Phosphorous acid functionalized polyacrylonitrile fibers with a polarity tunable surface micro-environment for one-pot C-C and C-N bond formation reactions

Article abstract of DOI:10.1039/c7gc02935g

The preparation and application of fiber catalysts have attracted much attention. However, research on the effect of the micro-environment of fiber catalysts on the catalytic activities though of special importance is limited. In this work, a novel strategy for the synthesis of phosphoric acid-functionalized polyacrylonitrile fibers with a polarity tunable surface micro-environment by hydrophobic groups for one-pot C-C and C-N bond formation reactions is reported. The special hydrophobic surface micro-environment of the fiber catalysts is proven to promote the catalytic activities impressively in cyclocondensation of β-ketoesters with 2-aminobenzamides, the Knoevenagel condensation as well as the multi-component Biginelli reactions in green solvents. Both the surface synergy of the catalytic sites and hydrophobic auxiliary groups (benzyl or n-butyl) in the surface of fiber catalysts and interface acceleration in reaction medium play an important role in the highly efficient promotion of catalytic activity. Thereby a surface synergistic mechanism is proposed to explain the micro-environment effect. In addition, the fiber catalysts could be simply separated from the reaction system using tweezers and directly used in the next cycle without further treatment. Importantly, even after 10 reaction cycles in water or ethanol, there is no significant loss in their catalytic activity. The results indicate that the phosphoric acid functionalized fibers show green and sustainable potential for industrial production.

Full text of DOI:10.1039/c7gc02935g

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Phosphorous Acid Functionalized Polyacrylonitrile Fiber with
Polarity Tunable Surface Micro-environment for One-Pot C-C and
C-N Bond Formation Reactions
Received 00th January 20xx,
Accepted 00th January 20xx
Gang Xu,a Lu Wang,a Mengmeng Li,a Minli Tao,a Wenqin Zhanga,b
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The preparation and application of fiber catalysts have attracted much attention, of special importance but limited
research is the effect of the micro-environment of fiber catalysts on the catalytic activities. In this work, a novel strategy
for the synthesis of phosphoric acid-functionalized polyacrylonitrile fiber with polarity tunable surface micro-environment
by hydrophobic groups for one-pot C-C and C-N bond formation reactions is reported. The special hydrophobic surface
micro-environment of the fiber catalysts is proven to promote the catalytic activities impressively in cyclocondensation of
β-ketoesters with 2-aminobenzamides, the Knoevenagel condensation as well as the multi-component Biginelli reactions
in green solvents. Both the surface synergy of the catalytic sites and hydrophobic auxiliary groups (benzyl or n-butyl) in the
surface of fiber catalysts and interface acceleration in reaction medium play an important role in the highly efficient
promotion of catalytic activity, thereby a surface synergistic mechanism is proposed to explain the micro-environment
effect. In addition, the fiber catalysts could be simply separated from reaction system using tweezers and directly applied
to the next cycle without further treatment. Importantly, even after 10 reaction cycles in water or ethanol, their catalytic
activity has no significant loss. The results indicate that the phosphoric acid functionalized fibers process a green and
sustainable potential for industrial production.
In the catalyst design context, various strategies such as
Introduction
adjusting pore size and structure of the catalyst,[5] regulating the
electronic effects of catalytic sites,[6] multifunctional catalytic
With the ever-increasing awareness of maximum catalytic
efficiencies and environmental problems, the immobilization of
organocatalysts on solid supports via stable covalent bonds has
become an urgent research topic for green and sustainable
chemistry due to its appealing advantages including excellent
reusability, good stability, easy separation and recovery from
reaction system etc.[1] Currently, both established and emerging
solid supports, such as silicas, zeolites, polymers, graphenes, MOFs
[2] etc. have been attempted and achieved fruitful results. However,
there are still many shortcomings to restrict its commercial process
especially when water is used as the reaction medium. For example,
a major drawback of zeolites is their low tolerance to hot water.[3]
Organic phase in aqueous solution is hard to enter the cavity of
some porous materials leading to a low catalytic efficiency. Besides,
regeneration is difficult and costly for the carbon materials.[4]
Hence, improving the catalytic performance of the supported
heterogeneous catalysts or applying more promising supports has
always been an important issue and still remains a challenge.
surface
synergies[7]
tuning
surface
polarity
or
hydrophilic/hydrophobic modification[8] etc. have been used to
enhance catalytic activity. Among them, hydrophilic/hydrophobic
regulation in the surface of supported-catalyst is one of the most
simple and effective methods. For example, Resasco et al[8b]
reported a hydrophobic zeolites to improve its stability in hot water
and catalytic activity in biofuel upgrading. It was proposed that the
hydrophobic functionalities on the external surface hindered its
direct contact with bulk water, thus preventing the collapse of the
framework during the reaction. Karimi et al[8c] synthesized a novel
catalyst comprising tungstate ions embedded into the hydrophobic
functionalized mesochannels of SBA-15, which exhibited significant
selectivity enhancement in oxidation of sulfides. More recently,
Khalil et al[8g] prepared surface-modified hematite nanoparticles.
The results showed that the catalytic activity could be improved by
slightly changing hydrophobicity of the nanoparticles. However, the
stronger hydrophobicity surface inevitably lead to the poor
dispersibility of these catalysts in water (especially for
nanoparticles, often floating on the surface of water). Besides,
hydrophobic modification of pore materials is primary on the
surface, and chemical modification in cavities is extremely difficult,
which in turn limits their catalytic efficiencies. This further
illustrates that the application of new supports for easier
modification and more widespread applications is very important
and necessary.
a.Department of Chemistry,School of Science, Tianjin University, Tianjin, 300072
P.R. China. E-mail: zhangwenqin@tju.edu.cn.
b.Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300072, P.R. China.
