l-Lysine Functionalized Polyacrylonitrile Fiber: A Green and Efficient Catalyst for Knoevenagel Condensation in Water

Article abstract of DOI:10.1007/s10562-017-2287-y

Abstract: The l-lysine functionalized polyacrylonitrile fiber (PAN_LF) was prepared by grafting the l-lysine into a commercially available polyacrylonitrile fiber and showed highly catalytic activity for Knoevenagel condensation reaction. With low temperature (45?°C) and short reaction time (1?h), the fiber catalyst was well applicable to Knoevenagel condensation of a wide range of aldehydes and the yields could reach up to 99%. Interestingly, only in water could the reaction take place smoothly (with a yield of 88%) and a polar micro-environment promoted reaction process had been proposed to explain this phenomenon. Besides, the fiber catalyst has advantages of easy preparation, high functional degree, strong mechanical strength and thermal stability, etc. And it can be reused at least 5 times without further treatment and performed well in scaled-up experiment (amplified 50 times) and flow chemistry experiment (no loss of catalytic activity after 48?h), which indicates its potential application in industry application. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2287-y

Catalysis Letters
https://doi.org/10.1007/s10562-017-2287-y
l-Lysine Functionalized Polyacrylonitrile Fiber: A Green and Efficient
Catalyst for Knoevenagel Condensation in Water
Pengyu Li1 · Yuanyuan Liu1 · Ning Ma1,2 · Wenqin Zhang1,2
Received: 22 October 2017 / Accepted: 25 December 2017
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
The l-lysine functionalized polyacrylonitrile fiber (PANLF) was prepared by grafting the l-lysine into a commercially avail-
able polyacrylonitrile fiber and showed highly catalytic activity for Knoevenagel condensation reaction. With low temperature
(45 °C) and short reaction time (1 h), the fiber catalyst was well applicable to Knoevenagel condensation of a wide range
of aldehydes and the yields could reach up to 99%. Interestingly, only in water could the reaction take place smoothly (with
a yield of 88%) and a polar micro-environment promoted reaction process had been proposed to explain this phenomenon.
Besides, the fiber catalyst has advantages of easy preparation, high functional degree, strong mechanical strength and thermal
stability, etc. And it can be reused at least 5 times without further treatment and performed well in scaled-up experiment
(amplified 50 times) and flow chemistry experiment (no loss of catalytic activity after 48 h), which indicates its potential
application in industry application.
Graphical Abstract
Keywords Polyacrylonitrile fiber · l-Lysine · Supported catalyst · Knoevenagel condensation · Flow chemistry
1 Introduction
Nowadays, more and more small organic molecules were
used as catalysts instead of enzymes and metals to catalyze
Electronic supplementary material The online version of this
organic reactions [1]. Among them, amino acids including
article (https://doi.org/10.1007/s10562-017-2287-y) contains
l-proline [2], phenylalanine [3], alanine and isovaline [4],
supplementary material, which is available to authorized users.
as natural and environmentally benign compounds, have
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

P. Li et al.
been demonstrated to be effective catalysts for many reac-
tions, such as aldol reaction [3, 5], Mannich reaction [6, 7],
Michael addition [8, 9], Knoevenagal condensation [10, 11]
and so on [1, 12].
2.2 Apparatus and Instruments
1H NMR (400 MHz) and 13C NMR (101 MHz) spectra
were recorded using CDCl3 as a solvent and TMS as
an internal standard on a Bruker Avance III (400 MHz)
spectrometer. FT-IR spectra were obtained using a Bruker
ALPHA spectrophotometer. Elemental analysis was
obtained from an Elementar vario EL analyzer. Scan-
ning electron microscope (SEM) (Hitachi, model S-4800)
was used to characterize the surface of the modified fib-
ers. Solar 969 Atomic Absorption Spectrometer (Amer-
ican Thermal Power Company) was employed to study
the structure of the fiber catalyst. Melting points were
acquired from a Yanagimoto MP-500 apparatus and were
not corrected.
Knoevenagal condensation, which is the condensation of
aldehydes or ketones with the compounds containing active
methylene under the base catalysis to create α,β-unsaturated
carbonyl compounds and their analogues, is one of the most
useful reactions in organic chemistry [13–19]. This reaction
is environment-friendly because water acts as the by-product
and it could proceed smoothly by the catalysis of ammonium
salt [20], bases [21], amino acids [22], organometallics [23]
and so on. What’s more, microwave radiation [24], lipase
[25] and ionic liquids [26, 27] were also used to promote
the reaction. However, homogeneous catalysis will cause
resource consumption and environmental pollution due to its
intrinsic disadvantage of difficult separation of the catalyst.
