Constructing mesoporous solid superbases by a dualcoating strategy

Article abstract of DOI:10.1039/c2ta00690a

Great attention has been paid to the development of mesoporous solid superbases due to their versatile catalytic activity under mild conditions. In this paper, a dualcoating strategy was designed to generate superbasicity on mesoporous silica SBA-15 by precoating ZrO₂ prior to the loading of base precursor (KNO₃). The ZrO₂ interlayer performs two functions by enhancing the guest-host interactions to promote the decomposition of KNO₃ at low temperatures, and by improving the alkali-resistance of the siliceous host, which simultaneously overcomes two weaknesses of mesoporous silicas. As a result, new dualcoating composites possessing both an ordered mesostructure and superbasicity were successfully fabricated. However, direct modification of SBA-15 with KNO₃ results in a sample with only weak basicity and a collapsed mesostructure. Interestingly, both the amount and dispersion degree of ZrO₂ play an important role in the generation of strong basicity. An intact layer can be formed on SBA-15 provided that the content of ZrO₂ is higher than 30 wt% and the ammonia/water-induced hydrolysis (AIH) method is employed. The mesostructure cannot be preserved if the content of ZrO₂ is lower than 30 wt% or if conventional coating methods (i.e. impregnation and grinding) are used. The obtained materials containing ZrO₂ are active in heterogeneous synthesis of dimethyl carbonate (DMC) and the yield of DMC can reach 28.4%, which is obviously higher than that over the material without a ZrO₂ coating (9.8%). The present strategy may open a new way for the design and synthesis of functional materials with strong basicity.

Full text of DOI:10.1039/c2ta00690a

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Materials Chemistry A
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Constructing mesoporous solid superbases by a
dualcoating strategy†
Cite this: J. Mater. Chem. A, 2013, 1,
1623
*
*
Xiao-Yan Liu, Lin-Bing Sun, Feng Lu, Tian-Tian Li and Xiao-Qin Liu
Great attention has been paid to the development of mesoporous solid superbases due to their versatile
catalytic activity under mild conditions. In this paper, a dualcoating strategy was designed to generate
superbasicity on mesoporous silica SBA-15 by precoating ZrO2 prior to the loading of base precursor
(KNO3). The ZrO2 interlayer performs two functions by enhancing the guest–host interactions to
promote the decomposition of KNO3 at low temperatures, and by improving the alkali-resistance of the
siliceous host, which simultaneously overcomes two weaknesses of mesoporous silicas. As a result, new
dualcoating composites possessing both an ordered mesostructure and superbasicity were successfully
fabricated. However, direct modification of SBA-15 with KNO3 results in a sample with only weak
basicity and a collapsed mesostructure. Interestingly, both the amount and dispersion degree of ZrO2
play an important role in the generation of strong basicity. An intact layer can be formed on SBA-15
provided that the content of ZrO2 is higher than 30 wt% and the ammonia/water-induced hydrolysis
(AIH) method is employed. The mesostructure cannot be preserved if the content of ZrO2 is lower than
30 wt% or if conventional coating methods (i.e. impregnation and grinding) are used. The obtained
materials containing ZrO2 are active in heterogeneous synthesis of dimethyl carbonate (DMC) and the
yield of DMC can reach 28.4%, which is obviously higher than that over the material without a ZrO2
coating (9.8%). The present strategy may open a new way for the design and synthesis of functional
materials with strong basicity.
Received 13th October 2012
Accepted 28th November 2012
DOI: 10.1039/c2ta00690a
www.rsc.org/MaterialsA
functionalization of mesoporous silicas is easy to realize by
either element incorporation into frameworks or modication
1 Introduction
on the surface.12–15 Therefore, the generation of strong basicity
on mesoporous silicas has been attracting extensive attention
ever since their discovery.
For the demands of sustainable development and green
chemistry, the use of heterogeneous catalysts instead of tradi-
tional homogeneous ones has received increasing attention.
