Water-Mediated proton conduction in a robust triazolyl phosphonate metal-organic framework with hydrophilic nanochannels

Article abstract of DOI:10.1002/chem.201402886

The development of water-mediated proton-conducting materials operating above 100 °C remains challenging because the extended structures of existing materials usually deteriorate at high temperatures. A new triazolyl phosphonate metal-organic framework (MOF) [La₃L₄(H₂O) ₆]Clx H₂O (1, L^2-=4-(4H-1,2,4-triazol-4-yl) phenyl phosphonate) with highly hydrophilic 1D channels was synthesized hydrothermally. Compound 1 is an example of a phosphonate MOF with large regular pores with 1.9 nm in diameter. It forms a water-stable, porous structure that can be reversibly hydrated and dehydrated. The proton-conducting properties of 1 were investigated by impedance spectroscopy. Magic-angle spinning (MAS) and pulse field gradient (PFG) NMR spectroscopies confirm the dynamic nature of the incorporated water molecules. The diffusivities, determined by PFG NMR and IR microscopy, were found to be close to that of liquid water. This porous framework accomplishes the challenges of water stability and proton conduction even at 110 °C. The conductivity in 1 is proposed to occur by the vehicle mechanism. Proton-conducting MOFs: Water-filled nanochannels are the basis for ionic conductivity in [La₃L₄(H₂O) ₆]Clx H₂O (see figure; L=triazolyl phenyl phosphonate). Reversible hydration/dehydration and proton conduction even above the boiling point of water are characteristic for the hydrothermally synthesized 3D coordination polymer. Crystal structure, diffusion, and impedance spectroscopy data are reported.

Full text of DOI:10.1002/chem.201402886

DOI: 10.1002/chem.201402886
Communication
&
Porous Materials
Water-Mediated Proton Conduction in a Robust Triazolyl
Phosphonate Metal–Organic Framework with Hydrophilic
Nanochannels
Salma Begum,[a] Zhaoyang Wang,[a] Anna Donnadio,[b] Ferdinando Costantino,[b]
Mario Casciola,[b] Rustem Valiullin,[c] Christian Chmelik,[c] Marko Bertmer,[c] Jçrg Kꢀrger,[c]
Jꢁrgen Haase,[c] and Harald Krautscheid*[a]
Dedicated to Professor Joachim Sieler on the occasion of his 75th birthday
polymers (CPs),[1,7] and porous organic solids.[8] The first study
Abstract: The development of water-mediated proton-
of proton conduction in a 2D copper- and dithioxamide-based
CP was reported in 1979 by Kanda et al.[9] Kitagawa et al.[10] ex-
tended this work by using derivatives of dithioxamides as or-
ganic linkers and reported the first systematic study on proton
conduction in CPs. Compared with carboxylate, phosphonate-
based MOFs are considerably scarce in the literature,[11] most of
the proton-conducting phosphonate MOFs have been report-
ed by Shimizu et al.,[12,13] with successful attempts to maintain
also uncoordinated phosphonate functionalities protruding to-
wards the pores as proton-conduction sites.[14] From the per-
spective of synthesizing novel, proton-conducting, porous ma-
terials, phosphonate-based MOFs are considered good candi-
dates owing to the acidity of the PꢀOH groups, to the high
chemical and thermal stability and to the high versatility to
crystallize in different structural architectures.[15,16] The design
of phosphonate MOFs with permanent porosity is still consid-
ered a challenge as the high number of PꢀO groups potential-
ly coordinating to metal atoms and the effect of noncovalent
interactions need to be combined for the achievement of the
desired porous compounds (Figure 1).
conducting materials operating above 1008C remains
challenging because the extended structures of existing
materials usually deteriorate at high temperatures. A new
triazolyl phosphonate metal–organic framework (MOF)
[La3L4(H2O)6]Cl·xH2O (1, L2ꢀ =4-(4H-1,2,4-triazol-4-yl)phenyl
phosphonate) with highly hydrophilic 1D channels was
synthesized hydrothermally. Compound 1 is an example
of a phosphonate MOF with large regular pores with
1.9 nm in diameter. It forms a water-stable, porous struc-
ture that can be reversibly hydrated and dehydrated. The
proton-conducting properties of 1 were investigated by
impedance spectroscopy. Magic-angle spinning (MAS) and
pulse field gradient (PFG) NMR spectroscopies confirm the
dynamic nature of the incorporated water molecules. The
diffusivities, determined by PFG NMR and IR microscopy,
were found to be close to that of liquid water. This
porous framework accomplishes the challenges of water
stability and proton conduction even at 1108C. The con-
ductivity in 1 is proposed to occur by the vehicle mecha-
nism.