Taking these into consideration, it was urgent to find a
catalyst which could overcome these disadvantages and real-
ize green chemical process. Thus, immobilized catalysts on
solid materials which could be both eco-friendly and eco-
nomic had attracted extensive attention. To synthesize them,
a variety of solid materials have been used, such as silica gel
[28], carbon materials [29, 30], metal oxides [31–34], zeolite
[35], MCM [36, 37] and montmorillonite [38]. However,
unsatisfactory catalyst loading, relatively higher temperature
and complicated preparation process may be inevitable.
Polyacrylonitrile fiber (PANF) is a cheap commercial
material with abundant cyano groups that could be easily
transformed into amide, carboxyl or other functional groups
[39–41]. Moreover, The PANF has excellent mechanical
strength and flexibility which is very suitable for industrial
application. In our previous reports, the PANF had been
used as a supporter successfully to synthesize effective
fiber catalysts for various organic reactions [42–44]. In this
work, l-lysine was firstly immobilized onto the PANF and
the obtained fiber catalyst could efficiently catalyze Knoev-
enagal condensation in water.
2.3 Preparation of the Fiber Catalysts
l-Lysine (2.19 g, 15 mmol), anhydrous sodium carbonate
(5.0 g), dried PANF (1.0 g) and deionized water (40 mL)
were added into a three-necked flask and the mixture was
stirred and refluxed for 13 h. Then the fiber was filtered
out and washed with water (60–70 °C) until the filtrate was
neutral. Finally, the modified fiber was dried overnight in
vacuum at 60 °C to obtain the fiber catalyst.
2.4 General Procedure for Knoevenagel
Condensation
A mixture of aldehyde (1 mmol), ethyl cyanoacetate
(1.5 mmol), fiber catalyst (0.1 mmol) and 15 mL of deion-
ized water or other solvents were put into a round-bottom
flask. Then the mixture was stirred at 45 °C for 1 h. The
fiber catalyst was filtered out and washed with ethyl ace-
tate until no organic compounds were tested by TLC in the
filtrate. The filtrates were combined, extracted three times
with ethyl acetate, dried by anhydrous sodium sulfate and
evaporated in vacuo. Finally column chromatography was
performed to purify the product with petroleum ether-ethyl
acetate as the eluent.
2 Experimental
2.1 Reagents
2.5 General Procedure for the Continual‑Flow
Process
PANF was purchased from Fushun Petrochemical Cor-
poration of China with the length of 10 cm and diameter
of 30 0.5 mm. All reagents including aldehydes, ethyl
cyanoacetate and organic solvents were analytical grade and
were not purified before being used.
The continual-flow reactor was prepared by filling PANLF
(0.5 g) into a silicone tube with 0.5 cm inner diameter
and 10 cm length. Then a solution of p-nitrobenzaldehyde
(1 mmol/50 mL) and ethyl cyanoacetate (1.5 mmol/50 mL)
in water/methanol (2:1) as solvent was pumped into the
glass reactor at a rate of 0.05 mL/min at room temperature.
After every 6 h, nitrobenzene as an internal standard was
1 3

l-Lysine Functionalized Polyacrylonitrile Fiber: A Green and Efficient Catalyst for…
Scheme 1 Preparation of the
fiber catalyst
Scheme 2 Possible mechanism of immobilizing H2N–R0 on PANF
added to the effluent reaction solution, and the yield was
determined by HPLC.
Table 1 Effect of different reflux time on the weight gain of fiber
Entry
Na2CO3 (g)
Time (h)
Weight gain (%)
1
2
3
4
5
6
7
8
5
5
5
5
5
–
–
–
10
11
12
13
14
22
44
48
4.5
8.6
12.5
15.4
18
–
3 Results and Discussion
3.1 Preparation of the Fiber Catalyst
Since the nucleophilic reactivity of two amino groups in
l-lysine are almost identical [45, 46], there are two existing
forms of PANLF (Scheme 1).
–
5
A possible mechanism of the immobilizing process is
descripted in Scheme 2. Initially, the amino in H2N–R0
attacked the C≡N in PANF to form an intermediate I which
turns to be intermediate II quickly. Then H2O was added
to C=NH in intermediate II and an intermediate III was
formed. After releasing a molecule of NH3, the H2N–R0 was
immobilized on PANF.
