Zirconium Metal-Organic Framework UiO-66: Stability in an Aqueous Environment and Its Relevance for Organophosphate Degradation

Article abstract of DOI:10.1021/acs.inorgchem.8b02360

Zirconium-based metal-organic frameworks were recently investigated as catalysts for degradation of organophosphate toxic compounds, such as pesticides or chemical warfare agents. The most utilized UiO-66 is considered as a stable material for these applications in an aqueous environment. However, the presented results indicate that the properties of UiO-66 are changing considerably in aqueous media under common conditions used for organophosphate degradations, and therefore its catalytic activity is not related to the number of structural defects created during the material synthesis. We delineate the stability of UiO-66 in water of various pHs, the in situ formation of new catalytic sites, and the correlation of these two parameters with the degradation rate of a model organophosphate pollutant, dimethyl-4-nitrophenyl phosphate (methyl-paraoxon). The stability was quantified using high-performance liquid chromatography (HPLC) by measuring the amounts of leached terephthalic acid, the linker of UiO-66, and monocarboxylic acids, the modulators bound at UiO-66 defects. We demonstrate that the HPLC analysis is a more suitable method for metal-organic frameworks stability assessment than commonly used methods, e.g., powder X-ray diffraction, adsorption isotherms, or electron microscopy.

Full text of DOI:10.1021/acs.inorgchem.8b02360

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Zirconium Metal−Organic Framework UiO-66: Stability in an
Aqueous Environment and Its Relevance for Organophosphate
Degradation
†,‡
Daniel Buzek,
†
Jan Demel,† and Kamil Lang*,†
̊
̌
̌
̌
Institute of Inorganic Chemistry of the Czech Academy of Sciences, 250 68 Rez, Czech Republic
‡
́
̌
́
́
̌
Faculty of the Environment, Jan Evangelista Purkyne University, Kralova Vysina 3132/7, 400 96 Ustí nad Labem, Czech Republic
S
* Supporting Information
ABSTRACT: Zirconium-based metal−organic frameworks were recently investigated as
catalysts for degradation of organophosphate toxic compounds, such as pesticides or
chemical warfare agents. The most utilized UiO-66 is considered as a stable material for
these applications in an aqueous environment. However, the presented results indicate
that the properties of UiO-66 are changing considerably in aqueous media under common
conditions used for organophosphate degradations, and therefore its catalytic activity is
not related to the number of structural defects created during the material synthesis. We
delineate the stability of UiO-66 in water of various pHs, the in situ formation of new
catalytic sites, and the correlation of these two parameters with the degradation rate of a
model organophosphate pollutant, dimethyl-4-nitrophenyl phosphate (methyl-paraoxon).
The stability was quantified using high-performance liquid chromatography (HPLC) by
measuring the amounts of leached terephthalic acid, the linker of UiO-66, and
monocarboxylic acids, the modulators bound at UiO-66 defects. We demonstrate that
the HPLC analysis is a more suitable method for metal−organic frameworks stability assessment than commonly used methods,
e.g., powder X-ray diffraction, adsorption isotherms, or electron microscopy.
material syntheses. Similar to missing-linker defects, missing-
cluster defects cause a deficiency of organic matter in the
framework. Regardless the type of the defects, the missing-
cluster/missing-linker dualism leads to the open-metal sites
which provide catalytic activity toward organophosphate
degradations.
