Size and function influence study on enhanced catalytic performance of a cooperative MOF for mild, green and fast C-C bond formation

Article abstract of DOI:10.1039/d0dt00433b

Tuning of pore function and size (surface area) are two key factors that play important roles in the performance of metal-organic-frameworks (MOFs) as catalysts. The catalytic performance of two bulk and nanosized MOFs with different functional groups suc

Full text of DOI:10.1039/d0dt00433b

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DOI: 10.1039/D0DT00433B
Size and Function Influence Study on Enhanced Catalytic Performance of a
Cooperative MOF for Mild, Green and Fast C-C Bond Formation
Najmeh Varnaseri†a, Farzaneh Rouhani†b, Ali Ramazania, c*, Ali Morsalib**
a Department of Chemistry, Faculty of Sciences, University of Zanjan, P.O. Box 45195-313, Zanjan, Iran
b Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14155-4838, Tehran,
Iran
c Research Institute of Modern Biological Techniques (RIMBT), University of Zanjan, P.O. Box 10
45195-313, Zanjan, Iran
† Najmeh Varnaseri and Farzaneh Rouhani contributed equally to this work.
Abstract
Tuning of pore function and size (surface area) are two key factors that play important roles
in metal-organic-frameworks (MOFs) performance as catalyst. The catalytic performance of
two bulk and nanosized MOFs with different functional groups such as Brønsted base and
Lewis acid were studied in line with catalyst sustainable development and green chemistry
principles. Bifunctional imine decorated TMU-33, ([Cd3(BDC)3(OPP)(DMF)2].2DMA)n
(TMU-33), (OPP:N,N'-(oxybis(4,1-phenylene)) bis(1-(pyridin-4yl) methanimine)), with
adjustable structure and amine functionalized TMU-40, [Zn(BDC)( L*)]. DMF, (L*: N1, N2-
Bis (pyridin-4-ylmethylene) ethane-1,2-diamine) , were evaluated in the C-C bonding
forming reaction in mild and green situation. The results show that nanosized samples of
bifunctional TMU-33 which simultaneously have imine and open metal site, have higher
performance as Knoevenagel catalyst. Furthermore, among nanosized samples, the nanoplate
TMU-33 with more access to open metal sites shows the highest catalytic activity without
any side product in water, room temperature at 5 min, that confirms the Lewis acid are
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effective catalyst for this reaction. The catalyst could be reused for at least three cycles
DOI: 10.1039/D0DT00433B
without significant loss of its activity. The performance of the structure indicates that tuning
of functionality of MOFs can be very promising route for the extension of green catalysts.
Key words: Metal-Organic Framework (MOF), Catalyst, Knoevenagel Condensation, Green
Solvent, Mild Situation
1. Introduction
The growing population has a crucial requirement for the production of fine chemicals. The
development of more efficient, selective and versatile catalysts can be the key to resolve the
demand1. Metal–organic frameworks (MOFs) have been noticed because of tunable porosity,
design ability, uniform and accessible catalytic centers and chemical and thermal stability2-5.
Therefore, they can be tunable and suitable catalysts for most of the organic reactions. Some
reactions have been catalyzed by MOFs as solid acid catalysts or catalyst supports such as
Knoevenagel condensation, trans esterification, aldol condensation, oxidation, epoxide ring-
opening reaction and so on 6-19
.
The Knoevenagel condensation of carbonyl groups (aldehyde and ketone) with active
methylene species is one of the methods to form C=C bonds, and it has numerous
applications in industrial organic synthesis 20. In literature, the reaction of aldehydes and their
derivatives, are reported frequently because the reaction of ketones with malononitrile need
to harsher conditions21. The Knoevenagel reaction has been reported by both basic22 and
acidic23 catalysts but activity of the acidic catalysts is lower 24. On the other hand, the polarity
of solvent has a remarkable effect on performance of MOFs as Knoevenagel reaction
catalyst25. Polar solvents actually stabilize the charged transition-state intermediate of the
Knoevenagel condensation26. A large diversity of homogeneous and heterogeneous catalysts
are applied to this reaction and they show different features27-28. Heterogeneous catalysis
offer several advantages over homogeneous ones29, because they have more recyclability,
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reusability easier separation and they minimized the undesired waste 30. MOFs can serve as
DOI: 10.1039/D0DT00433B
heterogeneous catalysts either by the metal ion or cluster as Lewis acid or functional ligands
as active sites that incorporated through direct synthesis or by post-synthetic modification31.
