (Table 1). Finally, the
acid properties of these catalysts have been tested in the isomerisa-
tion reaction of 1-butene. Table 4 display the catalytic results, which

4. 202 I. Jiménez-Morales et al. / Applied Catalysis B: Environmental 105 (2011) 199–205
Table 1
Textural parameters and acidity of SBA-15 and aluminum doped catalysts.
Sample SBET (m2
g−1
) Vp (cm3
g−1
) Total acidity (␮mol
NH3 g−1
)
Weak/total
acidity (%)
SBA-15 675 0.39 64 32.8
30-Al-SBA-350 500 0.37 397 29.7
30-Al-SBA-550 481 0.37 285 28.1
20-Al-SBA-350 443 0.36 459 30.1
20-Al-SBA-550 414 0.33 411 17.8
10-Al-SBA-350 373 0.35 996 32.3
10-Al-SBA-550 353 0.34 585 28.5
Table 2
Binding 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 57
20-Al-SBA-550 74.7 103.1 532.6 1458.7 1460.8 22 44
10-Al-SBA-550 74.9 102.9 532.5 1458.7 1461.2 11 18
a
By EDXRF analysis.
Table 3
Concentration 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
cB cL cB cL cB cL cB cL cB cL cB cL
100 60 10 101 44 91 22 120 128 92 41 128 70
200 64 0 70 25 68 5 69 31 58 0 113 31
300 41 0 49 18 44 2 57 14 40 0 81 33
reveal that all catalysts are quite active in this reaction with con-
versions ranged between 83.1% and 75.4%, although conversions
decrease when the activation of catalyst took place at 550 ◦C. Bear-
ing in mind that only Brönsted acid sites with medium and strong
strength are active in this reaction [30], this finding confirms that
some strong Brönsted acid sites have been eliminated after thermal
treatment at 550 ◦C with the concomitant formation of new Lewis
acid sites. For this reason, the iso-butene formation, which depends
on the presence of strongest acid sites, is clearly diminished when
the activation temperature is 550 ◦C. From the NH3-TPD, adsorption
of pyridine and the isomerisation reaction of 1-butene data, it can
be deduced that all the catalysts are acidic solids, confirming that
the post-synthesis alumination of SBA-15 silica is quite effective.
3.2. Transesterification of sunflower oil with methanol
These x-Al-SBA-T catalysts have been tested for the methanol-
ysis of sunflower oil, after activation at 350 and 550 ◦C, in order to
evaluate the influence of the dehydration degree of catalysts on the
catalytic performance. The experimental conditions were: amount
of oil = 15 g, methanol/oil molar ratio = 12, amount of catalyst with
respect to the oil weight = 5 wt%, stirring rate = 600 rpm, reaction
time = 6 h and reaction temperature = 200 ◦C. Under these experi-
mental conditions, both sets of catalysts were very active, leading
to 100% of sunflower conversion, with a biodiesel yield higher than
70 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 data
displayed in Fig. 3. Catalysts activated at 350 ◦C exhibit a decrease
in the biodiesel formation for increasing aluminum content, i.e. the
activity decreases when the total acidity is lower. However, when
the catalysts are activated at 550 ◦C, the FAME formation is always
higher than 80 wt%, being the maximum yield found for the 20Al-
SBA-550 catalyst (93 wt% of FAME yield). This sample exhibits the
maximum acidity, as determined from pyridine adsorption, hav-
ing also the highest concentration of Lewis acid sites. Taking into
account that after the thermal activation at 550 ◦C some acidic
–OH groups are eliminated with the concomitant formation of new
Lewis acid sites, the increase in the formation of biodiesel could
be related to the presence of these new acid centres and espe-
cially to the higher hydrophobic surface character, thus facilitating
the approaching of the hydrophobic triglycerides molecules to the
catalytic 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 that
the triglyceride molecules are chemisorbed. Once the formation
of this Lewis complex takes place, it is attacked by methanol in a
second step, producing the transesterification reaction [31].
Table 4
1-Butene isomerisation data for x-Al-SBA-T catalysts, at 400 ◦
C after 120 min of reaction time.
