Macroporous polystyrene degraded and functionalized chromium MPS-Cr(iii)-alen complex as a sustainable porous catalyst for CO2fixation under atmospheric pressure and selective oxidation of aromatic alkenes

Article abstract of DOI:10.1039/d0nj02972f

The benign synthesis of a macroporous polystyrene (MPS) supported chromium MPS-Cr(iii)-alen catalyst has been carried out precisely and characterized using different spectroscopic techniques such as Fourier-transform infrared (FT-IR) spectroscopy, ultraviolet visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The MPS-Cr(iii)-alen catalyst shows noteworthy performance for the sustainable production of cyclic carbonates from various substituted epoxides employing a CO2 balloon at room temperature under solvent free conditions. Additionally, the MPS-Cr(iii)-alen material was also evaluated for the selective oxidation reaction of aromatic alkenes in the presence of 30% aqueous H2O2. This material can be easily reused for several runs with almost consistent efficiency. This journal is

Full text of DOI:10.1039/d0nj02972f

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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
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DOI: 10.1039/D0NJ02972F
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Received 00th January 20xx,
Accepted 00th January 20xx
Macroporous Polystyrene Degraded and Functionalized
Chromium MPS-Cr(lll)-alen Complex as a Sustainable Porous
Catalyst for CO2 Fixation under Atmospheric Pressure and
Selective Oxidation of Aromatic Alkenes
DOI: 10.1039/x0xx0
www.rsc.org/
Priyanka Paula,c, Noor Salamb, Aslam Khane, Debasis Dasb , Seikh Mafiz Alamc* and Sk. Manirul
Islama*
Abstract
The benign synthesis of macroporous polystyrene supported chromium MPS-Cr(III)-alen catalyst has been carried out
precisely, which is characterized by different spectroscopic techniques like FT-IR, UV-Vis spectra, TEM, TGA and SEM. The
MPS-Cr(III)-alen catalyst shows noteworthy performance for the sustainable production of cyclic carbonates from various
substituted epoxides employing CO2 balloon at room temperature under solvent free condition. Additionally MPS-Cr(III)-
alen material has also elevated for the selective oxidation reaction of aromatic alkenes in presence of 30% aqueous H2O2.
This material can be easily reused for several runs with almost consistent efficiency.
metal complexes of iron,15 copper,16 manganese,17 and nickel18 are
reported for the oxidation reaction. However, the environmental
concern and other important factors associated with these reagents
limit their synthetic utility. The use of an eco-friendly, green and
mild oxidant like hydrogen peroxide19 would be more acceptable
over tert-butyl hydroperoxide17 and it is also more reactive than
another green oxidant, like molecular O2.20
Introduction
With dramatically increasing the environmental and societal
concerns about carbon emission, the utilization of carbon dioxide as
a sustainable carbon source for the synthesis of fine chemicals has
been gained remarkable interest for modern scientific community.
Indeed, there are sequestration methodologies to reduce
concentrated CO2 in the atmospheric layer and production of
valuable fine chemicals.1 Among them, the production of cyclic
organic carbonates via atmospheric CO2 fixation process are one of
the most promising strategies and desired compounds have
currently wide applicability in industrials, pharmaceuticals and fine
chemicals.2-4 Notable that in general the formation of cyclic
carbonates was carried out under homogeneous conditions under
mild conditions but they prevent from the separation of catalyst.
Meanwhile to overcome the limitations of this system,
heterogeneous catalytic system are still of great importance.5-8 Thus
design and synthesis of heterogeneous catalyst for CO2 fixation
reaction under green conditions is of enormous interest to reduce
global warming.
Thus, it is necessary to develop a novel heterogeneous catalytic
environment for benign synthesis of cyclic organic carbonates
employing CO2 and also selective oxidation reaction of aromatic
alkenes. Among the methods developed, different functionalized
polystyrene anchored catalytic systems are promising material as a
porous support material for this purpose due to their large pore,
easy separation from the reaction mixture, recyclability and good
thermal stability.21 Therefore, the polystyrene supported catalysts
containing ammonium salt or amino group, and the polar
macroporous adsorption resin, are efficient and reusable
heterogeneous basic catalysts for the synthesis of cyclic carbonates
employing CO2.2d There are also some heterogeneous catalysts
including polymeric ionic liquids, metal organic frameworks,
conjugated microporous polymer, polyamines, functionalized ion-
exchange resins, polystyrene-supported ionic liquids, etc. for the
cycloaddition of epoxide with CO2.6b
In this context, it is also highlighted that the catalytic oxidation
reactions of aryl alkenes have also received much attention in
recent years9 and these desired compounds are more significant
intermediates for the fine chemical industry and also
pharmaceuticals, etc.10-11 The traditional aromatic alkene oxidation
reactions require some metal oxides which are not safe and
environmentally hazardous.12 Hence an efficient atom-economical
and environment friendly process should necessarily be developed
for this purpose. Zeolite,13 alumina,11 MCM-41,14 etc. are reported
as the heterogeneous solid supports. Some polymer supported
In this context, keeping all these factors in mind we have
designed a new catalytic system composed of macroporous
polymer anchored chromium material as a novel recyclable catalyst
for the production of value added cyclic carbonates from epoxides
via chemical fixation of carbon dioxide and also selective oxidation
reaction of aromatic alkenes under ambient conditions.
Experimental
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India.
bDepartment of Chemistry, The University of Burdwan, Burdwan, , W.B., India.
cDepartment of Chemistry, Aliah University, Newtown, Kolkata-700156, W.B., India
dKing Abdullah Institute for Nanotechnology, King Saud University, Riyadh, 11451, Saudi
Arabia
Preparation of macroporous polymer supported chromium
catalyst as MPS-Cr(III)-alen
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1 gm of our prepared polymeric ligand (MPS-alen) was dissolved
Journal Name
The MPS-alen ligand and MPS-Cr(III)-alen catalystVaierweAsrthicolewOnnlinine
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in ethanol (20 ml) and then 7 ml of 1% (W/V) CrCl3 solution was Fig. 1. The elemental analysis (CHN analysis) data of porous MPS-
DOI: 10.1039/D0NJ02972F
poured drop wise into the resulting solution very carefully followed alen and MPS-Cr(III)-alen materials are shown in Table 1. The
by continuous stirring. After that, the resulting mixture was chromium metal loading of 2.75% on the MPS-Cr(III)-alen catalyst
subjected on the oil bath under refluxed condition for 12 h. Then was elevated by Atomic absorption spectroscopy. Elemental studies
the resultant solution was allowed to reach at RT and a light yellow of MPS-Cr(III)-alen by ICP-AES experiments afforded Cr loading of
coloured product named as MPS-Cr(III)-alen was obtained after 2.83 wt%.