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Recently, organic textile fibers such as polyacrylonitrile fiber leaching out or exchanged with some reaction component, which
(PANF), polypropylene fiber (PPF), polyamide fiber (nylon) etc. have will affect the reusability of the catalyst. Therefore, insights into
been proved to be good carrier materials due to their stable nature how to stabilize catalytic sites and adjust the micro-environment of
and environmentally friendly properties.[9] Among which, PANF is fiber surface for further improving their catalytic efficiency and
one of the most promising fibers with many advantages of low cost, recycling capacity are still desirable. Furthermore, the relationship
large surface areas, high mechanical strength and excellent between the surface micro-environment and the catalytic
flexibility. Furthermore, PANF contains an abundance of nitrile performance of the fiber is still not quite clear.
groups that can be easily transformed into various functional
moieties.[10] In our previous work, we have explored the merits and developed a new strategy to prepare a series of novel phosphoric
details of the functionalized PANFs as effective and reusable acid-functionalized PANFs by alternating the
Herein, as an important extension to our previous work, we
adsorbents and chemical sensors for heavy ions.[11] More hydrophilic/hydrophobic (polarity) micro-environment of the fiber
importantly, PANF is also proved to be an ideal catalyst support to surface or the catalytic active sites (Scheme 1). The hydrophobic
immobilize amines, DMAP, pyrrolidine, ionic liquids, and quaternary micro-environment of the fiber catalysts was tuned by introducing
ammoniums etc.[12] With the concept of green chemistry rooted in benzyl or alkyl groups. By virtue of the specific micro-environment
the heart of the people, reaction performed in green solvents such in the fiber surface, the surface synergy of the fiber catalysts and
as water and ethanol with obvious advantages of low cost and interface acceleration in reaction medium may be helpful to
minimal environmental impact has received much attention. In case improve their catalytic efficiency. C–C and C-N bond formation
of water, it can also positively influence reaction rates and product reactions represent the most important transformations for the
selectivity.[13] Hence, we are committed to the preparation of synthesis of complex natural products, bioactive molecules,
functionalized fiber catalysts by various methods to make them agrochemicals and functional materials.[14] And metal, oxidant and
more suitable for aqueous phase reactions. For example, a novel radiation-free strategy for these conversion in safe solvents would
ionic liquid functionalized fiber was used to promote Friedel–Crafts be a promising choice for greener and more sustainable processes.
alkylations of indoles in water.[12c] Then polyacrylonitrile fiber- In view of this, the catalytic activities of the prepared fibers were
supported quaternary ammonium salts were also prepared and tested by the cyclocondensation of β-ketoesters with 2-
applied to the catalytic reduction of aldehydes, ketones, azides, and aminobenzamides,
Knoevenagel condensation and Biginelli
benzyl halides in water using NaBH4 as the reductant.[12f] Although reactions in water or ethanol. The influences of the surface micro-
the prepared fiber catalysts were suitable for aqueous phase environments on their catalytic performances and the reusabilty of
reaction and achieved efficient catalytic activities for water- the fiber catalysts were also investigated.
insoluble reactants. The catalytic sites of these fibers are prone to
O
C N
O
C N
O
C N
OH
H
H
O
O
H
O
O
O
O
O
O
C NHR1
C NHR1
O
CN
O
C N
P
P
OH
OH
(1) POCl3, CH3CN
(2) H2O
O
OH
OH
C N
C N
H
H
H
O
O
C NHR1
O
C NHR1
O
CN
O
C N
O
O
O P OH
OH
O P OH
OH
C N
C N
H
H
H
R1=benzyl, Bn-PANEA
R1=n-butyl, Bu-PANEA
F
F
R1=benzyl, (b) Bn-PANEAP
R1=n-butyl, (c) Bu-PANEAP
F
F
(
1
(a) PANEAP
F
)
P
O
C
(
2
l
)
,
H
C
H
O
C
N
O
O
C N
O
OH
O P
C N
R2
COOCH3
OH
H
H
O
CN
O
CN
R2
CN
CN
Cl P O
(1)
O
OH
O
Cl
R2
R2
H2N
H2O
OH
O P
OH
C N
C N
H
H
(2) H2O
CN
O
CN
O
CN
CN
O
O P
OH
C N
C N
OH
H
H
R2=phenyl, (d) PANEAPF-Bn
R2=methyl, (e) PANEAPF-Bu
PANF
PANEA
F
Scheme 1. Fiber catalysts prepared in this work.
2 | J. Name., 2012, 00, 1-3
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Table 1. The weight gain of functionalized fibers and their acid exchange capacity, water uptake and mechanical properties.
Mechanical properties
Weight gain P contentb
Acid content
(mmol g-1)
Water
uptake (%)
Entry
Fiber
a (%)
(mmol g-1)
Breaking
Retention of breaking
strengthe(%)
strength (cN)
1
2
3
4
5
6
7
PANF
--
0
0
9.4
11.21
9.94
9.65
7.41
7.75
8.66
8.80
100
89
86
66
69
77
79
PANEAF
15.1
12.3
7.8
0
0
35.3
31.6
28.1
27.0
17.1
17.5
PANEAP
F
1.27
1.09
1.20
1.01
1.04
2.06
1.50
1.61
1.35
1.33
Bn-PANEAPFc
Bu-PANEAPFd
PANEAPF-Bn
PANEAPF-Bu
7.1
15.4
10.6
aThe weight gain refers to the phosphoric acid modification step, Weight gain = [(W2 − W1)/W1] × 100%, where W1 and W2 are the weights of initial fiber and
modified fiber, respectively. bDetermined by ICP-OES. cBenzyl functionality is 0.66 mmol g-1 according to the weight gain. dn-butyl functionality is 0.77 mmol
g-1 according to the weight gain. eBased on PANF = 100%.
modified by hydrophobic groups, the WCAs were notablely
increased to 78.3o and 79.3o (Fig. S1d-e), indicating the formation of
Results and discussion
more hydrophobic surface environment beneath the fiber catalysts.
All these results indicate that the hydrophilic and hydrophobic
Preparation and characterization of phosphorous acid
functionalized fiber Catalysts
environments in the surface layers of the fibers are successfully
constructed.
As shown in Scheme 1, the phosphorous acid functionalized fiber
catalysts, viz. PANEAPF, Bn-PANEAPF, Bu-PANEAPF, PANEAPF-Bn, and
PANEAPF-Bu were prepared via a simple two-step or three-step
procedure. The modification extent of the fiber catalysts
determined by weight gain, P content and acid content (Section 3 in
ESI †). As presented in Table 1, it is obvious that the phosphorus
functionalized fibers possess high content of acid (> 1 mmol g-1),
indicating the fibers are potential acid catalysts (Table 1, Entries 3-
7).
