Template-induced: In situ dispersion of enhanced basic-sites on sponge-like mesoporous silica and its improved catalytic property

Article abstract of DOI:10.1039/c6ra21236k

Catalytic performance of heterogeneous catalysis is strongly dependent on the dispersity of catalytic active sites, and especially a high exposure of the unit active phase is promising for the overall catalytic process. In this study, a novel strategy was developed to fabricate an unprecedented CaO-based mesoporous solid strong base catalyst. Relying on the physicochemical assembly of Ca²⁺ in the interface between a micelle and siliceous wall, thin-layer-like calcium oxide species were formed in situ and dispersed in the mesochannels of silica. Wherein, the gradual coverage of CaO on the mesoporous wall was controlled by adjusting the amounts of Ca²⁺ on alkylamine micelles. Interestingly, a novel sponge-like microscale structure of silica was discovered in the CaO-based mesoporous-composites for the first time, which completely differs from the reported mesoporous silica. More importantly, the introducing of a CaO solid base on the pore wall is nearly non-destructive for the textural properties of the mesoporous matrix. The direct template-induced mesoporous solid strong base not only received extremely dispersed and unexpected enhanced strong basic sites (CO₂-desorption temperature ≥718 °C), but also avoided repeated thermal processes for the degradation of the basic resource, and saved energy and time. This heterogeneous alkaline catalyst shows excellent catalytic activity for the synthesis of dimethyl carbonate under a milder reaction condition (30 °C, 25 min) and holds stability and reusability beyond comparison with the conventional catalysts. The dispersed and enhanced strong basic sites, combined with excellent mesoporous properties, are demonstrated to be responsible for such a high catalytic performance.

Full text of DOI:10.1039/c6ra21236k

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Page 1 of 28
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Template-induced in situ dispersion of enhanced basic-sites on
the sponge-like mesoporous silica and its improved catalytic
property
[
a]
[a]
[a], [b]
[a]
[a]
Fu Yang , Bangbang Wang , Shijian Zhou
a] State Key Laboratory of MaterialsꢀOriented Chemical Engineering, College of Chemical
Engineering, Nanjing Tech University, Nanjing 210009, Jiangsu, P. R. China
b] Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing
Tech University, Nanjing 210009, Jiangsu, P. R. China
, Xiaoning Yang , Yan Kong*
[
[

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Abstract: Catalytic performance of heterogeneous catalysis is strongly dependent on
dispersity of catalytic active sites, especially, high exposure of unit active phase is
promising for overall catalytic process. In the study, a novel strategy was developed to
fabricate an unprecedented CaOꢀbased mesoporous solid strong base catalyst. Relying
2+
on the physicochemical assembly of Ca in the interface between micelle and
siliceous wall, thinꢀlayerꢀlike calcium oxide species were in situ formed and dispersed
in the mesochannels of silica. Wherein, the gradual coverage of CaO on the
2
+
mesoporous wall was controlled by adjusting the amounts of Ca on alkylamine
micelles. Interestingly, a novel spongeꢀlike microscale structure of silica was
discovered in the CaOꢀbased mesoporousꢀcomposites for the first time, which
completely differs from the reported mesoporous silica. More importantly, the
introducing of CaO solid base on the pore wall is nearly nonꢀdestructive for the
textural properties of mesoporous matrix. The direct templateꢀinduced mesoporous
solid strong base not only received extremelyꢀdispersed unexpected enhanced strong
basic sites(CO ꢀdesorption temperature ≥718 °C), but also avoided repeated thermal
2
process for the degradation of basic resource, and saved energy and time. This
heterogeneous alkaline catalyst shows excellent catalytic activity for synthesis of
dimethyl carbonate under a milder reaction condition(30 °C, 25 min) and holds
stability and reusability that beyond comparison of conventional catalysts. The
dispersed and enhanced strong basic sites, in combined with excellent mesoporous
properties, is demonstrated to be responsible for such a high catalytic performance.
KEYWORDS: Calcium oxide, In Situ Dispersion, Templateꢀinduced, Mesoporous
Solid Base, Heterogeneous Catalysis, SpongeꢀLike

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1
. Introduction
Biodiesel, as sustainable energy, has been gradually applied into the practical
1
ꢀ3
production due to the consumption of limited fossil fuels. Wherein, the usage of
highlyꢀefficient heterogeneous alkaline catalyst in the transesterification of biodiesels
is an intriguing research orientation for obtaining many valuable chemicals from some
4
ꢀ6
biological sources as a substitution of limited fossil fuels. Recently, dimethyl
carbonate (DMC) as an ecoꢀfriendly chemical is now widely accepted due to its
negligible toxicity and efficient utilization of carbon dioxide in the synthesis process.
Generally, heterogeneous base catalysis is regarded as the interface reaction
process. Therefore, direct fabrication of high active heterogeneous solid base catalysts
with high exposure of unit catalytic active phase has been demonstrated to be
promising due to the extremely increasing accessibility of active sites. Many
endeavors on increasing the dispersity and surface exposure degree of active species
7
ꢀ10
to improve their catalytic activity have been continuously devoted to.
Mesoporous
solid bases have attracted great interests due to some individual advantages including
improved performance, simple separation, no corrosion, and negligible waste
11ꢀ13
production.
So far, increasing attention has been paid to developing
highlyꢀefficient mesoporous solid base catalyst possessing high surface area and
6
, 14ꢀ18
wellꢀdispersed strong basic sites.
Directly fabricating mesoporous solid base using basic metal sources as
mesoporous matrix is not fully reasonable due to the unavailability of massive
unexposed basic sites in the framework. In addition, the obtained mesoporous solid
19,
base also failed to give the desirable structure properties such as large surface area.
2
0
Hence, many postꢀtreatment methods have been developed by incorporating
alkaline sources on the supports with large surface area, such as mesoporous silica, to
obtain some preferable porous solid bases. For instance, through grafting special
organic bases or anchoring nitrogenꢀbearing species, some interesting lewis basic sites
14, 21ꢀ23
can be generated on the pore surface of mesoporous silica.
Despite all this, the
generated basicity on these materials is still relatively weak as compared to some

