A new redox strategy for low-temperature formation of strong basicity on mesoporous silica

Article abstract of DOI:10.1039/c5cc02502h

A redox strategy was designed to generate strong basicity on mesoporous silica by using the redox interaction of a precursor with methanol vapor. The formation of strongly basic sites was realized at 400 °C, which breaks the tradition of thermally induced decomposition that usually requires much higher temperatures (>600 °C).

Full text of DOI:10.1039/c5cc02502h

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A new redox strategy for low-temperature
formation of strong basicity on mesoporous
Cite this:DOI: 10.1039/c5cc02502h
silica†
Received 26th March 2015,
Accepted 12th May 2015
Li Zhu, Feng Lu, Xiao-Dan Liu, Xiao-Qin Liu and Lin-Bing Sun*
DOI: 10.1039/c5cc02502h
www.rsc.org/chemcomm
A redox strategy was designed to generate strong basicity on meso- temperatures are compulsory for such a well-known method. For
porous silica by using the redox interaction of a precursor with example, the decomposition of base precursor KNO3 to basic
methanol vapor. The formation of strongly basic sites was realized species K O on silica only occurred at temperatures higher than
2
7
at 400 8C, which breaks the tradition of thermally induced decom- 600 1C. Apparently, energy consumption in the decomposition
position that usually requires much higher temperatures (4600 8C). step is very high. Moreover, at such high temperatures the reactions
of strongly basic species with siliceous supports are almost inevit-
For the demands of sustainable development and green chem- able. As a result, the obtained materials only exhibit weak basicity
istry, increasing attention has been paid to the replacement of and the mesostructure of supports is seriously damaged. Despite
conventional homogeneous catalytic processes with hetero- the efforts, the development of a facile method to create strong
1
geneous ones. Among various heterogeneous catalysts, meso- basicity on mesoporous silica remains a great challenge to date.
porous solid strong bases are of great interest due to some
It is known that as frequently used precursors, nitrates also
remarkable advantages such as no corrosion, facile separation, and possess oxidizability. It is therefore possible to convert precursors to
2
negligible waste production. Their large pore openings favor mass their corresponding functional oxides through the interaction with
transport and avoid deactivation resulting from coke formation that appropriate reducing agents. Herein, we report for the first time
3
usually takes place in microporous catalysts. Furthermore, meso- the utilization of a redox strategy to construct strong basicity on
porosity plays a significant role in catalysis involving bulky substrates mesoporous silica, which breaks the tradition of thermally induced
and products (e.g. biomolecules). To date great efforts have been decomposition of base precursors. The first attempted reducing
devoted to fabricate strong basicity on mesoporous materials. Owing agent is methanol, whose vapor is designed to diffuse into the pores
to the low cost, good stability, and high synthetic controllability, of support and interact with predispersed KNO (Scheme 1). It is
3
mesoporous silica is the optimal choice of support among various fascinating that the redox interaction makes the conversion of KNO
3
4
candidates. Hence, the generation of strong basicity on mesoporous occur at the temperature of about 400 1C on mesoporous silica
silica has attracted massive interest ever since its discovery. SBA-15, which is obviously lower than that of the conventional
By grafting organic bases or incorporating nitrogen-containing thermal decomposition of KNO (ca. 600 1C). Accordingly,
3
species, some interesting basic sites can be formed on mesoporous strong basicity is successfully generated on mesoporous silica
5
silica. Nonetheless, the basicity of these materials is relatively weak and the ordered mesostructure is well maintained, which are
because of the inherent nature of basic species. To enhance the
base strength, the introduction of metal oxides with strong basicity
(
2
e.g. K O) was also tried; this method has been widely used for the
6
construction of strong basicity on a range of porous supports. The
preparation includes loading of base precursors (e.g. the alkali
metal nitrate, KNO
precursors to their corresponding metal oxides (e.g. K
3
) and subsequent calcination to decompose the
O). It is
2
worth noting that when silica is utilized as the support, quite high
State Key Laboratory of Materials-Oriented Chemical Engineering, College of
Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009,
China. E-mail: lbsun@njtech.edu.cn; Fax: +86-25-83587191; Tel: +86-25-83587177
Scheme 1 Conversion of the base precursor to strongly basic sites on
mesoporous silica through (A) the conventional thermal method as well as
(B) the redox strategy.