Table 1 also displays the mechanical properties of fiber catalysts.
The breaking strength of PANEAF decreases slightly from 11.21 to
9.94 cN and keeps 89% strength of the original fiber PANF (Table 1,
Entries 1-2). Furthermore, even after three steps of chemical
modification, the Bn-PANEAPF and Bu-PANEAPF maintain more than
66% mechanical strength of the PANF (Entries 4-5), which shows
that the functionalized fibers have high durability and strength for
further use.
The FTIR spectra of PANF, hydroxyethylated PANEAF and acid
catalysts are shown in Fig. 1 and Fig. S2. The FTIR spectrum of PANF
(Fig. 1a) shows absorption peaks at 2245 cm-1 and 1733 cm-1, which
are C≡N and C=O stretching vibrations evidencing the existence of
acrylonitrile and methyl acrylate units in the co-polymer.[16] After
hydroxyethylation of PANF (Fig. 1b), a broad absorption band
between 3600 and 3100 cm-1 is due to the stretching vibration of
the O-H and N-H groups. In the FTIR spectra of phosphorylated
functionalized fibers (Fig. 1c and Fig. S2a-d ), the new band at 982
cm-1 is assigned to P-OH stretching vibration,[17] and another two
absorption peaks at 1172 and 1080 cm-1 are correspond to the
stretching vibrations of PO2,[11c] which further demonstrate that the
phosphorous acid groups are successfully grafted in the fiber
support. Besides, after hydrophobic modification of benzyl group,
two new absorption peaks of the Bn-PANEAPF (Fig. 1c) and PANEAPF-
Bn (Fig. S2b) at 755 and 701 cm-1 correspond to the out-of-plane
bending vibration of phenyl moiety. Furthermore, the C≡N vibraꢀon
peak of all modified fiber weakens but still exists at 2245 cm-1,
showing that the C≡N groups in the inner part of the fiber don’t
participate in the reaction and the integrity of the fiber’s structure
is well maintained. More importantly, after being recycled once or
10 times (Fig. 1d-e), the spectra of Bn-PANEAPF-1 (the catalyst
In this work, benzyl and alkyl groups are used to tune the
hydrophobic surface micro-environment of the fiber catalysts. And
the water uptake content[15] is a key factor in determining the
hydrophilic property of the fiber surface. As shown in Table 1, the
compact surface and poor swelling behavior of PANF lead to a low
water absorption capacity of 9.4% (Table 1, Entry 1). And the -OH
and –P=O(OH)2 groups present strong hydrophilicity, hence, the
PANEAF and PANEAPF have high water uptake contents of 35.3% and
31.6% (Entries 2-3). However, when there numerous hydrophobic
groups such as benzyl and alkyl groups surround in the surface
layers of the fiber, the hydrophobicity of the Bn-PANEAPF and Bu-
PANEAPF is more greatly improved than that of PANEAPF (Entries 4-5).
Especially, when hydrophobic modification is performed directly at
the catalytic site, two more hydrophobic fiber PANEAPF-Bn and
PANEAPF-Bu are obtained (Entries 6-7). Besides, water contact angle
(WCA) was also measured in accordance with our previous
method[12h] to illustrate the surface hydrophilicity of the fiber
materials. As shown in Fig. S1, the WCA of original PANF was 64.2o
(Fig. S1a). After hydroxyethylation and phosphoric acid
modification, the WCAs of PANEAF and PANEAPF were obviously
decreased to 26.4o and 32.2o (Fig. S1b-c), which indicates that these
two fibers are more hydrophilc than PANF. When the fiber was
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recovered after the first cycle) and Bn-PANEAPF-10 (the catalyst
The elemental analysis was utilized to confirm the successful
recovered after the tenth cycle) are nearly the same as that of the grafting and the stability of the Bn-PANEAPF during application
Bn-PANEAPF, which indicates that the fiber catalyst can be reused process. The carbon and nitrogen contents of PANEAF decrease
more than 10 times without significant changes. Correspondingly, obviously and the hydrogen content increases as expected, since
the solid-state 31P NMR spectrum of phosphorylated functionalized ethanolamine has a lower carbon (39.34%), nitrogen (22.95%) and a
fibers (Fig. 2) shows signals at -0.2, -11.4 and -23 ppm, which are higher hydrogen (11.48%) content than the initial PANF (Table 2,
ascribed to organic phosphate analogue.[18] These signals can be Entries 1-2). Similarly, the Bn-PANEAF has a higher carbon content
assigned to P atoms in -P(=O)(OR)(-OH)2, -P(=O)(-OR-)2(OH) or - and lower nitrogen content than PANEAF due to the reaction with
P(=O)(-OR-)3 groups, respectively (R: organic bridging groups in benzylamine (Table 2, Entry 3). Besides, after immobilization of the
phosphonic acids), of which the dissociative P-OH groups are acid functional groups, the carbon, hydrogen and nitrogen contents
originated from the first two forms.
of the Bn-PANEAPF (Table 2, entry 4) all reduce due to the
introduction of the phosphorous-containing functional groups -
P(=O)(OR)n(-OH)3-n. Positively, the components of the Bn-PANEAPF-
10 are nearly the same as the Bn-PANEAPF-1 but slightly higher than
those of Bn-PANEAPF. The result is perhaps due to the tiny hydrolysis
of amide groups or phosphate esters, however, which can not be
completely avoided.[12d] It is worth mentioning that the nitrogen
content of Bn-PANEAPF-10 only increases by 0.6% based on Bn-
PANEAPF, this means that there is no significant out-flowing of the
functional groups from the fiber catalyst, which still maintains high
catalytic activity after being reused many times.
The surface area and porosity of the fibers were also measured
and the results are shown in Table S1. It is clear that the fibers
contain nanometer sized pores, and its average pore diameters are
3.19-3.51 nm. Besides, the specific surface areas of the fiber
catalysts are between 28 and 38.9 m2 g-1, and the larger specific
surface and pore diameter of the fibers are favorable for its
catalytic application.
