Polymer immobilized [Mg@PS-anthra] complex: An efficient recyclable heterogeneous catalyst for the incorporation of carbon dioxide into oxiranes at atmospheric pressure and Knoevenagel condensation reaction under solvent free condition

Article abstract of DOI:10.1016/j.jorganchem.2018.11.026

Chemical fixation of CO₂ into the organic molecules is very challenging from the perspective of greenhouse gas consumption and manufacturing of chemicals with C1 backbone. The process was developed with the synthesis of an efficient polystyrene

Full text of DOI:10.1016/j.jorganchem.2018.11.026

Accepted Manuscript
Polymer immobilized [Mg@PS-anthra] complex: An efficient recyclable
heterogeneous catalyst for the incorporation of carbon dioxide into oxiranes at
atmospheric pressure and Knoevenagel condensation reaction under solvent free
condition
Ranjan Kumar Mondal, Sk Riyajuddin, Aniruddha Ghosh, Swarbhanu Ghosh, Kaushik
Ghosh, Sk.Manirul Islam
PII:
S0022-328X(18)30879-9
DOI:
https://doi.org/10.1016/j.jorganchem.2018.11.026
JOM 20646
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 9 October 2018
Revised Date: 5 November 2018
Accepted Date: 16 November 2018
Please cite this article as: R.K. Mondal, S. Riyajuddin, A. Ghosh, S. Ghosh, K. Ghosh, S.M. Islam,
Polymer immobilized [Mg@PS-anthra] complex: An efficient recyclable heterogeneous catalyst for the
incorporation of carbon dioxide into oxiranes at atmospheric pressure and Knoevenagel condensation
reaction under solvent free condition, Journal of Organometallic Chemistry (2018), doi: https://
doi.org/10.1016/j.jorganchem.2018.11.026.
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ACCEPTED MANUSCRIPT
Polymer immobilized [Mg@PS-anthra] complex: An efficient recyclable
heterogeneous catalyst for the incorporation of carbon dioxide into
oxiranes at atmospheric pressure and Knoevenagel condensation reaction
under solvent free condition
Ranjan Kumar Mondala, Sk Riyajuddinb, Aniruddha Ghosha,¥, Swarbhanu Ghosha,¥,
Kaushik Ghoshb*, Sk. Manirul Islama*
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India
bInstitute of Nano Science and Technology, Mohali, Punjab-160062, India
¥These authors contributed equally.
Graphical Abstract

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Polymer immobilized [Mg@PS-anthra] complex: An efficient recyclable
heterogeneous catalyst for the incorporation of carbon dioxide into
oxiranes at atmospheric pressure and Knoevenagel condensation reaction
under solvent free condition
Ranjan Kumar Mondala, Sk Riyajuddinb, Aniruddha Ghosha,¥, Swarbhanu Ghosha,¥,
Kaushik Ghoshb*, Sk. Manirul Islama*
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India
bInstitute of Nano Science and Technology, Mohali, Punjab-160062, India
¥These authors contributed equally.
Abstract:
Chemical fixation of CO2 into the organic molecules is very challenging from the perspective
of greenhouse gas consumption and manufacturing of chemicals with C1 backbone. The
process was developed with the synthesis of an efficient polystyrene functionalized
Magnesium-anthranilic acid complex (Mg@PS-anthra). The catalytic system was
successfully explored as a heterogeneous catalyst for the synthesis of cyclic carbonates by
CO2 fixation at atmospheric pressure and Knoevenagel condensation reaction under room
temperature solvent free condition. The material was thoroughly characterized by diffuse
reflectance UV-vis, FTIR spectroscopy, thermal gravimetric analysis (TGA), scanning
electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and powder X-ray
diffraction (PXRD) techniques. The catalytic pathway has been investigated for the cyclo-
addition reaction of epoxides using atmospheric CO2 accompanied by the Knoevenagel
condensation reaction with a variety of different aldehydes over cost effective environmental
friendly Mg-catalyst. This is attributed to its high catalytic activity, recovery simplicity and
excellent five times recycling efficiency; without any substantial decrease in catalytic
performance.
Keywords: Heterogeneous Catalyst, Mg@PS-anthra complex, Cyclic Carbonate, CO2
fixation, Knoevenagel condensation.
_________________________________________________________________________
*Authors to whom correspondence should be addressed.
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India
bInstitute of Nano Science and Technology, Mohali, Punjab-160062, India
E-mail address: manir65@rediffmail.com (S.M. Islam), kaushik@inst.ac.in (K. Ghosh)

