Robust Mg(Ca)Zr-Doped Acid-Base Bifunctional Mesoporous Silica and Their Applications in the Deacetalization-Knoevenagel Reaction

Article abstract of DOI:10.1021/acs.inorgchem.1c00819

A series of Mg(Ca)Zr-doped acid-base bifunctional mesoporous silica were synthesized to study the impact of the one-step or two-step impregnation method on material structure. The two-step method seems to be a better way to synthesize metal-based functionalized catalyst and their catalytic performance is investigated using deacetalization-Knoevenagel reaction as the probe reaction. The coexisting dual active sites and suitable designing routes endowed highly efficient (Conv. >99.6%, Sel. >99.8%) and robust stability (10 consecutive cycles) of these materials. The present process succeeded in preparing catalysts decorated with acid-base sites by doping acidic and alkali metal species rather than grafting organic groups.

Full text of DOI:10.1021/acs.inorgchem.1c00819

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Robust Mg(Ca)Zr-Doped Acid−Base Bifunctional Mesoporous Silica
and Their Applications in the Deacetalization-Knoevenagel Reaction
Menglu Cai, Xiaozhong Wang, Yangyang Fang, Yingqi Chen, and Liyan Dai*
Cite This: Inorg. Chem. 2021, 60, 8924−8935
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ABSTRACT: A series of Mg(Ca)Zr-doped acid−base bifunctional mesoporous silica were synthesized to study the impact of the
one-step or two-step impregnation method on material structure. The two-step method seems to be a better way to synthesize metal-
based functionalized catalyst and their catalytic performance is investigated using deacetalization-Knoevenagel reaction as the probe
reaction. The coexisting dual active sites and suitable designing routes endowed highly efficient (Conv. >99.6%, Sel. >99.8%) and
robust stability (10 consecutive cycles) of these materials. The present process succeeded in preparing catalysts decorated with acid−
base sites by doping acidic and alkali metal species rather than grafting organic groups.
uneven distribution of active sites, complicated preparation
method, and poor universality caused by the deactivation
between acid and base sites.15
1. INTRODUCTION
The rising demand for an environmentally friendly and
economical synthetic process has attracted the attention of
both academia and the industry to develop an efficient catalytic
method for one-pot or tandem reactions.1−5 The main focus is
on finding a suitable catalytic protocol that potentially
minimizes the processing times, skips the costly separation
and purification steps, as well as achieves higher efficiency.
Some literature about tandem reactions has been published to
fulfill the aforesaid requirements,6−9 such as the development
of heterobimetallic catalyst application on metal-catalyzed
tandem reaction7 and the other research about autotandem
catalysis based on transition metal catalysts.9 Furthermore, a
bifunctional catalysis system seems to be more promising due
to the possibility of modification and enriched active sites.6
More specifically, owing to the easily deactivating behavior of
acid and base sites in a homogeneous catalytic system,
bifunctional heterogeneous catalysts have gradually emerged
as a major candidate and have exhibited excellent catalytic
performance in one-pot reactions.10−13 For example, an
efficient and robust hollow spherical structure bifunctional
catalyst was tailored by Gu’s group and exhibited high catalytic
performance in hydrolysis/Henry and hydrolysis/Knoevenagel
reactions.14 Nonetheless, the synthesis of desirable acid−base
bifunctional catalysts still has inherent problems, such as
Thus, there are many ongoing research studies on
decorating functionalized groups of the raw materials,16−18
among which finding a suitable support is an efficient and
hopeful solution to limit the interaction between the acid and
base sites, such as using silicas,19,21,22 polymers,23,24 metal−
organic frameworks,18,25,26 and graphene oxides10,20 as
supports to graft acid−base groups. Fan’s group fabricated an
organic amine-functionalized graphene oxide (GO) containing
sulfonic acid groups and the coexistence of acidic and basic
groups made excellent contribution to the catalytic perform-
ance in the one-pot reaction.20 Shanks et al. investigated the
acid−base cooperativity of the mesoporous silica and proposed
that the catalytic performance might highly rely on the
proximity of dual sites.21 Moreover, Gao et al. used a
Received: March 17, 2021
Published: June 8, 2021
https://doi.org/10.1021/acs.inorgchem.1c00819
© 2021 American Chemical Society
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postsynthetic modification method to prepare a new MIL-101
metal−organic framework functionalized with amino and sulfo
groups. This bifunctional catalyst had excellent performance in
the one-pot deacetalization-Knoevenagel reaction.25 However,
grafting acidic and amino groups on the matrix to accomplish
functionalization has several drawbacks. The preparation
process is quite complicated for grafting groups step by step,
and sometimes, a protecting step is needed to avoid self-
assembly,27,28 which increases the preparation steps. For
example, Lee et al. used −NO2, −SO3H as the terephthalate
linkers to prepare MIL-101-NH2-SO3H bifunctional catalyst,
and a reduction step was necessary.27 Alauzun et al.
