Mesoporous organosilicas with acidic frameworks and basic sites in the pores: An approach to cooperative catalytic reactions

Article abstract of DOI:10.1002/anie.200903985

Two In one: A periodic mesoporous phenylene silica (PMO) with acidic framework walls and basic pore channels was obtained by using protecting-group techniques. This approach allows location of the acidic groups in the hydrophobic benzene layers and of the amine groups in the hydrophilic silica layers. The afunctional material is an efficient catalyst for the tandem conversion of benzaldehyde dimethyl acetal into 2-nitrovinylbenzene (see scheme). chemical equation presentation 2010 Wiley-VCH Verlag GmbH &. Co. KGaA, Weinheim.

Full text of DOI:10.1002/anie.200903985

Communications
DOI: 10.1002/anie.200903985
Bifunctional Catalysis
Mesoporous Organosilicas with Acidic Frameworks and Basic Sites in
the Pores: An Approach to Cooperative Catalytic Reactions**
Sankaranarayanapillai Shylesh,* Alex Wagener, Andreas Seifert, Stefan Ernst, and
Werner R. Thiel*
Recently there has been significant progress in mimicking
natureꢀs multistep reaction cascades for the synthesis of
structurally complex organic molecules.[1] Well-controlled
multifunctionalization of solid supports can be an efficient
strategy for the design of cooperative catalytic systems.[2] This
approach requires that the relative concentrations and the
proper spatial arrangement of all functional groups are
controlled. Biocatalysts such as enzymes immobilize mutually
incompatible functional groups without destruction and allow
these functional groups to act independently or in a cooper-
ative manner.[3] To mimic such multistep reaction sequences
in one-pot reactions will be effective in terms of waste and
cost reduction.
Periodic mesoporous organosilicas (PMOs) derived from
organosilanes of the type (RO)3Si-X-Si(OR)3 are a unique
class of materials, as various organic functionalities can be
integrated into the stable inorganic frameworks in a well-
directed manner.[4] Development of these species has opened
up a wide area for the chemical design of novel nanoporous
materials. For instance, bifunctional mesoporous organosilica
materials in which one functional group is located in the pore
and the other in the framework were first reported by the
groups of Ozin and Markowitz.[5] Ozin and co-workers
described the formation of materials having both bridging
ethylene groups in the framework and terminal vinyl groups
in the channel pores, whereas the Markowitz group reported
organosilica materials having bridging ethylene groups in the
framework and a variety of other functional groups in the
channel pores, obtained by a one-step co-condensation
method. Thereafter, porous organosilica materials with
bifunctional character were considerably refined by integra-
tion of multiple functional groups into a single material.[6]
Among the class of organosilica materials, PMOs with
phenylene bridges (derived from (RO)3Si-C6H4-Si(OR)3) are
important owing to their quasi-crystalline pore walls, in which
hydrophobic benzene layers alternate with hydrophilic silica
layers with a periodicity of 7.6 ꢁ.[7] An important aspect of
the PMO synthesis is that the channel pores as well as the
framework can be functionalized, thus allowing the design of
bifunctional materials with distinguishable locations of the
functional groups.[2a] Recently, a few reports highlighted this
approach and described the immobilization of incompatible
acids and bases for one-pot acid–base reactions. For instance,
the combination of weak and strong acids such as silanol
groups, urea derivatives, or sulfonic acids with various organic
bases on solid supports was investigated, and synergistic
catalytic enhancements were observed.[8] However, the effi-
ciency and selectivity of these catalysts are relatively poor
owing to the lack of a continuous range of acidic and basic
catalytic sites. Besides, acidic and basic functions with
sufficient strength were necessary for the successful promo-
tion of both acid- and base-catalyzed reactions. Hence,
innovative synthetic efforts for the synthesis of heterogeneous
multifunctional catalysts have to be worked out, which
maintain and control the independent functionalities and
give high concentrations of acidic and basic sites.
Herein we report a synthetic procedure to generate a
successful cohabitation of two antagonistic functional groups
in a periodic mesoporous organosilica: the acidic groups
should be located in the framework walls and the basic groups
directed into the channel pores. To achieve this aim, 1,4-
bis(triethoxysilyl)benzene and 3-aminopropyltrimethoxysi-
lane were hydrolyzed in the presence of cetyltrimethylam-
monium bromide (CTAB), resulting in the amine-functional-
ized mesoporous phenylene-bridged silica material PMO-
NH2. In the next step, the amino groups were protected using
di-tert-butyl-dicarbonate, a strategy commonly used for
amino group protection.[9] This procedure yielded PMO-
NHBoC, which could be sulfonated at the bridging phenylene
units by simple treatment with chlorosulfonic acid, giving
PMO-SO3H-NHBoC.[10] Deprotection of the amino groups
by thermal treatment gave the bifunctional mesoporous
catalyst PMO-SO3H-NH2, in which the sulfonic acid groups
are located on the hydrophobic phenylene layers and the
propylamine groups are attached to the hydrophilic silica
layers (Scheme 1). Therefore the catalytically active sites are
separated and give rise to an organosilica sample containing
the two antagonistic functional groups.
[*] Dr. S. Shylesh, A. Wagener, Prof. Dr. S. Ernst, Prof. Dr. W. R. Thiel
Fachbereich Chemie, TU Kaiserslautern
Erwin-Schrꢀdinger-Strasse Geb. 54, Kaiserslautern (Germany)
Fax: (+49)631-205-4676
E-mail: shylesh19@gmail.com
thiel@chemie.uni-kl.de
The intermediates and the final bifunctional PMO-SO3H-
NH2 catalysts were characterized systematically by 13C CP-
MAS NMR, 29Si CP-MAS NMR, X-ray photoelectron spec-
troscopy (XPS), powder X-ray diffraction (PXRD), and N2
adsorption–desorption measurements.
Dr. A. Seifert
Institut fur Chemie, TU Chemnitz
Strasse der Nationen 62, Chemnitz (Germany)
[**] The Alexander von Humboldt Foundation is gratefully acknowl-
edged for a research grant to S.S.
The successful cohabitation of the acidic framework walls
and basic pore channels in PMO-SO3H-NH2 was confirmed
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903985.
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Angew. Chem. Int. Ed. 2010, 49, 184 –187