Sample Conversion (%) Yield (%)
Butane Cis-2-butene Trans-2-butene Isobutene
SBA-15 4.7 0 47.1 53.2 0
30-Al-SBA-350 80.7 8.4 23.7 30.7 37.3
30-Al-SBA-550 76.3 4.3 30.7 41.5 23.4
20-Al-SBA-350 82.2 9.7 23.1 30.5 36.6
20-Al-SBA-550 75.3 3.3 34.4 47.5 14.8
10-Al-SBA-350 83.1 11.5 22.4 29.8 36.3
10-Al-SBA-550 77.9 5.5 29.8 39.1 25.3

5. I. Jiménez-Morales et al. / Applied Catalysis B: Environmental 105 (2011) 199–205 203
Fig. 3. Evolution of the biodiesel formation in the transesterification of sunflower
oil with methanol for x-Al-SBA-T catalysts (reaction conditions: methanol/oil molar
ratio = 12, catalyst = 5 wt%, T = 200 ◦
C and reaction time = 6 h).
The influence of different experimental parameters on the
amount of biodiesel formation has been accomplished using the
20-Al-SBA-550 catalyst, which exhibited the maximum formation
of biodiesel amid the set of assayed catalysts. Firstly, the influence
of the temperature on the reaction was studied using 0.75 g of cat-
alyst at temperatures between 150 and 200 ◦C, and 6 h of reaction
time. From Fig. 4, it is deduced the important effect of the reac-
tion temperature. At 150 ◦C the formation of biodiesel is almost
nil, being only detected the formation of a 37 wt% mono and di-
glycerides. The maximum biodiesel yield is attained at 200 ◦C, with
a 92 wt% for a full conversion of the sunflower oil. For this rea-
son, 200 ◦C was the temperature chosen for the study of the other
experimental parameters.
On the other hand, the study of the influence of the reaction
time of reaction (Fig. 5) reveals that a biodiesel yield of 82% can be
reached after only 4 h of reaction time, with a 100% of sunflower
conversion. From these data, 4 h of reaction time was chosen for
the successive experiments. The contribution of the uncatalyzed
thermal reaction has been measured under these experimental
conditions (4 h of reaction at 200 ◦C), and the value found for the
biodiesel formation was 24 wt%. Under these experimental con-
ditions, a commercial Al2O3–SiO2 (8.8 wt% Al2O3, Grace Davison
Chemical, with a specific surface area of 467 m2 g−1) has been used,
reaching a lower biodiesel yield (65 wt%).
The optimization of the oil/methanol molar ratio on the FAME
yield is important since the recuperation of the excess of methanol
used in the biodiesel production has an important economic cost.
Fig. 6 shows the results obtained by varying this ratio from 1/4
Fig. 4. FAME formation as a function of the reaction temperature for the 20-Al-SBA-
550 catalyst (reaction conditions: methanol/oil molar ratio = 12, catalyst = 5 wt% and
reaction time = 6 h).
Fig. 5. Influence of the reaction time on the biodiesel formation in the methanolysis
of sunflower oil using the 20-Al-SBA-550 catalyst (reaction conditions: methanol/oil
molar ratio = 12, catalyst = 5 wt% and T = 200 ◦
C).
until 1/16. It can be observed that the yield slightly increases with
increasing the amount of methanol up to 1/12, giving 82 wt% of
biodiesel yield, but an ulterior increment maintains the FAME for-
mation constant. Therefore, a oil/methanol molar ratio of 1/12
was selected for the all the catalytic tests. From this result, it can
be 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. This excess
of methanol also extracts the reaction products from the catalyst
surface 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 sunflower
oil. 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 conversion
is almost complete in all the cases and that biodiesel formation
increases with the amount of catalyst, attaining 95.5% with the
maximum loading (10 wt%). The evolution of the FAME yields is a
lineal function of the catalyst weight, clearly indicating the absence
of 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
be reused as such or after regeneration. In the present study, the
spent 20-Al-SBA-550 catalyst after 4 h of reaction was reused, with-
out any pre-treatment, in two new catalytic cycles. Fig. 8 shows the
results 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).

