Multifunctional heterogeneous catalysts: SBA-15-containing primary amines and sulfonic acids

Article abstract of DOI:10.1002/anie.200602243

(Chemical Equation Presented) Dual functions: The simultaneous immobilization of primary amines and sulfonic acids on mesoporous silica SBA-15 (SBA-15-A/B; see scheme) creates a heterogeneous catalyst that exhibits cooperative catalytic behavior in the aldol condensation reaction. The respective chemical reactivities of the acid and base groups are maintained, whereas homogeneous analogues are neutralized in solution. TEOS = tetraethoxysilane.

Full text of DOI:10.1002/anie.200602243

Communications
Acid–Base Catalysis
DOI: 10.1002/anie.200602243
Multifunctional Heterogeneous Catalysts:
SBA-15-Containing Primary Amines and Sulfonic
Acids**
Ryan K. Zeidan, Son-Jong Hwang, and Mark E. Davis*
A current area of interest in the design and synthesis of solid
catalysts is the creation of materials that contain multiple
types of active centers. These functionalities may be used to
perform several steps in a reaction sequence or work in a
cooperative manner to alter the characteristics of a single
reaction, for example, rates, selectivities, and so forth.
Enzymes, such as aldolases, are interesting examples of
multifunctional catalysts as they are capable of conducting
multistep reactions. Enzymes immobilize mutually incompat-
[*] R. K. Zeidan, S.-J. Hwang, Prof. M. E. Davis
Division of Chemistry and Chemical Engineering
California Institute of Technology
Pasadena, CA 91125 (USA)
Fax: (+1)626-568-8743
E-mail: mdavis@cheme.caltech.edu
[**] Technical assistance from Stacey Zones, Allen Barton, and Ken Ong
of Chevron is gratefully acknowledged. This study was supported by
a grant from the US Department of Energy. SBA-15 is a mesoporous
silica (Santa Barbara Amorphous).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
6332
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 6332 –6335

