Water-stable fluorinated metal-organic frameworks (F-MOFs) with hydrophobic properties as efficient and highly active heterogeneous catalysts in aqueous solution

Article abstract of DOI:10.1039/c8gc02367k

Two new fluorinated metal-organic frameworks [Zn₂(hfipbb)₂(4-bpdh)]·0.5DMF (TMU-55) and [Zn₂(hfipbb)₂(4-bpdb)]·2DMF (HTMU-55) have been solvothermally synthesized by the reaction of two linear N-containing ligan

Full text of DOI:10.1039/c8gc02367k

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Water-stable fluorinated metal–organic
frameworks (F-MOFs) with hydrophobic properties
as efficient and highly active heterogeneous
catalysts in aqueous solution†
Cite this: Green Chem., 2018, 20,
5336
b
Monika Joharian,a Ali Morsali, *a Alireza Azhdari Tehrani, a Lucia Carlucci
and Davide M. Proserpio
b,c
Two new fluorinated metal–organic frameworks [Zn2(hfipbb)2(4-bpdh)]·0.5DMF (TMU-55) and
[Zn2(hfipbb)2(4-bpdb)]·2DMF (HTMU-55) have been solvothermally synthesized by the reaction of two
linear N-containing ligands as pillars (4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene and 4-bpdh =
2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene), and a flexible V-shaped fluorinated linker, H2hfipbb = 4,4’-
(hexafluoroisopropylidene) bis(benzoic acid). The two compounds were characterized by different tech-
niques such as X-ray crystallography, powder X-ray diffraction (PXRD), infrared spectroscopy (IR), contact
angle (CA), thermogravimetry (TGA), field emission scanning electron microscopy (FE-SEM), inductively
coupled plasma (ICP), and Brunauer–Emmett–Teller (BET) surface area analysis. Both compounds are
structurally similar and exhibit three dimensional (3D) coordination frameworks with hydrophobic pro-
perties. The catalytic activity of these isoreticular F-MOFs as efficient base heterogeneous catalysts
toward the Knoevenagel condensation reaction was tested and compared to each other. TMU-55 and
HTMU-55 exhibited good catalytic activity (with a minimum amount of catalyst, 2.5 mg (0.5 mol%), and
the shortest time, 5 min) in water as a green solvent with excellent conversions. Also, the results were
considerably higher in comparison with most of the values given in the literature for this reaction at ambient
temperature. The remarkable catalytic performance of TMU-55 and HTMU-55 in water can be attributed
to the presence of hydrophobic fluoro groups near the basic reaction center in the F-MOFs. These cata-
lysts maintain their crystalline frameworks after the reaction and are easily recovered and reused at least
for three cycles without significant loss in their catalytic activity. Furthermore, the conversion of benz-
aldehyde can be kept over 95% while the selectivity of the Knoevenagel reaction is kept at 100%.
Received 28th July 2018,
Accepted 16th October 2018
DOI: 10.1039/c8gc02367k
rsc.li/greenchem
number of organic ligands, inorganic metal ions/clusters, and
guests have been used to construct MOFs.
1. Introduction
Metal–organic frameworks (MOFs) have emerged as a unique
One of the especially interesting applications for MOFs is
class of crystalline materials that have attracted tremendous heterogeneous catalysis. In recent years, numerous studies have
attention due to their potential applications in different focused on heterogeneous catalysts based on MOFs owing to
research areas such as gas storage, catalysis, drug delivery, their high surface area, porosity, and chemical tunability.13
sensing, and removal of hazardous materials.1–12 A diverse Heterogeneous catalysis is superior to homogeneous catalysis in
terms of reusability, easier separation, and waste minimization.
On the other hand, by introducing suitable catalytic sites
into their structure, MOFs are the ideal candidates as hetero-
geneous catalytic materials for a wide range of reactions.14–17
Two major parts in the MOFs, which are very important in the
aDepartment of Chemistry, Faculty of sciences, Tarbiat Modares University, PO. BOX
14115-4838, Tehran, Iran. E-mail: Morsali_a@modares.ac.ir
bDipartimento di Chimica, Università degli Studi di Milano, Via C. Golgi 19, 20133
Milano, Italy
catalytic behaviour, can serve as the active centers: the unsatu-
rated metal site as a Lewis acid, and reactive functional groups
attached to any components (linkers) of the frameworks. In
order to have a catalytic material, the functional groups need
to be free and available to the substrates and should not be
directly coordinated to the metal ions of the MOFs.
