Highly Chemically Stable MOFs with Trifluoromethyl Groups: Effect of Position of Trifluoromethyl Groups on Chemical Stability

Article abstract of DOI:10.1021/acs.inorgchem.9b00088

Metal-organic frameworks (MOFs) are a class of advanced porous crystalline materials. However, numerous MOFs have poor chemical stability, significantly restricting their industrial application. The introduction of trifluoromethyl groups around clusters o

Full text of DOI:10.1021/acs.inorgchem.9b00088

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Highly Chemically Stable MOFs with Trifluoromethyl Groups: Effect
of Position of Trifluoromethyl Groups on Chemical Stability
Keke Wang,† Hongliang Huang,*,‡,§,∥ Xiaocong Zhou,† Qin Wang,† Guijie Li,† Haimin Shen,†
Yuanbin She,*,† and Chongli Zhong*,‡,§,∥
†State Key Laboratory Breeding Base of Green Chemistry−Synthesis Technology, College of Chemical Engineering, Zhejiang
University of Technology, Hangzhou 310014, China
‡School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China
§State Key Laboratory of Separation Membranes and Membrane Processes, National Center for International Joint Research on
Separation Membranes, Tianjin Polytechnic University, Tianjin 300387, China
∥Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing
100029, China
S
* Supporting Information
ABSTRACT: Metal−organic frameworks (MOFs) are a class
of advanced porous crystalline materials. However, numerous
MOFs have poor chemical stability, significantly restricting their
industrial application. The introduction of trifluoromethyl
groups around clusters of MOFs results in a shielding effect
caused by their hydrophobicity and bulkiness, thus preventing
guest molecules from attacking the coordination bonds. To
prove such a shielding effect, the position of the trifluoromethyl
groups is rationally adjusted, with trifluoromethyl groups at the
ortho positions of carboxyl groups significantly improving the
chemical stability of UiO-67. The prepared UiO-67-o-2CF3 remains intact after treatment with boiling water, 8 M HCl, 10 mM
NaOH, and 50 ppm of NaF aqueous solutions. As the control experiment, trifluoromethyl groups at the meta positions of
carboxyl groups have no shielding effect; hence, UiO-67-m-2CF3 has a stability that is lower than that of UiO-67-o-2CF3. In
addition, the shielding effect is also applied to other MOFs, including DUT-5-o-2CF3 and Al-TPDC-o-2CF3, confirming the
universality of this strategy.
polydimethysiloxane to enhance water stability.19 (IV)
1. INTRODUCTION
Generate framework interpenetration.21 (V) Prepare MOF
Metal−organic frameworks (MOFs) are widely studied
composites such as CNT@MOF-5 and MOF-5@SBA-15.22,23
functional porous materials, consisting of metal ions (or
Compared with other methods, the incorporations of high
metal clusters) and organic linkers which are linked through
oxidation state metals and hydrophobic groups are the most
coordination bonds to form extended porous crystalline
frameworks.1−3 High surface areas, proper pore structures,
facile and effective methods. The successful adoption of high
oxidation state metals generates highly chemically stable MOFs
and a variety of chemical compositions make MOFs extremely
such as UiO-66(Zr),24 MIL-53(Cr),25 and MIL-100(Fe).26
attractive for an array of applications, such as gas storage,4
separation,5 sensing,6 catalysis,7 and energy storage/conver-
However, the method is not applicable to all MOFs. For
sion.8 However, the application of MOFs in chemical
example, ZrMOF-BIPY is even sensitive toward acetone and
chloroform.27 As an alternative method, the introduction of
engineering is severely restricted by their poor chemical
stability.9
hydrophobic groups into MOFs can enhance the water
stability of the frameworks. For example, MOF-5 instability