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counterparts owe to the inherent nature of basic species. Fortunately, the decoration of
strong basic metal oxides (e.g. K O, CaO and MgO) on the surface of mesopore has
2
10, 24ꢀ27
also been demonstrated to be efficient for enhancing of base strength.
Typically,
the modification of basic metal oxides for mesoporous silica mainly involves the
introduction of basic metal sources in mesoporous silica and followed by calcination
to produce the basic sites on the pore surface of supports. However, noticeably, the
introduction of basic metal oxides renders the drastic descending of surface area and
pore size of supports by the conventional method. In addition, the introduction of
basic metal oxides in mesochannel is also uncontrollable and nonuniform, and
generally companied with the growth and sintering of large basic oxide particles,
hence failing to receive desirable solid base with full decoration of mesoporous
surface. More apparently, energy consumption in the repeated calcination including
removing of micelle template of porous material and decomposition of basic metal
salts is very high. Consequently, only weak basicity is obtained in the resulting
materials, meanwhile, accompanied with serious damage with regard to the
mesostructure of supports. Despite these efforts, the development of an efficient
method to create strong basic sites on mesoporous silica remains a great challenge to
date.
Herein, based on our previous works, we reported a simple templating method by
2+
directly introducing basic metal Ca in the interface of micelle and pore wall during
the assembly process and then in situ producing wellꢀdispersed and high exposure of
2+
enhanced CaO active basic species by the thermal process. Wherein, Ca not only
acted as base source, but also played a coꢀstructure directing role in the selfꢀassembly
2
+
process. The metallomicelles formed from the coordination of Ca
and
DDA(dodecylamine) micelle served as a cationꢀlike micelles template, which
facilitates to improve the structure of matrix due to the presence of stronger
countraꢀions interaction between the metallomicelles and siliceous oligomers. As a
result, thinꢀlayerꢀlike CaO strong basic species were in situ formed and dispersed on
the mesoporous wall of silica during the calcined process. The synthetic strategy is
schematically illustrated in Scheme 1. The involved catalysts were characterized using

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various efficient techniques to demonstrate the status of CaO and the structure of
material. Finally, the obtained solid base catalysts were applied for synthesis of
dimethyl carbonate to evaluate their catalytic performance, and the appeared special
basic sites were studied for this reaction. The catalytic reaction mechanism of
dimethyl carbonate synthesis over the mesoporous solid base was supposed and
clarified.
2
2
. Experimental
.1. Materials
Dodecyl amine (DDA, ≥98%) was obtained from Aladdin. Calcium acetate
monohydrate (Ca(CH COO) ·H O, ≥99%), tetraethylorthosilicate (TEOS, AR) and
3
2
2
methanol (MeOH, ≥99%) were purchased from Sinopharm Chemical Reagent Co.,
Ltd. Propylene carbonate (PC) was purchased from Aladdin. All chemicals were
commercially available and used without any purification.
2
.2. Preparation of xCaO-HMS(Hexagonal mesoporous silica)
2
+
The synthetic process of Ca ꢀparticipated assemble CaOꢀHMS was operated as
follows: DDA (1.85 g, 0.01 mol) was blended with ethanol (25 ml) and deionized
water (62.5 ml), and then the mixture was stirred at 45 °C for 30 min. Controllable
2 2
contents of Ca(Ac) ·H O (1, 2, 3 4 and 5 mmol) were added and resulted in the status
of solutions turning from lacte to translucence. After restored to room temperature,
TEOS (10.73 ml) was dropwisely added under the vigorous stirring with another 4 h
and subsequently kept static at 45 °C for 18 h. In the end, the resulting samples were
collected by centrifugation and washing with water and ethanol for three times. The
asꢀprepared samples were calcined at 550 °C for 6 h in dry air steam. The final
catalysts corresponding to gradually increasing calcium contents were labelled as
xCaOꢀHMS(x= 1, 2, 3, 4, 5), and their theoretical molar ratio of Ca/Si is 2.08, 4.17,
6
.24, 8.32 and 10.4 (%). The comparative catalyst was prepared based on the typical
wet postꢀimpregnated method involving introduction of calcium source in bare HMS
and followed by calcination, and then the resulting catalyst was designated as
5
CaOꢀHMS(p). The molar ratio of Ca/Si was controlled at about 10.4 %.
2
.3. Characterizations

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The XRD patterns of all of samples were collected using Smartlab TM 9 KW (Rigaku
Corporation, Tokyo, Japan) equipped with a rotating anode and Cu Kα radiation (λ =
0
.154178 nm).
The N adsorptionꢀdesorption tests were carried out using BELSORPꢀMINI
2
volumetric adsorption analyzer (BEL Japan, Osaka, Japan). The resulting samples
were outgassed in vacuum at 150 °C for 3 h and before measurements. The specific
surface areas and pore size distribution were calculated through
BrunauerꢀEmmetꢀTeller (BET) and BarrettꢀJoynerꢀHalenda (BJH) methods,
respectively.
2
The basicity of samples were examined by CO ꢀtemperature programmed desorption.
The samples were pretreated under a He stream at 200 °C for 1 h (100 mL/min).
Subsequently, temperature of system was cooled down to 100 °C, and a flow of pure
2 2
CO (50 mL/min) was subsequently introduced into the reactor. Desorption of CO
was performed from 100 to 800 °C (10 °C/min) under a He flow (30 mL/min), and
CO was detected with a TCD signals.
2
Fourier Transform Infrared (FTꢀIR) spectra of the samples were collected on Bruker
VECTOR22 resolution (Bruker, Bruker, Germany).
Scanning electron microscopic (SEM) images were obtained from a Hitachi S4800
Field Emission Scanning Electron Microscopy (Hitachi, Tokyo, Japan). The elemental
distribution of materials was determined with energy dispersive Xꢀray spectroscopy
(EDX).
Highꢀresolution transmission electron microscopy (HRTEM) images were collected
on a JEMꢀ2010 EX microscope (JEOL, Tokyo, Japan) equipped with EDX.
The Xꢀray photoelectron spectra (XPS) were recorded on a PHI 5000 Versa Probe
Xꢀray photoelectron spectrometer (ULVACꢀPHI, Kanagawa, Japan) equipped with Al
α
K radiation (1486.6 eV). The binding energy of C1s at 284.6 eV was utilized as the
reference.
The calcium contents in the catalysts were examined using a PE Optima 2000DV
Inductively Coupling Plasma emission spectrometer (ICP) (PerkinElmer, Waltham,
MA, USA). All the samples were pretreated and completely dissolved with

