Urea-functionalized mesoporous polymeric catalyst: A cooperative effect between support and secondary amine on water-medium Knoevenagel reactions

Article abstract of DOI:10.1039/c1nj20030e

Urea-functionalized mesoporous polymers (urea-MPs) were synthesized through the surfactant-directed urea-phenol-formaldehyde oligomers self-assembly approach. The as-prepared urea-MPs material exhibited superior catalytic activity than parent urea in water-medium Knoevenagel condensation reactions and could be used repetitively for seven times. The excellent activity could be attributed to the synergic effect derived from the secondary amine with the surface phenolic groups in the mesoporous support, which generated the acid-base cooperative catalytic behavior. Meanwhile, the urea functional groups embedded in the mesopore wall could inhibit the leaching of active species and thus resulted in the relatively good durability.

Full text of DOI:10.1039/c1nj20030e

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PAPER
Urea-functionalized mesoporous polymeric catalyst: a cooperative effect
between support and secondary amine on water-medium
Knoevenagel reactions
Ruixing Zhu, Jian Shen, Yongyi Wei and Fang Zhang*
Received (in Gainesville, FL, USA) 17th January 2011, Accepted 25th May 2011
DOI: 10.1039/c1nj20030e
Urea-functionalized mesoporous polymers (urea-MPs) were synthesized through the
surfactant-directed urea-phenol-formaldehyde oligomers self-assembly approach.
The as-prepared urea-MPs material exhibited superior catalytic activity than parent
urea in water-medium Knoevenagel condensation reactions and could be used repetitively
for seven times. The excellent activity could be attributed to the synergic effect derived
from the secondary amine with the surface phenolic groups in the mesoporous support,
which generated the acid–base cooperative catalytic behavior. Meanwhile, the urea
functional groups embedded in the mesopore wall could inhibit the leaching of
active species and thus resulted in the relatively good durability.
through surface chemical reaction.11a For example, Wu et al.
Introduction
prepared sulfonic acid modified MP through the SO3/H2SO4
Due to increased environmental and safety concerns, the demand
vapor sulfonation approach, which showed superior activity
for new green science and technologies when conducting
to the commercial acidic resins in liquid-phase Beckmann
chemical transformations has grown.1 General solutions to
rearrangement reaction.11b The same group also reported
these problems have become increasingly important in order
the synthesis of amino-functionalized MPs by the two-step
to develop economical and sustainable synthetic methodologies
chloromethylation/amination method. Such material displayed
that contribute to sustaining our limited resources.2 One
high catalytic performance in the Knoevenagel condensation
reactions.12 Moreover, the direct-synthesis approach was also
effective way to surmount these challenges is to discover and
use new highly active heterogeneous catalysts because they
investigated by several different groups, where the function-
generally allow for less waste, fewer toxic reagents, and easier
alized organic oligomers were directly assemble to the meso-
recycling of the catalyst.3 More interestingly, recent studies
structured polymers by the use of different templates.13
indicated that water could be used as green solvent for hetero-
Compared with post-synthesis technique, this method would
geneous catalytic systems, resulting in the exceptional advantage
likely obtain MPs with more uniform distribution and higher
of low cost and safety.4 However, unlike the enhancement of
loading of functional groups.14 Makowski et al. synthesized
catalytic performances induced by the supporting protein matrix
mesoporous polybenzimidazole (mp-PBI) by the polycondensation
in biological systems, a decreased activity was often observed
of an aromatic tricarboxylic acid ester with diaminobenzidine
while the efficient homogeneous catalysts were immobilized.5
in the presence of silica nanoparticles as template. As expected,
Therefore, novel approaches for preparing powerful hetero-
mp-PBI could be effectively employed as a basic catalyst
geneous catalysts in water-medium organic reactions are urgently
for the condensation of various aldehydes with malonic
derivatives.15 Antonietti’s group invented mesoporous C3N4
needed.
Recently, mesoporous polymers (MPs) with regular structural
materials through zinc chloride catalyzed polytrimerization of
characteristics have attracted a great deal of attention because
dicyanobenzene, which demonstrated the excellent activity in
of their high surface area and tunable pore size, which make
the various chemical transformations such as photocatalysis
or the oxidation of benzene to phenol.16 However, the ordered
it suitable as a solid support for various organic and/or inorganic
catalytic moieties.6–10 In general, post-synthesis modification
mesoporous structure of the above mesoporous polymer-
technique is the most common method to functionalize the
based catalysts through the direct-synthesis approach was
mesoporous polymer by attaching active sites to the pore surface
usually hard to obtain and furthermore usually acquired the
decreased reactivity in comparison with their homogeneous
counterparts.
