Layered zirconium phosphate chloride dimethyl sulfoxide as a two-dimensional exchanger of anionic ligands. Part I. Substitution of chloride with inorganic monodentate ligands

Article abstract of DOI:10.1021/ic010643v

Crystalline ZrPO₄Cl(CH₃)₂SO was prepared by direct precipitation in the presence of oxalic acid as a zirconium complexing agent. The structure of ZrPO₄Cl(CH₃)₂SO, refined with the Rietveld method using X-ray powder diffraction data, was confirmed to be close to that of the compound prepared using γ-zirconium phosphate as a precursor. Chloride anions directly bonded to zirconium were found to act as weak ligands; this made possible their replacement with other monodentate anionic ligands. The preparation and a preliminary characterization of a series of inorganic derivatives obtained by topotactic replacement of Cl with OH, Br, MSO₄ (M= H, NH₄, Na), NaMoO₄, and HCrO₄ anions is reported. The possibility of replacement of chloride also with organic anions, such as alkoxides and carboxylates, and the possibility of substituting also dimethyl sulfoxide with other neutral ligands, as shown by preliminary study, makes ZrPO₄Cl(CH₃)₂SO a useful and very flexible precursor for materials chemistry.

Full text of DOI:10.1021/ic010643v

Inorg. Chem. 2002, 41, 1913−1919
Layered Zirconium Phosphate Chloride Dimethyl Sulfoxide as a
Two-Dimensional Exchanger of Anionic Ligands. Part I. Substitution of
Chloride with Inorganic Monodentate Ligands
Giulio Alberti,* Silvia Masci, and Riccardo Vivani
Department of Chemistry, UniVersity of Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy
Received June 18, 2001
Crystalline ZrPO Cl(CH ) SO was prepared by direct precipitation in the presence of oxalic acid as a zirconium
4
3 2
complexing agent. The structure of ZrPO Cl(CH ) SO, refined with the Rietveld method using X-ray powder diffraction
4
3 2
data, was confirmed to be close to that of the compound prepared using γ-zirconium phosphate as a precursor.
Chloride anions directly bonded to zirconium were found to act as weak ligands; this made possible their replacement
with other monodentate anionic ligands. The preparation and a preliminary characterization of a series of inorganic
derivatives obtained by topotactic replacement of Cl with OH, Br, MSO (M ) H, NH , Na), NaMoO , and HCrO
4
4
4
4
anions is reported. The possibility of replacement of chloride also with organic anions, such as alkoxides and
carboxylates, and the possibility of substituting also dimethyl sulfoxide with other neutral ligands, as shown by
preliminary study, makes ZrPO Cl(CH ) SO a useful and very flexible precursor for materials chemistry.
4
3 2
Introduction
compound was obtained by Clearfield et al.8 by a direct
hydrothermal synthesis using dimethyl sulfoxide (dmso) as
solvent; the latter was prepared in our laboratory by
replacement of the bidentate anionic H2PO4- ligands of γ-ZP,
ZrPO4H2PO4, with couples of a monodentate anionic (Cl-)
and a neutral ligand (dmso).9 Both compounds crystallize
in the tetragonal system, space group P4/n. Their layer is
composed of two planes of zirconium atoms octahedrally
coordinated. Four equatorial positions, lying in the layer
plane, are occupied by four oxygen atoms of four different
tetrahedral PO4 groups. The remaining axial positions,
perpendicular to the layer, are occupied by fluoride or
chloride anions and dmso as a neutral ligand.
The layered structures of many metal phosphates can be
seen as formed by octahedra-tetrahedra building block
combinations in which the metal, usually octahedrally
coordinated, shares its corners or edges with tetracoordinated
oxyanions of phosphorus. By limiting our attention to
zirconium, which is the most investigated tetravalent metal,
two main layered structures are known, usually indicated as
R- and γ-zirconium phosphates (hereafter R- and γ-ZP).1-3
Their organic derivatives, also known as R- and γ-zirconium
phosphonates, have been prepared for long time,4,5 and their
cation exchange, intercalation, and proton conduction proper-
ties are now documented by an extensive literature.6,7
More recently, two new layered compounds, ZrPO4F(CH3)2-
SO and ZrPO4Cl(CH3)2SO, were reported. The former
Preliminary research performed in our laboratory clearly
showed that both anionic and neutral ligands of ZrPO4-
Cldmso can be topotactically replaced by a variety of other
ligands, generating thus a large family of derivative com-
pounds with the same type of layered structure. On the other
hand, we found that the fluoride derivative was unreactive
for similar topotactic replacements. The layered derivatives
obtained from the precursor ZrPO4Cldmso have been for-
* To whom correspondence should be addressed. E-mail: ric@unipg.it.
