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ARTICLE IN PRESS
MCAT-324; No. of Pages 7
Molecular Catalysis xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Tailored hybrid materials for biodiesel production: Tunning the base
type, support and preparation method for the best catalytic
performance
Ana Lúcia de Lima a , José S.C. Vieira b,c , Célia M. Ronconi d , Claudio J.A. Mota a,b,e,∗
a
Universidade Federal do Rio de Janeiro, Instituto de Química, Av. Athos da Silveira Ramos 149, CT Bl A, 21949-909 Rio de Janeiro, Brazil
Universidade Federal do Rio de Janeiro, Escola de Química, Av. Athos da Silveira Ramos 149, CT Bl E, 21949-909 Rio de Janeiro, Brazil
c
Instituto Federal do Maranhão − Campus Zé-doca, Rua da Tecnologia, 215, Vila Amorim, 65365-000 Maranhão, Brazil
d
Universidade Federal Fluminense, Instituto de Química, Campus do Valonguinho, Outeiro Sa˜o Joa˜o Batista s/n, Centro, 24020-150 Niteroı́i, RJ, Brazil
e
INCT Energia e Ambiente, UFRJ, Rio de Janeiro, Brazil
b
a r t i c l e
i n f o
Article history:
Received 26 July 2017
Received in revised form
21 September 2017
Accepted 29 September 2017
Available online xxx
Keywords:
Biodiesel
MCM-41
Transesterification
Base catalysis
a b s t r a c t
Hybrid materials were prepared by the functionalization of mesoporous silica MCM-41 with 1,5,7triazabicyclo [4.4.0] dec-5-ene (TBD) and 3-aminoquinuclidine (3AQ) by the co-condensation method,
resulting in the heterogeneous catalysts MCM-41-TBD, MCM-41-3AQ. Silica gel was also functionalized
with TBD and 3AQ using post-synthetic method upon the reaction with 3-chloro-propyl-triethoxy-silane
and then, undergoing nucleophilic substitution with TBD or 3AQ, resulting in SG-TBD and SG-3 AQ,
respectively. The successful anchoring of the organic bases on the silica supports has been shown by 29 Si
MAS/NMR and elemental analysis (CNH). The ordered structure of the MCM-41 support was damaged
upon base functionalization. The heterogeneous catalysts were tested in the transesterification of soybean oil to produce biodiesel. MCM-41-TBD catalyst converted 100% of the soybean oil to biodiesel within
2 h at 70 ◦ C, whereas the other catalysts required more severe reaction conditions to achieve conversion
in the range of 24–98%. The catalysts based on grafted TBD, regardless of the type of silica used, were
more active than those based on 3AQ, because of the higher basicity of the former amine. The MCM41-TBD presented slower deactivation rate, upon reuse, compared with the same material prepared by
post-synthetic methods The possible explanation is related with the higher incorporation of the base on
the silica support by the co-condensation synthetic method as well as its more uniform distribution on
the catalyst surface.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
In the face of global warming and consensus for the use of clean
energy sources, biodiesel has emerged as a promising alternative
to fossil diesel. It is usually obtained by transesterification of oils
and fats with short chain alcohols in the presence of basic or acid
catalysts. Biodiesel can be used in diesel engines without requiring
significant modifications [1–4]. In addition, there are advantages,
such as biodegradability, absence of sulfur and aromatics compounds and the carbon dioxide released in the combustion can
be absorbed by the plants [5–7]. However, the costs of production
depends on the raw material used [8,9].
∗ Corresponding author at: Universidade Federal do Rio de Janeiro, Instituto de
Química, Av Athos da Silveira Ramos 149, CT Bl A, 21949-909 Rio de Janeiro, Brazil.
E-mail address: cmota@iq.ufrj.br (C.J.A. Mota).
Typically, homogeneous catalysts, such as NaOH and NaOCH3 ,
are used in biodiesel production. They are highly active under mild
reaction conditions, but cannot be reused. In addition, expenditious washing procedures are necessary to remove catalyst residues
[10,11]. Heterogeneous catalysts, on one hand, can be easily separated from the medium and reused, but on the other hand, usually
require more drastic reaction conditions to achieve satisfactory
conversion [12]. The advantages and disadvantages of heterogeneous basic catalysis for biodiesel production were recently
addressed [13].
