Influence of Hydrogen Bond Donating Sites in UiO-66 Metal-Organic Framework for Highly Regioselective Methanolysis of Epoxides

Article abstract of DOI:10.1002/cctc.201902219

A Zr(IV)-based UiO-66 metal-organic framework (MOF) (named 1) was synthesized by employing 1-(aminomethyl)naphthalene-2-ol appended terephthalate linker and Zr(IV) salt via solvothermal method and subsequently characterized. Furthermore, the potential efficiency of activated (named 1′) form of as-synthesized MOF was investigated as an organocatalyst for the ring-opening of epoxide by methanol. The catalytic performance of 1 and 1′ was studied in the methanolysis of styrene oxide as a model substrate and the activity of 1′ was also examined with various alcohols. Under the optimized reaction conditions, the catalytic performance of 1′ reached 96 % conversion of styrene oxide to its corresponding product with 98 % regioselectivity. The reusability and stability of the catalyst were proved by recycling up to four runs in the methanolysis of styrene oxide. The Lewis acidity originating from metal nodes and hydrogen bond donating (HBD) sites in the linker is distributed homogeneously throughout the framework, thus playing crucial role in the activation of epoxide with easy accessibility.

Full text of DOI:10.1002/cctc.201902219

Accepted Article
Title: Influence of Hydrogen Bond Donating Sites in UiO-66 Metal-
Organic Framework for Highly Regioselective Methanolysis of
Epoxides
Authors: Aniruddha Das, Nagaraj Anbu, Mostakim SK, Amarajothi
Dhakshinamoorthy, and Shyam Biswas
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ChemCatChem
10.1002/cctc.201902219
FULL PAPER
Influence of Hydrogen Bond Donating Sites in UiO-66 Metal-
Organic Framework for Highly Regioselective Methanolysis of
Epoxides
[a]
[b]
[a]
[b]
Aniruddha Das, Nagaraj Anbu, Mostakim SK, Amarajothi Dhakshinamoorthy* and Shyam
Biswas*[a]
[
a]
A. Das, M. SK, Dr. S. Biswas
Department of Chemistry
Indian Institute of Technology Guwahati, Assam 781039, India.
E-mail: sbiswas@iitg.ac.in.
[
b]
N. Anbu, Dr. A. Dhakshinamoorthy
School of Chemistry,
Madurai Kamaraj University, Madurai, Tamil Nadu 625021, India.
E-mail: admguru@gmail.com
Supporting information for this article is given via a link at the end of the document.
Abstract
A Zr(IV)-based UiO-66 metal-organic framework (MOF) (named 1)
was synthesized by employing 1-(aminomethyl)naphthalene-2-ol
appended terephthalate linker and Zr(IV) salt via solvothermal
method and subsequently characterized. Furthermore, the potential
efficiency of activated (named 1′) form of as-synthesized MOF was
investigated as an organocatalyst for the ring-opening of epoxide by
methanol. The catalytic performance of 1 and 1′ was studied in the
methanolysis of epoxide using styrene oxide as a model substrate
and the activity of 1′ was also examined with various alcohols. Under
the optimized reaction conditions, the catalytic performance of 1′
reached 96% conversion of styrene oxide to its corresponding
product with 98% regioselectivity. The reusability and stability of the
catalyst were proved by recycling up to four runs in the methanolysis
of styrene oxide. The Lewis acidity originating from metal nodes and
hydrogen bond donating (HBD) sites in the linker is distributed
homogeneously throughout the framework, thus playing crucial role
in the activation of epoxide with easy accessibility.
