Radiochemical diagnostics of thermal hydrolysis of aluminum trichloride

Article abstract of DOI:10.1007/s11137-005-0017-6

Thermal hydrolysis of aluminum chloride was studied using ³⁶Cl and ³H tracers in combination with energy-dispersive and morphological (scanning electron microscopy) analyses. Pure partially hydrated AlCl₃ or mixtures of AlCl₃ and AlCl₃· 6H₂O were used as initial materials. It was found that Al ₇O₁₀Cl·2H₂O microtubes along with isometric microparticles are formed in the course of thermal hydrolysis due to evaporation and partial hydrolysis of the initial AlCl₃. In this case, AlCl₃·6H₂O acts as a source of water vapor, whereas chlorine atoms contained in this compound do not pass into the microtube material. The tests with water labeled with tritium also showed no incorporation of tritium in microtubes. Thus, thermal hydrolysis of AlCl ₃ proceeds predominantly by the recrystallization mechanism.

Full text of DOI:10.1007/s11137-005-0017-6

Radiochemistry, Vol. 46, No. 5, 2004, pp. 490 493. Translated from Radiokhimiya, Vol. 46, No. 5, 2004, pp. 454 457.
Original Russian Text Copyright 2004 by Baronov, Berdonosov, Baronova, Melikhov.
Radiochemical Diagnostics of Thermal Hydrolysis
of Aluminum Trichloride
S. B. Baronov, S. S. Berdonosov, Yu. V. Baronova, and I. V. Melikhov
Lomonosov State University, Moscow, Russia
Received November 4, 2003; in final form, January 28, 2004
3
Abstract Thermal hydrolysis of aluminum chloride was studied using 36Cl and H tracers in combination
with energy-dispersive and morphological (scanning electron microscopy) analyses. Pure partially hydrated
AlCl3 or mixtures of AlCl3 and AlCl3 6H2O were used as initial materials. It was found that Al7O10Cl 2H2O
microtubes along with isometric microparticles are formed in the course of thermal hydrolysis due to evapora-
tion and partial hydrolysis of the initial AlCl3. In this case, AlCl3 6H2O acts as a source of water vapor,
whereas chlorine atoms contained in this compound do not pass into the microtube material. The tests with
water labeled with tritium also showed no incorporation of tritium in microtubes. Thus, thermal hydrolysis of
AlCl3 proceeds predominantly by the recrystallization mechanism.
36
Hydrolysis of aluminum salts is widely used for
preparing materials, in particular, various radionuclide
sorbents based on Al O [1, 2]. Therefore, detailed
where A and M are the amounts of Cl and total
F
F
amount of chlorine atoms in the microtubes.
2
3
The A value at slow isotope exchange between the
F
study of the process and, especially, of the mechanism
of recently discovered [3, 4] self-organization of mi-
croparticles of X-ray amorphous aluminum oxide in
the course of gas-phase thermal hydrolysis is of par-
ticular importance. In the process, tubular particles
(microtubes) of the composition Al O Cl 2H O (I)
crystals and gas phase is given as
AF
=
0M01
,
(2)
1
and M is described by the following expression:
F
7
10
2
are formed.
MF
=
1M01
+
2M02,
(3)
In the course of thermal hydrolysis, recrystalliza-
tion and topochemical process pathways can be
realized simultaneously. In recrystallization, crystals
of the initial compound are evaporated and its vapors
interact with water vapor with subsequent condensa-
tion of the hydrolysis product. In the topochemical
process, crystals sorb water vapor, and the hydrolysis
product is formed on the surface and in the crystal
bulk without evaporation of the initial compound.
Both pathways can yield tubular particles due to
aggregation and growth of the product nanoparticles
formed by condensation (first pathway) or by detach-
ment of fragments from the crystals (second pathway).
36
where
= A /M , A is the amount of Cl atoms
0 01 0
0
added into the system; M and M are the amounts
of all the chlorine atoms in AlCl and AlCl 6H O
is the probability of transfer of
chlorine atoms from AlCl (i = 1) and AlCl 6H O
01
02
3
3
2
microcrystals;
i
3
3
2
(i = 2) into microtubes.
Chlorine can appear in the microtubes owing to
vaporization of AlCl molecules with their subsequent
3
conversion into AlOCl and addition to microtubes
(j = 1), vaporization of HCl molecules formed by the
topochemical reaction on the surface or in the crystal
bulk with subsequent sorption of HCl by microtubes
(j = 2), and by detachment of certain fragments of
the product of topochemical reaction with composi-
tion I and their attachment to the microtubes (j = 3).
The role of each pathway in microtube formation
can be evaluated by radiochemical analysis, similar to
the isotope dilution procedure. For this purpose, AlCl
crystals labeled with Cl are mixed with nonlabeled
3
36
Thus, the probability
can be given as
,
ij ij
i
crystals of AlCl 6H O, the mixture is heated in
3
2
3
dry air to complete conversion of the mixture into
product I, and the isotope ratio in the microtubes is
determined:
=
(4)
i
j= 1
where
and
are the probabilities of chlorine
ij
escape from the microcrystals and their subsequent
ij
= AF /MF,
(1)
F
1066-3622/04/4605-0490 2004 MAIK Nauka/Interperiodica

