Application of Ag/TFPG-DMB COF in carbamates synthesis via CO2 fixation reaction and one-pot reductive N-formylation of nitroarenes under sunlight

Article abstract of DOI:10.1016/j.mcat.2020.111050

We have designed mesoporous AgNPs decorated COF (Ag/TFPG-DMB COF) nanomaterial which has been formed by an easy ex-situ synthetic method. The synthesized material is characterized by FTIR, PXRD, UV–vis, N₂ adsorption–desorption studies, TEM, FESEM and XPS. The material showed the generation of identical mesopore at 3.9 nm. It is observed that the material can perform as both thermally and photochemically active catalyst for carbamate synthesis and one-pot reduction and N-formylation of nitroarenes respectively. The catalytic activity of the Ag/ TFPG-DMB COF nanomaterial is checked for green synthesis of carbamates from different amines and alcohols under 1 atmospheric pressure of CO₂ with excellent yield (upto 95 %) as well as with high TOF value (182 h^?1) and high selectivity. Additionally, the Ag/ TFPG-DMB COF nanomaterial is also applied as a potentially active photocatalyst for one-pot nitroarene reduction along with N-formylation reaction under sunlight irradiation in green reaction conditions with exceptionally high yield of formylated products upto 99 % as well as with high TOF value (762 h ^?1). The catalyst efficiently reduced and formylated para-nitrophenol, a potential water pollutant, which elaborates its scope as an efficient catalyst for water purification also. The catalyst recyclability is also checked for five reaction cycles for both the reactions and the Ag/TFPG-DMB COF material showed outstanding recycling ability without any noticeable leaching of active metal or catalyst degradation.

Full text of DOI:10.1016/j.mcat.2020.111050

MolecularCatalysis493(2020)111050
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Application of Ag/TFPG-DMB COF in carbamates synthesis via CO2 fixation
reaction and one-pot reductive N-formylation of nitroarenes under sunlight
Arpita Hazra Chowdhury, Ipsita Hazra Chowdhury, Surajit Biswas, Sk. Manirul Islam*
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Mesoporous
AgNPs
Covalent organic framework (COF)
CO2
Carbamates
Formylation
Photocatalysis
We have designed mesoporous AgNPs decorated COF (Ag/TFPG-DMB COF) nanomaterial which has been
formed by an easy ex-situ synthetic method. The synthesized material is characterized by FTIR, PXRD, UV–vis,
N2 adsorption–desorption studies, TEM, FESEM and XPS. The material showed the generation of identical me-
sopore at 3.9 nm. It is observed that the material can perform as both thermally and photochemically active
catalyst for carbamate synthesis and one-pot reduction and N-formylation of nitroarenes respectively. The
catalytic activity of the Ag/ TFPG-DMB COF nanomaterial is checked for green synthesis of carbamates from
different amines and alcohols under 1 atmospheric pressure of CO2 with excellent yield (upto 95 %) as well as
with high TOF value (182 h−1) and high selectivity. Additionally, the Ag/ TFPG-DMB COF nanomaterial is also
applied as a potentially active photocatalyst for one-pot nitroarene reduction along with N-formylation reaction
under sunlight irradiation in green reaction conditions with exceptionally high yield of formylated products upto
99 % as well as with high TOF value (762 h −1). The catalyst efficiently reduced and formylated para-ni-
trophenol, a potential water pollutant, which elaborates its scope as an efficient catalyst for water purification
also. The catalyst recyclability is also checked for five reaction cycles for both the reactions and the Ag/TFPG-
DMB COF material showed outstanding recycling ability without any noticeable leaching of active metal or
catalyst degradation.
1. Introduction
approach than the use of alkyl/aryl halides. There are reports on car-
bamate synthesis using homogeneous catalysts such as Cs2CO3, CsF,
In recent years, continuous increase of green house gas CO2 in the
atmosphere is one of the major environmental concerns. Consequently,
synthesis of industrially valuable chemicals utilizing the CO2 [1] in a
greener pathway is a promising research topic today, mainly CeN bond
formation for the synthesis of oxazolidinones, urea derivatives, carba-
mates and isocyanates by utilizing CO2 as feedstock [2]. One of the
industrially valuable chemicals is organic carbamates which are ex-
tensively used in the preparation of agrochemicals, herbicides, pesti-
cides, pharmaceutical agents and also for functional group protection,
like amine [2]. Carbamates are conventionally synthesized by using
phosgene which is an environmentally hazardous element [3]. Several
alternatine methods were also studied by researchers to avoid the use of
phosgene, such as substitution of urea, methoxycarbonylation of amine
etc [4,5,6]. One of the most promising approaches of carbamate
synthesis is utilizing CO2 as a reagent material. There are several re-
ports on cyclic carbonate and carbamate synthesis through incorpora-
tion of CO2 on amines with alkyl/aryl halides [7,8]. On the other hand,
carbamate synthesis using CO2, amines and alcohols is greener
Rb2CO3 with maximum carbamate yield upto 71 % but in presence of
dehydrating agents and under high pressure of CO2. In absence of de-
hydrating agents, the yield was limited upto 43 % [9]. Therefore, de-
velopment of potentially active heterogeneous catalyst is necessary for
carbamate synthesis using amines and alcohols, which can boost the
carbamate yield without using any dehydrating agent under atmo-
spheric pressure of CO2.
