Broad-Spectrum Liquid- and Gas-Phase Decontamination of Chemical Warfare Agents by One-Dimensional Heteropolyniobates

Article abstract of DOI:10.1002/anie.201601620

A wide range of chemical warfare agents and their simulants are catalytically decontaminated by a new one-dimensional polymeric polyniobate (P-PONb), K₁₂[Ti₂O₂][GeNb₁₂O₄₀]?19 H₂O (KGeNb) under mild conditions and in the dark. Uniquely, KGeNb facilitates hydrolysis of nerve agents Sarin (GB) and Soman (GD) (and their less reactive simulants, dimethyl methylphosphonate (DMMP)) as well as mustard (HD) in both liquid and gas phases at ambient temperature and in the absence of neutralizing bases or illumination. Three lines of evidence establish that KGeNb removes DMMP, and thus likely GB/GD, by general base catalysis: a) the k(H₂O)/k(D₂O) solvent isotope effect is 1.4; b) the rate law (hydrolysis at the same pH depends on the amount of P-PONb present); and c) hydroxide is far less active against the above simulants at the same pH than the P-PONbs themselves, a critical control experiment.

Full text of DOI:10.1002/anie.201601620

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Communications
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International Edition: DOI: 10.1002/anie.201601620
German Edition: DOI: 10.1002/ange.201601620
Decontamination
Broad-Spectrum Liquid- and Gas-Phase Decontamination of Chemical
Warfare Agents by One-Dimensional Heteropolyniobates
Weiwei Guo, Hongjin Lv, Kevin P. Sullivan, Wesley O. Gordon, Alex Balboa,
George W. Wagner, Djamaladdin G. Musaev, John Bacsa, and Craig L. Hill*
Abstract: A wide range of chemical warfare agents and their
simulants are catalytically decontaminated by a new one-
dimensional polymeric polyniobate (P-PONb), K12[Ti2O2]
[GeNb12O40]·19H2O (KGeNb) under mild conditions and in
the dark. Uniquely, KGeNb facilitates hydrolysis of nerve
agents Sarin (GB) and Soman (GD) (and their less reactive
simulants, dimethyl methylphosphonate (DMMP)) as well as
mustard (HD) in both liquid and gas phases at ambient
temperature and in the absence of neutralizing bases or
illumination. Three lines of evidence establish that KGeNb
removes DMMP, and thus likely GB/GD, by general base
catalysis: a) the k(H2O)/k(D2O) solvent isotope effect is 1.4;
b) the rate law (hydrolysis at the same pH depends on the
amount of P-PONb present); and c) hydroxide is far less active
against the above simulants at the same pH than the P-PONbs
themselves, a critical control experiment.
catalytic activities for hydrolyzing nerve agents and the
simulant dimethyl 4-nitrophenyl phosphate (DMNP).[7] How-
ever, the catalytic hydrolysis of DMNP by the Zr-based MOFs
often requires the addition of N-ethylmorpholine as a buffer
and a proximal base. These MOFs are of low activity
decontaminating the relatively inert nerve agent simulant,
DMMP.[8] Recently, a porphyrinic MOF that catalyzes
selective photooxidation of a sulfur mustard simulant was
also reported.[9]
Polyoxometalates (POMs), one polyoxoniobate (PONb),
[Nb6O19]8À and other Nb-based solid materials have also been
investigated for the degradation of CWAs and/or their
simulants.[10] POMs are a class of metal oxide clusters with
versatile applications in magnetism, medicine, electrochem-
istry and catalysis.[11] PONbs are a subclass of POMs with high
negative charges per polyanion oxygen and commensurately
basic surface oxygens.[12] We report here that a heterogeneous
one-dimensional polymeric heteropolyniobate: K12[Ti2O2]
[GeNb12O40]·19H2O (KGeNb) is as effective as any basic
heterogeneous CWA hydrolysis catalyst to date and the
mechanism is probed by kinetics and other methods.
In 2002, Nyman and co-workers reported the single crystal
structure of K12[Ti2O2][SiNb12O40]·22H2O (KSiNb).[13] In
2005, the structures of the sodium salts of [Ti2O2]
[SiNb12O40]12À and [Ti2O2][GeNb12O40]12À were inferred from
PXRD data.[14] However, given the remarkable basic reac-
tivity of these one-dimensional polyniobates, we sought
a high-resolution single-crystal structural determination of
these nano-thread-like materials in order to facilitate more
detailed experimental and computational study of their
surface-related reactions.