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1. Introduction
Carbon dioxide is one of the primary components of green-house gas emerging out through
various human and continuous industrial activities running in the environment. Accumulation
of CO2 is a very challenging aspect to the modern chemist for recompensing the utilization of
abundant CO2 in nature eventually. The excess concentration of carbon dioxide in the
atmospheric layers over the last few decades of human civilization is an obsessing problem in
our society [1]. The source of energy is fossil fuels upon burning leads to carbon dioxide,
which has the high capacity to heat storage that leads to the warming of atmosphere [2].
Carbon capture and separation (CCS) has recently been proved as the most fascinating
technique for regulating the unremitting release of greenhouse gases. Though, CCS meets
several operational and commercial limitations to develop on a large scale. One of the vital
economic drawbacks is that it is not a cost-effective movement that requires large capital
investment. Later, a carbon capture and utilisation (CCU) process has been introduced to
draw attention since it can transform emitting CO2 gas from power plants into valuable
products such as chemicals and fuels. The power station flue gas contains the very small
extent of CO2 than pure CO2 and is contaminated by numerous impurities and solid
particulate matter. Several functional and metal based catalysts have been designed for CO2
capturing and its exclusive separation with high CO2 recovery rate in a power plant flue gas
for the catalytic transformations. This area of current discussion will convey the scenario for
upcoming research on further practical based CO2 capture and separation for conversion into
cyclic carbonates directly from power plant flue gas [2,3]. Therefore, the advancement of an
efficient catalytic process for CO2 transformations into useful chemical compounds has
become an unveiling issue in the arena of benign and sustainable synthetic procedures [3,4].
Out of the several demanding and economic CO2 transformations of utilizing C1 backbone is
established to date, the most industrially efficacious process is a cyclo-addition reaction
between CO2 and epoxide [5].
The generation of cyclic carbonates have been accomplished through fixation of CO2
on epoxide over a wide range of homogeneous and heterogeneous catalysts such as crown
ether [6], mixed oxides [7], ionic liquids [8], alkali metal salts [9], Schiff-base or metal
complexes [10], organic bases [11], quaternary ammonium or phosphonium salts [12],
functionalized MOFs [13], zeolite [14] and titanosilicates [15].
The major concern for the chemical fixation of CO2 is the low energy level of the CO2
molecule itself which requires stringent conditions such as air sensitivity, high temperature

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and high pressure. Hence, it is a very challenging task to furnish an effective catalyst
embedded with suitable polymer moiety for the synthesis of fine chemicals through CO2
fixation process in the context of green chemistry. The five or six membered cyclic
carbonates are tremendous aprotic solvents of polar nature. The synthesized organic cyclic
carbonates have broadly appeared as intermediates for the production of pharmaceuticals,
carbamates, biomedical agents, engineering materials and can be applied as electrolyte
components in Li-ion batteries [16,17].
Apart from CO2 transformations in organic synthesis, the carbon–carbon bond
formation reactions have also significant industrial applications in high added value
chemicals synthesis. One such significant and largely useful C-C bond forming reaction is
Knoevenagel condensation between aldehydes and active methylene moieties. The reactions
are frequently catalyzed by bases [18] such as ammonia, Graphene based material, amines, or
sodium ethoxide in organic solvents. Ionic liquids [19], lewis acids [20], surfactants [21] and
zeolites [22] have also been applied to catalyze this reaction. In practise the environmental
friendly solvents like water in association with solvent-free reactions are expressively
convenient in terms of economical, benign and green chemical procedures [23]. The
advantage of using water or other green solvents not only eliminate the toxicity of organic
solvents but also enhance the reaction rate in organic synthesis. A variety of homogeneous
catalysts are exploited for cyclo-addition of epoxides via CO2 fixation and Knoevenagel
condensation reactions over several years [24]. Now the fact is there are certain difficulties in
homogeneous catalysts such as the catalysts are destroyed, and cannot be isolated or reused
further. From this perspective, the development of unique heterogeneous base catalysts has
gained extensive attention and is successfully applied throughout the synthesis to eliminate
the above limitations.
Therefore it is very necessary to design an efficient, low cost, eco-friendly, and highly
selective heterogeneous solid catalyst to attain the desired selective product. The
heterogeneous catalysts have been used for numerous organic transformations on account of
easy recovery and recycle of the catalyst from reaction mixture [25].
Regarding this research work, generally alkaline earth metal oxides like CaO, BaO
and MgO are commonly used as the active site enables to functioning as solid basic catalysts.
Particularly, MgO can potentially increase the catalytic activity due to their enormous basic
features [26-30]. Hence, it may be expected that a combined material prepared by the Mg
species coordinated with Merrifield resin (PS) tagged anthranilic acid ligand could play
efficient catalytic performance in the Knoevenagel condensation reaction. This could be