Department of Chemistry, Shanghai Normal University,
Shanghai 200234, China. E-mail: zhangfang@shnu.edu.cn;
Fax: +86 21-64322272; Tel: +86 21-64322642
In the present work we demonstrated the preparation of the
urea-functionalized mesoporous polymers (urea-MPs) from
c
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urea-phenol-formaldehyde oligomer via one-step surfactant-
directed self-assembly (EISA) approach. The obtained urea-MPs
materials possessed the highly ordered mesostructures, large
surface area, uniform pore size and large pore volume. In the
water-medium Knoevenagel reactions, the urea-MPs catalyst
exhibited superior catalytic activity than parent urea in homo-
geneous solution. This catalytic activity enhancement could
be attributed to the cooperative effect derived the unique
spatial orientation of intraframework urea active sites and
its neighboring phenolic –OH groups. Meanwhile, this hetero-
geneous catalyst can be reused for at least six times with slight
loss of catalytic activity.
25 ml ethanol solution composing of 1.2 g urea-RO and stirred
for 2 h to form a clear solution. The mixture was transferred
into dishes and exposed in air at 30 1C for 8 h to evaporate
ethanol and further thermo-polymerized at 120 1C for 24 h.
The as-made materials were calcined in a tubular furnace at
350 1C for 4 h in argon atmosphere to remove F127 template.
The final products were denoted as urea-MPs-1, urea-MPs-2
and urea-MPs-3, which involved with three corresponding
urea-RO precursor as functional building blocks. The nitrogen
content were determined by elemental analysis.
Characterization
Small-angle X-ray diffraction (XRD) patterns were conducted
on Rigaku D/maxr B diffractometer with Cu-Ka radiation. N2
sorption isotherms were measured at 77 K on Quantachrome
NOVA 4000e analyzer. Transmission electron microscopy
(TEM) experiments were performed on JEM2011 microscope
operated at 200 kV. Fourier transform infrared (FT-IR)
and solid nuclear magnetic resonance (NMR) spectra were
recorded on Nicolet Magna 550 IR spectrometer and Bruker
DRX-400 NMR spectrometer, respectively. The surface electronic
states were analyzed by X-ray photoelectron spectroscopy
(XPS, Perkin-Elmer PHI 5000C ESCA). All of the binding
energy values are calibrated by using C1s = 284.6 eV as a
reference. The nitrogen content was determined by inductively
coupled plasma optical emission spectrometer (ICP-OES,
Varian VISTA-MPX).
Experimental
Chemical
The solvents were of analytical quality and dried by standard
methods. Other materials were analytical grade and used as
purchased without further purification. Triblock copolymer
F127 was purchased from Sigma-Aldrich company. Other
chemicals were obtained from Shanghai Chemical Corp.
Synthesis of urea-derived resin oligomers (urea-ROs)
The urea-derived resin precursor was obtained through base-
catalyzed polymerization with urea, phenol and formaldehyde.17
In a typical synthesis, 1.6 g urea and 21 g 37 wt% formaldehyde
aqueous solution were mixed at 40 1C and then allowed the
mixture to reflux at 70 1C for 0.5 h. Following that, the
solution pH value was adjusted to 10.0 by adding certain
amount 10 wt% NaOH. Then, 9.8 g phenol was introduced in
the above solution and stirred for another 2.0 h. Finally, the
solution pH value was adjusted to 7.0 by using 1.2 M HCl and
then removed water by rotary evaporator to give urea-phenol-
formaldehyde resin oligomers. The final oligomers could be
defined as urea-ROs-1, urea-ROs-2 and urea-ROs-3, corres-
ponding to the molar ratio of urea/(urea+phenol) 10%, 20%,
and 40% in the initial mixture, respectively.
Activity test
Water-medium Knoevenagel condensation reaction was used
to evaluate the catalytic performances of the urea-MPs samples.
In each run of reaction, 0.20 mmol ethyl cyanoacetate,
0.24 mmol benzaldehyde, 0.15 mmol n-decane as internal
standard, 5.0 ml distilled water and a catalyst containing
0.05 mmol nitrogen amount were mixed and allowed to
stir at 50 1C for 4 h. After extracted with ethyl acetate, the
products were analyzed on a gas chromatograph (GC, Agilent
1790) equipped with a JWDB-5, 95% dimethyl 1-(5%)-diphenyl-
polysiloxane column and a FID detector. The reproducibility
was checked by repeating each result at least three times and
was found to be within Æ5%.
Synthesis of urea-functionalized mesoporous polymers
The urea-MPs were prepared by the surfactant-directed
evaporation induced self-assembly (EISA) approach using
urea-ROs precursor and block copolymer F127 (Scheme 1).
In a typical synthetic procedure, 1.0 g F127 was added into
In order to determine the catalyst durability, the catalyst
was centrifuged after each run of the reaction and the clear
supernatant liquid was decanted slowly. The residual solid
catalyst was reused with fresh charge of water and reactant for
subsequent recycle runs under the same reaction conditions.
