Microporous Nanotubes and Nanospheres with Iron-Catechol Sites: Efficient Lewis Acid Catalyst and Support for Ag Nanoparticles in CO2 Fixation Reaction

Article abstract of DOI:10.1002/chem.201802319

Fe^III-containing hyper-crosslinked microporous nanotubes (FeNTs) and nanospheres (FeNSs) are synthesized through the reaction of catechol and dimethoxymethane in the presence of FeCl₃ or CF₃SO₃H. Both FeNTs and FeNSs demonstrate excellent catalytic activity in Lewis acid catalysis (hydrolysis and regioselective methanolysis of styrene oxide) and tandem catalysis involving a sequential oxidation-cyclization process, which selectively converts benzyl alcohol to 2-phenyl benzimidazole. Apart from Lewis acidity, the FeNTs and FeNSs also showed CO₂ uptake capacities of 2.6 and 2.2 mmol g^?1, respectively, at a pressure of 1 atm and temperature of 273 K. Furthermore, Ag nanoparticles are immobilized successfully on the surfaces of FeNTs and FeNSs by the liquid-phase impregnation method to prepare Ag@FeNT and Ag@FeNS nanocomposites, which show high catalytic activity for the selective fixation of CO₂ to phenylacetylene to yield phenylpropiolic acid at 60 °C and 1 atm CO₂ pressure. Hence, Fe^III-catechol-containing hyper-crosslinked nanotubes and nanospheres have huge potential not only as Lewis acid catalysts, but also as excellent supports for immobilizing Ag nanoparticles in the design of a robust catalyst for the carboxylation of terminal alkynes, which has wide scope in catalysis and environmental research.

Full text of DOI:10.1002/chem.201802319

A Journal of
Accepted Article
Title: Microporous nanotubes and nanospheres with iron-catechol
sites: efficient Lewis acid catalyst and support for Ag
nanoparticles in CO2 fixation reaction
Authors: Arindam Modak, Piyali Bhanja, and Asim Bhaumik
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To be cited as: Chem. Eur. J. 10.1002/chem.201802319
Link to VoR: http://dx.doi.org/10.1002/chem.201802319
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FULL PAPERS
DOI: 10.1002/cctc.200((will be filled in by the editorial staff))
Microporous nanotubes and nanospheres with
iron-catechol sites: efficient Lewis acid catalyst
and support for Ag nanoparticles in CO2 fixation
reaction
Arindam Modak,*[a,b] Piyali Bhanja[a] and Asim Bhaumik*[a]
nanoparticles are successfully immobilized on the surfaces of FeNT
and FeNS by liquid phase impregnation method to prepare Ag@FeNT
and Ag@FeNS nanocomposites, which showed high catalytic activity
for selective fixation of CO2 to phenylacetylene to yield phenylpropiolic
acid at 60 oC and 1 atm CO2 pressure. Hence, Fe(III)-catechol
containing hyper-cross-linked nanotubes and nanospheres have huge
potential not only as Lewis acid catalysts but as an excellent support
in immobilizing Ag nanoparticles for designing a robust catalyst for the
carboxylation of terminal alkynes, which has wide scope in catalysis
and environmental research.
Fe(III)-containing hyper-cross-linked microporous nanotubes (FeNT)
and nanospheres (FeNS) have been synthesized through the reaction
of catechol and dimethoxymethane in the presence of FeCl3 or
CF3SO3H. Both FeNT and FeNS demonstrate excellent catalytic
activity in the Lewis acid catalysis (hydrolysis and regioselective
methanolysis of styrene oxide) and tandem catalysis involving a
sequential oxidation-cyclization process, which selectively converts
benzyl alcohol to 2-phenyl benzimidazole. Apart from Lewis acidity,
FeNT and FeNS also showed CO2 uptake capacity of 2.6 and 2.2
mmol g-1, respectively at 1 atm pressure and 273 K. Further, Ag
agglomerated solid without any special morphologies i.e.;
tubes/spheres. Thus, the development of HCPs with unique
shape and structure is quite challenging. Few previous reports
have demonstrated the synthesis of hollow tubular shaped
HCPs,[4] which motivated us to explore the research on various
reactive organic monomers, as chelating ligands that can easily
coordinate with the reactive metal ions in solution phase. Among
several chelating ligands, the “catechol” moiety act as a bidentate
precursor for the stabilization of metal complexes.[5] Therefore,
catechol functionalized high surface area porous support have
huge potential in designing efficient heterogeneous catalysts.
Although, there are few reports on porous polymers based on
single site catechol supports or from tetra-arm alkynes and
palladium catalysts,[6] these syntheses involved expensive
catalyst preparation steps and the use of harmful reagents.
Introduction
Hyper-cross-linked polymers (HCPs) have received increasing
research interest over the years as they possess high porosity,
robust nanostructure combined with ease of their synthesis and
diversity in selecting functional monomers.[1] Thus, HCPs provide
an invaluable platform for new functional polymers, which are
largely explored in gas storage, separation and catalysis.[2]
Therefore, based on the knitting condensation-polymerization
approach and by employing external linkers, a variety of high
surface area metal-grafted organic polymers can be synthesized.
