One-pot synthesis of bimetallic metal-organic frameworks (MOFs) as acid-base bifunctional catalysts for tandem reaction

Article abstract of DOI:10.1039/c9cy01940e

Designing and preparing effective acid-base tandem catalysts is one of the research hotspots at present because assembling antagonistic active sites in one catalyst is still a challenge. In this work, amine-functionalized bimetallic metal-organic frameworks (MIL-101(Al/Fe)-NH₂ (X)) were synthesized through a rapid reflux method and acted as acid-base bifunctional catalysts for tandem reaction (deacetalization-Knoevenagel condensation reaction). The bimetallic MOF catalysts exhibit enhanced catalytic activity when compared with monometallic catalysts either MIL-101(Al)-NH₂ or MIL-101(Fe)-NH₂. The superior performance of MIL-101(Al/Fe)-NH₂ (X) has been attributed to the synergistic effect between Al³⁺ and Fe³⁺ ions and the hierarchical pore structure. Moreover, the as-synthesised samples showed robust stability, excellent recyclability, and durable catalytic activity.

Full text of DOI:10.1039/c9cy01940e

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Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
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DOI: 10.1039/C9CY01940E
ARTICLE
One-pot Synthesis of Bimetallic Metal-Organic Frameworks (MOFs)
as Acid-Base Bifunctional Catalysts for Tandem Reaction
Yanjing Hu,a, + Jian Zhang,a, + Hang Huo,a Zhe Wang,a Xianzhu Xu,a Yulin Yang,a Kaifeng Lina,* and
groupReceived 00th January 20xx,
Accepted 00th January 20xx
Ruiqing Fana,*
DOI: 10.1039/x0xx00000x
Designing and preparing effective acid-base tandem catalysts is one of the research hotpots at present because assemble
antagonistic active sites in one catalyst is still a chanllage. In this work, amine-functionalized bimetallic metal-organic
frameworks (MIL-101(Al/Fe)-NH2(X)) were synthesized through a rapid reflux method and acted as acid-base bifunctional
catalysts for tandem reaction (deacetalization-Knoevenagel condensation reaction). The bimetallic MOFs catalysts exhibit
enhanced catalytic activity when compared with the monometal catalysts either MIL-101(Al)-NH2 or MIL-101(Fe)-NH2. The
suprrior performance of MIL-101(Al/Fe)-NH2(X) have been attributed to the synergistic effect between Al3+ and Fe3+ ions and
the hierarchical pore structure. Moreover, the as-synthesised samples showed robust stability, excellent recyclability, and
durable catalytic activity.
which displayed superior activities in tandem reactions, while
the synthetic method is tedious.10 Coincidentally, Sun’s group
fabricated an acid-base collaborative catalyst. This catalyst
Introduction
Acid-base sequential catalytic reactions, one of the most
important category of cascade reactions, occupy a dominant
position in the synthesis of industrial chemicals, fine chemicals
and pharmaceuticals.1,2 Recently, many kinds of mixed catalyst
and homogeneous acid-base catalysts have been utilized in the
sequential catalytic system. However, traditional homogeneous
acid-base catalysts cannot meet the requirement of rapid and
high-efficiency reaction, due to self-neutralization between acid
and base active sites. Besides, homogeneous catalytic reactions
suffer from the production contamination and non-reusability
of catalysts.3-5 Nowadays, heterogeneous catalysts have been
proved to avoid the above problems.6 For example, a
heterogeneous acid-base mesoporous silica material
synthesized by grafting and co-condensation methods showed
superb acid-base bifunctional catalytic performance.7 In
addition, Liu group constructed a phosphor-doped hexagonal
boron nitride nanosheets (BNP) metal-free heterogeneous
bifunctional catalyst for one-pot cascade reactions.8 Recently,
our group also reported hollow mesoporous silica nanospheres
with double-shelled structure exhibiting a good catalytic
performance for one-pot cascade reaction.9 Although many
successful cases on application of acid-base bifunctional
catalyst have been reported, the process of synthesis of these
catalysts is complex and cumbersome. For example, Wu and his
co-workers synthesized a novel acid-base bifunctional catalyst
