Reaction-dependent heteroatom modification of acid-base catalytic cooperativity in aminosilica materials

Article abstract of DOI:10.1016/j.apcata.2014.10.061

The effects of heteroatom substitution on the cooperative catalytic activity of a series of bifunctional acid-base aminosilica catalysts are probed in aldol and nitroaldol condensations. Three M³⁺ (B, Al, and Ga) and three M⁴⁺ (Ti, Zr, and Ce) heteroatoms are incorporated into different samples of SBA-15 silica and then grafted with aminosilanes to produce bifunctional acid-base catalysts. The catalytic activity of each material is measured in the aldol condensation of 4-nitrobenzaldehyde with acetone at 50 °C and the nitroaldol condensation of 4-nitrobenzaldehyde with nitromethane at 40 °C and compared to the catalytic activity of a heteroatom-free aminosilica catalyst. The heteroatom substitutions produce catalysts with larger amounts of strong Lewis acid sites compared to the heteroatom-free aminosilica catalyst. We rationalize these results in the context of the physical (e.g. surface area, pore diameter, particle size) and chemical properties (e.g. total number and strength of acid sites) of each material and the proposed catalytic mechanisms of the two reactions. The increase in the number of strong Lewis acid sites of each heteroatom material decreased its activity in the aldol condensation, though four heteroatom substitutions (B, Al, Ga, and Ti) increased the catalytic activity of the aminosilica catalyst in the nitroaldol condensation. The results suggest that inclusion of a small amount of Lewis acid sites in aminosilica materials can increase the cooperative catalytic activity of the materials in the nitroaldol condensation. The results also suggest that inclusion of Lewis acid sites in aminosilica materials decreases the cooperative catalytic activity of the materials in the aldol condensation.

Full text of DOI:10.1016/j.apcata.2014.10.061

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Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Reaction-dependent heteroatom modification of acid–base catalytic
cooperativity in aminosilica materials
Eric G. Moschetta, Nicholas A. Brunelli, Christopher W. Jones∗
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, GA 30332, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The effects of heteroatom substitution on the cooperative catalytic activity of a series of bifunctional
acid–base aminosilica catalysts are probed in aldol and nitroaldol condensations. Three M3+ (B, Al, and
Ga) and three M4+ (Ti, Zr, and Ce) heteroatoms are incorporated into different samples of SBA-15 silica
and then grafted with aminosilanes to produce bifunctional acid–base catalysts. The catalytic activity
of each material is measured in the aldol condensation of 4-nitrobenzaldehyde with acetone at 50 ◦C
and the nitroaldol condensation of 4-nitrobenzaldehyde with nitromethane at 40 ◦C and compared to
the catalytic activity of a heteroatom-free aminosilica catalyst. The heteroatom substitutions produce
catalysts with larger amounts of strong Lewis acid sites compared to the heteroatom-free aminosilica
catalyst. We rationalize these results in the context of the physical (e.g. surface area, pore diameter, par-
ticle size) and chemical properties (e.g. total number and strength of acid sites) of each material and
the proposed catalytic mechanisms of the two reactions. The increase in the number of strong Lewis acid
sites of each heteroatom material decreased its activity in the aldol condensation, though four heteroatom
substitutions (B, Al, Ga, and Ti) increased the catalytic activity of the aminosilica catalyst in the nitroaldol
condensation. The results suggest that inclusion of a small amount of Lewis acid sites in aminosilica mate-
rials can increase the cooperative catalytic activity of the materials in the nitroaldol condensation. The
results also suggest that inclusion of Lewis acid sites in aminosilica materials decreases the cooperative
catalytic activity of the materials in the aldol condensation.
Received 4 August 2014
Received in revised form 27 October 2014
Accepted 31 October 2014
Available online xxx
Keywords:
Cooperative catalysis
Aminosilica
Heteroatom
Brønsted acid
Lewis acid
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
organocatalysts [4–8], polymeric systems [9], carbon quantum
dots [10], and silica-immobilized systems [11–19]. The facile
ability to separate and recycle heterogeneous catalysts from
reactive fluids makes them attractive targets for incorporat-
ing acid–base cooperativity in new materials. While the results
from homogeneous catalysts can be used as a basis for design-
ing heterogeneous catalysts, incorporating cooperative catalytic
elements is considerably more challenging in heterogeneous sys-
tems such as those based on silica supports because the silanols
present on the surface, which act as organic functional group
anchoring points or as weakly acidic cooperative partners, are
typically not uniformly distributed [20–26]. Despite the com-
plexity of silica-based catalysts, numerous studies demonstrate
the ability to design materials that exhibit cooperative inter-
actions, providing considerable insights into optimizing such
catalysts. Early indications that silanols could act cooperatively
with amines were unambiguously defined through experiments
using thermally cleavable protecting groups and capping the sur-
face silanols with trimethylsilyl groups using hexamethyldisilazane
(HMDS) [27–31]. More recently, immobilizing amines with dif-
ferent linker lengths demonstrated that spatial positioning of
the amines with respect to the location of surface silanols was
Cooperative catalytic activity can be achieved in a variety of
ways and can be defined as the interaction between two dis-
tinct catalytic sites to achieve a rate of reaction that is greater
than the rate achieved with a single component [1]. Such reac-
tions can be achieved with bifunctional catalysts that contain two
distinct functional groups, most commonly an acid and a base,
that are spatially separated such that the functional groups do
not interfere with each other, yet can interact with a substrate(s)
cooperatively to increase the rate of reaction. The two distinct cat-
alytic units can individually activate substrates or act in concert to
enhance the reaction, among other modes of cooperative activa-
tion [1]. The design of bifunctional catalysts most often relies on
combining simple, inexpensive catalytic elements on chemically
functionalizable materials to enable cooperative interactions [2,3].
Examples of cooperative acid–base catalysts include homogeneous
∗
Corresponding author.
E-mail address: cjones@chbe.gatech.edu (C.W. Jones).
http://dx.doi.org/10.1016/j.apcata.2014.10.061
0926-860X/© 2014 Elsevier B.V. All rights reserved.
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crucial to optimizing the amine–silanol cooperativity [3,19]. While
many types of acidic species including sulfonic, phosphoric, and
carboxylic acids have been used as cooperative partners [11,12,18],
it was recently demonstrated that weaker acidic species, such as
silanols, provided more favorable cooperative interactions in the
form of superior catalytic activity for the aldol condensation of 4-
nitrobenzaldehyde with acetone in comparison to carboxylic acids
and other stronger acids [18]. Another study revealed that shorter
alkyl tethers (e.g. methyl and ethyl) limited cooperative interac-
tions, whereas alkyl linkers longer than propyl exhibited the same
catalytic rate for the aldol condensation of 4-nitrobenzaldehyde
with acetone [19]. Additionally, the choice of solvent can impact the
activity of the bifunctional catalyst. A recent report demonstrated
that primary amines tethered to silica were more active for the
aldol condensation of acetone and 4-nitrobenzaldehyde in water
than in hexane because water shifted the equilibrium in favor of
the aldol product and prevented the amine from forming an imine
with 4-nitrobenzaldehyde, which inhibited the reaction [32].
