The effect of acid-base pairing on catalysis: An efficient acid-base functionalized catalyst for aldol condensation

Article abstract of DOI:10.1016/j.jcat.2007.02.005

Acid-base bifunctionalized heterogeneous catalysts may be capable of exhibiting reactivity not achievable with homogeneous catalysts. We investigated the effect of pK_a of the acid component of mesoporous solids containing acid-base bifunctionality on their catalytic ability toward aldol condensation between 4-nitrobenzaldehyde and acetone. We found that higher levels of aldol product conversion were obtained when weaker acid components (phosphoric, carboxylic vs sulfonic) were used, an indication of the importance of the equilibrium between free acid and free base and the resulting neutralized ion pair in the catalytic capability of acid-base functionalized materials. A catalyst containing primary amine groups in combination with carboxylic acid groups (with similar chemical functionalities to proline) was found to be a superior catalyst in this aldol reaction with a number of different electrophilic components.

Full text of DOI:10.1016/j.jcat.2007.02.005

Journal of Catalysis 247 (2007) 379–382
Research Note
www.elsevier.com/locate/jcat
The effect of acid–base pairing on catalysis:
An efficient acid–base functionalized catalyst for aldol condensation
Ryan K. Zeidan, Mark E. Davis ∗
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
Received 11 December 2006; revised 30 January 2007; accepted 6 February 2007
Available online 7 March 2007
Abstract
Acid–base bifunctionalized heterogeneous catalysts may be capable of exhibiting reactivity not achievable with homogeneous catalysts. We
investigated the effect of pK of the acid component of mesoporous solids containing acid–base bifunctionality on their catalytic ability toward
a
aldol condensation between 4-nitrobenzaldehyde and acetone. We found that higher levels of aldol product conversion were obtained when weaker
acid components (phosphoric, carboxylic vs sulfonic) were used, an indication of the importance of the equilibrium between free acid and free
base and the resulting neutralized ion pair in the catalytic capability of acid–base functionalized materials. A catalyst containing primary amine
groups in combination with carboxylic acid groups (with similar chemical functionalities to proline) was found to be a superior catalyst in this
aldol reaction with a number of different electrophilic components.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Acid–base; Heterogeneous; Aldol reaction; Mesoporous materials; Bifunctional catalyst
1. Introduction
Interest in materials bearing sulfonic acids and amine groups
has been growing recently, due to these materials’ inherent acid
and basic properties. Corriu et al. reported a material bear-
ing sulfonic acid and amines isolated from one another [11],
whereas our group previously reported on the reactivity of im-
mobilized acid/base pairs wherein the sulfonic acid and amines
are incorporated together to create a material with accessible
sulfonic acid and amine sites exhibiting cooperative catalytic
behavior [10]. The nature of the equilibrium between the acid
and base groups in this material could be very important to the
tuning and understanding of its catalytic behavior; as reported
previously, a dramatic solvent effect was observed for this sys-
tem. In this particular reaction, acetone (the only component
bearing an acidic alpha-proton) is deprotonated to generate a
nucleophilic enolate (possibly assisted through imine forma-
tion), which then attacks the electron-deficient aldehyde to gen-
erate the aldol product. The proposed mechanism for acid/base
cooperative catalysis (Fig. 1) demonstrates the potential impor-
tance of acid/base interactions. Presumably, the regeneration of
the active catalytic species relies on the rate of proton exchange
between the acid and base groups, which would be facilitated
by surface silanols and water when immobilized on mesoporous
supports.
The design of multifunctional materials capable of carrying
out heterogeneous catalysis though the incorporation of organic
functional groups capable of cooperative catalysis has been of
growing utility. Numerous individual functional groups have
been incorporated into solids and have been shown to have
unique properties alone or in combination with other functional
groups [1–9]. Recently our group reported on mesoporous ma-
terials functionalized with incompatible acid and base groups
that showed reactivity not achievable by the organic group com-
bination in solution for an aldol condensation reaction [10].
The equilibrium between acids and bases when immobilized
together is not well defined but likely is very important in un-
derstanding the catalysis properties of such materials. In the
present study, we investigated the effect of the equilibrium be-
tween acid and base groups by varying the pKa of the acid
group.
*
Corresponding author.
E-mail address: mdavis@cheme.caltech.edu (M.E. Davis).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.02.005