RADIOCHEMICAL DIAGNOSTICS OF THERMAL HYDROLYSIS
491
transfer into the microtubes by one of the above path-
ways.
As seen from Eqs. (1) (4),
1
=
0(1 + M02Z/M01) ,
(5)
F
1
3
3
where Z =
.
1j 1j
2j 2j
j = 1
j = 1
In accordance with Eq. (5), the diagnostics of ther-
mal hydrolysis requires determination of the factor Z
using the functions
(M , M ) and its dependence
F
01 02
Z = Z( ) on the process conditions such as tempera-
i
ture T, concentration of H O and HCl in the vapor,
2
and parameters characterizing mutual arrangement of
AlCl and AlCl 6H O in the system. The Z( ) func-
Fig. 1. Electron micrograph of a microtube prepared using
AlCl layer containing no AlCl 6H O.
3
3
2
i
tion allows analysis of the
( ) and ( ) functions
ij
i
ij
i
3
3
2
characterizing the process mechanism; this problem is
considered in our paper.
vapor flow were formed on the surface layer (Fig. 1).
After cooling in an air flow, the boat was removed
In the first experimental series, we determined the
from the reactor and the radioactivity I of the result-
36
F
amount of Cl passing from AlCl crystals contain-
3
ing microtubes was determined. For this purpose, the
ing this radionuclide into the microtubes upon heating
of the initial material in tritium-free water vapor. In
the second series, a the mixture of Al Cl and AlCl
6H O crystals was thermally hydrolyzed, and, finally,
unlabeled AlCl was hydrolyzed in the atmosphere of
microtubes with weight m were placed into a slightly
F
acidified aqueous solution. The solution activity
was measured by liquid scintillation counting on a
TRI-CARB 2700 TR device (Packard, Australia); the
registration efficiency was 40%. We also measured
36
3
3
2
3
3
H O vapor.
2
the activity I of the solution obtained after the above
0
36
treatment of Al Cl crystals (m weight) and cal-
3
0
EXPERIMENTAL
culated the parameter B characterizing the stability of
the ratio of the number of radionuclide atoms to the
total number of atoms of the given element (isotope
ratio) in the course of thermal hydrolysis:
36
To prepare AlCl labeled with Cl, ultrapure grade
3
aluminum (99.9999%) was heated in a flow of gase-
ous HCl obtained from reaction of concentrated sul-
36
furic acid with solid NaCl containing Na Cl (pur-
B =
/
= (IF/I0)(m0/mF)(C0/CF),
0
(6)
F
chased from the Izotop Joint-Stock Company). The
resulting crystals were separated and stored in a dry
where I /I = A /A and C and C are the numbers
F
0
F
0
F
0
of chlorine atoms in 1 g of microtubes and AlCl
crystals.
box. The specific activity of these crystals was A
=
3
sp
6
1
2.3 10 Bq g , which corresponded to the isotope
3
ratio
= 7.0 10 . In our work we used crystalline
0
In the second series, thermal hydrolysis was per-
formed by the same procedure, but the mixture of
anhydrous AlCl and AlCl . 6H O was heated without
3
AlCl 6H O and tritium water H O produced by
Izotop (A = 9.0 10 Bq g ,
3
2
2
6
1
8
= 7.0 10 ).
sp
0
3
3
2
preliminarily exposure to the humid atmosphere. In
several tests, a bed of AlCl . 6H O was placed over
In the first series, a porcelain boat with a 4 5-mm
bed (bed weight m = 1.5 g) of labeled AlCl crystals
3
2
1
3
36
was placed in a closed chamber with a water vapor
pressure of 1.5 0.5 kPa. Then, the boat with the crys-
tals was transferred into a tubular reactor heated to
T = 473 K, which was purged with air at partial water
vapor pressure p = 1.0 1.5 kPa [3]. The gaseous HCl
liberated from the reactor was absorbed in a trap
packed with solid alkali.
a bed of Al Cl , and in the other tests, these com-
pounds were thoroughly mixed. The weight of AlCl .
6H O in the bed was m = 1.0 1.5 g.
3
3
2
2
In the third series, an aliquot of tritium water was
placed in a Petri dish over which (in a closed volume)
36
a boat with AlCl containing no Cl was kept for
3
4 h. Then, AlCl was thermally hydrolyzed by the
above procedure; the content of H in the microtubes
3
3
Upon heating, the microtubes growing toward the
RADIOCHEMISTRY Vol. 46 No. 5 2004

