A novel acid-base bifunctional catalyst (ZSM-5@Mg3Si4O9(OH)4) with core/shell hierarchical structure and superior activities in tandem reactions

Article abstract of DOI:10.1039/c6cc06779d

A hierarchically core/shell structured base-acid bifunctional catalyst, ZSM-5@Mg₃Si₄O₉(OH)₄, was successfully prepared through a simple hydrothermal reaction between the silica species on the ZSM-5 crystal surface and the Mg²⁺ source in basic solution. The obtained catalyst showed superior activity and stability in one-pot deacetalization-Knoevenagel condensation reaction.

Full text of DOI:10.1039/c6cc06779d

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DOI: 10.1039/C6CC06779D
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A Novel Acid-Base Bifunctional Catalyst of ZSM-5@Mg3Si4O9(OH)4
with Core/Shell Hierarchical Structures and Superior Activities in
Tandem Reactions†
Received 00th January 20xx,
Accepted 00th January 20xx
Darui Wang, Bo Wang, Yu Ding, Haihong Wu and Peng Wu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Hierarchically core/shell structured base-acid bifunctional catalyst base tandem reactions. In particular, Davis18 and Shanks
of ZSM-5@Mg3Si4O9(OH)4 was successfully prepared through a groups19 explored cooperative acid-base catalysts employing
simple hydrothermal reaction between the silica species on ZSM-5 an aminopropyl-functionalized SO3H-SBA-15 mesosilica.
crystal surface and the Mg2+ source in basic solution. The obtained Moreover, the amino group-functionalized metal-organic
catalyst showed superior activity and stability in one-pot framework, MIL-101(Al)-NH2, has been synthesized using a
deacetalization-Knoevenagel condensation reaction.
solvothermal method and employed as a bifunctional acid-
base catalyst for tandem deacetalization-Knoevenagel
condensation reaction.20 And the amine-functionalized
graphene oxide containing carboxylic acid was also prepared
by a facile one-step silylation approach and used as a
bifunctional catalyst.15 Although the fruitful achievements
have been obtained in this research field, however, they often
suffer from complicated protection-deprotection procedures
that are required to avoid neutralization. In addition, the
bifunctional catalysts with spatially well defined acid and base
sites are needed to develop with the purpose to improve the
unsatisfactory catalytic activity in existing systems due to the
enhanced steric hindrance.
Crystalline aluminosilicate zeolites possess integral channel
systems of well-defined microporosity and intrinsic solid
acidity.21 Considering these unique properties, incorporating
basic components into acidic zeolites can lead to excellent
acid-base bifunctional catalyst. Kubota et al22 once employed
the uncalcined Beta zeolite containing tetraethylammonium
(TEA+) cations to catalyze the Knoevenagel condensation. The
SiO− moieties located at the pore mouth and residual TEA+
cations were presumed to be the active basic sites.
Unfortunately, the small sized TEA+ cations were extracted
readily out of the channels of beta zeolites by acid treatment,
and suffered leaching problem in liquid-phase organic
reactions. On the other hand, supporting solid base alkali-
earth metal oxides on zeolites by conventional wetness
impregnation (IM) method is a simple preparation technology.
Various acid-base bifunctional catalysts, such as CaO/ITQ-2,23
MgO/MCM-2223,24 and MgO/ZSM-5,25 have thus been
successfully prepared. However, these catalysts may
encounter the drawbacks of basic active sites losing,
micropores blocking and unsatisfied catalytic activity because
of a relatively weak metal oxides-zeolite interaction.
