A layered mixed zirconium phosphate/phosphonate with exposed carboxylic and phosphonic groups: X-ray powder structure and proton conductivity properties

Article abstract of DOI:10.1021/ic502473w

A novel mixed zirconium phosphate/phosphonate based on glyphosine, of formula Zr₂(PO₄)H₅(L)₂·H₂O [L = (O₃PCH₂)₂NCH₂COO], was synthesized in mild conditions. The compound has a layered structure that was solved ab initio from laboratory PXRD data. It crystallizes in the monoclinic C2/c space group with the following cell parameters: a = 29.925(3), b = 8.4225(5), c = 9.0985(4) ?, and β = 98.474(6)°. Phosphate groups are placed inside the sheets and connect the zirconium atoms in a tetradentate fashion, while uncoordinated carboxylate and P-OH phosphonate groups are exposed on the layer surface. Due to the presence of these acidic groups, the compound showed remarkable proton conductivity properties, which were studied in a wide range of temperature and relative humidity (RH). The conductivity is strongly dependent on RH and reaches 1 × 10^-3 S cm^-1 at 140 °C and 95% RH. At this RH, the activation energy of conduction is 0.15 eV in the temperature range 80-140 °C. The similarities of this structure with related structures already reported in the literature were also discussed.

Full text of DOI:10.1021/ic502473w

Article
pubs.acs.org/IC
A Layered Mixed Zirconium Phosphate/Phosphonate with Exposed
Carboxylic and Phosphonic Groups: X‑ray Powder Structure and
Proton Conductivity Properties
†
‡
†
†
†
Anna Donnadio, Morena Nocchetti, Ferdinando Costantino, Marco Taddei, Mario Casciola,
§
,‡
́
Fabio da Silva Lisboa, and Riccardo Vivani*
†
‡
§
Dipartimento di Chimica, Biologia e Biotecnologie, Universita
Dipartimento di Scienze Farmaceutiche, Universita di Perugia, Via del Liceo 1, 06123 Perugia, Italy
Departamento de Química, Universidade Federal do Parana, Jardim das Americas CEP − 81.531-980, 19032 Curitiba, Brasil
S Supporting Information
̀
di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy
̀
́
́
*
ABSTRACT: A novel mixed zirconium phosphate/phospho-
nate based on glyphosine, of formula Zr (PO )H (L) ·H O [L
2
4
5
2
2
=
(O PCH ) NCH COO], was synthesized in mild con-
3 2 2 2
ditions. The compound has a layered structure that was solved
ab initio from laboratory PXRD data. It crystallizes in the
monoclinic C2/c space group with the following cell
parameters: a = 29.925(3), b = 8.4225(5), c = 9.0985(4) Å,
and β = 98.474(6)°. Phosphate groups are placed inside the
sheets and connect the zirconium atoms in a tetradentate
fashion, while uncoordinated carboxylate and P−OH
phosphonate groups are exposed on the layer surface. Due
to the presence of these acidic groups, the compound showed
remarkable proton conductivity properties, which were studied
−3
in a wide range of temperature and relative humidity (RH). The conductivity is strongly dependent on RH and reaches 1 × 10
−1
S cm at 140 °C and 95% RH. At this RH, the activation energy of conduction is 0.15 eV in the temperature range 80−140 °C.
The similarities of this structure with related structures already reported in the literature were also discussed.
INTRODUCTION
Chart 1. Molecular Structures of Glyphosine and PMIDA
■
Zirconium phosphonates are a class of materials characterized
by high insolubility and good ion exchange and intercalation
properties. These features make them suitable in many fields of
current interest such as fillers for polymeric nanocomposites,
1
molecular recognition, separation, and so on. They have also
been studied for their good proton conductivity properties in a
2
wide range of relative humidity and temperature. This
property is frequently accompanied by a remarkable chemical
and thermal stability. These joint features are intriguing owing
to the potential application of these materials in a variety of
devices, including fuel cells, electrolyzers, chemical sensors,
3
electrochromic displays, hydrogen pumps, and so on. In this
bidentate. The uncoordinated oxygen atom points toward the
interlayer region, forming a network of polar interactions with
carboxylic groups and hydration water molecules.
respect, the proton conductivity properties of a high number of
hybrid crystalline materials like coordination polymers and
metal−organic frameworks have been reported in the past few
4
This compound showed a good proton conductivity, which
years, many of which are metal phosphonates. Recently, we
reached the highest value (about 8 × 10− S cm ) at 140 °C
and 95% relative humidity (RH), which can be related to the
presence of these uncoordinated polar groups with different
acidity.