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filtered and allowed to wash with EtOH and then dried under
Table 1 Chemical composition of MPS-alen and MPS-Cr(III)-alen
vacuum (Scheme 1).
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materials
General experimental procedure for epoxide and CO2 coupling to
synthesize cyclic carbonate using MPS-Cr(III)-alen material
Compound
MPS-alen
MPS-Cr(III)-
alen
Colour
Yellow
Light
C %
83.74
H %
N % Cl % Cr %
8.36 5.82 1.68
-
To carry out the coupling reaction with epoxide and CO2, an
organic epoxide (2 mmol), tetra-n-butyl ammonium bromide (n-
Bu4NBr) (1 mmol) and catalyst (50 mg) were poured in a 25 ml
round bottom flask. After purge of CO2 balloon inside the flask
mixture was subjected to stir at room temperature. Upon
completion, the catalyst was filtered and washed thoroughly with
H2O followed by acetone for multiple times and dried out. The
extraction of filtrate part was performed with ethyl acetate and
then washed with water and the combined organic layer was
removed by anhydr. Na2SO4. The concentrated product was
analyzed by GC analysis to determine the yield of the desired cyclic
carbonate.
75.84
7.91 5.68 2.46 2.75
yellow
General experimental procedure for catalytic oxidation of
aromatic alkenes using MPS-Cr(III)-alen material
The oxidation of aromatic alkene was conducted by stirring 5
mmol of the substrate alkene with 30% aqueous H2O2 as oxidant, 50
mg MPS-Cr(III)-alen catalyst and 10 ml acetonitrile in a 50 ml R.B.
flask. The reaction was subjected to stir continuously for 8 h at 60
°C. After completion, the mixture was cooled at normal
temperature and diluted with ethyl acetate. After that the solid part
was filtered out from reaction medium and it was dried through
anhydrous salt Na2SO4 and again filtered. The corresponding
desired oxidized products were analyzed through GC technique.
Fig. 1 (A) MPS-alen ligand and (B) MPS-Cr(III)-alen material
UV-Vis spectroscopic studies
UV-Vis absorbance spectrum of MPS-alen ligand and MPS-
Cr(III)-alen catalyst are presented in Fig. 2. The electronic spectra of
the MPS-alen ligand (Fig. 2A) is appeared at 262 nm owing to the
transition arising from π→π* and another band at 325 nm (Fig. 2A)
is highlighted for the transition (n→π*) of the azomethine group. In
the electronic spectra of the MPS-Cr(III)-alen material, it shows new
absorption peak at the range of 225-550 nm due to π→π* and
n→π* bands which was shifted to slightly lower (Fig. 2B). This fact
clearly provided the metal ion grafted over the MPS-alen ligand.22b
Fig. 2 UV-Vis absorption spectra of MPS-alen and MPS-Cr(III)-alen
materials.
Scheme 1. Schematic route for the synthesis of MPS-Cr(III)-alen catalyst.
FT-IR spectra
Results and discussion
The FT-IR spectra of MPS-alen and MPS-Cr(III)-alen materials are
represented in Fig. 3. The figure indicates that there is a broad
absorption band in the MPS-alen ligand at 3420 cm-1, suggests the
presence of phenolic -OH stretching frequency. The IR spectrum of
Characterization of MPS-alen and MPS-Cr(III)-alen materials
Elemental analysis
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polymer grafted ligand MPS-alen exhibits a sharp peak at 1628 cm-1 mapping analysis shows presence of the various elements in
owing to C=N vibration.22 In MPS-alen ligand, the absorption band polymer matrix. The images confirm a uniDfoOrIm: 10d.1is0t3Vr9iieb/wDuA0tirNoticJnl0e2oO9fn7li2CnFer
at 1628 cm-1 which moved to lower at 1611 cm-1. It indicates the metal (Fig. 5I) over MPS-alen ligand (Fig. 5A-D) and the good
complexation of the ligand to Cr metal through azomethine group dispersion of all the elements present in the samples.
linkage. Two new bands appeared at 565 cm-1 indicating the
presence of Cr-N bond23 and another absorption band at 460 cm-1
signifies the Cr-O bond24 present in the catalyst MPS-Cr(III)-alen (Fig.
3 inset) respectively.
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Fig. 5 Elemental mapping of MPS-alen ligand [A] C, [B] Cl, [C] N, [D]
O and MPS-Cr(III)-alen catalyst [E] C, [F] Cl, [G] N, [H] O, [I] Cr
element.
Thermal stability
The TGA plots of the macroporous polymer grafted MPS-alen as
ligand and MPS-Cr(III)-alen catalyst are depicted in Fig. 6. The
thermal stability of the materials was measured over a range of 30
to 600 °C under N2 atmosphere at heating rate of 12 °C per minute.
It is demonstrated that thermal stability of the chromium metal
grafted macroporous polymer was fairly superior to that of
polymeric MPS-alen ligand. The thermal stability of the polymeric
ligand is observed up to 155 °C as shown in Fig. 6 and beyond this
temperature, decomposition of the ligand is started. The TG
Fig. 3 FT-IR bands of MPS-alen and MPS-Cr(III)-alen materials.
