Tuning Microenvironment of Quaternary Ammonium Salt and Tertiary Amine Bifunctionalized Polyacrylonitrile Fiber for Cooperatively Catalyzed Aza-Michael Addition

Article abstract of DOI:10.1007/s10562-020-03340-7

A series of novel polyacrylonitrile fiber catalysts modified with different ratio of quaternary ammonium salt (QA) to tertiary amine (TA) were screened for heterogeneously catalyzed aza-Michael addition in water. The best catalyst was found to be PAN

Full text of DOI:10.1007/s10562-020-03340-7

Catalysis Letters
https://doi.org/10.1007/s10562-020-03340-7
Tuning Microenvironment of Quaternary Ammonium Salt and Tertiary
Amine Bifunctionalized Polyacrylonitrile Fiber for Cooperatively
Catalyzed Aza‑Michael Addition
Xueyu Yuan1 · Huimin Du1 · Jingyan Zhao1 · Anyaegbu Ejike Chima1 · Ning Ma1,2 · Minli Tao1,3 · Wenqin Zhang1,2
Received: 6 June 2020 / Accepted: 26 July 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
A series of novel polyacrylonitrile fber catalysts modifed with diferent ratio of quaternary ammonium salt (QA) to tertiary
amine (TA) were screened for heterogeneously catalyzed aza-Michael addition in water. The best catalyst was found to be
PANPQ-1F with a QA:TA ratio of 1:1, which can lead to high yield, excellent selectivity of solvent and great recyclability
under optimized conditions. The special microenvironment of hydrophilic inner layer and hydrophobic surface layer of
PANPQ-1F is conducive to the leaving of the product and the high conversion of the reaction. Based on the fact that appro-
priate microenvironment combined with the catalytic sites promoted the reaction, cooperative efect of QA and TA in the
microenvironment of PANPQ-1F catalyst was proposed.
Graphic Abstract
Keywords Polyacrylonitrile fber · Cooperative catalysis · Microenvironment · Aza-michael addition
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-020-03340-7) contains
supplementary material, which is available to authorized users.
3
* Ning Ma
mntju@tju.edu.cn
National Demonstration Center for Experimental Chemistry
& Chemical Engineering Education, Tianjin University,
Tianjin 300350, China
1
Department of Chemistry, School of Sciences, Tianjin
University, Tianjin 300072, People’s Republic of China
2
Collaborative Innovation Center of Chemical
Science and Engineering (Tianjin), Tianjin 300072,
People’s Republic of China
Vol.:(0123456789)
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X. Yuan et al.
by introducing interchain auxiliary groups near the cata-
lytic sites [19, 34]. In this work, a novel microenviron-
ment regulation method was designed to synthesize a fber
catalyst with intrachain quaternary ammonium salt (QA)
moiety in the inner side of the catalytically active tertiary
amine (TA) sites; thus a series of catalysts (PANPQ-nF,
n = 0.1–1) with QA and TA bifunctionalized structure
were prepared (Scheme 1), in which a special microenvi-
ronment of hydrophobic inner and hydrophilic outer layer
were successfully constructed.
1 Introduction
For green chemical production, the recycling of catalysts
is of great importance. Recently, there have been exam-
ples of highly efcient small-molecule catalysts grafted
covalently onto solid supports, such as silica [1, 2],
zeolites [3, 4], metal oxides [5, 6], polymers [7, 8], and
metal–organic frameworks [9–11]. In this way, the state-
of-the-art heterogeneous catalysts showed the advantages
of easy separation and outstanding recovery capacity [12].
Because water is non-toxic, low-cost and easily available,
it’s widely regarded as a green solvent [13]. However,
many heterogeneous catalysts hardly perform well in aque-
ous media. For example, one of the drawback of zeolites
is their sensitivity to hot water, which may hinder their
full utilization in aqueous phase reactions [14, 15]. The
organic phase in aqueous solution is difcult to enter into
the pores of some mesoporous materials [16], which leads
to the decrease of catalytic efciency. Therefore, develop-
ment of new support materials to overcome this drawback
is important for green chemical production in both indus-
try and academia.