Recently, chemists have paid great attentions to the
development of one-pot and/or tandem organic reactions
owing to their natures of simple operations and process
intensification.1-3 Among the various systems designed to
satisfy this idea, many approaches are taking homogeneous
catalysis systems. Despite the high activity and selectivity,
homogeneous catalysts are difficult to recycle and reuse,
which will inevitably enhance the cost and even cause
pollution of the environment and products.4,5 Furthermore,
the one-pot reactions catalyzed by bifunctional acid-base
catalysts are hardly developed in the homogeneous phase
systems because the acid and base sites are easily deactivated
each other. Therefore, the development of heterogeneous
catalysts, especially acid-base bifunctional catalysts with
immobilized active sites that can effectively promote one-pot
reactions have currently received great interests.6-12
The combination of acid and base catalysts for the tandem
process is recognized as an attractive approach since the acidic
and basic functions can activate electrophiles and nucleophiles,
respectively.13-20 To date, acid-base bifunctional catalysts
hosted on the solid matrixes, such as silica, polymer and
grapheme oxide, have been reported.16 For instance, Angeletti
group17 developed a bifunctional system that the silanols
originating from MCM-41 mesosilica acted as acid sites
whereas the 1,8-bis(dimethylaminonaphthalene) functional
groups immobilized therein served as basic sites useful in acid-
Shanghai Key Laboratory of Green Chemistry and Chemical Process, School of
Chemistry and Molecular Engineering, East China Normal University , North
Zhongshan Rd. 3663, Shanghai 200062, China
E-mail: pwu@chem.ecnu.edu.cn
Fax: (+)86-21-62232292
†Electronic Supplementary Information (ESI) available: Details of experimental
procedures and material characterization. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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structure. Talc is composed of three infinite layeVrsiewfoArrtmicleeOdnlbinye
the sharing of oxygen ions at three corners of the silica
O
S
Mg
-OH
DOI: 10.1039/C6CC06779D
i
tetrahedron,
a
layer
of
octahedral-coordinated
magnesium/hydroxide ions holds two layers of tetrahedral-
coordinated silicon/oxygen ions together as a three-layer
sheet. And the crystals are held together by Van der Waals
forces acting across adjacent tetrahedral layers.28
Base
+
a
b
Boundary
Mg3Si4O9(OH)4
Acid
ZSM-5
200 nm
200 nm
Scheme
1 Schematic illustration of the preparation procedures for ZSM-
5@Mg3Si4O9(OH)4 and its application to acid-base one-pot reaction.
c
d
Mg3Si4O9(OH)4
0.456 nm
[020]
Herein, we communicate a novel method for preparing
hierarchically core/shell structured acid-base bifunctional
catalyst. As illustrated in Scheme 1, this process involved the
dissolving of silica species off the ZSM-5 crystal surface in base
solution and then in situ reaction with magnesium cations to
form the flower-like magnesium silicate, giving rise to a
ZSM-5
10 nm
10 nm
composite material of ZSM-5@Mg3Si4O9(OH)4 with ZSM-5 as Fig. 1 SEM image (a) and TEM images (b, c and d) of ZSM-5@Mg3Si4O9(OH)4.
core and Mg3Si4O9(OH)4 as shell. The special core/shell
structure was similar to that in the previous literatures.26,27
The SEM and TEM images showed that the pristine ZSM-5
Hydrothermal reaction between SiO2 and Mg2+ took place as was of the polycrystalline particles with a relatively smooth
follows: 4SiO2 + 3Mg2+ + 5H2O = Mg3Si4O9(OH)4 + 6H+. The surface and a regular size of 1 - 2 μm (Fig. S6, ESI†). After the
resultant catalysts showed a superior activity and stability in hydrothermal reaction in Mg2+ containing solution, the as-
one-pot deacetalization-Knoevenagel condensation reaction fabricated ZSM-5@Mg3Si4O9(OH)4 was evenly covered with a
with ZSM-5 acting as acid sites and Mg3Si4O9(OH)4 serving as shell made up of flower-like nanosheets, forming a core/shell
basic sites. This novel hydrothermal method was featured by structured materials resembling the origin shape (Fig. 1a). The
the simultaneous construction of a hierarchical structure and structure of thus prepared ZSM-5@Mg3Si4O9(OH)4 was further
spatially well-defined stable basic sites.
investigated by TEM investigation. The shell was composed of
Core/shell structured ZSM-5@Mg3Si4O9(OH)4 materials were flexible nanosheets with the thickness of about 120 nm (Fig.
