In situconstruction of phenanthroline-based cationic radical porous hybrid polymers for metal-free heterogeneous catalysis

Article abstract of DOI:10.1039/d0ta12018a

Rational design of multifunctional radical porous polymers with redox activity for targeted metal-free heterogeneous catalysis is an important research topic. In this work, we reported a new class of phenanthroline-based cationic radical porous hybrid polymers (Phen˙⁺-PHPs), which were constructed from the Heck reaction between a newly designed dibromo-substituted phenanthroline ionic monomer (iDBPhen) and a rigid building block, octavinylsilsesquioxane (VPOSS). For the first time, the stable phenanthroline-based radical cation was unexpectedly discovered in these polyhedral oligomeric silsesquioxane (POSS)-based porous hybrid polymers, probably undergoingin situreduction of the dicationic monomer iDBPhen during the alkaline reagent K₂CO₃-involved Heck reaction. The radical characters of the typical porous polymers Phen˙⁺-PHP-2 and Phen˙⁺-PHP-2Br were confirmed from the electron paramagnetic resonance (EPR) spectra and X-ray photoelectron spectra (XPS). The chemical structures and porous geometry were fully characterized by a series of advanced technologies. Surprisingly, the metal-free cationic radical polymer Phen˙⁺-PHP-2 exhibited high heterogeneous catalytic efficiency in the H₂O₂-mediated selective oxidation of various sulfides to sulfoxides with high yields under mild conditions, owing to the electron-accepting and redox ability of Phen-based dications and radical cations. Moreover, the extended sample Phen˙⁺-PHP-2Br prepared by post-treatment of Phen˙⁺-PHP-2 with aqueous HBr was also employed as a metal-free efficient heterogeneous catalyst in the conversion of CO₂with epoxides into cyclic carbonates under atmospheric pressure and low temperatures. The remarkable catalytic performance in CO₂conversion should be assigned to the synergistic catalysis of POSS-derived Si-OH groups and nucleophilic Br^?anions and N active atom-involved Phen cationic radical moieties within Phen˙⁺-PHP-2Br. These two catalysts can be facilely recovered and reused, also with stable recyclability in the above catalytic reaction systems, achieving the heterogeneous catalytic demands for multipurpose reactions.

Full text of DOI:10.1039/d0ta12018a

Journal of
Materials Chemistry A
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PAPER
In situ construction of phenanthroline-based
cationic radical porous hybrid polymers for metal-
free heterogeneous catalysis†
Cite this: J. Mater. Chem. A, 2021, 9,
556
7
Guojian Chen, * Yadong Zhang, Ke Liu, Xiaoqing Liu, Lei Wu, Hu Zhong,
Xuejing Dang, Minman Tong
and Zhouyang Long
*
Rational design of multifunctional radical porous polymers with redox activity for targeted metal-free
heterogeneous catalysis is an important research topic. In this work, we reported a new class of
+
phenanthroline-based cationic radical porous hybrid polymers (Phenc -PHPs), which were constructed
from the Heck reaction between a newly designed dibromo-substituted phenanthroline ionic monomer
(iDBPhen) and a rigid building block, octavinylsilsesquioxane (VPOSS). For the first time, the stable
phenanthroline-based radical cation was unexpectedly discovered in these polyhedral oligomeric
silsesquioxane (POSS)-based porous hybrid polymers, probably undergoing in situ reduction of the
dicationic monomer iDBPhen during the alkaline reagent K
2
CO
3
-involved Heck reaction. The radical
+
+
characters of the typical porous polymers Phenc -PHP-2 and Phenc -PHP-2Br were confirmed from the
electron paramagnetic resonance (EPR) spectra and X-ray photoelectron spectra (XPS). The chemical
structures and porous geometry were fully characterized by a series of advanced technologies.
+
Surprisingly, the metal-free cationic radical polymer Phenc -PHP-2 exhibited high heterogeneous
2 2
catalytic efficiency in the H O -mediated selective oxidation of various sulfides to sulfoxides with high
yields under mild conditions, owing to the electron-accepting and redox ability of Phen-based dications
+
and radical cations. Moreover, the extended sample Phenc -PHP-2Br prepared by post-treatment of
+
Phenc -PHP-2 with aqueous HBr was also employed as a metal-free efficient heterogeneous catalyst in
2
the conversion of CO with epoxides into cyclic carbonates under atmospheric pressure and low
temperatures. The remarkable catalytic performance in CO
2
conversion should be assigned to the
Received 11th December 2020
Accepted 15th February 2021
ꢀ
synergistic catalysis of POSS-derived Si–OH groups and nucleophilic Br anions and N active atom-
+
involved Phen cationic radical moieties within Phenc -PHP-2Br. These two catalysts can be facilely
DOI: 10.1039/d0ta12018a
recovered and reused, also with stable recyclability in the above catalytic reaction systems, achieving the
heterogeneous catalytic demands for multipurpose reactions.
rsc.li/materials-a
6–9
10–12
polymers (POPs) and covalent organic frameworks (COFs).
So far, most of the reported radical porous polymers mainly
Introduction
Multivariate porous organic materials, in which diverse organic focused on applications in magnets and electric energy
8–11
functional groups are incorporated into their skeletons, are storage.
However, few of such type porous polymers were
capable of catalysis, adsorption, separation, energy storage and applied to metal-free heterogeneous catalysis, which can afford
1,2
conversion. As kinds of attractive functional moieties, organic much greener, economical and sustainable catalytic processes.
radicals are active species with unpaired electrons, which have
At present, most of the reported radical-type POPs were
drawn great interest in many elds due to their unique prop- derived from neutral radicals such as triphenylmethyl (TPM)
erties in optical, electrical, magnetic, redox and catalysis and 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radicals or
3
–5
8,10,11
aspects.
considered as building blocks for achieving functional purely based polymers have been used as effective catalysts for oxida-
Recently, various organic radicals have been their derivatives.