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hydrofluoric acid.
Computational Details: All calculation results were based on the Gaussian 09 package.
Density of atom charge(Hirshfeld) was calculated with the Becke’s threeꢀparameter
hybrid exchange functional and the Lee−Yang−Parr correlation functional (B3LYP).
2
.4. Catalysis assess
The catalytic performance of series xCaOꢀHMS were assessed using
transesterification reaction of propylene carbonate and methanol. Before the reaction,
2
all the CaOꢀbased HMS materials were activated at 800 °C in a pure N flow for 1 h.
The reaction was carried out in a 25 mL flask under strong magnetic stirring at a
certain temperature with desirable time. The molar ratio of MeOH to PC in all the test
reactions was 4:1, the used catalyst amount was 0.50 wt% of MeOH. Finally, the
product was examined on a SPꢀ6890 gas chromatograph (Lunan Ruihong Chemical
Instrument Co. Ltd, China) with 0.32 mm × 30 m SEꢀ54 capillary column after
centrifugal separation. The conversion and selectivity were calculated by external
standard method.
In order to investigate the special effect of the appeared γ basic sites in the synthesis
2
of DMC, we conducted a designed experiment by the CO absorption and desorption
treatment at a specific temperature(550 °C) to cover γ basic sites and obtain
CaOꢀHMS containing only α and β basic sites(Scheme 4). The detail operation can
2 2
refer to the above CO ꢀTPD characterization section. The CO treated CaOꢀbased
HMS were further used to catalyze synthesis of DMC under the reaction conditions as
same as that(50 °C, 60 min) of 5CaOꢀHMS(p).
Finally, the stability of the catalysts was examined by the recycling experiments under
their respective optimized reaction conditions.
3
3
. Results and discussion
.1. Synthetic clarification
Synthetic procedure for the in situ dispersion of CaO on the mesoporous wall of silica
is illustrated in Scheme 1. The selfꢀassembly mechanism of material was proposed
2
+
28, 29
through a Ca ꢀparticipated templating process based on our previous works.
As
0
0
we know, the traditional HMS material prepared by neutral templating route(S I ) are
based on hydrogen bonding and selfꢀassemble interaction between neutral primary
0
0
amine surfactants(S ) and neutral inorganic precursors(I ). Herein, the introduced

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2
+
Ca cations served as important intermediary in this templating process setting up a
bridge between the DDA micelle template and siliceous species by the coordinated
2
+
and electrostatic interaction. In this process, Ca cations first coordinated on dodecyl
2
+
amine to generate a metallomicelle (Ca ꢀDDA) which is similar to a cationicꢀlike
2
+
micelle template. Massive cationic Ca were enriched on the surface of DDA
micelles. Therefore, negativelyꢀcharged silicate oligomers from the hydrolysis of
2
+
TEOS easily assembled on the surface of Ca modified DDA micelles by
countraꢀions intermediary interaction. As a result, this mesoporous HMS solid base
decorated with wellꢀdispersed CaO species on the surface would be in situ generated
after calcining to remove the template.
Scheme 1. Templateꢀinduced in situ dispersion of solid basic sites on mesoporous
silica (A) as well as the conventional thermal method (B).
3
.2. Characterizations for the special CaO-based solid base catalyst
Fig. 1a displays the lowꢀangle XRD patterns of series xCaOꢀHMS. As shown, all the
samples give a diffraction peak at 2ꢀ3° which attributed to the (100) diffraction peak
of mesophase. The presence of (100) feature peak demonstrates the mesostructure of
all the samples. In addition, an obvious shift to low angle for (100) diffraction of
samples from 1CaOꢀHMS to 5CaOꢀHMS can be observed, which indicates the
enlargement of d100 lattice spacing. This should be due to the increment of CaO basic
2
+
28
species on the pore wall with incorporation of more Ca in the assembly process.
2+
Interestingly, with the gradual introducing of Ca on the DDA micelles, the intensity

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of mesophase diffraction exhibits a gradual raise and subsequent drop process, and the
maximum of intensity is reached with regard to the sample of 2CaOꢀHMS. This result
suggests the variation of mesoporous regularity of series xCaOꢀHMS with different
2
+
CaO loadings, which is associated with the participation of Ca in the assembly
2+
process. The coordination of Ca on the DDA micelles results in the enrichment of
positive charges on the micelle template and the resulting metallomicelles would
more easily match negativeꢀcharge of SiꢀO oligomers in the assembly process. The
enhanced countraꢀion assemble interaction promotes the formation of improved
2+
structure. However, excessive gathering of Ca on micelles also may destruct the
pristine micelles’ structure due to mutual repulsion of positive charges on the
28
2+
micelles. Therefore, more introduction of Ca leads to the rational drop of
mesoporous regularity.
(a)
(b)
1CaOꢀHMS
2CaOꢀHMS
3CaOꢀHMS
4CaOꢀHMS
5CaOꢀHMS
5
CaOꢀHMS(p)
5
CaOꢀHMS
CaOꢀHMS
4
3
CaOꢀHMS
2CaOꢀHMS
1CaOꢀHMS
1
2
3
4
5
6
7
8
10
20
30
40
50
60
70
80
2Theta(deg)
2Theta(deg)
Fig. 1. Lowꢀangle(a) and wideꢀangle(b) XRD patterns of various samples.
The Fig. 1b shows the highꢀangle XRD patterns of all the samples. Each sample
shows a board peak around 20ꢀ30°, which is attributed to amorphous silica. However,
no crystal diffraction peak attributed to CaO is detected in all the curves of series
xCaOꢀHMS synthesized from templating route, which indicates that the Ca species
introduced may be uniformly dispersed on the pore wall and do not aggregate into
bulk CaO crystal. This result favors the formation of thinꢀlayerꢀlike dispersed solid
base on the mesoporous wall, which is also in accordance with TEM results(Fig. 3).
In contrary, it is obviously observed that three strong diffraction peaks present in the
curves of postꢀimpregnated sample 5CaOꢀHMS(p) typically attributed to cubic CaO