492
BARONOV et al.
Content of radionuclides in the microtubes at m0/MF = 75.8
Bed composition
Radionuclide
M02 /M01
(IF/I0) 103
B
AlCl3
AlCl3
36Cl
3H
0
0
0.83
1.38
<0.02
1.45
1.0 0.04
<0.04
1.00 0.04
Homogeneous mixture of AlCl3 and
AlCl3 6H2O
Two-layer mixture of AlCl3 and
AlCl3 6H2O
36Cl
36Cl
0.83
1.37
1.00 0.03
was determined immediately after completion of the
experiment. The atomic composition of the micro-
tubes was studied by energy-dispersive analysis and
= E /C was evaluated, where E is the number
were fulfilled:
=
0, Z << 1,
(7)
F
ac
F
F
F
i.e.,
of aluminum atoms in 1 g of microtubes of composi-
22
1
tion I and E = 1.0 10 g .
3
3
F
>>
.
2 j 2 j
(8)
1 j 1 j
j = 1
j = 1
RESULTS AND DISCUSSION
Expression (8) shows that transition of AlCl mole-
3
Our experimental data showed that, when exposed
cules into the gas phase on heating of AlCl . 6H O
3
2
to humid atmosphere for 1 h, the bed of AlCl crystals
3
crystals is negligible (
0). This suggests that
21
sorbs water in amounts of 4 mol per mole of AlCl .
3
AlCl . 6H O is hydrolyzed by the topochemical
3
2
Upon heating, this bed liberates gaseous HCl and
AlCl and H O vapors. The microtubes formed have
mechanism. In this case, crystals of aluminum chlor-
ide hexahydrate liberate HCl molecules, i.e., >> 0.
However, HCl molecules do not penetrate into the
microtubes ( 0) irrespective of distribution of
AlCl and AlCl . 6H O crystals (homogeneous or
3
2
22
the diameter from 20 to 500 m (5 50 m wall thick-
ness). The length of each tube increases at a rate of
22
1
0.25 mm s until the moment when the front of the
3
3
2
growing tube reaches the reactor wall or the initial
material is fully consumed.
two-layer system).
Low values indicate that sorption of HCl in the
22
In the first series, in which M = 0 and
(see table), i.e., the isotopic composition of each
microtubes is insignificant. If the sorption were strong,
01
F
0
then, in the presence of AlCl hexahydrate whose
3
system section is similar and constant, I determines
hydrolysis increases concentration of hydrogen chlor-
F
the number of all the chlorine atoms in the microtubes
ide in the gas phase near the microtubes,
decrease owing to an increase in the content of non-
radioactive chlorine, required by the ( ) function
related to the isotherm of HCl sorption in the micro-
would
F
of weight m :
F
22
j
MF = (I /I0)(A0 / ),
F
0
tubes. However, the probability
is too low to
22
which corresponds to the following values:
provide strong decrease in
. The probability of
F
detachment of hexahydrate crystal fragments with
their attachment to microtubes is also relatively small
= EF mF /MF = 0.15 0.01, CF /C0 = 0.106 0.007.
ac
0). Under the experimental conditions
23 23
studied, AlCl . 6H O acts only as a water source.
These values are similar to the results obtained us-
3
2
21
1
Taking into account that
0, we assume that
ing the energy-dispersive method (C = 1.37 10
g
23 23
F
is also very low, because the probability of
and thus C /C = 0.101 0.006 and
= 0.14 0.01).
13 13
F
0
ac
attachment of crystal fragments hardly depends on the
mechanism of their formation. Since the microtubes
do not sorb HCl, then
The parameters B evaluated in the second experi-
mental series are given in the table. As seen, for Cl,
36
B = 1.00 0.05 irrespective of the presence of AlCl .
3
6H O and distribution of AlCl and AlCl . 6H O
>>
,
.
13 13
(9)
2
3
3
2
11 11
12 12
crystals in the mixture. This fact indicates that, under
the experimental conditions, the following ratios
Expression (9) indicates that the microtubes grow
RADIOCHEMISTRY Vol. 46 No. 5 2004

