Efficient and reusable amine-functionalized polyacrylonitrile fiber catalysts for Knoevenagel condensation in water

Article abstract of DOI:10.1039/c2gc35483g

A series of new fiber catalysts has been synthesized by aminating a commercially available polyacrylonitrile fiber with polyamines such as diethylenetriamine and triethylenetetramine. These fiber catalysts can efficiently catalyze the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate in water (yields: 95-98%). The triethylenetetramine aminated fiber catalyst was used to catalyze the condensations of a wide range of aromatic aldehydes and active methylene compounds and in each case, this catalyst performed well with yields higher than 93%. This fiber catalyst also exhibited excellent reusability (up to 21 times) and photostability.

Full text of DOI:10.1039/c2gc35483g

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PAPER
Efficient and reusable amine-functionalized polyacrylonitrile fiber catalysts
for Knoevenagel condensation in water†
Guowei Li, Jia Xiao and Wenqin Zhang*
Received 30th March 2012, Accepted 14th May 2012
DOI: 10.1039/c2gc35483g
A series of new fiber catalysts has been synthesized by aminating a commercially available
polyacrylonitrile fiber with polyamines such as diethylenetriamine and triethylenetetramine. These fiber
catalysts can efficiently catalyze the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate
in water (yields: 95–98%). The triethylenetetramine aminated fiber catalyst was used to catalyze the
condensations of a wide range of aromatic aldehydes and active methylene compounds and in each case,
this catalyst performed well with yields higher than 93%. This fiber catalyst also exhibited excellent
reusability (up to 21 times) and photostability.
this method had some advantages in terms of improved yields
and shortened reaction times, the catalyst loading was unsatisfac-
1. Introduction
The Knoevenagel condensation is one of the most fundamental
condensation reactions in organic chemistry.1–7 The α,β-unsatu-
rated products have been widely used as intermediates in the
synthesis of fine chemicals,8 therapeutic drugs,9 natural pro-
ducts10 and functional polymers.11 In general, this type of con-
densation is performed under homogeneous conditions in the
presence of catalysts such as organic bases,12,13 solid bases,14,15
ionic liquids,16,17 amino acids,18,19 Lewis acids20,21 or organo-
metallic catalysts.22,23 However, most of the reported Knoevena-
gel condensations suffer from non-recoverable catalysts and use
organic solvents and toxic metals. In recent years, solvent- and
catalyst-free Knoevenagel condensations promoted by micro-
wave irradiation have been developed. Unfortunately, reactions
under these conditions often need high power microwaves and
are not applicable for large-scale synthesis.24,25
Nowadays there is an urgent need to develop green chemistry
processes, where nontoxic substances are used and the gener-
ation of waste can be avoided. From both environmental and
economic points of view, current research is focused on the
development of heterogeneous catalysts based on solid materials
and on using aqueous media to perform Knoevenagel
condensations.26–30 For example, different types of amine
immobilized catalysts based on polyacrylamide or silica gel have
been prepared to catalyze Knoevenagel condensation in
water.26,27 These amine supported catalysts gave moderate to
high yields but the catalyst loading was large (10–20 mol%).
A simple and rapid method for Knoevenagel condensation was
catalyzed by a DABCO-base ionic liquid in water.28 Although
tory (15 mol%). Zhang et al. immobilized diethyl aliphatic
amine basic ionic liquids onto hydroxyapatite-encapsulated
γ-Fe2O3 nanoparticles29 and used them as heterogeneous cata-
lysts for Knoevenagel condensation with short reaction times at
30 °C in water. However, most of the reaction yields were only
moderate and the reusability was limited to four times. Zhu et al.
synthesized a urea-functionalized mesoporous polymetic as a
catalyst for Knoevenagel condensation in water at 50 °C.30
However, this catalyst system only gave moderate yields with
long reaction times (4–24 h).
Although several supported catalysts have been reported for
Knoevenagel condensations in water, very few of them are based
on fabric materials. Therefore the design and synthesis of a cata-
lyst for Knoevenagel condensation in water based on a wearable
fiber is still a challenge. Polyacrylonitrile fiber (PANF) has been
widely used in the clothing industry as a fabric material, since it
is corrosion and mildew resistant and has excellent mechanical
strength. It also has lots of cyano groups (CuN) which can be
easily transformed into carboxyl, amide or amidoxime
groups.31–33 Therefore PANF should be a suitable starting
material for the preparation of a fiber catalyst. On the basis of
our previous work,34 we became interested in using a series of
polyamines such as diethylenetriamine and triethylenetetramine
as multifunctional compounds that could be immobilized onto
the fiber through their primary amine groups. The modified fiber
was expected to perform as a supported amine catalyst for base
catalyzed organic transformations such as Knoevenagel
condensation.
