Kinetics of degradation of sulfur mustard and sarin simulants on HKUST-1 metal organic framework

Article abstract of DOI:10.1039/c2dt31888a

The applicability of HKUST-1 for the degradation of sulfur mustard and sarin simulants was studied with and without coadsorbed water. Degradation was found to be via hydrolysis and dependent on the nucleophilic substitution reaction, vapour pressure and molecular diameter of the toxicants.

Full text of DOI:10.1039/c2dt31888a

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COMMUNICATION
Kinetics of degradation of sulfur mustard and sarin simulants on
HKUST-1 metal organic framework†
Anuradha Roy, Avanish K. Srivastava, Beer Singh,* Dilip Shah, Timir Haran Mahato and Anchal Srivastava
Received 3rd July 2012, Accepted 29th August 2012
DOI: 10.1039/c2dt31888a
and 0.804 cc g− . The average pore size and pore maxima were
found to be 20.27 Å and 5.7 Å respectively. The SEM image
(ESI, Fig. S2a†) of the synthesized HKUST-1 indicated the for-
mation of octahedral crystals while the phase purity was checked
by powder XRD and the spectra (ESI, Fig. S3†) obtained was in
accordance with the one reported in the literature for this specific
1
The applicability of HKUST-1 for the degradation of sulfur
mustard and sarin simulants was studied with and without
coadsorbed water. Degradation was found to be via hydroly-
sis and dependent on the nucleophilic substitution reaction,
vapour pressure and molecular diameter of the toxicants.
6
In the last few years the scientific community has shown an
interest in establishing rapid and reliable decontamination pro-
cesses for nerve agents, sarin (GB) and VX and for the blister
agent sulfur mustard (HD). The extensive reviews on decontami-
network.
TGA data (ESI, Fig. S4†) showed the first weight loss at
00 °C corresponding to the removal of physically adsorbed
water and a sharp decrease of weight at 350–400 °C was indica-
tive of decomposition of carboxyl groups and complete break-
1
1
2
nation procedures published in 1992 and 2007 were mostly
focused on liquid or soluble decontamination agents (mostly
alkaline and oxidizing solutions); minor attention was paid to
heterogeneous decontamination systems. Metal organic frame-
works (MOFs) comprising metal ions or metal ion clusters and
bridging organic ligands are referred to as porous coordination
down of HKUST-1 was observed at above 400 °C. The FTIR
spectrum (ESI, Fig. S5†) showing peaks at 1300–1700 cm−
1
,
was indicative of the coordination of trimesic acid with Cu.
−1
−1
−1
More precisely peaks at 1646 cm , 1599 cm and 1450 cm
,
1
374 cm− corresponded to asymmetric and symmetric vibration
1
3
polymers. In the last few years several groups have reported the
of coordinated carboxyl groups.
use of MOFs for guest molecule binding properties and adsorp-
The degradation kinetics was studied by exposing HKUST-1
to CEES, CEPS, DEClP and DECNP under ambient conditions.
Kinetic plots shown in Fig. 1 exhibited that initial degradation
of toxicants were faster followed by a steady state reaction indi-
cating the first order reaction. The fast initial reaction is attribu-
ted to the rapid adsorption and distribution of the toxicants
within the pores and its interaction with the accessible reactive
sites. The limited surface reaction occurs when the sites are
exhausted, obviously, replacing the initial fast reaction by a
steady state reaction. The result indicates that the degradation of
CEES is faster than CEPS. The half-lives for the degradation of
CEES and CEPS were found to be 16.11 and 75.27 min, respect-
ively (Table 1). DECNP was found to degrade relatively faster
4
,5
tion of various gases such as CCl , DMMP. Cu-BTC MOF,
4
HKUST-1 was one of the earlier MOFs which showed interest-
6
,7
8
ing applications such as gas storage nitric oxide adsorption
9
and removal of tetrahydrothiophene odorant from natural gas.
