Efficient solid-base catalysts for aldol reaction by optimizing the density and type of organoamine groups on nanoporous silica

Article abstract of DOI:10.1016/j.jcat.2009.04.018

A highly efficient solid-base catalyst for aldol condensation reaction was synthesized by grafting site-isolated secondary amines onto the channel walls of mesoporous silica, MCM-41, in a polar-protic solvent. These site-isolated organoamines pair up with the neighboring residual surface silanol (weak acid) groups to form optimized acid and base groups, which cooperatively catalyze the aldol condensation reaction with high TON (turn over number) and selectivity (alcohol products over alkene products). The organoamine samples grafted in a polar-protic solvent, isopropanol, showed higher catalytic efficiency toward aldol reaction than those grafted in a non-polar solvent, toluene, because the former gave a sample with less dense loading of grafted organoamines (or more silanols present in it). To elucidate the role of surface silanols as acidic sites and their ability to activate the substrates in aldol condensation, control experiment with diethylamine as a homogeneous catalyst in the presence of MCM-41, silica microspheres, methyl-capped MCM-41 or methyl-capped catalyst was carried out. MCM-41 resulted in significant enhancement of catalytic activity compared to the corresponding reactions conducted in the absence of MCM-41, or in the presence of methyl-capped catalysts or silica spheres. By testing materials with different grafted organoamine groups as catalysts, we also found that secondary amine functionalized sample produced the best acid and base pairs and most efficient catalytic activity in aldol reaction. This was followed by primary amines, while the tertiary amine functionalized samples showed negligible catalytic property.

Full text of DOI:10.1016/j.jcat.2009.04.018

Journal of Catalysis 265 (2009) 131–140
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Efficient solid-base catalysts for aldol reaction by optimizing the density
and type of organoamine groups on nanoporous silica
*
Youwei Xie, Krishna K. Sharma, Abhishek Anan, Gang Wang, Ankush V. Biradar, Tewodros Asefa
Department of Chemistry, Syracuse University, 111 College Place, Syracuse, NY 13244, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 December 2008
Revised 8 April 2009
Accepted 22 April 2009
Available online 21 May 2009
A highly efficient solid-base catalyst for aldol condensation reaction was synthesized by grafting site-iso-
lated secondary amines onto the channel walls of mesoporous silica, MCM-41, in a polar-protic solvent.
These site-isolated organoamines pair up with the neighboring residual surface silanol (weak acid)
groups to form optimized acid and base groups, which cooperatively catalyze the aldol condensation
reaction with high TON (turn over number) and selectivity (alcohol products over alkene products).
The organoamine samples grafted in a polar-protic solvent, isopropanol, showed higher catalytic effi-
ciency toward aldol reaction than those grafted in a non-polar solvent, toluene, because the former gave
a sample with less dense loading of grafted organoamines (or more silanols present in it). To elucidate the
role of surface silanols as acidic sites and their ability to activate the substrates in aldol condensation,
control experiment with diethylamine as a homogeneous catalyst in the presence of MCM-41, silica
microspheres, methyl-capped MCM-41 or methyl-capped catalyst was carried out. MCM-41 resulted in
significant enhancement of catalytic activity compared to the corresponding reactions conducted in
the absence of MCM-41, or in the presence of methyl-capped catalysts or silica spheres. By testing mate-
rials with different grafted organoamine groups as catalysts, we also found that secondary amine func-
tionalized sample produced the best acid and base pairs and most efficient catalytic activity in aldol
reaction. This was followed by primary amines, while the tertiary amine functionalized samples showed
negligible catalytic property.
Keywords:
Heterogeneous catalyst
Aldol reaction
Aldol condensation
Mesoporous catalyst
MCM-41
Catalysis
Organoamine grafting
Site-isolated catalyst
Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction
ammonia/amine surface reactions and surface nitridation of alu-
minophosphates, zeolites and mesoporous materials [12–14]. For
In nature, enzymes often employ sophisticated strategies to
achieve complex reactions or to drive many biological processes,
the complexity and efficiency of which are often unmatched by
man-made systems. The highly efficient catalytic abilities of en-
zymes often result from the collective efforts of several functional
groups or active catalytic sites [1,2]. A recent topic of interest in
synthetic methodologies to heterogeneous catalysts has been the
functionalization of solid-state materials with two or more func-
tional groups to produce efficient, cooperative catalysts [3–6] or
materials capable of catalyzing multi-step, one-pot tandem [7]
reactions.
Solid-base catalysts, which can be synthesized by functionaliza-
tion of various solid-state materials, constitute a class of heteroge-
neous catalysts that are capable of catalyzing various C–C bond
forming reactions such as the Henry, Knoevenagel, and aldol reac-
tions [8,9]. These catalysts are synthesized by a variety of synthetic
methods including surface functionalization of metal oxides with
base catalytic groups [10], co-assembly or organosilanes [11],
example, by exchanging alkali metal ions of zeolites with alkaline
metal ions, solid-base zeolites, whose basicity increases with
decreasing charge-to-radius ratio of the alkaline metal ion, can
be synthesized [15]. Similarly, by encapsulating alkali metal–oxide
clusters inside zeolites, solid bases with low degree of basicity have
been reported [16]. The substitution of framework oxygen atoms in
metal oxide materials by NHx groups via nitridation of amorphous
silica [17–21], aluminophospates [22–26], and zeolites [27–31]
with ammonia or nitrogen under high temperature also produces
solid-base materials. Depending on the nitridization temperature
and time, the nitrogen content as well as basicity of the resulting
materials can also be tuned.
Since their discovery in 1992 [32], mesoporous silicas have been
widely used as a host for catalytic moieties such as basic [33–35]
and acidic [36–38] groups to form active heterogeneous nanocata-
lysts. Because of their ordered nanopore structures compared to
amorphous silica and their large pores compared to zeolites, mes-
oporous silica materials are often more interesting as support
materials for making heterogeneous catalysts. As in zeolites, amor-
phous silica and aluminophosphates, the frameworks of mesopor-
ous materials can also undergo nitridization to form solid base,
* Corresponding author. Fax: +1 315 443 4070.