2. Experimental
Department of Chemistry, School of Sciences, Tianjin University, No.
92, Weijin Road, Tianjin 300072, P.R. China.
2.1. Reagents
E-mail: zhangwenqin@tju.edu.cn; Fax: +86-22-27403475;
Tel: +86-22-27890922
†Electronic supplementary information (ESI) available: 1H, 13C NMR
data and spectra of all compounds. See DOI: 10.1039/c2gc35483g
A commercially available polyacrylonitrile fiber (PANF) (93.0%
acrylonitrile, 6.5% methyl acrylate and 0.4–0.5% sodium styrene
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Scheme 2 Knoevenagel condensation of aromatic aldehydes with
active methylene compounds in water.
titration with 0.100 M NaOH. The exchange capacity was calcu-
lated based on the amount of acid consumed.
2.5. General procedure for Knoevenagel condensation
A mixture of aldehyde (5 mmol), ethyl cyanoacetate (5.5 mmol)
and fiber catalyst (0.25 mmol, the weight of the fiber catalyst =
0.25 mmol/A, where A is the acid exchange capacity of the fiber
catalyst in mmol g−1) in water or another solvent (15 mL) was
stirred at 50 °C. The reaction progress was monitored by TLC.
After completion of the reaction, the fiber catalyst was filtered
out and washed with ethyl acetate (3 × 10 mL). The filtrate was
separated and extracted three times with ethyl acetate. The com-
bined organic phases were concentrated and the crude product
was purified by column chromatography (petroleum ether–ethyl
acetate) (Scheme 2).
Scheme 1 Preparation of the amine functionalized fiber catalysts.
sulfonate) with a length of 10 cm and a diameter of 30 0.5 μm
(from the Fushun Petrochemical Corporation of China) was
used. All aldehydes, ethyl cyanoacetate, diethylenetriamine,
triethylenetetramine and all other reagents were analytical grade
and were used without further purification. Water was deionized.
2.2. Apparatus and instruments
The mechanical properties of the different fiber samples were
tested with an electronic single fiber strength tester (Laizhou
Electronic Instrument Corporation, China, model LLY-6).
Elemental analyses were performed on a vario micro cube analy-
zer (Elementar). X-Ray powder diffraction (XRD) spectra were
recorded with a BDX 3300 X-ray diffractometer (Peking Univer-
sity Instrument Factory) at 0.154 nm. A scanning electron micro-
scope (SEM) (Hitachi, model S-4800) was used to characterize
the surface of the modified fibers. Fourier-transfer infrared
(FTIR) spectra were obtained with an AVATAR 360 FTIR spec-
3. Results and discussion
3.1. Synthesis of the fiber catalysts
The extent of modification was determined using the weight gain
and acid exchange capacities of the produced fibers. The weight
gain was calculated as follows:
Weight gain ¼ ½ðW2 ꢀ W1Þ=W1ꢁ ꢂ 100%
where W1 and W2 are the weights of PANF and the aminated
fiber, respectively.
1
trometer (Thermo Nicolet). H NMR spectra were recorded on
an AVANCE III instrument (Bruker, 400 MHz) using TMS as the
internal standard. 13C NMR spectra were recorded on an
AVANCE III instrument (Bruker, 101 MHz) with complete
proton decoupling. Melting points were measured with a Yanagi-
moto MP-500 apparatus and were uncorrected.
The amination was found to be strongly influenced by the
concentration of polyamine, reaction time and temperature. For
example, in the case of triethylenetetramine, the acid exchange
capacity of PANTTF was effectively enhanced by increasing the
concentration of triethylenetetramine or by prolonging the reac-
tion time or by elevating the reaction temperature (see Fig. 1).
However a higher acid exchange capacity notably reduced the
mechanical strength of the fiber catalyst. Therefore, the best reac-
tion conditions to prepare triethylenetetramine aminated fiber
catalyst (PANTTF) were 20 mL of triethylenetetramine, 10 mL of
water and 1.000 g PAN fiber at 110 °C for 4 h.
The properties of four different fiber catalysts PANEDF,
PANDTF, PANTTF and PANTPF (aminated by ethylenediamine,
diethylenetriamine, triethylenetetramine and tetraethylenepenta-
mine, respectively) are shown in Table 1. Detailed procedures
for the synthesis of the different fiber catalysts are given in the
ESI.† For the Knoevenagel condensation between benzaldehyde
and ethyl cyanoacetate, the blank control reaction (no catalyst)
and the reaction with only PANF gave low yields of 5%
(Table 1, entries 1 and 2). However, all the fiber catalysts gave
high condensation yields (>95%, entries 3–6), which indicate an
efficient catalytic activity of the grafted amine groups. PANTTF
2.3. General procedure for the synthesis of the fiber catalysts
Dried PANF (1.000 g), the appropriate polyamine (20 mL) and
deionized water (10 mL) were added to a three-necked flask and
the mixture was stirred under reflux for the appropriate time.
