Catalytic Conversion of CO2 to Cyclic Carbonates through Multifunctional Zinc-Modified ZSM-5 Zeolite

Article abstract of DOI:10.1002/cjoc.201700573

The production of fine chemicals using CO₂ as C₁ building block through inexpensive heterogeneous catalysts with high efficiency under low pressure is challenging. Herein we propose for the first time the utilization of a multifuncti

Full text of DOI:10.1002/cjoc.201700573

Catalytic conversion of CO
2
to cyclic carbonates through multifunc-
tional zinc-modified ZSM-5 zeolite
b
a
a
a
a
a
Qing-Ning Zhao, Qing-Wen Song, * Ping Liu, Qian-Xia Zhang, Jun-Hua Gao and Kan Zhang *
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan,
30001, P. R. China
Department of Chemistry, Shanghai University, Shanghai, 200444, P. R. China
0
b
The production of fine chemicals using CO
with high efficiency under low pressure is challenging. Herein we propose for the first time the utilization of a mul-
tifunctional heterogeneous zinc-modified HZSM-5 (ZnHZSM-5) catalyst for upgrading CO by incorporation into
cyclic carbonates from CO and epoxides. Owing to the nice surface properties such as abundant Lewis acid,
Brønsted acid and Lewis base sites, large surface area, and plenty of micropores, CO could be concentrated and
well activated in ZnHZSM-5 verified by CO -TPD and TG-MS etc. Meanwhile, the epoxides were also activated
2 1
as C building block through inexpensive heterogeneous catalysts
2
2
2
2
through metal center and hydrogen bond. Therefore, the reaction can easily assemble at the catalyst interface and
show exceptional performance, affording the aimed products with high yield of up to 99% in the presence of com-
mercial tetra-n-propylammonium bromide (90% in kilogram scale with 0.004 mol% ZnHZSM-5 and 0.015 mol%
nPr
4
NBr).
Keywords Carbon dioxide utilization, Zeolite, Heterogeneous catalysis, Multifunctional activation, Heterocycle
Introduction
economic benefits.14 Therefore, how to utilize CO
with
2
inexpensive catalytic materials in an effective manner
with specific chemoselectivity will be of great signifi-
cance, appealing and urgently required. Through the
analysis of high-performance homogeneous catalytic
systems, several key points are indicated as follows: 1)
high contact frequency of catalyst with substrate; 2)
regulable active center; 3) catalytic framework with
multiple activation/catalytic sites. In this context, we
Cyclic carbonates are an important kind of valuable
heterocycles, which have been widely applied as polar
aprotic solvents, electrolytes in batteries, monomers in
the preparation of polycarbonates, and the intermediates
in the manufacture of fine chemicals. In view of sus-
tainable development and environmental protection,
chemical utilization of the abundant, easily available,
and renewable CO feedstock as an integral part of the
2
carbon cycle for producing cyclic carbonates has re-
ceived considerable attention, and it represents an ideal
for industry application. Here the cycloaddition of
epoxides with CO
1
2
envisioned that the efficiency of CO conversion could
be greatly improved through assembling above ad-
vantages of homogeneous catalysis into heterogeneous
catalysis, and high heterogeneous catalytic system
would be well developed. Therefore, development of
inexpensive functional inorganic catalysts with multiple
activation/catalytic sites together with high surface area
is proposed to be one route to construct
high-performance catalyst, and promising in both aca-
demia and industry.
2
2
provides an industrial route to cyclic
in place of phosgene and carbon
carbonates using CO
2
monoxide with both economic and environmental bene-
3
fits. Until 2015, approximately 0.18 million tons of
CO
bonates. Although great progress achieved, the utiliza-
tion of CO encounters major challenges that originate
2
per year is used for the production of organic car-
4
2
ZSM-5, generally synthesized via hydrothermal
method from aluminum compound and silicate, is one
from its high thermodynamic stability and kinetic inert-
ness. Therefore, numerous effective strategies are pro-
posed for the activation of CO and the epoxide to pro-
2
mote the cycloaddition, thus facilitating the reaction to
run under mild reaction conditions.
On the other hand, heterogeneous catalysis via inor-
2
ganic materials is an attractive subject in CO conver-
sion due to the good properties such as easily available,
simple preparation procedure, using aqueous media with
avoidance of any organic solvents, low cost and high
15
kind of important aluminosilicate zeolite, which has
many good properties such as easily available scale
preparation, tunable Si/Al ratio, and hydrothermal sta-
bility. Additionally, ZSM-5 in its hydrogen form
5
(
HZSM-5) or metal-modified HZSM-5 (MHZSM-5)
could be acquired with many functional active sites
Brønsted acid, Lewis acid and base). Besides, HZSM-5
or MHZSM-5 can undergo reversible physical and
chemical absorption of CO at the channel/surface of
(
2
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zeolites.16 Moreover, due to the unique pore structures,
high surface area, flexible frameworks, tunable acidity,
and controlled chemistry, they have been widely used in
o
+ 2
2020 M C adsorption apparatus at −196.5 C using N
as adsorbate. The specific surface area was calculated
following the BET method and the total pore volume
was determined at a relative pressure (P/P
17
heterogeneous catalysis as well as in the separation
0
) of 0.99.