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26
crystals,
which demonstrates the production of large CaO crystal in
postꢀimpregnated 5CaOꢀHMS(p). Associating with assemble mechanism of series
CaOꢀHMS materials, it suggests that the formation of extremelyꢀdispersed CaO
should be owe to the strong physicochemical interaction between metal calcium and
siliceous pore wall which efficiently inhibits the immigration and aggregation of CaO
during the highꢀtemperature thermal treatment process.
Fig. 1S presents the nitrogen sorption results and pore size distribution of series
2
xCaOꢀHMS. As exhibited in Fig. 1Sa, the N ꢀadsorption/desorption isotherms of all
the samples exhibit IV type curve and H1 hysteresis loop, indicating the typical
mesostructure of materials. In addition, it should be noted that all samples show high
3
adsorption capacity (ca. 2.1 cm /g) especially in the high relative pressure region of
0
.9ꢀ1.0. The sudden jump at high relative pressure indicates the presence of massive
stacked voids. This result reveals that the xCaOꢀHMS may be stacked with fineꢀsized
particles, and many voids exist between them. The further detailed textural parameters
calculated from nitrogen desorption data are summarized in Table 1S. All the samples
2
exhibit a similar pore size (ca. 2.6 nm) and high specific surface area (ca. 1000 m /g),
2
+
which manifests a fact that the incorporation of Ca in the assembly process show
little effect on the pore diameter and specific surface area. However, in comparison
with the postꢀsynthesized 3CaO/HMS(p) sample, all the CaOꢀHMS catalysts prepared
from the metalꢀassisted template method show an obviously superior textural
properties, which highlights the superiority of this selfꢀassembly process. In contrary,
this is very difficult to reach in the traditional method by introducing metal species
and followed calcined process, which is generally accompanied with large detriment
for the structure of matrix material. Note that the series xCaOꢀHMS samples even
exhibit a higher structural characteristic as compared to the pure HMS. This result
2+
manifests the introducing of Ca in the assembly process for obtaining high surface
area of solid base catalysts is meetly promising. It is suggested that the presence of
better structure should be attributed to the improved matching interaction by
countraꢀions intermediary during the assembly process of mesoporous silica, which
28
supported by the reported work. Another characteristic worth noting is that the

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calculated pore wall thickness of series xCaOꢀHMS exhibits a rising trend with
increment of CaO in the mesopore. Apparently, the increasing pore wall thickness,
herein, should be attributed to the CaO species in situ dispersed on the pore wall.
Fig. 2 displays the representative morphology and their partial characteristics of series
samples of xCaOꢀHMS from Scanning Electron Microscope. The lowꢀpower SEM
images show that the whole morphology of series xCaOꢀHMS consists of large
spherical particles and some broken fractions. Whereas, the higherꢀpower SEM
images show that the surface of these particles is in a fluffy status and seems to be
heaped up by the ultraꢀfine nanoparticles, and massive voids present in the sphere just
like sponge structure, which is a good explanation for the presence of high adsorption
2
capacity. This evidence is also in good accordance with N ꢀadsorption/desorption
results that the presence of high jump of adsorption volume at high relative pressure.
In addition, as compared to the status of microsphere in pure HMS sample (Fig. 2S),
the obvious difference can be found, and the surface fluffy status was absent in pure
2+
HMS. It can be concluded that the participation of Ca in the assembly process of
HMS have some special effects on the morphology.

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Fig. 2. Representative SEM images of series samples (a)1CaOꢀHMS, (b)2CaOꢀHMS,
(c)3CaOꢀHMS, (d)4CaOꢀHMS, (e)5CaOꢀHMS and (f)sponge photo.
The morphology and structural features of materials can be further examined by the
Transmission Electron Microscope. Fig. 3 shows the representative TEM images and
its elemental mapping images of sample 5CaOꢀHMS. Distinctly, the 5CaOꢀHMS
shows a special microsacle structure (multiꢀvoids spongeꢀlike characteristic), which
has not as yet been reported for the mesoporous silica materials. Typically, this
material consists of massive nanoscale particles, and the porosity of particles is also
observed based on the images of (a) and (b), which is similar to the reported
30, 31
wormholeꢀlike pore structure.
It should be noted that this structural characteristic
is universal in the overall material not only as a partial characteristic. In addition, the
observed results are in good agreement with the SEM surface characteristics. This is
also a reasonable explanation for the presence of high pressure’s jump of
2
adsorption/desorption curve in N ꢀadsorption/desorption isotherms. On the other hand,
the TEM results reveal that the visual CaO crystalline species is not observed,
however, the EDX spectrum demonstrates the existence of Ca element, suggesting

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that the dispersed CaO species should be incorporated in the mesochannel of silica in
the form of amorphous status. Meanwhile, elemental mapping results also confirm
that the resulting material is comprised of Si, O and Ca, and their uniform distribution
in the selected region of mesocomposite indicates the homogeneous dispersity of CaO
on the surface of silica. Finally, with respect to the formation of multiꢀparticles
stacked CaOꢀbased silica, a surfaceꢀenergyꢀdominated process is regarded to be the
result. The generation of metallomicelles during the assembly process may result in
their mutual repulsion among the micelles and thereby make the nucleated particles
assembled from the metallomicelles and siliceous oligomers hard to aggregate into a
larger bulk. The feasible assembly process is speculated and shown in scheme 2.
Fig. 3. Representative TEM images(a, b, c and d) and its elemental mapping images(e)
of spongeꢀlike 5CaOꢀHMS, the white scale bar represents 2 ꢁm.
Scheme 2. Formation of Spongeꢀlike mesoporous silica.
Subsequently, basicity of materials was evaluated with Temperature Programmed

