Facile, low-cost, and scalable fabrication of particle size and pore structure tuneable monodisperse mesoporous silica nanospheres as supports for advanced solid acid catalysts

Article abstract of DOI:10.1039/c5ra26432d

Monodisperse mesoporous silica nanospheres (MSN) have been emerging as one of the new frontiers in materials science and nanotechnology because of their potential medical and biological applications as well as heterogeneous catalysis. Although the synthesis of MSN with various morphologies and sphere size has been reported, the synthesis of MSN with monodisperse control below 200 nm by a facile, scalable and low-cost method with high tetraethylorthosilicate (TEOS) concentration still remains a challenge. Herein, this goal was achieved by a templating hydrothermal technique using cetyltrimethylammonium bromide (CTAB) as the templating surfactant and low-cost urea as mineralizing agent. The mesoporous feature and diameter of nanosphere of MSN can be efficiently adjusted. The high volume efficiency by using high TEOS concentration as Si sources and the low production cost by using urea as mineralizing agent for synthesizing MSN allow this novel technique to have great potential for industrial production. Furthermore, the advanced solid acid catalysts with superior catalytic activity and stability were prepared by supporting phosphotungstic acid (PTA) on MSN, ascribed to the high PTA dispersity and facilitated mass transfer by the short mesoporous channels in comparison with traditional mesoporous silica like MCM-41. This work presents an alternative method for overcoming low stability issue, a bottleneck problem for the industrial application of solid acid catalysts.

Full text of DOI:10.1039/c5ra26432d

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PAPER
Facile, low-cost, and scalable fabrication of particle size and pore
structure tuneable monodisperse mesoporous silica nanospheres
as supports for advanced solid acid catalysts†
Received 00th January 20xx,
Accepted 00th January 20xx
Zhongkui Zhao*a, Xianhui Wanga, Yanhua Jiaob, Boyuan Miaoa, Xinwen Guoa and Guiru Wanga
DOI: 10.1039/x0xx00000x
Monodisperse mesoporous silica nanospheres (MSN) have been emerging as one of the new fronties in materials science
and nanotechnology because of their potential medical and biological applications as well as heterogeneous catalysis.
Although the synthesis of MSN with various morphologies and sphere size has been reported, the synthesis of MSN with
monodisperse control below 200 nm by a facile, scalable and low-cost method with high tetraethylorthosilicate (TEOS)
concentration still remains a challenge. Herein, this goal was achieved by a templating hydrothernal technique using
cetyltrimethylammonium bromide (CTAB) as the templating surfactant and low-cost urea as mineralizing agent. The
mesoporous feature and diameter of nanosphere of MSN can be efficiently adjusted. The high volume efficiency by using
high TEOS concentration as Si sources and the low production cost by using urea as mineralizing agent for synthesizing
MSN allow this novel technique to have great potential for industrial production. Furthermore, the advanced solid acid
catalysts with superior catalytic activity and stability were prepared by supporting phosphotungstic acid (PTA) on MSN,
ascribed to the high PTA dispersity and facilitated mass transfer by the short mesoporous channels in comparison with
traditional mesoporous silica like MCM-41. This work presents an alternative method for overcoming low stability issue, a
bottleneck problem for the industrial application of solid acid catalysts.
www.rsc.org/
organosilanes,5 improve the process, resulting in better
particle size control. Without the addition of any additives,
control of pore size and structure is still challenging.6 Several
1. Introduction
Since the discovery of MCM-41 in 1992 by Exxon Mobil
Corporation, considerable progress has been made in the past
decade in the synthesis of mesoporous materials with defined
topology and morphology.1 The synthesis of monodisperse
mesoporous silica nanospheres (MSNs) has attracted special
attention owing to their potential applications in drug delivery,
cell imaging, separation, heterogeneous catalysis, adsorption
of pollutants and so forth.2 Chemical stability, homogeneous
pore architecture, non-poison and recovery from colloidal
solutions are required for some industrial applications of
MSNs.3
Soft-templating technique is one of the best and easiest
methods to synthesize MSNs because there is little
aggregation, involving a well-defined pore structure, uniform
morphologies, and particle size control. From references,3a,3b
the polymorphic and polydispersed MSNs are produced under
kinetic control using dilution and acid quenching. It has been
found that the growth inhibitor additives, such as a triblock
copolymer (F127),3c triethanol amines (TEAH3),4 and functional
groups have synthesized MSNs using microemulsion media.7
However, these structures are either too large (>200 nm) for
application in life sciences or too complex to be produced at a
large scale.8 Therefore, the development of a facile method for
the production of MSNs with tunable porosity and particle size,
particularly <200 nm, is highly desirable.