Among them, a great variety of TEMPO-
13
organic porous materials including radical-type porous organic tion conversion of organic groups. In contrast with neutral
radicals, cationic radicals are another example of radicals with
diverse redox-driven changes for exploring more catalytic
applications. Viologen radical cations are considered among
School of Chemistry and Materials Science, Jiangsu Key Laboratory of Green Synthetic
Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou, 21116,
the most stable organic radicals, which are obtained by means
Jiangsu, China. E-mail: gjchen@jsnu.edu.cn; longzhouyangfat@163.com
0
of chemical or electrical one-electron reductions of 4,4 -bipyr-
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ta12018a
14
idinium dications. Recently, viologen moieties have been
1
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1
5–24
incorporated into some POPs,
only some of which exhibit carbonates under mild conditions. The good catalytic perfor-
18,21,23
stable cationic radical characters.
ionic POPs were regarded as efficient heterogeneous catalysts POSS-derived Si–OH groups and Br anions and N active atoms
in the conversion of CO2 into cyclic carbonates,
rarely used in the liquid-phase catalytic oxidation reactions,
These viologen-based mance should be attributed to the synergistic catalytic effect of
ꢀ
19,20,22,23
but involving Phen radical cations within the catalyst.
24
which are an important and valuable class of organic trans-
formations. Therefore, it becomes an urgent need to develop
new-type redox-oriented cationic radical porous polymers with
versatile properties as metal-free catalysts for the targeted
reactions.
Results and discussion
Formation and characterization of Phenc -PHPs
+
Scheme 1 illustrates the synthetic process of Phen-based
+
cationic radical porous hybrid polymers Phenc -PHPs. First,
+
the typical Phenc -PHP-2 was fabricated from the one-pot Heck
Herein, we reported in situ construction of multifunctional
cationic radical porous polymers from ionic precursors via
a one-pot and one-step process. As depicted in Scheme 1, a new
class of phenanthroline-based cationic radical porous hybrid
reaction between iDBPhen and rigid VPOSS units. This newly
designed ionic precursor iDBPhen was prepared by the qua-
ternization reaction of 3,8-dibromo-1,10-phenanthroline and
1
,2-dibromoethane, which was found to be reduced into
+
polymers (denoted as Phenc -PHPs) were synthesized from the
+
cationic radical Phenc moieties within polymers due to the use
Heck reaction between a newly-designed phenanthroline ionic
monomer (iDBPhen) and a rigid unit octavinylsilsesquioxane
of the alkaline reagent K
2
CO
3
as an HBr absorbent in the Heck
23
ꢀ
reaction. Meanwhile, bromide anions (Br ) within the ionic
monomers can be also exchanged by the carbonate anions
(
VPOSS). To our delight, for the rst time, phenanthroline-
+
based radical cations were discovered in the obtained Phenc -
PHPs, which were formed by in situ reduction of iDBPhen
2
ꢀ
ꢀ
+
(CO
3
). Subsequently, Br anion-paired Phenc -PHP-2Br can
+
be obtained by acid post-treatment of the typical sample Phenc -
PHP-2 in 1 M HBr aqueous solution. The experimental and
characterization details for the synthesis of iDBPhen (Fig. S1†),
two control phenanthroline-based ionic monomers (iPhen-1
2 3
during the K CO -involved one-pot process. The core building
block VPOSS has an organic–inorganic hybrid structure with
inorganic siloxane cages with reactive vinyl groups, which not
only play an important part in fabricating robust porous
organic–inorganic hybrid polymers with a hydrophobic reaction
micro-environment but also afford POSS in situ derived Si–OH
+
and iPhen-2, Scheme S1, Fig. S2 and S3†) and Phenc -PHPs
(Scheme S2†) can be found in the ESI.† X-ray diffraction (XRD)
patterns reveal that both iDBPhen and VPOSS are crystalline
monomers (Fig. S4†), but the obtained polymers Phenc -PHPs
are amorphous forms (Fig. S5†), indicating the formation of
22,23,25
groups that can be benecial for the catalytic reactions.
+
Recently, some phenanthroline (Phen)-based neutral porous
organic polymers have been reported as porous ligands for
disordered three-dimensional network polymers by highly
26,27
coordinating metal complexes for heterogeneous catalysis,
23,25
crosslinking VPOSS with iDBPhen.
As a result, the above
but Phen-based ionic or radical polymers have not appeared as
yet for metal-free catalysis. In this work, the typical porous
+
Phenc -PHPs are insoluble in common organic solvents such as
tetrahydrofuran (THF), N,N-dimethylformamide (DMF) and
dimethyl sulfoxide (DMSO), indicating their robust and highly
cross-linked structures. As shown in the thermogravimetric
+
polymer Phenc -PHP-2 possesses the remarkable radical and
redox character, which was regarded as an efficient metal-free
heterogeneous catalyst in H O -mediated selective oxidation
2
2
+
analysis (TGA) curves (Fig. S6†), the typical samples Phenc -
of suldes to sulfoxides at low temperatures. Towards metal-
+
PHP-2 and Phenc -PHP-2Br exhibit good thermal stability up to
+
free catalytic CO
Br prepared by acid post-treatment of Phenc -PHP-2 with the
HBr aqueous solution afforded very high catalytic activities in
the conversion of CO with epoxides into various cyclic
2
xation, the extended catalyst Phenc -PHP-
ꢁ
3
00 C under an air atmosphere, even better than that of VPOSS.
+
2
+
+
Besides, Phenc -PHP-2 and Phenc -PHP-2Br show improved
hydrophilic properties compared with the hydrophobic VPOSS
by the introduced hydrophilic Phen ionic moieties, which can
2
+
+
Scheme 1 Synthetic routes to typical phenanthroline-based cationic radical porous hybrid polymers Phenc -PHP-2 and Phenc -PHP-2Br. (i)
ꢁ
Heck reaction conditions: VPOSS/iDBPhen ¼ 1/2, the catalyst Pd(PPh
)
3 4
, K
2
CO
3
, DMF/H
2
O, 120 C, 72 h; (ii) post-treatment by aqueous HBr at
room temperature for 12 h.
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obviously decreasing with the increasing of x values, contrib-
uting to about 42–51% of SBET values and only 6–16% of V
values. These results imply that Phenc -PHP-x series have
total
+
enriched mesoporosities, which are benecial to the mass
transfer of reactants to come into contact with the catalysts in
+
the reactions. Obviously, the sample Phenc -PHP-2 has a higher
2
ꢀ1
3
ꢀ1
S
BET value of 266 m g , the highest Vtotal of 0.881 cm g with
abundant large-size mesopores, and moderate N content
ꢀ
1
(
6.38 wt%) and Phen content (2.28 mmol g ), which would be
used as the desired heterogeneous catalyst. Aer the post-
ꢀ
treatment with aqueous HBr, the corresponding Br anion-
+
paired Phenc -PHP-2Br exhibits a slightly decreased S
value
BET
2
ꢀ1
3
ꢀ1
of 245 m g , a Vtotal value of 0.796 cm g and a Phen content
ꢀ
1
ꢀ
Fig. 1 (A) N
2
adsorption–desorption isotherms and (B) NLDFT pore of 1.88 mmol g , due to the introduction of bulky Br anions.
