Mechanically Stimulated Thermal Synthesis of Lithium Pentaaluminate from Lithium Carbonate and Aluminum Hydroxide

Article abstract of DOI:10.1134/S1070427217100056

It is shown that the phase composition of lithium aluminates formed when aluminum hydroxide in the form of gibbsite interacts with lithium carbonate in their mixture with Al: Li atomic ratio of 5: 1 depends on the duration of a preliminary mechanical activation of the mixture and on the temperature of the subsequent thermal treatment. A thermal treatment of the starting mixture at temperatures exceeding 800°C yields LiAl₅O₈ with a substantial admixture of α- and γ-LiAlO₂. Raising the duration of the mechanical activation to 5 min and more makes it possible to obtain highly dispersed single-phase LiAl₅O₈ with a specific surface area larger than 20 m² g^–1.

Full text of DOI:10.1134/S1070427217100056

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2017, Vol. 90, No. 10, pp. 1586−1591. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © V.P. Isupov, N.V. Eremina, I.A. Borodulina, 2017, published in Zhurnal Prikladnoi Khimii, 2017, Vol. 90, No. 10, pp. 1303−1308.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Mechanically Stimulated Thermal Synthesis of Lithium
Pentaaluminate from Lithium Carbonate
and Aluminum Hydroxide
V. P. Isupov*, N. V. Eremina, and I. A. Borodulina
Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch, Russian Academy of Sciences,
ul. Kutateladze 18, Novosibirsk, 630128 Russia
*e-mail: isupov@solid.nsc.ru
Received November 3, 2017
Abstract—It is shown that the phase composition of lithium aluminates formed when aluminum hydroxide in the
form of gibbsite interacts with lithium carbonate in their mixture with Al : Li atomic ratio of 5 : 1 depends on the
duration of a preliminary mechanical activation of the mixture and on the temperature of the subsequent thermal
treatment. A thermal treatment of the starting mixture at temperatures exceeding 800°C yields LiAl5O8 with a
substantial admixture of α- and γ-LiAlO2. Raising the duration of the mechanical activation to 5 min and more
makes it possible to obtain highly dispersed single-phase LiAl5O8 with a specific surface area larger than 20 m2 g–1
DOI: 10.1134/S1070427217100056
Highly dispersed lithium pentaaluminate LiAl5O8 is
used in catalysis as a catalyst support [1, 2] and as a matrix
for synthesis of phosphors [3–5]. The ceramic method
for synthesis of this compound, based on the calcination
of a mixture of lithium carbonate and aluminum oxide,
requires a high temperature (1100°C) and long reaction
duration (12 h) [6]. Another approach to synthesis of
this compound is based on the sol-gel method [7], which
includes mixing and hydrolysis with water of a mixture
of aluminum isopropylate and lithium nitrate, followed
by drying of the gel and its thermal treatment in air at
800°C. To a shortcoming of this method can be attributed
the use of expensive starting reagents and the formation
as a result of synthesis of by-products, which should be
utilized. One of promising ways to synthesize complex
oxides, which has been actively developed during the
last 10–15 years, is the mechanochemical method [8, 9].
As applied to lithium aluminates, this method has been
implemented in synthesis of γ-LiAlO2.Ahighly dispersed
γ-LiAlO2 with specific surface area larger than 10 m2 g–1
was obtained with a preliminary mechanical treatment of
a mixture of aluminum hydroxide (gibbsite) and lithium
carbonate in anAGO-2 planetary activator at a centrifugal
acceleration of 40 g, with the subsequent thermal
treatment of activation products in air at temperatures
exceeding 700°C [10]. This method attracts attention
because of the simple implementation, ecological
safety, and possibility of using readily available and
comparatively inexpensive salts of lithium carbonate and
aluminum hydroxide compounds.
The goal of our study was to examine the possibility
of performing a mechanochemical synthesis of lithium
pentaaluminate with mechanical activation of a mixture
of aluminum hydroxide (gibbsite) and lithium carbonate
and the subsequent thermal treatment of the activation
products in air.
