Transesterification of short chain esters using sulfonic acid-functionalized hybrid silicas: Effect of silica morphology

Article abstract of DOI:10.1016/j.cattod.2013.09.029

Sulfonic acid-functionalized hybrid silicas with different structure (amorphous, HMS, SBA-15) were synthesized by different methodologies, with a variable amount of organic moieties (propylSO₃H). The obtained catalysts, characterized by X-Ray photoelectron spectroscopy, low angle X-Ray diffraction, N₂ adsorption and acid capacity measurements, were tested in the transesterification reaction of short chain esters (from hexanoic to lauric ethyl ester). The optimized reaction was carried out under mild condition in the presence of 15% mol of the corresponding organic acid in order to reproduce a typical low-grade oil. A correlation between the catalytic activity of the materials and their acid capacities was found. The propyl-sulfonic SBA-15 catalyst presented the best performance in terms of activity and structural stability with no leaching of the sulfonic groups.

Full text of DOI:10.1016/j.cattod.2013.09.029

Catalysis Today 223 (2014) 115–121
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Transesterification of short chain esters using sulfonic
acid-functionalized hybrid silicas: Effect of silica morphology
Maria Luisa Testa∗, Valeria La Parola, Anna Maria Venezia
Istituto per lo Studio dei Materiali Nanostrutturati, UOS-PA CNR, via U. La Malfa 153, I-90146 Palermo, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 April 2013
Received in revised form 24 July 2013
Accepted 11 September 2013
Available online 15 October 2013
Sulfonic acid-functionalized hybrid silicas with different structure (amorphous, HMS, SBA-15) were syn-
thesized by different methodologies, with a variable amount of organic moieties (propyl SO3H). The
obtained catalysts, characterized by X-Ray photoelectron spectroscopy, low angle X-Ray diffraction, N2
adsorption and acid capacity measurements, were tested in the transesterification reaction of short chain
esters (from hexanoic to lauric ethyl ester). The optimized reaction was carried out under mild condition
in the presence of 15% mol of the corresponding organic acid in order to reproduce a typical low-grade
oil. A correlation between the catalytic activity of the materials and their acid capacities was found.
The propyl-sulfonic SBA-15 catalyst presented the best performance in terms of activity and structural
stability with no leaching of the sulfonic groups.
Keywords:
Transesterification
Acid catalyst
Sulfonic-acid silica
Mesostructured materials
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
biodiesel production facilitates the separation, regeneration and
recycling of the catalyst and reduces corrosion problems [2].
Transesterification and esterification are industrially important
reactions used in several fields, among them, biodiesel production.
Biodiesel is a renewable, biodegradable and non-toxic fuel. The
first generation biofuels using the edible parts of food plants as
feedstock are considered not fully sustainable and with a negative
impact on the food supply. On the other hand, second generation
biofuels using a low-grade oil feedstock, typically waste cooking
oil, represent a greener alternative. However, differently from the
refined oil containing less than 0.5% of free fatty acids (FFAs), the
low-grade oil has an FFA contents between 0.5–15% which leads
to the undesired soap formation [1]. In this case, a pretreatment,
including esterification, is necessary to reduce the amount of acids.
