Switching catalytic reaction conducted in pore void of mesoporous material by redox gate control

Article abstract of DOI:10.1039/b610444d

A molecular gate attached on the pore outlet of a mesoporous material, which is opened and closed by redox system of thiol groups, effectively switched the progress of a catalytic reaction promoted by the acidic site in the pore void. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b610444d

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Switching catalytic reaction conducted in pore void of mesoporous
material by redox gate control{
a
Masahiro Fujiwara,* Shigeki Terashima, Yasuko Endo, Kumi Shiokawa and Hiroyoshi Ohue
b
a
a
b
Received (in Cambridge, UK) 20th July 2006, Accepted 7th September 2006
First published as an Advance Article on the web 2nd October 2006
DOI: 10.1039/b610444d
A molecular gate attached on the pore outlet of a mesoporous
material, which is opened and closed by redox system of thiol
groups, effectively switched the progress of a catalytic reaction
promoted by the acidic site in the pore void.
The ordered pore voids of mesoporous matrices provide designed
systems at the molecular and nanometer levels. These pore
structures exhibit specific and interesting transport of molecules in
1
the pore voids. Recent research on molecular transfer using these
materials has focused on how to control the diffusion of molecules
from pore voids to the exterior. The main applications of this
2
technology are drug delivery and related systems. After studies on
3
the sustained release of molecules from mesopores, some
Fig. 1 Scheme of mesoporous materials modified with a redox-
responsive disulfide group.
4,5
responsive-gate systems were reported. The uniform pore sizes
of mesoporous materials such as MCM-41 are advantageous for
controlling the gating of pores, to achieve the effective control of
molecular release. Our previous report of photo-responsive
controlled release by coumarin-modified mesoporous silica is
5
related to this technology. In all of the cases mentioned above, the
Fig. 2 Dimerization of AMS and structure of DTT.
direction of molecular diffusion is from the pore interior to the
exterior. It is also expected that the approach of molecules from
the pore exterior to the interior is regulated by pore gating systems.
One possible application of this reverse molecular transport is the
control of catalytic reactions conducted in pore voids. This system
may provide a new and general method for controlling hetero-
geneous catalytic reactions using porous materials. In this
communication, we wish to briefly demonstrate a prototype for
the switching of the progress of a catalytic reaction in the pores of
a mesoporous material, where the redox reaction between disulfide
and thiol dominates the outlet gating of the pores (Fig. 1).
As mentioned later, we verified that no reaction occurred with
Si-MCM-41. Furthermore, simple dimerization with a single
reactant is advantageous for monitoring the approach of molecules
into the pore. An M41s-type of mesoporous silica (named MS)
obtained by hydrothermal treatment with hexadecyltrimethyl-
ammonium bromide and subsequent calcination was transformed
to Al-loaded (approximately 6 mol% of Al to Si) mesoporous silica
using Al(acac) (named Al-MS). Al-MCM-41-type materials are
3
8
reported to be active in catalyzing AMS dimerization. Although
the direct synthesis (one-pot synthesis) from a mixed gel of Si and
Al sources is common for the preparation of Al-MCM-41, this
The most common catalytic reaction using mesoporous silica
materials is an acid-catalyzed reaction by Brønsted acidic sites
6
method often decreases the structural regularity of materials.
9
Therefore, we employed a post synthesis method using Al(acac)
created with the isomorphous loading of different atoms.
3
.
Recently, Si-MCM-41 was claimed to promote some acid-
7
catalyzed reactions, which occur on the entire surface. Since the
Although the highly acidic catalytic site induced by Al atom is
thought to be distributed evenly on the surface of mesoporous
silica, the outer surface is negligible compared to the inner surface.