Additionally, reduction and formylation of nitroarenes into the
corresponding amines and formamides respectively are also very im-
portant transformations [10].The resulting amines are very useful for
manufacture of dyes, agrochemicals, polymers, pharmaceuticals etc
[11,12]. Formamides are also very useful for the synthesis of agro-
chemicals, dyes, pharmaceuticals, and fragrances [13,14]. In conven-
tional way, N-formylation of nitroarenes involves two distinctive steps,
i.e. reduction and formylation [15]. N-formyl derivatives from amines
conventionally synthesized by using toxic formylating agents, such as
CO, phosgene, chloral etc with immense waste production [16].
Therefore, green pathway for one-pot synthesis of N-formamides from
⁎ Corresponding author at: Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India.
E-mail address: manir65@rediffmail.com (S.M. Islam).
https://doi.org/10.1016/j.mcat.2020.111050
Received 30 March 2020; Received in revised form 20 May 2020; Accepted 28 May 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

A. Hazra Chowdhury, et al.
MolecularCatalysis493(2020)111050
Scheme 1. Synthetic scheme of Ag/TFPG-DM COF.
Fig. 1. Powder XRD patterns (a) small angle and (b) wide angle of Ag/TFPG-DMB COF sample.
Fig. 3. UV–vis spectra of the Ag/TFPG-DMB COF material.
Fig. 2. FTIR spectra of fresh and recycled Ag/TFPG-DMB COF sample.
H2 as a formylating agents even at high temperatures.
nitroarenes must be investigated by the researchers. So far, only few
works [17,18,19,20,21] have been reported on one-pot synthesis of N-
formamides from nitroarenes using HCOOH or its derivatives, or CO2/
Now days, sunlight derived catalysis is one of the most interesting
research topics to reduce energy consumption as well as CO2 emission.
Sunlight could be a potential substitute of fossil fuel. A. H. Chowdhury
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A. Hazra Chowdhury, et al.
MolecularCatalysis493(2020)111050
benign reaction conditions. According to the best of our knowledge, it
will be the first report in one-pot reduction and formylation of ni-
troarenes using formic acid in H2O with Ag/TFPG-DMB COF catalyst
under sunlight irradiation.
2. Experimental
2.1. Synthesis of TFPG-DMB COF
Synthesis of TFPG-DMB COF was carried out through a recently
developed solid state mixing procedure followed by hydrothermal
treatment. It was a modified route of a reported method [33]. Synthesis
was done by reacting 1,3,5-triformylphloroglucinol (TFPG) (189 mg,
0.9 mmol) and DMB (0.329 g,1.35 mmol) using 7.5 mmol (1426.5 mg)
of p-toluenesulfonic acid (PTSA). PTSA and DMB were homogeneously
mixed and grinded in a mortar-pestle for 10 min to obtain a sticky salt.
Then TFPG (0.9 mmol, 189 mg) was added to the reaction mixture
followed by further well grinding for 15 min to convert it into dough
like material. Few drops of DI (100−200 μL) were added to make the
mixture viscous. After that the COF precursors was taken in a Teflon-
lined steel autoclave and heated at 60 °C for first 6 h followed by 90 °C
for another 10 h in static condition. The resulting TFPG- DMB COF was
washed using N,N-dimethylacetamide (DMAc) and excess acetone to
remove the residues of the starting materials. The material was then
desiccated under vacuum for at room temperature 24 h to get the as-
synthesized red colored TFPG-DMB COF (83 % isolated yield).