It was previously reported that the nanospherical
mesoporous silica with an average size of 110 nm was
produced,9 but the extremely low TEOS concentration is
indispensible. This would lower the volume efficiency of this
technique, which depressed the scalable application of this
method. Recently, a great breakthrough has been made on
this issue, and the facile large-scale synthesis of MSNs with
sphere diameter less than 200 nm at
a high TEOS
concentration by using
a
templating sol-gel technique.8
However, the organic amines are required to be performed as
the mineralizing agent. The use of relatively high cost and
poison organic amines as mineralizing agent is contrary to the
sustainable chemistry, besides the high production cost, which
limits the large scale applications. It is highly attractive to
develop a facile and sustainable low-cost scalable method to
synthesize monodisperse MSNs with tunable particle size and
pore structure.
a. State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and
Engineering, School of Chemical Engineering, Dalian University of Technology,
Dalian 116024, P.R. China. E-mail: zkzhao@dlut.edu.cn; Fax:+86-411-84986354.
b. College of Material Chemistry and Chemical Engineering, Hangzhou Normal
University, Hangzhou 310036, P. R. China.
Herein,
a facile, low-cost and scalable approach is
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/cxraxxxxx. presented for synthesizing monodisperse MSNs with tuneable
This journal is © The Royal Society of Chemistry 20xx
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a
templating vacuum environment with heating at 128 oC. Then the
particle size and pore structure through
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o
hydrothermal technique using CTAB as the templating impregnated samples were dried at 10D5OIC: 10in.10a39ir/Co5vReAr2n64ig3h2Dt,
surfactant and urea as the mineralizing agent from the followed by calcinations in air at 300 oC for 3 hours, and the
Advanced catalytic Materials Research Group, DLUT. The PTA/MSN-x catalysts with a loading of 25 wt % were obtained.
particle size and pore structure can be adjusted by changing
the urea concentration. The application properties of the
Characterization of Samples
X-ray diffraction (XRD) profiles were collected from 10 to 80°
at a step width of 0.02° using Rigaku Automatic X-ray
Diffractometer (D/Max 2400) equipped with a CuKa source (λ=
1.5406 Å). Nitrogen adsorption and desorption isotherms were
determined on a Micromeritics apparatus of a model ASAP-
2050 system at -196 oC. The specific surface areas were
calculated by the BET method, and the pore size distributions
were calculated from an adsorption branch of the isotherm by
the BJH model. Transmission electron microscopy (TEM)
MSNs were measured in heterogeneous catalysis. The as-
synthesized MSNs with appropriate pore structure and particle
size could be employed as an excellent support to prepare
advanced solid acid catalysts. The catalytic stability is still a
bottleneck problem for the industrial application of solid acid
catalysts, the developed supported PTA catalyst on MSN
(PTA/MSN) demonstrates much superior catalytic activity and
stability for the alkenylation of p-xylene with phenylacetylene,
a model reaction for clean and atomic economic synthesis of
alpha-arylstyrenes,10 as well as outstanding catalytic efficieny
for diverse reactions like esterification, alkylation, and
benzylation. The improvement of coke-resistent stability
through shortening the length of mesopores is sapiential
strategy to solve the bottleneck problems of solid acid
catalysts for their industrial applications. Moreover, this work
presents a practical technique for synthesizing monodisperse
MSNs with less than 200 nm of particle size and the tuneable
pore structure, which can act as promising candidate for drug
delivery, cell imaging, separation, heterogeneous catalysis,
adsorption of pollutants and so forth.
images were obtained by using
a Tecnai F30 HRTEM
instrument (FEI Corp.) at an acceleration voltage of 300 kV. FT-
IR spectroscopy characterization of catalysts was performed at
150 oC under ultrahigh vacuum using a Bruker EQUINOX55
infrared spectrometer. NH3-TPD measurements were
performed to characterize the acidity of the samples. After
pretreatment of 50 mg samples in Ar (up to 300 oC with a ramp
rate of 10 oC min-1, and then kept for 0.5 h under 30 ml min-1
Ar flow), the samples were saturated with ammonia (10%
NH3–90% Ar) at 10 oC via the pulse injection of ammonia in an
Ar stream. The desorptions steps were carried out from 100
and 700 oC at a heating rate of 10oC min-1 and with an Ar flow
of 30 ml min-1. The NH3-TPD profiles were obtained via
monitoring the desorbed ammonia with a thermal conductivity
detector.
Experimental section
Synthesis of MSNs
Catalytic Performance Measurement
The synthesis was performed under mild conditions with a
typical composition of 1.0TEOS: 0.064CTAB: xurea: 300 H2O at
a molar basis, where x (urea/TEOS) varied from 0.01 to 8.0. In
a typical synthesis of MSN, 0.816 g cetyltrimethylammonium
Bromide (CTAB), the required amount of urea based on x
values were dissolved in 200 g H2O and stirred for 1 hour, and
then 7.3 g of tetraethylorthosilicate (TEOS) was added
dropwise into the above solution. The mixture was stirred at
The experiments on the alkenylation of aromatics with
phenylacetylene were performed in a stainless steel fixed-bed
continuous-flow reactor. A sample of 0.8 g PTA/MSN catalyst
with 20-60 mesh was loaded into the reactor for all the
reaction tests, and the remaining space of the reactor tube
was filled with 20-60 mesh quartz granules. Before the
introduction of feedstock, the catalyst was preactivated for 1 h
online in N2 flow. The liquid stream was introduced into the
fixed-bed reactor by a syringe pump. N2 (99.999% purity) was
used to maintain system pressure. In all cases, a time on
stream of 8 h was used. The PTA/MCM-41 and PTA/NSN were
aslo measured for comparison. Quantitative analysis of the
collected products was performed on a FULI 9790 II GC
equipped with an HP-5 column, 30 m × 0.32 mm×0.25μm, and
an FID detector. The phenylacetylene conversion was
calculated by weight percent of the consumed
phenylacetylene in the total phenylacetylene amount in the
feed; the selectivity to alpha-arylstyrene was calculated by
weight percent of desired alpha-arylstyrene in total products.
