Metalated-bipyridine-based porous hybrid polymers with POSS-derived Si-OH groups for synergistic catalytic CO2fixation

Article abstract of DOI:10.1039/d0dt01667e

Herein, we construct a new series of N-heterocyclic ligand bipyridine-based porous hybrid polymers (denoted Bpy-PHPs) from the Heck reaction of a rigid building unit octavinylsilsesquioxane (VPOSS) and 5,5′-dibromo-2,2′-bipyridine. Surprisingly, the typical sample Bpy-PHP-4 was found to be a metal-/halogen-free heterogeneous catalyst in the cycloaddition reaction of CO2 with a few epoxides under atmospheric pressure. After coordination with ZnBr2, the resultant ZnBr2@Bpy-PHP-4 afforded largely enhanced heterogeneous catalytic activities upon the conversion of carbon dioxide (CO2) and various epoxides into cyclic carbonates without using any co-catalysts under mild conditions. The moderate catalytic activities of Bpy-PHP-4 may be due to the presence of hydrogen bond donors (HBDs), i.e., polyhedral oligomeric silsesquioxane (POSS)-derived Si-OH groups and N active sites from Bpy linkers. In comparison, the high catalytic efficiency of ZnBr2@Bpy-PHP-4 should be attributed to the synergistic catalysis of Si-OH groups, N active atoms, and Bpy-coordinated ZnBr2. Moreover, the catalyst ZnBr2@Bpy-PHP-4 can be easily recovered and reused ten times without any significant loss of catalytic activities. This work affords an efficient metal-based porous hybrid polymer heterogeneous catalyst for the cycloaddition reaction of CO2 and epoxides under mild and co-catalyst-free conditions.

Full text of DOI:10.1039/d0dt01667e

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Douglas W. Stephen et al.
N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl,
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DOI: 10.1039/D0DT01667E
ARTICLE
Metalated-bipyridine-based porous hybrid polymers with POSS-
derived Si-OH groups for synergistic catalytic CO fixation
2
Received 00th January 20xx,
Accepted 00th January 20xx
Yadong Zhang‡, Niu Luo‡, Jingyu Xu, Ke Liu, Shengqi Zhang, Qinglin Xu, Rui Huang, Zhouyang
Long, Minman Tong and Guojian Chen*
DOI: 10.1039/x0xx00000x
Herein, we construct a new series of N-heterocyclic ligand bipyridine-based porous hybrid polymers (denoted Bpy-PHPs)
by the Heck reaction of a rigid building unit octavinylsilsesquioxane (VPOSS) and 5,5'-dibromo-2,2'-bipyridine. Surprisingly,
the typical sample Bpy-PHP-4 was found to be a metal-/halogen-free heterogeneous catalyst in the cycloaddition reaction
of CO
2
2 2
with a few epoxides under atmospheric pressure. After coordination with ZnBr , the resultant ZnBr @Bpy-PHP-4
afforded largely enhanced heterogeneous catalytic activities in the conversion of carbon dioxide (CO
2
) and various
epoxides into cyclic carbonates without using any co-catalysts under mild conditions. The moderate catalytic activities of
Bpy-PHP-4 may be due to the presence of hydrogen bond donors (HBDs), i.e., polyhedral oligomeric silsesquioxane (POSS)-
derived Si-OH groups and N active sites from Bpy linkers. In comparison, the high catalytic efficiency of ZnBr
should be attributed to the synergistic catalysis of Si-OH groups, N active atoms, and Bpy-coordinated ZnBr
the catalyst ZnBr @Bpy-PHP-4 can be easily recovered and reused for ten times without a significant loss of catalytic
activities. This work affords an efficient metal-based porous hybrid polymer heterogeneous catalyst for the cycloaddition
reaction of CO and epoxides under mild and co-catalyst-free conditions.
2
@Bpy-PHP-4
2
. Moreover,
2
2
heterogeneous catalytic systems often demand high
temperature and/or high CO pressure due to the lack of
suitable catalytic active sites. Therefore, it is highly desirable
to develop high-performing heterogeneous catalysts with
Introduction
The chemical fixation of CO
has been regarded as an effective mean to realize the
utilization of excessive CO
of useful chemicals.1 In the context, the cycloaddition of CO
2
,9
8
2
into high value-added products
multiple active sites for CO
As one type of heterogeneous catalysts, porous organic
polymers (POPs), are promising candidates for CO capture and
conversion, because of their diverse chemical functionalities,
2
conversion under mild conditions.
2
emission, and also affords a variety
,2
2
2
with epoxides to produce various cyclic carbonates is a
promising and atom-economic catalytic route to accomplish
8
-10
porous structures and excellent framework stability.
By
conversion.1 At present, extensive efforts have been paid
-3
CO
2
virtue of diverse synthetic strategies, numerous task-specific
catalytic active sites could be smoothly introduced into the
desired POPs. Recently, some effective metal-based POP
catalysts have been reported for the cycloaddition of CO , but
2
most of these catalytic systems still need extra homogeneous
cocatalysts, such as tetrabutylammonium bromide (TBAB), for
to the design and preparation of various heterogeneous
catalysts, due to their obvious advantages compared with
homogeneous ones, such as the facile recovery of the catalyst,
4
separation an purification of products. Moreover, a lot of
previous reports have revealed that heterogeneous catalysts
having nucleophile sites (halogen anions) and/or electrophile
sites (metal ions and hydrogen bond donors HBDs) could
greatly enhance the activation and conversion of epoxides and
1
1-16
the generation of cooperative activation.
were few examples of metal-based POP catalysts that can
achieve the cycloaddition of CO without adding co-catalysts,
but also the related catalytic system required harsh reaction
Indeed, there
5
-9
2
inserted CO
2
molecules.
So far, various heterogeneous
catalysts with the single nucleophile site or dual nucleophile-
electrophile sites have been reported for CO conversion.
2
Nevertheless, the example related to the simultaneous
combination of HBD groups, metal ions and halogen anions in
a heterogeneous catalyst is rather scarce. Besides, the current
1
7
conditions. Noticeably, ionic POPs (named iPOPs) with paired
halogen anions have recently emerged for catalytic conversion
4
,8,9
of CO
2
in absence of co-catalysts, but most of these reactions
1
8-22
are usually carried out under CO
2
pressured conditions.