Among various heterogeneous catalysts, mesoporous solid
superbases are highly attractive for applications in environ-
mentally friendly and economical catalytic processes. They can
catalyze diverse reactions under mild conditions and reduce
waste production.1–4 Their large pore sizes favor mass transfer
and avoid deactivation derived from coke formation that usually
happens in microporous catalysts.5–7 To date, many attempts
have been made to generate strong basicity on mesoporous
hosts. Since the discovery of mesoporous silica M41S by Kresge
et al.,8 a series of mesoporous materials have been synthesized
through the surfactant templating approach. Among various
candidates with a mesostructure, mesoporous silicas are the
best choice for a support, owing to their high stability, low
cost, and tuneable pore symmetry.9–11 Moreover, the
Graing organic basic groups onto silanols offers an inter-
esting method to form basic sites on mesoporous silicas.16–18
However, the basicity that is derived from organic groups (such
as amine groups) is generally weak. In addition, these organic–
inorganic hybrid materials can only be used at low tempera-
tures, owing to the degradation of organic building blocks at
elevated temperatures. By treating mesoporous silicas or Al-
substituted silicas with gaseous ammonia, the oxygen in the
frameworks can be partially displaced by nitrogen and generate
oxynitride frameworks, which produces a new kind of solid base
with a mesostructure.19–21 Recently, a methylation method was
also used to enhance the strength of nitrogen-doped meso-
porous silica, taking account of the donation of an electron
from the methyl group to the nitrogen atom.22 Nevertheless,
temperatures higher than 900 ꢀC have to be employed in the
process of nitrogen doping, and the strength of the basic sites is
still weak. To improve the base strength, alkaline metal oxides
with strong basicity (such as potassium and cesium oxides) have
been employed to modify mesoporous silica. These materials
were generally prepared by the introduction of precursors (such
State Key Laboratory of Materials-Oriented Chemical Engineering, College of
Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing
210009, China. E-mail: lbsun@njut.edu.cn; liuxq@njut.edu.cn; Fax: +86-25-
83587191; Tel: +86-25-83587177
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2ta00690a
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as KNO3 and CH3COOCs), followed by calcination at high of distilled water and 90 g of aqueous HCl solution (2 M) with
temperatures to decompose the precursors.23,24 Unfortunately, stirring at 40 C. Aer the formation of a homogeneous solu-
ꢀ
the obtained materials only exhibit weak basicity and the tion, 6.38 g of tetraethylorthosilicate (TEOS) was added and
mesostructure of the supports is severely deteriorated. This is stirred at this temperature for 24 h. Finally, the resulting gel was
ꢀ
because strongly basic species can react with silica, especially at transferred to a Teon-lined autoclave and kept at 120 C for
high temperatures. The modication of mesoporous silica by 24 h under static conditions. The as-prepared sample was
various alkaline and alkaline earth metal oxides was investi- recovered by ltration, washed with water, and air-dried at room
gated.24 The ordered mesoporous structure is always destroyed temperature. The removal of template was carried out at 550 ꢀC
À1
ꢀ
completely by the introduction of alkaline metal oxides. in an air ow for 5 h with a heating ramp of 2 C min
.
However, the introduction of CaO into mesoporous silica SBA- Coating ZrO2 on SBA-15. ZrO2 was introduced to SBA-15 by
15 produces a composite with superbasicity and an ordered three different methods, namely, AIH, impregnation, and
mesostructure. Further investigations show that solid strong grinding, in which ZrO(NO3)2 was used as the precursor for
bases can also be obtained from other alkaline earth metal ZrO2. The typical process for ZrO2 coating by the AIH method
oxide modication. It is known that alkaline metal oxides was as follows.28 The required amount of precursor was dis-
possess quite different properties as compared with alkaline solved in deionized water, followed by the addition of SBA-15
earth metal oxides. However, the generation of strong basicity under stirring for 24 h. The mixture was evaporated at 80 ꢀC
on mesoporous silica through alkaline metal oxide modica- followed by drying at 100 ꢀC for 4 h. The precursor-loaded SBA-
tion remains a challenge up to now.
15 was put in an open glass vial and kept inside an autoclave
Through an in-depth analysis of host and guest properties, containing NH3–H2O solution without direct contact between
we consider that there are two factors which hinder the gener- the solid and solution. The tightly closed autoclave was then
ꢀ
ation of strong basicity on mesoporous silica. The rst factor is heated to 60 C for 0.5 h. Finally, the solid was dried at room
the weak guest–host interactions between the base precursor temperature for 6 h and then at 100 ꢀC for 12 h, and calcined at
(e.g. KNO3) and silica, leading to difficulty in the conversion of 500 ꢀC for 5 h in air to get the sample denoted as ZS(A)-n, where
the base precursor to basic species. According to our previous n is the mass percentage of ZrO2. For impregnation, the
investigation, only a small amount of KNO3 can be decomposed required amount of precursor was dissolved in deionized water,
on silica, even if the sample was activated at a high temperature followed by the addition of SBA-15 under stirring for 24 h. The
of 600 ꢀC.25 The second factor is the poor alkali-resistance of mixture was evaporated at 80 C and then heated at 100 C for
mesoporous silica, which results in the collapse of the meso- 4 h. The solid was calcined at 500 ꢀC in an air ow for 5 h. The
porous structure aer the formation of strongly basic species.24 resultant material was denoted as ZS(I)-n, where n is the mass
Aiming to generate strong basicity on mesoporous silica, both percentage of ZrO2. In the case of solvent-free modication, the
ꢀ
ꢀ
of these two weaknesses must be overcome.