cSamara Center for Theoretical Material Science (SCTMS), Samara State Technical
University, Samara 443100, Russia
†Electronic supplementary information (ESI) available. CCDC 1852195. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8gc02367k
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A feature that has seldom been taken into consideration for lytic activity, thermal stability, higher gas affinity, selectivity,
improving the catalytic activity of MOFs is the hydrophobic etc.41,42 It is generally acknowledged that fluorination can
character of the pore surface. Recently, Farrusseng’s research impart a diversity of new properties to MOFs, such as
group attributed the increase in the reaction rate of the enhanced hydrophobicity, super acidity, low surface
Knoevenagel condensation to the modification of the environ- energy, and excellent optical and electrical properties.41
ment of the catalytic sites and, in particular, through the cre- Hydrophobicity, through the presence of alkyl or fluorinated
ation of a hydrophobic environment.18,19
The Knoevenagel condensation is a reaction between an water stability of MOFs.
(F, CF3, etc.) groups, is also an efficient way to improve the
active methylene compound and a carbonyl compound (alde-
Therefore we tried to synthesize such novel F-MOFs based
hydes or ketones). It is the most popular reaction used to investi- on fluorinated groups and to evaluate their properties in the
gate coordination polymers and MOFs containing basic function- catalytic reaction in the presence of water as an interesting
alized structures. Furthermore, it is one of the most important issue.
reactions for the formation of CvC bonds and for the synthesis
of several fine chemicals and medical intermediates; in general, bis(benzoic acid) (H2hfipbb) as a semi-rigid and long dicarboxy-
it is a highly useful reaction in synthetic chemistry.20–28
late ligand has been investigated for its bent geometry and
Among the various ligands, 4,4′-(hexafluoroisopropylidene)
Despite various solid catalysts having been employed for hydrophobic features. The –CF3 terminated surface was
the Knoevenagel condensation reaction, such as mesoporous reported to possess low free energy and the best hydrophobi-
materials,29–31 ionic liquids,32,33 organometallic catalysts,34 city,43,44 hence MOFs constructed from the H2hfipbb ligand
zeolites,35,36 etc., only a few sustainable methods that utilize may have hydrophobic features and better water stability,45
basic heterogeneous catalysts were reported.37 However, most and potential applications in hydrophobic materials.
of these methods have significant drawbacks, such as using
In this work, we have focused on the synthesis of two
hazardous and carcinogenic solvents, high catalyst loading or pillared F-MOFs from 4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-
non-recoverable catalysts that sometimes contain toxic metals. butadiene and 4-bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexa-
Therefore, there is a great need for new catalytic methods that diene, and the flexible fluorinated dicarboxylate building block,
do not have these problems.
H2hfipbb = 4,4′-(hexafluoroisopropylidene) bis(benzoic acid)
Hence, further development of appropriate and more (Scheme 1) to obtain novel, highly efficient, stable, and recycl-
impressive heterogeneous catalysts for the Knoevenagel reac- able Zn based heterogeneous catalysts. The incorporation of
tions is essential. There have been many reports with MOFs as such a hydrophobic organic linker in the MOF structures may
superior heterogeneous catalysts for the Knoevenagel also serve to develop heterogeneous organocatalysts with good
condensation38–40 and also MOFs with basic groups such as water stability and high catalytic activity in aqueous solution.
azines are efficient catalysts for this reaction.28 Most of the
Because the Knoevenagel reaction occurs with basic cata-
MOFs that have been investigated in the Knoevenagel conden- lysts we wish to use TMU-55 and HTMU-55 (TMU corresponds
sation reaction are not stable in water and need high tempera- to Tarbiat Modares University) containing azine groups as
ture, high amounts of catalysts or more time to complete the efficient heterogeneous catalysts.
reaction. To overcome these issues, we pursued the develop-
ment of more suitable MOFs as catalysts for this reaction.
Based on this assumption, the catalytic behaviour of these
hydrophobic frameworks for the Knoevenagel condensation
Recently researchers have paid attention to the possibility reaction of carbonyl compounds with malononitrile in the
of synthesizing fluorinated metal–organic frameworks presence of water as the appealing idea was investigated.