The reaction rate was calculated on the basis of molar amount
transformed reactants per mole of acidic sites per hour or
based on the transformed reactants per gram catalyst per hour.
o
80 C for 2 hours. After that the milky liquid was injected into
o
Teflon-lined autoclave and heated at 100 C for 20-60 hours.
The obtained solid product was recovered, washed, and then
dried at 105 oC for 20 h. The final MSNs were obtained by
o
calcining the above samples at 550 C for 6 h, and labelled as
MSN-0.01, MSN-0.025, MSN-0.1, MSN-2.5, and MSN-8,
respectively, based on the diverse x values for the mother
liquor composition.
Synthesis of PTA/MSN-x solid acid catalysts
According to our previously reported vacuum assisted wet
impregnation with heating method (IMPVH), the supported
PTA catalysts on MSN-x were synthesized. Typically, the as-
synthesized MSN-x power was sieved into 20-60 mesh
particles after tableting. The PTA was supported on MSN-x
supports via IMPVH method. The method exploited in our case
was carried out by impregnating MSN-x (1.0 g) with an
aqueous solution of PTA (the concentrations is 0.36 g ml-1) in a
Results and discussion
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Synthesis and Characterization of Particle Size and Pore Structure
Tunable MSNs
molar ratio of urea to TEOS, and average size increased from
37 to 130 nm as the x value rose from 0.01 to 0.1. The lower
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DOI: 10.1039/C5RA26432D
molar ratio of urea to TEOS (lower PH) results in smaller
particle size, and even the MSNs with less than 100 nm of
particle size can also be achived while the 0.01-0.1 of molar
ratio of urea to TEOS is used. The higher urea concentration
can produce more OH-, which enhances the hydrolysis rate of
TEOS. As a consequence, the more self-assembled silicate
micelles quickly form, and the MSNs particles with larger size
can be resulted, which is in consistent with the reported
results.8,11 More interestingly, the difference in urea
concentration can change the pore channel morphology. From
Fig. 1, the raspberry-like pores can be formed while the 0.01
of molar ratio of urea to TEOS was used; whereas the worm-
like pore channels can be formed while the ratio was not less
than 0.025. Scheme 1 demonstrates the schematic illustration
for the formation of the diverse pore channels led by the
change of molar ratio of urea to TEOS. From references,8,12 the
spherical mesoporous silica is formed through two steps: one
is the fomation of silicated mecelles, the other is the
aggregation of the as-formed silicated micelles. Using lower
urea concentration, the Br- counterions of CTAB surfactant
micelles are slowly substituted by the formed silicate.
Subsequently, the MSNs with spherical (raspberry-like) pore
channels is synthesized through the aggregation of the
The synthesis of MSMs was performed with a typical mother
liquor composition of 1.0SiO2:0.06CTAB:xUrea:300H2O at a
molar basis, where x (molar ratio of urea to TEOS) vaired from
0.01 to 8. Typically, 0.816 g of CTAB and the required amount
of urea based on x value was dissolved into 200 ml deioned
water, and then the 7.3 g of TEOS was added dropwise into the
above solution with stirring. The resulting mixture was
o
transferred into Teflon-lined autoclave and heated at 105 C
for 48 hours. The obtained solid product was recovered,
washed, and then dried at 105 oC for 20 h. The final MSNs
were obtianed by calcining the above samples at 550 oC for 6 h,
and labelled as MSN-0.01, MSN-0.025, MSN-0.1, MSN-2.5, and
MSN-8, respectively, based on the diverse x values for the
mother liquor composition. The detailed synthesis process can
be seen in the experimental section.
Fig. 1 presents the typical TEM images of the as-
synthesized MSN-x, and their particle size distribution is
depicted in Fig. S1 and Table 1. Fig. 1 clearly demonstrates
that the monodisperse spherical mesoporous silica particles
have successfully synthesized by the developed templating
hydrothermal method using CTAB as templating surfactant and
urea as mineralizing agent. From Fig. S1, 1, and Table 1, it can
be found that the particle size is strong dependent on the
spherical silicated micelles. Such
a rapberry-like pore
morphology was previously reported only in polymorphic and
polydispersed systems obtained by using both dilution and PH
quenching. In this work, the use of the urea as mineralizing
agent and the low urea concentration might provide thus
reaction environment, and therefore the MSNs with raspberry-
like pore morphology have been successfully synthesized. If
the urea concentration is increased to a certain value, i.e.
0.025 of molar ratio of urea to TEOS (the total amount of
water is fixed, the larger ratio means the higher urea
concentration), the large amount of silicate is quickly formed,
which can wrap the several surfactant micelles and then the
worm-like silicated micelles are formed. Then worm-like
silicated micelles-containing aggregates are produced. After
the subsequent calcination process, the MSNs with worm-like
mesopores were synthesized.
