Coating with mesoporous silica remarkably enhances the stability of the highly active yet fragile flower-like MgO catalyst for dimethyl carbonate synthesis

Article abstract of DOI:10.1039/c3cc42504e

Flower-like MgO is a highly effective catalyst for the synthesis of dimethyl carbonate through the transesterification method, and coating the catalyst with mesoporous silica significantly enhances the stability of the MgO catalyst.

Full text of DOI:10.1039/c3cc42504e

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Coating with mesoporous silica remarkably enhances
the stability of the highly active yet fragile flower-like
MgO catalyst for dimethyl carbonate synthesis†
Cite this: DOI: 10.1039/c3cc42504e
Received 7th April 2013,
Accepted 21st May 2013
Zhi-Min Cui,a Zhe Chen,b Chang-Yan Cao,b Wei-Guo Song*b and Lei Jiang*ab
DOI: 10.1039/c3cc42504e
www.rsc.org/chemcomm
Flower-like MgO is a highly effective catalyst for the synthesis of to be broken into pieces during stirring in the reaction flask.10
dimethyl carbonate through the transesterification method, and Thus it is vital to find a way to produce hierarchical structured
coating the catalyst with mesoporous silica significantly enhances catalysts with high specific surface area and strong mechanical
the stability of the MgO catalyst.
stability. Designing nanoarchitectures from the appropriate
nanoscale building blocks, including mesoporous materials,
Enhancing the structural and catalytic stability of an active is a promising approach.2,13,14
catalyst has always been at the heart of heterogeneous catalysis.
In this study, we developed a core–shell composite catalyst
This issue is especially important for nanostructured catalysts.1–3 that is composed of a flower-like MgO core and mesoporous
Delicacy and robustness are two properties that exclude each SiO2 (meso-SiO2) coating. The MgO@meso-SiO2 composite
other in hierarchical structured nanomaterials, which are catalyst shows remarkable stability over 10 consecutive runs
assembled from various nano-sized building blocks.4 These with conversion of ethylene carbonate (EC) constantly over 80%
hierarchical structured nanomaterials have desirable features and no morphology change, while un-coated flower like MgO
of catalysts, including large specific surface area, more cata- was broken into pieces after only one run.
lytically active sites and facile mass transportation pathways.5–11
The MgO@meso-SiO2 composite catalyst was designed as a
However, these nanostructured catalysts, especially metal oxide base catalyst for the synthesis of dimethyl carbonate (DMC),
based catalysts, lack the mechanical robustness for practical cata- which is a versatile reagent in many applications, such as fuel
lytic reactions. Though the basic building units of these materials, additives, methylating reagents, intermediates for manufacturing
i.e. nanoparticles, are usually crystalline and are resistant to polycarbonates, electrolytes in lithium ion batteries and aprotic
structural damage, the binding forces that keep the nanoparticles polar solvents.15–17 Recently, the direct synthesis of DMC from
together are weak interactions such as van der Waals force, methanol and CO2 has attracted much research interest due to
hydrogen bonds, and electrostatic attraction.12 During liquid its environmentally friendly nature and low cost.18 However,
phase reactions, the physical impact from stirring and colliding the yield of DMC is very low due to thermodynamic restrictions
frequently causes the structural damage of the hierarchical (DH = À16.74 kJ molÀ1).15,19,20
nanostructures, leading to complete breakdown of the overall
morphology.
An indirect but promising route for DMC production from
methanol and CO2 is the so-called transesterification method
For example, hierarchical flower-like MgO is composed of as shown in Scheme 1, in which a cyclic carbonate formed from
nanoparticles that self-assemble into leaf like building units, epoxide and CO2 is an intermediate, which is subsequently
which self-assemble into micron sized flower like spheres. Such transesterified by methanol to form DMC and alkylene glycol.
hierarchical structures retain high activities of the nano- The alkylene glycol can either be a useful reagent for synthesis
particles, and enable easier catalyst handling. But they tend of polyester, or be converted to epoxide again. There is no waste
or harmful byproduct in this route. The lack of a reliable catalyst
for the transesterification step is thwarting the whole process.
a Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of
Ministry of Education, School of Chemistry and Environment, Beihang University,
Beijing 100191, P. R. China
b National Laboratory for Molecular Science, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, P. R. China.
E-mail: wsong@iccas.ac.cn, jianglei@iccas.ac.cn
† Electronic supplementary information (ESI) available: Experimental details of
sample preparation; XRD patterns, SEM and TEM images of the related material.
See DOI: 10.1039/c3cc42504e
Scheme 1 Reaction process for the synthesis of dimethyl carbonate by the
transesterification method.
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Cyclic carbonate can be synthesized with conversion and selec- structure enables the MgO catalyst to have much more active
tivity close to 100% under moderate conditions, while in the sites than bulky MgO particles.
transesterification step DMC yield is only 50% at best.16,21
However, the hierarchical flower-like MgO catalyst is fragile
For the transesterification step, various homogeneous and as the petals are prone to break into pieces during stirring in
heterogeneous acid and base catalysts have been reported.19,22–24 the reaction flask. Fig. 1b and Fig. S4 (ESI†) show the catalyst