(F-MOFs) as a new class of advanced porous materials. Interestingly, high catalytic activities were observed even with a
Compared to metal–organic frameworks, F-MOFs with fluori- minimum amount of F-MOFs and at room temperature, as
nated channels or cavities are expected to have increased cata- well as, good reusability of the catalyst without significant
Scheme 1 Synthetic details of the two F-MOFs obtained with the use of a zinc salt in combination with H2hfipbb and 4-bpdh and 4-bpdb linkers.
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degradation in activity. Moreover, the utilization of harmful
Selected IR peaks (cm−1) for TMU-55: 3082(w), 2929(w),
solvents was avoided and solvents were environmentally 1687(s), 1641(vs), 1562(m), 1404(vs), 1247(s), 1212(w), 1170(m),
friendly.
964(m).
Selected IR peaks (cm−1) for HTMU-55: 3076(w), 2931(w),
1686(m), 1639(s), 1563(m), 1499(w), 1406(s), 1248(m), 962(m).
2. Experimental
2.3. Activation method
2.1. Materials and measurements
Before the catalysis process, for removing guest DMF mole-
cules, the crystals were immersed in 5 ml of CH3CN for 3 days
and fresh CH3CN was added every 24 h. After 3 days the
CH3CN solution was decanted and dried at 80 °C at least for
12 h.
All starting materials and solvents for the synthesis and ana-
lysis were commercially available from Aldrich and Merck
Companies and used as received. The ligands 1,4-bis(4-
pyridyl)-2,3-diaza-1,3-butadiene (4-bpdb) and 2,5-bis(4-
pyridyl)-3,4-diaza-2,4-hexadiene (4-bpdh) were prepared by the
reported method.46
2.4. Catalytic experiments
IR spectra were recorded using a Nicolet 100 Fourier
Transform IR spectrometer in the range of 500–4000 cm−1
using the KBr disk technique. The inductively coupled plasma
(ICP) analyses were performed on a Varian ICP-OES VISTA-PRO
CCD instrument. Melting points were measured on an
Electrothermal 9100 apparatus. The thermal behaviour was
measured with a PL-STA 1500 apparatus at a rate of 10 °C
min−1 under a static atmosphere of argon. Gas chromato-
graphy (GC) runs were performed on an Echrom GC A90 gas
chromatograph. The samples were also characterized by using
a field emission scanning electron microscope (FE-SEM)
TESCAN MIRA with gold coating. X-ray powder diffraction
(PXRD) measurements were performed using a Philips X′pert
diffractometer with monochromated Cu-Kα (λ = 1.54056 Å)
radiation. The contact angle was measured by using an OCA 15
plus apparatus.
X-ray diffraction structure determinations. Single crystals of
TMU-55 were selected and mounted in inert oil and trans-
ferred to the cold gas stream of a Bruker APEX-II CCD
diffractometer. The data were corrected for absorption based
on the multi-scan technique as implemented in SADABS. The
structures were solved by conventional methods and refined
using SHELX201647 implemented in a WinGX graphical user
interface.48 Anisotropic thermal parameters were refined for
non-hydrogen atoms and hydrogen atoms were calculated and
refined with a riding model. A DMF molecule was refined as a
disorder model on a 2-fold axis. CCDC 1852195† contains the
supplementary crystallographic data for this paper.
As shown in Scheme 2 catalytic experiments were carried out
in a 5 ml capped glass tube. In a typical procedure for the
Knoevenagel reaction, 0.5 mmol of malononitrile, 0.2 mmol of
an aromatic ketone, cyclohexanone, and four aldehyde com-
pounds, benzaldehyde, 4-methylbenzaldehyde, 4-nitrobenzal-
dehyde, 4-methoxybenzaldehyde, and 4-chlorobenzaldehyde,
and 2.5 mg of activated catalyst were added in 3 mL of
different solvents (Scheme 2). The resulting mixture was
stirred at room temperature for 5 min. Gas chromatography
(GC) analysis was employed to detect the reaction progress and
catalytic activity. When the reaction was completed, the cata-
lyst was recovered by centrifugation, while the supernatant
reaction mixture was passed through a 0.4 μ filter and sub-
sequently analyzed with a GC apparatus. The condensation
products were identified by comparison of their retention
times with those of authentic samples. The conversions are
based on the starting substrates. The results are summarized
in Fig. S4–S6† and Tables 1 and 2.