Further, the introduction of electrophile sites including metal
or HBD groups into iPOPs can afford metal-based iPOPs with
2
3-28
School of Chemistry and Materials Science, Jiangsu Key Laboratory of Green
Synthetic Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou
metal and halogen dual active sites,
sites,
or iPOPs with HBD
2
9-34
which have exhibited enhanced catalytic efficiency
2
21116, China. E-mail: gjchen@jsnu.edu.cn
‡
These authors contributed equally to this work.
under relatively mild conditions. As a result, the synergistic
catalytic effect of nucleophile and electrophile sites within one
Electronic Supplementary Information (ESI) available.
See DOI: 10.1039/x0xx00000x
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Page 2 of 11
ARTICLE
Journal Name
heterogeneous catalyst can be beneficial to promote the on an elemental analyzer Vario EL cubeVi.ew ACrthicelemOniclinael
catalytic performance under mild conditions without using co- compositions and electronic states ofDOtIh: 1e0.1s0a3m9/Dp0leDsT0w16e6r7eE
catalysts. Unfortunately, such desired “all-in-one” POP-based determined by the X-ray photoelectron spectroscopy (XPS,
heterogeneous catalysts simultaneous with Lewis acidic metal Thermo ESCALAB 250Xi). Fourier transform infrared
sites, halogen anions and HBD groups have not appeared for spectroscopy (FTIR) was recorded on a Bruker Vertex 80V FT-IR
-1
efficient CO
leaves a challenging task in this work.
With this target, we construct a new series of N-heterocyclic Q50 instrument under air atmosphere at a heating rate of 10
2
conversion under atmospheric pressure, which instrument (KBr discs) in the region 4000-400 cm .
Thermogravimetric analysis (TGA) was carried out with a TA
o
-1
ligand bipyridine-based porous hybrid polymers (Bpy-PHPs)
C min . X-ray diffraction (XRD) patterns were collected on the
with in-situ formed Si-OH groups by the Heck reaction Bruker D8 Advance powder diffractometer using Ni-filtered Cu
o
between
5,5'-dibromo-2,2'-bipyridine
(DBr-Bpy)
and Ka radiation source at 40 kV and 20 mA, from 5 to 80 with a
octavinylsilsesquioxane (VPOSS) units, as illustrated in Scheme scan rate of 0.2o s . Field emission scanning electron
-1
1
. The rigid building unit VPOSS not only plays a key role in microscope (FESEM, Hitachi SU8010) accompanied by Energy
fabricating porous organic-inorganic hybrid polymers but also dispersive X-ray spectrometry (EDS) was used to study the
3
2,35
provides useful Si-OH groups by destroying POSS cages.
morphology and the elemental distribution. Transmission
The typical Bpy-PHP-4 has a considerable surface area along electron microscopy (TEM) images were performed on the
with a high content of Bpy ligand, which is regarded as a JEOL JEM-2100F 200 kV field-emission transmission electron
metal-/halogen-free catalyst as well as a desired porous microscopes. N
polymeric platform to prepare the desired heterogeneous using a Quantachrome autosorb iQ2 analyzer, and the surface
catalyst ZnBr @Bpy-PHP-4 via a post-coordination process. area of samples was calculated using the Brunauer-Emmett-
Owing to the coexistence of abundant Si-OH groups, pyridinic Teller (BET) method and the pore size distribution was
2 2
and CO adsorption isotherms were measured
2
2
+
−
N atoms and Bpy-coordinated Zn and Br active sites, this determined by the nonlocalized density functional theory
o
“
all-in-one” catalyst ZnBr
2
@Bpy-PHP-4 exhibited significantly (NLDFT) model. All the samples were degassed at 150 C for 10
enhanced catalytic performance in the conversion of CO
2
and h in a high vacuum before analysis.
epoxides into cyclic carbonates without using any co-catalysts
under atmospheric pressure.
Synthesis of Bpy-PHPs
Bpy-PHPs were prepared by the Heck reaction of VPOSS with
DBr-Bpy. In a typical run, VPOSS (0.2 mmol, 0.1266 g) was
dissolved in 10 mL of dimethylformamide (DMF), and DBr-Bpy
(0.8 mmol, 0.2512 g) was dispersed in 10 mL of DMF. Then, the
mixture was transferred to a 25 mL Teflon-lined autoclave. At
Experimental
Materials
Octavinylsilsesquioxane (VPOSS), 5,5'-dibromo-2,2'-bipyridine,
tetrakis(triphenylphosphine)-palladium(0), K CO , ZnBr and
2 3 2
the required epoxide substrates and solvents are commercially
available and used without further purification.
the same time, K
water was poured into the mixture solution, followed by the
addition of Pd(PPh (0.02 mmol, 0.0232 g) with stirring for 30
2 3
CO (2 mmol, 0.2766 g) dissolved in 1 mL of
3 4
)
min. Next, the sealed Teflon-lined autoclave was placed in a
o
1
20 C constant temperature oven for 72 h. After reaction, the
Methods
solid product was recovered by filtration and thoroughly
1
Liquid-state H NMR spectra were performed on a Bruker DPX
washed with DMF, THF, H
2
O, and ethanol. The brown powder
1
3
5
00 spectrometer at ambient temperature. Solid-state
C
o
Bpy-PHP-4 was obtained by drying in vacuum for 12 h at 80 C.
A series of Bpy-PHP-x were also obtained by varying the molar
ratios of DBr-Bpy to VPOSS (x=1, 2, 3 and 4).
cross-polarization/magic angle spinning (CP/MAS) NMR and
2
9
Si MAS NMR spectra were performed on a Bruker AVANCE III
00 spectrometer. The CHN elemental analysis was performed
6
*
*
*
*
O
OSi
Si
O
O
N
N
S Oi Si O
N
N
Br
O Si OSi
N
N
Zn
O
O
O
Br
Si
Si
Zn
N
N
N
Br
O
O
Si
O
Si
O Si O Si
OH
O
Si
O
Br
OH
O
O
Si
O
VPOSS
Si
Si
O
O
O
Heck reaction
a)
ZnBr2
b)
O
O
Si
O
Si OSi
Si
Si
O
Si
+
O
O
O
Br
SiO SiO
OH OH
SiO SiO
OH OH
Br
Zn
(
(
Br Zn
N
N
N
Br
Br
Br
N
N
N
N
N
N
DBr-Bpy
Bpy-PHP-4
ZnBr2@Bpy-PHP-4
*
*
*
*
2
Scheme 1 Synthetic route to the typical bipyridine-based porous hybrid polymer Bpy-PHP-4 and the metalated catalyst ZnBr @Bpy-PHP-4 with plausible
o
o
3 4 2 3
chemical structures. Condition (a) the catalyst Pd(PPh ) , DMF/water, K CO , 120 C, 72 h; (b) DMF, 80 C, 24 h.