To enhance the chemical stability of MOFs, five strategies
have been proposed.10−14 (I) Improve coordination bond
toward moisture in air is easily rectified via the introduction of
methyl groups.17 However, compared with methyl groups,
strength by applying hard/soft acid/base rule. For example,
high oxidation state metals such as Zr4+, Cr3+, and Fe3+, as hard
trifluoromethyl groups are more hydrophobic and bulkier,
which increases their effectiveness in repelling water and their
acids, can form strong coordination bonds with carboxylate
steric hindrance.28,29 In addition, we envisage that the position
ligands (as hard bases).15,16 (II) Introduction of hydrophobic
groups such as an alkyl group into MOF structures.17,18 (III)
Coat the external surfaces of MOFs with hydrophobic
materials.19,20 For example, water-sensitive MOFs, including
MOF-5, HKUST-1, and ZnBT, are coated with hydrophobic
of the trifluoromethyl groups has significant influence on MOF
Received: January 10, 2019
© XXXX American Chemical Society
A
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a
Scheme 1. Shielding Effect of the Hydrophobic Groups in the Framework on Coordination Bonds of MOFs
a
(a) Unfunctionalized MOFs decompose after coordination bonds are attacked by guest molecules. (b) MOFs bearing hydrophobic groups far
from the coordination bonds decompose after attack by guest molecules (c) MOFs bearing hydrophobic groups close to the coordination bonds
remain intact after attack by guest molecules.
reported methods.33−35 The detailed synthetic procedures and
characterization data are presented in the Supporting Information.
2.2. Synthesis of UiO-67. ZrCl4 (0.117 g, 0.5 mmol), H2BPDC
(0.121 g, 0.5 mmol), benzoic acid (1.832 g, 15.0 mmol), and DMF
(20 mL) were added to a Teflon liner (100 mL). After 10 min of
sonication, the reaction mixture was kept at 120 °C for 24 h and then
cooled to room temperature. The resulting UiO-67 was collected by
centrifugation, washed with DMF and acetone five times, and dried
under vacuum at 120 °C for 12 h (0.115 g, yield: 65.3%).
2.3. Synthesis of UiO-67-o-2CF3. ZrCl4 (0.117 g, 0.5 mmol),
H2BPDC-o-2CF3 (0.190 g, 0.5 mmol), benzoic acid (0.610 g, 5.0
mmol), and DMF (20 mL) were added to a Teflon liner (100 mL).
After 10 min of sonication, the reaction mixture was kept at 120 °C
for 24 h and then cooled to room temperature. The resulting UiO-67-
o-2CF3 was collected by centrifugation, washed with DMF and
acetone five times, and dried under vacuum at 120 °C for 12 h (0.168
g, yield: 68.5%).
2.4. Synthesis of UiO-67-m-2CF3. UiO-67-m-2CF3 was synthe-
sized analogously by replacing H2BPDC-o-2CF3 with the equivalent
molar amount of H2BPDC-m-2CF3 (0.169 g, yield: 69.1%).
2.5. Synthesis of DUT-5. H2BPDC (0.260 g, 1.07 mmol),
Al(NO3)3·9H2O (0.525 g, 1.4 mmol), and DMF (30 mL) were added
to a Teflon liner (100 mL). After 10 min of sonication, the reaction
mixture was kept at 120 °C for 24 h and then cooled to room
temperature. The resulting DUT-5 was collected by centrifugation,
washed with DMF and acetone five times, and dried under vacuum at
120 °C for 12 h (0.283 g, yield: 93.2%).
2.6. Synthesis of DUT-5-o-2CF3, Al-TPDC, and Al-TPDC-o-
2CF3. DUT-5-o-2CF3 (0.408 g, yield: 90.6%), Al-TPDC (0.351 g,
yield: 90.9%), and Al-TPDC-o-2CF3 (0.480 g, yield: 90.3%) were
synthesized in a similar way to DUT-5 by replacing H2BPDC with the
equivalent molar amount of H2BPDC-o-2CF3, [1,1′:4′,1′′-terphenyl]-
4,4′′-dicarboxylic acid (H2TPDC), and H2TPDC-o-2CF3, respectively
stability.14 Moreover, the addition of such groups not only
improves hydrophobicity, but causes a shielding effect that
blocks destructive water molecules from MOF coordination
bonds, thereby effectively enhancing framework stability.