3. Results and discussion
3.1 Crystal structure analysis
HTMU-55 and TMU-55 were synthesized using the solvo-
thermal method by simply mixing Zn(NO3)2·6H2O and
H2hfipbb with 4-bpdb and 4-bpdh ligands, respectively. The
crystal structure of HTMU-55 has been confirmed by compari-
2.2. Synthesis of [Zn2(hfipbb)2(4-bpdh)]·0.5DMF (TMU-55)
and [Zn2(hfipbb)2(4-bpdb)]·2DMF (HTMU-55)
To prepare the single crystals of TMU-55 and HTMU-55, at first
H2hfipbb (0.039 g, 0.1 mmol), 4-bpdh (0.024 g, 0.1 mmol) or
4-bpdb (0.021 g, 0.1 mmol), Zn(NO3)2·6H2O (0.03 g, 0.1 mmol)
and N,N-dimethylformamide (DMF; 20 ml) were mixed. Then
the solution was divided in 3 ml glass vials and each filled up
to 2.5 ml. The vials were closed tightly and set in an oven and
heated for 3 days at 80 °C. After the crystals were formed, they
were recovered by filtration and dried at 80 °C in an oven over-
night. Yield: 83% for TMU-55 and 81% for HTMU-55
(Scheme 1).
Scheme 2 Schematic representation of the Knoevenagel condensation
reaction.
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Table 1 Knoevenagel condensation reaction performed in the pres-
ence of the two isoreticular F-MOF catalysts (TMU-55 and HTMU-55)
and in their absence
ligand (Fig. 1b). The 3D framework is constructed from
paddle-wheel Zn2(COO)4 clusters bridged by hfipbb bent
ligands to form 2D square grids (sql) further connected by the
pillar ligands (4-bpdh) (Fig. 1c). About the Zn atom, the four O
atoms are approximately in a plane, the Zn–O lengths are in
the range of 2.043(2)–2.048(2) Å, and the Zn–N length is
2.026(2) Å. The Zn⋯Zn distance across the 4-bpdh ligand is
15.2 Å.
Entry
Catalyst
Conversion (%)
Topological analysis50 reveals a six-coordinated underlying
net51 formed by the zinc paddlewheel as a 6-coordinated SBU
and the two different ligands as 2-coordinated linkers. The
single 3D framework is the rare self-catenated 6T9 net with a
(44·610·8) point symbol. The 6T9 topology, according to the
ToposPro database,52 is observed only in 13 isoreticular struc-
tures, including HUQFEG. Ten of them have the same general
formula and the same SBUs as TMU-55, namely [M2(bent
ligand)2(linear ligand)] where M = Zn, Cu, Co, Cd, and “bent
ligands” such as hfipbb and oba (4,4′-oxybisbenzoate).49,53–61
Moreover, the unit cell parameters are very similar and all crys-
tallize in the same monoclinic C2/c space group, showing a
strong propensity toward the 6T9 net.
The structure is formed by 2-fold interpenetrated square
layers of Zn2(hfipbb)2 that stack along [100] as ABAB. The
interpenetration of the sql layers62 is well known and fre-
quently observed only for bent ligands.63 Each single sql motif
forming the 2-fold interpenetration (sql(a) and sql(b)) is con-
nected above and below via the linear 4-bpdh or 4-bpdb
ligands in a way to obtain a single underlying net: sql(a)–
1
2
3
TMU-55
HTMU-55
—
99
87
5
GC yield, reaction conditions: benzaldehyde (0.2 mmol), malononitrile
(0.5 mmol), 2.5 mg (0.5 mol%) catalyst, solvent: H2O (3 ml), r.t., 5 min.
son of the powder pattern with that calculated from the known
structure (HUQFEG).49 Single crystal X-ray analysis of TMU-55
reveals that it is isoreticular with HTMU-55 and HUQFEG.
TMU-55 [Zn2(hfipbb)2(4-bpdh)]·0.5DMF crystallizes in the
monoclinic C2/c space group. Crystal data with data collection
and refinement parameters are summarized in Table S1,† and
selected bond distances and bond angles are given in Table S2.†
The ORTEP view of the asymmetric unit, consisting of one
zinc atom, one hfipbb and half 4-bpdh is shown in Fig. 1a.