The incorporation of the organic compounds on mesoporous silica to genarate hybrid materials has been widely reported in the
literature. MCM-41 mesoporous silica has high surface area and
their mesopores are arranged in a hexagonal shape. Silica gel, otherwise, has no organized structure, but present high surface area
and thermal stability. These different types of silica-based materials
http://dx.doi.org/10.1016/j.mcat.2017.09.032
2468-8231/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: A.L.d. Lima, et al., Tailored hybrid materials for biodiesel production: Tunning the base type, support
and preparation method for the best catalytic performance, Mol. Catal. (2017), http://dx.doi.org/10.1016/j.mcat.2017.09.032
G Model
MCAT-324; No. of Pages 7
ARTICLE IN PRESS
A.L.d. Lima et al. / Molecular Catalysis xxx (2017) xxx–xxx
2
are extensively used in catalysis, drug delivery systems, adsorbents
and biosensors [14–20].
Guanidine derivatives, such as 1,5,7-triazabicyclo [4.4.0] dec-5ene (TBD), are known for their strong basicity (pKa = 25.9). TBD, as
homogeneous catalyst, has catalytic activity comparable to NaOH
and KOH in the transesterification of soybeen oil [21]. In a previous report [22], we have shown that TBD anchored on the surface
of MCM-41, prepared by post-synthetic method, showed excellent catalytic performance for soybean transesterification with
methanol. Notwithstanding, the catalyst showed strong deactivation upon reuse, mostly due to neutralization by the free fatty acids
present in the oil.
Quinuclidine is a strong base (pKa = 11) formed by a bicyclic
system with a bridgehead nitrogen atom [23]. Quinuclidine derivatives with different basicities have been used as homogeneous and
hetogeneous catalysts in the Baylis-Hillman reaction [24,25]. The
products were obtained in moderate to high yields, indicating that
quinuclidine derivatives are promissing compounds for basic catalysis. To the best of our knowledge quinuclidine, or its derivatives,
has not been used as catalyst in the transesterification of vegetable
oils to produce biodiesel.
In this work we describe the synthesis of silica-based heterogeneous basic catalysts grafted with 1,5,7-triazabicyclo [4.4.0]
dec-5-ene (TBD) and 3-aminoquinuclidine (3AQ), for the transesterification of soybean oil with methanol. The catalysts based
on MCM-41 were obtained using the co-condensation method,
whereas the catalysts based on commercial silica gel were synthesized in two steps using the post-synthetic method. Grafting
of the organic bases on the mesoporous material by the postsynthetic method mainly occurs on the external surface of the
solid, and may lead to pore blockage and lower incorporation of
the organic moiety on the silica support. On the other hand, in the
co-condensation method the functionalization occurs inside and
outside the pores, resulting in a material in which the catalytic
sites are more homogeneously distributed and with higher incorporation of the organic moiety [26]. Thus, it would be expected
that MCM-41-TBD synthesized by co-condensation could be more
resistant toward deactivation than the same catalyst prepared by
post-synthetic methods.
at 5k Hz (29 Si). The spectra were obtained using magic angle spinning and cross polarization (CP ramp) with 2 ms contact time and
4 s repetition time for 13 C (13 C CPMAS); 4 ms contact time and 4 s
repetition time for 29 Si (29 Si CPMAS). X-ray diffraction were carried
out at the State University of Santa Cruz (UESC), Bahia, Brazil, using
a RIGAKU diffractometer model Miniflex 600, operating with Cu
K␣ radiation generated about 40 KV and 30 mA. The analyses of the
samples were carried out scanning from 1.5◦ to 10◦ (2 theta) with
an interval of 0.02◦ and scan of 1 min−1 . Textural analyses were carried out by N2 adsorption/desorption isotherms on an ASAP 2020
V304 e-serial 1200 at 77 K. The samples were pretreated at 120 ◦ C
for 10 h.