prepared readily by hydrothermal reaction at an optimal 1b). A representative HRTEM image clearly showed the
temperature of 120 oC for 3 h (Fig. S1 and S2, ESI†). The junction between the shell and core, in which the flower-like
coating of hierarchically flower-like Mg3Si4O9(OH)4 on ZSM-5 nanosheets and the crystalline ZSM-5 crystal grew connectedly
crystals by this novel method has been traced with the powder (Fig. 1c). As shown in the enlarged image of lamellar
XRD patterns and FT-IR spectra. The XRD pattern of the nanosheets (Fig. 1d), the distance of the adjacent lattice
pristine ZSM-5 was characteristic of a typical MFI structure (Fig. fringes was determined to be 0.456 nm, corresponding well to
S3a, ESI†). After the hydrothermal reaction in Mg2+ containing the d020 spacing value of Mg3Si4O9(OH)4 (JCPDS No. 03-0174).28
solution, the resultant ZSM-5@Mg3Si4O9(OH)4 showed
Solid state 29Si NMR spectroscopy is a powerful tool to study
additional diffractions around 19.5o, 34.5o, 36.2 o and 60.5o in the chemical environment and bonding patterns of the SiO4
high angle region (Fig. S3b, ESI†), which are indexed as the units. The pristine ZSM-5 showed the only peaks at -114 and -
[020], [200], [-133] and [-332] planes of magnesium silicate 106 ppm, corresponding to Q4 groups (Fig. S7, ESI†). This result
(Mg3Si4O9(OH)4, JCPDS No. 03-0174).28,29 In addition, no peaks indicated all Si atoms were interconnected by oxygen bridge.
related to other crystalline magnesium compounds were After hydrothermal reaction, the ZSM-5@Mg3Si4O9(OH)4
observed, indicating that Mg3Si4O9(OH)4 was the only phase showed a broad peak at 96 ppm, corresponding to Q3 groups,
newly developed. FT-IR spectroscopy (Fig. S4, ESI†) was utilized which was consistent with a talc-like crystal structure with SiO4
to characterize the formed functional groups. Compared with structural units forming layers through networking (Fig.
pristine ZSM-5, three new bands emerged at around 557 cm-1, 2A).32,33 The result agreed well with its Mg3Si4O9(OH)4
665 cm-1 and 1050 cm-1 for ZSM-5@Mg3Si4O9(OH)4, which are composition. Besides, a weak chemical shift at -87 ppm
ascribed to the Si-O-Mg stretching vibrations.30,31 This further assigned to Q2 was observed, which was due to the
verified the formation of magnesium silicate after amorphous Si-O species situated on the zeolite surface.
hydrothermal reaction. As shown in Fig. S5 (ESI†), the
The above results confirmed that magnesium silicate was
synthesized magnesium silicate was characteristic of the talc produced under basic conditions and showed the lamellar
2 | J. Name., 2012, 00, 1-3
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structure. The XPS data also provided the similar information theoretical loading amounts of 5.0, 7.5, 10.0, 1V2ie.5w Aartnicdle O1n5lin.0e
DOI: 10.1039/C6CC06779D
about the structure. As shown in Fig. S8 (ESI†), the binding wt%. ICP analysis indicated the Mg contents actually loaded on
energy of Si 2p was 102.8 eV for ZSM-5@Mg3Si4O9(OH)4, 0.6 zeolite were 5.1, 7.6, 10.0, 12.6 and 15.1 wt%, respectively.
eV lower than 103.4 eV for pristine ZSM-5. The difference in Thus, this method gave a utilization efficiency of Mg2+ source
binding energy for these two kinds of silicon species is in close to 100 % and easily controlled the Mg loading amounts in
agreement with the literature.34 In addition, the state of Al was a relatively wide range. Fig. S13 (ESI†) showed the SEM images
also investigated by 27Al MAS NMR (Fig. S9, ESI†), the of ZSM-5@Mg3Si4O9(OH)4 with different Mg contents. At lower
microenvironment of Al became more asymmetric in Mg loadings of 5.1 wt%, the rough surfaces were already
coordination states after incorporating Mg3Si4O9(OH)4.
observed clearly. Certainly, larger amount nanosheets were
formed when the Mg contents reached 15.1 wt%. In XRD
patterns (Fig. S14, ESI†), the intensity of peaks indexed as
Mg3Si4O9(OH)4 were enhanced with increasing Mg contents.
0.2
400
(A)
(B)
-114 (Q4)
0.1
300
0.0
b
a
10
100
Pore diameter (nm)
100
80
60
40
20
0
100
80
60
40
20
0
(A)
200
100
0
(B)
-96 (Q3)
-106 (Q4)
a
a
b
-87 (Q2)
b
-75
-90
-105
-120
0.0 0.2 0.4 0.6 0.8 1.0
c
Chemical shift (ppm)
Solid state 29Si NMR spectra of ZSM-5@Mg3Si4O9(OH)4 (A); N2
Relative pressure (P/P0)
c
Fig.