Additionally, metal coordinated HCPs are highly flexible and
robust in fixing metal complexes and/or small metallic
nanoparticles (NPs) at the pore surfaces.[3] Hence, inorganic
complexes and metallic NPs can be stabilized for prolonged use
through this porous framework, in contrast to their prompt
deactivation under homogeneous conditions. Further, due to the
fast polymerization reactions, HCPs are mostly formed as an
Therefore, we target
a
catechol-based hyper-cross-linked
polymer via cost-effective synthetic route.
On the other hand, ring opening of epoxides is an important
route for designing compounds with versatile 1,2-type functional
groups, like β-alkoxy alcohols, which are precursors for a broad
range of pharmaceuticals.[7] Generally, homogeneous acid/base
catalysts can effectively hydrolyze the epoxides to β-alkoxy
alcohols, but the strength of acid/base is insignificant in
controlling the regioselectivity of the product.[8] Thus, unwanted
polymerization and/or mixture of products are formed during the
epoxide ring opening. Although, several metal salts like Er(III),
Sn(II), Al(III), In(III) are used to catalyze such reactions, but these
are either toxic or less abundant, which restricts their catalytic
[a]
Dr. A. Modak, P. Bhanja, Prof. A. Bhaumik
Department of Materials Science
Indian Association for the Cultivation of Science
2A & B, Raja S.C. Mullick Road, Jadavpur, Kolkata− 700032, India
Fax: +91 33 2473 2805
E-mail: msab@iacs.res.in
[b]
Dr. A. Modak
S. N. Bose National Centre for Basic Sciences,
Block - JD, Sector-III, Salt Lake, Kolkata -700106,
India
potential.[9] Considering epoxide
hydration at
various
(water:substrate, W/S) ratio, it is essential to control the reaction
at low water concentration in order to avoid unwanted
polymerization and byproduct formation in this reaction.[10]
Similarly, Lewis acid catalyzed synthesis of benzimidazole has
E-mail: arindam_modak_2006@yahoo.co.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/cctc.200xxxxxx
1
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10.1002/chem.201802319
Chemistry - A European Journal
huge scope in pharmaceutical research, since substituted
benzimidazole are important building blocks for drugs having
antiviral, antifungal, antiulcer, antihypertensive, antihistaminic,
and anticancer properties.[11] However, one-step direct conversion
of benzimidazole from benzyl alcohol and o-diaminobenzene
often involves toxic oxidant together with expensive ruthenium[12]
and iridium catalysts.[13] Hence, cost-effective design of
nanocatalyst with desirable Lewis acidic sites at the surface is
essential.
Friedel-Craft condensation-polymerization between catechol
and dimethoxymethane (FDA) has been carried out in the
presence of different acid catalysts (FeCl3 and CF3SO3H) as
shown in Scheme 1. It is interesting to note that iron-catechol
nanotubes are observed from conventional HCP synthesis by
heating a mixture of catechol, FeCl3 and FDA in a chlorinated
solvent as dichloroethane. In this way, catechol moieties are in-
situ coordinated with iron sites in FeNT. On the other hand, when
the synthesis is carried out in absence of FeCl3 but by using
CF3SO3H as catalyst, we observed metal free catechol
nanosphere. This material was later post synthetically modified
through the FeCl3/THF treatment to obtain FeNS material.
Morphology of FeNT and FeNS are shown in Figure 1 (TEM) and
Figure S1 (FESEM), which suggested that FeNT is composed of
nanoparticles having micrometer long hollow tubes and their
dimeter (~100 nm) is similar with other microporous
nanotubes.[17,18] On the other hand FeNS consists of ~300-500
nm size nanospheres. Therefore, our synthetic strategy
suggested the merit of knitting polymerization to obtain iron-
On the other hand, CO2 sequestration into value added fuels
and chemicals are very demanding for the control and successful
use of this greenhouse gas.[14] The conversion of atmospheric
CO2 to enhance carbon chain products through C-C bond forming
reactions are generally accomplished by using different catalysts
with undefined particle morphologies.[15] Nanoparticles supported
heterogeneous catalysts offer excellent advantages for cost-
effective synthesis of desired products, but the size and
dispersion of NPs on the support often limit their catalytic
performances.[16] NPs supported on HCPs[17] are found to be quite
effective route in designing a robust and efficient heterogeneous
catalyst. Hence, the development of more efficient and robust
hyper-cross-linked porous polymer nanocatalyst based on
catechol linker are highly demanding for CO2 fixation reactions.
Herein, we first report the synthesis of hyper-cross-linked
polymers bearing iron-catechol moieties that can act as catalyst
for ring opening of epoxides and tandem oxidation-cyclization
reaction in one-step. Fe-catechol containing FeNT is quite
efficient for the hydrolysis and methanolysis of epoxides at both
low and high solvent concentration (2 to 20 M), which showed
high conversion (100%) and selectivity (99%) for the desired
products. Furthermore, this Fe-catechol based porous support
can be decorated with Ag NPs for the fixation of CO2 in terminal
alkynes to respective carboxylic acids under mild reaction
conditions, suggesting the merit of these HCP supports in
heterogeneous catalysis.
catechol
containing
porous
polymers
with
different
nanoarchitectures over other complicated synthetic strategies for
designing hyper-cross-linked polymers.[17a]
Results and Discussion
Synthesis and characterization of FeNT and FeNS
Figure 1. TEM images (a and b) FeNT and (c and d) FeNS; HRTEM images (e)
FeNT, (f) FeNS.