Then the fiber was filtered out and repeatedly washed with water
(60–70 °C) until the pH was neutral. The fiber was then dried
overnight under vacuum at 70 °C to give the fiber catalyst
(Scheme 1).
2.4. General procedure for determining the acid exchange
capacities of the fiber catalysts
The dried fiber catalyst (0.100 g) was immersed into 20 mL of
0.100 M HCl for 6 h. The treated fiber was then filtered out. The
HCl concentration of the remaining solution was determined by
exhibited
a
moderate acid exchange capacity, excellent
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Fig. 1 Effects of concentration, reaction temperature and time on the acid exchange capacity of the triethylenetetramine aminated fiber.
Table 1 Properties of PANEDF, PANDTF, PANTTF and PANTPF and
Table 2 The mechanical properties of PANTTF with different acid
exchange capacities and their activities in catalyzing a Knoevenagel
condensationa
their activities in catalyzing a Knoevenagel condensationa
Acid exchange
Weight
gain (%)
capacity
Yielde
(%)
Acid exchange
capacity
Retention of
breaking
Entry Catalyst
(mmol g−1
)
Breaking
strength (cN)
Yieldc
(%)
Entry
(mmol g−1
)
strengthb (%)
1
2
3
4
5
6
7
Blank controlb
—
—
13
20
26
40
—
—
—
5
5
96
95
98
97
86
PANFc
1
2
3
4
5
6
0.6
1.4
2.4
3.1
5.0
7.6
8.5
8.2
7.9
7.6
6.4
—
88
85
81
78
66
—
81
90
93
98
97
98
PANEDF
1.0
2.5
3.1
5.1
—
PANDT
PANTT
PANTP
Triethylenetetramined
F
F
F
a Reactions conditions: benzaldehyde (5 mmol), ethyl cyanoacetate
(5.5 mmol) and fiber catalyst (5 mol%, based on acid exchange
capacity) in H2O (15 mL) stirred at 50 °C for 1.5 h. b No catalyst. c With
unmodified PANF (0.081 g) as the catalyst. d With 0.0625 mmol
triethylenetetramine (0.25 mmol amine groups, 5 mol% based on
benzaldehyde) as the catalyst. e Isolated yield by column
chromatography.
a Reaction conditions: benzaldehyde (5 mmol), ethyl cyanoacetate
(5.5 mmol) and fiber catalyst (5 mol%) in H2O (15 mL) stirred at 50 °C
for 1.5 h. b Based on PANF = 100% (breaking strength 9.7 cN).
c Isolated yield by column chromatography.
3.2. Characterization of the fiber catalyst PANTT
F
3.2.1. Elemental analyses. The elemental analysis data of
PANF, PANTTF and PANTTF-1 (recovered after catalyzing one
Knoevenagel condensation reaction between benzaldehyde and
ethyl cyanoacetate) and PANTTF-21 (recovered after catalyzing
21 runs) are listed in Table 3 (The recyclability data is presented
in Section 3.3.4). Compared to the original PANF, the carbon
content of PANTTF decreased noticeably and the hydrogen
content increased as expected since triethylenetetramine has less
carbon and more hydrogen than the original PANF (Table 3,
entries 1 and 2). The decrease in nitrogen content in PANTTF is
due to the fact that some cyano groups underwent partial
hydrolysis to form primary amides or react with triethylenetetra-
mine in aqueous media resulting in the formation of secondary
amides and the release of ammonia. The released ammonia then
reacts with the –COOCH3 groups to form –CONH2 groups
(Scheme 1). Moreover, the low nitrogen content of PANTTF also
suggests that a cross-linking reaction induced by triethylenetetra-
mine and the hydrolysis of cyano groups is unavoidable
(Scheme 1). After both 1 and 21 uses as a catalyst in the Knoe-
venagel condensation, the carbon content of the recovered fiber
catalyst (PANTTF-1 and PANTTF-21) increased slightly and the
hydrogen and nitrogen contents decreased compared to PANTTF,
which indicates that some of the Knoevenagel product is
enwrapped by or grafts onto the fiber catalyst.
mechanical properties and the highest catalytic activity, so it was
selected for further research. When free triethylenetetramine was
used as the catalyst under the same conditions, the condensation
yield was only 86% (Table 1, entry 7). The fact that PANTTF pre-
sents a higher catalytic activity than the corresponding free
amine may be due to the fiber catalyst affording a special micro-
environment for the condensation.