E-mail address: tasefa@syr.edu (T. Asefa).
0021-9517/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2009.04.018

132
Y. Xie et al. / Journal of Catalysis 265 (2009) 131–140
mesoporous aminosilica, and silicon oxynitride materials [39–44].
Another more commonly used method to produce solid base mes-
oporous materials has been the grafting or co-assembly of amin-
oorganosilanes [3–5]. This method introduces organic base
catalytic groups on mesoporous silica framework at low tempera-
ture. Furthermore, it also allows the introduction of acidic groups
on the same material, affording a heterogeneous catalyst contain-
ing both acidic and basic functional groups (or cooperative cata-
lyst). For instance, Lin’s group reported the synthesis of a series
of bifunctional mesoporous silica nanosphere (MSN) materials
containing a general acid, ureidopropyl (UDP), and a general base,
3-[2-(2-aminoethylamino)ethylamino]-propyl (AEP), and showed
that the existence of the auxiliary acidic groups significantly en-
hanced the catalytic activity of the base-catalyzed aldol, Henry,
and cyanosilylation reactions [4]. Similar enhancement effect by
organic acid groups was also reported by Zeidan et al. [5b,5c],
who not only found a catalytic enhancement effect by the general
acid on the base-catalyzed aldol reaction, but also discovered the
dependence of the catalytic performance of the catalysts on the
pKa value of the auxiliary acidic groups. They further showed that
a weaker acid produces a lesser degree of neutralization and a
more efficient catalyst. Although the effect of different acidic
groups including weakly acidic surface silanols on the efficiency
of heterogeneous base-catalyzed reactions have been reported
[5,45], the effects of the type of basic sites and their relative surface
density along with surface silanols on the catalytic efficiency of
mesoporous silica-supported heterogeneous organocatalysts were
not well investigated. Recently, our group reported the synthesis
of highly efficient bifunctional catalyst for the Henry reaction by
simple post-grafting of site-isolated organoamine groups in po-
lar-protic solvents [46]. By grafting aminorganosilanes on meso-
porous silica in polar-protic solvent, we produced relatively less
density of surface organoamine and many residual silanol groups
compared to the materials we grafted in non-polar solvents under
the same condition. The primary amine and the surrounding weak
acid silanol groups in the former were proved to cooperatively cat-
alyze the Henry reaction more efficiently. However, this synthetic
strategy and its applications for catalysis have not been investi-
gated for reactions other than the Henry reaction so far.
Herein we report on the synthesis of highly efficient solid-base
catalysts containing grafted organoamine and significant numbers
of weakly acidic silanol groups for an aldol condensation reaction.
The materials were synthesized by grafting a series of aminoorg-
anosilanes in a polar-protic solvent, isopropanol. Isopropanol was
chosen because the grafting of aminorganosilane in a polar-protic
solvent produces less surface density and relatively greater site-
isolated organoamine groups that are surrounded by many resid-
ual surface silanols or weak acid groups. We find that the surface
silanol groups have significant cooperative catalytic activity with
the site-isolated organoamines toward aldol condensation. Com-
pared to the catalytic efficiency of other solid-base catalysts re-
ported previously [3–5], in which relatively strong acidic groups
were used, we find that the mildly acidic surface silanol groups
provide better acidic sites to cooperatively catalyze the aldol reac-
tion along with the amine groups possibly due to lesser degree of
neutralization between the antagonistic acidic and basic groups.
In order to optimize our catalytic system and to determine the best
acid and base pairs for aldol reaction, different types of amines
were tested and compared in our study. The results indicated that
secondary amine showed the highest cooperative catalytic effi-
ciency with silanols for aldol condensation reaction, followed by
primary amine; however, tertiary amine produced negligible cata-
lytic efficiency possibly because of its inability to form enamine
intermediates. In order to prove if the enhanced catalytic activity
was due to the surface silanol groups, MCM-41 and silica micro-
spheres were used as an additive during the homogeneous cataly-
sis of aldol reaction with diethylamine. While MCM-41 was found
to significantly enhance the catalytic ability of diethylamine, no
enhancement effect was observed on capping the surface silanols
of MCM-41 with trimethysilyl groups although the resulting mate-
rials still had large surface area and pore size. Silica microspheres
also gave no catalytic enhancement. We further synthesized and
investigated mesoporous catalysts whose silanols were capped
with methyl groups. These materials did also show lower catalytic
efficiency. This proved that the surface silanol groups of mesopor-
ous materials indeed enhanced the catalytic performance and
proved our hypothesis to utilize weakly acidic surface silanol
groups for producing the most effective solid-base catalysts for
the aldol reaction.
2. Experimental
2.1. Materials and reagents
Cetyltrimethylammonium bromide (CTAB), p-nitrobenzalde-
hyde (p-NO2), tetraethylorthosilicate (TEOS), 3-aminopropyltrime-
thoxysilane (APTS), ammonia solution (30%), and anhydrous
ethanol were obtained from Sigma–Aldrich. 3-(N-methylamino-
propyl)trimethoxysilane (MAPTS) and 3-(N,N-dimethylaminopro-
pyl)trimethoxysilane (DMAPTS) were obtained from Gelest, Inc.
Anhydrous toluene and isopropanol were purchased from Phar-
mco-AAEPR. Anhydrous acetone was purchased from Fischer Sci-
entific. All reagents were used as received without any further
purification.
2.2. Synthesis of mesoporous silica (MCM-41)
The synthesis of mesoporous silica, MCM-41, was performed as
reported previously [32,47]. Typically, 4.0 g (1.1 mmol) cetyltri-
methylammonium bromide (CTAB) was dissolved in 960 mL of
Millipore water and 14 mL of 2.0 M NaOH solution. After moder-
ately stirring the solution at 80 °C for 30 min, 22.6 mL
(101.2 mmol) of tetraethoxysilane (TEOS) was added into it. The
resulting solution was stirred for another 2 h at 80 °C. The hot solu-
tion was then filtered and the solid product was washed with Mil-
lipore water (4 Â 80 mL), followed by ethanol (4 Â 80 mL) and then
dried in oven at 80 °C. This resulted in as-synthesized mesostruc-
tured MCM-41. To remove the surfactant template, 6 g of the as-
synthesized MCM-41 was suspended in a mixture of 3 mL
(12.1 N) hydrochloric acid and 600 mL anhydrous ethanol and stir-
red at 50 °C for 5 h. The resulting solution was filtered and the solid
sample was washed with copious amount of Millipore water and
ethanol (3 Â 80 mL). The extracted mesoporous silica (MCM-41)
was dried in oven at 80 °C overnight before postgrafting it with
various aminoorganosilanes.