In an interesting finding, it was recently shown that the
observed trend for the alkyl linker lengths differs depending on
the specific reactions. For the nitroaldol (Henry) condensation of 4-
nitrobenzaldehyde with nitromethane, the rate of reaction steadily
increased with increasing alkyl linker length on SBA-15 silica with
a pore diameter of 6.5 nm [3]. Furthermore, that study also demon-
strated that changing the pore diameter of the silica varied the
catalytic activity by as much as an order of magnitude for the
nitroaldol condensation, while also shifting the distribution of opti-
mal amine linker lengths, providing another method for tuning the
amine–silanol cooperative interactions [3].
These previous studies have mainly focused on modifying the
organic components of the catalytic moieties and the physical
dimensions of the silica support, though the chemical modification
of the silica framework may also have positive effects on the activ-
ity of cooperative catalysts. Additionally, these previous studies
primarily used the surface silanols as the acidic component of the
cooperative interactions, but the inherent heterogeneity of the
distribution of silanols present on the surface makes controlling
the number of protic species with which the amines interact
difficult. An alternative method to modify the properties of the
silanol-covered surface is the incorporation of heteroatoms in the
silica matrix. M3+ elements will typically introduce a Brønsted acid
site that is a stronger acid than the silanols, while M4+ elements
will introduce new types of hydroxyl species such as titanols [33]
or even Lewis acidic sites in the case of coordinatively unsaturated
Zr4+ species [34,35] and tetrahedrally coordinated Ti sites [36].
At present, studies of the effects of heteroatom substitution on
cooperative acid–base catalysts have been limited mainly to
Al substitution for C–C coupling reactions and one-pot cascade
reactions [13,37–48]. In this study, we examine the effects of
heteroatom substitution with materials with similar pore diame-
ters, similar organic (amine) loadings, and approximately 5 mol%
heteroatom substitution. More specifically, we investigate the
effects of heteroatom substitution for two well-studied model
reactions (aldol vs. nitroaldol couplings), showing these reactions
are differently affected by the incorporation of heteroatoms into
the silica framework.
(ZrOCl·8H2O, 98%), ammonium fluoride (NH4F, 98%), and
nitromethane (96%) were obtained from Sigma-Aldrich. Acetone
(99.5%), aqueous ammonia (28 wt% NH3), and 4-nitrobenzaldehyde
(99%) were obtained from Alfa Aesar. Toluene (99.5%) and hexane
(98.5%) were obtained from BDH. Ethanol (100%) was obtained
from Koptec, (3-aminopropyl)triethoxysilane (99%) was obtained
from Gelest, 1,4-dimethoxybenzene (99%) was obtained from TCI,
and 12.1 M HCl was obtained from EMD. Deuterated chloroform
(99.8%) was obtained from Cambridge Isotope Laboratories, Inc.
2.2. Synthesis of SBA-15 and heteroatom-substituted SBA-15
All SBA-15 materials were prepared according to previous
reports [18,19,37,49–55]. The details of the synthetic procedures
are in the Supporting Information.
2.3. Grafting of organosilanes on SBA-15 materials
For each heteroatom material and bare SBA-15 silica, a 500 mg
sample was dried under vacuum at 100 ◦C overnight prior to func-
tionalization. The flask containing the 500 mg sample was purged
with UHP nitrogen and mixed with 12.5 mL of anhydrous toluene
and 58.5 ␮L of (3-aminopropyl)triethoxysilane (or 67.2 ␮L of (5-
aminopentyl)triethoxysilane synthesized in a Parr reactor from the
reaction of 5-bromopentyl triethoxysilane with ammonia) [19]. The
solution of toluene and silane was injected via syringe through
the rubber septum of the flask, at which point the mixture was
magnetically stirred at room temperature for 24 h. Next, the mix-
ture was heated to 80 ◦C and stirred for another 24 h. The solid
was washed and filtered with 100 mL each of toluene, hexane,
and ethanol, sequentially. The material was dried under vacuum
at 100 ◦C overnight and stored in a labeled container for later
use. Amine-functionalized heteroatom materials follow the same
naming convention, X-A#-SBA-15, in this manuscript, where X
refers to the heteroatom and A# refers to the number of carbon
atoms in the alkylamine linker (A1 for aminomethyl, A3 for amino-
propyl, A5 for aminopentyl). The amine-functionalized SBA-15
without heteroatom substitution is called A3-SBA-15 throughout
the manuscript.
2.4. Materials characterization
Nitrogen physisorption experiments were performed on a
Micromeritics Tristar 2030 at 77 K. All samples (approximately
100 mg each) were degassed under vacuum overnight at 110 ◦C
prior to the physisorption measurements. The surface area of
each material was determined using the Brunauer–Emmett–Teller
(BET) method and the total pore volume and pore diameter
were determined using the Broekhoff–de Boer method with the
Frenkel–Halsey–Hill (BdB–FHH) modification using the data from
the adsorption isotherm [56]. Scanning electron microscopy (SEM)
images were recorded on a Hitachi scanning electron microscope
(SEM SU 8010) using an acceleration voltage of 5 kV. A thin layer
of gold was sputtered onto each sample prior to scanning. Ammo-
nia temperature-programmed desorption (TPD) experiments were
performed on a Micromeritics Autochem II 2920. Approximately
125 mg of each sample was placed into a quartz U-tube and dried
under a flow of He at 10 mL/min at 500 ◦C for 1 h. Each sample was
cooled to 50 ◦C, at which point NH3 (2000 ppm in He) flowed over
the sample at a rate of 25 mL/min for 1 h. The sample was held at
50 ◦C, while the tube was purged with He for 30 min to remove
weakly adsorbed NH3. The desorption experiment started at 50 ◦C
and increased to 500 ◦C in a He flow of 10 mL/min with a tem-
perature increase of 10 ◦C/min. The desorbed NH3 was measured
by a thermal conductivity detector (TCD). All samples containing
organosilanes were sent to Atlantic Microlab (Norcross, GA) for
2. Experimental
2.1. Materials
Pluronic P123 block copolymer (EO20PO70EO20, Mn ∼ 5800),
tetraethyl orthosilicate (TEOS, 98%), tetramethyl orthosilicate
(TMOS, 98%), aluminum isopropoxide (Al(OiPr)3, 98%), titanium
tetraisopropoxide (Ti(OiPr)4, 98%), cerium(III) nitrate hexahy-
drate (Ce(NO3)3·6H2O, 99%), zirconium oxychloride octahydrate
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elemental analysis (EA) of the nitrogen content of each sample,
while all samples containing heteroatoms were sent to Galbraith
Laboratories, Inc. (Knoxville, TN) for EA for the heteroatom content.
