Fabrication of solid strong bases at decreased temperature by doping low-valence Cr3+ into supports

Article abstract of DOI:10.1016/j.apcata.2019.117153

Mesoporous solid strong bases (MSSBs) have gained tremendous research interest as environment-friendly catalysts in various reactions. Nevertheless, the fabrication of MSSBs remains an enormous challenge because a pretty high temperature is needed to produce strong basicity in traditional mesoporous materials. Here, we report a strategy of endowing supports with reducibility (ESWR) by doping low-valence Cr³⁺ into mesoporous Al₂O₃. The base precursors NaNO₃ can be converted to basic sites completely at the low temperature of 400 °C via the ESWR strategy. The examination of activation mechanism indicates that the redox interaction between Cr³⁺ and NaNO₃ is responsible for the low-temperature conversion of supported NaNO₃. As a result, MSSBs were synthesized successfully by the ESWR strategy, showing strong basicity and remarkable catalytic activity in the transesterification reaction. The catalytic activity is obviously higher than typical solid bases like MgO as well as a variety of reported basic catalysts.

Full text of DOI:10.1016/j.apcata.2019.117153

Accepted Manuscript
Title: Fabrication of solid strong bases at decreased
temperature by doping low-valence Cr3+ into supports
Authors: Song-Song Peng, Ju-Kang Wu, An-Zhong Peng,
Yu-Xia Li, Chen Gu, Ming-Bo Yue, Xiao-Qin Liu, Lin-Bing
Sun
PII:
DOI:
Article Number:
S0926-860X(19)30308-4
https://doi.org/10.1016/j.apcata.2019.117153
117153
Reference:
APCATA 117153
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
23 May 2019
13 July 2019
16 July 2019
Please cite this article as: Peng S-Song, Wu J-Kang, Peng A-Zhong, Li Y-Xia, Gu
C, Yue M-Bo, Liu X-Qin, Sun L-Bing, Fabrication of solid strong bases at decreased
temperature by doping low-valence Cr3+ into supports, Applied Catalysis A, General
(2019), https://doi.org/10.1016/j.apcata.2019.117153
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Fabrication of solid strong bases at decreased temperature by doping
low-valence Cr3+ into supports
Song-Song Penga, Ju-Kang Wua, An-Zhong Penga, Yu-Xia Lia, Chen Gua, Ming-Bo Yueb,
Xiao-Qin Liu*a and Lin-Bing Sun*a
aState Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic
Innovation Center for Advanced Materials (SICAM), College of Chemical Engineering, Nanjing Tech
University, 30 South Puzhu Road, Nanjing 211816, China
b
Key Laboratory of Life-Organic Analysis, School of Chemistry and Chemical Engineering, Qufu Normal
University, 57 West Jingxuan Road, Qufu 273165, China
*Corresponding author. Email: liuxq@njtech.edu.cn, lbsun@njtech.edu.cn.
Graphical Abstract
Highlights
 Mesoporous solid bases are prepared via endowing supports with reducibility by Cr3+
doping.
o
 Strong basicity is formed at 400 C, which is much lower than conventional method
(700 oC).

The obtained mesoporous solid strong bases show high activity in transesterification
reactions.
1

Abstract
Mesoporous solid strong bases (MSSBs) have gained tremendous research interest as
environment-friendly catalysts in various reactions. Nevertheless, the fabrication of MSSBs
remains an enormous challenge because a pretty high temperature is needed to produce strong
basicity in traditional mesoporous materials. Here, we report a strategy of endowing supports
with reducibility (ESWR) by doping low-valence Cr3+ into mesoporous Al2O3. The base
precursors NaNO3 can be converted to basic sites completely at the low temperature of 400 oC
via the ESWR strategy. The examination of activation mechanism indicates that the redox
interaction between Cr3+ and NaNO3 is responsible for the low-temperature conversion of
supported NaNO3. As a result, MSSBs were synthesized successfully by the ESWR strategy,
showing strong basicity and remarkable catalytic activity in the transesterification reaction.
The catalytic activity is obviously higher than typical solid bases like MgO as well as a
variety of reported basic catalysts.
Keywords: Activation temperature; Cr3+ doping; Mesoporous solid strong bases; Reducibility;
Transesterification
1. Introduction
In recent decades, as heterogeneous catalysts, mesoporous solid strong bases (MSSBs)
have drawn much attention in recent decades owing to their easy separation, high activities,
negligible corrosion, few disposal issues, etc.[1-5] These advantages make MSSBs potential
in diverse reactions such as transesterification, Michael addition, Knoevenagel condensation
and halogenation.[6-9] It is known that the pore structure of catalysts has a great effect on the
catalytic performance. In comparison with microporous catalysts, the presence of mesopores
benefits diffusion of reactants/products and prevents active sites from deactivation.[10, 11]
Additionally, the presence of mesoporous structure makes the reactions involving bulky
molecules become possible on MSSBs.[12-15]
Numerous attempts have been made to fabricate MSSBs on various mesoporous
supports.[2] As inorganic oxides with well-defined mesostructure and thermal stability,
2