18
and purification fields .
Before the adsorption measurements, the samples were
o
−4
Recently, MHZSM-5 has also emerged as a promis-
ing solid catalyst with superior catalytic performance in
dried in situ at 300 C for 3 h under 1 × 10 Pa. The
t-plot method was used to calculate the external surface
area and micropore volume. The surface morphology
was observed by the scanning electronic microscopy
(SEM, JSM 7001-F, JEOL, Japan) at room temperature.
The elemental analysis of the Si/Al ratios was con-
firmed by inductively coupled plasma atomic emission
spectroscopy (ICP-AES, Autoscan16, TJA) using an
19,20
the field of CO
2
conversion.
For example, lately,
FeHZSM-5 was reported as photo catalyst which could
2+
−
generate active species [Fe –O ]* and further promote
19
2
the reduction of CO to CO. Additionally, core-shell
catalyst Fe-Zn-Zr@HZSM-5–Hbeta developed by Tan
et al. was applied for the synthesis of isoalkanes via
20a
1
CO
2
reduction, in which, the coordination effect be-
Optima 4300 DV instrument (Perkin-Elmer). H NMR
tween metal (core) and HZSM-5 (shell) contributes to
the highly effective catalytic activity with nice selectiv-
ity of isoalkanes (iC4 + iC5+, 81.3%). To the best of our
knowledge, the use of HZSM-5 or MHZSM-5 as heter-
spectra was recorded on 400 MHz spectrometers using
CDCl
3
as solvent referenced to CDCl
3
(7.26 ppm). 13
C
NMR was recorded at 100.6 MHz in CDCl
3
(77.00
ppm). Multiplets were assigned as singlet, doublet, tri-
plet, doublet of doublet, multiplet and broad singlet.
2
ogeneous multifunctional catalyst in CO conversion for
the construction of valuable chemicals is rare and espe-
cially catalytic synthesis of cyclic carbonates from
Materials
2
epoxides and CO has not yet been reported.
Unless otherwise noted, carbon dioxide (99.99%),
commercially available epoxides. All starting materials
were obtained from TCI, Aladdin or Alfa Aesar and
used as received. All reactions were carried out without
any special precautions against air.
In this context, we hypothesized that ZnHZSM-5, as
a heterogeneous multifunctional catalyst bearing with
Brønsted acid (proton site), Lewis acid (non-framework
Al and Zn sites, Zn species is commonly modified with
low-cost and low-toxicity) and Lewis base, could effec-
tively activate the epoxide and CO
2
. Meanwhile, CO
2
is
General procedure for the synthesis of ZSM-5
concentrated at the surface by the unique pore frame-
work, and thus leading to simultaneous generation of
cyclic carbonates under mild conditions (Scheme 1). As
ZSM-5 precursor. MFI zeolite samples were hydro-
thermally synthesized using a stainless-steel autoclave
at 140 °C reference to the literatures.22 The synthesis
process was performed with sodium silicate as the silica
2
part of promising studies on CO activation and utiliza-
21
tion, we herein present an unprecedented heterogene-
ous multifunctional ZnHZSM-5 catalyst with multi-sites
activation to catalyze the cycloaddition of a variety of
2 4 3 2
source, and aluminum sulfate [Al (SO ) ·18H O] as an
alumina source. The Si/Al ratios of the starting mixtures
were fixed at 50 in the case. The zeolite products were
filtered, washed with deionized water, and dried at
2
epoxides and CO . As a result, an effective, easily
available and inexpensive multifunctional catalytic ma-
terial for the synthesis of cyclic carbonates is developed.
1
20 °C for 12 h. The zeolites were then calcined in stat-
ic air for 5 h at 550 °C for the removal of organic struc-
ture-directing agents. Subsequently, the zeolites were
Scheme 1 Multifunctional material catalysis/activation of CO
2
+
ion-exchanged into the NH
4
form three times in total
NO . NH -ZSM-5
was calcined at 550 C in static air for 5 h (temperature
and epoxide in this work.
