One-pot reaction cascades catalyzed by base- and acid-functionalized mesoporous silica nanoparticles

Article abstract of DOI:10.1039/b806664g

Mesoporous silica nanoparticles (MSNs) containing base (primary amine) and sulfonic acid inside the MCM-41 type porous channels were successfully used as compatible catalysts for one-pot reaction cascades. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b806664g

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LETTER
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One-pot reaction cascades catalyzed by base- and acid-functionalized
mesoporous silica nanoparticlesw
Yulin Huang, Brian G. Trewyn, Hung-Ting Chen and Victor S.-Y. Lin*
Received (in Montpellier, France) 18th April 2008, Accepted 10th June 2008
First published as an Advance Article on the web 23rd June 2008
DOI: 10.1039/b806664g
Mesoporous silica nanoparticles (MSNs) containing base
(primary amine) and sulfonic acid inside the MCM-41 type
porous channels were successfully used as compatible catalysts
for one-pot reaction cascades.
mesoporous silica materials have rendered several interesting
systems for cooperative catalysis.10–13 Given that Blum, Avnir
et al. have pioneeringly demonstrated that sol–gel silica can
serve as an effective matrix for entrapping opposing catalysts
for one-pot reaction cascades,2–6 we are interested in taking
advantage of the unique properties, i.e., homogeneous meso-
porous structure and tunable particle size and pore diameter,
of the MCM-type of mesoporous silicas for the site isolation of
opposing catalytic reagents as well as the pore size discrimina-
tion that can regulate the mass-transport properties of a given
reaction. By ‘‘hiding’’ one kind of functional group inside the
mesopores of a mesoporous silica particle, while ensuring the
other opposing reagent is situated inside another mesoporous
silica material with different particle and pore sizes, the
reaction kinetics could foreseeably be further manipulated.
To achieve this goal, a key prerequisite is to functionalize the
interior mesoporous surface with homogeneously distributed,
high concentrations of functional groups, along with precise
morphology control. We have recently developed an interfa-
cial designed co-condensation method under low surfactant
concentration condition for the synthesis of a series of orga-
nically functionalized mesoporous silica nanoparticle (MSN)
materials.13–18 Herein, we report on the synthesis and char-
acterization of two MCM-41 type MSN materials that are
functionalized with a 4-ethylphenylsulfonic acid (SAMSN)
and an aminopropyl functionality (APMSN) as depicted in
Fig. 1. We demonstrated that SAMSN and APMSN could
serve as acid and base catalysts, respectively, for the wolf-and-
lamb type of one-pot reaction cascades. As a proof of princi-
ple, we examined the catalytic conversion of 4-nitrobenzalde-
hyde dimethyl acetal (Compound A) to (E)-1-nitro-4-(2-
nitrovinyl)benzene (Compound C), which involved two sepa-
rate reactions, i.e., an acid-catalyzed deprotection to yield the
4-nitrobenzaldehyde denoted as Compound B, followed by a
base-catalyzed Henry reaction in nitromethane to generate the
final product (E)-1-nitro-4-(2-nitrovinyl)benzene (Table 1).
The SAMSN and APMSN materials were synthesized via the
previously described co-condensation of tetraethyl orthosilicate
(TEOS) and 4-chlorosulfonophneylethylene-trimethoxysilane
(CSTMOS) (or 3-aminopropyl-trimethoxysilane (APTMOS))
in the presence of cetyltrimethylammonium bromide (CTAB)
as template under basic conditions as detailed in the ESI.w The
CTAB-removed SAMSN and APMSN materials were charac-
terized with nitrogen sorption analysis, powder X-ray diffrac-
tion (XRD), scanning electron microscopy (SEM),
transimission electron microscopy (TEM), and the 13C and
29Si solid-state NMR. The TEM (Fig. 1a and b) and SEM
Nature’s strategy of employing multistep reaction cascades
for the synthesis of complex and bioactive organic molecules
in living systems has long been a goal for designing artificial
catalysts. While recent advancements in supramolecular chem-
istry, nanomaterial synthesis, and catalyst design have signifi-
cantly improved our ability in mimicking this ingenious
strategy of biocatalysis, the progress in constructing a compa-
tible multifunctional catalytic system that can operate syner-
gistically in one-pot sequential reactions is still relatively
limited. Nonetheless, the recently developed ‘‘site isolation’’
concept for synthesizing surface-supported catalysts with
multiple functionalities has led to many efficient biomimetic
catalysts. In these systems, the different and often incompa-
tible catalytic functionalities, such as acidic and basic groups,
are separately isolated within the supporting matrices. The
spatial separation of these chemical species that react avidly
with each other in solution prevents the undesired self destruc-
tion of the catalytic capability. A few recent literature reports
have highlighted the success of this approach. For example,
Cohen and co-workers first developed a ‘‘wolf and lamb’’ two-
stage reaction system, where a soluble reagent reacts first with
one polymeric reagent and the product with the second poly-
meric reagent.1 Blum, Avnir et al. further developed ‘‘wolf and
lamb’’ type one-pot reactions by encapsulating opposing
catalysts in a sol–gel.2–6 More recent investigations on using
soluble star polymers7 and other polymers8 as site-isolating
matrices also led to effective catalytic systems for one-pot
reaction cascades.
Ever since the discovery of MCM-41 mesoporous silica,9
