One-step synthesis of mesoporous H4SiW12O 40-SiO2 catalysts for the production of methyl and ethyl levulinate biodiesel

Article abstract of DOI:10.1016/j.catcom.2013.01.010

A novel one-step method for the synthesis of mesoporous H ₄SiW₁₂O₄₀-SiO₂ catalysts with tunable composition was successfully developed using non-ionic polyethylene glycol dodecyl ether (Brij 30) as the structure-directing template. Different loadings of H₄SiW₁₂O₄₀ (up to 30 wt.%) were effectively confined within the mesoporous channels of SiO₂, which would facilitate easy recycling and allow for the efficient mass transport of reactants and products. The resultant mesoporous H₄SiW ₁₂O₄₀-SiO₂ showed high catalytic activity for the production of methyl and ethyl levulinate biodiesel. The 20 wt.% H ₄SiW₁₂O₄₀-SiO₂ catalyst exhibited the best performance in the synthesis of both methyl levulinate (73% yield achieved at 79% conversion of levulinic acid) and ethyl levulinate (67% yield obtained at 75% conversion of levulinic acid).

Full text of DOI:10.1016/j.catcom.2013.01.010

Catalysis Communications 34 (2013) 58–63
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
One-step synthesis of mesoporous H SiW O -SiO catalysts for the
4
12 40
2
production of methyl and ethyl levulinate biodiesel
Kai Yan, Guosheng Wu, Jiali Wen, Aicheng Chen ⁎
Department of Chemistry, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel one-step method for the synthesis of mesoporous H SiW12O40-SiO catalysts with tunable composition
4
2
Received 19 November 2012
Received in revised form 6 January 2013
Accepted 8 January 2013
was successfully developed using non-ionic polyethylene glycol dodecyl ether (Brij 30) as the structure-directing
template. Different loadings of H SiW12 40 (up to 30 wt.%) were effectively confined within the mesoporous
channels of SiO , which would facilitate easy recycling and allow for the efficient mass transport of reactants
and products. The resultant mesoporous H SiW12 40-SiO showed high catalytic activity for the production of
methyl and ethyl levulinate biodiesel. The 20 wt.% H SiW12 40-SiO catalyst exhibited the best performance in
the synthesis of both methyl levulinate (73% yield achieved at 79% conversion of levulinic acid) and ethyl
levulinate (67% yield obtained at 75% conversion of levulinic acid).
© 2013 Elsevier B.V. All rights reserved.
4
O
2
Available online 20 January 2013
4
O
2
4
O
2
Keywords:
One-step synthesis
Mesoporous materials
4 2
H SiW12O40-SiO
Catalysis
Biodiesel
1
. Introduction
With the inevitable depletion of non-renewable petroleum resources
and beta, Y, ZSM-5 and mordenite zeolite catalysts for the conversion
of fructose to EL utilizing ethanol as the solvent and reactant. A
57% yield of EL was obtained on sulphonic acid functionalized
SBA-15 at 140 °C [19]. More recently, Baronetti et al. reported a
Wells–Dawson heteropoly acid that was supported on silicon gel for
the synthesis of EL from levulinic acid and ethanol [20]. A 76% yield
was achieved, where the weak heteropoly acid is the catalytic active
center [21,22].
There are great interests in the development of new and environ-
mentally benign technologies for the efficient synthesis of ML and
2
EL. Mesoporous SiO with suitable pore sizes has the capacity to
and growing environmental concerns associated with their use, renew-
able biomass is increasingly regarded as a promising alternative for the
sustainable supply of fuels and chemicals [1,2]. Substantial research
efforts have been invested in the conversion of biomass or biomass-
derived chemicals to biofuels and value-added chemicals [3–5]. Among
these investigations, it has been elucidated that methyl levulinate (ML)
and ethyl levulinate (EL) are very attractive candidates as novel miscible
diesel biofuels [6,7]. These biofuels may be synthesized from levulinic
acid and alkyl alcohol, where the levulinic acid is produced from renew-
able lignocellulosic biomass [1,8]. Due to their unique properties,
ML and EL are suitable for use as additives for gasoline and diesel
transportation fuels, which have manifold excellent performance
attributes, such as non-toxicity, high lubricity, flashpoint stability and
superior flow properties under cold conditions [9–12].