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Chemie
and ꢀ70.5) and to the propyl chain (d = ꢀ70.5 and ꢀ61.0),
respectively (see the Supporting Information, Figure S2).[12]
n
ꢀ
The absence of quaternary Q sites indicates that the C Si
bonds remain untouched during the gelation process, the
sulfonation reaction, and the thermal treatments, which we
anticipated would give a more hydrophobic nature and a
pseudo-crystalline structure to the phenylene silica PMO.
The amount of sulfonic acid was quantitatively deter-
mined by titration. The acid content of PMO-SO3H-NH2 was
estimated to be 0.81 mmolgꢀ1, which is almost equivalent to
the acid content in PMO-SO3H-NHBoC (0.92 mmolgꢀ1).
This finding shows that the deprotection of the amino groups
had only a minor impact on the acid character of the SO3H
groups. Besides, it is noteworthy that the N/S ratio obtained
from the elemental analysis does not change after thermal
treatment. PMO-SO3H-NH2 contains about 1.79 mmolgꢀ1
amine and 1.10 mmolgꢀ1 sulfonic acid. The low rate of
conversion of phenylene groups into sulfonated species in the
phenylene silica PMO may be related to a low reactivity of the
covalently bound phenylene groups in the siloxane network.[7]
Powder X-ray diffraction (PXRD) patterns showed the
characteristic reflections at low diffraction angles assigned to
the mesoporous structure of the material (see the Supporting
Information, Figure S3). Moreover, the PXRD patterns at
108 < 2q < 408 display peaks with d spacings of 7.6, 3.8, and
2.5 ꢁ (Figure 2), which can be assigned to the molecular-scale
periodicity in the pore walls with a spacing of 7.6 ꢁ.
Evidently, the structural integrity of the materials is main-
tained during the functionalization process.[12]
Scheme 1. Synthesis of the bifunctional PMO-SO3H-NH2 material.
by NMR spectroscopy. The 13C CP-MAS NMR spectrum of
PMO-NH2 shows the signals of both the phenylene units (d =
134 ppm) and the aminopropyl chains (CH2N d = 43.0, CH2
d = 23.7, CH2Si d = 10.3 ppm, Figure 1). In addition to these
Figure 1. 13C CP-MAS NMR spectra of a) PMO-NH2, b) PMO-NHBoC,
and c) PMO-SO3H-NH2. * in (a) denotes residual peaks of CTAB,
which disappear after the sulfonation reaction (c).
signals, the 13C NMR spectrum of PMO-NHBoC depicts
Figure 2. Powder X-ray diffraction patterns of a) PMO, b) PMO-NH2,
c) PMO-NHBoC, and d) PMO-SO3H-NH2.
ꢀ
sharp resonances at d = 28.0 (tert-butyl), 78.6 (C O), and
=
157.1 ppm (C O), which confirm the BoC protection of the
amine groups. In the 13C-CP-MAS NMR spectrum of PMO-
SO3H-NH2, the resonances associated with the BoC protec-
tion have disappeared, while the resonances associated with
the propyl spacers are still present. Furthermore, the phen-
ylene signal at d = 134 ppm becomes broadened, and a weak
shoulder at d = 141.3 ppm occurs, which, according to the
Nitrogen adsorption/desorption isotherms show type IV
behavior (Figure 3). The BJH method yields pore size
distributions typical for slightly disordered materials (see
the Supporting Information, Figure S4). The PMO-NH2
sample shows a BET surface area, total pore volume, and
pore size of 905 m2 gꢀ1, 0.962 cm3 gꢀ1, and 25.7 ꢁ, whereas
PMO-NHBoC and PMO-SO3H-NH2 had corresponding
values of 794 m2 gꢀ1, 0.735 cm3 gꢀ1, 25.1 ꢁ and 768 m2 gꢀ1,
0.676 cm3 gꢀ1, 24.2 ꢁ, respectively. As evidenced by these
investigations, the structure of the crystal-like pore walls is
literature,[11] can be assigned to the C SO3H carbon atom.
ꢀ
The presence of the sulfonic acid groups was also confirmed
by XPS analysis (see the Supporting Information, Figure S1).
The solid-state 29Si CP-MAS NMR spectra show three signals
at d = ꢀ80.6, ꢀ70.5, and ꢀ61.0 ppm. They can be assigned to
T2 and T3 species for silicon attached to phenylene (d = ꢀ80.6
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185