Although a number of crystalline porous phosphonate MOFs
has been prepared using low-valent cations,[17,18] trivalent and
tetravalent metal phosphonates are rarely crystalline[15] and
some of them possess interlayer porosity.[19] By functionaliza-
tion of phenyl phosphonate with a triazole ring as additional
donor group, it was possible to grow single crystals of 3D
porous phosphonate MOFs based on trivalent metal ions—
contrary to the established trend of phosphonates that often
precipitate as microcrystalline products. Availability of single-
crystal structure data of robust proton-conducting materials
could potentially fill the gap to establish a basis for the struc-
The fast-growing diverse field of proton-conducting metal–or-
ganic frameworks (MOFs) is currently explored for promising
applications as proton-exchange membranes, which is one of
the key components in fuel-cell technology.[1–3] Nafion and
Nafion-like polymer membranes are efficient proton conduc-
tors,[4,5] however the high costs and temperature limitations
have driven a number of strategies towards the design of alter-
native materials, such as mesoporous silica,[6] coordination
[a] S. Begum, Z. Wang, Prof. Dr. H. Krautscheid
Fakultꢀt fꢁr Chemie und Mineralogie
Universitꢀt Leipzig
[c] Dr. R. Valiullin, Dr. C. Chmelik, Dr. M. Bertmer, Prof. Dr. J. Kꢀrger,
Prof. Dr. J. Haase
Fakultꢀt fꢁr Physik und Geowissenschaften
Universitꢀt Leipzig
Linnꢂstrasse 5, 04103 Leipzig (Germany)
Johannisallee 29, 04103 Leipzig (Germany)
E-mail: krautscheid@rz.uni-leipzig.de
[b] Dr. A. Donnadio, Prof. Dr. F. Costantino, Prof. Dr. M. Casciola
Department of Chemistry
Supporting information for this article, including experimental details for
ligand and MOF syntheses, characterization by X-ray diffraction and TG/
1
University of Perugia
DTA-MS, H and 31P solid-state NMR, impedance spectroscopy, IR microsco-
Via Elce di Sotto n.8, Perugia (Italy)
py, and PFG NMR data, is available on the WWW under http://dx.doi.org/
10.1002/chem.201402886.
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1 was scrutinized initially by evacuating at 1ꢄ10ꢀ5 mbar and
408C for 24 h to remove the noncoordinating water molecules
while the evacuated samples were rehydrated by exposing
them to water. Keeping the samples for five days in refluxing
water did not change the diffraction patterns. The PXRD pat-
terns (Figure SI-2 in the Supporting Information) and TGA
graphs (Figure SI-3 in the Supporting Information) indicate
that the material is insoluble and stable in water. The TGA
curve of air-dried 1 shows a mass loss of 17 wt% in three
steps in the temperature range up to 4508C. The first two
steps are attributed to the loss of noncoordinated and coordi-
nated water molecules at about 908C and at 180–2108C, re-
spectively. The weight loss varies significantly with the drying
conditions, indicating that the capacity of 1 to accommodate
water molecules in nanochannels is strongly influenced by its
surrounding temperature and humidity. The first weight loss
(3.6%) of the evacuated sample corresponds to three H2O mol-
ecules per formula unit (calcd 3.7%).
Figure 1. Fragment of the crystal structure of 1,[21] view along [001]. The
arrow directs to the enlarged 1D channels (1.9 nm in diameter) filled with
noncoordinated (yellow) and coordinated (red) water molecules; phospho-
nate free oxygen atoms are shown in light blue.