+ +
size distributions of Phenc -PHP-x (x ¼ 1, 2, 4) and Phenc -PHP-2Br.
In addition, a control neutral porous hybrid polymer Phen-PHP-
(seen the porous structure in Fig. S7†) has a rather high SBET
value of 491 m g and an ultrahigh Vtotal value of 1.570 cm
2
2
ꢀ1
3
ꢀ1
+
stably absorb a small amount of water nearby ionic sites, which
g . In addition, the porous morphology of Phenc -PHP-2 and
+
is reected by the initial losses of weight (ca. 5–7 wt%) up to Phenc -PHP-2Br was observed by scanning electron microscopy
ꢁ
28–30
2
00 C in the TGA curves (Fig. S6†).
(SEM) and transmission electron microscopy (TEM). As shown
Porous textural properties and contents of Phen moieties for in Fig. S8,† both of them have the uffy cotton-like morphology
+
Phenc -PHP-x can be tailored by varying molar ratios of iDB- and abundant mesoporosity, which originated from the inter-
+
Phen to VPOSS (x ¼ 1, 2 and 4). The porous structures of Phenc - connected nanoparticles.
+
PHPs were analyzed by the
N
2
adsorption–desorption
The chemical structures and radical characters of Phenc -
measurement (Fig. 1A), and the textural properties, N contents PHPs were conrmed by a series of characterization studies
ꢀ
1
+
13
(
wt%) and Phen content (mmol g ) of Phenc -PHPs series are including solid-state C cross-polarization magic-angle spin-
+
29
summarized in Table 1. As depicted in Fig. 1A, all the Phenc - ning (CP-MAS) and Si MAS NMR spectra, UV-Vis spectra,
PHP-x exhibit type II and IV isotherms with certain uptakes (22– electron paramagnetic resonance (EPR) spectra, Fourier trans-
5 m g ) at low relative pressures (P/P
increasing ones at higher relative pressures (0.80 < P/P
indicating the coexistence of micropores and mesopores. The PHP-2 and Phenc -PHP-2Br, the multiple resonance peaks
3
ꢀ1
4
0
< 0.01) and obviously form infrared (FTIR) spectra and X-ray photoelectron spectros-
1
3
+
0
< 0.99), copy (XPS). In the C CP-MAS NMR spectra (Fig. 2A) of Phenc -
+
nonlocalized density functional theory (NLDFT) pore size centered at 128.3 and 129.2 ppm correspond to the aromatic
+
distributions (Fig. 1B) of Phenc -PHP-x further demonstrate carbons from Phen moieties and silicon-based ethylene carbons
23,25–27,31
their hierarchical porous structures with micropores narrowly (Si–CH]CH–) from VPOSS units.
The small peaks
centered at 1.47 nm and mesopores randomly appearing at appearing at around 159 ppm are assigned to N-bonded carbon
2
6,27
2
.65–4.75 nm. With the increase of x ¼ 1 to 4 in the synthesis atoms in Phen units.
menus, the BET surface areas (SBET) and total pore volume around 51.0 ppm and 41.2–42.6 ppm correspond to the ethylene
total) gradually decrease, while the N and Phen contents in carbons that can bridge the two N atoms in a Phen unit, which
The peaks with low chemical shis at
(V
+
13
Phenc -PHP-x remarkably increase from 3.84 to 9.95 wt% and can be found at similar positions in the liquid-state C NMR
.37 to 3.55 mmol g , respectively. Besides, the micropore spectra of iDBPhen and iPhen-1 (Fig. S1B and S2B†). These
ꢀ
1
1
surface areas (Smicro) and micropore volumes (Vmicro) are results demonstrate the successful formation of POSS-based
+
+
Table 1 Textural properties of Phenc -PHP-x series, Phenc -PHP-2Br and the control neutral polymer Phen-PHP-2
b
c
c
d
f
g
n(VPOSS) :
n(iDBphen)
S
BET
S
micro
V
micro
V
total
N content
(wt%)
Phen content
a
2
ꢀ1
2
ꢀ1
3
ꢀ1
3
ꢀ1
e
ꢀ1
Samples
(m g
)
(m g
)
(cm g
)
(cm g
)
D
p
(nm)
(mmol g
)
+
Phenc -PHP-1
1 : 1
1 : 2
1 : 4
1 : 2
1 : 2
267
266
142
245
491
135
119
60
121
285
0.064
0.054
0.027
0.056
0.130
0.401
0.881
0.375
0.796
1.570
1.47, 2.65
1.47, 2.77
1.47, 2.77
1.47, 2.77
1.23, 2.77
3.84
6.38
9.95
5.27
4.29
1.37
2.28
3.55
1.88
1.53
+
Phenc -PHP-2
+
Phenc -PHP-4
+
Phenc -PHP-2Br
h
Phen-PHP-2
a
+
b
Phenc -PHP-x was obtained from the Heck reaction of VPOSS with iDBPhen using different molar ratios (x ¼ 1, 2 and 4). BET surface area
c
d
calculated over the range P/P
calculated at P/P
0
¼ 0.05–0.20. Micropore surface area and pore volume calculated by the t-plot method. Total pore volume
e
f
0
¼ 0.99. The main pore diameters calculated using NLDFT theory. The N content (wt%) obtained by the CHN elemental
g
ꢀ1
h
analysis. Phen content (mmol g ) ¼ 0.5 ꢂ 1000 ꢂ N content (wt%)/14. For synthesis of Phen-PHP-2 (see the detailed process in Scheme S2),
the molar ratio of n(VPOSS) to n(DBPhen) is 1 : 2.
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broad absorbing bands in the range of 500–800 nm compared
with iDBPhen and iPhen-1 monomers, which are characteristic
of Phen radical cations. EPR spectra (Fig. 3A) provide direct
evidence for proving the existence of Phen radicals. The
+
+
samples Phenc -PHP-2 and Phenc -PHP-2Br both exhibit sharp
peaks with similar g factors (2.0026 and 2.0023), indicative of
the newfound Phen organic radicals with unpaired electrons,
12,18,21,23
which are similar to the common viologen radicals.
