Zn(ii)@TFP-DAQ COF: An efficient mesoporous catalyst for the synthesis of: N -methylated amine and carbamate through chemical fixation of CO2

Article abstract of DOI:10.1039/c9nj04673a

Selective N-methylation and carbamate formation reactions were demonstrated via the chemical incorporation of CO₂ using a Zn-loaded TFP-DAQ COF (covalent organic framework) as an active catalyst under mild reaction conditions. The selective N-methylation and N-formylation reactions were performed by simply varying the type of solvent. The Zn(ii)@TFP-DAQ COF catalyst was characterized via different characterization techniques such as PXRD, FTIR, UV-vis, N₂ adsorption-desorption studies, FESEM and TEM. The catalyst material showed pores in the mesoporous region with a high surface area of 1117.375 m² g^-1. The as-synthesized material was applied as a cheap catalyst for the N-methylation of secondary amines and in carbamate formation reactions with high yields of the desired products up to 98.5% and 97%, respectively, with >99% selectivity. The catalyst was found to be completely heterogeneous and reusable for multiple reaction cycles.

Full text of DOI:10.1039/c9nj04673a

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Zn(II)@TFP-DAQ COF: an efficient mesoporous
catalyst for the synthesis of N-methylated amine
and carbamate through chemical fixation of CO2†
Cite this:DOI: 10.1039/c9nj04673a
a
a
b
a
Priyanka Sarkar, Arpita Hazra Chowdhury, Sk. Riyajuddin,
Surajit Biswas,
b
a
Kaushik Ghosh and Sk. Manirul Islam
*
Selective N-methylation and carbamate formation reactions were demonstrated via the chemical
incorporation of CO using a Zn-loaded TFP-DAQ COF (covalent organic framework) as an active
2
catalyst under mild reaction conditions. The selective N-methylation and N-formylation reactions were
performed by simply varying the type of solvent. The Zn(II)@TFP-DAQ COF catalyst was characterized via
different characterization techniques such as PXRD, FTIR, UV-vis, N
2
adsorption–desorption studies,
Received 11th September 2019,
Accepted 28th November 2019
FESEM and TEM. The catalyst material showed pores in the mesoporous region with a high surface area
2
À1
. The as-synthesized material was applied as a cheap catalyst for the N-methylation of
of 1117.375 m
secondary amines and in carbamate formation reactions with high yields of the desired products up to
8.5% and 97%, respectively, with 499% selectivity. The catalyst was found to be completely
heterogeneous and reusable for multiple reaction cycles.
g
DOI: 10.1039/c9nj04673a
9
rsc.li/njc
Covalent organic frameworks (COFs) are new-age materials,
which are immaculate sets of covalently linked periodically
1
. Introduction
The removal of primary greenhouse gas carbon dioxide (CO
2
) to ordered crystalline porous two-dimensional (2D) materials
5
valuable fine chemicals is very important for environmental having a predictable network of molecular building blocks.
remediation as the continuous accumulation of CO
atmosphere is the major cause of the global environmental adsorption, heterogeneous catalysis, CO
2
in the The shape of a crystallite plays an important role in molecular
6
7
2
capture and conver-
1
8
problem worldwide. Therefore, CO capture and storage (CCS) sion, sensing of metal-ions/molecules, gas separation and
2
9
10a
10b
have turned out to be a promising and fast-growing research gas adsorption, light-harvesting,
pharmaceuticals
and as
1
1
topic for the depletion of the CO emissions from flue gasses. super-capacitors, due to their embedded porosity, crystallinity,
2
2
CO is an easily available and essential component in the global and a highly ordered and low density framework with highly
carbon cycle as well as non-toxic, non-flammable, renewable, accessible BET surface area and thermal stability.
and a low-cost C1 source, and it has great potential value as a
As COFs show high CO
C1 building block for C–C and C–N bond formations. The promising routes for CO
synthesis of fine chemicals by CO
2
adsorption capacity, one of the
utilisation is the synthesis of
2
2
2
capture and conversion N-substituted compounds (e.g., formamides, carbamates, and
could have a major encouraging influence on the worldwide methylamines) via the C–N bond formation from amines under
2
carbon balance. There are several high surface area porous mild conditions, preferably at atmospheric pressure (CO ) and
2
3
materials that support metallic nanoparticles and metal oxides
low temperature; however, this is more difficult and challen-
is thermodynamically stable and kinetically
carboxylate linkers, which were reported as reusable hetero- inert. Carbamates and N-methylamines are widely used as
geneous catalysts for CO fixation reactions for the large scale intermediates for the synthesis of organic dyes, medicines,
production of natural products, agrochemicals, fuels, and fragrances, and for the protection of the amine group.
as well as metal–organic frameworks (MOFs) containing multi- ging because CO
2
4
12
2
1
3
pharmaceutical compounds.
