D. Song et al. / Journal of Catalysis 333 (2016) 184–199
187
(
1
1.15 mmol) to ethanol (69.00 mmol) molar ratio of 1:60, and
.5 wt.% of catalyst; meanwhile, stirring was applied throughout
tubular particles. As increasing initial Si-to-Zr molar ratio to 0.75
and at P123-to-Si-to-Zr-to-HCl molar ratio of 0.0168:0.75:1:6, the
the reaction. The concentrations of the produced ethyl levulinate
and 2-(ethoxymethyl)furan were determined periodically on a Shi-
madzu 2014C gas chromatograph fitted with a HP-INNOWAX cap-
illary column and flame ionization detector. The injection port
temperature was 250 °C; the oven temperature was maintained
at 60 °C for 5 min and then raised to 180 °C for 10 min at a heating
rate of 8 °C min . The GC injector temperatures were 250 °C. Ethyl
laurate was applied as an internal standard. The catalytic activity
of all catalysts was evaluated quantitatively by the yield of ethyl
2
PW12/ZrO -Si(Et)Si nanohybrid exhibits both a disordered 3D inter-
connected mesostructure and a 1D hollow tubular nanostructure
(Fig. 1b). When the initial Si-to-Zr molar ratio is continuously
2
increased to 1.0, 2.0, and 3.0, the PW12/ZrO -Si(Et)Si nanohybrid
mainly exhibits a 1D hollow tubular nanostructure, and the esti-
mated inner diameter, shell thickness, and particle size of the tubes
are 4–5, 3, and 10–11 nm, respectively (Fig. 1c–e). At the same
amount of added P123 and initial Si-to-Zr molar ratio of 1.0 but a sig-
nificant decrease in the acidity, e.g., at a P123-to-Si-to-Zr-to-HCl
molar ratio of 0.0168:1:1:2, the morphology of the PW12/ZrO -Si
2
(Et)Si nanohybrid is transformed to a 2D hexagonal periodic
mesostructure (Fig. 1f). A similar morphology can also be found in
À1
À1
À1
levulinate (Y, %) and turnover frequency (TOF, h ). TOF (h ) =
À1
[
M
D
/(Atitration  m)]  t , where Atitration is the number of equiva-
+
lents of H determined by acid–base titration, m (g) is the mass
of the hybrid catalyst used in ethanolysis reaction, and t (h) is
the reaction time. The intermediates produced during the catalytic
processes were identified by mass spectrometry coupled with gas
chromatography (HP6890GC-5973MSD).
2 2
the PW12/ZrO -SiO nanohybrid prepared at a P123-to-Si-to-Zr-to-
HCl molar ratio of 0.0168:1:1:6 using TEOS rather than BTMSE as
the Si source (Fig. 1g). In this case, fabrication of 2D hexagonal
2 2
ordered mesoporous PW12/ZrO -SiO is realized at higher rather
than lower acidity. This is due to the fact that the organosilane pre-
cursorwithbridging ethylgroups has quite differentproperties from
TEOS, for example, the hydrolysis and condensation rate, the hydro
phobicity/hydrophilicity, and the rigidity [10]. However, in the
absence of a silicon precursor, neither tubular nor ordered nanos-
2
.6.3. Evaluation of the external mass-transfer limitation
To evaluate the external mass-transfer limitation in current sys-
tems, the influence of the stirring rate on the esterification and
ethanolysis activity of the PW12/ZrO -Si(Et)Si-NTs nanohybrids
was studied by selecting 12.1PW12/ZrO -Si(Et)Si-NTs1.0 as the rep-
resentative catalyst. The catalytic activity of the 12.1PW12/ZrO -Si
Et)Si-NTs1.0 toward both of the target reactions is hardly affected
2
2
tructure can be formed (i.e., 12.7PW12/ZrO , Fig. 1h), implying the
2
important morphology-adjusting function of the silicon precursor.
On the basis of these results, it is inferred that at a suitable con-
centration of P123 micelles, the Si-to-Zr molar ratio and acidity in
the initial gel mixture play the key roles in fabrication of
2
(
by changing the stirring rate from 400 to 800 or 1200 rpm (Fig. S1
of the electronic Supplementary Information). The result indicated
that the PW12/ZrO
nic acid and ethanolysis of furfuryl alcohol were free from the
external mass-transfer limitation.