General condition: PANF (1.0 g), l-lysine (2.19 g, 15 mmol), H2O
(40 mL) and anhydrous sodium carbonate (5 g or 0 g) were refluxed
for appropriate time
from Table 1 is that sodium carbonate plays a significant
role in the successful preparation of PANLF. Without sodium
carbonate, we obtained no weigh gain even refluxing 44 h
(Table 1, Entry 7). When the reflux time increased to 48 h,
we only obtained a weight gain of 5% (Table 1, Entry 8), and
the mechanical strength was too poor to use. Sodium carbon-
ate was essential in this reaction because it can promote the
hydrolysis of cyano groups in PANF, thereby facilitating the
formation of amide bonds [47].
The functional degree of the modified fiber was evaluated
by the weight gain which was calculated by the equation
below:
(
)
Weight gain = [ W2 − W1 W1] × 100%
in which W1 and W2 are the weights of PANF and the
modified fiber, respectively.
The weight gain was influenced by the reflux time signifi-
cantly. With the time prolonged from 10 to 14 h, the weight
gain was increased from 4.5 to 18% (Table 1, Entries 1–5).
However, it has a limitation because the mechanical strength
of the fiber catalyst could be weakened by the increase of
the weight gain. Taking this into consideration, we chose
13 h as the optimal reflux time. Besides, what we can see
In order to determine the structure of the catalyst, we used
AAS to determine the sodium ion content in the catalyst.
The results were listed in the Table S1. As the PANF also
contains sodium, we use the PANF as a blank control. After
modification with l-lysine, the difference of sodium between
PANLF and PANF is 0.23 mmol/g. And the theoretical value
which refers to the increase of the sodium content when
1 3

P. Li et al.
PANF is functionalized with l-lysine in the presence of
sodium carbonate is 0.25 mmol/g, which is almost consist-
ent with the obtained data. Therefore, it is believed that our
fiber catalyst is present as sodium salt (Scheme 1).
4.2 SEM
The SEM images of PANF, PANLF, PANLF-1, PANLF-5
are shown in Fig. 1. Compared to PANF, PANLF becomes
thicker and its surface becomes slightly crude. The diameter
of PANLF-1 and PANLF-5 have almost not changed and are
only slightly greater than PANF, which indicates that the
fiber catalyst was less damaged and could be reused in a
relatively favorable state.
4 Characterization of the Fiber Catalyst
4.1 Elemental Analysis (EA)
The elemental analysis data of PANF, PANLF and PANLF-1
(acquired after the first use in the Knoevenagel reaction
between benzaldehyde and ethyl cyanoacetate) and PANLF-5
(acquired after catalyzing 5 runs) are listed in Table S2.
Compared to the original PANF, the carbon content and
nitrogen content in PANLF were significantly reduced while
the hydrogen content increased as expected for the reason
that l-lysine has less carbon, less nitrogen and more hydro-
gen than the original PANF.
After PANLF was used to catalyze Knoevenagel con-
densation for the first time, the carbon and hydrogen con-
tent were increased and the nitrogen content in PANLF-1
was reduced compared to PANF, which may be due to the
adsorption of the intermediate or product on the fiber cata-
lyst. Compared to PANLF-1, the carbon, hydrogen and nitro-
gen content in PANLF-5 had a slight increase since there
may be some sodium ion loss.
Fig. 2 FT-IR spectra of a PANF, b PANLF, c PANLF-1, d PANLF-5
and e l-lysine
Fig. 1 SEM images of a PANF,
b PANLF, c PANLF-1, and d
PANLF-5
1 3

l-Lysine Functionalized Polyacrylonitrile Fiber: A Green and Efficient Catalyst for…
five times, the characteristic peaks of Na(I) were weakened
which might be due to the partial loss of Na+.
4.3 FT‑IR
Samples of PANF, PANLF, PANLF-1 and PANLF-5 were
pulverized by cutting and then prepared into KBr pellets.
