A simple strategy towards the preparation of a highly active bifunctionalized catalyst for the deacetalization reaction

Article abstract of DOI:10.1016/j.apcata.2015.12.036

A bifunctional catalyst containing both the hydrophobic methyl and hydrophilic sulfonic acid groups has been successfully fabricated by mild surface modification of the pre-prepared silica nanoparticles. To its credit, the methyl groups in the catalyst were applied to improve the surface wettability of silica solid nanoparticles, while the sulfonic acid groups were devoted to catalyzing the deacetalization of various acetal derivatives. The catalyst was characterized in detail by Scanning electron microscope (SEM), Transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR) and Thermogravimetric analysis (TGA). The catalytic results revealed that the bifunctional SiO₂-Me&SO₃H NPs exhibited superior activity for the deacetalization of 4-methoxybenzaldehyde dimethyl acetal and the product conversion could be >99% within 45 min in the toluene/water biphasic system. The main reason was probably due to the formation of Pickering emulsion, which could greatly improve the interface area between the oil and water and decrease the mass transfer resistance.

Full text of DOI:10.1016/j.apcata.2015.12.036

Applied Catalysis A: General 513 (2016) 47–52
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
A simple strategy towards the preparation of a highly active
bifunctionalized catalyst for the deacetalization reaction
Shuai Chena, Fengwei Zhangb,∗, Mengqi Yangc, Xincheng Lic, Helong Liangc, Yan Qiaoa,∗∗
,
Dongyan Liua, Weibin Fana
a Analytical Instrumentation Center, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China
b Institute of Crystalline Materials, Shanxi University, Taiyuan 030006, PR China
c School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A bifunctional catalyst containing both the hydrophobic methyl and hydrophilic sulfonic acid groups
has been successfully fabricated by mild surface modification of the pre-prepared silica nanoparticles.
To its credit, the methyl groups in the catalyst were applied to improve the surface wettability of sil-
ica solid nanoparticles, while the sulfonic acid groups were devoted to catalyzing the deacetalization
of various acetal derivatives. The catalyst was characterized in detail by Scanning electron microscope
(SEM), Transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR) and
Thermogravimetric analysis (TGA). The catalytic results revealed that the bifunctional SiO2 Me&SO3H
NPs exhibited superior activity for the deacetalization of 4-methoxybenzaldehyde dimethyl acetal and
the product conversion could be >99% within 45 min in the toluene/water biphasic system. The main rea-
son was probably due to the formation of Pickering emulsion, which could greatly improve the interface
Received 5 November 2015
Received in revised form
14 December 2015
Accepted 26 December 2015
Available online 30 December 2015
Keywords:
Bifunctional catalyst
Deacetalization reaction
Biphasic system
area between the oil and water and decrease the mass transfer resistance
© 2015 Elsevier B.V. All rights reserved.
Pickering emulsion
1. Introduction
food production, cosmetics, coating and catalysis to petroleum
industries [8–12].
Pickering emulsions (solid particles-stabilized emulsions) are
colloidal emulsions that are stabilized by solid particles instead of
traditional surfactant molecules [1,2]. In principle, the solid parti-
cles with appropriate hydrophilicity/hydrophobicity (wettability)
can be strongly adsorbed at the interface between two immiscible
fluids such as oil and water, creating an effective steric/electrostatic
shield for the emulsified droplets [3–5]. And in terms of the
wettability of the solid particles, the Pickering emulsion can be
classified into either oil-in-water (O/W) or water-in-oil (W/O)
type of emulsion. Generally, the hydrophilic particles tend to form
O/W emulsion, while the hydrophobic particles prefer to generate
W/O emulsion [6,7]. Beyond that, these solid particles-stabilized
emulsions also offer unique advantages over traditional surfactant
including reduced foaming, strong kinetic hindrance to droplet coa-
lescence and tunable interfacial permeability. More recently, the
researchers have demonstrated that the Pickering emulsions hold
great promise in a variety of potential applications ranging from
Particularly, the intense research has been fueled by the
development of Pickering emulsion catalyst for alcohol oxidation
[13], alkene epoxidation [14], nitroarenes hydrogenation [15,16]
and biofuel upgrade reactions [17–20]. Among them, the solid
nanoparticles-stabilized emulsion catalytic system, in which the
solid nanoparticles serve as both emulsifiers and catalysts, has
proven superior to traditional surfactant-stabilized emulsion due
to its easy separation and high recyclability [21,22]. To date, vari-
ous solid particles including SiO2 NPs, Fe3O4 NPs, TiO2 NPs, zeolites,
carbon nanotubes (CNTs) as well as their composites with appro-
priate wettability can be well located at the interface between
the oil/water two phase [23–28]. Moreover, after immobilizing the
catalytically active species onto the above support surface, they
are likely to seat at the amphiphilic catalyst-stabilized Pickering
emulsion droplet interface. In this regard, the resulting plentiful
micro-size nanoreactors not only greatly increase the oil/water
biphase contact area but also substantially reduce the transfer
resistance of organic substrates and avoid the self-aggregation and
leaching of catalytic active sites during the reaction [29].
The integration of amphiphilic solid nanomaterials and metal
nanoparticles has been a fruitful area in hydrogenation [30], oxi-
dation [31], biomass conversion [32] and esterification reactions
∗
Corresponding author.
Corresponding author.
E-mail addresses: fwzhang@sxu.edu.cn (F. Zhang), qiaoy@sxicc.ac.cn (Y. Qiao).
∗∗
http://dx.doi.org/10.1016/j.apcata.2015.12.036
0926-860X/© 2015 Elsevier B.V. All rights reserved.

