Ultrasonic-Assisted Linker Exchange (USALE): A Novel Post-Synthesis Method for Controlling the Functionality, Porosity, and Morphology of MOFs

Article abstract of DOI:10.1002/chem.201901554

The introduction of organic ligands into metal–organic frameworks (MOFs) with a specific topology and that cannot be attained by direct synthesis is a big challenge. To meet this challenge, different ligand exchange/incorporation methods have been employed. Here, a new method, called ultrasonic-assisted linker exchange (USALE), has been developed to overcome the above-mentioned problems. USALE is a novel method for ligand exchange based on the use of ultrasonic waves. The temperature and pressure caused by the USALE method in microscopic zones are so intense that the linker exchange process is much faster than with other methods. In addition to saving time during synthesis, the use of the USALE method leads to a higher surface area and pore volume compared with other methods such as solvent-assisted linker exchange (SALE). In this way, improved gas adsorption capacity has been achieved for daughter frameworks synthesized by the USALE method. By using the USALE method, we have transformed a nonporous and easy-to-synthesize TMU framework ([Zn(OBA)(BPDB)0.5]n?2DMF (TMU-4), in which H2OBA=4,4′-oxybis(benzoic acid) and BPDB=1,4-di(4-pyridyl)-2,3-diaza-1,3-butadiene) into another porous framework ([Zn(OBA)(H2DPT)0.5]n?DMF (TMU-34), in which H2DPT=3,6-di(4-pyridyl)-1,4-dihydro-1,2,4,5-tetrazine) that otherwise requires a relatively long time to synthesize. In addition to reducing the synthesis time for TMU-34 (in comparison with both direct sonochemical synthesis and the indirect SALE method), the data obtained revealed that the daughter TMU-34 framework synthesized by the USALE method has a higher surface area and accessible pore volume than TMU-34 frameworks synthesized by SALE and direct methods. The application of SALE-TMU-34 and USALE-TMU-34 in a catalytic Henry condensation reaction and Congo Red adsorption experiments showed that the higher porosity of USALE-TMU-34 leads to a higher turn-over frequency and saturation capacity compared with SALE-TMU-34.

Full text of DOI:10.1002/chem.201901554

A Journal of
Accepted Article
Title: Ultrasonic Assisted Linker Exchange (USALE): A Novel Post-
Synthesis Method for Controlling the Functionality, Porosity and
Morphology of the MOFs
Authors: Ali Morsali and Sayed Ali Akbar Razavi
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10.1002/chem.201901554
Chemistry - A European Journal
Ultrasonic Assisted Linker Exchange (USALE): A Novel
Post-Synthesis Method for Controlling the Functionality,
Porosity and Morphology of the MOFs
Sayed Ali Akbar Razavi, Ali Morsali*
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, Tehran, Islamic
Republic of Iran. P.O. Box 14117-13116
E-mail: morsali_a@modares.ac.ir.Tel: (+98) 21-82884416.
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Table of Content
In this work a novel method (ultrasonic assisted linker exchange (USALE)) related to fast and
efficient linker substitution had been described which is very quick compared to solvent assisted
linker exchange (SALE) method. Moreover, the increased amounts of surface area and pore
volume are higher rather SALE, too.
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Abstract
Introduction of organic ligands into the metal-organic frameworks (MOFs) structure with
specific topology and those that are unattainable through direct synthesis is a big challenge. To
solve this challenge different ligand exchange/incorporation methods had been applied. Here, a
new method called ultrasonic assisted linker exchange (USALE) introduced to solve mentioned
problem. USALE is novel method for ligand exchange based on the use of ultrasonic waves. The
temperature and pressure caused by USALE method in microscopic levels are so intense that the
linker exchange process is much faster than other methods. In addition to time saving during
synthesis method, the use of USALE method leads to higher surface area and pore volume rather
other methods like SALE. This observation leads to improved gas adsorption capacity for
daughter frameworks synthesized by USALE method. Using USALE method, we transform one
of non-porous and easy to synthesis TMU-frameworks ([Zn(OBA)(BPDB)0.5]n·2DMF (TMU-4)
where H2OBA = 4,4'-oxybis(benzoic acid) and BPDB = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-
butadiene) into another porous framework that requires relatively long time to synthesis
([Zn(OBA)(H2DPT)0.5]n.DMF (TMU-34) where H2DPT = 3,6-Di(pyridin-4-yl)-1,4-dihydro-
1,2,4,5-tetrazine). In addition to reduced synthesis time for TMU-34 (in comparison with both
direct sonochemical synthesis and indirect SALE method), the achieved data reveal that USALE
synthesized daughter TMU-34 framework has higher surface area and accessible pore volume
rather than direct and SALE synthesized TMU-34 frameworks. Application of SALE-TMU-34
and USALE-TMU-34 in catalytic Henry condensation and congored adsorption show that higher
porosity of USALE-TMU-34 leads to higher turn-over frequency and saturation capacity of the
USALE-TMU-34 rather SALE-TMU-34.