Fax: +39 075 585 5566.
(1) (a) Clearfield, A.; Costantino, U. In ComprehensiVe Supramolecular
Chemistry; Alberti, G., Bein, T., Eds.; Pergamon: Elsevier Science
Ltd.: Oxford, U.K., 1996; Vol. 7, p 107. (b) Alberti, G. In
ComprehensiVe Supramolecular Chemistry; Alberti, G., Bein, T., Eds.;
Pergamon: Elsevier Science Ltd.: Oxford, U.K., 1996; Vol. 7, p 151.
(2) Troup, J. M.; Clearfield, A. Inorg. Chem. 1977, 16, 3311.
(3) Poojary, D. M.; Zhang, B.; Dong, Y.; Peng, G.; Clearfield, A. J. Phys.
Chem. 1994, 98, 13616.
(7) Alberti, G.; Casciola, M.; Costantino U.; Vivani, R. AdV. Mater. 1996,
(4) Alberti, G.; Costantino, U.; Allulli, S.; Tomassini, N. J. Inorg. Nucl.
Chem. 1978, 40, 1113.
8, 291.
(8) Poojary, D. M.; Zhang, B.; Clearfield, A. J. Chem. Soc., Dalton Trans.
(5) Yamanaka, S. Inorg. Chem. 1976, 15, 2811.
(6) Clearfield A. In Progress in Inorganic Chemistry; Karlin, K. D., Ed.;
John Wiley & Sons: New York, 1998; Vol 47, p 374.
1994, 2453.
(9) Alberti, G.; Bartocci, M.; Santarelli, M.; Vivani, R. Inorg. Chem. 1997,
36, 3574.
10.1021/ic010643v CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/12/2002
Inorganic Chemistry, Vol. 41, No. 7, 2002 1913

Alberti et al.
dmso as solvent and maintained at 70 °C for 10 h under stirring.
After the second contact, the solid was washed with anhydrous
ethanol and dried at 60 °C.
mulated as ZrPO4(L L′), where L L′ is a generic couple of
monodentate anionic and neutral ligands.9 To better distin-
guish them from R- and γ-ZP derivatives, these compounds
will be named here λ-ZP derivatives, in which λ simply
means that they are derived by topotactic ligand substitution,
and hence, they are structure-related with the parent com-
pound ZrPO4Cldmso.
In this paper, a new method for the preparation of the
precursor ZrPO4Cldmso by direct precipitation will be
reported. The preparation of a sample of high crystallinity
allowed us to refine and confirm its structure.
The preparation and characterization of a series of
inorganic λ-ZP derivatives, obtained for the replacement of
Cl anions of ZrPO4Cldmso with OH, Br, MSO4 (M ) H,
NH4, Na), NaMoO4, and HCrO4 anions will then be
presented.
The difference between the reactivity of ZrPO4Cldmso and
ZrPO4Fdmso toward topotactic replacements is discussed on
the basis of structural and chemical considerations.
Anal. Found (calcd): % C 8.9 (9.4); % H 2.1 (2.3); Zr 2.5 ×
10-3 mol/g (2.6 × 10-3); PO4 2.6 × 10-3 mol/g (2.6 × 10-3); Br
2.6 × 10-3 mol/g (2.6 × 10-3). Weight loss at 1100 °C: 49.4%.
ZrPO4NH4SO4dmso. A 1 g portion of ZrPO4Cldmso was
suspended in 64 cm3 of a 0.473 M (NH4)2SO4 solution using a
33% (v/v) dmso/water mixture as solvent. The solid was maintained
in this solution for 24 h at 75 °C under stirring, and then it was
separated by centrifugation and washed three times with 100 cm3
of water. It was dried at 80° and then stored over P4O10.
Anal. Found (calcd): % C 6.7 (6.3); % H 2.5 (2.6); % N 3.4
(3.7); Zr 2.5 × 10-3 mol/g (2.6 × 10-3); PO4 2.6 × 10-3 mol/g
(2.6 × 10-3); SO4 2.4 × 10-3 mol/g (2.6 × 10-3). Weight loss at
1100 °C: 42.0%.
ZrPO4NaSO4dmso. A 1 g portion of ZrPO4Cldmso was
suspended in 100 cm3 of a 0.4 M Na2SO4 solution using a 25%
(v/v) dmso/water mixture as solvent. The synthesis was carried out
as described for ZrPO4NH4SO4dmso.