Usually, for the synthesis of formamides and methylamine
from CO and amines with H over metal-based catalysts, high
2
2
temperatures (4100 1C) and/or high pressures (0.5–5 MPa) are
a
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235,
required. However, in our experiment, instead of H , poly-
2
W.B., India. E-mail: manir65@rediffmail.com; Fax: +91-33-2582-8282;
Tel: +91-33-2582-8750
methylhydrosiloxane (PMHS) was used as the reductant since
the polar Si–H bond with a mild reduction potential is kineti-
b
Institute of Nano Science and Technology, Mohali, 160062, India
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj04673a cally more reactive than the H–H bond, which allows milder
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1
4
reaction conditions, and it is cheap, non-toxic, easy-to-handle 2.2. Procedure of N-methylation using the porous
1
5
and can be easily removed from the reaction.
Zn(II)@TFP-DAQ COF Catalyst
However, to date, there is little substrate scope mainly for
methylation, most probably due to the difficulty in obtaining
selectivity in methylation reactions, as formylation and methy-
lation take place in the same system. Identifying catalytic
systems with unique selective conditions for the two reactions
appears to be a key challenge. Moreover, for this, the develop-
ment of efficient, non-toxic, and recoverable catalytic systems
The N-methylation of the respective amine was performed with
a mixture of 2 mmol amines, PMHS (2 equiv.), 5 mL of solvent
and 15 mg of the Zn(II)@TFP-DAQ COF catalyst. The reaction
2
was processed in the presence of CO (1 atm) at 80 1C tem-
perature for 16 h. The COF catalyst was separated via filtration,
and the organic filtrate was washed using a saturated NaHCO3
solution, followed by drying over anhydrous Na SO . The yield
2
4
for the transformation of CO
highly desirable.
2
to N-substituted compounds is
1
of the preferred N-methylated product was analyzed via HNMR
spectroscopy.
Herein, we have developed an efficient Zn-based COF
(
Zn(II)@TFP-DAQ) heterogeneous catalyst, and we have repre- 2.3. Procedure carbamate synthesis using the porous
sented a new methodology using the Zn(II)@TFP-DAQ catalyst Zn(II)@TFP-DAQ COF catalyst
for the construction of C–N bonds from CO and amines to
2
Carbamate synthesis from aniline (2 mmol) and n-butyl bromide
2 mmol) was processed under a CO2 atmosphere at room
synthesize N-methylated amines and carbamate products under
mild conditions. Amusingly, the Zn(II)@TFP-DAQ catalyst
exhibited outstanding selectivity for both the formylation and
methylation of amines. The substrate scope covers numerous
different nitrogenous substrates for the reactions with 499%
selectivity. To the best of our knowledge, it is the first report on
(
temperature for 8 h utilizing 20 mg Zn(II)@TFP-DAQ COF as the
catalyst, and 10 mL DMSO as the solvent. After the completion of
the reaction, the catalyst was separated and the organic portion
was collected. The yield of the carbamate product was analyzed via
1
HNMR spectroscopy.
Zn(II)@TFP-DAQ COF as a catalyst utilized in these two CO
incorporation reactions.