2.3. Synthesis of solid-base mesoporous catalysts by postgrafting
The postgrafting of organoamines onto mesoporous silica,
MCM-41, was done by stirring 600 mg of MCM-41 in excess
(4.42 mmol) aminoorganosilanes (APTS, MAPTS, or DMAPTS) in
300 mL isopropanol (or toluene) at 80 °C for 5 h. Control samples
by grafting MCM-41 in excess (4.42 mmol) APTS and MAPTS in
300 mL toluene for 15 min and 30 min at 80 °C were also synthe-
sized. The hot solution was then filtered and the solid sample
was washed with dichloromethane (300 mL), followed by ethanol
(600 mL) and then dried in oven at 80 °C for overnight. The result-
ing materials synthesized from APTS, MAPS, and DMAPS in isopro-
panol were designated as MCM41-API, MCM41-MAPI, and
MCM41-DMAPI catalysts, respectively. While AP, MAP, and DMAP
are used to denote primary, secondary, and tertiary amines, respec-

Y. Xie et al. / Journal of Catalysis 265 (2009) 131–140
133
tively (the letter ‘‘I” represents the grafting solvent, isopropanol).
Similarly, samples grafted in toluene were labeled as MCM41-
APT, MCM41-MAPT, and MCM41-DMAPT catalysts, respectively
(the letter ‘‘T”, in these cases, indicates toluene as the grafting
solvent).
strate to catalyst (S/N) ratio were used in each of our reactions and
to make comparison of catalytic efficiency of the samples easier,
we used mesoporous catalysts containing the same number of cat-
alytic groups (2.5 lmol organoamines) in the case of samples
grafted in isopropanol (Table 2). The corresponding mass of cata-
lyst was calculated based on the wt.% N obtained from the elemen-
tal analysis data, which was found to be 25, 17, and 19 mg for
MCM41-API, MCM41-MAPI, and MCM41-DMAPI, respectively (Ta-
ble 2). In a typical reaction, the calculated mass of catalyst was
added to a solution containing 152 mg (1 mmol) of p-nitrobenzal-
dehyde in 10 mL of acetone, which is to serve both as a substrate
and solvent. The reaction mixture was stirred at 50 °C under nitro-
gen. Aliquots of ꢀ0.05 mL reaction mixture were taken out with a
syringe at different time intervals and analyzed by 1H NMR as re-
ported previously [4]. For catalysts grafted in toluene, the same
procedure was employed, except 25 mg of sample was used in each
2.4. Synthesis of solid-base mesoporous catalysts with capped silanols
To investigate the catalytic enhancement effects of surface sila-
nols on aldol reaction, the silanols were protected with trimethyl-
silyl groups by suspending 500 mg MCM-41 in a mixture of 100 mL
toluene and 5 mL hexamethyldisilazane (HMDS) [48,49]. The mix-
ture was stirred at room temperature under nitrogen atmosphere
for 24 h and then filtered. The solid sample was washed by follow-
ing a similar procedure mentioned above, resulting in a control
sample that was designated as MCM41-CH3. The samples were
characterized by various methods (Table 1). Similarly, MCM-API-
CH3 and MCM-MAPI-CH3 were synthesized.
case since the 2.5
mass of catalyst in these samples. The 25 mg of catalyst corre-
sponded to 64 and 58 mol of organoamine for MCM41-APT and
lmol organoamine corresponds to a rather low
l
2.5. Synthesis of silica microspheres
MCM-41-MAPT, respectively. The comparisons of catalytic efficien-
cies in this case were done based on catalytic turn-over-numbers
(TONs).
The synthesis of silica microspheres was carried out by follow-
ing the Stöber method [50,51]. TEOS (5.94 g) was added to 4 mL of
12.1 N ammonia solution in 100 mL of ethanol and 3.6 g of Milli-
pore water under stirring. The colloidal solution was then centri-
fuged and washed three times with 50 mL of ethanol and
redispersed in 50 mL ethanol. Ten millilitre of the colloidal solution
was centrifuged and washed three times with 50 mL of anhydrous
acetone and then redispersed in 10 mL of anhydrous acetone. The
corresponding colloidal solution was determined to have 30 mg
of silica microspheres in 1 mL of solution.
2.8. Enhancement effect tests and control experiments
In order to prove the enhancement effect by surface silanol
groups on the aldol reaction, MCM-41, MCM41-CH3, or silica
microspheres were added to a homogeneous catalytic system con-
taining diethylamine as a catalyst. Briefly, the homogeneous cata-
lyst, diethylamine (0.2 mmol), was added to a solution of 152 mg
p-nitrobenzaldehyde (1 mmol) in 10 mL of acetone, followed by
25 mg of MCM-41, (MCM41-CH3, or silica microspheres). The reac-
tion mixture was stirred at 50 °C under nitrogen atmosphere and
analyzed at different time intervals as mentioned above.
2.6. Probing site-isolation of organoamine catalytic sites
The degree of site-isolation of the organoamine catalytic sites in
the mesoporous materials was probed by following our recently
reported procedure [52], which involved complexation of Cu2+ ions
from dilute aqueous solution onto the organoamines and measure-
ment of the UV–Vis absorption spectra of the resulting Cu2+-amine
complexes. Briefly, 20 mg of the amine-functionalized sample
(MCM-API, MCM-APT, MCM-MAPI, or MCM-MAPT) was mixed
with 5 mL of 2 mM aqueous Cu2+ solution. The sample was soni-
cated for 5 min and then stirred for 1 h. The solution was then fil-
tered and the solid sample was washed with 5 mL ethanol and
dried under vacuum. The reflectance UV–Vis spectrum of the solid
sample was then obtained by placing the solid product between
two quartz plates.