Aza-Michael addition of nitrogen nucleophiles to elec-
tron-defcient alkenes is a common method for C-N bond
formation and has important applications in the synthesis
of N-containing compounds, such as β-aminocarbonyls and
N-containing heterocycles in many antibiotics and pharma-
ceutical intermediates [35, 36]. Aza-Michael addition can
be catalyzed by organic amine [37] or ionic liquids [38–40],
which indicates that the PANPQ-nF with both QA and TA
moieties has the potential to catalyze this reaction. There-
fore, diferent microenvironments in the fber surface were
investigated by using aza-Michael addition of chalcone and
aniline as a probe, and the efect of the ratio of QA to TA on
the catalytic activity and recyclability was elucidated.
The tuning of microenvironment near the catalytic
sites have been comprehensively proved to dramatically
enhance the activity of a catalyst [17, 18]. For heterogene-
ous catalysis in water, the mass transfer resistance between
aqueous and organic phases is also crucial [19–21]. There-
fore, the hydrophilicity/hydrophobicity near the catalytic
sites can be regulated to improve the catalytic activity
[22]. For example, a series of mesoporous silica catalysts
with diferent wettability have been prepared by Fu et al.,
which showed the catalytic activity in aqueous hydrogena-
tions fve times higher than that of the commercial Pd/C
catalyst [20].
2 Experimental
2.1 Synthesis of the Fiber Catalysts
The QA and TA bifunctionalized polyacrylonitrile fbers
PANPQ-nFs were synthesized via two steps (Scheme 1).
Step 1: The mixture of PANF (1 g), N-(2-aminoethyl)
piperazine (15 mL) and deionized water (15 mL)
was refuxed for 8 h. Then the fber was taken out
and washed with hot water (60–70 °C) until neu-
tral. The fber was dried overnight at 60 °C under
vacuum to gain PANPF. The weight gain of PANPF
based on PANF was 27% with the functionality of
1.6 mmol/g.
Polyacrylonitrile fber (PANF) is one of the support mate-
rials in which the surface microenvironment can be tuned to
make the catalytic sites highly efcient. PANF, widely used
in textile industry, has advantages such as low cost, easy
availability, high mechanical strength, corrosion resistance
and braid-ability [23, 24]. It contains sufcient cyano groups
that can form carboxyl, amido or other groups through fac-
ile reactions [25, 26]. In our previous reports, PANF has
been used to immobilize quaternary ammonium salt [25],
phosphorous acid [27], pyrrolidine [28], tertiary amine [29],
L-lysine [30], etc., making functionalized fber catalysts with
high activity, simple preparation and good recyclability [31,
32]. The immobilization process generates middle and deep
modifcation in the surface, so that a special microenviron-
ment surrounding active site is constructed, which helps to
improve the catalytic performance [26, 33].
Step 2: The mixture of PANPF (0.5 g), benzyl bromide
(5 g), acetonitrile (15 mL) and Na2CO3 (0.045 g)
was refuxed for 0.5–8 h to get diferent propor-
tions of QA and TA (Table 1). Afterwards, the
fber was taken out and washed with hot water
(60–70 °C). Then the fber was stirred in satu-
rated Na2CO3 solution overnight and washed with
hot water (60–70 °C) until neutral. The excess
benzyl bromide on the fber was extracted using
Soxhlet extraction apparatus and ethanol as sol-
vent for more than 6 h. The fber was dried over-
night at 60 °C under vacuum to gain PANPQ-nFs
(n=0.1–1).
However, in the abovementioned reports, the microen-
vironments of heterogeneous catalysts have been tuned
1 3

Tuning Microenvironment of Quaternary Ammonium Salt and Tertiary Amine Bifunctionalized…
Scheme 1 The general synthetic steps of fber catalysts
Table 1 Adjust the ratio of QA to tertiary TA of PANPQ-nFs
(Thermo Nicolet). The elemental analysis (EA) was per-
formed on a vario micro cube (Elementar). The X-ray
powder diffraction patterns (XRD) were obtained using a
D/MAX-2500 X-ray diffractometer (Rigaku Corporation).
