Full Papers
MOFs offer a promising platform for CO capture because they
2
Results and Discussion
have very high surface areas and gas sorption capacity as well
as a tailorable pore surface/environment that can facilitate se-
Crystal structure and general characterization
[
10]
lective CO binding. In addition to the crucial role of coordi-
2
natively unsaturated metal sites (CUSs) for CO2 adsorption,
given the polarizability and quadrupole moment of the acidic
oxide CO2 molecule, the polarizing and alkaline functional
Solvothermal reactions of Al(NO ) ·9H O and organic linker
3 3
2
(Sbpdc, bpdc, or bpydc) in DMF at 393 K yielded a white poly-
crystalline powder of Al(OH)(L) (L=Sbpdc, USTC-253; bpdc, EL-
MIL-53; bpydc, MOF-253). Powder X-ray diffraction (PXRD)
studies demonstrated their identical structural topology and
good crystallinity (Figure S3). Notably, the deviation from line-
arity (~1608) in Sbpdc does not affect the coordination modes
of the ligand, and this linker is able to form an isoreticular
[12]
groups involved in MOFs lead to enhanced CO2 capture.
Most recently, trifluoroacetic acid (TFA)/acetic acid and HCl
have been found to cause the structural defects in MOFs if
they are involved in the modulated synthesis. The defect sites
are able to enhance the CO adsorption and catalytic proper-
2
[
13]
III
ties significantly. In addition, most MOFs contain CUSs as
Lewis acid sites, bridging ÀOH groups with Brønsted acidity,
and other catalytically active sites on the organic linkers, which
structure with MOF-253 and EL-MIL-53. In the structure, Al is
coordinated octahedrally by four carboxylate O atoms in the
plane and two O atoms from hydroxyl groups located at the
axial positions. The AlÀOH chains, formed by the alternate
[11]
make MOFs applicable for heterogeneous catalysis.
The
III
high-density active sites distributed uniformly throughout the
framework are easily accessible, and the pore structure facili-
tates the transfer of substrates and products. Therefore, MOFs
with Lewis/Brønsted acid sites as well as basic sites may offer
array of Al and hydroxyl groups, are bridged by the organic
linkers to give a 3D network with rectangular channels of
2
around 1.1ꢁ1.1 nm (Figure 1).
excellent activity for CO conversion and utilization by cycload-
2
dition with epoxides. To date, several MOFs with Lewis acid
sites have been reported for the catalytic cycloaddition of CO2
with epoxides, most of which require high-pressure and/or
[
14]
high-temperature conditions.
Only two Cu-MOFs, typically
with mild stability, have been developed recently to accom-
[15]
plish such a conversion with 1 bar CO at room temperature.
2
It is agreed that stability is of prime importance for practical
implementation in heterogeneous catalysis. Moreover, to the
best of our knowledge, the correlation between the CO sorp-
2
tion performance and conversion by MOFs has not been ex-
plored yet.
Figure 1. Structure of USTC-253 (left) and the ligands involved in the struc-
tures of USTC-253, MOF-253, and El-MIL-53 (right).
With the aforementioned considerations, a stable Al-based
MOF, denoted as USTC-253 (USTC=University of Science and
Technology of China), has been synthesized by the reaction of
Al(NO ) ·9H O and 4,4’-dibenzoic acid-2,2’-sulfone (Sbpdc) in
3
3
2
DMF. For comparison, Sbpdc was replaced with 4,4’-biphenyldi-
carboxylic acid (bpdc) and 2,2’-bipyridine-5,5’-dicarboxylate
acid (bpydc) under the same conditions to afford elongated
It is well known that single crystals of Al-MOFs are extremely
III
difficult to obtain because the coordination between Al and
carboxylate ligands is rapid and the inert coordination bonds
[
16a]
[16b]
III
MIL-53 (EL-MIL-53)
and MOF-253,
respectively, which
between Al cations and carboxylate anions make ligand-ex-
have the same structural topology but substituted organic link-
ers. Remarkably, the introduction of TFA into USTC-253 leads
to USTC-253-TFA, which has almost the same structure as
USTC-253 with a certain amount of structural defects. The re-
sulting USTC-253 with polar sulfone groups exhibits a signifi-
change reactions slow, which is unfavorable for crystal growth.
A large amount of a small monocarboxylic acid, such as TFA,
which behaves as a modulator, is proposed to first coordinate
III
to the Al centers, which is followed by a relatively slow ligand
exchange between the acid and Sbpdc. As expected, with
such a modulated synthetic strategy, the TFA involved in the
synthesis endows the obtained USTC-253-TFA with a remarka-
bly improved crystallinity (Figures S3 and S4), which would be
cantly higher CO adsorption than of EL-MIL-53 and MOF-253.
2
The presence of defects leads to the generation of CUSs and
Lewis acid sites, which further enhances the CO capture capa-
2
[12b,17]
bility. More importantly, USTC-253-TFA with both Brønsted and
Lewis acid sites possesses a high activity and satisfactory recy-
beneficial for subsequent functional applications.
SEM
images indicate that the USTC-253-TFA particles are rod
shaped, of approximately 300 nm in size, and aggregate to
larger particles in the microscale, whereas USTC-253 nanoflakes
of approximately 200 nm in diameter have very few aggrega-
tions (Figure S5). In addition, similar to the synthesis of UiO-
clability toward catalytic CO cycloaddition under ambient con-
2
ditions. As far as we know, this work is the first report of the
synergistic improvement of CO uptake by the co-involvement
2
of polar functional groups and structural defects in the MOF. In
[13a,b]
addition, for the first time, the correlation between CO sorp-
66,
given the unavoidable incomplete ligand exchange,
2
tion performance and catalytic conversion has been proposed.
the participation of TFA as a modulator in the reaction will
lead to the formation of vacancies and missing-linker defects
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