2.2. Synthesis of the silylating agents modified with TBD and 3AQ
A solution of TBD (3.20 g, 22.9 mmol) or 3-aminoquinuclidine
(3AQ) dihydrochloride (3.57 g, 17.9 mmol) in THF was added dropwise to a suspension of NaH (0.88 g, 22.0 mmol for TBD or 2.55 g,
63.75 mmol for 3AQ) in THF under nitrogen atmosphere at 0 ◦ C. The
suspension was stirred for 2 h at room temperature. Then, a solution of CPTES in THF was added to the mixture at 0 ◦ C and stirred
for 24 h at 70 ◦ C. The resulting products were filtered and used in
the co-condensation synthesis of the hybrid catalysts.
2.3. Preparation of the functionalized MCM-41
The CTAB surfactant (1.0 g, 2.7 mmol), 3.5 mL of 2 mol L−1 solution of NaOH and 240 mL of deionized water were heated at 80 ◦ C
for 30 min 5 mL of TEOS was added together with the aminofunctionalized silylating agents, previouly synthesized as described
in Section 2.2. The reaction mixture was vigorously stirred at 80 ◦ C
for 2 h. The materials were filtered, washed with water and then
with methanol and dried under vacuum; the materials were named
MCM-41-TBD and MCM-41-3AQ.
The CTAB surfactant was removed by acid extraction at 60 ◦ C
for 3 h with 50 mL of MeOH and 0.3 mL of HCl. Dispersions of the
products (MCM-41-TBD and MCM-41-3AQ) in methanolic solutions of sodium carbonate were stirred at room temperature for
3 h to neutralize any remaining HCl. The resulting materials were
filtered, firstly washed with water and then methanol and dried
under vacuum.
2. Experimental section
2.1. Materials and characterization methods
The
following
reagents
were
purchased
from
Sigma-Aldrich and used without any prior treatment:
(3-chloropropyl)trimethoxysilane
(CPTES)
95%,
tetraethyl
orthosilicate (TEOS) 98%, 1,5,7-triazabicyclo[4.4.0]dec-5-ene
(TBD), 3-aminoquinuclidine (3AQ) dihydrochloride, hexadecyltrimethylammonium bromide (CTAB) and sodium hydride
(NaH). Ammonium hydroxide, anhydrous toluene, anhydrous
tetrahydrofuran (THF) and methanol (MeOH) were acquired from
Vetec, whereas a commercial silica gel was purchased from Merck.
Commercial refined soybean oil was used in the transesterification
reactions.
Fourier-transform infrared (FTIR) spectra were recorded in the
range from 400 to 4000 cm−1 on a Varian 660-IR FTIR spectrophotometer using KBr pellets. Elemental analyses (CHN) were carried
out on a Perkin-Elmer CHN 240C analyser at the Analytical Center
of the Institute of Chemistry of the University of São Paulo, Brazil.
13 C and 29 Si solid-state NMR were carried out on a Bruker Avance III
400 (9.4 T), operating at Larmor frequencies 100.65 and 79.51 MHz,
respectively. The analyses were performed in a 3.2 nm ZrO2 triple
channel probe, spinning at 10 kHz (13 C NMR) and in a 7.0 mm two
channel broad band probe with 7.0 mm rotors (Kel-Fcaps) spinning
2.4. Functionalization of the silica gel with
(3-chloropropyl)trimethoxysilane (CPTES)
9.0 g of commercial silica gel were dried at 150 ◦ C for 10 h
under vacuum. Then, the silica gel was dispersed in 150 mL of
dried toluene. To the dispersion, under reflux and N2 atmosphere,
9.0 mL of CPTES were added. After 1.5 h, 21 mL of the solvent was
withdrawn and further 9 mL of CPTES were added to the reaction
mixture. This procedure was repeated twice. After washing the
material in a Soxhlet with a mixture of 200 mL of diethyl ether and
200 mL of dichloromethane for 20 h, the solid was dried resulting
in a material named SG-Cl, which was subsequently used to anchor
the bases.