(ZSM-5@Mg3Si4O9(OH)4) with core/shell hierarchical structure
shows outstanding catalytic ability for synthesizing 5-Aryl-1H-
1,2,3-triazoles through the “one-pot” cyclization but the
synthetic process of catalyst taken up to three steps.11 In
general, the synthetic process of the most of traditional
heterogeneous acid-base bifunctional catalysts are at least two
or multiple steps. Tedious synthetic process usually aimed to
avoid the self-neutralization between acid and base active sites
during the synthetic process. Therefore, the design and
synthesis of acid-base bifunctional catalysts by one-step still
faces great challenges.5,7,12-14
Metal-organic frameworks (MOFs) are regarded as an
intriguing class of porous crystalline materials assembled with
controllable inorganic vertices (metal ions or metal clusters)
and numerous functionalized organic connectors.15-19 Due to
the tunable uniform porosity, tailorable surface, ultrahigh
surface area and functionalized networks, MOF materials have
been utilized for a wide diverse of applications, such as,
sensing,20 nonlinear optics,21-23 gas storage/separation,15,24-28
drug delivery,29-33 and catalysis,7,12,34-41 etc. Compared to the
conventional solid catalysts, MOFs can be tuned to give porous
frameworks containing both isolated acid and base active sites
through ingenious construction and effective post-synthesis.42-
44
Therefore, MOF-based catalysts exhibit advantages of both
homogeneous catalysts (high selectivity and ease of
modification) and heterogeneous catalysts (good recyclability
and easy product isolation) in the field of cascade reactions.45-48
For example, Zhou’s group constructed a multi-functional MOF
PCN-124 which displayed excellent catalytic activity in tandem
reaction.49 Matsuoka and his co-workers utilized Zr-MOF-NH2 as
a bifunctional MOF photocatalyst, which promoted sequential
photocatalytic oxidation and Knoevenagel condensation
a MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion
and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of
Technology, Harbin, 150001, China. E-mail: linkaifeng@hit.edu.cn,
fanruiqing@hit.edu.cn.
+ Yanjing Hu and Jian Zhang contributed equally to this work
Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C9CY01940E
Scheme 1 The schematic representation of the tandem reaction using a site-isolated heterogeneous catalyst. (a) The mesoporous structure of MIL-101(Al/Fe)-NH2 and
(b) the Lewis acidic sites (Al3+/Fe3+ centers) and Brønsted basic sites (-NH2 groups of organic linkers).
reaction under UV-light iradiation.50 Very recently, mixed-metal groups in the pore environment) can catalyse a Knoevenagel
MOF catalysts have attracted growing attentions, resulting from condensation (Scheme 1).59, 60, 62 Delightedly, the bifunctional
the unique synergistic effect.51-53 The electron transfer between catalyst exhibited superior catalytic performance than both of
the different metal ions in one cluster allows control over Fermi the monometal organic frameworks (MIL-101(Al)-NH2 and MIL-
level and provides admirable catalytic performance.16 Choosing 101(Fe)-NH2) due to the synergistic effect and their hierarchical
metal ions with the same coordination mode and constructing porous structures. The yield reach to nearly 100% after 90 min
isoreticular framework with similar organic linkers should be a reaction time. The physical and chemical performances of the
simple principle to build up the mixed-metal MOFs catalysts.16 as-synthesized samples were performed. Furthermore, the
Zaheer and co-workers synthesized Co/Fe MOFs as
a
reusability of the catalysts, and the catalytic mechanism were
multifunctional catalyst for catalytic reduction of nitroarenes investigated.
and dehydrogenation reactions.54 Tang’s group synthesized a
nanoporous bimetallic metal-organic frameworks (FeCo-BDC)
and used as a novel catalyst for efficient removal of organic
Experimental Section
contaminants.55 In addition, the bimetallic MOFs can be used as
an efficient electrocatalyst for oxygen evolution reaction56 and
can act a catalyst for degradation of sulfamethoxazole under
mild conditions.57 In the field of catalysis is full of such examples.
