Mechanochemical Process to Construct Porous Ionic Polymers by Menshutkin Reaction

Article abstract of DOI:10.1002/cssc.202101093

Abstract: The synthesis of porous ionic polymers (PIPs) via the Menshutkin reaction is intriguing because the reaction works smoothly in catalyst-free condition with 100 % atom utilization. However, the rotation of methane site, nonrigid knots, and charge

Full text of DOI:10.1002/cssc.202101093

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doi.org/10.1002/cssc.202101093
ChemSusChem
Very Important Paper
Mechanochemical Process to Construct Porous Ionic
Polymers by Menshutkin Reaction
[
a]
[a]
[b]
[a]
Shengtai. Hou, Minshan. Meng, Dandan. Liu, and Pengfei. Zhang*
Abstract: The synthesis of porous ionic polymers (PIPs) via the
Menshutkin reaction is intriguing because the reaction works
smoothly in catalyst-free condition with 100% atom utilization.
However, the rotation of methane site, nonrigid knots, and
charge interaction all may cause collapses of the channel, which
is detrimental to the synthesis PIP in solid-state conditions. In
this work, an inorganic salt (NaBr, NaCl: pollution-free and easy
to recycle) was rationally chosen as the hard template and
effectively prevented the intermolecular packing. Moreover, the
increased surface area dramatically promoted the catalytic
activity of PIP for cyclic carbonate synthesis. This work provides
a green and efficient strategy to construct PIPs via the
Menshutkin reaction.
mechanochemical process is supposed to be a high-potential
[2b,9,15]
candidate to construct PIPs.
The Menshutkin reaction,
known as one of the most common OPIs, works smoothly in
[16]
catalyst-free conditions with 100% atom utilization. Never-
theless, due to the intermolecular packing resulted from the
rotation of methane site, nonrigid knots, and charge
[17]
interaction, it is tough to realize the preparation of PIP simply
[5]
via common monomers by Menshutkin reaction.
According to recent reports, a rigid monomer was applied
to prevent the collapse of porous channel with Brunauer-
2
À 1 [9]
Emmett-Teller surface area (SBET) reaching 41 m g , seeming
to give a chance for the fabrication of PIPs. However, a series of
strict and complex treatments accompanied this method, such
as requirement for working in an autoclave at 120°C for 7 days
to form the porous structure. In our previous work, SiO was
2
employed as the hard template for directing porosity, yet still
[15]
Porous ionic polymers (PIPs) have gradually obtained scientific
suffered several deficiencies.
For example, severe environ-
[
1]
attention due to their various applications like separation,
mental problems would be caused by the excessive use of
[2]
[3]
[18]
catalysis, and gas storage. To date, several synthetic
strategies have been reported, including polymerization of ionic
one-step process of polymerization and
ionization, copolymerization of neutral monomer and ionic
building unit, and post-modification for producing ionic
groups. Actually, the vast majority of synthesis methods of
PIPs are limited to wet processes, which often demand
excessive use of organic solvent, long reaction time (e.g.,
12 h), accessible solubility of monomers, and inert gas
protection. Hence, developing a simple method to construct
PIPs is a matter of great urgency.
hydrofluoric acid to remove templates. In this regard, the
utilization of the Menshutkin reaction with rigid monomers to
construct PIPs may be a potential route.
[4]
building unit,
[
2b,5]
In this work, a mechanochemical strategy for PIP construc-
tion was provided via the Menshutkin reaction within 2 h
(Scheme 1). It is worth noting that NaBr was rationally chosen
as the salt template, which can be easily reused to reduce
environmental pollution. Interestingly, the inorganic salt pre-
vented the packing of intermolecular effectively. Meanwhile, a
moderate specific surface area was presented in PIPs with Br
[6]
[7]
[8]
[9]
[10]
>
[6c]
[19]
ions, revealing the possible use for catalysis.
Indeed, the
[11]
Mechanochemical process, possessing the advantages of
catalytic performance of the as-made PIP was almost twice that
of nonporous material for cyclic carbonate synthesis.
[12]
solvent-free operation and short reaction time,
exhibits a
promising prospect in the transformation of organic functional
groups and the synthesis of polymers. In principle, the one-
step process of polymerization and ionization (OPI) through
Firstly, the synthesis of PIPs was initiated via various
combinations of monomers by ball milling (Scheme 2; Table 1,
entries 1–6). The experimental data showed that no porous