INTRODUCTION
■
Metal−organic frameworks (MOFs) are crystalline coordina-
tion polymers with an open framework containing potential
voids. The structure is constructed from metal ions/clusters
forming nodes, often referred as secondary building units
(SBUs), which are linked together via coordination bonds with
organic linkers, frequently di-, tri-, and tetratopic carboxylic
acids.1,2 Due to the variability of metals and linkers, MOFs
provide a wide spectrum of structures and properties with high
porosity and surface area often exceeding 1000 m2 g−1.3 Those
properties make MOFs attractive candidates for gas storage
and separation,4,5 catalysis,6 drug delivery,7 sensing,8 removal
of hazardous compounds,9,10 light-harvesting,11 and photo-
catalysis.12
Since the discovery, the zirconium-based family of MOFs
have gained tremendous attention due to the high hydro-
thermal and chemical stability.13−15 The ideal structure of the
most used UiO-66 is composed of Zr6O4(OH)412+ SBUs as 12-
connected nodes linked together by terephthalic acids.16
Recent analyses using powder X-rays diffraction (XRD),
scanning electron microscopy (SEM), nitrogen adsorption
measurements, NMR, and thermogravimetric methods in-
dicated that the structure of UiO-66 contains a number of
defects (Scheme 1).15,17 Initially, missing-linker defects were
predicted.18,19 However, strong evidence has been collected in
last few years that missing-cluster defects prevail.20,21 The
amount of these defects strongly depends on the conditions of
Organophosphates or phosphate esters are a diverse group
of compounds, which constitute not only essential biomole-
cules, e.g., ATP, DNA,22 but also highly toxic compounds such
as pesticides (e.g., parathion, azinphos-methyl)23 and nerve
chemical warfare agents (sarin, soman, VX, etc.).24,25 Their
toxic effect is based on inhibition of acetylcholinesterase during
the neurotransmission leading to neuromuscular paralysis,
followed by the respiration and heart arrest, and therefore they
are extremely toxic toward insects and mammals.26,27 Due to
their huge toxic risks, the development of fast and effective
methods for organophosphate degradation is essential. In the
past, several decontamination techniques were developed
based on adsorption,28 electrochemical29 and photocatalytic
degradations,30 enzymatic biodegradation,31 and stoichiomet-
ric degradation with reactive sorbents.32,33 Despite the
progress in the field, the degradation efficacy of contemporary
materials still does not match the needs in protective
applications against organophosphate toxins. In recent years,
Received: August 21, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02360
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Scheme 1. Illustration of the UiO-66 Ideal (Non-Defect) Structure (Middle), and Missing-Cluster (Left) or Missing-Linker
(Right) Defective Structures with Compensating Acetic and Formic Acids
Figure 1. Properties of UiO-66. XRD patterns of as-prepared UiO-66 (A), and UiO-66 treated at pH 7.0 (B), and pH 11.0 (C) for 1 h. Inset:
Magnified area 10−60° (2θ). The diffraction lines labeled by (*) belong to a Mylar foil used as a support. SEM images of as-prepared UiO-66 (D),
and UiO-66 after 1 h treatment at pH 7.0 (E) and at pH 11.0 (F). Kinetics of dimethyl-4-nitrophenyl phosphate (DMNP) degradation (a) and 4-
nitrophenol (4-NP) formation (b) at natural pH 3.8 (G), pH 7.0 (H), and pH 11.0 (I). All concentrations are determined with less than 5% error.
XRD patterns (Figure S1), SEM images (Figure S2), N2 adsorption isotherms (Figure S5), and kinetic curves (Figure S9) for all investigated pH
values are presented in the Supporting Information.
the zirconium-based family of MOFs, mainly UiO-66 and its
derivates, were studied as catalysts for organophosphate
pesticide and nerve warfare agent degradation.34−39
The organophosphate degradation by UiO-66 is associated
with the reactivity of unsaturated sites on Lewis-acidic ZrIV
cations.34,36 Hupp et al.40,41 proposed that the catalytic
B
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degradation in a narrow pH range is related to the coordinated
−OH2 molecules at the zirconium SBUs, which can be easily
substituted by organophosphates. However, in light of recent
results, it seems that instead of the terminal −OH and −OH2
ligands at the missing-linker defects, the terminal ligands are
monocarboxylic acids, which were utilized as modulators
during the syntheses or were formed by degradation of the
solvent.20,21,42,43
Here we report on the influence of pH on the degradation
rate of a model organophosphate pollutant, dimethyl-4-
nitrophenyl phosphate (DMNP), catalyzed by zirconium
UiO-66. We studied the reaction in a broad pH range from
pH 3.8 to 11.0 and thoroughly analyzed reaction mixtures
using high-performance liquid chromatography (HPLC). This
approach revealed a strong correlation between the amount of
leached terephthalic acid from the UiO-66 structure and the
catalytic activity. In light of the recent findings concerning
defects in UiO-66,20 our observation provides evidence that
the reaction is initiated by zirconium-carboxylate bond
breakage leading to the formation of open-metal sites, whose
reactivity is responsible for the catalytic degradations.
accompanied by the release of water and carbon dioxide due to
the thermal decomposition of the modulator anions. At this
stage, UiO-66 is in the dehydroxylated form. The total thermal
decomposition of UiO-66 starts at approximately 460 °C and
is connected with the combustion of the terephthalate linkers
as indicated by CO2 evolution. The thermogravimetric analysis
gives a total weight loss of 55%.
