Formation of Layered Proton-Conducting Zirconium and Titanium Organophosphonates by Topotactic Reaction: Physicochemical Properties, Proton Dynamics, and Atomic-Resolution Structure

Article abstract of DOI:10.1021/acs.inorgchem.9b02755

A series of new zirconium and titanium phosphates-organophosphonates, in which the organophosphonate moiety is functionalized with a sulfo group, was prepared by a topotactic reaction involving the gamma modification of zirconium or titanium hydrogen phosphate with 2-bis(phosphonomethyl)amino-ethan-1-sulfonic acid (H₄TDP). H₄TDP represents a new type of functionalizing agent, which can be easily prepared by a Moedritzer-Irani reaction from taurine (2-aminoethanesulfonic acid). The gamma modification of zirconium hydrogen phosphate (γ-ZrP) with H₄TDP provides mixed phosphate-organophosphonate compounds with the formula Zr(PO₄)(H₂PO₄)_1-2x(H₂TDP)_x·yH₂O, where x = 0.15, 0.34, 0.45, and is controlled by the γ-ZrP/H₄TDP ratio in the starting mixture. On the contrary, by the topotactic gamma modification of titanium hydrogen phosphate (γ-TiP) with H₄TDP, only one product with the formula Ti(PO₄)(H₂PO₄)_1-2z(H₂TDP)_z·yH₂O, where z = 0.41 ± 0.01, was obtained regardless of the composition of the starting mixture. The synthesized compounds were characterized by elemental analysis, thermogravimetric analysis, energy-dispersive X-ray analysis, and infrared spectroscopy. The way the topotactic reaction proceeds and how the grafted organophosphonate groups are bonded to the layers of the host structure were suggested on the basis of the solid-state NMR data. It was found that the grafted moieties are spread evenly in the host layers among the hydrogen phosphate groups. The obtained solids are able to intercalate basic molecules, as was proved by the intercalation reactions of the zirconium series with butylamine. The amount of intercalated butylamine increases with increasing x. It is known that both host compounds, γ-ZrP and γ-TiP, are protonic conductors. It was found that the incorporation of H₂TDP increases conductivity of the zirconium compound when x = 0.15, but further incorporation of H₂TDP into the γ-ZrP host structure leads to a decrease of conductivity. This behavior is explained on the basis of the ¹H MAS and the ¹H-¹H EXSY NMR data.

Full text of DOI:10.1021/acs.inorgchem.9b02755

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Formation of Layered Proton-Conducting Zirconium and Titanium
Organophosphonates by Topotactic Reaction: Physicochemical
Properties, Proton Dynamics, and Atomic-Resolution Structure
,†,‡
‡
§
‡
J. Brus,† V. Zima,† L. Benes, J. Svoboda, L. Kobera,† and P. Kutalek
́
́
̌
́
K. Melanova,*
†Institute of Macromolecular Chemistry, Czech Academy of Sciences, Heyrovsky sq. 2, 162 06 Prague 6, Czech Republic
‡Joint Laboratory of Solid State Chemistry, Faculty of Chemical Technology, University of Pardubice, 532 10 Pardubice, Czech
Republic
§Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, 532 10 Pardubice,
Czech Republic
S
* Supporting Information
ABSTRACT: A series of new zirconium and titanium phosphates-
organophosphonates, in which the organophosphonate moiety is function-
alized with a sulfo group, was prepared by a topotactic reaction involving
the gamma modification of zirconium or titanium hydrogen phosphate
with 2-bis(phosphonomethyl)amino-ethan-1-sulfonic acid (H4TDP).
H4TDP represents a new type of functionalizing agent, which can be
easily prepared by a Moedritzer−Irani reaction from taurine (2-
aminoethanesulfonic acid). The gamma modification of zirconium
hydrogen phosphate (γ-ZrP) with H4TDP provides mixed phosphate-
organophosphonate compounds with the formula Zr(PO4)-
(H2PO4)1−2x(H2TDP)x·yH2O, where x = 0.15, 0.34, 0.45, and is controlled
by the γ-ZrP/H4TDP ratio in the starting mixture. On the contrary, by the
topotactic gamma modification of titanium hydrogen phosphate (γ-TiP)
with H4TDP, only one product with the formula Ti(PO4)(H2PO4)1−2z(H2TDP)z·yH2O, where z = 0.41 0.01, was obtained
regardless of the composition of the starting mixture. The synthesized compounds were characterized by elemental analysis,
thermogravimetric analysis, energy-dispersive X-ray analysis, and infrared spectroscopy. The way the topotactic reaction
proceeds and how the grafted organophosphonate groups are bonded to the layers of the host structure were suggested on the
basis of the solid-state NMR data. It was found that the grafted moieties are spread evenly in the host layers among the
hydrogen phosphate groups. The obtained solids are able to intercalate basic molecules, as was proved by the intercalation
reactions of the zirconium series with butylamine. The amount of intercalated butylamine increases with increasing x. It is
known that both host compounds, γ-ZrP and γ-TiP, are protonic conductors. It was found that the incorporation of H2TDP
increases conductivity of the zirconium compound when x = 0.15, but further incorporation of H2TDP into the γ-ZrP host
1
1
structure leads to a decrease of conductivity. This behavior is explained on the basis of the H MAS and the H−1H EXSY
NMR data.
