Capture of nerve agents and mustard gas analogues by hydrophobic robust MOF-5 type metal-organic frameworks

Article abstract of DOI:10.1021/ja2042113

In this communication, a series of observations and data analyses coherently confirms the suitability of the novel metal-organic framework (MOF) [Zn₄(μ₄-O)(μ₄-4-carboxy-3, 5-dimethyl-4-carboxy-pyrazolato)₃] (1) in the capture of harmful volatile organic compounds (VOCs). It is worthy of attention that 1, whose crystal structure resembles that of MOF-5, exhibits remarkable thermal, mechanical, and chemical stability, as required if practical applications are sought. In addition, it selectively captures harmful VOCs (including models of Sarin and mustard gas, which are chemical warfare agents), even in competition with ambient moisture (i.e., under conditions mimicking operative ones). The results can be rationalized on the basis of Henry constant and adsorption heat values for the different essayed adsorbates as well as H₂O/VOC partition coefficients as obtained from variable-temperature reverse gas chromatography experiments. To further strengthen the importance of 1, its performance in the capture of harmful VOCs has been compared with those of well-known materials, namely, a MOF with coordinatively unsaturated metal sites, [Cu₃(btc)₂] and the molecular sieve active carbon Carboxen. The results of this comparison show that coordinatively unsaturated metal sites (preferential guest-binding sites) are ineffective for the capture of VOCs in the presence of ambient moisture. Consequently, we propose that the driving force of the VOC-MOF recognition process is mainly dictated by pore size and surface hydrophobicity.

Full text of DOI:10.1021/ja2042113

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pubs.acs.org/JACS
Capture of Nerve Agents and Mustard Gas Analogues by Hydrophobic
Robust MOF-5 Type MetalꢀOrganic Frameworks
Carmen Montoro,† Fꢀatima Linares,† Elsa Quartapelle Procopio,† Irena Senkovska,‡ Stefan Kaskel,‡
Simona Galli,§ Norberto Masciocchi,§ Elisa Barea,*,† and Jorge A. R. Navarro*,†
†Departamento de Química Inorgꢀanica, Universidad de Granada, Av. Fuentenueva S/N, 18071 Granada, Spain
‡Department of Inorganic Chemistry, Dresden University of Technology, Mommsenstrasse 6, 01069 Dresden, Germany
§Dipartimento di Scienze Chimiche e Ambientali, Universitꢁa dell’Insubria, Via Valleggio 11, 22100 Como, Italy
S Supporting Information
b
ABSTRACT: In this communication, a series of observa-
tions and data analyses coherently confirms the suitability of
the novel metalꢀorganic framework (MOF) [Zn4(μ4-O)-
(μ4-4-carboxy-3,5-dimethyl-4-carboxy-pyrazolato)3] (1) in
the capture of harmful volatile organic compounds (VOCs).
It is worthy of attention that 1, whose crystal structure
resembles that of MOF-5, exhibits remarkable thermal,
mechanical, and chemical stability, as required if practical
applications are sought. In addition, it selectively captures
harmful VOCs (including models of Sarin and mustard gas,
which are chemical warfare agents), even in competition
with ambient moisture (i.e., under conditions mimicking
operative ones). The results can be rationalized on the basis
of Henry constant and adsorption heat values for the
Figure 1. Idealized view of a portion of the crystal structure of
different essayed adsorbates as well as H2O/VOC partition
[Zn4O(3,5-dimethyl-4-carboxypyrazolato)3] (1) with segregated Zn4O-
coefficients as obtained from variable-temperature reverse
(CO2)6 and Zn4O(pz)3 SBUs.
gas chromatography experiments. To further strengthen the
importance of 1, its performance in the capture of harmful
VOCs has been compared with those of well-known materi-
als, namely, a MOF with coordinatively unsaturated metal
sites, [Cu3(btc)2] and the molecular sieve active carbon
Carboxen. The results of this comparison show that coordi-
Scheme 1. Chemical Warfare Agents Isopropylmethylfluor-
ophosphate (IMFP, Sarin Nerve Gas), and Bis(2-chlor-
oethyl)sulfide (BCES, Mustard Vesicant Gas) and the
Corresponding Model Compounds Used in Our Studies
natively unsaturated metal sites (preferential guest-binding
(DIFP, Diisopropylfluorophosphate; DES, diethylsulfide)
sites) are ineffective for the capture of VOCs in the presence
of ambient moisture. Consequently, we propose that the
driving force of the VOCꢀMOF recognition process is
mainly dictated by pore size and surface hydrophobicity.
rotection against fortuitous or deliberate emissions of harm-
ful chemical compounds is of major social concern. Particu-
larly, protection against readily accessible chemical warfare
agents such as Sarin nerve gas and mustard vesicant gas is
definitely necessary. In this regard, the availability of high-
performance materials for sensing, capturing, or decomposing1,2
chemical warfare agents is of undoubted interest.