As
expected, the EPR signals for two control dicationic monomers
iDBPhen and iPhen-1 are silent, indicating the absence of
radicals in the dicationic precursors. These compared results
+
reveal that cationic radical Phenc -PHPs are formed by in situ
reduction of the dicationic iBPhen during the alkaline reagent
K
2
CO
reducing agents (e.g., Na
have given some clues to form viologen cationic radicals under
spectra of Phenc -PHP-2 and Phenc -PHP-2Br, (C) FTIR spectra of alkaline conditions, which are demonstrated again in the
3
-involved Heck reaction without using additional
12,36
2
S
2
O
4
).
Indeed, some recent reports
13
29
Fig. 2 The solid-state (A) C CP-MAS NMR spectra and (B) Si MAS
+
+
+
+
21,23
VPOSS, Phenc -PHP-2, Phenc -PHP-2Br, and (D) UV-Vis spectra of present Phen-based cationic radical polymers.
+
+
Phenc -PHP-2, Phenc -PHP-2Br, iDBPhen and iPhen-1.
+
The chemical states and compositions of Phenc -PHPs were
further conrmed using the XPS technique. The full survey XPS
spectra (Fig. 3B) show the elemental compositions of C, N, O
2
9
Phen-linked porous polymers. The Si MAS NMR spectra
+
+
and Si for Phenc -PHP-2, and C, N, O, Si and Br for Phenc -PHP-
n
m
(
Fig. 2B) indicate that T [CSi(OSi) (OH) ] and Q [Si(OSi) (-
OH)4 ] silicon units co-existed in the samples of Phenc -PHP-2
+
n
3ꢀn
m
2Br. To our surprise, the residual Br element in Phenc -PHP-2
+
ꢀm
was negligible, demonstrating that bromo-substituted iDB-
Phen monomers were fully consumed in the Heck reaction, and
Br anions within iDBPhen ionic monomers were also
exchanged by CO3 anions from K CO . The presence of CO
anions was also reected by a small FTIR adsorbed peak (O–C–
+
and Phenc -PHP-2Br. The main signals at ꢀ80.3 and ꢀ81.1 ppm
3
are assigned to silicon atoms of the retained cubic T
ꢀ
23,25,31
units;
the affiliated peaks at ꢀ70.8 and ꢀ71.6 ppm are
2ꢀ
2ꢀ
2
3
3
2
attributed to the newly-formed T units that arise from the
3
partial collapse of T units by the cleavage of Si–O–Si
For Phenc -PHP-2, obvious signals appearing at
100.5 and 110.2 ppm suggest the formation of Q and Q
which are caused by the cleavage of Si–C bonds
of T units and further self-condensation of Si–OH contained Q
ꢀ1
O) at 1460 cm (Fig. 2C) and a divided O–C–O peak at 286.9 eV
in the high-resolution C 1s XPS spectrum of Phenc -PHP-2
15,23,32
+
bonds.
+
3
4
ꢀ
37,38
(Fig. 3E).
Besides, very small amounts of residual metal Pd
31,32
silicon units,
+
+
(ca. 0.1 wt%) within Phenc -PHP-2 and Phenc -PHP-2Br were
3
3
detected by both XPS and inductively coupled plasma mass
spectrometry (ICP-MS), due to the lack of free N atom contain-
ing Phen ligands. The subsequent control experiments would
further indicate that the residual Pd has negligible inuence on
the targeted catalytic reactions, which are different from other
trapped trace metal Pd-catalyzed organic transformations.
Based on the aforementioned cationic radical characters for the
Phenc -PHPs, three electronic states for N atoms were recog-
nized from the divided peaks from the N 1s spectra (Fig. 3D).
For Phenc -PHP-2, the small peak located at 401.3 eV originated
from the N atoms of dicationic Phen moieties (Phen N ) and
the two relatively higher peaks at 399.9 and 398.8 eV can be
attributed to the N atoms in the cationic radical Phen species
1
5,22,33
n
units.
Besides, there are only slight differences of Q units
+
for Phenc -PHP-2Br, i.e., a small peak at ꢀ96.7 ppm is assigned
2
22,23
3
to the Q unit,
due to further decomposition of Q silicon
units by post-treatment in HBr aqueous solution. According to
the FTIR spectra in Fig. 2C, compared with one sharp peak for
the typical Si–O–Si asymmetric stretching vibration at
39,40
ꢀ1
33,34
1
110 cm for cubic VPOSS,
such peaks along with some
+
+
+
splits and broad shis for Phenc -PHP-2 and Phenc -PHP-2Br
appear at 1131, 1069 cm and 1135, 1074 cm , suggesting
the coexistence of T and Q silicon units.
Si–OH groups in Phenc -PHPs was indicated from the Si–OH
ꢀ
1
ꢀ1
+
n
n
22,23,25
The existence of
++
+
ꢀ
1
and Si–O stretching vibrations that appeared at 3649 cm and
ꢀ1
22
967 cm
.
In addition, the successful introduction of Phen
+
0
6,23
(Phen Nc ) and further reduced neutral Phen N , respectively.
parts was conrmed by the new bands located at around 1652
+
++
For the Phenc -PHP-2, the tting area ratios of Phen N , Phen
ꢀ
1
and 1602 cm assigned to C]N and C]C stretching vibra-
+
0
Nc and Phen N are 0.253, 0.378 and 0.369, implying that most
of the dicationic iDBPhen was in situ reduced to cationic radical
and neutral forms during the K CO -involved Heck reaction.
These two plausible reversible redox states related to Phenc and
Phen via one- and two-electron reductions of the model ionic
precursor iPhen-1 are described in Scheme S3,† similar to the
reversible redox processes of viologens and derivatives.
acid post-treatment of Phenc -PHP-2 with 1 M HBr aqueous
solution, as is well known, CO
27,35
2
2
3
tions.
As a result, the above formed T , Q and Q units
should be attributed to the destruction and distortion of cubic
POSS cages by the cleavage of Si–C and Si–O bonds during the
highly cross-linked process, just affording enriched Si–OH
groups for the subsequent catalytic reactions.
2
3
+
0
Furthermore, the solid-state UV-Vis and EPR spectroscopies
12,14,24
By
+
conrm the formation of Phen radical cations within Phenc -
+
+
PHP-2 and Phenc -PHP-2Br. In detail, UV-Vis spectra (Fig. 2D)
2ꢀ
3
anions were easily removed by
+
+
recorded for Phenc -PHP-2 and Phenc -PHP-2Br show obvious
+
ꢀ
reacting with excess acidic H and subsequently replaced by Br
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+
+
Fig. 3 (A) EPR spectroscopy of Phenc -PHP-2 and Phenc -PHP-2Br and their XPS spectra: (B) survey, (C) Br 3d, (D) N 1s, (E) C 1s, and (F) Si 2p.