The thermogravimetric analysis (TGA) of fiber samples
were measured by STA409PC TGA/DSC (Netzsch com-
pany, Germany). The mechanical properties of different
fiber catalyst samples were tested with an electronic sin-
gle fiber strength tester YG(B)001A (Wenzhou Darong
Textile Instrument Corporation, China). The X-ray photo-
electron spectra (XPS) were performed on PH1600 spec-
trometer (PERKIN ELMZR). The water contact angles
of fibers were determined using OCA15EC (DataPhysics
company, Germany). The fibers surface morphology was
characterized by scanning electron microscope (SEM)
(Hitachi, model S-4800). The analyses of HPLC were
Entry Benzyl
bromide
(g)
Solvent (15 mL) Tempera- Time (h) QA:TA
ture (°C)
1
2
3
4
5
5
5
5
5
5
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
80
80
80
80
80
0.5
3
4
5
8
0.1:1
0.2:1
0.3:1
0.5:1
1:1
PANPF (weight gain 27%, functional degree 1.6 mmol/g, 1 g)
Add Na2CO3 (0.17 g) into mixture
1
performed on Waters Model 510. The H NMR spectra
2.2 Characterization of the Fiber Catalysts
were obtained by BRUKER-AVANCE III (400 MHz)
instruments using TMS (tetramethylsilane) as the inter-
nal standard.
The Fourier transform infrared (FT-IR) spectra were
obtained using an AVATAR360 FTIR spectrometer
1 3

X. Yuan et al.
2.3 General Procedure for Aza‑Michael Addition
The mixture of chalcone (0.5 mmol), aniline (1 mmol), fber
catalyst (10 mol%) and water (3 mL) were stirred at 60 °C
for 10 h. After completion of the reaction, the mixture was
cooled to room temperature. The fber catalyst was taken
out with tweezers and washed with methylene chloride
(5 mL×3) and ethanol (5 mL×3). The fltrate was combined
with the remaining reaction mixture and a small aliquot was
taken out for HPLC analysis with the nitrobenzene as inter-
nal standard.
3 Results and Discussion
3.1 Preparation of PANPQ‑nFs
Fig. 1 FT-IR spectra of (a) PANF, (b) PANPF, (c) PANPQ-1F, (d)
PANPQ-1F-1, and (e) PANPQ-1F-6
The QA and TA bifunctionalized polyacrylonitrile fib-
ers (PANPQ-nFs) were synthesized by two simple steps
(Scheme 1). The functionality of PANPQ-nFs were deter-
mined by weight gain. The formula for weight gain is:
In the spectrum of PANPF (Fig. 1b), the appearance of a
peak at 1578 cm−1 is due to the bending vibration of N–H
in CONH, and the obvious peak at 837 cm−1 is attrib-
uted to the bending vibration of the N–H in the second-
ary amine. Both of them indicate the successful grafting
of N-aminoethylpiperazine on PANF. Furthermore, in the
spectrum of PANPQ-1F (Fig. 1c), the appearance of the
peaks at 744 cm−1 and 700 cm−1 can support the success-
ful grafting of benzyl groups in the PANPF; the decrease
of the peak at 837 cm−1 can also ndicate the consump-
tion of secondary amine. What’s more, the FT-IR spectra
of PANPQ-1F-1 and PANPQ-1F-6 show that the absorption
peaks of the catalyst PANPQ-1F remained unchanged after
being used once and six times, indicating that the fber
catalyst was stable enough for multiple use.
[
]
weightgain = (w2 − w1)∕w1 × 100%, in which w1 and
w2 are the weights of the original and functionalized fber,
respectively.
The weight gain of PANPQ-nFs can be easily adjusted by
changing the preparation conditions, including the function-
alization degree of PANPF, benzyl bromide concentration,
reaction temperature and time. After further balancing of
functionalization and mechanical strength, the weight gain
of PANPF about 27% was preferred. The ratio of quater-
nary ammonium (QA) to tertiary amine (TA) in the target
fber PANPQ-nFs were adjusted by varying the reaction time
of the selected PANPF fber (Table 1). With grafting time
more than 8 h, the proportion of QA could not be increased
further. To avoid the decrease of mechanical strength, the
grafting time was no longer prolonged.