of epoxides is
a well-established chemical transformation
Introduction
reaction that can be achieved by various homogeneous and
heterogeneous catalysts.24, 30-33 Alcohols having relatively poor
nucleophilicity are not preferred candidates due to the
production of lower yields in the ring-opening of epoxides
reaction.34 Hence, this type of reaction can be promoted by
enhancing the electrophilicity of epoxides. Traditionally, the
activation of epoxides was achieved by employing catalysts
possessing either LA or BA sites or both of them.11, 24,
Historically, the methanolysis of epoxides was attained by using
corrosive sulfuric acid whereas in recent times, few
homogeneous catalysts based on metal complexes and different
heterogeneous catalytic systems such as amberlyst-15, clays,
polymers and silica were also reported.24, 32, 36, 37 The major
problems arising during the use of homogeneous catalyst are
the difficulty in the separation from the reaction mixture and
inability to reuse.38, 39 In recent times, significant attempts have
been dedicated to develop MOF based heterogeneous catalysts
Metal-organic frameworks (MOFs) are composed of metal ions
or clusters and organic linker molecules in a highly ordered
manner resulting in three-dimensional framework solids with
well-defined porous struture.1-3 Within a short span of time,
porous MOFs have gained much attention in inorganic chemistry
35
4
due to their unique properties like gas storage, gas/liquid
separation, chemical sensing, molecular magnetism,7 drug
5
6
delivery,8 biomedicine,9 optoelectronics,10 heat transformation
process,11 host for metal nanoparticles and heterogeneous
12
catalysis.11,
13-15
Particularly, Zr-based MOFs have received
wider attention among other MOFs mainly due to their
exceptional features like high surface area, high thermal and
chemical stability,17 low toxicity,18 tunable pore size and shape,19
surface functionality and incorporation of a variety of functional
moieties in linker molecule.20 The unsaturated metal centers can
act as Lewis acidic (LA) sites as well as the inclusion of Lewis
acidic or Brønsted acidic (BA) functional group in linker side
1
6
possessing LA metal center or BA functional groups in the linker
like -SO
3
H and -OH for the methanolysis of epoxides.2
4, 29, 34, 40-44
1
1, 13,
However, these catalytic reactions still have many drawbacks
like unsatisfactory selectivity of product, robustness and
recyclability of catalyst and harsh reaction conditions.24, 25 Hence,
developing new heterogeneous catalysts to promote
methanolysis of epoxide with very high regioselectivity at very
high conversion of epoxide is of great interest. On other hand, it
is one of the benchmark reactions to determine the nature of
active sites in the as-synthesized catalysts.
enhances the catalytic activity in heterogeneous catalysis.
2
1-23
Epoxides are one of the most important intermediates in
organic synthesis that can undergo nucleophilic ring-opening
reactions by various nucleophiles including alcohols24,
25
to
produce compounds like β-alcoxyalcohols, 1,2-diols and other
compounds
importance.
having
pharmaceutical
and
agrochemical
2
6-28
Thus, methanol is one of the frequently used
The concept of double hydrogen bonding in activating the
oxygen in epoxides has emerged from epoxide hydrolase
nucleophiles for the ring-opening of epoxides due to its high
reactivity and importance of the resulting product.29 Alcoholysis
1
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ChemCatChem
10.1002/cctc.201902219
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enzyme where the existence of two phenolic hydrogen atoms of Results and Discussion
two tyrosine residues are effectively involved in the activation of
epoxides by establishing hydrogen bonding as shown in
Preparation
4
5
Scheme 1a. This concept was exploited by developing a series
of organocatalysts consisting of HBD sites for activation of
oxygen in epoxides by introducing functional groups in an
analogous fashion. Among these functional groups, urea,
thiourea, catechol and related derivatives have been effectively
employed as organocatalysts in homogeneous media for the
activation of epoxides as shown in Scheme 1b.46-48 The
development of organocatalysts with HBD motifs has become
one of the powerful strategies for the activation of oxygen in
epoxides by the formation of weak hydrogen bonding
interactions. In one of the early examples, the rate of reaction
was significantly higher with biphenylenediol as HBD type
organocatalysts as compared to phenol for the addition of
diethylamine to phenyl glycidyl ether. The superior activity of the
former compound was attributed to the formation of two H-bonds
with the oxygen atom of the electrophile.49 Although the concept
of HBD sites has been well explored in homogeneous catalysis
by developing a wide range of organocatalysts, there are only
limited examples of anchoring such types of catalysts in
heterogeneous motifs.
The synthesis and characterization details of 2-(((2-hydroxy
naphthalene-1-yl)methyl)amino)terephthalic acid are described
in synthesis of H
Figures S1-S3). Different possible reactions were performed
using ZrCl and H BDC-C11 10NO linker in DMF solvent with
2 2
BDC-NH-CH -Naph-OH linker section (SI,
4
2
H
various modulators (acetic acid, benzoic acid, formic acid and
trifluoroacetic acid) at different temperatures.50, 51 A highly
crystalline yellow colored precipitation of 1 was obtained upon
reacting a mixture of ZrCl , H BDC-C11H10NO and acetic acid in
4 2
1:1:30 molar ratio in DMF at 120 ºC for one day.
FT-IR study
The FT-IR spectra of free linker, 1 and 1′ were recorded (SI,
Figure S4) for checking the existence of carboxylate moiety of
linker molecule inside the framework of
1 and 1′. The
frequencies for asymmetric and symmetric stretching vibrations
-1 15,
of carboxylate in free linker were found at 1701 and 1454 cm .