Their FT-IR spectra are shown in Fig. 2. The 2242 cm−1
absorption band is attributed to C≡N vibrations and the
absorption peak at 1732 cm−1 is the stretching vibration
absorption peak of C=O in the ester group, and its presence
proves the presence of the second monomer methyl acrylate
in the PANF (Fig. 2a). After grafting the l-lysine, the 2243
and 1732 cm−1 peaks are slightly weakened (Fig. 2b), which
indicated that a portion of C≡N and CO2Me have partici-
pated in the amidation and the hydrolysis. For PANLF, the
new broad absorption band near 3455 cm−1 belongs to the
stretching vibrations of the N–H groups. Another new strong
peak appears at 1673 cm−1 corresponds to the stretching
vibration of C=O in the amide. In addition, the peak at
1655 cm−1 is assigned to the stretching vibration of C=O in
l-lysine (Fig. 2e), which indicate the success immobilization
of the l-lysine. After the first run, PANLF-1 (Fig. 2c) almost
showed the same FT-IR Spectrum as PANLF (Fig. 2b) except
a slight weakening at 1655 cm−1. After fifth run, the peak
at 1655 cm−1 becomes weaker, which indicates the slight
l-lysine loss during the reaction process.
4.5 XRD
The structures of different fibers were also characterized
by XRD (Figrue 4). The peaks at 2θ = 16.9° and 29.5° in
spectrum of PANF correspond to the strong diffraction of
(100) face and weak diffraction of (110) face, respectively
(Fig. 4a). For spectrum of PANLF, the peak at 2θ = 17° is
weaker than that of PANF, which indicates that the skeleton
structures of the fiber have been slightly damaged during the
modifying process. After being used five times in Knoeve-
nagel condensation of benzaldehyde and ethyl cyanoacetate,
the peak at 2θ=17° is much weaker. However, the skeleton
structure of the fiber is reserved.
4.6 Thermogravimetric Analysis
The TG/DSC curves of PANF and PANLF are given in
Figure S1 and Fig. 5, respectively. The maximum thermal
decomposition temperature (DSCmax) of the PANF was
determined to be 309.4 °C (Figure S1). After being modified
by l-lysine, the DSCmax of PANLF was declined to 298.6 °C
(Fig. 5), which shows that the structure of the fiber was only
destroyed slightly and the PANLF has the similar thermal
stability to PANF.
4.4 X‑Ray Photoelectron Spectroscopy (XPS)
The XPS spectra of PANF and PANLF are shown in Fig. 3.
For PANF, peaks at 1073.6, 499.2 and 64.8 eV could be
assigned to the characteristic binding energies of Na(I)
(Fig. 3a) [48–50]. This demonstrates the existence of sodium
4-vinylbenzenesulfonate. After being modified by l-lysine,
peaks at 1072.6, 499.2 and 64.8 eV are enhanced distinctly
(Fig. 3b), which shows the successful immobilization of
l-lysine in its sodium salt formation. After being reused
4.7 Mechanical Experiment
A fiber catalyst with higher mechanical strength will be more
suitable for its application as fixed bed material in industry.
The mechanical properties of PANF and PANLF have been
tested and the results are shown in Table 2. Compare to the
Fig. 4 XRD spectra of a PANF, b PANLF, c PANLF-5
Fig. 3 XPS spectra of a PANF, b PANLF, c PANLF-5
1 3

P. Li et al.
strength of the PANLF-1 only slightly declined to 6.7 and
6.4 cN, respectively (Table S3, entries 3, 4). Besides, from
the SEM images of PANLF-1 and PANLF-5 (Fig. 1c, d) we
can see that the basic fiber skeleton had not been damaged,
which also shows the well physical and chemical stability of
the fiber catalyst during the catalytic process.
5 Application of l‑lysine Functionalized
PANF in Knoevenagel Condensation
5.1 Catalytic Activity
We examined the Knoevenagel condensation of benzalde-
hyde with ethyl cyanoacetate using the l-lysine functioned
PANF as the catalyst. Primarily, we conducted two blank
experiments. In one experiment, only water was added as
the solvent and no catalyst was used; another involved PANF
as the catalyst. As expected, the yield of the former was
only 9% (Fig. 6), and the latter was 12% (Fig. 6). When
l-lysine was used as the catalyst, we obtained a yield of 33%
(Fig. 6). However, our fiber catalyst could obtain a yield of
63% (Fig. 6).