There has been enormous investigations on the catalytic performance of catalysis taking place
at the metal center32-33. However, only few studies have been reported on MOFs with
catalytically-active functional organic sites34. This is because, the functional groups need to
be free and accessible to the substrates and cannot be directly connected to the metal ions of
the MOF35-36. The numbers of heterogeneous catalysts including MOFs with various kinds of
basic sites have been offered to catalyze Knoevenagel condensation37-38. Several
possible mechanisms have been proposed for the reaction according to the catalysts structure
39-40. For catalysts with high base strength, the active methylene reactant is deprotonated by
basic sites and carbanion formed. The addition-elimination process is facilitated which occurs
between carbanion and the carbonyl carbon atom of aldehyde. For weaker bases such as
MOFs carrying amino groups at their structures, it has been proposed that the reaction occurs
between the surface amine sites and aldehyde molecule to form an imine intermediate and
one molecule of water21. Final product is yielded and regenerate the amine catalyst by a
molecular arrangement in methylene 41-42. The Knoevenagel condensation has usually been
considered as a classical test reaction to evaluate the basicity of catalysts43.
Although various catalytic systems have been reported to date, development of catalyst which
can promote this reaction without the formation of side products is still a challenge. In
addition to formation of side products, a reaction between the solvent and the aldehyde is
possible44. From a general point of view, the development of catalyst systems in non-toxic
solvent, at mild condition, methodology without environmental concern is very useful in the
field of green chemistry. In order to design the high effective catalyst, we used the TMU-33,
nanosized samples of TMU-33 and TMU-40 as catalyst. The TMU-33 framework has an
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open metal site and an imine functional group that can both act as catalysts sDeOpIa: 1r0a.1t0e3l9y/Do0DrT00433B
simultaneously45. Also, the structure of TMU-40 has active amines available for catalytic
reaction46. The results indicate that nanosized TMU-33 is a more effective catalyst due to the
simultaneous acid-base part of the (TMU-33) structure. Based on the results, the nanoplate
structure of TMU-33 with more abundant of the open metal site demonstrate the best catalytic
species for this reaction.
2. Experimental
2.1. Material and method
All chemicals without any purification were obtained from Sigma-Aldrich and Merck
companies. The progress of the catalytic reactions was monitored by an Echrom GC A90 gas
chromatograph. Synthesis methods of N,N'-(oxybis(4,1-phenylene))bis(1-(pyridin-4-
yl)methanimine) (OPP) and N1, N2-bis (pyridin-4-ylmethylene) ethane-1,2-diamine(L) ligand
expressed in S.I.
2.2. Synthesis of [Cd3 (BDC)3 (OPA)(DMF)2].2DMA (TMU-33)
TMU-33 was synthesized using the previously-reported method45. The mixture of
Cd(NO3)2·4H2O (1 mmol, 0.3048 g), OPP (1 mmol, 0.378 g ) and terephthalic acid (H2BDC)
(1 mmol, 0.0188 g ) added to 20mL DMF solvent and subjected to solvothermal reaction
conditions in glass vials for 72 h at 80°C. The generated precipitate was separated by
filtration, washed repeatedly with DMF and air-dried. Before the catalysis process, for
removing guest DMF molecules, the resulting precipitate was dispersed in acetonitrile for 72
h and the solvent exchanged three times. The solution was vacuum filtered and the solid was
dried in oven at 80 °C.
2.3. Synthesizes of [Cd(BDC)( L*)]. DMF (TMU-40)
L(1mmol,0.256 g), H2BDC (1mmol, 0.166 g) and Cd(NO3)2·4H2O (1 mmol, 0.3048 g), and
20 ml DMF was mixed and then heated to 120 °C for 72 h to form dark red crystals of TMU-
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40 46. After crystal formation, they washed with ethanol and dried. For obtaDinOiI:n1g0.10a3c9t/Div0DeT00433B
TMU-40, the crystals were immersed in dichloromethane for 24 h and the solvent exchanged
every 12h. Solvent was decanted and the powder dried in oven at 80 °C.
