On the synthesis and characterization of layered antimony(III) phosphate and its interaction with moist ammonia and amines

Article abstract of DOI:10.1016/S0025-5408(98)00094-4

Samples of antimony(III) phosphate, a layered material, were prepared at room temperature, using the two allotropic forms of Sb₂O₃, layered valentinite and cubic senarmontite. The compounds were characterized by infrared spectroscopy, X-ray diffraction, and thermogravimetric-differential thermal analysis. Solids exposed to ammonia, ethylenediamine, or hydrazine vapors also were studied. The interaction between SbPO₄ and the bases always led to the formation of stoichiometric amounts of Sb₂O₃ and hydrogen phosphate(-2) salts of the bases. With ammonia, the SbPO₄ raw materials gave rise to valentinite or senarmontite, depending on which Sb₂O₃ allotropic form was used to prepare antimony phosphate. This effect was modified when the SbPO₄ preparations were preheated at 600°C, in which case, a mixture of both forms was obtained. With ethylenediamine and hydrazine, SbPO₄ of whatever origin always gave rise to senarmontite. No evidence of intercalation in the interlayer region of SbPO₄ was obtained.

Full text of DOI:10.1016/S0025-5408(98)00094-4

Materials Research Bulletin, Vol. 33, No. 8, pp. 1221–1231, 1998
Copyright © 1998 Elsevier Science Ltd
Printed in the USA. All rights reserved
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025-5408/98 $19.00 ϩ .00
PII S0025-5408(98)00094-4
ON THE SYNTHESIS AND CHARACTERIZATION OF LAYERED
ANTIMONY(III) PHOSPHATE AND ITS INTERACTION WITH MOIST
AMMONIA AND AMINES
1
2
3
3
4
1
G. Alonzo *, N. Bertazzi , P. Galli , M.A. Massucci , P. Patrono , and F. Saiano
1
Dipartimento di Economia, Ingegneria e Tecnologie Agrarie (EITA), Universit a` di
Palermo, Viale delle Scienze, 90128 Palermo, Italy
Dipartimento di Chimica Inorganica, Universit a` di Palermo, Via Archirafi, 26, 90123
Palermo, Italy
2
3
Dipartimento di Chimica, Universit a` “La Sapienza,” P. le Aldo Moro, 5,
0
0185 Rome, Italy
4
IMAI-CNR, Area della Ricerca di Roma, Via Salaria, Km 29.6, 00016 Monterotondo
Stazione, Rome, Italy
(Refereed)
(Received August 19, 1997; Accepted December 2, 1997)
ABSTRACT
Samples of antimony(III) phosphate, a layered material, were prepared at
room temperature, using the two allotropic forms of Sb O , layered valentinite
2
3
and cubic senarmontite. The compounds were characterized by infrared spec-
troscopy, X-ray diffraction, and thermogravimetric–differential thermal anal-
ysis. Solids exposed to ammonia, ethylenediamine, or hydrazine vapors also
were studied. The interaction between SbPO and the bases always led to the
4
formation of stoichiometric amounts of Sb O and hydrogen phosphate(–2)
2
3
salts of the bases. With ammonia, the SbPO raw materials gave rise to
4
valentinite or senarmontite, depending on which Sb O allotropic form was
2
3
used to prepare antimony phosphate. This effect was modified when the
SbPO preparations were preheated at 600°C, in which case, a mixture of both
4
forms was obtained. With ethylenediamine and hydrazine, SbPO of whatever
4
origin always gave rise to senarmontite. No evidence of intercalation in the
interlayer region of SbPO was obtained. © 1998 Elsevier Science Ltd
4
*
To whom correspondence should be addressed.
1
221