Besides, suitable pore structure could be another key point to
design the MOF-based sequential catalysts. The large channels
and cavities can promote the reactions and allow easy product
separation. Previous studies have revealed that amino-
functionalized MIL-101 type MOFs can act as promising tandem
reactions catalysts due to plenty of active sites and pore
Materials
2-Aminoterephthalic acid (NH2-BDC, C8H7NO4, 99%) was
purchased from Shanghai Titan Scientific Co. Ltd. (Shanghai,
China). Aluminium chloride hexahydrate (AlCl3ꞏ6H2O, 97%) and
iron(III) chloride hexahydrate (FeCl3ꞏ6H2O, 99 %) were
obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai,
China). N,N’-dimethylformamide (DMF, C3H7NO, 99.5%) and
malonitrile (C3H2N2, 99%) were acquired from Aladdin Chemical
Reagent Co. Ltd. (Shanghai, China). Ethanol (EtOH, CH3CH2OH,
99.7%) and toluene (C7H8, 99.5%) were obtained from Tian in
Fuyu Fine Chemical Co. Ltd. (Tianjin, China). Benzaldehyde
dimethyl acetal (C8H12O2, 98%) was purchased from Energy
Chemical Co. Ltd. (Shanghai, China). Ethyl cyanoacetate
(C5H7NO2, 99%) was obtained from Shanghai Macklin
Biochemical Co. Ltd. (Shanghai, China). The internal standard
substance ethylbenzene (C8H10, 99.5%) was acquired from Inno-
chem (Beijing, China). Unless otherwise stated, all reagents
were of analytical grade and were used as supplied without
structure,58
such
as
photooxidation-Knoevenagel
condensation59 and Meinwald rearrangement-Knoevenagel
condensation.60 Li et al. reported the one pot reaction between
aromatic alcohols and active methylene compounds over MIL-
101(Fe)-NH2 via a coupled photooxidation and Knoevenagel
condensation.59 In addition, Shi’s group reported a bifunctional
acid-base MOF (MIL-101-SO3H-NH2) based on post-synthetic
modification, which is used for one-pot deacetalization-
nitroaldol reaction.61 However, mixed-metal MIL-101 type
MOFs in acid-base tandem reaction have not been reported.
Hence, a series of bimetallic functionalized MOF catalysts
named MIL-101(Al/Fe)-NH2 (X) were successfully synthesized by further purification. The solvents were dried with molecular
a one-pot reflux method. The bimetallic MOF catalysts were
assembled by 2-amine-1,4-benzenedicarboxylate ligands and
sieve before use.
trimeric Al3+/Fe3+ octahedral clusters which formed
a
Characterization
mesoporous MIL-101-NH2 series framework.58 Moreover, the
bimetallic metal-organic frameworks consisting of Lewis acidic
sites (the Al3+/Fe3+ triple metal clusters) can catalyze a
deacetalization reaction and Brønsed basic sites (the amino
The as-prepared samples were characterized by XRD (Powder X-ray
Diffraction) patterns on Bruker AXS D8 Advance diffractometer
equipped with Cu Kα (λ = 1.5406 Å) radiation at a voltage of 40
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kV and 40mA. (2 Theta of 0.6-50o). All samples were gently synthesized by similar conditions, and namVeiedw Aartiscle OMnlIinLe-
DOI: 10.1039/C9CY01940E
grinded to limit the effect of preferential orientation of 101(Al/Fe)-NH2 (15:1), MIL-101(Al/Fe)-NH2 (10:1) and MIL-
individual crystals. SEM (Scanning Electron Microscopy) 101(Al/Fe)-NH2 (5:1), respectively.
pictures were obtained by Hitachi S-4800. High-Angle Annular
All of products were centrifuged for 5 min at 8000 r/min, then
Dark Field-Scanning Transmission Electron microscopy (HAADF- washed repeatedly with DMF and ethanol for several times. In
STEM) images, mapping and Energy-dispersive X-ray spectrum addition, the solid powder was soaked and stirred in refluxing
(EDX) analyses were carried out under TECNAI F20 microscope DMF for 12 h, then repeated the process twice with fresh
operating at an accelerating voltage of 200 kV. XPS (X-ray ethanol. The final products centrifuged and dried at 80oC in a
Photoelectron Spectra) data were collected by PHI 5700 ESCA vacuum oven overnight.
