Investigation of pore-size effects on base catalysis using amino-functionalized monodispersed mesoporous silica spheres as a model catalyst

Article abstract of DOI:10.1016/j.jcat.2007.08.010

The effects of pore size on base catalysis have been studied using amino-functionalized monodispersed mesoporous silica spheres (NH₂-MMSS). By changing the surfactant that is used for the template and also the synthetic conditions, NH₂

Full text of DOI:10.1016/j.jcat.2007.08.010

Journal of Catalysis 251 (2007) 249–257
www.elsevier.com/locate/jcat
Investigation of pore-size effects on base catalysis using
amino-functionalized monodispersed mesoporous silica spheres
as a model catalyst
Tomiko M. Suzuki ∗, Masami Yamamoto, Keiko Fukumoto, Yusuke Akimoto, Kazuhisa Yano ∗
Toyota Central Research & Development Labs. Inc., Nagakute, Aichi 480-1192, Japan
Received 11 May 2007; revised 10 August 2007; accepted 11 August 2007
Abstract
The effects of pore size on base catalysis have been studied using amino-functionalized monodispersed mesoporous silica spheres (NH -
2
MMSS). By changing the surfactant that is used for the template and also the synthetic conditions, NH -MMSS with similar particle diameters
2
(560–600 nm) and different pore sizes (0–2.66 nm) were successfully prepared. Because the NH -MMSS that were obtained had the same shapes,
2
particle diameters and mesopore alignments, any differences in catalytic activity could be substantially attributed to differences in their pore sizes.
We employed the nitroaldol condensation between benzaldehyde derivatives and nitromethane as a base catalysis reaction. It was found for the first
time that the reaction mostly proceeds inside the mesopores (the effectiveness factor: 0.63) and that the optimum pore size for the NH -MMSS
2
was affected by changing the type and number of the substituent groups on the reactants. In addition, NH -MMSS was found to be an excellent
2
catalyst due to the radial alignment of the mesopores compared to the other types of mesoporous silica.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Mesoporous silica; Monodispersed spheres; Amino-functionalized; Pore size; Base catalyst
1. Introduction
of various MCM-41 with different pore sizes for the acetal-
ization of cyclohexanone with methanol. They showed that the
catalytic activity was strongly dependent on the pore size of the
catalysts, and was maximized when the pore size was around
1.9 nm. Because the diameters of cyclohexanone and the prod-
uct were 0.75 and 0.96 nm, respectively, the phenomenon could
not be explained by the concept of shape-selective catalysis.
They also reported that the catalytic activity was not dependent
on the molecular size of the reactant, but on the pore size of the
catalyst [2].
Tanchoux et al. demonstrated that a sharp increase in cat-
alytic activity for the double bond isomerization of 1-hexene
was observed for MCM-41 with a pore size of 3.4 nm. They
reported that this result was caused by a compensation effect be-
tween diffusion limitations and confinement activity enhance-
ment [3]. It became clear that MCM-41 had the optimum pore
size in acid catalytic reactions.
In recent years, various ordered mesoporous materials have
been synthesized by using surfactants as templates. Meso-
porous silica materials with uniform pore sizes, high surface
areas and high concentrations of surface hydroxyl groups are of
considerable research interest in fields such as catalysis, adsorp-
tion, separation, ion exchange and sensor design. In particular,
their application as heterogeneous catalysts is of great interest
from the view point of “green” chemistry. In general, hetero-
geneous catalysts have advantages in terms of separation and
regeneration from reaction solutions.
There have been several investigations into the effect of pore
size on catalytic activity using mesoporous silica with irregu-
lar shapes [1–3]. Iwamoto et al. studied the catalytic activity
*
Corresponding authors. Fax: +81 561 63 6507.
However, because the mesoporous silicas that have previ-
ously been used as catalysts have had irregular shapes and pore
E-mail addresses: tomiko@mosk.tytlabs.co.jp (T.M. Suzuki),
k-yano@mosk.tytlabs.co.jp (K. Yano).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.08.010

250
T.M. Suzuki et al. / Journal of Catalysis 251 (2007) 249–257
arrangements, it is quite difficult to accurately evaluate the ef-
fect of pore size on the catalytic activity without any influence
from other effects. In order to accurately study the effect of the
pore size of the mesoporous silica catalyst on catalytic activity,
it is essential to use porous materials that have the same shapes,
sizes and pore arrangements.