Furthermore, the dynamic nature of physisorbed water mol-
1
ture–activity relationship that has remained a challenge in the
field of noncrystalline electrolytes.[2,5,20]
ecules is confirmed by the comparison of solid-state H NMR
spectra of evacuated and hydrated samples of 1 (Figure SI-4a
in the Supporting Information). In the hydrated sample with
a chemical shift of 4.2 ppm (close to that of free water), the
resonance of the coordinated water molecules cannot be seen
as separate signal because of the fast exchange with the
mobile water molecules. In the evacuated sample this reso-
Herein, we report on the triazolyl phosphonate based La3+
MOF 3[La3L4(H2O)6]Cl·xH2O (1, L2ꢀ =4-(4H-1,2,4-triazol-4-yl)-
1
phenyl phosphonate) with wide hydrophilic nanochannels.
These channels with a unique structural arrangement of water
molecules make 1 interesting for investigation of proton-con-
ducting properties.
1
nance appears at 6.8 ppm. The H NMR signal intensity of a hy-
¯
Compound 1 crystallizes in the trigonal space group P3 (no.
drated sample corresponds to 84 protons referring to 42 water
molecules (78% of the overall intensity) in comparison to 24
protons (22%) belonging to the organic linkers (Figure SI-4a in
the Supporting Information). The comparison of 1H magic-
angle spinning (MAS) and 1H double quantum (DQ) filtered
NMR spectra (Figure SI-4b in the Supporting Information)
allows to differentiate the mobile species that contribute to
the proton transport from those bound to the MOF. The ab-
sence of a water-proton signal at 4.2 ppm in the DQ spectra in-
dicates that the dipole–dipole coupling among these protons
is weak and rapid isotropic molecular tumbling averages the
magnetic dipolar coupling to zero. Variable-temperature stud-
ies were performed in the temperature range from ꢀ4 to
658C. The line narrowing of the resonance at 4.2 ppm with in-
creasing temperatures agrees well with dynamic exchange
with an activation energy of 0.07 eV (Figure SI-5 in the Sup-
porting Information).
147) with four crystallographically independent linker anions
L2ꢀ and three La3+ ions in the asymmetric unit.[21] A charge-bal-
ancing chloride ion is detected in the elemental analysis, but
could only partly be localized in the structure refinement. With
LaꢀO bond lengths from 230 to 283 pm and LaꢀN bond
lengths from 270 to 278 pm the La3+ ions have coordination
number nine. Six of the 27 coordination sites for the three
La3+ ions are occupied by water molecules, the remaining
ones are occupied by four nitrogen atoms of three triazole
rings, five terminal and six bridging oxygen atoms of the phos-
phonate groups. The m4-, m5-, and m6-bridging triazolyl phenyl-
phosphonate linkers connect the La3+ ions into a 3D network;
the projection along [001] resembles a Kagomꢃ-type net
(Figure 1).
The coordinated water molecules and noncoordinating
phosphonate-oxygen atoms O3 form 1D chains through hy-
drogen bonds (O···O 264–316 pm), which extend along the
crystallographic c direction. Six such chains combine to highly
hydrophilic hexagonal channels that retain physisorbed water
molecules inside the pores. These channels are arranged like
a hexagonal packing of rods and show a visible open diameter
of 1.9 nm (Figure 1). Based on crystal-structure data a solvent-
accessible pore volume of 38% is calculated.[22] Regular pores
with a diameter of 1.8 nm in the isoreticularly constructed bis-
piperidine diphosphonate MOF by Wright et al. are the largest
among the phosphonate frameworks reported to date.[18]
Framework hydrostability is a special characteristic required
for proton-conducting hydrous materials in humid environ-
ments. The effect of dehydration and complete rehydration of
The ionic conductivity (s) of 1 was investigated by complex
impedance (Z*=Z’ꢀiZ“) measurements carried out on pellets
as a function of relative humidity (RH) at temperatures up to
110 8C. The conductivity of the pellets was calculated from the
analysis of Nyquist plots (Z” vs. Z’) as described in Supporting
Information. The dependence of s on RH at a constant temper-
ature is shown in Figure 2a: In particular, at 1108C and in the
RH range 20–98%, s increases with RH by three orders of mag-
nitude reaching 1.7ꢄ10ꢀ4 Scmꢀ1. To the best of our knowl-
edge, 1 is the first example of a hydrated MOF that allows
gradual increase in ionic conductance by raising the tempera-
ture up to 1108C. In contrast, conductivity studies on most
MOFs have not been reported at elevated temperatures.[1] At
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line inclined by 458 with respect to the Z’ axis; this is typical of
the Warburg impedance. Accordingly, Figure 3b shows that
the frequency response of the pellet–electrode assembly can