Fig. 1. The FTIR spectra of (a) PANF, (b) PANEAF, (c) Bn-PANEAPF, (d)
Bn-PANEAPF-1 (e) Bn-PANEAPF-10.
The XRD spectrum of PANF (Fig. 3a) shows a main diffraction
peak at 17° and a weak diffraction peak centered at 29.5°,
corresponding to the (100) and (110) crystallographic planes of the
PANF hexagonal lattice, respectively.[11d] After being modified by
ethanolamine (Fig. 3b) and further modification (Fig. 3c-g), the
spectra of which are nearly the same as that of PANF, indicating
that the polar interaction between molecular chains and the inner
crystalline structure of the PANF does not change obviously. These
results agree well with the SEM and TGA, which indicates that the
fiber catalysts keep good integrity to meet the requirements of
further application.
Fig. 2. The 31P NMR spectra of (a) PANEAPF, (b) Bn-PANEAPF, (c) Bu-
PANEAPF, (d) PANEAPF-Bn and (e) PANEAPF-Bu.
Table 2. The data of elemental analysis.
Entry
Sample
C (%)
66.19
61.76
63.20
56.35
56.89
57.99
H (%)
5.98
6.23
6.20
5.47
5.53
5.59
N (%)
24.38
20.44
18.60
16.49
16.57
16.60
1
2
3
4
5
6
PANF
PANEAF
Bn-PANEAF
Bn-PANEAPF
Bn-PANEAPF-1
Bn-PANEAPF-10
Fig. 3. The XRD spectra of (a) PANF, (b) PANEAF, (c) PANEAPF, (d) Bn-
PANEAPF, (e) Bu-PANEAPF, (f) PANEAPF-Bn and (g) PANEAPF-Bu.
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The SEM images of PANF, Bn-PANEAPF, Bn-PANEAPF-1, Bn-PANEAPF- more seams emerged, which is due to the influence of the
10 and other acid functionalized fibers are presented in Fig. 4 and mediated reaction under strong polar water. However, the
Fig. S3. For example as Bn-PANEAPF, the low magnification SEM integrities and fibrous structures are untouched. This is further
images (200 times) show that all of the fibers are continuous with confirmed by a mechanical strength test, which demonstrates that
few beads and the integrities of the fibers are well maintained, although after 1 time (or 10 times) of reuse, the strength of the
suggesting that the fibrous structures of the fibers are not fiber is 7.15 cN (or 6.44 cN) and still keeps 96% (or 87%) of the
destroyed during the grafting reaction and the application process original Bn-PANEAPF. The result indicates that Bn-PANEAPF is durable.
(
Fig. 4a-d). In the high magnification images (2000 or 20000 times), Moreover, the thermostability of the fiber catalysts was also
it’s clear that the surface of Bn-PANEAPF is smooth but much thicker investigated with thermogravimetry (Fig. S4). For the fiber catalysts,
and rougher than that of original PANF (Fig. 4a-b). In addition, many it is observed that no significant degradation occurred before 250
crack structures exist on the surface of the fibers, which consists oC (Fig. S4c-g). And the phosphorylated fibers have lower rate of
with the BET results. This would increase the contact area between mass loss and higher mass residue compared with the PANF and
o
the fiber and the reactants, thus improving the catalytic efficiency. PANEAF when heated at 800 C, so thermal stability of these fibers
Especially, after the sequential utilization, the surface layer of Bn- are enhanced. This further illustrates that the fiber catalysts are
PANEAPF-1 and Bn-PANEAPF-10 become increasingly dissilient and stable and have potential cycling capabilities.
Fig. 4. The SEM images of (a) PANF, (b) Bn-PANEAPF, (c) Bn-PANEAPF-1, (d) Bn-PANEAPF-10.
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Catalytic properties of heterogeneous phosphoric acid grafted hydrophilic micro-environment in PANEAPF, and good dissolution is
fiber catalysts
propitious to chemical mass transfer.
Cyclocondensation of β-ketoesters with 2-aminobenzamides
Next, the catalytic activities of different fiber catalysts in organic
As one of the most important N-heterocycles, quinazolinones are a solvents (taking the safest ethanol as an example) were further
privileged class of compounds with a variety of biological activities studied and the results are shown in Table S3. The catalytic
and therapeutic potential.[19] Conventionally, synthesis of activities of the fibers were different from that in water, the
quinazolinone
involved
condensation
reaction
of
2- hydrophobic fiber catalysts (Bn-PANEAPF and Bu-PANEAPF) exhibited
aminobenzamide with aldehydes,[20] alcohols,[21] methylarenes,[22] superior activities to hydrophilic fiber PANEAPF (Table S3, Entries 1-
amines,[23] keto alkynes,[24] or carboxylic acid derivatives.[25] 3). The low yields of the reaction promoted by PANEAPF-Bn and
Recently, the use of 2-bromobenzamide and multiform substrates PANEAPF-Bu in ethanol may be due to the low acidity of the di-
like aldehydes, alcohols, and methyl arenes was also performed.[26] phosphate ester groups onto the fiber (Table S3, Entries 4-5). In a
However, these methods suffered from certain drawbacks such as word, both the hydrophilic PANEAPF in water and hydrophobic Bn-
requirement of excess amounts of strong oxidants, high PANEAPF (Bu-PANEAPF) in ethanol could effectively promote the
temperature, difficult or unable to recover the catalyst, long reaction, and the hydrophobic functionalized fibers in ethanol
reaction time, using transition metal catalysts or toxic organic showed higher catalytic activity with lower reaction temperature
solvents etc. Here, new fiber-supported Brønsted acid catalysts with and shorter reaction time.
polarity tunable surface micro-environment were firstly used to Table 3. Optimization of the Reaction Conditionsa.
smoothly mediate cyclocondensation of
β-ketoesters with 2-
aminobenzamides via selective C-C bond cleavage for the synthesis
of quinazolinones under metal, oxidant and radiation-free
conditions, which shows a more economical and green synthetic
pathway.
