Direct fabrication of strong basic sites on ordered nanoporous materials: Exploring the possibility of metal-organic frameworks

Article abstract of DOI:10.1021/acs.chemmater.7b05102

Heterogeneous strong base catalysts possessing ordered nanoporous structure are highly expected due to their high activity and shape selectivity in diverse reactions. However, their fabrication remains a great challenge because quite high temperatures (600-700 °C) are compulsory for the generation of strong basicity on conventional ordered nanoporous materials (i.e., zeolites and mesoporous silicas). Here, we report for the first time direct fabrication of strong basic sites on metal-organic frameworks (MOFs) by using guest-host redox (GHR) interaction between base precursors and low-valence metal centers (e.g., Cr³⁺), which breaks the tradition of thermo-induced decomposition of base precursors. It is fascinating that base precursor KNO₃ can be converted to strong basic species on MIL-53(Cr) at 300 °C, which is much lower than that on zeolite Y (700 °C) and mesoporous silica SBA-15 (600 °C). The resultant solid base exhibits strong basicity, ordered nanoporous structure, and high activity and shape selectivity in base-catalyzed reactions.

Full text of DOI:10.1021/acs.chemmater.7b05102

Article
pubs.acs.org/cm
Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
Direct Fabrication of Strong Basic Sites on Ordered Nanoporous
Materials: Exploring the Possibility of Metal−Organic Frameworks
Wei Liu, Li Zhu, Yao Jiang, Xiao-Qin Liu, and Lin-Bing Sun*
State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced
Materials (SICAM), College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China
S
* Supporting Information
ABSTRACT: Heterogeneous strong base catalysts possessing ordered nanoporous structure are
highly expected due to their high activity and shape selectivity in diverse reactions. However,
their fabrication remains a great challenge because quite high temperatures (600−700 °C) are
compulsory for the generation of strong basicity on conventional ordered nanoporous materials
(i.e., zeolites and mesoporous silicas). Here, we report for the first time direct fabrication of
strong basic sites on metal−organic frameworks (MOFs) by using guest−host redox (GHR)
interaction between base precursors and low-valence metal centers (e.g., Cr3+), which breaks the
tradition of thermo-induced decomposition of base precursors. It is fascinating that base
precursor KNO3 can be converted to strong basic species on MIL-53(Cr) at 300 °C, which is
much lower than that on zeolite Y (700 °C) and mesoporous silica SBA-15 (600 °C). The
resultant solid base exhibits strong basicity, ordered nanoporous structure, and high activity and
shape selectivity in base-catalyzed reactions.
structure of supports is severely damaged. This is caused by the
reactions between strong basic species and supports at high
temperatures. To avoid such reactions, a dual-coating strategy
was designed in which mesoporous silicas were coated with an
alkali-resistant interlayer (e.g., Al2O3 and ZrO2).16,17 However,
the coating process is relatively complicated, and, after coating,
the surface areas of mesoporous silicas are obviously declined.
Vapor-induced conversion of nitrate precursors was also
reported by utilizing the reactions between vapors (e.g.,
methanol) and precursors.18,19 Basic sites can be produced at
relatively low temperatures (e.g., 400 °C by use of methanol),
and the degradation of siliceous supports is hindered to some
extent. Nevertheless, toxic organic vapors are involved in the
process of vapor-induced conversion. Although elaborate efforts
have been dedicated, direct fabrication of strong basic sites on
ordered nanoporous materials remains a great challenge.
Metal−organic frameworks (MOFs) are porous crystalline
materials assembled from organic linkers and metal ion (or
cluster) nodes. Because of their adjustable structure, diverse
functionality, and large surface areas, MOFs are highly
promising for applications in catalysis.20−22 MOFs possess
ordered nanopores with a size ranging from micropore to
mesopore, which bridges the gap between zeolites and
mesoporous silicas.23 The emergency of MOFs creates new
opportunities for the fabrication of solid strong bases with
ordered nanoporous structure. Nonetheless, the predominant
approach for generation of basicity on MOFs is still the
introduction of amino groups, either coordinatively grafted
INTRODUCTION
■
To satisfy the requirements of green and sustainable catalytic
processes, the replacement of conventional homogeneous
catalysts with their heterogeneous analogues is of significant
interest.1−4 Among heterogeneous catalysts, solid strong bases
have received growing attention due to some remarkable merits
over liquid bases, such as low corrosiveness, few disposal issues,
and easy recycle of catalysts.5 These solid strong bases can
catalyze diverse reactions including halogenation, Knoevenagel
condensation, Michael addition, and transesterification reac-
tion, to name just a few. It is known that selectivity of target
products is crucial for most reactions.6,7 To enhance the
selectivity, catalysts with ordered nanoporous structure are
frequently expected. Only reactants (or products) with a
matching dimension can access active sites in the pores of
catalysts (or escape from the pores), and selective synthesis is
thus realized. Zeolites and mesoporous silicas are two kinds of
representative ordered nanoporous materials, and are widely
used for the preparation of catalysts with enhanced
selectivity.8−11 Many attempts have also been made to create
basic sites on zeolites and mesoporous silicas, such as ion
exchange, nitrogen doping, and amino groups grafting. Some
interesting solid bases with ordered nanoporous structure can
be prepared, while most of these solid bases show only weak
basicity.12,13 To enhance the base strength, alkaline metal
oxides with strong basicity (such as K2O) have been attempted
to incorporate into zeolites and mesoporous silicas. The
preparative process includes introduction of a precursor (such
as KNO3), followed by calcination at a high temperature to
decompose the precursor.14,15 For example, 600 °C is required
to decompose KNO3 on mesoporous silica SBA-15. Unfortu-
nately, the obtained materials exhibit weak basicity, and the
Received: December 7, 2017
Revised: February 6, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.chemmater.7b05102
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
Figure 1. (a) Scheme depicting the conversion of KNO3 on the zeolite Y. (b) Scheme depicting the conversion of KNO3 on the mesoporous silica
SBA-15. (c) Scheme depicting the conversion of KNO3 on the metal−organic framework MIL-53(Cr). On the basis of TG analysis, the activation
temperatures for K@Y, K@SAB-15, and K@MIL-53(Cr) are 700, 600, and 300 °C, respectively.
onto open metal sites or covalently bonded to organic
linkers.24,25 Hence, only weak basicity can be formed on the
resultant materials. It is interesting to note that the frequently
employed precursors of strong bases, nitrates, own oxidizability.
Meanwhile, metal centers at low valence states possess
reducibility in many MOFs, which is absent in zeolites and
mesoporous silicas. By taking advantage of the redox reaction
between guest and host, it is expected that precursors can be
converted to strong basic sites at a low temperature. This may
lead to the fabrication of a new kind of solid strong bases with
ordered nanoporous structure, which is unlikely to be realized
on conventional support zeolites and mesoporous silicas.