X-ray diffraction (XRD) experiments were performed on a PAnalyt-
ical X’Pert diffractometer using a Cu K␣ source. Each sample was
placed on a sample holder using a parallel plate detector scanning
from 2Â of 0.6–3◦ with an incident grazing angle of 1◦ and a step size
of 0.02◦. Solid-state NMR spectra were recorded on a Bruker Avance
III 400 spectrometer at a sample spinning speed of 12 kHz. Samples
were hydrated prior to running the NMR experiments. Liquid-phase
1H NMR experiments were conducted on a Varian Mercury Vx
400. Diffuse reflectance UV–Vis spectra were recorded on a Cary
UV–Vis 500 using an internal reflectance cell and pure SBA-15
as the background. Pyridine IR experiments were conducted on a
Thermo Scientific Nicolet 8700 FTIR Spectrometer equipped with
an MCT/A detector. Samples were pressed into wafers and placed
in a stainless steel UHV chamber sealed with a fresh copper gasket
and evacuated at 200 ◦C for 1 h. The temperature of the cell was
decreased to 50 ◦C, at which point pyridine was admitted to the
cell at a pressure of 0.1 mbar and allowed to equilibrate for at least
20 min, followed by evacuation for 1 h to remove physisorbed pyri-
dine. Next, the IR spectrum of the sample was recorded to ensure
pyridine was chemisorbed to the acid sites prior to the desorption
experiments. The temperature of the cell was increased to 100 ◦C
and held for 1 h, then decreased to 50 ◦C and held for 10 min, then
increased to 200 ◦C for 1 h, then decreased again to 50 ◦C. Spectra
were recorded every 5 min. Each spectrum consisted of 64 scans at a
resolution of 4 cm−1. The total amounts of acid sites for each mate-
rial were obtained using the integrated peak areas (Brønsted sites
near 1540 cm−1, Lewis sites near 1450 cm−1) and molar extinction
coefficients for Brønsted acid sites (1.8 cm/ꢀmol) and Lewis acid
sites (1.3 cm/ꢀmol) from the literature [46,57,58].
Ce-A3-SBA-15
Zr-A3-SBA-15
Ti-A3-SBA-15
Ga-A3-SBA-15
Al-A3-SBA-15
B-A3-SBA-15
A3-SBA-15
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
P/P0
Fig. 1. Nitrogen physisorption isotherms for A3-SBA-15 and the heteroatom-
substituted SBA-15 materials.
vial. GC samples were run on a Shimadzu GC-2010 equipped with
a flame ionization detector (FID), an AOC-20i + s autoinjector and
autosampler, and a Restek Rtx-5 GC column (15 m, 0.25 mm inner
diameter, 0.1 ␮m film thickness). Product selectivity was deter-
mined by 1H NMR after 4 h of reaction time. The solutions were
concentrated in vacuo and dissolved in deuterated chloroform.
3. Results and discussion
2.5. Catalytic measurements
3.1. Physical characterization of materials
All reactions were performed in 25 mL two-neck round-bottom
flasks under pressure of UHP nitrogen, each fitted with a con-
denser, magnetic stir bar, and rubber septum. Stock solutions for
the aldol and nitroaldol condensations were prepared in advance
and stored in a refrigerator prior to use. Each solution consisted of
605 mg of 4-nitrobenzaldehyde, 553 mg of 1,4-dimethoxybenzene
as an internal standard, and either 80 mL of acetone (for the aldol
condensation) or 80 mL of nitromethane (for the nitroaldol con-
densation) as the reactant in excess, also acting as the solvent. For
each reaction, 1.0025 mL of the stock solution was added to the
flask, removing 25 ␮L as the initial sample for GC analysis. Next,
the supported catalyst was added to the flask such that 10 mol%
nitrogen (0.005 mmol) was present relative to the initial amount
of 4-nitrobenzaldehyde (0.05 mmol) in the flask. The flask was
immersed in an oil bath at 50 ◦C for the aldol condensation and 40 ◦C
for the nitrolaldol condensation, with magnetic stirring. Samples
were withdrawn through the septum with a syringe every 15 min
for 1 h and once an hour through 4 h. All reaction samples were
run through a short bed of silica gel in a cotton-plugged pipette to
remove the catalyst and washed with acetone directly into a GC
The amino-silanol catalysts were successfully synthesized,
each containing a series of different heteroatoms, including M3+
elements (B, Al, and Ga) and M4+ elements (Ti, Zr, and Ce). Nitrogen
physisorption experiments showed that the pore diameters are
approximately 7.5 nm (see Fig. 1 for the adsorption isotherms)
in all cases. It is critical to maintain the same pore diameters
among all catalysts because previous studies demonstrated that
the pore diameter affects the cooperativity between amines and
surface silanols [3,59–63]. The amine functionalization procedure
did not alter the pore diameter, indicating that the procedure was
sufficiently mild. Table 1 contains the surface areas, cumulative
pore volumes, pore diameters, and the average particle sizes of the
six amine-functionalized heteroatom materials and A3-SBA-15.
Fig. S1 in the Supporting Information shows the SEM images
for the amine–silanol catalysts. The particle morphologies differ
between the catalysts, most likely due to the different synthesis
conditions for each of the materials. The standard A3-SBA-15
material along with the B-, Ga-, Ti-, and Ce-A3-SBA-15 materials all
exhibit similar rod-like morphologies. The Zr-A3-SBA-15 catalyst
Table 1
Physical parameters of the amine-functionalized silica bifunctional catalysts.
Material
Surface area (m2/g)
Total pore volume (cm3/g)
Pore diameter (nm)
Particle size (␮m)
Amine loading (mmol/g oxide)
A3-SBA-15
930
918
1001
734
725
851
992
1.34
1.14
0.94
1.30
1.30
1.02
1.34
6.9
7.0
8.2
8.0
9.9
8.1
8.1
0.2
0.7
2.2
0.8
0.8
0.8
1.8
0.36
0.48
0.42
0.56
0.44
0.44
0.54
B-A3-SBA-15
Al-A3-SBA-15
Ga-A3-SBA-15
Ti-A3-SBA-15
Zr-A3-SBA-15
Ce-A3-SBA-15
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A3-SBA-15
B-A3-SBA-15
Al-A3-SBA-15
Ga-A3-SBA-15
Ti-A3-SBA-15
Zr-A3-SBA-15
Ce-A3-SBA-15
A3-SBA-15
Ti-A3-SBA-15
Zr-A3-SBA-15
Ce-A3-SBA-15
100
200
300
400
500
0.5
1.0
1.5
2.0
2.5
3.0
Temperature (ºC)
2θ (Degrees)
Fig. 4. TPD profiles of the M4+ heteroatom-containing materials. A3-SBA-15 is
shown for comparison.