confine the catalytic active species heteropoly acid, which facilitates
easy recycling, and allows for the efficient mass transport of reac-
tants and products. In this study, we have developed a novel
one-step method for the synthesis of H
using a new heteropoly acid (Keggin Unit, H
fined within the mesoporous channels of SiO
Brij30 was utilized for the first time as a structure directing agent
in the formation of mesoporous H SiW12 40-SiO . The resultant
mesoporous H SiW12 40-SiO catalysts were shown to be highly
4
SiW12
SiW12
. The surfactant
O
40-SiO
2
catalyst
4
O40) that is con-
2
Traditionally utilized catalysts for the production of esters
often focused on homogeneous mineral acids such as formic acid,
H
4
O
2
PO
3 4
and H
SO
2 4
[13–15]. Although high yields have been achieved,
4
O
2
the handling of mineral acids as well as corrosion and recycling issues
have been problematic. Due to their ease of recycling and separation,
heterogeneous catalysts have gained appeal. Zeolite and heteropoly
acid are the most often employed catalysts for esterification due to
benefits such as adjustable acidity, simple recyclability, economic
potential and environmental compatibility [16–18]. Recently, Riisager
et al. studied sulfonic acid functionalized SBA-15, sulfated zirconia
efficient for the production of ML and EL biodiesel.
2. Experimental
4 2
2.1. Synthesis and characterization of mesoporous H SiW12O40-SiO
Details on the reagents that were used in this study are described in
the supplementary information. The one-step synthesis was facilitated
in the presence of the surfactant Brij30. Firstly, 1.3 g Brij30 was dissolved
in a 25 mL HCl solution for 12 h under 40 °C. Secondly, a mixture of
⁎
Corresponding author. Tel.: +1 807 3438318.
E-mail address: aicheng.chen@lakeheadu.ca (A. Chen).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.01.010

K. Yan et al. / Catalysis Communications 34 (2013) 58–63
59
a
b
2
00 nm
2
00 nm
c
d
2
00 nm
200 nm
Fig. 1. SEM analysis of different H
4
SiW12O
40 loadings inside SiO
2
: (a) mesoporous SiO
2 4 2 4 2 4 2
; (b) 10 wt.% H SiW12O40-SiO ; (c) 20 wt.% H SiW12O40-SiO ; (d) 30 wt.% H SiW12O40-SiO .
2
.4 mL tetraethyl orthosilicate and 2.0 mL ethanol was added. Subse-
quently, a pre-determined amount of Keggin Unit tungstosilicic acid
hydrate (H SiW12 40), for instance 0.065 g for 10 wt.% loading, was
30 m DBWaxetr, FID) and high performance liquid chromatography
(Shimadzu LC10; column: organic acid resin; UV detector).
4
O
added to the Brij30/HCl solution. The resulting suspension was blended
at 40 °C for 24 h, and then heated at 100 °C for 24 h. Finally, the obtained
gel was rinsed with pure water, dried at 90 °C overnight and calcinated
at 350 °C for 8 h. The 10 wt.% tungstosilicic acid hydrate loading was
3. Results and discussion
3.1. Catalyst characterization
designated as 10 wt.% H
SiO was also prepared using the same procedure in the absence
of tungstosilicic acid hydrate. The synthesized mesoporous SiO
and H SiW12 40-SiO catalysts were characterized by X-ray diffraction
XRD), N adsorption/desorption (N -BET), transmission electron micros-
copy (TEM), scanning electron microscopy (SEM), Fourier transform
infrared spectroscopy (FT-IR), energy dispersive X-ray analysis (EDX)
and inductively coupled plasma-mass spectroscopy (ICP-MS). Details
are presented in the Supplementary information.