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1. Introduction
Owing to their permanent porosity and three dimensional (3D) crystalline and modular structure,
metal-organic frameworks applied enormously in synthesis of materials with tuned and tailored
characters.[1] MOFs extensively applied in different types of applications such as catalyst and
photocatalyst,[2] removal, detection, separation and degradation of pollutants,[3] gas storage and
separation,[4] construction of electrochemical and electronical devices,[5] bio application, drug
delivery[6] and magnetic materials.[7] Such widespread applications of MOFs has taken place due
to their unique properties such as high surface area and porosity, thermal and chemical stability,
chemically decorable nature, hybrid inorganic-organic nature and mild synthesis conditions.
MOFs had been synthesized using different methods such as conventional solvothermal,
microwave, mechanochemical, electrochemical and sonochemical.[8] Among these methods
sonochemical method is of importance because: (I) it is a green synthesis method to develop
MOFs,[9] (II) owing to its nature this method is very fast compared to conventional methods[10]
and (III) nano-sized MOF structures can be obtained using sonochemical method with ability to
control the size and morphology of the MOF particles.[10b, 11] In comparison with conventional
energy sources, ultrasonic irradiation provides rather unusual but useful and helpful reaction
conditions (a short duration of extremely high temperatures and pressures in liquids) which
cannot be provided by other MOF-synthesis methods. Sonochemistry and sonochemical
synthesis methods are based on acoustic cavitation which is the major physical phenomenon to
affect on a system in ultrasonic bath.[9, 12] Acoustic cavitation is consist of formation, growth, and
implosive collapse of bubbles in a liquid which leads to unleashing the concentrated energy
stored inside the bubble within a very short time (with a heating and cooling rate of >1010 K.s-
1).[13] Such bubble explosion leads to a temperature of ~5000 K and a pressure of ~1000 bar in
microscopic levels in the sonicated solution.[13]
Unfortunately although there are different methods for synthesizing MOFs, but direct synthesis
of MOFs with desirable function–structure properties is not always reliable because of some
reasons like formation of undesirable topologies, low solubility of building blocks, and loss of
functionality of the sensitive frameworks when synthesizing.[14] So, to overcome this problem
some new synthesis methods must be applied by using pre-synthesized frameworks. Solvent-
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assisted linker exchange (SALE) and solvent-assisted ligand incorporation (SALI) are among the
well-known and mostly applied method to overcome mentioned challenges.[14-15] These methods
are a kind of similar to conventional direct synthesis of MOFs in term of synthesis conditions.
For example in SALE and conventional synthesis method, the reactants should be placed in
suitable solvent in an oven and then give the reaction solution a relatively long time (from days
to weeks) to make the desirable reaction. This limitation in reaction time encouraged us to seek
for a new method for ligand exchange reactions.
Since ultrasonic synthesis of MOFs is a green and time-saving method during direct synthesis of
MOF powders, we applied ultrasonication method for fast, time-saving and green ligand
exchange procedure. So, we applied ultrasonic assisted linker exchange (USALE) method to
solve mentioned problems. The driving force for ligand exchange in USALE method provides by
using of ultrasonic waves. In this work, we transformed non-porous and easy to synthesize
mother TMU-4 framework (with formula [Zn(OBA)(BPDB)0.5]n·2DMF where H2OBA = 4,4'-
oxybis(benzoic acid) and BPDB = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) framework to
porous and easy to synthesize daughter TMU-34 frameworks ([Zn(OBA)(H2DPT)0.5]n.DMF
where H2DPT = 3,6-Di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine). Achieved results show that
USALE-synthesized TMU-34 (USALE-TMU-34) has higher surface area, higher accessible pore
volume and higher N2 (at 77 K) and CO2 (at 298 K) adsorption capacity rather SALE-
synthesized (SALE-TMU-34) and direct-sonochemical synthesized TMU-34 samples. Also,
owing to higher porosity, USALE-synthesized TMU-34 remove larger amount of congo-red
from aquoes solution and reaches maximum conversion (%) in henry condensation in relatively
shorter times owing to higher porosity and more comfortable diffusion of substrates into the
pores.
