Solvent Selective Polyacrylonitrile Fiber as a Recyclable Catalyst for the Knoevenagel-Michael Reaction in Water

Article abstract of DOI:10.1007/s10562-021-03593-w

Heterogeneous catalysis is an important branch of sustainable chemistry. In this work, a series of polyacrylonitrile fiber (PANF) fiber catalysts (PANF-E, PANF-EDs, and PANF-D) with different catalytic micro-environments are developed and characterized by

Full text of DOI:10.1007/s10562-021-03593-w

Catalysis Letters
https://doi.org/10.1007/s10562-021-03593-w
Solvent Selective Polyacrylonitrile Fiber as a Recyclable Catalyst
for the Knoevenagel‑Michael Reaction in Water
Pengyu Li1,2 · Yu Yang · Xueling Wu · Jiali Lu · Lingrui Hu · Weihua Chen · Wenqin Zhang
2
2
2
2
1
3
Received: 29 December 2020 / Accepted: 7 March 2021
©
The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
Heterogeneous catalysis is an important branch of sustainable chemistry. In this work, a series of polyacrylonitrile ꢀber
PANF) ꢀber catalysts (PANF-E, PANF-EDs, and PANF-D) with diꢁerent catalytic micro-environments are developed and
(
characterized by elemental analysis, Fourier-transfer infrared spectroscopy, x-ray diꢁraction, thermogravimetric/diꢁerential
scanning calorimetry analysis, scanning electron microscopy, and mechanical strength measurements to demonstrate the
successful immobilization of the diꢁerent amines as well as evaluate the physical strength and thermal stability of the ꢀber
catalysts at diꢁerent stages. The catalytic activities of the ꢀber catalysts are tested by one-pot three-component Knoevenagel-
Michael reaction to the synthesis of substituted 2-amino-4H-chromenes in which the inꢂuences of surface polarities, kinds
and proportions of functional groups on the ꢀber catalysts activities were investigated. Among the prepared catalysts, the
PANF-D with higher density of tertiary amino group and hydrophilic micro-environment exhibited the best catalytic activity
to eꢃciently catalyze the three-component reaction in water with excellent substrate suitability (92-98%). In addition, the
catalyst can be easily separated from the catalytic system and conveniently reused at least ten times. Moreover, the PANF-
D performs well in scaled-up experiment in a simple ꢀxed-bed reactor with a yield of 97% which allows it to have great
potential for further cleaner industrial applications.
Graphic Abstract
Keywords Green chemistry · Heterogeneous catalysis · Polyacrylonitrile ꢀber · Fixed-bed reactor
*
*
Pengyu Li
pyli@zut.edu.cn
Weihua Chen
chenweih@zzu.edu.cn
Extended author information available on the last page of the article
Vol.:(0
1
123456789)
3

P. Li et al.
1
Introduction
contaminants in water [19–21], synthesize phase change
[
22, 23], antimicrobial [24], and pH-sensitive materials
Chemistry has provided more than one hundred million
artificial chemical substances that are in widespread use
to maintain or even promote the living standards of our
society [1]. However, environmental problems like ozone
hole, river eutrophication, acid rain, heavy metal con-
tamination, and so on have also been caused by chemi-
cals over the last 50 years. The crisis also prompts peo-
ple to explore new energy sources and environmentally
friendly chemicals and chemical processes [2]. The “10
Key Green Chemistry Research Areas” [3] in Table 1 and
[18, 25], etc. However, the application of PANF based
heterogeneous catalysis in organic synthesis has been
very rare. In our previous works, PANF has well modi-
fied to prepare heterogeneous base [26, 27], acid [28,
29], phase transfer [30], metal [31], bifunctional catalysts
[32], etc. After the in-depth research, it was found that
the immobilization of the PANF is a high-density, deep-
seated modification, which constructs a specific micro-
environment by immobilized functional groups and poly-
mer block in the fiber catalyst. The micro-environment
influences the catalytic activity of the fiber catalyst to
a great extent by adsorption of organic substrate and
increasing their contact area. Besides, PANF can be
knitted into different shapes to suit for the application
of fixed bed in industry, which indicate that PANF is an
excellent support for heterogeneous catalyst.