Here, we report the direct fabrication of strong basic sites on
MOFs by using a guest−host redox (GHR) strategy, for the
first time, which breaks the tradition of thermal decomposition
of base precursors. As a proof of concept, MIL-53(Cr)
containing Cr metal centers at a low valence state (Cr3+) was
used as the host, and a typical base precursor KNO3 was
employed as the guest (Figure 1). MIL-53(Cr) is built up from
infinite chains of corner-sharing Cr3+O4(OH)2 octahedra,
interconnected by terephthalate groups to create a 3D
framework.26 It is intriguing that the GHR interaction makes
the conversion of KNO3 on MIL-53(Cr) occur at the
temperature of about 300 °C. Such a temperature is much
lower than the thermal decomposition of KNO3 on zeolite Y
(∼700 °C) and SBA-15 (∼600 °C) as well as all porous
supports reported until now. As a consequence, strong basicity
is directly generated on MIL-53(Cr), and the ordered
nanoporous structure of support is well maintained. This is
impossible to realize via the conventional thermal method,
because the structure of supports is seriously destroyed under
harsh conditions for the decomposition of precursors.The GHR
process can be confirmed by gaseous products as well as the
formation of Cr6+ from Cr3+ in MIL-53(Cr). We also
demonstrate that the resultant solid base K@MIL-53(Cr) is
active as heterogeneous catalysts in both Knoevenagel
condensation and transesterification reactions. The catalytic
results reflect the selectivity upon a series of reactants and
superior activity to various classic solid base catalysts.
EXPERIMENTAL SECTION
■
Chemicals and Materials. Tetragonal zirconium dioxide (t-ZrO2)
was prepared by the calcination of zirconium oxychloride octahydrate
ZrOCl2 8H2O (Aladdin, 98%) at 400 °C in an air flow for 4 h with a
heating rate of 2 °C min−1. Gases were all from Nanjing Ruier. Other
chemicals including EO20PO70EO20 (P123; Sigma-Aldrich, analytical
reagent), tetraethylorthosilicate (TEOS; Sigma-Aldrich, 98%), hydro-
chloric acid (Sigma-Aldrich, ≥99%), chromium nitrate nonahydrate
(Sigma-Aldrich, 99%), terephthalic acid (Aladdin, 99%), hydrofluoric
acid (Adamas-beta, 40%), zeolite molecular sieve (NaY; Aladdin,
>99%), silicon dioxide (Sinopharm, analytical reagent), γ-aluminum
oxide (Aladdin, analytical reagent), monoclinic zirconium dioxide (m-
ZrO2; Aladdin, analytical reagent), potassium nitrate (Sinopharm,
analytical reagent), benzaldehyde (Sinopharm, analytical reagent),
malononitrile (Sinopharm, analytical reagent), ethyl cyanoacetate
(Sinopharm, analytical reagent), ethyl acetoacetate (Aladdin, analytical
reagent), diethyl malonate (Aladdin, analytical reagent), ethylene
carbonate (Sinopharm, analytical reagent), phenolphthalein (Sino-
pharm, analytical reagent), and sodium hydroxide (Sigma-Aldrich,
98%) were used directly without any further purification. Deionized
water was generated by a Milli-Q integral pure and ultrapure water
purification system and used in all experiments.
Synthesis of SBA-15. Mesoporous silica SBA-15 was synthesized
according to the literature reported previously.18,27 First, 3 g of triblock
copolymer P123 (EO20PO70EO20) was dissolved in 90 g of aqueous
HCl solution (2 M) and 22.5 g of deionized water with stirring at 40
°C. Next, 6.38 g of silica source tetraethylorthosilicate (TEOS) was
added to the homogeneous solution and stirred at this temperature for
24 h. Finally, the as-prepared gel was moved to a Teflon-lined
autoclave and remained at 100 °C for 24 h. The resulting product was
recovered by filtration, washing, and drying at room temperature.
Calcination was carried out at 550 °C in an air flow for 5 h with a
heating rate of 2 °C min−1 to remove the template of pure SBA-15.
Synthesis of MIL-53 (Cr). MIL-53 (Cr) was hydrothermally
synthesized by using the published procedure.28 First, chromium
B
DOI: 10.1021/acs.chemmater.7b05102
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
nitrate nonahydrate Cr(NO3)3·9H2O (3.1972 g, 8 mmol), terephthalic
acid HO2C-(C6H4)-CO2H (1.3244 g, 8 mmol), hydrofluoric acid (0.16
g, 8 mmol), and H2O (40.2982 g) were mixed. Reactants were stirred
for 1 h under the airtight space before introducing the resulting
suspension in a Teflon-lined steel autoclave, and the temperature was
kept at 220 °C for 3 days. A grayish-green powder was obtained with
traces of terephthalic acid. Next, the free terephthalic acid was
evacuated by solvothermal exchange with DMF (1 g of MIL-53(Cr) in
50 mL of DMF) at 150 °C for 15 h. After being cooled to room
temperature, the powder was impregnated in the methanol for 3 days
and replaced three times to remove the DMF molecules. Eventually,
dichloromethane was replaced by the anhydrous methanol to obtain
the green product MIL-53 (Cr).
Preparation of Solid Bases. The base precursor KNO3 was
introduced by wet impregnation. An identical amount of KNO3 (10 wt
%) was put into the different supports. Typically, 0.1 g of KNO3 was
dissolved in 15 mL of deionized water, followed by addition of 0.9 g of
NaY (SiO2/Al2O3 = 4.86), SBA-15, MIL-53(Cr), SiO2, γ-Al2O3, m-
ZrO2, and t-ZrO2. After being stirred at room temperature for 2 h, the
mixture was evaporated at 60 °C and subsequently dried at 100 °C for
4 h. The obtained solid was denoted as K@Y, K@SBA-15, K@MIL-
53(Cr), K@SiO2, K@γ-Al2O3, K@m-ZrO2, and K@t-ZrO2. For the
conversion of base precursor KNO3 to basic sites, the samples K@Y,
K@SBA-15, and K@MIL-53(Cr) separately were calcined at 700, 300
and 600, 300 °C in an inert gas for 2 h. The final materials were
represented as K@Y-700, K@SBA-15-300 and K@SBA-15-600, K@
MIL-53(Cr)-300.
where phenolphthalein was employed as an indicator. The amount of
HCl consumed was used to calculate the basicity.
Knoevenagel Condensation Reaction. The catalytic perform-
ance of sample K@MIL-53(Cr)-300 was evaluated by Knoevenagel
condensation reaction.29 The catalysts (0.05 g) needed to experience
activation at 300 °C, and then it was placed in a 10 mL two-necked
glass flask containing a water-cooled condenser. The equimolar
quantities of benzaldehyde (1 mmol) and other four substrates
(malononitrile, ethyl cyanoacetate, ethyl acetoacetate, and diethyl
malonate) were added to the absolute ethyl alcohol (2.5 mL). After
the reaction mixtures were stirred at 70 °C for 1 h, the product was
cooled to 25 °C and the n-dodecane (0.4 g) as an internal standard
was injected. A centrifuge separates the supernatant, which was tested
by means of a gas chromatograph (GC, Agilent 7890A) equipped with
a FID and a JWDB-5 95% dimethyl-1-(5%)-diphenylpolysiloxane
column.