Therefore, in order to study the shielding effect of
hydrophobic groups, we designed a series of proof-of-concept
experiments. Since the UiO series of MOFs were pioneered by
Lillerud’s group in 2008,24 Zr-MOFs, especially UiO-66, have
attracted increasing interest due to their excellent stability. As a
Zr-MOF, UiO-67 has been intensively studied, showing
excellent performance in many areas including wastewater
treatment, catalysis, and drug delivery.30−32 However, in
contrast to UiO-66, UiO-67 possesses poor water stability
due to its reduced kinetic stability. Therefore, UiO-67 was
chosen as the model MOF material for our study. Herein, we
designed and synthesized two trifluoromethyl modified ligands,
3,3′-bis(trifluoromethyl)-4,4′-biphenyldicarboxylic acid
(H2BPDC-o-2CF3) and 2,2′-bis(trifluoromethyl)-4,4′-biphe-
nyldicarboxylic acid (H2BPDC-m-2CF3). H2BPDC,
H2BPDC-o-2CF3, and H2BPDC-m-2CF3, as organic ligands,
were coordinated with Zr(IV) clusters to form UiO-67, UiO-
67-o-2CF3, and UiO-67-m-2CF3, respectively. The chemical
stability of these materials was examined and compared
systematically and the shielding effect of the trifluoromethyl
groups proved. In order to further confirm the versatility of the
shielding effect, 3,3′′-bis(trifluoromethyl)-[1,1′:4′,1′′-terphen-
yl]-4,4′′-dicarboxylic acid (H2TPDC-o-2CF3) was synthesized.
H2BPDC-o-2CF3 and H2TPDC-o-2CF3 were used to obtain
two chemically stable Al-MOFs containing trifluoromethyl
groups at the ortho positions of carboxylate groups. Thus, the
shielding effect of the trifluoromethyl groups in MOFs can be
used to design and construct stable MOFs, which is ideal for
utilization under harsh conditions in chemical engineering.
3. RESULTS AND DISCUSSION
As shown in Scheme 1, the shielding effect may depend on the
distance between hydrophobic groups and coordination bonds
in MOFs. Once a limited distance between the hydrophobic
groups and coordination bonds is reached, the shielding effect
is not observed. Their paternal and functionalized frameworks
permit attack from guest molecules (Scheme 1a,b). However,
2. EXPERIMENTAL SECTION
2.1. Synthesis of Ligands. H2BPDC-o-2CF3, H2BPDC-m-2CF3,
and H2TPDC-o-2CF3 were synthesized according to previously
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Figure 1. Structures of (a) UiO-67, (b) UiO-67-m-2CF3, and (c) UiO-67-o-2CF3. Structures of (d) DUT-5-o-2CF3 viewed along the b-axis and c-
axis, and (e) Al-TPDC-o-2CF3 viewed along the a-axis and c-axis. Zr, green; Al, blue; C, black; O, red; H, white; F, turquoise.