In our laboratory, we have succeeded in the synthesis of
hexagonally ordered and well-defined highly monodispersed
mesoporous silica spheres (hereafter abbreviated as “MMSS”)
from tetramethoxysilane (TMOS) and n-alkyltrimethylammo-
nium halide (CnTMAX, X = Cl, Br) [4–6]. Because these
MMSS have radially aligned mesopores [6], it is expected that
accessibility for reactants and the release of products would be
enhanced if they were used as catalysts.
Recently, aminopropyl-functionalized mesoporous silica
materials such as MCM, FSM, and SBA have been reported
to be effective as base catalysts for Knoevenagel condensa-
tions [7–10], aldol condensations [11–14], Michael additions
[10,11,15] and epoxidation reactions [16].
We synthesized MMSS that were particles with almost the
same diameters and different pore sizes. These were function-
alized as base catalysts by incorporating an aminopropyl group
using a grafting method. The purpose of this work is to under-
stand the “pure” effect of pore size on base catalysis when using
amino-functionalized MMSS as a model catalyst.
When a reaction is conducted with micro/meso-porous cat-
alysts, diffusion could govern the reaction because the sizes of
reactants and pores are relatively very close [17]. To date, there
have been many reports using mesoporous silica as a catalyst.
However, in many cases, it has been expected that only a part
of mesopores works as a catalyst due to diffusion limitation.
Therefore, it is very interesting to know the effectiveness fac-
tor of mesoporous silica as a catalyst. This can be only realized
by using MMSS with the same pore size and different particle
diameter as a catalyst.
We have synthesized five types of amino-functionalized
MMSS with almost the same particle diameters and different
pore sizes by changing the template surfactant and the syn-
thetic conditions, and their base catalytic performances were
evaluated. It was found that the optimum pore size changed de-
pending on the molecular structure of the reactant. We have also
conducted a reaction with amino-functionalized MMSS with
the same pore size and different particle diameters. The effec-
tiveness factor of spherical mesoporous silica was evaluated for
the first time.
The amino-moieties were incorporated by a grafting method,
in which organically functional groups were attached onto the
silica surface via a reaction between the hydroxyl groups on the
surface of the mesoporous silica and silane compounds.
3-Aminopropyltrimethoxysilane (APTMS), 4-hydroxybenz-
aldehyde, 4-butoxybenzaldehyde, 4-n-octoxybenzaldehyde,
4-methoxybenzaldehyde, 2,4,6-trimethoxybenzaldehyde and
3,4,5-trimethoxybenzaldehyde were purchased from Aldrich.
Decyltrimethylammonium bromide (C10TMABr), tetradecyl-
trimethylammonium chloride (C14TMACl), hexadecyltrimeth-
ylammonium chloride (C16TMACl), octadecyltrimethylammo-
nium chloride (C18TMACl), and tetramethoxysilane (TMOS)
were purchased from Tokyo Kasei. Dococyltrimethylammo-
nium chloride (C22TMACl) was purchased from Lion Akzo
Co., Ltd. 1 M sodium hydroxide solution, methanol, hy-
drochloric acid, dry toluene, nitromethane, tetrahydrofuran
(THF), acetone-d6 and decane were purchased from Wako
Inc. Monodispersed silica spheres were purchased from Nip-
pon Shokubai Co., Ltd. All of the chemicals were used as-re-
ceived.
The syntheses were carried out under basic conditions, as
follows.
For example, in the case of C16-MS, 7.04 g of C16TMACl
and 6.84 ml of 1 M sodium hydroxide solution were dis-
solved in 1600 g of a methanol/water (50/50 = w/w) solution
(methanol ratio: 0.5). 5.28 g (34.7 mmol) of TMOS was added
to the solution with vigorous stirring at 298 K. After the addi-
tion of the TMOS, the clear solution suddenly turned opaque
and resulted in a white precipitate. After 8 h of continuous stir-
ring, the mixture was aged overnight. The white powder was
then filtered and washed three times with distilled water, after
which it was dried at 318 K for 72 h. The powder that was ob-
tained was calcined in air at 823 K for 6 h to remove the organic
species.
In the case of pore-expanded MMSS (C18PS-MS), 2 g of
C18-MS before calcination was suspended in 120 g of a 0.1 M
C22TMACl water–ethanol solution. The mixture was sealed and
placed in an oven at 353 K for 7 days without stirring. The
resulting white powder was filtered and washed with distilled
water and then dried. The powder that was obtained was cal-
cined in air at 823 K for 6 h to remove the organic species.