4
SiW12
O
40-SiO
2
. For comparison, mesoporous
The XRD patterns of pure H
4 2
SiW12O40, the synthesized SiO and
2
H
4
SiW12 40-SiO catalysts are presented in the Supplementary
O
2
2
information (Fig. S1). A broad diffraction peak that ranged from 20
to 30°, which can be attributed to the amorphous silica [23–26],
4
O
2
(
2
2
was observed when the H
30 wt.% (Fig. S1, b to e). However, the diffraction peaks correspond-
ing to H SiW12 40 were imperceptible, indicating that the loaded
SiW12 40 was effectively confined within the mesoporous
channels of the SiO . When the H SiW12 40 loading was increased
4
SiW12O40 loadings were lower than
4
O
H
4
O
2
4
O
to 40 wt.% (Fig. S1f), two small peaks appeared at 8.3° and 25.8°.
Meanwhile, the silica peak centered at ~24° became broader
2
.2. Catalytic synthesis of levulinate esters
and weaker, signifying that high H
might deform the silica skeleton and thus partially disorder the
mesoporous channels of SiO . In addition, high H SiW12 40 loading
might result in uneven dispersion or aggregation [23].
FT-IR analysis was further employed to identify the structural
evolution of the catalyst. The FTIR spectra of the mesoporous SiO and
20 wt.% H SiW12 40-SiO samples are shown in Fig. S2. The characteristic
bands of the mesoporous SiO (Fig. S2a) appeared at 820 (Si\O bending)
and 1063 cm (Si\O\Si stretching). Subsequent to the introduction of
20 wt.% H SiW12 40 in the mesoporous SiO , the catalyst displayed
characteristic bonds at 970 cm (W_O, terminal bonds), 920 and
4
SiW12O40 loading (40 wt.%)
The synthesis of ML and EL was performed in a flask that was
2
4
O
coupled with a thermometer and reflux condenser. In a typical run,
levulinic acid (205 mg) was dissolved in 2 mL methanol or ethanol,
followed by the addition of the H
and cyclohexane (5 mL) as the solvent. The flask was heated for
h at an agitation speed of 1000 rpm, where the temperature
4
SiW12
O40-SiO
2
catalyst (104 mg)
2
4
O
2
6
2
−
1
was maintained at 65 and 75 °C for the synthesis of ML and EL,
respectively. Following the reaction, the reactants and products
were analyzed by gas chromatography (Shimadzu GC 2014; column:
4
O
2
−
1

60
K. Yan et al. / Catalysis Communications 34 (2013) 58–63
a
b
c
d
e
Fig. 2. EDX analysis spectrum and EDX mapping of 20 wt.% H
4
SiW12
2 2 4 2 4 2
O40-SiO : (a) mesoporous SiO ; (b) 10 wt.% H SiW12O40-SiO ; (c) silica in 20 wt.% H SiW12O40-SiO ; (d) tungsten in
2
0 wt.% H
4
SiW12
O
40-SiO
2
; (e) oxygen in 20 wt.% H
4
SiW12
O
40-SiO
2
.
1
7
H
90 cm− (W\O\W bridges), respectively. This indicates that the
SiW12 40 maintained its Keggin Unit following the synthesis [20,27].
SEM and EDX were further employed to elucidate further
respectively. Strong oxygen and silicon peaks were observed and a num-
ber of tungsten peaks appeared in Fig. 2b, evidencing the existence
of tungsten in the sample. However, the strongest tungsten peak was
partially overlapped with the silica peak. EDX mapping was further
utilized to verify the homogeneous element distribution. A high
degree dispersion of the Si, W and O elements was found in the
4
O
morphological and compositional data of the synthesized catalysts.