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cc02502h
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extremely desirable for catalysis and unlikely to be realized noticeable that the band is hard to identify on the KS-R400-2
through traditional methods. Of course, the redox strategy is sample, indicating the complete conversion of KNO at 400 1C
3
capable of saving energy since the preparation temperature is using the redox strategy. Two crystalline phases (i.e. orthorhombic
evidently declined. We also demonstrate that the resultant meso- and hexagonal phases) of KNO were formed on KS as displayed in
3
porous solid bases exhibit excellent catalytic activity in hetero- the wide-angle X-ray diffraction (XRD) patterns (Fig. 1B). The
geneous transesterification reactions, which is much superior to diffraction lines are still visible in the KS-T400-2 sample, while they
the catalysts prepared through traditional methods as well as disappear in the KS-T600-2 sample that was treated thermally at
various typical solid strong bases.
600 1C. In the case of samples prepared using the redox strategy, the
The base precursor KNO was introduced into the mesoporous diffraction lines become weak gradually with the increase of treat-
3
silica SBA-15 by impregnation (see ESI† for experimental details), ment time even at 400 1C (Fig. 1B and Fig. S2, ESI†). Also, no
and the obtained material was denoted as KS. For the conversion of diffraction lines of KNO can be detected on the KS-R400-2 sample.
3
KNO using the redox strategy, KS was heated to 400 1C in a nitrogen The wide-angle XRD patterns thus confirm the IR results, pointing
3
flow containing the methanol vapor generated by bubbling. The out that base precursor KNO can be converted efficiently through
3
resultant materials were denoted as KS-R400-n, where n represents the redox strategy at the temperature as low as 400 1C.
the redox time varied from 0.5 to 2 h. In a similar process, conven-
The structural characterization of the samples was performed by
tional thermal decomposition of KNO was carried out. All the various techniques. In low-angle XRD patterns (Fig. 1C and Fig. S3
3
conditions were identical to that in the redox process except that and S4, ESI†), all of the samples from redox show an intense
no methanol vapor was involved. The materials obtained from diffraction line accompanied by two weak ones, which can be
thermal treatment at 400 and 600 1C were denoted as KS-T400-n indexed as the (100), (110) and (200) reflections corresponding to
and KS-T600-n, respectively, where n represents the treatment time. the 2D hexagonal pore regularity of the p6mm space group. This
Infrared (IR) spectroscopy was first employed to examine the suggests that the ordered mesostructure is well preserved. Due to the
À1
conversion of KNO
3
(Fig. 1A). An intense band at 1383 cm
3
limited decomposition of KNO , the diffraction lines in low-angle
8
assigned to nitrate can be observed on the sample KS, which is XRD patterns are also observable on the KS-T400-2 sample. However,
absent on pristine SBA-15 before the introduction of KNO . After no diffraction lines can be recognized on the KS-T600-2 sample,
3
thermal treatment at 400 1C for 2 h (KS-T400-2), the band is only implying the complete destruction of the mesostructure caused by
slightly weakened, reflecting the existence of a great deal of unde- the reactions between the formed strongly basic species and silic-
composed KNO . Further enhancing the treatment temperature to eous frameworks. Nitrogen adsorption data also give information on
3
600 1C (KS-T600-2), the intensity of the nitrate band decreases the structure of materials (Fig. S5–S8, ESI†). The samples from redox
evidently. That means, supported KNO can be thermally decom- present isotherms of type IV with an H1 hysteresis loop, indicating
3
posed at 600 1C, whereas the mesostructure is destroyed completely that the cylindrical mesopores of SBA-15 are well maintained.
as discussed later. Interestingly, the conversion of KNO is able to Nevertheless, the KS-T600-2 sample displays neglectable N uptake,
3 2
proceed smoothly at 400 1C by the use of the redox strategy. The which gives further evidence of the collapse of the mesostructure.
intensity of the nitrate band drops evidently and keeps declining Transmission electron microscopy (TEM) is another useful techni-
with the increase of treatment time (Fig. 1A and Fig. S1, ESI†). It is que to characterize the long-range channel ordering of mesoporous
materials (Fig. 1D and Fig. S9, ESI†). The images of KS-R400-2 show
that the periodic ordering of the mesostructure is well preserved.