Most of the industrial processes employ homogeneous acid or basic
catalysts. However, their use has several drawbacks. As concern the
homogeneous basic catalysts, they must work with refined oil poor
in FFA (<0.5%) to avoid soap formation and deactivation of the cata-
lyst. As concern the homogeneous acid catalysts, they promote the
esterification of FFA and transesterification simultaneously, thus
avoiding soap formation, but they are very corrosive and toxic for
the environment and are difficult to separate and recover. Employ-
ing new and sustainable heterogeneous solid acid catalysts for
A large number of heterogeneous solid acid catalysts, includ-
ing modified inorganic oxide such as zirconium [3], titanium [4]
and tin [5] oxides, sulfonic ion-exchange resins [6] and heteropoly-
acids, [7] have been reported in the literature for transesterification
reactions. Special attention has been devoted to those catalysts
containing sulfonic groups in particular organosulfonic-silica solids
that have been proved to be active at moderate temperatures
and pressure [1,8]. According to recent reports [9,10], an effi-
cient solid acid catalyst requires large pores to minimize diffusion
problems, high concentration of acid sites, high catalytic stability
against leaching and poisoning and the possibility to modify the
hydrophobicity of the surface to promote the preferential adsorp-
tion of substrates and the repulsion of highly polar compounds
which could cause deactivation. Due to their high surface area and
controlled porosity it is becoming very attractive to use ordered
mesoporous silicas (e.g., MCM-41, HMS, SBA-15) as supports [11]
for various catalytic applications. The pore sizes, larger than those
in zeolites, can be adjusted in the nanometer range by the appro-
priate choice of the surfactant templates, making these systems
suitable to host large molecules. Many reviews have covered sev-
eral aspects of the mesoporous materials related to their synthesis,
surface modification and practical applications [12–14]. The main
problems associated with such materials consists in their poor
hydrothermal stability and lower reactivity as compared to zeo-
lites [15]. Among ordered mesoporous silicas HMS and SBA-15
have quite different structure: whereas the former has a wormlike
porous structure the latter has an ordered close packed hexago-
nal arrangement of the pores forming straight channel. Moreover,
∗
Corresponding author at: Istituto per lo Studio dei Materiali Nanostrutturati
ISMN-CNR, via U. La Malfa 153, I-90146 Palermo, Italy. Tel.: +39 0916809253;
fax: +39 0916809399.
E-mail address: marialuisa.testa@ismn.cnr.it (M.L. Testa).
0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.09.029

116
M.L. Testa et al. / Catalysis Today 223 (2014) 115–121
O
The amorphous and the mesoporous grafted silica samples are
labeled as nAm G, nHMS G and nSBA G, where n corresponds to the
molar percentage of the SO3H groups.
S
OH
O
O
O
O
+
OEt
+
+
H2O
MeOH
OH +
EtOH
R
R
R
OMe
Catalyst
60°C
2.1.2. In situ oxidation method
R = hexanoic C6
caprylic C8
2.1.2.1. Synthesis of the propyl SO3H amorphous SiO2. The
propyl SO3H silica was synthesized by sol–gel technique accord-
ing to a published procedure [19]. TEOS (7.5 ml, 0.034 mol) was
dissolved in ethanol (5 ml) and stirred at 45 ◦C for 15 min. Then,
5 ml of aqueous solution of acetic acid at pH 5 was added to the
mixture, followed by addition of MPTMS, in amounts correspond-
ing to 2.5%, 5% and 10% mol/mol loadings, and hydrogen peroxide
(33% w/v solid:liquid ratio of 1:18). The solution was heated up to
80 ◦C and left at this temperature until formation of the gel. The
obtained wet gel was dried at 120 ◦C overnight.
capric C10
lauric C12
Scheme 1. Transesterification reaction in presence of acid on sulfonic acid-
functionalized hybrid silica.
the pore size of HMS support is smaller than the pore size of
SBA-15 (typically 3–4 nm vs. 6–8 nm). The reactivity of these meso-
porous materials is enhanced by the addition of functional groups.
In particular, organic groups with rather low reactivity, besides
increasing the surface hydrophobicity and avoiding surface hydrol-
ysis, are added to tailor the pores size. On the other hand, addition of
more reactive groups such as alkylamines and alkylthiol is pursued
for making innovative catalysts [16] or for environmental applica-
tion. Thiol containing organic groups are certainly among the most
interesting functional groups. Indeed, thiol functionalized silicas
(SiO2-SH) are generally oxidized to sulphonic acid silica materi-
als (SiO2-SO3H) for being used as acid catalysts for dehydration or
esterification reactions [17,18].
Sulfonic acid functionalized silicas have been recently explored
as esterification catalysts [19,20]. A recent comparison of different
solid acid catalysts in the acetylation of the glycerol has shown
the superiority of the sulfonic-functionalized hybrid amorphous in
terms of activity and stability [21].