It is pertinent to mention that the growth of nanotube like
particle morphology of the material is dependent on various
factors like molar concentrations of monomers and
dimethoxymethane
linker,
catalyst
(FeCl3),
solvent
(dichloroethane), reaction temperature, mechanical stirring, time
etc.[4] As for example, when the synthesis was carried out in the
absence of dimethoxymethane, resulting material did not show
any nanotube like particle morphology (Figure S2).
Scheme 1. Syntheses of iron-catechol based microporous hyper-cross-linked
organic polymers having hollow nanotube and nanosphere morphologies.
2
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10.1002/chem.201802319
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Stability of FeNT and FeNS materials are investigated by
TGA experiment. Figure 2a demonstrates the TGA plots, which
with a broad peak in the 2θ=15-30o range. Although the as-
synthesized FeNS sample showed some peaks corresponding to
suggested that these materials are stable upto 230 oC (wt loss c.a. some residual iron oxychloride impurity upon rigorous soxhlet
~20%) and this result agrees well with other microporous
polymers.[1,19] From FT-IR spectra (Figure 2b) we see the
presence of several bands for aromatic C=C (1600-1400 cm-1), C-
O / C-C stretching (1400-1200 cm-1), C-H bending (1200−1000
cm−1), which are correlating with the phenyl groups of catechol.
Again, we did not observe any characteristics band at ~1715 cm-1
for ketone sites, suggesting that catechol is not oxidized to
aromatic ketone. The broad band at 3400 cm-1 is possibly arised
from the hydrogen bonding in FeNT and FeNS. Moreover, from
the FT-IR spectra we did not observe any band at 576 cm-1 for
Fe-O stretching,[20] which suggested the absence of iron oxide
nanoparticles in the FeNT material before carbonization (Figure
S3). However, upon carbonization CFeNT sample displayed this
band due to Fe-o stretching. These result also agreed well with
HRTEM images (Figure 1; e and f). Interestingly, the sharp -OH
stretching of catechol at 3449 cm-1 disappears for iron
coordinated polymers and a broad stretching at 3468-3320 cm-1 is
observed, which could be because of free catechol as well as
metal-coordinated catechol sites (Figure S3 and Figure 2b)
extraction by refluxing in methanol could resulted amorphous
FeNS material (Figure 2d) without any impurity phase. On the
contrary, no diffraction peak corresponding to iron oxychloride
was observed for the XRD pattern of FeNT. In order to investigate
the coordination environment of iron present in FeNT and FeNS
materials, we have carried out the EPR analyses and the results
are shown in Figure 2e. At 100 K EPR measurement, FeNT
showed distinguishable strong peaks corresponding to several g
sites i.e. 8.2 and 4.6 values, suggesting the existence of high spin
Fe(III) in distorted rhombic environment.[21] However, the EPR
signal becomes weaker when the analysis temperature was
raised to 298 K, which clearly indicated the formation of
paramagnetic iron-catechol complex during the synthesis.
Additionally, very small peak at g = 2.003 signified the less
possibility for oxidation of catechol to o-semiquinone under the
synthesis conditions. In contrast, the post synthesis impregnation
with FeCl3 in NS did not exhibit any EPR signal for Fe(III) sites,
rather it showed a less significant signal at g = 3.4 on account of
organic radicals, suggesting that the complexation characteristics
of Fe(III) in FeNS is different than that of FeNT. Interestingly,
when FeNT is digested by treating with 12N HCl, we could not
observe any EPR signal, demonstrating that Fe(III) sites are
indeed showed strong EPR in FeNT. Existence of Fe(III) has
been further confirmed from XPS analysis, as shown in Figure 2f,
where the presence of two Fe 2p peaks at binding energies 711
eV (Fe 2p3/2) and 725 eV (Fe 2p1/2) is seen.[22] It is pertinent to
mention that Fe 2p in pure iron oxide showed peaks at 710 and
723.4 eV, corresponding to Fe2p3/2 and Fe2p1/2 respectively with
additional strong satellite peaks at 713.7 eV (2p3/2) and 728.3 eV
(2p1/2).[22] Although FeNT and FeNS showed two major Fe 2p
peaks, at 728.3 eV (2p1/2) and 713.7 eV (2p3/2), but their
corresponding satellite peaks are too weak to detect. However,
Fe 2p XPS of the carbonized FeNT is characteristically more
similar with iron oxide and distinctly different from FeNT (Section
S1 and Figure S4). Therefore, the FT-IR, EPR and XPS analysis
revealed the existence of iron(III) in FeNT and FeNS materials.
(b)
FeNT
FeNS
3500 3000 2500 2000 1500 1000
500
Wave number / cm-1
2
(c)
(d)
3
2
5
4
1 OH
OH
3
1
FeNT
OMe
4
*
5
*
*
*
FeNS
0
50
100
150
200
250
0
10
20
30
40
50
60
70
80
Chemical shift / ppm
Angle / 2

Fe2p3/2
(e)
Fe2p1/2
(f)
g = 4.3
Porosity and Surface area measurement
g = 8.2
1000
FeNT
0
2000
3000
4000
FeNS
FeNT
FeNS
740
730
720
710
700
3000
3200
3400
Magnetic field / Gauss
Binding energy / eV
Figure 2. TGA profile of FeNT, FeNS and NS (a); FT-IR spectra of FeNT, FeNS
and NS (b); solid state 13C CP-MAS NMR of NS (c); powder X-ray diffraction
patterns of FeNT, FeNS after soxhlet extraction (d); EPR spectra at 100 K (e)
and Fe 2p XPS spectra of FeNT and FeNS (f).