Anal. Found (calcd): % C 6.5 (6.3); % H 2.2 (1.6); Zr 2.5 ×
10-3 mol/g (2.6 × 10-3); PO4 2.6 × 10-3 mol/g (2.6 × 10-3); SO4
2.4 × 10-3 mol/g (2.6 × 10-3); Na 2.4 × 10-3 mol/g (2.6 × 10-3).
ZrPO4(HSO4)0.6(OH)0.4dmso. A 1 g portion of ZrPO4Cldmso
was suspended in 667 cm3 of a 0.252 M H2SO4 solution using a
90% (v/v) propanol/water mixture as solvent. The solid was
maintained in this solution for 14 h at 70 °C under stirring, and
then it was separated by centrifugation and washed twice with 100
cm3 of anhydrous ethanol. It was dried at 80° and then stored over
P4O10.
Experimental Section
Reagents. All reagents were C. Erba RPE reagents, except
zirconium oxychloride octahydrate, which was a Merck reagent.
They were used without further purification.
Preparation of the Precursor ZrPO4Cldmso. Two solutions,
named A and B, were separately prepared. Solution A was obtained
by mixing, at 80 °C, ZrOCl2‚8H2O (0.5 g, 1.55 × 10-3 mol)
previously dissolved in 37% HCl (13 cm3, Zr/Cl molar ratio )
1/100) and oxalic acid (3.5 g, 6.7 × 10-2 mol, Zr/H2C2O4 molar
ratio ) 1/43) previously dissolved in dmso (25 cm3). Solution B
was prepared by mixing 85% w/w H3PO4 (0.1 cm3, 1.48 × 10-3
mol, Zr/P molar ratio ) 1/0.95) and dmso (31 cm3). Solutions A
and B were then mixed at room temperature, and a clear solution,
having the following composition, was obtained: [Zr(IV)] ) 0.022
M, [HCl] ) 2.2 M, [H3PO4] ) 0.022 M, [H2C2O4] ) 0.93 M,
dmso ) 81% v/v.
Anal. Found (calcd): % C 6.9 (7.3); % H 2.4 (2.1); Zr 2.9 ×
10-3 mol/g (3.0 × 10-3); PO4 3.1 × 10-3 mol/g (3.0 × 10-3); SO4
1.8 × 10-3 mol/g.
ZrPO4HSO4dmso. A 1 g portion of ZrPO4OHdmso was
suspended in 570 cm3 of a solution obtained by adding 6.6 cm3
HSO3Cl to dmso. The solid was maintained in this solution for 14
h at room temperature under stirring, and then it was separated by
centrifugation and washed twice with 100 cm3 of anhydrous ethanol.
It was dried at 80° and then stored over P4O10.
The final solution was heated at 70 °C in a Teflon hydrothermal
bomb for 6 days. The product was filtered off, washed twice with
anhydrous ethanol, dried, and stored under P4O10.
Anal. Found (calcd): % C 6.3 (6.6); % H 2.1 (1.9); Zr 2.9 ×
10-3 mol/g (2.8 × 10-3); PO4 2.8 × 10-3 mol/g (2.8 × 10-3); SO4
2.8 × 10-3 mol/g; (2.8 × 10-3). Weight loss at 1100 °C: 37.3%.
ZrPO4NaMoO4dmso‚1.5H2O. A 1 g portion of ZrPO4Cldmso
was suspended in 56 cm3 of a 0.295 M Na2MoO4‚2H2O solution
using a 36% (v/v) dmso/water mixture as solvent. The initial pH
of this solution (8.1) was set at 7.1 by adding a few drops of
concentrated HNO3.
The solid was maintained in this solution for 8 h at 90 °C under
stirring; afterward, it was separated by centrifugation and washed
twice with 100 cm3 of formamide and finally with anhydrous
ethanol. It was then dried at 80°.
Anal. Found (calcd): % C 8.2 (8.0); % H 2.1 (2.0); Zr 3.2 ×
10-3 mol/g (3.3 × 10-3); PO4 3.2 × 10-3 mol/g (3.3 × 10-3); Cl
3.2 × 10-3 mol/g (3.3 × 10-3). Weight loss at 1100 °C: 33.8%.
Preparation of the Inorganic Derivatives of ZrPO4Cldmso.
ZrPO4Fdmso. A 1 g portion of ZrPO4Cldmso was contacted twice
with 110 cm3 of 0.03 M HF using dmso as solvent at 75 °C for 3
days for each contact. The solid was then washed with water and
dried at 80 °C.
Anal. Found (calcd): % C 8.2 (8.5); % H 2.1 (2.1); Zr 3.6 ×
10-3 mol/g (3.5 × 10-3); PO4 3.4 × 10-3 mol/g (3.5 × 10-3); F
3.5 × 10-3 mol/g (3.5 × 10-3). Weight loss at 1100 °C: 31.0%.