The structure of γ-ZrP consists of ZrO6 octahedra placed in
two different planes and joined to each other with a
tetradentate PO4 inside and H2PO4 groups outside these
planes.3 The H2PO4 groups can be replaced by phosphonate or
phosphite anions to obtain layered compounds with the
general formula Zr(PO4)(O2PRR′)·nS, where R and R′ can be
H, OH, or an organic group and S is an intercalated solvent.
These hybrid materials can be prepared by contacting γ-ZrP
with a solution of corresponding phosphonic acid in a 1:1
acetone−water mixture at 80 °C.4 This method was used for
the preparations of zirconium phosphate phosphite,4 benze-
nephosphonate,5,6 alkylphosphonate,7 and sulfoarylphospho-
INTRODUCTION
■
The research field of metal phosphonate chemistry has grown
considerably in the recent years due to an increased demand
for new functional solids for applications in many fields of
materials chemistry.1 One very important aspect of metal
phosphonates is represented by compounds having a layered
structure. Layered metal phosphonates are promising host
materials due to the variety of functional groups and the ease
of preparation from a metal salt and a corresponding
phosphonic acid. The most studied organophosphonates are
those of zirconium due to their outstanding stability.2 Most
research in the field of zirconium phosphonate chemistry has
been focused on layered α-zirconium phosphonate, but there
are also phosphonates derived from the structure of γ-
Zr(PO4)(H2PO4)·2H2O (γ-ZrP).
Received: September 19, 2019
© XXXX American Chemical Society
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nate.8 An analogous topotactic reaction involving the exchange
of H2PO4 groups with diphosphonate groups takes place when
γ-ZrP is contacted with a solution of a diphosphonic acid. In
this case, zirconium phosphate diphosphonates with the
general formula Zr(PO4)(HO3PRPO3H)·nS, where R is an
organic group and S is a solvent, are formed when the adjacent
layers are covalently bonded by the organic group belonging to
the diphosphonate moieties R.9−14 Recently, a series of
zirconium aminophosphonates has been prepared, usually by
direct synthesis, and their structures have been solved.2 In
most cases, these zirconium aminophosphonates contain some
fluorine ligands, which are coordinated to zirconium together
with phosphonate and/or phosphate ligands. Although γ-
titanium phosphate (γ-TiP) is isostructural with γ-ZrP,15 no γ-
titanium phosphate-phosphonates have been prepared up to
now.
phosphorous acid (16.4 g) in a 250 mL two-necked flask, and a 6
M HCl solution (100 mL) was added to this mixture. The reaction
mixture was heated to 100 °C, and a suspension of paraformaldehyde
(12.012 g) in 5 mL of water was injected into it in small doses within
2 h. After the addition of all the paraformaldehyde, the reaction
mixture was stirred overnight at 100 °C. The reaction mixture was
subsequently rotary evaporated at 80 °C to dryness. The crude
product was then dissolved in distilled water, the solution was heated
to boil, activated carbon (0.5 g) was added, and the suspension was
filtered into a storage bottle. The purity and exact concentration of
the obtained acid solution were then checked by titration with a
standard solution of sodium hydroxide. Yield 28.1 g (89%). 1H NMR
3
(500 MHz, D2O, 25 °C): δH = 3.93 (t, 2H, J(1H−1H) = 7.2 Hz,
CH2), 3.73 (d, 4H, 2J(1H−31P) = 13.3 Hz, CH2), 3.44 (t, 2H,
3J(1H−1H) = 7.2 Hz, CH2). 31P NMR (202 MHz, D2O, 25 °C): δP =
8.15. 13C NMR (126 MHz, D2O, 25 °C): δC = 41.42, 46.94, 48.06
ppm. HR-MALDI-MS (DHB): calc for C4H14NO9P2S ([M + H]+)
313.98590, found 313.98637; calc for C4H13NNaO9P2S ([M + Na]+)
335.96785, found 335.96832.
Reaction of γ-ZrP with H4TDP. γ-ZrP (0.319 g, 1 mmol) was
suspended in a mixture of acetone and water (25 mL + 5 mL), a
corresponding amount of 2.59 M H4TDP (see Table 1) was added,
and the reaction mixture was refluxed for 16 h. The solid product was
separated by centrifugation, washed three times with a mixture of
acetone and water (25 mL + 5 mL), and dried in air at room
temperature.
