Bifunctionalized mesoporous materials with site-separated Bronsted acids and bases: Catalyst for a two-step reaction sequence

Article abstract of DOI:10.1002/anie.201004572

Peaceful coexistence: Bronsted acids and bases were attached to different surfaces of a mesoporous silica nanoparticle. The internal surface was functionalized by using co-condensation, and postsynthesis grafting was used to functionalize the external surface. A two-step reaction sequence that cannot proceed with an acid and base in the same pot was accomplished using the bifunctionalized nanoparticle (see scheme). Copyright

Full text of DOI:10.1002/anie.201004572

DOI: 10.1002/anie.201004572
Mesoporous Materials
Bifunctionalized Mesoporous Materials with Site-Separated Brønsted
Acids and Bases: Catalyst for a Two-Step Reaction Sequence**
†
Yulin Huang,* Shu Xu, and Victor S.-Y. Lin
Mesoporous silica has been regarded as an ideal support for
heterogeneous catalysts because of its high surface area and
tunable pore size. This interest has increased since the
mesoporous silica materials functionalized with sulfonic acid
and amine groups; there were limited amounts of each acid
and base group to neutralize each other during the one-pot
[1]
[23]
discovery of the ordered mesoporous material MCM-41,
which led to a range of possibilities for the chemical design of
reaction. Mehdi and co-workers reported another bifunc-
tionalized mesoporous silica material having sulfonic acid
groups within its framework and basic groups within the
channel pores, but the sulfonic acid groups were not
[2]
novel heterogeneous catalysts. There has been rapid devel-
opment of immobilized, reusable organocatalysts, and one of
the major research interests is the functionalization of
mesoporous silica with organic functional groups. The func-
tionalization can be accomplished by using either postsyn-
[
40]
accessible to reactants. Thiel and co-workers also recently
reported a functionalized periodic mesoporous organosilica
(PMO) having the acidic groups within the framework walls
and the basic groups directed into the channel pores.
However, there is no report on selective dual-functionaliza-
tion of a single mesoporous silica nanoparticle with Brønsted
acid and Brønsted base groups on the external and internal
mesoporous silica surface, respectively; this is presumably a
result of the incompatibility of these groups and the difficulty
of independently controlling reactions on both the external
[
3–27]
thesis grafting or co-condensation.
The design and syn-
thesis of bi- or multifunctionalized mesoporous silica con-
taining multiple types of functional groups is appealing
because these functional groups might be used as catalysts
for a multistep reaction sequence requiring either a cooper-
[3,7,9–14,16,28]
ative or independent catalytic performance.
Actually, in biological systems, there are many interesting
[41–49]
examples of multifunctional catalysts—enzymes such as a-
and internal surfaces.
[29]
amylases,
as they can catalyze different reactions using
Herein we report two mesoporous silica nanoparticles
(MSNs) that were functionalized with both a Brønsted acid
and Brønsted base; one group was attached on the internal
surface of the MSN through co-condensation and the second
group was tethered onto the external surface of the MSN by
postsynthesis grafting. Both of these functional groups on one
particle could catalyze each step of a two-step reaction
sequence; for example, sulfonic acid catalyzed hydrolysis of 4-
nitrobenzaldehyde dimethyl acetal and the subsequent
amine-catalyzed Henry reaction of 4-nitrobenzaldehyde
with nitromethane, a sequence that cannot be achieved
when the catalysts are combined in a one-pot homogeneous
system. These novel materials were synthesized by co-
condensation of tetraethyl orthosilicate (TEOS) and 3-
aminopropyltrimethoxysilane (APTMOS) [or 3-mercapto-
propyltrimethoxysilane (STMOS)] in the presence of cetyl-
trimethylammonium bromide (CTAB) as a template under
basic reaction conditions, and subsequent post-treatment for
grafting another functional group onto the external surface.
Typically, these bifunctional mesoporous materials were
synthesized by co-condensation of one of the two functional
different catalytic active sites.
There are many examples of bifunctionalized mesoporous
material catalysts in which two different organic functional
[
16,30]
groups, such as amines with silanols,
amines with
[
31,32]
[3,7]
thiols,
acids,
amines with ureas,
amines with Lewis
and adjacent sulfonic
[
33–37]
[9,11,14]
sulfonic acid with thiols
[
28]
acid functional groups, are incorporated and are compat-
ible with each other. Recently different catalysts, each located
on a mesoporous silica nanoparticle, have been used as
[
38]
catalysts in a one-pot reaction sequence. As we know, many
enzymes can immobilize mutually incompatible catalytic
groups, such as a Brønsted acid and a Brønsted base, on a
single molecule in a site-separated manner that maintains
their independent function to catalyze one step in a multistep
reaction sequence. Up to now, there are only few samples of
mesoporous materials displaying two functional groups that
cannot otherwise coexist in solution. For example, both Davis
[10]
[39]
and co-workers and Thiel and co-workers reported on
[*] Dr. Y. Huang, Dr. S. Xu, Prof. Dr. V. S.-Y. Lin
[
4]
Department of Chemistry and
groups onto the internal channels and subsequent grafting
of the second group onto the external surface; since template
CTAB is still in the mesoporous channels only the external
Ames Laboratory – U.S. Department of Energy
Iowa State University, Ames, IA 50011 (USA)
E-mail: ylhuang@iastate.edu
[
48–51]
surface is exposed to the grafting reagent.
The bifunc-
[†] Deceased May 4, 2010.
tional mesoporous silica nanoparticle with sulfonic acid on its
internal surface and amine groups on its external surface was
labeled as SAMSN-AP (Figure 1). Another bifunctional
mesoporous silica nanoparticle with amine groups on its
internal surface and sulfonic acid groups on its external
surface was labeled as APMSN-SA (see Figure S1 in the
Supporting Information). For each of the bifunctionalized
[
**] This research at the Ames Laboratory was supported by the U.S.
DOE, office of BES, under contract DE-AC02-07CH11358. We also
thank Prof. Robert J. Angelici at Iowa State University for his
suggestions concerning this work.
Supporting information for this article (including synthetic proce-
dures) is available on the WWW under http://dx.doi.org/10.1002/
anie.201004572.
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661

