Heterogeneous catalysis with a coordination modulation synthesized MOF: Morphology-dependent catalytic activity

Article abstract of DOI:10.1039/c6nj04065a

We report control over morphology of a micro-porous three-fold interpenetration amide-functionalized Zn(ii)-based MOF, [Zn₂(oba)₂(bpta)]·(DMF)₃, TMU-22, (H₂oba = 4,4′-oxybis(benzoic acid); bpta = N,N′-bis(4-pyri

Full text of DOI:10.1039/c6nj04065a

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Heterogeneous Catalysis with Coordination Modulation
Synthesized MOF: Morphology-Dependent Catalytic Activity
† †
Massomeh Ghorbanlooa, b , Vahid Safarifardb and Ali Morsalib*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Abstract
www.rsc.org/
We report control over morphology of micro-porous three-fold interpenetration amide-functionalized Zn(II)-based MOF,
[Zn2(oba)2(bpta)]·(DMF)3, TMU-22, (H2oba = 4,4′-oxybis(benzoic acid); bpta = N,N′-bis(4-pyridyl)-terephthalamide) for the
Knoevenagel condensation through coordination modulation growth. Micro-rods and -plates of TMU-22 have been
prepared via coordination modulation method using acetic acid and pyridine as modulators, respectively. Several
experimental parameters including the effects of concentration of modulator, solvent and temperature of reaction on size
and morphology of the micro-crystals have been investigated. Catalytic performance of TMU-22 micro-sized MOF in
Knoevenagel condensation reaction was evaluated. Moreover, the catalytic activity of TMU-22 has been compared to two
isoreticular amide-functionalized MOFs, [Zn2(oba)2(bpfb)]·(DMF)5 (TMU-23), and [Zn2(oba)2(bpfn)]·(DMF)2 (TMU-24) (bpfb
= N,N'-bis-(4-pyridylformamide)-1,4-benzenediamine; bpfn = N,N′-bis-(4-pyridylformamide)-1,5-naphthalenediamine). It is
found that not only the incorporation of amide groups but also the morphologies of the MOF is crucial to obtain enhanced
catalytic performance.
restricted their use only under mild conditions, there have been
several reports that show MOFs can be excellent heterogeneous
catalysts for Knoevenagel condensation.7 MOF that its catalytic
1. Introduction
Metal-organic frameworks (MOFs) are constructed from metal
ions/clusters and organic ligands into crystalline solids with
uniformly porous structures.1 MOFs have attracted much interest
due to their large compositional and structural diversity.2 A myriad
of applications have been investigated so far, in domains such as
gas storage, separations, and catalysis.3 Preparation of function-
decorating MOFs by rational design has spurred a recent explosive
growth of interest in research into metal–organic frameworks.4 By
choosing the molecular structures and chemical properties of the
organic linkers used in pillared layer-MOFs, high porosity and large
internal surface areas can be achieved, while the size, shape, and
surface of the channels can be modified.5
activity was attributed to the functional groups of the organic ligand
(i.e., with active sites located at the organic molecule of the MOF
and not at the metal ion) still very scarce. This is because, in order
to have a catalytic material, the functional groups need to be free
and accessible to the substrates and should not be directly
coordinated to the metal ions of the MOF.8
The applications of MOFs not only depend on their composition and
chemical structure, but also rely on their size and morphology, since
in many cases the formation of MOF micro- and micro-particles
allows the properties to be tuned significantly without changing the
material composition.9 Micro- and micro-scale MOFs often exhibit
some novel, interesting physical and chemical properties that
cannot be observed in their bulk analogues.10 They can provide
combination features of micro-materials in addition to properties of
MOFs have begun to be explored as heterogeneous catalysts, owing
to their well-ordered porous structures, flexible and dynamic
behaviors in response to guest molecules and designable channel
surface functionalities.6 Although the low thermal and chemical
stability of MOFs as compared to their inorganic counterparts have
MOFs.11 To date, a number of efforts have been made to control
12
the size and morphology of MOF crystals. Capping agents
,
coordination modulators 13, solvothermal treatment or specific
synthetic techniques, such as microwave heating 15, ultrasonic
synthesis 16 and reverse microemulsion 17 methods, have been used
in attempts to achieve size and shape control of MOF crystals.
14
a.Department of Chemistry, Faculty of Science, University of Zanjan, 45371-38791
Zanjan, Iran
b.
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University,
P.O. Box 14115-175, Tehran, Islamic Republic of Iran
Modulating synthesis is
a method in which finely controlled
formation of MOFs can be obtained by adding capping reagents
(modulators) with the same chemical functionality as the linkers.