The mechanical properties and Knoevenagel condensation cat-
alytic activities of PANTTF with different acid exchange
capacities were studied and are shown in Table 2. As the acid
exchange capacities increased from 0.6 to 5.0 mmol g−1, the
breaking strength decreased from 8.5 to 6.4 cN and the retention
of breaking strength based on PANF (9.7 cN = 100%) decreased
from 88 to 66% accordingly (Table 2, entries 1–5). The breaking
strength of the PANTTF with an acid exchange capacity of
7.6 mmol g−1 was difficult to test because it was heavily
damaged during the amination reaction (Table 2, entry 6). For
the Knoevenagel condensation between benzaldehyde and ethyl
cyanoacetate, only moderate yields were obtained with PANTTF
with acid exchange capacities from 0.2 to 2.4 mmol g−1
(Table 2, entries 1–3). For PANTTF with acid exchange
capacities from 3.1 to 7.6 mmol g−1, higher condensation yields
were obtained (Table 2, entries 4–6). Considering the balance
between mechanical properties and catalytic activity, the
PANTTF with an acid exchange capacity of 3.1 mmol g−1 (78%
retention of breaking strength) was selected as the best catalyst
for further studies.
3.2.2. Mechanical properties. The breaking strengths of the
fiber samples were measured and are shown in Table 4. The
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Table 3 Elemental analysis data of PANF, PANTTF, PANTTF-1 and
PANTTF-21
Entry
Fiber sample
PANF
Weight gain (%)
C (%)
H (%)
N (%)
1
2
3
4
—
26
26
26
66.15
57.96
58.79
58.33
5.77
6.91
6.79
6.48
24.12
22.06
21.69
20.50
PANTT
F
PANTTF-1a
PANTTF-21b
a Recovered after the 1st use as the catalyst in the Knoevenagel
condensation of benzaldehyde with ethyl cyanoacetate as described in
Table 1. b Recovered after the 21st run of the reaction.
Table 4 Mechanical properties of PANF, PANTTF, PANTTF-1, and
PANTTF-21
Fiber
Entry sample
Breaking strength
(cN)
Retention of breaking
strength (%)
Fig. 2 XRD spectra of (a) PANF, (b) PANTTF, (c) PANTTF-1, and
(d) PANTTF-21.
1
2
3
4
PANF
PANTT
PANTTF-1
PANTTF-21 7.2
9.7
7.6
7.6
100
78
78
F
74
breaking strength of PANF was 9.7 cN (Table 4, entry 1). After
the amination, the breaking strength decreased to 7.6 cN
(Table 4, entry 2), i.e. 78% of the PANF breaking strength was
retained. The breaking strength of PANTTF was maintained
when used as the catalyst in the Knoevenagel condensation
between benzaldehyde and ethyl cyanoacetate (Table 4, entry 3).
The fiber catalyst maintained more than 74% of its physical
strength after being used 21 times in the Knoevenagel conden-
sation (Table 4, entry 4).
3.2.3. XRD. The XRD spectra of PANF, PANTTF, PANTTF-1
and PANTTF-21 are shown in Fig. 2. The intense reflection peak
at 2θ = 17° (Fig. 2a) corresponds to the (100) diffraction of the
hexagonal lattice formed by parallel close packing of the mol-
ecule rods, which demonstrates that PANF adopts a stiff rod-like
conformation due to the intermolecular repulsions of the nitrile
dipoles.35 After the amination, the reflection peak (2θ = 17°) of
PANTTF decreases obviously (Fig. 2b), indicating a decrease in
the polar interactions between the molecular chains and the
breakage of bonds in the inner crystal region during the reac-
tion.32 After the first run and the twenty-first run of the Knoeve-
nagel condensation, the recovered catalysts PANTTF-1 and
PANTTF-21 (Fig. 2c and d respectively) have XRD spectra that
are almost identical to the unused PANTTF (Fig. 2b) at 2θ = 17°,
which indicates that the inner crystal region of this fiber catalyst
is not further damaged after being repeatedly used.
Fig. 3 SEM photographs of (a) PANF, (b) PANTTF, (c) PANTTF-1, and
(d) PANTTF-21.
