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This publication was processed (scanned, OCR'ed and manually corrected) by Andrew Wong. It has not yet been proofread. The original is in Solid State Ionics, 63-1993, 391- Hydrothermal
synthesis of new metastable phases preparation and intercalation of a
new layered titanium phosphate Yingjeng James Li and M. Stanley
Whittingham Department
of Chemistry and Materials Research Center, State University of New
York at Binghamton, Binghamton, NY 13902-6000, USA We have investigated
the formation and reactivity of new metastable phases using mild hydrothermal
synthesis. Following the characterization of two new open structure
sodium tungsten oxides, we found that by using a small cationic template
we were able to synthesize a new layered structure titanium phosphate.
Preliminary chemical analysis of this compound indicates that its hydrogen
form has the formula TiO(OH)(H2PO4)·2H20.
31P MAS-NMR indicates that each phosphorus has two OH groups.
This solid acid is readily swelled on reaction with organic amines.
1.
Introduction There has been
much interest in recent years in generating a rational approach to the
synthesis of inorganic compounds, so that new structures with the desired
optimum chemical and physical properties can be formed on demand. Intercalation
chemistry, and "chimie douce" in general, is one such approach to rational
synthesis. We have recently been exploring the use of mild hydrothermal
synthesis, following the techniques of the zeolite chemist, to synthesize
new metastable structures of well known compounds such as the sodium
tungstates. By careful control of the synthesis conditions, such as
the pH, it is possible to control the solid structure finally formed.
Thus, when acidified sodium tungstate solutions are hydrothermally treated,
a hexagonal form is found for 1.5≤pH≤2.0, whereas a pyrochlore
structure is found in the pH range of 3.5 to 4.5 [1-3]. These relatively
open crystalline lattices are metastable, reverting to the well-known
high temperature phases on thermal treatment. We decided to extend these
hydrothermal studies to use larger templating cations, such as the tetramethyl
ammonium ion, to determine if this might lead to new interesting layer
structures as has been found in the molybdenum phosphates [4]. Our initial
effort has been focused on the tungsten oxide system [5] and on titanium
phosphates; in both cases, new crystalline forms were found, and we
report here some initial data on a titanium phosphate phase. Layered compounds
that undergo intercalative redox reactions and ion-exchange reactions
can be classified into three broad categories. The first category contains
the single sheet structures of graphite and boron nitride. The second
category comprises such compounds as the transition metal oxides and
dichalcogenides, MS2, which have readily reducible cations
and have found extensive use as electrodes for energy storage, in electrochromic
displays and as ionically reversible electrodes for electrochemical
measurements. The third group is much less prone to undergo redox behaviour,
but have an extensive ion-exchange and solvation chemistry. This category
contains such materials as the beta-aluminas, many naturally occurring
minerals such as mica, vermiculite and montmorillonite and synthetic
transition metal phosphates, arsenates or silicates such as uranyl phosphate
(HUP) [6] and zirconium arsenate (α-ZrAs) [7]. This third category
of layered compounds has been extensively investigated as potential
solid electrolytes and for ion exchange. Many of these materials tend
to have relatively open crystalline lattices or closer packed lattices
that are easily expanded. Several layered
phosphates of the group IVB transition metals have been reported [8]
and they crystallize in two general types of layered structures. Examples
are α-zirconium phosphate (α-ZrP) of composition Zr(HP04)2·H20
and γzirconium phosphate (γ-ZrP) of composition Zr(P04)•(H2PO4)•2H20.
The a phase compound has been well studied because it was synthesized
a long time ago and its structure is known [9]. By replacing the -OH
group of o-phosphoric acid with an organic species -R or -OR,
many derivatives of α-ZrP have been prepared [10-12]. Many members
of the α-ZrP family, including α-ZrP itself, have received
much attention because they show ion exchange, molecular intercalation,
hydrogen conductivity, catalytic or other properties [13-15]. The other
phase, γ-ZrP, has received less attention because its complete
structure remains unknown, although a model has been proposed [16,17].
The titanium analog, α-TiP [18], has the same type of structure
as α-ZrP. The gamma phase of titanium phosphate, γ-TiP [19],
was reported to have the same type of structure as that of γ-ZrP.
However, much less is known about the titanium compounds than those
of zirconium. The titanium compounds have a potentially richer chemistry,
because titanium exhibits an extensive redox behavior, whereas for zirconium
only the +4 state is readily attained with reduction going directly
in most cases to the metal. Here we report the preparation and preliminary
characterization of a new layered titanium phosphate phase, TiO(OH)(H2PO4)•2H20,
or named HTP (the hydrogen form of layered titanium phosphate) as it
was obtained through hydrogen cation exchange of a precursor compound,
named NMe4TP. 2.
Experimental NMe(sub)4TP was
synthesized in a mild hydrothermal process. Since the parameters in
a hydrothermal reaction are many, there is no unique condition for the
preparation of NMe4TP. A typical procedure is described as
follows: 7.292 g (0.020 mole) of 25% tetramethylammonium hydroxide solution,
0.799 g (0.0010 mole) of titanium dioxide in the anatase form and 5.765
g (0.050 mole) of 85% orthophosphoric acid were mixed and introduced
into a Teflon lined stainless steel sample preparation bomb (Parr 4744).
The bomb was sealed and heated in an oven at 160°C for three days. After being cooled,
the bomb was opened and its content filtered. The white polycrystalline
product was washed several times with distilled water and dried in air.