The addition of ligands with only one coordination site, which
compete with linkers for coordination to the metal cations, the so-
†
These authors contributed equally.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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called modulator approach 18, was recently introduced by using pyridylformamide)-1,5-naphthalenediamine) were synthesized
acetic acid as
a
modulator in the synthesis of according to previously reported method (See ESI).25
20
[Cu2(BDC)2(dabco)]·2DMF·2H2O19
,
[Zn2(oba)2(4-bpdb)]n·2(DMF)
,
2.2. Apparatus.
Dy(BTC)(H2O)12 and NH2-MIL-53(Al)21. Besides organic carboxylic The infrared spectra were recorded on a Nicolet Fourier Transform
acids, also organic bases or even inorganic modulators can exert IR, Nicolet 100 spectrometer in the range 500-4000 cm-1 using the
analogous effects.22 For instance, Cho et al. synthesized rod-, lump-, KBr disk technique. X-ray powder diffraction (XRPD) measurements
and disk-shaped porous coordination polymers in the presence of were performed using
a
Philips X’pert diffractometer with
pyridine as a blocking agent.23
monochromated Cu Kα radiation (λ=1.54056Å). Thermogravimetric
Recently,24 we successfully introduced and studied CO2 sorption analysis (TGA) were carried out in a Perkin Elmer Pyris 1 under N2
properties of three acylamide-containing threefold interpenetrated atmosphere and heating rate of 10 °C/min. Volumetric N2 sorption
pcu-MOFs;
([Zn2(oba)2(bpfb)]·(DMF)5), and TMU-24 ([Zn2(oba)2(bpfn)]·(DMF)2) Quantachrome Instruments. The samples were characterized with a
(H2oba 4,4′-oxybis(benzoic acid), bpta N,N′-bis(4- field emission scanning electron microscope (FE-SEM) Philips XL30,
TMU-22
([Zn2(oba)2(bpta)]·(DMF)3),
TMU-23 isotherms were collected at 77 K using the AutosorbIQ from
=
=
pyridinyl)terephthalamide, bpfb = N,N'-bis-(4-pyridylformamide)- TESCAN MIRA (Czech), and ZEISS SIGMA VP (Germany) with gold
1,4-benzenediamine, and bpfn = N,N′-bis(4-pyridylformamide)-1,5- coating. GC runs were performed on an Echrom GC A90 gas
naphthalenediamine, where DMF is N,N- dimethylformamide, chromatograph.
Scheme 1). The study revealed that not only the insertion of amide 2.3. Synthesis of TMU-22, TMU-23 and TMU-24 as Crystals.
functional groups in the MOFs but also the accessibility of the A mixture of Zn(NO3)2.6H2O (0.297 g, 1 mmol), H2oba (0.258 g, 1
functional groups could effect on their sorption and CO2/N2 mmol), and the corresponding amide ligand (0.5 mmol) and DMF
separation properties. Herein, we describe the catalytic activity of (50 mL) was sonicated until all solids were uniformly dispersed (≈ 3
three amide-functionalized
MOFs, TMUs-22/-23/-24, as minutes) and divided into 7 glass vials. The vials were heated at 120
heterogeneous catalysis in Knoevenagel condensation reaction °C for 3 days and then cooled to room temperature. Colourless
between aldehydes and active methylene compound. Furthermore, (TMU-22) and redbrown (TMU-23 and TMU-24) crystals were
we demonstrate how the correct use of modulators, like acetic acid obtained as pure phases, washed by DMF, and dried at room
and pyridine, can create even much more catalytic active materials temperature. Data for TMU-22. Yield: 44 %. FT-IR (cm−1): 1691 (m),
in TMU-22.
1595 (vs), 1504 (s), 1395 (vs), 1331 (m), 1298 (m), 1238 (vs), 1160
(vs), 1098 (m), 777 (m), 659 (m), 524 (m). Data for TMU-23. Yield:
36 %. FT-IR (cm−1): 1673 (vs), 1597 (vs), 1540 (m), 1510 (m), 1399
(vs), 1309 (m), 1220 (vs), 1159 (s), 1088 (m), 1067 (m), 1014 (m),
801 (m), 658 (m), 524 (m). Data for TMU-24. Yield: 38 %. IR (cm−1):
1667 (vs), 1595 (vs), 1570 (m), 1505 (s), 1386 (vs), 1235(s), 1158
(vs), 1089 (m), 1065 (m), 1015 (m), 878 (m), 659 (m), 522 (m).
