Synthesis and thermolysis of tris(triethoxysiloxy)aluminum and its complexes with potassium and sodium triethoxysilanolates

Article abstract of DOI:10.1023/A:1015443601131

Reaction of potassium (or sodium) triethoxysilanolate with AlBr ₃ in benzene in a 3:1 or 4:1 ratio yields, respectively, tris(triethoxysiloxy)aluminum Al[OSi(OEt)₃]₃ or potassium (or sodium) tetrakis(triethoxysiloxy)alumin

Full text of DOI:10.1023/A:1015443601131

Russian Journal of General Chemistry, Vol. 72, No. 3, 2002, pp. 394 397. Translated from Zhurnal Obshchei Khimii, Vol. 72, No. 3, 2002,
pp. 423 426.
Original Russian Text Copyright
2002 by Shcherbakov, Basova, Khorshev, Malysheva, Domrachev.
Synthesis and Thermolysis of Tris(triethoxysiloxy)aluminum
and Its Complexes with Potassium
and Sodium Triethoxysilanolates
V. I. Shcherbakov, G. V. Basova, S. Ya. Khorshev, I. P. Malysheva, and G. A. Domrachev
Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, Nizhni Novgorod, Russia
Received July 14, 2000
Abstract Reaction of potassium (or sodium) triethoxysilanolate with AlBr3 in benzene in a 3 : 1 or 4 : 1
ratio yields, respectively, tris(triethoxysiloxy)aluminum Al[OSi(OEt)3]3 or potassium (or sodium) tetrakis(tri-
ethoxysiloxy)aluminate M{Al[OSi(OEt)3]4} (M = K, Na). Single-stage thermolysis of tris(triethoxysiloxy)-
aluminum (200 C) and potassium tetrakis(triethoxysiloxy)aluminate (300 C), followed by annealing of the
solid residues at 1000 1250 C, yields ceramic materials, which were examined by X-ray diffraction. The
crystalline phase obtained from tris(triethoxysiloxy)aluminum is aluminum metasilicate Al6Si2O13 (mullite),
and the product obtained from potassium tetrakis(triethoxysiloxy)aluminate is a mixture of aluminum ortho-
silicate Al2SiO5 (kyanite) and feldspar aluminosilicate KAlSi3O8 (microcline).
Direct thermolysis of mixed aluminum and silicon
alkoxides (RO)2AlOSi(OR )3 (I) is a route to alu-
mina silica ceramic composites [1]. As a continuation
of our works [1, 2] on synthesis and study of such
dialkoxyaluminosiloxanes, we report in this paper the
synthesis and thermolysis of tris(triethoxysiloxy)alu-
minum Al[OSi(OEt)3]3 (II) and its complexes with
potassium and sodium triethoxysilanolates: potassium
(K{Al[OSi(OEt)3]4}, III) and sodium (Na{Al[OSi
(OEt)3]4}, IV) tetrakis(triethoxysiloxy)aluminates.
stances, in contrast to aluminosiloxanes I [2] and II
which are very viscous liquids. Their IR spectra are
practically identical and give little information. They
contain a set of the same characteristic bands originat-
ing from vibrations of the EtO group (as in tetraeth-
oxysilane) and a strong broad absorption band of the
SiOAl group (in the range of SiOSi vibrations);
for assignments, see [3 6].
We found previously [1] that dialkoxyaluminosil-
oxanes I start to decompose at 180 200 C; therefore,
thermolysis of aluminosiloxane II was also performed
at 200 C, with removal of volatiles in an evacuated
system. In a cooled trap we collected and identified
the following compounds (moles per mole of II):
ethylene (1.11), ethanol (1.38), tetraethoxysilane
(0.30), diethyl ether (0.27), and water (0.16). The dark
solid residue formed after such treatment, according
to the elemental analysis and the results of hydrolysis,
still contained alkoxy groups and therefore was sub-
jected to further thermolysis at 300 C. The following
compounds were additionally collected (moles per
mole of II): ethylene (0.49), ethanol (1.84), and water
(1.72). Also, we obtained a dark powdered residue IIa
in an amount of 44 wt % relative to II.
