Amine-bridged periodic mesoporous organosilica nanospheres as an active and reusable solid base-catalyst for water-medium and solvent-free organic reactions

Article abstract of DOI:10.1016/j.molcata.2012.07.015

An amine-bridged periodic mesoporous organosilica catalyst in uniform nanospheres (～85 nm) (NH-PMO-NS) was synthesized by surfactant-directed co-condensation between amine-bridged silane and tetraethoxysilane (TEOS). As-prepared NH-PMO-NS contained short and straight mesoporous channels with amine-groups integrally incorporated into silica walls. In water-medium Knoevenagel condensation and solvent-free Henry reactions, this catalyst exhibited much higher activity and selectivity than both the common amine-bridged PMO (NH-PMO-com) with irregular shape synthesized by traditional method and the amine-functionalized MCM-41 in nanospheres (NH-MCM-NS) prepared by grafting amine-groups onto the pore surface. In addition, it displayed comparable catalytic efficiencies with the homogeneous base-catalyst (diethylamine), and could also be easily recovered and used repetitively. Based on the structural characterizations and kinetic studies, the high catalytic activity and selectivity were ascribed to the short and straight mesoporous channels in nanospheres with amine-groups incorporated into silica walls. The accessible basic catalytic sites may facilitate the diffusion and thus, promoted the adsorption of organic reactant molecules, leading to the enhanced activity and selectivity.

Full text of DOI:10.1016/j.molcata.2012.07.015

Journal of Molecular Catalysis A: Chemical 363–364 (2012) 387–397
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Amine-bridged periodic mesoporous organosilica nanospheres as an active and
reusable solid base-catalyst for water-medium and solvent-free organic reactions
Fengxia Zhua,b, Didi Yangb, Fang Zhangb, Hexing Lib,∗
a Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huaian 223300, China
b Department of Chemistry, Shanghai University of Electric Power, Shanghai 200234, China
a r t i c l e i n f o
a b s t r a c t
Article history:
An amine-bridged periodic mesoporous organosilica catalyst in uniform nanospheres (∼85 nm) (NH-
PMO-NS) was synthesized by surfactant-directed co-condensation between amine-bridged silane and
tetraethoxysilane (TEOS). As-prepared NH-PMO-NS contained short and straight mesoporous channels
with amine-groups integrally incorporated into silica walls. In water-medium Knoevenagel condensa-
tion and solvent-free Henry reactions, this catalyst exhibited much higher activity and selectivity than
both the common amine-bridged PMO (NH-PMO-com) with irregular shape synthesized by traditional
method and the amine-functionalized MCM-41 in nanospheres (NH-MCM-NS) prepared by grafting
amine-groups onto the pore surface. In addition, it displayed comparable catalytic efficiencies with the
homogeneous base-catalyst (diethylamine), and could also be easily recovered and used repetitively.
Based on the structural characterizations and kinetic studies, the high catalytic activity and selectivity
were ascribed to the short and straight mesoporous channels in nanospheres with amine-groups incorpo-
rated into silica walls. The accessible basic catalytic sites may facilitate the diffusion and thus, promoted
the adsorption of organic reactant molecules, leading to the enhanced activity and selectivity.
© 2012 Elsevier B.V. All rights reserved.
Received 3 February 2012
Received in revised form 6 July 2012
Accepted 16 July 2012
Available online 25 July 2012
Keywords:
Amine-bridged PMO nanospheres
Short and straight mesoporous channels
Solid base-catalyst
Water-medium and solvent-free organic
reactions
Green chemistry
1. Introduction
Ir, Rh, and Os) in pharmaceuticals [15]. In fact, the development
of the metal-free and environmental friendly organocatalysts was
Since the discovery of M41S [1,2], a great number of efforts have
been devoted to designing organic-inorganic hybrid silicas with
ordered mesoporous structure due to their wide application poten-
tials in versatile fields, including catalysis, adsorption, separation,
etc. [3–6]. Previously, we reported that the organometallic catalysts
terminally bonded to the silica supports with ordered mesoporous
structure, exhibited higher catalytic efficiencies than those tradi-
tional immobilized catalysts owing to the enhanced dispersion of
the active sites [7–9]. Recently, we developed a new approach to
integrally incorporate organometals into the silica walls, leading to
the organometal-bridged periodic mesoporous organosilica (PMO)
[10–12], which further improved the catalytic performance owing
to the diminished diffusion limit and also improved the durability
of catalyst by inhibiting the leaching of metallic active sites [13,14].