3.2.2 Elemental Analysis (EA)
3.2 Characterization of the Fiber
As shown in Table S5, because of the lower carbon con-
tent and higher hydrogen content of N-(2-aminoethyl)
piperazine, the carbon content of the PANPF decreased
expectedly and the hydrogen content increased compared
with original fber PANF (Table S5, Entry 2). After fur-
ther treatment of PANPF with benzyl bromide, the hydro-
gen and nitrogen contents of PANPQ-1F decreased and the
carbon content increased in accordance with the carbon,
nitrogen and hydrogen contents of benzyl group (Table S5,
Entry 3). After the PANPQ-1F was used six times, there
was a slight change in the EA data of the PANPQ-1F-6
(Table S5, Entry 5). This change may be caused by the
slight adsorption of the reactants or product on the surface
of the fber catalyst.
3.2.1 Fourier Transform Infrared Spectroscopy (FT‑IR)
Before FT-IR measurement, the fber sample was shred-
ded and made into pellets with KBr. As shown in Fig. 1,
the FT-IR spectrum of PANF (Fig. 1a) has two charac-
teristic absorption peaks at 2244 cm−1 and 1734 cm−1
which attribute to the C≡N and C=O stretching vibrations,
respectively. Compared with PANF, the functionalized fb-
ers have a broad absorption peak around 3500 cm−1 caused
by the stretching vibration of N–H, which proves the suc-
cessful aminolysis of –C≡N associated with the weaken-
ing of the peak at 2244 cm−1. The peak at 1651 cm−1 is
attributed to the stretching vibration of C=O in amide.
1 3

Tuning Microenvironment of Quaternary Ammonium Salt and Tertiary Amine Bifunctionalized…
Fig. 2 SEM images of a PANF, b PANPF, c PANPQ-1F, d PANPQ-1F-1, and e PANPQ-1F-6
Table 2 Mechanical properties of diferent fbers
Entry
Fiber
BS (cN)
RBS (%)
1
2
3
4
5
PANF
PANPF
PANPQ-1
PANPQ-1F-1
PANPQ-1F-6
10.67
10.17
9.57
8.57
7.86
100
95
90
80
74
F
3.2.4 X‑ray Difraction (XRD)
The XRD pattern of PANF (Fig. 3a) shows a main difrac-
tion peak at 17° and a weak peak centered at 29.5°, corre-
sponding to the (100) and (110) crystallographic planes of
the PANF hexagonal lattice, respectively [27, 32, 41]. After
modifcation, the patterns of PANPF and PANPQ-1F remained
similar to that of PANF, which indicates that the grafting
of small molecules doesn’t change the crystal structure of
PANF. Even after being used six times, the difractogram of
PANPQ-1F-6 remained unchanged, proving that the crystal
structure of PANPQ-1F wasn’t damaged after reuse.
Fig. 3 X-ray difraction spectra of (a) PANF, (b) PANPF, (c)
PANPQ-1F, (d) PANPQ-1F-1, and (e) PANPQ-1F-6
3.2.3 Scanning Electron Microscope (SEM)
The SEM images of PANF, PANPF, PANPQ-1F, PANPQ-1F-1
and PANPQ-1F-6 are shown in Fig. 2. It can be seen from
the Fig. 2 that PANPF and PANPQ-1F become thicker com-
pared with PANF, which can prove the successful graft-
ing of small molecules (Fig. 2a, b, c). In addition, after
the fber was used 6 runs, the surface of PANPB-5F-6 was
rougher and had more cracks, but there was no serious
damage which indicated that the fber had good physical
tolerance during recycling (Fig. 2e).
3.2.5 Breaking Strength (BS)
The BS and retention of breaking strength (RBS) of fb-
ers are shown in Table 2. Compared with the original fber,
the breaking strength of PANPQ-1F decreased from 10.67 to
9.57 with 90% RBS (Table 2, Entry 3). After being used six
times, due to the long-time mechanical stirring and heating,
the PANPQ-1F-6 breaking strength was decreased from 9.57
1 3

X. Yuan et al.
to 7.86, but still maintained 82% breaking strength com-
pared with the PANPQ-1F (Table 2, Entry 5), indicating that
the functionalized fbers have good mechanical strength for
reuse.
at 399.6 and 402.1 eV, corresponding to N–C=O/N(C)3 and
N+ respectively (Fig. S2) [43]. To sum up, the XPS spectrum
of PANPQ-1F further demonstrates the successful grafting of
the fbers.