1
8
The absorption peaks at 1576 and 1574 cm-1 for 1 and 1′ were
present due to the existence of asymmetric stretching vibration
of carboxylate inside the framework of MOF, respectively. The
characteristic peaks for symmetric stretching vibration of
One of the main objectives of this work is to obtain a UiO-66
MOF with those linkers having HBD sites for mimicking epoxide
hydrolase enzyme. Herein, we report the solvothermal synthesis
of a Zr(IV)-based MOF of UiO-66 topology containing 2-(((2-
hydroxy naphthalene-1-yl)methyl)amino)terephthalic acid as a
linker molecule consisting of HBD sites. The as-synthesized
-1
carboxylate were found at 1435 and 1434 cm for 1 and 1′,
respectively. The presence of characteristic peaks of
carboxylate moiety for 1 and 1′ indicated that the linker
molecules were incorporated inside the framework.1
5, 18
The shift
in peak positions of symmetric and asymmetric carboxylate
stretching vibrations as compared free carboxylic acid ligand is
due to the binding of carboxylates with Zr(IV) ions. The peak at
1656 cm-1 in as-synthesized 1 is present due to the presence of
guest DMF solvent molecules which originated from the reaction
medium.52
solid 1 was characterized by XRPD measurements, TGA, N
sorption experiment and EDX analysis. The guest free form of 1
named 1′) was utilized as a heterogeneous catalyst for the
methanolysis reaction of epoxide. The incorporation of 1-
aminomethyl)naphthalene-2-ol moiety as side chain of linker
2
(
(
molecule helps to activate the epoxide molecules during the
methanolysis reaction. The experimental catalytic data revealed
that the metal nodes inside the framework act as LA and the -
OH and NH sites in the linker act as HBD sites. Both LA and
HBD sites participate in the reaction. However, HBD sites
influence the rate of reaction effectively in the methanolysis of
styrene oxide. Furthermore, the substrate scope as well as the
role of catalyst in methanolysis were checked by performing
control experiments in presence of the corresponding free linker
and various conventional catalysts. The stability of 1′ was
ascertained by performing reusability experiments up to four
cycles.
Figure 1. XRPD patterns of (a) Zr-UiO-66 (simulated), (b) as-synthesized 1
(experimental) and (c) activated 1′ (experimental).
XRPD analysis
The XRPD patterns of compound 1 and compound 1′ were
recorded, which are shown in Figure 1. From Figure 1, it is
concluded that as-prepared compound 1 has similar XRPD
pattern as UiO-66 MOF in terms of peak position and peak
Scheme 1. Activation of oxygen in epoxide (a) epoxide hydrolase enzyme and
(b) organocatalyst.
2
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intensity. Hence, 1 has same framework topology as the parent
UiO-66 MOF.18, 53 From Figure 1, it is also concluded that 1 is
stable enough under the activation conditions and it has
preserved the UiO-66 framework topology even after being
converted to 1′.18, 51, 53 To determine the unit cell parameters, the
indexing of the XRPD pattern of 1 was performed. The unit cell
parameters (SI, Table S1) confirm that the framework of 1 has
From (SI, Figure S6), it is observed the thermal treatment of 1
caused three weight loss steps. The first weight loss (2.4 wt%)
occurred in the temperature range of 25-160 °C (cald.: 2.3 wt%)
due to the loss of four water molecules. The second weight loss
was found to be 10.4 wt% due to the loss of 4.5 DMF molecules
(cald.: 10.6 wt%). The linker molecules start to be eliminated
from the framework of 1 beyond 285 °C. Therefore, it is
confirmed that 1 is stable up to 285 °C.
5
3
UiO-66 topology. Furthermore, the Pawley refinement of XRPD
pattern of 1 was carried out. The result obtained from Pawley fit
(
SI, Figure S5) is in good agreement with the simulated XRPD
pattern of Zr-UiO-66 MOF. Hence, these results also verify that
has UiO-66 framework topology.53
Chemical stability
To investigate the chemical stability of coordination bonds inside
the framework of MOF, material 1′ was immersed in water as
well as acidic (glacial acetic acid and 1 M HCl) and basic (0.1 M
NaOH) media. Material 1′ was stirred in these four solvent
systems for 10 h at room temperature. Afterward, the solid
powder was collected by vacuum filtration and dried at 80 °C
inside an oven. After drying, the XRPD patterns of these powder
materials were recorded. Figure S7 discloses high stability of
material 1′ in aqueous and acidic conditions. It is noteworthy that
material 1′ showed instability in a basic medium (0.1 M NaOH,
Figure S7). Hence, it is concluded that material 1′ shows high
chemical stability like the previously reported, analogous UiO-66
MOF systems.18, 55
1
Figure 2. Ball-and-stick representation of (a) H
2 2
BDC-NH-CH -Naph-OH linker,
(b) tetrahedral and (c) octahedral cages present within the framework structure
of 1. Color codes: Zr, green polyhedra; C, gray; O, red; N, blue; H, white. The
cavities are represented by yellow spheres. The structure of 1 was modeled
and drawn by using the Materials Studio (version 5.0, Accelrys Inc., San Diego,
2
N sorption analysis
To determine the BET surface area and nature of porosity, N
2
2009) software package.