The coordination geometry around Zn(II) can be described as a
distorted square pyramidal with the five coordination sites
occupied by four oxygen atoms of carboxylate groups from
hfipbb ligands and one pyridyl nitrogen atom from the 4-bpdh
Table 2 Knoevenagel condensation reaction catalyzed by the two isoreticular F-MOF catalysts (TMU-55 and HTMU-55) in the presence of different
aldehydes and ketones
Entry
1
Catalyst
Substrate
Substrate
Product
Conversion (%)
TMU-55
HTMU-55
99
87
2
3
TMU-55
HTMU-55
71
59
TMU-55
HTMU-55
65
50
4
5
TMU-55
HTMU-55
100
91
TMU-55
HTMU-55
99
90
6
7
TMU-55
HTMU-55
67
52
TMU-55
HTMU-55
61
48
8
TMU-55
HTMU-55
59
47
GC yield, reaction conditions: substrates (0.2 mmol), malononitrile (0.5 mmol), 2.5 mg (0.5 mol%) catalyst, solvent: H2O (3 ml), r.t., 5 min.
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Fig. 1 Crystal structure of [Zn2(hfipbb)2(4-bpdh)]·0.5DMF (TMU-55): (a) ORTEP view of the asymmetric unit. (b) {Zn2} paddlewheel building unit. (c)
View along the crystallographic b-axis showing the disposition of the clathrate DMF molecules along the helical channels.
(ligand)–sql(b). This converts the 2-fold 2D interpenetration in
the resulting single 3D self-catenated net called 6T9 (Fig. 2).
The structure shows narrow 1D channels along the crystallo-
graphic b-axis (Fig. 1c) filled with guest solvent molecules
(DMF for TMU-55 and water for HUQFEG). The walls of the
channels are formed by double Zn2(hfipbb) helices arising
from the 2-fold sql.
Powder X-ray diffraction (PXRD) of TMU-55 and HTMU-55
confirms that they are similar to each other and establishes
their isomorphism and isoreticularity (Fig. S1†). Therefore,
two basic azine-functionalized F-MOFs were synthesized which
are different in terms of structural hydrophobicity (Scheme 3).
TMU-55 shows limited porosity with the calculated void
space per unit cell for the guest-free framework of 14.5%.
TMU-55 has 1D narrow channels with approximately 1.8 ×
3.3 Å in cross-section (including van der Waals radii) that is
not large enough to be, in principle, accessible for N2 (kinetic
Scheme 3 Schematic representation of structural hydrophobicity.
diameters for N2: 3.75 Å) adsorption. Also, the BET measure-
ment shows that HTMU-55 is nonporous toward N2.
To confirm and evaluate the hydrophobicity of these
F-MOFs, the contact angles were measured and analyzed. Such
experiments show that both F-MOFs have relatively hydro-
phobic structures due to the presence of the fluoro groups in
their structures (TMU-55: 71.55 and HTMU-55: 65.6 degrees).
Moreover, TMU-55 has more hydrophobic character than
HTMU-55 due to the incorporation of methyl groups (Fig. S2†).
In order to activate the potential interacting sites of the
frameworks, the DMF molecules within the structure could be
exchanged with acetonitrile molecules, which are more easily
removed. The as-synthesized crystals of these frameworks were
activated by solvent exchange with acetonitrile for 3 days.
Then, the samples were filtered and dried at 80 °C for 4 h.
FT-IR spectroscopy (Fig. 3) confirmed the removal of guest
DMF molecules accommodated in the pores of the frameworks
Fig. 2 Topology of [Zn2(hfipbb)2(4-bpdh)]·0.5DMF (TMU-55). Left: View
of the underlying self-interpenetrated 6T9 net where the simplified
2-fold interpenetrated sql layers (black) are connected by the 4-bpdh
ligand (green). Right: Molecular (bottom) and simplified (top) views of
the 2-fold interpenetrating sql layers (in red and blue) in TMU-55.
due to the disappearance of the DMF peak at 1687 cm−1
.
Powder X-ray diffraction (PXRD) (Fig. 4) showed that the struc-
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Fig. 3 FT-IR spectra of the as-synthesized samples by the solvothermal method (in red) and of the activated samples (in blue) for TMU-55 (left) and
HTMU-55 (right).