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assigned to the distinctive chemical structure of Mg@PS-anthra complex along with the
tremendous basicity of the Mg species.
Cinnamic acid with its corresponding esters is extensively synthesized by Knoevenagel
condensation reactions. Very essential components such as perfumes, flavours, synthetic
indigo and pharmaceuticals are prepared from the carboxylic functional derivatives of
cinnamic acid [31,32].
In this work, we have simply demonstrated the preparation of efficacious heterogeneous base
catalyst namely Mg@PS-anthra complex by the reaction of Merrifield resin tagged with
anthranilic acid and Mg metal. Catalytic investigations on Mg@PS-anthra complex have
been accomplished for the cycloaddition reaction between different epoxides and carbon
dioxide at atmospheric pressure. Additionally, the catalyst was proved magnificent in the
Knoevenagel Condensation reaction using several aldehyde derivatives under solvent free
condition at room temperature. The recycling performance and the catalytic efficiency of
Mg@PS-anthra complex material were examined for the cycloaddition of oxirane along with
the Knoevenagel condensation reaction.
2. Experimental section
2.1. Materials
All chemicals and reagents were A.R. grade. Chloromethylated polystyrene was received
from Sigma-Aldrich Company (USA) and other chemicals were available from Alfa aesar
(India). Magnesium(II) chloride was purchased from Merck (India). The reagents were used
as received without further purification. Reagent grade solvents were distilled and dried
according to the standard methods before use.
2.2. Characterization Techniques
A Perkin Elmer, 2400C, USA elemental analyzer was operated to collect the
microanalytical data (C, H and N). The amount of metal content was determined by Optima
2100DV instrument (ICP-AES, Perkin Elmer). Scanning electron microscopy (SEM)
measurements of the synthesized materials were ensured with the help of a JEOL JEM 6700F
field-emission scanning electron microscope. Powder X-ray diffraction (PXRD) patterns of
the Mg@PS-anthra material was recorded through a Bruker D8 Advance X-ray
diffractometer using the Ni-filtered Cu Kα (λ = 0.15406 nm) radiation. FT-IR spectra of the
samples were monitored using KBr pellets in a Perkin-Elmer FT-IR 783 spectrophotometer
instrument. The Shimadzu UV-2401 PC doubled beam spectrophotometer connected to an
integrating sphere attachment was successively utilized in order to observe the UV-Vis

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spectra of the solid samples with a certain time. Mettler-Toledo TGA-DTA 851e instrument
provided the thermogravimetric analysis (TGA) of the material. The Atomic absorption
spectrophotometer (Model Varian AA240) was utilized to estimate the % content of
magnesium in the Mg@PS-anthra catalyst quantitatively. The Varian 3400 gas
chromatograph equipped with a 30 m CP-SIL 8CB capillary column and a flame ionization
detector precisely analyzed the reaction products obtained after completion of the reactions.
The products were identified by comparing with the corresponding authentic samples.
2.3. Preparation of Mg@PS -anthra complex catalyst
The Mg@PS-anthra complex was prepared according to the method described in the earlier
literature [33]. At the beginning, the Merrifield resin (1) (500 mg) was added to react with
anthranilic acid (250 mg) dissolved in DMF (5 mL) under the reflux condition for 30 h to
yield the polymer supported ligand (2). This polymer supported ligand (1 g) then reacted with
o
MgCl2.6 H2O (0.1 g) in ethanol (10 mL) for 9 h at 70 C to produce the resultant polymer
grafted Mg catalyst (3). The outline for the preparation of polymer supported metal complex
catalyst is given below (Scheme 1):
Scheme 1.Schematic representation of the synthesis of polymer supported Mg@PS-anthra
complex
The ICP-AES spectroscopy provided the elemental analysis for the synthesized Mg@PS-
anthra complex containing the Mg loading close to 4.26% by weight (Table 1). The elemental
analysis provided 3.78 wt% of Cl in the Mg@PS-anthra complex. The calculated molar ratio

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of Cl/Mg 1.1 is in line with the suggested structure (3) [(PS-anthra)MgCl]. Additionally, the
presence of 1.16 wt% of N corresponding to a molar ratio of N to Mg ~ 1.1 was computed by
the data acquired from elemental analysis. The value of N/Mg = 1 is within the range of
uncertainty in the elemental analysis (± 10%) and is almost identical with expected structure
(3). The above analysis substantially indicates the presence of 2.45 wt% Cl (i.e elimination of
1.33 wt% Cl in ligand from 3.78 wt% Cl in the complex) in Mg@PS-anthra complex (Table
1). This 2.45 wt% of Cl in Mg@PS-anthra complex comes exclusively from MgCl2.6H2O.
The obtained data for structure (3) was supportive to analyze the determination of molar
Cl/Mg ratio close to 1.1 [34,35].
Table 1. Chemical composition polymer-anchored ligand and its Mg catalyst.
Compound
Colour
C (%) H (%) N (%) Cl (%) Mg loading (%)
PS-anthra ligand
Pale yellow
Pale yellow
83.98
79.44
7.94
7.39
4.23
1.16
1.33
3.78
-
Mg@PS-anthra
4.26
complex
2.4. Typical procedure for synthesis of cyclic carbonates at 1 atmospheric pressure of
CO2
The reaction was executed by using epoxide (5 mmol), TBAB (Bu4NBr) (5 mol %), and
Mg@PS-anthra complex (30 mg) into a 100 ml of round bottom flask fitted with magnetic
stirring bar. After mixing the reactants one after the other in the round bottom flask, it was
followed by fitting with a CO2 balloon under the appliance of 1 atm pressure. The reaction
mixture was stirred at room temperature for 3-6 h. After completion of the reaction, the
polymer supported catalyst was filtered and the crude product was removed then dried in an
oven. The filtrate was extracted up to five times with ethyl acetate and washed several times
with water. The column chromatography was carried out with the isolated five-membered
organic carbonates.
2.5. General procedure for Knoevenagel condensation reaction
The heterogeneous catalyst Mg@PS-anthra was stirred with 2 mmol of ethyl cyanoacetate in
50 ml round bottle flask for 45 min at room temperature. The respective aldehyde (1 mmol)
was added to this mixture and the resulting mixture was stirred for the appropriate time at
room temperature. After the end of the reaction, the reaction mixture was treated with
dichloromethane followed by successive separation of organic and aqueous layers by using