Temp
(°C)
Time
(h)
Entry Catalyst
dosage Solvent
Yield (%)b
In this work, the reaction of 2-aminobenzamides with ethyl
acetoacetate in water was selected as a model to test the catalytic
activities of different fiber catalysts. The results are shown in Table
1
Blank
PANF
PANEAF
H3PO3
--
H2O
100
100
100
100
100
100
100
100
100
100
100
100
100
25
15
15
15
15
15
15
15
15
15
6
20
24
30
52
93
82
79
77
82
75
93
77
94
6
2
30 mg H2O
3
. Low yields (20-52%) were obtained without catalyst or in the
presence of PANF, PANEAF and H3PO3 Table 3, Entries 1-4), the
(
3
5%
5%
5%
5%
5%
5%
5%
5%
5%
2%
8%
5%
5%
5%
5%
5%
5%
5%
5%
5%
H2O
results indicate that -CN groups on the fiber does not influence the
goodness of the reaction in terms of yield. While with the existence
of phosphorous acid functionalized fiber catalysts (Entries 5-9), the
yields were greatly improved. It is worth mentioning that the most
hydrophilic PANEAPF gave the highest yields of 93%. Besides, after
further comparing the effects of reaction time, temperature, and
catalyst dosage on the reaction and considering the cost and
efficiency of the catalyst (Entries 10-15), the reaction was
performed with 5 mol% catalyst at 100 oC for 15 hours as the
optimum condition in water (Entry 5).
4
H2O
5
PANEAP
F
H2O
6
Bn-PANEAP
F
F
H2O
7
Bu-PANEAP
H2O
8
PANEAPF-Bn
PANEAPF-Bu
H2O
9
H2O
10
11
12
13
14
15
16
17
18
19
20
21
22
PANEAP
PANEAP
PANEAP
PANEAP
PANEAP
PANEAP
PANEAP
PANEAP
PANEAP
PANEAP
PANEAP
PANEAP
PANEAP
F
F
F
F
F
F
F
F
F
F
F
F
F
H2O
H2O
18
15
15
15
15
15
15
15
15
15
15
15
Solvents also played an important role in this reaction. The strong
polar solvents such as water, methanol, ethanol and acetonitrile
provided yields of 58%, 99% 69% and 89%, respectively (Entries 15-
18). In contrast, when the low polar solvents such as THF,
cyclohexane, toluene and 1,4-dioxane were used, low yields (0-
22%) were observed (Entries 19-22). Although the polarity of water
is the highest, it should note that the hydrophilic PANEAPF in organic
solvents with high polarity gives better catalytic activities than that
in water, which shows that the solubility of the reactants and
polarity of solvents have a superposition influence on the reaction
efficiency. To confirm this inference, the effect of the solvent
polarity and solubility of the reactants for the model reaction
catalyzed by hydrophilic PANEAPF were studied by adjusting the ratio
of water to ethanol (Table S2). As the polarity of the solvent
increasing, the yield decreased first, then almost maintained flat,
H2O
H2O
H2O
H2O
65
58
99
69
85
8
CH3OH
EtOH
CH3CN
THF
65
65
65
65
Cyclohexane
Toluene
1,4-dioxane
65
22
0
65
65
0
aReaction conditions: 2-aminobenzamide (1 mmol), ethyl acetoacetate (1.5
mmol), solvent (5 mL) and catalyst dosage based on acid content. b The yields
were obtained by HPLC using m-binitrobezene as an internal standard.
finally increased again (Table S2, Entries 1-7) This result suggests a
competitive relationship between polarity and solubility. The
solvent with large polarity and high solubility for substrate is
favorable for the reaction. This is because that large polarity of
reaction medium is beneficial for its contact with the surface
6 | J. Name., 2012, 00, 1-3
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Scheme 2 The catalytic modes of fiber catalysts in different reaction mediums.
According to the above experimental results, it’s apparent that substituted quinazolinone 1d was elaborated in 95% yield, with the
the catalytic activity relies on surface micro-environment properties strained cyclopropyl group untouched. For the further expansion of
of the fiber catalyst, reaction medium as well as the compatibility of 2-aminobenzamides, the N-methyl, chloro and methoxy-substituted
substrate and solvent. Hence, the micro-environment promoting substrates also produced the quinazolinones efficiently
mechanisms of the fiber catalysts for the reaction was proposed (yields>98%, 1g-1i). Besides, when aryl-substituted
β-ketoesters
and the diagrammatic sketch is shown in Scheme 2. In aqueous were used as substrates, a variety of 2-aryl functionalized
environment (Scheme 2a), hydrophilic PANEAPF fiber catalyst shows quinazolinones were readily synthesized in excellent yields (65-96%;
higher activity than these hydrophobic fibers. This is because the 1j-1n). However, the electron-withdrawing group of p-nitro phenyl
fiber with hydrophilic surface micro-environment infiltrates better substituted
β-ketoester gave a low yield of 45% (1o) for the
in water. And 2-aminobenzamide with strong polarity and ethyl corresponding quinazolinone, and the by-product without C-C
acetoacetate can well dissolve in water (especially in hot water). bonds cleavage gave 47% yield (1o’). In addition to quinazolinones,
Hence, the hydrophilic micro-environment advances the access of this strategy was also applied to the synthesis of other N-
reactants to the catalytic sites, thereby accelerating the reaction. heterocycles of benzimidazoles. Almost equivalent yields were
Furthermore, interface aggregation effect is another important obtained for the reaction between o-diaminobenzene and various
factor to accelerate the reaction.[27] The oil-water interface
obviously exists in aqueous organic reactions, the reactants are effect of the substituents (Scheme 3, 1p-1v).