Transesterification Reaction. With the prepared solid base
catalyst, the compound dimethyl carbonate (DMC) can be produced
via the transesterification of methanol and ethylene carbonate.14
Typically, methanol (0.5 mol), ethylene carbonate (0.1 mol), and
catalyst (0.5 wt % of methanol) were added to a three-necked glass
flask with a water-cooled condenser. The reaction was conducted at 65
°C with stirring for a given period of time. After the reaction was
completed, the reaction mixtures were recovered from the flask and
subjected to centrifuging. The obtained upper liquid was then analyzed
by use of a gas chromatograph (GC, Agilent 7890A) equipped with a
FID and a JWDB-5 95% dimethyl-1-(5%)-diphenylpolysiloxane
column.
Characterization and Measurement. X-ray diffraction (XRD)
patterns of materials were recorded using a Bruker D8 Advance
diffractometer with monochromatic Cu Kα radiation in the 2θ range
from 0.6° to 8° and from 5° to 80° at 40 kV and 40 mA along with the
scanning speed of 0.5°/min. To determine the surface area of the as-
synthesized materials, N2 adsorption−desorption isotherms were
measured by ASAP 2020 at −196 °C. The temperature selected to
perform the degasification step was different with regard to NaY, SBA-
15, and MIL-53(Cr), and the degasification time was 4 h. The
Brunauer−Emmett−Teller (BET) surface area was calculated with the
relative pressure ranging from 0.05 to 0.35. The total pore volume was
determined from the amount adsorbed at the relative pressure of about
0.99. The pore size distribution of various materials was calculated
from the adsorption branch by using the Barrett−Joyner−Halenda
(BJH) method. For exploring the conversion of KNO3, Fourier
transform infrared (IR) spectra of the samples diluted with KBr in a
good proportion of 1:150 were carried out on a Nicolet Nexus 360
spectrometer with a spectra resolution of 8 cm−1. Transmission
electron microscopy (TEM) images were captured to analyze the
structural characterization of the materials in a JEM-2010 UHR
electron microscope operated at 200 kV. The morphology of prepared
samples was examined by HITACHI S-4800 SEM, and the elemental
distribution of materials was determined by energy dispersive X-ray
spectroscopy (EDX). EDX was performed in the scanning electron
(SE) mode, and the accelerating voltage was 20 kV. TG analysis was
performed on a thermobalance (STA-499C, NETZSCH). Ten
milligrams of sample was heated from room temperature to 800 °C
at the rate of 10 °C min−1 in the flow of N2 (25 mL min−1). The
gaseous products derived from decomposition were analyzed by MS.
TP-MS combines with temperature programming and mass
spectrometry (HAL201, HIDEN), and its voltage is 70 eV. The
condition of determination is the same as TG-MS. UV−visible diffuse
reflectance (UV−vis) spectra of the samples were recorded with a UV-
2401PC spectrophotometer, and BaSO4 was used as an internal
standard. XPS measurement was done on a PHI5000VersaProbe
spectrometer (UIVAC-PHI) with monochromatic Al Kα radiation, and
the binding energies were referred to C1s at 284.6 eV.
RESULTS AND DISCUSSION
■
Aiming to create strong basicity, base precursor KNO3 was
introduced to three supports with ordered nanoporous
structure, the zeolite Y, the mesoporous silica SBA-15, and
the MOF MIL-53(Cr), by impregnation. The obtained samples
were denoted as K@Y, K@SBA-15, and K@MIL-53(Cr),
respectively. These samples were subjected to thermal
treatment at different temperatures to convert KNO3, which
results in the formation of K@Y-T, K@SBA-15-T, and K@
MIL-53(Cr)-T (where T represents the treatment temper-
ature). Strong basicity is anticipated to generate through the
conversion of KNO3 to alkaline metal oxide K2O. It is
noticeable that KNO3 exhibits a quite different conversion
behavior on different supports. Thermogravimetry-mass
spectrometer (TG-MS) technique was first adopted to examine
the conversion of KNO3. Four signals with m/z values of 30,
32, 44, and 46 were detected, and they stemmed from NO, O2,
N2O/CO2, and NO2, respectively (Figure 2). The intensity of
these signals is sort of different for the three samples, indicating
different pathways for KNO3 conversion. However, the signal
of NO (m/z = 30) that originated from KNO3 is predominant
in all samples. The signal of NO, in combination with the
weight loss, can be utilized to compare the conversion behavior
of KNO3 on different supports.
All samples give a weight loss at temperatures lower than 200
°C, which is assigned to desorption of physically adsorbed
water. In addition to this weight loss, K@Y presents another
weight loss centered at 700 °C ascribed to the conversion of
KNO3. In the case of K@SBA-15, the majority of KNO3 was
decomposed at about 600 °C with the minority at 300 °C. The
conversion of KNO3 to basic sites on zeolite Y and mesoporous
silica SBA-15 could be depicted respectively through the
following equations: 4KNO3 → 2K2O + 4NO + 2O2 and
6KNO3 → 3K2O + 4NO + N2O + 5O2. For the decomposition
of KNO3, the gaseous products include O2. Actually, there is
still a certain amount of residual carbon-containing compounds
To measure the amount of basic sites, 50 mg of sample after
activation was added to 10 mL of aqueous HCl and CH3COOH (0.05
M). The solid−liquid mixture was separated by a centrifuge after the
sample suspension was shaken for 24 h. The remaining acid in liquid
phase was titrated with a standard base (0.01 M aqueous NaOH)
C
DOI: 10.1021/acs.chemmater.7b05102
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
700 and 730 °C, respectively. However, the decomposition
temperature decreases to 400 and 470 °C on γ-Al2O3 and t-
ZrO2, respectively. There are octahedral vacancies on the
surface of γ-Al2O3 and t-ZrO2, and K+ of KNO3 is able to insert
in the vacancies in the preparation process.33 This enhances the
guest−host interaction and promotes the decomposition of
KNO3 at relatively low temperatures. Hence, KNO3 can be
decomposed at lower temperatures on γ-Al2O3 and t-ZrO2 than
that on SiO2 and m-ZrO2. In any case, the base precursor
KNO3 on MIL-53(Cr) can be converted at an unexpectedly
low temperature of 300 °C, which is unachievable on zeolites
and mesoporous silicas as well as any other porous supports
reported until now.