2
0
2
4
6
8
10 12 14 16
0
2
4
6
8
10
adsorption/desorption isotherm (B) of pristine ZSM-5 (a) and ZSM-5@Mg3Si4O9(OH)4
(b), insets showed the BJH pore size distribution.
Time (h)
Time (h)
Fig.
3 The kinetics of one-pot deacetalization-Knoevenagel condensation
reaction over ZSM-5@Mg3Si4O9(OH)4 (A) and MgO/ZSM-5 (B): conversion of
benzaldehyde dimethylacetal (a), yields of benzylidene ethyl cyanoacetane (b)
and benzaldehyde (c). Reaction conditions: catalyst, 50 mg; benzaldehyde
dimethylacetal, 5 mmol; ethyl cyanoacetate, 7.5 mmol; H2O, 5 mmol; toluene, 2
mL.
The N2 adsorption isotherms clearly revealed a significant
change in porosity after the hrdrothermal reaction. The
pristine ZSM-5 showed a typical type I isothermal (Fig. 2Ba),
characteristic of microporous materials. That of ZSM-
5@Mg3Si4O9(OH)4 was characteristic of both type I and type IV
(Fig. 2Bb), implying the presence of multiple pore systems. The
slowly increased uptake in the P/P0 region of 0.2 - 0.9 was
probably because of relatively irregular mesopores ascribed to
the disordered assembly of the flexible Mg3Si4O9(OH)4
nanosheets. The pore size distribution showed that ZSM-
5@Mg3Si4O9(OH)4 possessed the mesopores with 2 - 20 nm
diameters, which were absent in the pristine ZSM-5 sample. As
shown in Table S1 (ESI†), the mesopore surface areas
increased from 31 m2 g-1 to 193 m2 g-1 and mesopore volumes
increased from 0.10 cm3 g-1 to 0.26 cm3 g-1 when the Mg
content was 10.0 wt%. These novel materials, with confined
space and enhanced mass transfer rate, would have potential
applications as hierarchical catalysts.
The acidity and basicity were investigated by NH3-TPD and
CO2-TPD techniques, respectively. Compared with the pristine
ZSM-5 (Fig. S11a, ESI†), ZSM-5@Mg3Si4O9(OH)4 (Fig. S11b, ESI†)
possessed slightly lower total acid quantity of 0.38 mmol g-1.
And the strong acid quantity decreased form 0.143 mmol g-1 to
0.063 mmol g-1 while middle strong acid quantity increased
from 0.118 mmol g-1 to 0.268 mmol g-1, which may be due to
the sacrificing of ZSM-5 crystal or a partial cation exchange
between Mg2+ and H+ during hydrothermal reaction. On the
contrary, the total basic quantity was 0.24 mmol g-1 for ZSM-
5@Mg3Si4O9(OH)4 (Fig. S12b, ESI†), much higher than 0.14
mmol g-1 of pristine ZSM-5 (Fig. S12a, ESI†). Thus, the newly
formed magnesium silicate was speculated to be potential
basic active sites, and hierarchical ZSM-5@Mg3Si4O9(OH)4
could act as acid-base bifunctional catalyst in one-pot reaction.
To demonstrate the synthetic controllability, the products
were synthesized with different Mg contents, corresponding to
The potential use of ZSM-5@Mg3Si4O9(OH)4 as a bifunctional
acid-base catalyst for one-pot reactions was investigated next.
ZSM-5@Mg3Si4O9(OH)4 was employed to promote the
formation of benzylidene ethylcyanoacetane (BE) from the
reaction between benzaldehyde dimethylacetal (BD) and ethyl
cyanoacetate, which that takes place through sequential
deacetalization and Knoevenagel condensation processes (Fig.
S15, ESI†).35 Inspecting the time course of the process (Fig. 3A),
it is obvious that BE was efficiently generated from BD via a
pathway involving initial formation of benzaldehyde
intermediate, and the yield of BE reached 94.2% at 6 h. For
comparision, the pristine ZSM-5 and conventional acid-base
catalyst of MgO/ZSM-5 prepared by wetness impregnation
method were also applied for this one pot reaction. When the
pristine ZSM-5 was used as the catalyst (Fig. S16, ESI†), the
second step, involving Knoevenagel condensation of
benzaldehyde with ethyl cyanoacetate, did not take place
efficiently. As a result, only benzaldehyde was produced
because pristine ZSM-5 only served as an acid catalyst for the
deacetalization step. Also, the one-pot reaction took place
when conventional MgO/ZSM-5 was utilized (Fig. 3B), but the
reaction rate was much lower than that of the ZSM-
5@Mg3Si4O9(OH)4 catalyst, giving only 22.1% yield of BE at 6 h.