4
−1
reported on the preparation and characterization of a novel
layered zirconium phosphonate based on N,N-bis-
(
phosphonomethyl)glycine (glyphosine), a simple carboxy-
5
amino diphosphonate (Chart 1).
In this compound, due to the unusually high value of the P/
Zr molar ratio (8/3), half of the phosphonate groups
coordinate three zirconium atoms, while the other half is
Received: October 9, 2014
©
XXXX American Chemical Society
A
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Inorganic Chemistry
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In mid-1990s, Clearfield and co-workers explored the
reactivity of a phosphonic building block, N,N-phosphonome-
thylimino-diacetic acid (PMIDA), that has a molecular
geometry similar to that of glyphosine (Chart 1). They found
that zirconium is able to form a 1D structure when reacting
exclusively with PMIDA and a layered structure when
phosphoric acid is added to the reaction solution (ZPPMIDA).
In the latter, zirconium is coordinated with both phosphate and
PMIDA, and the interlayer region accommodates the two
Synthesis of Zr
3 2 2 2 2
2
(PO
4
)[(HO
3
PCH
2
)(O
3
PCH
2
)NHCH
2
COOH]-
[
(O PCH ) NHCH COOH]·H O (ZPGly). A total of 2.37 g (9
mmol) of glyphosine was solubilized in 93 mL of deionized water,
and then 6 mL of 1 M phosphoric acid was added to this solution. A
total of 1.93 g (6 mmol) of zirconium oxychloride octahydrate was
dissolved in 20.4 mL of 2.9 M hydrofluoric acid solution (59 mmol,
HF/Zr4 molar ratio = 10). These two solutions were mixed in a 500
mL Teflon bottle and placed in an oven at 90 °C. After 3 days, the
solid was filtered under vacuum, washed three times with deionized
water, and dried at 60 °C for 24 h (Yield: 37%). The anhydrous form
of ZPGly (hereafter ZPGly-a) was obtained by heating ZPGly at 100
+
6
carboxylate groups (Figure 1).
°
2
C. Anal. calc. for Zr P O C N H : Zr 22.3, P 19.4, C 11.7, N 3.4, H
2 5 21 8 2 19
3
1
.3%. Found Zr 23.1, P 18.5, C 10.4, N 3.0, H 2.2%. Quantitative P
NMR (D O/HF, 400 MHz): δ = 8.50 ppm (glyphosine, 4P), 1.07
2
ppm (phosphoric acid, 1P).
Analytical Procedures. Zirconium and phosphorus contents of
samples were obtained by inductively coupled plasma−optical
emission spectrophotometry (ICP−OES) using a Varian Liberty
Series II instrument working in axial geometry after the mineralization
of samples with hydrofluoric acid. Carbon, nitrogen, and hydrogen
contents were determined by elemental analysis using an EA 1108
CHN Fisons instrument. PXRD patterns for structure determination
and Rietveld refinements were collected with Cu Kα radiation on a
PANalytical X’PERT PRO diffractometer and PW3050 goniometer
equipped with an X’Celerator detector. The long fine focus (LFF)
ceramic tube was operated at 40 kV and 40 mA. To minimize
preferential orientations of the microcrystals, the samples were
carefully side-loaded onto an aluminum sample holder. The sample
was kept at 100 °C by means of a custom-made Peltier equipped
sample holder during the data collection. Thermogravimetric (TG)
measurements were performed using a Netzsch STA490C thermoa-
−
1
−1
nalyser under a 20 mL min air flux with a heating rate of 5 °C min .