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separating funnel. Na2SO4 was added to the organic layer to remove the aqueous content. All
the yields of individual esters were calculated from the isolated product.
3. Results and discussion
3.1. Characterization of Mg@PS-anthra complex
3.1.1. Power X-ray diffraction analysis
The Mg@PS-anthra complex exhibits a broadband at the range of 2θ (10-30) clearly
suggesting the amorphous nature of the catalyst [36]. Consequently, there was no indication
of any nanocrystalline nature of the MgO material detected in the synthesized Mg@PS-anthra
complex during the introduction of Mg-metal into the ligand (Fig. 1).
5000
Mg@PS-anthra complex
4000
3000
2000
1000
0
10 20 30 40 50 60 70 80
2θ (degree)
Fig. 1. PXRD of Mg@PS-anthra complex
3.1.2. Microscopic analysis
Scanning electron micrographs (SEM) support to ascertain the morphological changes
between polymer-supported ligand and the Mg@PS-anthra complex on the polymer matrix.
The SEM images of the polymer-supported ligand (a) and the Mg@PS-anthra complex (b)
are described in Fig. 2. SEM images provided a qualitative approach about the alteration in
morphology between PS-anthra ligand and the Mg@PS-anthra complex (Fig. 2b). It was
clearly observed the Mg@PS-anthra complex that quite roughening of the ligand attached
polystyrene surface appeared when coordinated to magnesium metal. In addition, the EDX
data also facilitates to determine the attachment of metal on the polymer surface (Fig. 3).
There observed no remarkable changes in morphology pattern between the synthesized
Mg@PS-anthra complex and the reused Mg@PS-anthra catalyst.

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Fig. 2. FE-SEM picture of PS-anthra ligand (a) and Mg@PS-anthra complex (b) respectively
Fig. 3. EDX spectra of PS-anthra ligand and Mg@PS-anthra complex.
3.1.3. UV-visible spectra
Another evidence for the presence of Mg supported on polymer material was attained with
the help of UV-vis spectra. The polymer attached Mg catalyst has some limitations of its
solubility in known organic solvents. As a consequence, the UV-vis spectra of the catalyst

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has been achieved in reflectance diffused spectrum mode using BaSO4 as a reference. The
UV-vis reflectance diffused spectra of the produced PS-anthra ligand and the Mg@PS-anthra
complex has been indicated in Fig. 4. The UV-vis absorption spectrum of the Mg@PS-anthra
complex was revealed near 255 nm wavelength. The respective absorption bands at 232, 255
and 344 nm for the pure ligand are assigning the characteristic π-π* electronic transitions
responsible for the aromatic ring. When PS-anthra ligand was coordinated to Mg, a new band
observed at 350 nm. It may be due to the charge transfer transitions between coordinated
atoms of the ligand and the central Mg atom [37].
Fig. 4. The UV-visible absorption spectrum of PS-anthra ligand and Mg@PS-anthra complex
3.1.4. Thermogravimetric analysis
The PS-anthra ligand and Mg@PS-anthra complex were taken in an air atmosphere at a
heating rate of 10 oC min-1 to study the thermogravimetric analysis (TGA). As realized from
o
Fig. 5 the synthesized Mg@PS-anthra complex is reasonably stable up to 470 C, however,
PS-anthra ligand decomposes at slight lower temperature 420oC. The thermal stability of
Mg@PS-anthra catalyst is sufficiently improved as a result of the coordination followed by
complexation with magnesium metal. The corresponding studies of DTA support that the
stages of decomposition are exothermic in nature (not shown).

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Fig. 5. Thermogravimetric analyses of PS-anthra ligand and Mg@PS-anthra complex.
3.1.5. FT IR spectra
The binding mode of anthranilic acid and magnesium metal onto the Merrifield resin has
been established by infrared spectral data in 4000-400 cm-1 regions. The FT-IR spectrum of
the PS-anthra ligand shows a very weak band or absence of C-Cl peak (due to –CH2Cl
groups) near 1,264 cm-1. The appearance of a strong band at 3382 cm-1 (ν-NH, the secondary
amine) and disappearance of (ν-C-Cl) band upon reacting with anthranilic acid implies the
formation of covalent bond through N atom. The other distinctive peaks at 1702 (ν C=O),
3449 (ν -OH), 3364 (ν-NH), 1403 (νsym COO) and 1552 cm-1 (νasym COO) confirmed also the
formation of PS-anthra ligand [38]. When PS-anthra ligand coordinated to magnesium metal,
the shifting of peaks towards the lower wavenumber regions (1407 for νsym- COO, 1562 for
ν
asym -COO and 3282 for ν N-H respectively) implies the participation of the -NH2 and
COOH functional groups of anthranilic acid in the development of the Mg@PS-anthra
complex (Fig. 6).