be accurately modelled by an equivalent circuit (Figure SI-7 in
the Supporting Information) where the electrode–electrolyte
interface is represented by a Warburg element connected with
a parallel combination of a resistance and a constant phase el-
ement, which accounts for the charge-transfer process. The
strong reduction of the low-frequency tail indicates the pres-
ence of mobile protonic species (H3O+, OHꢀ), which take part
in the electrode reactions and concentration of these species
may be enhanced either by deprotonation of water molecules
coordinated to La3+ or by protonation of triazole groups and/
or free phosphonate oxygen atoms. According to Archer,[25] the
appearance of the Warburg impedance when N2 is replaced by
H2 is consistent with the presence of two charge carriers, one
of which is blocked at the electrodes. These findings and pres-
ence of partially disordered chloride ions in the crystallograph-
ic data suggest that a fraction of the charge-balancing chloride
ions might be located in the MOF channels and partially con-
tribute to the ionic conduction together with the protonic spe-
cies. When the impedance measurements are carried out with
an applied DC voltage of 0.1 and 0.2 V (Figure SI-9 and 10 in
the Supporting Information), the arc representing the frequen-
cy response of the pellet remains substantially unchanged,
whereas, with increasing the DC voltage, a progressive growth
of the impedance of the electrode–electrolyte interface is ob-
served; this can be ascribed to electrode-polarization phenom-
ena.
Figure 2. Proton-conductivity data for 1. a) As a function of RH at different
temperatures and b) Arrhenius plot of the activation energy at 98% RH for
the temperature range 25–1108C.
a constant RH of 98%, the activation energy for conduction
(Ea =0.7 eV), calculated from the Arrhenius plot (Figure 2b), is
significantly higher than those reported for proton-conducting
phosphonate MOFs, such as PCMOF-3[12] (Ea =0.17 eV), PCMOF-
5[14] (Ea =0.16 eV), or ZrCyclo[23] (Ea =0.23 eV), but lower than re-
cently reported values for two tetracarboxylate-based lantha-
nide MOFs (Ea =0.9 eV).[24]
To get insight into the interaction energy of water molecules
in 1D confinement, diffusion studies were carried out by IR mi-
croscopy[26] and pulsed field gradient (PFG) NMR.[27] To estimate
the degree of pore filling at each chosen pressure and temper-
ature, equilibrium adsorption isotherms were recorded in the
temperature range of 25–1108C (Figure 4a). Due to the pres-
sure limitations of the instrumental setup only at 258C and
408C full pore filling is reached, as evidenced by the sharp
condensation steps in the isotherms occurring at 7 and
18 mbar, respectively. No change in the adsorption capacity
and uptake rates was evidenced during a series of experiments
with the same crystal; this testifies again the excellent stability
of 1 in a humid environment and at temperatures up to
110 8C.
To investigate the nature of the charge carriers in 1, impe-
dance measurements were performed at 258C under a flow of
humidified N2 and H2 (see the Supporting Information for de-
tails): In the presence of hydrogen the Pt electrodes are rever-
sible towards the redox process involving protons, whereas
they are ideally blocked in the presence of nitrogen. As expect-
ed, the Nyquist plot collected under N2 (Figure 3a) shows
a large capacitive tail at low frequency; this indicates that the
electrodes are substantially blocked. Under H2, the low-fre-
quency tail shrinks by a factor of about 70 (Figure 3b), giving
rise to a small arc followed, at low frequencies, by a straight
To cover the full range of pore fillings the loading depend-
ence of the transport diffusivity was investigated at 408C (Fig-
ure 4b). Two regions can be distinguished. At intermediate-to-
high pore fillings, the transport diffusivity is roughly constant
at about DT =1.5ꢄ10ꢀ9 m2 sꢀ1. At low-to-intermediate loadings,
we find a strongly increasing transport rate. This is typical for
systems with strong interaction sites at the pore walls (type-III
concentration dependence of diffusivities[26b,28] observed, e.g.,
with water and ammonia in zeolite NaX[29] and ammonia in
MFI-type zeolites).[30] The number of free sites decreases with
increasing coverage, while, in turn, the mobility of the guest
water molecules increases. The existence of strong interaction
sites is supported by the particularly high activation energy for
diffusion at zero loading of about Ea =0.48 eV, which was esti-
Figure 3. Nyquist plots for 1 at 258C a) under humidified N2 at 100% RH
and b) under humidified H2 at 100% RH. Z1, Z2, and CPE3 are the elements
of the equivalent circuit (Figure SI-7 in the Supporting Information) used to
model the frequency response of the pellet.