3.2.4. FTIR. Samples of PANF, PANTTF, PANTTF-1 and
PANTTF-21 were pulverized by cutting and then prepared into
KBr pellets. Their FTIR spectra are shown in Fig. 4. For the
FTIR spectrum of PANF (Fig. 4a), the 2242 cm−1 absorption
band is attributed to CuN vibrations and the 1731 cm−1 CvO
vibration peak indicates the existence of methyl acrylate units in
the copolymer. The broad absorption bands at 3700–3150 cm−1
in PANTTF (Fig. 4b) correspond to the stretching vibrations of
the N–H groups. The 2242 cm−1 CuN vibration peak still exists
in PANTTF, which suggests that not all the CuN groups partici-
pate in the amination and the hydrolysis. In PANTTF, the CvO
vibrational peak at 1731 cm−1 decreases noticeably, since
triethylenetetramine preferentially reacts with methyl esters. For
PANTTF, the strong absorption band in the region of 1650 cm−1
is assigned to the amide I bands. The new broad peak at
1560 cm−1 is attributed to the overlap of the amide II bands, i.e.
C—N stretching vibrations and N—H bending vibrations. After
a comparison with the CuN groups in ethyl cyanoacetate
3.2.3. SEM. The SEM photographs of PANF, PANTTF,
PANTTF-1 and PANTTF-21 are shown in Fig. 3. Commercially
available PANF has a very smooth surface (Fig. 3a). After amin-
ation, the surface of the modified fiber (Fig. 3b) becomes
rougher. The surfaces of the recovered fiber catalysts (after 1 and
21 cycles) gradually become even rougher but no major flaws
are seen (Fig. 3c and d), which indicates that this fiber catalyst
can be reused many times to catalyze Knoevenagel
condensation.
(2260 cm−1 36
)
and the CuN groups in the Knoevenagel
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from ethyl cyanoacetate, the fiber catalyst and the CvN imine
form a cyclic transition state with a hydrogen bond. The acti-
vated CvN imine is then attacked by the anion of ethyl cyano-
acetate to produce the corresponding adduct. After capturing a
proton again, a similar cyclic transition state is formed. When
the product is released from the surface of the fiber, the fiber cat-
alyst returns back to its original structure. For the mechanism
catalyzed by the secondary amine moieties (Scheme 3B), the
benzaldehyde molecules and the secondary amine groups first
form the CvN+ iminium and release hydroxyl ions. Then a
process similar to that for the primary amine occurs. During the
catalytic process, some of the Knoevenagel products may be
grafted onto the fiber catalyst to form an amide (Scheme 3C) or
they may be enwrapped in the fiber, which is supported by the
elemental analysis and FTIR results (Fig. 4).
3.3.2. Optimization of the Knoevenagel reaction conditions.
To optimize the reaction conditions, the condensation of benzal-
dehyde and ethyl cyanoacetate was used as a model reaction
(Table 5). With benzaldehyde (5 mmol), ethyl cyanoacetate
(5 mmol) and PANTTF (5 mol%), the Knoevenagel condensation
gave a yield of 90% at 50 °C for 1.5 h (Table 5, entry 3). When
higher amounts of benzaldehyde or ethyl cyanoacetate were
used, higher yields were obtained within 1.5 h (Table 5, entries
1, 2, 4 and 5). To balance catalytic activity and economic
efficiency, a benzaldehyde to ethyl cyanoacetate ratio of 1 : 1.1
was selected for further research (Table 5, entry 4). Using this
optimal molar ratio of substrates, the effect of reaction tempera-
ture was then studied. When the reaction temperature was
increased from 20 to 50 °C, the yields increase from 76 to 98%
(Table 5, entries 6–8 and 4). However, no further increase in
yield was found with a temperature of 60 °C (Table 5, entry 9).
So 50 °C was determined to be the best temperature for this
Knoevenagel condensation.
Fig. 4 FTIR spectra of (a) PANF, (b) PANTTF, (c) PANTTF-1, and
(d) PANTTF-21.
products (2221 cm−1),37 the new peaks at 2205 cm−1 in the
recovered catalysts (PANTTF-1 and PANTTF-21) are attributed to
the cyano groups of the Knoevenagel products grafted onto the
fiber catalyst (Fig. 4c and d). The increase in the peak at
1731 cm−1 (CvO) in the PANTTF-1 and PANTTF-21 spectra
further indicates that some of the Knoevenagel products are
enwrapped in the fiber catalyst (the CvO stretching vibration of
α,β-unsaturated Knoevenagel products is at 1733 cm−1 and at
1745 cm−1 for saturated products).37,38 These results agree well
with the elemental analysis data.