Fig. 2. XRD patterns of A3-SBA-15 and the six heteroatom-functionalized materials.
shows a spherical shape, while the Al-A3-SBA-15 catalyst shows
an irregular morphology.
the TPD profile revealed a low-temperature peak at 86 ◦C and a
high-temperature peak at 222 ◦C. All six heteroatom substitutions
produced materials with more total acid sites compared to the
standard A3-SBA-15, because the total amount of adsorbed NH3
was higher in each case after heteroatom substitution. Heteroatoms
of M3+ character can incorporate themselves tetrahedrally into the
silica framework to introduce Brønsted acidity if proton balances
the framework negative charge [64]. Additionally, M3+ cations may
contribute Lewis acid sites to the overall framework when coordi-
natively unsaturated metal sites are present. The B-A3-SBA-15 and
Al-A3-SBA-15 heteroatom materials exhibit sharp peaks at 95 and
99 ◦C, respectively, while both materials have high-temperature
peaks at 300 ◦C (the peak area for B-A3-SBA-15 is small). The Ga-A3-
SBA-15 heteroatom material exhibits a sharp peak at 108 ◦C with
a broad shoulder appearing after the low-temperature peak and
continuing to approximately 500 ◦C. This broad shoulder for Ga-A3-
SBA-15 indicates both a larger total number of acid sites and a range
of acid strengths (addressed further below) for the Ga3+ cations
dispersed throughout the silica. Palomino et al. synthesized MCM-
41 silica doped with Ga and observed Brønsted and Lewis acidity
via FTIR spectra of adsorbed pyridine and CO and attributed the
Brønsted acidity to hydroxyls bridging Si and Ga atoms and Lewis
acidity to coordinatively unsaturated Ga3+ ions [65]. Al-substituted
SBA-15 materials exhibit Brønsted and Lewis acidity under the
same conditions [37,66]. Eswaramoorthi and Dalai synthesized
B-substituted SBA-15 and used 11B triple quantum magic-angle
spinning (MAS) NMR to observe tri- and tetracoordinated B atoms
in the silica matrix and diffuse reflectance infrared Fourier trans-
form (DRIFT) spectroscopy to observe increased Brønsted and Lewis
acidity as the degree of B substitution increased [67].
Fig. 2 shows the XRD patterns for the six heteroatom materi-
als compared to A3-SBA-15. In all cases, the large (100) diffraction
peak appears from 2Â = 0.6–1.2 and the two smaller (110) and (200)
diffraction peaks appear from 2Â = 1.3–2.0, verifying the retention
of the expected 2D hexagonal mesoporous structure. For larger het-
eroatoms such as Zr and Ce, the (110) and (200) diffraction peaks
are also larger.
3.2. Chemical characterization of materials
Fig. 3 shows the NH3 TPD profiles for the A3-SBA-15 silica
compared to the M3+ heteroatom materials and Fig. 4 shows the
TPD profiles for the M4+ heteroatom materials. For A3-SBA-15,
A3-SBA-15
B-A3-SBA-15
Al-A3-SBA-15
Ga-A3-SBA-15
Among the M4+ heteroatom materials, the Ti-A3-SBA-15 and
Ce-A3-SBA-15 heteroatom materials showed single peaks at 108
and 92 ◦C, respectively, while Ti-A3-SBA-15 displayed a small
high-temperature peak at 325 ◦C. The Zr-A3-SBA-15 heteroatom
material exhibited a peak at 118 ◦C and a broad shoulder after-
ward, all indicating an increased total number of acid sites for
the heteroatom material compared to A3-SBA-15. Heteroatom
substitutions in silica materials using Ti, Zr, and Ce as the het-
eroatoms are known to contribute Lewis acidity to the overall
material [53–55,68]. For Zr substitutions, it is possible to observe
100
200
300
400
500
Temperature (ºC)
Fig. 3. TPD profiles of the M3+ functionalized heteroatom-containing materials. A3-
SBA-15 is shown for comparison.
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Table 2
Initial TOFs and heteroatom content of the six heteroatom materials and aminopropyl-functionalized SBA-15 for the nitroaldol and aldol condensations.
Material
Heteroatom (wt%)
Aldol
Nitroaldol
TOF (h−1
)
Relative rate
TOF (h−1
)
Relative rate
A3-SBA-15
−
2.9
2.0
1.6
2.2
0.9
0.4
1.4
−
2.8
5.0
5.05
3.5
4.2
1.3
3.0
−
B-A3-SBA-15
Al-A3-SBA-15
Ga-A3-SBA-15
Ti-A3-SBA-15
Zr-A3-SBA-15
Ce-A3-SBA-15
0.11
0.05
3.13
0.13
5.24
1.47
0.69
0.56
0.74
0.30
0.13
0.47
1.79
1.82
1.25
1.53
0.46
1.08
The relative rate is the ratio of the TOF of the heteroatom material to the A3-SBA-15 material.
Brønsted acidity as well [53]. Zr substitutions in silica materials
also introduce Zr–O–Si linkages, which may distort the silica struc-
ture and induce charge imbalances that are offset by weakening
the existing O–H bonds of the silanols in the vicinity of the Zr–O–Si
linkage, thereby creating a slight increase in Brønsted acidity at
these weakened silanols [69]. Ti substitutions in silica materials
introduce Ti–O–Si linkages, which may require the charge balanc-
ing of tetrahedral Si and octahedral Ti through a hydroxyl group if
water vapor is present [70,71]. Kataoka and Dumesic used Pauling’s
electrostatic valence rule to show the oxygen in a Ti–O–Si linkage
was electronically undersaturated by 0.33 valence units (v.u.) if the
Ti was octahedrally coordinated in the presence of water vapor,
thereby creating a Brønsted acid site at the undersaturated oxy-
gen [71]. Other reports state that tetrahedral Ti does not introduce
Brønsted acid sites in silica-rich frameworks, as expected for Ti4+
substitution of silicon [70,72].
Table 2 contains the elemental analysis for the six amine-
functionalized heteroatom materials and A3-SBA-15. The results
indicate that the incorporation of the heteroatom depended on
the nature of the synthesis. In all cases, the amount of heteroatom
present is less than the theoretical amount as outlined in Section
2 of the Supporting Information for each material. However, the
larger amounts of Ga and Zr merit further characterization to deter-
mine whether the heteroatoms truly are incorporated into the silica
20
10
0
-10
-20
Chemical Shift (ppm)
Fig. 6. Solid-state 11B NMR spectrum of the B-A3-SBA-15 material.
spectrum of Ga-A3-SBA-15. The lone peak at 144 ppm corresponds
to tetrahedral Ga incorporated into the silica framework [74,75].
Okumura et al. [76] synthesized Ga-substituted MCM-41 and
observed a peak at −24 ppm that they attributed to octahedral Ga
external to the silica structure, a peak that is absent in our sample.
framework or if they are present as bulk oxide domains.