Fig. 4. N2 adsorption–desorption isotherm of the Ag/TFPG-DMB COF material
at 77 K. Pore size distribution is shown in the inset.
et al. have reported mesoporous SnO2 as a potential photocatalyst for
CO2 reduction [22]. Typical TiO2 based photocatalyst materials have
been broadly investigated [23,24]. The drawback of these photo-
catalysts is that they can only absorb UV light but almost half of the
solar energy is conserved by the visible part of the spectrum. It is re-
nowned that Cu, Ag and Au nanoparticles efficiently absorb visible-light
as a result of SPR (surface plasmon resonance) effect [25,26,27]. Ad-
ditionally, the inter band transition of 4d electrons to the 5sp band
makes silver NPs capable of substantial UV light absorption [28,29].
Therefore, AgNPS can utilize the complete solar spectrum. On the other
hand, covalent organic frameworks (COFs) have come out as a new
generation photoactive material for light-induced reactions [30,31].
COFs have in plane π-electron conjugation as well as axial stacking
through π-orbital overlaps which make COFs capable of transporting
charge in both in plane and axial direction [32,33].
Herein, keeping the advantages of silver nanoparticles and COF
materials in mind, we have developed AgNPS decorated COF (Ag/
TFPG-DMB COF) nanomaterial as both thermally and photochemically
active new generation heterogeneous catalyst. It is applied as a ther-
mally active catalyst for green synthesis of carbamates from amines and
alcohols under 1 atmospheric pressure of CO2 with excellent yield (upto
95 %) and high selectivity. Additionally, the Ag/TFPG-DMB COF na-
nomaterial is also applied as a potentially active photocatalyst for one-
pot nitroarene reduction and formylation reaction under sunlight irra-
diation in green reaction conditions with exceptionally high yield of
formylated products upto 99 %. The catalyst material also shows high
recyclability for multiple reaction cycles for both the reactions without
any considerable catalyst deactivation, supporting its industrial ap-
plicability as a potential heterogeneous catalyst in environmentally
2.2. Synthesis of silver nanoparticles decorated mesoporous TFPG-DMB
(Ag/TFPG-DMB) COF material
Silver nanoparticles were prepared by using 10 ml aqueous solution
of 0.5 mmol AgNO3 in a round bottom flask with 10 ml solution of 0.1 g
PVP. The mixture was stirred for 30 min. Then 5 ml 1 (M) aqueous
solution of NaBH4 was added as a reducing agent followed by 10 min
stirring of the reaction mixture. The prepared silver nanoparticles were
then isolated through centrifugation and washed with DI water and
ethanol for multiple times. The Ag/TFPG-DMB COF material was pre-
pared by ex-situ synthetic method (Scheme 1). 200 mg of TFPG-DMB
COF material was sonicated in 10 ml ethanol for 1 h followed by ad-
dition of 5 mg of prepared silver nanoparticles. The mixture was then
stirred for 1 h at room temperature and the synthesized solid material
was collected through centrifugation. The product was dried at 60 °C in
vacuum for 2 h to obtain the final Ag/TFPG-DMB COF material.
2.3. Synthetic procedure for carbamate production using mesoporous Ag/
TFPG-DMB COF nanomaterial
Carbamate synthesis in a green pathway under 1 atm pressure of
CO2 using amine (2 mmol) and alcohol (200 mmol) was performed at
80 °C temperature in a round bottom flask, attached with a reflux
Fig. 5. FE-SEM images of Ag/TFPG-DMB COF material at two different magnifications.
3

A. Hazra Chowdhury, et al.
MolecularCatalysis493(2020)111050
Fig. 6. TEM images (a, b, c), lattice fringes (d), SAED pattern (e) and EDAX pattern (f) of Ag/TFPG-DMB COF material.
condenser for 8 h in presence of 10 mg of Ag/TFPG-DMB COF catalyst
and Cs2CO3 (0.1 g). The work up was done with ethyl acetate after
isolating the solid catalyst through centrifugation. The washing was
done with DI H2O and the collected organic part containing the car-
bamate product was dried over anhydrous Na2SO4. The isolated car-
bamate product was characterized by 1HNMR and 13CNMR spectro-
scopy.
2.4. Procedure of photocatalytic N-formylation of nitroarenes using
mesoporous Ag/TFPG-DMB COF nanomaterial as photocatalyst
The photocatalytic N-formylation of nitroarenes (2 mmol) was
executed by using HCO2H (6 mmol), H2O (1 mL) at room temperature
in presence of Ag/TFPG-DMB COF as an active photocatalyst under
sunlight irradiation for 2 h. The progress of the reaction was monitored
through gas chromatographic analysis and the N-formylated products
were characterized by 1HNMR and 13CNMR spectroscopy.