mesoporous silica was used for the preparation of MSSBs. Some representative approaches
include doping nitrogen and grafting amino groups; however, the obtained materials have
only weak basic sites.[16-18] To enhance the base strength, a lot of alkali metal oxides (such
as K2O) have been loaded on mesoporous silica.[19-22] Nevertheless, the mesostructure of
supports is destroyed due to the reactions between alkali metal oxides and silica during
activation. In addition to mesoporous silica, mesoporous metal oxides have also been used to
fabricate MSSBs by loading alkali metal oxides. For example, mesoporous Al2O3, as a
common support owing to its good availability, can be loaded with alkali metal oxides.[22] In
order to create strong basicity, a base precursor like KNO3 is first introduced to mesoporous
Al2O3 followed by thermal activation to decompose the precursor to basic sites.[23, 24] It
should be stated that quite high temperature (ca. 700 oC) is required to decompose KNO3 on
Al2O3. The high-temperature activation consumes lots of energy and leads to the degradation
of mesoporous structure. This limits the applications of MSSBs seriously. Therefore, it is
highly expected to develop new methods for the generation of strong basicity on mesoporous
materials at a low temperature.
Herein, we report the preparation of MSSBs through a strategy of endowing supports with
reducibility (ESWR) by doping low-valence Cr3+ into mesoporous Al2O3 (Scheme 1). Upon
Cr3+ doping, the base precursor NaNO3 can be converted to strong basic sites on mesoporous
Al2O3 at the temperature of 400 oC. This is much lower than the temperature (700 oC) for
thermal decomposition of NaNO3 on undoped Al2O3. We also demonstrate that the
low-temperature conversion of NaNO3 is caused by its redox interaction with Cr3+, and Cr6+ is
yielded quantitatively as the oxidation product. Our strategy breaks the convention of thermal
pyrolyzation of base precursors that extensively reported in literature. The obtained MSSBs
exhibit strong basicity, well-defined mesostructure, and good catalytic activity in
transesterification, which is superior to typical solid bases such as MgO as well as a variety of
reported basic catalysts containing alkali metal and alkaline earth metal oxides.
3

2. Experimental section
2.1 Chemicals
P123 [(EO)20(PO)70(EO)20], chromium nitrate nonahydrate (Cr(NO3)3·9H2O), sodium
hydroxide (NaOH), and aluminum isopropoxide were purchased from Sigma-Aldrich.
Sodium nitrate (NaNO3), nitric acid, ethanol, ethylene carbonate, methanol, ethanediol,
phenolphthalein were purchased from Sinopharm. All chemicals were used directly without
any further purification. Deionized water was used in all experiments.
2.2 Materials synthesis
2.2.1 Synthesis of Cr-doped mesoporous Al2O3.
Cr-doped mesoporous Al2O3 was synthesized by evaporation-induced self-assemble (EISA)
strategy as follows. In a typical synthesis procedure, 2.0 g P123 was dissolved in absolute
ethanol 40.0 mL with vigorous magnetic stirring. Then, quantitative Cr(NO3)3·9H2O and
aluminum isopropoxide were sequentially added. And then 3.2 mL nitric acid (67.0 wt %) was
added. The obtained mixture was treated at 60 oC for 48 h to volatilize ethanol after stirring
for 5 h at 30 oC. The bluish green materials were ground to powders and calcined in an air
flow at 700 oC for 4 h. The obtained Cr-doped mesoporous Al2O3 were named as MCA. The
molar ratio of Cr/Al fixed in 1/5. The mesoporous Al2O3 without Cr doping was also made by
the same method without the addition of Cr(NO3)3·9H2O, and the obtained material was
labeled as MA.
2.2.2 Preparation of Solid Bases
The base precursor of NaNO3 was loaded onto MA by wet impregnation method. The
procedures are as follows, 0.2 g of NaNO3 (20 wt %) was dissolved into 15 g deionized water
with magnetic stirring. Then 0.8 g of MA was added and after stirring 2 h, the mixture was
evaporated at about 75 oC in a water bath kettle. After that the material was dried at 80 oC for
24 h. The obtained solid was labeled as 20NaMA. By using MCA as the support and adjusting
4

