Chem.Commun.
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Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Iodine Capture by Hofmann-type clathrate
NiII(pz)[NiII(CN)4]
Giovanni Massasso,a Jérôme Long, a Julien Haines, a Sabine Devautour-Vinot,a Guillaume
Maurin, a Agnès Grandjean,b Barbara Onida, c Bruno Donnadieu, d Joulia Larionova,a*
Christian Guérin a and Yannick Guaria
DOI: 10.1039/x0xx00000x
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The thermally stable Hofmann-type clathrate framework
NiII(pz)[NiII(CN)4] (where pz = pyrazine) was investigated for
the efficient and reversible sorption of iodine (I2) in the
gaseous phase and in solution with a maximum adsorption
capacity of 1 mole of I2 per mole of NiII(pz)[NiII(CN)4].
Various processes from nuclear industry produce an important
quantity of radioactive effluents that has to be treated in order to
minimize their impact on environment. Among those, radioactive
iodine isotopes produced during the fission of 235U, and particularly
the -rays emitter 129I with a long half-life, are dangerous for health
due to their volatility and their persistence in the environment. For
this reason, the efficient and selective iodine capture during fuel
reprocessing or in the case of an accidental release has attracted a
great deal of attention in the recent years. Several processes
consisting in the precipitation of stable and insoluble compounds
like NaI in the scrubbing with alkali solutions, HgI2 in the
“Mercurex process” or Ba(IO3)2 in the “Iodox process” are currently
used in industry. However, they generally possess multi-steps
procedures and cannot be used directly for gas capture. Such
treatments also generate an important volume of radioactive liquid or
solid wastes, which should be then retreated.1 Other industrial
process uses silver sorbent, like silver exchange zeolite. In this case,
the iodine diffusion inside the zeolite is slow and limits the
adsorption process; moreover the stability of silver-exchange zeolite
is poor.
One of the most promising strategies to direct sequestration of
radioiodine from the exhaust gas is to design new thermally stable
nanoporous materials able to selectively and efficiently capture I2. In
this line of thought the capture of I2 with zeolites,2 Ag-loaded
zeolites,3 zeolite-related structures (“zeoballs”)4 or Metal-OrganicFrameworks (MOFs)5-8 have recently been reported. In particular,
these later solids present an increasing interest for their high iodine
sorption efficiency due to their higher porosity in comparison with
the zeolite-like materials. One can cite the tetrazole-based threedimensional [CuII(btz)]n MOF (with H2btz = 1,5-bis(5-tetrazolo)-3oxapentane) able to absorb 0.5 I2/CuII and a series of aluminium
This journal is © The Royal Society of Chemistry 2012
carboxylate-based functionalized MOFs investigated for an iodine
sorption in cyclohexane with a maximum capacity of 0.71 I2/Al for
MIL-100-NH2.5 The remarkable properties of Zn2+-based zeolitic
imidazolate framework (ZIF-8)6 for capturing volatile iodine with a
maximal capacity up to 0.76 I2/Zn as well as of [Cu3(btc)2]n (btc =
1,3,5-benzene tricarboxylate) MOF7 with a maximum capacity of 1.1
I2/Zn for competitive iodine sorption from humid gas streams have
also been reported. Recently, the iodine entrapment by another
suitable and promising nanoporous coordination polymer family
called Hofmann-like clathrates has been patented.9 However to the
best of our knowledge the detailed studies of the iodine sorption by
these materials were never performed.
Hofmann clathrates are a wide family of cyano-bridged coordination
polymers of transition metal ions exhibiting cages in their two- or
three- dimensional networks and thus able to capture small guest
molecules. The first clathrate of formula Ni(CN)2(NH3)·C6H6 having
a guest benzene molecule has been reported in 1897 by Hofmann
and coll.10 Henceforth, numerous compounds of this family of
different composition and dimensionality, capable to capture various
tailored molecules have been investigated.11 Among those,
FeII(pz)[MII(CN)4] (pz = pyrazine, M2+ = Ni2+, Pd2+, Pt2+) networks
have been reported as chemo-responsive porous coordination
polymers presenting a switching of FeII spin state through guest
adsorption processes at room temperature. In such cases, the guest
molecules i.e. gases (N2, O2, CO2, SO2, H2) or vapours of alcoholic
(MeOH, EtOH), aromatic (toluene, benzene) or miscellaneous
solvents (CS2, CH3C(O)CH3, CH3CN) are confined into the pores,
interacting weakly with the pyrazine pillar ligands or the FeII
centres.12 Surprisingly, only one article reports so far the potential
aptitude of such networks to host the I2 molecule with the formation
of a FeII(pz)[PtII/IV(CN)4](I) complex synthesized directly in the
presence of iodine and leading to an oxidative addition with the
formation of a PtIV-I bond.13 In this communication, we report the
iodine adsorption investigation by NiII(pz)[NiII(CN)4] network that
for the first time demonstrates the efficiency of Hofmann clathrates
as high-capacity iodine absorbents in liquid and gaseous phases.