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A. Hazra Chowdhury, et al.
MolecularCatalysis493(2020)111050
Fig. 7. (a) XPS survey scan, XPS curve of (b) C 1s, (c) Ag 3d (d) N1s and (e) O1s for Ag/TFPG-DMB COF.
3. Results and discussion
AgNPs from Scherrer's equation is 14.89 nm.
3.1. Characterization
3.1.2. FTIR analysis
The FTIR spectra of synthesized Ag/TFPG-DMB COF before and
after catalytic application are shown in Fig. 2. The peaks at
∼1270 cm−1and ∼1569 cm−1 are due to –(CeN) and –(C]C) bonds,
respectively and this observation confirms the occurrence of the β-ke-
toenamine linked COF structure. The sharp peak at 2919 cm−1 relates
to the –(C–H) stretching frequency. The wide peak at 3410 cm-1 cor-
responds to stretching vibration of –(NeH) groups in the aromatic ring
[35]. The stretching vibration band of hydroxyl group -(OeH) is sup-
posed to be appear at 3200−3600 cm−1 [36], which may be over-
lapped with the broad –(NeH) band. The similarities in both the FTIR
3.1.1. Powder X-ray diffraction (PXRD) analysis
The formation of synthesized Ag/TFPG-DMB COF was confirmed by
PXRD analysis showed in Fig. 1. The characteristic peak at 3.5 corre-
sponds to the diffraction plane (100) (Fig. 1a) and peak at 26.39 cor-
responds to the plane (001) which arises from the π-π stacking of (001)
planes (Fig. 1b) [34]. The characteristic peaks of metallic Ag (JCPDS
no.04−0783) at 2θ values of around 38.17, 44.56, 64.31 and 77.31
indicating the presence of crystal planes (111), (200), (220) and (311),
respectively were observed (Fig. 1b). The estimated crystallite size of
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A. Hazra Chowdhury, et al.
MolecularCatalysis493(2020)111050
Scheme 2. Carbamate synthesis from benzylamine using methanol and CO2.
Table 1
Optimization chart for carbamate synthesis reactiona.
Entry
Amount of Cs2CO3 (g)
Catalyst
Temper-ature
Yieldb (%) (Carba-mate)
(A)
Yieldb (%) (Dibenzyl-urea)
(B)
0
(
C)
1c
2
3
4
5
0
Ag/ TFPG-DMB COF
Ag/ TFPG-DMB COF
Ag/ TFPG-DMB COF
Ag/ TFPG-DMB COF
Ag/ TFPG-DMB COF
Ag/ TFPG-DMB COF
TFPG-DMB COF
AgNPs
80
80
80
60
50
RT
80
80
80
80
3
trace
< 5
–
1
5
25
6
–
< 1
–
0.05
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0
40
> 97
64
27
< 5
5
–
2
–
6
7
8c
9c
10c
–
–
a
Reactions carried out using 10 mg of catalyst, 2 mmol benzylamine, 200 mmol CH3OH and Cs2CO3, under CO2 balloon (1 atm), 8 h.
GC yield.
Reaction time : 24 h.
b
c
Table 2
Ag/TFPG-DMB COF catalysed carbamate synthesis from amines, alcohols and CO2.a.
Entry
1
Amines
Alcohol
Carbamate products
Yieldb (%)
95
TON
TOF (h−1
)
MeOH
EtOH
14.61 × 102
182
2
3
92
93
14.15 × 102
14.30 × 102
177
179
MeOH
4
EtOH
88
13.53 × 102
169
5
6
MeOH
EtOH
94
90
14.46 × 102
13.84 × 102
181
173
a
Reactions carried out using 10 mg of catalyst, 2 mmol benzylamine, 200 mmol alcohol and Cs2CO3 (0.1 g), under CO2 balloon (1 atm), 80 °C, 8 h.
isolated yield.
b
curves confirmed that the sample remained unchanged after the cata-
lytic reaction.