Fourier transform infrared (IR) spectra of different samples
were recorded to further examine the conversion of KNO3. The
samples K@Y, K@SBA-15, and K@MIL-53(Cr) all display a
strong characteristic band at 1383 cm−1 that originates from the
asymmetric stretching vibration of N−O in nitrate.34 According
to TG-MS results, K@Y was activated at 700 °C, and after
treatment the characteristic band of KNO3 disappeared (Figure
S5), indicating the decomposition of KNO3. Similarly, thermal
treatment at 600 °C led to vanishing of the characteristic band
of KNO3 on K@SBA-15 (Figure S6). Because TG-MS results
show that KNO3 is partially decomposed at 300 °C, the IR
spectrum of K@SBA-15-300 was measured as well. However,
the characteristic band of KNO3 in K@SBA-15-300 is slightly
weaker than that in K@SBA-15. This means that only a small
amount of KNO3 on SBA-15 is decomposed at 300 °C. In the
IR spectrum of MIL-53(Cr), two intense bands at 1537 and
1385 cm−1 attributed to symmetrical and asymmetric stretching
vibrations of carbonyl groups are detected (Figure S7).35
Interestingly, the characteristic band of KNO3 overlaps the
asymmetric stretching vibration of carbonyl groups. Despite the
difficulty, it is possible to identify the conversion of KNO3 by
comparing the relative intensity of the band at 1385 cm−1 with
that at 1537 cm−1. The relative intensity of the band at 1385
cm−1 first increases after loading KNO3 and then decreases
after activation, revealing the conversion of KNO3 upon
thermal treatment at 300 °C.
Various techniques were employed to characterize the
structure of resultant materials. Characteristic peaks of zeolite
Y are observed in the wide-angle X-ray diffraction (XRD)
patterns of pristine Y and K@Y, while only a broad peak
ascribed to amorphous frameworks is visible in K@Y-700
(Figure 3a). This mirrors that the structure of zeolite is retained
after the introduction of base precursor KNO3, and is
completely destroyed after activation at 700 °C aiming to
convert the precursor to basic sites. In the case of SBA-15 with
mesostructure, no diffraction peaks appear in wide-angle XRD
patterns due to the pore size at mesoporous scale (Figure S8).
The periodic pore arrangement can be revealed by three
diffraction peaks in the low-angle XRD patterns, which are
indexed as the (100), (110), and (200) reflections (Figure 3d).
Because only a small number of KNO3 are decomposed on
SBA-15 at 300 °C, the diffraction peaks are partially weakened
for the sample K@SBA-15-300. When the activation temper-
ature is increased to 600 °C, all diffraction lines disappear,
which indicates the complete degradation of structure for the
sample K@SBA-15-600. Interestingly, the activated sample K@
MIL-53(Cr)-300 exhibits almost the same XRD patterns as that
of pristine MIL-53(Cr) and K@MIL-53(Cr) before activation
(Figure 3g). This means that the ordered structure of MIL-
53(Cr) is well preserved after the formation of basic sites,
Figure 2. Conversion of KNO3 on different porous supports: (a) K@
Y, (b) K@SBA-15, and (c) K@MIL-53(Cr) monitored by TG-MS. C
represents the conversion of KNO3, and D represents the degradation
of support. The MS signals 30, 32, 44, and 46 can be ascribed to NO,
O2, N2O/CO2, and NO2, respectively.
in SBA-15 after the removal of template.30,31 Such carbon-
containing compounds can react with O2, leading to the
formation of CO2. As a result, the signal of O2 is not observed
in the TG-MS data. Noteworthily, K@MIL-53(Cr) shows two
weight losses at 300 and 500 °C, which correspond to two
different signals with m/z values of 30 and 44 in MS curves,
respectively. NO is the unique origin of the signal of m/z = 30
and is thus associated with the conversion of KNO3. However,
the signal of m/z = 44 at 500 °C comes from CO2 due to the
degradation of MOFs.32 Thus, the predominant products from
the decomposition of KNO3 include NO, N2O, and O2 as
shown in the following equation: 24KNO3 + 8Cr3+ → 12K2O +
12NO + 6N2O + 8Cr6+ + 21O2. This reflects that KNO3 is
converted at 300 °C on MIL-53(Cr), and the structure of
support is intact at such a low temperature. Nevertheless, the
conversion of KNO3 on zeolite Y and mesoporous silica SBA-
15 takes place mainly at 700 and 600 °C, which is much higher
than that on MIL-53(Cr). On the basis of TG-MS results, the
activation temperature of KNO3 supported on different porous
materials can be identified.
It is surprising that the conversion of KNO3 on MIL-53(Cr)
takes place at a temperature as low as 300 °C, which has never
been reported on any supports to our knowledge. Several
classic porous supports, SiO2, γ-Al2O3, and ZrO2 in monoclinic
and tetragonal crystallite (m-ZrO2 and t-ZrO2), were employed
for comparison (Table S1 and Figures S1−S4). KNO3 loaded
on SiO2 and m-ZrO2 is decomposed at high temperatures of
D
DOI: 10.1021/acs.chemmater.7b05102
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
conversion, the structure of zeolite Y and mesoporous silica
SBA-15 is seriously destroyed. However, the structure of MIL-
53(Cr) remains intact when supported KNO3 is entirely
converted to basic sites.
The basic properties of obtained materials were charac-
terized. To measure the base strength, Hammett titration was
employed. The base strength (H−) of K@Y-700 and K@SBA-
15-300 is less than 9.3, and that of K@SBA-15-600 is 15.0. In
the case of MIL-53(Cr)-300, it is difficult to observe the color
change of indicator due to the dark green color of the sample.
The amount of basic sites was measured as well. For the sample
K@Y-700, the amount of basic sites is only 0.04 mmol g−1 due
to the reaction of basic species with the zeolite support (Figure
S21). When SBA-15 was used as the support, the amount of
basic sites for K@SBA-15-300 is just 0.06 mmol g−1 due to the
incomplete conversion of KNO3, while that for K@SBA-15-600
is 0.39 mmol g−1 because of the reaction between basic species
and silica support. Remarkably, in the case of K@MIL-53(Cr)-
300, the amount of basic sites reaches 0.92 mmol g−1, which is
close to the theoretical value (0.99 mmol g−1). Acetic acid was
also used to measure basicity of samples in consideration that
HCl is a strong acid. The amount of basic sites for the samples
K@Y-700, K@SBA-15-300, K@SBA-15-600, and K@MIL-
53(Cr)-300 is 0.05, 0.07, 0.38, and 0.95 mmol g−1, respectively,
which is in good agreement with the values obtained by using
HCl. The experimental error for basicity using both HCl and
acetic acid varies between 0.01 and 0.02 mmol g−1. On the basis
of these results, it is obvious that the precursor KNO3 can be
fully converted to basic sites on MIL-53(Cr).