On these premises and as a continuation of our current research
on solid acid catalyst, sulfonic acid-functionalized hybrid silicas
both amorphous and with ordered mesopority of the HMS and SBA-
15 type and different loadings of organic moieties (propyl SO3H)
were synthesized by different procedures and tested in the trans-
esterification reaction of short chain esters (from hexanoic to lauric
ethyl ester, as described in Scheme 1).
2.1.2.2. Synthesis of the propyl SO3H HMS silica. The propyl SO3H
HMS was synthesized according to a previous published procedure
[25]. Tipically, TEOS (6 ml, 0.027 mol), MPTMS (0.5 ml, 0.0026 mol)
and hydrogen peroxide (33% w/v solid:liquid ratio of 1:18) were
added to a stirred solution of ethanol (15 ml), water (15 ml), and
1-dodecylamine (1.84 ml, 0.008 mol). The mixture was stirred at
room temperature for 24 h. The white precipitate obtained was fil-
tered under vacuum and washed with de-ionized water (36 ml) and
ethanol (36 ml). The organic template was extracted by suspending
the solid product in ethanol and refluxing for 24 h. The final product
was dried at 100 ◦C overnight.
2.1.2.3. Synthesis of the propyl SO3H SBA-15. The propyl SO3H
SBA-15 was prepared according to the procedures described before
[21,24,26]. Pluronic P123 (4.0 g) was dissolved in a solution of 2 M
HCl (125 ml). To this solution TEOS (8.2 ml, 0.037 mol), MPTMS
(0.69 ml, 0.0037 mol) and hydrogen peroxide (33% w/v solid:liquid
ratio of 1:18) were added and stirred overnight at 40 ◦C in a 250 ml
one neck flask. The milky suspension was aged at 100 ◦C for 24 h
in a closed polypropylene bottle. The solid product was filtered,
washed with ethanol. The surfactant was removed by ethanol
washing under reflux for 24 h. The final product was dried at 120 ◦C
overnight.
The catalytic behavior of the functionalized silicas was studied
under different experimental conditions. Fresh and spent materi-
als were analyzed by means of X-Ray photoelectron spectroscopy,
X-Ray diffraction, N2 adsorption isotherms and acid capacity mea-
surements.
The amorphous and the mesoporous silica samples function-
alized by the in situ oxidation method are labeled as nAm Ox,
nHMS Ox and nSBA Ox, where n corresponds to the molar percent-
age of the SO3H groups.
2. Experimental
2.2. Catalyst characterization
2.1. Synthesis of sulfonic-acid functionalized materials
Acid capacity and concentration of sulfonic groups of samples
were determined by titration with 0.01 M NaOH (aq) [27]. In a typi-
cal experiment, 0.1 g of solid was added to 10 ml of deionized water.
The resulting suspension was allowed to equilibrate and thereafter
was titrated by dropwise addition of 0.01 M NaOH solution using
phenolphthalein as pH indicator.
The textural properties were obtained using a Carlo Erba Sorp-
tomat 1900 instrument. The fully computerized analysis of the N2
adsorption isotherm at 77 K allowed to obtain, through the BET
method in the standard pressure range 0.05–0.3 P/P0, the specific
surface areas of the samples. The total pore volume, Vp was evalu-
ated on the basis of the amount of nitrogen adsorbed at a relative
pressure of about 0.98 [28].
X-ray photoelectron spectroscopy analyses were performed
with a VGMicrotech ESCA 3000 Multilab, equipped with a dual
Mg/Al anode. The spectra were excited by the unmonochromatized
Al K ␣ source (1486.6 eV) run at 14 kV and 15 mA. The analyser
operated in the constant analyser energy (CAE) mode. For the indi-
vidual peak energy regions, a pass energy of 20 eV set across the
hemispheres was used. Survey spectra were measured at 50 eV pass
Amorphous and mesostructured (HMS and SBA-15) SiO2 were
functionalized with propyl sulfonic groups adopting two different
procedures: grafting and in situ oxidation.
All the compounds used in the synthesis were purchased from
commercial suppliers (Aldrich, Fluka) and used without further
purification.
2.1.1. Grafting method
The different types of silica, the amorphous, the HMS and the
SBA-15 were synthesized by sol–gel technique according to pub-
lished procedures [22–24].
