Efficient cycloaddition of epoxides and carbon dioxide over novel organic-inorganic hybrid zeolite catalysts

Article abstract of DOI:10.1039/c4cc07620f

Organic-inorganic hybrid zeolites with the MFI-type lamellar structure serve as efficient solid Lewis base catalysts for solvent-free synthesis of a variety of cyclic carbonates from corresponding epoxides and carbon dioxide. The ion-exchange with iodide, in particular, renders these materials an excellent catalytic activity and good recyclability.

Full text of DOI:10.1039/c4cc07620f

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Efficient cycloaddition of epoxides and carbon
dioxide over novel organic–inorganic hybrid
zeolite catalysts†
Cite this:DOI: 10.1039/c4cc07620f
Received 26th September 2014,
Accepted 28th October 2014
Chen-Geng Li, Le Xu, Peng Wu,* Haihong Wu and Mingyuan He*
DOI: 10.1039/c4cc07620f
www.rsc.org/chemcomm
Organic–inorganic hybrid zeolites with the MFI-type lamellar structure against leaching in an organic solution and rigid enough
serve as efficient solid Lewis base catalysts for solvent-free synthesis of against degradation under severe reaction conditions of high
a variety of cyclic carbonates from corresponding epoxides and carbon temperature and pressure. It is also important that these
dioxide. The ion-exchange with iodide, in particular, renders these active sites located inside zeolite pores should be reachable
materials an excellent catalytic activity and good recyclability.
by guest molecules. Recently, Ryoo et al. developed a series of
Gemini-type surfactants that could lead to mesoporous MFI
Organic structure-directing agents (SDAs) are widely used in the zeolites composed of ordered microporous nanosheets.11
synthesis of microporous zeolites, directing the formation of With one quaternary ammonium inside 10-membered ring
various crystalline frameworks with unique porosities. Zeolites (MR) channels of MFI zeolite sheets, the long alkyl groups and
usually suffer difficulties in catalyzing reactions involving bulky branch groups of SDAs are then firmly immobilized against
molecules. Such reactions cause severe diffusion problems leaching. The other ammonium cations located in interlayer
when taking place inside micropores, limiting the application mesostructured spaces are thus accessible from outside. In
of zeolites.1–3 One of the alternative approaches to solve this fact, we demonstrated that the organic–inorganic lamellar
problem is to design new SDAs that are capable of leading to ZSM-5 composite exhibited bifunctionalities as solid acid–
new zeolite structures containing nanometer-scaled meso- base catalysts.12
pores, if possible.4–6
The cycloaddition of carbon dioxide and epoxides is an
Despite high manufacturing costs, SDAs are always elimi- important reaction for synthesizing value added cyclic carbon-
nated by calcination after completion of crystallization, render- ates, widely used in the polymer industry, pharmaceuticals and
ing the zeolite pores open and to have a large surface area. The biomedical fine chemical synthesis,18 and it is expected as an
calcination may force the framework of zeolite precursors to effective way for reducing carbon dioxide emission as well.13
destruct in some cases.7–8 Davis et al. first proposed a protocol The activation of CQO bonds is an essential issue in utilizing
for reusing the SDA molecules occluded in as-synthesized ZSM- carbon dioxide via cycloaddition. By properly designing the
5 zeolites through unique processes of degradation and reas- catalysts such as metallic complexes,19–21 phosphines,22,23
sembling of SDAs.9 In this way, the calcination is avoided and organic bases,24 ionic liquids,25,26 metal oxides,27–29 mild con-
the fragile zeolite structures might be well preserved.
ditions have been optimized for this reaction. Here, we com-
Rather than extracting the SDAs out of zeolites, it is more municate that an organic SDA embedded layered MFI zeolite
economical and environmentally friendly if SDAs can be used cooperated synergistically with the post incorporated halogen
directly as active sites for catalyses. Little attention has been anions in the cycloaddition reactions (Scheme 1), producing
paid on utilizing the SDAs directly.10 To serve as useful cata- carbonates actively and selectively in a heterogeneous way.
lysts, the SDA-containing zeolite composite materials should
The XRD patterns of lamellar MFI zeolites (LMFI-AS)
À
satisfy the preconditions that these organic species are stable as-synthesized with [C18H37Me2N+(CH2)6N+Pr3]Br2 (C18-6-3
)
showed a typical layered structure that was different from
conventional three-dimensional MFI (Fig. 1 and ESI,† Fig. S1).