these structurally ordered materials have been regarded as the
ideal solid support for various catalysts due to their high
g
surface areas (4800 m2 À1) and tunable pore sizes (2–20
nm). Several recent reports on the multifunctionalization of
Department of Chemistry, U.S. Department of Energy Ames
Laboratory, Iowa State University, Ames, IA 50011, USA.
E-mail: vsylin@iastate.edu; Fax: +1 515-294-0105;
Tel: +1 515-294-3135
w Electronic supplementary information (ESI) available: Syntheses of
APMSN and SAMSN, nitrogen adsorption/desorption isotherms and
pore size distributions, powder small angle XRD, SEM images, 29Si
solid state NMR spectra of APMSN and SAMSN. See DOI: 10.1039/
b806664g
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Fig. 2 13C CP-MAS spectra of SAMSN (above) and APMSN
(bottom).
and chemcial structures of the desired organic acid and base
functionalities were quantatively verified by the 13C solid-state
CP-MAS NMR spectra (Fig. 2). The loading of sulfonic acid
was determined to be 0.32 mmol gÀ1 of SAMSN, whereas the
loading of amine was 0.40 mmol gÀ1 of APMSN.
Fig. 1 Mesoporous silica nanoparticles functionalized with an ethyl-
phenylsulfonic acid (SAMSN) and an aminopropyl group (APMSN).
The transmission electron micrographs (TEM) of SAMSN (a) and
APMSN (b). Scale bar = 50 nm.
(Fig. S1 of the ESIw) images of SAMSN and APMSN showed
that both materials have the typical MCM-41 type, highly
ordered parallel channel-like porous structure packed in a
hexagonal symmetry. The XRD diffraction patterns (Fig.
S2w) further confirmed the MCM-41 type mesoporous structure
with the d100 = 42.5 A and 40.9 A (SAMSN and APMSN,
respectively). As illustrated in Fig. S3,w the N2 surface sorption
analysis showed very high total surface areas (SAMSN: 827.9
In order to test the compatibility of these mesopore-
confined acid and base solids in catalyzing the one-pot
reaction cascade, different molar ratios of SAMSN : APMSN
were applied to the chemical transformation of 4-nitrobenzal-
dehyde dimethyl acetal (Compound A) to (E)-1-nitro-4-
(2-nitrovinyl)benzene (Compound C). Reactions without any
catalysts, with either free acid or free base, and with non-
functionalized MSN were also performed as control experi-
ments (Table 1).
m2 À1; APMSN: 789.0 m2 À1) and narrow pore diameter
g g
distributions (SAMSN: 25.4 A; APMSN: 22.3 A).
As shown in entries 1–4 in Table 1, the acid-catalyzed
deprotection of A, which is the first step of this two-step
cascade reaction, was completed within 24 h in the presence
of SAMSN (1.0 mol%) and APMSN (1.0–6.0 mol%). The
result suggested that the sulfonic acid functionality of SAMSN
was not affected by the presence of APMSN. Interestingly, the
conversion of B to the final product C of the second Henry
The 29Si solid-state crossed polarization magic angle spining
(CP-MAS) NMR (Fig. S4 and S5w) of these materials con-
firmed the covalent linkage between the organic functional
groups to the silica surfaces as indicated by the T2 and T3
peaks, which are derived from (RSiO)2Si(OH)R and
(RSiO)3SiR, respectively.19–21 Furthermore, the presence
Table 1 One-pot reaction cascade catalyzed by SAMSN and APMSN
Entry
SAMSN (mol%)
APMSN (mol%)
Conversion of A (%)
Yield of B (%)
Yield of C (%)
1
2
3
4
5
6
7
8
9
10
1.0
1.0
1.0
1.0
1.0
0
1.0
1.0
2.0
4.0
6.0
0
1.0
99.9
100
100
100
98.1
0
0
0
0
0
56.4
22.0
5.0
2.3
98.1
0
0
0
0
0
43.5
78.0
95.0
97.7
0
0
0
0
0
1.0 (tert-butylamine)
1.0
1.0 (tert-butylamine)
Pure MSN
1.0 (para-toluenesulfonic acid)
1.0 (para-toluenesulfonic acid)
Pure MSN
0
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reaction was significantly enhanced from 43.5 to 97.7% as the
amount of base-catalyst (APMSN) increased from 1.0 to 6.0
mol%. Also, different amounts of compound B (56.4–2.3%)
were isolated at the end of 24 h in these reactions (entries 1–4)
of various quantities of APMSN. Apparently, the more
APMSN introduced to the reaction mixture, the faster the
kinetics that could be achieved in the Henry reaction.
Furthermore, the desired Henry adduct C did not form in
entries 5–6, where only one of the two MSN catalysts was
present in the reaction. In entries 7 and 8, a molecular base
(tert-butylamine) and an acid (para-toluenesulfonic acid) that
are structurally similar to the corresponding organic groups
immobilized in APMSN and SAMSN, respectively, were used
to replace the solid catalysts. As predicted, these molecular
substitutes could freely diffuse into the mesopores of APMSN
(or SAMSN) and reacted to the surface-anchored acidic/basic
functional groups. The deactivated solids could no longer
catalyze the reaction cascade. This homogeneous acid–base
neutralization-induced destruction was confirmed by mixing
both tert-butylamine and para-toluenesulfonic in the reaction
solution (entry 9).
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In conclusion, we have demonstrated that by confining an
organic acid and base inside mesoporous silica nanoparticles,
these opposing reagents can be isolated and can serve as
effective catalysts for a one-pot reaction cascade that requires
incompatible catalysts. We envision that this approach can be
further developed into a general design principle for mimick-
ing biological systems, in which a series of reactions are
catalyzed by different enzymes in a precise sequence.
The authors thank the Office of Basic Energy Sciences of the
U.S. Department of Energy (DOE) under Contract No.
DE-AC02-07CH11358 for providing financial support of this
research.
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