In this paper, novel zirconium and titanium phosphate-
phosphonates prepared by a topotactic reaction of γ-ZrP or γ-
TiP with 2-bis(phosphonomethyl)amino-ethan-1-sulfonic acid
(see Scheme 1) are reported and their physicochemical
Scheme 1. 2-Bis(phosphonomethyl)amino-ethan-1-sulfonic
Acid
Butylamine intercalates of exchanged γ-ZrP were prepared by
shaking the hosts (0.1 g) in butylamine (5 mL) at room temperature
for 5 days. The intercalates were washed with acetone and dried in air
at room temperature.
Reaction of γ-TiP with H4TDP. A mixture of γ-TiP (0.276 g, 1
mmol), a corresponding amount of 2.59 M H4TDP (see Table 1),
and water (9 mL) was placed into a Teflon-lined 23 mL Parr acid
digestion bomb and heated under an autogenous pressure at 100 °C
for 7 days. The solid product was separated by centrifugation, washed
three times with a mixture of acetone and water (25 mL + 5 mL), and
dried in air at room temperature.
properties, such as intercalation behavior and macroscopic
ionic conductivity, are discussed. Structure and dynamics at an
atomic resolution level are investigated in detail by a range of
advanced techniques of contemporary solid-state NMR
spectroscopy and, on the basis of the obtained results, the
mechanism of topotactic reactions and proton conductivity is
proposed.
1
NMR Spectroscopy. H, 13C, and 31P NMR spectra of H4TDP
were recorded at 500, 126, and 202 MHz, respectively, with a Bruker
AVANCE 500 instrument at 25 °C equipped with a Prodigy
CryoProbe.
1H, 13C, 31P, and 91Zr solid-state NMR (ss-NMR) spectra were
measured at 11.7 T using a Bruker Avance III HD 500 US/WB NMR
spectrometer with 2.5, 3.2, and 4 mm probeheads (Karlsruhe,
Germany, 2013). The following techniques were applied: (1) one-
1
dimensional (1D) H and 31P MAS and 13C CP/MAS NMR; (2)
EXPERIMENTAL SECTION
■
two-dimensional (2D) 1H−13C and 1H−31P FSLG HETCOR
NMR;17 (3) 2D 1H−1H spin-exchange (EXSY) NMR; (4) 2D
31P−31P double-quantum/single-quantum18,19 (DQ/SQ) SPC5 re-
coupled20 NMR; and (5) 1D 91Zr WURST-QCPMG21 NMR. The
frictional heating22 of the spinning samples was compensated by
active cooling, and the temperature calibrations were performed with
Pb(NO3)2. (For the detailed experimental parameters see Supporting
Information, Section SI1.)
Preparations of γ-Zirconium and γ-Titanium Phosphates. A
well-crystallized γ-modification of Zr(PO4)(H2PO4)·2H2O (further
denoted as γ-ZrP) was obtained according to Poojary et al.3 A γ-
modification of Ti(PO4)(H2PO4)·2H2O (further denoted as γ-TiP)
was prepared by a hydrothermal treatment of amorphous titanium
phosphate.16
Preparation of 2-Bis(phosphonomethyl)amino-ethan-1-sul-
fonic Acid (H4TDP). Taurine (12.514 g) was mixed with
Table 1. Denotation, Formulas, and Basal Spacing of the Samples
denotation
formula
n H4TDP [mmol]
basal spacing [Å]
γ-ZrP
Zr-A
Zr-B
Zr-C
γ-TiP
Ti-A
Ti-B
Ti-C
Ti-D
Zr(PO4)(H2PO4)·yH2O
0
12.26
13.34
15.42
17.12
11.59
16.29 + 11.59
16.05
15.80
16.21
Zr(PO4)(H2PO4)0.70(H2TDP)0.15·2H2O
Zr(PO4)(H2PO4)0.32(H2TDP)0.34·2H2O
Zr(PO4)(H2PO4)0.10(H2TDP)0.45·1.5H2O
Ti(PO4)(H2PO4)·2H2O
0.5
1.0
2.0
0
0.52
1.03
1.55
2.06
two-phase system
Ti(PO4)(H2PO4)0.20(H2TDP)0.40·1.3H2O
Ti(PO4)(H2PO4)0.16(H2TDP)0.42·1.5H2O
Ti(PO4)(H2PO4)0.18(H2TDP)0.41·1.5H2O
B
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Mass Spectrometry. High-resolution MALDI MS spectra were
measured on a MALDI mass spectrometer LTQ Orbitrap XL
(Thermo Fisher Scientific, Bremen, Germany) equipped with a
nitrogen UV laser (337 nm, 60 Hz).
Energy-Dispersive X-ray Analysis. Energy-dispersive X-ray
analysis (EDX) was done using an electron scanning microscope
JEOL JSM-5500LV equipped with an energy-dispersive X-ray
microanalyzer IXRF Systems (detector GRESHAM Sirius 10). The
accelerating voltage of the primary electron beam was 20 kV. The
content of carbon, hydrogen, nitrogen, and sulfur was determined
using organic elemental analysis.