6. 204 I. Jiménez-Morales et al. / Applied Catalysis B: Environmental 105 (2011) 199–205
Fig. 7. Variation of the FAME yield in the methanolysis of sunflower oil as a func-
tion of the catalyst amount over the 20-Al-SBA-550 catalyst (reaction conditions:
methanol/oil molar ratio = 12, T = 200 ◦
C and reaction time = 4 h).
catalyst maintains its catalytic activity, especially in the two last
cycles where the biodiesel yield is 76 wt%. The small loss of activity
with respect to the first reaction cycle (7%) could be due to the acid
site blockage by some adsorbed intermediates or reaction product
more polar than the reactants [7] and/or carbon deposits [33,34]. In
fact, CNH analysis of this spent catalyst, rinsed several times with
hexane, indicates 7.2 wt% of carbon. Simultaneously with the reuti-
lization test, it was evaluated the possible leaching of aluminum
ions into the reaction medium. For this purpose, the catalyst was
separated by filtration and the resulting liquid was analyzed by ICP
to determine the presence of aluminum ions. The analytical result
reveals that the aluminum concentration is nil, thus indicating the
great stability of the catalyst under the experimental conditions
of the transesterification reaction and confirming that aluminum
ions are grafted on the pore surface forming a stable structure with
strong acidic properties.
It is well known that a more sustainable biodiesel production
can be achieved using either fried oils or non edible oils with a high
content of free fatty acids (FFAs). In this sense, the effect of the
presence of these FFAs on the catalytic performance of the 20-Al-
SBA-550 catalyst has been evaluated using in the transesterification
reaction a sunflower oil to which different amounts of oleic acid
(3◦, 6◦ and 9◦) were added. The catalytic results indicate that this
catalyst accomplished both the esterification of FFAs and the trans-
esterification of sunflower oil in the same extent, i.e. a biodiesel
formation close to 82 wt% even in the presence of 9◦ of oil acidity.
This result was expected because the 20-Al-SBA-550 catalyst with a
Fig. 8. Reusability test of the 20-Al-SBA-550 catalyst in the methanolysis of
sunflower oil (reaction conditions: methanol/oil molar ratio = 12, catalyst = 5 wt%,
T = 200 ◦
C and reaction time = 4 h).
strong superficial acidity is stable against a weak acid as is oleic acid
and, as a consequence, no leaching of aluminum ions toward the
solution liquids was found. On the other hand, at 200 ◦C the ester-
ification of oleic acid with methanol is quite fast [35]; thus, under
these experimental conditions, oleic acid is fully transformed into
FAME. Finally, the influence of the presence of 5 wt% of water in
the oil was investigated, and a reduction of 11% in the biodiesel
formation was observed. This fact can be explained by considering
the adsorption of water molecules on the strongest acid sites of the
catalyst, which could decrease the hydrophobic character of these
centres leading to a slight decrease in the biodiesel formation.
The influence of co-solvent adding was studied by incorporating
10 and 15 v/v% of toluene to the reagents. Using the 20-Al-SBA-550
catalyst (5 wt%), after 4 h of reaction at 200 ◦C, the biodiesel for-
mation was 78 and 77 wt%, respectively, values slightly lower than
that found in the reaction without co-solvent. Taking into account
that toluene is a good solvent for vegetable oils and miscible with
methanol, it was expected a little increase in the biodiesel yield
when this co-solvent is added. Therefore, the slight inhibitory effect
observed on the reaction due to the presence of toluene can only
be 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 reaction
the yield values of biodiesel found are slightly lesser.
4. Conclusions
Post-synthesis alumination of a SAB-15 silica affords a easy and
low cost method to synthesize mesoporous acid solids with differ-
ent Si/Al molar ratios, showing interesting acidic properties due to
the incorporation of aluminum ions on the pore walls, where are
forming Si–O–Al bonds. The total acidity of these solids depends
upon the aluminum concentration and the thermal treatment. After
calcination at 550 ◦C, the remaining acid sites are manly of Lewis
type, which are able to produce the transesterification of sunflower
oil with methanol at 200 ◦C, with a low percentage of catalyst after
only 4 h of reaction time. All the catalysts activated at 550 ◦C give
rise to FAME yields higher than 80 wt%, although the best result are
achieved with the 20-Al-SBA-550 catalyst. With only 10 wt% of this
catalyst, 96 wt% of biodiesel yield is attained. This catalyst is stable
against oils with acidity even of 9◦, and no leaching of aluminum
ions into the reaction liquids is found; moreover it is able to simul-
taneously accomplish the oil transesterification and esterification
of the FFAs. The presence of 5 wt% of water or the use of a co-solvent
hardly affect to the biodiesel formation.
Acknowledgements
The authors are grateful to financial support from the Span-
ish Ministry of Science and Innovation (ENE2009-12743-C04-03
Project) and Junta de Andalucía (P09-FQM-5070) and FEDER
founds. I.J.M. would like to thank the Agencia Estatal CSIC for a
JAE-Predoctoral Grant.
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