In order to quantify the amount of monocarboxylate ligands,
which compensate for the defects in the structure, UiO-66 was
dissolved in 1 M NaOH/D2O followed by 1H NMR
1
measurements (Figure S4). The H NMR spectrum shows
typical peaks of formate (8.3 ppm), terephthalate (7.7 ppm),
and acetate anions (1.7 ppm). In addition, traces of
dimethylamine (2.1 ppm) were also identified. Formate anions
and dimethylamine are the products of DMF thermal
decomposition occurring during the solvothermal synthesis.
The fact that both components were not removed by thorough
washing of the material indicates that they are bonded to the
UiO-66 framework; dimethylamine is probably ion-paired with
the terminal terephthalate ligands and formate anions
compensate for the defects similarly to intentionally added
acetates. A terephthalate/acetate molar ratio of 1/0.18
1
ascertained from H NMR measurement is in line with the
RESULTS AND DISCUSSION
■
reported results,20 indicating that minimally 15% of zirconium
coordination sites are occupied by acetates. A molar
terephthalate/formate ratio of 1/0.13 is quite high probably
due to relatively low activation temperature (90 °C) used in
our case. This result documents the high content of formates
as an in situ produced modulator. In total, the measured
terephthalate/monocarboxylate molar ratio is 1:0.31.
Synthesis and Characterization. UiO-66 was obtained
by the solvothermal reaction of zirconium chloride with
terephthalic acid in the presence of acetic acid as a modulator
in 95 mol equiv to terephthalic acid. The resulting powder was
then recovered from the synthesis solutions via centrifugation
and washed with DMF and acetone. Water was not used for
washing in order to eliminate all hydrolytic/leaching processes
(see below). To confirm the identity of as-prepared UiO-66,
On the basis of the results described above, we conclude that
our sample behaves very similarly to those analyzed by Lillerud
et al.,20 where the material is defective with predominant
missing-cluster defects (reo phase) with compensating
monocarboxylic acids bonded at the defective sites.
Stability of UiO-66 in Water and Its Dependence on
pH. The assessment of the UiO-66 stability in aqueous
solutions can enable rationalizing the catalytic activity at
different pHs. Therefore, we first investigated the acid−base
potentiometric titration of UiO-66 suspensions (Figure 2).
The natural pH value of the suspensions is 3.8. The titration
curve shows features similar to those described earlier.45 The
titration curve can be divided into three specific regions: (i) a
steep pH increase from pH 3.8 to 8.5; (ii) a gradual increase
from pH 8.5 to 9.5; (iii) finally, a rapid increase from pH 9.5
1
we employed XRD, SEM, H NMR, nitrogen adsorption, and
thermogravimetric measurements.
The XRD pattern of as-prepared UiO-66 matches well with
the pattern of microcrystalline UiO-66 (Figure 1, Figure S1).16
The only extra line is a broad diffraction at 1.9−6.5°, indicating
the presence of structural defects (Figure S1).20 SEM images
show octahedral crystals with a broad size distribution varying
from 0.5 to 2.0 μm (Figure 1, Figure S2). The BET surface
area determined from N2 adsorption isotherm is 1391 m2 g−1,
which is higher than that of the ideal (nondefect) UiO-66
structure (i.e., 1125 m2 g−1).44 (Table 1). The thermal
behavior has features similar to those described in the literature
(Figure S3).20 The first broad endothermic peak between
room temperature and 230 °C with a weight loss of 10%
corresponds to the release of surface and bound water
molecules. An exothermic peak at approximately 400 °C is
Table 1. Effect of pH on the Equilibrium Concentration of
the Leached Terephthalate Linker and Surface Area after 1
h Treatment
pH
terephthalic acid/mg L−1
surface area/m2 g−1
a
1391
b
3.8
5.5
7.0
8.5
9.5
< LOD
1.2
51
114
422
424
1331
1315
1285
1283
9
11.0
c
a
b
As-prepared UiO-66. LOD is the limit of detection (0.8 mg L−1).
Figure 2. Acid−base titration curve of UiO-66 (a) and its first
derivation (b).
c
Below detection limit.
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Figure 3. (A) Concentrations of terephthalic (a), acetic (b), and formic (c) acids leached immediately at different pHs from 50 mg of UiO-66 in 50
mL of water compared with the solubility of terephthalic acid in water (d). All concentrations are presented in Table S1. (B) Kinetics of the
terephthalic acid release at pH 7.0; the equilibrium concentration is reached within 3 min.
up to more than 11.0. The titration curve has three equivalence
points at pH 6.2 (1.04 mL 0.1 M NaOH), pH 7.9 (1.28 mL),
and pH 10.1 (4.49 mL).