2. Experimental
2.1. Chemical and methods
Materials. All required chemicals were purchased from commercial suppliers and were used
without further purification unless otherwise noted. Especially, (4,4'-oxybis(benzoic acid))
H2OBA was purchased from Aldrich company.
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Instrumentation. Ultrasonication was carried out in a Sonica-2200 EP ultrasonic bath
(frequency 40 KHz). X-ray powder diffraction (XRD) measurements were performed using a
Philips X’pert diffractometer with monochromated Cu-Kα radiation. Elemental analyses were
carried out on a Thermo Scientific Flash 2000 CHNS elemental analyzer. Adsorption studies
were performed using a TriStar II 3020 surface area analyzer from Micromeritics Instrument
Corporation with N2 at 77 K and CO2 at 298 K. UV-Vis absorbance spectra were measured on a
Varian Cary 50 UV-Vis spectrophotometer equipped with a single-beam facility with a spectral
resolution of 0.2 nm. Luminescence spectra were recorded with a PerkinElmer LS-55
fluorescence spectrometer at room temperature. Infrared spectra were recorded using Thermo
Nicolet IR 100 FT-IR. Thermal behavior was measured with a PL-STA 1500 apparatus at a rate
of 10 °C.min-1 under a static atmosphere of nitrogen. 1H NMR spectra were recorded on a Bruker
500 NMR spectrometer. The samples were characterized with a scanning electron microscope
(SEM) ZEISS SIGMA VP (Germany) with gold coating.
2.2. Synthesis of pillars and frameworks
Synthesis of pillar spacers. 3,6-Di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine (H2DPT) and 1,4-
bis(4-pyridyl)-2,3-diaza-1,3-butadiene (BPDB), used as spacers, were synthesized according to
the techniques reported previously.[4d, 16]
Synthesis of TMU-4. The mixture of H2OBA (0.128 g, 0.5 mmol), BPDB (0.105 g, 0.5 mmol)
and Zn(CH3COO)2·2H2O (0.07 g, 0.3 mmol) in 20 mL DMF were ultrasonicated in an ultrasonic
bath at ambient temperature and atmospheric pressure for 60 min. The resulting yellow powder
was isolated by centrifugation, washed with DMF three times and dried at 80◦C. Yield: 83%.IR
data (KBr pellet, cm−1): selected bands: 445(w), 523(w), 659(m), 692(m), 776(m), 875(m),
1021(w), 1089(m), 1159 (s), 1241(vs), 1389(vs), 1412(vs), 1500(s), 1568(s), 1608(vs), 1679(vs),
2926(w)
and
3414(w-br).
Elemental
analysis
(%)
calculated
for
[Zn(C14O5H8)(C12H10N4)0.5]·(C3NOH7)2: C: 55.3, H: 4.0, N: 8.4; Found: C: 55.8, H: 4.2, N:
8.5.2.5
Synthesis of TMU-34. TMU-34 powder was synthesized from
a
mixture of
Zn(CH3COO)2·2H2O (0.22 g, 1 mmol), H2OBA (0.26 g, 1 mmol), and H2DPT (0.24 g, 1 mmol)
in DMF (30 mL). The mixture was sonicated for 160 min at ambient temperature and
atmospheric pressure. It was then centrifuged and the resulting powder was washed with DMF
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and dried at room temperature. Yield: 0.34 g (78% based on OBA). IR (KBr pellet , cm−1):
selected bands: 661 (m), 778 (m), 872 (m), 1162 (s), 1241 (vs), 1407 (vs), 1607 (vs), 1674 (s),
2928 (m), 3273; Elemental analysis (%) calculated for [Zn(C14O5H8)(C12N6H10)0.5]·(C3ONH7):
C 53.8, N 10.9, H 3.9; found: C 54.1, N 11.4, H 4.1.
2.3. Linker exchange procedures
For transformation of non-porous and easy to synthesize mother TMU-4 framework to daughter
TMU-34 frameworks, SALE and novel USALE linker exchange method had been performed.
Solvent assisted linker exchange (SALE). A number of 10 ml screw cap glass vial was used for
SALE procedure. 0.3 g activated TMU-4 added to each one of glass vials and followed by
addition of 10 ml H2DPT ligand solution which had been prepared by dissolvation of 1 g H2DPT
in 100 mL of DMF (10 g.L-1 ligand solution). Sampling for monitoring the exchange process was
carried out at intervals of every 12 hours until the ligand exchange is complete (0, 12, 24, 36, 52
1
hours). Exchange procedure monitored using HNMR and photoluminescence spectroscopy
techniques.