using a 1 M aqueous solution of NH
4
3
4
o
o
-1
ramp 5 C min ) in order to obtain H-ZSM-5. The de-
tailed procedure is listed as follows:
ZSM-5 zeolite was synthesized with the molar com-
position of SiO
BuNH :0.60 H
2 2
2
:4.50 Al
O:68, from the mixture of sodium hy-
droxide (NaOH, 12 g, 0.30 mol), sodium silicate (Si:
4.17 wt%, Na: 7.52 wt%, 550.00 g, 4.50 mol), alumi-
num sulfate [Al (SO ·18H O, 15.2 g, 0.045 mol),
n-butylamine ( BuNH , 56.0 mL, 0.60 mol), and 1.20 L
O. The mixture was conducted at 120 C for 72 h in a
2 3 2
O :0.045 Na O:4.65
n
1
2
4
)
3
2
n
2
Experimental
o
H
2
General analytic methods
5 L stainless steel autoclave under rotation (260 r/min).
The solid products were recovered by filtration under
vacuum condition and washing with deionized water
X-ray diffraction (XRD) patterns were recorded with
a Rigaku MiniFlex II desktop X-ray diffractometer. Two
repeatedly until the mother liquid showed a pH value of
o
o
o
theta angles ranged from 5 to 80 with a speed of 6
o
7
–8, followed by drying at 110 C overnight and calci-
-
1
min operating with monochromated Cu Kα radiation
154.06 pm, 30 kV, and 15 mA). BET surface area and
pore volume were measured on a Micromeritics ASAP
o
nation at 550 C for 5 h in air. The ZSM-5 zeolite in
hydrogen form (HZSM-5) was obtained through
(
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ion-exchanging with aqueous NH
4
NO
3
solution (1 M,
Representative procedure of cycloaddition reaction
for the synthesis of cyclic carbonates
o
7
50 mL × 3) at 80 C for 5 h and subsequent calcination
o
at 550 C for 5 h.
In a typical reaction, the cycloaddition reaction of CO
2
Zn doped ZSM-5. The zinc doped ZSM-5 catalysts
were prepared using impregnation with water as the
1
2
3
with propylene oxide (R = CH , R = H) was carried
3
23
out in a stainless steel autoclave (50 cm inner volume)
with a stir bar. ZnHZSM-5 (50 mg, zinc content of 2.93
wt%), TPAB (13.3 mg, 0.5 mol%) and propylene oxide
solvent. In order to obtain catalysts with a final zinc
loading of 1 to 16 wt%, the desired amount of zinc pre-
.
cursor Zn(NO
3
)
2
2
6H O was dissolved in water (2 mL)
(0.58 g, 10 mmol) was successively introduced and
and mixed with zeolite HZSM-5 (ca. 2 g) under vigor-
ous stirring. The amount of zeolite was adjusted to
compensate the metal assay for each precursor. The re-
sealed at room temperature. The pressure was adjusted
to 1 MPa at the preset temperature and the autoclave
was heated at that temperature for 8 h. After the reaction
was completed, the reactor was cooled in ice-water bath,
o
sulting slurry was stirred and heated up slowly to 120 C.
o
Each catalyst was dried at 120 C for 12 h, and calcined
o
o
2
and then excess of CO was carefully vented. The mix-
at 550 C for 4 h in static air (temperature ramp 5 C
-
1
ture was diluted with ethyl acetate, and the yield of cy-
clic carbonate was determined by gas chromatograph
(Agilent 6890) equipped with a capillary column (HP-5
min ).
Catalyst characterization
3
0 m × 0.25 μm) using a flame ionization detector using
NH
NH
3
-TPD: Temperature programmed desorption of
(NH -TPD) was performed on an AutoChem
biphenyl (40 mg) as the internal standard. Then, the
residue was obtained by removing the solvent under
vacuum and further purified by column chromatography
with ethyl acetate-petroleum ether as the eluent to ob-
tain the desired product. The products were further
identified by NMR. All cyclic carbonates have been
reported previously and the characterization data are all
3
3
TP-5080 chemisorption analyzer (Tianjin Xianquan In-
strument Co., Ltd.), and carried out in a quartz tubular
reactor. Approximately 100 mg (0.60-0.40 mm sieve
fraction) of the zeolite sample was used in each meas-
urement, which was first pretreated at 300 C for 2 h in
an argon stream (30 ml/min) and then cooled down to
room temperature. Saturated adsorption of NH
then achieved by introducing NH (30 ml/min) into the
tube for 30 min. After that, the physically adsorbed NH
was removed by flushing the sample tube with an argon
flow (30 ml/min) for 2 h. To get the NH -TPD profile,
the zeolite sample was subsequently heated to 700 C at
3
a ramp of 10 C/min; the amount of NH released during
the heating for desorption was measured by a thermal
conductivity detector (TCD).
o
5d
in good agreement with literature value ([2a, 2c-h],
3
was
12e
9
[
2b], [2i] ).
3
4
-Methyl-1,3-dioxolan-2-one (2a) Colorless liquid.
3
1
3
H NMR (CDCl
3
, 400 MHz) δ 1.49 (d, J = 6.0 Hz, 3H),
3
3
4
4
.04 (t, J = 8.4 Hz, 1H), 4.58 (t, J = 8.4 Hz, 1H),
.84-4.92 (m, 1H) ppm. C NMR (CDCl , 100.6 MHz)
3
3
13
o
δ 19.4, 70.7, 73.7, 155.1 ppm.