2
| J. Name., 2012, 00, 1-3
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ARTICLE
Synthesis of ZnBr
Using a typical molar ratio of ZnBr
PHP-4 (2:1), the catalyst ZnBr @Bpy-PHP-4 was prepared in
the following process: Bpy-PHP-4 (0.2786 mmol, 0.14 g) was
suspended in 20 mL of DMF with adding ZnBr (0.5572 mmol,
2
@Bpy-PHP-4
View Article Online
DOI: 10.1039/D0DT01667E
2
to the Bpy ligand in Bpy-
2
2
0
8
.1255 g), and the resultant mixture solution was heated to
o
0 C for 24 h. The solid was filtered and washed with DMF and
o
ethanol and then dried at 80 C under vacuum for 12 h. A
control catalyst ZnBr
using a different molar ratio of ZnBr
control catalyst Zn(OAc) @Bpy-PHP-4 was prepared by using
2
@Bpy-PHP-4 (1:1) was also obtained by
2
to Bpy (1:1). Besides, the
2
2
the molar ratio with Zn(OAc) to Bpy (2:1).
Catalytic tests
Fig.
1
(A)
N
2
adsorption-desorption isotherms and (B) NLDFT pore size
2
The CO cycloaddition reactions were performed using a 25 mL
glass tube with a magnetic stirrer and 1,2-epoxyhexane (EH) as
a model substrate under mild conditions. In a typical run, EH (2
2
distributions of Bpy-PHP-x series (x=1, 2, 3 and 4) and ZnBr @Bpy-PHP-4.
existence of both micropores and mesopores within Bpy-PHP-
mmol) and the catalyst ZnBr
charged in a Schlenk tube connected with a CO
2
@Bpy-PHP-4 (0.05 g) were
balloon (0.1
3
2,35
x.
As shown in Fig. 1B, the NLDFT pore size distributions
2
confirm that Bpy-PHP-x not only possess narrowly distributed
micropores centered at 1.41 nm but also display some wide-
range mesopores appeared at 2.77, 5.44, 12.79 and 20.06 nm.
As summarized in Table 1, as molar ratios of DBr-Bpy to
VPOSS increase from x=1 to 4, the BET surface areas (SBET),
micropore surface areas (Smicro), and total pore volume (Vtotal
increase initially and decrease afterwards, while N contents in
Bpy-PHPs continuously increase from 0.81 to 5.57 wt%. When
MPa), the mixture was stirred at the targeted temperature
with a stirring for a certain time. After reaction, the resulting
mixture was diluted with 3 mL ethyl acetate and stirred for 0.5
h. After then, the catalyst was separated by centrifugation and
the remaining solution was obtained by concentrating under a
low-temperature reduced pressure, and then directly analyzed
)
1
by H NMR spectroscopy to determine the yield of the cyclic
carbonate. For other substrates, similar reaction procedures
and analytical method were also carried out. The solid catalyst
was separated by centrifugation or filtration, washed with
ethyl acetate, dried in the vacuum and then was directly
reused in the next run.
2
x=2, the sample Bpy-PHP-2 has the largest SBET value of 483 m
-1
3
-1
g with an ultrahigh pore volume of 1.86 cm g ; when x=4,
the sample Bpy-PHP-4 gives the highest N content of 5.57 wt%
with the corresponding Bpy content of 1.99 mmol g , and also
provides a desired SBET value of 384 m g and a high Vtotal of
-1
2
-1
3
-1
1
.14 cm g . Besides, it is worthy to point out that the Smicro
2
-1
values of Bpy-PHP-x ranging from 104 to 175 m g provide
low contribution ratios of 32.3 to 36.2 % for total SBET values,
and the corresponding micropore volumes (Vmicro) ranging
Results and discussion
Construction of Bpy-PHPs and ZnBr @Bpy-PHP-4
2
3
-1
from 0.047 to 0.078 cm g only contribute to low ratios of 3.3
to 6.1 % for Vtotal values. The above results indicate that these
porous polymers have enriched mesoporosities, which would
benefit to facilitate mass transfer of epoxides in the catalytic
reaction. Obviously, pore structures and N contents of Bpy-
PHPs can be easily tailored by varying the molar ratios of DBr-
Bpy to VPOSS. As a result, the sample Bpy-PHP-4 having the
highest Bpy content with the moderate surface area and
enriched mesoporosity would be more suitable to be used as a
As shown in Scheme 1, N-heterocyclic ligand bipyridine-based
porous hybrid polymers (Bpy-PHPs) were synthesized directly
via the Heck reaction between VPOSS and DBr-bpy monomers
o
in a DMF/water (v/v: 10/1) mixture solution at 120 C for 72 h
under solvothermal conditions. Subsequently, the metal-based
porous hybrid polymer ZnBr
post-coordination of a typical sample Bpy-PHP-4 with ZnBr
a desired molar ratio of ZnBr to Bpy (2:1) in the solvent of
DMF. As a result, the typical samples of Bpy-PHP-4 and
ZnBr @Bpy-PHP-4 were fully performed by following various
2
@Bpy-PHP-4 was obtained by
2
in
2
catalyst or catalyst support in the conversion of CO
Bpy-PHP-4 was chosen as a porous platform for coordinating
ZnBr to prepare heterogeneous catalyst ZnBr @Bpy-PHP-4.
The obtained ZnBr @Bpy-PHP-4 has certain a decreased
surface area of 251 m g and pore volume of 0.86 cm g
Table 1), attributed to the successful introduction of ZnBr
2
. Further,
2
characterization techniques.