3.1.3. UV–vis analysis
The number of electron–hole pair generated from absorption of light
depends on the intensity of incident photons with energy exceeding or
equaling the band gap energy. Fig. 3 depicts the solid state UV–vis
6

A. Hazra Chowdhury, et al.
MolecularCatalysis493(2020)111050
3.1.5. Microscopic analysis
The FESEM images (Fig. 5) of the synthesized Ag/TFPG-DMB COF
showed sheet like morphology of the sample, which is also confirmed
by the TEM images (Fig. 6). The porous nature of the sample is also
confirmed from the high magnified TEM image as shown in Fig. 6b and
c. The Ag NPs dispersed on the COF surface is shown by white doted
circles in Fig. 6c. The particle size of AgNPs calculated from the TEM
images is found to be 10−20 nm. The HR-TEM, selected area electron
diffraction (SAED) patterns and energy dispersive X-ray spectroscopy
(EDS) are shown in Fig. 6d–f. The D-spacing of 0.23 nm calculated from
the HRTEM image (Fig.6d) corresponds to the (111) plane of Ag. The
SAED pattern of the sample indicated the polycrystalline nature of the
particles (Fig. 6e).The bright spots of the concentric diffraction rings
corresponding to (111) and (220) planes of AgNPs as obtained from
XRD. The EDS spectra (Fig.6f) confirmed the presence of C, N, O and Ag
with given weight percentages.
3.1.6. XPS analysis
The XPS survey (Fig. 7a) scan indicates the presence carbon, ni-
trogen, oxygen and silver in the synthesized material. The high-re-
solution XPS spectrum of C 1s region (Fig. 7b) on deconvolution depicts
peaks at binding energies 284.4, 286.0, and 288.2 eV which suggest
that the material contains CeC and C–H, CeN, and CeO / C]O bonds
respectively [39]. Fig. 7c shows the high resolution XPS curve of Ag 3d
region. The binding energy of Ag 3d5/2 and Ag 3d3/2 centred at 368.6
and 374.6 eV, respectively with the separation of peak at 6.0 eV in-
dicated the presence of metallic Ag in Ag/TFPG-DMB COF sample [40].
The N 1s band (Fig. 7d) appeared at 400.2 eV could be related to the
existence of the nitrogen atoms of the amino groups of the COF. The O
1s band (Fig. 7e) confirms the existence of the oxygen atoms in the
synthesized material.
Fig. 8. Probable formation mechanism of acyclic carbamate from amine and
alcohol.
Scheme 3. Formylation of nitrobenzene under sunlight irradiation.
absorbance spectra of the synthesized Ag/TFPG-DMB COF catalyst. The
material showed a broad absorption peak at 406 nm. The absorption
spectra include many transitions in the highly ordered conjugation
systems [37]. The band gap energy (Eg) is calculated from the formula,
Eg (eV) = 1240/λ (nm), which is found to be 3.05 eV.
3.2. Catalytic activity
3.2.1. Carbamate synthesis
Using Ag/TFPG-DMB COF nanohybrid as catalyst we pursued
acyclic carbamate synthesis reaction using benzylamine in green sol-
vent (CH3OH) medium at 80 °C temperature under 1 atm pressure of
CO2 in presence of Cs2CO3 (Scheme 2) for 8 h. The probable products
from the reaction are methyl benzylcarbamate (A) and dibenzylurea
(B). There could also be a side product formed from methanol car-
boxylation is dimethyl carbonate (C). We have focused on the synthesis
of carbamte product only.
To find out the ideal reaction condition we performed several re-
actions varying the crucial reaction parameters. When the reaction was
performed in absence of CS2CO3, only 3 % yield of the carbamate
product (A) was found even after a long reaction period (Table 1, entry
1). Table 1 suggests that with increasing the amount of CS2CO3 the
yield of carbamate product also increases (Table 1, entries 1–3)
and > 97 % yield was obtained by using 0.1 g of CS2CO3 (Table 1, entry
3). The effect of reaction temperature was also studied (Table 1, entries
4–6). Noticeably, when the reaction was performed at room
3.1.4. Surface area measurement
The N2 adsorption–desorption isotherm of the synthesized Ag/
TFPG-DMB COF catalyst showed in Fig. 4. It shows type IV isotherm
with H2 hysteresis loop which indicates mesoporous characteristic of
the sample with ink-bottle-like pore constrictions, where pore mouth is
smaller than the pore body [38]. The subsequent pore size distributions
(PSD) resulting from the respective adsorption data is given in the index
of the isotherm plot. From the PSD curve, it can be observed that the
Ag/TFPG-DMB COF sample showed pore diameter within 3−15 nm
with peak pore diameter at 3.9 nm which falls in the mesoporous region
(2−50 nm). The estimated BET surface area and total pore volume of
the COF are observed to be 651.64 m²/g and 0.36 cm3 g−1, respec-
tively.
Table 3
Optimization chart for N-formylation of nitrobenzenea.