Fig. 4 Simulated and experimental PXRD patterns for the as-synthesized, activated, treated in water, and used as catalyst samples of TMU-55 (left)
and HTMU-55 (right). We used the HUQFEG structure to simulate the spectrum for HTMU-55.
tures were retained after activation. The only places available which PXRD analysis of the isolated sample revealed retention
and active for the catalytic reaction are the basic nitrogen of the original framework structure (Fig. 4). This remarkable
centers that emerged within the F-MOF structure walls on the water stability of F-MOFs could be attributed to the hydro-
surface.
phobic nature of the framework surface due to the CF3
As shown in Fig. S3,† thermogravimetric analysis (TGA) has groups.45,47,65
been carried out in the range of 25–800 °C. The TGA curve of
TMU-55 exhibits a first weight loss, from room temperature up
to 200 °C of 3.16%, that could be associated with the loss of
0.5 DMF molecules, a second weight loss in the temperature
range of about 370–470 °C that refers to the degradation of the
framework (62.08%), and finally, a complete decomposition of
the compound to ZnO (Fig. S3†). The TG analysis of HTMU-55
shows a curve similar to that of TMU-55 apart from a more
evident weight loss at the first step (from 25 to 200 °C) of
10.94% that can be attributed to the loss of 2 DMF molecules.
The degradation of the framework (weight loss of 53.58%) is
complete within 470 °C giving a final residue of ZnO (Fig. S3†).
3.3 Catalytic studies
The application of MOFs in heterogeneous catalysis is one of
the most widely investigated topics and is of great interest to
researchers. Many studies were performed on the hetero-
geneous catalysts including MOFs for the Knoevenagel con-
densation reaction. This condensation is usually catalyzed by
Lewis bases or acids. The design and synthesis of non-toxic,
reusable, easily separable, and low-cost catalysts has become
an important area of research in the field of green chemistry.
In the present study, the Knoevenagel condensation reac-
tion was chosen as a probe to study the catalytic activity of the
compounds [Zn2(hfipbb)2(4-bpdh)]·0.5DMF (TMU-55) and
[Zn2(hfipbb)2(4-bpdb)]·2DMF (HTMU-55).
3.2 Water stability of F-MOFs
F-MOFs exhibiting stability in water have several potential
The Knoevenagel condensation reaction is well known not
applications as hydrophobic materials.64 In this work, the only as a weak base catalyzed model reaction but also as a reac-
stability of TMU-55 and HTMU-55 in water was investigated. tion that generates a CvC bond (Scheme S1†).
The powder of these frameworks was immersed in water with
The use of a suitable solvent can significantly increase the
vigorous stirring in a screw-capped glass vial for 3 d, after yield of the reaction. So in the first step of our experiments, in
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order to optimize the reaction solvent, a range of solvents were mediates of the condensation reaction or any side products
selected (Fig. S4†). In the experiments 2.5 mg catalyst, were not observed.
0.2 mmol benzaldehyde and 0.5 mmol malononitrile in 3 ml
We used various ketones and also aldehydes as substrates
of different solvents such as methanol, ethanol, H2O, and to compare the catalytic activity of the F-MOFs in the presence
toluene at room temperature were employed. It can be con- of electron-donating or electron-withdrawing groups. The
cluded from these results that higher yields were achieved in results showed that the catalytic activity was enhanced by
polar protic solvents, while the reaction occurred with increased N-donor ligand basicity (Table 2 and Fig. S6†). In
difficulty in solvents with lower polarity such as toluene.
To achieve a high yield, the Knoevenagel condensation reac- of aldehydes, such as benzaldehyde derivatives, is investigated
tion in H2O was selected as a probe reaction. because the ketones were reacted with malononitrile under
The reaction with benzaldehyde in H2O and in the absence more severe conditions.
most of the papers for the Knoevenagel reaction, the reaction
of a catalyst gave 5% conversion as obtained by GC (Table 1),
In addition, these catalyst F-MOFs can be recycled and are
whereas in the presence of 2.5 mg TMU-55 and HTMU-55 as reused at least for 3 runs without significant loss of their
the catalyst for 5 min at room temperature, the reaction pro- efficiency. The reason for the 8% decrease in the percentage of
ceeded with 99% and 87% conversions, respectively. This conversion after the third run is most probably the de-
observation clearly revealed the catalytic effect of the syn- activation of the active sites. The destruction of the structure is
thesized F-MOFs. In addition, the same reaction runs were per- unlikely to happen before the fourth reaction run as shown by
formed for the optimization of the catalyst amounts. PXRD and ICP (Fig. 5). A leaching test was carried out for the
Malononitrile (0.5 mmol), benzaldehyde (0.2 mmol) and H2O zinc ions by ICP analysis and it was found that less than 1% of
(3 mL) were mixed into three glass vessels equipped with a the zinc atoms from the frameworks passes into the reaction
magnetic stirrer bar. Then 1, 2, and 2.5 mg of catalysts were solution; thus more than 99% of the zinc metal center does
added to each vessel, respectively. The reactions were moni- not leach into the reaction mixture under the reaction con-
tored by GC (the results are shown in Fig. S5†). During the ditions. So these analyses prove the stability of the base cata-
survey of the reaction conditions, the best results were lyst during the reaction in the H2O solvent. Moreover, the Zn
obtained by using 2.5 mg of the catalysts at r.t in 3 ml of H2O.