48
S. Chen et al. / Applied Catalysis A: General 513 (2016) 47–52
[33]. These studies proved that the solid catalyst materials with
amphiphilic properties can not only significantly accelerate the
reaction rate but also optimize the product selectivity to a certain
extent. The possible reason was that they could greatly improve
adsorption of substrates on the catalytic activity sites at the
oil–water biphasic interfaces and exclude the product from the cat-
alyst surface simultaneously. But as far as we know, the amphiphilic
metal-free solid nanoparticles for the heterogeneous catalytic reac-
tion in oil–water biphasic systems are still rarely reported [34].
In the current work, we present a simple strategy towards the
preparation of a highly active metal-free bifunctionalized cata-
lyst for the deacetalization reaction. The catalyst was prepared
via the simultaneous modification of the silica nanoparticles sur-
face with the hydrophobic methyl and hydrophilic sulfonic acid
organosilanes. The deacetalization results demonstrated that the
amphiphilic SiO2 Me&SO3H catalyst possessed excellent catalytic
activity and stability in toluene/water biphasic system.
SiO2 Me&SO3H catalyst was added to a 25 mL of 1.0 M NaCl aque-
ous solution, and the resulting suspension was stirred at room
temperature for 48 h until equilibrium was reached. The filtrate
was titrated with 50 mM NaOH and phenolphthalein was applied
as indicator. The acid amount of SiO2 Me&SO3H catalyst was deter-
mined to be 0.14 mmol g−1, which was highly consistent with the
elemental analysis result.
2.5. General procedure for the hydrolysis reaction of acetal
derivatives
The hydrolysis reaction of acetal derivatives was performed in
a 10 mL of round bottom flask. A typical reaction process was as
follows: 45 mg of SiO2 Me&SO3H NPs (6.3 ␮mol of SO3H groups)
was put into the reactor, 3.0 mL of toluene and 3.0 mL of deion-
ized water were added as solvent. After stirring for 5 min, 1 mmol
of benzaldehyde dimethyl acetal was added into above mixture by
syringe. The hydrolysis reaction was carried out at 30 ◦C for 45 min.
After the reaction, the product was obtained through centrifuga-
tion and then the supernatant was determined by Agilent 7890A
GC. Then the catalyst was recovered and washed several times
with deionized water and ethyl acetate, dried at 50 ◦C for 2 h, and
subsequently used in the next cycles.
2. Experimental
2.1. Materials and methods
2-(4-Chlorosulfonylphenyl) ethyltrichlorosilane in methylene
chloride (CSPETS) was purchased from J&K Co., Ltd. Methyl-
trichlorosilane (MTCS), tetraethyl orthosilicate (TEOS) and aqueous
ammonia (25 wt%) were purchased from Aladdin Chemical Reagent
Co., Ltd. All other chemicals and reagents were analytical grade and
used without further purification. Deionized water was used in the
whole experiment.
3. Results and discussion
The morphology and structure of SiO2 NPs and SiO2 Me&SO3H
NPs were firstly elucidated by SEM. The representative SEM images
of the as-prepared SiO2 NPs and SiO2 Me&SO3H NPs are shown
in Fig. 1a and b, the low magnification SEM image illustrated the
large-scale formation of uniform monodisperse spherical SiO2 NPs
and SiO2 Me&SO3H NPs with an average diameter of ∼150 nm. The
TEM image (Fig. 1c and d) showed that the SiO2 Me&SO3H NPs
have a rough surface compared with the pure SiO2 NPs, indicat-
ing that each nanosphere consists of SiO2 and Me&SO3H organic
silane and the presence of Me&SO3H affected the morphology of
SiO2, which was consistent with the FT-IR results.
Fig. 2 shows the digital photographs and optical microscopy
images of SiO2 Me&SO3H NPs-stabilized Pickering emulsions. The
formation process of Pickering emulsions was as follows: 30 mg
of SiO2 Me&SO3H NPs, 3 mL of toluene and 3 mL of water were
added into a 10 mL of glass vial. The mixture was sonicated for
2 min and then violently stirring for another 5 min, and then stand-
ing for 1 h for optical microscope analysis. As can be seen in Fig. 2a,
the whole materials were participated into the production of the
emulsion and the height of the emulsion layer was about 0.95 cm,
while the height of the upper toluene was approximately 0.44 cm.
Thus, it can be speculated that the emulsification efficiency reached
up to 68%. As for Fig. 2b, the optical microscopy image revealed that
the SiO2 Me&SO3H NPs-stabilized Pickering emulsions with ellip-
soidal shape and the droplets size were below 300 ␮m, which were
well-dispersed on the glass slide. More importantly, the obtained
emulsion could greatly improve the interface area between the oil
and water and decreased the mass transfer resistance.
2.2. Preparation of the solid silica nanoparticles (SiO2 NPs)
The SiO2 NPs were synthesized by the classical Stöber method
and with slight modification. Typically, 20 mL of aqueous ammo-
nia (25 wt%) was added into the mixture of 400 mL of ethanol and
50 mL of deionized water via ultrasonication. After vigorously stir-
ring for 30 min, 20 mL of TEOS was added slowly, and the mixture
was stirred for another 6 h to obtain the uniform silica nanparticles.
The resulting products were recovered through centrifugation and
washed several times with deionized water and ethanol, and then
dried at 50 ◦C under vacuum for 12 h.
2.3. Preparation of methyl and sulfonic acid bifunctionalized SiO2
NPs (SiO2 Me&SO3NPs)
The synthesis of SiO2 Me&SO3H NPs was based on the co-
condensation of 2-(4-chlorosulfonylphenyl) ethyltrichlorosilane
(CSPETS) and methyltrichlorosilane (MTCS) in the presence of the
silica nanoparticles, which was based on our previously reported
work [35]. In a typical procedure, 1.0 g of SiO2 NPs were dispersed
into 50 mL of dry toluene via ultrasonication, then MTCS (0.4 mL,
3.0 mmol) and CSPETS (0.25 mL, 0.5 mmol) were slowly added into
above mixture. The resulting mixture was stirred for 24 h at room
temperature, and then it was washed three times with ethanol and
each time was 15 mL. Subsequently, the solid samples were sus-
pended in 1 M H2SO4 (50 mL) solution for 4 h, washed with a large
amount of deionized water and dried at 50 ◦C under vacuum for 12 h
to give the corresponding SiO2 Me&SO3H NPs. Elemental analy-
sis result of the as-prepared catalyst revealed that the content of
carbon, hydrogen and sulfur was 4.05 wt%, 1.69 wt% and 0.45 wt%,
respectively.
FT-IR spectroscopy was employed to verify the successful mod-
ification of Me&SO3H groups on the as-synthesized SiO2 NPs. As
shown in Fig. 3a, FT-IR spectra of the SiO2 NPs around 1110 cm−1
and 795 cm−1 corresponded to the antisymmetric and symmetric
stretching vibrations of Si
O Si bond in oxygen–silica tetrahe-
dral, respectively. The presence of the anchored alkyl groups was
confirmed by the aliphatic weak C H stretching vibrations appear-
ing at 2975 cm−1 and 2884 cm−1 in the SiO2 Me&SO3H NPs. An
increased intensity and broadening of the band at 3000–3500 cm−1
in the samples suggested the existence of more OH groups on
the SiO2 Me&SO3H NPs. Taken together, these results indicated
2.4. Determination of the SiO2 Me&SO3catalyst’s acidity
The concentration of sulfonic acid groups was determined
by ion-exchange pH analysis protocol [36]. Typically, 50 mg of