In a typical synthesis by grafting, a mixture of 2.00 g of calcined
silica in 35 ml of dry toluene and 3-mercaptopropyltrimethoxy
silane (MPTMS) (3.33 mmol) was refluxed for 24 h at 110 ◦C. The
materials were then recovered by filtration, washed several times
with toluene and dried at 120 ◦C overnight. Thereafter, the mercap-
topropyl groups were oxidized to sulfonic groups with hydrogen
peroxide (33% w/v solid:liquid ratio of 1:18) at room temperature
during 24 h. After filtration the solid was dried at 120 ◦C overnight.

M.L. Testa et al. / Catalysis Today 223 (2014) 115–121
117
energy. The sample powders were analyzed mounted on a double-
1200
1000
800
600
400
200
0
sided adhesive tape. The pressure in the analysis chamber was of
the order of 10−8 Torr during data collection. The constant charg-
ing of the samples was removed by referencing all the energies to
the C 1s previously calibrated at 285.1 eV, arising from the adven-
titious carbon. The invariance of the peak shapes and widths at
the beginning and at the end of the analyses ensured absence of
differential charging. Analyses of the peaks were performed with
the software provided by VG, based on non-linear least squares
fitting program using a weighted sum of Lorentzian and Gaussian
component curves after background subtraction [29,30]. Atomic
concentrations were calculated from peak intensity using the sensi-
tivity factors provided with the software. The binding energy values
are quoted with a precision of 0.15 eV and the atomic percentage
with a precision of 10%.
10SBA_Ox
10HMS_Ox
10Am_Ox
0,0
0,2
0,4
0,6
0,8
1,0
P/P0
2.3. Transesterification reaction
Fig. 1. Nitrogen adsorption–desorption isotherms of 10SO3H functionalized sup-
ports synthesized by in situ oxidation method. The curves have been shifted along
Y-axes for clarity.
The best operative reaction conditions were chosen after pre-
liminary tests on the selected catalysts, 10SBA Ox, 10HMS Ox and
10Am Ox, under different reagents concentration and different cat-
alyst loading. Indeed, the reaction of ethyl hexanoate was carried
out with the ratio methanol: ester equal to 2:1, 4:1 and 10:1. By
using a molar ratio 4:1 the yield of methyl ester was double the
amount obtained with molar ratio 2:1. Above the molar ratio 4:1 no
significant increase in the catalytic activity was detected. Concern-
ing the effect of the catalyst loading, the reaction was carried out
varying the amount of the selected catalysts, using 50 mg, 200 mg,
300 mg for 25 mmol of ester. It was observed that with the increase
in the catalysts amount from 50 to 200 mg, the conversion increased
from 12 to 44% and thereafter remained almost constant.
According to the above optimization tests, in a typical experi-
ment, 25 mmol of a single ethyl ester (C6–C12, Aldrich) or a mixture
of them were added to a suspension of propyl SO3H silica catalyst
(200 mg) in methanol (4 ml). The methanol/ester molar ratio was
4:1. The reaction was carried out in a 25 ml round bottom flask
equipped with a water-cooled condenser, refluxing at 60 ◦C. The
temperature was maintained using an oil-bath connected with a
thermostat. The reaction mixture was continuously stirred during
the reaction using a magnetic stirrer set at 300 rpm. The products
were analyzed by a GC–mass spectroscopy with a GCMS-QP5050A
Shimadzu mass spectrometer with ionization energy of 70 eV in a
Zebron ZB-5 column 60 × 0.25 × 0.25. The GC–MS program used an
injection and interface temperature of 200 ◦C, a starting temper-
ature of 40 ◦C with a rate of 7.0 ◦C/min until 100 ◦C. The analysis
of the products were conducted after 1, 3, 4, 24 h by GC–mass
spectroscopy and their chromatograms were compared with those
obtained from reference samples. The final solution was then fil-
tered to recover the catalyst that was dried at 100 ◦C overnight and
reused in further reactions. The transesterification reaction of ethyl
esters was also investigated in the presence of the corresponding
free acids in amount of 15% mol with respect to the esters in order
to reproduce a typical low-grade oil.
methanol in order to clean the catalyst surface, dried at 120 ◦C
overnight and reused. After filtration the catalysts were analyzed by
acid capacity measurements and by XPS technique yielding quan-
titative information on sulfur amount.