H2TDP groups cannot be done in this way. The H2TDP
groups can be incorporated into γ-TiP by a hydrothermal
treatment at 100 °C. It was found that the amount of the
incorporated H2TDP groups does not depend on the amount
of H4TDP in the reaction mixture. Only in the case of the
lowest amount used (sample Ti-A, about 0.5 mmol per 1
mmol of γ-TiP), the exchanged phase was accompanied by a
small amount of unreacted γ-TiP.
Powder X-ray patterns of the zirconium compounds
prepared are shown in Figure 1. As can be seen from the
Thermogravimetric Analysis. The thermogravimetric analysis
was done using a homemade apparatus constructed of a computer-
controlled oven and a Sartorius BP210 S balance. The measurements
were carried out in air between 30 and 960 °C at a heating rate of 5
°C min−1.
X-ray Diffraction Analysis. Powder X-ray diffraction data were
obtained with a D8-Advance diffractometer (Bruker AXE, Germany)
with Bragg−Brentano θ−θ geometry (40 kV, 30 mA) using CuKα
radiation with a secondary graphite monochromator. The diffraction
angles were measured at room temperature from 2 to 65° (2θ) in
0.02° steps with a counting time of 10 s per step. The samples were
stored at various relative humidities and the intercalates with
butylamine were covered with a protection foil to avoid spontaneous
dehydration or a loss of intercalated butylamine.
Infrared Spectroscopy. Infrared spectra in the range 400−4000
cm−1 were recorded at 64 scans per spectrum at 2 cm−1 resolution
using a fully computerized Thermo Nicolet NEXUS 870 FTIR
Spectrometer equipped with a DTGS TEC detector. Measurements
of the powdered samples were performed ex situ in the transmission
mode in KBr pellets. All spectra were corrected for the presence of
moisture and carbon dioxide in the optical path.
Figure 1. Powder X-ray pattern of the compounds prepared.
Impedance Spectroscopy. The sample for the conductivity
measurements was prepared by pressing the powdered sample (0.15
g) to form a rectangular pellet having thickness L of 3 mm, to which
0.25 cm2 electrodes were painted using a carbon paste.
figure, the basal spacing of the zirconium compounds prepared
increases with an increasing amount of the H2TDP groups
present. The diffraction patterns contain a very intensive (00l)
line, which is significantly broadened in comparison with that
of parent γ-ZrP. In the case of titanium compounds, the basal
spacings of all exchanged compounds prepared are very similar,
ranging from 15.80 to 16.29 Å (see Table 1).
All exchanged compounds contain an interlayer water. They
decompose in two steps (see Figure 2 for the zirconium
derivatives). In the first step, the interlayer water is released,
and in the second one the phosphonate anions are
decomposed and a condensation of the phosphate groups
occurs. The end product of thermal decomposition is ZrP2O7
The conductivity measurements were carried out at room
temperature with a Metrohm Autolab PGStat12 instrumentation in
a frequency range from 0.1 Hz to 1 MHz and with a signal amplitude
of 100 mV. The impedance data that had the shape of a semicircle
with a low-frequency linear tail in a complex impedance plot were
analyzed by an equivalent circuit approach using a ZSimpWin
software.23 The chosen equivalent electrical circuit used for fitting
consisted of a parallel arrangement of the resistance R and a constant
phase element (CPE), as defined by Barsoukov and MacDonald.24
The fit provides the value of resistance R. From the relationship σ =
L/RA, where A is the area of the electrodes and L is the distance
between them (thickness of the sample), the conductivity σ of the
samples was calculated.
RESULTS AND DISCUSSION
■
Preparation and Characterization of the Samples.
The samples containing H2TDP groups were prepared using
delamination of γ-ZrP in a mixture of water and acetone and a
subsequent exchange of H2TDP groups for H2PO4. All the
samples prepared are white solids. The formulas of the
samples, based on elemental analysis and EDX measurement,
together with the amount of H4TDP used for the preparation,
are shown in Table 1. As can be seen from the formulas, the
amount of H2TDP groups increases with the increasing
amount of H4TDP used for the reaction. If the stoichiometric
amount of the H4TDP is used, only 30% of the dihydrogen
phosphate anions were exchanged. To exchange nearly all
dihydrogen phosphonate anions, 2 mmol of acid per 1 mmol of
the parent γ-ZrP must be used.
In contrast to γ-ZrP, γ-TiP does not delaminate by refluxing
in the mixture of acetone and water, and therefore, the
replacement of dihydrogen phosphonate anions for the
Figure 2. Thermogravimetric curves of the compounds prepared
together with a parent γ-ZrP.
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(JCPDS No. 04-009-1102) or TiP2O7 (JCPDS No. 04-002-
9495).25 The found and calculated weight losses for all
compounds prepared are given in the Supporting Information
SI2 (Table S1).