First, the porous structures of Bpy-PHPs and ZnBr @Bpy-
2
PHP-4 were characterized by N -sorption experiments at 77 K.
2
Porous structures and the contents of Bpy moieties for these
2
2
2
2
-1
3
-1
(
2
Bpy-PHP-x can be adjusted by changing their molar ratios (x=1,
into Bpy-PHP-4 and the occupation of certain porous channels.
The porous structures and morphologies of Bpy-PHP-4 and
2
, 3 and 4) of DBr-Bpy to VPOSS. For Bpy-PHP-x series, Type II
and IV sorption isotherms were observed for these polymers
ZnBr
images. As shown in Fig. 2A and 2C, SEM images of Bpy-PHP-4
and ZnBr @Bpy-PHP-4 present fluffy spongy-like morphology
2
@Bpy-PHP-4 were further observed by the SEM and TEM
(Fig. 1A). All the polymers exhibited obvious adsorbing uptakes
3
-1
0
(42~73 cm g ) at low relative pressures (P/P < 0.01) and
significantly increasing ones at higher relative pressures (0.80
P/P < 0.99) with H1 type hysteresis loops, indicating the
2
that assembled by numerous interconnected nanoparticles
<
0
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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View Article Online
Table 1 Textural properties, N and Bpy contents of Bpy-PHPs and ZnBr
2
@Bpy-PHP-4.
DOI: 10.1039/D0DT01667E
b
c
c
d
e
n(VPOSS):
n(DBr-Bpy)
S
BET
2
S
micro
V
micro
3
V
total
3
D
p
N content
Bpy content
(mmol g )
Samples a
-1
2
-1
-1
-1
(wt%) f
-1
f
(m g )
(m g )
(cm g )
(cm g )
(nm)
Bpy-PHP-1
Bpy-PHP-2
Bpy-PHP-3
Bpy-PHP-4
1:1
1:2
1:3
1:4
1:4
301
104
0.047
1.42
1.41, 2.77
0.81
0.29
1.05
1.76
1.99
1.56
483
421
384
251
175
152
124
44
0.078
0.066
0.054
0.018
1.86
1.09
1.14
0.86
1.41, 2.77
1.41, 2.77
1.41, 2.77
1.41, 3.17
2.94
4.94
5.57
4.38
2
ZnBr @Bpy-PHP-4
a
Bpy-PHP-x was obtained by the Heck reaction of DBr-Bpy with VPOSS in different molar ratios (x=1, 2, 3 and 4); ZnBr
2
@Bpy-PHP-4 was obtained by
=0.05~0.20. c Micropore
metalation of Bpy-PHP-4 with ZnBr using a molar ratio of ZnBr
2
2
to Bpy (2:1). b The BET surface area calculated over the range P/P
0
surface area and pore volume calculated by the t-plot method. d Total pore volume calculated at P/P
=0.99. e The main pore diameters calculated by the
0
f
-1
NLDFT theory. The N content (wt%) was obtained by the CHN elemental analysis. Bpy content (mmol g ) = 0.5*1000*N content (wt%)/14.
To confirm the successful preparation of the targeted
1
3
29
chemical structure, solid-state C and Si NMR spectra for
Bpy-PHP-4, and FTIR and XPS spectra for Bpy-PHP-4 and
1
3
2
ZnBr @Bpy-PHP-4 were conducted. From the C CP/MAS
NMR of Bpy-PHP-4 (Fig. 3A), the appearance of obvious peaks
at 131.0 and 153.2 ppm, assigned to aromatic carbons and N-
1
1,38
bonded carbons (Cbpy=N) within Bpy units,
demonstrating
the successful introduction of the desired Bpy ligands into Bpy-
PHPs. Besides, the signals observed at 120.2 and 146.5 ppm
3
9,40
come from silicon-bonded ethylene carbons (Si-CH=CH-),
demonstrating the formation of ethylene linkers by the Heck
reaction between VPOSS and DBr-Bpy units. The POSS-derived
2
9
Si-OH groups were confirmed by the solid-state Si MAS NMR
spectrum of Bpy-PHP-4 (Fig. 3B). Two signals at -73.1 and -80.3
2
ppm in the spectrum are attributed to the silicon atoms of T
3
n
32,35,41,42
and T units [T : CSi(OSi)
n
(OH)3-n], respectively.
The
2
Fig. 2 SEM and TEM images of (A, B) Bpy-PHP-4 and (C, D) ZnBr @Bpy-PHP-
2
3
affiliated T units come from the partial collapse T cubic cages
by the partial cleavage of Si-O bonds during the synthesis,
4
.
2
41,42
3
giving some T -type Si-OH groups.
0.3 ppm confirms the preservation of most of cubic POSS
cages, which are consistent with previously reported POSS-
The main T peak at -
with very small sizes about 20-30 nm. It can be observed that
the bright sections in TEM images (Fig. 2B and 2D) reflect the
nanovoids among interconnected nanoparticles. These results
8
3
9-43
based porous polymers.
The affiliated peak at -102.0 ppm
2
imply that Bpy-PHP-4 and ZnBr @Bpy-PHP-4 possess abundant
3
3
demonstrates the formation of Q units [Si(OSi)
3
OH] with Q -
mesoporosities, which are consistent with the results in N
2
-
3
type Si-OH groups, caused by the cleavage of Si-C bonds of T
sorption analyses.
3
1,32,41,42
4
silicon units.
4
The Q units [Si(OSi) ] appeared at -110.9
Moreover, XRD patterns of Bpy-PHP-4 and ZnBr
(Fig. S1) show broad diffraction peaks at 22.4~25.4 ,
demonstrating their amorphous nature of porous polymers.
2
@Bpy-PHP-
ppm should be resulted from self-condensation of Si-OH
o
4
3
31,32,41
groups in the formed Q units.
destruction can be explained by the distortion of cubic POSS
cages connected by rigid Bpy units and the addition of K CO
The partial collapse and
3
6
Besides, no characteristic peaks appeared for ZnBr
ZnBr @Bpy-PHP-4, indicating that the introduced ZnBr
highly dispersed in the porous skeleton of Bpy-PHP-4 by
2
in the
2
3
2
2
was
4
0,42
as the HBr absorbent.