EXPERIMENTAL
The starting mixture was prepared from the following
reagents: crystalline aluminum hydroxide Al(OH)3
(gibbsite) of analytically pure grade [GOST (State
Standard) 11841–76] with specific surface area of
0.2 m2 g–1 and lithium carbonate Li2CO3 of chemically
pure grade [TU (Technical Specification) 6-09-3728–83]
with specific surface area of 0.7 m2 g–1. The starting
reagents were mixed in a Li2CO3 : Al(OH)3 molar ratio
of 1 : 10, necessary for obtaining LiAl5O8, and then
1586

MECHANICALLY STIMULATED THERMAL SYNTHESIS
1587
H
H
C
H
H
H
H
H
H
H
H
H
H
H
C
C
H
H
H
C
H
C
C
C
starting
τMA, min
2θ, deg
Fig. 1. Dependence of the specific surface area Ssp of
(1) mixtures of aluminum hydroxide and lithium carbonate
and (2) products of their thermal treatment on the duration
Fig. 2. XRD data for products of the mechanical activation of a
mixture of aluminum hydroxide and lithium carbonate at process
durations of 1, 5, and 10 min. (2θ) Bragg angle; the same for
Figs. 5–9. (H) Al(OH)3 and (C) Li2CO3.
τMA of mechanical activation. Thermal treatment conditions:
900°C, 2 h, air.
were activated in an AGO-2 planetary centrifugal mill at
acceleration of 40 g for 1, 5, and 10 min. For the treatment
durations of 5 and 10 min, the activation was performed
discretely, during 2.5 min with a break in between. The
activation was performed in air in 100-mL steel drums
with steel balls 5 mm in diameter. The ratio between the
substance mass and the mass of the ball load (200 g)
was 1 : 20.
the profile of the XRD pattern by the Rietveld method
with Topas 4.2 software (Bruker AXS, Germany). A
thermogravimetric analysis was made in air on a modified
MOM derivatograph (Hungary) in the temperature range
20–800°C in corundum crucibles at a heating rate of
10 deg min–1. The specific surface area was estimated
by the chromatographic method on the basis of argon
desorption with the use of a standard sample. The
dispersity was measured with a Microsizer-201A laser
particle dispersity meter, with ethanol serving as the
dispersion medium.
The samples obtained after the mechanical activation
of the mixture of the starting reagents and their heating in
air were examined by X-ray diffraction (XRD) analysis,
in situ XRD analysis, thermal analysis, and measurement
of the specific surface area and particle size. The XRD
patterns were recorded with a D8Advance diffractometer
with CuKα radiation, measurement range 2θ = 10–70°
with a step of 0.02° and accumulation duration of 35 s.
High-temperature in situ XRD measurements were
made in an Anton Par HTK 1200N chamber (Austria)
with a corundum cuvette. The heating was performed in
air in stages from room temperature to 100°C at a rate
of 12 deg min–1. Then, an XRD pattern was recorded
at this temperature at angles in the range 2θ = 10–60°.
After the recording was complete, the sample was heated
to 200°C and an XRD pattern was recorded, etc. The
thermal treatment of a mechanically activated mixture
was performed in air in a SNOL muffle furnace at a
heating rate of 10 deg min–1. After 900°C was reached,
the mixture was kept at this temperature for 2 h. A
quantitative analysis of phases was made by modeling
RESULTS AND DISCUSSION
Mechanical activation of a mixture ofAl(OH)3 and
Li2CO3. The dependence of the specific surface area of
the mixture on its activation duration is nonmonotonic
(Fig. 1). The specific surface area increases when the
activation duration is up to 5 min. Further, it decreases
with increasing time of mechanical activation. The
significant (by two orders of magnitude) increase in the
specific surface area is indicative of a strong dispersion
of reagents to give submicrometer particles of aluminum
hydroxide and lithium carbonate. The mechanical
activation leads not only to the dispersion of reagents,
but also changes their microstructure and structure.
For example, even after 1-min activation, the integral
intensity of the reflections of aluminum hydroxide and
lithium carbonate strongly decreases and their width
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1588
ISUPOV et al.
P, %
(a)
(b)
D, μm
D, μm
Fig. 3. Particle size (D) distribution P for (a) mixture of aluminum hydroxide and lithium carbonate and (b) products of a thermal
treatment of a mechanically activated mixture. Duration of mechanical activation (min): (1) 0, (2) 1, (3) 5, (4) 10; the same for Fig. 9.
Study of processes occurring in thermal treatment
of the starting and mechanically activated mixture of
aluminum hydroxide and lithium carbonate. Three
stages in the change in the mass of the substance can
be distinguished in the thermogram of the unactivated
mixture. The first stage begins at temperatures higher
than 220°C and ends at 320–350°C (Fig. 4).As indicated
by in situ XRD data, this stage is associated with the
dehydration of aluminum hydroxide to give boehmite and
χ-aluminum oxide (Fig. 5). The second stage begins at a
temperature higher than 450°C and ends at 550–570°C. It
is mostly associated with the decomposition of boehmite
to give gamma aluminum oxide (Fig. 4). The third
stage, observed at temperatures higher than 570°C, is
T, °C
Fig. 4. Thermograms of (1) starting mixture of aluminum
associated with the interaction of lithium carbonate with
aluminum oxides. This is evidenced by the decrease in
the integral intensity of lithium carbonate reflections
and by the appearance at 600°C of broadened reflections
characteristic of α-LiAlO2. LiAl5O8 reflections start to
appear, together with the α-LiAlO2 reflections, at 700°C,
and their intensity grows with increasing temperature.