2.4. Synthesis of TMU-33 Nanostructures
TMU-33 nanostructures synthesize via sonochemical reaction in ultrasonic
bath. TMU-33 nanostructures were synthesized by a concentrations of [H2BDC] =
[Cd(OAc)2.4H2O] = [OPP ligand] of [0.001] and optimal concentrations of benzoic acid and
pyridine as capping reagents (modulator), respectively at room temperature for 20 min.
These capping agents are chosen due to similarity of their coordination sites to H2BDC and
OPP ligand, respectively. The "r" factor is ratio of [capping reagent]/ [OPP], for nanorod
TMU-33, "r" factor is defined as the ratio [benzoic acid]/ [H2BDC] were 10, 5 and 2, and r
factors for [pyridine]/ [OPP ligand] about nanoplates also were 10, 5 and 2. The solid was
washed with DMF then filtered under vacuum and dried in oven at 100°C 45.
2.5. Catalytic experiments
The catalytic reactions were carried out following green conditions, with 1 mg of catalysts at
room temperature using water as solvent. In a typical catalytic experiment, aromatic aldehyde
with different structure (0.1mmol) and malononitrile (0.25mmol) in 3 ml water was stirred at
room temperature for 5 min with 1 mg of the activated catalysts. The reaction was monitored
by gas chromatography until completion of the reaction.
3. Results and discussion
TMU-40 (CCDC number: 1811813) was synthesized by the reaction of 1,2-N1,N2-
bis(pyridin-4-ylmethylene)ethane (ligand L) Cd(NO3)2.6H2O and H2BDC in DMF at 120°C
for 72 h. The ligand L*= 5,6-dipyridin-4-yl-1,2,3,4-tetrahydropyrazine synthesized through an
in-situ C=C formation. Such aldimine C=C coupling reactions catalyzed by cyanide ion
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reported previously by Strain in 182447-48 .The steps of L* formation are schematically
DOI: 10.1039/D0DT00433B
illustrated in Figure 1 and the proposed mechanism are presented as Figure S1.
NH
2O
H
N
N
NH2
O
reflux ,3h
N
H
N
N
N
L
H
N
N
HN
NH
Oxidation
N
N
N
N
L**
L*
Figure 1. Synthesis rout of L and C=C coupling reaction of L to L**.
The 3D framework of the TMU-40(Cd) with expecting 4-connected sql tetragonal plane net
crystallizes in the orthorhombic space group Pbca.. The structure of [Cd(H2BDC)(L*)].DMF
extended in three dimensions through hydrogen bonding. Pore sizes evaluated form single-
crystal X-ray structure is 5.5×7.9 Å and has 13 Å3 free voids (Figure 2A). The
thermogravimetric analysis (TGA) has been carried out on TMU-40 in the range of 25–600
°C. The thermogravimetric curve of TMU-40(Cd) shows a weight loss of 13.1% at about
200°C, corresponding to removal of trapped solvent in the pores. As can be seen in Figure
2B, the structure is stable at temperatures up to 360 °C.
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DOI: 10.1039/D0DT00433B
Figure 2. (A) The schematic pictures of constructive part and 3D expansion of TMU-40, (B)
Thermogravimetric analysis of TMU-40. Color code: C: black; O: red; N: blue, Cd: green and
H: yellow.
On the other hand, solvothermal reaction of Cd(NO3)2·4H2O, V-shaped imine based ligand
(OPP) and H2BDC in DMF at 100°C for 72 h afforded a MOF by the following framework
formula: [Cd3 (BDC)3 (OPA) (DMF)2].2DMA (TMU-33) (CCDC number: 1522499). The
imine based ligand OPP was readily synthesized by a nucleophilic substitution reaction of
4,4′-oxydianiline and 4-pyridine carboxaldehyde in good yield and gram scale. The OPP is a
typical flexible ligand with rotating around a single bond (one sp3 hybrid atom on its
backbones) linking to a quaternary carbon atom. In particular, 4 BDC2- anions are in the
bridging mode and two others have two-coordination position. More crystallography data are
presented in our previous paper45. The 3D framework of TMU-33 with rectangle channels
(6.6×6.4 Å) along the b axis contains of two non-coordinated DMA molecules which are
formed by partial hydrolysis of solvent (DMF) during synthesis procedures. The activation
process was done by immersion of obtained samples in CH3CN for 72h. The fresh solvent
was added every 12 h. The double interpenetration of TMU-33 with formed 6.6×6.4 Å
rectangular pores is presented in Figure 3A. The TGA results shown in Figure 3D also
illustrate the loss of two free dimethylamine (DMA) molecules and two coordinated DMF
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molecules in the temperature range of 25-260°C. Further increase of temperatureDOlIe: 1d0.1t0o3V9ile/oDw0sADsrtiTc0le0O4n3l3inBe
of the coordinated OPP ligand and the disintegration of the framework at about 300. The TG
analyses of TMU-33 crystal, as synthesized sample in green, TMU-33 after coordinated DMF
removal in black and after DMA in red are shown in Figure 3D.