Figure 3. (a) Wide-angle XRD patterns of the zeolite Y, K@Y, and
K@Y-700 samples. (b and c) TEM images of the zeolite Y and K@Y-
700. (d) Low-angle XRD patterns of SBA-15, K@SBA-15, K@SBA-15-
300, and K@SBA-15-600 samples. (e and f) TEM images of SBA-15
and K@SBA-15-600. (g) Wide-angle XRD patterns of MIL-53(Cr),
K@MIL-53(Cr), and K@MIL-53(Cr)-300 samples. (h and i) TEM
images of MIL-53(Cr) and K@MIL-53(Cr)-300.
whereas that of zeolite Y and mesoporous silica SBA-15 is
destroyed completely.
The new solid base K@MIL-53(Cr)-300 was first applied to
catalyze Knoevenagel condensation. Knoevenagel condensation
is a well-known reaction between an aromatic aldehyde and a
compound containing active methylene group. This reaction is
useful for the formation of a C−C bond and usually catalyzed
by relatively weak or medium strong base.36,37 In this study,
Knoevenagel condensation of benzaldehyde with different
active methylene compounds was examined (Table 1). Under
the same conditions, the reaction of benzaldehyde with
N2 adsorption and electron microscopy give further
information on the structure of materials. For the activated
samples K@Y-700 and K@SBA-15-600, the adsorption amount
of N2 declines dramatically in contrast to pristine zeolite Y and
mesoporous silica SBA-15 (Figures S9 and S10). Moreover, the
BET surface area decreases from 745 m2 g−1 for Y zeolite to 40
m2 g−1 for K@Y-700 (Table S2). Similarly, the BET surface
area of K@SBA-15-600 is 70 m2 g−1, which is much lower than
that of SBA-15 (943 m2 g−1). Unlike zeolite and mesoporous
silica, the adsorption amount of N2 only decreases slightly after
incorporating basic species to MIL-53(Cr) (Figure S11). A
similar tendency is also observed for BET surface areas of the
samples based on MIL-53(Cr). In both scanning electron
microscopy (SEM) and transmission electron microscopy
(TEM) images of pristine zeolite Y (Figures S12a and 3b),
cubic crystals can be identified. However, the cubic crystals
disappear, and amorphous phases become visible in the images
of K@Y-700 (Figures S12b and 3c). SBA-15 presents a rod-like
morphology in the SEM image, which is partially degraded on
K@SBA-15-300 and completely destroyed on K@SBA-15-600
(Figure S13). Similarly, ordered mesopores can be seen from
the TEM image of SBA-15, while the damage of mesostructure
takes place in K@SBA-15-300 and K@SBA-15-600 (Figures
3e,f and S14). On the contrary, the original crystalline
morphology of MIL-53(Cr) is well retained after incorporating
basic K species from both SEM and TEM images (Figures 3h,i
and S15). EDX and elemental mapping analysis indicate the
existence of K that uniformly distributed in the samples K@Y,
K@SBA-15, and K@MIL-53(Cr), despite the presence or
absence of ordered structure after activation (Figures S16−19).
On the basis of the results from different characterization
techniques, it is safe to say that at the temperature for KNO3
Table 1. Knoevenagel Condensation of Benzaldehyde with
Different Active Methylene Compounds Catalyzed by K@
a
MIL-53(Cr)-300
a
Reaction conditions: 1 mmol of benzaldehyde, 1 mmol of active
methylene compounds, 50 mg of catalyst, 2.5 mL of ethanol, 70 °C, 1
h. EWG = electron-withdrawing group such as CN, COOR, and COR.
E
DOI: 10.1021/acs.chemmater.7b05102
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
malononitrile gives a high yield of 94.8%. In the case of ethyl
cyanoacetate, the yield decreases to 81.6%. It is worth noting
that the yield is only 20.1% for the reaction of benzaldehyde
with ethyl acetoacetate, and the yield is less than 1.0% for the
reaction involving diethyl malonate. Two factors are considered
to account for the significant difference in reactivity. The first
factor is the acidity of active methylene compound. The acidity
decreases in the order of malononitrile (pKa = 11.1) > ethyl
cyanoacetate (pKa = 13.1) > ethyl acetoacetate (pKa = 14.3) >
diethyl malonate (pKa = 16.4), which is in good agreement with
the reactivity. Because the active methylene compound
functions as a donor molecule in base-catalyzed Knoevenagel
condensation, the reactivity declines with the increase of pKa
values of the donor molecule. The other factor is steric
hindrance. The yield of target products generally decreases with
the increase of reactant/product molecular dimension. For the
reactions of benzaldehyde with ethyl acetoacetate and diethyl
malonate, the yields are obviously lower than those with
malononitrile and ethyl cyanoacetate. Taking into account that
the pore size of MIL-53(Cr) is 7.85 Å,28 the shape selectivity
will become obvious when reactant/product molecules possess
comparable sizes. According to the molecular sizes shown in
Table 1 and Figure S22, the Knoevenagel products of
malononitrile (11.492 Å × 7.739 Å) and ethyl cyanoacetate
(13.931 Å × 7.891 Å) are able to transport from the pores of
catalyst considering molecular vibration. Nevertheless, trans-
portation of Knoevenagel products of ethyl acetoacetate
(12.585 Å × 9.648 Å) and diethyl malonate (12.733 Å ×
9.261 Å) is apparently difficult. The results reflect that the yield
of diethyl 2-benzylidenemalonate is rationally reduced on
account of the limited space in the K@MIL-53(Cr).
Besides benzaldehyde, the aromatic aldehyde with larger
molecule size, 1-naphthaldehyde, was applied to react with four
methylene substrates (Figure 4). The yield is 30.3% for the
reaction of 1-naphthaldehyde with ethyl cyanoacetate (30.3%),
and is much lower than that of benzaldehyde (81.6%).
Similarly, the yield for the reaction of 1-naphthaldehyde with
malononitrile, ethyl acetoacetate, and diethyl malonate is
77.6%, 2.0%, and 0.2%, respectively, which is lower than that
of benzaldehyde (94.8%, 20.1%, and 0.8%, respectively).
Moreover, the reaction between ethyl cyanoacetate and two
different steric substrates benzaldehyde and 1-naphthaldehyde
was carried out in one pot. The reaction with benzaldehyde
leads to a high yield of 61.8%, which is obviously higher than
that with 1-naphthaldehyde (14.6%). These results can thus
demonstrate the shape selectivity of the present solid base K@
MIL-53(Cr)-300.
Figure 4. (a) Knoevenagel condensation of benzaldehyde or 1-
naphthaldehyde with different active methylene compounds over K@
MIL-53(Cr)-300. (b) Filtration test of the catalyst K@MIL-53(Cr)-
300 for the reaction between benzaldehyde and ethyl cyanoacetate. (c)
Recycling experiments of K@MIL-53(Cr)-300 for the reaction
between benzaldehyde and ethyl cyanoacetate.
catalysis. The K/Cr ratio of K@MIL-53(Cr)-300 before and
after reaction was tested as well. On the basis of the SEM-EDX
spectra (Figure S20), the K/Cr ratio of K@MIL-53(Cr)-300 is
0.23 after reaction, which is comparable to the value before
reaction (0.21). This mirrors that there is no leaching of K in
the reaction processes.