-Ethyl-1,3-dioxolan-2-one (2b) Colorless liquid.
H NMR (CDCl , 400 MHz) δ 0.98 (t, J = 7.6 Hz, 3H),
.71-1.77 (m, 2H), 4.03-4.07 (1H), 4.50 (t, J = 8.2 Hz,
H), 4.60-4.67 (m, 1H) ppm. 13C NMR (CDCl
, 100.6
o
4
1
3
1
1
CO
ments were performed at 50 C. Temperature pro-
grammed desorption of CO (CO -TPD) was similar to
the NH -TPD. Prior to the adsorption of CO , the cata-
lyst sample (0.20 g, 0.40-0.20 mm sieve fraction) was
heated at 300 C for 2 h by Ar flowing continually. After
2
-TPD: Carbon dioxide chemisorption measure-
3
o
MHz) δ 8.3, 26.7, 68.9, 77.9, 155.1 ppm.
-n-Butyl-1,3-dioxolan-2-one (2c) Colorless liquid.
4
2
2
1
3
H NMR (CDCl
3
, 400 MHz) δ 0.93 (t, J = 7.2 Hz, 3H),
3
2
3
1
.26-1.48 (m, 4H), 1.64-1.87 (m, 2H), 4.08 (dd, J = 7.3
2
3
o
Hz, J = 8.0 Hz, 1H), 4.54 (t, J = 8.0 Hz, 1H),
.68-4.75 (m, 1H) ppm. 13C NMR (CDCl
, 100.6 MHz)
δ 13.7, 22.2, 26.4, 33.5, 69.4, 77.1, 155.1 ppm.
o
4
3
cooling down to 50 C, the catalyst was saturated with a
-
1
CO
2
(30 mL min ) at this temperature for 1 h. After-
1
4
-Phenyl-1,3-dioxolan-2-one (2d) White solid. H
ward, the TPD experiment was started with a heating
rate of 10 C min under Ar flow (30 mL min ), and the
desorbed CO was monitored by a thermal conductivity
3
o
-1
-1
3
NMR (CDCl , 400 MHz) δ 4.35 (t, J = 8.4 Hz, 1H,
3
3
OCH
2
), 4.80 (t, J = 8.4 Hz, 1H, OCH
2
), 5.70 (t, J = 8.0
), 7.44 (d,
) ppm. C NMR (CDCl , 100.6
MHz) δ 66.2, 68.8, 74.1, 114.6, 121.9, 129.6, 154.7,
57.7 ppm.
-Chloromethyl-1,3-dioxolan-2-one (2e) Colorless
2
3
Hz, 1H, OCH), 7.36 (d, J = 7.6 Hz, 2H, C
6 5
H
detector (TCD).
3
13
J = 6.4 Hz, 3H, C
H
6 5
3
Pyridine-IR: Pyridine adsorption Fourier Transform
Infrared IR (Py-FTIR) was recorded on a Bruker Ten-
sor27 FT-IR spectrophotometer. Initially, the zeolite
sample was pressed into a self-supported wafer. Prior to
the measurement, the sample cell was evacuated to
1
4
1
3
3
liquid. H NMR (CDCl , 400 MHz) δ 3.81 (dd, J = 3.2
Hz, J = 12.0 Hz, 1H), 3.82 (dd, J = 5.2 Hz, J = 12.0
2
3
2
-1
o
5
×10 Pa at 120 C for 0.5 h; FT-IR background spectra
3
2
Hz, 1H), 4.43 (dd, J = 6.0 Hz, J = 8.4 Hz, 1H), 4.60 (t,
o
were then recorded at 60 C. To get the FT-IR spectra
for pyridine adsorption, pyridine vapor was introduced
into the cell at 60 C for 5 min; the spectra were then
3
13
J = 8.4 Hz, 1H), 4.99-5.05 (m, 1H) ppm. C NMR
CDCl , 100.6 MHz) δ 44.0, 67.0, 74.4, 154.4 ppm.
-i-Propoxy-1,3-dioxolan-2-one (2f) Colorless
(
3
o
4
o
recorded at 60 C after evacuation (desorption for 15
1
3
liquid. H NMR (CDCl
Hz, 6H), 3.58–3.70 (m, 3H), 4.38 (dd, J = 8.0 Hz, J =
1
3
, 400 MHz) δ 1.16 (t, J = 6.4
min under 5×10- Pa) at 120 C for 0.5 h and cooling
1
o
3
2
down.
3
5.6 Hz, 1H), 4.50 (t, J = 8.0 Hz, 1H), 4.82 (m, 1H)
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ppm. 13C NMR (CDCl
7.1, 72.9, 75.3, 155.2 ppm.