Amino moiety grafting was performed on calcined and evac-
uated (323 K) samples of MMSS, on FSM [18] and on non-
porous monodispersed silica spheres (SS).
For example, in the case of C16-MS-AP, 0.7 g of calcined
C16-MS was added to a mixture of 70 g of dehydrated toluene
and 1.88 g of APTMS, and the solution was then refluxed at
363 K for 15 h. The product was dried overnight at 318 K after
filtration.
Hereafter in this paper, the samples that were obtained are
abbreviated as CxPS-MS, where x is 10, 14, 16 or 18. x de-
notes the number of carbon atoms of alkyl chain in the sur-
factant that was used as a templating agent, and PS indicates
that the pore size of the material was expanded by the sur-
factant exchange method [19]. Amino-functionalized materi-
als are denoted by attaching “AP” to the end of the sample
names.
2. Experimental
2.1. Chemicals and catalyst synthesis
We synthesized MMSS with the same particle diameters
and different pore sizes by using n-alkyltrimethylammonium
halide (CnTMAX: n = 10–18, X = Cl, Br) as a templating
agent and a tetramethoxysilane (TMOS) as a silica source [5,6].
The particle sizes of the MMSS were adjusted by changing the
methanol/water ratio in the solvent.

T.M. Suzuki et al. / Journal of Catalysis 251 (2007) 249–257
251
2.2. Sample characterization
Powder X-ray diffraction measurements were carried out
on a Rigaku Rint-2200 X-ray diffractometer using CuKα ra-
diation. Scanning electron micrographs (SEMs) were obtained
using a Sigma-V (Akashi Seisakusho). The surfaces of the sam-
ples were coated with gold before the measurements. The av-
erage particle diameter was calculated from the diameters of
50 particles in an SEM picture. The standard deviation was also
calculated, from which the distribution of the particle diameters
was derived. The nitrogen adsorption isotherms were measured
using a Belsorp-mini II (BEL Japan) at 77 K. The samples
were evacuated at 373 K under 10−3 mmHg before measure-
ment. The pore diameters were calculated using the Barrett–
Joyner–Halenda (BJH) method. By considering the linearity of
a Brunauer–Emmett–Teller (BET) plot, the specific surface ar-
eas were calculated using adsorption data in the P/P0 range
from 0.05 to 0.13. Transmission electron micrographs were ob-
tained using a Jeol-200CX TEM at an acceleration voltage of
200 kV. 29Si magic-angle-spinning (MAS) nuclear magnetic
resonance (NMR) and 13C cross-polarization (CP) NMR analy-
ses were carried out on a Bruker AVANCE 400 spectrometer at
79.49 MHz for 29Si and at 100.61 MHz for 13C. The 29Si MAS
NMR spectra were measured at 60 s repetition delay and 3 µs
pulse width. The 13C CP MAS NMR spectra were measured at
2 s repetition delay, 2 ms contact time, and 2.8 µs 1H 90◦ pulse.
The chemical shifts for 29Si and 13C NMR were referenced to
tetramethylsilane and glycine, respectively. For both measure-
ments, the spinning rate was 5 kHz. N elemental analyses (EA)
were carried out on an Elementer vario EL elemental analyzer.
Fig. 1. Relationship between the methanol ratio and average particle diameter
of MMSS obtained using quaternary ammonium salts with different alkyl-chain
lengths. (a) C TMABr, (b) C TMACl, (c) C TMACl and (d) C TMACl.
10
14
16
18
3. Results and discussion
3.1. Synthesis of MMSS with the same particle diameters and
different pore sizes
In order to clarify the relationship between the pore size of
a mesoporous silica catalyst and its catalytic activity, the pro-
vision of MMSS samples with the same particle diameters and
different pore sizes is necessary.
We have previously succeeded in the synthesis of MMSS
with different pore sizes by using various surfactants as tem-
plates. However, no attempts have been made to synthesize
MMSS with the same particle diameters. From a previous re-
port [6], it is obvious that the particle diameter of MMSS
tends to increase as the number of carbon atoms in the sur-
factant increases. To overcome this tendency, the requirement
for precise control of the reaction conditions during MMSS
synthesis is essential. Because the particle diameter of the
MMSS is significantly affected by the methanol ratio in the
solvent [5], syntheses were conducted under various methanol
ratios. Fig. 1 shows the relationship between the methanol ratio
and the average particle diameter of MMSS samples obtained
from quaternary ammonium salts with different alkyl-chain
lengths (C10TMABr, C14TMACl, C16TMACl, C18TMACl).