Fig. 5 shows the 27Al solid-state NMR spectrum of Al-A3-SBA-
15. The peak at ∼0 ppm corresponds to octahedral Al and the peak
at ∼54 ppm corresponds to tetrahedral Al [66]. The integrated peak
areas produce a ratio of 1.6 octahedral Al atoms to every tetra-
hedral Al atom, indicating more potentially Lewis-acidic Al was
substituted into the silica framework than Brønsted acidic species.
Fig. 6 shows the 11B solid-state NMR spectrum of B-A3-SBA-15.
The peak at −3.4 ppm corresponds to tetrahedral B only, indicative
of Brønsted acidity [67,73]. Fig. 7 shows the 71Ga solid-state NMR
250
200
150
100
50
0
-50
200 150 100 50
0
-50 -100 -150 -200
Chemical Shift (ppm)
Chemical Shift (ppm)
Fig. 5. Solid-state 27Al NMR spectrum of the Al-substituted SBA-15 material.
Fig. 7. Solid-state 71Ga NMR spectrum of the Ga-A3-SBA-15 material.
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6
absorption near 320 nm, indicative of octahedral Ti [80,81]. The Zr-
A3-SBA-15
0.6
0.5
0.4
0.3
0.2
0.1
0.0
A3-SBA-15 material exhibited an absorption near 195 nm, which
may correspond to isolated tetrahedral Zr, and another absorption
near 300 nm, which corresponds to octahedral Zr [82]. Further-
more, the lack of a peak near 240 nm for Zr-A3-SBA-15 indicates
there is no bulk ZrO2 on the silica surface [82]. Other reports
confirm that isolated Zr4+ species absorb at shorter wavelengths
(blue-shift) than bulk Zr4+ species because the bulk Zr4+ species
exist in the monoclinic phase which has a lower ligand-to-metal
charge-transfer (LCMT) energy compared to the isolated Zr4+
species [83–85]. The amine-functionalized Ce material exhibited an
absorption near 300 nm, indicative of tetracoordinated Ce4+ species
in the silica framework, though the smaller peak near 240 nm is
indicative of the presence of a small fraction of Ce3+ [54,55].
Fig. 9 shows the amount of adsorbed pyridine (confirming the
presence of Brønsted and Lewis acid sites) in each of the heteroatom
materials and the standard A3-SBA-15 material as determined by
IR spectroscopy measurements, while the amounts of each type of
acid site are listed in Table 3. IR spectra for all samples are located
in the Supporting Information (Figs. S2–S8). All six heteroatom
materials had higher amounts of Lewis acid sites than Brønsted
acid sites at all desorption temperatures. A3-SBA-15 had weakly
physisorbed pyridine near 1450 cm−1 at 50 ◦C, but otherwise had
no discernible Brønsted or Lewis acid sites at all desorption temper-
atures, in agreement with the literature [58]. With the exception
of Ce, higher loadings of heteroatoms led to materials with more
total acid sites. In particular, the Ga- and Zr-A3-SBA-15 materials
had the strongest and largest amounts of both types of acid sites,
which can be attributed to the higher heteroatom loadings of these
two materials. Furthermore, the broad TPD curves observed for Ga-
and Zr-A3-SBA-14 are consistent with the pyridine IR results. The
A3-SBA-15 and Al- and Ti-SBA-15 materials did not exhibit peaks
Ti-A3-SBA-15
Zr-A3-SBA-15
Ce-A3-SBA-15
200
300
400
500
600
Wavelength (nm)
Fig. 8. Diffuse reflectance UV–Vis spectra of Ti-A3-SBA-15, Zr-A3-SBA-15, and Ce-
A3-SBA-15 materials. The standard A3-SBA-15 sample is shown for reference.
Additionally, no peaks above the baseline noise corresponding to
bulk Ga2O3 domains appear (25, 40–56, 200 ppm), indicating that
all Ga was incorporated into the silica framework [77–79].
Fig. 8 shows the diffuse reflectance UV–Vis spectra of Ti-
A3-SBA-15, Zr-A3-SBA-15, and Ce-A3-SBA-15 materials. The
aminopropyl-functionalized Ti heteratom material exhibited an
absorption near 230 nm, indicative of tetrahedral Ti, and an
50 oC
200
175
150
125
100
75
100 oC
200 oC
μ
50
25
0
SBA
B
Al
Ga
Ti
Zr
Ce
Heteroatom Material
Fig. 9. Total amounts of acid sites (Brønsted and Lewis) for the six heteroatom materials and the standard A3-SBA-15 material for which only physisorbed pyridine was
observed. The data represent the amounts of acid sites present after desorption of pyridine at various temperatures. For each material, the left bar refers to a desorption
temperature of 50 ◦C, the middle bar refers to 100 ◦C, and the right bar refers to 200 ◦C. Note that A3-SBA-15 had weakly physisorbed pyridine (see Fig. S2) where weak
interaction with surface silanols would be expected [58], indicative of very weak Brønsted acid sites.
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Table 3
Total amounts of Brønsted and Lewis acid sites for the six heteroatom materials and the standard A3-SBA-15 material as determined via pyridine IR experiments.
Material
Brønsted acid sites (␮mol/g silica)
Lewis acid sites (␮mol/g silica)
50 ◦
C
100 ◦
C
200 ◦
C
50 ◦
C
100 ◦
C
200 ◦
C
A3-SBA-15
0
2
0
23
0
25
3
0
2
0
11
0
22
1
0
0
0
5
0
0
55
18
85
39
0
30
8
58
16
134
4
0
12
5
38
6
B-A3-SBA-15
Al-A3-SBA-15
Ga-A3-SBA-15
Ti-A3-SBA-15
Zr-A3-SBA-15
Ce-A3-SBA-15
13
0
182
10
89
4
the highest activity for the aldol condensation of all three M3+
heteroatom materials, though B-A3-SBA-15 has a very similar
rate. While the Ga-substituted silica has the highest heteroatom
content of the M3+ materials, it also has the strongest and largest
number of Lewis acid sites. The decrease in catalytic activity for
the array of M3+ heteroatom materials may be attributed to the
increased total number and strength of the acid sites present in the
silica supports, as a previous report showed that stronger Brønsted
acid sites inhibit the aldol condensation [18]. This is likely due
to non-productive, strong amine–acid interactions, removing the
amine from the pool of sites available to activate the reagents.