Encouraged by the adsorption and guest molecule binding
properties of MOFs, in this communication, we report the appli-
cability of HKUST-1 for the degradation of simulants of sulfur
mustard and sarin such as 2-chloroethylphenyl sulfide (CEPS),
2
-chloroethyl ethyl sulfide (CEES) and diethyl chloro phosphate
(DEClP), diethyl cyano phosphonate (DECNP) respectively.
Herein we have synthesized HKUST-1 by a solvothermal
method and thereafter, characterized it using a surface area ana-
lyzer, a scanning electron microscope, a powder X-ray diffracto-
meter, a Fourier transform infrared spectrophotometer and a
thermogravimetric analyzer. Thereafter HKUST-1 was studied
for the degradation kinetics of sulfur mustard and sarin
simulants.
N BET adsorption isotherm was type-I and the surface area
2
2
−1
(
ESI, Fig. S1†) was found to be 1645 m g . The total pore
volume and N -DR micropore volume was measured to be 0.833
2
Defence Research and Development Establishment, Jhansi Road,
Gwalior 474 002, (M.P.), India. E-mail: beerbs5@rediffmail.com;
Fax: +91-751-2233482; Tel: +91-9893040985
†
Electronic supplementary information (ESI) available: Synthetic
method of Cu (BTC) and characterization and reaction conditions
details. See DOI: 10.1039/c2dt31888a
3
2
Fig. 1 Kinetics of degradation of CEES, CEPS, DEClP and DECNP.
12346 | Dalton Trans., 2012, 41, 12346–12348
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Table 1 Kinetics parameters for the degradation of CEES, CEPS,
DEClP and DECNP on HKUST-1
Table 2 Vapor pressure and molecular diameter of CEES, CEPS,
DEClP and DECNP on HKUST-1
Toxicants
CEES
CEPS
DEClP
DECNP
Toxicants
CEES CEPS DEClP DECNP
Rate constant (k) (min−
Half life (t1/2) (min)
1
)
0.043
16.11
0.009
75.27
0.007
60.18
0.011
42.98
Vapor pressure (mm Hg/25 °C) 3.789
Molecular diameter (Å) 6.98
0.043
8.6
0.100
12
0.158
10
Table 3 Reusability of HKUST-1 for degradation of CEES
BET surface area
Micropore volume
% of
degradation
2
−1
−1
No. of cycle
(m g
)
(cc g
)
1
2
3
st
nd
rd
1645
441
43
0.804
0.214
0.018
92
70
50
The high surface area of the MOF was found to facilitate the
faster degradation of toxicants. The role of the surface area and
porosity was further confirmed by reaction of CEES with copper
acetate monohydrate which has a similar coordination environ-
ment to that of HKUST-1 but a very low surface area (>5 m g ).
Results indicated an insignificant degradation (approx. 7%) of
CEES over 3 h by copper acetate monohydrate whereas MOF
degraded more than 91% in 40 min (Fig. 1).
The effect of coadsorbed moisture on the degradation of sulfur
mustard and sarin simulants was also studied. The degradation
obtained for CEES and CEPS with HKUST-1 containing 10 μL
of water was found to be 67 and 38% only whereas, activated
MOF having negligible moisture indicated 83 and 72% degra-
dation of CEES and CEPS respectively.
Scheme 1 Reactions products of CEES and CEPS.
2
−1
than DEClP with a half-life of 42.98 min (Table 1). The order
for degradation of studied stimulants of sulfur mustard was:
CEES > CEPS and for sarin was: DECNP > DEClP. No degra-
dation of toxicants was observed with blank solution without
MOF (Fig. 1).
Kinetic studies indicated that the degradation of CEES is
faster than CEPS which is in accordance with the neighboring
group participation phenomenon. The slow degradation of CEPS
is because of resonance involvement of the lone pair of the
sulfur atom in the phenyl ring, which in turn decreases the neigh-
boring group participation hence decreasing the leaving group
The reason for the decreased degradation of CEES and CEPS
by the MOF containing moisture is due to the blockage of the
active sites of the MOF by the pre-adsorbed water molecules.