SEM and EDAX analysis
The SEM images of porous MPS-alen ligand and MPS-Cr(III)-alen
analysis of MPS-Cr(III)-alen complex shows that the decomposition
catalyst are shown in Fig. 4. As seen from the images that the MPS-
of the catalyst is started from 245 °C which explains the
[Cr(III)alen] catalyst depicts a smoothening of the rough polymeric
comparatively more thermal stability of the complex than its
surface of the ligand due to decoration of metal on the solid matrix.
supporting ligand. Beyond 445 °C temperature, final decomposition
This indicates the presence of some irregular particles architectured
of the catalyst occurs as observed from Fig. 6.
on the smooth surface of the ligand.
Fig. 4 SEM images of (A) MPS- alen ligand and (B) MPS-Cr(III)-alen
catalyst.
Moreover, the EDAX pattern of polymer anchored MPS-alen ligand
Fig. 6 TGA plot of MPS-alen ligand and MPS-Cr(III)-alen catalyst.
and its MPS-Cr(III)-alen material are depicted in Fig. S1. EDAX
analysis of catalyst also provided the presence of Cr metal over the
polymeric matrix. Thus the result provided that Cr metal containing
on the surface of MPS-[Cr(III)alen] catalyst. The elemental mapping
images of ligand MPS-alen and catalyst MPS-Cr(III)-alen are given in
Fig. 5. To investigate the morphological properties, EDAX elemental
TEM Analysis
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TEM images of the macroporous polymer grafted MPS-alen resolution O 1s spectrum, the peak at around 531 eVViieswaAsrctircilbeeOdnlitnoe
ligand and MPS-Cr(III)-alen catalyst are given in Figure 7 to identify the oxygen atoms of Cr-O. Additionally, the peak is located at the
DOI: 10.1039/D0NJ02972F
the morphological properties of the materials.
binding energy of 533.2 eV, which is ascribed to O of C-O.
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Fig. 7 The UHR-TEM images of MPS-alen ligand at three different
magnifications: 500 nm (a), 200 nm (b) and 50 nm (c); and of MPS-
Cr(III)-alen catalyst at five different magnifications: 500 nm (d) 200
nm (e), 100 nm (f), 50 nm (g) and 5 nm (h).
Fig. 8 XPS survey scan measurement on MPS-Cr(III)-alen catalyst
(a), the XPS spectrum of MPS-Cr(III)-alen material illustrating the
bands corresponding to chromium featuring certain oxidation state
(Cr3+) (b), C 1s narrow scan (c), N 1s narrow scan (d) and O 1s
narrow scan measurement (e) on MPS-Cr(III)-alen catalyst.
As noticed from the TEM images it is clearly seen the well-
dispersed chromium metal particles (black dots) over polymer
matrix.
Catalytic activity of MPS-Cr(III)-alen material
The macroporous polymer supported Schiff base complex MPS-
Cr(III)-alen has been employed as a heterogeneous catalyst, for the
synthesis of organic substituted carbonates from various epoxides
employing CO2 under atmospheric pressure and also in the catalytic
oxidation reaction of aromatic alkenes using aqueous H2O2 as a
green oxidant.
XPS analysis has been conducted to get a clear approach about the
oxidation state of Cr site and different binding sites with the ligand.
Figure 8 demonstrates the XPS spectra of MPS-Cr(III)-alen catalyst,
which suggested the presence of several components such as N 1s,
C 1s, O 1s and Cr 2p. Figure 8b displays the XPS spectrum of Cr 2p of
MPS-Cr(III)-alen catalyst having two several binding energies at
577.3 eV and 586.9 eV, corresponds to two parts of splitting (i.e.
2p3/2 and 2p1/2), respectively. These results suggested the presence
of Cr(III) in MPS-Cr(III)-alen catalyst.
Carbon dioxide fixation with epoxides catalyzed by MPS-Cr(III)-
alen material
The catalytic performance of the porous catalyst MPS-Cr(III)-
alen was explored by considering the coupling reaction of
epichlorohydrin employing carbon dioxide under atmospheric
pressure as a model reaction (Scheme 2). The performance of the
chromium-catalyzed chemical fixation is governed by various
reaction parameters like the amount of the cocatalyst (TBAB), time
period of reaction and catalyst amount.
As noticed from XPS spectrum of MPS-Cr(III)-alen catalyst, the
high-resolution picture of C 1s spectrum on deconvolution analyzes
the bands at 286.4, and 284.6 eV binding energies which
corresponds to the respective binding energies of C of C-N/C-O, and
C=C/C-C/C-H.24b As noticed from N 1s image of MPS-Cr(III)-alen
catalyst, the band from N 1s at 400.0 eV corresponds to –N=, which
can bind to the active Cr(III). Of note, this high-resolution N 1s
image displays an additional band, located at binding energy of
404.3 eV, being referenced to  excitation. Observed from the high-
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conditions are 50 mg of catalyst, 1 mmol of n-Bu4NBr over CO2
fixation reaction under atmospheric conditions at RT fVoierw2A4rthic.le Online
DOI: 10.1039/D0NJ02972F
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The controlled experiments for the fixation of CO2 with epoxides
were carried out by varying chromium support and TBAB and the
results are shown in Table S1. In presence of TBAB, better
conversion of epoxide was observed when Cr(III)-alen was used as a
catalyst compared to the absence of TBAB (Table S1, entries 1 and
2). It is also detected that if in absence of TBAB as co catalyst Cr
metal free macroporous polymer was used as catalyst, then
negligible conversion was found (Table S1, entry 3). Further table
S1, entry 4 shows that in absence of cocatalyst TBAB, negligible
conversion occurs when MPS-Cr(III)-alen was used as a catalyst. The
best result was obtained when the experiment was carried out with
1 mmol of TBAB (Table S1, entry 5).
Scheme 2. Production of cyclic carbonate from epichlorohydrin and
CO2
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Initially, the reaction conditions were optimized by considering
various amount of n-Bu4NBr as co-catalyst shown in Table 2. It is
observed that in absence of co-catalyst, i.e., n-Bu4NBr; the
negligible conversion was found (Table 2, entry 1) and with
increasing the amount of TBAB, conversion of the model substrate
increases (Table 2, entries 2 and 3). It is shown that the best result
was obtained when the experiment was carried out with 1 mmol of
n-Bu4NBr (Table 2, entry 4).