+
anions via anion-exchange, and the obtained Phenc -PHP-2Br heterocyclic organic salts have been reported as homogeneous
ꢀ
45–47
has obviously increased Br anions, which was displayed by organocatalysts for the oxidation of suldes,
there are few
Br 3d3/2 and Br 3d5/2 at 68.9 and 67.8 eV in the Br 3d spectrum examples of metal-free heterogeneous catalysts for achieving
2
3,25
+
24,48,49
(
Fig. 3C).
Phenc -PHP-2Br still exhibits the radical character such oxidations,
making this work challenging. First, the
+
+
with multiple Phen N forms, similar to its precursor Phenc - typical sample Phenc -PHP-2 was used as a metal-free hetero-
PHP-2. The tting ratios of these three Phen N forms change geneous catalyst in selective oxidation of the model substrate
slightly, that is, some Phen Nc and Phen N return to Phen N
+
0
++
methyl phenyl sulde (MPS) with 30 wt% H O . Very surpris-
2 2
21
aer HBr acid treatment. The molar ratio of dicationic Phen ingly, the product methyl phenyl sulfoxide (MPSO) was
+
+
ꢀ
N
paired with Br anions is about 0.269, while the value for smoothly obtained with both a high yield and selectivity of 99%
+
Phen Nc is 0.376. Besides, the C 1s spectra (Fig. 3E) can be (Table 2, entry 1) under mild conditions (i.e., near room
mainly tted to three peaks at 285.9 (C–N), 284.8 (C–C/C–H) and temperature 40 C for only 1 h) along with a high turnover
84.6 eV (Si–C), which are assigned to the carbon atoms in Phen frequency (TOF) value of 21.7 h . Besides, the catalyst Phenc -
moieties, methylene groups and POSS units.
ꢁ
ꢀ
1
+
2
23,25
Three types of PHP-2 can be used at least three times without obvious loss of
electronic states of silicon atoms were conrmed from the high- catalytic activities, indicating its heterogeneous nature and
+
ꢀ
+
resolution Si 2p spectra (Fig. 3F). Taking Phenc -PHP-2Br as the good stability. It was found that Br anion-paired Phenc -PHP-
example, the Si 2p peak at 102.5 eV can be assigned to the Si 2Br also achieved good conversion of MPS to MPSO with
3
23,25,41
ꢀ1
atoms within T units (i.e., O –Si–C);
the main peak a high yield of 99% and a high TOF value of 26.3 h as well as
3
+
located at 102.9 eV should be attributed to Si–OH groups Phenc -PHP-2. This result suggests that the type of counter
derived from T , Q , and Q silicon units; the Si 2p peaks at the anions has no obvious inuence on catalytic oxidation activi-
higher value of 103.6 eV are assigned to Q silicon atoms ties. In order to determine the exact catalytically active site
3
2
3
4
23,25,31
(SiO
2
).
These multiple electron states of Si atoms again responsible for the oxidation activity, several control catalysts
n
n
demonstrate the co-existence of T and Q units with Si–OH were prepared. When the neutral polymer Phen-PHP-2 and
+
groups in Phenc -PHPs, which can be regarded as hydrogen Phen monomer were used as catalysts in the reaction (Table 2,
bond donors (HBDs) for enhancing catalytic efficiency in CO
conversion.
entries 3 and 4), MPSO yields of 56% and 58% were obtained,
but these values were very close to the yield of 54% in a blank
2
23,25
run without using catalysts and only adding the oxidant H
Table 2, entry 10). These equal results indicate that neutral
Phen moieties have negligible contributions to accelerate the
2 2
O
(
+
Metal-free oxidation of suldes catalyzed by Phenc -PHPs
+
Owing to the radical and redox characters of Phenc -PHPs, they catalytic oxidation activity. In contrast, dicationic monomers
were applied as metal-free heterogeneous catalysts in the iDBPhen and iPhen-1 (see in Scheme S1†) with ethylene bridges
oxidation of suldes using hydrogen peroxide (H
oxidant, which can produce many important ne chemicals.
2 2
O ) as a green can efficiently catalyze this oxidation, affording high yields of
42
98% and 87% and desired TOF values (Table 2, entries 5 and 6).
Most of the effective catalysts for these oxidation reactions However, the monocationic monomer iPhen-2 tethered with an
mainly depend on metal active species such as poly- ethane group only gave a low yield of 56%, close to the value in
43,44
oxometalates.
Although
several
electron-decient
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2 2
Table 2 Selective oxidation of methyl phenyl sulfide (MPS) to methyl Table 3 Substrate scope for selective oxidation of sulfides with H O
+
+
a
phenyl sulfoxide (MPSO) catalyzed by Phenc -PHPs and control over the catalyst Phenc -PHP-2
catalysts
a
T
( C)
t
(h)
ꢁ
b
Entry
1
Substrate
Product
Yield (%)
40
2
1
99
b
b
c
ꢀ1
2
3
40
40
99
99
Entry Catalyst
Yield (%) Selectivity (%) TOF value (h )
+
d
1
2
3
4
5
6
7
8
9
1
Phenc -PHP-2
99/98/95
99
99
99
99
99
99
99
99
99
99
21.7
26.3
—
+
Phenc -PHP-2Br 99
Phen-PHP-2
56
58
98
87
56
52
58
54
2
e
Phen
—
e
iDBPhen
21.6
19.1
—
—
—
e
iPhen-1e
iPhen-2
4
5
60
60
6
6
93
96
f
K
2
CO
3
g
Pd(PPh
3
)
4
h
0
Blank
—
a
Reaction conditions: MPS (1 mmol), 30 wt% H
2
O
2
(2 mmol), the
ꢁ
catalyst (20 mg), the solvent MeOH (4 mL), temperature (40 C), time
b
1
(1 h). Yield and selectivity of MPSO were determined by H NMR
c
analyses (see the spectrum in Fig. S9). Turnover frequency (TOF) ¼
6
60
6
90
[
mmol (product)]/[mmol (Phen content in the catalyst) ꢂ reaction
d
time (h)]. The yields were obtained by the recovery of the catalyst for
e
three runs. The dosages of ionic Phen (iPhen-1 and iPhen-2) and
1
,10-phenanthroline (Phen) monomers were calculated based on Phen
a
Reaction conditions: sulde (1 mmol), 30 wt% H
2
O
2
(2 mmol), the
+
f
contents of Phenc -PHP-2 and Phen-PHP-2. The dosage of K
2
CO
was 0.02 mg based on the Pd
content in Phenc -PHP-2. The blank experiment was conducted
without using catalysts only using 30 wt% H
3
was
+
catalyst Phenc -PHP-2 (20 mg), the solvent MeOH (4 mL), temperature
g
1
0 mg. The dosage of Pd(PPh
3
)
4
ꢁ
b
1
(
T ¼ 40–60 C), time (t ¼ 1–6 h). Yields were determined from
H
+
h
NMR spectra (Fig. S10–S15) of the crude sulfoxides. The selectivities
2 2
O .