1
222
G. ALONZO et al.
Vol. 33, No. 8
KEYWORDS: A. inorganic compounds, B. chemical synthesis, C. infrared
spectroscopy, C. thermogravimetric analysis (TGA), C. X-ray diffraction
INTRODUCTION
Layered inorganic ion-exchangers, such as clays and insoluble acid phosphates of tetravalent
metals, can generally be inserted between the layers of transition metal ions, coordination
compounds, polyoxocations, and molecules with polar character [1]. The main interest in
producing these materials is to obtain either catalysts or catalyst precursors to be used in
different processes (e.g., petrochemical, oxidative dehydrogenation, carbonylation) or mate-
rials for gas separation, ionic conduction, etc. Other layered inorganic compounds without
ion-exchange properties (e.g., VOPO ⅐nH O, n ϭ 0–2) behave as hosts for various polar
4
2
organic molecules, and materials or composites are produced with new interesting properties,
such as anisotropic electrical conduction [2].
During our investigations on the preparation of layered compounds to be used as such or
suitably modified (chemically and/or physically) in catalysis [1,3] or other processes, our
interest was attracted by the recent claim [4] of successful intercalation reactions of a number
of amines into the layers of antimony(III) phosphate, SbPO , and the possibility of antimo-
4
ny(III) being directly involved in bonding interactions with the guest molecules. To gain
further insight into the intercalation behavior of SbPO , we planned an investigation of this
4
potential guest material by studying its interaction with a simple base such as ammonia,
following the process by means of various techniques. Because we found that the behavior
toward NH was such as to suggest conclusions different from those reported in ref. 4, we
3
deemed it appropriate to consider experiments with other bases (e.g., ethylenediamine and
hydrazine), which were among the amines studied by Saadaoui et al. [4].
EXPERIMENTAL
Materials. Sb O is usually sold without any reference to the particular allotropic form. We
2
3
found the Sb O supplied by Carlo Erba to be valentinite (layered, orthorhombic [5]),
2
3
whereas that supplied by Fluka appeared to be senarmontite (cubic [6]). Solid H PO (99%)
3
4
and H PO (85%) and NH (30%) solutions were from Carlo Erba ACS RPE, all of the
3
4
3
highest purity. Ethylenediamine (en) (Merck “pro analyst,” 99.5%) and hydrazine (hy)
(monohydrate, Aldrich, 98%) were used.
Antimony(III) phosphate was prepared at room temperature (rt) according to a general
method [7,8] and following our specifications: 21 g of Sb O are mixed with 240 mL of a
2
3
Ϫ3
H PO 6 mol dm solution (PO /Sb molar ratio ϭ 10) and the slurry is left to react, under
3
4
4
continuous stirring, for 5 days. The suspension is filtered off and the solid is washed with
distilled water until a pH of 4.5–5 is reached and then dried in an oven at 130°C.
Both valentinite and senarmontite dried at 130°C gave rise to SbPO samples for which the
4
infrared (IR) spectra, thermogravimetric–differential thermal analysis (TG-DTA) curves, and
X-ray diffraction (XRD) data appeared identical. However, a careful comparison with the
X-ray pattern of SbPO reported in ref. 7 gave evidence of significant differences in the peak
4
intensities. When our samples were heated at 600°C for 24 h, these differences were
cancelled out and the results thus obtained agreed well with the literature data [7]. To
differentiate the various SbPO materials, those prepared from valentinite or senarmontite
4