Field emission-scanning electron microscopy (FE-SEM) images were
collected with a LEO 1525 ZEISS instrument working with an
acceleration voltage of 15 kV. Transmittance mid-Fourier transform
infrared (FT-IR) measurements were carried out with a JASCO FT/IR
Figure 1. Schematic representation of the structure of ZPPMIDA.
The similarity of glyphosine molecular structure with
PMIDA suggested to us to further investigate the zirconium−
glyphosine system, also adding phosphoric acid, in the hope to
obtain, in analogy to ZPPMIDA, a solid structure containing a
considerable number of free acidic phosphonate and carbox-
ylate groups in the interlayer region, which could effectively
contribute to the protonic conduction.
As a result of this investigation, a novel microcrystalline
compound, built from glyphosine and phosphate was
synthesized. Its structure was solved ab initio from powder X-
ray diffraction (PXRD) data, showing that although it is related
with that of ZPPMIDA and rich in uncoordinated carboxylate
and phosphonate groups our expectations were not totally
complied. However, due to its structure, the obtained material
showed increased proton conductivity properties as compared
with the parent zirconium−glyphosine derivative.
4
000 spectrophotometer. The spectral range collected was 400 to 4000
−1
−1
cm , with a spectral resolution of 2 cm acquiring 100 scans. The
samples were dispersed on anhydrous KBr pellets.
Conductivity measurements were carried out on pellets of pressed
powder by impedance spectroscopy with a Solartron Sl 1260
impedance/gain phase analyzer in the frequency range of 10 Hz−1
MHz at a signal amplitude of ≤100 mV. Pellets, 10 mm in diameter
and 1−1.5 mm thick, were prepared by pressing ∼200 mg of material
2
at 40 kN/cm . The two flat surfaces of the pellet were coated with a
thin layer of pressed platinum black (Aldrich) mixed with the powder
in a 3:1 ratio. RH was controlled by using stainless steel sealed-off cells
consisting of two communicating cylindrical compartments held at
different temperatures. The cold compartment contained water, while
the hot compartment housed the pellet under test. RH values were
calculated from the ratio between the pressures of saturated water
vapor (p) at the temperatures of the cold (T ) and hot (T )
c
h
compartment: RH = p(T )/p(T ) × 100.
c
h
Water uptake (WU) at controlled temperature and RH was
determined as described in ref 5. Specifically, the cell for water uptake
had the same size and shape as the conductivity cell and differed from
that mainly because the MEA holder is replaced by a glass container
hosting the sample (≈ 0.2 g). The cell was equipped with a device that
allowed the sample container to be closed with a Teflon plug without
opening the cell. After a suitable equilibration time (usually 1 day) at
the desired temperature and RH, the sample container was closed,
extracted from the cell, and weighed. The water content was
determined on the basis of the weight of the material dried at 120
°C for 5 h, taking into account the amount of water trapped in the
sample container at the temperature and RH of the experiment. The
error on the determination of WU was estimated to be ±0.2 water
molecules per unit formula.
EXPERIMENTAL SECTION
Chemicals. All of the chemicals were purchased from Sigma-
Aldrich and were used as received.
■
Synthesis of N,N-Bis(phosphonomethyl)glycine (Glypho-
sine). Glyphosine was prepared following the Moedritzer-Irani
7
method; 5 g of glycine (67 mmol) were dissolved in 50 mL of 6
M HCl, together with 11 g of H PO (133 mmol). This mixture was
3
3
heated to reflux, and 8 g of paraformaldehyde (266 mmol) dispersed in
0 mL of water were slowly added within 2 h. After the last addition of
1
paraformaldehyde, the solution was refluxed for 1 more hour and then
the solvent was evaporated. The raw mixture was treated with 2-
propanol, yielding a white solid that was filtered under vacuum and
dried in an oven at 60 °C; 13.6 g of glyphosine was recovered. (Yield =
7
(
8
4%). 1H NMR (D O, 400 MHz): δ = 4.29 (singlet, 2H), 3.55
Structure Determination and Refinement for ZPGly-a. The
crystal structure of ZPGly-a was solved ab initio from PXRD data.