The M4+ heteroatom substitutions also lowered the rate of
reaction relative to A3-SBA-15. This behavior is surprising for Ce
because it is known to have more basic character, and the aldol con-
densation is more active in the presence of weaker acids [86]. The
behavior of Zr-A3-SBA-15 is expected because the Zr heteroatoms
serve as Lewis acid sites, which bind Lewis bases, and the acid–base
interaction would reduce the catalytic activity [53,87,88]. The TPD
and pyridine IR results for Zr-A3-SBA-15 indicate that it has the
largest number of Lewis acid sites (most at 50 ◦C) and the largest
number of strong acid sites (most at 200 ◦C) of all the amine–silanol
catalysts, which explains why the other Lewis-acidic catalysts such
as B-, Al,-, and Ti-A3-SBA-15 are more active than Zr-A3-SBA-15
in the aldol condensation. The results for Ga-A3-SBA-15 and Zr-
SBA-15 suggest that the physical geometry of the heteroatom in
the silica framework that creates the Lewis acid site may affect the
catalytic activity. As mentioned above, coordinatively unsaturated
Ga3+ species create Lewis acid sites in Ga-substituted silica [65]
while Zr–O–Si linkages create Lewis acid sites in Zr-substituted sil-
ica [69]. These Zr–O–Si linkages distort the silica framework and
induce an increased Brønsted acidity in the silanols, which could
explain why the larger number of strong Lewis acid sites of Zr-
A3-SBA-15 inhibit the aldol condensation more severely compared
to Ga-A3-SBA-15, which also has a large number of strong Lewis
acid sites. Trukhan et al. [33] synthesized a series of Ti-substituted
silicates, including Ti-SBA-15, and concluded via FTIR spectra of
adsorbed CD3CN that the addition of Ti to the silica materials
increased the strength of the surface Brønsted acid sites (titanols),
though not as much when compared to Al-doped silica materi-
als. Furthermore, they observed differences in acid strength for
the materials depending on the nature of the synthetic conditions,
the extent of surface hydroxylation, and Ti dispersion in the sil-
ica framework [33]. Titanols on the surface of mesoporous silicas
provide stronger Brønsted acidity than silanols, which explains the
decreased activity of Ti-A3-SBA-15 compared to the A3-SBA-15 for
the aldol condensation, though the pyridine IR results for both the
A3-SBA-15 and Ti-A3-SBA-15 indicated very weak Brønsted acidity
[48,89].
in the Brønsted acid region of the IR spectra because the strengths
of these acid sites were likely not significant enough to measure.
Additionally, the 27Al NMR results indicated the likelihood of more
total Lewis acid sites in Al-A3-SBA-15 because of the larger amount
of octahedral Al present in the sample. The pyridine IR results sug-
gest that the nature of the synthesis of the heteroatom material,
as well as the chemical and physical properties of the heteroatom
itself, contributed to the strengths and amounts of the Brønsted and
Lewis acid sites present in each material.
3.3. Catalytic behavior of heteroatom functionalized materials
For the model aldol condensation reaction of 4-
nitrobenzaldehyde with acetone, each heteroatom-substituted
catalyst is observed to be less active than the standard silica
material A3-SBA-15 (Fig. 10). The initial turnover frequencies
(TOFs) are reported for all materials in Table 2. This observation is
consistent with published reports using a heteroatom-substituted
aminosilica, whereby the authors observed lower catalytic rates
when using Al as a heteroatom in the silica matrix [13,39]. For
the M3+ heteroatoms, each material has a similar catalytic activity
(between a factor of 0.6 and 0.75 of the catalytic activity of A3-
SBA-15). Given the variability of acid strength and total number
of acid sites for the heteroatom-incorporated silicas, this behavior
is surprising, especially considering the high Ga loading for Ga-
A3-SBA-15. These similarities could reflect the low heteroatom
substitution of the B- and Al-SBA-15 materials, though these
results suggest it would be disadvantageous to further increase
the B and Al heteroatom content. The Ga-A3-SBA-15 material has
80
70
60
50
40
30
20
10
0
A3-SBA-15
B-A3-SBA-15
Al-A3-SBA-15
Ga-A3-SBA-15
Ti-A3-SBA-15
Zr-A3-SBA-15
Ce-A3-SBA-15
In addition to the effects of the heteroatoms themselves, the
optimal alkyl linker length may have changed after heteroatom
substitution due to changes in the silica microstructure, despite
the large pore volume. For small pores (2.3 nm), long alkyl link-
ers resulted in lower rates of reaction for the aldol condensation
of 4-nitrobenzaldehyde with acetone at 50 ◦C over silica supports
0
1
2
3
4
Time (h)
Fig. 10. Conversion of 4-nitrobenzaldehyde as a function of time for each of the
heteroatom materials in the model aldol condensation. The conversion profile of
A3-SBA-15 is provided for reference.
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A3-SBA-15
Al-A1-SBA-15
Ti-A1-SBA-15
100
90
80
70
60
50
40
30
20
10
0
A3-SBA-15
B-A3-SBA-15
Al-A3-SBA-15
Ga-A3-SBA-15
Ti-A3-SBA-15
Zr-A3-SBA-15
Ce-A3-SBA-15
60
50
40
30
20
10
0
0
1
2
3
4
0
1
2
3
4
Time (h)
Time (h)
Fig. 11. Conversion of 4-nitrobenzaldehyde as a function of time for aminomethyl-
functionalized materials Al-A1-SBA-15 and Ti-A1-SBA-15 in the aldol condensation.
The conversion profile of A3-SBA-15 is provided for reference.
Fig. 12. Conversion of 4-nitrobenzaldehyde as a function of time for each of the
heteroatom materials in the model nitroaldol condensation. The conversion profile
of A3-SBA-15 is provided for reference.
the observed catalytic activity in the nitroaldol condensations,
while it played a more modest role in the aldol condensation. For
the aldol condensation on SBA-15 silica with a 6.5 nm pore diame-
ter, it was shown that the catalytic activity increased as the length
of the alkyl linker increased from one carbon (C1) to three carbons
(C3), but further increasing the linker length to C5 or even C11
did not significantly increase or decrease the activity compared to
the C3 linker [3,19]. Conversely, for the nitroaldol condensation
on the same series of SBA-15 silica materials with a 6.5 nm pore
diameter, the activity continued to increase as the linker length
increased from C1 to C5, though a slight decrease in activity for C11
relative to C5 was observed, most likely because the longer linker
could not interact with the silanols as efficiently as the shorter
C5 linker [3]. Knowing that the nitroaldol condensation was more
sensitive to the alkyl linker length than the aldol condensation
and that the proposed nitrolaldol mechanism [3] relies on two
acid sites compared to one acid site for the aldol mechanism [19],
it is interesting to note that increasing the number of Lewis acid
sites available to the amines for cooperative interactions increases
the activity of four of the heteroatom-containing materials for
the nitroaldol condensation. Thus, the nitroaldol condensation is
more sensitive to amine–silanol site pairing and the presence of
additional Lewis acid sites, whereas the aldol condensation is less
affected by these variables.