Very recently, Corma et al. reported that gold nanoparticles collected after one run. All flower-like MgO are broken into
promote the catalytic activity of CeO2 for the transalkylation of pieces. In the second run using the recycled catalyst, EC
propylene carbonate to DMC and a DMC yield of 35% was conversion dropped sharply to 52% (Fig. 3b), and the catalyst
obtained.21 MgO or the mixture of MgO and CeO2 show promising was damaged severely, making it very difficult to be recycled for
features as active catalysts for the synthesis of DMC, but conver- a third run.
sion of cyclic carbonate is still too low for practical application.19,25
Thus an active and stable catalyst is much desired.
Since weak bonding among nano building blocks is the key
factor for structural damage of the MgO flower, a rational
The hierarchical flower-like MgO catalyst was synthesized approach is to enhance the binding forces among the nano-
according to a modified ethylene-glycol-mediated method.10 After particles with external binders. We envisioned that when a
heating the synthesis mixture in a sealed Teflon-lined stainless continuous layer was coated on the surface of hierarchical
steel autoclave at 180 1C for 5 h, a uniform MgO precursor with structures, with strong binding between the coating and nano-
the diameter of about 1 mm was obtained as shown in ESI† particles, the coating will glue the surface nanoparticles
(Fig. S1a). Every MgO precursor particle is elegantly flower like, together and help to keep the whole structure intact. The
with a smooth surface and flat flower petals.
coating would also shield the nanoparticles from direct physical
After calcination at 500 1C for 2 h, the morphology of the impact to further help in maintaining the structural stability.26
resulted MgO flower remains the same as the precursor, but the Mesoporous silica has attracted numerous research interest
surfaces of the flower petals become rough (Fig. 1a). TEM due to medium-sized pores and a large specific surface area.
images show that the petals are composed of MgO nano- Most studies have focused on the application of meso-SiO2 in
particles with the diameter of about 5 nm (Fig. 1a and adsorption, as a catalyst support, as a drug-delivery carrier,
Fig. S1b, ESI†). The XRD pattern demonstrates that the flower- etc.27–30 Using meso-SiO2 coating to protect an effective but
like MgO is highly crystalline (Fig. S2, ESI†). The Brunauer– fragile catalyst was largely ignored, as mesoporous silica itself
Emmett–Teller (BET) gas sorptometry analysis shows that suffers from poor mechanical stability.
MgO with big flower petals has a specific surface area of
The precursor of flower-like MgO is then coated with meso-
141.4 m2 gÀ1. Bian et al. reported that hierarchical MgO with SiO2 using hexadecyltrimethyl ammonium bromide as a sur-
a specific surface area of 45 m2 gÀ1 showed high catalytic factant and TEOS as a silica source, followed by calcination
activity for the Claisen–Schmidt condensation reactions due at 500 1C to remove all organic species. In contrast to the
to its hierarchical structure.10 Thus we envision a higher rough surface of the flower-like MgO (Fig. 1a), the obtained
activity for the hierarchical flower-like MgO with a larger sur- MgO@meso-SiO2 composite has a relatively smooth surface,
face area in this work.
while the overall morphology and the size of the composite
The transesterification reaction of EC with methanol was remain the same as original flower-like MgO (Fig. 2a), indicating
carried out at 140 1C in a flask at atmospheric pressure for 6 h that meso-SiO2 is coated evenly on the surface of flower-like MgO.
using hierarchical flower-like MgO as the catalyst. EC conver-
sion in the first run was 92%, much higher than the 51% EC
conversion obtained from commercial MgO (Fig. S3, ESI†)
under identical conditions. The flower-like MgO also exhibited
excellent product selectivity with a 96% DMC selectivity. The
flower-like MgO consisted of petal like sub-units, which further
consisted of MgO nanoparticles of 5–6 nm. Such a hierarchical
Fig. 1 (a) SEM image of hierarchical flower-like MgO, inset shows the TEM
image of a MgO flower; (b) SEM image of the flower-like MgO after one run in
the transesterification reaction between methanol and EC, the flower-like MgO
was broken into pieces.
Fig. 2 (a) SEM image of MgO@meso-SiO2 composite; (b) TEM image of
MgO@meso-SiO2 composite; (c) TEM image of the meso-SiO2 layer after MgO
is removed, inset shows the HRTEM image of the SiO2 layer; (d) SEM image of
MgO@meso-SiO2 after 10 consecutive runs.
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In summary, we produced an MgO@meso-silica composite,
which is not only very active in the transesterification reaction
to produce DMC, but also very stable. Meso-SiO2 coating is
the key design in enhancing the mechanical stability of the
hierarchical MgO. Experiments to improve the stability of other
fragile catalysts by meso-SiO2 are underway.
We thank the financial support from the National Basic
Research Program of China (MOST 2009CB930400), the National
Natural Science Foundation of China (NSFC 21273244,
21121063), the Fundamental Research Funds for the Central
Universities and the Chinese Academy of Sciences.
Fig. 3 (a) The N2 adsorption–desorption isotherms of MgO, MgO@meso-SiO2 and
the outer SiO2 coat; (b) the catalytic performance of the hierarchical flower-like MgO
and MgO@meso-SiO2 in the transesterification of EC in 10 consecutive runs.
Notes and references
The TEM image in Fig. 2b also shows that flower-like morphology
is well preserved. However, there is no visible boundary between
the MgO core and the meso-SiO2 shell due to the low contrast
between MgO and SiO2.
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