material was obtained, even using a rigid monomer (Table 1,
entries 2, 4, and 6). Subsequently, to achieve a porous polymer
by solid-to-solid replication, NaBr was consequently used as the
salt template. The PIP showed a moderate specific surface area
of 55 m g (Table 1, entry 7), which further illustrated the
necessity of templates. Subsequently, a systematic research of
additives was conducted to determine the dosage and type of
salt templates (Table 1, entries 7–11). These results confirmed
strongly the feasibility to obtain a PIP via the Menshutkin
reaction in solid-state conditions.
[13]
[14]
[
a] Dr. S. Hou, M. Meng, Prof. P. Zhang
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University
2
À 1
Shanghai, 200240 (P. R. China)
E-mail: chemistryzpf@sjtu.edu.cn
b] D. Liu
[
Key Laboratory for Advanced Materials and
Research Institute of Industrial Catalysis
School of Chemistry and Molecular Engineering,
East China University of Science and Technology
Shanghai 200237 (P. R. China)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cssc.202101093
To determine the structural characteristics (Scheme 3), the
PIP was characterized by solid-state C NMR spectroscopy. As
shown in the spectrum (Figure 1a), the polymer produced
obvious peaks in the region of 157.3–124.1 ppm, which are
ascribed to aromatic carbon atoms (A, C–F). The signal of
13
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Scheme 1. Schematic representation of mechanochemical synthesis of PIP.
scopy (HRTEM) for these two types of networks (IP, PIP)
provided morphological information. As shown in Figure S1 and
Figure 2a, b, the data showed broad peaks and amorphous
aggregations, suggesting its amorphous structure. The non-
porous IP showed irregular spherical particles with a diameter
of 1–3 μm (Figure 2a). A coral-like porous structure was revealed
by the SEM and TEM images of PIP (Figure 2b, d, Figure S2),
proving the role of NaBr for constructing PIP. Then, the
permanent porosity of IP and PIP was measured by nitrogen
physisorption measurements (77 K). As expected, the salt
template endowed the ionic polymer with moderate surface
2
À 1
areas up to 70 m g (Figure 2e). All above results offered
evidence that a salt template is indispensable for constructing
the PIP via the Menshutkin reaction.
Scheme 2. Chemical formulas of monomers.
The thermal stability of PIPs was investigated by thermogra-
[
a]
Table 1. Screening of the optimal solid-state synthesis condition of PIP.
vimetric analysis (TGA) under N atmosphere. The PIPs remained
2
Entry
Monomer 1
Monomer 2
Additive
Yield
%]
S
BET
2
stable up to 200°C and decomposed rapidly at 265°C (Fig-
À 1
[
[m g
]
ure 2f). Meanwhile, the elemental analysis data also showed a
high content of Br (31.57%), which was close to the theoretical
À
1
2
3
4
5
6
1a
1a
1b
1b
1c
1c
1b
1b
1b
1b
1b
2a
2b
2a
2b
2a
2b
2b
2b
2b
2b
2b
–
–
–
–
–
–
93
91
94
88
90
87
94
92
93
80
86
<5
<5
<5
<5
<5
<5
55
34
70
67
62
value (Table S1). These inherent properties played a decisive
[22]
role in application.
Considering the intrinsic halogen anions in the structure,
the catalytic performance of the cyclic carbonate synthesis was
[
[
[
b]
[c]
7
8
9
1
1
2×M
[23]
b,d]
b]
initially investigated.
Propylene oxide was selected as the
2×M
5×M
10×M
20×M
model substrate to optimize the reaction conditions. As
[
[
b]
b]
0
1
predicted, the PIP was active towards conversion of CO into
2
cyclic carbonates, and the conversion was almost twice that of
the nonporous material (IP) further exhibiting the advantage of
the porous material (Table 2, entries 1 and 2). Next, we
increased the reaction time to 24 h, which allowed us to reach
[
[
a] Reaction conditions: the monomers were mixed by ball milling for 1 h.
b] Monomer 1 and additive (NaBr) were ball-milled for 0.5 h, then
monomer 2 was added to the system with ball milling for 0.5 h. [c] 2×M:
Twice the total mass of monomers. [d] Additive: NaCl.
8
9% GC yield (Table 2, entries 3–6). Subsequently, when the
dosage of catalyst was increased to 100 mg, almost all of the
0 mL propylene oxide was converted into target product
2
carbon chemical shift in 60.2 ppm was assigned to methylene
(Table 2, entry 7). Under the optimized conditions, the reuse
and recovery of PIP were estimated by recycling test. Excellent
catalytic performance was well preserved during four rounds of
recycling test (Figure 3). Hence, the PIP seemed to be a good
alternative solid catalyst for cyclic carbonate synthesis.
Encouraged by the catalytic performance in conversion of
CO2 into cyclic carbonates, the substrate compatibility was