In other control experiments the surface silanol groups of MCM-
API and MCM-MAPI were capped with trimethysilyl groups, or
samples MCM-API-CH3 and MCM-MAPI-CH3, respectively were
used as catalysts and their catalytic properties were then tested
by following the same procedure as the one conducted for MCM-
API and MCM-MAPI (see Section 2.7 for details).
2.9. Instrumentation
The TEM (transmission electron microscopy) images were taken
by using an FEI Tecnai G-20 TEM/STEM microscope working at an
accelerating voltage of 120 KeV. The XRD (powder X-ray diffrac-
tion) patterns were obtained by using a Scintag powder diffractom-
eter. The BET gas adsorption–desorption measurements were
carried out on Micromeritics Tristar 3000 volumetric adsorption
analyzer after degassing the samples at 160 °C for 12 h. The so-
lid-state 13C (75.5 MHz) and 29Si (59.6 MHz) NMR spectra were ac-
quired on a Bruker AVANCE 300 NMR spectrometer. For 13C CP-
MAS NMR experiments, 7.0 kHz spin rate, 5 s recycle delay, 1 ms
2.7. Catalytic tests
An aldol reaction was carried out by following a procedure re-
ported previously [4], but by using a higher substrate to catalyst
ratio. To ensure that the same mole of catalyst and the same sub-
Table 1
Wt.% N content and structural information of the mesoporous catalysts.
Catalyst
Wt.% C
Wt.% N
Number of N (nm2)
BET surface area (m2/g)
BJH pore diametera (Å)
Pore volumea (cm3/g)
MCM-41
0
0
0
1069
849
873
886
899
875
973
30.2
27.8
27.1
26.4
27.1
27
1.04
0.75
0.63
0.62
0.57
0.53
0.66
MCM41-API
MCM41-MAPI
MCM41-DMAPI
MCM41-APT
MCM41-MAPT
MCM41-CH3
9.15
11.35
12.4
10.37
12.31
10
1.44
2.16
1.87
3.7
3.43
0
0.72
1.06
0.91
1.79
1.70
0
27.2
a
The BJH pore diameter and the pore volume were calculated from the desorption branch of the isotherm.

134
Y. Xie et al. / Journal of Catalysis 265 (2009) 131–140
Table 2
Catalytic reaction and results of the aldol condensation reaction catalyzed by acid–base mesoporous catalysts.
OH
O
O
CHO
O
Catalyst
50 °C
+
+
O2N
O2N
O2N
A = Alcohol Product
B = Dehydrated Product
Acid–base mesoporous catalysta
mg (l
mol) of catalyst used
S/Nb
% Yield in 6 hc
% Yield in 20 h
TON at 6 hc
TON at 20 h
A/B
MCM-41d
MCM41-API
MCM41-MAPI
MCM41-DMAPI
MCM41-APT
25 (0)
0
0
NAe
94.6
NAe
5.1
0
NAe
38.0
NAe
1.9
NAe
9.0/1
9.0/1
25 (25)
17 (25)
19 (25)
25 (64)
25 (58)
18 (25)
40/1
40/1
40/1
15.6/1
17.1/1
40/1
43.8
97.0
0
15.1
95.6
93.2
17.5
38.8
0
2.35
16.4
34.0
NAe
NAe
NAe
NAe
NAe
NAe
9.2/1
10.5/1
9.4/1
MCM41-MAPT
MCM41-MAPT-15 min Graftingf
a
The grafting of the samples was performed by stirring the organosilanes in isopropanol or toluene for 5 h unless mentioned otherwise.
The S/N (substrate to amine) ratio was calculated based on the elemental analysis and the quantity of catalysts used, the quantity of catalysts grafted in isopropanol was
b
chosen to make S/N content as 40/1, while the mass of other catalysts was chosen to be constant (25 mg).
c
Reaction was run twice for each catalyst with little variation in percentage yield and TON (mole of starting materials converted into products per mole of amines), and the
results shown here are the averages of two runs.
d
Parent material before grafting.
NA = Not available.
e
f
The grafting of this sample was performed for 15 min, which has produced similar density of grafted organoamine groups as sample MCM41-MAPI. The mmol of catalytic
sites for these samples was calculated from TGA traces and not from elemental analysis.
contact time,
p
/2 pulse width of 5.6
l
s, and 600 scans using TPPM
tionalization of the mesoporous silica with organoamines,
indicating the stability of the mesostructure during the chemical
modification process. The TEM images of the mesoporous materials
or catalysts before and after the chemical modification also
showed well-ordered mesoporous structures. The low and high
magnification TEM images of MCM-41 are shown in Fig. 1b and
c, respectively, while a high magnification TEM image of a repre-
sentative organoamine functionalized sample, MCM41-API, is
shown in Fig. 1d. Mesoporous particles with spherical morphology
are clearly observed under low magnification while their well-or-
dered mesoporous structures are seen under high magnification.
Similar images were also obtained for the remaining materials that
were modified with secondary and tertiary amine groups (not
shown).
1H decoupling were employed. For the 29Si CP-MAS NMR experi-
ments, 7.0 kHz spin rate, 10 s recycle delay, 10 ms contact time,
p
/2 pulse width of 5.6 l
s, and 600 scans by using TPPM 1H decou-
pling were employed. The solution 1H NMR was measured by using
a Bruker DPX-300 NMR spectrometer. The elemental analysis was
performed at Quantitative Technologies, Inc.