Figure 3. (A) Schematic picture of OPP, H2BDC and Cd3 SBU, (B) 3D expansion and (C) two
fold interpenetration TMU-33, (D) Thermogravimetric analyses of TMU-33 (green), TMU-33 after
DMF removal (black) after removal DMA (red). Color code: C: black; O: red; N: blue, Cd: green and
H: yellow.
Nanoplate and nanorods samples of TMU-33 were synthesized in ultrasonic bath by using
pyridine and benzoic acid as capping reagents, respectively. These capping agents are chosen
due to similarity of their coordination sites to H2BDC and OPP ligand, respectively. The "r"
factor is ratio of [capping reagent]/ [OPP], where benzoic acid and pyridine are capping
reagent in nanorod and nanoplate morphology, respectively.
For the study of the effect of concentration of the initial reagents on size and morphology of
nano-structured TMU-33, the above processes were performed with concentrations of [BDC]
= [Cd (OAc) 2 .4H2O] = [OPA ligand] of [0.05], [0.01] and [0.005] M. For TMU-33 nanorods
and nanoplates, the above processes were performed with concentrations of [BDC] = [Cd
(OAc) .4H2O] = [OPA ligand] of [0.05] by adding different concentrations of benzoic acid
2
and pyridine as capping reagents (modulator) at ambient for 20 min. The resulting powder
was isolated by centrifugation, washed with DMF three times and dried in air for
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characterization. The morphology of the MOF samples was determined by SEMD(OFI: i1g0.u10r3Ve9ie/Dw40A)Dr.tiTc0le0O4n3l3inBe
Apparently, the morphologies and dimensions of the product are strongly dependent on the
reaction conditions. It is found that the sonication time, concentration of starting reagents and
sonication power plays the dominating roles in the final morphology of product (in order to
control of nucleation). As a result, for the preparation of optimal nanorod and nanoplate
samples, concentration of [0.001] M of each reactant, 20 minutes for sonication time and
24W as sonication power using as synthesis conditions.
Figure 4. SEM images of nanorod morphology with r=2 (A), r=5 (B) and r=10 (C) of TMU-
33, and nanoplate morphology with r=2 (D), r=5 (E) and r=10 (F) of TMU-33.
Powder X-ray diffraction (XRD) measurements demonstrate that nano-sized samples of
TMU-33 were structurally similar to TMU-33 prepared via solvothermal (Figure S2). A
homogeneous distribution of active sites, microenvironments, transmission channels and the
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presence of functional groups on the inner surface of the MOF voids and channelDsOlIe: 1a0d.10t3o9/Dth0DeT00433B
distinct catalytic properties of MOF catalysts49. It is clear that accessibility of both metal
nodes and functionality of ligands make MOFs great candidates for application in catalytic
area. To select the best condensation reaction catalyst, we divided our studies into three
sections. We first used the crystalline structure of TMU-33 which has the open metal site and
imine vacancy and TMU-33 witch has an amine as the catalytic site.
To examine the catalytic activity of the TMU-33 and TMU-40 frameworks which have
different size and function groups (amine, imine and open metal site), the Knoevenagel
condensation as a base-catalyzed model reaction was carried out. 0.1 mmol benzaldehyde,
0.25 mmol malononitrile and 1mg catalyst were added into a glass reactor with 3 ml of
different solvents at room temperature. A range of solvents were employed to optimize the
reaction solvent leading to increase the yield. The higher yields of target products were
obtained in polar protic solvents (Table 1). However, the reaction under solvent-free
conditions was slow. Therefore, water was selected as reaction solvent.
Table 1. Control reactions for finding the optimal condensation for TMU-33 and TMU-40 as
Knoevenagel condensation reaction with 4-chlorobenzaldeyde and malononitrile.
