Cerium(IV) vs Zirconium(IV) Based Metal-Organic Frameworks for Detoxification of a Nerve Agent

Article abstract of DOI:10.1021/acs.chemmater.6b04835

A study was conducted to demonstrate cerium(IV) vs zirconium(IV) based metal-organic frameworks (MOFs) for detoxification of a nerve agent. The rate of hydrolysis observed using a dehydrated version of NU-1000, where ligated water and hydroxide were removed with thermal treatment, was much faster than that of as-synthesized NU-1000. Experiments of this kind showed that Ce-BDC and UiO-66 had similar densities of defect sites. DRIFTS spectra revealed that the sharp peak at 3674 cm^-1 corresponding to the hydroxyl groups on the Zr₆ node of UiO-66 was shifted to 3646 cm^-1 in Ce-BDC, which suggested weaker O-H bonds in the case of Ce₆ node.

Full text of DOI:10.1021/acs.chemmater.6b04835

Communication
pubs.acs.org/cm
Cerium(IV) vs Zirconium(IV) Based Metal−Organic Frameworks for
Detoxification of a Nerve Agent
§
§
§
‡
‡
Timur Islamoglu, Ahmet Atilgan, Su-Young Moon, Gregory W. Peterson, Jared B. DeCoste,
‡
§
,§,⊥
Morgan Hall, Joseph T. Hupp, and Omar K. Farha*
§
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
‡
Edgewood Chemical Biological Center, U.S. Army Research, Development, and Engineering Command, 5183 Blackhawk Road,
Aberdeen Proving Ground, Maryland 21010, United States
⊥
Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
S Supporting Information
*
arious materials and methods for the destruction and
deactivation of toxic chemicals, such as chemical warfare
agents (CWAs), have been investigated for over 1 century.
findings, it is of interest to study MOFs containing different
metal ions that can provide similar connectivity to that of the
V
1
,2
Zr cluster but different ligand exchange rates and/or Lewis
6
Organophosphate nerve agents, a subclass of CWAs, are volatile
liquids that are designed to interfere with the human nervous
acidity while maintaining a similar oxidation state. Although
there is no direct comparison of ligand exchange rates on
Zr(IV) versus Ce(IV), it has previously been reported that
Ce(IV) complexes show superior rates of phosphonate ester
bond cleavage compared to Zr(IV) analogues in homogeneous
3
,4
system by shutting down the activity of acetylcholinesterase.
This results in an accumulation of the neurotransmitter5,
acetylcholine, and ultimately leads to death by asphyxiation.
The selective, hydrolytic cleavage of the PF bond of
organophosphate nerve agents has been shown to be effective
7
,24−26
form.
The unprecedented activity of Ce(IV) has been
attributed to the role of the Ce(IV) 4f orbitals that can mix with
the orbitals of the PO bond to form hybrid orbitals. This
gives rise to a penta-coordinate intermediate that is susceptible
6
for chemical neutralization of these agents. PF bond
cleavage with cationic Lewis acids is particularly promising as
phosphate ester hydrolysis can be accelerated by polarizing the
27
to nucleophilic attack. Although this is very promising,
Ce(IV) ions tend to form polymeric metal−hydroxo
precipitates at pH > 4, which limits the potential of
7
PO bond. In addition, metal ions can form metal
coordinated hydroxyl groups by lowering the pK for water
a
7
from their inner hydration sphere. In turn, the ligated
28
homogeneous Ce(IV) catalysts. Recently, Stock and co-
hydroxide may (in some cases) serve as the attacking
workers reported a Ce(IV) based UiO-66 analogue referred to
8
nucleophile for PF bond disruption. Therefore, bimetallic
29
as Ce-BDC (Figure 1). Because Ce(IV) ions are stabilized in
complexes may show even further enhancement in hydrolysis
rates compared to mononuclear counterparts as they may allow
12+
a [Ce O (OH) ]
node, Ce-BDC should not suffer from
6
4
4
metal-hydroxo precipitation. We postulated that Ce-BDC
would therefore show enhanced hydrolysis rates for DMNP
compared to Zr-BDC (UiO-66) given that Ce(IV) shows
superior activity to Zr(IV) under homogeneous conditions.
Ce-BDC was synthesized according to a published
9
both of these steps to happen in close proximity.
Metal−organic frameworks (MOFs) have shown great
potential as heterogeneous catalysts due to their tunable
1
0,11
textural (i.e., porosity, pore size etc.)
and chemical (i.e.,
12
functional groups, reactivity etc.) properties. In particular,
zirconium-based MOFs have gained a great deal of attention as
potential heterogeneous catalysts due to their exceptionally
29
procedure with slight modifications (see Supporting
Information). N isotherms collected at 77 K revealed that
2
2
−1
13,14
Ce-BDC has a BET surface area of 1250 m g , which is in line
with literature report and comparable to UiO-66 (Figure S3).
For UiO type MOFs, surface areas can be related to defect
densities, a finding that is relevant here because defects (sites
exposed on nodes due to missing linkers) are known to be the
active sites for catalysis of hydrolysis reactions. Potentiometric
pH titrations offer a more direct way of assaying defect
densities in MOFs. Experiments of this kind show that Ce-
BDC and UiO-66 (HCl synthesis) have similar densities of
defect sites (Figure S7 and Table S1). DRIFTS spectra revealed
that the sharp peak at 3674 cm− corresponding to the hydroxyl
high thermal, chemical and mechanical stability.
We
recently demonstrated that Zr -cluster-containing MOFs in
6
1
5,16
17
18
the UiO family
as well as NU-1000 and MOF-808 have
30
unprecedented hydrolysis rates toward organophosphate based