C
hemical warfare agents (CWAs), used in past wars and
recent terrorist attacks, represent a significant threat to
humankind.[1] Among all of the classes of chemical weapons,
vesicants (such as sulfur mustard) and nerve agents are the
most common.[2] Sulfur mustard is one of the most effective
CWAs used in modern warfare.[3] Nerve agents are organo-
À
phosphorus compounds containing P X bonds (X = F, CN,
SR etc.),[4] that rapidly inactivate acetylcholinesterase, the
enzyme which facilitates hydrolysis of the neurotransmitter
acetylcholine in the nervous system, leading to a range of
incapacitating states and in higher concentrations, death.[5]
There remains a need to develop materials and methods to
rapidly, fully and catalytically decontaminate all the main
CWAs under mild conditions.
Both organic and inorganic materials have been devel-
oped to catalyze hydrolysis of nerve agents and their
simulants.[2,6] A series of Zr-based metal-organic frameworks
(MOFs, especially UiO-66, NU-1000 and MOF-808) that
contain strongly acidic ZrIV sites, have shown significant
Although significant efforts failed to produce single
crystals of the sodium salt of the germanium-based hetero-
polyanion, [Ti2O2][GeNb12O40]12À, the corresponding potas-
sium salt, KGeNb,[15] was readily synthesized under hydro-
thermal conditions from Nb2O5·xH2O, tetraethoxygermane
and tetraisopropyltitanium in an aqueous solution of KOH.
Not surprisingly considering the similar size and properties of
the GeIV and SiIV heteroatoms, KGeNb and KSiNb are
isostructural. These nano-scale threads are composed of
silico/germano-dodecaniobate Keggin ions connected by
two edge-sharing TiO6 octahedra ([Ti2O2]4+) forming one-
dimensional infinite chains (Figure 1a).[14] The K+ counter
cations are situated between the 1D polyanions, and Fig-
ure 1b shows that two of the K+ ions reside in pockets defined
by adjacent Keggin units in each polyanion polymer. The rate
of nerve agent decomposition by [Nb6O19]8À is affected by
counter cations.[10a] Although the specific counter cation
effect in KGeNb is complicated, the short bond lengths
[*] W. Guo, K. P. Sullivan, Dr. D. G. Musaev, Prof. C. L. Hill
Department of Chemistry, Cherry L. Emerson Center for Scientific
Computation, Emory University
1515 Dickey Dr., Atlanta, Georgia 30322 (USA)
E-mail: chill@emory.edu
W. Guo, K. P. Sullivan, Dr. J. Bacsa, Prof. C. L. Hill
X-ray Crystallography Center, Emory University
1515 Dickey Dr., Atlanta, Georgia 30322 (USA)
W. O. Gordon, A. Balboa, G. W. Wagner
U.S. Army Edgewood Chemical Biological Center
APG, MD 21010-5424 (USA)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201601620.
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meric polyniobate species, with KGeNb being slightly more
soluble than KSiNb, or c) an intrinsically more reactive
polymeric polyanion for KGeNb than for KSiNb. Scenario (a)
is not likely because examination of KGeNb and KSiNb by
optical microscope shows similar crystallite sizes for both
(Figure S6). Furthermore, both of these P-PONbs were
ground to make particles that were smaller and quite uniform
prior to use in the decontamination reactions. Scenario (b) is
very unlikely from five lines of evidence: 1) there is no
detectable activity in the supernatant after suspensions of
KGeNb or KSiNb are used for DMMP hydrolysis (the
suspensions were filtered and the filtrate assessed for activity
after use; Figure S7); 2) elemental analysis of the filtrate
indicates negligible amounts of Si, Nb or Ge are present;
3) the weighable amount of KGeNb and KSiNb is the same
before and after reaction; 4) the KGeNb collected after
reaction, washed extensively with deionized water, dried
under vacuum and re-used in successive DMMP hydrolysis
runs shows comparable activity to the first run (Figure S8);
and 5) the FT-IR of KGeNb and KSiNb before and after the
reaction with DMMP remain identical (Figures S9 and S10).
All these argue for the heterogeneous nature of the catalytic
hydrolysis. Thus, scenario (c) is possible and might derive
from the subtle effects on the negative charge density of the
polyanion oxygens by larger size of the central GeO4
tetrahedron versus the central SiO4 tetrahedron.
Figure 1. a) Polyhedral representation of the one-dimensional polyan-
ion chain of KGeNb. Color code: green: NbO6; blue: TiO6; pink: GeO4.
b) Precise structural environments of two potassium counter cations
interacting with the polymeric polyanion unit (bond lengths in Š)
(2.621–3.329 Š) including one to the Ti-O-Ti oxygen (O4
from the X-ray structure) implicates a non-innocent role for
these counter cations in the organophosphate compound
degradation mechanism.