The obtained solid base K@MIL-53(Cr)-300 was further
used for the synthesis of dimethyl carbonate (DMC) via the
transesterification of ethylene carbonate and methanol, which is
catalyzed by materials with strong basicity.19 DMC is a versatile
green chemical and has been utilized as a carbonylating and
methylating agent as well as a promising fuel additive.38,39
Traditionally, DMC is synthesized in the presence of
homogeneous catalysts, whereas increasing attention has been
paid to the development of heterogeneous catalysts. As listed in
Table 2, the yield of DMC increases with the prolonged
reaction time over all catalysts. Under the catalysis of K@MIL-
53(Cr)-300, the yield of DMC reaches 26.9% after the reaction
for 4 h. It is interesting to compare the catalytic activity of K@
MIL-53(Cr)-300 with some well-known solid bases. Under the
same reaction conditions, the yield of DMC is only 7.6% over
the benchmark solid base MgO. For the strongly basic zeolite
KL, the yield of DMC is 4.0%. The zeolite CsX with even
stronger basicity than KL was also employed. CsX gives a DMC
yield of 6.1%, which is higher than that over KL but much lower
To prove the heterogeneous nature of K@MIL-53(Cr)-300,
hot-filtration during catalysis to remove the catalyst was
conducted. In the heterogeneous case, it should be no catalytic
activity after the removal of catalysts. The catalyst K@MIL-
53(Cr)-300 was removed after 10 min for the reaction between
benzaldehyde and ethyl cyanoacetate. The yield was 34.9% in
10 min and kept at this level in 60 min. However, the yield
reached 81.6% for the reaction without filtration of catalyst.
These results can thus demonstrate the heterogeneous nature
during catalysis. Meanwhile, recycle experiments were con-
ducted to test the catalytic efficiency. The recovered catalyst
K@MIL-53(Cr)-300 shows similar activity as compared to the
fresh one in the reaction between benzaldehyde and ethyl
cyanoacetate. The yield remains at a high level of 79.1% at the
fifth run, which is comparable to that of the first run (81.6%).
This indicates no degradation of K@MIL-53(Cr)-300 during
F
DOI: 10.1021/acs.chemmater.7b05102
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
Table 2. Transesterification Reaction of Methanol with
a
Ethylene Carbonate Catalyzed by Various Solid Bases
a
Reaction conditions: 0.5 mol of methanol, 0.1 mol of ethylene
carbonate, 50 mg of catalyst, 20 mL of methanol, 65 °C.
than that over K@MIL-53(Cr)-300. On the basis of these
results, it is safe to say that the present material K@MIL-
53(Cr)-300 owns strong basicity in addition to shape selectivity
in catalysis.
By use of the GHR strategy, KNO3 can be converted to basic
sites at a low temperature of 300 °C on MIL-53(Cr). This
strategy is obviously different from thermal decomposition of
KNO3 on conventional supports including zeolite and
mesoporous silica, which is usually conducted at quite high
temperatures (600−700 °C). The low-temperature conversion
of KNO3 on MIL-53(Cr) can be ascribed to the redox
interaction between KNO3 and Cr metal centers at a low
valence state (Cr3+), which is absent in conventional supports.
To verify the mechanism, the K-containing MIL-53(Cr) was
analyzed by X-ray photoelectron spectroscopy (XPS) and
compared to pristine MIL-53(Cr). The Cr 2p region was
studied in detail. For pristine MIL-53(Cr), the Cr 2p3/2 and Cr
2p1/2 spectra at the binding energies (BE) of 577.6 and 587.1
eV are observed (Figure 5a), which are attributed to Cr3+
species.40 In the case of K@MIL-53(Cr)-300, two shoulder
peaks at BE values of 579.3 and 588.7 eV ascribed to Cr6+
appear (Figure 5b), indicating that Cr3+ in MIL-53(Cr) is
partially oxidized to Cr6+ during the reduction of KNO3 to basic
species. To confirm the generation of Cr6+, diffuse reflectance
UV−vis spectra of the support MIL-53(Cr) and solid base K@
MIL-53(Cr)-300 were recorded. MIL-53(Cr) presents three
pronounced absorption bands at about 295, 455, and 596 nm,
which are assigned to the d−d transition of Cr3+ species (Figure
5c). For the sample K@MIL-53(Cr)-300, a new peak at 263
nm becomes visible along with the blue shift of the peak at 295
nm caused by ligand-to-metal charge transfer, implying the
formation of Cr6+.41 Taking into consideration that the content
of KNO3 is 10 wt % in the sample, 8.4% of Cr3+ should be
oxidized to Cr6+ provided that all KNO3 is reduced to K2O.
The quantitative analysis of XPS shows that 9.2% of Cr6+ is
yielded, which is in line with the theoretical value. The organic
ligands in MIL-53(Cr) with an empty antibonding orbital
(unsaturated hydrocarbons and double bonds) can accept
electrons from metal ions. In the redox process, electron
transfer from the d orbital of metal ions to the π* orbital of
ligands takes place, leading to the decline in the peaks of 455
and 596 nm.42,43 Hence, the generation of basic sites on MIL-
53(Cr) is caused by the guest−host redox interaction, in which
KNO3 is reduced to K2O and Cr3+ is oxidized to Cr6+. This
makes the formation of basic sites on MIL-53(Cr) at a quite
Figure 5. XPS of (a) MIL-53(Cr) and (b) K@MIL-53(Cr)-300
samples. (c) UV−vis spectra of MIL-53(Cr) and K@MIL-53(Cr)-300
samples.
low temperature, and the ordered nanoporous structure is well
maintained.
CONCLUSION
■
We have demonstrated direct fabrication of strong basic sites
on MOFs by using the GHR strategy between base precursors
and metal centers at a low valence state, for the first time. This
breaks the tradition of thermo-induced decomposition of base
precursors. As a result, base precursor KNO3 can be converted
to strong basic species on MIL-53(Cr) at 300 °C, which is
much lower than the thermal decomposition of KNO3 on
representative nanoporous supports zeolite Y (∼700 °C) and
mesoporous silica SBA-15 (∼600 °C) as well as any porous
supports reported to date. Strong bases with ordered
nanoporous structure are successfully fabricated from MOFs,
while weak basicity and collapsed structure are observed on
zeolites and mesoporous silicas due to the reactions between
strong basic species and supports at high temperatures. The
resultant new solid base exhibits high activity and shape
selectivity in Knoevenagel condensation and transesterification
reactions, and the activity is higher than various classic solid
base catalysts. The present strategy may open an avenue for the
fabrication of various functional materials by use of guest−host
interaction.
G
DOI: 10.1021/acs.chemmater.7b05102
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
of Lithium Sites and High Basic Catalytic Activity. Chem. Commun.
2014, 50, 11299−11302.