Angewandte
Chemie
Table 1: Acid–base aldol catalysis.
ible functional groups in a manner that maintains their
independent functionality, and as such, are able to carry out
multistep reaction sequences with functionalities that would
not be tolerated together in solution.
Previous work on bifunctional solid-catalyst synthesis
largely involved the use of metal centers with acid sites.
Recently, attention has been focused on combinations of
organic functional groups. Immobilized organic amines (base)
have been used with supported silanols (acid)[1] to perform
bifunctional catalysis, and amine functional groups have been
immobilized together with thiols.[2,3] Dufaud and Davis[4]
reported on the positioning of adjacent sulfonic acid func-
tional groups, and Zeidan et al.[5] have also incorporated
sulfonic acids and thiols that exhibit a cooperative effect in a
condensation reaction. Also, bifunctional catalysis with
different active centers on separate supports[6–9] and hydro-
gen-bonding ureas with amine bases[10] have been investi-
gated. However, no reports of a material containing an
organic acid and organic base that are incorporated into the
same material have appeared, presumably as a result of the
incompatibility of these groups. Herein, we report a solid
material that contains both organic acid and base groups that
together exhibit a reactivity in the aldol condensation of
acetone with 4-nitrobenzaldehyde that is not achievable with
these same groups in solution.
Entry
1
Catalyst (10 mol%)
A [%]
B [%]
Conv. [%][a]
62
45
17
2
8
8
16
3
4
5
25
30
0
8
14
0
33
44
0
SBA-15-A/SBA-15-B[b]
SBA-15
6
0
0
0
7
8
3
3
5
1
8
4
A sulfonic acid and amine were incorporated into SBA-15
(Santa Barbara Amorphous) through a “one-pot” synthesis
using poly(ethylene glycol)-block-poly(propylene glycol)-
block-poly(ethylene glycol) (P123), HCl, H2O, and tetra-
ethoxysilane (TEOS; Scheme 1) to prepare the dual-func-
tionalized mesoporous catalyst SBA-15-A/B.
1
[a] Total conversion. Yields determined through H NMR spectroscopic
analysis with THF as the internal standard. [b] 1:1 mixture of sulfonic
acid functionalized SBA-15 (SBA-15-A) and amine-functionalized SBA-15
(SBA-15-B).
and/or sulfonic acid functional groups was kept
constant at 0.05 mmol. Significant production of
both products occurred and gave a total conversion
of 62%. Use of SBA-15 functionalized only with a
sulfonic acid or amine functionality gave signifi-
cantly lower levels of conversion (Table 1, entries 2
and 3). A physical mixture of acid-functionalized
Scheme 1. Catalyst synthesis.
SBA-15 and amine-functionalized SBA-15 showed
an intermediate level of conversion that was signifi-
cantly lower than the dual-functionalized acid/base
This material (as well as SBA-15 functionalized with
sulfonic acid alone and propylamine alone) was analyzed by
13C and 29Si CP/MAS solid-state NMR spectroscopy, X-ray
diffraction (XRD), N2 adsorption, and thermogravimetric
analysis (see Supporting Information).
The characterization data show that the functionalized
materials have surface areas of between 600 and 800 m2 gÀ1
and pore sizes on the order of 40–60 Š, typical of SBA-15-
type materials.[4,5] The 13C NMR spectra indicate the presence
of the intact organic functional groups, and the 29Si NMR
spectrum indicates the incorporation of organic functional
groups.
The immobilized catalysts were tested for activity in the
aldol condensation of 4-nitrobenzaldehyde with acetone to
give the resulting aldol addition and dehydration products.
SBA-15-A/B was used as a catalyst for this reaction (Table 1,
entry 1), and in all experiments the total amount of amine
catalyst (Table 1, entry 4). The support alone caused no
conversion (Table 1, entry 5). Interestingly, the homogeneous
analogues of the sulfonic acid and amine showed no
conversion of the starting materials into either product, as
these functionalities apparently neutralize each other and salt
out of the reaction mixture (Table 1, entry 6). Also, the
individual acid and base homogeneous analogues gave rise to
very low conversions (Table 1, entries 7 and 8), much less than
the acid/base bifunctional catalyst. The observed cooperative
effect indicates some level of acid/base pairing in this
material. With the two functional groups being dispersed
randomly in the synthesis, it was expected that there would be
acid sites neighboring acid sites, base sites neighboring base
sites, and some acid sites in proximity to base sites. If the
catalytic activity is due to single acid or base sites, or
neighboring acid/acid or base/base sites, the level of con-
version in entries 2 and 3 (Table 1) would be the maximum
Angew. Chem. Int. Ed. 2006, 45, 6332 –6335
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6333