successfully obtained bifunctional mesoporous materials
through several steps, specifically, a reduction of disulfide
unites and subsequently oxidation to the −SO3H group under
amino groups protection.28 A complex preparation process
definitely hindered their application. Meanwhile, the prepared
catalyst needs to face the risk of inferior catalytic performance
due to the mutual quenching as well as the steric hindrance
effect.29
Fortunately, introducing acidity/basicity metal sources into
the matrix has been regarded as a promising method to avoid
the shortcomings of the synthesis of the acid−base bifunctional
catalysts mentioned above. Fan et al. synthesized a series of
MIL-101(Al/Fe)-NH2(X) catalysts using a rapid reflux
method, where the Al3+/Fe3+ metal clusters reacted as Lewis
acid sites and the amino groups played as Brønsted basic
sites.30 Due to the property of MOFs, such as the high specific
surface area, this method avoided the group protection and
reduced the possibility of acid/base neutralization. Benefitting
from the dual active sites, the functionalized bimetallic catalysts
achieved enhanced catalytic activities in the deacetalization-
Knoevenagel reaction compared to the monometallic catalysts.
The above achievements encourage us to get a new insight
into the acid−base materials, that is, doping acidic or/and
alkali metal species onto high-surface-area support materials.
Notably, the mesoporous silica has several merits that can
satisfy the requirements of “modification”. The higher specific
surface area of the materials guaranteed the good dispersion of
acid/base sites as well as provided abundant room for the
subsequent dispersion of base/acid sites. Therefore, the use of
mesoporous materials to avoid the self-association of metal
particles as well as adjusting the acid−base property has been
highly appreciated.
An alternative route for synthesizing acid−base bifunctional
materials is to incorporate other metal species onto acidic or
alkaline supports. It is well known that doping acidic metal
species (e.g., Al3+, Ti2+, and Zr4+) into the silica skeleton could
prepare the acidic mesoporous silica directly. Meanwhile,
alkaline zeolites could be obtained by inducing basic metal
species (e.g., Ca2+ and Mg2+) into the silicon precursor via the
hydrothermal method, where, due to the high specific surface
area of zeolites, the acidic metal species could be well dispersed
on the surface to avoid the neutralization between acid and
base sites.31
Herein, a facile way to synthesize functionalized mesoporous
material based on metal oxides has been proposed (Scheme 1).
The high surface area of the support circumvented the
interaction between the acid and base sites, and we found
anchoring acid/base species onto the support step by step can
prevent agglomeration to a certain degree. Moreover, using
acidic and alkali metal species rather than grafting organic
acidic and basic groups as the origin of dual sites enabled the
Scheme 1. Synthesis Procedure of Mg-Zr/Ti-HMS-T: (a)
General View of Ti-HMS Skeleton and (b) Morphology of
Mg-Zr/Ti-HMS-T
tailoring of the active sites as well as shortened the preparation
steps. The catalytic performance and robustness of the
fabricated bifunctional catalysts were demonstrated in a one-
pot deacetalization-Knoevenagel reaction, and excellent
reaction results were obtained.
2. EXPERIMENTAL SECTION
2.1. Catalyst Preparation. The Ti-containing mesoporous silica
(Ti-HMS, Si/Ti = 100:1) material was prepared by a sol−gel method
using a customary quaternary ammonium ion template according to
the literature.32−34 The catalysts Mg(Ca)-Zr/Ti-HMS-T were
obtained by a wet-impregnation method. Forty milliliters of
Zr(NO3)4·5H2O (1 mmol) aqueous solution containing Ti-HMS
(1.0 g) was treated under vigorous stirring for 4 h. After removing the
excess water at 70 °C, the sample was calcined at 350 °C for 2 h to get
Zr/Ti-HMS. Afterward, 40 mL of Mg(NO3)2·6H2O or Ca(NO3)2·
4H2O (1 mmol) aqueous solution containing Zr/Ti-HMS was treated
under vigorous stirring for 4 h to form a slurry. Then, the slurry was
dried at 70 °C and the resulting solid was calcined at 550 °C for 5 h to
give the target Mg(Ca)-Zr/Ti-HMS-T catalyst. For convenience, the
prepared samples were named as yMg(Ca)-Zr/Ti-HMS, where “y”
represents the mole ratio of Mg(Ca) to Zr. For example, Mg(Ca)-Zr/
Ti-HMS meant that the mole ratio of Mg to Zr was 1, and the catalyst
was prepared by a two-step wet-impregnation method (Mg-Zr/Ti-
HMS-T). For comparison, Mg(Ca)-Zr/Ti-HMS-O catalysts were
prepared using a one-step method to investigate the mutual
neutralization and agglomeration effects.
2.2. Characterizations. X-ray diffraction (XRD) spectra of the
prepared samples were recorded on a PANalytical X’ Pert
diffractometer using a Cu Kα as a radiation source (λ = 1.5406 Å).