The methanol ratios with which the monodispersed particles
were obtained changed drastically depending on the type of
surfactant that was used. Although C16-MS (MMSS synthe-
sized using C16TMACl as a template) could be obtained over
a wide methanol ratio range (0.45–0.6), C14-MS and C18-MS
were only obtained in narrow methanol ratio ranges. It was
found that by precisely controlling the methanol ratio, MMSS
samples with almost the same particle diameters could be ob-
tained using quaternary ammonium salts with different alkyl-
chain lengths.
2.3. Catalytic reactions
Reactions were carried out in a round-bottomed flask
equipped with a reflux condenser. The nitroaldol condensa-
tion was typically carried out as follows: a reaction mixture
consisting of 50 mg of the catalyst and 0.61 g (5.0 mmol) of
4-hydroxybenzaldehyde in 10 ml of nitromethane was heated
at 363 K with constant stirring for 1 h. The mixture was filtered
and the catalyst on the filter was washed thoroughly with chlo-
roform. The filtrate was evaporated under reduced pressure. The
solid that was obtained was completely dissolved in a sufficient
amount of acetone-d6. An internal standard, THF (≈10 mmol),
was added into the acetone-d6 solution. The product was ana-
1
lyzed by studying its H NMR spectra on a Jeol JNM-LA500
NMR spectrometer [14].
In case when 4-methoxybenzaldehyde was used as a reac-
tant, the reaction was monitored periodically by withdrawing
an aliquot of solution from the solution in order to determine
a reaction rate. The samples were filtered and analyzed by gas
chromatograph (Agilent, 6890) provided with a 30 m capillary
column of DB-1, using decane as an internal standard for mass
balance. These products were characterized by MS (Agilent,
6890-5973) provided with a 30 m capillary column of DB-5MS.
In order to compare reaction rates of various MMSS, we eval-
uated TOF (turn over frequency) from the data for the reaction
time of 10 min.
3.2. Characterization of Cx-MS-AP
Amino-functionalized MMSS samples were obtained by
grafting APTMS onto Cx-MS. Fig. 2 shows SEM images of
Cx-MS-AP. As can be seen from Fig. 2, the particles were
highly monodispersed, and their average particle diameters

252
T.M. Suzuki et al. / Journal of Catalysis 251 (2007) 249–257
Fig. 3. XRD patterns of the samples: (a) C -MS-AP, (b) C -MS-AP, (c) C -
16
10
14
MS-AP, (d) C -MS-AP and (e) C PS-MS-AP.
18
18
isotherms of the samples are similar to those without organ-
ically functional groups (MMSS) [6], but C10-MS-AP hardly
adsorbed any nitrogen. This result agreed well with the result
from the XRD measurements. Most of the mesopores in C10-
MS-AP were filled with amino-moieties. As with the results of
the XRD measurements, the pore sizes of Cx-MS-AP increased
as the alkyl-chain length of the surfactant increased. C18PS-
MS-AP that was synthesized from pore-expanded C18PS by the
surfactant exchange method had the largest pore size of 2.66 nm
(Table 1).
The N contents of the samples are also shown in Table 1. Al-
though the N content of C18PS-MS-AP (which has the greatest
pore size) was a little larger (2.1 mmol/g), most of the samples
had almost the same N content (1.71–1.82 mmol/g), indicating
that nearly equal amounts of amino groups were incorporated
into MMSS samples with different pore diameters.
For comparison, grafting of amino-moieties was conducted
for FSM-16, which is mesoporous silica having irregular shape,
and monodispersed silica spheres without pores. They are de-
noted as FSM-16-AP and SS-AP hereafter. FSM-16-AP had the
pore size of 1.98 nm which was little bit larger than C18-MS-
AP and N content of 1.74 mmol/g. SS-AP had almost the same
particle diameter of MMSS samples (550 nm).
Fig. 2. Scanning electron micrographs of (a) C -MS-AP, (b) C -MS-AP,
10
14
(c) C -MS-AP, (d) C -MS-AP and (e) C PS-MS-AP. Scale bar represents
16
18
18
1.1 µm.
were between 560 and 600 nm. Table 1 summarizes the syn-
thetic conditions and properties of the particles. The regularity
of the mesopores was studied by X-ray diffraction measure-
ments (Fig. 3). The peak intensities of d100 for all samples
except for C10-MS-AP were over 10,000, indicating that the
Cx-MS-AP had well-ordered hexagonal regularity. In the case
of C10-MS-AP, the intensity of d100 was very low. It is con-
sidered that because most of the mesopores of C10-MS were
filled with amino-moieties, the contrast between the mesopores
and the silica walls became less, leading to low d100 intensity.