S. Chen et al. / Applied Catalysis A: General 513 (2016) 47–52
49
Fig. 1. The SEM and corresponding TEM images of (a, c) SiO2 NPs and (b, d) SiO2 Me&SO3H NPs.
Fig. 2. (a) The digital photograph and (b) the corresponding optical microscopy image of SiO2 Me&SO3H NPs-stabilized Pickering emulsions.
that the functional groups were successfully grafted on the sur-
face of the SiO2 Me&SO3H NPs. TGA was performed to analyze
the grafting amount of organosilanes on the SiO2 Me&SO3H sam-
ple, and the results were shown in Fig. 3b. It can be seen that the
SiO2 NPs exhibited two main weight losses. The first rapid weight
loss (∼2.0 wt%) could be attributed to the removal of physisorbed
water molecules on the sample surface when temperature was up
to 150 ◦C. The second weight loss (∼6.15 wt%) was due to decom-
position of the oxygen-containing functional groups from SiO2
NPs between 150 and 650 ◦C. In comparison, the weight losses
of SiO2 Me&SO3H NPs were 1.6 wt% below 150 ◦C and 8.75% at
temperature range of 150–800 ◦C, respectively. This clearly indi-
cated that much more functional groups from SiO2 Me&SO3H NPs
were decomposed at the second weight loss period than SiO2 NPs.
This remarkable weight loss could be attributed to the existence of
Me&SO3H functional groups on the SiO2 Me&SO3H NPs because
the organosulfonic acid required a higher decomposition temper-
ature to achieve constant weight status.
To further determine the surface chemical composition of the
SiO2 NPs and SiO2 CH3&SO3H catalyst, X-ray photoelectron spec-
troscopy (XPS) measurement was also carried out. As shown in
Fig. 4, the XPS spectrum of the SiO2 CH3&SO3H catalyst indicated
that the specimen consisted of Si, O, C and S atoms as compared
with pure SiO2 NPs, whereas there was no S emission could be