2
3
. Results and discussion
3.1. Characterization
2. Experimental
3.1.1. Powder X-ray diffraction (PXRD) analysis. The pro-
2
.1. Synthetic procedure for Zn(II)@TFP-DAQ COF
Initially, 1 g of the as-synthesized TFP-DAQ COF and 0.1 g of
anhydrous ZnCl salt were taken in a 50 mL round bottom (RB)
duction of TFP-DAQ COF was established via the PXRD analysis,
as depicted in Fig. 1. The characteristic peaks present at 3.6
and 27.2 correspond to the (100) and (001) diffraction planes,
respectively. The peak at 27.11 arises from the p–p stacking of the
2
flask containing 25 mL ethanol. The RB flask was fitted with a
condenser and a magnetic stirrer. The reaction was processed
for 10 h at 70 1C temperature. Then, the resulting Zn-loaded
TFP-DAQ COF, i.e., Zn(II)@TFP-DAQ, was obtained through
filtration and washing with ethanol to remove any unreacted
16
(001) plane. The PXRD pattern of Zn(II)@TFP-DAQ COF was
similar to that of the AA-eclipsed stacking model, and this results
16
matched with a previously reported literature.
.1.2. FTIR analysis. The FTIR spectra of TFP, DAQ,
TFP-DAQ COF, and Zn(II)@TFP-DAQ COF catalyst are shown
3
2
ZnCl and finally, it was vacuum dried for 12 h at room
temperature (Scheme 1).
À1
À1
in Fig. S1 (ESI†). The peaks at B1266 cm and B1554 cm
are ascribed to the –(C–N) and –(CQC) bonds. The peaks in
À1
the FTIR spectra of DAQ in the range of 3418–3200 cm
correspond to the –(N–H) bonds, which completely disappeared
Fig. 1 Powder XRD patterns of the TFP-DAQ COF and Zn(II)@TFP-DAQ
Scheme 1 Synthetic route of Zn(II)@TFP-DAQ COF.
COF samples.
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Fig. 2
2
N adsorption–desorption isotherm of Zn(II)@TFP-DAQ COF at
7
7 K. Pore size distribution plot (inset).
Fig. 3 FE-SEM images of the TFP-DAQ COF (a and b) and Zn(II)@TFP-DAQ
COF (c and d) samples at different magnifications.
in the FTIR spectra of TFP-DAQ COF, thus confirming the
complex formation between TFP and DAQ. The sharp peak at
À1
1
–
609 cm in the spectra of TFP-DAQ COF is ascribed to the
À1
(CQO) bonds. The disappearance of the 1609 cm peak in
the spectra of Zn(II)@TFP-DAQ COF catalyst indicates the
coordination of –(CQO) to the Zn metal.
3.1.3. UV-vis analysis. Fig. S2 (ESI†) illustrates the solid-
state UV-visible absorbance spectra of the TFP-DAQ COF and
Zn(II)@TFP-DAQ COF catalysts. Both the materials showed
broad absorption spectra in the range from 200 to 460 nm with
a shoulder band at 490 nm. The absorption spectra include
1
7
many transitions in the highly ordered conjugation systems.
.1.4. Surface area measurement. The N adsorption–
3
2
desorption isotherms of the Zn(II)@TFP-DAQ COF catalyst is
depicted in Fig. 2. The subsequent pore size distribution (PSD)
resulting from the respective desorption data is given in the
index of the respective isotherm plot. The Zn(II)@TFP-DAQ COF
catalyst has a narrow peak at the 3.5 nm pore diameter and has
some broad peaks of pores in the range from 4.6 nm to 35 nm,
which confirms the mesoporous nature of the catalyst.
The mean pore size was found to be 14.8 nm. The estimated
BET surface area and total pore volume of the Zn(II)TFP-DAQ
Fig. 4 EDS mapping images of Zn(II)@TFP-DAQ COF: (a) electron
microscopic image, C (b), N (c), O(d), Zn (e) element EDS mapping.
2
À1
3
À1
COF were observed to be 1117.375 m g and 1.31 cm g
respectively.
,
3.1.5. Microscopic analysis. The FESEM images (Fig. 3)
of the TFP-DAQ COF (Fig. 3a and b) and Zn(II)@TFP-DAQ
COF (Fig. 3c and d) samples show the disordered fibrous
morphology.
The elemental mapping images of carbon, nitrogen, oxygen,
and Zn of the catalyst material are shown in Fig. 4a–e. The
interparticle porosity of the mesoporous COF material is con-
firmed by the TEM images (Fig. 5a and b). The elemental
composition of the TFP-DAQ COF and Zn(II)@TFP-DAQ COF
materials are confirmed by the EDAX patterns (Fig. 6a and b) of
the materials.
Fig. 5 TEM images of Zn(II)@TFP-DAQ COF sample.