Nitrogen adsorption-desorption isotherms of the as-
synthesized MSNs are presented in Fig. 2a. All of the
isotherms are of type IV according to IUPAC classification and
exhibit H1 hysteresis with a featured capillary condensation in
the mesopores, indicating the presence of mesopores.13 It’s
interesting that the isotherms present two clear capillary
condensation steps: one is at 0.15-0.50 of a lower relative
pressure (p/p0) and the other is at 0.8-1.0 of a higher relative
pressure region, implying the pore feature of MSNs with
bimodal mesopores.14 The bimodal “mesopores”are also
confirmed by the BJH adsorption pore szie distribution. As can
be seen from Fig. 2b, the as-synthesized MSNs contain two
types of pores with small mesopores (the center of the pore
distribution from the adsorption branch is 2.8 nm, also can be
clearly observed in the TEM images shown in Fig. 1) and large
Fig. 1. TEM images of the as-synthesized MSN-x with diverse molar ratios of
urea (mineralizing agent) to TEOS (silica source). a) x=0.01, b) x=0.025, c) x=0.1, d)
x=2.5, and e) x=8. The insets in the Fig. 1a and 1b are the magnified regions.
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Scheme 1. Schematic illustration for the formation of monodisperse mesoporous silica nanospheres (MSN) with diverse pore structures depending on the molar ratio
of urea to TEOS.
mesopores (25-250 nm, based on IUPAC classification, the
latter may be assigned to larger mesopores or large pores).
The smaller mesopores are thought to be generated by the
removal of templating surfactant, whereas the larger
mespores or larger pore channels can be originated from the
accumulation of MSNs. The Brunauer-Emmett-Teller surface
area (SBET), Barrett-Joyner-Halenda pore volume (VBJH),
micropore surface area by T-plot method (Smic), micropore
pore volume by T-plot method (Vmic), and the BJH pore
diameters (DBJH) from the adsorption branch of isotherms the
series of MSNs (MSN-x, x=0.01, 0.025, 0.1, 2.5, 8) are
summarized in Table 1. The specific surface area of MSNs
increases with the increase in urea concentration. However,
the micropores appear while the molar ratio of urea to TEOS is
not less than 0.1, which can be confirmed by the sharp
increased adsorbed amount with the increase in relative
pressure (p/p0) at lower p/p0 region shown in Fig. 2a. But it is
concluded that much more micropore come into reality as the
concentration of urea increase. That may because under
alkaline conditions (pH > 7.0), silicates with a high negatively-
charged density can only assemble with the cationic
surfactants through strong electrostatic interaction. The fast
silica condensation and strong electrostatic interactions
between silica and cationic surfactants induce the fast
simultaneous assembling-growth of the silica-surfactant nuclei,
which also leads to the increase in particle size of MSN-x with
the increasing x value.11 By deducting the micropore surface
area, no obvious change in the left surface area can be
observed as the x value increases. As the x value increases, no
large change in BJH pore diameters generated from the
removal of templating surfactant take places (it increases from
Fig. 2. N2 adsorption-desorption isotherms (a) and the Barrett-Joyner-Halenda
(BJH) pore size distribution from adsorption branch of the as-synthesized
mesoporous silica nanosphere MSN-x with diverse x values.
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Table 1 Textural properties of the as-prepared MSN-x samples.
a
b
c
d
e
Sample
SBET
VBJH
Smic
Vmic
DBJH
PSD
[nm]f)
37±11
53±9
63±8
113±4
130±14
[m2 g-1] [cm3 g-1] [m2 g-1] [cm3 g-1]
[nm]
0
0
0.02
0.13
0.21
MSN-0.01
MSN-0.025
MSN-0.1
MSN-2.5
MSN-8
98
104
125
273
395
0.68
0.75
0.75
0.54
0.49
0
0
24
173
274
2.8/35
2.8/45
2.8/50
2.9/70
3.01/--
a denoted as the specific surface area. b denoted as the BJH pore volume from the
c
d
adsorption branch. denoted as micropore surface area by T-plot method.
denoted as micropore volume by t-plot method. e Denoted as is the center of the
pore distribution from the adsorption branch.
f
Denoted as particle size
distribution, obtained from TEM images shown in Fig. 1.
2.8 to 3.0 nm as the x rises from 0.01 to 8), whereas the pore
diameter of the accumulated pore increases as the x value
increases from 0.01 to 2.5. No accumulated pore can be
observed as the 8 of molar ratio of urea to TEOS can be used.
Moreover, the increase in urea concentration leads to a visible
decrease in BJH pore volume. The textural features can affect
the acidic properties of the supported PTA catalysts on MSN-x,
besides affect the mass transfer for solid acid catalyzed
transformations. As a consequence, they can affect the
catalytic properties of PTA/MSN-x.
Catalytic Performance Measurement of the PTA/MSN-x Solid Acid
Catalyst
Based on the above work, a series of MSNs with diverse
particle sizes and pore structures have been successfully
synthesized. In the synthesis process, the low-cost and non-
poison urea was employed as mineralizing agent to replace the
previously reported high-cost and poison organic amines like
such as triethanolamine, 2-amino-2-(hydroxymethyl) propane-
1,3-diol, and triethyleneamine. Moreover, from previous
reports, the MSNs with less than 200 nm of particle size could
be a promising candidate for diverse applications including
heterogeneous catalysis, medical, and biological fields.