The U.S. Army nominated DMMP as an ideal model
chemical for toxicology and carcinogenesis studies.[8] In this
study, KSiNb and the new more reactive P-PONb, KGeNb,
hydrolyze DMMP to methyl
The initial rate for reaction of DMMP with KGeNb is
approximately first order with respect to DMMP (Fig-
ures S11) and the quantity of insoluble KGeNb present (at
constant pH 10, Figures S12), which is strong evidence for
phosphonic acid (MP) under
mild conditions (with only
water at room temperature (Fig-
ure 2a and 2b). Hydrolysis of the
phosphate ester bonds was
observed and monitored by
31P NMR spectroscopy (Fig-
ure S4 in the Supporting Infor-
mation (SI)). The extent of con-
version was calculated as the
ratio of the integrated 31P NMR
peak for MP (the only hydrolysis
product) to that of DMMP.
KGeNb converts 54% of this
quite inert simulant over the
time course of 264 h. The rates
of DMMP hydrolysis by KGeNb
or KSiNb slow down after ca.
100 h due to the production of
MP which inhibits the reaction
(Figure S5).
The higher hydrolytic activity
of KGeNb relative to KSiNb
could be explained by a) smaller
Figure 2. a) Hydrolytic decomposition pathway of DMMP to MP. b) DMMP decomposition using
KGeNb and KSiNb; conditions: [DMMP]=15.5 mm, 18 mmol KGeNb or KSiNb, 0.5 mL D2O and 1.0 mL
H2O at room temperature. c) DECP decomposition using KGeNb and KSiNb; conditions:
[DECP]=100 mm, 11 mmol KGeNb or KSiNb, 600 mL of DMF and 50 mL of H2O at room temperature.
d) Hydrolytic decomposition pathway of DECP to DEHP.
crystallites of the former than
the latter, b) slight dissolution of
these P-PONbs to form hydro-
lytically active soluble mono-
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a general base-catalyzed mechanism. We note the well-
and thus self-inhibiting hydrolysis product, nor with strong
established qualification that getting the order in a heteroge-
neous catalyst is subject to more error than for homogeneous
reactants where the concentration is precisely defined. We
also measured the kinetic solvent isotope effect for DMMP
hydrolysis.[16] The ratio of reaction rate constants, k(H2O)/
k(D2O), was determined to be 1.4, which is also consistent
with a general base-catalyzed reaction.[17] A third argument
for general base catalysis is that the rate of DMMP hydrolysis
by hydroxide alone is far lower than by P-PONbs at that same
pH (pH 10) (Table 1). The collective data indicate that
mixing or other means to increase the TON, this level of
reactivity under ambient conditions is of interest for human
protection in some real-world environments. Keep in mind
that the deployed decontaminating technologies in use by the
U.S. Army are essentially stoichiometric. Any catalytic
method that could use ambient water (humid air) for
hydrolysis or ambient O2 (air) for oxidative decontamination
remains of considerable interest. KGeNb and KSiNb both
contain many water of hydration (hydrogen-bonded to the
basic polyniobate oxygens) and thus should be able to
catalytically remove many equivalents of nerve agent per
repeating unit in their as-synthesized forms.
Table 1 summarizes the activities of KGeNb, the reported
Zr-based MOFs, the monomeric polyniobate, [Nb6O19]8À, as
well as simple hydroxide, against DMMP and DECP under
the same mild conditions. The half-life, t1/2 (50% conversion)
is also listed.[6m] UiO-66 or MOF-808 show no detectable
hydrolysis of DMMP under the same conditions, although
under other conditions some of these MOFs do hydrolyze
DMMP.[7c] Control experiments using Nb2O5, NbO2, TiO2 or
MgO show no degradation of DMMP whatsoever,[2c] and
a lower activity towards DECP by MgO or TiO2 compared
with KGeNb is observed (Table 1). KGeNb is also more
active than [Nb6O19]8À towards DMMP or DECP under the
same conditions. Since the P-PONbs under the reaction
conditions in this study are effectively insoluble, the local
negative changes of the polymeric polyanions consequently
appear to be vital for efficient basic nerve agent simulant
hydrolysis.
KSiNb was investigated for its activity towards the
degradation of live agent, GD (Table 1 and Figure S13).
After 1 h, 43% of the GD is consumed with continuous
stirring, a point consistent with the oxygens of this polymer
being more reactive those those of [Nb6O19]8À. The control
reaction (same conditions but no P-PONb added) shows
negligible reaction of GD within 1 h. Degradation of GB by
KGeNb and KSiNb was also studied in the gas phase (GB
vapor passed over the P-PONb) using diffuse reflectance
infrared Fourier transform spectroscopy (DRIFTS). Fig-
ures S14 and S15 indicate that GB is partially hydrolyzed on
KGeNb and KSiNb at room temperature (see SI for details
and spectral characterization).