This catalytic reaction is considered to occur exclusively in the
pore, and is dominated by a pore gating system.
aim of this study was to control a catalytic reaction by a pore
gating system, only catalytic reactions that occur inside the pore
are suitable. Therefore, we chose the dimerization reaction of
8
a-methylstyrene (AMS) shown in Fig. 2. While this reaction
The steps for the transformation of mesoporous materials are
summarized in Fig. 1 and experimental procedures are described in
detail in the ESI.{ A disilane–disulfide compound (1) readily
produced by the direct reaction of 6-hydroxy-2-naphthyl disulfide
with 3-(triethoxysilyl)propyl isocyanate was grafted (approxi-
mately 3 wt% according to TGA measurement) on Al-MS to
form a disulfide-modified mesoporous material [(SS)Al-MS]. Our
readily proceeds with a strong acid, no weak acids catalyzed this.
a
Kansai Center, National Institute of Advanced Industrial Science and
Technology (AIST), 1 Midorigaoka, Ikeda, Osaka 563-8577, Japan.
E-mail: m-fujiwara@aist.go.jp.; Fax: +81 72 75 9628;
Tel: +81 72 751 9652
b
Osaka Institute of Technology, 5 Ohmiya, Asahi-ku, Osaka 535-8585,
Japan
5
previous reports revealed that the density of coumarin substituent
{ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b610444d
on the surface of mesoporous silica is a crucial factor in a
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a
Table 1 Profiles and catalytic activities of mesoporous materials
Yields (%)
b
SSA /m g
2
21
c
PPD /nm
d
PV /ml g
21
Catalyst
d
100/nm
A
B
MS
Al-MS
3.96
3.86
3.87
3.84
3.84
909
797
769
742
709
2.74
2.52
2.52
2.52
2.52
937
779
722
739
727
0.1
,0.1 (8.6)
,0.1
.99.0 (85.5)
e
e
f
f
(
(
(
SS)Al-MS
SH)Al-MS
SS)*Al-MS
0.4 (0.9)
1.7 (59.7)
,0.1 (,0.1)
97.1 (37.5)
,0.1 (0.2)
e
e
f
f
0.1 (1.5)
c
a
b
AMS 1 mmol, catalyst 20 mg, toluene 5 mL, 120 uC, 2 h. Specific surface area by the BET method. Peak pore diameter by the BJH
d
e
f
method. Pore volume by the BJH method. 80 uC, 2 h. 120 uC, 6 h.
successful gating system. Dense grafting of coumarin substituent
promotes its photo-dimerization to form crosslinking, resulting in
pore closure. However, a sparse coumarin substituent is unfavor-
able for dimerization and does not induce closure of the pore. Our
current conditions using a disilane–disulfide compound 1, which
forms crosslinking directly in the pore, is expected to be more
effective for construction of a gating system. The results of AMS
dimerization using various catalysts are summarized in Table 1.
Mesoporous silica (MS; similar to Si-MCM-41) had no catalytic
activity due to the absence of highly acidic sites. On the other
hand, an AMS dimer (product B) formed by an intramolecular
Friedel–Crafts reaction was obtained quantitatively by Al-MS at
promoted the dimerization reaction as well as Al-MS. Therefore,
the redox processing did not influence the catalytic activity of
Al-MS for AMS dimerization. We also modified the sample MS
(mesoporous silica without Al) using a disilane–disulfide com-
pound 1, and a series of treatments in a similar fashion to Al-MS
was performed. In all catalytic reactions using these Al-free three
samples corresponding to (SS)Al-MS, (SH)Al-MS and (SS)*Al-
MS, no AMS dimers were formed. Therefore, no catalytic
activities of disilane substituents themselves (in both disulfide
and thiol forms) were confirmed as well.
There are no clear differences in the d100 interplanar spacing of
XRD patterns among the various samples in Table 1. Fig. 3A
shows BJH pore-size distribution plots of some samples [Al-MS,
(SS)Al-MS, (SH)Al-MS and (SS)*Al-MS]. The decrease of pore
volume was observed from Al-MS to (SS)Al-MS. However, there
120 uC for 2 h. When the disulfide-modified Al-MS [(SS)Al-MS]
was used for this reaction, no dimers of AMS were obtained under
the same reaction conditions, and most of the AMS was recovered.