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Fig. 6. FT IR spectra of PS-anthra ligand and Mg@PS-anthra complex
3.2. Catalytic Activity
3.2.1. Synthesis of Cyclic carbonates catalysed by polymer supported Mg@PS-anthra
complex
CO2 insertion on oxiranes initiates the formation of a five membered organic cyclic
carbonates by polymer supported Mg-based catalyst in presence of TBAB (Bu4NBr) (Scheme
2).
Scheme 2. Preparation of organic cyclic carbonates from CO2 and epoxides
Application of CO2 fixation and/or conversion is potentially important for the fruitful
utilization of abundant greenhouse CO2 gas into value added chemicals. In order to optimize
the yield of the five-membered cyclic carbonates, we have tested all the reaction parameters
such as time, amount of Bu4NBr (co-catalyst). The production of styrene carbonate from the
cyclo-addition reaction of CO2 with styrene oxide is selected as the model example for this
reaction (Table 2). From Table 2 it is clear that among the different amount of Bu4NBr used,

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5 mol% exhibited the best conversion (entry 10). Further, we have also checked the influence
of reaction time on the yield of the products (Table 2, entries 1-10). The best output was
achieved when carrying out the reaction with 5 mol% of Bu4NBr in presence of 30 mg of the
catalyst at 1 atmospheric pressure and RT for 6h.
Table 2. Optimization of reaction conditions for the synthesis of styrene carbonate a
Entry
Bu4NBr
(mol %)
Time
(h)
CO2 pressure
Yieldb
(%)
45
54
60
52
64
75
82
(atm)
1
2
3
4
5
5
5
5
3
4
5
5
5
0
5
1
2
3
3
3
4
5
6
1
1
1
1
1
1
1
1
1
1
6
7
8c
9
Trace
Trace
97
6
6
10
b
aReaction conditions: Styrene oxide (5 mmol), catalyst (30 mg), RT; GC yield of cyclic
carbonate, cwithout catalyst.
Reactions experienced with smaller and higher than the actual reaction completion time
consequences to the incomplete styrene oxide conversions and relatively lower yield of the
product (Table 2, entry 1-3). For the lower amount of Bu4NBr, the reaction did not proceed
completely and hence resulting in a lower yield of the product (Table 2, entries 4 and 5). In
presence of Bu4NBr alone without catalyst and in absence of Bu4NBr with catalyst, it was
observed that trace amount of products were obtained in the same conditions (Table 2, entry
no. 8 and 9).
In order to scrutinize the optimization reaction conditions, we have thoroughly examined the
activities of Mg@PS-anthra complex for the generation of cyclic carbonate through CO2
fixation reaction. A long range of oxiranes such as styrene oxide, allyl glycidyl ether,
propylene oxide, epichlorohydrin, aryloxy epoxides (1, 2-epoxy-3-phenoxy propane) and
glycidol (epoxides with substituted functional groups) have been used as substrates. The
experimental results are summarized in Table 3. Most of the oxiranes could be easily

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converted to organic five-membered cyclic carbonate derivatives via CO2 fixation with
reasonable selectivity and high yield. It is worth mentioning that the electron-releasing (Table
3, entries 1-3) and electron-withdrawing groups (Table 3, entry 4) attached on the terminal
epoxides has no effect on the yield of the products. Additionally, we have performed our
experiments using aromatic oxirane derivatives like 2-(4-fluorophenyl) oxirane and 2-(4-
chlorophenyl) oxirane (Table 3, entry 7,8). The results of this substituted aromatic oxiranes in
this cyclic carbonate synthesis show that there is no noticeable change in yield percentage of
product compared to the other substrates. Due to the low boiling point (34 °C) of the
substrate propylene oxide, it offered comparatively lower yields (Table 3, entry 1).
Fig.7. Effect of reaction time on the conversion of SO to SC over Mg@PS-anthra complex
The time of reaction has been varied under optimized reaction conditions to explore the
complete potential of Mg@PS-anthra catalyst in the cycloaddition reaction until the full
conversion of SO is completed. The summary for the reaction time with conversion and
selectivity has been represented in Fig. 7. From the graph, it is found that the conversion of
styrene oxide linearly increases over time from the initial to 6 h and then in the remaining 2 h
it retained the 97% conversion level, whereas the selectivity of styrene carbonate remained
nearly 100% during the entire period of reaction. From the Fig. 7 it is evident that 6 h
reaction time is sufficient to complete this CO2 fixation reaction at room temperature.