Although a great number of metal-based catalysts have been suc-
cessfully synthesized for organic synthesis, their applications are
now gradually limited by the environmental and health consider-
ations. A new pharmaceutical guideline in the United States has
dramatically lowered the limits of trace metal impurities (e.g., Ru,
in direct response to these new regulations [16]. Up to now, many
amine-based organocatalysts have been reported as alternatives of
traditional metal-based systems for a variety of C C coupling reac-
tions (e.g., Aldol condensation [17], Mannich reaction [18], Michael
addition [19], Knoevenagel condensation [20] and Henry reaction
[21]). Most studies are focused on homogeneous organocatalysts,
but they are difficult to be reused, which inevitably adds cost and
even causes the pollution on either the environment or the products
[22]. Immobilized organocatalysts could be easily recycled [23],
but they usually display lower efficiencies than the correspond-
ing homogeneous analogues [24–27]. Organocatalysts bonded to
traditional mesoporous silica supports, including MCM-41, SBA-15
and PMO [28–31] showed significant improvement on the catalytic
activity owing to the enhanced dispersion of active sites and the
reduced diffusion limit [12,32]. However, the irregular shape and
long mesoporous channels might disturb the uniform distribution
of active sites and also display steric hindrance for the diffusion of
reactant molecules, especially for those with large molecule size,
which must be harmful for the catalytic activity and selectivity.
Both theoretical and experimental studies predict that the short
channels favor the molecule diffusion. Hao et al. reported synthesis
of amino-functionalized mesoporous silica in uniform nanospheres
(MSN) with amine-groups terminally bonded onto the pore surface
∗
Corresponding author. Tel.: +86 21 64322272; fax: +86 21 64322272.
E-mail address: Hexing-Li@shnu.edu.cn (H. Li).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.07.015

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F. Zhu et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 387–397
Scheme 1. Illustration of the preparation of NH-PMO-NSby co-condensation (upper) and NH-MCM-NS by grafting method (bottom).
[33]. However, those pendant amine-groups might still block the
pore channels. Herein, we reported a novel amine-bridged peri-
odic mesoporous silica in uniform nanospheres with amine-groups
integrally incorporated into silica walls (NH-PMO-NS), which could
be used as an active and reusable solid base-catalyst for water-
medium Knoevenagel condensation and even solvent-free Henry
reactions. It exhibited comparable efficiencies with the diethy-
lamine homogeneous catalyst owing to both the diminished pore
blockage by amine-groups and the short pore length, which facili-
tated the diffusion and adsorption of reactant molecules.
washed completely with toluene and ethanol, followed by drying
for 10 h at 80 ◦C under vacuum. Meanwhile, the common amine-
bridged PMO (NH-PMO-com) was also prepared by traditional
surfactant-directed co-condensation method. Briefly, CTAB (0.36 g)
was dissolved in H2O (24 mL) containing concentrated ammonia
(28 wt%) (15 g) at 30 ◦C, followed by adding TEOS (9.2 mmol) and
BTEA (0.40 mmol) simultaneously. After being stirred at 30 ◦C for
0.5 h, the solution was transferred into an autoclave and kept at
100 ◦C for 15 h. The white precipitate was filtrated and treated
according to the procedure used in preparing NH-PMO-NS.
2.2. Characterization
2. Experimental
Surface morphologies and mesoporous structures were
observed through field emission scan electronic microscopy
(FESEM, Hitachi S-4800) and transmission electronic microscopy
(TEM, JEOL JEM2011). Small-angle X-ray powder diffraction (XRD)
patterns were obtained on a Rigaku D/maxr B diffractometer
with Cu Ka. N2 adsorption–desorption isotherms were mea-
sured at −196 ◦C using a Quantachrome NOVA 4000e analyzer
after the samples being degassed at 100 ◦C for more than 10 h,
based on which, the specific surface area (SBET), average pore
diameter (DP) and pore volume (VP) were calculated by apply-
ing multiple-point Brunauer–Emmett–Teller (BET) model and
Barrett–Joyner–Halenda (BJH) model on adsorption branches,
respectively. Fourier transform infrared (FT-IR) spectra were col-
lected on a Nicolet Magna 550 spectrometer using the KBr method.
Solid NMR spectra were recorded on a Bruker DRX-400 NMR
spectrometer. Thermal gravimetric analysis (TGA) was carried
out with a Perkin-Elmer Pyris Diamond TG analyzer under air
atmosphere with a heating ramp of 5 ◦C/min. The N loading was
determined by elemental analysis on a Heraeus CHNS elemental
analyzer. The surface electronic states were analyzed by X-ray pho-
toelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C ESCA). All
the binding energy values were calibrated by using C1S = 284.6 eV
as a reference.
