Communication
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ABSTRACT: Three-dimensional covalent organic frameworks (3D-COFs) are emerging as designable porous materials because of
their unique structural characteristics and porous features. However, because of the lack of 3D organic building units and the less
reversible covalent bonds, the topologies of 3D-COFs to date have been limited to dia, ctn, ffc, bor, rra, srs, pts, lon, stp, acs, tbo,
bcu, and fjh. Here we report a 3D-COF with the ceq topology utilizing a D -symmetric triangular prism vertex with a planar
3h
triangular linker. The as-synthesized COF displays a twofold-interpenetrated structure with a Brunauer−Emmett−Teller surface area
2
−1
of 1148.6 m g . Gas sorption measurements revealed that 3D-ceq-COF could efficiently absorb CO , CH , and H under a
2
4
2
moderate surface area. This work provides new building units and approaches for structural and application exploration of 3D-COFs.
ovalent organic frameworks (COFs) −10 have made great
1
of 2D-COFs for the following reasons: (1) In contrast to
MOFs, in which the metal-containing units are generally three-
dimensional multidots, the building units in COFs are straight-
line bidots or planar tri-, tetra-, or hexadots because of the
organic compounds’ topological limitations. Lack of three-
dimensional building units creates inherent difficulty in the
exploration of 3D-COFs. (2) The formation reactions of COFs
are less reversible than the coordination reactions in the
formation of MOFs, and the strong covalent linkages make
COFs more stable; however, the trade-off is decreased
crystallinity and thus more challenges in constructing the
crystalline frameworks.
C
progress in structure development and application
exploration for the past decade. Most research is focused on
two-dimensional (2D) COFs; the development of three-
dimensional (3D) COFs materials has been relatively slow. In
D-COFs, various topological design and construction
approaches have been developed, including [3 + 2], [3 + 3],
2
6
,11
[
4 + 2], [4 + 4], [6 + 2], and [6 + 3].
These planar
networks are generally stacked layer by layer via π−π
interactions and eventually produce one-dimensional channels
with a limited number of pore shapes (hexagonal, tetragonal,
rhombic, and trigonal cylinders) and pore environments
mostly the hydrogens of the aromatic rings). In contrast, in
D-COFs, most π planes of the building units are unstacked
and contribute to the pores, which leads to higher theoretical
surface area, lower density, larger pore volume, and more
adsorption sites than 2D-COFs.
Because of the limitation of 3D organic building units and
the less reversible covalent bonds, exploring new topologies to
enrich the 3D-COF family remains a challenge in the field.
Accordingly, the topologies of 3D-COFs to date are limited to
(
3
2
3
12
12
24
25
26
27
28
29
30
dia, ctn, bor, ffc, rra, srs, pts, lon, stp, acs,
3
1
32
33
tbo, bcu, and fjh. Here we report the first example of a
3D-COF with the ceq topology (denoted as 3D-ceq-COF)
that uses a novel three-dimensional hexadot building unit. 3D-
ceq-COF exhibits excellent CO , CH , and H uptake
The distinct pore features in 3D-COFs are highly desired for
porous materials in the applications of gas sorption,
1
2
1
3−16
15,17−19
20
catalysis,
ionic conduction,
and sensing. For
2
4
2
instance, the interconnected pore structures of 3D-COFs are
attractive for mass-transfer-related applications such as
catalysis, ionic conduction, and chemical sensing. At the
same time, the twisted pore geometries and exposed aromatic
rings in 3D-COFs create more efficient adsorption sites, which
will benefit the storage of hydrogen, methane, and carbon
dioxide. Specific surface area is a prerequisite for gas
adsorption; however, the surface area of materials cannot be
increased indefinitely. Meanwhile, for COF and metal−organic
framework (MOF) materials, the higher specific surface are is
often accompanied by greater possibilities of crystal collapse
and decreased chemical stability (due to the highly reversible
performance under the limited specific surface area, partly
attributed to its 3D framework and the unique triptycene-
based building unit.
34−36
Triptycenes
are a unique family of aromatic com-
pounds composed of three independent aromatic hydrocarbon
units fused through a bicyclo[2.2.2]octane skeleton. Their 3D
structure provides a well-defined spatial arrangement and
considerable internal free volume (IFV) for scaffolding.
Triptycene (9,10-dihydro-9,10[1′,2′]-benzenoanthracene) is a
Received: October 27, 2020
21,22
formation reactions).
Thus, exploring new topologies and
structures with efficient adsorption sites would provide another
way to enhance the gas adsorption properties.
Despite the structural characteristics and potential applica-
tions, the development of 3D-COFs is much slower than that
©
XXXX American Chemical Society
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
A