required amount of precursor was manually ground together
In the present study, we developed a dualcoating strategy to with 1 g of SBA-15 sample in a mortar at room temperature for
generate strong basicity on mesoporous silica. Mesoporous about 0.5 h. The resulting homogeneous powder was calcined at
silica SBA-15 was precoated with a layer of ZrO2 prior to the 500 ꢀC in an air ow for 5 h and denoted as ZS(G)-n, where n is
introduction of the base precursor KNO3. The ZrO2 interlayer the mass percentage of ZrO2.
plays a double role by enhancing the interaction of the guest
Introduction of potassium species. The base precursor,
with the host and improving the alkali-resistance of the sili- KNO3, was introduced by wet impregnation. An identical
ceous host. Our results show that the decomposition tempera- amount of KNO3 (25 wt%) was used for all samples. Typically,
ture of the base precursor was lowered apparently, and 0.25 g of KNO3 was dissolved in 15 mL of deionized water, fol-
superbasic sites were successfully formed on mesoporous silica lowed by the addition of 0.75 g of host. Aer stirring atꢀ room
SBA-15. More importantly, we demonstrated that the dispersion temperature for 24 h, the mixture was evaporated at 80 C and
degree of ZrO2 had an important effect on the mesostructure subsequently dried at 100 ꢀC for 4 h. The obtained solid was
and basicity of the resulting materials. By use of the ammonia/ calcined at 550 ꢀC in a N2 ow for 200 min to decompose KNO3.
water-induced hydrolysis (AIH) approach, a well-dispersed ZrO2 The nal samples were respectively represented as KS, KZS(A)-n,
interlayer can be created, and the dispersion degree is obviously KZS(I)-n, and KZS(G)-n for potassium species supported on
higher than that derived from conventional methods (i.e. SBA-15, ZS(A)-n, ZS(I)-n, and ZS(G)-n, where n is the mass
impregnation and grinding). The structural and basic proper- percentage of ZrO2.
ties of the obtained materials were characterized by various
methods. The basic catalytic performance of these new dual-
coating materials was also evaluated.
2.2 Materials characterization
X-ray diffraction (XRD) patterns of the materials were recorded
using a Bruker D8 Advance diffractometer with Cu Ka at 40 kV
2 Experimental section
2.1 Materials synthesis
and 40 mA. The N2 adsorption–desorption ꢀisotherms were
measured using a Belsorp II system at À196 C. The samples
Synthesis of SBA-15. Mesoporous silica SBA-15 was synthe- were degassed at 300 ꢀC for 4 h prior to analysis. The pore
sized according to the reported method.26,27 Three grams of diameter was calculated from the adsorption branch by using
triblock copolymer P123 (EO20PO70EO20) was dissolved in 22.5 g the Barrett–Joyner–Halenda (BJH) method. Transmission
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electron microscopy (TEM) images of the materials were despite the modication methods (i.e. AIH, impregnation, and
captured in a JEM-2010 UHR electron microscope operated at grinding). In general, the intensity of the (100) reection should
200 kV. Fourier transform infrared (IR) spectra of the samples decline with the introduction of metal oxides because of the
diluted with KBr were performed on a Nicolet Nexus 470 spec- decreasing scatter contrast between the pore walls and pore
trometer. UV-visible diffuse reectance (UV-vis) spectra of the space. However, the (100) reection of the samples containing
samples were recorded using a UV-2401PC spectrophotometer, 10–30 wt% of ZrO2 derived from the AIH method is obviously
and BaSO4 was used as an internal standard.
more intense than that of parent SBA-15. This can be attributed
The base strength (H ) of the materials was detected by to the formation of a smooth ZrO2 layer on the internal walls of
À
using a series of Hammett indicators.29 The amount of basic SBA-15, similar to what happened in CuO- or Y2O3-coated SBA-
sites was measured by standard acid–base titration.30 Temper- 15.31–33 Such a smooth layer contributes to the spatial order,
ature-programmed desorption of CO2 (CO2-TPD) was performed giving rise to the diffraction line itself. Interestingly, the
using a BELSORP BEL-CAT-A instrument. The samples were intensity of the (100) reection decreases in the order ZS(A)-30 >
pretreated at 550 ꢀC for 200 min under N2 ow prior to the ZS(I)-30 > ZS(G)-30, despite the same Zr content. That means
adsorption of CO2 at room temperature. Aer the physically that ZrO2 coated by the AIH method has the highest dispersion
adsorbed CO2 was purged by a He ow (99.999%) at room degree, while aggregated ZrO2 may be formed in the sample
temperature, the sample was heated to 800 ꢀC, and the CO2 from grinding, as can be proven by the wide-angle XRD results.
liberated was detected by mass spectrometry.
As shown in Fig. 1B, only a broad peak at 2q of 23ꢀ, originating
from amorphous silica, is detected for parent SBA-15. In the
case of ZS(A) and ZS(I), no new diffraction peak emerges.