2.1. Catalyst preparation
The amine-bridged PMO in nanospheres (NH-PMO-NS, where
NS refers to nanospheres) was synthesized by surfactant-directed
co-condensation as illustrated in Scheme 1 (upper). In a typ-
ical run of synthesis, hexadecyltrimethylammnonium bromide
(CTAB) (0.89 g) was dissolved in 428 mL aqueous solution con-
taining 6.2 mmol NaOH. Then, the desired amounts of tetraethyl
silicate (TEOS) and bis [3-(triethoxysilyl)propyl] amine (BTEA)
were added. The initial molar ratio in the mother solution is
Si:CTAB:NaOH:H2O = 1:0.23:0.12:0.31:1189, where Si refers to the
total amount of silica source. After being stirred for 2 h at 80 ◦C, the
gel solution was aged at 80 ◦C for 20 h. The white precipitate was
filtrated, washed thoroughly with ethanol and deionized water, fol-
lowed by drying under vacuum at 80 ◦C for 10 h. Subsequently, the
surfactant template was removed by refluxing the solid product
twice in 1000 mL ethanol at 80 ◦C for 24 h and then the product
was dried under vacuum at 80 ◦C overnight. The amine content was
adjusted by changing the molar ratio of BTEA to TEOS in the initial
solution, leading to NH-PMO-NS-n series samples.
For comparison, the amine-functionalized MCM-41 in
nanospheres (NH-MCM-NS) was also synthesized by grafting
method as illustrated in Scheme 1 (bottom). Firstly, 0.89 g CTAB
was added into 428 mL aqueous solution containing 6.2 mmol
NaOH. After all the reagents completely dissolved at 80 ◦C,
20 mmol TEOS was added dropwise into the solution, followed
by stirring for 2 h and aging at 80 ◦C for 20 h. The precipitate
was filtered and dried under vacuum at 80 ◦C for 10 h. Then, the
surfactant template was removed by refluxing the solid product
twice in 1000 mL ethanol at 80 ◦C for 24 h. Then, the solid product
was washed thoroughly with ethanol and water, followed by
drying at 80 ◦C under a vacuum overnight, leading to the MCM-41
nanospheres (MCM-NS). Finally, 0.50 g MCM-NS was dispersed in
5.0 mL toluene containing 1.0 mmol BTEA. After being stirred at
70 ◦C for 24 h, the solid product was recovered by filtration and
2.3. Activity test
The catalytic performance was evaluated by using Knoevenagel
condensation (Reaction 1) and Henry (Reaction 2) reactions (see
Schemes 2 and 3) as probes. Reaction 1 was carried out in a 10 mL
flask containing a catalyst with desired N content, aromatic alde-
hyde (0.50 mmol), ethyl cyanoacetate (0.60 mmol), H2O (3.0 mL),
and n-decane (100 ␮L) as an internal standard. After reaching
the maximum conversion, the reaction mixture was extracted
with ethyl acetate and followed by analysis on a gas chromato-
graph (GC, Agilent 1790) equipped with a FID and a JWDB-5 95%

F. Zhu et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 387–397
389
irregular shapes with much larger particle size. The HRTEM images
further demonstrated that all those samples comprised of straight
mesoporous channels with high ordering degree regardless of the
different preparation methods.
R
R'
R'
CN
RCHO
CN
As shown in Fig. 3a, all the as-prepared samples displayed
typical type IV N2 adsorption-desorption isotherms with H4-type
hysteresis loops indicative of the mesoporous structure. The small-
angle XRD patterns in Fig. 3b revealed that all these samples
displayed an intense diffraction around 2ꢀ = 2.2◦ of characteristic
(1 0 0) and two weak peaks at larger 2ꢀ corresponding to (1 1 0) and
(2 0 0) diffractions. These diffraction patterns represent the pres-
ence of the ordered 2D p6mm hexagonal mesoporous structures
[34,35]. With the increase in amine-content from NH-PMO-NS-1
to NH-PMO-NS-4, the principal XRD peak gradually weakened, and
the other two small peaks gradually disappeared, indicating the
decrease in ordering degree of the mesoporous structure since the
addition of a large amount of BTEA molecules might disturb the
surfactant self-assembly process [36].
The specific surface area (SBET), pore volume (VP)
and the pore diameter (DP) were calculated by applying
multiple-point Brunauer–Emmett–Teller (BET) model and
Barrett–Joyner–Halenda (BJH) model on absorption branches
of the N2 adsorption–desorption isotherms, respectively. As
shown in Table 1, the NH-MCM-NS exhibited much lower SBET, DP
and VP than either the NH-PMO-NS-2 or the NH-PMO-com with
the similar N-loadings since the amine-groups in the NH-MCM-NS
were terminally bonded onto the pore surface rather than incor-
porated into the pore walls (see Scheme 1), which might partially
block the pore channels. The NH-PMO-NS-2 showed equivalent DP
and VP to the NH-PMO-com owing to their similar periodic meso-
porous structures. The slightly higher SBET of the NH-PMO-NS-2
than that of the NH-PMO-com could be attributed to the uniform
nanospheres in comparison with the large and irregular particles.
With the increase of the N-loading from the NH-PMO-NS-1 to the
NH-PMO-NS-2, no significant change in either SBET or DP or VP was
observed, obviously owing to the incorporation of amine-groups
into the silica walls. Further increase in the N-loading caused an
abrupt decrease in SBET, DP and VP, evidently due to the damage of
the ordered mesoporous structure.