From the XRD results shown in Table 1, it was confirmed that
the value of d100 increased with increasing alkyl-chain length
of the surfactant.
The nitrogen adsorption–desorption isotherms were mea-
sured and the pore-size distributions were analyzed by the BJH
method. Fig. 4 shows the results for Cx-MS-AP samples. The
Table 1
Properties of aminopropyl-functionalized MMSS
Sample
Surfactant
concentra-
tion (mM)
Methanol
ratio
Average
diameter
(nm)
Standard
deviation
(%)
XRD
Nitrogen adsorption
N
content
(mmol/g)
d
100
Intensity
(cps)
Pore
size
S
Pore
volume
(ml/g)
BET
2
(nm)
(m /g)
(nm)
C
C
C
C
C
-MS-AP
-MS-AP
-MS-AP
-MS-AP
PS-MS-AP
13.7
13.7
13.4
13.8
–
0.25
0.44
0.50
0.59
–
590
600
580
560
590
–
10.1
5.4
6.2
6.3
5.6
–
2.56
3.18
3.36
3.67
4.09
3.71
3400
10,100
12,500
14,500
16,200
8900
–
280
730
870
850
810
600
0.03
0.31
0.40
0.50
0.74
0.41
1.71
1.79
1.78
1.82
2.10
1.74
0.13
10
14
16
18
18
1.34
1.53
1.89
2.66
1.98
a
FSM-16-AP
SS-AP
–
–
550
4.0
a
This was obtained by aminopropyl grafting on pore-expanded MMSS [19].

T.M. Suzuki et al. / Journal of Catalysis 251 (2007) 249–257
253
(A)
13
Fig. 6. C CP MAS NMR spectrum of C -MS-AP. (* Signals from residual
16
ethoxy group. ** Signals from residual methoxy group.)
n = 2–4, Q4: δ = −110 ppm, Q3: δ = −100 ppm, Q2: δ =
−92 ppm) and two peaks derived from Tm (Tm = RSi(OSi)m-
(OH)3−m, m = 1–3, T3: δ = −65 ppm, T2: δ = −57 ppm) were
observed. The spectrum that was obtained is the same as those
of organically functionalized SBA-15 or MCM-41 obtained by
the grafting method [20,21]. The presence of the Tm peaks
indicated the incorporation of amino-moieties into the silica
skeleton. In the 13C CP MAS NMR spectrum, peaks derived
from the C atoms of the incorporated aminopropyl groups were
observed. This result also supports the conclusion that amino-
moieties were successfully incorporated into the mesopores.
Other than main peaks, there are some small peaks observed.
Two small peaks at δ = 59, 16 ppm were assigned to the ethoxy
carbon atoms assumed to be generated by the partial esterifi-
cation of the surface silanol with ethanol which was used as
solvent for washing [22]. A small peak at 50 ppm was assigned
to unreacted methoxy carbon atoms of organic trimethoxysi-
lane [23].
(B)
Fig. 4. (A) Nitrogen adsorption–desorption isotherms and (B) pore-size distri-
butions of (a) C -MS-AP, (b) C -MS-AP, (c) C -MS-AP, (d) C -MS-AP
10
14
16
18
and (e) C PS-MS-AP.
18
The radial alignment of the mesopores was confirmed by
TEM observations (Fig. 7). It was observed that the hexago-
nal mesopores were arrayed in a radial pattern [19,24].
These experimental results led us to the conclusion that five
types of amino-functionalized MMSS with nearly the same par-
ticle diameters and different pore sizes had been successfully
prepared.
3.3. Evaluation of catalytic activity
The catalytic activities of Cx-MS-AP were studied by the ni-
troaldol condensation of nitromethane and benzaldehyde deriv-
atives. Firstly, 4-methoxybenzaldehyde was used as the reac-
tant. Fig. 8 shows time course of yields of nitroaldol condensa-
tion of nitromethane and 4-methoxybenzaldehyde using C14-
MS-AP, C18-MS-AP, and FSM-16-AP as a catalyst. Table 2
29
Fig. 5. Si MAS NMR spectrum of C -MS-AP.
