F. Zhu et al.
flammable, which could be considered as an alternative to
expensive organic solvents and an attractive media for the
development of environmentally benign chemical processes.
Furthermore, separation of water-insoluble organic com-
pounds fromthe aqueous phase is veryeasy[5]. Ingeneral, the
water-medium Knoevenagel condensation is carried out
homogenously using nitrogenous molecules such as aliphatic
amines, urea and piperidine or their corresponding ammo-
niumsaltsandaminoacids[6, 7]. Though the high activity and
selectivity, the use of such catalysts inevitably adds cost since
large amount of organocatalysts and even causes the pollution
on either the environment or the products [8]. Immobilized
organocatalysts could be easily recycled [9], but they usually
display lower efficiencies than the corresponding homoge-
neous analogues [10–13]. The development of novel solid
organocatalyst is a major research topic either in academy or
industry. Up to now, many amine immobilized catalysts have
been prepared to catalyze Knoevenagel condensation in water
[14, 15]. However, the irregular shape of most supports are
harmful for the catalytic performances due to the enhanced
diffusion limit.
reaction tool relies on the power of the microwaves to
reduce the times of reaction and the energy consumption
increasing the yields and selectivity of the investigated
reactions in many cases [33, 34]. Microwave assisted
methodologies also provide a more benign and environ-
mentally compatible approach to organic reactions.
In regards of these viewpoints, we report herein one-pot
synthesis of amine-functionalized mesoporous silica nano-
spheres (NH2-MSNS) and their catalytic performance in the
water-medium Knoevenagel condensation of aromatic alde-
hydes with ethyl cyanoacetate under microwave irradiation.
2 Experimental
2.1 Catalyst Preparation
The NH2-MSNS was synthesized by surfactant-directed co-
condensation. In a typical run of synthesis, 0.36 g hex-
adecyltrimethylammnonium bromide (CTAB) and 0.050 g
NaOH were dissolved in 190 mL water. Then, the desired
amounts of aminopropyltrimethoxysilane (APTMS) and tet-
raethyl silicate (TEOS) were added. The initial molar ratio in
the mother solution is Si:CTAB:NaOH:H2O = 1:0.23:0.12:
0.31:1.189, where Si refers to the total amount of TEOS and
APTMS. After being stirred for 2 h at 80 °C, the mixture was
kept static for another 20 h. The white precipitate was filtrated
and washed thoroughly with water, following by vacuum
drying at 80 °C overnight. The surfactant and other organic
residues were then removed by refluxing in ethanol for 24 h,
leading to NH2-MSNS. The nitrogen loading was adjusted by
changing the molar ratio of APTMS/TEOS in the initial
mixture, corresponding to NH2-MSNS-1, NH2-MSNS-2 and
NH2-MSNS-3. The nitrogen content in the materials was
determined by elemental analysis.
The mesoporous silicas with various organic functional
groups are of great interest because of their potential for new
applications in separation, adsorption, catalysis, sensor
design, drug delivery, and nanotechnology [16–19]. Among
these, amino group functionalized mesoporous materials are
well known for their use in base-catalyzed reactions, waste-
water treatment, and absorption of heavy metal ions, etc.
[20]. Incorporation of functional groups into mesoporous
materials can be commonly achieved by two strategies,
namely direct synthesis (co-condensation reaction) and post-
synthesis (grafting). Compared to grafting method, the direct
synthesis of mesoporous materials, organic silica is an
attractive alternative that produces functionalized materials
with high loadings and homogeneous surface coverages of
the specific functionality [21, 22].
For comparison, the amine-functionalized MCM-41 was
also synthesized according to the literature [35]. Briefly,
0.36 g CTAB was dissolved in 24 mL H2O containing 15 g
concentrated ammonia (28 wt%) at 40 °C, followed by
adding 9.0 mmol TEOS and 1.0 mmol APTMS imulta-
neously. After being stirred at 40 °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 NH2-MSNS.
The as-prepared NH2-functionalized silica with mesopor-
ous structure similar to the MCM-41 was denoted as NH2-
MCM-41. The nitrogen content in the materials was
determined by elemental analysis.
Amine group bonded to silica supports with ordered
mesoporous channels, such as MCM-41, SBA-15 and PMO
etc. prepared by co-condensation method [23–25] showed
significant improvement on the adsorption and diffusion of
reactant molecules, leading to the enhanced activity [26,
27]. However, their irregular shape and long pore channels
are detrimental for the catalytic performances due to the
enhanced diffusion limit. Recently, mesoporous silica
nanospheres (MSNS) with an average diameter around
100 nm have been reported [28]. Obviously, such silica
supports should be more favorable for the diffusion and
adsorption of reactant molecules owing to the short pore
channels and the regular nanospheres.
Microwaves have been the subject of extensive studies
in the last few years and a wide range of publications can
be found on various applications of microwave assisted
organic synthetic protocols [29–32]. This useful novel
2.2 Characterization
The N content in all samples was determined by elemental
analysis on a Heraeus CHNS elemental analyzer. Fourier
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