Scandium-containing layered hydroxides

Article abstract of DOI:10.1134/S0036023607110046

A scandium analogue of hydtotalcite and a mixed sample containing both scandium and aluminum have been synthesized and characterized. It is known that both samples have a layered structure; the unit cell parameters of the scandium-containing samples are slightly higher than the respective values of an ordinary aluminum-containing sample. The mixed-metal scandium-aluminum sample easily regains its layered structure after a dehydration/rehydration cycle, while the scandium analogue after this treatment is restored only partially.

Full text of DOI:10.1134/S0036023607110046

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 11, pp. 1662–1665. © Pleiades Publishing, Inc., 2007.
Original Russian Text © O.A. Vorontsova, R.N. Saenko, O.E. Lebedeva, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 11, pp. 1770–1773.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Scandium-Containing Layered Hydroxides
O. A. Vorontsova, R. N. Saenko, and O. E. Lebedeva
Belgorod State University, Belgorod, Russia
e-mail: OLebedeva@bsu.edu.ru
Received October 19, 2006
Abstract—A scandium analogue of hydtotalcite and a mixed sample containing both scandium and aluminum
have been synthesized and characterized. It is known that both samples have a layered structure; the unit cell
parameters of the scandium-containing samples are slightly higher than the respective values of an ordinary alu-
minum-containing sample. The mixed-metal scandium–aluminum sample easily regains its layered structure
after a dehydration/rehydration cycle, while the scandium analogue after this treatment is restored only par-
tially.
DOI: 10.1134/S0036023607110046
THEORETICAL ANALYSIS
were synthesized through precipitation from aqueous
salt solutions, followed by thermostating, ion exchange
for carbonate ions, washing, and drying. For reference,
magnesium aluminum (Mg/Al) layered hydroxide was
prepared by the same method. The formulas of the
compounds were derived from chemical analysis for
scandium, aluminum, and magnesium. Crystal water
was estimated from DTA–TG analysis.
Layered hydroxides were identified and some their
properties were studied using powder X-ray diffraction
(a DRON-3 X-ray diffractometer; 2θ scan steps, 0.5°;
CuKα radiation).
Layered double hydroxides (LDHs) mean com-
pounds that are, strictly speaking, hydroxo salts. Inas-
much as hydrotalcite (magnesium aluminum hydroxo-
carbonate) is the best known natural LDH, term “hydro-
talcite-like compounds” is sometimes used. This class of
materials has been in the focus of research in the last few
decades because of their high potential for use as adsor-
bents [1], anion exchangers [2], catalysts, and their pre-
cursors [2–5]. The ability of LDHs to regain their layered
structure after dehydration/rehydration cycles (the mem-
ory effect) is also being studied.
The idealized formula of layered hydroxide is
2+
1 – x
M
M3x+(OH)2[(An–)x/n · mH2O], where å2+ and å3+
RESULTS AND DISCUSSION
are metal cations and Än– is virtually any anion. The
unit cell of an LDH is described by an octahedron cen-
tered by a cation with six oxygen atoms in the vertices
[2, 6–8]. Octahedra are linked into a network and form
layers. The isomorphic substitutions of triply charged
for doubly charged cations generate an excess positive
charge, which is balanced by interlayer anions.
A wide range of natural and synthetic LDHs with
various doubly charged cations (zinc, nickel, copper,
magnesium, and others) is known [2, 6, 9]; the synthe-
sis of LDHs with a singly charged (lithium) cation has
been described [10]. Triply charged (chromium, iron)
cations can also substitute for aluminum [2, 11].
The samples synthesized have X-ray diffraction pat-
terns typical of well-crystallized layered hydroxides
(Fig. 1). Their unit cell parameters ‡ and Ò and grain
sizes were calculated from X-ray diffraction data
(Table 1).
Most literature sources use the parameter Ò as the
main characteristic of layered hydroxides. This param-
eter characterizes the interlayer spacing and is calcu-
lated as the tripled d/n value for the first peak corre-
sponding to plane 003 on the X-ray diffraction pattern
[2]. The parameter ‡ shows the distance between the
nearest-neighboring cations in a brucite-like layer; this
parameter is equal to the doubled interplanar spacing
for reflection (110) [2].
Table 1 makes it clear that in the Mg/Sc sample the
distance between metal-hydroxide layers is greater than
in magnesium aluminum hydrotalcite, in agreement
with the ionic radii of the relevant cations [12]: the
crystal-chemical radius of the scandium cation (0.82 Å)
is far greater than the aluminum cation radius (0.57 Å).
This work intended to synthesize and characterize
scandium-containing layered hydroxides. Hydrotal-
cite-like scandium-containing compounds have not
been documented.
EXPERIMENTAL
Magnesium scandium (Mg/Sc) and magnesium Interestingly, the greatest interlayer spacing is in the
scandium aluminum (Mg/AlSc) layered hydroxides sample in which scandium partially substitutes for alu-
1662