sorption experiment was carried out with 1′ at -196 °C. Upon N
2
sorption measurement, the specific BET surface area was found
-1
Structure description
For the structural characterization of material 1, the XRPD
pattern of 1 was compared with the parent UiO-66 MOF (Figure
to be 946 m2 g . The micropore volume at p/p
0
= 0.5
3
-1
corresponded to 0.51 cm g . Remarkably, the observed surface
area is high in spite of having a large functional moiety in the
side chain of the linker molecule. The typical BET surface area
of un-functionalized Zr-UiO-66 MOF falls in the range of 850-
1). The XRPD data is well matched with the parent UiO-66 MOF
in terms of peak positions and peak intensities. In addition, the
indexing of XRPD pattern of material 1 was also performed to
evaluate the unit cell parameters (SI, Table S1). It was observed
that the unit cell parameters obtained after indexing the XRPD
pattern of 1 are very similar as the pristine UiO-66 MOF.
2
-1 56, 57
2
-1
1050 m g .
But, 1′ has a BET surface area of 946ꢀ m g in
spite of having large side chain attached with the parent linker
(BDC) molecule. This type of high BET surface area is generally
observed due to the presence of ‘missing linker defects/missing
Furthermore, the Pawley fit of XRPD pattern of 1 was carried out, cluster defects’ inside the MOF framework.58-60 The N
sorption
curve reveals microporous nature of the compound (SI, Figure
2
as shown in (SI, Figure S5). From this Figure, it can be observed
that the XRPD pattern of 1 is nicely matched with the XRPD
pattern of parent UiO-66 MOF. The observed values of Rwp and
S8). The BET surface area as well as micropore volume are in
good agreement with the reported UiO-66 MOF.54, 61, 62 The pore
size distribution curve is displayed in (SI, Figure S9), which
shows that the microspores of 1′ are centered at 15∙4 Å.
R
p
obtained from the Pawley fit are 5.18 and 3.79, respectively.
All these data corroborate that the framework of 1 is isostructural
with UiO-66. Hence, the framework of contains
Zr (OH)
]12+ clusters as secondary building units (SBUs) and
these SBUs are interconnected with the carboxylate ends of
BDC-NH-CH -Naph-OH linker. From Figure 2, it can be seen
1
[
6
O
4
4
CATALYTIC STUDIES
H
2
2
Study of catalytic activity
that the framework of 1 has both octahedral as well as
tetrahedral cages. Each zirconium atom resides in a square anti-
prismatic coordination environment.
The catalytic activity of 1′ was studied in the ring-opening
reaction of the of styrene oxide by methanol to obtain 2-
methoxy-2-phenylethanol as the only product. The reaction was
optimized in terms of catalyst loading and temperature. A blank
test in the absence of catalyst showed around 10% conversion
of styrene oxide in methanol at 60 °C while 96% conversion was
achieved with 1′ as a catalyst in methanol at 60 °C. The time
conversion plot for the ring-opening of styrene oxide in methanol
with 1′ as a catalyst is shown in Figure 3. Further, the effect of
catalyst loading for the ring-opening of epoxide was performed
at three catalyst dosages and the observed results are given in
Figure 4. These results indicate that the conversion of styrene
oxide effectively enhanced upon increasing the catalyst dosage
from 3 to 6 mg and a further increase in catalyst loading affords
Thermal stability
Thermal stability of a MOF is one of the crucial issues in
determining its applications. Hence, the thermal stability of 1′
was investigated. For the study of thermal stability of MOF in the
solid phase, the sample was placed in a thermogravimetric
analyzer and heated up from 25 to 700 °C at a constant heating
rate of 10 °C/min in argon atmosphere. From (SI, Figure S6), it
is inferred that both 1 and 1′ are stable up to 285 °C. The
thermal stability of compound 1 and 1′ is comparable with
existing UiO-66 type MOFs.18, 54
3
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the quantitative conversion of styrene oxide. On the other hand,
a similar trend was observed in the effect of temperature. The
conversion of styrene oxide was gradually increased upon
increasing the reaction temperature from room temperature to
Comparative catalytic study and reaction mechanism
In order to gain an understanding of reaction mechanism in the
methanolysis of styrene oxide using 1′ as solid catalyst under
the present reaction conditions, the activity of 1′ was compared
5
0 °C and the maximum conversion was attained at 60 °C. The
with UiO-66(Zr) and the linker (H
catalytic performance of 1′ was compared with UiO-66(Zr) and 1
Figure S10, Supporting Information). The observed results
2 2
BDC-NH-CH -Naph-OH). The
observed catalytic data are presented in Figure 5.