3. Results
3.1. Synthesis and characterization of mesoporous base catalysts for
aldol condensation reaction
We employed a synthetic strategy to produce efficient solid-
base catalysts containing site-isolated organoamine groups along
with many residual silanols for aldol condensation reaction. Meso-
porous silica, MCM-41, was grafted with organosilanes containing
primary, secondary, or tertiary amine in isopropanol producing
MCM41-API, MCM-41-MAPI, and MCM41-DMAPI catalysts, respec-
tively. Similarly, MCM-41 was grafted with organosilanes contain-
ing primary, secondary, or tertiary amines in toluene producing
MCM41-APT, MCM41-MAPT, and MCM41-DMAPT catalysts,
respectively. The samples grafted with aminoorganosilanes in iso-
propanol are expected to have relatively more site-isolated amine
catalytic groups while samples grafted in toluene are anticipated to
have more aggregated organoamine groups [46]. Control samples
were also synthesized by grafting the surface silanols of MCM-
41, MCM-API, and MCM-MAPI with methyl groups, which pro-
duced MCM41-CH3, MCM-API-CH3, and MCM-MAPI-CH3, respec-
tively (see Section 2 for details).
All the mesoporous materials and catalysts were characterized
by XRD, TEM, elemental analysis, solid-state 13C and 29Si NMR
spectroscopies, and nitrogen gas adsorption. The low angle XRD
patterns (Fig. 1a) showed three well-defined peaks with 2h values
that can be indexed as (100), (110), and (200) Bragg reflections,
which are typical of the P6mm honeycomb structure of mesopor-
ous materials [48]. These peaks remained observable after func-
The results from elemental analysis (Table 1) showed that after
aminoorganosilane grafting, the materials showed a significant in-
crease in the wt.% of N and C indicating the functionalization of
mesoporous silica material with the organoamines. Samples
grafted with aminorganosilanes in toluene had much higher wt.%
N (e.g., 3.7 wt.% for MCM41-APT) than the corresponding samples
grafted in isopropanol (e.g., 1.44 wt.% for MCM41-API). This result
is consistent with the data reported previously [46,52]. The func-
tionalization of the materials with the organoamine groups was
further confirmed by 13C CP-MAS NMR and 29Si CP-MAS NMR spec-
troscopies (Supplementary Fig. 1S). The 13C CP-MAS NMR spectra
showed typical chemical shifts that can be assigned to the carbon
atoms of the different organoamine groups containing primary,
secondary, and tertiary amines (see Supplementary Fig. S1). The
assignment of the peaks was similar to that reported previously
in Ref. [48]. The 29Si CP-MAS NMR spectra showed T peaks cen-
tered at around À50 ppm, which correspond to silicon atoms that
are attached to the organoamine groups. Furthermore, additional
peaks were also observed between À80 and À115 ppm on the
29Si CP-MAS NMR spectra, which correspond to silicon atoms of
the parent MCM-41 framework (SiOx(OH)4Àx).
The surface areas and pore diameters of the materials were
characterized by nitrogen gas adsorption. The samples were de-
gassed at 160 °C for 12 h before the measurement. The nitrogen

Y. Xie et al. / Journal of Catalysis 265 (2009) 131–140
135
Fig. 1. (a) Powder X-ray diffraction (XRD) spectra of MCM-41 and different organoamine-functionalized catalysts (the XRD patterns were vertically spaced for clarity). (b and
c) Low and high magnification TEM images of MCM-41. (d) High magnification TEM image of MCM41-API.
BET Isotherm
50
40
30
20
10
0
40
35
30
25
20
15
10
5
(b)
(a)
MCM-41
MCM41-API
MCM41-MAPI
MCM41-DMAPI
MCM41-APT
MCM41-MAPT
MCM41-CH3
MCM41
MCM41-API
MCM41-MAPI
MCM41-DMAPI
MCM41-APT
MCM41-MAPT
MCM41-CH
3
0
0.0
0.2
0.4
0.6
0.8
1.0
20
30
40
50
60
70
80
90
100
Relative Pressure (P/P0)
Pore Width (Angstroms)
Fig. 2. (a) BET gas adsorption isotherms and (b) BJH pore-size distributions MCM-41 and different organoamine-functionalized catalysts and control samples. The pore size
distributions were obtained from the desorption branch of the isotherms. All the graphs were vertically spaced on the y-axis by six data points in (a) and by five data points in
(b) for clarity.
gas adsorption and desorption measurements resulted in type IV
isotherm for all the catalysts, indicating further that all the samples
have mesoporous structures (Fig. 2 and Table 1). While all the
materials showed high surface areas (Table 1), upon modification
by the organoamine groups, the surface area, pore width, and pore
volume decreased compared to the unmodified MCM-41. The sur-
face area of all the organoamine functionalized samples (Table 1,
entry 2 through entry 6) fell in the range between 850 and
900 m2/g, with only marginal differences in the values. Although
the choice of solvents isopropoanol and toluene was made to pro-
duce different loading and surface density of organoamines on the
mesoporous materials [46,52], no significant difference in surface
area between the samples grafted in isopropanol and toluene
was observed (see Table 1). However, the pore size and pore vol-
ume decreased more significantly in samples grafted in toluene
compared to the corresponding samples grafted in isopropanol.
The relative degree of site-isolation of the primary and second-
ary organoamine groups was probed by using the copper-complex-
ation method as we have reported recently [52]. Representative
reflectance UV–Vis spectra of solid mesoporous samples, after
being treated with dilute aqueous Cu2+ solution, are shown in Sup-
plementary Fig. 2S. Sample MCM41-MAPI, which was grafted in
isopropanol for 5 h, showed a red-shifted absorption maximum
at 717 nm, which indicated the presence of CuO5N1 structure and
relatively less densely populated or more site-isolated organo-
amine groups in it. However, the corresponding sample grafted
in toluene for 5 h, MCM41-MAPT, showed a blue-shifted absorp-
tion maximum at 676 nm, which indicated the presence of CuO4N2

136
Y. Xie et al. / Journal of Catalysis 265 (2009) 131–140
structure and more densely populated (or more aggregated)
organoamine groups in it (Fig. S2). On the other hand, the control
samples synthesized by grafting 3-(N-methylaminopropyl)tri-
methoxysilane (MAPTS) in toluene for relatively short periods of
times of 15 and 30 min showed similar low density of grafted
organoamine groups as the corresponding sample grafted in iso-
propanol for 5 h as observed on thermogravimetric analysis (see
Fig. S3). These samples further exhibited red-shifted absorption
maxima at ꢀ700 upon complexation with Cu2+ ions (Fig. S4). This
indicates that samples grafted in toluene for shorter periods of
time can indeed produce less densely populated, more spatially
distributed amine groups and CuO5N1 structures as much as the
samples grafted in isopropanol (Fig. S4). While we have used this
copper-complexation method successfully to monitor the degree
of site-isolation of primary amine groups in mesoporous materials
previously [55], our experiment here further proved of the meth-
od’s versatility in probing the degree of site-isolation of secondary
organoamine groups for the first time.
and a TON value of 17.5 in 6 h while the corresponding sample
MCM41-APT afforded a total conversion of 15.1% and a TON value
of 2.4 in 6 h. It is also worth noting that the catalytic efficiency ob-
tained for MCM41-API in aldol reaction, i.e. a total conversion of
94.6% and a TON value of 38 in 24 h, is much higher than those re-
cently reported for acid–base bifunctional catalysts by Zeidan et al.