β-ketoesters, regardless of the steric hindrance and electronic
more inclined to the surfaces of the fiber to facilitate the
The acid functionalized fiber catalysts show many advantages
occurrence of reaction. For those more hydrophobic fiber catalysts, when compared with small Brønsted acid[28] and some silica gel,
although the oil−water interface is much clearer, the specific polymer, graphene and metal nanoparticle supported-catalysts[29]
hydrophobic surface has repulsive effect on water, which results in reported in the literature. Although many progresses have been
poor contact between the catalytic sites and the reactants dissolved made, these methods have one or more defects. For example, low
in water, thus leading a low catalytic activity. In organic solvents yields were obtained for the substrates with bulky substituents
(
Scheme 2b), it is obvious that the catalytic activity of PANEAPF is when used the strong Brønsted acid TsOH, and the catalysts can not
related to the polarity of the solvent, the yield increases with rising be recycled, resulting in waste and environmental pollution.
the polarity of solvents. According to the principle of “like dissolves Besides, the supported-catalysts are mainly supported by metal and
like”
，the catalytic sites with polar/hydrophilic micro-environment metal analogues, they also possess shortcomings such as excessive
are able to access the solvents with high polarity, as well as the alkali additives, poor reusability and catalytic site loss which
organic reactants, leading to a high catalytic efficiency. In the case adversely affect the economics as well as the eco-friendly nature. In
of low polarity solvents which have poor contact to the surfaces of contrast to the reported methods, this fiber-catalyzed system
fiber catalyst, the reactants disperse in the solvent but can not shows many advantages of metal, oxidant and radiation-free
access to the active sites, which causes the unsuccess of the conditions, green reaction solvent, high compatibility with aryl-
reaction. Besides, the superior activity of hydrophobic Bn-PANEAP
F
substitutes and excellent cycle potential. To the best of our
and Bu-PANEAPF in ethanol may be attributed to the synergistic knowledge, this is the first Brønsted acid-supported catalyst for this
effects of the catalytic sites and auxiliary groups (benzyl and n- cyclization reaction.
butyl). The auxiliary groups are hydrophobic, which are able to get Knoevenagel condensation
in touch with organic phase and subsequently constrain organic Knoevenagel condensation is one of the most classic methods for
reactants, increasing the reactant population around the catalytic the synthesis of
sites (Scheme 2c).
pharmaceutical, functional material and cosmetic industry etc.[30]
As shown in Scheme 3, the acid functionalized fiber catalysts Numerous supported-base catalysts based on silica gel, graphene,
could be applied to a variety of -ketoesters to produce the MOFs, metal particle have been well established for the
corresponding quinazolinones
in high yields via selective Knoevenagel condensation.[31] However, the utilization of Brønsted
α,β-unsaturated compounds which widely used in
β
1
R(CO)−C(α) bond cleavage. Different functional groups such as alkyl, acid functionalized supported-catalysts for this conversion has been
chloro and ether etc. were well tolerated with excellent yields of very rare.[32] Especially, finding a more efficient, eco-friendly and
85-99% (Scheme 3, 1a-1f). It is worth noting that 2-cyclopropyl-
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Journal Name
Scheme 3. Fiber Catalyzed Cyclocondensation Reactions of 2-Aminobenzamides with β .
-ketoestersa,b
o
aReaction conditions: 2-aminobenzamide (1 mmol),
β
-ketoester (1.5 mmol), fiber catalyst PANEAPF (5 mol%) and H2O (5 mL) at 100 C.
c
o
d
bIsolated yield. 2-aminobenzamide (1 mmol),
β
-ketoester (1.2 mmol), fiber catalyst Bn-PANEAPF (5 mol%) and EtOH (5 mL) at 80 C. o-
substituted aniline (1 mmol),
β
-ketoester (1.1 mmol), fiber catalyst PANEAPF (5 mol%) and H2O (5 mL) at 80 oC.
stable catalyst to promote the condensation reaction in water catalytic performance of the fiber catalyst is better than those
under more moderate conditions is still of great value and MOF,[30, 31e] silica gel,[33] microporous carbon,[34] polystyrene,[31d]
significance. Hence, the catalytic performance of the fiber catalysts metal nanoparticle[31c] and metallic oxide[35] etc. supported-catalytic
on the Knoevenagel condensation was studied.
systems (Table S4). Most of these reactions were carried out in
As shown in Table 4, the hydrophobic Bn-PANEAPF and Bu- organic solvents such as toluene, THF and acetonitrile etc,[30, 31e, 33-
34]
34-35]
PANEAPF display higher catalytic activity than other fiber catalysts
required larger catalyst loading,[31d,
or needed higher
31c, 33-35]
(
Table 4, Entries 4-8). Then we choose Bn-PANEAPF for further reaction temperature and longer reaction time.[30,
research. Positively, the optimal conditions are obtained in water in Positively, the fiber catalyst can be performed in water under mild
the presence of only 5 mol% catalyst loading for 2 h at room conditions,
temperature (Entries 9-12). It is comforting to observe that the
8 | J. Name., 2012, 00, 1-3
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ARTICLE
Table 4. Optimization of the Reaction Conditionsa.
Knoevenagel condensation is determined by the polarity of the
catalytic sites. We have applied amine functionalized fibers to
catalyze the Knoevenagel condensation. Tertiary amine
functionalized fibers have excellent catalytic activity in various types
of solvents such as methanol, ethanol, acetonitrile, ethyl acetate,
Dosage
(mol%)
Time
(h)
Yield (%)b
36]
cyclohexane etc.[12a,
However, when primary amines and
Entry Catalyst
Solvent
secondary amines functionalized fiber catalysts are used, the
reaction can not be happened in the solvents with polarity less than
ethanol and reacts very well in water and methanol.[12b] The polarity
of tertiary amines is less than that of primary amines, secondary
amines and phosphate groups, so we can draw the conclusion that
the catalytic sites of great polarity are suitable for reactions in polar
proton solvents like water and ethanol. Besides, it is worth
mentioning that the acid functionalized fibers possess higher
catalytic activity with lower catalyst dosage under room
temperature than the amine functionalized fiber catalysts.