Previously [1, 2], aluminosiloxanes I and R2AlO
Si(OR)3 were prepared by exchange of aluminum bro-
mide (EtO)2AlBr or i-Bu2AlBr with potassium (or
sodium) triethoxysilanolate. A similar reaction of
AlBr3 with alkali metal triethoxysilanolate in a 1 : 3
ratio yields II. At a 1 : 4 ratio, complexes III and IV
are obtained in one stage. These complexes can also
be prepared by addition of an equimolar amount of
potassium (or sodium) triethoxysilanolate to alumino-
siloxane II. The scheme of the reactions is shown
below.
3MOSi(OEt)3
AlBr3
Al[OSi(OEt)3]3
3MBr
II
4MOSi(OEt)3
3MBr
MOSi(OEt)3
Similarly, in thermolysis of III at 300 C, we col-
lected and identified the following compounds (moles
per mole of III): ethylene (2.20), tetraethoxysilane
(0.85), ethanol (2.63), and water (0.18). The amount
of the dark solid residue IIIa was 53 wt % relative
to III.
M{Al[OSi(OEt)3]4}
III, IV
M = K (III), Na (IV).
Complexes III and IV are solid nonmelting sub-
1070-3632/02/7203-0394$27.00 2002 MAIK Nauka/Interperiodica

SYNTHESIS AND THERMOLYSIS OF TRIS(TRIETHOXYSILOXY)ALUMINUM
395
Amorphous powdered residues IIa and IIIa were
annealed in air at 1000 and 1250 C for 1 h, and the
resulting crystalline phases were identified by X-ray
phase analysis. After annealing of IIa at 1000 C, a
dark powder IIb was obtained. The Debye pattern of
IIb exhibits a set of well-defined reflections corre-
sponding to Al6Si2O13 (mullite). Residue IIIa, when
annealed at 1000 C, undergoes caking and remains
dark. No phases can be identified in its Debye pattern.
On annealing at a higher temperature, 1250 C, a col-
orless fused ceramics IIIb is obtained. In its Debye
pattern, three phases can be identified: mullite, alumi-
num orthosilicate Al2SiO5 (kyanite modification),
and potassium-containing feldspar aluminosilicate
KAlSi3O8 (microcline modification). The two latter
phases are detected by several most intense rings,
characteristic for each phase, in the complex diffrac-
tion pattern.
starting compound) is too low, and the analytical data
for V are also inconsistent with the composition of
Va. The number of KO groups in V determined by
acid titration was also only 80% of the amount calcu-
lated for Va. Apparently, the composition of V is
more complex, and at present we cannot identify this
product.
EXPERIMENTAL
We used absolute ethanol, dry solvents [7], and
freshly distilled tetraethoxysilane. All manipulations
with aluminum bromide (analytically pure grade) were
performed in a glove box with a dry atmosphere. The
compositions and yields of the gaseous and liquid
products of the thermal reactions were determined by
GLC on a Tsvet-104 chromatograph (thermal conduc-
tivity detector, carrier gas helium, steel columns 3 mm
in diameter). Gaseous products were determined at
50 C on a 2-m column packed with silica gel S-3.
Tetraethoxysilane and ethanol were determined at
70 C on a 2-m column packed with Chromaton-N +
15% Reoplex-400. Water and ethanol were separated
at 100 C on a 1-m column packed with Polysorb-1.
The IR spectra were taken on a Perkin Elmer-577
spectrometer (mineral oil, KBr). The X-ray phase
analysis of annealed powders was performed on an
IRIS-1 device equipped with a Debye camera; CuK
radiation. The crystalline phases were identified by
comparing the Debye patterns of the samples with the
reference data from the ASTM data-base. Analysis of
II IV and solid residues IIa and IIIa for C and H by
combustion in an oxygen flow was performed in the
presence of PbO.
The colorless cake IIIb obtained by direct thermo-
lysis of III with subsequent annealing of the residue
at 1200 1250 C shows a very strong adhesion to the
ceramic support. Presumably, with the sodium analog
IV the annealing temperature required for preparing
a ceramic material will be lower, because the melting
point of the sodium-containing feldspar aluminosili-
cate is lower. Thus, compounds II IV can be consid-
ered as possible precursors for one-stage preparation
of ceramic materials by thermolysis in air.