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Desorption of CO
2
(CO
2
2
ꢀTPD) (Fig. 4). The desorption peaks of CO is provisionally
divided into three components designated as α, β and γ at ca. 350, 470 and 720 °C,
respectively, assigned to weak, medium, and strong basic sites. Obviously, all the
curves of CaOꢀbased HMS exhibit three kinds of different basic sites, suggesting the
coꢀexistence of weak, medium, and strong basic sites. It can be found that CaO
concentrations’ effect on basic intensity is weak. However, it is noticeable that, the
detected basic amounts in sample shows an obvious enhancing trend with increasing
Ca concentration in the sample. When loadings of Ca in 5CaOꢀHMS reach the
maximum, the γ basic sites observed at the high temperature of 718 °C are very
abundant, indicating the presence of plentiful strongꢀbase sites in the
templateꢀinduced CaOꢀbased solid bases. It was reported that, in some traditional
CaOꢀbased mesoporous silica materials, the appeared weak basic sites with desorption
32
temperature at ca. 150 °C are attributed to SiO
2
matrix, which is accorded with
results from 5CaOꢀHMS(p)(Fig. 3S), and the medium basic sites appearing at 220 °C
32
corresponds to surface CaSiO species. However, in the desorption profiles of series
3
xCaOꢀHMS, it can be observed that the present weak α basic sites at ca. 350 °C give
an obviously higher basicity than those medium basic sites in traditional CaOꢀbased
materials. Herein, these weak α basic sites with obvious enhanced basic intensity
should be attributed to the CaO species directly connected on the siliceous pore wall,
3 2
which is similar to CaSiO species. Additionally, as expected, no any CO desorption
peak attributed to parent silica is detected, suggesting that the exposed silica surface is
low due to the modification of dispersed CaO species, which favors the
templateꢀinduced full coverage of CaO on the mesoporous wall. On the other hand, a
relatively minor β peak at ca. 470 °C is observed and indexed to the medium basic
sites of CaO species on the siliceous pore wall modified with calcium. According to
33
some reports, generally, the highest CO
2
desorption temperature of pure CaO,
33
34, 35
9, 26
CaO/C, CaO/ZrO
2
and some other CaOꢀbased silica materials
is only ca.
6
00 °C. However, herein, with respect to the series samples of xCaOꢀHMS, the
desorption of CO persists up to 800 °C and even a higher temperature, which is
compared to, not inferior to, some solid superbases, thereby demonstrating the
2

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presence of extraordinarily strong basicity in samples. It is concluded that, as
compared to those reported Caꢀbased mesoporous materials prepared by doping and
postꢀdegradation method, we demonstrated that the templateꢀinduced Ca solid basic
sites in this material are the strongest on basis of their desorption temperature.
γ
a
ß
718
°C
3
60
°C
4
69
91
78
°
C
676 °C
4
°C
3
62
40
30
°C
5CaOꢀHMS
720 °C
4
°C
3
°C
4CaOꢀHMS
750 °C
452
°C
3
°C
3CaOꢀHMS
2
CaOꢀHMS
660 °C
474 °C
1CaOꢀHMS
339 °C
100
200
300
400
500
600
700
800
Temperature( °C )
Fig. 4. Temperatureꢀprogrammed desorption profiles of CO
2
over series of
xCaOꢀHMS mesoporous solid base.
ꢀ1
ꢀ1
Fig. 5 displays the FTꢀIR spectra in the range from 1200 cm to 500 cm of series
samples of xCaOꢀHMS. The feature variations for the FTꢀIR spectra of xCaOꢀHMS
can be obviously observed with increasing loadings of CaO on the surface. As shown,
ꢀ1
the feature absorption band attributed to the surface SiꢀOH groups at 962 cm
exhibits a gradual drop trend in intensity with increasing incorporation of CaO species
on the mesoporous surface. This is typical evidence for the consumption of silanol
groups in the process of incorporation owing to the strong interaction between the
,
pore wall’s silanol groups and incorporated CaO. The almost disappeared silanol
groups indicate the nearly complete coverage of CaO on the internal surface of
11, 28
mesoporous wall.
This is well coincided with the CO ꢀTPD results and is a strong
2
evidence for supporting the presence of thinꢀlayerꢀlike CaO on the surface of
28
mesoporous wall.

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5
CaOꢀHMS
CaOꢀHMS
CaOꢀHMS
CaOꢀHMS
4
3
2
ꢀ1
958 cm
1CaOꢀHMS
SiꢀOH
1200
1100
1000
900
800
700
600
500
-1
Wavenumbers(cm )
Fig. 5. FTꢀIR spectra of series samples of xCaOꢀHMS.
To accurately study the elemental chemical status and natural environment in samples
and confirm the presence of Ca on the surface of DDA micelles, highꢀresolution XPS
spectra for N1s of uꢀ5CaꢀHMS and HMSꢀDDA, Ca2p of uꢀ5CaꢀHMS and
5
CaOꢀHMS are recorded in Fig. 6a and Fig. 6b, respectively. As observed in Fig. 6a,
the N1s spectrum of the HMSꢀDDA shows a main peak at 399.60 eV and a shoulder
36
at 401.3 eV, respectively, assigned to free amino groups of DDA and its amino
2
8
groups interacted with mesoporous silicate surface by hydrogenꢀbonding. Whereas,
the N1s spectrum from uꢀ5CaꢀHMS gives two characteristic peaks at 399.71 and
4
01.68 eV, which departs obviously from that interacted by hydrogenꢀbonding with
siliceous wall. A slight shifts (0.11 eV) for main component, an obvious shifts (0.38
eV) for its shoulder to higher binding energy and the enhanced intensity for its
shoulder are discerned in the curve. This result obviously indicates the difference of
amino groups of DDA in chemical state and coordination environment before and
2+
after introducing Ca in assembly process. This should be attributed to the
2+
coordination of Ca on the N atoms of DDA micelles in which the electronꢀenriched
nitrogen atoms donated electrons to Ca ions, therefore, electron cloud densities from
32
nitrogen atoms are reduced This result confirms the presence and introducing of
2
+
Ca in the interface of micelle and siliceous wall. On the other hand, the present solid
basic sites of Ca in the materials are investigated by the Ca2p XPS spectra. The