-((Allyloxy)methyl)-1,3-dioxolan-2-one (2g) 1H
NMR (CDCl , 400 MHz) δ 3.59-3.71 (2H), 4.05-4.06
2H), 4.38-4.52 (2H), 4.79-4.84 (1H), 5.21-5.30 (2H),
, 100.6 MHz) δ 21.8, 21.9, 66.4,
Furthermore, the acidity of the different zeolite cat-
alysts was characterized by NH -TPD (Fig. 1B). Two
3
6
3
4
desorption peaks can be observed on the profile of pure
zeolite HZSM-5, and the peak located in the range of
3
o
(
100–180 C (denoted as peak I) can be attributed to the
13
5
.82-5.90 (m, 1H) ppm. C NMR (CDCl
δ 66.3, 68.8, 72.6, 75.0, 118.0, 133.6, 154.9 ppm.
-Phenoxymethyl-1,3-dioxolan-2-one (2h) White
3
, 100.6 MHz)
weak acidic sites, while the peak located within the
o
range of 200–450 C (denoted as peak II), belongs to
4
strong acid site. For the ZnHZSM-5 catalysts, peak II
shift respectively to a higher temperature and with a
lower concentration of strong acid sites while the posi-
tion and intensity of I peak have no difference. This re-
sult indicates that, compared with pure zeolite HZSM-5,
the obvious interaction between zinc species and strong
acid sites after the modification. In addition, the above
interaction was also detected through pyridine-IR
method which gives more detailed information in the
surface of ZnHZSM-5. IR profile showed that the area
o
1
3
solid, mp: 99-100 C. H NMR (CDCl , 400 MHz) δ
4
3
3
2
3
.16 (dd, J = 4.4 Hz, J = 10.8 Hz, 1H), 4.24 (dd, J =
2
3
2
.6 Hz, J = 10.8 Hz, 1H), 4.55 (dd, J = 8.4 Hz, J = 6
3
Hz, 1H), 4.62 (t, J = 8.4 Hz, 1H), 5.03 (m, 1H), 6.91 (d,
3
3
3
J = 8.0 Hz, 2H), 7.02 (t, J = 7.4 Hz, 1H), 7.31 (t, J =
13
8
6
3
.0 Hz, 2H) ppm. C NMR (CDCl , 100.6 MHz) δ 66.3,
6.9, 74.1, 114.7, 122.1, 129.7, 157.8 ppm.
Hexahydrobenzo[1,3]dioxol-2-one (2i) Slight
1
yellow liquid. H NMR (CDCl
3
, 400 MHz) δ 4.70 (s,
-1
2
1
H), 1.91 (d, J = 5.4 Hz, 4H), 1.69-1.52 (m, 2H),
of various peaks, namely weak acid (1545 cm ), mod-
13
-1
-1
.51-1.34 (m, 2H) ppm. C NMR (CDCl
3
, 100.6 MHz)
erate acid (1490 cm ) and strong acid (1450 cm ), be-
comes smaller with the increase of zinc content, sug-
gesting an adjustable acid control both the strength and
amount of the total acid.
δ 155.4, 75.8, 26.8, 19.2 ppm.
Catalyst recycling studies
After the reaction, the product was collected. Then, the
precipitated catalyst was filtrated and washed with eth-
o
anol, and further dried at 120 C for 2 h. The catalytic
cycle was started with the same amount of epoxide as
described above with replenishment of TPAB. The XRD
and SEM analysis was taken for the recovered catalyst
and compared with the fresh sample. In the kilogram
scale experiment, after separating the product by distil-
lation under vacuum, the residues could be directly used
as catalyst in the next run
Results and discussion
To begin with, we prepared the HZSM-5 and
ZnHZSM-5 with different zinc content (For detailed
experimental data and characterization data, see the
Supporting Information), and further studied their
properties.
N
2
adsorption analysis revealed that
HZSM-5 has a high BET surface area (429 m²/g) with
average pore width 0.58 nm. In addition, the surface
area of ZnHZSM-5 decreases with increase of zinc
content in ZnHZSM-5 due to the overlay of zinc species
in the surface of zeolite (Table S1 in Supporting
Information).
All ZnHZSM-5 zeolites prepared by the method of
impregnation exhibit similar characteristic diffraction
peaks of MFI structure (Fig. 1A, a-e vs f), suggesting
that the introduction of zinc species has little influence
on the framework of the parent HZSM-5. Under the low
content of zinc (<5 wt%), only very weak diffraction
o
o
o
o
o
o
peaks for ZnO at 31.7 , 34.4 , 36.2 , 47.3 , 56.5 , 62.7
o
and 67.7 can be discriminated from the XRD patterns
24
of ZnHZSM-5, suggesting that the zinc species are
highly dispersed in the parent HZSM-5. Whereas, it is
difficult to disperse ZnO completely in monolayer on
HZSM-5 surface through the high concentration modi-
fication of HZSM-5 as illuminated from the distinct
diffraction peaks (Fig. 1A, d and e).