The results of energy dispersive X-ray spectroscopy (EDX) elemental
mapping demonstrate the presence of silicon, potassium, and
oxygen, and potassium is uniformly dispersed (Fig. S10, ESI†). That
2
means, well-dispersed K O can be formed over the silica support. On
the basis of aforementioned results, it is conclusive that the redox
strategy benefits the preservation of the ordered mesostructure and
that thermal treatment at 600 1C results in the complete damage of
the mesostructure although supported KNO can be decomposed.
3
The basicity of resultant materials was first characterized by the
amount of basic sites (Fig. S11, ESI†). As for the samples prepared
through the thermal method, the amount of basic sites is only
À1
À1
0
.52 mmol g (KS-T400-2) and 0.02 mmol g (KS-T600-2), owing to
the incomplete decomposition of KNO and/or the reaction of
3
strongly basic species with siliceous support. Remarkably, for the
sample after the redox treatment for 0.5 h (KS-R400-0.5), the amount
À1
of basic sites increases sharply to 1.29 mmol g . The amount of
basic sites keeps increasing with the increase of treatment time and
reaches 1.87 mmol g in the case of the sample treated for 2 h (KS-
Fig. 1 (A) IR spectra, (B) wide-angle XRD patterns, and (C) low-angle XRD
patterns of SBA-15 as well as KS before and after thermal or redox treatment.
À1
R400-2). Notably, this value is quite close to the theoretical value
(1.98 mmol g ), thus suggesting the nearly complete conversion of
,
andE denote KNO
3
of orthorhombic and hexagonal phases, respectively.
À1
(D) A typical TEM image of the KS-R400-2 sample.
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2
Fig. 2 CO -TPD profiles of KS-R400-2, KS-T400-2, and KS-T600-2.
Fig. 3 The yield of DMC via the transesterification of ethylene carbonate
and methanol under the catalysis of different samples.
KNO
desorption of CO
basicity (Fig. 2). The desorption peaks can be tentatively divided into respectively. Strongly basic metal oxides were loaded on famous
three parts denoted as a, b, and g at approximately 100, 350, and porous supports Al and ZrO , producing solid strong bases
00 1C, respectively, which are attributed to weak, medium, and Li O/Al O , Na O/Al O and CaO/ZrO . The three catalysts give the
3
to basic sites via the redox strategy. Temperature programmed
2
(CO -TPD) was employed to further evaluate the basicity, namely KL and CsX, give the DMC yield of 4.0% and 6.1%,
2
2
O
3
2
7
2
2
3
2
2
3
2
strong basicity. Both the samples derived from thermal treatment DMC yield of 18.8%, 28.2%, and 15.1%, respectively. CaO has been
KS-T400-2 and KS-T600-2) give rise to desorption peaks originated regarded as a promising candidate among alkali and alkaline earth
from weak and medium basicity. It is noticeable that the sample metal oxides for generation of strong basicity on mesoporous
(
9b
prepared by the redox strategy (KS-R400-2) shows an intense peak silica. However, over CaO/SBA-15 the yield of DMC is only 13.4%.
caused by strongly basic sites in addition to weak and medium ones. Two mesoporous solid bases prepared using the dual coating
9
a
7
Moreover, the desorption of CO
which is comparable to, if not higher than, some solid superbases,
2
temperature persists up to 800 1C, strategy, i.e. K
2
O/C/SBA-15 and K
2 2
O/ZrO /SBA-15, present 38.6%
9
and 28.4% DMC yield, respectively. Based on these results, it is clear
thus revealing the presence of unusually strong basicity on the KS- that the obtained materials show excellent catalytic performance,
R400-2 sample. CO -TPD profiles of samples derived from different which is superior to a series of well-known solid strong bases and
2
redox times are presented in Fig. S12 (ESI†). With the increase of even superbases with various pore structures.