further investigated. As shown in Scheme 4, substrates with
halogen atom or hydroxy groups gave good yields. In the case
of large inert substrates, substrates with functional groups such
[20]
carbon (B) linking the benzene ring and pyridium. Addition-
ally, the Fourier transform infrared (FTIR) spectroscopy of
monomers and PIP further confirmed the success of SN2
substitution-based polymerization. As shown in Figure 1b, the
À 1
signal at 1645 cm could be attributed to the characteristic
+
[21]
band of C=N .
All those results indicated that the SN2
reaction works smoothly in ball milling conditions.
Powder X-ray diffraction (P-XRD), scanning electron micro-
scopy (SEM), and high-resolution transmission electron micro-
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Scheme 3. Chemical formulas of IP and PIP.
1
3
Figure 1. (a) Solid-state C NMR spectrum of PIP.( b) FTIR spectra of monomer 1b, monomer 2b, and PIP.
Figure 2. SEM images of (a) IP and (b) PIP. TEM image of (c) IP and (d) PIP. (e) Nitrogen adsorption isotherms (77 K) of PIP and IP. (f) TGA data of PIP and IP.
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[a]
Table 2. Optimization of reaction conditions.
[b]
Entry
Catalyst
t
Yield
[%]
Selectivity
[%]
[
h]
1
2
3
4
5
6
7
8
9
IP
PIP
PIP
PIP
PIP
PIP
PIP
PIP-Cl
PIP-OTf
4
4
8
16
16
24
24
4
18
31
45
83
84
89
97
7
>99
>99
>99
>99
>99
>99
>99
>99
>99
[
[
[
[
[
c]
c]
c,d]
e]
f]
4
26
[a] Reaction conditions: 2-methyloxirane (20 mL), catalyst (60 mg), CO
2
pressure (2 MPa), 120°C. [b] Yield was determined by GC. [c] Continuous
Scheme 5. Proposed mechanism of conversion of CO into cyclic carbonates
2
CO
2
injection to maintain 2 MPa. [d] Catalyst (100 mg) [e] Halogen anion:
À
À
Cl . [f] Anion: OTf.
To gain mechanistic insight into the reaction, control PIP
À
catalysts with different anions were prepared. The Br of PIP
À
À
was substituted by Cl and OTf, respectively. Compared with
PIPÀ Br, PIPÀ Cl and PIP-OTf showed lower catalytic activity
(
Table 2, entries 8 and 9). Based on the experimental results, a
[
24]
possible pathway for synthesis of propylene carbonate
is
proposed in Scheme 5. Firstly, the β-C atom of the substrate
À
was attacked by the Br anion (step 1). Then, CO was inserted
2
into the intermediate (step 2), and the carbonate was formed
by intermolecular nucleophilic substitution reaction (step 3).
In summary, we elucidated a facile solid-state method to
construct a PIP by the Menshutkin reaction. In this article, NaBr
was rationally chosen as the solid template, which endowed the
materials with porosity because of its inhibition on initial charge
interaction. The obtained pores contributed to mass transfer
and exposure of active site. Indeed, PIP was active towards the
Figure 3. Recycling performance of PIP for synthesis of cyclic carbonates
conversion of CO into cyclic carbonates, with an activity almost
2
twice that of the nonporous material. This method provided a
promising alternative for constructing the PIPs with excellent
performance.
Acknowledgements
The work was supported by National Natural Science Foundation
of China (Grant No. 21776174), Shanghai Rising-Star Program
(
20QA1405200), Shanghai Jiao Tong University Scientific and
Technological Innovation Funds (No.2019QYB06), National Key R
D Program Plan (2020YFB0606400) and Inner Mongolia Erdos
&
Scheme 4. Investigation of substrate scope. 4b–d: GC yields; 4e,f: isolated
yields.
Group.
Conflict of Interest
as styrene oxide and phenyl glycidyl ether could also be
transformed into cyclic carbonates efficiently. However, allyl
glycidyl ether as the functional group inhibited the catalytic
activity.
The authors declare no conflict of interest.
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Keywords: mechanochemistry
polymers · porous ionic polymer · salt template
·
Menshutkin reaction
·
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Version of record online: ■■■, ■■■■
2
2
8, 5747–5754.
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These are not the final page numbers!

COMMUNICATIONS
Dr. S. Hou, M. Meng, D. Liu, Prof. P.
Zhang*
1
– 6
Mechanochemical Process to
Construct Porous Ionic Polymers
by Menshutkin Reaction
PIPs for peak performance: This
system provides a facile way to syn-
thesize porous ionic polymers (PIPs)
via the Menshutkin reaction. NaBr as
the salt template prevents the inter-
molecular packing resulting from the
rotation of methane site, nonrigid
knots, and charge interaction. The
obtained PIP is active towards the
conversion of CO into cyclic carbo-
2
nates, with an activity almost twice
that of the nonporous material.