[5b,5c] as well as by Huh et al. [4] (see Supplementary Table 1S).
It is also worth noting that our amine functionalized materials
gave much more selective alcohol product than the dehydrated,
olefin product compared to those reported by Davis group
[5b,5c]. The aldol reaction can produce significant amount of dehy-
drated, olefin products along with an alcohol product in aldol reac-
tion, with ratios of dehydrated olefin : alcohol products as high as
49%:42% and 45%:17% [5b,5c]. The olefin product from aldol reac-
tion is most likely resulted from dehydration of the alchol product.
So, it also appears that some of the olefin products we observed are
also products of dehdydration of the alcohol product under the
reaction conditions we used, although produced less so than the
ones observed in Refs. [5b,5c]. This further makes our materials
not only efficient catalysts for aldol reaction but also more suitable,
selective catalysts for producing a higher relative ratio of alcohol/
olefin. This may be due to the fact that our reaction goes to almost
completion within 6 h and the tendency of the alcohol products to
undergo dehydration over this relatively shorter time period is less
than it would be over 24 h, which is a reaction time employed in
Zeidan et al. [5b,5c].
3.2. Catalysis tests
The catalytic activity of all the amine-functionalized materials
and control samples were then investigated in aldol reaction. We
describe catalysis results for all the samples containing primary,
secondary, and tertiary amine groups. But, in order to simplify
our arguments, we will discuss in more detail the samples contain-
ing primary and secondary amines (and their corresponding con-
trol samples), which we have found to be the most efficient
catalysts for aldol reaction. Since we also have found that the sam-
ples containing tertiary amines were found to be very inefficient in
the aldol reaction, the results for these materials are only briefly
discussed. From Table 2 and Fig. 3, we observe that the catalysts
that were grafted with primary amines in isopropanol and toluene
(MCM41-API and MCM41-APT, respectively) showed different cat-
alytic performances. Sample MCM41-API exhibited a more efficient
catalytic activity than MCM41-APT (Table 2 and Table 1S). For
example, sample MCM41-API gave a total conversion of 43.8%
After demonstrating the highly effective catalysis in aldol reac-
tion by the primary amine functionalized material, we tested the
two other samples containing secondary and tertiary amines. N-
Methylaminopropyl
(MAP)
and
N,N-dimethylaminopropyl
(DMAP)-functionalized catalysts were synthesized in isopropanol
and toluene and their catalytic properties were compared with
the corresponding samples containing primary amine groups
(Fig. 3). Interestingly, the secondary amine grafted samples,
MCM41-MAPI and MCM41-MAPT, showed more efficient catalytic
activity than the corresponding primary amine grafted samples,
MCM41-API and MCM41-APT. On the other hand, both MCM41-
DMAPI and MCM41-DMAPT exhibited negligible catalytic activity
(Fig. 3). Interestingly, samples grafted in toluene for shorter times
gave similar wt.% organoamine groups as the corresponding sam-
ple grafted in isopropanol for 5 h. However, the former still showed
weaker catalytic efficiency than the latter (Table 2 and Fig. S5). For
instance, the sample grafted with APTS in toluene for 15 min,
which has similar number of amine (or silanol groups) as sample
MCM41-API, showed significantly lower catalytic efficiency than
the latter (Table 2 and Fig. S5).
One can speculate that the higher catalytic activity of MCM41-
API and MCM41-MAPI compared to that of MCM41-APT and
MCM41-MAPT, respectively, might be due to the fact that the den-
sely populated organoamine groups grafted in the latter reduce the
surface area and pore size of the material, which subsequently may
result in lower catalytic efficiency [53,54]. However, nitrogen
adsorption–desorption experiment on samples grafted in isopropa-
nol versus toluene did not show significant differences in surface
areas, pore size, and pore volume between the two samples, de-
spite the samples had significant differences in organoamine load-
ing (Table 1). Although the latter difference is expected to cause
some differences in catalytic efficiency, it alone could not explain
the significant difference in catalytic efficiency between the two
samples, especially after normalizing the catalytic efficiency per
unit surface area [55]. In other words, there must be reasons such
as possible cooperative catalytic activity [3–5,46,52,55] that may
have resulted in the higher catalytic performance in aldol reaction
by the samples grafted in isopropanol compared to those grafted in
toluene. We recently reported that the primary amine functional-
ized samples that were synthesized by grafting in isopropanol for
Aldol Reaction
100
(a)
MCM-API
80
60
40
20
0
MCM-MAPI
MCM-DMAPI
MCM-APT
MCM-MAPT
0
1
2
3
4
5
6
Time (h)
Aldol Reaction
40
(b)
35
30
25
20
15
10
5
MCM-API
MCM-MAPI
MCM-DMAPI
MCM-APT
MCM-MAPT
0
0
1
2
3
4
5
6
Time (h)
Fig. 3. Time-dependent study of the catalytic performance of the mesoporous
catalysts in an aldol reaction. (a) Percentage conversion and (b) TON (moles of
starting materials converted to products per mole of amines).