3.2.6 Thermogravimetric Analysis (TGA)
3.3 Catalytic Properties of Bifunctionalized Fiber
Catalysts in Aza‑Michael Addition
Thermogravimetric analysis is used to evaluate the ther-
mal stability of the functionalized fbers. The samples were
heated in nitrogen gradually from room temperature to
800 °C. As shown in Fig. S1, from room temperature to
300 °C, PANF had almost no weight loss, indicating that
the fber support has high thermal stability. The PANPF and
PANPQ-1F had slight mass loss (<6%) below 200 °C due to
possible dehydration, which implies that the functional fb-
ers have good thermal stability and can be applied in a wide
temperature range. Compared with PANPQ-1F, PANPF had
more mass loss at 200 °C, mainly because it absorbed more
water via stronger hydrogen bonding between amine N–H
and water molecule. The functional fbers were decomposed
when the temperature was higher than 200 °C. It was prob-
ably caused by decomposition of QA and TA moieties and
structural damage of fber matrix.
To investigate the microenvironments of fber catalysts,
the reaction of aniline with chalcone in water was used as
a model reaction. Without any catalyst or in the presence
of PANF and PANPF, the yields of the reaction were very
low, which indicated that neither cyano groups in PANF nor
amido and amino groups in PANPF could catalyze the reac-
tion (Table 3, Entry 1–3). However, the yield by PANPQ-0.1
F
catalysis was signifcantly improved (Table 3, Entry 4). The
ratio of QA to TA changed gradually by prolonging the
grafting time of PANPF (Table 1). The results showed that
the catalytic performance increased gradually as the content
of QA moiety increased (Table 3, Entry 4–8). When the ratio
of QA to TA equals 1: 1, the catalytic performance was the
best (Table 3, Entry 8). Using the fber catalyst PANEF with
only TA groups, the catalytic activity decreased obviously
and the yield was only 32% (Scheme 1; Table 3, Entry 9).
Using the fber PANPD-BrF with only QA groups as a cata-
lyst, the yield decreased to 19% (Scheme 1; Table 3, Entry
10). Therefore, QA and TA showed cooperative efect in
PANPQ-1F-catalyzed aza-Michael addition.
3.2.7 X‑Ray Photoelectron Spectroscopy (XPS)
The fiber surface composition can be characterized by
XPS. The XPS spectra of PANF, PANPF, PANPQ-1F and
PANPQ-BrF (Scheme S1) are presented in Fig. 4. The high
resolution C 1s spectrum for the PANPQ-1F can be decon-
voluted into three species, near 284.7, 286.5, and 288.3 eV,
which are attributed to carbon from C–N/C–O, C≡N, and
N–C=O, respectively (Fig. S2) [27, 32, 42, 43]. The high
resolution N 1s spectrum for the PANPQ-1F has two peaks
The cooperative catalysis can also be demonstrated by
the PAND-QF (Scheme 1). The yield was 88% in the pres-
ence of PANDF (Table 3, Entry 11). The PANDF was further
reacted with benzyl bromide to obtain PAND-QF, which had
both QA and TA moieties. The PAND-QF catalytic yield was
up to 98%, indicating that both QA and TA were involved
in the cooperative catalysis (Table 3, Entry 13). After con-
verting all the TA groups of the PANDF into QA to obtain
the PANQF, the yield decreased to 19%, indicating that the
catalytic activity was poor when only QA groups was pre-
sent (Table 3, Entry 12). However, the product was densely
adsorbed onto PAND-QF surface and was difcult to separate
after the reaction was completed, which didn’t happen in
PANPQ-1F catalysis.