3. Results and discussion
In Table 1, the textural properties in terms of specific surface
area and pore volumes are listed for the bare amorphous and meso-
porous silicas and the corresponding functionalized oxides. The
amorphous silica samples, functionalized by in situ oxidation, are
characterized by high surface area, comparable with the area of
the amorphous SiO2. As already reported, the differences in the
surface areas of the similarly functionalized materials are inde-
pendent of the propylsulfonic groups loading [19]. The sample
10Am G, originated from amorphous silica but modified by the
grafting followed by H2O2 oxidation has lower surface area. The
difference between the samples prepared by the two methods, is
attributed to the gas evolution occurring in the in situ oxidation
during the step of the structure formation. Differently from the
amorphous silica case, the HMS and SBA-15 materials function-
alized with propylsulfonic groups by the in situ oxidation have a
significantly lower surface areas as compared to the corresponding
bare supports characterized by the high surface area of 997 m2/g
and 755 m2/g, respectively. Partial filling of the mesopores by the
functional groups may explain the surface area decrease. Accord-
ing to literature, pore sizes of the HMS and SBA samples equal to 3
and 6 nm respectively were obtained [31]. In Fig. 1 the adsorption-
desorption isotherms relative to 10SO3H functionalized supports
synthesized by in situ oxidation method are shown. According to
the curves, and in comparison with the corresponding bare sup-
ports, the in situ functionalization procedure allowed the formation
of the typical SBA-15 mesoporous structure, but did not produce the
mesostructured porosity of the HMS silica.
As a model reaction the transesterification of ethyl hexanoate
with methanol was considered. Then blank reaction without any
catalyst, homogeneous reaction of p-toluensulfonic acid (pTSA)
and heterogeneous reaction of Amberlyst-15 were carried out. The
amounts of pTSA and Amberlyst-15 were chosen to have the same
amount of mmol SO3H with respect to the most active catalyst,
10SBA Ox.
In Fig. 2 the XRD patterns of the synthesized materials recorded
at low angles are shown. The SBA functionalized materials exhibit
the typical pattern with the three well defined peaks between 0.9◦
and 3◦ 2Â which are indexed as (100), (110), and (200) Bragg reflec-
tions of the hexagonal (p6 mm) SBA-15 [31]. With respect to the
sample prepared by grafting, the sample 10SBA Ox, still maintain-
ing the hexagonal structure, is less ordered as evidenced by the
low intensity of the reflections. The HMS functionalized material
diffraction patterns are displayed in Fig. 2b. The 10HMS G sample
shows a pattern typical of the HMS structure with the reflection at
2.4. Catalyst recycling test
Recycling experiments were performed over the most active
catalysts of the series, represented by 10Am Ox and 10SBA Ox.
After the reaction, the catalysts were filtered, washed with

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M.L. Testa et al. / Catalysis Today 223 (2014) 115–121
Table 1
Textural and acid capacities properties corresponding to propyl sulfonic-silica.
a
Sample
Surface area (m2/g)
Pore volume (cm3/g)
XPS
Acid capacities (mmol H+ g−1
)
S 2p
S/Si
Amorphous (Am) SiO2
2.5Am Ox
5Am Ox
10Am Ox
10Am G
HMS SiO2
10HMS Ox
10HMS G
SBA-15 SiO2
10SBA Ox
10SBA G
514
493
480
640
265
997
520
n.d.b
755
489
n.d.b
1.73
0.44
0.41
2.60
0.28
1.51
0.30
n.d.b
2.40
1.10
n.d.b
0.04
170.0
170.6
171.1
170.2
0.04
0.06
0.13
0.04
0.52 (0.385)
0.81 (0.77)
2.09 (1.44)
0.27 (1.44)
0.14
0.52 (1.44)
0.36 (1.44)
0.16
170.2
170.7
0.06
0.02
170.1
170.5
0.08
0.02
1.54 (1.44)
0.40 (1.44)
a
The expected acid capacity values are given in parentheses.