The FTIR spectra in Fig. 4 revealed that the NH-PMO-NS-2,
NH-PMO-com and NH-MCM-NS displayed no significant signal
at 2937 cm−1, indicating that the CTAB surfactant template had
been completely removed [37]. The strong peak around 1640 cm−1
was mainly from the bending vibration of adsorbed H2O [38].
The absorbance bands around 1080, 795 and 468 cm−1 could be
assigned to the vibrations from the Si O bond while the peak
around 953 cm−1 could be attributed to the vibration from the
Si OH groups [39]. In comparison with the pristine MCM-41, the
NH-PMO-NS-2, NH-PMO-com and NH-MCM-NS displayed an addi-
tional band around 692 cm−1 characteristic of the secondary amine,
suggesting that the amine-groups had been successfully anchored
to the silica support [40]. The N H vibration bands around 795 and
1640 cm−1 were overlapped by adsorbed and Si O vibration bands.
Fig. 5 shows the solid NMR spectra of NH-MCM-NS, NH-
PMO-NS-2 and NH-PMO-com. The 29Si CP MAS NMR spectra
revealed that all these samples displayed three resonance
peaks corresponding to Q4 (ı = −111 ppm), Q3 (ı = −102 ppm)
and Q2 (ı = −93 ppm), where Qn = Si(OSi)n(OH)4−n, n = 2–4, and
two other peaks corresponding to T3 (ı = −67 ppm) and T2
(ı = −60 ppm), where (Tm = RSi(OSi)m(OH)3−m, m = 1–3) [41,42]. The
presence of the Qn could be attributed to the self-condensations
between TEOS. The appearance of Tm peaks confirmed the
cross co-condensation between TEOS and BTEA, leading to the
incorporation of organic moieties into the silica network. Mean-
while, the 13C MAS NMR exhibited three prominent peaks
at ı of 6.6, 17, and 47 ppm indicative of three C atoms in
Scheme 2. Chemical equation of the Knoevenagel condensation reaction (Reaction
1).
NO2
CHO
CH3NO2
O2N
O2N
Scheme 3. Chemical equation of Henry reaction (Reaction 2).
dimethyl-1-(5%)-diphenylpolysiloxane column. The column tem-
perature was programmed from 80 to 250 ◦C at a ramp of 10 ◦C/min.
N2 was used as carrier gas. The conversion was calculated based on
an aldehyde since the other reactant was excess. Reaction 2 was
performed in the similar way by stirring a mixture containing a
catalyst with desired N content, p-nitrobenzaldehyde (0.50 mmol),
nitromethane (5.0 mL) and n-decane (100 ␮L) as an internal stan-
dard. After reaction for 24 h under refluxing condition at 90 ◦C, the
solid was removed from the solution using a centrifuge, and the
separated liquid was analyzed by GC. The conversion was calculated
based on p-nitrobenzaldehyde since nitromethane was excess. The
reproducibility was checked by repeating each result at least three
times and was found to be within acceptable limits ( 5%).
To determine the catalyst durability, the catalyst was allowed
to centrifuge after each run of reactions, and the clear super-
natant liquid was decanted slowly. The catalysts were washed
thoroughly with distilled water and ethanol, followed by drying at
80 ◦C overnight under vacuum condition. Then, the catalysts were
reused with fresh charge of reactants for subsequent recycle under
the identical reaction conditions.
2.4. Adsorption test
To examine the adsorption behavior, catalyst (50.0 mg) was
soaked in water (200 mL) and oscillated at 25 ◦C for 12 h, fol-
lowed by adding aqueous solution (50.0 mL) containing 2.50 mg
rhodamine B (RhB). The solution was sampled at given time inter-
vals and the concentration of the left RhB in the solution was
determined on an UV-visible spectrophotometer (UV 7504/PC) at
the characteristic adsorption wavelength of 523 nm.
3. Results and discussion
3.1. Structural characteristics
The N-loadings in different catalysts were determined by ele-
mental analysis and summarized in Table 1. As shown in Fig. 1,
the FESEM and HRTEM images revealed that both the NH-PMO-
NS-2 and the NH-MCM-NS displayed uniform nanospheres with
the average diameter around 85 nm (see the particle size distri-
bution diagrams in Fig. 2). However, the NH-PMO-com showed
Table 1
Some structure parameters of different samples.a
Catalysts
N loading (wt%)
SBET (m2/g)
DP (nm)
VP (cm3/g)
NH-PMO-NS-1
NH-PMO-NS-2
NH-PMO-NS-3
NH-PMO-NS-4
NH-PMO-com
NH-MCM-NS
0.33
0.83
1.1
1.6
0.81
0.89
980
917
776
517
845
728
2.7
2.6
2.4
2.1
2.7
2.1
0.86
0.97
0.69
0.51
0.87
0.70
a
DP and VP were calculated by applying BJH model on the adsorption branches.