system equipped with Al Kα radiation and the spectra were
calibrated with the C 1s peak at 284.6 eV as an internal
Catalytic reactions
standard. FT-IR (Fourier Transform Infrared Spectroscopy) were The one pot deacetalization-Knoevenagel reaction was
carried out in the range of 400-4000 cm-1 on a Thermo Scientific conducted in the liquid phase under atmospheric pressure and
Nicolet iS5. EPR (Electron Paramagnetic Resonance) spectra a temperature of 90oC. Prior to use, the catalysts were activated
were recorded at low temperature (110 K) on an EPR at 150oC for two hours. In a typical process, a mixture of
spectrometer (JEOL-FE1X) operating in the X-band frequencies benzaldehyde dimethyl acetal (1 mmol), malononitrile (1.5
with a field modulation of 100 kHz. The metals amount was mmol), internal standard ethylbenzene (100 μL) and solvent
detected by ICP-AES (Inductively Coupled Plasma Atomic methylbenzene (3 mL) were added in a two-necked round-
Emission Spectroscopy) (The samples were weighed accurately bottomed flask, equipped with
a
condenser and
a
and digested with hydrochloric acid and perchloric acid at high thermometer, and provided with a stirrer and heater. Do not
temperature, the obtained clarification solution was volumetric add the catalyst after the temperature went up to the set
in volumetric flask for testing.) TGA (Thermogravimetric reaction temperature, meanwhile, started timing the time. The
Analysis) were recorded on TGA SDTQ-600, which was reaction was sampled for a period time with a pipette, and the
measured in air atmosphere at 5oC/min from RT (room liquid was centrifuged (12000 r/min, 5min). The supernatant
temperature) to 800oC, and the samples were active at 150 oC was analysed by gas chromatography (Agilent 7820A) equipped
for
2
hours to remove the guest molecules. N2 with FID and Agilent 6980 inert XL MSD (GC-MS).
For the 2nd, 3rd, 4th and 5th catalytic run catalysts were
adsorption/desorption experiments were carried out on a
Micrometrics ASAP 2020 M volumetric gas sorption instrument. separated by centrifugation and washed with acetone several
After the sample was degassed in a vacuum at 150oC for 10 h, times. Then the samples were activated at 150oC before used.
the nitrogen adsorption and desorption isotherms were For comparison, we also check the catalytic activities of
measured at −
196oC. Agilent 7820A gas chromatography with compounds under the same conditions with the temperature
FID and Agilent 6980 inert XL MSD (GC-MS) were adopted to increases to 110 oC for 3 times.
detect the contents of product and the intermediates in the
solution.
Results and discussion
Synthetic procedures
Characterization of catalysts
MIL-101(Fe)-NH2 was synthesized according to a modified
reported literature63 through dissolving iron(III) chloride
hexahydrate (FeCl3ꞏ6H2O, 2.492 g, 9.22 mmol) in 200 mL DMF,
and kept stirring thirty minutes. Subsequently, 2-
aminoterephthalic acid (NH2-BDC, 1.670 g, 9.22 mmol) was
added to the above solution, then continued reflux stirring for
half an hour at 150oC to get the reddish-brown solid. The
synthesis method of MIL-101(Al)-NH2 is similar to MIL-101(Fe)-
NH2, which was obtained by a rapid reflux method just replacing
FeCl3ꞏ6H2O with AlCl3ꞏ6H2O (2.225 g, 9.22 mmol) with the
same reaction time.