2.5. Preparation of the SG-TBD and SG-3AQ
A solution of TBD (2.50 g, 17.9 mmol) or 3-aminoquinuclidine
(3AQ) dihydrochloride (2.86 g, 14.3 mmol) in THF was added dropwise to a suspension of NaH (0.68 g, 28.3 mmol for TBD or 2.61 g,
65.3 mmol for 3AQ) in 15 mL of THF under N2 atmosphere and 0 ◦ C.
The suspension was stirred for 2 h at room temperature. Then, 3.0 g
of the synthesized SG-Cl, as described in section 2.4, in THF was
added to the suspension at 0 ◦ C. The dispersion was stirred for 24 h
at 70 ◦ C. Subsequently, the suspension was filtered and the solid
Please cite this article in press as: A.L.d. Lima, et al., Tailored hybrid materials for biodiesel production: Tunning the base type, support
and preparation method for the best catalytic performance, Mol. Catal. (2017), http://dx.doi.org/10.1016/j.mcat.2017.09.032
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3
Fig. 1. Schematic synthesis of the hybrid heterogeneous basic catalysts.
Table 1
Textural properties and nitrogen content of the catalysts.
3. Results and discussion
Catalysts
SBET (m2 g−1 )
Vp (cm3 g−1 )
N content (mmol g−1 )
MCM-41
MCM−41-TBDa
MCM−41-TBDb
MCM-41-3AQ
SBA-15
SBA-15-TBD
SBA-15-3AQ
SG
SG-Cl
SG-TBD
SG-3AQ
949
5.4
0.9
8.0
630
111
0.3
177
179
60
27
0.86
0.05
N. A.
0.02
0.46
0.02
N. A.
N. A.
N. A.
0.09
0.15
–
2.93
2.17
2.41
–
2.09
1.34
–
–
0.94
0.42
N.A. – Not available.
a
Prepared by co-condensation – this work.
b
Prepared by post-synthetic method – ref. [22].
washed with diethyl ether and dichloromethane in a Soxhlet for
20 h and dried under vacuum. The resulting products were named
SG-TBD and SG-3AQ.
MCM-41 was also prepared according to procedures described
in the literature. [26]
2.6. Catalytic evaluation
Methanol and the heterogeneous catalyst were stirred at room
temperature for 30 min. Then, soybean oil was added and the system was stirred at the temperature and time shown in Table 3.
At the end, the catalyst was filtered and the biodiesel phase separated from the glycerin phase. The yield of biodiesel was measured
by HPLC (High Performance Liquid Chromatography, Agilent 1200)
using a previously described procedure [22,27].
3.1. Synthesis of the heterogeneous basic catalysts
Fig. 1 shows the schematic routes for the synthesis of the MCM41 and SG-based heterogeneous basic catalysts. The catalysts based
on mesoporous silica nanoparticles MCM-41 were obtained by the
co-condensation method. In this procedure, the silylating agent,
(3-chloropropyl)trimethoxysilane (CPTES), was reacted through
bimolecular nucleophilic substitution (SN 2) with deprotonated
1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) or 3-aminoquinuclidine
(3AQ). Then, these silylating agents were added to the synthesis
medium of the mesoporous MCM-41 yielding the desired heterogenous basic catalysts. The catalysts based on silica gel were prepared
by post-synthetic methods. Commercial silica gel was firstly functionalized with the silylating agent CPTES and then, reacted with
deprotonated TBD or 3AQ.