The amount of the interlayer water, and also the basal
spacing of all three zirconium compounds, depends on the
relative humidity (RH) of the atmosphere (see Figure 3). The
and 637 cm−1 correspond to δ(P−OH) and δ(O−P−O),
respectively, and a band of water deformation vibration δ(H−
O−H) occurs at 1625 cm−1.26 All these bands are present also
in the spectra of exchanged compounds, only the intensity of
the band at about 1240 cm−1 is significantly lower. In addition,
new bands at 504, 1406, 1458, 2860, and 2962 cm−1 appear,
corresponding to rocking CH2 vibration, symmetric and
asymmetric CH2 deformation vibrations, and symmetric and
asymmetric stretching C−H vibrations, respectively.27 The
intensity of these bands increases with an increasing number of
the H2TDP groups present in the compound.
Intercalation Behavior. Intercalation of butylamine was
done by shaking the solid hosts with an excess of the guest at
room temperature. Powder X-ray diffraction patterns of as-
prepared intercalates, measured with an excess of butylamine,
contain two or three (00l) lines and are shown in Figure 5.
Figure 3. Dependence of the basal spacing d and water content y on
relative humidity.
samples were stored over solutions with a defined RH for 1
week before the thermogravimetric and X-ray diffraction
measurements. Over phosphorus pentoxide, i.e., at 0% RH,
all three samples lost all their interlayer water. The water
content and basal spacing of samples Zr-A and Zr-B changed
very slightly in the region of 11−70% RH. For the sample with
the highest content of the H2TDP groups, the water content
and the basal spacing increase with increasing RH. Above 70%
RH, the water content and also the basal spacing increase
significantly for all three samples.
IR spectra of the exchanged zirconium compounds together
with parent γ-ZrP are shown in Figure 4. A very broad band at
3400−3500 cm−1 in the spectrum of γ-ZrP can be ascribed to
valence vibration ν(PO−H); the broad band with maxima at
1142 and 1022 cm−1 corresponds to symmetric and
asymmetric P−O valence vibrations. Sharp bands at 1244
Figure 5. Powder X-ray pattern of the intercalates of Zr-A, Zr-B, and
Zr-C with butylamine. Black lines correspond to intercalates
measured with an excess of butylamine, and gray lines correspond
to washed and dried intercalates.
Their basal spacings are very similar, ranging from 28.0 to 28.3
Å (see black patterns). The intercalates prepared are unstable,
and their basal spacing decreases if they are exposed to at
ambient air, probably due to a partial loss of intercalated
butylamine. A similar decrease of basal spacing was observed
after washing and drying (see gray patterns in Figure 5). The
formulas of these washed intercalates derived from thermog-
ravimetric analysis are Zr(PO4)(H2PO4)0.70(H2TDP)0.15·1.25-
(BuNH2)·2H2O, Zr(PO4)(H2PO4)0.50(H2TDP)0.25·1.3-
(BuNH2)·1.5H2O, and Zr(PO4)(H2PO4)0.16(H2TDP)0.42·1.4-
(BuNH2)·H2O.
Impedance Measurements. The gamma modification of
zirconium hydrogen phosphate represents a layered material
with significant protonic conductivity,28 which might be
influenced by topotactic reactions with phosphonates contain-
ing a sulfonic group.29,30 Therefore, it was of interest to study
the protonic conductivity of the materials prepared in this
work. For this purpose, we used measurements of impedance
on solid samples of the studied materials prepared from their
powder form. The complex impedance plots of the impedance
data measured in the frequency range from 0.01 to 106 Hz have
a shape of a semicircle in the high-frequency region with a
Figure 4. IR spectra of zirconium derivatives together with parent γ-
ZrP.
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linear tail in the low-frequency region, as shown in Figure 6 for
Zr-C as an example.
Topotactic Modification of γ-ZrP and γ-TiP Systems
Monitored by 31P and 91Zr ss-NMR. As follows from the
analysis of 1D and 2D 31P and 91Zr ss-NMR spectra (Figures
8a,b and Supporting Information SI3, Figures S1a, S2a, and
S3a), the parent ZrP system consists of P(OZr)4 and
P(OZr)2(OH)2 phosphate units regularly arranged in the
crystal lattice of the γ-ZrP framework. This framework is
accompanied by a small amount of structural defects and a
minor fraction of the α-ZrP form.31−34 By the topotactic
reactions with 2-bis(phosphonomethyl)amino-ethan-1-sulfonic
acid (H4TDP), the highly regular structure of the parent γ-ZrP
is considerably modified as reflected (1) by the significant low-
frequency shift of the signal of interlayer P(OZr)2(OH)2
groups, (2) by the appearance of new high-frequency 31P
resonances at 11.0 and −0.5 ppm, and (3) by the broadening
of all the detected 31P NMR signals (Figure 8a). The newly
appeared 31P resonances reflect two distinct phosphonate
motifs evolved during the topotactic replacement of the
P(OZr)2(OH)2 units with the H2TDP groups. On the basis of
the analogy with the previously observed changes of 31P NMR
signals during the condensation of phosphoric acid to a ZrP
Figure 6. Complex impedance plot for Zr-C.