“
12 Green Chemistry Principles” [4, 5] show the main
research areas of green chemistry in recent years and, to
some extent, indicate the direction of green chemistry
in the future. Among them, the 10 key catalyst immo-
bilization without significant loss in kinetics principles
is still an important direction for the research of green
chemistry.
In recent years, 2-amino-4H-chromenes have amused
increasing interest due to their pervasive biological
activities such as anticancer, antibacterial, anticoagu-
lant, antidiabetic and antianaphylaxis activities [33–35].
A number of catalysts have been developed for the syn-
thesis of 2-amino-4H-chromenes, including ionic liquids
[36], metal bromide [37, 38], metal complex [39], amines
[40], piperazine-functionalized nickel ferrite nanoparti-
cle [41], Schiff base complex functionalized-magnetic
nanoparticles [42]. Despite their potential utility, those
methods have limitations, like long reaction time, haz-
ardous solvents, tedious handle approaches, and unsat-
isfactory yields, which restrict their further practical
applications. Therefore, it is still of extreme necessity to
explore simpler, greener, and efficient catalytic systems
for the synthesis of 2-amino-4H-chromenes.
Nowadays, varieties of heterogeneous catalysts had
been developed to realize green chemistry and multifari-
ous materials like zeolite [6, 7], metal oxides [8, 9], poly-
mer [10, 11], silica [12], biochar [13, 14] and so on had
served as supports. Due to the unique microenvironment,
polymer-based heterogeneous catalysts often show supe-
rior catalytic activity, excellent regioselectivity, and many
other unique properties [15–17].
Polyacrylonitrile fiber (PANF) with advantages of
light density, high strength, and low cost, is a common
and widely used material in the industry and our daily
life. Moreover, the abundant cyano groups in PANF can
be easily transformed into aminocarbonyl, carboxyl, and
other functional groups [18]. To date, various modi-
fied PANF have been prepared and applied to remove
Table 1 Key green chemistry research areas
Rank
Research area
1
2
3
4
5
6
7
8
9
Development of eꢁective and versatile methodology utilizing cheap/sustainable metals.*
General methods for catalytic/sustainable (direct) amide or peptide formation.
Aliphatic and aromatic C–H activation, using green oxidants and giving predictable site selectivities.
Amide reductions avoiding LiAlH and diborane.
4
Direct substitution of alcohols.
Catalyst immobilization without signiꢀcant loss in kinetics.*
Asymmetric hydrogenation of unfunctionalized oleꢀns/enamines/imines.
Improved ꢂuorination/triꢂuoromethoxylation.
‡
Wittig chemistry without Ph PO.
3
1
0
Alternatives for oxidations, C–O or C–N redox processes.
*
New research area
‡
New research area, but on the 2006 long list
1
3

Solvent Selective Polyacrylonitrile Fiber as a Recyclable Catalyst for the Knoevenagel-Michael…
In this work, based on our previous researches on pol-
yacrylonitrile fiber catalysts, two 1,4-diazabicyclo[2.2.2]
octane (DABCO) derivate functionalized polyacryloni-
trile fiber catalysts were developed for the synthesis of
substituted 2-amino-4H-chromenes by catalyzing one-pot
Knoevenagel-Michael multi-component tandem reac-
tions. The influences of surface polarities, kinds, and
proportions of functional groups on the fiber catalysts
activities were detailedly investigated. Furthermore,
scaled-up experiment in simple fixed-bed reactor dem-
onstrates the industrial application potential of the fiber
catalyst.
round-bottom flask and the mixture was stirred and
refluxed at 105 °C for the appropriate time, then the fiber
was filtered out and washed repeatedly with deionized
water (60–70 °C) until the pH of the filtrate was neutral.