P
In this communication, we report a proof of concept of the use
of hydrophobic MOFs to capture chemical warfare agents.
Indeed, the novel robust MOF [Zn4O(3,5-dimethyl-4-carboxy-
pyrazolato)3] (1) (Figure 1) is able to capture model compounds
of Sarin nerve gas and Mustard vesicant gas (Scheme 1) from the
gas phase.
The selective adsorptive3 and catalytic4 properties of porous
metalꢀorganic frameworks (MOFs) are very attractive for the
capture and/or decomposition of harmful volatile organic com-
pounds (VOCs).5,6 Nevertheless, to the best of our knowledge,
only one report on the possible utility of MOFs for chemical
warfare protection, dealing with the solidꢀliquid adsorption
process of an anionic organophosphate nerve agent surrogate,
has appeared in the scientific literature.7
Received: May 7, 2011
Published: July 14, 2011
r
2011 American Chemical Society
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|

Journal of the American Chemical Society
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Figure 2. Chemical stability tests on 1 suspended in water at room
temperature and in boiling organic solvents for 24 h.
1 can easily be prepared in large amounts and high yields (up
to 20 g per batch) as a microcrystalline material by the direct
reaction of Zn(NO3)2 with 3,5-dimethyl-4-carboxypyrazole
(H2dmcapz) in a basic ethanol medium under reflux. 1 crystal-
lizes in the cubic Fm-3m space group.8 Within its three-dimen-
sional porous framework, Zn4O6+ tetranuclear nodes are
connected to six symmetry-related ones by (50% orientationally
disordered) N,N0,O,O0-exo-tetradentate 3,5-dimethyl-4-carboxy-
pyrazolato spacers. Rather narrow windows (less than 4 Å ꢁ 4 Å)
give access to pores with diameters of ∼6 Å9 comprising, after
solvent removal, a void volume of ∼47%. The protrusion of the
methyl groups into the pores is reasonably responsible for the
high hydrophobicity featured by 1 (see below) and possibly
contributes to its chemical inertness by protecting the MꢀO-
(carboxylate) bonds from hydrolysis. When the symmetry is
reduced to the F23 space group (i.e., when the disorder is
eliminated), the framework possesses a rock salt-type topology
in which M4O(CO2)6 and M4O(pz)3 secondary building units
(SBUs) are joined by the dmcapz spacers (Figure 1). Accord-
ingly, the structure of 1 is reminiscent of that of the renown
[Zn4O(1,4-benzenedicarboxylato)3] system (MOF-5),10 the
disorder observed in 1 being retraceable to the nearly equal
coordination features of the N- and O-ends of the dmcapz ligand.
The utility of MOF-5 for practical applications is hampered by
the easy hydrolysis of the ZnꢀO coordinative bonds, which
makes the whole framework extremely sensitive to moisture.11
This drawback can be limited by substituting the carboxylate
ligands with structurally related N-donor spacers (tetrazolates,
triazolates, pyrazolates), which afford materials with enhanced
thermal and chemical stability as a consequence of the greater
robustness of the MꢀN bonds.6,12,13 In the case of 1, which
contains a hybrid N,O-donor ligand, we assessed its robustness
by performing a series of thermal assays as well as chemical and
mechanical stability tests. Significantly, 1 resisted key treatments
such as suspension in distilled water or a slightly acidic medium
(0.001 M HNO3) at room temperature or in a boiling organic
solvent (methanol, benzene, or cyclohexane) for a prolonged
period of time (Figure 2). Only after severe heating in water at
80 °C for 8 h was the formation of a (still uncharacterized) dense
material formulated as [Zn3(OH)2(dmcapz)2] (2) observed. On
the other hand, the thermal analyses performed on 1 [thermo-
gravimetric analysis, differential scanning calorimetry, and ther-
modiffractometric measurements (TXRPD)] in a reactive atmo-
sphere of air indicated its outstanding thermal robustness, with
decomposition beginning only at 773 K. TXRPD experiments
Figure 3. (a) N2 (77 K, blue diamonds) and CO2 (195 K, red triangles)
adsorption isotherms for 1; (b) H2O physisorption isotherm at 298 K
(first cycle, black circles; second cycle, brown triangles) for 1. Solid and
open symbols denote adsorption and desorption, respectively.