Y. Xie et al. / Journal of Catalysis 265 (2009) 131–140
137
6 h were proved to have enhanced catalytic performance in the
24
18
12
6
(a)
MCM-API
Henry reaction than the corresponding samples grafted in toluene
for the same period of time because of the improved cooperative
effect between the site-isolated amine groups and many residual
silanols in the former [56]. Based on this previous report for pri-
mary amine functionalized samples for the Henry reaction, we
have hypothesized that the samples grafted with the different
amines in isopropanol for 5 h here also should have many surface
residual silanol groups surrounding amine groups and produce
cooperative catalytic activity in the aldol reaction compared to
the corresponding samples grafted in toluene for 5 h. Similarly,
the samples grafted in toluene for shorter times of 15 and 30 min
may have similarly higher numbers of ungrafted silanol groups,
which result in higher catalytic efficiency. This hypothesis is rea-
sonable considering the fact that surface silanol groups are weak
acids, and base-catalyzed aldol reactions are activated by acidic
groups [3–5]. Furthermore, mesoporous silica (MCM-41) has been
shown to produce enhanced catalytic effect on base-catalyzed al-
dol reaction in homogeneous catalysis [45].
MCM-API-Capped
0
0
1
2
3
4
5
6
Time (h)
40
35
30
25
20
15
10
5
(b)
MCM-MAPI
MCM-MAPI-
CAPPED
MCM-MAPT
0
In order to prove the enhancement effect of silanols in aldol
reaction by the samples grafted in isopropanol for 5 h and samples
grafted in toluene for 15 and 30 min, we did several control exper-
iments. We first performed an experiment similar to the one done
by Kubota et al. [45] by adding MCM-41 in the reaction mixture.
We also did reactions by using samples MCM41-CH3, MCM-API-
CH3, and MCM-MAPI-CH3 whose silanols were functionalized with
trimethylsilyl groups but have large surface area and pore diame-
ter as their corresponding unfunctionalized samples (see Section 2
for details). When MCM-41 was added to the reaction mixture in
which diethylamine was used as a homogeneous catalyst, the cat-
alytic ability was significantly improved. However, when the sur-
face silanol groups were capped with trimethylsilyl groups
(MCM41-CH3), no enhanced catalytic effect was observed (Table
3). Furthermore, the catalytic activity of the amine-functionalized
catalysts decreased after functionalization of their silanols with
methyl groups (Fig. 4). For instance, upon grafting the silanol
groups of MCM-API (or upon using MCM-API-CH3), the catalytic
efficiency decreased significantly (Fig. 4), confirming the role of
surface silanol groups as a general acid. Although not significant,
similar decrease in catalytic efficiency was also observed for sam-
ple MCM-MAPI-CH3 compared to its unfunctionalized counterpart,
MCM-API (Fig. 4b). Interestingly the sample grafted with APTS for
15 min, which gave similar density of organoamine (or silanol
groups) as sample MCM-API (MAPTS-grafted sample in isopropa-
nol for 5 h), showed significantly lower catalytic efficiency than
sample MCM-API (Table 2 and Fig. S5). However, the former
showed higher catalytic efficiency than the corresponding sample
grafted in toluene for 5 h.
0
1
2
3
4
5
6
hrs
Time (h)
Fig. 4. Time-dependent study of the catalytic performance of the mesoporous
catalysts in an aldol reaction before and after capping with silanol groups of the
catalysts: Samples with (a) primary amine and (b) secondary amine groups.
From the results shown in Table 2, Table 1S, Figs. 3 and 4, and
Fig. S5, three trends in catalytic activity can be drawn, i.e. (1) the
catalytic efficiency of catalysts grafted in isopropanol > corre-
sponding catalysts grafted in toluene; (2) the catalytic efficiency
of catalysts grafted with secondary amine > primary amine > ter-
tiary amine; and (3) the catalytic activity was significantly reduced
after capping the silanols with methyl groups in the case of pri-
mary amine containing sample and in a moderate degree in the
case of the sample containing secondary amine groups (Fig. 4).
The lower degree of catalytic activity by the sample containing ter-
tiary amine groups was consistent with what was reported previ-
ously [45], suggesting that the enamine formation may be the
key intermediate in the catalysis by our materials as well and that
the tertiary amines have difficulty in forming enamine
intermediates.
Based upon all the discussion above, a mechanistic explanation
for the difference in catalytic performance between the catalysts
grafted in isopropanol and those grafted in toluene is proposed
in Fig. 5, where MCM41-MAPI and MCM41-MAPT were used as
examples. Samples grafted in isopropanol, which have relatively
more site-isolated organoamines that are surrounded by many sil-
Table 3
Catalytic activity of diethylamine with and without additives.
OH
O
O
CHO
+
Diethylamine
O
+
Additive, 50 °C
O2N
O2N
O2N
A = Alcohol Product
B = Dehydrated Product
Additive used
No additive
MCM-41
MCM41-CH3
Silica microspheres
Percentage yield at 1 ha
% Yield at 6 ha
NAb
10.5
9.6
36
NAb
9.9
NAb
8.7
a
% Yield is the sum of A and B.
b
NA = Not available since the product peaks were so small in the NMR spectra that it could not be integrated to get reliable data. All reactions were run twice and the
obtained data were similar.

138
Y. Xie et al. / Journal of Catalysis 265 (2009) 131–140
(a)
O
O
O
O
O
O
O
O
O
O
O
H
O
O
H
O
O
O
O
O
Si
Si
Si
OH
Si
Si
Si
Si
Si
OH
Si
Si
O
O
O
H
O
N
HN
NH
HN
O2
N
(b)
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
NH
NH
NH
HN
NH
N
NH
HN
H
O
O
NO2
Fig. 5. Mechanistic-based explanation for the difference in catalytic efficiency between MCM41-MAPI (a) and MCM41-MAPT (b).
anol groups [46], not only activate the acetone molecules to form
enamines but also help the aldehyde to be more susceptible for
nucleophilic addition [51]. This combined effect produces more
efficient catalysis. Since the samples grafted in toluene produce
more aggregated organoamines [46,55] and have only few surface
silanol groups surrounding the amines, this cooperative catalytic
activity by the surface silanols to activate both acetone and benz-
aldehyde in the aldol reaction is reduced significantly (Figs. 4 and
5).