Communications
yields, which indicates the cooperative behavior of the
bifunctionalized catalyst PMO-SO3H-NH2. Addition of
equivalent amounts of structurally similar free acid (p-
toluenesulfonic acid) or base (tert-butylamine) to solid
PMO-SO3H-NH2 stops the activity, as these homogeneous
species diffuse into the pore channels and destroy the catalytic
sites, presumably by formation of ion pairs. Thus the
formation of the nitroaldol product 3 was anticipated to
take place by a sulfonic acid catalyzed deacetalization
reaction and a subsequent nitroaldol reaction of nitrome-
thane with benzaldehyde. These results confirm that a well-
designed localization of acid and base functionalities in
mesoporous solids will lead to an effective catalyst for one-
pot reaction cascades.
The recyclability of the bifunctional catalyst PMO-SO3H-
NH2 was examined by isolating it from the reaction mixture
(centrifugation, washing with ethanol and dichloromethane,
and drying). Owing to the tight covalent anchoring and the
spatial separation of the organic functional groups, the
catalyst showed almost no loss in activity in the third run.
Furthermore, the 1H NMR spectroscopic analysis of the
reaction filtrate gave no hint of leaching of the immobilized
organic groups, and the elemental analysis of the recovered
catalyst confirmed the retention of the organic content on the
mesoporous surface.
Figure 3. Nitrogen adsorption–desorption isotherms of a) PMO-NH2,
b) PMO-NHBoC, and c) PMO-SO3H-NH2.
ꢀ
retained, and the C Si bonds of the phenylene silica PMO
survive the functionalization reactions.
The accessibility of the free amino and sulfonic acid
groups in PMO-SO3H-NH2 was evaluated in a tandem
reaction, the catalytic conversion of benzaldehyde dimethyl
acetal (1) into 2-nitrovinyl benzene (3). This reaction
sequence involves two separate steps: an acid-catalyzed
deprotection to give the intermediate benzaldehyde (2) and
the subsequent base-catalyzed nitroaldol (Henry) reaction to
yield 2-nitrovinyl benzene (3, Table 1).[8u]
Remarkably, the bifunctionalized PMO-SO3H-NH2
sample converts 1 into 3 in almost quantitative yields after
20 h. This result demonstrates that the reactivity of the
aminopropyl groups is not affected by the presence of sulfonic
acid groups in the sample. In the absence of amine groups (in
PMO-SO3H-NHBoC), benzaldehyde (2) is the only product,
while formation of a negligible amount of 3 was noted with
PMO-NH2. Thus, each catalyst (acid and base) on its own is
unable to promote the conversion 1!2!3 in quantitative
In summary, bifunctional mesoporous organosilicas pos-
sessing organic amines and sulfonic acid groups were
successfully generated and used in a cooperative catalytic
transformation. Compared to earlier reports, the current
methodology benefits from a precise location and concen-
tration of the active functional groups in a mesoporous
phenylene silica with crystalline pore walls, in which the acidic
groups reside mainly on hydrophobic benzene layers and the
basic amino groups on hydrophilic silica layers for coopera-
tive effects. Further investigation is currently underway
regarding enhancement of the acid–base properties of the
materials, additional catalytic enhancements, and advanced
applications.
Received: July 20, 2009
Revised: September 9, 2009
Published online: December 2, 2009
Keywords: bifunctional catalysts · domino reactions ·
heterogeneous catalysis · mesoporous materials · silicates
.
Table 1: One pot deacetalization-nitroaldol reaction cascade.[a]
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Entry Catalyst
Conv. of
1 [%]
Yield of
2 [%]
Yield of
3 [%]
1
2
3
4
PMO-SO3H-NH2
PMO-SO3H-NHBoC
PMO-NH2
PMO-SO3H-NH2 +tert-butyl trace
amine
100
100
trace
2.5
100
trace
trace
97.5
0
trace
trace
5
PMO-SO3H-NH2 +p-toluene- 100
sulfonic acid
100
trace
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CH3NO2 (5 mL), 908C, 20 h.
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