the loading amount of NaNO3, yNaMCA can be obtained, where y represents the mass ratio of
NaNO3 (varied from 5 to 20 wt %). Then the products were activated at a certain temperature
(400 or 700 oC) in an Ar flow for 3 h to convert the supported NaNO3 to basicity. The
obtained samples were labeled as yNaMCA-t (or yNaMA-t), where t denotes the activation
temperature.
2.3 Materials Characterization
The X-ray diffraction (XRD) patterns of the obtained samples were recorded on a Bruker
D8 Advance diffractometer with monochromatic Cu Kα radiation in the 2θ range of 0.6–8°
and 5–80° at 40 kV and 40 mA. The N2 adsorption-desorption isotherms were measured using
an ASAP 2020 system at –196 oC. Prior the test, the samples were evacuated in the degas port
at 200 oC for 4 h. The multipoint Brunauer-Emmet-Teller (BET) surface area was estimated at
the relative pressure ranging from 0.04 to 0.25.[25, 26] The total pore volume was calculated
by using the amount adsorbed at a relative pressure of 0.99. The pore-size distributions of the
mesoporous materials was calculated by the adsorption branch. Fourier transform infrared (IR)
spectra measurements of the materials were recorded on a Nicolet Nexus 470 spectrometer by
means of the KBr pellet technique. The X-ray photoelectron spectroscopy (XPS) spectras
were done on ESCAlab250, Thermal Fischer, and the binding energies were calibrated by
setting the C1s to 284.6 eV.[27] Transmission electron microscopy (TEM) images of the
samples were performed on HT7700 Exalens electron microscope operated at 200 kV. The
field-emission scanning electron microscopy (SEM) was examined by JSM-7001F running at
2 kV. Thermogravimetry-mass (TG-MS) spectrometer was carried out on a thermobalance
(STA-499C, NETZSCH). 10 mg of sample were heated from 25 to 900 oC with the heating
rate of 5 oC min-1 in Ar (20 mL min-1). The gaseous products derived from pyrolyzation of
samples were analyzed by MS. The different samples were characterized by UV-visible
(UV-vis) diffuse reflectance spectra (UV-2401PC), and BaSO4 was used as the standard for
these measurements.[28, 29] The basicity and the basic strength distribution of the catalysts
were characterized by temperature programmed desorption of carbon dioxide (CO2-TPD)
experiments by using a BELSORP BEL-CAT-A apparatus. Before the test, 50 mg of sample
5