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representative samples of uꢀ5CaꢀHMS and 5CaOꢀHMS were examined by
highꢀresolution XPS and displayed in Fig. 6b. Primarily, with respect to uꢀ5CaꢀHMS,
the binding energy from Ca2p transition splits at 346.95 and 350.5 eV for the 2p3/2
and 2p1/2 are observed, respectively. While, in the profiles of 5CaOꢀHMS, the binding
energy from Ca2p transition splits at 347.6 and 351.4 eV corresponding to the 2p3/2
26
and 2p1/2 are observed, respectively, which is typically assigned to the CaO.
Additionally, the obvious difference in binding energy from Ca atoms incorporating
HMS before and after calcination suggests the changing of chemical state of Ca
atoms. This result manifests that the Ca species are not stably located in framework of
28
silica and surrounded by chemicallyꢀinteracted SiꢀO species. In fact, the Ca species
should be introduced on the surface of mesopore, supporting the fact of the
templateꢀinduced dispersion.
(
a)
399.71 eV
(b)
5CaOꢀHMS
2p3/2
N 1s
Ca 2p
347.6 eV
401.68 eV
uꢀ5CaꢀHMS
2p1/2
351.4 eV
2
p3/2
3
46.95 eV
399.60 eV
uꢀ5CaꢀHMS
2
p1/2 350.5 eV
HMSꢀDDA
401.3 eV
3
92
394
396
398
400
402
404
406
408
410
340
342
344
346
348
350
352
354
356
358
Binding Energy(eV)
Binding Energy(eV)
Fig. 6. N1s spectra(a) of uꢀ5CaꢀHMS(it represents uncalcined 5CaOꢀHMS) and
HMSꢀDDA(it represents uncalcined HMS) and Ca2p spectra(b) of 5CaOꢀHMS and
uꢀ5CaꢀHMS.
3
.3. Study of catalytic performance
Scheme 3 Transesterification reaction of PC and MeOH over CaOꢀHMS.
Just as reported in the previous works, the transesterification process of PC and

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methanol(Scheme 3) could be effectively catalyzed with CaOꢀbased bioactive catalyst
at a lower temperature than other reported solid base catalysts needed, and the
37
reaction equilibrium even can be reached in 30 min.
Herein, the catalytic performance of CaOꢀbearing HMS materials was evaluated with
transesterification reaction of PC and methanol, and the DMC and propylene
glycol(PG) as the main products were directly synthesized in this reaction. Firstly,
under the temperature of 50 °C, the effect of time on the transesterification reaction
was investigated and the catalytic results were shown in Fig. 7a. It is noticeable that
the templateꢀinduced 5CaOꢀHMS affords the faster reaction rate and higher
conversion in the transesterification in comparison to the postꢀsynthesized
5
CaOꢀHMS(p). With regard to the superior catalytic activity of 5CaOꢀHMS, the
appeared large surface area and high exposure of basic sites should be responsible for
the results. On the other hand, to investigate the effect of temperature, the two
catalysts were respectively used to catalyze the reaction under the different reaction
temperatures ranging from 10ꢀ50 °C. The resulting catalytic data was displayed in the
Fig. 7b and 7c. The catalytic results reveal that the reaction over 5CaOꢀHMS can
receive the optimized catalytic conversion under a lower reaction temperature(~30 °C)
in a shorter time. In contrast, the optimized catalytic conversion of postꢀsynthesized
5
CaOꢀHMS(p) requires a higher reaction temperature and longer reaction time. The
short reaction time based on the 5CaOꢀHMS is benefited from the large surface area
and plentiful exposed active sites providing more diffused channels and more contact
chances for the chemicals. While the lower reaction temperature may be resulted from
the appeared enhanced basic intensity in the templateꢀinduced 5CaOꢀHMS, which is
3
8
well supported by the reported work.

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6
5
4
3
2
1
0
0
0
0
0
0
0
(a)
5
5
CaOꢀHMS
CaOꢀHMS(p)
0
10
20
30
40
50
60
Reaction time(min)
9
0
90
80
70
60
50
40
30
Con.(%)
Sel. (%)
80
(c)
(
b)
Con. (%)
Sel. (%)
7
0
60
5
4
3
2
1
0
0
0
0
0
10
20
30
40
50
10
20
30
40
50
60
Reaction Temperature(
℃
)
Reaction temperature(
℃
)
Fig. 7. Effect of reaction time based on 5CaOꢀHMS(p) and 5CaOꢀHMS (a):
temperature: 50 °C. Effect of temperature on PC conversion and DMC selectivity
with 5CaOꢀHMS, reaction time: 30 min (b), and 5CaOꢀHMS(p), reaction time: 60
min (c). All the reactions above were conducted under the conditions: atmospheric
pressure(AP), MeOH/PC=4:1, catalyst amount =0.50 wt% of MeOH.
Subsequently, different calciumꢀcontaining HMS catalysts were tested under the
optimized reaction condition. The relationship of DMC yield versus different reaction
time is shown in Fig. 8a. The detailed catalytic data using different catalysts are
summarized in Table 1. As shown, the pure HMS gives negligible reaction activity
even after the reaction for 30 min, which indicates the pure mesoporous silica is
nearly inactive in the transesterification of PC. On the other hand, other
calciumꢀcontaining solid basic catalysts exhibit certain catalytic activity, suggesting
that calcium oxide should be the catalytic active centers in this reaction. It is obvious
that the catalytic activity of series xCaOꢀHMS increases with increasing calcium
loading in HMS, and the maximum yield(52.3%) of DMC is reached at the highest
loading. This result demonstrates that the exposed calcium oxides on the mesoporous