Alkali metal triethoxysilanolates used for synthe-
sizing II IV were prepared in solution of tetraethoxy-
silane taken in a slight excess, with subsequent vacu-
um evaporation of the solvent at room temperature
and drying of the residue in a vacuum at 35 40 C.
It should be noted that at temperatures higher by only
20 30 C potassium triethoxysilanolate decomposes
with release (moles per mole of the initial compound)
of tetraethoxysilane (0.23) and small amounts of
ethanol (0.06) and ethylene (0.01), and the powdered
residue V is insoluble in tetraethoxysilane. The IR
spectrum of the residue in the range of OEt absorption
undergoes significant changes: the bands at 1160 and
Potassium and sodium triethoxysilanolates. Tet-
raethoxysilane (4.5 ml) was added in portions with
vigorous stirring to 0.5 g of powdered KOH, and the
resulting mixture was heated at 35 40 C, avoiding
agglutination of the precipitate, until KOH dissolved
completely. Excess tetraethoxysilane and the released
ethanol were vacuum-evaporated to dryness, and the
solid residue was dried in a vacuum at 35 40 C to
constant weight. Potassium triethoxysilanolate, 1.97 g,
1
1
940 cm disappear, and a strong band at 760 cm
characteristic of potassium triethoxysilanolate is split
1
was obtained. IR spectrum, , cm : 1160, 930, 760
1
in two very weak broad bands at 760 and 700 cm .
(EtO); 1080, 470 (SiOC). Sodium triethoxysilanolate
was prepared similarly using powdered NaOH.
This means that the decomposition product V contains
no (EtO)3Si groups but can contain (EtO)2Si groups.
Its hydrolysis yields ethanol, and the balance with
respect to EtO groups between the initial compound
and decomposition products is 98%. It could be ex-
pected that thermolysis of potassium triethoxysilano-
late would be accompanied by disproportionation to
tetraethoxysilane and potassium diethoxysilanediolate
(EtO)2Si(OK)2 (Va). However, the amount of the
released tetraethoxysilane ( 1 mol per 4 mol of the
Thermolysis of potassium triethoxysilanolate.
A 1.57-g portion of potassium triethoxysilanolate was
heated in an evacuated system at 80 C for 3 h. The
volatile decomposition products were condensed in a
trap cooled with liquid nitrogen. Noncondensable
gases were virtually absent. The liquid phase (0.37 g)
consisted of tetraethoxysilane (92.0%), ethanol (5.3%),
and dissolved ethylene (2.7%). The residue was a
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 3 2002

396
SHCHERBAKOV et al.
colorless amorphous powder (1.2 g). IR spectrum, ,
cm : 1040, 460 (SiOC); 760, 700 (EtO). Found, %:
ampule was sealed, and the solution was heated at
70 C for 10 h, after which the solvent was vacuum-
evaporated at room temperature. Evacuation was con-
tinued until the residue became solid. Compound III
was obtained; yield 3.1 g (96%). The product charac-
teristics are identical to those of the sample prepared
by procedure a.
1
C 29.75; H 5.03. Calculated for potassuim diethoxy-
silanediolate, C4H10K2O4Si, %: C 21.04; H 4.41.
A 0.9-g portion of the solid residue was hydrolyzed in
an evacuated sealed glass ampule (1 ml) with acidified
water (pH 4) at 100 C for 1 h. In the hydrolysis
products, 0.47 g of ethanol was detected by GLC,
which corresponds to 0.62 g of ethanol in the whole
amount of the residue.
c. Similarly, from 2.18 g of tris(triethoxysiloxy)-
aluminum and 0.78 g of sodium triethoxysilanolate,
we prepared 2.89 g (98%) of Na{Al[OSi(OEt)3]4}(IV)
as a colorless solid. The product does not melt when
heated to 300 C. Its IR spectrum is similar to that of
III. Found, %: C 36.84; H 7.20; Al 3.58; Na 3.05; Si
14.92. C24H60AlNaO16Si4. Calculated, %: C 37.58;
H 7.88; Al 3.52; Na 3.00; Si 14.65.