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F. Zhu et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 387–397
Fig. 1. FESEM (left) and HRTEM (right) images of NH-PMO-NS-2 (a and b), NH-MCM-NS (c and d) and NH-PMO-com (e and f).
the Si CH2 CH2 CH2 NH CH2 CH2 CH2 Si group [38]. These
results demonstrated that the amine-groups had integrally grafted
into the silica network. The incorporation of amine-groups could
also be confirmed by TG/DTA analysis.
and NH-PMO-com samples, which further confirmed the complete
removal of CTAB surfactant template [30]. The other the weight loss
peak found between 300 ◦C and 700 ◦C could be attributed to the
combustion of amine-groups incorporated into the silica network
[40]. The NH-PMO-NS-2 and the NH-MCM-NS exhibited similar
weight loss (∼10 wt%) at 130 ◦C, which was much higher than that
observed in the NH-PMO-com (∼5 wt%), suggesting they adsorbed
much more water or other solvents than the NH-PMO-com,
As shown in Fig. 6, an exothermic peak around 120 ◦C could
be attributed to desorption of adsorbed water or other solvents.
No significant exothermic peak and weight loss were found in the
range from 130 to 300 ◦C for all the NH-PMO-NS-2, NH-MCM-NS
Fig. 2. Particle size distribution diagrams.

F. Zhu et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 387–397
391
Fig. 3. N2 adsorption–desorption isotherms (a) and low-angle XRD patterns of different samples (b).
possibly owing to higher surface energy of the small nanospheres
than that of the large irregular particles. All three samples exhib-
ited a similar weight loss (∼9 wt%) between 300 ◦C and 700 ◦C since
they contained almost the same amount of amine-groups.
The XPS spectra (Fig. 7) revealed that all the NH-PMO-NS-
2, NH-MCM-NS and NH-PMO-com samples contained secondary
amine groups, corresponding to the binding energy around
399.4 eV in N1S level [43]. In comparison with the BTEA
[(CH3CH2O)3 Si (CH2)3 NH (CH2)3 Si (CH3CH2O)3] and the
diethylamine (CH3CH2NHCH2CH3), no significant shift of the bind-
ing energy was observed, suggesting that the amine-bridged silanes
were directly integrated into the silica framework without chang-
ing the chemical microenvironment of the nitrogen atom.
A key consideration was the location of the amine-groups incor-
porated into the silica walls of the NH-PMO-NS catalyst. That is to
say, were they chemically accessible to the reactant molecules dur-
ing catalytic reactions? Firstly, the CO2 temperature programmed
desorption (CO2-TPD) was conducted. As shown in Fig. 8, the NH-
PMO-NS-2 organocatalyst before pre-adsorption of CO2 displayed
only two signals around 300 and 525 ◦C, respectively. This could
be attributed to the combustion of the organic residues due to
the incomplete hydrolysis of silicon sources and the organic frag-
ments as a part of silica walls in the PMO, leading to the formation
of CO2. The NH-PMO-NS-2 organocatalyst after pre-adsorption of
CO2 exhibited an additional signal around 100 ◦C indicative of CO2
desorption from the Bronsted base active sites [33], which demon-
strated that the amine-groups incorporated into the silica walls
were chemically accessible.
Based on the XPS spectra and the elemental analysis, the molar
ratio of the surface amine species to the total amine species (N%)
could be estimated. It is well known that the XPS spectra could esti-
mate the content of surface amine by measuring the peak intensity
indicative of nitrogen (I) while the elemental analysis could give
the total content of amine (NTotal). Taking into account that the
NH-MCM-NS displayed 100% surface amine species since all those
amine-groups were terminally bonded onto the pore surface rather
than incorporated into silica walls, the percentage of exposed
amine-groups in the NH-PMO-NS-2 was evaluated as 95% by using
the following equation, which further confirmed that nearly all the
amine-groups in the NH-PMO-NS organocatalyst were chemically
accessible for catalytic reactions.
INH-PMO-NS-2 × NTotal
NH-MCM-NS
INH-MCM-NS × NTotal
N% =
NH-PMO-NS-2
3.2. Catalytic performances
The water-mediated organic reactions should be carefully
conducted since they contained three phases. Firstly, the reaction
conditions were optimized by testing the effects of various factors
(temperature, time, catalyst and water amount) on the reaction
conversion and selectivity. As shown in Table 2, the Knoevenagel
condensation displayed nearly 100% selectivity toward the target
product under different conditions. The benzaldehyde conversion
increased gradually with the reaction time. However, it could
not reach 100% benzaldehyde conversion even after reaction for
a very long period, possibly due to the poor solubility of organic
Table 2
Effect of reaction conditions on Knoevenagel condensation efficiency.a
Temperature
(◦C)
Catalyst
(mmol)
Water
(mL)
Time
(h)
Conv.
(%)
Select.