2.4. Synthesis of Bulk TMU-22.
In a typical experiment, TMU-22 was obtained by mixing H2oba (0.5
mmol), bpta (0.25 mmol), and Zn(OAc)2·4H2O (0.5 mmol) in 20 mL
DMF in a round-bottom flask at R.T. for 24 h. The resulted white
powders were isolated by centrifugation, washed with DMF 3 times,
and dried in air for characterization. Yield: 54 %. FT-IR (cm−1): 1689
(m), 1593 (vs), 1500 (s), 1398 (vs), 1332 (m), 1300 (m), 1241 (vs),
1161 (vs), 1095 (m), 774 (m), 662 (m), 526 (m).
2.5. Synthesis of TMU-22 Microstructures.
The same process was applied for preparing TMU-22 micro-rods
and -plates, by adding different concentrations of acetic acid and
pyridine as capping reagents (modulator), respectively, at 100 °C
and R.T. for 24 h in the presence of different solvents. For TMU-22,
r factors (where r is defined as the ratio of concentration of
modulator to ligand) for acetic acid were 20, 10, 5 and 0.5, and for
pyridine were 30, 15, 5, and 1. Yield ~ 45 %. FT-IR (cm−1): 1693 (m),
1598 (vs), 1501 (s), 1392 (vs), 1333 (m), 1295 (m), 1236 (vs), 1162
(vs), 1097 (m), 777 (m), 654 (m), 523 (m).
Scheme 1. Schematic representation of the pillared-layer structures of
[Zn2(oba)2(L)].(DMF)x. H2oba = 4,4′-oxybisbenzoic acid; TMU-22: L = bpta, x =
3; TMU-23: L = bpfb, x = 5; TMU-24: L = bpfn, x = 2.
2. Experimental Section
2.1 Materials and Physical Techniques.
2.6. Activation Method.
All compounds were activated using a very similar protocol that
started with a solvent-exchange step. The MOFs were immersed in
Zinc(II) acetate dehydrate, zinc(II) nitrate dehydrate, acetic acid,
pyridine, and 4,4′-oxybis(benzoic acid) (H2oba) were purchased
from Aldrich and Merck Companies and used as received. The
ligands bpta (N,N′-di(pyridin-4-yl)terephthalamide), bpfb (N,N'-bis-
(4-pyridylformamide)-1,4-benzenediamine) and bpfn (N,N′-bis-(4-
dichloromethane for
3 days, during which the solvent was
exchanged once daily. Note here that previous stability tests
showed that all synthesized compounds were stable in the solvent
(Fig. S1 in the ESI) for at least 24 h at R.T. Then, each MOF was
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evacuated for 24 h under vacuum at 120 °C prior to gas sorption
and catalytic activity. It is noteworthy that PXRD diagrams collected
after the activation process confirm that the frameworks retain
their crystallinity and remained unaltered after both processes (Fig.
S2 in the ESI). These observations were further corroborated by
TGA, in which only the weight loses around 400 oC corresponding to
their decomposition were observed (Fig. S3 in the ESI).
2.7. Low-Pressure Sorption Measurements.
Volumetric N2 sorption isotherms were collected at 77 K using an
ASAP 2020 HD (Micromeritics). Temperature was controlled by
using a liquid nitrogen bath. The total pore volumes (Vt) were
calculated at P/P0 = 0.95.
2.8. Knoevenagel Condensation.
To a mixture of malononitrile (1.1 mmol) and benzaldehyde (1
mmol) was added 4 mol % of activated catalyst. The resulting
mixture was stirred at room temperature for a desired time. On
completion of the reaction, the catalyst was recovered by
centrifugation and the supernatant liquid was collected and
evaporated to dryness. Pure product was obtained by
recrystallization from EtOH. 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 catalyst was washed repeatedly with EtOAc, dried
and reused when required.
Fig. 1. Layers of Zn(II)-oba (in green) pillared by bpta, bpfb and bpfn in (a)
TMU-22, (b) TMU-23 and (c) TMU-24, respectively. The packing of the
structure leads to three-fold interpenetrated (in blue, red and green)
frameworks possessing 1D pore in (d) TMU-22,(e) TMU-23 and (f) TMU-24.
Representation of the pores highlighting the amide groups in (g) TMU-22,
(h) TMU-23 and (i) TMU-24 (presented in space-filling) pointing toward the
pores of the frameworks. All hydrogen atoms and the disordered guest
molecules are omitted for clarity.
3. Results and Discussion
3.1. Catalytic activity of TMU-22, TMU-23 and TMU-24.
TMUs-22/-23/-24 were synthesized via solvothermal reactions
between Zn(NO3)2·6H2O, H2oba, and the corresponding pillar ligand
in DMF at 120 °C for 3 days. The simulated (derived from the single
crystal structures) and experimental powder X-ray diffraction
(PXRD) patterns were consistent (Fig. S2 in the ESI), confirming that
all synthesized MOFs can be obtained as pure phases; as also
corroborated by Fourier transformed infrared spectroscopy.