However, it should be noted that the increased activity observed
in the nitroaldol condensation occurred only for four of the six
heteroatom materials evaluated (B-, Al-, Ga-, and Ti-A3-SBA-15),
and a dramatic decrease in catalytic activity for Zr-A3-SBA-15 was
observed relative to the pure silica case. The B-, Al-, and Ti-A3-
SBA-15 materials were the most active catalysts and had smaller
amounts of Lewis acid sties. The Ce-A3-SBA-15 material had the
least amount of Lewis acid sites and thus behaved nearly identi-
cally to the A3-SBA-15 catalyst. The Zr-A3-SBA-15 material had the
most Lewis acid sites of the six heteroatom materials and was the
least catalytically active, with a rate below that of the heteroatom-
free material. The materials with the second largest number of
Lewis acid sites, Ga-A3-SBA-15, was found to be slightly more active
than the A3-SBA-15 catalyst. These results suggest that adding very
small amounts of Lewis acid sites (Ce-A3-SBA-15) had a negligible
effect on catalytic activity for the nitroaldol condensation, while
adding modest amounts of Lewis acid sites (B-, Al-, and Ti-SBA-15)
[3]. However, the increased number of acid sites and morpholo-
gies of the heteroatom materials may alter the required distances
of the acid–base cooperative interactions. To examine this possibil-
ity, two materials were synthesized with aminomethyl (A1) groups,
rather than aminopropyl moieties, tethered to the surface: one
was substituted with Al (M3+) and the other was substituted with
Ti (M4+). Interestingly, both aminomethyl-functionalized materials
exhibited the same behavior as the pure silica material function-
alized with aminomethyl groups [19]. The apparent conversion of
4-nitrobenzaldehyde reached ∼10% after 4 h (Fig. 11), consistent
with a quantitative reaction of the aldehyde with the surface amine
to produce an imine. In the previously proposed mechanism, the
nucleophilic amine attacks the silanol-activated acetone creating
an enamine, which is followed by reaction with the acid-activated
aldehyde [19]. For the aminomethyl catalysts, cooperative acti-
vation of the acetone was hypothesized to be inefficient due to
the short linker length, and stoichiometric imine formation was
observed, which was suggested to represent an off-cycle adduct
that formed because the catalyst cannot efficiently activate the ace-
tone and turnover catalytically [19]. Thus, in this work, as well as
in the previous study, the amines behaved as if they were isolated
from Brønsted acidic silanol species. These results indicate that
the aminomethyl group provides an amine site that is effectively
isolated from acidic surface hydroxyls.
In contrast to the above results, the observed catalytic activities
changed significantly for the model nitroaldol condensation reac-
tion and actually increased for all M3+ heteroatom substitutions (Al,
Ga, and B) and for the Ti-substituted material (Fig. 12). This increase
in catalytic activity could reflect the influence of the Lewis acid
sites of the heteroatom materials compared to A3-SBA-15, result-
ing in more efficient catalytic cooperativity with the proposed [3]
two surface acid sites (for the heteroatom materials, the Lewis acid
sites may serve the same role as the hydroxyl groups) that are sug-
gested to play a role in the reaction. Another proposed mechanism
for the nitroaldol reaction requires only basic catalytic moieties
and involves imine formation as an intermediate [90,91]. However,
the base-only mechanism, if it operates at all, is slower than the
acid-assisted mechanism.
In previous work using heteroatom-free silica materials, the dis-
tance between the acid sites and basic amines strongly influenced
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increased the catalytic activity. Continuing to increase the number
of Lewis acid sites (Ga-A3-SBA-15) decreased the activity some-
what, though was still beneficial compared to A3-SBA-15. Inclusion
of too much Lewis acidity (Zr-A3-SBA-15) was detrimental to the
activity of the heteroatom material. At this stage, the observed
trends in catalytic activity for all the heteroatom materials are ten-
tatively attributed to the total number of Lewis acid sites, rather
than the strength of the acid sites. With the varying amounts of
heteroatoms in each of the materials, definitive conclusions cannot
be drawn about the strengths of the Lewis acid sites based on the
type of heteroatom present. However, incorporating even modest
amounts of Lewis acid sites relative to the heteroatom-free A3-SBA-
15 material appears to have a significant effect on the cooperative
catalytic behavior of these materials, so at this stage, the trends
in catalytic activity cannot be definitively ascribed to nature or
the strength of the sites, and a correlation with total number of
sites is most appropriate. Thus, just as there was an optimal amine
linker length for the nitroaldol condensation in the previous work
[3,19], there appears to be an optimal amount of Lewis acidity for
increasing the reaction rate in the nitroaldol reaction.
100
90
80
70
60
50
40
30
20
10
0
A3-SBA-15
Al-A3-SBA-15
Al-A5-SBA-15
Ti-A3-SBA-15
Ti-A5-SBA-15
0
1
2
3
4
Time (h)
The effect of amine linker length was also probed for selected
heteroatom-containing materials. Fig. 13 shows the results of the
nitroaldol condensation for Al- and Ti-substituted materials func-
tionalized with aminopentyl (A5) groups instead of aminopropyl
species. The initial rate of reaction for the longer alkyl amine in
the nitroaldol condensation was identical to the rate of reaction for
the aminopropyl-functionalized heteroatom materials, though the
reaction yields were lower at longer reaction times when using the
longer linkers. In the case of the nitroaldol condensation, the incor-
poration of Al or Ti heteroatoms in the material provided a similar
increase in catalytic activity as increasing the length of the alkyl
linker from C3 to C5 on pure silica support materials with a large
(6.5 nm) pore diameter, as discussed previously [3]. However, the
data in Fig. 13 demonstrate that the rate is not further increased
over the Ti and Al heteroatom containing materials by using the
C5 linker, and thus the rate enhancements associated with longer
linkers and heteroatoms do not appear to be additive.
Fig. 13. Conversion of 4-nitrobenzaldehyde as a function of time for Al- and Ti-
substituted materials functionalized with aminopentyl (A5) groups. The conversion
profiles of A3-SBA-15, Al-A3-SBA-15, and Ti-A3-SBA-15 are provided for reference.
Table 4 shows the reaction schemes, products, and selectivity
after 4 h toward each product for the six heteroatom materials and
A3-SBA-15 for the aldol and nitroaldol condensations. For the aldol
condensation, the standard A3-SBA-15 catalyst favored formation
of the aldol product (1, 77% selectivity) compared to the unsatu-
rated dehydration product (2, 23% selectivity). This ratio of 3:1 in
favor of the aldol product is in agreement with Zeidan and Davis’s
original report for an aminosilica catalyst with no heteroatom con-
tent [11]. The heteroatom materials with fewer total acid sites (e.g.
B-, Al-, Ti-, and Ce-A3-SBA-15) were selective toward the aldol
product while the heteroatom materials with more total acid sites
(e.g. Ga- and Zr-A3-SBA-15) were selective toward the unsaturated
Table 4
Reaction schemes and product selectivity for the aldol and nitroaldol condensations on A3-SBA-15 and the six heteroatom materials after 4 h. Product distributions were
determined using 1H NMR spectroscopy. Water is lost upon formation of 2 and 5 (4 is formed from 5).