13C CP-MAS-NMR spectrum is shown in Figure 2c, which
suggested the presence of bands at 128.6 and 143 ppm,
corresponding to aromatic carbon atoms of the catechol groups.
Additionally, the crosslinking is evidenced from aliphatic signals at
26, 14, 44 ppm from several alkyl and alkoxy species.[19]
Therefore, the 13C NMR investigation revealed the successful
polymerization of catechol units in FeNT and FeNS. Powder X-
ray diffraction pattern of these materials are shown in Figure 2d,
which suggested the formation of amorphous polymeric material
Figure 3. (a) N2 adsorption (closed circles) and desorption (open circles)
isotherms of FeNT, FeNS and NS; (b) Pore size distribution from density
functional theory (DFT) model; CO2 adsorption isotherms of FeNT, FeNS and
NS (c, d).
3
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Porosity and surface areas of the FeNT, FeNS and NS
materials are measured from N2 sorption analysis and these are
shown in Figure 3a. All three materials showed high N2 uptake at
low P/P0 region (type I isotherm), corresponding to the presence
of micropores.[23] In Table 1, we have summarized the physical
parameters of these supported HCPs, which suggested that iron
coordinated polymers have lower BET surface area than iron-free
NS. The low surface area of FeNT and FeNS could be attributed
to the presence of Fe(III) in the framework (higher atomic mass
than carbon). DFT pore size distribution plots derived from these
N2 sorption isotherms are shown in Figure 3b, which suggested
the presence of both micropores (< 2nm) and mesopores (2-6
nm). Nevertheless, all sorption isotherm shows hysteresis
because of swelling characteristics of these soft polymeric
frameworks.[24] The quantity of iron present in FeNT and FeNS
materials are given in Table 1. Additionally, microporous nature of
NS, FeNT and FeNS together with their good pore volumes have
motivated us to measure their CO2 uptake ability. Results are
shown in Figure 3c and d, which suggested that NS has the
highest CO2 uptake ability (3.8 mmol g-1) over the FeNT (2.6
mmol g-1) and FeNS (2.2 mmol g-1) materials at 273 K and 1 atm
pressure. This is because NS possess significantly large amount
of free –OH sites as well as higher BET surface area and pore
volume than Fe containing FeNT and FeNS materials. It is
noteworthy to mention here that, these HCPs showed
comparable CO2 uptake capacity compared with other polymer
tubes (CMPN-2; 1.8 mmol g-1),[25] conjugated polymers,[26] and
other hydroxyl-rich microporous adsorbents,[27] suggesting their
promising future in CO2 sequestration. In Table S1 we compared
CO2 uptake ability of NS, FeNS and FeNT with other state-of-the-
art porous adsorbents like MOFs and COFs.
Table 1. Physical parameters of FeNT, NS and FeNS as estimated from BET analysis
[a]
[b]
Name
SBET
m2 g-1
400
SLangmuir
m2 g-1
547
Vtotal
Vmicro
Micropore size
nm[c]
Fe amount
cm3 g-1
0.45
cm3 g-1
0.1
(μmol g-1)[d]
FeNT
NS
0.9 -1.30
0.5 -1.31
0.5 -1.26
7.5
-
545
734
0.35
0.2
FeNS
352
475
0.22
0.12
4.2
[a]Measured from N2 sorption isotherm at 0.99 P/P0. [b]t-plot micropore
[c
volume calculated at 77 K from N2 sorption isotherm. DFT micropore size
distribution from N2 sorption isotherm. [d]Calculated from ICP analysis.
Lewis acid catalysis of FeNT and FeNS
We have evaluated the potential of FeNT and FeNS as
solid acid catalyst. In this context, we have investigated the
hydrolysis/methanolysis reaction of styrene oxide as well as
tandem oxidation-cyclization reaction. The ring opening
hydrolysis and methanolysis of styrene oxide were carried out
o
o
at 25 C and at 60 C, considering various H2O/substrate, as
shown in Figures a, b, and d, respectively. The
4
c
corresponding hydrolysis and methanolysis products were
obtained in good to excellent yield, where selectivity for 1-
phenylethane-1,2-diol was observed at 99% as the hydrolysis
product. We observed the regioselective synthesis of 2-
methoxy-2-phenylethanol as the major methanolysis product.
Apart from methanol, we also tried ring opening reactions
in other solvents like ethanol, 2-propanol etc, where we found
the reactivity decreases in the order: MeOH>EtOH>n-PrOH,
with full conversion has been observed in methanol within 6 h,
followed by 8 h in ethanol and 20 h in n-PrOH. This could be
attributed to the higher steric effects as imparted by heavier
alcohols.[28] It is noticeable that hydrolysis of styrene oxide
showed faster kinetics when W/S is 20 at 25 oC, which later
decreases with decreasing the H2O/substrate ratio. Since
solvent plays an effective role in controlling this reaction, we
then investigate the role of catalyst. For this reason, the blank
experiment in the absence of FeNT and FeNS has been
carried out and it showed no hydrolysis/methnolysis product
even after 20 h at W/S 20. This result suggested that the
reaction is truly catalytic in nature. As seen from Figure 4a and
4b, FeNT showed faster hydrolysis kinetics than FeNS that
could be attributed to the coordinated Fe-catechol moieties in
FeNT. Considering FeNT as catalyst, 95% conversion of
styrene oxide is seen even at W/S = 2 and at 60 oC.