Table 4. Optimization of the amount of catalyst for the synthesis of
epichlorohydrin carbonatea
Table 2. Optimization of the amount of n-Bu4NBr for the synthesis
Conversion (%)b
of epichlorohydrin carbonatea
Entry
Amount of catalyst (mg)
1
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0
Trace
73
Amount of n-Bu4NBr
Entry
Conversion (%)b
(mmol)
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50
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0
Trace
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0.5
90
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99
1.0
99
aReaction conditions: epichlorohydrin (2 mmol), co-catalyst (1
mmol), CO2 gas (1 atm.), catalyst, room temperature, time 24 h.
bDetermined by GC.
aReaction conditions: epichlorohydrin (2 mmol), n-Bu4NBr, catalyst
(50 mg, 2.75 wt% Cr loading), CO2 gas (1 atm.), time 24 h, room
temperature. bDetermined by GC.
Table 5. The scopes of various substituted epoxides in presence of
CO2 catalyzed by polymeric MPS-Cr(III)-alen catalysta
It is undoubtedly seen that the synthesis of organic carbonate is
significantly influenced by reaction time shown in Table 3. It is
noted that with increase in reaction time, the conversion of the
model substrate was improved (entries 1-3). When the reaction was
continued for 24 h in presence of n-Bu4NBr, the conversion of
epichlorohydrin was reached to a maximum value of 99% (entry 4).
Yieldb
(%)
Entry
Substrate
Product
1
97
Table 3. Optimization of reaction time for epichlorohydrin
carbonate synthesisa
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93
Entry
Time (h)
Conversion (%)b
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99
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24
aReaction conditions: epichlorohydrin (2 mmol), n-Bu4NBr (1
mmol), catalyst (50 mg, 2.75 wt% Cr loading), CO2 gas (1 atm.),
room temperature. bDetermined by GC.
Table 4 shows the influence of the catalyst amount on the reaction.
In a controlled experiment involving the sole use of cocatalyst, the
reaction did not proceed without the catalyst (entry 1). The
progress in the conversion of the model reaction was detected with
increase in the amount of catalyst (Table 4, entries 2-4) and
maximum conversion was obtained when catalyst amount was 50
mg (Table 4, entry 5). But no further increase in the conversion
occurred on addition of excess catalyst. Therefore, after screening
of all the parameters it is concluded that the optimization reaction
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aReaction conditions: substrate (2 mmol), cocatalyst n-Bu4NBr (1
mmol), catalyst (50 mg, 2.75 wt% Cr loading), CO2 gas (1 atm.),
room temperature. bDetermined by GC.
To further examine the catalytic activity of MPS-Cr(III)-alen catalyst
in presence of the co-catalyst, the fixation of CO2 with epoxides was
carried out with several typical substituted epoxides under
optimized reaction conditions (Table 5). Various aliphatic and
aromatic terminal epoxides substitutes were examined.
Epichlorohydrin gave excellent yield of desired five-membered
organic cyclic carbonate under optimized reaction conditions. The
other aliphatic epoxides (Table 5, entries 2-5) also showed high to
excellent yields of the substrates to form the corresponding desired
cyclic carbonates. After that remarkable successful application of
our presented methodology in case of aliphatic substrates we tried
to extend in the aryl terminal epoxides like styrene oxide, glycidyl
phenyl ether and benzyl glycidyl ether which gave the respective
carbonates with moderate-to-good yields (entries 6-8). From this
examination over all parameters we have concluded that MPS-
Cr(III)-alen gives the excellent catalytic performance for CO2
fixation.
Scheme 3. Proposed mechanism of MPS-Cr(III)-alen catalyzed cyclic
carbonates production
Catalytic oxidation of aromatic alkenes by MPS-Cr(III)-alen
material
The oxidation reaction of MPS-Cr(III)-alen complex was
examined over the aromatic alkenes under mild reaction condition
by the use of hydrogen peroxide as a green oxidant. In this context,
we investigated the oxidation reaction of the aromatic alkenes
considering styrene as a model substrate (Scheme 4). Now, to
determine the optimal reaction conditions, some major parameters
including oxidant, solvent, styrene to H2O2 molar concentration
ratio, temperature were checked.
From some previously reported works,1c,3e it is observed that in the
reaction mechanism regarding the synthesis of cyclic carbonate
employing CO2 with epoxides, the most effective catalyst combine a
Lewis-acid to activate the epoxide with a nucleophile to ring open
the epoxide and subsequently act as a leaving group and a Lewis- or
Brønsted-base to activate the CO2. Another report1d on the
mechanism for similar reaction shows that for metal-based
catalysts, the reaction proceeds via direct CO2 activation and
activation of high energy co-reactants, e.g., epoxides.
The plausible reaction mechanism of the carbon dioxide fixation
reaction over epoxides to synthesize organic carbonates is
represented in Scheme 3. In step I, the active site of the catalyst
MPS-Cr(III)-alen initiates the coupling reaction by coordination with
O atom of the epoxide for the activation of epoxy ring for the
reaction. In Step II, the bromide ion (originated from n-Bu4NBr)
attacks C atom of the coordinated epoxide and the ring is opened to
form a halo-alkoxide. The O- anion of the opened epoxy ring reacts
with CO2 to produce an intermediate, i.e., an alkyl carbonate anion
in step III. Then this intermediate in step IV takes place through
ring-closer to generate the desired cyclic carbonate and
simultaneously the catalyst is regenerated. Hence the actual reason
behind this coupling reaction to occur under mild conditions is the
synergistic effect of the catalyst and co-catalyst tetra-n-butyl
ammonium bromide (n-Bu4NBr). Activation of the nucleophile is not
necessary as long as the epoxide substrate is activated by binding to
the Lewis-acidic Cr(III) center.25
Scheme 4. Oxidation of styrene catalyzed by MPS-Cr(III)-alen
material
The effect of different oxidants like 70% aq. TBHP, 30% aq. H2O2, O2
(1 atm.) on the styrene conversion and selectivity of desired
product is shown in Table 6. Notable that the reaction was not
underwent without an oxidizing agent under the optimized
conditions (entry 1). In presence of 30% aq. H2O2, the styrene
conversion of 48% was detected with 69% selectivity of
benzaldehyde (entry 2). Furthermore, when the reaction was
conducted in presence of other oxidizing agent like 70% aq. TBHP,
then selectivity of benzaldehyde decreases from 69% to 49% (Table
6, entry 3). Styrene conversion was observed very low when O2 (1
atm.) was used instead of H2O2 (entry 4). In addition, it is noted that
when the reaction underwent without the catalyst then conversion
of styrene was observed in trace (entry 5).