Communications
2
9
2
Figure 2. Si solid-state NMR spectrum of SAMSN-AP. (T : À59 ppm;
3
2
3
4
T : À68 ppm; Q : À90 ppm; Q : À100 ppm; Q : À110 ppm).
Figure 1. Syntheses of bifunctional mesoporous silica nanoparticles
having sulfonic acid groups on the internal surface and organic amine
groups on the external surface.
MSNs (Figures S2–S5), the functionalization of the external
surface through postsynthesis grafting was successful because
the mesopores of the material were blocked by surfactant
during the grafting process, and it is even possible that partial
displacement of surfactant during the grafting was limited
because of the lost condensation steps around P/P = 0.3
0
(
P/P : relative pressure; insets of Figures S2–S5).
0
13
Figure 3. C solid-state NMR spectrum of SAMSN-AP. (chemical shift
These materials [MSN, SAMSN (sulfonic acid function-
at d=29.5 ppm was from CTAB; chemical shift at d=26.0 ppm was
from the 3-mercaptopropyl group of the starting material).
alized MSN), APMSN (amine-functionalized MSN)] were
analyzed by N adsorption/desorption, X-ray powder diffrac-
tion (XRD), and C and Si solid-state NMR spectroscopy.
2
1
3
29
The N adsorption/desorption measurements for SAMSN-
AP and APMSN-SA showed type IV isotherms, which have
(Table 1). In all experiments, the amount of either the
amine or sulfonic acid functional groups was kept at
2.3 mol%. After the two-step reaction sequence had been
run using either SAMSN-AP or APMSN-SA as the catalyst,
the conversion of the starting material was 100% and more
than 97% of the mixture was the desired product C (Table 1,
entries 1 and 2). These results were consistent with the result
2
very clear H -hysteresis loops at a relatively high pressure,
1
characteristic of mesoporous materials, having BET surface
2
À1
2
À1
areas over 853 m g for SAMSN-AP, and 934 m g for
APMSN-SA; additionally the total pore volume for SAMSN-
3
À1
3
À1
AP was 0.8 cm g , and for APMSN-SA was 0.9 cm g .
There was also a very narrow pore-size distribution centered
at 2.8 nm for SAMSN-AP, and at 2.6 nm for APMSN-SA
Table 1: One-pot reaction cascades composed of acid-catalyzed hydrol-
(
Figures S6 and S7, and Table S1). Small-angle X-ray scatter-
[
a]
ysis and base-catalyzed Henry reaction.
ing patterns indicated highly ordered structures with
d100 values of 4.1 nm and 4.2 nm for SAMSN-AP and
APMSN-SA, respectively (Figure S8 and Table S1). The
TEM images in Figure S10 confirmed the mesoporous
structures as having parallel channels as well as a uniform
pore size.
Entry
Catalyst
B [%]
C [%]
Conv. of A [%]
2
3
29
T
and T peaks in the Si solid-state NMR spectra
1
2
3
4
5
6
7
8
9
10
SAMSN-AP
APMSN-SA
SAMSN/APMSN
SAMSN
APMSN
SAMSN-AP/AP
SAMSN-AP/PTSA
APMSN-SA/AP
APMSN-SA/PTSA
MSN
2.3
1.9
4.5
100
0
0
100
0
97.7
98.1
95.5
0
0
0
0
0
0
100
100
100
100
0
0
100
0
(
Figure 2 and Figure S11) indicated the incorporation of
1
3
sulfonic acid and amine groups. The C solid-state NMR
spectra (Figure 3 and Figure S12) indicated the presence of
the intact organic functional groups and the removal of the
surfactant. Elemental analyses of SAMSN-AP and APMSN-
À1
SA showed that each of the materials contained 0.35 mmolg
À1
of sulfur and 0.35 mmolg of nitrogen, which means the
100
0
100
0
concentration of sulfonic acid on MSN particles is equal to
that of the amine, and has a sulfur/nitrogen ratio around 1.0.
The activities of these immobilized bifunctional catalysts
were tested in a one-pot reaction sequence involving the
hydrolysis of an acetal and subsequent Henry reaction
0
[
a] Reaction conditions: Catalyst: A (100.0 mg, 1.5 mmol), H O
2
(
1.5 mmol) CH NO (1.0 mL), 808C, 48 h. Conversion and yields were
3 2
determined using GC data. AP: 1-aminopropane, PTSA: p-toluenesul-
fonic acid.
6
62
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in entry 3 wherein the amine and sulfonic acid were each
located on different mesoporous silica nanoparticles, SAMSN
and APMSN. However, neither SAMSN nor APMSN showed
any conversion of the reactant A into the product C (Table 1,
entries 4 and 5) even though SAMSN could catalyze the first
step of this two-step sequence. Interestingly, no conversion of