3.2. Characterization of the heterogeneous basic catalysts
Fig. 2 shows the FTIR spectra of the synthesized materials. The
broad bands centered at 3452 cm−1 are attributed to the stretching O H vibration of silanol groups present on the surface of the
silica and/or due to remaining adsorbed water. The shoulders centered at 900–965 cm−1 are also due to the silanol groups. The
intense bands centered at approximately 1100 cm−1 are assigned
to siloxane groups (Si O Si). The functionalized materials also
show bands at 2900 and 2851 cm−1 assigned to the C H stretching
vibrations related to the organic moiety. The bands at 1470 cm−1
and 1625 cm−1 present in the amino-functionalized catalysts correspond to C N and C N stretching vibrations [22]. The weak bands,
at approximately 696 cm−1 , are attributed to N H bending vibra-
Please cite this article in press as: A.L.d. Lima, et al., Tailored hybrid materials for biodiesel production: Tunning the base type, support
and preparation method for the best catalytic performance, Mol. Catal. (2017), http://dx.doi.org/10.1016/j.mcat.2017.09.032
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Fig. 3. 29 Si CP-MAS/NMR spectra of the materials: (A) MCM-41, (B) MCM-41-TBD,
(C) MCM-41-3AQ, (D) SG, (E) SG-Cl, (F) SG-TBD, (G) SG-3AQ.
Fig. 2. FTIR of the heterogeneous basic catalysts and their precursors.
tions. In the SG-Cl material, a weak band at 700 cm−1 is assigned to
the C Cl vibration.
The 29 Si CP-MAS/NMR spectra of the materials are shown in
Fig. 3. MCM-41 shows signals at −86 and −96 ppm due to SiO2 (OH)2
(Q2 sites) and SiO3 (OH) (Q3 sites) silanol groups − Fig. 3(A). A signal at −110.5 ppm, due to siloxane group SiO4 (Q4 sites), is also
observed. This spectrum pattern is typical of MCM-41 [17,18,22].
Upon functionalization, the intensity of the Q2 and Q3 signals
decreases relative to that of the Q4 signals − Fig. 3(B) and (C)
[18,19,22]. These results are consistent with the binding of the
organic moieties on the silanol groups. In addition, signals at −40 to
−60 ppm, ascribed to C Si(OSi)2 OH groups (T2 sites), and at −60 to
−70 ppm, associated to C Si(OSi)3 (T3 sites), could be observed in
the spectra, Fig. 2(B) and (C). The presence of T2 and T3 sites in the
spectra of Fig. 3 is a strong evidence of the amino functionalization
of the silica materials [17,18,22].
The BET surface area of the materials was obtained by N2
adsorption/desorption isotherms (Table 1). The surface area of the
MCM-41 is within the range reported in the literature [17,22,28,29].
The surface area decreases with functionalization, which suggests
that the mesoporous structure of MCM-41-TBD and MCM-41-3AQ
might has been damaged or the pores were significantly blocked.
However, the surface area of the MCM-41-TBD synthesized by
co-condensation was higher than the area of the same material
prepared by post-synthetic method [22]. The surface area of the
silica gel also significantly decreased upon functionalization with
TBD or 3AQ.
The nitrogen content of the catalysts indicaded that MCM41-TBD prepared by co-condensation achieved a higher degree
of functionalization of base per gram of catalyst than the same
catalysts prepared by post-synthetic method [22] (Table 1). It
also shows the highest amine surface density, followed by SGTBD (Table 2). These results indicate that the funcionalization
with TBD base was more effective than functionalization with 3aminoquinuclidine (3AQ), in terms of amine surface density, on
the MCM-41 support. The silica gel-based catalysts showed lower
degree of functionalization per gram of material. This might be
due to the lower surface area of this support compared to MCM41. In addition, the incorporation of the base was carried out by
post-synthetic method, wich may also explain the lower degree of
funcionalization.
To investigate the integrity of the mesoporous structures of
MCM-41 upon functionalization, powder X-ray diffraction analyses (PXRD) were carried out (Fig. 4). The PXRD pattern of pure
MCM-41 dispays three low-angle reflections, typical of a hexagonal array that can be indexed as the (100), (110) and (200) Bragg
peaks [18,22]. The absence of peaks at angles higher than those
observed indicates a noncrystalline material. The PXRD patterns
of MCM-41-TBD and MCM-41-3AQ show no reflections. The loss
of the d100 peak indicates that the functionalization with TBD and
3AQ damaged the mesoporous structure of MCM-41[18,22]. The
PXRD patterns of the silica gel materials show a typical broad halo
assigned to non-crystalline silica (Fig. 4) [30].
