metallurgical silicon and low alcohols. As a result
, the reactions were carried out,
at best, in a continuous
-
cyclic mode for low reaction rates, and the process
consumes a lot of energy per unit of product.
As known, the induction period of the direct reaction of
metallurgical silicon
with alcohol can be
from several hours to tens of hours. The
main cause of the
induction period is an oxide film on the silicon surface due to rapid oxidation of
silicon with atmospheric oxygen. In order to reduce the induction period duration
an additional step of reaction medium activation was proposed to be introduced
into the alkoxysilane synthesis process [Pat.US 5783720] where the activation is
made at temperatures up to 400° C and hydrogen and nitrogen are proposed to be
used as an activating agent. The authors state th
at the activation process in practice
takes much time from 5 to 10 hours, which affects the technology efficiency. To
shorten the induction period, silicon in form of powder was recommended to be
preliminary treated with hydrofluoric acid to remove an oxid
e film from the
surface of particles [Pat. JP511692, Pat. US5177234]. These solutions lead to
additional complications because after treatment with hydrofluoric acid
the silicon
powder must be cleaned and dried in an inert atmosphere, which dramatically
complicates the synthesis process.
Some papers
proposed to make the activation of
the reaction mass by holding it at higher temperature in the atmosphere of nitrogen,
argon and others [US5177234, US4727173] and to preliminarily mix silicon with a
catalyst in
the inert atmosphere for 8 hours [US4487949]. To activate silicon
before synthesis, haloids:
chlorine
alkily, hydrogen chlorides, ammonium chlorides
[US5177234] or NH4HF
2
[ER517398] were proposed to be introduced.
It should be noted that introduction of substances such as haloids or
alkylhaloids into the reactor before synthesis, there is an additional operation of
cleaning of the target product from these impurities, for example by distillation,
which reduces productivity and complicates the technology of
alkoxysilane
production. Thus, the analysis shows that there is no
clear understanding of the
causes and nature of the induction period during the direct synthesis of
alkoxysilanes, especially when there are no effective solutions of this problem. The
prop
osal of additional reagents to be used in synthesis naturally requires their
removal from the final product, but i
t leads to additional operations in the
manufacturing process, which complicates the technology of alkoxysilane
production and inevitably increases the cost of the final product.
We have proposed to exclude
the causes of this problem
–
no oxide film
formation on the surface of just ground particles of silicon during the milling
process.
There is also a problem of moisture. The point is that the
silicon powder
is very hygroscopic and intensively absorbs moisture from the environment. In the
reaction this moisture behaves, on the one hand, as a factor blocking the start of the
target reaction and, on the other one, causes undesirable side reactions. Thus, the
preparation of silicon in a protective environment,
in our case in a solvent,
eliminates not only from oxide film formation, but also from excess of moisture in
the reaction medium.
It is also known that besides the main reactions of alkoxysil
ane synthesis in
the reactor there are side reactions leading to formation of oligoalkoxysiloxane,
103
water and other by-products that are gradually accumulated in the reaction medium
and reduces the process rate [US5783720, JP511692, US6090965, US4931578].
Many of these side reactions are catalyzed by metals, most of which is present in
initial silicon as impurities. The used silicon mass comprised by initial silicon,
impurities and alkoxyisiloxane is accumulated in the reactor,
which reduces the
reaction rat
e. These processes make it necessary to regenerate the solvent for
further use in the synthesis of alkoxysilanes.
Thus, a key challenge in the practical implementation of the alkoxysilane
process is to develop a method allowing the synthesis reaction with high
conversion in silicon and alcohol, regulation of a
composition of the reaction
products and multiple recovery of solvent and its reuse. This problem can be
solved by eliminating or minimizing the induction period of the synthesis reaction,
activating
the reaction medium, simplifying the technology and running the
reaction of alkoxysilane synthesis in a continuous mode.
As a result, a new technology of alkoxysilane synthesis
by direct reaction of
metallurgical
silicon and low alcohols (methanol or ethanol) has been developed.
The physical basis of the proposed technical solutions of how to activate the
reagents is that the milling of raw silicon is performed in the environment of a high
boiling solvent, not in the air, which makes it possible to avoid
a layer of natural
oxide SiOx inevitably formed on the surface of just ground particles of
metallurgical
silicon during their contact with atmospheric oxygen. This reaction
runs at any temperature, including room one, and is independent of chemical purity
of silicon. A scheme of the alkoxysilane synthesis process is presented in Fig. 1.1.
and
General view of the experimental setup for the implementation of the process
of synthesis of monosilane
is presented in Fig. 1.
2.
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