In comparison to ZSM-5@Mg3Si4O9(OH)4, MgO/ZSM-5 showed
significantly decrease in the micro/mesopore surface area and
micro/mesopore volume due to the pores blocking (Fig. S10
and Table S1, ESI†). In addition, MgO/ZSM-5 (Fig. S11c and 12c,
ESI†) possessed less acid/base active sites than ZSM-
5@Mg3Si4O9(OH)4. These observations clearly demonstrated
that ZSM-5@Mg3Si4O9(OH)4 possessed both hierarchical pores
and plenitudinous acid/base active sites that served as an
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J.-C. Wasilke, S. J. Obrey, R. T. Baker and G. C. Bazan, Chem.
effective catalyst for respective deacetalization and
Knoevenagel condensation reactions.
View Article Online
Rev., 2005, 105, 1001.
M. J. Climent, A. Corma and S. Iborra,DCOhIe:m10..1R0e39v/.,C26C01C10,6717191D
,
In addition, when the condensation reactant ethyl
cyanoacetate was substituted by malononitrile, the final
product yield increased obviously on ZSM-5@Mg3Si4O9(OH)4.
The BE conversion reached 100% and the yield of benzylidene
malononitrile (BM) was 99.7% after 90 min (Fig. S17b, ESI†),
and also showed faster reaction rate than MgO/ZSM-5 (Fig.
S17c, ESI†) due to its special hierarchical structure and more
active sites. In a following step, the stabilities of ZSM-
5@Mg3Si4O9(OH)4 and MgO/ZSM-5 were checked by recycling
the catalyst (Fig. S18, ESI†). No notable changes in activity
were observed over eight cycles for ZSM-5@Mg3Si4O9(OH)4.
However, MgO/ZSM-5 lost about 18.3% of its initial activity
and the BE yield decreased from 91.8% to 60.5% after eight
reaction runs. The activity loss was presumed to be mainly due
to loss of basic active sites and coke formation. For the reused
ZSM-5@Mg3Si4O9(OH)4 catalyst, the framework structure was
maintained well (Fig. S19, ESI†), and the Mg content loss was
prevented, according to ICP analysis, we can calculate that Mg
contents were 9.8 wt% and 4.3 wt% for ZSM-5@Mg3Si4O9(OH)4
and MgO/ZSM-5 after eight recycling runs, respectively.
Moreover, the coke formation was also effectively prevented
for ZSM-5@Mg3Si4O9(OH)4 due to the hierarchical structure,
only 4.1 wt% coke was observed by TG analysis, but coke
content reached up to 7.4 wt% for conventional MgO/ZSM-5.
This result was caused by the shorter diffusion path length in
the hierarchical catalyst. The stability was further checked by
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Importantly, the synthetic strategy is quite general. We have
successfully prepared other acid-base bifunctional catalysts by
varying the types of zeolites in the hydrothermal reaction,
such as mordenite@Mg3Si4O9(OH)4, Beta@Mg3Si4O9(OH)4 and
MCM-22@Mg3Si4O9(OH)4 (Fig. S21, ESI†).
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In summary, we have developed a facile route for preparing
hierarchically core/shell structured base-acid bifunctional
catalyst, ZSM-5@Mg3Si4O9(OH)4 was successfully prepared
through a simple hydrothermal reaction in base solution. The
flower-like Mg3Si4O9(OH)4 nanosheets can readily grow on the
surface of zeolite to form a hierarchically core/shell structure
without any pretreatment or binding agent. Furthermore, the
obtained ZSM-5@Mg3Si4O9(OH)4 catalyst showed super activity
and stability in tandem deacetalization-Knoevenagel
condensation reaction. In particular, we found that this simple
method was also applicable to other zeolites (e.g. mordenite,
Beta and MCM-22) with different framework topologies.
The authors gratefully acknowledge the financial supports
from NSFC of China (21533002, 21373089, 21373088), and
China Ministry of Science and Technology under contract of
2016YFA0202804.
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