framework,31,34 and with experimental support by the H−31P
1
From the impedance data, the conductivity σ was calculated
by the procedure described in the Experimental Section. The
conductivity of the samples γ-ZrP, Zr-A, Zr-B, and Zr-C is
1.90 × 10−5, 5.30 × 10−5, 2.42 × 10−5, and 1.06 × 10−5 S cm−1,
respectively. The dependence of conductivity on the amount of
grafted H2TDP groups is shown in Figure 7. All compounds
FSLG HETCOR NMR spectra (Figure 8c), the signal at −0.5
ppm, which strongly correlates with CH2 protons, reflects
completely condensed H2TDP phosphonate −C-P(-OH)(-
OZr)2 units, whereas the signal at 11.0 ppm corresponds with
H2TDP phosphonates containing residual hydroxyl groups
−C-P(-OH)2(-OZr). With increasing the extent of the
topotactic reactions, the signal intensities of these H2TDP
phosphonate groups gradually increase, while the signals of the
P(OZr)2(OH)2 units correspondingly decrease (Figure 8a,
compounds Zr-B and Zr-C).
Subsequently, the mechanism of the topotactic reactions was
monitored by 2D 31P−31P SQ/DQ MAS NMR spectroscopy
(Figure 8d and Supporting Information SI3, Figure S2). Even
at low conversions (Zr-A), the incorporated H2TDP
phosphonate fragments strongly correlate with the framework
P(OZr)4 phosphate units and with the P(OZr)2(OH)2 groups,
while the autocorrelation signal H2TDP···H2TDP is very weak
(Figure 8d). These spectral features then indicate that the
individual H2TDP phosphonate groups, −C-P(-OH)(OZr)2,
are uniformly dispersed in the host layers and surrounded by
the P(OZr)2(OH)2 groups. With an increasing degree of the
topotactic substitution (compounds Zr-B and Zr-C), the
31P−31P autocorrelation signals of the P(OZr)2(OH)2 groups
gradually disappear while the autocorrelation signals of the
H2TDP phosphonate units increase. Simultaneously, the new
correlation signals between two subpopulations of the H2TDP
phosphonate groups appear (Figure 8d, red box). This
phenomenon shows that the H2TDP fragments gradually
become more and more densely incorporated in the modified
γ-ZrP layers, forming at first pairs and small clusters that
gradually grow to finally form a continuous phase. This process
is accompanied by a loss of the long-range periodic
arrangement, which is reflected by an observed broadening
of 31P MAS NMR resonances and the decrease in intensities in
the 91Zr WURST-QCPMG NMR spectra (Supporting
Information SI3, Figure S3). The well-evolved correlation
signals between the P(OZr)4 units and the H2TDP
phosphonate groups, however, confirm that the net layered
structure of γ-ZrP is basically retained.
Figure 7. Conductivity σ as a function of the amount of H2TDP
groups (x).
prepared show ionic conductivity. Their conductivity decreases
with x, but compounds with x = 0.15 and 0.34 (Zr-A and Zr-B,
respectively) are more conductive than parent γ-ZrP.
The observed conductivity behavior can be explained in the
following way. In the original γ-ZrP, the extent of protonic
conductivity is given by the net of hydrogen bonds stemming
from the interlayer water molecules and OH groups of outer
H2PO4 groups. By a partial replacement of the H2PO4 groups
with the sulfophosphonate groups, this regular net is disturbed,
which leads to an increased mobility of the protons in the
interlayer space of Zr-A. Further replacement of the H2PO4
groups with the sulfophosphonate groups leads, on the
contrary, to a decrease of the concentration of these labile
protons and also to a decrease of their mobility. Thus, we
observe an increase of conductivity for the sample in which
only 30% of the H2PO4 groups is replaced by the
sulfophosphonate groups but a decrease for samples in which
more H2PO4 groups are replaced. These considerations are
It is worthy to note that the analysis of 2D 31P−31P SQ/DQ
MAS NMR spectra also provides information on the
1
supported by H NMR results below.