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molecules, noncoordinating phosphonate oxygen atoms, tria-
zole groups, and plenty of uncoordinated water molecules) ac-
complishes the challenges of water stability and proton con-
duction even at 1108C. The proton conductivity increases sig-
nificantly with increasing temperature and relative humidity by
facilitating charge formation and maintaining high charge den-
sity, respectively. MAS and PFG NMR confirm the dynamic
nature of the water molecules and that the water molecules
are free to diffuse for long distances of at least 20 mm along
1D channels in the framework of 1.
Acknowledgements
We thank Dr. M. Kischel for assistance with TG-MS, Dr. H. J.
Stꢀrk (UFZ Leipzig) for performing the chloride analysis, M.
Lange and Dr. J. Mçllmer (INC Leipzig) for adsorption measure-
ments and helpful discussions and Experimental Physics Insti-
tutes (Universitꢀt Leipzig) for support with the Avance
750 MHz NMR spectrometer. Financial support from Deutsche
Forschungsgemeinschaft (SPP 1362—Porçse metallorganische
Gerꢁstverbindungen, KR 1675/7-2), Universitꢀt Leipzig (PbF-1)
and the graduate school BuildMoNa is gratefully acknowl-
edged. S. Begum acknowledges financial support from Higher
Education Commission Pakistan and a fellowship by DAAD.
Figure 4. a) Adsorption isotherms of water in 1 at different temperatures
measured by IR microscopy. All curves are normalized to the maximum load-
ing reached for water vapor pressure at 258C to indicate the degree of pore
filling. b) Diffusivity of water in 1 as obtained from IR microscopy measure-
ments; dependence of the transport diffusivity DT on the degree of pore fill-
ing at 408C.
Keywords: diffusion studies
· impedance spectroscopy ·
metal–organic frameworks · microporous materials · proton
conduction
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1416.2 gmolꢀ1, crystal size 0.8ꢄ0.1ꢄ0.1 mm , trigonal, space group P3
¯
(no. 147), a=3057.3(4), c=1352.1(3), Z=6, T=180(2) K, V=10944(3)ꢄ
106 pm3, 1calc =1.323 gcmꢀ3
,
m=1.904 mmꢀ1
,
l=71.073 pm (MoKa),
T
R
min =0.72, Tmax =0.89, 24423 measured, 13678 independent reflections,
int =0.056, 8183 with I>2s(I), 580 parameters, two phosphonate li-
gands and coordinating water molecules disordered, H atoms of phos-
phonate ligands in idealized positions, R1 (observed reflections)=0.045,
wR2 (all data)=0.101, max/min residual-electron-density peaks 1.89/
ꢀ1.28eꢀ/106 pm3. CCDC 934648 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif
[31] M. Holz, S. R. Heil, A. Sacco, Phys. Chem. Chem. Phys. 2000, 2, 4740–
4742.
[22] A. L. Spek, J. Appl. Crystallogr. 2003, 36, 7–13.
Received: April 1, 2014
[23] F. Costantino, A. Donnadio, M. Casciola, Inorg. Chem. 2012, 51, 6992–
7000.
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
5
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
Proton-conducting MOFs: Water-filled
nanochannels are the basis for ionic
conductivity in [La3L4(H2O)6]Cl·xH2O (see
figure; L=triazolyl phenyl phospho-
nate). Reversible hydration/dehydration
and proton conduction even above the
boiling point of water are characteristic
for the hydrothermally synthesized 3D
coordination polymer. Crystal structure,
diffusion, and impedance spectroscopy
data are reported.
&
Porous Materials
S. Begum, Z. Wang, A. Donnadio,
F. Costantino, M. Casciola, R. Valiullin,
C. Chmelik, M. Bertmer, J. Kꢀrger,
J. Haase, H. Krautscheid*
&& – &&
Water-Mediated Proton Conduction in
a Robust Triazolyl Phosphonate
Metal–Organic Framework with
Hydrophilic Nanochannels
&
&
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
6
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!