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wall are increased with the appeared Ca in sample, coinciding with ICP result in Table
. It is also worth noting that the sequential increasing of calcium loading in HMS
1
leads to the slow increment of DMC yield at high loading, indicating the nearly
saturated status of surface calcium oxides on mesoporous wall, thereby confirming the
aforementioned FTꢀIR analysis. In addition, the variation of DMC yield with different
reaction time can be observed in the Fig. 8a. The reaction equilibrium is almost
reached in 25 min with respect to the series xCaOꢀHMS, indicating the quick reaction
rate over the series xCaOꢀHMS catalysts. This is obviously attributed to the presence
of large surface and massive voids in sample which promotes the rapid diffusion and
reaction of chemicals. For comparison, the catalytic data of postꢀimpregnated
5
CaOꢀHMS(p) under 50 °C and 60 min is displayed in Table 1. However, even with
higher reaction temperature and longer reaction time, only the poor catalytic
activity(30.4% of yield) is received with respect to the postꢀtreatment one, which is
obviously lower than that of templateꢀinduced CaOꢀbased material. Based on the
highꢀangle XRD result of 5CaOꢀHMS(p), we can safely make a conclusion that a
large amount of calcium oxides should be aggregated into large particles during the
calcined process. Therefore, the exposed active sites on the support is limited even
with a high loading. The result demonstrates that the high dispersion and exposure of
active sites are extremely useful in heterogeneous catalytic reaction.
On the other hand, the reported reference catalysts were compared with the
templateꢀinduced CaOꢀbased HMS and their corresponding catalytic results for the
synthesis of DMC with PC are listed in Table 1. Just as we observed, the CaOꢀbased
catalysts provided is based on different supports including supportꢀfree, carbon and
2
ZrO , however, these reported CaOꢀbased catalysts even with relatively high
dispersity of active species still exhibited the relatively poor catalytic activity in
comparison to the templateꢀinduced CaOꢀbased HMS. Associating with
aforementioned analysis, the appeared basic intensity of templateꢀinduced CaOꢀHMS
is obviously superior to that of CaO/C even the pure CaO, we suggest that the
appeared enhanced base sites may be responsible for the higher catalytic activity as
compared to their counterparts. On the other hand, based on the same support, the

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2 2
Li O and K O modified mesoporous silica reported still do not show a higher yield for
DMC, which may be due to their weaker basic sites compared to CaOꢀHMS,
supporting above analysis.
3
.4. Catalytic study of special basic sites
In addition, to investigate the special effect of the appeared γ basic sites in the
synthesis of DMC, we conducted a designed experiment by the CO absorption and
2
desorption treatment at a specific temperature(550 °C) to cover γ basic sites and
obtain CaOꢀHMS containing only α and β basic sites(Scheme 4). Wherein, desorption
2 2
temperature of CO is dependent on the CO ꢀTPD results and is lower than the
desorption point of γ basic sites. The amount of γ basic sites is determined based on
the titration method by subtracting the amount of α and β basic sites(from treated
sample under 550 °C) from the total basic amount(from activated sample under
8
2
00 °C). Due to the coverage of enhanced γ basic sites, the CO treated series
xCaOꢀHMS were further used to catalyze the synthesis of DMC under the higher
reaction temperature and longer reaction time(50 °C, 60 min). The received catalytic
results are displayed in Fig. 8b. Just as we observed, the CO
xCaOꢀHMS show the obviously poor catalytic performance in comparison with the
untreated series xCaOꢀHMS. Associating with the aforementioned CO ꢀTPD results,
it can be known that the γ basic sites have been covered during the CO
2
treated series
2
2
absorption/desorption process. Therefore, herein, the catalytic performance of the CO
2
treated series xCaOꢀHMS is merely attributed to the left α and β basic sites on the
mesoporous wall. In addition, relationship between the calculated DMC yields
contributed from γ basic sites and the amount of γ basic sites(per g catalyst) is plotted
and shown in Fig. 8c. This result demonstrates that the appeared special γ strong basic
sites are very significant and contribute to the main catalytic activity in the synthesis
of DMC. In addition, it is worth noting that, with the increment of calcium amounts in
the samples, the CO
which should be due to the increasing amount of the remaining α and β basic sites on
the pore wall. On the other hand, it should be noted that CO treated 5CaOꢀHMS(p)
exhibits a higher catalytic activity as compared to the CO treated 5CaOꢀHMS. This is
2
treated xCaOꢀHMS also show increasing catalytic activity,
2
2

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due to that the strongest basic sites on the 5CaOꢀHMS(p) are much weaker than that
of 5CaOꢀHMS(Fig. 3S). Therefore, the low temperature desorption of CO treated
2
5
5
CaOꢀHMS(p) did not result in the whole coverage of the strongest basic sites of
CaOꢀHMS(p). As a result, the more exposed basic sites give the higher catalytic
activity with respect to CO treated 5CaOꢀHMS(p).
2
Scheme 4. Coverage of γ strong basic sites and test of catalytic performance of
remaining α and β basic sites.
3
.5. Stability test
Finally, the catalyst of 5CaOꢀHMS as the optimized one is used for stability test by
the recycled experiments, and its result is shown in Fig. 8d. Just as shown, the
5
CaOꢀHMS exhibits a high stability and durable catalytic activity(≥ 47% of DMC
yield) with six recycling experiments. However, as compared to the 5CaOꢀHMS, the
postꢀsynthesized sample 5CaOꢀHMS(p) fails to parallel catalytic activity and stability
in the synthesis of DMC. According to the aforementioned analysis, the obtained high
stability may be attributed to the strong interaction produced between CaO and
mesoporous wall. Wherein, in this special templating method, the Ca atoms should be
anchored on the silanol of siliceous wall by certain physicochemical interaction [28].
Apart from that, the finally recycled 5CaOꢀHMS sample was examined by the
lowꢀangle XRD, ICP, TEM and SEM techniques and their results are shown in Fig. 4S.
The results reveal that the structure of 5CaOꢀHMS has not been obviously destructed
from the XRD, TEM and SEM results. However, the molar ratio of Ca/Si decreases
slowly from 8.87% to 7.32% detected by ICP results, which may be the
reason(leaching of CaO) resulting in the slight drop of catalytic activity.
a
Table 1 Catalytic results for synthesis of dimethyl carbonate with different catalysts.