species leached out from the solid are not able to catalyze the
The evaluation of the conversions with 100% selectivity in condensation on their own; a test with zinc(II) nitrate salts
all cases shows that only one product is formed. Any inter- gives only 30% conversion in 4 h. Also we tested the residual
Fig. 5 Simulated and experimental PXRD patterns after the first and fourth catalytic cycles of TMU-55 (top left) and HTMU-55 (top right). Bottom:
Bar chart (blue for TMU-55, yellow for HTMU-55) of the GC % conversion yield in the four catalytic cycles for the reaction conditions: benzaldehyde
(0.2 mmol), malononitrile (0.5 mmol), 2.5 mg (0.5 mol%) catalyst, solvent: H2O (3 ml), r.t., 5 min.
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Table 3 A comparison of the catalytic activity of various catalysts in the Knoevenagel condensation reaction of benzaldehyde and malononitrile
Entry
Catalyst
Amount
Time
Solvent
Temp. [°C]
Conversion (%)
Ref.
1
2
3
4
5
6
7
8
9
Pb(cpna)2·2DMF·6H2O
[Cd(4-btapa)2(NO3)2]·6H2O·2DMF
[Cd(bipd)2(DMF)2]·(ClO4)2·(2DMF)
[Gd2(tnbd)3(DMF)4]·4DMF·3H2O
Au@Cu(II)-MOF(1)
{[Ni3(TBIB)2(BTC)2(H2O)6]·5C2H5OH·9H2O}n
Fe3O4@ZIF-8
3 mol%
5 mol%
4 mol%
10 mol%
3 mol%
5 mol%
4 mol%
0.5 mol%
0.5 mol%
24 h
12 h
30 min
20 min
23 h
2 h
3 h
5 min
5 min
CH3CN
C6H6
C6H6
C6H6
Toluene/MeOH
DCM
Toluene
H2O
H2O
r.t.
r.t.
r.t.
r.t.
r.t.
60 °C
r.t.
r.t.
r.t.
100
98
93
96
99
100
94
99
66
67
68
69
70
71
72
TMU-55
HTMU-55
This work
This work
87
activity of the solution without the solid and the reaction con- 4-bpdh that enhance the basicity and hydrophobicity of the
tinued very slowly. nitrogen active centers. This study demonstrates the important
SEM images obtained on crystals of the two F-MOFs before role of the basicity of azine functionalized frameworks, as well
and after a catalytic cycle, shown in Fig. S7,† indicate the as that of the hydrophobicity of the frameworks on the cata-
maintenance of the frameworks after the Knoevenagel conden- lytic performance.
sation reaction.
A comparison of the catalytic activity of TMU-55 and
HTMU-55 with that of other heterogeneous catalysts previously
Conflicts of interest
used in such a reaction is reported in Table 3. To demonstrate
the advantages of our catalyst over other heterogeneous cata-
lysts used in the Knoevenagel condensation reaction between
There are no conflicts of interests.
benzaldehyde and malononitrile, we compared the obtained
results based on the amount of the catalyst, solvent used, and
the reaction time (Table 3).
Acknowledgements
The results obtained in this work were considerably higher
in comparison with most given values in the literature for this
reaction conducted at ambient temperature.
In this case, the reaction proceeds well in H2O solvent
through the role of hydrophobic groups (CF3) over the hfipbb
Support of this investigation by Tarbiat Modares University is
gratefully acknowledged. L. C. and D. M. P. thank Università
degli Studi di Milano (Piano di Sviluppo di Ateneo, azione B,
progetti
di
interesse
interdisciplinare
PSR2015-
ligands. The results show that TMU-55 due to methyl groups
near the basic center reaction in the 4-bpdh ligand increases
the basicity and hydrophobicity and better interaction between
azine groups (as Lewis base) in the catalyst and substrate and
so has higher catalytic activity than HTMU-55 in aqueous
media.
1716FDEMA_07) for financial support.
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