as this distance is reduced, the hydrophobic groups can
effectively hinder guest molecules and consequently enhance
chemical stability (Scheme 1c). To prove this hypothesis, we
designed and synthesized trifluoromethyl-functionalized UiO-
67. As shown in Figure 1a, UiO-67 possesses Zr6O4(OH)4
clusters interconnected with 12 4,4′-biphenyldicarboxylate
(BPDC) linker molecules, generating octahedral and tetrahe-
dral micropores. By incorporating H2BPDC-o-2CF3 and
H2BPDC-m-2CF3 instead of H2BPDC, two trifluoromethyl-
functionalized MOFs, labeled UiO-67-o-2CF3 and UiO-67-m-
2CF3, were obtained, respectively. The yields of UiO-67, UiO-
67-o-2CF3, and UiO-67-m-2CF3 were 65.3%, 68.5%, and
69.1%, respectively, which indicates that the trifluoromethyl
groups have no significant effect on MOF yields. Unfortu-
nately, even though numerous attempts were made, a single
crystal of UiO-67-o-2CF3 and UiO-67-m-2CF3 was not
obtained. To confirm the crystal structures of UiO-67, UiO-
67-o-2CF3, and UiO-67-m-2CF3, they were characterized by
powder X-ray diffraction (PXRD). UiO-67-o-2CF3 and UiO-
67-m-2CF3 structures (Figure 1b,c) were determined by
Rietveld refinement (Figures S16 and S17). Their initial
models were obtained by functionalizing the UiO-67 frame-
work in Materials Studio software.36 This approach has been
widely applied by many groups.18,37 Figure S18 shows that the
trifluoromethyl group’s fluorine atoms are near oxygen atoms
of the carboxylate groups; hence, the Zr metal sites are not
covered up.
C
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Figure 2. (a) PXRD patterns of UiO-67, UiO-67-o-2CF3, and UiO-67-m-2CF3. PXRD patterns of (b) UiO-67 treated with water, 1 M HCl, and 10
mM NaOH (pH = 12) for 1 day; PXRD patterns of (c) UiO-67-o-2CF3 and (d) UiO-67-m-2CF3 treated with water, 1 M HCl, and 10 mM NaOH
(pH = 12) for 1 day and treated with water for 60 days.
Figure 3. PXRD patterns of (a) UiO-67-o-2CF3 and (b) UiO-67-m-2CF3 treated with boiling water, 4 M HCl, 8 M HCl, and 50 ppm of NaF
aqueous solutions for 1 day. N2 adsorption−desorption isotherms at 77 K for (c) UiO-67-o-2CF3 and (d) UiO-67-m-2CF3 treated with boiling
water, 4 M HCl, 8 M HCl, and 50 ppm of NaF aqueous solutions for 1 day.
As shown in Figure 2a, the similarity between the
experimental and simulated patterns of UiO-67 confirms its
formation. Meanwhile, the PXRD patterns of UiO-67-o-2CF3
and UiO-67-m-2CF3 are in good agreement with those of UiO-
67. In the case of UiO-67-o-2CF3, an obscure shoulder peak at
10.8 degrees overlapped partially by the intense and broad
peak at 11.2 degrees is observed. The consistency of PXRD
patterns shows that UiO-67, UiO-67-o-2CF3, and UiO-67-m-
2CF3 are topologically equivalent. This phenomenon of
functioned Zr-MOFs maintaining parent structures has been
observed in the literature.38,39 In addition, no redundant peak
is observed in the PXRD patterns, indicating that the MOFs
are pure crystals. To confirm retention of the trifluoromethyl
groups in the frameworks, UiO-67-o-2CF3 and UiO-67-m-
2CF3 were characterized by FT-IR. As shown in Figure S19,
the absorption peaks at 1129 and 1137 cm−1 for UiO-67-o-
2CF3 and UiO-67-m-2CF3, respectively, are attributed to C−F
stretching vibrations of the trifluoromethyl groups.40
To study their porosity, UiO-67, UiO-67-o-2CF3, and UiO-
67-m-2CF3 were characterized by N2 adsorption at 77 K
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Figure 4. SEM images of UiO-67-o-2CF3 treated with water, boiling water, 1 M HCl, 4 M HCl, 8 M HCl, 10 mM NaOH (pH = 12), and 50 ppm
of NaF aqueous solutions for 1 day and treated with water for 60 days.