3.3. Catalyst evaluation for biodiesel production
The soybean oil used in this study contained 0.16% of humidity
and 0.13% of FFA, meeting the required specifications for using in
basic transesterification.
Please cite this article in press as: A.L.d. Lima, et al., Tailored hybrid materials for biodiesel production: Tunning the base type, support
and preparation method for the best catalytic performance, Mol. Catal. (2017), http://dx.doi.org/10.1016/j.mcat.2017.09.032
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Table 2
Transesterification of soybean oil with methanol using different heterogeneous basic catalysts and their precursors. Results of biodiesel yield after 8 h and molar ratio
(oil:methanol) of 1:12.
Catalysts
Catalyst loading (g)
Surface densityb (N atoms/nm2 )
Temp. (◦ C)
Yield (%)
MCM-41
SG
MCM−41-TBDa
MCM-41-3AQ
SG-TBD
SG-3AQ
0.15
0.15
0.10
0.15
0.15
0.15
–
–
326.4
181.6
9.4
9.4
140
140
70
140
140
140
4
3
100
32
98
24
a
b
Reaction caried out at 70 ◦ C and 2 h.
Calculated taken the N content obtained by chemical analysis, mutiplied by the Avogrado number and divided by the measured surface area (in nm2 ).
significant conversion to biodiesel. Therefore, all other catalysts
were tested at more severe conditions (140 ◦ C and 8 h).
The TBD-functionalized silica supports presented the highest
yield of biodiesel. This result can be attributted to the higher basicity of the TBD (pKa = 25.9), compared to the basicity of quinuclidine
(pKa around 11.0) [21,31]. The higher pKa value of the TBD conjugated acid is related to the stability of its guanidinium-type
cation, in which the positive charge can be delocalized, through
resonance, on the three nitrogen atoms. SG-TBD did not present
significant biodiesel conversion at 70 ◦ C and 2 h. The explanation
for this behaviour may be associated with its lower surface density,
expressed in terms of nitrogen atoms per nm2 . Although we cannot
completely rely on these numbers, as we used the total chemical
analysis, instead of the surface one, to calculate the density, it is certainly an indication, especially when big differences are considered.
The lower incorporation of the organic moiety on the surface, compared to the MCM-41, suggests that a minimum loading of catalyst
may be necessary for observing proper conversion to biodiesel at
less severe reaction conditions. The same was observed for 3-AQgrafted catalysts, where MCM-41-3AQ showed a higher biodiesel
yield than SG-3AQ at the same reaction conditions. Therefore, the
best catalytic performance of the TBD anchored on MCM-41 might
be related to its intrinsic basicity and to the higher number of amino
groups per square nanometer on the silica supports.
3.4. Comparison of MCM-41-TBD synthesized by co-condensation
and post-synthesis
Fig. 4. PXRD of the heterogeneous basic catalysts and their precursors.
Table 3
Transesterification of soybean oil with methanol at 70 ◦ C using MCM-41-TBD synthesized by co-condensation.
Entry
Time (h)
Catalyst loading (g)
Oil:methanol Molar ratio
Yield (%)
1
2
3
1
1
2
0.05
0.15
0.10
1:15
1:9
1:12
84
99
100
Table 2 shows the results of the transesterification of soybean
oil with the heterogeneous basic catalysts synthesized in this work.
Control experiments were also carried out and showed that the silica supports exhibited almost negligible conversion to biodiesel at
the same reaction conditions. At 70 ◦ C, only MCM-41-TBD showed
We have previously shown that MCM-41-TBD prepared by the
post-synthetic method gave 99% biodiesel yield at 70 ◦ C, 3 h and
1:9 molar oil to methanol ratio [22]. Therefore, we decided to test
the catalysts prepared by co-condensation at similar reaction conditions. Table 3 shows the results at different conditions. It can be
seen that at 70 ◦ C, 1:9 oil to methanol molar ratio and 0.15 g of catalyst loading, the biodiesel yield was 99%, similar to what was found
with the same type of catalyst, but prepared by the post-synthetic
method [22]. Thus, the preparation method does not affect the
activity of the catalyst.