Table S1). The abrupt release of terephthalic acid is evidently
related to its pKa values (pKa 3.81 and 4.82), indicating its
dissociation above pH 5.0, which sharply increases the
solubility in water (Figure 3A). This massive release also
correlates with the course of the titration curve, which shows
the maximal buffer capacity in the region between pH 8.5 and
9.5. These findings can be further validated by the mass
balance. The weight loss, attributed to the release of
terephthalic and monocarboxylic acids at pH 11.0, is
approximately 46%. The theoretical mass loss, calculated
using the formula of ideal UiO-66 and simplifying the final
product as ZrO2, is approximately 45%, which matches the
experimental result well. These data are also consistent with
the thermogravimetric analysis described above. A total weight
loss of 55%, from which approximately 10% is assigned to the
water release, suggests that approximately 45% weight loss is
connected with the combustion of the organic matter. The
consistency of these results signifies that UiO-66 decomposes
due to the complete release of the terephthalate linker at pH
values above 9.5. In addition, the concentration of leached
zirconium species to solutions is in all cases below the
detection limit of the ICP-MS technique. Evidently, the
hydrolysis of the Zr4+ species after the linker release is quite
fast and leads to amorphous zirconium oxide hydroxide.
A terephthalate/acetate molar ratio of 1/0.19 measured by
HPLC at pH above 9.5 is in very good agreement with the
result of NMR-based dissolution analysis (i.e., 1/0.18). On the
other hand, a terephthalate/formate molar ratio of 1/0.03 is
much lower than the ratio of 1/0.13 obtained by the
dissolution of UiO-66. The discrepancy originates in high
concentration of NaOH needed for complete release of formic
acid (i.e., 1 M NaOH). It follows from the repetition of the
NMR-dissolution experiment using 0.1 M NaOH instead of 1
M NaOH. Under these conditions, a terephthalate/formate
molar ratio was 1/0.04, i.e., coincident with the result of the
leaching experiments measured using HPLC. It documents
that only strongly basic conditions allow for the complete
release of formate anions from hydrolyzed zirconium oxide-
hydroxide. In contrast, the acetate ligands are easily substituted
by the hydroxide ligands at common pHs.
In order to unravel the structural changes of UiO-66 after
contact with aqueous media of different pHs, UiO-66 was
separated from the suspensions after 1 h at given pH and
analyzed by powder XRD, nitrogen adsorption isotherms, and
SEM. Using these standard techniques, the UiO-66 structure
seems to remain nearly unchanged up to pH 8.5; however, the
MOF structure completely collapses at pH 9.5 and above
(Figure 1, Figure S1). The surface area of UiO-66 slightly
decreases from 1391 m2 g−1 for the as-prepared material to
1283 m2 g−1 for the sample treated at pH 8.5, and finally, the
treatment at greater pHs generates an abrupt drop (Table 1,
Figure S5). Interestingly, SEM images do not indicate any
morphological changes even at pH values above 9.5 (Figure 1,
Figure S2), documenting the preservation of the particle
morphology even after removal of the organic matter.
Unfortunately, the broad size distribution of the original
material masks a probable particle downsizing. To conclude,
the used standard techniques indicate only minor structural
changes of UiO-66 after the treatment at the pH between 3.8
and 8.5. The analysis of the SEM images is not conclusive even
at greater pH values.
Thus, to track the changes to UiO-66 occurring in aqueous
media at pH below 8.5, analytical methods of high sensitivity
have to be utilized. In this context, the HPLC technique offers
high accuracy for the analyses of leached terephthalic and
monocarboxylic acids in concentrations down to approximately
1 ppm (1 mg L−1) (Table 1, Table S1). During the course of
the potentiometric titration, the concentrations of formic and
acetic acids, measured immediately after reaching the given
pH, gradually grow from 1 and 12 mg L−1 up to approximately
3 and 29 mg L−1 at pH 6.5, respectively, and then plateau at
higher pHs (compare Figure 2 with Figure 3). On the other
hand, terephthalic acid is not detected below pH 5.0; however,
its immediate concentration is 22 mg L−1 at pH 7.0, which is
approximately 5% of the total linker content in as-prepared
UiO-66 (Figure 3A, Table S1). At this point, we would like to
note that the release is very fast, and the equilibrium
concentration of terephthalic acid (approximately 13% of the
linker content) is reached within 3 min (Figure 3B, Table 1).