3.3. Application of PANTTF in Knoevenagel condensation
To study the effect of the catalyst loading, reactions were
carried out with various amounts of PANTTF (from 1 to 6 mol%)
using benzaldehyde (5 mmol) and ethyl cyanoacetate
(5.5 mmol) in H2O (15 mL) at 50 °C for 1.5 h. As can be seen
from the results in Table 5 (entries 10–12 and 4), there was a
sharp increase in the yield from 82 to 98% when the catalyst
loading increased from 1 to 5 mol%. When the amount of cata-
lyst was increased from 5 to 6 mol%, no further increase in yield
was observed. Therefore a dosage of 5 mol% PANTTF was
selected for all subsequent reactions.
To investigate the effect of reaction time on the yield of the
product, the reaction mixture was stirred at 50 °C for different
periods of time (from 0.5 to 2 h) (Table 5, entries 14–17 and 4).
The yield increased from 85 to 98% when the reaction time was
increased from 0.5 to 1.5 h. After 1.5 h, the yield of the conden-
sation reached its maximum level. So stirring at 50 °C for 1.5 h
was chosen as the optimal reaction time for the condensation of
aromatic aldehydes with ethyl cyanoacetate.
3.3.1. Catalytic activity. Knoevenagel condensation is
mainly a base catalyzed reaction. PANF did not show any
activity for the condensation of benzaldehyde and ethyl cyano-
acetate in water (compare Table 1, entries 1 and 2). When the
functionalized PANF (PANTTF) was used as the catalyst, the
condensation yield was 98%, indicating a highly efficient cataly-
tic activity of the grafted amine groups. When our previous ter-
tiary-amine functionalized fiber34 was used as a catalyst for the
Knoevenagel condensation in water at the same conditions, the
yield of the condensation between benzaldehyde and ethyl
cyanoacetate was only 78% and the fiber catalyst was destroyed
after the reaction. So the PANTTF offered a higher catalytic
activity and stability for Knoevenagel condensation in water.
Fibers are known to have larger specific surface areas than granu-
lar materials. Compared to macroporous resins and molecular
sieves, fibers also have good kinetic properties due to the short
diffusion path of the reactants into the fibers. The PANTTF is
grafted with primary and secondary amine moieties, which can
function as a highly reactive base catalyst under mild conditions
in water. The possible mechanisms for the Knoevenagel conden-
sation are shown in Scheme 3. For the mechanism catalyzed by
the primary amine moieties (Scheme 3A), the benzaldehyde
molecules and the primary amine groups form the CvN imine
species and release water molecules. After capturing a proton
3.3.3. The influence of solvent on Knoevenagel condensation.
The influence of solvent on the Knoevenagel condensation cata-
lyzed by PANTTF was also investigated using the same model
reaction between benzaldehyde and ethyl cyanoacetate and the
results are shown in Table 6. For solvents such as cyclohexane,
benzene, CHCl3, THF, EtOAc, 1,4-dioxane and acetone, which
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Scheme 3 Possible mechanism of Knoevenagel condensation catalyzed by PANTTF.
are less polar than ethanol, the Knoevenagel condensation did
not occur within 1.5 h at either 50 °C or at reflux (Table 6,
entries 1–7). In C2H5OH and CH3CN, the reaction did not occur
or only gave trace yields at 50 °C (Table 6, entries 8 and 11).
However, the same reaction proceeded well at reflux conditions
(Table 6, entries 9 and 12). In DMF, the reaction time was pro-
longed to 6 h, but at 50 °C only a low yield of 65% was obtained
(Table 6, entry 13). DMSO was also a poor solvent for the Knoe-
venagel condensation at 50 °C for 1.5 h, which produced only a
trace amount of product (Table 6, entry 15). When carried out at
80 °C in DMF or DMSO, the Knoevenagel condensation was
complete within 1 h and high yields were obtained (Table 6,
entries 14 and 16). But to our disappointment, the fiber catalyst
PANTTF became fractured at 80 °C in DMF and DMSO after
1 h. Only in CH3OH and H2O, could the reactions reach com-
pletion within 1.5 h with high yields (Table 6, entries 10 and
17). Considering reaction temperature and environmental impact,
H2O is the best solvent for the model Knoevenagel condensation
catalyzed by PANTTF. Compared to our published tertiary-amine
fiber catalyst (yield, 78%),34 the triethylenetetramine moieties on
PANTTF are more hydrophilic than tertiary-amine, which may
promote the catalytic activity for the heterogeneous Knoevenagel
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Table 5 Optimization of the Knoevenagel reaction conditionsa
Entry
Benzaldehyde (mmol)
Ethyl cyanoacetate (mmol)
Catalyst loading (mol%)
Temp. (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
6
5.5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5.5
6
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5
5
5
5
5
5
5
5
5
1
3
4
6
5
5
5
5
50
50
50
50
50
20
30
40
60
50
50
50
50
50
50
50
50
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.5
1
96
91
90
98
98
76
80
94
98
82
91
94
98
85
90
92
98
9
10
11
12
13
14
15
16
17
1.25
2
a Reactions were carried out with benzaldehyde and ethyl cyanoacetate in the presence of PANTTF in H2O (15 mL). b Isolated yield by column
chromatography.