was activated at 400 oC with the heating rate of 10 oC min-1 in Ar (20 mL min-1) for 3 h. When
the temperature drops to room temperature, the sample begin absorbing CO2 at 50 oC for 1 h.
After the physical adsorbed CO2 was purged by a He flow (99.999%) at room temperature for
30 min, the sample was heated to 900 oC with the heating rate of 10 oC min-1 and the CO2
liberated was detected by a mass spectrometer (HAL201, HIDEN). To measure the amount of
total basic sites of these samples, 50 mg of catalyst was added into 10.0 mL of
certain concentration aqueous HCl (0.05 mol L-1 ). The mixed liquid was vibrated for 24 h
and the undissolved portion was separated by centrifuge. The serum was titrated with 0.01
mol L-1 aqueous NaOH for determining the excess of acid.
2.4 Catalytic tests
Dimethyl carbonate (DMC) was synthesized via the transesterification reaction between the
ethylene carbonate and methanol.[30-32] The reaction procedures and conditions are as
follows, methanol (16 g), ethylene carbonate (8.8 g), and the catalyst (50 mg) were added into
a 50 mL three necked glass flask with a water cooled condenser device. The reaction was
conducted at 70 oC with magnetic stirring for 4h. After the reaction, the obtained upper liquid
mixture was centrifuged and analyzed via an Agilent 7890A gas chromatography equipped
with a flame ionization detector (FID).
3. Results and Discussion
3.1 Conversion of the base precursor NaNO3
Aiming to manufacture strong basicity, the base precursor of NaNO3 was loaded on the
samples Al2O3 and Cr-doped Al2O3. Strongly basic sites are anticipated to produce from the
conversion of NaNO3 to Na2O. TG-MS was first adopted to characterize the conversion of
NaNO3 on different supports. As shown in Fig. 1a, 20NaMA shows an obvious weight loss at
around 700 oC ascribed to NaNO3 decomposition, indicating the high temperature for the
formation of basicity on undoped mesoporous Al2O3. Interestingly, on Cr-doped support MCA,
the conversion of NaNO3 occurs at about 400 oC (Fig. 1b). This reflects that the presence of
6

Cr lowers the temperature for NaNO3 conversion significantly. The gaseous products liberated
during NaNO3 conversion were monitored by MS. Four MS signals were monitored with the
m/z values of 30, 32, 44 and 46; they are assigned to NO, O2, N2O and NO2, respectively (Fig.
1). The decomposition of NaNO3 on MA predominantly produce NO, O2, N2O and NO2. On
the support MCA, however, the main products are NO and NO2 along with a small amount of
N2O. The different gaseous products derived from thermal treatment of NaMA and NaMCA
indicate different pathways for NaNO3 conversion.
Wide-angle XRD patterns were recorded to characterize the samples. As shown in Fig. 2a,
MA only exhibits XRD peaks originated from amorphous frameworks.[33] After the
introduction of base precursor, the diffraction peaks at 2θ of 29.3o ascribed to NaNO3 can be
observed (JCPDS No. 07-0271). It is noticeable that after activation at 400 oC, the sample
20NaMA-400 still gives intense diffraction peaks of NaNO3, indicating that only small quality
of NaNO3 is decomposed. In the case of 20NaMA-700 activated at 700 oC, the diffraction
peaks of NaNO3 disappear, proving that the decomposition of NaNO3 only takes place at high
temperatures on MA. In addition, new intense diffraction peaks of Na2Al2O4 become visible
due to the guest-host reaction at high temperatures, which leads to the degradation of
mesostructure as evidenced later. As shown in Fig. 2b, the doping of Cr on mesoporous Al2O3
leads to the formation of evident XRD peaks at 2θ of 24.7, 33.9, 36.5, 55.4 and 65.7o, which
correspond to (012), (104), (110), (116) and (300) crystal faces of Cr2O3 (JCPDS No.
84-0315),[34] respectively. After the loading of NaNO3, the diffraction peaks of NaNO3
(JCPDS No. 07-0271) appear, and the intensity increases with the enhancement of NaNO3
loading (Fig. 2b). As shown in Fig. 2c, the diffraction peaks of NaNO3 disappear after
activation at 400 oC on the support MCA regardless of NaNO3 content, which is much lower
o
than that on undoped MA (700 C). This proves that the presence of Cr2O3 promotes the
conversion of NaNO3.
Fig. 3 exhibits the FT-IR spectra of different samples before and after activation. After the
loading of NaNO3, 20NaMA gives an obvious IR band at 1384 cm-1 attributed to N-O
vibration of nitrate.[35, 36] After activation at 400 oC, the sample 20NaMA-400 still has a
strong vibration band of nitrate, which indicates that NaNO3 is seldom converted. For the
Cr-doped samples, after the introduction of NaNO3, the N-O vibration at 1384 cm-1 increases
7