However, the diffraction peaks ascribed to tetragonal ZrO2
(JCPDS no. 50-1089) become visible for ZS(G),29 which reects
the dispersion degree of ZrO2 in different samples and conrms
the low-angle XRD patterns.
Fig. S1† presents the N2 adsorption–desorption results for
different samples. The isotherms of all Zr-containing samples
are of type IV with an H1 hysteresis loop, similar to that of
parent SBA-15. This reects that the mesoporous structure is
well preserved aer the introduction of ZrO2. It is worth noting
that the samples ZS(I)-30 and ZS(G)-30 give a tail in the
isotherms. The hysteresis remains open up to a relative pressure
of 0.50 for ZS(I)-30 and 0.44 for ZS(G)-30, which is apparently
delayed in contrast to ZS(A)-30, which closes at a relative pres-
sure of 0.64. This indicates that the introduction of ZrO2 by
the AIH method can form a smoother layer, resulting in less-
blocked mesopores.34
2.3 Catalytic tests
Dimethyl carbonate (DMC) was synthesized from the trans-
esterication of ethylene carbonate and methanol. In a typical
process, methanol (0.5 mol), ethylene carbonate (0.1 mol), and
catalyst (0.5 wt% of methanol) were added to a 50 mL three-
necked glass ask with a water-cooled condenser. All catalyÀst1s
were pretreated at 550 ꢀC in a N2 ow (99.999%, 30 mL min
)
for 200 min prior to reaction. The reaction was conducted at
65 ꢀC with stirring for a given period of time. Aer the reaction
was nished, the products and unreacted substrates were
recovered from the ask and subjected to centrifuging. The
obtained upper liquid was then analyzed by a Varian 3800 gas
chromatograph equipped with a ame ionization detector.
3 Results
3.1 Coating ZrO2 on mesoporous silica
UV-vis spectroscopy is known to be a very sensitive probe for
the characterization of metal oxide dispersion and metal ion
coordination environment. Valuable information can be
obtained from the position of absorption bands. As displayed in
Fig. 2, the sample ZS(A)-30 exhibits an absorption band at
205 nm, which corresponds to ligand-to-metal charge transfer
Fig. 1A shows the low-angle XRD patterns of parent SBA-15 and
ZrO2-coated SBA-15 samples. The (100), (110), and (200) reec-
tions can be well discerned aer the introduction of ZrO2,
indicating that the ordered mesostructure is well preserved
Fig. 1 (A) Low-angle and (B) wide-angle XRD patterns of (a) SBA-15, (b) ZS(A)-5,
(c) ZS(A)-10, (d) ZS(A)-20, (e) ZS(A)-30, (f) ZS(A)-40, (e1) ZS(I)-30, and (e2) ZS(G)-30
samples.
Fig. 2 UV-vis spectra of (a) reference ZrO2, (b) ZS(I)-30, (c) ZS(A)-30, and (d)
ZS(G)-30 samples.
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from an O2À to a highly dispersed Zr4+ ion.28 In the case of ZS(I)- functionalized samples. In the absence of ZrO2, the mesoporous
30, the band appears at 210 nm. This red-shi of absorption structure of pure SBA-15 is destructed by strongly basic K2O
mirrors a decrease in dispersion of Zr species and an increase in formed in the process of activation. Similarly, coating 5–20 wt%
the coordination number, probably owing to the formation of of ZrO2 by the AIH method fails to protect the siliceous host as
some Zr–O–Zr bonds. On the contrary, reference ZrO2 shows an well, and the mesostructure of SBA-15 seldom remains. When
intense and broad band at 230 nm due to full connectivity of the amount of ZrO2 reaches 30 wt%, the main peak for a
Zr–O–Zr linkages,35 revealing the aggregation of Zr species in hexagonal lattice emerges in the low-angle XRD pattern.
bulk oxide. A broad band at 210–230 nm can be observed in the Apparently, the existence of a ZrO2 interlayer on SBA-15 avoids
spectrum of ZS(G)-30, indicating that at least part of the ZrO2 is the contact of strongly basic potassium species with the sili-
aggregated in the sample. On the basis of the aforementioned ceous frameworks, preventing the mesostructure from
results, it is concluded that the dispersion degree of ZrO2 in the corroding. For the three samples with an identical Zr content,
samples derived from different methods decreases in the order the mesostructure decreases in the order KZS(A)-30 > KZS(I)-30 >
ZS(A) > ZS(I) > ZS(G).