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Figure 1. Characterization of pure HZSM-5 and zinc-modified
zeolites A) XRD patterns; B) NH -TPD profiles; C) Pyridine-IR
profiles: a) HZSM-5; b) wt% ZnHZSM-5; c) wt%
zinc modification for catalyst preparation has conspic-
uous effect on the strength of the basic site.
In this work, the TG-MS method was also employed to
3
1
3
ZnHZSM-5; d) 4 wt% ZnHZSM-5; e) 8 wt% ZnHZSM-5; f) 16
wt% ZnHZSM-5.
monitor the desorption process of CO
be seen that the rate of desorption of CO
the increment of the temperature from 80 to 160 C in
TG diagram. Thus, the decreased CO became sluggish
with further increase in temperature due to less amount
of CO desorbed in the strong basic site (total absorbed
CO is about 45 mgCO2/gcat in TG-MS compared with 35
mgCO2/gcat in CO -TPD, and the difference is due to the
trace water absorbed).
2
in Fig. 3B. It can
2
was fast with
o
2
The SEM images of HZSM-5 and ZnHZSM-5 zeo-
lites shown in Fig. 2 suggest that the introduction of
zinc species by impregnation method also has little in-
fluence on the crystal sizes and the morphology of the
parent HZSM-5 zeolite, which has a crystal size around
2
2
2
3
00 nm aggregated from small crystals of around 100
nm. On the other hand, the SEM picture of ZnHZSM-5
zeolite reveals the presence of high zinc content with a
slightly of crystal aggregation (Fig. 2e and 2f, ZnO par-
ticles around 30 nm), consistent with the XRD results.
a
c
e
b
d
f
Figure 3. A) CO
ZnHZSM-5 (up, 3 wt%) catalysts; B) TG profile of the
CO -desorbed ZnHZSM-5 (3 wt%).
2
-TPD profiles of the HZSM-5 (down) and
2
After the detailed research of ZSM-5 zeolite, initially,
the cycloaddition of propylene oxide (PO) as the model
reaction with various catalysts was investigated as de-
picted in Table 1. No product was detected when only
NaZSM-5 or HZSM-5 was used (entries 1 and 2). On
the other hand, tetra-n-propylammonium bromide
(TPAB) promoted the cycloaddition to afford propylene
carbonate (PC) in a low yield (entry 3). Interestingly,
combination of NaZSM-5 or HZSM-5 with TPAB as a
catalyst rendered PC yield remarkably up to 41% and
Figure 2. SEM pictures of the different ZSM-5 catalysts: (a, b)
HZSM-5, (c, d) 3 wt% ZnHZSM-5, and (e, f) 16 wt%
ZnHZSM-5.
2
Furthermore, before the study on CO conversion via
zeolite catalysis, we explored the activation mode
through functional ZnHZSM-5 catalyst as shown in Fig.
7
0%, respectively (entries 4 and 5). On the one hand,
the results of the comparison experiments indicated that
the concentration and activation of CO by zeolite con-
3
. Two CO
α) and high temperature (peak β) can be observed for
the functional ZnHZSM-5, corresponding to CO de-
sorbed from a weak basic site and a strong basic site
total: 35 mgCO2/gZnHZSM-5), respectively (Fig. 3A). What
2
desorption peaks at low temperature (peak
2
tributes to the effective promotion of the transfor-
mations. On the other hand, it is one of the most effec-
2
tive methods for CO
2
conversion through the activation
(
strategy of the epoxide using Brønsted acid (hydrogen
25
is more, the temperature and absorption amount of α
peak for the two catalysts (HZSM-5 and ZnHZSM-5)
bond donating group activation mode ), and TPAB
generally acted as nucleophilic reagent in the synergistic
catalytic process.
has noticeable change (the peak position shifts from 120
o
to 210 C and the amount of desorbed CO
2
increases
In addition, tetra-n-butylammonium bromide and
significantly from 0.7 to 1.9 (a.u.)), indicating that the
This article is protected by copyright. All rights reserved.

2
ZnBr were also screened (entries 6-8), and moderate or
even lower yield was obtained which also revealed the
importance of the coordination between nucleophile and
cation. Whereas, an increase of PC yield was observed
when zinc-modified HZSM-5 (low metal loaded
ZnHZSM-5 selected without obvious aggregation) was
used (entries 9-11), suggesting multifunctional synergis-
What is more, the results from ZnNaZSM-5 and
ZnO catalysis further confirmed the promotion effect of
zinc species (entries 9 and 10). Totally, the order of cat-
alytic performance is ZnHZSM-5 > HZSM-5 >
ZnNaZSM-5> NaZSM-5. Moreover, other metals such
as Fe(III), Co(II), and Ni (II) modified ZSM-5 were also
screened but low yields were obtained (35%, 53% and
33%, respectively, Table S3, entries 1-3). Additionally,
with the further increase of zinc loading (> 3 wt%), the
yield was unchanged under the same reaction conditions
which was in accordance with the pyridine-IR profile
and also supported the above synergistic argument (For
detailed experimental data, see the Supporting Infor-
mation, Scheme S2). Moreover, with the increase of the
TPAB loading to 1 mol%, almost quantitative yield of
PC was acquired under the identical conditions (entry
2
tic activation on the epoxide and CO through hydrogen
+
bonding (-OH, ZnOH ) and Lewis acid/base and thus
further enhances catalytic efficiencies.