Entry Catalyst
Amount of
catalyst
Solvent Temperature(˚C)
Time
(min)
Yield
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
TMU-33
TMU-33
TMU-33
TMU-33
TMU-33
TMU-40
TMU-40
TMU-40
TMU-40
TMU-40
---
1 mg
1 mg
1 mg
1 mg
1 mg
1 mg
1 mg
1 mg
1 mg
1 mg
---
H2O
EtOH
MeOH
CH3CN
Toluene
H2O
r.t
r.t
r.t
r.t
r.t
r.t
r.t
r.t
r.t
r.t
r.t
r.t
r.t
r.t
5
5
5
5
5
5
5
5
5
5
5
5
5
5
99
28
24
11
4
92
21
27
9
EtOH
MeOH
CH3CN
Toluene
H2O
EtOH
MeOH
CH3CN
4
17
10
14
2
---
---
---
---
---
---
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DOI: 10.1039/D0DT00433B
15
16
17
18
19
---
---
Toluene
H2O
H2O
H2O
H2O
r.t
r.t
r.t
35
45
5
5
5
>5
2
0
TMU-33
TMU-33
TMU-33
TMU-33
0.5 mg
2 mg
1 mg
1 mg
75
99
99
99
To probe the optimal amount of catalyst, a solution of 4-chlorobenzaldehyde (0.1mmol) and
malononitrile (0.25 mmol) was loaded into two glass vessels. Then 1 mg of catalysts were
added to each one and allowed to react with active methylene compounds. The stirring
continued at room temperature and progress of the reaction is monitored by GC. Also,
different amount of catalyst and higher temperature were studied. The catalytic results
indicate the reactions could be done on 1 mg of the catalysts at room temperature in 3 ml of
H2O and one product was obtained in the best yield (Table 1). Survey of results reveals that
one product is formed and no by-products were observed in our analyses. As shown in Table
2, we investigated aldehydes with different substituents as substrates to compare the catalytic
activity of the MOFs. The structure and purity of the products were analyzed using 1H
NMR spectroscopy (Figure S3-S11). Reactions progress was monitored by GC (Figure S12-
S22). About all of the reagents, the catalytic performance of TMU-33 is better than that of
TMU-40 which could be attributed to better functionality of open metal site or imine as
catalyst.
Table 2. Knoevenagel condensation reaction catalyzed by TMU-33 and TMU-40 in the
presence of various aldehydes (reaction conditions: substrates (0.1mmol), malononitrile
(0.25mmol), 1mg catalyst, solvent: H2O (3ml), r.t., 5 min)
Substrate
Substrate
Product
Yield (%)
Entry
1
Catalyst
TMU-33
TMU-40
CN
CN
74
61
NC
CN
O
H
CN
CN
2
TMU-33
TMU-40
58
67
NC
CN
O
H
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DOI: 10.1039/D0DT00433B
CN
CN
Br
3
TMU-33
TMU-40
88
79
NC
CN
O
Br
H
Cl
CN
CN
4
5
6
TMU-33
TMU-40
98
92
NC
NC
NC
CN
CN
CN
O
H
Cl
CN
CN
O2N
HO
TMU-33
TMU-40
92
90
O
O2N
H
CN
CN
TMU-33
TMU-40
85
73
O
HO
Cl
H
Cl
Cl
O
7
8
TMU-33
TMU-40
75
63
CN
CN
NC
NC
CN
CN
H
Cl
OCH3
OCH3
O
TMU-33
TMU-40
79
71
CN
CN
H
H3CO
NC
H3CO
CN
CN
9
TMU-33
TMU-40
94
90
NC
CN
O
H
NC
H
O
10
11
TMU-33
TMU-40
82
80
O2N
CN
CN
O2N
NC
NC
CN
CN
Cl
O
Cl
TMU-33
TMU-40
68
56
CN
CN
H
In the second part of study, to evaluate the size effect, nanosized samples of TMU-33 with
different morphologies were used as catalyst. In nanoplate sample, the relative accessibility
of Lewis acid on the external surface of the catalyst is more than that of Brønsted base. On
the other hand, in nanorod sample, there is more relative accessibility to Brønsted base
(Table 3).
Table 3. The catalytic sites in different catalysts.