The different catalytic phenomena and activities
between PANPQ-1F and PAND-QF can be attributed to the
diferent fber microenvironments (Fig. 5). In the microen-
vironment constructed with separated interchain QA and
TA moieties into PAND-QF surface, hydrophobic product
molecules accumulated around hydrophobic active sites
of TA, which inhibited the recovery of active sites for
catalytic cycle. However, by decorating QA and TA in the
same molecular chain, the PANPQ-1F had a special micro-
environment with hydrophobic outer layer and hydrophilic
inner layer, which was benefcial to the swelling of the
Fig. 4 XPS spectra of PANF, PANPF, PANPQ-1F and PANPQ-Br
F
1 3

Tuning Microenvironment of Quaternary Ammonium Salt and Tertiary Amine Bifunctionalized…
Table 3 Catalytic activities of diferent fber catalysts
Entry
Catalyst
Dosage
Temperature (°C)
Solvent
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
Blank
PANF
PANPF
PANPQ-0.1
PANPQ-0.2
PANPQ-0.3
PANPQ-0.5
PANPQ-1
PANEF
–
50
50
50
50
50
50
50
50
50
50
50
50
50
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
12
12
12
12
12
12
12
12
12
12
24
24
24
2
3
9
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
F
F
F
F
57
84
89
93
99
32
17
88
19
98
F
9
10
11
12
13
PANPD-Br
PANDF
PANQF
F
PAND-Q
F
Reaction conditions: chalcone (0.5 mmol), aniline (1 mmol) in 3 mL of solvent and the fber catalyst dosage was based on TA (1.3 mmol/g)
aMeasured by HPLC with nitrobenzene as the internal standard
Fig. 5 The microenvironment of PANPQ-1F and PAND-Q
F
fber in water. The reactants were easily enriched in outer
layer during stirring, and they interacted with the cata-
lytic TA sites to yield the addition product. Afterward, the
hydrophilic inner layer of PANPQ-1F had a certain repul-
sion to hydrophobic product molecule, which facilitated
its movement out of the microenvironment, and made the
active TA sites free and highly exposed to next catalytic
cycle. As such, tuning of microenvironment near the active
sites afected their catalytic efciency via diferent desorp-
tion (Fig. 6).
1 3

X. Yuan et al.
Fig. 6 Possible microenvironment of fber for the aza-Michael addition catalyzed by PANPQ-1
F
After that, the efect of diferent anions of the fber cata-
lyst on the activity was investigated. As shown in Scheme
S4, fve fber catalysts with diferent anions had been pre-
pared. The experimental results showed that changing the
anion of the fber catalysts had a great efect on their perfor-
mance. When the anion was CO32−, its catalytic activity was
the highest (Table S3, Entry 1–6).
time. However, there was no target product when the substitu-
ent –CH3 was in the ortho position, indicating that the reaction
is greatly afected by the steric hindrance. When the substitu-
ents were electron-withdrawing groups, diferent phenomena
were observed. When the substituent was –Cl at the para posi-
tion, the yield could reach more than 94% in a short time, and
the good yield could be obtained by prolonging the reaction
time with meta-Cl, indicating that the position of the substitu-
ent had a signifcant efect on the reaction. When the substitu-
ent was –COOCH3, the reaction was conducted at 60 °C for
above 36 h to get an 80% yield. By increasing the reaction
temperature to 80 °C, a yield of 97% could be achieved in 10 h.
However, when the substituent was –NO2, the target product
couldn’t be obtained when the temperature was increased or
the reaction time was prolonged. It was probably because the
strong electron uptake of –NO2 led to the decrease of amino
activity. In summary, the PANPQ-1F can efectively catalyze
aza-Michael addition with wide substrate scope and no by-
products were observed.
Moreover, the reaction conditions of catalytic aza-
Michael addition were optimized (Table 4), and diferent
solvents were screened (Table 4, Entry 1–10). Because of
special microenvironment in the fber surface, the PANPQ-1
F
had good solvent selectivity, and the reaction had high yields
only in water (Table 4, Entry 1). The optimal conditions for
the reaction were 10 mol% PANPQ-1F, 60 °C, 10 h, and water
as the solvent (Table 4, Entry 20).