KZS(G)-30. These results demonstrate that the effect of ZrO2 on
The dispersion of ZrO2 can also be related to the amount of the protection of the host structure is not only related to the
residual silanol groups (Si–OH) in the samples aer ZrO2 amount, but also to the dispersion degree.
modication.31,32 The bending vibrations of Si–OH give rise to
Fig. 4 presents the N2 adsorption–desorption data of the
the band at 960 cmÀ1, which can be easily identied in parent samples aer potassium loading. For the samples coated with
SBA-15 (Fig. S2†). The introduction of ZrO2 by the AIH method 0–20 wt% of ZrO2, the N2 uptake in the mesoporous region is
leads to an obvious decrease of the band at 960 cmÀ1. Such a negligible, indicative of the damage to the mesostructure. It is
band was almost invisible when the ZrO2 loading exceeds 20 wt worth noting that an obvious hysteresis loop is observed in the
%. This is due to the interaction between ZrO2 and SBA-15, isotherm of KZS(A)-30, which reects the preservation of the
leading to the consumption of silanol groups and subsequently mesoporous structure aer precoating 30 wt% of ZrO2 through
the dispersion of ZrO2 on the surface.32,36 It is noticeable that the AIH method. In the case of KZS(I)-30, the shape of the
the amount of silanol groups is different for ZS(A)-30, ZS(I)-30, isotherm changes and the hysteresis extends to high pressures,
and ZS(G)-30 in spite of the same Zr content. The band at suggesting partial destruction of the ordered mesoporous
960 cmÀ1 is obviously more intense in the IR spectrum of ZS(G)- networks. According to the isotherm of KZS(G)-30, it is clear that
30, which indicates that fewer silanol groups were consumed in its mesostructure was severely destroyed, in spite of it having
the process of grinding, leading to the aggregation of ZrO2. The the same Zr content as KZS(A)-30 and KZS(I)-30.
IR results thus provide further evidence of the dispersion degree
TEM is an important technique to characterize mesostruc-
of ZrO2-coated SBA-15 prepared by the three different methods ture. The images of typical samples aer potassium loading
(i.e. AIH, impregnation, and grinding).
were taken and shown in Fig. 5. The sample KZS(A)-30 exhibits a
highly ordered mesostructure, implying that the structure of
parent SBA-15 is well maintained. In comparison with KZS(A)-
30, the periodic ordering of KZS(I)-30 is worse and the meso-
structure is partially damaged. For the sample KZS(G)-30, the
pore system of SBA-15 is completely destroyed, while some large
pores are generated due to the corrosion of the siliceous host by
strongly basic species. It is worth noting that sporadic meso-
structure in a small area can be observed in the sample KS
without a ZrO2 coating. This is due to the fact that the majority
3.2 Structural characterization of mesoporous solid base
To obtain mesoporous solid bases, KNO3 was introduced to
SBA-15 precoated with ZrO2. Strongly basic species K2O are
supposed to be generated from the decomposition of
KNO3 during activation. Structural characterization of the
resultant materials was performed by various techniques. Fig. 3
illustrates the low-angle XRD patterns of the potassium-
Fig. 4 (A) N2 adsorption–desorption isotherms and (B) pore size distributions of
(a) KS, (b) KZS(A)-5, (c) KZS(A)-10, (d) KZS(A)-20, (e) KZS(A)-30, (f) KZS(A)-40, (e1)
KZS(I)-30, and (e2) KZS(G)-30 samples.
Fig. 3 Low-angle XRD patterns of (a) KS, (b) KZS(A)-5, (c) KZS(A)-10, (d) KZS(A)-
20, (e) KZS(A)-30, (f) KZS(A)-40, (e1) KZS(I)-30, and (e2) KZS(G)-30 samples.
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diffraction lines are still evident. That means that the decom-
position of KNO3 on pure mesoporous silica is quite difficult,
even at high temperature of 550 ꢀC. Similar results were also
reported previously for KNO3 supported on bulk silica.25
Coating 5 wt% of ZrO2 leads to a sharp decrease of crystalline
KNO3. When the content of ZrO2 is higher than 20 wt%, the
diffraction lines of KNO3 disappear for the samples derived
from both AIH and impregnation methods. However, the
characteristic peaks for KNO3 are still visible in the XRD pattern
of KZS(G)-30 prepared by grinding, revealing the existence of
residual KNO3. Apparently, the ZrO2 layer modies the surface
properties of SBA-15 and subsequently promotes the decom-
position of supported KNO3. Moreover, the effect of an inter-
layer strongly depends on the dispersion degree of ZrO2.
IR spectra of different samples before and aer activation at
550 ꢀC were recorded to examine the decomposition of KNO3. As
shown in Fig. S3,† all samples before activation reveal an
obvious band at 1383 cmÀ1 that can be attributed to the
asymmetric stretching vibrations of N–O in nitrate.38–40 Activa-
tion only slightly weakens the band at 1383 cmÀ1 in the sample
KS, reecting that a large amount of KNO3 remains undecom-
posed in the sample. In the case of the KZS(I)-30 and KZS(G)-30
samples, the intensity of such a band decreased signicantly
Fig. 5 TEM images of (A) KS, (B) KZS(A)-30, (C) KZS(I)-30, and (D) KZS(G)-30.