Indexing was performed using both the TREOR and the DICVOL06
2
31
doublet, J = 6.0 Hz, 4H) ppm. P NMR (CDCl , 400 MHz): δ =
3
.13 ppm.
B
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8
programs. Space group was assigned using the CHEKCELL
RESULTS AND DISCUSSION
9
■
program. A reliable structural model was determined using the real
Structure Description. ZPGly-a crystallizes into the
space global optimization methods implemented in the FOX program.
Trial structures were generated using the parallel tempering algorithm.
Rietveld refinement of the structural model was performed using the
GSAS program. First, zero-shift, cell, background, and profile shape
parameters were refined. A corrected pseudo-Voigt profile function
monoclinic C2/c space group, and its structure is composed
of the packing of complex hybrid layers along the a-axis. These
layers are built from the connection of ZrO octahedra, PO
10
6
4
tetrahedra, and PO C phosphonate tetrahedra belonging to the
3
(
six terms) with two terms for the correction of asymmetry at the low-
glyphosine ligands. The unit cell contains two stacked layers
with an ABA sequence. A polyhedral representation and
asymmetric unit of ZPGly-a are depicted in Figure 3.
angle region was used. Then, atomic coordinates were refined by
restraining the bond distances to the following values: Zr−O = 2.05(5)
Å, P−O = 1.56(5) Å, C−C = 1.54(5) Å, and C−N = 1.45(5) Å. The
statistical weight of these restraints was decreased as the refinement
proceeded. At the end of the refinement procedure, the shifts in all
parameters were less than their standard deviations.
Table 1 reports the crystallographic data and refinement details for
ZPGly-a. Figure 2 shows the Rietveld plot for ZPGly-a.
Table 1. Structural Data and Refinement Details for ZPGly-a
compound
ZPGly-a
empirical formula
formula weight
crystal system
space group
a (Å)
C N O P5 Zr2
8
2
20
781.4
Monoclinic
C2/c
29.925(3)
8.4225(5)
9.0985(4)
90
b (Å)
c (Å)
α (deg)
β (deg)
98.474(6)
90
γ (deg)
Figure 3. (a) Polyhedral representation of ZPGly-a viewed along the c-
volume (Å)
2268.2(3)
4
axis. ZrO octahedra are in purple. PO tetrahedra are in gray. PO C
6
4
3
Z
tetrahedra are in green. (b) Asymmetric unit of ZPGly-a. Extended
connectivity is represented in light color. Hydrogen bonds are
represented as dashed lines.
T (°C)
100
calculated density (g cm−3)
data range (2θ deg−1)
wavelength (Å)
no. of data points
reflections collected, unique
no. of parameters
no. of restraints
2.29
3−100
1.54056
5705
Each layer is about 15 Å thick. The structure of each layer is
constituted of a double plane of ZrO octahedra, which are
6
1157
connected by tetradentate PO groups lying in an intermediate
4
85
plane. Tetrahedra and octahedra are connected by sharing their
vertices. Phosphate groups are placed in special positions inside
the layer and link four different zirconium atoms. The
glyphosine moieties are placed on the external part of the
58
a
Rp
0.0458
0.0619
0.0815
2.67
b
Rwp
c
R 2
F
layers, connecting the zirconium atoms through the PO C
3
d
GOF
groups. One PO C tetrahedron (P1) is placed inside the ZrO
3
6
a
b
2
2
1/2
c
2
o
R = ∑|I -I |/∑I . R = [∑w(I -I ) /∑wI ] . R 2 = ∑|F
−
sheet, and it is triply connected to three crystallographically
equivalent zirconium octahedra, generating eight-membered
p
o
c
o
wp
o
c
o
F
F |/∑ |F |. GOF = [∑w(I -I ) /(N − N )]1/2.
2
2
d
2
c
o
o
c
o
var.