with the enhancement of NaNO3 loading. After activation at 400 oC, the band of nitrate drops
sharply, and there only left the peak at 1384 cm-1 derived from the support, which indicates
the entire conversion of NaNO3 in the presence of Cr (Fig. 3). XPS spectra were further
collected to characterize the samples. The presence of Na, Cr, N, and Al elements in the
samples can be verified from the spectra (Figs. 4, S1-S3). Especially, the conversion of
NaNO3 can be monitored by N1s XPS spectra as shown in Figs. 4a and 4b. Before activation,
-
the N1s spectra exhibit an XPS peak derived from NO3 at the binding energy of 407 eV.[8]
After activation at 400 oC, the N1s peak disappears in the samples 5NaCA-400, 10NaCA-400
and 20NaCA-400, indicating that the base precursor of NaNO3 is completely converted.
Based on the above results, it is obvious that the doping of low-valence Cr3+ into MA
facilitates the conversion of base precursor NaNO3 significantly. The complete conversion of
NaNO3 can be achieved at the temperature of 400 oC, which is pretty lower than the
decomposition temperature for NaNO3 on undoped MA (700 oC). The existence of Cr has an
important effect on the conversion of NaNO3, which is different from the traditional thermal
decomposition of the base precursor.
3.2. Proposed mechanism for basicity generation
The conversion of NaNO3 on MCA should be attributed to the redox interaction
considering the reducibility of Cr3+ and the oxidizability of NaNO3. The transformation of Cr
valence during redox was thus explored. The Cr2p XPS spectra of the samples before and
after activation were shown in Figs. 4c and 4d. As shown in Fig. 4c, for the samples 5NaMCA,
10NaMCA and 20NaMCA, there are two peaks at the binding energy of 586.5 eV and 577.2
eV associated with Cr3+ species.[7, 37] After activation at 400 oC, two new peaks appear at
the binding energy values of 589.4 and 580.4 eV, which can be ascribed to Cr6+ species (Fig.
4d).[38] This demonstrates that Cr3+ in MCA was oxidized to Cr6+ during the conversion of
NaNO3. In addition, the content of Cr6+ species gradually increase with the enhancement of
NaNO3 loading.
To further investigate the valence transformation of Cr, diffuse reflectance UV-vis spectra
of the sample before and after activation were recorded. As shown in Fig. S4, for the sample
8

20NaMCA, two broad light absorption peaks arise in UV range at about 460 and 600 nm,
which corresponds to Cr3+ species.[39-41] After activation, in the case of 20NaMCA-400, two
new UV bands appear at 270 and 360 nm, which can be ascribed to Cr6+ species.[42] In the
meanwhile, the peaks of Cr3+ species decline obviously, indicating that a large amount of Cr3+
are converted to Cr6+. It is known the transformation of the valence state of metal is often
accompanied by the change in color. The digital photos of different samples with diverse
loading of NaNO3 before and after activation at 400 oC were taken and shown in Fig. 5. The
color of 5NaMCA, 10NaMCA and 20NaMCA is green black. Interestingly, after activation at
400 oC, the color of these samples changes to yellow green little by little due to the valence
variation of Cr3+ to Cr6+.
From TG-MS results of the sample 20NaMA (Fig. 1a), the decomposition of NaNO3 on
MA predominantly produces NO, O2, NO2 and N2O. Hence, the pyrolyzation of NaNO3 to
basic sites on MA could be depicted through the following equations: 6NaNO3 → 3Na2O +
2NO + N2O + 2NO2 + 4O2. In the presence of the Cr, however, the main products are NO,
N2O and NO2 (Fig. 1b). Therefore, by combining XPS and UV-vis data, the conversion of
NaNO3 to basic sites on MCA could be depicted through the following equation: 24NaNO3 +
8Cr3+ → 12Na2O + 8Cr6+ + 5NO + 7N2O + 5NO2 + 19O2. Quantitative analysis of the
consumption of Cr3+ for the conversion of NaNO3 in the NaMCA series samples was
conducted. The quantitative analysis of XPS presents that 48.7% of Cr3+ was oxidized to Cr6+
for sample 10NaMCA, which is on the verge of the theoretical value (44.1%) provided that all
supported NaNO3 is converted. The amount of Cr3+ oxidized to Cr6+ is 15.4% for 5NaMCA
and 87.8% for 20NaMCA, which is in line with the theoretical values (22.0 and 88.2%).
The aforementioned results demonstrate that the generation of basic sites at low
activation temperature in Cr-doped Al2O3 is caused by the guest-host redox interaction. In the
redox process, NaNO3 is reduced to Na2O and low-valence Cr3+ is transformed to Cr6+. This
makes the formation of strong basic sites on Cr-doped mesoporous Al2O3 at a quite low
temperature. The generation of Cr6+ has been evidenced by various approaches including XPS
and UV-vis spectra as well as color change. Quantitative analysis shows that Cr3+ in NaMCA
series samples is sufficient for the conversion of supported NaNO3. The theoretical values are
in good agreement with the measured ones according to various characterizations.
9