(%)
40
40
40
30
50
40
40
40
40
0.030
0.045
0.055
0.045
0.045
0.045
0.045
0.045
0.045
3.0
3.0
3.0
3.0
3.0
2.0
4.0
3.0
3.0
4
4
4
4
4
4
4
3
5
79
93
98
67
98
78
89
73
95
∼100
∼100
∼100
∼100
∼100
∼100
∼100
∼100
∼100
a
Fig. 4. FTIR spectra of MCM-NS (a), NH-PMO-NS-2 (b), NH-PMO-com (c) and NH-
NH-PMO-NS-2 catalyst, 0.50 mmol benzaldehyde, 0.60 mmol ethyl cyanoac-
MCM-NS (d).
etate.

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F. Zhu et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 387–397
Fig. 5. 29Si MAS NMR (left) and 13C MAS NMR (right) spectra of NH-PMO-NS-2 (a and b), NH-PMO-com (c and d) and NH-MCM-NS (e and f).
reactants in water. With catalyst amount increasing from 0.030
to 0.045 mmol, the benzaldehyde conversion increased from 79%
to 93%, obviously owing to increased basic sites. Further increase
in the catalyst amount displayed no significant promoting effect
on the conversion, possibly due to the poor dispersion of the
catalyst. Meanwhile, it was also found that the benzaldehyde
conversion first increased and then decreased with the increase of
water amount, and the optimum water amount was determined
as 3.0 mL. The protic water might activate benzaldehyde with
weak acidity, which favored the base-catalysis [44]. However, the
very large amount of water was harmful for the reaction since
the amine active sites in the mesoporous channels were covered
by water molecules [45]. Furthermore, the reaction temperature
also exhibited strong influence on the catalytic efficiency, and the
reason would be discussed below.
Table 3 summarized the catalytic efficiencies of different
catalysts in the water-medium Knoevenagel condensation and
solvent-free Henry reactions under the optimized conditions. Con-
sidering the NH-PMO-NS series catalysts prepared by the same
co-condensation method, one could see that, with the increase
of the N-loading, the activity first increased from NH-PMO-NS-1
to NH-PMO-NS-2 and then decreased gradually from NH-PMO-
NS-2 to NH-PMO-NS-4. Meanwhile, the selectivity in the Henry
reactions first increased from the NH-PMO-NS-1 to the NH-PMO-
NS-2 and then remained nearly constant from the NH-PMO-NS-2
to the NH-PMO-NS-4. Taking into account that both Knoevenagel

F. Zhu et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 387–397
393
Scheme 4. A plausible catalytic cycle in Knoevenagel reaction.
Scheme 5. A plausible catalytic cycle in Henry reaction.
Fig. 6. TG/DTA curves of different samples.
Fig. 7. XPS spectra of different samples.

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F. Zhu et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 387–397
Fig. 8. CO2–TPD curves of the NH-PMO-NS-2 organocatalyst before (a) and after (b)
pre-adsorption of CO2.
condensation and Henry reactions involved the cross-coupling of
two reactant molecules adsorbed on two neighboring amine active
sites (see Schemes 4 and 5), the NH-PMO-NS-1 with very low
N-loading displayed low activity and selectivity could be mainly
attributed to the long distance between two neighboring amine
active sites. The NH-PMO-NS-3 and NH-PMO-NS-4 exhibited lower
activity than the NH-PMO-NS-2 due to the partial damage of
ordered mesoporous structure, leading to the low SBET and narrow
DP, which disfavored the diffusion and adsorption of the reactant
molecules. Although the NH-PMO-NS-3 and NH-PMO-NS-4 exhib-
ited lower activities, they showed almost the same selectivity as
the NH-PMO-NS-2 owing to the enough short distance between
two neighboring owing to the enough short distance between two
neighboring amine active sites.
Fig. 9. Effect of the reaction temperature on the benzaldehyde conversion within 1 h
(a) and dependence of benzaldehyde conversion on the reaction time (b). The inset is
the change of ln(Cbenzaldehyde) with the reaction time. Reaction conditions: a catalyst
containing 0.045 mmol N, 0.50 mmol benzaldehyde, 0.60 mmol ethyl cyanoacetate,
3.0 mL H2O, reaction temperature = 40 ◦C.