Not determined.
b
Fig. 2. XRD patterns of mesoporous supports SO3H functionalized by in situ oxidation and by grafting methods.
2–3.5◦ 2Â. Similar to the SBA case, the decrease of the peak intensity
of the 10HMS Ox catalyst indicates deterioration of the structural
order [32].
As reported in Table 1 and shown in the spectra of Fig. 3,
all samples exhibited the S 2p photoelectron peak centered at
170.5 0.3 eV typical of SO3− indicating that the H2O2 treatment
allowed complete oxidation of mercapto to sulphonic groups as
confirmed also by the absence of the S 2p component, typical of a
mercapto group, lying at ca. 164 eV [33]. The S/Si atomic ratio, as
obtained by quantitative XPS analysis, are included in Table 1 along
with the acid capacity measured by titration. The two series of val-
ues exhibit similar trend. Generally the in situ oxidation samples
of the different type of silica, with the same mercapto group load-
ing, gave rise to larger S/Si atomic ratios and larger acid capacity as
compared to the grafted samples. The correlation between the two
quantities obtained by the two techniques is indicative of a rather
uniform distribution of the sulfo-groups on the surface.
The catalytic results of the transesterification reaction of the
ethyl hexanoate, in terms of methyl ester conversion, are plotted in
Fig. 4 for the different silica catalysts characterized by the different
propylsulfonic group loading and by the different silica morphol-
ogy. According to the data in Fig. 4, the two more active materials
were the 10Am Ox and 10SBA Ox achieving 63% and 44% yield of
methyl ester, respectively. It is important to note that the func-
tionalization method played a very important role in the activity of
the catalysts in the transesterification reaction. In particular, when
the reaction was carried out with the catalysts prepared by in situ
oxidation method, a considerable increase in the conversion was
obtained with respect to the grafted ones. As shown by the plot
in Fig. 5 the activity of these catalysts correlated well with their
acid capacities. Sulfonic amorphous and mesoporous silicas with
Fig. 3. Si 2s and S2p region of (a) 10Am Ox; (b) 10HMS Ox; (c) 10SBA OX.

M.L. Testa et al. / Catalysis Today 223 (2014) 115–121
119
80
70
60
50
40
30
20
10
0
Homogeneous
In situ oxidation
Grafting
10Am_Ox
10SBA_Ox
25
20
15
10
blank 2,5Am 5Am
10Am 10SBA 10HMS
5
SO3H Functionalised Silica Catalysts
1
2
3
4
Fig. 4. Screening of sulfonic silica catalysts in the transesterification of
ethyl hexanoate. Reaction condition: molar ratio esters:methanol = 1:4; catalyst
amount = 200 mg; T = 60 ◦C; t = 4 h.
Cycles
Fig. 6. Recycling catalytic runs of 1 h each, of the transesterification of ethyl hex-
anoate over 10% sulfonic amorphous and SBA silica catalyst.
70
10Am_Ox
a 10% loading of propyl-sulfonic moieties and acid capacities of
2.09 and 1.54 mmol/g, respectively, showed the best activity in the
transesterification of ethylhexanoate.
60
50
10SBA_Ox
In order to evaluate the stability of 10Am Ox and 10SBA Ox sam-
ples, catalytic tests of 1 h each were repeated four times (Fig. 6).
In the case of the 10Am Ox catalyst a decreasing level of activity
was already observed after the first cycle. By using 10SBA Ox, the
activity in the first cycle, lower with respect to the amorphous cata-
lyst, was maintained almost constant for all the subsequent cycles.
Both spent catalysts were then analyzed by XPS and acidic capacity
measurements. As shown by the XPS spectra in Fig. 7, the disap-
pearance of the S 2p peak in the 10Am Ox sample analyzed after
the recycling tests indicates that the catalysts underwent leach-
ing of the sulfo-groups, contrary to the sample 10SBA Ox which
suffer only a slight decrease in the S/Si ratio from 0.08 to 0.07.