Communications
achievable. The enhancement of reactivity beyond those cases
is likely due to interactions between neighboring acid and
base sites (see Supporting Information for the mechanism).
It is expected that the acid and base groups in SBA-15-
A/B would be in equilibrium between the free acid and base
and the ion pair that results from neutralization, and that the
solvent would have a dramatic effect on this equilibrium, that
is, protic solvents would confer different properties to aprotic
solvents as a result of differing abilities to stabilize the
neutralized ion pair. The equilibrium would lie towards the
ion pair in polar, protic solvents, as proton exchange would be
rapid and the protic solvent would stabilize the ion pair the
most. Nonpolar, aprotic solvents would cause slower
exchange of the protons and would stabilize an ion pair
much less than a protic solvent, thus forcing the equilibrium in
favor of the free acid and base. To investigate these
possibilities, reactions were carried out in a variety of
cosolvents to examine the effect of solvent polarity on
conversion. The conversion into both products is nearly
30% in polar, protic solvents, such as water and methanol
(Table 2, entries 1 and 2). The conversion more than doubles
1
Figure 1. Solvent effect on the line shape of H MAS NMR spectra.
Sharp lines are due to incomplete deuteration of the solvents.
Table 2: Solvent effect on the catalysis.
restricted motion of the two groups as the acid and base
functional groups associate to avoid unfavorable interactions
with nonpolar solvents. The trend is in full agreement with the
catalysis data as reactions in nonpolar solvents cause the polar
acid and base groups to associate to a greater extent, thus
allowing for an increased cooperative effect. The groups are
more mobile in polar solvents as they favorably interact with
the solvent and are less likely to interact with one another for
cooperative catalysis to occur. Taken together, the catalytic
and NMR spectroscopic data suggest that the catalytic
behavior of SBA-15-A/B is clearly dictated by multiple
factors, two of which seem to be the state of the equilibrium
between the acid and base and the ion pair and the
interactions between the two functional groups as caused by
polar/nonpolar interactions with the solvent.
Entry
Cosolvent (1:1)[a]
A [%]
B [%]
Conv. [%][b]
1
2
3
4
5
6
H2O
MeOH
Et2O
CHCl3
hexane
benzene
27
14
52
60
75
62
7
9
13
9
13
12
34
23
65
69
88
74
[a] 50:50 mixture of acetone/cosolvent. [b] Total conversion. Yields
determined through 1H NMR spectroscopic analysis with THF as the
internal standard.
To probe the bifunctionality of the catalyst, SBA-15-A/B
was treated with acid to neutralize the amine functional
groups, thus leaving only the sulfonic acid sites. SBA-15-A/B
was washed with 1m HCl or aqueous para-toluenesulfonic
acid (p-TSA; Table 3, entries 1 and 2, respectively), and it is
clear that the material behaves as the immobilized sulfonic
acid alone (Table 1, entry 2). Likewise, SBA-15-A/B was
treated with propylamine (Table 3, entry 3) and the resulting
material, in which the sulfonic acid sites are neutralized,
behaved the same as the amine-functionalized catalyst
(Table 1, entry 3). These results illustrate the bifunctionality
of SBA-15-A/B and the coexistent acidity and basicity. These
experiments highlight the interesting properties achievable by
immobilization of multiple functional group types. The amine
and sulfonic acid salt out in solution and give rise to no
conversion (Table 1, entry 6), and when the heterogeneous
catalyst SBA-15-A/B is treated with homogeneous acids or
bases the opposing functional group salts out and the
respective behavior is lost. On the other hand, the two
opposing functional groups do not lose catalytic activity when
the amine and sulfonic acid are immobilized on SBA-15-A/B
to about 70% upon moving to polar, aprotic solvents, such as
diethyl ether and chloroform (Table 2, entries 3 and 4).
Nonpolar, aprotic solvents, such as hexane and benzene
(Table 2, entries 5 and 6), give further improvements in
conversion, which approach 90% in the case of hexanes
(Table 2, entry 5). Thus, there is a clear effect on conversion
that is related to solvent polarity, thereby suggesting that the
nature of the solvent does affect the equilibrium of proton
exchange and hence the effective concentration of free acid
and base.
Solid-state CP/MAS NMR spectroscopic analysis was
used to probe the local motion of the bifunctional catalyst.
SBA-15-A/B was fully deuterated and then slurried in
different deuterated solvents to study the changes in motion
of the functional groups in these different environments
(Figure 1). The resolution of the 1H NMR resonances in D2O
and CD3OD (from non-exchanging protons in the organic
functional groups) sharpened, thus indicating a more mobile
structure. However, the resonances broadened in the non-
polar solvents CD3Cl and C6D6, thus suggesting more
6334
www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 6332 –6335