MIL-101(Al/Fe)-NH2 (X) (X indicates the molar ratio between
Al3+ and Fe3+ in the precursor) composites were synthesized
through rapid reflux method, which were similar to that of MIL-
101(Al)-NH2 and MIL-101(Fe)-NH2. NH2-BDC (1.670 g, 9.22
mmol) was dissolved in 200 mL DMF, then AlCl3ꞏ6H2O (2.120 g)
and FeCl3ꞏ6H2O (0.119 g) were added to the solution (The total
molar is 9.22 mmol). Other synthesis process was the same as
the MIL-101(Al)-NH2 mentioned above. The product was
denoted as MIL-101(Al/Fe)-NH2 (20:1). The MIL-101(Al/Fe)-NH2
with different molar ratio of Al3+/Fe3+ (15:1, 10:1, 5:1) were
A
series of bimetallic metal-organic frameworks MIL-
101(Al/Fe)-NH2 (20:1/15:1/10:1/5:1) were synthesised by one-
pot rapid reflux method with various Al3+/Fe3+ molar ratio. The
morphologies of as-synthesized samples were investigated by
the scanning electron microscope (SEM). As shown in the Fig. 1a
and Fig. S1, the bimetallic MIL-101(Al/Fe)-NH2 series displayed
different morphologies with the MIL-101(Al)-NH2 and MIL-
101(Fe)-NH2 which made up by uniform nanoparticles (particle
size about 30nm) rather than blocky structure. The
morphologies in comparison to the MIL-101 can be attributed
to the different hydrolyzing condition of trivalent aluminum
ions and iron ions. Investigate the crystallinities and structures
of the as-synthesized samples, they were tested by powder X-
ray diffraction (PXRD). The appearance of peaks of MIL-
101(Al/Fe)-NH2 (X) displayed analogous to those of MIL-101(Al)-
NH2 and MIL-101(Fe)-NH2, indicating the iso-structure and same
topology of monometal MOF frameworks (Fig. 1c). In addition,
no additional peaks were found in the XRD patterns, showing
the high crystallinity and purity of as-synthesized products.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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There are no characteristic peaks of ferric trichloride, proving samples were conducted and revealed the massViepweArrtcicelenOtanlginee
DOI: 10.1039/C9CY01940E
that there was no residual ferric chloride in these structures.
was only 0.17 % in the MIL-101(Al/Fe)-NH2 (15:1) (in Table S1).
The electron paramagnetic resonance (EPR) measurements
of MIL-101(Al/Fe)-NH2 (15:1), MIL-101(Fe)-NH2 and MIL-
101(Al)-NH2 were also recorded to detect the location of iron
ions. As shown in Fig. 1e, there is no signal could be detected in
the spectrum of MIL-101(Al)-NH2. Nevertheless, a sharp signal
at g = 4.2 could be seen in the both MIL-101(Al/Fe)-NH2 (15:1)
and MIL-101(Fe)-NH2 indicating the presence of Fe ions,
consistent with previous reported works.69-71 In addition, two
weak resonance signals at g = 6.4 and 3.8 in the spectra of MIL-
101(Fe)-NH2 should be attributed the high contents of iron
ions.69 In addition, the distribution of elements of the MIL-
101(Al/Fe)-NH2 (15:1) was further investigated by HAADF-STEM
and EDX as shown in Fig. 2. Elemental mapping shows that the
elements C, O, N, Al and Fe are homogeneously distributed
throughout the whole particle (Fig. 2). Moreover, it was clearly
seen that the element Fe dispersed in the skeleton structure
(Fig. 2e). As noticed in EDX spectra shown in Fig. 2g, the weak
intensity of the Fe-K signal confirmed the presence of Fe ions.
All the results revealed the bimetallic MIL-101(Al/Fe)-NH2(X)
with high crystallinity and thermal stability were successfully
synthesized.