(Figure S20). At the adsorption branch, with an increase in
relative pressure, the N2 adsorption amounts on all three
MOFs rapidly reach equilibrium (P/P0 < 0.05), which
indicates that they are microporous materials.41 The surface
areas of the studied materials were obtained using the
Brunauer−Emmett−Teller (BET) method (Table S3). The
BET surface areas of UiO-67-o-2CF3 and UiO-67-m-2CF3 are
1198 and 1193 m2 g−1, respectively, and their total pore
volumes calculated from their adsorption isotherms at P/P0 =
0.95, are 0.53 and 0.50 cm3 g−1, respectively. Similar surface
area and pore volume values of UiO-67-o-2CF3 and UiO-67-m-
2CF3 are attributed to their comparable ligand length, same
functional groups, and topological structures. Furthermore,
these values are much lower than those of UiO-67 (BET
surface area of 1857 m2 g−1; total pore volume of 0.79 cm3
g−1), which is due to the bulk size of trifluoromethyl groups in
the frameworks, leading to reduced free space and increased
framework weight.
To study the effect of the trifluoromethyl groups on
chemical stability, UiO-67, UiO-67-o-2CF3, and UiO-67-m-
2CF3 were immersed in water, 1 M HCl, and 10 mM NaOH
(pH = 12) for 1 day. After which, they were characterized by
PXRD to assess their stability. As shown in Figure 2b, the
structure of UiO-67 completely collapses when exposed to
these conditions, resulting in amorphous solids. However,
UiO-67-o-2CF3 and UiO-67-m-2CF3 PXRD patterns remain
unchanged (Figure 2c,d), confirming their stability. More
importantly, when UiO-67-o-2CF3 and UiO-67-m-2CF3 are
immersed in water for 60 days, their structural integrity
remains unaffected. The enhanced chemical stability is
attributed to the hydrophobicity of trifluoromethyl groups,
which is consistent with the literature.42,43 Figure S21 shows
that the contact angles of water in UiO-67-o-2CF3, UiO-67-m-
2CF3, and UiO-67 are 142.4°, 135.3°, and 43.3°, respectively.
However, the position of the trifluoromethyl groups may lead
to variations in MOF stability even though they have similar
hydrophobicity.
To prove the presence of the shielding effect of
trifluoromethyl groups, UiO-67-o-2CF3 and UiO-67-m-2CF3
were immersed in boiling water, 50 ppm of NaF, 4 M HCl, and
8 M HCl for 1 day. As shown in Figure 3, after these
treatments, PXRD peak intensities of UiO-67-m-2CF3
significantly decrease due to structure decomposition and
loss of crystallinity. However, PXRD peak intensities of treated
UiO-67-o-2CF3 are still comparable with those of untreated
MOF (Figure 3a), which highlights UiO-67-o-2CF3 undeterred
crystallinity under the same conditions. In addition, in order to
determine whether the surface areas of MOFs decrease after
exposure to these conditions, treated UiO-67-o-2CF3 and UiO-
67-m-2CF3 were characterized by N2 adsorption at 77 K. As
shown in Figure 3c, Figure S22, and Table S4, UiO-67-o-2CF3
retains its surface area after these treatments. Meanwhile, the
crystallite surfaces of treated UiO-67-o-2CF3 are still smooth
(Figure 4). In the case of UiO-67-m-2CF3, after treatment with
water, 1 M HCl, and 10 mM NaOH, the BET surface area and
crystallite surface remain practically unchanged (Figures S23
and S24). However, after treatment with boiling water, 4 M
HCl, 8 M HCl, and 50 ppm of NaF aqueous solutions, the
BET surface area significantly drops from 1193 to 383, 423,
354, and 163 m2 g−1, respectively (Figure 3d and Table S4).