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1
Figure 8. 31P MAS NMR spectra of parent γ-ZrP, Zr-A, Zr-B, and Zr-C (a); 91Zr WURST-QCPMG NMR spectrum of γ-ZrP (b); 2D H−31P
FSLF HETCOR NMR spectrum of Zr-A (c); 2D 31P−31P SQ/DQ MAS NMR spectra of Zr-A and Zr-C (d); and the idealized structure of γ-ZrP
modified with H4TDP acid (e).
conformation of the incorporated H2TDP fragments. As each
H2TDP fragment consists of two phosphonate groups, the
observed low intensity of the autocorrelation signals of the
H2TDP fragments reflect rather long 31P···31P interatomic
distances between these groups. This arrangement is reached
in the stretched conformation of the H2TDP fragments
incorporated into the γ-ZrP framework demonstrated in
Figure 8e, in which the 31P···31P distance between the
phosphonate groups is larger than ca. 5.3 Å.
confirms a high degree of the topotactic reactions, when the
H2TDP phosphonate units are extensively and regularly
incorporated into the γ-TiP framework structure and strongly
interact with its P(OTi)4 phosphate groups. The high density
of the H2TDP molecular fragments in the layers is then
definitely confirmed by a strong autocorrelation signal at about
0 ppm (Figure 9b, orange box).
1H NMR Spectroscopy and Proton Conductivity.
When considering the proton conductivity, we focused on
the analysis of the 1H MAS NMR spectra. As demonstrated in
Analogously as for the γ-ZrP systems, the recorded 31P ss-
NMR spectra (Figure 9) confirm extensive topotactic reactions
also for γ-TiP (Figure 9a). It is clear that the P(OTi)2(OH)2
phosphate groups, which resonate at ca. −10 ppm, are almost
completely substituted by the H2TDP groups, the built-in
fragments of which are indicated by the new resonances in the
region from 20 to 0 ppm. The multiple resonances detected in
this region then reflect the coexistence of distinctly different
surface structures of the phosphonate groups, which, similarly
as in the previous case of the γ-ZrP systems, result from
incomplete condensation reactions. The corresponding 2D
31P−31P SQ/DQ MAS NMR spectrum (Figure 9b) then
1
Figure 10a, the H MAS NMR spectrum of the parent γ-ZrP
system shows two signals reflecting (1) acidic protons (10.8
ppm) and (2) weakly hydrogen-bonded hydroxyls of P-
(OZr)2(OH)2 and water molecules (6.8 ppm). These protons
mutually communicate by the chemical exchange, as revealed
1
by the variable-temperature H−1H EXSY MAS NMR spectra
(Figure 10b) in which the intensity of the correlation signals
increases with the elevated temperature. The high lability of
these acidic protons is further confirmed by a fully reversible
disappearance of the corresponding 1H NMR signal above 310
K (see Supporting Information SI4, Figure S4). All these facts
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Article
surface bonding. This finding thus explains the increase in
macroscopic proton conductivity observed for the slightly
modified γ-ZrP system (Zr-A). The subsequent progress in the
topotactic reactions, however, leads to a decrease in surface
density and mobility of these labile protons, which is indicated
by the broadening and the high-frequency shift of the
corresponding 1H MAS NMR signals to ca. 8 ppm.
Consequently, the macroscopic proton conductivity of the
extensively modified γ-ZrP systems is reduced (Zr-B and Zr-
C).
CONCLUSION
■
The gamma modifications of zirconium dihydrogen phosphate
and titanium dihydrogen phosphate were successfully modified
with 2-bis(phosphonomethyl)amino-ethan-1-sulfonic acid by a
topotactic reaction. For the first time the preparation of this
acid and its characterization are described in scientific
literature.
In the case of zirconium dihydrogen phosphate, the amount
of incorporated sulfonic acid can be controlled by the ratio of
starting components. In the case of titanium dihydrogen
phosphate, the amount of incorporated sulfonic acid remains
practically the same regardless the ratio of the starting
components. The basal spacing of the zirconium compounds
increases with the amount of incorporated sulfonic acid and is
also influenced by the amount of interlayer water. In the
titanium compounds, the amount of the incorporated acid and
the basal spacing of the products are roughly the same in all
cases.
Figure 9. 31P−31P SQ/DQ MAS NMR spectra of parent γ-TiP
system (a) and the modified compound Ti−B (b).
The prepared layered solids are able to intercalate basic
compounds, as was proven by intercalation of butylamine. The
amount of intercalated amine increases with the increasing
content of the incorporated 2-bis(phosphonomethyl)amino-
ethan-1-sulfonic acid.
thus can be considered as a basis for the observed macroscopic
proton conductivity of the parent γ-ZrP system.
Upon the topotactic reactions, the high-frequency signal at
10.8 ppm disappears, while the signal at 6.8 ppm is
considerably narrowed (Figure 10b). These phenomena reflect
enhanced proton mobility and decreased strength of the
The results obtained from the analysis of 2D 31P−31P SQ/
DQ MAS NMR spectra provided sufficient structural data to
propose the mechanism of topotactic reactions in the γ-ZrP
Figure 10. 1H MAS NMR spectra of parent γ-ZrP and Zr-A, Zr-B, and Zr-C (a), and 2D 1H−1H EXSY NMR spectra of γ -ZrP measured at 303
and 308 K (b). On the right side, the slices through the correlation exchange signal at 10.8 ppm are displayed (b).