Figure 4. Kinetic studies for styrene oxide ring opening hydrolysis reaction
catalyzed by FeNT (a) and FeNS (b); methanolysis reaction of styrene oxide
by FeNT (c) and FeNS (d).
4
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Table 2. Tandem oxidation-cyclization of benzylalcohol with o-
phenylenediamine [a]
Entry
Catalyst Additive
Solvent
T/
oC
Time/ Yield[
b]
h
%
1
2
3
4
FeNT
FeNS
-
TBHP
TBHP
TBHP
-
CH3CN
CH3CN
CH3CN
mesityl
ene
60
60
60
120
24
24
24
18
70
55
10
90
Ir
(2
wt%)
/TiO2-
500[32]
Fe-
5
TBHP
CH3CN
60
24
68
PPOP[1
0]
Figure 5. TEM (a), STEM (b) images of Ag@FeNT and Ag@FeNS
respectively. Powder XRD (c) and XPS (d) patterns of Ag@FeNT and
Ag@FeNS.
[a]Reactions were carried out by 1.5 mmol benzyl alcohol, 0.5 mmol o-
diaminobenzene, 0.004 mmol iron site and 70% TBHP (2.5 mmol, 1.5 mmol
and 0.5 mmol) at an interval of 15 min, 12 h and 20 h
[b]Confirmed from 1H and 13C NMR analysis, given in S5.
Additionally, the Lewis acidity of FeNT and FeNS were
also explored in one-step synthesis of benzimidazole from
benzyl alcohol and o-phenylenediamine and results are shown
in Table 2. The catalytic data suggested that in the presence of
TBHP as oxidant the Fe(III) sites of FeNT and FeNS are
useful for oxidation of benzyl alcohol to benzaldehyde as
Similarly, methanolysis of styrene oxide at different
MeOH/styrene oxide are given in Figures 4c and 4d, where the
formation of 2-methoxy-2-phenylethanol is considered as the
major regioisomer, seen from the results of liquid NMR (Figure
S6). When MeOH/Styrene oxide is 5, FeNT shows 92%
conversion of styrene oxide to 2-methoxy-2-phenylethanol at
60 oC and 21 h, whereas FeNS showed 84% conversion under
same condition. Mechanistically it can be said that the
formation of major regioisomer is dependent on the propensity
of attack by H2O or methanol to the less sterically hindered
epoxide carbon which is activated by Lewis acid FeNS and
FeNT. For this reason, styrene oxide shows faster hydrolysis
rate compared with its methanolysis rate. Additionally, we also
compare the catalytic activity of FeNT with other conventional
catalysts, shown in Table S2.
Again, microporous FeNT and FeNS also contain
physisorbed water (seen from TGA), which can influence the
hydrolysis kinetics of styrene oxide. In order to investigate
such effect, we used the catalyst after drying at 130 oC for 2 h
in vacuum, which shows the non-existence of 1-phenylethane-
1,2-diol as product, indicating that physisorbed water is less
significant in this study. Nevertheless, Lewis acid sites are
indeed essential for this reactions since no hydrolysis product
is observed in absence of Fe(III). For heterogeneous catalysts
that has been studied, Fe(III)-based mesoporous silica,[29]
FeCl3 supported on silica gel,[Error! Bookmark not defined.] [Fe(BTC)]
(BTC: 1,3,5-benzenetricarboxylate) based metal-organic
framework (MOF),[30] Fe-complex containing tridentate
polymers,[31] showed comparable catalytic activity with FeNT
and FeNS. It is noteworthy, the reaction without FeNT and
FeNS but in presence of iron chloride solution did not give any
reaction products while carrying out the reaction of styrene
oxide and methanol at room temperature.
intermediate,
followed
by
cyclization
with
o-
phenylenediamine.[32] FeNT shows 70% yield of benzimidazole
o
o
at 60 C, 24 h, which is milder than 135 C reaction condition,
reported by Reddy et al.[33] For oxidant free synthesis of
benzimidazole, Tateyama et al. prepared Ir on TiO2 support,
which shows very high activity, but it requires high reaction
temperature and complicated procedure for the catalyst
activation with H2 at 500 oC. On the contrary, few other porous
polymers with iron sites are found useful for tandem cyclization
under mild conditions, whereas other shows only oxidation
catalysis.[34] Reusability of FeNT as Lewis acid catalyst was
checked for ring opening reaction of styrene oxide, indicating
that FeNT can be recycled for at least four consecutive cycles
without much loss in yield and selectivity of 1-phenylethane-
1,2-diol (Figure S7).