Aldol
Nitroaldol
Material
1 (%)
2 (%)
3 (%)
4 (%)
5 (%)
A3-SBA-15
77
77
65
44
74
35
58
23
23
35
56
26
65
42
0
0
0
23
23
24
22
22
15
17
77
77
76
78
78
50
51
B-A3-SBA-15
Al-A3-SBA-15
Ga-A3-SBA-15
Ti-A3-SBA-15
Zr-A3-SBA-15
Ce-A3-SBA-15
0
<1
35
32
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dehydration product. Zeidan and Davis also observed a higher pref-
erence for the aldol product when using weaker organic acids such
as phosphoric and carboxylic acids compared to a stronger sulfonic
acid [12]. While incorporating inorganic heteroatoms into the silica
framework decreased the activity for the aldol condensation, doing
so also increased the selectivity toward the unsaturated product,
which could prove useful in future applications if that product is
desired.
interactions are sequential. The reaction data shown here for the
nitroaldol condensation is consistent with the previous hypothe-
sis [3] that multiple hydroxyls (or Lewis acid sites) are involved
in the reaction, suggesting that this mechanism involves two acid
sites simultaneously activating the aldehyde and the nitro group of
nitromethane in a concerted fashion [3]. This difference between
the two mechanisms may also be responsible for the differences
in catalytic activity for the two reactions when substituting het-
eroatoms into the surface of the material, though at this stage,
this remains speculative. Regardless, the nitroaldol condensation
is clearly more sensitive to acid–base pairing than the aldol con-
densation.
The nitroaldol condensation yielded three different products:
the ␤-nitroalcohol (3), the 1,3-dinitroalkane (4), and the ␤-
nitrostyrene (5). The 1,3-dinitroalkane is formed via a Michael
addition of the excess nitromethane to the ␤-nitrostyrene
[43,92,93]. The standard A3-SBA-15 material exhibited 77% selec-
tivity toward the ␤-nitrostyrene and 23% selectivity toward the
1,3-dinitroalkane. No ␤-nitroalcohol was present in the 1H NMR
spectrum. The results are consistent with literature reports where
primary amines on mesoporous silica materials favored the forma-
tion of ␤-nitrostyrene with yields similar to those observed here
while secondary and tertiary amines favored the formation of the
␤-nitroalcohol [94,95]. The four most active heteroatom materi-
als for the nitroaldol condensation (B-, Al-, Ga-, and Ti- SBA-15)
all exhibited the same selectivity toward the ␤-nitrostyrene and
the 1,3-dinitroalkane as the A3-SBA-15 material, though Ti-A3-
SBA-15 formed a trace amount of the ␤-nitroalcohol (less than 1%
selectivity). Conversely, the two least active heteroatom materials
for the nitroaldol condensation, Zr- and Ce-A3-SBA-15, exhibited
appreciable selectivity (32 and 35%, respectively) toward the ␤-
nitroalcohol. These results suggest that primary amines could be
used with Zr- and Ce-substituted silicas to create catalysts that
favor the formation of ␤-nitroalcohols if the amounts of het-
eroatoms can be tuned to increase the catalytic activity compared
to the results obtained here without sacrificing selectivity. How-
ever, increasing the amount of Lewis acid sites did not appear to
affect the selectivity of the nitroaldol condensation, given that Ga-
A3-SBA-15 had the same selectivity as A3-SBA-15, which does not
have Lewis acid sites. Additionally, the large difference in total acid
sites between Zr-A3-SBA-15 and Ce-A3-SBA-15 further supports
the notion that the number of acid sites did not affect the selectiv-
ity. It is more likely that the electronic character of the heteroatoms
controlled the selectivity. None of the M3+ elements (B, Al, Ga)
formed ␤-nitroalcohol, but all three M4+ elements (Ti, Zr, Ce) did,
though Ti-A3-SBA-15 formed ␤-nitroalcohol in a trace amount. Cat-
alysts containing Brønsted acid sites are known to be more selective
toward the dehydration of secondary alcohols while catalysts con-
taining Lewis acid sites are more selective toward dehydration
of primary alcohols [96,97]. The large number of Brønsted acid
sites for the Zr-A3-SBA-15 material seems to contradict this notion
because this catalyst formed the ␤-nitroalcohol, a secondary alco-
hol, with moderate selectivity (32%). However, the varying amounts
of Brønsted acid sites among the heteroatom materials used here
make it difficult to determine the cause of the alcohol dehydration.
At this stage, the exact reason for the formation of ␤-nitroalcohol of
the M4+ catalysts remains unclear, but future studies of M3+ and M4+
heteroatom materials containing different amounts of heteroatoms
could prove useful in elucidating the switch in selectivity.
4. Conclusions
A series of amine-functionalized silica bifunctional catalysts
with various heteroatom substitutions was prepared to probe the
effects of the substitutions on the cooperative catalytic behavior
of these modified materials in the aldol and nitroaldol conden-
sations. Ammonia TPD of the heteroatom materials demonstrated
the increased total number of acid sties of the heteroatom substi-
tuted materials in all cases. It was determined that the increased
number and strength of the Lewis acid sites of each heteroatom
material decreased the catalytic activity compared to the standard
aminosilica catalyst A3-SBA-15 for the aldol condensation of 4-
nitrobenzaldehyde with acetone, which is inhibited by stronger
acids. For the nitroaldol condensation of 4-nitrobenzaldehyde with
nitromethane, the increased number of Lewis acid sites of four
heteroatom materials (B-, Al-, Ga-, and Ti-A3-SBA-15) increased
the catalytic activity compared to heteroatom-free A3-SBA-15. The
catalytic activity in the nitroaldol condensation for Ti and Al con-
taining aminopentyl-functionalized materials was not enhanced
compared to the aminopropyl case, showing that rate enhance-
ments observed over heteroatom-free silica catalysts with longer
alkyl linkers and rate enhancements associated with moderate
Lewis acidity associated with these two heteroatoms were not
additive. These results further support the notion that the aldol
and nitroaldol reactions proceed through different mechanisms.
Acknowledgments
This work was supported by the Department of Energy
Office of Basic Energy Sciences through Catalysis Contract No.
DEFG02-03ER15459. We thank Dr. Johannes Leisen for conduct-
ing solid-state NMR experiments, Mr. Kiwon Eum for collecting
SEM images, Mr. Guo Shiou Foo for his help in performing pyri-
dine IR experiments, and Ms. Caroline Hoyt for her help with 1H
NMR experiments.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2014.10.061.
The combined results of the aldol and nitroaldol condensations
on the heteroatom materials provide strong evidence that each
reaction proceeds via significantly different pathways. Our previ-
ous work, in conjunction with homogeneous mechanisms for the
two reactions reported in the literature, suggests that the aldol
condensation proceeds with two important steps facilitated by a
hydrogen bonding or proton exchange interaction [98,99]. These
steps are sequential with one step being the initial dual activation
of the ketone (acetone) and the second step being a transition-
state stabilized acid–base interaction. The same hydroxyl can be
used for both stabilizations because the aforementioned acid–base
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