Fig. 5 TG/DSC curves of PANF
original PANF (its break strength is 11.0 cN), break strength
of the PANLF is declined to 6.8 cN (Table S3, entries 1,
2). That was caused by a certain extent of destroy to fiber
structure at high temperature. After being used once and five
times, because of the mild reaction conditions the breaking
Table 2 Optimization of the Knoevenagel condensation
Entry
Solvent
Temp (°C)
Reactant ratio
Yielda (%)
1
2
3
4
5
6
7
8
Water
Water
Water
Water
Water
Water
Water
Methanol
Acetonitrile
1,4-Dioxane
Ethanol
Ethyl acetate
Toluene
Water
Water
Water
Water
0
25
35
45
55
75
95
45
45
45
45
45
45
45
45
45
45
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
32
64
68
77
77
77
71
16
0
0
5
0
0
9
10
11
12
13
14
15
16
17
1:1
1:1.25
1:1.5
1:1.75
1:2
82
88
90
88
Reaction conditions: benzaldehyde (1 mmol), ethyl cyanoacetate, PANLF (10 mol%) in each solvent (15 mL) for 1 h
aObtained yield by HPLC
1 3

l-Lysine Functionalized Polyacrylonitrile Fiber: A Green and Efficient Catalyst for…
70
60
50
40
30
20
10
0
were used to test the effect of solvents on Knoevenagel con-
densation when PANLF acted as the catalyst. As we can see
from the Table 2 (Entries 4, 8–13), the polarities of solvents
have significant impacts on the reaction yields. We could
obtain a higher yield of 77% if a highly polar solvent like
water was used (Table 2, Entry 4). When the polarity of the
solvent was reduced and methanol was used, a low yield
of 16% was got (Table 2, Entry 8). And with the use of
less polar solvents, such as acetonitrile, 1,4-dioxane, etha-
nol, ethyl acetate and toluene, the reaction could not occur
effectively (Table 2, Entries 9–13). This phenomenon can be
explained by the relative polarity of the surface of PANLF,
reactants, and solvent. A highly polar micro-environment
promotion process has been proposed to make this phenome-
non easy to understand (Figure S2). The PANLF with highly
polar surface micro-environment can’t be infiltrated well and
constrictive in weak polar solvents, the low polar organic
reactants are well dissolved in weak polar solvents (Figure
S2a), and hardly access to the active catalytic sits in the high
polar micro-environment of the fiber catalyst. The PANLF
was infiltrated well in high polar solvent like water, and it’s
surface micro-environment was unfolded (Figure S2b). On
the other hand, low polar reactants can hardly dissolve in
water and are compelled to adhere on the surface of fiber
catalyst. In this way, the reactants can access to the micro-
environment of the PANLF and be catalyzed by active cata-
lytic sites to generate the product. It is specific and favorable
highlight for PANLF with high selectivity to strong polar
solvents (especially for water, which is an environmentally
friendly solvent).
-
PANF
L-lysine
PANLF
Catalyst
Fig. 6 The catalytic activities of PANF, l-lysine and PANLF in cata-
lyzing Knoevenagel condensation
5.2 Optimization of the Knoeveagel Condensation
Conditions
We chose the condensation of benzaldehyde and ethyl
cyanoacetate as a model reaction to optimize the reaction
conditions. First, we studied the influence of reaction time.
When the time was increased from 5 min to 2 h, the yield
was improved in 1 h and nearly stabilized from 1 to 2 h
(Fig. 7a). Then, the catalyst amount was taken into con-
sideration. From 2.5 to 10 mol%, the yield was increased.
However, the yield was nearly constant from 10 to 20 mol%
(Fig. 7b).
What’s more, in order to increase the yield, we tried to
change the proportion of reactants, namely increase the
amount of ethyl cyanoacetate. When the ratio of benzal-
dehyde to ethyl cyanoacetate was 1:1.25, the yield was
increased to 82% (Table 2, Entry 14). A higher yield of 88%
Additionally, we also investigated reaction temperature.
From 0 to 45 °C, the yield continued to rise and it was
nearly unchanged when the temperature was raised from 45
to 95 °C (Table 2, Entries 1–7). Besides, different solvents
Fig. 7 Effects of a reaction time and b catalyst amount on the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate
1 3

P. Li et al.
Scheme 3 Possible mechanism
for the Knoevenagel reaction
catalyzed by PANLF
was obtained when the ratio reached 1:1.5 (Table 2, Entry
15). The yield was nearly constant even the amount of ethyl
cyanoacetate was improved to 1.75 or 2 equiv. (Table 2,
Entries 16–17) In a word, the acquired optimum reaction
conditions were: aldehyde to ethyl cyanoacetate 1:1.5,
15 mL of water, 10 mol% of catalyst dosage, 45 °C, 1 h.
carbon atom, increase its electrophilicity and increase the
reactivity of the aldehyde. Therefore, when the aromatic
ring contains strong electron-withdrawing groups such as
nitro, trifluoromethyl, the yield can reach more than 95%
(Table 3, Entries 2–4). For o-nitrobenzaldehyde, it was
even up to 99%. o-Chloro, o-bromo and p-chlorobenzal-
dehyde also obtained high yields (Table 3, Entries 5–7).