3.3. Structural properties of the obtained solid bases
Various techniques were used to examine the structure of the obtained mesoporous solid
bases, since mesoporosity is important for the materials. The N2 adsorption-desorption
isotherms and the pore size distributions of the NaMCA-400 series samples are presented in
Fig. 6. The textural properties such as surface area and pore volume are displayed in Table 1.
All of the Na-modified samples show a type-IV curves with H1 hysteresis loop, which
confirms the preservation of the mesostructure after activation at 400 oC.[43] The isotherms
have a clear condensation step at a relative pressure of p/p0=0.5-0.9. This indicates uniform
meso-structure with an average mesoporous size of around 8 nm. For the NaMCA-400
samples, the hysteresis loop and pore volume exhibit a decreasing tendency with increasing
NaNO3 content, and the mesostructure remains intact in these solid catalysts. The NaMA
sample without Cr doping was also investigated as a comparison. In order to decompose
NaNO3, the NaMA sample was activated at 700 oC. As shown in Fig. S5 and Table 1, it is
notable that the sample 20NaMA-700 presents relatively low N2 uptake and has a quite low
surface area of 39 m2·g−1, which provides strong proof of the serious damage of mesoporous
structure.
Low-angle XRD patterns was also applied to characterize the mesostructure of the MSSBs.
As shown in Fig. 7, for the sample 20NaMA-700 without Cr doping, no diffraction peak is
detected despite that the support MA shows an ordered mesostructure, suggesting that the
mesostructure is degraded after activation. In the case of Cr doped samples, the support MCA
has a diffraction peak at 2θ around 1o along with a weak one at about 1.7o, corresponding to
the crystal faces of (100) and (110).[34] This indicates an ordered two-dimensional hexagonal
mesoporous structure. These diffraction peaks are also observed in NaMCA-400 series
samples regardless of the NaNO3 content, demonstrating that the mesoporous structure was
preserved after NaNO3 loading and subsequent activation.
In order to further characterize and verify the presence of ordered meso-structure, TEM
micrographs of the samples are presented in Fig. 8. As exhibited in Fig. 8, for the support MA,
periodic mesoporous structure is observed from TEM images (Fig. 8a). Nonetheless, the
loading of NaNO3 and activation results in the obvious change of pore structure. Instead of
10