ZrPO4OHdmso‚1.5H2O. A 1 g portion of ZrPO4Cldmso was
suspended in 350 cm3 of a 5% (v/v) dmso/water mixture and
maintained at room temperature under stirring for 24 h. The solid
was then separated by centrifugation and stored under 75% relative
humidity.
Anal. Found (calcd): % C 5.4 (5.1); % H 1.6 (1.9); Zr 2.1 ×
10-3 mol/g (2.1 × 10-3); PO4 2.0 × 10-3 mol/g (2.1 × 10-3);
MoO4 1.9 × 10-3 mol/g (2.1 × 10-3); Na 1.9 × 10-3 mol/g
(2.1 × 10-3).
ZrPO4HCrO4dmso‚1.5H2O. A 1 g portion of ZrPO4Cldmso was
suspended in 100 cm3 of a 0.103 M K2CrO4 solution using 28%
(v/v) dmso/water mixture as solvent. The initial pH of this solution,
close to 10, was set at 3.2 by adding a few drops of concentrated
HNO3. The solid was maintained in this solution for 19 h at 70 °C
under stirring; afterward, it was separated by centrifugation and
washed twice with 100 cm3 of formamide and finally with
anhydrous ethanol. It was then dried at 80°.
Anal. Found (calcd): % C 7.4 (7.8); % H 2.4 (3.2); Zr 3.2 ×
10-3 mol/g (3.2 × 10-3); PO4 3.3 × 10-3 mol/g (3.2 × 10-3).
Weight loss at 1100 °C: 35.4%.
ZrPO4Brdmso‚0.5dmso. A 1 g portion of ZrPO4OH dmso‚
1.5H2O prepared as described was contacted twice with 357 cm3
of a 3 M HBr and 3 M LiBr solution using a 50% v/v ethanol/
1914 Inorganic Chemistry, Vol. 41, No. 7, 2002

Zirconium Phosphate Chloride Dimethyl Sulfoxide
Anal. Found (calcd): % C 5.7 (5.9); % H 2.7 (2.5); Zr 2.4 ×
10-3 mol/g (2.4 × 10-3); PO4 2.4 × 10-3 mol/g (2.4 × 10-3);
CrO4 2.2 × 10-3 mol/g (2.4 × 10-3).
Analytical and Instrumental Procedures. The zirconium
content of samples was determined gravimetrically by dissolving
a weighed amount (0.150 g) in a few drops of concentrated HF,
followed by precipitation with cupferron and subsequent calcination
to ZrO2.
Anions (phosphate, fluoride, chloride, bromide, sulfate, chromate,
and molybdate) were determined by ion chromatography. About
0.100 g of sample was refluxed for 3 h with 10 cm3 of 1 M NaOH;
after filtration and suitable dilution, the solution was injected into
a Dionex series 2000 i/sp instrument, using an IonPack AS4A
column and a buffer solution having the following composition:
1.7 × 10-3 M in NaHCO3, 1.8 × 10-3 M in Na2CO3 (3.5 × 10-3
M for fluoride ions) as eluent.
Chromates were also determined as chromium by atomic
absorption spectroscopy using a Perkin-Elmer 3100 spectrometer.
Sodium was determined by atomic emission spectroscopy using
a Perkin-Elmer 3100 spectrometer.
Coupled TGA and DTA curves were obtained by a Stanton
Redcroft STA 780 thermoanalyzer, at heating rates between 1 and
5 °C/min.
Figure 1. Structure of ZrPO4Cldmso. Methyl groups of dmso indicated
with an asterisk have occupancy 0.5.
by γ-ZP,9 attempts to prepare the precursor by direct
synthesis were carried out.
The strategy adopted is a modification of the thermal
zirconium fluoro complex decomposition12,13 that essentially
consists of leaving these complexes to thermally decompose
in a solution containing the suitable reagents. However, under
the synthesis conditions used here, the presence of fluoride
ions in the reaction solution caused the formation of the inert
compound ZrPO4Fdmso; we therefore used oxalic acid as a
weaker complexing agent.
Carbon and hydrogen elemental analysis was performed by a
Carlo Erba 1106 analyzer.
Hydration water was determined by TG analysis, heating the
sample at 110 °C until a constant weight was reached.
FT-IR spectra were recorded using a Bruker JFS V FT-IR
spectrometer, in the 400-7000 cm-1 spectral range. The samples
were prepared as pressed pellets in anhydrous KBr.
SEM micrographs were recorded by a Philips XL30 scanning
electron microscope, fitted with an LaB6 electron gun.