After that, the modified fiber was dried overnight under
vacuum at 60 °C to give the PANF-E.
2.1.3 Synthesis of the Fiber Catalysts PANF‑ED and PANF‑D
A mixture of dried PANF-E (0.5 g), DABCO-COOMe (15
eq. of the amino group in PANF-E), Na CO (0.5 g), and
2
3
water (30 mL) was added into a 100 mL round-bottom ꢂask
and the mixture was stirred and reꢂuxed at 105 °C for the
appropriate time, then the ꢀber was ꢀltered out and washed
repeatedly with deionized water (60–70 °C) until the pH of
the ꢀltrate was neutral. After that, the modiꢀed ꢀber was
dried overnight under vacuum at 60 °C to give the PANF-
ED or PANF-D.
2
Experimental Section
2.1 Synthesis of the PANF Catalysts
2.1.1 Synthesis of the DABCO Derivate (DABCO‑COOMe)
2
.2 General Procedure for the PANF‑D Catalyzed
One‑Pot Knoevenagel‑Michael Multicomponent
Reaction
DABCO (0.1 mol) was dissolved into 200 mL EtOAc,
.08 mol methyl chloroacetate was dropped into the solu-
0
tion above, and the mixture was stirred at RT for 24 h.
After the reaction was completed, the mixture was filtered
and the white solid was washed by anhydrous ether (30
mL × 3). Then, the white solid was dried overnight at
A mixture of aldehyde (1.0 mmol), malononitrile
(1.0mmol), 1-naphthol, or 5,5-dimethyl-1,3-cyclohexane-
dione (1.0 mmol), H O (20 mL), and PANF-D (15 mol%,
2
6
0 °C to give the DABCO derivate of DABCO-COOMe
based on DABCO group) were stirred and refluxed for
the appropriate time. Then, the fiber catalyst was filtered
out and washed with EtOAc (3 × 10 mL). The organic
phase was combined and concentrated, and the crude
product was purified by recrystallization from EtOH to
afford the pure product. For the recyclability test, the
recycled catalyst was dried and used directly for the next
cycle.
(
Scheme 1).
2.1.2 Synthesis of the Ethylenediamine Functionalized
Polyacrylonitrile Fiber (PANF‑E)
A mixture of dried PANF (1.0 g), ethylenediamine
10 mL) and water (20 mL) was added into a 100 mL
(
Scheme 1 Synthesis of diꢁerent ꢀber catalysts
1
3

P. Li et al.
Table 3 Elemental analysis data of diꢁerent ꢀbers
3
Results and Discussion
Entry
Fiber
C (%)
H (%)
N (%)
S (%)
3
.1 Syntheses of Fiber Catalysts
1
2
3
4
5
PANF
70.75
62.93
63.22
63.29
65.63
5.94
6.32
6.14
6.17
6.28
26.12
22.87
20.36
20.30
18.74
0.09
0.08
0.06
0.07
0.08
PANF-E
PANF-D
PANF-D-1
PANF-D-10
The PANF-E was synthesized by directly refluxing the
mixture of PANF, water, and ethylenediamine. Then, the
PANF-ED was easily prepared by the reaction of PANF-E
with DABCO-COOMe for the appropriate time. When the
amino groups are consumed completely and the PANF-D
is obtained (Scheme 1). The functional degrees of the ꢀber
catalysts are determined by weight gain and acid exchange
capacity, and the results are listed in Table 2. Compared with
PANF-E, the preparation of PANF-ED is a time-consum-
ing process caused by the low concentration of DABCO-
COOMe (Table 2, entries 2-5).
Compared to those of PANF-E, EA data of PANF-D has
similar changes, which demonstrate the successful immobi-
lization of DABCO (Table 3, entry 3). After being used one
time, the EA data of PANF-D-1 almost the same as those
for PANF-D (Table 3, entry 4). When the PANF-D was used
ten times, the EA data of PANF-D-10 only slightly changed
which may be caused by the absorption of some intermedi-
ates or products during the reaction process.