of the sulfoxides are over 99%.
the blank reaction. These interesting compared results reveal
that the ethylene bridges within dicationic Phen organic salts
are vital for electron transfer in reversible redox states between
+
Phenc -PHP-2 showed good catalytic activities in the oxidation
3 3
of MPS analogues with electron-donating groups (–CH , –OCH
and –OH) located in para-positions, giving the corresponding
sulfoxides with high yields of 99% (Table 3, entries 1–3) under
very mild conditions at 40 C for 1–2 h. Especially, the relatively
inert suldes with electron-withdrawing groups (–F, –Cl and
Br) could be also transformed into the corresponding sulfox-
ides with desired yields of 90–96% (Table 3, entries 4–6) at
a higher temperature of 60 C for 6 h. As a result, Phenc -PHP-2
has been proved as an efficient metal-free heterogeneous cata-
lyst for achieving the oxidation conversion of various suldes to
sulfoxides under mild conditions.
Based on previous ndings in H
suldes over metal-free organocatalysts such as pyrazinium or
heteroarenium salts,
catalyst Phenc -PHP-2 is proposed in Scheme 2. First, when the
catalyst Phenc -PHP-2 reacted with the oxidant H
vated H transforms into active species cOOH by an electron
transfer process of Phen or Phenc with the formation of Phen
radical cations or neutral forms, thus leading to the formation
+
+
+
Phen and Phen Nc species (Scheme S3†), but the ethane
group cannot play such a role of electronic bridges for the redox
reactions. Besides, the control reagent K CO and Pd(PPh3)4
ꢁ
2
3
have not obviously improved catalytic activities (Table 2, entries
–
8
and 9) compared with the blank experiment, revealing the
2
ꢀ
absence of catalytic roles of CO
3
anions and residual trace Pd
ꢁ
+
+
within Phenc -PHP-2. As a result, these above comparisons
denitely conrm that the true active sites for catalyzing such
an oxidation reaction are dicationic or cationic radical Phen
moieties. To the best of our knowledge, it was the rst time to
discover that dicationic and cationic radical Phen salts can be
regarded as metal-free catalysts for the oxidation reactions. In
2 2
O -mediated oxidations of
45–47
a possible reaction mechanism for the
+
a word, the catalyst Phenc -PHP-2 exhibited the optimal cata-
+
lytic activity in H
2 2
O -mediated oxidation of MPS under mild
+
2 2
O , the acti-
conditions, which was much better than that of a recently re-
2 2
O
24
ported viologen-based heterogeneous catalyst PIN-1, and was
even comparable to many reported metal-based heterogeneous
+
+
+
43,44
catalysts (see Table S1† for details).
24,45–47
of a Phen-hydroperoxide adduct.
sulde to sulfoxide was achieved by oxygen transfer from the
Phen-hydroperoxide active species to the substrates.
the catalyst Phenc -PHP-2 was regenerated by the elimination of
Then, the oxidation of
From the view of green chemistry, the metal-/halogen-free
+
catalyst Phenc -PHP-2 was more suitable for the selective
45–47
Finally,
2 2
oxidation of suldes with H O . To evaluate the substrate scope
+
+
of Phenc -PHP-2, catalytic oxidation of various suldes was
carried out under mild conditions (Table 3). The catalyst
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yield of 37% for the product 4-(chloromethyl)-1,3-dioxolan-2-
one, but could afford a moderate yield of 83% using a longer
reaction time of 72 h (Table 4, entries 1 and 2). For comparison,
ꢀ
+
the Br anion-paired catalyst Phenc -PHP-2Br provided an
ꢁ
excellent yield of 99% and a perfect selectivity of 99% at 60 C
for 48 h (Table 4, entry 3) and still gave a desired yield of 93% at
ꢁ
a lower temperature of 40 C for 96 h (Table 4, entry 4). The
ꢀ
above results suggest that nucleophilic Br anions are very
Scheme 2 Proposed reaction mechanism for oxidation of sulfides important to remarkably enhance the catalytic activity in CO
2
+
2 2
with H O catalyzed by Phenc -PHP-2.
conversion. In order to understand the true catalytic active sites
+
+
within these two different catalysts Phenc -PHP-2 and Phenc -
PHP-2Br, another control metal-/halogen-free catalyst neutral
ꢁ
a hydroxyl group from a pseudobase and also release of the by- Phen-PHP-2 was carried out under similar conditions. At 60 C
product water.
45,47
for 48 h and 72 h, the catalyst Phen-PHP-2 also gave moderate
yields of 28% and 78% (Table 4, entries 5 and 6) that were
slightly lower than those of Phenc -PHP-2. In order to assess the
possible catalytic roles of POSS-derived Si–OH groups and N
active sites, some well-designed control experiments were
carried out as follows. Compared with the catalytic behaviors of
Phenc -PHP-2 and Phen-PHP-2, the control anhydrous 1,10-
phenanthroline (Phen) gave relatively lower yields of 13% and
0% at 60 C for 48 h and 72 h (Table 4, entries 7 and 8), while
an interesting catalyst 1,10-phenanthroline monohydrate
+
+
2
Metal-free catalytic CO xation over Phenc -PHP-2Br
The chemical xation of CO with epoxides into value-added
2
cyclic carbonates is a promising, atom economic and energy-
saved non-redox route to achieve the utilization of such C1
+
50–52
resources.