Table 1 Effect of reaction parameters on the cycloaddition of CO
and propylene oxidea
2
1
4 vs 10). The results disclosed that the activity of the
Catal.
Zn wt%)
T
P
Yield
(%)b
Entry
Co-catal.
binary system originates from a synergistic effect be-
tween ZnHZSM-5 and TPAB.
(
(°C) (MPa)
1
2
3
4
5
6
7
8
9
NaZSM-5
HZSM-5
-
-
-
120
120
120
120
120
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
As revealed in the previous study, high pressure and
temperature could accelerate the cycloaddition reaction,
and the theory also applies to this system (entries 10 and
0
1
5-19, and Table S3 in Supporting Information). Be-
nPr
NBr
NBr
NBr
24
41
70
57
20
2
4
4
4
sides, the adsorption of CO
2
in the zeolite increases with
16a
nPr
the rise of gas phase pressure which further improve
the reaction rate in the surface of catalyst. Much im-
portantly, considering the reaction temperature used
NaZSM-5
HZSM-5
HZSM-5
HZSM-5
-
nPr
o
o
2
here (140 C or 160 C), the CO absorption-desorption
nBu
NBr 120
4
process in the weak basic cites (Fig. 3A, peak α) seems
to be rather related to the balance of physical and
chemical absorption, and the later contributes to the ac-
ZnBr
2
120
120
120
120
120
120
120
120
tivation of CO
meanwhile verified by CO
2 2
. This means that the activated CO
ZnBr
2
o
2
-TPD and TG-MS: < 200 C)
ZnHZSM-5(1) nPr
ZnHZSM-5(3) nPr
ZnHZSM-5(4) nPr
74
85
84
55
27
4
NBr
4
NBr
4
NBr
4
NBr
4
NBr
4
NBr
4
NBr
4
NBr
4
NBr
4
NBr
4
NBr
is reasonably beneficial to the effective transformation
under mild conditions. Notably, the recovered
ZnHZSM-5 catalyst also performed the same catalytic
activity in the second run under the same conditions
1
0
11
(
entry 18).
To further investigate the wide applicability of the
newly developed binary-catalyst system
nPr
1
1
1
1
1
1
1
2
ZnNaZSM-5
ZnO
3c
4d
5
nPr
ZnHZSM-5/TPAB, a series of terminal and internal
epoxide substrates were examined. As listed in Table 2,
a variety of terminal epoxides with either aliphatic or
aromatic group could be smoothly converted to the cor-
responding cyclic carbonates with high yield and excel-
lent selectivity (entries 1–4). Delightedly, representative
terminal epoxide substrates bearing with different func-
tionalized group were also demonstrated, and corre-
sponding products were also afforded in good yields
ZnHZSM-5(3) nPr
>99
72
ZnHZSM-5(3) nPr
120 0.5
ZnHZSM-5(3) nPr
6
120
80
2
1
1
1
97
7
ZnHZSM-5(3) nPr
ZnHZSM-5(3) nPr
ZnHZSM-5(3) nPr
47
8
140
160
>99(>99)e
99
(
entries 5–8). But, the internal epoxides such as cyclo-
hexene oxide exhibited a little lower reactivity probably
as a result of the steric effect, and meanwhile the effi-
ciency is greatly improved through further raising the
reaction temperature (entry 9).
1
9
a
Reaction condition: propylene oxide 1a (0.58 g, 10 mmol), zeo-
b
lite catalyst (50 mg), co-catalyst (0.5 mol%), 8 h. Determined by
GC using biphenyl as the internal standard. ZnO (1.9 mg, on
basis of Zn content in 3 wt% ZnHZSM-5). dnPr
mol%). Recovered ZnHZSM-5 catalyst (3 wt%) for the second
run.
Potential application was tested in a scale-up ex-
periment (Table 3). As seen from the results, excellent
yield could be still achieved on hundred gram or kilo-
gram scale even using a lower catalytic loading (Table 3,
c
4
NBr (26.6 mg, 1
e
-1
Entry 2: TON = 8,476, TOF: 848 h ; Entry 3: TON =
This article is protected by copyright. All rights reserved.

-
1
2
1,400, TOF: 892 h ). Thus, the present protocol could
be potentially applied in the large-scale chemical fixa-
tion of CO to cyclic carbonates under mild reaction
peated reactions show that no significant loss of cata-
lytic activity was proved at least 3 times, while the ex-
cellent selectivity of PC is nearly unchanged (Figure
S3).