Fourier transform infrared (FT-IR) spectra were collected in an
IRAffinity-1S (Shimadzu, Japan) spectrometer with KBr pellets from
4000 to 400 cm−1. Transmission electron microscopy (TEM) and
high-resolution TEM (HRTEM) measurements were recorded to
observe the morphology of the catalyst on HT-7700 operating at 120
kV. Field emission scanning microscopy (FESEM) images were
recorded on a Hitachi SU-70 microscope equipped with an energy-
dispersive spectrometry (EDS) detector (X-MaxN 80T, Oxford
Instruments, U.K.). The surface chemical composition of the prepared
samples was examined by X-ray photoelectron spectroscopy (XPS) on
a Thermo Scientific ESCALAB 250Xi spectrometer using Al Kα
radiation (1486.6 eV). The binding energy of the C 1s peak (BE =
284.8 eV) was used as the reference to calibrate the XPS data. A N2
adsorption−desorption test was conducted on a Micromeritics ASAP
2020 analyzer at −196 °C to examine the specific surface areas, pore
size, and volume of the prepared catalysts using the Brunauer−
Emmett−Teller (BET) method and the BJH method, respectively.
The acid−base property of the obtained materials was investigated by
the temperature-programmed desorption (TPD) of NH3 and CO2
measurements (AutoChem II 2920, Micromeritics). Thermogravi-
metric analysis was performed on a TGA Q500 V20.13, and the
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Figure 1. FESEM images of (a) Ti-HMS, (b) Zr/Ti-HMS, (c) Mg-Zr/Ti-HMS-T, and (d) Ca-Zr/Ti-HMS-T.
samples were heated at 50 to 800 °C at a heating rate of 10 °C min−1
amorphous silica structure. A broad peak between 1.5 and 3°
was a characteristic reflection (100) of the hexagonal structure
of the materials, and no other peaks were observed at higher
angles, indicating a long-range orderliness rather than a short-
range one in the obtained materials.35,36 After the incorpo-
ration of acidic or basic metal species, the crystallinity was
decreased as the corresponding diffraction peak (100)
weakened or disappeared, or perhaps the partial damage of
the order of the support surface.25,37 Interestingly, the different
tendency of the low-angle XRD pattern in the Mg(Ca)-Zr/Ti-
HMS-T and Mg(Ca)-Zr/Ti-HMS-O catalysts might be caused
by the hole occurrence of the support surface, as verified by the
SEM measurements.
in air.
2.3. Activity Tests. The deacetalization-Knoevenagel reaction was
carried out to evaluate the catalytic performance of the prepared
catalysts. Typically, 2 mmol of benzaldehyde dimethylacetal, 60 mg of
catalyst, 8 mL of toluene, and 20 μL of deionized water were added
into a three-necked glass reactor (25 mL) equipped with a reflux
condenser. Under magnetic stirring, 3 mmol of malononitrile was
injected into the mixture solvent, then the temperature was heated up
to 90 °C and maintained for 9 h. The solid catalyst and solution were
separated by centrifugation (3000 rpm, 3 min) when the reaction was
complete; then, the products were analyzed by Agilent 6820 equipped
with OV-1701 capillary column and further reidentified by GC-MS
(Agilent 6890−5973).
Considering the possibility of the destruction of the
mesoporous surface in Mg(Ca)-Zr/Ti-HMS-T, the TEM
measurement was further used to detect the wormhole
structure in the prepared samples to investigate whether the
microstructure was influenced after modification. As shown in
Figure S3, Mg(Ca)-Zr/Ti-HMS-T retained the typical worm-
hole-like mesostructure,38,39 suggesting that the incorporation
of magnesium (calcium) and zirconium metal species step by
step did not cause the collapse of the silica framework.36
The FT-IR spectra of the prepared samples are presented in
Figure 2. The absorption at 459 cm−1 is related to the silanol
bending,39 and the vibration of asymmetric and symmetric
stretching vibrations of Si−O−Si are assigned to the bands at
1084 and 801 cm−1, respectively.39−41 These characteristics
also indicated the retention of the original structure, meaning
the prepared bifunctional catalysts can preserve the merits of
the mesoporous silica supports. Meanwhile, a band at 969
cm−1 for the obtained samples could be explained as the
framework Si−O−M vibrations, reflecting the incorporation of
the metal species into the silica framework.42,43 The band at
1647 cm−1 corresponded to the hydroxyl deformation mode
on the material surface.44
3. RESULTS AND DISCUSSION
A series of metal-doped bifunctional silica materials were
synthesized by the wet-impregnation method. We performed
the one-step and two-step synthesis methods to investigate the
modification sequence impact on material structure and
catalytic activities.
3.1. Characterization. In this work, the morphology of the
prepared materials was characterized by SEM. The Mg(Ca)-
Zr/Ti-HMS samples exhibited similar sheet structures as that
of Ti-HMS in Figure 1. For the one-step impregnation
method, Mg(Ca)-Zr/Ti-HMS-O showed a slight agglomer-
ation compared with Mg(Ca)-Zr/Ti-HMS-T, but regular
shapes still existed in all samples, whether as small flakes or
layered structures, indicating the stability of the Ti-HMS
support during the impregnation process (Figure S1).