hydroxide and lithium carbonate and products of its mechanical
activation for (2) 1, (3) 5, and (4) 10 min. Heating rate
10 deg min–1, air as atmosphere. (Δm/m) Loss of mass and
(T) temperature.
grows (Fig. 2). Raising the activation duration to 5 min
leads to a further decrease in the integral intensity and
broadening of the reflections of these compounds. Finally,
10-min activation is accompanied by the formation of a
nearly X-ray-amorphous product.
Apreliminary mechanical activation of the mixture for
1 min results in that the onset temperature of the thermal
decomposition of aluminum hydroxide decreases by more
than 100°, the loss of mass in the stage of aluminum
hydroxide dehydration (100–350°C) increases, and
the loss of mass in the stage of boehmite dehydration
(450–570°C) decreases (Fig. 4). The amount of boehmite
formed in the stage of aluminum hydroxide dehydration
is substantially smaller, compared with the unactivated
mixture (Fig. 6). The lithium carbonate reflections start to
decrease in intensity in the temperature range 400–500°C.
At 500°C, there appear broadened reflections peaked
Most part of the starting mixture contains aggregates
30 to 150 μm in size, which predominantly belong to
aluminum hydroxide (Fig. 3a). Activation of the mixture
for 1 min leads to a decrease in the share of the coarse
fraction and makes higher the share of fine fractions with
sizes less than 30 μm. Raising the mechanical treatment
duration to 5 min affects only slightly the particle size
distribution. Activation for 10 min results in that the
particle size distribution changes in the <30 μm range.
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MECHANICALLY STIMULATED THERMAL SYNTHESIS
H
1589
H
H
H
H
H
H
С
С
С
B
С
B
С
B
B
P
P
P
A
2θ, deg
2θ, deg
Fig. 5. In situ X-ray diffraction patterns of the starting mixture of aluminum hydroxide and lithium carbonate in air. T (°C): (1) 30,
(2) 200, (3) 300, (4) 400, (5) 500, (6) 600, (7) 700, (8) 800, (9) 900; the same for Figs. 6 and 8. Reflections: (H) Al(OH)3; (C) Li2CO3;
(B) AlO(OH); (α) α-LiAlO2; (P) LiAl5O8; (χ) χ-Al2O3; (γ) γ-Al2O3.
at 2θ = 37.4 and 45°, which can be attributed both to
χ-Al2O3 (37.4°) and to LiAl5O8 (37.4, 45.4°). Raising
the temperature leads to an increase in the intensity and
decrease in the width of the LiAl5O8 reflections.At 800°C
and more, weak reflections of γ-LiAlO2 are recorded
together with those of lithium pentaaluminate.
Raising the activation duration of the mixture to
5 min intensifies the dehydration of the hydroxide
(Fig. 4). The change in mass (30%), observed in the
temperature range 100–350°C, corresponds to the nearly
full dehydration of aluminum hydroxide (31.6%) in the
mixture. The dehydration of aluminum hydroxide does
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
B
P
P
P
P
P
P
P
P
P
P
g
P
P
2θ, deg
2θ, deg
Fig. 7. In situ X-ray diffraction patterns of a mixture of aluminum
hydroxide and lithium carbonate, mechanically activated for
5 min, in air. T (°C): (1) 30, (2) 200, (3) 300, (4) 500, (5) 600,
(6) 700, (7) 800, (8) 900. Reflections: (H)Al(OH)3; (С) Li2CO3;
(P) LiAl5O8.
Fig. 6. In situ X-ray diffraction patterns of a mixture of
aluminum hydroxide and lithium carbonate, mechanically
activated for 1 min, in air. Reflections: (H) Al(OH)3;
(C) Li2CO3; (B) AlO(OH); (α) α-LiAlO2; (P) LiAl5O8.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 90 No. 10 2017

1590
ISUPOV et al.