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R.K. Zeidan, M.E. Davis / Journal of Catalysis 247 (2007) 379–382
Fig. 1. Proposed catalytic cycle.
In the case of sulfonic acids and amines, the equilibrium
EtOH (4 × 500 mL), then dried overnight on the aspirator to
obtain a dry white solid, which was further dried at 80 ◦C under
vacuum for 24 h.
would be expected to lie heavily in favor of the inactive, neu-
tralized ion pair, with only small amounts of the nonneutralized
acid/base species containing free sulfonic acids and amines.
Sulfonic acids, being strong acids, should protonate the primary
amine, readily causing the equilibrium to lie heavily in the di-
rection of the neutralized ion pair.
Solids were characterized by solid-state NMR on a Bruker
200-MHz NMR using CP/MAS with spinning at 4 kHz. TGA
analysis was carried out on the dry solids on a Netzsch STA
449C with a heating rate of 10 ◦C/min. N2 adsorption was
carried out after drying the catalysts at 90 ◦C for 24 h on a
Micromeritics 2010 instrument. The organic loading per func-
tional group was 0.55 mmol/g for SBA-15-S.A., 0.57 mmol/g
for SBA-15-P.A., and 0.62 mmol/g for SBA-15-C.A. Charac-
terization data for SBA-15-S.A. was reported previously [1].
2. Experimental
2.1. “One-pot” catalyst synthesis
Siloxane starting materials were purchased from Gelest. Tri-
block copolymer poly(ethylene glycol)-block-poly(propylene
glycol)-block-poly(ethylene glycol) P123 (4.0 g, 0.688 mmol,
Mn 5800) was weighed into a Teflon bottle. Then 2.0 M HCl
(120.0 mL, 240.0 mmol) and H2O (6.0 mL, 333.0 mmol)
were added and stirred at 40 ◦C until the P123 was fully dis-
solved. TEOS (8.2 mL, 37.8 mmol) was then added to the
reaction, and the mixture was stirred at 40 ◦C for another
45 min prehydrolysis time. Then 2-(4-chlorosulfonylphenyl)-
ethyltrimethoxysilane (0.65 g, 50/50 w/w solution in dichloro-
methane, 1.0 mmol) was added, to allow for hydrolysis of the
sulfonyl chloride, followed by 3-aminopropyltrimethoxysilane
(0.19 mL, 1.0 mmol). The respective acid component was sub-
stituted for the sulfonyl chloride for different acid functional-
izations. This mixture was stirred at 40 ◦C for 20 h, then aged at
100 ◦C for 24 h. The mixture was then cooled to room temper-
ature, and the resulting solid was filtered and rinsed repeatedly
with excess H2O (4 × 500 mL), then allowed to dry overnight
on an aspirating filter. The dried solid was then extracted with
EtOH (400 mL per g) by refluxing in EtOH for 24 h to remove
P123. The solid was again filtered and rinsed repeatedly with
2.2. Catalytic experiments
The following general procedure was used for the aldol con-
densation. First, 4-nitrobenzaldehyde (76 mg, 0.5 mmol) was
dissolved in acetone (10 mL), and the respective catalyst was
added (0.05 mmol total amines and/or acids). The reaction was
then sealed under Ar and heated at 50 ◦C for 20 h. The reac-
tion was then filtered to remove the solid catalysts, and washed
with acetone and then chloroform (3 × 10 mL each). The fil-
trate was concentrated in vacuo, and the resulting product was
analyzed by 1H NMR in CDCl3 with THF as an internal stan-
dard. The aldol addition product was quantified on the basis of
the triplet occurring at 5.3 ppm, and the aldol dehydration prod-
uct was clearly distinguished by the olefin doublet at 6.9 ppm
in comparison with the internal standard.
3. Results and discussion
When examining this equilibrium, we reasoned that a care-
ful choice of acids to pair with the amino group could allow