Entry
Catalyst
Solvent
Time (h)
Conversionb (%)
Selectivityb (%) (A)
Selectivityb (%) (B)
1
2
3
4
5c
6
–
H2O
H2O
H2O
H2O
H2O
–
12
12
12
2
12
12
0
0
100
100
0
0
0
0
0
AgNPs
TFPG-DMB COF
Ag/ TFPG-DMB COF
Ag/ TFPG-DMB COF
Ag/ TFPG-DMB COF
< 3
> 99
0
> 97
< 1
0
30
32
68
a
Reactions carried out using 10 mg of catalyst, 2 mmol nitrobenzene, 6 mmol of HCOOH and 1 ml H2O, RT under sunlight irradiation.
Conversion and selectivity were evaluated by GC.
Dark.
b
c
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A. Hazra Chowdhury, et al.
MolecularCatalysis493(2020)111050
Table 4
Ag/TFPG-DMB COF catalysed N-formylation of nitroarenesa.
Entry
1
Nitroarenes
Products
Yieldb (%)
99
TON
TOF
(h−1
)
15.23
x 102
762
754
746
2
3
98
97
15.07
x 102
14.92
x 102
4
96
14.77
x 102
739
5
98
32
15.07
x 102
754
246
6c
4.92
x 102
a
Reactions carried out using 10 mg of Ag/COF catalyst, 2 mmol nitrobenzene, 6 mmol of HCOOH and 1 ml H2O, RT under sunlight irradiation for 2 h.
Isolated yield.
Irradiation time: 5 h.
b
c
Fig. 9. Probable photocatalytic reduction and formylation mechanism of nitroarenes.
temperature (RT) dibenzylurea (B) was formed as major product with
moderate yield and methyl benzylcarbamate was formed as minor
product with very poor yield (Table 1, entry 6). Interestingly, the car-
bamate product was found to be the major product with increasing the
reaction temperature with highest yield (> 97 %) at 80 °C temperature
(Table 1, entry 3). Reaction performed with only TFPG-DMB COF ma-
terial showed very poor yield of both the products (Table 1, entry 7).
Furthermore, when the reaction was carried out in presence of only
AgNPs, there was no progress of the reaction was found even after
longer reaction time (Table 1, entry 8). Therefore, it is confirmed that
the synergistic effect of AgNPs along with TFPG-DMB COF made the
CO2 fixation reaction happen to produce acyclic carbamate from amine
and alcohol. To understand the role of catalyst, we performed the
Fig. 10. Recyclability chart of Ag/TFPG-DMB COF catalyst.
8

A. Hazra Chowdhury, et al.
MolecularCatalysis493(2020)111050
reaction time (Table 3, entry 2). After that we have ran the reaction in
presence of 10 mg of only TFPG-DMB COF catalyst and there was found
to be 100 % conversion of nitrobenzene, where aniline (B) product
selectivity was > 97 % and that of N-phenylformamide (A) was < 3%
(Table 3, entry 3). Therefore, in presence of TFPG-DMB COF catalyst,
the reduction of nitrobenzene to aniline occurred but the formylation
reaction was found to be very slow. When the reaction was performed
in presence of 10 mg of Ag/TFPG-DMB COF catalyst 100 % conversion
of nitrobenzene was occurred within only 2 h of reaction time with the
formation of N-phenylformamide as the major product (Table 3, entry
4). Therefore, we can conclude that the synergistic effect of AgNPs
along with TFPG-DMB COF play a major role to enhance the reduction
as well as the formylation reaction of nitrobenzene. When the reaction
was performed under dark condition there was no conversion of ni-
trobenzene was found even after 12 h of reaction period (Table 3, entry
5), which confirms that light plays an important role to activate the
catalyst to initiate the reduction and formylation reaction of ni-
trobenzene. It is very interesting to note that when the reaction was
performed in absence of water, only limited amount conversion (30 %)
of nitrobenzene was happened (Table 3, entry 6) which is may be due to
H2O not only acts as a solvent but also acts as a sacrificial electron
donor which plays a crucial role to reproduce the catalyst for further
reaction cycles.
After setting up the ideal reaction conditions, the performance of
the Ag/TFPG-DMB COF catalyst was assessed on variety of nitroarene
substrates (Table 4). Interestingly, all the substrates including the
substrates containing electron donating (Table 4, entry 2) as well as
electron withdrawing groups (Table 4, entries 3–5) were efficiently
converted to their corresponding formylated products with high yields.