Ag nanoparticle loaded FeNT and FeNS as CO2 conversion
catalyst
Owing to significant Lewis acid property and high surface
area of FeNT and FeNS, we believe these polymers might be
an ideal platform for CO2 conversion after loading with Ag
NPs,[35] because CO2 can be activated with Ag NPs and
converted to carboxylic acids from alkynes.[36] However, there
remains a strong challenge for efficient and cost-effective
development of stable support materials, which to our
knowledge is mainly limited to MOF-based supports. But the
limitation of utilizing MOFs as stable support renders the full
usefulness of HCPs as polymeric support of Ag NPs[37] for
exploring CO2 fixation with Ag@FeNT and Ag@FeNS. We
characterized these two Ag catalysts by TEM, XRD and XPS
analysis. As shown from TEM and STEM observation (Figure
5a and b), ~7-10 nm Ag NPs are homogeneously distributed
on the support without any agglomeration. Various crystal
planes [(111), (200), (220) and (331)] of Ag is clearly seen at
5
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10.1002/chem.201802319
Chemistry - A European Journal
2θ = 38o, 44.2o, 64.4o and 77.4o, as evidenced from wide angle
powder XRD from Figure 5c. Nevertheless, the mean size of
Ag particles as observed from TEM analysis is correlated from
the particle size (~7.8 nm), calculated from Debye-Scherer
equation.[38]
results for the synthesis of phenylpropiolic acid. As seen from
this table that unlike Ag deposited conventional supports, iron-
catechol based porous support is indeed more reactive in
activating CO2 towards alkynes. This result suggested crucial
role of Ag sites located in close proximity to the Lewis acidic
iron-catechol sites. We also found that solvent plays crucial
role in this CO2 fixation reaction, where the product
phenylpropiolic acid yield increases with increasing polarity of
the solvent. DMSO was found to be the best solvent among all
other solvents like CH3CN, THF, 1,4-dioxane, DMF. Among
several bases, Cs2CO3 showed high yield of the desired
carboxylic acid, which could be because of its ability to
Table 3. CO2 fixation to alkynyl carboxylic acid.[a],[b]
En Catalyst
try
T/
oC
Base
PCO2
/ atm
Solvent
Yiel
d
[b]
(%)
70
1
Ag@FeNT
60 Cs2CO3
1
DMSO
2
3
Ag@FeNS
Ag@FeNT
60 Cs2CO3
60 K2CO3
1
1
CH3CN
DMSO
62
68
4
Ag@FeNT
60 Cs2CO3
5
DMSO
89
40
5
6
Ag@FeNT
Ag@FeNT
25 Cs2CO3
60 Cs2CO3
1
1
DMSO
Dioxane
trac
e
-
7
FeNT
60 Cs2CO3
1
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
[c]
Ag/Al2O3
8
50 Cs2CO3 40
50 Cs2CO3 60
50 Cs2CO3 40
<10
48
Figure 6. Recycling efficiency of Ag@FeNT for the carboxylation of
phenylacetylene (1.0 mmol, 1.5 mmol Cs2CO3, 0.05 g Ag@FeNT catalyst, 5
mL anhydrous DMSO) under 1 atm CO2 pressure and kept at 60 oC for 15 h
[d]
Ag/SiO2
9
Ag/C[e]
10
11
12
<5
50
Ag@NT[f]
60 Cs2CO3
60 Cs2CO3
1
1
deprotonate the terminal alkyne for accelerating the formation
of silver acetylide intermediate. Strong polar solvents like
DMSO favors in the dissolution of Cs2CO3. Therefore we saw
that polar solvent and strong bases are mandatory in order to
achieve successful fixation of CO2 onto the terminal alkynes.[40]
On the other hand, small size Ag NPs (~7-10 nm) are crucial
for the promotion of this reaction (size distribution of Ag, Figure
S10).[41] Compared with FeNT, the BET surface area of
Ag@FeNT appreciably decreases to 305 m2 g-1, as obtained
from the respective N2 sorption isotherm (Figure S11). The
total pore volume also decreases to 0.3 cm3 g-1, which
suggested that it might be possible for Ag NPs to occupy the
cavity of the host framework. Since FeNT contains large
mesopores and small micropores, therefore the size of Ag
permits to occupy the mesoporous cavity of FeNT. When
loading of Ag increases to 6.9 wt%, BET surface area slightly
increases to 340 m2 g-1, because the agglomerated Ag cannot
occupy the pores, rather exists at the outer surface. This
investigation demonstrates that optimum loading of Ag on
FeNT support is necessary for this CO2 fixation reaction.[41]
The merit of Ag@FeNT was further examined from its
recyclability experiment (Figure 6). Product yields as shown in
this figure after several cycles suggested that the Ag@FeNT
catalyst can be recycled for at least four reaction cycles. After
the fourth reaction cycle, almost 5% decrease in catalytic
activity is observed possibly because of little agglomeration of
Ag NPs at the surface of the polymeric nanotubes. Moreover,
XPS analysis of recycled Ag@FeNT showed little shift of
binding energy maximums (Ag3d5/2; 367.6 to 367.1) than fresh
catalyst (Figure 5d). This can be due to the partial oxidation of
Ag(0) during the reaction in DMSO. In order to investigate the
probability of leaching of Ag to the reaction mixture, the liquid
phase of the reaction mixture was collected by hot filtration
Ag@FeNT[
52
g]
[a]Phenylacetylene (1.0 mmol), Cs2CO3 (1.5 mmol), 0.05 g Ag catalyst, 5 mL
anhydrous DMSO after 15 h reaction. [b]Isolated product yield calculated
from NMR. [c 5 wt% Ag in Al2O3. [d]20 wt% Ag in SiO2. [e]5 wt% Ag in carbon.