R.K. Zeidan, M.E. Davis / Journal of Catalysis 247 (2007) 379–382
381
Scheme 1. Catalyst syntheses.
for a change in the equilibrium toward an increased number of
free acid/base sites and potentially lead to higher catalytic ac-
tivity if less strong acids with a higher pKa were chosen. The
pKa of sulfonic acids is approximately −2; weaker phosphoric
acids have a pKa near 3, and even weaker carboxylic acids have
a pKa around 5. These three acids of varying strengths were in-
corporated into SBA-15 though a one-pot synthesis procedure
under acidic conditions with P123 as a structure-directing agent
(Scheme 1).
These different acid-functionalized materials were used in
the aldol condensation between acetone and 4-nitrobenzalde-
hyde. The catalysis data are given in Table 1. The hypothe-
sized trend in catalytic behavior is clearly realized and illus-
trated by these results in that the conversion increases signifi-
cantly with decreasing acid strength. The combination of sul-
fonic acid with amines (entry 1, Table 1) gives rise to 62%
conversion of aldol product, illustrating cooperative catalysis
between these groups. When using a slightly weaker acid in
the phosphoric acid/amine catalyst (entry 2, Table 1) a signif-
icant improvement in conversion (to 78%) is observed. When
using an even weaker acid, the carboxylic acid/amine mater-
ial (entry 3, Table 1), further enhancement is observed, and the
reaction is nearly quantitative. Entries 4–7 serve as control ex-
periments to illustrate the catalytic ability of these functional
groups when used independently and to exemplify the type of
improvements than can be achieved by immobilizing different
functional groups that have the ability to behave in a coopera-
tive manner.
The data from kinetic experiments run on the carboxylic acid
catalyst (entry 3) show that the conversion is linear with time
and gives a constant selectivity throughout the course of the
reaction (see Supplementary material).
To examine the effect of the electron density of the alde-
hyde partner on conversion, we chose a variety of aldehyde
partners for the aldol condensation and used SBA-15-C.A. as
the catalyst. Entries 1–4 in Table 2 illustrate the importance of
an electron-withdrawing group at the para position for this reac-
tion. The conversion varies with the electron-donating ability of
the para substituent. When a chloro group is at position 4 (en-
try 1), poor conversion to the aldol product is observed. How-
ever, when switching to a fluoro-, nitro-, and trifluoromethyl
(entries 2–4) groups, conversion improves significantly, accord-
ing to these groups’ electron-withdrawing capability (Cl < F
< NO2 < CF3). When these electron-withdrawing groups are
moved to the ortho position, where they may be able to impart a
steric blockage of the aldehyde position, conversions decrease
significantly, but in this case they seem to track with the steric
bulk of the substituent rather than the electron-withdrawing na-
ture (i.e., F > NO2 > CF3).
In conclusion, we have prepared a catalytic material capa-
ble of providing excellent conversion toward the aldol reaction
that is rationalized by the proper pairing of weaker acid groups
with amine groups. Mesoporous materials functionalized with
carboxylic acids and amines (similar to proline) provide a very
efficient catalyst capable of near-quantitative conversion in the
aldol condensation. This particular pair of functional groups

382
R.K. Zeidan, M.E. Davis / Journal of Catalysis 247 (2007) 379–382
Table 1
Table 2
Catalysis data
Substrate scope
Entry
1
Catalyst (10 mol%)
% A
45
% B
17
Total conversion
62
Entry Ketone Aldehyde
Alcohol
product
(% A)
Olefin
product
(% B)
Total
conversion
(%)
1
2
12
12
24
49
42
91
2
3
62
75
16
24
78
99
3
4
75
66
24
33
99
99
4
5
0
0
0
0
0
0
5
6
40
32
20
14
60
46
6
7
8
8
8
16
33
7
26
8
34
25
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the case of stronger acids paired with amines and gives rise to
dramatically enhanced reactivity. The detailed mechanism and
characterization of the neutralization equilibrium are under in-
vestigation using NMR techniques and will be reported in due
course.
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