This reaction is conducted under various types of solvent like protic
and aprotic over MPS-Cr(III)-alen catalyst keeping the other
parameters constant shown in Table 7. Solvent has a decisive role in
the behaviour of a catalyst for a catalytic reaction. Both aprotic and
6 | J. Name., 2012, 00, 1-3
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protic solvents like isopropanol, n-Butanol, acetone, acetonitrile Thus the optimum styrene/H2O2 molar ratio of 3 was chosen to
and ethanol were used as the reaction medium to explore the achieve the best result for styrene oxidationDreOaI:c1t0io.1n0.3V9ie/wDA0rNticJl0e2O9n7li2nFe
effect of solvent over the styrene conversion and selectivity of
To explore the catalytic performance of MPS-Cr(III)-alen material,
major product benzaldehyde.
the oxidation of styrene in presence of H2O2 was screened in the
temperature range of 40-80 °C (Table 9).
Table 6. Optimization of oxidant on the reactiona
8
9
Table 8. Optimization of styrene to H2O2 molar concentration ratio
Conversion Selectivity
for styrene oxidationa
Entry
Catalyst
Oxidant
(%)b
(%)b
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Molar ratio
Selectivity
Entry
Conversion(%)b
MPS-Cr(III)-
alen
(%)b
1
2
3
4
5
Nil
0
0
Styrene:H2O2
MPS-Cr(III)-
alen
30% aq.
H2O2
1
2
3
4
1
2
3
4
15
29
48
51
76
71
69
62
48
65
~5
~8
69
49
68
72
MPS-Cr(III)-
alen
70% aq.
TBHP
MPS-Cr(III)-
alen
O2 (1
atm.)
aReaction conditions: styrene (5 mmol), 30% aq. H2O2, acetonitrile
(10 mL), catalyst (50 mg), 60 °C, 8 h. bDetermined by GC.
30% aq.
H2O2
Nil
The experimental results show that with rise in temperature,
conversion of styrene increases along with the decrease in the
selectivity of the major product benzaldehyde. At temperature
range of 40-60 °C, styrene conversion increased remarkably with
aReaction conditions: styrene (5 mmol), oxidant, acetonitrile (10
ml), catalyst (50 mg), 60 °C, 8 h. bDetermined by GC.
In presence of n-Butanol, 23% conversion of styrene was observed the enhancement of temperature (entries 1-3). But 48% conversion
and gave 55% selectivity of desired product (benzaldehyde) (Table of styrene was observed with 69% selectivity of benzaldehyde at 60
7, entry 1). It is observed that rate of conversion of styrene also °C (entry 3). In addition, styrene conversion increases from 48-55%
increases when reaction was conducted with increasing the polarity and the benzaldehyde selectivity diminishes from 69-57% (entries
of solvent (entries 2-4). Consequently it revealed that the maximum 3-5). In this case, with temperature enhancement the selectivity of
conversion of styrene with excellent selectivity of desired product other by-products increases while that of the major product
was observed when highly polar aprotic solvent like acetonitrile was benzaldehyde declines. Thus for styrene oxidation the optimum
used (entry 5).
reaction temperature was 60 °C.
Table 7. Influence of solvent over the styrene oxidationa
Table 9. Optimization of temperature for the styrene oxidationa
Selectivity
(%)b
Temperature
Entry
Solvent
Conversion(%)b
Entry
Conversion(%)b
Selectivity (%)b
(°C)
40
50
60
70
80
1
2
3
4
5
n-Butanol
Isopropanol
Acetone
23
26
35
39
48
55
58
50
63
69
1
2
3
4
5
30
42
48
53
55
85
81
69
64
57
Ethanol
Acetonitrile
aReaction conditions: styrene (5 mmol), styrene:H2O2 molar ratio 3, aReaction conditions: styrene (5 mmol), styrene:H2O2 molar ratio 3,
acetonitrile (10 mL), MPS-Cr(III)-alen (50 mg), 60 °C,
8
h. acetonitrile (10 mL), catalyst (50 mg, 2.75 wt% Cr loading), time 8 h.
bDetermined by GC.
bDetermined by GC.
Furthermore, we have also investigated the effect of oxidant (30% Table 10. Substitute scopes of aromatic alkenes for oxidation
aq. H2O2) amount by varying the styrene/H2O2 molar ratio, reaction catalyzed by MPS-[Cr(III)alen] under optimized conditions a
presented in Table 8. A sharp increase of styrene conversion was
observed with increment of the styrene/H2O2 molar ratio from 1 to
3 (entries 1-3) but decrease in the major product benzaldehyde
selectivity. At substrate/oxidant molar ratio 3, 48% of the
conversion of styrene was obtained and gave 69% selectivity of the
major product (entry 3). After that, a significant decrease of
benzaldehyde selectivity was detected when the molar ratio was 4
(entry 4). This suggests that if excessive amount of oxidant (H2O2) is
present in the reaction mixture then it will facilitate the production
of other by-products instead of the major product benzaldehyde.
Conversion
/Selectivity
(%)b
Entry
Substrate
Product
48/69
1
2
39/65
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3
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8
h.
View Article Online
EpichlorohydrDinOI: 10.1039/D0NJ02972F
(10 mmol), 1.0
41/67
37/61
23/58
Alkylated
Zn(salpyr)
MPa CO2, catalyst
(0.5 mol %), 80 °C,
18h.
38
86.9
75
28
29
30
4
5
9
Epichlorohydrin
(5 mmol), catalyst
(43 mg),
CO2, 140 °C, 14 h.
