Synthesis of high-purity calcium hydride

Article abstract of DOI:10.1023/B:RJAC.0000044107.80122.61

A procedure for synthesizing high-purity calcium hydride in high yield was suggested. The admixture composition of the resulting CaH₂ was determined by laser mass spectrometry.

Full text of DOI:10.1023/B:RJAC.0000044107.80122.61

Russian Journal of Applied Chemistry, Vol. 77, No. 6, 2004, pp. 875 877. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 6, 2004,
pp. 887 889.
Original Russian Text Copyright
2004 by Bulanov, Troshin, Balabanov.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Synthesis of High-Purity Calcium Hydride
A. D. Bulanov, O. Yu. Troshin, and V. V. Balabanov
Institute of Chemistry of High-Purity Substances, Russian Academy of Sciences, Nizhni Novgorod, Russia
Received December 4, 2003
Abstract A procedure for synthesizing high-purity calcium hydride in high yield was suggested.
The admixture composition of the resulting CaH2 was determined by laser mass spectrometry.
CaH is used in laboratory practice as a desiccant
decreases the yield of SiH , and, therefore, there is
2
4
that can take away water from crystal hydrates [1]. It
is highly efficient in drying of hydrocarbons, ethers,
and other solvents. Calcium hydride has found wide
application as a reducing agent in preparing powders
of certain metals (Ti, Zr, Nb, Ta, Mo, and W) from
their oxides and halides [2, 3]. A method is known for
converting silicon halide compounds (silicon tetra-
chloride, trichlorosilane, and silicon tetrafluoride) into
no point in using commercial-grade CaH . Moreover,
2
an admixture of metal calcium in CaH may result
2
in thermal reduction of SiF to give monoisotopic
4
elementary silicon [12]. Therefore, development of
a procedure for synthesizing active CaH in high yield
2
is a matter of current interest.
The reaction of Ca with H becomes noticeable at
2
o
298
1
1
70 250 C [10], Hf (CaH ) = 45.1 kcal mol
2
silane by their reaction with CaH [4]. In this case,
2
[13]. For practical use, temperatures in the range from
admixtures of F , H O, O , N , AsH , and PH are
2
2
2
2
3
3
2
50 to 700 C are recommended [14]. The hydrogena-
removed from SiH via their adsorption on a metal
4
tion of solid Ca to give a solid product is a topochem-
ical process. Kinetic studies [15] revealed three stages
hydride [5]. In [6], calcium hydride was used as
a reducing agent to obtain high-purity hydrides of
2
8
29
30
of the reaction: formation of centers of CaH crystal-
monoisotopic silicon ( SiH , SiH , and SiH4)
2
4
4
lization on the surface of a sample (induction period),
from monoisotopic SiF by the reaction
4
formation of a CaH surface layer precluding a direct
2
contact of H with Ca, and diffusion of H with the
SiF4 + 2CaH2
SiH4 + 2CaF2.
(1)
2
2
subsequent reaction at the Ca CaH interface (dis-
2
solution or formation of a chemical bond). The rate of
The choice of CaH was determined by the fact
2
Ca hydrogenation depends on temperature, H pres-
that, unlike NaF in the reaction with NaH, CaF does
2
2
sure, linear dimensions of a sample, internal structure
of the metal, and admixtures present in calcium and
hydrogen [15 17].
not react with monoisotopic SiF to give CaSiF in
4
6
synthesis of silane [7, 8]. Moreover, CaF reacts with
2
a BF admixture in silicon fluoride to give a stable
3
Ca(BF ) complex [4], which favors purification of
4
2
The admixtures of O and H O in hydrogen de-
crease the hydrogenation rate, because CaO formed on
2
2
the resulting SiH to remove boron-containing ad-
4
mixtures. At the same time, CaH is the safest in
2
the surface of calcium hinders the diffusion of H . It
2
handling, compared with other hydrides and metal
aluminum hydrides.
was shown in [18] that 0.5% oxygen decelerates the
reaction by a factor of 2.5, and 0.5% H O, by a factor
2
The compound CaH is a light gray crystalline
of 6.
2
substance that does not react with oxygen up to 400
5
00 C [9] and reacts with water to give CaO,
EXPERIMENTAL
Ca(OH) , and hydrogen [10]. In synthesizing SiH4
2
from SiF , the reactions of SiF with the products of
To obtain high-purity CaH , we used distilled Ca
4
4
2
CaH hydrolysis are thermodynamically allowed [11].
It has been established experimentally that presence of
several percent CaO in calcium hydride substantially
in the form of 1.5-mm-thick shavings [TU (Technical
Specifications) 95.768 80, manufactured by Mashino-
stroitel’nyi zavod Open Joint-Stock Company (Elekt-
2
1
070-4272/04/7706-0875 2004 MAIK Nauka/Interperiodica