Interestingly, the stacked layered structures of the prepared
Mg(Ca)-Zr/Ti-HMS-T materials (Figure 1c,d) were much
clearer than those of the Zr/Ti-HMS sample (Figure 1b). We
guess that during the preparation process, the basic metal
species would lubricate flakes to form the present stacked
layered materials, or perhaps corrode the surface of modified
supports and result in a porous catalyst.
The N2 adsorption−desorption test and the detailed textural
parameters of the prepared samples are displayed in Table 1.
The prepared catalysts still have a large specific surface area
(305.3−485.8 m2 g−1), significantly less than that of the parent
material (1103.2 m2 g−1). The relatively reversible adsorption
The low-angle and wide-angle XRD patterns were employed
to investigate the difference in the samples prepared through
the impregnation method (Figure S2). Similar profiles of the
samples in wide-angle XRD patterns demonstrated an
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of metal species into the mesoporous structure. If we
considering the radius effects, the pore size in Ca-Zr/Ti-
HMS should be larger than that in Mg-Zr/Ti-HMS; this is in
agreement with the sample value prepared in the two-step
preparation method. Also, the degree of replacement of
defective sites would decide the improvement in SBET and
pore volume.48
X-ray photoelectron spectra were used to illustrate the
information of the elemental composition and their oxidation
states in the prepared samples. Considering the destruction
possibility of the mesoporous structures in the prepared
materials via the one-step method, we characterized and
evaluated the materials prepared in the one/two-step method.
The shift in the binding energy region of O 1s (532.6 eV) in
Mg(Ca)-Zr/Ti-HMS and Mg(Ca)-Zr/Ti-HMS-O might be
attributed to the presence of basic metal species, as shown in
Figure 4a.49 Due to the low content of Ti in parent materials,
the intensity of Ti is quite low. However, the change in the
binding energies of Si 2p and Ti 2p (Figure 4b,c) indicated
that the electronic interaction between Ti and Si (Si−O−Ti)
Figure 2. FT-IR spectra of all of the catalysts prepared by the
impregnation method.
a
Table 1. Textural Parameters of the Prepared Samples
sample
Ti-HMS
Mg-Zr/Ti-HMS-T
Ca-Zr/Ti-HMS-T
Mg-Zr/Ti-HMS-O
Ca-Zr/Ti-HMS-O
SBET (m2 g−1
)
Vp(cm3 g−1
)
rp (nm)
1103.2
485.8
397.1
305.3
478.0
0.649
0.355
0.305
0.249
0.360
2.55
2.86
3.12
3.10
2.57
50
for the band at 103.2 eV was attributed to SiOx. The
existence of Mg(Ca), Zr, Ti, Si, and O elements was identified
by the wide-range XPS spectra in Figure 4d.
The chemical stability tests of Mg(Ca)-Zr/Ti-HMS were
performed by soaking them in different solvents including
CH3OH, dimethylformamide (DMF), acetonitrile, toluene,
and water for 24 h. The XRD patterns were unaltered both in
Mg-Zr/Ti-HMS and Ca-Zr/Ti-HMS after soaking in different
solvents, confirming that the prepared bifunctional materials
had chemical stability and could maintain the silica structure
during the tests (Figure 5a). A thermogravimetric test was also
performed to investigate the thermal stability, and the prepared
samples gave similar curves (Figure 5b); all samples showed a
rapid weight loss region from 50 to 120 °C because of the
physical absorption of water.37 When the temperature was
increased to 800 °C in air, the weight residues of Ti-HMS, Mg-
Zr/Ti-HMS, and Ca-Zr/Ti-HMS were 92.3, 90.1, and 88.6%,
respectively, indicating the excellent thermal stability of the
prepared materials.
3.2. Modulation Possibility of the Acid/Base Bifunc-
tional Property via the Wet-Impregnation Method. We
found that the bifunctional catalysts prepared by a one-step or
two-step method had some structural differences during the
SEM measurements and the N2 adsorption−desorption tests,
and the materials prepared via a two-step method seemed
closer to our original assumption. Hence, CO2-TPD and NH3-
a
Determined from N2 adsorption−desorption at −195.99 °C and
calculated using the BET, BJH (SBET and rp), and t-plot methods (Vp).
isotherms in Figure 3a correspond to type IV isotherms, with
H1 hysteresis loops observed at an intermediate relative
pressure, inferring the existence of the mesoporous structures
of the parent material.45 Meanwhile, the loop at the
intermediate relative pressure P/P0 = 0.4−0.6 is a characteristic
of the regular mesoporous structure, indicating that the
prepared materials except for Ca-Zr/Ti-HMS-O have uniform
pore sizes and structures.39 Usually, the loss of the specific
surface area was explained as the partial plugging of the pore or
the destruction of the framework during the modulation
process,46,47 while the similar distribution of the pore size for
all samples, ranging from 2.5 to 4 nm, indicated that the
mesoporous structure was not destroyed (Figure 3b). Thus, we
inferred that the change in the surface area was mainly caused
by the agglomeration of active species and the plugging of the
pores. The pore size distribution in functionalized materials
slightly shifted to a larger value compared to that in Ti-HMS.