SCANDIUM-CONTAINING LAYERED HYDROXIDES
1663
I/I
0
(003)
(009)
(006)
(015)
(018)
(113)
(110)
‡
b
c
4
16
24
32
40
48
56
64
2θ, deg
Fig. 1. X-ray diffraction patterns for LDH samples: (a) Mg/Sc, (b) Mg/AlSc, and (c) Mg/Al.
I/I
0
1
2
3
‡
b
c
7
16
24
32
40
48
56
64
2θ, deg
Fig. 2. X-ray diffraction patterns for LDH samples calcined at 823 K: (a) Mg/Sc, (b) Mg/AlSc, and (c) Mg/Al. Notations: (1) scan-
dium oxide Sc O , (2) magnesium oxide MgO, and (3) aluminum oxide Al O .
2
3
2 3
minum in hydrotalcite; probably, this is due to the deionized water with stirring at room temperature and
structure distortion induced by a combination of triply
charged cations with different radii in a layer. The grain
size and the unit cell parameter ‡ rise systematically
with increasing scandium proportion in the sample, sig-
nifying an increase in the unit cell.
drying at 372 K. As expected, the samples dehydrated
during calcination, losing their layered structure. The
X-ray diffraction patterns of the calcined samples dis-
play reflections that can be unambiguously assigned to
metal oxides (Fig. 2).
To study the memory effect, we calcined samples at
823 K for 2 h, then rehydrating them for 2 days in layered structure during rehydration, which is demon-
The Mg/Al and Mg/Sc samples easily regain their
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 11 2007

1664
VORONTSOVA et al.
I/I
0
‡
b
6
16
24
32
40
48
56
62
2θ, deg
Fig. 3. X-ray diffraction patterns for restored samples: (a) Mg/AlSc and (b) Mg/Al.
I/I
0
1
8
16
24
32
40
48
56
64
2θ, deg
Fig. 4. X-ray diffraction pattern for a rehydrated Mg/Sc sample. Notation: (1) magnesium oxide.
strated by single-crystal X-ray diffraction data: the scandium-containing system is close in its composition
restored samples have the set of basal reflections that to hydrotalcite.
are intrinsic to the hydrotalcite structure (Fig. 3). The
Magnesium scandium layered hydroxide after dehy-
dration–rehydration cycling only partially regains its
layered structure: reflections from magnesium hydrox-
ide in the X-ray diffraction pattern of the rehydrated
unit cell parameters and grain sizes for the restored
samples are listed in Table 2. The restored samples far
less differ in their unit cell parameters than the starting
samples; this is likely because the layered hydroxide
formed during dehydration–hydration cycling in the samples are rather strong (Fig. 4).
Table 1. Unit cell parameters for LDH samples synthesized
Sample
Mg/Sc
Formula unit
Ò, Å
‡, Å
V, Å3
Mg0.677Sc0.323(OH)2(CO3)0.162 · 0.805H2O
Mg0.819Al0.167Sc0.014(OH)2(CO3)0.091 · 0.783H2O
Mg0.780Al0.220(OH)2(CO3)0.110 · 0.947H2O
23.61
23.91
23.28
3.16
3.08
3.06
204.2
196.4
188.8
Mg/AlSc
Mg/Al
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SCANDIUM-CONTAINING LAYERED HYDROXIDES
1665
Table 2. Unit cell parameters for restored samples
2. F. Cavani, F. Trifiro, andA. Vaccari, Catal. Today 11, 173
(1991).
V, Å3
3. A. A. Eliseev, A. V. Lukashin, A. A. Vergetel, et al., Dokl.
Sample
Ò, Å
‡, Å
Akad. Nauk 387 (6), 777 (2002).
4. Z. Tong, T. Shichi, G. Takagi, and K. Takagi, Res. Chem.
Intermed. 29 (3), 335 (2003).
Mg/AlSc
Mg/Al
23.49
23.49
3.08
3.07
193.0
191.7
5. B. F. Sels, D. E. De Vos, and P. A. Jacobs, Catal. Rev. 43
(4), 443 (2001).
6. R. V. Prikhod’ko, M. V. Sychev, I. M. Astrelin, et al.,
Zh. Prikl. Khim. 74 (10), 1573 (2001).
7. V. R. L. Constantino and T. J. Pinnavaia, Inorg. Chem.,
No. 34, 883 (1995).
CONCLUSIONS
8. M. Bellotto, B. Rebours, O. Clause, and J. Lynch,
J. Phys. Chem., No. 100, 8527 (1996).
Our studies show that, in spite of the significant dif-
ference between the aluminum and scandium cation
radii, a scandium analogue of hydrotalcite can be syn-
thesized, as well as scandium/aluminum mixed-metal
hydrotalcite-like material. The latter can fully regain its
layered structure after dehydration–rehydration
cycling, while magnesium scandium layered hydroxide
after this treatment is restored only partially.
9. I. Kostov, Mineralogy (Oliver and Boyd, Edinburgh
(U.K.), 1968; Mir, Moscow, 1971).
10. T. A. Tripol’skaya, G. P. Pilipenko, I. V. Pokhabova, and
E. G. Ippolitov, Zh. Neorg. Khim. 47 (2), 212 (2002)
[Russ. J. Inorg. Chem. 47 (2), 168 (2002)].
11. M. P. Nikiforov, M. V. Chernysheva, A. V. Lukashin, et al.,
Dokl. Akad. Nauk 391 (1), 53 (2003).
12. Properties of Inorganic Compounds. Handbook, Ed. by
A. I. Efimov et al. (Khimiya, Leningrad, 1983) [in Rus-
sian].
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1. Y. Seida andY. Nakano, Water Res. 36 (5), 1306 (2002).
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