The reusability of MCM-41-TBD was investigated in five consecutive reaction cycles at 70 ◦ C for 2 h, 1:12 oil to methanol molar
ratio and 0.10 g of the catalyst loading. After each reaction cycle, the
catalyst was separated by filtration and reused without any treatment. After five consecutive runs the yield of biodiesel decreased to
31% (Fig. 5). The result is significantly better when compared with
the same catalyst prepared by post-synthetic method, which presented only 0.8% of biodiesel yield after five concecutive runs [22].
This result may be due to the better dispersion of the base on the
silica surface, differently from the catalyst prepared by the postsynthetic method, which favors functionalization at the external
surface or at the entrance of the pores [32–34], leading to rapid
deactivation. In addition, the higher incorporation of the base, as
well as the slightly higher surface area of the material prepared by
co-condensation may contribute to this better result.
Please cite this article in press as: A.L.d. Lima, et al., Tailored hybrid materials for biodiesel production: Tunning the base type, support
and preparation method for the best catalytic performance, Mol. Catal. (2017), http://dx.doi.org/10.1016/j.mcat.2017.09.032
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The MCM-41-TBD was the most active catalyst, showing 100%
biodiesel yield at 70 ◦ C, 1:12 oil to methanol molar ratio, 0.15 g of
catalyst loading and 2 h of reaction time. Comparison with the same
material prepared by post-synthetic method indicated a slower
deactivation. After five consecutive runs the biodiesel yield was
31% for the catalyst prepared by co-condensation and 0.8% for
the catalyst synthesized by post-synthesis. These results indicated
that co-condensation method produces a catalyst with a better
dispersion of the amine group, preventing the fast deactivation
by neutralization by the free fat acids present in the soybean oil.
The material prepared by post-synthesis probably concentrates the
amine groups on the outer surface, being more prone to deactivation.
Ackowledgements
The authors gratefully acknowledge FAPERJ, CNPq, FINEP and
CAPES for financial support. The also ackonwledge Dr Miriam Sanae
Tokumoto by XRD analysis, Thiago C. dos Santos for the assistance
with N2 adsorption/desorption isotherms measurements and Dr.
Rosane A. S. San Gil for CP/MAS NMR measurements.
References
Fig. 5. Comparison of the reusability of the MCM-41-TBD catalyst prepared by
co-condensation (red filled bars) and by post-synthesis (purple filled bars) in the
transesterification of soybean oil at 70 ◦ C, 2 h (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
To check the extent of leaching of the MCM-41-TBD prepared by
co-condensation, we stirred the catalyst with methanol for 2 h at
70 ◦ C. Then, the catalyst was filtered and the remaining methanolic
solution was added to a flask containing the soybean oil. The system
was kept, under stirring, at 70 ◦ C for 2 h, but the yield of biodiesel
was not significant, indicating that leaching of the amino compound
is not the main cause of deactivation. In fact, chemical analysis
of the MCM-41-TBD revealed the presence of 2.40 mmol g−1 of
nitrogen atoms, indicating a loss of 18.5% nitrogen atoms upon
five consecutive uses. Therefore, although we can observe some
leaching of the base after 5 concecutive uses, the main cause of
deactivation is probably the neutralization of the basic sites by the
free fat acids present in the oil, as previouly observed in the study
with the catalyst prepared by post-synthetic method [22].
4. Conclusions
Basic heterogeneous catalysts of organic amines grafted on
MCM-41 were synthesized by the co-condensation method and
characterized by several techniques. The CP-MAS/NMR analysis
showed that the organic groups were covalently bound on the
silica-based support. The area is drastically reduced upon functionalization of the mesoporous support, probably due to damage of the
structure of the MCM-41 support or pore blocklage. The ordered
structure of the MCM-41 material was also lost with amine grafting,
as shown by X ray analysis.
The TBD-derived catalysts were more active than the correspondent 3-AQ ones, which may be associated with the higher bacisity of
the former amine. For a same silica support, MCM-41 showed the
best performance because of the higher surface density of nitrogen atoms (basic sites) per nm2 , providing more catalytic sites per
surface area.
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