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S. Chen et al. / Applied Catalysis A: General 513 (2016) 47–52
Fig. 3. (a) The FTIR spectra and (b) the TGA curves of SiO2 NPs and SiO2 Me&SO3H NPs.
the synergistic effect of the SO3H groups and CH3 groups. The
SO3H groups behaved as the catalytic centers to release hydrogen
ions, while the hydrophobic CH3 groups could greatly enrich the
substrates around the catalytic active sites owing to the formation
of Pickering emulsion.
To explore the generality of SiO2 Me&SO3H catalyst for deac-
etalization reaction, we examined conversion efficiency of a series
of different substrates. As shown in Table 1, 4-methoxy benzalde-
hyde dimethyl acetal was transferred to 4-methoxy benzaldehyde
and showed excellent conversion above 99% within 60 min. 1,1-
dimethoxycyclohexane converted into cyclohexanone with 92.2%
conversion within 60 min. 4-bromo-benzaldehyde dimethyl acetal
deacetalized to 4-bromo-benzaldehyde at the conversion of 86.4%.
Butanal dimethyl acetal was transferred to butanal with the con-
version of 86.9% within 75 min. Moreover, the conversion of
4-methyl-2-phenyl-1,3-dioxolane was also reached up to 91.3% at
60 ◦C for 90 min. The possible reaction mechanism was presented in
Fig. 5. Therefore, theSiO2 Me&SO3Hcatalysthasexcellentcatalytic
ability for the deacetalization reaction.
Fig. 4. The XPS spectra of SiO2 NPs and SiO2 CH3&SO3H catalyst (the insert image
In an attempt to investigate the solvent effect on the cata-
lyst activity, we conducted the deacetalization of benzaldehyde
dimethyl acetal in different solvent. As shown in Fig. 6a, the conver-
sion of benzaldehyde dimethyl acetal was only 48.7% within 90 min
in the toluene system, while in the toluene-water biphasic system
it reached up to 92.7% within 45 min. It suggested that the toluene-
water biphasic system was a better solvent for the reaction. Fig. 6b
revealed that the SiO2 Me&SO3H catalyst displayed the higher
catalytic activity when compared to SiO2 SO3H catalyst, and the
SiO2 NPs almost without any activity for the deacetalization. We
next examined the influence of SiO2 Me&SO3H catalyst amount
on the deacetalization reaction (Fig. 6c). Three parallel experimen-
tal groups with catalyst amount of 30, 45 and 60 mg were tested,
respectively. It could be found that the highest reaction conver-
sion occurred when catalyst amount was 45 mg. That was probably
because the over-dosage (60 mg) led to the quenching of active cen-
ters of catalyst while the low-dosage (30 mg) could not provide
sufficient active centers.
Last but not least, the recyclability of SiO2 Me&SO3H
catalyst was examined in the deacetalization reaction of 4-
bromobenzaldehyde dimethyl acetal under the described reaction
conditions. Upon completion of each reaction, the reaction mixture
was centrifuged and the upper toluene phase was withdrawn from
the system by a syringe, and then a fresh of reactant and toluene
was added into the above-mentioned catalyst aqueous solution for
the subsequent reaction under the same reaction conditions. As can
be seen in Fig. 7, no obvious change in the activity of the catalyst
is the corresponding S2p curves).
detected in the spectrum and the signal of C1s also had a remarkable
increase from SiO2 CH3&SO3H. In the inset image, the high resolu-
tion XPS spectrum showed a peak at 169 eV from SiO2 CH3&SO3H
sample which is assigned to S 2p from SO3H group [37]. As a result,
XPS data further confirmed that the CH3 and SO3H groups were
well-decorated on SiO2 surface.
The deacetalization of aromatic and aliphatic acetal derivatives
was chosen as the model reaction for the investigation of the
catalytic performance of different catalysts (Table 1). The deacetal-
ization of acetals to produce aldehyde derivatives represents an
important pretreatment step in the production of pharmaceutical
intermediates and fine chemicals based on aldehyde derivatives.
Herein, four types of catalysts were initially carried out to illus-
trate the influence factors on the benzaldehyde dimethyl acetal, of
which the toluene/water biphasic system was used as reaction sol-
vent. When taking SiO2 NPs and SiO2 Me NPs as the catalyst, the
benzaldehyde conversion was only 4.5% and 6.4% within 45 min,
showing extremely low catalytic activity. We attributed the low
catalytic activity to the lack of catalytic active sites. In contrast, the
conversion reached up to 64.5% in the presence of SiO2 SO3H NPs,
thus indicating that the sulfonic acid groups played a critical role in
the deacetalization of acetal. Encouragingly, the benzaldehyde con-
version reached up to 97% with the assistance of SiO2 Me&SO3H
catalyst. (Table 1, entry 4). We reasoned that the excellent catalytic
performance of SiO2 Me&SO3H catalyst was mainly attributed to