16
The 29Si and 13C CP MAS NMR spectra of C16-MS-AP
were measured and its chemical structure was investigated
(Figs. 5 and 6). In the 29Si MAS NMR spectrum, three clear
summarizes initial rate and TOF (turnover frequency, min−1
)
determined from data for 10 min reaction, and TON (turnover
number) determined from data for 1 h reaction for various cat-
resonance peaks derived from Qn (Qn = Si(OSi)n(OH)4−n
,

254
T.M. Suzuki et al. / Journal of Catalysis 251 (2007) 249–257
Fig. 8. Changes in the activity of various types of catalysts in nitroaldol con-
densation reaction.
alysts. From Fig. 8 it is obvious that most of the reactions
finished just before 1 h. These results indicate that compari-
son of their catalytic activity by the use of TON determined
from the data for 1 h reaction is appropriate for this reaction
system. In order to determine reaction rates, we used GC for
analysis. However, yields determined by GC fluctuated and we
used NMR for determining TON, because the data obtained
from NMR is more reliable than the data from GC due to poor
volatile property of the product. It turned out that the initial TOF
and TON largely affected by the pore size of MMSS. C16-MS-
AP (pore size: 1.53 nm) and C18-MS-AP (pore size: 1.89 nm)
showed high catalytic activity. Initial TOF values of these sam-
ples were 1.3 and 1.2 min−1, which were higher than the val-
ues for the other catalyst. The final yields of these samples
were also higher than those of the other catalysts, indicating
that initial TOF and TON correlated well for Cx-MS-AP sam-
ples. Meanwhile, amino-grafted FSM-16 (pore size: 1.98 nm)
showed smallest initial TOF of 0.8 min−1. However, yield in-
creased gradually, and the final TON became almost the same
value with that of C18-MS-AP. FSM-16 is mesoporous silica
having an irregular shape, and most of mesopores are aligned
in the same direction. This means that most of its outer surface
is covered with silica walls, leading to limited access for the re-
actants. On the contrary, in MMSS, the mesopores are aligned
radially from the center to the outside of a particle. Therefore,
reactants can enter into the mesopores from all directions. It
may be concluded that Cx-MS-AP is one of the best catalysts
for the nitroaldol reactions of benzaldehyde derivatives among
other mesoporous silicas.
Fig. 7. Transmission electron micrographs of (a) C -MS-AP and (b) C PS-
MS-AP. Scale bar represents 50 nm.
16
18
Table 2
Results of nitroaldol condensations of 4-methoxybenzaldehyde with ni-
tromethane in the presence of catalyst (363 K)
a
b
Catalyst
Initial rate
Initial TOF
TON
−1
−1
−1
(mmol g min
)
(min
)
(yield, %)
cat
C
C
C
C
C
-MS-AP
-MS-AP
-MS-AP
-MS-AP
PS-MS-AP
0
0
3 (5)
10
14
16
18
18
1.8
2.3
2.2
2.2
1.3
1.0
1.3
1.2
1.0
0.8
41 (74)
54 (96)
52 (94)
44 (92)
52 (90)
It turned out that the catalytic activity was affected by the
pore size of MMSS, that is, the reaction is not in totally dif-
fusion control. If sizes of mesopores are close to the sizes of
reactants, a reaction could be mainly controlled by diffusion. In
this case, a product is generated mostly on the surface of a cata-
lyst. Therefore, it is very important to know what percentage of
mesopores is effectively used for the reaction. Then, the effec-
FSM-16-AP
a
Turnover frequency = mmol product/mmol organic groups during initial
10 min. TOF was determined by GC analysis.
b
Turnover number = mmol product/mmol organic groups during 1 h. TON
was determined by NMR analysis.

T.M. Suzuki et al. / Journal of Catalysis 251 (2007) 249–257
255
Fig. 9. Changes in the activity of MMSS catalyst having different particle di-
ameter in nitroaldol condensation reaction.
Table 3
Results of nitroaldol condensations of 4-substituted benzaldehyde with ni-
tromethane in the presence of C -MS-AP catalyst (363 K, 1 h)
x
Fig. 10. Relationship between the pore size of particles and turnover number
for nitroaldol condensation. The pore size of C -MS-AP was estimated to be
10
zero from the nitrogen adsorption measurement.