In this work, the properties of diꢁerent ꢀber catalysts
were determined by water uptake content, which is a key
factor in determining the hydrophilic properties of the ꢀber
surface [43]. The PANF has low water uptake content of
1
0.9% for the dense surface (Table S1, entry 1). Because of
3.3 Fourier‑Transform Infrared Spectroscopy (FTIR)
the primary, tertiary amines, and quaternary ammonium are
hydrophilic groups, the water uptake contents of PANF-E,
PANF-ED, and PANF-D rise to 32.6%, 54.7%, and 66.9%
Samples of diꢁerent ꢀbers are pulverized and mixed into KBr
pellets and the spectra are shown in Fig. 1. For the FTIR spec-
−
1
(
Table S1, entries 2–4), respectively as expected.
trum of PANF (Fig. 1a), the peak at 2942 cm is assigned
to the C–H stretching vibration of methyl and methylene in
−
1
3.2 Elemental Analyses (EA)
PANF. The 2245 cm absorption band is attributed to the
stretching vibration of –C≡N. The adsorption peak at 1733
−1
The elemental analysis data of PANF, PANF-E, PANF-D,
PANF-D-1 (the PANF-D have been used one time) and
PANF-D-10 (the PANF-D have been used ten times) were
tested three times and the average values were calculated
cm indicates the existence of methyl acrylate units. After
−
1
modiꢀed by ethylenediamine (Fig. 1b), the peaks at 2245 cm
−
1
and 1733 cm are reduced in size as expected for the reac-
tion of -CN and -COOMe groups with -NH in ethylenedi-
2
−1
(
Table 3). For the ethylenediamine has lower carbon content
39.97%), higher hydrogen and nitrogen contents (13.42%
amine. The broad peak that appears at 3404 cm is assigned
(
to N–H in the –CONH– and –NH moieties formed during
2
and 46.61%, respectively), the carbon content of PANF-E is
decreased, and the hydrogen content is increased as expected
(
Table 3, entry 2). The nitrogen content is decreased to
2
2.87% may be caused by the hydrolysis of -CN to -CONH .
2
Table 2 Preparation of diꢁerent ꢀber catalysts
Entry Raw ꢀber Time (h) WG (%)a F (mmol/g) Catalyst
b
1
2
3
4
5
.
PANF
3
7
8
9
8.21
6.72
9.92
13.37
16.79
1.06
0.36
0.53
0.71
0.99
PANF-E
c
c
c
c
PANF-E
PANF-E
PANF-E
PANF-ED2/1
PANF-ED1/1
PANF-ED1/2
PANF-D
PANF-E 10
a
Weight gain = [(W2–W1)/W1] × 100%, in which W1 and W2 are
the weights of original and modiꢀed ꢀbers, respectively
Determined by titration
b
Fig. 1 FTIR spectra of a PANF, b PANF-E, c PANF-D, d PANF-
c
Determined by weight gain
D-10
1
3

Solvent Selective Polyacrylonitrile Fiber as a Recyclable Catalyst for the Knoevenagel-Michael…
the aminolysis of the cyano and ester groups with ethylen-
−1
ediamine. In addition, two new bonds at 1639 cm and 1563
−1
cm are superpositions of C=O, C–N stretching vibration and
N–H bending vibration in –CONH– and –CONH . For the
2
spectrum of PANF-D (Fig. 1c), the peaks at 3404, 1639, and
−
1
1
563 cm are all enhanced which indicates the successful
grafting of DABCO on PANF. After used ten times, the spec-
trum of PANF-D-10 only has minor changes, which suggests
the excellent recoverability of the ꢀber catalyst (Fig. 1d).