[f]Catalyst is devoid of iron (Section 1, supporting information). [g]6.9 wt% Ag.
Different electronic state of Ag(0) are clearly seen from
XPS result (Figure 5d), which means the existence of two
bands corresponding to Ag3d3/2 and Ag3d5/2 of Ag
nanoparticles. All these result justify the formation of Ag
nanoparticles on FeNT and FeNS. Initially to explore the
catalytic property of these two catalysts, we choose the
carboxylation of a terminal alkyne phenylacetylene as the
model reaction under the optimized conditions (60 oC, 1 atm
CO2 and DMSO solvent for 15 h). This reaction produces
phenylpropiolic acid in ~70% yield over Ag@FeNT, and ~62%
yield over Ag@FeNS. The formation of phenylpropiolic acid
from phenylacetylene was confirmed from NMR results (Figure
S8 and S9). On the contrary, the non-catalyzed reaction
showed only 2.5% yield of phenylpropiolic acid. When the
reaction has been carried out with FeNT itself, it also showed
very poor yield of phenylpropiolic acid. This investigation
actually suggests the necessity of Ag(0) for activation and
conversion of CO2. However, when Fe-sites are removed the
catalytic activity of Ag@NT deceases considerably (entry 11),
suggesting the Lewis acid sites as present in FeNT play
significant role for this reaction and this is also observed for
other supports.[39] In Table 3 we have summarized our catalytic
6
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10.1002/chem.201802319
Chemistry - A European Journal
PLASAMSPEC-II inductively coupled plasma atomic emission
spectrometry (ICP-AES). Powder X-ray diffraction (PXRD) analysis
has been carried out on a Rigaku D/Max2500 PC diffractometer
with Cu Kα radiation (λ=1.5418 Å) over the 2θ range of 5-80o at a
scan speed of 5o per min at room temperature. Bruker DPX-300
through the intermediate stage of the reaction and the filtrate
was analyzed by inductively coupled plasma-mass
spectrometry (ICP-MS). A very low amount of dissolved Ag
(less than 0.2% of the total Ag) was detected in the solution at
the end of the reaction. This result suggested that Ag NPs are
strongly bound to the surface of the catalyst and they do not
leach out during the liquid phase catalytic reactions. Thus,
partial poisoning and pore blocking of Ag@FeNT catalyst could
be responsible for little decrease in the catalytic activity after
four reaction cycles.
1
NMR spectrometer was used to measure the H and 13C NMR of
the reaction products in liquid state. Particle sizes were determined
from Nano Measurer 1.2 software.
Synthesis of FeNT
A 50 mL two-neck round bottom flask containing magnetic stirrer
and reflux condenser was charged with 0.2 g (2 mmol) catechol,
0.6 g (4 mmol) FeCl3 and 15 mL dichloroethane. The mixture was
stirred and to it 0.34 mL (0.3 g, 4 mmol) dimethoxymethane was
added to initiate the polymerization reaction and then heated in an
Conclusion
Iron(III) containing hyper-cross-linked microporous
polymers with well-defined nanotube (FeNT) and nanosphere
(FeNS) morphologies can be synthesized via acid catalyzed
o
oil bath at 80 C for 20 h. At the end, the dark mixture was diluted
with methanol and filtered. The solid residue was thoroughly
washed with ethanol, THF, methanol and kept for 30 h under
soxhlet extraction with refluxing methanol. Finally, the dark brown
solid was dried under vacuum, which gives 80% yield of the
product. CHN elemental analysis of FeNT suggested the presence
of C = 39.7 %, O = 44.5 %, H = 4 %.
condensation-polymerization
of
catechol
and
dimethoxymethane in the presence of FeCl3. The resulting
polymers exhibit high activity as Lewis acid catalyst.
Furthermore, FeNT and FeNS can be utilized as support
material for the successful immobilization of Ag NPs and the
resulting materials act as efficient catalysts for the one-step
selective fixation of CO2 onto terminal alkynes to the
respective carboxylic acid. Nevertheless, the utility of catechol
as functional moiety in building the porous support has huge
potential in catalysis due to its strong binding affinity as
chelating ligand for stabilization of inorganic metal ions and
metallic nanoparticles. Utilizing catechol as cheap chemical,
the bottom-up chemistry route for the synthesis of Fe-
Synthesis of FeNS
In a 50 mL two-neck round bottom flask containing magnetic stirrer
and reflux condensor, 0.2 g (2 mmol) catechol was dissolved in 15
mL dichloromethane. Then 0.6 g (8 mmol) dimethoxymethane was
added slowly and stirred well for 30 min at room temperature (25
oC). Next, 0.003 mol (1.2 equivalent with catechol)
trifluoromethanesulfonic acid (CF3SO3H) was added very slowly
and continued the addition for 30 min. The whole mixture was kept
containing nanomaterials with
0 and 1D nanostructures
o
o
under stirring at 25 C for 1 h and then heated at 60 C for 24 h.