Owing to the multiple active sites including Phen
ꢀ
cationic radical sites paired with Br anions and POSS-derived
ꢁ
6
+
Si–OH groups, Phenc -PHP-2Br was further investigated as
a metal-free heterogeneous catalyst in CO conversion. First, the
2
Phen$(H O) afforded an equal yield of 82% and a slightly
decreased selectivity of 97% (Table 4, entry 9). As a result, one
2
+
+
catalytic performances of both Phenc -PHP-2 and Phenc -PHP-
Br were assessed in the cycloaddition of CO with a model
2
2
water molecule in Phen$(H O) has a positive effect in enhancing
2
substrate epichlorohydrin (ECH) without using any solvents
the catalytic activity and only has a slight inuence on the
and co-catalysts under atmospheric pressure (0.1 MPa) using
selectivity. These valuable comparisons indicate that N active
atoms in Phen or Phenc moieties in the catalysts have certain
catalytic activities in such reactions. Moreover, the abundant
OH groups from both Si–OH groups and a small amount of
absorbed water molecules play a vital hydrogen bond donor
ꢁ
+
a CO
2
balloon (Table 4). At a low temperature of 60 C for 48 h,
+
the metal-/halogen-free catalyst Phenc -PHP-2 only gave a low
+
Table 4 The CO
under different conditions
2
cycloaddition with ECH catalyzed by Phenc -PHPs
(HBD) role in enhancing the catalytic activity; meanwhile, they
a
do not obviously affect the product selectivity via a possible
hydrolysis process, which has been reected in the result using
the control catalyst Phen$(H O) and also revealed by some
2
recent reports related to Si–OH or water-promoted HBD catal-
2
3,25,53–56
ysis.
Besides, no product was found over the control
(entry 11), which could exclude the possible
catalyst Pd(PPh )
3 4
T
( C)
t
(h)
inuence on this catalytic reaction. Therefore, moderate cata-
ꢁ
b
b
Entry
Catalyst
Yield (%)
Selectivity (%)
lytic activities over metal-/halogen-free heterogeneous catalysts
+
+
1
2
3
4
5
6
7
8
9
Phenc -PHP-2
60
60
60
40
60
60
60
60
60
60
60
48
72
48
96
48
72
48
72
72
48
48
37
83
99
93
28
78
13
60
99
99
99
99
99
99
99
99
97
99
—
Phenc -PHP-2 and Phen-PHP-2 should be attributed to the
+
+
Phenc -PHP-2
synergistic combination of Phen-based free N or Nc active sites,
+
Phenc -PHP-2Br
POSS-derived Si–OH groups and a small amount of absorbed
+
Phenc -PHP-2Br
23,56–58
water within the catalysts.
Phen-PHP-2
Phen-PHP-2
In fact, the present metal-/halogen-free heterogeneous cata-
c
+
Phen
lysts Phenc -PHP-2 and Phen-PHP-2 still gave considerable
c
Phen
catalytic activities in the conversion of CO
2
under ambient
c
Phen$(H
2
O)
82
91
conditions, which could be comparable to those of recently re-
c
1
1
0
1
iPhen-1
59–62
d
ported metal-/halogen-free catalysts.
To attain much better
Pd(PPh
3
)
4
Trace
catalytic efficiency under milder conditions, the improved
a
Reaction conditions: ECH (2 mmol), CO
2
ꢁ
pressure (0.1 MPa), the
+
Phenc -PHP-2Br was the more suitable heterogeneous catalyst,
catalyst (40 mg), temperature (T ¼ 40–60 C), time (t ¼ 48–96 h).
b
even superior to the catalytic behavior over a homogeneous
Yield and selectivity of the cyclic carbonate determined by both GC
1
and H NMR (see the typical spectrum in Fig. S16); a small amount of ionic monomer iPhen-1 (Table 4, entry 10). These results also
by-product is 3-chloro-1,2-propanediol. The dosages of iPhen-1, Phen
O) are calculated based on the Phen content of Phenc -
PHP-2Br and Phen-PHP-2, respectively. The dosage of Pd(PPh
c
conrm that Si–OH groups and inherent absorbed H
2
O within
Phenc -PHP-2Br work together with Br anions to achieve the
good catalytic performance in CO conversion with ECH, which
+
and Phen$(H
2
+
ꢀ
d
3
)
4
was
+
0
.04 mg based on the trace Pd content in Phenc -PHP-2Br.
2
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Fig. 4 Substrate scope for the cycloaddition of CO
epoxides catalyzed by Phenc -PHP-2Br. Reaction conditions:
2
with various
+
ꢁ
substrate (2.0 mmol), catalyst (40 mg), CO
2
(0.1 MPa), 60–100 C, 48–
1
72 h. The yields of cyclic carbonates were determined from the
H
NMR spectra of crude products (Fig. S17–S24†).
Scheme
conversion of CO
PHP-2Br.
3
A
plausible synergistic catalytic mechanism for the
+
2
with epoxides over the “all-in-one” catalyst Phenc -
was superior to those of many reported porous ionic polymers
1
9,20,63–66
(PIPs)
and was also comparable to those of metal-free
22,23,25,56,67–70
PIPs with HBD groups (see Table S2† for details).
23,25
sites.
2
Furthermore, the moderate CO adsorption capacities
+
The optimal catalyst Phenc -PHP-2Br was further explored in
the cycloaddition of CO2 with various epoxides under atmo-
spheric pressure. As shown in Fig. 4, the ECH analogue epi-
bromohydrin 1a could be smoothly converted to its cyclic
+
ꢀ1
+
of Phenc -PHPs (i.e., 1.28 mmol g
.89 mmol g
for Phenc -PHP-2 and
ꢀ1
+
0
for Phenc -PHP-2Br at 273 K and 0.1 MPa,
Fig. S29†) also favor the capture and enrichment of CO within
2
porous solid catalysts during the reaction.
ꢁ
carbonate 2a with a high yield of 99% at 60 C for 48 h. The
Based on previous reports, our experimental results and
above analyses, a plausible synergistic catalytic mechanism was
commonly used substrates 1b–1d (styrene oxide, glycidyl phenyl
ether and allyl glycidyl ether) were also efficiently converted into
the corresponding cyclic carbonates (2b–2d) with desired yields
of 91–96% at a higher temperature of 80 C. Notably, efficient
transformation of long carbon chains with inert epoxides (1e–
2
afforded for CO conversion with epoxides over the catalyst
+
Phenc -PHP-2Br, as shown in Scheme 3. First, the epoxide
substrate was together activated by multiple active sites
including POSS-derived Si–OH groups, a small amount of
absorbed water and some active radical N atoms from Phen
moieties, greatly leading to the ring opening of the epoxide via
ꢁ
1h) could be well achieved in high yields (95–99%) of products
(2e–2h) under different reaction conditions. The longer the
carbon chains of substrates, the higher the temperatures or
longer the reaction times needed in the reactions. In a word,
23,25,56,57
hydrogen-bond interaction and C–O bond polarization.