The high moisture in MOF results in the formation of a thin film
of water over the surface of the MOF, which worked as a barrier
for the adsorption of lipophilic CEES and CEPS and caused the
−
tendency of Cl . In the case of CEES, the ethyl group attached
to the sulfur atom acts as an electron releasing group which
increases the electron density on the sulfur atom. The increased
electron density on the sulfur atom results in an increase in
−
11
neighboring group participation and makes Cl a better leaving
slower degradation of CEES and CEPS. The degradation of
group as shown in Scheme 1. Kinetic studies also indicated that
DECNP degraded faster than DEClP. The fast degradation of
DEClP and DECNP were found to be 60, 83% on MOF with the
coadsorbed water and 48, 78% respectively on activated MOF
containing negligible moisture. Here the degradation of the nerve
agent simulants is enhanced on the MOF having coadsorbed
water. The increased reactivity of the MOF containing water is
attributed to the hydrophilic character of the nerve agent
simulants.
−
DECNP is due to the better leaving tendency of CN as com-
−
pared to Cl of DEClP. Moreover, the order of degradation of the
toxicants are in line with their vapour pressure (Table 2).
Toxicants with higher vapour pressure will diffuse more rapidly
and result in a faster rate of degradation.
On the other hand the effect of molecular size of the toxicant
on the adsorption over MOF cannot be ruled out. The reason for
the slower degradation of CEPS than CEES and DEClP than
DECNP lies in the fact that the larger the size of the molecule
the slower will be the adsorption (Table 2). Molecular diameter
was calculated using the equation in ref. 10. The molecular size
of CEPS is greater than CEES and DEClP is greater than
DECNP. Therefore, toxicants having a bigger size cause slower
adsorption and degradation of CEPS and DEClP relative to their
counterparts due to steric hindrance. Kinetic studies also indi-
cated that the degradation of DECNP is faster than CEPS which
is not in accordance with their molecular diameter based adsorp-
tion. The fast degradation of DECNP is attributed to a better
Based on the faster degradation of toxicants, CEES was
selected for checking the reusability of the HKUST-1. Reusabil-
ity tests indicated that the MOF can degrade CEES efficiently up
to the 4th cycle. The fresh samples showed 92% degradation in
2 h whereas in 2nd, 3rd and 4th cycle degradation it was found
to decrease up to 70, 50 and 45% respectively. Deactivation of
MOF is attributed to the strong adsorption of products, quite
evident from the reducing surface area with increased reaction
cycles (Table 3) and EDAX spectra (ESI, Fig. S6†) indicating
the presence of sulfur and chlorine. Another reason for the de-
activation is the irreversible collapse of the structure; evident
from SEM images (ESI, Fig. S2b–c†) due to the migration of the
framework metal ions because of reaction with hydrochloric acid
resulted from hydrolysis of CEES with the increased reaction
cycles.
−
leaving tendency of CN which results in fast hydrolysis of the
DECNP.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 12346–12348 | 12347

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better leaving tendency of CN− as compared to Cl . The mol-
ecular diameter and vapour pressure of the toxicants was also
found to influence the adsorption properties of the MOF as toxi-
cants having a bigger size and lower vapour pressure caused by
steric hindrance and slow diffusion which in turn results in slow
adsorption as well as degradation. The study clearly indicates
that HKUST-1, is a potential material for degradation of toxi-
cants via hydrolysis, therefore, HKUST-1 can be effectively used
for in situ degradation of nerve and blister agents. This study
may lead to the development of an efficient NBC filtration
system and decontamination formulation based on MOFs.
Caution: Since the CW agents are highly toxic in nature,
these experiments should only be performed by trained per-
sonnel using appropriate protective gear in a high-quality
fuming hood.
−
Scheme 2 Reactions products of DEClP and DECNP.
In order to understand the reaction pathways the reaction pro-
ducts were analyzed using GC-MS (Schemes 1 and 2). GC-MS
data of the reaction products of CEES, CEPS (ESI, Fig. S7–S8†)
1,12
indicated the formation of the hydrolysis products
-hydroxyethyl ethyl sulfide (HEES) (m/z at 47, 53, 75, 96, 106)
and hydroxy ethyl phenyl sulfide (HEPS) (m/z 45, 59, 73, 91,
03, 137, 167, 211, 226). A similar reaction pathway was
i.e.,
2
1
observed for the degradation of DEClP and DECNP on
HKUST-1. GC-MS data (ESI, Fig. S9†) showed the formation of
diethyl phosphate (DEP) (m/z 45, 65, 81, 96, 113, 127, 147, 155,
Acknowledgements
We thank Prof. (Dr) M.P. Kaushik, Director, DRDE for his kind
support and useful suggestions.