3
According to the FTIR spectra in Fig.
2
@Bpy-PHP-4 still behave typical Si-O-
C, Bpy-PHP-4 and ZnBr
2
8
coordination interaction. The thermal stability of the porous
Si asymmetric stretching vibration along with some splits and
o
hybrid polymers was evaluated by TGA in air up to 800 C (Fig.
-
1
-1
broad shifts appeared at 1130, 1077 cm and 1122, 1066 cm ,
S2). The samples of Bpy-PHP-4 and ZnBr
the excellent thermal stability up to 350 C, even superior to
2
@Bpy-PHP-4 show
−
1
in contrast to the sharp peak at 1110 cm for cubic VPOSS,
due to the coexistence of T and Q silicon units.
bands located at around 3645 and 3653 cm plus 956 and 960
cm are attributed to the Si-OH and Si-O stretching vibrations,
further confirming the existence of Si-OH groups in the hybrid
o
n
n
32,35
Two
that of VPOSS monomer. Besides, the initial losses of weight
-1
o
(
ca. 4 wt%) up to 200 C indicate the existence of a small
-1
amount of adsorbed water in the samples that can present a
positive effect on the catalytic activity in CO conversion.
2
3
7
3
1,32,35
networks.
Besides, new bands appeared near 1649 and
4
| J. Name., 2012, 00, 1-3
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View Article Online
DOI: 10.1039/D0DT01667E
Fig. 3 Structural characterizations of Bpy-PHP-4 and ZnBr
2
@Bpy-PHP-4: (A) 13C CP/MAS NMR, (B) 29Si MAS NMR, (C) FTIR spectra, (D) XPS spectra survey,
(E) Si 2p, (F) C 1s, (G) N 1s, (H) Zn 2p, and (I) Br 3d.
4
1
650 cm-1 are ascribed to C=N stretching vibration from Bpy at higher values of 104.1 and 103.4 eV are assigned to Q
4
4,45
moieties, indicating the successful incorporation of Bpy ligands silicon atoms (SiO
2
).
These multiple states of silicon atoms
into the porous hybrid polymer. From the view of synthesis characterized by the XPS Si 2p analyses are consistent with the
of ordered polymeric materials, the destroy of cubic POSS result of Si MAS NMR (Fig. 3B). The C 1s spectra (Fig. 3F) of
cages is not perfect, however, just these defective POSS cages Bpy-PHP-4 and ZnBr @Bpy-PHP-4 exhibit three characteristic
afford two kinds of Si-OH groups from T and Q silicon units, peak components at binding energy 286.2/285.7, 285.2/284.8
which could be used as HBD active groups for enhancing and 284.3/284.2 eV, which are assigned to C-N of Bpy units, C-
3
5
2
9
2
2
3
catalytic CO
The chemical states of the successfully introduced Si-OH carbons of POSS units.
groups, Bpy ligands and N-coordinated ZnBr were further Bpy-PHP-4 just gives one peak at 398.9 eV, assigned to
verified by the XPS technique. The full survey XPS spectra (Fig. pyridinic N atoms in Bpy moieties. For ZnBr @Bpy-PHP-4,
D) display the contents of C, N, O and Si elements for both apart from the free N 1s peak (398.4 eV) in Bpy ligands, there
2
conversion.
C/C-H of ethylene and aromatic carbon groups, and Si-C
3
5,44
The scan on N 1s region (Fig. 3G) of
2
2
3
Bpy-PHP-4 and ZnBr
2
@Bpy-PHP-4, especially exhibit the newly was a new peak appeared at 399.5 eV, which should be
@Bpy-PHP-4. The reasonably attributed to ZnBr -coordinated N atoms (Zn-N2) in
In the Zn 2p XPS spectrum (Fig. 3H), the
DBr-Bpy monomers were completely consumed by the Heck peaks of Zn 2p1/2 and Zn 2p3/2 at 1044.7 and 1021.7eV clearly
introduced Zn and Br elements for ZnBr
residual Br element was negligible in Bpy-PHP-4, indicating the Bpy moieties.
2
2
1
1,38
2
+
reaction. In Si 2p XPS spectra of Bpy-PHP-4 and ZnBr
2
@Bpy- demonstrate that Zn ions were introduced into Bpy-PHP-
PHP-4 (Fig. 3E), the Si 2p peaks located at the binding energy 4.2
7,28,47
The Br 3d core level spectra (Fig. 3I) are fitted to Br
(
BE) of 102.1 and 102.2 eV, which correspond to O -Si-C (i.e., 3d3/2 and Br 3d5/2 at 70.8 and 69.8 eV, indicative of the
3
3
44-46
−
2+
27,28
RSiO3/2) in T -type POSS units.
1
groups originated from T and Q units.
The Si 2p peaks occurred at presence of Br anions paired with Zn cations.
According
2
+
03.2 and 102.7 eV should be attributed to the emerging Si-OH to the XPS analysis, the atomic (at) concentrations of Zn and
2
3
32,35,44
−
2
The Si 2p peaks Br in ZnBr @Bpy-PHP-4 are 2.68 at% and 5.26 at%. The above
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results suggest that the N-coordinated ZnBr
Bpy-PHP-4 by strong coordination interactions between Zn
2
stably existed in and 99% and high selectivities of 99% under same conditions
View Article Online
DOI: 10.1039/D0DT01667E
2
+
(Table 2, entries 2 and 3). Obviously, the catalyst ZnBr
2
@Bpy-
2
+
and Bpy ligands, as well as ionic interactions between Zn
PHP-4 with a higher content of ZnBr gave the optimal catalytic
2
−
cations and Br anions. Based on the post-coordination process, activity compared with the undecorated Bpy-PHP-4, which
the introduced ZnBr
the surface of the porous polymer, rather in the bulk phase, and Bpy-coordinated ZnBr
which may be more suitable for heterogeneous catalysis. The Further, a series of control experiments were carried out to
elemental mapping based on energy-dispersive X-ray reflect the vital roles of Si-OH groups and ZnBr active sites. In
spectroscopy (EDS) analysis further revealed that the existence the reaction system with a mixture of Bpy-PHP-4 and ZnBr as
and homogeneous distributions of C, O, N for Bpy-PHP-4 and the co-catalysts (Table 2, entry 4), the given yield was much
C, O, N, Zn and Br for ZnBr @Bpy-PHP-4 (Fig. S3). Based on the higher than those of individual Bpy-PHP-4 or ZnBr (Table 2,
above full characterizations, we have successfully constructed entries 1 and 5), as high as that of ZnBr @Bpy-PHP-4 (Table 2,
the targeted ZnBr @Bpy-PHP-4 with abundant Si-OH groups entry 3). In details, only using ZnBr as a catalyst, the yield was
and Bpy-coordinated ZnBr sites, which would be regarded as very low (7%, entry 5). The dramatically enhanced yields were
an efficient synergetic heterogeneous catalyst for CO fixation. both obtained when ZnBr worked together with Bpy-PHP-4 in
2
species ought to be mainly supported on may be attributed to the synergistic catalysis of Si-OH groups
2
.