(
clearly suggest that 1′ shows a two-fold enhanced activity than
UiO-66(Zr) catalyst and higher activity than pristine 1. This
superior behavior of 1′ than UiO-66(Zr) and 1 is due to the
existence of dual active sites like LA and HBD sites.24 The LA
originates from metal centers while the HBD sites arise due to
the availability of OH and NH groups in the linker. The lower
activity of UiO-66(Zr) is due to the lack of these HBD sites. The
notable difference in the catalytic behavior between 1 and 1′ (SI,
Figure S10) suggest the crucial role of activation in removing the
loosely bound solvent molecules by lowering the diffusion
limitation to reach the active sites. This hypothesis was
confirmed by performing an additional control experiment with
the linker (H
styrene oxide with
2
BDC-NH-CH
2
-Naph-OH). The conversion of
-Naph-OH linker as
H
2
BDC-NH-CH
2
homogeneous catalyst was almost identical to 1′, but with higher
initial activity. This enhanced initial activity shown by the linker is
due to the availability of HBD sites which can readily interact
with oxygen of styrene oxide through weak hydrogen bond
interactions and these sites are readily accessible than 1′. On
Figure 3. Time conversion plots for the methanolysis of styrene oxide (a) in
the presence of 1′, (b) hot filtration test and (c) blank control experiment.
the other hand, the use of ZrCl
4
as a homogeneous solid
catalyst afforded the desired product in 97% conversion after 1 h
under identical conditions. This superior activity is due to the
inherent Lewis acidity and efficient interaction of styrene oxide
with Zr(IV) sites without experiencing diffusion limitation. Further,
pyridine was used as a catalyst poison to quench the active sites
2 2
in UiO-66(Zr), H BDC-NH-CH -Naph-OH linker and 1′. The
observed results are shown in Figure 6. The catalytic data with
and without pyridine for all these catalysts clearly indicate that
pyridine interacts strongly with the LA/HBD sites, thus exhibiting
a long induction period (6 h) while it is absent in the absence of
pyridine. These catalytic data confirm that pyridine strongly
inhibits the interaction of styrene oxide with these active sites.
Furthermore, the interaction of styrene oxide with the LA/HBD
sites activates the adjacent carbon for nucleophilic attack as
shown in Scheme 1b.
Figure 4. Effect of catalyst loading for the methanolysis of styrene oxide (a) 3
mg, (b) 6 mg and (c) 10 mg of 1′.
The ring-opening of styrene oxide by methanol using 1′ as
catalyst required 18 h at 60 °C. The activity of 1′ was compared
with other reported catalysts like Fe(BTC) and Cu
identical conditions. Styrene oxide conversion was 98% after 12
h using Fe(BTC) while Cu (BTC) exhibited 90% conversion
3 2
(BTC) under
3
2
after 1 h. These results are shown in Figure 7. The results
suggest that the activity of a solid catalyst mainly depends on
the nature and accessibility of active sites. The inherent LA of
Cu2+ in Cu
3 2
(BTC) favors to reach high conversion of styrene
oxide within 1 h without diffusion limitation while Fe(BTC) offers
much lower activity due to comparatively lower LA sites. On the
other hand, the activity of 1′ is slightly lower with these two
catalysts and this may be attributed to the steric crowding
caused by the bulky linker. In any case, the active sites are
accessible by reactants and 1′ can promote this reaction. These
comparative studies indicate that although the activity of 1′ is
lower than other MOFs, the study provides sufficient evidences
that LA and HBD sites can be distinctly installed in the MOF
structures and these sites can be effectively employed for
Figure 5. Effect of temperature for the methanolysis of styrene oxide: (a) room
temperature (b) 50 °C and (c) 60 °C using 1′ as solid catalyst.
4
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FULL PAPER
catalytic reaction. In addition, this work also provides alternative
types of active sites like HBD sites than the very often used -
SO H moiety as BA sites for this reaction.