X-ray powder diffraction (XRD) patterns for structure determi-
nation and Rietveld refinement of ZrPO4Cldmso were collected
according to the step scanning procedure in the range 6°-130° 2θ
(0.01° step size and 30 s counting time) with Cu KR radiation on
a Philips APD X’PERT diffractometer, PW3020 goniometer
equipped with a bent graphite monochromator on the diffracted
beam; 0.5° divergence and scatter slits, 0.1 mm receiving slit were
used. The LFF tube operated at 40 KV, 40 mA. To minimize
preferred orientations, the sample was carefully sideloaded onto
an aluminum sample holder with an oriented quartz monocrystal
underneath.
A series of experiments allowed us to optimize the
synthesis. As expected, the oxalate/zirconium molar ratio in
the solution was found to affect the degree of crystallinity
of the product obtained. This ratio ranged from 20 to 100,
giving products with increasing crystallinity. Unfortunately,
the higher crystallinity was accompanied by a dramatic
decrease in the yield of the reaction and a consistent increase
of the reaction time. The C2O42-/Zr molar ratio was finally
set to 43 as the best compromise.
The compound, obtained as described in the Experimental
Section, consists of a white powder of platelike microcrystals,
with homogeneous dimensions around 10 µm diameter and
about 1 µm thick, as assessed by SEM images (see Figure
2). Figure 3a shows a thermogravimetric curve of the
compound. The weight loss starts at about 200 °C, with the
simultaneous loss of chloride and dmso. At the end of the
analysis, at 1100 °C, the formation of an equimolar mixture
of zirconium oxide and zirconium pyrophosphate was
assumed, according to the following reaction of decomposi-
tion:
Other XRD patterns were recorded using a 0.03° 2θ step size
and 1 s counting time in the 2°-40° 2θ range.
Structure Refinement of ZrPO4Cldmso. XRD pattern and unit
cell parameters (determined with the TREOR90 program10) are close
to those first reported: the cell is tetragonal, a ) 5.696 Å, and c )
10.242 Å. A Rietveld refinement was therefore performed (with
the GSAS program11) using, as starting model, the structural
parameters previously determined.9 The refinement substantially
confirmed the structure previously reported. Figure 1 shows the
refined structure.
2ZrPO4Cldmso + nO2 f
Results and Discussion
ZrO2 + ZrP2O7 + gaseous products
Direct Synthesis of ZrPO4Cldmso. To improve the
degree of crystallinity of ZrPO4Cldmso, previously obtained
The calculated % weight loss is 35.2, in agreement with that
experimentally found (33.8).
(10) Werner, P. E.; Eriksson, L.; Westdhal, M. J. Appl. Crystallogr. 1985,
18, 367.
(11) Larson, A.; Von Dreele, R. B. GSAS, Generalized Structure Analysis
System; Los Alamos National Laboratory: Los Alamos, NM, 1988.
(12) Alberti G.; Torracca, E. J. Inorg. Nucl. Chem. 1968, 30, 317.
(13) Alberti, G.; Costantino, U.; Giulietti, R. J. Inorg. Nucl. Chem. 1980,
42, 1062.
Inorganic Chemistry, Vol. 41, No. 7, 2002 1915

Alberti et al.
Figure 4. van der Waals surface of a λ-layer. The anionic and neutral
ligands have been removed to better evidence “mountain” and “valley”
positions.
In the case of ZrPO4Cldmso, the anionic ligands are
expected to be more reactive than the neutral ones, because
they occupy the more accessible “mountain” site. Accord-
ingly, the precursor was found to be very reactive to anionic
replacement, and a series of inorganic derivatives with
different anions in the “mountain” position were easily
prepared by topotactic reactions, as described later.
Figure 2. SEM micrographs of ZrPO4Cldmso.
In agreement with a very low reactivity of its “valley”
sites, attempts to replace dmso with other neutral ligands
were unsuccessful. However, recent experiments have shown
that dmso can be replaced by neutral ligands with a high
affinity for zirconium, such as amines. It seems, however,
that the total reaction also involves the “mountain” sites,
because the Cl- ions are also eluted; after the reaction, the
“mountain” sites were occupied by the amine, while elec-
troneutrality was preserved by hydroxyl groups found in the
“valley” positions. The complex mechanism of this reaction
will be discussed elsewhere.14
Figure 3. TGA of (a) ZrPO4Cldmso, (b) ZrPO4OHdmso‚1.5H2O, (c)
ZrPO4HSO4dmso, and (d) ZrPO4Brdmso‚0.5dmso. Static atmosphere,
heating rate 5 °C/min.