Scale bars represent 50 nm.
of KNO3 cannot be decomposed during activation under the aer activation under the same activation conditions, while the
same conditions, as will be discussed later. On the basis of the amount of residual KNO3 in KZS(G)-30 is larger than that in
above-mentioned results, it is clear that the mesostructure KZS(I)-30. It is worth noting that the characteristic band of
decreases in the order KZS(A)-30 > KZS(I)-30 > KZS(G)-30. This nitrate disappears in the sample KZS(A)-30, indicating that
order is in good agreement with the dispersion degree of ZrO2 in KNO3 is completely decomposed by activation. The difference
SBA-15, pointing out that a well-dispersed ZrO2 layer is bene- spectra of the samples before and aer activation were provided
cial for the protection of mesostructure.
in Fig. S4.† The decomposition of KNO3 on different supports is
well reected by the difference spectra. A large absorbance
change at 1383 cmÀ1 is observed for the sample KZS(A)-30
before and aer activation, which is in agreement with the
complete decomposition of KNO3. On the contrary, the differ-
ence between the sample KS before and aer activation is very
small, indicating that the majority of KNO3 does not decom-
pose. Therefore, the decomposition of KNO3 on different
supports is in line with the dispersion degree of ZrO2, namely
KZS(A)-30 > KZS(I)-30 > KZS(G)-30.
3.3 ZrO2-promoted decomposition of KNO3
Fig. 6A shows the wide-angle XRD patterns of potassium-con-
taining samples before activation. The characteristic diffraction
lines of KNO3 in the orthorhombic phase (JCPDS no. 74-2055)
and/or hexagonal phase (JCPDS no.76-1693) are detected.25,37
Activation leads to the conversion of the orthorhombic phase to
the hexagonal one in the sample KS (Fig. 6B), while the
Fig. 6 Wide-angle XRD patterns of (a) KS, (b) KZS(A)-5, (c) KZS(A)-10, (d) KZS(A)-
20, (e) KZS(A)-30, (f) KZS(A)-40, (e1) KZS(I)-30, and (e2) KZS(G)-30 samples (A)
before and (B) after activation. Pand A denote KNO3 of orthorhombic and
hexagonal phases, respectively.
Fig. 7 (A) TG and (B) DTG curves of (a) KS, (b) KZS(A)-30, (c) KZS(I)-30, and (d)
KZS(G)-30 samples before activation. TG and DTG curves are plotted offset for
clarity.
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Aiming to have deep insight into the decomposition
The amount of basic sites on parent SBA-15 is 0 mmol gÀ1
.
behavior of KNO3 on ZrO2-coated SBA-15, the TG technique was Coating ZrO2 leads to a slight increase in the amount of basic
utilized. Fig. 7 depicts the TG and DTG curves of KS, KZS(A)-30, sites due to the amphoteric properties of ZrO2. For instance, the
KZS(I)-30, and KZS(G)-30 samples before activation. All sup- amount of basic sites for the sample ZS(A)-30 is 0.20 mmol gÀ1
.
ported samples give a weight loss centered at around 100 ꢀC For the sample KZS(A)-30, the amount of basic sites reaches
ascribed to the desorption of physically adsorbed water. For the 2.54 mmol gÀ1, which is consistent with the theoretical value
unsupported sample, the decomposition oꢀf bulk KNO3 gives a (2.47 mmol gÀ1) supposing that all supported KNO3 is converted
single weight loss centered at around 700 C (Fig. S5†). In the to K2O. However, the amount of basic sites on KZS(I)-30 and
case of KNO3 supported on SBA-15, the decomposition can be KZS(G)-30 is only 2.18 and 0.85 mmol gÀ1 due to the reaction of
tentatively assigned to two stages, while the DTG peak at about strongly basic species with silica and/or incomplete decompo-
700 ꢀC is much more intense. This indicates that the loading of sition of KNO3.
KNO3 on SBA-15 has no obvious effect on the decrease of
The CO2-TPD technique was employed to further examine
decomposition temperature. However, the coating of ZrO2 the basicity of the potassium-modied samples (Fig. 8). An
results in an apparent decrease of the decomposition temper- obvious desorption peak at about 790 ꢀC emerges for the sample
ature of KNO3. The decomposition occurs at around 500, 530, KZS(A)-30. The desorption of CO2 at such a high temperature
and 560 ꢀC for KNO3 supported on ZS(A)-30, ZS(I)-30, and demonstrates the existence of superbasic sites, which conrms
ZS(G)-30, respectively. Obviously, the coating of a ZrO2 layer the results for base strength. In addition, a broad desorption
ꢀ
promotes the decomposition of KNO3 at much lower temper- peak at 150 C was observed in the sample KZS(A)-30. Such a
atures. Among various supports, KNO3 supported on ZS(A)-30 peak is also detected on the other two samples containing ZrO2.
derived from the AIH method has the lowest decomposition The samples KZS(I)-30 and KZS(G)-30 show a desorption peak at
temperature, which is 200 ꢀC lower as compared with KNO3 on high temperatures as well, whereas the desorption tempera-
pure SBA-15.
tures are obviously lower as compared with KZS(A)-30. The
sample KS only shows a weak, broad desorption peak centered
at about 250 ꢀC, owing to the reaction of strongly basic species
with silica.