Table 6 The influence of solvent on Knoevenagel condensation of
entries 16 and 18), which may be due to hydrogen bonding
between the fluorine atom and the amine functional groups
which lowers the activity of the CvN imine species and pre-
vents the removal of the intermediates (Scheme 3). In methanol,
the reactions gave high yields at 70 °C at 6 and 2 h (Table 7,
entries 17 and 19). Compared to ethyl cyanoacetate, malononi-
trile was more reactive and the reaction could be carried out at
room temperature with an aldehyde to malononitrile ratio of 1 : 1
and the reaction time decreased to 1 h to give the condensation
products in yields above 96% (Table 7, entries 20–24). However,
the Knoevenagel condensation of 4-nitrophenylacetonitrile with
aromatic aldehydes did not take place due to the poor solubility
of 4-nitrophenylacetonitrile in water. So methanol was added to
the solvent. The Knoevenagel condensation between aromatic
aldehydes and 4-nitrophenylacetonitrile then proceeded
smoothly at 80 °C in the mixed solvent (H2O–CH3OH = 1 : 1,
v/v) and high yields were obtained (Table 7, entries 25–27).
Compared with the reported NMR data,13,16,24,28 all the
α,β-unsaturated products have the structure with the aryl group
cis to the CN group which reduces any steric effects.
benzaldehyde and ethyl cyanoacetatea
Entry
Solvent
Temp. (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
Cyclohexane
C6H6
CHCl3
THF
EtOAc
1,4-Dioxane
Acetone
C2H5OH
C2H5OH
CH3OH
CH3CN
CH3CN
DMF
50/81
50/81
50/62
50/66
50/78
50/102
50/59
50
78
50
50
81
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
9
6
1
1.5
1
1.5
nr
nr
nr
nr
nr
nr
nr
Trace
97
96
nr
95
65
95
Trace
98
98
9
10
11
12
13
14
15
16
17
50
80
50
80
DMF
DMSO
DMSO
H2O
50
a Reaction conditions: benzaldehyde (5 mmol), ethyl cyanoacetate
(5.5 mmol), PANTTF (5 mol%) in each solvent (15 mL). b Isolated yield
by column chromatography.
3.3.5. Reusability of the catalyst PANTTF. To evaluate the
reusability of the catalyst, 0.081 g PANTTF (5 mol%) was used
to catalyze the condensation of benzaldehyde (5 mmol) with
ethyl cyanoacetate (5.5 mmol) in water (15 mL) at 50 °C. After
completion of each cycle (1.5 h), the fiber catalyst was filtered
out and washed three times with ethyl acetate, and then the
recycled catalyst was directly used for the next cycle without any
additional treatment. The yields of the Knoevenagel conden-
sation decreased by only 4% with the regenerated catalyst after
20 cycles (see Table 8). After recycling twenty times, the reac-
tion time was prolonged to 3 h and to our surprise, the conden-
sation yield was as high as that of the first cycle (Table 8, entries
1 and 21). Compared to currently used supported catalysts which
can only be used less than ten times,27–30 PANTTF offers excel-
lent reusability (21 times).
condensation in water. In addition, water media can stabilize the
cyclic transition state with hydrogen bond (Scheme 3) during the
catalyzed reaction.