parallel pore channels observed in the supports, disordered pore arrangement is detected for
20NaMA-700, which provides the evidence of the damage of mesoporous structure. However,
TEM images of 20NaMCA and 20NaCA-400 show an ordered structure with long-range
ordered mesopores.[44-46] This suggests that the introduction of NaNO3 and subsequent
conversion to basic sites do not damage the mesoporous structure. The SEM picture and the
corresponding EDX-mapping images of different samples were taken and shown in Fig. 9, S6,
and S7. The surface of the samples 5NaMCA-400, 10NaMCA-400 and 20NaMCA-400 are
honeycombed, this surface topography not only increases the surface area but also enhances
adsorption capacity for NaNO3.The existence of Na, Cr, O and Al atoms is evidenced, and
these atoms uniformly distributed in the samples.
From the results mentioned above, it is found that the doping of Cr atoms result in the
formation of solid base with good mesoporous structure at obviously low temperatures. On
the contrary, Al2O3 without Cr doping shows collapsed mesostructure after the introduction of
basic sites. This indicates the importance of Cr doping in the preservation of ordered structure
of solid base catalysts.
3.4. Basicity and catalytic performance
The basic properties of different samples were first measured by acid-base titration. As
shown in Table 1, the supports MA and MCA give tiny amount of basic sites, indicating the
negligible basicity of supports. Interestingly, for the sample 20NaMA-400 activated at 400 oC,
the amount of total basic sites is only 0.36 mmol g-1, which is comparable to the support and
indicates that very small amount of NaNO3 is decomposed on MA. Further increasing the
activation temperature to 700 oC, the sample 20NaMA-700 presents 1.06 mmol g-1 of the
amount of basic sites, which is obviously lower than the theoretical value (2.36 mmol g-1)
caused by the guest-host reaction at such a high temperature. However, in the case of
5NaMCA-400 activated at 400 oC, the amount of basic sites is 0.54, which is in consistent
with the theoretical value (0.59 mmol g-1). With the NaNO3 amount loaded increasing, the
amount of basic sites continue to increase. The amount of basic sites reaches to 1.13 mmol g-1
for 10NaMCA-400 and 2.30 mmol g-1 for 20NaMCA-400, which is in line with the
11

theoretical values (1.18 and 2.36 mmol g-1). This demonstrates the full decomposition of
NaNO3 to basic sites in Cr-doped supports at relatively low temperatures.
The TPD profiles of CO2 were applied to monitor the basicity distribution of the obtained
materials (Fig. 10). The CO2 desorption peaks can be classified as three parts named as α, β,
and γ at about 100, 200, and 530 oC, which are originated from weak, medium, and strong
basic sites, respectively. For the sample MCA, only a desorbed peak of CO2 attributed to weak
basicity is detected, manifesting that there are poor basicity on MA. In the case of
5NaMCA-400, a big CO2 desorption peaks derived from weak basicity and a small desorption
from strong basicity are observed, indicating the increase of basicity after introducing NaNO3.
With increasing NaNO3 loading, the desorption peaks at β and γ parts increase obviously,
suggesting the enhancement of medium and strong basic sites. Especially, a sharp CO2
desorption peak is observed at high temperature in the sample 20NaMCA-400; this reflects
that a large amount of strong basicity is formed for the sample with high NaNO3 loading.
These results confirm the above titration data and suggest that the resultant solid bases are
promising candidates for catalysis.
The obtained strong base materials were applied to synthesize DMC via the
transesterification of methanol and ethylene carbonate. As a versatile green chemical, DMC
can be utilized as a fuel additive, carbonylating agent and methylating agent.[47-49] Much
consideration has been focused on the synthesis of DMC by using heterogeneous catalysts
instead of conventional homogeneous ones.[31, 50, 51] As shown in Fig. 11, the supports
sample MCA without NaNO3 loading exhibit a DMC yield less than 10% after the reaction
for 4 h. The limited amount of basic sites on that sample could be the reason of the low yield
of DMC. The introduction of NaNO3 leads to an obvious increase of catalytic activity. The
yield of DMC is 17.1% over the sample 5NaMCA-400. Further enhancement of NaNO3
loading results in the increase of catalytic activity, and the yield of DMC reaches 33.8% over
the sample 20NaMCA-400. For the sample 20NaMA-700, the yield of DMC only is 18.8%,
which is much lower than 20NaMCA-400 (Fig. S8). This is due to the strong basic sites of
Na2O reacted with supports producing Na2Al2O4 at high temperatures (Fig. 2a). A variety of
catalysts used for the catalysis of the same reaction were employed for comparison as shown
in Table S1. Nevertheless, the DMC yield is only 15.1% under the catalysis of CaO/ZrO2.
12