TMU-22, TMU-23 and TMU-24 are neutral threefold
interpenetrated MOFs formed by {Zn(oba)} layers connected
through the N,N-pillar ligands. In these structures, each nonlinear
dicarboxylate oba ligand is coordinated to three Zn(II) centers
forming the {Zn(oba)} layers along the ab plane. Because of the
dinuclear nature of the metal cluster, these layers are formed by
two sheets of parallel oba ligands that form π−π stacking
interactions between them (Fig. 1). These {Zn(oba)} layers are then
pillared by the linear acylamide-functionalized ligands, extending
the structure in three dimensions with 6-connected net and pcu
topology. Although all compounds are three-fold interpenetrated,
they still possess apparent 1D pore channels functionalized with
acylamide groups (∼7.0 × 6.5 Å2, ∼6.8 × 6.4 Å2 and ∼6.7 × 5.5 Å2-
including van der Waals (vdW) radii-for TMU-22, TMU-23 and TMU-
24, respectively) running along the ⟨101⟩ direction and noticeable
free space (33%, 32% and 30% for TMU-22, TMU-23 and TMU-24,
respectively) (Fig. 1).
MOFs with amide groups are candidate compounds that have
guest-accessible functional organic sites as guest interaction sites.26
Amide is a fascinating functional group attributed for its dual
interaction sites through the −NH moiety and the C=O group.
Specifically, the –NH moiety of the amide group can act as electron
acceptor whereas the –C=O group can act as electron donor.27
These multifunctional moieties of amide groups tend to form
hydrogen bonds among themselves and interact negligibly with
guest molecules after construction of MOFs. If a MOF could be
prepared without forming amide-amide interactions, these groups
would constitute attractive interaction sites for catalysis inside the
channel.28 In our previous study, the CO2 sorption properties of
these three MOFs and their selectivity for CO2 over N2, from single
gas sorption isotherms to breakthrough measurements, have been
investigated. The results revealed that not only the incorporation of
amide groups but also their accessibility is crucial to obtain
enhanced CO2 sorption and CO2/N2 selectivity. TMU-24 with more
accessible amide groups showed a CO2/N2 selectivity value of ca.
10, which was ca. 500% and 700% of the selectivity values observed
for the other amide-containing TMU-22 and TMU-23,
respectively.24
We also expected the –C=O moiety in the channels of the TMUs to
provide guest interaction sites as base catalyst. The channels of
TMU-22-24 are big enough to give access for the reactant
molecules of appropriate sizes. In combination with their
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Table 1. Knoevenagel reaction of various aldehydes and
malononitrile with TMUs-22/-23/-24.
insolubility in most organic solvents (Fig. S1 in the ESI), these
characteristics of the TMUs pave the way for carrying out
heterogeneous Lewis base-catalyzed reactions in the pores. To
characterize the base-type properties of the TMUs, Knoevenagel
condensation reactions catalyzed by TMU-22-24 were performed.
The Knoevenagel reaction is well known, not only as a weak base-
catalyzed model reaction but also as a reaction that generates a C-C
bond (Scheme 2).29
Conversion (%)c
Entry
Substrate
TMU-22a
TMU-23b
TMU-24b
1
2
Benzaldehyde
4-MeO-Benz
99.1a
82.7b
22.0a
15.3b
80.0a
99.5
39.0
99.8
48.0
Scheme 2. Reaction mechanism for the Knoevenagel condensation.
3
4
3-Me-Benz
4-Br-Benz
82.0
99.8
95.0
99.8
99.7 a
94.8 b
43.0 a
26.0b
Our experiments employed a 1 : 1.1 molar ratio of benzaldehyde
and malononitrile in different solvents. As shown in Table S1, 4
mol% catalyst TMU-23 leads to a 99.5% yield in EtOH as the solvent.