3.3.4. Knoevenagel condensation of various aromatic alde-
hydes with active methylene compounds. This fiber catalyst was
then applied to reactions of several aromatic aldehydes with
active methylene compounds (Table 7). For most of the alde-
hydes with ethyl cyanoacetate, the yields of the condensations
were above 95%, indicating that the fiber catalyst has a high
level of activity (Table 7, entries 1–15). In the condensations of
2- and 3-trifluoromethyl benzaldehyde and ethyl cyanoacetate in
water at 70 °C, the starting materials disappeared after 4.5 and
3 h, respectively. However, the yields were moderate (Table 7,
2240 | Green Chem., 2012, 14, 2234–2242
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Table 7 Knoevenagel condensation of aromatic aldehydes with active methylene compoundsa
Entry
Ar
R
Solvent
Temp. (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
C6H5
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
CNc
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
70
70
70
70
20
20
20
20
20
80
80
80
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
3
98
99
99
99
99
99
99
90
96
90
97
95
99
99
99
54
98
80
94
99
99
99
98
96
95
99
96
2-MeOC6H4
3-MeOC6H4
4-MeOC6H4
3-HOC6H4
4-HOC6H4
4-(CH3)2NC6H4
3,4-(CH3)2C6H3
3,4-(CH3)2C6H3
2-ClC6H4
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
1.5
3
2-ClC6H4
4-FC6H4
1.5
1.5
1.5
1.5
4.5
6
3-O2NC6H4
4-O2NC6H4
2-Furanyl
2-F3CC6H4
2-F3CC6H4
3-F3CC6H4
3-F3CC6H4
C6H5
2-MeOC6H4
4-MeOC6H4
3-HOC6H4
4-FC6H4
CH3OH
H2O
CH3OH
H2O
H2O
H2O
3
2
1.5
1.5
1.5
1.5
1.5
3
CNc
CNc
CNc
H2O
H2O
CNc
C6H5
4-MeOC6H4
3-HOC6H4
4-O2NC6H4
4-O2NC6H4
4-O2NC6H4
H2O–CH3OHd
H2O–CH3OHd
H2O–CH3OHd
4
6
a Reaction conditions: aromatic aldehyde (5 mmol), ethyl cyanoacetate (5.5 mmol), PANTTF (0.081 g, 5 mol%) in solvent (15 mL). b Isolated yield by
column chromatography. c Reactions were carried out with aromatic aldehyde (5 mmol) and malononitrile (5 mmol). d Reactions were carried out with
aromatic aldehyde (2 mmol) and 4-nitrophenylacetonitrile (2 mmol) in the mixed solvent (10 mL, H2O–CH3OH = 1 : 1, v/v).
Table 8 Reusability of the fiber catalyst PANTTFa
Entry
No. of recycling runs
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
3
98
98
98
98
97
98
97
97
97
97
96
96
97
96
96
96
95
95
94
94
98
Scheme 4 Large-scale Knoevenagel condensation catalyzed by
PANTTF in water.
3.3.6. A large-scale Knoevenagel condensation catalyzed by
PANTTF. In order to investigate the catalytic activity of PANTT
F
9
9
on a larger scale, the Knoevenagel condensation was performed
with 50 mmol of benzaldehyde and 55 mmol of ethyl cyanoace-
tate in 100 mL H2O at 50 °C for 1.5 h. In this case, the yield of
ethyl (E)-2-cyano-3-(3-nitrophenyl)-2-propenoate was 93%
(Scheme 4), which demonstrates that PANTTF is a potential
green catalyst which may be applied to industrial-scale
productions.
10
11
12
13
14
15
16
17
18
19
20
21
10
11
12
13
14
15
16
17
18
19
20
21
3.3.5. Photostability of the fiber catalyst PANTTF. The fiber
catalyst PANTTF was exposed to direct sunlight in air for 30
days, and then the PANTTF (0.081 g, 5 mol%) was used to cata-
lyze the Knoevenagel condensation between benzaldehyde
(5 mmol) and ethyl cyanoacetate (5.5 mmol) in water (15 mL).
a Reaction conditions: benzaldehyde (5 mmol), ethyl cyanoacetate
(5.5 mmol), PANTTF (0.081 g, 5 mol%) in H2O (15 mL) at 50 °C.
b Isolated yield by column chromatography.
This journal is © The Royal Society of Chemistry 2012
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View Online
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4. Conclusions
A series of fiber catalysts for Knoevenagel condensation were
prepared by aminating polyacrylonitrile fiber with polyamines
such as diethylenetriamine and triethylenetetramine. The cataly-
tic activities of these fiber catalysts were evaluated and the
triethylenetetramine aminated fiber (PANTTF) was selected for
detailed study. Knoevenagel condensation proceeded smoothly
in the presence of the fiber catalyst. Among all the tested sol-
vents, water was the best since it is an environmentally friendly
solvent which gives a high yield (98%) and it is low cost and
safe to use in production processes. After the completion of the
reaction, the catalyst was easily separated from the reaction
system by filtration and could be conveniently recycled and
reused. This catalytic system was applicable to a wide range of
aromatic aldehydes and active methylene compounds. Moreover,
this catalyst performed well in a large-scale Knoevenagel con-
densation with a high yield (93%). PANTTF also offered excel-
lent reusability (up to 21 times) and was very photostable
(exposed to direct sunlight in air for 30 days, Knoevenagel yield
95%). These attractive features of this fiber catalyst could help
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catalysts for use in green and continuous chemical processes.
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Acknowledgements
The authors are grateful for the financial support from the Key
Program of the National Natural Science Foundation of China
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2242 | Green Chem., 2012, 14, 2234–2242
This journal is © The Royal Society of Chemistry 2012