KGeNb and KSiNb were also investigated for the
degradation of HD using DRIFTS (Figure 3 and S16).
Difference IR spectra collected in situ of the KGeNb during
HD exposure did not show peaks associated with intact HD
adsorbed to the surface of the material. Instead, perturbation
of water molecules and/or hydroxo groups of KGeNb and
KSiNb was noted by a negative feature at 3630 cmÀ1. A
multiplet of peaks was observed from 1230 to 1050 cmÀ1
which likely derives from the formation of the hydrolysis
products: half mustard (HM) and thiodiglycol (TDG).
Negative peaks in the 930–820 cmÀ1 range are due to
depletion of modes of KGeNb and KSiNb. The DRIFTS
results suggest that HD largely hydrolyzes to HM and TDG
upon exposure to these P-PONbs, even under a dry He flow.
Reactivity of these 1D materials under humid conditions
would likely increase.
Table 1: Comparison of DMMP and DECP decomposition by different
materials at room temperature under the same conditions.[a]
Materials
Substrate
Time
Conversion
t1/2 [h]
KGeNb
UiO-66
MOF-808
NaOH[b]
K8Nb6O19
KGeNb
UiO-66
MOF-808
K8Nb6O19
MgO
DMMP
DMMP
DMMP
DMMP
DMMP
DECP
DECP
DECP
DECP
DECP
DECP
GD
24 h
24 h
24 h
24 h
25%
<1%
<1%
4%
54
>1343
>1343
336
269
0.1
1.3
0.4
0.2
0.2
24 h
5%
30 min
30 min
30 min
30 min
30 min
30 min
1 h
100%
32%
50%
90%
90%
87%
43%
37%
<1%
TiO2
KSiNb
0.2
1.2
1.4
31
Cs8Nb6O19
GD
GD
1 h
1 h
N/A[c]
[a] Conditions for DMMP hydrolysis: [DMMP]=15.5 mm, 50 mg of
materials for each entry, 0.5 mL D2O and 1.0 mL H2O. Conditions for
DECP hydrolysis: [DECP]=100 mm, 30 mg of material for each entry,
600 mL of DMF and 50 mL of H2O. [b] NaOH was prepared as
a homogeneous phase with a pH of 10. [c] No MOF, POM or other active
material added.
generation of hydroxide by reaction of the P-PONb with
water in an initial pre-equilibrum (specific base catalysis)
does not account for the great majority of the nerve agent
simulant hydrolysis by these basic polymeric nano-scale
thread-like materials. A general-base hydrolysis mechanism
dictates that the slow step of the overall hydrolysis mechanism
involves deprotonation of a water molecule by the P-PONb
while the incipient forming hydroxide simultaneously attacks
its electrophilic partner, the nerve agent simulant phosphorus
atom. This is also consistent with the mechanism proposed for
the decomposition of diisopropyl fluorophosphates by
[Nb6O19]8À [10a]
The subsequent charge neutralization of prod-
.
uct species by proton transfer is fast and does not contribute
to the observed rate.
Figure 2c and 2d show the time profile and the reaction
pathway for the hydrolytic degradation of another CWA
simulant, diethyl cyanophosphonate (DECP) to diethyl
hydrogen phosphate (DEHP) by KGeNb and KSiNb.
Hydrolysis of the P-CN bond was observed and monitored
by 31P NMR spectroscopy. Within 30 min, all DECP is
removed. The addition of KGeNb or KSiNb to the mixture
greatly accelerates the reaction. A TON of ca. 6 is achieved in
30 min. Without addition of extra base to remove the acidic
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Keywords: catalysis · chemical warfare agents ·
decontamination · heteropolyniobates · hydrolysis
How to cite: Angew. Chem. Int. Ed. 2016, 55, 7403–7407
Angew. Chem. 2016, 128, 7529–7533
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In summary, the newly prepared one-dimensional P-
PONb, KGeNb, is as active as any known base for hydrolysis
of nerve agents and their simulants. Importantly, KGeNb is
far more reactive than hydroxide alone toward the simulants
at the same pH in water. Experimental and calculated data
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DMMP, and by extension, likely the live nerve agents
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BB11PHM156 and BA13PHM210) gratefully acknowledge
support by the Defense Threat Reduction Agency. We thank
Bryan Schindler for help on the DRIFTS experiments.
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Received: February 15, 2016
Revised: March 17, 2016
Published online: April 9, 2016
Angew. Chem. Int. Ed. 2016, 55, 7403 –7407
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 7407