ZnTCPP-
(Br−)Etim-
UiO-66
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1 bar
aReaction conditions: styrene (5 mmol), styrene:H2O2 molar ratio 3,
acetonitrile (10 mL), catalyst (50 mg, 2.75 wt% Cr loading), 60 °C, 8
h. bDetermined by GC.
Epichlorohydrin
(1.66
catalyst
mol%g),
CO2, 25 °C, 24 h.
Cr(salphe
n)
complex
mmol),
(1.5
We have explored the generality of the oxidation reaction of
various substituted aromatic alkenes by porous MPS-Cr(III)-alen
material under the optimized conditions (Table 10). The model
aromatic alkene styrene showed 48% substrate conversion with
69% benzaldehyde selectivity (Table 10, entry 1). It is seen that
styrene containing electron donating methyl group provided 39%
conversion of 4-methyl styrene with 65% selectivity of 4-methyl
benzaldehyde (entry 2). But 4-chloro styrene and 4-nitro styrene
both having electron attracting chloro and nitro groups, were
oxidized upto 41% and 37% conversion of the substrates
respectively along with 67% and 61% selectivity of the
corresponding aldehyde products respectively (entries 3-4). A
comparatively lower conversion of 23% was detected for trans-
stilbene with 58% benzaldehyde selectivity (entry 5). From this
observation over all features we have concluded that MPS-Cr(III)-
alen material shows the excellent catalytic activity for oxidation
reaction. The plausible reaction mechanism of selective oxidation
reaction of aromatic alkenes is represented in Scheme S1.
1
bar
Epichlorohydrin
(2 mmol), n-
Bu4NBr co-
MPS-
Cr(III)-
alen
catalyst (1
This
study
97
mmol), 1 atm.
CO2, catalyst (50
mg), Room
Temperature.
From the Table 11 and 12, it is proved that this study shows better
catalytic activity of our MPS-Cr(III)-alen material under optimised
reaction conditions compared to the other reported systems.10, 26-33
Table 12. Comparison of catalytic reactivity of aromatic alkenes
oxidation with previously reported systems
Comparison of catalytic activity of MPS-Cr(III)-alen material with
other reported systems
Conversion/
Reaction
The catalytic activity of our present MPS-Cr(III)-alen material has
been compared with some other previously reported works
towards the CO2 fixation reactions over epoxides for the synthesis
of cyclic carbonates and the oxidation of aromatic alkene (Table 11
and 12).
Reaction
Catalyst
Selectivity
(%)
Ref.
condition
Styrene (20
mmol),
Table 11. Comparative study of catalytic activity of cyclic carbonate
synthesis with different reported catalyst
catalyst (0.15
g), TBHP (30
mmol), 6 h, 65
°C.
ZSM-5(60)
45.3/87.2
31
Reaction
condition
Yield
(%)
Reaction
Catalyst
Ref.
Epichlorohydrin
(1.66
catalyst (2.5 mol
%), Bu4NBr co-
catalyst (2.5 mol
%), 1 bar CO2, °C,
24 h.
Styrene/H2O2
molar ratio of
0.45, catalyst
(0.2 g), 7 h, 60
°C.
mmol),
Chromium
(III)
Oxidation
of styrene
91
87
26
salophen
chloride
Cr-ZSM-5
44.2/66.5
32
33
Synthesis of
cyclic
carbonates
Styrene
(12.5
THABr co-catalyst
(0.90 mmol),
catalyst (71 mg),
oxide
mmol),
Styrene (10
mmol),
PS-
[MoO2(fsal-
ohyba)·
DMF]
H-MCrPN
27
catalyst
17.5/64.01
CO2
(balloon),
(0.040
H2O2
g),
Room
Temperature, 48
(20
8 | J. Name., 2012, 00, 1-3
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1
2
3
4
mmol), 6 h, 80
°C.
View Article Online
DOI: 10.1039/D0NJ02972F
5
6
7
n(styrene)/n(
H2O2)
=
3,
8
9
m(catalyst)/m
Cr-MSU-1
23.6/60.2
10
(styrene)
5.0%, 333K,
7h.
=
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 9B Recycling efficiency of MPS-Cr(III)-alen catalyst for the
oxidation of styrene in presence of 30% aq. H2O2
Styrene
mmol),
(5
styrene:H2O2
MPS-Cr(III)- molar ratio 3,
This
48/69
alen
ACN (10 mL),
catalyst (50
mg), 60 °C, 8
h.
study
Recyclability of MPS-Cr(III)-alen material
The heterogeneous catalysts are widely used because they can
be easily recycled which makes a heterogeneous catalytic reaction
more cost-effective. Here we have studied the recyclability of MPS-
Cr(III)-alen catalyst considering the reaction of epichlorohydrin with
CO2 for the production of cyclic carbonate and also for the styrene
oxidation under the optimization conditions. The catalyst was
collected from the reaction mixture with the help of centrifugation
process. After that MPS-Cr(III)-alen catalyst was washed with
acetone to remove any unreacted chemicals and underwent to dry
in air oven. The regenerated complex was used for the next run.
The data given in Fig. 9A and 9B depicts that the MPS-Cr(III)-alen
catalyst can be recycled up to five times without any significant
decay in its catalytic activity that proves MPS-Cr(III)-alen material is
entirely heterogeneous in nature. The MPS-Cr(III)-alen material was
characterized by FT-IR spectroscopic studies (Fig. 10), which
indicates a similarity between the freshly prepared catalyst and
reused catalyst.
Fig. 10 FT-IR spectra of reused MPS-Cr(III)-alen catalyst
Heterogeneity test
The nature of heterogeneity of the catalyst was established by
the controlled reaction of epichlorohydrin with CO2 for the
production of cyclic carbonate. For this, a hot filtration test was
subjected to a standard reaction of epichlorohydrin with carbon
dioxide under optimal reaction condition to test the leaching of
chromium metal into the reaction medium. It was stopped after 8 h
and the catalyst was collected from the mixture medium by
applying filtration, it was allowed to continue for additional 16 h.