3.4 Scanning Electron Microscopy (SEM)
The SEM images of PANF, PANF-E, PANF-D, and
PANF-D-10 are illustrated in Fig. 2. Compare to the
SEM image of PANF, the surface of PANF-E and PANF-
D become slightly rougher, and their diameters increase
remarkably, which caused by the swelling effect dur-
ing the modification processes (Fig. 2a–c). After being
used ten times (Fig. 2d), the fiber catalyst PANF-D was
swelled further and its surface becomes rougher. At last,
the basic structure of the PANF-D is still retained with
considerable mechanical strength (Table 3) to meet the
need for further applications (Fig. 3).
Fig. 2 SEM images of a PANF, b PANF-E, c PANF-D, d PANF-
D-10
3.5 X‑Ray Diꢀraction (XRD)
XRD spectra of PANF, PANF-D, and PANF-D-10 are
exhibited in Fig. 4. For PANF (Fig. 3a), two peaks at
2
θ = 16.9°, 29.4° are corresponded to strong diffrac-
tion of (100) face and weak diffraction of (110) face,
respectively. The hexagonal lattice formed by parallel
close packing of the molecule rods caused the peak at 2θ
=
16.9°, which indicates that PANF adopts a stiff rod-
like conformation due to the intermolecular repulsion
of the nitrile dipoles. After being modified by DABCO-
COOMe (Fig. 3b), the reflection peak at 2θ = 16.9° was
decreased, which demonstrates that part of the crystalline
Fig. 3 XRD spectra of a PANF, b PANF-D, c PANF-D-10
Fig. 4 TG/DSC curves of a PANF-D, b PANF-D-10
1
3

P. Li et al.
Table 4 Mechanical strength of diꢁerent ꢀbers
of the PANF-D is 309.4 °C [32]. After a two-step modi-
ꢀ
cation, the DSCmax of PANF-D is decreased to 300.2
Entry
Fiber
BS (cN)
RBS (%)
°
C (Fig. 4a), which indicates little structural damage hap-
1
2
3
4
PANF
11.0
10.2
9.5
100
93
86
pening on the ꢀber catalyst. When the ꢀber catalyst was
used ten times, the PANF-D-10 was further swelled and
its DSCmax was declined to 281.2 °C (Fig. 4b), which can
still meet the thermal stability requirement for the one-pot
Knoevenagel-Michael multi-component tandem reaction.
PANF-E
PANF-D
PANF-D-10
8.9
81
phase of the fibers has been damaged during the modified
processes. When the fiber catalyst was used ten times
3.7 Mechanical Strength
(
Fig. 3c), the peak at 2θ = 16.9° was further weakened
for the continuous breakage of bonds in the inner crystal
region. However, the basic structure of the PANF-D-10
is still retained (Fig. 2d).
Appropriate breaking strength (BS) is an important param-
eter for the ꢀber catalyst as ꢀxed-bed material in the indus-
try. The mechanical strengths of diꢁerent ꢀbers are tested
and the results are listed in Table 4. The PANF has a BS
of 11.0 cN and its retention of breaking strength (RBS)
is 100% (Table 4, entry 1). After modiꢀed by ethylenedi-
amine, the breaking strength of PANF-E decreases to 10.2
cN with 93% RBS (Table 4, entry 2), and the mechanical
strength of PANF-D is further declined to 9.5 cN BS and
86% RBS (Table 4, entry 3). When the ꢀber catalyst was
3.6 Thermal Stability Analysis
Thermal stabilities of PANF-D and PANF-D-10 are tested
by TG/DSC and the results are presented in Fig. 4. The
DSCmax (maximum thermal decomposition temperature)
Table 5 Optimization of the
three-component condensation
a
reaction reaction
o
Time (min) Yield (%)b TOF (h-1)
Entry
Fiber
PANF
Dosage (mol%)
T ( C)
80
1
2
----
15
60
60
20
46
----
3.1
PANF-E
80
8
8
8
0
0
0
3
4
5
PANF-ED1/2
PANF-ED1/2
PANF-ED1/2
c
c
c
15
15
15
60
60
60
61
69
78
4.1
4.6
5.2
6
7
PANF-D
PANF-D
15
15
80
90
60
60
84
93
5.6
6.2
8
9
PANF-D
PANF-D
PANF-D
PANF-D
15
15
10
20
refluxed
refluxed
refluxed
refluxed
60
80
60
60
98
98
81
98
6.5
4.9
8.1
4.9
1
0
11
a
Reaction conditions: 4-chlorobenzaldehyde (1.0 mmol), malononitrile (1 mmol) and 1-naphthol (1
mmol) in the presence of PANF-D in water (10 mL) are reꢂuxed for appropriate time
b
Isolated yield after recrystallization with ethanol
The subscripted number indicates the proportion of primary amine to DABCO groups
c
1
3

Solvent Selective Polyacrylonitrile Fiber as a Recyclable Catalyst for the Knoevenagel-Michael…
a
b
Fig. 5 Knoevenagel-Michael at a high polar solvent, b low polar solvent.