2
2
2
2
2
2
2
2
2
2
a physical mixture or coordinated with Bpy-PHP-4 in a
heterogeneous catalyst. These remarkable comparisons
Catalytic performance in CO
Considering the abundant Si-OH groups and active N atoms of
Bpy-PHP-4 and additional Bpy-coordinated ZnBr sites within
ZnBr @Bpy-PHP-4, they were assessed as heterogeneous
catalysts in the cycloaddition of CO with epoxides into high
value-added cyclic carbonates in the absence of any solvents
and co-catalysts. To explore the suitable conditions for
synthesis of cyclic carbonates, 1,2-epoxy hexane (EH) was
selected as a typical substrate for this transformation under
2
fixation
demonstrate that synergistic effect of Si-OH and ZnBr
2
plays
the vital role in enhancing the catalytic performance.
2
Besides, a control bromide-free heterogeneous catalyst
Zn(OAc) @Bpy-PHP-4 gave a low yield of 20% (Table 2, entry
), indicating that the nucleophilic Br anions from ZnBr
an important role in the catalytic process. It was also noted
that Zn(OAc) @Bpy-PHP-4 without enough free N atoms gave
2
2
2
−
6
2
play
2
much lower catalytic activity than that of Bpy-PHP-4, also
revealing the possible co-catalytic role of free N atoms in Bpy
ligands. A straightforward control test over Bpy gave certain a
yield of 15% for the product (Table 2, entry 7), suggesting the
catalytic role of Lewis base N atoms in Bpy. Meanwhile, this
much lower yield than that of Bpy-PHP-4 again presents the
important role of Si-OH HBD active groups in Bpy-PHP-4. As a
result, under atmospheric pressure and co-catalyst-free
2
atmospheric pressure (0.1 MPa) using a CO balloon and the
results of the catalytic reactions were summarized in Table 2.
First, the reaction was directly carried out at a relative low
o
temperature of 80 C for 72 h, the metal-/halogen-free catalyst
Bpy-PHP-4 gave a low catalytic activity with the yield of 60%
(
Table 2, entry 1), which could be considered as a moderate
result only using a Si-OH containing catalyst. As a contrast, the
all-in-one” metal-based catalysts ZnBr @Bpy-PHP-4 (1:1) and
ZnBr @Bpy-PHP-4 gave significantly enhanced yields of 86%
2
conditions, the remarkable catalytic behavior of ZnBr @Bpy-
“
2
PHP-4 was superior to those of many reported metal-based
polymer heterogeneous catalysts that required high
temperature and/or high pressure and extra co-catalysts
2
2
Table 2 Catalytic performances in the cycloaddition of CO with EH over
various catalysts.a
1
1,12,14
(Table S1, entries 1-6),
and comparable to those of
2
3,26,47-51
metal-based iPOPs (Table S1).
In a word, this
O
satisfactory heterogeneous catalytic performance of
O
O
CO2 balloon
Catalyst
ZnBr @Bpy-PHP-4 can be attributed to the synergistic catalysis
2
O
C H
4
9
C H
4 9
of POSS-derived Si-OH, pyridinc N active atoms and ZnBr
2
Lewis sites in this “all-in-one” catalyst.
Both the modest catalytic activity of metal-/halogen-free
catalyst Bpy-PHP-4 and good catalytic performance of
o
Y (%) f
S (%) f
Entry
Catalyst
T ( C)
t (h)
1
2
3
4
5
6
7
Bpy-PHP-4
80
72
72
72
72
72
72
72
60
86
99
98
7
99
99
99
99
99
99
99
ZnBr
and difference in catalytic conversion of other epoxides. The
cycloaddition reactions of CO were carried out using different
2
@Bpy-PHP-4 encouraged us to explore the applicability
ZnBr
ZnBr
Bpy-PHP-4+ZnBr b
ZnBr c
2
@Bpy-PHP-4 (1:1)
80
80
80
80
80
80
2
@Bpy-PHP-4
2
2
epoxide substrates (1a-1h), such as epichlorohydrin (ECH),
epibromohydrin (EBH), glycidyl phenyl ether (GPE), allyl
glycidyl ether (AGE), styrene oxide (SO) and large-sized
substrates of long-chain alkyl epoxides, and the contrastive
results were presented in Fig. 4. Surprisingly, the ECH and EBH
could be converted into the corresponding products 2a and 2b
with high yield of 99% over the catalyst Bpy-PHP-4 as well as
ZnBr @Bpy-PHP-4. These unexpected good catalytic activities
2
may be assigned to the in-situ formation of ionic liquid-based
catalytic active sites via the quaternarization reaction between
2
@Bpy-PHP-4 d
Zn(OAc)
2
20
15
Bpy e
a
Reaction conditions: EH (2 mmol), CO
temperature (T), time (t); b Bpy-PHP-4 (0.05 g), ZnBr
0.022 g); d, e The dosage of Zn(OAc)
@Bpy-PHP-4 and 2,2'-bipyridine (Bpy)
2
balloon (0.1 MPa), catalyst (0.05 g),
(0.022 g); c ZnBr
2
2
(
2
was based on the Bpy-PHP-4; f Yield (Y) and selectivity (S) of the cyclic
1
carbonate determined by H NMR (see the typical spectrum in Fig. S14).