Tris(triethoxysiloxy)aluminum (II). A solution of
1.25 g of AlBr3 in 5 ml of benzene was added in a
vacuum with vigorous stirring to a solution of sodium
triethoxysilanolate, prepared as described above from
0.56 g of NaOH, in 10 ml of benzene. After heat
evolution ceased, the ampule was sealed, and the mix-
ture was heated at 40 C for 30 min with intermittent
shaking. Then the mixture was centrifuged, and the
transparent benzene solution was separated by decant-
ing and vacuum-evaporated to leave 2.6 g (98%) of
compound II as a viscous liquid hydrolyzing in air.
Thermolysis of tris(triethoxysiloxy)aluminum
(II). A 2.0-g portion of tris(triethoxysiloxy)aluminum
was heated for 2 h in an evacuated system at 200 C.
The volatile decomposition products were condensed
in a trap cooled with liquid nitrogen. Ethylene (88 ml)
and a liquid fraction (0.52 g) were obtained; the latter
consisted of ethanol (43.2%), tetraethoxysilane
(43.1%), diethyl ether (13.6%), and water (2.0%). The
residue (1.38 g) was a dark powder containing 13.58%
C and 1.74% H. This substance was heated at 300 C
for 1 h. Ethylene (39 ml) and a liquid fraction (0.41 g)
were obtained; the latter consisted of ethanol (73.2%)
and water (26.8%). The residue (0.88 g, IIa) was a
black powder. Its annealing at 1000 C for 1 h gave a
dark powdered ceramics IIb; the weight loss was 7%
relative to IIa. Annealing of IIb at 1250 C for 1 h
does not result in sintering, decolorization, or weight
loss.
1
IR spectrum, , cm : 1100 1070 (AlOSi); 1150,
950, 780, 470 (EtO). Found, %: C 36.51; H 8.6; Al
4.80; Si 14.89. C18H45AlO12Si3. Calculated, %: C
38.28; H 8.03; Al 4.78; Si 14.92. The precipitate was
washed with benzene (3 5 ml) and dried in air; it
was identified as sodium bromide (1.42 g, 98%).
Potassium (III) and sodium (IV) tetrakis(tri-
ethoxysiloxy)aluminates. a. A solution of potassium
triethoxysilanolate, prepared from 1.07 g of KOH, in
20 ml of benzene was placed in a three-necked flask
equipped with a stirrer, a dropping funnel, and a re-
flux condenser. A solution of 1.28 g of AlBr3 in
10 ml of benzene was added dropwise over a period
of 15 min with vigorous stirring to the solution of
potassium triethoxysilanolate, and the mixture was
refluxed for 7 h. Then the mixture was placed in an
ampule and centrifuged to separate KBr, and the col-
orless benzene solution was decanted and vacuum-
evaporated; 3.32 g (88%) of K{Al[OSi(OEt)3]4} (III)
was obtained as a solid substance hydrolyzing in air.
The product does not melt when heated to 300 C.
Its IR spectrum is identical to that of II. Found, %:
C 37.50; H 7.70; Al 3.55; K 5.13; Si 14.80. C24H60
AlKO16Si4. Calculated, %: C 36.80; H 7.72; Al 3.44;
K 4.99; Si 14.35. The precipitate was washed with
benzene (4 5 ml) and dried in air; it was identified
as potassium bromide (1.65 g, 96%).
Thermolysis of potassium tetrakis(triethoxysil-
oxy)aluminate (III). A 1.90-g portion of III was
heated in an evacuated system at 300 C for 2 h. The
gas phase (119 ml) consisted of ethylene, and the
liquid phase (0.73 g), of tetraethoxysilane (58.8%),
ethanol (40.1%), and water (1.1%). The solid residue
(1.01 g, IIIa) was a black powder. Found, %: C 3.97;
H 1.10. Annealing of IIIa at 1000 C for 1 h gave a
weakly sintered black product; its repeated annealing
at 1250 C for 1 h gave a colorless ceramic cake IIIb.
The weight loss relative to IIIa was 7.4%.
ACKNOWLEDGMENTS
The authors are grateful to I.L. Vasilevskaya for
X-ray diffraction measurements.
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