Moreover, the crystallite surfaces of the treated MOFs are
severely corrupted (Figure S24), which indicates that UiO-67-
m-2CF3 is unstable toward such harsh conditions. These
results are consistent with PXRD analysis. Although UiO-67-o-
2CF3 and UiO-67-m-2CF3 have similar hydrophobicity, UiO-
67-o-2CF3 has enhanced chemical stability, which is attributed
to the position of trifluoromethyl groups, which are closer to
Zr−O bonds between Zr(IV) and carboxylate groups
effectively blocking guest molecules (such as H2O, H+, and
F−) from attacking the coordination bonds (Scheme 2a). In
the case of UiO-67-m-2CF3, although UiO-67-m-2CF3 is stable
toward water and 1 M HCl, the trifluoromethyl groups are far
from the coordination bonds, leaving them unprotected and
open to attack in boiling water, 4 M HCl, 8 M HCl, and 50
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with final Rwp values of 5.02% and 4.34% for DUT-5-o-2CF3
and Al-TPDC-o-2CF3, respectively (Figures S25 and S26). As
shown in Figure 1d,e and Figure S27, in the frameworks, every
Al atom is linked to six O atoms, forming a distorted
octahedral structure. The Al chains are connected by BPDC,
BPDC-o-2CF3, TPDC, and TPDC-o-2CF3 linkers to obtain
three-dimensional frameworks, namely, DUT-5, DUT-5-o-
2CF3, Al-TPDC, and Al-TPDC-o-2CF3, respectively. As
shown in Figure S28, the PXRD pattern of DUT-5-o-2CF3 is
in good agreement with that of DUT-5, which demonstrates
that DUT-5-o-2CF3 is topologically equivalent to DUT-5. As
shown in Figure S29, the PXRD pattern of Al-TPDC-o-2CF3 is
in good agreement with that of Al-TPDC. To study their
porosity, the N2 adsorption−desorption isotherms at 77 K
were measured (Figures S30−S32). The BET surface areas of
DUT-5, DUT-5-o-2CF3, Al-TPDC, and Al-TPDC-o-2CF3 are
1479, 1366, 553, and 1203 m2 g−1, respectively.
To confirm the excellent chemical stability of DUT-5-o-
2CF3 and Al-TPDC-o-2CF3, they were immersed in water, 10
mM HCl (pH = 2), and 1 mM NaOH (pH = 11) for 1 day. As
shown in Figure 5, after treatment, the PXRD patterns of
DUT-5-o-2CF3 and Al-TPDC-o-2CF3 remain practically
unchanged. PXRD peaks of treated Al-TPDC-o-2CF3 at high
angle are broad, which may be attributed to a decrease in
particle sizes.44 This phenomenon was previously re-
ported.37,45,46 In addition, a reduction in N2 uptake of the
treated samples at 77 K is not observed (Figures S41 and S42).
However, even when treated with water, DUT-5 and Al-TPDC
structures decompose (Figures S43 and S44). For DUT-5-o-
2CF3 and Al-TPDC-o-2CF3, the enhanced chemical stability is
attributed to the trifluoromethyl groups at the ortho positions
of the carboxyl groups. These results reveal that our strategy is
effective in the synthesis of chemically stable MOFs viable
under harsh conditions in chemical engineering. In addition,
we speculate that tetra-trifluoromethyl groups at the ortho
positions in the ligand can further enhance the chemical
stability of MOFs. In the near future, the synthesis of MOFs
with enhanced chemical stability will be examined.
Scheme 2. The Maintenance of the UiO-67-o-2CF3
Structure and the Collapse of the UiO-67-m-2CF3
Structure
a
a
(a) The maintenance of the UiO-67-o-2CF3 structure after treatment
with boiling water, 4 M HCl, 8 M HCl, and 50 ppm of NaF aqueous
solutions; (b) the collapse of the UiO-67-m-2CF3 structure after
treatment with boiling water. Zr, green; C, gray; O, red; H, white; F,
light blue.
