A simple modification creates a great difference: New solid-base catalyst using methylated N-substituted SBA-15

Article abstract of DOI:10.1021/ja2070522

A simple modification, methylation of the nitrogen-substituted mesoporous silica SBA-15, enhances the basicity of a solid-base catalyst. The methyl group donates an electron to the nitrogen atom in the silica framework. This catalyst accelerates Knoevenag

Full text of DOI:10.1021/ja2070522

Communication
pubs.acs.org/JACS
A Simple Modification Creates a Great Difference: New Solid-Base
Catalyst Using Methylated N-Substituted SBA-15
Kotaro Sugino, Nobuhiro Oya, Naoko Yoshie, and Masaru Ogura*
Institute of Industrial Science, The University of Tokyo, Tokyo, Japan
*
S Supporting Information
ABSTRACT: A simple modification, methylation of the
nitrogen-substituted mesoporous silica SBA-15, enhances
the basicity of a solid-base catalyst. The methyl group
donates an electron to the nitrogen atom in the silica
framework. This catalyst accelerates Knoevenagel con-
densation using benzaldehyde and diethyl malonate, which
conventional solid-base catalysts reported to date cannot
do. This report demonstrates a possible new type of base
catalyst using nitrogen-substituted mesoporous silica
materials.
Figure 1. Knoevenagel condensation using benzaldehyde with active
methylene compounds.
esoporous silica materials, first reported almost two
M
decades ago, have become the focus of a great deal of
current research in the field of nanostructured materials
because of their regular structures, uniform and large pore
methylation of nitrogen-substituted SBA-15 (Me-NSBA) to
improve its basicity by a simple modification, and we
demonstrate its catalytic performance as a new, stronger base
catalyst.
Me-NSBA was synthesized as shown in Scheme 1. The
methyl group is expected to increase the basicity of the amine
1
sizes, and high surface areas. Furthermore, many researchers
envision using these materials as catalysts because they can be
2
easily separated from a reaction mixture. Therefore, they are
very desirable for industrial processes in terms of cost and
environmental loads.
A new class of nitrogen-substituted solid-base materials using
Scheme 1. Methylation of NSBA
MCM-41 and SBA-15 (SBA) with high specific surface area was
3
recently developed. The basic sites in such materials arise from
the presence of nitrogen atoms in their frameworks. These
nitrogen species are considered to be as strongly basic as
conventional solid-base catalysts such as alkali- or alkaline-
earth-metal oxides, alkaline-ion-exchanged zeolites, or meso-
4
porous silica materials impregnated with basic salts. Before
these conventional base catalysts are used, pretreatment at high
temperatures (>873 K) is essential because their catalytic sites
4a,c
tightly adsorb ambient water or carbon dioxide.
On the
part by donating an electron to the nitrogen atom. Nitrogen-
substituted SBA-15 (NSBA) was prepared essentially as
described in a previous work, but the flow rate of ammonia
other hand, our group previously reported that nitrogen-
substituted mesoporous silica (NS-MSM) materials do not
require such pretreatment, which is a compelling reason to
pursue research on NS-MSM and also suggests that these5
materials would be useful and practical for industrial processes.
The performance of solid-base catalysts is generally evaluated
by a Knoevenagel condensation in which benzaldehyde (1) is
reacted with active methylene compounds such as malononi-
5
was changed to 1 L/min to enhance nitridation. NSBA was
then methylated by using methyl iodide with potassium
carbonate (NSBA:MeI:K CO molar ratio = 1:15:3). The
2
3
reaction was conducted in anhydrous ethanol at 350 K for 48 h
with a reflux condenser, after which the product was filtered,
washed, and dried at 373 K. After the catalyst was synthesized,
its basicity was evaluated by means of Knoevenagel
condensation of 1 and 2c. The substrate (2 mmol) and catalyst
2b,4,5
trile (2a) or ethyl cyanoacetate (2b).
However, because the
nitrogen species is a secondary amine, its basicity and catalytic
activity are limited, so Knoevenagel condensation using 1 and
diethyl malonate (2c) cannot be catalyzed by conventional base
catalysts (Figure 1). To the best of our knowledge, no one has
reported successful a Knoevenagel condensation catalyzed by
conventional base catalysts or NS-MSM. Here we report the
Received: July 27, 2011
Published: November 21, 2011
©
2011 American Chemical Society
20030
dx.doi.org/10.1021/ja2070522 | J. Am. Chem.Soc. 2011, 133, 20030−20032