3.1.6. XPS analysis. Fig. 7 shows the XPS spectra of Zn in
the catalyst sample, where the Zn 2p3/2 and Zn 2p1/2 peaks splitting distance at 23 eV indicated the Zn presence in the
+2
18
appeared at 1022.1 eV and 1045.1 eV, respectively. The spin–orbit catalyst system as the Zn state.
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Table 1 Effect of solvent and temperature on the N-methylation of
a
N-methyl aniline
b
b
Temperature Yield (%) (A)
Yield (%) (B)
Entry Solvent
(1C) [selectivity (%)] [selectivity (%)]
1
2
3
4
5
6
7
8
Acetonitrile 80
1,4-Dioxane 80
499 [499]
98.5 [499]
—
—
—
—
THF
Toluene
DMF
80
80
80
498 [499]
—
—
—
—
—
—
Acetonitrile 50
Acetonitrile r.t.
57 [499]
11 [499]
—
Fig. 6 EDAX pattern of the TFP-DAQ COF (a) and Zn(II)@TFP-DAQ COF
c
—
80
(
b) samples.
a
2
Reactions performed at atmospheric pressure (1 atm) of CO using
N-methyl aniline (2.0 mmol), Zn(II)@TFP-DAQ COF catalyst (15 mg),
b
c
PMHS (2 equiv.), solvent (5 mL), 16 h. GC yield. Without solvent.
Scheme 3 Probable products of the N-methylation reaction of N-methyl
aniline.
Fig. 7 XPS spectra of Zn 2p peak of Zn(II)@TFP-DAQ COF.
reaction showed N-formylated product as the major product, with
a high yield (Table 1, entry 3). Interestingly, in the presence of
different solvents, either the N-methylated or the N-formylated
products was obtained with high selectivity (499%). In the
presence of toluene and DMF, there was no reaction even at high
temperatures (Table 1, entries 4 and 5). At room temperature, in
the presence of acetonitrile, only 11% product yield of the
N-methylated product was found (Table 1, entry 7). With the
increase in temperature, the product yield was also increased and
the highest yield was obtained at 80 1C temperature (Table 1,
entry 1). It is very important to note that the selectivity of the
desired products was always maintained 499% for all the reactions
studied to optimize the reaction. There was no product found in
the absence of any solvent, even at 80 1C temperature (Table 1,
entry 8).
3
.2. Catalytic activity
.2.1. N-Methylation of amines catalyzed by the Zn(II)@
TFP-DAQ COF. The chemical fixation of CO via the N-methyla-
3
2
tion of amine over the mesoporous Zn(II)@TFP-DAQ COF
catalyst in the presence of hydrosilane under 1 atmospheric
pressure of CO at 80 1C temperature is depicted in Scheme 2.
2
The yield of the preferred product was optimized using
different kinds of solvents and varying the reaction temperature
(Table 1). Both of the factors showed a noticeable effect on the
product yield. The probable side product of this N-methylation
reaction is the N-formylated amine (Scheme 3B). Therefore,
we have studied the selectivity of the methylation reaction.
The N-methylation reaction was studied in the presence of
different solvents and different reaction temperatures (Table 1).
Therefore, we have used different kinds of solvents, such as
acetonitrile, 1,4-dioxane, THF, DMF, and toluene, to increase
the yield of the product. Among the used solvents, acetonitrile
The reaction was also standardized by varying the catalyst
loading and amount of PMHS (Table 2). The amount of PMHS
was varied from 0.5 equiv. to 2 equiv. (Table 2, entries 1–3).
was found to be the most efficient solvent for the synthesis of the Table 2 Effect of catalyst loading and the amount of PMHS on the
a
N-methylation of N-methylaniline
N-methylated product with a high yield of 499% (Table 1, entry 1).
1
,4-Dioxane also showed a high yield of the N-methylated product
b
b
Catalyst
loading (mg)
Amount of
PMHS (equiv.)
Yield
Yield
(Table 1, entry 2). Moreover, in the presence of THF as a solvent, the Entry
(%) (A)
(%) (B)
1
2
3
4
5
6
7
15
15
15
10
5
—
15
0.5
1
2
2
2
2
—
Trace
25
499
53
23
—
—
—
—
—
—
—
—
c
d
—
a
2
Reactions performed at atmospheric pressure (1 atm) of CO using
N-methylaniline (2.0 mmol), Zn(II)@TFP-DAQ COF catalyst, PMHS,
b
c
d
Scheme 2 N-Methylation of amines through the chemical fixation of 80 1C, 16 h, acetonitrile (5 mL). GC yield. Without catalyst. Without
CO
2
.
PMHS.
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Fig. 8 Reaction rate study in the presence and in the absence of the
catalyst.
Fig. 9 Proposed mechanistic pathway for the N-methylation of amine
over the Zn(II)@TFP-DAQ COF catalyst.
By using 1 equiv. of PMHS, only 25% of the methylated product
was obtained (Table 2, entry 2), and the yield excellently
increased to 499% when using 2 equiv. of PMHS (Table 2,
entry 3). With the increase in the amount of the catalyst loading
from 5 mg to 15 mg, the methylated product yield also increased
from 23% to 499% (Table 2, entries 3–5). No product was
obtained in the absence of a catalyst (Table 2, entry 6) and also,
no reaction occurred in the absence of PMHS (Table 2, entry 7).
We have studied the reaction rates with time in the presence
as well as in the absence of the Zn(II)@TFP-DAQ COF catalyst
COF catalyst, 2 equiv. of PMHS under CO2 balloon at 80 1C
temperature in the presence of acetonitrile. Most of the secondary
amines showed excellent yield (490%) of the corresponding
N-methylated products (Table 3, entries 1–4) except N-phenyl-
benzylamine, which showed a slightly poor yield (83%) (Table 3,
entry 5) and that could be due to the steric hindrance of the bulky
benzyl group.
1
9,20
The probable mechanistic route
for the synthesis of
N-methyl amine via CO2 incorporation over the Zn(II)@TFP-
DAQ COF catalyst is shown in Fig. 9. The N-methylated product
was obtained, when the Zn metal activated the Si–H bond of
(Fig. 8). It was clearly observed that when the reaction was
performed in the presence of the catalyst, the reaction rate
sharply increased with a high slope of the curve and finished
within 16 h. However, when the reaction was performed in the
absence of the catalyst, a very slow increase in the reaction rate
was observed, and a yield of o5% was found even after 18 h of
reaction time.
2
hydrosiloxane (PMHS), followed by the CO incorporation into
the Si–H bond. As PMHS has multiple Si–H bonds, it effectively
supplies hydrogen to the reaction system for the carbonyl group
reduction to obtain the N-methylated amine.
3.2.2. Alkyl and aryl carbamate synthesis catalyzed by
The N-methylation reaction was performed on several types of
secondary amines (2 mmol) using 15 mg of the Zn(II)@TFP-DAQ
Zn(II)@TFP-DAQ COF. The mesoporous Zn(II)@TFP-DAQ COF
material was used as a catalyst for the carbamate synthesis
reaction (Scheme 4) in the absence of any base and temperature.
The reaction condition was optimized using the synthetic
procedure of butyl N-phenyl carbamate (Scheme 5). The effect
of parameters, such as reaction time (Table 4, entries 1–4) and
different solvents (Table 4, entries 4–6), were studied. Among
the solvents, DMSO and DMF (Table 4, entries 4 and 5) showed
a better yield of the needed carbamate product than toluene
(Table 4, entry 6). DMSO showed the best yield of the product.
Therefore, we have chosen DMSO as the reaction solvent for the
a
Table 3 N-Methylation catalysed by Zn(II)@TFP-DAQ COF
b
À1
Entry Amine
1
Product
Yield (%) TON
TOF (h )
2
98.5
93
4.86 Â 10 30.4
2
2
3
4
4.59 Â 10 28.7
2
94
4.64 Â 10 29.0
2
96
4.74 Â 10 29.6
2
Scheme 4 Synthesis of carbamates through the chemical fixation of CO .
2
5
83
4.10 Â 10 25.6
a
2
Reactions performed at atmospheric pressure (1 atm) of CO using
N-methylaniline (2.0 mmol), Zn(II)@TFP-DAQ COF catalyst (15 mg),
PMHS (2 equiv.), 80 1C, 16 h, acetonitrile (5 mL). Isolated yield.
b
2
Scheme 5 Butyl N-phenyl carbamate synthesis via CO incorporation.
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Table 4 Effect of solvent and reaction time on carbamate synthesis carbamate synthesis reaction. In the absence of any solvent,
a
reaction
only 32% of the product was obtained (Table 4, entry 7). In the
b
presence of DMSO as the solvent, it was interesting to note
that with an increase in the reaction time from 1 h to 8 h, the
yield of the product also increased from 14% to 98% (Table 4,
entries 1–4).
We have studied the reaction rates with time in the presence
as well as in the absence of Zn(II)@TFP-DAQ COF catalyst
(Fig. 10). It was clearly observed that when the reaction was
carried out in the presence of the catalyst, the reaction rate
Entry
Solvent
Reaction time (h)
Yield (%) [selectivity (%)]
1
2
3
4
5
6
DMSO
DMSO
DMSO
DMSO
DMF
Toluene
—
1
3
5
8
8
8
8
14 [499]
32 [499]
69 [499]
98 [499]
85 [499]
72 [499]
32 [499]
c
7
a
2
Reactions performed under CO (1 atm) at room temperature using
0 mg of catalyst, 2 mmol amine, 2 mmol of alkyl/aryl bromide and sharply increased with a high slope of the curve and finished
0 mL solvent. GC yield of carbamate. Without solvent.
2
1
b
c
within 8 h. However, when the reaction was performed in the
absence of the catalyst, a very slow increase in the reaction rate
was observed and a yield of o5% was found even after 10 h of
the reaction time.
The carbamate synthesis reaction was studied under an
optimized reaction condition on aniline, p-anisidine, p-chloro-
aniline and N-methylaniline with butyl bromide and benzyl
bromide (Table 5, entries 1–8). Aniline and p-chloroaniline
showed a very good yield of the carbamate products with the
alkyl bromide (Table 5, entries 1 and 3) as well as with the aryl
bromide (Table 5, entries 5 and 7). Moreover, p-anisidine and
N-methylaniline showed a relatively poor yield of the carbamate
products with the alkyl and aryl bromides (Table 5, entries 2,
4, 6 and 8).
3.3. Heterogeneity test
The Zn loading percentage (%) was studied in Zn(II)@TFP-DAQ
Fig. 10 Reaction rate study in the presence and in the absence of the
catalyst.
COF via atomic absorption spectroscopy (AAS), which was
a
Table 5 Carbamate synthesis catalyzed by Zn(II)@TFP-DAQ COF
b
À1
Entry
1
Amines
Bromide
Products
Yield (%)
TON
TOF (h
)
2
2
97
84
95
80
3.6 Â 10
45.0
2
3
4
3.1 Â 10
3.5 Â 10
2.9 Â 10
38.8
43.7
37.0
2
2
2
2
5
6
7
96
80
3.5 Â 10
2.9 Â 10
44.4
37.0
2
2
93
72
3.4 Â 10
2.6 Â 10
43.0
33.3
8
a
2
Reactions performed under CO (1 atm) at room temperature using 20 mg of catalyst, 2 mmol amine, 2 mmol of alkyl/aryl bromide and 10 mL
b
DMSO, 8 h. Isolated yield of carbamate.
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the Zn(II)@TFP-DAQ COF catalyst was completely hetero-
geneous in nature.
3.4. Recyclability of the Zn(II)@TFP-DAQ COF catalyst
One of the most important characteristics of heterogeneous
catalysts is the recyclability of the material. We have checked
the recyclability of the catalyst for five consecutive cycles for
both the reactions via collecting the catalyst by centrifugation
after completing each cycle. The result is shown in Fig. 12,
which indicates that there was almost no considerable change
in the yields of the products, suggesting no catalyst deactiva-
tion. The FESEM image of the reused catalyst (Fig. S3, ESI†)
suggested that the catalyst retained its structural characteristics
after recycling. We have also studied the structural details and
pore structure of the reused catalyst after five reaction cycles via
XRD (Fig. S4, ESI†) and nitrogen adsorption/desorption study
Fig. 11 Heterogeneity test suggesting the minimal leaching of the active
metal into the filtrate solution.