[
O−P−O−Zr] square rings that have been frequently found in
2
11
other similar zirconium phosphonates. The distances
Zr(14)−O(2) and Zr(14)−O(16) are identical [2.08(1) Å],
and they are comparable to those found in similar systems. The
second PO C tetrahedron (P2) is connected to only one Zr
3
atom in a monodentate fashion, with the free P−O groups
pointing toward the interlayer space where the organic residue
also resides. Thus, a very polar interlayer region in which free
carboxylic, P−O, and amino groups are facing each other is
found. A number of close contacts between these groups is
observed, suggesting that a hydrogen-bond network, extended
both inside each layer and between adjacent layers, may be
present, as depicted in Figure 4. One P−O group interacts with
the oxygen atoms of neighboring carboxylates (P7−O8···
O13#1 = 2.99(3) Å, #1 = 0.5 − x, 0.5 − y, 1 − z; P7−O8···
O12#2 = 2.73(2) Å, #2 = 0.5 + x, 0.5 − y, 0.5 + z), while the
remaining phosphonate oxygen (O9) points toward O12 (P7−
O9···O12#3 = 2.58 Å, #3 = x, 1 − y, 0.5 + z) and the aminic
nitrogen atom belonging to the adjacent glyphosine moiety
Figure 2. Rietveld plot of ZPGly-a. Inset shows an enlargement of the
0−100° 2θ region.
4
C
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Figure 5. Comparison between the structures of ZPGly-a (a) and
ZPPMIDA (b), see ref 12. ZrO6 octahedra are in purple. PO4
tetrahedra are in gray. PO C tetrahedra are in green.
3
Thermal Behavior. Figure 6 shows the TG and DTA
curves for ZPGly equilibrated over saturated NaCl solution
(
75% RH).
Figure 4. Detail of the H-bond interactions (represented as red dashed
lines) between the sheets of ZPGly-a. ZrO octahedra are in purple.
6
PO tetrahedra are in gray. PO C tetrahedra are in green.
4
3
(
P7−O9···N5#3 = 2.94(2) Å). This last interaction was
frequently found in many other zirconium aminophospho-
1
2
nates. Very likely the proton resides on the nitrogen atom, as
suggested by FT-IR analysis, discussed herein. FT-IR analysis
also shows that the carboxylic groups are protonated. For
electroneutrality requirements, the P−O8 bond is in turn a
double bond and a single bond with protonated oxygen; for
best describing this situation, a less symmetrical space group
could have been used, such as P2 /c, in order to work with an
1
asymmetric unit containing two distinct glyphosine moieties.
However, in all our attempts, the use of this space group did
not lead to any improvement of the structure refinement, at the
cost of a larger asymmetric unit. Therefore, as C2/c was found
to be the largely best fitting space group, we chose to perform
structure solution and refinement in this space group.
On the basis of the almost identical PXRD patterns, we can
argue that the structure of the as-synthesized ZPGly is very
similar to that of ZPGly-a (Figure 1S, Supporting Information),
but we were not able to identify the position of the water
molecule by difference Fourier maps. This is likely due to the
fact that there is only one-half water molecule per asymmetric
unit, probably disordered in different positions.
Figure 6. TG and DTA curves of ZPGly conditioned at 75% RH.
The first endothermic weight loss (2.2%) is associated with
the loss of one water molecule per formula unit (calculated:
2
.2%). The second exothermic weight loss (11.1%), occurring
between 150 and 350 °C, corresponds to the loss of two CO2
molecules per formula unit due to the decarboxylation of the
glycine moieties (calculated: 11.0%). The following exothermic
step (13.0%), between 350 and 860 °C, is due to the
combustion of the organic part to yield ZrP O and P O . The
2
7
2
5
last weight loss can be attributed to the gradual loss of excess
P O . Finally, the solid recovered from the TG was
The connectivity of the different building blocks of ZPGly-a
is related to that of ZPPMIDA (Figure 5).
2
5
characterized by PXRD, and only cubic ZrP O was detected.