Therefore, this novel technique developed in this work has
more practical applications in industry.
The supported PTA solid acid catalysts on mesoporous
supports have attracted great attention in diverse
transformations owing their excellent catalytic performance.15
Herein, we evaluated the application properties of MSNs in
heterogeneous catalysis by employing them as supports to
prepare the supported PTA solid acid catalysts for diverse
Fig. 3. TEM images of the supported PTA catalysts on the as-synthesized MSN-x
silica nanospheres with different x values. a) PTA/MSN-0.01, b) PTA/MSN-0.025,
c) PTA/MSN-0.1, d) PTA/MSN-2.5, e) PTA/MSN-8. The image of f) MSN-2.5
support is included for comparison.
Table 2 Textural properties of the supported PTA catalysts on the diverse
MSN-x supports.
a
b
c
d
e
SBET
VBJH
Smic
Vmic
DBJH
Acidity
(mmol g-1)
0.37
0.25
0.24
Sample
[m2 g-1] [cm3 g-1] [m2 g-1] [cm3 g-1] [nm]
0
0
0
PTA/MSN-0.01
PTA/MSN-0.025
PTA/MSN-0.1
PTA/MSN-2.5
PTA/MSN-8
124
91
0.79
0.72
0.50
0.40
0.29
0
0
0
2.8/35
2.8/70
2.8/70
2.8/90
2.8/--
58
98
41
118
0.03
0.08
0.22
0.21
179
a denoted as the specific surface area. b denoted as the BJH pore volume from the
c
d
adsorption branch. denoted as micropore surface area by T-plot method.
denoted as micropore volume by T-plot method. e Denoted as is the center of the
pore distribution from the adsorption branch.
f
Denoted as particle size
distribution, obtained from TEM images shown in Fig. 3.
transformations
including
alkenylation,
esterification,
alkylation, and benzylation reactions. Fig. 3 presents the
typical TEM images of the supported PTA catalysts on diverse
MSN-x (x=0.01, 0.025, 0.1, 2.5, 8). From Fig. 3, the similar
morphology feature of the supported PTA catalysts on MSN to
that of the supports shown in Fig. 1 can be observed, except
for the existence of black dots assigned to PTA aggregate
within the MSNs shown in Fig. 3d-f. Moreover, although the
surface area of the MSNs supports increases with the
increasing x value, the PTA dispersity decreases, and even the
PTA aggregate can be clearly seen in the TEM images of the
supported catalysts on MSN-2.5 and MSN-8 (Fig. 3d and e).
The decreasing PTA dispersity can be ascribed to the decrease
in BJH pore volumes of mesopores in the MSNs. Higher PTA
dispersity would result in more acidic sites (Fig. S2 and Table
2).
The textural features of the PTA/MSN-x catalysts were
characterized by N2 adsorption-desorption analysis. Fig. S3
presents the N2 adsorption-desorption isotherms of the
PTA/MSN-x catalysts, and the textural features are
summarized in Table 2. From Fig. S3 and Table 2, the
PTA/MSN-x catalysts have the similar isotherms and pore size
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distributions. By comparing the textures of MSN supports mesopores of PTA/MSN-0.025 catalyst confirmed by TEM
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(Table 1) and the PTA/MSN-x catalysts (Table 2), it can be images and N2 adsorption analysis aDreOI:n1o0.t103w9e/Cll5-RoAr2d6e4r3e2dD.
found that, except for the PTA/MSN-0.01 catalyst, the However, the disordered feature of the mesopores doesn’t
decreasing surface area and BJH pore volume can be observed. depress the catalytic performance of PTA/MSN-0.025, which
More interestingly, the decreases in surface area (SBET and Smic
)
can be partially confirmed by the comparison of its catalytic
and pore volume (VBJH and Vmic) towards the samples with 0.1 performance with that of PTA/MCM-41 catalyst presented in
to 8 of x values are quite dramatic. Combining the visible PTA Table 3. Under the same reaction conditions, the developed
aggregate on the supported PTA catalysts on MSNs with larger PTA/MSN-0.025 exhibits superior catalytic performance to
x value, it can be proposed that the above decrease in surface PTA/MCM-41 catalyst. 99.4% and 83.7% conversion of
area and pore volume can be mainly ascribed to the block of phenylacetylene can be obtained over the PTA/MSN-0.025 and
pore channels, especially the block of micropores on the PTA/MCM-41, respectively. Furthermore, now that PTA/MSN-
samples with 0.1-8 of x values. Moreover, towards the MSN- 0.025 only has disordered mesopores, we also have a doubt
0.01 support and PTA/MSN-0.01 catalyst, the surface area and that the small size of PTA/MSN-0.025 may be the reason for it
pore volume don’t decrease but increase, might be ascribed to exhibiting the outstanding catalytic performance, although this
the formed pores from the dispersed PTA on the surface of the issue has been primarily confirmed by the instance that
accumulated mesopores (35 nm), whereas the accumulated PTA/MSN-0.025 has a larger particle size but exhibits higher
pores on the samples with 0.1-8 of x values are too large catalytic activity in comparison with PTA/MSN-0.01. We
(larger than 50 nm) to form the mesopores. Moreover, From compared the catalytic performance of PTA/MSN-0.025
Fig. 2b, Fig. S3b, Table 1, and Table 2, the pore volume catalyst with supported PTA catalyst on the smaller silica
corresponding to accumulated pores after supporting PTA on nanospheres without mesopores (PTA/NSN). According to
MSNs supports, might be resulted from the block of PTA. From reference,16 the monodisperse non-porous silica nanospheres
Table 1 and 2, the center of accumulated pores of PTA/MSNs is with less than 15 nm of particle size (NSN) were synthesized.