After the first 8 h and the next 16 h, the conversion of
epichlorohydrin was almost same. After 8 h, no leaching of the
catalyst was observed in the filtrate from which the catalyst was
separated out. It is also observed that further reaction was not
proceeded after filtration of the catalyst from mixture. Similar
experiment was done for the selective oxidation of aromatic alkene
considering styrene as a model substrate. Initially the reaction was
carried out in presence of catalyst for 3 h; then another 5 h the
reaction was continued in absence of catalyst. After the first 3 h and
the next 5 h, the conversion of styrene was almost same. From this
leaching limit studied with the help of AAS instrument, it is
observed that after multiple reaction runs, a very inconsiderable
quantity of Cr was leached out in our experiment. Further from the
ICP-AES experiments of the filtrate and the recovered MPS-Cr(III)-
alen catalytic system, no noticeable Cr trace was detected in the
filtrate and in MPS-Cr(III)-alen catalyst; it retains almost the same Cr
loading as that of fresh catalyst (≈2.80 wt%). Hence the results
suggest that no noteworthy leaching of the metal was occurred.
Therefore, our investigation obviously demonstrates that our
porous MPS-Cr(III)-alen material was stable and completely
heterogeneous in nature.
Fig. 9A Recycling efficiency of MPS-Cr(III)-alen catalyst for
the synthesis of cyclic carbonate
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ARTICLE
Furthermore, for the reaction of epichlorohydrin with CO2 for
Journal Name
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4
5
6
7
8
3707-3728; (d) G. Fiorani, W. Guo and A. W. Kleij,
View Article Online
Green Chem., 2015, 17, 1375-1389; (e) A. H.
the production of cyclic carbonate, after 8 h the reaction was
stopped and the filtrate was collected by removing the catalyst
from the mixture by filtration. Here it was observed that after hot
filtration test, with the filtrate at approximately 55% yield was
obtained, which was essentially stopped at this point; i.e., no
further significant improvement of yield of the reaction was noticed
with further increment of time. It strongly suggests that the
reaction likely proceeded on the heterogeneous surface, as shown
in Fig. S3. Similar hot filtration test was reported in the literature by
DOI: 10.1039/D0NJ02972F
Chowdhury, A. Das, S. Riyajuddin, K. Ghosh, S. M.
Islam, Catal. Sci. Technol., 2019, 9, 6566-6569; (f) R.
Khatun, S. Biswas, M. S. Islam, I. H. Biswas, S.
Riyajuddin, K. Ghosh, S. M. Islam, ChemCatChem,
2019, 11, 1303-1312; (g) A. H. Chowdhury, S. Ghosh
and S. M. Islam, New J. Chem., 2018, 42, 14194-14202;
(h) A. H. Chowdhury, U. Kayal, I. H. Chowdhury, S.
Ghosh, S. M. Islam, ChemistrySelect, 2019, 4, 1069-
1077; (i) S. Ghosh, S. Riyajuddin, S. Sarkar, K. Ghosh, S.
M. Islam, ChemNanoMat, 2020, 6, 160-172; (j) S. Ghosh,
A. Ghosh, S. Riyajuddin, S. Sarkar, A. H. Chowdhury, K.
Ghosh, S. M. Islam, ChemCatChem, 2020, 12, 1055-
1067; (k) P. Sarkar, A. H. Chowdhury, S. Riyajuddin, S.
Biswas, K. Ghosh and S. M. Islam, New J. Chem., 2020,
44, 744-752; (l) P. Sarkar, S. Riyajuddin, A. Das, A. H.
Chowdhury, K. Ghosh, S. M. Islam, Mol. Catal., 2020,
484, 110730; (m) N. Salam, P. Paul, S. Ghosh, U. Mandi,
A. Khan, S. M. Alam, D. Das and S. M. Islam, New J.
Chem., 2020, 44, 5448-5456; (n) I. H. Chowdhury, A. H.
Chowdhury, A. Das, A. Khan and S. M. Islam, New J.
Chem., 2020, DOI 10.1039/D0NJ01147A; (o) R. Khatun,
S. Biswas, I. H. Biswas, S. Riyajuddin, N. Haque, K.
Ghosh, S. M. Islam, J. CO₂ Util., 2020, 40, 101180 (p) S.
Biswas, R. Khatun, M. Dolai, I. H. Biswas, N. Haque, M.
Sengupta, M. S. Islam, S. M. Islam, New J. Chem.,
2020,44,141-151.(q) S. Biswas, D. Roy, S. Ghosh, S. M.
Islam, J. Organomet. Chem., 2019, 898 ,120877.
9
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59
60
other research group 6d
.
Conclusions
In conclusion, we have decorated an benign and atom-
economic novel synthetic route for synthesized macroporous
polymer supported chromium MPS-Cr(III)-alen catalyst. The
material shows efficient catyalytic pathway towards the synthesis of
cyclic carbonates of the various substituted epoxides employing
carbon dioxide as carbon source at room temperature and solvent
free condition under atmospheric pressure. Furthermore, the
material also acts as outstanding catalytic system for the oxidation
of aromatic alkenes in presence of an eco-friendly oxidant. More
importantly, the heterogeneous MPS-Cr(III)-alen catalyst can be
easily recovered and recycled, which may provide new opportunity
in environmental and sustainable catalytic research in future.
Acknowledgements
(a) X. Zhang, X. Zhang, H. Dong and Z. Zhao, Energ.
Environ. Sci., 2012, 5, 6668-6681; (b) K. M. Yu, I.
2.
SMI acknowledges the Department of Science and Technology
DST-SERB (project referenceno. EMR/2016/004956), New Delhi,
Govt. of India, the Board of Research in Nuclear Sciences (BRNS),
Project reference No: 37(2)/14/03/2018-BRNS/37003, Govt. of
India and the Council of Scientific and Industrial and Research, CSIR
(project reference no 02(0284)2016/EMR-II) New Delhi, Govt. of
India for providing financial support. NS wish to acknowledge
University Grants Commission, New Delhi for Dr. D. S Kothari
Postdoctoral Fellowship (Award letter no. F.4-2/2006(BSR)/CH/17-
18/0176). PP also acknowledges to Aliah University, Kolkata for
providing financial assistance. AK thanks the Researchers
Supporting Project (Grant no. RSP-2019/127), King Saud University,
Riyadh, Saudi Arabia for financial support. We also thankful to the
Department of Science and Technology (DST) and University Grant
Commission (UGC) New Delhi, India for providing support to the
Department of Chemistry, the University of Kalyani and University
of Burdwan under FIST and SAP program.