2
7
In both of them, the PO groups and tridentate PO C
4
3
FT-IR. The FT-IR spectrum of ZPGly is shown in Figure 7.
−1
tetrahedra play the same structural role, However, ZPGly has a
higher P/Zr ratio than ZPPMIDA (2.5 vs 1.5) because ZPGly
has an additional phosphonate tetrahedron due to the use of
the diphosphonic glyphosine instead of the monophosphonic
PMIDA moieties. The second phosphonate tetrahedron of
glyphosine, being monodentate, replaces the apical water
molecule coordinated to zirconium in ZPPMIDA. Therefore,
this phosphonate group, although not totally protruding into
the interlayer region, as observed for the carboxylates in
ZPPMIDA, has two of the three P−O groups that are
uncoordinated and free to participate in the building of the
highly polar interlayer region.
At 3424 cm , a broad band due to the O−H bond stretching
in water is observed, due to the water molecules present in the
−1
interlayer space. At 1640 cm , the O−H bending of water is
also visible. The bands at 3002 and 2995 cm− are due to the
1
−1
C−H stretching. The broad band centered at 2600 cm is
+
typical of a R −NH group, evidencing the presence of
3
−1
protonated amino groups. The band at 1739 cm is related to
the CO stretching in a −COOH group, accompanied by the
−
1
bands at 1427 (O−H bending) and 1244 cm (C−O
stretching), further evidence of the presence of protonated
−1
−COOH groups. The bands from 950 to 1200 cm are related
−1
to the P−O stretchings, whereas in the region below 900 cm ,
D
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Figure 8. Conductivity of a ZPGly pellet as a function of temperature
at 95% RH.
conductivity dependence on RH at 100 °C is shown in Figure
Figure 7. FT-IR spectrum of ZPGly-a.
9
.
a number of bands are found, which are not straightforward to
be assigned (various bending modes, Zr−O stretchings).
Proton Conductivity. In the unit formula of ZPGly, there
+
are five protogenic functions including R−NH and the
hydroxyls belonging to carboxylic and monodentate phos-
phonic groups. ZPGly is therefore expected to conduct protons,
and the conductivity of ZPGly pellets was investigated by
complex impedance (Z* = Z′+ iZ″) measurements both as a
function of temperature at constant RH and as a function of
RH at constant temperature. At low RH values, the Nyquist
plot (Z″ versus Z′) consists in a single depressed semicircle,
representing the frequency response of the pellet, followed by a
linear region on the low frequency side associated with the
electrode−pellet interface (Figure 2S, Supporting Information).
With increasing RH, the decrease in the pellet resistance results
in a progressive shift of the points of the Nyquist plot from the
semicircle to the linear region so that the arc disappears at the
highest RH values. In all cases, the pellet conductivity (σ) is
calculated from the extrapolation (R) of the linear region to the
Z′ axis, taking into account the thickness (d) and the flat
surface area (A) of the pellet: σ = d/(A × R). The PXRD
patterns of the material collected before and after the
conductivity measurements did not show significant differences
Figure 9. Conductivity of a ZPGly pellet as a function of RH at 100
C.
°
For RH in the range 35−95%, the logarithm of the
conductivity is a linear function of RH, and σ increases from
1.4 × 10− to 5.8 × 10 S cm . As already observed for
6
−4
−1
2e
zirconium arylphosphonates, the strong conductivity depend-
ence on RH is likely to be due to the weak acid character of the
protogenic groups of ZPGly. It is interesting to observe that
when the impedance data collected at different RH values are
normalized by dividing Z′ and Z″ by the pellet resistance (R)
the points of the normalized Nyquist plots gather around the
same semicircle (Figure 10).
(Figure 3S, Supporting Information), thus indicating that no
irreversible transformations take place during the conductivity
tests. Measurements at 95% RH were carried out by decreasing
temperature from 140 to 80 °C; concomitantly, the water
uptake was determined on separate ZPGly pellets under the
same experimental conditions. The conductivity decreased
This suggests that the pellet conductivity originates from
only one type of proton transport because in general different
types of transport (e.g., bulk, surface, or grain boundary
transport) are expected to respond differently to RH changes,
thus determining changes in the shape of the Nyquist plots as
shown in ref 2j.
from 1 × 10− to 5 × 10 S cm (Figure 8), while the
hydration degree was three water molecules per unit formula in
the whole temperature range.