We believe that the activation effect to the carbonyl carbons via
the single hydrogen bonding would be significant enough to ex-
plain the observed difference in catalytic efficiency (or catalytic
promotional effect of silanols) between the sample grafted in iso-
propanol (or that containing more silanols) versus the one grafted
in toluene (or that containing less silanols). This hydrogen bonding
makes that carbon more electrophilic or more susceptible to nucle-
ophilic attack [3c] and more so in samples containing more density
of silanols than in those samples containing fewer silanols. Fur-
thermore, as shown in Fig. 5, the activation occurs to both reac-
tants (p-nitrobenzaldehyde and acetone) making the activation
or promotional effect by silanols to be even more significant.
either of the above two cases, one could expect low catalytic effi-
ciency. Our group recently developed a new method to achieve
more site-isolated organoamine basic groups and many residual
silanol weak acid groups in the pores of mesoporous materials by
using polar-protic solvents such as isopropanol [46,55]. The advan-
tage of this approach has been the weakly acidic silanol groups,
which are barely capable of ‘‘killing” the site-isolated organoamine
basic groups, leaving both acidic and basic sites intact in close
proximity within the material. We have probed the degree of
site-isolation of the organoamine catalytic groups colorimetrically
and demonstrated an enhanced catalytic activity of the material for
the Henry reaction [46,55].
Here we employed a similar strategy to achieve site-isolated ba-
sic groups along with many residual silanols for aldol condensation
reaction. Furthermore, by a systematic change of the basic sites,
the most optimum and efficient site-isolated catalyst for the reac-
tion was determined. While grafting different aminoorganosilanes
in isopropanol was proved to result in site-isolated organoamine
and many weakly acidic silanol catalytic groups, samples grafted
in toluene produced more aggregated organoamine groups and
fewer silanols. The samples grafted with primary and secondary
amines showed significant catalytic activity in aldol condensation
reaction, with the one with secondary amine being the most effi-
cient. The latter observation is interesting compared to our previ-
ous study for the Henry reaction where the sample with primary
amine was obtained to be the most effective cooperative catalyst
[57]. The sample grafted with tertiary amines showed negligible
catalytic ability whether it has site-isolated organoamine groups
or not. Control samples synthesized by grafting the surface silanols
of MCM-41, MCM41-API, and MCM41-MAPI with methyl groups
exhibited less catalytic activity, proving the importance of the sil-
anol groups (Fig. 4).
Generally, the samples that were synthesized in isopropanol
simply contained the surface silanol groups in large numbers
around site-isolated organoamines, enabling a cooperative cata-
lytic activity in aldol reaction to take place efficiently. The unique-
ness in our materials is the presence of many residual surface
silanol groups, which are weakly acidic, around the site-isolated
organoamine groups. There are several advantages of using surface
silanol groups to produce acid–base bifunctional catalysts over the
different organic acid groups that were employed previously [3–5]:
(1) weak acidity results in less degree of neutralization with the
amine groups, giving higher catalytic activity; (2) weak acids favor
less dehydration of the alcohol product (A) into dehydrated prod-
uct (B) during the aldol reaction, giving higher selectivity toward
4. Discussion
Recently, extensive efforts toward the synthesis of heteroge-
neous catalysts containing multiple functional groups that are
capable of cooperatively catalyzing various chemical processes
have continued [3–5,46,52,55]. Enhancement of catalytic activity
by acidic groups in base-catalyzed reactions has been reported
[3–5,46,55,56]. For example, Zeidan et al. [5b,5c] recently reported
enhanced base catalysis in an aldol condensation by placing organ-
ic acid and organic base groups on the same solid support. They
further found that the efficiency of the catalysts was dependent
on the pKa value of the acid. A weaker organic acid group provides
a better acid and base pair to catalyze the reaction owing to low-
ered degree of neutralization between the acidic and basic groups
[5b,5c]. However, during the synthesis of these catalysts, the acidic
and basic groups are not protected from each other [3–5]; there-
fore, neutralization cannot be completely avoided. In another
work, attempts were made to isolate the acidic and basic groups
by administering them separately in the pores and within the
frameworks of the mesoporous materials [56]. This, however, pro-
duces greater distance between the acid and base groups, making
them not capable of exercising effective cooperative activity. In

Y. Xie et al. / Journal of Catalysis 265 (2009) 131–140
139
A as shown in Table 2 and Supplementary Table 1S, which is a
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5. Conclusion
A highly efficient solid-base catalyst for the aldol condensation
reaction was synthesized by placing site-isolated organoamine
groups along with many residual silanols on mesoporous materi-
als. The synthesis was carried out by grafting aminoroganosilanes
in a polar-protic solvent. The mesoporous catalysts were character-
ized by various methods. The surface silanol groups, which were
proved to be more in samples grafted in isopropanol compared
to the corresponding samples grafted in a non-polar solvent, tolu-
ene, were found to be responsible for the enhanced catalytic activ-
ity in aldol reaction. Control experiments were carried out to
investigate the importance of silanol groups for the cooperative
catalytic activity in base-catalyzed aldol reaction. By systemati-
cally investigating a series of organoamines including primary, sec-
ondary, and tertiary amines, the most optimum basic catalyst for
the reaction was determined to be the one with a secondary amine
group that was grafted in isopropanol (MCM41-MAPI). This obser-
vation is interesting compared to our previous study for the Henry
reaction where the sample with primary amine was obtained to be
the most effective cooperative catalyst [57]. Upon comparison of
the catalysis on the basis of catalytic yield and turn-over-number,
sample MCM41-MAPI showed the highest acid–base cooperative
catalytic activity and efficiency for aldol reaction, even when com-
pared to other materials reported previously. A mechanism was
proposed revealing the cooperative catalytic activity between the
site-isolated organoamine groups and residual silanols, where the
surface silanol groups not only participate in the formation of en-
amine intermediate but also facilitate the nucleophilic addition.
This new solid-base catalyst, which contained many residual sur-
face silanols, has enhanced cooperative catalytic activity and selec-
tivity while its synthesis involves no need of additional secondary
organic acid group and proceeds through a less costly procedure.
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