The concentration of released terephthalic acid steeply
increases to more than 400 mg L−1 at pHs above 9.5,
indicating the total release of the linker (Figure 3A, Table 1,
With thus validated methodology, we decided to study the
origin of the acidic pH of UiO-66 suspensions in water. To our
surprise, 50 mg of UiO-66 suspended in 50 mL of deionized
water released 12 mg L−1 of acetic acid and less than 1 mg L−1
of formic acid. No terephthalic acid was detected. Logically, we
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Scheme 2. Formation of Open-Metal Sites at Zr(IV) SBUs of UiO-66 after the Release of the Terephthalate Linkers, and
a
Coordinated Acetate and Formate Ligands
a
These sites act as catalytic centers of DMNP degradation.
a
Table 2. Degradation of DMNP Catalyzed by UiO-66 at Different pHs
pH
conversion/%
k
DMNP/min−1
τ1/2(DMNP)/min
k
4‑NP/min−1
τ1/2(4‑NP)/min
3.8
5.0
5.5
6.0
7.0
8.5
9.5
11.0
76
75
80
0.035
0.032
0.039
0.049
0.105
0.116
n.a.
20
22
20
14
7
6
n.a.
n.a.
0.032
0.030
0.034
0.039
0.094
0.104
n.a.
22
23
18
18
7
7
n.a.
n.a.
81
100
100
37
17
n.a.
n.a.
a
Conversion of DMNP is given after 60 min of the reaction; kDMNP and τ1/2(DMNP) are the first-order rate constant and half-time of the DMNP
degradation, respectively; k4‑NP and τ1/2(4‑NP) are the first-order rate constant and half-time of the 4-NP formation, respectively.
found the direct proportion between the suspended amount of
UiO-66 and concentrations of the released monocarboxylic
acids (Figure S6). At this point, we emphasize that as-prepared
UiO-66 was thoroughly washed with DMF, the solvent in
which both formic and acetic acids are well soluble, and
therefore, the released acids were not occluded impurities
within the UiO-66 structure. Evidently, the coordinated
monocarboxylates, mainly acetates, are exchangeable with
water molecules and released acid increases the acidity to pH
3.8.
In order to prove this assumption, as-prepared UiO-66 was
treated with deionized water in five consecutive cycles (Figure
S7). The concentrations of both acids gradually decrease as
their supply in the structure is depleted. In concert with it, the
natural pH value increases from pH 3.8 in the first cycle to pH
5.2 after the fifth cycle as the concentration of the released
acetic acid decreased considerably. During this experiment, we
did not observe any terephthalic acid leaching. On the other
hand, five consecutive treatments of UiO-66 at pH 7.0 resulted
in the repetitive leaching of terephthalic acid with decreasing
equilibrium concentrations from one treatment to the other
(Figure S8). On the basis of these results, we conclude that the
leaching of the UiO-66 linker is a dynamic process, depending
on the actual content of terephthalic acid in the sample and on
the pH value. In addition, the equilibrium concentrations are
reached quickly as documented by corresponding kinetics
presented in Figure 3B. These findings point to the importance
of the washing procedure on the resulting composition and
properties of UiO-66.
DMNP leading to 4-NP and dimethyl phosphate at different
pH values. Because dimethyl phosphate partially adsorbs onto
UiO-66 (see the Supporting Information for details), we
followed the course of the reaction by analyzing DMNP and 4-
NP concentrations (Table 2, Figure 1, Figure S9). The
reaction curves follow first-order kinetics, and the fitting to the
monoexponential function gives corresponding rate constants
and half-times. The blank reactions were evaluated in the
absence of UiO-66. As expected, DMNP itself is stable in the
broad range of pHs, and only a subtle conversion of
approximately 3% at pH 11.0 after 60 min of the reaction
was noticed.