876
BULANOV et al.
Results of analysis by laser mass-spectrometry
We synthesized CaH by the following procedure.
2
A stainless-steel flange reactor was charged with 350
Content, at. %
Element
400 g of Ca, connected it to the installation, evacuated
to 10 1 mm Hg, and heated to 360 C within 0.5 h.
According to [16], melted Ca in the form of shavings
Ca
CaH2
starts to react with H at 400 C; hence, we hydro-
genated Ca in the temperature range 400 450 C. We
2
6
2
5
3
3
5
3
<1 10 4
B
C
P
<8 10
<3 10
<1 10
2
8
1
1
1
4
2
2
5
7
3
10
10
10
10
10
10
10
10
10
10
filled the reactor with purified H to a pressure of
2
4
3
3
5
4
2
4
4
3
4
3
4
5 atm and kept it for about 5 min. During this time,
F
7
1
5
2
10
10
10
the pressure of H in the reactor decreased first grad-
2
Mg
Al
Si
Cl
Fe
Mn
Cu
Zn
Sr
S
ually and then abruptly to 0.05 0.1 atm. The increase
in the rate of H absorption is accounted for by the
2
10
high rate of the hydrogenation reaction on the surface
of Ca. In this stage of hydrogenation, we controlled
0.12
10
10
10
10
3
3
4
4
2
3
1
1
6
2
3
1
the flow of H into the reactor in such a manner that
2
the pressure of H in the reactor did not exceed 1 atm,
2
since an elevated pressure of H results in a rise in
2
<1 10
temperature.
10
10
5
3
10
10
A rise in temperature may lead to caking of CaH2,
which impedes its withdrawal from the reactor. After
a layer of CaH is formed on the surface of Ca, dif-
2
fusion of H to the Ca CaH interface begins, and
2
2
rostal’)] and H purified by diffusion across a pal-
2
the rate of H absorption decreases. To complete
2
ladium membrane. The content of admixtures in cal-
cium, as determined by laser mass spectrometry, is
listed in the table.
the hydrogenation, we raised the H2 pressure in
the reactor to 10 atm, and the temperature to 480 C.
We judged the completion of the hydrogenation reac-
The content of O and H O admixtures in purified
tion from the cessation of the H absorption.
2
2
2
4
hydrogen does not exceed 10 mol %.
The yield of CaH was calculated as the ratio of
2
We synthesized CaH on an installation shown
2
the weight of CaH obtained in the experiment to its
2
schematically in the figure. The main units of the in-
theoretical value. The yield of CaH was 99.6 0.1%,
2
stallation are a cylinder with H 1, a H purification
2
2
i.e., the synthesis procedure results in an almost com-
plete hydrogenation of Ca. The output capacity of
unit 2, and a flange reactor 3. We controlled the pres-
sure of H during the process with vacuum gages 4
2
the pilot installation for CaH synthesis is 150 g of
2
and 5. The temperature of the resistance furnace 6
was set with an R-133 temperature control unit and
monitored with a Chromel Copel thermocouple 7.
CaH per hour.
2
The resulting CaH was analyzed by laser mass
2
spectrometry for the content of admixtures. The re-
sults obtained are listed in the table. It can be seen
that the main admixtures in CaH are C and Cl,
2
whereas the content of metal admixtures is at the level
of 10 3 at.%.
After the process was complete, we placed the
cooled reactor in a box purged with dried nitrogen
and then ground the CaH obtained to powder with
2
particle size of less than 0.6 mm. The powdered
CaH was charged into the reactor in a flow of puri-
2
fied H in order to obtain silane.
2
An effective procedure for removal of carbon from
metals [19] is their washing with purified H2.
The admixtures of hydrocarbons can be formed in
Schematic of the installation for synthesis of CaH .
the reaction of H with admixtures of metal carbides
present in calcium:
2
2
(
1 7) For explanations, see text; (8) receiver.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 6 2004

SYNTHESIS OF HIGH-PURITY CALCIUM HYDRIDE
M C (solid) + 2mH = nM(solid) + mCH (gas). (2) REFERENCES
877
n
m
2
4
In [20], CaH was obtained in the reaction of
1. Hurd, D.T., An Introduction to the Chemistry of Hy-
drides, New York: Wiley, 1952.
2. Meerson, G.A. and Kolchin, O.P., Atomnaya Ener-
giya, 1957, vol. 2, no. 3, pp. 253 259.
2
calcium carbide with H at 100 1000oC under an
2
elevated pressure (up to 100 atm):
CaC2 + 5H2
CaH2 + 2CH4.
(3)
3. Bichara, M., Ann. Chim., 1956, vol. 1, p. 399.
4
. RF Patent 2077483.
Therefore, side reactions of hydrogenation of car-
bon and its compounds to give hydrocarbons can
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6
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2
In this study, we blew CaH with a flow of purified
2
7
8
9
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1
H at 250 C for 8 h in order to diminish the content of
2
carbon admixtures. It was shown that, after contact
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2
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allowed us to diminish the content of the most dif-
ficultly removable admixture of ethylene in silane by
an order of magnitude.
1
1
1
1. Opalovsky, A.A., Labkov, E.U., Torosyan, S.S., and
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CONCLUSIONS
1
1
2. US Patent 4446120.
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dov (Chemistry of Hydrides), Leningrad: Khimiya,
(1) High-purity calcium hydride was obtained by
hydrogenation of distilled metallic calcium with
purified hydrogen.
1
969.
1
4. Johnson, W.C., Stubbs, M.F., and Pechukas, A., J. Am.
Chem. Soc., 1939, vol. 61, no. 2, p. 318.
(
2) The main admixtures in CaH are C and Cl,
2
whereas the content of metal admixtures is at the
15. Krestovnikov, E.P., Trudy UNIKHIM, 1958, no. 5,
level of 10 3 at.%. The procedure of washing of
pp. 118 135.
CaH was used to decrease the content of carbon
admixture in it.
16. Efremkin, V.V. and Fefelova, G.F., Trudy UNIKHIM,
2
1958, no. 5, pp. 136 151.
1
7. Teplenko, V.G., Kudinova, K.G., and Shikhano-
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ACKNOWLEDGMENTS
The study was financially supported by the 6th
Competition-examination of scientific projects of
young scientists of the Russian Academy of Sciences,
in the field of fundamental and applied research (grant
no. 165).
18. US Patent 2702740.
19. Devyatykh, G.G., Yushin, A.S., and Osipova, L.I.,
Metally osoboi chistoty (Special-Purity Metals), Mos-
cow: Nauka, 1976, pp. 105 113.
20. Jpn. Patent Application 61-178401.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 6 2004