The coordination polymer NiII(pz)[NiII(CN)4]H2O was
obtained as a microcrystalline powder according to a slightly
Chem. Commun., 2013, 00, 1-3 | 1
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modified published procedure (see Electronic Supporting
Information (ESI) and Fig. S1).12 The X-Ray Powder
Diffractograms (XRPD) (Fig. 1) indicate that the compound is
isostructural to the previously reported FeII(pz)[PtII(CN)4]
analogue.13 Lebail analysis carried out to model the XRPD data
indicates that the materials crystallize in a tetragonal P4/m
space group with the cell parameters values a = b = 7.1762(3)
Å, c = 7.0316(5) Å, α =  =  = 90.0° (Fig. S2, ESI). The
thermogravimetric analysis shows that the investigated material
is thermally stable up to 400 °C (Fig. S3, ESI).
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loading was analysed by means of FT-IR, Raman, XPS and
XRPD experiments. First, FT-IR spectra of samples after iodine
loading show small shifts of the bands related to the pyrazine
ligand (1160, 1129, 1088, 810 cm-1). In addition the cyanide
stretching vibration shifts from 2173 to 2166 cm -1 upon iodine
insertion indicating an increase in the back-donation d- caused
by an interaction between the iodine and the tetracyanonickelate
moiety (Fig. S6, ESI). Besides, XPS experiments further show
no change in the oxidation state of the nickel(II) ion after iodine
loading that clearly indicates an absence of redox processes
involving metal centres (Fig. S7, ESI). The appearance of a
broad band in the range 200-210 cm-1 characteristic of a weak
charge transfer between iodine and the pyrazine ligand was also
observed by Raman spectroscopy (Fig. S8, ESI).14
Fig.1. X-Ray powder diffractograms for NiII(pz)[NiII(CN)4] before,
after maximal iodine adsorption in cyclohexane and after I2 desorption.
The arrows denote the modifications upon iodine insertion.
The iodine capture by thermally activated material at 80 °C
was performed first in cyclohexane by measuring the residual
concentration of iodine in solution using UV-Vis electronic
spectroscopy (see ESI). The I2 sorption kinetic curves
performed for the 3.10-3 M iodine cyclohexane solution show
that the process is quite rapid with the equilibrium reached after
2 hours (Fig. S4, ESI). As for most processes involving
chemisorption on a solid, a pseudo-second order kinetic is
observed. The iodine adsorption isotherm performed at room
temperature is shown Fig. 2. The curve is concave to the
concentration axis, which reflects the ability of the materials for
the I2 sorption in a wide range of concentration. The curve
exhibits also a sudden increase at low iodine concentrations
indicating a high affinity of the clathrate for I2. Logically, the
material changes its colour to dark brown after I2 filling (Inset
Fig. 2). The maximum adsorption capacity (Qmax) which is
indicative of the efficiency of the materials to capture I2
determined from the plateau of the isotherm is equal to 3.28 and
corresponds to the adsorption of about 1 mole of I2 per mole of
clathrate.
The iodine capture by this clathrate network was also
investigated in gas phase. The thermally activated compound
was exposed to I2 vapour at 80 °C (iodine vapour pressure
about 1 kPa) in a closed adsorption chamber during 72 h. The
so-obtained material was then thoroughly washed by
cyclohexane in order to remove physisorbed iodine at the
surface. The I2 uptake from gas is somewhat lower than the one
in solution and equal to 1.99 mmol.g-1 that corresponds to
uptake of 0.60 mole of I2 per mole of NiII(pz)[NiII(CN)4]. Note
that the iodine adsorption capacities in both, solution or in gas
phase are comparable to what has already been observed for
MOFs materials.5-8
In order to give a microscopic insight into the iodine
sorption, NiII(pz)[NiII(CN)4]@I2 sample with maximal I2
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Fig.2. Iodine adsorption isotherm for NiII(pz)[NiII(CN)4]. The solid line
represents the fit with the Langmuir model. The found from the model Qmax
are close to the experimental values and equal to 3.34 mmol.g-1. Inset: from
left to right, photographs of thermally activated powders before and after
contact with iodine at maximal loading.