An oriented crystal growth made missing the corresponding
reflections related to the b-axis. The lamellar material was also
featured by a diffraction in the low-angle region (2y = 1.41),
which was attributed to the mesostructure consisting of zeolite
nanosheets and occluded organic micelle layers. The SEM
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department
of Chemistry, East China Normal University, North Zhongshan Road 3663,
Shanghai 200062, China. E-mail: pwu@chem.ecnu.edu.cn, hemingyuan@126.com;
Fax: +86-21 62232292
† Electronic supplementary information (ESI) available: Details of experimental
procedures, material characterization and catalyst reusability and stability. See
DOI: 10.1039/c4cc07620f
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Fig. 2 N2 adsorption/desorption isotherms (A) and pore diameter distri-
bution (B) of LMFI-AS (a) and LMFI-I (b).
Scheme 1 Cycloaddition of epoxides and CO2 over halogen anion-
exchanged organic–inorganic hybrid zeolite catalysts.
treatment in comparison to [C22H45Me2N+(CH2)6N+Me3]Br2
À
(C22-6-3), which was less stable against acid washing (ESI,†
Fig. S3).
The compositions of C, H, N and I of lamellar zeolites were
quantified by elemental analysis or ion chromatography (ESI,†
Table S1). The C/N ratios were slightly higher than the theore-
tical value of C18-6-3 compounds, probably as a result of a partial
coke formation in hydrothermal synthesis. The difference in
elemental composition before and after acid treatment was
mainly due to the loss of those surfactant species that did not
serve as SDAs, but simply physisorbed on the crystal surface.
The acid treatment easily swept this part of SDAs off the zeolite
crystals. Nevertheless, by employing a bulky Gemini-type diqua-
Fig. 1 SEM and HRTEM images as well as XRD patterns of LMFI-AS (a) and
LMFI-HI (b) materials.
image showed that the morphology of LMFI-AS was of flower- ternary ammonium surfactant containing propyl groups, stable
like spheres composed of primary palette crystals (Fig. 1a). The hybrid composites were obtained even after acid washing.
high resolution TEM image further confirmed the presence of a
The N2 adsorption isotherms showed that the acid treatment
uniform two-dimensional lamellar structure with the quatern- changed the adsorption capacity of lamellar MFI zeolites
ary ammonium head groups buried within zeolite channels (Fig. 2).
while the long alkyl chain groups pillaring the sheets. A
The LMFI-AS material adsorbed a limited amount of nitro-
repeated acid treatment with a 1 M HI/EtOH solution made gen molecules and exhibited almost no micropore distribution,
the mesostructure almost intact, as the XRD reflection at small because the SDA molecules filled up both intralayer micropores
angles remained at the same position (ESI,† Fig. S1c), and a and most of interlayer mesopores. After extracting those remo-
well-preserved lamellar structure was still visible in the TEM vable SDA species, a part of mesopores became open, and the
image (Fig. 1a and b). The anchoring of the diammonium specific surface area increased from 80 m2 gÀ1 for LMFI-AS to
groups inside zeolite channels firmly prevents the SDA from 239 m2 gÀ1 for LMFI-I (ESI,† Table S1), making interlayer spaces
leaching by acid washing. Thus, the organic–inorganic hybrid more accessible to guest molecules. However, the adsorption
zeolites were relatively stable in chemical composition, which capacity of micropores did not change so much. This could also
rendered them possible applications as catalysts.
be a proof that the SDAs were firmly embedded inside zeolite
The quantity of organic species was measured by thermal channels and not removable by acid washing. Both LMFI-AS
gravimetric analysis (ESI,† Fig. S2). The massive loss before and LMFI-I showed a narrow mesopore distribution at 3.7 nm,
100 1C corresponded to water physically adsorbed on the which was approximately the length of the alkyl tail in SDAs. It
material and that over 500 1C was because of the condensation is noteworthy that the interlayer distance was not a fixed value
of silanol groups. The materials showed the DTG peaks in the due to the flexibility of linear organic SDAs.
