Angewandte
Chemie
Thermogravimetric analysis (TGA) and IR spectroscopy
were therefore used to qualitatively assess the hydrophobic
character of these solids. TGA measurements have shown
that the samples exhibit almost no loss of free water
(Supporting Information Figure S18).
140s are highly stable with a slightly higher thermal stability,
of around 5008C under air, against 4508C for the UiOs. As
a further step, the hydrothermal and mechanical stability of
the MIL-140 series were explored and compared to the
corresponding UiO-66 analogues.
Optical water vapor adsorption isotherms have been
collected (Figure S29) and show that these solids can be
considered as being more hydrophobic than the UiO-66
counterpart which is fully loaded with water at intermediate
humidity rate (P/P0 = 0.5):[11] Only a little adsorption occurs at
low pressure, particularly for the smaller pore size MIL-140A
and B solids, probably corresponding to coordination to the
metal sites; at higher pressure, a filling of the pores starts, as
typically observed for some activated carbons or siliceous
zeolites,[26] even if no sharp adsorption step is observed.
Albeit not as hydrophobic as fluorinated MOFs,[11] the MIL-
140s can be considered as rare examples of slightly hydro-
phobic porous solids that have a significant amount of Lewis
acid sites.
The industrial use of an adsorbent requires both moisture
(or hydrothermal) and mechanical stability to avoid structural
distortion or framework collapse under hydrostatic compres-
sion during the shaping steps.[30,31] First, it appears, based on
XRPD analysis, that MIL-140 compounds are hydrothermally
stable, that is, after an overnight dispersion in deionized water
at 1008C (Figure 3), no loss of crystallinity occurs, even for the
upper analogues. The UiO-66(Zr) solid also retains its
crystallinity upon the same treatment, but this is not the
case for the upper UiO analogues, for which only poorly
crystalline samples are recovered (Figure S28).
As a consequence of the 1D pore system, the pore size and
volume of the MIL-140s are smaller than for the UiO series,
for which experimental and/or theoretical specific surface
areas[27] range from 1000 to 3500 m2 gÀ1 (Figure S26, S27). The
pore size distributions (PSD) of the MIL-140A–D structures
were estimated by the method reported by Gelb and
Gubbins.[28] The pore sizes are roughly 3.2, 4.0, 5.7 and
6.3 ꢀ respectively (Figure S20). The experimental BET sur-
face areas were further determined at 415(10), 460(10),
670(20) and 701(20) m2 gÀ1, respectively for the activated
MIL-140A–D samples. Values for MIL-140C and D are lower
than the theoretical BET surface areas from their simulated
adsorption isotherms of N2 at 77 K, obtained by Grand
Canonical Monte Carlo simulations (841 and 875 m2 gÀ1,
respectively, Figures S24 and S25).[27] Such a deviation can be
explained by the presence of residual Zr oxide as evidenced
by TGA and elemental analysis (Figure S19, Table S4).
Further, while the theoretical BET values are similar to the
accessible surface areas for the MIL-140C and D, a situation
that validates the use of the geometric method for these solids
with medium pore sizes, the situation drastically differs for the
small pore MIL-140A and B structures. Their theoretical BET
surface areas deduced from experimental isotherms (360 and
429 m2 gÀ1, respectively) are much more reliable than the
accessible surface areas (0 and 149 m2 gÀ1) and fit well the
experimental value. This result can be related to previous
studies[29] which have emphasized that the side pockets of the
mordenite type zeolite cannot be probed using the geometric
method. Thus, the geometric approach is not suitable for
adsorbents with a pore size comparable to those of N2. The
consideration of the pore volume is an alternative way to
circumvent such limitations and further characterize the
ultra-small pore sizes structures. The thermodynamic pore
volumes calculated (Table S9) lead to values of 0.10, 0.21,
0.35, 0.36 cm3 gÀ1 for MIL-140A, B, C, and D (exp: 0.18, 0.18,
0.27, 0.29 cm3 gÀ1, respectively).
Figure 3. XRPD of MIL-140A, B, C, and D (from bottom to top)
(lCu =1.5406 ꢂ). Before water treatment (solid line), after hydrothermal
treatment (1008C, 15 h; dotted line).
Interestingly, no peaks from the recrystallized dicarbox-
ylate linkers are observed after the hydrothermal treatment.
Considering the poor solubility of these linkers in water and
the position of the remaining diffraction peaks of the sample
after hydrothermal treatment, close to the main peaks of the
initial phase, this data suggest a loss of long range order only.
Thus, either the hydrothermal treatment leads to a partial
degradation of the starting UiO material or to the formation
of poorly crystalline MIL-140 solids through a dissolution
recrystallization process. To assess this hypothesis, the reac-
tivity of the Cl2H2ABDC/Zr4+ pair in DMF was explored at
various temperatures (150–2008C; Figure S1). While UiO
solid is formed at low temperature (150–1608C), a further
increase of the temperature leads first to a dissolution of the
phase and the recrystallization of MIL-140D at 1808C.
Similarly, UiO-66 and MIL-140A are obtained in DMF at
1508C or 2208C, all other conditions (reactants, time, etc.)
being kept identical. Thus, it can be estimated that UiOs and
their polymorph MIL-140s are the kinetic or thermodynamic
phases, respectively. To our knowledge, this is the first time
that a series of isoreticular extended solids exhibits a hydro-
thermal stability. The higher hydrothermal stability of the
MIL-140s, compared to the UiOs, could be due to the
inorganic building units, that is, infinite Zr oxide chains versus
isolated Zr6O4(OH)4 oxoclusters for the UiO structures,
To further assess the thermal stability of the two sets of
porous Zr MOFs, that is, the UiO-66 and MIL-140 series,
combined TGA and X-ray powder diffractometry analysis
were carried out (Figures S18 and S19). It appears that MIL-
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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