1
67) as the hydrolysis product of DEClP and DECNP.
The formation of the reaction products can be explained by a
detailed understanding of the structure of HKUST-1. The struc-
ture is composed of large hydrophilic type central cavities (dia-
meter 9.0 Å) surrounded by small pockets (diameter 5.0 Å), these
pores are less hydrophilic as they are encircled by four benzene
Notes and references
1 Y. C. Yang, J. A. Baker and J. R. Ward, Chem. Rev., 1992, 92, 1729.
13
2
S. S. Talmage, A. P. Watson, V. Hauschild, N. B. Munro and J. King,
Curr. Org. Chem., 2007, 11, 285.
rings. The average pore diameter of the synthesized HKUST-1
was found to be 20.27 Å. Each metal corner has two copper
atoms bonded to the oxygen of four BTC linkers. In the as-syn-
thesized material, each copper atom is also coordinated to one
water molecule that is, two water molecules for each paddle-
wheel metal corner, corresponding to 8 wt% water loading. The
presence of water molecules in the first coordination sphere of
3
C. T. Chen and K. S. Suslick, Coord. Chem. Rev., 1993, 128, 293.
4 S. Calero, A. M. Calvo, S. Hamad and E. García-Pérez, Chem. Commun.,
2011, 47, 508.
5
6
7
F. J. Ma, S. X. Liu, C. Y. Sun, D. D. Liang, G. J. Ren, F. Wei, Y. G. Chen
and Z. M. Su, J. Am. Chem. Soc., 2011, 133, 4178.
P. Krawiec, M. Kramer, M. Sabo, R. Kunschke, H. Frode and S. Kaskel,
Adv. Eng. Mater., 2006, 8, 293.
2
+
O. M. Yaghi and J. L. C. Rowsell, J. Am. Chem. Soc., 2005, 128, 1304.
Cu ions has suggested the possibility of obtaining a coordina-
2
+
8 B. Xiao, P. S. Wheatley, X. Zhao, A. J. Fletcher, S. Fox, A. G. Rossi,
I. L. Megson, S. Bordiga, L. Regli, K. M. Thomas and R. E. Morris,
J. Am. Chem. Soc., 2007, 129, 1203.
9 U. Mueller, M. Schubert, F. Teich, H. Puetter, K. Schierle-Arndt and
J. Pastre, J. Mater. Chem., 2006, 16, 626.
tive vacancy on the Cu species. The partial positive charge on
the metal sites in HKUST-1 enhances the adsorption proper-
1
4
ties. Therefore, toxicant molecules were attracted by the attrac-
tive forces and adsorbed in the pores of the MOF by
physisorption. Thereafter adsorbed toxicant molecules reacted
with chemisorbed water molecules and resulted in the formation
of the hydrolysis products.
10 W. D. Harkins and G. Jura, J. Am. Chem. Soc., 1944, 66, 1366.
11 G. W. Wagner, G. W. Peterson and J. J. Mahle, Ind. Eng. Chem. Res.,
2012, 51, 3598.
12 H. Tang, Z. Cheng, H. Zhu, G. Zuo and M. Zhang, Appl. Catal., B, 2008,
79, 323.
In summary, kinetic studies indicated that the degradation of
CEES is faster than CEPS because of the neighboring group par-
ticipation effect. DECNP degraded faster than DEClP due to a
13 S. S. Y. Chui, S. M. F. Lo, J. P. H. Charmant, A. G. Orpen and
I. D. Williams, Science, 1999, 283, 1148.
14 L. C. Rowsell and O. M. Yaghi, Angew. Chem., Int. Ed., 2005, 44, 4670.
12348 | Dalton Trans., 2012, 41, 12346–12348
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