General Considerations on the Topotactic Reactions
of ZrPO4Cldmso. The topotactic replacement of ligands in
a generic ZrPO4LL′ compound can be written as
Fluoride Derivative. It is known that the tendency of F-
anions to be coordinated to zirconium is very high; accord-
ingly, a high value of SF/Cl (7.8) was found.9
ZrPO4LL′ + L′′ h ZrPO4LL′′ + L′
To avoid a dissolution-recrystallization mechanism caused
by the fluoride solution, the conversion of ZrPO4Cldmso into
the fluoride derivative was achieved by carefully maintaining
a very low F- concentration during the reaction. Sequential
contacts of the parent compound with dilute F- solutions
were therefore performed. The high affinity of zirconium
for fluoride was confirmed with a full conversion into ZrPO4-
Fdmso. The derivative obtained had the same XRD pattern
and composition of that prepared by Clearfield et al. by direct
synthesis.8
where L′ and L′′ are a couple of anionic or neutral ligands.
The relative affinity (or selectivity) coefficient can be
defined as SL′′/L′ ) (XhL′′XL′)/(XhL′XL′′), where Xh and X are the
molar fractions of the two ligands in the solid phase and in
solution, respectively.
In a given solvent, the value of SL′′/L′ coefficient will
depend on the relative tendency of these ligands to be
coordinated to the tetravalent metal ion as well as on the
environment of the sites of exchange that can sterically hinder
the replacement.
Hydroxyl Derivative. It is known that zirconium exhibits
a high affinity for OH-. Thus, Cl- of the precursor is
expected to be completely exchanged even with a very low
concentration of OH-. Experiments have shown that ZrPO4-
Cldmso is not stable in pure water.9 However, we found that
a small amount of dmso in water (at least 5% v/v) is
sufficient to stabilize the neutral ligand in the solid phase,
at room temperature. In this solvent, only the Cl/OH anionic
exchange was observed, with the formation of a compound
of composition ZrPO4OHdmso‚1.5H2O. The reaction pro-
ceeds very quickly and is almost complete in about 2 h,
Before discussing the reactivity of this precursor, let us
examine some structural peculiarities. In its layers, zirconium
atoms are arranged in two parallel planes (Figure 1). Both
chloride and dmso ligands face the layer surface, but they
occupy two nonequivalent positions, as shown by the van
der Waals surface of a λ-layer, in which the ligands have
been omitted (Figure 4). This picture clearly shows that the
layer is not flat but undulated. The anionic site is placed on
a relief, and it will be called the “mountain” site, while the
neutral site is placed in a hollow, and it will be called the
“valley” site. The free area available for a couple of neutral-
anionic ligands is 43.5 Å2 (i.e., larger than in R- and γ-type
structures, 24.0 Å2 and 35.3 Å2, respectively).
(14) Alberti, G.; Masci, S.; Vivani, R. To be published.
1916 Inorganic Chemistry, Vol. 41, No. 7, 2002

Zirconium Phosphate Chloride Dimethyl Sulfoxide
Figure 7. FT-IR spectra of (a) ZrPO4Cldmso, (b) ZrPO4OHdmso‚1.5H2O,
and (c) ZrPO4OHH2O‚(H2O)3.
Figure 5. % of chloride replacement as a function of the time when ZrPO4-
Cldmso is suspended in 5% dmso/water mixture.
The Cl/OH exchange reaction is reversible: by contacting
the solid with a 1.5 M HCl solution in dmso at 75 °C for 24
h, the original chloride phase is restored.
Figure 7 compares FT-IR spectra of ZrPO4Cldmso, ZrPO4-
OHdmso, and ZrPO4OHH2O‚(H2O)3. Note that the bands at
2920, 3000, and 3020 cm-1 related to symmetric and
asymmetric stretching of dmso methyl groups and those at
1420 and 1320 cm-1 related to symmetric and asymmetric
bending of the same groups are present in Cl-dmso and
OH-dmso spectra and no longer visible in OH-H2O, where
dmso was replaced by water.
Bromide Derivatives. The replacement of Cl- with Br-
ions was attempted in order to obtain a compound with a
higher interlayer distance and weaker interactions between
the anions and the inorganic backbone and which is,
therefore, more reactive than ZrPO4Cldmso. Unfortunately,
our attempts to obtain such material by contacting the
chloride form with solutions of bromidric acid in various
conditions were unsuccessful. Therefore, we decided to use
the hydroxyl form, ZrPO4OHdmso‚1.5H2O prepared as
described in the Experimental Section, as precursor, in the
hope that a possible neutralization reaction between the OH-
from the solid and the strong acid from the solution could
force the Br- exchange according to the following reaction:
Figure 6. XRD patterns for (a) ZrPO4OHdmso‚1.5H2O, (b) ZrPO4Brdmso‚
0.5dmso, (c) the same as in (b) after the loss of intercalated dmso when
heated at 140 °C, and (d) ZrPO4HSO4dmso.
as shown in the kinetic curve of Figure 5. At low pH values,
the reaction is strongly inhibited, because of the lowered OH-
concentration. We found that the presence of 0.1 M HCl
completely inhibits such exchange.