Some studies discussed that the increase in the pore size might
be caused by changes in the skeleton, such as the introduction
Figure 3. (a) N2 adsorption−desorption isotherms and (b) pore size distributions of the parent material and catalysts.
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Figure 4. High-resolution XPS spectrum of (a) O 1s, (b) Si 2p, and (c) Ti 2p and the XPS survey (d) of the prepared samples.
Figure 5. (a) XRD patterns of the samples soaked in different solvents for 24 h and (b) TGA curves of Ti-HMS, Mg-Zr/Ti-HMS, and Ca-Zr/Ti-
HMS.
Figure 6. (a) NH3-TPD and (b) CO2-TPD profiles of the prepared samples via a two-step method.
TPD measurements were employed to gain a deep insight into
whether the acid/base property can be modulated by metal
oxides and the profile of each material was deconvoluted to
several peaks based on the desorption temperature. As shown
in Figure 6a, the dominant peak at 110−115 °C could be
assigned to the surface weak acid sites.48 After the modulation,
the number of acid sites in the prepared materials was
decreased while the number of base sites seemed to increase
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Table 2. Number of Acid and Base Sites According to NH3-TPD and CO2-TPD Patterns
the number of acid/base sites (mmol g−1
)
ratio of acid to base site
BS
a
b
2
sample
ANH3
Aw
AM
AS
BCO
Bw
BM
Mg-Zr/Ti-HMS
Ca-Zr/Ti-HMS
2Mg-Zr/Ti-HMS
2Ca-Zr/Ti-HMS
0.138
0.135
0.121
0.135
0.071
0.057
0.068
0.079
0.058
0.055
0.051
0.032
0.009
0.023
0.002
0.024
0.082
0.082
0.097
0.123
0.024
0.024
0.028
0.025
0.018
0.017
0.026
0.040
0.041
0.043
0.053
1.68
1.65
1.25
1.10
0.045
a
b
The number of weak (AW), moderate (AM), and strong acid sites (AS) calculated by TPD of NH3. CO2 uptakes measured by TPD of CO2. BW,
BM, and BS are the number of weak, moderate, and strong base sites, respectively.
consistently based on the basicity of MgO and CaO (Table 2).
With a continuous increase in the loading amount of basic
metal species, the total number of a weak site (110−130 °C),
moderate site (180−250 °C), and even strong base site (above
400 °C) were increased (Figure 6b)51,52 but the number of
acid sites changed abnormally.
Table 3. Catalytic Performance of the Prepared Catalysts in
the Deacetalization-Knoevenagel Reaction
a
Based on the TPD studies, the acidity was superior to
basicity as we calculated the ratio of acid to base site (Table 2).
Compared with that in Mg-Zr/Ti-HMS (0.138 mmol g−1), the
decrease of the total acid quantity in 2Mg-Zr/Ti-HMS (0.121
mmol g−1) might be due to the mutual neutralization; in other
words, two species were incompatible and tended to react with
acid sites rather than being loaded on the surface in the
situation of insufficient spare room. Specifically, catalysts
prepared via the two-step impregnation method would also
decrease the surface acidity of the samples, but this might
depend on the left load space for the latter metal species.
However, the total number of acid sites was the same in Ca-
Zr/Ti-HMS and 2Ca-Zr/Ti-HMS, seemingly not affected by
the increased number of base sites, which was beyond our
expectations. SEM was further used to detect the morphology
of those catalysts, and a certain amount of CaO was found to
be aggregated on the surface of 2Ca-Zr/Ti-HMS (Figure S4),
meaning that the strong basicity metal species might be out of
control and were apt to agglomerate during the process. If we
consider the dehydration and dehydrogenation properties of
the acid/base sites in bifunctional catalysts,53,54 the deficiency
of the acid sites would hinder the first step process in the DK
reaction and the uneven dispersion of the functional
component led to the failure of the bifunctional catalyst
preparation even though it had the acid/base property.