Even with a longer reaction time (6 h), no increase in product
yields was detected. It is thought that the disulfide substituent
prevented AMS from accessing the acidic site inside the pore,
resulting in no formation of AMS dimers. The molecular length of
compound 1 was estimated to be approximately 2.8 nm, which is
too large to penetrate deeply into a pore of about 2.5 nm in
diameter. Although the intimate arrangements of grafted sub-
stituents remain unknown, the disulfide substituent was likely to be
attached mainly to the outer surface and in the vicinity of the pore
outlet. These substituents effectively cover pore outlets to interfere
with the access of AMS to catalytic sites.
Dithiothreitol (DTT) is often used for the cleavage of disulfide
1
0
bonds to two thiol groups in protein and other chemistry.
SS)Al-MS bearing a disulfide group was treated with DTT
(
aqueous solution. Before this process, we had confirmed no
activity of DTT for this dimerization. Using the obtained material
[
(SH)Al-MS], AMS dimerization proceeded in high yield at 120 uC
97.1%). However, the yield of Friedel–Crafts AMS dimer (B) at
0 uC (37.5%) was considerably lower than that of Al-MS (85.5%).
(
8
8
Friedel–Crafts dimer B is produced sequentially via dimers A.
Next, this sample [(SH)Al-MS] was treated with an I solution of
O/AcOH to convert the thiol groups into a disulfide bond
(SS)*Al-MS]. As residual I and HI promote the dimerization
2
11
H
2
[
2
reaction, this sample was washed with water and ethanol
thoroughly before use. AMS dimerization reaction using this
(SS)*Al-MS resulted in the poor formation of dimers even in a
longer reaction. Thus, AMS dimerization was effectively controlled
by redox treatments.
A series of control experiments was also examined using Al-MS.
Al-MS was treated with DTT in a similar manner to (SS)Al-MS.
Then, this sample was mixed in the I
2
solution of H O/AcOH.
2
Fig. 3 BJH pore-size distribution plots (A) and DR-UV spectra (B) of
These two catalysts thus treated without disilane substituent
Al-MS, (SS)Al-MS, (SH)Al-MS, (SS)*Al-MS.
4
636 | Chem. Commun., 2006, 4635–4637
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were no significant differences among three grafted catalysts
(SS)Al-MS, (SH)Al-MS and (SS)*Al-MS]. Similar observations
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are also found in mesoporous silica samples with a coumarin gate
5
system. The access of nitrogen into the pores is not influenced by
2
004, 16, 4160.
the state of the gate substituent. Therefore, the situations of the
whole pore voids did not change by redox treatments. Fig. 3B
shows diffuse reflectance UV spectra of four samples. In modified
samples, UV absorptions derived from disilane substituents were
observed from 250 to 400 nm. The samples (SS)Al-MS and
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respectively. These absorptions are assigned to disulfide or thiol
1
2
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SS)Al-MS and (SH)Al-MS. In (SS)*Al-MS, the absorption of the
(
S–H bond disappeared and that of the S–S bond was regenerated
at around 250 nm.
All results mentioned above support our concept shown in Fig. 1
that for the progress of a catalytic reaction in the pore, voids can
be switched by the outlet gating of the pores. The high yields of the
dimers observed in the reaction using (SH)Al-MS were caused by
the change of the gate substituents. The cleavage of the disulfide
substituent by DTT opened the outlet of the pore, which permitted
the reactant to penetrate the catalytic site. The modest decrease of
the catalytic activity of (SH)Al-MS might be induced by
substituents in thiol form that still remain on the surface. Even
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mately 1.4 nm long interfered with the access of AMS to impair
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2
thiol groups of (SH)Al-MS were oxidized by I to disulfide. The
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