Strong effects of solvent polarity have been observed for catalytic
Knoevenagel reactions. To establish the solvent effects for TMU-23,
we performed the reaction in different solvents such as ethanol,
dichloromethane, tetrahydrofuran and toluene at room
temperature (Table S2 in the ESI). It proved that the catalyst has the
highest activity (>99% conversion within 3 h) in EtOH. Ethanol
(protic, ε = 24.3) is a good medium for the reaction. This suggests
that besides the increased rate of proton transfer, the different
solvation power plays an important role.30 The amphiprotic
properties of ethanol may induce the electrophilic polarization of
the carbonyl group of benzaldehyde (due to its moderated acid
character) and enhance the reaction rate.31 By contrast, nonprotic
and less polar solvents, including DCM (ε = 9.1), THF (ε = 7.5) and
toluene (ε = 2.4), are not good media (Table S2, entries 2–4 in the
ESI). The effects can be generally explained by the assumption that
polar solvents help to stabilize the charged transition-state complex
of the reaction. A nearly apolar solvent like toluene seems to affect
negatively the catalytic performance of the MOFs, in clear contrast
with results reported for other solid supported basic catalysts.32 The
solvent effects are similar to those observed for some basic
organocatalysts33 and amine-functionalized MOF catalysts, IRMOF-
334 and NH2-MIL-101(Al).35
5
Naphthaldehyde
35.0
27.0
a
Reaction conditions: catalyst 4 mol %, temperature 25 ±1 ˚C, substrate 1
b
mmol, solvent 3 mL, reaction time 6 h, Reaction conditions: catalyst 4 mol
%, temperature 25 ±1 ˚C, substrate 1 mmol, solvent 3 mL, reaction time 3 h,
c Conversions are based on the starting substrates.
As shown in Table 1, the Knoevenagel condensation reactions
catalyzed by TMU-23 catalyst have higher conversions compare to
TMU-22. As mentioned earlier, the Knoevenagel reaction is typically
catalyzed by base, and amide group has been reported as a weak
base for this reaction.37 The carbonyl group of the pillars are
bonded to phenyl/pyridyl groups as an electron-donating/-
withdrawing groups in TMU-23 and TMU-22, respectively,
suggesting that increased TMU-23 activity, as a basic catalyst,
should be mostly related to its more basic carbonyl group.30 On the
other hand, the phenyl group in bpfb pillar of TMU-23 is replaced
with naphthalene, which has higher electron releasing effect than
phenyl, in bpfn pillar of TMU-24. In addition, TMU-24 has more
lipophilic pore walls that can cause better interaction with aromatic
compounds.38 As a result, TMU-24 with the most basic and
accessible carbonyl group showed the best catalytic performance
for the Knoevenagel condensation reactions compare to TMUs-
22/-23 (Table 1, entries 1–4). Interestingly, in the case of
naphthaldehyde, due to the pore size of TMU-24 (6.8 × 5.6 Å2), the
conversion was lower than two other catalysts (Table 1, entry 5).
This decrease in the activity is considered as indicating the role of
steric factors limiting the accessibility of reactants inside the pores
and proves again that highly efficient reactions are those taking
place within the cavities of the porous materials.
The effect of electron-withdrawing/-donating groups on
benzaldehyde derivatives were tested for Knoevenagel
condensation over TMU-23 using malononitrile as an active
methylene compound. Since the nucleophilic addition is the rate-
determining step in the Knoevenagel condensation, the electron-
deficient derivative, such as 4-bropmobenzaldehyde, gave the
corresponding products with higher conversion (99.8%). On the
contrary, the obtained yields were lower for the electron-rich
derivatives, such as 3-methylbenzaldehyde (82%) and 4-
methoxybenzaldehyde (39%) (Table 1).
In addition, TMUs-22/-23/-24 could be reused for at least 3 runs
without any loss in their activity. The integrity of the frameworks
are confirmed by the PXRD data (Fig. S2 in the ESI). To further verify
the heterogeneity of system, procedures similar to that of Sheldon
et al. were performed for the catalysts.39 After the reaction
mixtures were stirred for 3 hour, removal of each catalyst via
filtration resulted in the complete shutdown of the reactions and
hence, the systems are totally heterogeneous in nature with no
leaching under the reaction conditions. Therefore, these
compounds containing effective guest-accessible functional organic
sites, amide groups, can be considered to exhibit a sufficient
catalytic activity with base property.