Vol. 33, No. 8
LAYERED ANTIMONY PHOSPHATE
1223
and dried at 130°C are denoted as SbPO (v, 130) and SbPO (s, 130), respectively, while
4
4
SbPO (600) denotes the antimony(III) phosphate of both origins, but heated at 600°C.
4
In order to unambiguously identify (enH )HPO in the IR and X-ray patterns of the SbPO
2
4
4
samples treated with ethylenediamine, we prepared this salt by mixing the base and solid
H PO in a 1:1 molar ratio. The white compound obtained was ground to a fine powder and
3
4
submitted to XRD and IR analysis for identification.
The amine-reacted materials were obtained by exposing the SbPO samples to the vapors
4
of aqueous concentrated solutions of the bases in a closed glass container. The system was
initially evacuated and then left to react until completion (i.e., until the total disappearance
of SbPO from the solid).
4
Chemical Analyses and Physical Measurements. The PO /Sb molar ratio was determined
4
as follows: 100 mg of the solid were refluxed in 25 mL of concentrated H SO until complete
2
4
dissolution and then diluted to 100 mL with distilled water. Antimony(III) was determined
Ϫ3
by redox potentiometric titration with a KBrO 0.05 mol dm solution, and the phosphate
3
ions were analyzed by conventional absorption spectrophotometry as phosphomolybdates
[9]. Amine-loaded materials were washed with distilled water, and the soluble phosphate was
determined spectrophotometrically. A Metrohm Erisau potentiograph model E-536 was used
for the potentiometric titrations of antimony. The phosphate determination was performed on
a Perkin-Elmer 550S spectrophotometer at a wavelength of 430 nm.
A Philips diffractometer was used to monitor phase changes. Nickel-filtered Cu K␣
radiation was used, and the measurements of 2␪ are believed to be accurate to 0.05°. A
Stanton Redcroft simultaneous TG-DTA STA-781 model was used to study the thermal
behavior of the various materials and to evaluate their amine and water content (heating rate
Ϫ1
5
°C min. , ignition up to 900°C to constant weight, in an air flow).
Infrared measurements were performed using KBr pellets on a Bruker IFS 25 spectropho-
tometer equipped with KBr optics.
RESULTS AND DISCUSSION
Antimony(III) phosphate is a layered solid with the layers stacked along the a axis of a
monoclinic cell with an interlayer spacing of 5.0868 Å [10]. Antimony atoms are located on
the outermost side of the layers and bisphenoidally coordinated by four oxygens of different
PO tetrahedra. If a stereoactive lone pair of electrons is considered on the interlayer side, the
4
overall coordinating geometry about Sb can be described as distorted trigonal bipyramidal
with equatorial lone pair. The availability (in principle) of the metal at the interlayer renders
this phosphate matrix atypical and attractive, raising the question of whether Lewis-base
guests could directly interact with the metal or whether the copresence of other guests (e.g.,
water) is required.
The infrared spectrum of SbPO (s, 130) (Fig. 1) shows the characteristic features of
4
crystalline ortophosphates dominated by the group of strong ␯(P–O) bands centered at about
Ϫ1
Ϫ1
1
050 cm (1145, 1052, and 973 cm ) and by the absence of absorptions in the 900–650
Ϫ1
Ϫ1
cm region [11]. The lack of bands at frequencies higher than 1150 cm is in agreement
with the crystal structure [10], showing all the phosphate oxygens engaged in P–O–Sb
bridges; this indicates that no phosphoric acid impurities were trapped in the material during
the synthesis. Contributions from ␯(SbO) and ␦(PO ) are expected to produce the lower
4

1
224
G. ALONZO et al.
Vol. 33, No. 8
FIG. 1
Infrared spectrum of SbPO (s, 130).
4
Ϫ1
frequency pattern (664, 590, 500, 475, and 372 cm ). The thermal treatment of SbPO (s,
4
1
30 or v, 130) up to 900°C did not appear to affect its IR spectra.
Reaction of SbPO with NH –SbPO (v, 130) and SbPO (s, 130). Samples exposed to
4
3
4
4
ammonia vapors reacted immediately (few hours). The IR spectra of the products are shown
Ϫ1
in Figure 2. A new feature, assigned to ␯(P ϭ O), appears at 1199 cm , together with bands
Ϫ1
characteristic of the salt (NH ) HPO (860 s, 558 m, 534 w, and 445 m cm ), and the region
4
2
4
Ϫ1
at about 1050 cm appears deeply modified. Worthy of note is the appearance of a new band
at 690 cm (Fig. 2a, band indicated with v) for the material obtained from SbPO (v, 130)
and at 735 cm (Fig. 2b, band indicated with s) for that obtained using SbPO (s, 130).
These two absorptions at 690 and 735 cm , assigned to n(SbO), characteristically differ-
Ϫ1
4
Ϫ1
4
Ϫ1
entiate the valentinite and senarmontite allotropic Sb O forms, respectively [12]. What was
2
3
observed points to antimony phosphate matrices retaining a preference to restore the starting
antimony oxide, a preference which was lost when the experiment was repeated with SbPO4
(600). In this case, however, the IR spectrum of the ammonia-treated material always showed
both the v and s bands, with approximately equal intensity (Fig. 2c). These findings were
fully confirmed by the XRD data collected for the same materials and shown in Figure 3. In

Vol. 33, No. 8
LAYERED ANTIMONY PHOSPHATE
1225
FIG. 2
Infrared spectra of SbPO samples after exposure to ammonia vapors: (a) SbPO (v,130); (b)
4
4
SbPO (s,130); (c) SbPO (600).
4
4