the starting material to the final product was observed when
either of the homogeneous analogues of the sulfonic acid or
amine was used with the SAMSN-AP or APMSN-SA
internal or external surfaces. Therefore, a series of Henry
reactions catalyzed by five different 3-aminopropyl-function-
alized MSNs with different concentrations of the surface-
bound amines (Figures S13–S15) were investigated (Scheme 1
and Figure 5). From the fitted curve of TOF versus the
(
Table 1, entries 6–9); this behavior results from the fact
Scheme 1. Henry reaction catalyzed by APMSN.
that the functionalities neutralized each other. The pure MSN,
as an experimental control, led to no conversion (Table 1,
entry 10).
These bifunctionalized MSNs (SAMSN-AP and APMSN-
SA) can be recycled five by simple filtration after each use
without any detectable decrease in catalytic activity
(
Table S2), thus confirming that these two functional groups
were quite stable and appropriately site-separated on the
different MSN surfaces.
Although the one-pot reaction sequence including acetal
hydrolysis and a Henry reaction was studied to establish proof
of our site-separation of a Brønsted acid and base on a single
mesoporous silica nanoparticle, we investigated the kinetics
of these catalysts (Figure 4) to provide a comparison to our
Figure 5. Fitted curve of base activity versus base concentration on the
APMSN surface (reaction conditions were the same as those used in
Table 1).
concentration of the base on the MSN surface (mmol amine
per square meter surface), the catalytic activity (TOF)
decreased dramatically when the surface coverage of catalyst
was increased; the same trend was observed in the APMSN-
catalyzed Henry reaction at different reaction times (Figur-
es S16–S19). Furthermore, the relationship between the TOF
and the functional-group concentration was investigated for
the one-pot reaction sequence catalyzed by the bifunctional
materials (Table S3); overall, the yield of the final product
was increased and the TOF decreased when the concentration
of basic sites was increased.
In conclusion, by combining co-condensation to function-
alize the internal surface of MSNs and postsynthesis grafting
to functionalize the external surface, we have shown that site-
separation of Brønsted acid and Brønsted base sites on a
single mesoporous silica nanoparticle was successful. As a
result of this ideal site-separation, reaction sequences requir-
ing two or more catalysts, which are incompatible with each
other in a homogenous solution, were carried out successfully
using our bifunctionalized particle. This model is useful for
systems in which a series of reactions are catalyzed by only
one multifunctional enzyme or catalyst. At the same time, we
also demonstrated that the activity of the catalyst on the
Figure 4. Turnover frequency of the acid and base catalysts that are
located on either the internal or external surfaces of SAMSN-AP and
APMSN-SA.
[
38]
earlier reported results.
The reactivity (TOF: turnover
frequency) of both the acid and base decreased with increas-
ing reaction time because of the decreased concentration of
the reactant. Although silanol groups on the external surface
of MCM-41 were more kinetically accessible than those on
[
44,52,53]
the internal surface,
both the acid and base introduced
by co-condensation methods onto the internal surface of the
MSNs showed higher reactivity (TOF) than their counter-
parts, wherein the groups were grafted onto the external
surface of the MSNs.
These kinetic results indicate that 1) there may not be any
diffusion limitation in our MSN-based catalysts and 2) the
reactivity of the acid and base might be related to the
dispersion or surface coverage of the catalytic sites which
differs for the acid or base catalysts that are located on either
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www.angewandte.org
663

Communications
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the MSN surface and can envision better control of the
kinetics and efficiency of a catalyst by changing the number of
catalytic sites on every unit surface of MSN.
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