As shown in Fig. 1, the representative SEM images of the mesoporous
SiO
displayed similar morphologies with an interconnected pore/network
structure. However, when the H SiW12 40 loading was 30 wt.%
Fig. 1d), some aggregation was formed on the surface, which is consis-
tent with the aforementioned XRD results. Fig. 2a and b presents the
2 4 2 4 2
(a), 10 wt.% H SiW12O40-SiO (b) and 20 wt.% H SiW12O40-SiO (c)
20 wt.% H
Fig. 3 displays the representative TEM images of 20 wt.%
SiW12 40-SiO . As seen from Fig. 3a, the synthesized catalyst
4 2
SiW12O40-SiO sample (Fig. 2c–e).
4
O
(
H
4
O
2
exhibited the interconnected porous-network structure with high
porosity and no notable aggregation was detected. The high-
2 4 2
EDX spectra of the mesoporous SiO and 10 wt.% H SiW12O40-SiO ,
a
b
10 nm
2
00 nm
4 2
Fig. 3. TEM analysis of the selected 20 wt.% H SiW12O40-SiO : (a) at 200 nm; (b) at 10 nm.

K. Yan et al. / Catalysis Communications 34 (2013) 58–63
61
9
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
00
0
9
8
00
00
a
b
700
600
5
4
3
2
1
00
00
00
00
00
0
0
.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
P/P0
P/P0
1
.6
.4
.2
.0
.8
.6
.4
.2
3
3
2
2
1
1
.5
c
d
1
.0
1
1
0
0
0
0
.5
.0
.5
.0
0.5
0
.05
0.10
0.15
0.20
0.25
0.30
0.05
0.10
0.15
0.20
0.25
0.30
P/P0
P/P0
Fig. 4. Surface area and pore size analysis of mesoporous SiO
2
and 20 wt.% H
4
SiW12
O
40-SiO
2
: (a) N
2
adsorption and desorption curves of mesoporous SiO
SiW12 40-SiO
2 2
; (b) N adsorption and
desorption curves of 20 wt.% H SiW12 40-SiO ; (c) BET plot of mesoporous SiO
4
O
2
2
; (d) BET plot of 20 wt.% H
4
O
2
.
magnification TEM image (Fig. 3b) showed that the interplanar
spacing of the H SiW12 40 nanoparticles was ~0.360 nm, which is
in good agreement with the d value of the (222) planes of the body
centered cubic H SiW12
The introduction of H
the porous structure of the resultant samples [23,28]. Therefore, further
-BET analysis was performed. The N -isotherm and BET plots of two
selected samples (mesoporous SiO and 20 wt.% H SiW12 40-SiO ) are
depicted in Fig. 4. A typical IV curve of mesoporous materials was
observed in the N -sorption isotherm. For the mesoporous SiO
(Fig. 4a), the steps on the adsorption isotherms were relatively wide,
which exhibited capillary condensation at a relative pressure of ~0.58.
In the case of 20 wt.% H SiW12O40-SiO (Fig. 4b), the steps on the adsorp-
4 2
tion isotherms were relatively narrow and revealed the capillary conden-
sation at a relative pressure of ~0.75. BET plots (Fig. 4c and d) were used
to evaluate the surface area and pore size of the resulting samples. In both
4
O
4
O
40
.
4
SiW12O40 into the silica network may influence
N
2
2
cases the a
confirmed that the fitting of linear and a
surface areas and pore sizes of the resulting samples are presented in
Table S1. With the exception of 40 wt.% H SiW12 40-SiO (Table S1, No.
s
plots revealed that the R-square was over 0.999, which
2
4
O
2
s
plots were highly reliable. The
2
2
4
O
2
4 2
Scheme 1. Synthesis of methyl levulinate (ML) and ethyl levulinate (EL) using the mesoporous H SiW12O40-SiO catalysts.