It was also found that, with the similar N-loading, the NH-
PMO-NS-2 exhibited much higher activity and better selectivity
than either the NH-PMO-com or the NH-MCM-NS in both Kno-
evenagel and Henry condensation reactions. The higher activity
and selectivity of the NH-PMO-NS-2 could be explored, which
was based on the kinetic studies. Firstly, the effect of the reac-
tion temperature (T) on the catalytic activity was examined in the
Knoevenagel condensation reaction. As shown in Fig. 9a, the liner
relationship between catalytic activity and 1/T was obtained for
influence of the benzaldehyde concentration (Cbenzaldehyde) on the
activity of the NH-PMO-NS-2 was investigated, which showed lin-
ear relationship between ln(Cbenzaldehyde) and the reaction time
(see Fig. 9b). The remarkable deviation from the linear relationship
could be attributed to the extremely low benzaldehyde concentra-
tion. Taking into account that the ethyl cyanoacetate was excess in
the present reaction, we could reasonably conclude that the Kno-
evenagel condensation reaction was first order with respect to the
benzaldehyde concentration (r = kCb1enzaldehyde). This meant that
the different catalysts. By using Arrenius Equation (dC/dt = ke−E /RT),
a
the apparent activation energy (Ea) could be calculated from the
line slope. All the NH-PMO-NS-2, NH-PMO-com, NH-MCM-NS and
diethylamine showed equivalent Ea values, indicating the simi-
lar chemical microenvironments of the active sites, which was
consistent with the aforementioned XPS analysis. Meanwhile, the
the adsorption of the benzaldehyde molecules on the catalyst was
not saturated and thus, the adsorption rate played a key role in
determining the catalytic activity.
Fig. 10 showed the adsorption behaviors of different cata-
lysts measured by using RhB as a model adsorbent, which could
be easily determined by UV spectrometer analysis. Besides the
higher adsorption capacity owing to the larger SBET, the NH-
PMO-NS-2 also displayed much more rapid adsorption than the
NH-PMO-com, which confirmed that the short straight meso-
porous channels efficiently diminished the diffusion limit and thus
facilitated the adsorption. The NH-MCM-NS displayed the lowest
adsorption capacity and the slowest adsorption rate due to the
low SBET and especially, the pore blockage by the amine-groups
terminally bonded onto the pore surface rather than incorpo-
rated into the silica walls. Obviously, rapid adsorption promoted
the reaction, leading to the higher activity of the NH-PMO-NS-2
than that of either the NH-PMO-com or the NH-MCM-NS, which
could also account for the higher activity of the NH-PMO-com
than that of the NH-MCM-NS. Meanwhile, the higher selectiv-
ity of the NH-PMO-NS-2 than that of either the NH-PMO-com
Table 3
Catalytic efficiencies in Knoevenagel condensation and Henry reaction.
Catalysts
Knoevenagelcondensationa
Henry reactionb
Conv.
(%)
Select.
(%)
Conv.
(%)
Select.
(%)
NH-PMO-NS-1
NH-PMO-NS-2
NH-PMO-NS-3
NH-PMO-NS-4
NH-PMO-com
NH-MCM-NS
Diethylamine
89
93
88
78
83
85
98
∼100
∼100
∼100
∼100
∼100
∼100
∼100
84
91
88
80
86
83
96
80
87
87
88
79
82
84
a
Reaction conditions: a catalyst containing 0.045 mmol N, 0.50 mmol benzalde-
hyde, 0.60 mmol ethyl cyanoacetate, 3.0 mL H2O, 40 ◦C, 4 h.
b
Reaction conditions:
a catalyst containing 0.047 mmol N, 0.50 mmol p-
nitrobenzaldehyde, 5.0 mL nitromethane, 90 ◦C, 24 h.

F. Zhu et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 387–397
395
Fig. 11. Recycling test of the NH-PMO-NS-2 catalyst in Knoevenagel condensation
reaction. Reaction conditions are given in Table 2.
Fig. 10. Adsorption test of different catalysts. Adsorption conditions: 50.0 mg
catalyst, 250 mL aqueous solution containing 2.50 mg RhB, adsorption tempera-
ture = 25 ◦C.
adsorption saturation. The rapid diffusion of p-nitrobenzaldehyde
molecules inside the mesoporous channels could promote their
adsorption on N active site of the amine-catalyst, leading to the
cross-coupling reaction between the nitromethane anion and the
adsorbed p-nitrobenzaldehyde to form target product. Otherwise,
the fraction of the reaction occurring at the mesopore entrance
between the free p-nitrobenzaldehyde in the solution and the
adsorbed nitromethane anion would increase, corresponding to the
decrease in selectivity.
Table 4 shows that the NH-PMO-NS-2 exhibited similar reac-
tion conversions to the homogeneous diethylamine catalyst in
other Knoevenagel condensation reactions with various substrates.
To make more precise comparison, the turnover frequency (TOF)
at an early stage of the reaction was also calculated based on
the following equation, taking into account that the Knoevenagel
or the NH-MCM-NS in Henry reaction could also be explained
based on the adsorption rate. Scheme 3 illustrated a plausible
mechanism of Henry reaction. The target product 1-nitro-4-(2-
nitrovinyl)benzene was formed through the reaction between
p-nitrobenzaldehyde and nitromethane anion both adsorbed on
the N active sites of the amine-catalyst, which mainly occurred
inside mesoporous channels. While, the side product 2-nitro-1-
(4-nitrophenyl) ethanol was produced by direct attract of the
carbonyl group in p-nitrobenzaldehyde present freely in the solu-
tion with the nitromethane anion adsorbed on the amine-catalyst.