2
-Si(Et)Si-NTs-catalyzed esterification of levuli-
PW12/ZrO
influence of the initial Si-to-Zr molar ratio on the morphological
evolution of the PW12/ZrO -Si(Et)Si nanohybrids is explained in
terms of the hydrolysis/condensation rate and hydrophobicity/
hydrophilicity of the precursors (e.g., BTMSE and Zr(OC
10]. On one hand, P123 is capable of self-assembly into single rod-
like micelles in strong acidic media. The Zr(OC precursor with
a faster hydrolysis/condensation rate can produce more hydro-
2
-Si(Et)Si nanohybrids with different morphologies. The
2
3
. Results and discussion
4 9 4
H ) )
[
3.1. Preparation and characterization of the catalyst
4 9 4
H )
+
A
series of PW12/ZrO
2
-Si(Et)Si organic–inorganic hybrid
40 loadings (3.7, 7.0, 12.1,
4 9 2 x
lyzed species (e.g., Zr(OC H )4Àx(OH ) ) to condense around P123
nanocatalysts with various H
3
PW12
O
single rodlike micelles, which in turn results in the aggregation
of single rodlike micelles and the formation of 3D interconnected
mesostructure. However, the hydrolysis/condensation rate of
and 14.8 wt.%) and initial Si-to-Zr molar ratios (1.0, 2.0, and 3.0)
were successfully fabricated by a well-designed P123 single-
micelle-templated sol–gel co-condensation route. The procedure
includes one-step co-condensation of bridging organosilane (1,2-
bis(trimethoxysilyl)ethane) and zirconium n-butoxide around tri-
block copolymer surfactant (P123) micelles in the presence of
H PW12O40. After formation of the Keggin units and ZrO bifunc-
3 2
tionalized ethane-bridged organosilica framework, it underwent
hydrothermal treatment to further fasten the linkage of the
4 9 4
BTMSE is much slower than that of Zr(OC H ) , and increasing
the initial Si-to-Zr molar ratio (e.g., 1.0, 2.0, or 3.0) can slow the
hydrolysis/condensation rate of the precursors, which effectively
inhibits the aggregation of P123 single rodlike micelles and readily
forms a 1D hollow tubular nanostructure. On the other hand, the
morphology of the hybrid materials is related to the hydrophobic-
ity/hydrophilicity of the precursors. BTMSE with bridging ethyl
organosilica framework with the Keggin units and ZrO
PW12/ZrO -Si(Et)Si was formed after removal of P123 by washing
with boiling ethanol (Scheme 1).
Amphiphilic copolymer surfactants (e.g., P123) with both hydro-
philic (e.g., PEO) and hydrophobic (e.g., PPO) blocks can readily self-
assemble into micelles via hydrogen bonding and hydrophobic/
hydrophilic interactions under acidic conditions. The micelles can
further aggregate to form lyotropic liquid crystal structures with
various morphologies such as rodlike and cylindrical shapes via
careful adjustment of preparation conditions. Here, the morpholog-
2
. Finally,
4 9 4
groups is a hydrophobic precursor compared with Zr(OC H ) . For-
2
mation of P123 single rodlike micelles can be encouraged by the
hydrophobic unhydrolyzed BTMSE, since the charge density on
the surface of P123 micelles is decreased owing to the penetration
of the hydrophobic unhydrolyzed BTMSE into the micelles. With
increasing initial Si-to-Zr molar ratio, the enhanced hydrophobicity
in the preparation system leads to the formation of a 1D hollow
tubular morphology. Therefore, it is concluded that changing initial
Si-to-Zr molar ratios in the preparation systems can adjust the
hydrolysis/condensation rate and hydrophobicity/hydrophilicity
2
ical evolution of the PW12/ZrO -Si(Et)Si nanohybrids with 3D inter-
2
of the precursors, which in turn leads to PW12/ZrO -Si(Et)Si
connected mesostructure, 2D hexagonal periodic mesostructure,
and 1D hollow tubular nanostructure is realized by tailoring prepa-
ration conditions includingthe concentration of silica and zirconium
precursors (or initial Si-to-Zr molar ratio) and acidity in the initial
gel mixture. As illustrated in Scheme 1 and displayed in Fig. 1a, at
nanohybrids with a 3D interconnected mesostructure or a 1D hol-
low tubular nanostructure.
Additionally, at the same added amount of P123 and initial Si-
to-Zr molar ratio but a significantly decreased concentration of
HCl, e.g., a P123-to-Si-to-Zr-to-HCl molar ratio of 0.0168:1:1:2,
a
P123-to-Si-to-Zr-to-HCl molar ratio of 0.0168:0.5:1:6, the
PW12/ZrO -Si(Et)Si nanohybrid mainly displays a disordered 3D
interconnected mesostructure, accompanied by a few 1D hollow
2
the morphology of PW12/ZrO -Si(Et)Si transforms to a 2D hexago-
nal periodic mesostructure. The result suggests that the acidity of
the preparation system also plays an important role in the
2