Over the benchmark solid base MgO, about 7.6% of DMC is yielded. For the Li2O/SBA-15,
the yield of DMC is only 12.2%, which is obviously lower than Na2O-modified catalysts.
Apparently, our materials show remarkable catalytic performance, which is better than various
traditional solid strong bases in DMC synthesis. Reusability is a key index to evaluate the
performance of the catalyst. The sample 20NaMCA-400 also shown ideal reusability for
catalyzing DMC synthesis (Fig. S9). After four cycles, the DMC yield of sample
20NaMCA-400 remained at around 32%, indicating the stability of the catalysts.
Apparently, the characterization of basicity shows that the obtained solid bases possess
strong basicity, and the basicity increases when the amount of NaNO3 is enhanced. The strong
basicity, along with the ordered mesoporous structure, makes the present catalysts active in
the transesterification reactions of synthesis of DMC. The catalytic activity of 20NaMCA-400
is superior to a variety of catalysts with active sites ranged from alkali metal and alkaline
earth metal oxides.
4. Conclusions
In summary, we have developed a redox strategy to fabricate MSSBs via doping
low-valence Cr3+ into MA. In the process of activation, the base precursor NaNO3 is
converted to strongly basic sites of Na2O while low-valence Cr3+ is oxidized to high-valence
Cr6+. This strategy breaks though the tradition of thermal induced decomposition of base
precursors. Consequently, the strongly basic sites can be produced on MCA at 400 oC, which
is unexpectedly lower than that on mesoporous Al2O3 (700 oC). In addition to be
energy-saving, the activation at low temperature is beneficial to the preservation of ordered
mesoporous structure, which is different from the collapse of structure occurred in undoped
supports MA. The obtained materials with strong basicity and ordered mesostructure show
high activity in transesterification reactions, and is more active than various reported solid
strong base catalysts. The present work may open up a new way for the fabrication of
functional materials by tailoring the surface redox characteristic of supports.
13

Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21878149,
21676138, 21722606, and 21576137), Six Talent Plan (2016XCL031), and the Project of
Priority Academic Program Development of Jiangsu Higher Education Institutions.
14

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16

Figure captions
Fig. 1. Conversion of NaNO3 on different samples (a) 20NaMA and (b) 20NaMCA. The MS
signals 30, 32, 44 and 46 can be ascribed to NO, O2, N2O and NO2, respectively.
Fig. 2. Wide-angle XRD patterns of (a) the NaMA series samples and NaMCA series samples
with different NaNO3 loading (b) before and (c) after activation at 400 oC.
17

Fig. 3. FT-IR spectra of the samples with different amounts of NaNO3 before and after
activation at 400 oC.
18

Fig. 4. XPS spectra of different samples. (a) N1s and (c) Cr2p before activation as well as (b)
N1s and (d) Cr2p after activation.
19

Fig. 5. Digital photos of the samples before and after activation.
Fig. 6. (a) N2 adsorption-desorption isotherms and (b) pore size distributions of MCA and
NaMCA series samples.
Fig. 7. Low-angle XRD patterns of the samples MA, 20NaMA-700, MCA, 5NaMCA-400,
10NaMCA-400 and 20NaMCA-400.
20

Fig. 8. TEM images of the samples (a) 20NaMA, (b) 20NaMA-700, (c) 20NaMCA and (d)
20NaMCA-400.
Fig. 9. (a) SEM and (b-f) EDX-mapping images of the sample 20NaMCA-400.
21

Fig. 10. CO2-TPD profiles of the samples MCA, 5NaMCA-400, 10NaMCA-400 and
20NaMCA-400.
Fig. 11. Transesterification reactions of methanol with ethylene carbonate catalyzed by MCA,
5NaMCA-400, 10NaMCA-400 and 20NaMCA-400.
22

Scheme 1. Scheme depicting the conversion of NaNO3 on the supports MA and MCA.
23

Table 1. Physicochemical characteristics of different samples
Amount of basic sites (mmol g-1)
Sample
SBET (m2 g-1)
Vp (cm3 g-1)
Theoreticala
Measured
0.28
MA
156
177
113
39
0.37
0.40
0.28
0.17
0.25
0.23
0.20
0
MCA
0
0.22
20NaMA-400
20NaMA-700
5NaMCA-400
0
0.36
2.36
0.59
1.18
2.36
1.06
164
0.54
10NaMCA-400 115
20NaMCA-400 85
1.13
2.30
aTheoretical values were calculated based on the amount of NaNO3 loaded.
24