Results and discussion
Elemental analysis revealed that the nitrogen content in the
urea-MPs samples was in the range from 0.68 wt% to 2.1 wt%,
indicating the existence of the organic urea species. The
chemical composition of the representative urea-MPs-2 was
further investigated by solid NMR spectroscopy. As shown
in Fig. 1, the 13C CP MAS NMR spectrum displayed two
peaks around 150 ppm and 127 ppm, indicative of the –OH
substituted carbon and non-substituted carbon of the phenyl
ring, respectively.8 More importantly, the two shoulder peaks
at 48 ppm and 38 ppm in the amplified spectrum (inset) could
Scheme 1 Illustration of the preparation of the urea-MPs.
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Fig. 3 XRD patterns (a) and N2 sorption isotherms (b) of urea-MPs
samples with different urea contents.
Fig. 1 13C CP MAS NMR spectrum of urea-MPs-2 sample.
be assigned to the methylene carbon in the urea units
(–NH–CH2–NH–) and the methylene carbon between urea
and phenolic rings (–Ph–CH2–NH–), demonstrating successful
incorporation of the urea functional group as secondary amine
in the mesoporous polymeric framework.18 Meanwhile, the
XPS analysis (Fig. 2) demonstrated that nitrogen species in the
urea-MPs-2 was present in negative trivalence state, corres-
ponding to the binding energy of 399.8 eV in N1s level.19 As
expected, the binding energy value of the urea-MPs-2 was in
excellent with that of the parent urea, suggesting the chemical
microenvironment of urea could be well retained after the
co-polymerization and assembly process.
The degree of structural order of urea-MPs was investigated
by XRD measurement, as shown in Fig. 3a. All three samples
displayed an intense (100) diffraction peak around and two
(110) and (200) peaks in the range of 1–31, indicating the
ordered 2D p6mm hexagonal mesoporous structure.20 The
slight decrease of the peak intensity with the increase of urea
content could be observed, which revealed that more urea
units in the precursor maybe disturbed the effective assembly
between the surfactant and the hydrophilic phenolic –OH
groups. The TEM morphologies further confirmed urea-MPs
samples exhibited a two-dimensional hexagonal arrangement
of one-dimensional channels with uniform size (Fig. 4).6 Even
after assembling with 40% urea containing resin oligomer,
Fig. 4 TEM images of urea-MPs samples with different urea contents.
the mesoporous structure of urea-MPs-3 retained long-range
ordering, which demonstrated this approach could effectively
assemble different organic building blocks to form highly
ordered mesostructure.
The pore structure of these samples was further charac-
terized using nitrogen sorption experiments. All the urea-MPs
samples exhibited type IV isotherms with H1 hysteresis loop
characteristic of mesoporous structure with the narrow range
pore size distribution (Fig. 3b).13 In the region of low relative
pressure, the adsorption and desorption branches could not
Fig. 2 XPS spectra of parent urea and urea-MPs-2 sample.
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Table 1 The catalytic performances of different catalysts and structural parameters of urea-MPs samples with different urea contents
Catalyst
Amine (wt%)
SBET (m2 gÀ1
)
VP (cm3 gÀ1
)
Dp (nm)
Yield (%)
urea-MPs-1
urea-MPs-2
urea-MPs-3
urea
urea + MPs
urea-MPs-2-600
0.68
1.1
2.1
1.1
1.1
1.0
401
380
258
/
/
403
0.43
0.40
0.19
/
/
0.29
3.84
3.82
3.60
/
/
2.80
89
94
66
15
58
71
have a complete closure owing to the intrinsic property of
polymer materials.6 The BET surface areas, average pore size,
and pore volume of these functionalized mesoporous polymers
were summarized in Table 1. Increase with urea content
resulted in the decrease of SBET, DP and VP due to the
decreased ordered degree of the mesoporous structure.21 This
phenomenon was maybe due to the reduced hydrophilic –OH
groups in the urea-ROs with more urea units, which was
unfavorable for –OH groups to interact with the triblock
copolymer templates to form the final ordered mesostructures
during the solvent evaporation.17
examined the catalytic activity of urea-MPs-2 after calcination
at 600 1C. Accordingly, the yield decreased to 71%, which was
significantly lower than that of the as-prepared urea-MPs-2.