3
Scheme 2 shows the proposed mechanism for the ring opening
of styrene oxide by methanol involving the contribution of HBD
sites. The observed catalytic data has shown that the influence
of HBD sites are much stronger than LA sites. The presence of
NH and OH groups in the linker coordinates with oxygen of
styrene oxide by forming weak hydrogen bonds thus decreasing
the electron density in the adjacent carbon containing phenyl
groups. Thus, methanol readily attacks this carbon to initially
form an intermediate which upon hydrogen transfer leads to the
final product.
One of the general tendencies in heterogeneous catalysis is to
ascertain the heterogeneity of the catalytic reaction. Hence, hot
filtration test was performed to ensure the absence of leached
active species in the solution from the solid catalyst. This test
was performed by filtering the solid after 1 h under the reaction
temperature and the resulting reaction mixture was stirred for
the remaining period. The time conversion plot (Figure 3)
indicates that the reaction is completely inhibited upon removal
of catalyst from the reaction, thus suggesting the absence of
Scheme 2. A proposed mechanism for the ring opening of styrene oxid
e by methanol involving HBD sites.
active sites in the solution.
Thus, this test proved the
heterogeneity of the reaction. Further, ICP-AES analysis of the
reaction mixture indicated the absence of Zr, thus ruling out the
possibility of metal leaching.
Performing the reusability test in consecutive cycles is another
way to confirm the catalyst stability in heterogeneous catalysis.
In this aspect, the reusability of 1′ was performed in the ring-
opening of styrene oxide by methanol at 60 °C. After the
reaction, the solid was washed with methanol, dried and reused
in consecutive cycles with the fresh reactants. The attained
catalytic results exhibited in Figure 8 indicate that the catalyst
retains its activity up to four cycles with no appreciable decay in
its activity. An identical activity was achieved in four consecutive
cycles upon repeated uses of 1′, suggesting that the density of
active sites remains unaltered under the present reaction
conditions. The structural stability of 1′ was examined by the
XRPD study, FT-IR, EDX and FE-SEM analyses. The XRPD
study (Figure 9) suggested that catalyst 1′ is structurally stable
enough through the catalytic reaction up to four consecutive
cycles. The FT-IR spectra of the recovered MOF after the first
and fourth catalytic cycle of reaction are shown in Figure S4
Figure 6. Time conversion plots for the methanolysis of styrene oxide using
(●) H
mmol) as catalyst poison with (∆) Zr-UiO-66-NH-CH
and (○) H BDC-NH-CH -Naph-OH linker.
2
BDC-NH-CH
2
-Naph-OH linker, (▲)1′, (■) Zr-UiO-66 and pyridine (0.25
2
-Naph-OH, (□) Zr-UiO-66
2
2
(
Supporting Information). These spectra clearly reveal that there
are peaks at 1572 and 1434 cm-1 which are due to the
asymmetric and symmetric stretching vibrations of carboxylates
4
+
bonded with Zr inside the MOF catalyst, respectively. The
presence of these peaks confirms that the MOF catalyst is
stable enough even after the fourth catalytic cycle. Furthermore,
EDX studies (SI, Figures S11-S14) showed that the atomic
percentage of the elements (C, N, O and Zr) present in 1′ before
and after four cycles of catalysis is almost close to each other. In
addition, the FE-SEM analyses (SI, Figures S15-S16))
confirmed that catalyst 1′ has similar morphology for fresh and
reused (for four cycles) samples.
Figure 7. Time conversion plots for methanolysis of styrene oxide by (●)
Cu (BTC) , (■) Fe(BTC) and (▲) 1′.
3
2
5
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Catalytic studies with various substrates
With the optimized reaction conditions in hand, the scope of 1′
was screened with other epoxides and alcohols, and the
observed results are discussed in Table 1. The ring-opening of
styrene oxide by methanol using 1′ as solid catalyst afforded
96% conversion after 18 h at 60 °C. Under identical conditions,
the conversion of styrene oxide was slightly decreased with
ethanol as the nucleophile. Further, the reactivity of styrene
oxide with 2-propanol and 1-butanol was further decreased to 78
and 74% conversions, respectively, compared to methanol. This
lower conversion of styrene oxide upon the increase of chain
length in alcohol is probably due to the lower nucleophilicity and
diffusion limitations imposed by the bulkier nature of these
alcohols. The observed catalytic data are in good agreement
with earlier results in the literature.24, 31, 36, 63-65 Interestingly, our
hypothesis of the steric hindrance of alcohol was proved by
observing distinct reactivity between 1-butanol and t-butanol with
styrene oxide affording 74 and 35% conversions, respectively,
under identical conditions.