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Fig. 8. Effect of the amount of catalyst for the CO2 fixation over Mg@PS-anthra complex
The quantity of catalyst has a significant role in this reaction (Fig. 8). When the
catalyst amount was used in 10 mg and 20 mg during the reaction the conversions were 30%
and 60% respectively. It was noticed that optimum conversion was observed when 30 mg of
catalyst was used. Nevertheless, no further increase in the product yield with a further rise in
the catalyst amount exceeding 30 mg.
Table 3 Preparation of organic cyclic five-membered carbonates over Mg@PS-anthra
catalyst under optimized conditions.
Entry
Epoxide
Product
Time
(h)
Yielda
(%)
1
6
6
6
86
94
95
2
3
4
6
90

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5
6
6
6
87
97
7
8
6
6
81
78
Reaction conditions: Styrene oxide (5 mmol), Catalyst (30 mg), Co-catalyst TBAB (5 mol
%), CO2 pressure (1 atm), ayield was determined by GC.
3.2.2. Possible mechanistic pathway for the synthesis of cyclic carbonates
In the first step, the CO2 molecule is efficiently activated by the Mg@PS-anthra
complex to afford a three membered Mg-active intermediate [39] (Scheme 3). The Mg centre
presents in the Mg@PS-anthra catalyst surface can act as facile electron donors [40] for the
activation of the CO2 molecules. The subsequent step is the attack of substrate oxirane
through the coordination of O atom and Mg centre of the polymer attached catalyst followed
by ring opening of epoxide. The produced Mg-coordinated epoxide intermediate is initiated
by the nucleophilic attack through Br- ion from quaternary salt on the less hindered side of
epoxide, leads to an oxyanion species. The oxyanion generated from ring opening then
attacks the carbon atom of the carbon dioxide molecule to produce another oxyanion Mg-
species. At this stage Br- group leaves the product by which the respective cyclic carbonate is
formed, and it desorbed from the catalyst surface as soon as the Mg@PS-anthra catalyst gets
back to its initial state [41]. The kinetic studies of the corresponding reactions have been
executed by varying the concentration of epoxide and CO2 pressure, keeping the other
parameters unaltered. The reactions followed first-order reaction kinetics as monitored from
the kinetics measurements. The rate of the cycloaddition of CO2 on epichlorohydrin could be
determined by altering the concentrations of catalyst, tetrabutylammonium bromide and CO2

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[42]. Hence the rate of this reaction can be written as: rate = k[epoxide]1[CO2]1[[Mg@PS-
anthra complex]1[Bu4NBr]2.
Scheme 3. Proposed mechanistic pathway for the synthesis of cyclic carbonates catalyzed by
Mg@PS-anthra complex.
3.2.3. Knoevenagel condensation reaction
The catalytic activity of the Mg@PS-anthra complex was scrutinized for the Knoevenagel
condensations between benzaldehyde and ethyl cyanoacetate to prepare benzylidene ethyl
cyanoacetate as the predominant product (Scheme 4).
Scheme 4. Knoevenagel condensation reaction of benzaldehyde with ethylcyanoacetate using
the Mg@PS-anthra catalyst
Romero et. al. [43] reported the amine-functionalized SBA-15 catalyst contributing 98%
o
conversion in the Knoevenagel condensation reaction at 105 C. However, at room
temperature amine-functionalized MCM-41 showed 81% conversion as reported by Sugi et.

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al. [44]. The typical drawbacks came upon with these methods were either higher
temperature or requirement of long reaction time to yield the corresponding products.
However, in this present study, we have got fascinating results attributed to the presence of
basic sites in the synthesized Mg@PS-anthra complex. According to the principles of green
chemistry, we are highly motivated to perform the reaction under room temperature and
solvent free conditions. Initially, we have monitored that no reaction found in the absence of
Mg@PS-anthra complex, whereas Mg@PS-anthra complex effectively catalyzed the
Knoevenagel condensation reactions leading to the high yield of benzylidene
ethylcyanoacetate. This fact is accounted for the presence of basic sites in Mg@PS-anthra
complex. After this experiment, we have examined the effects of various reaction parameters
on the condensation between benzaldehyde with ethyl-cyanoacetate to produce benzylidene
ethyl-cyanoacetate using Mg@PS-anthra as a catalyst. The molar ratio of benzaldehyde to
ethyl-cyanoacetate for the condensation reaction is 1:4, 1:3, 1:2, and 1:1 respectively (Fig. 9).
At a benzaldehyde to ethyl-cyanoacetate molar ratio of 1:2, the catalyst gives higher yield, 97
% in 2 h. The selectivity towards the product benzylidene ethyl-cyanoacetate was 98%.
Fig. 9. The effect of molar ratios of benzaldehyde and ethylcyanoacetate towards
Knoevenagel condensation reaction
Further increasing the concentration of ethylcyanoacetate, the conversion remains almost the
same but the selectivity got decreased. This is due to the rise in ethylcyanoacetate
concentration in the reactant mixture; the self-condensation process may occur within the
ethylcyanoacetate molecules.
Throughout the condensation reaction, the effect of reaction time on conversion and
selectivity of the product under the Mg@PS-anthra catalyst are shown in Fig. 10. It has been