larger than that of MSNs, owing to the change of accumulate From Table 4, the NSN nanoparticle has much larger surface
feature in the supporting PTA process.
area than MSN (260 and 104 m2 g-1 for NSN and MSN,
The catalytic performance of PTA/MSN-x with diverse respectively). The former doesn’t have mesopores, and the
molar ratio of urea to TEOS for alkenylation of p-xylene with much larger surface area of NSN than MSN is owing to the
phenylacetylene, as a model reaction, was measured. The smaller silica particle size of NSN. From Table 3, PTA/NSN
results are demonstrated in Fig. S4. The conversion of exhibited much lower catalytic activity than PTA/MSN-0.025,
phenylacetylene decreases with the increasing x from 0.01 to although PTA/NSN has much higher surface area (176 m2 g-1) in
0.025, and reaches the maximum over the PTA/MSN-0.025. comparison with PTA/MSN-0.025 (91 m2 g-1). This presents an
The decreasing conversion can be observed as the x value is instance that the existence of the worm-like mesopores in the
further increased from 0.025 to 8. No visible change in MSN-0.025, although they are disordered mesopores,
selectivity towards alpha-arylstyrene can be observed as the x significantly favors the catalytic reaction. Furthermore, the
value rises from 0.01 to 8 (Fig. S4). The decreasing catalytic acidic properties of the supported PTA catalysts on MSN-0.025,
activity with the increase in x value from 0.025 to 8 can be MCM-41, and NSN (with the same PTA loading) were
ascribed to the increase in length of mesopores led by the measured by NH3-TPD (Fig. 4 and Table 4). The much higher
larger particle size, besides the lowering acidic sites shown in surface area and larger pore volume of MCM-41 allows the
Fig. S2 and Table 2. More interestingly, the PTA/MSN-0.01 PTA/MCM-41 catalyst to have 0.52 mmol g-1 of highest acidic
demonstrates lower catalytic activity in comparison with concentration owing to the improved PTA dispersity confirmed
PTA/MSN-0.025, although the former has more acidic sites and by XRD analysis (Fig. S6), whereas the PTA/NSN has lowest
smaller particle size than the latter. This may be ascribed to acidic concentration owing to its non-porous feature (0.20
the worse mass transfer over the PTA/MSN-0.01 catalyst than mmol g-1). The developed PTA/MSN-0.025 demonstrates much
that over PTA/MSN-0.025 led by different pore channels high TOF (8.9 h-1) for alkenylation reaction than the other two
morphologies. The raspberry-like mesopores in PTA/MSN-0.01 catalysts (3.6 and 5.6 h-1 over PTA/MCM-41 and PTA/NSN,
but worm-like ones in PTA/MSN-0.025 have been confirmed in
Fig. 1 and 3. Therefore, we can safely say that the catalytic
performance of the supported PTA catalysts on MSN-x
supports is strongly dependent on the pore morphology,
besides dependent on the acidic properties. It can be also
concluded that the worm-like mesopores favor the catalytic
reaction owing to more favorable mass transfer in comparison
with the paspberry-like ones. The supported PTA catalyst on
MSN-0.025 support with worm-like pore feature and smaller
particle size exhibits the best catalytic performance among all
of the PTA/MSN-x (x=0.01, 0.025, 0.1, 2.5, 8) catalysts.
Table 3 Reaction result of the alkenylation of p-xylene with phenylacetylene over the
supported PTA catalysts on the developed MSN-0.025, as well as on the traditional
mesoporous silica MCM-41 and the non-porous silica nanosphere NSN.a
Product distribution (%)
Catalyst
Con. (%)
α-ASb
91.8
95.1
92.6
APc
1.8
1.0
1.8
HPd ISe OLf
0.1 2.5 3.8
0.1 0.8 3.0
0.1 4.0 1.5
PTA/MCM-41
PTA/MSN-0.025
PTA/NSN
83.7
99.4
49.7
a Reaction conditions: catalyst 0.8 g, np-xy/Phen 25:1, Tr 150 oC, Ps 1.0 MPa, VHSV 7.2
d
ml h-1 g-1 cat; TOS 8 h. b denoted as α-arylstyrenes. c denoted as acetophenone.
denoted as hydrogenation products. denoted as Isomers.
e
f
denoted as
Moreover, no clear peak on the low-angle XRD pattern of
PTA/MSN-0.025 (Fig. S5) can be well-resolved, suggesting the
oligomers.