In contrast, when an electron-donating group was attached
to the aromatic ring, the activity of the aldehyde carbonyl
was weakened and the yield decreased (Table 3, Entries
8–10). However, o-methoxybenzaldehyde still gave 85%
yield (Table 3, Entry 11). For some aldehydes with hetero-
cycles, such as furan formaldehyde and thiophene formal-
dehyde, PANLF also has a good catalytic effect on them,
providing 99% and 91% yields, respectively (Table 3,
Entries 12–13). Aliphatic aldehyde like 3-cyclohexenyl-
formaldehyde could also give a yield of 85% (Table 3,
Entry 14). The superior results suggest that the PANLF is
a highly active catalyst for the Knoevenagel condensation
in water. Because of the high activity of the malononitrile,
the reaction of malononitrile with different aldehydes can
be finished at RT with yields of 97–99%. However, the
reaction of benzaldehyde with acetylacetone only obtained
a yield of 36% due to the low activity of the acetylacetone.
After refluxing 6 h, no product was detected for the ethyl
2-cyanopropanoate, which indicates that steric hindrance
has great influence on the reactivity of the substrate.
5.3 Possible Mechanism of the Knoevenagel
Reaction
A possible mechanism of the Knoevenagel reaction cata-
lyzed by PANLF is shown in Scheme 3. Initially, the PANLF
removes an α-proton from ethyl cyanoacetate to generate a
carbanion A− and the lysine moiety on PANLF exists as an
inner salt. Then carbanion A− attacks the carbonyl carbon
of aldehyde to form an intermediate C which then captures
a proton from PANLF to produce an intermediate D. After
an intramolecular dehydration of D, the α,β-unsaturated car-
bonyl compounds were released and the fiber catalyst was
recovered to promote the next reaction cycle.
5.4 Knoevenagel Condensation of Various
Aldehydes with Ethyl Cyanoacetate
Under the optimized conditions, the Knoevenagel con-
densation of different aldehydes with ethyl cyanoacetate
was studied. The results of the experiments are shown in
the Table 3. When the aromatic ring does not have any
substituent (benzaldehyde), the yield was 81% (Table 3,
Entry 1). The electron withdrawing groups in aromatic
rings can reduce the electronic cloud density of carbonyl
1 3

l-Lysine Functionalized Polyacrylonitrile Fiber: A Green and Efficient Catalyst for…
Table 3 Reaction of aldehydes with methylele active compounds
Entry
1
1
2
Product
Yield (%)a
3a
81
2
3b
99
3
3c
3d
96
95
4
5
3e
93
6
3f
81
7
3g
88
8
3h
66
9
3i
50
10
3j
76
85
11
3k
12
13
14
3l
3m
3n
99
91
85
15b
16b
17b
3o
3p
3q
98
98
97
18b
3r
3s
99
36
19c
20d
3t
0
1 3

P. Li et al.
Table 3 (continued)
Reactions were carried out with benzaldehyde (1 mmol), ethyl cyanoacetate (1.5 mmol), 15 mL water in the presence of 10 mol% PANLF under
45 °C in 1 h
aIsolate yield by column chromatography
bAt RT for 1 h
cAt 80 °C for 6 h
dRefluxed for 6 h
5.5 Reusability of the Catalyst PANLF
PANLF
The recyclability of the fiber catalyst PANLF was tested
by catalyzing the Knoevenagel condensation of benzalde-
hyde (1 mmol) and ethyl cyanoacetate (1.5 mmol) in water
(15 mL) at 45 °C. After the completion of each cycle, the
fiber catalyst was taken out and then washed with ethyl
acetate to remove the adsorbed reagents. The recycled
catalyst was dried and reused directly in the next cycle
without any additional treatment. The obtained yields of
the first five cycles were 81, 80, 73, 62 and 60%, respec-
tively, which indicated that the PANLF could be reused at
least five times.
pump
5.6 Comparison of PANLF with Other Catalysts
Fig. 8 Schematic diagram of the continual-flow reaction system
Hexamethylenediamine aminated fiber (PANHF) and
l-Lysine functionalized graphene oxide (GOL) were pre-
pared similarly to catalyze the Knoevenagel condensation,
respectively. The reaction conditions were: benzaldehyde
(1 mmol), ethyl cyanoacetate (1 mmol), 15 mL water
10 mol % PANLF, 45 °C and 1 h. We obtained yields of 55%
(PANHF) and 39% (GOL), respectively, which were lower
than that of PANLF (77%). Obviously, our catalyst PANLF
has better catalytic effect.
and the results are shown in Figure S3. After 48 h, the yield
of the Knoevenagel condensation reaction was basically
unchanged, which indicates high potential application of
this fiber catalyst in industry.