On the other hand, using TMU-23 catalyst, under the same
experimental conditions when the size of the substrate increased,
for example naphthaldehyde, a slightly elongated molecule (7.2 ×
5.9 Å2), the relative conversions decreased to 39%. This decrease in
the activity has been considered as indicating the role of steric
factors limiting the accessibility of reactants inside the pores and
hence highly efficient reactions are those taking place within the
cavities of the porous material.36
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3.2. Shape control of TMU-22 by modulation synthesis
Here, we report a simple strategy, the coordination modulation
method, which can be easily scalable using acetic acid and pyridine
as capping reagents to achieve efficient control over the nucleation
process and, thus, control over the size of MOF. Furthermore, the
catalytic activity of MOF with different morphologies in the
Knoevenagel condensation reaction of malononitrile with
benzaldehyde has been investigated. The possibility to terminate
the growth of micro- and micro-particles by coordination
modulation has been successfully demonstrated by Kitagawa and
co-workers using [Cu3(btc)2] as a model system. This competition
can direct crystal growth along the particular direction. 10b
Among these three MOFs, TMU-22 which has the lowest catalytic
activity in crystal form was selected for modulation synthesis due to
study about morphology effect on Knoevenagel condensation
reaction rate. Microstructures of TMU-22, was synthesized by
simply mixing
a constant concentration of initial reagents
(Zn(OAc)2.2H2O, H2oba and bpta) using different concentrations of
acetic acid and pyridine as capping reagents. Monocarboxylic acid,
acetic acid, was chosen because it has the same carboxylate
functionality as the H2oba ligand and is expected to efficiently
prevent the coordination interactions between zinc and H2oba
ligand. Also, the nucleation rate can be controlled by acetic acid
through controlling of deprotonation of H2oba during the synthesis
and therefore moderate size and aspect ratio of MOFs. On the
other hand, an amine containing a nitrogen atom with a lone pair,
pyridine,40 was chosen to create competition with the N-donor
pillar ligands for coordinating to Zn(II) centers. Moreover, zinc(II)
acetate was chosen because upon mixing with H2oba and N-donor
ligands at room temperature it readily forms a withe precipitate of
TMU-22, while with zinc(II) nitrate, high temperatures and longer
reaction times are needed before the appearance of precipitate is
observed.41 Various “r” ratios (where r is defined as the ratio of
concentration of modulator to ligand) have been used to produce
rod- and plate-like morphologies.
Fig. 2. Field-emission scanning electron microscopy (FE-SEM) images of
TMU-22
samples
obtained
with
various
combinations
of
modulator/temperature; r represents the ratio between acetic acid and oba:
(a) r = 0.5, [oba] = 0.025 M, T = RT, solvent = H2O/DMF; (b) r = 5, [oba] =
0.025 M, T = RT, solvent = H2O/DMF; (c) r = 10, [oba] = 0.025 M, T = RT,
solvent = H2O/DMF; (d) r = 20, [oba] = 0.025 M, T = RT, solvent = H2O/DMF;
(e) r = 10, [oba] = 0.0125 M, T = RT, solvent = H2O/DMF; (f) r = 10, [oba] =
0.0125 M, T = RT, solvent = DMF; (g) r = 10, [oba] = 0.0125 M, T = 100 °C,
solvent = DMF. (h) Bulk powder of TMU-22, [oba]=0.025 M, T = RT, solvent =
DMF.
It has been found that when the ratio between acetic acid and
H2oba was increased from r = 0.5 to r = 20 at room temperature, in
constant concentration of H2oba (c = 0.025 M), rods with decreased
size have been obtained (Fig. 2a−d). Thus, the increase in the acetic
acid concentration significantly decelerated the rate of crystal
growth and facilitated the formation of microrods.18 Also, larger
amount of the modulators enables a larger external surface area of
the microcrystals to be capped.18 Moreover, upon decreasing
concentration of reagents, from 0.025 M of H2oba (Fig. 2c) to
0.0125 M (Fig. 2e), micro-rods of TMU-22 have been obtained in
which the aspect ratio of length to width is increased, (compare Fig.
2c and e). Also, for investigate about solvent effect, in r = 10
([H2oba] = 0.0125 M), DMF has been used as sole solvent instead of
H2O/DMF, micro-rods of TMU-22 have been obtained (Fig. 2f). In
fact, in the presence of water, N,N-dimethylformamide (DMF) easily
absorbs water and decomposes to form formic acid.42 The presence
of formic acid in the synthesis obviously accelerated the formation
of the crystalline Zn-MOF, with the highest equivalent of formic acid
giving the largest crystals. Finally, increasing temperature from RT
to 100 °C in 24 h and r = 10 ([oba] = 0.0125 M), leads to larger
crystals sizes due to increase crystal growth rate43 (Fig. 2g).
On the other hand, plate-like morphologies have been acquired
using pyridine as a modulator (Fig. 3). Similar to acetic acid, sizes of
plates have been reduced upon increasing r from 1 to 30 (Fig. 3a-d).
For investigate about the solvent effect, DMF has been used as sole
solvent (Fig. 3e and f) instead of H2O/DMF (Fig. 3a). In r = 30 micro-
plates of TMU-22 have been obtained, in which the morphology is
changed and the particle size is decreased. We postulated that DMF
decomposition into dimethylamine and carbon monoxide or formic
acid played a role in influencing the shapes of the resulting
crystals.44 Finally, higher temperature leads to micro-plates of TMU-
22 (Fig. 3g and h compare with Fig. 3e and f). For the rest of study,
micro-rods (r = 10, Fig. 2f) and micro-plates (r = 30, Fig. 3f) prepared
in R.T. and with sole DMF as solvent were used.