The catalyst efficiently reduce and formylated para-nitrophenol
(Table 4, entry 5) which is a potential water pollutant. On the other
hand, nitrobutane showed very poor yield of the corresponding for-
mylated product (Table 4, entry 6), even after longer irradiation time.
The probable formation mechanism is depicted in Fig. 9, inspired
from previously reported literatures [42]. Here, Ag/TFPG-DMB COF
acts as a potentially active photocatalyst. Both electron trapping effi-
ciency of Ag to enhance electron–hole pair separation, and high surface
area and pore size of Ag/TFPG-DMB COF which provide higher surface
active sites played a major role to increase the photocatalytic efficiency.
On the other hand, the sheet-like morphology of the catalyst provides
more geometrical area which leads to higher surface active sites for
heterogeneous catalysis. Sliver doping in COF semiconductor can en-
hance its photocatalytic property as Ag can efficiently absorb the full
solar spectrum due to its SPR, interband charge transfer properties and
its electron trapping efficiency [25,26,27,28,29]. Photoexcited Ag/
TFPG-DMB COF generated from sunlight absorption transport energy to
the surface adsorbed HCOOH molecules and activate them to split into
CO2 and H2 on the catalyst surface [43,44] leading to the reduction and
formylation of nitroarene molecules. HCOOH can be adsorbed on the
active sites of the catalyst and the lone pair of amine makes the
Fig. 11. Heterogeneity test suggesting the absence of leached active metal into
the filtrate solution.
reaction in absence of any catalyst and found that there were very poor
yield of both products A and B (Table 1, entry 9) even after much longer
reaction time. In absence of any catalyst and CS2CO3, the reaction did
not even proceed even after 24 h of reaction time (Table 1, entry 10).
After setting up the ideal reaction conditions, the performance of
the Ag/TFPG-DMB COF catalyst was assessed on variety of aromatic
amine substrates in presence of methanol and ethanol (Table 2). All the
substrates showed slightly lower yields of the catbamate products in
presence of ethanol (Table 2, entries 2, 4, 6, 8) than that of methanol
(Table 2, entries 1, 3, 5, 7). The benzylamine substrates containing
electron donating (Table 2, entries 3, 4) as well as electron withdrawing
groups (Table 2, entries 5, 6) were efficiently converted to their cor-
responding carbamate products with high yields.
The probable formation mechanism is depicted in Fig. 8. The re-
action is promoted through the NeH bond activation of amines with the
help of Ag/TFPG-DMB COF nanomaterial in presence of Cs2CO3 base
followed by CO2 insertion to form an intermediate [A]. The inter-
mediate forms the corresponding isocyanate [B] through a thermal
dehydration process. The isocyanate forms the carbamate or urea de-
rivative on further condensation with an alcohol or an amine respec-
tively reproducing the catalyst material [41].
3.2.2. N-formylation of Nitroarenes
Using Ag/TFPG-DMB COF nanohybrid as photoactive catalyst we
pursued N-formylation of nitrobenzene using HCOOH as reducing as
well as formylating agent in green solvent (H2O) medium at room
temperature (RT) (Scheme 3).
Sunlight irradiation of 1 ml aqueous solution of 6 mmol HCOOH
containing 2 mmol nitrobenzene in absence of any catalyst was per-
formed for 12 h (Table 3, entry 1) and it was observed that there was no
conversion of nitrobenzene even after such a long reaction period.
Again, when the reaction was carried out in presence of only AgNPs,
there was also no progress of the reaction was found even after long
Table 5
Relative study of Ag/TFPG-DMB COF catalyst with other reported catalytic systems.
Reaction
Catalyst
CeO2
Reaction Condition
Time Yield (%) Reference
Carbamate Synthesis
Benzylamine (5.0 mmol), CH3OH
(900 mmol), Catalyst (0.17 g), CO2
(5 MPa), 423 K.
12 h
92
[48]
Ag/TFPG-DMB
COF
Benzylamine (2.0 mmol), CH3OH
(200 mmol), Catalyst (10 mg), Cs2CO3 (0.1 g), CO2
(1 atm), 80 °C
8 h
95
This Study
N-Formylation Reaction AgPd@g-C3N4
Nitrobenzene (1 mmol), Formic acid (2 mmol), Water (1 mL) Catalyst (25 mg), 40 W domestic 2 h
99
[47]
bulb
Co@NPC
Nitrobenzene (0.25 mmol), HCOONH4 (10 eqiv.), Water THF (2.5 mL) Catalyst (4.8 mol% of 12 h
Co), 120 °C
Nitrobenzene (2 mmol), Formic acid (6 mmol), Water (1 mL) Catalyst (10 mg), sunlight 2 h
> 99
99
[49]
Ag/TFPG-DMB
COF
This Study
9

A. Hazra Chowdhury, et al.
MolecularCatalysis493(2020)111050
nucleophilic attack to the carbon atom of HCOOH. Finally, the dehy-
dration from the complex leads to the formation of the N-formylated
product [45].
catalysis.