not only highlighted that 1 preserves its structural features and
high crystallinity up to pyrolysis but also were indicative of the
framework rigidity: the unit cell volume underwent a very small
decrease of ꢀ0.5% upon heating [see the Supporting Informa-
tion (SI)]. The mechanical stability of 1 is also remarkably high,
as this material remained practically unaffected after the
application of pressures up to 0.2 GPa, which induced a
decrease of ∼20% in the specific surface area. In contrast,
under similar conditions, the highly porous MOFs [Cr3F-
(H2O)2O(1,4-benzenedicarboxylato)3] (MIL-101)14 and [Cu3-
(1,3,5-benzenetricarboxylato)2] ([Cu3(btc)2])15 underwent sig-
nificant amorphization, with loss of up to 90% of the original
porosity (see the SI).
The accessibility of guest molecules into the porous network
of 1 was assessed by solidꢀgas adsorption experiments involving
different probe gases, namely, N2 (77 K), CO2 (195 K), and H2O
(298 K) (Figure 3). The N2 and CO2 adsorption isotherms
exhibit type-I behavior, which is typical of crystalline micropor-
ous materials, with BET specific surface areas of 840 and 740 m2
g
ꢀ1, respectively. The adsorption capacity of the micropores is
quite high: ∼12 mmol of N2 gꢀ1, corresponding to a micropore
volume of 0.45 cm3 per cm3 of 1. It is noteworthy that H2O
adsorption at 298 K takes place only at very high relative
pressures (p/po > 0.7), which is indicative of a highly hydro-
phobic material that is possibly adequate for the adsorption of
VOCs. The total pore volume calculated from the water adsorp-
tion isotherm amounted to 0.40 cm3 per cm3 of 1. The high
reproducibility of the H2O adsorption isotherms in multiple
cycles further supports the high stability of 1 toward hydrolysis.
The hydrophobic nature of 1 prompted us to investigate its
utility for the capture of harmful VOCs. These studies were
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Journal of the American Chemical Society
COMMUNICATION
Table 2. Calculated Heats of Adsorption ΔHads (kJ molꢀ1),
Henry Constants KH (cm3 mꢀ2), and partition coefficients Kr
for [Cu3(btc)2] and Carboxen with Various VOCs and Water
a
Adsorbent
VOC
ꢀΔHads
KH
Kra
[Cu3(btc)2]
[Cu3(btc)2]
[Cu3(btc)2]
Carboxen
DES
DIFP
H2O
DES
DIFP
H2O
78.1
48.4
43.9
49.0
38.3
2.7
6570
116
67.9
56.3
1170
76.9
0.02
1.2
ꢀ
58000
3800
ꢀ
Carboxen
Carboxen
a Calculated values at 298 K. Values of Kr for VOCs vs H2O were
obtained from variable-temperature zero-coverage gas chromatography
experiments on [Cu3(btc)2] and Carboxen.
ꢀR[∂(ln Vg)/∂(1/T)].18 The direct relation between the reten-
tion volume (Vg) and the Henry constant (KH) also permitted
values of the Henry constant under ambient conditions (298 K)
to be calculated (Table 1). The high values of ΔHads and KH in
the case of benzene and the essayed chemical warfare agent
models further indicate the strength of their interaction with 1.
On the other hand, the low affinity of this material for water
vapor gives rise to very high VOC/H2O partition coefficients
(Kr), suggesting that ambient moisture should not hamper the
incorporation of VOCs into 1. As a matter of fact, breakthrough
curve measurements performed on 1 with streams of benzene
under both dry and humid conditions supported the effective-
ness of this material for VOC capture at room temperature (293
K) even in the presence of moisture (see the SI): The use of a
10 mL minꢀ1 He stream containing 130 ppm benzene under dry
conditions gave rise to an adsorption of 130 mg of benzene per
gram of 1. When 1 was previously exposed to open air for 48 h,
thus mimicking actual operating conditions, flowing a stream of
He containing 130 ppm benzene and 50% moisture still induced
a significant adsorption of this VOC (60 mg gꢀ1), confirming
that ambient moisture is not a strong competitor.