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Fig. 4. Powder X-ray diffraction patterns of TMU-22 microstructures. For
rod-shaped, conditions are r = 10, T = RT °C, 24 h, solvent = DMF; and for
plate-shaped, conditions are
r = 30, T = RT, 24 h, solvent = DMF.
Microstructures of TMU-22 are retained after the activation processes and
three runs catalytic processes, confirming that all the frameworks are
robust. Simulated (black), as-synthesized crystal (red), as-synthesized bulk
(brown), micro-rod (pink), micro-rod recovered after 3 runs (green), micro-
plate (blue) and micro-plate recovered after 3 runs (purple).
Fig. 3. Field-emission scanning electron microscopy (FE-SEM) images of
TMU-22
samples
obtained
with
various
combinations
of
The coordination modulation method with pyridine not only allows
one to prepare finely micro-plate of TMU-22 crystals, but it also
provides materials with improved crystallinity.45 The adsorption
isotherms for nitrogen of the solvent-extracted micro-rods and -
plates and aggregated bulk powder prepared without modulators
clearly indicate the amounts of nitrogen adsorbed by the micro-
plates were significantly more than amounts adsorbed by the
micro-rods and as-synthesized bulk powder (Fig. 5). For the micro-
plate sample, in addition to the type I isotherm indicative of a
microporous structure, a typical type IV isotherm with a H4
hysteresis loop at relatively high P/P0 (P/P0 = 0.4-0.9) was observed.
This is related to the capillary condensation associated with
mesopore channels, indicating the presence of an additional
mesoporous structure in this sample.46 The total surface area (BET
surface area (ABET) = 290 m2.g-1 and pore volume (Pv) = 0.15 cm3.g-1)
of micro-plates TMU-22 are comprised of both types of pores and
are significantly higher than those obtained for the micro-rods
sample (BET surface area (ABET) = 60 m2.g-1 and pore volume (Pv) =
0.03 cm3.g-1), due to the presence of the mesostructure. The
mesopore diameter distribution was calculated using the Barrett-
Joyner-Halenda (BJH) method (inset of Fig. 5), indicating a narrow
mesopore diameter distribution with an average value of 4.5 nm.
However, as reported previously, N2 isotherm at 77 K on single
crystal TMU-22, prepared via solvothermal method, showed a
characteristic type I behavior of microporous materials, as expected
modulator/temperature; r represents the ratio between pyridine and 4-
bpta: (a) r = 1, T = RT, solvent = H2O/DMF; (b) r = 5, T = RT, solvent =
H2O/DMF; (c) r = 15, T = RT, solvent = H2O/DMF; (d) r = 30, T = RT, solvent =
H2O/DMF; (e and f) r = 30, T = RT, solvent = DMF; (g and h) r = 30, T = 100 °C,
solvent = DMF.
The structure of samples was further studied by X-ray powder
diffraction (XRPD). As shown in Fig. 4, the XRPD patterns of rods
and plates are similar to the simulated pattern of TMU-22 indicating
the unchanged structures. Similar effects had been observed in the
synthesis of UiO-66 and UiO-67 when benzoic acid or acetic acid
was used as modulators.42a It is also noted that for the micro-rods
the relative intensities of (020) and (11-1) reflections are,
respectively, smaller and larger than that in the simulated pattern.
This is attributed to crystal growth along the [101] direction and
prevention of crystal growth along the [010] direction in the
presence of acetic acid (Fig. 4). On the contrary, for micro-plates the
reflection intensity becomes reversed because pyridine is used (Fig.
4).
1
from its crystal structure (BET area (ABET) = 680 m2.g− ; pore volume
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1
(Vt) = 0.28 cm3.g− ).24 The lower sorption amount of the micro- d reaction time 90 min;.
plates and -rods and aggregated bulk sample is believed to be
related to the lower crystallinity and critical number of defects that This slightly different catalytic performance can be attributed to
can block the channels and/or may be collapse of framework in the more accessible pore apertures and higher specific surface area in
micro-plates and -rods and as-synthesized bulk powder.12, 18
the micro-plate morphology and also more available amide groups
on the surfaces of micro-plates rather than micro-rod morphology.