Ultrasonic assisted linker exchange (USALE). The general condition for USALE is same as
SALE procedure, but monitoring the exchange process was carried out at intervals of every 30
1
minutes (0, 30, 60, 90 and 120 minutes). Exchange procedure monitored using HNMR and
photoluminescence spectroscopies.
2.4. General procedure for Henry condensation.
To examine the roles of porosity and functionality of synthesized frameworks, TMU-4, TMU-34,
SALE-TMU-34 and USALE-TMU-34, Henry condensation had been conducted. All samples
activated before catalytic tests as reported procedure.[4d]
A 6 ml screw cap glass vial with magnetic stir bar was used to carry out the catalytic test
reaction. 0.03 g of activated frameworks (0.034 mmol of catalytic sites and 1.7 mol% of catalyst)
added to the glass vial and followed by addition of 5 ml aqoues solution of benzaldehyde (5
mmol, excess reagent) and nitromethane (2 mmol, limiting reagent). The mixture stirred at 60 °C
and atmospheric pressure for 10 hour. Then the catalyst and solution separated by centrifugation
(6000 rpm). The catalyst dried and applied for XRD and N2 adsorption analysis. The above
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filtrate solution applied for GC analysis to calculate the catalytic reaction conversion (%).
Recyclability test conducted for 5 times in the same procedure.
2.5. General procedure for dye adsorption.
For evaluation the roles of porosity and functionality of synthesized frameworks, TMU-4, TMU-
34, SALE-TMU-34 and USALE-TMU-34, dye adsorption experiment had been conducted. Here,
all samples activated before adsorption tests, too. 100 ppm solution of congo-red (CR) was
chosen as a model pollutant solution to evaluate the adsorption capacity of the frameworks. CR
adsorption experiments were carried out in an ultrasonic bath in different volumes in the
presence of 50 mg activated frameworks at ambient conditions and sonicated for 10 minutes.
Small portion of samples for UV-Vis analyses were taken from the suspension at specified time
intervals (0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90 minutes) to monitor adsorption
process by UV-Vis analysis.
3. Results and Discussion
3.1. Synthesis and Characterization
Here, ultrasonication method had been applied for both direct-synthesis and post-synthesis of
MOFs. TMU-4 and TMU-34 had been directly synthesized by sonochemical method using
Zn(CH3COO)2.2H2O, H2OBA and related pillar spacers; BPDB for TMU-4 and H2DPT for
TMU-34. PXRD patterns show that direct sonochemical synthesized TMU-4 and TMU-34 have
same PXRD pattern as their simulated and crystalline structure (Figure S1). As a nonporous
(toward N2) and easy-to-synthesis mother framework, TMU-4 applied in SALE and USALE
methods to became converted into the SALE-TMU-34 and USALE-TMU-34 daughter
frameworks using a solution of H2DPT in DMF (N,N’-dimethylformamide).
3D structure of pillar-layered TMU-4 framework is developed based on linking of 2D sheets by
BPDB N-donor pillar spacer. 2D sheets are constructed by deprotonation of H2OBA and
coordination of free carboxylate groups of OBA2- to Zn2+ metal ions to generate non-
symmetrical Zn#1-Zn#2 secondary building blocks with distorted tetrahedral and square-planner
geometries. TMU-34 is containing of azine decorated 1D pores with 10.8 × 10.7 Å theoretical
maximum open pore size (TMOPS) based on crystallographic data (Scheme 1). Similarly, TMU-
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34 is based on H2OBA, H2DPT and Zn2+ building blocks, Zn2(COO)2 2D sheets with non-
symmetrical Zn#1-Zn#2 secondary building blocks, 3D pillar layered structure by H2DPT and
dihydro-tetrazine decorated 1D pores with 10.8 × 10.5 Å TMOPS (Scheme 1).
Scheme 1. Structural representation of TMU-frameworks. (a) molecular structure of O and N-donor ligands. (b)
Non-symmetrical Zn#1-Zn#2 secondary building blocks (Zn2(COO)2). (c) One-dimensional azine decorated pores of
TMU-4. (d) One-dimensional dihydro-tetrazine decorated pores of TMU-34.
Although TMU-4 and TMU-34 are isostructure (Figure S2) with close structural parameters
(Table S1), but their differences based on applied N-donor pillar spacers (BPDB for TMU-4 vs.