used ten times (Table 4, entry 4), the PANF-D-10 still has
of catalyst dosage, reꢂuxed 60 min at 80 °C in water were
selected as the optimal reaction conditions.
8
.9 cN BS and 81% RBS mechanical strength, which indi-
cates the ꢀber catalyst has excellent mechanical strength
for further researches.
Furthermore, the solvent eꢁect on the Knoevenagel-
Michael reaction had been studied in this work. As we can
see from Table S2, the polarities of solvents have signiꢀ-
cant impacts on the reaction yields. The reaction can only
3.8 Catalytic Activities
proceed eꢁectively in strongly polar solvents like H O
2
The catalytic activities of different fiber catalysts were
tested using the template reaction of 4-chlorobenzaldehyde,
malononitrile, and 1-naphthol in water (Table 5). When the
reaction was carried out at 80 °C for 60 min with PANF as
catalyst, only 20% yield was obtained (Table 5, entry 1).
In the case of PANF-E as catalyst, the yield was 46%. For
PANF-ED, the yields were grown from 61 to 84% as pro-
portions of the DABCO group in the ꢀber catalysts were
increased (Table 5, entries 3–6), which suggests that the
DABCO group is the dominating catalytic unit. With PANF-
D as catalyst, the yields were increased from 84 to 98% as
the reaction temperature enhanced from 80 °C to reꢂuxing
condition (Table 5, entries 7 and 8). Reaction time is another
reaction parameter, when the reaction time was prolonged
to 80 min, the yield was kept unchanged (Table 5, entry 9).
When dosage of the catalyst was reduced to 10 mol%, the
reactants were consumed incompletely, and the yield was
reduced to 81%. However, the yield was unaꢁected if the
catalyst dosage was increased to 20 mol%. Finally, 15 mol%
which can get a yield of 98% (Table S2, entry 1). CH OH
3
solvent, whose polarity is 6.6, the yield was declined to
12% rapidly. However, when the PANF-D was performed
in solvents with less polarity than CH OH, like CH CN,
3
3
1,4-dioxane, C H OH, and toluene, the reaction even can’t
2
5
occur (Table S1, entries 3-6). It is a strange phenomenon
for common organic reactions. In order to understand this
phenomenon, we proposed a high polar micro-environment
promote mechanism (Fig. 5). This phenomenon can be
explained by the relative polarity of solvent and the sur-
face of ꢀber catalyst. The tertiary amine group, quaternary
ammonium group, and polymer block construct a high
polar micro-environment in ꢀber catalyst. In high polar
solvent like water (Fig. 5a), the micro-environment can
be stretched, and the mixture of solvent and reactants can
access the active catalytic sites in the micro-environment
to generate the product. On the contrary, the high polar
micro-environment will be shrunk in a less polar solvent
(Fig. 5b), the mixture of solvent and reactants can hardly
access to the active catalytic sits, resulting in the failure of
1
3