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noted that the conversion and selectivity are increasing up to 2 h. Later the reaction became
slower and reached a maximum. The % of conversion and product selectivity values did not
vary considerably with a further increase in reaction time.
Fig. 10. Effect of reaction time on Knoevenagel condensation reaction
Table 4. Knoevenagel condensation of different aldehydes with ethylcyanoacetate catalyzed
by polymer supported Mg@PS-anthra complex
Entry
1
Aldehydes
Ethylcyanoacetate Time
Product
Yielda
(%)
97
(h)
2
2
3
2
2
82
84
4
5
2
2
98
92
6
7
2
2
85
96

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8
9
2
86
74
2
2
10
72
Reaction conditions: ethylcyanoacetate (2 mmol), aldehyde (1 mmol), catalyst (0.05 g),
solvent-free, at room temperature. ayield was determined by GC.
The catalyst Mg@PS-anthra exhibits a reasonable activity in the Knoevenagel
condensation reaction due to its structural basicity. We have executed several substituted
aldehydes and active ethylcyanoacetate with the molar ratio of 1:2 to explore the
functionality of Mg@PS-anthra in Knoevenagel condensation. In this reaction
ethylcyanoacetate and the substituted benzaldehydes containing electron releasing groups e.g.
-CH3, decreased the conversion rate and yield of the product although the electron
withdrawing substituents like -NO2 on the aromatic ring significantly increased the
conversion and contributed high product yield (Table 4). The scope of the reaction was
elaborated using heterocyclic aldehydes e.g. furan-2-carbaldehyde and thiophene-2-
carbaldehyde (Table 4, entry 9, 10). The data represented in table 4 suggests that yield of the
respective heterocyclic products is slightly lower than that of the aromatic aldehydes.
The plausible mechanism [45] for the Knoevenagel condensation reaction has been shown in
Scheme 5. It is assumed that Mg centre facilitates the Knoevenagel type coupling by co-
ordinating to oxygen atom of carbonyl groups. Initially, by the attack of the base catalyst
Mg@PS-anthra complex, the ethylcyanoacetate molecule loses its acidic proton to form an
anionic reactive enolate intermediate. In this process, the Mg centre of the Mg@PS-anthra
can activate ethylcyanoacetate so that deprotonation occurs from the ethylcyanoacetate
moiety. The next step is the nucleophilic attack of this anion at the electron deficient carbonyl
centre of benzaldehyde. Simultaneous abstraction of the proton takes place from the
protonated Mg@PS-anthra catalyst accordingly the production of the Mg-coordinated
β-
hydroxyl species together with the regenerated catalyst. The final desired products can be
achieved via the dehydration of the β-hydroxyl intermediate.

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Scheme 5. Proposed reaction mechanism for the Knoevenagel condensation reaction
3.2.4. Evaluation of catalytic activity of Mg@PS-anthra catalyst with the earlier
reported systems
The evaluation of efficiency for the Mg@PS-anthra catalyst with the other reported catalyst
for the present reactions is assigned in Table 5. The following comparison reveals that the
polymer tagged Mg-catalyst is more worthy and well enough than the earlier catalyst. From
the tally of the related former catalytic systems involving production of organic five-
membered cyclic carbonates under 1 atm CO2 pressure along with Knoevenagel condensation
products, it has appeared that our Mg@PS-anthra complex executed greater efficiency under
ambient conditions.
Table 5. Comparison of catalytic activity of the Mg@PS-anthra catalyst in the cyclic
carbonate reaction and Knoevenagel condensation with other reported systems
Reaction
Catalyst
Reaction conditions
Yield (%) [Ref.]
TiZiF
Styrene oxide (10 mol), CO2 (25
psi), catalyst (1.0 mol%),100 oC, 8 h
Styrene oxide (1.66 mmol), 10 bar of
98
46
Aluminium
83
47
complexes of a CO2, catalyst (42.1 mg), Bu4NBr
series
of (26.7 mg,0.083 mmol), RT, 24 h
bis(pyrazol-1-
yl)methane
MCM-41-IPr- Styrene oxide (50 mmol), CO2 (2.0
CO2 MPa/19.72 atm), catalyst (0.25
Cyclic
carbonates
87
48