6
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0DT01667E
O
O
O
CO balloon
O
2
R
Catalysts
R
1a-1h
2a-2h
O
O
O
(
A)
(B)
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Cl
Br
Cl
Br
2b': 99% (80°C) 2c': 89% (100°C)
2
a: 99% (80°C) 2b: 99% (80°C) 2c: 60% (120°C)
2a': 99% (80°C)
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Bpy-PHP-4
ZnBr
2
@Bpy-PHP-4
2d: 74% (120°C)
2e: 69% (120°C)
2d': 95% (100°C)
2e': 98% (100°C)
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
C12H25
C6H13
C10H21
C6H13
C10H21
C12H25
2f: 44% (100°C) 2g: 14% (120°C) 2h: 12% (120°C)
2f': 91% (80°C)
2g': 93% (120°C) 2h': 89% (120°C)
Fig. 4 Substrate scope for the CO
2
2
cycloaddition with various epoxides catalyzed by (A) Bpy-PHP-4 and (B) ZnBr @Bpy-PHP-4. Reaction conditions:
o
1
substrate (2.0 mmol), catalyst (0.05 g), CO
2
2
(0.1 MPa), 80~120 C, 72 h. The yields of all the products were determined by H NMR analyses of crude
products and the detailed spectra over ZnBr
@Bpy-PHP-4 were provided in Figs. S9-S17.
pyridinic N atoms and halogen-tethered ECH and EBH.52-54 For
Bpy-PHP-4, the conversion of 1c-1e (SO, PGE and AGE) could
also give moderate yields (60-74%) for the products 2c-2 under
o
a higher temperature of 120 C, whereas the inert substrates
(1f-1h) of long-chain alkyl epoxides showed much lower
conversions. Even so, as a metal-/halogen-free heterogeneous
catalyst, Bpy-PHP-4 still behaved moderate catalytic activities
under atmospheric pressure, superior to those of the examples
5
5-59
of metal-/halogen-free heterogeneous catalysts.
As a
contrast, the catalyst ZnBr @Bpy-PHP-4 was investigated using
2
various epoxides without adding any co-catalysts. As shown in
Fig. 4B, the significantly enhanced catalytic activities with high
yields of 89-98% were provided for the products 2c'-2f' under
mild conditions. Even facing the inert long-chain substrates
(
1g-1h), excellent yields (89-93%) for 2g'-2h' were also
o
obtained by increasing temperature to 120 C. As a result, Fig. 5 Catalytic reusability of ZnBr @Bpy-PHP-4 in the cycloaddition of CO
2
2
compared with the undecorated Bpy-PHP-4, the improved with EH. Reaction conditions: EH (2 mmol), CO pressure (0.1 MPa), the
2
catalyst ZnBr
various epoxides.
For heterogeneous catalysis, the recovery and recyclability (Figs. S4-S7). Especially getting from XPS spectra (Fig. S4), the
2
@Bpy-PHP-4 exhibits much better applicability to catalyst ZnBr @Bpy-PHP-4 (0.05 g), 80
o
C, 72 h.
2
2
+
−
of catalysts are very important to practical applications. The catalytic active sites Zn , Br anions and Si-OH groups were
heterogeneous catalyst ZnBr
@Bpy-PHP-4 could be easily relatively well-retained compared with the fresh one.
recovered from the reaction system by a simple centrifugation. Moreover, the reused ZnBr @Bpy-PHP-4R had similar Zn and
As shown in Fig. 5, ZnBr
@Bpy-PHP-4 behaved excellent Br contents to those of the fresh catalyst (Table S2), indicating
reusability in terms of yield and selectivity only with negligible the negligible leaching of ZnBr into the reaction system. All
losses after ten cycles in the cycloaddition of CO
with EH, due the results confirm the good recyclability and heterogeneous
2
2
2
2
2
to inevitable operational mass losses during the repeated character of ZnBr
2
@Bpy-PHP-4.
recovery of the catalyst, also indicating its good structural
Insights into the catalytic reaction process
stability. Further evidences in support of the good stability of
the reused catalyst ZnBr @Bpy-PHP-4R were obtained from It was reported that nucleophiles (halogen anions) and
2
2
the characterization results of XPS, FTIR, SEM, and N -sorption
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DOI: 10.1039/D0DT01667E
HO
HO
HO
HO
HO
HO
(A)
OH
(B)
OH
OH
POSS
OH
POSS
O
O
POSS
Si
POSS
Si
N
N
N
N
O
OH
O
O
Zn
OH
O
R
O
HO
O
HO
OH
OH
R
Bpy-PHP-4
R
R
Br
Br
ZnBr2@Bpy-PHP-4
HO
HO
HO
HO
HO
HO
HO
OH
HO
HO
HO
HO
HO
OH
OH
O
POSS
OH
OH
OH
POSS
OH
O
POSS
OH
POSS
POSS
Si
N
N
POSS
Si
POSS
Si
O
OH
POSS
Si
N
N
N
N
N
N
O
OH
Zn
OH
Zn
OH
O
O
O
H
H
O
O
O
H
O
H
O
Br
H
O
H
Br
R
H
H
O
O
O
Br
Br
R
R
R
CO2
HO
HO
HO
HO
O
C O
O
C
O
HO
HO
OH
OH
OH
CO2
OH
O
POSS
OH
POSS
POSS
Si
POSS
Si
N
N
N
N
Zn
OH
O
O
O
H
H
H
Br
O
O
H
R
R
Br
O
C O
Scheme 2 Comparison of plausible catalytic mechanisms for CO
2
cycloaddition with epoxides over the catalysts (A) Bpy-PHP-4 and (B) ZnBr
2
@Bpy-PHP-4.
electrophiles (metal salts and HBD groups) are all beneficial to carbonate and regenerated the catalyst Bpy-PHP-4. However,
5
,28,48,60
activate the epoxides.
combination of halogen anions, metal ions and Si-OH HBD free Bpy-PHP-4 cannot provide satisfactory catalytic activities
groups and active N atoms into a heterogeneous catalyst in the conversion of CO with some inert epoxide substrates.