The XRPD of NiII(pz)[NiII(CN)4]@I2 sample was treated with
Lebail analyses in order to model the evolution of the lattice
parameters of the unit cell with iodine filling. A well-defined
structure with a P4/m space group and cell parameters a = b =
7.2605(6) Å, c = 7.0072(5) Å, α =  =  = 90.0° indicates essentially
a slight increase (i.e. 0.084 Å) of the a and b cell parameters (Fig. 1,
Fig. S2, ESI). This fact is consistent with the incorporation of iodine
into the pores of the coordination network through the formation of
weak interactions with pyrazine and tetracyanonickelate as
suggested by vibrational and XPS spectroscopies.
A computational approach based on an energy minimization
procedure15 was employed to determine a plausible crystal structure
of the fully loaded NiII(pz)[NiII(CN)4]@I2 sample. Grand Canonical
Monte Carlo (GCMC) simulations were further performed at 25 °C
to calculate the I2 adsorption isotherm. Similarly to previous studies
on MOFs,7 the iodine/host interactions included only short-range van
der Waals terms with parameters taken from the Universal Force
Field (UFF)16 (see ESI). These calculations revealed that the optimal
maximal uptake is 1 mole of I2 by unit cell, confirming the
experimentally achieved saturation capacity of the material. The
analysis of the preferential arrangements of iodine issued from these
simulations evidenced that the iodine molecules: (i) interact with
both, the Ni2+ ion of the [Ni(CN)4]2- moiety (distances ranging from
3.32 to 3.71 Å) and the pyrazine (3.52-3.74 Å), in agreement with
the experimental observation reported above, and (ii) tend to
orientate along the direction of the tunnel with distances separating
each other over 4 Å (Fig. 3).
Finally, the iodine adsorption reversibility by Hofmann clathrates
was checked. As the fully loaded materials NiII(pz)[NiII(CN)4]@I2 is
treated during 0.5 h at 300 °C under a continuous flux of argon, I2 is
This journal is © The Royal Society of Chemistry 2012
Journal Name
totally removed from the framework giving the empty clathrate. The
spectroscopy data as well as the PXRD obtained after the heat
treatment is in accordance with such a reversible desorption process
(Fig. 1, Fig. S3, ESI).
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Electronic Supplementary Information (ESI) available: synthesis,
caracterisations, iodaion loading conditions in cycloxehane and gaze. See
DOI: 10.1039/c000000x/
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Fig.3. Grand Canonical Monte Carlo (GCMC) optimized distribution of
iodine in NiII(pz)[NiII(CN)4]@I2 corresponding to a loading of 1
molecule/u.c., C. Colour code: green, Ni; grey, C; blue, N, purple, I.
In summary, we have presented here for the first time the study
of I2 capture within the Hofmann clathrate framework
NiII(pz)[NiII(CN)4] from either the cyclohexane solution or from
the gaseous phase, with a maximum capacity of 1 mole of I2 for 1
mole of NiII(pz)[NiII(CN)4]. Structural and spectroscopic
investigations combined with molecular simulations indicate that
the adsorption of iodine is mainly due to the insertion of I2
molecules within the network cages through synergetic
interactions with both the pyrazine ligand pillars and the
cyanometallate moiety occurring without any redox processes.
The iodine uptake process is fully reversible by applying heat
treatment recovering the starting material and several I 2
sorption/desorption cycles maybe performed without material
decomposition. This study demonstrates that the Hofmann’s
clathrate family is an appropriate thermally stable porous material
for iodine capture in liquid and gas phase which opens
interesting perspectives in relation to the many possible
variations in chemical composition of these materials for iodine
decontamination.
The authors thank the ANR (ANR-AA-RMNP-003-01),
University of Montpellier 2 and CNRS for financial support. We
also thank Mme D. Granier, (PAC Chimie Balard, Montpellier), Ms.
E. Celasco (Politecnico di Torino) for PXRD and XPS
measurements.
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Notes and references
a
Institut Charles Gerhardt Montpellier, UMR5253, CNRS-UM2ENSCM-UM1, CMOS (G. M., J. Lo, C. G., Y. G. J. La), C2M (J. H.),
DAMP (S. D.-V., G. M.), MACS (G.L). Université Montpellier II, Place
E. Bataillon, 34095, Montpellier Cx 5, France. E-mail:
joulia.larionova@um2.fr.
b
Institut de Chimie Séparative de Marcoule, UMR5257, CEA-CNRSUM2-ENSCM, BP17171, 30207, Bagnols sur Cèze, France.
c
Institute of Chemistry, Department of Applied Science and Technology,
Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
d
Féderation de Recherche Chimie Balard- FR3105, Université Montpellier II,
Place E. Bataillon, 34095 Montpellier cedex 5, France.
This journal is © The Royal Society of Chemistry 2012
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