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226
G. ALONZO et al.
Vol. 33, No. 8
FIG. 3
XRD patterns of SbPO samples after exposure to ammonia vapors: (a) SbPO (v, 130); (b)
4
4
SbPO (s, 130); (c) SbPO (600).
4
4
all cases, the XRD patterns show the presence of the (NH ) HPO (with the two most intense
4
2
4
reflections at 2␪ ϭ 17.55 and 17.95), together with valentinite (Fig. 3a, the most intense
reflection at 2␪ ϭ 28.49, d ϭ 3.14 Å), or senarmontite (Fig. 3b, 2␪max ϭ 27.72, d ϭ 3.22 Å),
or mixed valentinite and senarmontite (Fig. 3c).
The three NH -treated materials washed with distilled water completely lost (NH ) HPO ,
3
4 2
4
and the phosphate ions were found in the expected amount in the contact solutions. Pure
valentinite, senarmontite, or a mixture of both was the only recovered residual solid, as
shown by the XRD and IR analyses.

Vol. 33, No. 8
LAYERED ANTIMONY PHOSPHATE
1227
FIG. 4
TG-DTA of SbPO (600) after exposure to ammonia vapors.
4
The reaction between SbPO and NH vapors, with water necessary to the whole process,
4
3
can be written as
2
SbPO ϩ 4NH ϩ H O ϭ Sb O ϩ 2(NH ) HPO
4
(1)
4
3
2
2
3
4
2
Thermal analysis data recorded for all the SbPO –ammonia reacted materials are in accord
4
with reaction 1. Figure 4 shows the TG-DTA curves for SbPO (600) after contact with NH .
4
3
The DTA curve shows three endothermic effects up to 400°C. The first two, evidenced by
sharp, well-resolved peaks with maxima at 150 and 180°C, respectively, correspond to the
FIG. 5
XRD spectra of SbPO (600) after exposure to ethylendiamine vapors.
4

1
228
G. ALONZO et al.
Vol. 33, No. 8
FIG. 6
IR spectra of SbPO (600) after exposure to ethylenediamine vapors.
4
loss of ammonia, as is confirmed by the absence of the stronger (NH ) HPO reflections in
4
2
4
the XRD spectrum of the material heated at 200°C and the presence of those of Sb O in both
2
3
allotropic forms. At this temperature the formation of SbPO from Sb O and H PO begins
4
2
3
3
4
and its stronger XRD reflections can be observed, although at a very low intensity. The
presence of SbPO means that at 200°C water begins to be lost, although maximum water
4
elimination occurs in the range 250–400°C; i.e., in correspondence with the third broad
endothermic peak. The TG curve indicates an overall mass loss of 22.1%, in accord with the
calculated loss of 21.57% for the elimination of water and ammonia in reaction 1. The small
exothermic peak at 480°C is attributed to the recrystallization of SbPO , which was recov-
4
ered at the end of the experiment. Thus, during heating, the reverse of reaction 1 is
accomplished; hence, the process can be seen as an easy solid-state route to SbPO (600)
4
formation.
Reaction of SbPO with Ethylendiamine and Hydrazine. Exposure of SbPO to the
4
4
vapors of ethylenediamine results in a mixture of Sb O and the hydrogen phosphate(–2) salt
2
3
of the base, although long contact times (60 days) are required for the reaction to reach
completion. Antimony(III) oxide was invariably found as senarmontite regardless of the