ZnBr @Bpy-PHP-4 achieved the synergistic catalytic conversion By coordination with ZnBr , the resultant ZnBr @Bpy-PHP-4
of CO under mild conditions. Besides, the high surface area actually improved the catalytic efficiency for various epoxides.
and enriched mesoporosity of ZnBr @Bpy-PHP-4 make for the Different from Bpy-PHP-4, the plausible synergistic catalytic
sufficient exposure and high dispersion of multiple active sites, mechanism for the CO cycloaddition with epoxides over the
which can enhance mass transfer of epoxides to the catalyst catalyst ZnBr @Bpy-PHP-4 was exhibited in Scheme 2B.
and thus accelerate catalytic efficiency. Further, the desired Initially, the epoxide was together activated by electrophile
In this work, the simultaneous without the participation of nucleophiles, the metal-/halogen-
2
2
2
2
2
2
2
2
2
+
hydrophobic reaction micro-environment constructed from sites including Si-OH groups, Lewis acidic metal Zn cations
POSS building blocks is beneficial to make large-sized and also some uncoordinated free N atoms from Bpy ligands,
hydrophobic substrates easily contact the catalytic active greatly promoting the ring opening of the epoxide via C-O
species, which can explain their high catalytic activities for bond polarization.
long-chain alkyl epoxides.
capacities of Bpy-PHP-4 and ZnBr
mmol g for Bpy-PHP-4, 0.67 mmol g for ZnBr
at 273 K, 0.1 MPa, Fig. S8) can also favor the sufficient contact oxyanion intermediate interacted with the introduced CO
between activated substrates, CO
catalytic active sites.3
2
7,32,35,48
Then, the free nucleophilic Br−
attacked the less hindered β-carbon atom of
@Bpy-PHP-4 (e.g., 0.71 the activated epoxide to produce an oxyanion intermediate
3
2,35
2 2
Finally, moderate CO adsorption anions from ZnBr
2
-1
-1
32,35,61
2
@Bpy-PHP-4, that stabilized by Si-OH HBD groups.
Subsequently, the
to
2
6
1
2
molecules, and multiple generate bromoalkyl-carbonate anion intermediate. Finally,
the product cyclic carbonate was formed and the catalyst
5
2
Based on our experimental results and previous reports, the ZnBr @Bpy-PHP-4 was regenerated by the successive
comparison of plausible catalytic mechanisms was proposed to intramolecular ring closure of carbonate anion intermediate
−
explain good catalytic activities of Bpy-PHP-4 and ZnBr
2
@Bpy- and breakage of the C-Br bond with the release of Br anion.
PHP-4 in the cycloaddition of CO with epoxides. As described
2
in Scheme 2A for Bpy-PHP-4, the epoxide was initially
activated by the H-bonding interactions between the O atoms
Conclusions
of epoxides and POSS-derived Si-OH groups plus small amount
In summary, a new series of N-heterocyclic ligand bipyridine-
based porous hybrid polymers with POSS-derived Si-OH groups
were constructed by the Heck reaction. By virtue of the
desired multiple characters including high surface areas with
abundant mesoporosities, Si-OH groups and N active sites, the
typical Bpy-PHP-4 could be served as a metal-/halogen-free
of absorbed water.3
1-33,61,62
Meanwhile, the pyridinc N atom of
Bpy moieties interacted with the less hidered β-carbon atom
of the epoxide via its lone pair of electrons, weakening the C-O
bond and leading to ring opening of the epoxide and formation
of the C-N bond between the β carbon and the pyridinc N
atom.5
8,60,63
Furthermore, the proton from Si-OH groups
catalyst for CO
resultant ZnBr
catalytic activities for conversion of CO
2
conversion. After coordination with ZnBr
@Bpy-PHP-4 exhibited significantly enhanced
with various epoxides
2
, the
migrated to the epoxide O atom with the partial negative
charge and transformed to anionic intermediate that stabilized
2
2
6
0,63
by H-bonding interactions.
Subsequently, the inserted CO
2
into cyclic carbonates without using any co-catalysts under
mild conditions, which should be attributed to the synergistic
catalysis of Si-OH groups, N active atoms and Bpy-coordinated
molecule was coupled with the negative epoxide O atom and
changed to a more stable epoxide-carbonate intermediate
5
8,63
accompanied by the migration of the proton.
At last, the
2
ZnBr . This work provides a new porous ligand platform for
intramolecular ring-closing of epoxide-carbonate intermediate
and the breakage of the C-N bond afforded the product cyclic
8
| J. Name., 2012, 00, 1-3
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Journal Name
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2
2
2
2
2
0 J. Li, D. Jia, Z. Guo, Y. Liu, Y. Lyu, Y. Zhou and J.VWiewaAnrgtic,leGOrenleinne
preparing metal-based porous polymer catalysts for CO
fixation and extended heterogeneous catalysis.
2
Chem., 2017, 19, 2675-2686.
DOI: 10.1039/D0DT01667E
1 H. Zhong, Y. Su, X. Chen, X. Li and R. Wang, ChemSusChem,
2
017, 10, 4855-4863.
2 Z.-W. Liu, B.-H. Han, Curr. Opin. Green Sustainable Chem.
019, 16, 20-25.
3 T.-T. Liu, J. Liang, Y.-B. Huang and R. Cao, Chem. Commun.,
Conflicts of interest
There are no conflicts to declare.
2
2
016, 52, 13288-13291.
4 Y. Leng, D. Lu, C. Zhang, P. Jiang, W. Zhang and J. Wang,
Chem. Eur. J., 2016, 22, 8368-8375.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21603089, 21706106 and 21503098), the
25 Y. Chen, R. Luo, Q. Xu, J. Jiang, X. Zhou and H. Ji,
ChemSusChem, 2017, 10, 2534-2541.
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6 D. Ma, J. Li, K. Liu, B. Li, C. Li and Z. Shi, Green Chem., 2018,
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the Natural Science Foundation of Jiangsu Higher Education
Institutions of China (16KJB150014), the Postgraduate
Research & Practice Innovation Program of Jiangsu Province
and the Top-notch Academic Programs Project of Jiangsu
Higher Education Institutions (TAPP).
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Dalton Transactions
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DOI: 10.1039/D0DT01667E
The Table of Contents Entry
ZnBr metalated-bipyridine porous hybrid polymers with POSS-derived Si-OH as “all-in-one”
2
heterogeneous catalysts for synergistic catalytic CO fixation.
2