3.4 Basicity and catalytic performance of dualcoating
materials
The obtained solid bases were also applied to catalyze the
Aer the decomposition of supported KNO3 to K2O, strongly synthesis of DMC via the transesterication of ethylene
basic sites are supposed to be produced. The basic properties of carbonate and methanol.43,44 As a versatile green chemical, DMC
the resultant materials were rst characterized by the detection of has been proposed as a methylating agent instead of methyl
the base strength and amount of basic sites. Parent SBA-15 halides and dimethyl sulfate, which are toxic and corrosive.
exhibits a base strength (H ) of less than 9.3. Coating ZrO2 on DMC is also used as a carbonylating agent, polar solvent, and
À
SBA-15 does not affect such a base strength. Aer functionaliza- fuel additive. Conventionally, the synthesis of DMC is carried out
tion with potassium species, the resulting materials derived from in the presence of homogeneous basic catalyst, whereas the
different supports show quite different base strengths. The development of heterogeneous catalysts attracts much attention
samples KS, KZS(A)-5, and KZS(A)-10 possess a base strength of recently. Fig. 9 shows the catalytic performance of different
9.3. For the sample KZS(A)-20, a base strength of 15.0 is detected. samples. Under the catalysis of KS, the yield of DMC is only 9.8%
That means that no strongly basic sites are formed when the at a reaction time of 4 h. However, the yield of DMC reaches
content of ZrO2 is lower than 20 wt%. Interestingly, basic sites 28.4% on KZS(A)-30 under the same reaction conditions. This
with a high strength of 27.0 emerge on samples KZS(A)-30 and demonstrates the importance of a ZrO2 layer coated by the AIH
KZS(A)-40. According to the denition,41,42 a solid material that method. About 22.5% and 15.7% of DMC is yielded over KZS(I)-
has basic sites with a strength higher than 26.5 can be regarded 30 and KZS(G)-30, which is lower than that over KZS(A)-30, but
as a solid superbase. This is direct evidence of the generation of
superbasicity on potassium-functionalized SBA-15 precoated
with ZrO2 by the AIH method. Such a high strength is also
detected on the sample KZS(I)-30 prepared by impregnation,
while a base strength of 18.4 is measured on the sample KZS(G)-
30 derived from grinding. In terms of the results above, it is clear
that the amount of ZrO2 should be large enough to form an intact
interlayer between the basic species and silica. In other words,
the ZrO2 layer should cover the surface of silica completely;
otherwise, the reaction of strongly basic species with silica may
take place, leading to the formation of materials with weak
basicity and a collapsed mesostructure. Although the sample
KZS(G)-30 contains the same amount of ZrO2 as KZS(A)-30,
aggregation takes place for ZrO2 introduced by grinding. Some
silica surface uncovered by ZrO2 is thus exposed, and the reaction
of strongly basic species with silica is still unavoidable.
Fig. 8 CO2-TPD profiles of (a) KZS(A)-30, (b) KZS(I)-30, (c) KZS(G)-30, and (d) KS
samples.
1628 | J. Mater. Chem. A, 2013, 1, 1623–1631
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Journal of Materials Chemistry A
Fig. 9 The yield of DMC as a function of reaction time on (a) KZS(A)-30, (b)
Scheme 1 Comparison of potassium species loaded on SBA-15 as well as on
ZrO2-coated SBA-15 prepared through different approaches.
KZS(I)-30, (c) KZS(G)-30, and (d) KS samples.
higher than that over KS. On the basis of the results above, it is
evident that the catalytic performance of different samples is
strongly dependent on their mesostructure and basicity.
KZS(A)-30. The use of ZrO2 as a promoter to enhance the guest–
host interactions is associated with its electronegativity.
According to the reported method,46 the electronegativity of the
cations in ZrO2 and SiO2 can be calculated to be 11.97 and 17.10,
respectively. With the reduction of electronegativity, the metal–
oxygen interactions become weaker and the coordination ability
4 Discussion
Conventionally, a solid strong base can be prepared by intro- of lattice oxygen increases.47 Because the electronegativity of Zr4+
duction of a base precursor to a host and subsequent calcina- is obviously smaller than that of Si4+, the coordination ability of
tion to decompose the precursor, forming strongly basic sites. oxygen in ZrO2 with the precursor should be higher. Therefore,
The same approach has also been attempted to generate strong coating mesoporous silica with ZrO2 enables the surface to
basicity on mesoporous silicas. However, two difficulties hinder provide enhanced guest–host interactions, weakening the bond
the application of such a classic approach when mesoporous of K+ and NO3À and promoting the decomposition of KNO3. As a
silicas are employed as hosts. On the one hand, the decompo- result, the decomposition of KNO3 can take place at lower
sition of the precursor is quite difficult because of the weak temperatures on SBA-15 by precoating ZrO2.