ppm of NaF. Once the coordination bonds cleave, Zr metal
sites are exposed, which may coordinate H2O molecules in
boiling water (Scheme 2b). In 4 M (8 M) HCl and 50 ppm of
NaF, chlorine and fluorine atoms provide charge compensation
after coordination bond cleavage (Schemes S1 and S2).27
To confirm that the shielding effect is valid for MOFs with
other metal ions and ligands, we further increased the ligand
length to generate H2TPDC-o-2CF3. H2BPDC, H2BPDC-o-
2CF3, H2TPDC, and H2TPDC-o-2CF3 were used to synthesize
isoreticular Al-based MOFs, namely, DUT-5 (Al(OH)-
(BPDC)), DUT-5-o-2CF3 (Al(OH)(BPDC-o-2CF3), BPDC-
o-2CF3 = 3,3′-bis(trifluoromethyl)-4,4′-biphenyldicarboxy-
late), Al-TPDC (Al(OH)(TPDC), TPDC = [1,1′:4′,1′′-
terphenyl]-4,4′′-dicarboxylate), and Al-TPDC-o-2CF3 (Al-
(OH)(TPDC-o-2CF3), TPDC-o-2CF3 = 3,3′′-bis-
(trifluoromethyl)-[1,1′:4′,1′′-terphenyl]-4,4′′-dicarboxylate),
respectively. The yields of DUT-5, DUT-5-o-2CF3, Al-TPDC,
and Al-TPDC-o-2CF3 based on the ligands are 93.2%, 90.6%,
90.9%, and 90.3%, respectively. In accordance with the DUT-5
structure, the initial structures of DUT-5-o-2CF3 and Al-
TPDC-o-2CF3 were obtained by substituting BPDC with
BPDC-o-2CF3 and TPDC-o-2CF3 linkers in Materials Studio
software followed by geometry optimization.36 Then, the
structures of DUT-5-o-2CF3 and Al-TPDC-o-2CF3 were
determined by Rietveld refinement. The final profile fits
show good agreement with the observed diffraction patterns,
4. CONCLUSION
By harnessing the advantageous hydrophobicity and bulkiness
of trifluoromethyl groups, we synthesized trifluoromethyl-
functionalized UiO-67 (namely, UiO-67-o-2CF3 and UiO-67-
m-2CF3) to enhance the chemical stability of UiO-67. In
contrast to UiO-67, UiO-67-o-2CF3 and UiO-67-m-2CF3 are
stable toward water, 1 M HCl, and 10 mM NaOH. In addition,
although UiO-67-o-2CF3 and UiO-67-m-2CF3 have similar
Figure 5. PXRD patterns of (a) DUT-5-o-2CF3 and (b) Al-TPDC-o-2CF3 before and after treatment with water, 10 mM HCl (pH = 2), and 1 mM
NaOH (pH = 11).
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hydrophobicity, due to the shielding effect of the trifluor-
omethyl groups at the ortho positions of the carboxyl groups,
UiO-67-o-2CF3 is more chemically stable. Moreover, we
determined that the shielding effect is also valid for Al-based
MOFs of varying ligand length. We believe that the shielding
effect of the hydrophobic groups provides a promising
opportunity to prepare chemically stable MOFs with industrial
applications.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00088.
Additional experimental procedures, characterization
techniques, PXRD patterns, NMR spectra, SEM images,
TGA data, nitrogen sorption isotherms, FT-IR spectra,
contact angles of water, and structural data (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: huanghongliang@tjpu.edu.cn (H.H.).
*E-mail: sheyb@zjut.edu.cn (Y.S.).
*E-mail: zhongchongli@tjpu.edu.cn (C.Z.).
■
ORCID
Hongliang Huang: 0000-0001-9690-9259
Guijie Li: 0000-0002-0740-2235
Yuanbin She: 0000-0002-1007-1852
Chongli Zhong: 0000-0002-7921-9923
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21808201, 21878229, 21536001,
21606007, 21476270, and 21776259), the National Key
Projects for Fundamental Research and Development of
China (2016YFB0600901), and the Science and Technology
Plans of Tianjin (17PTSYJC00040 and 18PTSYJC00180).
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