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b
c
d
Samples
Ca/Si (%) Conv. (%)
Sel. (%)
Yield(%) Ref.
Pure HMS
—
4
—
—
This work
This work
1
2
3
4
5
5
CaOꢀHMS
CaOꢀHMS
CaOꢀHMS
CaOꢀHMS
CaOꢀHMS
1.84
3.56
5.23
7.11
8.87
15.9
27.7
39.8
51.6
56.8
34.9
53
92
14.6
25.8
36.2
46.4
52.3
30.4
46
93
This work
This work
This work
This work
This work
91
90
92
e
CaOꢀHMS(p) 10.2
87
33
33
34
39
40
CaO
—
—
—
—
—
87
CaO/C
CaO/ZrO2
—
—
42.9
—
45
—
Li
2
O/SiO
2
32.4
44.3
—
—
K
2
O/SiO
2
—
—
a
Reaction conditions: 30 °C, 30 min, atmospheric pressure(AP), MeOH/PC=4:1, catalyst amount
b
c
=
0.50 wt% of MeOH. The molar ratio of Ca/Si is determined from the result of ICP.
d
Conversion = mmol (PC) converted/mmol (initial PC) × 100%. Selectivity of DMC = mmol
e
(
DMC)/mmol (PC converted) × 100%. Reaction conditions: 50 °C, 60 min, MeOH/PC=4:1,
catalyst amount =0.50 wt% of MeOH.
6
5
4
0
0
0
60
50
40
30
20
10
0
1
2
CaOꢀHMS
CaOꢀHMS
untreated xCaOꢀHMS
(b)
(a)
CO treated xCaOꢀHMS
2
3CaOꢀHMS
4
5
CaOꢀHMS
CaOꢀHMS
HMS
30
20
10
0
0
5
10
15
20
25
30
35
Reaction time(min)

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4
3
3
2
2
1
1
0
5
0
5
0
5
0
5
60
(d)
5CaOꢀHMS
CaOꢀHMS(p)
(c)
5
50
40
30
20
10
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
1
2
3
4
5
6
Amount of (
γ
) basic sites(mmol/g)
Run times
Fig. 8. The yield of DMC versus the reaction time over the different samples (a):
temperature: 30 °C; Catalytic performance of untreated CaOꢀbased HMS under their
respective optimized conditions and that of CO treated CaOꢀbased HMS under
2
temperature: 50 °C, time: 60 min (b); Relationship between catalytic activity and
amount of γ basic sites in xCaOꢀHMS samples (c); Stability test based on 5CaOꢀHMS
and 5CaOꢀHMS(p) under their respective optimized conditions (d). All the reactions
above were conducted under the conditions: atmospheric pressure(AP),
MeOH/PC=4:1, catalyst amount =0.50 wt% of MeOH.
3
.6. Possible mechanism of reaction
Transesterification reaction between PC and methanol catalyzed with baseꢀcatalyst is
37
typical nucleophilic substitution process. In this transesterification process,
methanol acts as nucleophilic regent, which would be activated by the CaO basic sites
dispersed on mesoporous wall. Subsequently, the activated methanol attacks the ester
bond of PC and substitutes the opening sites of PC, in which the methanol would
produce the absorption and dissociation interaction with the CaO species. The
electronegative oxygen atom bonding to Ca atom would capture the hydrogen ion of
methanol, meanwhile, the positivelyꢀcharged Ca atom would stabilize the alkoxy
anion whose hydrogen ions snatched away. Wherein, hydroxypropyl methyl carbonate
(HPMC) is supposed to be the intermediates when the activated methanol attracts the
PC. To further confirm the process of reaction, we calculated the charge distribution
of methanol, PC and HPMC and the distribution of Hirshfeld charge density and
Coulomb potential patterns are shown in Fig. 5S. Charge density of several special
atoms are listed in Table 2. Hydroxyl oxygen atom carrying the charge in methanol
and HPMC are ꢀ0.222 and ꢀ0.208, respectively. The charge of carbonyl carbon atom

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in the HPMC and PC are 0.227 and 0.219, respectively. Thus, the reaction free energy
of methanol and HPMC are significantly lower than the corresponding free energy of
methanol and PC. Therefore, HPMC as intermediates is regarded to be reasonable. On
basis of these analysis, we proposed the possible reaction mechanism of DMC
synthesis and its depiction is displayed in scheme 5.
Scheme 5. Reaction process of PC to DMC and its possible reaction mechanism.
＊
Table 2 Hirshfeld charge distribution of MeOH, HPMC and PC Molecule.
Charge of atoms
Molecules
O
a
H
b
C
c
MeOH
HPMC
ꢀ0.222
ꢀ0.208
—
0.151
0.148
—
—
0.227
0.219
PC
＊
The charge distribution calculated from the Guassion 09 package. The atoms O
illustrated as follows:
a b c
, H and C are
4
. Conclusions
In summary, we successfully fabricated an unprecedented strong basicity of
spongeꢀlike mesoporous silica by the in situ incorporation of thinꢀlayer CaO inside its
mesopore based on a simple templatingꢀinduced method. An unexpected strong

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basicity is obtained and obviously superior to that of traditional CaOꢀbased silica
2
+
material. Additionally, the addition of Ca in the assembly process resulted in the
formation of new spongeꢀlike microscale structure of silica. Based on its appeared
superior physicochemical properties, a high catalytic activity and stability for the
synthesis of DMC was received. It was discovered that the enhancement of base
strength could remarkably reduce the reaction temperature needed, and the appeared
special γ basic sites are responsible for the main catalytic activity.
Acknowledgments
The authors acknowledge the project Funded by the financial support of the National
Natural Science Foundations of China (21276125, 20876077 and 21476108), the
Priority Academic Program Development of Jiangsu Higher Education Institutions
(PAPD).
Notes and references
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Well-dispersed and enhanced CaO were directly formed on sponge-like mesoporous
silica, improved activity is received at a lower reaction temperature.