As far as we know, the appropriate pore sizes should be a
main advantage of MOF-based catalysts. The N2 sorption
properties of as-synthesized materials were analyzed by the BET
(Brunauer-Emmett Teller) modal along with the pore size
distribution which was calculated by H-K (Horvaih-Kawazoe)
and BJH (Barrett-Joyner-Halenda) methods (Fig. 1f and Fig. S3,
Fig. 1 (a) and (b) The SEM images of MIL-101(Al/Fe)-NH2 (15:1) and MIL-101(Al)-
NH2, respectively; (c) The XRD patterns; (d) The TGA curves ( the detailed residual
percentages were shown in inset figure) of as-synthesized samples; (e) The EPR
spectra of MIL-101(Al/Fe)-NH2 (15:1), MIL-101(Fe)-NH2 and MIL-101(Al)-NH2; (f)
The N2 adsorption-desorption isotherms of MIL-101(Al/Fe)-NH2 (20:1), MIL- S4). Monometal samples MIL-101(Al)-NH2 and MIL-101(Fe)-NH2
101(Al/Fe)-NH2 (15:1), MIL-101(Al/Fe)-NH2 (10:1) and MIL-101(Al/Fe)-NH2 (5:1),
respectively. (For sake of clarity, spectra of MIL-101(Al/Fe)-NH2 (20:1), MIL-
101(Al/Fe)-NH2 (15:1), MIL-101(Al/Fe)-NH2 (10:1) are offset along the vertical axis
by 260, 160 and 80 cm3/g, respectively).
showed I type absorption/desorption isotherms, while the MIL-
101(Al/Fe)-NH2 (X) displayed IV type absorption/desorption
isotherm. The absorption/desorption isotherms and the pore
The TGA curves of all samples were shown in Fig. 1d. The
primary turning points of MIL-101(Al/Fe)-NH2 (X) and MIL-
101(Al)-NH2 showed the similar trend, suggesting the similar
thermal stabilities of MIL-101(Al/Fe)-NH2 (X) and MIL-101(Al)-
NH2. But the monometal MIL-101(Fe)-NH2 exhibits a poor
thermal stability and the decomposition temperature below
300oC of the skeleton, which is consistent with previous
report.64 In addition, the detailed residual percentages were
shown in inset figure. Due to their low iron content, the residual
difference is very small. The attenuated total reflectance-
Fourier transform infrared reflection (ATR-FTIR) spectrums and
the X-ray photoelectron spectroscopy (XPS) spectra were also
performed to study the composition of MIL-101(Al/Fe)-NH2 (X),
MIL-101(Al)-NH2 and MIL-101(Fe)-NH2, respectively. As shown
in Fig. S2, the bands at 3498.5 cm-1 and 3370.5 cm-1 should be
referred to the primary amine groups (-NH2) on the organic
linker.65-68 Furthermore, the peaks below 1000 cm-1 were
corresponding to Al-O and Fe-O bonds vibration, respectively
(Fig. S2b). Unfortunately, there were no obvious Fe-O stretching
vibration in the bimetallic MIL-101(Al/Fe)-NH2 (15:1) as well as
the high resolution XPS spectra of iron 3d orbital due to the
extremely little content of iron atoms. To further investigate the
Fig. 2 HAADF-STEM analysis of (a) the selected region in MIL-101(Al/Fe)-NH2 (15:1)
content of Fe, the Inductively coupled plasma (ICP) of acidolysis
with elemental mapping analysis for (b) Al (green), (c) Fe (blue), (d) N (orange), (e)
4 | J. Name., 2012, 00, 1-3
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a
O (yellow) and (f) C (red), respectively; (g) Energy-dispersive X-ray spectrum of
101(Al/Fe)-NH2 (15:1), the enlarged area of Fe element was shown in inset figure.
Reaction conditions: A (1.0 mmol), D (1.5 mmol), Ethylbenzene (Internal
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standard) (100 μL), Toluene (3 mL), catalyst (20mg). The reactions were conducted
for 90 min at 90 oC. b Yield are according to the GC measurements.
DOI: 10.1039/C9CY01940E
size distribution results revealed that all of samples are inherent
microporous/mesoporous hierarchical pore materials. By
introducing Fe ions into the synthetic process, not only can form the
mixed metal clusters but also affect the pore environment. Therefore,
these bimetallic MOFs material contained larger mesopores than
that of monometal samples in the skeleton, and the mesoporous size
about 3.7 nm (Fig. S4) which will promote the reaction. These
hierarchical pores will not only promote the accessibility of the
reactants to the active sites of the catalysts but also accelerate the
rapid separation of the product away from the active sties. Therefore,
the bimetallic MIL-101(Al/Fe)-NH2(X) should be an ideal catalyst for
the size-effect organic reactions.