Furthermore, bulk sample of TMU-22 is also tested as a reference
to shed light on the effect of the MOF morphology on catalytic
activity. The catalytic activity of the bulk sample is similar after 6 h
(99.5% yield). The catalyst MOFs could be reused for at least 3 runs
without any loss in activity, so their PXRD patterns measured after
reactions show the same profile as the simulated pattern of TMU-
22, indicating the unchanged coordination framework. Although the
crystallinity seems to be weakened due to broadening of diffraction
peaks (Fig. 4a).
Table 3. Recycling of TMU-22 in Knoevenagel reaction a
Number of
cycle
Conversion
Micro-rods
Micro-plates
Fig. 5. Nitrogen adsorption-desorption isotherms at 77 K: (a) the solvent-
extracted micro-plates TMU-22 (r = 30, T = RT, 24 h, solvent = DMF) displays
a type I isotherm at relatively low P/P0 pressures for a micropore structure
and a typical type IV isotherm with a H4 hysteresis loop associated with
mesopore channels (inset: the mesopore diameter distribution for the
micro-plates), (b) micro-rods TMU-22 (r = 10, T = RT °C, 24 h, solvent = DMF)
exhibits a type I isotherm characterizing a micropore structure with a
smaller pore volume, (c) TMU-22 bulk powder prepared without modulator.
Fresh
99.6
99.4
99.8
99.4
99.3
99.0
99.8
1
2
3
99.1
a Reaction conditions: catalyst 4 mol %, temperature 25 ±1 ˚C, substrate
3.3. Morphology dependent catalytic performance
The rod- and plate-like TMU-22 catalysts were used as
heterogeneous catalysts in the Knoevenagel condensation reaction.
According to our results presented here, micro-plate morphology of
TMU-22 with 99.6% yield after 90 min exhibited higher reaction
rate compared to other morphologies for the condensation of
benzaldehyde with malononitrile (See Table 2). Catalytic activity
recorded for micro-rod morphology (95.0% and 99.3% yields after
90 and 150 min, respectively) of TMU-22 is to some extent slower
than micro-plate morphology.
1 mmol, solvent 3 mL; reaction time 150 min;
Micro-rods and -plates of TMU-22 could be reused for at least 3
runs without any loss in their activity (Table 3). The integrity of the
frameworks are confirmed by the PXRD data (Fig. 4a). To further
verify the heterogeneity of system, micro-plates TMU-22 catalyst
were tested by the ‘hot filtration method’ according to the reported
procedures.39, 47 After the reaction mixture was stirred for 30 min
(77% conversion), removal of the catalyst via filtration resulted in
the complete shutdown of the reactions and hence, the systems are
totally heterogeneous in nature with no leaching under the reaction
conditions.
Table 2. Morphology dependent catalytic performance of TMU-22
in Knoevenagel reaction a
Substrate
Conversion
Crystals Micro-rods
Micro-
plates
Bulk
4. Conclusions
powder
In
summary,
three
acylamide-functionalized
three-fold
interpenetration MOFs, TMUs-22/-23/-24 were successfully
employed as heterogeneous catalysis in Knoevenagel condensation
reaction between different aldehydes and malononitrile. Thanks to
the pore shape of TMU-24 with more accessible amide groups
because of using a bulky, central naphthalene core, the catalytic
activity of this framework was much higher than the other amide-
decorated MOFs. Furthermore, we have presented rare example of
morphology-dependent catalytic activity of coordination
modulation synthesized MOFs. Modulation synthesis was used to
99.6d
99.1b
80.7d
99.3c
95.0d
99.5b
Benzaldehyde
81.5d
a Reaction conditions: catalyst 4 mol %, temperature 25 ±1 ˚C, substrate
1 mmol, solvent 3 mL; b reaction time 6 h, c reaction time 150 min;
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Safarifard, S. Beheshti and A. Morsali, CrystEngComm,
2015, 17, 1680-1685.
produce micro-rods and plates of TMU-22, using acetic acid and
pyridine as modulators, respectively. Interestingly, it is found that
not only the incorporation of amide groups but also the
morphologies of the MOF and thus accessibility of functional groups
is crucial to obtain enhanced catalytic performance. Therefore,
micro-plates of TMU-22 showed the catalytic activity (99.6% in 90
min) which is much better than the values observed for the other
morphologies of TMU-22 and also other amide-containing MOFs
(the best result is 99.8% in 3h for crystals of TMU-24). This study
clearly unveils the dependency of the catalytic performance of MOF
with its morphology.
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Acknowledgements
The authors are thankful to the University of Zanjan and
Tarbiat Modares University for financial support of this study.
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Heterogeneous Catalysis with Coordination Modulation Synthesized MOF:
Morphology-Dependent Catalytic Activity
Massomeh Ghorbanlooa, b†, Vahid Safarifardb† and Ali Morsalib*
Graphical Abstract: