Immobilization of ionic liquids to covalent organic frameworks for catalyzing the formylation of amines with CO2 and phenylsilane

Article abstract of DOI:10.1039/c6cc03058k

We presented the immobilization of ionic liquids on the channel walls of COFs using a post-synthetic strategy. The ionic [Et₄NBr]_50%-Py-COF afforded a high CO₂ adsorption capacity of 164.6 mg g^-1 (1 bar, 273 K) and was developed as an effective heterogeneous catalyst for the transformation of CO₂ into value-added formamides under ambient conditions.

Full text of DOI:10.1039/c6cc03058k

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Immobilization of Ionic Liquids to Covalent Organic Frameworks
for Catalyzing Formylation of Amines with CO2 and Phenylsilane
Bin Dong,a Liangying Wang,a Shang Zhao,a Rile Ge,a Xuedan Song,b Yu Wang,a Yanan Gaoa,*
Received 00th April 2016
Accepted 00th April 2016
DOI: 10.1039/x0xx00000x
www.rsc.org/
We presented the immobilization of ionic liquids on the channel
walls of COFs by post-synthesis strategy. The ionic [Et4NBr]50%-Py-
COF afforded a high CO2 adsorption capacity of 164.6 mg/g (1 bar,
273 K) and was developed as an effectively heterogeneous
catalyst for transformation of CO2 to value-added formamides
under ambient conditions.
As an emerging class of porous and crystalline materials, covalent
organic frameworks (COFs) are synthesized based on the principles
of reticular chemistry, where organic building blocks with pre-
designed geometries and radicals are linked through the formation
of covalent bonds to give extended networks.1–6 These materials
exhibit periodic architectures, low densities, and high porosity,
rendering them ideal candidates for various applications in
catalysis,7–10 gas storage and separation,11–18 and optoelectronics.19–
22
The nanosized and charged pores in the ionic architectures
exhibit special properties at adsorption and separation towards gas
molecules.23,24 The ionization approach or chemical modification
with ionic compounds for the COF materials may afford an
opportunity to fine-tune their structures and properties, through
which the frameworks are charged either positively or negatively, Scheme 1 Synthesis of [Et4NBr]X%-Py-COFs with the ionization of
chanel walls through the Williamson ether reaction of [HO]X%-Py-
COFs with (2-bromoethyl)triethylammonium bromide. The [HO]X%
coupled with counter ions. Due to the restriction of available
building blocks, the reports on ionic COFs were limited.8,25
-
Py-COFs were synthesized by the condensation of PyTTA with DHPA
and PA at various molar ratios.
facilitates mass transfer with chemical reactions.7–10 Therefore, the
targeted synthesis of two dimensional (2D) COFs can afford
recoverable, metal-free catalytic systems for the transformation of
CO2.
CO2 contributes to global warming but also is a cheap, abundant
C1 resource. The chemical fixation of CO2 is significant since it
allows the transformation of waste CO2 to value-added products.26–
28
Formamides are important chemicals, generally produced
through the formylation of amines. Using CO2 for the formylation of
N-H bonds is attractive yet challenging due to the thermodynamic
stability of CO2.29–33 The reported systems basically suffered from
drawbacks such as difficulty in catalyst recovery, requiring metal
catalysts, or rigorous reaction conditions. Ordered one-dimensional
(1D) open channels found in two-dimensional covalent organic
frameworks are beneficial to the diffusion of substances, which
Herein, we described a postsynthetic strategy for the ionization
of the channel walls in 2D COFs and presented its effectiveness in
CO2 adsorption and transformation to highly value-added
formamides. Mesoporous imine-linked pyrene COFs were utilized as
the scaffold, which were initially synthesized by a three-component
condensation system (Scheme 1). 4,4′,4′′,4′′′-(Pyrene-1,3,6,8-
tetrayl) tetraaniline (PyTTA) was used as the vertices and two
a.Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
China. E-mail: ygao@dicp.ac.cn
linearly
shaped
dialdehyde
monomers,
namely
2,5-
b.State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian
116024, China.
dihydroxyterephthalaldehyde (DHPA) and 1,4-phthalaldehyde (PA),
at various molar ratios were used as edges. The COFs with different
phenol contents on their channel walls were fabricated and
† Electronic Supplementary Information (ESI) available: Materials, characterization,
and additional data. See DOI: 10.1039/x0xx00000x
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nominated as [HO]X%-Py-COFs (X = 0, 25, 50, 75, 100), where X%
represented the molar percentage of DHPA present in the
dialdehyde blend. The immobilization of ionic liquids onto COFs was
directly accomplished through the Williamson ether reaction
between the phenol group in [HO]X%-Py-COFs and (2-
bromoethyl)triethylammonium bromide ionic liquid (Scheme 1).34
The ionization degree on the channel walls was modulated by
controlling the content of phenol groups. The resulting ionic COFs
were termed as [Et4NBr]X%-Py-COFs (X = 25, 50). To the best of our
knowledge, this is the first example that COFs have been
successfully decorated with catalytically-active ionic compounds.
The crystalline nature of all the COFs was confirmed by powder
X-ray diffraction (PXRD) measurements. The [HO]0%-Py-COF sample
exhibited strong XRD peaks at 3.72, 5.23, 7.47, 8.67, 11.29, 15.17,
and 23.69°, which were assignable to (110), (020), (220), (030),
(040), (060), and (001) facets, respectively (Fig. 1a), similar to the
reported [HO]100%-Py-COF.35 The identical diffraction patterns were
also observed for other [HO]X%-Py-COFs (Fig. S1). The [Et4NBr]X%-Py-
COFs (X = 25, 50) exhibited XRD patterns (Fig. 1b) similar to those of
[HO]X%-Py-COFs, indicating that they owned the similar crystal
structures. Nevertheless, the [Et4NBr]X%-Py-PAFs (X = 75, 100)
exhibited the amorphous frameworks (Fig. S2), indicating that the
crystal structures were retained after modification only when the
ionization degree on the channel walls was moderate. The following
Fig. 2 (a) N2 sorption isotherms measured at 77 K, (b) BET surface
area, (c) CO2 adsorption isotherms under 273 K (solid symbol) and
298 K (opean symbol), and (d) the respective isosteric heats of
adsorption for CO2 of [HO]X%-Py-COFs and [Et4NBr]X%-Py-COFs (X =
25, 50).
[Et4NBr]X%-Py-COFs (X = 25, 50) with controlled ion density were
confirmed by several technologies. Fourier transform infrared (FTIR)
spectroscopy (Fig. S4) and solid-state 13C cross-polarization/magic-
angel spinning (CP/MAS) NMR spectroscopy (Fig. S5) supported the
grafted ionic liquids onto [Et4NBr]X%-Py-COFs (X = 25, 50), implying
the validity of this modification strategy. The contents of bromide
ions across [Et4NBr]X%-Py-COFs (X = 25, 50) were determined by ion
chromatography (4.6% and 8.9%, respectively). The obtained values
were lower than the theoretical values (8.1% and 13.2%,
respectively), resulting from the incomplete conversion of the
Williamson ether reaction. Thermogravimetric analysis (TGA)
revealed that the [Et4NBr]X%-Py-COFs (X = 25, 50) exhibited high
thermal stability with the decomposition temperature over 300 °C
(Fig. S6).
examinations
were
focused
on
the
COF
materials
unless otherwise stated. The use of lattice modeling and Pawley
refinement processes led to an eclipsed AA stacking model that
could reproduce the PXRD results in the peak position and intensity
with Rwp of 8.8% and Rp of 6.7% (Fig. S3). The unit cell was created
with a PMMN space group of a = 33.7188 Å, b = 32.6690 Å, c =
3.8206 Å, and α = β = γ = 90°. By contrast, an alternatively staggered
AB model did not match the observed data.
The three-component condensation reaction was allowed for
the integration of phenol groups in [HO]X%-Py-COFs with
a
controlled manner (Table S1, Fig. S4,S5).16,17 After modification, the
The porous properties of the COFs were determined by nitrogen
sorption isotherms measured at 77 K. The [HO]X%-Py-COFs exhibited
typical type-IV isotherms characteristic of mesoporous nature (Fig.
2a and Fig. S7), while the [Et4NBr]X%-Py-COFs (X = 25, 50) exhibited
type-I isotherms (Fig. 2b). The Brunauer–Emmett–Teller (BET)
surface area, pore volume, and pore size were all decreased, as
expected, when [HO]X%-Py-COFs were modified through
immobilization of ionic liquids (Fig. S8 and Table S2). These results
indicated that the tetraalkylammonium cations together with
counter anions occupied the pore space. Note that these COFs
basically possessed one main type of pore in the framework as
shown in the pore size distribution profiles (Fig. S9), revealing that
the tetraalkylammonium cations were randomly integrated onto
the pore walls of [Et4NBr]X%-Py-COFs (X = 25, 50).
The CO2 adsorption experiments were then performed at
pressures up to 1 bar at 273 and 298 K (Fig. 2c). As listed in Table
S3, the [Et4NBr]X%-Py-COFs (X = 25, 50) exhibited elevated CO2
uptake capacities and higher isosteric heats of adsorption (Qst) (Fig.
2d and Fig. S10) than those of [HO]X%-Py-COFs, supporting the
effectiveness of pore-wall functionalization for CO2 capture. The
[Et4NBr]50%-Py-COF owned the CO2 uptake capacity of 164.6 mg/g at
Fig. 1 PXRD patterns of (a) [HO]X%-Py-COFs (X = 0, 25, 50) and (b)
[Et4NBr]X%-Py-COFs (X = 25, 50). Right: Top views of [HO]50%-Py-COF
and [Et4NBr]50%-Py-COF. Carbon, cyan; nitrogen, blue; and hydrogen
atoms and counter anions were omitted for clarity.
2 | Chem. Commun., 2016, 00, 1-4
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bar and 273 Table
1 The [Et4NBr]50%-Py-COFs catalyzed
formylation of amines 1 with CO2 and PhSiH3 at room temperature.
Entry
1
Amine substrates (1)
(1a)
Product (2)
Yield (%)a
94
2a
Scheme 2 Possible reaction mechanism of the [Et4NBr]50%-Py-COF
catalyzed formylation of amines using CO2 and PhSiH3 at room
temperature.
2
3
4
5
(1b)
(1c)
(1d)
(1e)
2b
2c
2d
2e
91
92
new signal at 8.24 ppm was attributed to formoxysilane,
demonstrating that the reaction of PhSiH3 with CO2 catalyzed by
[Et4NBr]50%-Py-COF. In addition, the chemical shift of N-H in amine
was observed in the 1H NMR spectra of N-methylaniline with
[Et4NBr]50%-Py-COF (Fig. S13), probably caused by the hydrogen
bond between the amine and bromide ion of [Et4NBr]50%-Py-COF. All
these results suggested that the [Et4NBr]50%-Py-COF behaved as a
bifunctional catalyst, which activated PhSiH3 to react with CO2
yielding formoxysilane, and activated the amine through the
hydrogen bond (Scheme 2). It can be assumed that the ordered
open channels and high CO2 uptake capacity of [Et4NBr]50%-Py-COF
allows formylation reactions to be conducted under mild
conditions. As a heterogeneous catalyst, the [Et4NBr]50%-Py-COF can
be easily recovered from the reaction mixture by a simple filtration
and reused for four times (Fig. S14). As evidenced by the decrease
92
88b
aIsolated yield. bReaction time (48 h)
K, higher than the most reported COFs (Table S4). The high uptake
and Qst for CO2 of [Et4NBr]X%-Py-COFs can be mainly ascribed to the
following factors: (i) after immobilization of ionic liquids on pore
walls, the pore sizes are further reduced to be micropores, which
in the BET surface area and in the crystallization of the [Et4NBr]50%
-
enhances the performance of CO2 capture and separation since Py-COF after the fourth cycle (Fig. S15,S16), the slightly decreased
activity was due to the pore channels blocked after repetitive
recycling.
micropores or ultramicropores dominate the low-pressure
adsorption via pore-filling process;36,37 (ii) the high positive charge
density of the tetraalkylammonium cations strengthens their
interaction with CO2.38
In summary, we developed a postsynthetic approach to the
immobilization of ionic liquids on the channel walls of COFs and
demonstrated a metal-free method for the formylation of amine
with CO2 and PhSiH3 catalyzed by the as-synthesized ionic COF.
Different types of formamides were produced in excellent yields
under 1 bar CO2 and room temperature. The ordered open channels
and high CO2 uptake capacity of the ionic COF allows formylation
With the ionic liquids grafted on the channel walls, the ionic
COFs were explored to act as the heterogeneous catalyst for the N-
formylation reactions of different amines. In the initial experiments,
the formylation of N-methylaniline (1a) with CO2 and phenylsilane
(PhSiH3) was used as a model reaction. With the amount of 5 mol%
[Et4NBr]50%-Py-COF, the formylation of 1a can occur at 1 bar CO2
and 30 oC with DMF as the solvent. The solely formylated product
2a was obtained with an isolated yield of 94% (Table 1, entry 1).
Nevertheless, the [HO]50%-Py-COF showed a very low catalytic
activity with the isolated yield of 32% under the same reaction
conditions. Using the [Et4NBr]50%-Py-COF as the catalyst, N-
methylanilines with electron-donating (methyl methoxyl) or
electron-withdrawing groups (4-choloro- and 4-fluoro-) were well
tolerated, producing the formamides in excellent isolated yields
(Table 1).
reactions to be conducted under mild conditions. As
a
heterogeneous catalyst, the ionic COF can be reused at least four
times without evident activity loss. We expect the ionic COFs would
be modified through other charged guest molecules by means of
ion exchange, making them a promising platform for various
applications.
The authors gratefully acknowledge financial support from the
National Natural Science Foundation of China (21273235, 21303076
and 21473196), the 100-Talents Program of Chinese Academy of
Sciences, the Liaoning Provincial Natural Science Foundation &
Shenyang National Laboratory for Materials Science (2015021018),
To gain insight into the role of the [Et4NBr]50%-Py-COF, the 1H
1
NMR analysis was performed. From the H NMR spectra of PhSiH3 and the Youth Innovation Promotion Association of the Chinese
and its mixture with [Et4NBr]50%-Py-COF in DMF (Fig. S11), the signal
assigning to Si-H of PhSiH3 was shifted from 4.16 to 4.12 ppm,
suggesting the interaction between PhSiH3 and [Et4NBr]50%-Py-COF.
In other words, the Si-H bond of PhSiH3 was activated by
[Et4NBr]50%-Py-COF, which favored the insertion of CO2. To confirm
this, the mixture of PhSiH3 and [Et4NBr]50%-Py-COF in DMF was
prepared at 30 oC and 1 bar CO2, which was examined by 1H NMR
analysis after being stirred for 10 h and CO2 release (Fig. S12). A
Academy of Sciences.
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