So, such reactions mainly occurred at the entrance of meso-
porous channels. Since the nitromethane was excess and rapid
diffused into the mesoporous channels owing to the smaller molec-
ular size than the p-nitrobenzaldehyde, it could easily reach the
Table 4
The water-medium Knoevenagel condensation with different substrates.a
b
Substrate 1
Substrate 2
Product
NC
Time (h)
4
Conv. (%)
93 (98)
TOF (h−1
)
CN
CO2Et
NC
CHO
4.7 (4.3)
18.4 (17.6)
4.5 (4.1)
CO2Et
H
CN
CO2Et
CO2Et
O2N
CHO
CHO
1
4
99 (98)
90 (84)
O2N
CO2Et
H
NC
CN
H3CO
H3CO
CO2Et
H
NC
H
CN
CO2Et
1
4
1
99 (99)
94 (96)
99 (94)
18.7 (18.0)
4.9 (4.9)
CHO
O
CO2Et
O
CN
CN
CHO
CO2Et
CO2Et
NC
CN
CN
CN
CHO
18.3 (17.7)
H
a
Reaction conditions: the catalyst NH-PMO-NS-2 containing 0.045 mmol of N, 0.50 mmol Substrate 1, 0.60 mmol Substrate 2, 3.0 mL H2O, reaction temperature = 40 ◦C.
The values in parentheses are obtained by using 0.045 mmol diethylamine as a homogeneous catalyst under the same reaction conditions.
b
TOF values were determined based on conversion of Substrate 1 after reaction for 20 min.

396
F. Zhu et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 387–397
Fig. 12. N2 sorption isotherm (a), low-angle XRD pattern (b) and HRTEM image (c) of the NH-PMO-NS-2 organocatalyst after being used repetitively for 5 times in Knoevenagel
condensation reaction.
condensation reactions exhibited almost 100% selectivity to the
target products.
more than five times during Knoevenagel condensation reaction
without a significant decrease in activity. According to the ele-
mental analysis, no significant decrease of the N-loading in the
used NH-PMO-NS-2 catalyst was detected, indicating that the
leaching of amine active sites could be neglected. Obviously, the
incorporation of amine-groups into the silica walls could effec-
tively stabilize amine-groups against leaching. Meanwhile, the
N2 adsorption–desorption isotherm, low-angle XRD pattern and
HRTEM image in Fig. 12 clearly demonstrated that the NH-PMO-
NS-2 catalyst still displayed uniform nanospheres and the straight
and ordered mesoporous channels were remained after being used
repetitively for five times, showing the excellent hydrothermal sta-
bility.
Conversion (%) × moles of Substrate
(1/3 h) × moles of amine × 95%
1
TOF (h−1) =
where all the conversions were calculated based on the consump-
tion of Substrate 1 after reaction for 20 min, i.e., 1/3 h. The 95%
was used because, according to the aforementioned XPS analy-
sis, only 95% amine groups in the NH-PMO-NS-2 catalyst were
chemically accessible. Obviously, the TOF values calculated from
the NH-PMO-NS-2 catalyst were similar to those from the homo-
geneous diethylamine catalyst. These results clearly demonstrated
that the NH-PMO-NS-2 catalyst displayed comparable activity with
the homogeneous diethylamine catalyst.
To ascertain whether the amine anchored to the silica support
or homogeneous amine leached from the support was the real
organocatalyst, the following experiments were carried out accord-
ing to the procedure proposed by Sheldon et al. [46] After reaction
for 2 h with the conversion exceeding 60% in the Knoevenagel con-
densation reaction, the solid NH-PMO-NS-2 catalyst was filtered
out, and the liquid solution was allowed to react for another 3 h
under identical reaction conditions. No significant change in either
the benzaldehyde conversion or the yield of product was observed,
indicating that the active phase was not the amine species leached
from the NH-PMO-NS-2. Therefore, it is reasonable to conclude that
the present catalyst was really heterogeneous in nature.
4. Conclusions
This work developed a facile approach of surfactant-directed
co-condensation for preparing
a novel amine-bridged peri-
odic mesoporous organosilica in uniform nanospheres with
amine-groups integrally incorporated into the silica walls. This
heterogenized organocatalyst exhibited comparable activities and
selectivities in various water-medium Knoevenagel condensation
reactions and even solvent-free Henry reactions due to the reser-
vation of chemical micro-environment of active sites and the
diminished diffusion limit. More importantly, it could be eas-
ily recycled and used repetitively. These studies demonstrate
the significant potential of its practical applications in organic
synthesis.
Besides the high activity and selectivity, the NH-PMO-NS-
2 could be easily recycled and used repetitively. As shown in
Fig. 11, the NH-PMO-NS-2 catalyst could be used repetitively for

F. Zhu et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 387–397
397
Acknowledgments
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of China (20825724) and Shanghai Local Government (S30406,
10dj1400100, 08GG13), Education Ministry Key Lab of Resource
Chemistry, and Shanghai Key Laboratory of Rare Earth Functional
Materials.
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