In acidic pHs, the degradation kinetics of DMNP is quite
slow with a half-time of 20 min at pH 3.8 and 14 min at pH
6.0. The half-times of the 4-NP formation are the same within
the experimental error, which is expected for a simple
monomolecular degradation mechanism (Scheme 2). As
described above, this pH region is characterized by the
predominant release of acetic acid from UiO-66, whereas the
leaching of the terephthalate linker is a minor process. The
release of acetic acid is accompanied by active sites formation
catalyzing the organophosphate hydrolysis. At pHs between
7.0 and 8.5, the catalysis accelerates significantly, leading to the
quantitative transformation of DMNP to 4-NP within 60 min
with half-times of 7 min. These high reaction rates are
evidently associated with the significant release of the
terephthalate linker (Figure 3A), thus opening many new
catalytic sites at the Zr6 SBUs without the disruption of the
UiO-66 structure (Scheme 2). Further increase of the pH
above 9.5 leads to the deactivation of the catalyst within
approximately 5 min and quite low conversions. Clearly,
massive dissolution of the terephthalate linker from UiO-66
causes complete destruction of the UiO-66 structure, and the
low conversions are due to a low catalytic activity of
Catalytic Activity of UiO-66 at Different pHs. As shown
in the literature, the degradation of organophosphates is
catalyzed by open-metal sites on Zr SBUs (Scheme 2).41 In
order to correlate the stability of UiO-66 (see above) with the
catalytic activity, we investigated the hydrolytic degradation of
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amorphous zirconium oxide hydroxide,46 formed by the
decomposition of UiO-66.
and the signals were collected at 272 and 300 nm for DMNP and 4-
NP, respectively. Terephthalic acid was analyzed on a Hydrosphere 5
μm C18 column (YMC HPLC COLUMN, Japan, 150 mm × 4.6
mm) under isocratic elution (1 mL min−1) with a 30/70 (v/v)
acetonitrile/water/0.1% HCOOH mobile phase. The time of analysis
was set to 5 min, and the signal was collected at 240 nm. Formic and
acetic acids were quantified using a Synergi 4 μm polar-RP 80 A
column (Phenomenex, USA, 100 mm × 4.6 mm) with a water/0.1%
HCl mobile phase (0.5 mL min−1). The time of the analysis was set to
5 min followed by equilibration for 5 min, and the signals were
collected at 210 nm. The concentrations of all compounds were
evaluated using the calibration curve method. The concentration of
leached zirconium was measured on an ICP-MS Agilent 7700
equipped with an argon burner, ion optics, collision reaction ORS 3
cell, and hyperbolic quadrupole mass analyzer with a DDEM detector.
The samples (1 mL, pH 3.8−11.0, filtration using 0.1 μm microfilters)
were measured under the helium and no-gas mode, and the detector
was set on the mass of 90 and 91 for 90Zr and 91Zr, respectively.
1H NMR spectra were measured using a JEOL 600 MHz NMR
The release of terephthalate linkers and monocarboxylate
ligands at given pHs indicates the creation of coordinatively
unsaturated sites in situ during the course of the reaction. In
order to evaluate the effect of the number of created defects,
UiO-66 was pretreated at pH 7.0 for 30 min, and its catalytic
activity was investigated at lower pHs afterward (Figure S10).
Clearly, the conversion of DMNP to 4-NP at pH 6.1 catalyzed
by pretreated UiO-66 is comparable with that of as-prepared
UiO-66 performed at pH 7.0. These results confirm the
importance of the in situ defect formation for the catalytic
reaction and suggest the ways for the improvement of the
catalytic activity of UiO-66 at lower pHs, i.e., under conditions
of higher stability of this material.
EXPERIMENTAL SECTION
■
1
spectrometer. The chemical shifts are referenced to the residual H
Materials. Zirconium(IV) chloride (99.99% anhydrous), tereph-
thalic acid, dimethyl-4-nitrophenyl phosphate (methyl-paraoxon,
abbreviated hereafter as DMNP), 4-nitrophenol (abbreviated here-
after as 4-NP), methanol and acetonitrile (both for HPLC, gradient
grade, Sigma-Aldrich), N,N-dimethylformamide (Penta, Czech
Republic; abbreviated as DMF), acetic acid (99.8%), formic acid
(98%), acetone, NaOH, HCl (35%) (all Lach-Ner, Czech Republic)
were used as received.
signal of D2O. Thermal analyses were carried out on a Setaram
SETSYS Evolution-16-MS instrument coupled with a mass
spectrometer. The measurements were performed in synthetic air
(30 mL min−1) from 20 to 800 °C with a heating rate of 5 °C min−1.