H2DPT for TMU-34) had led to different properties. In addition to different functionality (azine
for TMU-4 vs. dihydro-tetrazine for TMU-34), TMU-4 is non-porous toward N2 molecules
(Figure 1) with short sonochemical synthesis time (Table 1). Inversely, TMU-34 is porous
toward N2 molecules (Figure 1) with relatively long sonochemical synthesis time (Table 1).
Despite TMU-4 which is nonporous toward N2 molecules, TMU-34 shows type I isotherm
relating to microporous solids. But, the isotherms for SALE-TMU-34 and USALE-TMU-34
samples differ (Figure 1). SALE-TMU-34 and USALE-TMU-34 show type IV isotherms
indicating mesoporosity character of the adsorbents (Figure 1). Such difference in porosity
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nature of direct-synthesized TMU-34 framework and daughter SALE-TMU-34 and USALE-
TMU-34 frameworks show that the essences of the created defects in the daughter frameworks
after exchange procedures are based on mesoporous nature.
Table 1. Gas adsorption properties of direct and post synthesized frameworks.
TMU-4
TMU-34
197
SALE-TMU-34
USALE-TMU-34
N2 (cm3.g-1)-77 K
BET (m2.g-1)
(nonporous)
224
720
0.34
38
243
830
0.38
50
-
540
Pore Volume (cm3.g-1)
CO2 (cm3.g-1)-298 K
Synthesis Time (min)
-
0.28
30
39
30
160
3120
120
Figure 1. N2 adsorption properties of direct and post synthesized frameworks.
Scanning electron microscopy (SEM) images show that TMU-4 and TMU-34 represent different
morphologies (Figure 2). TMU-4 morphology is based on micro-plates with 200-250 nm
thickness. But in case of TMU-34, morphology is based on completely aggregated particles
without specific shape with thickness varying from several hundred nanometers to several
micrometers.
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Figure 2. SEM images of TMU-4 (a) and TMU-34 (b).
3.2. Ligand Exchange Procedure
Considering the advantages of TMU-4 (easy and short synthesis time, uniform morphology with
smaller particle size) and TMU-34 (porosity toward N2 and dihydro-tetrazine function efficiency
as H-donor[3e], H-bond donor[3g] and Lewis basic site[4d]) frameworks, we tried to combine and
improve these beneficial characters into the one framework. So, we used linker exchange
methods, solvent-assisted linker exchange (SALE) and ultrasonic-assisted linker exchange
(USALE) methods, in a way that TMU-4 applied as parent framework and TMU-34 as targeted
daughter framework. According to mentioned procedure in section 2.3, a solution of H2DPT in
DMF was poured on TMU-4 powder and then linker exchange using SALE and USALE
methods performed in preheated 120 °C oven and ultrasonic bath, respectively.
1
Linker exchange kinetic in both SALE and USALE procedures monitored by H-NMR and
1
photoluminescence spectroscopies. For H-NMR analysis, approximately 5 mg of TMU-4 in
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different time intervals was placed in an NMR tube and dissolved in D2SO4 (100 mL) and
[D6]DMSO (0.6 mL) with the aid of sonication. Once a homogeneous solution had been
1
1
obtained, H-NMR spectra were obtained by locking the sample to [D6]DMSO. Prior to H-
NMR, linker exchange treated TMU-4 powder was soaked for 4 h in fresh DMF to remove
residual ligands. For PL analysis, approximately 2 mg of TMU-4 in different time intervals had
been sonicated for 10 min and once a homogeneous and dispersed solution had been obtained,
PL measurement carried out.
Based on 1H-NMR spectrum, the most distinct differences in TMU-4 and TMU-34 spectrum are
according to dihydro-tetrazine (−NH) peak for daughter TMU-34 framework around 9.5 ppm
and azine (H−C=N) peak for parent TMU-4 framework around 8.4 ppm (Figure S3). Linker
1
exchange procedure by H-NMR technique monitored using these two peaks (Figure 3).
1
Sampling for H-NMR had been done at every 30 min for USALE and 12 hour for SALE
procedures. According to the ratio of BPDB/H2DPT pillar spacers (Table S2) it is clear that
SALE and USALE procedures are completed after 52 hour (3120 min) and 120 min,
respectively. The end of exchange procedure, SALE and USALE, is considered at a time when
the TMU-4 peak around 8.4 ppm related to (H−N=C) is completely disappeared. It is clear that
by continuing the exchange procedure the intensity for TMU-4 peak (~ 8.4 ppm, blue) is reduced
and the intensity for TMU-34 peak (~ 9.5 ppm, red) is increased (Figure 3).