The XPS analyses are also in accord with the acidic capacity mea-
surements giving a decrease of the capacity value from 2.09 to
0.72 mmol H+ g−1 for the 10Am Ox, and a much lower decrease
from 1.54 to 0.98 mmol H+ g−1 for the 10SBA Ox. The leaching of
40
30
5Am_Ox
20
10HMS_Ox
10
0
10SBA_G
2.5Am_Ox
10HMS_G
10Am_G
0.0
0.5
1.0
1.5
2.0
Acid Capacities mmol/g
Fig. 5. Correlation between the activity of investigated catalysts and their acid
capacity.
Fig. 7. XPS Si2s and S2p region of 10Am Ox and 10SBA Ox fresh and after recycling runs.

120
M.L. Testa et al. / Catalysis Today 223 (2014) 115–121
80
70
60
50
40
30
20
10
0
as indicated in Scheme 1. The results are summarized in Fig. 9. In
10SBA_Ox
pTSA
this case, an increment of obtained methyl ester was observed, due
to the simultaneous transesterification and esterification reactions
of free acid and a corresponding decrease of acid component of
the mixture. Although, as reported in literature [34] the reactivity
of long chain fatty acid toward esterification should decrease with
chain length, in the presence of the 10SBA Ox catalyst and with the
mixture of acids C6–C12 it was observed that, after 3 h of reaction,
all the acid was transformed into ester regardless the chain length.
Amberlyst-15
4. Conclusion
The present study has shown that the activity of sulfonic-
acid hybrid silica used as catalysts for transesterification reaction
depended on the morphology of the silica and also on the prepa-
ration procedure. The functionalized SBA silica and the in situ
oxidation procedures yielded the most stable catalyst 10SBA Ox, as
confirmed by recycling experiments, in which only a slight leach-
ing of sulfonic groups was observed. As concern the amorphous
silica, its higher activity seems to be linked to the contribution
of homogeneous species. The main role in the catalytic behavior
was played by the catalyst acid capacities which correlated well
with the activities. The 10SBA Ox catalyst was able to perform, in
the presence of 15% of FFA, typical of a low-grade oil, a simul-
taneous transesterification of ethyl esters with the esterification
of the corresponding free fatty acid. Moreover, the chosen mate-
rial compared quite favorably with recent literature data. Further
investigation on the role of the silica morphology and textural prop-
erties on the stability of the functional propyl sulfonic groups is in
progress.
0
1
2
3
4
Reaction Time (h)
Fig. 8. Conversion to methyl hexanoate catalyzed by different acid catalysts (ꢀ
10SBA Ox); (᭹ p-TSA); (ꢁ Amberlyst-15). Reaction condition: molar ratio esters:
methanol = 1:4; T = 60 ◦C; the amount of catalysts producing the same acid capacity
for all the catalysts was used.
sulfonic groups in amorphous silica explains its higher activity
probably linked to a contribution of homogeneous species. On the
other hand, because of its good stability in recycling experiments,
functionalized SBA catalyst was selected for the next reactions.
For comparison purposes, p-TSA and commercial Amberlyst-
15, as sulfonic acid reference catalysts of the homogeneous and
heterogeneous type respectively, were also used in the esterifica-
tion reaction of ethyl hexanoate. The amount of these catalysts was
chosen to have the same amount of mmol of H+ as the 10SBA Ox
catalyst. As shown in Fig. 8 the mesoporous sulfonic silica is five
times more active as compared to Amberlyst-15 catalyst.
In order to evaluate the activity of 10SBA Ox in the transesteri-
fication for biodiesel production, the heterogeneous reaction was
also studied on a mixture of esters with large number of carbon
atoms (C6–C12). At variance with the results reported by Martin-
Alonso et al. [6] who found a decrease of yield of methyl esters with
the increase of chain length of ethyl esters, in the present case no
significant influence of chain length on the reaction was observed.
Additionally, in order to mimic the reaction with a low-grade
oil, the selected catalyst 10SBA Ox was also tested in a long run
reaction of a mixture of C6–C12 ethyl esters and C6–C12 acids (15%)
Acknowledgment
Support by the European Community, COST Action CM0903 is
acknowledged.
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