the results obtained from the reaction between nitromethane
and 4-hydroxybenzaldehyde (3a). MMSS without any amino
groups (C16-MS) showed no catalytic activity, suggesting that it
is the amino-moieties that act as catalysts. The yield increased
as we increased the pore size of the Cx-MS-AP, and reached
a maximum (86%) for C16-MS-AP. C10-MS-AP showed very
low catalytic activity; its yield was 2% and its TON was 1. Be-
cause the mesopores of C10-MS-AP are considered to be filled
with amino-moieties, the reactants could not reach the active
sites (= amino-moieties) inside the mesopores. When C14-MS-
AP (pore size: 1.34 nm) or C16-MS-AP (1.53 nm) were used as
the catalysts, the TON increased to 29 or 48, respectively. It be-
came clear that the mesopores require a certain capacity for the
enhancement of catalytic activity. However, when C18-MS-AP
(pore size: 1.89 nm) or C18PS-MS-AP (2.66 nm), which have
larger mesopores, were used as catalysts, the TON decreased
to 43 or 36, respectively. Meanwhile, when SS-AP, which has
nearly the same particle diameter with MMSS samples and no
mesopores, was used as a catalyst, the yield was as low as 5%.
This result strongly supports the assumption that the reaction
occurred mainly in mesopores, but outer surfaces. Fig. 10 shows
the relationship between the pore size of the samples and the
TON for the reaction. The pore size of C10-MS-AP was es-
timated to be zero from the result of the nitrogen adsorption
measurement. It can be seen from Fig. 10 that there is an op-
timum pore size for the reaction. In Fig. 10, results obtained
when 4-methoxybenzaldehyde was used as a reactant were also
shown and the tendency was the same with the results for the
other reactants.
a
Catalyst
Yield (%)
TON
a
b
c
a
b
c
C
10
C
14
C
16
C
18
C
18
C
16
-MS-AP
-MS-AP
-MS-AP
-MS-AP
PS-MS-AP
-MS
2
52
4
49
4
1
29
48
43
36
0
2
2
55
92
86
91
–
27
ꢀ56
54
47
–
31
52
47
43
–
86
78
75
0
100
98
98
–
SS-AP
5
–
–
38
–
–
a
Turnover number = mmol product/mmol organic groups during 1 h.
tiveness factor of MMSS was evaluated from the kinetic data
measured by use of catalysts with different particle diameters
but the same pore size. Fig. 9 shows time course of yields of
nitroaldol condensation of nitromethane and 4-methoxybenz-
aldehyde using C16-MS-AP (particle diameter: 580 nm) and
C16-MS-AP-730nm (particle diameter: 730 nm). Initial rates
determined from the reaction time of 10 min for C16-MS-AP
and C16-MS-AP-730 nm were 2.3 and 1.8 mmol g−ca1t min−1
,
respectively. From these rate and particle diameter data, the
effectiveness factor of MMSS was evaluated to be 0.63. This
result suggests that MMSS with smaller particle diameter could
be a candidate for an ideal catalyst or catalyst support because
higher effectiveness factor can be expected for smaller particles.
3.4. Influence of molecular size
In order to investigate the effect of the molecular size of
the reactant on the catalytic activity, firstly, para-benzaldehyde
derivatives incorporating alkoxy groups with different alkyl-
chain lengths were examined. Products for 1 h reaction were
analyzed, and yields and TON were determined. Table 3 shows
Then, the study was conducted by using para-substituted re-
actants with longer substituents. The reactants that were used
were 4-butoxybenzaldehyde (3b) and 4-n-octoxybenzaldehyde

256
T.M. Suzuki et al. / Journal of Catalysis 251 (2007) 249–257
Table 4
Results of nitroaldol condensations of multi-substituted benzaldehyde with nitromethane in the presence of C -MS-AP catalyst (363 K, 1 h)
x
a
Catalyst
Yield (%)
TON
4-OMe
2,4,6-OMe
3,4,5-OMe
4-OMe
2,4,6-OMe
3,4,5-OMe
a
b
c
a
b
c
C
10
C
14
C
16
C
18
C
18
-MS-AP
-MS-AP
-MS-AP
-MS-AP
PS-MS-AP
5
74
96
94
92
21
69
62
83
75
1
27
77
88
90
3
41
54
52
44
12
39
35
46
36
1
15
43
48
43
a
Turnover number = mmol product/mmol organic groups during 1 h.