6 Conclusions
In summary, l-lysine functionalized polyacrylonitrile fiber
has been designed and synthesized. As a heterogeneous cata-
lyst, it was used to catalyze the Knoevenagel condensation of
various aldehydes and ethyl cyanoacetate highly efficiently
to afford the corresponding α, β-unsaturated cyanoesters.
The fiber catalyst has advantages of easy preparation, high
functionality degree, strong mechanical strength and thermal
stability, etc. After optimization, with water as the solvent,
a high weight gain, good strength and high catalytic activity
fiber catalyst was obtained. When it was used to catalyze the
Knoevenagel condensation, the catalyst had a high selectiv-
ity with water as the solvent, which met the requirements
of green chemistry. A series of aldehydes reacted smoothly
with cyanoacetate under the catalysis of PANLF to afford the
corresponding products in moderate to excellent yields from
50 to 99%. The catalyst could be easily separated from the
reaction system and reused at least five times without any
other treatment. It also performs well in scaled-up and flow
5.7 Scaled‑Up Experiment of the Knoevenagel
Condensation Catalyzed by PANLF
Under the optimized conditions, the Knoevenagel condensa-
tion was scaled up by 50 times and the reaction can smoothly
undergo with a yield of 88% in 1 h, which indicated that the
PANLF had a potential application in industry.
5.8 Flow Chemistry of the Knoevenagel Reaction
Catalyzed by PANLF
Furthermore, in order to study the potential application of
the fiber catalyst in industry. PANLF was employed in a
simple continual-flow reactor by filling PANLF into a sili-
cone tube and the Knoevenagel condensation reaction was
used as a template reaction to test the continual-flow reac-
tion (Fig. 8). A solution of p-nitrobenzaldehyde and ethyl
cyanoacetate was pumped into the silicone reactor at 45 °C
1 3

l-Lysine Functionalized Polyacrylonitrile Fiber: A Green and Efficient Catalyst for…
24. Radi M, Bernardo V, Bechi B, Castagnolo D, Pagano M, Botta M
(2009) Tetrahedron Lett 50:6572–6575
chemistry experiments, which have potential applications
in industry.
25. Lai YF, Zheng H, Chai SJ, Zhang PF, Chen XZ (2010) Green
Chem 12:1917–1918
Acknowledgements We gratefully acknowledge the support from the
National Natural Science Foundation of China (No. 21572156, No.
21777111 and No. 21306133).
26. Xu DZ, Liu Y, Shi S, Wang Y (2010) Green Chem 12:514–517
27. Ying A, Ni Y, Xu S, Liu S, Yang J, Li R (2014) Ind Eng Chem
Res 53:5678–5682
28. Trilla M, Pleixats R, Man MWC, Bied C (2009) Green Chem
11:1815–1820
29. Sun Y, Cao C, Huang P, Yang S, Song W (2015) RSC Adv
5:86082–86087
References
30. Rana S, Jonnalagadda SB (2017) Catal Commun 92:31–34
31. Zhang Y, Xia C (2009) Appl Cat A 366:141–147
32. Ying A, Qiu F, Wu C, Hu H, Yang J (2014) RSC Adv
4:33175–33183
1. Mukherjee S, Yang JW, Hoffmann S, List B (2007) Chem Rev
107:5471–5569
2. Ward DE, Jheengut V, Akinnusi OT (2005) Org Lett 7:1181–1184
3. Hajos ZG, Parrish DR (1974) J Org Chem 39:1615–1621
4. Pizzarello S, Weber AL (2004) Science 303:1151