Vol. 33, No. 8
LAYERED ANTIMONY PHOSPHATE
1229
FIG. 7
TG-DTA of SbPO (600) after exposure to ethylenediamine vapors.
4
origin of SbPO . Figure 5 shows the X-ray pattern of SbPO (600) exposed to the base
4
4
vapors. The characteristic reflections of senarmontite are accompanied by those of
enH )HPO , the most intense of which are at angles 2␪ ϭ 20.69, 23.70, and 25.31. The IR
(
2
4
Ϫ1
spectrum given in Figure 6 clearly shows the senarmontite characteristic band at 735 cm
the ␯(NH) stretchings, and, in the phosphate region, the bands relative to the salt (enH )HPO
at 1026 m, 961 m, 898 s, and 827 m cm . The phosphate salt of the base was removed when
the reacted material was washed with distilled water, leaving the solid senarmontite. The TG
analysis gave an overall mass loss of 29.74%. This agrees with the decomposition of a solid
of the type Sb O ⅐2(enH )HPO , which loses ethylenediamine and water in an amount
,
2
4
Ϫ1
2
3
2
4
calculated as 28.67%, to give SbPO . The advanced reaction scheme implying a di-proto-
4
nation of ethylenediamine is as follows:
2
SbPO ϩ 2en ϩ 3H O ϭ Sb O ϩ 2(enH ) HPO
4
(2)
4
2
2
3
2
The thermal behavior of the product of reaction 2 is shown in Figure 7. It appears rather
complex, presenting four steps of mass loss in the TG curve and the same number of thermic
effects in the DTA. The two endothermics in the ranges 150–280 and 280–350°C are related
to the removal of ethylenediamine. Of the two exothermics (430–550 and 550–700°C), the
first is linked, as for ammonia, to the recrystallization of SbPO and the second to the
4
elimination of the carbonaceous organic residuals.
The complete reaction between SbPO and hydrazine vapors required 10 days equilibra-
4
tion of the solid over N H ⅐H O and, as in previous cases, the resulting material was a
2
4
2
mixture of Sb O and the hydrogen phosphate(–2) salt of the base. SbPO (v, 30) was chosen
2
3
4
for the experiment, and Sb O was found only in the senarmontite modification, as observed
2
3
Ϫ1
from IR spectra (showing the strong feature at 735 cm ). The XRD pattern of the reacted
material corresponds to that of pure senarmontite [6], shown in Figure 5 without the
(
enH )HPO reflections, pointing to an amorphous hydrazine–hydrogen phosphate salt.
2 4
Water completely dissolved the latter, and pure senarmontite was recovered. The TG analysis
of the reacted compound (Fig. 8) gave a total mass loss of 29.48%, in agreement with the

1
230
G. ALONZO et al.
Vol. 33, No. 8
FIG. 8
TG-DTA of SbPO (v, 130) after exposure to hydrazine vapors.
4
presence of a monoprotonated hydrazine phosphate salt, (hyH) HPO , for which the calcu-
2
4
lated loss is 30.22%. The decomposition occurred in two main stages, both accompanied by
endothermic effects. The percent values of the partial mass losses correspond to the elimi-
nation of one mole of hydrazine per mole of final compound in the range 50–180°C and a
second mole of hydrazine plus one mole of water per mole of compound in the range
1
80–400°C. The advanced reaction scheme is thus
SbPO ϩ 2hy ϩ 3H O ϭ Sb O ϩ 2(hyH) HPO
4
2
(3)
4
2
2
3
2
The monoprotonation of hydrazine appears justified after considering the very weak second
protonation constant, pK ϭ 15.
CONCLUSIONS
Interaction of ammonia, ethylenediamine, and hydrazine, in the vapor phase, with SbPO (v,
4
1
30), SbPO (s, 130), and SbPO (600) always resulted in Sb O and hydrogen phos-
4 4 2 3
phate(–2) salt of the corresponding base, with adventitious water molecules playing a key
role in the process. Although it cannot be excluded that an initial reaction stage involving the
intercalation of these bases in the interlayer region of whatever SbPO samples give rise to
4
species (we were unable to detect) that are very reactive toward moisture, we do not have any
evidence from chemical, thermal, IR, or XRD data about the formation of new layered
materials.
The structural difference between SbPO (v, 130) and SbPO (s, 130) was not detectable
4
4
by IR or XRD techniques and was only observed in their reaction with ammonia, which
restores the original different oxides. This matter is further complicated by the behavior of
SbPO (600), which, in the same reaction, gives a mixture of both valentinite and senar-
4
montite and the fact that interaction of ethylenediamine and hydrazine with SbPO material,
4
regardless of its origin, always leads to the formation of senarmontite.

Vol. 33, No. 8
LAYERED ANTIMONY PHOSPHATE
1231
The XRD patterns of the solids resulting from the interaction of SbPO with ethylenedi-
4
amine and hydrazine reported in ref. 4 appear to have been misinterpreted, because they are
very similar to ours and show unambiguously the presence of Sb O in the senarmontite
2
3
modification. The latter, often contaminated by unreacted SbPO , seems to be present in
4
solids obtained from experiments with the other organic bases reported in ref. 4.
ACKNOWLEDGMENTS
The authors thank the Ministero dell’Universit a` e della Ricerca Scientifica e Tecnologica
(MURST), Rome, for financial support.
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