62
K. Yan et al. / Catalysis Communications 34 (2013) 58–63
1
00
However, on the other hand, it would simultaneously enhance the
side reactions, for instance the etherification of methanol; thus the
selectivity of ML would be reduced. With 40 wt.% H SiW12 40-SiO as
the catalyst, an only 61% yield was obtained. This is likely due to the
significant decrease of the surface area (Table S1). For the catalytic
synthesis of EL, the best catalytic performance was also observed
with the 20 wt.% H SiW12O40-SiO catalyst (Fig. 5b): a 67% yield of
4 2
EL was achieved at 75% conversion of levulinic acid. The same trend
for the production of ML and EL was observed in terms of the
Conv./%
Yield/%
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
a
4
O
2
yield: 20 wt.% H
SiW12 40-SiO >meso-SiO
In order to test recycling properties, the spent catalyst (20 wt.%
SiW12 40-SiO ) was separated through centrifugation following
4 2 4 2
SiW12O40-SiO >40 wt.% H SiW12O40-SiO >10 wt.%
H
4
O
2
2
.
H
4
O
2
the reaction, dried at overnight at 120 °C, and then employed again
for the catalytic tests. As shown in Table S2, a slight decrease of
the performance (Run 2) was observed in both cases compared to
the fresh catalyst (Run 1). During the second run, in the case of
ML synthesis, a 60.2% yield was achieved at 74% conversion. For
the EL synthesis, a 56% yield was obtained at 73% conversion. This
0
10 wt%
20 wt%
30 wt%
40 wt%
Different Loading of H SiW O - SiO
4
12 40
2
1
00
Conv./%
Yield/%
decrease was likely due to the leaching of H
deposited on the surface of the mesoporous SiO
showed that ~1.0% of the H SiW12 40 was leached during Run 1. In
the consecutive third and fourth runs, stable activity was achieved
Runs 3 and 4). No further leaching of tungsten was detected by ICP
analysis, confirming that the resultant catalysts maintained stability.
4
SiW12O40 that was
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
b
2
. Our ICP analysis
4
O
(
4
. Conclusion
Mesoporous H
4
2
SiW12O40-SiO catalysts with tunable compositions
were successfully fabricated through a one-step method in the presence
of a Brij 30 surfactant. The resultant catalysts were efficient for the synthe-
sis of methyl levulinate and ethyl levulinate. The best performance of
methyl levulinate (73% yield achieved at a 79% conversion) and ethyl
levulinate (67% yield at a 75% conversion) was obtained using the
0
10 wt%
20 wt%
30 wt%
40 wt%
4 2
20 wt.% H SiW12O40-SiO catalyst. The recyclability tests demonstrated
Different Loading of H SiW O - SiO
2
that the resultant catalysts maintained stable catalytic activities, which
is promising for general applications in the production of biodiesel fuels.
4
12 40
Fig. 5. Catalytic activities of H
and ethyl levulinate (b) under the following conditions: m(levulinic acid)=205 mg;
V(cyclohexane)=5 mL; (catalyst)=104 mg; and t=6 h. To synthesize ML:
V(methanol)=2 mL and T=65 °C. To produce EL: V(ethanol)=2 mL and T=75 °C.
4 2
SiW12O40-SiO in the synthesis of methyl levulinate (a)
Acknowledgments
m
This work was supported by a Discovery Grant from the Natural
Sciences and Engineering Research Council of Canada (NSERC) and the
Centre for Research and Innovation in the Bio-Economy (CRIBE). K. Yan.
appreciates the Ontario Postdoctoral Fellowship. A. Chen acknowledges
NSERC and the Canadian Foundation of Innovation (CFI) for the Canada
Research Chair Award in Material and Environmental Chemistry.
5
), the remaining four samples exhibited a high BET surface area in the
2
4
range of 130–650 m /g. When the H SiW12O40 loading was increased,
from 0 to 40 wt.% (No. 1 to No. 5), the surface areas became smaller
and smaller. However, their pore sizes significantly increased after the
introduction of H
4
SiW12O40 up to 30 wt.% as it was expected that the
confined H
channel.
4
SiW12
O
40 nanoparticles would swell the mesoporous SiO
2
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2013.01.010.
3
.2. Catalytic performance
The mesoporous H
synthesis of ML and EL (Scheme 1):
4 2
SiW12O40-SiO catalysts were evaluated in the
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4 2
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acid. In contrast, in a comparative reaction that utilized only the
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