region of 200–500 1C, which belonged to the SDA species inside
Table 1 shows the catalytic performance of lamellar MFI
channels. Those physically adsorbed on the external surface zeolites in the cycloaddition of carbon dioxide and different
were decomposed in a lower temperature range of 200–250 1C. epoxides under solvent-free conditions. No carbonate products
The weight loss of LMFI-AS was 28.1%. The acid treatment were obtained when the reactions were carried out in the
effectively swept the SDAs physisorbed or loosely occluded on absence of catalysts for all the epoxides investigated at 413 K
the external surface, resulting in a weight loss of 14.4% for and 2.0 MPa CO2 pressure for 4 hours, indicating that non-
LMFI-I. These remaining SDA organic species are presumed to catalytic activation of carbon dioxide was extremely difficult
be stuck firmly inside zeolite channels. The stability of C18-6-3 even under severe conditions. With the aid of LMFI-I catalysts,
containing propyl groups was confirmed by a multi-step acid the epoxides were converted significantly, most of which
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Table 1 Cycloaddition of different epoxides with CO2 over LMFI-Ia
catalysts in the cycloaddition reaction. Table 2 compares the
performance among various catalysts. Without the presence of
catalysts, the epoxide was slightly consumed, and it was pre-
dominately hydrolysed to corresponding diols (No. 1). The S-1-I
catalyst, prepared from conventional silicalite-1 as-synthesized
with TPAOH and HI/EtOH washing, showed an ECH conversion
of 44.6% and a carbonate selectivity of 73.6% (No. 2). The
relatively high selectivity toward carbonates on S-1-I also
demonstrated that the iodide effectively catalysed the reaction.
However, its performance was obviously inferior to that of LMFI
materials (No. 3–6). Possessing a three-dimensional MFI topo-
logy, the S-1-I material containing the TPA species inside
Entry Substrate
Product
Conv. (%) Sel. (%) TON
1
90.1
73.8
95.0
96.0
589
483
2
3
4
99.5
99.4
29.5
97.6
95.7
652
650
193
channels possessed a low surface area of 80 m2
g
À1. The
cycloaddition took place only on TPA+IÀ located at the pore
entrance and on the external surface of zeolite crystals. As the
conversion of epoxides to carbonates was low, the possibility of
hydrolysis of epoxide molecule diols became obvious, which
lowered the carbonate selectivity. Thus, the lamellar zeolite
composites had the advantages by containing simultaneously
mesopores and quaternary ammonium species.
5
100
a
Reaction conditions: LMFI-I, 100 mg; substrate amount, 10 mmol;
CO2 pressure, 2.0 MPa; temp., 413 K; time, 4 h; no solvents.
Among the LMFI materials ion-exchanged with various
halogen anions (IÀ, ClÀ and BrÀ), the iodides proved to be
the most efficient catalytic sites (Table 2, No. 4). The higher
activity of iodides is attributed to their larger anion radius,
which weakens the bonding of the valence electron with the
nucleus. As a result, iodide easily attacks the a-C on epoxides as
in the first step of the catalytic circulation in comparison to
chloride or bromide (ESI,† Scheme S1). After the organic SDA
species were eliminated completely by calcination, the LMFI-I-cal
catalyst showed no activity to carbonate formation even though it
had the highest surface area (No. 7). C18-6-3I2 as a homogeneous
catalyst was also highly active to the reaction (No. 8), verifying
that the organic ammonium iodide in LMFI-I was the reactive
active site.
showed a conversion of over 90% and a carbonate selectivity of
over 95%. The byproducts were only diols formed from ring-
opening of epoxides. The high carbonate selectivity suggested
that the pairs of quaternary ammonium cations and halide
anions were efficient catalytic active sites for cycloaddition.
Styrene oxide showed a lower conversion likely because its
bulky molecular size proposed a steric hindrance when
approaching the active sites. Since the interlayer mesopores
were partly blocked by the alkyl tail of SDAs, the diffusion
limitation then retarded the reaction to a great extent. The
specific catalytic activity or the turnover number (TON) was
calculated by referring the amount of converted ECH to that of
IÀ active sites used. LMFI-I gave a TON in the range of 193–652,
indicating a high catalytic efficiency.
