Introduction
Thermal treatment of inorganic phosphate substances
has a great synthetic potential as it may turn simple
compounds into advanced materials, such as ceramics,
catalysts and glasses. The mechanism and kinetics
studies of solid-state reactions are needed in order to
take advantage of this potential [1–3]. In this respect,
amorphous AlPO
4
has attracted the interest of many
researchers due to the realization that AlPO
4
is com-
pletely iso-structural with silica and exhibits parallel
polymorphic transformation [4]. However, the rea-
sons of high catalytic activity remain a point of con-
tention. Thus, in the last few years many works have
undertaken a series of research studies on the synthe
-
sis, characterization and catalytic activity of different
AlPO
4
[4]. The discovery of aluminum phosphate mo
-
lecular sieves [5] generated a flurry of activity in molec
-
ular sieve synthesis. As new structural types began to
emerge, there was renewed interest in family of alumi
-
num phosphate referred to as the H series discovered by
D’Yoire four decades earlier [6]. Structural character
-
ization of the aluminum phosphates H1 through H4,
which were generally all thought to be molecular
sieves [5], is incomplete. The aluminum phosphate
AlPO
4
·H
2
O-H4 was first reported in 1961 [6], how
-
ever, the crystal structure was determined only re
-
cently [5]. AlPO
4
H
2
O-H4 is not a molecular sieve but
rather an interesting condensed aluminum phosphate
phase having half of the aluminum atoms four coordi
-
nates and the other half six coordinates. The key fac-
tor in obtaining AlPO
4
with different properties has
been the use of different preparative methods. In the
literature, AlPO
4
have been prepared with thermal de-
composition of AlPO
4
·2H
2
O, AlPO
4
·H
2
O-H1-3 and
AlPO
4
-21 precursors [7–10].
In the present study, the formation of AlPO
4
from AlPO
4
·H
2
O-H4 was followed using differential
thermal analysis-thermogravimetry (TG-DTG/DTA),
X-ray powder diffraction (XRD), Fourier transform-
infrared (FTIR) and Fourier transform-Raman spectro-
scopy (FT-Raman). The kinetics analysis of the
non-isothermal results for the aluminum phosphate
decomposition steps were carried out using the iso
-
conversional methods of Ozawa [11] and
Kissinger-Akahira-Sunose (KAS) [12] and the
possible conversion functions had been estimated
through the comparative methods. In the literature
there is no report on the thermal decomposition
kinetics of AlPO
4
·H
2
O-H4. So the aim of this work is
to investigate the kinetic parameters of dehydration of
AlPO
4
·H
2
O-H4.
Experimental
AlPO
4
·H
2
O-H4 was prepared according to the previous
methods [5, 13] by stirring 1.096 g of Al
2
O
3
,2.27mL
of 86.4 mass/mass% H
3
PO
4
and 9.9 mL of deionized
water for 2 h followed by the addition of 0.77 mL of
1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary
© 2008 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands
Journal of Thermal Analysis and Calorimetry, Vol. 91 (2008) 2, 511–516
SYNTHESIS OF AlPO
4
AND KINETICS OF THERMAL DECOMPOSITION
OF AlPO
4
·H
2
O-H4 PRECURSOR
B. Boonchom
1,2
, S. Youngme
1
, T. Srithanratana
1
and C. Danvirutai
1*
1
Department of Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand
2
King Mongkut’s Institute of Technology Ladkrabang Chumphon Campus, 17/1 M. 6 Pha Thiew District, Chumphon 86160, Thailand
The thermal decomposition of aluminum phosphate monohydrate AlPO
4
·H
2
O-H4 was investigated in air using TG-DTG/DTA. The
AlPO
4
·H
2
O-H4 decomposes in one step and final decomposition product (AlPO
4
) was studied by X-ray powder diffraction, FTIR
and FT-Raman spectroscopy. The activation energies of dehydration reaction of AlPO
4
·H
2
O-H4 were calculated through the
isoconversional methods of Ozawa and Kissinger–Akahira–Sunose (KAS), and the possible conversion functions have been esti
-
mated through the comparative methods. The activation energy calculated for the decomposition of AlPO
4
·H
2
O-H4 by different
methods and techniques were found to be consistent. The kinetic model that better described the reaction of dehydration for
AlPO
4
·H
2
O-H4 was the F
n
model as simple n-order reaction and the corresponding function is f(a)=(1–a)
2.75
and
g(a)= –[1–(1–a)
–1.75
/(1.75)].
Keywords: AlPO
4
·H
2
O-H4, aluminum phosphate, non-isothermal dehydration kinetics
* Author for correspondence: chanai@kku.ac.th