S. Chen et al. / Applied Catalysis A: General 513 (2016) 47–52
51
Table 1
The deacetalization of acetal derivatives by SiO2 Me&SO3H catalyst.a
Entry
Catalyst
Substrate
Product
T ( ◦C)
t (min)
Conv. (%)b
1
2
3
4
5
6
7
8
SiO2
30
30
30
30
30
30
30
50
45
45
45
60
60
60
60
75
4.5
SiO2 Me
6.4
SiO2 SO3H
64.5
97.0
>99
92.2
86.4
86.9
SiO2 Me&SO3H
SiO2 Me&SO3H
SiO2 Me&SO3H
SiO2 Me&SO3H
SiO2 Me&SO3H
9
SiO2 Me&SO3H
60
90
91.3
a
Reaction conditions: 1.0 mmol substrate, 45 mg catalyst (6.3 ␮mol, S/C = 160), 3.0 mL toluene and 3.0 mL H2O.
The conversion was determined by GC (average conversion for three times), and the selectivity of the corresponding aldehydes >99%.
b
Fig. 5. The possible deacetalization mechanism of 4-methyl-2-phenyl-1,3-dioxolane.
Fig. 6. The yield of benzaldehyde in the presence of (a) single and biphasic system, (b) different types of catalysts and (c) different amount of SiO2 Me&SO3H catalyst.
Reaction conditions: 1.0 mmol of benzaldehyde dimethyl acetal, 3.0 mL of toluene and 3.0 mL H2O.

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Conflict of interest
The authors declare no competing financial interest.
Acknowledgements
This work was supported by the Scientific Research Start-up
Funds of Shanxi University (023151801002) and Natural Science
Foundation for Young Scientists of Shanxi Province (2015021051).