3.3. Catalytic Activity in a One-Pot Reaction. To
evaluate the catalytic activity, a typical acid−base-catalyzed
one-pot reaction was chosen as a probe reaction, and then the
acid−base property was investigated according to the catalytic
performance. In brief, the prepared materials (60 mg) were
evaluated in the reaction between benzaldehyde dimethylacetal
(2.0 mmol) and malononitrile (3 mmol). As listed in Table 3,
all of the catalysts afforded good catalytic performance of
benzaldehyde dimethylacetal 1, except for Ca-Zr/Ti-HMS-O,
and 2Mg(Ca)-Zr/Ti-HMS (entries 5, 7, and 8). They gave less
than 50% selectivity of the target product benzylidene
malononitrile 3, even inferior to the monocomponent catalyst
Zr/Ti-HMS (Conv. 99.9%, Sel. of 3, 89.1%). When the parent
material alone participated in the one-pot reaction, due to
weak alkalinity, the selectivity of product 3 was quite low. The
selectivity of product 3 was decreased when catalyzed by the
Mg-Zr/Ti-HMS-O (Conv. 99.5%, Sel. of 3, 86.6%) catalyst
compared with Mg-Zr/Ti-HMS-T (Conv. 99.6%, Sel. of 3,
99.8%). Combined with the SEM results that agglomeration
occurred easily in the one-step preparation method, the
different catalytic performance indicated the importance of
conv. (%)
1
sel. (%)
entry
sample
Ti-HMS
Zr/Ti-HMS
Mg-Zr/Ti-HMS-O
Mg-Zr/Ti-HMS-T
Ca-Zr/Ti-HMS-O
Ca-Zr/Ti-HMS-T
2Mg-Zr/Ti-HMS-T
2Ca-Zr/Ti-HMS-T
none
2
3
1
2
3
4
5
6
7
8
9
99.9
99.9
99.5
99.6
79.1
99.9
99.9
99.9
34.4
70.0
10.9
13.4
trace
56.1
trace
67.4
54.0
86.1
30.0
89.1
86.6
99.8
43.9
99.9
32.6
46.0
13.9
a
Reaction conditions: substrate 1 (2 mmol), malononitrile (3 mmol),
60 mg of catalyst, toluene (8 mL), H2O (20 μL), reaction time of 9 h,
and reaction temperature of 90 °C.
good dispersion. Normally, the number of base sites are
essential for the Knoevenagel condensation reaction to
proceed; the poor catalytic performance of 2Mg(Ca)-Zr/Ti-
HMS may be ascribed to the potential acid−base neutraliza-
tion and the active species agglomeration during the
preparation process, thus failing to achieve good catalytic
performance. Also, a strong basic environment during the
preparation process might corrode the hole of the materials
and subsequently prevent the transmission of the reactants and
the products.55 The reaction results clearly revealed the
intimate relations between the acid−base neutralization and
the catalytic performance, which could provide some guidance
for preparing bifunctional catalysts.
We further investigated the influence of the reaction time to
examine the formation of product 3 and intermediate product
2. As presented in Figure 7a,b, a trace conversion of substrate 1
was detected in the first 1 h and then the conversion increased
rapidly to provide the target product 3. When a certain amount
of intermediate product 2 accumulated, the Knoevenagel
reaction proceeded smoothly to form more target product 3.
Meanwhile, if we focus on the catalytic performance or rather
the catalytic efficiency, the conversion of substrate 1 reached
98.3% within 5 h in the Mg-Zr/Ti-HMS catalyst (Figure 7a),
which is a higher catalytic activity than that of Ca-Zr/Ti-HMS.
Based on the investigation of the time effect, we also filtered off
the catalyst from the system after 2 h and made the filtrate to
further react for 7 h. As shown in Figure 7c, no significant
increase in the conversion of benzaldehyde dimethylacetal was
observed, indicating heterogeneous catalysis in nature. The
yield of product 2 slightly decreased and product 3 increased
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Figure 7. Effect of reaction time on the deacetalization-Knoevenagel reaction of (a) Mg-Zr/Ti-HMS and (b) Ca-Zr/Ti-HMS. Hot filtration tests
for (c) Mg-Zr/Ti-HMS and (d) Ca-Zr/Ti-HMS in the deacetalization-Knoevenagel reaction. Reaction conditions: substrate 1 (4 mmol),
malononitrile (6 mmol), 120 mg of catalyst, toluene (16 mL), H2O (40 μL), reaction time of 9 h, and reaction temperature of 90 °C.
Figure 8. Recycling experiments of (a) Mg-Zr/Ti-HMS and (b) Ca-Zr/Ti-HMS in the deacetalization-Knoevenagel reaction. Reaction conditions:
substrate 1 (4 mmol), malononitrile (6 mmol), 120 mg of catalyst, toluene (16 mL), H2O (40 μL), reaction time of 9 h, and reaction temperature
of 90 °C.
due to the natural occurrence of the condensation reaction.
However, the conversion of benzaldehyde dimethylacetal was
gradually increased after filtering Ca-Zr/Ti-HMS (Figure 7d),
indicating slight leaching of active catalytic species.