(13) Chen, S. Y.; Huang, C. Y.; Yokoi, T.; Tang, C. Y.; Huang, S. J.;
Lee, J. J.; Chan, J. C. C.; Tatsumi, T.; Cheng, S. Synthesis and
Catalytic Activity of Amino-Functionalized SBA-15 Materials with
Controllable Channel Lengths and Amino Loadings. J. Mater. Chem.
2012, 22, 2233−2243.
(14) Sun, L. B.; Liu, X. Y.; Li, A. G.; Liu, X. D.; Liu, X. Q. Template-
Derived Carbon: An Unexpected Promoter for the Creation of Strong
Basicity on Mesoporous Silica. Chem. Commun. 2014, 50, 11192−
11195.
(15) Liu, X. Y.; Sun, L. B.; Liu, X. D.; Li, A. G.; Lu, F.; Liu, X. Q.
Low-Temperature Fabrication of Mesoporous Solid Strong Bases by
Using Multifunction of A Carbon Interlayer. ACS Appl. Mater.
Interfaces 2013, 5, 9823−9829.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-
ter.7b05102.
■
S
Experimental details and additional data (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: lbsun@njtech.edu.cn.
ORCID
Yao Jiang: 0000-0002-2316-8274
Lin-Bing Sun: 0000-0002-6395-312X
Notes
■
(16) Sun, Y. H.; Sun, L. B.; Li, T. T.; Liu, X. Q. Modulating the Host
Nature by Coating Alumina: A strategy to Promote Potassium Nitrate
Decomposition and Superbasicity Generation on Mesoporous Silica
SBA-15. J. Phys. Chem. C 2010, 114, 18988−18995.
The authors declare no competing financial interest.
(17) Liu, X. Y.; Sun, L. B.; Lu, F.; Li, T. T.; Liu, X. Q. Constructing
Mesoporous Solid Superbases by A Dualcoating Strategy. J. Mater.
Chem. A 2013, 1, 1623−1631.
(18) Jiang, W. J.; Yin, Y.; Liu, X. Q.; Yin, X. Q.; Shi, Y. Q.; Sun, L. B.
Fabrication of Supported Cuprous Sites at Low Temperatures: An
efficient, Controllable Strategy Using Vapor-Induced Reduction. J. Am.
Chem. Soc. 2013, 135, 8137−8140.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21722606, 21576137, and 21676138)
and the Project of Priority Academic Program Development of
Jiangsu Higher Education Institutions.
(19) Zhu, L.; Lu, F.; Liu, X. D.; Liu, X. Q.; Sun, L. B. A New Redox
Strategy for Low-Temperature Formation of Strong Basicity on
Mesoporous Silica. Chem. Commun. 2015, 51, 10058−10061.
(20) Dhakshinamoorthy, A.; Asiri, A. M.; Garcia, H. Metal-Organic
Frameworks Catalyzed C-C and C-Heteroatom Coupling Reactions.
Chem. Soc. Rev. 2015, 44, 1922−1947.
(21) Chughtai, A. H.; Ahmad, N.; Younus, H. A.; Laypkov, A.;
Verpoort, F. Metal-Organic Frameworks: Versatile Heterogeneous
Catalysts for Efficient Catalytic Organic Transformations. Chem. Soc.
Rev. 2015, 44, 6804−6849.
(22) Valvekens, P.; Jonckheere, D.; De Baerdemaeker, T.; Kubarev,
A. V.; Vandichel, M.; Hemelsoet, K.; Waroquier, M.; Van Speybroeck,
V.; Smolders, E.; Depla, D.; Roeffaers, M. B. J.; De Vos, D. Base
Catalytic Activity of Alkaline Earth MOFs: A (Micro)Spectroscopic
Study of Active Site Formation by the Controlled Transformation of
Structural Anions. Chem. Sci. 2014, 5, 4517−4524.
(23) Sorribas, S.; Gorgojo, P.; Tellez, C.; Coronas, J.; Livingston, A.
G. High Flux Thin Film Nanocomposite Membranes Based on Metal-
Organic Frameworks for Organic Solvent Nanofiltration. J. Am. Chem.
Soc. 2013, 135, 15201−15208.
(24) Zhu, L.; Liu, X. Q.; Jiang, H. L.; Sun, L. B. Metal-Organic
Frameworks for Heterogeneous Basic Catalysis. Chem. Rev. 2017, 117,
8129−8176.
(25) Choluj, A.; Nikishkin, N. I.; Chmielewski, M. J. Facile Post-
Synthetic Deamination of MOFs and the Synthesis of the Missing
Parent Compound of the MIL-101 Family. Chem. Commun.
(Cambridge, U. K.) 2017, 53, 10196−10199.
(26) Kolokolov, D. I.; Jobic, H.; Rives, S.; Yot, P. G.; Ollivier, J.;
Trens, P.; Stepanov, A. G.; Maurin, G. Diffusion of Benzene in the
Breathing Metal−Organic Framework MIL-53(Cr): A Joint Exper-
imental−Computational Investigation. J. Phys. Chem. C 2015, 119,
8217−8225.
(27) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G.
H.; Chmelka, B. F.; Stucky, G. D. Triblock Copolymer Syntheses of
Mesoporous Silica with Periodic 50 to 300 Angstrom Pores. Science
1998, 279, 548−592.
REFERENCES
■
(1) Zhao, M.; Yuan, K.; Wang, Y.; Li, G.; Guo, J.; Gu, L.; Hu, W.;
Zhao, H.; Tang, Z. Metal-Organic Frameworks as Selectivity
Regulators for Hydrogenation Reactions. Nature 2016, 539, 76−80.
(2) Mondal, B.; Acharyya, K.; Howlader, P.; Mukherjee, P. S.
Molecular Cage Impregnated Palladium Nanoparticles: Efficient,
Additive-Free Heterogeneous Catalysts for Cyanation of Aryl Halides.
J. Am. Chem. Soc. 2016, 138, 1709−1716.
(3) Sun, L. B.; Liu, X. Q.; Zhou, H. C. Design and Fabrication of
Mesoporous Heterogeneous Basic Catalysts. Chem. Soc. Rev. 2015, 44,
5092−5147.
(4) Molinari, V.; Giordano, C.; Antonietti, M.; Esposito, D. Titanium
Nitride-Nickel Nanocomposite as Heterogeneous Catalyst for the
Hydrogenolysis of Aryl Ethers. J. Am. Chem. Soc. 2014, 136, 1758−
1761.
(5) Buss, F.; Mehlmann, P.; Muck-Lichtenfeld, C.; Bergander, K.;
Dielmann, F. Reversible Carbon Dioxide Binding by Simple Lewis
Base Adducts with Electron-Rich Phosphines. J. Am. Chem. Soc. 2016,
138, 1840−1843.
(6) Blumel, M.; Crocker, R. D.; Harper, J. B.; Enders, D.; Nguyen, T.