The TG curve of the hydroxyl derivative is reported in
Figure 3b. Thermal decomposition proceeds first by the loss
of the intercalation water, then the loss of dmso; finally, the
condensation of phosphates to pyrophosphates is observed.
If we assume the formation of an equimolar mixture of
zirconium oxide and zirconium pyrophosphate at 1100 °C,
the calculated % weight loss at that temperature is in good
agreement with that experimentally found (37.0 vs 35.5).
According to the hypothesis of a topotactic process, the
replacement of chlorides with hydroxyl groups should not
change appreciably the structure of the parent compound.
Although the XRD pattern, reported in Figure 6a, is very
broadened and not suitable for structural analysis, we found
that it is consistent with the following tetragonal unit cell:
a ) 6.56 Å; c ) 9.90 Å. Furthermore, the peaks relative to
the layer framework are still well evident (the 110 and 200
reflections at d ) 4.65 and 3.28 Å, respectively), indicating
that the layers maintained the λ-structure. When the com-
pound is heated at 150 °C and intercalation water is lost,
the interlayer distance decreases from 9.8 to 9.2 Å, a value
close to that of ZrPO4Fdmso. Structural models can show
that in both compounds the interlayer distance depends on
the dmso dimensions because of the small size of both
hydroxyl and fluoride anions.
ZrPO4OHdmso + HBr h ZrPO4Brdmso + H2O
This strategy was successful, and a compound of formula
ZrPO4Br0.8(OH)0.2dmso‚0.5dmso was first prepared by con-
tacting ZrPO4OHdmso‚1.5H2O with a 2 M HBr solution in
dmso.
The full exchange was finally achieved by forcing the
reaction with the use of 3 M HBr and 3 M LiBr in ethanol/
dmso solvent (see Experimental Section). A compound of
formula ZrPO4Brdmso‚0.5dmso was obtained.
Figure 3d shows the TG curve for the bromide derivative.
The dmso is lost in two steps: 0.5 mol of dmso per formula
weight, corresponding to the intercalated solvent in the
interlayer space, is lost at lower temperatures (about 100 °C).
A further 1.0 mol per formula weight, corresponding to the
dmso bonded to zirconium, is lost at about 200 °C. The
compound has an interlayer distance of 13.4 Å, which
decreases to 11.9 Å when heated at 140 °C, because of the
loss of interlayer dmso (Figure 6b,c). Experiments are in
progress to compare the reactivity of such material with that
of the parent compound.
Inorganic Chemistry, Vol. 41, No. 7, 2002 1917

Alberti et al.
Figure 8. XRD pattern for ZrPO4NH4SO4dmso.
The experimental % weight loss at 1100 °C (49.4)
perfectly agrees with that calculated if we assume the
formation of an equimolar mixture of zirconium oxide and
zirconium pyrophosphate.
Figure 9. XRD pattern for (a) ZrPO4(HCrO4)dmso, (b) ZrPO4NaMoO4-
dmso, and (c) ZrPO4NaMoO4dmso after heating at 500 °C in oxygen
atmosphere.
F20 ) 24), and it is undoubtedly λ-type, confirming the
topotactic nature of the anion ligand exchange reaction
involved.
Chromate and Molybdate Derivatives. For potential
applications as catalysts, the preparation of derivatives
containing chromate and molybdate were also investigated.
The reactions
Sulfate Derivatives. Materials containing acid sulfate
groups on their surface are of interest for potential applica-
tions as catalysts or as protonic conductors. The Cl/HSO4
exchange was therefore investigated. Because of the low
affinity of acid sulfate groups for zirconium, it was difficult
to complete the reaction by contacting the parent compound
directly with sulfuric acid. In most of our attempts, we found
that the hydroxyl group was in strong competition with
ZrPO4Cldmso + HCrO4- h ZrPO4HCrO4dmso + Cl-
and
-
HSO4 . The best results achieved led to a 60% replacement
-
of HSO4 , the remaining chlorides being replaced by
hydroxyl groups, ZrPO4(HSO4)0.6(OH)0.4dmso, and attempts
to use more drastic conditions resulted in an increase of OH-
content in the sample.