During the addition of CF3SO3H, the initial solution became reddish,
which later turn to brown while prolonging the reaction time. The
final product was collected by filtration and washed thoroughly with
water, ethanol, THF, acetone and dried in vacuum. Yield of NS
was 70%. For loading iron in NS, 0.1 g NS was dispersed in 15 mL
reported herein has huge potential in designing bifunctional
catalysts comprising both Lewis acidic and metal nanoparticles
as active sites, which can be explored in other tandem organic
reactions in future.
o
THF and the whole mixture was stirred with 0.3 g FeCl3 at 50 C.
After 24 h, the mixture was filtered and the residue was collected,
washed with copious amount of water, ethanol, THF, acetone until
the filtrate shows no color. The dark brown product has been
collected and purified by rigorous soxhlet extraction for 72 h in
refluxing methanol and this sample is designated as FeNS.
Experimental Section
Chemicals and reagents
All analytical grade reagents were used as received unless
otherwise stated. Catechol, dimethoxymethane, dichloromethane,
Synthesis of Ag@FeNT and Ag@FeNS
trifluorosulfonic
acid,
styrene
oxide,
benzylalcohol,
phenylacetylene were purchased from Sigma-Aldrich Company,
Ltd. (USA). For the CO2 fixation reaction, CO2 cylinder with high
purity CO2 (99.99%) was used. All solvents and chemicals required
for catalysis were brought from Merk and Spectrochem Pvt. Ltd.
Ag nanoparticles were loaded onto FeNT and FeNS surface by
NaBH4 reduction method. For this, we at first prepare colloidal Ag
NPs by mixing 100 mg AgNO3 with 10 mL acetonitrile containing
0.5 mmol TRIS [2-Amino-2-(hydroxymethyl) propane-1,3-diol] and
stirred for 5-10 min. Then, 10 mL NaBH4 solution (0.8 mmol mL-1)
in ethanol was added and stirred for additional 20 h and the
resulting colloidal Ag NPs was stored at 4 oC. Next, 0.5 g
FeNT/FeNS was dispersed in 10 mL of TRIS-stabilized Ag NPs
and stirred for 12 h at 25 oC. The Ag loaded polymers were
collected by centrifugation and washed with distilled water, ethanol
until no Ag ions are detected in the filtrate. It was found that
Ag@FeNT contains 3.4 wt% Ag and Ag@FeNS showed 3 wt% Ag
loading, as estimated from inductively coupled plasma atomic
emission spectroscopic (ICP-AES) analyses.
Instrumentation
N2 adsorption-desorption isotherms are measured at -196 oC
by using a Micromeritics ASAP2020 adsorption analyzer. Samples
were degassed at 120 oC for 6 h before analysis. Pore size
distribution was measured by using density functional theory (DFT)
model using N2 at 77 K on carbon, slit pores. Thermogravimetry
analysis (TGA) has been carried out by NETZSCH STA 449F3
analyzer starting from 30 to 800 oC with a heating rate of 10 oC
min-1 in N2. X-ray photoelectron spectroscopic (XPS) data was
recorded on a VG ESCALAB MK2 apparatus by using AlKα (hλ =
1486.6 eV) as the excitation light source. Solid state 13C NMR
spectra were obtained with a Bruker 500 MHz spectrometer
equipped with a magic-angle spin probe using a 4 mm ZrO2 rotor
and the signals were referenced to glycine (C2H5NO2). Fourier
transform infrared spectra (FT-IR) were recorded on a Nicolet
Nexus 470 IR spectrometer (KBr pellets were prepared).
Inductively coupled plasma (ICP) analysis was conducted on
Ring opening reaction of Styrene oxide
For the epoxide ring opening reaction, the catalyst FeNT (10 mg =
0.075 μmol Fe) was added into a solution of styrene oxide (0.25 g,
1 mmol) and water. The amount of water was adjusted according
to various water/styrene oxide ratio (20, 5 etc.) and the solution
was stirred at room temperature or at elevated temperature to get
7
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10.1002/chem.201802319
Chemistry - A European Journal
the product. The progress of the reaction was monitored by a
Agilent gas chromatography (GC) fitted with a J&W GC HP-5
capillary column (30 m x 0.32 mm x 0.25 mm) and FID detector.
Similarly, methanolysis reactions were also studied at various
methanol/styrene oxide ratio, while other conditions are same as
used during hydrolysis reactions.
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autoclave equipped with a magnetic stirrer. For a typical
reaction, phenylacetylene (1.0 mmol), Cs2CO3 (0.489 g, 1.5
mmol), 0.05 g Ag catalyst were taken in an autoclave along
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Keywords: Nanotubes; immobilization; Lewis acid
catalysis; tandem catalysis; CO2 fixation.
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Table of Content
FULL PAPER
Arindam Modak,*[a,b] Piyali Bhanja and Asim Bhaumik*[a]
Microporous nanotubes and nanospheres with iron-
catechol sites: efficient Lewis acid catalyst and support
for Ag nanoparticles in CO2 fixation reaction
Fe-catechol POP for acid catalysis and CO2 fixation.
Fe-catechol-based microporous polymers with different
morphologies are synthesized under different synthesis
conditions and they demonstrated efficient Lewis acid
catalysis and support of Ag NPs for CO2 fixation reaction
for the carboxylation of terminal alkynes.
10
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