(3c). The results of the reactions are also shown in Table 3
and Fig. 10. In all cases, C16-MS-AP exhibited the best cat-
alytic performance among those tested, as seen for 4-hydroxy-
benzaldehyde and 4-methoxybenzaldehyde. It is surprising that
the optimum pore size remained unchanged for the reaction,
even though the molecular sizes of the reactants changed dras-
tically. Based on the Molecular Mechanics model, the sizes
of the molecules along their longer axes are about 0.67 nm
for 4-hydroxybenzaldehyde, 0.82 nm for 4-methoxybenzalde-
hyde, 1.15 nm for 4-butoxybenzaldehyde, and 1.70 nm for
4-n-octoxybenzaldehyde. The pore size of C16-MS-AP as de-
termined from the adsorption measurement was 1.53 nm, which
is smaller than the molecular size of 4-n-octoxybenzaldehyde
on its longer axis. However, all of the benzaldehyde deriva-
tives had the same molecular size, ca. 0.43 nm, for their shorter
axes. It is assumed that the benzaldehyde derivatives entered
into mesopores parallel to the pore directions and reacted with
the amino groups that existed inside mesopores. The pore size
of C16-MS-AP is considered to be the most suitable for the re-
action of amino groups and para-substituted aldehyde moieties
from the view point of optimal space and density of the amino-
moieties.
The size of the substituent group affected the catalytic activ-
ities of the Cx-MS-AP samples. When 4-butoxybenzaldehyde
(3b) was used as the reactant, the value of TON was the highest
in many cases, that is, Cx-MS-AP exhibits some selectivity for
the reaction [25]. It is thought that hydrophobic environment
was formed in the mesopores of MMSS by the incorporation of
organically functional groups, leading to easy infiltration of re-
actants with an alkyl-chain into the mesopores. However, given
the diffusion of the reactant, there is an optimum size for the re-
action to take place. The optimum pore size was not affected at
all by the para-substitution of the reactant.
aldehydes such as 2,4,6-trimethoxybenzaldehyde (5b) and
3,4,5-trimethoxybenzaldehyde (5c) were examined. It is ex-
pected that introduction of substituents on meta or ortho po-
sition leads to the change in minimum kinetic diameter [17,26],
which brings to the change in an optimum pore size for the
reaction. Table 4 and Fig. 11 show the results of the catalytic
activities for the reaction. When multi-substituted aldehydes
were used as a reactant, C18-MS-AP, which has a pore size of
1.89 nm, exhibited the highest catalytic activity. The optimum
pore size for the nitroaldol condensation shifted to higher value
by increasing the number of substituent groups in the reactant.
The molecular size of the reactants became more bulky as the
number of methoxy groups increased, and then the catalytic
activity of C16-MS-AP fell sharply. On the contrary, the ac-
tivity of C18-MS-AP and C18PS-MS-AP hardly changed. The
dimensions of the molecule between the 1-position and the 4-
position for the longer axis is about 0.82 nm and the shorter
axis is 0.43 nm for 4-methoxybenzaldehyde. The shorter axis
increases to 0.82 nm for the multi-substituted aldehydes be-
cause the methoxy groups are disposed along both sides of
the benzene ring. We assume from the foregoing experiments
that the aldehydes entered into the mesopores parallel to the
pore directions. Larger pores are considered to be necessary to
enable more-bulky reactants to enter smoothly into the meso-
pores.
4. Conclusions
Precise control of the reaction conditions led to the success-
ful synthesis of amino-functionalized monodispersed meso-
porous silica spheres with nearly the same particle diameters
and different pore sizes or with the same pore size and different
particle diameters. By using the amino-functionalized monodis-
persed mesoporous silica spheres as catalysts, the effect of pore
size on base catalysis has been studied. It was found for the
In order to investigate further the effects of the molecular
size of the reactant on the catalytic activity, multi-substituted

T.M. Suzuki et al. / Journal of Catalysis 251 (2007) 249–257
257
Acknowledgments
The authors thank Dr. Yasutomo Goto for assistance with
NMR measurements. The authors also thank Dr. Takao Masuda
and Dr. Teruoki Tago of Hokkaido University for fruitful dis-
cussion on the reaction rate of MMSS catalysts.
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first time that an optimum pore size exists, which changes de-
pending on the size and shape of the reactants. In addition, by
changing particle diameter, the effectiveness factor of MMSS
was evaluated to be 0.63. Amino-functionalized monodispersed
mesoporous silica spheres were found to be excellent catalysts
for nitroaldol condensation due to the radial alignment of their
mesopores when compared to other types of mesoporous silica,
such as FSM-16. Because the amino-functionalized monodis-
persed mesoporous silica spheres were identical particles ex-
cept for the sizes of their mesopores, other external causes, such
as particle diameter and shape, and mesopore alignment, can be
ignored. It is believed that mesoporous materials such as these
will be useful as basic research materials for studies of catal-
ysis if a reaction is not under diffusion control. Moreover, by
designing the pore properties of the MMSS through the intro-
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this material can now be considered as a candidate for a new
type of substrate-selective catalyst.
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