2
conditions through an inexpensive heterogeneous cata-
lytic method.
Table 3 Scale-up experimentsa
Table 2 Substrate scopea
ZnHZSM-5 nPr
4NBr
1
a
T
2a
/gc
Yield
/%
g
2a/(gcat
h)d
Entry
/mol%b
g)
.02
/mol%b
Entry
Substrate 1
Product 2
Yield (%)b
99 (97)d
/g
/h
(
(g)
0
0.076
(0.36)
0.038
(0.36)
0.015
(0.72)
1
2
3
103
212
1032
12
10
166
323
92
89
90
11.8
32.3
29.1
c
1
(0.81)
0.01
1
a
2
a
(
0.81)
.004
(1.62)
0
24 1634
2
3
4
5
6
7
8
9
87
>99 (98)d
>99 (98)d
91
1
b
2
b
aReaction condition: ZnHZSM-5 (3 wt%), initial 140 °C, 3 MPa,
rotation rate 300 r/min, reactor volume of entries 1-3 is 0.5, 2 and
b
5
c
litres, correspondingly. Zn mole content on the basis of 1a.
c
d
1
c
Isolated yield by vacuum distillation. mcat (g) = m(ZnHZSM-5)
2c
n
+
m( Pr
4
NBr). TON = moles of 2a/moles of Zn content; TOF =
TON/time.
c
1
d
2
d
2
Based on previous mechanistic investigation and
experimental results in this work, a tentative mechanism
for the ZnHZSM-5/TPAB-catalyzed fixation of CO
with epoxides is illustrated in Scheme 2. Initially, the
functional sites in ZnHZSM-5 interact with the epoxides
2
1
e
e
2
+
3+
through multiple Lewis acid (Zn , Al ) and Brønsted
+
acid (ZnOH ), and epoxides were activated. Notably,
98
the activated epoxide concurrently undertakes
ring-opening step upon nucleophilic attack by the bro-
mide anion from the less-steric-hindrance carbon atom
a
1
f
2
f
(
2
step 1). Meanwhile, the CO molecule was bound by
89(92)d
>99c
the zeolite pore and basic sites, and concentrated in the
surface and further activated. Subsequently, the active
site-stabilized alkoxide intermediate A formed. Then,
nucleophilic attack of the intermediate A at the zeo-
1
g
2
g
lite-activated CO
2
generates the alkyl carbonate anion B
1
h
2
h
(
step 2). Finally, cyclic carbonate is produced via the
intramolecular cyclization with regeneration of the cat-
alytic species (step 3). Notably, the release way of Br
from intermediate B is same, but the rate is different due
-
34(82)d
1
i
2
i
to the distinct electron environment of activation center.
aReaction conditions: epoxides 1 (10 mmol), ZnHZSM-5 (50 mg,
3
nPr
wt%, 0.22 mol% Zn content on the basis of substrate 1),
b
4
NBr (13.3 mg, 0.5 mol%), 140 °C, 1 MPa, 8 h. Isolated yield.
cDetermined by GC using biphenyl as the internal standard.
d160 °C, 1 MPa, 4 h.
In addition, after the reaction, the ZnHZSM-5 cata-
lyst was recovered and analysed through XRD diffrac-
tion, SEM measurements and elemental analysis. The
same characteristic diffraction peaks of MFI structure
and unchanged crystal sizes and the morphology of used
ZnHZSM-5 is completely coincident with the fresh cat-
alyst, thus indicating the good stability of the zeolite
catalyst (Supporting Information, Figure S1 and S2).
ICP detection revealed a slight loss of zinc content (en-
tries 7 and 8, Table S1). Moreover, the results of re-
This article is protected by copyright. All rights reserved.

Scheme 2 Plausible reaction mechanism.
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We are grateful to the National Natural Science
Foundation of China (21602232), the Natural Science
Foundation for Youths of Shanxi (201701D221057), and
Project of “Utilization of Low Rank Coal” Strategic
Leading Special Fund, Chinese Academy of Sciences
,
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2
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(
XDA-07070800, XDA-07070400). Qing-Ning Zhao
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thanks the Joint Training Project of Shanghai University
and Institute of Coal Chemistry, Chinese Academy of
Sciences.
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Entry for the Table of Contents
Page No.
2
Catalytic conversion of CO access to cy-
clic carbonates through multifunctional
zinc-modified ZSM-5 zeolite
Qing-Ning Zhao, Qing-Wen Song,* Ping
Liu, Qian-Xia Zhang, Jun-Hua Gao, Kan
Zhang*
2
The production of cyclic carbonates via CO implant through multifunctional heteroge-
neous zeolite catalysis with excellent efficiency is described.
.
This article is protected by copyright. All rights reserved.