Stability Testing of UiO-66. Concentrations of terephthalic,
formic, and acetic acid released from UiO-66 in water were
determined at a natural pH value of 3.8 and at pHs between 4.0
and 11.0 with a step of 0.5. The procedure was as follows: 50 mg of
UiO-66 was mixed with 50 mL of water, shortly sonicated, and the pH
values were adjusted by adding 0.1 M NaOH using the automatic
Titrator 794 Basic Titrino (Metrohm, Switzerland). Aliquots of 0.2
mL were taken immediately after reaching the respective pH values,
filtered through Whatman microfilters (0.2 μm, PTFE), and analyzed
by HPLC as described above. The immediate solubility of
terephthalic acid was measured in the stirred dispersion of 50 mg
of terephthalic acid in 50 mL of water using the same procedure. The
equilibrium concentrations of terephthalic acid at given pH values
were measured in the dispersions after 1 h of stirring. The kinetics of
terephthalic acid release was measured at pH 7.0 with sampling from
0.5 to 240 min. All presented concentrations are the mean values of
three independent experiments. The standard deviations are lower
than 10%.
Synthesis of UiO-66. UiO-66 was synthesized by the modified
solvothermal procedure16 using a ZrCl4/terephthalic acid molar ratio
of 1:1. Zirconium chloride (106 mg, 0.455 mmol) was dissolved in
DMF (20 mL) in a 40 mL Wheaton vial under sonication. Then,
terephthalic acid (75.6 mg, 0.455 mmol) was added, followed by
sonication to obtain a transparent solution. The solution was mixed
with acetic acid (99.8%, 2.5 mL), and the vial was sealed. The
crystallization was carried out in an oven at 120 °C for 24 h without
stirring. The precipitate was separated by centrifugation (Hettich
Rotina 35, 10 000 rpm, 5 min), washed four times with DMF and five
times with acetone. The resulting white powder was air-dried
overnight and activated at 90 °C under a dynamic vacuum for 24 h.
Instrumental Methods. Powder X-ray diffraction (XRD) was
recorded using a PANalytical X’Pert PRO diffractometer in the
transmission setup equipped with a conventional Cu X-ray tube (40
kV, 30 mA). Qualitative analysis was performed with the High-
ScorePlus software package (PANalytical, Almelo, The Netherlands,
version 3.0) and the JCPDS PDF-2 database.47 The porosity was
probed by measuring nitrogen adsorption isotherms at 77 K using a
Belsorp max II instrument (Microtrac Bel). Prior to the adsorption
experiments, the samples were evacuated at 90 °C for at least 24 h.
The surface area was determined using the Brunauer−Emmett−Teller
(BET) method. High-resolution scanning electron microscopy (SEM)
was performed using a FEI Nova NanoSEM equipped with a circular
backscatter detector in the backscattered electron mode. An
accelerating voltage was set to 5 or 10 kV. UiO-66 was suspended
in acetone in an ultrasonic bath and deposited onto a silicon wafer
chip, and the deposit was air-dried overnight.
The acid−base titration of UiO-66 was performed using an
automatic Titrator 794 Basic Titrino (Metrohm, Switzerland). 50 mg
of UiO-66 was transferred to a beaker and 50 mL of water was added,
followed by 15 min sonication leading to the milky-like suspension.
The pH was adjusted to 3.0 using 0.1 M HCl, and the stirred
suspension was titrated by 0.1 M NaOH with a rate of 0.04 mL min−1
up to pH 11.3. The titration experiments were repeated three times
with excellent reproducibility.
The concentrations of DMNP and 4-NP were measured using an
HPLC-DAD DIONEX UltiMate 3000 instrument equipped with a
diode array detector, 20 μL sampling loop, and a Kinetex 2.6 μm C18
column (Phenomenex, USA, 50 mm × 3 mm). Methanol/water was
used as a mobile phase (0.5 mL min−1, both solvents contain 0.1%
HCOOH) with a gradient starting from 40/60 to 95/5 (v/v). The
time of analysis was set to 5 min followed by equilibration for 2 min,
Liquid 1H NMR spectra were measured after the dissolution of 20
mg of UiO-66 in 0.8 mL of 1 M NaOH in D2O. UiO-66 was dissolved
overnight at room temperature similar to the procedure developed by
Lillerud et al.20
Degradation of DMNP at Different pHs. In a typical
experiment, 2 mg of activated UiO-66 (24 h, 90 °C under a dynamic
vacuum) was mixed with 1.75 mL of water in a 4 mL vial and
sonicated to form a turbid suspension. In degradation experiments at
the natural pH (pH 3.8), 0.25 mL of 8 mM DMNP aqueous solution
was added. The pH was adjusted by consecutive additions of 0.1 M
NaOH to 5.0, 5.5, 6.0, 7.0, 8.5, 9.5, and 11.0 using the automatic
Titrator 794 Basic Titrino followed by the addition of 8 mM DMNP.