Deacetalization-Knoevenagel condensation cascade reaction
Recently, the bifunctional heterogeneous MOFs catalysts for
the one-pot cascade reactions has aroused extensive attention.
The MIL-101(Al/Fe)-NH2(X) contained unsaturated Al/Fe metal
centers, which can provide Lewis acidic sites for catalyzing the
deacetalization reaction and the −NH2 groups in the framework
can be used as Brønsted basic sites for the Knoevenagel
condensation reaction. Therefore, deacetalization-Knoevenagel
condensation cascade reaction by a series of bimetallic catalysts
MIL-101(Al/Fe)-NH2 (20:1), MIL-101(Al/Fe)-NH2 (15:1), MIL-
101(Al/Fe)-NH2 (10:1) and MIL-101(Al/Fe)-NH2 (5:1) were
investigated. Briefly, the catalytic performances of these
materials were evaluated in the presence of 20 mg of the
catalysts, where in benzaldehyde dimethyl acetal (a) (1.0 mmol)
reacts with malononitrile (1.5 mmol) in toluene (3 mL) at 90oC
for 90 min. The relevant conversions and yields were monitored
by gas chromatography (GC) and showed in Table 1. As shown
in Fig. 3a, the monometal catalysts MIL-101(Al)-NH2 and MIL-
101(Fe)-NH2 showed good catalytic activity for the cascade
reaction with yields up to 90 % and 70% in 90 min. Furthermore,
all of the bimetallic catalysts MIL-101(Al/Fe)-NH2 (X) displayed
superior catalytic performance than the monometal products,
and the yields reach nearly 100% after 90 min. Especially, the
Fig.3 (a) Time dependence of reaction yield for the cascade reaction of different
catalysts; (b) Recycle capacity tests of MIL-101(Al/Fe)-NH2(15:1) catalyst for the
tandem deacetalization-Knoevenagel reaction.
MIL-101(Al/Fe)-NH2 (15:1) exhibits the optimum performance
and rapidly catalytic activity, the calculated yield near to 35 %
only after 10 min. For the comparison, the mix (which means
mixed MIL-101(Al)-NH2 and MIL-101(Fe)-NH2 with the same iron
content as MIL-101(Al/Fe)-NH2 (15:1)) was also used as catalyst
with the same condition. The catalytic performance dropped
significantly and the efficiency of monometal catalyst should be
affected by introducing the other component. Thus, the
enhanced catalytic performance could be attributed to the
synergistic effect of bimetallic MIL-101(Al/Fe)-NH2 (X) series
catalysts. To certify the synergistic effect in the metal clusters
rather than in the pore, the MIL-101(Al)-NH2@Fe3+ (which
means soaked MIL-101(Al)-NH2 in FeCl3 ethanol solution and
containing the isolated iron ions in the framework) was also
prepared. As shown in Fig. 3a, the MIL-101(Al)-NH2@Fe3+
displayed the worst catalytic performance, indicating no
interaction between the isolated Fe3+ ions in the pore and the
Al-O clusters in the skeleton. On the other hand, the channels
Table 1. Deacetalization-Knoevenagel Condensation Reactiona
OCH3
OCH3
(D)
O
Cat
Cat, CH2(CN)2
CN
CN
H
H
-H2O
-2CH3OH
(B)
(A)
(C)
Catalyst
Conv of Yield of Yield of
A (%)
100
100
100
100
90.5
76.6
97.2
47.5
~0
Bb (%)
Trace
Trace
Trace
Trace
3.5
8.6
6.8
26.1
Trace
Cb (%)
99.8
99.9
99.7
99.8
85.1
67.7
89.7
20.2
Trace
MIL-101(Al/Fe)-NH2(20:1)
MIL-101(Al/Fe)-NH2(15:1)
MIL-101(Al/Fe)-NH2(10:1)
MIL-101(Al/Fe)-NH2(5:1)
MIL-101(Al)-NH2
MIL-101(Fe)-NH2
Mix
MIL-101(Al)-NH2@Fe3+
No catalyst
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were occupied by Fe3+ ions, which affect the transmission of the
raw materials and the production.