P. Li et al.
Scheme 2 The possible reaction mechanism
the reaction. The high selectivity to strongly polar solvents
PANF-D was then applied to the Knoevenagel-Michael
cascade reaction of diꢁerent aldehydes, malononitrile with
1-naphthol or 5,5-dimethyl-1,3-cyclohexanedione under
the optimal conditions mentioned above, and the results are
summarized in Table 6. Aldehydes with electron-donating or
electron-withdrawing groups are all completed the reaction
smoothly. However, the activity of 2-methoxybenzaldehyde
is lower than that of 4-methoxybenzaldehyde which may be
caused by the larger steric hindrance of the former (Table 6,
E7, E8, F6, and F7). Furthermore, the three-component con-
densation reaction can also proceed successfully when the
1-naphthol was replaced by 5,5-dimethyl-1,3-cyclohexanedi-
one (Table 6, E1-10, F1-10), which demonstrate the PANF-
D has excellent substrate suitability for the Knoevenagel-
Michael cascade reaction.
(
especially for water, which is an environmentally friendly
solvent) is one of the highlights for PANF-D.
A possible mechanism of the Knoevenagel-Michael cascade
reaction catalyzed by PANF-D is depicted in Scheme 2. The ring
structure of the DABCO group does not only make the lone pair
electrons on the nitrogen atoms more exposed but also makes
them more alkaline. For this three-component one-pot reaction,
PANF-D acts as the Brønsted proton scavenger to promote the
Knoevenagel reaction of aldehydes with malononitrile, and the
intermediate product M is formed. At the same time, the 1-naph-
thol is activated by PANF-D to form the intermediate N. Then,
the C=C bond in the intermediate product M is attacked by the
intermediate N to form another intermediate O. In the end, after
an intramolecular ring closure reaction facilitated by PANF-D,
the ꢀnal product E is produced.
1
3

Solvent Selective Polyacrylonitrile Fiber as a Recyclable Catalyst for the Knoevenagel-Michael…
Table 6 Three-component reaction of aldehydes, malononitrile and ethyl cyanoacetatea
a
Reaction conditions: aldehydes (1.0 mmol), malononitrile (1 mmol) and 1-naphthol (1 mmol) in the presence of PANF-D (15 mol%) in water
(
10 mL) are reꢂux for appropriate time. Yield of isolated product
3
.9 Scaled‑Up Experiment in Simple Fixed‑Bed
amplified 50 times in a simple fixed-bed reactor (250
mL, with the catalyst PANF-D intertwined on the stirring
paddle, Fig. 6). The three-component one-pot reaction
can be finished without any extension of the reaction
time to obtain high product yields of 97% (16.1 g), which
could meet the further preindustrial application research.
Reactor
To demonstrate the potential industrial application of
the fiber catalyst, the Knoevenagel-Michael reaction of
4
-chlorobenzaldehyde, malononitrile and 1-naphthol was
1
3

P. Li et al.
4
-chlorobenzaldehyde, malononitrile, and 1-naphthol under
the optimal reaction conditions (Fig. 7). After each cycle,
the PANF-D is separated out with tweezers and rinsed by
EtOAc (3 × 10 mL). Then the ꢀber catalyst can be directly
used for the next cycle and the results are shown in Fig. 6.
After ten cycles, the reaction can still reach a yield of 90%,
which indicates excellent recoverability of the ꢀber catalyst
PANF-D.