To assess the significance of these results, we studied the
performance of cornerstone porous materials toward the capture
of the essayed VOCs. To this purpose, we selected a MOF with
coordinatively unsaturated metal centers, [Cu3(btc)2], and an
activated carbon material, Carboxen adsorbent from Sigma-
Aldrich. The rationale behind this selection was the following:
[Cu3(btc)2] is recognized as an outperforming material for the
capture of the sulfur-containing VOC tetrahydrothiophene un-
der dry conditions as a consequence of sulfur coordination to the
unsaturated Cu centers.5 The activated-carbon Carboxen adsor-
bent possesses pore sizes of 4ꢀ8 Å, which are adequate for the
selective capture of the studied VOCs, and a specific surface area
(485 m2 gꢀ1) similar to that of 1. The variable-temperature zero-
coverage adsorption measurements on [Cu3(btc)2] and Carbox-
en are summarized in Table 2. The zero-coverage adsorption
heats and values of the Henry constant for Carboxen are very
similar to those obtained for 1, thus suggesting similar adsorption
processes dominated by the small size and apolar nature of the
pores in the two materials. At variance, the ΔHads and KH values
for [Cu3(btc)2] when probed using DES, DIFP, and H2O are
remarkably higher than those obtained for 1 and Carboxen as a
consequence of the interaction of these probe molecules with the
coordinatively unsaturated metal centers through their S- and
O-donor sites, respectively. It is worthy of note that the high
affinity of water toward Cu(II) in [Cu3(btc)2] induces very low
Figure 4. Variable-temperature pulse gas chromatograms for (a) DIFP
and (b) DES passed through a chromatographic column packed with 1
using a He flow of 30 mL minꢀ1. The insets represent the variation of the
retention volume Vg (in cm3
temperature (383ꢀ513 K).
g
ꢀ1) as a function of the adsorption
Table 1. Calculated Heats of Adsorption ΔHads (kJ molꢀ1),
Henry Constants KH (cm3 mꢀ2), and Partition Coefficients Kr
for 1 with Various VOCs and Water
a
VOC
ꢀΔHads
KH
Kra
DES
DIFP
BZ
50.1
44.8
48.2
9.4
2850
9950
275
71.2
1090
0.07
0.26
4170
CH
0.24
H2O
17.9
ꢀ
a Calculated values at 298 K. Values of Kr for VOCs vs H2O were
obtained from variable-temperature zero-coverage gas chromatography
experiments on 1. Abbreviations: DIFP, diisopropylfluorophosphate;
DES, diethylsulfide; BZ, benzene; CH, cyclohexane.
carried out by means of pulse gas chromatography16 and
measurement of breakthrough curves.17 The variable-tempera-
ture pulse gas chromatography studies revealed high retention
times for benzene and the chemical warfare agent models DIFP
and DES (Figure 4 and Table 1), indicative of a strong interac-
tion of this type of adsorbate with the porous framework of 1. In
contrast, cyclohexane and water vapor were characterized by very
low retention times. The weak interaction of cyclohexane with 1
is probably related to its average molecular diameter (>6 Å),
which is larger than the small pore windows of 1, while the weak
interaction of H2O with 1 agrees once again with the hydro-
phobic nature of this MOF (see above).
To quantify the strength of the interaction of these guest
molecules with 1, we calculated their zero-coverage adsorption
heats (ΔHads) from the variation of the retention volumes (Vg)
as a function of temperature according to the equation ΔHads
=
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Journal of the American Chemical Society
COMMUNICATION
VOC/H2O partition coefficients: this unfortunate effect, mani-
fested by the unequivocal color change from deep-purple to light-
blue when [Cu3(btc)2] pellets are exposed to ambient moisture,
definitely hampers the usefulness of this material under ambient
conditions. Accordingly, when hydrated [Cu3(btc)2] was placed
in the chromatographic column, neither DES nor DIFP were
retained under ambient conditions. It should be noted that
DES retention by [Cu3(btc)2] should be favored by its high
DES/H2O Kr; since this was not the case, we can conclude that
thermodynamic equilibrium was not reached under the essayed
dynamic conditions.6 In contrast, as expected on the basis of their
hydrophobic nature, both Carboxen and 1 did retain DES under
humid conditions.
Andalucía, and the EU (NanoMOF-228604) is acknowledged.
The authors are very grateful for the valuable comments raised by
the reviewers.
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’ ASSOCIATED CONTENT
(19) Gulf War and Health, Volume 1: Depleted Uranium, Sarin,
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S
Supporting Information. Experimental methods, ther-
b
mal analyses, chemical stability tests, mechanical stress tests, and
gas adsorption breakthrough curves. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
ebaream@ugr.es; jarn@ugr.es
’ ACKNOWLEDGMENT
Generous support by the Spanish MCINN (Grant CTQ2008-
00037/PPQ and Ramon y Cajal Contract to E.B.), the Junta de
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dx.doi.org/10.1021/ja2042113 |J. Am. Chem. Soc. 2011, 133, 11888–11891