1
Blank
PANF
--
H2O
2
30
33
46
46
52
50
38
29
94
96
81
96
44
0
2
30 mg
5%
5%
5%
5%
5%
5%
5%
5%
2%
8%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
H2O
2
2
1
1
1
1
1
2
4
2
2
2
2
2
2
2
2
2
2
2
2
2
3
PANEAF
H2O
4
PANEAP
F
H2O
5
Bn-PANEAP
F
F
H2O
6
Bu-PANEAP
H2O
7
PANEAPF-Bn
PANEAPF-Bu
Bn-PANEAPF
H2O
8
H2O
The Bn-PANEAPF fiber catalyst was then used to synthesise various
9
H2O
α,
influence of the electronic nature and the position of different
substituted benzaldehyde with excellent yields of 90-99% (Table 5
Entries 1-10). However, the trifluoromethyl-substituted
benzaldehyde provided a low yield of 67% (Entry 11). Besides,
heterocyclic aldehydes, -unsaturated aldehyde and ethyl
β-unsaturated compounds (Table 5). There was no significant
10
11
12
13
14
15
16
17
18
19
20
21
22
Bn-PANEAP
Bn-PANEAP
Bn-PANEAP
Bn-PANEAP
Bn-PANEAP
Bn-PANEAP
Bn-PANEAP
Bn-PANEAP
Bn-PANEAP
Bn-PANEAP
Bn-PANEAP
Bn-PANEAP
Bn-PANEAP
Bn-PANEAP
F
F
F
F
F
F
F
F
F
F
F
F
F
F
H2O
,
H2O
H2O
CH3OH
CH3CN
EtOH
DMSO
DMF
α,β
cyanoacetate substrates all gave high yields when prolonged the
reaction time or increased the reaction temperature (Entries 12-
16).
54
trace
0
Table 5. Fiber catalyzed Knoevenagel condensation of aromatic
aldehydes with active methylene compoundsa,b
.
CH2Cl2
THF
0
0
Cyclohexane
Toluene
1,4-dioxane
Acetone
0
Entry Ar
R
Time (h) Product
Yield (%)
94
trace
0
1
C6H5
CN
2
2
2
2
2
2
2
2
2
2
4
3
6
5
4
1.5
2a
2b
2c
2d
2e
2f
2
2-NO2C6H4
3-NO2C6H4
4-NO2C6H4
3-MeOC6H4
4-MeOC6H4
4-(CH3)2NC6H4
4-OHC6H4
4-ClC6H4
CN
96
23
0
3
CN
99
a
Reaction conditions: Benzaldehyde (1 mmol), malononitrile (1.1 mmol), 10
4
CN
99
b
mL solvent and catalyst dosage based on acid content. The yields were
obtained by HPLC using naphthalene as an internal standard.
5
CN
94
showing excellent application potential in the chemical industry.
The excellent catalytic activity of the hydrophobic fibers in water is
easy to understand, the solubility of aromatic aldehydes in water is
poor, the hydrophobic blocks of the fiber catalyst are able to allure
organic reactants from aqueous medium. Thus, the reactants are
readily enriched at the catalytic site to accelerate the reaction.
Besides, the Knoevenagel condensation catalyzed by the
hydrophobic Bn-PANEAPF is greatly affected by solvents (Entries 13-
23). Under the above optimal conditions, it is found that the
reaction can only be performed in the proton solvents such as
methanol, ethanol and water. However, when those aprotic
solvents such as acetonitrile, DMSO, DMF THF, and toluene etc.
were used, the reaction even failed to occur.
6
CN
90
7
CN
2g
2h
2i
94
8
CN
99
9
CN
96
10
11
12c
13
14
15d
3-BrC6H4
CN
2j
99
2-CF3C6H4
2-Cl-3-quinolinyl
2-thienyl
CN
2k
2l
67
CN
99
CN
2m
2n
2o
2p
95
cinnamyl
CN
85
C6H5
COOEt
COOEt
93
As discussed earlier, polar catalytic sites are favorable for highly
polar reactants (such as 2-aminobenzamide) to react in polar
environments, especially in methanol and ethanol. The polarity of
benzaldehyde is not high enough. Therefore, the interesting
experimental phenomenon of the solvent selectivity for the
16d
4-NO2C6H4
99
a
Reaction conditions: aromatic aldehyde (1 mmol), methylene compound
(1.1 mmol), Bn-PANEAPF 5 mol% in H2O (10 mL). bIsolated yield. cThe
reaction temperature is 50 oC. d The reaction temperature is 80 oC.
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Table 6. Fiber catalyzed Biginelli reactionsa,b
.
substrate (Scheme S1). Unfortunately, the target product was not
obtained, and almost equivalent Knoevenagel condensation
product (3n) of p-methoxybenzaldehyde with ethyl benzoylacetate
was acquired. To the best of our knowledge, substrate extension of
Biginelli reactions using ethyl benzoylacetate as substrate was also
not reported in the literature. This may be due to the fact that the
large conjugated system of the Knoevenagel intermediate is too
stable to make it difficult to further react with urea.
Entry
1
Ar
R
Product
3a
Yield (%)
90
C6H5
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
Me
At last, different catalytic systems for the Biginelli reactions in the
literature were compared and the results are summarized in Table
S5. It’s important to recall that the hydrophobic Bn-PANEAPF shows
many advantages in term of catalyst dosage, reaction yield,
substrate range and stability of catalyst. In addition, the greener
radiation, oxidant, metal-free process make it very attractive for
safe production of DHPMs in chemical industry.
2
4-CH3C6H4
4-MeOC6H4
3-MeOC6H4
2-MeOC6H4
3-CF3C6H4
4-NO2C6H4
4-ClC6H4
3b
3c
91
3
94
4
3d
3e
98
5
92
6
3f
91
Recyclability of the fiber catalysts
7
3g
88
Recyclability is an important factor for heterogeneous supported-
catalysts. Hence, the reusability of the Bn-PANEAPF for the
cyclocondensation of 2-aminobenzamide with ethyl acetoacetate in
ethanol and the Knoevenagel condensation of benzaldehyde with
malononitrile in water were all studied in details. After each run,
the fiber catalyst was simply taken out using tweezers and washed
with ethanol, then it was directly used for next circulation. As
shown in Fig. 5, both of the fiber catalyst in ethanol and water
retains its high activity after 10 times of recycling, which is
significantly superior to many existing catalytic systems.[29d, 31b, 32,
8
3h
3i
91
9
3-BrC6H4
2-thienyl
cinnamyl
4-MeOC6H4
4-MeOC6H4
90
10
11
12
13
3j
93
3k
94
3l
85
OCH2C6H5
3m
96
aReaction conditions: aromatic aldehyde (1 mmol), methylene compound
(1.2 mmol), urea (1.2 mmol), Bn-PANEAPF 5 mol% in EtOH (5 mL) 80 oC for 12
h. bIsolated yield.