6 | J. Name., 2012, 00, 1-3
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RSC Advances
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Journal Name
ARTICLE
over PTA/MSN-0.025 can efficiently improve the catalytic
stability. It is generally accepted that the depressed catalytic
View Article Online
DOI: 10.1039/C5RA26432D
Table 4 Textural feature, average crystalline size of PTA, acidic properties,
and turn of frequency (TOF) of the developed PTA/MSN-0.025 catalyst as
well as PTA/MCM-41 and PTA/NSN catalysts with same PTA loading are
included for comparison.
stability is the bottleneck problem for the application of solid
acid catalysts in industry. The deactivation of solid acid catalyst
mainly results from the coke deposition led by acid catalyzed
polymerization. The too strong acidic sites and the long staying
time of reactants or products can enhance this coke deposition
process.17 Especially for the direct alkenylation of aromatics
with alkyne for producing alpha-arylstyrenes, the coke issue of
solid acid catalyst is particularly acute owing to the tendency
to polymerization of both reactant (alkyne) and the formed
product (alpha-arylstyrenes).10,18 If the formed alpha-
arylstyrenes can’t be timely removed through diffusion, the
solid acid catalyst would suffer from serious deactivation by
coke deposition. Besides the development of efficient
methods such as alkaline earth and rare earth modification to
eliminate the too strong acidic sites,19 the intensification of
mass transfer would be a sapiential strategy. It was previously
reported from our ACM research group that,10 the
mesoporous solid acid demonstrated much better coke-
resistent stability in comparison with zeolite owing to
facilitated mass transfer. Although the enlargement of pore
diameter from micropore to mesopore efficiently improves the
stability of solid acid catalysts,10,20 this kind of improvement is
of a limited strategy owing to the poor hydrothermal stability
and lower surface area led by too larger mesopore or the
macropore.21 Especially, towards the H2O-including reaction
systems, like esterification, dehydration, hydration, etc., the
hydrothermal stability is also important issue. Therefore, the
development of new strategy to enhance the diffusion is highly
desirable.
The shortening of length of mesopore in the solid acid
catalysts would be a sapiential strategy.22 This strategy was
examined by employing the alkenylation of p-xylene with
phenylacetylene as a model reaction. From Fig. 5, the
PTA/MSN-0.025 catalyst demonstrates much higher activity
and stability along with time on stream. Scheme 2 illustrates
the schematic for this enhancing effect. Owing to the stronger
acidic sites of PTA/MSN-0.025 catalyst in comparison with
PTA/MCM-41, the improvement in the stability by the
suppressed coke formation doesn’t come from a decrease in
strong acidic sites. Owing to only 53 nm of average particle size,
the worm-like mesopores in the MSN-0.025 is much shorter
than the hexagonal mesopores in MCM-41 with several
micrometers of particle size. The formed alpha-arylstyrenes
could quickly leave from the solid acid catalysts before they
would suffer from ploymerization process. As a result, the
PTA/MSN-0.025 solid acid catalyst demonstrates superior
coke-resistent stability in comparison with PTA/MCM-41. The
presented approach by shorting length of mesopore of the
mesoporous solid acid catalysts can be considered as a
sapiential strategy for improving coke-resistent stability of
solid acid catalysts. Moreover, the developed PTA/MSN-0.025
could be a promising solid acid catalyst for the production of
alpha-arylstyrenes through direct alkenylation of diverse
aromatics with alkynes.
CSa
Acidity
SBET
Vtotal
DBJH
TOFb
Sample
(nm) (mmol g-
(m2 g-1) (cm3 g-1) (nm)
(h-1)
1
)
PTA/MSN-0.025
MSN-0.025
PTA/MCM-41
MCM-41
91
104
519
1054
176
260
0.7
0.7
0.6
1.1
--
2.7
2.7
2.9
2.9
--
16.7
--
15.8
--
--
--
0.25
--
0.52
--
0.20
--
8.9
--
3.6
--
5.6
--
PTA/NSN
NSN
--
--
a
Average crystalline size of PTA supported on diverse supports including the
developed mesoporous silica nanosphere (MSN-0.025) and the traditional
mesoporous silica (MCM-41) as well as the non-precious silica nanosphere (NSN).
b
Denoted as the mmol product per mmol of acidic sites. Reaction conditions:
catalyst 0.8 g, np-xy/Phen 25:1, Tr 150 oC, Ps 1.0 MPa, VHSV 7.2 ml h-1 g-1 cat; TOS 8 h.
respectively), although it has lowest surface area. This can be
ascribed to their unique pore features. Although both
PTA/MSN-0.025 and PTA/NSN have accumulated pores, the
former has mesopores in nanospheres but the latter has no
mesopores in silica nanosphere. This result presents a clear
instance that the presence of disordered worm-like mesopores
in PTA/MSN-0.025 plays an important role in catalyzing the
alkenylation reaction. In comparison with PTA/MCM-41, the
PTA/MSN-0.025 shows 2.5 times of much higher TOF for
alkenylation, ascribed to their different mesopore features.