4
Conclusion
In summary, a series of PANF ꢀber catalysts (PANF-E,
PANF-ED , PANF-ED , PANF-ED , and PANF-D)
2
/1
1/1
1/2
with diꢁerent functional groups are developed and applied
to the synthesis of substituted 2-amino-4H-chromenes by
three-component one-pot Knoevenagel-Michael reaction
in water. The inꢂuences of surface polarities, kinds, and
proportions of functional groups on the ꢀber catalysts
activities were investigated. The experiments testify that
the catalytic activity and solvent selectivity of synthesized
PANF support catalyst is mostly dependent on the strong
basicity and micro-environment polarity of the ꢀber cata-
lyst. Among the ꢀber catalysts, the PANF-D has shown
the best catalytic activity which can complete the Knoev-
enagel-Michael reaction with high yields of 92–98% in 1
h. Moreover, the PANF-D has excellent recyclability (at
least ten times) and performs well in scaled-up experiment
with a simple ꢀxed-bed reactor (97%). It turns out that the
Fig. 6 Diagrammatic sketch of the simple ꢀxed-bed reactor for Kno-
evenagel-Michael reaction
3.10 Recyclability Test
From the view of economical and sustainable chemistry,
recyclability is one of the characteristic advantages of
the heterogeneous catalyst. Therefore, the recyclability of
PANF-D is tested with the Knoevenagel-Michael reaction of
Fig. 7 Recyclability of PANF-D
in the Knoevenagel-Michael
reaction
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3

Solvent Selective Polyacrylonitrile Fiber as a Recyclable Catalyst for the Knoevenagel-Michael…
catalyst has many advantages, such as easy preparation,
highly eꢃcient activity, broad substrate scope, green sol-
vent, and excellent recyclability. It is believed that more
new applications of PANF-D will be developed in value-
added chemistry.
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Acknowledgements We gratefully acknowledge the support from
the Natural Science Foundation of Henan Province (212300410318).
Fundamental Research Funds of Zhongyuan University of Technol-
ogy (K2020QN008), National Natural Science Foundation of China
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2
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9. Parlayıcı Ş, Yar A, Pehlivan E, Avcı A (2019) J Anal Sci Technol
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0:24
0. Tao R, Yang S, Shao C, Li X, Li X, Liu S, Zhang J, Liu Y (2019)
ACS Appl Nano Mater 2:3081–3090
(
21802034). Collaborative Innovation Centre of Henan Textile and
Clothing Industry, Innovation Scientists and Technicians Troop Con-
struction Projects of Henan Province (Grant Nos. 164100510007 and
CXTD2015018), and Zhengzhou University (Grant Nos. 1421316035
and 2016xjxm258), Program for interdisciplinary Direction Team in
Zhongyuan University of Technology, China.
1. Xu W, Zheng W, Wang F, Xiong Q, Shi X-L, Kalkhajeh YK, Xu
G, Gao H (2021) Chem Eng J 403:126349
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Min X, Huang Z (2019) Mater Res Express 6:095502
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Li S (2016) Energy Convers Manage 110:176–183
Compliance with Ethical Standards
24. Chang L, Zhang X, Shi X, Zhao L, Liu X (2014) Fibers Polym
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5. Shen X, Ji Y, Wang J (2008) J Appl Polym Sci 110:313–320
6. Li P, Du J, Xie Y, Tao M, Zhang W-Q (2016) ACS Sustain Chem
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Conflict of interest There has no conꢂict of interest for each contribut-
ing author.
2
7. Xu G, Wang L, Li M, Tao M, Zhang W (2017) Green Chem
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Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional aꢃliations.
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P. Li et al.
Authors and Aꢀliations
Pengyu Li1,2 · Yu Yang · Xueling Wu · Jiali Lu · Lingrui Hu · Weihua Chen · Wenqin Zhang
2
2
2
2
1
3
1
3
College of Chemistry, Zhengzhou University,
Zhengzhou 450001, People’s Republic of China
Collaborative Innovation Centre of Chemical Science
and Engineering (Tianjin), Tianjin University,
Tianjin 300072, People’s Republic of China
2
School of Material and Chemical Engineering,
Centre for Advanced Materials Research, Zhongyuan
University of Technology, Zhengzhou 450007,
People’s Republic of China
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3