ZrPO4Cldmso + NaMoO4- h ZrPO4NaMoO4dmso + Cl-
Another approach was therefore used. A 0.1 M chloro-
sulfonic solution in dmso was left to react with ZrPO4-
OHdmso. The complete conversion into ZrPO4HSO4dmso
was achieved by the following reaction:
were carried out successfully.
Figure 9a,b shows XRD patterns of the prepared com-
pounds, respectively. These two patterns can be easily
indexed with the following tetragonal unit cells: a ) 6.616
Å, c ) 12.098 Å for the chromate derivative, and a ) 6.610
Å, c ) 27.83 Å for the molybdate derivative. Note that these
unit cells are λ-type; only, the c parameter for molybdate
derivative is doubled with respect to its interlayer distance.
To confirm the topotactic nature of the anion exchange, the
reactions were followed by XRD analysis. We found that
they proceeded by a progressive increase of the final products
and a simultaneous decrease of the peaks of the parent
compound, without the appearance of any other phases.
The presence of dmso in the structure is also assessed by
FT-IR spectra that show the characteristic absorption bands
of such species.
Both samples showed a peculiar thermal behavior: be-
cause of the redox properties of chromate and molybdate
groups, TG curves were different whether the analyses were
carried out in reducing or oxidizing atmosphere (Figure 10).
Especially for the molybdate derivative, thermal analysis in
oxygen shows an initial weight loss and a characteristic
weight increase, because of an oxidation process leading to
a compound that is thermally stable from 400 to 600 °C.
The XRD pattern of this compound is shown in Figure 9c.
ZrPO4OHdmso + ClSO3H h ZrPO4HSO4dmso + HCl
Figure 3c shows the TG curve for the obtained compound,
while Figure 6d shows its XRD pattern.
At the end of TG analysis, at 1100 °C, the formation of
â-Zr2(PO4)2SO4 was observed, as assessed by the comparison
of the XRD pattern with that from ref 15.
Sulfate derivatives are very sensitive toward hydrolysis:
the presence of 5% v/v of water in dmso is sufficient to
reconvert the solid into the OH-dmso form.
The use of ammonium or sodium sulfate for the ion
exchange allowed us to prepare other sulfate derivatives, in
which the acidic proton of the sulfate group was replaced
by the respective cation. In this case, the topotactic reactions
were performed directly on λ-ZrPO4Cldmso. Probably be-
cause of the presence of the cationic moiety in the interlayer
region, these derivatives were obtained with a relatively high
degree of crystallinity. The XRD pattern of ZrPO4NH4SO4-
dmso (shown in Figure 8) was indexed with the TREOR90
program,10 and cell parameters were determined: the cell is
tetragonal, a ) 6.609(1) Å, c ) 12.660(5) Å (M(20) ) 18,
Conclusion
The present study clearly shows that the chlorides present
on the reactive “mountain” sites of ZrPO4Cldmso can be
(15) Piffard, Y.; Verbaere, A.; Kinoshita, M. J. Solid State Chem. 1987,
71, 121.
1918 Inorganic Chemistry, Vol. 41, No. 7, 2002

Zirconium Phosphate Chloride Dimethyl Sulfoxide
environment around functional groups introduced in these
compounds. This feature could be of interest for the
preparation of tailor-made materials for application in
catalysis, ionic or molecular recognition, and ionic or
protonic conductivity. Finally, preliminary experiments
showed that dmso can be also replaced with other neutral
ligands. This opportunity, along with the possibility to
introduce a large number of anionic organic ligands with
specific properties, makes λ-ZP a very flexible and intriguing
compound for materials chemistry.
Acknowledgment. This work was supported by MURST
Progetti Nazionali 1999 and CNR.
Figure 10. TG curve for (a) ZrPO4NaMoO4dmso in oxygen, (b) ZrPO4-
NaMoO4dmso in nitrogen, (c) ZrPO4(HCrO4)dmso in oxygen, and
(d) ZrPO4(HCrO4)dmso in nitrogen. Gas flux 40 cm3/min; heating rate
5 °C/min.
Supporting Information Available: Rietveld refinement de-
tails, Rietveld plot, tables of crystallographic data, structural
parameters, and selected bond lengths and angles for ZrPO4Cldmso.
This material is available free of charge via the Internet at
http://pubs.acs.org.
topotactically replaced by a variety of inorganic ligands, thus
originating a large family of λ-inorganic derivatives. Because
the layered structure does not change appreciably during
anionic ligand exchange, it is possible to predict the
IC010643V
Inorganic Chemistry, Vol. 41, No. 7, 2002 1919