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mmol), CH2Cl2, 120 oC, 48 h
Mg@PS-anthra Styrene oxide (5 mmol), CO2 (1
atm), catalyst (30mg), TBAB (5
mol %), RT, 6 h
97
This
work
94
LDH-OH
(Mg3Al)
Benzaldehyde (10 mmol), ethyl
cyanoacetate (10 mmol), catalyst
(150 mg), Water, 60 oC, 10h
49
50
51
52
Knoevenagel
condensation
LDH-
Benzaldehyde (2 mmol), ethyl
95
>99
82
diisopropylamid cyanoacetate (2 mmol), catalyst (50
e
mg), DMF, RT, 2h
Aminopropyl-
functionalized
SBA-15
Mg–Ga/Al-
MMO
Benzaldehyde (10 mmol), ethyl
cyanoacetate (10 mmol), catalyst
(150 mg), Cyclohexane, 82 oC, 1h
Benzaldehyde (50 mmol), ethyl
cyanoacetate (50 mmol), catalyst (1
wt%) , 60 oC, Solvent-free, 4 h
Mg@PS-anthra Benzaldehyde (1 mmol), ethyl
cyanoacetate (2 mmol), catalyst
97
This
work
(0.05 g), solvent-free, RT, 2h
3.3. Recyclability of Mg@PS-anthra complex catalyst
After completion of the reaction, the Mg@PS-anthra catalyst was separated out from the final
reaction mixture by applying gentle centrifugation process. Then the isolated Mg@PS-anthra
was thoroughly washed with water, acetone to get rid of all impurity if any and dried out in
0
an air oven at 70 C up to 6-7 h. With the separated catalyst the next successive runs were
accomplished for both cyclo-addition of epoxide and Knoevenagel condensation reactions
under the optimized condition. The Mg@PS-anthra complex can be efficiently recovered and
recyclable up to five times without any significant decay in yields which clearly suggests that
the Mg@PS-anthra complex is entirely heterogeneous in nature (Fig. 11). The prepared
Mg@PS-anthra complex was successfully committed to the excellent recycling efficiency for
both the synthesis of the cyclic carbonate and Knoevenagel condensation reaction.

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Fig.11. Recyclability chart of Mg@PS-anthra complex catalyst under the optimized reaction
conditions.
In our recycling experiments, we have simultaneously studied the leaching of
magnesium (if any) in our synthesized Mg@PS-anthra catalyst and the data are represented in
table 6. From this leaching limit study by AAS instrument we can conclude that after the first
reaction cycle, no leaching of magnesium was observed. The AAS analysis recorded that
after several reaction runs, a very negligible quantity of magnesium was leached out in our
experiment compared to the other reported systems.
Table 6. Leaching limit of the Mg@PS-anthra catalyst in recycling experiments for both the
cyclo addition of oxiranes and Knoevenagel condensation reactions
Leaching of Magnesium (ppm)
Serial
No.
Reaction
cycle
Catalyst name
Cyclo addition of
oxiranes
Knoevenagel
condensation
1.
2.
3.
4.
5.
1
2
3
4
5
0.00
0.004
0.009
0.016
0.026
0.00
0.002
0.006
0.009
0.015
Mg@PS-anthra
catalyst
4. Conclusions
In summary, the polystyrene immobilized heterogeneous Mg catalyst (Mg@PS-anthra) has
been synthesized. The catalyst was precisely characterized by powder XRD, SEM, IR, UV-
vis, TGA and elemental analysis. An outstanding activity was achieved by the catalyst for the
production of organic five-membered cyclic carbonates from epoxides in presence of CO2
under atmospheric pressure at room temperature. It is shown that Mg@PS-anthra complex

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can also be used as a heterogeneous catalyst for Knoevenagel condensation reaction. The
synthesized Mg@PS-anthra complex was evidenced as a very operative for the Knoevenagel
condensation reaction using a variety of aldehydes with electron repelling and attracting
groups providing high yields. The catalyst can be recovered and reused for five reaction
cycles without any drop in its function and selectivity for both the reactions. These data
nicely illustrate that the catalyst retains its stability under the optimized reaction conditions.
The method described here is very simple and convenient to synthesize a series of cyclic
carbonates and Knoevenagel condensation products under mild reaction conditions.
Remarkably, all the reactions performed in presence of Mg@PS-anthra catalyst were found to
undergo smoothly. The targeted products were obtained in satisfactory to excellent yields
without the formation of unwanted products. In addition, it is expected that the current
method offers many merits like cost-effectiveness, short reaction time and commercially
important compounds without generation of any harmful waste at the end of the reaction.
Acknowledgements
S.M.I. acknowledges the Department of Science and Technology (DST-SERB, Project No.
EMR/2016/004956), New Delhi, Govt. of India, and Board of Research in Nuclear Sciences
(BRNS), Project reference No: 37(2)/14/03/2018-BRNS/37003, Govt. of India, for funding.
AG expresses his sincere gratitude to the SERB-NPDF, DST, New Delhi, India (File No.
PDF/2017/000478) for the National Post-Doctoral Fellowship. SMI is also thankful to
Council of Scientific and Industrial and Research, CSIR (project reference no
02(0284)2016/EMR-II dated 06/12/2016) New Delhi, Govt. of India and Department of
Science and Technology, West Bengal (DST-WB, Sanction No. 811(sanc.)/ST/P/S&T/4G-
8/2014 Dated: 04.01.2016) for providing financial support. SMI acknowledges University of
Kalyani, India for providing personal research grant. We sincerely acknowledge the DST and
UGC, New Delhi, Govt. of India, for providing grant under FIST, PURSE and SAP program
to the Department of Chemistry, University of Kalyani.

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