V. N-Heterocyclic Olefins as Efficient Phase-Transfer Catalysts for
Base-Promoted Alkylation Reactions. Chem. Commun. 2016, 52,
7958−7961.
(7) Chaudhary, R.; Dhepe, P. L. Solid Base Catalyzed Depolymeriza-
tion of Lignin into Low Molecular Weight Products. Green Chem.
2017, 19, 778−788.
(8) Moliner, M.; Martinez, C.; Corma, A. Multipore Zeolites:
Synthesis and Catalytic Applications. Angew. Chem., Int. Ed. 2015, 54,
3560−3579.
(9) Liang, J.; Liang, Z.; Zou, R.; Zhao, Y. Heterogeneous Catalysis in
Zeolites, Mesoporous Silica, and Metal-Organic Frameworks. Adv.
Mater. 2017, 29, 1701139−1701160.
(10) Awala, H.; Gilson, J. P.; Retoux, R.; Boullay, P.; Goupil, J. M.;
Valtchev, V.; Mintova, S. Template-Free Nanosized Faujasite-Type
Zeolites. Nat. Mater. 2015, 14, 447−451.
(28) Serre, C.; Millange, F.; Thouvenot, C.; Nogues, M.; Marsolier,
G.; Louene, D. Ferey, a. G. Very Large Breathing Effect in the First
Nanoporous Chromium(III)-Based Solids: MIL-53 or CrIII(OH)·
{O2C−C6H4−CO2}·{HO2C−C6H4−CO2H}x·H2Oy. J. Am. Chem. Soc.
2002, 124, 13519−13526.
(11) Zhang, P.; Wang, L.; Yang, S.; Schott, J. A.; Liu, X.; Mahurin, S.
M.; Huang, C.; Zhang, Y.; Fulvio, P. F.; Chisholm, M. F.; Dai, S. Solid-
State Synthesis of Ordered Mesoporous Carbon Catalysts via A
Mechanochemical Assembly through Coordination Cross-linking. Nat.
Commun. 2017, 8, 15020−15028.
(12) Sun, L. B.; Shen, J.; Lu, F.; Liu, X. D.; Zhu, L.; Liu, X. Q.
Fabrication of Solid Strong Bases with A Molecular-Level Dispersion
(29) Hasegawa, Shinpei; Horike, Satoshi; Matsuda, Ryotaro;
Furukawa, Shuhei; Mochizuki, Katsunori; Kinoshita, Yoshinori;
H
DOI: 10.1021/acs.chemmater.7b05102
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
Kitagawa, a. S. Three-Dimensional Porous Coordination Polymer
Functionalized with Amide Groups Based on Tridentate Ligand:
Selective Sorption and Catalysis. J. Am. Chem. Soc. 2007, 129, 2607−
2614.
(30) Kleitz, F.; Schmidt, W.; Schuth, F. Calcination Behavior of
̈
Different Surfactant-Templated Mesostructured Silica Materials.
Microporous Mesoporous Mater. 2003, 65, 1−29.
(31) Bae, Y. K.; Han, O. H. Removal of Copolymer Template from
SBA-15 Studied by 1H MAS NMR. Microporous Mesoporous Mater.
2007, 106, 304−307.
(32) Wang, C.; Liu, D.; Lin, W. Metal-Organic Frameworks as A
Tunable Platform for Designing Functional Molecular Materials. J. Am.
Chem. Soc. 2013, 135, 13222−13234.
(33) Sun, L. B.; Gu, F. N.; Chun, Y.; Yang, J.; Wang, Y.; Zhu, J. H.
Attempt to Generate Strong Basicity on Silica and Titania. J. Phys.
Chem. C 2008, 112, 4978−4985.
(34) Sun, L. B.; Yang, J.; Kou, J. H.; Gu, F. N.; Chun, Y.; Wang, Y.;
Zhu, J. H.; Zou, Z. G. One-pot Synthesis of Potassium-Functionalized
Mesoporous Gamma-Alumina: A Solid Superbase. Angew. Chem., Int.
Ed. 2008, 47, 3418−3421.
(35) Manna, K.; P, Ji; Greene, F. X.; Lin, W. Metal-Organic
Framework Nodes Support Single-site Magnesium-Alkyl Catalysts for
Hydroboration and Hydroamination Reactions. J. Am. Chem. Soc.
2016, 138, 7488−7491.
(36) Valvekens, P.; Vandichel, M.; Waroquier, M.; Van Speybroeck,
V.; De Vos, D. Metal-Dioxidoterephthalate MOFs of the MOF-74
Type: Microporous Basic Catalysts with Well-Defined Active Sites. J.
Catal. 2014, 317, 1−10.
(37) Garrabou, X.; Wicky, B. I.; Hilvert, D. Fast Knoevenagel
Condensations Catalyzed by an Artificial Schiff-Base-Forming Enzyme.
J. Am. Chem. Soc. 2016, 138, 6972−6974.
(38) Bansode, A.; Urakawa, A. Continuous DMC Synthesis from
CO2 and Methanol over a CeO2 Catalyst in a Fixed Bed Reactor in the
Presence of a Dehydrating Agent. ACS Catal. 2014, 4, 3877−3880.
(39) Shukla, K.; Srivastava, V. C. Synthesis of Organic Carbonates
from Alcoholysis of Urea: A Review. Catal. Rev.: Sci. Eng. 2017, 59, 1−
43.
(40) Liang, R.; Jing, F.; Shen, L.; Qin, N.; Wu, L. MIL-53(Fe) As A
Highly Efficient Bifunctional Hotocatalyst for the Simultaneous
Reduction of Cr(VI) and Oxidation of Dyes. J. Hazard. Mater.
2015, 287, 364−372.
(41) Wei, Z.; Gu, Z. Y.; Arvapally, R. K.; Chen, Y. P.; McDougald, R.
N., Jr.; Ivy, J. F.; Yakovenko, A. A.; Feng, D.; Omary, M. A.; Zhou, H.
C. Rigidifying Fluorescent Linkers by Metal-Organic Framework
Formation for Fluorescence Blue Shift and Quantum Yield Enhance-
ment. J. Am. Chem. Soc. 2014, 136, 8269−8276.
(42) Zhao, H.; Song, H.; Xu, L.; Chou, L. Isobutane Dehydrogen-
ation over the Mesoporous Cr2O3/Al2O3 Catalysts Synthesized from A
Metal-Organic Framework MIL-101. Appl. Catal., A 2013, 456, 188−
196.
(43) Shi, L.; Wang, T.; Zhang, H.; Chang, K.; Meng, X.; Liu, H.; Ye,
J. An Amine-Functionalized Iron(III) Metal-Organic Framework as
Efficient Visible-Light Photocatalyst for Cr(VI) Reduction. Adv. Sci.
2015, 2, 150006−1500014.
I
DOI: 10.1021/acs.chemmater.7b05102
Chem. Mater. XXXX, XXX, XXX−XXX