Journal of the American Chemical Society
20 mg) were mixed in ethanol (2.13 mL), and the mixture was
Communication
(
heated at 353 K with a reflux condenser. An aliquot of the
reactant solution was analyzed by gas chromatography
afterward.
Figure 3. Effect of methylation of nitrogen on the Knoevenagel
condensation.
The results for each sample are summarized in Figure 3. It is
noteworthy that only Me-NSBA catalyzed the reaction, whereas
the other samples showed almost no catalytic ability. These
results clearly show that the addition of methyl groups to
nitrogen atoms in the framework greatly increased the basicity
of the catalyst and that the catalytic activity was significantly
increased by methylation. On the basis of the nitrogen
adsorption isotherm on MeNSBA, methylation did not result
in mesopore blockage. This means that the methylation was not
limited to sites near the mesopore mouth. The catalytic
performance might suggest that the catalytically active N
species attached to methyl group is inside the mesopore, which
leads to slow product formation by diffusion.
Figure 2. (a) PXRD results and (b) FT-IR spectra.
Powder X-ray diffraction (PXRD) revealed three typical
peaks from SBA-15 (Figure 2a). After methylation, Me-NSBA
maintained its two-dimensional hexagonal periodic structure.
Fourier transform IR spectra of the samples were also measured
(
Figure 2b). The peak at 3736 cm−1 observed for all the
6
a,b
samples is attributed to the OH bond of silanol.
After
−
1
nitrogen substitution, a wide band at 3356 cm corresponding
Another mesoporous silica, MCM-41, was used as the
support of methylated nitrogen in the silica framework. First of
all, it was quite difficult to conduct nitridation and methylation
on MCM-41 because the periodic mesoporous structure was
easily destroyed by those treatments. This occurred because
MCM-41 has a much thinner mesopore wall than SBA-15.
Thus, SBA-15 is superior as the substrate for the methylated
nitrogen group on silica inside a confined mesospace. Finally,
Me-NMCM-41 was obtained successfully by nitridation at 973
K for 12 h followed by methylation at 350 K for 4 h (Figure S1
in the Supporting Information). The contents of nitrogen and
carbon in Me-NMCM-41 are shown in Table S1. The amount
of doped nitrogen in MCM-41 was much less than that in SBA-
15; this is due not only to the low temperature for nitridation
but also to the smaller amount of silanol on the surface of
MCM-41. Knoevenagel condensation using 1 and 2c was also
carried out (Figure S2). The catalytic activity of Me-NMCM-41
was comparable to that in Me-NSBA, and the smaller reaction
rate is due to the smaller number of catalytic active sites. There
was no induction period, as the reaction product was seen in
the reactant mixture of Me-NMCM-41, while there existed an
induction in that of Me-NSBA-15. It is also interesting to note
that the C/N ratio in Me-NMCM-41 was close to 1, as shown
in Table S1; almost all of the N atoms could be methylated,
which was never done in Me-NSBA. This is due to the location
of the methylated N atom along within a shorter mesopore of
Me-NMCM-41, while N atoms also exist in the micropores of
SBA-15, resulting in less methylation of N atoms because of
slow diffusion of methyl iodide in the micropore.
6a,b
to the NH stretching vibration appeared. The sharp peak at
−
1
−1
2
978 cm and the small peak at 2900 cm appearing after the
methylation reaction are assigned to CH stretching of methyl
6c
groups connected to nitrogen atoms, providing powerful
evidence that NSBA was methylated. However, two bands at
−1
2
850 and 2935 cm , which could be assigned to the stretching
vibration of CH bonds connected to O atoms, were also
detected. This indicates that a portion of the silanol might also
have been methylated.
Table 1. Elemental Analyses of NSBA and Me-NSBA
N
C
sample
NSBA
Me-NSBA
wt %
mmol/g
wt %
mmol/g
24.2
18.4
17.3
13.1
−
3.81
−
3.17
To clarify the number of carbon and nitrogen atoms in the
structures of the samples, elemental analysis was performed
(Table 1). Carbon derived from the methyl group was detected,
although the amount of nitrogen decreased after methylation.
Because the amine bridge is weak against water, it was
hydrolyzed even though an anhydrous solvent was used. The
ratio of methyl groups to nitrogen was limited to C/N = 0.3,
even though methylation was carried out under several
conditions.
Knoevenagel condensation using 1 and 2c was performed to
investigate the catalytic performance of Me-NSBA. Ethanol was
used as the solvent for this reaction because a protic solvent is
5
As previously reported, the microporous space was too small
7
known to assist proton transfer in the reaction. To clarify the
effect of the methylated nitrogens, a catalyst that was
methylated but contained no amine bridges was also used.
to react benzaldehyde with ethyl cyanoacetate. It is also worth
mentioning that methyl iodide for the methylation of the N
atom in the silicate cannot penetrate into micropores of N-
2
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dx.doi.org/10.1021/ja2070522 | J. Am. Chem.Soc. 2011, 133, 20030−20032

Journal of the American Chemical Society
Communication
silicalite. From these results, we can stress that a mesospace that
is at least larger than the microporous space obtained in zeolites
is needed to promote such a reaction on a basic surface. More
bulky substrates are normally used in most of the reactions
required for organosynthesis using basic catalysts; thus, we can
conclude the potential application seen in our newly developed
samples.
In summary, we have successfully synthesized a new solid-
base catalyst, methylated nitrogen-substituted SBA-15, by a very
simple method and demonstrated Knoevenagel condensation
using benzaldehyde and diethyl malonate with this catalyst. The
results indicate that the methyl group enhances the basicity of
the amine part and enables this Knoevenagel condensation,
which is impossible using known conventional solid-base
catalysts.
ASSOCIATED CONTENT
Supporting Information
■
*
S
XRD patterns, N and C contents, and catalytic performance of
MCM-41, NMCM-41, and Me-NMCM-41. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
oguram@iis.u-tokyo.ac.jp
■
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