View Article Online
DOI: 10.1039/C9CY01940E
Obviously, there were scarcely product could be obtained
without as-synthesized samples after 90 min (Table 1),
indicating the irreplaceable role of catalysts. In addition, the
catalytic activity of bimetallic MOF was deeply investigated in
either the deacetalization reaction or Knoevenagel
condensation, respectively. The results displayed in Fig 4a and
4b. The bimetallic MOF materials not only enhance the
deacetalization reaction but also greatly improved the
Knoevenagel condensation reaction. This result was
contributed to the synergic catalytic effect between Al/Fe metal
clusters and anime groups. A leaching test was conducted and
indicated that no active materials was left into the reaction
solution (Fig. S8). In addition, the same reaction was carried out
at 110oC and the conversion/yield was very high, the reaction
completes within thirteen minutes (Fig. S7). After the reaction,
the catalysts could be easily removed by hot filtration method,
then reused for more than five cycles without significant
decrease in activity (Fig. 3b). The recycled catalyst MIL-
101(Al/Fe)-NH2 (15:1) remains its original crystal structure and
no obvious change happened even after recycling five times (Fig.
S9), indicating a good recyclability and superior stability.
Furthermore, the possible mechanism was shown in Fig. 5.
The active sites commonly located on the structure are likely to
be Al(III) and Fe(III) together with -NH2 groups. Therefore, it is
reasonable to believe that the reaction occurs on the
unsaturated species and NH2 group in the framework. Typically,
unsaturated Al and Fe species were employed as acid sites
adsorb benzaldehyde dimethyl acetal, catalyzing hydrolysis
reaction and forming benzaldehyde. Meanwhile, Al and Fe ions
synergistically adsorb benzaldehyde through the coordination
Fig.4 Comparison of catalysis (a) the deacetalization reaction at 90oC; (b) the
Knoevenagel condensation reaction at 90oC.
Fig.5 The Schematic representation of the mechanism for the deacetalization-
Knoevenagel condensation reaction catalysed by MIL-101(Al/Fe)-NH2 (X).
of the oxygen in the metal clusters, which activates the
dehydrogenation of carbonyl making it partially negative
charged and be easily nucleophilic attacked by the
malononitrile. In addition, the derivative could be activated by
the coordination of the metal ions (such as Fe ions) with the N
atoms of the malononitrile, inducting the deprotonation of the
-CH2- by amine groups, in which the α-H of the methylene group
6 | J. Name., 2012, 00, 1-3
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is much more positively charged.72 At last, the C=C bond is
formed together with losing a molecule of water, obtaining the
final products.
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DOI: 10.1039/C9CY01940E
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Conclusions
In conclusion, we used a rapid reflux method constructed a
series of bimetallic MIL-101(Al/Fe)-NH2(X) catalysts, which have
hierarchical pore structures in the skeleton. Acting as acid-base
bifunctional catalysts, the MIL-101(Al/Fe)-NH2 exhibit higher
catalytic properties for one-pot deacetalization-Knoevenagel
condensation reaction under relatively low temperature.
Compared with the monometallic frameworks (MIL-101-Al-NH2
and MIL-101-Fe-NH2), the bimetallic catalysts possess the
enhanced reaction rate and superior cycling stability, due to the
synergistic effect between the Al3+ ions and Fe3+ ions and the
larger mesoporous pores. This study further demonstrates that
MOFs can offer extraordinary opportunity of achieving site-
isolated acid-base catalysts, which are desirable for more
efficient and more facile organic synthesis. This work will open
a new path for designing the novel MOF-based bifunctional
sequential catalysts.
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Conflicts of interest
There are no conflicts to declare
Acknowledgements
We gratefully acknowledge the National Science Foundation of
China (NSFC No.21873025, 21571042, 51603055 and
51472062).
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DOI: 10.1039/C9CY01940E
Table of Contents
TOC figure
Bimetallic MIL-101(Al/Fe)-NH2 exhibit enchanced acid-base bifunctional catalytic activity due
to the synergistic mechanism and hierarchical pore system.
Keywords: bimetallic MOFs, tandem reaction, acid-base catalyst, synergistic effect