Since TMU-4 and TMU-34 have different excitation wavelength (338 nm for TMU-4 and 482
for TMU-34 in water), emission band(430-520 nm for TMU-4 and 570-675 nm for TMU-34)
and maximum emission peak(465 for TMU-4 and 615.5 for TMU-34), PL measurement is
another helpful method to monitor exchange procedure (Figure S4). Similar results had been
achieved by monitoring the SALE and USALE procedures using PL measurement (Figure 4).
SALE and USALE procedure became completed after 52 hour (3120 min) and 120 min
1
respectively. H2DPT/BPDB ratio at every interval time is very close to those achieved by H-
NMR method (Table S3). The end of exchange procedure, SALE and USALE, is considered at a
time when the TMU-4 emission peak band (430-520 nm) had been turned-off completely.
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Figure 3. 1H-NMR spectrum for SALE (a) and USALE (b) procedure.
Figure 4. PL emission spectrum for SALE (a) and USALE (b) procedure.
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Since parent TMU-4 framework and sonochemically synthesizedTMU-34 represent same PXRD
patterns, we anticipate that daughter SALE-TMU-34 and USALE-TMU-34 frameworks show
same PXRD patterns (Figure S5). Figure 5 compares the synthesized TMU-4, TMU-34, SALE-
TMU-34 and USALE-TMU-34.
Figure 5. Synthesized powder of the frameworks and related N-donor ligands. (a) BPDB pillar spacer for
construction of TMU-4. (b) As-synthesized powder of TMU-4. (c) H2DPT pillar spacer for construction of TMU-34.
(d) As-synthesized powder of TMU-34. (e) As-synthesized powder of SALE-TMU-34. (f) As-synthesized powder
of USALE-TMU-34.
Gas adsorption analysis (N2 at 77 K and CO2 at 298 K) had been conducted to evaluate porosity
and surface area of daughter SALE-TMU-34 and USALE-TMU-34 frameworks and compare
with direct synthesized TMU-4 and TMU-34 frameworks. Interestingly, Table 1 and Figure 1
represent that the daughter framework synthesized by USALE method have higher adsorption
capacity (cm3.g-1) toward N2 and CO2, higher pore volume (cm3.g-1) and surface area (m2.g-1).
This noticeable difference in adsorption properties and exchange kinetic is related to nature of
ligand exchange driving force in SALE and USALE methods. In USALE method, cavitation
collapse at the interface of liquid (H2DPT solution)-solid (TMU-4 parent framework) leads to
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faster removing of BPDB ligand from the framework which results in linker-based substantial
defects inside the parent framework.[10a, 13, 17] On the other hand, using ultrasound waves makes
the penetration of the H2DPT ligands inside the frameworks kinetically easier. As results, ligand
replacement can be performed more quickly in free spaces created by linker-based substantial
defects. This phenomenon allows the exchange of ligands in much less times (120 min for
USALE vs. 3120 min for USALE). On the other hand, USALE-TMU-34 framework with
improved porosity and adsorption capacity can be synthesized in smaller time even in
comparison with direct sonochemically synthesized TMU-34.
It is recognized that: (I) using ultrasonic method in synthesis of nano-materials leads to improved
porosity and surface area of targeted materials because of reduction in particle size and defect
creator nature of this method[9, 10b] and (II) using SALE leads to higher surface area and porosity
owing to (partial) eliminating the interpenetration and linker-based substantial defects.[18] So, it
make sense that using ultrasonic method in linker exchange process leads to higher surface area
for USALE-daughter framework rather SALE synthesized one. Thermogravimetric analysis
(TGA) clearly show that USALE-TMU-34 sample carries much more solvent molecules inside
the pores rather SALE-TMU-34 sample (6.71% vs. 17.52 in Figure S6). As mentioned, USALE-
TMU-34 higher porosity and accessible pore volume is owing to partial elimination of 2-fold
interpenetration and created linker-based substantial defects during exchange reaction.
Figure 6. CO2 adsorption at 298 K.
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SEM images display that unlike the sonochemically direct-synthesized TMU-34 sample with
aggregated morphology and large particle size, daughter USALE-TMU-34 and SALE-TMU-34
frameworks have uniform micro-plate morphologies. Similar to TMU-4 parent framework,
particle thickness for SALE-TMU-34 is mainly in range of 200-250 nm (Figure 7). But for
USALE-TMU-34 sample particle thickness is mainly lower than 200 nm.
Figure 7. SEM images of SALE-TMU-34 (a) and USALE-TMU-34 (b) samples.