redox time, some new peaks at medium and high temperatures
3
To investigate the pathway for the conversion of KNO , gaseous
become visible gradually. This clearly demonstrates that the redox products formed in the process were analyzed using a mass spectro-
time affects not only the amount of basic sites but also their meter (MS). The signals with m/z values of 2, 18, 28, 30, and 44 were
strength. According to these results, it is safe to say that the redox detected and can be attributed to H
strategy is pretty efficient for the fabrication of basic sites with regard respectively, regardless of the methods used for KNO
to both amount and strength. (Fig. 4). Two additional signals with m/z values of 31 and 32 were
Our materials were also utilized to catalyze the synthesis of also monitored, and their origins are dependent on the methods.
dimethyl carbonate (DMC) via the transesterification of ethylene For the thermal conversion of KNO , the m/z signal of 32 comes
. However, for the redox conversion of KNO , the m/z signals
been proposed as a polar solvent, methylating and carbonylating of 32, together with 31, should have originated from methanol. As
2
, H
2
O, CO, NO, and CO
2
,
3
conversion
3
10
carbonate and methanol. As a versatile green chemical, DMC has from O
2
3
11
agent, as well as a fuel additive. Traditionally, DMC is synthesized shown in Fig. 4B, both signals (31 and 32) present the same
by the use of homogeneous strong base catalysts, while increasing changing trend. In terms of these MS data, the pathway for the
3
attention has been paid to the development of heterogeneous conversion of KNO can be illustrated. Thermal decomposition of
catalysts recently. As shown in Fig. 3, no DMC is produced at all KNO on SBA-15 mainly takes place at about 600 1C, and the
3
even after the reaction for 4 h over pristine SBA-15. Under the predominant products include NO and O as shown in eqn (1).
2
catalysis of KS-T400-2 and KS-T600-2, the yield of DMC is 12.3%
and 3.2%, respectively. It is worth noting that the samples from
2
KNO
3
+ CH
3
OH - K
2
O + 2NO + CO
2
+ 2H
2
O
(1)
redox exhibit excellent catalytic performance. Even over the sample It is interesting to note that the presence of reducing agent
treated for only 0.5 h (KS-R400-0.5), 26.3% of DMC can be produced methanol leads to a quite different pathway for the conversion
(
Fig. S13, ESI†). With increasing treatment time, the yield of DMC of KNO
3
. The conversion of KNO
3
occurs at the temperature as
2
O become the main gaseous
continues to increase. Surprisingly, when KS-R400-2 is used as the low as 400 1C, and NO, CO
2
, and H
catalyst, the yield of DMC reaches 44.3%, which is much higher products. When the temperature is higher than 500 1C, the
than that over KS-T400-2 and KS-T600-2 despite the identical decomposition of methanol can be observed, as the signals of
potassium content. In order to elucidate the catalytic performance H2 and CO increase sharply while the signals of methanol
of our materials deeply, some well-known solid bases were applied decrease noticeably. In this regard, the redox conversion of
to make a comparison (Table S1, ESI†). Only 7.6% of DMC is KNO
3
on SBA-15 should be operated at temperatures lower than
yielded over the typical solid base MgO. Two zeolites with strong 500 1C. On the basis of these results, it is justified that the redox
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This work was supported by the Distinguished Youth Foun-
dation of Jiangsu Province (BK20130045), the Fok Ying-Tong
Education Foundation (141069), the National High Technology
Research and Development Program of China (863 Program,
2013AA032003), the National Basic Research Program of China
(973 Program, 2013CB733504), and the Project of Priority
Academic Program Development of Jiangsu Higher Education
Institutions.
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3
5
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À1
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2
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1
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À1
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1
3
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À1
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À1
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1
3
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