3
−4
−1
As a consequence, the change in conductivity reflects the
only change in temperature. This allows us to calculate the
To get further insight into the physical origin of proton
transport, water uptake determinations were carried out on
pellets of ZPGly as a function of RH at 100 °C in the same RH
range explored for conductivity measurements. The hydration
of pellets (Figure 11) increases from ∼2 to 3 water molecules
per unit formula for RH going from 35% to 95%, being always
larger than the hydration of the material at room temperature.
This is not surprising because, RH being the same, the water
vapor pressure at 100 °C is by more than 40 times higher than
that at 20 °C.
activation energy (E ) of proton conduction on the basis of the
a
Arrhenius equation: σT = σ exp(−E /kT), where σ is a
0
a
0
temperature independent pre-exponential factor, and k and T
have their usual meaning. The E value, determined from a
a
least-squares fit of log(σT) as a function of 1/T (Figure 4S,
Supporting Information), turns out to be 0.15 eV and is typical
of a transport mechanism of the Grotthuss type, which is
2j
characterized by E values in the range of 0.1−0.4 eV. The
a
E
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Inorganic Chemistry
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CONCLUSIONS
■
A novel layered zirconium phosphate diphosphonate with
aminocarboxylic groups has been synthesized in mild condition.
Due to the analogy of glyphosine and PMIDA moieties, the
structure of this new compound shows some similarity with
that of the previously reported ZPPMIDA: its layered nature
and the presence of tetradentate phosphate and tridentate
phosphonate groups with a similar structural role. However, the
additional phosphonate group in glyphosine induces the
presence of an increased number of P−O groups uncoordi-
nated to metal atoms on the surface of layers, as compared to
ZPPMIDA. These acidic groups, together with carboxylates, are
responsible for the measured protonic conductivity. The
conductivity behavior, studied as a function of temperature
and relative humidity, and the concomitant determination of
the hydration degree led to conclude that the conductivity is
mainly due to surface proton transport. Future research will be
addressed to the development of similar materials containing
free acidic groups with an increased acidic strength in
comparison with carboxylic groups, which could more
efficiently enhance the proton transportation.
Figure 10. Normalized Nyquist plots at 100 °C and at the indicated
RH values for a ZPGly pellet.
ASSOCIATED CONTENT
Supporting Information
■
*
S
Crystallographic information file and complete crystallographic
parameters. PXRD patterns of ZPGly as synthesized and heated
at 100 °C, Nyquist plots for ZPGly at 100 °C and different RH,
Arrhenius plot of the conductivity for ZPGly pellet, PXRD
patterns of ZPGly as synthesized, and after conductivity
measurements. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 11. Hydration as a function of RH % at 100 °C for a ZPGly
pellet.
AUTHOR INFORMATION
Corresponding Author
E-mail: riccardo.vivani@unipg.it.
■
*
Notes
Comparison of Figures 9 and 11 shows that in the RH range
of 35−50% the conductivity increases by a factor of 3, while the
hydration keeps nearly constant. Similarly, in the RH range of
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
75−95%, the conductivity increases by a factor of 9, while the
This work was supported by MIUR − Project FIRB 2010 no.
RBFR10CWDA_003 and Fondazione Cassa di Risparmio di
Perugia Project no. 2014.0100.021.
hydration goes from 2.9 to 3 water molecules per unit formula.
These findings suggest that the conductivity of ZPGly is not
significantly affected by changes in the bulk hydration of ZPGly,
but it reflects only the changes in the hydration of the surface of
the ZPGly microcrystals. Therefore, in agreement with the
invariance of the shape of the normalized Nyquist plots toward
RH changes, it can be concluded that the conductivity of ZPGly
pellets arises mainly from surface proton transport.
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■
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