Phase composition of products formed after the thermal
treatment of mechanically activated mixtures of lithium
carbonate and aluminum hydroxide. Temperature of the thermal
treatment, 900°C; treatment duration 2 h
not yield boehmite (Fig. 7). Analysis of the behavior of
lithium carbonate under heating of the activated mixture
is difficult because of the broadening of the reflections
of this compound. Broadened reflections of lithium
pentaaluminate start to appear in the XRD pattern
beginning at 600°C. Raising the temperature leads to a
decrease in the width and increase in the intensity of the
reflections associated with this compound. Raising the
duration of the mechanical activation from 5 to 10 min
weakly affects both the TG curves and the nature of phase
transformations observed in the thermal treatment of a
mechanically activated mixture (Figs. 4 and 8).
Weight percent
MA duration,
min
α-LiAlO2
γ-LiAlO2
LiAl5O8
0
1
8.3
0.3
–
2.8
1.5
–
88.9
98.2
100
100
5
10
–
–
To study how the phase composition of the lithium
aluminates being formed depends on the duration of
the mechanical activation, we performed experiments
on the thermal treatment of activated samples at 900°C
for 2 h. As also shown by in situ experiments, the
thermal treatment of an unactivated mixture of reagents
yields a mixture containing LiAl5O8 with admixture
of α-LiAlO2 and γ-LiAlO2 (see the table, Fig. 9). The
content of α-LiAlO2 and γ-LiAlO2 impurity decreases
with increasing duration of the mechanical activation,
and upon a mechanical activation for 5 min, a single-
phase lithium pentaaluminate is formed with specific
surface area of 23 to 27 m2 g–1 (Fig. 1b). The average
size D of “primary” LiAl5O8 particles, calculated by
the formula S = 6/ρS, where ρ is the substance density
(3.6 g cm–3) and S is the specific surface area, falls within
the range 60–70 nm. Thus, the lithium pentaaluminate
we synthesized is composed of submicrometer particles
forming aggregates with micrometer and larger dimen-
sions (Fig. 3). Noteworthy is the polymodal particle size
distribution, which nearly coincides with the particle
distribution in the mechanically activated mixture
(Figs. 3a and 3b).
H
H
P
P
P
P
P
P
2θ, deg
Fig. 8. XRD in situ analysis of a mixture of aluminum hydroxide
and lithium carbonate, mechanically activated for 10 min, in
air. Reflections: (H) Al(OH)3; (P) LiAl5O8.
The available experimental data suggest the following
scheme of processes occurring in the mechanochemical
synthesis of lithium pentaaluminate. In the thermal
treatment of an unactivated mixture of aluminum
hydroxide and lithium carbonate in the temperature
range 220–500°C, aluminum hydroxide decomposes
to give a mixture of crystalline aluminum oxides
γ-Al2O3 and χ-Al2O3. Above 550°C, these oxides react
with lithium carbonate to give α-LiAlO2, with lithium
pentaaluminate formed at higher temperatures. The
appearance of α-LiAlO2 in the first stage may occur for
two reasons. In γ-Al2O3 and χ-Al2O3, aluminum cations
are predominantly in the octahedral coordination [11, 12].
P
P
P
P
P
P
P
P
P
P
P
P
2θ, deg
Fig. 9. XRD patterns of a mixture of lithium carbonate and
aluminum hydroxide, activated during a varied time and
calcined at 900°C for 2 h in air. Reflections: (P) LiAl5O8;
(g) γ-LiAlO2; (α) α-LiAlO2.
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MECHANICALLY STIMULATED THERMAL SYNTHESIS
1591
Therefore, formation of α-LiAlO2 in which all of the
aluminum atoms are in octahedral oxygen coordination
is presumably preferable to formation of LiAl5O8 in
which the fraction of octahedrally coordinated aluminum
is substantially smaller [14]. Another possible reason
why α-LiAlO2 is formed is the presence in the mixture
of unactivated reagents of local regions with increased
content of lithium carbonate. The appearance of an
admixture of gamma-lithium monoaluminate is due to
the phase transition of α-LiAlO2 being formed to the
γ-LiAlO2 form [15].
Siberian Branch, Russian Academy of Sciences,
N.V. Bulina, V.R. Khusnutdinov, andA.K. Tret’yakov for
performing the X-ray diffraction analysis, measuring the
particle size and making a thermal analysis.
The study was carried out under the State assign-
ment to the Institute of Solid-State Chemistry and
Mechanochemistry, Siberian Branch, Russian Academy
of Sciences (project no. 0301-2016-0015).
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The mechanical activation of the mixture leads to
dispersion and mixing of the reagents, i.e., to a decrease
in the percentage of local regions with increased content
of lithium carbonate, which is a reason why the content
of α-LiAlO2 and γ-LiAlO2 decreases with increasing
activation duration. The mechanical activation of the
mixture is accompanied not only by the dispersion and
mixing of the reagents, but also by the amorphization of
aluminum hydroxide and lithium carbonate. The thermal
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