5. Davies SG, Sheppard RL, Smith AD, Thomson JE (2005) Chem
Commun 30:3802–3804
33. Ying A, Wang L, Qiu F, Hu H, Yang J (2015) C R Chim
18:223–232
34. Parida KM, Mallick S, Sahoo PC, Rana SK (2010) Appl Catal A
381:226–232
35. Nguyen LTL, Le KKA, Truong HX, Phan NTS (2012) Catal Sci
Technol 2:521–528
6. Ibrahem I, Zou W, Engqvist M, Xu Y, Cordova A (2005) Chem
Eur J 11:7024–7029
36. Rana S, Maddila S, Pagadala R, Jonnalagadda SB (2015) J Porous
Mater 22:353–360
7. Cheng L, Wu X, Lu Y (2007) Org Biomol Chem 5:1018–1020
8. List B, Pojarliev P, Martin HJ (2001) Org Lett 3:2423–2425
9. List B (2004) Acc Chem Res 37:548–557
37. Dong X, Hui Y, Xie S, Zhang P, Zhou G, Xie Z (2013) RSC Adv
3:3222–3226
10. Venkatanarayana M, Dubey PK (2014) J Heterocycl Chem
51:877–882
38. Varadwaj GB, Rana S, Parida KM (2013) Dalton Trans
42:5122–5129
11. Tan H, Chen X, Chen H, Liu H, Qiu S (2015) Eur J Org Chem
2015:4956–4963
39. Liu RX, Zhang BW, Tang HX (1999) React Funct Polym 39:71–81
40. Moroi G (2004) Polym Degrad Stab 84:207–214
41. Vatutsina OM, Soldatov VS, Sokolova VI, Johann J, Bissen M,
Weissenbacher A (2007) React Funct Polym 67:184–201
42. Li G, Xiao J, Zhang W (2012) Green Chem 14:2234–2242
43. Zhang W, Ma L, Yuan L, Xu C, Li G, Tao M (2012) Synthesis
45:45–52
12. Xu LW, Lu Y (2008) Org Biomol Chem 6:2047–2453
13. Choudary BM, Lakshmi Kantam M, Neeraja V, Koteswara Rao
K, Figueras F, Delmotte L (2001) Green Chem 3:257–260
14. Dandia A, Parewa V, Jain AK, Rathore KS (2011) Green Chem
13:2135–2145
15. Hong BC, Dange NS, Ding CF, Liao JH (2012) Org Lett
14:448–451
44. Li P, Du J, Xie Y, Tao M, Zhang WQ (2016) ACS Sustain Chem
Eng 4:1139–1147
16. Clemens JJ, Asgian JL, Busch BB, Coon T, Ernst J, Kaljevic L,
Krenitsky PJ, Neubert TD, Schweiger EJ, Termin A, Stamos D
(2013) J Org Chem 78:780–785
45. Gong B, Wang Y, Sun Y (2002) Chin J Chem 20:63–67
46. Su J, Sheng X, Li S, Sun T, Liu G, Hao A (2012) Org Biomol
Chem 10:9319–9324
17. Palao E, Agarrabeitia AR, Banuelos-Prieto J, Lopez TA, Lopez-
Arbeloa I, Armesto D, Ortiz MJ (2013) Org Lett 15:4454–4457
18. Srinivas V, Koketsu M (2013) J Org Chem 78:11612–11617
19. Xu DZ, Yu YQ, Song LL, Yang C (2017) Synthesis 49:1641–1647
20. Mase N, Horibe T (2013) Org Lett 15:1854–1857
21. Zhang Y, Chen C, Wu G, Guan N, Li L, Zhang J (2014) Chem
Commun 50:4305–4308
47. Su J, Su F, Ma M, Li S, Xing P, Hao A (2014) Synth Commun
44:1111–1121
48. Lesiak B, Zemek J, Jiricek P, Gedeon O, Jozwik A (2008) Surf
Interface Anal 40:1507–1515
49. Mekki A, Khattak GK, Gondal MA (2011) Int J Surf Sci Eng
5:434–445
50. Nesbitt HW, Bancroft GM, Ho R (2017) Surf Interface Anal
49:1298–1308
22. List B, Castello C (2001) Synlett 2001:1687–1689
23. Ogiwara Y, Takahashi K, Kitazawa T, Sakai N (2015) J Org Chem
80:3101–3110
Affiliations
Pengyu Li1 · Yuanyuan Liu1 · Ning Ma1,2 · Wenqin Zhang1,2
1
* Ning Ma
Department of Chemistry, School of Science, Tianjin
University, Tianjin 300072, China
mntju@tju.edu.cn
2
* Wenqin Zhang
Collaborative Innovation Center of Chemical Science
and Engineering (Tianjin), Tianjin 300072, China
zhangwenqin@tju.edu.cn
1 3