As mentioned in several studies, the intrinsic bifunctional
activity of a catalyst is often related to the relative separation
between the acid and base sites. Because a large surface area
could promote mass transfer and anchor the acid/base sites,
the inferior catalytic performance of Ca-Zr/Ti-HMS (397.1 m2
g−1) compared to that of Mg-Zr/Ti-HMS (485.8 m2 g−1)
makes sense. Meanwhile, the occupation of the channels in
materials by Ca2+ ions might influence the transmission of the
reactants and the products.30
tory stability as there was no significant loss of activity during
the reusability test (Figure 8a). However, the Ca-Zr/Ti-HMS
catalyst had poor selectivity during the reusability test (Figure
8b). Based on the characterization of the reused catalyst, the
structural integrity of the reused catalyst Ca-Zr/Ti-HMS
remained but with some differences in morphology (Figures
S5, S6). The BET measurements showed that the surface area
of the reused Mg-Zr/Ti-HMS and Ca-Zr/Ti-HMS was
decreased and the pore size was increased to different degrees
(Figure S7 and Table S1). Materials with a high surface area as
supports could maintain the good dispersion of metal particles,
thus guaranteeing good catalytic performance;23 the inferior
selectivity of Ca-Zr/Ti-HMS in the recycling tests was quite
possibly caused by the agglomeration of some original base
sites. Meanwhile, the possibility of leaching of active metal
species in this catalyst as detected in hot filtration tests (Figure
The reusability of the prepared materials was studied to
confirm the stability of the obtained materials. During the
recycling tests, the Mg-Zr/Ti-HMS catalyst showed satisfac-
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Figure 9. Corresponding elemental distribution mapping of (a) Mg-Zr/Ti-HMS and (b) Ca-Zr/Ti-HMS.
7d) and the loss of the catalyst during centrifugation (Table
S2) were other reasons for the inferior catalytic performance in
each run.
Scheme 2. Possible Reaction Mechanism of the One-Pot
Deacetalization-Knoevenagel Reaction
Elemental mapping was further carried out to characterize
the distribution of Mg(Ca), Zr, Ti, and Si elements in the
Mg(Ca)-Zr/Ti-HMS catalyst to have a deep insight into the
relationship between the active component separation and
catalytic performance. As shown in Figure 9, Mg-Zr/Ti-HMS
had a more homogeneous distribution than Ca-Zr/Ti-HMS,
especially the zirconium that of the species. Combined with
the inferior catalytic performance when catalyzed by Ca-Zr/Ti-
HMS, the results also indicated that good dispersion was
obviously necessary for an acid−base bifunctional catalyst.
3.4. Insights into a One-Pot Cascade Reaction over
Bifunctional Catalyst. This reaction involved two sequential
steps: an acid-catalyzed deacetalization reaction and the
reaction of the resulting benzaldehyde with CH2(CN)2 to
generate benzylidene malononitrile 3 through a base-catalyzed
Knoevenagel reaction. Based on the catalytic performances and
the characterization of the prepared samples, a possible
reaction mechanism was surveyed to give a deep insight into
the acid/base property in bifunctional catalysts. More
specifically, the acid sites mainly originated from the M−O−
Si bridges of the parent material, and the base sites were
derived from the basic metal species.56,57 In the reaction,
benzaldehyde dimethylacetal was absorbed on the surface acid
sites first and the carbonyl group was attacked by water to
generate the benzaldehyde via the release of methanol
(Scheme 2).26,58 Subsequently, the CO bond of benzalde-
hyde was polarized by the acid sites to allow a nucleophilic
attack at the C atom.59 Meanwhile, the base sites activated α-H
of CH2(CN)2 to produce carbanion and further formed the
target product 3.60−62
1.10, respectively, and the total acid amount in each sample is
almost equal. The first step to form benzaldehyde is smooth
according to the reaction mechanism, which accounts for the
excellent conversion of the reactant in those materials.
Logically, the higher the number of base sites, the easier it is
to conduct the condensation reaction. However, the next step
to obtain the target product 3 is hindered in 2Mg(Ca)-Zr/Ti-
HMS-T. During the preparation process, the active component
agglomerated in 2Mg(Ca)-Zr/Ti-HMS-T as observed by SEM
(Figure S4).
The catalytic performance of different catalysts showed that
the systematic modulation of the parent material with a high
surface area by adjusting the ratio of metal species was surely
important. The transmission of the reactants and the products
seemed to be sluggish with the reduced dispersion of active
sites, meaning that even though the catalysts had a certain
acid/base property, it failed the preparation process of
bifunctional catalysts.
As shown in Table 3, Mg(Ca)-Zr/Ti-HMS-T has similar
excellent catalytic performance but 2Mg(Ca)-Zr/Ti-HMS-T
has poor selectivity in the DK reaction even though the
conversion of benzaldehyde dimethylacetal is up to 99.9%.