On the other hand, the ring-opening reaction between styrene
oxide with 1-octanol showed 62% conversion after 18 h, and no
further significant change in the conversion was seen even after
doubling the reaction time. Ring-opening of styrene oxide by
cyclohexanol showed 31% conversion, and this lower activity
may be due to the steric hindrance of reactants. On the other
hand, ring-opening of cyclohexene oxide by methanol showed a
significantly higher conversion of 82% under similar conditions,
showing the facile diffusion of these reactants than to the
previous case. The enhanced reactivity was observed for 2-
methylpropene oxide than to 2-chloromethyloxirane by
methanolysis under similar conditions, and this reactivity
difference is due to the difference in the diffusion ability of these
epoxides to interact with the active sites in 1′. The later substrate
finds more difficulty in reaching the active sites while the former
substrate with its smaller dimension can easily penetrate the
pores to interact with active sites easily, thus enhancing the
electrophilicity in epoxide for methanol attack. These catalytic
data clearly confirm that the reaction occurs within the pores of
the solid 1′, thus supporting the size selective catalysis.
Figure 9. XRPD patterns of 1′ (a) before catalysis, (b) after the 1st cycle and
(c) after the 4th cycle of catalysis.
The diffusion limitation exerted by 1′ in the catalytic
performance in the opening of epoxide was further verified by
conducting additional experiments with epoxies possessing
larger kinetic diameters. The observed results are shown in
Table 1 and Figures S36-S37 (Supporting Information). 4-
Bromostyrene oxide afforded 52% conversion with methanol
using 1′ under identical conditions with high regioselectivity of
type I isomer. Further, 4-t-butylstyrene oxide exhibited trace
conversion (3%) while cyclooctene oxide showed no reactivity
under identical conditions, thus clearly suggesting the influence
of substrate kinetic diameter to diffuse inside the pores of 1′. On
the other hand, the reaction of cyclooctene oxide with methanol
in the presence of UiO-66(Zr) afforded around 6% conversion to
the desired product with complete selectivity under identical
conditions. But, this lower conversion is due to the lack of HBD
sites and the nature of cyclic epoxide. These catalytic results
prove the size selective catalysis by 1′ and demonstrating the
diffusional limitations due to the functionalization of the linker.
Figure 8. Reusability profile for the methanolysis of styrene oxide using 1′ as a
solid catalyst.
6
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ChemCatChem
10.1002/cctc.201902219
FULL PAPER
Conclusion
Table 1. Alcoholysis of epoxides in the presence of 1′ as hydrogen-bonding
organocatalyst.
a
Conv.b
(%)
To summarize, a new functionalized Zr-UiO-66 MOF with HBD
sites in the linker has been developed as a heterogeneous
catalyst for the methanolysis of epoxides. We have judiciously
designed and successfully synthesized catalyst 1′ by traditional
solvothermal method followed by the activation procedure. The
catalyst 1′ was characterized by FT-IR, XRPD, TGA, BET, EDX
and FE-SEM analyses. The catalytic activity of 1′ was tested in
the methanolysis of epoxide with styrene oxide as a model
substrate and the activity of catalyst 1′ was also examined with
various alcohols. The catalytic reaction was optimized in terms
of catalyst loading and temperature. Catalyst 1′ showed 96%
conversion of styrene oxide to its corresponding product with
Ent
ry
Epoxide
Alcohol
Ti
me
Sel. %
Isomer
Isomer
II
(h)
I
1
2
3
methanol
ethanol
18
96
98
2
18
86
90
98
95
2
5
3
6
6
2
4
5
9
97
2-
18
24
78
84
94
94
98% regioselectivity. A series of control experiments indicated
propanol
that catalysis occurs within the pores of 1′ and the participation
of dual active sites like LA and HBD sites simultaneously,
among which the later sites predominant in the catalytic activity.
Catalyst 1′ showed similar catalytic performance in four cycles.
A comparison of the fresh and four times used 1′ revealed that
the crystalline nature and morphology of the used solids match
well with the fresh solid, thus exhibiting high stability under these
experimental conditions. Finally, the present catalytic data
clearly suggest that the design of this new functionalized MOF
possessing HBD sites provides the possibility of developing
heterogeneous solid catalyst to mimic the enzyme as well as
organocatalysts behavior in MOFs, thus experiencing dual active
sites (LA and HBD) within the framework to accelerate the
reaction rate in the alcoholysis of epoxide.