3.4 Aza‑Michael Addition of Diferent Amines
and Chalcone
To investigate the reaction scope, aza-Michael addition of dif-
ferent amines and chalcone was explored under the optimal
reaction conditions. As shown in Table 5, most products were
obtained with excellent yields. The electron-donating groups
on benzene ring of aniline such as –CH3 and –OCH3 were
favorable, and more than 93% yields could be obtained in short
3.5 Possible Mechanism of PANPQ‑1F‑Catalyzed
Aza‑Michael Addition
A possible mechanism of aza-Michael addition of chal-
cone with aniline catalyzed by PANPQ-1F was described
1 3

Tuning Microenvironment of Quaternary Ammonium Salt and Tertiary Amine Bifunctionalized…
Table 4 Optimization of the aza-Michael addition
Entry
Catalyst
Dosagea
Temperature (°C)
Solvent
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
PANPQ-1
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
20 mol%
10 mol%
5 mol%
50
50
50
50
50
50
50
50
50
50
25
50
60
70
80
100
60
60
60
60
60
H2O
10
10
10
10
10
10
10
10
10
10
6
91
12
16
3
1
4
1
1
2
28
33
77
84
86
85
49
99
99
77
99
89
EtOH
CH3OH
CH3CN
EtOAc
DMF
THF
1,4-Dioxane
Cyclohexane
Toluene
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
9
10
11
12
13
14
15
16
17
18
19
20
21
6
6
6
6
6
12
12
12
10
8
10 mol%
10 mol%
Reaction conditions: chalcone (0.5 mmol), aniline (1 mmol), H2O (3 mL)
aBased on TA
bMeasured by HPLC with nitrobenzene as the internal standard
in Scheme 2, indicating the cooperative catalysis of QA
and TA. Initially, the TA in PANPQ-1F acted as a nucleo-
phile, in which N atom attacked electron-defcient alkene
to form an active intermediate A. Then the CO32− moved
outward to activate aniline through hydrogen bonding
[44–46], which improved the nucleophilicity of aniline,
catalytic sites were highly exposed for the next catalytic
cycle (Fig. 6). Therefore, QA in PANPQ-1F has two func-
tions: N cation constructs the inner hydrophilic layer asso-
2−
ciated with CO3 to facilitate the desorption of product
molecule and enhance the catalytic efciency of active TA
sites; carbonate ion activates aniline nucleophile and also
mediates proton transfer. To the best of our knowledge,
this type of tuning method of microenvironment around
active sites and the resulting cooperative catalysis mode of
QA with TA have not been reported, although there have
been scarce reports on cooperative heterogeneous cataly-
sis by QA and TA, for example, Yin et al. reported that
the cooperative efect of QA and TA in GO-immobilized
urotropin derivative for catalytic CO2 cycloaddition [47].
then promoted the key nucleophilic substitution of inter-
2−
mediate A and the proton transfer from aniline to CO3
.
Subsequently, TA on the fber catalyst was released to form
the enolate intermediate in B, which accepted proton from
HCO3− and underwent keto-enol tautomerization to form
the addition product. Eventually, the product molecule
was expelled out of the microenvironment by hydrophilic
inner layer, especially with the aid of CO32−. The surface
microenvironment of PANPQ-1F was recovered and the
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X. Yuan et al.
Table 5 PANPQ-1F catalyzed aza-Michael addition of various amines and chalcone
Reaction conditions: chalcone (1 mmol), aniline (2 mmol), and catalyst dosage of 10 mol% based on tertiary amine content (PANPQ-1F)
Isolated yield
Scheme 2 Possible mechanism of the aza-Michael addition catalyzed by PANPQ-1
F
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Tuning Microenvironment of Quaternary Ammonium Salt and Tertiary Amine Bifunctionalized…
low cost and environmental friendliness, showing great
practical potential in chemical industry. The unique micro-
environment with outer hydrophobicity and inner hydrophi-
licity was proposed to explain the diferent desorption of
the formed product, and novel cooperative catalysis mode
of tertiary amino, quaternary ammonium and carbonate ion
was elucidated.
Acknowledgements This work was financially supported by the
National Natural Science Foundation of China (No. 21777111).
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