38b, 40a, 42]
The stability of fiber catalyst in the application process is
tested by FTIR, EA, SEM and mechanical properties above, which
proves that the loss of catalytic sites can be neglected. All these
results indicate that the acid functionalized fiber catalyst processes
excellent reusable capacity.
Biginelli reactions
We also examined the catalytic activity of Bn-PANEAP
F for
multicomponent reactions (MCRs). Among MCRs, the Biginelli
reaction which involves a one-pot condensation of an aldehyde, β-
ketoester and urea is used to the direct synthesis of 3,4-
dihydropyrimidin-2-(1H)-ones
(DHPMs).[37]
Considering
the
attractive biological activity of DHPMs, different supported
heterogeneous catalysts including ionic liquid,[38] organic sulfonic
acid,[39] phosphomolybdic acid[40] and transition metal[41] based on
silica gel, polymer and nanoparticle etc. were used to improve the
reaction yield. Nevertheless, there still evidently exists defects such
as hazardous organic solvent, long reaction time, high catalyst
dosage. In addition, the main drawback of some ionic liquid or
metal supported catalytic system is the loss of catalytic sites, which
will bring difficulties to the post process and affect the recycling of
the catalyst. Hence, a greener and sustainable process under more
amenable conditions is still of great value.
As shown in Table 6, various substituted aromatic aldehydes
were reacted with ethyl acetoacetate, acetylacetone or benzyl
acetoacetate and urea in the presence of 5 mol% Bn-PANEAP
F
loading in ethanol to prepare a wide range of DHPMs with excellent
yields of 85-98% (Entries 1-13). It is obvious that the position of
substituents had little effect on the reaction, however, the
aldehydes with electronic donating afforded slightly higher yields
than that with electron withdrawing groups. It is worth noting that
Fig. 5. The reusability of the Bn-PANEAPF for (a) cyclocondensation
of 2-aminobenzamide with ethyl acetoacetate in ethanol; (b)
Knoevenagel condensation of benzaldehyde with malononitrile in
water.
stronger electron withdrawing of
R group is beneficial to the
reaction (Entry 3 and Entries 12-13). Besides, we also attempted to
synthesize DHPM derivatives using ethyl benzoylacetate as
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ARTICLE
(1) The preparation of PANEAPF, Bn-PANEAPF and Bu-PANEAP
F
Dried 1.0 g PANEAF (Bn-PANEAF or Bu-PANEAF), POCl3 (2 mL) and
acetonitrile (20 mL) were placed in a three-necked flask, and the
o
mixture was heated at 80 C for 4 h. After cooling the mixture to
room temperature, the fiber was taken out with tweezers. Then it
was put into ice water and stirred for one hour. The fiber was
filtered out and washed with water (60-70
°C) until neutral, and
then dried at 60 C overnight to get the corresponding PANEAPF, Bn-
°
PANEAPF and Bu-PANEAPF, respectively.
(2) The preparation of PANEAPF-Bn and PANEAPF-Bu
A mixture of POCl3 (20 mol), TEA (20 mmol) and acetonitrile (20 mL)
o
was added in a single neck bottle. After the mixture cooled to 0 C,
20 mmol of 3- phenyl propanol (n-butanol) and acetonitrile (15 mL)
in a dropping funnel was slowly added in the above mixture for 20
min, then the reaction was continued for 1 hour. After the
triethylamine hydrochloride was filtered out, the acetonitrile
solution of 3-phenylpropyl dichlorophosphate (or butyl
dichlorophosphate) and dried 1.0 g PANEAF were put in a 50 mL of
Scheme 4. Gram-scale synthesis of 1a, 2a and 3a catalyzed by Bn-
PANEAPF.
Gram-scale synthesis
Gram-scale reactions of each process for the synthesis of 1a, 2a and
o
single neck bottle, and the mixture was heated at 80 C for 4 h.
3a were performed to evaluate the practicability of the fiber
catalyst. As shown in Scheme 4, all the reactions could be
proceeded efficiently without the increase of catalyst dosage and
reaction time. After the reaction completed, high yields of 93%,
95% and 91% can be obtained simply by recrystallization in ethanol.
The results further illustrate the remarkable catalytic performance
of the fiber catalyst.
Then the same post-treatment method as PANEAPF was used to
obtain the PANEAPF-Bn (PANEAPF-Bu).
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
Conclusions
This work was financially supported by the National Natural Science
Foundation of China (Nos. 21572156 and 21777111), Tianjin
Research Program of Application Foundation and Advanced
Technology (No. 14JCYBJC22600).
In summary, we have developed a serious of novel phosphoric acid-
functionalized polyacrylonitrile fibers as highly efficient solid acid
catalysts with alternating hydrophilic or hydrophobic surface micro-
environment. The catalytic performances of the functionalized
fibers with different surface micro-environment are tested by the
cyclocondensation of
β-ketoesters with 2-aminobenzamides,
Notes and references
Knoevenagel condensation and Biginelli reactions. On the basis of
several compelling observations, the benzyl or n-butyl modified
hydrophobic fibers (Bn-PANEAPF or Bu-PANEAPF) show much higher
catalytic activities, which illustrates surface synergies of the catalyst
and interface acceleration effect in reaction medium, thus
promoting catalytic activities impressively in the C-C and C-N bond
formation reactions. Besides, a surface synergistic mechanism is
proposed to explain the micro-environment effect. More
importantly, the fiber catalysts do not demonstrate significant
leaching of acid catalytic active sites and can be reused over 10
times without decrease in catalytic activity. Given many other
advantages such as simple preparation, metal, oxidant and
radiation-free process, low catalyst dosage, green solvents, wide
range of applications, economical and environmental-friendlines,
the phosphoric acid-functionalized fiber catalysts are very attractive
for industrial production. Meanwhile, such a surface tuning strategy
opens up a new possibility for designing efficient fiber catalysts for
greener and more sustainable in the future.
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Green Chemistry
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DOI: 10.1039/C7GC02935G
TOC
Novel hydrophilic/hydrophobic surface micro-environment modified phosphoric acid-
functionalized PANFs are prepared for one-pot C-C and C-N bond formation reactions with high
efficiencies and preeminent reusability in environmental-friendly application process.