interactions between the precursor and silica. The majority of
In addition to promoting the decomposition of KNO3, ZrO2
KNO3 cannot be decomposed on SBA-15, even at a high coating also improves the alkali-resistance of the host. The
temperature of 550 ꢀC, as described above. The increase of formation of a ZrO2 interlayer avoids the direct contact of basic
temperature to 700 ꢀC can lead to the decomposition of KNO3, sites with the siliceous frameworks, thus preventing the host
while such a high temperature will accelerate the reaction of from being corroded by strongly basic species. As a result, the
strongly basic species with silica and destroy the mesostructure mesoporous structure of the resultant materials is well
completely. On the other hand, the alkali-resistance of silica is preserved and mesoporous solid superbases are successfully
poor, which is different from zirconia and alumina which are prepared. It should be stated that the amount of ZrO2 is of great
well-known hosts for solid strong bases.29,42,45 The mesoporous importance for the protection of the mesoporous structure. The
structure can be severely damaged due to the reaction between amount of ZrO2 should be large enough to form an intact layer
strongly basic species and silica, even if the amount of basic (at least a monolayer) on the surface of SBA-15. Otherwise, the
species is quite small, as shown above. Hence, it is difficult to reaction of strongly basic species with silica is still inevitable
generate strong basicity on mesoporous silicas by the classic due to the exposed silica surface that is not covered by ZrO2.
approach, namely direct incorporation of precursor followed Assuming that each ZrO2 molecule exists in the tetragonal
by calcination.
form with a unit cell constant (a0) of 0.5128 nm, its coverage is
To overcome the difficulties in basicity generation on meso- 0.2630 nm2. Therefore, for the formation of a monolayer on
porous silica, a dualcoating strategy was developed in the SBA-15 with a surface area of 690 m2 gÀ1, about 35 wt% of ZrO2
present study. A layer of ZrO2 was precoated on mesoporous is required.48 Taking into consideration that some small
silica before loading the base precursor KNO3 (Scheme 1). The micropores are inaccessible to zirconium compounds, the
ZrO2 interlayer plays a double role by enhancing the guest–host amount of ZrO2 for the formation of a monolayer should be
interactions to promote the decomposition of the precursor and lower than 35 wt%. According to the present investigation, the
by preventing the siliceous frameworks from corroding by the mesostructure can be preserved when the content of ZrO2
formed basic species. In comparison with unmodied SBA-15, exceeds 30 wt% for the AIH method, which is consistent with
the decomposition temperature of KNO3 was obviously lowered the theoretical value. Hence, aiming to effectively protect
on ZrO2-coated SBA-15. All of the loaded KNO3 can be converted the host structure, the amount of ZrO2 should be higher than
to the strongly basic species K2O aer activation for the sample 30 wt% from the viewpoint of forming an intact layer.
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Paper
It is interesting to note that the dispersion degree of ZrO2 the amount of ZrO2 is higher than 30 wt%, an intact layer can
plays an essential role in the preparation of mesoporous solid be produced by use of the AIH method. Nonetheless, aggre-
bases. Besides the AIH method, the introduction of ZrO2 by gation takes place if ZrO2 was introduced by impregnation or
impregnation and grinding was also carried out. However, the grinding, which is not benecial for the protection of the
resultant samples aer potassium functionalization exhibit mesostructure.
quite different properties. As described above, the sample
KZS(A)-30 exhibits an ordered mesostructure. Nevertheless, the
mesoporous structure of KZS(I)-30 was partially damaged and
Acknowledgements
that of KZS(G)-30 was destroyed completely despite an identical We acknowledge nancial support of this work by the National
ZrO2 content. Correspondingly, their basicity and catalytic Science Foundation of China (nos. 21006048 and 20976082), the
performance decrease in the same order. In terms of the results Specialized Research Fund for the Doctoral Program of Higher
from various characterization techniques, it is clear that the Education of China (no. 20093221120001), China Postdoctoral
dispersion degree of ZrO2 in the materials derived from Science Foundation (no. 20110491406), Jiangsu Planned
different methods declines in the order ZS(A) > ZS(I) > ZS(G). Projects for Postdoctoral Research Funds (no. 1101155C), and
The good dispersion of ZrO2 by the AIH method is related to the the Priority Academic Program Development of Jiangsu Higher
pre-hydrolysis of the precursor ZrO(NO3)2 in the presence of Education Institutions.
ammonia/water vapor. In such a low-temperature process, the
intermediate Zr(OH)4 tends to distribute inside the pores
evenly, resulting in a composite with a uniform coating of ZrO2
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