(Fig. S5, ESI†). The XRD showed no considerable changes in the
reused catalyst. The nitrogen adsorption/desorption study of
the reused catalyst showed a decrease in the BET surface area
2
À1
(
700.545 m g ) with a relatively smaller pore size (3.2 nm) due
to blocking of the pores because of the use of PMHS in the
reaction. We have also done the EDS mappings (Fig. S6, ESI†) of
the recovered catalyst, which showed the presence of silicon,
suggesting the existence of a by-product derived from the
PMHS on the catalyst surface. Furthermore, we have done the
EDAX analysis of the reused catalyst, which also confirmed
the presence of silicon from the PMHS in the reused catalyst
Fig. 12 Recyclability diagram of the mesoporous Zn(II)@TFP-DAQ COF (Fig. S7, ESI†).
catalyst.
From the comparative chart (Table 6), it can be clearly
observed that the mesoporous Zn(II)@TFP-DAQ COF catalyst
showed a relatively improved yield of the desired N-methylated
and carbamate products with much higher TOF (h ) values
than the other reported catalysts under 1 atmospheric pressure
2
of CO .
observed to be 1.76%. The heterogeneous property of the
Zn(II)@TFP-DAQ COF catalyst was studied via a hot filtration
test. The test was conducted on the N-methylation reaction of
N-methylaniline. After conducting the reaction for 8 h, a 56%
yield was obtained. Then, the catalyst was isolated from the
reaction mixture, and the reaction was conducted again in
the absence of the catalyst for another 10 h. It was observed that
À1
4. Conclusions
the yield of the product remained the same (approximately 56%) A mesoporous Zn(II)@TFP-DAQ COF catalyst was synthesized
after separating the catalyst (Fig. 11), which implied that there with a high surface area. This cheap catalyst material showed
was no further reaction happening after isolating the catalyst. efficient catalytic performance for CO incorporation reactions
2
There was no leaching of the active metal found via the ICP-AES such as the N-methylation of secondary amines and carbamate
study of the filtrate, implying minimal zinc metal leaching from synthesis under atmospheric pressure of CO with high yield as
2
the porous TFP-DAQ COF support throughout the reaction, and well as high selectivity of the needed products. In addition, the
Table 6 Comparison chart of the mesoporous Zn(II)@TFP-DAQ COF catalyst with other reported catalysts
Yield (%)/
À1
Reaction
Catalyst
Reaction condition
Time (h) TOF (h
)
Ref.
21
N-Methylation Rhodium perimidine-based
N-Methyl aniline (0.5 mmol), PhSiH
3
(2 mmol, 4 equiv.), 16
498/24.5
NHC complex
catalyst (5 mol%), 90 1C, 2 mL toluene, 1 atm of CO
2
Mesoporous Zn(II)@TFP-DAQ N-Methyl aniline (2 mmol), PMHS (2 equiv.),
16
98.5/30.4
This study
COF
catalyst (15 mg), 80 8C, 5 mL, acetonitrile, 1 atm of CO
2
Carbamate
synthesis
Zeolite-beta
Aniline (10 mmol), n-butyl bromide (10 mmol),
4
52.8/5.9
22
catalyst (150 mg), CO
Mesoporous Zn(II)@TFP-DAQ Aniline (2 mmol), n-butyl bromide (2 mmol),
COF catalyst (20 mg), 10 mL DMSO, RT, 1 atm of CO
2
(3.4 bar), 353 K
8
97/45.0
This study
2
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reusability of the catalyst material was observed multiple times,
signifying the mesoporous Zn(II)@TFP-DAQ COF material as a
potentially active, new and cheap heterogeneous catalyst for
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CO incorporation reactions.
2
Conflicts of interest
There are no conflicts to declare.
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(a) J. Yu and P. B. Balbuena, J. Phys. Chem. C, 2013,
0
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financial support. P. Sarkar is thankful to CSIR, New Delhi, Govt. of
India, for her CSIR-JRF fellowship. A. Hazra Chowdhury is thankful
to the University of Kalyani, India for providing her URS fellowship.
S. Biswas acknowledges the University Grants Commission for his
D. S. Kothari Post Doctoral Fellowship (Award letter no. F.4-2/2006
41017–41024; (d) Y. Wu, H. Xu, X. Chen, J. Gao and
D. Jiang, Chem. Commun., 2015, 51, 10096–10098; (e) V. S.
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Chemistry, the University of Kalyani under PURSE, FIST and SAP
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