HTP was obtained by placing NMe4TP in concentrated hydrochloric
acid at room temperature for five days. A Scintag X-ray
diffractometer with Cu Kα radiation was used to characterize the
compounds formed. Thermogravimetric analysis was performed at 1°C/min in a nitrogen atmosphere on
a Perkin-Elmer TG7 instrument. Quantitative analysis for Ti and P was
carried out using a Fison's 55-7 DCP-AES. 3.
Results and discussion The powder X-ray
diffraction (XRD) pattern of the NMe4TP material, fig. 1a,
shows (00l) reflections with peaks of alternating intensity.
The interlayer distance was calculated to be 11.4 Ǻ using linear
regression from all the (00l) reflections. The HTP phase, obtained
by immersing NMe4TP in concentrated hydrochloric acid, had
a smaller repeat distance of 10.0 Ǻ. Its powder XRD pattern, fig.
1b, also differs from that of the NMe4TP in that the (00l)
reflections have intensities decreasing monotonically with l. Thermogravimetric
analysis (TGA) of the HTP phase shows various amount of weight loss
below 120°C. The amount of weight loss depends
on the condition in which the compound was stored. Thus, fully hydrated
samples (materials stored under 100% relative humidity for longer than
a month) show two steps of weight loss, as indicated in fig. 2. Each
corresponds to one water molecule per formula unit; one molecule of
water comes out below 400C and the other between 60°C and 120°C. The lattice repeat distance decreased
to 8.9 Ǻ as these two water molecules were lost. The DCP-AES analysis
indicated that HTP contains 27.1% wt. of titanium and 17.8% wt. of phosphorus
in the dehydrated sample indicating that the mole ratio of Ti:P is 1:1.
The 31P magic angle spinning solid state NMR spectrum, fig.
3, of the HTP sample has a resonance of about -7.5 ppm#1
clearly Fig.
1. Powder XRD pattern of (a) NMe4TP; (b) HTP and (c) octylamine
intercalated HTP. showing
that two of the oxygens in the P04 groups are present as
OH groups free from structural framework [20,21]. This is in contrast
to the α- and γ-phosphates, which show shifts at -18 and -32.5
ppm respectively as listed in table 1. Based on the above and the following
observations the chemical formula of HTP is best proposed as TiO(OH)(H2P04)•2H20:
(1) ortho-phosphoric acid does not tend to condense into pyro-phosphate
under hydrothermal condition at temperatures below 160°C; (2) the TGA weight loss on TGA
does Fig.
3. 31P magic angle spinning solid state NMR spectrum of HTP. not
correspond to thermal decomposition of N(CH3)4+
suggesting that the tetramethylammonium ion has been entirely exchanged
by protons during the preparation of HTP; and (3) the energy dispersive
spectrum on an electron microprobe as well as precipitation with silver
cation showed no trace of chloride indicating that chloride ions are
not incorporated into HTP during the ion exchange in hydrochloric acid. Similar to that of many other solid
acids with layer structures, HTP was found to react readily with organic
bases giving molecular intercalates. Thus, for example n-alkyl
amines with carbons numbers be- Table
1 31P NMR shifts in titanium phosphates.
tween
5 and 14, i.e.. 5≤n≤14, can be easily intercalated
into HTP by direct contact of the pure amines with the solid at 75°C for around three days. The powder
XRD patterns of these intercalates show (00l) reflections, as
exemplified by the octylamine complex in fig. 1c. The interlayer distances
of the intercalates are listed in table 2, and as shown in fig. 4 the
interlayer distances increase linearly with the Table
2 Repeat
distances of n-alkylamine intercalated HTP.
Fig.
4. Relation between n-alkylamines and the interlayer distances
of n-alkylamine intercalated HTPs. number
of the carbons in the amine. This degree of expansion is consistent
with a bilayer amine conformation with the chains tilted at an angle
of 54.9° with respect to the layer basal
plane. The titanium to phosphorus ratio
as well as the MAS-NMR and XRD results clearly indicate that HTP is
a new layered titanium phosphate phase that differs from both α-TiP
and γ-TiP. As HTP possesses two active sites on each phosphorus
atom, it promises the possibility of preparing pillared derivatives
by substituting -R or -OR for one or more hydrogen(s) on H3P04
during the synthesis of HTP. We recently prepared a compound with an
interlayer distance of about 15 Ǻ using phenylphosphinic acid instead
of ortho-phosphoric acid. Therefore, a new family of layered titanium
phosphate can be expected and applications on them might follow. We
are presently investigating more of the fundamental properties of HTP
and proposing a model for its structure. #1
In fact, two peaks, at -6.47 and -8.39 ppm, were observed. This might
be explained as due to chemically similar but crystallographically inequivalent
phosphorus atoms. Acknowledgement We appreciate the fruitful discussions
with Professors G. Alberti of Perugia University and M.E. Thompson of
Princeton University. Thanks are also due to the help from M.E. Thompson
for the 31P MAS, solid state NMR measurement, and D.R. Naslund
for the use of the DCP-AES. This work was supported in part by the National
Science Foundation, DMR-8913849, and the Petroleum Research Fund, administered
by the American Chemical Society. References [1] K.P. Reis, A. Ramanan and M.S. Whittingham,
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and F.W. Lai, Science 246 (1989) 1289. [5] J. Guo, work in progress. [6] V. Pekarek and M.J. Benesova, Inorg.
Nucl. Chem. 26 (1964) 1743. [7] E. Torracca, U. Costantino and M.A.J.
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8 (1987) 1877. This page was last
updated on 14-Feb-2001 by Arne
Hessenbruch. |
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