Curcic, J. Gabriel and S.C. Tsang, Chem Sus Chem.,
2008, 1, 893-899; (c) P. Markewitz, Energ. Environ. Sci.,
2012, 5, 7281-7305; (d) Y. Du, F. Cai, D.-L. Kong and
L.-N. He, Green Chem., 2005, 7, 518-523; (e) S. S.
Islam, S Riyajuddin, R. A. Molla, N. Yasmin, K Ghosh,
S. M. Islam, New J. Chem., 2020,44,1979-1987;(f) N.
Salam, P. Paul, S. Ghosh, U. Mandi, A. Khan, S. M.
Alam, D. Das, S. M. Islam , New J. Chem., 2020,44,5448-5456.
(g) S. Ghosh, S. Riyajuddin, S. Sarkar, K. Ghosh, S. M.
Islam, ChemNanoMat, 2020, 6(1), 160-172.
(a) T. Sakakura, J. C. Choi and H. Yasuda, Chem. Rev.,
2007, 107, 2365-2387; (b) I. Omae, Catal. Today, 2006,
115, 33-52; (c) I. Omae, Coord. Chem. Rev., 2012, 256,
1384-1405; (d) M. Aresta and A. Dibenedetto, Dalton
Trans., 2007, 2975-2992; (e) M. North, R. Pasquale and
C. Young, Green Chem., 2010, 12, 1514-1539.
(a) D. H. Gibson, Chem. Rev., 1996, 96, 2063-2096; (b)
K. M. Yu, I. Curcic, J. Gabriel and S. C. Tsang, Chem
Sus Chem., 2008, 1, 893-899; (c) R. K. Mondal, S.
Riyajuddin, A. Ghosh, S. Ghosh, K. Ghosh, S. M. Islam,
J. Organomet. Chem., 2019, 880, 322-332; (d) M.
Sengupta, A. Bag, S. Ghosh, P. Mondal, A. Bordoloi, S.
M. Islam, J. CO₂ Util., 2019, 34, 533-542 (e) P. Sarkar,
A. H. Chowdhury, S. Riuajuddin, S. Biswas, K. Ghosh,
S. M. Islam, New J. Chem., 2020,44,744-752; (f) S
Ghosh, R. A. Molla, U. Kayal, A. Bhaumik, S. M. Islam,
Dalton Trans, 2019, 48, 4657-4666;(g) S. Ghosh, P.
Bhanja, N. Salam, R. Khatun, A. Bhaumik, S. M. Islam,
3.
4.
* Author to whom correspondence should be addressed.
Prof. Sk. Manirul Islam, Department of Chemistry, University of
Kalyani, Kalyani, Nadia, 741235, W.B., India. Phone: +91-33-2582-
8750, Fax: +91-33-2582-8282, E-mail: manir65@rediffmail.com
Dr. Seikh Mafiz Alam, Department of Chemistry, Aliah University,
Newtown, Kolkata-700156, West Bengal, India, Phone: +91-
8902214131,
Fax:
+91-33-2986-0252,
E-mail:
mafizchem@gmail.com
Catal Today, 2018, 309,253-262;(h) R. A. Molla, P.
Notes and references
Bhanja, K. Ghosh, S. S. Islam, A. Bhaumik, S. M. Islam,
Chem Cat Chem, 2017, 9, 1- 9.(i) A. H. Chowdhury, P.
Bhanja, N. Salam, A. Bhaumik, S. M. Islam, Mol
Catal,2018,450, 46-54.(j) S. Ghosh, P. Mondal, D. Das,
K. Tuhina, S. M. Islam, J organomet, Chem,2018, 866,1-
(a) M. Peters, B. Kchler, W. Kuckshinrichs, W. Leitner,
P. Markewitz and T. E. Müller, Chem Sus Chem, 2011, 4,
1216-1240; (b) M. Aresta, A. Dibenedetto and A.
Angelini, Chem. Rev., 2014, 114, 1709-1742; (c) Q.-W.
Song, Z.-H. Zhou and L.-N. He, Green Chem., 2017, 19,
1.
10 | J. Name., 2012, 00, 1-3
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2
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5
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8
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12.(k) R. Khatun, S. Biswas, M.S. Islam, I. H. Biswas, S.
Riyajuddin, K. Ghosh, S. M. Islam, Chem Cat Chem.,
2019, 11(4), 1303-1312.
Ding, RSC Adv., 2017, 7, 2836-2841; (c) J.VGiewaoA,rticQle.-OWnl.ine
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View Article Online
DOI: 10.1039/D0NJ02972F
Graphical Abstract
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Macroporous Polystyrene Functionalized Chromium MPS-Cr(lll)-alen
Complex as a Sustainable Porous Catalyst for CO2 Fixation under
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atmospheric pressure and Selective Oxidation of Aromatic Alkenes
Priyanka Paula,c, Noor Salamb, Aslam Khand, Debasis Dasb, Seikh Mafiz Alamc* and Sk.
Manirul Islama*
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India.
bDepartment of Chemistry, The University of Burdwan, Burdwan, , W.B., India.
cDepartment of Chemistry, Aliah University, Newtown, Kolkata-700156, W.B., India.
dKing Abdullah Institute for Nanotechnology, King Saud University, Riyadh, 11451, Saudi Arabia.
The benign synthesis of porous polystyrene supported chromium catalyst has been carried out
for the production of cyclic carbonates via CO2 fixation under atmospheric pressure and also for the
selective oxidation reaction of aromatic alkenes under sustainable conditions.
Macroporous
60 °C, 8 h, H2O2
MPS-Cr(lll)-alen
recyclable catalyst