So, our attempt to synthesis of TMU-34 framework with increased porosity, surface area and
optimized porosity was successful through synthesis of USALE-TMU-34 sample. Interestingly,
USALE-TMU-34 sample can be synthesized in shorter time rather sonochemically direct-
synthesized TMU-34 framework (160 min vs. 120 min). Moreover, USALE-TMU-34 sample
shows improved properties rather SALE-TMU-34. USALE-TMU-34 show increased porosity
and surface area (15%), pore volume (10%) and decreased synthesis time (96%) rather SALE-
TMU-34 sample. All these descriptions reveal that USALE is a very effective method for
improving the porosity and adsorption capacity as well as optimization of morphology of MOFs.
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10.1002/chem.201901554
Chemistry - A European Journal
Since adsorption and catalysis applications are highly depended on the functionality and porosity
of the host frameworks, we applied all four frameworks including TMU-4, sonochemically
direct-synthesized TMU-34 (TMU-34), daughter SALE-TMU-34 (SALE-TMU-34) and daughter
USALE-TMU-34 (USALE-TMU-34) frameworks in nitroaldol condensation and congored
removal to evaluate the roles of functionality and improved porosity in mentioned applications.
3.3. Nitroaldol Condensation
Nitroaldol condensation is a reaction between nitroalkane and carbonyl containing α-hydrogen to
form new C-C bond between α-carbon of nitroalkane and carbonyl C atom of carbonyl.
Nitroaldol condensation can be catalyzed by Lewis basic catalysts. Since all four mentioned
frameworks are decorated with Lewis basic organocatalyst sites, azine and dihydro-tetrazine, we
conducted this reaction using the TMU-4, TMU-34, SALE-TMU-34, USALE-TMU-34 Lewis
basic heterogeneous MOF-based catalysts. In addition to differences in functionality, these four
MOFs also differ in porosity and surface area. Such similarities and differences allow to be
investigated the role of functionality and porosity of these four frameworks in nitroaldol
condensation.
Nitroaldol condensation procedure using TMU-4, TMU-34, SALE-TMU-34 and USALE-TMU-
34 had been conducted as reported in section 2.4 and the results are gathered in Table S4. The
relevant data based on optimized conditions are presented in Table 2. Due to the sharp
differences in conversion(%) of TMU-4 and other frameworks (more than 4 times), it is possible
to decelerate that dihydro-tetrazine function inside the pore-walls of TMU-34, SALE-TMU-34
and USALE-TMU-34 is more effective Lewis basic heterogeneous organocatalyst site rather
azine group inside the framework of TMU-4. But there is still a difference between these three
structures. Although TMU-34, SALE-TMU-34 and USALE-TMU-34 frameworks are decorated
with dihydro-tetrazine function, but they had different porosity and surface area in a way that the
catalyst with higher porosity and surface area show higher conversion(%), TON and TOF. This
observation can be due to the fact that by increasing the porosity and surface area, more reactant
molecules can penetrate inside the framework cavities which act as nano-reactors functionalized
with Lewis basic organocatalytic dihydro-tetrazine sites. As a result, the reaction proceeds more
rapidly and maximum conversion time can be achieved is shorter time as well as higher TOF and
TON numbers.
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10.1002/chem.201901554
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Table 2. Results of nitroaldol condensation in optimized condensations.*
BET (m2.g-1)
518**
540
Function
Conversion (%)
Selectivity (%)***
TON****
TOF*****
TMU-4
Azine
20
75
82
82
83
~12
~54
~56
~59
1.2
5.4
5.6
5.9
TMU-34
Dihydro-tetrazine
Dihydro-tetrazine
Dihydro-tetrazine
91
SALE-TMU-34
720
95
USALE-TMU-34 830
100
*: Optimized catalytic reaction conditions (0.03 g activated catalyst, stirring at 60 °C, 5cc methanol, 5 mmol benzaldehyde
(excess), 2 mmol nitromethane (limited), 10 hour.
**: Calculated by CO2 adsorption at 195 K.
***: Selectivity toward product 1.
****: Turn-over number (= (mmol limited reactant)/(mmol of catalyst))
*****: Turn-over frequency (ratio of TON over reaction time (hour))
Evaluation of catalytic activity of USALE-TMU-34 as optimized catalysts in filtration test
(Figure S7), Stability test (Figure S8), porosity (Figure S9) and reusability test (Figure S10)
show that presence of USALE-TMU-34 for progressing the nitroaldol reaction is essential and
this framework can save its catalytic activity and structural stability and porosity even after 3
cycles.