nerve agents. We attributed this unusual activity to the Zr
OHZr-containing nodes which potentially mimics two-
metal-ion catalysis in enzymes where MOHM serves as
31
1
5,19
the active site (e.g., phosphotriesterase).
We discovered
2
0
21
that altering the pore size, particle size, chemical
2
2
23
functionality and Zr-site accessibility in a series of Zr6-
MOFs can favorably affect the rate of catalytic hydrolysis of
dimethyl 4-nitrophenyl phosphate (DMNP), a nerve-agent
simulant. Additionally, the rate of hydrolysis observed using a
dehydrated version of NU-1000, where ligated water and
hydroxide were removed with thermal treatment, was much
1
−1
groups on the Zr node of UiO-66 is shifted to 3646 cm in
6
Received: November 12, 2016
Revised: March 3, 2017
Published: March 8, 2017
17
faster than that of as-synthesized NU-1000. In light of these
©
XXXX American Chemical Society
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DOI: 10.1021/acs.chemmater.6b04835
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Chemistry of Materials
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worth noting that particle size plays a significant role in
21,23
hydrolysis rates, especially for UiO-66 type MOFs.
The Ce-
BDC used in this example consists of larger particles compared
to UiO-66 (Figure S6). Therefore, it is expected that the actual
difference in hydrolysis rate might be even more drastic if the
particle size could be easily decreased.
After observation of the fast rate of DMNP hydrolysis using
Ce-BDC, the next step was to test Ce-BDC as a catalyst for the
hydrolysis of the actual CWA, soman (O-pinacolyl methyl-
phosphonofluoridate, also known as GD) (Figure 3). The half-
Figure 1. Chemical structures of Ce-BDC (a), linear-polyethylenimine
(PEI) (b), and hydrolysis reaction of phosphonate-based nerve agents
(O-pinacolyl methylphosphonofluoridate, GD and a simulant
(dimethyl 4-nitrophenyl phosphonate, DMNP) (c). Carbon (gray),
oxygen (red) and cerium (purple).
Ce-BDC, which suggests weaker O−H bonds in the case of Ce6
node. (Figure S5).
Given the toxicity of organophosphate based nerve agents,
we first investigated the catalytic activity of Ce-BDC using the
less toxic simulant, DMNP. In general, 3 mg (1.5 μmol, 6 mol
Figure 3. Hydrolysis profiles of GD with Ce-BDC in N-ethylmorpho-
line buffer solution.
%
catalyst loading based on Ce cluster) of Ce-BDC was added
6
life for the degradation of GD using Ce-BDC was observed to
to a 1 mL aqueous solution, buffered at pH 10 with 0.45 M N-
ethylmorpholine. To this solution was added 4 μL of DMNP
be 3 min whereas UiO-66 showed ∼4 min half-life under the
20
same conditions. It is important to note that DMNP is
hydrolyzed at the P−O bond whereas GD is hydrolyzed at the
P−F bond, which leads to differences in hydrolysis rate
(
25 μmol), and the resulting mixture was agitated for 15 s
31
before being transferred to an NMR tube. The P NMR
spectrum of the mixture was collected every minute for 60 min
and conversion was tracked by integrating the peaks associated
with DMNP versus the dimethyl phosphate product obtained
from cleavage of the P−O bond (Figure 1).
Conversion profiles for both Ce-BDC and UiO-66 are shown
in Figure 2. Initial rate calculations reveal that Ce-BDC
substantially reduces the DMNP half-life, from 19 to 8 min. It is
20,32
between the simulant and agent.
Although Ce-BDC presents promising hydrolysis rates for
DMNP as well as GD in an N-ethylmorpholine buffering
solution, the use of liquid buffer must be eliminated for
32
applications such as protective suits and masks. We recently
demonstrated that polyethylenimine (PEI), a linear polymer
with repeating units composed of amine groups and carbon
spacers, can be mixed with a MOF catalyst and employed as a
heterogeneous buffer resulting in hydrolysis rates comparable
33
to those found using N-ethylmorpholine solutions. With
these results in hand, we prepared the PEI/Ce-BDC mixtures
with different amounts of PEI to optimize the amine loading
(
Figure 4 and Figure S8). We previously showed that the
34
hydrolysis reaction with PEI alone in water is negligible.
Amazingly, under optimized conditions (Figure S8), a shorter
half-life was observed using PEI compared to N-ethylmorpho-
line (4.5 vs 8 min). This highlights the role of primary amines
in the hydrolysis mechanism and points to a role for the buffer
beyond just controlling the solution hydroxide concentration.
In conclusion, we demonstrated, for the first time, that
Ce(IV) based MOFs can be used to achieve fast hydrolysis of a
phosphate-ester-based nerve agent, soman, as well as a
simulant, DMNP. Hydrolysis is observed to be faster with the
Ce(IV) MOF than with previously investigated Zr(IV)
analogue (UiO-66). Importantly, Ce-BDC induces even faster
hydrolysis rates in a MOF/PEI solid than in an N-ethyl-
Figure 2. Hydrolysis profiles of DMNP with Ce-BDC (red circle) and
UiO-66 (blue circle).
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DOI: 10.1021/acs.chemmater.6b04835
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-
ter.6b04835.
■
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Timur Islamoglu: 0000-0003-3688-9158
Jared B. DeCoste: 0000-0003-0345-2697
Joseph T. Hupp: 0000-0003-3982-9812
Omar K. Farha: 0000-0002-9904-9845
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BDC, benzene-1,4-dicarboxaldehyde; DMF, dimethylforma-
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DOI: 10.1021/acs.chemmater.6b04835
Chem. Mater. XXXX, XXX, XXX−XXX