Angewandte
Chemie
of the cosolvent studies, 10 mL of a 50:50 mixture of acetone and
Table 3: Catalyst quenching experiments.
cosolvent was used.
Received: June 5, 2006
Revised: July 5, 2006
Published online: August 28, 2006
Entry Catalyst
1
A [%] B [%] Conv. [%][a]
Keywords: aldol reaction · amines · heterogeneous catalysis ·
.
silica · sulfonic acids
7
7
14
[1] J. D. Bass, A. Solovyov, A. J. Pascall, A. Katz, J. Am. Chem. Soc.
2006, 128, 3737 – 3747.
2
3
8
7
7
15
38
[2] D. Coutinko, S. Madhugiri, K. J. Balkus, J. Porous Mater. 2004,
11, 239 – 254.
[3] J. D. Bass, A. Katz, Chem. Mater. 2006, 18, 1611 – 1620.
[4] V. Dufaud, M. E. Davis, J. Am. Chem. Soc. 2003, 125, 9403 –
9413.
[5] R. K. Zeidan, V. Dufaud, M. E. Davis, J. Catal. 2006, 239, 299 –
306.
[6] F. Gelman, J. Blum, D. Avnir, Angew. Chem. 2001, 113, 3759 –
3761; Angew. Chem. Int. Ed. 2001, 40, 3647 – 3649.
[7] B. Voit, Angew. Chem. 2006, 118, 4344 – 4346; Angew. Chem. Int.
Ed. 2006, 45, 4238 – 4240.
31
[a] Total conversion. Yields determined by 1H NMR spectroscopic
analysis with THF as the internal standard.
[8] K. Motokura, N. Fujita, K. Mori, T. Mizugaki, K. Ebitani, K.
Kaneda, J. Am. Chem. Soc. 2005, 127, 9674 – 9675.
[9] B. Helms, S. J. Guillaudeu, Y. Xie, M. McMurdo, C. J. Hawker,
J. M. J. Frechet, Angew. Chem. 2005, 117, 6542 – 6545; Angew.
Chem. Int. Ed. 2005, 44, 6384 – 6387.
[10] S. Huh, H. T. Chen, J. W. Wiench, M. Pruski, V. Lin, Angew.
Chem. 2005, 117, 1860 – 1864; Angew. Chem. Int. Ed. 2005, 44,
1826 – 1830.
but rather function in a cooperative manner to give the most
active catalyst used in this study.
In summary, a new approach to carrying out heteroge-
neous catalysis with functional groups that are incompatible
and incapable of catalyzing reactions in solution has been
reported. By immobilizing functional groups that can not
coexist in solution as active catalytic sites, it has been
demonstrated that reactivity can be restored and a cooper-
ative, catalytic effect is observed between the functional
groups. Advanced solid-state NMR experiments are in
progress to study the proton motion and spacing of functional
groups in these materials, and further catalytic applications
are being investigated.
Experimental Section
“One-pot” synthesis of catalyst: HCl (120.0 mL, 240.0 mmol, 2.0m)
and H2O (6.0 mL, 333.0 mmol) were added to triblock copolymer
P123 (4.0 g, 0.688 mmol, Mn = 5800) and stirred at 408C until the P123
was fully dissolved. TEOS (8.2 mL, 37.8 mmol) was then added, and
the reaction mixture was allowed to stir at 408C for 45 min. Then 2-(4-
chlorosulfonylphenyl)ethyltrimethoxysilane (0.65 g) in dichlorome-
thane (1.0 mmol, 50/50 (w/w)) was added followed by 3-amino-
propyltrimethoxysilane (0.19 mL, 1.0 mmol). The reaction mixture
was then stirred at 408C for 20 h, and then left at 1008C for 24 h. The
resulting solid was filtered and rinsed with excess H2O (4 ” 500 mL)
until the slurry reached a neutral pH value. The solid was then
allowed to dry overnight on a filter. The dried solid was then extracted
with EtOH (400 mL per gram) by refluxing in EtOH for 24 h to
remove the P123. The solid was filtered and rinsed repeatedly with
EtOH (4 ” 500 mL), washed with copious amounts of water until
neutral, and dried overnight to obtain a dry white solid. The solid was
then further dried at 808C under vacuum for 24 h.
Catalytic experiments (using the following general procedure for
the aldol condensation):[10] Catalyst was added (0.05 mmol total
amines and/or sulfonic acids) to a solution of 4-nitrobenzaldehyde
(76 mg, 0.5 mmol) in acetone (10 mL), and the reaction flask was then
sealed under Ar and heated at 508C for 20 h. Acetone was then
removed in vacuo, and the product was analyzed by 1H NMR
spectroscopy in CDCl3 with THF as an internal standard. In the case
Angew. Chem. Int. Ed. 2006, 45, 6332 –6335
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6335