ꢀ
Then, the nucleophilic Br anion attacked the less hindered b-
+
Phenc -PHP-2Br has good substrate scope in the conversion of
carbon of the activated epoxide to create an oxyanion interme-
CO
2
with various epoxides. Besides, the robust recyclability and
23,53
diate stabilized by multiple –OH HBD groups.
Subsequently,
+
heterogeneous nature of the catalyst Phenc -PHP-2Br were
the oxyanion intermediate interacted with the inserted CO
molecules to form the bromoalkyl-carbonate anion interme-
2
investigated by a ve-cycle assessment in CO conversion with
2
ECH that exhibited no obvious loss of catalytic activity
22,23,53
diate.
Finally, the product cyclic carbonate was formed by
(
Fig. S25†). The chemical composition and structural
morphology of the reused catalyst were assessed by character-
ization studies such as FTIR, XPS, N sorption and SEM
Fig. S26–S28†), conrming its good structural stability.
Previous reports have revealed that nucleophiles (halogen
intramolecular ring-closure of the carbonate anion interme-
ꢀ
diate with the release of Br anions, following with the regen-
2
eration of the catalyst.
(
Conclusions
anions) and electrophiles (e.g., HBD groups) are benecial for
5
0,51,58
the activation and conversion of epoxides with CO2.
In this
In summary, a new series of phenanthroline cationic radical
porous hybrid polymers were constructed by the Heck reaction
between newly-designed phenanthroline ionic monomers and
+
work, the designed “all-in-one” catalyst Phenc -PHP-2Br
possesses multiple catalytically active sites including Phen-
ꢀ
based cationic radical sites with Br anions and N active
+
VPOSS units. The obtained Phenc -PHPs have desired porous
atoms, POSS-derived Si–OH groups and H-bonded water mole-
cules, which together account for the high catalytic activity by
synergistic catalytic effect. Besides, the catalyst also has
a moderate surface area, abundant mesoporosity and a suitable
POSS-constructed hydrophobic reaction micro-environment,
which can improve the mass transfer of large-sized long-chain
hydrophobic epoxides to smoothly contact catalytic active
structures with moderate surface areas and possess multiple
catalytically active sites, which could be regarded as metal-free
heterogeneous catalysts for both oxidation of suldes and
chemical xation of CO . Owing to its unique cationic radical
2
+
redox character, the typical catalyst Phenc -PHP-2 exhibited
remarkable heterogeneous catalytic activities in the selective
oxidation of suldes to sulfoxides using a green oxidant H O
2 2
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+
under mild conditions. Besides, the extended catalyst Phenc - 15 G. Chen, Y. Zhou, X. Wang, J. Li, S. Xue, Y. Liu, Q. Wang and
PHP-2Br could be also used as a highly efficient metal-free J. Wang, Sci. Rep., 2015, 5, 11236.
heterogeneous catalyst for the conversion of CO2 with epox- 16 P. Zhang, Z.-A. Qiao, X. Jiang, G. M. Veith and S. Dai, Nano
ides into cyclic carbonates under atmospheric pressure, which Lett., 2015, 15, 823–828.
is attributed to the synergistic catalysis effect of nucleophilic 17 G. Chen, W. Hou, J. Li, X. Wang, Y. Zhou and J. Wang, Dalton
ꢀ
Br anions and N active atoms within Phen radical moieties and
Trans., 2016, 45, 4504–4508.
HBD active sites including Si–OH groups and H-bonded water. 18 C. Hua, B. Chan, A. Rawal, F. Tuna, D. Collison, J. M. Hook
This work affords a general route to construct more new
and D. M. D'Alessandro, J. Mater. Chem. C, 2016, 4, 2535–
multifunctional cationic radical porous polymers for diverse
2544.
applications in metal-free heterogeneous catalysis and related 19 O. Buyukcakir, S. H. Je, D. S. Choi, S. N. Talapaneni, Y. Seo,
energy storage and conversion.
Y. Jung, K. Polychronopoulouc and A. Coskun, Chem.
Commun., 2016, 52, 934–937.
2
0 O. Buyukcakir, S. H. Je, S. N. Talapaneni, D. Kim and
A. Coskun, ACS Appl. Mater. Interfaces, 2017, 9, 7209–7216.
Conflicts of interest
There are no conicts to declare.
21 G. Das, T. Skorjanc, S. K. Sharma, F. Gandar ´a , M. Lusi,
D. S. S. Rao, S. Vimala, S. K. Prasad, J. Raya, D. S. Han,
R. Jagannathan, J.-C. Olsen and A. Trabolsi, J. Am. Chem.
Soc., 2017, 139, 9558–9565.
Acknowledgements
This work was supported by the National Natural Science 22 G. Chen, X. Huang, Y. Zhang, M. Sun, J. Shen, R. Huang,
Foundation of China (21603089, 21706106 and 21503098), the
Natural Science Foundation of Jiangsu Province (BK20160209),
M. Tong, Z. Long and X. Wang, Chem. Commun., 2018, 54,
12174–12177.
the Natural Science Foundation of Jiangsu Higher Education 23 Y. Zhang, K. Liu, L. Wu, H. Zhong, N. Luo, Y. Zhu, M. Tong,
Institutions of China (16KJB150014), the Postgraduate Research
Practice Innovation Program of Jiangsu Province
Z. Long and G. Chen, ACS Sustainable Chem. Eng., 2019, 7,
16907–16916.
&
(KYCX20_2357 and KYCX20_2354) and the Top-notch Academic 24 S. Hou, N. Chen, P. Zhang and S. Dai, Green Chem., 2019, 21,
Programs Project of Jiangsu Higher Education Institutions
TAPP).
1455–1460.
(
25 G. Chen, Y. Zhang, J. Xu, X. Liu, K. Liu, M. Tong and Z. Long,
Chem. Eng. J., 2020, 381, 122765.
2
6 C.-A. Wang, K. Nie, G.-D. Song, Y.-W. Lia and Y.-F. Han, RSC
Adv., 2019, 9, 8239–8245.
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5
6
7
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