Combined with the TPD results (Table 2), the ratios of acid to
base sites in the catalysts mentioned are 1.68, 1.65, 1.25, and
3.5. Discussion and Guidance to Synthesize Acid−
Base Bifunctional Mesoporous Materials. In this work, we
investigated the influence of modification sequence on the wet-
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Scheme 3. Synthesis Protocols for Bifunctional Mesoporous Materials
Scheme 4. Assumption for Synthesizing Controllable Acid/Base Bifunctional Mesoporous Materials
impregnation method of material structure and achieved a
satisfying result. We found that anchoring acid/alkali metal
species on the support step by step was a good method to
prepare acid−base bifunctional mesoporous materials (Scheme
1).
alkaline mesoporous materials and expect the metal species to
be anchored inside the materials, such as the silica skeleton.31
With the help of the high specific surface area, acidic metal
species then could be easily loaded on the surface. The
neutralization seemed difficult to occur on the surface due to
the spatial isolation of acid and base sites, and the high surface
area assisted the good dispersion of acid sites, making the
modulation of the acid/base property possible.
Synthesizing bifunctional catalysts according to design 1a
would result in the agglomeration of acid and base sites as
mentioned in the acid−base neutralization theory, and the
method in design 1b can prohibit the agglomeration and
neutralization to some degree (Scheme 3). Because the uneven
dispersion of active sites would certainly weaken the catalytic
activity of the obtained materials (Table 3), anchoring one
active species on mesoporous support first and followed by the
insertion of other species into the empty room seemed
reasonable. The first step in design 1b was easy control and
could guarantee the good dispersion of acid sites due to the
high surface area of the supports. However, the insertion of a
second component relied highly on the remaining surface area.
The deficient loading area would cause the agglomeration of
metal oxides and sometimes destroy the original acidity,
consistent with the SEM and TPD results.
Hence, encouraged by the characterization and evaluation of
the prepared samples, we proposed another strategy for
synthesizing bifunctional materials based on the deep
investigation of the two design methods (Scheme 3,
Assumption). The main problem now is how to adjust the
acid/base amounts of bifunctional catalysts, and the protocol
of spatial isolation seems inevitable.63,64 As shown in Scheme
4, we could control the acidity/basicity when preparing acidic/
4. CONCLUSIONS
The robust bifunctional mesoporous catalysts were successfully
prepared based on the modulation of metal species on
mesoporous silica. During the research, we found that a high
surface area would guarantee the good dispersion of acid/base
sites and better inhibit the acid/base neutralization, which
ensured bifunctional catalysis. Mg-Zr/Ti-HMS-T was one of
the materials that exhibited high catalytic performance and
remarkable stability in the one-pot deacetalization-Knoevena-
gel reaction, indicating the importance of the high surface area
and the synthesis method. Therefore, the present work
illustrated a possible and facile strategy to synthesize high-
performance bifunctional mesoporous silica, and a study on the
synthesis of controllable acid/base bifunctional mesoporous
materials is ongoing.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00819.
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SEM images of Mg(Ca)-Zr/Ti-HMS-O, 2Mg(Ca)-Zr/
pubs.acs.org/IC
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AUTHOR INFORMATION
Corresponding Author
■
Liyan Dai − College of Chemical and Biological Engineering,
Zhejiang University, Zhejiang Provincial Key Laboratory of
Advanced Chemical Engineering Manufacture Technology,
Hangzhou 310027, China; orcid.org/0000-0001-6141-
6278; Email: dailiyan@zju.edu.cn
Authors
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Functionalized GO as an Active and Reusable Acid−Base Bifunctional
Catalyst for One-Pot Cascade Reactions. ACS Catal. 2014, 4, 394−
401.
Menglu Cai − College of Chemical and Biological Engineering,
Zhejiang University, Zhejiang Provincial Key Laboratory of
Advanced Chemical Engineering Manufacture Technology,
Hangzhou 310027, China
Xiaozhong Wang − College of Chemical and Biological
Engineering, Zhejiang University, Zhejiang Provincial Key
Laboratory of Advanced Chemical Engineering Manufacture
Technology, Hangzhou 310027, China; Institute of Zhejiang
University-Quzhou, Quzhou 324000, China
Yangyang Fang − College of Chemical and Biological
Engineering, Zhejiang University, Zhejiang Provincial Key
Laboratory of Advanced Chemical Engineering Manufacture
Technology, Hangzhou 310027, China
Yingqi Chen − College of Chemical and Biological
Engineering, Zhejiang University, Zhejiang Provincial Key
Laboratory of Advanced Chemical Engineering Manufacture
Technology, Hangzhou 310027, China
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00819
Author Contributions
M.C. performed the investigation, formal analysis, method-
ology, and writing of the original draft, X.W. was responsible
for visualization and resources. Y.F. performed data curation
and reviewing and editing of the manuscript. Y.C. handled the
resources and reviewing and editing of the manuscript. L.D.
performed the supervision, resource manipulation, and
reviewing and editing of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Zhejiang Provincial Key
Laboratory of the Advanced Chemical Engineering Manufac-
ture Technology (No. 2017E10001), Zhejiang Province
(China).
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ACS Appl. Mater. Interfaces 2014, 6, 933−941.
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Wang, J. Synergistic catalysis of one-pot cascade reactions by acidic
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