The initial concentration of DMNP was 1 mM. During the reaction,
the pH values were kept constant using the automatic titrator. The
reaction was performed under stirring at room temperature; 50 μL
samples were taken at regular intervals (2, 5, 10, 15, 30, 45, and 60
min) and diluted with 1 mL of 0.1% HCOOH to stop the reaction.
Finally, the samples were filtered using a 0.2 μm PTFE Whatmann
microfilter and analyzed by HPLC. Pretreated UiO-66 was prepared
by the mixing of 2 mg of as-prepared UiO-66 with water adjusted to
pH 7.0. After 30 min of stirring, solid UiO-66 was separated by
centrifugation and washed twice with water until the absence of
terephthalic acid in the solution. Then, the solid was used in
degradation experiments as described above without pH adjustment.
Blank experiments were performed by the analogous procedure in the
absence of UiO-66. The presented results are the average values of
three independent experiments. In all cases, the standard deviations
are lower than 5%.
F
DOI: 10.1021/acs.inorgchem.8b02360
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Structural Changes of UiO-66 at Different pHs. For the
purpose of structural analyses, the degradation reaction of DMNP was
scaled-up to 50 mg of UiO-66 in 43.75 mL of water and 6.25 mL of 8
mM DMNP, leading to a total volume of 50 mL and DMNP
concentration of 1 mM at pH 3.8 (natural), 5.5, 7.0, 8.5, 9.5, and 11.0.
The pH value was kept constant during the course of the reaction
using the automatic titrator. After 1 h under stirring, solid UiO-66 was
separated by centrifugation and DMNP, 4-NP, and terephthalic acid
concentrations were measured using HPLC. Solid UiO-66 was
washed three times with water and three times with acetone, air-dried
for 24 h, and analyzed by XRD, SEM, and by nitrogen adsorption.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: lang@iic.cas.cz.
ORCID
̊
̌
Daniel Buzek: 0000-0002-7387-9461
Jan Demel: 0000-0001-7796-6338
Kamil Lang: 0000-0002-4151-8805
Notes
The authors declare no competing financial interest.
CONCLUSIONS
■
ACKNOWLEDGMENTS
■
In previous reports, the potentiometric acid−base titration was
suggested as a method for evaluation of acidity of protons of
the −OH2 and −OH ligands, which compensate for missing
terephthalate linkers.45 Evaluation of the MOF acidity by the
potentiometric titration would be valid only if the structure
and composition of the titrated material would not change
during the titration. Our quantitative analyses indicate that this
assumption is not correct.
The work was supported by the Czech Science Foundation
(No. 16-02098S) and by the Student Grant Agency of the
University of Jan Evangelista Purkyne in Ustı nad Labem. The
authors acknowledge the assistance provided by the Research
Infrastructure NanoEnviCz, supported by the Ministry of
Education, Youth and Sports of the Czech Republic under
́
̌
́
̌
Project No. LM2015073. We thank Petr Bezdicka for XRD,
́
Slavomır Adamec for ICP-MS, and Jan Hynek for NMR
We summarize the results as follows: (i) A number of −OH2
and −OH ligands in unsaturated coordination sites of as-
prepared UiO-66 are limited as these sites are verifiably
occupied by modulator anions including formates generated in
situ from DMF during the synthesis.20,43 (ii) In aqueous
dispersions of UiO-66, coordinated acetates and formates
released into water are responsible for its acidity (i.e., natural
pH 3.8). It points to the importance of the washing procedure
on the composition and properties of UiO-66. (iii) The release
of the terephthalate linkers and monocarboxylate ligands
during the titration evidently creates new defects (i.e.,
unsaturated coordination sites), not originally present in the
structure (Scheme 1). This fact can be used for the intentional
defect formation. (iv) Characterization methods generally used
for quantification of MOF stabilities (SEM, XRD, and
adsorption studies) are not conclusive. The subtle changes to
the structure, indicated by released structural components,
have to be analyzed by sensitive techniques such as HPLC. (v)
New catalytic sites toward the dimethyl-4-nitrophenyl
phosphate degradation are formed after the release of the
linker or other compensating monocarboxylate ligandsthe
more massive release without the disruption of the structure,
the higher catalytic activity of UiO-66. These results point to
the dynamic nature of Zr-based MOFs where coordination/
decoordination of ligands can occur at mild conditions and can
significantly alter the resulting MOF properties. (vi) The
leaching of terephthalate linker from UiO-66 shows that UiO-
66 it is not a good material for flow reactors in neutral and
basic environments.
measurements.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the
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