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framework. These NMR spectra indicate that the H2TDP
moieties bonded to the γ-ZrP layers are uniformly distributed.
This fact can be explained by the following two-step
mechanism. During the first step, the H4TDP molecules
enter the interlayer space and are uniformly distributed
between the γ-ZrP layers. This intercalation of H4TDP causes
an increase of the basal spacing and the loss of regular crystal
structure. In the second step, the topotactic substitution
reactions take place and the internal surface is uniformly
modified.
Analysis of H MAS and H−1H EXSY NMR spectra also
provided a base for explaining macroscopic proton con-
ductivity of the compounds derived from γ-ZrP. We found
that, although all the prepared ZrP systems exhibit an ionic
conductivity, the highest conductivity shows in the system with
the smallest amount of the incorporated acid. In this regard,
the applied 1H ss-NMR spectroscopy techniques revealed
considerably enhanced proton mobility and a decreased
strength of proton surface bonding for this ZrP system.
However, with the subsequent progress in topotactic reactions,
surface density and mobility of these labile protons consistently
decrease, which results in reducing macroscopic proton
conductivity.
1
1
ASSOCIATED CONTENT
■
(10) Alberti, G.; Murciamascaros, S.; Vivani, R. Preparation and
characterization of zirconium phosphate diphosphonates with the
gamma-structure - a new class of covalently pillared compounds.
Mater. Chem. Phys. 1993, 35, 187−192.
S
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b02755.
(11) Alberti, G.; Vivani, R.; Mascaros, S. M. First structural
determination of layered and pillared organic derivatives of gamma-
zirconium phosphate by X-ray powder diffraction data. J. Mol. Struct.
1998, 470, 81−92.
Experimental parameters to solid-state NMR spectros-
copy, results of EDX, elemental, and thermogravimetric
analyses of the exchanged samples, details to 31P and
91Zr solid-state NMR spectroscopy of γ-ZrP framework
and modified systems, variable-temperature 1H MAS
NMR spectroscopy of γ-ZrP framework, and tables of
observed parameters (PDF)
(12) Alberti, G.; MurciaMascaros, S.; Vivani, R. Pillared derivatives
of gamma-zirconium phosphate containing nonrigid alkyl chain
pillars. J. Am. Chem. Soc. 1998, 120, 9291−9295.
(13) Alberti, G.; Boccali, L.; Dionigi, C.; Vivani, R.; Kalchenko, V. I.;
Atamas, L. I. Preparation and first characterization of a layered
gamma-zirconium phosphate derivative containing benzo 15-crown-5
groups covalently attached to inorganic layers. Supramol. Chem. 1996,
7, 129−135.
(14) Alberti, G.; Brunet, E.; Dionigi, C.; Juanes, O.; delaMata, M. J.;
RodriguezUbis, J. C.; Vivani, R. Shaping solid-state supramolecular
cavities: Chemically induced accordionlike movement of gamma-
zirconium phosphate containing polyethylenoxide pillars. Angew.
Chem., Int. Ed. 1999, 38, 3351−3353.
(15) Christensen, A. N.; Andersen, E. K.; Andersen, I. G. K.; Alberti,
G.; Nielsen, M.; Lehmann, M. S. X-Ray-powder diffraction study of
layer compounds - the crystal-structure of alpha-Ti(HPO4)2·H2O and
a proposed structure for gamma-Ti(H2PO4)(PO4)·2H2O. Acta Chem.
Scand. 1990, 44, 865−872.
(16) Krogh Andersen, A. M.; Norby, P. Structural aspects of the
dehydration and dehydroxylation of gamma-titanium phosphate,
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(17) van Rossum, B. J.; Forster, H.; de Groot, H. J. M. High-field
and high-speed CP-MAS C-13 NMR heteronuclear dipolar-
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Goldburg homonuclear decoupling. J. Magn. Reson. 1997, 124,
516−519.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: klara.melanova@upce.cz.
ORCID
́
́
K. Melanova: 0000-0001-7163-2476
J. Brus: 0000-0003-2692-612X
V. Zima: 0000-0002-2456-2628
J. Svoboda: 0000-0001-7948-4746
L. Kobera: 0000-0002-8826-948X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Czech Science Foundation (K.M., V.Z.,
and L.B.: Grant No. 17-10639S; J.B. and L.K.: Grants No. 18-
12925S and 19-05259S) for financial support.
(18) Schnell, I.; Brown, S. P.; Low, H. Y.; Ishida, H.; Spiess, H. W.
An investigation of hydrogen bonding in benzoxazine dimers by fast
magic-angle spinning and double-quantum H-1 NMR spectroscopy. J.
Am. Chem. Soc. 1998, 120, 11784−11795.
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for the elucidation of structure and dynamics of molecular,
macromolecular, and supramolecular systems. Chem. Rev. 2001, 101,
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