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spectively. The apparent Brunauer–Emmett–Teller (BET)
surface area and the total pore volume of UiO-67-Rs dropped
from 1303 to 349 m2 gÀ1 and from 0.48 to 0.21 mLgÀ1
,
respectively, as the length of the alkyl chain increased, and
all values were smaller than those of UiO-67 (2761 m2 gÀ1,
0.97 mLgÀ1), whereas the pore sizes (11.7–11.2 ꢀ) calculated
by the DFT method remain comparable (see Figure S12 and
Table S1 in the Supporting Information). These results are in
accordance with the introduction of bulky alkyl chains into
the MOF pores with the framework unchanged.
Thermogravimetric analysis (TGA) indicated that all as-
prepared MOFs contained a large amount of solvent, which
could be evacuated by heating (see Figure S13). Variable-
temperature PXRD measurements indicated that UiO-67-Rs
had thermal stability up to 3008C, which is lower than that of
UiO-67 owing to the presence of C8 chains linked by alkoxy
oxygen atoms (see Figure S14). To investigate the chemical
stability of these MOFs, fresh samples of UiO-67 and UiO-67-
Rs were immersed in aqueous solution with different pH
values. PXRD revealed that UiO-67-Et retained high crys-
tallinity after exposure to a harsh chemical environment in
solution at pH 1 and 1008C for 5 days, whereas UiO-67-But,
UiO-67-Hex, and UiO-67-Oct remained stable even longer
for at least 20 days. In contrast, UiO-67 was degraded after
only 3 h (see Figure S15). When immersed in solution at
pH 12 and 1008C, UiO-67-Et maintained its crystallinity for
3 days, UiO-67-But remained unchanged for 11 days, and
UiO-67-Hex and UiO-67-Oct survived for at least 20 days, in
contrast to UiO-67, which lost crystallinity in only 2 h (see
Figure S16). In a solution at pH 13 at room temperature, UiO-
67-Et, UiO-67-But, UiO-67-Hex, and UiO-67-Oct still sur-
vived for 2, 3, 18, and 21 h, respectively (see Figure S17).
When treated with fresh water at 1008C, UiO-67-Et retained
crystallinity for 5 days, whereas UiO-67-But, UiO-67-Hex,
and UiO-67-Oct remained intact for at least 20 days and were
thus much more stable than UiO-67, which degraded after
10 h (see Figure S18). The good porosity retained after water
and acid/base treatment was verified by N2 adsorption
isotherms of the UiO-67-Oct sample (see Figure S19). The
above results highlight the high chemical stability of UiO-67-
Rs toward acidic/basic aqueous solutions and their suitably
for application under harsh conditions.
Figure 3. Water adsorption isotherms of UiO-67 and UiO-67-Rs at
298 K.
between increasing hydrophobicity of the MOF pores and the
alkyl-chain length. PXRD patterns after water adsorption
indicated that the crystallinity of UiO-67-Rs remained more
intact for the MOFs with longer alkyl chains, in contrast to
UiO-67, which degrades obviously (see Figure S20). In
particular, the water adsorption behavior of UiO-67-Oct
maintained perfect robustness for three successive cycles (see
Figure S21).
Convincing proof for the hydrophobic performance of
UiO-67-Rs was obtained by measuring the water contact
angle (CA), which according to criteria for superhydropho-
bicity must be no less than 1508.[2] The powder of UiO-67 is
readily wetted by water, indicating a hydrophilic nature
(Figure 4; see also Figure S22). For comparison, a water
droplet can roll over the UiO-67-R powders as over a lotus
leaf, thus demonstrating their good hydrophobicity. The CA
of a water droplet on UiO-67-R powders quickly increases
from 121.1 to 154.08 as the alkyl chains are extended from
ethyl to n-octyl, clearly confirming that the introduction of
alkyl chains on the organic linkers can transform the surface
character of UiO-67 from hydrophilicity to hydrophobicity,
and even superhydrophobicity with longer n-hexyl and n-
octyl chains. On the other hand, UiO-67-Oct is completely
wettable with toluene, n-hexane, and dichloromethane, sink-
ing in these organic solvents but floating on water, thus
demonstrating its intrinsic superhydrophobicity and super-
oleophilicity (see Figure S23).
Intrinsic MOF Hydrophobicity and Crystal Growth and Analysis
It is reasonable that the alkyl chains are associated with
hydrophobicity, favoring a decrease the surface energy of the
materials to enhance hydrophobicity.[1] However, CA meas-
urements of the pure H2L3–8 ligands revealed that these
organic linkers are actually hydrophilic owing to the carbox-
ylate acid groups (see Figure S24). Hence materials simply
containing alkyl chains are not automatically endowed with
hydrophobicity, especially as C2–C8 alkyl chains are not as
long as those commonly used to coat or modify MOFs for
hydrophobicity.[24,27,30,31] As the UiO-67-Rs are constructed
from essentially hydrophilic Zr6O8 clusters and dicarboxylate
linkers, we may rationalize that the formation of the porous
and ordered framework plays a critical role, whereby the alkyl
chains are universally disposed in an appropriate way to
To check the hydrophobicity of UiO-67-Rs as related to
the MOF structures, we first performed water vapor adsorp-
tion at 298 K. The saturation water uptake of UiO-67 reached
259.3 mLgÀ1, which is much higher than those of UiO-67-Rs
(180.5, 174.5, 144.9, and 83.8 mLgÀ1 for UiO-67-Et, UiO-67-
But, UiO-67-Hex, and UiO-67-Oct, respectively; Figure 3;
see also Figures S20–S21), thus indicating a significant de-
crease in water adsorption capacity with an increase in alkyl-
chain length. As an estimation of overall hydrophobic
behavior, the starting points for abrupt water uptake were
found at relative pressures P/P0 of 0.21, 0.22, 0.50, 0.65, and
0.70 for UiO-67, UiO-67-Et, UiO-67-But, UiO-67-Hex, and
UiO-67-Oct, respectively; thus, showing a positive correlation
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ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 2 – 10
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