3
0
91
74
96
97
4
3
4
1-butanol
18
2
4
6
83
91
96
96
4
4
3
5
6
1-butanol
1-octanol
18
35
41
51
62
91
90
90
97
9
10
10
3
2
4
6
3
18
36
67
31
97
94
3
6
7
cyclohexa
nol
18
Experimental Section
3
6
47
82
98
92
8
-
Synthesis of [Zr
6 4 4 6 2
O (OH) (BDC-C11H10NO) ]∙4H O∙4.5DMF (Zr-UiO-66-
8
9
methanol
18
100
100
1-(aminomethyl) naphthalen-2-ol, 1).
30
-
ZrCl
4
(28 mg, 0.12 mmol) and 2-(((2-hydroxy naphthalene-1-
BDC-C11 10NO) (40 mg, 0.12
yl)methyl)amino)terephthalic acid (H
2
H
methanol
methanol
18
90
98
8
7
92
93
mmol) was mixed in 3 mL N,N-dimethylformamide (DMF) inside a sealed
tube. This reaction mixture was sonicated for 20 min and then 210 µL of
acetic acid was added. The total reaction mixture was allowed to heat
onto a pre-heated block heater at 120 ºC and heating was continued up
to 24 h. After 24 h, the tube was cooled to room temperature and the
solid precipitation was filtered through membrane filter paper. The
powdered material was dried inside an oven at 60 ºC for 2 h. The yield
30
1
0
1
18
29
58
100
98
-
36
2
1
was 35 mg (0.011 mmol, 56%) based on zirconium tetrachloride salt.
methanol
methanol
18
18
52
-
96
-
4
-
-1
127.5 10.5
Elem. anal. calcd for C127.5H N O46.5Zr
6
(3098.26 g mol ): C, 49.43;
H, 4.15; N, 4.75%. Found: C, 49.26; H, 4.03; N, 4.51%. FT-IR (KBr,
cm−1): 3369 (br), 1656 (vs),1573 (vs), 1496 (w), 1435 (s), 1385 (s), 1259
1
2
3
(vs), 1157 (w), 1102 (w), 1022 (w), 965 (w), 796 (vs), 767 (vs), 664 (vs),
571 (w), 481 (vs).
1
Activation of compound 1
methanol
18
3
97
3
The as-synthesized compound 1 (100 mg) was stirred in 30 mL of
methanol for one day. Afterward, the solid was filtered through
membrane filter paper and the powder was dried at 70 ºC for 2 h. In the
next step, this oven-dried powder was activated by heating at 120 ºC for
one day under vacuum conditions. In this process, the activated
compound (1′) was achieved.
[
a] Reaction conditions: epoxide (0.25 mmol), alcohol (2 mL), Zr-UiO-66-NH-
.
CH
2
-Naph-OH (10 mg), 60 °C . [b] Conversion and selectivity were determined
by GC. Selectivity of isomer
I indicates 2-alkoxy-2-phenylethanol and
selectivity of isomer II represents other isomers.
7
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Reaction procedure for catalysis
A 15 mL oven-dried Schleck tube was charged with 1′ (10 mg) and
styrene oxide (0.25 mmol) and HPLC grade methanol (2 mL) was added
to this mixture. Then, this heterogeneous slurry was stirred at 60 °C for
the required time, as shown in Table and Figures. The conversion of
styrene oxide was measured by collecting samples from the reaction
mixture at different time intervals. The samples were analyzed in Agilent
7820A gas chromatography (GC) (SI, Figures S17-S37). Conversion and
selectivity were determined by the same instrument using the internal
standard method. The reusability test was performed for 1′ as follows: at
the end of the reaction, the solid catalyst was filtered, washed three times
with fresh methanol (3 × 5 mL) and dried at 60 °C for 3 h. The activity of
this dried catalyst was examined in the next cycle with fresh styrene
oxide and methanol.
Acknowledgements
AD and SB are grateful for financial assistance from Science
and Engineering Research Board, New Delhi via grant no.
EMR/2016/006500 and EEQ/2016/000012, respectively.
Keywords: Metal-organic framework • UiO-66 • Catalysis •
Hydrogen bonding site • Regioselectivity.
8
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Table of Contents
A reusable Zr(IV)-based UiO-66 MOF was synthesized by employing 1-(aminomethyl)naphthalene-2-
ol appended terephthalate linker and Zr(IV) salt via solvothermal method. The catalytic performance of
the activated MOF was examined in the ring opening of epoxides by methanol. The observed
experimental catalytic data indicated that the superior activity of activated MOF compared to as-
synthesized MOF and UiO-66(Zr) is due to the presence of dual active sites comprising Lewis and
hydrogen bond donating sites.
1
1
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