The influence of temperature on the cycloaddition of ECH
and CO2 over LMFI-I was investigated (ESI,† Fig. S4A). The ECH
conversion increased progressively with temperature and
reached over 90% at 413 K. The activation of CQO bonds in
CO2 required high temperatures. On the other hand, the cyclo-
addition of CO2 and epoxides is an exothermic reaction,14 which
is inhibited at high temperature. Balancing these factors, 413 K
was chosen to be the optimal temperature for cycloaddition.
A gas–liquid phase reaction often happens in extreme
circumstances since the gas needs to dissolve into a liquid
mixture at a high pressure. In solvent-free synthesis of carbon-
ates, carbon dioxide played a crucial role since it acted as both
the reactant and the solvent. In the pressure range from 0.5 to
2.0 MPa, both the ECH conversion and carbonate selectivity
increased with increasing pressure (ESI,† Fig. S4B). At low
pressures, few CO2 was dissolved in ECH liquid, limiting the
reaction equilibrium. A part of epoxide was hydrolysed to diol
side reaction. The diol formed may also then react with carbon
Herein, we employed epichlorohydrin (ECH) as the substrate
to systematically investigate the catalytic properties of LMFI
Table 2 Cycloaddition of carbon dioxide to ECH over different catalystsa
X contentb
(%)
SBET
c
Conv.
(%)
Sel.
(%)
No.
Cat.
(m2 gÀ1
)
1
2
3
4
5
6
7
8
No cat.
S-1-I
LMFI-AS
LMFI-I
LMFI-Cl
LMFI-Br
18.2
44.6
62.7
90.0
49.4
71.0
37.0
99.4
0
0
—
—
1.94
1.92
0.49
—
—
—
d
73.6
88.8
95.0
87.0
88.1
0
2
d
80
239
118
214
837
—
d
LMFI-I-cal
e
C18-6-3 2
I
97.0
a
Reaction conditions: cat., 100 mg; ECH, 10 mmol; CO2 pressure, dioxide to give the same carbonate product, but the two-step
b
2.0 MPa; temp., 423 K; time, 4 h; no solvents. X = Cl, Br or I,
reactions were less efficient than direct cycloaddition. Thus, a
c
quantified by ion chromatography. Measured by N2 adsorption at
d
e
low CO2 pressure will limit the reactivity of both epichloro-
hydrin and diols, resulting in low ECH conversion and
77 K. Not determined. 10 mg of C18-6-3 2
I was used, with the same
iodide content in 100 mg LMFI-I.
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carbonate selectivity. Nevertheless, the reaction carried out at zeolites with a lamellar mesostructure. The ammonium cations
too high CO2 pressures would cause negative effects. It was of SDAs and the iodide anions cooperate synergistically for the
once reported that a ring-like intermediate will be produced at selective production of carbonates in the absence of solvents and
high CO2 pressure.15 Thus, we choose 2.0 MPa as an optimal co-catalysts. With a good stability and reusability, this novel
pressure for the LMFI-I catalyst in the present study.
The reaction time contributed more to the yield of carbonates tion of cyclic carbonates in an environmentally friendly way.
than to the conversion of epoxides in the cycloaddition over the The authors gratefully acknowledge the financial support
material is a promising heterogeneous catalyst for the produc-
LMFI-I catalyst. At 2 h, the ECH conversion already reached 90% from the NSFC of China (20973064, 20925310, U1162102), MOST
with a carbonate yield of 78% (ESI,† Fig. S4C). Further prolonging (2012BAE05B02), STCSM (12JC1403600), SMEC (13zz038) and
the reaction time to 4 h, the conversion increased little while the the SLAD Project (B409).
carbonate yield progressively increased to 85%. This could be due
to two-step reaction pathways, which also produced the carbonate,
that is, partial hydrolysis of ECH to diols by the trace amount of
Notes and references
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there would be a delay in comparison to direct cycloaddition of
epoxides and CO2. This explains the rise in the carbonate yield
after three hours of reaction.
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