3.4. Dye Adsorption Experiment
Adsorption capacity of MOF is highly depended to their porosity and surface area and
application of TMU-4, TMU-34, SALE-TMU-34 and USALE-TMU-34 in N2 adsorption clearly
show that higher surface area and porosity leads to higher adsorption capacity toward gas
molecules. Considering this point, we applied four mentioned framework in adsorption
experiments in liquid phase to investigate the ability of these framework in liquid phase. 50 cc of
100 ppm aqoues solution of congored was selected as model pollutant for adsorption experiment
in gas phase. Here we choose congored because: (I) the adsorption capacity of the adsorbent
MOFs toward congored molecules is dominantly determined by pore volume and surface area[19]
and (II) shape and dimensionality of molecular skeleton of congored molecules is suitable for
into the 1D pores of the frameworks (Figure S11).
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10.1002/chem.201901554
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Adsorption experiments conducted as section 2.4 and the results are gathered in Figure S12 and
Table 3. The results show that the type of functional group does not have much effect on the
adsorption process. On the other side, it is easy to understand that increasing the surface area and
pore volume will increase the adsorption capacity as well as the time to reach maximum capacity
(Figure 8). Stability and reusability tests show that USALE-TMU-34 save its capacity, porosity
and crystallinity at least after 3 cycles (Figure S13-S15)
Figure 8. Time-dependent adsorption capacity for TMU-frameworks. (a) Concentration curve. (b) Maximum
capacity curve.
Table 3. Congored adsorption data at equilibrium.
BET
(m2.g-1)
518*
540
Function
Removal Efficiency Maximum Capacity
Equilibrium time
(%)
38
47
56
69
(mg.g-1)
(min)
35
TMU4
Azine
72
TMU-34
Dihydro-tetrazine
Dihydro-tetrazine
Dihydro-tetrazine
94
40
SALE-TMU-34
USALE-TMU-34
720
112
138
45
830
50
*: Calculated by CO2 adsorption at 195 K.
Finally in this work we show that through ultrasonic-assisted exchange reaction we were able to
improve the porosity and surface area of TMU-34 as well as modified morphology and
decreasing the particle size. Moreover, the synthesis time had been decreased (25%). Application
of USALE-TMU-34 sample with improved surface area in catalytic nitroaldol condensation
show that by increasing of the surface area and pore volume of the catalyst, the diffusion rate of
the reactants into the Lewis basic decorated pores increase which results in higher number of
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10.1002/chem.201901554
Chemistry - A European Journal
TOF factor. Also adsorption capacity in both gas and liquid phase show that by increasing the
pore volume, adsorbed amount of N2 and congored molecules increased.
Conclusion
In this work we synthesized two samples of MOF powders, TMU-4 and TMU-34, by direct
sonochemical method for 30 and 160 min, respectively. TMU-4 is an easy to synthesis MOF
which nonporous toward N2 at 77 K along with uniform micro-plate morphology. TMU-34 is
porous toward nitrogen at 77 K with aggregated non-uniform morphology with particle size from
several hundred nanometers to several micrometers. Since, the only difference in building block
of TMU-4 and TMU-34 is related to the N-donor pillar spacers, BPDB for TMU-4 and H2DPT
for TMU-34, we applied linker exchange methods to replace BPDB (inside the parent framework
of TMU-4) with H2DPT (inside the daughter framework of TMU-34) to afford dihydro-tetrazine
decorated TMU-34 framework with modified morphology, improved porosity and surface area.
We applied SALE and novel USALE methods. Interestingly we observe that USALE method is
much more useful rather SALE method in which USALE-TMU-34 sample shows improved
properties rather SALE-TMU-34. USALE-TMU-34 show increased porosity and surface area
(15%), pore volume (10%) and decreased synthesis time (96%) rather SALE-TMU-34 sample.
Also, synthesis time for USALE-TMU-34 is shorter than direct synthesized TMU-34 (120 min
vs. 160 min). Application of TMU-34, SALE-TMU-34 and USALE-TMU-34 in catalytic and
adsorption experiments show that owing to its functionality and improved porosity, USALE-
TMU-34 is more effective that both SALE-TMU-34 and TMU-34. More studies about USALE
method are underway.
AUTHOR INFORMATION
Corresponding Authors
E-mail: morsali_a@modares.ac.ir.Tel: (+98) 21-82884416
ORCID
Ali Morsali: 0000-0002-1828-7287
Notes
The authors declare no competing financial interest.
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10.1002/chem.201901554
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ACKNOWLEDGMENT
Support of this investigation by Tarbiat Modares University is gratefully acknowledged.
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