Declaration of Competing Interest
3.2.3. Recyclability and heterogeneity test of Ag/TFPG-DMB COF catalyst
The Ag/TFPG-DMB COF catalyst was found to be an easily re-
coverable and reusable heterogeneous catalyst. The catalyst showed
high yields of the carbamate and formylated products even on fifth
reaction cycles for both the CO2 incorporation and formylation reac-
tions (Fig. 10). We have studied the conversion rates for both the re-
actions with time in presence of Ag/TFPG-DMB COF catalyst and from
the obtained data the kinetic curves (Figure S2 and Figure S3) have
been drawn. It was clearly observed that the conversion rate was much
faster for the formylation reaction (Figure S3) than that of the carba-
mate synthesis (Figure S2). The conversion rates of different recycling
runs for both the reactions have also been studied and compared
(Figure S4 and Figure S5). Those figures suggested that there were
minimal decrease in the conversion rates after five recycling runs in-
dicating the Ag/TFPG-DMB COF catalyst was reusable with similar
activity for both the reactions. The FTIR data (Fig. 2), PXRD pattern
(Figure S6) and FESEM image (Figure S7) of the recycled catalyst
confirmed that there were no significant structural and morphological
changes in the reused catalyst, respectively. We have also analyzed the
pore size distribution plot of the reused catalyst (Figure S8). It sug-
gested that there was small decrease in peak pore diameter (3.18 nm),
which was may be due to blocking of pores by product material. This
blocking of pores may cause the decrease in yield (%) of desired pro-
ducts for reused catalyst.
To understand the stability of the Ag/TFPG-DMB COF catalyst, hot
filtration test [46] was performed on the CO2 fixation reaction by using
ICPAES analysis. The reaction was performed for 6 h in ideal reaction
conditions with 10 mg of catalyst. Then we stopped the reaction and
isolated the catalyst from the reaction mixture and further ran the re-
action for another 4 h in absence of catalyst. Fig. 11 confirms that there
was no further progress of the reaction after isolating the catalyst.
Additionally, no significant presence of leached Ag metal in the ICPAES
analysis of the filtrate also confirmed the stability of the catalyst in
which the porous COF material strongly supports the Ag nanoparticles
on its surface.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
CRediT authorship contribution statement
Arpita Hazra Chowdhury: Data curation, Formal analysis,
Investigation, Methodology, Writing - original draft, Writing - review &
editing. Ipsita Hazra Chowdhury: Data curation, Investigation.
Surajit Biswas: Data curation, Writing - review & editing. Sk. Manirul
Islam: Supervision, Conceptualization, Methodology, Writing - review
& editing.
Acknowledgments
AHC is thankful to University of Kalyani, India for providing her
URS fellowship. SMI is thankful to the Department of Science and
Technology, (DST-SERB project reference no. EMR/2016/004956),
New Delhi, Govt. of India, the Board of Research in Nuclear Sciences
Government of India, (BRNS project reference no. (37(2)/14/03/2018-
BRNS/37003) and Council of Scientific and Industrial Research, (CSIR
project reference no. 02(0284)2016/EMR-II dated 06/12/2016) New
Delhi, Govt. of India, for giving financial support. I.H.C. is thankful to
CSIR, India (09/106 (0181) 2019 EMR-I) for her fellowship. Dr S.
Biswas acknowledges University Grants Commission (UGC) for fi-
nancial support Post-Doctoral Fellowship (Award letter no. F.4-2/
2006(BSR)/CH/16-17/0026). We truthfully acknowledge the UGC and
DST, New Delhi, Govt. of India, for allotting grant under FIST, PURSE
and SAP program to the Department of Chemistry, University of
Kalyani.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.111050.
The relative study of our present work with other reported catalytic
systems is summarized in Table 5 which suggests that Ag/TFPG-DMB
COF showed efficient yield of the Carbamate and Formamide products
with greener and economically benign reaction conditions. We have
performed the reduction and formylation reaction of nitroarenes using
only 10 mg of 1.44 % Ag loaded TFPG-DMB COF catalyst much more
efficiently than other reported catalyst [47].
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