Now that MCM-41 generally has several micrometers of
particle size, the length of mesopores in MSN-0.025 is much
shorter than the length of hexagonal ones in MCM-41. This
could efficiently facilitate mass transfer by shortening diffusion
distance in the alkenylation process over the PTA/MSN-0.025
solid acid catalyst in comparison with PTA/MCM-41. In
comparison with the reported results, the developed
PTA/MSN-0.025 catalyst demonstrates much better catalytic
performance.10,18
Besides the improved activity over PTA/MSN-0.025 in
comparison with PTA/MCM-41, the facilitating mass transfer
Fig. 4. NH3-TPD profiles of the developed 25 wt.%PTA/MSN-0.025 catalyst, and
the supported PTA caatlysts on MCM-41 and NMSN are included for comparison.
Furthermore, besides for above alkenylation, the catalytic
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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ARTICLE
Journal Name
considered as a promising solid acid catalystViefworArtidcleivOenrlisnee
DOI: 10.1039/C5RA26432D
transformations.
Conclusions
In summary, In conclusion, this work has presented a facile,
low-cost, and scalable templating hydrothermal method for
synthesizing monodisperse mesoporous silica nanospheres
with tunable particle size and pore structure using CTAB as
templating surfactant and urea as mineralizing agent. The
particle size and pore structure are strongly dependent on the
molar ratio of urea to TEOS. This novel technique has practical
applications in industry. By employing MSNs as support, the
advanced solid acid catalyst can be synthesized. Besides higher
activity, the supported PTA catalyst on MSN-0.025 presents
much superior coke-resistent catalytic stability for alkenylation
of aromatic with alkyne, a model reaction, in comparison with
PTA/MCM-41. This shows that the shortening length of
mesopores of solid acid catalysts can be considered as a
sapiential strategy for improving catalytic stability through
suppressing coke deposition, which presents a practical
approach for solving the bottleneck problems of solid acid
catalysts. The PTA/MSN-0.025 solid acid catalyst demonstrates
outstanding catalytic performance for diverse transformations.
Fig. 5. A comparison between PTA/MSN-0.025 and PTA/MCM-41 on the
catalytic stability for the alkenylation of p-xylene with phenylacetylene. Reaction
conditions: catalyst 0.8 g; np-xy/Phen 25:1; Tr 150 oC; Ps 1.0 MPa; VHSV 7.2 ml h-1 g-
1cat; TOS 8 h.
Acknowledgments
This work is financially supported by the National Natural
Science Foundation of China (grant no. 21276041, U1261104
and 20803006), also by the Chinese Ministry of Education via
the Program for New Century Excellent Talents in Universities
(grant no. NCET-12-0079) and the Natural Science Foundation
of Liaoning Province (grant no. 2015020200), and the
Fundamental Research Funds for the Central Universities (grant
no. DUT15LK41), and the Science and Technology
Development Program of Hangzhou (20130533B14).
Scheme 2. Schematic for improved catalytic performance in alkenylation of p-
xylene and phenylacetylene over the PTA/MSN in comparison with traditional
PTA/MCM-41 through intensifying mass transfer.
Table 5 Reaction rate for the model reactions over the diverse solid acid
catalysts.
Reaction rate (mmol h-1 g-1cat)
Catalyst
Esterificationa
Alkylationb
20.5
Benzylationc
Notes and references
PTA/MSN-0.025
PTA/MCM-41
HY
Amberlyst-15
Blank
19.9
16.4
8.2
2.8
0.8
11.0
8.9
4.3
2.1
0.2
State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry
and Engineering, School of Chemical Engineering, Dalian University of
Technology, Dalian 116024, P.R. China. E-mail: zkzhao@dlut.edu.cn;
Fax:+86-411-84986354
† Electronic Supplementary Information (ESI) available: Particle size
distribution of MSN-x NH3-TPD profiles, extra N2 physisorption results, low-
angle XRD patterns, XRD, and extra reaction results. See
DOI: 10.1039/c0ra00000x/
17.3
10.5
2.4
0.3
a
Reaction condition: acetic acid 128 mmol, ethanol 1mol, catalyst 50 mg,
Tr=70 oC, t=5 h. b Reaction condition: styrene 128 mmol, benzene 500 mmol，
catalyst 50 mg, Tr=70 oC, t=5 h. c Reaction condition: benzyl alcohol 50 mmol，
benzene 625 mmol, catalyst 200 mg, Tr=110 oC, t=5 h.
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DOI: 10.1039/C5RA26432D
Facile, low-cost, and scalable fabrication of particle size and pore structure tuneable monodisperse
mesoporous silica nanospheres as supports for advanced solid acid catalysts†
Zhongkui Zhao*a, Xianhui Wanga, Yanhua Jiaob, Boyuan Miaoa, Xinwen Guoa and Guiru Wanga
a State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, School
of Chemical Engineering, Dalian University of Technology, Dalian 116024, P.R. China. E-mail:
zkzhao@dlut.edu.cn; Fax:+86-411-84986354.
b College of Material Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou
310036, P. R. China.
This work presents a facile, low-cost, and scalable strategy for fabricating monodisperse mesoporous silica
nanospheres (MSN) with tuneable particle size and pore structure. The high volume efficiency by using high
TEOS concentration as Si sources and the low production cost by using urea as mineralizing agent allow it
to have great potential for industrial production. Using MSN as a promising support, the advanced solid
acid catalysts with much superior catalytic activity and stability were prepared, presenting an alternative
method for overcoming low stability issue, a bottleneck problem for the industrial application of solid acid
catalysts.