RADIOCHEMICAL DIAGNOSTICS OF THERMAL HYDROLYSIS
493
hexahydrate. The vapors of AlCl are mixed with
3
water vapor to form product particles in two mor-
phological forms: microtubes and isometric particles,
whose certain fraction is precipitated on the microtube
surface as aggregates of various shapes (Fig. 2). The
recrystallization pathway of AlCl thermal hydrolysis
3
is morphologically branched. At the same time, the
results of radiometric and energy-dispersive analyses
show that the composition of the microtubular form is
constant. This can be explained by the fact that the
microtubes are composed by cyclic clusters of com-
position I; under given experimental conditions, the
probability of their formation is significantly higher
than that of the clusters of the other compositions.
In these clusters, the mobility of chlorine atoms is
small and they do not participate in the isotope ex-
change with the gas phase, which is confirmed by
expression (7).
Fig. 2. Electron micrograph of a microtube end. Particle
aggregates are seen to cover the microtube surface.
owing to the sublimation and hydrolysis of AlCl
vapor, i.e., thermal hydrolysis of aluminum chloride
proceeds by the recrystallization mechanism.
3
ACKNOWLEDGMENTS
The third series data (table) confirm that sorption
of HCl in the microtubes is rather small. Otherwise,
The authors are grateful to S.N. Kalmykov,
Yu.A. Sapozhnikov, M.A. Prokof’ev, and D.A. Emel’-
yanov for their assistance in this work.
I
for tritium in this experimental series would be
F
very high. Small I values also confirm the absence of
HCl sorption in the microtubes.
F
The work was financially supported by the Russian
Foundation for Basic Research (project nos. 02-02-
33176 and 03-03-06563).
If microtubes acquire composition I directly while
growing, then B for tritium determined immediately
after their formation is close to zero. In the further
process, however, tritium atoms escape from the
microtubes as a result of the isotope exchange with
water vapor on cooling of the reactor and dissolution
of the microtubes for radioactivity measurements. As
a result, by the moment of measurements B = 0. How-
ever, our experimental data do not allow us to rule out
incorporation of water molecules in the microtubes by
sorption after the nanotube formation, rather than
during the nanotube growth.
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3
3
2
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3
2
compose with liberation of H O and HCl molecules
2
and formation of amorphous Al O areas on the sur-
2
3
face of each crystal and their further growth into the
crystal bulk. Simultaneously, AlCl crystals sublime
3
and sorb water vapor generated by aluminum chloride
RADIOCHEMISTRY Vol. 46 No. 5 2004