Meanwhile, the only minor reduction in SBET and Dp shown in
N2 sorption results suggested that the decreased activity of the
calcinated urea-MPs-2 was due to the lack of surface phenolic
–OH groups rather than the resistance to internal mass
transfer or the blockage of secondary amine sites.23 Based
upon the control experiment results, the reactivity enhance-
ment was attributed to the site isolation effect induced by the
heterogenization of urea functional group in the framework of
urea-MPs-2. Meanwhile, the phenolic groups in the support
surface also provided the cooperative effect to improve the
catalytic reactivity.24
The water-medium Knoevenagel condensation reaction was
used to evaluate the catalytic performance of urea-functionalized
mesoporous polymers. As summarized in Table 1, a series of
urea-MPs displayed highly catalytic activity which indicated
urea functional group was indeed catalytically active. Sub-
sequently, we investigated the effect of nitrogen content on the
catalytic activity in the condensation of benzaldehyde with
ethyl cyanoacetate. The results showed that the yield of
ethyl a-cyanocinnamate increased from 89% to 94% with
the increase of nitrogen content from to 0.68 to 1.1 wt%.
However, the yield drastically dropped to 66% when nitrogen
content increased to 2.1 wt%. The inferior catalytic capability
of urea-MPs-1 may be due to the lower active site density
that reduced the requisite substance contact. In addition, the
decreased ordered degree of pore structure of urea-MPs-3
disfavored the transport limit and/or the interaction between
the amine species and the substances, and thus resulted in
the reduced catalytic efficiency. Furthermore, we examined the
difference in catalytic activity between the urea-MPs-2 and
the homogeneous counter parts. Interestingly, we found that
the urea-MPs-2 exhibited superior catalytic reactivity to
parent urea, which only has 15% yield in the same conditions.
To get a better insight into the higher catalytic efficiency of the
urea-MPs-2 catalyst, we also examined the catalytic activity of
the physical mixtures of urea and non-functionalized meso-
porous polymer. As shown in Table 1, this physically combining
catalytic system obtained improvement in the reactivity
with the yield of 58%. This result indicated the addition of
mesoporous polymers into the reaction system could enhance
the catalytic efficiency of parent urea, which maybe due to
the mesoporous polymer support could inhibit the urea self-
assembled supramolecular aggregate.22 Nevertheless, the activity
of the physical mixture still lower than that of urea-MPs-2.
Thus, the urea functional group embedded in the mesoporous
framework may adapt a spatial orientation, in which the surface
hydrogen bonding between the urea and phenolic –OH groups
generated the synergic effect for the enhanced catalytic
capability of urea-MPs-2. To scrutinize this hypothesis, we
With the aim of examining the possibility of the wide use of
the urea-MPs-2, we tested a series of Knoevenagel conden-
sation reactions and the results were listed in Table 2. Firstly,
the electron donor/withdrawing capability of substituent
(entries 1–2) in the benzene ring of derivatives was investi-
gated. As expected, a benzaldehyde derivative with electron
withdrawing substituent in its benzene ring was conducive to
the Knoevenagel condensation and 99% yield of the conden-
sation product was obtained. On the contrary, the existence of
electron donor substituent led to lower 72% yield. Further-
more, different activated methylene compounds including
1,3-di-ketone, b-keto-ester and 1,3-diester were also tested by
Table 2 Results of various substances and benzaldehyde catalyzed by
urea-MPs-2a
Entry
1
Reaction equation
t (h)
Yield (%)
72
4
2
4
99
21
3
4
24
24
24
57
76
5
a
Reaction conditions: 63.4 mg urea-MPs-2, 0.20 mmol activated
methylene compound, 0.24 mmol aldehyde compund, 5.0 ml distilled
water, T = 50 1C.
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ordered mesoporous structure of the urea-MP-2 was well
preserved after being reused for 6 times, as a result of the
excellent stability in water-medium chemical transformations.
Conclusion
In summary, we demonstrated the synthesis of urea-functionalized
mesoporous polymers through one-step EISA approach. This
heterogeneous basic catalyst showed high activity and stability
in the water-medium Knoevengel reaction, presumably as a
result of the cooperative effect between the urea active sites
located in the mesopore wall and its neighboring phenolic
–OH groups in the mesoporous support. This facile assembly
route is of great potential for the preparation of other function-
alized mesoporous polymeric catalysts for various chemical
transformations.
Fig. 5 The recycling tests of the urea-MPs-2 catalyst during water-
medium Knoevenagel reaction between benzaldehyde and ethyl
cyanoacetate.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (50943048), Shanghai Government
(11YZ88, S30406, 07dz22303, 10PJ1408200 and ssd10014).
the urea-MPs-2 catalyst (entries 3–5). The results showed that
the urea-MPs-2 could catalyze all three methylene compounds
to react with benzaldehyde and obtained their corresponding
condensation product. Meanwhile, we also found the ester-
substituted compounds were more easily to activate than the
ketone-substituted compounds and thus the yield of 3-benzyl-
idenepentane-2,4-dione from pentane-2,4-dione and benzaldehyde
has only 21% after reacting 24 h at 50 1C. However, the
urea-MPs-2 catalyst could not promote the Knoevenagel
condensation between acetaldehyde and benzaldehyde. These
results agreed well with the previous report in the other amine-
catalyzed Knoevenagel condensation.25
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