Entry
Catalyst
TMU-40
TMU-33
Active site
amine group
imine group + open metal site
imine group more accessible than open metal site
1
2
3
Nanorod TMU-33
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DOI: 10.1039/D0DT00433B
NanoplateTMU-33
4
open metal site more accessible than imine group
The presented results in Table 4 demonstrate that the nanostructures are more effective
catalyst due to the higher accessibility of catalytic sites in compare of bulk structure.
Furthermore, the catalytic performance of the nanoplate sample is better than that of the other
samples regardless electron donating or electron withdrawing groups of the aldehyde.
This indicates that the nanoplate-TMU-33 catalyst is a high performance catalyst in
Knoevenagel condensation reaction.
Table 4. Knoevenagel condensation reaction catalyzed by bulk and nanosized TMU-30.
Substrate
Yield (%)
Entry
1
Catalyst
TMU-33
Nanorod TMU-33
NanoplateTMU-33
74
96
100
O
H
Cl
TMU-33
Nanorod TMU-33
NanoplateTMU-33
98
95
99
2
O
H
H
O
TMU-33
92
92
96
79
89
90
O2N
Nanorod TMU-33
NanoplateTMU-33
TMU-33
Nanorod TMU-33
NanoplateTMU-33
3
4
OCH3
O
H
H3CO
Cl
O
TMU-33
Nanorod TMU-33
NanoplateTMU-33
75
92
92
5
H
Cl
In order to investigate the reusability of TMU-33 and TMU-40, the catalyst was separated
from the reaction product by centrifugation. The PXRD patterns of catalysts studied after 3
run reactions. The intact patterns show that the recycled catalyst can be reused at least 3 runs
without any loss of intrinsic catalytic activity (Figure S23). A comparison of the catalytic
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activity of nanostructure TMU-33 and TMU-33 with that of other heterogeneous cataVliyewsAtsrticle Online
DOI: 10.1039/D0DT00433B
previously used in Knoevenagel condensation reaction is reported in Table 5.
Figure 5. PXRD of nanoplate TMU-33 (left) and TMU-40 (right) recovered after the first
and fourth catalytic cycles.
Table 5. Knoevenagel condensation reactions of benzaldehyde and malononitrile catalyzed by
different catalysts.
Temp
[°C]
Entry
1
Catalyst
Amount
5 mol%
Time
2h
Solvent
DCM
Yield
100
Ref
49
{[Ni3(TBIB)2(BTC)2(H2O)6]·5C2H5O
H·9H2O}n
60
2
3
Au@Cu(II)-MOF(1)
3 mol%
5 mol%
23h
4h
Toluene/MeOH r.t.
99
99
50
51
ZIF-9
Toluene
r.t.
60
{[Zn(Py2TTz)(2-NH2
BDC)]·(DMF)n
4
2mol%
6h
Free solvent
99.9
52
[Gd2(tnbd)3(DMF)4]·4DMF·3H2O
20
min
7h
5
6
7
10 mol%
25 mg
C6H6
EtOH
MeOH
r.t.
40
80
96
53
44
CAU-1-NH2
100
100
NH2(50%)-MIL-53
5.3 mg/mL
5h
54
55
8
9
Schiff base supported MCM-41
Nanoplate TMU-33
5mg
6h
H2O
H2O
r.t.
r.t.
99
This
work
10mg
5min
100
4. Conclusion
The catalytic performance of four bulk and nanosized MOFs with different functionality such
as Brønsted base and Lewis acid and different frequencies of functional group studied for
Knoevenagel reaction. By nanosized TMU-33 as catalyst, the conversion easily reached more
14

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than 90% under mild condition with a low amount of catalyst and the short time. Between
DOI: 10.1039/D0DT00433B
four used catalysts, nanoplate sample in which the relative accessibility of open metal site on
the external surface of the catalyst is more than that of Brønsted base is better than the other
samples regardless electron donating or electron withdrawing groups of the aldehyde. The
results show that open metal site as Lewis acid is better catalyst for Knoevenagel
condensation. Indeed this study highlights the important role of size and accessibility of
catalytic site in the catalytic performance.
Conflicts of interest
There are no conflicts to declare.
Acknowledgments
This work was supported by the “Iran National Science Foundation: INSF”, “Tarbiat
Modares University” and “University of Zanjan”.
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